Radiation Chemistry and Its Applications

TECHNICAL REPORTS SERIES No. 84

Radiation Chemistry and its Applications

?J INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1968

RADIATION CHEMISTRY AND ITS APPLICATIONS The following States are Members of the International Atomic Energy Agency:

AFGHANISTAN GERMANY, FEDERAL NORWAY ALBANIA REPUBLIC OF PAKISTAN ALGERIA GHANA PANAMA ARGENTINA GREECE PARAGUAY AUSTRALIA GUATEMALA PERU AUSTRIA HAITI PHILIPPINES BELGIUM HOLY SEE POLAND BOLIVIA HUNGARY PORTUGAL BRAZIL ICELAND ROMANIA BULGARIA INDIA SAUDI ARABIA BURMA INDONESIA SENEGAL BYELORUSSIAN SOVIET IRAN SIERRA LEONE SOCIALIST REPUBLIC IRAQ SINGAPORE CAMBODIA ISRAEL SOUTH AFRICA CAMEROON ITALY SPAIN CANADA IVORY COAST SUDAN CEYLON JAMAICA SWEDEN CHILE JAPAN SWITZERLAND CHINA JORDAN SYRIAN ARAB REPUBLIC COLOMBIA KENYA THAILAND CONGO, DEMOCRATIC KOREA, REPUBLIC OF TUNISIA REPUBLIC OF KUWAIT TURKEY COSTA RICA LEBANON UGANDA CUBA LIBERIA UKRAINIAN SOVIET SOCIALIST CYPRUS LIBYA REPUBLIC CZECHOSLOVAK SOCIALIST LUXEMBOURG UNION OF SOVIET SOCIALIST REPUBLIC MADAGASCAR REPUBLICS DENMARK MALI UNITED ARAB REPUBLIC DOMINICAN REPUBLIC MEXICO UNITED KINGDOM OF GREAT ECUADOR MONACO BRITAIN AND NORTHERN EL SALVADOR MOROCCO IRELAND ETHIOPIA NETHERLANDS UNITED STATES OF AMERICA FINLAND NEW ZEALAND URUGUAY FRANCE NICARAGUA VENEZUELA GABON NIGERIA VIET-NAM YUGOSLAVIA

The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is "to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world".

Printed by the IAEA in Austria April 1968 TECHNICAL REPORTS SERIES No. 84

RADIATION CHEMISTRY AND ITS APPLICATIONS

REPORT OF A PANEL ON RADIATION CHEMISTRY: RECENT DEVELOPMENT AND REVIEW OF RANGE OF APPLICABILITY OF EXISTING SOURCES, HELD IN VIENNA, 17-21 APRIL 1967

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1968 RADIATION CHEMISTRY AND ITS APPLICATIONS (Technical Reports Series, No. 84)

ABSTRACT. The report of a panel convened by the IAEA and held in Vienna, 17-21 April 1967. The meeting was attended by 15 specialists from ten countries. Contents: Introduction: Summary of discussions; General observations and recommendations; Reports; Communications and comments. The ten reports cover such subjects as application of ionizing radiation to polymer chemistry, the status of chemonuclear reactors and radiation chemical processing, synthesis and decomposition induced by ionizing radiation, and the use of low-energy electron accelerators for the curing of paints and thin films. Each report is in its original language (9 English and 1 French) and is preceded by an abstract in English with a second one in the original language if this is not English. The rest of the publication is in English.

(182 pp., 16 X 24 cm, paper-bound, 23 figures) (1968) Price: US S4.00; £1.13.4

RADIATION CHEMISTRY AND ITS APPLICATIONS IAEA, VIENNA, 1968 STI/DOC/ 10/84 FOREWORD

In recent years considerable progress has been made in understanding the fundamental chemical reactions that occur when materials are irradiated. This has followed from the development of new techniques for studying these reactions. There have also been significant advances in source technology and in the design of accelerators for carrying out irradiations. Parallel to these developments there has been an increasing interest in the industrial application of chemical effects of radiation, particularly in work on polymers. The International Atomic Energy Agency held a Panel on Radiation Chemistry in Vienna on 17 - 21 April 1967 to review the current status of various sources, new techniques in radiation chemistry, and their applications. Fifteen specialists attended from 10 countries. The main sources mentioned by the Panel were isotope sources, electron accelerators, and chemonuclear reactors. Among the basic techniques discussed were pulsed radiolysis, flash photolysis, fast ESR methods, irradiation at liquid helium temperatures, electric discharge methods and far ultra-violet methods. Interesting industrial applications were discussed, such as the development of wood-plastic combinations, and a paper was given on the curing of paints and thin films. Summaries of the presentations and discussions, together with the recommendations and the written contributions, are given in this report. It is hoped that the publication will be useful to radiation chemists and to those responsible for establishing and operating programmes based on radiation chemical processes.

CONTENTS

I. INTRODUCTION 1 II. SUMMARY OF DISCUSSIONS 1 III. GENERAL OBSERVATIONS AND RECOMMENDATIONS 17 IV. REPORTS 23 Application of ionizing radiation to polymer chemistry (PL- 236/1) 23 A . Danno Characteristics and range of applicability of accelerators for radiation chemical processes (PL-236/10) 43 N. W. Holm Status report on chemonuclear reactors and radiation chemical processing (PL-236/4) : 53 M. Steinberg Safety for large irradiation facilities (PL-236/2) 63 A. Danno Low-temperature irradiations (PL-236/5) 67 Z. P. Zagorski and S. Mine Pulse radiolysis (PL-236/6) 89 D. F. Sangster Le développement de la radiochimie: quelques aspects de la conjoncture (PL-236/8) 93 P. Lévêque et J.R. Puig Synthesis and decomposition induced by ionizing radiation (PL-236/7) 97 Silvia Ionescu Radiation-induced polymerization: mechanisms and industrial aspects (PL-236/9) 125 D. O. Hummel, Christel Schneider, R. C. Potter, G. Ley, J. Denaxas, D. Widdershoven and M. Ryska The use of low-energy electron accelerators for the curing of paints and thin films (PL-236/3) 165 F.L. Dalton V. COMMUNICATIONS AND COMMENTS 171 List of participants 181

I. INTRODUCTION

The use of radiation in industry has increased greatly during the last few years. Of importance has been the use of radiation to modify polymers giving cross-linked and shrinkable plastic film and tubing. It has also been demonstrated that chemicals can be manufactured by radiation chemical processes. More recently radiation grafting of polymers on to various substrates and the manufacture of copolymers have both been under investigation. In these cases, to obtain optimum conditions, research is directed to maximize yields. In any atomic energy programme there will be many examples of materials exposed to ionizing radiation. Reactor moderator, coolant, chemical reprocessing solutions and radioisotopes may be instanced. In these cases it is necessary to minimize the deleterious effects of radiation. Fundamental investigations in the field of radiation chemistry have been directed towards elucidating the reaction mechanisms obtained in systems subjected to radiation, and measurements of the reaction parameters. This has led to a better understanding of chemical reactions, in general, as well as of those particularly associated with radiolysis. The field covered by the Panel was broad in its scope ranging from fundamental studies through research and development of processes of potential industrial importance to commercial enterprises. Because of this wide scope, this report cannot claim to have covered each area exhaustively. Rather, an attempt has been made to define the field and to put each part into perspective in relation to each other part. At the same time it has been possible on some subjects to comment in some detail where the expert knowledge of Panel members made this possible. The report of the Panel comprises four parts: summary of presentations and discussions on each topic discussed at the Panel; general observations that reflect the view of the participants on each topic, and recommendations regarding the Agency1 s activities; the ten written contributions to the Panel; finally, comments on various selected topics.

II. SUMMARY OF DISCUSSIONS

1. Sources

1.1. Various existing sources

Steinberg (United States of America) reviewed chemonuclear fission- fragment energy sources1. Uranium containing glass fibre and U02-coated platinum sources has the disadvantages of low melting point and radiation damage. Uranium-palladium solid solutions have been developed to the point where reliable fission-fragment radiation chemistry experiments

1 STEINBERG, M., "Status report on chemonuclear reactors and radiation chemical processing", this Report, PL-236/4.

1 2 DISCUSSIONS can be performed. Platinum-coated U-Pd improves the resistance to radiation damage and should prevent spallation of uranium atoms. Nitrous oxide and carbon dioxide gas dosimetry allows the determination of fission- fragment energy deposition efficiency which is in conformity with analytical range-geometrical calculations. The 0.1 mil (2.5 /urn) foil has been fabricated into honeycomb arrays for insertion into the in-pile chemonuclear loop. Brookhaven is continuing to develop bonded coextruded 60Co sources. 137Cs sources of caesium chloride in stainless-steel envelopes are being tested. 90Sr sources of microspheres in stainless-steel needles are being used in small-scale experiments. Larger multicurie sources of 90Sr which would have high integrity have yet to be developed. 137Cs has the advantage of long half-life and availability, but large-scale application depends primarily on the development of a high integrity source. Holm (Denmark) reviewed his work on d. c. and linear accelerators2. The main points included increased technological development of machine radiation sources based on continued development of accelerators for research purposes. The economics of the various accelerators, beam characteristics, penetration characteristics,, routine process control and energy conversion efficiencies of electron beam to X-ray radiation were stressed. He especially pointed out the benefits of PVC film dosimetry and a special calorimeter consisting of a thermistor buried in styrofoam (foamed polystyrene) inserted in a petri dish filled with water. In the discussion the importance was pointed out of current density and power in the radiation intensity factor available from electron machine radiation sources. Hummel (Federal Republic of Germany) reviewed the radiation sources and pilot plant facilities being designed and developed at the University of Cologne? Some of the main design features include a coronoid source with an activity of not more than 105 Ci, a variable but uniform dose distribution, a reaction volume of not more than 100 litres, facilities for transport of heavy equipment and installation of safety devices. Several designs submitted by different manufactures were shown. Costs of US $125 000 to US $200 000 for typical facilities were mentioned. Mrs. Ionescu (Romania) reviewed the alpha radiation source facility developed in her laboratory4. The (n, a) reaction on lithium is used. Foils of Li inserted in a reaction vessel of about 1 litre capacity are placed inside nuclear reactors with a maximum flux of 1013 n/cm2 s. Gases are passed over this foil and the products analysed. When methane gas was passed over the irradiated lithium foil, a decomposition of 40% of the methane was experienced. This should be compared with a conversion of 3% when no lithium was present, and seems to indicate a LET effect. Danno (Japan) reviewed the design features and development of large- scale 60Co sources5 . Some of the characteristics of machine and rod,

2 HOLM, N. W., "Characteristics and range of applicability of accelerators for radiation-chemical processes", this Report, PL-236/10.

3 HUMMEL, D. O., SCHNEIDER, C., POTTER, R. C., LEY, G., DENAXAS, J., WIDDERSHOVEN, D., RYSKA, M., "Radiation-induced polymerization : mechanisms and industrial aspects", this Report, PL-236/9.

4 IONESCU, S., "Synthesis and decomposition induced by ionizing radiation", this Report, PL-236/7.

5 DANNO, A., "Application of ionizing radiation to polymer chemistry", this Report, PL-236/1. 3 DISCUSSIONS plate, cylindrical and honeycomb radioisotope type sources were mentioned. A major design consideration is heat generation from the intensity of the source and the exothermic heat of polymerization of monomer. In the case of accelerators, the beam is used in a scanning mode. Thus power only is an indication of output, not energy dissipation rate. Danno pointed out that if it were possible to increase the dose-rate of gamma sources to 10 Mrad/h, processing of all polymeric systems would be made possible with radioisotope sources. 85Kr is being studied as a possible homogeneous radiation source for liquid- and gas-phase reactions. Conta- mination of samples and leakage into facilities are two important problems encountered in using 85Kr. Puig (France) reviewed the main radiation sources and facilities operating in France6. A total of 700 000 Ci of 60Co are being used in industry, government and university laboratories and plants, and the mobile irradiator IRMA uses 400 000 Ci 137Cs. A linear electron beam accelerator (7 MeV-10 kW) is operated at the SRTI irradiation plant. A panoramic- type facility at Saclay provides extremely flexible arrangements for experimental irradiation. The Nuclear Centre at Grenoble is being used to study the technology of irradiation both on an experimental and a theoretical basis. The commercial facilities are used primarily for sterilization. Dalton (United Kingdom) listed the industrial and development sources in the United Kingdom7. These include 60Co sources of Johnson, Gillette, Ethicon, Swan-Moreton and an accelerator of Smith and Nephew. Two large pilot-scale plants using 60Co and a 4-MeV accelerator are available in Wantage, and at Harwell there is a spent fuel assembly using elements from the DIDO reactor. A 5-15 MeV accelerator and a 300-kV, 100-mA accelerator are under construction at Wantage. It was pointed out that during pilot and development work each process must be treated in a specific manner. Wantage has tended to use large concrete-shielded caves with simple radiation sources and build the necessary equipment round the source even if this involves a low efficiency of source utilization. Sangster (Australia) mentioned that in addition to the Westminster Carpets Source which is used for sterilization of goat hair and pharmaceuticals and some research work in Melbourne, a spent fuel pond is available at Lucas Heights. A large range of dose-rates is possible; however, the fuel element facility is not a commercial proposition because the need to reshuffle fuel elements requires repeated dosimetry measurements. Getoff (Austria) mentioned the use of radon as an alpha source and some interesting enhancing effects of LET in organic acid reaction. He stressed the application of X-ray and vacuum u. v. sources for research work, pilot-plant experiments and pulsed radiolysis. Silverman (United States of America) mentioned two types of machine sources. One is the variable-energy positively charged particle accelerator for studying LET effects in solid and liquid systems, and the other is the high-intensity electron beam accelerator that is being increasingly

6 LEVEQUE, P., PUIG, J. R., "Le developpement de la radiochimie : quelques aspects de la conjoncture", this Report, PL-236/8.

' DALTON, F. L., "The use of low-energy electron accelerators for the curing of paints and thin films", this Report, PL-236/3. 4 DISCUSSIONS applied in radiation chemical systems. The latter can be relatively low- energy electron machines. Relatively low-cost electron gun accelerators are becoming available, borrowing technology from electron evaporator, electron-beam-welding and TV-tube-manufacturing techniques. LET and high-energy basic radiation chemistry require much further study for elucidating effects previously noted.

1. 2. Other remarks

Fission-chemonuclear processes look economically most attractive on the basis of US $/kWh of radiation energy but pose large problems in, for example, contamination. Research is still in the early stages so that costs cannot be realistic. The question is whether countries with reactors can carry out capsule experiments to advance the necessary research and development work. Along these lines Steinberg mentioned that the Brookhaven chemonuclear in-pile research loop, which is a highly versatile, unique research facility, could possibly be made available for an international effort where scientists from all parts of the world could go to Brookhaven to experiment with this facility. The criterion of a radiation process must be the cost of product in US $/kg and what it can be sold for. There is no clear indication of what source should be used unless process in considered as a whole and specifically. In the case of low-energy accelerators window technology must be advanced. Considerable discussion took place on the use of 137Cs as a gamma source. Hummel mentioned the drawback of the softer gamma regarding penetration and cost. Steinberg pointed out the advantage of half-life and low maintenance cost for a fractional power intensity dependence of rate. A source of high integrity must be developed. Dalton and Silverman quoted costs of American 137Cs ranging from 10 to 15 £/Ci of 137Cs unencapsulated. Puig mentioned a cost of 1.2 F. Fr. /Ci encapsulated for French material. The dual-purpose electrochemonuclear system using power for electrolytic cells was mentioned by Steinberg as nominally competitive with chemonuclear reactors. The Brookhaven chemonuclear in-pile loop facility and the Japanese JAERI chemonuclear loop facility now under construction may be considered unique, and IAEA could act as a clearing house for the possible use of these facilities by various research and development groups. Singer (Denmark) spoke of the possibility of using mixed pile radiation for wood-plastic research and development similar to what is being done by Lockheed-Georgia in the United States of America. Steinberg mentioned that 252Cf spontaneous fission sources that are becoming more readily available in the United States of America can be a useful tool for investigating high LET radiation chemistry. A question was raised concerning the use and status of liquid metal circulating loops in reactors as high-intensity short-lived gamma irradiators, for example, the gamma-emitting indium-gallium loops, as mentioned by the Russians in Salzburg in 1963 at the Conference on the Application of Large Radiation Sources in Industry, but no reply was forthcoming. 5 DISCUSSIONS

2. Dosimetry

2.1. Electron and gamma dosimetry

A substantial part of the papers and discussions was devoted to the dosimetry of gammas and fast electrons. Holm (Denmark) gave an out- line of the PVC film dosimeter used at Ris^ for fast electron dosimetry, and explained how such a film system could be calibrated by means of a cheap and simple water calorimeter. The accuracy of the calorimeter was reported to be ± 1-2%. The accuracy of the PVC film used was about ± 3-5% for a calibrated sheet and ± 10% when the batch only was calibrated. The PVC used was a 0.25-mm "GENOTHERM" film from Kalle, Federal Republic of Germany. Puig (France) reviewed the methods used for electron dosimetry in France. At Saclay an aluminium calorimeter and at SRTI a water-flow calorimeter for total power measurement are being used. For routine use at the CIRCE accelerator, PVC was used after calibration against an aluminium-plate calorimeter. Corrections had to be made for back-scattering and bremsstrahlung production. The reproducibility of the PVC dosimeter was reported to be poor and there was an indication of an energy-dependence. Puig stated that in general one should always attempt to use dosimeters of the same atomic composition as that of the materials to be irradiated, and that physical conditions should be carefully simulated when dosimetry surveys were made. Silverman (United States of America) pointed out that very little was known concerning absorption of electrons in complex targets. Some work had been done at Ris^ [1] and in Japan [2], and some calculations had been done by Berger[3] and Spencer [4], Silverman had recently made some such calculations and presented them at meetings of the American Physical Society and in Japan [5], It was stressed that more efforts should be devoted to this subject. Holm remarked that some more information on dose distributions was given at the International Conference on Radiation Research, Natick, Mass. , United States of America, in January 1963. A discussion followed on the behaviour of PVC and other film systems, particularly with regard to their response to gamma and electron radiation. It was concluded that these systems had to be calibrated under the conditions where they were to be used, and that inconveniences such as heat treatment (PVC) or humidity control (Red Perspex) had to be tolerated. Much more, however, should be known about the reaction mechanisms of these systems. For example, for PVC the measured effect was most probably due to impurities and this explained why different brands differed in their reproducibility and sensitivity. It was generally agreed that several systems, such as PVC, Blue Cellophane, Red Perspex and Clear Perspex were equally usable. When dealing with thin coatings or films, calculations could support or be a substitute for the measurements. Sangster (Australia) expressed the need for reliable chemical dosimeters usable at high doses. Holm described a number of dosimeters useable beyond the upper limit of the Fricke dosimeter, such as oxalic acid, ferrous-cupric-sulphate, de-aerated ferrous sulphate, McLaughlin's Dye system, and the water dosimeter developed at Argonne. The latter should be especially well suited to high dose-rates. It was emphasized that dosimeters in general need not be "simple and easy to operate" but they should be accurate. There should always be time to deal in depth 6 DISCUSSIONS with this important problem by doing accurate measurements and then substituting the routine dosimetry with a good process control. The discussion was further developed on the Fe-Cu system, and Silverman and Dalton (United Kingdom) reported very satisfactory results with this system. Holm mentioned that they used the system as described by Hart in 1954 but redetermined the G-value to be 0.70 [6]. Unless freshly prepared solutions were made every day, it was recommended to pre-irradiate the stock solution to 300 krad, thus stabilizing the solution for several days.

.2.2. In-pile dosimetry

In-pile dosimetry was covered in a presentation given by Puig. In France, three different types of calorimeters have been developed for dosimetry in high dose-rate fields. The isothermal calorimeters are operated submerged in water pools. The use of the conduction-type calorimeters does nor rely on the knowledge of the specific heat of the calori- metric bodies. They give a continuous indication of the instantaneous dose-rate, and they are absolute (in the sense of built-in calibration facilities), fast and sensitive.

2.3. Fission-product particle dosimetry

The dosimetry of fission-product particle radiation, as used in reactor loops to irradiate gases, was dealt with in the paper given by Steinberg (United States of America). He used the N20-system, as described in the literature [7], measuring by gas chromatography the N2 produced. The upper temperature limit of that system is 150°C; there seem to be neither LET nor surface effects, as shown by the good agreement with calculated dose values. A small contribution from (n, p) reactions may be suppressed (according to a recent publication by Harteck and Dondes) by using 15N20[8], This is, however, a rather costly procedure.

2. 4. General remarks

Some contributions were made concerning co-operation and exchange of information on dosimetry. Hara (IAEA) described the activities of the IAEA, which had sponsored three symposia and three panels on this subject. A handbook on fluence measurements and one on absorbed dose measurements are being prepared under the auspices of the IAEA, and a Panel held in 1966 recommended that the Agency should sponsor an intercompari- son programme for in-pile calorimeters and should set up a working group on dosimetry. Steinberg reported on the BNL dosimetry workshop and the activities in the United States of America with regard to procuring standard methods. He recommended the IAEA to exchange information with organizations like NBS, ASTM and the BNL workshop mentioned above. Sangster mentioned that the techniques used by hospital radio- logists in measuring doses accurately might be helpful to radiation chemists. The reports issued by the ICRU should be looked into. Puig recommended that the physics of absorption of high-energy electrons should be studied more extensively, and that the chemistry of thin film dosimeters should be looked at more systematically with special reference to dye-stuffs. 7 DISCUSSIONS

Co-operation between groups working on dosimetry was also recommended. Holm observed that several groups within the IAEA already dealt with dosimetry, and that it should not be the task of the Panel to stimulate the formation of another such group but rather to bring to the attention of the existing groups problems that were of particular interest to this Panel.

3. Fundamentals

3.1. Pulsed radiolysis

Sangster (Australia) described the value of pulsed radiolysis in the direct observation of the kinetics of species produced shortly after the absorption of ionizing radiation8. While most of the work reported to date is in the microsecond region, some results have already been obtained in the nanosecond region at Argonne National Laboratory. Much of the research effort has been in aqueous systems; however, there is a significant body of information on organic liquids and some work in gaseous systems. The results not only yield the decay times of transient species produced shortly after the radiation event but also, in the case of nanosecond studies, may show the existence of spurs. Discussion revealed that work on ice systems has been published by Soviet scientists. Similar studies are in progress in Denmark. Sangster added information regarding work in the gas phase on NH3 and on CO2. Imthe latter system, evidence of the carbon atom has been observed. Silverman (United States of America) remarked that pulsed radiolysis can be used to observe prompt production of unusual stable species (as opposed to transient ones) such as diene formation in alkanes; the prompt production of this species would involve the almost simultaneous removal of four hydrogen atoms. Hummel (Federal Republic of Germany) referred to studies on styrene and styrene-water systems performed by Schneider and Swallow. In addition to the free radical species reported by them earlier, they now find-transients they attribute to radical anions. They found using repetition pulsing (50 p/s) that some of the polymerizing radicals can survive 102-103 pulses. Also the chain length of the polymers produced in the microsecond pulses is far greater than one can account for by free radical kinetics. Dalton (United Kingdom) made the same observation regarding chain length in studies at Wantage of the polymerization of styrene (and other monomers) by means of a pulsing linear accelerator. Silverman obtained similar results at Ris^. It was observed that other methods and instruments can be used for the detection and measurements of short-lived intermediates.

3.2. Low temperature techniques

Zagorski and Mine (Poland) presented a comprehensive review of low-temperature radiation chemistry9, and summarized the present situation in radiation chemistry. Stressing the present achievements in different branches of radiation chemistry, it was also pointed out that

8 SANGSTER, D.F., "Pulse radiolysis", this Report, PL-236/6. 9 ZAGORSKI, Z.P., MINC, S., "Low-temperature irradiations", this Report, PL-236/5. 8 DISCUSSIONS both the technique of low-temperature irradiations and their interpretation are in need of supplementary investigations. The most important are all measurements characterizing the frozen sample: densities, solubilities of gases, homogeneity even of one-component systems, crystal size, structure of glasses, presence, identity and concentration of defects of different kinds. The additional measurements are in many instances more complicated than the radiation-chemical part of the investigation. Nevertheless, these measurements have to be done to justify the conclusions drawn, and must be integrated into the whole picture of radiation-induced phenomena in condensed phases. One can consider low-temperature radiation chemistry as the source of applications for the rather distant future. One of the negative factors is of an economic nature: bringing the sample into low temperature means an additional cost. This situation may not be as disadvantageous if the processing demands the application of low temperature, for instance in some methods of food irradiation. Another negative factor is connected with diminution of radiation yields due to the enhanced emission of light in the course of irradiation and thawing. Nevertheless, there is justified optimism as to the possibility of obtaining quite unusual polymers of very high value and specific properties in the process of low-temperature irradiation. Hummel (Federal Republic of Germany) pointed out the advantage of performing cationic polymerizations in such systems, especially in commercially attractive alternating copolymers (see Footnote 3). This sparked off considerable discussion of the potential practical value of low- temperature processes. The Polish contribution stressed the large energy losses in frozen systems due to luminescence and also pointed out the cost of freezing. Hummel mentioned that energy losses in long chain polymerizations are not a serious cost element in the ionic copolymerizations of potential practical value. Danno (Japan) observed that some of the solid-state processes were regarded with great interest at the Japan Atomic Energy Research Institute. Zagorski pointed out that there are marked differences between solid-state polymerization processes near room temperature and deep-frozen systems. The guarded pessimism expressed in the paper referred to the latter. Some remarks were made regarding the difficulties in obtaining reliable kinetics of solid-state polymerization because of post-effects. Silverman (United States of America) reported on his and other published work with frozen styrene under conditions where post-effects were rigidly excluded. He made two observations: the existing data do not fit any published theory, and the nature of the solid can be adjusted to give a wide variety of results. The complex, discontinuous pattern of the post- irradiation of acrylamide crystals observed by Adler at Brookhaven was advanced to show that homogeneous polymerization kinetics should not be applied to solids. Dalton (United Kingdom) reported similar observations in experiments at Wantage with solid acrylonitrile. He concluded that solid-state polymerization studies should be done only on those systems where the nature of the solid can be sharply defined. In response to a question by Steinberg (United States of America) about the radiation chemistry of multiphase liquids at low temperatures, Zagorski mentioned that nothing unusual was observed in single-phase liquid systems 9 DISCUSSIONS at low temperatures except in chain reactions where viscosity effects occur; when a second phase is introduced (solid or liquid), light emission and secondary photochemical processes are frequently observed; furthermore, in addition to the complications, interesting catalytic effects frequently of a promising nature arise from the introduction of solids. Mine and Zagorski mentioned that the radiation-induced or modified catalysis was not dealt with. In connection with the discussion on the two- phase systems at low temperature, it has to be stressed that further investigation on surface phenomena in the radiation field has to be made. This may help to enhance the reactivity of boundaries between phases and seems to be important also to the so-called radiation catalysis.

3. LET

Puig (France) reported on recent results on the LET dependence of the decomposition of terphenyl by mixed gamma ray and fast neutron fluxes. The dependence of the degree of conversion on the concentration of ter- plienyls was determined and found to be proportional to a power of the concentration of about 3/2. Initial G-values for the disappearance of terphenyls as a function of LET can then be calculated for all available data: 0.2 for pure gamma rays and about 0.9 for pure fast neutron fluxes. Silverman described the large variation in reported neutron-to-gamma G-value ratios for this system ranging from 2 to 20. This arises from the treatment of the heat, gamma and neutron effects in a linear manner. It seemed to him highly unlikely that a system of such complexity could be linear. He described open discussions with MIT and ORGEL scientists and referred to his paper on additivity of radiation effects challenging the earlier work of these groups [9]. The main point was that apparent over- all G-values in a complex sequence of reactions are useful for engineering purposes but it is difficult to derive a -mechanistic meaning from such data and dangerous to extrapolate from them. Thus the high ratio (~4) reported by Puig, while derived from useful engineering data, is still too high than one might expect from other systems, and its basic chemical significance is still uncertain. Silverman also raised the question of equivalence of electron beam results compared with gamma effects produced by the same dose at the same dose rate. Reviews by Motz, Olsen and Koch of the US National Bureau of Standards suggest this as an important area of research [10].

3.4. u. v. techniques

Getoff (Austria) gave a short presentation on the vacuum u. v. radiation (X < 2000 A) as a link between u. v. light in its usual sense (X > 2000 A) and ionizing radiation. In addition to a general description of vacuum u. v. radiation sources and their operation, data concerning resonance emissions for a number of gases and specifications of vacuum u. v. transmitting materials were reported. The vacuum u. v. radiation is energetic enough to effect photodissocia- tion, and in some cases also photoionization of molecules. These two processes lead to the formation of free radicals, radical ions and ions in exactly the same way as they are produced by ionizing radiation. From 10 DISCUSSIONS

this point of view the vacuum u. v. radiation represents a powerful tool for investigating reaction mechanisms caused by radiation. It was made clear that vacuum u.v. light, while capable of ionizing and exciting a wide variety of substances, is different from gammas in that the former is a much more selective tool for research purposes, e.g. in the production of specific ion molecules. Silverman (United States of America) added that Schlag [11] used far u.v. and an electric field to demonstrate conclusively that isobutene cations initiate the polymerization of the monomer but that negative and neutral species produced in the energy absorption process do not.

3. 5. Other techniques

Mine mentioned that among new sources of high energy for chemical purposes, low-temperature plasma (about 20 000°C) is gaining importance. At the same time the radiation chemistry of gases is more and more promising, and connection between these two branches of high energy chemistry may result in the development of processes for, for example, acetylene, ethylene and other products of industrial importance. On the other hand, in both cases, interesting intermediates are formed, and any differences or similarities may lead to the better understanding of the mechanisms. Silverman emphasized the value of atomic bombardment techniques, particularly those involving high-frequency discharge, in distinguishing between ionic and free radical effects. Broida's classical work [12] on the gas discharge was cited as the basis for obtaining atoms in good yield without detectable contamination by exciting an ionized EPR species. Silverman described results on polyethylene film with hydrogen atoms showing its similarities with gamma-irradiated polyethylene and some differences. Steinberg suggested that irradiated gases may produce higher concentrations of atoms than gas discharge techniques.

3.6. General fundamental considerations

Using polyolefins as an example, Silverman illustrated the advances and failures in understanding and exploiting radiation effects: the free radicals have been considered in terms of potentially practical grafting processes, the cross-linking of polyethylene is already an important practical process, unsaturation (which is as abundant in yield as cross- linking) has not found a practical value, and neither have the ions. Steinberg expressed concern over the inability to obtain more than a half of the theoretical maximum G-value for nitrogen fixation in N2-O2 systems. He felt that energy transfer studies in solid-gas systems might hold the clue to the solution of this problem. Citing further potential advantages in heterogeneous systems, Silverman described the results of Snow at the Sinclair Oil Co. in the radiation polymerization of ethylene-catalyst systems [13], and the work of Lampe and Johnston on dimethyl amine gas with copper [14], 11 DISCUSSIONS

3. 7. Dryness and purity

Hummel (Federal Republic of Germany) made a survey of the advances and problems in emulsion polymerization and suspension polymerization (see Footnote 3). He described the breakdown of the Smith-Ewart theory, particularly in systems other than styrene. Dalton reported on an earlier study of the emulsion polymerization of styrene in which the dose-rate dependence on monomer concentration showed an apparent deviation from the Smith-Ewart theory. The unusual behaviour he ascribed to two different reducing radicals, the hydrogen atom and the solvated electron. Puig cited the work of Magat on pressure effects on polymerizing systems and detailed physical characterization of grafts. This is an area of practical importance that has received too little attention. Hummel stressed the importance of purity and dryness, especially in vinyl polymerizations and copolymerizations. Large increases in yield arise from rigorous purification of the styrene (as shown by the work at Wantage, United Kingdom, Brookhaven, United States of America, and Kyoto University, Japan). The results indicate the need for further extensive studies in the above and related systems.

4. Applications

4. 1. Polymerization and other reactions

Work carried out at BNL was reported by Steinberg (United States of America) (see Footnote 1). The formation of nitrogen compounds, ozone, hydrogen, hydrogen peroxide and hydrazine was investigated in ampoule irradiation by recoil fission fragements from uranium. As a result of a study of the economics of nuclear reactors using directly the recoil fission energy for chemical conversion, the nitrogen fixation would be economical provided that a G-value of 6 is reached for conversion. This has not yet been obtained. A loop is being constructed to evaluate the process further. Polymerization reactions of gases were also studied using 60Co gamma irradiation. The ethylene reaction was studied in static and dynamic devices with temperatures up to 200°C and pressures reaching 2000 kg/cm2 . Steinberg stressed the safety problems raised by these experiments during which explosions occurred associated with fast reaction

rates (G(-C2H4)~ 10®). Exothermic reactions like C2H4 polymerization require therefore careful chemical engineering design to tackle the heat generation. The economics of the radiation production of low-density polyethylene was studied, and it was shown that the most favourable cost of conversion of ethylene into polyethylene was about 1

Contrary to the findings of other authors, no remarkable differences were found in the isomar composition of the hexachlorocyclohexane compared with the reaction product of the u.v. -initiated chlorination. Steinberg asked if Mrs. Ionescu could expand her reference to the oxidation of sulphides. Her reply was that the work had been published in detail, but that the process was not being used industrially. Hummel remarked that this result shows once more that, in the case of long-chain reactions, the kind of initiation does not affect the composition of the reaction products. Only with short-chain reactions, because of the different species produced by the initiation reaction, slight differences in the composition of the reaction products might be observed. Referring to Dalton's optimistic picture of the use of ionizing radiation for the curing (polymerization or cross-linking) of lacquers for curved (uneven) surfaces. Hummel remarked that one of the biggest markets in this field is that of car enamels. There, however, the hydrophilic baking enamels (phenolics, acrylic copolymers, urea and melamine combinations) with their easy application (electrophoretic coating) and good qualities might be very strong competitors. Dalton replied that the technique applied only to industrial finishing and that there were large areas where the resins mentioned by Hummel were not appropriate. Puig gave an introduction to the present main developments in radiation processing in France: (a) Gamma-ray-crosslinked polyethylene film is ready for commercialization. (b) Wood plastic compounds have been taken up by a private company with interesting prospects for the immediate future. (c) Pre-curing of natural rubber latex by electron beam irradiation is being actively investigated in co-operation with private industry. (d) Polymerization of trioxane in the solid state is also being investigated. Other items are referred to in the paper presented by Puig10. Steinberg underlined Hummel's remarks on the importance of making copolymers of materials that would not homopolymerize. He then asked Puig why cross-linking of polyethylene film was becoming a 60Co rather than a machine process in France. Puig mentioned that this was a decision of the producers on which he could not comment. He suggested that it would be useful to persuade electricity companies to sell high voltage d. c. Dalton mentioned that while this possibility was naturally being considered, it was at present not feasible in the United Kingdom.

4. 2. Wood-plastics

Singer (Denmark) presented a statement concerning the actual status of the irradiated wood-polymer combinations, and pointed out that there still remain unsolved problems, in impregnation technology such as the evaluation of types of polymers and in radiation technology such as the type of radiation source. He claimed that the application of electron accelerators for this process has not been sufficiently investigated.

10 LEVEQUE, P., PUIG, J. R., "Le developpement de la radiochimie : quelques aspects de la conjuncture", this Report, PL-236/8. 14 DISCUSSIONS

Puig mentioned that one French company is now offering irradiated combinations of beech and methyl-methacrylate on a commercial scale, and that this company has based its production on an extensive market analysis covering more than 100 wood manufacturers and consumers. Steinberg agreed with the views expressed by Singer and added that he had been able to load pine with polyethylene to 100% by weight. Dalton pointed out that commercial 60Co plants for fabricating wood-plastic have to be rather large to be economically attractive, and this puts a limit on possible interesting products. Silverman referred to the starting-up of other radiation-technical processes such as medical sterilization and polyethylene cross-linking, which from very small beginnings have increased to very important production. Wood-plastic products could probably in the beginning find application in high-cost specialized articles for a number of smaller enterprises. Such enterprises could very well be started in different countries using the increasing number of production and pilot-plant irradiation installations offering these services to interested wood- manufacturers. In this connection Singer referred to some experience regarding sporting articles, where the production price would have no practical influence on the final price of the product, and where transport of the prefabricated products to a service irradiation plant and back is no major problem.

5. Information

It was generally agreed that there is a need for further exchange of information. Whereas meetings on fundamental aspects are held rather frequently, meetings on applications are more regional and restricted. Therefore further meetings of a clearing-house type are required. Hara (IAEA) described the two functions of the IAEA as regulatory functions, such as safeguards and safety, and promoting functions, such as technical assistance, scientific information and promotion of particular research. To implement these functions, the organization of symposia, panels, etc. ,. was mentioned. Beswick (IAEA) pointed out that the IAEA was compiling a world list of large radiation source facilities, and that this would include all facilities with more than 20 000 Ci of activity. It was hoped that all Member States would co-operate in supplying information so that the details in the list would be as complete as possible. A Consultants' Meeting on Bibliographies on Industrial Applications of Radioisotopes in April 1966 had resulted in tentative agreement from some Member States to supply regularly to the Agency, on punched-cards or tape, references from literature published in each country. It was hoped to start publication of a quarterly bibliography later this year, and this would contain references on radiation chemistry, including patents. A Study Group on Irradiated Wood Plastic Combinations was to be convened in Bangkok in November 1967 to discuss the economic and technical significance of this material in Asia and the Far East. A large number of countries in this region were carrying out research on this material, and it was hoped that this research could be co-ordinated. 15 DISCUSSIONS

Several Panel members pointed out that pseudo-technical information released to the press often created a better response from industry than articles in the scientific literature. Puig described the information work that had arisen from co-operation between the European Common Market countries during the past three years: (a) Investigation of the situation in the textile industry in the six countries by visits of nuclear experts. The result was to be made public at the beginning of July 1967 in an international symposium. (b) The gathering of information relating to the various techniques of irradiation. (c) The gathering of information relating to wood technology. (d) The centralization of scientific, technical and patent literature using computer storage. The CEA in France is developing mechanographic centralization of references including patents. More general information is disseminated by organizing a one-year course on "preparation and modification of polymers" for industrialists and researchers at the University of Paris. A compilation of papers dealing with applications carried out in France together with a list of main irradiation facilities is issued annually.

6. Non-technical problems including safety

Danno stressed that with increasing source dimensions the potential hazard is greatly increased. The installations have to be safe against radiation, pressure, fire or explosion and corrosion. Whereas the problem of hazards of nuclear reactors is well taken care of, similar activities should be promoted in radiation chemistry. Steinberg mentioned a number of safety problems relating to the use of experimental and development radiation facilities for woTking with combustible and explosible materials. Radiation biological hazards are fairly covered, but the additional load from combustion, explosion and chemical hazards should be treated on a broader basis. Exchange of information concerning the experiences of various groups and their design standards would be very valuable. Underwater facilities are very safe for use when dealing with explosives in vessels; however, they present great problems in building and operating flow equipment. Closed cave facilities make experimental and development operations much more convenient but they present greater hazard problems. Hummel stated that sources should be designed to withstand pressure and shock waves. He preferred corona-type source arrangements doubly contained in stainless steel and cells of "open roof" design. Puig described some accidents occurring during measurement of source in dry irradiation cells, and mechanical failures in swimming- pool irradiation units. Silverman suggested that the safety problems should be treated separately for research laboratories, testing and pilot plants units, and industrial plants. Dalton supported this with the additional remark that hazards can be reduced on the laboratory scale by a reduction of the quantity of material, and in industrial plant use should be made of background knowledge of experts for conventional chemical plants. To a remark from Getoff concerning the explosion hazard involved in irradiations 16 DISCUSSIONS of liquid nitrogen, Zagorski stated that neither in Poland nor in the USSR had this problem occurred during their experiments. Puig mentioned that some of the problems, and in particular irradiations at agogenic temperatures, have already been experienced and solved in research reactor loops. Dalton pointed out that besides the safety aspects three other problems arise during the transfer steps from laboratory scale to industrial dimensions: reliability (600-800 h/a with predicted shutdowns), guaranteed supply of sources, and economic assessment based on industrial standard methods. Hahn described in detail safety aspects of radiation technology using machines where the production of the machine, personnel and precautions against reaction accidents have to be considered. Proper interlocking systems can be provided but special aspects have to be considered also, such as residual changes in capacitors for linears. Concerning the terms of reference of this Panel, Puig stressed that marketing was outside its scope and that in France this activity is confined to industry. Steinberg agreed in principle, stating that recommendations should nevertheless be made, and he proposed that industrialists should be invited to future meetings.

REFERENCES

[1] BRYNJOLFSON, A., THAARUP, B., Ristf Rep. No. 53, Danish A. E. K. (1963). [2] NAKAI, Y., Jap. J. Phys. 2 (1963) 743. [3] BERGER, M.J., "Monte Carlo calculation of the penetration and diffusion of fast charged particles", Methods in Computational Physics 1 (ALDER, B., FERNBACH, S., ROTENBERG, M., Eds), Academic Press, New York (1963); BERGER, M.J., "Tables of energy losses and range of electrons and positions". Paper 10, Publication 1133, US National Academy of Sciences, National Research Council (1964). [4] SPENCER, L. V., Energy Dissipation by Fast Electrons, NBS Monograph No. 1, National Bureau of Standards, Washington (1959). [5] KNIEDLER, M., SILVERMAN, J., Paper A/M-Z, Seventh Japan Conference on Radioisotopes, 1966 (in press). [6] HART, E.J., Radiat. Res. 1 (1954) 342. [7] DONDES, S., Int. Conf. peaceful Uses atom. Energy (Proc. Conf. Geneva, 1955) 14 UN, New York (1956) 176; HEARNE, J. A., Radiat. Res. 15 (1961) 254; STEINBERG, M., Radiation Processing

Report No. 1, Gamma Irradiation Experiments in the N2-Oz System, BNL Rep. 612, Brookhaven National Laboratory (1960). [81 BROWN, R.D., DONDES, S., HARTECK, P., Some Studies of the Ionizing Radiation Induced Isotopic Exchange in Gaseous Nitrogen, Progress Rep. USAEC RPI 321-9, Rensselaer Polytechnic Institute, Troy, N. Y. (1966). [9] SILVERMAN, J., Trans. Am. nucl. Soc. 7 (1964) 442. [10] MOTZ, J.W., OLSEN, H., KOCH, A.W., Rev. mod. Phys. 36 (1964) 881. [11] SCHLAG, E.W., SPARAPANY, J.J., J. Am. chem. Soc. 86 (1964) 1875. [12] BROIDA, H.P., MOYER, J.W., J. opt. Soc. Am. 42 (1952) 37; BASS, A.M., BROIDA. H.P. (Eds). Formation and Trapping of Free Radicals, Academic Press, New York (1960); FEHSENFELD, F. C., EVENSON, K. M., BROIDA, H.P., Rev. scient. Instrum. 36 (1965) 294. [13] SNOW, A., Private communication. [14] LAMPE, F. W., JOHNSTON, W. H., Paper B/3-2, Seventh Japan Conference on Radioisotopes, 1966 (in press). OBSERVATIONS AND RECOMMENDATIONS 17

III. GENERAL OBSERVATIONS AND RECOMMENDATIONS

1. Sources

Sources for use in commercial radiation chemical processing might comprise: 1. Isotope sources, e.g. gamma-emitting sources like 60Co and 137Cs or beta emitters such as 90Sr. 2. Electron accelerators, such as d. c.-machines, and linear accelerators with or without X-ray conversion devices. 3. Chemonuclear reactors utilizing, for example, fission fragments or ionization radiation from loops. 4. Spent fuel element facilities. The deliberations of the Panel concentrated on the source types where an early industrial application could be foreseen, and the Panel did not discuss in depth all the possibilities mentioned above.

1.1. Isotope sources

With regard to isotope sources it was observed that abundant information was available on the construction and operation of 60Co plants. The most important development seems to be the construction of the bonded sources, which offer an improved source integrity. Much interest was shown in the application of 137Cs in radiation chemical processing. The advantages include lighter shielding (mobile irradiators), possible higher source utilization efficiency, cheaper source material as 137Cs is produced from reactor waste products, and a longer half-life. Prices for unencapsulated 137Cs might be expected to be about 10 - 15 ^/Ci, when large separation plants are becoming operational. Problems include improved encapsulation techniques as these sources must have a ionger lifetime, and suitable irradiation techniques need to be developed.

1.2. Electron accelerators

With regard to electron accelerators the Panel observed that a special situation arose because such machines had to be designed for a certain, quite high power output in order to be economical. In addition, assembly line production was necessary for obtaining low prices on such equipment. It was therefore suggested that national laboratories or service facilities made irradiation services available, so that enterprises that could not justify a large plant on their own could be introduced on a share basis. It was emphasized that such equipment must meet the same reliability requirements as conventional process equipment, and that further development work needs to be done in this respect. The Panel observed that very low-energy accelerators (20 keV) might be applicable to certain processes, if a suitable irradiation technique can be worked out.

1.3. Chemonuclear reactors

The application of chemonuclear reactors is still at the early experimental stage. The continuation of such work is justified as chemonuclear

2 18 OBSERVATIONS AND RECOMMENDATIONS reactors could provide the cheapest ionizing energy foreseeable. It should be emphasized, however, that such plants need to be very large to be economical, so that only products for which there is a large demand (such as nitrogen fertilizers) should be considered. With regard to the application of reactor loops, it was observed that these could mainly be considered interesting when it was convenient to integrate the chemical factory with a multipurpose nuclear power plant. To perform experimental work on this subject, it was suggested that in-pile research loop facilities, such as the Brookhaven chemonuclear in-pile loop, should be made available to investigators from all countries.

1.4. Spent fuels

In general, spent fuel elements were considered impractical for radiation chemical processing at present.

1.5. Safety

The safety of large irradiation facilities may be divided into the safety aspects of design and the safety of chemical reactions under irradiations, and it was observed that information on the safety of both laboratory- scale experiments and that of plant-scale development is not widely available. It was pointed out that appropriate measures for guidance must be established on the safety of large irradiation facilities, and more efforts are needed to disseminate information on this subject.

2. Dosimetry

It was observed that very many organizations were dealing already with dosimetry problems, and that efforts should be directed to co- ordinate these existing activities rather than to create new ones. In general, it was pointed out that work should be pursued on dose distributions in complex targets, and that fundamental information should be obtained on the reaction mechanisms of plastic film dosimeters. The Panel agreed that it was very important, when establishing a radiation process, to carry out an accurate dosimetry survey, but whenever possible to substitute for this a routine dosimetry programme with an extensive process control.

3. Fundamentals

The task of defining subjects of research for practical purposes is always difficult. The topics cited are of much wider interest than the practical end to which this Panel was mainly devoted. Nevertheless, the Panel found some topics of particular relevance and it directs the attention of radiation scientists towards them. The fundamental tasks were divided into three categories: research problems, techniques, and reactions of potential practical interest. 19 OBSERVATIONS AND RECOMMENDATIONS

3.1. Research problems

The phenomena that stimulated the extensive discussion were as follows: (1) LET effects (including fission fragment chemistry) (2) High dose-rate effects (3) Low-temperature effects (4) High purity (5) Heterogeneous reactions (6) Correlation of the effects of radiation chemistry and of low- temperature gas discharge (7) Effects of transient species other than free radicals in the ground state. The first four are parameters of normal interest to the radiation chemist. The Panel mentioned them explicitly for the following reasons. In the case of LET, it is increasingly apparent that studies involving this parameter are often incomplete and ambiguous. Knowledge on fission fragment chemistry is inadequate. Furthermore, there is some information to suggest that electron and gamma-ray effects may not, as is often assumed, be equivalent. Similarly, effects at very high dose- rates have been quite unexpected on the basis of low dose-rate results; they merit further study. Results at low temperature have been used to advance mechanism theories. The discussions showed that many of these studies were based on systems in which the nature of the solid was too poorly defined. The question of purity is always of concern but the vastly different results obtained in the radiation chemistry of ultra-dry and ultra-pure systems now obliges radiation scientists to re-examine several systems that had been considered well-known, particularly since some of the newest results point to highly increased yields in polymerization processes of commercial interest. Just as trace homogeneous contaminants sometimes poison important and interesting reactions, heterogeneous additives sometimes strongly enhance such reactions. Applied research in heterogeneous systems is still too meagre and should be encouraged. The Panel also took notice of the advances in gas discharge techniques and recommends continuing and increased attention to this subject. Gas discharge may turn out to be a strong competitor of radiation in commercial applications, and a comparison of the effects should be made in the synthesis of chemicals that can be made by both techniques. The last item on the list of recommended topics for research reflects the Panel's observation that most of the practical applications under consideration involve the use of radiation as a generator of free radicals in the ground state all through a target. This is far too limited, and the Panel encourages more research into applications of the other species produced by radiation.

3.2. Basic techniques

The basic techniques discussed were as follows: (1) Pulsed radiolysis (2) Flash photolysis (including pulsed laser beams) 20 OBSERVATIONS AND RECOMMENDATIONS

(3) Fast ESR methods (4) Irradiation at liquid helium temperatures (5) Electric discharge methods (6) Far u. v. methods. The first three techniques reflect the Panel's view that information on prompt effects are useful not only in pure research but also in the understanding of such diverse applied problems as highly increased polymerization rates of ultra-pure vinyl monomers, damage to reactor coolants and moderators, and the build-up of unsaturation of polyolefins. Liquid helium freezes almost all molecular motion, and can be used to preserve many early radiation effects without more expensive pulsing equipment. The gas discharge method is of interest not only as a technique that is potentially competitive with radiation, but also as a generator of gaseous atoms for research problems that distinguish between free radical and other effects in radiation chemistry. Far u. v. techniques can be used to generate a sharply defined range of atomic, molecular and ionic species in selected states of excitation and thus serve a similar end.

3.3. Reactions of potential practical interest

The list that follows is not intended to cover promising process applications but it reflects the Panel's view that these topics deserve increased industrial interest: (1) Polymerization and copolymerization of ultra-pure systems (2) Heterogeneous polymerization and telomerization reactions (3) Chain curing reactions at high dose-rates.

4. Applications

The Panel considered that the present position regarding radiation chemical processing could be divided into three categories: (1) Industrial radiation chemical processes now in existence, an example of which is crosslinking of polyethylene; (2) Processes currently in advanced stages of development, such as wood-plastic combination; (3) Subjects where further study may lead to economical processes and products.

4.1. Industrial radiation chemical processes in operation

The Panel agreed that there was no point in extensively discussing processes in this category.

4.2. Processes in advanced developmental stage

In this category a number of these processes are being studied in detail by groups involving commercial aspects including patents and proprietory know-how. In many instances there is insufficient background knowledge to support these programmes. In particular it was felt that kinetics of reaction in high intensity electron beams and in dynamic systems could usefully be the subject of more intensive studies. Also 21 OBSERVATIONS AND RECOMMENDATIONS the kinetics and mechanism of chain reactions with respect to high purity systems should be considered. The effect of LET is also far from clear in many cases of practical interest.

4.3. Future potential processes

The Panel recognized that the choice of subject depends to a large extent on personal opinion and local conditions. It may, however, be relevant to point out that there is relatively little experimental data on in-pile reactor and heavy-charged particles radiation chemistry. Also, in chain reactions that do not lead to polymers there have been few systematic studies. In polymerization further activity may be considered such as emulsion and suspension polymerizations. The Panel agreed that in general there is increasing emphasis on technology and engineering, including safety, and that this should be fostered.

5. Recommendations to the IAEA

Radiation chemistry and radiation chemical processes cover a wide area, and already a number of organizations have been making an extensive effort. In carrying out any programme, the IAEA should pay attention to the activities of other organizations to avoid unnecessary overlapping, and also to the commercial aspects of any proposed activities.

5.1. Sources

The Panel encourages the IAEA in its efforts to compile a list of radiation sources in member countries, and suggests that the IAEA investigate which of these sources are available for service irradiations for other member countries. This list should indicate which sources are available for non-profit and/or commercial organizations.

5.2. Dosimetry

The Panel recommends that the IAEA should draft procedures for selected dosimeters, and arrange for testing these at a number of laboratories in member countries. It further recommends that the IAEA should consider the activities of various organizations in this subject with the aim of integrating its activities accordingly.

5.3. Fundamentals

The Panel recommends that the IAEA should view with favour proposals made regarding the topics of research cited in Section III. 3. 1, particularly the effects of LET (including fission fragment chemistry), high dose-rate, purity and heterogeneous systems. The IAEA should avoid committing funds to building new pulsed radiolysis facilities except where it is demonstrated that ready access to such facilities is generally not possible for qualified scientists. 22 OBSERVATIONS AND RECOMMENDATIONS

However, IAEA provision of funds to support travel to, and work at, existing facilities is encouraged by the Panel. The IAEA should set up advanced courses and seminars in radiation chemistry, and develop appropriate courses and educational materials.

5.4. Applic ations

The Panel recommends to the IAEA that it should undertake to review and evaluate any and all radiation chemical processes and products that are likely to be of potential importance. For instance, the IAEA should undertake the publication of monographs dealing with some advanced aspects of applied radiation chemistry and technology. These evaluating monographs should include safety aspects. The IAEA might arrange for specialized existing research and development facilities to be made available to scientists from other countries, in particular access to chemonuclear loops and large accelerators could be of great interest.

5. 5. Advisory service to developing countries

The Panel feels that it is desirable to promote radiation chemical work where such work is related to local needs. However, in promoting such work, attention must be paid to the economic aspects. Patent aspects of work already carried out in advanced countries, especially those held by commercial firms, must also be considered. When experts are sent to countries needing them, e.g. a developing country, to make an on-the-spot assessment, consideration should be given to carrying out any necessary research and development in another country with more appropriate resources of scientists and facilities.

5.6. Co-ordination of activities

With regard to chemonuclear process flow systems, there will be two loops available, and the establishment of co-ordinated effort on these loops would contribute to progress in this sphere.

5.7. Safety

Information on the safety of laboratory-scale and plant-scale experiments is limited in its availability. The Panel felt that appropriate guiding principles should be established on safety in irradiations with large sources.

5.8. Panel

The IAEA should organize a Panel meeting of scientists actively engaged in developing radiation chemical processes with more restricted terms of reference than the present Panel, namely, to evaluate such processes. IV. REPORTS

APPLICATION OF IONIZING RADIATION TO POLYMER CHEMISTRY

A. DANNO TAKASAKI-SHI RADIATION CHEMISTRY RESEARCH ESTABLISHMENT, JAPAN ATOMIC ENERGY RESEARCH INSTITUTE, TAKASAKI-SHI, JAPAN

Abstract

APPLICATION OF IONIZING RADIATION TO POLYMER CHEMISTRY. Promising fields for radiation chemical processes in polymer chemistry are modification of polymer, radiation-induced graft copolymerization, radiation-induced polymerization, and radiation-synthesis of chemicals. There are many special and difficult problems in the practical application of radiation chemistry. Some technological problems are discussed based on the radiation chemical processes now being investigated at the Takasaki Radiation Chemistry Research Establishment. Examples are production of foamed polyethylene, grafting of styrene on to cellulosic fibre, polymerization of ethylene in the gas phase, solid-state polymerization of trioxane and radiation-initiated chlorination of hydrocarbons. The following points on technological problems regarding Ihe use of radiation sources are discussed: comparison between radioisotope sources and electron beam accelerators, radioisotope sources used on a large scale, and the dose and dose-rate required for radiation treatment.

1. INTRODUCTION

The practical application of radiation chemistry in industry has progressed rather slowly. Today, however, we have accumulated much basic information in polymer chemistry, and industrial applications of radiation chemistry have come to look more promising. These are radiation modification of plastics [1], radiation-induced polymerization and graft copolymerization [2], Great attention has been given to the application of irradiated polyethylene, which was one of the first commercial products and had the qualities of heat resistance and heat shrinkability. Unfortunately new applications of radiation chemistry in the polymer industry have not progressed as rapidly as was originally expected. Recently, however, the applications have been re-examined at many laboratories where experiments on a pilot scale or at advanced stages are being carried out. Promising fields in which radiation chemical processes in polymer chemistry are already in use or appear to be promising are listed in Table I [3]. New applications of radiation chemistry in industry may be expected because of the development of radiation engineering, which has never been experienced in the conventional chemical processes, as well as the accumulation of basic information on radiation chemistry. There are many special and difficult problems in the practical application of radiation chemistry. In this review paper, some technological problems are discussed based on the radiation chemical processes now being investigated.

23 24 DANNO

TABLE I. INDUSTRIAL CHEMICAL PROCESSES USING RADIATION

Chemical process Location Status Source

Modification of polymer

Polyethylene tape General Electric Commercial Accelerator (U. S. A.) Sumitomo Electric Commercial Accelerator (Japan) and several others

Polyethylene film W.R. Grace Commercial Accelerator (U.S. A.) and several others

Polyethylene oxide Union Carbide Design Co-60, 100 kCi (U.S.A.) plant

Plastic moulded parts Raychem Commercial Accelerator (U.S.A.)

Polyethylene foam Toyo Rayon Commercial Accelerator (Japan) Sekisui Chemical Commercial Accelerator (Japan)

Polyethylene wire Several Commercial Accelerator covering (U. S. A., France, Japan, etc.)

Natural rubber latex Saclay Pilot Accelerator (France)

Graft copolymerization

Polyethylene with Dow Chemical Pilot Electron, 2 MeV acrylic acid (U.S.A.)

Polyethylene with Electrical Communication Pilot Co-60 butadiene Lab. (Japan)

Polyethylene with Takasaki Research Establishment Pilot Co-60 butadiene and styrene (Japan)

Poly v in ylchloride Takasaki Research Establishment Pilot Co-60, 36 kCi with butadiene (Japan)

Polyvinylchloride Dow Chemical Pilot Co-60, 18 kCi with styrene (U.S. A.)

Polyester and cotton blend Deering-Milliken Commercial Accelerator fabric with vinyl monomer (U.S. A.)

Cellulosic fibre with Takasaki Research Establishment Pilot Electron, 2 MeV styrene (Japan)

Polyethylene films with Sekisui Chemical Pilot Accelerator vinyl monomers (Japan)

Polymerization

Ethylene Brookhaven Pilot Co-60, 10 kCi (U.S. A.) Takasaki Research Establishment Pilot Co-60, 108 kCi (Japan) and several others (USSR, France, Italy) PL-236/1 25

TABLE I (cont)

Chemical process Location Status Source

Ethylene, vinyl Farbwerke Hoechst Experim ental Co-60, 11 kCi compounds (Fed.Rep. Germany)

Trioxane H. B.N.P. C. Pilot X -rays (France) Takasaki Research Establishment Pilot Electron, 3 MeV (Japan)

Coating, polyesters Radiation Dynamics Commercial Electron, 0.3 MeV with styrene and acrylics (U.S.A.)

Polyestere in glass-fibre Ford Motor Commercial Electron, 0.3 MeV laminate (U.S.A.)

Wood-plastics American Novawood Commercial Co-60 combination (U.S.A.) Lockheed-Georgia Pilot Co-60 (U.S.A.) West Virginia University Experimental Co-60 (U.S.A.)

Chemicals

Bromination of hydro- Dow Chemical Commercial Co-60, 3 kCi carbons (ethyl bromide) (U.S.A.)

Sulphoxidation of hydro- Esso Research Pilot plant Co-60 carbons (SAS) (U. S.A.)

Chlorination of hydro- Takasaki Research Establishment Pilot Co-60 carbons (Japan)

Chlorination of benzene Wantage Research Lab- Experimental Co-60 (BHC) oratory (United Kingdom)

Oxidation of paraffin (Romania) Industrial Gamma rays

Oxidation of benzene Takasaki Research Establishment Experimental Co-60 in aqueous solution (Japan)

Cracking of hydrocarbons Institute of Petroleum and Pilot Electron, 2 MeV Chemical Synthesis (USSR)

2. APPLICATION OF RADIATION CHEMISTRY

Ionizing radiation has not yet found a commercially important industrial application. However, the application of radiation is already being used in some industrial processes, which may be divided into four sections, as seen in Table I. The use of high-energy radiation for the modification of polymers is a very interesting application. This is illustrated by the production of radiation cross-linking polyethylene on an industrial scale. Radiation- induced graft copolymerization is another example in which superior polymeric materials can be produced. One of the examples is a commercial product, a new fabric in which heat-set and soil-release properties are imparted to a polyester-cotton-blend fabric. Radiation-induced polymerization is also a direct application of radiation. Unfortunately, com- 26 DANNO mercial application of this process has not yet been reported, but pilot- scale experiments are being carried out at several laboratories. Radiation synthesis of chemicals is a technologically interesting field. The production of ethyl bromide by means of radiation was an initial process that demonstrated how radiation chemistry could be applied in industry, using relatively small amounts of radioisotopes.

2. 1. Radiation modification of plastics

The formation of cross-linking is one of the most important chemical changes brought about by radiation on polymers. Since cross-linked polymers lead to beneficial changes in some of their properties, such as heat resistance, tensile strength, cold flow, etc., the possibility of radiation effects on polymers has attracted a number of applications. Polyethylene converted by irradiation of high-energy radiation into cross-linked polyethylene can be considered a new kind of plastic exhi- biting a number of modified properties. At present, several kinds of irradiated polyethylene are commercially available - tapes and films of various sizes, wrapping foils and insulation tubing as well as moulding products. Recently, foamed polyethylene sheets were produced commercially in Japan. The method of producing foamed polyethylene is as follows. A blowing agent mixed with polyethylene is decomposed under the action of radiation, thus bringing about foaming and cross-linking of the polymer at the same time. Polyethylene foams are produced by expanding the irradiated material to 10 to 40 times the original dimensions under raised temperatures. The product has excellent characteristics, as shown in Table II. These results indicate that polyethylene foam is suitable for a large variety of uses such as in cushions, shock absorbers, heat insula- tors, floats and decorations. The radiation dose used for the cross-linking of plastics is in the range of 4-40 Mrad, because the G-value of the cross-linking for polyethylene is about 3. This means that high-dose irradiation is necessary to produce the irradiated materials. Electrons from accelerators are therefore most often used to produce these materials. Moreover, the samples to be irradiated are usually in the form of films, tapes and sheets that are suitable for continuous treatment by electron beam irradiation. To reduce the optimum dosage, the addition of a sensitizer has been investigated. For example, natural rubber latex [4] has been converted by irradiation into a cross-linked material. Rubber films obtained from pure latex irradiated with 13 Mrad had excellent mechanical properties. The optimum dose can be reduced by a factor of about ten when chloroform solutions are used as the sensitizer.

2.2. Radiation-induced graft copolymerization

A number of studies on the preparation of graft copolymers have been published. The results already available have attracted considerable interest in polymer chemistry, because it has been found that radiation chemical methods for the preparation of graft copolymers are often easier in practice than conventional chemical methods. Moreover, these methods PL-236/1 27

TABLE II. PROPERTIES OF POLYETHYLENE FOAM PRODUCED BY THE RADIATION PROCESS

Apparent density (g/cra3)

Properties 0.035 0. 070 0.11

Mechanical properties

Tensile strength (g/cm2) 4.71 7.51 10.21

Tensile elongation (%) 167 289 290

Tensile modulus (kg/cm2) 9.5 27 36

Compression modulus (kg/cm!) 0.8 1.0 1.5

Compression permanent strain ("7°) 22 14 11

Resiliance (%) 53 38 19

Thermal properties

Tolerable temperature for

continuous use (°C) 80 80 80

Maximum temperature for use C) 100 100 100

Minimum temperature for use (°C) -70 -70 -70

Thermal conductivity (kcal/ml deg C) 0.038 - -

Ignition temperature (°Q 470 470 470

Flash point ("q 360 360 360

Electrical properties

Dielectric strength (kV/mm) 4 - -

Dielectric constant 1. 05 - -

Power factor 0. 0002 - -

Volume resistivity (ohm cm) 1. 5 X 10"

are very general, in principle, and can be used to prepare any desired combination of polymers. The possibility of using radiation-grafting to modify polymers is a very interesting application. The graft can either be made homogeneously throughout the polymer material, or be limited to a layer of any desired 28 DANNO thickness at the surface. The advantages and possibilities of graft polymerization point towards the economic utility of radiation grafting. Ionizing radiation, such as gamma radiations or high-energy electrons, may be used to prepare graft copolymers. Several methods for the preparation have been developed and can be divided into four main methods: (a) Direct grafting of vinyl monomer on to a polymer by irradiating the polymer in the presence of a monomer; (b) Grafting on radiation-preoxidized polymers; (c) Grafting initiated by trapped radicals; (d) Combination of two different polymeric radicals. In the case of (b) and (c), pre-irradiation of the polymer is first made and then grafting of the monomer follows. The choice of these methods is dependent on the character of the polymers, the reactivity of monomers and the properties of graft copolymers. Grafting of vinyl monomers on to polyethylene is a good example. A pilot-scale experiment for grafting of acrylic acid on to polyethylene [5] has been studied in the United States of America. The graft copolymer is greatly improved in adhesion properties. Grafting of butadiene on to polyethylene [6] has been investigated in Japan. In the former case, the pre-irradiation method was used and the polymer was irradiated to 0.8-2.0 Mrad of total irradiation dose using electron beams from the accelerator. In the latter case, the direct-irradiation method was used in the gas phase, and polyethylene was irradiated to 0.2-1.0 Mrad by gamma rays from a 60Co source. There is no distinct rule for taking either electron beams or gamma rays as the radiation source. Some idea of what type of radiation can be used for grafting is determined by several factors, such as inhibition of the homopolymer in grafting material and the process of grafting on a large scale. In the pre-irradiation method electron-beam irradiation is appropriate for polyethylene-acrylic acid, cellulose-styrene and polyester cotton fabric-acrylic acid systems. In these systems, relatively large doses are necessary to produce a sufficient amount of reactive sites in the polymers. After irradiation, the polymer reacts with the monomer in a suitable solution to make graft copolymers. On the other hand, in the direct irradiation method, gamma-ray irradiation is appropriate for the polyethylene-butadiene and polyvinylchloride-butadiene systems. In these systems, the monomer is grafted in the gas phase or the liquid phase and the formation of homopolymer is very slight. Low dose-rate irradiation is more suitable because of the diffusion of the monomer into the polymer. Moreover, since samples are usually in the shape of powders, grains or bulks, a high penetrating power of radiation is required. Many works on pre-irradiation grafting on to fibre have been reported, but little seems to have been done on preparation on a large scale. At the Takasaki Radiation Chemistry Research Establishment, grafting of styrene on to pre-irradiated cellulosic fibres [7] has been studied to obtain the basic data for scaling up the process. The fibre used was staple fibre, mostly polynosic rayon, in the form of webs. Pre-irradiation was carried out in air by means of 2 MeV electrons (from EBG) to doses of between 0.3 and 7 Mrad. The monomer was emulsified in distilled water with emulsifiers. The standard conditions chosen for styrene were: 2 Mard pre- irradiation of polynosic fibre, 10% monomer concentration in emulsion, PL-236/1 29

polyethylene-sorbitan-monolaurate as emulsifier, 200 ml emulsion/1 g of fibre, 200 ppm oxygen content in the reaction atmosphere, a reaction temperature of 50°C, and a reaction time of 1 h. The grafting reaction occurred in two stages, i. e. the reaction first proceeded very rapidly, and was followed by a rather slow stage. As expected, the reaction rate was increased by increasing the total absorbed dose as shown in Fig. 1, but no effect of the monomer concentration was seen above a critical value. The effect of the oxygen content in the monomer emulsion atmosphere was rather small compared with graft polymerization carried out in a solution of monomer.

2. 3. Radiation-induced polymerization

The radiation-induced polymerization of vinyl monomers is a direct application of radiation. In general, ionizing radiation can initiate radical polymerization. The activation energy for the initiation is supplied only by ionizing radiations. Once the initiation radicals are formed, the propagation reaction proceeds in a similar way to the free radical chain reaction. In certain specific cases, radiation-induced polymerization also initiates ionic polymerization.

100

REACTION TIME (min)

FIG. 1. Effect of reaction time on grafting of cellulosic fibre; 10% styrene emulsion, 200 ppm oxygen, 50"C reaction temperature. A= polynosic rayon; 0= cotton; • = refined faced rayon.

Ionizing radiation is finding a valuable use in the special system of polymerization. Radicals are generated at a wide range of rates in any chemical system at-any temperature and in any phase. Therefore, compared with the conventional polymerization, no restrictions on temperatures, pressures and phases are observed in radiation-induced polymerization. Basic research on radiation-induced polymerization has indicated that ethylene was polymerized by radiation at lower temperatures and at relatively lower pressures. The products obtained include oily liquid, wax-like materials and tough, solid plastics, depending on the operating conditions. The high-molecular weight products are produced at higher pressures and at temperatures slightly above room temperature. 30 DANNO

A pilot-scale experiment is being carried out at several laboratories. At the Brookhaven National Laboratory in the United States of America advanced stage experiments on polymerization of ethylene [8] at constant conditions in a non-flow system have been studied. At the Takasaki Radiation Chemistry Research Establishment in Japan, pilot-scale experiments [9] in a flow system are being carried out under higher pressures and at temperatures below the melting point of polyethylene. Figure 2 shows the flow sheet of this process.

VACUUM PUMP

COOLER

FIG. 2. Flow sheet of ethylene polymerization pilot plant.

The operating conditions were determined to obtain a powder-like polyethylene. The reaction vessel was a stainless-steel (SUS-32) cylinder with a capacity of 10 litres. The vessel was designed to operate at a maximum pressure and a maximum temperature of 440 atm and 150°C respectively. Irradiation was carried out using a cylindrical 60Co source of 108 000 Ci located outside the vessel. The radiation intensity inside the vessel was 3.7X105 rad/h. The polyethylene produced in this pilot plant has a density and a crystallinity intermediate between those of conventional "low-density" and "high-density" polyethylene. The molecular weight of the polymers varied in a wide range from 20 000 to 300 000 depending on the reaction conditions. Figure 3 shows the relation between the molecular weight and the reaction time as a function of the reaction temperature [9], It is clearly seen how at lower temperatures the molecular weight increases with reaction time, and tends to saturate increasingly earlier as the reaction temperature is raised. The distinctive nature of the polyethylene produced by radiation consists of very low methyl branching and of very low un- saturations providing good properties for electrical materials. PL-236/1 31

0 2 3 5

REACTION TIME (h)

FIG. 3. Relationship between molecular weight and reaction time at various reaction temperatures; pressure 400 kg/cm!, dose-rate 2.5 x 104 rad/h.

These results indicate that the radiation process has many advantages in the production of polyethylene such as: (a) The molecular weight of the polymer can be controlled by selecting the radiation intensity, reaction pressure, reaction temperature and reaction time; (b) The crystallinity and melt-index of the polymer can be changed in a wide range using the same plant. Another advantage of radiation-induced polymerization is the possibility of initiating polymerization in the solid state at lower temperatures of vinyl monomers and cyclic compounds. A number of studies on solid- state polymerization have been published. These results have indicated a considerable interest in polymer chemistry because it was found that ionic polymerization took place in some specific system. Solid-state polymerization of trioxane has been studied from the points of view of both research and application. Scale-up experiments of this process have been carried out in France and Japan, but details have not been published. Some preliminary work [10] carried out at the Takasaki Radiation Chemistry Research Establishment is described as follows: Trioxane used in the experiment is purified by distillation over calcium hydride in a flow of nitrogen followed by crystallization and is then crushed to a certain size. Irradiations are carried out in air using electrons from a Cockcroft-Walton-type accelerator of 3 MeV. After irradiation, polymerization is carried out in a pressurized vessel at temperatures below 50°C, the melting point of trioxane. The polymer yield is dependent on the pre-irradiation dose, post-polymerization temperature and reaction time. When samples were irradiated with 1.3 X 106 rad in air, and were afterwards polymerized in air at different temperatures, polymer yields finally reached a conversion of about 80%, except in the case of the 58°C temperature. The saturation conversion yield depends on the pre-irradiation dosage, as shown in Fig. 4. In the range between 0.05 and 0.14 Mrad, the 32 DANNO

100

a 55 °C 96 h

• 50 °C 96 h O 45 °C 96 h X 55°C 3 h

5 6 IxlO Jx10 1x10

PRE-IRRADIATION DOSE (Mrad)

FIG. 4. Effect of total dose on conversion of post-polymerization of trioxane at various temperatures. saturation yield is proportional to the irradiation dosage, and above 0.20 Mrad the yield reaches about 80% conversion. The advantages of radiation-induced solid-state polymerization are that pure trioxane is easily obtained and the heat of reaction in the polymerization process is smaller compared with polymerization of formaldehyde. The distinctive nature of polyoxymethylene produced by radiation- induced polymerization of trioxane means that the polymer consists of highly three-dimensionally ordered crystals and has twin configuration.

2.4. Radiation-synthesis of chemicals

There has been a growing interest in the radiation-synthesis of chemicals. There are many fields in which research has been carried out with some success. These are the production of chemicals by halogenation, oxidation, sulphoxidation, and many effects in catalytical reactions. The application of radiation to produce ethyl bromide [11] was a beginning in demonstrating what kind of applications can be made by a radiation chemical industry, and how a relatively small radiation source can initiate a reaction. Ionizing radiation is generally useful for initiating free radical reactions. During the lifetime of the free radicals, they can undergo a number of types of reactions, such as abstraction, addition and recombination reactions. There are many examples of free radical chain reactions. In the case of free radical polymerization, since the product radicals are similar to the reactant, only one type of propagation is needed. There are many free radical chain reactions in which addition- abstraction combinations regenerate the original radical that provides cyclic paths to long chains. These examples are bromination of ethylene PL-236/1 33

(Dow-process), chlorination of hydrocarbons (in Japan), sulphoxidation of hydrocarbons (Esso-process) and oxidation of paraffin (in Romania). Radiation-initiated chlorination of hydrocarbons [12] has been studied at the Takasaki Radiation Chemistry Research Establishment. Direct chlorination by gamma radiation is one of the outstanding examples of radiation-initiated free radical reaction. It was found that the chain reaction can be initiated and maintained if chlorine is fed continuously. The following reactions are thought to occur in the reaction of a compound of CnHmClp, where n, m, and p are integers and m + p = 2n + 2:

CnHmClp-—> " CnHm Clp-i + • CI

C H ' n m-l Clp + -H

• CnHm-i Clp + Cl2 CnHm-i Clp + 1 + • CI

The above reactions occur only under irradiation and are not affected by temperature or pressure. As soon as the chlorine radicals are formed the chlorination proceeds by the following chain reactions:

• CI + Cn HmClp -» • CnHm_! Clp + HC1

" CnHm-iClp + CI2 - CnHm-iClp + i + - CI

Hydrocarbons are thus chlorinated successively to more highly chlorinated hydrocarbons and with extremely high yields. The G-values for the radiation-initiated chlorination of 1, 2-dichloroethane (DCE) were measured as functions of radiation intensity, flow-rate of chlorine gas and reaction temperature. The effects of the dose-rate and the flow-rate of chlorine gas on the G-value of conversion of 1,2- dichloroethane are shown in Table III. It is seen that G(-DCE) decreases with increasing dose-rate and increases with increasing flow-rate of Cl2.

TABLE III. EFFECTS OF DOSE-RATE AND FLOW-RATE OF Cl2 ON CHLORINATION YIELD

Dose-rate3 CI,, flow-rate^ G(-DCE) G(-DCE) CIO3 rad/h) (ml/min)

250 4. 9 X 103 300 6.7 X 103

3 120 8. 6 X 10 500 8. 9 X 103

22 4. 5 X 10* 700 12.4 X 103

4 8,6 11.4 X 10 • 1200 16.0 X 103

a Temperature,20°C; Cl2 flow-rate, 500 ml/min.

b Temperature,20°C; dose-rate, 1.2 x 105 rad/h. 34 DANNO

o 2.2 S li. 2.0 °

0 0 20 AO 60 eo 100 120

REACTION TIME (min)

FIG. 5. Changes of product components from the chlorination of dichloroethane (EDC) as a function of 5 reaction time at 1.2 X 10 rad/h, 500 ml of Cl2/min, and 20°C.

O = dichloroethane V = pentachloroethane = 1,1,1,2-isomer A = trichloroethane • hexachloroethane = 1,1,2,2-isomer O = tetrachloroethane chlorine

A positive temperature coefficient was observed, dependent on the degree of chlorination. Figure 5 shows changes in the component of chlorinated compounds in the chlorination of 1,2-dichloroethane as a function of irradiation time. At the beginning of the reaction trichloroethane was first produced, then its concentration reached about 40 mole %, and production of tetrachloroethane began. Two isomers of tetrachloroethane were formed. The maximum yield of trichloroethane was obtained at about 70 mole %. The amount of chlorine reacting at this yield was 1.0-1.1 mole/1.0 mole of 1,2-dichloroethane. By increasing the reaction time, concentration of higher chlorinated ethane increased, as shown in Fig. 5.

3. RADIATION SOURCES USED IN RADIATION PROCESSES

The possibility of radiation chemical processes becoming competitive with conventional processes will largely depend on the radiation sources in economical use. The radiation sources now widely used are radioisotopes and particle accelerators. Radioisotopes employed in industrial irradiation plant are mainly 60Co with some 137Cs. On the other hand, accelerators used in radiation chemical processes are mainly electron beam accelerators of 0.5-3 MeV and of several kilowatts, with some X-ray generators of 0.2-0.5 MeV and a few kilowatts. It is difficult to choose the most useful types of radiation sources in industrial plant, because the choice depends on the nature of each chemical process.

3' PL-236/1 35

For industrial treatment, a 80Co source seems to be most appropriate for large objects, such as packaged goods, and for processes such as polymerization where low dose-rate irradiation is important. These sources can be extremely reliable, and once installed require little specialist supervision. For a special case, a 137Cs source (0.67 MeV) appears to be suitable because it requires less shielding for the radiation. The advantages and disadvantages of using radioisotopes in industrial production are as follows:

(1) Advantages

(a) Ratio of output energy to the weight of source is very large, so that an irradiation facility can be of a compact size. (b) The half-life of radioisotopes is longer for 60Co and 137Cs, so that the maintenance of an irradiation facility is easy. (c) In general, the induced activity of a product by gamma rays does not occur below 2 MeV of gamma radiation. (d) Lower dose-rate irradiation is possible, and dose-rate dependence on chemical reaction can be reduced. (e) Variation of dose-rate is small, so that operation of facilities is extremely reliable. (f) Radiation is emitted in all directions from the source, and irradiation can be made at any position.

(2) Disadvantages

(a) In the case of gamma rays, the amount of shielding is large so that the cost of shielding per output energy of radiation is high. (b) Radiation cannot stop when the source is not in use. (c) Care must be taken to prevent contamination of product and facility from the leakage of radioisotopes. (d) Higher dose-rate irradiation is not possible. (e) The dose-rate decreases over a long period, so that a certain amount of radioisotope must be charged each year. (f) Shielding of the radiation is necessary in all directions so that the cost of shielding increases.

The number of electron beam accelerators has greatly increased in the paist few years. They include electrostatic generators (Van de Graaff type), pulsed generators (linear accelerator type), transformer systems (reso- nant transformer type and ICT type) and capacity devices (Cockcroft- Walton type and Dynamitron type). At present the highest power output, of about 10 kW or more, is available from transformer devices, operating at somewhat lower voltages and particularly suitable for thin materials. As design progresses, the power from these machines has been growing very rapidly. Electron-beam accelerators can provide high dose-rate irradiation in a relatively small volume and for processes such as cross- linking where a high dose is necessary.

3.1. Large-scale radioisotope sources

For industrial treatment large sources comprising many thousands of curies of radioactive element are used. A 60Co source is at present 36 DANNO

available for industrial purposes, and is preferred for a number of industrial applications where continuous working and high penetration are important features. To reduce the shielding requirement, a ^Cs source is used in special cases such as movable irradiation equipment. There are several types of large radiation sources for industrial use. They are, for convenience, divided into the rod type, plate type, cylindrical type and honeycomb type sources. Characteristics of these sources are described below.

3. 1. 1. Rod-type source

This type is usually constructed in a stainless-steel tube in which the pellets, coins or rods sources are encapsuled. To obtain uniform radiation intensity along the axis of the rod, two small pieces of rod with high activity are arranged on both ends. The rod-type source makes handling simple and easy. Maximum radioactivity of the single rod-type source is limited by heat generation and self-absorption, which are due to absorption of radiation in the rod. At present, several thousand curies of 60Co sources are available.

3. 1. 2. Plate-type source

Plate-type sources are composed of a number of rod-type sources assembled in parallel with each other to form a plate. The plate-type source is most appropriate for industrial treatment because there is no limitation on samples as regards shape and volume. Samples to be irradiated are moved in parallel along the surface of the plate. A large number of samples can be treated by moving them successively. To make irradiation uniform, samples are moved between a pair of plates, or on both sides of the surface of a plate. In the latter case, the irradiation efficiency of the plate source increases remarkably.

3. 1. 3. Cylindrical-type source

This type has been widely used as an experimental source. A cylindrical-type source is composed of a number of rod-type sources assembled in a circle to form a cylinder. This source appears to be suitable not only for research but also for production. The maximum radiat-ion intensity is available in the centre of the cylinder where high dose-rate irradiation experiments are possible. The lower radiation field outside the cylinder is also used for irradiation of materials in large volume. At present, about a hundred thousand curies of 60Co source is being used in a pilot-scale experiment.

3. 1.4. Horieycomb-type source

A honeycomb-type source is composed of a number of rod-type sources assembled in a hexagonal to form a honeycomb structure. Because of the special character of this source it is possible to obtain an almost uniform radiation field in the centre of each hexagonal. This source may be used for irradiation of gaseous or liquid samples in a flow system. PL-236/1 37

J 4.304 x10 — 5.1B9x'l0 '| 2.725 x 10* ' x io;| w^T3.787 x 102

7.610x10* 8.767 xlO2 6.598 x I0!

CYLINDRICAL TYPE PLATE TYPE HONEYCOMB TYPE

FIG. 6. Radiation intensity distribution of plate, cylindrical and honeycomb type sources. Each source is composed of 42 rods having the same amount of activity. Figure 6 shows a comparison between radiation fields in the plate-, cylindrical- and honeycomb-type sources [13]. Each source is composed of 42 rod-type sources having the same amount of activity, and has nearly the same volume inside the source. It is shown that the radiation field obtained from the pair of plates is lowest and the field obtained from the honeycomb-type source is highest when the same amount of radioisotopes is installed. Moreover, the uniformity of radiation field inside the source is in the order honeycomb, cylindrical^plates.

3. 2. Dose and dose-rate required for radiation treatment

The dose and dose-rate required for radiation chemical processes are summarized in Table IV. Radiation doses required for chemical processes are dependent on irradiation conditions, such as kind of radiations, dose-rates, reaction temperatures, reaction pressures and addition of sensitizers. In the case of cross-linking of plastics and radiation- induced grafting, they are also dependent on the properties of the product to be produced. Moreover, the dose-rate has an influence on the rate of reaction and the G-value of the product. Therefore, very close attention must be paid to the use of radiation sources.

3. 2. 1. Influence on rate of reaction and G-value

The following relationships are obtained for radiation chemical reactions: Rate of reaction x In G-value I""1 where I denotes dose-rate and n is a certain constant determined by the reaction kinetics of the chemical process. In general, the value of n is in the range of 0.5-1.0. 38 DANNO

TABLE IV. THE DOSE AND DOSE-RATE REQUIRED FOR RADIATION CHEMICAL PROCESSES

Dose Dose-rate Chemical process Radiation3 (Mrad) (rad/h)

Nitrogen fixation 550 - 10" F.S.

(N2 + o2 = NOz)

Cross-linking of 4 - 40 10s - 1010 e polyethylene

Vulcanization of 10 - 50 106 - 1010 X, e neutral rubber (with sensitizer) 3 - 5

Curing of polyester 0.1-5 105 - 1010 X, e

Graft copolymerization 0.1 - 5 104 - 109 X, e (in general)

Polymerization 0. 1 - 10 104 - 10® X, e (in general)

Polymerization of 0. 05 - 2. 0 104 - 108 X, e trioxane

Ethyl bromide 0.24 - 105 X

(C2H4 + HBr = C2 H5 Br)

Sulphoxidation of 0.2 - 105 X hydrocarbons

Oxidation of 0.25 - 104 X paraffin

a F. S. = fission fragment, e = electrons, X = gamma rays or X-rays.

No problem on the dose-rate effect is observed when n = 1. However, when n is smaller than unity, with increasing dose-rate the G-value decreases and the radiation dose required for the process increases. On the other hand, with decreasing dose-rate, the rate of reaction decreases and the volume of the reaction vessel must be enlarged to keep a constant rate of production. Therefore, an optimum economic dose-rate can be determined for the given process. In the case of graft copolymerization by the direct method, when the dose-rate is greater than the certain limiting value, the value of n may sometimes be smaller than 0.5 because of the diffusion of the monomer from the surface to the interior of the material. PL-236/1 39

3. 2. 2. Influence on the molecular weight

The molecular weight of the polymer obtained from the radiation- induced polymerization is inversely proportional to In. For example, in the case of radiation-induced polymerization, the effect of the dose- rate on the molecular weight [14] is shown in Fig. 7, which shows the effect of the dose-rate in the range from 5.8X 104 to 4.7X 105 rad/h on the intrinsic viscosity, rj< at pressures of 680 and 340 atm respectively. Each experiment was irradiated for a total dose of l'.OX 105 rad at a temperature of about 20°C. At the conditions indicated the intrinsic viscosity is inversely proportional to the intensity.

TOTAL DOSE 1x 105 rad REACTION TEMPERATURE 20 °C

0 2 3 4 5 6

DOSE-RATE (105 rad/h)

FIG. 7. Effect of dose-rate on intrinsic viscosity of polyethylene produced by radiation.

The same effect may be observed in the case of graft copolymerization in the direct method. The number of grafting sites and length of grafted polymer (or molecular weight) may be dependent on the dose-rate. There- fore, to produce a given molecular weight of grafted polymer or the appropriate properties of graft copolymer, the choice of dose-rate is a very important factor. 40 DANNO

3. 2. 3. Generation of heat and accumulation of charges

When materials are irradiated with high dose-rate using electrons from accelerators, generation of heat and accumulation of charge in the irradiated materials bring about serious damage. Melting of the exit window on the accelerating tube and discharge of electrons accumulated in the irradiated material are reported. Generation of heat, Q (cal/g min), absorbed by radiation in the irrra- diated materials, is calculated by the following equation:

Q = 2.4 I

and temperature rise, T (deg C/g min), of the irradiated material is given by T = 2.4 I/C

where I is the dose-rate in Mrad/min and C (cal/g deg C) is the heat capacity of the material. Figure 8 shows the relationship between the dose-rate and heat generated in the irradiated material. In the case of polymers, since the upper limit of heat generated may be determined from the melting point of the polymer, the limit is reasonably to settle the value of 10 cal/g min. The heat generated by gamma rays is very low because the maximum radiation intensity by gamma-ray sources is lower than 10® rad/min (6 Mrad/h). On the other hand, the heat generated by elec-

DOSE-RATE (rad /h g)

FIG. 8. Heat generation from absorption of radiation. PL-236/1 41 tron irradiation is so high that the beams must be spread over a large area by the scanning technique. Another source of heat generation induced by chemical reaction must be taken into consideration. In pilot-scale experiments, it is a serious problem to eliminate heat produced by radiation and by chemical reaction from the irradiated materials. Therefore, irradiation with a high dose- rate is not appropriate in large-scale experiments. For example, when materials are irradiated by scanning with an electron accelerator at 10 kW, the beams being 50 cm wide and the sample moving at a speed of 10 cm/min, the absorbed dose deposited in the material is

D (rad/min) = 1.2 X 105 (rad/min cm2) and heat generation by absorption of radiation, assuming the thickness of sample is 0.5 cm and the density is 1.0 g/cm3, is

Q (cal/min) = 6.4X 10"1 (cal/min g)

On the other hand, the heat of reaction in the case of radiation-induced graft copolymerization is considerable, provided that the G-value of polymerization is 104 and the heat of polymerization 15 kcal/mole,

Q1 (cal/min) = 37.4 (cal/min g)

Comparison of the heat generation of Q and with that of Q' shows that heat generated from the chemical reaction is several times greater than that from radiation.

REFERENCES

[1] CHARLESBY, A., Atomic Radiation and Polymers, Pergamon, Oxford (1960). [2] CHAPIRO, A., Radiation Chemistry of Polymeric Systems, Interscience Publishers, Chichester (1962). [3] CHEMICAL WEEK, 17 Dec. (1966) 84. [4] LAMM, A., LAMM, G., Application des Rayonnements en France 1 (1965) 139. [5] RIEKE, J. K., HART, G. M., SAUNDER, F. L., J. Polym. Sci. C_4 (1963) 589. [6] FURUHASHI, A., KADONAGA, M., Rev. elect. Commun. Lab. , Tokyo 12 (1964) 775. [7] GOTODA, M., KAGEYAMA, E., NOZAKI, F., UENO, T., MATSUDA, O., UDAGAWA, A., Int. Symp. Macromolecular Chemistry, Paper 5/2/06, Tokyo Kyoto, 1966. [8] STEINBERG, M., COLOMBO, P., EL KUKACKA, L., CHAPMAN, R.N., ADLER, G., Rep. BNL-740 (T-238) (1963), [9] MACHI, S., HAGIWARA, M., MITSUI, M., KAGIYA, T., Int. Symp. Macromolecular Chemistry, Paper 4/2/08, Tokyo Kyoto, 1966. [10] SAKAMOTO, M., ISHIGAKI, I., SHIMIZU, A., KUMAKURA, K., NISHI, M., YAMASHINA, H., IWAI, T., ITO, A., Int. Symp. Macromolecular Chemistry, Paper 4/2/16, Tokyo Kyoto, 1966. [11] HARMER, D.E., BEALE, J. S., PUMPELLY, C. T., WILKINSON, B.W., in Industrial Uses of Large Radiation Sources (Proc. Conf. Salzburg, 1963) 2, IAEA, Vienna (1963) 205. [12] DANNO, A., TSUCHIHASI, G., SODA, T., 18th Annual Meeting of Chem. Soc. of Japan (1965). [13] DANNO, A., J. atom. Energy Soc. Japan 2 (1960) 554. [14] COLOMBO, P., FONTANA, J., KUKACKA, L. E., STEINBERG, M., Rep. BNL-9043 (1965).

CHARACTERISTICS AND RANGE OF APPLICABILITY OF ACCELERATORS FOR RADIATION CHEMICAL PROCESSES

N.W. HOLM ACCELERATOR DEPARTMENT, RESEARCH ESTABLISHMENT RISØ, ROSKILDE, DENMARK

Abstract

CHARACTERISTICS AND RANGE OF APPLICABILITY OF ACCELERATORS FOR RADIATION CHEMICAL PROCESSES. A general description of the characteristics of d. c. -accelerators and linear accelerators is given. Some comments are made on the economics of radiation processing, beam characteristics, penetration characteristics, process control and conversion efficiency.

GENERAL REMARKS ON ACCELERATORS

As little practical process information is available at present on the characteristics and range of applicability of accelerators for radiation chemical processing, the subject has to be dealt with in very general terms. Two basic types of electron accelerator are capable of radiation chemical processing: d. c.-accelerators and linear accelerators. Most of the d. c.-machines operate in a comparatively low energy range, 0.3-4.0 MeV. When the electrons are applied directly, the penetrating ability sets narrow limits to the applicability, the useful range being about 1-15 mm of water-equivalent material. These machines are therefore primarily useful for irradiation of thin materials like plastic tubes and films, textiles, fibres and coatings, or for other surface treatments, particularly in sterilization. The chemistry behind these applications will not be discussed here. Linear accelerators have lately gained in popularity after a somewhat slow start as far as industrial application is concerned. Manufactured for use at higher particle energies, they can handle much bulkier materials and have been used particularly for radio-sterilization of medical products. There is at present no linear accelerator (except those at multi-purpose or service installations) in operation on a strictly radiation chemical process. It is likely that this situation will change for several reasons. One is the construction of the two-mile linear accelerator at Stanford, California. At a later stage this accelerator will supposedly be fed by approximately 1000 klystrons; about 300 are used at present. Such a plant brings with it more powerful and much cheaper power sources, i.e. klystrons, and it also introduces vital parts such as accelerator wave guides on a true factory assembly line. It is clear that this event could mean an economic breakthrough for process linear accelerators. Another reason is tö be found in the neat and tidy directional emission of radiation energy from the linac compared with isotope sources. It is simply easier to obtain a good utilization of the radiation.

43 TABLE I. CHARACTERISTICS OF VARIOUS TYPES OF ELECTRON ACCELERATORS [ 1]

Approximate Purchase Maintenance number Energy Current Power cost cost installed (MeV) (MA) (kW) ($/kW) ($/kWh) or on order

0.3a 25 7.5 6.700 1.5 10 15 6.000 0.30 Dynamitron 15 (RDI) 3.0 10 30 4.400 0.25

Insulating 0.5 20 10 10.000 0.30 - 0.60 core 44 transformer (3.0) (20) (60) (6.200) 0.08 - 0.15 (HVEC)

5.0 6.0 30 8.000 (5.0) (50.0) (250) (2. 000) Linac 63 (several 10.0 0.5 5 50.000 6.00 companies) (10.0) (2. 5) (25) (22.000) (2. 00)

Resonance 1.0(peak) 5.0 - 4 17.750 1.00 200 transformer (GE) 2.0 (peak) 6.0 10 13.100 0.50

1.5 1.6 2.5 24.000 2.00 - 3.00 Van de Graaff 200 (mainly HVEC) 4.0 1.0 4.0 39.000 3.00 - 4.00

0.6 10 6 - - BBCa 2 3.0 10 30

a Information insetted by N.W. Holm. PL-236/10 45

The applicability of both types of machine can be vastly expanded by conversion of the electrons into the more penetrating bremsstrahlung. This method of irradiation of very bulky items, however, does not seem to offer serious competition to gamma-emitting isotope sources, at least not at present. After treating the subject in the most general terms, it may be of interest to comment in more detail on some selected accelerator characteristics as these may reflect other aspects of the applicability. The subjects dealt with are the economics of radiation processing, beam characteristics, penetration characteristics, process control and conversion efficiency.

ECONOMICS OF VARIOUS TYPES OF ACCELERATORS

As the economic factors are often given top priority, they will be covered first. This is a somewhat delicate subject as most available information is obtainable only from the manufacturers because many of the users consider their operational experiences as proprietary information. To explain this, let us take a typical "case story" as evaluated by the sales division of a company. These people will most often amortize the equipment over a ten-year period. Every customer with whom the subject was discussed stated unhesitatingly that with the fast development in this field they would not even consider an amortization period longer than five years. An assumption also frequently made in these budgets is that you can run on full load - and proceed on this load - from the very minute you push the green button the first time, an assumption that is not always correct. A dilemma is found in the estimated cost compared with the guaranteed lifetime of costly vital spare parts such as klystrons, electron guns, accelerator tubes and the like. On the one hand the company has to be conservative in its guarantees, on the other hand, to demonstrate the good economic aspects, it will have to point out that such components last much longer than the guaranteed period. These remarks are not intended to be purely critical, but merely emphasize that such budgets can be misleading when applied to other situations than those originally assumed. Table I is based on a manufacturer's information and compiled by Koch and Eisenhower [1], to which a few data have been added. It is regrettable that a more up-to-date compilation cannot be presented. It is noticeable that the prices per kilowatt decrease drasti- cally with increasing total power. The majority of the machines referred to in the table are used in research. Industrial accelerators accounted for about 150 kW at that time (1964), and machines good for a total about 450-500 kW are used in radiation processing today; this number as well as the number of applications is rapidly increasing. It should be added that several other companies produce accelerators, d. c.-machines are, for example, made also by Haefely (Switzerland) and Sames (France); linear accelerators are produced by, for example, ARCO (United States of America), CSF (France), Mullard (United Kingdom), Varian (United States of America) and Vickers (United Kingdom). It is possible that such machines are also made in Japan and the USSR. 46 HOLM

BEAM CHARACTERISTICS

The beam characteristics are, of course, mainly determined by the design principles of the particular type of machine. The d.c. -machines generate the full potential required to accelerate the electrons to their final energy, and are therefore in practice limited to energies about 4 MeV. The practical lower limit is to some extent determined by the absorption in the beam exit window. This window, often made of aluminium or titanium, separates the vacuum in the accelerator tube from the atmosphere. For an energy of, e.g. 0.3 MeV, about 15% of the beam power is lost as heat in the window. This also puts a limit on the beam current and thereby on the total power output. The lower the electron energy and the higher the current, the more heat is generated by beam absorption in the window. This heat problem is normally accounted for by forced air cooling and by scanning and/or defocusing the beam before it hits the window. It is thought that the beam characteristics shown in Table I do not represent the power limit of such machines. The scan width may be further increased, cooling systems may be improved, or several heads may be adapted to each generator unit. Turning our attention to linear accelerators, we may say that the available energy range is, in principle, unlimited. At the lower end the limit is set by economics, as electrons accelerated by microwaves are substantially more expensive than electrons accelerated in d.c. -machines. One therefore seldom finds linacs for operation below 4-6 MeV. The upper limit has in practice been set by the induced activity in the product. It is, for example, not likely that irradiation of foods will be cleared for energies above 10 MeV in the near future. Depending on the system to be irradiated, however, much higher energies could safely be applied to non-consumable products, which would increase the useful penetration range and the applicability. The beam current is a function of the klystron power, the conversion efficiency of the wave guides and the energy at which the machine is to operate. With a klystron giving an average output power of, for example, 25 kW, a beam power of 10-13 kW may be obtained. It is obvious that there is no direct constructional limitation to performance, and the windpw problem is much less significant for the higher energies at which the linacs operate, as much less of the beam energy is absorbed in the window. Another respect in which the linac is distinguished from the d. c. - ' accelerator is the pulsed mode in which it operates. The pulse characteristics are mainly determined by the type of klystron; typical figures are pulse lengths of 3-7 /us and repetition rates of 200-600 p/s. All these parameters are important for the matching of equipment and process. Another factor deserves attention: the dose-rate. In pulsed machines, where the beam is "on" perhaps only 0.1% of the time, the peak dose- rate - in the pulse - is evidently very high compared with the average dose-rate, and thus with the dose-rate of a d. c. -machine of the same output and about the same energy. This factor may have a very substantial influence on the yield for a number of radiation chemical processes, and should therefore be carefully considered. It should be noted, however, that for d. c. -machines in the lower part of the energy range, e.g. 0. 3. MeV, the major part of the beam power may be absorbed in a 1-mm PL-236/10 47 layer compared with a 35-mm layer for a 10-MeV linac, i.e. the dose- rate of such a machine may also be high.

PENETRATION CHARACTERISTICS

The penetration characteristics of radiation at a given energy have to be examined when an irradiation process is planned, to evaluate a good combination of dose homogeneity and radiation economy. This requirement is normally best met when the exit dose equals the entrance dose. Some dose distribution curves for broad electron beams of various energies impinging on a unit density target (water) are shown in Fig. 1. These curves can provide a rough estimate of the optimum material thickness to be irradiated at a given energy. For irradiation of homogeneous materials in low-energy machines the information may in many cases be most easily obtained by calculative methods as dose distribution measurements in thin samples require quite sophisticated dosimetry techniques. A simple method of calculating the conditions under which the exit dose equals the entrance dose for various energies and various low-Z materials has been developed by Cheek and Linnenbom [2] . At higher energies it is probably preferable to run a dosimetry survey for

FIG. 1. Dose distribution in water at different electron energies.

It should be kept in mind that these dose distribution curves do not necessarily represent a completely unchangeable situation. The dose distribution may be influenced by various tricks. Figure 2 [3] demon- strates how the application of a reflector plate can improve the dose distribution in a material of a given thickness. Figure 3 [4] shows how the maximum on the dose distribution curve can be flattened by using 48 HOLM

FIG. 2. Dose distribution in water at different depths.

FIG. 3. Effect on dose distribution curve caused by a scatter plate.

a scatter plate between the beam exit window and the sample. A combination of such devices can certainly lead to both better dose homogeneity and better beam utilization.

ROUTINE PROCESS CONTROL

When all evaluations are made of beam and penetration characteristics with regard to the process to be carried out, the routine process PL-236/10 49

FIG.4. System for controlling machine parameters.

control must be considered. The information we want is how to maintain and record the irradiation parameters chosen for the process in the minute-to-minute and day-to-day operation. This problem has been dealt with in a chapter on dosimetry in industrial radiation processing in a forthcoming new edition of Hine and Brownell's Radiation Dosimetry. The routine at the linear accelerator facility at Ris^ has been described by Brynjolfsson et al. (1963). Figure 4 gives an idea of how the machine parameters are at present controlled. Two secondary emission collectors are located in the scanner housing. The beam energy is derived from the current to the bending magnet necessary to bend the beam to pass symmetrically between the two collectors. When it does so, the electrons reflected from the exit window will cause identical signals from the two collectors. Otherwise one of the collectors will pick up more current than the other, and this can be made to trigger an alarm or an energy- regulating circuit. The same set-up provides a means of current control as the number of reflected electrons picked up by the collectors is proportional to the current of the primary beam. At Ris^ this signal is fed back to the conveyor drive so that a decrease in current is compensated by a simultaneous decrease in conveyor speed; thus a constant surface dose is maintained. The scanning operation is also controlled by a feed- back system from a pick-up coil at the magnet back to the power supply. When combined into a single signal, these features should take care of the necessary process control in a constant irradiation geometry. The general principles of this system should be applicable to any kind of irradiation process.

CONVERSION EFFICIENCIES

It was stated earlier that conversion of the electron beam into bremsstrahlung by letting the beam impinge on a heavy metal target would lead to much more penetrating radiation. This method has been advocated by several people as a very tempting idea. As it might be of

4 Ol TABLE II. CALCULATED EFFICIENCES FOR X-RAY PRODUCTION [1] o

Aluminium target Tungsten target

Total Forward Total Forward E • Range Range, efficiency efficiency efficiency efficiency (MeV) (g/cm2) (g/cm2) C7°) m (7»)

10.0 5.8 7.7 6.3 6.1 30 19

5.0 3.1 4.0 3.1 3.6 19 11

3.0 1.9 2.5 1.9 2.3 14 6.2

2.0 1.2 1.8 1.3 1.6 10 4.1

1.0 0.55 0.9 0.7 0.76 6 2.0

0.5 0.23 0.4 0.3 0.33 3 0.9 PL-236/10 51 interest for some special applications, and, at a later stage, when electrons may be even cheaper than today, for a much broader use, some conversion efficiency data reported by Koch and Eisenhower (1965) are given in Table II. It should be noted that only X-rays in the forward direction can be utilized and that only some fraction of these can be absorbed in the product. The complex energy spectrum of the bremsstrahlung will make it difficult to obtain a good dose homogeneity in the product unless multipass conveyor systems (or, in the case of a liquid, multipass flow systems) are provided. It is almost certain that this method will come into use some day, but it is at present not likely to be chosen for a given process. Because of lack of suitable information, practical radiation chemical processes have not been covered here. Little material has been released on such processes, but the whole field is in a state of very rapid development, and two major breakthroughs were reported during 1966 (the Deering-Millikan textile process and the Ford coating technique).

REFERENCES

[1] KOCH, H.W., EISENHOWER, E.H., Accelerators for Food Processing, N. A.S.-N.R.C. Rep. 1273 . (1965). [2] CHEEK, C.H., LINNENBOM, V.J., Calculations of Absorbed Dose, NRLRep.5448 (1960). [3] BRYNjdLFSSON, A., HOLM, N.W., THARUP. G., SEHESTED, K., "Industrial sterilization at the electron linear-accelerator facility at Ris^", Industrial Uses of Large Radiation Sources (Proc. Conf. Salzburg, 1963) 2, IAEA, Vienna (1963) 281. [4] BRYNjdLFSSON, A., THARUP, G., Determination of Beam Parameters and Measurements of Dose Distribution in Materials Irradiated by Electrons in the Range of 6 MeV to 14 MeV, Rise! Rep. No. 53 (1963).

STATUS REPORT ON CHEMONUCLEAR REACTORS AND RADIATION CHEMICAL PROCESSING

M. STEINBERG BROOKHAVEN NATIONAL LABORATORY, UPTON, N.Y., UNITED STATES OF AMERICA

Abstract

STATUS REPORT ON CHEMONUCLEAR REACTORS AND RADIATION CHEMICAL PROCESSING. The use of nuclear energy for the synthesis and production of industrial chemicals is reviewed. Fission-fragment chemonuclear reactors are used for basic endothermic reactors in large capacities. These are applicable to the fixations of nitrogen, synthesis of ozone and synthesis of carbon monoxide from low-cost, readily available, raw materials. The development of fissiochemonuclear fuel elements, fissiochemistry, the contamination problem, the in-pile loop experiments and the economics of chemonuclear reactors are reviewed. The use of isotopic and machine sources is mainly reserved for exothermic chain-type reactions. These sources range from 6°Co, 13,Cs, 9°Sr to liquid 24Na and In-Ga loops to 0.3 to 20 MeV accelerators. Polymerization, partial oxidation and halogenation have partially reached commercial status. New areas for research include heterogeneous radiation chemistry, cryogenic radiation chemistry, electrode reactions, plastic material combinations, copolymerization and ionic polymerization.

INTRODUCTION

The possibilities of the use of high-energy radiation for the synthesis and production of industrial chemicals and chemical processing purposes were first considered in the later 1940s. Serious research and development work began in the early 1950s and has grown through several cycles and degrees of interest and support to the present day. High-energy radiation chemical processing has attracted considerable research expendi- ture; however, the practical application to operating processes has been relatively slow in developing, and relatively few large-scale radiation- induced processes exist today. The potential, however, still exists, and considering that the subject is comparatively new, it appears to many to be worth much further exploration. Research and development in the early days, were based on a trial- and-error basis in a wide variety of systems. This was mainly carried out in the United States of America by private corporations who quickly became disillusioned because they found that radiation was not the panacea for all their existing conventional chemical processing problems. The research then reverted to more fundamental studies on the effects of the interaction of high-energy radiation with matter. During this time the USAEC began to support an expanding programme for applying radiation to industrial processes and this has continued to the present day. The USAEC has also strongly supported a very fundamental research programme in basic radiation chemistry. In addition to the studies and programmes in the United States of America, work has been going on in maijy countries throughout the world and comprehensive programmes have

53 54 STEINBERG been particularly developed in France, Japan and the Soviet Union. I believe that today we have a better understanding as to where and how the various kinds of high-energy radiation could apply, and because of this certain patterns of development can be traced. In reviewing the status of radiation chemical processing, a discussion is presented, starting first with a major classification of radiation sources. Then, under each category, the type of process that best fits the source is reviewed. This is followed by a discussion of the development of the design and fabrication of the source. Finally, the processes that either have been developed, are in advanced stages of development, or are in the early stages of research and development, are discussed.

I. FISSION-FRAGMENT AND REACTOR RADIATION PROCESS DEVELOPMENT

The use of fission-fragment recoil energy for inducing chemical changes in matter is a direct conversion of nuclear to chemical energy. This type of energy is primarily made available in a uranium fission

NONCHAIN REACTIONS £ o ENOOTHERMIC ., CHAIN REACTIONS-EXOTHERMIC—^ z>

O.I1 L. _l I I I 1 \ 1 0.1 * 0.1 1.0 10 100 1000 10 000 100000 1000 000 G-VALUE - MOLECULES/lOOeV

FIG. 1. Chemonuclear reactors: power requirement versus G-value for average product MW = 50. reactor. For large capacities, it appears to be the lowest cost form of high-energy radiation. Because of the large capacity, the application of this type of radiation is usually reserved for endothermic, non-chain, low yield, low G-value reactions, as shown in Fig. 1.

I.l. Fission-fragment source and fuel development

Because of the low range of fission fragments in solids, usually <10 jum, it becomes necessary to design fission-fragment solid sources with at least one dimension less than this range. This is because a significant fraction of the energy of the fission fragment can be released PL-236/1 55 in the chemical reactant. Initially uranium containing glass fibre of 3 and 6 jum in diameter was developed. Energy deposition efficiency of about 50% was available. The stability of this material to thermal and radiation damage, however, is limited. Work has been performed by several investigators on development of metal and cermet fibre fuel in micron diameter sizes. The most advanced form of fission-fragment chemonuclear fuel, developed at Brookhaven National Laboratory (BNL), is a 2. 5-/um thin foil of an alloy of uranium in palladium and coated with platinum. This shows good characteristics for mechanical, thermal, chemical and radiation stability. A fission-fragment energy deposition efficiency of 35% has been obtained with this element. A multiple array honeycomb fuel element of this material was fabricated. The foil has been used in many in-pile capsule irradiation experiments.

1.2. Fission-fragment radiation chemistry and yield values

The main chemical reactions that have been investigated using fission-fragment energy in capsule experiments are as follows:

(1) Formation of nitrogen oxides (fixed nitrogen as N02 and NzO) from nitrogen and oxygen. (2) The synthesis of carbon monoxide (CO) by the decomposition of carbon dioxide.

(3) The synthesis of ozone (03) from oxygen.

(4) The formation of hydrogen (H2) and hydrogen peroxide (H2O2) by decomposition of water. (5) The synthesis of hydrazine from liquid ammonia. (6) The synthesis of hydrocyanic acid (HCN) from nitrogen and methane.

(7) The synthesis of ethylene glycol (CH2OH)2 from methanol. The most extensive work with fission-fragment energy has been done in the homogeneous irradiation of gas-phase nitrogen and oxygen for the fixation of nitrogen in static capsule experiments. One can convert a large fraction of the nitrogen and oxygen, forming high concentrations of oxides of nitrogen. The problem has been in obtaining yields that would be economically attractive. For example, a G-value of 2 for N02 formation can be readily obtained; however, it is necessary to obtain a G-value of 6 to compete economically with competitive nitrogen fixation processes. Recent work at BNL has indicated that there is a possibility that a two-phase gas-liquid aqueous system may bring improvement in the yields. Because of the huge fertilizer market, this system continues to attract continual research studies. The yields of carbon monoxide and the formation of ozone appear to be, for the moment, economically more attractive. The carbon monoxide system is of great interest for chemical synthesis gas and pipeline gas production, whereas the ozone system is of interest for possible water purification and chemical oxygenation processes. The other systems mentioned above suffer either from smaller market demand or values that do not provide the incentive for continued research and development expenditures to bring the processes to an economically competitive status. In the case of hydrazine, some capsule fission- fragment chemistry was performed and a liquid ammonia in-pile loop was built; however, since the market demand for hydrazine is low, the 56 STEINBERG work was discontinued. In the case of hydrocyanic acid, the yields obtained did not appear to be of sufficient value to continue the development. An interesting point concerning the fundamental radiation chemistry of fission-fragment radiation is that in very few instances has it been found that the high linear energy transfer (LET) radiation from the heavy fission fragments yields results that are different from the low LET gamma- and beta-type radiation. This means that the radiation chemistry of fission fragments can be carried on more economically and conveniently with non-reactor radiation, and these results can then later be verified by reactor experiments.

1.3. Contamination problem

The price one pays for obtaining large quantities of low-cost high- energy radiation is the possibility of contaminating the product with radioactive fission products. Work to date in capsules indicates that the gaseous products are contaminated with halogen and noble gas fission fragments. This contamination can be eliminated by standard chemical engineering operations. However, this has yet to be demonstrated on larger quantities of material produced with fission-fragment energy. The determination of yields from radiation chemistry studies still remains the main area for investigating the development of chemonuclear processes. Unless sufficiently high yields are obtained, a large effort in developing decontamination studies is not warranted.

1.4. In-pile loops

A slurry loop of uranium oxide in liquid ammonia was operated by the Aerojet-General Corporation in the Materials Testing Reactor at Idaho Falls at various times in 1965. The loop was constructed to synthesize hydrazine, and some yield and concentration data were obtained. The loop has since been removed and dismantled. A chemonuclear in-pile loop or small circulating reactor radiation pilot plant is nearing completion at the site of the Brookhaven Graphite Research Reactor. With this loop it should be possible to investigate all the chemical systems so far considered for chemonuclear processes. Pressure conditions up to 68 atm, temperatures up to 540°C, and fission- fragment energies up to 5000 W are the design capabilities of this loop. Chemical analysis can be made with a sophisticated continuous in-line chemical instrumentation system.

1.5. Status of chemonuclear reactors

No fission-fragment reactor has been built to date. Research and development has indicated that this form of high-energy radiation is most applicable to the production of large-capacity low-cost chemicals that feed on low-cost raw materials. The potential seems at present to be mainly limited to the following processes: (1) The fixation of nitrogen from air for fertilizer production. (2) The production of carbon monoxide from carbon dioxide for synthesis and pipeline gas production. (3) The synthesis of ozone from oxygen and air for water purification and chemical oxygenation processes. PL-236/1 57

Other systems that have potential but are of less immediate interest are those that yield hydrogen peroxide from water and hydrazine from ammonia. Table I indicates some of the capacities that are needed to make these systems economically competitive under US conditions. Fission-fragment chemonuclear reactors essentially compete with electrochemical processes. The energy required for these processes can be derived either from conventional fossil fuel plants or from nuclear power reactor sources. In fact, in some of the systems considered above are dual cycle processes whereby both direct nuclear-to-chemical and indirect nuclear-to-power-to-chemical production schemes are combined to obtain an economically optimum system. The development of the radiation chemistry and the determination of radiation chemical yields in both static and flow systems remain the greatest and most fertile research area for developing reactor radiation for production purposes. As the means for obtaining increased yields are developed, the less there is a need for high efficiency fission-fragment fuel, and the closer one gets to a conventional power reactor type fuel with increased thickness. A greater utilization is thus made of the reactor neutron and gamma energy. In the United States of America, research and development effort in chemonuclear processes is mainly supported by the USAEC in universities and national laboratories.

II. ISOTOPIC AND MACHINE RADIATION PROCESS DEVELOPMENT

As can be seen in Fig.l, isotopic and machine-produced radiation sources are mainly suitable for exothermic, chain-type, high-yield, high G-value reactions. These reactions usually involve such processes as polymerization, oxidation and halogenation.

II. 1 .Sources

The most highly developed gamma source is 60Co. Bonded, doubly encapsulated, stainless-steel strip sources of 60Co have been developed at BNL, and can be made available commercially. Cylindrical pellets, wire and other shaped 60CQ sources are available in various countries throughout the world. Sources of 137Cs in stainless-steel slabs have been developed at BNL and are under test. Other forms of 137Cs sources of integrity are available in France. Small 9c5r beta needle sources are also commercially available. The larger 90Sr clad sources have yet to be developed for commercial use. Shorter-lived liquid metal loops coupled to reactors for supplying gamma energy have had less application in the United States of America than in the Soviet Union. The 24Na system in the Hallam, Nebraska, reactor did not materialize because the reactor was shut down. However, several liquid-metal loops (i.e. In-Ga) have been reported to have been operated in the Soviet Union. Electron accelerators for beta energy have been mainly popular in the 2- to 3-MeV energy range. The bulk of the radiation energy used in industry today, mainly for sterilization and cross-linking purposes, is derived from electron accelerators. A new generation of machines providing energies in the 0. 3-MeV range, mainly for treating thin coatings, TABLE I. CHEMONUCLEAR PRODUCTION COST ESTIMATES 01 CO

Fixed Production Reactor Total Plant Energy0 charge Product3 System G-value'5 cost power plant cost capacity (kWh(th)/lb) rate V P(MW(th)) ($ million) T (tons/d) f

1. Fixed nitrogen as Dual 6 11.1 0.07 $30/eq 2000 128 2150 N

N in (NH3+ N02) with cycle ton NH3 Oj by-product $4/ton Oj 5750 02

2. Fixed nitrogen as Dual 11 8.5 0.14 330/eq 3300 220 4650 N

N in (NH3 + NOz) cycle ton NH3

3. Fixed nitrogen as Dual 6 29.0 0.07 $25/eq 1300 98 540 N

N in (N02) and purpose ton NH3 electrical power 4. 0 mils/ 400 MW(e) by-product kWh(e) cn -3 4. Synthesis gas Dual 20 2.9 0.07 $0.30/10sft3 1250 98 5900 ra 12 3 t—t (CO + H;) with cycle (HjtCO) (160 X 10 ft ) Z Oj by-product til $4/ton O; (CO + Hj) 5000 0 2 o 5. Synthesis gas Dual 10 4.0 0.07 $0.60/106ft3 1890 135 6300 3 (CO + H2) with cycle (H2+ CO) (171 X 10" ft ) O by-product z $4Aon O, (CO+Hj) 8000 Oj

6. Ozone, 03 Single 6 8.5 0.14 $47/ton 423 38 600 purpose

7. Hydrogen peroxide, HjOj Single 1.7 42.0 0. 25 $0.15/lb 700 51 200 purpose

8. Hydrazine, N2H., Single 2.0 38.0 0.25 $0.25/lb 100 16 30 purpose (excludes NH3 cost)

® Raw material for all products are air and water. b G-value for fission-fragment energy. c Energy requirement is total for product and by-products, includes fission-fragment and electrical energy.

Basis: tj0 - fission-fragment energy deposition efficiency = 0.50. ijp = thermal-to-electrical power efficiency = 0.311. 6 3 Energy for H2 electrolytic cells = 140 kWh(e) 10 ft H2 PL-236/1 59 has recently been receiving considerable attention in the United States of America.

II. 2. Chemical processing

The processes that either have gained commercial status or have been near commercialization using isotopic sources are as follows: (1) The Dow Process for producing ethyl bromide by the hydrobromination of ethylene. This process remains one of the few outstanding commercial radiation chemical process successes in existence today. (2) The sulphoxidation of hydrocarbons for the production of biodegradable detergents alkane sulphonates. Although reported to be in advanced stages of development, both by Esso Research and Farbwerke Hoechst, it is doubtful whether the process is now commercially practised. (3) The oxidation of paraffins has been reported to be practised on an industrial scale in Romania.

II. 3. Polymerization

Radiation-induced polymerization continues to be one of the most active areas for possible large-scale radiation processing.

II. 3.1. Polymerization of ethylene

Work at BNL has indicated that the 60Co gamma-radiation-induced polymerization of ethylene can be competitive with the high-pressure thermocatalytic process. A small-scale, 100-cm3 pilot flow plant is in the process of being operated under conditions above the melting point of the polymer where film-grade-type material is produced. The Takasaki Radiation Research Establishment is operating a pilot plant under conditions below the melting point of polyethylene where a low melt index powder is formed. The Soviet Radiation Processing Laboratory at Obninsk is reported to be investigating the radiation polymerization of ethylene. A Farbwerke Hoechst group in the Federal Republic of Germany is also involved in this kind of investigation.

II. 3. 2. Solid-state polymerization

An experimental pilot plant has been built and operated by the Takasaki Laboratory for the polymerization of crystalline trioxane with machine radiation.

II. 3. 3. Curing of coatings

There has been a recent intense interest in the United States of America in the use of machine radiation for the curing of polyester and other monomer coatings on various materials. The advantages of machines over other types of radiation and over curing by conventional thermal means are the relatively low investment cost of the system and the high rate of production. The Ford Motor Company, The Boise Cascade Company, and the Radiation Dynamics Company are leaders in this field in the United States of America. 60 STEINBERG

II. 3.4. Wood-plastic combinations

The West Virginia University, with support from the USAEC, has pioneered in developing plastic-impregnated wood. The American Novawood Company and the Lockheed Georgia Company have since acquired the capability of turning out relatively large quantities of wood- plastic combinations for test and commercial purposes. Advantages of the radiation process over thermocatalytic processes relate to the poor thermal conductivity of wood and the ability of gamma radiation to polymerize and operate at low enough temperatures to prevent vapori- zation of the monomer, thus providing a more uniform impregnation.

II. 3. 5. Graft copolymerization

The grafting of monomers to various fibres has attracted considerable' interest in the United States of America. These processes allow improved qualities of "permanent press", easy removed of soil by washing (referred to as "soil release"), and improvements in dyeability and improved "wash- and-wear" properties to be built into textiles and clothing made of radiation-induced graft-copolymerized fibres. The firms of Dow, Deering-Milliken, Burlington Industries and Takasaki Laboratories are actively working in this field.

II. 3. 6. Polymer irradiation

The irradiation cross-linking of polyethylene for producing improved electrical insulating tape and food wrapping film has been a successful commercial venture. The foaming of polyethylene has been reported to be commercialized in Japan. The cross-linking of natural rubber latex has been reported in the pilot plant stage at Saclay in France. A radiation process for cross-linking of polyethylene oxide to increase its viscosity and improve other properties has been reported by the Union Carbide Company.

III. NEW AREAS FOR RESEARCH

Investigations of new radiation processes are undoubtedly continuing in many private laboratories. Some of these could have significant applied value in the not too distant future. The following are areas of research currently being investigated, mainly at BNL.

III.l. Heterogeneous radiation chemistry

Chemical systems are being investigated to determine improved radiation utilization efficiencies and energy transfer effects. Studies on the heterogeneous radiation-induced decomposition of CO2 and formation of oxides of nitrogen over solid oxide substrates over a range of pressures and temperatures are being investigated. These could have application to reactor irradiation processes, particularly in an attempt to obtain energy transfer from fission-fragment sources. PL-236/1 61

A study on the effect of 6<>Co gamma radiation on the high-pressure thermocatalytic synthesis of ammonia indicated that there were either no improved yield effects or even slightly negative effects on the yield of ammonia.

III. 2. Effects of radiation on electrode reactions

There have been indications from various sources that higher LET radiation reduces the oxygen over-voltage in electrolytic cells. This could be of some significance in reducing energy requirements in such large electrochemical processes as those for the production of caustic and chlorine. A study is being performed at BNL on the effect of 60Co gamma and other particle radiation on the oxygen over-voltage at oxygen electrodes.

III. 3. Cryogenic radiolysis

The ability to produce a reactive radical or ionic species under almost any conditions by means of high-energy radiation has prompted an investigation at BNL on the possibility of producing thermally unstable materials by exothermic reactions. For example, the reaction of hydrogen and oxygen at liquid oxygen temperatures may produce significant yields of hydrogen peroxide. Another example is that of the irradiation of hydrogen in liquid nitrogen for the synthesis of exothermic ammonia. It is the in situ production, quenching and stabilization of reactive free radicals that one is seeking in these systems. Chain reactions would be needed to make the technique economically attractive.

III. 4. Radiation-induced oxidation process for water purification

A mine drainage waste problem is being investigated concerning the possibility of the chain oxidation of ferrous to ferric iron in the presence of an organic impurity for eventually removing iron from the mine drainage waste for disposal to ground. The production and utilization of ozone by a low-cost radiation process could also be of interest in water purification treatments. It is felt that other possibilities for water purification using radiation-induced oxidation and sterilization reactions exist, but have hitherto remained essentially unexplored. There has been relatively little research performed on this subject.

IIL5. Plastic-impregnated concrete

It has been demonstrated at BNL that concrete can be fully impregnated with polymer by irradiating with 60Co gamma radiation a piece of concrete soaked with monomer. The compressive strength has been increased by more than a factor of two and the permeability has been almost completely eliminated. There are many possible applications of this new material. Plastic-impregnated concrete would prevent the "freeze-thaw" cracking of concrete. It could improve the corrosion resistance of concrete towards chemicals and brines for desalination and chemical equipment manufacture. The full potential of this material has yet to be explored. It has also been preliminarily demonstrated that leathers can be impregnated with polymer using in situ radiation polymerization techniques. 62 STEINBERG

III. 6. Polymerization

It has been found that radiation-induced polymerization of ultrapure monomers such as styrene probably proceeds by means of an ionic mechanism. The rates of polymerization may be as much as two to three orders of magnitude higher than those of conventional free-radical polymerization. Radiation-induced ionic polymerization thus may bring economic benefits in the form of lower cost processes for the manufacture of a number of large market conventional plastics. Purity of this monomer raw material, especially with respect to oxygen and water, is of prime consideration in this case. Development work in this field could be expanded.

III.7. Copolymerization

New high-polymer compositions have been formed from very inexpensive monomers by using 60Co gamma-initiated polymerization techniques. Ethylene has been copolymerized with carbon monoxide forming stable high-molecular weight polyketones. Ethylene has been copolymerized with sulphur dioxide, forming high-molecular weight polysulphones. Further investigation of the properties and processing characteristics is required to find proper applications for these materials. The field of radiation-induced copolymerization of two and more component systems is a fertile field for investigation.

III. 8. Material purification

A relatively untouched area is the possibility of radiation-induced purification of gases, liquids and solids. For example, if it is necessary to remove small traces of oxygen from organic streams such as ethylene, it is possible to irradiate the stream, thus forming high-molecular weight oxygenated compounds that can be condensed and more easily removed from the system. The radiation-induced desulphurization and dehydro- sulphurization of petroleum raw materials have been mentioned as possible research areas in the Soviet literature.

III. 9. Radiation chemical reactor and process design studies

Both conceptual designs and experimental work on radiation chemical processes are being carried out at BNL for developing general design criteria and principles for making radiation processing a standard chemical engineering unit operation.

BIBLIOGRAPHY

STEINBERG, M., Chem. Engng Prog. 62 (1966) 105. STEINBERG, M., Chemonuclear and Radiation Chemical Process Research and Development, Brookhaven National Laboratory, Informal Rep. BNL-10020 (1966). HARMER, D. E., Symposium on Radiation Processing, American Institute of Chemical Engineers National Meeting, Detroit, November 1966 (in press). LAWRENCE, I.H., MANOWITZ, B., LOEB, B.S., Radioisotopes and Radiation, McGraw-Hill, New York (1964) 103-29. Annual and Bimonthly Reports of the Nuclear Engineering Department, Brookhaven National Laboratory, Upton, N.Y. 1966-67. SAFETY FOR LARGE IRRADIATION FACILITIES

A. DANNO TAKASAKI-SHI RADIATION CHEMISTRY RESEARCH ESTABLISHMENT, JAPAN ATOMIC ENERGY RESEARCH INSTITUTE, TAKASAKI-SHI, JAPAN

Abstract

SAFETY FOR LARGE IRRADIATION FACILITIES. Safety for large irradiation facilities on a pilot scale or in industrial use is very important not only from the point of view of radiation, but also from the standpoint of radiation chemical processing. The regulations cover safety for radiation, safety for fire and explosion, safety for high pressure and safety for corrosion by chemicals. An example is given of the polymerization of ethylene on a pilot scale at the Takasaki Radiation Chemistry Research Establishment.

INTRODUCTION

Large irradiation facilities on a pilot scale or in industrial use must naturally comply with all the safety regulations covering conventional industrial installations and must be safe from the point of view of radiation. These regulations cover safety for radiation, safety for fire and explosion, safety for high pressure and safety for corrosion by chemicals. These considerations fundamentally affect the design of an irradiation plant and reaction vessel [1]. An appreciation of these will assist in understanding the reasons behind the design.

1. SAFETY FOR RADIATION

Maximum acceptable levels of radiation have been agreed interna- tionally for different categories of persons. These are: General public 0. 25 mrad/h Non-classified workers 0. 7 5 mrad/h Classified workers 2. 5 mrad/h The plant designer will have to design the plant for a maximum dose- rate of less than 0. 7 5 mrad/h in any area to which plant operation or maintenance men can gain access. As is obvious to any person working in radiation chemistry, no induced radiation is created by treatment of materials in gamma radiation. Provision for the remote possibility that contamination from the source might be spread has, however, been taken, A nitrogen purge stream is utilized to warn of equipment failure that could result in product contamination and a radiation hazard [2], The nitrogen purge streams pass through very retentive filters to concentrate any possible radioactive contamination. The tube of a sensitive Geiger counter is located in the operation building near the product return line and the filters.

63 64 DANNO

2. SAFETY FOR FIRE AND EXPLOSION

Fire precautions may have to be taken when inflammable materials are being irradiated, such as ethylene in radiation-induced polymerization or isopropyl alcohol in the packing of sources. Then an automatic C.O2 fire-extinguishing system is needed in the irradiation cell, labyrinth and conveyor-loading room. This system must be interlocked with the detector of inflammable gas so that the system can operate as soon as the gas leaks into the cell. The sensitivity of the detector is adjusted to operate when the concentration of gas reaches above one-third of the limiting concentration of explosion. All electric equipment used in the cell or near gas stream lines must be provided with explosion-protection devices. These are electric motors, compressors, magnetic switches, etc.

3. SAFETY FOR PRESSURE [3],

Pressure vessels including reaction vessels, pressure relief tanks, product receivers, filters and heat exchangers must be designed with a minimum safety factor of three based on ultimate tensile strength. Piping, valves and compressors are designed in accordance with rules such as the ASME pressure vessel rules and so have a minimum safety factor of four. The rupture disc on the reaction vessel is set to vent to the pressure relief tank at a pressure of 10% increase in operating pressure. The system pressure would, therefore, at all times be kept below the equipment design pressure even in the event of decomposition or explosion of reactant gases. The pressure relief tank is always filled with inert gas before plant start-up. Escape of combustible gas through a rupture disc or break in the process equipment would therefore not lead to combustion and decomposition. A thermal-conductivity-type gas analyser should be located within the cell to continuously monitor the leakage of inflammable gas into the inert atmosphere. The monitor, whose sensitivity is about 500 ppm ethylene by volume, will require about 200 cm3 /min of gas sample. An ethylene concentration signal will be retransmitted to a control room meter and alarm.

4. AN EXAMPLE FOR POLYMERIZATION OF ETHYLENE ON A PILOT SCALE [4]

Radiation-induced polymerization of ethylene in a pilot-scale experimental facility at the Takasaki Radiation Chemistry Research Establish- ment passed a very strict inspection conducted by Government officials with regard to control regulations before the start-up of the facility in September 1965. These regulations related to inflammable gases, high- pressure gases and radiations. Irradiation was carried out in a concrete-shielded cell. A 60Co source with 108 000 Ci was stored at the bottom of a, pool of water about PL-236/2 65

6 m deep. A reaction vessel of 10 litres capacity and two stainless-steel reservoirs with a capacity of 45 litres are located in the centre of the cell. Before the start-up, the source is lifted above the water level and is then placed outside the reaction vessel. Experimental conditions are: maximum pressure, 410 kg/cm2, normal temperature, about 150° C, current velocity, 5-15 kg C2H4/h. To implement the above safety features, and to provide protection against other foreseeable hazards, the following equipment is provided: (a) Rupture discs or safety relief valves are attached to all vessels and process equipment. (b) Safety relief valves located on high-pressure systems are designed in accordance with the control regulations on high-pressure gases. (c) The rupture disc on the reaction vessel is set to vent at 440 kg/cm2 so that the system pressure can be kept below the operating pressure of 410 kg/cm2 even in the events. The rupture disc is made of aluminium foil so that it can be easily ruptured. (d) Interlocks and relays are installed to automatically shut off the ethylene supply and Dowtherm heater in the event of either an increase in system pressure or a sudden increase in temperature. (e) Ethylene pressure in the high-pressure line is measured at each checking point and is recorded, and an alarm is given when the pressure limit is exceeded. (f) Analysis of ethylene gas can be provided to monitor ethylene decompositions by a batch-type sampling system. (g) A carbon-dioxide fire-extinguishing system is located in the cell so that fires of any origin can be extinguished. (h) Two types of gas monitors are located inside and outside the cell to continuously monitor the leakage of ethylene gas. (i ) The inside and the outside of the cell are ventilated during operation. (j ) All electric equipment is provided with explosion-protection devices. (k) In the external source reaction vessel, the source would not be damaged in the event of an accident, as it is doubly encapsulated in stainless-steel. (1) The cell is made of high-density concrete providing both radiation shielding and protection in the event of an explosion.,

REFERENCES

[1] BAINES, B. D., Application of Large Radiation Sources in Industry, Elsevier, Amsterdam, (1964) 119. [2] • HARMER, D. E., BEALE, J. S., PUMPELLY, C. T., WILKINSON, B. W., in Industrial Uses of Large Radiation Sources (Proc. Conf. Salzburg, 1963) 2, IAEA, Vienna (1963) 205. [3] STEINBERG, M., et al., Rep. BNL-796 (T-305) (1963) [4] Cited from the Government official inspection for the safety of an ethylene pilot plant (1965)

LOW-TEMPERATURE IRRADIATIONS

Z. P. ZAGÓRSKI AND S. MINC DEPARTMENT OF RADIATION CHEMISTRY, INSTITUTE OF NUCLEAR RESEARCH, WARSAW, POLAND

Abstract

LOW-TEMPERATURE IRRADIATIONS. Radiation chemistry of deep frozen materials is reviewed from the point of view of phenomena characteristic of this particular form of radiolysis. Differences of chemical end-points in the liquid and frozen state are stressed. Special attention is paid to the identity and migration of intermediate species at the temperature of irradiation and at higher temperatures (thawing). The importance of luminescent phenomena is shown by examples. Low-temperature radiation chemistry of particular systems (from water to those occurring in outer space chemistry) is reviewed. In experimental considerations the role of the structure of frozen solids is discussed and the need for additional investigations put forward.

1. INTRODUCTION

Looking back on the papers devoted to low-temperature irradiations one is impressed by the variety of reasons that led the investigator to that particular and difficult condition of experiment. At the same time one may easily be astonished by the reckless conclusions drawn from some experiments on frozen systems whose structures are practically unknown. Unfortunately, some papers do not treat the investigation of the deep-frozen state as the "radiation chemistry of deep-frozen materials" but as a supplementary experiment carried out in addition to the "radiation chemistry of liquids". We shall try to show that the radiation chemistry of deep-frozen substances is a separate branch of radiation chemistry, and that treating this technique as a supplementary one may lead to erroneous results. We should like also to stress the need for basic research on the frozen state, on basic physical and physicochemical data such as densities, solubilities of gases, crystalline structures and size of crystals. It is obvious that we cannot review all the published literature on low- temperature irradiations. We shall omit, for example, E. S. R. measurements performed after low-temperature irradiation, quoting some papers only when they contribute to the question of true "frozen-state radiation chemistry" and are not restricted to the identification of the free radical from its signals. It is worth emphasizing that nowadays E. S. R. interpretations tend to treat the free radical not as an entity isolated from the matrix, but as an integral constituent of the system. During the past few years a large number of inorganic radicals have been identified by the E.S. R. technique in irradiated, usually frozen, solids. Many unstable species are satis- factorily described as small molecules and the electron spin resonance results are well interpreted by simple molecular orbital theory for such molecules. In other instances, the magnetic centre cannot be described

67 68 ZAGORSKI and MINC as a small molecule since it remains part of the bulk material. In yet other instances positive holes or electrons are trapped in the lattice. It is associated with a particular property of the crystal and bears no re- semblance to an ordinary molecular fragment [1]. The latter cases belong clearly to the field of "low-temperature radiation chemistry". We shall not deal separately with optical absorption methods, because they are used as an analytical tool. As in the case of E.S.R., the method has special features distinguishing it from spectrophotometry in liquid, but these will be mentioned only in the final section devoted to experimental details. The results obtained by measurement of ultra-violet, visible and infra-red absorption are very important for low-temperature radiation chemistry, and will be quoted in appropriate places in this review. The measurement of light emission is more closely bound to the low- temperature radiation chemistry and radiation chemistry of the solid state so that a special section is devoted to these problems. The number of publications on low-temperature irradiations is increasing at present more rapidly than in other branches of radiation chemistry. The progress may be estimated by a comparison of annual reports where low-temperature problems are treated in separate sections or in special surveys (e.g. Refs [2,3]).

2. RADIATION CHEMISTRY OF DEEP-FROZEN SUBSTANCES AS A SEPARATE BRANCH OF RADIATION CHEMISTRY

2.1. General considerations

A survey of the situation will first be presented, based on papers in which low-temperature irradiation is treated not as a supplementary technique but as a separate branch of radiation chemistry. There is a marked difference in the effects if we consider the mode of bringing the sample into the solid state. The first group consists of samples that are solid at room temperature before low temperature is applied, namely solid samples obtained by crystallization from the melt or from the solution, then substances normally solid because of high molecular weight. The second group consists of liquid or even gaseous samples shock-frozen to the solid state. We encounter here the kind of solid state hardly comparable with the state we usually deal with at room temperature and higher. Shock-freezing results very often in glassy state formation on which there is not much information. The same applies to the question of defects in the deep-frozen state, an important problem even in respect to the frozen samples, solid even at room temperature. The question of defects has to be understood in a broad sense, e. g. in terms of susceptibility of the structure from potential wells (low-energy electrons may dig traps in the matrix). The uniqueness of low-temperature radiation chemistry is very often due to the uniqueness of frozen and deep cooled-down structures. We should have liked to make the section on the role played by structure the largest one, but unfortunately at the present time there is not enough information. PL-236/1 69

2. 2. The chemical end-point of radiolysis in the liquid and frozen state

All irradiated systems, gaseous, liquid or solid, reach a certain chemical composition after irradiation. This chemical end-point is the state of equilibrium of stable compounds and appears some time after the flux of ionizing radiation ceases. The period of time varies considerably, from microseconds to weeks. The time of attaining the equilibrium is dependent on the viscosity of the medium if the reactions demand transport of reacting species, or are totally independent of the state of the sample if the reaction consists in intramolecular change, e.g. in the decay of a triplet state. In radiation chemistry we deal with a very wide spectrum of reactivi- ties and mobilities of intermediate species, and therefore the time of attaining equilibrium is highly dependent on the phase and the temperature of the sample. Even in the liquid state we encounter the reactions either occurring with maximum speed caused by reactions at every collision, or, on the other hand, occurring slowly, e. g. between the molecular product of radiolysis of water-hydrogen peroxide and iron (II) ions present in the system. In the frozen state the situation is more complicated. We notice reactions which still take place at the low temperature long after the radiation has been stopped, then we observe the nebula of reactions in the course of heating and thawing. Even after reaching the liquid state the reactions may proceed further. There is nothing remarkable in the fact that the results of irradiation of the same compound in the liquid and solid state differ. After all we are conversant with the phenomenon called the phase effect, occurring in irradiation of samples just below and just above the melting point. However, in low-temperature chemistry the differences are bigger than in the so-called phase effect [4], because the final effect is influenced not only by the phase of the sample but also by the wide range of temperatures. The differences are of a quantitative or even of a qualitative nature. The first group (different G-values) is, of course, larger, because from the point of view of thermodynamics the probability of formation of quite different compounds, due only to the changed mode of degradation of supplied energy, is highly improbable in the given system. Therefore in the majority of systems only the yields of particular products are changed, not the qualitative course of reactions. Thus the frozen Fricke system gives iron (III), although the smaller the yield, the lower the temperature [5, 6], The phenomenon is so closely connected to the local temperature that the tritiatedsystem cooled down to a temperature of 77°K gives iron (III) with a G-value characteristic rather for a higher temperature, due to local thermal spikes. The second group, of qualitatively different results in comparison with liquid irradiation, is a smaller one, even if systems are incorpo- rated which at low temperatures give substantial amounts of a compound but at room temperature in liquid state nil, or vice versa. Although the difference seems to be qualitative, the contrast between high yield to G-value equal zero has the character of a quantitative difference. This was discovered by Zagorski and Weimann [7] in the simple alkaline system. Polarography applied to the molten samples helped to reveal the basic difference between the radiolysis of the liquid aqueous sodium 70 ZAGORSKI and MINC hydroxide solution and the solidified one; in the liquid-state irradiation, no hydrogen peroxide may be detected, in addition to there being no hydrogen. The situation resembles one of pure water radiolysis. Only after adding an appropriate scavenger (in our case sulphite, SO|", was investigated), molecular hydrogen is obtained in the basic yield, generally accepted, of 0.45 [8]. In solidified alkaline solution irradiated at a temperature of 77°K, we have found appreciable amounts of hydrogen peroxide with a radiation yield of about 0.1 for the dose 5X 1018 eV/ml, already without scavengers. The final result of radiolysis (chemical end-point), both in the liquid or in the frozen state, may be nil, which means that the system is coming back to the original composition and structure but only at a slightly increased temperature, equivalent to the energy of absorbed radiation. That may be the case either when there is no possibility of the formation of a stable compound (e.g. irradiation of gaseous hydrogen, gamma irradiation of metals), or if one group of products of radiolysis is destroying another (e.g. radiolysis of pure water). Solid systems are interesting in this respect: quite a number of them, on the way from the temperature of irradiation to the melting point, lose the deposited energy, partially as light, and they reach the melting point in an annealed state. As examples may serve all systems used for thermoluminescent dosimetry or alkali halides. The latter, however, may be dissolved before thermal or photochemical annealing (irradiated NaCl produces chlorine, hydroxyl ions [9]), offering insight into the nature of frozen intermediates. Unfortunately, a similar experiment in the case of deep-frozen systems seems to be difficult if not impossible. As far as we know such an idea has not been realized yet because of the lack of the appropriate solvent, liquid and able to dissolve at liquid nitrogen temperature. The concept of dissolving the irradiated sample at possibly low temperatures without reaching the melting point was realized by Falconer and Salovey [10]. They have observed post-irradiation suppression of the dimer; when irradiated (at the solid CO2 temperature) solid n-hexadecane was dissolved at temperatures below its melting point in solvents such as 2 -methylpentane containing dissolved iodine. In each case G (dimer) was reduced to virtually the same level as when dispersed iodine was present during irradiation.

2.3. Ionic species in the irradiated frozen systems

Most papers dealing with charged species in frozen samples are concerned with electrons, their trapping and reactions. The formation of electrons is obvious: the first act of ionization is almost independent of the sample's temperature and phase. Comparatively little attention is paid to charged species of opposite positive charge. Although positive species or holes are not as mobile as electrons, their role is equally important. The question of positive charged species was considered recently in the case of frozen water and aqueous solutions by Moorthy and Weiss [11]. These authors claim to provide evidence that the reactions of the radiation produced positive holes apart from the reactions of the electrons. E.S. R. measurements show that radiation-produced electrons react with acidic solutes to form H-atoms and with group lib metal ions to give corresponding PL-236/1 71 univalent radical ions, while the holes can react with anions such as SO4" and H2PO4, giving the radical ions SO4 and HPO4. Continuing the concept of polarons developed by Weiss et al. , the authors think that electrons and holes are coupled with each other and may exist in irradiated pure ice primarily in an exciton-like bound state. The electrons are distributed among a number of exciton levels covering a wide energy range. The electrons bound in levels that are separated from the conduction band by energies less than the thermal energy can be thermally excited into the conduction band. Following such an excitation the electrons and holes can undergo radiative re-combination, thus causing the observed thermoluminescence of irradiated frozen ice (see Section 2. 5). The separation of electrons and positive ions has far-reaching consequences. It is clear from investigations on migration of electrons (see Section 2.4) that in frozen glass there may be enough time, even in the region of the spur, for the positive ion to react with neighbouring molecules before neutralization occurs. Positive ions may be formed in secondary processes. Dainton, Salmon and Teply [12] demonstrated that in the radiolysis of methanolic glasses at 77°K, the CH3OH2 ion is formed by a proton transfer process, and that this ion, upon neutralization, yields a hot hydrogen atom that reacts with methanol to produce CH2OH.

2. 4. Migration of intermediate species in low-temperature radiolysis

2.4.1. General

The question what is diffusing, or more generally, migrating, how far and with what consequence, remains the centre of attention in experiments and theoretical considerations on radiation chemistry in the liquid state. The same questions are still worth answering in low-temperature radiation chemistry although it is obvious that migration of intermediate species cannot resemble the situation in the liquid state, both in respect to the identity of species and to the mode and distance of migration. Migration in frozen systems may be roughly divided into the phenomenon taking place during, or very shortly after, irradiation, then into the migration that proceeds without the increase of the temperature of the sample, and, finally, into the large and complicated group of phenomena during warming and thawing of the irradiated sample. The second group includes phenomena generally called "bleaching". If the irradiated sample is treated afterwards with ultra-violet (sometimes even visible light is sufficient), E.S. R. and optical spectrum may be changed. Apparently the trapped, but unstable, species obtain energy of absorbed light quanta and may therefore travel to another trap or acceptor, or even to the site of origin. The classical example here is the optical bleaching of F-centres in irradiated crystals of alkali halides (violet KCI, yellow NaCl, etc., become colourless again). In some cases, induced phenomena of migration still take place at the low temperature of irradiation, but belong clearly to the third group. These are all cases in which local increase of the temperature takes place, e. g. due to infra-red irradiation or to the internal source of high LET irradiation. The last-mentioned case is represented by internal tritium irradiation in which local thermal spikes cause the irradiation to take 72 ZAGORSKI and MINC place always at a higher temperature than the average, measured temperature of the sample. Contrary to the liquid state, the differences between the diffusion constant of particular species are larger than in the liquid state. At temperatures of 77°K and lower, practically only electrons and hydrogen atoms may diffuse, all other species being large enough to render diffusion in the rigid matrix impossible. In this section we refer to free diffusion and do not take into consideration the migration of excitations which may simulate the transport of atoms or molecules. Also we are not considering here short-range derealizations.

2. 4. 2. Migration of intermediates in frozen solids at low temperatures

Most of the papers that deal with the migration of intermediates during irradiation are concerned with electrons, hydrogen atoms and some positive ions. As was expected, electrons were shown to be able to migrate for fairly long distances in frozen systems, if no molecules of an appropriate acceptor are in their way. Hamill and his co-workers [13] demonstrated that in low-temperature glassy solids, electrons can migrate distances of several tens of angstroms from the site of the ionization process, and can then either be trapped physically in the matrix, forming a trapped

electron (ex-) or become attached to a molecule of a suitable electron acceptor. Hamill's work was performed on systems consisting of 2-methyltetrahydrofuran, 3-methylpentane, methylcyclohexane ethanol and isopentane as solvents, and biphenyl, naphthalene, nitrobenzene, benzophenone, tetracyanoethylene, styrene and carbon tetrachloride as solutes whose anion absorption spectra are recorded. The solvent spectrum can be selectively bleached, with a concomitant increase in the solute anion absorption bands. Investigation of the influence of the acceptor concentration may make it possible to calculate the range. Dainton and Salmon [14] also used 2-methyltetrahydrofuran (MTHF) to investigate the range of migration of electrons. This has excellent compatibility with glass, methanol and concentrated glassy solutions of NaOH and KOH. The addition of solutes (capable of capturing electrons) to any of these systems, reduces the extent of electron trapping in the matrix and in some cases an identifiable product of electron capture is produced. Electron capture can result either in the formation of a stable anion or in dissociation of the capturing molecule. The products of these reactions are immobilized in glassy materials at 77°K and can therefore be examined by E.S. R. and optical spectroscopy. At the same time, thermalized electrons that escaped the reaction with acceptors are also frozen and show their presence in the E.S. R. spectrum. In the case of methanol, Dainton used as acceptors carbon tetrachloride, sulphuric acid, naphthalene and benzyl chloride; electron capture efficiencies were in the ratio 2.2:1.7:1.2:1.0. The thermalizing electron encounters on the average 450 methanol molecules before reacting with a carbon tetrachloride molecule or about 1000 before reacting with a molecule of benzylchloride. Similar results have been obtained with anhydrous ferric chloride as the electron acceptor in MTHF, which suggests that electrons are displaced through an average linear distance of about 35 A in this system before reacting with FeCl3. PL-236/1 73

The experimental approach described is generally of no use, because many glassy systems do not trap electrons as such because either there is no significant number of trapping sites available, as in the case of alkane glasses (e.g. 3-methylpentane), or electrons are able to react with a component of the glassy material itself, as in the case, of the acid aqueous glasses. Nevertheless, using specific electron acceptors, Dainton's researchers were able to estimate the distances of migration of electrons as about 50 A in 5 M glassy sulphuric acid at 77°K. The next step involves the investigation of migration as a function of the energy of electrons. In these experiments the release of different energy electrons is helpful; this may be achieved by thermal or optical bleaching. In MTHF glass, for example, thermally released electrons

do not react with FeCl3 whereas this solute does reduce the formation of trapped electrons and hence must capture electrons liberated by gamma irradiation. It seems probable that in MTHF electrons travel approximately 150 A before being thermalized and trapped. However, an electron

cannot react with a FeCl3 molecule if it has less energy than it possesses after migrating 35 A from its parent positive ion. The distances of electron migration found by Dyne and Miller [15] also in methyltetrahydrofuran glass are even longer. They noted that the quantum yield of bleaching electrons depends on the fraction of electrons bleached and not on their concentration. The authors cited conclude Otha • t about half the electrons are trapped close to the positive ion (10-80 A), whereas the other half are trapped homogeneously after travelling up to 1000 A. A remarkable fact was also demonstrated by Dainton who stated that in some systems electrons are more mobile in the glassy phase than in either the liquid or crystalline phases. In the case of sulphuric acid, the formation of acid glasses is thought to be due to the condensation of the acid into long chain polymers, and it may be that electrons are able to migrate preferentially along these chains. Apart from conclusions concerning the fate of electrons, the consequence of long-range migration of electrons affects also speculations on the fate of positive ions, which may not be neutralized for a relatively long time and are able to reach the neighbouring molecules. More details about positive ions may be found in Section 3. 3.

2. 4. 3. Migration of intermediates at temperatures higher than the temperature of irradiation

In low-temperature radiation chemistry we encounter two periods of enhanced chemical changes: the first, during the irradiation, where the migration of small and mobile intermediates follows the primary effects, and the second, during warming up, when frozen intermediates are mobilized and react, reaching the chemical end-point with a rate related to the rate of heating. In between there is a period where some slow processes take place, in spite of keeping the temperature at the low level, the same or lower than at the irradiation. The period between the irradiation and thawing may be practically indefinitely long, providing there is no increase in temperature and no absorption of energy (u. v., visible light, i. r. ) by the sample. 74 ZAGORSKI and MINC

The stormy and extremely complicated stage of warming up is not quite clear as far as the mechanisms involved are concerned, because several changes are taking place in parallel: structural changes of the matrix, and vibrations of the elements of the structure, which may provide the activation energy for the trapped species, then diffusion of intermediates to the sites of reaction and several phenomena of less importance. Some of these effects are connected with the evolution or consumption of thermal energy that contributes to the heterogeneity of the system, complicating the interpretation. It is obvious that a proper analytical method is the best tool for the identification of changes in the sample during melting and localization of products on the temperature axis. The most important method is E. S. R., providing information about changes of free radical both qualitatively and quantitatively. This method does not cover the total chemical composition and is supported by absorption spectrophotometry in different parts of the spectrum. Some examples will illustrate the experimental approach. The application of i. r. spectroscopy offers interesting possibilities. The paper of Sukhov et al. [16] may serve as an example. They deal with different caoutchoucs, irradiated in vacuum, with 1-2 MeV electrons, at a temperature between 85-90°K, then analysed by i. r. in the range 700-1700 cm-1 and 1600-3000 cm-1 consecutively at temperatures of, for instance, 90, 120, 250, 310°K. As the i. r. spectroscopy is not very sensitive, the dose applied had to be rather high - up to 600 Mrad. Re- sults show that most important reactions proceed at the temperature of irradiation: 48% of the isoprene double bonds in natural caoutchouc disappeared at 120°K. About 2% turned into transvinyl double bonds through migration. The rest (46%) of the isoprene double bonds formed cross- linkings. During further heating to 2 50°K transvinyl unsaturation doubles because of the effect of the secondary transfer of the free radical centre on the chain and the resulting migration of the double bond. Other secondary processes increase the degree of amorphism. The chemical end-point differs from that in room temperature irradiation.

2. 5. Luminescence, phosphorescence and thermoluminescence in irradiated frozen systems

The role played in radiation chemistry by radiation-induced luminescence and phosphorescence increased and widened after irradiations at low temperatures had been extended. Radiation-induced luminescence of liquids is very weak and usually amounts to about 3% of total emission of gamma-irradiated aqueous solutions. The remainder is Cherenkov radiation, which is easy to distinguish from luminescent light. Only in the case of specific compounds present in the system is the absorbed energy of ionizing radiation emitted as light. This effect reaches its maximum in the case of deliberately chosen liquid scintillators. Very weak luminescence emitted by most solutions was seldom interpreted because of the intensive Cherenkov background, but both sources of light had to be taken into account [17] in optical measurements [18] performed in the radiation field. PL-236/1 75

The effect of emission of light both as luminescence and phosphorescence is more common in the case of solids. Most intensive luminescence is given by scintillators, organic and inorganic. Comparatively high conversion is reached also in the case of ordinary fused silica. As far as we know, no use is made of it; on the contrary, intensive light emitted is a complicating factor in optical measurements. Dondes [19].who had experience of this, estimated the radiation yield of light quanta to have a G-value of about 1 (1 quantum of visible or u. v. light per 100 eV of absorbed energy of ionizing radiation). The glowing of ordinary quartz is very intensive, which led us in our earlier work to abandon this material totally, and to use cells and other equipment only of very pure SiOj that does not darken nor emit light in the ionizing radiation field [17]. Ordinary silica also emits light as phosphorescence of long duration. Both radioluminescence and phosphorescence of silica depend on the quality and quantity of impurities as well as on the structure of the sample. The third phenomenon connected with the emission of light, namely thermoluminescence, was investigated mainly from the point of view of dosimetry. Substances like CaF2, LiF, CaS04 doped with Mn or Sm, and SrS doped with Sm, proved to be excellent accumulators of absorbed ionizing energy, releasing comparatively large portions of it as visible light when heated (for a recent review of thermoluminescent dosimetry see Ref. [20]). Most work in this field has been performed with samples irradiated at room temperatures, and does not belong to low-temperature radiation chemistry (but see Section 3.6). One can say generally that moving towards lower temperatures of the samples the conversion to light is enhanced. Frozen liquids that were not emitting in the liquid state, now show the phenomenon of radioluminescence and very often also phosphorescence lasting for hours, because of the slow intramolecular transformations. For instance, Brocklehurst, Porter and Yates [21] took spectra of the light emitted during the warm-up of a gamma-irradiated solution of naphthalene in an alkane mixture at 77°K. The phosphorescence emitted by naphthalene was about six times more intense than the fluorescence. It was therefore concluded that a triplet excited state of naphthalene was produced by ion recombination. Most of the papers in this section deal with phosphorescence stimulated by u. v. , visible or i. r. radiation applied to the frozen sample after high- energy (usually gamma or e~) irradiation. The effects of i. r. illumination are usually similar to those obtained in warming up, which results in thermoluminescence. Usually not only frozen species that are absorbing in this spectral region are mobilized, but also the matrix may be less rigid in the effect of local heating. The technique of the recombination-luminescence stimulation by illumination of frozen irradiated solutions proved useful, especially in investigations on ionic processes in gamma-irradiated organic solids. Thus Skelly and Hamill [22] measured the phosphorescence emitted when a gamma-irradiated dilute solution of triphenylamine (TPA) in 3-methyl-pentane is bleached with i. r. light (0.8 - 2 nm) at 77°K. Optical absorption spectroscopy provides evidence for colour centres tentatively identified as TPA+, TPA" as well as solvent-trapped electrons (e"). i. r. bleaching of e" and TPA" induces phosphorescence of TPA with corre- 76 ZAGORSKI and MINC sponding decrease of TPA+. Addition of organic halide (e~ trap) decreases TPA~ and e", and ethanol (hole trap) decreases TPA+, while both decrease phosphorescence which is attributed to ion recombination.

In the USSR) Tochin et al. [23] have worked on aromatic compounds irradiated by 1.6 MeV electrons at 77°K and then illuminated by mono- chromatic light of constant flux, but changing continuously from 350 nm to 2.3 /um. Luminescence peaks appear, being interpreted by neutralization of both negative and positive ions. Before the illumination these ions are stabilized in the process of irradiation. According to the authors cited, positive charge is stabilized in molecules of the main constituent of the system. To prove this, systems with low yield of radicals were chosen (crystalline benzene, diphenylamine, glassy cumene, and both glassy and crystalline toluene, ethylbenzene and triphenyl methane). The dose applied was usually about 0.3 Mrad. Spectral composition of the emitted light does not depend on the wave-length of the exciting illumination. All investigated hydrocarbons showed a maximum at 1 /um of stimu- lating light, and one about 450 nm. In addition, there were some peaks characteristic for particular compounds. The luminescence occurring at the last-mentioned wave-length was interpretedas the release of electrons trapped by radicals. The long-wave excitation seems to be connected with the release of positive ions stabilized on a cluster of non-ionized molecules, similar to the mechanism proposed by Shida and Hamill [24] in the case of CCI4 stabilization on CCI4 molecules. Some papers have dealt with the question of the mechanism of lumino- sity itself rather than using it only as a sign of reactions taking place. The existence of thermoluminosity of substances irradiated at low temperature becomes a general rule. After investigating several, mostly different, groups of compounds we may say that if only the narrowest spectral window (transmittence in the u. v. or visible region) exists in the investigated sample, it will emit light after irradiation at at least liquid nitrogen temperature. According to the regulations of an IAEA Panel we cannot give our unpublished data concerning the universality of phenomena and their importance to the reactions in the frozen state. For these reasons we shall not discuss papers dealing with radiolysis of compounds adsorbed on the surface of silicaneous materials, irradiated at the temperature of liquid nitrogen (e. g. Ref. [25]). We are also conscious of the fact that virtually all solids occurring in nature, except those freshly formed, inherit some radiation changes in their structures. This applies especially to old mineral deposits that from the cooling-down stage to the present time were absorbing energy of ionizing radiation (mainly protons from outer space and radiation from environmental radioactivity). The energy stored in this way may be most easily released as thermoluminescence. It is unnecessary to emphasize how important this phenomenon is for geology (cf. Section 3.6). The investigation of thermoluminescence alongwith E.S. R. and optical spectra, and general confrontation with the chemistry of the investigated system, hashelpedto enrich radiation chemistry with facts formerly be- longing to radiation physics. For example, there is a tendency to consider radiation-induced changes in ionic solids as pure physical phenomena. The best proof that they are not is demonstrated in various experiments involving the dissolution of irradiated alkali halides. If the dissolution in water of irradiated sodium chloride takes place in the dark- PL-236/1 77 ness, products of reaction between electrons and of free chlorine atoms with water may be easily found [9]. What is more, during dissolution luminescence appears, an interesting phenomenon presented in the series of papers by Ahnstrom.

2. 6. The structure of frozen sample and low-temperature radiation chemistry

It is surprising that such little information is available on the structure of frozen liquids or even deep-frozen crystalline substances obtained at higher temperatures. The second group may be only apparently less complicated. Investigators of low-temperature radiation chemistry usually divide rapidly frozen structures into crystalline and glassy structures on the basis of visual inspection. In some systems it is possible, either by changing the rate of the temperature drop, or by conditioning or pressure treatment, to pass from the glassy to the crystalline state, and to compare the radiation-induced effects in both states. For such cases there is some information available in the literature. The influence of the structure of frozen glasses and, moreover, of changes that take place in the state was shown, among others, by Burton and co-workers. This author [26] has shown that gamma-irradiated, pure 3-methylpentane glass at 77°K emits a characteristic phosphorescence of several hours duration, and that its intensity is decreased [27] by per- mitting the sample to relax at 77°K between vitrification (complete after 3 min) and gamma irradiation (up to several hours). The occurrence of some cracks in the annealed glass indicated that the glass relaxation or structural rearrangement had taken place. Janssen [28] has shown that cracks in the annealed glasses are not necessarily required for the annealing effect to be observed, although they apparently accelerate the glass relaxation markedly. It was also shown that the luminescence spectra of isopentane differ for the glassy and poly crystalline states. Voevodskii et al. [29] have tried to investigate the influence of the physical state on the result of radiation-induced reactions. They have investigated alcohols and hydrocarbons, and obtained results indicating that the free radical yields (determined by the E. S. R. method) in the same compound are always higher in the amorphous (glassy) sample than in the crystalline. Also differences in the relative concentrations of the different radicals have been observed. According to Voevodskii this effect seems to be connected with the influence of structural defects on the primary processes of radiolysis. It is evident from the lengthy paper by Teply [30] that it is difficult to generalize on the role played by the crystallinity of the sample. Looking more closely at the problem of structure we may easily come to the conclusion that simple division into crystalline and amorphous samples produces an imprecise simplification of the situation. If there is a difference in behaviour between crystalline and glassy samples, the size of the crystals as well as the size of the aggregates or micelles in a glassy sample must be different too. Henriksen [31] has written on the effect of crystal dimensions on the yield of radiation-induced radicals in organic substances. 78 ZAGORSKI and MINC

It is important to realize that complications implied by the structure are not restricted to those samples which are liquids at room temperature and shock-frozen to the temperature of irradiation. Low-temperature radiation chemistry is also complicated in samples that are solid at room temperature and merely cooled. In this case also phase changes are possible, and the forces developed between crystals may reach values so high that on warming a tribothermoluminescent peak is developed, simulating radiation-induced thermoluminescence. Therefore, especially in the case of optical measurements on deep-frozen crystals involving thawing, a blank experiment without irradiation must be performed. Tri- boluminescent peaks are known in thermoluminescent dosimetry [32], The question of "crystalline or glassy" is important in a one- component system, but is even more complicated in two- and more component systems. Most investigators hope that frozen samples that are apparently glassy are totally uniform in spite of their multicomponent composition. If the sample is crystalline, there is no such guarantee at all. In some cases there is total separation into individual crystals that exhibit their own radiation chemistry; in others the samples are perfectly homogeneous, which means every part of the system on the molecular level has the same composition as the average of the systems. A paper by Dyne and Denhartog [33] provides a striking example. They investigated a classical cyclohexane-benzene system known for its characteristic curve of hydrogen yield, deviating from the additivity law because of the protective action of benzene. The investigation was performed at a temperature of 77°K. If solid mixtures were prepared simply by plunging liquid samples into liquid nitrogen the effect of the protective action of benzene would be hardly visible: hydrogen yield was almost linear - cyclohexane seemed to be diluted with a different substance (cf. Ref. [-34]). There was, however, a suspicion that there had been some segregation of the liquid mixture into two phases, solid benzene and solid cyclohexane. This segregation would eliminate the interactions that occur in the radiolysis of the liquid mixtures, and the yields would lie close to the mixture line. To test this. Dyne prepared other samples by the slow condensation of a stream of the premixed vapours on to a surface cooled by liquid nitrogen. Segregation into two phases was inhibited at these temperatures, the samples were homogeneous, and the yields were much lower than from the frozen liquid samples and did not differ qualitatively from those found in the liquid, and did not obey the mixture law but demonstrated the protection effect.

2. 7. Low-temperature radiation chemistry and solid-state concepts

It is obvious that several authors have tried to approach the interpretation of low-temperature radiolysis from the point of view of solid- state concepts, especially in view of the success of radiation physics with ionic crystals and semiconductors, solid at room temperatures. Radio- lysis of frozen liquids may be also treated in terms of solid-state concepts. According to these, the ionization process is equivalent to the excitation of an electron into the conduction band continuum leaving behind a positive hole in the valence band. In the band picture neither the electron nor the hole is localized on any specific molecule. Further, they PL-236/1 79 can bring about electronic polarization of the surrounding medium and can exist in the polaron state [11]. The attractive potential between such electron and hole polarons determines a series of stable bound states in the energy gap between the valence and conduction bands. In the absence of reactive solutes in the system, the electrons subsequently drop into one of the stable exciton states, and this process rather than the annihi- lation of electrons and holes is considered to be the first-order recombination process discussed by the cited authors. The solid-state approach would be a fruitful one in the field discussed, if more experimental data on frozen irradiated systems could be found. E.S. R. , light absorption and emission give insufficient data from the point of view of solid-state chemistry and physics. More experiments connected with electrical properties of frozen irradiated systems are needed. The fact that they are not performed in the range needed may be explained by experimental difficulties connected with electrical measurements at low temperatures and different parameters of the experiment in comparison with room temperature measurements, due to the different energy levels, mobilities, etc., involved.

2.8. Investigations related to the low-temperature radiation chemistry

Considering the basic radiation chemistry of frozen samples one cannot confine one's attention only to investigations involving ionizing radiation. Investigations carried on from other points of view and using different techniques may produce a valuable new outlook, especially in the case of difficulties in interpretation. Thus the papers and concepts of Riehl [35] presented at the Energy Transfer Symposium are worth mentioning. This author is interested in protonic conductivity in ice or other compounds forming hydrogen bridges. At temperatures sufficiently low to preclude thermal dissociation, he was able to establish space-charged limited proton currents using proton-injecting contacts and high electrical fields. Pure ice single crystals were cooled to 77°K, and fields between powdered Pd electrodes saturated with hydrogen were applied up to 450 kV/cm. Leaving aside the question of proton traps, amounting roughly to 3.5X 1012/cm3 density, Riehl came to an interesting conclusion: the proton mobility increases with decreasing temperature. This indicates that the mobility of protons in ice must be considered as a process similar to the motion of electrons in a conduction band and not as a "hopping" mechanism. Although this finding may not be quite ready for generalization, it is worth considering and, we believe, does not contradict the results from the radiation chemistry of ice.

3. SURVEY OF LOW-TEMPERATURE RADIATION CHEMISTRY OF PARTICULAR SYSTEMS

3.1. General

Until now, practically all the most important liquid systems have been investigated in the frozen state also. The results undoubtedly en- 80 ZAGORSKI and MINC riched liquid radiation chemistry, contributing to a more complete picture of radiation chemistry. The importance of low-temperature radiation chemistry does not finish with a supplementary role for liquids. The implications of this technique and generalized facts stretch from basic research on the atomic level through molecular biology to the new concepts of chemical effects in outer space. Thus low-temperature chemistry has increased the contributions to other branches of science. It is very difficult to gain a whole picture of phenomena in radiation chemistry of particular importance today, because, as a rule, every author gives a somewhat biased picture of phenomena, stressing one particular aspect and neglecting others. As stressed frequently in this survey, the frozen state offers so many structural and energetic possibilities that all phenomena observed are understandable. The relative importance of phenomena and mechanisms remains to be investigated, as is already done in other branches of radiation chemistry of solids, namely those that are solid at room temperature, e.g. in the case of alkali halides.

3. 2. Water and aqueous systems

In the first place, the study of the radiolysis of ice and frozen aqueous solutions helped to elucidate the views put forward to explain the radiation- induced chemical changes in water and aqueous solutions. Early, but most important, papers [36-41] presented proofs for the presence of H and OH radicals in deep-frozen irradiated ice by E.S.R. and optical spectroscopy. Basic radiation yields established then by Siegel, G/H/= G/OH/ = 0.8 - 0.9 at 4°K or G/H/ = 0 and G/OH/ = 0.6 at 77°K, need now to be corrected, and according to Moorthy and Weiss [11] are too high by a factor of about two. If the basic yields may be the source of some controversy, especially in cases concerning very low temperature and pure water, so the registra- tion of phenomena in frozen aqueous solutions seems to agree within satisfactory limits. Thus early measurements by Schulte-Frohlinde [42] of absorption spectra of gamma-irradiated alkaline ice (which is deep blue) were many times confirmed, for example, recently by Ershov £Lnd Pikaev [43]. Also many papers on E.S.R. spectra of different frozen solutions, for example, Henriksen [44] in the case of alkaline solutions, seem to agree. Nevertheless, the controversy begins usually at the stage of interpretation. Every new experimental fact, perhaps the conventional analysis of the molten sample, or the introduction of the new parameter of the sample measured (cf. Refs [7, 45] in the case of alkaline solutions), causes a change in the interpretation.

3.3. Simple organic compounds

Low-temperature radiation chemistry of simple organic compounds is already very rich. In this short section we shall stress only the question of positive ions. Experimental demonstration of its presence and behaviour in the frozen sample distinguishes the situation clearly from radiation chemistry of the same compounds in the liquid state. PL-236/1 81

The series of papers by Hamill [24, 46, 47] is devoted to molecular ions and ionic processes in organic glasses. Gamma-irradiated (at 77°K) polycrystalline CCI4 shows the presence of positive holes absorbing at about 400 nm. The solvent band is depressed by aromatic amines or other additives that act as hole traps with the appearance of the corresponding amine cation absorption. Photo-bleaching or brief heating to 143°K reduces the 400 nm band with an increase in solute cation absorption. A kinetic treatment that assumes that the only labile species in the system is a migrating positive charge can explain the experimental results in a satisfactory way. A rough estimate of the total available positive charge is 1.9/100 eV of absorbed energy. The addition of aromatic hydrocarbons to the polycrystalline matrix of CCI4 leads to the formation of molecular monopositive ions of, for example, benzene, toluene, biphenyl, terphenyl, naphthalene and stilbene. The authors interpret these results as resonance charge transfer from CCI4 to CCI4, terminated by transfer from CCI4 to the solute. Positive-hole migration has also been found in 3-methylpentane (also by Hamill). In the system consisting of this hydrocarbon and a small addition of compounds undergoing charge transfer or proton transfer reactions, the mobile positive hole is transferred from 3-methylpentane to the additive, and trapped. Although we stress the importance of positive ions in the radiolysis of organic compounds, which is shown clearly in low-temperature radiation chemistry, we do not claim that it is the sole mechanism. Parallel or later processes are going on in which free radicals occur (cf. Ref. [48] for the case of 3-methylpentane). The low-temperature radiation chemistry of the same compound has been mentioned also elsewhere in this review. The full picture of the radiolysis of organic compounds is far from complete, if we take all experimental facts and all ranges of time into consideration.

3.4. Organic polymers

The problems of low-temperature radiation chemistry connected with polymers are again too widely represented in the literature to be covered fully in this review. This field may be divided into three groups: radiation- induced polymerization, where the starting point is the monomer, the second group of radiation-induced degradation and cross-linking of irradiated polymers, and the third intermediate group where both polymer and monomer are irradiated, resulting in radiation-induced grafting. The first group, radiation-induced polymerization, has very much in common with the radiation chemistry of simple organic molecules, which in practically all cases produces polymers in its wide spectrum of products, in spite of the fact that no typical monomer was used as an irradiated sample. In the case of a typical monomer the yields are usually much larger. Radiation-induced polymerization proceeds in 3. frozen state that is different in comparison with the liquid state. In particular, reactions proceeding with very high radiation yields of evident chain mechanisms connected with the diffusion of intermediates, proceed at a different low temperature. Up to now most interest has been paid to those cases of radiation- induced polymerization in the frozen state in which the ionic mechanism 82 ZAGORSKI and MINC of reaction was suspected. Convincing proof for the ionic mechanism in the frozen state could be an argument for a similar explanation for polymerization in the liquid state, competing with a generally accepted radical mechanism. The theory of these reactions is not yet fully agreed. Many intervening steps are involved between initial energy absorption and final end-points [49], There is some hope that polymers may be obtained, characterized by unusual properties, which may be of industrial importance. In the second group - low-temperature radiation chemistry of polymers - the intermediate effects are more similar to the effects at the room temperature than was the case with liquid monomers irradiated at room temperature and in the frozen state. In polymers the mobility is restricted even at room temperature if not so severely as in liquid nitrogen. The most interesting possibilities offered by low-temperature radiation chemistry in the field of radiation interaction on polymers consist in localization of particular effects on the temperature axis. These measurements are performed with the help of E. S. R. spectroscopy, and also with i. r. [16] and u. v. absorptiometry (e. g. Ref. [50]), conductivity and thermoluminescence. The last mentioned field is at the stage of intense development. Poly- mers irradiated at liquid nitrogen temperatures emit comparatively intense light when warmed. Glow curves, easy to obtain even without a photomultiplier, may be used for the interpretation of phenomena occurring, and are also proposed for identification purposes and quality control. Thus Mozisek [51] found the peaks on the glow curve to be directly connected with the mobility variations of the molecules or their segments, and cor- respond to the structural transition temperatures. This technique is therefore proposed to investigate polymer structure.

3. 5. Compounds of biological importance

As in other sub-sections of Section 3, it is impossible to give a full account of the role played by low-temperature irradiations. We shall therefore confine ourselves to one problem only connected with radio- protective agents. An important role is played by low-temperature irradiation in the field of the chemistry of radio-protective agents, which is a separate branch of the energy transfer group of problems. For example, cysteamine (NH2 . (CH2)2> SH, called MEA) is a well-known radio-protective agent at all levels, from the whole animal to individual chemicals. As the mechanisms involved are not clear, several attempts were made to investigate it, including irradiations and E. S. R. measurements at 77°K. Conclusions from low-temperature irradiation of proteins with cysteamine (Refs [52-58]) may be summarized in an important statement, that MEA has little or no effect on the E.S.R. spectrum at this temperature, and therefore this compound does not influence free radical formation. At higher temperatures it was found that the protein or nucleoprotein radicals migrated to the sulphydryl group of the MEA to give the radical -CH2-S" . The proposed mechanism is damage to the protein molecule RH:

RH/WWV^R-

6* PL-236/1 83 and the reaction R- + -SH -» RH + -S', called "repair". This repair reaction could account for the protective action of MEA. In the presence of oxygen, MEA and oxygen compete for the free radicals, giving protection and sensitization respectively. Oxygen reacts with the sulphur radicals formed by the repair reaction, thus preventing the damaging back reaction.

3. 6. Outer space chemistry

As was already mentioned in Sections 2.2 and 2.5, radiation-induced changes may be frozen in different deposits and minerals occurring in the earth crust. The temperature of the deposits found does not go down to the temperatures common in low-temperature radiation chemistry. Even rocks from Antarctica would be too "warm" to store intermediate species usually frozen at liquid nitrogen temperature. Outside the earth, however, there are conditions of extreme cold, extreme heat, various kinds of radiation and sufficient time that make the reactions of cold radiation chemistry possible. Sun [59] believed that ionizing radiation reaching the surface of the moon and lunar temperature changes provide ideal conditions for thermoluminescence, the release of stored-up energy in the form of visible light during a rapid temperature rise. The dark side of the moon, at a temperature of ~-150°C, and which is bombarded by protons, stores energy almost like samples of silicous materials, gamma-irradiated under liquid nitrogen in the laboratory. The revolving moon brings dawn after a two-week lunar night and the stored energy is released in the form of visible light. Thus, in a strip less than 100 miles wide alongside the lunar terminator the moon emits light of its own, which may be almost as intense as its reflected sunlight, if we take into account that the "day" temperature of the surface of the moon reaches about +120°C, and that the radiation yields of thermoluminescence may be very high (cf. Section 2.5). One has to keep in mind that the lunar surface has no protective atmosphere, so that in vacuo even 1 keV protons reach the moon. The fluorescent materials may receive a dose of about 103 Mrad during the dark period, at about 200 A. The rate of temperature rise is about 4.5 degC/min, which means an increase from -150 to +120°C in about 1 h. Sun's theory seems to be a reasonable explanation for the fact that the band of moonlight near the terminator is brighter than the rest of the moon's daytime surface. As we see, the moon's surface is possibly a site where periodically reaction induced by radiation at low temperature happens. Outer space may in general be considered as a gigantic "cold radiation chemistry" laboratory. The meteorites are very probably a source of formation of free radicals, especially when they collide with a planet not large enough to hold an atmosphere and far away from the sun, so that it is at the temperature of outer space (about 2°K) [60]. Collision produces a localized temperature sufficiently high to vaporize many substances and disrupt chemical bonds with subsequent stabilization of free radicals, ions and electrons. No work has been done in the laboratory at this temperature, but experience from low-temperature irradiation at a slightly higher temperature of 4°K (boiling point of helium) shows the possibility of manifold reactions with important consequences for cosmic chemistry. 84 ZAGORSKI and MINC

4. EXPERIMENTAL CONSIDERATIONS

4. 1. Preparation and handling of frozen samples

It is well known that the preparation of deep-frozen samples (e.g. in liquid nitrogen) for gamma irradiation and subsequent investigation by the E.S. R. method (for the presence of species with unpaired electrons) for absorption and emission of light, structural investigation, etc., is not an easy procedure. The technique employed must allow a convenient transfer of samples from the irradiation vessel into the sample holder. The most simple way, i.e. the irradiation in thin-wall tubes, has many disadvantages: (i) Most materials (e.g. glass) emit radiation in the E.S. R. spectrum or in other methods (absorption and emission of light and others) because of the formation of intermediate or semi-stable products of radiolysis. (ii) Some materials do not give signals in the same region as the sample. However, even in favourable cases (e.g. special glass that does not give the signals after irradiation) the presence of additional substances surrounding the sample in the magnetic cavity or other device lowers the sensitivity of measurement. (iii) Most materials for tubings, especially organic ones, may con- taminate samples during pouring and freezing. Very often the investigation of the addition of an active solution is needed. Several techniques have been proposed to overcome these difficulties. The most important are:

(i) Irradiation of rod-shaped samples in sealed glass tubings several times longer than the sample After the irradiation, the lower part of the tubing, with the sample, is kept in liquid nitrogen, while the upper end is heated to anneal the radiation changes. Afterwards the entire tube is cooled again, turned round and the sample pushed into the regenerated part.

(ii) Irradiation, or shaping in a thin-wall tubing made of organic material, resistant to low temperature Plastic material may be destroyed and discarded before or after the irradiation. This technique entails the danger of contamination of the sample with organic material because the investigated solutions are usually active [61].

(iii) Application of internal sources of low-range radiation that are unable to penetrate considerably the material of the vessel or tubing This technique introduces significant changes into the radiation- induced process, because the radiation from the internal source has, as a rule, an LET value higher than the LET of gamma radiation and causes . thermal spikes in the system [62].

(iv) Direct irradiation of the sample in liquid nitrogen Because of troubles causedby the construction materials of the vessel, the direct irradiation of the sample in liquid nitrogen is sometimes applied. For example, in the case of aqueous solutions (Ref.[63]), the solutions were rapidly frozen in 2 mm i. d. Pyrex tubes in liquid nitrogen to give opaque cry- PL-236/1 85 stalline solids. The required lengths were cut off from these, and the ice cylinders were pushed out into 4 mm i. d. Pyrex tubes for irradiation. The specimens were in contact with liquid nitrogen during irradiation. Rods of the sample may be prepared also in a different way: Casting of the sample of the desired shape in the mould, made from solidified mercury, is possible. This operation may be performed in a simple device [64], The steel cylinder is filled to the appropriate level with mercury, and a steel rod inserted and positioned. The rod is shaped according to a strict pattern according to the shape and dimensions of the desired frozen sample to be obtained. The filled device is lowered into liquid nitrogen and kept there until the mercury is solidified. Because of the shrinkability of mercury, the rod is easily removed and the whole device thoroughly frozen with liquid nitrogen. The room previously occupied by the stick is then filled with the investigated liquid in which, at the final phase of operation, the platinum eye is fixed. Further experimental problems concern the purification of samples. Low-temperature radiation chemistry has revealed new kinds of impurities. Johnson and Albrecht [65], for instance, report that CO2 is an efficient electron trap in 3-methylpentane at 7 7°K. Usually the presence of CO2 is not prevented by conventional vacuum degassing techniques.

4. 2. Additional data needed for experimental work and interpretation in the field of low-temperature radiation chemistry

In this survey the point has been frequently stressed that the re- searcher in this field is lacking many data. These may be divided into two groups: the first is connected with the technique of irradiation, the second with interpretation. In the first group we shall put data connected with the physicpchemical properties of frozen liquids. There are practically no data on densities of samples. For instance, it is known that aqueous solutions of sodium hydroxide behave like water up to the concentration of about 6N, namely the density of alkaline ice is lower than the liquid phase. At higher concentrations the density is higher, which means that the liquid is shrinking at freezing. Similar phenomena occur in other liquids; this complicates the preparation of samples and makes the exact calculation of doses impossible. There are also no data on the behaviour of dissolved gases in the process of freezing, both with respect to the homogeneity of the frozen sample and to the formation of porosity (voids). In the second group we encounter all the information on the frozen state connected with the interpretation of results. As was stressed in previous sections one encounters a serious lack of information on the structure, both on the molecular and the macroscopic level, homogeneity and phase changes during warming. The equilibria at low temperatures and how far our system is from the equilibrium, what forces are active in the frozen system, are also unknown. Consequently, the kinetics of reaching the equilibrium are usually not known, in addition to the role of defects of different kinds, the possibilities of formation and annealing of defects caused by irradiation, etc. The lack of measurements is not caused by lack of appropriate experimental methods. The existing techniques may be adapted, and in 86 ZAGORSKI and MINC

many cases are already adapted, for measurements at low temperatures, and are waiting for experimental realization. We are inclined to put first electron microscopy, X-ray structural analysis, neutron diffraction, differential thermal analysis, optical microscopy, autoradiography, all of which are connected with methods of making defects and macroscopic structural details visible. There is no doubt that structural investigation at low temperature is not simple, and in future will demand considerable experimental skill and the development of totally new approaches. Unfortunately they are only additional to the main problems of low-temperature radiation chemistry and therefore not very attractive. Nevertheless, if low-temperature radiation chemistry is to be developed they have to be tackled.

REFERENCES

SYMONS, M. C. R., Symposium II, Paper No. 8, 3rd Int. Congress on Radiation Research, Cortina d'Ampezzo, 1966. MATHESON, M. S., Nucleonics 19 10 (1961) 57. SHARPATYI, W.A., Usp.Khim. 32 (1963) 737. BURR, I. G., Nucleonics 19. 10 (1961) 49. KROH, J., SPINKS, J., Roczn. Chem. 36 (1962) 563. KROH, J., CZERWIK, Z., Bull. Acad. pol. Sci. S6r. Sci. chim. 14 (1966) 245. ZAG6RSKI, Z.P., WEIMANN, L., IBJ Rep. 549/XVII (1964). ZAGORSKI, Z.P., Proc.Symp.Radiation Chemistry, Tihany, Hungary (1962) 405. TAYLOR,, E. H., J. chem. Educ. 36 (1959) 396. FALCONER, W. E. , SALOVEY, R. , J. chem. Phys. 46 (1967) 387. MOORTHY, P.N., WEISS, J.J., in Solvated Electrons (HART, E.J., Ed.), American Chemical Soc. Publications, Washington, D. C. (1965) 180. DAINTON, F.S., SALMON, G. A., TEPLY, J., Proc. R. Soc. A 286 (1965) 27. RONAYNE, M.R., GUARINO, J.P., HAMILL, W. H. , J. Am. chem. Soc. 84(1962) 4230. DAINTON, F. S., SALMON, G. A., in Energy Transfer in Radiation Processes (PHILLIPS, G. O., Ed.), Elsevier, Amsterdam (1966) 85. DYNE, P.J., MILLER, O. A., Can.J. Chem. 43 (1965) 2696. SUKHOV, F.F., IL'ICHEVA, Z.F., SLOVOKHOTOVA, N.A., MARGOLIN, D.M., TEREKHOV, V. D., Khimiya vys. Energii 1 (1967) 58. ZAGORSKI, Z. P., KOSEK, S., IBJ Rep. 431/XVII (1963). MINC, S., ZAG6RSKI, Z. P., Nature,Lond. 193 (1962) 1290. DONDES, S., HARTECK, P., KUNZ, C., Radiat. Res. 27 (1966) 174. SPURNY, Z., Atom. Energy Rev. 3 2 (1965) 61. BROCKLEHURST, B., PORTER, G., YATES, J. M., J.phys. Chem. 68 (1964) 203. SKELLY, P.W., HAMILL, W. H., J. chem. Phys. 43 (1965) 3497. TOCHIN, V. A., NIKOL'SKH, V. G., BUBEN, N. Ya., Khimiya vys. Energii 1 (1967) 71. SHIDA, T., HAMILL, W. H., J. chem. Phys. 44 (1966) 2369. SOROKIN, Yu.A., KOTOV, A. G., PSHEZHETSKII, S.Ya., Zh.fiz.khim. 40 (1966) 2277. BURTON, M., DILLON, M., REIN, R., J. chem. Phys. 41 (1964) 2228. FUNABASHI, K., HARLEY, P.J., BURTON, M., J.chem.Phys. 43 (1965) 3939. JANSSEN, O., FUNABASHI, K., J. chem. Phys. 46 (1967) 101. VOEVODSKII, V. V., ERMOLAEV, V. K., SOLOVYCH, N. A., in Radiation Chemistry 2 (DODO, J., HEDVIG, P., Eds), Akademiai Kiado, Budapest (1967). TEPLf, J., JANOWSKY, Rep. UJV-1739 (1967). HENRIKSEN, T., Acta chem. scand. 20 (1966) 2898. KARZMARK, C.J. , FOWLER, J.F., WHITE, J. T., Int. J. appl. Radiat. Isotopes 17 (1966) 161. DYNE, P.J., tlENHARTOG, J., Nature, Lond. 202 (1964) 1105. KROH, J., KAROLCZAK, S., Nature, Lond. 201 (1964) 66. RIEHL, N., in Energy Transfer in Radiation Processes (PHILLIPS, G.O., Ed.), Elsevier, Amsterdam (1966) 95. PL-236/7 87

[36] SMALLER, B., MATHESON, M.S., YASAITIS, E. L., Phys. Rev. 94 (1954) 202. [31] LIVINGSTON, R. , ZELDES, H., TAYLOR, E. H., Phys. Rev. 94 (1954) 725. [38] LIVINGSTON, R. , ZELDES, H. , TAYLOR, E.'H. , Discuss. Faraday Soc. 19 (1955) 166. [39] GHORMLEY, J. A., STEWART, A. C. , J. Am. chem. Soc. 78 (1956) 2934. [40] SIEGEL, S., BAUM, L. H., SKOLNIK, S., FLOURNOY, J. M., J. chem. Phys. 32 (1960) 1249. [43] SIEGEL, S. , FLOURNOY, J. M., BAUM, L. H., J. chem. Phys. 34 (1961) 1782. [42] SCHULTE-FROHLINDE, D., EIBEN, K. , Z.Naturf. 17A (1962) 445. [43] ERSHOV, B. G. , PKAEV, A. K. , Khimiya vys. Energii 1 (1967) 29. [44] HENRIKSEN, T., Radiat. Res. 23 (1964) 63. [45] ZAG6RSKI, Z.P., MINC, S., Paper No. 950 , 3rd Int. Congress on Radiation Research, Cortina d'Ampezzo, 1966. [46] SHIDA, T., HAMILL, W. H., J. chem. Phys. 44 (1966) 2375. [47] GALLIVAN, J. B., HAMILL, W. H., J. chem. Phys. 44 (1966) 2378. [48] TSUJI, K., YOSHIDA, H. . HAYASHI, K., J. chem. Phys. 46 (1967) 810. [49] CHARLESBY, A., Paper No. 49, 3rd Int. Congress on Radiation Research, Cortina d'Ampezzo, 1966. [50] DOLE, M., BODILY, D. M., Paper No. 50, 3rd Int. Congress on Radiation Research, Cortina d'Ampez^o, 1966. [51] MOZISEK, M., in Radiation Chemistry 2 (DOD6, J., HEDVIG, P., Eds), Akademiai Kiad<5, Budapest (1967). [52] ORMEROD, M. G., ALEXANDER, P., Nature, Lond. 193 (1962) 290. [53] ORMEROD, M. G., ALEXANDER, P., Radiat. Res. 18 (1963) 495. [54] HENRIKSEN, T., SAUNER, T., PIHL, A., Radiat. Res. 18 (1963) 163. [55] ALEXANDER, P., ORMEROD, M. G., "Repair of the primary chemical lesion: a unitary hypothesis for radiosensitization by oxygen and protection by sulphydryl compounds", Biological Effects of Ionizing Radiation at the Molecular Level (Proc. Symp. Brno, 1962), IAEA, Vienna (1962) 399. [56] SINGH, B. B., ORMEROD, M. G., Nature, Lond. 206 (1965) 131. [57] SINGH, B.B. , ORMEROD, M. G., Biochim. biophys. Acta 109 (1965) 204. [58] BAKER, A., ORMEROD, M. G. , in Energy Transfer in Radiation Processes (PHILLIPS, G. O. , Ed.), Elsevier, Amsterdam (1966) 160. [59] SUN, K. H. , GONZALES, J. L., Nature, Lond. 212 (1966) 23. [60] RICE, F.O. . Am. Scient. 54 (1966) 158. [61] SCHULTE-FROHLINDE, D., EIBEN, K., Z. Naturf. 18A (1963) 99. [62] KROH, I., GREEN, B., SPINKS, J. , Nature,Lond. 189 (1961) 655. [63] KEVAN, L. .MOORTHY, P.N., WEISS, J.J., J. Am. chem. Soc. 86_ (1964) 771. [64] ZAGfiRSKI, Z. P. , Roczn. Chem. (in press). [65] JOHNSON, M. M., ALBRECHT, A. C., J. chem. Phys. 44 (1966) 1845.

PULSE RADIOLYSIS

D.F. SANGSTER AUSTRALIAN ATOMIC ENERGY RESEARCH ESTABLISHMENT, LUCAS HEIGHTS, N S.W., AUSTRALIA

Abstract

PULSE RADIOLYSIS. The technique of pulse radiolysis enables one to examine the early species formed in radiolysis and to study their reactions. There remains much work to be done in the microsecond to millisecond region. Recent advances include techniques with a time resolution of less than one nanosecond. This enables one to study the "primary species" present in the so-called spur region. These reactions could be important in the concentrated solutions that are likely to be encountered in practical applications.

Pulse radiolysis is the technique whereby a system is subjected to an intense pulse of ionizing radiation, usually an electron beam, for a very short time - microseconds or nanoseconds. The ensuing chemical reactions can then be followed during the pulse or over a further interval of time with detectors capable of very rapid response times. Since the chemical species present during the very earliest stages of the reaction can thus often be identified, measured and followed, this has proved a most valuable method of determining the mechanism of radiolytic reactions. Review articles [1, 2] are available on the subject, and two recent conference proceedings have been published in book form [3, 4] . The kinetics of the reactions of the hydrated electron have been studied extensively, and a number of reaction-rate constants have been measured. These have been tabulated by Anbar and Neta [5] . A number of other transient species, usually radical species or excited states, have been identified and characterized.

PRINCIPLES

Essentially, a short pulse of ionizing radiation is delivered to the system being investigated, and the appearance and disappearance of chemical species studied over the ensuing time interval. Usually the radiation source is an electron accelerator of energy 1-16 million volts and a current of 1 -200 mA. The pulse duration is 0.1-10/LIS. The system can be solid, liquid or gaseous, or a solution. Of course, the experimental arrangements may be different in each case. Any means can be used to detect the species formed, provided it is sensitive to small concentrations or small changes in concentration, and its time of response short enough to follow the changes. Optical absorption, conductivity and, recently, polarography have been used. Electron spin resonance has a limited application, but the scan time is rather long for convenience; however, it can give valuable information on the structure of the radical species formed. Conductivity is not very selective because any ionic species will give a signal. Ultra-violet and visible absortiometry

89 90 SANGSTER has been the most widely used technique, and it can give some information on the probable structure of the species formed. A great number of the species formed show absorption bands.

RECENT DEVELOPMENTS

An idea of the variety of problems and systems that have already been studied can be obtained from the references and from the chemical literature. The number and nature of the ancillary techniques that can be used with pulse radiolysis are constantly being extended. Under a high pressure of hydrogen gas (100 atm) all the oxidizing radicals in aqueous solution are converted to reducing radicals, and this simplifies the problems considerably. Gaseous systems have also been studied. A start has been made on studying the photochemistry of the short-lived transient species - a combination of pulse radiolysis with flash photolysis. Over the past few months measurement times have become increasingly shorter as machines have been commissioned giving intense electron pulses lasting about 1 ns. One of the most recent utilizes the field emission principle as a source of electrons. The importance of this short time interval is that one can now study "spur" reactions - the reactions that are likely to predominate in the more concentrated "practical" systems that would be expected in industrial applications. We can expect to see further extensions of the techniques in the near future. Perhaps lasers will find an application here in following some of the species. Faster i.r. detectors are being developed. The kinetics of various free radical and ion-molecule reactions will be worked out, and their transition complexes positively identified. We can expect to have spectral tables to assist in identifying all manner of short-lived species.

ACCESS

Pulse radiolysis facilities are expensive both in capital cost and operation, although some existing accelerating machines have been modified at reasonable cost. Custodians of these facilities have been extremely generous in allowing other groups to use them. The usefulness of pulse radiolysis is such that all investigators should consider whether it can assist in solving their problems.

INSTALLATIONS

In some centres there are pulse radiolysis facilities that have been operating for some years: Argonne National Laboratory; Mount Vernon Hospital, London; Paterson Laboratory, Manchester Hospital; University of Notre Dame, United States of America; Brookhaven National Laboratory; Du Pont Research Laboratory; Mellon Institute; CEN, Saclay. PL-236/7 91

Others commissioned more recently are: Cookridge Radiation Centre, Leeds; Oak Ridge National Laboratory; Ohio State University; Ris^, Denmark; Hahn-Meitner Institut, Berlin; General Atomics, San Diego; and Natick Research Center.

VALUE OF PULSE RADIOLYSIS

The value of the technique lies in the fact that one is able to obtain an understanding of the early, and previously inaccessible, reactions. Formerly, inductive reasoning had to be used. The radiation chemist, knowing how the final products altered as conditions were changed, argued back to the intermediate and primary species. Now it is possible to identify and measure many of the transient species present during the first few millionths of a second of reaction when radiation interacts with matter. Even less is understood about what happens in non-polar liquids.

Biological and biochemical

Experiments in radiation biology show the sort of problem involved, but the simplest biological system is so complex that even a partial solution is most unlikely to be found. An understanding of the radiation chemistry of simpler chemicals (model compounds) is becoming possible. This understanding can be expected to be extrapolated to simple biological cells along such paths as that through proteins and nucleic acids.

Aqueous and polar solvents

Despite the great amount of work that has been done on aqueous systems, the nature, yield and spatial distribution of the earliest physicochemical species is not certain. Rather more is known about the later chemical entities (so-called "primary species") but even here there are discrepancies. Very little is known about yields and reactions in alkaline solutions (pH13), and independent investigators have arrived at somewhat different values. This reflects the inadequacy of our knowledge of the early events. The early work was done on dilute solutions because these are somewhat simpler but recently the work on more concentrated solutions has increased. It is expected that many interesting phenomena will be found. Concentrated solutions are closer to what can be expected in industrial practice. Of the polar solvent systems other than aqueous systems, the more intensively investigated ones are methanolic and ethanolic solutions. The older data on yields, etc., are being re-investigated at present.

Polymers

Polymers have been dealt with in detail during this Panel meeting. It is still debatable whether ionic or radical mechanisms operate in the various polymerization reactions. Solid-state polymerization induced by radiation is an interesting field. 92 SANGSTER

CONCLUSION

The full impact of the technique of pulse radiolysis is only now beginning to be felt. At last we have a method of determining what is happening at the very beginning of the radiation chemical reactions, and of studying the reactions of the transient species. This opens up what is virtually a new world of chemistry that will go far beyond the bounds of present-day radiation chemistry.

REFERENCES

[1] DORFMAN. L.M., MATHESON, M.S., in Progress in Reaction Kinetics 3 (PORTER, G.. Ed.) Pergamon, Oxford (1965) 237. [2] SANGSTER, D.F., Atomic Energy in Australia 8 3 (1965) 13. [31 The Solvated Electron, Advances in Chemistry Series, No.50, American Chemical Society (1965). [4] EBERT, M., KEENE, J.P., SWALLOW, A.J., BAXENDALE, I.H. (Eds), Pulse Radiolysis, Academic Press, London (1965). [5] ANBAR, M., NETA, P., Int. J. appl. Radiat. Isotopes 16 (1965) 227; Israel Atomic Energy Commission, Rep. I.A. 1079 (1966). LE DEVELOPPEMENT DE LA RADIOCHIMIE Quelques aspects de la conjoncture

P. LEVEQUE ET J. R. PUIG CENTRE D'ETUDES NUCLEAIRES DE SACLAY, FRANCE

Abstract — Résumé

DEVELOPMENTS IN RADIOCHEMISTRY: SOME ASPECTS OF THE PRESENT SITUATION. The situation in radiochemistry during 1966-67 in France is described. The principal features are the appearance and ecorfomic exploitation of powerful irradiation facilities, and the increased emphasis being placed on the industrial nature of development studies. Basic research is being maintained, particularly in macromolecular chemistry.

LE DEVELOPPEMENT DE LA RADIOCHIMIE; QUELQUES ASPECTS DE LA CONJONCTURE. Les auteurs présentent une analyse de la conjoncture française en 1966-1967 dans le domaine de la radiochimie. Cette conjoncture se caractérise par l'apparition et l'utilisation économique de puissantes installations d'irradiation, ainsi que par des études techniques dont le caractère industriel est de plus en plus accusé. La recherche fondamentale est toujours soutenue, notamment dans le domaine de la chimie macromoléculaire.

Depuis quelques années la physionomie de la situation en matière de chimie des radiations subit une évolution profonde. Une prise de conscience semble se faire dans l'industrie. Des secteurs nouveaux voient dans cette technique un moyen de production et d'innovation à ne pas négliger. Des procédés nouveaux sont développés. Sur le plan international la coopération s'instaure, soit de façon bilatérale, soit par le truchement d'organismes internationaiux. Dans quelles conditions cette évolution se fait-elle? C'est ce que nous essaierons d'analyser. Il nous paraît que, si des réalisations à une échelle beaucoup plus grande que par le passé sont mises en oeuvre, les succès enregistrés sont autant de cas particuliers qui rendent difficile tout pronostic d'ordre général autre que celui d'une accélération des recherches au cours des années à venir. L'apparition d'installations d'irradiation à grande échelle au cours des dernières années est un phénomène remarquable qui se fait sentir aussi bien dans la recherche que dans l'industrie de l'irradiation. Nous énumérerons ci-dessous les principales installations françaises.

1) L'installation du Centre lyonnais d'application atomique. Cette installation déjà ancienne a été rénovée en 1966 et dotée de deux systèmes de convoyeurs: un système à balancelles permettant le chargement continu depuis une zone externe et un convoyeur semi-continu dit convoyeur carré, semblable à celui d'IRMA. La source de cobalt-60 atteint actuellement environ 100 000 Ci.

2) L'irradiateur mobile autonome (IRMA). Cet appareil a déjà, été décrit. L'exploitation s'en poursuit de façon satisfaisante avec la source de 170 000 Ci de césium-137 dont il est pourvu.

93 94 LEVEQUE et PUIG

Ces deux installations sont gérées par la société Conservatome Industrie. Deux traitements radiochimiques y sont réalisés en plus des traitements de radio pasteurisation, radiostérilisation et radio- inhibition; ce sont la fabrication d'un film de polyethylene réticulé, le «Girolène», dont le marché se développe, et la fabrication de «bois- plastique» pour lequel cette société a fait une enquête de marché appro- fondie et qui laisse entrevoir des débouchés sérieux.

3) Le CARIC (Centre d'application des radiations ionisantes de Corbeville). Ce centre est géré par une entreprise privée (SRTI) et mobilise l'accélérateur linéaire CIRCE., Cette installation fonctionne depuis l'été 1966 et donne toute satisfaction. CIRCE fournit un faisceau de 8 à 10 kW d'électrons de 7 MeV et est utilisé principalement pour la stérilisation médicale. De nombreuses irradiations de recherche y sont effectuées sous contrat, notamment une étude de développement de la radiovulcanisation du latex.

4) Le Commissariat à l'énergie atomique est en train de s'équiper de nouveaux moyens puissants qui viendront prendre la suite de l'unité relativement modeste qui fonctionne à Saclay depuis 1960 (accélérateur linéaire Mévadyne de 4 MeV et 0, 2 kW, irradiateur panoramique de 10 kCi PAGURE). Les programmes d'extension concernent: a) Un laboratoire de technologie de l'irradiation, qui a commencé à fonctionner au Centre de Grenoble. Il comprend une casemate sur piscine d'une capacité de 100 000 Ci de cobalt-60. Ce laboratoire étudie expérimentalement les problèmes de configuration et de fabrication des sources; il a mis au point des méthodes de calcul FORTRAN donnant les solutions d'une variété de problèmes de configuration et d'autoabsorp- tion. Cette installation abrite aussi des expériences de génie radiochimique, notamment une boucle destinée à la polymérisation en phase liquide à 300°C et une boucle destinée à la polymérisation en phase gazeuse à pression élevée. b) Un centre d'application des rayonnements ionisants, qui sera implanté au Centre de Saclay et possédera essentiellement deux casemates sur piscine d'une capacité de 106 Ci de cobalt-60, un accélérateur d'électrons de 3 MeV et 3 kW et un accélérateur fournissant des impul- sions uniques de quelques nanosecondes. Par ailleurs, PAGURE sera renforcé, l'activité de sa source passant à 20 kCi au cours du premier semestre de 1967. Un certain nombre de projets qui, il y a seulement cinq ans, auraient paru extrêmement ambitieux sont donc en cours de réalisation et même d'exploitation. Ceci n'a évidemment été possible que grâce à certains succès de la recherche qui ont coïncidé avec une situation commerciale favorable.

Dans ce qui suit, nous décrivons succinctement les activités déployées en France dans les différents secteurs de la recherche ou de l'exploitation en radiochimie. La recherche radiochimique en France remonte, on le sait, au début des années 50. On peut constater aujourd'hui que, insensibles aux enthousiasmes ou aux scepticismes irraisonnés, des chercheurs ont accumulé, en dépit des nombreux aléas auxquels ils ont dû faire face. PL-236/8 95 une quantité de résultats dont certains présentent des attraits commerciaux et constituent donc des domaines de valorisation économique de la radiochimie. Divers degrés d'avancement des travaux ont toutefois été atteints:

Commercialisation - Girolène

Dé veloppem ent - Bois-plastique - Latex radiovulcanisé - Polymérisation (trioxane, oléfines)

Recherche appliquée - Radiosynthèse de l'hydrazine - Greffages divers, textiles ou autres polymérisations - Synthèses diverses - Reticulation des polymères

Recherche de base - Polymérisation en phase solide cristalline ou vitreuse (laboratoire du Dr Chapiro) - Greffage (laboratoire du Pr Magat - laboratoire dû Dr Chapiro)

Construction d'installation d'irradiation - Conservatome Industrie (Co-60, Cs-137) - Compagnie de télégraphie sans fil(CSF), accélérateur linéaire du type CIRCE - SAMES (accélérateur électrostatique)

La situation actuelle offre donc aux radiochimistes quelques motifs de satisfaction, mais si l'on considère que nous voyons là l'aboutisse- ment d'une quinzaine d'années d'efforts, il apparaît que le développement de la radiochimie a été relativement lent, et cela en raison de difficultés de tous ordres.

La principale difficulté que rencontrent en France les promoteurs de la chimie des radiations tient ci l'ignorance ou au scepticisme de l'industrie joints au reflexe de crainte que provoque toujours l'évocation des rayonnements ionisants. Il faut bien le dire aussi, de nombreuses tâches priori- taires ont empêché pendant longtemps les entreprises publiques et privées et les centres de recherches publics eux-mêmes de consacrer des moyens importants en personnel et en capitaux à un domaine de la recherche où les possibilités d'innovation demandent un effort d'imagination que ne vient aiguillonner aucune concurrence étrangère. Aujourd'hui, ces facteurs tendent à disparaître. En outre, depuis deux ans les actions stimulantes se font plus efficaces. En France, le Commissariat à l'énergie atomique s'est appliqué à définir une politique d'association avec l'industrie en prenant à sa charge la plus grande partie des risques et des charges financières. Par ailleurs, il a multiplié les collaborations par des contrats bilatéraux 96 LEVEQUE et PUIG

avec des firmes intéressées. L'Association technique pour l'énergie nucléaire a constitué, sous l'impulsion du CEA, une commission où se réunissent des industriels et des chercheurs qui participent au développe- ment de la radiochimie, au nombre d'une trentaine environ. Le CEA collabore, par des échanges de personnel et d'informations et par une coordination des efforts, avec le Japan Atomic Energy Research Institute. Il a mis sur pied au début de l'année un cours sur la préparation et la modification des polymères par l'irradiation qui est dispensé à des responsables de l'industrie et à des universitaires par le Dr Chapiro et le Pr Hayashi (Université de Kyoto). Enfin, le CEA participe active- ment aux actions entreprises à l'échelle européenne par le bureau Eurisotop de l'Euratom pour la diffusion des connaissances et des techniques radiochimiques. Actuellement ces actions commencent à porter leurs fruits, notamment dans le domaine des textiles et du bois. Comme on le voit, il semble bien que les conditions soient maintenant réunies pour un essor rapide mais raisonné des applications radiochimiques. Cependant, il existe encore de nombreux obstacles, qui, en tout état de cause, gênent un développement que des instances internationales souhaitent encourager; elles pourraient le faire en contribuant - ci la diffusion des connaissances et des techniques dans l'industrie (enquêtes, sondage), - au soutien, en capitaux et surtout en personnel, des projets à l'étude dans les centres bien équipés, - à la centralisation des renseignements et k leur diffusion par des méthodes rapides et efficaces (littérature, brevets, traductions), - à l'organisation et au soutien de l'enseignement des disciplines radiochimiques, - ^ la rédaction de monographies couvrant des domaines avancés de la technique radiochimique, - à la coopération internationale. Il ne s'agit là que de suggestions qui ne se veulent pas limitatives et dont nous espérons qu'elles apporteront une contribution positive aux travaux du Groupe d'étude. SYNTHESIS AND DECOMPOSITION INDUCED BY IONIZING RADIATION

SILVIA IONESCU INSTITUTE OF ATOMIC PHYSICS, BUCHAREST, ROMANIA

Abstract

SYNTHESIS AND DECOMPOSITION INDUCED BY IONIZING RADIATION. Syntheses and decompositions that have been studied most are shortly reviewed. Ozone formation and decomposition, hydrazine synthesis of ammonia, nitrogen fixation, hydrocyanic acid formation, radiosynthesis with sulphur, products

obtained from the radiolysis of COZl products from radiolysis of hydrocarbons, chlorination reactions, radiochemical oxidations, radiolytic carboxidation, ethylene glycol synthesis and nitration of aromatic compounds are described simply. The methods employed to increase the G-value of methane transformation in unsaturated hydrocarbons are discussed.

INTRODUCTION

Ionizing radiation offers a new form of energy for chemical technology. At present the development of a technology based on the use of ionizing radiation is expensive and requires special studies. Whether such an industry will in fact develop is a problem of economics. A comparison between chemical processes induced by radiation and those accomplished by thermal and electrical methods shows many possibilities of exploiting any advantages inherent in the use of radiation. For this comparison the consumed energy is expressed in watts. We can represent the conversion of watts into curies for gamma radiation of different intensities. In this case the source power is expressed in watts too. The number of transformed molecules for a watt is

2-25IOO°22G=2-25X1O20G

The number of kilograms for a watt is

2.25X1020 G.M Q 373X10-6Q M 6.02X1023 X 1000 °-d7dX10 G-M

2.78X106 where M is the molecular weight and 1 W/kg= ^ and the same value for 1 kW/t. We can expect that in the near future nuclear energy will be available for chemical processes. For such an event we have to calculate the limits of its applicability. If G = 10 molecules/100 eV and the molecular weight

97 7 98 IONEISCU

M = 100, the energy necessary for the production of 1 kg of this material will be 2'7°^106 = 2700 (or 0.044 $/kg). 103 kg 6' For G = 105 and M = 100, the irradiation cost will be 0. 27 W/kg or 0. 27 kW/t, which makes it very attractive. For G-values of 10 or higher, or for very high molecular weight materials, industrial application is a possibility. In this case we require a chain reaction to give a high G, or a process in which a small percentage of chemical change brings the desired large molecular weight change in the product. For an industry to be based on radiation chemistry the synthesis of expensive materials, or those which could not be prepared by any other method and where radiation replaces the catalyst, would be needed. The penetrating power of ionizing radiation allows more uniform reactions in large volumes of reactants, and makes it possible to carry out the reactions at relatively low temperatures and at high pressures. The radiation-induced reactions may eliminate some stages of a synthesis, and give rise to products of very great purity. Unfortunately, the ionizing radiation is not usually absorbed in a selective manner; in the irradiated system a variety of products appear. Their determination often requires difficult analyses. On the other hand, the resulting products react themselves or with initial molecules, hindering the primary reactions. One may achieve a steady state, where the concentration of the products does not vary. It is an equilibrium state characterized by an equality of the formation and the decomposition rate of the product. In this steady state the increase in the G-value ceases. From the great number of reactions that have been closely studied only a few will be selected of particular interest to industry. The most thoroughly studied syntheses and decompositions induced by ionizing radiation will be reviewed here.

OZONE FORMATION IN THE RADIOLYSES OF OXYGEN

The radiolytic synthesis of ozone by alpha particles begun by Lind [1] and continued by various researchers [2-4] has resolved the mechanism of the formation and decomposition of ozone in this case. In a more recent work, Buneev et al. [5] irradiated liquid and gaseous oxygen with electrons of 200 keV and with 60Co gamma rays. The quantity of ozone formed is proportional to the total dose and to the electron energy. Plotting the ozone concentration [03]/t as function of the contact time with the vessel, the curve shows the decrease of ozone concentration, which tends to a steady-state concentration of 0. 18%. The same experiments, repeated with liquid oxygen in static conditions, give a steady-state value of 0. 8% of ozone. In gaseous oxygen irradiated by electrons of 200 keV by a flow of seven minutes contact, a G-value of 1. 5 molecules/100 eV is obtained. 3 3 For the steady state of gaseous oxygen, [03]/[02] = IX 10" -^ 2X10" , and for the liquid state, this ratio is 8X10-3. These values show that in the gaseous oxygen the reaction rate of ozone decomposition is 500- 1000 times greater than the formation rate, and in liquid 125 times greater.

T PL-236/7 99

Because the ratio of yields of ozone in the liquid and the gaseous state is 6, one may deduce that this number represents the ratio of the two rate constants of the reactions in both media. The greater rate of the ozone formation in liquid oxygen is to be explained because at the low temperature of liquid nitrogen the excited molecules cannot easily disactivate. The collision number between molecules is smaller and the ozone formed decomposes extensively. The influence of the vessel wall on the recombination of oxygen atoms is greater in the gaseous state. The experimental values of G obtained by various workers are listed in Table I.

TABLE I. EXPERIMENTAL VALUES OF G .

Type of G(O ) Date Ref. radiation a

Alpha 1.5 1911 [1]

Alpha 3.1-3.8 1925 [2]

Alpha 3.1 1926 [3]

Alpha 3.1-5.2 1927 [4]

Electrons 62 - 124 1925 [6]

Electrons 6.2-9.3 1928 [7]

Electrons 3.1 1936 [81

Gamma 9 1960 [9]

Gamma 12.8 1963 [10]

Taking this and other later results as a basis, F. W. Lampe et al. [11]

calculated G(03) values and steady-state concentrations using a mechanism that includes the participation of ions and excited molecules. Omitting the two extreme cases [1] and [5], the authors concluded that

G(03) for particles is 3-5, for fast electrons 3-9 and for gamma rays 9-13. The primary ionization reactions of oxygen radiolyses are + + O2-JW—•0 + e" (1) or 02-jmr—»02 + e" (2) (positive ions) 02+e" ->nAr—"O^" >0" + 0 (3).

Only if collides with 02 before dissociation could stable O2 be found. However C^* dissociates in 10_13s, and collision time at the pressure of 1 atm is 10"10 s. Neutralization reactions are

02 + e"—.O + O (4) or O2+O"—+0 (5) 100 IONEISCU

Ozone is formed by

0 + 02 + 02 >03+02 (6) or

°2 +°2 >03 + 0

If ozone is formed by methods (3) (5) and (7), four ozone molecules per ion pair are formed. W(02 ) = 32. 5 eV gives 3. 08 ion pairs, 3. 08X4 = 12. 3 molecules of ozone/100 eV and G(03 ) ~ 12. If the ozone formation takes place via (4) and (7), only two ozone taolecules per ion pair are formed, and G(03) = 6; G(Oa) would increase as LET decreases. For the steady state of ozone formation, the decomposition of ozone after a significant ozone concentration must be taken into account:

O + Og »201

0+ surface > |02

03—ilWC—'dissociation

Taking the rate of ozone formation and decomposition that are equal at the steady state, and the rate values from experimental work [12], Lampe and co-workers calculated steady-state ozone formation for different pressures at different temperatures. At low temperatures and high pressures the conversion of 02 to 03 is great (Fig. 1). Taking into account the coefficient of the surface destruction of ozone conversion, the conversion of ozone, at the steady state, is very small, in fact many orders of magnitude smaller than that calculated without surface destruction (Fig. 2). The ozone is decomposed by transfer of energy stored in the surface. To test these calculations F. W. Lampe and co-workers have constructed a beta irradiator, a 600-Ci 90Sr source.

HYDRAZINE SYNTHESIS FROM AMMONIA

Various papers [13-16] have been concerned with radiolysis of ammonia which decomposes by irradiation according to

..NH' + H" NH 3-rfinr—> NH' +H2

The radicals act between themselves leading to hydrazine:

NHs+NHj >N2H4 and NH' + NH3 >N2H4 At the same time, hydrazine decomposes:

H- + N2H4 > N2H3+H2

2N2Hj —~N2+2NH3

H' + N2H4 —> NH3 +NH2 PL-236/7 101

0.7 100 •/. YIELD CONVERSION OF 02 TO 03

0 100 200 300

TEMPERATURE (°K) FIG. 1. Effect of temperature on conversion of 02 to 03.

Shortly after the irradiation of ammonia a steady state is reached where the hydrazine concentration is very small. The ammonia was irradiated with alpha particles [13], fast electrons [14], dark discharges [15] and with 200-keV electrons [16], In all these experiments, the G-value for the formation of hydrazine is between 1. 27 and 2. 5 molecules for 100 eV. The decrease of temperature does not increase the hydrazine yield; greater densities do not permit long radical lives. In more recent works [17-19] the ammonia is irradiated with gamma rays. Reported G-values are very small, 0.5-0.9 molecules for 100 eV absorbed. White [20] has studied the hydrazine formation in the fission-fragment radiolysis of liquid ammonia, and obtained a G-value of 1.5. This value decreases rapidly with increasing dose and approaches a G-value of approximately 0. 1 at high doses. A recent work of Sorokin et al. [21] studied the hydrazine synthesis from ammonia absorbed on synthetic zeolite. Experiments were performed at 0 and -196°C using a 1017 eV/g s dose-rate. Table II shows the different yields of hydrazine and G-values that reach 38 molecules/100 eV but this large value corresponds to a very small quantity of ammonia TABLE II. ENERGETIC YIELD VALUES OF THE AMMONIUM RADIOLYSIS PRODUCTS

Dose-rate 1.35X 10" eV/g s Dose = 4.85 x 10!0 eV/fc

MOO eV ^heterogeneous = m°leculi Product radiolysis Conversion Gads = moleculesAOO eV adsorbed Irradiation quantities % NH, (wt.%) 19 adsorbed by NH, (adsorbed by NH,} temper aiure <"C) <10" moleculcs/g NH3) NHS into NsH(

N N,H4 N.H, "A >

0 0.33 18.5 14.3 38.1 29.6 - 0.14 0.11 1.056

0 0.65 9.2 10. 0 - 19.0 20.6 - 0.14 0.15 - 0.520

0 1.12 7.7 8.0 - 15.9 16.5 - 0.21 0.21 - 0.436

0 4.95 1.95 2.3 0.5 4.05 4.8 0.97 0.23 0.27 0.05 O.UO 0 11.9 1.96 1.62 1.0 4.05 3.3 2.06 0.50 0.41 0.25 0.112

0 100.0 0.03 0.32 0.08 0.07 0.65 0.17 0.07 0.65 0.17 0.0019

-106 0.31 9.2 12.4 0 19.0 25.5 0 0.07 0.09 0 0.52 -196 1.05 6.5 5.2 0 13.4 10.7 0 o.ie 0.13 0 0.368 0.324 -196 1.42 5.7 11.8 - 0.19 - - 0.264 -196 1.76 4.8 9.8 " - 0.20 - - -196 11.90 0.8 1.5 0.3 1.7 3.1 0.6 0.20 0.37 0.07 0.046 0.22 0.053 -196 14.00 0.9 0.8 1.9 1.6 - 0.27 - -196 100.0 0.04 0.15 0.03 0.08 0.32 0.07 0.08 0.32 0.07 0.002 PL-236/7 103 absorbed by zeolite, namely 0. 33%. This makes the method very unattractive. From these different results, it may be concluded that the best method of obtaining hydrazine from ammonia is offered by dark discharges and not by ionizing radiation.

NITROGEN FIXATION

The well-known experiments of Harteck and Dondes [22] show that along a fission fragment track concentrations of ions and radicals is so great that their recombination takes place before these entities can leave the track by diffusion. The facts are more evident at great pressure.

Experiments carried out in a nuclear reactor with N2 + 02 mixtures of different percentages sometimes yield large concentrations of N02 exceeding the saturation pressure and N204 condensed during the irradiation. The most important mechanism proposed involves the following reactions;

N2-dW—>N2 + e" (1)

+ N2+02 • NO + NO (2)

N2+N2 ' N4 I3)

+ N| + N2 » N 4+e" (4)

+ (N2 f + N2 'N3+N (5)

+ + N3 +02 >N02+N2 (6)

+ Ng + 02 • N20* + NO (7)

N 3+°2 >N0 + N20 (8)

= In practice, radiation does not decompose N02(GNC,2 075) (2) but nitrogen atoms react with NOZ forming

—> NO + NO+ (9)

NOZ+N2 >N20 + N0 (10)

NG2+N ^2+02 (11)

N20'" resulting from (7) is transformed:

N20* • N + NO

N2O • N2+O

N20*+M •NP + M* 104 IONEISCU

and the neutralization of

NO+ + e" > N + O

The G-values of N20, NO and N02 are strong, depending on the proportion of the initial mixtures, pressure and temperature. G(-N2)~7 in the reactor by fragment-fission irradiation [22], Later experiments by Dmitriev and Pshezhetskii [23] started from the fact that the excitation and ionization of nitrogen atoms leads to the formation of atoms which then react further with oxygen. They studied the mechanism of the nitrogen oxidation by electrons of energies from 15 to 400 eV and by small pressures. Later [24] the same authors, working with mixtures of 02:N2 as 1:1, by atmospheric pressure and irradiating with electrons of 200 eV, found a G(-N2) of 2 molecules/100 eV and for air G = l. 3 molecules/100 eV. The same authors [25] studied these reactions under the action of the 90 electrons of Sr beta rays. The primary ions are identified as N2, 02, N and O . The yields depend on nitrogen molar fraction; the greater the molar fraction, the smaller the yields. The nitrogen ions result from the radiolytic oxidation of N. The latest work of Dmitriev and- Sorokin [26] studied these reactions at high temperatures between 0 and 1000°C and pressures from 1 atm to 150 atm. The determinations were made in a glow discharge for voltages <15 kV. As the temperature increases from 0 to 200°C, the rate and energy yield of the reaction increase owing to a decrease in the recombination coefficient of the ions. The effective energy of activation varies with pressure from 1. 5 to 2 kcal/mol. From 200°C the rate of the reaction decreases because a reverse reaction takes place. A s the temperature is increased from 200 to 700°C the energy yield decreases from 3. 3 molecules/100 eV at 200°C to 1. 1 molecules/100 eV at 600°C because of the thermal decomposition of NOz . At temperatures between 700-1000°C, the reaction rate increases because of the additional factor of the thermal oxidation of nitrogen.

RADIATION SYNTHESIS OF HYDROCYNANIC ACID

Hydrocynanic acid results from mixtures of unsaturated hydrocarbons with nitrogen or ammonia [27-29], The mechanism proposed by Dzantiev and Trubnikov is the following:

N|-JWP—»N' + N"

C^a + N' >HCN + CH'

CH' +N2 > HCN + N"

The mixtures irradiated with 600 keV electrons show large G-values, between 300-900, depending on composition, pressure and additives (Ar or He) where growth increases the yields. Various mixtures of C2H4, C2H2, CH4, C2Hg, C3H8 and C4H10, and NH3 were sealed in quartz tubes and irradiated with neutrons in a nuclear PL-236/7 105 reactor at integral doses of 0. 35-2. 7X1012 n/cm2 s. The quantities of HCN_, spectrophotometrically determined, G(HCN). are proportional to the dose-rate, hydrocarbon-ammonia ratio, and pressures (in the range 517 500 for 35 1-5 atm). G^CN) % C2H4, 65% NH3 and also for the mixture 15 30% C2H6, 70%"NH3 at a total dose of 5X10 eV/g. Furthermore, the radiochemical conversion of thiocyanate in acetic acid was also studied [30], Solutions of KCNS and NH4 CNS in water with 60Co gamma rays at dose-rates of 1014-1016 eV/ml s in the presence of He and O2 were irradiated. Among the radiolysis products are SO2*, CN and K + . The radiochemical yield G, in a 0. 1 M solution of KNCN, irradiated with 1018 eV/ml, is approximately 360. It decreases with increase in the dose. The G-dependence of the dose-rate shows chain reactions.

RADIOSYNTHESIS WITH SULPHUR

Radiosynthesis of sulphuric acid is one of the earliest studies in this field [31], Later the radiolysis of S02 + 02 +H20 was taken up in different variants. One suggestion [31] was to irradiate a saturated sulphur dioxide solution with 60Co gamma rays. The non-irradiated part of the solution contained more sulphuric acid than the irradiated one. A gaseous mixture of S02 + 02 was irradiated at higher temperatures up to 115°C. A few percents of SOa were formed, because S03 decomposes during the irradiation [32], Finally S02 was previously irradiated at 260°C and afterwards was introduced into a chemical reactor that contained oxygen at a high pressure. Neither of these methods can give substantial yields of sulphuric acid. Careful quantitative studies on this subject were carried out by Schrader and Sch&nherr [33-36] on the radiolytic oxidation of sulphurous acid. Aqueous solutions saturated with S02 and 02 were irradiated with X-rays from RCntgen tubes of 180 kW and 15 mA and with gamma radiation from a 0. 8-Ci 60Co source. In all these cases the dose-rate was about 3. 1016 eV/ml min. The G-values depend on the dose-rate, dose and temperatures. The highest value of G, at the beginning of the reaction, was 510+30. The dose-rate dependence is related to G(H2so4) = 3.1 X 1010 XD"0-47 where D is the dose-rate. 2 In our laboratory a mixture of SO2+O2 + H2O vapour was subjected to the gamma rays of a 800-Ci 60Co source [37], Static experiments were carried out. The composition, pressure, temperature, dose-rate and dose were varied. Stoichiometric composition of the mixture S02+02 gave higher yields. Water was always in excess. Although greater pressures facilitated the reaction the experiments were carried out at two atmospheres in the temperature ranges 60-150°C and for 6 atm at 26 - 74°C. The G-values showed a linear dependence on the inverse square root of the dose-rate, showing that the reaction is a chain reaction. The extrapolation of G at zero time gave a maximum value of ~800 molecules/100 eV. Hirakadze et al. [38] irradiated with X- and gamma rays some solutions of sulphides, thiocyanates, mercaptans, thiophenols and thiourea. 106 IONEISCU

Aqueous solutions of 0. 005 to 0.5 M of Na2S, KHS, and Na HS were oxidized to the corresponding sulphates with G = 60. In these cyanate solutions the conversion was only 30 to 40% with a G-value of 20 to 30 that decreased with the temperature. The sulphur from butyl, amyl and hexyl mercaptans was oxidized to S and S02~ in non-aqueous and aqueous solutions respectively. Thiophenols produced the corresponding disulphides with a G-value of 20. An oxidation-decomposition occurred in thiourea with a G-value of 180 and a 15-20% conversion. It was observed that irradiation of sulphur compounds produces directed changes in their state of oxidation. The same authors [39-42] studied also radiolytic oxidation of other sulphur organic compounds such as aliphatic mercaptans, aliphatic mercaptides, thiophenols in alkaline media and thiourea solutions. They obtained radiation chemical yields reaching 600-700 molecules/100 eV. The processes involved are considered to be of a chain character.

PRODUCTS OBTAINED FROM THE RADIOLYSIS OF C02 AND VARIOUS MIXTURES CONTAINING C02

The stability of pure gaseous carbon dioxide to ionizing radiation is well known. In the presence of additives that can react with oxygen atoms, C02 decomposes with G-values in the range of 9 [43] to 3. 5 [44], The CC^ is more stable to radiation in the gas than in the liquid phase.

The products of gamma radiolyses of liquid C02 are CO, 02 and 03 with G-values G^coj= 5-3. 5, 0 - 0. 6 and 0. 7 in the presence of such additives as C2H2 , C2H4, C4H10, N02, N20 and H2 , at -42°C [45],

In gaseous C02 these values are smaller. Since the carbon dioxide is a favourable coolant in the reactor, its decomposition in carbon monoxide and oxygen was achieved by introducing small amounts of nitrogen dioxide [4 3], Recently, the radiolytic behaviour of C02 by pursuing an isotopic 13 ls exchange reaction between C02-02-CO, using labelled C and O, has been studied [46], CO is formed as a primary product with a G-value of 11. C^ resulted as secondary product from the decomposition of COj. The first step in the radiolysis of C02 may be 2 CO2-JUIP—>2 C0+02. The radiolysis of C0/C02 mixtures was studied in the reactor [47] at 130-150°C. A solid of approximately the formula C^gO was found. The CO is destroyed with a G-value of about 5. A simultaneous reaction takes place to re-form CO.

Radiolysis of a mixture of C02 and H2 [48, 49] forms initially CO and water vapour in equal quantities together with small traces of methane. As the reaction progresses, the rate of CO formation decreases rapidly and an orange solid, with the empirical formula C2H301-1 , is formed in the reaction. Variation in relative concentrations of the reactants shows that the activated species from H2(H2 or H) contributed only to CH4 = formation (GCH4 0. 3) with little or no effect on the formation of CO and 1700/RT H20. (G(CO) = G(H20) =17.3 e" molecules/100 eV.)

The reduction of C02 by H2 , initiated by 115 keV electrons,gave-CO, glyoxal and organic acids [50], PL-236/7 107

Carbon dioxide results in many chemical industries as an unusual product. A rational use was discussed by Harteck and Dondes [51], They irradiated in the reactor with fission fragments the mixture

co2 +| N2 +O2 >NO2 + N2O +CO

If C02 is irradiated, one obtains CO and 02. The ionizing radiation transforms CO in suboxide with G = 2. 5. This polymerizes; the polymer can react with water or alcohols to give anhydride or other organic function which can serve in a different synthesis. Harteck and Dondes calculated that 1 g of 335U, in the reactor, in favourable conditions, could generate a tonne of suboxides from CO.

PRODUCTS RESULTING FROM ALCAN RADIOLYSES

Gamma radiolysis of alcans does not yield chain reactions. For this reason the energetic balance is bad. Thus, the radiolytic yield is 7-10 molecules/100 eV for radicalic products and ~3 for unsaturated products. Considering an average value of 3 eV per chemical bond, it follows that only 30% of the absorbed energy is used to produce chemical reactions, 70% being lost in collisions and energy transfer. Considering the selectivity of bond breaking, 80% of products result from breaking of C-H bonds and 20% from C-C bonds, giving rise to saturated and non- saturated hydrocarbons [5 2], In the radiolysis of alcans, the main products are H2 and CH4-CH4 - percentage increases with CH3/CH2 ratio [53]. For instance, GCH4 2 in n-hexadecane radiolysis and 3.2 in iso-hexane radiolysis respectively. Cyclopentane does not form methane as an irradiation product, but izo- octane yields = 37% of CH4. The product-yield dependence on dose is linear in the range of small doses (3. 1018- 20. 1018 eV/ml) for n-hexane and similar hydrocarbons. The deviation from linearity at high doses (1-30. 1021 eV) is caused by secondary reactions. The hexane radiolysis in the small dose range [54-58] gives the following products: hydrogen with a G-value of 5. 1, methane, 0.41, ethane, 0.82, ethylene, 0.62, propane, 0.68, butane, 0.65, 1-butene, 0.24, 1-hexane, 0.25, vinylidene structures, 0.29, hexadienes, 0.008, pentanes, heptanes and heptanes non-determined, nonanes, 0. 17, decanes, 0.19, undecanes, 0.03, including 4. 5 diethyloctane, 0.39, 4 ethyl-5-methylnonane, 0.85, dimethyldecane, 0.52, 5-methylundecane + n-ethyl decane, 0. 4, n-dodecane, 0. 03. In the high dose range up to 21 ~10. 10 eV/ml was found: H2 4.5, CH4, 0.2, C2-C5 saturated hydrocarbons, 1.6, C2-C5 unsaturated hydrocarbons, 0.4, liquid unsaturated compounds, ~2.0. From infra-red spectra, even at doses of 1. 1021 eV, approximately 20% of the liquid compounds are non-saturated hydrocarbons containing more than ten carbon atoms per molecule. The mass spectrometric analysis of these compounds shows also a number of saturated hydrocarbons (Cg -C28 )• However, the majority are non-saturated compounds. 108 IONEISCU

The practical uses of these radiolytic processes are unimportant. Even hydrogen production does not exceed a G-value of 5 molecules/100 eV.

Alcan radiolysis in the presence of oxidic catalysts

In several studies [59] a linear relationship was established between the radiolytic rate of the reaction and the number of electrons contained in a monolayer of the catalyst surface and the absorbed hydrocarbon. The constant ratio of these two quantities shows that the energy transfer is possible only in the adsorbing monolayer. In the bulk, the reaction is considered to be homogeneous. A relation between catalyst composition and this catalytic effect has been demonstrated by E.S.R. spectra. It is presumed that during the irradiation excitation or charge acceptors are formed on the catalyst surface. The sudden decrease of the reaction rate in the heterogeneous radiolysis at high doses is ascribed to a rapid polymerization process of the unsaturated compounds on the catalyst surface. In this case, the catalyst efficiency is very great, because the polymerizations are chain reactions. Therefore we believe that polymerization reactions on catalysts have the most promising future in the radiation chemistry of heterogeneous systems.

Temperature influence on alcan radiolysis

Alcan radiolysis at higher temperatures does not give a much greater number of products than at ordinary temperatures, but the radiolytic yields are perceptibly increased. High temperatures, together with radiation, raise the number of initiations and of chain propagation centres. This fact led to the study of radiation thermal cracking of hydrocarbons [60-62] in view of its industrial application. Thermal cracking of alcans usually takes place at temperatures of 500-600°C and consists of two steps: initiation of the reaction, i.e. the creation of radicals from the molecule dissociation, which need an activation energy of 80 kcal/mol [53], and the chain propagation whose activation energy is ~20 kcal/mol. The average energy of 60 kcal/mol is the reason for the high temperature of the thermal cracking. The use of the irradiation together with the heating decreases temperatures by 150- 200 deg C to obtain the same effect, because the irradiation influences only the initiation step that consumes the greater part of the energy. The efficiency of the thermo-radiolytic cracking was studied in static and continuous experiments. Both tests gave similar results. Up to 550°C the thermal component is very small and the radiation influence gave an important initiation to the process. Even over 550°C, when an intensive thermal cracking takes place, the radiolytic component is four times greater than the thermal one at a certain dose-rate. In this way, the use of irradiation with the thermal cracking process has a favourable effect on the decrease of cracking temperature and increases appreciably the quantity of products. The promising results obtained by radiothermal cracking represent an initial step in the investigation. To complete the work requires supplementary studies on pressure influence on catalyst presence and on the best dynamic factors. PL-236/7 109

THE RADIOLYSIS OF UNSATURATED HYDROCARBONS

Studies on the radiolysis of unsaturated hydrocarbons seem to indicate some possible directions. The simplest compounds represent the most suitable systems for studies of fundamental processes. For this reason, ethylene, the first representative of unsaturated hydrocarbons, was mostly studied [63-71], On this substance, photochemical [72-75], free radicals [76] and mass-spectrometric [77-83] studies were made. Sauer and Dorfman [74], Okabe and McNesby, as well as Dzantiev and co-workers [75], carried out photolitic reactions on ethylene with hydrogen and methyl addition to form ethane and butane. Yang and Manno [66] disagreed with radicalic formation of butane. Meisels and Sworski [69,71], after detailed studies made to elucidate the role of hydrogen atoms and of intermediates in ethylene radiolysis, came to the conclusion that the ethyl radical was yielded in ethylene radiolysis with a G-value of 6.7 radicals/100 eV. Meisels considered that higher hydrocarbons are formed by ion-molecule reactions, and it is not necessary to consider radicals as precursors in the formation of these compounds. N-propyl, sec-propyl, sec-butyl are formed in small quantities by ion molecule reactions. The kinetic treatment of this problem is based on three hypotheses. The steady-state concentration of radicals is determined by the hydrogen and by radicals formed out of it. The initial quantity of radicals is constant, and all radicals are added to ethylene or combine with ethyl radicals. The experimental data agree with the radical mechanism of the formation of higher homologues. At small dose-rates and high pressures, ethylene polymerizes. Meisels established for the G-value of acetylene polymerization a relationship based on a radicalic mechanism that coincided perfectly with the empirical relationship of Hayward and Bretton [84], The role of methyl radicals, obtained in the radiolysis of unsaturated hydrocarbons, was determined by labelling them with 14C [85, 86]. Un- saturated hydrocarbons, labelled with 2H, showed that allylic hydrogen is lost eleven times more readily than a primary hydrogen. The irradiation of butadyene, isoprene, piperilene diisopropenyl and cyclopropenyl [64] gave rise to products such as H2, CH4, C3H6 , C2H2 . The results of butene irradiation are C2H4, 03!% , C2H2, CH4 and H2 and saturated hydrocarbons with long chains [87], Similar products are obtained by the radiolysis of cis-2-butene. In presence of Ar or Xe the main product is C2H4 , and others are H2 and CH4 . The radiolytic isomerization of stilbene [88, 89] was performed in a solution of saturated hydrocarbons and in benzene. In irradiated solutions of saturated hydrocarbons decomposition products were found, whereas in benzene solutions an isomerization takes place [88,89], Irradiation of binary solutions, such as cyclopentane with cyclohexane, or cyclopentane and cyclohexane, gave decomposition products also [90], Acetylene was irradiated as a gas and in aqueous solutions. The chief product obtained in the gaseous phase was benzene [91, 92], Its yield was a pressure function; at 20-30 Torr, it passed through a maximum that rose with the temperature increase. In this case, the G-values were 5. 1-13.0. The quantity of benzene did not depend on dose-rate, but at 110 IONEISCU

6X 12-42X 1012eV/cm3 s fell suddenly [91, 93]. The irradiated solutions of acetylene gave polymers in the presence of N2 and H2 . In the presence of 02 the polymerization was inhibited and high yields of glyoxal were found. Aromatic hydrocarbons are resistent to irradiation. Fast neutrons decompose them more rapidly than gamma radiations [94, 95]. Alpha particles, resulting from a 10B(n,a)7Li reaction, cause higher G-values [96], A variation of G(H2) in this radiolysis caused by protons, deuterons and alpha particles is a function of LET [97, 98], Orthoterphenyl and di- phenyl in this case showed a variation of G(H2) and G(-MJ on LET also [94], Burns and Jones [99] discussed these results for benzene. The radicals formed in the radiolysis of benzene [100] and the formation of cyclohexadyene [101] have been studied. The hydrogen yield obtained by the irradiation in the cyclotron is greater than that obtained by fast electron irradiations [102], The irradiated mixtures of benzene and methyl and ethyl cyclohexane [103] show a lower hydrogen yield. Much work has been devoted to the mixtures of aromatic and saturated hydrocarbons [103-105], Solutions of benzylchloride or benzalchloride in saturated hydrocarbons by irradiation give up their radicals and eliminate the chlorine atoms [106, 107], The radiolysis of antracene oils has been studied by Proksch and Bildstein [108],

CHLORINATION REACTIONS

At an IAEA Conference in 1959 in Warsaw, an economic evaluation by I.Rosen [109] concluded that in chlorination as in sulphochlorination reactions, gamma radiation acts as a free radical initiator, and the same products as in photochemical reactions are obtained. The energy cost of the photochemical initiation is negligible, and cannot be reduced by the use of high energy radiation sources. The chlorination rate, for large dose-rates, depends only on the rapidity of dissolution and the quantity of chlorine. This rate is little changed by changing the temperature. It is therefore more suitable to work at room temperature.

FIG. 3. Radiation plant for ethyl bromide synthesis. PL-236/7 111

It is generally considered that the selective preparation of n-alkyl chlorides by the chlorination of oleffins is difficult; recent Japanese work [2] found that both n-butyl chloride and ethyl-chloride were efficiently (G = 35 000) formed by a gamma-ray-induced reaction of ethylene and hydrogen chloride. The G-values depend on both the dose and the dose- rate. The reaction is presumed to be a vapour phase radical chain reaction; and the proposed mechanism agrees with experimental results. Initiation HCHUT—H' + Cl'

c2H4 + cr >c2H4cr

C2H4C1" +HC1—> C2H5C1 + C1' chain c2H4cr +C2H4 > c4H8cr

c4H8cr + HCI

ci' + cr >ci2

From the gamma radiolysis of mixtures of CC14, CHC13 , CH2C12 and cycloalkene one obtains a synthesis of intricate compounds [2] as trichloromethyl cycloalkanes and others. These are used to obtain tracers containing 14C in the molecule. The gamma radiolysis of o- and m-dichlorbenzene [3] yields benzene substitution derivatives from C6H4Cl2 to C6H2C14 and oligomers. The G-values are between 0. 9 and 7.1. For this purpose accelerated electrons from a Van de Graaf generator (1.5 MeV) were also employed. The chlorination of kerosene by gamma 60Co-irradiation [5] takes place rapidly (15 times more rapidly than that effected by u. v. irradiation), G = 20X105 and the product includes 60% chlorine.

Hydrogen bromide addition to ethylene

Ethyl bromide preparation is the first industrial application of a radiolytic synthesis (Dow Chemical Company). The gaseous ethylene and hydrogen bromides are introduced in a chemical reactor through g (Fig.3). The 6Cto-source (1800 Ci) is in d. The reaction is presumed to be a chain reaction with the following mechanism;

HBr-dW—'H' + Br'

Br' + CH2 = CH2 •BrCH2 - CH£

Br CH2 - CHj + HBr >BrCH2 -CH3 +Br'

These reactions take place at -2°C and at 0. 15X106 rad/h. The G- value is 3X9X104 molecules/100 eV. The mixture flows with a speed that permits a great transformation of the gases. The remaining gases are conducted again into the circuit. The hydrogen bromide formed is eliminated, cooled, distilled, neutralized and dried. The chemical reactor 112 IONESCU is built of nickel. Seventy per cent of the gamma-source radiation is used. The method requires clean and dried HBr for radiolytic synthesis. The radiation is cheap (<$l/d). The Dow Chemical Company synthesized 500 tons/yr. It serves as an anaesthetic and as an intermediate in organic synthesis.

RADIOCHEMICAL OXIDATIONS OF ORGANIC SYSTEMS

Radiation may be used to accelerate normal thermal oxidations, although it is difficult in such systems to gain any understanding of the fundamental radiolytic processes. If it is assumed that the initial radiolytic decomposition of the hydrocarbon is primarily to give radicals, then from the knowledge of the elementary processes occurring in thermal oxidations the following sequence of reactions may be proposed [112]:

RH-IAJIP—>R ' + H'

RH-rfinr—>R i+R2

H+O2 *HO2

R- + O2 -RO2

H' + RH -H2+R'

HOJ + RH —— H202 + R'

ROG + RH ——'ROO H + R'

H02 + H02-—• H2O2 + o2

R02+R02- >ROOR + 02

RO2 + HO2 - >ROOH + OJ

RO2+RO2- > ketone + alcohol + O;

Saturated hydrocarbons

The radiolytic oxidation of methane was originally studied in the Wantage Research Laboratory for possible industrial application. An attempt was made to induce thermal oxidation (which occurs via a chain reaction at about 350°C) to occur at lower temperatures by gamma irradiation. Failure to obtain a chain reaction led to a study of radiolytic oxidation at room temperature, and a note was published [113] on the variation in methyl hydroperoxide yield on 60Co gamma radiolysis of 0.1, 1. 0 and 10% O2-CH4 mixtures. The yields of methyl hydroperoxide and formaldehyde from the 4-MeV electron and 60Co gamma radiolyses of 10% 02-CH4 mixture at 25°C have been studied [114] in the first part of PL-236/7 113 the work over pressure and dose-rate ranges of 1-76 cm Hg, and 1. 82X1014 - 9. 62 X 1017 eV/ml min, normalized to 20°C and 76 cm Hg. G(CH3OOH) increased from 0. 9 to 6. 0 with increasing dose-rate (at atmospheric pressure), and increases from 1. 7 to 6.0 with increasing pressure (at the highest dose-rate). G(CH20) is about half G(CH3OOH), and behaves similarly with respect to dose-rate and pressure. In the second part [115] of the report the formation of the alcohols, esters, aldehydes, ketones, ethers and hydrocarbons is described. A study of the reaction mechanism based on radical-radical and atom-radical reactions was carried out. The product formation G-values were unimportant and the G-value for the methane disappearance was G(-CH4) = 14. Of all the radiation-induced hydrocarbon oxidation systems, the cyclohexane-oxygen system has been the most widely investigated. By using both paper and gas chromatography it has been established [116] that the organic products of the oxidation of cyclohexane are cyclo- hexyl hydroperoxide, dicyclohexyl peroxide, cyclohexanone and cyclohexanol. The product yields obtained in Ref. [116] are compared in Table III with those of Bakh [117] , Dewhurst [118] and McCarthy and MacLachlan [119], The absolute yields of these products are still not definitely established. The carbonyl yield in pure chlorocyclohexane is considerably greater than in cyclohexane, and this is consistent with the higher radical yield observed in chlorocyclohexane.

Unsaturated hydrocarbons

The difference in behaviour of unsaturated hydrocarbons compared with saturated hydrocarbons in irradiation-induced oxidation may be partly attributed to the weaker alpha C-H bond so that the reaction

R02 + RH > R02H + R' may take place readily at room temperature, and thus give rise to chain- induced oxidation, and partly to the possibility of the addition of radicals to the double bond. When induced-radiation oxidation of acetylene is carried out, a yellow polymer resembling cuprene is obtained, with -M/N= 19.8. The mechanism of these reactions has not yet been elucidated [119], In the presence of significant concentrations of oxygen, the oxidation of the ethylene takes place [120], The product yields obtained for carbon dioxide, organic acids, formaldehyde, peroxides, glycolaldehyde and butene are not greater than 3.5. For the induced-radiation oxidations of other unsaturated hydrocarbons as propylene, cyclohexane, diisobutylene, the product yields were also unimportant.

Oxidation of other organic compounds

Formaldehyde and hydrogen were proved to be major products of the oxidation of m ethanol. 60 Co -gamma radiolysis of aerated methanol was investigated by Hayon and Weiss [121] and Lichtin et al. [122], Hayon and Weiss obtained G(CH20) = 4. 28 and G(H202) = 2. 89 at a dose-rate of 114 IONEISCU

TABLE III. PRODUCT YIELDS FOR CYCLOHEXANE OXIDATION TEMPERATURE: 25°C

Product G-value

Ref. [116] Ref. [119] Ref. [118] Ref. [117]

Cyclohexanone 2.02 2.28 3.5 0.6

Cyclohexanol 1.55 2.40 3. 7 -

Cyclohexylhydroperoxide 1.05 1.1 0 1.0

Dicyclohexylperoxide 0.31 - - 0.2

Hydrogen peroxide 0.10 - - 0

Acid - 0 - 0.2

13fcs 6-MeV 0.8-MeV 70-kV Irradiation source rays electrons electrons X-rays

19 n Dose-rate 1.6 X 10" eV/ Pulsed 1. 6 x 10 eV/ 10 eV g"1 g"1 g"1 irradiation s"1 s"1 s-1

21 Total dose 2XlO20 eV/ 2xl021 eV/ 6xl021 eV 10 eV 1 g"1 g"1 G" g"1

1.6X1016 eV/ml min, and a total dose of (1-2)X 1018 eV/ml. On the other hand, Lichtin et al. found that aerated methanol yields formaldehyde and peroxide with G = 10+1 and 2. 8, at a dose-rate of2X1017 eV/ml min, and a total dose of the same order as given by Hayon and Weiss. Imamura and Seki [123] in a more recent work reinvestigated this system extensively, and revealed the pronounced effects of dose and oxygen concentration upon the yield of formaldehyde. From a study of the effect of 60Co gamma radiation on benzyl alcohol-02 solutions, it was shown that a high yield of peroxides (G = 50) was obtained [124], This clearly indicated the existence of a chain mechanism, presumably because peroxy radicals can abstract a hydrogen atom from benzyl alcohol.

Phenol from benzene

Many researchers have tried to obtain phenol from a benzene-water mixture in the presence and in the absence of air'[125]. PL-236/7 115

In O2-saturated solutions and at 220°C the G-value for the phenol formation was 50 molecules/100 eV; this method of obtaining phenol may therefore be considered convenient for industrial application. Proskurninetal.[126] studied the effects of temperature, total dose, dose-rate and 02-pressure on the radiochemical oxidation of benzene in aqueous solutions. Christensen [127] carried out the radiolysis of aqueous 0.1 N alkaline solutions of benzene with 60Co-gamma rays in the presence of various inorganic oxides such as CuO, Cr203, ZnO, Ti02, Th02 , U02. When C203 gel, ZnO or Ti02 were added, a 9-13% increase in the yield of phenol was obtained, and the addition of U02 and ThOz caused a 31 and 39% increase.

P- xylene oxidation

A p-xylene solution containing 1% Co naphthenate was irradiated [128] with fast neutrons and gamma rays in the presence of oxygen. After 8 h we obtained a yield of 12-22% p-toluic acid. The dose rate was: 1.25X1013 eV/g s; the fast neutron beam was 107 n/cm2 s; temperature was 60-130°C. The oxidation of p-xylene at 25°C induced by 60Co-gamma radiation [129] is accelerated by the addition of organic bromine compounds. CBr4 is the most efficient sensitizer and the rate of absorption of oxygen accelerates to a maximum at about 2% oxidation and then decreases. The observed kinetics are consistent with a mechanism in which the auto- retardation and the post-irradiation oxidation result from the reaction ROOH + HBr •RO' + HsO+Br between the hydroperoxide and hydrogen bromide formed as intermediates.

Oxidation of paraffin

The authors [130] used the fission products in a pilot set-up to initiate the oxidation of paraffin. They described the set-up for the irradiation of paraffin without air bubbling, the results relating to the oxidation of pre-irradiated paraffin, the optimum work conditions for the source used and data on the doses absorbed in the course of the experiments. Oxidation was carried out in the presence of air and at 135°C, From 300g oxidized products, 87. 5g fatty acids were obtained. After a vacuum distillation at 1 mm Hg three characteristic fractions were obtained.

Ethers

Irradiation of pure disopropyl ether [131] by 200-kV X-rays in the absence of oxygen gives G(carbonyl) = 10 and Glalcohol)^ 0.7. The carbonyl compounds are acetaldehyde, acetone and a long chain of methyl- ketone. In the presence of oxygen, the carbonyl yield is = 25 and formaldehyde is formed in addition to acetaldehyde. New products include acids (G = 1. 5) and peroxides (G = 20). Similar results were obtained using diethyl ether, G(carbonyl) = 20 and G(peroxide)= 15. 116 IONEISCU

Halogen compounds

The difficulty of obtaining pure compounds appears to be one of the problems in this field. The oxidation of chloroform has been shown [132] to proceed via a chain mechanism with G(-CHCl3>- 100 for a dose-rate of 1015 eV/g s using 60Co gamma rays at room temperature.

Tetrafluorethylene when irradiated in an equimolar mixture with 02, at 1 atm total pressure, with 50-kV X-rays at room temperature, is oxidized by a chain process [133], The following yields were found:

G(COF2) = 3300 and G(CF2)p = 1090.

RADIOLYTIC CARBOXILATION

Simple organic substances can be ca'rboxilated by carbon dioxide under the influence of ionizing radiation [134-136], In the irradiated C02 aqueous solutions formaldehyde, formic acid and oxalic acid are formed by gamma- and alpha-particle irradiation. The process is strongly dependent on the pH of the solution, dose-rate and dose. In a formic acid aqueous solution, oxalic acid is produced with a G-value of 1. 8 at a dose of 2X1018 eV/ml. At increasing pH values up to 10.8 the chemical yield (9. 1 molecules/100 eV) is increased too. The quantities of oxalic acid are strongly dependent on the dose rate. At 140 krad/h the curve passes through a maximum. This means that for the formation of oxalic acid many reactions take place, i. e. it is a chain reaction. The iron-II ions react as scavengers for intermediate radiolitic products of water, and reduce the yield of formic acid in C02 aqueous solutions.

In methanol solutions saturated with C02, under the influence of gamma rays, glycolic acid is produced. The proposed mechanism is:

CH3OH + H' 'CH2OH + H 2

CH3OH + OH' >' CH2OH + I^O

COOH ' CH2OH + ' COOH » in acidic solutions CH2OH

CH2OH 'CH2OH + CO2 in neutral and alkaline solutions COO"

In dilute aqueous solutions the radicals ' CH2OH and 'COOH are formed (G = 2. 95). PL-236/7 117

By the combination of two radicals, tartaric acid was formed according to:

CH2OH ' CHOH | + H • + H r COOH COOH

CH2OH 'CHOH + OH' + H2O COOH COOH

'CHOH COOH (CHOH)2 COOH COOH

At high concentrations of methanol no more oxalic acid exists, and only ethylene glycol is formed [137],

FISSION FRAGMENTS INITIATE ETHYLENE GLYCOL SYNTHESIS

The reaction of methanol to produce ethylene glycol apparently involves primarily the combining of 'CH2OH radicals that arise either when + H' is ejected from a neutralized parent ion CH3OH or when a free radical (H' , CHj, CH2, etc.) extracts hydrogen from methanol:

H' Ho

CH2OH+ - ->'CH2OH+ - CH, CH,

2 CH2OH' >(CH2OH)2

In the irradiated material, hydrogen extraction must compete with recombination of radicals such as H' , CHg and CH2, which reduces the ethylene glycol yield. ' Fission fragments, each having about 20-22 unit charges at the moment of fission, produce short but dense ionization tracks. The molecules within this track are ionized or broken into ionized fragments, which react directly or combine with electrons to form neutral but extremely reactive fragments. These fragments then react by typical free-radical mechanism, as we have described above. Irradiation of methanol by fission fragments [138] gives 0. 1 lb of ethylene glycol per kilo'watt hour with 65% chemical conversion (plus an additional 10% conversion to formaldehyde). These results represent the irradiations carried out in two different types of reaction vessels in two nuclear reactors. Although these yields are high, they can probably be increased by further research on the use of diluents (such as water or H2) and chain transfer agents in irradiations above 150°C [139], 118 IONEISCU

Methanol irradiation experiments were designed to show the effects of temperature, dilution with water and addition of carbon tetrachloride.

RADIATION-INDUCED NITRATION OF AROMATIC COMPOUNDS

The nitration of aromatic compounds induced by radiation has already been studied for neutral [140] or acidic [141] solutions of either benzene [140-142] or benzoic and salicylic acids [141], In these investigations the observed products were those of nitration (as nitrobenzene) or nitration and oxidation (as the corresponding nitrophenols). In a recent work [143] the radiation-induced nitration of benzene and phenol in acidic, neutral and alkaline aqueous solutions has been investigated. A "Gammacell 220" 60Co-source was used, giving a dose-rate of 2.96X1017 eV/ml min. Solutions were irradiated at natural pH or in the presence of sulphuric acid or sodium hydroxide up to concentrations of 0. 05 N and 0. 5 N respectively. The corresponding G-values of the o-nitrophenol formation under the conditions mentioned above were G = 0.41; 0.68; 0.38, NO nitrobenzene or picric acid was formed. Referring to the mechanism, it seems probable that this process may proceed simultaneously by oxidation of the primary formed nitro-compounds as well as by a direct radical reaction.

DYES

The dyes undergo colour changes under the radiation, either by oxidation or by reduction reactions. They therefore may be used in chemical dosimetry. Interest in the behaviour of dyes under radiation began in 1931 [144], Discolouration of dyes is not a sensitive enough reaction to be used for low-dose determinations. Methylene blue, which is easily reduced by ionizing radiations to the colourless form, is the dye most studied [145, 146], Methylene blue discolours in the absence of oxygen and in the presence of some organic substances such as albumin, ethanol and glucose. The colour recovers entirely in the presence of an appreciable amount of oxygen, proving that the discolouration is due to a reduction reaction. This reaction may be employed in the chemical dosimetry by using air-free solutions. A large number of dyes behave like methylene blue under irradiation. The radiochemical discolouration of seven dyes in the presence and absence of 2. 5-dichlorohydroquinone was examined in the presence of air [147], In the presence of the hydroquinone discolouration occurs because of the action of HO2 radicals. H02 radicals do not attack safranine T and neutral red, whereas methylene blue, acridine orange NO and the indigo dyes are discoloured. The G-value of discolouration by HO2 radicals depends strongly on the dye concentration. Some dye syntheses were performed experimentally under radiation, in particular that of thionine [148], A hydrochloric solution of p-phenylene diamine was irradiated with a 60Co source in the presence of CUSO4 and PL-236/7 119

"S(NH4)2 with different pH values. Most irradiations were carried out at 20°C, at dose-rates from 14 000 to 150 000 rad/h. It seems that the reaction yield may reach 2.

DISCUSSION

Endothermic chemical reactions occur by the radiolytic process where the radiative energy is partially transformed into chemical energy. Many syntheses have been produced in different laboratories in the hope that they can be applied in industry. From the large number of radiochemical reactions proposed (apart from the polymers), only the ethylene bromide synthesis has been used in industry. A detailed analysis of radiolytic processes is therefore needed. The chemical reactions induced by ionizing radiation give rise to a large number of products, some of them in quantities comparable with those obtained by normal methods. After a certain time, the desired product reaches a steady state, the product decomposing at the same rate as it is formed. If the secondary products can be removed, the reverse reaction will be decreased and thus the possibility of a greater conversion will be created. The greater part of the radiation energy is consumed in primary processes to form ions and excited molecules. The activation energy for ion or radical recombination is negligible compared with that consumed for the primary processes. From this arises the difficulty of directing the subsequent processes towards forming the desired product. In the radiolysis of oxygen to obtain ozone, the wall effect causes the recombination of atomic oxygen with itself or with ozone molecules producing a considerable decrease of the ozone yield. Linde's calculations show that the decrease of this effect produces a considerable increase in ozone conversion. If in the hydrazine radiosynthesis the hydrogen could be removed by diffusion, the hydrazine conversion would be much greater. The radiolysis of methane offers an even clearer example. The methane being irradiated with alpha particles [63], with 2 MeV electrons [149], deuterons. X-rays or gamma rays [150], with slow electrons [151], with fission recoil fragments [152], with electric discharges [153], with fast electrons from a linear accelerator [154] or with 60Co-gamma radiation, gives rise to the same products: gaseous and saturated hydrocarbons of C1-C6 , and an oily liquid composed of both unsaturated and saturated hydrocarbons, of C16-C28 [150], Because the condensation is related to the primary acts of radiolysis, the ratio C/H gives 72% CnH2n and 28% CnH2n+ 2. The nature of the products is independent of the initial gas pressure, the dose-rate and temperature up to 100°C. But the methane conversion varies from 10 to 100%. The G-values decrease at high doses, beyond 50 Mrad. According to Lampe, the polymer quantity increases proportionally to the dose. Moreover, the identity G(H2) = G(.CH4Jis found. Wexler and Yesse [155] gave a mechanism for the formation of different ions by ion-molecule reactions, and showed that the passage to 120 IONEISCU

higher homologues, and the concomitant formation of ethylene and acetylene, occur as follows:

CH4+ CH4 »CH5 + CH3

CH*+e' » CHj +H2 »CH2+H2 + H

+ CH3 +CH4 ' (C2H7V »C2H5+H2

C2H7+e" » C2Hg +H2 »C2H4 + H2 + H'

+ C2H5 + e —^ C2H * • C2H4 + H •

The formation of ethylene also takes place by radical recombination in the case of excited molecules:

CH4 -TWT- CH^ »CHG + H' ; 2CH3 »C2H6

CH3 «— ' CH2+H 2CH2 »C2H4

CH3 + CH2 C2H5

C2H,- < 1 CH - CHG + HG

CH2 - CH'i=?CH = CH + H' nCH = CH vpolymer

The condensed fraction, which is not yet well known, constitutes the only interesting product resulting from all the numerous works concerning methane radiolysis. It might be avoided if the condensation or the polymerization of unsaturated hydrocarbons could be hindered. Farley and Lloyd [152] and Miguel and Chirol [153], who worked by means of dark discharges, showed the initial formation of small quantities of ethylene and traces of acetylene, which disappeared after a short time of irradiation. In recent work by Hummel [115, 156], small quantities of oxygen were added to methane. In this case, the ethylene appeared with a G-value of 0. 65 oxygen, probably inhibiting partially the condensation or the polymerization. When the static experiments were replaced with dynamic ones, and the radiolysis products passed through a trap cooled with liquid nitrogen, the G-value of ethylene was high.

Hummel showed that the processes invoked for CH4-radiolysis had to explain the ethylene and acetylene formation, and many of the products, previously ascribed to the ion-molecule processes, resulted from ethylene and acetylene reactions taking place in the primary step. The energy content of an excited molecule is so great that it decomposes to give a methylene radical and reacts with other CH4 molecules to give ethylene and acetylene directly. A comparison with conventional classical methods of acetylene formation from methane, taking into consideration that the ionizing radiation replaces thermal or electric energies, gives some interesting conclusions. PL-236/7 121

In the electric discharge the methane remains only 1/1000-1/100 s for the radical formation and afterwards the mixture is soon cooled for the recombination of radicals. Perhaps if methane passes rapidly through a strong radiation field that offers a certain dose-rate, and the final mixture can be circulated to condense the unsaturated hydrocarbons, it will be possible to obtain great quantities of ethylene and acetylene from methane. Another problem is to remove the hydrogen formed (contributing to the reverse reactions) by diffusion. These examples show that this new form of energy is not used well. The basic investigations with ionizing radiations have given us an understanding of the natural phenomena (the role of the solvated electron in solutions, charge, energy and proton transfer, the type of ions and excited molecules, etc.). The application of this knowledge to economic use needs much more work.

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[129] VERDIN, D., HYDE, S. M., NEIGHBOUR, F., I. phys. Chem. 69(1965) 1992. [130] IOANID, G., DRAGUT, A., DRIMUS, J., DUMITRESCO, V, STOIAN, I., in Large Radiation Sources in Industry (Proc. Conf. Warsaw, 1959) 2, IAEA, Vienna (1960) 51. [131] BAKH, N.A., MEDVEDOVSKY, V.I., SARAEVA, V.V., in Int. Conf. peaceful Uses atom. Energy (Proc. Conf. Geneva, 1958) 29, UN, Geneva (1958) 128. [132] SCHULTE, I.W., SUTTLE, J.F., WILHELM, R., J. Am. chem. Soc. 75 (1953) 2222. [133] CAGLIOTI, V., LENZI, M. , MELE, A., Nature, Lond. 201 (1964) 610. [134] GETOFF, N., Discuss. Faraday Soc. 36 (1963) 314. [135] GUTLBAUER, F., GETOFF, N., Radiochim. Acta 3 (1964) 1. [136] GUTLBAUER, F., GETOFF, N., Int. I. appl. Radiat. Isotopes 16 (1965) 199. [137] GETOFF, N., GUTLBAUER, F., SCHENK, G.O., Int. ]. appl. Radiat. Isotopes 1]_ (1966) 341. [138] WIEBE, A.K., CONNER, W. P., KINZER, G.W., Nucleonics (Feb. 1961) 50. [139] CONNER, W. P., DAVIS, W.E., Canadian Patent No. 576 979, 2 lune, 1959; German Patent No. 1045 , 403, 27 May, 1959; U.K. Patent 770. 594, 20 March, 1957. [140] BROSZKIEWICZ, R., MINC, S., ZAGORSKI, Z. P., Institute of Nuclear Research, Warsaw, Rep. 123/ChR (1959); BROSZKIEWICZ, R., MINC, S., ZAGC5RSKI, Z. P., Bull. Acad. pol. Sci. Sgr. Sci. chim. 8 (1960) 103. [141] CHERNOVA, A. L . OREKHOV, V. D., PROSKURNIN, M.A., in Tr. 2-90 (Vtorogo) Vses. Soveshch. po Radiats. Khim., 1960, Akad. Nauk SSSR, Otd. Khim. Nauk, Moscow (1962)233. [142] SUGIMOTO, K., ANDO, W., OAE, S., Bull. chem. Soc. lapan 36 (1963) 124. [143] BROSZKIEWICZ, R., Nature, Lond. 209 (1966) 1235. [144] CLARK, G.L., FITCH, K.R., Radiology r7 (1931) 285. [145] DAY, M.J., STEIN, G., Radiat. Res. 6 (1957) 666. [146] HA YON, E., SCHOLES, G., WEISS, I., I. chem. Soc. (1957) 301. [147] BALES TIC, F., MAGAT, M., in Large Radiation Sourcesin Industry (Proc. Conf. Warsaw, 1959) 2, IAEA, Vienna (1960) 149. [148] RAKINTZIS, N., PAPACONSTANT1NOU, E., Z. phys. Chem. 44(1965) 257. [149] LAMPE, F.W., J. Am. chem. Soc. 79 (1959) 1055. [150] MAURIN, ]., I. Chim. phys. 59 (1962) 15. [151] WILLIAMS, R., J. phys. Chem. 63 (1959) 776. [152] FARLEY, C. W. , LLOYD, W.A., Trans. Am. nucl. Soc. 5 (1962) 1. [153] MIGUEL, R., CHIROL, M., Bull. Soc. chim. Fr. 8^9(1962) 1677. [154] HUMMEL, R. W., Nature, Lond. 192 (1961) 1178. [155] WEXLER, S., IESSE, N., I. Am. chem. Soc. 84 (1962) 3425. [156] HEARNE, J. A., HUMMEL, R. W., Rep. AERE-R-4581 (1964). RADIATION-INDUCED POLYMERIZATION: MECHANISMS AND INDUSTRIAL ASPECTS

D.O. HUMMEL. CHRISTEL SCHNEIDER, R.C. POTTER, G. LEY, J. DENAXAS, D. WIDDERSHOVEN INSTITUT FÜR PHYSIKALISCHE CHEMIE UND KOLLOIDCHEMIE, ABTEILUNG FÜR STRAHLENCHEMIE, UNIVERSITY OF COLOGNE, COLOGNE, FEDERAL REPUBLIC OF GERMANY AND M. RYSKA INSTITUTE FOR MACROMOLECULAR CHEMISTRY, ACADEMY OF SCIENCES, PRAGUE, CSSR

Abstract

RADIATION-INDUCED POLYMERIZATION: MECHANISMS AND INDUSTRIAL ASPECTS. Radiation- induced polymerization is one of the main fields in radiation chemistry that has an important practical value. This paper is intended to give a current picture of radiation-induced polymerization in both fundamental and applied research and development. The subjects covered include suspension polymerization, emulsion polymerization, copolymerization, graft copolymerization, ionic polymerization of liquid monomers, and solid-state polymerization. A description is given also of a pilot-plant investigation into certain radiation chemical processes.

INTRODUCTION

There is no doubt that the study of radiation-induced polymerization reactions has enlarged our knowledge of the mechanism of this type of reaction to a considerable degree. Well-known systems have been reinvestigated, and completely new aspects have been revealed of the kinetics of emulsion polymerization, ionic polymerization and solid-state polymerization. Furthermore, polymerization reactions have been initiated under conditions where "classical" initiators have previously failed. Finally, quite a number of polymers have been synthesized, which cannot, ordinarily, be prepared by conventional processes. There has been, and there still is, much scepticism on the industrial applicability of radiation-initiated polymerization. Undoubtedly the idea of large radiation-sources - not to speak of nuclear reactors - as vital components of industrial syntheses is anathema to many chemists. The main reasons for this attitude may be the uncertainties in the calculation of the price of the final product, fear for the potential hazards when radiation is involved, or merely the considerable red-tape that has to be overcome before an irradiation unit can be installed. Nevertheless, it is un- realistic to believe that radiation chemical processes have no industrial future; and this is especially true for radiation-initiated polymerizations or other radiation-initiated chain reactions.

125 126 HUMMEL et al.

One of the main problems is lack of facilities on a pilot-plant scale. This situation has led a group of radiation chemists in this laboratory to plan such,a pilot plant to study continuous or discontinuous polymerization reactions under a large variety of factors such as dose-rate,| temperature and pressure. This, is not yet an ideal plan, however. We also know of a small number of similar facilities being planned or already in operation; for instance, in the Brookhaven National Laboratory, in Wantage (U.K. A. E. A.) and in Saclay (France). Essentially, then, the main concern of this review is to revitalize the discussion on industrial applications of radiation-initiated polymerization. The description of radiation-initiated polymerization reactions being investigated in laboratories throughout the world is not by any means exhaustive. In fact, such a discussion would fill a large volume, and so we decided to make a judicious and, it is hoped, a pragmatic selection.

I. SUSPENSION AND EMULSION POLYMERIZATION

Two types of reactions are well differentiated by their physics and kinetics: homogeneous and heterogeneous polymerization. The latter can be divided into two further groups: (1) Systems that are initially homogeneous and that become heterogeneous during the polymerization reaction. (2) Systems that consist initially of two or more different phases. The first group is represented by polymerizations in the gas phase, precipitation polymerizations and solid-state polymerizations. Suspension and emulsion systems belong to the second group. Homogeneous systems are comparatively simple in their kinetics. On the other hand, rapidly polymerizing monomers in homogeneous systems may cause severe problems of heat transfer and molecular weight control. In such cases, the use of proper solvents may help to overcome these problems. Strictly speaking, no polymerization can be considered as homogeneous in a kinetic sense since the polymer itself can be considered as a new phase, even though phase separation is not observable. This is especially true in the presence of poor solvents. By definition, in a suspension polymerization the monomer is dispersed in a non-solvent by the use of a dispersing agent that does not form micelles in the solvent phase; in these cases, the initiator is usually soluble in the monomer but relatively insoluble in the second phase (usually water). Emulsions are formed when the emulsifier molecules aggregate and the monomer is partly occluded in these micelles; here the initiator is soluble in the continuous phase (usually water) and comparatively insoluble in the monomer. A third phase may be formed by the monomer droplets, which are orders of magnitude larger than the monomer-swollen micelles. The preferred loci for the polymerization reaction, in this case, are the micelles, and not the monomer droplets. The critical factor determining the physics and kinetics of the system is, in both cases, the type of dispersing agent (protective colloid) or emulsifier that is employed. (Water-soluble monomers cannot be considered in this respect.) Thus, in the case of radiation initiation we can observe the analogous formation of kinetically "real" emulsion or dispersion systems, depending on the kind of protective colloid or emulsifier, although the PL-23 6/9 127 formation of radicals is not restricted to one of the phases as in the case of chemical initiators. Emulsions or suspensions may be of the types "oil-in-water" or "water-in-oil". Usually the monomer is the dispersed phase; however, during the last few years inverse systems have been investigated in which the monomer is the continuous phase [1].

1. Radiation-initiated suspension polymerization

The first and, up to now, the last study on suspension polymerization in the aqueous phase was published in 1959 by Chapiro and Maeda [2] . For each experiment, 5.5 ml of styrene was dispersed in 45 ml of water with 0.09 g of polyvinyl alcohol as a protective colloid. Each batch was de- gassed, agitated and polymerized under the influence of gamma radiation. 65 The reaction rate was proportional to l<>- , an(j ^he molecular weight was proportional to I"0-5; Up to 60% of the conversion was proportional to the reaction time. Comparing the conditions, the over-all reaction rate of the suspension polymerization was four times the rate of the bulk polymerization, probably because a large fraction of the initiating radicals is formed in the aqueous phase and, from there, enters the dispersed monomer droplets. In this way the radiation-initiated suspension polymerization differs remarkably from the conventional suspension polymerization, which is initiated by radicals of an oleophilic initiator dissolved in the monomer. This reaction can therefore be regarded as a water-cooled bulk polymerization. Chapiro and Maeda observed an over-all reaction rate of 0.007% min"1 (or 6.7 X 10"6 mole kg"1 min"1) at a dose-rate of 8.7 X 1015 eV g"1 min"1. The (viscosimetric) molecular weight of the polymer was 2 X 104 under the above conditions, and the polymer was only partially soluble in benzene. Very likely the benzene-insoluble fraction (20%) was grafted by polyvinyl alcohol, which, in aqueous solution, is readily degraded by gamma radiation. Nakashio et al. [3] were the first the investigate a radiation-initiated suspension polymerization in a non-aqueous system. They dispersed liquid formaldehyde at -80°C in n-heptane or methylcyclohexane (28% HCHO in the hydrocarbon), in the presence of sorbitane sesquioleate as a dispersing agent. The solubility of formaldehyde in n-heptane at -80° C is less than 2%. The polymerization was initiated by gamma radiation, and the suspension in the evacuated vessel was agitated during the subsequent reaction. The over-all reaction rate is, initially, almost independent of the concentration of the dispersing agent, and also of the size and number of monomer di'oplets. On the other hand, the reaction rate increased almost proportionally with the dose-rate (I0-9) and also showed a dependency on the monomer concentration in the system. The average molecular weight of the polymer was about 105 and was nearly independent of the dose-rate. The initial reaction rate, with a monomer concentration of 28% and a dose-rate of 5.5 X 1016 eV g"1 min"1, was 0.35% min"1 (or 3.27 X 10"2 mole kg"1 ( suspension) min The resulting polyoxymethylene is pearl-shaped and has an average particle diameter of 50-100 /Lim. The same authors also investigated the radiation-initiated polymerization of formaldehyde in toluene or ethyl bromide solution [4]. From their results the authors conclude that the polymerization follows an ionic 128 HUMMEL et al. mechanism, with the molecular weight of the polymer primarily determined by chain transfer to the monomer. Finally, Chapiro and Jendrychowska-Bonamour [5] investigated a suspension polymerization at low temperatures in a solid medium. Their results are discussed below. Although not much is known concerning radiation-initiated suspension polymerization, this technique is undoubtedly of considerable industrial interest. Because of the high fraction of initiation from the aqueous phase, the rates should be higher than in homogeneous polymerizations. If dispersing agents with a good radiation resistance are used, the resulting polymers .should contain fewer impurities than conventional suspension polymers.

2. Radiation-initiated emulsion polymerization

Most of the work concerned with this type of polymerization reaction was carried out by the following seven groups of radiation chemists: (1) D.S. Ballantine etal. (Brookhaven National Laboratory). (2) S.S. Medvedev etal. (Karpov Institute, Moscow). (3) P.E.M. Allen etal. (University of Birmingham, United Kingdom). (4) J.W. Vanderhoff at al. (Dow Chemical Company). (5) G.J.K. Acres and F. L. Dalton (U.K.A.E.A., Wantage Laboratory). (6). V.T. Stannett et al. (Research Triangle Institute, Durham, N. C. ). (7) D.O. Hummel etal. (University of Cologne, Federal Republic of Germany). Since radiation-initiated emulsion polymerization (EP) is industrially promising we shall consider this subject in some detail.

2.1. D.S. Ballantine et al. [6] were the first to study gamma-radiation- initiated EP. The monomer (styrene) was emulsified in 1 and 3% aqueous solutions of Duponol G (probably an amine salt of a fatty alcohol sulphate), and was irradiated at a dose-rate of 6 X 1016 eV g"1 min"1. The authors observed over-all reaction rates 10 to 100 times higher and molecular weights approximately 10 times higher than those obtainable by bulk polymerization under comparable conditions. With a monomer concentration of 1 mole kg"1 (emulsion), at 25°C an over-all reaction rate of 5.7 X 10~3 mole kg"1 (emulsion) min"1 and molecular weights of approximately 1 X 10® were observed. The reaction rate appeared to be dependent on the temperature of the system, and an over-all activation energy for the reaction of 3.7 kcal mole"1 was calculated.

2.2. L. P. Mezhirova et al. [7] have studied the EP of styrene and methyl methacrylate in the presence of sodium laurate, "MK" (Ci5H31SOsNa), Nekal (alkyl naphthalene sulphonate) and cetylpyridinium bromide as emulsifiers (3% aqueous solutions). The monomer-water ratio was about 1 : 3, the temperatures varied between 25 and 87°C, and the dose-rate was kept constant at about 2.4 X 1017 eV g^min"1. The over-all reaction rate was 100-300 times higher than for a bulk polymerization under comparable conditions; it was approximately independent of temperature, but depended on the type of emulsifier. The highest rates were obtained with sodium laurate, the lowest ones with the cationic emulsifier, cetylpyridinium bromide. The absolute over-all reaction rate for an emulsion PL-23 6/9 129

with 3% "MK" was 7.7 X 10"4 mole kg (emulsion)"1 min"1. The average molecular weight of the polymer was about 1.6 X 106, which is roughly 10 times higher than the molecular weight obtained by a bulk polymerization under comparable conditions. The authors observed a considerable "after- effect": after the interruption of the radiation, the polymerization continued for about 60 - 65 min and the conversion during this period was approximately 15 - 20% of the total monomer. This after-effect seemed to increase with decreasing temperature. The authors assume that the after- effect can be explained by long-lived radicals in emulsion systems, or by the presence of H^D2 formed during the radiolysis of water. The rate constant for the termination reaction was estimated to be 10-103 litres/ mole"1 s"1 . The over-all reaction rate during the EP of methyl methacrylate was much higher than for the EP of styrene: at 25°C, and under the conditions given above, a conversion of 63% was obtained in 12 min. The maximum over-all reaction rate under the described conditions was 13.8% min"1, i.e. 3.4 X 10"1 mole kg (emulsion)"1 min"1. The molecular weight of the polymer was 1.3 X 106.

2.3. P.E.M. Allen et al. [8] have investigated the radiation-induced grafting of styrene and methyl methacrylate on polyvinyl acetate (films, latexes, and finely divided solid polymer). In this connection the authors also studied the radiation-induced EP of vinyl acetate and methyl methacrylate. An emulsion of 1.25 ml monomer and 10 ml of 3.75% aqueous sodium dioctyl sulphosuccinate was irradiated with a dose-rate of 6 X 1016 eV g"1 min"1. The results of these authors are shown in Table I.

TABLE I. EMULSION POLYMERIZATION OF VINYL ACETATE (VAc) AND METHYL METHACRYLATE (MMA) [8]

Temperature Particles Reaction rate kp Monomer rc> (ml"1 latex) (mole l'V) (1 mole"1 s"1)

VAc 15 4.85 X 1015 5.77 230

VAc 14 8.44 X 1015 9.8 230

VAc 28 9.32 X 1015 10.1 220

MMA 5 10.4 X1015 4.2 92

2.4. J.W. Vanderhoff et al. [9] have compared the EP of styrene initiated by potassium persulphate with that by gamma radiation. They used emulsions with two monomer concentrations (20% and 40%) and four emulsifier concentrations (1.5, 1.75, 2.0 and 2.25% of sodium dihexyl sulphosuccinate). The dose-rate (gammaradiation) was 4.42 X 1016eV g"1 min"1 and the temperatures were 30, 50 and 70°C. It was found that the number of particles N increased with the square of the emulsifier concentration, whereas according to the theory of Smith and Ewart, N should increase with the 0.6th power of (E). This deviation from the theory and from the results of Gerrens et al. is, according to the

10 130 HUMMEL et al. authors, a consequence of the different emulsifiers. The values for the propagation and termination constants (kp and kt) were calculated to be 41 litres/mole"1 s'Mkp, 30°C), and 104-105 litres/mole'1 s'1^, 50°C). Finally, the authors investigated the "competitive growth" of latexes of known particle size, and determined the corresponding results for radiation-induced and catalytically induced EP, The only significant difference between the two types of initiation became obvious in the temperature dependence of particle formation. For persulphate-initiated systems, the particle number N increases with increasing temperature; radiation-initiated systems, on the other hand, show the opposite effect. This is explained as follows. The rate of radical formation (p) for radiation-initiated processes is, within certain limits, independent of the temperature, whereas kp increases with the temperature. The rate of decomposition of persulphate increases with the temperature and, therefore, has a positive temperature coefficient. Since a monomer-polymer particle would grow faster at higher temperatures, more emulsifier is used for its stabilization. Consequently, more micelles have to be broken up for this process, and are thus unable to form new monomer-polymer particles.

2.5. G.J.K. Acres and F. L. Dalton [10] studied the EP of styrene and methyl methacrylate, using a dilatometric method. Sodium dioctyl sulphosuccinate was used as an emulsifier; the dose-rate was 3.8 X 1016 eV g"1 min"1, and the temperature 21.5°C. The authors found inhibition periods of from 1 to 8 min, depending on the dose-rate and the monomer concentration. They claim that these inhibition periods are characteristic for emulsion polymerizations and cannot be overcome "by the most exhaustive purification techniques which have been employed". This conclusion does not agree with the results of Ley et al., who did not find any inhibition periods when the system was carefully freed from oxygen and other radical scavengers. Acres and Dalton found a dose-rate dependency for the over- all reaction rate, which was considerably lower than those previously observed. Ac- cording to these authors, the dose-rate exponent "a" has values between 0.2 and 0.35, depending mainly upon the monomer concentration. Only with very small monomer concentrations is the value of "a" said to approach the theoretical value of 0.4 (according to Smith and Ewart). This is explained by the role of H-atoms formed during the radiolysis of water. These H-atoms, according to Acres and Dalton, have two possibilities of reaction: (1) To initiate a polymerization by entering a micelle or a monomer- polymer particle, or (2) To react with dissolved styrene molecules in the aqueous phase "to produce a somewhat unreactive radical whose nature remains unspecified". The unusually low values of the exponent "a" are thus explained by the inactivation of primary radicals. It is doubtful, however, whether such a strict differentiation between H and OH radicals for the initiation reaction is reasonable. For a monomer concentration of 1.525 moles of styrene per litre of emulsion and 2.78% sodium dioctyl sulphosuccinate in the aqueous phase, an over-all reaction rate of 1.5 X 10~3 mole kg"1 (emulsion) min"i was PL-23 6/9 131 found. For a monomer concentration of 2.77 moles of MMA per litre emulsion and 1.39% emulsifier in the aqueous phase, an over-all reaction rate of 7.2 X 10"2 mole kg"1 (emulsion) min"1 was found.

2.6. V.T. Stannett [11] investigated the EP of styrene and vinyl acetate with lauryl sulphate as an emulsifier and gamma-radiation as an initiator. Each batch contained 75 g of water, 25 g of monomer, and 0.5 g of emulsifier. The dose-rate was 4.5 X 1016 eV g"1 min"1 at temperatures of 0 and 60°C, respectively. For styrene the authors observed a considerable difference in the pH values of the final latexes, depending upon the type of initiation (by radiation or by potassium persulphate):

Radiation Persulphate Styrene 7.9.-9.2 3.8 Vinyl acetate 3.1.-3.8 3.1.-3.8

The latexes obtained by radiation-initiation showed a smaller distribution of particle sizes than the latexes obtained by persulphate initiation. The viscosimetric molecular weight of the polystyrene obtained by radiation-initiation was measured and found to be 4.13 X 105 (0°C) and 1.66 X 106 (60°C). Under comparable conditions, persulphate-catalyzed polystyrene exhibited a molecular weight of 2.7 X 106. The molecular weight distribution was about the same for all polymer samples. The viscosimetric molecular weights of the polyvinyl acetate samples were as follows:

initiation Temperature M (°C) v Persulphate 60 4,9 XlO5 .Radiation 60 1.06 XlO6 Radiation 0 1.9 XlO6

The over-all reaction rates (vbr) were measured by a dilatometric method. For styrene with radiation initiation, the following values were determined:

vbr Temperature % min"1 mole kg"1 min"1 (°C) 60 1.85 1. 5 X 10"2 40 1.01 8.6 XlO"3 0. 7 0.231 3.2 X 10"3

From the temperature dependence vbr, an over-all activation energy of 3.2 kcal mole"1 was calculated. The EP of isoprene and MMA was studied only with respect to possible changes in the microstructure of the polymers. Unfortunately, isoprene polymerizes only very slowly in emulsion; reaction rates were not given TABLE II. MAXIMUM REACTION RATES OF VARIOUS MONOMERS, TOGETHER WITH EXPERIMENTAL PARAMETERS

Monomer Concentration of Absolute Relative concentration sodium dodecyl reaction reaction Temp. Dose -rate 1 Monomer -1 (moles kg" sulphate rate rate CO (eV g" min ) (emulsion)) (moles kg 1 (g min"1 kg"1 (styrene = 1) (water)) (emulsion))

Methacrylonitrile 25 17 .5 X 1014 2.229 18.75 0 0127 0 0404

45 17 .5 X 1014 2.229 18.75 0 .1384 0 234

14 Decyl methacrylate 25 17 5 X 10 0.662 17.5 0 281 0 310

Styrene 25 17 5 X 1014 1.288 50.76 0 963 1 000

Acrylonitrile 25 17 5 X 1014 2.890 17.8 0 287 1 335

Vinylidene chloride 5 17 5 x io14 2.441 18.3 0 3565 1 833

14 5 17 5 X 10 2.436 54.5 0 8476 1 829

14 Methyl methacrylate 25 8 75 X 10 1.449 53.4 1 8i92 2 316

14 Butyl methacrylate 25 8 75 X 10 1.095 53.16 2 655 2 475

14 25 8 75 X 10 1.0396 17.8 1 652 4 464

14 Chloroprene 5 17 5 x io 2.13 54.8 1 267 2 67

4 Butyl acrylate 25 4 4 X 10' 1.578 18.75 5 749 11 20

14 Ethyl acrylate 25 4 4 X 10 2.032 18.72 6. 769 16. 90

14 Methyl acrylate 25 4 4 X 10 2.228 18.75 7. 944 23. 06 PL-236/7 133

by the authors. The tacticity of the PMMA samples was studied by a NMR method:

Polymerization Syndiotactic Heterotactic temperature °C) (%) (%) 60 59.4 40.6 0 67.2 31.8 The increase in syndiotactic structures with decreasing polymerization temperatures fits in well with the known behaviour of polymerizing MMA.

2. 7. D.O. Hummel et al. [12] investigated the kinetics of the EP of a series of vinyl monomers. Sodium lauryl sulphate, cetyl pyridinium bromide and cetyl trimethyl ammonium chloride were used as emulsifiers. Table II shows the maximum reaction rates with different monomers, together with experimental parameters. For an easier comparison, the reaction rates for different monomers were also related to the maximum reaction rate of a styrene emulsion system under similar conditions. All rate measurements were performed by an automatic recording dilatometric apparatus of extreme sensitivity (1-mm pen deflection corresponding to 0.1-mg polystyrene g"1 (emulsion)). Inhibition periods were eliminated by a careful purification of all substances, and by a virtually complete exclusion of oxygen. The latexes obtained by radiation-initiated polymerization were rather stable. The particle concentration in the latexes was found in the usual range: 1014- 1016 particles per ml latex. Because of the relatively low dose-rates (between 500 and 2000 rad/h), the molecular weights of the polymers were rather high, being in the range of 106 (provided that no regulators were added). The rate-time functions obtained for the different monomers had the "classical" form (Gerrens) only in a very few cases. Generally, the kinetic behaviour of the monomers is much more complicated than that predicted by the theory of Smith and Ewart. Measurements during the non-stationary state of the systems led to the conclusion that radicals are able to escape the monomer-polymer particles, even in the classical styrene system. Half-lives for radicals, and rate constants for the termination reaction between growing chains ("interparticle" termination), have been determined for a series of systems. The termination constants in emulsion systems are lower by two to four orders of magnitude than the corresponding termination constants in homogeneous systems. This explains the high reaction rates in emulsion systems and, also, the high molecular weights of the polymers. A complete kinetic description and a plausible quantitative interpretation of the behaviour of these complicated systems is not possible according to the existing theories.

3. Conclusions

This short review of our present knowledge on the behaviour of emulsion and suspension systems, under the influence of ionizing radiation, has 134 HUMMEL et al. perhaps conveyed some of the theoretical and industrial potential in this field of research. Up to the present time a considerable fraction, if not the majority, of industrial polymers are produced by suspension and emulsion processes. Perhaps the day is not far off when radiation-initiated emulsion and suspension processes will supplant conventional processes.

II. COPOLYMERIZATION

Much work has been done on radiation-initiated copolymerization, both in homogeneous and heterogeneous systems. Table III gives a survey of papers published during the last few years only (beginning in 1963). Below are discussed in some detail only systems having industrial interest.

1. Ethylene copolymers [13, 14]

Numerous monomers will copolymerize with ethylene under the influence of gamma radiation. Some monomers that are practically in- capable of homopolymerization under the influence of ionizing radiation, such as propene and 1-butene, easily copolymerize with ethylene. TableIV shows results of Steinberg and Colombo [13] obtained at 20°C and with a total pressure of 680 atm.

2. Tetrafluoroethylene (TFE) copolymers

2.1. With vinyl chloride (VC) [17]

The copolymerization was investigated at -78°C. The reaction was inhibited by DPPH. The reactivity parameters were determined as 7.75 (q, VC), and 0.03 (R2, TFE). The rate of copolymerization decreases with increasing concentration of TFE in the monomer mixture; the most active species in the reaction chain are the VC radicals.

2.2. With ethylene [18]

At -78°C.and a dose-rate of 2 X 105 rad/h, and with a molar concentration of 44.7% of ethylene in the monomer mixture, the conversion amounted to 9% within 42 h. The rate of copolymerization decreases sharply with increasing ethylene concentration in the monomer mixture. The melting point of the statistical copolymers decreased with increasing ethylene concentration from 320°C (~ 10 mole % ethylene) to 240°C 78 mole% ethylene).

2.3. With propene [21]

The reaction was studied at -78°C and with a dose-rate of4.0 X 105rad/h. With a molar concentration of 33.6% propene in the monomer mixture, and an irradiation time of 100 h, the conversion was 7%. The rate of copolymerization increases with increasing concentration of TFE in the monomer mixture. Radical scavengers inhibit the reaction. PL-236/7 135

2.4. With isobutene [19, 20]

This system shows a remarkable behaviour. At -78°C, and over a wide range of compositions of the monomer mixture, the copolymer being formed always has an almost alternating structure. The copolymer contains 45 mole% of TFE. The copolymerization rate is proportional to the square root of the dose-rate, and the conversion is directly proportional to the radiation dose. The reaction is inhibited by radical scavengers (pyrogallol, p-benzoquinone).

3. Styrene copolymers

3.1. With isobutene [31]

The reaction was studied at 0°C in the presence of ZnO. The reactivity parameters were 3.3 (isobutene) and 0.2 (styrene). The reaction mechanism is supposed to be partially ionic.

3.2. With methyl methacrylate [32]

The reaction was studied in the presence of 7-AI2O3 . The reaction is of the radical type at temperatures of 30 and 0°C, whereas at -78°C it is said to be ionic.

4. Acrylonitrile copolymers

4.1. With styrene [23, 25]

The reaction was studied at temperatures of 15, 0, -20 and -78°C. At a temperature of 15°C and at conversions up to 6-7%, the copolymerization rate is directly proportional to the dose. The reaction rate increases with increasing concentration of AN in the monomer mixture. The dependency of the copolymerization parameters rj (AN) and r2 (styrene) on the reaction temperature is shown in Table V. The copolymer obtained by irradiating a solid monomer mixture at -78°C has an alternating structure. Simulta- neously with the copolymerization, homopolymerization of styrene takes place in a second solid phase.

4.2. With methyl methacrylate [23, 26]

This system was investigated in a range of temperatures between 52.5 and -78°C, and with dose-rates of 2 X 104, 3 . 5 X 104, 6.1 X 104, and 11. 0 X 104 rad/h. At a temperature of 15°C and conversions of up to 6-8%, the reaction rate was directly proportional to the dose. At higher conversions, the reaction rate had a higher dependence on the dose-rate. This is explained by a Trommsdorff effect. The rate of copolymerization has a positive temperature coefficient. At 15°C, the copolymerization rate decreases with increasing concentration of AN in the monomer mixture. An opposite effect is observed at a temperature of -78°C. Here, the reaction rate increases approximately with the molar concentration of AN in the (solid) monomer mixture. Apparently, the reactivity of the monomers is completely different in the solid state than in the liquid state, probably 136 HUMMEL et al.

TABLE III. SURVEY OF PAPERS ON RADIATION-INITIATED COPOLYMERIZATION (1963-66)

Comonomer Remarks Ref.

Ethylene

Styrene [1] Methyl methacrylate [1] Vinyl acetate [1] Acrylonitrile [1] Isobutylene [1, 2] Chlorotrifluoroethylene [1] Trans-2-butene [1] Cis-2-butene [1] 1-Butene [1] Methyl acrylate [1] Isoprene [1] Propylene [1.2] Vinyl chloride [1. 2] 1-Octene [1, 2]

Propylene

Chlorotrifluoroethylene Alternating 1:1 copolymer [3] Vinyl chloride [4]

Tetrafluoroethylene

Vinyl chloride Melts higher than PVC [5] Ethylene Crystalline copolymer [6] Isobutylene Alternating copolymer [7, 8] Propylene [9]

Butadiene

Isobutylene [10]

Acrylonitrile

Styrene The copolymer obtained by solid-state copolymerization has an alternating structure Methyl methacrylate [11, 14] Ethylene oxide [15] Propylene oxide [15] Vinyl chloride [16] Formaldehyde [17]

Vinylidene chloride

Isobutylene [18] PL-23 6/9 137

Styrene

Isobutylene [19] Methyl methacrylate Anionic mechanism at -78 °C [20, 21] Unsaturated polyesters [22]

Vinyl acetate

Isopropenyl acetate [23]

Acrylic acid

AUylamine The copolymers are polyampholytes [24]

Methacrylic acid

Allylamine [24, 25]

Formaldehyde a-Methylstyrene [17] Methyl methacrylate [17] Acrylonitrile [17] Acetaldehyde [17] n-Butyraldehyde [17] Styrene The copolymer has a constant [26]

composition (90% CH20) Carbonyl halide [27]

Maleic anhydride

Ethylene Alternating copolymer [28] Methyls tyrene Alternating copolymer [29] Styrene Alternating copolymer [30]

Carbon monoxide

Ethylene [1, 31, 32] Propylene Polyketones with alternating [33] 1-Butylene structure [33] Isobutylene [33]: Ethyleneimine Alternating, crystalline copolymer [34] Propyleneimine [34]

Sulphur dioxide

Propylene Alternating 1 : 1-polysulphones [35] Butadiene Alternating 1 : 1-polysulphones [36,37] (1, 4-cis and -trans) Isoprene 1 : 1 [37] 2,3-Dimethylbutadiene 1 : 1 [37] Styrene 2 : 1 at 30°C, 1 : 1 at -78°C [38-40] Vinyl acetate 1 : 1. ceiling temp, of -20°C [41,42] Allyl acetate 1 : 1 [43] Allyl alcohol 1 : 1 [44] Vinyl chloride 2 : 1 at 25°C, 1.5 : 1 at -78°C [45] Cyclohexane 1 : 1 [46] Chloroprene Composition of the copolymer depends [37] on reaction temperature and composition of monomer mixture 138 HUMMEL et al.

TABLE IV. RESULTS FROM REF. [13] AT A TEMPERATURE OF 20°C AND A TOTAL PRESSURE OF 680 atm

Composition of batch (g) Dose Percentage Comonomer (Mrad) conversion Ethylene Comonomer

MMA 42.6 23.5 8.8 96.9

Vinyl acetate 42.8 25.2 18.9 89.1

Isobutylene 41.8 15.0 15.6 22.4

Propylene 43.1 11.8 14.7 28.5

Vinyl chloride 40.1 30.0 0.92 47.1

1-Butene 46.5 8.3 0.92 8.2

Styrene 40.5 23.6 19.5 24.3

Acrylonitrile 33.2 52.9 14.0 19.9

TABLE V. DEPENDENCY OF PARAMETERS r^AN) and r2 (STYRENE) ON THE REACTION TEMPERATURE

Temperature State VAN) r2(styrene) CC)

15 liquid 0.03 0.33

0 liquid 0.03 0.33

-20 liquid 0.28 0

-78 Solid 0 0

TABLE VI. C OPOL YMERIZ ATI ON PARAMETERS OF AN(rx) AND MMA (r2) AT DIFFERENT REACTION TEMPERATURES

Temperature VAN) r2 (MMA) CC)

-78 0.01 0.13

-20 0.10 1.30

0 0.10 1.30

15 0.08 1.10

52 0.15 1.15 PL-236/7 139 because of a change in the polymerization mechanism. Table VI shows the copolymerization parameters of AN (rj) and MMA (r2) at different reaction temperatures.

4.3. With vinyl chloride [28]

The reaction was investigated at temperatures of 30 and -78°C, and at a dose-rate of 1. 1 X 104rad/h. At both temperatures the copolymerization rate decreases with increasing percentage of vinyl chloride in the monomer mixture. Only at very high concentrations of VC does the rate again increase rapidly. Apparently AN acts in small concentrations as a retarder for the polymerization of VC.

4.4. With ethylene oxide and propylene oxide [28]

The reaction was studied at temperatures of 30, -78 and -196°C, and at dose-rates between 2 X 104and 1.8 X 106 rad/h. For the copolymerization of AN with ethylene oxide the rate decreases with an increasing percentage of ethylene oxide in the monomer mixture. This effect is strongest at 30°C and weakest at -196°C. At normal temperatures (0-30°C), the percentage of the copolymer of ethylene oxide is almost independent of the composition of the monomer mixture, and is about 5-8%. At low temperatures, the percentage of ethylene oxide units in the copolymer reaches a maximum of 60-70% (with a molar ratio EO/AN of 10 in the monomer mixture). The copolymerization of acrylonitrile with propylene oxide was studied at -78°C (liquid phase) with a dose-rate of 2 X 104 rad/h. The rate of copolymerization is increased by polar solvents such as chlorinated hydrocarbons or dimethylformamide. Furthermore, the copolymer yield is directly proportional to the radiation dose up to rather high conversions. This is evidence that the copolymerization proceeds by an ionic mechanism. In a manner somewhat different from the EO/AN copolymerization, the copolymerization rate in this case increases with decreasing temperature.

5. Vinyl acetate - isopropen.yl acetate copolymers [35]

The copolymerization of VAc with isopropenyl acetate was studied at 30, 0 and -78°C, with a dose-rate of 1.1 X 104 rad/h. The copolymerization rate decreases with an increasing percentage of VAc in the monomer mixture. T'he conversion of a 1 :1 (by volume) monomer mixture after a radiation dose of 3. 3 X 104 rad was found to be 0. 35%. The reaction rate at - 78°C was higher than that at 0°C.

6. Formaldehyde copolymers [38]

Under irradiation, formaldehyde copolymerizes with the following monomers (in order of decreasing copolymerization rates): a-methylstyrene, styrene, methyl methacrylate, acrylonitrile, and ethylene oxide. The copolymers with styrene have been equally thoroughly studied. The reaction was performed at -78°C with doses up to 107 rad. The 140 HUMMEL et al. reaction rate increases with an increasing percentage of formaldehyde in the monomer mixture. Until 70 mole% of CH20 in the monomer mixture is reached, the copolymer has a constant composition of 90 mole% oxymethylene and 10 mole% styrene. Above 70 mole% CH20 in the monomer mixture, the percentage of oxymethylene in the copolymer approaches 100. At 170°C the copolymers begin to decompose. A few styrene units in polyoxymethylene improve the heat stability of this material considerably.

7. Maleic anhydride - ethylene copolymers [40]

This was studied at temperatures of 30, 40 and 65°C, at a dose-rate of 3.0 X 105 rad/h. The composition of the copolymer is almost equimolecular, and practically independent of the composition of the monomer mixture. The rate-determining step is the addition of ethylene to a polymer chain with a terminal maleic anhydride radical. During the copolymerization, the reaction becomes heterogeneous; the over-all activation energy of the radiation-initiated copolymerization was found to be 1.8 kcal/mole; for the copolymerization initiated with azo-bis(iso- butyronitrile), a value of 27. 5 kcal/mole was found. This considerable difference is a consequence of the fact that the formation of radicals by radiation requires no activation energy.

8. Carbon monoxide copolymers

8.1. With ethylene [13, 43, 44]

Carbon monoxide is easily copolymerizable with ethylene and forms copolymers (ethylene polyketones) with rather interesting properties. Copolymers with a low carbonyl content are formed only with a very high excess of ethylene in the monomer mixture. Only 5% of CO in the monomer mixture leads to the formation of copolymers containing 45% CO. The reaction rate shows a broad maximum at concentrations between 27. 5% and 39.2% CO in the monomer mixture (20°C) [13, 43). The melting point of the copolymers increases, with increasing carbonyl content, from 111°C (27% CO) to 242°C (50% CO). An alternating copolymer begins to decompose at about 250°C. Carbon monoxide is not homopolymerizable. Peculiarly, copolymers have been obtained with considerably more than 50% CO. These copolymers consequently contain either 1, 2-dicarbonyl groupings or oxygen-containing groups other than the carbonyl group. This question has not yet been resolved.

8.2. With ethylene imine or propylene imine [46]

The copolymerization of cyclic amines with carbon monoxide is an interesting method for the synthesis of polyamides. The copolymers with ethylene imine or propylene imine have an alternating structure almost independent of the composition of the monomer mixture. The copolymerization of ethylene imine with carbon monoxide yields polyamide-3, which shows the same infra-red spectrum as the product obtained by the poly- addition of acrylamide. The polymer melts (with decomposition) at 322-335°C. PL-236/7 141

9. Sulphur dioxide copolymers (polysulphones)

9.1. With propene [47]

The reaction was studied at temperatures between 25 and -196°C. The monomer ratio in the polymer is almost unity, independent of the composition of the monomer mixture. The copolymer has an alternating structure, since SO2 is not homopolymerizable. The copolymerization rate increases with decreasing propene concentration in the monomer mixture.

9.2. With conjugated dienes [48]

Butadiene yields alternating polysulphones with a monomer ratio of unity, independent of the composition of the monomer mixture and of temperature (25 to -78°C). The same result was observed with isoprene as a comonomer. The reaction rate shows a maximum at -45°C and a monomer ratio of 1:2 (isoprene: SO2). 2, 3-dimethylbutadiene also yields alternating copolymers with a monomer ratio (in the polymer) of unity. The reaction rate is much lower than that for butadiene and isoprene. 2-chlorobutadiene (chloroprene) behaves differently in that it is able to

form polysulphones with more than one chloroprene unit per S02 -unit. High reaction temperatures favour the formation of poly(chloroprene sulphones) with a higher percentage of chloroprene units.

9.3. With styrene [52]

The reaction was investigated at 30 and -78°C, at a dose-rate of 5.2 X 10s rad/h. The polysulphones have an alternating structure. At

30°C, the molar ratio styrene/S02 is almost exactly two. Mass spectrometric investigation of the pyrolysis products revealed the alternating structure. At -78°C, polysulphones are formed with a molar ratio of almost unity (1.1:1)

9.4. With vinyl chloride [57]

The composition of the poly(vinyl chloride sulphones) is practically independent of the composition of the monomer mixture. It does depend, however, on the reaction temperature. Between room temperature and about -50°C, the copolymers have a molar ratio VC/S02 of two. Still lower temperatures seem to favour equimolecular copolymers. The poly- sulphone obtained at -78°C showed a molar ratio VC/SO2 of 1. 5. The reaction rate showed a maximum at -10°C, with 72% VC in the monomer mixture.

III. RADIATION-INDUCED GRAFT COPOLYMERIZATION

Just as alloys comprise a specific area of compositional studies in metallurgy, so, too, graft copolymers occupy the analogous area in macromolecular chemistry. Although in the last few years several new techniques for catalytic grafting have been developed, the importance of radiation grafting remains undiminished, which is a direct consequence of the 142 HUMMEL et al. decidedly simpler techniques afforded by radiation. These radiation techniques may be conveniently divided into the following four groups: (i) Direct irradiation of a mixture of monomer with polymer, (ii) Grafting of monomer to a radiation-peroxidized polymer. (iii) Grafting of monomer to a polymer containing trapped radicals. (iv) Cross-linking of a mixture of two or more polymers.

(i) Upon irradiating the monomer-polymer mixture, reaction centres (radicals) are formed in both the polymer structure and the monomer. Thus by this method a graft copolymer can be formed simultaneously with a homopolymer, the latter constituting an unintentional by-product. How- ever, the yield of graft copolymer will exceed that of homopolymer by an amount dependent upon how low the monomer concentration is, and how much greater Gr(polymer) is than Gr(monomer). On this basis one can theoretically predict from a knowledge of Gr values for monomer and polymer whether or not the method might be suitable for a specific system. The values of Gr for the most commonly used monomers and polymers may be found in Tables VII and VIII [59, 60]. The direct method is the most technically feasible and usually gives the highest yields. The homopolymerization difficulty is by-passed, in most cases, by irradiating monomer-swollen polymer. Sometimes, however, it is necessary to irradiate the polymers in the presence of gaseous monomers. Often the polymerization process becomes diffusion controlled (similar to the situation that occurs for homopolymerization in a very viscous medium), and the gel effect may appear. Other complications may arise with the onset of the coalescence effect, and this may or may not be favourable depending on the system.

(ii) This method, whereby grafting occurs via radiation-peroxicjized polymer, is in reality a two-stage process. First, the polymer is irradiated in an oxygen-containing atmosphere so that peroxy compounds are formed. Then in the presence of monomer these compounds may be thermally degraded into radical initiators. This method is somewhat impractical compared with method (i), but it results in graft copolymers qf a higher purity. The long life-time of the peroxides is especially useful from a technical standpoint. The two stages, peroxidation and grafting, may be carried out in different places. For those monomers and solvents with relatively high transfer constants, this method finds less utility since an excess of homopolymer is formed.

(iii) Grafting with trapped radicals is of technical interest in those cases where very pure graft copolymers are desired. Although the yields are usually low, they may be enhanced by employing a lower temperature during the irradiation of the polymer.

(iv) For technical reasons, the cross-linking of a mixture of two polymers can only rarely be utilized. The yields are low, radiation sources of high intensity are required, and the preparation of the homogeneous mixture is difficult. Various types of radiation sources have been used for these methods of graft copolymerization, including electron accelerators [61-63], mixed PL-23 6/9 143

TABLE VII. RADICAL YIELDS OF SELECTED MONOMERS

Monomer Gr Method of determination

Butadiene Very small

Styrene 0.69 Kinetics and DPPH

Ethylene 4 Kinetics

Acrylonitrile 5.6 Kinetics

5 DPPH

Methyl methacrylate 11.5 Kinetics

5.5-6 DPPH

Methyl acrylate 6.3 DPPH

Vinyl acetate 12 Kinetics

9.6 DPPH

Vinyl chloride Presumably close to 10

TABLE VIII. EXPECTED RADICAL YIELDS OF SELECTED POLYMERS

Polymer Expected Gr

Polybutadiene 2-4

Polyisoprene 2-4

Polystyrene 1.5 - 3.0

polyethylene 6-8

Polyisobutylene 6-8

Poly(methyl methacrylate) Polyvinyl acetate) 6 or 12 Poly(methyl acrylate)

Silicone 3.6 or 7.2

Cellulose 10 Polyvinyl alcohol)

Polyvinyl chloride) 10-15 Poly(vinylidene chloride)

Polyamides Probably high Fluoro polymers 144 HUMMEL et al. radiation in nuclear reactors, gamma radiation from 60Co [59] and secondary radiation from the nuclear reaction 10B(n, a )7Li [64], For technical irradiation, large beta sources using 90Sr, 147Pm and 144Co have been suggested [65]. In the literature there are hundreds of scientific publications and patents concerned with the radiation grafting of different materials and synthetic substances with every conceivable monomer. Nearly all these are of technical interest because grafting is not only a source of completely new products, but also provides a method for altering the characteristics of polymers in desired directions. Because of the very large number of results, a drastic selection must be made. The scope of this summary has therefore been narrowed to give a brief picture only of those systems that are especially interesting either because of the methods employed or because of unusual characteristics in the resultant copolymers.

1. Polyethylene

Either high- or low-pressure polyethylene is often used in conjunction with methods (i) and (ii). Styrene has proved to be a very good monomer because of its good swelling properties and low G-values. For low radiation dose-rates, the yield is proportional to the dose-rate, whereas in the region of 5-72 rad/s, the grafting rate is independent of the dose-rate [66]. The yields can be increased by the addition of 2-propanol [66] and methanol [68], and, in fact, a maximum yield was observed for a 50-60% concentration of methanol. It has also been found that the grafting is not homogeneous. The amorphous regions of the polyethylene are grafted more easily than the crystalline regions, so that the graft polymer is concentrated in the amorphous areas [66] . Generally, the product resulting from the system is harder and more resistant to the effects of solvents. The system may also be used to produce cation-exchange membrane by the simple expedient of sulphonating a grafted film [69] . In acrylonitrile, polyethylene does not swell quite as well. The yields are lower in this case, but the resulting graft copolymer has interesting optical properties [70] . The grafting of methyl methacrylate is hindered by a strong gel effect, although a high degree of grafting can be achieved with pulsed radiation [71]. Thus, films of high-pressure polyethylene having a thickness of 0.031. . . 0.05 mm were irradiated with a total dose of 3 X 104rad. This dose was delivered in 30 single pulses, each having a duration of 2 min. Between pulses, a period of 28 min was allowed for the diffusion of monomer. It was found that the optimum temperature for the reaction was 25°C, at which point the rate of grafting is 2000 times greater than the rate of homopolymerization. In addition, several other monomers have been grafted to polyethylene by means of radiation. These include acrylic acid [72], vinyl carbanole [69], vinyl pyridine [73], vinyl stearate [73], vinyl acetate [63], vinyl pyrrolidone [74] and vinylidene chloride [63], Fluorine-containing and polyfunctional monomers have been used for the cross-linking of polyethylene [75, 76]. PL-23 6/9 145

2. Polypropylene

This has been grafted by the usual methods [72]. The peroxide method proves to be faster in this case than for polyethylene [77, 78].

3. Polyisobutylene

Treated with styrene according to method (i), polyisobutylene yields a mixture of graft copolymer and homopolymer. The removal of graft copolymer is accomplished by extraction with cyclohexane [79].

4. Polystyrene

Because of its low Gr value, polystyrene gives only low yields when grafted by methods (i) or (ii). Nevertheless, the radiation yield of peroxides may be increased by carrying out the irradiation in radiation-sensitive solvents such as chloroform [80].

5. Poly(methyl methacrylate)

This can easily be grafted with acrylonitrile [81] and methyl aery late [82] . A transparent product is obtained when acrylonitrile is grafted from a 25% methanol solution. The product resulting from a graft with vinyl pyrrolidone can be swollen in water [83]. Graft copolymers of poly (methyl methacrylate) with vinyl acetate have been prepared in emulsion systems, and also polyvinyl acetate with butyl methacrylate, po"ly(butylmethacrylate) and polyvinyl acetate with styrene, and polyvinyl acetate with acrylonitrile and methyl methacrylate [84],

6. Polyvinyl alcohol

This is often radiation grafted with styrene, methyl methacrylate and other monomers [85, 86] to improve its hydrophobic properties and to provide better processing at higher temperatures. Grafting with aery lamide and 2-methyl-5-vinylpyridine or 4-vinylpyridine improves the dyeability of polyvinyl alcohol fibres [87], Yields may be increased by the addition of water [86]. No graft copolymers are formed using solutions of acrylonitrile in benzene or dimethyl formamide because of their hydrophobic properties. For grafting, it has been shown that it is better to use a 6% solution of acrylonitrile in water [85].

7. Nylon

Nylon fibres have been grafted with methyl methacrylate, acrylic acid, acrylonitrile, vinyl acetate, methyl acrylate and ethyl acrylate [88] . Again, the presence of water increases the yields. By means of grafting, nylon's dynamic characteristics at different temperatures have been altered. To improve the dyeability of nylon-6 fibres, the direct method of grafting may be used with acrylamide. In addition, the electrical properties of polyamides have been shown to undergo modification by grafting [89]. Several other fibres have been grafted using the direct method (e.g. natural silk and polycaprolactam [90]). The dyeability of polyterophthalate

10 146 HUMMEL et al. fibres has been improved by radiation grafting with 4-vinylpyridine. Fibres prepared in this way were dyeable with acid dyes. By treating the fibres afterwards with a 5% solution of dimethyl sulphate in methanol, the surface resistance is lowered so that the build-up of static charge is hindered [80].

8. Poly(vinyl pyrrolidone)

Grafted with acrylonitrile, poly(vinyl pyrrolidone) finds use as an ion- exchange membrane [91] .

9. Polyvinyl chloride

This can easily be grafted with many different monomers because it is swellable in virtually all monomers and has ahigh Gr value. From a technical point of view, the grafts with polyfunctional monomers are of particular interest (e. g. with diallyl maleinate, diallyl sebacinate, diallyl phthalate, ethylene glycol dimethacrylate and triallyl cyanurate [92]). These monomers are used as plasticizers for PVC; the sheets are coated and then irradiated with a dose of from 106. . . 108 rad. Below the softening point, the resultant product has a rubber-like character. By grafting PVC films with acrylonitrile, a transparent film has been obtained [93] . Trans- parent, hard films can also be prepared by grafting with styrene and methyl methacrylate [94, 95],

10. Teflon

Teflon grafts differ from those of other polymers because of the low swelling of Teflon in solvents and monomers [73, 81, 96], Therefore, under normal conditions, the grafting occurs only near the surface. How- ever, at low dose-rates and higher temperatures it is possible to induce grafting in the deeper layers of Teflon film [97] . There has been also some success with grafting in Teflon emulsions [93] . The maximum yield for an emulsion system with styrene has been found to be 8% (80°C); similarly, there is an 8% maximum yield for butyl methacrylate (55°C). Polyfluoroethylene films that have been 3. . . 5% surface grafted with styrene have good adhesive properties [97]. Teflon is often grafted to improve its dyeability [98] . Grafts with acrylic acid yield ion-exchange membranes [99].

11. Cellulose

Cellulose grafting has been thoroughly studied over the last few years, particularly by the Japanese [100]. The radiation grafting of cellulose acetate with vinyl acetate lowers the softening point; here vinyl acetate acts as an internal plasticizer [101] . By the addition of small amounts of bifunctional monomers, one obtains an easily formable product. Indeed, this method has been used in industry since 1957 [102] . Cellulose fibres have often been grafted to increase their hydrophobic characteristics. The monomers normally used in this application are styrene and, in particular, acrylonitrile [102-105]. In this case, the radiation yields were found to be critically affected by monomer and solvent concentration, the ratio of monomer and cellulose concentration, the initial form of the cellulose and the presence of traces of oxygen [104] . Vinyl and allyl PL-23 6/9 147 compounds have also been grafted to other textile fibres not only to improve their hydrophobic properties but also to increase their resistance to biological damage [106, 107],

12. Natural rubber

This has been grafted with p-chlorostyrene and 2,5 dichlorostyrene. By this means the rubber becomes more susceptible to radiation vulcanization [108]. Methyl methacrylate is graftable only to extracted rubber. It is also possible to graft the latex itself [109-111] .

13. Other uses

Radiation grafting has also been used for dyeing natural and synthetic substances. Dyes with polymerizable groups have been used, including diazotated 2-amino-4-sulpho-4-acryloyl aminodiphenylsulphone or 3-sulpho-7-amino- 1-naphthol [112]. By means of grafting with special monomers, products with particularly good adhesive properties have been obtained. Thus, polypropylene grafted with vinyl-di-(2-chloroethyl)phosphate has good adhesion with several metals [113]. The adhesion of ink to polyethylene was improved by grafting a mixture of polyethylene and sulphur dioxide [61]. Radiation grafting is additionally used for the improvement of the mechanical properties of semi-conducting polymers. A semi-conducting material with good electrical and mechanical properties was prepared by a graft of acetylene monomers and cyanoethylene with glass fibres and subsequent thermal treatment [67].

IV. POLYMERIZATION UNDER UNUSUAL CONDITIONS

1. Ionic polymerization of liquid monomers

It has been remarked that "research in radiation chemistry is more productive of publications than any other field, because every paper as- serting a mechanism has to be followed by another in which the assertion is retracted" [114] . One need examine only the tortuous history of radiation- initiated polymerization to appreciate the aptness of this quip. For many years the universally accepted view prevailed that radiation- initiated polymerization in liquid monomers was uniquely a free radical process. This conclusion had been based on the kinetic behaviour exhibited by these systems by using criteria developed from years of experimental data amassed in studies of chemically and photochemically initiated reactions. In 1957, however, the first fissures appeared in the free radical shibboleth with the discovery of the low-temperature ionic polymerization of liquid isobutylene [115]. By 1960, a host of monomers including styrene [116], butadiene [117], formaldehyde [118] and acrylonitrile [119] had been identified as being susceptible to ionic polymerization. Without exception, however, these polymerizations occurred under what must be termed "extreme" conditions; that is, they were carried out at very low temperature (to favour ionic over free radical propagation), and sometimes with an added second phase (to act as an electron trap) or an 148 HUMMEL et al. homogeneously distributed second component of reasonably high dielectric constant (to promote ion stabilization). Since 1960, the schools of Metz, Williams and Okamura have steadily hammered at the last bastion of the radical hypothesis by demonstrating that ionic propagation species are responsible for the radiation polymerization of several monomers under "normal" conditions (i.e. bulk and solution polymerization at room temperature). The elucidation of this completely unsuspected and absolutely fundamental fact was brought about by simply employing highly purified monomers. In particular, water content has been shown to be the critical factor [120-124]. Thus, under sufficiently anhydrous conditions the normal free radical propagation is vastly overshadowed by a superimposed ionic contribution for such monomers as styrene [120- 122, 125-130], o-methylstyrene [123 , 124, 129, 131, 132], j3-pinene [123, 124, 131 ], isobutyl vinyl ether [131], cyclohexene oxide [134], and others. The evidence for ionic mechanisms has been manifested by very high rates of polymerization [120-138], linear dependence of rate of polymerization on dose rate of radiation [120, 121, 127-129], non-dependence of molecular weight on dose-rate [120-122, 125, 126, 128], molecular weights that vary inversely with reaction temperature [120-122], temperature coefficients of low magnitude [120-122, 128], sensitivity to ionic scavengers [120 - 122] and direct observation of ionic species by pulse radiolysis [136], The explanation currently in vogue for the sudden appearance of this ionic phenomenon is that a substantial radiolytic yield of reasonably long- lived free ions is always generated in the reaction system, regardless of purity. However, unless the system has been stringently dried, the growing chains are terminated in the oligomer stage by a very efficient reaction with molecules or, perhaps, ions of water. As to the nature of these polymeric ions in anhydrous systems, it is generally believed that they are cationic, although there is no agreement as to whether they are carbonium ions or carbonium radical ions. Indeed, in certain systems such as o-methylstyrene and styrene there is evidence for the simultaneous propagation of carbonium ions and carbanions (in addition to a free radical contribution) [120, 128, 129, 134], And now the relevance of the opening remark can be fully understood: for the moment, the state of the art has come full circle and we are living with Lind's theories of forty years ago. Generally, the various purification schemes for preparing these dry monomers have four steps in common, all of which are performed in a vacuum environment: distillation of monomer, bake-out of preparative apparatus, degassing of monomer, and contacting of monomer with a drying agent such as activated silica gel [120, 121, 123, 125-129, 131, 135], Na-K alloy [122, 130] or barium oxide [137]. As might be expected for an ionic polymerization, these elaborate schemes do not necessarily yield reproduceable results. However, for the styrene and o-methylstyrene systems, reproducibility has been attained [120, 121, 127-129], For this case, it is interesting to note that styrene at 0°C, when exposed to ^Co- gamma rays at a dose-rate of 1 Mrad/h, polymerizes at a rate of 94%/h, compared with the classical free radical value of 0.2%/h for these conditions. Armed with the above facts and a reasonable imagination, one can see many possibilities for experimental and theoretical work in this area of pure, dry monomers. For example, all the previous free radical work should be repeated under the more stringent conditions - including the PL-23 6/9 149 radiolysis of hydrocarbons. It would also be logical to investigate copolymerization. For comonomer systems of normal purity, anddepending upon the type of initiation, the copolymerization proceeds exclusively by either an ionic mechanism or a free radical mechanism. Depending on the mechanism, each pair of monomers has a characteristic set of reactivity ratios. Thus, despite the fact that there are infinitely many possible sequential and over-all compositions, only a few of these copolymers are actually accessible. In a radiation-initiated ultra-pure system, however, the polymerization proceeds via the simultaneous operation of two or more mechanisms. Further, the degree of the ionic contributions can be controlled by the water content. These factors lead to the conclusion that systems of pure monomers open a pathway to copolymers of radically different structure and properties. For those with an even more intrepid imagination, it is possible to discuss the practical application of these techniques. In the past, industry had shunned radiation because polymerization rates were low and the cost of radiation was high. The situation has now been reversed, and the reaction rates are extraordinarily high, while radiation costs have lessened. For such well-entrenched technologies as styrene polymerization, it is probably fruitless to consider seriously the development of a radiation process. However, there are some intriguing possibilities for those monomers that have been, or will be, neglected because of thermal sensitivity or difficulties in catalyst neutralization. Modifications slanted towards cost reduction are the necessary and sufficient conditions for feasibility. It has already been demonstrated that the presence of a drying agent is not absolutely necessary, and that adsorbed water on the glass reaction surfaces is the primary contributor to ion destruction in radiation polymerization systems [120, 122, 128] . One can envisage a continuous flow system in which, before start-up, removal of water from the apparatus surfaces would be accomplished by a super- heated stream of dry nitrogen. During operation the dry reactor, coupled with a reasonably good distillation, would ensure the stabilization of ionic species. Of course, the source of dry nitrogen would also be used to dis- place dissolved oxygen in the monomer, and as a substitute for a vacuum environment. Periodically, as the water concentration approached a critical level, the reaction system would require "reactivation" with the heated nitrogen purge. As a specific possibility, more than one industrial firm has investigated the irradiation of liquid trioxane to form polyoxymethylene, thus hoping to by-pass the considerable process difficulties that are encountered in a conventional catalytic system. The unfortunate result was that radiation simply would not polymerize liquid trioxane. However, it has recently been announced that liquid trioxane, when subjected to the new purification technique, is indeed susceptible to radiation polymerization [138]. It should also be borne in mind that if research produces the previously mentioned unique copolymers (or terpolymers), then a radiation process would be the sole production possibility.

2. Solid-state polymerization

The advent of stereospecific polymerization by chemical initiators (Ziegler-Natta catalysts) led to an increased interest in the possibilities 150 HUMMEL et al. of preparing polymers by the irradiation of the solid monomer. It was expected that the structure of the polymer might reproduce the ordered, crystalline form of the solid monomer. The eventual discovery that only a few monomers readily polymerized to crystalline products led to a more fundamental investigation of the polymerization mechanisms. The subsequent muddled aspect of radiation polymerization in the solid state, if nothing else, is a clear demonstration that the scientific world is not always more comfortable in the solid phase than in the liquid phase. The reactions in the solid state appear to be quite different from those in the corresponding liquid monomers, being largely dominated by the crystalline environment and orientation of the monomer units. No single pattern of behaviour emerges from the very numerous monomeric systems studied, although a classification of monomers has been suggested based on the variation of the rate of polymerization when changing from the liquid to the solid state [139] . Most of the various systems that have been investigated are believed to proceed by an anionic mechanism (e.g. cetyl methacrylate [140], trioxane [141], acrylonitrile [142 ], diketene [143], (3-propiolactone [144]). Such a mechanism would depend on electron capture by monomer molecules. Although this would imply the existence of stabilized carbonium ions, the possibility of cationic propagation has not been seriously considered, even in the case of trioxane. Other systems have been assigned a free radical mechanism, including the thoroughly studied monomer acrylamide [145], and also the acrylic acid[146], and methyl and ethyl acrylate systems [147], One very interesting theory is that all solid-state polymerizations proceed via chain-carrying excited molecules [148] . In several instances, these assignments of mechanisms have been made on the basis of rather dubious logic. For example, many ESR studies have shown the existence of free radicals in an irradiated solid monomer. All too often, it has not been realized that these are not necessarily the species responsible for initiation or propagation, particularly if they are ion radicals. Other methods of reasoning have attempted to utilize liquid phase kinetics for the interpretation of solid-state phenomena. For example, the observation that oxygen or other free radical scavengers do not retard certain solid-state reactions has often been interpreted as evidence for an ionic mechanism, even if there was no possibility of oxygen diffusing through the crystal structure. And in spite of the dearth of knowledge about solid-state reactions, a first-order dependence of reaction rate on dose-rate has been considered as strong evidence for ionic chain carriers. It is hardly surprising, then, that in the solid-state literature one often finds the assignment of different mechanisms to the same monomer by different authors. Undoubtedly more effort should be devoted to the quantitative evaluation of results from the more secure vantage point of solid-state chemistry and physics. In this way, it might be hoped that some of the present embarass- ments would be eliminated. (The supreme example of this is methyl methacrylate, this monomer having been reported to be both completely unpolymerizable [149, 150] and decidedly polymerizable [151, 152].) Moreover, it would be possible for solid-state polymerization to become a valuable tool in itself for investigating the mobility of organic molecules within crystalline and amorphous solids, the nature and distribu- PL-23 6/9 151 tion of defects and dislocations, and the influence of temperature and pressure on their behaviour. Of course, there are some extenuating circumstances for the present confusion, one of which is the prevalence of post-irradiation effects. Such after-effects as polymerization, chain-scission, degradation and oxidation can occur because of the trapping of reactive species in the crystal lattice. An example of the situation created by after-effects is typified by styrene. The anionic solid-state polymerization that was initially reported [153] was subsequently shown to be a post-polymerization occurring during the extraction of unreacted monomer [154]. Nevertheless, reaction rates have been successfully measured by the techniques of differential thermal analysis [155], dilatometry [156], optical and electron microscopy [157-159], and optical birefringence [160, 161]. Sometimes these reaction rates are considerably enhanced by monomer orientation within the lattice, and stereoregular polymers may be formed directly in suitable crystals (notably, the cyclic compounds [162]). In other monomers, crystallinity impedes the reaction, which then tends to be initiated at crystalline defects to give an amorphous polymer within the original crystalline lattice (e.g. acrylamide). Very different reaction rates may be observed in the same monomer, depending on its crystalline or amorphous environment (e.g. N-vinyl-caprolactam [163], calcium acrylate, and barium methacrylate [164]). Other experiments have emphasized the effect of external stress, such as magnetic field (thus enhancing the rate of polymerization of formaldehyde [165]) or pressure (thus enhancing the rate of polymerization of trioxane [166] and acrylamide [167]). Still others have concentrated on the influence of crystallite size and perfection, e.g. trioxane [168] and other ring compounds [162] polymerized from large crystals are better oriented. Also, the effect of non-reactive groups within the solid has been explored (e.g. methyl methacrylate, vinyl acetate, and styrene in solid mixtures with paraffin oil [169]). In binary systems that form solid solutions, a certain number of copolymerizations have been carried out (e.g. acrylamide-methacrylamide [170], 3-chloromethyl-3-ethyloxetane with 3-fluoromethyl-3-ethyloxetane and 3,3-bis(chloromethyl)-oxetane [171]). In addition, mixed crystals (3-chloromethyl-3-ethyloxetane with diketene and trioxane [171]) and eutectic-forming systems (acrylamide-acenaphthylene [170] and acrylic monomers with crotonic acid [172]) have been investigated. Copolymers have also been formed in glass-like mixtures of non-eutectic-forming monomer pairs with paraffin oil [173]. Typically, the previously mentioned erudite complexities have caused industry no great inconvenience, as can be deduced by the flurry of solid- state patent literature over the last'few years. (An excellent example is trioxane [174-178].) Perhaps the most promising industrial opportunities are to be found in the ordered polymers of cyclic monomers. Evaluation of the physical and mechanical properties of the polymer from 3, 3-bis (chloromethyl) - cyclo oxabutane (Penton) suggests that the polymers from cyclic monomers might be useful for the production of oriented films and for injection mouldings [179]. Polymers prepared from diketene and j3-propiolactone in the solid state have been shown to have a higher melting point and better crystallinity than the polymers obtained by ionic catalysts 152 HUMMEL et al.

[162], Also, the recently synthesized high-melting N-substituted polyi- mides of maleic acid (approximately 300°C) [ 180] could have considerable utility. Similar to the biological sciences, the use of canal complexes as molecular templates for carrying out selective and stereospecific solid- state polymerization by radiation has been investigated [181, 182], Thus, high-melting trans 1:4 polymers of 2,3-dimethylbutadiene, 2,3-dichloro- butadiene, 1,3-cyclohexadiene, and cyclohexadiene monoxide have been prepared [181] . This highly imaginative method holds promise for producing many interesting new polymers via solid-state irradiation.

V. PILOT-PLANT INVESTIGATIONS

1. Basic aspects of radiation-induced chain reactions

Single-step radiation-chemical reactions have G-values up to about 10 (i.e. 10 molecules changed per 100 eV absorbed radiation energy). If the full radiation energy of a 60Co-source with an activity of 104Ci (1.60 X 1024eV/h) were to be absorbed in a chemical system producing a certain molecular type with a G-value of 3, then the yield of this compound would be approximately 0.1 mole/h. When the reactive species are re- produced during each step, a chain reaction will take place. The yields of radiation-induced chain reactions are 102 to 10® times larger than the yields of single-step reactions, depending on the system. Theoretically, at least, any reaction chain is infinitely long. The reaction products may be small molecules or polymers; in the latter case, the products of each step form a chain. In many cases the species necessary for the initiation of a chain reaction (radicals, radical ions, ion molecules, and excited molecules) can also be produced in the system by conventional chemical means. Radiation initiation offers, however, the following principal advantages: (1) Without disturbing the system, the initiation rate can be varied within wide limits, even during the reaction. (2) In the case of radical processes, the chain length can be controlled by the dose-rate. The use of modifiers is therefore unneccessary. (3) Since no chemical initiators are present, the final product contains no foreign end groups or other impurities introduced by the initiator. (4) Many chain reactions can be initiated under conditions where chemical initiators fail (low temperatures, solid monomers). One-step or short-chain reactions are attractive for industry only when the cost of radiation is low ("chemical reactor"), or when the profit on the reaction product is high. To the latter type belongs the radiation modification of plastics (cross-linking and oxidation). It can be shown that an activity of 5 X 104Ci 60Co will be sufficient to study radiation-initiated chain reactions on a pilot-plant scale. Let us assume that the full radiation energy of this source is absorbed by the chemical system. This would then mean an energy absorbance of 8 X 1024 eV/h. If a certain reaction product is formed with a G-value of 1, then 0.133 moles of this compound are formed each hour. To obtain the yield of a chain reaction in grams per hour, we have to multiply this PL-23 6/9 153

value with the actual G-value of the reaction, with the molecular weight of the reaction product (or of the monomer unit in the case of polymers) and with the fraction of the radiation actually absorbed by the system. The highly temperature-resistant, alternating ethylene-carbon monoxide copolymer is formed with a G-value of at least 103. Let us assume that one-fifth of the total energy emitted by the source is absorbed by the system. Then the yield of this process would be at least 0.2 X 0.133 X 103 X 56 = 15 0 0 g of copolymer per hour. From this example, it can be seen that the actual chain length of the reaction has a very strong influence on the economy of the process. Since the frequency of termination reactions is not only influenced by the dose-rate, but also by the purity of the system and the size of the reaction vessel, it is useless to investigate industrially interesting systems on a test-tube scale. An example for a chain-reaction with low-molecular weight reaction products is the sulphoxidation of paraffins. A G-value of 103 for an alkylsulphonic acid of molecular weight 300 is easily obtainable. This process does not require high pressures and, consequently, no thick- walled vessels, so that a radiation yield of 50% should be within reach. The yield of alkylsulphonic acid would be about 20 kg/h.

2. Suggestions for a radiation chemical pilot plant

The irradiation unit is the heart of the pilot plant. We have proposed the following specifications to obtain optimum performance and wide variability: (1) The dose-rates within the chemical system should be rather uniform. It was therefore decided to construct a corona source with 12 or 16 single rods, preferably consisting of pencil-type 60Co with a total activity of 5 X 104Ci. (2) With fixed coronoids, the volume of the inner cylindrical part should not exceed 100 litres. To achieve maximum dose-rates it was desirable to make the diameter of the crown of sources variable. (3) For lowest dose-rates, and to reach as many intermediate dose- rates as possible, it was desirable to make the source pencils independently movable. Also, it was necessary to graduate the activities of the single sources according to a symmetrical scheme. Four companies have made proposals for the construction of an irradiation unit of this type. The results are discussed below. The architectural problems were solved by the Architekturbflro Heier & Monse, Cologne, Federal Republic of Germany. A total view of the whole pilot plant is given in Fig. 1. Next to the source building (3) there are two large rooms (4) for the preparation and purification of the raw materials, as well as for the handling of the reaction products. Op- posite these rooms, beyond a small courtyard, is a building (5) containing storage rooms and the central energy supply. A large gangway, the "spine", connects all parts of the complex. To the left of the technical buildings are conventional laboratories (2) for the evaluation of the polymers and for routine investigations. The administrative offices of the plant (1) borders on the street. On the other side of the complex, and at some distance from the laboratories, there is a house for the caretaker of the plant. The 154 HUMMEL et al.

FIG. 1. Lay-out of a radiation pilot plant. 1, Administration; 2, Laboratories; 3, Source building; 4, Technical buildings; 5, Warehouse; 6, Caretaker's house. whole complex was planned in detail, but the details, however, will not be given here. In the following paragraphs, we shall present a short discussion of the proposals of four companies (in alphabetical order) for a pilot-plant irradiation unit.

(1) Atomic Energy of Canada, Ltd., Ottawa, Canada

This company offers an interesting irradiation unit (Fig. 2, with two independent ways of operating the sources). Two standard containers with 8 cobalt rods each are combined to form a unit with 16 rods. The diameter of this corona source is variable between 355 mm and 760 mm. The unit can, alternatively, be built with a pneumatic drive where the sources ride on a cushion of air, or with two independent driving mechanisms, one pneumatic and one mechanical. The latter is rather complicated but has a certain safety advantage. The standard containers can also be used for shipping the sources.

(2) H.S. Marsh Ltd., Reading, England

The corona source consists of 16 single sources with a fixed diameter for the crown of 70 cm (Fig. 3, oil-flow system). The single sources are PL-23 6/9 155

FIG. 2. Proposal for a radiation unit from Atomic Energy of Canada, Ltd. Combined air-flow and mechanical driving system.

1. -Free access to inner irradiation space. Diameter 68.5 cm. 2. Adjustment of diameter of the source-pipe ring. 3. Intermediate conducting pipes- 4. Source holder with lead filling. 5. N. B. A false base or a raised platform round the assembly can be set up in the chamber at any required height. 6. Duct through chamber wall. 7. Air extraction pipe and mechanical emergency withdrawal system for the sources. 8. Iris-pattern variation of the source pipes. Min. diameter 35.5 cm. Max. diameter 76 cm. 9. Free space for emergency evacuation system. Diameter 20 cm. (Evacuation system to be constructed by client) of the pencil type and can be operated independently. In their lowest position they are shielded by a lead container that also can be used as a transport container. It bears a central boring with a diameter of 10 cm which can be used for continuous reactions and as a discharge opening for discontinuous processes, especially in cases of emergency. The sources are moved in stainless-steel tubes. Two driving mechanisms are offered: (a) A system where the cobalt sources in their stainless-steel canning "swim" in an oil stream. They are kept in their working position as long as the oil is streaming. When the oil stream is stopped, the sources sink to their shielded position. (b) A system where the sources are fixed on top of hydraulically moved steel rods.

(3) Gebrtider Sulzer AG, Winterthur, Switzerland

This company offers a very versatile unit. The source container is fixed on top of or within the ceiling of the irradiation cell (Fig. 4). A corona assembly of steel tubes hangs from the ceiling; the single sources are moved by a "Teleflex" drive. The diameter of the corona source is variable between 340 mm and as much as 2340 mm, so that the dose-rate can be varied within wide limits. 156 HUMMEL et al.

FIG. 3. Proposal for a radiation unit from H.S. Marsh & Co. Air-flow system.

The unit can hold a maximum of 20 single-source pencils with a total activity of 105 Ci of 60Co. A more modest model possesses 12 source rods with a total activity of 5 X 104 Ci of ^feo. Irradiations can also be performed in an inner tube of the source container. This is important for flow systems that require very high dose-rates because of the short irradiation times. Figure 4 shows, also, a horizontal cross-section of the unit and the labyrinth. Two manipulators are provided. The irradiation chamber can be observed through a lead glass window. The Sulzer unit is adaptable also for industrial production. Polymers being formed with G-values in the range of 104- 105 (which are easily attainable) could be produced on a 1000 ton per year scale.

(4) Hans WSlischmiller, Meersburg, Lake Constance, Federal Republic of Germany

A cross-section of the unit is shown in Fig. 5. The source is of the corona type with 16 single rods. The diameter is variable between 200mm and 1000 mm. In the rest position, the cobalt rods are singly shielded in pie-wedge segments. These segments can be used as transport containers. The construction of the source also allows the loading of the PL-23 6/9 157

SECTION ttn

1 RADIATION ROOM 5 STORAGE- AND COOLING 9 MANIPULATORS UNIT FOR SOURCES 10 2 CONTROL ROOM 6 STEEL SHIELDING CONVEYOR SYSTEM

11 CRANE 61 3 LABYRINTH ? QQNJROL CONSOLE AND "^MOVABLE

4 SOURCE GUIDE TUBES SOURCE DRIVE UNIT 12 CRANE 51

8 LEAD GLASS WINDOW ALL DIMENSIONS IN METRES

FIG.4. Proposal for a radiation unit from Gebruder Sulzer AG. Teleflex system. 158 HUMMEL et al.

FIG. 5. Proposal for a radiation unit from Hans Walischmiller. Mechanical drive system and hydraulic lift. 1, Lead shield; 2, Source in shielded position; 3, Source travelling tube; 4, Concrete; 5, Push rods; 6, Hydraulic lift.

source in situ. The single sources are fixed on top of steel rods driven hydraulically or electromechanically. An attractive feature of this unit is a hydraulic lift within the cylindrical irradiation volume. To insert reaction vessels, the platform is raised to the upper plane of the crown of the source. After securing the vessel (which may weigh as much as several hundred kilograms), the platform is lowered to the working position. Finally, the source also has an axial boring for continuous processes and for emergency unloading. Each of the units described has a number of advantages and, also, a few disadvantages. Nevertheless, the number of possible improvements is limited. Consequently, we believe that we have described reasonable possibilities for the construction of an irradiation unit for a pilot plant. To reach a final decision, the following aspects may be of importance: (1) Variability of dose-rates is a crucial point. Thus, the diameter of the corona source should be variable, or the sources must be individually mobile. The best, but most expensive, solution is a combination of both. PL-23 6/9 159

(2) Transport and handling of heavy equipment is easier and safer by "rolling transport" than by "hanging transport". Therefore, hanging sources have a certain advantage. Proposal (4), having the movable platform within the source, offers a reasonable compromise. (3) The driving mechanism for the sources should be as safe as possible. Any shut-down of the plant due to source trouble (sticking of a source, corrosion of the drive mechanism) is extremely costly. No material susceptible to radiation damage should, therefore, be used in the construction of the unit. Most problematic is the use of hydraulic systems with organic media. Even stainless steel may present problems in the presence of oxygen and water, since.passivity is suspended in a radiation field.

3. Suggestions for large-scale radiation-induced chain reactions

It is not the purpose of this report to give detailed data for promising radiation chemical processes. Despite that we are inclined to give at least succinct hints on interesting chain reactions.

3. 1. Chain reactions with low-molecular products

There are hundreds of publications on this subject. The following processes seem to have an industrial interest:

3.1.1. Oxidation of aromatic hydrocarbons. Here, the catalyzed oxidation of benzene in aqueous phase yields phenol (G(phenol) = 70) j If it were possible to lengthen the chain, the process would be competitive with other well-established syntheses.

3. 1.2. Chlorination of aromatic hydrocarbons. Hexachlorocyclohexane is formed with rather high G-values (up to 4 X 105). It is not certain, as suggested in earlier papers, that the yield of the gamma component is higher compared with the photochemical process.

3.1.3. Sulphochlorination of aliphatic hydrocarbons. G-values up to 107 have been reported.

3.1.4. Sulphoxidation of aliphatic hydrocarbons. The highest G-values seem to be in the range of 6 X 103.

3.1.5. Carbochlorination of aliphatic hydrocarbons. Aliphatic acid chlorides are formed by the irradiation of CO and Cl2 with paraffins in a chain reaction of medium length.

3.1.6. Hydrobromination of ethylene. This process has already been developed by the Dow Chemical Co. , Midland. The G-value of the reaction is 3.9 X 104, and the capacity of the Dow plant a few years ago was 500 tons per year. 160 HUMMEL et al.

3.1.7. Hydrochlorination of olefins. Depending upon the olefin and the conditions, the G-values are between 102 and 103 (short-chain reaction).

3.2. Radiation-initiated polymerization

Several suggestions on this subject have already been made in this paper, so that only a few summarizing remarks will be given here.

3.2.1. Polymerization in the gaseous phase. The polymer precipitates in liquid or solid form. The polymerization of ethylene has been most intensely studied, especially by an American group (Steinberg et al., BNL) and an Italian group (Munari et al. ). More than 60 publications have appeared in the last few years.

3.2.2. Solid-state polymerization and copolymerization. This technique is promising. Unfortunately, the reactions are well understood. On the other hand, the polymers formed by a solid-state process are often highly ordered and often have outstanding properties.

3.2.3. Polymerization in the presence of special additives. Both the reaction rate and the mechanism of certain polymerizations are strongly influenced by certain inorganic compounds (ZnO, t-A1^D3, silver salts) that act as rate promoters.

3.2.4. Copolymerization. Observing certain conditions of temperature, and of the purity of the monomers as well, radiation-initiated copolymerization may yield useful copolymers that cannot be prepared by conventional processes.

3.2.5. Grafting of polymers. This technique holds promise in the textile industry. Many industrially important qualities of fibres (such as cotton, wool and polypropylene) are greatly enhanced by grafting with a proper monomer. This technique has been studied by (among others) the School of Textiles of the Research Triangle Institute (Raleigh, N. C.), and by numerous Japanese authors.

3.2.6. Emulsion polymerization and copolymerization. This technique does not yield new products. It may, however, have some advantages over the conventional processes.

ACKNOWLEDGE MENTS

We are very grateful to Atomic Energy of Canada, Ltd. , H.S. Marsh, Ltd., Gebriider Sulzer AG, and Mr. Hans WSlischmiller for providing construction plans and proposals without charge. (These plans are strictly proprietary.) We are also grateful to the Bundesministerium fur wissen- schaftliche Forschung and to the Deutsche Forschungsgemeinschaft for granting fellowships to M. RyskaandR.C. Potter respectively. Finally, we thank the BMWF for financing the pilot-plant study. PL-23 6/9 161

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[138] UENO, K., HAYASHI, K., OKAMURA, S., Paper presented at the First Joint Japanese-American Conference on Radiation Polymerization, Honolulu, May 1966. [139] CHAPIRO, A., USAEC Rep. TID-7643 (1962). [140] HARDY, Gy., NYITROY, K., KOVACS, G., FEDOROVA.N., Actachim. hung. 43 (1965) 121. [141] HAYASHI, K., OKAMURA, S., Makromolek. Chem. £7 (1961) 230. [142] SOBUE, H.. TABATA, Y.. J. Polym. Sci. 43 (1960) 459. [143] HAYASHI, K. etal., Isotopes and Radiat., Japan 3 (1960 ) 346. [144] HAYASHI, K. etal., Isotopesand Radiat., Japan 3 (1960) 510. [145] ADLER, G. etal.. J. Polym. Sci. 48 (1960) 195. [146] BALLANTINE, D.S. etal., Ricerca scientifica25A (1955) 178. [147] WYCHERLEY, V., unpublished observations. [148] SEMENOV, N.N., Khimiya Tekhnol. Polim. No.7-8 (1960) 136. [149] BOVEY, F.A., J. Polym. Sci. 46 (1960) 59. [150] AMAGI, Y.. CHAPIRO, A., J. Chim. phys. 59 (1962) 537. [151] LIPSCOMB, N.T., WEBER, E.C., J. Polym. Sci. (in press). [152] BARKALOV, I.M. etal.. Paper presented at IUPAC Symposium on Macromolecular Chemistry, Paris, Aug.1963. [153] CHAPIRO. A., STANNETT. V., J. Chim. phys. 57 (1960) 35. [154] CHAPIRO, A., Radiation-Initiated Polymerization of Solid Monomers, Fourth Japanese Forum on Radioisotopes, Kyoto (1961). [155] CHACHATY, C. etal., J. Polym. Sci. 48 (1960) 139. [156] HOFFMANN, A.S., J. Polym. Sci. 34 (1959) 241. [157] ADLER, G. etal., J. Polym. Sci. 48 (1960) 195. [158] ADLER, G., REAMS, W., J. chem. Phys. 32 (1960) 1698. [159] SELLA, C., C.r. hebd. Seanc. Acad. Sci., Paris 253 (1961) 1511. [160] ADLER. G., J. chem. Phys. 31 (1959) 848. [161] FEE, J.G. etal., J. Polym. Sci. 33 (1958) 95. [162] OKAMURA, S., HAYASHI, K., KITANISHI, Y., J. Polym. Sci. 58 (1962) 925. [163] HARDY, Gy., NYITRAI, K., VARGA, J., KOVACS, G.. FEDOROVA, N., Paper presented at IUPAC Symposium on Macromolecular Chemistry, Paris, Aug. 1963. [164] LANDO, J., Dissertation, Polytechnic Institute of Brooklyn, June 1963. [165] TABATA, Y., KIMURA, H., SOBUE, H., OSHIMA, K., Bull. chem. Soc. Japan 37 11 (1964) 1713. [166] RAO, H., BALLANTINE, D.S., J. Polym. Sci.. Pt A3 (1965) 2579. [167] TABATA, Y., SUZUKI, T.. Makromolek. Chem. 81 (1965) 223. [168] MARANS, N.S., WESSELS, F.A., J. Polym. Sci. 9 (1965) 3681. [169] CHAPIRO. A., PERTESSIS, M., J. Chim. phys. 61 (1964) 991. [170] NISH1I, M., TSUKAMOTO, H.. Nippon Hoshasen Kobunshi Kenkyu Kyokai Nempo 5 (1964) 115. [171] MIURA, M., HIRA1, T.. KAWAMATSU, S., Bull. chem. Soc. Japan 38 (1965) 344. [172] KAETSU, I., HAYASHI, K., Chemy high Polym. 23 (1966) 125. [173] CHAPIRO, A., BONAMOUR, A., ROUSSEL, D., C.r. hebd. Seanc. Acad. Sci., Paris 258 (1964) 914. [174] W. R. GRACE & Co., French Patents 1,310,141; 1,350,349; 1, 351, 326; 1. 348, 294; 1,352,671; 1, 386,213; British Patents 983,075; 983, 673; 984,097; 984,322; 984,394. [175] HOUILLERES DU BASSIN DU NORD ET PAS DE CALAIS, French Patents 1, 292, 224;1, 352, 998; 1,353, 999; Belgian Patents 641, 714;637,434; British Patents 939,498:984, 874. [176] JAPANESE ASSOCIATION FOR RADIATION RESEARCH, British Patent 923,154. [177] TOYO RAYON, French Patent 1. 330, 650. [178] KANEGAFUCHI SPINNING Co. Ltd.. British Patent 939,475. [179] SANDIFORD, D.J.H.. J. appl. Chem.. Lond. 8 3 (1958) 188." [180] IVANOV, V.L., SMIRNOVA, V.K., BORYAS, V.N., Paper presented at IUPAC International Symposium on Macromolecular Chemistry, Moscow, July 1965. [181] BROWN, J.F., WHITE, D.M., J. Am. chem. Soc. 82 (1960) 5671. [182] WHITE. D.M., J. Am. chem. Soc. 82 (1960 ) 5678. THE USE OF LOW-ENERGY ELECTRON ACCELERATORS FOR THE CURING OF PAINTS AND THIN FILMS

F.L. DALTON WANTAGE RESEARCH LABORATORY, ATOMIC ENERGY RESEARCH ESTABLISHMENT, WANTAGE, BERKS, UNITED KINGDOM

Abstract

THE USE OF LOW-ENERGY ELECTRON ACCELERATORS FOR THE CURING OF PAINTS AND THIN FILMS. Considerable industrial interest has been aroused in the use of accelerators based on transformer- rectifier sets operating in the 150-300 kV range, and in insulating core transformers above 300 kV, for the curing of paint films and the initiation of reactions taking place in thin surface coatings. An indication is given here of the position that has been reached in the commercialization of such processes, and some problems are set out that need to be solved before large-scale exploitation can be achieved. Some general remarks on the advantages of radiation curing and the cost of equipment are also offered.

INTRODUCTION

A considerable amount of industrial interest has been aroused in the last few years by the possibility of using electron accelerators operating in the range 150-500 kV for the treatment of thin films and surface coatings. Such machines can be made much more cheaply than accelerators with energies of 2 MeV or more, since a simple transformer rectifier system can supply the necessary voltage up to 300 kV, and the use of an insulated core transformer extends this well above the range of our present interest. Most surface coating applications do not involve thicknesses of more than 20 thousands of an inch, and the transformer rectifier system is therefore adequate as a voltage supply. High currents, leading to dose-rates in the sample from 100 to 1000 Mrad/min, may be obtained by producing electrons from a heated filament, accelerating them in vacuo, and allowing them to emerge from the equipment through a thin window. After acceleration the electron beam is scanned in a manner analogous to that used in a TV tube so that a beam of convenient width for practical use with flat sheet or continuous material is obtained. Beams 4 ft wide are obtained on commercially available equipment at the present time. Figure 1 shows the mean range of electrons in material of unit density as a function of energy. Because of increased absorption of electrons by the window and by the air gap between the window and the products to be irradiated, it seems unlikely that energies below 150-200 kV will be used in practice even for the irradiation of extremely thin films.

SCOPE OF LOW-ENERGY ACCELERATORS

The greatest potential in the method lies in the possibility of curing paint films and other surface coatings at a higher speed and more

165 166 DALTON

.r u

FIG. 1. Electron mean range in unit density material.

cheaply than can be done in conventional ovens. The range of paints suitable for electron curing is limited by the chemical nature of the reactions induced by the method. These reactions are free radical chain reactions, and the hardening of the paint is caused by polymerization of a vinyl compound which in the course of its polymerization reacts with the resin content of the paint to form a three-dimensional network. This means that total-forming (solvent-free) paints should be used, and the largest existing class of such paints is the unsaturated polyester monomer type used for wood finishing. The limitation in the use of these paints has been due largely to difficulty in curing, and the electron curing technique could well lead to an extension of the formulations of the paints and the purposes for which they are used. The curing of other major classes of industrial paints is based on condensation reactions, which are not induced by electron bombardment, and solvent evaporation. This means that some modification of acrylic, vinyl, epoxy and urethane paints is essential before radiation curing can be used with them, and insufficient attention has been given to this aspect of radiation curing although some significant work now being carried out will be mentioned later. Other fields in which the technique may be used are in the cross- linking of thin plastic films and the modification of the properties of textiles. Such developments are also mentioned below. In general, any free radical reaction that needs to be carried out rapidly and at room PL-236/3 167 temperature in a thin layer is a potential target for low-energy electron treatment, and this has led to specific interests covering a wide range of projects being aroused in many companies. A demand for treating greater thicknesses of material, e.g. for polyester pre-impregnated glass fibre mats, at very high intensities has also arisen, but this requires more expensive and specialized equipment than that used in the thin film work, and is outside the scope of this review.

ADVANTAGES OF RADIATION CURING

Speed of cure

Curing times of one-tenth of a second or less can be readily achieved; on continuous strip speeds of 20-200 ft/min can be obtained with paints requiring average doses to cure. Still higher speeds should be obtainable with relatively little development work. As well as the obvious advantages of rapid cure, large savings are often made by shortening the time during which a coated surface is liable to dust damage, since considerable re- coating is often necessary when long drying times are used under factory conditions.

Space

Some 5 or 6 ft of the production line are needed to handle the throughput of ovens which may be 100-200 ft in length.

Use on heat-sensitive substrates

Because the curing process takes place at room temperature, the curing of coatings on hardboard, plywood and plasterboard can be carried out Continuously without damage to the base material. The curing of high gloss polyester lacquers on paper is also feasible. Difficulties of change of colour of paint at oven temperature are also eliminated.

Elimination of catalyst from paint

Since the curing reactions are induced by electron bombardment, no catalyst or accelerator is needed in the paint, and this leads to an unlimited pot life and eliminates the need for two-pack systems and regular machine cleaning.

Elimination of solvent

Although the need to use solvent-free paints for radiation curing has made considerable development work necessary, the use of such formulations eliminates the need for solvent recovery equipment and reduces atmospheric pollution problems.

Speed of operation

Switch-on and switch-off are instantaneous whereas ovens may take hours or even days to heat up and cool down. This gives electron curing 168 DALTON a large advantage, particularly on lines where unforeseen breakdown is possible.

COST OF RADIATION CURING

It is difficult to give accurate cost figures for the radiation curing process for several reasons. American equipment has to date had only a limited sale, while in the United Kingdom we are still in the development stage. A large proportion of the capital investment is in the 300-kV transformer rectifier set, and the price of these is dependent to a small extent only on their output current, so that over-all cost is not simply related to throughput. To give some general ideas of cost, however, it may be said that a 30 kVA accelerator, capable of treating 100-120 million ft2 of material in an 8000-hour year if an average curing dose is assumed, would cost approximately £30 000 installed in the United Kingdom if built as a "one-off" item. Professional staff would not be needed for routine running of the equipment although expert servicing two or three times a year is desirable. Much more accurate estimate of costs for equipment to treat a specific throughput of a specific product can naturally be made if detailed information is available.

EXISTING EQUIPMENT'

The first commercially available accelerators were the High Voltage Corporation's I.C.T. 300 and 500; these machines operate at 300 and 500 kV respectively, and have scanning heads up to 4 ft wide. They use insulated core transformers to generate the accelerating voltage and this means that the capital cost of the 300 kV equipment is rather high. The maximum output of the 500 kV machine is 20 mA though not more than 15 mA can be used with one scanning head. Irradiation of samples is carried out by the High Voltage Corporation for interested companies on a commercial basis. The first 300-kV equipment based on a transformer rectifier system designed specifically for curing paint films and similar coatings was the "Dynacote", introduced by Radiation Dynamics Inc. This provides a 1-25 mA beam which may be scanned over 2 or 4 ft. A double power supply is available if two heads are required. This company also makes radiation facilities available at commercial rates. The Texas Nuclear Corporation offers 300-kV equipment but details are not yet to hand. The Ford Motor Company has installed its own equipment and has done a considerable amount of research and development work although little detailed information is available. Patents filed by the company cover developments in machine design and also in paint formulation; considerable emphasis has been laid on the development of radiation curing of acrylic resins. Ford technical knowledge relevant to wood finishing has been licensed to Boise Cascade, who are said to be starting pilot-scale production in April 1967. Low-energy electron accelerators have been used for some years by the Cryovac Division of W.R. Grace & Company in the production of their shrinkable polythene packaging film Cryovac L. Deering Millikan have recently announced that similar equipment is being used in a new method of producing crease- and soil-resistant finishes for polyester cotton fabrics. PL-236/3 169

No large-scale demonstration equipment is at present available in Europe, although a number of experimental assemblies exist. In particular, the Wantage Research Laboratory has two experimental 150-kV machines, one treating samples up to in. wide and the other samples up to 6 in. In view of the interest aroused by the results obtained on these accelerators, a 300-kV 100-mA installation having a 4-ft head has been developed, which it is hoped to commission in July 1967. The machine will be used in the Wantage research programme, but time will be made available to interested companies. Technically, this equipment should provide a suitable pilot facility for all the applications of electron curing envisaged so far.

TYPES OF PAINT SUITABLE FOR RADIATION CURING

As has already been indicated, the most widely studied paints to date are those based on unsaturated polyester monomer mixtures, the monomer in most cases being styrene. The radiation-induced curing of such mixtures was reported by Callinan [1] and studied in more detail by Charlesby and his co-workers [2] who, in addition to studying a range of physical properties, showed that the curing required relatively low total doses, and pointed out that rate of cure was proportional to the square root of the radiation intensity. More recently, Burlant and Hinsch [3] , working at higher intensities, have reported rates directly proportional to, or independent of, intensity, and work at Wantage has shown that the intensity exponent is, in fact, a function of intensity, the precise form of this function depending on the detailed formulation of the system. At a practical level it is clear that, although some claims for improved adhesion have been made, radiation-cured polyester monomer systems give finishes similar to those obtained by conventional curing, and that these cari be obtained at doses in the 2-10 Mrad range. Whereas most reported work has been carried out on simplified systems, a number of U.K. companies working in collaboration with the Wantage Research Laboratory have obtained data on fully compounded resin systems covering a wide range of specifications. Polyester paints are not at present used on a very wide scale industrially, but the new curing method may well open up new fields for them. The main objections to their use have been difficulty in curing, the need for a two-pack system that limits shelf life and necessitates frequent cleaning of equipment, together with the difficulty of obtaining "tack-free" surfaces. None of these problems arises if radiation curing is used, and this makes unsaturated polyesters much more attractive materials to the paint manufacturer. This is leading to new types of formulation for use where polyesters have previously been considered unpractical. A second group of paints studied in detail by the Ford Motor Company in the United States of America are the acrylics. The Ford Motor Company has announced the development of practical radiation- cured acrylics, but no details have been released. In the United Kingdom, work at the Wantage Research Laboratory has concentrated on the development of solvent-free acrylic formulations and considerable success has been achieved, although again no details have yet been released. Less detailed work has been carried out in various laboratories on a range of other paint systems. Unmodified epoxies are not amenable to radiation 170 DALTON curing but epoxy-acrylics have been studied by Radiation Dynamics in the United States of America and at the Wantage Research Laboratory in the United Kingdom, Radiation Dynamics have also reported working on PVC plastisols and plastisols with styrenated polyesters.

AREAS OF INTEREST IN WHICH FURTHER DEVELOPMENT WORK IS REQUIRED

Accelerators

An improvement in the design of accelerators for treating flat surfaces is possible and necessary, as is the development of a range of instruments of varying output to cover varying commercial requirements. Little work has yet been carried out on the treatment of shaped surfaces, and the production of machines capable of irradiating car bodies and other large volume items could lead to very large-scale use of the radiation curing method.

Paint systems

The development of radiation curing paints of various types to the stage at which a range of commercially acceptable finishes can be offered is urgently needed if machine technology is not to develop more rapidly than paint technology. With the exception of the polyester paints, very little effort has been put into this. The requirement extends beyond paints to materials such as printing inks and textile finishes.

CONCLUSIONS

The low-energy electron irradiation technique appears to offer great advantages in a number of surface coating applications, and a rapid development programme is necessary to improve the range of machines available and to produce many more types of coating suitable for electron curing. In Europe, the construction of pilot-scale facilities is essential before commercial exploitation can be achieved, and the equipment under construction at the Wantage Research Laboratory of the U.K.A.E.A. represents a major step in this direction.

REFERENCES

[1] CALLINAN, T.D. , ONR Symposium, Rep. ACR-2 (1954) 24; Electl Engng, N.Y. 74 (1955) 510; Electl Equip. (July 1956); Insulation, Lond. (August 1956). [2] CHARLESBY, A. , WHYCHERLEY, V., Int. J. appl. Radiat. Isotopes 2 (1957) 26; CHARLESBY, A., WHYCHERLEY, V. , GREENWOOD, T.T., Proc. R. Soc., Ser. A, 244 (1958) 54. [3] BURLANT, W., H1NSCH, J. , J. Polym. Sci. A 3 (1965) 3587; I. Polym. Sci. A 2 (1964) 2135. V. COMMUNICATIONS AND COMMENTS

SUMMARY OF REMARKS ON THE

ADDITIVITY OF SIMULTANEOUS RADIATION EFFECTS

J. Silverman When a radiation experiment is performed in mixed neutron-gamma fields, it is frequently assumed that the effects are additive in a linear manner. To obtain the net neutron effect, some investigators perform the experiment in a pure gamma field and subtract the gamma effect from the result obtained in the mixed field. This procedure can lead to serious errors and has apparently done so in the case of the radiolysis of the polyphenyls. The principle of non-linearity is an old one, well recognized by almost all photochemists, some polymer chemists and a few radiation scientists. For example, in the polymerization of vinyl monomers by heat and gammas the proper expression for the radiation-induced free radical reaction rate is given by

Rn = R2t - Rb where Rn = rate of radiation-induced polymerization, Rt = over-all rate including radiation and thermal effects, and Rb = rate of thermal polymerization alone.

Similarly, if the same reaction is observed in a mixed radiation field, it is wrong to conclude that

Rn = Rn, y " Ry

where the three symbols are the neutron-induced rate, the mixed field rate and the gamma-induced rate respectively. The proper expression is

R n - r 2 R :

A significant thermal effect complicates the kinetics still further. In a complex reaction (such as the decomposition of polyphenyls), it is often unreasonable to use a model of the following type:

+ R Rn,y = knIn + Mf b where kn and k^ refer to the yields of the neutron and gamma respectively, the I's refer to the dose rates, A is the power of the dose-rate dependence, and Rb is the thermal blank. If A is not equal to unity, the expression is certainly invalid. If it is unity at low dose rates, it may still be invalid at higher dose rates where saturation effects occur.

171 172 COMMUNICATIONS AND COMMENTS

VACUUM ULTRA-VIOLET RADIATION

N. Getoff

Introduction

The vacuum ultra-violet (v.u.v.) light is an electromagnetic radiation with a wavelength < 2000 A This light is strongly absorbed by oxygen, leading to the formation of ozone, and is therefore involved in several photochemical processes in the atmosphere. Since the energy per quantum in the wavelength range is relatively high, i.e. 6.6 eV/quantum at X =o1849 A, 8.4 eV/quantum at X = 1470 A and 10. 1 eV/quantum at A. = 1236 A, the v.u.v. light represents a link between u. v. light in its usual sense (X > 2000 A) and ionizing radiations. In fact, it was found very recently that hydrocarbons, i.e. n-butane, can be ionized, .using the argon resonance lines at 1067 and 1048 A [1, 2]. Apart from this, by irradiating water in the liquid state with v.u.v. light, solvated electrons ^-~e-aq) are formed with a quantum yield, 4> —e-aq) = 0.03atl849 A [3] and0.06at 1470 A [4]. The v.u.v. light as a new tool of irradiation has become increasingly important in the last 10 years. One can now achieve an efficient light intensity (1014 - 1015 quanta/s) to produce measurable amounts of products. v.u.v. radiation sources

Gas filling The v.u.v. light sources are electrodeless discharge lamps containing a rare gas or gas mixture of about 1 mm pressure, powered by microwave energy. Some resonance emissions for atoms are presented in Table I. The v.u.v. light obtainable has wavelengths ranging from 584.4 to about 2000 A. Usually a lamp is filled with a rare gas and consists of a Pyrex tube to which a quartz finger containing a Ba-Al-Ni alloy (getter) is attached. The getter absorbs the impurities in the rare gas and one can achieve a chromatic purity in the lamp emission. Instead of getter the same result can be obtained by using a refrigerant for the gas impurities, i. e. liquid oxygen in the case of a xenon resonance lamp or liquid nitrogen for krypton filling, etc.

Window material A special "window" is needed through which the v.u.v. light produced by the gas discharge can be propagated to the sample. In the case of wavelengths above 1050 A the requirement of transparency is achieved by the windows listed in Table II. For v.u.v. irradiation of gas an LiF window is stuck on to the one end of the lamp. In this case, both lamps and reaction vessels usually require such a window. The sapphire has the advantage of making possible a graded seal to the Pyrex and withstands temperatures from 77 to 700° K.

After prolonged use of the lamp the window of LiF, CaF2 , etc., becomes coloured from the creation of "F" centres. The colouration. 173 COMMUNICATIONS AND COMMENTS

TABLE I. SOME CURRENT RESONANCE EMISSIONS FOR ATOMS [5-7]

Gas \ Gas ( * (in A) (in A)

Hydrogen 1215. 7 1149 Radon 1786.1

Deuterium 1215.3 1582 Bromine 1633

Helium 584.4

735. 9 1799 Neon 743. 7 Iodine 1844 2064

1048.2 Mercury 1849 Argon 1066.7 (low -pressure) 2537

1164. 9 20 vol. % Nitrogen "1 1495 Krypton 1235. 8 80 vol. % Argon J 1743

1295. 6 Xenon 1469.6

TABLE II. VACUUM ULTRA-VIOLET TRANSMITTING MATERIALS WITH A THICKNESS OF 1 mm [5, 6, 8]

v. u. v. light transmission Transmitting material above 10% for X in A

Lithium fluoride 1050

Lithium fluoride (X-rayed) 1200

Calcium fluoride 1230

Strontium fluoride 1280

Barium fluoride 1350

Sapphire 1425

Cultured crystal quartz ~ 1450

Suprasil 1600 174 COMMUNICATIONS AND COMMENTS

GAS IN

s ST w

FIG. 1. Vacuum ultra-violet irradiation apparatus;

A, antenna; LO, liquid oxygen; S, solution; St, stirrer; W, window.

however, can be bleached by irradiation with an ordinary medium pressure mercury arc. For wavelengths below 1050 A the v.u.v. lamps are windowless and the rare gas pressure is kept by differential pumping systems [9], Recently we have constructed modified v.u.v. lamps, which allow not only gas, but liquids, solutions or solid substances to be irradiated also [ 10]. For the irradiation of solids argon is passed through the sample container during the irradiation. When solutions have to be irradiated a sapphire window, whichis submerged in the solution (Fig. 1), is used.

Microwave generator As mentioned above, the gas discharge in the lamp is generated by microwaves from a generator supplied with an appropriate antenna. We use a "Radarmed 12 T 201/9" generator (2450 Mc/s). Its highest power output is usually adjusted so that the discharge in the lamp is about 1-2 cm from the window to limit the reabsorption of the radiation.

Actinometry The intensity of a v.u.v. lamp is determined by using well-established

actinometer systems such as C02, O2, etc. [5],

REFERENCES

[1] DOEPKER, R. D., AUSLOOS, P., J. chem. Phys, 42 (1965) 3746. [2] AUSLOOS, P., LIAS, S. G., J. chem. Phys. 45 (1966) 524. [3] GETOFF, N., Mitt. Inst. Radiumforch., Wien (in press). [4] GETOFF, N., SCHENCK, G. O., J. phys. Chem. (in press) [5] McNESBY, J. R., OKABE, H., in Advances in Photochemistry 3, (NOYES, W. A., Jr., HAMMOND, G. S., PITTS, J. N., Jr., Eds) Interscience, New York (1964). [6] FRIEDMAN, H., in Physics of the Upper Atmosphere, (RATCLIFFE, J. A., Ed.) Academic Press, New York (1960). [7] OKABE, H., J. opt. Soc. Am. 54 (1964) 478. [8] KOLLER, L. R., Ultraviolet Radiation, .Wiley, New York (1965). [9] BACK, R. A., WALKER. D. C., J. chem. Phys. 37 (1962) 2348. [10] GETOFF, N., SCHENCK, G. O. (in preparation). 175 COMMUNICATIONS AND COMMENTS

AQUEOUS SYSTEMS

D.F. Sangster

Radiation chemistry research covers more than attempts to discover new radiation chemical processes. More radiation chemical investigations have been carried out on water and aqueous solutions than on any other class of system. The rate of publication is still increasing. This is a reflection not only of the interesting fundamental problems which exist in aqueous solutions but also of their importance in any atomic energy programme .

Importance of aqueous systems

In any atomic energy programme there are many examples of water or aqueous solutions subjected to ionizing radiation. In such cases radiation chemical effects can be expected and it may be necessary to minimize or enhance these effects as appropriate. Examples are:

(1) Research or power reactors that are water or heavy-water moderated or cooled (This review is confined to the condensed state so phenomena with the steam-cooling of reactors are excluded. ) Fortunately, the net water radiolytic decomposition in reactors using completely enclosed fuel is minimized by back reactions reforming water provided the water is maintained pure and the free gas space is not large. The use of new alloys and other materials in water reactors raises the possibility of corrosion and similar effects under irradiation. Homogeneous reactors in which the fissile material is dissolved in the moderator and in solution raise further and serious problems.

(2) Reprocessing of irradiated fuel The most favoured fuel recovery and fissile material separation processes are still those based on aqueous systems using either solvent extraction or ion exchange techniques. In operation any radiation chemical effects have been minimized to a satisfactory level by limiting as far as possible the time of contact, by including additional separation stages or procedures and by the addition of reagents to counteract the effects. A great deal of ad hoc work has been done to arrive at these conditions, but they are not necessarily the best means of achieving the required results.

(3) Chemical analytical methods Solutions that must be analysed to maintain chemical control over reprocessing and other operations can sometimes be extremely radioactive. There have been very few studies reported on the effect of radiation on analytical methods. Some interesting and useful work might be done on this subject. 176 COMMUNICATIONS AND COMMENTS

(4) Radioisotope production Generally, it is possible to choose solid inert targets for isotope production but in some special instances this is not possible. Radiation chemical effects could be quite severe in the high radiation fields that must be used.

(5) Self decomposition of labelled compounds Radioisotopes are often prepared and used in aqueous solution and are consequently stored in aqueous solution. Some work has been done on radiation chemical effects in these solutions occasioned by the radiation emitted by the radioactive elements in the labelled compound. Some anomalies are found when the results are compared with those from 60Co irradiation.

(6) Sterilization of pharmaceuticals At present, most surgical goods such as syringes, catheters and dressings are dry materials and consequently very few radiation chemical effects are encountered. Where topical application is intended side effects are probably not important but for parenteral injection it is necessary to ensure that no undesirable by-products are formed which could endanger the patient. The reported effect of irradiated dextrose solutions on carrot and mammalian cells shows that careful biological testing is needed, and it may be that radiation chemical studies will be able to show ways of minimizing the formation of toxic products without interfering unduly with the sterilization.

(7) Food preservation Radiation spoilage can be a big problem when radiation is used in an attempt to disinfest or preserve foodstuffs. This is a complicated biological and biochemical problem, and it may be that a solution can be found only by an intensive study of model systems of increasing complexity. Perhaps the formation of peroxides or hydroperoxides are important.

(8) Radiation biology As such, radiation biology is outside the scope of this review. However, the problems that exist might have a better chance of being solved if more were known about the behaviour of less complex compounds- model systems. Many biological systems can be considered as concentrated aqueous solutions but it is important to distinguish direct interaction of radiation with cell constituents from the indirect action of species from the radiolysis of water.

(9) New compounds Just as the study of natural products by the organic chemists has led to the discovery and eventual application of many new and novel compounds, so radiation could be a means of discovering new and potentially valuable organic chemicals and biochemicals.

(10) Fine chemicals There appear to be no existing applications of radiation chemical processes in the manufacture of fine chemicals from aqueous solutions. 177 COMMUNICATIONS AND COMMENTS

It is attractive to consider the possibility of being able to achieve a complicated organic chemical synthesis in a single step. Although some radiation reactions are surprisingly selective, usually a range of products is produced and the main problem becomes one of separation. Highly selective separation processes are now available.

(11) Heavy chemicals Similar remarks apply to the manufacture of heavy chemicals. This has been considered from time to time and has not proved economically or technically attractive in most cases. These appraisals have often had to be based on scant information, and there has been no indication that there is good reason to undertake the extensive study required to enable a thorough assessment to be made. A radiation chemical process must be better economically and technically than any alternative before it will be used.

(12) Polymers There has been a considerable amount of work on the polymerization of monomers in aqueous solution. In application, other solvents are generally preferred but in some specific cases aqueous solutions Could be preferable.

Problems

The most extensively and intensively investigated system in radiation chemistry is probably the ferrous sulphate-sulphuric acid solution used in the Fricke dosimeter. Yet there still remains work to be done on this system. In very few systems have all the important products been identified.

Other areas

A set of examples similar to the above can be drawn up for each area of radiation chemistry - gases, organic compounds and mixtures, solids, polar solvents, etc.

Impact on chemical science

The more recent findings resulting from radiation chemistry research have elucidated chemical reaction mechanisms in general, but particularly those involving free radicals or free radical intermediates, ion-molecule reactions or excited electronic states of molecules. Radiation can sometimes be a controllable source of such species and can help us interpret chemical events in other fields.

Conclusion

It is hoped that these remarks will illustrate that radiation chemistry has a purpose and a function other than that of discovering and developing new radiation chemical processes for industrial use. 178 COMMUNICATIONS AND COMMENTS

SUMMARY OF REMARKS ON THE STATUS OF RADIATION PROCESSES

J. Silverman Since their earliest days, radiation process applications have been plagued by extravagant claims that do not survive close scrutiny. The credibility gap that faces the applied radiation chemist and engineer is far too great. This persistent problem is particularly unfortunate at this time when the subject can boast of genuine accomplishment and real promise. There has not been enough time during this Panel meeting to make an evaluation of the many process applications that have been the subject of widespread publicity in the past few years. In the view of almost all Panel members, this is unfortunate. Several applications were discussed during the Panel's deliberation. The absence of some definite statements as to current status may lead the reader to draw the inference that nearly all of them have great merit. I have therefore presented to the Panel my personal judgements regarding some applications. Although there were many expressions of agreement, I repeat that the views may not represent those of the entire Panel simply because there was insufficient time to discuss them.

1. Cross-linking of polyolefins This process is a demonstrable success. Heat shrinkable packaging film, heat shrinkable electrical fittings, foamed insulating tape, and foamed padding for bathing suits and lingerie are some of the increasing number of products made by this radiation process.

2. Curing of paints and finishes

This is one of the most promising of all processing applications. Several major paint companies have produced paint formulations that cure to an excellent finish with small doses of radiation. The development of reliable low-voltage machines'is at hand and wide industrial exploitation is almost a certainty.

3. Ethylene polymerization

The success of pilot-scale experiments (particularly those at the JAERI laboratory in Takasaki, and at Brookhaven) has led to significant industrial investment in private facilities for further study. On the other hand, the published data on cost and product quality suggest that the product will be sold to speciality markets and in much smaller quantities than the polyethylene now available.

4. Wood-plastic combinations

Although the prospects of producing widely applicable super-wood products at competitive prices are not encouraging, there is some hope that an irradiated wood-plastic product may be obtainable whose sole advantage is reduced water uptake and therefore better dimensional stability. Also there are strong indications that a few private companies will soon offer for sale speciality items in small quantities. 179 COMMUNICATIONS AND COMMENTS

5. Graft polymerizations

By this technique, an inexpensive base polymer can be modified to have premium qualities by a radiation-cum-monomer treatment that is inherently cheap and simple. Industry has long been aware of the method and its many attractive accomplishments in the laboratory such as dyeable polypropylene, etc. However only two products, a styrene graft to Teflon, and a battery-separator material based on polyolefins, have been offered for sale, bothbyRAI Research. The first item has been withdrawn but the second has been successful and is sold in small but increasing volume. Recently Deering-Milliken announced that a textile product modified by radiation grafting would be offered for sale; industrial interest in the success or failure of this venture is very high.

6. Sulphoxidation of alkanes

A plant using this method of producing biodegradable detergents was to have been erected by Esso. The project is dormant and will continue to be as long as non-radiation products dominate the market.

7. Nitrogen fixation

Steinberg is his most effective competitor. His dual-purpose chemonuclear process for nitrogen fixation is much more expensive than the one in which he proposes to use electrolysis at the site of a power reactor. Furthermore, even if the chemonuclear process achieves the maximum theoretical G-value and the formidable engineering obstacles are overcome, it may face comparable advances in the technology of electrolysis. Thus, while chemonuclear processes may produce public benefits that justify the current level of research and development, the outlook is not very clear.

WOOD-POLYMER COMBINATIONS K. Singer At present, two commercial companies in the United States of America and one company in France offer irradiated wood-plastic products, but as far as I know they use only one type of polymer, namely polymethyl methacrylate, and a few types of wood. It is still uncertain how large the market for these products may become, and whether the most economical and practical method of production will be one applying irradiation or one with conventional catalysts. It seems to me that a number of problems in the fabrication process still require a good deal of research effort, not only in impregnation technology (to obtain homogeneous products), but also in useful polymers and radiation technology, for instance concerning the type of radiation sources. I should like to consider these three fields in the light of the following question: which properties of wood are most important to improve? 180 COMMUNICATIONS AND COMMENTS

I think the following order of priority could be given: (1) Dimensional stability; (2) Surface properties such as hardness and abrasion resistance; (3) Fire resistance, rod resistance and mechanical strength. Concerning dimensional stabilization, it remains to be clearly demonstrated whether grafting of the polymer on to the wood substances is of any importance or not. Also the number and chain-length of the grafted polymer molecules may have an influence on this property. Furthermore, the location of the polymer in the wood, i.e. in the cell wall or in the cell cavity, is probably of importance, and this location depends mostly on the chemical properties of the impregnated monomer, but also to some extent on the impregnation technique applied. In any case, it is of the utmost importance to obtain a homogeneous impregnation throughout the wood, and based on our experience, which is supported by recent practical investigations in the United States of America, there still remain problems on how to obtain large reproduceable and homogeneous wood-polymer samples. To improve surface properties, it is necessary to develop a good surface impregnation technique. We shall here have to deal with the surface effect, which normally is considered to be caused by monomer evaporation during the fabrication. Whether this explanation is sufficient still needs to be proved. Finally, there is much work to do in using different additives to the monomer systems to obtain fire resistance, rod resistance or dyed products. The possible inhibiting effect on the radiation polymerization which these additives may cause can be rather surprising. I should like to add a few words concerning radiation technology. Today 60Co gamma sources are normally considered to be most suitable for this process, but as some potential applications would involve products of limited thickness, for instance veneers, knife handles and even flooring, high-energy electron accelerators might be considered as a potential radiation source for wood-plastic production. There is not much published work concerning the application of high-energy electrons, and here is certainly a subject that should be seriously investigated. We published some results at the Radiation Chemistry Conference held in Tihany in 1966, indicating that by suitable combinations of wood, monomer, temperature and water content of the wood it would be possible to obtain complete conversion of monomer to polymer with radiation doses below 10 Mrad. One of the most important problems in the eventual applications of high-energy electrons is, of course, the heat generation caused by the reaction, and how this will influence the size of wood items and the radiation technique. But also in connection with 60Co sources there remain some problems regarding the best principles of plant construction, and what the dose requirements would be when the process is scaled up. Based on our experience with the Ris^ 10 kCi 60Co facility we should consider, for instance, the necessary dose for methyl-methacrylate conversion to be 3 Mrad, whereas most recently published economic evaluations are based on a dose requirement of 1. 5-2 Mrad. I believe that the necessary dose will be the result of an economic optimization. These few remarks are intended to draw the Panel's attention to some of the problems which must be further investigated before wood-plastic products, in spite of commercialization having already begun, can really be evaluated as a new group of materials. LIST OF PARTICIPANTS

D. SANGSTER A. A. E. C. Research Establishment, (Chairman) Private Mail Bag, Sutherland, N. S. W., Australia

H. BILDSTEIN The Chemical Institute, Osterreichische Studiengesellschaft fur Atomenergie, Reaktorzentrum Seibersdorf, 2444 Seibersdorf, Austria

E.L. DALTON Radiation Branch, Isotope Research Division, Wantage Research Laboratory, Wantage, Berks, United Kingdom

A. DANNO Takasaki Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gun-maken, Japan

N. GETOFF Institut fur Radiumforschung und Kernphysik, University of Vienna, Boltzmanngasse 3, 1090 Vienna, Austria

W. HOLM Ris^ Research Establishment, Roskilde, Denmark

D. HUMMEL Institut fur Physikalische Chemie und Kolloidchemie, University of Cologne, Severinswall 34, Cologne, Federal Republic of Germany

S. IONESCU Institute of Atomic Physics, B.P. No. 35, Bucharest, Romania

S. MINC Instytut Badan Jadrowych, Ulica Dorodna 16, Warsaw, Poland

181 182 LIST OF PARTICIPANTS

E. PROKSCH The Chemical Institute, Osterreichische Studiengesellschaft fur Atomenergie, Reaktorzentrum Seibersdorf, 2444 Seibersdorf, Austria

J. R. PUIG Centre d1 Etudes Nucleaires de Saclay, B.P. No. 2, 91 Gif-sur-Yvette (Seine et Oise), France

J. SILVERMAN University of Maryland, Silver Spring, Md., United States of America

K. SINGER Chemistry Department, Ris^ Research Establishment, Roskilde, Denmark

M. STEINBERG Brookhaven National Laboratory, Upton, Long Island, N. Y. 11973, United States of America

Z.P. ZAG6RSKI Instytut Badan Jadrowych, Ulica Dorodna 16, Warsaw, Poland

Scientific Secretaries

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