Gamma Irradiation of Hexafluorobenzene1 R
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JOURNAL OF RESEARCH of the National Bureau of Standards-A. Physics and Chemi stry Vol. 64A, No. 4, July- August 1960 Gamma Irradiation of Hexafluorobenzene1 R. E. Florin, 1. A. Wall, and D. W. Brown r, (March 27, 1960) Mi xtures of hexafluorobenze ne a,nd benzene were irradi ated in li quid phase by means of a C060 cram ma ource at 20° a nd a t '2 18° C. P crfiuorohep tane a nd vari ous binar.v mixt ures involvi~ g perfluorohepta nc, hexaflu or obe ll ze ne, be nze ne, and cyclo hexane were a lso in'adi ated at 20° C. H exafluorobenzene rese mbled benzene very closely in its behavior upon r adio\ysis. Generall y t he fluorocarbon-hydrocarbon mixt ures evolved much mo rc SiF4 (indicatin g the formation of J-IF, which r eacts with the glass vessel) than the pure flu oro carbon components. The polymer from hexaflu o robenzene-benzrne mixtures was p r oba~ly rich in cyclohexadiene and cyclohexene un its, I' csembiing t hat from pu re benzene, a nd Its co mposition ratio exhibited a strong "altern atin g" tendency. The res ults a re discussed in terms of free-radical and excited-state mechanisms. r\ t 218 0 C hexaflu orobenzene a nd a lso its mixtures with benze ne showed qualitative difl'erences from their behavior a t 20° C, a ltho ugh t he G va lues for SiF4 and polymer rema ined moderate. 1. Introduction more resistant than hydrocarbons to heat, and also ftromatic compounds more resistant than aliphatic. Fully fluorinated aromatic compounds have onl y U nder ionizing radiation, ali phatic hydrocarbons recentlv become accessible. Because of their com give off relatively large amounts of hydrogen; bination of C- F bonds and aromatic reso nan ce aromatic hydrocarbons lose very l,ittle hy~roge~, structure the\- Iln.ve attrn.cted iuterest as possible but form polymers in moderate ywld . Ahphat!c heat-res istanl; mnterials. It has beC'n observed thn.t fluorocarbons su fl' er breaks in the carbon cham certain polyphenyls (0 6F4)X are stable at hi gh and lose slig ht to moderate amounts of fluorine in temperatures [1] 2 }wel that the mass . spectrum ionic form f7 to 9]. The arOlll<ttic hydrocarbons of t hese compouncis shows relatnrely little frag biphenyl and terphen~T l are r es istan t enough . to men tation under electron impn.ct [2]. The In,tter radiation at high temperatures to be of s?me H~ observation is remilliscent of benzell e itself, n.nd tcrest ItS reltctor coola nts f18]. The order of magl1l con t rH,sts strongly wi t h the extensive fragmentation tude of the radiation :vielel , G, in molec ules per 100 of satumted fillorocarbons in the mass spectrometer ev absorbed, is indiclttecl in table 1 for the several [3 , 4]. .. processes. In t his paper , we report some observ~ }ti1 Sin ce electrons arc Importan t In termeduttc tions on the irradi}ttion of C6F 6 , C 7F 16, and theil' in the action of ionizing radiation, it can be expected mixtures with other materials. that hexftfluorobenzene derivatives, like their hydro carbon an,dogs, will be relatively resistan t to TABLE 1. Radiation yields .r0!' several prar esses radiation ,uld may surpass t hem ullder some com (l value bin ations' of hi gh temperature }wcl mdifttion. An~T Process improvement ill materials for use under such eoncb C ol-I, tions would be desirable. The radiation chemistry of fluorocMbo ns has bee n c- c scission. ___________________________ _ 0.02 1.7 11 2 Or Fz________ _______ __ __________ _ . 0'1 . 1 studied very little un til very recently, except for a ])olymer OJ' crosslinks ____________________ _ 1 o few polymers and monomers [5 to 14] . !Ial?gen compounds, including a few monoAuoro denvatIves, hftve been investigated and generally have been 2 . Experimental Procedure fou nd to exhi bit a very high sensitivity to radiation In general, samples were prepared on a vacuum f15]. Benzene has been studied very extensivel}:' , line and irradiated to doses of about 100 to 300 both because of its low sensitivity and t he POSSl megaroentgens C~1r). Products volatile at - 80 0 bility that it can exhibit a "protective" effect in and 25 0 C were analyzed by mass spectrometer. mixtures. Likewise, a few higher aromatic hydro Nonvolatile residues were isolated by distillation carbons have been studied f16 to 18] . In radiation of the liquid and characterized by infrared spectra chemical studies in general, atom and free-radical and elemental analysis. The hexafiuorobenzene mechanisms have bee n fairly well accepted f19], used was from a specially purified center cut of though seldom unequivocally proven. Besides the material synthesized in our laboratory l30] and great wealth of data on hydrocarbon radical re su bj ected to repeated fractional freezing. Its con actions f20], there have recently been a numb er~f stants were bp= 80.5° C/759 mm, nf3 = 1.3781. The studies on reactions of aliphatic fluorocarbon radI benzene used was the commercial product (c.p.) , cal [21 to 29]. not further purified. The perfluoroheptane used The present situation is that fluorocarbons seem was the product of Minnesota Mining & Manu 1 This wo rk was spo nsored by the Aeron autical Research Laboratory, "\'right facturing Co. and was used as received. The Air D evelopment Center, U.S. Ail' Force. ' Figures in brackets indicate t he literature references at t he end of thiS pa per. cyclohexane used was an NBS standard sample, 269 For the irradiations at room temperature, the from ref [31] . Typical factors in ev g-INIr-l X 10- 20 liquids were dried several weeks over P 20 S in break were: C nH 2n, 0.623; C6H 6 , 0.589 ; CnF 2n) 0.526; and seal tubes and distilled on the vacuum line into C6F 6, 0.530 . weighing bulbs of 30-ml capacity. These bulbs were After irradiation, which required several weeks, fitted with standard taper joints and Hoke bellows the samples were brought to the mass spectrometer valves connected via copper or Kovar glass seals. whenever the design of the vessel was appropriate, Samples were made up by distilling in vacuo into and mass spectra taken with contents at -80° and irradiation vessels of several kinds, then degassing, + 25° C . Where the container was not adapted for cooling with liquid nitrogen , opening for about ] this, the contents were first transferred on an minute to a measured pressure of a few centimeters auxiliary vacuum line. :YIany of the Kovar-seal of argon, and sealing or closing off. 'iVeights of com containers in which hydrocarbon-fluorocarbon mix ponents of the mixture, obtained by difference, were tures were e(Cposed to high doses were broken during checked against the total weight of the radiation irradiation or subsequ ent handling because of cor vessel. Discrepancies of a few milligrams were rosion at the glass-metal seals. To calculate the common, probably because of absorption of the mass spectrometer results for gaseous products, volatile liquids in stopcock grease. reliance was placed upon the argon in troduced as The radiation vessels for use at 218° C were 3-mm an internal standard. For liquid products, the thicl,-walled glass tubes. At room temperature examination at 25 ° C yielded only relative values , nickel capsules of about 10- to 15-ml capacity were distorted by fractionation effects. The opened used. They were silver-soldered to 1. 5-mm x 40-mm tubes were emptied by suction, and the liquid con nickel tubes and then through 4.7-mm o.d. Kovar tent was frozen and sublimed at reduced preSSUTe to seals to long 3-mm o.d. Pyrex end sections which isolate the nonvolatile residue. The empty tubes were designed for openin g into the mass spectrometer wer e rinsed with benzene repeatedly, yielding small inlet system . In one irradiation experimcn t under addition al amounts of residue. The residues were a high pressure of hydrogen , a third type of radiation analyzed 3 for C, H , and F , and infrared spectra 4 vessel was used, consistin g of a Monel bomb, 12-mm were also taken in film s and Nujol mulls. o.d. x 6-mm i.d. x 300 mm in length, closed by a In the mass spectrometric procedure, bulbs con Hoke bellows valve. The simple capillary tube had taining a large liquid sample were connected to the the advantages of strength and small size but had a inlet system of the mass spectrometer before break large potentially reactive wall surface and also could ing the seal. Anal~7 ses of volatiles were then made not be opened for analysis without some exposure to with the sample bulb first cooled to - 80° C and then air. The composite capsules were mostly corrosion warmed to 25° C. The analysis of material volatile resistant, vacuum tight, and easily fittcd to the mass at - 80° C should give a reasonably accurate esti spectrometer but were very fragile after use. The mate of gaseous products for all samples in whi ch the existence of a partial internal glass surface had some liquid was compl etel ~7 frozen at that temperature. advantages as well as drawbacks. Any HF formed This includes all samples containing hexafluoro was converted to the easily measured SiF4, while any benzene, benzene, and cyclohexane only . However , reactive fluorocarbon intermediates of short life perfluorohep tane, although the reported freezin g probably underwen t further reaction in the liquid point is - 55 °, is often still liquid at - 80° C and system before they could diffuse to the glass surJace.