J. Riibb. Res. Inst. Malaya, 22(2), 214-224 (1969)

Symbiotic Relations Between Natural and

JAMES D. D'lANNI The Goodyear & Rubber Company, Akron, Ohio, U.S.A.

The relations between natural and synthetic rubbers are truly of a symbiotic nature. Many of the rubber products essential to world economy today are based on blends of both types selected to provide optimum properties for the specific end use. In each case, of course, economic considerations help to determine the final composition. Tyres probably represent the ideal case, since each part is specifically engineered, from the standpoint of materials and design, to provide optimum performance. The quantify of in tyres varies greatly, depending upon the type and intended use. Industrial rubber products are designed with The same concepts in mind, as well as foam rubber. The rubber industry has grown vigorously because the symbiotic relations between natural and synthetic rubbers have been most rewarding. The future of the industry will be beneficially influenced if these relations remain favourable.

Symbiosis may be broadly defined as a mu- synthesis are met. Moreover, through the re- tually beneficial partnership between two or- search efforts of the Rubber Research Institute ganisms of different kinds. Many examples of Malaya and similar institutions, remarkable exist in nature, such as the lichens (alga-fungus increases in yield are being attained by hybridi- pairs). The tubercle-forming bacteria sym- sation, so that in the foreseeable future a ten- biotically associated with the roots of legumi- fold increase in the 1967 world average yield nous plants, in some way not yet clear, capture of 380 pounds per acre (430 kilograms per and fix free atmospheric nitrogen. Symbiosis hectare) seems possible (INTERNATIONAL RUB- is more widespread than is generally realised, BER STUDY GROUP, 1967). This improvement and may occur between two plants, two ani- must surely rank as one of the most econo- mals, or a plant and an animal. Thus the con- mically significant achievements in plant breed- cept of living together in mutual aid, assistance ing of all time. and support is well established in nature. A parallel can be drawn in the complex In contrast, synthetic rubbers are based on relations between natural and synthetic rub- petrochemicals from fossil fuels which are bers which have become such an important slowly but surely being depleted. New dis- part of our modern industrial economy. Both coveries of petroleum and gas continue to add similarities and differences exist which will be to the world's proved reserves, but consump- described in this paper to emphasise the inter- tion also continues to increase dramatically, dependency that underlies many of their appli- and some day in the future may outstrip new cations, despite their differing origins, sources. Most of these fossil fuels are used to A contrast may be drawn in the sources of generate energy. Since the amount converted the two major classes of rubber. Natural rub- into synthetic rubbers is probably less than ber is a true product of chemurgy. It is ob- 0.1 per cent of the total consumption of petro- tained from a plant, Hevea brasiliensis, and as leum products, we should not become unduly it is harvested, it is being constantly reple- alarmed about future sources of raw materials nished, so long as the requirements for photo- for synthetic rubbers.

COMMUNICATION 514 214 JAMES D. D*!ANNI: Symbiotic Relations Between Natural and Synthetic Rubber

Symbiosis is superbly illustrated in the in- TABLE 1. STRUCTURES OF POLYISOPRENES stance of blends of natural and synthetic rubbers. Many factors, such as cost and qua- Polymer Or-1,4 Trans-1,4 3,4- 1,2- lity considerations, determine the types and Natural rubber proportions of the various elastomers used in (Hevea) 98 — 2 - a particular rubber product. In many cases, Synthetic poly- the optimisation process requires constant re- isoprene evaluatioa of these factors to arrive at the (Natsyn type) 97 _ 3 _ particular composition used at any particular Synthetic poly- time — in other words, a dynamic equilibrium isoprene is maintained with a balancing of costs, pro- (Shell Li type) 94 _ 6 _ perties and performance as the critical criteria. Natural trans- In fact, the cases of synergism where blends polyisoprene are superior to either component alone out- (Balaia) 98 2 . number those where only one is indicated, Synthetic irans- particularly for today's tyre, which we shall polyisoprene discuss in more detail later. (Vanadium A case for symbiosis can also be made in catalysed) — 98 2 - the synthesis of polyisoprene with natural rub- ber properties by the use of stereospecific view, namely, that the polyisoprenes in natural catalysts to polymerise isoprene in solution. rubber, balata and their synthetic counterparts Perhaps this success encouraged Professor (with the exception of the lithium-catalysed F. Lynen, as well as researchers at N.R.P.R.A., polymer) consist only of 1,4 type structures. to work out a biochemical synthesis of rubber. However, differences in polymer chain struc- The success of their efforts, in turn, has en- ture such as head-to-tail type variation as well couraged us to believe that stereoregular poly- as cyclic structure and branching may exist mers can be synthesised in aqueous systems. between the natural and synthetic polyiso- prenes. But as it stands now, these are more MOLECULAR STRUCTURE OF or less speculations pending future experi- ISOPRENE POLYMERS mental verification. Unravelling the microstructure of natural rub- An interesting point on the chemical struc- ber and other isoprene polymers has challenged ture of natural rubber has been made by the analytical chemist for decades. Until re- SEKHAR (1961). There is evidence that natural cently, based on infrared absorption measure- rubber contains 5 to 12 aldehyde groups per ments, the generally accepted structures were million molecular weight. The storage harden- as follows (Table 1): ing of natural rubber is attributed to cross- The difference in chemical structure of poly- linking reactions involving these functional isoprene between the natural polymer and its groups. Reaction with hydroxylamine hydro- synthetic counterpart, if there is any, is very chloride converts them to oxime groups which slight and occurs mainly in the content of the cannot participate in cross-Unking reactions. 3,4- addition unit. However, the validity of Rubber so treated maintains a viscosity level the method for determination of the 3,4- unit on storage equal to that of the original planta- in polyisoprenes containing small amounts of tion rubber. We have no evidence that syn- this structure has been disputed since the thetic polyisoprene (Natsyn type) contains method was first developed. It now appears such functional groups, supported by the addi- that the numbers in the 3,4- column may all tional fact that it does not harden on storage. be 2 - 3 per cent high. Both near infrared and The close similarity in structure between the nuclear magnetic resonance measurements by natural and synthetic polyisoprenes does not, CHEN (1966) of our laboratories and others however, imply that their physical properties strongly substantiate this recently proposed must be entirely alike. Other factors besides

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COPYRIGHT © MALAYSIAN Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969 microstructure play a major role in the physi- advantage holds for both polybutadiene and cal properties of polymer. These include gel SBR, whereas polyisoprenes are somewhat at content, molecular weight and the distribution a disadvantage because of their ready chain of molecular weights. Chain scission occurs so scission. readily in polyisoprene that the exact values In most cases, the stereospecific polybuta- of these macromolecular physical properties dienes do not have the excellent mill behaviour depend to a great extent upon the exposure of natural rubber. However, the plasticising to heat, light and mechanical forces prior to effect of styrene in SBR does contribute to the time of measurement. processability, so that this copolymer is easily handled, especially after years of effort to PROCESSING CHARACTERISTICS develop varieties which provide maximum ease It is axiomatic that processability is a key con- of processing without sacrifice in quality. sideration in the fabrication of virtually every Polybutadiene is also characterised by ex- rubber product. Whether processing is excel- treme lack of nerve and tack, and a tendency lent, good or poor is often difficult to define to 'bag' on the mill. Despite this appearance of because the standards of the consumer may 'nervelessness', polybutadiene stocks do not vary a great deal depending upon his facilities flow easily in high speed extrusions, tending to and the nature of the end use. give torn edges and high die swell. Thus it was Natural rubber has been highly regarded inevitable that polybutadienes find their major because of its high initial Mooney viscosity, applications in blends with other rubbers. In its easy breakdown to a usable plasticity, its mixtures with SBR or with Hevea (or both), tack and green strength, and, generally, its processability of the stock is improved to adaptability to the fabrication techniques of acceptable levels. the rubber factory. However, natural rubber also has significant deficiencies. Not all con- Most polybutadienes also suffer from poor sumers find that the breakdown characteristics tack, and nothing short of adding poly- suit their special needs. Some would prefer isoprene really solves this problem. There is, of better retention of compounded Mooney visco- course, extensive use of 'tackifiers*, but most sity, whereas others would prefer a faster processors will only agree that these make breakdown without peptisers. All consumers rubber 'stickier'. There is a clear distinction be- would like to reduce the costs associated with tween a strong, cohesive merging of like rub- pre-mastication. bers — which we call tack — and the surface The synthetic polyisoprenes do offer the softening, and weakening, that tackifiers pro- advantage of lower initial Mooney viscosity to vide. eliminate the breakdown portion of the pro- The ethylene-propylene terpolymers (EPDM) cessing cycle, but they still have the easy break- possess a different combination of processing down that is so characteristic of polyisoprene characteristics. On the mill, they undergo a molecules. On the other hand, the synthetic thermoplastic softening without breakdown polyisoprenes do not yet have quite the high which helps that phase of factory operations. green strength that is typical of natural rubber, But the complete lack of building tack, the although it is adequate for many applications. relatively slow rate of cure and the lack of Greater green strength will be tomorrow's cure compatibility with highly unsaturated development. elastomers are all processing disadvantages Polybutadiene has a processing advantage, which EPDM has had to contend with in its or disadvantage, depending on the point of efforts to gain acceptance in tyres. Considerable view, in that it is quite resistant to breakdown progress has been made toward solving some on the mill. Thus a high molecular weight of these problems, but much more effort is polymer can be plasticised with extending oil needed. and then processed, without losing the inherent In the years ahead, we may see dramatic advantage of high molecular weight. This changes in rubber processing resulting from

216 JAMES D. D'IANNI: Symbiotic Relations Between Natural and Synthetic Rubber

advances in polymer synthesis. One example TABLE 2. TRANSITION IN ELASTOMER is the Thermolastic polymers commercially in- USAGE IN TYRES (U.S.A.) troduced by Shell (BAJLBY et a/., 1966). These low molecular weight polybutadiene or poly- Elastomer 1940 1950 1960 1965 1975 isoprene rubbers, with blocks of polystyrene at each end of the rubbery chain, have some similarities to cured rubbers below the soften- Natural 100 60 40 30 15 ing point of the styrene blocks. Above this tem- SBR 40 60 55 50 perature, they are very soft, injection mould- able materials. Such polymers can be processed Stereo 15 30 without the conventional compounding and Speciality 5 curing steps, and readily lend themselves to automated operations. % Man-made 0 40 60 70 85 Similarly, we have seen the liquid, castable capturing larger markets as a result of their simple processing, despite mate- Some reasons for the changing use of rials costs far in excess of those of the usual elastomers in tyres are indicated in this com- rubbers. parison of tyre properties (Table 3): If this trend continues, we can foresee the TABLE 3. TYRE POLYMER COMPARISON day when inexpensive liquid hydrocarbons will be end-linked and cured — in a manner physi- Poly- Poly- Property Natural SBR buta- EPDM cally analogous to that for thermoelastics and isoprene diene polyurethanes — so that the mills and Ban- bury mixers of today may no longer be the trademark of rubber processing. Treadwear 100 100 115 150 115 Resilience 100 100 75 95 85 RUBBERS IN TYRES Heat In tyres, we see symbiosis at work to a greater durability 100 100 150 150 150+ degree than anywhere else in the industry. Where once a single rubber dominated the entire Cold scene, we have observed the evolution of a flexibility 100 100 90 120 100 complex tyre, containing several rubbers in Weather differing blends, each blend carefully designed resistance 100 100 115 100 150 to accomplish a particular task, and each com- pound perfected to provide its own contri- bution to the whole. Moreover, the inter-rela- Although there are differences between tionships with the fabric in the carcass and in natural rubber and synthetic polyisoprene in the belt (where used), as well as design features the raw polymers and in their handling charac- in the tread and other sections of the tyre teristics, these are largely levelled out during justify the statement that the tyre is one of vulcanisation. Thus these two rubbers compete the most highly engineered products on the primarily on a processing and cost basis — in consumer market today. other words, on their total all-in cost after pro- It appears that these evolutionary changes cessing into a tyre. will continue as we learn to take fuller advan- The other rubbers offer unique advantages tage of the diverse properties of newer rubbers over the polyisoprenes. SBR has long been and as the polymer chemist generates still recognised for treadwear improvement and newer types for evaluation. Today, rubber resistance to tread cracking, and it, in turn, usage in U.S.A. tyres is more than 75 per cent has had to bow to a partial replacement by synthetic, and predictions have ranged up to polybutadiene. Additionally, the high resili- 85 per cent for 1975 (Table 2): ence of polybutadiene, coupled with its heat 217 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969 resistance and flexibility, has made it popular passenger tyres are seldom used. Instead, the as a carcass component. tyres are made to seal at the rim without air As for EPDM, its outstanding weather resis- leakage, and the tyre is lined with a compound tance has moved it into tyre sidewalls in spite resistant to air diffusion. Low air diffusion is, of its poor compatibility with the highly un- of course, the primary Hner requirement. How- saturated rubbers normally used in tyres. ever, since this characteristic must last for the Perhaps the best way to picture the symbiotic life of the tyre, resistance to reversion in the relations of these rubbers is to examine in presence of hot air under pressure becomes some detail the construction of a passenger very important. Such a liner may be com- tyre. pounded from blends of chlorobutyl and poly- The carcass of a tyre is the main strength isoprene rubbers, with oil and resins as addi- member — it is the mass of cord plies and rub- tional ingredients. ber that must flex and twist to transform The sides of a tyre also receive considerable engine energy into vehicle motion and to ab- punishment, so it is necessary for some protec- sorb the shocks and vibrations of that motion. tion to be provided for the carcass. This is the The carcass compound must provide an true function of the sidewall. There is an es- excellent bond to the fabric, and, because of thetic factor as well, since the tyre must be the rise in temperature, heat durability is a sold in a highly competitive market. Thus, the prime consideration. The current answer to tyre must present a pleasing appearance and these requirements is a low-black compound must resist the effects of long-term weathering. with a substantial amount of natural rubber or More importantly, however, it must continue equivalent polyisoprene, along with SBR and to protect the carcass against scuffs and bruises, polybutadiene. The polyisoprene contributes and it must not fatigue from constant flexing. good processing, resilience and strength. The Hot SBR, with its freedom from proi-oxidant SBR and polybutadiene distinctly improve catalysts, has been the preferred rubber for ageing, and the polybutadiene also helps tyre sidewalls. Recently, EPDM has made its resilience. initial entry into tyres on a commercial scale The tyre tread is that part of the tyre with by providing added weathering protection, which we are most familiar. If the average which is further enhanced by the use of sub- motorist knows little else about the tyres on stantial amounts of anti-ozonants and waxes. his automobile, he usually is quite concerned Thus in the whole tyre, there is now an about the rate at which the tread is wearing. assortment of rubbers, used in a variety of This factor — abrasion resistance — is the compounds, each designed to do its own job, prime consideration in a tread compound. But yet each engineered to energistically balance there must also be freedom from groove crack- with its neighbours for the elimination of ing. And, of course, the principal function of points of stress within the structure. Each com- the tyre is traction, which the motorist seldom pound must also be so designed that proces- thinks of — except when it is missing. sing is reasonably easy, that building can be These requirements are satisfactorily met by conveniently accomplished, and that each part compounds that are higher in black and ex- of the tyre will reach optimum cure at the same tended with oil. The use of blends of SBR and time. polybutadiene in passenger tyre treads has Our attention has been directed largely to become widespread in recent years. The poly- the passenger tyre, but similar considerations butadiene content has ranged up to 40 or 50 apply to other types, such as truck, off-the- per cent of the total rubber, but such treads road and aeroplane tyres. Natural rubber is do not have acceptable side-slip characteristics used to a larger extent in these types, although on wet roads. The compromise that gives recent developments permit the use of syn- superior treadwear without side slip seems to thetic rubber in the treads of aircraft tyres for be near the level of 25 per cent polybutadiene. a greater number of landings before retreading In the United States today, inner tubes for becomes necessary. 218 JAMES D. D'IANNI: Symbiotic Relations Between Natural and Synthetic Rubber

Tyre engineering has become a highly sophis- the problems encountered in production. Later, ticated activity from the standpoint of both became available in ample quantity, materials and design. Substantial improve- but EPDM appears to be holding on to some ments in treadwear and road-handling charac- of the applications acquired during the Neo- teristics have been obtained recently by wide prene shortage. tread bias/belted constructions with fibreglass Although synthetic rubbers with exceptional in the belt and polyester or other cord in the properties, such as oil resistance, weathering carcass. Such constructions appear to offer a resistance, high-temperature resistance, etc., happy compromise which includes the best have found many specific applications, it should features of both the bias-ply and the radial- also be recognised that natural rubber conti- ply constructions, and provide the motorist nues to be a very important factor in the indus- with a substantial reduction in tyre cost per trial rubber products area. The following are mile. illustrations of such applications in which sub- stantial amounts of natural rubber are used: RUBBER IN INDUSTRIAL PRODUCTS 1. A roll of conveyor belting weighing Rubbers of all varieties are employed in that 32000 pounds and standing nearly 13 broad category commonly called industrial feet in height. It contains more than a rubber products, which include hose, belts, quarter of a mile length of inch-thick, gaskets, seals, rubberised fabrics and many 42-inch wide belting to carry crushed others. Natural rubber is widely used in many limestone. of these products. In some products, the more 2. A network of conveyor belting which is expensive synthetic rubbers are used in whole stockpiling various sizes of sand and or in part because certain performance specifi- cations can be met in no other way. gravel at a Texas aggregate plant. With the low cost of natural rubber that has 3. A rubber conveyor belt used in the prevailed during the past year, one might ex- cookie manufacturing tine of a Cali- pect that many products now made with syn- fornia bakery. thetics, especially the non-oil-resistant types, 4. Belts 450 feet in length used as moving would be converted to the use of natural rub- sidewalks to transport thousands of ber. Such changes are more difficult to make passengers daily at the San Francisco than it appears. Industrial products are estab- International Airport. They are the lished on the basis of tests and specifications longest moving sidewalks in the United to define their performance. Before a change States. can be made, a great deal of evaluation by the 5. Belts used as moving sidewalks at the manufacturer and the customer is in order. Festival of Gas Pavilion at the recent Many processing changes may be involved, New York World Fair. Eighteen of such as in compounding, milling, extruding, them were used to move millions of mould lubrication and a host of others. visitors from place to place. An interesting recent example of the com- petition between rubbers is that which involved 6. A complicated conveyor system built Neoprene and EPDM. Because of a shortage and assembled at our new Marysville, of supply of Neoprene a couple of years ago, Ohio plant, for sorting huge quantities a number of rubber products were changed of limestone rock used in road building. over to EPDM as the basic rubber. These 7. A collapsible container, called a Van- included automobile trunk strips, window Tank, filled with 1100 gallons of latex channel strips, irrigation ditch liners and which is being loaded aboard a freighter others. The working out of a proper cure of at Baltimore Harbour. the new synthetic was difficult, but its lower 8. Rubber sleeves manufactured in one of price and suitable properties for these applica- our Akron plants to be used as flexible tions offered considerable incentive for solving connections between pipes. 219 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

9. Rubber treads for military vehicles such cyclisation,hydrochlorinationandchlorination. as tanks, manufactured in our plants at Cyclised natural rubber (Pliolite) is used as a St Marys, Ohio and Muncie, Indiana. moisture-resistant paper coating and rubber 10. Natural rubber mountings for hay rake hydrochloride (Pliofilm) as a food-packaging teeth, with a several hundred percentage film. Chlorinated rubber (e.g., Parlon) enjoys increase in life span. success in speciality paint applications. Des- 11. Rubber dock fenders, 19 feet in length, pite the large amount of effort to develop to protect both ship and pier at Port other rubber derivatives, these three are the Everglades, Florida. only ones to have enjoyed some commercial 12. Flexible highway posts, which will take success. bumps from a car with little or no da- mage to either the car or the post. APPLICATIONS IN FOAM RUBBER 13. Fenders to protect the bow and sides of The foam rubber industry, which started out a deep sea tug. The ship illustrated was about thirty-five years ago with natural rubber fitted with more than 16 000 pounds of latex as its basic raw material, represents an fenders. excellent example of symbiosis. Most latex 14. Truck seat and air-springs in combina- foam articles today are made from a blend of tion to provide a comfortable ride for natural and synthetic rubber latices, designed long periods. Neoprene and natural rub- to provide the desired properties at the lowest ber are used together in the air-spring, cost. The hot SBR latices available in the early with the natural rubber providing 1950's did not penetrate this field to any great superior flex properties. extent. Foam rubber required the high stress/ These are just a few of the illustrations one strain values and excellent wet gel characteris- could mention of the use of natural rubber in tics of natural latex to make a foam of good a variety of rubber products. With all the com- quality, and could tolerate only 10 to 20 per pounding experience that has accumulated cent of the lower cost synthetic. However, the over the many decades that natural rubber has situation was changed when improved syn- been used, it would be surprising to run into thetic latex became available from the low- an entirely new compounding problem. How- temperature polymerisation process. ever, in the example of tank treads just cited, Urethane foam, which had its early develop- the plant chemists called to our attention a ment in Germany, started to make inroads porosity condition that existed in the centre of into the higher-priced latex foam. The rapid the rubber tracks. Many formulations from the increase in use of urethane foam was en- available technical literature were evaluated couraged by the dramatic decrease in price of with very little success. Finally, a team of re- its raw materials, especially TDI (tolylene di- search chemists and physicists tackled the isocyanate), which dropped from more than problem and evolved a new concept of sulphur- $1.00 per pound in 1955 to less than 40 cents. accelerator ratio that not only produced the Substantial savings were also realised by sub- desired uniformity in cure but also improved stituting lower cost polyethers for the more ex- other physical properties. pensive polyesters recommended in many of The unique shear and damping characteris- the original formulations. tics of vulcanised rubber are responsible for As one might expect, companies that sup- its application as bearings for bridges and plied latex foam also began to manufacture highway structures. Natural rubber now shares urethane foam, and in several cases dropped with Neoprene the position of having been latex foam completely and either went out of accepted for this application by the American the 'cushioning market' completely or turned to Association of State Highway Officials. urethane foam. Goodyear was one company The transformation of natural rubber into that maintained its interest in latex foam and useful derivatives by chemical reactions has also manufactured urethane foam. The interest been carried out commercially in three areas — in latex foam was strongly supported by the 220 JAMES D. D'IANNI: Symbiotic Relations Between Natural and Synthetic Rubber timely development in their research laboratory uniform gelling, which is a processing advan- of a new 'cold' polymerised, large particle size tage, is attributed to the synthetic component. SBR latex which sold at a lower price than The important physical properties of moul- natural latex at the time, and made latex foam ded latex foam made of 100 per cent natural more nearly competitive with urethane foam. rubber, 100 per cent SBR and a 50/50 blend of This latex, known commercially as Pliolite natural rubber and SBR, have been sum- Latex 5352, was also sold to other manu- marised in Table 4. facturers of latex foam to provide them with a lower cost raw material. In the United States TABLE 4. PHYSICAL PROPERTIES OF natural rubber latex, which sold at a substan- MOULDED LATEX FOAM tially higher price, lost its hold on the foam rubber market. SBR/ Foam manufacturers in Europe did not have Property Natural Natural SBR these lower-priced, large particle size synthetic latices readily available and did not convert to them as quickly as did American manufac- Stock efficiency 100 100 100 turers. The price of natural rubber latex then began to drop, so there was less incentive to Odour 90 95 100 change, and today foam rubber produced in Tensile/Elongation 100 75 50 Europe contains a higher percentage of natural rubber. In America the natural/synthetic ratio Permanent set 95 97 100 in latex foam averages 15/85, but in Europe Cigarette burn resistance 100 100 95 it is approximately 50/50. Flex life 100 100 100 One questions why a rubber foam manufac- turer does not shift his raw materials more quickly to take advantage of lower cost latex. One very important consideration is mould Ratings of 100 for different physical properties size, which is related to shrinkage during gel- are found in each of the three types. The ten- ation and subsequent vulcanisation. The excel- sile/elongation rating of foam rubber contain- lent wet gel strength of natural rubber, which ing 100 per cent natural is twice that of the is a plus factor in almost all of its applications, 100 per cent SBR. In a few applications, this is a negative factor in this case. In order to property is a significant advantage, such as change to natural rubber latex, a foam producer when foam slab is being pulled around a rigid has to increase the size of his moulds at sub- base in order to cushion the edge. However, stantial additional expense. Around the 50/50 for practically all applications the stress/strain natural/synthetic ratio, some adjustment up- properties of the 100 per cent synthetic foam ward to a 70/30 ratio is possible with little are adequate. effect on the shrinkage ratio. Table 5 lists some of the processing charac- Foam rubber manufacturers would probably teristics of latices when converted to foam rub- agree that a 50/50 natural/synthetic blend is ber. The volume shrinkage of SBR is less than optimum, but costly mould replacement would that of natural, a property which allows easier be required for those now making foam with stripping from core type moulds. The other a higher synthetic content to make the change. properties, especially uniformity of processing, After the foam industry learned to use the favour synthetic. 'cold'polymerised large particle size SBR latex, Urethane foam outsells latex foam by an the foam was found to have certain advantages approximate 2 to 1 margin based on volume, such as better ageing, improved permanent set though the ratio is more than reversed, based properties, and improved odour. Natural rub- on tonnage or weight. However, the higher ber is retained for improved cigarette burning quality cushioning properties of latex foam resistance and higher stress/strain values. More have enabled it to hold a substantial portion 221 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

TABLE 5. PROCESSING CHARACTERISTICS tion of shock-absorbent foam for this appli- OF MOULDED LATEX FOAM cation. Although these cushions are relatively small, an unusually high foam density is re- Properties Natural SBR/ SBR quired for satisfactory performance. The higher Natural density also requires a higher accelerator level to give a satisfactory cure in the same factory equipment as that used for flexible cushioning Volume shrinkage, % 30 20 15 foam. Cure rate Several other products utilise natural rubber (min at 100°Q 14 12 10 latex in spite of the increasing competition Maturing of from synthetic sources. Rubber thread for latex compound Yes Yes No clothing is one area which urethane, with its excellent ageing resistance, threatened to cap- De-ammoniation Yes Yes No ture a few years ago. Natural rubber thread, Processing uniformity Good Very Excellent with low permanent stretch and more constant good modulus, is maintaining a substantial share of this market. Scrim adhesives for carpeting are made of this market and this situation is likely to with both natural and synthetic latices, in an continue. application where maximum pigment loading A new development relating to latex foam is an important requirement. Natural rubber rubber has made its appearance in several latex enjoyed all of this business until a few American automobiles. This is an impact-resis- years ago when it was found that certain syn- tant foam rubber cushion that is positioned thetic latices, especially the carboxylic types behind the front seat of the automobile and permitted even higher loadings of pigments and which, in case of an accident, protects the thus lowered overall costs — symbiosis at its heads and faces of people riding in the rear best. seat. Although latex foam rubber is well noted High density foam rubber as carpet backing for its excellent resilience, this new type of is becoming a high volume application in foam has a completely opposite characteristic America. Natural latex blended with synthetic — it is shock-absorbent rather than resilient, and 100 parts or more of low-cost pigment and this property extends over a wide range comprise the basic ingredients for this stock, of temperatures. The rebound characteristics which has a finished density of 10 to 11 pounds change very little as the temperature varies per cubic foot (160 to 177 kilograms per cubic from -20°F (-29°C), which is not too un- meter). This is an excellent example of the common in the northern regions of North blending of the best qualities of both types — America, up to 120°F (49°C) which the interior the mechanical stability of the synthetic latex of a closed automobile may reach in the sum- and its ability to bind a large quantity of low- mer. The shock-absorbing ability of this latex cost pigment, combined with the excellent tack foam is much superior to that of its competitor, and superlative tensile/elongation values of the urethane foam. Although at present the compo- natural component. sition of the shock-absorbing foam is 100 per Fire and flame resistance will be increasingly cent synthetic, the formulation can be adjusted needed in many foam rubber articles of the to accommodate up to 50 per cent natural future. The world-wide trend toward greater rubber latex. safety in products used in the home and auto- Another application for this foam is for mobile will require flame-resistant cushioning head rests. Resilient foams are now used for materials, and development of satisfactory this purpose but because of serious neck and formulations by compounding is well under back injuries resulting from 'backlash' in auto- way. The work of the Natural Rubber Pro- mobile collisions, there is a move in the direc- ducers' Research Association reported several

222 JAMES D. D'lxNNi: Symbiotic Relations Between Natural and Synthetic Rubber years ago on the reaction of natural rubber then coated and baled. Similar finishing tech- latex with trichlorobromomethane is also niques and equipment are used for cw-poly- significant (COCKBAIN et al, 1963). Although butadiene and in some cases for SBR. foam rubber can be made flame-resistant by About ten years ago, our natural rubber this modification, the cost is high and ageing technologists decided to study processing tech- properties are adversely affected. niques of this type and their possible appli- cation on the plantation. The first experiments RUBBER PROCESSING TECHNIQUES in Akron were with natural rubber made from It is encouraging to note the accelerated pace preserved latex and from quantities of wet cup- with which natural rubber producers are ex- lump sent from Sumatra and Costa Rica. panding their output of new process, tech- Natural rubber was found to be tougher and nically specified forms of natural rubber. It is built up more heat going through the drier. a sure indication of a magnificent job in im- Synthetic polymer passed through the screw proving technically to meet competition. BATE- quite easily, but natural rubber tended to MAN (1968) has estimated that these improved adhere in spots and build up into oxidised natural rubbers will top the 100 000-ton mark lumps. Working with several engineering com- in 1968. panies, we finally developed a machine that The symbiotic relations between natural and would commercially dry natural rubber effec- synthetic rubber are well illustrated in our new tively. This equipment, preceded by a washing venture in Sumatra which employs finishing and dirt-removal unit, is now in operation at equipment originally designed and supplied for the Dolok Merangir Estate in Sumatra. our production of cw-polyisoprene and other stereopolymers made by solution polymerisa- AID TO MEDICAL RESEARCH tion processes. Cw-polyisoprene is prepared in an aliphatic Before terminating this discussion, I feel that hydrocarbon solvent and the viscous polymer I should inject one additional facet of rubber solution is mixed into hot water with vigorous symbiosis, namely, that man has done much agitation to volatilise the solvent and unreacted to make rubber better through the years, and monomer. At this stage the rubber is in large, rubber has in turn improved man's existence. loosely agglomerated lumps containing about At Goodyear Research, we have one small 50 per cent water, resembling natural rubber but important project in which our rubber as it comes off the washer. experts contribute their know-how in rubber The wet rubber is passed through a dewater- compounding and fabrication to aid our ing machine which consists of a barrel with nation's medical research teams. This work longitudinal slits which allow the water to dates back to some of the pioneering efforts escape. The rubber is forced through the on developing artificial hearts, arteries and machine by screw action and passes through heart valves, starting about 1960. I might a perforated plate at the end. The squeezing illustrate two or three projects. and mechanical working of the rubber at this Perhaps the smallest and simplest of these stage raises the temperature to 200 -220T prosthetic devices is a tiny T-tube, about the size (95 - 105°C). It leaves the machine as small of a dime (or sixpence). These tubes are intended lumps having a reduced water content of to assist in providing a new blood supply to about 10 per cent. damaged heart muscle when a blood clot has The rubber now goes through a devolatiliser, caused a heart attack. which is a screw-type extruder operating at The idea is to quickly insert the tube in the about 400°F (205°C). The rubber is exposed heart muscle as soon after the heart attack to these conditions for only a few seconds, and as possible, so that blood from the ventricle then is forced through another perforated plate may flood the damaged area, until the body where it virtually explodes, expelling the re- can supply a new system of blood vessels. maining water as steam. The rubber pellets are The unit is now being tested in animals.

223 Journal of the Rubber Research Institute of Malaya, Volume 22, Part 2, 1969

Another tiny but very important contri- modulus urethane is used to provide a tough bution deals with clot prevention during the outer shell. This total replacement heart com- implanting of small arteries. Todate, there bines two ventricles, plus appropriate connec- has been no possible replacement for these tions for pressure monitoring. The heart has arteries, because the small tubes quickly deve- been tested in animals, and it performs well. lop clots which flake off and plug the arterial system involved. Our people made a tube of ACKNOWLEDGEMENT conductive rubber with an inner liner of Dacron velour. The velour, with its velvet-like The author wishes to acknowledge the assistance pile surface, successfully builds a neo-intima of many members of the Goodyear Research lining from the blood stream without clot Division, and several Development Divisions, formation. in assembling and organising the data in the Artery segments of this size have previously preparation of this review paper. developed clots within three or four hours of implantation in dogs. Now, the medical re- REFERENCES searchers replace these arteries with confidence BAILEY, J.T., BISHOP, E.T., HENDRICKS, W.R., HOLDEN, that clotting will not be a problem. G. AND LEGGE, N.R. (1966) Thermoplastic elasto- The heart itself is, of course, the target for mers—physical properties and applications, Rubb. complete replacement by these doctors. You Age, 98(10), 69, BATEMAN, L. (1968) Natural rubber—a changing in- will recall the heart function, involving alter- dustry in a changing Commonwealth. Rubb. Dev., nating relaxation and compression action, 2KD, 4. which brings venous blood into the right ven- CHEN, H.Y. (1966) Microstructures of polyisoprenes tricle, then pressures it into the lungs, next by high-resolution nuclear magnetic resonance. accepts the oxygenated blood into the left J. Polym. Sci. B. Polym. Letters, 4(11), 891. COCKBAIN, E.G., PENDLE, T.D., POLE, E.G. AND ventricle, and finally pressures it into the arterial TURNER, D.T. (1963) Flame-resistant halogenated system. rubber, Proc. 4th Rubb. Technol. Conf. London This same type of action is accomplished by 1962, 498. ventricle constructions where compressed air INTERNATIONAL RUBBER STUDY GROUP (1967) Table 54 squeezes the blood-containing inner sac. Here, Area under plantation rubber (acres). Rubb. statist. Bull., 21(11), 44. we have a combination of natural and ure- SEKHAR, B.C. (1961) Inhibition of hardening in natural thane rubbers. The natural rubber provides rubber. Proc. nat. Rubb. Res, Conf. Koala Lumpur the soft, flexible inner chamber, while a high 1960, 512.

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