Symbiotic Relations Between Natural and Synthetic Rubber
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J. Riibb. Res. Inst. Malaya, 22(2), 214-224 (1969) Symbiotic Relations Between Natural and Synthetic Rubber JAMES D. D'lANNI The Goodyear Tire & 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 natural rubber 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 215 COPYRIGHT © MALAYSIAN RUBBER BOARD 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.