Synthetic polyisoprene rubbers development

Between the 1920s when the initial commercial development of mbbery elastomers based on 1,3-dienes began (5—7), and 1955 when transition metal catalysts were fkst used to prepare synthetic polyisoprene, researchers in the U.S. and Europe developed emulsion polybutadiene and styrene—butadiene copolymers as substitutes for natural mbber. However, the tire properties of these polymers were inferior to natural mbber compounds. In seeking to improve the synthetic material properties, research was conducted in many laboratories worldwide, especially in the U.S. under the Rubber Reserve Program. 

The discovery by Ziegler that ethylene and propylene can be polymerized with transition-metal salts reduced with trialkyl aluminum gave impetus to investigations of the polymerization of conjugated dienes (7—9). In 1955, synthetic polyisoprene (90—97% tij -l,4) was prepared using two new catalysts. A transition-metal catalyst was developed at B. E. Goodrich (10) and an alkaU metal catalyst was developed at the Ekestone Tke Rubber Co. (11). Both catalysts were used to prepare tij -l,4-polyisoprene on a commercial scale (9—19). 

With the availability of polymerization catalysts, extensive efforts were devoted to developing economical processes for manufacture of isoprene. Several synthetic routes have been commercialized. With natural rubber as an alternative, the ultimate value of the polymer was more or less dictated by that market. The first commercial use of isoprene in the United States started in 1940. It was used as a minor comonomer with isobutylene for the preparation of butyl mbber. Polyisoprene was commercialized extensively in the 1960s (6). In the 1990s isoprene is used almost exclusively as a monomer for polymerization (see Elastomers, synthetic-polyisoprene). 

The development of the Ziegler-Natta catalysts has affected rubber production as well. Eirst, it facilitated the synthesis of all-c/s polyisoprene and the demonstration that its properties were nearly identical to those of natural rubber. (A small amount of synthetic natural rubber is produced today.) Second, a new kind of synthetic rubber was developed all-c/s polybutadiene. It now ranks second in production after styrene-butadiene rubber. 

In view of the wide application of Py—GC in industry and research, the development of techniques and equipment for automatic analysis by this method is of great practical interest. An automatic Py—GC system was developed by Coulter and Thompson [69] for Curie-type cells with a filament for specific application in the tyre industry. A typical analysis involves the identification and determination of polymers in a tyre material sample. The material of a tyre is essentially a mixture of polymers, most often natural rubber (polyisoprene), synthetic polyisoprene, polybutadiene and butadiene-styrene copolymer. A tube is normally made of a material based on butyl rubber and a copolymer of isobutylene with small amounts of isoprene. In addition to the above ingredients, the material contains another ten to twelve, such as sulphur, zinc oxide, carbon black, mineral oil, pine pitch, resins, antioxidants, accelerators and stearic acid. In analysing very small samples of the tyre material, the chemist must usually answer the following question on the basis of which polymers is the tyre made and what is their ratio The problem is not made easier by the fact that cured rubber is not soluble in any solvent. 

Dr. N.R. Legge, 1987 Charles Goodyear medalist and, at that time, director of the synthetic polyisoprene program at Shell Development Company, recalled that it was in attempting to provide a solution to this problem of poor "green strength in alkyl lithium polymerized polyisoprene that the styrenic block copolymers were first synthesized. The functional use of the first block copolymers was not as an identifiable monolithic rubber structure but provided a vital function to another identifiable material and lost its identity in this process. 

Before the advent of infra-red analysis, natural rubber (and gutta percha) was identified by the Weber test which involved bromination. The rubber was reacted with bromine to form a dibromide which was then treated with a solution of phenol in carbon tetrachloride. A violet colouration of the residue developed after gentle boiling of the mixture. (This test also gives positive results for synthetic polyisoprenes and butyl rubber.)... 

We will now apply to natural rubber, i.e., NR, biosynthesis what we have learned so far. As mentioned before, in 2005 approximately 10 million tons of NR was produced worldwide for commercial use, from which about 15% was consumed in the United States. While the United States is self-sufficient in S5mthetic mbber production, with substantial export activities, no NR is produced domestically. The development of a U.S.-based supply of NR is recognized in the Critical Agricultural Materials Act of 1984 (Laws 95-592 and 98-284). The Act recognizes that NR latex is a commodity of vital importance to the economy and the defense of the nation. It is important to emphasize that synthetic polyisoprene does not match the performance of imported Hevea NR in several applications, so NR remains irreplaceable. 

Up to the present time, the market for synthetic polyisoprene has been only a small fraction of the total rubber market. The low cost of SBR and the ready availability, up until now, of natural rubber, have rather discouraged developments with this material. It has therefore tended to be used for those applications where the marginal difference in properties between the synthetic material and natural rubber are most significant. It is probably fair to say that in the 1970s the greatest interest in synthetic polyisoprene was in... 

The main objective of this chapter is to inform rubber technologists how and when to use synthetic high cw-polyisoprene and to attempt, where possible, to link certain observed behaviour with studies made by rubber scientists concerning molecular structure. Emphasis is placed on situations where technical advantage is established for the synthetic polymer, since it is almost invariably sold at a premium to natural rubber and its value is appreciated by rubber manufacturers. Selective usage is the pattern that has developed and correctly so, since synthetic polyisoprene is unlikely to be a cheap replacement for natural rubber in the foreseeable future. 

Natural rubber invariably contains fatty acids which have an activating effect in accelerated sulphur vulcanisation systems. This type of activator is absent in synthetic polyisoprenes and must be added in sufficient quantity (1 -5-2 0 phr) to ensure full crosslink development. Alternative activators of a similar chemical nature, such as zinc stearate or zinc 2-ethylhexanoate, have been used at lower levels (see Section 7). 

The differences between torque values developed by the various synthetic polyisoprenes parallel differences in modulus values and reflect cross-linking efficiency. The higher torque development for natural rubber compared to Natsyn 2200 is believed to be due partly to a different viscoelastic behaviour since, for example, hardness is higher for Natsyn 2200 than for natural rubber and estimates of crosslink densities by swelling measurements show a close similarity. 

The revolutionary development of stereospecific polymerization by the Ziegler-Natta catalysts also resulted ia the accomplishment ia the 1950s of a 100-year-old goal, the synthesis of i7j -l,4-polyisoprene (natural mbber). This actually led to the immediate termination of the U.S. Government Synthetic Rubber Program ia 1956 because the technical problem of dupHcating the molecular stmcture of natural mbber was thereby solved, and also because the mbber plantations of the Far East were again available. 

Other polymeric binders, natural and synthetic, may be found as paints or varnishes in modern artworks and installations. Artists very easily adopt resins developed as industrial coatings or for specialized applications, and use them according to their creative needs. Natural rubber latex is a water dispersion of 1,4-ds-polyisoprene particles where pigments can be added to give coloured paints. By means of Py-GC/MS the presence of these paints can be easily assessed. As shown in Figure 12.13, the principal marker peaks in the pyrogram are those of isoprene, limonene and other cyclic dimers. 

As of this date, there is no lithium or alkyl-lithium catalyzed polyisoprene manufactured by the leading synthetic rubber producers- in the industrial nations. However, there are several rubber producers who manufacture alkyl-lithium catalyzed synthetic polybutadiene and commercialize it under trade names like "Diene Rubber"(Firestone) "Soleprene"(Phillips Petroleum), "Tufdene"(Ashai KASA Japan). In the early stage of development of alkyl-lithium catalyzed poly-butadiene it was felt that a narrow molecular distribution was needed to give it the excellent wear properties of polybutadiene. However, it was found later that its narrow molecular distribution, coupled with the purity of the rubber, made it the choice rubber to be used in the reinforcement of plastics, such as high impact polystyrene. Till the present time, polybutadiene made by alkyl-lithium catalyst is, for many chemical and technological reasons, still the undisputed rubber in the reinforced plastics applications industries. 

The hydrogenation of the centre block of SBS copolymer produced oxidation stable thermoplastic elastomer. This product was commercialized by the Shell Development Company under the trade name of Kraton G. The field of thermoplastic elastomers based on styrene, 1-3-butadiene or isoprene has expanded so much in the last 10 years that the synthetic rubber chemist produced more of these polymers than the market could handle. However, the anionically prepared thermoplastic system is still the leader in this field, since it produced the best TPR s with the best physical properties. These TPR s can accommodate more filler, which reduces the cost. For example, the SBS Kraton type copolymer varies the monomer of the middle block to produce polyisoprene at various combinations, then, followed... 

In 1954, 1,4-cA-polyisoprene, the synthetic equivalent of natural rubber, was obtained in the laboratories of Goodrich-Gulf [22] by isoprene polymerisation with new catalysts developed by Natta, and later on 1,4-trans-polyisoprene, a synthetic analogue of gutta percha, was obtained by Natta et al. [23]. 

Significant developments in synthetic rubber began at this time. Outstanding were the introduction of polychloroprene (neoprene) by Carothers, and of the oil-resistant polysulfide rubber Thiokol by Patrick. These were soon followed by styrene-butadiene copolymers, nitrile rubber, butyl rubber, and various other types, some of which were rushed into production for the war effort in the early 1940s. The stereospecific catalysts researched by Ziegler and Natta aided this development, including synthesis of true rubber hydrocarbon (polyisoprene). Since 1935 synthetic rubbers have been referred to as elastomers. 

Rubber is one of the few examples where chemical synthesis succeeded in a nearly identical performance copy of a natural polymer (polyisoprene) - albeit with a completely different chemical composition (styrene-butadiene-rubber, SBR). Regarding sustainable development, the complete imbalance of the early rubber history has emanated during recent years into equilibrium between natural and synthetic rubber. 

As described in Section 1.1, the first commercial polymers, which were naturally occurring, were polyisoprenes (natural rubber and gutta-percha) and subsequently cellulose derivatives. From the early twentieth century, various totally synthetic polymers were introduced. Farbenfabrrken Bayer introduced bulk polymerized totally synthetic elastomers in 1910. Poly(dimethyl butadiene) synthetic rubber was produced commercially by Bayer in Leverkusen during World War I. The 1920s saw the commercial development of polystyrene (PS) and poly(vinyl chloride) (PVC). In 1934, the IG Farbenindustrie (a combine of Bayer, BASF, Floechst, and other firms) began to commercially manufacture butadiene-acrylonitrile copolymer (N BR) as an oil resistant rubber and in 1937 butadiene-styrene copolymer (SBR) intended for pneumatic tires. 

---