Natural rubber reaction efficiency

Figure 1.4 Vulcanization of natural rubber with sulfur, (a) Linear polyisoprene (natural rubber), (b) An idealized structure produced by vulcanization with sulfur. The number (x) of sulfur atoms in sulfide cross-linkages is 1 or 2 in efficient vulcanization systems but may be as high as 8 under conditions where cyclic and other structures are also formed in the reaction, (c) The effect of cross-linking is to introduce points of linkage or anchor points between chain molecules, restricting their slippage.

The second example of a polymer reaction is the industrial cross-linking of rubber by vulcanization sketched in Fig. 3.50. The process was invented already in 1839 by C. N. Goodyear without knowledge of its chemical stracture. Natural rubber is cis-poly(l-methyl-1-butenylene) or polyisoprene with a low glass transition temperature of about 210 K. Its structure and those of other rubbers are given in Fig. 1.15. The addition of sulfur in the form of Sg rings and heating causes the vulcanization. Of the listed cross-hnks in Fig. 3.50, only the left example is an efficient network former. The sulfur introduces about 1 cross-link for each of 50 S-atoms used. Modem vulcanization involves activators and accelerators for increased efficiency. The detailed mechanism is rather complicated and not fully understood. 

Few monomeric radicals are lost by coupling with polymeric radicals when dialkyl peroxides are used as the curative. Also, if the elastomer is properly chosen, the scission reaction is not excessive [82-88]. For dicumyl peroxide in natural rubber, the crossUnking efficiency has been estimated at about 1.0. One mole of crosslinks is formed for each mole of peroxide crossUnking is mainly by the coupling of two polymeric radicals [89,90]. One peroxide moiety gives two monomeric free radicals, which react with rubber to give two polymeric radicals that couple to form one crosslink. 

In comparison with the developments in new rubbers that have occurred this century, developments concerned with the chemistry of the reactions of the already-formed rubbery polymers have been less immediately spectacular. It has already been pointed out that for about ISO years sulphur has been the dominant vulcanizing agent, almost exclusively used with diene rubbers. It must however be stressed that the efficiency with which the sulphur is used and the quality of the vulcanizates is today vastly superior. In part this is due to systematic semi-empirical studies which led to the development of a wide range of accelerators of vulcanization. It is also in part due to the excellent scientific studies undertaken by many chemists throughout the world but particularly by the Malaysian Rubber Producers Research Association (and its forerunners the Natural Rubber Producers Research Association and the British Rubber Producers Research Association). As a consequence of this work the mechanism of vulcanization and its control, at least in the major diene rubbers, is reasonably well understood. 

In recent years there has been increasing interest, particularly by research workers at the Malaysian Rubber Producers Research Association, (MRPRA), in certain reactions that involve the direct reaction between molecules and do not involve active species such as ions and free radicals as intermediates. Among the advantages of such reactions are the fact that catalysts are not involved and that they proceed smoothly, predictably and in a reasonably efficient manner. The reactions of particular interest are those which involve highly substituted ethylenes. These are of particular relevance to polyisoprenes such as natural rubber, which may be considered to be a poly-trisubstituted ethylene, as a means of introducing interesting groups into the polymer structure. 

The vulcanisation of natural rubber with the efficient accelerator N-cyclohexyl-2 benzothiazolesulphenamide, in the presence of zinc oxide, stearic acid, and 4,4 -dithiobismorpholine (DTBM), was studied with a disc oscillating curemeter to determine the causes of the pronounced plateaus in the cure curves observed with this and similar systems. It was determined that this behaviour was not caused by the formation of an inhibitory byproduct but was due to the occurrence of at least two distinct crosslinking reactions. DTBM was shown to act as a sulphur donor and not, as initially suspected, as an inhibitor. 14 refs. 

Plasticizers can be classified according to their chemical nature. The most important classes of plasticizers used in rubber adhesives are phthalates, polymeric plasticizers, and esters. The group phthalate plasticizers constitutes the biggest and most widely used plasticizers. The linear alkyl phthalates impart improved low-temperature performance and have reduced volatility. Most of the polymeric plasticizers are saturated polyesters obtained by reaction of a diol with a dicarboxylic acid. The most common diols are propanediol, 1,3- and 1,4-butanediol, and 1,6-hexanediol. Adipic, phthalic and sebacic acids are common carboxylic acids used in the manufacture of polymeric plasticizers. Some poly-hydroxybutyrates are used in rubber adhesive formulations. Both the molecular weight and the chemical nature determine the performance of the polymeric plasticizers. Increasing the molecular weight reduces the volatility of the plasticizer but reduces the plasticizing efficiency and low-temperature properties. Typical esters used as plasticizers are n-butyl acetate and cellulose acetobutyrate. 

A wide variety of redox reactions between metals or metal compounds and organic matter may be employed in this context. Because most of them are ionic in nature, they may be conveniently carried out in aqueous solution and occur rather rapidly even at relatively low temperatures. As a consequence, redox systems with many different compositions have been developed into initiators that are very efficient and useful, particularly for suspension and emulsion polymerization in aqueous media [2], which is dealt with in detail in Chapter 6. The low-temperature (at 5°C) copolymerization of styrene and butadiene for the production of GR-S rubber was made possible with the success of these catalytic systems. 

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