Rubber, Gutta, and Chicle

With the multiplicity of elastomers now available, a definition for rubber is difficult to formulate. One that many would accept is that developed by the American Society for Testing and Materials (2). 

A rubber in its modified state, free of diluents, retracts within 1 min to less than 1.5 times its original length after being stretched at room temperature (18° to 29 °C) to twice its length and held for 1 min before release.  

The ability to put rubber into solution was followed up by a Scotsman, Macintosh, who made a waterproof fabric suitable for raincoats (14). He dissolved rubber in benzene and naphtha, then placed the dried sheet between two layers of fabric. 

By 1900, rubber acreage was estimated at 5000 acres in Southeast Asia, mostly on estates. World rubber consumption was about 54000 tons by 1915 this had grown to an estimated 162000 tons and to 838000 tons in 1930 (3). About 66% of this came from plantations or estates, the remainder from smallholders. These are private individuals who farm 40 hectares or less (approximately 100 acres). In the same period, there were wide fluctuations in its price. The average spot price in the London market for a popular grade -RSS 1 was as low as 47.7 Malaysian cents per kilogram in 1930 from a high of 969.7 in 1910 (3). 

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Rubber, gutta, and chicle are 1,4-polyisoprenes of different molecular weights and geometries. These products are of great commercial significance and are discussed in some detail in Chap. 10. 

The latex of the Sapota achras yields a thermoplastic material, chicle, consisting of about 17.4% hydrocarbon, 40% acetone soluble resin and 35% occluded water. The hydrocarbon appears to contain both trans- and c/s-polyisoprene. Although originally introduced as gutta pereha and natural rubber substitutes, deresinated chicle has become important as the base for chewing gum. Like other polyisoprenes, it is meeting competition from synthetic polymers. 

Chromenes, Benzofurans, Benzopyrans, and Precocenes Polyterpenes Biosynthesis Biological Activity Natural Rubber Hevea brasiliensis Guayule Gutta Percha Chicle... 

Chicle, a mixture of Z- and -isomers of polyisoprene is isolated from Manilkara Achras) sapota (Sapotaceae). The biosynthesis of this material does not appear to have been examined. There is little doubt that there are two stereospecific enzyme systems metabolizing IPP in chicle latex, one producing rubber (2) and the other yielding gutta (Chari-wood and Banthorpe, 1978). Chicle is one of the few known examples where a mixture of the two structures occurs (Archer and Audley, 1973). Chicle, which is commonly used in chewing gum, has been partially replaced by synthetic substitutes (Loomis and Croteau, 1980). 

Of the polyterpenes rubber, gutta-percha, and balata, our discussion will be restricted to the technically most important product, rubber. About 2,000 species of higher plants produce rubber but only a few of them, mainly from the families Apocynaceae, Asclepiadaceae, Compositae, Euphorbiaceae, and Moraceae, do so in sufficient quantity as to make its extraction technically worthwhile. A few of the more important species are listed in Table 5. The main source of rubber is the rubber tree Hevea brasiliensis. Achras sapota, the chicle of which provides the basic ingredient of chewing gum, is also included in the list. 

Cellulose not only provides the support substance of wood, but also that for all other plants. Flax stalk husks provide linen and hemp is obtained from the leaves of the sisal plant. Cotton fibers are recovered from the seed hairs of the cotton bush. What are known as vegetable gums, which are also polysaccharides, are to be found in other seeds and algae. Starch is recovered from grain seeds, and starch is a mixture of the polysaccharides amylose and amylopectin. Pectin, an acidic polysaccharide, is provided by fruits. Latices containing the polyprenes natural rubber, balata, gutta percha, or chicle are obtained from certain trees and plants. 

Polyprenes are oligomers and polymers of isoprene that occur in over 2000 plants in nature. The naturally occurring l,4-/ran5-poly(isoprene) is known as gutta percha or balata, and the l,4-cw-poly(isoprene) is called natural rubber. Chicle is a mixture of l,4-/ran5-poly(isoprenes) and triter-penes which are substances consisting of six isoprene units. 

Rubber is all-cw-polyisoprene of mixed MW, from less than 10 to about 4 X 10 (Archer and Audley, 1967). Gutta is traw5-polyisoprene of lower MW (Archer and Audley, 1973). Chicle, the traditional chewing gum base, is a mixture of low-MW cis- and tra 5-polyisoprenes, together with acetone-soluble resins (Archer and Audley, 1973). 

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