Rubber tree

Natural rubber is obtained from the bark of the rubber tree, Hevea brasiliensis, grown on enormous plantations in Southeast Asia. 

Rubber—an unusual name for an unusual substance—is a naturally occurring aikene polymer produced by more than 400 different plants. The major source is the so-called rubber tree, Hevea brasiliensis, from which the crude material is harvested as it drips from a slice made through the bark. The name rubber was coined by Joseph Priestley, the discoverer of oxygen and early researcher of rubber chemistry, for the simple reason that one of rubber s early uses was to tub out pencil marks on paper. 

Although rubber originally meant a natural thermoset material obtained from a rubber tree, with the development of plastics it identifies a thermoset elastomer (TSE) or thermoplastic elastomer (TPE) material. Different properties identify the elastomers such as strength and stiffness, abrasion resistance, solvent resistance, shock and... 

FIGURE 19.8 Collecting latex from a rubber tree in Malaysia, a principal producer of latex. 

Rubber is a polymer of isoprene (15). Natural rubber is obtained from the bark of the rubber tree as a milky white liquid, which is called latex (Fig. 19.8) and consists of a suspension of rubber particles in water. The rubber itself is a soft white solid that becomes even softer when warm. It is used for pencil erasers and was once used as crepe rubber for the soles of shoes. 

Natural rubber latex, obtained from rubber trees, is converted to its final form by a process known as vulcanization, first discovered by Charles Goodyear in 1839. Vulcaiuzation is basically a crosslinking reaction of double bonds in the latex structure with sulfur. The polymerization of butadiene with itself or with other vinyl monomers results in a material that like natural latex, still contains double bonds. Thus, synthetic rubber made from butadiene can be processed and vulcanized just like natural rubber. 

Isoprene may be the naturally occurring alkene with the greatest economic impact. This compound, a major component of the sap of the rubber tree, is used to make the long-chain molecules of natural rubber (polyisoprene). As we describe in Chapter 13. the synthetic rubbers that make up most of today s tires are made from other alkenes. 

The first alkene polymer to be used in society was polyisoprene, a natural product extracted from the sap of rubber trees. See our Box for a description of the history of rubber. The monomer from which this polymer is constructed... 

The hydroxynitrile lyase (HNL)-catalyzed addition of HCN to aldehydes is the most important synthesis of non-racemic cyanohydrins. Since now not only (f )-PaHNL from almonds is available in unlimited amounts, but the recombinant (S)-HNLs from cassava (MeHNL) and rubber tree (HbHNL) are also available in giga units, the large-scale productions of non-racemic cyanohydrins have become possible. The synthetic potential of chiral cyanohydrins for the stereoselective preparation of biologically active compounds has been developed during the last 15 years. 

Neoprene, Carothers first practical invention, was made reluctantly, as a kind of side issue to his scientific investigation of polymers. Synthetic rubber was of great commercial interest. The car-happy United States used half the world s natural rubber, and demand had outstripped the supply from wild rubber trees in the Amazon. Price fluctuations on British rubber plantations in Southeast Asia provided further incentive for the development of synthetic substitutes. Du Pont had been trying without success to... 

Hasslacher, M., Schall, M., Hayn, M. et al. (1997) High-level intracellular expression of hydroxynitrile lyase from the tropical rubber tree Hevea brasiliensis in microbial hosts. Protein Expression and Purification, 11, 61-71. 

In 1994, the worldwide consumption of rubber was approximately 14.5 million tons a year, of which about 40% consisted of natural rubber. Natural rubber is produced as latex by tropical rubber trees (Hevea brasiliensis). It is processed locally and therefore the quality of natural rubber fluctuates remarkably [ 140]. Due to increasing demand for rubbers, combined with a decreasing production capacity in Asia and a vast increase in labor costs, the price of natural rubber is still rising sharply. In 1990-1994, the average price of natural rubber was about 0.38 /lb, while in 1996 it was already over 0.80 /lb. The remaining 60% of the articles were manufactured from synthetic petroleum-based rubbers such as isoprene rubber, styrene-butadiene rubber, chloroprene rubber and polyurethanes. The quality of synthetic rubbers is constant, and their price varies between 2 and 5 US per kilogram [137-140]. 

A term applied to clones of Hevea trees which give a much higher yield of latex than ordinary rubber trees. The average yield in the 1950s was under 500 kg of rubber per hectare per annum. It is now more common to achieve a two- to threefold increase on these figures from modem planting material. 

The opening of a second tapping panel on rubber trees at a height of about 2.5 m above the ground, access to which requires the use of a ladder or long-handled tapping knife. 

The rubber may be natural, in which case the latex is produced by the rubber tree. Latex of the main synthetic rubbers is produced by the technique of emulsion polymerisation. The term latex has been broadened in recent years and a general definition is now a stable dispersion of a polymeric substance in an aqueous medium . Latices may be classified as natural (from trees and plants), synthetic (by emulsion polymerisation) and artificial (by dispersion of the solid polymer in an aqueous medium). They may also be classified according to the chemical nature of the polymer, e.g., SBR, nitrile, polychloroprene, etc. 

A disease of rubber trees affecting the pods and leaves. The resulting pod-rot and leaf-fall seriously reduces the latex yield. It is common in most rubber growing countries with, until recently, the exception of Malaysia, where urgent... 

The annual leaf loss suffered by rubber trees. It coincides with the dry season in February and March. Since the trees are then using their reserves for refoliation there is a temporary drop in yield, and changes occur in the composition of the latex. 

The application of plant hormones to the bark of rubber trees, or of copper sulphate placed in holes bored in the trunk, can produce a consistent increase in yield without any damage to the tree or any deleterious effect on either the latex or the rubber produced from it. 

Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

Sharpies, A., Diseases and Pests of the Rubber Tree, Macmillan, London,... 

Weir, J. R., Pathological Survey of the Para Rubber Tree (Hevea brasiliensis)... 

In 1838 Macintosh and Hancock at Goodyear discovered how to take tacky naturai rubber from rubber trees and react it with suifur in the presence of heat to vuicanize the rubber to a nonstick compound that couid be usefui for items such as boots, rain coats, and tires. Synthetic rubber research started between Worid Wars I and II and progressed very quickly after World War II. The modern birth of soiid synthetic poiymers for commerciai products may be traced to Hyatt in 1868. He discovered how to react cellulose nitrate and camphor to produce a hard piastic that was used to fabricate billiard balls because ivory had become scarce. 

Rubber comes from trees that grow in the tropics.The rubber tree has a type of sap, called latex, which oozes out of the bark and can be collected without hurting the tree. Rubber has been used by Indians in North America, Central America, and South America for hundreds of years to make balls and waterproof clothes. When Europeans came to the Americas, they used rubber for the same things.They also used rubber to rub out pencil marks, which is where the name comes from. 

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