Hevea braziliensis rubber, natural

Natural rubber is obtained by cutting (tapping) the bark of Hevea braziliensis, a wild tree originally found in South America. The rubber is in a milky form containing about 35% latex. The latex is separated by coagulation with a weak acid and it is soluble in a range of solvents, including hydrocarbons, chlorinated hydrocarbons, certain ketones, esters and carbon disulphide. 

NR belongs to a class of compounds known as elastomers [5]. NR was used for centuries by the Mayans in the Western hemisphere before it was introduced into Europe by Columbus. The term rubber was, however, coined by Joseph Priestly. NR is indispensable in our daily lives. The main uses of NR are concentrated in four key areas, namely medical devices, industrial products, domestic and recreational goods, and foremost automobile products. The current elastomer consumption in the world is 18 million t per year [6]. NR supplies about one-third of the world demand for elastomers. It is also used as an industrial raw material. NR is a naturally occurring elastomeric polymer of isoprene (2-methyl-l,3-butadiene). It can be extracted from latex of only one kind of tree, Hevea braziliensis. Hevea rubber is produced in many tropical regions of Southeast Asia, Africa and Central and South America. There is practically only one other potential source of NR, that is the guayule shrub (Parthenium argentatum). 

Section 6.3 deals with purely phenomenological theories. In this section the predictions of a theory based on the microstructure of a rubber are considered. By 1788 at the latest the term rubber was being applied to the material obtained from the latex of the tree Hevea braziliensis because of its ability to remove pencil marks from paper. The first printed account of this use for wiping off from paper the marks of black lead pencil was given by Joseph Priestley as early as 1770. This material is now called natural rubber and its chemical structure is shown in fig. 6.9. 

Natural rubber is found in the latex of the tree Hevea braziliensis its principal constituent is polyisoprene ... 

Of course, the first manmade crosslinked polymer was Charles Goodyear s vulcanized rubber with low crosslink density. Since there were only a few sulfur crosslinks between the polyisoprene chains, Charles Goodyear s product, which he called vulcanite, was elastic but had a higher modulus than the original linear Hevea braziliensis. However, Nelson Goodyear s product, called ebonite, produced by the addition of larger amounts of sulfur (25-30%) to natural rubber, was a nonelastic intractable solid. 

An elastomer may be defined as a natural or synthetic material that exhibits the rubberlike properties of high extensibility and flexibility. Although the term rubber originally meant the TS elastomeric material obtained from the para rubber tree (Hevea braziliensis) it now identifies any thermoset elastomer (TSE) or thermoplastic elastomeric (TPE) material. Such synthetics as neoprene, nitrile, styrene butadiene, and butadiene are now grouped with natural rubber. 

In the past rubber meant a natural thermoset elastomeric (TSE) material obtained from a rubber tree, hevea braziliensis. The term elastomer developed with the advent of rubber-like synthetic materials. Elastomers identify natural or synthetic TS elastomers (TSEs) and thermoplastic elastomers (TPEs). At room temperature all elastomers basically stretch under low stress to at least twice in length and snaps back to approximately the original length on release of the stress, pull, within a specified time period. 

Natural rubber is obtained as a latex from a tree called Hevea Braziliensis. It consists predominantly of cis-lA-polyisopropene (Fig. 3.1). The word rubber is derived from the ability of this material to remove marks from paper, which was noted by Priestley in 1770. Rubber materials are not, however, restricted to natural rubber. They include a great variety of synthetic polymers of similar properties. An elastomer is a polymer which exhibits rubber elastic properties, i.e. a material which can be stretched to several times its original length without breaking and which, on release of the stress, immediately returns to its original length. That is to say, its deformation is reversible. 

Natural rubber (NR), the most fascinating material known to mankind, is extracted from the latex of Hevea braziliensis. NR is a linear, long-chain polymer chemically known as cis-l,4pdyi P i - Like other high polymers, it is made up of molecules of different sizes with molecular weight ranging from 30000 to about 10 miUion. Polyisoprene exists naturally in the form of two stereoisomers, namely cix-1,4-polyisoprene and tranx-l,4-polyisoprene (Figure 8.1). 

Carbon-chain polymers are normally associated with synthetic polymers made by the polymerisation of vinyl compounds but many naturally occurring polymers contain iminterrupted sequences of carbon atoms. The most studied and best understood of these is natural cw-polyisoprene (NR), synthesised by the rubber tree Hevea Braziliensis, which is chemically identical to synthetic c/s-IR (cw-polyisoprene mbber). 

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