Natural rubber
Commercial natural rubber is almost produced from the hevea tree. Natural rubber latex (about 10%) can be got directly by cutting the bark of the tree. Concentrated natural rubber latex (up to 60%) is obtained by centrifuging. The concentrated natural rubber latex is often used in glove industries. Usually, natural latex is coagulated into solid natural rubber by addition of formic or acetic acid. Typically, solid raw natural rubber contains about 95% cis-1, 4-polyisoprene with 2.5% protein, and about 2.5% lipids etc. The chemical structure of natural rubber is shown in Figure 9.

Many different grades of natural rubber are marketed, depending on processing method. Ribbed smoked sheet and pale crepe and technically classified rubber and Standard Malaysian Rubber are the main solid rubbers. A small amount of modified natural rubber is also available.

Properties
Solid rubber is normally amorphous but form crystals once cooling down to 0 oC. The maximum crystallizing temperature is -25 oC and the melting temperature is 25 oC. Raw natural rubber is a sticky, yellow and weak material. It fluids at about 120 oC. The sticky natural rubber can be crosslinked by sulfur using some accelerators such as mercaptobenzothiazole. The crosslink reaction is called <vulcanization>. The typical formulation of the vulcanization compounds includes natural rubber (100 parts), carbon black fillers (0-150 parts), zinc oxide (activator, 5 parts), stearic acid (activator, 2 parts), N-isopropyl-N'-phenylparaphenylenediamine (antioxidant, 2 parts), sulfur (cross-linker, 0-4 parts) and N-cyclohexyl-2-benzothiazyl sulfenamide (accelerator, 0.5-3 parts). [Andrew Ciesielski, An Introduction to Rubber Technology, Rapra Technology Limited, UK, 2000, p32.] 

Processing
Rubber is masticated by high shear force to decrease the molecular weight of the rubber and thus reduce the melt viscosity of the material. The processing machines may be extruders, banbury mixer or rollers. 

Future
Natural rubber is an important raw material for rubber industries due to its excellent properties such as low cost, low hysteresis, high strength and high resilience etc., despite competition from the many more recently developed synthetic rubbers. Its major uses are in tire and hose industries, and in mechanical uses, such as shock absorbers.
The three largest producers in Asia are Thailand, Indonesia and Malaysia, which produce about 95% of world production. Recently, more and more natural rubber were consumed by Asia countries themselves. 

Recommend books and references:

Barlow, F. W. Rubber Compounding Principles, Material and Technology; Marcel Dekker Inc.: New York, 1993.

Mark, J. E.; Erman, B.; Eirich, F. R., Eds.;Science and Technology of Rubber; Academic Press: CA, 1994.

Andrew Ciesielski, An Introduction to Rubber Technology, Rapra Technology Limited, UK, 2000.