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Natural Rubber

Q: Where and when do the earliest references to natural rubber occur?

A: The earliest references to natural rubber occur in accounts of the second voyage of the discovery of the New World by Christopher Columbus in 1495–96: there, he is reported to have seen American Indians playing with balls that bounced and were made from the juice of trees. However, the first samples of the new substance were not to be sent to Europe for another 240 years when a French scientist, Charles de la Condamine, visited Ecuador and gave rubber its French term ‘caoutchouc’, derived from an Inca word with the literal meaning ‘weeping tree’.

Q: When was natural rubber first cultivated and by whom?

A: The first attempt at cultivation was made in 1876 when an Englishman, Henry Wickham, collected seeds from Brazilian trees with the botanical name Hevea brasiliensis and sent them to Kew Botanic Gardens in London. Seedlings from the germinated seeds were sent to Ceylon (now Sri Lanka) but the results were disappointing; however, the following year, a few seedlings were sent on to Singapore, destined to be the basis of the natural rubber industry in Southeast Asia.

Q: What were the early uses of natural rubber?

A: In 1770, the English chemist, Joseph Priestley, noted that the substance was able to rub out lead pencil marks and, as a result, small cubes of it were soon put on sale as erasers, which became popularly known as ‘India rubbers’ with reference to the material’s origin in the West Indies. There was also a considerable trade in rubber shoes produced in the Amazon and exported to America and Europe. Following these early and minor applications of rubber, it was not until 1820 that other products started to appear: this was when Thomas Hancock invented a mastication machine to mix and soften rubber, enabling it be shaped, and also set up a rubber goods manufacturing factory in London. Three years later in Glasgow Charles Macintosh developed a process of coating cloth with a solution of rubber in benzene and started producing the waterproof garments to which he gave his name.

However, there was still a major problem to be overcome: in warm weather rubber would soon become sticky and in cold weather it became brittle. The great breakthrough - apparently made by accident - that overcame this problem occurred in 1839 when Charles Goodyear, an American, discovered that by heat treating rubber with sulphur the resultant material was no longer affected by temperature. He patented this process, known as vulcanization, five years later.

Q: How is natural rubber produced and under what conditions?

A: Hevea brasiliensis can only be grown in areas with similar conditions to the Amazon rain forests, which effectively restricts production to regions 15 to 20 degrees latitude north or south of the equator. It takes five to eight years for a rubber tree to mature to the girth at which it can be tapped and its economic life will then be 20 to 30 years. At the end of its life rubberwood provides a valuable end product as a medium density tropical hardwood.

The rubber is extracted in the form of latex, a white, milky fluid which is held in cells found in the inner layers of the bark of the trees, using a method known as ‘tapping’, which involves paring away a thin slice of bark without damaging the growing layer in a series of half-spiral cuts, usually on alternate days, using a special knife. The latex then oozes from the cut and flows into a collecting cup for a period of several hours or more until it begins to coagulate and the flow ceases.

After collection the latex, which at this point is about 70% water, may be taken to a processing plant, where it may be sieved to remove extraneous matter, blended, coagulated, rolled into sheets and then dried in ‘smokehouses’ to produce ‘ribbed smoked sheets’ (RSS). Alternatively, after coagulation, it may be washed, shredded and granulated under controlled conditions before being dried in deep-bed driers to form a ‘block’ rubber known as Technically Specified Rubber (TSR). Whichever process is used, the rubber is then pressed into bales and wrapped into polythene bags for despatch. Finally, a small proportion of natural rubber is also processed and sold as latex concentrate; water is removed by centrifuging, creaming or evaporation to give a product containing around 60% rubber.

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