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Natural rubber

An elastomer with exceptional qualities

Rubber, a generic term designating an elastomer, is a material with unique mechanical properties due to its elasticity, the result of cross-linking, known as vulcanisation in the rubber industry. It can be produced from natural or synthetic products.


Rubber - Cao (wood) and Tchu (to weep) - "Wood that weeps"


The real history of rubber starts well before the end of the fifteenth century, when, following major discoveries, the Europeans started to observe in Central and South America the secular use made by indigenous populations of a material then unknown in Europe, which they called Cao Tchu (wood that weeps, the latex being produced by bleeding different plants - including hevea and guayule. The
Amerindians produced everyday objects, made by using clay moulds. Balls, coated fabrics and torches which they made water-proof by passing them through smoke.
They associated it with the myths of creation, of the motion of the earth : in “juego de pelota" ( a pre-Colombian ball game), the rubber ball and its incessant bouncing mimic the motion of the sun. Rubber thus became a sacred material.
The Conquistadores, more interested in Eldorado, brought some samples back to Europe but they were relegated to "collections of curios", there being no obvious applications, since latex :

  • is sticky when exposed to sun ;
  • melts at high temperature ;
  • becomes brittle at low temperatures ;
  • goes brown and coagulates when exposed to air.


Today, the term "rubber" is used to designate :

  • the raw material : natural rubber, obtained from the hevea tree, or synthetic rubber, produced by the petrochemical industry
  • mixtures made from natural or synthetic rubbers with additives
  • the final product obtained after shaping and vulcanisation.

Harvesting


Natural rubber occurs in numerous plant species but today it comes mainly from the hevea tree (Hevea brasiliensis) found in the tropical zones of south-east Asia (Vietnam, Indonesia, Thailand) and in Africa (Cote d’Ivoire).



Natural rubber is marketed in two forms :

  • liquid : after harvesting, the latex, treated with a few drops of ammonia to prevent it coagulating, is centrifuged to remove a part of the water which it contains. Chemical stabilisers help to preserve it. Possible applications : gloves, condoms
  • solid : latex can be treated in several ways : either by natural coagulation or by controlled coagulation (by treatment with acid)



The key vulcanisation stage


Raw natural rubber is highly sensitive to heat : hard and pliable when cold, it becomes sticky and soft at high temperatures and so unusable. The perfecting of vulcanisation by the American, Charles Goodyear, in the middle of the nineteenth century, made considerable improvements to its mechanical properties and also its ability to withstand variations in temperature. This step was the start of many applications developed by the rubber industry.



 


Sources : Wikipedia and http://www.direccte.gouv.fr/IMG/pdf/Etude_caoutchouc_1ere_partie.pdf

mage courtesy of SNCP / Fotolia / Trelleborg
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