Introduction
Whilst most rubber is now synthesised from petroleum, around one-quarter of the world's rubber comes from a natural source ­ a tropical tree known as Hevea brasiliensis, which is native to the tropical Americas.
H. brasiliensis grows best at temperatures of 21-28 degrees, in areas with an annual rainfall of just under 2000mm. These features, and its preferred altitude of around 600 metres mean that its prime growing area is around 10 degrees on either side of the equator, although it is also cultivated further north in China, Mexico, and Guatemala.

Historical Origins

Rubber has long been used by the South American natives before Europeans began to take advantage of its properties. In 1736 it was treated with great curiosity when several rolls of it were sent to France, along with a number of other rubber products. In 1791 rubber manufacturing, albeit on a small scale, became commercial when the Englishman Samuel Peal discovered a method of waterproofing cloth by treating it with a solution of rubber in turpentine.

Vulcanisation
By the 1830's, rubber was popular in the United States, but its drawbacks included becoming brittle in cold weather, and soft and bad smelling in warm weather. These negative properties could be removed by a processs known as vulcanisation. First developed in 1839 by the inventor Charles Goodyear, it involved cooking rubber with sulphur, which increased its strength, elasticity, and decreased its sensitivity to temperature change. Vulcanised rubber is also resistant to chemical action, water abrasion, impermeable to gases, and will not conduct electricity. Such properties meant that it could be used in a wide variety of products.

Further Developments
In modern manufacturing, rubber is mechanically ground in a process known as mastication. The resulting soft sticky rubber is treated with various compounding ingredients and mixed before being moulded and shaped. It is then vulcanised under high pressure. Another significant development was the discovery of a process known as acid reclamation. Introduced in 1877, American Chapman Mitchell found he could recycle old rubber for use in the manufacuring of new rubber products.

Rubber Plantations

In 1876, Sir Henry Wickham collected some 70 000 seeds from Brazil, which he germinated and shipped to the East Indies, where they began today's great rubber plantations. Today Malaysia, Indonesia, and Thailand are the major producers of natural rubber, producing around 90% of the world's natual rubber.
New plantations have been established in Africa, The Phillipines, and Europe to make up for insufficient replanting (and consequently less rubber output) in Indonesia.

Seeds and Planting
Edgar (1947) states that freshly collected seed loses around 50% of its moisture in the first three days. It has also been found that germination rate decreases with the loss of moisture, and therefore seeds should be planted as soon as possible after collection. If planting is not possible straight away, they can be placed in a tin of charcoal to help retain moisture levels.
Seeds are planted after being raised in special germination beds, around 90cm wide, with spaces between to allow room for walking.
They must be planted horizontally to avoid twisted shoots, and generally in numbers ranging from 5-9 seeds (Edgar;1947), allowing for some failure although 20% failure is still regarded as being good. Heavier seeds tend to produce more vigorous seedlings whilst those seeds which fail to germinate within 14 to 21 days should not be used as they will tend to produce weak seedlings.

Rubber trees produce seed around once a year which can be collected from clonal trees, or seedling trees. Those seeds collected from clonal trees are termed selected, whilst those that are collected from seedling trees are termed unselected or ordinary seeds. Clonal seeds consist of two types; monoclonal and polyclonal, the latter benefiting from their hybrid nature. They are carefully selected after experimentation over time to pinpoint the trees with the most desirable traits before being recommended for commercial propagation. This process may sometimes take decades, an example being the Malaysia 700 series which are only now being recommended after experimentation began back in the 1950's. Edgar (1947) states that non-clonal seeds should be taken from an area carrying a stand of apparently healthy trees. Whilst this common theory forms the basis for the improvement of genetic stock, there have been many advancements over the last half century.

For further reading in such developments, termed 'advanced planting materials' go to:
http://www.irrdb.org/agronomy/propagat.htm

Tapping for Latex

Hevea brasiliensis, like many plants produces latex which oozes from injuries to the stem in the form of a milky sap. Latex is produced by special cells called laticifers and is thought to be a defence against insect pathogens (http://www.ucs.byu.edu/bioag/aghort100/rubber/htm.) and possibly a site for the depositing of metabolic waste of the tree. Not all latexes are elastic, but those that are, including the latex of H. brasiliensis, contain long hydrocarbon chains.

Tapping a tree is the first step in obtaining latex. Edgar (1947) states that latex flows from the living parts of a rubber tree in response to wounding, and that tapping in fact is 'controlled wounding' ­ using the sap for the manufacture of rubber without seriously damaging the tree.
On a reasonable site, a tree will be tapped once it has reached maturity, usually at an age of around seven years. The bark is cut periodically on a 25-30 degree angle from the top left to the bottom right in order to expose the maximum number of latex vessels. The 'excision' method of reopening the same cut to create channels in which the latex runs before being collected in a cup is favoured because it leaves the trees relatively unharmed, and can therefore be repeatedly used. The main aim of this method is to make the smallest cut possible to obtain the maximum amount of latex, and this is thought to be the main reason why Hevea plantations have been so successful.

The native method of tapping in the Amazon Jungle involved using a hatchet and creating a number of cuts at equal distance around the trunk in a ring. At each tapping a new ring of cuts would be made at a vertical distance of between 5 and 30cm from the first ring. Using this method the tree became so damaged that tapping became increasingly difficult. Tapping occurred 4-5 hours a day for six months of the year, with the other six months left to allow the trees to recover (Gomez; 1983).

Today, skilled tappers can tap the same trees for around 25 years because damage to the tree is minimised. Once cut, the latex will flow from vessels for 1-3 hours before the vessels become plugged by coagulum. The stimulant edaphron is used as a means of delaying plugging, although its effects are not maintained in the long-term. Because edaphron increases yield in the short-term, it is seen as an ideal way to reduce labour costs as the trees need to be tapped far less regularly.

Puncture tapping is also a method widely used in Malaysia. One puncture a week is made on a tree in an area of scraped bark. It is then treated with edaphron and left for two to three days to allow the latex to run down a spout to collect in a cup. Tapping is usually carried out early in the day, when the rate of latex flow is higher due to a higher turgor pressure. Each tree will yield up to three-quarters of a cup of latex at every tapping. Once the latex has collected in the cup it is coagulated in metal pans using a dilute acid. The resulting cake of raw rubber must be washed to remove any acid and rolled to remove excess moisture. After rolling the rubber is dried on a rack and smoked over a wood fire to stabilise it, before it is finally sold to rubber manufacturers.

Conclusion
Rubber has been used for centuries as the favoured material for the manufacturing of many products. Manufacturing methods have evolved considerably towards achieving a greater efficiency, and overall volume of rubber. This process first began when rubber became commercialised, and has steadily progressed into becoming a large worldwide industry. Hevea brasiliensis is still a major source of the world's rubber, raised and harvested in many plantations centered mainly around Malaysia, Indonesia, and Thailand. Although petroleum is now by far favoured as the main source for rubber manufacture, it is a non-renewable resource which will eventually become depleted. Hevea brasiliensis may therefore become more important in the future.

References

Edgar, A.T. (1947) "Manual of Rubber Planting (Malaya)" The Incorporated Society of Planters (Malaya) London

Gomez, J.B. (1983) "Physiology of Latex (Rubber) Production" Malaysia Rubber Research and Development Board, Rubber Research institute of Malaysia.

http://www.pa.msu.edu./~science/ask_st/090193.html

http://www.irrdb.org/hevea/intro.htm

http://www.bio.ilstu.edu/armstrong/syllabi/rubber/rubber.htm

http://www.encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=01307000&hs=rubber+trees

http://ucs.byu.edu/bioag/aghort100/rubber.htm

http://www.irrdb.org/agronomy/propagat.htm

http://www.irrdb.org/agronomy/tapping.htm