Para rubber
Family Euphorbiaceae
Hevea brasiliensis, para rubber (H. brasiliensis 1), is a quick growing tree, up to 25 m high, with trifoliate leaves and small yellow unisexual flowers borne in axillary panicles. The male flowers are 5 mm long, with 10 stamens; the female ones 8 mm long, with pubescent ovary with 3 stigmas. The fruit is a capsule, 3-5 cm in diameter, containing 3 seeds. There is white latex in all parts. Fresh latex consists of a suspension of rubber particles in water, together with proteins, sugars, resins, tannins, alkaloids and mineral salts.
Distribution
Para rubber grows wild in the tropical forests of the Amazon basin.
Use
After the discovery of the use of latex by the Indians, pieces of rubber were brought to Europe as a curiosity. In 1770 the English chemist Priestly discovered that rubber would rub out pencil marks on paper, and in 1820 waterproof raincoats were made by coating fabric with dissolved rubber. In 1839 the process of vulcanisation was discovered, in which rubber was heated with sulphur. This resulted in the formation of rubber that could be made into useful shapes, and this lead to a rapid increase of the demand for raw rubber resulting in the founding of rubber plantations in the East in the 1860s. When Dunlop invented the pneumatic tyre in 1888, followed by the rapid growth of the automobile industry, the demand for rubber increased tremendously. Today, about three quarters of the rubber consumption is in the manufacture of tyres and tubes, but from the remainder quarter tens of thousands of articles are made, ranging from footwear, insulation material, washers, shock absorbers, belts and hoses, household and hospital devices, to contraceptive devices and mattresses.
To meet the increasing demand for rubber, 70.000 seeds were collected in 1876 and shipped to England. From this collection 2800 young plants were raised and sent to the Botanical Gardens of Sri Lanka, Singapore and Bogor on Java. Finally, mainly due to the efforts of Ridley in Singapore, there was a good supply of planting material in the Far East and eventually over 7 million seeds were distributed. Around 1910 the cultivated rubber production started on large-scale plantations in Asia. Until that time all rubber was obtained from wild trees in Brazil.
The latex vessels occur in the phloem (bark), and a sloping cut (H. brasiliensis 2) is made to collect the latex in a metal cup (H. brasiliensis 3). It is necessary for the cut to be as near the cambium as possible, but without damaging it. Tapping is done early in the morning when the turgor pressure in the latex vessels is greatest. The first tapping begins when the trees are 5-7 years old, and take place twice a week. In the factory the latex is coagulated (H. brasiliensis 4) by adding acetic or formic acid, and thick sponge-like sheets are produced which are rolled and smoked. These sheets of semi-transparent rubber are exported, mainly to the US, Russia, Britain and Japan.
The seed kernels contain a semi-drying oil, which can be used for the manufacture of soap.
The loss of para rubber from Indonesia and Malaysia in World War II led to the successful development of synthetic rubber. After the war the boom of the natural rubber cultivation was over, but still there is a production of about 4 million tons annually, mainly in Malaysia, Indonesia (H. brasiliensis 5) and Thailand. The average annual yield is about 200 kg of sheet rubber per ha.