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382 Rubber, Natural

system had to be devised to remove the rubber latex from tapping, and longevity. All contain more resin, that is,
the tree without permanent damage to the tree. By 1900, acetone-soluble material. For example, the Russian dan-
the modern rubber industry had been born. Today, natural delion koksagys contains only 10% rubber hydrocarbon,
and synthetic rubbers enjoy a symbiotic relationship. gathered from the roots. Goldenrod is reputed to have
about 8% rubber in its stems. During World War II, sev-
eral of these species were utilized when the Asian sources
I. INTRODUCTION
were cut off. Guayule has recently been reactivated in a
joint program by Mexico and the United States. Its ideal
Natural rubber is a high-molecular-weight polymer of
aim is freedom from dependency on overseas sources.
isoprene, whose chemical structure is 2-methyl-1,3-
In practical reality, it is a program by the U.S. Depart-
butadiene. Natural rubber most commonly is obtained
ment of Agriculture and the U.S. Bureau of Indian Affairs
from the latex of the Hevea brasiliensis tree. The Hevea
to provide employment in Mexico and the southwestern
tree is indigenous to South America, especially in the
United States.
Amazon valley. However, for the past hundred years, it
has been cultivated in many tropical areas around the
A. Historical Background
world. Principally, this species is grown in Southeast
Asia, especially Malaysia and Indonesia. As a rule, Naturalrubberhasbeentracedbacktotheeleventhcentury
◦
regions that are 5–10 latitude north or south of the when the Mayan Indians made simple articles from it. First
equator can grow rubber trees. Heavy rainfall over the scientific observations were made by the French, Charles
full year, temperatures holding between 70 and 90 F, de la Condamine and C. F. Fresneau. They identified the
◦
and elevations preferably no higher than a thousand feet rubber trees as the species Hevea Brasiliensis.
above sea level are necessary for commercial growth. Economic growth of natural rubber was hampered by
More than 14 million acres, currently containing more its thermoplastic behavior. In 1939, independent discovery
than 2 billion rubber trees, had to be cleared out of dense of vulcanization with sulfur was independently made in
jungles and rain forests in Southeast Asia. Conversely, the the United States by Charles Goodyear and in England by
Hevea brasiliensis tree is practically no longer cultivated Thomas Hancock.
in its South American birthplace, because of its suscep- In 1888, the Englishman, John Dunlop developed the
tibility to leaf blight disease. The latter is caused by a pneumatic tire, for bicycles. With the advent of the au-
fungus, Dothidella ulei. Some progress has been made tomobile, the current rubber industry was born. However,
recently in controlling the disease, but not yet on a com- natural rubber was native to only Central and South Amer-
mercial scale. Two Hevea species are known that are re- ica, where it was highly susceptible to yellow leaf blight
sistant to the leaf blight—Hevea benthaniama and Hevea disease.
pauciflora. Efforts have been made to cross-breed Hevea In 1876, Sir Henry Wickham gathered 70,000 Hevea
brasiliensis in order to develop high-yielding progeny and rubber seeds in Brazil. These were ultimately planted as
high-quality rubber. seedlings throughout Southeast Asia. From these plants,
Natural rubber is present in varying degree in at least 75% of the total cultivated rubber in the world has sprung.
200 different species of plants, including such common Plantation rubber became a reality in 1888. Henry N.
weeds as goldenrod and dandelion (Table I). All these Ridley, director of the Botanic Gardens in Singapore, dis-
species fall short of Hevea in yield, purity, frequency of covered the current method of “tapping” a tree to obtain
the latex. He is known as the father of the modern planta-
tion industry.
TABLE I Some Common Rubber-Bearing Plant Within a decade, Indonesia, Sri Lanka, and Malaya
Species
started rubber plantations. Ridley later showed that rub-
Name Habitat ber trees could be planted in regular rows in clear areas,
that trees could be tapped every few days, and that the
Funtumia elastica Africa trees would produce latex efficiently for at least 30 years.
Landolphia Africa Rapid growth of rubber production followed (Table II).
Castilloa elastica Mexico
Manihot glaziovii (Ceara) South America
II. AGRICULTURE
Ficus elastica India, Burma
Taraxacum koksagys Russia
A. Botanical Studies
Parthenium argentatum Mexico, United States
(Guayule) (California)
The Hevea tree is a tall tree, averaging 60 ft in height,
Cryptostegia grandiflora United States (Florida)
and reaching as high as 120 ft. The bark is smooth and