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Rubber and Other Latex Products
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Rubber Hevea Rubber
Wild Rubber Plantation Rubber (East) Plantation Rubber (West) Castilla Rubber
Ceara Rubber
Assam Rubber Lagos Silk Rubber Landolphia
Rubber Guayule Rubber Dandelion Rubber
Minor Sources of Rubber Other Latex Products Gutta-Percha Balata Jelutong
Chicle
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Rubber is acquired from the milky fluid, or latex, of various erect or
climbing woody plants of the tropics or subtropics. Most of the rubber plants belong to the Moraceae,
Euphorbiaceae, or Apocynaceae.
Although more than 50 species are available as sources, only a few
have been important commercially, with Hevea brasiliensis being the
most prominent. Originally only wild
trees were the source of rubber, but later cultivated Hevea trees, the
“plantation rubber” furnished about 98 percent of the supply (Hill 1952).
Rubber is the most recent of the major crops of the world. The industry by 2003 was about 150 years old,
but cultivation has been carried on only about 100 years. The increase in the production of
plantation rubber was from 800 long tons in 1900 to 305,000 tons in 1920 and
1,395,000 tons in 1940. This
development of the rubber-growing industry has not been without
drawbacks. Overproduction had
seriously affected the industry financially by 1950, and many attempts have
been made to regulate its supply. The
British and Dutch, in particular, tried to restrict production and exert
other methods of control in Malaya, Java, Sumatra and other plantation rubber
centers within their spheres of influence.
The successful development of synthetic substitutes for rubber, after
many years of experimentation, further jeopardized the natural rubber
industry. Nevertheless, while these
substitutes are superior for some purposes, such as the conduction of oil,
natural rubber is still preferred for vehicle tires, which utilize about
3/4ths of the rubber output.
Latex occurs in special cells or in a series of special vessels, which
permeate the bark, leaves, and other soft parts of the tree. Usually only the latex from the lower part
of the trunk is of importance commercially.
Latex is a gummy white liquid full of minute globules. It consists of a mixture of water,
hydrocarbons, resins, oils, proteins, acids, salts, sugar and caoutchouc, the
substance used as the source of rubber.
The significance of latex to the plant is obscure, but it is know to
be of some value in the healing of wounds, and it may serve for protection,
nutrition, and the transport of materials or as a fluid reservoir.
The properties of rubber were known by primitive cultures in Central
America and Peru. Their word cauchuc
has been changed to the present caoutchouc (caucho in Spanish). Columbus was the first to report the
existence of rubber to Europeans, but it was not introduced into Europe until
1735. Priestley first applied the
name “rubber” in 1770, due to the fact that caoutchouc could be used for
removing pencil marks. Mackintosh in
1823 developed a process for waterproofing cloth, but it was not until 1830
when Goodyear made the discovery of the vulcanizing process, that rubber
really became popular. From then on
the rubber industry developed rapidly, and today rubber has a vast and
constantly increasing number of uses and industrial applications. The properties that make it so valuable
include its plasticity and elasticity, its resistance to abrasion and to
electrical currents, and the fact that it is impermeable to liquids and
gasses.
The Hevea or Para rubber tree (Hevea brasiliensis) is the
normal source of 95-98% of the rubber produced throughout the world. The tree is native to the hot damp forests
of the southern affluents of the Amazon River in South America. It was estimated in 1952 that that there
are over 300-million trees in the vast area traversed by the Amazon and the
Orinoco. Within this region the
optimum conditions for its development are found. This includes a uniform climate with a temperature range from
75-90 deg. Fahrenheit and a rainfall of 80-120 inches. The trees may reach an age of at least 200
years and may attain a height of 60-140 ft.
The leaves are 3-lobed, and the flowers are small and
inconspicuous. The fruits have three
seeds, which contain 23-32 % of fatty oil.
This oil is sometimes extracted and used as a drying oil. The resulting oil cake is rich in proteins
and is used as a stock feed. Although
rubber trees are natives of swampy floodlands, they grow best on deep,
fertile, well-drained upland soils at an elevation of 600-1500 ft. above sea
level. The latex occurs in a series
of vessels in the cortex. Several
other species of Hevea are tapped, including Hevea benthamiana,
which has high-quality latex, but their production is insignificant compared
with H. brasiliensis.
In the beginning of the industry only wild rubber trees are available
as a source of rubber. The maximum
production was reached in 1910 with an output of 83,000 long tons. After that with the development of
plantation rubber, wild rubber production steadily declined to as low as
8,500 tons in 1932. Since that time
the output has fluctuated, depending on price and demand. During World War II wild rubber was again
actively exploited.
The methods employed in obtaining and preparing the latex have changed
little over the years. The collection
of the latex is in the hands of local residents, called “seringuieros” who
are usually in the employ of absentee owners (Hill 1952). Each seringuiero is responsible for a
single tapping circuit, which contains from 35-180 trees, about 2 or 3 to the
acre. When a new tree is located, it
is tapped at a point about 3 ft. above the ground by cutting several short
downward 30-degree panelike incisions in the bark with a special knife. The cuts are deep enough to sever the
latex vessels but do not extend far enough to injure the cambium. Cups are placed below the incisions to
receive the latex, which flows for several hours. The seringuiero makes the round of his trees each day. Successive tappings consist of entirely
new incisions made slightly below the previous ones. The latex is collected from the cups and
carried to the camp for coagulation.
This is done by coating a pole with latex and suspending it over a
fire made of palm seeds or special woods.
These yield a dense smoke containing acetic acid, creosote, and tars
which coagulates the latex, forming a layer of crude rubber. The process is repeated until balls
weighing 125-200 lb. are obtained. In
some areas paddles are dipped in the latex and held in the smoke. The balls are shipped to mills for
processing. Plantation Rubber in the
Eastern Hemisphere
Henry Wickham transported 70,000 Hevea seeds from the Amazon to
England in 1876. These were grown at
Kew, and enough seedlings were obtained to start a rubber industry in the
eastern tropics, at first in British Malaya and Ceylon and later in Java,
Sumatra and other areas (Hill 1952).
There had been earlier attempts at rubber cultivation, but plantation
rubber did not become permanently established until the beginning of the 20th
Century. In 1910, when Amazon rubber
production was at its peak, the East produced only 11,000 tons. Four years later it had outstripped its
rival, and by the outbreak of World War II, it was responsible for 98% of the
world’s output of 1,500,00 tons, with nine million acres under cultivation.
The rapid development of plantation rubber in the East may be
attributed to a favorable climate, absence of diseases, low labor costs, and
the extensive research program launched by the British. Every step in the production of the crop
was rigidly supervised, and the best methods of planting, cultivation,
tapping and coagulation were discovered.
A selection and breeding program facilitated an increase in
yield. The discovery of bud grafting
allowed that buds of high-yielding plants be grafted on ordinary
seedlings. The resulting clones
differed greatly, and only those with the most desirable traits were
retained. Using such approved clones
began in 1925. By 1945 10% of the
plantations had trees that were the result of grafting, and they obtained a
yield of 1500 lb per acre. The 90%,
which still had a random mixture of trees, had a yield of only 450-500
lb/acre. Where controlled crosses of
high-yield clones were made, yields as much as 2000 lb. per acre were
possible. Not all plantation rubber
was produced on the large estates.
The acreage devoted to rubber grown by small growers was produced in
small family gardens and was actually more than that grown on estates. Plantation Rubber in the
Western Hemisphere
Following the imposition of export restrictions on eastern rubber in
1924, the United States favored the establishment of plantations in other
parts of the world. The Firestone
Corporation selected Liberia, but Ford Motor Co. and Goodyear turned to
tropical America, the native area of Hevea. The 2,500,00-acre Fordlandia was started in Brazil in 1928,
while Goodyear chose Panama and Costa Rica.
All the American efforts failed after a promising beginning, due
primarily to the ravages of leaf blight, Dothidella ulei, to which
plantation rubber in the Western Hemisphere was very susceptible. It was obvious that rubber could not
succeed as a crop in the American tropics without adequate disease
control. A research program did
produce satisfactory results, however.
An effective spray, of copper and sulfur fungicides with various
spreaders and stickers, was developed and could be used on seedlings. Yield and resistance experiments were
conducted on thousands of wild trees, with the most desirable being selected
for propagation. Fortunately there
was a successful importation of approved eastern clones from a Goodyear
plantation in the Philippines. When
these were budded on resistant stock from the jungles, desirable combinations
of characters were obtained. Double budding
also proved satisfactory. This
involves budding a high-yielding eastern clone onto a native root as soon as
the plant is large enough. This is
grown in a nursery under spray control until it is 6-10 ft. tall. Then the top is budded with a disease-resistant
strain. The resulting rubber tree has
a native root system, a high-yielding eastern clonal trunk, and a
disease-resistant crown. Long-term
breeding programs were continued well into the mid 1900’s, as well as
extensive explorations in the jungle, with the goal of obtaining strains that
combined disease resistance and high yield and which would reduce the
expensive spraying and double-budding practices.
The cooperation of the United States with governments in Latin America
resulted in rubber being grown successfully in many areas. Emphasis was placed on small family
plantings rather than on large plantations.
The rubber trees were usually set out at intervals of 12-24 ft. in
tows 20 ft. apart. Other crops were
grown between the rows. Thus, from
1/3rd to 1/2 of the acreage was resting; the balance was tapped on some
alternating program. At each
successive tapping a thin shaving of bark was sliced off the original panel
until it almost reached the ground.
One-third, 1/2 and full spirals were used.
Following the collection of latex, a small amount of ammonia or some
other anticoagulant was often added to keep it liquid until it reached the
processing area, where it was concentrated and made into sheet rubber. Cleaning the latex and pouring it into large
pans accomplished this. A coagulant,
such as formic or acetic acid, was added, and in several hours a mass of soft
rubber resulted. Blocks of this were
washed and run between rollers to form thin sheets. These were smoked and baled for shipment. Other types of crude rubber were also
prepared. “Worm rubber” consists of
irregular wormlike pieces cut from sheets with a pair of shears. “Crepe rubber” was made by passing washed
and bleached coagulated latex through a creping machine, which turned out long,
thin, perforated strips of rubber. In
the preparation of “sprayed rubber” latex was dropped on whirling disks and
little particles of rubber were thrown off.
Any moisture quickly evaporated.
This kind of rubber was exceedingly pure and clean. Also, steadily increasing amount of
liquid latex was evaporated. This
required immediate processing in the field and was feasible only for large
plantations
Panama or Castilla rubber is from Castilla
elastica, a tall tree native to southern Mexico and Central America. There are many names for this tree in the
indigenous languages of the region.
It was the chief source of rubber from 1794-1850. Other species of Castilla may also
be used, among which are C. ulei of the Amazon, the source of caucho
rubber. Castilla elastica is found in deep loamy soil on high ground
and may reach a height of 150 ft. It
requires temperatures above 60 deg. F. and requires shade when young. Often the natives cut these trees down in
order to obtain the large possible yield of latex, which could ultimately
lead to this species extinction.
Trees are tapped when 8-10 years of age, with adult trees yielding a
maximum of 50 lb. of latex. This is
coagulated with plant juices, alum and by boiling or exposure to the
air. The crude rubber is shipped in
flat cakes. Castilla rubber was once
extensively cultivated in Central America in plantations, but it could not
compete successfully with Hevea rubber.
The plantations were abandoned.
The surviving trees were the source of some of the emergency supply of
Castilla rubber during World War II (Hill 1952).
Ceara or Manicoba rubber is obtained from Manihot
glaziovii, a small tree native to drier regions in Brazil. It grows well in dry rocky ground so that
it may be utilized in areas unsuitable for other types of rubber. The tree grows rapidly, reaching its
maximum height of 30 ft. in a few years.
Ceara rubber is also grown in Ceylon, India and other tropical
countries. The trees are tapped when
4-5 years of age and yield a good grade of rubber. The latex is coagulated by exposure to air or smoke. The crude rubber is exported as blocks or
flat cakes.
Assam or India rubber as it is often
called, is obtained from Ficus elastica, a native tree of northern
India and Malaya. The tree requires a
hot climate and a high rainfall. It
often begins life as an epiphyte. It
grows to a great height, developing huge buttresses or prop roots. The roots as well as the stem are tapped. The latex is allowed to drip onto bamboo
mats, where it coagulates. A large
amount coagulates on the trunk as well.
This crude rubber is scraped off, cleaned and dried. The native procedures for tapping the wild
trees have been very wasteful, so the tree is cultivated in some areas. The yield is low and the trees do not
mature until about 50 years of age.
Assam rubber is of low grade and has been of little or no commercial
importance.
Lagos silk rubber is from Futumia elastica, a large tree of
tropical West Africa. Discovered in
1894, it was immediately exploited to the point of near extermination. In 1898 over 6 million pounds were
exported. The tree has been
cultivated and an attempt made to reestablish the industry. However, plantations of Hevea rubber are
the chief source of rubber in West Africa.
Other former sources of African rubber were several woody
climbers in the genus Landolphia.
The most important were L. kirkii on the east coast and L.
heudelotis and L. owariensis on the west coast. The huge vines, sometimes 6 in. in
diameter. Were pulled down and cut into small pieces. The latex that exuded was coagulated with
plant juices or the heat of the sun, sometimes even on the bodies of the
native collectors (Hill 1952). The
most wasteful methods were employed, for a large immediate yield was
desired. There was very barbaric
treatment of the natives involved in this harvest. Operations in the Belgian Congo during the reign of Leopold II
remain some of the most awful pages in history. Although there is still a considerable amount of wild
landolphia rubber, there is apparently no future commercial value for it. Cultivation of this plant is not
practical.
The guayule, Parthenium argentatum, a native of America, has
been utilized sine 1910 as a minor source of rubber. This low semi-shrubby plant grows in the
arid regions of Mexico and the southern United States. The early Amerindians who obtained the
caoutchouc by chewing the plants knew it.
Unlike the other sources of rubber, there is no latex present in the
guayule plant. There are small
granules of caoutchouc scattered all through the tissues. These are extracted mechanically or by
means of solvents. The entire plant
is pulled up and chopped up or ground with water. The particles of caoutehouc float to the surface and are
removed. They are then dried and
pressed into slabs. If solvents
remove the resinous materials the product is as good as the best Hevea
rubber. Guayule rubber is especially
good for mixing with synthetic rubber substitutes. Guayule has been adapted to plantation culture and it was
extensively investigated as part of the Emergency Rubber Program during World
War II. The yield has been increased;
the life cycle shortened, and improved cultural, harvesting, and processing
machinery developed. Production costs
are still high so that it does not compete with Hevea rubber except in
emergencies. The importance of
guayule for the future lies in the fact that it constitutes a living
stockpile of rubber on land which otherwise would be idle (Hill 1952).
The dandelion of Russia, Taraxacum kok-saghys, is a close
relative of the common dandelion. In 1931
Russian botanists discovered it while searching for new economic plants. Rubber is present in considerable
quantities in latex tubes in the long taproots. The yield has been increased by selection and breeding, and the
plant was grown on some 2 million acres during the time of the Soviet
Union. Seeds were brought to the
United States in 1942 and planted experimentally in 42 states. Vigorous plants with greater root weight
and rubber content were segregated.
Hill (1952) believed that a yield of 400-500 lb. per acre could be
achieved through continued research.
The plant is well adapted to the northern United States and Canada,
where it can be harvested the year it is planted. Under the most favorable conditions it might be profitable to
grow this species as a minor source of rubber, and indeed was cultivated for
this purpose in Argentina.
Other species of plants have been considered as a possible source of
rubber. Among these are intisy (Euphorbia
intisy). This leafless shrub of
arid regions of Madagascar contains latex that has had considerable use
locally. The rubber coagulates on the
surface of the plant in long elastic strands and is of high grade. Attempts have been made to cultivate
intisy in the United States because of its adaptation to desert conditions.
Cryptostegia grandiflora and C. madagascariensis are
ornamental woody climbers from Madagascar.
They are now found in the tropics and subtropics of both hemispheres
and were seriously considered during World War II as a new source of
rubber. Both species have been grown
for many years in India as a source of high-grade palay rubber. They are abundant in Mexico, occuring
there in a wide variety of soils and climates. They seem the hardiest and fastest growing of all rubber
plants, attaining harvest age within six months. They seem to offer considerable future promise, but attempts in
Haiti to grow them on a large scale failed due to difficulties of extracting
the rubber.
Other rubber-bearing plants of local importance include the mangabeira (Hancornia speciosa), a shrub or small
tree of Bolivia, Brazil and Paraguay; a species of Micrandra, which
yields the caura rubber of Venezuela; and various species of Sapium in
northern South America. Also a few
native North American plants have rubber-contining latex. Among those, which have been given some
attention, are the desert milkweed (Asclepias
subulata); the Indian hemp (Apocynum scannabinum); several goldenrods, in particular Solidago leavenworthii,
which was investigated by Thomas A. Edison; species of the rabbit brush (Chrysothamnus), the source of
chrysil rubber; and species of Cnidoscolus from which chilte rubber is
obtained. Other Latex Products
Gutta-percha is a nonelestic rubber obtained from the grayish-white
latex of several members of the Sapolaceae.
It has been known since 1842.
The chief source is Palaguium gutta, a tree of Malayan origin,
which has been grown in Borneo, Sumatra, the Philippines and other tropical
countries. The latex is produced in sacs, which occur in the cortex, phloem,
pith and leaves. It is obtained by
making incisions from which the milky juice runs out very slowly, or by felling
the trees. The latter is the usual method. The bark is removed in strips one inch in
with and one foot apart, and the latex is collected in coconut shells or in
palm of plantain leaves. The latex
soon coagulates into grayish-yellow masses of a hard substance, which is
odorless and heavier than water. This
crude product contains several resins and other impurities and is purified by
washing in hot water. The whole mass
is boiled and then kneaded into blocks, or it is chopped or sliced up and the
pieces are washed, strained and kneaded, and then rolled into thin sheets
(Hill 1952). The value of
gutta-percha depends on the amount of a hydrocarbon, gutta that is present.
Gutta-percha is hard at ordinary temperatures. It deteriorates very rapidly when exposed
to air through oxidation and should be kept under water. It softens at 77 degrees Fahrenheit, can
be kneaded at 122 deg. F., and melts at 248 deg. F. Because it is an exceedingly poor conductor of electricity it
has been used for insulation. For
many years no other material could replace gutta-percha and the similar
balata in the construction of submarine cables, which require a substance
that is resistant to salt water, pliable and with just the right amount of
rigidity. Other uses included
splints, supports, pipes, golf balls, speakingtubes, telephone receivers,
waterproofing and adhesives. It has
also been utilized for protecting wounds and in dentistry.
Balata is a nonelastic rubber that is obtained from the latex of Manilkara
bidentata, formerly known as Mimusops balata, and other species of
the genus. It is a native of Trinidad
and South America. This magnificent
tree grows to a height of over 100 ft.
When mature its purplish wood is very hard and durable and is much
used for ties and building purposes under the name of bully wood or
bulletwood. The fruit is also
edible. The latex is obtained by
tapping the trees three times each year.
It flows freely and readily coagulates in the air. A tree 3 ft. in circumference will yield
50-100 lbs. of dry balata. After
coagulation it is cleaned and molded into cakes. Balata contains about 50% gum.
It serves the same purpose in industry as gutta-percha. It is particularly well adapted for
machine beltings as it grips tightly and never stretches. It is also used as a substitute for
chicle. Balata has been known since
1859, but unlike gutta-percha, it has never been cultivated.
Several inferior types of balata are found in the Amazon region, chief
of which is abiurana or coquilana obtained from Ecclinusa balata.
Jelutong is obtained mainly from Dyera costulata and related
species from Malaya. These trees have
a copious flow of latex, greater than all the other latex species
combined. From 1910-1015 this was
exploited as a source of rubber.
However, the latex is combined with many gums, resins and other
impurities, and it yielded a poor quality of rubber. Jelutong has been used mainly as a
substitute for chicle.
Sapodilla or naseberry (Achras
zapota) is a tall evergreen tree native to the Yucatan Peninsula of
Mexico. It is cultivated in tropical
America and Florida for its edible fruit.
The bark contains latex, 20-25% of which consists of a gum, or chicle,
similar to that of gutta-percha. It
is the basis of the chewing-gum industry but is also used in making surgical
tape and dental supplies.
The primitive method of obtaining chicle involves tapping the trunk
and then scraping the thickened exudates from the bark. The crude or leaf chicle consists of pink
or reddish-brown pieces mixed with 25-40% impurities. In southeastern Mexico and British
Honduras where the industry occurs most abundantly, the native collectors or
chicleros are more careful. Zigzag
gashes are cut in the trunk with a machete, up to a height of 30 ft. The latex runs to the base of the tree
where it is collected in rubberized bags, leaves or even hollows in the
earth. This accounts for the grains
of sand that are oftentimes found in chewing gum. The flow of latex lasts for several hours and the yield may be
as much as 55 liters. In order to
conserve the supply, plantations were established. However, these were not practicable because the trees can be
tapped only every 2-3 years.
The hardened chicle is boiled; a process that requires considerable
skill, for the chicle must be poured off when the moisture content reaches
33%. It is then molded into blocks
for shipment. Raw chicle contains resin,
gutta, arabin, calcium, sugar and various soluble salts. Purification involves breaking the blocks
into small pieces, washing in a strong alkali, neutralizing with sodium acid
phosphate, washing again and finally drying and powdering. The final product is an amorphous
pale-pink powder, which is insoluble in water and forms a very sticky mass
when heated. Ageing partially
oxidizes the mixture that then turns brown and becomes very brittle. The final steps in the manufacture of
chewing gum involves cleaning, filtering, sterilizing and compounding with
various flavoring materials. Thirteen
pounds of the final product yields about 5000 pieces of chewing gum. A piece of gum usually contains about 15%
chicle, the remainder being chicle substitutes, sugar and flavoring
substances. Attempts to find
substitutes have not been very successful, but inferior latex from other
sapodillas, balata and jelutong are all utilized to some extent. Sorva or leche caspi, obtained from Couma
macrocarpa, a large tree of the upper Amazon, was exploited during World
War II. The tree, which has very abundant
latex, is felled, ringed and the latex is collected in cups made from palm
leaflets. The latex is then
coagulated by boiling and is shipped in the form of large blocks. The white wood of this species is also
used for furniture, and the fruits have a mucilaginous but edible pulp.
By 1952 the United States was the greatest chewing-gum market and used
about the entire output of chicle.
The supply was chiefly from Mexico, Guatemala and British Honduras. Rubber Plantation <bot713> Hevea brasiliensis plantation {Brazil}
[CLICK to enlarge)]
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