rubber
, elastic substance obtained from the exudations of certain tropical plants (natural rubber) or derived from petroleum and
natural gas
(synthetic rubber). Because of its elasticity,
resilience
, and toughness, rubber is the basic
constituent
of the tires used in automotive vehicles, aircraft, and bicycles. More than half of all rubber produced goes into automobile tires; the rest goes into mechanical parts such as mountings, gaskets, belts, and hoses, as well as consumer products such as shoes, clothing, furniture, and toys.
The main chemical
constituents
of rubber are
elastomers
, or “elastic polymers,” large chainlike molecules that can be stretched to great lengths and yet recover their original shape. The first common
elastomer
was
polyisoprene
, from which natural rubber is made. Formed in a living organism, natural rubber consists of solids suspended in a milky fluid, called
latex
, that circulates in the inner portions of the bark of many tropical and subtropical trees and shrubs, but predominantly
Hevea brasiliensis
, a tall softwood tree originating in Brazil. Natural rubber was first scientifically described by
Charles-Marie de La Condamine
and Francois Fresneau of France following an expedition to
South America
in 1735. The English chemist
Joseph Priestley
gave it the name rubber in 1770 when he found it could be used to rub out pencil marks. Its major commercial success came only after the
vulcanization
process was invented by
Charles Goodyear
in 1839.
Natural rubber continues to hold an important place in the market today; its resistance to heat buildup makes it valuable for tires used on racing cars, trucks, buses, and airplanes. Nevertheless, it
constitutes
less than half of the rubber produced commercially; the rest is rubber produced synthetically by means of chemical processes that were partly known in the 19th century but were not applied commercially until the second half of the 20th century, after
World War II
. Among the most important
synthetic rubbers
are
butadiene rubber
,
styrene-butadiene rubber
,
neoprene
, the
polysulfide
rubbers (thiokols),
butyl rubber
, and the
silicones
.
Synthetic
rubbers, like natural rubbers, can be toughened by vulcanization and improved and modified for special purposes by reinforcement with other materials.
Essential properties of the polymers used to produce the principal commercial rubbers are listed in the
table.
Britannica Quiz
Building Blocks of Everyday Objects
Properties and applications of commercially important elastomers
polymer type
|
glass transition temperature (°C)
|
melting temperature (°C)
|
heat resistance*
|
oil resistance*
|
flex resistance*
|
typical products and applications
|
*E = excellent, G = good, F = fair, P = poor.
|
polyisoprene (natural rubber, isoprene rubber)
|
?70
|
25
|
P
|
P
|
E
|
tires, springs, shoes, adhesives
|
styrene-butadiene copolymer (styrene-butadiene rubber)
|
?60
|
|
P
|
P
|
G
|
tire treads, adhesives, belts
|
polybutadiene (butadiene rubber)
|
?100
|
5
|
P
|
P
|
F
|
tire treads, shoes, conveyor belts
|
acrylonitrile-butadiene copolymer (nitrile rubber)
|
?50 to ?25
|
|
G
|
G
|
F
|
fuel hoses gaskets, rollers
|
isobutylene-isoprene copolymer (butyl rubber)
|
?70
|
?5
|
F
|
P
|
F
|
tire liners, window strips
|
ethylene-propylene monomer (EPM), ethylene-propylene-diene monomer (EPDM)
|
?55
|
|
F
|
P
|
F
|
flexible seals, electrical insulation
|
polychloroprene (neoprene)
|
?50
|
25
|
G
|
G
|
G
|
hoses, belts, springs, gaskets
|
polysulfide (Thiokol)
|
?50
|
|
F
|
E
|
F
|
seals, gaskets, rocket propellants
|
polydimethyl siloxane (silicone)
|
?125
|
?50
|
G
|
F
|
F
|
seals, gaskets, surgical implants
|
fluoroelastomer
|
?10
|
|
E
|
E
|
F
|
O-rings, seals, gaskets
|
polyacrylate elastomer
|
?15 to ?40
|
|
G
|
G
|
F
|
hoses, belts, seals, coated fabrics
|
polyethylene (chlorinated, chlorosulfonated)
|
?70
|
|
G
|
G
|
F
|
O-rings, seals, gaskets
|
styrene-isoprene-styrene (SIS), styrene-butadiene-styrene (SBS) block copolymer
|
?60
|
|
P
|
P
|
F
|
automotive parts, shoes, adhesives
|
EPDM-polypropylene blend
|
?50
|
|
F
|
P
|
F
|
shoes, flexible covers
|
The rubber tree
Commercially, natural rubber is obtained almost exclusively from
Hevea brasiliensis
,
a tree
indigenous
to South America, where it grows wild to a height of 34 metres (120 feet).
Cultivated
in plantations, however, the tree grows only to about 24 metres (80 feet) because carbon, necessary for growth, is also an essential constituent of rubber. Since only atmospheric
carbon dioxide
can supply carbon to the plant, the element has to be rationed between the two needs when the tree is in active production. Also, with foliage limited to the top of the tree (to
facilitate
tapping), the intake of carbon dioxide is less than in a wild tree. Other trees, shrubs, and herbaceous plants produce rubber, but, because none of them compares for
efficiency
with
Hevea brasiliensis,
industry botanists have concentrated their efforts exclusively on this species.
In the cultivation of
Hevea,
the natural
contours
of the land are followed, and the trees are protected from wind. Cover crops planted
adjacent
to the rubber trees hold rainwater on sloping ground and help to fertilize the soil by fixing atmospheric nitrogen. Standard horticultural techniques, such as nursery growing of hardy rootstocks and grafting on top of them, hand pollination, and
vegetative propagation
(cloning) to produce a genetically uniform product, are also employed.
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Hevea
grows only within a well-defined area of the tropics and subtropics where frost is never encountered. Heavy annual rainfall of about 2,500 mm (100 inches) is essential, with emphasis on a wet spring. As a consequence of these requirements, growing areas are limited.
Southeast Asia
is particularly well situated for rubber culture; so too are parts of
South Asia
and
West Africa
. Cultivation of
Hevea
in Brazil, its native habitat, was virtually destroyed by
blight
early in the 20th century.