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Characteristics

Description and technical features
Origin and history

Description and technical features

Kinds of rubber

Rubbers are polymeric materials endowed with the properties of flexibility and extensibility: with the application of force, the molecules straighten out in the direction in which they are being pulled; on release from being extended, they spontaneously recover their normal, random arrangements.

Rubbers include natural rubbers (NRs) and synthetic rubbers (SRs). Natural rubber is a naturally occurring substance obtained from the exudations of certain tropical plants (see below sources of natural rubber). Synthetic rubber is artificially derived from petrochemical products. Among the most important synthetics are styrene-butadiene, polybutadiene, and isoprene (commonly classified as the "general purpose" SRs), as well as ethylene-propylene rubbers (often referred to as "specialty rubbers"). The prices of these synthetics have been historically in the range of natural rubber prices and their markets have, although to varying degrees, overlapped those of natural rubber. Synthetic rubbers are materials with ab origine distinctive chemical structures, whereas the emphasis with natural rubber lies on different types and grades within one single broad category.

Source: UNCTAD secretariat

Composition, types, and properties of natural rubber

Natural rubber (polymer designation cis-1-4 polyisoprene, empirical formula (C5H8n) is obtained from the sap ("latex") of several rubber-yielding plants (e.g., Hevea Brasiliensis and Parthenia argentatum) by coagulation with chemicals, drying, electrical coagulation, and other processes.

Latex is a polydispersed colloidal system of rubber particles in an aqueous phase. With Hevea latex, the dry rubber content varies between approximately 28-40%, although it may rise to 45-50% after a long period of non-tapping (notice that, for statistical purposes, figures for natural rubber may include the dry rubber content of latex). The rubber produced from latex contains, besides the hydrocarbon, relatively small quantities of protein, carbohydrates, resin-like substances, mineral salts, and fatty acids.

Composition of latex

Natural rubber is either exported as "latex concentrates" or processed into "dry/solid rubber" (in sheet, crepe or block forms). Latex concentrate is made from freshly tapped field latex, uncoagulated; dry rubber is prepared from coagulated field latex (sheet rubbers and pale crepes) or remilled rubber sheets (estate brown, thin brown, thick blanket, and flat bark crepes). Dry rubber is either visually graded (sheets and crepes) or classified by instruments (block rubber). Over the years, technically specified rubber in block form has become increasingly important at the expenses of other rubbers (please refer to the market section for information on NR exports by types and grades).

Types of NR

Source: UNCTAD secretariat

Natural rubber has outstanding resilience and tensile strength, as well as low heat build-up. In addition, natural rubber latex has excellent tack (that is, the ability to stick to itself and to other materials), which makes it best suited for pressure-sensitive adhesives, and excellent water resistance (whereas some synthetics absorb water). However, a drawback is natural rubber moderate environmental resistance to factors such as oxidation and ozone; so too for its scarce resistance to chemicals, including gasoline, kerosene, hydraulic fluids, degreasers, synthetic lubricants, and solvents. In addition, latex contains proteins that can cause severe allergic response in a small percentage of the population and among medical professionals following extensive exposure.

The composition, structure and properties of natural rubber make it an important industrial polymer, suitable for use in many applications.

Properties of NR

Name	Natural Rubber (NR)	Natural polyisoprene
Molecular behaviour	Glass transition temperature (° C)	-70
	Melting temperature (° C)	25
	Hardness range (Shore A)	30-100
	Maximum tensile strength (at 70 F, psi)	4000
	Maximum elongation (at 70 F, %)	750
Advantages	Physical resistance	Excellent resilience
		Exc. tear strength
		Exc. abrasion resistance
		Exc. impact strength
		Exc. cut growth resistance
		Good compression set
	Environmental resistance	Exc. water resistance
		Good-exc. low temperature flexibility
		Fair-good oxidation resistance
	Chemical resistance	Good resistance to alcohols and oxygenated solvents
		Fair-good resistance to acids
Limits	Environmental resistance	Poor ozone resistance
		Poor sunlight resistance
		Very little flame retardance
	Chemical resistance	Poor oil and gasoline resistance
		Poor resistance to (aliphatic and aromatic) hydrocarbon solvents

Source: UNCTAD secretariat (adapted from H. Long, Engineering Properties of Elastomers, The Roofing Industry Educational Institute)

For a selection of readings on the structure, composition and properties of natural rubber, please refer to the Rubber-Stichting Information Center for Natural Rubber (select "Articles from Natuurrubber" on the left menu).

Sources of natural rubber

Commercially, natural rubber is obtained almost exclusively from Hevea brasiliensis (genus Hevea, family Euphorbiaceae), a tall softwood tree indigenous to Brazil. Industry botanists have concentrated their efforts mostly on this species. The reader is referred to the crop section for further information on rubber production from Hevea.

Other trees, shrubs, and herbaceous plants produce natural rubber. Among the species which have received commercial attention are the Ficus elastica (Moraceae family), the guayule bush (of the family Asteraceae), and the Russian dandelion (Asteraceae, alt. Compositae). Guayule (Parthenium argentatum), a rubber-containing desert shrub of the family Asteraceae, native to the north-central plateau of Mexico and the Big Bend area of Texas, is the only other plant under cultivation as a commercial rubber source.
In recent years, these and other species have attracted increasing attention, because of diminishing acreage of Hevea plantations, increasing demand for high quality rubber with specialty applications, and severe allergic response to the latex based products of the Hevea. Guayule has attracted increasing interest as a commercial alternative for hypoallergenic latex. In a number of Latin American countries, hybridisation between Hevea benthamiana and Hevea brasiliensis has provided relatively SALB-free sources of rubber. The molecular mechanism of rubber biosynthesis in other plants (for example, in the fig tree -Ficus carica) is being investigated with a view to developing alternative rubber crops.

Hevea brasiliensis and most of the other rubber-yielding species grow only within a well defined area of the tropical and sub-tropical countries, though there are also some rubber-bearing species whose distributional range includes temperate areas (e.g., Taraxacum kok-saghyz, native to the Soviet Middle Asia, i.e. Kazakhstan and Kyrgyzstan, and China, as well as other species of the family Compositae).

For statistical purpose, rubbery materials extracted from species other than Hevea Brasiliensis, including guayule (Parthenium argentatum) and gutta-percha (Palachium gutta), are conventionally referred to as "Natural Gums".

Selected rubber-yielding species

Scientific name	Common name	Distributional range
Castilla elastica Sessé	Panama rubber tree	AMERICA (Mexico; Central America; Western South America) widely naturalized in tropics
Ficus vogelii (Miq.) Miq.	West African rubber tree	AFRICA (Macaronesia; Northeast Tropical Africa; East Tropical Africa; West-Central Tropical Africa; West Tropical Africa; South Tropical Africa; South Africa; Western Indian Ocean)
Funtumia africana (Benth.) Stapf	Lagos silk rubber tree	AFRICA (East Tropical Africa; West-Central Tropical Africa; West Tropical Africa; South Tropical Africa)
Hevea brasiliensis (Willd. ex Adr. Juss.) Muell. Arg.	Rubber tree	SOUTHERN AMERICA (Brazil; Bolivia; Colombia ; Peru) also cultivated & naturalized elsewhere
Holarrhena floribunda (G. Don) Durand & Schinz	False rubber tree	AFRICA (West-Central Tropical Africa; West Tropical Africa)
Funtumia elastica		AFRICA (Northeast Tropical Africa; East Tropical Africa; West-Central Tropical Africa; West Tropical Africa) also cultivated elsewhere
Ficus elastica	Indian rubber plant	ASIA-TROPICAL (India; China; Malaisia) widely cultivated elsewhere
Parthenium argentatum	Guayule	NORTHERN AMERICA (South-Central U.S.A.; Mexico)
Taraxacum koksahgyz	Russian dandelion	ASIA-TEMPERATE Former Soviet Union; China

Source: UNCTAD secretariat (Links: USDA, NRCS. 2005. The PLANTS Database, Version 3.5. Data compiled from various sources by Mark W. Skinner. National Plant Data Center, Baton Rouge, LA 70874-4490 USA).

For more detailed information, please refer to: the National PLANTS Database of the United States Department of Agriculture (USDA, NRCS. 2005. The PLANTS Database, Version 3.5 (http://plants.usda.gov);
Further information is available at the International Rubber Research & Development Board (IRRDB)'s website- and in particular the subsections rubber tree and other sources of rubber.

Origin and history

Natural rubber from the latex yielding trees Hevea and Castilla was known to South American Indians in early times (pre-Columbian era). Latex was allowed to evaporate naturally and processed into usable articles (such as balls used in ritualistic games). "Caoutchouc", the world first used to describe the condensed juice of the Hevea tree, was the French spelling of an indigenous term for "weeping wood".

Natural rubber was first scientifically described in 1735 by C.-M. de la Condamine, a member of a French geographic expedition to South America. In 1770 the English chemist Joseph Priestley found that it could be used to rub out pencil marks (the term "rubber" was thus coined). Other applications gradually developed for waterproof shoes and clothes.

The world rubber industry began to develop in the 1800s, with the invention of the masticator and the vulcanization process (see the table below for an overview of major technical achievements in the rubber industry). Demand for rubber grew rapidly with the invention of the solid and later the pneumatic rubber tire and the demand for rubber insulation by the electrical industry.

With a view to spreading rubber cultivation, seeds of the Hevea brasiliensis were smuggled from Brazil to the United Kingdom in 1876 at the instigation of the British India Office. Seedlings were raised at Kew Gardens and then shipped to Ceylon (Sri Lanka) and Singapore, where H.N. Ridley, director of the Singapore Botanic Gardens, introduced new horticultural and tapping methods. Seeds of the rubber tree were later brought to other tropical regions, especially the Malay area and Java and Sumatra, marking the origin of the rubber plantation industry in Asia.

After Ridley, the next important progress was made in the Netherlands colonial territory of the East Indies (Indonesia), where more sophisticated breeding techniques (selecting high-yielding trees as a source of seedlings for new plantings and budgrafting -that is, vegetatively propagating chosen materials) were introduced.

A notable fact in the early 1900s was also the emergence of production by smallholders, which produced rubber as one amongst several crops. Despite its original commercial development as a plantation crop, by the mid-1930s natural rubber production was evenly split evenly between estates and smallholdings.

The monopoly position of natural rubber as the only kind of industrial elastomer remained virtually unchallenged until World War II (WWII). The origins of synthetic rubber can be traced to the 19th century, when research on the chemistry of natural rubber led to the isolation of isoprene, the chemical compound from which natural rubber is polymerised. During World War I, under the stimulus of the blockade by the Allies, German scientists produced a crude synthetic rubber, and during the 1920s and '30s several polymerising processes were fisrt developed in Germany, in the Soviet Union, the United Kingdom, and the United States. However, it was not until WWII that synthetics started to be produced on a large scale. Western Europe and the United States were cut off from their main sources of NR, and turned to the domestic chemical industry to increase SR capacity. By the early 1960s synthetic rubbers had overtaken natural rubber in volume.

Milestones in the history of natural rubber

1735	Rubber samples sent to Europe by Charles-Marie de La Condamine
1763	François Fresneau discovered that rubber would dissolve in turpentine
1803	The first rubber factory was established near Paris
1823	Charles Macintosh patented a method to avoid the sticky surface of garments treated with rubber
Early 1820s	Hancock invented the masticator, a machine that shredded rubber scraps, allowing rubber to be recycled after being formed into blocks or rolled into sheets
1839	Invention of vulcanisation by Charles Goodyear
1845	The first patent for a pneumatic tire was issued to Robert William Thomson in England
1888	John Boyd Dunlop, a veterinary surgeon of Belfast, obtained patents on a pneumatic tire for bycicles. Pneumatic tires were first applied to motor vehicles by the French rubber manufacturer Michelin & Cie
1858	The first patent on an integral pencil and eraser was issued in the United States to Joseph Rechendorfer of New York City
1876	Seeds of the hevea brasiliensis were shipped from the upper Orinoco basin in Brazil to Kew Garden, London, at the instigation of Sir Henry Wickham
1888-1911	Henry Ridley, Director of the Singapore Botanic Gardens, introduced modern horticultural and tapping methods for rubber

For more detailed information on the history of NR, please refer to:
- History of natural rubber (the International Rubber Research and Development Board);
- "Historical perspectives" at bouncing-ball.com;
- "Natural Rubber", the newsletter of the R-S Information Center for Natural Rubber (articles listed under the heading "history").