RUBBER(NATURAL), INDIA RUBBER OR CAOUTCHOUC LATEX[HEVEA BRASILIENSIS]

LISTING DESCRIPTION

Natural rubber, also called India rubber or caoutchouc, as initially produced, consists of polymers of the organic compoundisoprene, with minor impurities of other organic compounds plus water. Malaysia is a leading producer of rubber.

DETAILED DESACRIPTION

 Forms ofpolyisoprene that are used as natural rubbers are classified as elastomers. Natural rubber is used by many manufacturing companies for the production of rubber products. Currently, rubber is harvested mainly in the form of the latex from certain trees. The latex is a sticky, milky colloid drawn off by making incisions into the bark and collecting the fluid in vessels in a process called "tapping". The latex then is refined into rubber ready for commercial processing. Natural rubber is used extensively in many applications and products, either alone or in combination with other materials. In most of its useful forms, it has a large stretch ratio and high resilience, and is extremely waterproof.^[1]

Uses

Compared to vulcanized rubber, uncured rubber has relatively few uses. It is used for cements; for adhesive, insulating, and friction tapes; and for crepe rubber used in insulating blankets and footwear. Vulcanized rubber, on the other hand, has numerous applications. Resistance to abrasion makes softer kinds of rubber valuable for the treads of vehicle tires and conveyor belts, and makes hard rubber valuable for pump housings and piping used in the handling of abrasive sludge.

The flexibility of rubber is often used in hose, tires, and rollers for a wide variety of devices ranging from domestic clothes wringers to printing presses; its elasticity makes it suitable for various kinds of shock absorbers and for specialized machinery mountings designed to reduce vibration. Being relatively impermeable to gases, rubber is useful in the manufacture of articles such as air hoses, balloons, balls, and cushions. The resistance of rubber to water and to the action of most fluid chemicals has led to its use in rainwear, diving gear, and chemical and medicinal tubing, and as a lining for storage tanks, processing equipment, and railroad tank cars. Because of their electrical resistance, soft rubber goods are used as insulation and for protective gloves, shoes, and blankets; hard rubber is used for articles such as telephone housings, parts for radio sets, meters, and other electrical instruments. The coefficient of friction of rubber, which is high on dry surfaces and low on wet surfaces, leads to the use of rubber both for power-transmission belting and for water-lubricated bearings in deep-well pumps.

Varieties

The major commercial source of natural rubber latex is the Pará rubber tree (Hevea brasiliensis), a member of the spurge family, Euphorbiaceae. This species is widely used because it grows well under cultivation and a properly managed tree responds to wounding by producing more latex for several years.

Congo rubber, formerly a major source of rubber, came from vines in the genus Landolphia (L. kirkii, L. heudelotis, and L. owariensis).^[2] These cannot be cultivated, and the intense drive to collect latex from wild plants was responsible for many of the atrocities committed under the Congo Free State.

Many other plants produce forms of latex rich in isoprene polymers, though not all produce usable forms of polymer as easily as the Pará rubber tree does; some of them require more elaborate processing to produce anything like usable rubber, and most are more difficult to tap. Some produce other desirable materials, for example gutta-percha(Palaquium gutta)^[3] and chicle from Manilkara species. Others that have been commercially exploited, or at least have shown promise as sources of rubber, include the rubber fig (Ficus elastica), Panama rubber tree (Castilla elastica), various spurges (Euphorbia spp.), lettuce (Lactuca species), the related Scorzonera tau-saghyz, various Taraxacumspecies, including common dandelion (Taraxacum officinale) and Russian dandelion (Taraxacum kok-saghyz), and perhaps most importantly for its hypoallergenic properties,guayule (Parthenium argentatum). The term gum rubber is sometimes applied to the tree-obtained version of natural rubber in order to distinguish it from the synthetic version.^[1]

Discovery of commercial potential

The Para rubber tree is indigenous to South America. Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736.^[4] In 1751, he presented a paper by François Fresneau to the Académie (eventually published in 1755) which described many of the properties of rubber. This has been referred to as the first scientific paper on rubber.^[4] In England, Joseph Priestley, in 1770, observed that a piece of the material was extremely good for rubbing off pencilmarks on paper, hence the name "rubber". Later, it slowly made its way around England.

South America remained the main source of the limited amounts of latex rubber used during much of the 19th century. The trade was well protected and exporting seeds from Brazil was said to be a capital offense, although there was no law against it. Nevertheless, in 1876, Henry Wickham smuggled 70,000 Pará rubber tree seeds from Brazil and delivered them to Kew Gardens, England. Only 2,400 of these germinated after which the seedlings were then sent to India, Ceylon (Sri Lanka), Indonesia, Singapore, and British Malaya. Malaya (now Peninsular Malaysia) was later to become the biggest producer of rubber. In the early 1900s, the Congo Free State in Africa was also a significant source of natural rubber latex, mostly gathered by forced labor. Liberia and Nigeria also started production of rubber.

In India, commercial cultivation of natural rubber was introduced by the British planters, although the experimental efforts to grow rubber on a commercial scale in India were initiated as early as 1873 at the Botanical Gardens, Calcutta. The first commercial Hevea plantations in India were established at Thattekadu in Kerala in 1902.

In Singapore and Malaya, commercial production of rubber was heavily promoted by Sir Henry Nicholas Ridley, who served as the first Scientific Director of the Singapore Botanic Gardens from 1888 to 1911. He distributed rubber seeds to many planters and developed the first technique for tapping trees for latex without causing serious harm to the tree.^[5]Because of his very fervent promotion of this crop, he is popularly remembered by the nickname "Mad Ridley".^[6]

Properties

Rubber exhibits unique physical and chemical properties. Rubber's stress-strain behavior exhibits the Mullins effect and the Payne effect, and is often modeled as hyperelastic. Rubber strain crystallizes.

Due to the presence of a double bond in each repeat unit, natural rubber is susceptible to vulcanisation and sensitive to ozone cracking.

The two main solvents for rubber are turpentine and naphtha (petroleum). The former has been in use since 1764 when François Fresnau made the discovery. Giovanni Fabbroni is credited with the discovery of naphtha as a rubber solvent in 1779. Because rubber does not dissolve easily, the material is finely divided by shredding prior to its immersion.

An ammonia solution can be used to prevent the coagulation of raw latex while it is being transported from its collection site.

Elasticity

On a microscopic scale, relaxed rubber is a disorganized cluster of erratically changing wrinkled chains. In stretched rubber, the chains are almost linear. The restoring force is due to the preponderance of wrinkled conformations over more linear ones. For the quantitative treatment see ideal chain, for more examples see entropic force.

Cooling below the glass transition temperature still permits local conformational changes but a reordering is practically impossible because of the larger energy barrier for the concerted movement of longer chains. "Frozen" rubber's elasticity is low and strain results from small changes of bond lengths and angles: this caused the Challenger disaster, when the American Space Shuttle's flattened o-rings failed to relax to fill a widening gap.^[7] The glass transition is fast and reversible: the force resumes on heating.

The parallel chains of stretched rubber are susceptible to crystallization. This takes some time because turns of twisted chains have to move out of the way of the growing crystallites. Crystallization has occurred, for example, when, after days, an inflated toy balloon is found withered at a relatively large remaining volume. Where it is touched, it shrinks because the temperature of the hand is enough to melt the crystals.

Vulcanization of rubber creates disulfide bonds between chains, which limits the degrees of freedom and results in chains that tighten more quickly for a given strain, thereby increasing the elastic force constant and making the rubber harder and less extensible.

Chemical makeup

Latex is the polymer cis-1,4-polyisoprene – with a molecular weight of 100,000 to 1,000,000 daltons. Typically, a small percentage (up to 5% of dry mass) of other materials, such as proteins, fatty acids, resins, and inorganic materials (salts) are found in natural rubber. Polyisoprene can also be created synthetically, producing what is sometimes referred to as "synthetic natural rubber", but the synthetic and natural routes are completely different.^[1] Some natural rubber sources, such as gutta-percha, are composed of trans-1,4-polyisoprene, a structural isomer that has similar, but not identical, properties.

Natural rubber is an elastomer and a thermoplastic. Once the rubber is vulcanized, it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both; i.e., if it is heated and cooled, it is degraded but not destroyed.

The final properties of a rubber item depend not just on the polymer, but also on modifiers and fillers, such ascarbon black, factice, whiting, and a host of others.

PRICE

$180/KG

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