INTRODUCTION OF NATURAL RUBBER

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0 [$ @& L. `/ }! d  I. INTRODUCTION
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5 Q4 _+ G1 a1 U2 O Natural rubber production begins with the collection of latex, a milky white substance produced by the cells of several plants. Through an initial cut and selective removal of bark, a rubber tree will yield approximately 1.8 kg (about 4 lb) of dry crude rubber annually.
Encarta Encyclopedia
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Rubber, natural or synthetic substance characterized by elasticity, water repellence, and electrical resistance. Natural rubber is obtained from the milky white fluid called latex, found in many plants; synthetic rubbers are produced from unsaturated hydrocarbons.

  II. NATURAL RUBBER
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In its natural state, rubber exists as a colloidal suspension in the latex of rubber-producing plants (see Colloid). The most important of these plants are the tree Hevea brasiliensis of the spurge family, and other species in the same genus, which were the sources of the original South American rubber, the commercially important Para rubber. The term Para rubber was then also applied to the product of H. brasiliensis trees cultivated in the rubber plantations of Indonesia, the Malay Peninsula, and Sri Lanka. These trees produce about 90 percent of all the new natural rubber consumed.

Crude rubber from other plant sources is generally contaminated by an admixture of resins that must be removed before the rubber is suitable for use. Such crude rubbers include gutta-percha and balata, which are products of various tropical trees in the sapodilla family, Sapotaceae. Other, nontropical sources of rubber, which were cultivated for economic reasons during World War II (1939-1945), include two shrublike plants: guayule, Parthenium argentatum, native to Mexico, and the Russian dandelion, Taraxacum kok-saghyz, native to Western Turkistan.

  A. Collection of Latex
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Rubber Production  
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Rubber Production
- A1 [. A3 E& @7 @! x9 bProducing rubber involves several stages. Workers on a plantation begin by drawing latex from trees in a method called tapping. Once the milky liquid latex is tapped and collected, it is placed in a treatment tank where the liquid will begin to gel, or coagulate, into a more solid form. Then, in a roller mill, the gel is pressed into sheets called crepe. Finally, the rubber is smoked, dried, and baled for shipping to manufacturers.
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To gather the latex from plantation trees, a diagonal cut angled downward is made through the bark; this cut extends one-third to one-half of the circumference of the trunk. The latex exudes from the cut and is collected in a small cup. The amount of latex obtained on each tapping is about 30 ml (about 1 fl oz). Thereafter, a thin strip of bark is shaved from the bottom of the original cut to retap the tree, usually every other day. When the cuttings reach the ground, the bark is permitted to renew itself before a new tapping panel is started. About 250 trees are planted per hectare (100/acre), and the annual yield for ordinary trees is about 450 kg/hectare (400 lb/acre) of dry crude rubber. In specially selected high-yield trees, the annual yield may range as high as 2225 kg/hectare (2000 lb/acre), and experimental trees that yield 3335 kg/hectare (3000 lb/acre) have been developed. The gathered latex is strained, diluted with water, and treated with acid to cause the suspended rubber particles within the latex to clump together. After being pressed between rollers to consolidate the rubber into 0.6-cm (0.25-in) slabs or thin crepe sheets, the rubber is air- or smoke-dried for shipment.

7 |4 c! f. j5 O5 `! V$ Y0 {  B. Chemical and Physical Properties
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Pure crude rubber is a white or colorless hydrocarbon. The simplest unit of rubber is isoprene, which has the chemical formula C5H8. At the temperature of liquid air, which is about -195° C (about -319° F), crude rubber is a hard, transparent solid; from 0° to 10° C (32° to 50° F) it is brittle and opaque, and above 20° C (68° F) it becomes soft, resilient, and translucent. When rubber is mechanically kneaded, or is heated above 50° C (122° F), it becomes plastic and sticky; above 200° C (392° F) it decomposes.
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Crude rubber is insoluble in water, alkali, and weak acid; it is soluble in benzene, gasoline, chlorinated hydrocarbons, and carbon disulfide. It is oxidized readily by chemical oxidizing agents, and slowly by atmospheric oxygen.
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5 s+ f2 H5 }8 C+ F, w9 e$ E5 d) `2 H  C. Historical Origins
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Some of the properties and uses of rubber were discovered by the Native South Americans long before the voyages of Columbus in 1492 made the knowledge available to Europe. For many years, the Spaniards tried to duplicate the water-resistant products (shoes, coats, and capes) of the Native South Americans, but they were unsuccessful. Rubber was merely a museum curiosity in Europe for the next two centuries.

( K  [  ?) l% ?1 K9 m2 @In 1731 the French government sent the mathematical geographer Charles Marie de La Condamine to South America on a geographical expedition. In 1736 he sent back to France several rolls of crude rubber, together with a description of the products fabricated from it by the people of the Amazon Valley. General scientific interest in the substance and its properties was revived. In 1770 the British chemist Joseph Priestley discovered that rubber can be used to erase pencil marks by rubbing, the property from which the name of the substance is derived. In 1791 the first commercial application of rubber was initiated when an English manufacturer, Samuel Peal, patented a method of waterproofing cloth by treating it with a solution of rubber in turpentine. The British inventor and chemist Charles Macintosh, in 1823, established a plant in Glasgow for the manufacture of waterproof cloth and the rainproof garments that have since borne his name.
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( C7 u3 x" T' j  v; ~' T  D. Rubber Plantations

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Rubber Tree Plantation  
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! ]4 k' Y. B+ t8 L* E/ j" k* BAlmost all plantation rubber comes from southeastern Asia, although rubber trees have been successfully cultivated elsewhere. Natural rubber is produced from the latex of various plants, most often members of the spurge family. Leaf blight frequently strikes rubber trees grown in tropical zones of the western hemisphere, so these regions may elect to produce synthetic rubber instead of depending on the natural source. Here, healthy trees in Ghana undergo routine tapping.
Encarta Encyclopedia
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6 r/ `. z6 j. V# j# K8 iThe wild rubber trees of the South American jungles continued to be the main source of crude rubber for most of the 19th century. In 1876 the British explorer Sir Henry Wickham collected about 70,000 seeds of H. brasiliensis, and, despite a rigid embargo, smuggled them out of Brazil. The seeds were successfully germinated in the hothouses of the Royal Botanical Gardens in London, and were used to establish plantations first in Ceylon (now Sri Lanka) and then in other tropical regions of the eastern hemisphere. Similar plantations have since been established, largely within a narrow belt extending about 1100 km (about 700 mi) on both sides of the equator. About 99 percent of plantation rubber comes from southeastern Asia. Attempts to establish significant rubber plantations in the tropical zone of the western hemisphere have failed because of widespread tree loss as a result of a leaf blight.

' ]0 G& x& M' e+ ^& J* M" K4 {  E. Development of Production Processes
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6 ^, j. H, x) j8 zIn the United States, rubberized goods had become popular by the 1830s, and rubber bottles and shoes made by the Native South Americans were imported in substantial quantities. Other rubber articles were imported from England, and in 1832, at Roxbury, Massachusetts, John Haskins and Edward Chaffee organized the first rubber-goods factory in the United States. However, the resulting products, like the imported articles, became brittle in cold weather, and tacky and malodorous in summer. In 1834 the German chemist Friedrich Ludersdorf and the American chemist Nathaniel Hayward discovered that the addition of sulfur to gum rubber lessened or eliminated the stickiness of finished rubber goods. In 1839 the American inventor Charles Goodyear, using the findings of the two chemists, discovered that cooking rubber with sulfur removed the gum's unfavorable properties, in a process called vulcanization. Vulcanized rubber has increased strength and elasticity and greater resistance to changes in temperature than unvulcanized rubber; it is impermeable to gases, and resistant to abrasion, chemical action, heat, and electricity; vulcanized rubber also exhibits high frictional resistance on dry surfaces and low frictional resistance on water-wet surfaces.
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  1. Reclamation of Scrap
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