natural or synthetic substance characterized by elasticity, water repellence, and electrical resistance. Natural rubber is obtained from the milky white fluid called LATEX, (q.v.), found in many plants; synthetic rubbers are produced from unsaturated hydrocarbons.

NATURAL RUBBER

In its natural state, rubber exists as a colloidal suspension in the latex of rubber-producing plants. The most important of these plants are the tree Hevea brasiliensis of the SPURGE, (q.v.) family, and other species in the same genus, which were the sources of the original native 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 guttapercha 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, include two shrublike plants: GUAYULE, (q.v.), Parthenium argentatum, native to Mexico, and the Russian dandelion, Taraxacum koksaghyz, native to Russian Turkestan.

Collection of Latex.

To gather the latex from plantation trees, a cut is made through the bark; this cut extends one-third to one-half of the circumference of the trunk and is made in the shape of a chevron. The latex exudes from the cut and is collected in a small cup, the amount of latex obtained on each tapping being 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 and 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/ha (400 lb/acre) of dry crude rubber. In specially selected high-yield trees, the annual yield may range as high as 2225 kg/ha (2000 lb/acre), and experimental trees that yield 3335 kg/ha (3000 lb/acre) have been developed. The gathered latex is strained, diluted with water, and treated with acid to coagulate the colloidal particles of rubber. After being pressed between rollers to consolidate the coagulum into 0.6-cm (0.25-in) slabs or thin crepe sheets, the rubber is air- or smoke-dried for shipment.

Chemical and Physical Properties.

Pure crude rubber is a white or colorless hydrocarbon that has, as its simplest unit, isoprene, C5H8. At the temperature of liquid air, 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; below 200° C (392° F) it decomposes.

Crude rubber is insoluble in water, alkali, and weak acid; it is soluble in benzene, gasoline, chlorinated hydrocarbons, and carbon bisulfide. It is oxidized readily by chemical oxidizing agents, and slowly by atmospheric oxygen. When crude rubber is heated with sulfur to a temperature between 120° and 160° C (248° and 320° F) it becomes vulcanized, that is, it has more strength and elasticity and greater resistance to changes in temperature; it becomes impermeable to gases; resistant to abrasion, chemical action, heat, and electricity; and exhibits high frictional resistance on dry surfaces and low frictional resistance on water-wet surfaces.

Historical Origins.

Some of the properties and uses of rubber were discovered by the Indians of tropical South America long before the voyages of Columbus made the knowledge available to the Old World. For many years, the Spaniards tried to duplicate the water-resistant products (shoes, coats, and capes) of the Indians, but they were unsuccessful. Rubber became merely a museum curiosity in Europe for the duration of the next two centuries.

In 1731 the French government sent the mathematical geographer Charles Marie de La Condamine (1701–74) 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 Indians of the Amazon Valley. General scientific interest in the substance and its properties was revived, and ways were sought to dissolve the gum—which hardens rapidly after gathering—so that it might be worked at a distance from its natural source. Many scientists labored on the problem, and 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.

Rubber Plantations.

The 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 (1846–1928) 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 either side 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.

Development of Production Processes.

In the U.S., rubberized goods had become popular by the 1830s, and rubber bottles and shoes made by the South American Indians were imported in substantial quantities. Other rubber articles were imported from England, and in 1832, at Roxbury, Mass., John Haskins and Edward M. Chaffee (1806–72) organized the first rubber goods factory in the U.S.; however, their products were, like the imported articles, brittle in cold weather, and tacky and malodorous in summer. In 1834 the German chemist Friedrich Ludersdorf and the American chemist Nathaniel Hayward (1808–65) 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, accidentally found that cooking sulfur-treated rubber removed the gum's unfavorable properties; the process, called vulcanization, is still the basis of rubber manufacture.

Reclamation of scrap.

Shortly after the invention of the pneumatic tire in 1877, the American manufacturer Chapman Mitchell founded a new branch of the industry by introducing the acid-reclamation process for scrap rubber. This process used hot sulfuric acid to destroy fabric incorporated in the scrap and heat treatment to render the scrap sufficiently plastic to incorporate in batches of crude rubber. About 1905 the alkaline-recovery process was invented by the American chemist Arthur H. Marks, who also established the first rubber-factory laboratory and placed rubber research on a scientific basis. His alkaline-recovery process permitted the use of large quantities of reclaimed rubber without seriously impairing the quality of the finished product. In the following year the American chemist George Oenslager (1873–1956), working in Marks's laboratory on the problem of using low-grade rubber in manufacturing processes, discovered organic accelerators of vulcanization, such as aniline and thiocarbanilide. These accelerators not only reduced the time of heating necessary for vulcanization by 60 to 85 percent, but also increased the quality of the product, although crude rubbers of inferior grades were used as raw materials.

Prolonged rubber life.

The next great advance in rubber technology came a decade later with the invention of the accelerated-aging oven for measuring rubber deterioration. This oven duplicated, in a few days, the results of years of normal use. It enabled rubber technologists to measure rapidly the deterioration caused by various conditions, especially exposure to atmospheric oxygen, and to adopt the use of agents, called antioxidants, that notably prolonged the useful life of heavy rubber articles, such as automobile tires. Within a few years, new chemical compounds were created that markedly slowed the deterioration of such soft rubber goods as gloves, sheeting, tubing, and other sanitary articles.

Other developments.

Another development in rubber technology involved the use of uncoagulated latex. Methods were developed of extruding rubber in fine threads for use in the fabrication of textile materials, such as those used in the manufacture of elastic undergarments, and also of electroplating rubber on metals and other materials.

Modern Manufacturing Processes.

In the modern manufacture of natural-rubber articles, the crude rubber is treated with compounding ingredients in several mixing machines. The mixture is then applied mechanically to a base or is shaped, and the coated object or shaped mixture is placed in molds and vulcanized.

The principal sources of crude rubber are the sheets, slabs, or biscuits produced on rubber plantations from the latex of Hevea trees, or, in certain manufacturing operations, the uncoagulated latex. Such crude rubber is used as shipped; secondary grades are washed to remove sand, bark, and other foreign material, and are treated to remove excess resin. Reclaimed rubber, treated by heating with alkali for 12 to 30 hours, may be used as an adulterant of crude rubber to lower the price of the finished article. The amounts of reclaimed rubber that are used depend on the quality of the article to be manufactured. Scrap rubber, consisting of unvulcanized trimmings from previous processes, is used as available.

Compounding ingredients.

For the majority of applications, the raw rubber is mixed with a variety of compounding ingredients to modify its characteristics. Fillers that stiffen the rubber in the final product, but that do not materially increase its strength, include such agents as whiting, or calcium carbonate, and barite, or barium sulfate. Reinforcing fillers add materially to the strength of the finished product; they include such materials as carbon black, zinc oxide, magnesium carbonate, and various clays. Pigments include zinc oxide, lithopone, and a number of organic dyes. Softeners, which are necessary when the mix is too stiff for proper incorporation of the various ingredients, consist usually of petroleum products, such as oils or waxes, of pine tar, or of fatty acids.

The principal vulcanizing agent continues to be sulfur; selenium and tellurium are also used, but generally with large proportions of sulfur. In the hot process of vulcanizing, which is used for most rubber goods, the sulfur is ground and mixed with the rubber at the same time as the other dry ingredients. The proportion of sulfur to rubber varies from 1:40 in soft rubber goods, to as much as 1:1 in hard rubber. Cold vulcanization, used chiefly for soft, thin rubber goods such as gloves and sheeting, is accomplished by exposing the uncured articles to the vapor of sulfur chloride, S2Cl2. Accelerators of vulcanization at first included only metallic oxides, such as white lead and lime. Since the discoveries of Oenslager, however, they include a wide variety of organic amines. The age-resisting agents are mostly secondary amino compounds.

Masticating machines.

Before the compounding ingredients are mixed with the raw rubber, it is subjected to a mechanical grinding process called mastication, to make it soft, plastic, and sticky. In such conditions, it mixes and blends more easily and more thoroughly with the various fillers, pigments, vulcanizers, and other dry ingredients, or with the various solvents used in the production of cements. Masticators are of two types. One type is the rubber mill, consisting of two power-driven steel rollers, which rotate at different rates in a trough to shear and knead the rubber until it is broken down to a soft and pliable condition. The rollers are hollow to permit the circulation of cold water or steam to control the temperature of the operation. After the 1920s the rubber mill was largely replaced by the Gordon plasticator, which consists of a power-driven variable-pitch screw (like the common household food chopper) operating in a jacketed cylinder. The churning action of the screw on the rubber generates temperatures of 182° C (360° F). The heat, rather than the mechanical action, breaks down the rubber.

Mixing machines.

After the masticator, the next machine in the production line is the mixer. Mixers may resemble the masticator in having two rollers, but in the mixer the rollers rotate in opposite directions, whereas in the masticator the rollers rotate in the same direction at different speed. Closed-cylinder mixers are also used, especially in the preparation of rubber solutions and cements by the admixture of solvents. Such liquid preparations of rubber are used in the waterproofing of fabrics and in the manufacture of articles, such as rubber gloves, that are shaped by dipping a form into the solution. In most cases, however, the ingredients are dry-mixed for later calendering, extrusion, or other fabrication in the preparation for final vulcanization.

Calendering.

After crude rubber has been plasticized and mixed with compounding ingredients, it undergoes either calendering or extrusion, depending on the use for which it is intended. Calenders are machines consisting of three to five rolls of equal diameter, arranged to be adjustable for clearance between the various rolls, and for operating at the same or at different speeds, depending on the product desired. Calenders are used for sheeting (the production of sheets of compounded raw rubber, with or without impressed designs such as tire-tread markings); for frictioning (the squeezing of rubber into the texture of fabrics or cords); or for coating, with a layer of rubber, fabrics or cords that had previously been frictioned or coated. The products of the calender are generally further fabricated, as in the processes involved in the manufacture of automobile tires, before they are vulcanized.

Extrusion.

Extrusion presses are used to force the rubber compound through dies to form flat, tubular, or specially shaped strips, which are used in the manufacture of rubber tubing, hose, inner tubes, and such stripping as channels for setting windows or sealing doors. Specially designed extrusion heads are used for coating tubular fabric in making pressure hose and mixing colors in variegated or striped patterns.

Vulcanization.

After fabrication is complete, most rubber products are vulcanized under high temperature and pressure. Many articles are vulcanized in molds that are placed under compression in hydraulic presses, or are subjected to external or internal steam pressure during heating. Certain types of garden hose, for example, are given a coating of lead, and are vulcanized by passing live steam through the opening in the hose, the rubber hosing being compressed against the lead housing during the process. After the process is complete, the lead is stripped from the hose and is melted down for reuse. Tin sheathing is used in the same way in processing certain types of high-grade electrical insulation.

Foam rubber and dip goods.

Foam rubber is manufactured directly from latex by using emulsified compounding ingredients. The mixture is then whipped mechanically in a frothing machine into a form containing millions of air bubbles. The foam is poured into molds and vulcanized by heating to form such articles as mattresses and seat cushions.

Latex can be shaped into such products as toys or gloves by dipping forms made of porcelain or plaster of paris into concentrated latex. A coating of latex adheres to the form and is stripped off after vulcanization.

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, based on its varied properties. Resistance to abrasion makes softer kinds of rubber valuable for the treads of vehicle tires and conveyor belting, and hard rubber valuable for pump housings and piping used in the handling of abrasive sludges.

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 air hoses, balloons, balls, cushions, and other pneumatic articles. The resistance of rubber to water and to the action of most fluid chemicals has led to its use for 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 numerous articles, such as telephone housings and parts for radio sets, meters, and other electrical instruments. The coefficient of friction of rubber, being high on dry surfaces and low on water-wet surfaces, leads to such divergent uses as its employment for power-transmission belting and for water-lubricated bearings in deep-well pumps.

Rubber Production.

The area devoted to the cultivation of rubber had peak activity in the years immediately preceding World War II. In British possessions in India, Sri Lanka, the Malay Peninsula, and the Malay Archipelago, about 1,820,000 ha (about 4.5 million acres) were under cultivation. Another 1,420,000 ha (3.5 million acres) of rubber plantations in the Netherlands East Indies (now Indonesia) completed the bulk of the world acreage of some 3,640,000 ha (9 million acres) that existed before the destruction of many of the plantations in the Far East during World War II.

The political and economic significance of natural rubber became evident when, during World War II, the supply from the Far East was terminated. The acute rubber shortage accelerated the development of synthetic rubber in various countries, notably the U.S. Synthetic rubber eventually may eliminate the need for natural rubber.

World production of crude natural rubber totaled 6 million metric tons a year in the mid-1990s. Thailand, Indonesia, and Malaysia together accounted for more than 70 percent of the world output. The annual U.S. consumption of natural rubber in the late 1990s was about 1.04 million metric tons, compared to 2.28 million metric tons of synthetic rubber.

SYNTHETIC RUBBER

Any artificially produced substance that resembles natural rubber in essential chemical and physical properties may be called synthetic rubber. Such substances, also called elastomers, are produced by chemical reactions, known as condensation or polymerization, of certain unsaturated hydrocarbons. The basic materials of synthetic rubber are monomers: compounds of relatively low molecular weight that form the building units of huge molecules called polymers (see POLYMER,). After fabrication, the synthetic rubber is cured by vulcanization.

Development.

The origin of synthetic-rubber technology can be traced to 1860, when the British chemist Charles Hanson Greville Williams (1829–1910) determined that natural rubber was a polymer of the monomer isoprene—formula CH2:C(CH3)CH:CH2. Many efforts were made during the next 70 years to synthesize rubber in the laboratory by using isoprene as the monomer. Other monomers also were investigated, and during World War I German chemists polymerized dimethylbutadiene—formula CH2:C(CH3)C(CH3):CH2—producing an elastomer called methyl rubber, which was of limited usefulness.

A breakthrough in synthetic-rubber research did not occur, however, until about 1930, when the American chemist Wallace Hume Carothers (1896–1937) and the German scientist Hermann Staudinger (1881–1965) placed polymer chemistry on a theoretically sound scientific basis. The work of these scientists contributed greatly to present-day knowledge that polymers are huge molecules with a chainlike structure, arising from chemical reactions of large numbers of monomers, and that synthetic rubber can be prepared from monomers other than isoprene.

Synthetic-rubber research initiated in the U.S. during World War II led to the synthesis of a polymer of isoprene identical in chemical composition with natural rubber.

Types of Synthetic Rubber.

Various types of synthetic rubber are in production, their composition determined by the specific use for which they are intended.

Neoprene.

One of the first successful synthetic rubbers resulting from Carothers's research was neoprene, which is the polymer of the monomer chloroprene, CH2:C(Cl)CH:CH2. The raw materials of chloroprene are acetylene and hydrochloric acid. Developed in 1931, neoprene has high resistance to heat and such chemicals as oils and gasoline and is used in hose for conveying gasoline and as an insulating material for cables and in machinery.

Buna rubbers.

In 1935 German chemists developed the first of a group of synthetic rubbers called Buna, produced by copolymerization, that is, the polymerization of two monomers, called comonomers. The name Buna is derived from the initial letters of BUTADIENE, (q.v.), used as one of the comonomers, and natrium (sodium), which was used as a catalyst. One of these products, Buna-N, uses acrylonitrile, CH2:CH(CN), as the other comonomer. Acrylonitrile is produced from cyanide. Buna-N is valuable for uses requiring resistance to the action of oils or abrasion.

During World War II a Buna-type rubber called GR-S (Government Rubber-Styrene) was designated as the general-purpose rubber for the U.S. war effort. The basic rubber produced by the present-day U.S. synthetic-rubber industry, GR-S is a copolymer of butadiene and styrene. The various grades of GR-S are classified in two categories, regular and cold, depending on the temperatures of copolymerization. Cold GR-S types, which exhibit superior properties, are prepared at 5° C (41° F), compared to temperatures of 50° C (122° F) in the preparation of regular GR-S rubbers. Cold GR-S is used to make longer-wearing tires for automobiles and trucks.

Butyl rubber.

Butyl rubber, produced initially in 1940, is prepared by copolymerization of isobutylene with butadiene or isoprene. It is plastic and can be compounded like natural rubber, but is difficult to vulcanize. Although butyl rubber is not as resilient as natural rubber and other synthetics, it is extremely resistant to oxidation and the action of corrosive chemicals. Because of its low permeability to gas, butyl rubber is used widely for inner tubes in automobile tires.

Other specialty rubbers.

Many other types of synthetic rubber have been developed for purposes requiring particular advantageous properties. One such specialty rubber, called Koroseal, is a polymer of vinyl chloride, CH2:CHCl. Vinyl chloride polymers are heat-, electric-, and corrosion-resistant and are unaffected by exposure to light or by long storage. Koroseal cannot be vulcanized, but, when not subjected to high temperatures, it is more resistant to abrasion than natural rubber or leather.

Another specialty rubber is Thiokol, produced by copolymerization of ethylene dichloride CHCl:CHCl, and sodium tetrasulfide, Na2S4. This type, which can be compounded and vulcanized like natural rubber, is resistant to the action of oils and to organic solvents used for lacquers, and is useful for electrical insulation because it does not deteriorate when exposed to electrical discharge and to light.

Many other types of synthetic rubber are produced in the U.S., mostly by methods similar to those described above. Changes in the process or the polymerization recipes often succeeded in improving quality as well as reducing production costs. In one outstanding development, petroleum oil was used as an additive; it lowered the cost by conserving a substantial amount of synthetic-rubber stock. Tires made from such oil-extended rubber give excellent service. Other important advances include the development of synthetic foam rubber, used mainly for upholstery, mattresses, and pillows; and cellular-crepe rubber, used by the shoe industry.

Production.

Before World War II, synthetic rubbers were expensive and were used only when special properties were required. With the outbreak of the war, which cut down the supply of natural rubber available to the U.S., necessity forced the government to embark on a program costing more than $700 million for the establishment of synthetic-rubber plants. By 1952 annual plant capacity reached 1 million tons of high-quality rubbers at prices comparable to that of natural rubber. Because of the high productive level achieved by the synthetic-rubber industry, and because the U.S. had accumulated a natural-rubber stockpile sufficient to ensure national security, the U.S. Congress in 1953 authorized disposal of the 29 government-owned plants to private industry. In the late 1990s the annual consumption of synthetic rubber in the U.S. averaged more than twice the annual consumption of natural rubber.        S.Z.L., SEYMOUR Z. LEWIN, M.S., Ph.D.

See also PLASTICS,.

For further information on this topic, see the Bibliography, sections 416. Polymer chemistry, 635. Rubber.