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Properties and Uses of Rubber Properties Uses Natural Rubber Cultivation Tapping Collecting and Processing Latex Synthetic Rubber SBR Polybutadiene Rubber Neoprene, or Polychloroprene Butyl Rubber Reclaimed Rubber The Mark Process The Open Steam Process Making Rubber Products Masticating The Rubber Mill The Banbury Mixer The Gordon Plasticizer Mixing Extrusion Molding Electrodeposition Dipping Other Processes History of Rubber Natural Rubber Synthetic Rubber Properties and Uses of Rubber Properties Rubber is not only elastic, but is also waterproof and is a good electrical insulator. Natural rubber is resilient and is resistant to tearing. Some types of rubber are resistant to oils, solvents, and other chemicals. In a raw state, natural and synthetic rubber become sticky when hot and brittle when cold. The vulcanization process modifies rubber so that these changes will not occur. In the typical vulcanization process, sulfur and certain other substances are added to raw rubber and the mixture is then heated. The process tends to increase rubber's elasticity and its resistance to heat, cold, abrasion, and oxidation. It also makes rubber relatively airtight and resistant to deterioration by sunlight. The molecules that make up rubber are long, coiled, and twisted. They are elongated by a stretching force and tend to resume their original shape when the force is removed, giving rubber the property of elasticity. Vulcanization sets up chemical linkages between the molecules, improving rubber's ability to return to its original shape after it is stretched. Uses Rubber is made into articles as diverse as raincoats and sponges, bowling balls and pillows, electrical insulation and erasers. People ride on rubber tires and walk on rubber heels. Rubber is also used in toys, balls, rafts, elastic bandages, adhesives, paints, hoses, and a multitude of other products. The single most important use of rubber is for tires. Most tires contain several kinds of rubber, both natural and synthetic. Radial automobile tires contain a greater percentage of natural rubber than other types of automobile tires because radial tires have flexible sidewalls that tend to produce a buildup of heat, to which natural rubber has a superior resistance. Either natural or synthetic rubber is suitable for most uses, and price determines which is used. Natural Rubber Rubber can be obtained from a large number of plants, but almost all commercially produced natural rubber comes from the rubber tree, Hevea brasiliensis. This tree, sometimes called the Par$aA rubber tree, is native to the Amazon Basin but today is grown on plantations in many other tropical areas. The tree grows best in warm climates with an annual rainfall of at least 75 inches (1,900 mm) well distributed throughout the year. The leading rubber-producing countries are Thailand, Indonesia, and Malaysia. Rubber is also produced in India, China, and Sri Lanka; in several African countries; and in tropical America. Brazil, homeland of the rubber tree, still produces rubber, but on a much smaller scale than in the past. Other natural sources of rubber include the guayule plant; the gutta-percha tree; the Russian dandelion, or kok-saghyz; and the rubber plant. Hevea brasiliensis is a member of the spurge family, Euphorbiaceae. Cultivation Rubber trees are not grown directly from seed, for trees thus grown vary greatly in production of latex, and the average yield is low. To obtain trees of high production, buds from high-producing trees are grafted on year-old seedlings. After the bud has sprouted, the top of the seedling is removed, and the shoot from the bud is allowed to replace it. The resulting tree has the properties of that from which the bud was taken. The best budded trees produce up to 2,000 pounds of dry rubber per acre (2,240 kg per hectare). Budded seedlings are transplanted at the rate of about 100 to 150 per acre (250 to 370 per hectare). They grow rapidly, and at about six years of age, when their trunks are about six inches (15 cm) in diameter, they can be tapped. Tapping is done with a special tool. A shallow, diagonal cut extending about halfway around the trunk is made in the bark. A spout leads from the bottom of the incision to a cup into which the latex drips. Trees are usually tapped every other day. The first cut is begun about five feet (1.5 m) above the ground. In making the next cut, the worker first strips off a layer of coagulated (thickened or solidified) latex and then makes a single cut below the previous incision. By the time the channel nears the ground, bark has grown over most of the upper incisions. Work then begins on the other side of the tree. Thus managed, trees produce for 25 years or longer. Collecting and Processing Latex Tapping begins early in the morning, when the air is relatively cool. After tapping a tree the worker puts a little ammonia in the cup to keep the latex from coagulating. After the last tree has been tapped, the worker returns to the first tree and collects the latex. The latex consists of about 35 per cent rubber. The rubber is in the form of tiny particles suspended in a liquid. Latex is prepared for shipment at a central plant, where it is either concentrated or coagulated. It is concentrated by whirling it in a centrifuge similar to a cream separator. A preservative is added to the concentrated latex to keep it in liquid form during shipment in tank cars and ships. Most latex is coagulated before shipment. This is done by putting it in tanks and adding formic acid or some other acid to it. The acid causes the rubber to separate from the liquid, just as the curd separates from the whey in cheesemaking. The curd of rubber is removed and passed through corrugated rollers, which squeeze it to remove the liquid. Pale crepe rubber is made by adding a bleach—usually sodium bisulfite—to latex before it is coagulated. Pale yellow in color, pale crepe rubber is used to make articles in which whiteness or transparency is desirable. Most natural rubber is processed to meet certain specifications and is generally graded and shipped in bales. Some rubber is shipped as ribbed smoked sheets. This rubber is prepared by hanging sheets of rubber in a smokehouse, where they are exposed to the smoke of a wood fire and kept at a temperature of about 120° F. (49° C.) for several days. Smoking preserves the rubber from molds, bacteria, and oxidation. Synthetic RubberMany types of synthetic rubber have been developed during the 20th century. Some are used as substitutes for natural rubber. Others have properties that make them superior to natural rubber for certain uses. Among the synthetic rubbers most used in the United States are SBR (styrene-butadiene rubber) and polybutadiene, neoprene, and butyl rubber. Synthetic rubber represents about two-thirds of the total worldwide production of new rubber. Synthetic rubber is made by a process called polymerization, the process of changing monomers (compounds of relatively low molecular weight and simple structure, such as isoprene) into polymers. SBR (formerly called GR-S [government rubber—styrene]) is durable and resistant to abrasion, heat, and cold. It is used mainly in tires, shock absorbers, gaskets, hoses, heels, and adhesives. SBR is produced from two monomers—butadiene and styrene. Butadiene is obtained from petroleum, butane, or ethyl alcohol. Styrene is obtained from petroleum or benzene. These monomers are converted to SBR by the emulsion polymerization process. The monomers are mixed with water and soap to form an emulsion and with a catalyst to begin the process. Other substances are added to control the molecular weight of SBR. The monomers are usually polymerized at about 40° F. (4° C). (SBR formed at this temperature is known as cold rubber.) The process produces a latex. The latex is used as is (for example, in paints and in adhesives), or it is coagulated with an acidified brine solution or an aluminum sulfate solution to concentrate the rubber. An antioxidant is added to the rubber before coagulation to prevent oxidation during drying and storage. The coagulated rubber forms fine crumbs, which are washed, dried by heat, and pressed into bales for shipment. Polybutadiene Rubber closely resembles natural rubber in many properties. It is highly elastic and resistant to abrasion, heat, and cold. It is mainly blended with natural rubber or with SBR in tires. Polybutadiene is produced from butadiene by the solvent polymerization process. The monomer is dissolved in a hydrocarbon solvent (such as kerosine), then polymerized at about 120° F. (49° C), using an alkylaluminum catalyst. This process does not produce a latex. Instead, it forms a thick, pastelike solution. An antioxidant is added to the rubber. The rubber is dried by heat to remove the solvent, then baled. Neoprene, or Polychloroprene is resistant to heat, weathering, abrasion, oils, and solvents. It is used in adhesives, paints, hoses, linings for chemical containers, and surgical and other rubber gloves. Neoprene is produced from chloroprene, which is obtained from petroleum or acetylene. The monomer is polymerized in an emulsion, using potassium persulfate as a catalyst. Neoprene is processed similarly to SBR, except that neoprene latex is usually coagulated by freezing. Butyl Rubber is resistant to acids, oxygen, and heat. Because it is highly impermeable to gases, butyl rubber is used mainly in tire linings and other gas-retaining applications. It is also used in electrical insulation, hoses, and adhesives. Butyl rubber is produced from isobutylene and isoprene, which are obtained from petroleum. The monomers are dissolved in a chlorinated solvent and polymerized at about -150° F. (-101° C.), using aluminum chloride as a catalyst. The process does not produce a latex. The rubber forms fine particles. An antioxidant is added to the rubber. The rubber is then washed to remove the solvent, dried by heat, and baled. Reclaimed RubberRubber salvaged from tires and other rubber products is used to make hard rubber goods, garden hose, floor mats, and a variety of other rubber goods. The used rubber is shredded and passed under a magnet that removes pieces of metal. The rubber is then processed by either of two methods: The Mark Process The rubber is cooked in a caustic-soda solution in a pressure cooker, or digester, at 375° F. (191° C.) for 12 to 20 hours. This step destroys any fabric that might be in the rubber and softens the rubber itself. Next, the rubber is washed, dried, and screened to remove foreign substances. It is then passed through a series of rollers that reduce it to a paper-thin sheet. Finally, the rubber is formed into a roll and sliced preparatory to milling. The Open Steam Process is used to salvage rubber that does not contain fabric. The rubber is ground, mixed with oils and caustic soda, and then placed in pans that are heated with steam. Further processing, beginning with washing and drying, is as in the Mark process. Making Rubber ProductsIn the manufacture of most rubber products, the rubber is kneaded, mixed with various substances, formed, and then vulcanized. The vulcanization process usually involves heating rubber to which a vulcanizing agent such as sulfur has been added. Masticating Rubber is first cut into small pieces and then masticated (kneaded to make it workable) in a rubber mill, a Ban-bury mixer, or a Gordon plasticizer. The Rubber Mill consists of two rollers that rotate at different speeds. The rollers thoroughly knead the rubber and make it plastic. The Banbury Mixer is a water-cooled chamber within which two rollers fitted with blades rotate in opposite directions. The Banbury mixer is the most commonly used masticating device. The Gordon Plasticizer is a long barrel within which a screw rotates. As the rubber is kneaded by the screw, its temperature is raised as high as 360°F. (182° C.). It is this heat, rather than mechanical kneading, that plasticizes the rubber. Mixing Various substances are added to produce rubber with desired properties. Sulfur is mixed with rubber to be vulcanized. Carbon black is added to tire-tread rubber and other kinds of rubber that must resist abrasion and exposure to light. Zinc oxide gives rubber a light color. Selenium makes rubber less likely to crack when repeatedly stretched. Paraffin helps protect rubber from deterioration by ozone. Other substances preserve rubber from oxidation, give it various colors, make it harder or softer, or give it other properties. Extrusion Many rubber products, including gaskets and sheeting, are made by extrusion—by being squeezed out of machines just as toothpaste is squeezed out of a tube. Molding Rubber heels, hot water bags, and a number of other products are shaped by molding. In compression molding, the proper quantity of rubber is placed in the lower part of a mold and the upper part is squeezed over it by a press. In transfer molding, fluid rubber is forced into hollow molds by hydraulic pressure. In injection molding, a screw mechanism forces fluid rubber into hollow molds. Electrodeposition Rubber gloves, overshoes, rubber-coated metals, and a number of other products are made in the same way that metals are plated with silver. A metal mold is connected to the anode, the electrode is connected to the positive pole of a source of direct current, and the mold is submerged in latex. When the current is turned on, the negatively charged latex forms a coating on the mold. When the coating reaches the proper thickness, it is stripped from the mold and vulcanized. Dipping Some articles, such as toys, balloons, and rubber gloves, are made by dipping a mold in liquid latex. The latex is then dried and stripped from the mold. Other Processes Tires are manufactured by the series of processes shown in the illustrations accompanying the article TIRE. Sponge rubber is produced by bubbles formed by gases released from certain ingredients by the heat of vulcanization. Hard rubber, known as ebonite and vulcanite, is made by adding 30 to 50 per cent sulfur to rubber before vulcanizing it. Hard rubber was once widely used for such products as bowling balls, telephone receivers, and storage battery cases, but in many instances it has been replaced by plastics. History of RubberNatural Rubber Long before the voyages of Columbus, Indians in tropical America used rubber to waterproof shoes and fabrics and to make bottles and balls. Europeans brought back samples and made many efforts to produce waterproof rubberized fabrics, but the rubber turned gluey in hot weather and became brittle in cold. Their first practical use of rubber, which was then called by its Indian name of caoutchouc ("weeping tree"), was as a pencil eraser, or rubber. It is from this use that the English chemist Joseph Priestley in 1770 named the substance rubber. In 1820 Thomas Hancock, of England, made an important advance by inventing the masticating process. However, the rubber products still became tacky or brittle with temperature changes. About 1832 Charles Goodyear, of the United States, began to seek a means of keeping rubber pliable and solid in all weather. He was unsuccessful until 1839, when he heated a mixture of rubber and sulfur. He found that the leathery rubber thus produced was unaffected by the weather. Thus vulcanization (named for the Roman god of fire) was discovered. (The story that Goodyear discovered vulcanization accidentally in his wife's kitchen is doubted by historians.) In 1841 Goodyear gave some vulcanized rubber to an English visitor, who passed it on to Hancock. Hancock discovered how it was made, and in 1843 took out a British patent for vulcanizing rubber. Goodyear did not take out a United States patent until the following year. Demand for rubber increased. In 1876 Henry Wickham, of England, sent several baskets of Brazilian rubber seeds to England. The seeds were planted in Kew Gardens, and about 2,000 of the plants that sprouted from the seeds were shipped to Ceylon (Sri Lanka). Later, seeds and plants were distributed to Malaya and other countries of Southeast Asia. In this way the growing of rubber was transferred from tropical America to plantations in the Far East. Synthetic Rubber In 1826 Michael Faraday, of England, discovered that rubber is a hydrocarbon, composed of five parts of carbon to eight parts of hydrogen. Another Englishman, Greville Williams, in 1860 changed solid rubber into a liquid he called isoprene. A Frenchman, Gust$aAve Bouchardat, treated isoprene with hydrochloric acid and obtained a rubbery substance in 1879. In 1892 Sir William Tilden, of England, produced a synthetic rubber from isoprene obtained from turpentine. His process, however, was not commercially practicable. Butadiene was first made in 1863 by the French chemist J. B. Caventou. Butadiene was polymerized to rubber in 1910. In World War I, German scientists developed a synthetic rubber that was satisfactory for hard rubber goods but not for tires and other articles of soft rubber. Two Russian scientists working in the United States, I. Ostromislensky and A. T. Maximoff, made butadiene by a new process in 1922, and in 1923 they produced a synthetic rubber from it. In 1926 German scientists began efforts to make synthetic rubber from butadiene. They first produced Buna-S from butadiene and sodium, and they later produced Buna-N from butadiene and acrylonitrile. Neoprene was developed in the United States in 1931. World War II greatly stimulated the search for a good synthetic rubber, and GR-S (SBR) and a number of other satisfactory substitutes were discovered. Beginning in 1950, new methods for making polymers and rubbers were discovered, and by 1959 an isoprene synthetic rubber identical to natural rubber was commercially available. A similar rubber made from butadiene came into use a few years later. |