Rubber is an elastomer—that is, a polymer that has the ability to regain its original shape after being deformed. Rubber is also tough and resistant to weathering and chemical attack. Elastomers can be naturally occurring polymers, such as natural rubber, or they can be synthetically produced substances, such as butyl rubber, Thiokol, or neoprene. For a substance to be a useful elastomer it must possess a high molecular weight and a flexible polymer chain.
Natural rubber is one of nature's unique materials. The Native Americans of tropical South America's Amazon basin knew of rubber and its uses long before Christopher Columbus's explorations brought it to the attention of Europeans. The Indians made balls of rubber by smoking the milky, white latex of trees of the genus Hevea that had been placed on a wooden paddle, to promote water evaporation and to cure the substance.
Spanish navigator and historian Gonzalo Fernández de Oviedo y Valdes (1478–1557) was the first European to describe these balls to a European audience. In 1615 a Spanish writer enumerated the practical uses of rubber. He reported that the Indians waterproofed their cloaks by brushing them with this latex and made waterproof shoes by coating earthen molds with it and allowing these coatings to dry.
In 1735 interest was revived in this unusual substance when French mathematical geographer and explorer Charles-Marie de La Condamine (1701–1774) sent several rolls of crude rubber to France with an accompanying description of products made from it by the South American natives. Although it met with some use in waterproofing boots, shoes, and garments, it largely remained a museum curiosity. Crude rubber possessed the valuable properties of elasticity, plasticity, strength, durability, electrical nonconductivity, and resistance to water; however, products made from it hardened in winter, softened and became sticky in summer, were attacked by solvents, and smelled bad.
Rubber, sometimes called "gum-elastic," was known to the Indians by the name of caoutchouc (from caa, "wood," and o-chu, "to flow or to weep"). In 1770 English chemist and Unitarian clergyman Joseph Priestley (1733–1804), the discoverer of oxygen, proposed the name "rubber" for the substance because it could be used to erase pencil marks by its rubbing on paper in lieu of previously used bread crumbs.
In 1791 rubber was first used commercially when English manufacturer Samuel Peal patented a method for waterproofing cloth by treating it with a solution of rubber in turpentine. In 1820 the modern rubber industry began when English coachmaker and inventor Thomas Hancock (1786–1865) established the first rubber factory. He was the first to compound rubber with other materials to be shaped into molds, a common modern industrial practice.
In 1823 Scottish chemist and inventor Charles Macintosh (1766–1843) began to manufacture double-textured rainproof garments known as "mackintoshes." He made these by introducing a coal tar naphtha solution of rubber between two pieces of fabric, thus circumventing the sticky (when warm) and brittle (when cold) surfaces associated with single-textured rubber-containing garments.
Composition and Structure
Crude rubber is primarily hydrocarbon in nature. In 1826 English chemist Michael Faraday (1791–1867) analyzed natural rubber and found it to have the empirical (simplest) formula C5H8, along with 2 to 4 percent protein and 1 to 4 percent acetone-soluble materials (resins, fatty acids, and sterols). In 1860 English chemist Charles Hanson Greville Williams (1829–1910) confirmed Faraday's analysis and in 1862 distilled natural rubber to obtain the pure monomer, which he named isoprene . He determined isoprene's vapor density and molecular formula, and he showed that it polymerizes to a rubbery product—an observation that led to the notion that rubber is a linear polymer of isoprene, proposed in 1910 by English chemist Samuel Shrowder Pickles (1878–1962).
The molecular weights of rubber molecules range from 50,000 to 3,000,000. Sixty percent of the molecules have molecular weights of greater than 1,300,000. The repeating unit in natural rubber has the cis configuration (with chain extensions on the same side of the ethylene double bond), which is essential for elasticity. If the configuration is trans (with chain extensions on opposite sides of the ethylene double bond), the polymer is either a hard plastic (naturally occurring gutta-percha, obtained from the leaves of Palaquium, a species of sapotaceous Malaysian and East Indies trees) that was used for wire and cable coating during the nineteenth century; or a substance like gutta-percha (balata, obtained from Mimusops globosa, trees native to Panama and South America), used for modern golf ball covers.
Because there are few (if any) cross-links in the chains of rubber molecules, natural rubber is thermoplastic; that is, it becomes soft and sticky in summer and hard and brittle in winter. It is also malodorous and softened or dissolved by various solvents, as noted. These undesirable properties of natural rubber were not overcome until 1839, when American inventor Charles Goodyear (1800–1860), at the end of five years of constant experimentation, accidentally placed a sample of rubber mixed with sulfur and litharge (lead oxide, PbO) on a hot stove in Woburn, Massachusetts. The operation converted rubber into a heavily cross-linked, and therefore insoluble and infusible, thermosetting polymer or "thermoset." William Brockedon, a friend of Hancock's, named Goodyear's curing process "vulcanization" (after Vulcan, the ancient Roman god of fire and metalworking). Goodyear later used the term, but only reluctantly.
In practice, vulcanization was so simple that many persons used it without paying royalties, and Goodyear spent much of his time contesting approximately sixty infringements of his patent. He died a pauper and left behind debts estimated at between $200,000 and $600,000. His name lives on in Goodyear tires and Goodyear blimps.
Paradoxically, neither Goodyear nor any of his family members or descendents were involved with the Goodyear Tire and Rubber Company, whose founder, Frank A. Seiberling, named it to honor one of America's most famous inventors and the founder of an industry that is indispensable to modern life. In 1851 Goodyear's brother Nelson used sulfur to convert natural rubber into ebonite, the first thermosetting plastic.
The Modern Rubber Industry
Vulcanization marked the birth of the modern rubber industry, and although later discoveries have somewhat modified Goodyear's original procedure, today it remains essentially the same as his process of 1839. Vulcanization is still an imperfectly understood chemical reaction between rubber and sulfur. It results in cross-linking between linear chains of rubber molecules and prevents slippage of the chains as the material retains the desired elasticity.
Temperatures of 140–180°C (184–356°F) are used for modern vulcanization, and additives other than sulfur are often used. Accelerators permit the reaction to occur at lower temperatures and in less time, and antioxidants prolong the life of rubber products by reducing the deterioration that is caused by atmospheric oxygen (or ozone), which breaks covalent bonds and lowers the molecular weight. Reinforcing agents (e.g., carbon black) increase stiffness, tensile strength, and resistance to abrasion. Coloring agents and fillers are sometimes added.
The Search for Substitutes
The earliest synthetic polymers were synthetic rubbers. Before 1920 natural rubber was the only available elastomer, but constant attempts, with varying degrees of success, to develop commercial rubber substitutes had been made previously, especially by English and German chemists, who competed with each other in the search.
As mentioned, natural rubber is a polymer consisting of repeating units of isoprene, its "mother substance." Scientists at first sought an exact chemical equivalent. But they attained their first success in preparing a suitable substitute only when they abandoned their attempts to synthesize rubber from isoprene, butadiene, or other dienes (hydrocarbons with two double bonds) and tried to synthesize an original polymer that possessed the physical properties of natural rubber.
The development of a synthetic rubber was a slow process, because it was almost impossible for the early synthetic products to compete economically with cheap natural rubber and because they were not as good as natural rubber for most uses. The driving force in the search for synthetic rubber was the shortages created by wartime needs.
CARL "SPEED" MARVEL (1894–1988)
During World War II, the United States was almost entirely blockaded from its rubber suppliers. Carl Marvel became a part of the successful effort to meet the demand for synthetics. Along with others, he worked to increase the efficiency and production of existing rubber syntheses.
During World War I German chemists, whose country was cut off from its sources of natural rubber by the British blockade, polymerized 3-methylisoprene (2,3-dimethyl-1,3-butadiene) units, (CH 2 = C(CH 3 )C(CH 3 ) = CH 2 ), obtained from acetone, to form an inferior substitute called methyl rubber. By the end of the war Germany was producing 15 tons (13.6 metric tons) of this rubber per month. The USSR (Union of Soviet Socialist Republics), which built a pilot plant at Leningrad (now St. Petersburg) in 1930 and three factories in 1932 and 1933, was the first country to institute a fullscale synthetic rubber industry.
Two Serendipitous Discoveries
During World War II the United States, cut off from India, Ceylon (now Sri Lanka), Malaysia, and the Dutch East Indies (areas which, since the late nineteenth century, had replaced South America as the main suppliers of natural rubber), developed several superior synthetic rubbers. The U.S. synthetic rubber industry originated from two discoveries that were serendipitous; that is, they occurred while the researchers were searching for something else.
In 1922 independent inventor and physician Joseph C. Patrick (1892–1965) was trying to make ethylene glycol (HOCH 2 CH 2 OH) to be used as antifreeze. Instead he discovered Thiokol (a trade name that has become generic), a rubbery polysulfide condensation product of ethylene dichloride and sodium tetrasulfide. This early product is still used for gaskets, sealants, sealer adhesives, and hoses because it is resistant to oil and organic solvents.
In 1931 Arnold Collins, a chemist in the Du Pont research group of Wallace Hume Carothers (1896–1937), the discoverer of nylon, discovered neoprene accidentally while studying the by-products of divinylacetylene (H 2 C = CH−C=CH). There are several types of neoprenes. They have high tensile strength, high resilience, and excellent resistance to oxygen, ozone, other chemicals, and oil. They also resist heat, flame, and tearing. They are good general-purpose rubbers, but they are limited to uses requiring rubbers with special properties because of their high cost.
Other Synthetic Rubbers
In 1937 Robert McKee Thomas (1908–1986) and William Joseph Sparks (1904–1976) at the Standard Oil Development Company (now Exxon) synthesized butyl rubber via the copolymerization (polymerization of a mixture of monomers) of isobutylene (2-methylpropene (CH 3 ) 2 C = CH 2 ) with a small amount of isoprene.
By 1929 the German firm I. G. Farben developed a series of synthetic rubbers similar to those produced in the USSR. They were called Buna rubbers ("Bu" for butadiene, one of the copolymers, and "na" for sodium, the polymerization catalyst ). They included the oil-resistant Buna S (S for styrene) and Buna N (N for nitrate). Buna S, styrene butadiene rubber, is currently called SBR, and it is produced at about twice the volume of natural rubber, making it the most common synthetic rubber. Buna N, acrylonitrile-butadiene rubber, is now called NBR. During World War II the United States produced these rubbers for the American war effort.
While earlier attempts to produce satisfactory synthetic rubber from isoprene were unsuccessful, in 1955 American chemist Samuel Emmett Horne Jr. (b. 1924) prepared 98 percent cis -1,4-polyisoprene via the stereospecific polymerization of isoprene. Horne's product differs from natural rubber only in that it contains a small amount of cis -1,2-polyisoprene, but it is indistinguishable from natural rubber in physical properties. First produced in 1961, BR (for butadiene rubber), a rubberlike polymer that is almost exclusively cis -1,4-polybutadiene, when blended with natural or SBR rubber, has been used for tire treads.
Polyurethane (PU) was first synthesized in the 1930s by German chemist Otto Bayer (1902–1982), who was trying to prepare a nylonlike fiber. PU is a versatile polymer that is used for rigid and flexible foams, bristles, coatings, fibers, and automobile parts, such as bumpers. Other synthetics are used in products such as stretchable fabrics and binders for paints.
After the end of World War II the American synthetic rubber industry declined sharply. However, by the early 1950s, as better and more uniform synthetic rubbers became available, it underwent a renaissance. By the early 1960s the amount of synthetic rubber produced worldwide equaled that of natural rubber, and it has increased steadily ever since. Although natural rubber performs well for most uses, some of the newer synthetics are superior to it for specialized purposes. Today rubber is indispensable for a variety of products and industries, and our modern world, with its many necessities and luxuries, would be unthinkable without it.
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