05 March 2009
The invention: The first practical synthetic rubber product developed, Buna inspired the creation of other other synthetic substances that eventually replaced natural rubber in industrial applications. The people behind the invention: Charles de la Condamine (1701-1774), a French naturalist Charles Goodyear (1800-1860), an American inventor Joseph Priestley (1733-1804), an English chemist Charles Greville Williams (1829-1910), an English chemist A New Synthetic Rubber The discovery of natural rubber is often credited to the French scientist Charles de la Condamine, who, in 1736, sent the French Academy of Science samples of an elastic material used by Peruvian Indians to make balls that bounced. The material was primarily a curiosity until 1770, when Joseph Priestley, an English chemist, discovered that it rubbed out pencil marks, after which he called it “rubber.” Natural rubber, made from the sap of the rubber tree (Hevea brasiliensis), became important after Charles Goodyear discovered in 1830 that heating rubber with sulfur (a process called “vulcanization”) made it more elastic and easier to use. Vulcanized natural rubber came to be used to make raincoats, rubber bands, and motor vehicle tires. Natural rubber is difficult to obtain (making one tire requires the amount of rubber produced by one tree in two years), and wars have often cut off supplies of this material to various countries. Therefore, efforts to manufacture synthetic rubber began in the late eighteenth century. Those efforts followed the discovery by English chemist Charles GrevilleWilliams and others in the 1860’s that natural rubber was composed of thousands of molecules of a chemical called isoprene that had been joined to form giant, necklace- like molecules. The first successful synthetic rubber, Buna, was patented by Germany’s I. G. Farben Industrie in 1926. The success of this rubber led to the development of many other synthetic rubbers, which are now used in place of natural rubber in many applications.From Erasers to Gas Pumps Natural rubber belongs to the group of chemicals called “polymers.” Apolymer is a giant molecule that is made up of many simpler chemical units (“monomers”) that are attached chemically to form long strings. In natural rubber, the monomer is isoprene (dimethylbutadiene). The first efforts to make a synthetic rubber used the discovery that isoprene could be made and converted into an elastic polymer. The synthetic rubber that was created from isoprene was, however, inferior to natural rubber. The first Buna rubber, which was patented by I. G. Farben in 1926, was better, but it was still less than ideal. Buna rubber was made by polymerizing the monomer butadiene in the presence of sodium. The name Buna comes from the first two letters of the words “butadiene” and “natrium” (German for sodium). Natural and Buna rubbers are called homopolymers because they contain only one kind of monomer. The ability of chemists to make Buna rubber, along with its successful use, led to experimentation with the addition of other monomers to isoprene-like chemicals used to make synthetic rubber. Among the first great successes were materials that contained two alternating monomers; such materials are called “copolymers.” If the two monomers are designated Aand B, part of a polymer molecule can be represented as (ABABABABABABABABAB). Numerous synthetic copolymers, which are often called “elastomers,” now replace natural rubber in applications where they have superior properties. All elastomers are rubbers, since objects made from them both stretch greatly when pulled and return quickly to their original shape when the tension is released. Two other well-known rubbers developed by I. G. Farben are the copolymers called Buna-N and Buna-S. These materials combine butadiene and the monomers acrylonitrile and styrene, respectively. Many modern motor vehicle tires are made of synthetic rubber that differs little from Buna-S rubber. This rubber was developed after the United States was cut off in the 1940’s, during World War II, from its Asian source of natural rubber. The solution to this problem was the development of a synthetic rubber industry based on GR-S rubber (government rubber plus styrene), which was essentially Buna-S rubber. This rubber is still widely used.Buna-S rubber is often made by mixing butadiene and styrene in huge tanks of soapy water, stirring vigorously, and heating the mixture. The polymer contains equal amounts of butadiene and styrene (BSBSBSBSBSBSBSBS). When the molecules of the Buna-S polymer reach the desired size, the polymerization is stopped and the rubber is coagulated (solidified) chemically. Then, water and all the unused starting materials are removed, after which the rubber is dried and shipped to various plants for use in tires and other products. The major difference between Buna-S and GR-S rubber is that the method of making GR-S rubber involves the use of low temperatures. Buna-N rubber is made in a fashion similar to that used for Buna- S, using butadiene and acrylonitrile. Both Buna-N and the related neoprene rubber, invented by Du Pont, are very resistant to gasoline and other liquid vehicle fuels. For this reason, they can be used in gas-pump hoses. All synthetic rubbers are vulcanized before they are used in industry. Impact Buna rubber became the basis for the development of the other modern synthetic rubbers. These rubbers have special properties that make them suitable for specific applications. One developmental approach involved the use of chemically modified butadiene in homopolymers such as neoprene. Made of chloroprene (chlorobutadiene), neoprene is extremely resistant to sun, air, and chemicals. It is so widely used in machine parts, shoe soles, and hoses that more than 400 million pounds are produced annually. Another developmental approach involved copolymers that alternated butadiene with other monomers. For example, the successful Buna-N rubber (butadiene and acrylonitrile) has properties similar to those of neoprene. It differs sufficiently from neoprene, however, to be used to make items such as printing press rollers. About 200 million pounds of Buna-N are produced annually. Some 4 billion pounds of the even more widely used polymer Buna-S/ GR-S are produced annually, most of which is used to make tires. Several other synthetic rubbers have significant industrial applications, and efforts to make copolymers for still other purposes continue.