As U.S. citizens this week begin feeling the effects of the nation’s reduced supplies of natural rubber, they will also begin hearing more & more about synthetic rubbers. Shortage or no shortage, synthetic rubbers will soon be making war news, for in many respects they are better than tree-bled rubber—notably for making the gas-retaining, sun-resisting barrage balloons which soon may be floating over such “air-raided” cities as San Francisco and New York.
Strictly speaking, there are no real synthetic rubbers, chemically identical with natural rubber, yet artificially made. Instead there are only 1) substitutes, and 2) kindred stretchy substances which chemists prefer to call “elastomers.”
Natural rubber can be thought of as a long hydrocarbon chain, composed essentially of a cramped-up chain of molecules of methyl butadiene or isoprene. When the rubber is stretched this chain unfolds; when the rubber contracts, it doubles up again. So the problem of synthesizing rubbers is 1) to find basic chain-units similar to methyl butadiene, 2) to build these up into larger, stringy, stretchy molecules. Best way of classifying synthetic rubbers is by their basic materials.
From almost any vegetable material—molasses, potatoes, scrap wood—but best of all from coal, gas or oil can be derived plain butadiene, a gas which is easily liquefied under pressure to form the basic building-blocks of most synthetic rubbers. Butadiene molecules were first polymerized—or built up into larger molecules—with the help of metallic sodium, making a stretchy substance which its German inventors about 1927 called Buna (Bu for butadiene, Na for sodium). It was not a very satisfactory synthetic: but better than the methyl rubber (dimethyl butadiene) of World War I, when it was said German Army trucks often had to be jacked up overnight so that their solid tires would not flatten out permanently under their weight. German chemists soon discovered ways to make superior products by combining butadiene with other substances and co-polymerizing, making a rubber called Buna-N or Perbunan and another called Buna-S, widely used in Germany for auto tires. Both are now made in the U.S. by Firestone.
The Germans realized that butadiene could be obtained far more cheaply from petroleum than from coal. So, hard up for oil, they exchanged their patents for others to Standard Oil of New Jersey, which licensed them in turn to several U.S. rubber processors. Chemists of the U.S. Rubber Co. discovered that polymerization of butadiene was easier when it was emulsified in soapy water and converted by pressure into a milky, latex-like dispersion. This method is now used in Germany. Goodrich’s Ameripol rubber is made by a similar emulsion process.
Whatever their various trade names, these rubbers are all essentially,Bunas. Many of them are not subject to natural rubber’s major drawbacks: deterioration under the influence of 1) oils, 2) high temperatures, 3) sunlight. But some of them still heat up more than natural rubber when subjected to flexing and, though elastic, are less snappy.
From acetylene is made Du Pont’s neoprene (known in an earlier, smellier form as DuPrene). Acetylene gas is made into monovinylacetylene, which reacts with hydrochloric acid to form a liquid called chloroprene. Heat and pressure polymerize this substance into a tough, elastic product which looks much like crude natural rubber, but far surpasses it in resistance to age, heat, sunlight and gases. Thus neoprene is an excellent material for coating the 1,000,000 square yards of cotton in every U.S. barrage balloon. With remarkable foresight the U.S. Army last spring placed orders or laid plans with every large rubber processor in the country for production of hundreds of such balloons.
From petroleum alone is made Standard Oil of N.J.’s Butyl rubber. Its building-blocks are olefins (unsaturated hydrocarbons like ethylene) but polymerizing agents remain secret. It is a superior synthetic rubber except that it is not oil-resistant.
Substitutes for rubber, rather than rubberlike elastomers, are Goodrich’s Koroseal, Union Carbide & Chemical’s Vinyon, etc. Most of these are synthetic resins, i.e., plastics flexible enough for use in hose, fuel-tank seals, etc. Thiokol, made by Dow Chemical Co., is used as a barrage-balloon coating by the Vulcan Proofing Co. of Brooklyn, N.Y.
Today all these synthetics and substitutes cost from two to four times as much as natural rubber (1941 price: 22½¢ per lb.), chiefly because of small volume. But of imminent significance is the fact that they are all cheaper than natural rubber was in 1923, when it hit $1.23 per lb. To produce synthetically the 600,000 tons of rubber consumed yearly in the U.S. would require new plants costing from $100,000,000 to $200,000,000—the cost of one or two battleships.
But final perfection of synthetic rubber, thoughtful rubbermen admit, will demand a further outlay of at least $30,000,000 for scientific research alone. And because this research would probably make present techniques obsolete, they are—unless Japan’s blockade becomes morbidly effective—in no hurry to make a premature, war-inspired effort to capture natural rubber’s markets.
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