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.