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Four hundred years have passed since explorers of South America found natives playing with peculiar bouncing balls and using unbreakable bottles, waterproof shoes, and cloth. Three hundred years later (1884) Goodyear took out a patent on the production of a superior quality of rubber by heating with sulfur. From then on there was an explosive expansion in the use and production of rubber which now runs into a million tons per year. In addition, a number of polymers have appeared on the market which help to satiate our hunger for rubber-type materials, and, through well-directed synthesis, the new compounds are endowed with properties accurately suited to the problem at hand (1). Although the manufacturer has a wide choice from natural and synthetic materials compounded in many various ways, there are always unfulfilled wishes, and this symposium deals with one of them. Rubber is an organic compound, and as such it is sensitive to a number of chemical influences— oxidation, reduction, and substitution —which readily break up the molecule or cause radical changes in the desirable properties of the rubber. In most cases, good judgment will determine where a particular rubber may be used. There is, however, one agent or, rather, a set of conditions to which all rubbers have to be exposed, namely, the atmosphere. Its components are not reacting violently with the rubber, but its slow attack is just as trouble-some. Some rubber compounds become tacky when exposed to the oxygen of the atmosphere. The addition of special inhibitors has overcome these unpleasant symptoms of degradation. Nevertheless, oxidation by molecular oxygen continues, although with greatly reduced rate. Just as the oils in paints slowly oxidize with a breakdown of the outer molecular layers, thereby losing their protective action, so does the rubber hydrocarbon lose its elastic properties through oxidation, resulting in a chalking of the surface. It is well established that olefin oxidation by molecular oxygen attacks the carbon atom adjacent to the double bond, forming hydro-peroxides and further oxidation products (2–4). These oxidations proceed at a considerable rate only after a certain induction period. There is experimental evidence that during the initial period peroxides are formed which catalyze further rapid uptake of oxygen. In order to prevent this buildup of peroxides, inhibitors such as phenols are added to the rubber, overcoming some of the bad features of the oxidizing effects. A special case of this oxidation is the effect of light, whereby numerous minute shallow cracks are formed. It is most likely that under the influence of the radiation radicals are formed which combine with the abundant supply of oxygen giving rise to chain-initiating radicals.
Haagen-Smit, A. J.
Professor of Bio-organic Chemistry, California Institute of Technology, Pasadena, Calif.