The Science and Technology of Silicone Rubber

This is a review of silicone rubber and challenging it is since the silicones, a relatively new group of completely synthetic materials, are growing very rapidly even when viewed against today's rapidly advancing technology. The silicones bring together something of the properties of inorganic materials with their silicon-oxygen-silicon polymeric backbones combined with a sheath of organic groups, and the properties which result have won for the silicones an important position in our modern array of new materials. By silicones we mean structures which contain silicon-oxygen polymeric backbones with organic groups attached to the silicon. The name arose when Professor F. S. Kipping and thers in the early 1900's began to work with these materials which they thought at first to be analogous to the organic ketones. For this reason they called them “silicoketones” or “silicones” for short. These materials are not at all, however, related to the organic compounds from which they derive their name for they do not contain any silicon double bonds. Instead the polymers are linked together in giant networks of linear and crosslinked polymers by single bonded silicon-oxygen-silicon bonds. Although the silicones have only recently become important they had a relatively early beginning. It was approximately one hundred years ago, in the same year that Kekule proposed the cyclic structure for benzene, that Friedel and Crafts made the first material which could be classified as a silicone. In the intervening years Professor F. S. Kipping published a monumental series of papers on the chemistry of the silicone intermediates up through the year 1937. However, Professor Kipping was mostly interested in the chemistry of the small molecules and placed very little emphasis on the high molecular weight polymers which were to become so important later on. There was a stirring of interest in the industrial potentials for silicones in the 1930's. Dr. Winton Patnode initiated work at General Electric on silicon compounds containing organic groups as did Dr. J. Franklin Hyde at Corning Glass. Meanwhile, in Russia, B. N. Dolgov and K. A. Andrianov were also working in the field of organo-silicon chemistry. In the early 1940's these efforts finally lead to the blossoming of the commercially important silicones. The Direct Process was discovered by Dr. E. G. Rochow and this important process coupled with other basic rubber technology formed the basis for commercial silicone rubber. Since that time the technology has advanced very rapidly and although prior to 1940 there were virtually no patents on silicones and mostly Kipping's papers from the academic point, there are now approximately 10,000 publications throughout the world including U.S. and foreign patents. Activity has expanded at such a rate that the majority of this literature has been generated in the last few years. The scope of the silicone industry is indicated to some extent by the products as shown in Figure 1. Each of the types of products shown here include many individual products so that there are many hundreds of actual products manufactured and sold. The total business at this time in the United States represents approximately $75 million and silicone rubbers account for a substantial fraction of this total.