360 BELL SYSTEM TECHNICAL JOURNAL 



Early Status of GR-S Synthetic Rubber 



The process by which GR-S type synthetic rubber is made is known as 

 the emulsion polymerization process. In it, butadiene and styrene in the 

 proper proportions are emulsified in water with small amounts of catalysts 

 and substances called modifiers which serve to control the plasticity of the 

 polymer. During the reaction period of from ten to twenty hours about 

 three-fourths of the butadiene and styrene are converted into the synthetic 

 rubber. The reaction occurs in such a way that very minute particles are 

 formed and the resulting synthetic latex is suggestive of natural latex. To 

 obtain the rubber itself the latex is coagulated with acid and sodium chloride 

 or with aluminum sulfate and the coagulum washed. After drying the 

 rubber crumbs are baled and shipped to the fabricating factories. The above 

 brief sketch of course does not provide an idea of the many complexities 

 which arise in practice nor of the many process variations which can be used 

 to control the final properties of the rubber. A complete treatment of this 

 subject falls outside the scope of this paper. 



\\Tien the Baruch Committee advised "bulling through" the synthetic 

 program on the basis of Buna S type rubber, it fixed the chemical composi- 

 tion of the product to a very great extent. We knew then, or shortly after- 

 ward, that we would be required to use approximately 690,000 tons of 

 butadiene and 197,500 tons of styrene per year to produce the coplymer 

 rubber. Whatever other components might be employed would be available 

 in only insignificant quantities by comparison. One element of choice 

 remained as far as chemical composition was concerned, namely the propor- 

 tions in which the two components might be used. German Buna S is 

 supposed to consist of 75 parts by weight of butadiene to 25 parts of styrene 

 but, as we shall see later, this ratio does not determine the ratio actually 

 present in the final copolymer which is a function of reaction variables as 

 well as the initial ratio of the ingredients. Consequently it was necessary 

 to examine the composition of the final copolymer and to control it at the 

 proper ratio of butadiene to styrene. The chemical composition was not 

 the only factor to be controlled, however, since as we have seen, the proper- 

 ties of polymers unlike ordinary chemical compounds depend as much if not 

 more on the chain structure. This is of course not only dependent on the 

 nature of the starting ingredients but also on the manner in which they are 

 combined into the chain. 



At the time intensive work was undertaken in this country on Buna S 

 type synthetic rubber little attention had been given to its characterization 

 by physico-chemical means. The usual physical testing jirocedures invoh-- 

 ing the preparation of compounds by mixing in pigments and vulcanizing 

 were of course being employed to supply useful information about the 



