SYNTUETIC RUBBER RESEARCH 379 



and molding. Several different types of machines are employed including 

 mills, calenders, Banbury mixers and extrusion machines. At every stage 

 and particularly in Banbury mixing, where carbon black is generally mixed 

 in, the copol>Tner undergoes changes which affect its performance in the 

 finished product. This is especially true if, as nearly always happens in 

 practice, considerable heat is developed during the operation. Indeed, it 

 is frequently true that processing operations have more to do with the 

 ultimate rubber properties than do factors in the production of the copoly- 

 mer itself^^ It is only fairly recently that this point has been sufficiently 

 emphasized and considerable progress made in controlling the processing 

 steps to the same extent as the polymerization is now controlled. 



To see what happens during processing of the copolymer let us assume we 

 subject two extreme types of GR-S, one containing no gel and one contain- 

 ing gel of rather low swelling volume, to a typical hot processing treatment 

 consisting of hot mastication and Banbury mixing in of carbon black. ^^ In 

 addition, in order to exhibit differences in processing let us consider the 

 effects of cold processing on the same two samples. As is evident from Fig. 

 10 which summarizes the results,^* hot processing tends to build up gel and 

 decrease its swelling volume in both of the rubber samples. The dilute 

 solution viscosity on the other hand falls. This behavior although of 

 advantage to subsequent extrusion and calendering operations is definitely 

 opposed to securing the best mechanical properties in the final rubber. 

 Cold processing has the opposite effect on the samples. Thus, the copolymer 

 not containing gel is little affected, whereas the gel in the other is broken 

 down and gives rise to a higher dilute solution viscosity. 



In processing, therefore, important changes in the chain structure of the 

 copolymer are brought about. Under certain circumstances, these are 

 beneficial, but since most processing involves considerable heat development 

 the changes are usually detrimental. It is of the utmost importance there- 

 fore that a type of copolymer is produced which is compatible with the type 

 of processing machinery already installed in industry. In addition, uni- 

 formity of the copolymer is of very great significance if control of processing 

 operations is to be achieved, for such control cannot be attained with a 

 variable starting material. 



The processing step which involves the mixing in of carbon black (or 

 other pigment) is perhaps the most important. Unfortunately, the presence 

 of the carbon black makes it impossible to employ the usual sol-gel analysis 

 because a new kind of insolubility enters.'^- -^ In addition to the primary 

 valence gel discussed up to this point, a secondary valence combination 

 involving the carbon black and the large sol molecules forms. This is not 

 immediately distinguishable from the first type of gel unless we have other 

 reason to know that the latter is absent and does not form during the 



