Some Observations on Rubbers with Low Nitrogen Content

Rubber has been prepared protein-free, although not entirely nitrogen-free, by digesting latex with caustic soda, as recommended by Pummerer and Pahl. Rubbers with a nitrogen content of from 0.004–0.0096 per cent have been prepared by a slight modification of the original procedure. This rubber can be compounded and cured to give good quality vulcanizates, which compare very favorably with the controls. Protein-free rubber, when acetone-extracted, becomes nitrogen-free. This can still be vulcanized, although it cures slowly. The rate of cure of rubber from protein-free latex is affected very little by the pH of the coagulating medium whereas with full-nitrogen rubber rate of cure varies considerably with change in pH value at the time of coagulation. It was also found that protein-free rubber could be racked. Data on the preparation, coagulation and nitrogen content as well as the vulcanization results of protein-free rubber are presented. The observations lead to the conclusion that protein is not the key to the explanation of the physical properties of rubber. Rubber cannot be prepared absolutely nitrogen-free by the method of Pummerer and Pahl, even though the treatment of the latex with caustic is continued twice as long as they recommend. The residual 0.004 to 0.009 per cent of nitrogen is not protein nitrogen, since it is removable by acetone extraction and the amount of nitrogen falls so low as to be unmeasurable with accuracy by the exact method used in this research—i. e., colorimetric determination with Nessler???s solution. The authors note with satisfaction that Pummerer has come to exactly the same conclusion. Comparison of the results of vulcanizing protein-free rubber with those of the controls and with the results of the previous research shows that the rate of cure is constant under different conditions of coagulation and that the quality of the nitrogen-poor vulcanizates compares very well with the controls and with the earlier mentioned work described by Dinsmore. It appears, therefore, that the variation in cure and quality as noted by Dinsmore was due entirely to some particular condition of the rubber protein brought about by the varying pH values existing at the time of coagulation. Protein-free rubber, after acetone extraction, is practically nitrogen-free and cures slowly to give vulcanizates of lower quality than is obtained with the protein-free rubber. It should be remembered, however, that, as stated above, this effect may not be due entirely to the absence of nitrogen. Definite conclusions on this phase of the question must be withheld until more experiments are made. The evidence presented shows that protein-free rubber can be compounded and cured to give good quality products. Hence the protein is not an essential factor in producing the physical properties of vulcanized rubber. The condition of the protein does affect the rate of cure and so exerts an indirect effect upon the quality of the rubber.