Experiments on the Preparation of Transparent Vulcanized Rubbers

1. It has been found possible, by the use of combinations of organic accelerators, to prepare accelerated rubbers which contain sufficient zinc oxide for activation but show no trace of pigmentation by the oxide, and are quite as transparent as a mixture containing only rubber and sulfur, and often lighter in color than the latter mixture. These accelerated rubbers have excellent tensile properties. The aging properties vary considerably according to the accelerators used, but in most cases are very good, even if the rubber is over-cured. It is, therefore, possible to make nonblooming rubbers without danger of bad aging. 2. Tests with other activators (zinc carbonate, zinc oleate, and zinc stearate plus magnesium carbonate) showed that these are less effective than zinc oxide, for the vulcanizates obtained had inferior mechanical properties and, in the case of zinc carbonate and oleate, showed a strong tendency to bloom. 3. A combination of accelerators which has been stated to work well without activator was shown to vulcanize very quickly, but to give poor mechanical properties, 4. The tendency of various accelerators to give rubbers, the surfaces of which are easily marked by scratching, presumably owing to efflorescence of accelerator, was noted. 5. The most promising of the mixtures tested for making transparent vulcanized rubbers was one accelerated with diphenylguanidine plus mercapto- benzothiazole, with zinc oxide as activator (mixture F). 6. A mixture is described (mixture K) which gives better aging when vulcanized beyond the optimum tensile properties than when vulcanized short of the optimum cure. 7. Rubbers can be toughened by the use of organic accelerators so as to possess greater resilience and lower permanent set than if toughened by the usual inorganic reinforcing agents. The aging tests described above were all made in the absence of light. For transparent rubber goods which are exposed to light during use, the addition of antioxidants and (or) softeners might be found necessary to give resistance to deterioration by light. In connection with fancy goods, the staining of paper in contact with the rubber may be of importance, and in such cases the choice of accelerators and antioxidants must be made with this in mind. The staining effects of numerous accelerators and antioxidants have already been described. In conclusion, it should be added that the experiments were directed in the first instance towards the production of brown transparent rubbers, for which reason smoked sheet was used. Where a light-colored vulcanizate is required, and especially when it is desired to obtain colored rubbers by adding organic dyes, the use of pale crepe is essential.