The Plasticization of Buna

Abstract

Abstract Recently oxidative thermal softening of Buna has come to the fore, a process which has been called “degradation”, a not too happily chosen term which suggests technical deterioration in quality. Properly speaking, there is loss in quality, just as there is considerable deterioration in quality when natural rubber latex is converted to dry rubber, a deterioration which is made still more extensive by mastication. Thus, while it is possible under laboratory conditions to make tread compounds having tensile strengths of 300–350 kg. per sq. cm. from natural rubber and latex compounds having tensile strengths of 450–500 kg. per sq. cm., the values obtainable with milled natural rubber are considerably lower. Despite this, one would not wish, except in the rarest instances, to dispense with the milling of natural rubber, since its processibility depends on the plasticization which is obtained by mastication. Thermal softening of Buna-S corresponds to the mastication of natural rubber. It has been possible to produce treads of unsoftened Buna-S with tensile strengths of 300 to 320 kg. per sq. cm. from compounds prepared in the laboratory, but these results cannot be obtained on factory mills because of the poor dispersion of fillers. With thermal softening, the tensile strengths of tread compounds made on laboratory mills would, to be sure, be somewhat lower, viz., 280–290 kg. per sq. cm. In mechanical compounds, the improved dispersion of fillers, compared with unsoftened Buna-S, would cause an increase in tensile strength. Oxidative softening is justified, not only from a processing and technical point of view, but above all because of the improvement in the finished product—provided, of course, that the degree of softening is suited to the intended application and that the optimum formula has been worked out. It would, therefore, be better to replace the derogatory word “degradation” by “oxidation” or “thermal softening”.