The Heat of Swelling of Rubber and Its Relation to the Previous History of the Rubber and to the Nature of the Swelling Agent

Abstract

Abstract The swelling method developed by Hock and Bostroem for determining calorimetrically changes in the state of energy of raw rubber (the Joule heat effect on stretching, the fiber structure of Hock, and the filler effect of Wiegand) has been developed further, especially by Hock and Hartner, with the object of obtaining information about the boundary surface energy between rubber and these fillers by means of the heat of swelling first of all of unvulcanized rubber mixtures containing active fillers such as carbon black and zinc oxide. The “activity” of fillers of this kind evidently depends upon the effect of boundary surface forces, which increase the mechanical work of rupture of rubber, A=∫p.dl, after their admixture in the rubber. To ascertain to what extent and in what quantitative way this mechanical “reënforcement” of rubber is related to the boundary surface energy, where there is mutual wetting, is a problem which is attractive both from a theoretical and technological point of view, and to the solution of which the works mentioned offer the first contributions. In continuing the experiments along the same general direction, on the one hand from the point of view of a critical examination of methods and on the other hand with the object of increasing the precision of the measurements, it was soon realized that it would be necessary to carry out special preliminary experiments. In fact, when it was once decided to replace benzine, which had been particularly suitable in experiments on the swelling of rubber mixtures because of certain unique properties, by benzene and other liquids, it became necessary to determine the heat of rubber as an objective in itself, whereas in the earlier experiments this had been only a means to an end.