Changes in the Electrical Properties of Natural Rubber/Carbon Black Compounds during Vulcanization

To conclude, let us sum up the advantages of direct current or low frequency measurement of the electrical properties of natural rubber compounds, when operating inside the mold in which vulcanization is occurring. In this way, it is possible to follow the changes in properties during heating or vulcanization more closely and accurately than when operating with samples outside the mold. An orientation of the colloidal structure of compounds above 10% by volume of MPC black is thus easily demonstrated. The dielectric properties of the compounds depend, in this case, on the size of the samples, since this orientation is a function of the dimensions, especially of the thickness of the molded compound. On the other hand, from the behavior of vulcanizable and nonvulcanizable compounds, a distinction may be made between the respective effects of both the vulcanization and the special state of agglomeration of the black colloidal particles in the compounds. This ‘structure’ of the black affects apparently both the losses and the dielectric constant through an electronic polarization phenomenon which can, in principle, be analyzed through the Maxwell-Wagner general theory of heterogeneous dielectrics. The effect of ‘structure’ on low-frequency losses depends essentially upon the nature and ratio of the black and varies like direct current conductivity. This ‘structure’ seems to be influenced also—although to a much smaller extent—by vulcanization, since the level of losses during heating is lower than with a nonvulcanizable compound containing the same ratio of black. When this ratio does not exceed 10% by volume, vulcanization governs the dielectric properties more directly, both by a fixation of polar sulfur to molecular chains, and by an ionic dissociation of vulcanizing ingredients. In the range of this study the only effect of molecular polarization is a rather limited increase of the dielectric constant; the losses are not affected. The sudden decrease of losses noted during the vulcanization of a gum compound is explained by a disappearance of the ions formed by the vulcanization reactions. This ionic polarization of the Maxwell-Wagner type plays no marked part in Compounds containing large amounts of black, no doubt because the ions are adsorbed by the black. By extending this study, as it is hoped, into the field of radio frequencies, it should be possible to improve the theory of the evolution of dielectric properties during vulcanization. From the practical point of view, such work would provide useful experimental data for a rational application of radio-vulcanization.