Theory of Filler Reinforcement in Rubber

Abstract

Abstract There are many active filler substances capable of improving to a considerable extent the mechanical properties of rubber mixes. The reinforcing effect of fillers is rather a complex problem and is known to depend, not only on the surface area and size of the particles involved, but also the nature and shape of the particles and the nature of the material in which they are situated. The influence of the surface forces between the filler and matrix has been discussed from a theoretical point of view, particularly by Hock and his collaborators. However, a closer perusal of the available experimental material reveals that the surface energy cannot be the only important factor in the reinforcing effect. It is clear that a particle of a foreign substance embedded within the bulk of the rubber will, in general, have a different volume from the actual space in the material available for it. If it is held firmly enough by the surface forces, the particle will either compress or expand the surrounding medium, depending on whether its size is larger or smaller than the hole in the matrix available for it. Consequently, this particle will become the center of a more or less symmetrical stress (compressional or tensional) field. In a discussion of filler reinforcement, the elastic energy generated by the particles must be taken into account. In the following it will be attempted to give a brief discussion of this effect and its relation to the previously treated surface effect of the particles. Although this should not be considered a complete theory, it introduces a new feature into the discussion of this subject, provides a basis for further experiments, and might be helpful in assessing the usefulness of fillers in rubber mixes.