Abstract
Many finely divided solids have been added to elastomers to effect reinforcement of the cured stocks. It is generally agreed that the more finely divided the solid and the greater the degree of physical adhesion (wetting) between the surface of the filler and the polymer chains, the greater will be the general overall reinforcing effect. However, recent work on the mechanism of reinforcement has indicated that an important, if not limiting, property of the filler, in at least modulus reinforcement, is the immobilization of polymer segments on the solid surface. The immobilization is effected by the formation of strong chemisorptive bonds, if not covalent bonds, between the polymer and the filler. The application of this concept to carbon blacks has been discussed in detail by the authors, and similar conclusions have been reached by Blanchard and Parkinson and by Barton, Smallwood, and Ganzhorn. Up to the present time carbon blacks are unique as reinforcing agents in Hevea and GR-S type polymers. The numerous inorganic fillers which have been studied are generally inferior to the carbon blacks in one or more respects. The rubber chemist and technologist is chiefly interested in such properties as modulus, tensile strength, abrasion resistance, and resilience of the compounded stock. Tensile strength and resilience as commonly measured are primarily a function of the extent of surface, the particle-size distribution, and the degree of dispersion of the added pigment or filler. The chemical nature of the solid surface appears to be of secondary importance. Abrasion resistance is a difficult quantity to measure, especially in the laboratory. Therefore, in studying the mechanism of reinforcement and the reinforcing effect of different types of fillers, measurements have been confined to an evaluation of the so-called “equilibrium modulus”, from which an apparent work of retraction may be calculated. It has been found that the work of retraction is highly sensitive to changes in the chemical nature of the solid surface and the results may be interpreted in terms of the kinetic theory of elasticity.