Elastic Modulus and Swelling of Butyl Vulcanizates. Influence of Fillers

Abstract

Abstract The reinforcing of elastomeric vulcanizates by filler particles has been studied with Butyl rubber, a polymer of limited unsaturation. This limited unsaturation enables a comparison to be made of filler reinforcement in materials having a constant concentration of chemical cross-links, because excess curing agents can be added without increasing the state of cure byond a certain limit. Under such conditions differences in the adsorptive powers of fillers do not result in variations in the ultimate state of vulcanization. Reinforcement in this paper was defined as the ability of a pigment to enhance the elastic or equilibrium stress (at constant elongation) of a vulcanized specimen. Equilibrium stress at 50 per cent elongation was determined on a triple-beam balance, and the internal energy and entropy components of stress were determined. The entropy component of stress was found to conform more closely to the classical viscosity analog equation for reinforcement, where only the volume concentration of filler and not the surface area is the controlling factor. It was observed that carbon blacks, especially those of finer particle size, produced a greater internal energy contribution to stress than did mineral fillers, which appeared as a function of the total pigment surface added. The increase in stress observed with mineral fillers was due largely to entropy contributions. Another manifestation of reinforcement was shown to be the restriction of volume swelling of a network (in a good solvent) in the presence of pigments. Carbon blacks showed the greatest restriction of swelling capacity, and in distinction to stress behavior, this restriction was solely a function of the total surface area of carbon black present, regardless of type. With mineral fillers, far less restriction was observed, and in one case an increase in swelling was noted. This can be attributed to poor bonding, which results in voids. The photomicrograph of clay particles in a thin stretched section of Butyl shows this effect in Figure 21. The role of surface forces in the case of carbon blacks has been further demonstrated through measurements of the elastic properties of swollen rubber. The reinforcing nature of carbon black is maintained in a swollen network, while the weak bonds between the polymer and a mineral filler are destroyed when the chains are separated by the imbibed solvent, and the stress then approaches that of an unloaded compound.