Behavior of Unfilled and Filled Rubbers in Shear in the Glass Transition Region

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Abstract
Rubbers filled with high amounts of a hard inorganic filler still show typical mechanical properties of a high polymer, viz., a glass transition region, a glassy state and a rubbery state. Effects of filler characteristics on the glass transition are discussed, chiefly on the basis of the course of shear modulus at constant frequency as a function of temperature. Effect of volume content of filler on the glass transition of composite materials consists chiefly in a change of the levels of shear modulus in the glassy and in the rubbery state. This change may be described by a simple macroscopic model due to van der Poel. Predictions by this theory could be confirmed for rubbers filled with single filler fractions in the filler size range 30 to 500 µm, and in the range of filler content 0 to 50 vol%. At smaller particle sizes, an effect of filler size was observed, which points to increase in the transition temperature and to increase in the level of modulus in the rubbery state with decreasing filler size. Preparation of composite materials with high filler contents (>55 vol%) is only possible by using a filler with a bimodal size distribution. In this case, moduli depend on filler content and mixing ratio of coarse to fine filler fraction; the van der Poel theory then gives predictions which are too high in comparison with experimental results.

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