Mechanical behavior and dilatation of particulate‐filled thermosets in the glassy state

Abstract Dilatation of specimens is measured during tensile tests to investigate the mechanical response of particulate‐filled amorphous networks in the glassy state. The effects of particle size, volume fraction of filler, coupling agents, and crosslink density of the matrix on the mechanical‐dilatational behavior are studied on model composites of glass‐bead‐filled polyurethanes. It is found that the stress‐strain response of composites with untreated glass beads shows nonlinearity and subsequent yielding due to dewetting of particles from the matrix. In contrast, composites containing particles coated with a comupling agent fracture in a brittle manner, showing no significant nonlinearity and dewetting. Coated particles provide a higher tensile strength, but a lower strain at fracture, than uncoated particles. The volume fraction of the filler has an effect on Young's modulus, which is independent of the degree of coupling between the matrix and the filler. Tensile strength and strain at break decrease with increasing filler content for coated and uncoated particles. No strong effect of particle size is observed on either the tensile modulus or the dilatational behavior in the 25 μm to 160 μm diameter range. However, strain at break increases with decreasing particle size. When the accompanying yield phenomena shift to smaller strains, and a transition to brittle fracture takes place at high crosslink densities.