Additives for improving the strength, stiffness, and ductility of epoxy resins

Abstract

Abstract A range of additives at 7 to 20 wt% loading can increase the stiffness and strength of crosslinked epoxy resins by up to 60%, yet the tensile specimens also fail in a ductile fashion. In additive formulations where little chemical bonding occurs between the additive and the epoxy matrix (e.g., the reaction product of 4‐hydroxyacetanilide and 1,2‐epoxy‐3‐phenoxypropane), the increase in modulus is directly related to the decrease in free volume available for segmental mobility. The increase in strength results from a combination of the increased modulus and an increased fracture energy, A ductile mode of failure occurs because the cured plastic exhibits a large increase in free volume on straining (low Poinsson's ratio). The strain‐induced increase in free volume effectively takes the sample through the glass to rubber transition. In more highly reactive formulations (e.g., the reaction product of 4‐hydroxyacetanilide and vinyl cyclohexene dioxide), chemical bonding effects complicate this simple free volume interpretation, but the occurrence of a ductile failure mode is again related to the free volume increase with strain.