Physical Aging of Epoxy Polymers and Their Composites

Exposure to extended periods of sub-Tg temperatures causes physical changes in the molecular structure of epoxy resins and epoxy-based materials to occur. These physical aging mechanisms include the reduction in free volume and changes to the molecular configuration. As a result, mechanical, thermodynamic, and physical properties are affected in ways that can compromise the reliability of epoxy-based engineering components and structures. In this review, the physical changes in the molecular structure of epoxies are described, and the influence of these changes on the bulk-level response is detailed. Specifically, the influence of physical aging on the quasistatic mechanical properties, viscoelasticity, fracture toughness, thermal expansion coefficient, volume relaxation, enthalpy relaxation, endothermic peak temperature, fictive temperature, and moisture/solvent absorption capability are reviewed. Also discussed are relationships between relaxation functions, crosslink density, composite reinforcement, and epoxy/co-polymer blending and the physical aging response of epoxies. Finally, the concepts of thermal and mechanical rejuvenation are discussed.