A review on aging effects of thermoset prepregs

Thermoset prepreg materials have emerged as the composite material of choice for demanding structural applications due to their easy processability and attainable superior part quality. However, the one-part nature of common precured thermoset resins renders prepregs susceptible to undesired premature curing, which may entail deterioration of properties. This article reviews the current state of literature on thermoset prepregs with regard to aging effects that originate before the final cure cycle of composite parts, i.e., during freezer storage and upon temporary storage or processing at room temperature. Therefore, the aging-related evolution of the physio-chemical prepreg state (conversion, T_g, water uptake, viscosity) is examined in terms of its impact on processing (forming properties, tack), applied cure cycles (temperature, dwell time, pressure) and post-cure laminate quality (porosity, mechanical performance). The economic and ecological implications of prepreg aging are discussed in conjunction with recycling strategies for out-of-spec material. Finally, approaches proposed to mitigate out-time effects of prepregs (latent curing agents, cure cycle modification, vitrimer matrices) are presented.