Fast curing unidirectional carbon epoxy prepregs based on a semi-latent hardener: The influence of ambient aging on the prepregs Tg0, processing behavior and thus derived interlaminar performance of the composite

Abstract The aging behavior of a fast-curing epoxy resin (EP) was examined over three months in unidirectional carbon fiber prepregs for the first time. The semi-latent, 120 °C curing one-pot formulation was developed based on industry-grade diglycidyl ether of bisphenol A (DGEBA) resins without the necessity of B-staging, to fully monitor the property change by aging without any post-processing influence. To extend the shelf-life compared to standard wet EP systems, the latent dicyandiamide (DICY) and a semi-latent urea-based catalyst were used, leading to full conversion within 15 min. The conversion was monitored by increasing Tg0 and rheology. High quality unidirectional carbon prepregs were manufactured and then investigated over three months regarding tackiness and resin flow. In parallel, laminates were autoclave-manufactured to evaluate the aging influence on the interlaminar properties. Thus, a correlation between decreasing gel-time, interlaminar adhesion and resulting multiple crack growth was observed via GIC and interlaminar shear strength (ILSS) testing. The investigations show a clear influence of the consolidation quality of the prepregs based on ILSS values and GIC values. A decrease of 19% in ILSS and combined with an erroneous increase of 63% of the GIC due to multiple delamination was observed in the worst case.