Cure behaviour and void development within rapidly cured out-of-autoclave composites

This study investigates the effect of slow and fast heating rates (1.5 and 10 °C/min) on the formation of voids during the out-of-autoclave curing process of an aerospace composite (HexPly 8552). The cure cycles were interrupted at pre-defined stages for each heating rate enabling the in situ behaviour of the resin, void content and growth of voids to be studied. It was found that the morphology and content of voids remained unchanged up to the second heating stage of the cure cycle, regardless of heating rate. Thereafter, differences to minimum resin viscosity for the faster heating rate (5.2 Pa s compared to 32.5 Pa s) and a higher gelation temperature (177 °C compared to 160 °C) caused a slight increase to void growth for the rapid curing conditions. The causes of voids were the result of moisture volatiles contained within the prepreg, identified by mass spectrometry. Overall, the faster heating rate reduced the cure cycle time by 32% without any effect on the final degree of cross-linking (88.4%) or overall void content, which remained below 2%.