Hygroscopicity in epoxy powder composites

Epoxy powders offer a low-cost way of manufacturing thick-section composite parts, such as those found in wind and tidal turbines. Currently, their processing cycle includes a lengthy drying stage (≥15 h) to remove ambient moisture. This drying stage prevents void defect formation and, thereby, a reduction in mechanical properties; however, it constitutes up to 60% of the processing time. Little research has been published which studies the drying stage or its optimisation. In the present work, experimental and simulated analyses are used to investigate the effects of hygroscopicity in epoxy powder composites. Tests are performed to quantify the void content of dried and undried laminates and to measure its impact on transverse flexural strength. Dynamic vapour sorption analysis is used to study the sorption behaviour of the epoxy powder. It is shown that the epoxy powder is slightly hygroscopic (1.36 wt%) and exhibits sorption behaviour that is characteristic of glassy polymers. This results in up to 4.8% voids (by volume) if processed in an undried state, leading to a 43% reduction in transverse flexural strength. A modified linear driving force model is fitted to the desorption data and then implemented in existing process-simulation tools. The drying of a thick epoxy powder composite section is simulated to investigate the influence of powder sintering on the duration of the drying stage. Process simulations reveal that a standard drying cycle prematurely sinters the powder, which inhibits moisture release. By maintaining the powder state, simulations show that the drying cycle can be reduced to 5 h.