Effect of room-temperature out-time on tow impregnation in an out-of-autoclave prepreg

Tow impregnation as a function of material out-time was investigated for an out-of-autoclave carbon fiber–epoxy prepreg. Prepreg was aged at ambient temperature for 56 days. Every 7 days, laminates were laid up and cured using vacuum bag only processing. Void content was calculated through image analysis of polished sections. Experimental results were used to validate an analytical model for tow impregnation. Model predictions were based on flow kinetics during processing conditions, taking into account increasing degree of cure and evolution of resin viscosity as a function of ambient aging time. The study found that no significant tow porosity occurred within the material’s stated out-life, that tow porosity increased once this out-life was exceeded and eventually stabilized due to the room-temperature vitrification of the resin. The model’s predicted trends were consistent with experimental results, suggesting that an increase in resin viscosity is indeed the main cause of out-time induced tow porosity and providing a means of predicting laminate quality as a function of room temperature aging time.