Room temperature-curable VARTM epoxy resins: Promising alternative to vinyl ester resins

The objective of this investigation is to characterize various room temperature (RT)-curable epoxies for vacuum-assisted resin transfer molding (VARTM) of large structure manufacturing. Six epoxy candidate resins: X-40, 780-33, 780-35, 8601/8602, 8602, 8603, and two vinyl ester resins (VE), 411-350 and 411-510A, are physically and thermochemically characterized. All the resins are cured at RT with extended period of time. The degree of cure for 24-h RT-cured samples ranges from 70 to 85% for epoxies and is comparable with the baseline VE systems (75%). After 1 year at RT, the degree of cure increases from 90 to 98%. Most of the epoxies show a single transition in dynamic mechanical analysis and differential scanning calorimetry. However, two heterogeneous transitions are observed for the VE systems. The glass transition temperature increases monotonically with exposure time, except X-40, that rapidly achieves a plateau and remains constant. The degree of cure for the majority of the systems increases logarithmically with RT curing time with excellent fitting (R² varies from 0.92 to 1). Consistent with the increase in degree of cure, the storage modulus increases and (tan δ)max decreases with time of exposure. A negative correlation between the curing temperature range and the total heat of reaction is observed among the epoxy systems. However, the VE systems show the reverse trend. RT curing epoxy resin (X-40) shows promising overall result to VE system and can be a viable alternative to VE for VARTM processing.