Erasure behavior of isothermal physical aging effect below glass transition temperature in a fully cured epoxy resin. Differential scanning calorimetry measurement

A mixture of diglycidyl ether of bisphenol A (DGEBA) epoxy resin and amine curing agent with a stoichiometric ratio was fully cured at 220 degreesC for 30 h (T-ginfinity = 179 degreesC). The fully cured specimen was isothermally aged at various aging temperatures (T-a = 90-150 degreesC) for aging time (t(a)) of 1000 min, and effect of the isothermal physical aging was investigated by differential scanning calorimetry (DSC). The effect of the physical aging appeared as a sharp endothermic peak in the glass transition temperature (T-g) region at higher aging temperatures but a small broad peak below T-g at lower aging temperatures. In this work, erasure below T-g of the isothermal physical aging effect was examined for a specimen aged at T-a = 150 degreesC by heating to different erasure temperatures (T-er = 155-170 degreesC) above the aging temperature but below the glass transition temperature for different erasure times (t(er) = 10-60 min). Enthalpy relaxation was obtained from area difference of DSC thermograms between aged and de-aged specimen, and used to investigate the effect of heating to T-er. It was found that the heating to T-er affects differently to the previous aging at T-a = 150 degreesC. When the aged specimen was heated at T-er = 155 degreesC near T-a, the heating leads to additional aging to the previous aging. However, the previous aging effect was partially erased with residual aging during heating at higher erasure temperatures. Amount and rate of the erasure became larger with increasing T-er. After the erasure, a small amount of further aging occurred with erasure time. This result indicates that there is competition between aging and de-aging at an aging temperature between aging temperature and glass transition temperature.