Interplay between chemical and physical aging in diethanolamine cured diglycidyl ether of bisphenol-A (DGEBA) epoxy

Abstract

Thermomechanical properties during isothermal aging of diethanolamine (DEA) cured diglycidyl ether of bisphenol-A (DGEBA) were tracked using uniaxial compression and differential scanning calorimetry. The epoxy was cured at 70 °C for 24 h, following standard procedure, and then aged at one of four temperatures, T: 55 °C, 65 °C, 76 °C or 105 °C. The initial glass transition of cured DGEBA/DEA was 70 °C and increased proportionally with the logarithm of time indicating that additional crosslinks had formed. Similarly, the yield stress, σy, increased, by up to 80%, proportionally with the logarithm of aging time until approximately 600 h. After that time period, the increase in σy slows significantly. Aging also narrowed the yield peak and increased the post-yield flow stress. The changes in σy due to physical aging were separated from those due to chemical aging by testing samples combining both physical and chemical aging and comparing the results to those from (only) chemically aged samples. The proposed method for separating out the physical contributions can be used to predict the mechanical changes due to physical aging in slowly chemically evolving materials. The aging data are also utilized along with data on σy as a function of strain rate for unaged material to generate a tentative expression for σy as a function of strain rate, aging time and temperature. The derivation of this expression requires only the common assumption that the rate of change of σy/T with respect to log(strain rate) is independent of temperature and of aging time.