Epoxy resins (DGEBA): The curing and physical aging process

Abstract Differential scanning calorimetry (DSC) and infrared spectroscopy (IR) were used to monitor the degree of cure of partially cured epoxy resin (Epon 828/MDA) samples. The extent of cure, as determined by residual heat of reaction, concurred with that determined by monitoring the infrared radiation absorbance of the epoxide group near 916 cm −l . The fictive temperature T f, g was found to increase with the degree of cure, increasing rapidly during cure until reaching a value near the cure temperature T c of 130°C (approximately 80% cure) where the material vitrified. The greatly reduced reaction rate during the final 20% of cure was not only a consequence of vitrification but, as revealed by infrared spectroscopy, the result of the depletion in the number of reactive epoxide groups. The endothermic peak areas and peak temperatures evident during the DSC scans were used as a measure of the extent of “physical aging” which took place during the cure of this resin, and after, fully cured samples were aged 37°C below their ultimate glass temperature for various periods of time. The rate of physical aging slowed as the temperature increment (T t,g − T c ) increased. Although an endothermic peak was evident after only 1 h of cure ( T f, g = 138.3°C), such a peak did not appear until fully cured samples were aged for 16 h or more. Enthalpy data revealed that for partially cured material, the fictive temperature T f, a , reflecting physical aging, increased with curing time. In contrast, the T f, a , for fully cured samples decreased with sub‐ T g aging time. The characteristic jump in the heat capacity Δ C p which occurred at the T f, g decreased as curing progressed. This decrease appears to be dependent upon the rotational and vibrational degrees of freedom of the glass. Finally, a graphical method of determining the fictive temperature T f, a , of partially and fully cured epoxy material from measured endothermic peak areas was developed.