Contributions of the side groups to the characteristics of water absorption in cured epoxy resins

Novolac resin and the esterified novolac resins with CH3COO-, CH3CH2CH2COO-, and C6H5CH2COO- side groups are used as curing agents for epoxy resin. The cured resins are referred to as EP, EPA, EPB and EPP for the respective agents. The influence of the side groups on the water absorption behavior of the cured epoxy resins was investigated using a gravimetric method, differential scanning calorimetry (DSC), positron annihilation (PA) and Brunauer-Emmett-Teller (BET) adsorption. The experimental results show that at 20°C the equilibrium water content (M∞) of the cured resins is in the following order: EP (2.13%) > EPA (0.77%) > EPP (0.65%) > EPB (0.39%); however, the order of the diffusion coefficients of water in the resins (D) is: EPB (1.81 x 10-8 cm2 · s-1) > EPA (9.5 × 10-9 cm2 · s-1) > EPP (8.4 × 10-9 cm2 · s-1) > EP (2.4 × 10-9 cm2 · s-1). But at 60 and 80°C, the sequence of D changes into, EPB > EPP > EPA > EP. Furthermore, the M∞ of EP varies slightly with the temperature, ranging from 20 to 80°C. However, for the modified resins, M∞ increases significantly with temperature. The results suggest that the higher the polarity, the higher the M∞ and the lower the D; and the better the flexibility, the lower the M∞ and the higher the D. There is no evidence for a dependence of M∞ on the volume of the nanopores, as measured by the BET surface area.