Comparison of the effects of acetoxy and hydroxy groups on the water uptake into the cured epoxy resin

A comparison of the effects of acetoxy and hydroxy groups on the water upta ke into the cured epoxy resin was made using the gravimetric method, DSC an d corrosion test. The experimental results show that the replacing of the h ydroxy groups with acetoxy groups in the resin causes less water uptake, bu t increases the diffusibility of water in the resin. For example, at 30 deg rees C, the equilibrium water sorption of the o-cresol novolac epoxy resin cured with phenol novolac acetate resin(EPA) is 0. 84% (mass fraction), whi le the epoxy resin cured with phenol novolac resin(EP) is 1. 97%; the activ ation energies for diffusion of water in EPA and EP are 40 and 48 kJ . mol( -1) respectively. The effects may be mainly due to the hydrophobic methyl g roup of the acetoxy groups. The methyl group prevents the uptake of water, but makes the acetoxy group more flexible, which causes water diffusion les s difficult. Moreover, the results show that the unique structure of the in terphase of the EPA coating/Al substrate even causes further less water upt ake, and improves the effectiveness of corrosion protection for the coating accordingly.