An assessment of the effect of externally applied mechanical stress and water absorption on the electrical tree growth behaviour in glassy epoxy resins

The type of electrical tree growth between point and plane electrodes in glassy epoxy resins and times to failure are greatly affected by sample age, namely the time between casting and testing with a 50 Hz electrical stress. The effect has been attributed to structural relaxation in the polymer, characterized by the magnitude of the residual internal mechanical stress (RIMS). It is now shown that (i) varying the RIMS by the application of an external mechanical stress (tensional or compressional) has no significant effect on the rate of tree growth and (ii) storing samples in water affected the RIMS and the rate of tree growth. It appears that water absorption has a greater effect than does mechanical stress on electrical tree growth in these epoxy resins.