A new series of Interpenetrating polymer networks (IPN) was prepared by using both free radical and cationic photoinitiators simultaneously under ultraviolet radiation. The systems studied were castor oil based ure'thane acrylates (Network I) and epoxy resin (Network II) (UV-IPN). In order to compare the effect of different crosslinking operations

urethane-acrylate/ amine cured epoxy resin (AC-IPN) and urethane-acrylate/epoxy-acrylate conetwork (EA-CPN) systems were also synthesized and their dynamic mechanical and stress-strain properties were examined. The results show that UV-IPN exhibits only one Tan δ peak

while two Tan δ peaks can be seen in AC-IPN systems. This suggests that the former is much more compatible than the later.The stress at break was found to increase as epoxy resincomponent increased in both UV-IPN and AC-IPN

while appeared to decrease in EA-CPN as epoxy-acrylate increased.