Internal stress of epoxide resin modified with spiro ortho-ester type resin

Bisphenol-A type epoxide resin modified with various amounts of spiro ortho-ester (SOE) resin was cured with an imidazole. Internal stress of the modified systems decreased with increasing fraction of SOE resin in the cured resins. In particular, a drastic reduction of the internal stress was observed in the systems modified with more than 33 mol% SOE resin. In addition, heterogeneous structure was observed with modifier content over 33 mol%, and the elastic modulus of these systems decreased step-wise with increasing ambient temperature. On the other hand, the systems in which the modifier content was less than 20 mol% had homogeneous structure and thus the modulus was considerably higher than that of the former systems. Consequently, it was concluded that the drastic reduction of the internal stress in the systems modified with more than 33 mol% SOE resin depends on the low elastic modulus caused by the formation of heterogeneous structure.