Study on novolac cyanate ester co-cured with epoxy E-51

The blends of novolac cyanate ester and bisphenol A epoxy resin were expected to attain the high thermal resistance of novolac cyanate ester and the excellent processsablity of epoxy. Little work has been done on novolac cyanate ester/epoxy systems before because the high purity novolac cyanate esters was very difficult to synthesize. In this work, a high purity novolac cyanate ester (M-n = 900) and a bisphenol A epoxy ( E-51) with an equal molar ratio co-cured system was selected to study the reaction pathway and mechanism by FTIR trace method in the molten state. Isothermal cure conditions include 150, 180, 200, 220 degrees C were used to conduct the co-cured reactions. The reaction was found to be complex and involve several pathways. When cured below 150 degrees C, trimerization of novolac cyanate ester was the main reaction and the trimerization rate was accelerated after the addition of epoxy. An open ring co-polymerization between epoxy and carbamate, which is an impurity contained in novolac cyanate ester or induced by reaction with water from environment or from epoxy, can always occur. This made a slow dissipation of epoxy in the conversion-time plot. No reactions between epoxy and novolac cyanate ester was found in this stage. When temperature was raised up to and over 180 degrees C, oxazolidinone ring was formed and such a reaction increased the dissipation of epoxy function. As in the case of cyanate function,the transformation of triazine rings quickly reaches it's maximum value and then descent. It was found that increased temperatures facilitated the formation of oxazolidinone. The transformation rate of triazine ring can easily reach 1.0 while the alpha that value of oxazolidinone formation is less than 0.5.