Adhesive and Thermal Properties of Epoxy-Imide Resins Obtained from Different Diimide-Diacids: Structure-Property Correlations

Different epoxy-imide resins were prepared through the reaction of epoxy resins namely, Araldite® GY 250 (difunctional; DGEBA) and Araldite® EPN 1138 (polyfunctional; novolac epoxy) with diimide-diacids such as 4,4′-bis(4-carboxyphthalimido)diphenylmethane, 4,4′-bis(4-carboxyphthalimido)diphenylsulphone, 3,3′-bis(4-carboxyphthalimido)diphenylsulphone, 4,4′-bis(4-carboxyphthalimido)diphenylether, 2,2-bis[4-(4-trimellitimidophenoxy)phenyl]propane, 1,13-bis(4-carboxyphthalimido)-4,7,10-trioxatridecane, and 1,6-bis(4-carboxyphthalimido)hexane in 1:1 carboxyl equivalent to epoxy equivalent ratio. The adhesive and thermal properties of these systems were evaluated to arrive at structure–property correlations. It is observed that epoxy-imides with more aliphatic moieties and (or) ether linkages give higher room temperature adhesive strength when compared to those with more aromatic moieties. But the latter systems give higher retention of room temperature adhesive strength at elevated temperatures when compared to that of the former systems. Thermogravimetric analysis shows that epoxy-imide systems are stable up to 360°C and char residues of GY 250- and EPN 1138-based systems at 800°C fall in the range of 14–38% and 23–48%, respectively, in nitrogen atmosphere. This has been attributed to the higher crosslinking possible for the latter systems due to the polyfunctional nature of EPN 1138.