Cure kinetics of a high epoxide/hydroxyl group‐ratio bisphenol a epoxy resin—anhydride system by infrared absorption spectroscopy

Abstract An infrared absorption spectroscopy study of the curing (gelation and postcure) kinetics of a high (4.7) epoxide/hydroxyl group‐ratio diglycidyl ether of bisphenol A (DGEBA)–mixed anhydride epoxy resin system is reported. Peak assignments to molecular vibrational modes are given for the range 400–4000 cm −1 , and the optical density behavior of all peaks during reaction is discussed in detail. Chemical reaction was found to follow consecutive‐step addition esterification and simultaneous addition etherification. Epoxide hydroxyl‐group and carboxylic acid dimer hydrogen bonding was found to occur. The gelation phase of reaction is complex, exhibiting rapid initial hydroxyl–anhydride reactions followed by S‐shaped kinetics approaching an incompletely reacted limit. Postcure exhibits functional group kinetic behavior similar to that occurring in low epoxide/hydroxyl group‐ratio bisphenol A epoxy resin–phthalic anhydride systems and produces similar final chemical structures. The reaction behavior of low and high epoxide/hydroxyl group‐ratio bisphenol A epoxy resin–anhydride systems arises from an hydroxyl group‐limited inhomogeneous reaction mechanism involving bisphenol A epoxy resin molecular aggregates. The importance of free hydroxyl group content is discussed.