Interphase reaction of isocyanates with epoxy resins containing functionalgroups of different reactivity

Interphase reactions between cured epoxy resins and isocyanates are investi gated. The epoxy resins contain secondary hydroxyl groups as reactive group s only or secondary hydroxyl plus amine. The isocyanate diffuses into the e poxy resin forming an interphase with a thickness of some micrometers. Depe nding on the functional groups available in the epoxy resin, urethane and u rea groups are formed in the interphase. If a monofunctional isocyanate is used, no difference between both kinds of epoxy resin could be detected reg arding the formation of urethane. If the epoxy resins react with bifunction al isocyanates, a crosslinked interphase is formed. Due to the higher react ivity between amine and isocyanate compared to hydroxyl and isocyanate, the urea is formed first. The resulting cross-links restrict the further diffu sion of isocyanate into the epoxy resin. The consequence is a lower urethan e content in the interphase and a thinner interphase compared to the epoxy resin containing hydroxyl only. If a prepolymer with isocyanate end groups is used as isocyanate the formation of the interphase is slower compared to the low molecular weight isocyanate. This is due to the reduced mobility o f the prepolymer.