Interphase reaction of isocyanates with epoxy resins containing functional groups of different reactivity

Interphase reactions between cured epoxy resins and isocyanates are investigated. The epoxy resins contain secondary hydroxyl groups as reactive groups only or secondary hydroxyl plus amine. The isocyanate diffuses into the epoxy resin forming an interphase with a thickness of some micrometers. Depending on the functional groups available in the epoxy resin, urethane and urea groups are formed in the interphase. If a monofunctional isocyanate is used, no difference between both kinds of epoxy resin could be detected regarding the formation of urethane. If the epoxy resins react with bifunctional isocyanates, a crosslinked interphase is formed. Due to the higher reactivity between amine and isocyanate compared to hydroxyl and isocyanate, the urea is formed first. The resulting cross-links restrict the further diffusion of isocyanate into the epoxy resin. The consequence is a lower urethane 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 of the prepolymer.