POLYURETHANE-DUCTILIZED EPOXY-RESINS

Amine-cured epoxy resins were modified to improve their impact propert ies. Urethane prepolymers (PUs), in which terminal isocyanate groups w ere blocked with nonylphenol (NP) for easy handling, were used as modi fiers. The synthesis of the elastomers were carried out at different N CO : OH ratios: 1 : 1, 2 : 1, and 3 : 1 (PU1, PU2, and PU3). Character ization of these materials by GPC and FTIR indicated that PU1 has a ne gligible amount of NCO-terminated chains and no unreacted toluenediiso cyanate (TDI). PU2 and PU3 have free-blocked TDI in the mixture, even after distillation under a vacuum. The molecular weight and polydisper sity of the prepolymer increases as PU3 < PU2 < PU1. Copolymerization was carried out by crosslinking with a mixture of cycloaliphatic amine s, which react with the epoxy ring and with the NCO groups by deblocki ng and urea formation. Dynamic mechanical tests were used to measure t he glass transition temperatures (T-g) of the copolymers. Two T-g were found if PU1 was the epoxy modifier, indicating that phase separation took place. This was correlated with a structure of PU1 of linear cha ins with a negligible amount of reactive groups. Flexural and compress ion properties showed negligible changes for PU2- and PUB-modified epo xy, but the critical strain energy release rate (G(1C)) was improved i f PU2 was the modifier. This behavior was explained by the Linkage of elastomeric chains into the epoxy network. The PU1-epoxy copolymer sho wed a completely different behavior, with the bending modulus (E-b) re duced to almost one-half with respect to that of the epoxy matrix and with largely improved impact properties. This difference was attribute d to the separation of an elastomeric phase, which favors the formatio n of shear bands in the epoxy matrix. (C) 1998 John Wiley & Sons, Inc.