ROOM-TEMPERATURE CURING EPOXY ADHESIVES FOR ELEVATED-TEMPERATURE SERVICE .2. COMPOSITION, PROPERTIES, MICROSTRUCTURE RELATIONSHIPS

Low temperature curing epoxy formulations for elevated temperature service have been previously developed and studied (Part II). Balanced performance with respect to shear and peel properties have been obtained for a system composed of a tetra and trifunctional epoxy blend crosslinked by a mixture of multifunctional amine and an amino-terminated elastomer. In continuation of the previous study, the present one is aimed at investigation the effect of substitution of difunctional epoxy resin and curing agent for trifunctional ones on the developing microstructure and resulting mechanical properties. Furthermore, a new type of amino-terminated-acrylonitrile (ATBN) and an epoxy-terminated silane were included in the present investigation. Experimental results show that while reduction in the overall functionality of the reactants results in a lower lap shear strength, it gives rise to enhancement in peel strength. The same effect was observed when the new ATBN was used. Thermal analysis of the polymerization processes, taking place during curing of the various low temperature curing formulations, indicates that the curing activation energies are appreciably lower compared with high temperature curing systems. Addition of silane, ATBN and substitution of the multifunctional amine curing agent by a lower functional one, resulted in a moderate increase in the activation energy. The basic formulation, comprising a tetra-and trifunctional resin blend and a multifunctional amine and ATBN crosslinking mixture, developed a typical two-phase matrix-rubber microstructure. A third phase was observed when the trifunctional epoxy resin or the multifunctional curing agent was substituted by lower functional ones. A similar three-phase morphology was obtained when the epoxy-terminated silane was added to the basic treta-and trifunctional reactant system. © 1990, Taylor & Francis Group, LLC. All rights reserved.