A new dual-stage multicomponent thermosetting polymer system

Abstract

A new polymer combining the processing advantages of epoxy resins with the superior thermal performance of the dicyanate resins was developed for use in bonding aramid honeycomb core composites. Dual-stage curing to meet the unique requirements of the honeycomb manufacturing process was achieved with a tricomponent mixture of epoxy and dicyanate monomers and a carboxylic acid anhydride hardener. In the first stage of curing the anhydride hardener advances the resin mixture to the appropriate tack condition. Heat-activated latent curing reactions complete the polymer crosslinking at a later manufacturing stage. The effect of variations in the levels of cyanate and anhydride on the bonding strength to the aramid paper and the time to reach tack-free conditions were studied.Characterization of the polymers chemical network development through kinetic investigations, thermal analysis and FT-IR spectroscopy was also undertaken. The reaction between cyanate ester groups and anhydride rings produced linear polyimidocarbamates that were identifiable by their FT-IR spectrum as one of the principal polymerization pathways. In the presence of an epoxy resin and tertiary amine catalysts, these imidocarbamates further reacted to form polyimides with possible organic carbonate branching. The resulting polymers showed good thermal stability with a single glass transition temperature that indicates the formation of an interconnecting polymer with significant crosslinking. Little if any polycyanurate was detected in the final product.Polymerization kinetics were studied using dynamic differential scanning calorimetry (DSC). The imidocarbamate forming reaction was found to be of the same order as the competing epoxy/acid anhydride esterification reaction and significantly faster than the dicyanate/epoxy or dicyanate cyclotrimerization reactions. Autocatalytic kinetic behavior was observed for the imidocarbamate reaction. When all three components were reacted in a combined system, the extent of epoxy esterification appeared to be largely over predicted by the isolated DSC results. The overall reactive system appears to be more complex than can be modeled by a simple combination of the individual reactions.