Air Stable and Latent Single-Component Curing of Epoxy/Anhydride Resins Catalyzed by Thermally Liberated N-Heterocyclic Carbenes

Abstract

Bisphenol A diglycidyl ether (BADGE) is cured thermally using phthalic acid anhydride (PhA) or hexahydrophthalic anhydride (HHPA) as hardener in the presence of different protected N-heterocyclic carbenes (NHCs), from which the catalytically active NHCs are generated in situ upon heating. It is found that the curing reactions proceed in a well-defined manner, delivering highly cross-linked, high-Tg-thermosets using low catalyst loadings (0.1–1 mol % of NHC precursor). The polymerizations can be conducted under air without loss of activity, employing mild curing temperatures (120–160 °C) and short reaction times. By contrast, at room temperature, polymerizations proceed only very slowly and the mixtures remain processable for weeks, enabling formation of a true single-component composition suitable for applications where large processing windows or storage are required. The curing process was followed in situ by DSC as well as by rheological measurements. On the basis of these observations, the structure of the NHC precursor is correlated with its polymerization activity with regard to latency, temperature profile and polymerization kinetics. The robust and fully homogeneous system consisting of the protected NHC, BADGE, and HHPA was successfully tuned both in terms of activity and pot life by choosing the appropriate protected NHC out of 12 different precatalysts. The most rapid polymerization was effected by N,N′-bis(2,4-dimethoxyphenyl-)tetrahydropyrimidinium-2-carboxylate (6-OMe-CO2), while a dimeric zinc-based NHC-complex (6-Mes-ZnCl2) displayed the longest pot times.