Room Temperature Ionic Liquids as Thermally Latent Initiators for Polymerization of Epoxy Resins

Abstract

Latent curing agents are designed to remain inactive at ambient conditions and to undergo controlled reaction upon exposure to external stimuli, such as elevated temperature or radiation that leads to cure of the resin. The desired inertness increases storage and handling capabilities. Latent curatives have been developed using a variety of hardeners in either multicomponent or one-pot formulations. In particular, the initiation and polymerization of epoxy resins are of commercial interest due to the great versatility in application of these systems. They are used in adhesives, insulation materials, paints, and fiber reinforced composites. Some examples of thermally initiated curing agents are based on nitrogen (amines, imidazole), anhydrides, phenolic derivatives, and metal complexes. Onepot epoxy formulations typically use imidazole or dicyandiamide (Dicy) and exhibit complex initiation and propagation reaction schemes. Dicy is thermally latent because it is a solid at room temperature but melts and dissolves into the epoxy resin at elevated temperatures to initiate polymerization. This need for a phase transformation results in practical drawbacks during manufacture and processing, such as difficulty dispersing the curing agent and inhomogeneous cure. A miscible liquid initiator with a similar range of latent properties would solve these problems and provide significantly improved systems. Ionic liquids (ILs) have melting points below room temperature, low viscosities, and highly tunable physicochemical properties showing promise in several polymers such as in separation membranes, as solvents, lubricants, porogens or novel composite membranes. There is one communication in the literature wherein the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate has been shown to harden epoxies. However, the degree to which reaction occurs, the mechanism of reaction and resulting mechanical properties remain unclear, and initiation might be through a mechanism similar to BF3 · amine complexes. The following report demonstrates that, out of many ionic liquids tested (see Supporting Information), 1-ethyl-3-methylimidazolium dicyanamide ([emim]N(CN)2) not only can be used to initiate polymerizations of epoxy resins but also can be used in formulations which exhibit excellent miscibility and long-term room temperature latency, in addition to producing a cross-linked structure with good thermomechanical properties once cured.