Epoxy Resin Highly Loaded with an Ionic Liquid: Morphology, Rheology, and Thermophysical Properties

An epoxy resin can be crosslinked with an imidazole-based ionic liquid (IL), whose excess, provided its high melting temperature, can potentially form a dispersed phase to store thermal energy and produce a phase-change material (PCM). This work investigates the crosslinking of diglycidyl ether of bisphenol A (DGEBA) using 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) at its mass fractions of 5, 10, 20, 40, and 60%. The effect of [EMIM]Cl on the viscosity, curing rate, and curing degree was studied, and the thermophysical properties and morphology of the resulting crosslinked epoxy polymer were investigated. During the curing, [EMIM]Cl changes its role from a crosslinking agent (an initiator of homopolymerization) and a diluent of the epoxy resin to a plasticizer of the cured epoxy polymer and a dispersed phase-change agent. An increase in the [EMIM]Cl content accelerates the curing firstly because of the growth in the number of reaction centers, and then the curing slows down because of the action of the IL as a diluent, which reduces the concentration of reacting substances. In addition, a rise in the proportion of [EMIM]Cl led to the predominance of the initiation over the chain growth, causing the formation of short non-crosslinked molecules. The IL content of 5% allowed for curing the epoxy resin and elevating the stiffness of the crosslinked product by almost 7 times compared to tetraethylenetriamine as a usual aliphatic amine hardener (6.95 GPa versus 1.1 GPa). The [EMIM]Cl content of 20-40% resulted in a thermoplastic epoxy polymer capable of flowing and molding at elevated temperatures. The formation of IL emulsion in the epoxy matrix occurred at 60% [EMIM]Cl, but its hygroscopicity and absorption of water from surrounding air reduced the crystallinity of dispersed [EMIM]Cl, not allowing for an effective phase-change material to be obtained.