Examining the effects of storage on the initiation behaviour of ionic liquids towards the cure of epoxy resins

Four structurally related ionic liquids (1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium dicyanamide, and 1-ethyl-3-methylimidazolium thiocyanate) are examined for their storage characteristics and the effect on their ability to initiate the cure of epoxy resins. At ambient temperature, epoxy formulations containing 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium thiocyanate display marked colour changes to yield dark red samples with greatly increased viscosity after one day; after six days both samples have undergone vitrification. The epoxy formulation containing 1-ethyl-3-methylimidazolium acetate continued to polymerise even at sub-zero temperatures. Storage in dark bottles retarded the reaction during the 30-minute period that the sample is removed from the freezer prior to an aliquot being taken, but once the autocatalytic low temperature reaction has started, the dark glass no longer provides effective protection. Samples of 1-ethyl-3-methylimidazolium dicyanamide/epoxy were also stored and sampled in the same manner, but no differences were exhibited between the samples in clear and dark brown glass bottles. Infrared and nuclear magnetic resonance studies confirmed that the hygroscopic ionic liquids pick up water readily (coordinating to the H atom at the 2-position on the imidazolium ring), but once dried the initiating ability is lost.