Investigating the mechanism through which ionic liquids initiate the polymerisation of epoxy resins

Abstract

The mechanism of reaction between 1-ethyl-3-methylimidazolium acetate and the difunctional diglycidyl ether of bisphenol A (DGEBA) is explored using thermal and spectroscopic methods. Investigation of the 1,3-dialkylimidazolium based ionic liquids comprising the common cation (1-ethyl-3-methylimidazolium) and different anions (acetate, diethyl phosphate, dicyanamide or thiocyanate) via thermogravimetric analysis revealed 1-ethyl-3-methylimidazolium acetate to be the least thermally stable, both in air and nitrogen, and 1-ethyl-3-methylimidazolium dicyanamide to be the most thermally stable. Dynamic differential scanning calorimetry reveals the formulations comprising DGEBA and ionic liquid where it was revealed that the lowest and highest temperature for the onset of reaction were observed for formulations with 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium dicyanamide respectively. 1-Ethyl-3-methylimidazolium acetate was shown, via nuclear magnetic resonance (NMR) spectroscopy and residual gas analysis, to degrade at 150 °C to yield dealkylated products including methyl acetate and ethyl acetate as well as 1-methylimidazole and 1-ethylimidazole. The dealkylated imidazole ring is proposed as a route for initiation of the epoxy ring. Adduct formation between 1-ethyl-3-methylimidazoloium acetate and benzaldehyde at room temperature was observed leading to the proposal of the generation of a carbene species as a route for initiation of the epoxy ring in formulations with the acetate anion. NMR analysis of formulations comprising 1-ethyl-3-methylimidazolium thiocyanate and epoxy are believed, at room temperature, to initiate via reaction of the thiocyanate anion with the epoxy ring. At elevated temperatures, it is proposed that a second, competing reaction, involving deprotonation of the imidazolium ring, also becomes active. The three proposed reaction pathways, namely the carbene route, the imidazole route, and the counter-ion route, are all proposed to occur when an ionic liquid is used to initiate an epoxy resin.