Study of the curing reaction of epoxy resin by near-infrared laser-excited FT-Raman spectrometry

In the present investigation we examined the curing reaction of epoxy resin using acid anhydride as a curing agent by near-infrared laser-excited FT-Raman spectrometry. The sample system was as follows. Diglycidyl-ether-bisphenol A (DGEBA) was used as the main agent, methyl nadic anhydride (MNA) as the curing agent, and 2-ethyl-4-methylimidazole (EMI) as the curing catalyst. Though this sample system is widely used in practice, quantitative or semi-quantitative examinations of the curing reaction have not yet been sufficiently developed. This situation is assumed to result from the fact that in infrared spectrometry the characteristic bands of the main agent severely overlap with those of the curing agent. On the other hand, in the usual Raman spectrometry using laser light in the visible region, a very strong fluorescent background is generated from this sample system illuminated by laser beam, and the spectra are hidden in this background. It was shown that when FT-Raman spectrometry is performed with a near-infrared laser as an excitation source very clear spectra consisting of sharp bands can be obtained, and the characteristic signals of each of the chemical species involved in the curing reaction in question can be recognized. On the basis of these characteristic Raman signals, the effect of the quantities of the curing catalyst on the curing phenomena was examined. It was proven that when the quantity of EMI is in the concentration range from 0.2 wt% to 2 wt%, EMI promotes a polymerization reaction between the main agent DGEBA and the curing agent MNA, which is the situation that we had expected to realize. On the other hand, when the concentrations of EMI are larger than 2 wt%, it is clear that the reaction scheme changes, and that the reaction between DGEBA and EMI takes priority over the desired curing reaction. It was concluded that the thus-obtained information is very important for the optimum curing characteristics of epoxy resin.