Analytical Sciences

Pulsed NMR Study of the Curing Process of Epoxy Resin

Hiroki KIMOTO, Chikako TANAKA, Michiko YAGINUMA, Emi SHINOHARA, Atsushi ASANO, and Takuzo KUROTSU

Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan

To analyze a curing process of epoxy resin in terms of molecular motion, we adapted a pulsed NMR method. Three kinds of ¹H spin-spin relaxation times (T_2L (long), T_2S (short) and T_2M (intermediate)) were estimated from observed solid echo train signals as the curing process proceeded. A short T_2S value below 20 µs suggests the existence of a motion-restricted chain, that is, cured elements of resin, and its fraction, P_S, sigmoidally increased with the curing time. On the other hand, the fraction of T_2L, P_L, decreased with the reaction time reciprocally against P_S, suggesting the disappearance of highly mobile molecules raised from pre-cured resin. The spin-lattice relaxation time, T₁, was also measured to check another aspect of molecular motion in the process. T₁ of the mixed epoxy resin and curing agent gradually increased just after mixing both of them. This corresponds to an increment of a less-mobile fraction, of which the correction time is more than 10^-6 s, and also means that the occurrence of a network structure whose mobility is strongly restricted by chemically bonded bridges between the epoxy resin and curing agent. The time courses of these parameters coincided with those of IR peaks pertinent to the curing reaction. Therefore, pulsed NMR is a useful tool to monitor the hardening process of epoxy resin in real time non-distractively in terms of the molecular motion of protons.

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