Study on the reactivity and kinetics of primary and secondary amines during epoxy curing by NIR spectroscopy combined with multivariate analysis

Abstract

The temperature of epoxy curing reaction and the change of amino reactivity for the amine curing agent have an effect on the three-dimensional network structure of the product, which in turn affect the performance of the epoxy resin. The study on the reactivity and kinetics of primary and secondary amines in epoxy curing process is of great significance to deeply understand the curing mechanism and realize the control of product quality. In this paper, an effective method for studying the reactivity and kinetics of primary and secondary amines in epoxy curing process by near-infrared (NIR) spectroscopy combined with multivariate analysis was proposed. The curing system of bisphenol A epoxy resin (E51) and M-xylylenediamine (MXDA) was studied, and the resins were isothermally prepared at 50, 60, 70, 80, 90 °C, respectively. The NIR spectra of the system for each curing reaction process were continuously collected online. The characteristic absorption bands of the primary and secondary amines overlap heavily in the wavenumber range of 6700–6330 cm−1. The wavelet-based second derivative together with multivariate curve resolution(MCR) were employed to resolve their respective absorption peaks. Then, the curves of conversion and conversion rate for primary and secondary amines were obtained. The study shows that the kinetics of the curing reaction follows the self-catalytic Kamal model in the early stage, and it follows the revised model of incorporating the diffusion factor after vitrification, in which the hydroxyl group plays a self-catalytic role in the reaction process; the reactivity of primary and secondary amines is different during epoxy curing process. There is a competitive reaction between them, and its competitiveness can be expressed by k2(SA)/k2(PA); the reactivity of primary and secondary amines varies with curing temperature, and k2(SA)/k2(PA) increases significantly when curing temperature exceeds 60 °C, which has a significant influence on the formed resin network structure.