Evidence for kinetic inhomogeneity in the curing of epoxy using the near-infrared multispectral imaging technique

The kinetics of curing of an epoxy resin by amine was studied using a near-infrared (NIR) multispectral imaging spectrometer. This imaging spectrometer is capable of sensitively and rapidly recording NIR spectral images of a sample because it was constructed with an acousto-optic tunable filter and an InGaAs focal plane array NIR camera. The high sensitivity and fast scanning ability of the spectrometer make it suitable for kinetic determination of fast reactions. Additionally, it has features that conventional NIR spectrometers cannot offer, namely, its ability to provide kinetic information at different positions within a sample. Furthermore, the high spatial resolution and sensitivity of the InGaAs camera make it possible to determine the kinetics from data collected by a single pixel in the camera. The kinetics of curing of epoxy by amine, determined by this multispectral imaging instrument, show that the reaction rates within the sample are very inhomogeneous. Because of this kinetic inhomogeneity, differences in the degrees of cure at different positions within the sample can be as high as 37% when data from only a single pixel were used for calculation. The inhomogeneity was not be observed if an average of a large number of pixels were used.