Near Infrared-spectroscopy in resin transfer molding – determination of the degree of cure

Abstract

Near infrared (NIR) spectroscopy is employed to monitor the degree of cure of a composite material directly during the curing phase in the resin-transfer-molding (RTM) process. The composite material used consists of natural fibers and an epoxy/amine resin. The quantitative determination of the degree of cure, α, from the NIR spectra is realized by partial least square (PLS) regression in conjunction with various pre-treatments of the spectral data. As a reference, the degree of cure is also determined by isothermal DSC experiments. The best PLS model that could be obtained for α is characterized by a determination coefficient of prediction (R²P) of 0.980 and root-mean-square error of prediction (RMSEP) of 4.4 %. While for every RTM trial the spectra during curing change corresponding to literature, major differences can be observed between the spectra of different RTM trials. The reasons for this are discussed in detail. The findings show the potential of inline NIR spectroscopy for monitoring α in the RTM process.