In situ monitoring of polyurethane cure using fibre-optical FT-NIR spectroscopy

In-line monitoring of the reaction extent of polyurethane during a reactive injection moulding (RIM) process is carried out using fibre-optic near infrared (NIR) spectroscopy. Up to 250 transmission spectra are recorded during the reaction. Univariate and multivariate analysis of transmittance spectra were used to calculate the chemical conversion. A good agreement is observed between first principal component of principal component analysis (PCA), and univariate (Beer-Lambert) results. It is observed that, in this case, the PCA method can provide a good practical estimation of the time-concentration profile during the reaction, without the need of the time-consuming calibration methods. The scores of PC1 are merely linearly correlated to the level of conversion and contain enough information for the quantitative analysis. As expected interactions and hydrogen-bonding play an important role. Hence the spectral region of PCA analysis has to be carefully selected to obtain a good agreement with the Beer-Lambert law. The NIR spectroscopy and the PCA are easy-to-use techniques for on line monitoring of polyurethane reactions and these results open up a low cost effective opportunity for monitoring the fast RIM process.