Process Control in Ultraviolet Curing with in-line near Infrared Reflection Spectroscopy

Abstract

Near infrared reflection spectroscopy was used for in-line monitoring of the conversion and the coating thickness of thin UV-cured coatings which have a typical thickness in the range of some micrometers only. Quantitative analysis of the spectral data was carried out by PLS-based multivariate calibration methods. In addition, univariate procedures were tested for some applications. The conversion, in particular its dependence on the applied irradiation dose, was followed in both acrylic coatings, which were cross-linked by free radical polymerisation and in epoxy / vinyl ether blends, which were cured according to a cationic reaction mechanism. The thickness of acrylate coatings was studied in a range between 5 μm and 100 μm. It was shown that quantitative data with high precision and time resolution can be also obtained in pilot scale investigations, even if the coating line is operated at high speed.