Monitoring of thickness and conversion of thick pigmented UV-cured coatings by NIR hyperspectral imaging

Abstract

Hyperspectral imaging using a near-infrared (NIR) camera was used for in-line monitoring of thickness and conversion of white pigmented UV-cured acrylate coatings. In particular, it was applied for the quantitative determination of both parameters as well as for the evaluation of the homogeneity of their spatial distribution. Quantitative data were obtained by means of chemometric approaches. Calibration models were built up on the basis of the partial least squares (PLS) regression algorithm using reference data of coatings with thicknesses in the range from about 20–200 μm and with a conversion between about 15 and 80%. Reference values for the conversion were determined by NIR reflection spectroscopy utilizing the acrylate band at 1620 nm. The resulting accuracies (in terms of the root mean square error of prediction, RMSEP) were found to be 8.7 μm for the prediction of the layer thickness and 2.5% for the acrylate conversion. The developed methods were also used for the visualization of the spatial distribution of both parameters across the surface of the UV-cured coating and the detection of inhomogeneities within the layers. The performance of this approach was demonstrated with coatings that formed a pronounced heterogeneous surface structure after UV curing resulting from shrinkage. All NIR measurements were carried under conditions that were similar to those in technical coating processes. Consequently, the results of this study prove that hyperspectral imaging has considerable potential for in-line process and quality control.