In-line determination of the thickness of UV-cured coatings on polymer films by NIR spectroscopy

We report on the development of a measuring method based on near-infrared (NIR) spectroscopy, which is able to determine the thickness of UV-cured coatings and which can be used for in-line monitoring in technical coating processes. In particular, acrylate coatings, which were applied to transparent polymer films with a typical thickness of 5-35 μm, were investigated. NIR spectra were recorded in transflection mode. Quantitative analysis of the spectral data was carried out with partial least square (PLS) regression. In-line measurements were performed on a pilot-scale roll coating machine at web speeds up to 50 m/min. It was shown that quantitative data with excellent precision (i.e. with a standard deviation lower than ±1 μm) and high time resolution (2.5 spectra/s) can be obtained.