Effect of changes of the coating thickness on the in-line monitoring of the conversion of photopolymerized acrylate coatings by near-infrared reflection spectroscopy

Near-infrared (NIR) reflection spectroscopy was used for in-line monitoring of the conversion and the thickness of thin UV-cured acrylate coatings applied to polymer foils. Quantitative analysis of the spectroscopic data was performed either with the aid of PLS-based chemometric models or by band integration according to the Beer-Lambert law. Unintended changes of the thickness of the coating, e.g. caused by variation of the web speed, were found to preclude the correct analysis of the conversion by chemometric methods. In order to correct the conversion data for such changes, NIR spectra were recorded before and after UV irradiation. The conversion was determined from the ratio of the band integrals of the overtone of the acrylic double bond at 1620 nm. It was shown that quantitative conversion data with high precision were achieved in this way. The method was used for in-line monitoring of the conversion in clear and pigmented coatings, which were applied to OPP foil by roll coating at line speeds up to 120 m min−1.

Graphical abstractDownload full-size image