In-line monitoring of printing processes in an offset printing press by NIR spectroscopy: Correlation between the conversion and the content of extractable acrylate in UV-cured printing inks

Near-infrared (NIR) reflection spectroscopy was used for in-line monitoring of the conversion in printed layers of a UV-curable cyan printing ink at an offset printing press. Quantitative analysis of the spectra was based on the calibration of the method with chemometric approaches using reference data for the conversion obtained by FTIR/ATR spectroscopy. In this way, the conversion in ink layers with a thickness of 0.8 g m⁻² was determined with a precision of 5%. Moreover, it was shown that the specific migration of the acrylate components in the binder of the printing ink was linearly dependent on the conversion in the studied range between about 80% and 100%, which allowed an estimation of the specific migration from conversion data. The strict linear relation between both parameters was used for indirect in-line monitoring of the acrylate migration during the printing process by predicting it from the conversion data derived from the NIR spectra. For comparison, off-line measurements on random samples by HPLC were carried out. The error of the predicted migration data in comparison with the results of the analytic measurements was found to be about 0.03 mg dm⁻². The proposed indirect prediction of the migration will allow a better and more direct control of technical printing processes.