The effect of different gloss levels on in-line monitoring of the thickness of printed layers by NIR spectroscopy

Abstract

Near-infrared (NIR) reflection spectroscopy was used for monitoring the thickness or rather the coating weight of thin printed layers of transparent oil-based offset printing varnishes in a range from 0.5 to 5 g m(-2). Quantitative analysis of the spectral data was carried out with partial least squares regression. Surface properties such as the gloss were found to strongly affect the prediction of the coating weight. This influence was minimized by the development of calibration models, which contained spectra of layers with a broad range of gloss levels. The prediction error of these models was in the order of 0.12 to 0.16 g m(-2). In-line measurements were carried out at a sheet-fed offset printing press in order to test the performance of the models under real process control conditions. Varnishes were applied to paper at printing speeds of 90 or 180 m min(-1). A close correlation between the predictions from in-line NIR spectra and the reference data from gravimetry was observed regardless of the specific degree of gloss of the layers (errors between 0.15 and 0.17 g m(-2)). The results clearly prove the efficiency of NIR reflection spectroscopy for quantitative investigations on thin layers in fast processes such as printing and demonstrate its analytical potential for quality and process control.

In-Line Monitoring of the Thickness of Printed Layers by Near-Infrared (NIR) Spectroscopy at a Printing Press

“…The influence of such factors, e.g., variation of the composition of the acrylate formulation, the substrate, or the degree of gloss of the printed layers, will be discussed in forthcoming papers. 34,35 Future work will also focus on the mounting of the probe head in the delivery unit of the printing press, which requires considerable technical effort due to the complex sheetconveying system. However, this kind of mounting of the probe head would not only allow measurements of the thickness of dried or cured layers, but also monitoring of the conversion of radiation-curable varnishes achieved after UV irradiation.…”

In-Line Monitoring of the Thickness of Printed Layers by NIR Spectroscopy: Elimination of the Effect of the Varnish Formulation on the Prediction of the Coating Weight

“…It has been demonstrated in previous investigations that these kinds of individual calibration models that were developed for layers made up of specific varnishes can be effectively used for in-line monitoring of printing processes. − However, it is obvious that their application range is strictly limited to that formulation for which the model had been set up. To extend this range and to improve in this way the practical usability of chemometric models in process control, universal calibration models were developed.…”

“…37 However, the composition of varnish formulations may vary to a large extent. It is obvious that the composition also has a strong impact on the spectrum of the varnish and consequently on the prediction of the thickness of the printed layers.…”

“…Nevertheless, it was shown that, in principle, the thickness of printed layers can be determined under in-line conditions. However, it was also found that the specific properties of the printed layers such as their gloss strongly influence the precision of the predictions. Moreover, the influence of the specific substrate (e.g., papers with different surface finishes) on the measurements was studied in detail .…”

Monitoring of the Degree of Condensation in Alkoxysiloxane Layers by NIR Reflection Spectroscopy

“…Near-infrared (NIR) reflection spectroscopy is widely used for process control for a long time. , Recently, it has been shown, that it is also a powerful tool for in-line monitoring of the application and UV curing of thin acrylic coatings and printed layers with a thickness in the range of a few micrometers only. − In particular, parameters such as the conversion after UV curing, thickness (or coating weight) of the layer, drying state, etc., can be determined in real-time and directly in the coating machine or printing press. Quantitative data are obtained by use of chemometric approaches on the basis of the partial least-squares (PLS) algorithm using suitable reference methods.…”