In-line Monitoring of Printing Processes by NIR Reflection Spectroscopy

Introduction
In the last years, the use of printing inks and varnishes increased rapidly. In case of UV-curable inks, the processing and application properties of printed layers strongly depend on the final conversion that is achieved during irradiation. The conversion, in turn, is influenced by various technical parameters and ambient conditions. Unfortunately, only some of them can be easily controlled in technical curing processes. Hence, in order to avoid a negative effect of an insufficient conversion on the properties of the layers, an efficient process control based on continuous monitoring of the conversion is necessary.
Another important parameter in printing technology is the coating weight, which can be only determined for printing inks with standard colors so far (by color density measurements). For in-line monitoring of the coating weight of clear printing varnishes and inks with special colors, an alternative method is required.

Experimental
Near-infrared (NIR) reflection spectroscopy has already been proven to be a powerful tool for in-line monitoring of the conversion and the thickness of coatings (with a typical thickness range of ~10 g/m²). It will be shown in this presentation that this approach is also suited for printing processes despite the much thinner layers and the higher web speeds. A special reflection probe head was developed for installation in the delivery unit of a large-scale sheet-fed offset printing press. It was linked to a process spectrometer equipped with an InGaAs photodiode array detector covering a spectral range from 1100 to 2200 nm. Chemometric approaches such as PLS were applied for quantitative analysis of the spectral data. Their calibration was carried out with reference values obtained by FTIR spectroscopy or gravimetry, respectively.

Results and Discussion
The new method was developed for in-line characterization of thin printed layers of inks and varnishes with a thickness between 0.8 and 3 g/m² (conversion) or 0.5 and 7 g/m² (thickness). Spectra for calibration were recorded with a setup that mimics the specific conditions in the printing press. During in-line monitoring, it was demonstrated that the conversion in printed layers can be determined with a precision of ~ 4 to 5 % at printing speeds at least up to 180 m/min. Moreover, the coating weight of conventional oil-based, UV-curable, and water-based dispersion inks and varnishes was determined with prediction errors (RMSEP) between 0.12 and 0.2 g/m² depending on the specific ink and substrate. The printing speed was found to have no significant influence on the precision of the measurements. Moreover, data predicted in-line show an excellent correlation with reference values obtained off-line by FTIR spectroscopy or gravimetry.