13 October 2013
Year: 2013Price: 10.00
UV-curable printing inks and varnishes play an important role in printing technology since they allow high printing speeds and fast further processing and provide an excellent optical appearance [1]. Therefore, package printing is one of their main application areas, where they are particularly used for high-quality goods such as cosmetics. Most UV-curable printing inks and lacquers are based on acrylate formulations, which cure in a radical reaction. The processing and application properties of printed layers such as scratch or wipe resistance, migration behavior, blocking, surface appearance, and other parameters do not only depend on the composition of the ink or varnish formulation, but also substantially on the level of the final conversion, which is achieved during UV curing. The conversion is influenced by various physico-chemical and technical factors. The most important one is the irradiation dose, which is applied to the printed layers and which depends on both the irradiance and the printing speed. Moreover, ambient conditions such as temperature and inertization can influence the final conversion. Most of these parameters can be easily controlled in order to ensure a sufficient level of conversion. However, some other influencing factors are hardly to control, for example the different absorption of inks with different colors or the pollution and aging of the UV lamps, which can strongly decrease their emittance and consequently affect the con-version in the inks reached during UV irradiation.
In order to meet the increasing quality requirements, it is therefore essential to control the conversion continuously and directly in the printing process. One of the most common analytical methods used for process control is near-infrared (NIR) spectroscopy [2-4], which has found numerous applications in agriculture, food manufacturing, chemistry, pharmaceutics, and many others. The investigation of thin polymer coatings is still a rather new area, which is probably due to the challenging analytic requirements with respect to sensitivity. In the past, we reported on investigations on the conversion [5-9] and the thickness [10-12] of UV-cured acrylate coatings by NIR spectroscopy. It was demonstrated that NIR spectroscopy can be used for continuous quantitative in-line monitoring of both pa-rameters in acrylate and epoxy/vinyl ether coatings during roll coating at line speeds at least up to 120 m min-1. A broad range of different substrates and coating materials as well as the influence of various ambient and technical parameters on the precision of the measurements were studied. Typically, the prediction errors were found to be in the order of 2 to 3 % for the conversion and 0.5 to 1 μm for the thickness. Moreover, it was also shown that both parameters can be determined simultaneously [13].
In case of printed layers, the thickness is lower than that of coatings by about one order of magnitude. Consequently, the requirements to the sensitivity of the analytical equipment are even higher. Nevertheless, it was demonstrated that the coating weight of thin printed layers (which is used in instead of the thickness in printing technology) can be determined with high precision by this method [14-17]. The error of the prediction of the coating weight of printed varnish layers measured in an offset press was about 0.15 to 0.2 g m-2.
The objective of the present work consisted in the development of methods for the in-line measurement of the conversion in UV-cured printed layers. This also included the development of a new small probe head for NIR spectroscopy because the available space in the delivery unit of the offset printing press was very limited and several other restrictions had to be considered. Therefore, the probe head developed previously for the determination of the coating weights of printed layers, which was carried out at another, more easily accessible position of the press, could not be used in the present study.
In case of printed layers, the thickness is lower than that of coatings by about one order of magnitude. Consequently, the requirements to the sensitivity of the analytical equipment are even higher. Nevertheless, it was demonstrated that the coating weight of thin printed layers (which is used in instead of the thickness in printing technology) can be determined with high precision by this method [14-17]. The error of the prediction of the coating weight of printed varnish layers measured in an offset press was about 0.15 to 0.2 g m-2.
The objective of the present work consisted in the development of methods for the in-line measurement of the conversion in UV-cured printed layers. This also included the development of a new small probe head for NIR spectroscopy because the available space in the delivery unit of the offset printing press was very limited and several other restrictions had to be considered. Therefore, the probe head developed previously for the determination of the coating weights of printed layers, which was carried out at another, more easily accessible position of the press, could not be used in the present study.
