Measurement system for monitoring the paper coating process

It is widely known that drying conditions affect the quality of the coated paper, and especially the printing properties. However, in the paper coating process, typically only moisture before coating, moisture after coating and coat weight are measured. The power of the dryers is controlled by these measurements, but the details of the drying process are not controlled. The studies of the paper coating process have concentrated on theoretical simulations on the one hand, and on the experimental determination of the immobilisation points on the other hand. Conventionally, the immobilisation of paper coating has been described mainly by the first and second critical points, which can be determined with gloss and scattering measurements. Furthermore, it is crucial to understand the behaviour of water that is removed from the coating layer in two ways: through surface evaporation and through penetration into the base paper. These processes depend on the drying conditions, but also on the physical and chemical properties of the coating colour and base paper. A new viewpoint was added to the research by introducing a measurement system and method to study the paper coating process from point of view of water absorption and penetration into the paper. The system is based on a near-infrared hyperspectral imaging spectrograph, a sophisticated fibre optic light source and fibre optic measurement probes. The system allows the measurement of gloss, scattering, and moisture with multiple geometries, all of these from seven different measurement positions. A wealth of experimental data was collected during pilot coating trials. It was possible to follow the penetration of water into the base paper, and the drying of the paper. The results suggest that the first critical point may not be well defined in many cases or it is so close to the application that the gloss is decreasing all the time. The second critical point is very clearly seen both in the gloss and in the scattering measurement and it is usually after the first IR dryers or the first air foil dryer. The drying strategy in the coating process is found to cause clear differences in water penetration into the paper and in the immobilisation of the coating. Water of the coating layer penetrates to the base pa per despite the drying intensity until the second critical point is reached. According to the bottom reflection measurement, the levelling of the water content in the base paper continues still after the second critical point. The research will lead to a better understanding of the coating process, and will give means to control the printability of the paper already during coating.