Moisture depth profiling in paper using near-infrared spectroscopy

A near-infrared spectroscopic method and measurement system for moisture depth profiling is presented in this thesis. The measurement system is based on a fibre-optic light source and probes, and a hyperspectral imaging spectrograph. The probes feature six different measurement geometries, including a specular reflection measurement, and diffuse reflection and transmission measurements, all of which are measured in seven positions simultaneously. A theoretical analysis of the depth profiling capabilities of the system is presented, as well as laboratory tests of the system performance. Based on these, the analysis depth of the reflection moisture measurement in a 75 μm thick uncoated paper is estimated to be about 40 μm. It is also shown that the depth profiling properties depend on the average moisture content of the paper, and that the inclusion of a coating layer complicates the depth profiling. The use of the theoretical analysis results in the interpretation of the measurement results is also discussed. As the main application, the measurement system was installed on a pilot-scale paper coating machine. The feasibility of semi-quantitative moisture depth profiling during the coating process is shown. The determination of the first and second critical concentrations of the coating colour consolidation process is also discussed. Detailed results from the extensive programme of trial runs are presented, along with discussion of the physics behind the measurements. It is found that the water transport characteristics measured during the coating process are not in accordance with mass and heat transfer theories in some respects. Also, the specular reflection measurement results do not behave as expected, making the determination of the first critical concentration impossible in some cases. Instead, the second critical concentration is easily detected, but does not always behave as expected. Reasons for the unexpected behaviour of the measurements are discussed, and conclusions drawn about the water transport during the coating process. Copyright © VTT Technical Research Centre of Finland 2007.