Abstract
Near-infrared (NIR) reflection spectroscopy was used for the determination of
the thickness or the coating weight, respectively, of white-pigmented acrylic
coatings and layers of printing inks. The thickness of coatings was studied in the
range from 5 to 60 μm, whereas the coating weights of the printed layers covered a
range between 1 to 5 g m<sup>–2</sup>. Quantitative analysis of the spectral data
relied on partial least squares (PLS) regression. A thickness gauge or gravimetry,
respectively, were used to obtain reference data. Calibration models were typically
based on six factors. The corresponding root mean square errors of prediction
(RMSEP) were found to be on the order of 0.87 for coatings and 0.38 for printed
layers. Monitoring of the coating thickness under process conditions was carried out
on a pilot-scale roll coating machine. In order to simulate thickness changes during
a coating process, either the nip between the applicator rolls or the web speed was
varied. Data with high precision (standard deviation ∼1 μm for coatings, ∼0.4 g
m<sup>–2</sup> for printed layers) and an excellent correlation with off-line
reference data were obtained. The investigations have shown that NIR spectroscopy
can be used for process control in coating and curing technology.
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