21 Applications of NIR Techniques in Polymer Coatings …
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of the conversion has been demonstrated by in-line monitoring experiments at a roll
coating machine [11].
Alternatively, the variation of the thickness of the coating can be included as an
additional parameter into the chemometric calibration models, which is certainly the
most sophisticated approach. This can be achieved by use of the PLS2 algorithm,
which is able to predict two or even more parameters from the same input data. Of
course, this implies much higher efforts for calibration, since the calibration samples
have to cover the full range of both conversion and thickness that can occur during
process control. On the other hand, the higher complexity of the calibration process
is offset by the fact that simultaneous prediction of both conversion and coating
thickness can be achieved while using one NIR probe head only.
The suitability of this approach was demonstrated during application of clear
and pigmented acrylate varnishes to PP film at a roll coating machine and their
cross-linking by UV irradiation [24]. Reference samples for the setup of the PLS2
model covering a broad spectrum of combinations of conversion and thickness were
characterized by FTIR spectroscopy and a thickness gauge. During roll coating, the
line speed was increased in incremental steps from 20 to 100 m/min, which inevitably
led to changes of the thickness as well as the conversion due to the corresponding
decrease of the irradiation dose. Both parameters were predicted from the recorded
NIR spectra using the PLS2 model. Results of one specific coating experiment are
shown in Fig. 21.9. For quantitative evaluation of the predicted data, reference values
were determined off-line for each step of the line speed after the end of the coating
trial. These values are included in Fig. 21.9 as well. The data clearly prove that the
conversion was predicted with a precision of 2–3% even under the conditions of
changing thickness, whereas the error in the in-line measurement of the thickness
was found to be about 0.5 to 1 μm. These error margins correspond to those found in
independent predictions of both parameters by PLS1 models (see paragraphs 21.2.2.
and 21.2.3.1.).
21.2.4 Hyperspectral Imaging of UV-Cured Coatings
In the studies reported so far, the parameters of interest (thickness, coating weight,
conversion etc.) have been monitored by conventional NIR spectroscopy. Since the
spot of most NIR spectrometers has a diameter of a few millimeters only, this means
that these parameters can be determined in a small stripe only during monitoring a
running process. However, for coatings as two-dimensional objects the spatial distribution of the parameter under investigation might be of interest for a comprehensive
process control. For example, UV lamps have rod-like shape, which may lead to a
decay of the intensity at their ends due to aging, pollution etc. In UV LED systems,
single elements may drop out. Both effects lead to a reduced conversion in the corresponding region, which might be possibly insufficient for the intended application.
Also in case of the application weight, local deviations may occur, e.g., due to running
out of the varnish formulation in the roll gap or the reservoir of the coating machine,
491
of the conversion has been demonstrated by in-line monitoring experiments at a roll
coating machine [11].
Alternatively, the variation of the thickness of the coating can be included as an
additional parameter into the chemometric calibration models, which is certainly the
most sophisticated approach. This can be achieved by use of the PLS2 algorithm,
which is able to predict two or even more parameters from the same input data. Of
course, this implies much higher efforts for calibration, since the calibration samples
have to cover the full range of both conversion and thickness that can occur during
process control. On the other hand, the higher complexity of the calibration process
is offset by the fact that simultaneous prediction of both conversion and coating
thickness can be achieved while using one NIR probe head only.
The suitability of this approach was demonstrated during application of clear
and pigmented acrylate varnishes to PP film at a roll coating machine and their
cross-linking by UV irradiation [24]. Reference samples for the setup of the PLS2
model covering a broad spectrum of combinations of conversion and thickness were
characterized by FTIR spectroscopy and a thickness gauge. During roll coating, the
line speed was increased in incremental steps from 20 to 100 m/min, which inevitably
led to changes of the thickness as well as the conversion due to the corresponding
decrease of the irradiation dose. Both parameters were predicted from the recorded
NIR spectra using the PLS2 model. Results of one specific coating experiment are
shown in Fig. 21.9. For quantitative evaluation of the predicted data, reference values
were determined off-line for each step of the line speed after the end of the coating
trial. These values are included in Fig. 21.9 as well. The data clearly prove that the
conversion was predicted with a precision of 2–3% even under the conditions of
changing thickness, whereas the error in the in-line measurement of the thickness
was found to be about 0.5 to 1 μm. These error margins correspond to those found in
independent predictions of both parameters by PLS1 models (see paragraphs 21.2.2.
and 21.2.3.1.).
21.2.4 Hyperspectral Imaging of UV-Cured Coatings
In the studies reported so far, the parameters of interest (thickness, coating weight,
conversion etc.) have been monitored by conventional NIR spectroscopy. Since the
spot of most NIR spectrometers has a diameter of a few millimeters only, this means
that these parameters can be determined in a small stripe only during monitoring a
running process. However, for coatings as two-dimensional objects the spatial distribution of the parameter under investigation might be of interest for a comprehensive
process control. For example, UV lamps have rod-like shape, which may lead to a
decay of the intensity at their ends due to aging, pollution etc. In UV LED systems,
single elements may drop out. Both effects lead to a reduced conversion in the corresponding region, which might be possibly insufficient for the intended application.
Also in case of the application weight, local deviations may occur, e.g., due to running
out of the varnish formulation in the roll gap or the reservoir of the coating machine,