482
T. Scherzer
control of the thickness of the applied layers in technical coating processes is mostly
carried out off-line by gravimetric determination of the coating weight. However, it
is apparent that this approach is not able to respond to sudden changes of the thickness, and thus it is poorly suited for process control. However, it was shown that
thickness or coating weight can be followed by NIR reflection spectroscopy under
the conditions of technical coating processes [16, 17].
Clear acrylate coatings with thicknesses in the range from 5 to 100 μm were
studied [16]. Although calibration samples were prepared with different thicknesses
for PLS modeling with a set of various Baker applicators with well-defined gaps, the
resulting thickness had to be measured after UV curing mainly due to the well-known
shrinkage of acrylate formulations during the cross-linking reaction and spreading of
the applied wet coating. Shrinkage and other effects affecting the thickness may lead
to a decrease of the thickness up to 30%. Instead of direct thickness measurements
(e.g., with a thickness gauge), gravimetry may be used alternatively to provide reference data. Depending on the range covered by the specific PLS model, prediction
errors were found to be in the order between less than one and about 4 μm [16, 17].
Similar results were also found for white-pigmented coatings containing 10 wt%
titanium dioxide [18]. For example, the thickness of such coatings with a thickness
from 5 to 60 μm was predicted with a precision of about 1 μm.
Moreover, in-line monitoring of the thickness was carried out at a pilot-scale roll
coating machine. Quantitative data were recorded at line speeds up to 50 m/min. Very
close correlation between data predicted from NIR spectra and reference data determined off-line after the end of the coating trials was observed. Prediction errors were
found to be very similar to those obtained in the external validation tests although
the experimental conditions at the coating machine were less optimal than in the
lab. In particular, in-line measurements were exposed to disturbing factors such as
vibrations of the coating machine including the mount of the probe head and flutters
of the moving film web. In order to simulate abrupt changes of the thickness during
a real coating process, the nip between the applicator rolls was stepwise reduced
and increased. An example for clear coats is shown in Fig. 21.2 [17]. Similar investigations were also carried out for pigmented coatings [18]. It was demonstrated
that changes in layer thickness lower than 1 μm can be clearly detected by NIR
spectroscopy.
The thickness of clear and pigmented UV-cured coatings was also monitored at
different line speeds up to 100 m/min [16–18]. A marked increase of the thickness
with increasing speed was observed up to about 50 m/min, which was followed by a
decrease at even higher web speeds. Although this behavior seems to be somewhat
surprising, it is well-known in coating technology [19, 20]. In fact, it is due to a very
complex interaction of several physical and technological parameters of the coating
process. In particular, acrylate formulations are typical non-Newtonian fluids, i.e.,
their viscosity depends on the shear rate. In-line NIR spectroscopy offered for the
first time the unique possibility to monitor the thickness changes upon variation of
the line speed directly during the coating process in a quantitative manner.
T. Scherzer
control of the thickness of the applied layers in technical coating processes is mostly
carried out off-line by gravimetric determination of the coating weight. However, it
is apparent that this approach is not able to respond to sudden changes of the thickness, and thus it is poorly suited for process control. However, it was shown that
thickness or coating weight can be followed by NIR reflection spectroscopy under
the conditions of technical coating processes [16, 17].
Clear acrylate coatings with thicknesses in the range from 5 to 100 μm were
studied [16]. Although calibration samples were prepared with different thicknesses
for PLS modeling with a set of various Baker applicators with well-defined gaps, the
resulting thickness had to be measured after UV curing mainly due to the well-known
shrinkage of acrylate formulations during the cross-linking reaction and spreading of
the applied wet coating. Shrinkage and other effects affecting the thickness may lead
to a decrease of the thickness up to 30%. Instead of direct thickness measurements
(e.g., with a thickness gauge), gravimetry may be used alternatively to provide reference data. Depending on the range covered by the specific PLS model, prediction
errors were found to be in the order between less than one and about 4 μm [16, 17].
Similar results were also found for white-pigmented coatings containing 10 wt%
titanium dioxide [18]. For example, the thickness of such coatings with a thickness
from 5 to 60 μm was predicted with a precision of about 1 μm.
Moreover, in-line monitoring of the thickness was carried out at a pilot-scale roll
coating machine. Quantitative data were recorded at line speeds up to 50 m/min. Very
close correlation between data predicted from NIR spectra and reference data determined off-line after the end of the coating trials was observed. Prediction errors were
found to be very similar to those obtained in the external validation tests although
the experimental conditions at the coating machine were less optimal than in the
lab. In particular, in-line measurements were exposed to disturbing factors such as
vibrations of the coating machine including the mount of the probe head and flutters
of the moving film web. In order to simulate abrupt changes of the thickness during
a real coating process, the nip between the applicator rolls was stepwise reduced
and increased. An example for clear coats is shown in Fig. 21.2 [17]. Similar investigations were also carried out for pigmented coatings [18]. It was demonstrated
that changes in layer thickness lower than 1 μm can be clearly detected by NIR
spectroscopy.
The thickness of clear and pigmented UV-cured coatings was also monitored at
different line speeds up to 100 m/min [16–18]. A marked increase of the thickness
with increasing speed was observed up to about 50 m/min, which was followed by a
decrease at even higher web speeds. Although this behavior seems to be somewhat
surprising, it is well-known in coating technology [19, 20]. In fact, it is due to a very
complex interaction of several physical and technological parameters of the coating
process. In particular, acrylate formulations are typical non-Newtonian fluids, i.e.,
their viscosity depends on the shear rate. In-line NIR spectroscopy offered for the
first time the unique possibility to monitor the thickness changes upon variation of
the line speed directly during the coating process in a quantitative manner.