21 Applications of NIR Techniques in Polymer Coatings …
493
PUR foam is widely used for cushioning and sound insulation. For faster and easier
mounting in end-use applications such as automotive engineering, it may be provided
with an adhesive layer forming a semifinished product. Due to the mostly coarse
surface structure of such foam plastics, the adhesive cannot be applied as melted mass.
Rather, melt-spun fibrous webs of thermoplastic adhesives such aliphatic polyamide
or polyester are laminated to the surface of the foam mat. Higher thicknesses may be
achieved by application of several adhesive webs. In this way, applications weights of
the adhesive between 20 and 125 g/m
2 were achieved. The development of calibration
models for the prediction of the coating weight is significantly impeded by the fact
that calibration samples with well-defined homogeneous thickness can be hardly
prepared due to the numerous open bubbles at the foam surface as well as due to
the fibrous structure of the adhesive webs. Moreover, some of the PUR substrates
are rather dark (e.g., dark gray), which leads to rather low reflectance. In order to
consider a possible non-uniform distribution of the coating weight across the samples,
the surface of each sample was divided into 20 rectangular regions by defining a grid
of 5 columns × 4 rows. Spectra from each rectangle (several thousands in each) were
averaged. The averaged spectra were allocated alternatingly, that is, according to a
chessboard pattern, to the calibration and the validation set. This procedure resulted
in RMSEP values of only 1.5–3.5 g/m
2 despite the heterogeneity of the adhesive web
and the PUR foam substrates [29, 30].
Using these calibration models, the application weight of adhesive layers was
quantitatively monitored with a hyperspectral camera mounted above a conveyor.
At low coating weights, the adhesive layers shows significant local thickness variation (i.e., for samples with up to two adhesive webs), whereas it becomes more
homogeneous at higher coating weights [29]. This effect mainly results from surface
structure of the PUR foam. During the melting process of the web, the adhesive
might partly flow into the open bubbles at the surface of the foam leading to an
uneven distribution. For an overall evaluation of the quantitative prediction results,
the individual values predicted from the spectra across the complete surface of each
sample were averaged and compared to gravimetric values. Deviations were found
to be less than 2–3 g/m
2 (the fact that some values are lower than RMSEP is related
to the averaging process).
Spectral imaging was also used for the detection of inhomogeneities and coating
errors, which may occur during the lamination process. During melting, the adhesives
form continuous glossy transparent layers on the foam substrates, which usually
prevents the detection of coating errors by visual inspection. In contrast, spectral
imaging is able to highlight them. As an example, Fig. 21.10 shows the image of a
coated foam sample, where the corner of one adhesive web was fold over during the
lamination process [30].
Thickness and conversion of UV-cured acrylate coatings were studied by hyperspectral imaging as well. In particular, white-pigmented coatings were investigated.
Calibration to the thickness was carried out for coatings with thicknesses up to
200 μm, although linearity between thickness and NIR signal was even found for
much thicker coatings [21]. Calibrations to the conversion resulted in RMSEP values
between 2 and 3% [14, 21]. However, specific calibration models were required for
493
PUR foam is widely used for cushioning and sound insulation. For faster and easier
mounting in end-use applications such as automotive engineering, it may be provided
with an adhesive layer forming a semifinished product. Due to the mostly coarse
surface structure of such foam plastics, the adhesive cannot be applied as melted mass.
Rather, melt-spun fibrous webs of thermoplastic adhesives such aliphatic polyamide
or polyester are laminated to the surface of the foam mat. Higher thicknesses may be
achieved by application of several adhesive webs. In this way, applications weights of
the adhesive between 20 and 125 g/m
2 were achieved. The development of calibration
models for the prediction of the coating weight is significantly impeded by the fact
that calibration samples with well-defined homogeneous thickness can be hardly
prepared due to the numerous open bubbles at the foam surface as well as due to
the fibrous structure of the adhesive webs. Moreover, some of the PUR substrates
are rather dark (e.g., dark gray), which leads to rather low reflectance. In order to
consider a possible non-uniform distribution of the coating weight across the samples,
the surface of each sample was divided into 20 rectangular regions by defining a grid
of 5 columns × 4 rows. Spectra from each rectangle (several thousands in each) were
averaged. The averaged spectra were allocated alternatingly, that is, according to a
chessboard pattern, to the calibration and the validation set. This procedure resulted
in RMSEP values of only 1.5–3.5 g/m
2 despite the heterogeneity of the adhesive web
and the PUR foam substrates [29, 30].
Using these calibration models, the application weight of adhesive layers was
quantitatively monitored with a hyperspectral camera mounted above a conveyor.
At low coating weights, the adhesive layers shows significant local thickness variation (i.e., for samples with up to two adhesive webs), whereas it becomes more
homogeneous at higher coating weights [29]. This effect mainly results from surface
structure of the PUR foam. During the melting process of the web, the adhesive
might partly flow into the open bubbles at the surface of the foam leading to an
uneven distribution. For an overall evaluation of the quantitative prediction results,
the individual values predicted from the spectra across the complete surface of each
sample were averaged and compared to gravimetric values. Deviations were found
to be less than 2–3 g/m
2 (the fact that some values are lower than RMSEP is related
to the averaging process).
Spectral imaging was also used for the detection of inhomogeneities and coating
errors, which may occur during the lamination process. During melting, the adhesives
form continuous glossy transparent layers on the foam substrates, which usually
prevents the detection of coating errors by visual inspection. In contrast, spectral
imaging is able to highlight them. As an example, Fig. 21.10 shows the image of a
coated foam sample, where the corner of one adhesive web was fold over during the
lamination process [30].
Thickness and conversion of UV-cured acrylate coatings were studied by hyperspectral imaging as well. In particular, white-pigmented coatings were investigated.
Calibration to the thickness was carried out for coatings with thicknesses up to
200 μm, although linearity between thickness and NIR signal was even found for
much thicker coatings [21]. Calibrations to the conversion resulted in RMSEP values
between 2 and 3% [14, 21]. However, specific calibration models were required for