21 Applications of NIR Techniques in Polymer Coatings …
485
controlling the application process and the quality of the applied layers, they developed a method based on FT-NIR spectroscopy. Several parameters such as coating
weight, gloss and smoothness were predicted from appropriate PLS models. In case
of the coating weight, reference data were obtained by gravimetry before and after
dissolution of the layers. RMSEP was found to be 0.054 g/m
2 with a coefficient of
determination (R
2 ) of 0.98. Convincing results were also obtained for the smoothness, whereas the method was of limited suitability only for the determination of the
gloss. It was demonstrated that all three parameters could be also predicted in-line
during the coatings process.
21.2.3 Conversion of UV-Cured Coatings
21.2.3.1 Acrylic Coatings
The conversion that is achieved during UV irradiation is the most important parameter of any UV-cured coating. As outlined above, it determines all functional and
handling properties of the coatings, which are relevant for their application or further
processing. Consequently, continuous monitoring of the conversion during a coating
process would be highly desirable. However, until recently no efficient analytical
method was available, which allowed direct in-line monitoring of this important
parameter. At-line or off-line determination of the conversion is state of the art,
which is often carried out by FTIR spectroscopy. But IR spectroscopy in the midinfrared (MIR) region is not suited for in-line control, since non-contact reflection
measurements over a rather large measuring distance are difficult or even impossible.
In contrast, vibrational spectroscopy in the near-infrared range offers this possibility, which provides an opportunity for monitoring the conversion during the coating
process. The overtone band of the C-H stretching vibration of the carbon double bond
at 1620 nm is well separated from the corresponding bands of other C-H structures
(methyl, methylene etc.). Hence, it might be used directly for the determination of
the conversion in acrylates and methacrylates using band integration. However, this
requires recording of some spectra of the uncured coating before irradiation, which
is often difficult to realize in complex coating processes. Moreover, this approach
involves considerable dangers resulting from unintended but inevitable changes of the
thickness of the coating as discussed in paragraph 21.2.1.2. For this reason, chemometric modeling is the more time-consuming, but also the more reliable approach.
Depending on the specific varnish (e.g., clear or pigmented), its thickness and other
parameters, MIR (in transmission or using ATR) [15, 24] or NIR spectroscopy (band
integration) [14, 21], Raman spectroscopy or other analytical methods [15] can be
used as reference method for calibration. Even secondary parameters that depend
directly on the conversion (e.g., the hardness of the coating) may be used as reference data for calibration models. A typical PLS-based calibration model for the
conversion in an acrylic clear coat is shown in Fig. 21.4.
485
controlling the application process and the quality of the applied layers, they developed a method based on FT-NIR spectroscopy. Several parameters such as coating
weight, gloss and smoothness were predicted from appropriate PLS models. In case
of the coating weight, reference data were obtained by gravimetry before and after
dissolution of the layers. RMSEP was found to be 0.054 g/m
2 with a coefficient of
determination (R
2 ) of 0.98. Convincing results were also obtained for the smoothness, whereas the method was of limited suitability only for the determination of the
gloss. It was demonstrated that all three parameters could be also predicted in-line
during the coatings process.
21.2.3 Conversion of UV-Cured Coatings
21.2.3.1 Acrylic Coatings
The conversion that is achieved during UV irradiation is the most important parameter of any UV-cured coating. As outlined above, it determines all functional and
handling properties of the coatings, which are relevant for their application or further
processing. Consequently, continuous monitoring of the conversion during a coating
process would be highly desirable. However, until recently no efficient analytical
method was available, which allowed direct in-line monitoring of this important
parameter. At-line or off-line determination of the conversion is state of the art,
which is often carried out by FTIR spectroscopy. But IR spectroscopy in the midinfrared (MIR) region is not suited for in-line control, since non-contact reflection
measurements over a rather large measuring distance are difficult or even impossible.
In contrast, vibrational spectroscopy in the near-infrared range offers this possibility, which provides an opportunity for monitoring the conversion during the coating
process. The overtone band of the C-H stretching vibration of the carbon double bond
at 1620 nm is well separated from the corresponding bands of other C-H structures
(methyl, methylene etc.). Hence, it might be used directly for the determination of
the conversion in acrylates and methacrylates using band integration. However, this
requires recording of some spectra of the uncured coating before irradiation, which
is often difficult to realize in complex coating processes. Moreover, this approach
involves considerable dangers resulting from unintended but inevitable changes of the
thickness of the coating as discussed in paragraph 21.2.1.2. For this reason, chemometric modeling is the more time-consuming, but also the more reliable approach.
Depending on the specific varnish (e.g., clear or pigmented), its thickness and other
parameters, MIR (in transmission or using ATR) [15, 24] or NIR spectroscopy (band
integration) [14, 21], Raman spectroscopy or other analytical methods [15] can be
used as reference method for calibration. Even secondary parameters that depend
directly on the conversion (e.g., the hardness of the coating) may be used as reference data for calibration models. A typical PLS-based calibration model for the
conversion in an acrylic clear coat is shown in Fig. 21.4.