480
T. Scherzer
In stationary applications of NIR spectroscopy, the conversion may be obtained
from the ratio of a specific band of the corresponding functional group before and after
irradiation. In case of acrylates, methacrylates, vinyl ethers, etc. the band of the first
overtone of the C-H stretching vibration of the carbon double bond at 1620 nm can
be used for quantification. This band is sufficiently separated from the corresponding
absorptions of other C-H bonds (~1670–1750 nm), which usually allows integration
without complex pretreatments such as band separation. In contrast, no specific band
is available for the epoxide groups in cylcoaliphatic epoxy resins [10]. However,
this approach comes to its limits in most in-line process control applications. For
example, variations of the thickness during the process due to changes of the line
speed [11] or the inherent polymerization shrinkage (e.g., of acrylic coatings [12])
distort the band ratio and prevent a correct determination of the conversion. Moreover,
the precision of the integration method decreases at very high conversions. Therefore,
almost all applications of NIR spectroscopy in process control are based on efficient
chemometric methods.
The preparation of photopolymerized samples with a predefined conversion,
which are required for calibration, is hardly possible. Therefore, the conversion
resulting from the application of different UV doses and/or the use of different
amounts of photoinitiator in the samples has to be characterized independently.
Usually, the preferred reference method for the conversion is FTIR spectroscopy
in the mid-infrared range. In case of (meth)acrylates, quantification is obtained by
band integration of the peak of the = CH 2 twisting vibration of the acrylic bond at
810 cm
−1 . However, infrared spectra obtained by FTIR-ATR spectroscopy reflect
the chemical state close to the surface only due to the limited penetration depth of
IR radiation into the layer, which is typically a few microns if a diamond is used
as ATR crystal (~ 1-3 μm) [13]. However, many coatings in UV curing technology
are much thicker (tens of μm). Moreover, UV-cured coatings often show a strong
gradient of the conversion with increasing depth due to the limited penetration of UV
light. Thus, ATR spectra do not provide precise information about the conversion
across the profile of a coating.
In contrast, it is well known that NIR radiation penetrates to much greater depths
of organic coatings as a consequence of the lower molar absorptivity in this spectral region, in particular in transparent systems. Thus, NIR spectra give an average
response of the whole depth profile of the coating. Therefore, conversion data
obtained from the ratio of band integrals before and after irradiation represent an
average value of the conversion within the coating. Consequently, they are better
suited for the calibration of chemometric models [14].
21.2.1.3 Interference Suppression in the Spectra of Thin Polymer Films
In order to be able to monitor the properties of coatings on both transparent and
intransparent substrates with the same equipment, NIR spectra are taken almost
exclusively in reflection in process control. Due to the low reflectance of transparent
substrates such as thin polymer films, the spectra of coatings on those materials are
T. Scherzer
In stationary applications of NIR spectroscopy, the conversion may be obtained
from the ratio of a specific band of the corresponding functional group before and after
irradiation. In case of acrylates, methacrylates, vinyl ethers, etc. the band of the first
overtone of the C-H stretching vibration of the carbon double bond at 1620 nm can
be used for quantification. This band is sufficiently separated from the corresponding
absorptions of other C-H bonds (~1670–1750 nm), which usually allows integration
without complex pretreatments such as band separation. In contrast, no specific band
is available for the epoxide groups in cylcoaliphatic epoxy resins [10]. However,
this approach comes to its limits in most in-line process control applications. For
example, variations of the thickness during the process due to changes of the line
speed [11] or the inherent polymerization shrinkage (e.g., of acrylic coatings [12])
distort the band ratio and prevent a correct determination of the conversion. Moreover,
the precision of the integration method decreases at very high conversions. Therefore,
almost all applications of NIR spectroscopy in process control are based on efficient
chemometric methods.
The preparation of photopolymerized samples with a predefined conversion,
which are required for calibration, is hardly possible. Therefore, the conversion
resulting from the application of different UV doses and/or the use of different
amounts of photoinitiator in the samples has to be characterized independently.
Usually, the preferred reference method for the conversion is FTIR spectroscopy
in the mid-infrared range. In case of (meth)acrylates, quantification is obtained by
band integration of the peak of the = CH 2 twisting vibration of the acrylic bond at
810 cm
−1 . However, infrared spectra obtained by FTIR-ATR spectroscopy reflect
the chemical state close to the surface only due to the limited penetration depth of
IR radiation into the layer, which is typically a few microns if a diamond is used
as ATR crystal (~ 1-3 μm) [13]. However, many coatings in UV curing technology
are much thicker (tens of μm). Moreover, UV-cured coatings often show a strong
gradient of the conversion with increasing depth due to the limited penetration of UV
light. Thus, ATR spectra do not provide precise information about the conversion
across the profile of a coating.
In contrast, it is well known that NIR radiation penetrates to much greater depths
of organic coatings as a consequence of the lower molar absorptivity in this spectral region, in particular in transparent systems. Thus, NIR spectra give an average
response of the whole depth profile of the coating. Therefore, conversion data
obtained from the ratio of band integrals before and after irradiation represent an
average value of the conversion within the coating. Consequently, they are better
suited for the calibration of chemometric models [14].
21.2.1.3 Interference Suppression in the Spectra of Thin Polymer Films
In order to be able to monitor the properties of coatings on both transparent and
intransparent substrates with the same equipment, NIR spectra are taken almost
exclusively in reflection in process control. Due to the low reflectance of transparent
substrates such as thin polymer films, the spectra of coatings on those materials are