21 Applications of NIR Techniques in Polymer Coatings …
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are accessible to this technique. However, there is one significant potential obstacle
of NIR-based techniques with respect to the characterization of coatings: the extinction coefficients in the near-infrared are much lower than in the mid-infrared range.
Whereas this characteristic is useful in most other application areas of NIR spectroscopy in process control, where samples are typically rather compact objects with
thicknesses of several millimeters or even more, it is rather unfavorable in case of the
investigation of thin layers with a thickness in the range of some micrometers only.
This constellation makes very high demands on the sensitivity of the spectroscopic
equipment used for in-line monitoring as well as to the efficiency of the chemometric
methods applied for quantitative analysis of the data.
Possibly, it is attributed to these experimental challenges that only very few studies
dealing with the spectroscopic monitoring of coating processes can be found in the
scientific literature. Most papers on the analytics of “coatings” by NIR spectroscopy
are related to coatings on tablets or microspheres in pharmaceutics, which will not
be discussed in this chapter. Studies on NIR-based monitoring of coating and curing
processes with respect to the application of technical coatings are primarily focused
on UV-cured coatings and printed layers. Therefore, this paragraph will mainly give
an outline about the characterization of such coatings and layers.
21.2.1.2 Coatings Made by UV Photopolymerization
UV-curable coatings are advantageous with respect to saving of energy, environmental protection and waste reduction since UV curing consumes less energy than
thermal drying or curing. Furthermore, solvent-free varnishes are widely used in UV
curing technology, which may reduce environmental pollution. Due to the almost
instantaneous cross-linking of the coating during the short irradiation with UV light
(typically some tens of milliseconds), UV curing is a highly efficient coating technology. Moreover, the wide variety of acrylic and other monomers grants access to
coatings with a broad spectrum of functional properties.
The most important parameter of UV-cured coatings is the conversion, which
determines mechanical properties such as abrasion and scratch resistance or hardness, but also the content of extractables as well as their migration, chemical stability,
weathering resistance, etc. Furthermore, sufficient conversion is also required for
further processing of the coatings. For instance, wipe resistance must be achieved
before stacking or winding. The conversion depends on a large number of chemical and technical parameters. Apart from the chemistry of the specific monomers
(radical or cationic polymerization), the most important factor is the applied irradiation dose, which is given by the line speed and the irradiance of the incident UV light.
Variations in the composition of the reactive formulation or the ambient conditions
(e.g., temperature, humidity, inertization) may affect the conversion as well. Unfortunately, only some of these parameters can be easily controlled under the conditions
of technical UV curing, whereas it is too complex for other influences. Consequently,
monitoring of UV-induced photopolymerization reactions is an important issue.
479
are accessible to this technique. However, there is one significant potential obstacle
of NIR-based techniques with respect to the characterization of coatings: the extinction coefficients in the near-infrared are much lower than in the mid-infrared range.
Whereas this characteristic is useful in most other application areas of NIR spectroscopy in process control, where samples are typically rather compact objects with
thicknesses of several millimeters or even more, it is rather unfavorable in case of the
investigation of thin layers with a thickness in the range of some micrometers only.
This constellation makes very high demands on the sensitivity of the spectroscopic
equipment used for in-line monitoring as well as to the efficiency of the chemometric
methods applied for quantitative analysis of the data.
Possibly, it is attributed to these experimental challenges that only very few studies
dealing with the spectroscopic monitoring of coating processes can be found in the
scientific literature. Most papers on the analytics of “coatings” by NIR spectroscopy
are related to coatings on tablets or microspheres in pharmaceutics, which will not
be discussed in this chapter. Studies on NIR-based monitoring of coating and curing
processes with respect to the application of technical coatings are primarily focused
on UV-cured coatings and printed layers. Therefore, this paragraph will mainly give
an outline about the characterization of such coatings and layers.
21.2.1.2 Coatings Made by UV Photopolymerization
UV-curable coatings are advantageous with respect to saving of energy, environmental protection and waste reduction since UV curing consumes less energy than
thermal drying or curing. Furthermore, solvent-free varnishes are widely used in UV
curing technology, which may reduce environmental pollution. Due to the almost
instantaneous cross-linking of the coating during the short irradiation with UV light
(typically some tens of milliseconds), UV curing is a highly efficient coating technology. Moreover, the wide variety of acrylic and other monomers grants access to
coatings with a broad spectrum of functional properties.
The most important parameter of UV-cured coatings is the conversion, which
determines mechanical properties such as abrasion and scratch resistance or hardness, but also the content of extractables as well as their migration, chemical stability,
weathering resistance, etc. Furthermore, sufficient conversion is also required for
further processing of the coatings. For instance, wipe resistance must be achieved
before stacking or winding. The conversion depends on a large number of chemical and technical parameters. Apart from the chemistry of the specific monomers
(radical or cationic polymerization), the most important factor is the applied irradiation dose, which is given by the line speed and the irradiance of the incident UV light.
Variations in the composition of the reactive formulation or the ambient conditions
(e.g., temperature, humidity, inertization) may affect the conversion as well. Unfortunately, only some of these parameters can be easily controlled under the conditions
of technical UV curing, whereas it is too complex for other influences. Consequently,
monitoring of UV-induced photopolymerization reactions is an important issue.