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T. Scherzer
21.2 Polymer Coatings and Printed Layers
21.2.1 Specific Challenges of the Analysis of Coatings
and Other Thin Layers by NIR Spectroscopy
21.2.1.1 Spectroscopic Characterization of Polymer Coatings
Polymer coatings are present everywhere in our daily life. They are applied to
numerous materials for an extremely broad spectrum of functions ranging from
decorative purposes only via protection against various external influences up to
highly specialized coatings that provide the material with certain functional properties. Coatings can be made from a multitude of polymers, and also the methods of their
application show a wide variety. Except for spray coating with solvent-containing
varnishes or polymer dispersions one of the most widespread methods for the preparation of coatings is the application of resinous materials such as viscous monomer
or oligomer formulations, which may be cross-linked by thermosetting (e.g., epoxy
resins), UV photopolymerization (e.g., acrylates/methacrylates) or as reactive twocomponent system (e.g., PUR varnishes, polyester resins). Such coatings resulting
from cross-linking reactions are characterized by high durability, excellent resistance
to various impacts (mechanical, chemical, moisture, weathering, etc.), good adhesion
and many other beneficial properties. However, it is obvious that the properties of
such systems strongly depend on the degree of cross-linking that is achieved during
curing. In particular, this applies for many protective and functional properties. If
such coatings are applied in continuous processes such as roll coating, insufficient
cross-linking may have fatal consequences because the high production speed will
rapidly lead to large amounts of rejects. Consequently, monitoring the application
process and the actual state of the applied coating would be highly useful for an
efficient process control because it enables rapid intervention in case of serious
deviations from the specification, e.g., the degree of cure.
Surprisingly, only very few attempts have been made so far to control technical
coating and curing processes by analytical methods. Undoubtedly, this is related
to the high experimental requirements. Appropriate measuring methods must have
very high sensitivity since the conversion has to be determined in thin layers with
a thickness in the range of some microns only. They must be able to record data at
high sampling rates because of the usually high web speeds in coating technology.
Moreover, high reproducibility and reliability of the data as well as robustness of
the instrumentation to withstand the conditions in technical environments (dust,
variations in temperature, vibrations, etc.) are required. The experimental method
must not damage the material, hence measurement in a non-contact mode is desired.
The most promising analytical method for in-line monitoring of curing reactions in coating technology is NIR spectroscopy. It is non-destructive and possesses
sufficient time resolution and sensitivity as well as comprehensive analytical potential for quantitative monitoring of chemical reactions. Measurements can be easily
carried out in reflectance mode, which strongly expands the range of samples that
T. Scherzer
21.2 Polymer Coatings and Printed Layers
21.2.1 Specific Challenges of the Analysis of Coatings
and Other Thin Layers by NIR Spectroscopy
21.2.1.1 Spectroscopic Characterization of Polymer Coatings
Polymer coatings are present everywhere in our daily life. They are applied to
numerous materials for an extremely broad spectrum of functions ranging from
decorative purposes only via protection against various external influences up to
highly specialized coatings that provide the material with certain functional properties. Coatings can be made from a multitude of polymers, and also the methods of their
application show a wide variety. Except for spray coating with solvent-containing
varnishes or polymer dispersions one of the most widespread methods for the preparation of coatings is the application of resinous materials such as viscous monomer
or oligomer formulations, which may be cross-linked by thermosetting (e.g., epoxy
resins), UV photopolymerization (e.g., acrylates/methacrylates) or as reactive twocomponent system (e.g., PUR varnishes, polyester resins). Such coatings resulting
from cross-linking reactions are characterized by high durability, excellent resistance
to various impacts (mechanical, chemical, moisture, weathering, etc.), good adhesion
and many other beneficial properties. However, it is obvious that the properties of
such systems strongly depend on the degree of cross-linking that is achieved during
curing. In particular, this applies for many protective and functional properties. If
such coatings are applied in continuous processes such as roll coating, insufficient
cross-linking may have fatal consequences because the high production speed will
rapidly lead to large amounts of rejects. Consequently, monitoring the application
process and the actual state of the applied coating would be highly useful for an
efficient process control because it enables rapid intervention in case of serious
deviations from the specification, e.g., the degree of cure.
Surprisingly, only very few attempts have been made so far to control technical
coating and curing processes by analytical methods. Undoubtedly, this is related
to the high experimental requirements. Appropriate measuring methods must have
very high sensitivity since the conversion has to be determined in thin layers with
a thickness in the range of some microns only. They must be able to record data at
high sampling rates because of the usually high web speeds in coating technology.
Moreover, high reproducibility and reliability of the data as well as robustness of
the instrumentation to withstand the conditions in technical environments (dust,
variations in temperature, vibrations, etc.) are required. The experimental method
must not damage the material, hence measurement in a non-contact mode is desired.
The most promising analytical method for in-line monitoring of curing reactions in coating technology is NIR spectroscopy. It is non-destructive and possesses
sufficient time resolution and sensitivity as well as comprehensive analytical potential for quantitative monitoring of chemical reactions. Measurements can be easily
carried out in reflectance mode, which strongly expands the range of samples that