476
T. Scherzer
21.1 Introduction
Polymers have become one of the most important classes of materials today, which
is mainly due to the amazingly broad range of properties that may be achieved
with this kind of materials. Accordingly, there is almost no range in our daily life,
which goes without polymers. A multitude of different polymer materials has been
developed so far and is commercially available. The widespread use and commercial importance of polymers makes also great demands on the analytics of such
materials. With respect to the production of polymers, their converting, finishing,
recovery or disposal, powerful and versatile analytical methods are required for characterization, identification, process and quality control etc. In particular, techniques
used for sorting and control applications have to comply with special requirements
such as robustness, reliability, durability, safety, significance of the data etc. [1–3].
Furthermore, in most cases they should be able to record data in a non-contact mode.
Near-infrared spectroscopy satisfies these specifications to a great extend. Moreover, due to its specific measurement principle NIR spectroscopy has high sensitivity
to typical molecular structures in organic chemistry that are relevant for synthetic
polymers. Consequently, NIR spectroscopy is particularly well suited for applications
in polymer production and processing and related areas [4–9].
Apart from the underlying measurement principle, the great potential of NIR
spectroscopy for its use in science and technology is mainly related to
(i) The high transmission of quartz and several glasses for radiation in this spectral
range, which allows the use of conventional windows, lenses, prisms and last
but not least optical fibers in experimental setups, which provides easy access
to the region of interest,
(ii) The possibility to measure in reflection or transflection mode, which is
advantageous in case of intransparent materials and
(iii) The rather low extinction coefficients of most organic matter in the near-infrared
in comparison to other spectral ranges used in analytics such as mid-infrared
or UV, which allows transmission measurements on samples with rather high
thickness or optical path lengths in the range of several millimeters or even
more without the need of dilution, cutting, pressing or other special sample
preparation steps.
A quite recent application of NIR spectroscopy in polymer technology is the
in-line analysis of polymer coatings. In particular, the conversion in coatings of
radiation-curable monomers and oligomers such as acrylates, methacrylates, vinyl
esters, thiol-ene systems, cycloaliphatic epoxies and vinyl ethers that is achieved
after UV or electron beam (EB) irradiation is of great interest, since it strongly
determines both the application and processing properties of such coatings. However,
in-line analysis of rather thin coatings with thicknesses between a few up to several
hundreds of micrometers is a considerable challenge for several reasons such as
the required sensitivity and time resolution. Consequently, only very few attempts
have been made in the past to monitor technical coating and curing processes by
T. Scherzer
21.1 Introduction
Polymers have become one of the most important classes of materials today, which
is mainly due to the amazingly broad range of properties that may be achieved
with this kind of materials. Accordingly, there is almost no range in our daily life,
which goes without polymers. A multitude of different polymer materials has been
developed so far and is commercially available. The widespread use and commercial importance of polymers makes also great demands on the analytics of such
materials. With respect to the production of polymers, their converting, finishing,
recovery or disposal, powerful and versatile analytical methods are required for characterization, identification, process and quality control etc. In particular, techniques
used for sorting and control applications have to comply with special requirements
such as robustness, reliability, durability, safety, significance of the data etc. [1–3].
Furthermore, in most cases they should be able to record data in a non-contact mode.
Near-infrared spectroscopy satisfies these specifications to a great extend. Moreover, due to its specific measurement principle NIR spectroscopy has high sensitivity
to typical molecular structures in organic chemistry that are relevant for synthetic
polymers. Consequently, NIR spectroscopy is particularly well suited for applications
in polymer production and processing and related areas [4–9].
Apart from the underlying measurement principle, the great potential of NIR
spectroscopy for its use in science and technology is mainly related to
(i) The high transmission of quartz and several glasses for radiation in this spectral
range, which allows the use of conventional windows, lenses, prisms and last
but not least optical fibers in experimental setups, which provides easy access
to the region of interest,
(ii) The possibility to measure in reflection or transflection mode, which is
advantageous in case of intransparent materials and
(iii) The rather low extinction coefficients of most organic matter in the near-infrared
in comparison to other spectral ranges used in analytics such as mid-infrared
or UV, which allows transmission measurements on samples with rather high
thickness or optical path lengths in the range of several millimeters or even
more without the need of dilution, cutting, pressing or other special sample
preparation steps.
A quite recent application of NIR spectroscopy in polymer technology is the
in-line analysis of polymer coatings. In particular, the conversion in coatings of
radiation-curable monomers and oligomers such as acrylates, methacrylates, vinyl
esters, thiol-ene systems, cycloaliphatic epoxies and vinyl ethers that is achieved
after UV or electron beam (EB) irradiation is of great interest, since it strongly
determines both the application and processing properties of such coatings. However,
in-line analysis of rather thin coatings with thicknesses between a few up to several
hundreds of micrometers is a considerable challenge for several reasons such as
the required sensitivity and time resolution. Consequently, only very few attempts
have been made in the past to monitor technical coating and curing processes by