510
T. Scherzer
21.3.4 Lamination of Textiles
In several technical applications, textiles are used as laminates in order to combine
the properties of different materials. For example, combinations of a fabric as top
layer for the optical appearance and a hidden nonwoven material for sound insulation are widely used for interior design in automotive engineering (e.g. for door
and roof linings or rear panel shelfs) and similar applications. Both textile layers
are bonded to each other by laminating hot-melt adhesives. Such adhesives can be
applied as powder, hot melted mass or melt-spun fibrous webs. During application
of the adhesive and the subsequent lamination of the two webs, several deficiencies
may occur depending on the specific adhesive system such as over/underdosage or
uncoated areas for powders and melts as well as tears, holes or buckle formation in
case of adhesives webs. Generally, lamination defects lead to deficient bond strength
or even delamination, impairment of the visual appearance etc. Therefore, continuous monitoring of the homogeneity of the inside adhesive layer would be essential
for an efficient control of the lamination process. However, visual inspection of the
hidden layers or use of a corresponding camera is prevented by their invisibility and
inaccessibility from outside.
Near-infrared radiation is well-known to penetrate to a certain degree into polymer
materials, at least for some tens of micrometers depending on the specific material, its
color and morphology. Materials for interior design in automotive engineering often
contain a top layer made up of polyester piqué fabrics with weights between about
100 and 200 g/m
2 . Colors may cover a broad range, but the majority of materials is
beige, gray or black. Due to their weave structure, such fabrics appear to be slightly
translucent. On the other hand, the fibrous structure of the materials may lead to
diffuse scattering of incident radiation. Therefore, investigations were accomplished
in order to clear up if adhesive layers hidden in textile laminates can be visualized and
quantitatively analyzed by hyperspectral imaging. Generally, spectral analysis was
carried out from the top side, since the nonwoven is usually thicker than the fabric and
shows much stronger scattering of the probe light. In fact, it was demonstrated that
the reflection spectra of such three-layer laminates show significant contributions of
the adhesive layers based on aliphatic polyester [30] (see Fig. 21.21). In particular,
a specific absorption was observed around 1410 nm, which is attributed to a C-H
combination band in aliphatic hydrocarbons (2νCH 2 and δCH 2 ) [51], as well as in
the range above 1700 nm. Similar results were obtained for laminates containing
PA-based adhesives webs. Even reflection spectra of corresponding black textile
laminates can be recorded in high quality and with explicit effect of the coating
thickness of the adhesive layers [52], although the optical conditions are far from
being ideal for this kind of measurements.
Reflection spectra were found to correlate with the application weight of the hot
melt adhesive. A principal component analysis (PCA) of the NIR spectra of the laminates showed that the scores plot of the first two components (PC1 versus PC2) only
allowed a discrimination between samples with and without adhesive layer, whereas
T. Scherzer
21.3.4 Lamination of Textiles
In several technical applications, textiles are used as laminates in order to combine
the properties of different materials. For example, combinations of a fabric as top
layer for the optical appearance and a hidden nonwoven material for sound insulation are widely used for interior design in automotive engineering (e.g. for door
and roof linings or rear panel shelfs) and similar applications. Both textile layers
are bonded to each other by laminating hot-melt adhesives. Such adhesives can be
applied as powder, hot melted mass or melt-spun fibrous webs. During application
of the adhesive and the subsequent lamination of the two webs, several deficiencies
may occur depending on the specific adhesive system such as over/underdosage or
uncoated areas for powders and melts as well as tears, holes or buckle formation in
case of adhesives webs. Generally, lamination defects lead to deficient bond strength
or even delamination, impairment of the visual appearance etc. Therefore, continuous monitoring of the homogeneity of the inside adhesive layer would be essential
for an efficient control of the lamination process. However, visual inspection of the
hidden layers or use of a corresponding camera is prevented by their invisibility and
inaccessibility from outside.
Near-infrared radiation is well-known to penetrate to a certain degree into polymer
materials, at least for some tens of micrometers depending on the specific material, its
color and morphology. Materials for interior design in automotive engineering often
contain a top layer made up of polyester piqué fabrics with weights between about
100 and 200 g/m
2 . Colors may cover a broad range, but the majority of materials is
beige, gray or black. Due to their weave structure, such fabrics appear to be slightly
translucent. On the other hand, the fibrous structure of the materials may lead to
diffuse scattering of incident radiation. Therefore, investigations were accomplished
in order to clear up if adhesive layers hidden in textile laminates can be visualized and
quantitatively analyzed by hyperspectral imaging. Generally, spectral analysis was
carried out from the top side, since the nonwoven is usually thicker than the fabric and
shows much stronger scattering of the probe light. In fact, it was demonstrated that
the reflection spectra of such three-layer laminates show significant contributions of
the adhesive layers based on aliphatic polyester [30] (see Fig. 21.21). In particular,
a specific absorption was observed around 1410 nm, which is attributed to a C-H
combination band in aliphatic hydrocarbons (2νCH 2 and δCH 2 ) [51], as well as in
the range above 1700 nm. Similar results were obtained for laminates containing
PA-based adhesives webs. Even reflection spectra of corresponding black textile
laminates can be recorded in high quality and with explicit effect of the coating
thickness of the adhesive layers [52], although the optical conditions are far from
being ideal for this kind of measurements.
Reflection spectra were found to correlate with the application weight of the hot
melt adhesive. A principal component analysis (PCA) of the NIR spectra of the laminates showed that the scores plot of the first two components (PC1 versus PC2) only
allowed a discrimination between samples with and without adhesive layer, whereas