Biennial Report 2016/2017

Selected Results
Near-Infrared Chemical Imaging of Functional Finishes on Textiles
O. Daikos, G. Mirschel, T. Scherzer
The permanently increasing demands on product
quality and homogeneity pose a continuous
challenge to measuring and process control
technologies that are indispensable to meet these
requirements. In case of the production or processing
of web-like or planar materials such as
paper, polymer films, textiles, plates, sheets etc.,
the homogeneity of the material and the spatial
distribution of quantitative values of parameters
resulting from finishing steps such as coating,
impregnation, varnishing, lamination, etc. become
more and more important. The advent of large
hyperspectral cameras working in the NIR range
broadened the potential of analytical methods for
process control considerably. In the framework of
this research area, new approaches for
monitoring of technical processes are developed
[1]. This contribution is focused on applications of
NIR chemical imaging in textile surface finishing
technology.
Agents applied as finishes provide the textile
fabric with special functional features. For
example, such formulations include stiffening
agents, optical brighteners, flame retardants,
hydrophilic or hydrophobic agents, anti-static, and
anti-microbial finishes etc. Depending on the
specific substrate, agent, and intended application
they may be applied with application weights in
the range from less than 1 g/m² to several tens of
g/m². The rather low application weight of some
finishes makes very high demands on the
sensitivity of the detection method.
Figure 1: Chemical image of a polyester fabric provided with
different amounts of a flame retardant. Gravimetric
application weights: 15.5 g/m² (left) and 26 g/m² (right).
Figure 2: Chemical image of the distribution of a PVAc stiffening
agent on cotton fabric (gravimetric application weight:
12.4 g/m²).
Generally, all NIR methods require previous calibration
with reference data by means of powerful
chemometric techniques such as the partial least
squares (PLS) algorithm for quantitative
evaluation of the spectral data. The application of
such approaches finally results in quantitative distribution
patterns of the parameter of interest
such as application weight, thickness, moisture
content etc. in the context of this project. It has
been shown that the application weight of finishes
can be determined with typical prediction errors
(RMSEP) in the order of 1 to 2 g/m² under in-line
conditions, whereas more sophisticated
calibration models developed for very thin layers
of sizes allow the recording of chemical images
with RMSEP values lower than 0.5 g/m².
Figures 1 and 2 show some examples of chemical
images of finished textiles. Both the flame
retardant and the stiffening agent show significant
inhomogeneities of the application weight
resulting from the specific preparation and drying
processes. Nevertheless, the textiles appear
completely homogeneous in the visible range of
the spectrum since both finishing agents form
colourless layers.
The precision of the predictions was crosschecked
by integration of the individual data
across the surface of each finished fabric (~10 4
…10 5 ) and comparison of the resulting average
value with gravimetric reference data. Typically,
the deviation correlated quite well with the
prediction error of the calibration model (i.e. 1…2
g/m²).
Further investigations by hyperspectral imaging
were directed towards the in-line analysis of UVcured
acrylate coatings (degree of cure,
thickness), thin printed layers (homogeneity), release
coatings (detection of surface
discontinuities), adhesive layers inside textile
laminates (lamination defects) etc.
Literature
[1] G. Mirschel, O. Daikos, T. Scherzer, C. Steckert, Anal.
Chim. Acta 932 (2016) 69-79 (doi 10.1016/j.aca.2016.
05.015).
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