Katja Heymann

Simultaneous in-line monitoring of the conversion and the coating thickness in UV-cured acrylate coatings by near-infrared reflection spectroscopy.

Near-infrared (NIR) reflection spectroscopy was used for in-line analysis of the conversion and the coating thickness (5-20 μm) of UV-cured clear and pigmented acrylate coatings. The quantitative evaluation of the recorded spectra was carried out by partial least-squares (PLS) regression, in particular with the PLS2 algorithm, which allows simultaneous prediction of both parameters. The efficiency of this method was investigated in roll coating experiments at line speeds up to 100 m min(-1). It was shown that the method is able to compensate for the effect of accidental variations of the coating thickness, which inevitably occur upon changes of the line speed, on the prediction of the conversion. Accordingly, the conversion could be determined with a precision of ±2...3%, whereas the error in the measurement of the thickness was found to be about 0.5-1 μm.

Monitoring of the thickness of ultraviolet-cured pigmented coatings and printed layers by near-infrared spectroscopy.

Near-infrared (NIR) reflection spectroscopy was used for the determination of the thickness or the coating weight, respectively, of white-pigmented acrylic coatings and layers of printing inks. The thickness of coatings was studied in the range from 5 to 60 μm, whereas the coating weights of the printed layers covered a range between 1 to 5 g m−2. Quantitative analysis of the spectral data relied on partial least squares (PLS) regression. A thickness gauge or gravimetry, respectively, were used to obtain reference data. Calibration models were typically based on six factors. The corresponding root mean square errors of prediction (RMSEP) were found to be on the order of 0. 87 for coatings and 0. 38 for printed layers. Monitoring of the coating thickness under process conditions was carried out on a pilot-scale roll coating machine. In order to simulate thickness changes during a coating process, either the nip between the applicator rolls or the web speed was varied. Data with high precision (standard deviation ∼1 μm for coatings, ∼0.4 g m−2 for printed layers) and an excellent correlation with off-line reference data were obtained. The investigations have shown that NIR spectroscopy can be used for process control in coating and curing technology.

In-line monitoring of the thickness of printed layers by near-infrared (NIR) spectroscopy at a printing press.

In this work, it is demonstrated that the coating weight of printed layers can be determined in-line in a running printing press by near-infrared (NIR) reflection spectroscopy assisted by chemometric methods. Three different unpigmented lacquer systems, i.e., a conventional oil-based printing lacquer, an ultraviolet (UV)-curable formulation, and a water-based dispersion varnish, were printed on paper with coating weights between about 0.5 and 7 g m−2. NIR spectra for calibration were recorded with a special metal reflector simulating the mounting conditions of the probe head at the printing press. Calibration models were developed on the basis of the partial least squares (PLS) algorithm and evaluated by independent test samples. The prediction performance of the developed models was examined at a sheet-fed offset printing press at line speeds between 90 and 180 m min−1. Results show an excellent correlation of data predicted in-line from the NIR spectra with reference values obtained off-line by gravimetry. The prediction errors were found to be ≤ 0.2 g m−2, which confirms the suitability of the developed spectroscopic method for process control in technical printing processes.

Determination of the Thickness of Silazane-Based SiOx Coatings in the Submicrometer Range by Near-Infrared Reflection Spectroscopy

The thickness of thin silica layers in the submicrometer range, i.e., between about 150 and 700 nm, was determined by near-infrared (NIR) reflection spectroscopy. Silica layers were prepared by spin-coating of perhydropolysilazane (PHPS) on silicon wafers or poly(ethylene terephthalate) (PET) foil and subsequent conversion of the PHPS into SiOx by vacuum ultraviolet (VUV) irradiation at 172 nm. Since the NIR spectra of the inorganic layers do not show overtone and combination bands, analysis is based on tiny differences in reflectance of samples provided with layers of different thicknesses. Quantitative investigations were carried out by use of chemometric approaches on the basis of the partial least squares (PLS) algorithm. Optimization of the chemometric models was achieved by systematic variation of the preprocessing of the spectra before application of the PLS regression. The root mean square error of prediction (RMSEP) and the coefficient of determination R2 were used for the evaluation of the various pretreatment strategies. Reference data for the calibration procedures were obtained by means of gravimetry. The maximum error for the determination of the thickness was estimated to be on the order of 20%. The method was used to monitor the homogeneity of the thickness of silica layers made by use of a pilot scale coating machine. Thickness profiles recorded by NIR spectroscopy showed clear differences between layers with uniform or non-uniform quality of the application. Moreover, a close correlation of the profiles with the average coating weights determined by gravimetry was found.

Process control in ultraviolet curing with in-line near infrared reflection spectroscopy

Near infrared reflection spectroscopy was used for in-line monitoring of the conversion and the coating thickness of thin UV-cured coatings which have a typical thickness in the range of some micrometers only. Quantitative analysis of the spectral data was carried out by PLS-based multivariate calibration methods. In addition, univariate procedures were tested for some applications. The conversion, in particular its dependence on the applied irradiation dose, was followed in both acrylic coatings, which were cross-linked by free radical polymerisation and in epoxy / vinyl ether blends, which were cured according to a cationic reaction mechanism. The thickness of acrylate coatings was studied in a range between 5 μm and 100 μm. It was shown that quantitative data with high precision and time resolution can be also obtained in pilot scale investigations, even if the coating line is operated at high speed.

Peculiarities of the photoinitiator-free photopolymerization of pentabrominated and pentafluorinated aromatic acrylates and methacrylates

Pentabrominated and fluorinated aromatic (meth)acrylates as well as their non-halogenated counterparts have been studied with the aim to avoid conventional photoinitiators and to overcome some negative consequences related to their use. Therefore, RTIR spectroscopy, laser flash photolysis and GC/MS were utilized. Even low concentrations (1 to 5 wt%) of brominated (meth)acrylates in the model varnish lead to initiation of a photopolymerization reaction under exposure to UV light with λ > 300 nm. This is due to the fact that excitation of the aryl moiety leads to the homolysis of bromine-phenyl bonds with a high quantum yield of ∼0.15-0.3. Both, bromine radicals released from either ortho, meta or para position as well as the corresponding tetrabromoaryl radicals, may initiate the polymerization of brominated aromatic (meth)acrylates. In contrast, fluorinated aromatic (meth)acrylates undergo α-cleavage of the carboxyl group (as in the case of non-halogenated aromatic (meth)acrylates), if excitation of the acrylic double bonds is done with UV-C light (λ < 280 nm). Radical formation occurs with a comparable quantum yield of 0.1-0.22 (fluorinated) and 0.16-0.36 (non-halogenated compounds), despite the different pathway of fragmentation. Thus, in all cases the efficiency of initiation is comparable to conventional photoinitiators. Quantum chemical calculations of orbitals involved and of the Gibbs free energy of transients and products support the suggested reaction pathway.