Tom Scherzer

REAL-TIME FTIR-ATR SPECTROSCOPY TO STUDY THE KINETICS OF ULTRAFAST PHOTOPOLYMERIZATION REACTIONS INDUCED BY MONOCHROMATIC UV LIGHT

Abstract Real-Time FTIR–ATR spectroscopy was used to study the kinetics of photopolymerization reactions induced by monochromatic UV light. Various photoinitiator systems were tested for their efficiency to start the curing reaction of acrylates on irradiation at 313 or 222 nm. The effect of physical and chemical factors such as photoinitiator concentration, light intensity, temperature, monomer functionality and inertization on kinetic parameters like polymerization rate, induction period and final conversion was studied. The contribution of the postcuring to the final conversion was determined by following the decay of the double bonds during and after irradiation with single or multiple short UV flashes with a duration of 50–200 ms. Finally, some investigations of samples from practical applications such as UV-curable powder coatings, printing inks, and release coatings of silicone acrylates will be reported.

UV curing of pressure sensitive adhesives studied by real-time FTIR-ATR spectroscopy

Abstract The UV-induced crosslinking of a pressure sensitive adhesive based on an acrylic resin with copolymerized benzophenone groups was studied by real-time FTIR-ATR spectroscopy in order to simulate technical curing processes. The dependence of the conversion on the applied UV dose was investigated by irradiation with single or multiple UV pulses and with variable light intensities. Depth profiling of the degree of cure during the UV curing of thick adhesive coatings revealed a strong gradient of the conversion due to the inner filter effect of the photoreactive groups. Temperature was found to have a significant influence on cure rate and conversion. Results from FTIR spectroscopy were related to the adhesive properties of the cured coatings.

Depth Profiling of the Degree of Cure during the Photopolymerization of Acrylates Studied by Real-Time FT-IR Attenuated Total Reflection Spectroscopy

Real-time Fourier transform infrared attenuated total reflection (FT-IR/ATR) spectroscopy was used to study the depth profile of the conversion during photopolymerization reactions. The method is based on the UV irradiation of coatings of different thicknesses on the ATR crystal. Due to the finite depth of penetration of the infrared probe signal into the sample, conversion is analyzed only in a narrow layer at the bottom of the coating. The method was used to characterize the effect of photo-initiators on the depth of cure in acrylate coatings. The self-screening of a strongly absorbing morpholino ketone photo-initiator and the photobleaching of a bisacylphosphine oxide initiator are demonstrated. Moreover, depth profiling studies on the filter effect of UV absorbers and on the influence of pigmentation in white-pigmented systems will be reported.

Kinetic investigations on UV-induced photopolymerization reactions by real-time FTIR-ATR spectroscopy: the efficiency of photoinitiators at 313 and 222 nm

Abstract Real-time FTIR-ATR spectroscopy was used to study the kinetics of photopolymerization reactions induced by monochromatic UV light. Various photoinitiators with a broad range of extinction coefficients were tested for their efficiency to start the curing reaction of a diacrylate on irradiation at 313 or 222 nm. The effect of the absorption and initiation properties of the photoinitiators on the polymerization rate was analyzed. At 222 nm, the acrylate was found to polymerize even without photoinitiator.

Depth profiling of the conversion during the photopolymerization of acrylates using real-time FTIR-ATR spectroscopy

Real-time FTIR-attenuated total reflection (FTIR-ATR) spectroscopy was used to investigate the depth profile of the conversion during the photopolymerization of acrylate coatings. Layers of different but well-defined thickness were deposited on the ATR crystal and irradiated by UV light. Depth profiling studies were carried out on the self-screening of a strongly absorbing photoinitiator, the filter effect of an UV absorber and on the influence of pigmentation in a white-pigmented printing ink.

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.

Direct initiation of the photopolymerization of acrylates by short-wavelength excimer UV radiation

Abstract Investigations on the direct initiation of the photopolymerization of acrylates by irradiation with short-wavelength monochromatic UV light using the 222 nm emission of a KrCl * excimer lamp are reported. The reactivity of various acrylates was studied by real-time FTIR-ATR spectroscopy. Laser photolysis experiments and accompanying quantum chemical calculations were performed in order to propose a possible mechanism of initiation.

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.

The effect of temperature on the induction period in the photoinitiated polymerization of tripropylene glycol diacrylate

Abstract The effect of temperature on the kinetics of the photopolymerization of tripropylene glycol diacrylate was studied by real-time FTIR–attenuated total reflection (ATR) spectroscopy. The temperature behaviour of the polymerization rate was found to be dependent on the photoinitiator used. The induction period strongly decreases with increasing temperature. It was shown by quantitative determination of the oxygen concentration that the decay of the induction period is solely due to the decreasing solubility of oxygen in the acrylate.