NIR‐laser based detection of the photopolymerization state of acrylate coatings suitable for in situ measurements

The present article describes a method for the determination of the radical photopolymerization conversion of acrylic coatings suitable for an in situ monitoring during the coating process. Acrylate based coatings are increasingly used in many kinds of industrial coating processes, because of advantages of the solvent‐free application process. The applied method is based on 1620 nm light absorption of the acrylate. To overcome the disadvantages of a common NIR spectrometer, only the diffuse reflection of the relevant discrete wavelength at 1620 nm and of a reference wavelength is measured. A sophisticated sensor setup including an optical spatial filter and lock‐in amplified signal processing is used to achieve a sufficient signal to noise ratio. The capability of the sensor to discriminate between polymerized and unpolymerized coatings on metal substrates down to a coating thickness of 16 μm is demonstrated. Furthermore, the information on polymerization from larger depth in TiO₂ pigmented coatings is investigated. The results can be analytically modeled in analogy to the Lambert–Beer's law, resulting in a detection limit of a maximum pigment concentration of 15 wt % with 100 μm coating thickness. The presented sensor design is suitable to be used in an industrial production environment for example in screen printing applications as a monitoring and quality control tool. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012