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18 June 2007 Depth tracing the influence of oxygen on UV curing
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The degree of cure of the coatings should be known for quality control and design of the curing system. Often the radiation part of the curing installation is oversized, because the spectral irradiances needed for a certain conversion of carbon double bonds were not determined beforehand. The usual testing methods for the quality of a coating, as for instance ATR spectroscopy, are only sensitive to the attributes of the surface. In our investigations, we measured the degree of conversion locally at different depths of the layer by means of confocal Raman spectroscopy with a spatial resolution of approximately 1&mgr;m. For the kinetic studies, we measured the change of Raman scattering of 532 nm Laser radiation. Which induces a transition on a vibration level of the carbon double bond corresponding to a wavenumber of 1620 cm-1. The change was normalized with respect to the CH2 deformation mode. Furthermore, we investigated the effect of different spectral distributions of radiation on the local conversion at different depths. By this it could be shown that most of the radiation power is needed to harden the surface. This is caused by the inhibiting effect of oxygen which hinders the generation of radicals. The measured depth profile of conversion reveals that oxygen is effective up to depths of 30 &mgr;m. The results of confocal Raman spectroscopy could also be used to optimize a curing system with inertization so that the radiation power could by reduced by 91%.
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Stefan Pieke and Wolfgang Heering "Depth tracing the influence of oxygen on UV curing", Proc. SPIE 6616, Optical Measurement Systems for Industrial Inspection V, 66163X (18 June 2007);

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