Photochemical Treatment of Powder Coatings and VOC-Free Coatings with NIR Lasers Exhibiting Line-Shaped Focus: Physical and Chemical Solidification

NIR-sensitized (NIR = near-infrared) radical photopolymerization can be employed to chemically cure VOC-free coatings; that is, either a powder coating or liquid monomer coating system with no significant components of volatile organic compounds (VOC). A NIR laser with line-shaped focus was applied to cure the coatings. This system operated in multi-wavelength mode where preferentially the emission wavelengths lambda = 808 nm and 980 nm of the laser system could be employed to solidify and crosslink the starting components as a transparent film; that is, chemical drying by photonic events. Cyanines absorbing at lambda = 980 nm and 808 nm functioned as heat generators to melt the powder and as sensitizers (Sens) to generate initiating radicals in combination with a radical initiator (RI). The radical initiator was either the iodonium salt RI-1 ([(t-C4H9-Ph)(2)[(-)][(CF3SO2) N-2(-)]) or triazine A (RI-2). Photoinduced electron transfer between the excited states of Sens and RI was found to control the efficiency of initiating radicals. Line-shape-focused laser experiments demonstrated that both liquid VOC-free coatings and solid powder coatings can be cured. Curing of solid powder coatings occurs with a curing rate of 3.6 mmin(-1), opening the possibility to introduce NIR laser curing as a new technology in coating sciences. Finally, rheological measurements and differential scanning calorimetry (DSC) were applied to characterize the flow properties of the molten powder and the glass transition temperatures of the crosslinked materials, respectively. The molten powder coating behaved like a liquid up to 160 8C as concluded from the complex shear modulus (G*), while the phase shift (partial derivative) documents crosslinking above this temperature.