Near infrared LEDs and diode lasers for manufacture of new materials
Bernd Strehmel, Hochschule Niederrhein, Krefeld, GermanyChristian Schmitz, Niederrhein University of Applied Sciences, Krefeld, GermanyThomas Brömme, Niederrhein University of Applied Sciences, Krefeld, Germany
Exposure of a sensitizer (Sens) with polymethine pattern with near infrared (NIR) emitted by either NIR-LEDs and/or NIR-lasers in combination with a radical initiator (In) results in generation of both initiating radicals and cations. Radical initiators were selected from iodonium salts (ISX) bearing different anions (X). Photoinduced electron transfer from the excited state of Sens to ISX controls formation of initiating species. Furthermore, irradiation of the photoinitiator system comprising Sens and In results in bleaching at the excitation wavelength and opens the possibility of large curing depth.
Embedding of NIR photoinitiator systems in lithographic materials results in fast imaging systems. The computer to plate technology (CtP), known as digital exposure technique, was applied to study the sensitivity of these systems in solid films exhibiting a sensitivity between 50-300 mJ/cm2 depending on the anion of the iodonium salt. Lithographic plates were exposed at 830 nm.
Furthermore, NIR photoinitiator systems comprising Sens and In were investigated in model coatings derived from reactive solvents and/or industrial coatings based on urethane acrylates. The solubility of both Sens and In was up to 5-10 g/L in some cases of Sens and larger than several 100 g/L using iodonium salts with bis(trifluoromethyl)sulfonyl imide (NTf2-) as anion, respectively. Nevertheless, the highest reactivity was observed for those iodonium salts exhibiting a high conductivity in coatings. This was in many cases the B(CN)4- anion in reactive solvents showing that a good solubility does not causes always a high reactivity.
Furthermore, we demonstrate in some examples the feasibility of how line-shape focused lasers can be used to cure large coating areas. Particularly, powder coatings were in the focus. We used a laser system that continuously generated a focused line with an emission at both 808 and 980 nm. The photoinitiator system working at 808 nm generates initiating species according to a photonic mechanism while the NIR dye absorbing at 980 nm functions as absorber, which releases enough heat by non-radiative deactivation to melt the powder. This occurs in a short time frame while chemical solidification (crosslinking) of the molten powder coating occurs at 808 nm.