Digital Imaging of Lithographic Materials by Radical Photopolymerization and Photonic Baking with NIR Diode Lasers

Abstract

Photo-initiated cross-linking of multifunctional acrylic esters in polymeric binders was investigated based on digital imaging using the Computer-to-Plate (CtP) technology applying laser exposure in the near-infrared (NIR). Generation of initiating radicals occurs by electron transfer from the excited state of the NIR-sensitizer to the radical generator, an onium salt. Iodonium salts derived from several borates and those with the bis(trifluoromethylsulfonyl)imide anion resulted in lithographic materials with high sensitivity. Photo-induced electron transfer plays a major function to generate initiating radicals by a sensitized mechanism but thermal events also influence sensitivity of the coating. Internal conversion was the major deactivation pathway while a certain fraction of NIR-dye fluorescence was also available. A line shape focused laser system with emission in the NIR was successfully used to bake the materials.

Comparison between NIR and UV-Sensitized Radical and Cationic Reactivity of Iodonium Salts Comprising Anions with Different Coordination Behavior

“…In this point, UV and NIR systems mostly differ. NIR systems based on polymethine sensitizers do not form triplet states and the yield for deactivation by fluorescence is relatively low (<15% [12]). Thus, most of the radiation absorbed by the sensitizers would be released as heat.…”

“…Due to the short wavelength absorption of iodoium salts (<300 nm), both radical and/or cation generation has been often pursued by a sensitized mechanism [4,5], where traditional mercury lamps [4,7], LEDs [4,[8][9][10][11] or NIR lasers [12][13][14][15] have been applied to generate initiating species. This requires to use a sensitizer exhibiting a significant absorption in the emission area of the light source.…”

“…Photochemical generation of initiating species occurs by electron transfer. This is tuned from a thermodynamic point of view by the oxidation potential of the sensitizer, the reduction potential of the iodonium cation and the excitation energy of the sensitizer [4,8,[12][13][14][15].…”

Advances of Near Infrared Sensitized Radical and Cationic Photopolymerization: from Graphic Industry to Traditional Coatings

“…Highly light sensitive materials are needed for such procedures because the time frame to expose one pixel covers only several s/pixel. Nowadays, some feasibility studies successfully showed the working principle of NIR initiated photopolymerization for coatings [3][4][5][6][7]. The majority of radiation curing relies on the well established UV-curing [8].…”

“…One reason can be seen in insufficient solubility of the initiator components Sens and RI in the high viscous resins. Many NIR absorbers based on cyanines and radical initiators based on iodonium salts possess an insufficient solubility in common industrial coatings [3][4][5][6]. There is a need for materials exhibiting an improved solubility in such surroundings either.…”