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

Abstract

This contribution summarizes recent progress in the field of near-infrared (NIR) initiated photopolymerization. The photoinitiator system consists of a cyanine as sensitizer (Sens) and an iodonium salt with distinct structural pattern of both the cation and anion as radical initiator. Both, photonic and thermal events are discussed as the main source for formation of initiating species. Electron transfer between the excited state of Sens (Sens*) and the iodonium salt can be seen as the main source for formation of initiating species such as radicals and protons/electrophiles. Furthermore, the ion mobility as probed by the electric conductivity possesses a major function to tune the reactivity of the photopolymer system. The reactivity of these systems was studied in different applications such as Computer to Plate (CtP), LED curing, photonic baking, and curing of powder coatings with NIR lasers exhibiting line shape focus.

Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0

“…These facts advantageously demonstrate the benefits of cyanines where tailor-made synthesis of the unsaturated cation and the selection of an appropriate anion facilitated sufficient solubility with many matrix materials [5,6]. In comparison, cyanines depict from this point of view outstanding properties since the aforementioned benefits enabled their use in photopolymerization to pattern light sensitive coatings [11][12][13][14], to cure liquid coatings [15], and powder coatings [16,17] or to initiate either a chemical and/or thermal curing process [58].…”

“…Nevertheless, a variation between the cation of the cyanine with distinct anions may move such materials to applications where either an aqueous surrounding being available in biological applications [7][8][9] or a dipolar aprotic matrix takes the function of the matrix with absorber embedded. Typically, digital imaging in Computer to Plate technology (CtP) [10][11][12][13][14], curing of liquid coatings [5,15], powder coatings [16,17], laser drying [18][19][20], laser welding [21][22][23][24][25], or laser marking of plastics [26][27][28][29][30][31] represent some practical examples. Nevertheless, cyanines have held a long history regarding their practical use, which started to use such materials as sensitizers in silver halide photography [32][33][34] until the point when electronic media took their place in imaging sciences to save pictures.…”

“…A summary of determined and almost published photophysical and electrochemical data should complement available data available [3,13,14,[59][60][61][62] to provide a brief idea regarding their use in applications based on sensitized photoinduced electron transfer. This can be NIR-sensitized photopolymerization resulting in formation of initiating radicals and conjugate acid [5,6,[13][14][15]63,64]. Recently, the use of NIR-LEDs exhibiting high excitation intensity brought more light in this field and helped to understand the function of existing intrinsic barrier in such systems comprising cationic cyanines in photoinduced electron transfer systems [65].…”

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

“…Alternative anions may be fluorinated phosphates with fluorinated alkyl groups ([PF 6-x (C n F 2n+1 ) x ] -; x>0, x<6) showing from our best knowledge no HF release under conditions where this phenomenon was observed in the case of PF 6 interesting alternative anion in iodonium salts. It exhibited in some cases a giant solubility in many monomers while the reactivity in NIR-photopolymers was remarkable [8,19] --anion as counter ion.…”

“…Iodonium salts (ISs) have been used for a long time either as photo acid generator or radical initiator [1][2][3][4][5][6]. 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.…”