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

Abstract

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 λ=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 λ=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)2I+][(CF3SO2)2N−]) 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 m min−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 °C as concluded from the complex shear modulus (G*), while the phase shift (∂) documents crosslinking above this temperature.

New High‐Power LEDs Open Photochemistry for Near‐Infrared‐Sensitized Radical and Cationic Photopolymerization

“…Furthermore, cyanines with absorption in the NIR also generate heat that is needed to overcome activation processes . Cyanines also opened the feasibility to melt powder coatings by application of a NIR laser with line‐shaped focus emitting at both 980 nm and 808 nm . Thus, one could generally use them for inline manufacturing instead of ovens .…”

“…[11] Cyanines also opened the feasibility to melt powder coatings by application of aNIR laser with line-shaped focus emitting at both 980 nm and 808 nm. [12] Thus,o ne could generally use them for inline manufacturing instead of ovens. [13] Thei mportance of heat generated by non-radiative deactivation was additionally studied.…”

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

“…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].…”

Nahinfrarot‐sensibilisierte photoinduzierte ATRP mit einer Kupfer(II)‐Katalysatorkonzentration im ppm‐Bereich

“…Während Photopolymersysteme aus einem NIR-Sensibilisator und einem Elektronenakzeptor seit mehr als 15 Jahren zur Initiierung der radikalischen Polymerisation industriell eingesetzt werden, [11] wurde der NIR-Bereich bis vor kurzem nicht beachtet. [15] Kürzlich berichteten LaleveØ und Mitarbeiter über Initiatorsysteme zur radikalischen Polymerisation mit NIR bei geringer Lichtintensitäti nG egenwart von Sauerstoff. Solche Laser kommen in vielen digitalen Arbeitsabläufen zum Einsatz, um Photopolymerfilme in "Industrie 4.0"-Anwendungen zu belichten.…”