Combination of NIR and UV-LEDs enables physical and chemical drying of aqueous coating dispersions as new green technology

Heptamethine based cyanines, namely 1,3,-trimethyl-2-(2-2[2-phenylsulfanyl-3-[2-(1,3,3-trimethyl-1,3,3-trithyl-1,3-dihydro-indol-2-ylidene)-ethylidene]cyclohex-1-enyl]vinyl)-3H-indolium chloride (S1) and 2-[2-(2-chloro-[2-[1,1-dimethyl-7-sulfo-3-(4-sulfobutyl)-1,3-dihydro-benzo[e]indol-2-ylidene]-ethylidene]cyclopent-1-enyl]vinyl]-1,1-dimethyl-7-sulfo-3-(4-sulfobutyl)-1Hbenzo[e]indolium hydroxide, inner salt, triethylammonium salt (S2), efficiently result in physical drying of an aqueous dispersion comprising a polyurethane binder. S2 possesses a water solubility of 40 g/L. A high-intensity near-infrared-LED emitting at 820 nm with an intensity of 1 W/cm² served as light source. The cyanine converted the light absorbed into heat by internal conversion needing less drying time compared to conventional drying. Water content after film formation showed less then 1%. In the second step, ultraviolet (UV) exposure with a LED emitting at 395 nm resulted in formation of semi-interpenetrating polymer networks by crosslinking of the multifunctional (meth)acrylate operating as reactive diluent. Ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate-L served as effective UV-photoinitiator. Furthermore, the UV-exposure together with Norrish Type I and Type II photoinitator systems results in a very efficient bleaching of the green physical dried film. This contribution shows for the first time a new photonic hybrid technique describing successful replacement of an oven-based process by a photonic based step that generates heat needed for drying.