Combination of NIR and UV-LEDs Enables Physical and Chemical Drying of
Aqueous Coating Dispersions as New Green Technology
Abstract
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 NIR-LED emitting at 820 nm with an intensity of 1 W/cm
2 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, 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. TPO-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.