Photoinitiated radical and cationic curing with new NIR-LED device

 
We introduce a new LED device for near-infrared (NIR) curing of coatings applying photopolymerization technology. Suitable initiator systems comprise heptamethine cyanines as NIR absorber and an iodonium salt as coinitiator bearing anions with different nucleophilicity. This generates initiating radicals and acidic cations. Internal conversion of the cyanine mainly directs the excited state towards non-radiative deactivation as the most likely deactivation process whereas the chemical reaction of the absorbers excited state possesses an important function for chemical curing and solidification of coatings. Thus, this device introduces heat into a system by conversion of radiation into heat by non-radiative deactivation, which may help to replace ovens, and to initiate photochemical processes by exposure of large areas. The latter can be assigned to radical and cationic polymerization using (meth)acrylates and cationic crosslinkable monomers, respectively. Our prototype exhibits an emission maximum at 805 nm with a power of 2.5 W/cm 2. This device temporary achieves a temperature of about 150°C depending on the absorber embedded and its concentration. It activates thermal processes needed for efficient curing.