NIR Light for Initiation of Photopolymerization
NIR light represents an interesting alternative for initiating both radical and cationic polymerization. It also offers the opportunity to provide non-radiative heat generated by the NIR sensitizer, resulting in initiation of physical processes, such as melting of powder coatings or decreasing the viscosity of liquids, which is needed to improve the spreading on certain substrates. This was concluded from the photophysical data of the NIR absorbers/sensitizers (cyanines). Generation of initiation species such as radicals and acidic cations follows both a phototonic and a thermally activated route, indicating the hybrid function of such photoinitiator systems. Iodonium salts with low coordinating anions function well as co-initiators/acceptors. Alternatively, upconverting nanoparticles (UCNPs) and upconversion phosphors represent an interesting alternative since these materials possess the capability to convert absorbed NIR laser light either into visible or UV light, resulting in excitation of the respective photoinitiator systems. Computer-to-plate (CtP) technology applying digital imaging of lithographic plates has mostly benefited from development of NIR-sensitized photopolymerization. Pioneering work was additionally pursued to cure both radical- and cationic-polymerizable multifunctional monomers. Formation of acidic cations was probed with Rhodamine B lactone. The curing of powder coatings applying NIR lasers instead of ovens can be seen as a breakthrough in the entire field of coatings. In addition, NIR photopolymerization can be applied for tailor-made synthesis of copolymers exhibiting a small polydispersity following the route of photo ATRP.