NIR Light for Initiation of Photopolymerization

Abstract

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.

Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0

“…From this point of view, they appear more or less as efficient molecular ovens, which was practically approved by melting of powder coatings applying lasers with line-shaped focus [16]. Thus, these absorbers possess large potential to replace thermal processes based on furnaces by introduction of such molecular ovens in combination with modern light sources [64,65,67]. The development of modern LED systems with emission in the NIR will enforce such developments [64].…”

“…Experimentally, there is often no reaction under ambient light conditions. However, strong sources such as lasers with line-shaped focus or high-power NIR-LEDs helped to initiate photochemi-cal reactions [64,65,67] . Recent studies confirmed these findings [85][86][87][88].…”

“…References [11][12][13][14][63][64][65] provide more details regarding the mechanistic details. Such systems have been successfully applied in CtP systems for more than 20 years [11,12].…”

“…A summary of determined and almost published photophysical and electrochemical data should complement available data available [3,13,14,[59][60][61][62] to provide a brief idea regarding their use in applications based on sensitized photoinduced electron transfer. This can be NIR-sensitized photopolymerization resulting in formation of initiating radicals and conjugate acid [5,6,[13][14][15]63,64]. Recently, the use of NIR-LEDs exhibiting high excitation intensity brought more light in this field and helped to understand the function of existing intrinsic barrier in such systems comprising cationic cyanines in photoinduced electron transfer systems [65].…”

“…Many examples of cyanines focus on the use of absorption in the NIR [64]. Among of the disclosed applications, photopolymerization depicts one representative method where the absorber uptakes the function of a sensitizer to generate initiating radicals and conjugate acid [13][14][15][16][17]58,65].…”