Brief introduction to photoinitiators

Photoinitiator, as a light-absorbing substance, is a key component of the photopolymerization system, so it is also known as photosensitizer or photocuring agent. It is a kind of compound that can absorb a certain wavelength of energy in ultraviolet light or visible light, and then produce free radicals, cations, etc., which leads to monomer polymerization, cross-linking and curing. Photoinitiator is mainly used in UV curing technology in radiation curing industry, which has a great influence on the curing rate of UV system and plays an important role in UV curing system. Photoinitiators can be divided into two categories according to the active intermediates produced by the absorption of light energy, among which the most widely used is the free radical photoinitiator, according to the type of absorbed radiation, it can be divided into ultraviolet light initiator and visible light initiator.
Action of photoinitiator
In the whole photopolymerization system, the proportion of photoinitiator is small, but it is an indispensable part. The wavelength, molar extinction coefficient and photoreactivity of photoinitiator have obvious influence on the photoinitiation performance. In addition, the photoinitiator has a great influence on curing speed, yellowing tendency and production cost. After light energy is absorbed by photoinitiator molecules, triplet states are excited to generate active species mainly through related chemical interactions, which leads to system polymerization and the formation of cross-linking network structure.

Photoinitiator and UV LED applications
The more overlapping of the absorption peak of the photoinitiator and the main emission band of the light source, the higher the intensity of the effective radiation of the photoinitiator, and the better the effect of improving the curing rate of the ink. Due to the narrow wave peak of UV LED, the radiation energy can only be concentrated in a narrow UV spectrum segment, resulting in UV curing can only be concentrated in a narrow range of the UV spectrum. At present, the wavelength of UV LED curing light source mainly includes 365nm, 375nm, 385nm, 395nm and 405nm, and the emission bandwidth of each wavelength light source is about 10nm. However, most photoinitiators on the market have strong absorption performance in the range of 300-370nm, but poor absorption performance in the range of greater than 370nm. As a result, the emission spectrum of the light source in the UV LED curing system does not match the absorption spectrum of the traditional photoinitiator, which ultimately affects the initiation efficiency of the photoinitiator. In addition, due to the low power of UV LED, especially shortwave UV LED, the output light intensity and luminescence efficiency are low, which reduces the sensitivity of photoinitiator to it, and also affects the initiation efficiency of photoinitiator.