In photo-curing technology, the wavelength and light intensity of light source are two important physical factors of photo-curing reaction, which determine whether photo-curing reaction can take place, what reaction mechanism and reaction speed, etc., and ultimately affect the properties of materials. The wavelength of light source must match the absorption wavelength of photoinitiator to initiate photo-curing reaction.

1. Free radical light curing

Light-curing of free radicals is that free radicals are generated by free radical photoinitiators after being illuminated, and the free radicals initiate double bond polymerization to cure. The free radical curing reaction speed is fast, and it is widely used in traditional UV industry, with high technology maturity, various types of monomers, resins and initiators, and has been applied on a large scale, which not only has more selectivity, but also has relatively low material cost.

Commonly used free radical photocuring monomers include acrylate, vinyl and vinyl ether monomers; Functionality can be 1, 2, 3, multi-official, etc.

Light-curable resins include polyurethane acrylate, epoxy acrylate, polyester acrylate, polyether acrylate, etc. Generally speaking, polyurethane acrylate has good flexibility and wear resistance, but due to the existence of a large number of amine ester bonds and a large number of hydrogen bonds within or between molecules, the viscosity of the resin is usually high. Epoxy ester has fast curing speed and high strength, but it is brittle and easy to yellow. Polyester acrylate has good curing quality and wide adjustable range of resin properties. Because there are a lot of ether bonds in the molecular chain of polyether acrylate, the molecular chain has good flexibility and yellowing resistance, but the material is soft, and the mechanical strength, hardness and chemical resistance are poor. The functionality of free radical curing resin is usually greater than 2.

There are many kinds of free radical photoinitiators, including cracking photoinitiators and hydrogen abstraction photoinitiators.

2. Cationic light curing

Photo-curing is that cationic initiator generates super-strong protonic acid with active center under illumination, and protonic acid initiates ring-opening curing of epoxy, oxetane or curing of vinyl ether materials. Or epoxy oxetane ring-opening curing has the advantages of small volume shrinkage and no oxygen polymerization inhibition. However, cationic photocuring has a relatively long induction period, slow reaction speed, relatively little application in traditional photocuring field, few types of monomers, resins and initiators, less selectivity, and the cost of formula system is higher than that of free radical system.

The monomers that can be used for cationic photocuring include epoxy, oxetane and vinyl ether monomers. The commonly used resin is mainly epoxy resin. Initiators mainly include sulfonium salt, iodonium salt and sensitizer.