There are inherent shortcomings in free radical photopolymerization and cationic photopolymerization initiation technologies , such as high volatility, large oxygen inhibition, high shrinkage or slow curing speed, serious influence of moisture, etc. Hybrid systems are a method that can improve these shortcomings.
The cationic UV system diluent can be a mixture of a free radical diluent containing double bonds and a cationic diluent. However, there are also some monomers such as vinyl compounds that are electron-rich. Under the action of strong acid, vinyl compounds can undergo cationic polymerization, which can be used for free radical curing and can also be used in cationic curing systems. The polymer has a strong dilution effect.
The absorption wavelength of diaryliodonium salts and triarylsulfonium salts is shorter. In order to match the near-ultraviolet light source, some sensitizers are usually added. Free radicals generated by photolysis of benzoyl peroxide (BPO), anthraquinone derivatives, a-hydroxyketone, etc. can reduce onium salts and ultimately generate cations and free radicals, respectively triggering cationic and free radical polymerization reactions and photosensitization. The agent plays an indirect electron transfer role on the onium salt, extending the absorption wavelength and accelerating the curing reaction.
Polyhydroxy compounds are introduced into the cationic UV curing system, and the hydroxyl groups act as chain transfer agents to accelerate polymerization .
During free radical polymerization, the distance between monomers or oligomers changes from the van der Waals force distance before curing to the distance between covalent bonds after curing, so the volume shrinks significantly, resulting in disadvantages such as large internal stress and poor adhesion. .When epoxy compounds are ring-opening polymerized, on the one hand, the distance between epoxy monomers changes from the van der Waals force distance before curing to the distance between covalent bonds after curing. This process causes volume shrinkage; on the other hand, On the other hand, when the epoxy monomer is polymerized, the size of the structural unit formed by opening the ring on the monomer is larger than the monomer molecule. The overall result of the two is to reducethe volume shrinkage of the ring-opening polymerization reaction of the epoxy compound. Therefore, the size of the epoxy compound Hybrid polymerization consisting of cationic ring-opening polymerization and free radical polymerization of acrylate monomers can control the volume change during curing and reduce the volume shrinkage, thereby reducing internal stress and enhancing adhesion properties.