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In the world of coatings and inks, traditional solvent-based players always have a group of "organic followers" - that is, organic solvents, whose main function is to dissolve solid components and adjust the "viscosity" of the entire system. However, these "little followers" do not participate in the film-forming process. They quietly slip away behind the scenes and evaporate into the air, which not only pollutes the environment, but also may bring certain safety hazards.
For radiation curing systems, since most of the UV resins used in the formula have a relatively high viscosity, solvents or diluents are also needed to adjust the viscosity. However, unlike the "organic followers" of traditional solvent-based coatings and inks, the diluents in radiation curing systems can usually participate in the curing and film-forming process and rarely evaporate into the air during the coating process. This makes the radiation curing system environmentally friendly and become a superstar in the environmental protection field.
Compared with organic solvents, this type of diluent that can participate in the photocuring film-forming reaction is called a reactive diluent. It is an organic small molecule containing a polymerizable functional group, so people usually call it a monomer. In the early photocuring system, the reactive diluent used was a general addition polymerization monomer, such as styrene, N-vinyl pyrrolidone, methyl acrylate, isooctyl acrylate, etc. Since these monomers have a low boiling point, a strong odor, and high toxicity, they are rarely used now. Some monomers are highly viscous liquids or even solids at room temperature and do not have a diluting effect, so it is more appropriate to call them monomers rather than reactive diluents.
According to the number of reactive groups contained in each molecule, monomers can be divided into monofunctional, bifunctional and multifunctional. Monofunctional monomers refer to monomers with only one group that can participate in the curing reaction in each molecule. The most common monomers are hydroxyethyl methacrylate (HEMA), isobornyl acrylate (IBOA), tetrahydrofuran acrylate (THFA), etc.; bifunctional monomers refer to monomers with two groups that can participate in the curing reaction in each molecule, such as tripropylene glycol diacrylate (TPGDA), 1,6-hexanediol diacrylate (HDDA); multifunctional monomers refer to monomers with three or more groups that can participate in the curing reaction, such as trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate, propoxylated glycerol triacrylate, etc. Theoretically speaking, the more functional groups there are, the faster the curing rate, the better the film-forming property, the higher the hardness, and the higher the cross-linking density; at the same time, its molecular weight also increases accordingly, the interaction between molecules increases, so the viscosity also increases, and the dilution effect decreases.
According to the type of functional group, monomers can be divided into acrylates, methacrylates, vinyls, vinyl ethers, epoxies, etc.
According to the curing mechanism, monomers can be divided into free-radical and cationic type. Acrylates, methacrylates, and vinyls are free radical types; epoxies are cationic types; vinyl ethers can participate in both free-radical polymerization and cationic polymerization, so they can be used as monomers for both photocuring systems.
Currently, no matter if it is UV coatings, UV inks, or UV adhesives, they are mainly based on free radical curing systems, and most of the monomers used in this system are acrylates.
