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Overview of UV Monomers

Release time:

2025-01-10 17:02

In the world of coatings and inks, traditional solvent-based formulations always come accompanied by a group of “little organic sidekicks”—the organic solvents themselves. Their primary role is to dissolve solid components and adjust the overall viscosity of the system. However, these “little sidekicks” don’t actually take part in the main performance of film formation; instead, they quietly slip away behind the scenes, evaporating into the air. As a result, not only do they add to environmental pollution, but they may also pose certain safety risks.

For radiation-curable systems, since most of the UV resins used in formulations have relatively high viscosities, it is also necessary to add solvents or diluents to adjust the viscosity. However, unlike the “organic little companions” found in conventional solvent-based coatings and inks, the diluents used in radiation-curable systems typically participate in the curing and film-forming process itself and rarely volatilize into the air during application. This inherent environmental friendliness has earned radiation-curable systems a glowing reputation in the environmental protection community, making them true superstars in the field of green chemistry.

Compared to organic solvents, diluents that can participate in photocuring film-forming reactions are called reactive diluents. These are small organic molecules containing polymerizable functional groups, and thus they are commonly referred to as monomers. In early photocuring systems, the reactive diluents used were typically conventional addition-polymerization monomers, such as styrene, N-vinylpyrrolidone, methyl acrylate, and 2-ethylhexyl acrylate. However, due to their low boiling points, strong odors, and high toxicity, these monomers are now rarely employed. Moreover, some individual monomers are highly viscous liquids or even solids at room temperature and do not possess any diluting effect; therefore, it is more appropriate to refer to them as monomers rather than reactive diluents.

Based on the number of reactive functional groups contained in each molecule, monomers can be classified into monofunctional, difunctional, and polyfunctional types. A monofunctional monomer has only one reactive group per molecule that can participate in the curing reaction; the most common examples include hydroxyethyl methacrylate (HEMA), isobornyl acrylate (IBOA), and tetrahydrofuran acrylate (THFA). A difunctional monomer contains two reactive groups per molecule that can participate in the curing reaction, such as tripropylene glycol diacrylate (TPGDA) and 1,6-hexanediol diacrylate (HDDA). A polyfunctional monomer has three or more reactive groups per molecule that can participate in the curing reaction, such as trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate, and propoxylated glycerol triacrylate. In theory, the greater the number of functional groups, the faster the curing rate, the better the film-forming properties, the higher the hardness, and the greater the cross-linking density. At the same time, the molecular weight also increases accordingly, leading to stronger intermolecular interactions and thus higher viscosity, while the diluting effect is reduced.

According to the type of functional group, monomers can be classified into acrylate-based, methacrylate-based, vinyl-based, vinyl ether-based, epoxy-based, and other categories.

According to their curing mechanisms, monomers can be classified into two major categories: free-radical type and cationic type. Acrylates, methacrylates, and vinyl compounds belong to the free-radical type; epoxy compounds, on the other hand, belong to the cationic type. Vinyl ethers, since they can participate in both free-radical polymerization and cationic polymerization, can serve as monomers for either of these two types of UV-curable systems.

Currently, whether it's UV coatings, UV inks, or UV adhesives, the dominant curing system remains the free-radical curing system, and most of the monomers used in this system are acrylates.

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Monofunctional UV monomer

Hydroxyacrylate monomer