Monofunctional UV monomers

In the previous issue, we introduced the definition, functions, and classification of UV monomers. UV monomers can be divided into monofunctional, difunctional, and multifunctional types according to the number of reactive groups contained in each molecule. Today, we will discuss monofunctional UV monomers.

Monofunctional UV monomers refer to those containing only one group capable of participating in the curing reaction per molecule. The types of functional groups include acrylates, methacrylates, vinyls, vinyl ethers, epoxies, etc.

Monofunctional UV monomers, due to their low molecular weight, exhibit high volatility, leading to issues such as strong odor and high irritancy. Besides these, what other characteristics do monofunctional UV monomers possess?

1. Low viscosity

Most monofunctional UV monomers exhibit low viscosity, typically below 30 cps, thus possessing strong diluting capacity.

2. Slow curing rate

Due to the presence of only one photocurable reactive group in monofunctional UV monomers, their photocuring rate is slow.

3. Low volume shrinkage upon curing

During the process of free radical addition polymerization, carbon-carbon double bonds are converted to single bonds, resulting in reduced molecular spacing and increased density, which leads to volume shrinkage—analogous to the freezing of water into ice. Owing to the low double bond content in monofunctional UV monomers, their volume shrinkage is relatively minimal. This characteristic renders such UV monomers suitable for applications requiring low shrinkage, such as UV adhesives.

4. Low crosslinking density

Due to the presence of only one photocurable reactive group, monofunctional UV monomers do not form crosslinking points during radiation curing. While this can enhance the flexibility of the cured film and reduce brittleness, it simultaneously decreases properties such as hardness, abrasion resistance, and solvent resistance. Therefore, they are typically used in combination with multifunctional UV resins and UV monomers to maintain sufficient crosslinking density.

5. High conversion rate

Monofunctional UV monomers exhibit a high conversion rate due to their low double bond content and low viscosity, which facilitate their participation in polymerization reactions.

The following introduces some common monofunctional UV monomers.

1. Methyl acrylate/butyl acrylate types

These monomers, characterized by low viscosity and excellent diluting properties, were used as diluents in early photocuring formulations. However, due to their high volatility, strong odor, and flammability, they are now rarely used.

2. HEA/HEMA/HPA/HPMA

These four UV monomers contain hydroxyl groups, which are beneficial for enhancing adhesion to polar substrates.

Due to their high irritancy, HEA and HPA may cause issues such as skin allergies and eye irritation when used in high-temperature environments. Therefore, they are generally not directly used as reactive diluents in formulations, but rather in the synthesis of polyurethane acrylates.

Hydroxy methacrylates, with an additional methyl group on the carbon of the double bond, exhibit reduced odor and skin irritancy, but their reactivity is consequently decreased. HEMA and HPMA are currently widely used as reactive diluents in nail gels.

3. IBOA/THFA/CTFA/PHEA

All four monomers feature cyclic structures in their molecular architectures.

IBOA: Isobornyl acrylate

IBOA is a UV monomer featuring a bicyclic isobornyl group, characterized by high boiling point, high Tg (glass transition temperature), low shrinkage, and excellent adhesion to various substrates, offering exceptional comprehensive performance. The sole drawback is its inherent odor, which may vary in acceptability among different users.

THFA: Tetrahydrofurfuryl acrylate

THFA contains a polar tetrahydrofuran ring in its molecule, which exhibits certain corrosiveness to substrates, particularly plastics, thereby enhancing adhesion. It has a low Tg, resulting in a relatively soft cured film.

CTFA: Cyclotrimethylolpropane formal acrylate

CTFA is a rare monofunctional monomer with low odor and low irritancy. It exhibits certain adhesion to most plastics and metals, featuring good toughness and excellent abrasion resistance, and is also a monomer with outstanding comprehensive performance.

PHEA: 2-Phenoxyethyl acrylate

PHEA is a low-viscosity monomer characterized by excellent diluting ability, high reactivity, high refractive index, low shrinkage, and good flexibility. However, it inherently has a phenolic odor, which limits its application in formulations with low odor requirements.

4. ACMO/DMAA

These two monomers belong to the acrylamide category.

ACMO: Acryloyl morpholine

ACMO is suitable for LED-UV curing, featuring fast curing speed, low odor, low irritancy, low viscosity, and excellent diluting ability. It exhibits faster reactivity compared to ordinary monofunctional monomers, and its cured film possesses good flexibility along with a certain degree of hardness. It is miscible with water and can be used in water-based UV systems.

DMAA: N,N-Dimethylacrylamide

DMAA has low viscosity and exhibits excellent dilution effects on UV resins. It demonstrates good adhesion to plastics and glass, forms a film with high hardness, and offers good chemical resistance.

5. St/NVP

These two monomers belong to the vinyl monomer category.

St: Styrene

Styrene was first used in combination with unsaturated polyesters in the first generation of photocurable wood coatings. It is characterized by low cost, low viscosity, and strong diluting ability. However, due to its high volatility, strong odor, flammability, and high toxicity, it is now rarely used as a monomer.

NVP: N-Vinylpyrrolidone

NVP features low viscosity, low skin irritancy, strong diluting ability, and high reactivity. However, its odor and carcinogenicity have limited its use, and it is now rarely used in photocurable formulations.

The above are common monofunctional UV monomers.