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Monofunctional UV monomer

Release time:

2025-01-10 17:02

Based on the number of reactive functional groups contained in each molecule, UV monomers can be classified as monofunctional, difunctional, and polyfunctional. Today, let’s talk about monofunctional UV monomers.

A monofunctional UV monomer refers to a molecule that contains only one functional group capable of participating in the curing reaction. The types of functional groups include acrylates, methacrylates, vinyl compounds, vinyl ethers, and epoxies, among others.

Due to their lower molecular weight, monofunctional UV monomers tend to be highly volatile, leading to issues such as strong odors and significant irritation. Beyond that, what other characteristics do monofunctional UV monomers possess?

I. Low Viscosity

Most monofunctional UV monomers have low viscosity, typically below 30 cPs, and therefore exhibit strong diluting capabilities.

II. Slow curing rate

Since monofunctional UV monomers contain only one photopolymerizable functional group, their photocuring rate is slow.

3. Low volumetric shrinkage upon curing

During free-radical addition polymerization, the carbon-carbon double bond is converted into a single bond, reducing the intermolecular distance and increasing the density, which leads to volume shrinkage—much like water freezing into ice. Since monofunctional UV monomers have a low content of double bonds, their volume shrinkage is relatively minor. This characteristic makes these UV monomers particularly suitable for applications that demand low shrinkage, such as UV adhesives.

4. Low cross-linking density

Since a single photopolymerization functional group is present, monofunctional UV monomers do not generate crosslinking points during radiation curing. Although this approach can enhance the flexibility of the cured film and reduce its brittleness, it also tends to lower the film’s hardness, abrasion resistance, and solvent resistance. Therefore, monofunctional UV monomers are typically used in combination with polyfunctional UV resins and UV monomers to maintain an adequate crosslinking density.

5. High conversion rate

Single-functional UV monomers have a low double-bond content and low viscosity, making them easy to participate in polymerization and thus achieving high conversion rates.

The following introduces some common monofunctional UV monomers.

I. Methyl acrylate/Butyl acrylate series

These monomers have low viscosity and good diluting properties, and were initially used as diluents in early light-curing formulations. However, due to their high volatility, strong odor, and flammability, they are now largely discontinued.

II. HEA/HEMA/HPA/HPMA

These four UV monomers contain hydroxyl groups, which help enhance adhesion to polar substrates.

Due to their relatively strong irritancy, HEA and HPA may cause skin allergies and eye irritation when used in environments with higher temperatures. Therefore, they are generally not used directly as active diluents in formulations but rather employed in the synthesis reaction of polyurethane acrylates.

Hydroxyethyl methacrylate (HEMA) has one additional methyl group attached to the carbon atom of the double bond, which reduces its odor and skin irritation potential but also lowers its reactivity. Currently, HEMA and hydroxypropyl methacrylate (HPMA) are widely used as reactive diluents in gel nail polishes.

III. IBOA/THFA/CTFA/PHEA

The molecular structures of these four monomers all contain cyclic structures.

IBOA: Isobornyl acrylate

IBOA is a UV monomer featuring a bicyclic isobornyl group. It boasts a high boiling point, a high glass transition temperature (Tg), and low shrinkage. It exhibits excellent adhesion to various substrates and demonstrates outstanding overall performance. Its only drawback is that it has a noticeable odor, and different users have varying degrees of tolerance toward this odor.

THFA: Tetrahydrofuran Acrylate

The THFA molecule features a polar tetrahydrofuran ring, which exhibits some corrosive effects on substrates—especially plastics—thereby enhancing adhesion. With a low Tg, its cured film tends to be softer.

CTFA: Cyclo-trimethylolpropane acrylate

CTFA is a rare low-odor, low-irritation monomer among monofunctional monomers. It exhibits good adhesion to most plastics and metals, boasts excellent toughness, and demonstrates superior wear resistance. It is a monomer with outstanding overall performance.

PHEA: 2-phenoxyethyl acrylate

PHEA is a low-viscosity monomer characterized by excellent diluting properties, high reactivity, high refractive index, low shrinkage, and good flexibility. However, it has an inherent phenol odor, which limits its use in formulations with low odor requirements.

4. ACMO/DMAA

These two monomers belong to the acrylamide class of monomers.

ACMO: Acrylamide morpholine

ACMO is suitable for LED-UV curing and features rapid curing, low odor, low irritation, low viscosity, and excellent dilutability. It reacts more quickly than conventional monofunctional monomers, and the cured film exhibits both good flexibility and a certain degree of hardness. It is water-soluble and can be used in water-based UV systems.

DMAA: N,N-Dimethylacrylamide

DMAA has low viscosity, excellent UV resin dilution performance, good adhesion to plastic and glass substrates, high film hardness, and excellent chemical resistance.

V. St/NVP

These two monomers belong to the vinyl class of monomers.

St: Styrene

Styrene was first used in combination with unsaturated polyester in the first-generation UV-curable wood coatings. It boasts advantages such as low cost, low viscosity, and strong diluting ability. However, due to its high volatility, strong odor, flammability, and significant toxicity, it is now rarely used as a monomer.

NVP: N-Vinylpyrrolidone

NVP features low viscosity, low skin irritation, strong diluting ability, and high reactivity. However, due to its strong odor and carcinogenic properties, its use is limited, and it is now rarely employed in UV-curable formulations.

The above are common monofunctional UV monomers.

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