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How to Choose the Right UV Monomer

Release time:

2025-01-10 17:02

In UV-curable formulation systems, in addition to UV resins and photoinitiators, UV monomers are also a crucial component. UV monomers not only help adjust the viscosity of the system but can also impart or enhance various properties of the cured film, such as improving adhesion, enhancing flexibility, and increasing abrasion resistance. Therefore, the judicious selection and use of different monomers is an essential aspect of formulation design.

When selecting UV monomers appropriately, one should take into account comprehensively the field to which the development project belongs, the type of substrate, and the performance requirements. The goal is to balance as much as possible the various demands and skillfully combine different monomers to achieve optimal results. Below, we’ll introduce some factors that should be considered when choosing UV monomers for practical applications.

I. Price

With the continuous advancement of light-curing technology and the ever-increasing saturation of the market, raw material suppliers have sprung up like bamboo shoots after rain, making the competitive landscape increasingly fierce. In this environment, the rapid expansion of production capacity has led to an oversupply in the market, intensifying the price war. The price of UV monomers directly affects their proportion in formulation systems, thereby influencing the final selling price and profit margins of the finished products—and ultimately impacting the products’ competitiveness in the market. In a year like this one, characterized by intense internal competition, price has become the primary factor that manufacturers consider when selecting UV monomers.

II. Odor and Irritancy

As people’s awareness of environmental protection continues to grow, the demands for the odor and irritancy of formulated products are also increasing. Different UV monomers, due to their varying chemical structures, exhibit distinct differences in odor and irritancy. Generally speaking, UV monomers with lower molecular weights tend to have stronger odors; though this is not an absolute rule. The odor and irritancy of UV monomers are primarily mediated through their vapor phase. Therefore, we should assess these properties based on their vapor pressures: the higher the vapor pressure, the more readily the UV monomer volatilizes. Once the concentration in the air reaches a certain level, it can cause discomfort to sensitive areas such as the skin and eyes.

Generally, the odor and irritancy of UV monomers rank as follows: mono-functional > di-functional > poly-functional.

In the production process of UV monomers, optimizing the process—such as performing ethoxylation and propoxylation modifications—can reduce the monomer’s odor and irritancy.

Methacrylate monomers have lower odor and less irritancy than acrylate monomers. In certain applications that are particularly sensitive to odor and irritation—such as nail gel polishes—HEMA and HPMA are typically chosen as active diluents.

3. Viscosity and Dilutability

The primary function of UV monomers in a formulation is to reduce the viscosity of the system. If a particular UV monomer exhibits strong viscosity-reducing capabilities, its dosage in the formulation can be minimized, thereby maximizing the influence of the formulation’s main component—UV resin—on the performance of the cured film. It should be noted, however, that a monomer with low viscosity does not necessarily have strong viscosity-reducing ability. Even for the same monomer, its diluting capacity often varies depending on the specific formulation system. Generally, high-viscosity monomers are seldom used solely for viscosity reduction; instead, they are typically employed to enhance other properties of the cured film.

IV. Reactivity

Another role of UV monomers in a formulation is to influence the curing speed of the system. The higher the reactivity of a particular UV monomer, the greater its ability to accelerate the curing process of the entire formulation. It’s important to note, however, that there isn’t a strictly linear relationship between the double-bond content of a UV monomer and its reactivity. Among the major functional-group classifications mentioned in the previous video, (methyl)acrylate monomers exhibit the highest reactivity; among these, acrylate monomers are more reactive than methyl acrylate monomers. This difference arises from the electron-donating effect and steric hindrance caused by the methyl group attached to the double bond in methyl acrylates, which reduce their reactivity. Although monofunctional UV monomers have relatively low polymerization rates, given sufficient energy exposure, their final conversion rate can often reach over 80% to 90%. On the other hand, polyfunctional UV monomers, despite having higher photopolymerization rates, may leave behind a higher residual concentration of reactive groups at the end of the reaction.

V. Curing Shrinkage Rate

In the photopolymerization process, free-radical systems typically experience volume shrinkage, which occurs because double bonds are converted into single bonds during curing, thereby reducing the overall distance between atoms. In most application scenarios, this shrinkage is an undesirable factor. When coatings are cured on substrates such as metals, glass, and plastics, significant shrinkage can lead to poor adhesion. For applications on soft substrates—such as paper or thin films—shrinkage during curing can cause the substrate to wrinkle or curl.

The higher the functionality of a UV monomer, the more double bonds it has to participate in the curing reaction, and thus the greater its shrinkage. In thermally cured formulations, the stresses generated during curing have sufficient time to dissipate, resulting in better adhesion. In contrast, photopolymerization is much faster than thermal curing; the stresses generated during photopolymerization do not have enough time to release, thereby adversely affecting the adhesion performance of the coating to the substrate.

6. Surface Tension

Surface tension is an important factor influencing the performance of light-curing systems. The wettability, adhesion, permeability, and leveling properties of coatings and inks are all related to surface tension. Liquids with low surface tension can wet and spread effectively on surfaces with high wetting energy; conversely, when a liquid’s surface tension exceeds the substrate’s wetting energy, it cannot wet or spread effectively. Selecting monomers with appropriate surface tension as reactive diluents is one effective way to adjust the surface tension of the system.

In practical applications, it is essential to conduct a comprehensive analysis and consideration based on the viscosity, curing rate, adhesion, and physical and mechanical properties of the cured film required by the development project. If a single monomer cannot meet these requirements, two or more monomers with different functional groups should be selected and combined to achieve an optimal formulation for the UV-curable product with the best overall performance.

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Multifunctional UV monomer