How to Select UV Monomers

In the photo-curing formulation system, apart from UV resins and photoinitiators, UV monomers also serve as a vital component. UV monomers not only adjust the viscosity of the system but also impart or enhance different properties of the cured film, such as enhancing adhesion, improving flexibility, and increasing wear resistance. Therefore, the rational use of various monomers is also an important link in formulation design.

The rational selection of UV monomers should be comprehensively considered according to the application field of the project developed, the type of substrate, and the performance requirements, so as to balance various requirements as much as possible, and skillfully combine various monomers to achieve the best results. The following are some factors to be considered when selecting UV monomers in practical applications.

1. Price

With the continuous progress of photo-curing technology and the increasing saturation of the market, raw material suppliers have emerged like mushrooms, and the competition has become increasingly fierce. In this environment, the sharp expansion of production capacity has led to an oversupply in the market, and price wars have intensified. The price of UV monomers affects their proportion in the formulation system, thereby affecting the selling price and profit of the finished product in the market, that is, affecting the final market competitiveness of the finished product. In this highly competitive environment this year, price has become the first factor to consider when selecting UV monomers.

2. Odor and Irritation

With the enhancement of people's environmental awareness, the requirements for the odor and irritation of formulated products are also getting higher and higher. Different UV monomers have certain differences in odor and irritation due to different chemical structures. Generally speaking, UV monomers with smaller molecular weights have stronger odors, but this is not absolute; the odor and irritation of UV monomers take effect through their vapor, which can be assessed by their vapor pressure. The larger the vapor pressure, the more volatile the UV monomer is. When the concentration in the air reaches a certain level, it will cause discomfort to sensitive parts such as the skin and eyes.

Under normal circumstances, the order of odor and irritation of UV monomers is as follows: monofunctional > difunctional > polyfunctional.

In the production process of UV monomers, the odor and irritation of monomers can be reduced by optimizing the process, such as ethoxylation and propoxylation modification.

Methacrylate monomers have smaller odor and irritation than acrylate monomers. In some fields with strict requirements on odor and irritation, such as nail gel, HEMA and HPMA are usually selected as reactive diluents.

3. Viscosity and Dilutability

The main role of UV monomers in the formulation is to reduce the viscosity of the system. If a UV monomer has a strong ability to reduce viscosity, its dosage in the formulation can be minimized, so that the main body of the formulation, UV resin, can maximize its influence on the properties of the cured film. It should be noted that low-viscosity monomers do not necessarily have strong viscosity reduction ability. Even for the same monomer, its diluting ability often varies in different formulation systems. High-viscosity monomers are generally less used for viscosity reduction and are usually used to improve other properties of the cured film.

4. Reactivity

Another role of UV monomers in the formulation is to affect the curing speed of the system. If a UV monomer has higher reactivity, it will have a greater promoting effect on the curing of the entire formulation system. It should be noted that the double bond content of UV monomers is not completely proportional to the reactivity. In the classification of several major functional groups mentioned in the previous video, (meth)acrylate monomers have the highest reactivity, among which the reactivity of acrylate monomers is higher than that of methacrylate monomers, which is because the electron effect and steric hindrance of the methyl group on the double bond of methacrylate lead to a decrease in its activity. Although the polymerization rate of monofunctional UV monomers is low, as long as sufficient energy is given, their final polymerization conversion rate can often reach more than 80-90%. However, although the photopolymerization rate of polyfunctional UV monomers is relatively high, the residual amount of reactive groups may also be relatively high.

5. Curing Shrinkage

Free radical systems are usually accompanied by volume shrinkage during the photo-curing process, which is because the double bonds are converted into single bonds during the curing process, resulting in a shortening of the distance between atoms in general. Shrinkage is a disadvantage in most application scenarios. When coating and curing on substrates such as metal, glass, and plastic, large curing shrinkage will lead to poor adhesion. For applications on soft substrates, such as coating on paper and film, the shrinkage during curing will cause the substrate to curl.

The higher the functionality of the UV monomer, the more double bonds involved in the curing reaction, and the greater the shrinkage. In the thermal curing formulation system, the stress generated during the curing process has enough time to be eliminated, so the adhesion is better. However, the curing speed of photo-curing is much faster than that of thermal curing, and the stress generated during curing has no time to be released, thus affecting its adhesion to the substrate.

6. Surface Tension

Surface tension is an important factor affecting the performance of photo-curing systems. The wettability, adhesion, permeability, and leveling of coatings and inks are all related to surface tension. Liquids with low surface tension can wet and spread on surfaces with high surface energy; on the contrary, when the surface tension of the liquid is greater than the surface energy of the substrate, it cannot effectively wet and spread. Selecting monomers with appropriate surface tension as reactive diluents is one of the effective means to adjust the surface tension of the system.

In the actual application process, comprehensive consideration and analysis should be carried out according to the required viscosity, curing rate, adhesion, and physical and mechanical properties of the cured film of the development project. If a single monomer cannot meet the above requirements, two or more monomers with different functional groups should be selected and used in combination to obtain a photo-curing product formulation with the best comprehensive performance.