Oxygen inhibition problem and solution in UV curing

UV curing technology is a high-efficiency, energy- saving and environmentally friendly high-tech curing technology developed since the 1960s. It refers to the process in which the photoinitiator in the system absorbs ultraviolet light to generate free radicals under the irradiation of high-energy ultraviolet light as the curing energy, triggering a chain polymerization reaction between UV resin and monomer, causing the liquid phase system to polymerize, cross-link and cure instantly.

Since most photocuring processes are carried out in an air environment, oxygen molecules have a non-negligible inhibitory effect on free radical polymerization. Oxygen inhibition can cause the lower layer of the photocuring coating to be cured, but the surface is still uncured and sticky. As a manufacturer of photocuring resins, Bossin will talk to you about oxygen inhibition today.

The ground state of general substances is singlet state, but oxygen molecules are an exception. Its ground state is triplet state, which is essentially a diradical. Therefore, it has strong addition activity to the active free radicals produced in photoinitiation, forming peroxyl radicals that are inactive to vinyl monomers. This process is fast and can compete with the addition reaction of active free radicals to monomers, significantly hindering the polymerization process.

Does it feel hard to understand? Let me give you an example to explain. In the movie The Treasure Box, the Fire Cloud Evil God showed how to hold a bullet with his hand, but the bullet shot through his palm. If we compare oxygen molecules to bullets, fingers to active radicals, the process of holding the bullet with the hand is like the competition between oxygen inhibition and active radicals. The bullet passes through the fingers, just like oxygen inhibition wins. So how to resist oxygen inhibition? The same scene of holding a bullet with the hand, the Fire Cloud Evil God succeeded in Kung Fu. Just like what is described in the clip, as long as the speed is fast enough, the bullet can be held with the hand.

Back to what we said about UV curing, as long as the curing rate of the formula system is fast enough, it will be completely cured without waiting for the oxygen molecules to react, which means that some raw materials such as resin monomers and photoinitiators with fast curing speeds should be used in the formula. However, these raw materials usually make the cured film brittle and hard, and cannot meet the requirements of flexibility, etc. Are there any other methods of anti-oxidation and inhibition?

First, from the formula

1. Adding active amine

Active amines that can be used as co-initiators are generally tertiary amines with at least one α-H. The active amine alkyl radicals regenerated by the hydrogen abstraction reaction initiate polymerization. The alkoxy radicals released by the decomposition of alkyl hydroperoxide also have certain initiating activity for vinyl monomers, but its further hydrogen abstraction reaction seems to be more dominant. In some thin coating systems and processes with fast coating line speeds, such as paper varnish, adding active amines has become an important means to overcome oxygen inhibition in free radical photocuring formulas. However, active amines tend to yellow after curing, which is a major disadvantage of using active amines as an anti-oxidant inhibition method.

2. Adding mercaptan

Thiol is a general term for compounds containing -SH functional groups. As a hydrogen donor, thiol can capture peroxyl radicals while generating sulfur radicals, which can continue to initiate polymerization. Therefore, the addition of thiol can inhibit oxygen inhibition during free radical photocuring. In LED UV curing systems such as nail polish sealant and crystal glue, a certain proportion of thiol is added to improve surface drying and deep curing. However, thiol itself has a certain odor, poor storage stability with UV system, and relatively high price, which have become factors limiting its widespread use.

3. Cationic formula

Cationic polymerization uses the proton acid generated by cationic photoinitiators under light to catalyze the ring-opening polymerization of epoxy groups or the cationic polymerization of electron-rich carbon-carbon double bonds. Compared with free radical photocuring systems, cationic photocuring systems do not have the problem of oxygen inhibition. Cationic polymerization has been used in SLA 3D printing systems. However, the problems with cationic curing systems are slow photocuring speed, few raw material varieties, high prices, and great influence by temperature and alkaline atmosphere.

Second, from the perspective of technology

1. Lamination method

When the coating system is completed, a thin film is closely covered on it, and after UV light exposure, the film is peeled off. This layer of film isolates the air from the solidified material. Without oxygen, oxygen inhibition will naturally disappear. Currently, there is a mature application of UV laminating matte oil. However, this process requires special laminating equipment and should be used in a vacuum environment as much as possible, otherwise it will affect the film quality of the system.

2. Nitrogen protection

A relatively closed space is established in the UV curing system, and nitrogen is filled into it, which greatly reduces the proportion of oxygen, thereby reducing oxygen inhibition and increasing the light curing rate. It is used in 172nm excimer skin-feel system and EB electron beam curing system. Of course, this method also has the cost of equipment modification and nitrogen consumption.

The above are the most common methods of anti-oxidation and anti-polymerization. Each method has its advantages and disadvantages. The one that suits you is the best choice.