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The Oxygen Inhibition Problem in UV Curing and Its Solutions
Release time:
2025-01-10 17:01
Ultraviolet (UV) curing technology is a high-tech curing method developed since the 1960s. It is an efficient, energy-saving, and environmentally friendly process in which, under irradiation by high-energy UV light serving as the curing energy source, photoinitiators within the system absorb the UV light and generate free radicals, which in turn trigger a chain polymerization reaction between UV-curable resins and monomers, causing the liquid-phase system to undergo polymerization, crosslinking, and curing in an instant.
Since most light-curing processes are carried out in an air environment, oxygen molecules exert a significant inhibition effect on free-radical polymerization. This oxygen-induced inhibition can result in a situation where the lower layers of the light-cured coating have already solidified, while the surface remains uncured and sticky. As a manufacturer of light-curable resins, Boxing New Materials would like to take this opportunity today to discuss oxygen inhibition with you.
The ground state of most substances is a singlet state, but the oxygen molecule is an exception—it has a triplet ground state and is essentially a diradical. Consequently, it exhibits strong additivity toward active free radicals generated by photo-initiation, forming peroxy radicals that are inert toward vinyl monomers. This process occurs at a relatively high rate and can compete with the addition reaction of active free radicals to monomers, thereby significantly hindering the polymerization process.
Does it feel hard to understand? Let me give you an example to illustrate: In the movie "The Legend of the Condor Heroes," Huo Yun Xie Shen demonstrates the feat of catching a bullet with his hand—but the bullet actually pierces right through his palm. If we liken oxygen molecules to bullets and our fingers to active free radicals, then the act of catching the bullet with one’s hand is akin to the competition between oxygen inhibition and active free radicals. The bullet passing through the finger is just like oxygen inhibition emerging victorious in this battle. So how can we counteract oxygen inhibition? Take the same scenario of catching a bullet with your hand again—this time, in kung fu, Huo Yun Xie Shen succeeds. As depicted in the clip, as long as your reaction speed is fast enough, you can indeed catch a bullet with your bare hand.
Returning to the topic of light curing, as long as the formulation system has a sufficiently fast curing rate—fast enough that oxygen molecules don't have time to react before the curing process is complete—this means that the formulation must incorporate certain raw materials, such as fast-curing resin monomers and photoinitiators. However, these raw materials typically result in a cured film that’s relatively brittle and rigid, making it unable to meet requirements for flexibility and other properties. Are there any other methods available to counteract oxygen inhibition and polymerization?
First, from the perspective of formulation:
1. Add active amine
Active amines that can serve as co-initiators are generally tertiary amines with at least one α-H. The active amine alkyl radicals generated through hydrogen-abstraction reactions initiate polymerization. Alkoxy radicals released from the decomposition of alkyl peroxides also exhibit some initiating activity toward vinyl monomers; however, their subsequent hydrogen-abstraction reactions appear to be more dominant. In certain thin-coat systems and high-speed coating processes—such as paper varnishing—adding active amines has become an important approach for overcoming oxygen inhibition in free-radical photopolymerization formulations. Nevertheless, active amines tend to yellow easily after curing, which represents a significant drawback when using them as an anti-oxygen-inhibition method.
2. Add thiol
Thiols are a general term for compounds containing the -SH functional group. As hydrogen donors, thiols capture peroxide radicals while generating sulfur radicals, which can in turn continue to initiate polymerization. Therefore, the addition of thiols can effectively inhibit oxygen inhibition during free-radical UV curing. In LED UV-curing systems such as nail polish topcoats and crystal glue, a certain proportion of thiols is added to enhance surface drying and deep-cure performance. However, thiols themselves have a distinct odor, exhibit poor storage stability in UV systems, and are relatively expensive—factors that have thus far limited their widespread use.
3. Cationic formulation
Cationic polymerization utilizes a cationic photoinitiator that, upon irradiation, generates a proton acid to catalyze the ring-opening polymerization of epoxy groups or the cationic polymerization of electron-rich carbon-carbon double bonds. Compared with free-radical photopolymerization systems, cationic photopolymerization systems do not suffer from oxygen inhibition. Cationic polymerization has already been employed in SLA 3D printing systems. However, cationic curing systems face several challenges: slow curing speed, limited availability of compatible raw materials, relatively high costs, and significant sensitivity to temperature and alkaline atmospheres.
Second, from a process perspective.
1. Laminating Method
After the coating system has been fully applied, a thin film is tightly adhered over it. Once exposed to UV light, the film is removed. This film effectively isolates the air from the cured material; without oxygen present, oxygen-induced polymerization inhibition naturally ceases. Currently, UV-coated matte oils have found mature applications. However, this process requires specialized coating equipment and ideally should be carried out in a vacuum environment; otherwise, the film-forming quality of the system could be compromised.
2. Nitrogen protection
In UV curing systems, establishing a relatively sealed space and filling it with nitrogen significantly reduces the oxygen concentration, thereby minimizing oxygen inhibition and enhancing the rate of photo-curing. This approach is applied in 172nm excimer laser curing systems and EB electron-beam curing systems. Of course, this method also incurs costs associated with equipment modifications and nitrogen consumption.
The above are the commonly used methods for antioxidant and polymerization inhibition. Each method has its own advantages and disadvantages; the best choice is the one that suits you best.
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