A large portion of UV-curing formulations cure through a free radical curing process. In this process, UV waves from a light source trigger a photoinitiator to release free radicals, which then start a chain reaction with reactive acrylate groups on monomers and oligomers to create a cross-linked polymer.
However, these free radicals can also react with oxygen in the air and form very stable peroxy radicals, preventing the radicals from starting the crosslinking chain reaction. This phenomenon, known in the industry as oxygen inhibition, can result in a coating or adhesive that cures beneath the surface where the amount of diffused oxygen is low but remains tacky at the surface where it is exposed to oxygen-rich air. So, if this is going on, why is UV curing still such a rapidly growing technology? Despite this technical challenge, chemists can use different formulation and process tricks to reduce this effect and still obtain good surface cure.
Some chemistries can interact with oxygen and/or peroxy radicals in different ways to reduce the effect of the oxygen inhibition or surface cure. Some of these are listed below:
Thiols or mercaptans
Thiols or mercaptans are the most effective, pound for pound. They can reduce water absorption and may help adhesion. Thiols can also be used to reduce hardness, lower modulus, lower tensile strength and increase elongation. Unfortunately, thiols contain sulfur and the sulfur can impart an unpleasant smell.
Amines are slightly less effective than thiols at reducing oxygen inhibition but are low cost and may help adhesion. The effectiveness of amines is very dependent on the photoinitiator package used. Some challenges with amines are the risk of yellowing during curing and after curing, the potential to increase moisture sensitivity, and the possibility of residual odors.
Ethers are the least effective and need to be used in larger quantities, which can impact the coating properties. They can also reduce the temperature and water resistance.
Higher functional components react quicker, giving the free radicals less time to interact with the oxygen. Also, acrylate groups react quicker than methacrylate groups. Both will affect the properties of the final coating. Higher functionality will generally increase the hardness, reduce flexibility, and increase shrinkage. Switching from methacrylate to acrylate will lower the Tg and increase shrinkage.
More photoinitiator can generate excess free radicals, creating a sacrificial amount to form peroxy radicals. Increasing the photoinitiator concentration can also generate free radicals faster than the oxygen can diffuse back into the coating. This can improve the surface cure, but too much photoinitiator can reduce physical properties and can be cost-prohibitive. Another approach is to look at alternate photoinitiators that may be less prone to oxygen inhibition.
Combinations of these formulation approaches may be used to limit the negative effects of oxygen inhibition.
By modifying the application and curing process, you can overcome some of the negative effects of oxygen inhibition.
The interaction of the UV-source wavelength and the different photoinitiators used can affect the surface cure. Long wavelengths are absorbed into the coating and provide better depth of cure. Short wavelengths are absorbed at or near the surface, which can improve the surface cure speed and reduce the impact of oxygen inhibition. If the curing system being used has broad-spectrum lamps, the photoinitiators in the system must be tuned to absorb in the shorter wavelengths, i.e. 290 – 370nm range. Because LED lamps emit longer wavelengths in the UVA range – 365, 385, 395, or 405nm – using these systems can still present challenges to achieving a good surface cure.
The faster the coating cures, the less time the oxygen has to interfere with the curing reaction. By increasing the light intensity, the reaction proceeds more rapidly. Rearranging the curing process to decrease the distance from the lamp source to the formulation being cured can increase intensity. However, in most cases, an increase in intensity can only be accomplished with more lights or higher intensity lights, which can be cost-prohibitive.
Removing oxygen from the environment where the curing is occurring is the most effective method of eliminating the inhibition, but it is the most expensive and can be difficult to accomplish. The most common way to achieve this is to provide a localized nitrogen blanket over the cure zone.
Another way to exclude oxygen is to cover the surface with a transparent film before curing. This technique works well, but the film can restrict the amount of light or certain wavelengths of light passing through. The film must be removed or left on as part of the product. Waxes have been used to accomplish this exclusion, but they must migrate to the surface to be effective. They also will affect the performance of the coating.
Different combinations of these formulation and process tips can be used to help prevent or reduce the impact of oxygen inhibition.