Most UV cured polymers cure through a process called free radical polymerization, in which highly reactive free radicals drive a chain-growth reaction. However, early chain termination can occur when radical molecules react with airborne oxygen, resulting in an incomplete cure at air exposed surfaces.
This incomplete surface cure is commonly known as oxygen inhibition. Oxygen inhibited surfaces are normally tacky or sticky, which could present issues for sealants, encapsulants, and coatings.
This post is dedicated to the concept of oxygen inhibition and specifically aims to address the following topics:
- Why oxygen inhibition occurs
- Overcoming oxygen inhibition with changes in formulation
- Overcoming oxygen inhibition with production methods
- Overcoming oxygen inhibition with cationic alternatives
Why Oxygen Inhibition Occurs
In order to fully understand why oxygen inhibition occurs, lets first quickly review how free radical polymerization occurs. Free radical polymerization involves three stages: initiation, propagation and termination.
- Initiation of the polymerization process can be performed by a various types of compounds. In the case of UV curing products, this is compound is a photoinitiator. Photoinitiators produce free radicals (basically just molecules that have an unpaired electrons and are therefore highly reactive) upon exposure to radiation such as UV. The free radical that is produced during the initiation step reacts with the nearby monomers, converting them into free radicals as well
- As the monomer is now a free radical itself, it will also react with other monomers, forming a longer radical chain. This process is called propagation, in which monomers are constantly added so that the polymer chain becomes longer and longer.
- Termination is the final step in the free radical polymerization process and will halt the growth of the polymer chain. Termination can occur in many ways, such as the combination of two radical chains, or the transfer of the radical from the growing chain to the monomer. Termination can also occur when radical chains react with other molecules in the environment, such as oxygen.
It is important to recognize that termination is a natural component of free radical polymerization, however, early termination is an issue.
And of course oxygen is a major cause of early termination.
Molecular oxygen easily reacts with any free radical, including the initiator molecules. This quenching results in the initiator losing its radical and reactive state, which subsequently lowers the number of free radicals produced by the initiator, less initiated polymer chains, and of course a fewer number of final polymer chains produced.
The reaction of the initiator with molecular oxygen also produces an oxygen free radical. This oxygen free radical reacts with already existing propagating radicals, converting the propagating chains to oxygen based free radicals (this is the scavenging reaction). This resulting type of radical is less reactive, which further inhibits the polymerization process by producing lower molecular weight polymers.
As the presence of oxygen greatly deters free radical polymerization, oxygen inhibition has long since been considered to be a problem for polymers such as coatings that are cured through free-radical polymerization. In cases like this, an oxygen inhibition layer forms on the surface of these polymers, mainly composed of uncured monomers, which is why it is frequently referred to as a surface cure problem.
However, because oxygen inhibition is such a well known and addressed issue, many methods exist to overcome oxygen inhibition of UV curing products:
- Changes in formulation - UV curing adhesives and sealants can be formulated to reduce oxygen inhibition (some products are substantially better than others)
- Specific production methods - production methods exist that allow end users to control the degree of oxygen inhibition during production/ assembly.
- Cationic alternatives - UV curing products exist that cure via a cationic polymerization reaction. Cationic polymerization is not sensitive to oxygen.
The follow sections deal with each of these above methods.
Overcoming Oxygen Inhibition With Changes In Formulation
Free radical curing adhesives and sealants can be formulated to contain thiols, amines, or ethers, all of which contain easily abstractable hydrogen atoms that will react with the peroxy radicals formed by oxygen, reducing scavenging.
However, each of these chemicals have their own advantages and drawbacks:
- Thiols improve the thermal resistance of the compound, reduce the absorption of moisture, while also improving the adhesion. Unfortunately, using thiols results in the product having the distinct dislikeable odor that sulfur compounds possess.
- Amines, on the other hand, are quite inexpensive and also improve adhesion. However, after curing, formulation with amines may result in a yellowish color, as well as some residual odor, and the resulting product is quite sensitive to moisture.
- Ethers can be used in large volumes, however, they normally affect some of the properties of the coating, and they lower the temperature and water resistance.
Other options include the use of waxes, which migrate to the surface and form a barrier. Waxes, however, though quite inexpensive, also affect the final properties of the coating, and it takes quite some time for the wax to migrate to the surface of the coating.
An indirect way to reduce the effects of oxygen inhibition would be to increase the concentration of the photoinitiator, which of course, is easy to do, but then this results in more residuals and by-products being produced, as well as reduced coating properties.
The point of all this is: free radical polymer formulations are highly versatile, and, depending on your application, a product that meets your application requirements and exhibits a low level of oxygen inhibition probably either already exists or can be formulated.
Overcoming Oxygen Inhibition With Production Methods
From the end-user's perspective, oxygen inhibition can be reduced by controlling environmental variables, and manipulating the intensity/ wavelength of the UV light source:
- controlling environmental variable - Oxygen can be removed from the UV curing zone by performing the curing process in the presence of an inert gas, such as nitrogen, but then again, this is usually expensive and difficult to implement.
- UV light intensity - Increasing the light intensity also directly increases the amount of free radicals present, and very well does not affect the properties of the coating. However, doing this method usually energy intensive and costly.
- UV wavelengths - varying wavelengths can also be used to indirectly decrease oxygen inhibition during the curing process. The concept behind is that specific wavelengths tend to be absorbed at different sections of the coating. On the surface, shorter wavelengths, such as UVC rays, are commonly absorbed, whereas in the inner portion, near the substrate, longer wavelengths such as UVAs tend to be taken in. Following this trail of thought, mid-wavelength UVB rays are usually absorbed near the middle of the coating. Using these various wavelengths would therefore increase the overall exposure of the product to radiation, which also increases the amount of free radicals produced.
Although the above methods are generally more costly to the end-user, UV curing processes are still overall less expensive in many cases.
Overcoming Oxygen Inhibition With Cationic Alternatives
Last but not least, a final option would be to use cationic based products (such as epoxies), rather than use free radical polymerization based products (such as acrylate adhesives and sealants). Cationic polymerization products are produced in relatively the same way as free radical polymerization, involving the same steps of initiation, propagation and termination. However, the polymerization is initiated by a compound that produces cations, which attack the monomer units, and is propagated either by the continuous production of cationic chains. In this case, molecular oxygen does not interfere with the reaction process.
One requirement, however, is that the counterion produced by the initiator is non-nucleophilic. This is the reason why, even though these types of products are not affected by atmospheric oxygen, they can still be disrupted by the presence of moisture, which is a relatively strong nucleophilic compound.