3.10 UV Cure Light Emitting Diode (LED) nail gel Unit.
3.5.3 Methods for Mitigating Oxygen Inhibition During UV Cure
Numerous techniques have been used to mitigate oxygen inhibition. As shown in Table 3.1 these oxygen mitigation techniques offer several methods that work only to a certain degree [6].
Figure 3.11 Chain termination of the free radical initiator (see PI in Figure 3.2.) by oxygen. When the free radical is formed, oxygen in the environment quenches the PI to an unexcited non-reactive state. This quenching lowers the number of polymeric chains and cross-linking within the system. As can be seen in the first step the reaction forms an oxygen based free radical which then seeks another free radical and then chain terminates as shown in the second step. This classically results in the formation of an uncured surface.
Table 3.1 Methods to Mitigate Oxygen Inhibition showing Advantages and Disadvantages. These techniques can include, but are not limited to, the following methods: uses of inert gases, waxes, coatings, increased PI concentration, increased light intensity and use of thiols, amines and ethers.
| Method | Advantages | Disadvantages |
| Inert Gas | Does not adversely affect coating properties | Expensive; difficult to implement |
| Waxes | Inexpensive | Affects final coating properties; time needed for migration |
| Films | Good solution when film becomes part of product | Cost/disposal of film when not part of product |
| Increased PI Concentration | Easy to implement | Increased residuals/ by-products; reduced coating properties |
| Increased Light Intensity | May not affect coating properties | Part of existing equipment; cost |
| Thiols | Improved thermal resistance; reduced moisture absorption; improved adhesion | Odor |
| Amines | Inexpensive; possible improved adhesion | Yellowing upon or after cure; residual odor; moisture sensitivity |
| Ethers | Can be used in large quantities | Affects coating properties; reduced temperature resistance; possible reduced water resistance |
Inert gases work quite well since the coating is in absence of oxygen so no oxygen inhibition can occur. In fact, electron beam curing will only work when done in an inert atmosphere. Paraffinic waxes work well also but complicate the issue with gloss since paraffinic waxes migrate to the surface and are low gloss. Two additional steps are needed to bring the UV cured coating back to its original gloss by first grinding away the wax and then buffing and polishing the surface back to a high gloss. Barrier coatings work but again add an additional step in the process and are problematic on contoured surfaces. Increasing the PI concentration is also an easy fix but it is costly and can reduce the coating properties. Also increasing the UV light intensity can help override the oxygen inhibition issues but could result in coating degradation. Thiolene based acrylate (mercapto acrylate) is currently being used in the UV cure industry but results in ‘post odor’ of the UV cured coating. Amines are also used but result in yellowing and ‘post odor’ of the UV cured coating. Ether acrylate-based chemistries are used but might result in poor performance properties of the cured coating.
3.5.4 Combinatorial Chemistry Technique Used to Mitigate Oxygen Inhibition for Low Energy UV-A Cure Resulting in Tack-Free Surfaces
As can be seen from above that oxygen inhibition is an issue that needs to be reconciled if one is going to cure UV nail gels. As was shown in Table 3.1 the increase of PI concentration can help override oxygen inhibition. However, Arceneaux does not mention the most efficient type of PI that should be used to obtain the maximum efficiency of the PI as well as overriding the oxygen inhibition issue [6]. Besides the PI issue, the types and variety of oligomers and monomers used in the formulations can have a dramatic effect on cure. Such a complex problem was essentially solved by researchers in the UV cure automotive refinish industry where the same issue was faced in developing a UV cure product using only UV-A light sources and formulations that could override the oxygen inhibition issues.
As can be seen in Table 3.2 a screening was done using 6 independent factors, namely UV curable resins, reactive diluents, PI, PI levels, irradiation time, and distance from the lamp. In this evaluation the so-called Edisonian method meets the combinatorial world of chemistry. In this analysis, over 15,000 coatings were evaluated looking for the sweet spot and synergistic effects between these 6 independent factors.
It is obvious from Figure 3.12 that the only way to obtain the full surface cure (and the minimization of oxygen inhibition) of a UV-A system is to run massive amounts of tests to see if a combination or synergistic effect can be found. Figure 3.12 x-axis shows the PI and PI combinations that were evaluated. Also in Figure 3.12 the y-axis shows the oligomers and oligomer combinations that were evaluated. In this test protocol the R2/R5 (oligomer) along with IRGACURE®819 showed a synergistic effect. Even though the test resins were evaluated in the past, the fact they cured as well as they did without surface tack (i.e. surface inhibition by oxygen) was a surprise. All of this testing was done on clear coatings followed by evaluation of pigment systems. The pigmented systems were evaluated at a P/B (Pigment to Binder ratio) = 0.8, ≤75 µm dry, 2 minutes under a 250 W UV-A lamp and 25 cm distance. The resultant primers had no surface tack and required no surface solvent wipe before the traditional sanding step.
Table 3.2 Factors and levels covered in the search for formulations exhibiting tack-free surfaces (due to oxygen inhibition) when UV cured using a low intensity 250 W UV-A lamp. The screening was done using the following 6 independent factors: UV curable resins, reactive diluents, PIs, PI levels, irradiation time and distance from lamp.
The development work also revealed in these papers [7, 8] that showed the development of a one-component (1K; komponente; Ger.) UV-A curable clear coat. Issues that these papers raised regarding developing this 1K