Surface Science and Adhesion in Cosmetics. Группа авторов
rutile (TiO2). d= Lucirin TPO absorbs above 365 nm to allow proper through-cure of the coating due to high pigment loading by rutile (TiO2)."/>
Figure 3.6 Absorption curves for photoinitiators used. a=Benzoinether absorbs in the 280 to 360 nm range, b=Irgacure 651 absorbs in the 280 to 360 nm range, c=Darocur 1173 absorbs in the 280 to 360 nm range and is blocked by the rutile (TiO2). d= Lucirin TPO absorbs above 365 nm to allow proper through-cure of the coating due to high pigment loading by rutile (TiO2).
The ability to cure coatings that contain up to 30% rutile ((TiO2)) has an impact on through-cure. The higher the pigmentation the less the chance for through-cure. In addition, a thicker pigmented coating also results in no through-cure. Using the same UV cure formulation, researchers have shown that by just increasing the energy density one can get better through-cure as shown in Figure 3.7 (a & b) using UV arc lamps at 200W and 300 W. By utilizing the addition of TL03 UV Lamp (Gallium-doped low wattage long wavelength fluorescent; Phillips 60 W) one enhances the deep penetration of the UV light into the coating which results in better through-cure and hardness development. Even better performance results are shown in Figure 3.7 (E-Gallium Doped & F-Iron Doped) and the use of high performance (300 W and 600 W) UV light sources results in the best through-cure and hardness development [4].
Figure 3.7 UV through-cure of various rutile ((TiO2)) formulations; Pendulum Hardness Values, sec vs. P/B (pigment to binder) ratio vs. thickness, µm. These formulations were compared against (a) UV Arc @ 200W & (b) 300W, (c) TL03 (GA-FL; 60W) @ 200W & (d) TL03(GA-FL; 60W) @ 300W, (e) Electrodeless V (GA) & D (Fe) @ 300W and (f) Electrodeless V (GA) & D (Fe) @ 600W UV light sources.
In the UV nail gel market the first UV light sources were gallium-doped low-wattage long wavelength fluorescent bulbs (GA-FL) that are close to Figure 3.7 (c & d) UV wavelength and energy levels except for the use of the UV arc lamp bulbs. Early work with these GA-FL bulbs allowed the formulator to cure oligomeric chemistries in about 3 minutes.
3.4 UV Cure Oxygen Inhibition Issues
As mentioned earlier, oxygen inhibition issues with UV cure will result in coatings that have poor surface cure. Work done on early UV cure systems looked at the depth of penetration that ambient air had on the problem of oxygen inhibition.
It is obvious from Figure 3.8 that oxygen in the uncured coating will result in less favorable cure potentially all the way down to 50 μm [5]. This potential problem resulted in development of several new techniques to solve this issue of oxygen inhibition. Some of the earliest techniques utilized paraffinic waxes that were of low density and would rise from within the coating to the surface, blocking the ambient oxygen. This technique works but the paraffinic wax needs to be ground and polished to result in a high gloss finish. Another old technique shown in Figure 3.8 was the use of free radical PIs in combination with allyl ethers in the presence of oxygen to form hydroperoxides to prevent oxygen inhibition at the surface. Amines and thiol-enes will also work in mitigating oxygen inhibition. However, amines result in yellow color shades especially in light colored coatings. Thiol-ene based coatings emit strong odor before, during and after UV curing and are not a preferred chemistry for UV cured nail gels. Other method to prevent oxygen inhibition is the use of inert gases such as nitrogen, argon and carbon dioxide.
Figure 3.8 Depth of penetration by ambient oxygen in a UV cure coating. This oxygen migration into the coating is the source of oxygen inhibition by quenching the chain propagation of the free radical. The depth of oxygen penetration can be as high as 50 µm.
3.5 Special Considerations for the Use of UV Nail Gel Technology Due to Oxygen Inhibition
As was mentioned in the previous section the concept of UV nail gels has the following concerns: oxygen inhibition, low energy UV light sources, and industrial hygiene (IH).
3.5.1 UV Nail Gel Cure Units: GA-FL and LED
Oxygen inhibition is even more difficult to overcome in the UV nail gel industry since the UV cure light units liberate only very low levels of UV whether it is FL or Light Emitting Diode (LED). The power output for GA-FL units is very low. The unit is pictured in Figure 3.9.
Wattage output for the GA-FL units is as low as 40W. In addition, the newer technique for UV curing of nail gels is the use of LED units that emit an even lower level of UV energy than the GA-FL UV cure units.
Power output for these UV cure LED nail gel units (Figure 3.10) is at 48W. Since now it has an even lower amount of UV energy to free-radically cure the coating, the oxygen inhibition issue becomes even more important.
3.5.2 UV Cure and Free Radical Oxygen Inhibition
To overcome these issues the formulator can look at the several chemical techniques to override the oxygen inhibition. As seen in Figure 3.11 when the free radicals are formed (see PI reaction in Figure 3.2) oxygen in the environment quenches the PI to an unexcited non-reactive state. This quenching lowers the number of polymeric chains and thus lowers crosslinking within the system. As can be seen in the first step (Figure 3.11) of the reaction, an oxygen-based free radical is formed which then seeks another free radical and chain terminates as shown in the second step. This classically results in an uncured coating surface [3].
Figure 3.9 Gallium-doped low-wattage long wavelength fluorescent bulb (GA-FL).