(1,12-Dodecane Dimethacrylate), (f) HA (Hexylacrylate) (g) DEGEEA (Diethylene Glycol Ethylether Acrylate), (h) PPGDA (Poly (Propylene Glycol) Diacrylate), (i) EGMEA (Ethylene Glycol Methylether Acrylate) and (j) DDA (1,10-Decane Diacrylate).
As one can see in Figure 3.14 the best choice for both nail gel UV-A cure light units (GA-FL and LED) is the bis-acylphosphine oxide (BAPO) PI. The BAPO PI cleaves to give a two-photon photo-bleachable free radicals once exposed to either the UV-A GA-FL or LED unit. This PI has wide wavelength overlap and will function at 365 nm, 380 nm and 390 nm. The photobleaching enhances the optical penetration and results in lower delta E (lower yellowing of the cured coatings) values. In addition to this performance shown in Figure 3.6 and Figure 3.7 the BAPO also helps in the curing of pigmented coatings as shown in Figure 3.15. Since BAPO PI activates at 365 nm, 380 nm and 390 nm colored pigmented coatings will through-cure since the pigments absorbance is below these wavelengths.
The ability to cure the systems that are pigmented results in selection criteria where high percent transmission is needed to penetrate the coating all the way to the substrate. This so-called through-cure needs to be understood especially when using different pigments. These pigments under this test criterion showed that through-cure can be rated as follows (easiest to hardest): white, red & blue, yellow and black [10].
It is also well known that selection of pigmentation that is transparent to UV-A energy will enhance the through-cure process.
Figure 3.14 BAPO (bis-acylphosphine oxide) as a two-photon photo-bleachable PI. The BAPO PI cleaves to give a two-photon photo-bleachable free radicals once exposed to either the UV-A- GA-FL or LED unit. This type of PI is the preferred PI since it activates at 365 nm, 380 nm and 390 nm that the UV nail gel uses in the GA-FL or LED light source.
Figure 3.15 Pigment selection. The through-cure window is important for UV nail gels in that pigmentation will prevent the through-cure. Since the GA-FL and LED UV cure units operate at 380 nm and above the BAPO PI will through-cure these pigmented systems. These pigments under this test criterion showed that through-cure can be rated as follows (easiest to hardest); white, red & blue, yellow and black [10].
3.7 Formulation of UV Nail Gels with 100% Solids UV Cure Oligomers and Monomers
We can now delve into the actual formulation of the so-called 100% solids UV cure oligomers and monomers. The following would be considered a baseline formulation that is pigmented or unpigmented.
From the formulation in Table 3.3 one can see that the (meth)acrylate polymer is used as a high molecular weight material that will free-radically cure within the system. The urethane (meth)acrylate oligomer has high molecular weight and high viscosity and will free-radically cure within the system. The mixtures of hydroxyalkyl (meth) acrylate, cycloalkyl (meth) acrylate and tri((meth)acrylate are all low viscosity and low molecular weight monomers that are used to dilute acrylate oligomers to the ‘use viscosity. These monomers will then all free-radically cure within the nail gel coating. When the PI is subjected to either the UV energy emitted from the GA-FL or LED light source the PI forms free radicals like those shown in Figure 3.14. These free radicals propagate the formation of the cross-linked coating with all of the (meth)acrylate species listed in Table 3.3. The proper PI selection criterion as described earlier will be a PI that is activated within the 320 nm to 420 nm range. This matching of the wavelength to the proper PI also has benefits that when the formulation described in Table 3.3 is pigmented the PI shown in Figure 3.14 will function properly above the absorbance of the colored pigmentation [11].
Table 3.3 UV nail gel formulation from patent WO 2017/217/217983. The base formulation for a so-called 100% solids formulation is based on (Meth) acrylic polymer (monomer), urethane (meth) acrylic oligomer, mixture of hydroxyalkyl (meth) acrylate and cycloalkyl (meth) acrylate, photoinitiator, antioxidant and colorant.
| Chemical name | wt % |
| (Meth)acrylic polymer | 50-60 |
| Urethane (meth)acrylate oligomer | 25-35 |
| Mixture of hydroxyalky1 (meth)acrylate and cycloalkyl (meth)acrylate | 10-15 |
| Tri((meth)acrylate) | 1 |
| Photoinitiator | 2 |
| Anitoxidant | <1 |
| Colorant | <0.05 |
3.7.1 Formulation of a UV Nail Gel Using a UV Cure Polyurethane Dispersion (UV-PUD)
Formulation of UV nail gel with UV cure polyurethane dispersions (UV-PUDs) is a newer technology that has evolved in the UV nail gel industry which relies on water-based technology. This technology has its roots in the development that instead of using (meth)acrylate monomers that were described in Figure 3.13 it uses water to reduce the ‘use viscosity’ of the system. This technology can be described as a UV-PUD and is unique in that its molecular weight can be greater than 200,000 daltons and yet has extremely low viscosity when diluted into water.
UV-PUDs are manufactured similarly to the 100% solids UV cure oligomers except for additional dispersion step and chain extension. In Figure 3.16 we show an aliphatic diisocyanate reacting with an aliphatic polyester polyol in combination with a diol dispersant, forming a diisocyanate prepolymer. This isocyanate prepolymer is then dispersed with a volatile tertiary amine in water followed by chain-extending the isocyanate prepolymer. This then results in the formation of an aqueous polyurethane dispersion. The final step is the removal of the acetone by way of a special distillation technique that separates the acetone from the water. These UV-PUDs are typically 40% solids in water [12]. Several patents and patent applications have been granted for the use of UV-PUD technology in UV nail gel industry. Two of these patents take the UV-PUD and combine it with a non-UV cure PUDs and limit the amount of TPO/BAPO PI described in Figure 3.15 [13, 14]. The benefits of these two patents are the low-level use of
