Surface Science and Adhesion in Cosmetics. Группа авторов
clear coat are: 1) lack of flexibility in polymers based on radical polymerization, 2) keep the unreacted double bonds very low(≤10%) due to post-cure issues, 3) formulation and coating color due to visible light PI, and 4) primer, base coat and clear coat compatibility [7, 8].
Curing a UV nail gel with low wattage GA-FL or LED UV certainly needs more in-depth laboratory work as shown by these researchers [7, 8].
3.6 How to Formulate a UV-A Cure Nail Gel
With the obvious issues stated in section 5 the formulator needs to be cognizant of the following questions,
1 a. Which oligomers should one use to give the best performance during cure with the least amount of oxygen inhibition?
2 b. Which monomers should one use to give the best performance with the minimum amount of oxygen inhibition?
3 c. Which PIs should one use to utilize the wavelength emitted by the GA-FL and LED light sources?
4 d. From an industrial hygiene (IH) standpoint which are the safest acrylate monomers that should be evaluated in developing a nail gel UV-A cured system?
As was shown earlier in Figure 3.12 the type of oligomer used will have a strong impact on cure and performance. Targeting oligomers that respond best to low wattage GA-FL and LED units should be the formulation chemists top priority.
Figure 3.12 High throughput primary screening results based on the evaluation of over 25,000 coatings followed by a statistical analysis. The x-axis shows the PI and PI combinations that were evaluated. The y-axis shows the oligomers and oligomer combinations that were evaluated. Shown is the average predicted surface cure for all resin-photoinitiator combinations after curing using a 250 W UV-A light source. The average is taken over all other parameters screened in this experiment. Thus, each circle represents an average of 48 values (3 reactive diluents, 2 photoinitiator concentrations, 4 irradiation times and 2 lamp distances). The bigger the circle the better the surface cure.
3.6.1 Formulating with (Meth) Acrylate Monomers
Selection of the acrylate monomer needs to be carefully considered. As can be seen in Figure 3.13 a multitude of acrylated monomers are available to the formulator.
It was found [9] that compared to acrylates, methacrylates are much less sensitive to oxygen inhibition. Model compound systems were evaluated to determine the effect of ether groups on polymerization inhibition. They found that the reduction of oxygen inhibition occurs by a series of chain transfer/oxygen scavenging reactions [9].
3.6.2 Formulating with the Proper Photoinitiator
The ability to cure the coating with low intensity UV-A light sources requires PIs that operate within the wavelength of the GA-FL and LED units available in the cosmetic marketplace. As was shown in Figure 3.6 and Figure 3.7 the selection criteria for pigmented UV-A formulations require that the PIs used operate at 380 nm and above.
Figure 3.13 Chemical structures of methacrylates and acrylates that could be considered in UV nail gel technology. The following structures are: (a) DEGDMA (Di (Ethylene Glycol) Dimethacrylate), (b) TEGDMA (Tri (Ethylene Glycol) Dimethacrylate), (c) PEGDMA (Poly(Ethylene Glycol) Dimethyacrylate), (d) PPGDMA (Poly(Propylene Glycol) Dimethacrylate, (e) DDDMA (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.