mechanical behavior of the nail is another important factor in understanding how to coat the human nail. Grigale-Sorocina et al [21] evaluated the mechanical properties of the human nail. The formulations in the test conformed to the requirements of the EU Cosmetic Regulations. The formulations included the following: tri-functional urethane acrylate oligomer and a hexa-functional urethane acrylate oligomer. The viscosity was achieved by the addition of the following acrylate monomers for the proper ‘use viscosity’ at various levels: tertisobutyl cyclohexyl acrylate (TBCHA), ethylene glycol dimethacrylate (EGDMA) tetrahydrofurfuryl acrylate (THFA) and hydroxypropyl methacrylate (HPMA). The PI used was TPO that has been shown in Figure 3.15. The UV nail gel coating should match the natural nail plate mechanical properties so that there is no loss of adhesion. Mechanical stress due to shrinkage in the applied UV nail gel coating is the normal cause for this adhesion loss.
As can be seen in Table 3.7 Grigale-Sorocina et al. [21] evaluated different monomers and looked at the gel fraction, surface gloss and micro-hardness of the UV cured coatings. They varied the monomer concentration by 30 and 40%. Several of the formulations met the nail coating application viscosity of 3,000 to 4,500 cPa.s. Degree of conversion as shown as gel content was the highest with the formulation using monomer EGDMA with gel fraction (GF) of 96.4% to 98.5%.
Table 3.6 Surface free energy of untreated, hydrated and abraded nail plates.
| Liquid combination used | State of the nail | Total surface energy (mJ/m2) γ | Surface energy components (mJ/m2) | |||
| Lifshitzvan der Waals γLW | Acid-base (polar) γAB | Acid i.e. electron acceptor γ+ | Basic i.e. electron donor γ | |||
| WFD | untreated | 35.5 ± 4.7 | 34.0 ± 3.9 | 1.6 ± 4.0 | 0.4 ± 0.9 | 11.0 ± 7.0 |
| hydrated | 34.2 ± 3.6 | 33.6 ± 3.8 | 0.7 ± 3.3 | 0.5 ± 1.0 | 11.8 ± 8.7 | |
| abraded | 39.2 ± 3.9 | 37.0 ± 4.2 | 2.2 ± 3.9 | 0.7 ± 1.1 | 9.5 ± 6.5 | |
| WGD | untreated | 32.6 ± 6.2 | 34.1 ± 3.9 | -1.3 ± 5.6 | -0.1 ± 1.3 | 13.7 ± 7.6 |
| hydrated | 31.5 ± 6.0 | 33.4 ± 3.8 | -1.9 ± 4.9 | -0.2 ± 0.6 | 15.0 ± 8.2 | |
| abraded | 32.9 ± 4.8 | 37.0 ± 4.0 | -4.0 ± 5.4 | -0.1 ± 1.4 | 15.4 ± 9.2 | |
Table 3.7 Monomers effect on the gel fraction (GF), surface gloss (G) and micro-hardness (HV) of the UV cured cross-linked coatings. Monomers 1-TBCHA, 7-HPMA and 8-HPMA fall into the ɛβ (elongation at break) range of 20-50% so that they exhibit reduced brittleness. AGF=GF0 (is after 72 hours of storage) –GF (is immediately after cure).
| Monomer | Conc. | Gel fraction | Surface gloss | Microhardness | |||||||
| GF | GF0 | AGF*/GF0 x 100 | G | G0 | AG**/G0 x 100 | HV | HV0 | AHV***/HV0 x 100 | |||
| % | % | % | % | GU | GU | % | MPa | MPa | % | ||
| 1 | TBCHA | 30 | 97,4 | 98,0 | 0,61 | 46 | 68 | 32 | 104 | 144 | 38 |
| 2 | TBCHA | 40 | 94,0 | 94,1 | 0,11 | 51 | 72 | 29 | 69 | 83 | 20 |
| 3 | EGDMA | 30 | 98,5 | 99,3 |
0,81
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