* AGF = (GF0 – GF)
** AG = (G0 – G)
*** AHV = (HVQ – HV)
**** Characteristics in bold are in required range of close to that
Figure 3.17 Elastic modulus (E)and (%) elongation at break (ɛB) values for the coatings with 30% and 40% of monomers measured immediately after cure (0) and after 3 days of storage.
They also found that the human nail at 55% relative humidity has a Young’s modulus of 2.32 GPa. Systems containing TBCHA that they tested showed the most stable E values with the post-cure having no effect on the post-cured coatings. So, for the natural nail the coating should have at least an ɛB (elongation at break) value of 0.6%. As can be seen in Figure 3.17 TBCHA and HPMA fall within the range of the recommended 20-50% for a coating performance that was not too brittle yet not too soft for a coating to perform on the nail plate [21].
3.9 Adhesion of UV Nail Gel to the Human Nail Plate
Adhesion is another area of concern for the application of UV nail gel polish. In one aspect the user wants high adhesion for 2-4 weeks when subjected to dish washing detergents, hand cleaners, hand creams and a whole host of other chemicals that will cause the UV nail gel to lose adhesion. On the other hand, the user wants the UV cured coating to be removed quickly when fashion changes require it. Testing was accomplished by doing soak-off (in solution of 95% acetone and 5% water) that was performed for 72 hours and the results can be seen in Figure 3.18.
Figure 3.18 Correlation of gel fraction data (after 72 hours of soak-off) of the compositions with the adhesion loss time from the substrate in a solvent medium (95% acetone and 5% water).
The results correlated with the GF data obtained after the 72-hour of soak-off. The researchers found that the formulation containing the 30% THFA showed the quickest adhesion loss in 3 minutes. This result of the 30% THFA also correlates with the smallest GF value [21].
A very recent US Patent issued [22] teaches this new technique for improved adhesion to the human nail. As can be seen in Table 3.8 the improvement over the control shows that the addition to the base UV nail gel formulation of the polyhedral oligomeric silsesquioxane (POSS) increased the adhesion from 71% to 125%. The adhesion test was performed utilizing the Cross-Hatch Adhesion Test according to ASTM D3359 [22].
Table 3.8 Improved adhesion to the human nail using a UV curable nail gel formulation with the addition of polyhedral oligomeric silsesquioxane (POSS). Testing was performed using ASTM D3359 Cross-Hatch Adhesion Test.
| Formulation # | Average adhesion score | Improvement over Control |
| 1 | 3.0 | 71% |
| 1 Control | 1.75 | – |
| 2 | 2.25 | 125 |
| 2 Control | 1.0 | – |
3.10 Removal of the UV Nail Gel From the Human Nail Plate
UV nail gel technology has an issue with the removal from the human nail. All the technology developed that we have discussed so far relates to the ability to create a heavily crosslinked coating that can remain on the human nail for over two weeks without chipping or peeling [23]. However, this heavily cross-linked UV cured coating now results in a coating that is difficult to remove either by using a solvent soak or by mechanically grinding it away. Cook [24] has developed technology that allows the coating to UV crosslink but utilizes an ‘Achilles heel’ in the polymer
