Over the last few years, the nail polish industry has really opened up its beauty box of tricks, and on the market today, to name but a few, we see nail polishes with “magnetic, gel, matte, glitter, crackle, stamping, quick-dry, caviar, sponge, color changing, leather and sandy” effects. There are also nail polishes with kale, silk and nylon extracts! It really has turned into an art form. However, in order for us to appreciate the wonders of these magical nail polishes, there must be some exciting science behind it all?
I have always wondered about and been fascinated by these different effects and thought it would be a good idea to finally look into why stamping polish “stamps”, why crackle polish “crackles” and why UV nail polish “cures”…
Believe it or not, there are varying degrees of curing gel polishes; those that are displayed in your local retailer as ‘daylight curing’ polish, those ‘hybrid’ gels that can be bought from your local shopping center, and then, of course, those ‘soak-off’ gels in potted jars found only at your local beauticians! Both the hybrid and soak-off gels require the use of a UV lamp to cure. The difference being that the hybrid gels incorporate similar solvents and additives as regular nail polish does, allowing them to soak off faster, with a lower viscosity for easier application. Traditional gels have a higher degree of cross-linking, resulting in a greater resistance to acetone .
The Kinexus got its first manicure when we used it to study different colored UV-curing nail gels (of the potted, soak-off variety) in the glorious shades of gold glitter, red, pink and black. By applying a fixed intensity of UV light to the gels for 30 seconds we could monitor the curing profile and consequently change in modulus (stiffness) in these materials over time. Distinct differences were seen between gels with different pigments (see Figure 1).
The clear gel with gold glitter particles cured the fastest of all four gels, exhibiting a much higher modulus (~7.5 x 107 Pa) in comparison to the black gel, which cured more slowly, resulting in a much lower shear modulus (~3.7 x 105 Pa).
Figure 1. The curing profiles of different colored nail gels with time.
This result was confirmed by conducting a post-cure amplitude sweep on the gels. This measurement probed the linear viscoelastic region (LVER) of the samples by applying an increasing strain and determining the point at which the structure in the material was broken down – the onset of non-linearity. Figure 2 shows the results from this experiment and it can be clearly seen that the black gel has a longer LVER than the glitter gel.
“What does this mean?” I hear you ask….
Well, really this just means that the black gel will be more flexible on the nail and is more likely to be easily removed. The glitter gel will exhibit more brittle properties and may put up a fight when it comes to removal! It is instances like these where rheology can be employed as a useful tool in determining differences and characterizing products that will be noticeable and important to consumers!
Figure 2. Post-cure amplitude sweep of a gold glitter and black gel
This blog has focused on curing nail polish and the influence of pigment. However, the effects of rheology on nail polishes are never-ending; just like paints, nail polishes exhibit thixotropic properties, the properties that most importantly give you that nice smooth and even surface finish. Stay tuned for more information on this diverse application!
- Article “The Science of gels” by Sree Roy
- Webinar “Thixotropy! What is it? Why is it important? How do I measure it?” by Howard Barnes