92 TESTING Stringiness Purple gel Pink polish
4000 3500 3000 2500 2000 1500 1000 500 0
0 500 1000 1500 Time (sec) Figure 5: Gel is more stringy than polish during drying, but dries faster.
is driven down into the sample, applying force until rupture. Stress and strain are recorded, from which the sample’s mechanical properties can be determined. The higher the force required to puncture the film, the stronger and stiffer it is. Toughness can be assessed by measuring the area under the force / time curve. Some manufacturers may be interested in testing the same batch of nail polish daily over the course of a week (or more) to ascertain how mechanical properties change over time. It is not uncommon for strength to increase, the longer the polish has been out of the bottle. The ideal nail polish formulation will have consistent toughness values over time, low stiffness and a high failure strain.
Film features One of the most recent products to enter the nail care category is the adhesive nail wrap. Quickly gaining in popularity thanks to their creative designs and longevity, nail wraps must be honed in terms of both their design and functionality. High initial adhesion
2000 2500
600 400 200 0
-200 -400 -600 -800 -1000 -1200 -1400 -1600
Time (sec) Figure 6: Comparison of adhesive forces of two nail wraps.
is vital to ensuring the manicure will last, and there must be no weak spots in the adhesive. Adhesive force, work of adhesion and adhesive stringiness can be measured – and different samples compared – using a multi- hole indexing plate attached to an adhesive indexing system. Ten wraps are applied to the underside of the plate, and a 7 mm domical probe is brought into contact with the first one. A specific load is applied for three seconds, after which the probe is quickly pulled away and the adhesion forces are measured. Graphs showing the adhesive force of two different nail wrap samples are shown in Figure 6. The higher adhesive force of the black sample shows it is less likely to fail during wear. Wraps’ tensile properties should also be
measured to give an indication of their flexibility, likelihood of tearing during application and strength. Rubber-faced tensile grips provide a surface with a good grip to prevent the wrap slipping during testing. A 10 mm wide sample is loaded into the grips with a 5 mm spacing. The
2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0
-200 0 5 10 Figure 7: Tensile test on adhesive nail wrap. PERSONAL CARE EUROPE April 2018 15 Time (sec) 20 25 30
grips then pull apart, initially stretching the wrap and ultimately breaking it. Results of a typical test are shown in Figure 7.
Conclusion
As innovation continues apace in nail cosmetics, it is more important than ever to develop and launch quality products quickly. Maintaining that consistent quality is critical to repeat purchases, brand loyalty and sales. Texture analysis is a proven and trusted science, already adopted by some of the world’s leading cosmetic brands, that ensures standards are maintained and products succeed in this diverse, dynamic market.
. PC
References 1. Global cosmetics market - by product type,
ingredient, geography, and vendors - market size, demand forecasts, industry trends and updates, supplier market shares 2014- 2020, Oristep Consulting, June 2015
2. World Nail Polish Market research report, ICRWorld, January 2017
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