In a vast global cosmetics market forecast to reach US$ 675 billion by 2020,1 only the fittest can survive. That means outstanding consumer insight, creativity, constant NPD and a sharp focus on quality.

While these attributes are required throughout the category, it is especially the case when it comes to nail cosmetics. A longstanding aspect of the beauty industry, nail care has become a top priority for many consumers in today’s celebrity-obsessed, selfie-taking society. With demand growing incessantly, it is more important than ever to develop the perfect product quickly and ensure quality standards are never compromised. Using objective analysis techniques is a reliable and proven way to optimise formulations and maintain quality and consistency, so improving consumer experience, boosting loyalty and growing long-term sales.

Challenging formulations

Today’s nail care products have to meet a very demanding brief. Appearance is, of course, critical, but, ease of application, durability and comfort must also be taken into consideration. The attributes of the final nail care product are more aspirational than ever, and while new ingredients open up new opportunities, equally it can be difficult to develop products for some growing segments, such as ‘free-from’ and natural, due to limitations on the ingredients and components that can be used.These factors further emphasise the need for reliable and objective analysis techniques.

Traditional ASTM or ISO methodologies are difficult to apply to nail products as there are limitations on sample preparation and test methods. Instead, customised, innovative tests must be used to closely imitate both application and use. The latest techniques are carried out using a desktop texture analyser, which is fitted with probes appropriate to the tests being undertaken.

Nail polish: the star of the nail care category

Forecast to grow at a CAGR of nearly 7% to 2021,2 nail polish holds widespread appeal throughout the global cosmetics sector. ‘Fast drying’ is a property increasingly sought by demanding consumers but smudging is still a common complaint, especially in polishes with fewer volatile solvents. The Nail Polish Adhesion Rig enables formulators to measure drying time accurately, so helping identify the ingredients that will give a flawless, smudge-free finish.

Assessing nail polish performance at the time of application

Nail polish is poured into a 20 cm channel to a depth of 0.5 mm and subjected to ten successive adhesive tests. A 2.5 cm ball probe holds a specific force on the polish to allow a bond to form, and is then quickly withdrawn, breaking the nail polish bonds.

The force required to do this for each of the ten test sites is measured. The unique ball probe, a Community Registered design, has been designed to be rotated, while mounted, to the next available clean test surface providing efficiency and repeatability by avoiding the need to clean and dry between the ten tests. The adhesiveness of each sample clearly shows how quickly and thoroughly the polish dries. Results of a typical test are shown in Figure 1.

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Three important conclusions can be drawn from the graph generated by the texture analyser’s software:

• Adhesiveness (stickiness) of the polish: demonstrated by the maximum force needed to withdraw the probe from the sample. Figure 1 shows the first sample, which is still wet, has a very low adhesive force. As time passes, stickiness increases until it peaks, then drops off as the polish dries. After the highest adhesive force peak, polish is usually touch-dry.
• Polish softness: defined by the time the probe takes to sink into the surface of the polish before the force reaches 5 g. Softness decreases with drying time until it reaches a plateau, at which time is it touch dry.
• Stringiness: the extent to which polish adheres to the probe. If the polish and the probe lose contact immediately after the probe is withdrawn, there is no stringiness. A stringy sample will leave a force on the probe for a longer time.

These properties are depicted in Figure 2, which shows a zoomed in section of Figure 1.

Figures 3 – 5 show these properties for two different nail product samples – a pink polish and a purple gel. The gel is more desirable than the polish in every case: it dries aster and hardens more quickly, so will be less prone to film damage when a wet layer of polish remains under the touch dry surface.

Evaluating dry nail polish

Once nail polish has been applied and is being worn, stiffness and toughness are the two textural characteristics that matter most. If the product is too stiff and brittle, it will break when the nail bends – opening a soft drink can, for instance. As the nail bends, the polish must be flexible enough to follow it, and must also show high ‘strain to failure’ to avoid breaking under deformation.

These properties can be measured using a biaxial tension method. A 3 cm x 3 cm film sample is cut from a strip that has been set in a purpose-built mould. The sample is placed in a film support rig, with a circular clamp surrounding a 1 cm diameter circular aperture. A 5 mm stainless steel ball probe 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 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 multihole 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.

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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 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

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