TESTING 91 Indentation softness Force (g) Stringiness Stickiness
Softness=time to reach 5g
5g Time (s) Hold force Force hold time
Delay before first cycle
Cycle lenght
900 800 700 600 500 400 300 200 100 0
0 500 1000 1500 Time (sec)
Figure 2: Nail polish properties shown on a force / time graph generated by Exponent software following adhesive tests.
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. 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
Stickiness Purple gel Pink polish 2000 Figure 3: Nail gel is harder than polish when first applied. 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 If the polish and the probe lose contact immediately after the probe is withdrawn, there is no stringiness.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
faster 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
2000 2500 Purple gel Pink polish
1500
1000
500
0 0 500 1000 1500 Time (sec) Figure 4: Polish remains stickier for longer than gel. April 2018 PERSONAL CARE EUROPE 2000 2500
Stickiness (g)
Softness (ms)
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