Surface Science and Adhesion in Cosmetics. Группа авторов
is stretched twice manually in both the X and Y directions. A visual assessment of the sample for cracking is made.
Figure 1.7 demonstrates what would classify as performing well with the stretch test (left), as compared to that which performed poorly (right). This test is used to mimic consumer experience through the movement of their lips to determine if the deposited film would result in cracks and other undesirable visual effects. Such movements may include talking, yawning, and smiling which could result in the lipstick wearing off. Cracking is rated on a scale from 1 to 5 depending on the severity of crack formations that were observed.
Figure 1.7 Lipstick stretch test results indicating no cracking (left) and severe cracking (right) after manual stretching.
1.3.2.4 Tack Testing
Tack is used to identify the quality of the formulation after application to a substrate and relates to what the consumer may feel in regards to stickiness either after application or after eating oily foods that may cause the formulation to degrade, in turn causing the formulation to run off or have a tacky feel. This method uses the Texture Analyzer and calculates the removal force of a 1/2” stainless steel ball probe after contact with the dried formulation [54]. The maximum force detected is considered to be the tack. This measurement is conducted five times on a sample and averaged in order to find the average tack force for the sample. This test can also be conducted after exposure to olive oil and saliva to determine the impact on the formulation, relating to product performance after eating or after the consumer licks her lips. Long-wear lipsticks that have a wide stick have a higher tack force as a result of higher evaporation of the volatile solvent.
1.3.2.5 Thermal Analysis of Lipsticks
Differential Scanning Calorimetry (DSC) measures the melting of the fats and oils in the sample to provide an idea of product performance [61]. This technique, referred to as fingerprinting, provides information regarding product wear and performance as a direct relation to peak shape, size, and temperature after being run on a single furnace, heat flux DSC [61]. We can look at the signature peaks from the DSC readout to help characterize the type of wax and expected consumer sensory. This method is ideal for long-wear sticks as the isododecane helps keep the stick soft and acts as a solvent for the different waxes in the formulation. Typically, liquid format long-wear lipsticks are not preferred for this method as there is a very small amount of wax in the formulation, making it hard to fingerprint.
A sample profile can be seen in Figure 1.8. In this experiment, 30mg of samples for lipstick formulations A, B, C, and D were placed in hermetically sealed aluminum pans and were ramped from -100°C to 100°C at a rate of 2°C/minute to obtain the full DSC profile of the sample [61]. The figure demonstrates that there are melting peaks between 25°C to 50°C, indicating that the samples will spread on warm surfaces, such as the lips [61]. The location of the peaks can help to identify the type of wax used in the formulation; for example, in Formulation D in Figure 1.8, we can conclude that different waxes that have a higher melting temperature and must be harder compared to Formulations A-C due to the location and intensity of the peaks. Peaks located at higher temperature indicate that the formulation may be more difficult to apply to the lip due to its lower temperature, but once applied will stay longer [61].
This provides further insight into the application process experienced by consumers. Softer formulations will melt on the lip during application but will still be hard enough to resist snapping when used; this provides a positive sensory experience for the consumer.
1.3.3 Visual Properties of Lipstick Formulations
1.3.3.1 Transfer Resistance Test
The transfer resistance test, also known as the “kiss” test, is a commonly used evaluation method in order to determine the transfer of product from the lip after lipstick application. This provides an indication as to what may be experienced by consumers in terms of transferring color to undesired places such as glassware, clothing, silverware, or other lips [3]. Not only does this issue result in color appearing in unwanted locations, but it also ruins the appearance of the consumer’s lips, resulting in an uneven coating of the lipstick. This method is an effective way to rank the transfer-resistant ability of long-wear lipsticks.
Figure 1.8 DSC profiles of lipstick formulation samples A-D [61].
The formulation is applied to a keratinous material, such as the human lips, and left to dry for about fifteen minutes. After this time, another substrate such as skin, artificial skin, lips, or glass is kissed. The amount of formulation transferred to the substrate is reviewed and ranked on a scale from little or no transfer to complete transfer. Should the formulation be transfer-resistant, then a majority will remain on the lips [4].
Figure 1.9 demonstrates the results from the kiss test by a sensory panel. In this study, lipstick was applied to a consumer panel in order to confirm the lastingness of the lipstick being investigated; after drying, four prints were made on a piece of white paper [58]. As seen in Figure 1.9, Formulation A had a much larger transfer amount compared to Formulations B and C; therefore, one could conclude that Formulation A is not as transfer-resistant.
Such studies help identify key differences between formulations and the impact these components have on durability, transfer, and lastingness. For example, the study above indicates the impact that the wax, butter, and copolymer type have on performance [58]. Not only does this information help formulators further optimize their compositions, but it also helps in regards to comparisons with competitor products. This method is a relatively quick and efficient way to help identify transfer in ideal conditions, without the influence of water or oil.
1.3.3.2 Gloss Measurement
Currently, most single-step long-wear lipsticks have a matte finish simply due to formulation components, but fashion and beauty trends may result in the consumer desiring other options for lipstick appearance; for example, some view glossy lips as an indication of health or youthfulness. During the formulation process, the level of gloss can be measured using a glossmeter, which measures the reflection from a surface by shooting a beam of light (the incident light) at a specified angle. The reflected light is then measured and the ratio of reflected light to incident light is taken as a measure of gloss.
In using this meter, the formulation is spread between fifty to five hundred micrometers in thickness on either a Leneta contrast card or BYK Opacity Chart and left to dry for twenty-four hours at room temperature [4]. Once dry, the sample is measured at 60° on a white background using a BYK Gardner glossmeter [4]. This measurement is conducted three times and averaged. In the event that the measured value obtained at 60° is too large, therefore meaning the sample is very glossy, the angle is adjusted to 20° in order to allow for an accurate reading. This analysis can also be completed after the application of olive oil or saliva to determine the impact on formulation appearance.