use [55]. Hardness can be measured according to two common ASTM methods: ASTM D-1321 and ASTM D-937. These methods utilize needle penetration and cone penetration, respectively, in order to calculate formulation firmness in a quantifiable measure [56, 57]. Previous studies have indicated the use of a texture analyzer using a 500N load cell, needle penetration of 3.6 mm, velocity of 1 mm s-1, and a trigger force of 0.05N [58]. Although this method is able to provide some quantitative data, it lacks in its relation to product performance when in use by consumers. Human panels are typically used in order to subjectively assess the residual left on the lips and product feel after application. Because this value is subjective, it is subject to change and result in reproducibility issues, making it even more difficult to correlate lipstick hardness with customer satisfaction and performance. In efforts to alleviate this issue, a method and apparatus has been developed to better correlate hardness and deposition to user experience. This method studies different deposition rates of semisolid lipsticks when applied to a substrate under controlled conditions and relates the drag and deposition to consumer experience [59]. It is claimed that the results from this method relate to the consumers ‘mouth feel’ through the relation of consumer preference in terms of lipstick coverage with the deposition accomplished by the invented apparatus [59]. As a result, this creates an in vitro method which better correlates with results from human panels.
Hardness testing is only conducted for formulations that are in the stick format. In long-wear lipstick formulations, the isododecane acts as a good solvent for polyethylene wax; this wax is used to help soften the stick and is difficult to crystallize. These long-wear lipstick sticks, such as Revlon ColorStay and Maybelline SuperStay Ink Crayon Lipstick, are made slim to keep the stick softer without compromising ease of application and preventing breakage of the stick. Should a stick be wide and kept soft, it would not be able to support the stress of application and would be easy to break. Additionally, sticks with a larger diameter enable more evaporation of the volatile solvent due to its larger surface area, making the formulation tacky. In the hardness testing described above, a slim stick should have a readout of 70-100 gForce hardness, making it ideal to deposit a coating onto the lips that is not too thick.
1.3.2.2 Lipstick Wear Assays
1.3.2.2.1 Adsorption by Contact Angle Measurements
Adsorption testing of lipsticks provides an understanding of how oil and saliva adsorb onto the lipstick film over a period of time by looking at the spreading and wetting on the formulation sample. Such testing requires the use of a tensiometer, which is a machine used to measure the surface tension of liquids by dropping the liquid of interest (in this case olive oil or saliva) onto the film surface in the form of a sessile drop. This provides us with an idea of how resistant to oil or liquid the formulation may be; if there is a low value for the contact angle (Θ), the film surface is considered to be highly wetting. When the drop placed is water and the formulation has a contact angle greater than 90°, it is hydrophobic; a value less than 90° indicates the formulation has an affinity with water and can be characterized as hydrophilic. A contact angle less than 90° when testing with oil indicates the formulation is lipophilic and has an affinity with silicone.
1.3.2.2.2 Rub Test with Oil and Water
This test method is commonly used in order to determine a lipstick’s durability in regards to oils and water. Should a consumer eat greasy foods, the lipstick will become tacky or removed from the lips. In this case, the oil in the greasy food essentially acts as a make-up remover and breaks down the formulation. While resistance to grease is improved through the use of the MQ resin, products on the market today are not yet producing satisfactory outcomes when interacting with such oils. In vitro tests can be used to help predict the formulation resistance to such oils. Lipstick formulation is drawn down with 25 micrometer draw down bar on BIOSKIN, which is a urethane elastomer that is made to resemble human skin[60]. This substrate is commonly used to evaluate cosmetics due to its similarity, in terms of resiliency and feel, to human skin. The drawn down sample is left to dry at 37°C and 35%RH. In order to test oil resistance and waterproof qualities, a droplet with a controlled quantity of oil and water is then placed on the sample for ten minutes; after this point, the sample is manually wiped with a Kimwipe fifteen times, and a visual inspection is made in regards to the quality of the sample after wiping. The formulation passes the assay if there is no visual removal of the formulation after wiping with the Kimwipe. Figure 1.6 (left) shows what is considered to be a good and a bad sample after the test is completed. This method correlates to the challenges associated with eating and drinking in regards to lipstick durability. Long-wear lipstick formulations should have no removal after being wiped as no formulation should transfer.
1.3.2.2.3 Adhesion with Scotch® Tape
To determine the adhesion property of the formulation, the wear assay can also be completed with a Scotch tape, in which the tape is placed on top of the sample and removed at a 90° angle. A similar visual assessment is made to analyze the quality of the sample. These visual inspections are then translated to a ranked scale. Figure 1.6 (right) shows what is considered to be a good and a bad sample after the test is completed. The long-wear lipstick formula would ideally have a higher adhesion to the substrate rather than to the tape, preventing any formulation from being removed.
Figure 1.6 Wear assay results indicating satisfactory (left) and unsatisfactory (right) resistance to oil and formulation adhesion (tape test).
1.3.2.2.4 Adhesion Testing
Adhesion testing measures the adherence quality to a specific substrate after formulation application. This indicates how well a formulation will remain on a consumer’s lips over a period of time. The formulation is applied onto the BIOSKIN surface and left to dry. ASTM crosshatch tape is applied onto the film surface and removed at a 180° angle [54]. The amount of film removed with the tape is rated on a scale from 1-3, where 1 indicates no removal, 2 is some removal, and 3 is severe removal [54]. This scale relates back to how formulations are rated in consumer sensory testing. There are other means to measure the film adhesion; rather than using the ASTM crosshatch tape, two pieces of BIOSKIN can be used in contact with the formulation located in between. One piece of BIOSKIN can be peeled off either manually and the adhesion is evaluated by the ratings described above. Alternatively, a texture analyzer can be used to peel apart the BIOSKIN in order to quantify the force needed for removal.
1.3.2.3 Lipstick Cohesion Test
This method utilizes lipstick formulation drawn down on the BIOSKIN substrate using a 25µm or 75µm drawdown bar. The sample dries for about one to two hours at 37°C and 35%RH. Once dry, the BIOSKIN substrate 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.
