Lips are structurally very susceptible to drying. While lipstick is a makeup product, moisturizing performance is a high priority for most consumers. Kao conducted a survey and found that most women judge the moistness of their lipstick by how slippery and thick it feels when they rub their lips together. This inspired us to take a new approach to enhancing the moist feel of lipstick. Evaluating various commercial preparations, we identified some that imparted this kind of moist feel, but all of them had the fatal flaw of spinnability, a tendency to form viscous threads when pulled apart. In rheological terms, they exhibit the first normal stress difference (N1) in the high shear rate region, a property commonly illustrated by the Weissenberg effect (Figure 1). At Kao, we set about designing and developing a polymer with a higher N1 than any of the commercial preparations, yet exhibiting low spinnability. Taking advantage of the formation and destruction of intramolecular associations, we developed a long alkyl acylated cellulose (RCE) that behaves like an ultra-high-molecular-weight compound in that it has a high normal force, providing a moist feel, yet behaves like a low-molecular-weight compound in that the molecular associations break when subjected to large shear deformations (Figure 2).
This technology won an award from the Society of Rheology, Japan, in 2016, and it has been applied in our Sofina lipstick line.
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