Surface Science and Adhesion in Cosmetics. Группа авторов
potential irritants from lipstick is mainly by swallowing, such as after a consumer licks their lips. Currently, color additives must have FDA approval for its intended use, as many can contain traces of lead as an impurity. Following an investigation effort in 2007, the FDA determined that up to 10 parts per million (ppm) of lead in lipsticks would not pose a health risk.
The long-wear lipstick market can be classified into four key categories, each of which has its own benefits and appeals to consumers: long-wear, gloss, lasting lip gloss, and lip care. Long-wear lipstick, which was also advertised as transfer-resistant, was introduced in the cosmetics market by Shiseido in 1986 as a solution to problems associated with wear and movement experienced by a majority of lipstick users [2]. Functionally, lipsticks are expected to bring instant gratification in regards to the user’s appearance, regardless of style and color. To accomplish this, an ideal lipstick is expected to be non-drying, provide sun protection, and have great wear, color, and shine. Wear of lipsticks shall be mentioned further throughout the chapter and can hereby be formally defined as the user’s experience as a whole, consumer perception of performance and comfort, and formulation lastingness. Lipsticks should also be easy to apply on the lips and leave a thin film deposit. Early iterations of lipstick technology did not withstand the challenges associated with consumer use and wear, which, in turn, left the consumers desiring a formulation that was able to weather their lifestyle without reduced performance or need for re-application. As a result, long-wear lipstick, substantiating claims of non-transfer, lasting color, and no smearing/smudging, was introduced to the public in the form of a two-part kit. In this kit, the first layer, a pigmented lipstick, was intended to deposit color on the lips and was to be covered by a secondary overcoat containing film-forming polymers [3]. It is important to note the key difference between lipsticks described as long-wear versus transfer-resistant. Long-wear lipstick refers to the coloring remaining the same or visually consistent from the time of application over a period of time [4]. Transfer-resistance relates to the ability of the product not to be transferred onto a secondary surface upon contact. This would relate to the formulation being removed from the consumer’s lips after touching another surface, such as a glass. However, most of the time these terms are used interchangeably with consumers.
A key challenge in lipstick development is attaining an acceptable adhesion to the skin and semi-mucous areas around the lip region. After application, products can tend to migrate into cracks in the lip due to poor adhesion, creating an uneven coating and coloring over time [5]. Efforts to alleviate this problem resulted in the use of a pressure-sensitive silicone resin with MQ units (trimethyl endcap and four-way branch point units). This MQ resin, as seen in Figure 1.1, is commonly used to boost lip adherence, but the material itself is brittle, thus requiring a plasticizer in order to allow for film formation [2, 3]. This plasticizer contributes to the formation of a pliable film. Interactions with other components of the formulation, such as pigments, solvents, and fillers, also enhance product adhesion to the skin [2]. Most long-wear cosmetics utilize MQ resin technology because its surface free energy is similar to that of the human skin [2]. Skin surface free energy is a key factor for a cosmetic’s wettability, adsorption, and adhesion [6]. This key characteristic is also what permits the technology to allow for better wear and adhesion to the consumer’s lips. In formulations of products like Revlon ColorStay and CoverGirl, the MQ resin is used in conjunction with poly(dimethylsiloxane) (PDMS) gum to enhance the tack of the final product [2]. Additionally, initial versions of these non-transfer technologies utilized a volatile solvent with low viscosity PDMS or high viscosity gum in order to deposit a non-transferrable film upon solvent evaporation [2]. L’Oreal further developed the non-transfer MQ resin technology by introducing the use of a thermoplastic elastomer as a replacement for the PDMS gum and oil, in turn enhancing the product durability and adhesion to the lip [2]. This was incorporated into Maybelline Superstay 24 and L’Oreal Infallible lipsticks, permitting a wider range of motion of the lips and longer contact with a secondary surface, such as a drinking glass, without sacrificing the durability and appearance of the formulation [2].
Figure 1.1 Structure of the MQ resin used to enhance formulation adhesion [2].
The use of the MQ resin created quite a tradeoff for formulators: while the MQ resin improves long-wear qualities, it also created issues in terms of application, feel, and shine/gloss capability [4]. The MQ resin also contributes to dryness experienced by consumers post application; this is because the silicone derivative in combination with other lipstick ingredients would result in a dry and tightening effect on the lips [7]. The enhanced adhesion to the skin is beneficial in terms of product appearance, durability and consumer experience, but time and time again consumers have criticized the long-wear formulations due to their drying tendency and discomfort. With no hydrating property, application of the formulation to the lips stratum corneum, which is also relatively low in water content, reduces consumer comfort [2]. This effect is also caused by the film itself: a continuous film applies stress on the skin’s keratinous layer which, in turn, translates to the discomfort experienced by the user [8]. A secondary top layer was introduced to the two-step kit in order to help address consumer complaints in regards to durability and performance [2]. A second benefit to the two-step application was the freedom for the consumer to decide what appearance they would like. Rather than solely having a matte option, a shiny topcoat could be used to achieve a satin or sheer effect. Although beneficial, there are limitations to this technology; in order to provide an acceptable level of shine and appearance, the topcoat may need to be re-applied throughout the course of the day and, therefore, cannot truly be classified as long-wearing [4]. At this point, consumers needed to make a tradeoff between appearance and comfort versus the need for constant re-application to achieve the desired shine. This further drove the need for a simple application method that met consumer product requirements through formulation optimization.
A second issue raised by formulations incorporating the MQ resin related to the pigment appearance. Although the resin had enhanced lipstick water repellence, the color was found to run once applied to the lips. This is because the MQ resin was soluble in the same phase as the pigments within the formulation, causing the formation of a discontinuous film that allowed the color to run due to detached pigments [3]. To alleviate this issue, the oil within the formulation was replaced with a volatile oil. Not only did this help the color remain in place, but it introduced an additional benefit of enhanced oil resistance [3]. Even with these formulation improvements, consumers still desired a simple, single-step application. Modern long-wear formulations include a complex silicone, which provides the lipstick’s transfer-resistant characteristics, and a volatile oil which contributes towards ease of spreading during application and evaporates to leave behind the benefits of the silicone [3]. From this point on, continuous adjustments to the long-wear lipstick formulation have been made to enhance product performance and perception.
As the long-wear lipstick formulation was being optimized, the consumer need for a single-step application with varying gloss levels arose. The current trend leaned towards glossy, which signified a youthful and healthy appearance. In order to meet this consumer need, the “silicone plus” polymer, a product from 3M known for being a water-resistant film-former with good bioadhesion, was incorporated, which brought shine to the lipstick while also providing long-wear benefits [3]. These tweaks to the long-wear lipstick formulation enabled better performance experienced by the consumer and brought the industry to where products on the market are today. Currently, there are a number of products on the market with claims of long-wear benefits that last between eight and twenty-four hours. Some examples of such products can be seen in Figure 1.2.
Further tweaks to formulations have been made to bring about additional benefits to the consumer. Guerlain Paris, a cosmetics company, introduced a long-lasting lipstick under their KissKiss brand in 1995 that incorporated ceramides and a sun filter [9]. In doing so, the lipstick not only was long-lasting and transfer-resistant, but also introduced additional benefits