Polyester is an important category of cosmetic ingredients. The International Dictionary and Handbook of Cosmetic Ingredients lists more than 200 different types of polyesters that serve a variety of functions, including film formers and conditioners for skin and hair. 2 Polyester is a polymer chain consisting of repeating units linked by ester bonds, as shown in Figure 1. The polyester backbone is polar due to the presence of ester bonds, and the degree of polarity is determined by increasing/decreasing the backbone and the aliphatic content in the polymer chain end groups. Generally, polyesters are hydrophobic and insoluble in water, but can be made water-dispersible or water-soluble by introducing hydrophilic ionizable groups into the backbone. Polyesters can be linear, branched and/or cross-linked, with differences in structure modulating key physical properties such as viscosity and film-forming properties. The chemical structure of polyesters can also be changed to adjust solution and surface compatibility, solubility and dispersion. This article will discuss two polyester molecules. The primary structures of both molecules are shown in Figure 2, and structural changes are cited in the various sections about this polymer.
LexFeel™ Vibrant (palmitic acid/adipic acid/pentaerythritol crosspolymer) is a branched/crosslinked polyester according to Figure 2 with long carbon chain (C16-C18) end groups. This specific polyester structure results in a stiff, waxy ingredient structure that can co-crystallize with wax to provide structure and texture in cosmetic formulations. Compared to using only traditional waxes, such as beeswax and stinky wax, LexFeel’s vibrant polyester maintains structure while controlling negative effects like sweating (Figure 3). In addition, the polyester is an amorphous and low Tg polymer compared to crystalline waxes, providing thickening of the oil phase without causing haze or turbidity. This results in an improved formulation background of the pigments, giving the product brighter, more vivid colors both in and after use. As can be seen in Figure 4, the result is higher yield and more vibrant impactful color when applied – ideal for delivering bold colors in the tinted cosmetics market.
LexFeel 699 (pentaerythritol adipate/capric acid/octanoic acid/heptyl enanthate) is a branched/cross-linked polyester shown in Figure 2, with shorter carbon chain (C6-C10) end groups. The resulting polymer is a fluid with a velvety texture and provides similar functionality to castor oil in cosmetic formulations. The combination of the polar backbone and the hydrophobic character of the end groups provides interaction with the pigment, resulting in uniform dispersion and prevention of flocculation. This effect was demonstrated in two pigment dispersion tests, in which viscosity was measured as an indicator of how well the base fluid wets the pigment. Lower viscosity measurements indicate better pigment wetting, which translates into improved pigment dispersion in the formulation. Figure 5 shows the results of a pigment dispersion test using red calcium lake No. 7 at a 1:1 pigment:dispersant loading.
The viscosity of the 699 dispersion is lower than that of castor oil, polyglyceryl-2 triisostearate, and diisostearate malate dispersions. Pigment dispersion testing of micronized zinc oxide was also performed using pigment powder:dispersant at a loading of 1:2. The viscosity of the 699 dispersion is 8,000 cP, which is lower than trioctyldodecyl citrate (10,000 cP), pentaerythritol tetraisostearate (90,000 cP), and tridecyl trimellitate (235,000 cP ) and dipentaerythritol trimerhydroxystearate (23,000 cP). After about 3 minutes of manual mixing, the smooth and evenly dispersed appearance of micronized zinc oxide in 699 is shown in Figure 6. The dispersion uniformity during application was evaluated for 699 and isotridecyl isodecanate with and without coated titanium dioxide at a loading of 2:1 pigment:dispersant (Figure 7). Pigments dispersed with 699 have a smoother, more uniform appearance when using both variations of titanium dioxide.
