Skin Care Polymer Developments
Here’s an update on polymers that are based on natural or modified natural materials, as well as a look at advances in silicone elastomer and polymer synthesis.
Robert Y. Lochhead and Virginia Smith
The Institute for Formulation Science,
The School of Polymers & High Performance Materials
The University of Southern Mississippi
Surveillance of recently published patent applications can provide insight into trends in our industry. In preparing this article we surveyed recently published U.S. patent applications and attempted to identify emerging trends. We identified clear trends in advances in polymer synthesis, the emergence of precise natural and modified naturals and advances in silicone elastomers. In this précis, the trends that we identified are summarized.
Controlling Molecular Properties
The precise properties of polymer can be tailored best if the molecular weight of the material is closely controlled. In general, synthetic polymers have a broad distribution of molecular weights but modern polymerization is changing this situation with techniques such as “living” free radical polymerization that have the propensity to make batches in which each polymer molecule is almost the same size.
The distribution of molecular weight of a polymeric material is conventionally represented by its molar mass distribution (or polydispersity index) which is defined as the ratio of the weight average molecular weight to number average molecular weight. The importance of controlling the polydispersity index of a polymer is illustrated by a recent patent application that is directed to the use of polyethylene waxes.
Waxes provide consistency and influence the hardness of many cosmetic products. They are also used in liquid cream-like and gel compositions to improve the cohesion and water resistance of the composition. In general, natural waxes such as beeswax and carnauba wax have been preferred for these applications but there is a desire to use polyethylene wax.
Unfortunately, low molecular weight linear polyethylene waxes are brittle and have low adhesion to skin.1 Polymerization of polyolefin waxes using metallocene catalysts are claimed to provide a polydispersity in the range of 1.5 to 10. This polydispersity is still quite broad but the patent application discloses that it provides improved properties that bring the properties of the polyolefin waxes closer to those of the natural waxes.2
Natural & Modified Natural
Pectin is a standard gellant in the food industry. Pectin is an abundant polymer that is often regarded as a waste-product of fruit and vegetable processing. It occurs in a macromolecular network with cellulose and hemicellulose in plant cell walls and this biopolymer complex bestows rigidity on the cells of the plant. This complex, called propectin, is also the “glue” that attaches the pectin to the other cell wall components. Pectin is obtained by the acid or alkaline hydrolysis of propectin. The pectin is obtained as a linear polymer of D-galacturonic acid and its methyl ester in which the monomer units are linked by 1, 4 glycosidic bonds. Pectin is classified as high ester or low ester pectin, having degrees of esterification of greater than or less than 50%, respectively. A method of preparing a more homogeneously de-esterified pectin involves swelling pectin-containing plant material in aqueous neutral salt solutions and then base hydrolysis with ammonia.3 The pectin so produced is claimed to have a viscosity that is at least 2.5 times that of conventionally pectin products. This improved pectin is a product that is worthy of the cosmetic formulator’s attention The glycosaminoglycan, hyaluron, can be crosslinked with divinylsulfone to produce highly cohesive, adhesive and elastic gels. These gels are claimed to be useful for soft tissue augmentation.4
Rinse-off skin conditioners are usually compositions containing high levels of emollient and low levels of surfactant stabilized by rheology modifiers such as carbomer or hydroxyethylcellulose. They are applied in the shower after washing and rinsed off to leave a thin layer of emollient oil deposited on the skin. Recently it has been revealed that the inclusion of a fatty acid and a non-pregelatinized starch (such as Pure-Gel starches from Grain Processing Corporation and National, Novation and Purity starches from National Starch and Chemical Corporation) can improve the deposition and structure the compositions to enhance the viscosity and stability of rinse-off skin conditioners.5 Natural starch granules are usually insoluble below about 40°C due to intermolecular hydrogen-bonding within each granule. The starch only becomes useful as a thickener after the starch granules are heated and swell in an aqueous environment; a process called gelatinization.
Above a critical temperature, called the gelatinization temperature, the intermolecular hydrogen bonding is thermally disrupted and the granules swell irreversibly. Cold-water thickening starches can also be produced by chemical modifications of the natural starch. The starches most preferred for these skin-conditioning products are hydroxypropyl starch phosphates with gelatinization temperatures between 30°C and 70°C to permit easy, low viscosity mixing at room temperature and thickening and structuring with modest heating.
Polyquaternium 24 is a hydrophobically-modified cationic hydroxyethylcellulose. This type of polymer, having a degree of polymerization of 4,000 to 10,000 anhydroglucose units and a degree of hydrophobic substitution of 0.0008 to 0.08 moles per anhydroglucose ring, exhibits enhanced depositions of cosmetic oil from a bodywash formulation.6 This polymer is an improvement on similar polymers with lower degrees of polymerization (400 to 1,600 anhydroglucose units) that display less desirable substantivity. The new polymers also supersede polymers with degrees of substitution of more than 0.11 to 0.25 moles of hydrophobe and 0.05 to 0.5 moles of cationic substitution respectively per anhydroglucose ring that give good foaming but poor substantivity and a viscosity that is so high that it renders the product impractical.
Benefits of Zinc
Zinc is the second most abundant trace metal in the body and, due to its inclusion in metalloenzymes, it catalyzes nearly every biochemical process in the human system.7 Dietary deficiency of zinc can cause dermatitis, alopecia, anorexia and stunted growth. Zinc has been used for more than 3000 years to treat skin disorders. Consumer acceptance of surfactant-based, zinc-containing topical products is affected by the rheology of the product and it is desirable to add thickening agents to achieve the desired rheological profile. Anionic thickeners are preferred for many topically-applied personal care products; however, such anionic thickeners bind with the metal. Therefore, cationic or nonionic thickeners are preferable in this case. A recent patent application is directed to the use of cationic and nonionic thickeners as rheology modifiers for zinc-containing surfactant products.8 The preferred cationic polymers for this purpose are polyquaternium-10 or polyquaternium-24 and preferred nonionic polymers are disclosed as being PEG and Polyox polymers from Union Carbide.
Peptides formed from protein hydrolysis are commonly used as skin conditioning agents if they can form films that are substantive to the surface of skin. These films can protect and lubricate the skin and moisturize it by reducing transepidermal water loss. However, films of these hydrolyzed protein fragments may lack the durability to confer long-lasting effects. Dupont researchers have tackled the challenge of enhancing peptide substantivity by applying modern biochemical combinatorial techniques to identify and isolate peptides with specific affinity to hair, skin and nails. They use combinatorial phage-peptide techniques to identify and isolate peptides that have a high binding affinity to hair, skin or nails.9 The phage peptide library technique is a biochemical approach that has revealed details of the sequencing and conformation of peptide amino-acids that are essential to antibody-antigen interaction.10
Using this approach, Dupont scientists exposed hair, skin or nail samples to a “library” of combinatorial generated phage peptides. This library is actually a “soup” containing an enormous number of different peptides, each having specific amino acid sequences. The phage peptides that have affinity for the target tissue—skin for example—adhere to this tissue and this allows them to be fished out of the library. This process is sometimes called “biopanning” by analogy to the panning process in gold prospecting. The selection of strongly-bound peptides is refined by serial biopanning steps with elution of weakly-bound peptides at each stage. The refined phage peptide complex is then used to infect bacterial host cells that are cultured in a suitable cell growth medium. The desired phage-peptide complexes with high affinity for skin, hair and nails are then harvested from this medium.
This Dupont research has resulted in the isolation of block dipeptides in which one block is a peptide that binds strongly to hair, skin or nails and the other block is either a conditioning agent or a coloring agent. Triblock polymers having a spacer between the two functional blocks have also been produced. The peptide-based skin conditioners are produced by covalently bonding a conditioning agent to a skin binding peptide block. For this purpose, conditioning agents are defined as astringents, exfoliants, emollients, humectants and occlusives, and preferred conditioning agents are polysalicylates, propylene glycol, glycerin, alphahydroxy acids, salicylic acid and propanediol. The attachment of coloring agents to the skin-binding peptide sequences are useful for substantive decorative color and also sunless tanning agents with prolonged durability.
Formulating with Silicones
Water-in-oil emulsions can exhibit the apparently paradoxical attributes of creamy texture and easy spread on the skin without tacky or greasy feel if at least two different emulsifying silicone elastomers are employed.11 In one embodiment, the compositions comprise at least one polyoxyalkenated silicone elastomer and at least one polyglycerolated silicone elastomer. Suitable polyoxyalkenated silicone elastomers can be selected from Shin-Etsu’s KSG -20 and 30 series and Dow Corning’s DC 9000 series and suitable polyglycerolated silicone elastomers can be selected from Shin-Etsu’s KSG -70 and 800 series.
The silicone elastomers are used as “soft focus” powders in foundations. A soft focus powder is a pigment that provides a natural finish with good coverage to camouflage skin blemishes.12 The soft focus powder functions by having high Total Luminous Transmittance (Tt) and Diffuse Luminous Transmit- tance (Td). When both these values are high, the powder exhibits a high transparency that provides a natural finish. The second criterion for soft focus is a relatively high Haze value. The Haze value is (Td x100)/Tt. High Haze values provide contrast between the illuminated area of the skin and the shaded area of the skin and, as a result, they hide the appearance of pores and wrinkles. The Tt, Td and Haze values are measured according to ASTM D 103-00, “Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.” Soft focus silicone elastomers are particularly effective on the skin because they provide a low matte level. Either emulsifying or non-emulsifying silicone elastomers may confer soft finish on a foundation.
It has been claimed that the physicochemical characteristics of personal care compositions can be improved by incorporating a silicone gel polymer that is made by polymerizing a polyorganohydrosiloxane having a molecular weight of 3500 to 4000 and 6-7 Si-H groups per molecule with a lower alkyl terminated polydimethylsiloxane of molecular weight 20,000 to 25,000 at low concentrations in a low viscosity medium.13 Colloid–milling of the product produces a silicone polymer that is clear and can be easily formulated into clear compositions without further milling or the need to add any additional solvent.
Self-assembled molecular networks can be tailored to be solid or highly viscous below a desired temperature, yet become fluid and flowable above that temperature. Clearly such systems are of value to formulators who wish to produce products with good aesthetics and storage stability combined with lubricious application on the skin. Suitable materials can be produced by blending silicone derivatives that can self-assemble intermolecularly by Lewis acid-Lewis base interaction. Examples of network systems that display this type of thermal stimulus-response behavior are blends of aminopropyl-substituted dimethicones and benzoic acid terminated dimethicones.14
…And Soap Keeps Advancing
A long time ago, soap was used to just clean the body surface. Later modifications added skin comfort as another desirable attribute. Today, consumers desire cleansing bars that are designed to cleanse, exfoliate, condition, moisturize, pander to individual preferences and deliver aesthetic sensual signals such as fragrances. In order to meet those needs, a variety of fibrous structures have been included in cleansing bars. Researchers recently discovered that the inclusion of wear-promoting agents leads to improved lathering, exfoliation, aesthetics and sensory perception by the user.15
Wear promotion is defined as increase in the weight loss of the bar as measured by a wear test that is considered to simulate real use conditions. The wear promotion agents include humectants such as glycerin, sorbitol, propylene glycol, poly(ethylene glycol), polyglyceryl esters, urea and hydrophilic plant extracts. Hydrophobic emollients can also be used to promote wear; these include silicone oils, fats, oils, waxes, hydrophobic plant extracts, hydrocarbons, long-chain fatty acids, alcohols and esters, and essential oils. One can conclude that in our consumer-based society, faster consumption is an aesthetically-pleasing goal!
1. Hermann, H.F.; Lukasch, A.; Hohner, G.; Michaelis, H.; Lachmann, A.; Cometic, pharmaceutical and dermatological preparations comprising homopolymer and/or copolymer waxes of the monomers ethylene and propylene, US Patent Application 20070031361, Feb. 8, 200 7; assigned to Clariant.
2. Hermann, H.F.; Lukasch, A.; Hohner, G.; Michaelis, H.; Lachmann, A.; Cometic, pharmaceutical and dermatological preparations comprising homopolymer and/or copolymer waxes of the monomers ethylene and propylene, US Patent Application 20070031361, Feb. 8, 2007; assigned to Clariant.
3. Larsen, P.F.; Mathiasen, M.J.; “Method for preparing fibre-containing pectin and products and uses thereof,” US Patent Application 20070031572 A1, Feb. 8, 2007; assigned to KMC Kartoffelmelcentralen AMBA.
4. Leschiner, A.K; Konowicz, P.; Chang, M.Y.G.; Vasilyeva, V.; “Polymeric materials, their preparation and use,” US Patent Application 20070036745 A1, Feb. 15, 2007.
5. Patel, R; Sabine, R; Paredes, R; “Rinse-off conditioner comprising non-pregelatinized starch and fatty acid system for improved properties,” US Patent Application 20070032393 A1, Feb. 8, 2007: assigned to Conopco, Unilever.
6. Kreeger, R.L.; Zhou, S: “Cellulose ethers;” US Patent Application 20070031362 Al, Feb. 8, 2007; Union Carbide.
7. Neibauer, M. F.; Lane, B.S.; Schwarz, J.R.; Warnke, D.T.; “Personal care compositions comprising a non-binding thickener with a metal ion,” US Patent Application 20070009472, Jan. 11, 2007; assigned to Procter & Gamble.
8. Neibauer, M. F:; Lane, B.S.; Schwarz, J.R.; Warnke, D.T.; “Personal care compositions comprising a non-binding thickener with a metal ion,” US Patent Application 20070009472, Jan. 11, 2007; assigned to Procter & Gamble.
9. Huang, X.; Wu.Y.; Wang, H; “Peptide-based conditioners and colorants for hair, skin and nails,” US Patent Application 20070048236 A1, March 1 , 2007. E.I. Du Pont De Nemours.
10. Daniels, D.A.; Lane, D.P.; “Phage Peptide Libraries,” Methods, 1996, 9, 494.
11. Themens, A.; “Cosmetic compositions comprising at least two emulsifying silicone elastomers,” US Patent Application 20070020217, Jan 25, 2007.
12. Taniguchi, T; Sako, T.; Fujiwara, Y. ; Fujii, K, Watanabe, N., Solid skin care composition comprising multiple layers, US Patent Application 20070060666, March 15, 2007; Procter & Gamble.
13. Slavashevic, P.; Carson, J. C.; Zofchak, A.A.; “Applications of cross-linked silicone gel in personal care products.” US Patent Application 20070048240, March 1, 2007.
14. Knaijansky, S.; Zhong, H.; “ Energy responsive composition and associated method,” US Patent Application 20070032610 A1, Feb 8, 2007, assigned to General Electric Corporation.
15. Keenan, D. M; Grissett, G.A.; Macedon F.A.; Williams, D.R.; “Rapid dissolving bar soap with fibrous assembly,” US Patent Application 20070049512, March 1, 2007, assigned to Conopco , Unilever.