The cosmetics market is driven by innovation, performance and uniqueness. Formulating chemists have to come up with new products to captivate the imagination of consumers and differentiate products from those of the competition. However, the toolbox of conventional chemicals is becoming exhausted; thus, many formulations tend to contain the same ingredients while savvy consumers look for novel product offerings.
To satisfy this need, in recent years there has been a drive to use more exotic ingredients such as oligopetides, plant extracts and innumerable derivatives of natural products. This article focuses on a new rangea of synthetic chemicals known as polyhedral oligomericsilsesquioxanes (POSS).
This family of nanostructuredb molecules offers new performance possibilities. While traditional silicones are well-known, POSS additives provide different properties due to their rigid “silica” core and multi-armed structure.
POSS are a spin-off technology from the US Air Force Research Laboratories at the Edwards Air Force Base in California and were commercialized in the last decade by Hybrid Plastics Inc. They were first noted for their potential in peak performance plastics (see Commercialized POSS).
Beyond the plastics field, POSS have been investigated for lubricants,1 semi-conductor photoresists,2, 3 optical applications, coating additives, dental adhesives4, 5 and more. The unique structure of POSS offers numerous options, as evidenced by thousands of papers and patents in many disparate fields.
Solubility and Miscibility
It is apparent from their structure that POSS have a hybrid composition whereby an inorganic, silica-like core—i.e., a core reminiscent of silica but not identical to it—has been fused to organic side arms. One of the simplest forms of POSS is one in which only hydrogens are attached to the cage, or octahydro POSS (see Figure 1),
but these substances are neither stable nor readily soluble and thus of limited interest for cosmetics. As organic groups become larger, POSS become soluble. So for example, octa-butyl POSS shown in Figure 2 can be dissolved in nonpolar organic solvents and oils.
Even longer side groups such as iso-octyl form liquid POSS types. If the formulation is more polar, alternative pendant arms would be chosen to achieve the desired compatibility. For instance, PEG POSS is soluble in polar media such as acetone or ethyl acetate. These POSS with eight equivalent functionalities are known collectively as molecular silicas. They are inert and do not normally react, even under harsh conditions. Their acid stability is exceptional, remaining unaltered under prolonged exposure to bases. This stability is a prerequisite for creating viable cosmetic formulations.
The most polar POSS variants are readily soluble in water. Examples include octatetra-methyl ammonium POSS and octaammonium POSS. The latter is polycationic and has been reported to have biocidal properties.6 One new POSS type is N-phenylaminopropyl POSS, a liquid that is water soluble at acidic pH values and shows metal complexing activity.
Perhaps the most interesting new POSS for cosmetics is PEG POSS (INCI naming is in progress as of press). Polyethylene glycols (PEG) are well-known and versatile ingredients for cosmetics.7 PEG is safe and exhibits an unusual property in that it can dissolve in both water and in organic liquids. Additionally, PEG can bind water and metal cations (Li+, Na+ and K+) the way that crown ethers do. This ability to bind water makes it an ideal humectant.
The PEG POSS is designed in two versions that differ by the length of the PEG arms. This allows formulation tuning. Normally higher molecular weight PEGs crystallize and become solid. The multi-armed PEG POSS has a high molecular weight but crystallization is prevented by the 3D star structure (see Figure 3).
Thus the PEG POSS provides a PEG type that is a nonvolatile liquid that does not thicken like high molecular weight PEG/PEO does. Example formulations have incorporated PEG POSS to replace glycerol in moisturizers (data not shown). The POSS can be predissolved in either the oil or water phase, depending on the polarity and thus solubility of the POSS, or it may be incorporated into a pre-formed emulsion using a high-speed mixer.
The POSS types known as molecular silicas tend to be inert because in those instances, the core cage is nonreactive and the arms are simple alkyl groups that are recognized as inert to most types of chemistry. However, other POSS types are designed specifically to be functional. Octaacrylate POSS and octamethacrylate POSS are polyfunctional cross-linkers that provide hard coatings with fast cure time, due to the multifunctional nature of the POSS monomer, and high scratch-resistance because of the hardness imparted by the rigid POSS cage.8
As one might imagine, these materials have aroused interest for nail polish products. Octaepoxy POSS are another sort of cross-linker for durable coatings and adhesives that are VOC-free.
The list of functionalities available is wide-ranging, including: carboxylic acids, sulfonic acids, alcohols, amines, thiols, alkyl halides, imides, isocyanates, norbornenyls, olefins and silanes. Custom compounds also can be manufactured. The functional group generally is attached either on each arm or just on one corner of the cage. Of course, other configurations are also possible. Proteins and peptides also can be tethered to POSS.9
Fluoroalkyl POSS compounds have been of increasing interest lately because research has shown that they can give ultra hydrophobic and ultraoleophobic surfaces with extreme contact angles.10 Such surfaces illustrate the Lotus Effect, whereby water beads up and rolls off the surface, taking the dirt with it (self-cleaning); in addition, they repel oily contaminants. The “holy grail” is to create surfaces that stay eternally clean and shiny.
Another aspect of POSS is that they can act as effective dispersants. POSS trisilanols are key ingredients for such purposes11 because they have one silicon atom removed from the corner of the cage, leaving three reactive Si-OH groups (see Figure 4). These trisilanols are the only known well-defined and stable silanol compounds. The so-called organosilanes are well-established dispersants for fillers and pigments, finding use in coatings, plastics and cosmetic products alike.12, 13
Reactive POSS types also act as adhesion promoters by bonding to both surfaces. For instance, silanols bond to most silicate minerals (silica, mica, wollastonite, kaolin, etc.)14, 15 so that when a reactive group such as vinyl, epoxy or methacrylate is also attached to the POSS cage, the latter group can bond to the matrix polymer or coating,16 thereby strengthening the interface. Whereas conventional trialkoxyorganosilanes give off VOCs when they react, usually as methanol or ethanol, POSS trisilanols give off only traces of water instead, making for an odor-free, environmentally sound formulation.
POSS trisilanol treated pigments17 have been commercializedc and this area is expected to expand into printer pigments, light emitting polymers and other related areas. Interestingly other POSS trisilanols have been reported to assist skin healing without scarring; and even regrowth of strong, dense skull bone has been reported.18, 19
POSS additives are diverse and unique with a variety of types to fulfill various roles. The rigid silica-like core imparts novel hybrid inorganic-organic properties. Interest in POSS is spreading, as is evident by the more than 2,300 articles and 800 patents20 generated during the last decade or more. Commercialization has already taken place in the plastics and electronic fields with a steady stream of new applications. Work is ongoing in the optical and medical fields.18,19
Most recently, the cosmetics industry has discovered POSS technology.21–24 Perhaps that is hardly surprising since cosmetics is one of the most dynamic and creative industries and POSS represents a new set of tools for the formulator to craft enhanced products for superior performance.
1. R Misra, K Rollins and SE Morgan, Polymer Preprints 2008, 49(1) 517 (2008)
2. US Patents 6,759,460 and US 2004/0138355, assigned to Asahi Chemical (2004)
3. US Patent US 7,217,683, Lubrication via nanoscopic polyhedral oligomericsilsesquioxanes, assigned to R Blanski, SH Phillips, SL Rodgers, JD Lichtenhan and JJ Schwab (2007)
4. E Tegou, V Bellas, E Gogolides and P Argitis, Polyhedral oligomericsilsesquioxane (POSS) acrylate copolymers for microfabrication: Properties and formulation of resist materials, Microelectronic Engineering, 73–74, 238–243 (2004)
5. H-M Lin et al, Polyhedral oligomericsilsesquioxane containing copolymers for negative-type photoresists, Macromol Rapid Commun, 27 1550–1555 (2006)
6. US Patent 7,160,941, Dental composite materials and method of manufacture thereof, assigned to S Jin and W. Jia, Pentron Clinical Technologies Inc. (2007)
7. US Patent 7,226,960 Self-etching primer and method of use thereof, W. Jia, assigned to Pentron Clinical Technologies Inc. (2007)
8. World Pat WO 2007/025272, Biocidal premixtures, NA Merrill, JE Garft and JD Clay, assigned to Honeywell International Inc. (2007)
9. R Schueller and P Romanowski, Beginning Cosmetic Chemistry Second Edition, Allured Publishing Corp.: Carol Stream, IL USA (2003)
10. FM Capaldi et al, The mechanical properties of crystalline cyclopentyl polyhedral oligomericsilsesquioxane, J Chem Phys 124 214709 (2006)
11. N Alobaid et al, Nanocomposite containing bioactive peptides promote endothelialisation by circulating progenitor cells: An in vitro evaluation, Eur J Endovasc Surg 16466940 (Feb 5, 2006)
12. A Tuteja et al, Designing super-oleophobic surfaces, Science 318 1618 (2007)
13. US Patent US 2007/0225434, POSS nanostructured chemicals as dispersion aids and friction reducing agents, JD Lichtenhan et al. (2007)
14. R Rothon in Particulate-Filled Polymer Composites 2nd Edition, RAPRA, UK (2003)
15. C DeArmitt and R Rothon, Fillers and Surface Treatment, Plastics Additives and Compounding, Elsevier: Oxford, UK 4 5 12–14 (2002)
16. World Patent WO 2006/081512, Surface modification with polyhedral oligomericsilsesquioxanessilanols, JD Lichtenhan, JJ Schwab, Y-Z An and W. Reinerth, assigned to Hybrid Plastics Inc (2006)
17. PA Wheeler, R Misra, RD Cook and SE Morgan, Polyhedral oligomericsilsesquioxanetrisilanols as dispersants for titanium oxide nanopowder, J Appl Polym Sci 1082503–2508 (2008)
18. T Inage, Skin regeneration using nanotechnology, Advanced Materials Symposium, Hattiesburg, MS USA (2007)
19. T Inage Evaluation of bone regeneration utilizing POSS, Advanced Materials Symposium, Hattiesburg, MS USA (2007)
20. POSS Literature Repository, Hybrid Plastics Web site, available at: www.hybridplastics.com (Accessed Jun 4, 2008)
21. World Patent WO 2006/116404, Biomimetic materials comprising polyhedral oligomericsilsesquioxanes, J. Schwab, assigned to Hybrid Plastics Inc. (2006)
22. US Patent 2004/0202622, EPOSS-containing cosmetics and personal care products, SAM Quadir, L’Oréal (2004)
23. WO2004/082611, POSS and EPOSS-containing cosmetics and personal care products, SAM Quadir, L’Oréal (2004)
24. US Patent US 2008/0081022, POSS-containing cosmetic compositions having improved wear and/or pliability and methods of making improved cosmetic compositions, WH Yu and SAM Quadir, L’Oréal (2008)