When formulating many types of personal care products, it’s important to make stable anhydrous gels. These products include lip glosses, waterproof mascaras, and blushers as well as formulations for soft antiperspirant and skin care gels using water unstable actives.
All thickening mechanisms involve solvent binding or immobilization. These mechanisms include absorption/adsorption, intermolecular hydrogen bonding (fiber network forming), hydrophobe/hydrophobe interaction, and electrostatic repulsion (the latter is only relevant in water-based gels).
The various types of anhydrous gels that can be formulated include oil in polyol emulsions, polyol in oil emulsions, and oil in oil emulsions. Anhydrous polyol gels and emulsions are mostly commonly based on fatty acid metal salts, particles or fiber forming technology. Other types of gels include polar/nonpolar oil gels made using polymeric waxes, fatty acid metal salts, soluble/swellable polymers, particles, lamellar gel networks and fiber forming materials.
Anhydrous lamellar gel network technology
The technology typically utilizes crystalline nonionic emulsifiers with a hydrophilic-lipophilic balance (HLB) of 6 to 20, depending on the choice of polyol.
The lamellar gel network forms when the surfactant/polyol is heated above the melting point of the surfactant and cooled, forming the lamellar phase which can bind the polyol solvent. If an oil phase is added when hot, one can form an oil in polyol emulsion. Recommended polyols are Propanediol, Glycerin, Butylene Glycol, or PEG-4 or PEG-8. Recommended emulsifiers include Polyglyceryl-6 Dipalmitate, Steareth-2, Beheneth-5, and Steareth-20.
Fatty acid metal salts
Sodium stearate is widely used to form clear deodorant gels or sticks using glycols like Propanediol, Propylene Glycol, PEG-4 or PEG-8, and Butylene Glycol. These can be formulated to be hydrous or anhydrous. Aluminum Distearate can be used to form polar or nonpolar gels. The disadvantage using Aluminum Distearate is the high temperature necessary to dissolve material (typically over 100°C, depending on the polarity of the oil used).
Carbomers can be used to form anhydrous polyol gels using propanediol, Propylene Glycol, Glycerin, PEG-4 or PEG-8, and Butylene Glycol. Unneutralized Carbomer will form thick gels by hydrogen bonding with the polyol. You can also neutralize using an organic amine like Triethanolamine to generally get higher viscosities. An oil phase can also be added to the gel when added with high shear to form an oil in polyol emulsion. FlexiThix (PVP-Ashland) will gel medium/polar oils using heat and approximately 3-5 percent polymer. Dow Corning EL-8050 ID Silicone Organic Elastomer Blend (Isododecane (and) Dimethicone/Bis-Isobutyl PPG-20 Crosspolymer) can also form clear, stable gels which have good nonpolar/polar oil compatibility.
Low molecular weight polyethylene (400-500 Daltons) is widely used to form heat stable gels and sticks using non polar/medium polarity oils. Intelimer® 13-1 and Intelimer® 13-6 Polymer (Poly C10-30 alkyl acrylate – Air Products and Chemicals Inc.) can also be used to form heat stable, non-tacky gels using nonpolar/medium polarity oils. Rheopearl WX (Dextrin Palmitate/Ethylhexanoate – Chiba Flour Milling Ltd/Miyioshi) will also produce stable soft gels.
Other options include the Oleocraft™ (Croda) polyamide based polymers which can form clear soft gels and sticks using a broad range of emollients. HP-31 (Polyamide 3) is good for high polar oils, HP-32, MP-30 and 31 (Polyamide 3) for medium polarity oils, and LP-20 (Polyamide 8) for low to medium polarity oils.
Fiber forming ingredients
These are highly efficient materials that can form clear, soft gels using <1 percent active material. AJKOD 2046 (Octyldodecanol-80%, Dibutyl Lauroyl Glutamide-12%, Dibutyl Ethylhexanoyl Glutamide-8% – Ajinomoto) is an easy-to-use pre-gel for making sticks/soft gels using nonpolar to medium polarity oils. The use of Octyldodecanol enables gels and sticks to be made below 100°C. Clear formulations are also possible.
Particles used to form oil gels include organically modified clays, fumed Silica, hydrophobically modified fumed Silica, and Aerogels. Organically modified clays are typically produced by reacting a cationic surfactant with a Sepiolite, Montmorillonite, Hectorite or Bentonite clay. They are sold as preformed gels or as powders.
Garamite 7308 XR (Quaternium-90 Sepiolite, Quaternium-90 Montmorillonite-Eckart America) is a good choice because only low shear processing is needed to form oil gels and no polar additive like Ethanol or Propylene Carbonate is required. One can also use a broad range of polarity oils.
Silicas can be used to form clear, heat stable gels with a broad range of polarities. However, high shear is needed to fully develop the viscosity. The most efficient Silica is Dow Corning VM-2270 Aerogel Fine particles (Silica Silyate-Dow Corning) which is a hydrophobically modified aerogel.
Viscous gels can be produced using around 5-6 percent. A less efficient option would be to use Aerosil® R 972 (Silica Silyate-Evonik) which is a hydrophobically modified fumed silica at around 8-10 percent.
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