Rheological agents are multi-functional, ubiquitous ingredients found in most formulations that can significantly impact stability and performance. Nothing is more fundamental to a formulation than creating structure. Learning how to choose the correct technology for your formulation can dramatically improve in use product performance and stability.
Rheological Agent Benefits
Key benefits of using rheological agents include:
- Create viscosity to improve product stability, dispensability and functionality
- Stabilize suspensions, anhydrous gels, and emulsions
- Disperse and stabilize solids
- Modify skin feel and product in use attributes
- Improve product efficacy (SPF, moisturization, deposition, foam/lather)
- Can function as primary emulsifiers in oil in water formulations
- Can reduce irritation
All thickening mechanisms involve solvent binding or immobilization. These include absorption, adsorption, hydrogen bonding, electrostatic repulsion, and hydrophobe/hydrophobe interaction. The various technologies used to thicken solvents include soluble/swellable polymers and natural polymers which can contain a mixture a soluble/swellable polymers.
Other technologies include fiber forming networks, and particles, which can be porous or charged. Colloidal structures such as lamellar/liquid crystalline phases, elongated micelles, and high internal phase water in oil or oil in water emulsions containing over 50% emulsified phase can also be very effective at creating structure.
The typical technologies that are used to thicken water continuous formulations include swellable/soluble polymers, lamellar crystalline phases, elongated micelles, and particles. The classes of soluble polymers include starches, polysaccharides and cellulosics. All of these can be anionic, cationic, or nonionic in nature.
Swellable polymers include alkali or acid swellable anionic or cationic polymers, hydrophobically modified alkali swellable anionic polymers, and hydrophobically modified nonionics (polyols, fatty alcohols, ethoxylated urethanes). The most common rheological agents used in oil in water formulations are anionic swellable polymers, soluble polysaccharide based polymers, and lamellar crystalline phases used alone or in combination with swellable or soluble polymers.
The most common thickeners for surfactant based products include salt, amides and amphoteric surfactants which promote thickening by forming elongated micelles. Hydrophobically modified nonionics or acrylates function by cross-linking micelles.
Technologies used to thicken various polarity oils include Aluminum Distearate, waxes, network forming fibers, particles, and oil soluble/swellable polymers. Fiber formers include Glutamide esters.
Waxes used to create structure include long chain hydrocarbons, fatty alcohols, triglycerides, and esters. Many natural based waxes often contain a mixture of various chemical classes.
Common viscosifying particles include silicone elastomers, Aerogels, hydrophobically modified Aerogels, fumed Silica, hydrophobically modified Silica, and cationic surfactant modified clays. Oil soluble polymers used include Polyethylene, Ethylene/Acrylic acid copolymers, Synthetic wax, Dextran fatty esters, and alkyl modified acrylates.
Rheological Agent Formulation tips
- Determine the desired critical properties (e.g. PH, electrolyte tolerance, solubility, clarity, compatibility, rheology profile, processing restrictions, cost, skin feel).
- Think value-added: use thickeners that can provide additional benefits.
- Think combinations, especially by using thickeners that work by different mechanisms. Rarely is the optimum rheological system one ingredient.
- Always determine a viscosity/rheological system dose response for your formulation.
- Thickening systems that maintain viscosity at elevated temperatures will give you better heat stability.
- Thin, sprayable formulations with suspended solids require systems with a high yield value and shear thinning behavior.
- Develop multiple options; some will fail.
- Work to minimize the amount of thickener used. Generally, the best rheological system is the one that is the most efficient.
- Swellable polymers generally provide higher low shear viscosity and yield then soluble polymers.
- Using only swellable polymers can cause syneresis in some formulations containing high density suspended solids. Combinations with soluble polymers can improve performance.
- Divalent or trivalent salts have a significantly greater impact on swellable polymer viscosity than monovalent ones.
- Small changes in the thickener level can have a profound effect. For most polymers, viscosity increases exponentially with concentration. A solution could be thin at .5% and a viscous gel at 1%. Flattening this viscosity response by using combinations of materials will make your formulation more robust.
Rheological Agent Trends
- Growing use of acrylate and polyurethane dispersions to thicken shampoos and shower gels.
- Growing use of natural emollient-based materials for thickening shampoos and shower gels.
- Less use of ethoxylated nonionics as thickeners for shampoos and shower gels.
- Development of more hydrophobically modified polymers both as thickening emulsifiers and to improve salt tolerance.
- Development of crème gel formulations. These are sterically stabilized oil in water or water in oil emulsions that contain no emulsifiers.
Did you know the Knowledge Center is home to a four-part video series on Rheology? See Part 1, and get links to the others here.
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