Dispersants are invaluable personal care ingredients that are broadly used in any formulation in which solids need to be dispersed. These include antiperspirants, colored cosmetics, and metal oxide based sunscreen formulations.
Dispersants are materials which improve the separation of particles in a suspension or colloidal dispersion and reduce settling/agglomeration and can be surfactants or polymeric materials. The three stages of the particle dispersion process include wetting, particle separation/dispersion (particle size reduction), and suspension stabilization.
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Wetting
- A good dispersant can displace gases, liquids, and other contaminants from a pigment surface while increasing the surface area available for wetting.
- This helps reduce the amount of mechanical energy needed to make the dispersion.
- Eliminating air can also provide a significantly lower dispersion viscosity and improve product stability.
Particle separation/dispersion (particle size reduction)
- Most powders contain both aggregate and agglomerate particles.
- Aggregates are primary particles that are strongly bound together and normally need very high shear to break up. Roller and ball mills are typically used to impart this type of shear.
- Agglomerates are loosely bound particles that can normally be broken up using roto stator mills or even propeller mixing with a good dispersing agent.
- A good dispersant will wet the particle surface quickly, reduce the agglomerate particle size, and prevent the agglomerate from reforming.
Particle stabilization
- A good dispersant will keep pigment particles separated by adsorbing onto the pigment particle providing a barrier against particle reagglomeration.
- All dispersants work by charge (electrostatic), steric, or electrosteric stabilization (combination of both). The type of dispersant you need is to use is dependent on whether you are working in an aqueous or non-aqueous system.
- Electrostatic stabilization occurs only in an aqueous environment and is based on charge repulsion for maintaining particle separation.
- Steric stabilization can be used in both aqueous and non-aqueous formulations. Anchoring groups on the polymeric dispersing agent provide a strong specific interaction with the pigment surface. The polymeric stabilizing group extends into the oil or water phase preventing the pigment from reagglomerating. Steric stabilization also works well in non-aqueous formulations.
- Electrosteric polymeric stabilization works the best in aqueous formulations. Side chain alkyl groups on the polymer backbone provide steric stabilization and electrostatic stabilization is provided by neutralized charged groups (carboxyl, sulfate, sulfonate, amino).
Types of dispersants
- The simplest dispersants are surfactants. These typically have poor adsorption on the particle surface and are not very efficient at suspension stabilization because they form a small steric barrier. This steric barrier is the density of the dispersant on the particle surface.
- Polymeric dispersants are very efficient providing high surface absorption and a large steric barrier which prevents reagglomeration. Benefits of using polymeric dispersants include;
- Improved color development
- Significant viscosity reduction
- Faster dispersion, reduced energy consumption, and reduced cost
- Less settling of dispersed particles
- Easier redispersion if settling occurs
- Higher solids in the suspension
- Higher SPF, less skin whitening (ZnO, TiO2)
Recommended nonaqueous dispersants
- Castor oil was one of the first dispersants used in lipsticks and is still quite popular. Castor oil is a polar oil comprised of over 90% Glyceryl Triricinolate. Ricinoleic acid is a unsaturated hydroxy fatty acid that provides excellent steric stabilization and many polymeric dispersants are based on this chemistry.
- Polyhydroxystearic acid is widely used in dispersing pigments and metal oxides. It is produced from Hydroxystearic acid which is made by hydrogenating Castor oil-derived Ricinoleic acid and polymerizing the hydroxyl and carboxyl groups.
- Polyricinoleic acid
- Polyglyceryl 6 Polyricinoleate
Recommended aqueous dispersants
- Sodium Polyaspartate
- Sodium Polyacrylate
- Butyl PVP
- Sodium Polynaphthalene Sulfonate
Formulation tips
- Use surface-treated pigments to produce more uniform dispersions.
- Use a roto stator mill for making most dispersions. Some cosmetic color dispersions may require a roller or ball mill. Reduction in viscosity and improved stability are good predictors of dispersion quality.
- Evaluate increasing levels of dispersing agents until the dispersion viscosity reaches its lowest viscosity. Typically you need to use ~5-7% dispersant based on the percent solids being dispersed.
- For pigments, increased color development is a useful indicator of dispersion quality
- Try combinations of dispersants that work by different mechanisms
- Polymeric dispersants normally work better than surfactants whether working in oil or water.
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