Carbomers are an important class of materials every formulator should get familiar with. They are high molecular weight crosslinked polymers of acrylic acid with molecular weights of up to 3 – 4 billion Daltons. Carbomers are polymerized in ethyl acetate, cyclohexane and typically used crosslinking agents are pentaerythritol polyallylether and polyallyl sucrose.
Powdered carbomers have a dry particle agglomerated size of 2-7 microns (primary particle size 50-200 nm). Dispersions are acidic with a pH of 3 approximately and when neutralized to a pH of 6-10, the particles swell to around 1000 times their initial volume and the viscosity dramatically increases due to charge repulsion. Salts can decrease viscosity by reducing the charge repulsion. Carbomers can also produce clear gels in water and ethanol due to refractive index matching. Extensively crosslinked carbomers are commonly used as super absorbers in disposable diapers.
Functions in cosmetics
- Very efficient water thickening using <1% polymer (no electrolytes).
- Can make aqueous or alcoholic clear gels.
- Very efficient at suspending solids and stabilizing emulsions due to the high yield value of gels.
- Can make stable water in oil in water emulsion.
- Excellent skin feel (<.5%) and shear thinning rheology.
Carbomer formulating tips
- Carbomers can easily be added to emulsions by addition to the oil phase prior to emulsification.
- Easy to disperse powdered grades are preferred when dispersing directly into water.
- Adding electrolyte or small amounts of acid to the water phase prior to Carbomer addition significantly improves its dispersion by reducing solution viscosity. Up to 5% dispersions of Carbomer in water can typically be made with this approach.
- Use Triisopropanolamine to neutralize carbomer when gelling up to 90% ethanol.
- Carbomers are UV sensitive. You need to incorporate a UV absorber into gels when using clear packaging.