Rheology Modifiers: Carbomer and Xanthan

Thickeners are chosen late in a project, often to fix a texture that a marketer described in one word. That is the wrong order. The rheology modifier decides whether pigment stays suspended, how the product pours, and whether a pump can deliver a dose. This note compares the main families and the failure modes each one brings.

1. Structure, not just thickness

A thickener builds a three-dimensional network that resists flow until a stress threshold is crossed. That threshold is the yield point, and it is what holds a bead of scrub or a suspended pearl in place. A formula can be thick and still fail to suspend, which is why a single viscosity number tells you so little.

2. Carbomer

Carbomer is a cross-linked polyacrylic acid supplied as a powder or a pre-dispersed liquid. It builds viscosity only after neutralisation to about pH 6 to 7, usually at 0.2 to 0.6 percent. The gel is clear, bright and shear-thinning, which reads well in a jar. It is also intolerant of electrolytes: 1 percent sodium chloride can collapse the structure.

3. Xanthan gum

Xanthan is a fermentation polysaccharide used at 0.2 to 0.5 percent. It is pseudoplastic, tolerant of salt and acid, and stable across a wide pH range, which makes it the default for scrubs, clay masks and anything with a high solids load. Its weakness is feel: above about 0.5 percent it turns stringy and slippery, and it can leave a tacky film.

4. Cellulose and associative polymers

Hydroxyethylcellulose gives a soft, non-tacky gel that spreads well but drops viscosity when hot. Associative acrylates and polyurethane thickeners build structure through hydrophobic association, tolerate electrolytes and give a cleaner rub-out. They cost more per kilogram and can be sensitive to surfactant and solvent levels, so they are usually paired rather than used alone.

5. Yield point and suspension

To keep a particle suspended, the yield stress needs to exceed the gravitational stress on that particle, which grows with its size and density difference. A 200 micrometre pearl in a low-density emulsion needs noticeably more structure than a 20 micrometre pigment. If a product shows sediment in three months, measure yield stress before increasing dose.

6. Combining two thickeners

Carbomer with a small amount of xanthan often gives better suspension than either alone, because the polysaccharide adds a secondary network. The reverse pairing, a high xanthan load plus carbomer, tends to feel heavy and can produce a gel that will not level in a jar. Test combinations at three ratios before fixing a formula.

7. Neutraliser choice

Sodium hydroxide gives a firm, clear gel and is cheap; triethanolamine gives a softer gel and buffers better; arginine is used where the label needs an amino acid. The neutraliser also sets the final pH, so it interacts with actives and preservatives. Add it slowly with low shear, because over-neutralisation past pH 8 usually thins the gel again.

8. Shear history and recovery

Structure that is broken by pumping does not always rebuild to the same value. A gel that reads 40,000 centipoise in the tank can read 28,000 after passing a lobe pump and a narrow nozzle. Measure viscosity after simulated filling, and give the sample 24 hours to recover before judging the result, since many systems continue to build overnight.

9. Common failure modes

Thinning over three months usually means pH drift or an electrolyte leak from another ingredient. Gritty texture points to poorly dispersed carbomer or undissolved xanthan. “Fish eyes” come from adding powder to water too fast. A gel that weeps clear liquid on standing is a sign of network collapse, often from an overloaded thickener rather than too little.

10. Where it fits

Rheology decisions show up first in serums and creams. Compare cream formats and facial essence, or read the encyclopedia on emulsion design.