Thickening

The property customers judge by, and the one most sensitive to small changes.

Three ways to thicken

Salt, into a suitable surfactant system. Cheap, simple and sharply non-linear.

Polymer thickeners, which build structure and are usually pH-dependent.

Surfactant structuring, using the surfactant blend itself to build viscosity.

They behave differently under shear, in the cold, and over time, so the choice is not only about the number on a viscometer.

Salt thickening and its peak

Add salt to an anionic surfactant solution and viscosity rises, reaches a maximum, then falls sharply.

The peak is the trap. A formula sitting on it is at its most viscous and at its least stable: a small variation in surfactant lot or salt level moves viscosity a long way in either direction.

Formulate deliberately on the low-salt side of the peak, where the curve is flatter, and accept a slightly lower viscosity for a far more reproducible product.

And note that salt also lowers the cloud point, so a salt-thickened product is more likely to go cloudy in the cold.

Polymers

Dispersion technique matters more than dose. Added wrong they form fish-eyes that never fully hydrate, and the batch looks thin while containing all the thickener you paid for.

Most are pH-dependent and build viscosity only after neutralization, which is why order of addition matters so much here.

They are also food for microbes, which is the link between a thinning product and a preservative failure.

Getting it consistent

Measure viscosity on every batch, with the same method, at the same temperature, after the same standing time.

Standing time matters: many formulas keep building for hours after mixing, so a reading taken at the end of mixing is not the product.

Record a target and a tolerance, and treat a batch outside it as a signal rather than as something to correct into range.

References