Pigment Dispersion: Grind Size and the White Cast Problem

Pigment dispersion is the step that decides whether a foundation looks smooth or grainy, whether a shade matches the approved standard, and whether a mineral sunscreen leaves a cast. The pigment dose gets the attention; the dispersion gets the complaints.

1. Agglomerates

Pigment arrives as agglomerates of primary particles. Until those are broken down, the effective particle size is far above the intended one, and the product reads as grain, specks or an uneven cast. A dispersion below 20 microns is generally invisible on skin; above 40 microns it is visible in a mirror.

2. Wetting

Wetting the pigment with a surfactant or a dispersant before milling prevents the particles from re-agglomerating after the mill. Dry pigment added directly to an emulsion will re-agglomerate within days. The wetting step is small in cost and decisive in outcome.

3. Mill type and energy

A triple-roll mill suits high-viscosity pigment pastes and is the standard for lipstick and pressed powder. A bead mill suits low-viscosity emulsions and gives finer dispersion with less heat. The energy input, expressed as passes or residence time, belongs in the batch record.

4. Temperature

Milling raises temperature, and heat can degrade actives or volatilise a fragrance. A bead mill running too long can push a sensitive emulsion above 50 degrees and change its texture. Cool the mill or shorten the residence time rather than accepting the heat.

5. The white cast

A mineral sunscreen cast comes from particles scattering visible light, which is a function of size and load, not of dispersion quality alone. Better dispersion reduces the patchiness of the cast and makes the film more even, which reads as a lighter cast even at the same dose.

6. Shade matching

Two batches with the same pigment dose can read half a shade apart if the dispersion differs. Measure on a spectrophotometer at a fixed film thickness, and hold the mill parameters fixed across batches. Colour complaints in foundation are dispersion complaints more often than pigment complaints.

7. Pigment loading limits

Above 20 percent pigment in a liquid foundation, the viscosity rises and the dispersion becomes difficult to hold. The practical ceiling is 15 to 18 percent for a flowable product. A higher-coverage product needs a different vehicle rather than more pigment.

8. Visual checks

Draw down a film on a glass plate at 25 microns and inspect under a lamp. Speckles, streaks and a grainy edge are all visible before the batch is filled. The draw-down takes two minutes and catches most dispersion failures that a lab measurement would miss.

9. Stability of the dispersion

Dispersed pigment can settle or flocculate over shelf life, producing a shade that reads darker at the bottom of the bottle. A yield value in the vehicle and a dispersant that stays active prevent it. Shake-before-use instructions are a workaround, not a fix.

10. Supplier variation

Two pigment suppliers can deliver the same CI number with different surface treatment and different dispersion behaviour. A brand that switches supplier without re-running the dispersion trial will see a shade shift. Keep the supplier in the specification.

11. Where it fits

Dispersion runs across the foundation range; the shade question continues in the shade matching entry.