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Pigment dispersion

Also known as
pigment wetting, grind

Pigment dispersion is the process of breaking up clumps of dry pigment powder and distributing the individual particles evenly and stably in a liquid such as a paint or ink. It involves wetting the pigment, breaking apart agglomerates and stabilizing the particles against re-clumping.

What is pigment dispersion?

Pigments give coatings and inks their color, opacity and some protective properties. As supplied, pigment powders are made of tiny primary particles stuck together in clumps, called agglomerates, with air trapped between them. A coating performs as intended only if those clumps are broken up and the particles stay evenly spread through the liquid. Achieving that is pigment dispersion.

The three stages

  1. Wetting. The liquid must displace air from the pigment surface and from the spaces inside agglomerates. Wetting agents and surfactants lower surface tension so the liquid can reach those surfaces.
  2. Deagglomeration. Mechanical energy from mixers, mills and grinders breaks agglomerates into smaller particles.
  3. Stabilization. Dispersants adsorb on the particles and keep them apart by electrical repulsion or a physical polymer barrier, preventing them from clumping again, called flocculation.

Signs of poor dispersion

  • Weaker color strength and color shifts between batches.
  • Reduced gloss and a hazy appearance.
  • Settling or hard sediment in the container.
  • Floating and flooding, where pigments separate in the wet film.
  • Specks and defects in the dried film.

Formulators check dispersion with a fineness-of-grind gauge, color strength measurements and a rub-out test, in which rubbing the wet film reveals flocculated pigment by a color change.

Why it matters

Pigments are often among the most expensive ingredients in a coating, and poorly dispersed pigment does not contribute its full color or hiding power. Good dispersion therefore affects cost as well as appearance and durability. Moving to waterborne and low-VOC formulas often makes pigment wetting harder, which puts more weight on the wetting and dispersing additives.

Pigment dispersion and Locus biosurfactants

Locus supplies its Amphi biosurfactants to coatings formulators, describing them as improving substrate and pigment wetting, dispersion stability and film formation.

Frequently asked questions

A wetting agent helps the liquid displace air and wet the pigment surface. A dispersant keeps the separated particles from clumping back together. Some additives do both.

Particles that are not well stabilized attract one another and form loose clusters, especially when the formulation changes, such as on dilution, mixing with other components, or aging.

  • Dispersant

    A dispersant is a substance that keeps solid particles or droplets evenly separated in a liquid and prevents them from clumping together or settling out. It works by adsorbing on particle surfaces and creating electrical or physical repulsion between them.

  • Wetting

    Wetting is the ability of a liquid to spread over and stay in contact with a solid surface. It depends on the balance between the liquid's attraction to the surface and to itself, and it is usually measured by the contact angle a droplet makes with the surface.

  • Leveling (coatings)

    Leveling is the ability of a wet coating to flow out after application and form a smooth, even film, eliminating brush marks, roller texture and other surface irregularities before it dries. It is driven by surface tension and opposed by the coating's viscosity.

  • Cratering

    Cratering is a coating defect in which small, round depressions form in a wet film as it dries, sometimes exposing the surface underneath. It is usually caused by a low-surface-tension contaminant, such as a droplet of oil or silicone, that makes the surrounding coating flow away from it.

Sources

  1. Coatings technology references, e.g. Paint and Coating Testing Manual, ASTM International
  2. Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012