How to Select Dispersing Agents for Paints and Coatings

Dispersion is the process where a non-soluble solid is suspended in a liquid and stabilised against settling, separation, agglomeration, or aggregation. The concentration of solids in the liquid must be uniform during mixing, and once mixing is removed, the mixture must remain stable. For paints and coatings, pigments are dispersed into a resin/solvent matrix, and stability against dispersion or flocculation is an essential formulation requirement.

All dispersing agents have two key parts: an anchoring group and a soluble tail. Anchoring groups adsorb to the surface of pigments and fillers, forming strong physical bonds with the chemical groups present on the particle surface — the so-called anchor sites. Soluble tails create a stabilising layer around the solid particles, preventing dispersed particles from coming together again and ensuring colloidal stability.

If there is no affinity between the anchor groups of the dispersant and the anchor sites of the solid particles, bonding fails and the dispersant cannot fulfil its role in the formulation. Chemical functional groups at the surface of pigments and fillers vary considerably. Some surfaces are surface-treated and others are not, which is why the dispersant offering available to formulators is so wide. For solvent-based systems, steric stabilisation is the primary mechanism. For water-based systems, both electrostatic and steric stabilisation can prove useful, and some modern dispersants offer electro-steric stabilisation in a single molecule.

There are three main types of dispersing agents: Conventional Dispersing Agents based on polyesters, polyamides, polyglycols, and fatty acid chemistry (Mol wt. 500–2000 g/mol) that provide mainly electrostatic stabilisation; Polymeric Dispersing Agents typically based on polyacrylates, polyester, polyether, or polyurethane chemistry (Mol wt. 5000–50,000 g/mol) that are very effective for long-term stabilisation; and Ionic and Non-Ionic Dispersing Agents based mainly on alkyl phenol ethoxylate chemistry (Mol wt. 300–1,000 g/mol) providing mainly electrostatic stabilisation. Selecting the right dispersant requires careful evaluation of the complete viscosity-vs-concentration curve to ensure both performance and processability across production tolerances.