There is no electrostatic stabilisation to rely on
In water, a dispersant can stabilise pigment particles electrostatically, sterically, or through a combination of both. In a solvent-borne system the dielectric constant is low and the double-layer mechanism effectively disappears. Stabilisation is almost purely steric: the dispersant adsorbs onto the pigment through anchor groups and presents a layer of solvated chains that keeps particles apart.
This changes the selection logic. The two variables that matter most are the strength of adsorption onto the pigment surface and the thickness and solvation of the chain layer extending into the binder solution.
Match anchor chemistry to the pigment
Inorganic pigments and extenders offer polar sites that accept a wide range of anchors; polyester and polyurethane dispersants with amino or acidic groups adsorb readily. Organic pigments and carbon black present fewer, less polar sites and require dispersants with multiple anchor groups along the molecule.
A dispersant that desorbs during storage, displaced by binder or by another pigment, produces the familiar late failures: viscosity rise, hard settlement and loss of colour strength. Multi-point anchoring is the practical defence.
Chain compatibility with the binder solution
The solvated chains must be compatible with the solvent blend and the binder. Chains that are insoluble collapse onto the pigment and provide no stabilising layer; chains that are too compatible can promote interactions between pigment and binder and destabilise the millbase.
Polarity matching between dispersant, resin and solvent is therefore part of the selection, not an afterthought, and it is why universal solvent-borne grades are a compromise rather than a universal solution.
Verify on the full formulation
Grinding performance in the millbase is only the first gate. Resin let-down frequently shifts the equilibrium of adsorbed species, and rub-out, gloss and viscosity after let-down reveal failures that a millbase test misses.
Screen candidates at the dosage calculated on pigment weight, measure fineness and colour strength after grinding, then re-check after let-down and after accelerated storage. The dispersant that survives all three stages is the one that works in the product.
Apply this to your own formulation
Send us the binder system, substrate and the defect you are seeing. We will suggest a specific grade and dosage range.