Choosing a dispersant for pigment concentrates

Most dispersion problems get blamed on the mill. The mill is rarely the cause. Grinding breaks agglomerates apart for a few seconds; what happens next is chemistry, and the dispersant decides it. This is the selection logic, in the order that keeps a millbase from being re-formulated twice.

Three steps, and the dispersant owns two

Dispersing a pigment is three operations in sequence.

Wetting. The liquid phase has to displace air and adsorbed moisture from the pigment surface and penetrate the agglomerates. Surface tension governs this, and it happens before any energy goes in.

Deagglomeration. Mechanical. The mill or the disperser separates the wetted agglomerates down toward primary particles.

Stabilization. The freshly created surface is high energy and wants to come back together. Something has to sit on it and keep the particles apart, permanently, through storage, tinting, and let-down.

The mill only does the middle one. If wetting is poor the mill grinds slowly and never reaches full color development. If stabilization is poor the pigment re-agglomerates on the way to the can, and the result looks exactly like a grind that was cut short. That is why longer milling so often fails to fix a dispersion problem.

Two ways to keep particles apart

Once particles are separated, they are held apart by charge, by polymer chains, or by both.

Electrostatic. The dispersant gives every particle the same surface charge, and they repel. This works in water and it is cheap, but it is fragile: raise the ionic strength with an electrolyte, or move the pH toward the point where the charge disappears, and the repulsion collapses. Salt tolerance is the first thing to check on a purely electrostatic system.

Steric. The dispersant anchors on the particle and dangles polymer chains into the medium. When two particles approach, the solvated chains resist being compressed. This works in water and in solvent, and it is far less sensitive to pH and electrolyte. The requirement is that the dangling chains actually be solvated by the medium; a chain that collapses in the carrier gives no barrier at all.

Electrosteric. Both at once: a polymeric backbone with charged anchoring groups. This is the workhorse for modern waterborne concentrates, and it is why polymeric dispersants displaced simple surfactants for pigment work.

Match the anchor to the pigment surface

The anchoring group is the part of the molecule that decides whether the dispersant stays put. Different pigments give it very different surfaces to grip.

Titanium dioxide and other treated inorganics. Pigmentary grades are surface treated, typically with alumina and silica, so they present a polar, hydroxylated surface. Acidic anchoring groups adsorb onto it readily. Specific surface area is low, roughly ten to twenty square meters per gram, so the dispersant demand per gram of pigment is the lowest of the three families. Iron oxides behave similarly.

Carbon black. The hard case. Surface area runs from tens up to several hundred square meters per gram depending on grade, the surface is largely non-polar, and there are few obvious anchor sites unless the black has been oxidized. Simple surfactants do not hold. What works is a high-molecular-weight polymeric dispersant carrying many anchoring groups along one chain, so that even weak individual adsorptions add up to an anchor that does not desorb. Dispersant demand is the highest of the three by a wide margin, and jetness and gloss are the first things to suffer when the dose is short.

Organic pigments. The most variable. Surface areas sit between the other two, surfaces are low in polarity, and the crystal chemistry differs from one pigment class to the next, so a dispersant that is excellent on a phthalocyanine blue can be mediocre on a quinacridone. This is where pigment synergists earn their place: a molecule built on the pigment's own chromophore adsorbs strongly onto the crystal and presents a group the dispersant can anchor to, effectively giving a difficult surface a handle.

Dose from surface area, not from weight percent

A dispersant covers surface. Two pigments at the same weight loading can differ by an order of magnitude in how much surface they present, which is why a dose carried over from one concentrate to the next so often lands wrong.

Work in milligrams of dispersant solids per gram of pigment, or per square meter of pigment surface if you have the BET number. Then find the level empirically: run a ladder of three to five dispersant loadings on the same pigment and resin, mill each the same way, and measure millbase viscosity, color strength, and gloss on a drawdown. Viscosity drops steeply as coverage improves and then flattens. That plateau is where the surface is covered. The working dose sits at the start of it, not well past it.

Overdosing is not free. Excess dispersant stays in the film as a mobile, often water-sensitive component: it can raise foam, soften the film, hurt water resistance and adhesion, and in a tinting system it migrates and causes problems in a base paint it was never tested in.

What under-dispersion looks like on the line

Flocculation is reversible re-agglomeration, and it announces itself in a handful of ways worth recognizing early.

Viscosity that climbs on standing. A millbase that thickens overnight is building a particle network. This is often the first signal, and it shows up before any color problem does.

Color strength short of target. Flocculated pigment scatters and absorbs less efficiently, so the batch reads weak and the reflex is to add pigment, which raises cost and makes the flocculation worse.

A rub-out that changes shade. The direct test. Draw down the paint, then rub a small area with a finger as it starts to set. Rubbing shears loose flocculates apart, so if the rubbed patch develops more strength or a different shade than the film around it, the pigment was flocculated. It takes ten seconds and needs no instrument.

Float and flood in mixed-pigment systems. Pigments of different size and density separate as the film dries and convection carries them, giving streaks or a uniform shade shift on the surface. It shows up in tinted systems where one pigment is stabilized well and the other is not.

Gloss loss, haze, and seeding. Particles that never came apart, or came apart and rejoined, sit proud of the film surface and scatter light.

Water, solvent, or universal

The carrier sets which stabilization mechanism is available. In waterborne systems electrosteric dispersants dominate, and the pH window matters: check that the dispersant holds through the range the paint will actually see, including after tinting. In solventborne systems stabilization is steric only, and the chains have to be solvated by that specific solvent blend, so a dispersant that performs in an ester can disappoint in an aliphatic.

Universal tinting is its own problem. A colorant that has to go into both waterborne and solventborne bases needs a dispersant with solubility in both, which is a genuine constraint on the chemistry and worth stating up front rather than discovering at the second base.

The regulatory layer

Alkylphenol ethoxylates were the default nonionic for pigment wetting for decades, and they are being designed out. Nonylphenol and its ethoxylates are restricted in the EU under REACH Annex XVII: entry 46 covers cleaning and a set of industrial uses at 0.1 percent by weight, and entry 46a covers textile articles that can reasonably be expected to be washed in water at 0.01 percent, which applied from 3 February 2021. Downstream customers increasingly ask for APEO-free regardless of whether the restriction reaches their own article.

Treat the swap as a formulation change. Alcohol ethoxylates and biobased alternatives differ in cloud point, foam, wetting rate, and electrolyte tolerance, so the millbase needs re-optimizing rather than substituting. Ask for readily biodegradable classification, aquatic toxicity data, and inventory status at the sample stage, so the compliance answer arrives with the performance answer instead of six weeks later.

What to send us

The fastest route to a short list is four facts: the pigment or filler and its grade, the resin system, the carrier, and what is going wrong now. Millbase viscosity data and a drawdown help more than a description.

P.A.T. Products supplies the Lankem surfactant and dispersant range in North America, including the LANSPERSE polymeric and biobased dispersants across polyacrylate, polyester, polyether, and polyurethane backbones, and grades soluble in both water and solvent for universal tinting work. The full line sits on the industrial surfactants and dispersants page, and the manufacturer background is on the Lankem partner page.

If your question is the color itself rather than the dispersion, that is a different desk: pigments and color covers the liquid color and masterbatch side, and color match covers matching to a standard. The choice between the two delivery forms is covered in masterbatch vs liquid color.

Frequently asked

What is the difference between a wetting agent and a dispersant?

A wetting agent lowers surface tension so the liquid displaces air and moisture from the pigment surface and gets into the agglomerates. A dispersant is normally a higher-molecular-weight polymer that anchors onto the particle after it has been broken down and holds particles apart so they do not come back together. Wetting happens before the mill; stabilization has to hold after it. Many products do both, which is why the term wetting and dispersing agent exists.

Why does carbon black need more dispersant than titanium dioxide?

Surface area and surface chemistry. Pigmentary titanium dioxide is surface treated with alumina or silica, giving a polar surface that acidic anchors grip, at roughly ten to twenty square meters per gram. Carbon black runs from tens to several hundred square meters per gram with a largely non-polar surface and few anchor sites, so it needs much more dispersant and a high-molecular-weight polymer carrying many anchoring groups per chain.

How much dispersant should I use?

Dose against pigment surface area, not total formulation weight. Express it as milligrams of dispersant solids per gram of pigment, run a ladder of three to five loadings, and measure millbase viscosity, color strength, and gloss. The curve flattens once the surface is covered, and the working dose sits at the start of that plateau. Past it you buy water sensitivity and foam rather than stability.

How do I tell whether a pigment has flocculated?

Rub out the wet film. Draw down the paint and rub a small area with a finger as it begins to set. Rubbing breaks loose flocculates, so a rubbed patch that gains strength or shifts shade means the pigment was flocculated. Millbase viscosity rising on standing, weak color strength, gloss and haze problems, and float and flood patterns all point the same way.

Is switching to an APEO-free dispersant a drop-in change?

Rarely. Nonylphenol ethoxylates are restricted under REACH Annex XVII, entry 46 for cleaning and several industrial uses and entry 46a for textiles washed in water, which applied from February 2021. Alcohol ethoxylate and biobased replacements differ in cloud point, foam, wetting rate, and electrolyte tolerance, so the millbase needs re-optimizing. Ask for biodegradability and aquatic-toxicity data and inventory status at the sample stage.

Selecting a dispersant?

Send the pigment, the resin, the carrier, and the failure mode. We will recommend a grade to trial, ship a sample, and bring the manufacturer's technical team in where the millbase needs work.