Dispersants for Coatings: Selection, Chemistry and Dosage Optimisation

---

A practical selection guide for formulators: how dispersants work, how to match chemistry to pigment type, how to find the optimal dosage, and how to avoid the mistakes that quietly cost you colour strength, gloss, and shelf life.

Why the Dispersant Decides Everything Downstream

A dispersant is a small part of the formulation — typically 0.5–3% on pigment — but it is the additive with the longest reach. Every property the customer sees depends on how well the pigment was wetted, separated, and stabilised:

• Colour strength — flocculated pigment develops less colour, so you compensate with more pigment
• Gloss — agglomerates scatter light and flatten the surface
• Hiding power — poorly dispersed TiO₂ hides less, so you add more of the most expensive raw material in the formula
• Storage stability — inadequate stabilisation shows up as settling, viscosity drift, and colour float weeks after the batch shipped
• Weatherability — agglomerates create weak points where the film degrades first

This is why dispersant selection deserves more rigour than almost any other additive decision. Get it right and the rest of the formulation becomes easier. Get it wrong and you spend the next two years compensating with raw material cost.

---

The Three Stages of Dispersion

Dispersion is not one process. It is three, and a dispersant contributes to each of them differently.

Stage 1: Wetting

The pigment surface arrives coated in air and adsorbed moisture. Before anything else can happen, the liquid phase must displace that air and make contact with the solid surface.

Wetting is driven by surface tension. If the liquid's surface tension is too high relative to the pigment's surface energy, the liquid will not penetrate the agglomerate pores and the mill simply grinds dry powder in a wet medium.

Stage 2: Separation (grinding)

Mechanical energy — bead mill, high-speed disperser, basket mill — breaks agglomerates into aggregates and, ideally, into primary particles. This is the energy-intensive stage and the one most often blamed when dispersion quality is poor.

In practice, the mill is rarely the problem. If separation is slow, the usual cause is that stage 1 was incomplete or stage 3 is failing, so particles re-agglomerate as fast as the mill separates them.

Stage 3: Stabilisation

Freshly separated particles have high surface energy and want to re-agglomerate. The dispersant must create a barrier that prevents them from approaching close enough for van der Waals attraction to take over.

This is where the dispersant does its most important work, and where cheap products fail. A surfactant can wet a pigment. Only a properly designed dispersant can keep it apart for twelve months in a warehouse.

---

Two Stabilisation Mechanisms

Electrostatic stabilisation

The dispersant imparts charge to the particle surface. Like charges repel, keeping particles separated.

• Works well in water, where the dielectric constant supports charge separation
• Fails in solvent-borne systems — low dielectric constant collapses the double layer
• Sensitive to pH and to electrolytes; adding a salt or shifting pH can flocculate the whole batch
• Typical chemistry: polyacrylates, polycarboxylates, polyphosphates

Steric stabilisation

The dispersant anchors to the pigment surface and extends polymer chains into the liquid phase. When two particles approach, the chains are compressed, which is entropically unfavourable, so the particles push apart.

• Works in both water and solvent
• Insensitive to pH and electrolytes
• Requires a polymer with a genuine anchor group and a genuinely solvated chain — this is what separates a real dispersant from a surfactant
• Typical chemistry: polyurethanes, modified polyacrylates, polyester-polyamines

Why modern water-based systems need both

Classical polycarboxylates rely on charge alone. That is why they lose performance as pigment loading rises, as pH drifts, or as freeze-thaw cycles disturb the double layer.

Polymeric hyperdispersants provide steric stabilisation with an electrostatic contribution. The result is higher pigment loading, lower viscosity, better freeze-thaw stability, and much less sensitivity to formulation changes. In high-performance water-based coatings they have largely displaced the classical chemistries, and the gap widens as pigment loading increases.

---

Anatomy of a Polymeric Dispersant

Understanding the structure explains most selection decisions.

The two matching rules that drive most selection decisions:

1. Anchor must match the pigment. Basic pigment surfaces (most inorganics, treated TiO₂) need acidic anchors. Acidic surfaces (many organics, carbon black) need basic anchors. Get this backwards and adsorption is weak, so stabilisation fails regardless of how good the rest of the molecule is.

2. Solvated chain must match the medium. A polyether chain works in water and in polar solvents; a polyester chain works in medium-polarity systems; a polyolefin chain works in aliphatic solvents. If the chain is not properly solvated it collapses onto the particle and provides no steric barrier at all.

This is also why a dispersant that performs beautifully in one resin system can fail completely in another with the same pigment.

---

Selection by Pigment Type

Titanium dioxide

The workhorse and the largest cost item in most white formulations. Surface-treated TiO₂ (alumina, silica) presents a mildly basic, high-energy surface that is comparatively easy to wet.

• Chemistry: acidic anchor groups, polyether-modified polymer
• Priority: viscosity reduction at high loading, which is what enables higher solids
• Target: Hegman 7+ within normal grind time
• Typical dosage: 0.3–0.8% on pigment
• ASTRA: DISP-1306 (analogue DISPERBYK-103), DISP-1206 (analogue DISPERBYK-180)

Inorganic colour pigments (iron oxides, chrome, ultramarine)

Larger particles, lower surface area, generally straightforward. Iron oxides are the easiest common pigment class to disperse.

• Chemistry: acidic anchor, moderate molecular weight
• Priority: anti-settling, since these are dense pigments
• Typical dosage: 0.3–1.0% on pigment
• ASTRA: DISP-1106 (analogue DISPERBYK-108), DISP-1506

Organic pigments

High surface area, small primary particle size, strong tendency to flocculate. This is where dispersant quality becomes visible immediately.

• Chemistry: high-molecular-weight polymeric hyperdispersant, basic anchor groups
• Priority: colour strength development and gloss
• Typical dosage: 15–40% on pigment for tinting pastes; 2–6% in a pigmented topcoat
• ASTRA: DISP-1806 (2K PU, acrylate, polyester, UV inks), DISP-1706 (medium-polarity systems)

Cost note: organic pigment dispersant loadings look alarming until you compare them with the pigment price. Underdosing a phthalocyanine blue to save on dispersant is one of the most expensive economies available to a formulator.

Carbon black

The hardest common pigment to disperse. Very high surface area, strongly hydrophobic, and highly sensitive to inadequate stabilisation — jetness collapses long before you see obvious agglomerates.

• Chemistry: very high molecular weight, multiple anchor points, aromatic-compatible chain
• Priority: jetness (My value), blue undertone, viscosity control
• Typical dosage: 30–100% on pigment for high-jetness systems
• ASTRA: DISP-1006 (analogue DISPERBYK-130), DISP-1806

Effect pigments (aluminium, pearlescent)

Not a dispersion problem in the usual sense — the platelets must not be broken or bent, and orientation in the film matters more than particle size.

• Chemistry: low-shear compatible, orientation-controlling additive
• Priority: platelet integrity and parallel orientation
• Process: never mill; stir in gently at let-down
• Note: avoid water contact in the dispersing stage for non-passivated aluminium — gassing risk

Matting agents and fillers

Silica needs acidic anchors; wax needs non-polar compatibility. Matting agents are shear-sensitive and lose matting efficiency if over-ground.

• ASTRA: DISP-1706 (matting agent dispersion)

---

Finding the Optimal Dosage: the Adsorption Isotherm

The single most useful test in dispersant work, and the one most often skipped.

Every dispersant/pigment pair has a characteristic adsorption isotherm — a curve of how much dispersant the pigment surface adsorbs before saturation.

Below saturation: part of the pigment surface is unprotected. Particles flocculate, viscosity is high, colour strength is low, and storage stability is poor.

At saturation: minimum viscosity, maximum colour development, best stability. This is the target.

Above saturation: excess dispersant remains free in the liquid phase, where it can cause:

• Foam stabilisation, particularly in water-based systems
• Water sensitivity and poor wet adhesion
• Intercoat adhesion failure
• Viscosity drift during storage
• Increased cost with no performance benefit

The three-point dosage test

You do not need a full isotherm. A three-point test finds the working optimum in a single day:

1. Prepare grind pastes at 80%, 100%, and 120% of the intended dosage
2. Mill all three under identical conditions and record grind time to target Hegman
3. Measure viscosity, Hegman fineness, gloss, and colour strength
4. Run 40 °C storage for 7 days and re-measure viscosity

The lowest viscosity at equivalent fineness is the practical saturation point. If 120% gives no viscosity improvement over 100%, you are already at or above saturation and can often reduce the dosage.

This is also why 1:1 substitution between suppliers rarely works. Two dispersants with the same active chemistry can have materially different isotherms because of differences in molecular weight distribution, anchor density, and active content. Always run the three-point test when you change product.

---

Dispersant Selection Workflow

Work through these in order. Each step eliminates options, so the sequence saves time.

Step 1 — System. Water-based, solvent-borne, UV-curable, or solvent-free? This eliminates most of the catalogue immediately, because solvated chain requirements differ fundamentally.

Step 2 — Pigment. Inorganic, organic, carbon black, effect, or matting agent? This determines the anchor group.

Step 3 — Resin. Acrylic, alkyd, polyurethane, epoxy, polyester, amino? Check compatibility, particularly for hydroxyl- or amine-functional resins that can interact with the dispersant.

Step 4 — Application priority. What matters most: maximum colour strength, minimum viscosity at high solids, storage stability, or lowest cost? Ranking these prevents the common mistake of optimising for the wrong property.

Step 5 — Constraints. APEO-free? Low VOC? Food-contact compliance? Regional regulatory requirements? Apply these as filters, not afterthoughts.

Then run the three-point dosage test on the two or three candidates that survive.

---

Seven Dispersant Mistakes That Cost the Most

1. Using a wetting agent as a dispersant. A surfactant lowers surface tension and helps wetting, but has no anchor group and no meaningful steric chain. Dispersion looks acceptable on day one and flocculates in the drum. If the label says "wetting agent" and nothing about stabilisation, it is not a dispersant.

2. Applying solvent-borne logic to water. Electrostatic stabilisation behaves completely differently in the two media. A dispersant optimised for a solvent-borne alkyd will underperform in a water-based acrylic even with the same pigment.

3. Dosing on total formulation instead of on pigment. Dispersant demand scales with pigment surface area, not with batch size. Reformulate the pigment package and the dispersant dosage must be recalculated — this is the most common cause of "the formula suddenly stopped working."

4. Accepting Hegman 5 when Hegman 7 is available. The difference is invisible in a drawdown but shows up in hiding power. Going from Hegman 5 to 7 typically reduces TiO₂ demand by 5–10%, which is worth far more than the dispersant costs.

5. Overdosing "to be safe." Excess dispersant is not neutral. It stabilises foam, increases water sensitivity, and causes intercoat adhesion failures that surface only after the customer applies a second coat.

6. Skipping storage stability testing. Flocculation, settling, viscosity drift, and colour float develop over weeks. A dispersion that passes on the day of manufacture can fail in the customer's warehouse. Minimum protocol: 40 °C for 30 days, plus five freeze-thaw cycles for water-based systems.

7. Testing dispersant changes on a problematic formulation. If you want to know whether a new dispersant works, test it on a formula you understand completely. Introducing a new variable into a formula that already has issues produces results nobody can interpret.

---

Test Protocol for Qualifying a New Dispersant

A structured evaluation takes roughly three weeks and removes almost all of the risk.

Lab screening — week 1

Stability — weeks 2–3

Production trial — week 3+

Scale to 100–500 kg on a standard, well-characterised formulation using normal production equipment. Confirm grind time, viscosity, and colour against lab results before committing to a portfolio switch.

---

ASTRA DISP® Product Range

ASTRA DISP® covers 97 grades across water-based, solvent-borne, and radiation-curable systems.

Additional grades cover water-based architectural systems, APEO-free requirements, and pigment concentrate production. Full TDS and SDS documentation is available for every grade.

Trademark notice: DISPERBYK, BYK, TEGO and other product names are the property of their respective owners. References are for identification only and do not imply affiliation or endorsement. "Analogue" indicates comparable chemistry and property range, not identity. Always validate with your own testing.

---

Key Takeaways

• Dispersion is three processes — wetting, separation, stabilisation — and stabilisation is where cheap products fail
• Match the anchor group to the pigment surface and the solvated chain to the medium; these two rules drive most selection decisions
• Polymeric hyperdispersants outperform classical polycarboxylates in water-based systems, and the gap widens as pigment loading rises
• Dose on pigment, never on total formulation, and re-optimise whenever the pigment package changes
• Run the three-point dosage test (80/100/120%) on every dispersant change — 1:1 substitution between suppliers rarely lands on the optimum
• Hegman 5 to 7 typically cuts TiO₂ demand by 5–10%, which pays for a better dispersant many times over
• Overdosing causes foam stabilisation, water sensitivity, and intercoat adhesion failure — more is not safer
• Storage stability testing at 40 °C for 30 days is the minimum insurance against complaints that appear weeks after shipment