Pigment Dispersion
Breaking down pigment agglomerates into primary particles and stabilizing them in a vehicle — the single most important factor in color development, gloss, transparency, and storage stability.
What Is Pigment Dispersion?
Dispersion is the process of breaking down pigment agglomerates (clusters of primary particles bound by van der Waals forces) into individual primary particles and uniformly distributing them in a binder or vehicle. Effective dispersion is the foundation of all pigment performance — a poorly dispersed pigment will never achieve its rated color strength, gloss, transparency, or chemical resistance, regardless of its inherent quality.
The Three Stages of Dispersion
Stage 1 — Wetting: Replacing air and moisture on the pigment surface with the binder/vehicle. Determined by surface tension of the vehicle vs. surface energy of the pigment. Poor wetting = air pockets trapped in agglomerates, preventing mechanical separation.
Stage 2 — De-agglomeration: Applying mechanical energy to break agglomerates into primary particles. Achieved via high-speed dissolver, bead mill, triple-roll mill, or ultrasonic dispersion. The amount of energy required depends on pigment hardness (cohesive energy density) and specific surface area.
Stage 3 — Stabilization: Preventing re-agglomeration through electrostatic repulsion (zeta potential > ±30mV), steric hindrance (polymeric dispersant adsorbed layer), or a combination of both. Unstable dispersions result in color drift, settling, and viscosity increase over time.
Quality Metrics & Test Methods
| Metric | Test Method | What It Measures | Target Range |
|---|---|---|---|
| Fineness of Grind (Hegman) | ASTM D1210 | Largest agglomerates in the dispersion (µm) | Coatings: Hegman 5-7 (12-40µm); Inks: Hegman 6-7.5 (5-20µm); Inkjet: <0.5µm (not measurable by Hegman) |
| Particle Size D50/D90 | ISO 13320 (Laser Diffraction) | Median and upper-decile particle diameter | D50 should approach primary particle size; D90/D50 ratio < 3 indicates narrow distribution |
| Color Strength Development | Spectrophotometer, 1:10 TiO₂ reduction | Color strength vs. grind time — plateaus when fully dispersed | >95% of maximum within target grind time |
| Gloss (20°/60°/85°) | ISO 2813 | Surface smoothness — undispersed particles scatter light | 20° gloss > 80 GU for high-gloss coatings |
| Transparency | Contrast ratio on black/white chart | Degree of transparency — affected by particle size | Process inks: contrast ratio < 0.15 |
| Viscosity Stability | Brookfield / Cone & Plate | Viscosity change over time (indicates flocculation) | <15% increase after 28 days at 50°C |
Pigment Dispersion Difficulty by Type
| Pigment (CI) | Primary Particle Size (nm) | SSA (m²/g) | Dispersion Difficulty | Recommended Mill Type | Target Hegman |
|---|---|---|---|---|---|
| PB15:3 — Phthalo Blue | 50–100 | 40–80 | Hard | Bead mill (0.3–0.8mm beads) | 7+ |
| PG7 — Phthalo Green | 30–80 | 50–90 | Very Hard | Triple-roll mill or bead mill | 7+ |
| PR254 — DPP Red | 50–150 | 30–60 | Moderate | Bead mill | 7 |
| PV19 — Quinacridone Violet | 50–120 | 30–50 | Moderate | Bead mill | 6–7 |
| PY150 — Benzimidazolone | 80–200 | 15–30 | Moderate | Bead mill or dissolver | 6–7 |
| PY12 — Diarylide Yellow | 50–80 | 20–40 | Easy | High-speed dissolver | 6–7 |
| PR57:1 — Lithol Rubine | 80–150 | 15–25 | Easy | High-speed dissolver | 6–7 |
| PR101 — Iron Oxide Red | 100–500 | 5–15 | Very Easy | High-speed dissolver | 5–6 |
| PBk7 — Carbon Black | 13–75 | 25–260 | Very Hard | Bead mill | 7–8 |
| PW6 — TiO₂ (Rutile) | 200–300 | 7–15 | Easy | High-speed dissolver | 6–7 |
Dispersant Selection by Pigment Chemistry
| Pigment Chemistry | Surface Character | Recommended Dispersant Type | Typical Dosage (% on Pigment) |
|---|---|---|---|
| Phthalocyanine (PB15, PG7) | Non-polar, aromatic surface | Copper phthalocyanine sulfonic acid derivatives (synergists), polyurethane-based polymeric dispersants | 10–30% (polymeric), 2–5% (synergist) |
| DPP (PR254) | Polar carbonyl groups | Polyacrylate-based, polyester-based polymeric dispersants | 15–25% |
| Quinacridone (PV19, PR122) | Polar N-H and C=O groups | Polyacrylate, polyurethane dispersants with amine anchoring groups | 15–30% |
| Azo (PY12, PR57:1, PR170) | Moderately polar | Polymeric dispersants (polyacrylate, polyester), fatty acid derivatives | 10–20% |
| Carbon Black (PBk7) | Non-polar with oxidized surface groups | Polyurethane-based, polyacrylate with aromatic anchoring groups | 20–50% (high SSA grades) |
| Iron Oxides (PR101, PY42) | Hydrophilic, charged surface | Polyacrylate-based, polyphosphate esters | 5–10% |
| TiO₂ (PW6) | Hydrophilic with surface treatment | Polyacrylate, polyphosphate, amino alcohol-based | 2–5% |
Frequently Asked Questions
How do I know if my pigment is fully dispersed?
Three diagnostic methods: (1) Hegman gauge — grind readings should plateau at the target value; (2) Color strength development curve — measure color strength (1:10 TiO₂ reduction) at increasing grind times; when it plateaus (<2% increase with additional grinding), dispersion is complete; (3) Microscope inspection at 400x — no visible agglomerates >1µm. For inkjet-grade dispersions (<0.2µm), particle size analyzer (Malvern) is the only reliable method.
Why does PB15:3 need bead milling while PY12 only needs a dissolver?
PB15:3 (phthalocyanine blue) has a very high specific surface area (40-80 m²/g) and strong inter-particle cohesive forces due to its planar aromatic structure. Breaking these agglomerates requires the high shear and impact energy of a bead mill. PY12 (diarylide yellow) has lower SSA (20-40 m²/g) and weaker agglomerates — the shear from a high-speed dissolver (tip speed 15-25 m/s) is sufficient. Using a dissolver for PB15:3 will never achieve full color strength — you’ll leave 20-40% of the potential color strength undeveloped.
What’s the target Hegman reading for printing inks vs coatings?
Offset/letterpress paste inks: Hegman 6-7 (10-20µm) — critical for gloss and transparency. Flexo/gravure liquid inks: Hegman 7-8 (5-10µm) — critical for print quality on smooth substrates. Industrial coatings: Hegman 5-6 (20-30µm) — sufficient for most applications. Automotive topcoats: Hegman 7 (10-15µm). The finer the dispersion, the higher the gloss, color strength, and transparency — but costs increase exponentially with grind time.
How do I calculate the correct dispersant dosage?
The C.A.D. (Concentration at Agglomerate Demand) method: prepare a series of pigment dispersions with increasing dispersant concentration (e.g., 5%, 10%, 15%, 20%, 30% on pigment weight). Measure viscosity of each dispersion — it will decrease as dispersant increases, then plateau or increase. The dosage at the viscosity minimum is the optimal C.A.D. As a formula: Dispersant (g) = Pigment Weight (g) × SSA (m²/g) × Anchor Group Coverage (mg/m²) / 1000. Typical anchor group coverage: 2-5 mg/m² for phthalos, 1-3 mg/m² for azo pigments.
Pro Tip
Dispersion quality affects everything downstream — color strength, gloss, transparency, rheology, storage stability, and even chemical resistance. Investing an extra 15 minutes of grind time on a bead mill can save hours of reformulation later. For hard-to-disperse pigments (PB15:3, PG7, PBk7), always pre-wet with solvent/vehicle for 10-15 minutes before starting mechanical dispersion to displace air from the pigment surface.
Need help optimizing your pigment dispersion?
Our technical team can recommend the right dispersant and milling conditions for your specific pigment-vehicle system.