Dispersant Selection and Bead Mill Process Parameters for Color Paste Manufacturing
A correctly chosen dispersant will still fail if the grinding parameters do not match, and the reverse is equally true.
A correctly chosen dispersant will still fail if the grinding parameters do not match, and the reverse is equally true. This article separates color paste process selection into two blocks. Block one covers dispersant selection and dosage: which class to use, which anchoring group it requires, which pigment and system it suits, how it affects fineness and viscosity, and which standard governs acceptance. Block two covers bead mill parameters: media material and diameter, tip speed, number of passes, residence time, slurry temperature and fill ratio. The values given are general engineering ranges, not fixed process cards. Final settings must be confirmed by trial runs on the actual equipment and pigment system. Fineness is measured to GB/T 6753.1 (scraper method), viscosity to GB/T 9751, and particle size distribution to ISO 13320 (laser diffraction).
1. Dispersant class, anchoring group and applicable system
Dispersants fall into three anchoring mechanisms: acidic anchoring for metal oxides and inorganic pigments, alkaline or polyamine anchoring for acidic carbon black surfaces, and non-ionic or general wetting types.
Table 1 — Dispersant selection matrix
| Dispersant class | Primary anchoring group | Suitable pigments | Suitable systems | Effect on fineness and viscosity | Acceptance test |
|---|---|---|---|---|---|
| Acidic anchoring (carboxylic, phosphoric, phosphonic) | –COOH, –PO4H2, phosphonic | Titanium dioxide, iron oxide and other metal oxide and inorganic pigments | Solvent-based (acrylic, polyester, alkyd); UV/EB (epoxy and polyurethane acrylate) | Lowers initial viscosity and shortens grinding time; overdose raises viscosity after heat storage and reduces film adhesion | Fineness GB/T 6753.1; viscosity GB/T 9751; PSD ISO 13320; heat storage stability |
| Alkaline / polyamine anchoring | Primary, secondary and tertiary amine, polyamine, amide | Carbon black, high surface energy organic pigments | Solvent-based and general industrial pastes; UV systems require assessment of yellowing and cure interference | Effective at opening carbon black agglomerates; excess free amine raises viscosity and causes UV yellowing | Fineness GB/T 6753.1; viscosity GB/T 9751; D50/D90 to ISO 13320; yellowing and adhesion after UV cure |
| Non-ionic / general wetting | Polyether, esterified or neutral anchor | General and mixed pigment systems | Solvent-based; water-borne dual use requires verification; UV/EB by compatibility | Fast wetting and moderate viscosity reduction; usually insufficient alone for hard-to-disperse carbon black or titanium dioxide | Fineness GB/T 6753.1; viscosity GB/T 9751; settling in storage |
| Polymeric hyperdispersant (two-segment) | Above anchoring end plus compatible solvated chain | High specific surface area pigments, high solids pastes | High-solids solvent-based; true solvent-free UV/EB (FLUV) | Thick steric layer and good anti-flocculation; solvated chain must be compatible with resin or monomer | Fineness GB/T 6753.1; viscosity GB/T 9751; PSD ISO 13320; cold and hot storage cycling |
Selection principle: match the anchoring group to the pigment surface acidity or basicity first, then match the solvated chain to the resin or monomer phase. Estimate dosage from oil absorption or BET specific surface area and confirm it by a five-point gradient. Do not apply a fixed percentage.
2. Dosage: estimate, gradient, heat storage
Step 1 — Estimate the starting anchor quantity from pigment oil absorption or BET specific surface area. Step 2 — On the same mill, with the same media and tip speed, run five gradients at 50, 75, 100, 125 and 150 percent of the estimate. Measure fineness to GB/T 6753.1, viscosity to GB/T 9751 and particle size distribution to ISO 13320. Step 3 — Select the point with the lowest viscosity, stable fineness and the smallest change after seven days at 50 °C.
For true solvent-free UV/EB systems (FLUV; the EB route uses no photoinitiator), yellowing and adhesion after cure must be verified in addition. System classification matters: RFNC is resin-free but still contains solvent; FLTY contains approximately 20 percent PMA and must not be simplified as a solvent-free system.
3. Bead mill process parameters
Table 2 — Bead mill parameter ranges
| Parameter | General range | Basis and effect | Control point |
|---|---|---|---|
| Media material | Yttria-stabilised zirconia (YSZ, high wear resistance), zirconium silicate, glass beads; zirconia for high hardness and nano grade | Zirconia has high density, strong energy transfer and low wear; glass is lower cost but wears faster and contaminates | Use high-purity zirconia for light and white pastes to avoid metal or glass contamination causing shade shift |
| Media diameter | Coarse grinding 0.8–2.0 mm; fine grinding 0.3–0.8 mm; nano grade 0.1–0.3 mm | Smaller media gives more contact points and finer final fineness, but raises energy density and temperature | Two-stage media loading, coarse then fine; a finer target D50 requires smaller media |
| Rotor tip speed | Approximately 8–14 m/s; lower for coarse, higher for fine nano grinding | Tip speed sets energy input; excessive speed raises temperature and risks shear heating of resin or monomer | Use the lower half of the range with enhanced cooling for UV/EB and heat-sensitive systems |
| Passes and residence time | Multi-pass circulation (2–5 passes) or continuous passage, depending on equipment | More passes or longer residence gives finer fineness up to a limit; over-grinding is uneconomical | Sample each pass; stop when fineness no longer decreases |
| Slurry temperature | Typically held at or below 40–60 °C depending on system | High temperature causes solvent loss, premature reaction of UV monomers or oligomers, and resin viscosity drop | Link jacket cooling water flow to inlet temperature; temperature control is critical for UV systems |
| Media fill ratio | Approximately 60–85 percent of effective chamber volume | Too low reduces efficiency; too high raises heat and media wear | Adjust from equipment recommendation and paste viscosity |
| Slurry viscosity and solids | Matched to pigment oil absorption and dispersant dosage, kept pumpable | Too high impairs pumping and raises temperature; too low gives insufficient shear | Monitor inlet viscosity by GB/T 9751, inline or by sampling |
The table is a general range, not a fixed process card. Specific tip speed, pass count, slurry temperature and fill ratio must be confirmed against equipment model, media batch and pigment system.
4. How parameters govern dispersion efficiency and final fineness
Grinding is the application of shear and impact by the media to pigment agglomerates. Smaller media, higher tip speed and a moderate fill ratio increase the number of effective collisions per unit volume and drive the agglomerates toward the target fineness. The same energy input, however, becomes heat. Rising slurry temperature narrows the operating window of solvent-based systems and can trigger premature reaction or yellowing in UV/EB systems. Faster is therefore not automatically finer. When fineness by GB/T 6753.1 and D50/D90 by ISO 13320 show no further reduction over two consecutive passes, the formulation and equipment have reached their practical limit; further grinding adds energy and heat without benefit. Industrial practice is to break soft agglomerates in a coarse stage with larger media at higher tip speed over few passes, then bring the fineness down in a fine stage with smaller media at moderate tip speed under temperature control.
5. Scale-up from laboratory to production
The difference between laboratory bead milling and a production horizontal mill is not only throughput. A laboratory batch has long residence time and good heat removal; a production line runs continuous passage with short residence time and a different specific energy density. Scale-up should hold the energy delivered per unit mass of pigment approximately constant rather than copying tip speed directly. Media batch, inlet viscosity and slurry temperature must be monitored continuously, and the gradient trial must be repeated whenever the pigment or dispersant grade changes. Automated and inline fineness or viscosity feedback is being introduced gradually, but manual sampling with laboratory re-check remains the main practice. The maturity of unattended operation should not be overstated.
6. Frequently asked questions
Q1. What is the first principle of dispersant selection?
Match the anchoring group to the pigment surface acidity or basicity — acidic or phosphoric anchors for metal oxides, polyamine anchors for carbon black — then match the solvated chain to the resin or monomer phase, and finally set the dosage by oil absorption or BET through a gradient trial.
Q2. Is finer media always better?
Smaller media is required for nano-grade fineness, but it raises energy density and temperature faster. Two-stage loading, coarse then fine, balances efficiency against temperature control.
Q3. Why must slurry temperature be controlled tightly?
Excessive temperature causes solvent loss, premature reaction or yellowing of UV/EB monomers, and resin viscosity drop, which affects storage stability and cured film properties. UV systems should run at moderate to low tip speed with enhanced cooling.
Q4. How many passes are enough?
Sample each pass. When fineness to GB/T 6753.1 and D50/D90 to ISO 13320 show no further reduction over two consecutive passes, the limit has been reached; additional passes are uneconomical.
Q5. Is passing the fineness test sufficient for release?
No. Fineness, viscosity to GB/T 9751, particle size distribution to ISO 13320 and heat storage stability must all pass together. UV systems additionally require yellowing and adhesion testing after cure.
Key conclusions
- Dispersant selection proceeds in two steps: match the anchoring group to the pigment surface, then match the solvated chain to the resin or monomer. Dosage is estimated from oil absorption or BET and confirmed by gradient and heat storage trials rather than a fixed ratio.
- Bead mill parameters — media material and diameter, tip speed, passes and residence time, slurry temperature, fill ratio — start from engineering ranges and are confirmed by trial. Fineness, viscosity and particle size distribution are assessed to GB/T 6753.1, GB/T 9751 and ISO 13320 respectively.
- System classification must be kept accurate: FLUV is true solvent-free UV/EB with no photoinitiator on the EB route; RFNC is resin-free but not solvent-free; FLTY contains approximately 20 percent PMA and must never be described as solvent-free. Scale-up and material changes require a fresh trial.