Dr E. Ramanathan PhD
[gtranslat]
To grind pigments in an attritor for manufacturing acrylic white emulsion paint, follow this structured step-by-step process:
🔹 1. Prepare the Mill Base (Pigment Paste)
White emulsion paint mainly uses Titanium Dioxide (TiO₂) as the white pigment. Prepare the mill base using:
Typical Ingredients:
- Deionized water: balance
- Dispersant (e.g., sodium polyacrylate) : 0.3–1%
- Antifoam: 0.1–0.3%
- TiO₂ (rutile): 15–25%
- Extenders (e.g., CaCO₃, talc): optional, 5–15%
- Thickener (optional): for viscosity adjustment during grinding
- Wetting agents: 0.2–0.5%
🔹 2. Premix with High-Speed Disperser
Before feeding into the attritor:
- Premix all above components in a high-speed disperser (Cowles) for 15–20 min.
- Target a particle size of <50 µm for uniform dispersion.
🔹 3. Charge the Attritor
Add the premixed mill base into the attritor tank along with grinding media.
Typical grinding media:
- Material: Zirconia or glass beads
- Size: 1.0–2.0 mm
- Media loading: 60–70% of attritor volume
🔹 4. Grinding Operation Parameters
| Parameter | Value |
|---|---|
| Speed | 200–400 rpm (depends on viscosity) |
| Grinding time | 30–90 minutes |
| Temperature control | Maintain below 50 °C (use water cooling jacket if needed) |
| Viscosity of slurry | 500–1500 cP during grinding |
- Monitor the particle size using Hegman gauge.
- Target 5–7 Hegman (10–20 µm) for high opacity and finish.
🔹 5. Media Separation and Let-down
- After grinding, separate grinding media using a mesh or screen.
- Transfer the pigment paste to a mixing tank for let-down with:
- Acrylic emulsion binder (40–50%)
- Coalescent (butyl glycol, Texanol)
- Thickener (HEC or ASE-type)
- Preservative
- Water to adjust final solids/viscosity
🔹 6. Final Adjustments
- Adjust pH to 8.0–8.5 with ammonia or AMP-95.
- Check viscosity (Krebs or Brookfield), gloss, and opacity.
- Filter through 100-mesh to remove coarse particles before packing.
Grinding Media For Attritor
For attritor mills, both striated ceramic balls and steel shots are used as grinding media, depending on the application. Here’s a comparison to guide selection:
🔹 1. Striated Ceramic Balls
| Parameter | Details |
|---|---|
| Material | Zirconia, alumina, or steatite |
| Size range | 0.5 mm to 5 mm |
| Surface | Striated (grooved) for better shearing and friction |
| Best for | Fine grinding of pigments, sensitive to metal contamination |
| Applications | Water-based paints, high-gloss acrylic emulsions, nano dispersions |
| Advantage | Chemically inert, non-contaminating, long life |
| Disadvantage | Higher cost, brittle under impact |
🔹 2. Steel Shots
| Parameter | Details |
|---|---|
| Material | Hardened carbon steel or stainless steel |
| Size range | 0.8 mm to 3 mm |
| Surface | Smooth, may have some roughness depending on grade |
| Best for | Rapid grinding, especially for solvent-based or industrial-grade paints |
| Applications | Alkyd paints, primer dispersions, non-white pigmented pastes |
| Advantage | High density → faster grinding |
| Disadvantage | Can cause metal contamination, unsuitable for white or water-based systems |
🔹 Selection Guide:
| Application | Recommended Media |
|---|---|
| White / Glossy Acrylic Emulsions | Striated Zirconia/Alumina Ceramic balls |
| Industrial primers / fast grind | Steel shots (cost-effective) |
| Solvent-based paints | Steel or ceramic (based on contamination tolerance) |
| Water-based sensitive systems | Ceramic only |
