Bead-Mill Dispersion of TiO2 in Printing Inks
Ink TiO2 must reach Hegman 7+ in 30–60 minutes of bead-milling. Here's how to set up the process.
Printing ink TiO2 dispersion is the bottleneck in white ink production. Mill time directly determines ink plant throughput; poor dispersion shows up as anilox blinding, dot gain on press, and inconsistent print color.
Bead-mill mechanics: Modern ink-mill bead mills use small zirconia or steel beads (typically 0.3–1.0 mm) in a high-shear chamber. The pigment-binder slurry is pumped through; mechanical shear breaks up TiO2 agglomerates and disperses individual particles into the binder.
Typical equipment: - Horizontal bead mill (Netzsch ZETA, Buhler PolyMill): production workhorse, 50–500 L/hr - Vertical attritor mill (Eiger, Premier): laboratory and small production, 1–50 L/hr - Basket mill (Bachofen, MicroCer): batch processing for small volumes
Bead size selection: - 0.3–0.5 mm beads: fastest dispersion to Hegman 7+ (typical 30–45 min). Use for production. - 0.6–0.8 mm beads: slightly slower but lower mechanical wear on equipment. - 1.0–1.5 mm beads: too coarse for TiO2 ink; only for primer applications.
Pre-mix step: Before bead-milling, pre-mix TiO2 + binder + dispersant + solvent at low shear (Cowles disk, 10–15 m/s) for 5–10 min. This wets the powder and creates a uniform paste. Skipping pre-mix leads to lumping and dramatically extends bead-mill time.
Dispersant selection by ink type:
| Ink type | Dispersant | Loading | |
|---|---|---|---|
| Offset oil-based | Soya lecithin or polymeric | 1–2% | |
| Solventborne flexo | Acrylic block copolymer | 1–3% | |
| Waterborne flexo | Polyacrylate or APEO-free | 1–2% | |
| Solventborne gravure | Polyester dispersant | 1–2% | |
| UV ink | Photoinitiator-compatible polymeric | 1–3% | |
| Screen ink | Sodium dispersant | 1–2% |
Insufficient dispersant = flocculation = poor Hegman. Excess dispersant = print quality issues (foaming, slow ink drying, anilox release problems).
Mill time targets: - Hegman 7+: standard quality, achievable in 30–60 min for SEMITI INK-1 - Hegman 8+: premium quality, requires 60–90 min and tighter bead size - Below Hegman 7: undispersed agglomerates visible — reject or remill
SEMITI INK-1 specifically: SEMITI INK-1 is pre-treated with a small amount of dispersant at the factory. This: - Wets faster in pre-mix (reduces 5–10 min) - Disperses to Hegman 7+ in 30–45 min (vs 60–90 for general-purpose TiO2) - Produces narrower final PSD = less anilox blinding
The pre-treatment is an industry-standard approach for ink-grade TiO2 (Ti-Pure R-931 uses similar approach).
Common dispersion problems: 1. Lumping in pre-mix: dispersant added too late; fix by adding before TiO2 2. Anilox blinding: coarse PSD tail; fix by extending mill time or switching to ink-grade TiO2 3. Foaming: too much dispersant or wrong dispersant chemistry; fix by adding anti-foam 4. Slow color shift on press: pigment-binder flocculation; fix by recalibrating dispersant level 5. Pinhole in screen mesh: agglomerates >40 μm; fix by improving mill or pre-filtering
Quality control: - Hegman gauge readout — manual but standard - Particle size (laser diffraction) — D90 < 0.7 μm for ink quality - Mill drop test — pour milled ink through 400-mesh screen, residue indicates dispersion failure