Plastic Masterbatch Dispersion Fundamentals
Poor TiO2 dispersion in masterbatch shows up as fish-eye defects in film, nozzle deposits in injection molding, and screen blinding in extrusion. Here's how to get it right.
Producing high-quality white masterbatch requires more than just mixing TiO2 with carrier polymer. The dispersion quality at the masterbatch stage determines the visible quality of the final plastic product.
Twin-screw extruder setup: Modern masterbatch production uses co-rotating twin-screw extruders. Typical specifications for 50–60% TiO2 loading masterbatch: - L/D ratio: 36–44 - Screw diameter: 25–135 mm (production scale varies) - Screw speed: 300–500 rpm - Output: 50–500 kg/hr depending on size - Zone temperature profile: 180–230°C for PE carrier, 230–260°C for PP carrier
Screw configuration: - Zone 1–2: feeding section (TiO2 + carrier + dispersant) - Zone 3–5: kneading blocks (intensive mixing, breakdown of TiO2 agglomerates) - Zone 6: distributive mixing elements (uniform pigment distribution) - Zone 7: vacuum vent (remove residual moisture) - Zone 8: die
The kneading blocks at zones 3–5 are the critical dispersion section. Inadequate kneading = inadequate dispersion.
Dispersant selection: Pure TiO2 in pure polymer carrier is impossible to disperse to high quality — the polymer is too viscous and the pigment has too much surface energy. A dispersant is essential.
Common dispersants for TiO2 masterbatch: - Calcium stearate: cheap, universal, 1–3% loading. Standard for commodity PE/PP MB. - Polyethylene wax: lubricant + dispersant, 1–3% loading. Pairs well with calcium stearate. - Polymeric dispersants (e.g., Lubrizol Solplus): premium, 0.5–2% loading. For high-load MB (65%+). - Maleic anhydride grafted PE/PP: for engineering plastics, 1–2% loading. Improves TiO2-polymer interface.
Side-feeding vs main-feeding: For high-load masterbatch (60%+), the typical approach is: - Feed carrier polymer + dispersant at main hopper (zone 1) - Side-feed TiO2 at zone 4 (after carrier is fully melted) - This prevents TiO2 from being abraded by feed-section screw flights, reducing wear
Vacuum venting: TiO2 surface treatment carries 0.2–0.5% moisture. At extrusion temperature, this moisture evaporates and must be removed via vacuum venting — otherwise it creates voids in the strand and surface defects in downstream film. The vacuum vent at zone 6–7 should pull at least 20 inHg below atmosphere.
Dispersion quality measurement: 1. Fish-eye count: Extrude 50 μm PE blown film at 50% loading of the MB. Count visible defects under transmitted light per 100 cm². Target: < 5 defects. 2. Pressure filter test: Force compound through 200-mesh screen at 250°C. Pressure rise over 1 hr indicates large agglomerates. 3. Optical microscopy: Direct observation of dispersion at 200x magnification. Reveals incomplete kneading.
Common dispersion failures: 1. Inadequate kneading section length — fix by adding more kneading block elements 2. Wrong dispersant chemistry for the polymer system — fix by switching dispersant type 3. Too-cold extrusion temperature for surface treatment to wet — fix by raising zone 3–5 temperature 4. Moisture in TiO2 not vented — fix by improving vacuum vent 5. Inadequate residence time — fix by reducing screw speed or extending barrel
TiO2 grade selection for masterbatch: - Commodity PE/PP MB: SEMITI 996, 2310 - Engineering plastics MB: SEMITI 960 (amine-compatible, HALS-friendly) - Outdoor PVC: SEMITI 2160 (durable rutile) - High-load (70%+) MB: SEMITI 2190 (premium dispersion)