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 — fish-eye defects, nozzle deposits, and screen blinding all trace back to inadequate dispersion at the masterbatch stage. A well-made white masterbatch running at 50–60% TiO2 loading should produce film with fewer than 5 fish-eyes per 100 cm² and pressure filter values well within spec over a 30-minute production window.
Twin-screw extruder setup for TiO2 masterbatch
Modern masterbatch production uses co-rotating twin-screw extruders. The co-rotating design provides both distributive and dispersive mixing — essential for breaking TiO2 agglomerates (dispersive) and uniformly spreading them through the polymer melt (distributive). Counter-rotating twin screws are sometimes used for PVC, but co-rotating is standard for PE/PP masterbatch.
Typical specifications for 50–60% TiO2 loading masterbatch production: - L/D ratio: 36–44. Longer barrels allow more residence time and better dispersion; 40:1 is the common sweet spot for high-load white MB. - Screw diameter: 25 mm (laboratory development), 52–70 mm (pilot scale, 50–200 kg/hr), 90–135 mm (production scale, 200–600 kg/hr) - Screw speed: 300–500 rpm. Higher speed increases shear and dispersion quality but also generates more heat — balance speed against temperature limits of the polymer and dispersant system. - Output: 50–500 kg/hr depending on size and loading - Zone temperature profile: 170–195°C for LDPE carrier, 195–220°C for HDPE carrier, 220–250°C for PP carrier, 190–210°C for EVA carrier
Pre-drying TiO2: if the TiO2 is stored in humid conditions or is a hygroscopic grade, pre-drying at 105°C for 2–4 hours reduces moisture content to < 0.05%. Moisture above 0.1% generates steam in the melt zone that causes strand voids and surface defects in downstream blown film.
Screw configuration and kneading zone design
The screw configuration determines where and how intensely TiO2 agglomerates are broken up. Getting this right is the single largest process lever for dispersion quality.
Recommended screw configuration for high-load TiO2 MB: - Zone 1–2 (feeding section): conveying elements with relatively open pitch. This section transports the solid blend toward the melt zone — low shear, no dispersive work yet. - Zone 3 (pre-melt transition): reverse kneading block or narrow-pitch conveying elements to begin building pressure and temperature. - Zone 4–6 (primary kneading zone — critical): series of kneading blocks, typically 45°–60° stagger angle. This is where TiO2 agglomerates are broken to primary particles. Insufficient kneading block length here is the most common cause of fish-eye defects in downstream film. - Zone 6 (distributive mixing): neutral or forward kneading blocks or mixing elements (ZME, SME) to uniformly distribute the now-dispersed TiO2 throughout the melt. - Zone 7 (vacuum vent): open vent port with vacuum pump pulling 20–26 inHg below atmosphere to remove moisture and entrained air. Critical for strand quality. - Zone 8 (die): gear pump (optional but recommended for high-load MB) and strand die with underwater pelletizer.
The kneading blocks at zones 4–6 are the critical dispersion section. A common upgrade for quality improvement is adding 1–2 additional kneading block elements to this zone — this alone can reduce fish-eye count from 15–20/100 cm² to < 5/100 cm² in a poorly dispersing formulation.
Dispersant selection and side-feeding strategy
Pure TiO2 in pure polymer carrier is essentially impossible to disperse to commercial quality — the polymer melt viscosity is too high and TiO2 particle surface energy is too great without wetting assistance. A dispersant is essential, and dispersant chemistry must match both the TiO2 surface treatment and the carrier polymer.
Common dispersants for TiO2 masterbatch and typical loadings: - Calcium stearate: 1–3% on TiO2 weight. Cheap, universal, provides both internal lubrication and wetting. Standard for commodity PE/PP MB where cost dominates. Limitation: can bloom to the surface of film or fiber if overloaded. - Polyethylene wax (Honeywell AC-6, BASF Luwax A): 1–3% loading. Lubricant and dispersant combined. Pairs well with calcium stearate for PE masterbatch. Low cost, good processing. - Polymeric dispersants (Lubrizol Solplus D510, BYK Disperbyk-111): 0.5–2% loading. Premium for high-load MB (65%+). Provides steric stabilization that prevents re-agglomeration after initial dispersion. More expensive but essential for loading > 65% where simple wetting agents are insufficient. - Maleic anhydride grafted PE or PP (MAH-g-PE, MAH-g-PP): 1–2% loading. Creates covalent bonding between TiO2 surface and polyolefin carrier. Best for engineering plastics MB where long-term interface stability matters. Commonly used in outdoor PP/PE with HALS packages. - Titanate coupling agents (Kenrich LICA-12, KR TTS): 0.5–1% loading. Surface-modify TiO2 in-situ to improve polymer compatibility. Premium cost; justified for very high-load (70%+) or specialty applications.
Side-feeding vs main-feeding strategy: For standard-load masterbatch (40–55% TiO2), main-feeding all components together at zone 1 is acceptable — the conveying section is long enough to partially melt the carrier before reaching the kneading zone.
For high-load masterbatch (60%+), side-feeding TiO2 is strongly preferred: - Feed carrier polymer + dispersant at main hopper (zone 1). The carrier melts completely in zones 1–3. - Side-feed TiO2 at zone 4 (directly into the already-molten carrier). This means TiO2 enters a low-viscosity melt, which wets pigment surfaces faster and gives the kneading zone more mechanical energy to spend on dispersion rather than on melting solids. - Side-feeding also reduces abrasion of the feed-section screw flights by the dense TiO2 powder, meaningfully extending screw life.
Vacuum venting: TiO2 surface treatment carries 0.2–0.5% moisture at typical warehouse humidity. At 190–250°C, this moisture flashes to steam within the extruder melt zone. Without vacuum venting, steam bubbles are trapped in the strand and appear as voids in the pellet or surface bubbles in downstream blown film. The vacuum vent at zone 7 should maintain at least 20 inHg below atmosphere; 24–26 inHg is preferred for high-moisture TiO2.
Dispersion quality measurement and SEMITI grade selection
Dispersion quality measurement — standard methods:
1. Fish-eye count (primary test): Extrude a 25–50 μm PE blown film at 25–50% dilution of the masterbatch into neat polymer. Collect 1 m² of film. Count visible white specks (undispersed agglomerates) under transmitted light. Target: < 5 fish-eyes per 100 cm² for premium MB; < 20 for commodity MB.
2. Pressure filter value (PFV) / screen test: Force the compound melt through a 200-mesh (74 μm) wire screen at 250°C at 10 kg/cm² pressure. Record the pressure rise over 30–60 minutes. A clean, well-dispersed MB shows PFV < 2 bar/g. High PFV indicates large agglomerates surviving dispersion and fouling fine screens.
3. Optical microscopy: Mount a thin section of the MB in a microtome and examine at 200–500× magnification. Reveals agglomerate size distribution and incomplete wetting that fish-eye counts alone cannot quantify. Useful for diagnosing which stage of dispersion failed.
4. Melt flow index (MFI): Run MFI of the MB per ISO 1133. TiO2 loading increases viscosity and reduces MFI. Sudden MFI shifts batch-to-batch indicate inconsistent TiO2 loading or dispersant variation.
Common dispersion failures and systematic fixes: 1. Fish-eye count > 20/100 cm²: most likely insufficient kneading block length in zone 4–6. Add 1–2 kneading block elements before requesting material change. 2. High PFV with normal fish-eye count: large agglomerates (50–200 μm) present but transparent in thin film. Extend kneading section or reduce screw speed to increase specific energy input. 3. Wrong dispersant chemistry: calcium stearate works for PE/PP but causes delamination in PA/PET carrier. Switch to MAH-g-PP for nylon masterbatch. 4. Moisture voids in strand: increase vacuum vent suction; pre-dry TiO2 to < 0.05% moisture; check vacuum vent seal integrity. 5. MFI variation batch-to-batch: inconsistent TiO2 side-feed metering. Calibrate gravimetric feeder; check for bridging in TiO2 hopper.
TiO2 grade selection by masterbatch application: - Commodity PE/PP blown film and injection molding MB: SEMITI 996 (chloride rutile, universal) - Outdoor PP/PE MB for garden furniture, automotive trim, agricultural film: SEMITI 960 (amine-compatible, HALS-friendly surface treatment) - Universal plastics MB including rigid and flexible PVC: SEMITI 2310 (optimized surface treatment for broad polymer compatibility) - Outdoor PVC window profiles and siding: SEMITI 2160 (durable rutile with zirconia overcoat) - High-load MB (65–70% TiO2) for premium applications: SEMITI 2190 (finer PSD, optimized surface for high-concentration dispersibility) - Engineering plastics (PA, PET, PC) MB: SEMITI 960 with MAH-g-PP dispersant for interface compatibility