Powder Coating Extrusion with TiO2

Powder coatings are solid coatings applied as dry powder and cured by heat. They are the dominant coating for steel furniture, appliances, architectural aluminum, automotive trim, and many industrial applications. Production involves twin-screw extrusion of the dry powder formulation, followed by cooling, grinding, and classification to final powder size.

Why powder coating is demanding for TiO2: The TiO2 in powder coatings is exposed to high temperatures twice: 1. Extrusion: 100–130°C for 30–60 sec in twin-screw extruder 2. Cure: 180–200°C for 10–20 min in oven

The TiO2 surface treatment must survive both without degradation that would cause yellowing or coupling to other formulation components.

TiO2 grade selection: - SEMITI 706 — premium chloride rutile, gold standard for architectural and appliance powder - SEMITI 826D — heavy-duty for outdoor-service powder (architectural cladding) - SEMITI 2310 — universal plastics rutile, acceptable for general powder applications - SEMITI 902 — universal coatings rutile, acceptable for interior powder

Process basics:

Step 1: Pre-blend (dry mix) TiO2 + binder + crosslinker + extender + colorant + additives are dry-blended in a high-speed mixer (Henschel-type) at room temperature, 5–10 min. The pre-blend is uniform powder that flows freely.

Step 2: Twin-screw extrusion The pre-blend is fed to a twin-screw extruder (Buss Kneader or modern co-rotating). Process conditions: - Zone temperatures: 60–100°C entry, 120–130°C melt section, 80–100°C exit - Screw speed: 200–500 rpm - Residence time: 30–60 seconds

The extruder melts the binder (which has Tg below 50°C typically), disperses the pigment and additives, and discharges hot strand.

Step 3: Cooling and crushing The hot strand is rolled flat on cooling belts, then broken into chips. The chips are cooled to room temperature.

Step 4: Grinding and classification Chips are ground in air-classifying mills (ACM) to produce powder with target PSD (typical D50 30–50 μm for spray application). Oversize is recycled to grinding.

TiO2 dosage in powder: - Standard white architectural powder: 25–32% TiO2 - Premium white appliance: 28–35% TiO2 - Off-white / pastel: 5–25% TiO2 + colored pigments - Metallic effects: 5–15% TiO2 + aluminum flake

Cure chemistry: Modern powder coatings use various crosslinker systems: - TGIC-polyester (legacy, phasing out): TGIC + polyester, cure at 200°C - TGIC-free polyester / HAA (hydroxyalkylamide): modern standard, 180–200°C cure - Epoxy-polyester hybrid: economical, indoor only, 180–200°C cure - Epoxy (functional industrial): chemical resistance, 180°C cure - PUR-PIR: premium, 180–200°C cure

All systems require thermally stable TiO2 — chloride rutile is essential.

Common defects: 1. Crater / fish-eye: contamination or moisture in TiO2; predry TiO2 to <0.2% moisture 2. Color drift batch-to-batch: TiO2 ΔE > 0.5; specify tighter color tolerance from supplier 3. Yellowing at cure: insufficient thermal stability; switch from sulfate to chloride TiO2 4. Poor flow on extruder: TiO2 surface treatment incompatible with binder; verify dispersant + binder system 5. Particle agglomeration in powder: ACM grinding too aggressive; adjust feed rate and classifier speed

Quality testing for finished powder: - Gel permeation chromatography (GPC) — verify cure completeness after extrusion - Particle size distribution — D50 30–50 μm, D90 < 100 μm - Flow rate (Hosokawa cup) — > 30 sec for 100 g standard - Application test on metal panel — verify gloss, color, flow at standard cure conditions