Powder Coating Extrusion with TiO2
Powder coating TiO2 must survive 100–130°C extrusion + 180–200°C cure without yellowing. Premium chloride rutile is the standard.
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 uniquely demanding for TiO2
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
The powder coating production process
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, cure chemistry, and common defects
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
Surface treatment requirements and HALS interaction in powder coatings
Powder coating binders — typically polyester, epoxy, or polyester-urethane — are polar resins with hydroxyl and carboxyl groups. TiO2 surface treatment must be compatible with these functional groups to achieve adequate wetting and dispersion during extrusion.
Alumina + silica (Al2O3 + SiO2) treatment is the standard for interior and general-purpose powder coatings. The alumina layer provides binder wetting; silica suppresses photocatalytic yellowing at cure temperature. SEMITI 706 carries this treatment and is the workhorse for appliance and general architectural powder.
Al2O3 + SiO2 + ZrO2 triple-layer treatment (SEMITI 826D) is required for outdoor architectural cladding and other exterior powder applications. Zirconia adds incremental photocatalysis suppression that becomes critical for 10–20 year outdoor service. The additional ZrO2 cost is justified for premium outdoor specifications.
HALS compatibility in powder: Outdoor powder coatings for architectural aluminum (e.g., QUALICOAT Class 2 specifications requiring 2000+ hours QUV-B stability) rely on HALS (hindered amine light stabilizers) to protect the polyester binder. Some TiO2 surface treatments — particularly high-alumina grades — adsorb HALS molecules, reducing their effective concentration in the binder matrix. SEMITI 826D uses an amine-compatible surface treatment that avoids HALS deactivation, preserving weathering performance in premium outdoor specifications.
Outdoor durability standards for powder coatings
Powder-coated architectural aluminum panels must meet one of several international weathering standards, which impose strict color and gloss retention requirements over QUV or Florida exposure:
| Standard | Gloss retention | Color ΔE | Duration | |
|---|---|---|---|---|
| QUALICOAT Class 1 | ≥ 50% at 60° | ≤ 5 | 1000 hr QUV-A | |
| QUALICOAT Class 2 | ≥ 50% at 60° | ≤ 3 | 2000 hr QUV-B | |
| AAMA 2604 | ≥ 35% at 60° | ≤ 5 | 2000 hr QUV | |
| AAMA 2605 | ≥ 50% at 60° | ≤ 5 | 4000 hr QUV | |
| GSB Class Elite | ≥ 50% | ≤ 2 | 3000 hr QUV-B |
SEMITI 826D with amine-compatible ZrO2 treatment consistently achieves QUALICOAT Class 2 and AAMA 2604 in standard polyester/HAA systems. For AAMA 2605 (the most demanding North American specification), SEMITI 826D combined with PVDF or super-durable polyester binder is the standard approach.
Troubleshooting powder coating defects related to TiO2
Common defects in finished powder coatings often trace to TiO2 quality or process conditions. Crater and fish-eye defects appear as small depressions in the cured film, typically caused by moisture in TiO2 (even 0.1–0.2% moisture volatilizes during extrusion at 120°C) or silicone-based contamination. Pre-dry TiO2 in a vacuum oven to <0.2% moisture before compounding; verify feed material sources are silicone-free. Color drift batch-to-batch indicates incoming TiO2 variation (ΔE > 0.5 per batch) — specify tighter color tolerance from supplier and implement incoming QC colorimetry. Yellowing at cure suggests sulfate-process TiO2 or incompatible surface treatment; always confirm chloride-process grade (SEMITI 706, 826D) is being used. Poor powder flow indicates TiO2 surface treatment incompatibility with binder chemistry; test via small-scale extrusion trial (500 g batch) before full-scale production. Agglomeration in the ground powder suggests ACM mill speed is too aggressive for the TiO2 grade — reduce classifier speed and extend residence time to improve particle separation without over-milling.