Rubber Compounding with White TiO2

Rubber compounding with TiO2 produces the white components of tires (decorative sidewall stripes), white EPDM weather strips, food-grade conveyor belts, and various consumer rubber products. The mechanical and chemical demands of rubber compounding differ significantly from coatings and plastics — crystal hardness, antioxidant compatibility, and curing system chemistry all affect grade selection. A grade change that looks straightforward on paper can cause unexpected equipment wear issues, cure system interference, or color drift in the finished compound.

Why anatase is preferred over rutile for most rubber

Two distinct technical reasons drive the preference for anatase over rutile in most rubber applications, and both are significant enough to override the hiding-power advantage of rutile.

Crystal hardness: Anatase (Mohs 5.5–6.0) is softer than rutile (Mohs 6.0–6.5). This half-Mohs difference sounds small, but at the high mechanical shear of Banbury mixing — rotor tip speed 15–25 m/s, pressures exceeding 5 bar, temperatures 80–120°C — the abrasive effect of a harder pigment accumulates significantly over equipment lifetime. In a rubber compounding plant running production 5 days per week with 3–4 Banbury cycles per hour, switching from rutile to anatase typically reduces rotor weight loss by 20–30% per year. Drop doors, seals, and downstream extruder die faces also show reduced wear. Over a 5-year equipment life cycle, this can represent $200,000–$500,000 in reduced maintenance and replacement cost for a mid-size compounding operation.

Color characteristics: Anatase has a bluer undertone (CIE b* typically 1.0–1.5) compared to rutile (b* 1.5–2.5). In white tire sidewall styling, the cooler, bluer white of anatase is the preferred aesthetic — it reads as "pure white" while rutile gives a slightly creamy tone. For high-visibility white rubber goods (footwear, sports equipment, industrial gloves), the anatase undertone is typically specified.

The exception is outdoor-service rubber: EPDM weather strips on automotive windows, exterior cosmetic rubber body parts, rooftop membrane rubber. For these, rutile is preferred because anatase's photocatalytic activity degrades the rubber polymer surface under UV exposure, causing surface chalking and discoloration within 12–24 months. Rutile with zirconia or silica surface treatment (SEMITI 996) provides adequate UV stability for typical automotive weather-seal service life requirements.

Banbury mixing fundamentals and dosage by application

The internal Banbury mixer (or modern co-rotating or counter-rotating equivalents from Farrel, HF Mixing, or Kobe Steel) is the workhorse for rubber compounding. Understanding the mixing cycle helps optimize TiO2 incorporation for dispersion quality.

Standard intensive mixer operating conditions for TiO2-containing compounds: - Fill factor: 65–80% of chamber volume - Rotor speed: 50–80 rpm (lower speed for better filler wetting, higher for faster throughput) - Ram pressure: 4–8 bar - Temperature rise from friction: 25–40°C above initial temperature - Drop temperature: 130–150°C (depends on cure system; must be below activation temperature of accelerators)

Compounding cycle sequence for maximum TiO2 dispersion: 1. Mastication (~30 sec): rubber polymer added alone, broken down to reduce viscosity 2. First addition (60–90 sec): TiO2 + ZnO + stearic acid + plasticizer + reinforcing fillers. Adding TiO2 early (before fillers) with the low-viscosity rubber and plasticizer gives best dispersion 3. Ram lift / cooling (if needed): for heat-sensitive compounds, lift ram to allow heat dissipation before curative addition 4. Curative addition (~30 sec): sulfur + accelerators + antioxidants — always added last to prevent premature scorch 5. Drop and sheet: discharge to open mill, sheet to cooling conveyor

TiO2 dosage by application: - White tire sidewall compound: 5–15 phr (parts per hundred rubber). Higher loading for opaque solid-white stripe; lower for tinted or transparent-effect sidewall. - EPDM automotive weather strip: 3–8 phr. Combined with ZnO 5 phr + amine antioxidant for peroxide cure compatibility. - Food-grade conveyor belt (food/pharma): 8–15 phr + FDA-compliant antioxidant package. SEMITI A100 or A200 recommended for FDA 21 CFR compliance. - General white rubber goods: 5–12 phr depending on color depth requirement. - Silicone rubber (HTV, LSR): 5–20 phr. Require surface-treated TiO2 compatible with platinum cure catalyst; verify compatibility before use. - Synthetic fiber delustering (PET, PA, PP melt spinning): 0.3–0.5% TiO2 in spinning melt. Entirely different demands from bulk rubber compounding — see below.

Antioxidant and curing system compatibility

Antioxidant compatibility: Rubber vulcanization at 145–165°C requires antioxidants to prevent thermo-oxidative and ozone degradation. The TiO2 surface treatment must not interfere with antioxidant function — a concern primarily when alumina-heavy surface treatments adsorb amine-type antioxidants.

• ZnO + amine antioxidants (TMQ, 6PPD, IPPD): standard package for natural and synthetic rubber. All SEMITI rubber grades (A100, A200, 996) are compatible.
• Phenolic antioxidants (Wingstay L, Vulkanox BKF): premium package for non-staining applications (white goods, food contact). Compatible with all SEMITI grades.
• HALS in peroxide-cured EPDM: hindered amine light stabilizers for outdoor EPDM. Test TiO2 compatibility — some alumina-heavy rutile grades adsorb HALS. SEMITI 996 amine-compatible treatment avoids this.

Curing system compatibility: - Sulfur/accelerator vulcanization: standard for NR, SBR, NBR, EPDM. Compatible with all TiO2 grades — sulfur does not react with TiO2 surface treatment. - Peroxide cure (DCP, DBPH, Vulcup R): used for EPDM, silicone rubber, and specialty applications. Verify TiO2 surface treatment doesn't include peroxide-quenching organics (some stearic acid treatments have limited quenching effect). SEMITI A100 and A200 are peroxide-cure compatible. - Resin cure (alkyl phenol-formaldehyde resin): specialty system for butyl and halobutyl rubber. TiO2 grade compatibility should be verified case-by-case; generally not problematic.

SEMITI rubber-grade recommendations and fiber delustering

SEMITI rubber-grade recommendations by application: - SEMITI A200 — premium anatase, preferred for white tire sidewall, EPDM, and white rubber goods requiring the highest whiteness. Ishihara TIPAQUE A-100 equivalent. - SEMITI A100 — universal anatase, cost-effective choice for most rubber applications and the standard for fiber delustering. - SEMITI 996 — chloride rutile, only for outdoor-service rubber (automotive EPDM weather strips, exterior cosmetic rubber) where UV stability outweighs equipment wear concerns.

Fiber delustering specifics — very different requirements: Synthetic fiber (PET, PA6, PA66, PP) melt spinning at 280–295°C requires fine, ultra-pure TiO2 to delustre without breaking fibers at high draw ratios (3–5× for polyester). The requirements diverge sharply from bulk rubber compounding:

• Loading: 0.3–0.5% TiO2 in the spinning melt (not 5–15 phr as in rubber)
• Particle size: D50 < 0.20 μm, with no agglomerates above 0.5 μm. Coarse particles or agglomerates cause fiber breaks at the spinneret or draw stage
• Purity: Pb < 3 ppm, no spinneret-fouling metals (no Cu, no Co, no Mn compounds). Metal contamination causes spinneret clogging within hours
• Surface treatment: minimal organic content (< 0.3% LOI), hydrophobic preferred to minimize moisture carryover at spinning temperature
• Pre-dispersion: TiO2 must be pre-dispersed in polymer carrier (usually in the same polymer as the fiber) before addition to the spinning melt. Direct addition of TiO2 powder to the spinning line causes immediate agglomeration and spinneret fouling.

SEMITI A100 is the standard choice for polyester and nylon fiber delustering, typically supplied as pre-dispersed masterbatch at 20–30% TiO2 loading in the carrier polymer.

Common compounding issues and fixes: 1. Excessive equipment wear: switch from rutile (SEMITI 996) to anatase (SEMITI A200 or A100); inspect Banbury rotor chrome-hard facing condition; rebalance rotors if uneven wear 2. Spinneret fouling in fiber: TiO2 too coarse or agglomerated; switch to A100 verified at D50 < 0.18 μm; improve pre-dispersion protocol; check for batch-to-batch PSD variation 3. Stickiness / "blooming" on calender rolls: dispersant migrating from TiO2 surface to rubber surface; reduce dispersant level; switch to mineral-oil-compatible dispersant grade 4. Poor cure or undercure in peroxide system: TiO2 surface treatment quenching peroxide radicals; verify surface treatment composition with supplier; switch to peroxide-compatible grade 5. Yellow discoloration in white compound over time: residual metallic impurity catalyzing oxidation; specify Pb < 5 ppm, Fe < 50 ppm on CoA; switch to chloride-process grade with higher purity