Rubber Compounding with White TiO2
White rubber compounds use TiO2 for opacity. Crystal hardness, surface treatment, and antioxidant compatibility drive grade selection.
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 from coatings and plastics.
Why anatase is preferred for most rubber: Two reasons: 1. Crystal hardness: Anatase (Mohs 5.5–6.0) is softer than rutile (Mohs 6.0–6.5). At the high mechanical shear of Banbury mixing, rutile causes significantly more wear on rotors, drop doors, and downstream extruder dies. Over a 5-year rubber compounding operation, switching from rutile to anatase can save 30% on equipment wear costs. 2. Color characteristics: Anatase has a bluer undertone, often preferred for "pure white" rubber aesthetics in tire sidewall styling.
The exception is outdoor-service rubber (EPDM weather strips, exterior cosmetic rubber). For these, rutile is preferred for photochemical stability.
Banbury mixing fundamentals: Standard intensive mixer (Banbury or modern equivalent) at 80–120°C, 60–90 rpm rotor speed. Compounding cycle: 1. Mastication: rubber polymer added, broken down at high shear (~30 sec) 2. First addition: TiO2 + carbon black (if any) + reinforcing fillers + plasticizer (~60 sec) 3. Curative addition: sulfur + accelerators + antioxidants (~30 sec) 4. Drop temperature: 130–150°C (depends on cure system) 5. Output: sheeted, cooled, pelletized or extruded direct
TiO2 dosage by application: - White tire sidewall: 5–15 phr (parts per hundred rubber) - EPDM weather strip: 3–8 phr + ZnO + amine antioxidant - Conveyor belt (food/pharma): 8–15 phr + FDA-compliant antioxidant - White rubber goods: 5–12 phr - Synthetic fiber delustering (PET, PA, PP melt spinning): 0.3–0.5%
Antioxidant compatibility: Rubber vulcanization at 145–165°C requires antioxidants to prevent thermo-oxidative degradation. The TiO2 surface treatment must not interfere with antioxidant function: - ZnO + amine antioxidant (TMQ, 6PPD): standard package, all SEMITI rubber grades compatible - Phenolic antioxidants (Wingstay L): premium package, compatible with all SEMITI grades
Curing system compatibility: - Sulfur cure (vulcanization): standard for tire and most rubber; compatible with all TiO2 - Peroxide cure (Vulcup R, DBPH): used for EPDM and silicone rubber; verify TiO2 surface treatment doesn't inhibit peroxide - Resin cure: specialty; case-by-case verification
SEMITI rubber-grade recommendations: - SEMITI A200 — universal anatase for tire sidewall, EPDM, white rubber compounds (Ishihara TIPAQUE A-100 equivalent) - SEMITI A100 — universal anatase, secondary choice for rubber and primary for fiber delustering - SEMITI 996 — rutile, only for outdoor-service rubber needing UV stability (acceptable EPDM)
Fiber delustering specifics: Synthetic fiber (PET, PA, PP) spinning at 280°C+ requires fine, clean TiO2 to delustre without breaking fibers at high draw ratios: - Loading: 0.3–0.5% TiO2 in spinning melt - Particle size: D50 < 0.20 μm, no agglomerates - Purity: Pb < 3 ppm, no spinneret-fouling metals - Surface treatment: minimal or hydrophobic (avoid water carryover)
SEMITI A100 with proper pre-dispersion in the polymer carrier is the typical choice.
Common compounding issues: 1. Equipment wear (with rutile): switch to anatase; check Banbury rotor condition 2. Spinneret fouling (fiber): too coarse TiO2 or impurities; switch to A100-FB sub-grade 3. Stickiness on calender rolls: TiO2 dispersant migrating; reduce dispersant level or change chemistry 4. Poor cure (peroxide system): verify TiO2 surface treatment doesn't have residual organic peroxide-quencher