Precipitated Silica as Reinforcing Filler in Rubber Compounds
Precipitated silica has displaced carbon black as the reinforcing filler of choice in high-performance rubber applications — most visibly in passenger tire tread (the "green tire" technology), but also in industrial EPDM seals, NR conveyor belts, and SBR sole compounds. The primary drivers are lower rolling resistance (≥20% reduction vs. carbon black), improved wet traction, and the ability to achieve high whiteness for colorable compounds — none of which carbon black can deliver.
ZEMSIL PS-160 (BET 155–170 m²/g, CTAB 145–160 m²/g) and PS-200 (BET 185–210 m²/g, CTAB 175–195 m²/g) are the primary reinforcing precipitated silica grades for rubber. PS-160 is preferred in EPDM and NR compounds where processability and tear resistance are paramount; PS-200 provides higher modulus and abrasion resistance, making it the choice for tire tread and high-wear industrial articles.
Silane Coupling Agents for Silica-Rubber Bonding
The critical limitation of silica as a rubber filler — compared to carbon black — is poor inherent compatibility with the rubber matrix. Silica surfaces are covered with silanol groups (Si–OH) that are strongly hydrophilic. In a non-polar rubber matrix, these surface silanols cause:
- Silica-silica filler networking (Payne effect), raising compound viscosity and making mixing difficult
- Poor stress transfer at the rubber-filler interface, limiting reinforcement efficiency
- High hysteresis → heat buildup in dynamic applications
Bifunctional organosilane coupling agents solve this by converting the hydrophilic silica surface to a hydrophobic, rubber-compatible one. The standard industrial silane systems are:
| Silane | Chemical Name | CAS | Primary Rubber Use |
|---|---|---|---|
| Si-69 (TESPT) | Bis[3-(triethoxysilyl)propyl] tetrasulfide | 40372-72-3 | Passenger tire tread, EPDM |
| Si-75 (TESPD) | Bis[3-(triethoxysilyl)propyl] disulfide | 56706-10-6 | Truck tire, lower scorch risk |
| MPTES | 3-Mercaptopropyltriethoxysilane | 14814-09-6 | Accelerated recipes, lower sulfur |
| KH-570 (CS-570) | γ-MPTMS | 2530-85-0 | Non-sulfur functional silica treatment |
TESPT (Si-69) remains the reference silane for passenger tire tread: the tetrasulfide groups release sulfur atoms during vulcanization, participating directly in the crosslink network (bifunctional coupling mechanism). Processing requires 140–150 °C mixing temperature in a Banbury/internal mixer for silane silanization to proceed, followed by controlled cool-down to prevent premature crosslinking (scorch).
Mixing Protocols for Silica Compounds
Silica/silane rubber compounding is significantly more complex than carbon black compounding. Standard protocol for a tire tread-type compound:
Stage 1 (masterbatch): Add rubber + 50% silica + all silane → mix at 150–160 °C for 5–7 min. Critical: the silane must react with silica (silanization step) before the silica is fully dispersed. Dump temperature 150–155 °C. Allow remill at 100 °C to improve dispersion.
Stage 2 (remill): Add remaining 50% silica + processing oil → mix at 140–150 °C for 4–5 min. This two-stage silica addition minimizes mixer torque peaks.
Stage 3 (final): Add curative (sulfur + CBS/TBBS accelerator) below 110 °C to prevent scorch. Short mix time (2–3 min) to avoid premature crosslinking.
Key compound parameters for a silica-TESPT compound (vs. carbon black reference):
| Parameter | CB Compound | Silica/TESPT | Delta |
|---|---|---|---|
| Mooney viscosity ML(1+4) 100°C | 60–75 MU | 70–90 MU | Higher — silica filler network |
| tan δ at 0°C (wet traction proxy) | 0.18–0.25 | 0.30–0.45 | Higher = better wet grip |
| tan δ at 60°C (rolling resistance proxy) | 0.10–0.15 | 0.06–0.10 | Lower = less heat buildup |
| Shore A hardness (vulcanized) | 60–70 | 58–68 | Similar |
| Abrasion resistance (DIN 53516) | Reference | +10–25% improvement | Silica advantage in high-NR |
Silicone Oil as Processing Lubricant
Silicone oil — specifically low-viscosity PDMS (50–200 cSt, ZEMSIL DM-50 to DM-200) — is used as an internal lubricant and processing aid in rubber compounding at 1–3 phr. Its roles are distinct from silane:
- Reduces compound viscosity during mixing and extrusion without acting as a plasticizer (does not depress Tg)
- Improves mold flow and surface finish of extruded profiles
- Acts as a release agent at the compound-mill roll interface during open mill processing
- In silicone rubber (HTV, LSR): PDMS is the base polymer, not an additive — crosslinking density controls hardness
For EPDM automotive seals and weather-strips, 1–2 phr PDMS reduces extrusion pressure and minimizes surface roughness ("sharkskin" melt fracture). In NR and SBR, PDMS is compatible at low levels but can reduce tensile strength if overdosed above 5 phr by acting as a diluent.
Quality Control Parameters
Critical incoming quality specifications for PS-160/PS-200 precipitated silica:
- BET surface area (ISO 5794): ±10 m²/g of nominal; higher BET = higher reinforcement but more difficult dispersion
- CTAB surface area: measures accessible surface to rubber polymers; CTAB/BET ratio >0.90 indicates low microporosity
- pH (5% slurry): 6.0–7.5; pH <6 indicates residual acid that retards accelerated vulcanization
- DBP oil absorption: indicates structure; high DBP = high void volume = better filler wetting
- Moisture content: <7.0% (Karl Fischer); excess moisture generates steam during mixing, causing porosity
Related Reading
Precipitated Silica — PS-160 / PS-200 · Silane Coupling Agents — CS-570 · Silicone Oil — DM Series · Applications: Gasket and O-Ring · Applications: Reinforcement