Precipitated Silica BET 115 m²/g

What Is BET 115 m²/g Precipitated Silica?

Precipitated silica with a BET surface area of 115 m²/g is the entry-level reinforcing grade in the commercial precipitated silica product range. BET 115 m²/g sits at the lower end of the surface area spectrum, which translates directly into properties that make this grade the practical choice for applications where easy processability, manageable compound viscosity, and white color are the primary requirements.

The BET (Brunauer–Emmett–Teller) surface area of 115 m²/g means that each gram of this silica has approximately 115 square meters of internal and external surface area available for interaction with the rubber polymer and silane coupling agent. Compared to a BET 220 m²/g grade, the 115 grade has roughly half the total surface area, which has predictable and significant effects on how the material behaves in compound.

BET Surface Area and What It Means for Compound Performance

Understanding the surface area–performance relationship is essential for selecting the right grade. Higher BET surface area means:

1. More contact surface for polymer adsorption — greater potential reinforcement
2. More silanol groups to react with silane — higher silane demand at full conversion
3. Higher DBP absorption — more porous structure absorbs more plasticizer, raising compound viscosity
4. More demanding dispersibility requirements — agglomerates are harder to break down

For BET 115 m²/g, all of these effects are moderate. The grade disperses readily in NR/SBR compounds even with standard mixing sequences. Compound Mooney viscosity increase is manageable. Silane coupling agent demand is low (Si-69 at 4–6 phr is sufficient for most shoe-sole and industrial rubber applications). The tradeoff is that the maximum reinforcement ceiling is lower than higher-BET grades.

Performance in Rubber Compounding

Tensile strength: In NR-based compounds at 50 phr loading, BET 115 m²/g precipitated silica delivers tensile strength of 15–20 MPa, which is adequate for shoe sole requirements (ASTM D412 test). This is significantly higher than an unfilled NR compound (typically 10–12 MPa after aging) and comparable to medium-reinforcing carbon black grades.

Elongation at break: High, typically 400–600%, giving the flexibility needed for shoe sole flex cycles. The lower structure of the 115 grade (DBP 180–210 mL/100g) contributes to better elastic recovery versus higher-DBP grades.

Compound viscosity: The Mooney viscosity increase from adding 50 phr BET 115 silica to NR is approximately 15–25 ML(1+4) units above the base gum, depending on oil and plasticizer levels. This is substantially lower than the viscosity increase from BET 200 or 220 grades, enabling easier mill mixing and extruder processing.

Dispersion index: BET 115 m²/g disperses readily with standard mixing sequences (one-pass or two-pass). Dispersion index values (ASTM D3053 or equivalent) of 70–80 are achievable without specialized mixing equipment.

Abrasion resistance: DIN abrasion (ISO 4649) in NR compounds with BET 115 silica gives volume loss of 120–180 mm³, which is adequate for shoe sole applications. Higher-BET grades would give better abrasion resistance but at the cost of higher viscosity and more demanding processing.

Recommended Compound Systems

Shoe sole NR/SBR compound (50 phr silica):

• NR RSS3: 100 phr
• Precipitated silica BET 115: 50 phr
• Si-69 silane: 4 phr (optional — improves wet grip and aging)
• Zinc oxide: 5 phr
• Stearic acid: 2 phr
• Sulfur: 2 phr
• CBS accelerator: 1.5 phr

Industrial rubber EPDM compound (40 phr silica):

• EPDM (ENB type, ML 1+4 at 125°C ≈ 65): 100 phr
• Precipitated silica BET 115: 40 phr
• Process oil (paraffinic): 30 phr
• ZnO: 5 phr, Stearic acid: 1 phr
• Sulfur/CBS cure system

Processing Tips

Mixing sequence for shoe-sole compounds: Add silica and silane together in the first drop at 80–100°C. Dump temperature should reach 150–160°C to allow silane reaction time. If using Si-69, allow at least 2–3 minutes at dump temperature. A second-pass mixing step is not usually required for BET 115 grades.

Moisture management: Silica absorbs atmospheric moisture. If stored bags are opened in humid conditions, pre-heat the silica at 105°C for 2 hours to reduce moisture below 5%. Excess moisture causes steam generation in the mixer and batch rejection.

Oil loading: BET 115 m²/g has moderate oil absorption (180–210 mL/100g). In high-oil shoe-sole compounds, the silica will absorb some of the processing oil — account for this in plasticizer balance to achieve target compound hardness.

FAQ: BET 115 m²/g Grade

Q: Can I use BET 115 for green tire compounds? A: Not recommended. Green tire tread requires BET 175–220 m²/g grades with HD dispersibility to achieve the rolling resistance and wet grip targets required by EU tire label regulations. BET 115 does not provide sufficient reinforcement density.

Q: What is the equivalent Evonik or Solvay grade? A: Evonik Ultrasil VN3 (BET ~175 m²/g) is in a higher surface area class. The closest Western equivalents in the 115 m²/g class include PPG Hi-Sil 135 or older general-purpose grades. Chinese BET 115 grades are direct equivalents in surface area and performance.

Q: Do I need a silane coupling agent with BET 115? A: Optional for many applications. In dry NR/SBR shoe-sole compounds where wet grip is not critical, silane can be omitted. Adding 4 phr Si-69 improves aging retention and wet abrasion resistance. For white shoe soles exposed to outdoor conditions, silane is recommended.

Q: What is the shelf life? A: 2 years from manufacture date when stored in original sealed packaging in a dry warehouse below 70% RH. Once opened, use within 6 months.