HJSIL® Fumed Silica — Hydrophilic and Hydrophobic Grades, CAS 112945-52-5

Overview

Fumed silica is amorphous synthetic SiO₂ produced by flame hydrolysis of silicon tetrachloride at ~1,800°C. Non-porous nanosized primary particles fuse into 3D aggregate chains, delivering two procurement-critical properties: thixotropic rheology control in liquid systems and reinforcing filler performance in elastomers.

HJSIL® is the Henan Minmetals East fumed silica line — 8 commercial grades spanning 150–380 m²/g BET (hydrophilic) and three surface-modification chemistries (hydrophobic). Functionally equivalent to Evonik Aerosil 150 / 200 / 300 / 380 / R972 / R974 / R104 / R208, Wacker HDK N20 / T30 / N20P / H17, and Cabot Cab-O-Sil M-5 / EH-5 / TS-720 / TS-530 at a 30–45% landed-cost discount. REACH-registered, ISO 9001 + ISO 14001 manufactured; every batch ships with CoA (BET, tapped density, LOI, pH at 4%, Karl Fischer water).

For other silica forms see MEGEL® Aerogel and HMMAT® Matting Agents.

Chemistry and Surface Structure

Fumed silica is amorphous a-SiO₂ — distinct from precipitated silica (wet route), crystalline quartz, and colloidal silica. Flame hydrolysis gives non-porous 7–40 nm primary particles that fuse into 100–500 nm aggregates, which in turn associate via hydrogen bonding into reversible bulk-powder agglomerates.

Surface chemistry centres on two functional groups: silanol (Si–OH) — 3–4.5 OH/nm² on hydrophilic grades, driving H-bonding, water adsorption and the inter-aggregate thixotropy network in polar systems; and siloxane bridges (Si–O–Si) — low-energy, chemically stable, increasing as a fraction after surface treatment.

Hydrophobic grades cap a controlled fraction of silanols via post-treatment of the hydrophilic base with chlorosilanes or silazanes:

Mechanism of Action

Thixotropic thickening (liquids): at rest, aggregates connect via H-bonded chains into a percolated 3D network that supports a yield stress (no sedimentation, no sag, no two-component segregation). Under shear the network breaks; on shear removal it reforms in seconds — the signature thixotropic recovery absent in non-recoverable thickeners. Network strength scales with BET: 200 m²/g grades give ~2× the thickening efficiency of 150 m²/g at the same loading, which is why 200/300 dominate polyester, epoxy, and adhesive systems.

Elastomer reinforcement (HTV / RTV-2 silicone, fluoroelastomers, EPDM): rigid aggregates increase modulus via hydrodynamic stiffening, silanol surface H-bonds with the siloxane backbone forming a partial polymer–filler network, and above 20 phr a filler–filler percolation network bears additional stress. Hydrophilic gives strongest reinforcement but worst processing (structure formation, high mix torque). Hydrophobic trades modest reinforcement loss for dramatically improved processability — the standard choice for LSR and addition-cure RTV-2.

Powder anti-caking: hydrophobic at 0.1–0.5% w/w on powders forms a "ball-bearing" film that breaks inter-particle capillary bridges. Hydrophobicity is essential — hydrophilic grades adsorb atmospheric moisture and become the caking agent.

Applications and Formulation Guidance

Procurement, Storage, and QC

Packaging, MOQ, and lead time:

Hydrophobic grades: sealed dry storage <25°C, 24-month shelf life. Hydrophilic adsorbs atmospheric moisture irreversibly — once opened, decant to sealed container or use within 60 days (12 mo sealed / 6 mo opened). Bulk density 30–50 g/L makes air freight rarely cost-effective.

QC documentation (every batch CoA includes):

• BET surface area — single-point N₂ adsorption, ISO 9277
• Tapped density — ISO 787-11
• Loss on Ignition at 1,000°C × 2 hr — ISO 3262-20
• pH of 4% aqueous slurry — ISO 787-9
• Karl Fischer water content at 105°C
• Carbon content (hydrophobic grades only)
• Heavy metals (Pb / As / Cd / Hg — cosmetic / personal care customers)

Regulatory: REACH-registered (synthetic amorphous silica), self-classified non-hazardous per CLP, no SVHC. FDA 21 CFR 175.300 for indirect food contact / polymer adjuvants. Not crystalline silica (no IARC Group 1), standard powder PPE (P100 respirator + eye protection), OSHA PEL 5 mg/m³ respirable. Non-hazmat for road / sea / air.

Specification Table — All 8 HJSIL Grades

Hydrophilic Series

Hydrophobic Series

All values are typical batch ranges; batch-specific CoA is the committed specification.

Selection Guide

FAQ

Is HJSIL the same as Aerosil, and how do I switch?

Functionally yes — HJSIL is produced by the same flame-hydrolysis route as Evonik Aerosil, each grade matched to its Aerosil counterpart's BET, surface chemistry, and carbon content within Evonik's own batch variation. Default first trial is 1:1 direct substitution; validation compares (1) viscosity build-up curve at design loading, (2) yield-stress recovery after high-shear, (3) cured-state mechanical properties (tensile / tear / hardness for elastomers, flexural / Charpy for filled resins). In nine of ten silicone-sealant qualification trials shipped, HJSIL R272 / R110 substitution requires zero formulation adjustment. Exception: high-clarity applications where C >1.0% can introduce a faint yellow tint — use R272 (lower carbon) instead of R974-grade alternatives. Cost differential is 30–45% landed (Shanghai FOB → US/EU DDP).

What's the difference between HJSIL 200 and HJSIL R274?

HJSIL 200 is hydrophilic (native Si–OH surface). HJSIL R274 has the surface DMDS-modified, capping silanols with trimethylsilyl groups. Use R274 when (1) the continuous phase is non-polar (silicone oils, PDMS), (2) shelf life requires moisture exclusion, or (3) the application is exposed to humid air. Use 200 in polar liquids (water, alcohols, polyester resin) where it gives stronger thickening per unit weight.

Can HJSIL R110 be used in food-contact and pharmaceutical applications?

Yes. HMDS-treated HJSIL R110 is eligible under FDA 21 CFR 175.300 for indirect food contact (polymer adjuvants in food-contact materials). EU food contact is covered under the EFSA opinion on synthetic amorphous silica (E-551), though host-polymer migration must be verified per final formulation. For direct food / pharmaceutical use, request the FCF (food-contact-fit) variant with additional heavy-metals and residual-silazane purity controls.

What is the typical lead time and MOQ?

Hydrophobic trial quantities MOQ 5 × 10 kg bags (50 kg). Commercial supply MOQ 1 pallet (1.2 t). Container-load shipments available multi-grade. Lead time 4–6 weeks peak season (Mar–Oct), 3–4 weeks off-season, FOB Shanghai or CIF/DDP via agency. HJSIL 200 / R272 / R274 / R110 are typically in stock.

How is fumed silica different from precipitated silica?

Both are amorphous SiO₂ but the process differs. Fumed (HJSIL) — flame hydrolysis of SiCl₄ at ~1,800°C, non-porous 7–40 nm primary particles, very high surface energy. Precipitated — acid precipitation from sodium silicate, porous 10–50 nm with internal pore volume, lower surface energy. Procurement consequence: fumed gives 1.5–3× the thickening efficiency per unit weight in liquids, but precipitated is significantly cheaper and is the right pick for tire compounding, shoe sole, and toothpaste abrasive. For precipitated silica applications, see HPSIL® Healthcare Silica (toothpaste / food / cosmetic) and Shoe Sole Silica (footwear compounding).

Related & Sister Sites

• MEGEL® Silica Aerogel — thermal-insulation, ultra-low conductivity, ship/pipeline/oil-refining.
• HMMAT® Matting Agents — silica flatting agents for paint, ink, leather coatings.
• HPSIL® Healthcare Silica — FDA-compliant precipitated silica for toothpaste and food-contact.
• MESIL® Specialty Fumed Silica — high-purity specialty grades.
• Silicone Oils — PDMS, amino-functional, methylhydrogen and reactive silicone fluids.
• For silane couplers and reactive surface-treatment chemistries, see chemzip.com/products/silane-couplers/. For combined fumed silica + curing catalysts + TiO₂ in coatings and inks, see coatingsink.com matting-agent hub. For aerogel-blanket form factors in furnace / refractory installation, see thermaleast.com.