Is Fumed Silica a Nanomaterial? EU Definition, Aggregate Reality & What It Means for Formulators
Fumed silica primary particles are 5–50 nm, but commercial product ships as fused aggregates well above the 100 nm nano threshold — classification hinges on which particle state regulators measure.
The European Commission’s 2022 recommendation (2022/C 229/01) defines a nanomaterial as any material containing 50% or more of constituent particles in the 1–100 nm size range by number distribution. This replaced the 2011 definition and removed the previous exemption for aggregates and agglomerates — meaning regulators now consider whether primary particles within a fused structure fall in the nano range, not just the outer dimensions of the shipped powder.
For fumed silica manufacturers, this distinction is critical. The updated definition explicitly states that particles in aggregates or agglomerates count toward the 50% threshold if their internal constituent particles are 1–100 nm. REACH dossiers filed before 2022 that classified fumed silica as non-nano based on aggregate sizing may require reassessment.
Fumed silica is produced by flame hydrolysis of chlorosilanes at 1000–1800 °C. Primary particles of 5–50 nm form in the flame zone and immediately fuse into branched aggregates of 100–500 nm through sintered Si–O–Si bonds. These aggregates are permanent — they cannot be broken apart by dispersion energy. Agglomerates of aggregates range from 1–100 μm and are held by van der Waals forces, easily disrupted by shear.
The regulatory question becomes: do you measure primary particles (nano) or aggregates (non-nano)? TEM imaging resolves primary particles at 5–50 nm, confirming nano status. Laser diffraction measures agglomerates at 1–30 μm, suggesting non-nano. BET surface area (50–400 m²/g) indirectly confirms nanoscale primary structure — a 200 m²/g grade implies ~12 nm equivalent spherical diameter.
Under the updated definition, most commercial fumed silica grades qualify as nanomaterials based on primary particle analysis. This triggers specific REACH obligations: nano-form registration annexes (Annexes I, III, VI–XII per Regulation 2018/1881), Safety Data Sheet nano-labeling in Section 3, and exposure scenario documentation for nano-specific hazard endpoints.
Suppliers like Evonik (AEROSIL), Cabot (CAB-O-SIL), and Wacker (HDK) have updated REACH dossiers to include nano-form characterization. Formulators receiving fumed silica should verify that their supplier’s SDS reflects the 2022 definition and includes nano-form particle size distribution data — typically reported by TEM as number-weighted primary particle diameter.
For B2B formulators using fumed silica as a thixotrope, reinforcing filler, or anti-settling agent, the nano classification does not change the product’s performance. A grade with 200 m²/g BET surface area and 12 nm primary particles behaves identically regardless of regulatory label. What changes is documentation: your REACH compliance files, workplace exposure assessments, and potentially customer-facing product information must reflect nano status.
When specifying grades, request the supplier’s nano-form characterization data — primary particle size (TEM), aggregate size (DLS or disc centrifuge), BET surface area, and surface treatment type (hydrophilic vs. hydrophobic). Hydrophobic grades (e.g., dimethyldichlorosilane-treated, BET 100–140 m²/g) and hydrophilic grades (BET 150–400 m²/g) are both classified as nanomaterials under the 2022 definition.
The table below compares common fumed silica grades across nano-relevant parameters. All grades listed have primary particles in the 5–50 nm range, confirming nano classification under EU 2022/C 229/01. Aggregate sizes vary with BET surface area — higher surface area grades have smaller primary particles and correspondingly smaller aggregates.
| Grade Example | BET (m²/g) | Primary Particle (nm) | Aggregate Size (nm) | Surface | Nano Status (EU 2022) |
|---|---|---|---|---|---|
| Hydrophilic 150 | 150 ± 15 | ~14 | 200–350 | Untreated (–OH) | Yes |
| Hydrophilic 200 | 200 ± 25 | ~12 | 150–300 | Untreated (–OH) | Yes |
| Hydrophilic 300 | 300 ± 30 | ~7 | 100–200 | Untreated (–OH) | Yes |
| Hydrophobic R972 | 110 ± 20 | ~16 | 200–400 | DDS-treated | Yes |
| Hydrophobic R812 | 260 ± 30 | ~7 | 100–250 | HMDS-treated | Yes |
| Hydrophilic 380 | 380 ± 30 | ~5 | 100–180 | Untreated (–OH) | Yes |
All commercial fumed silica grades are nanomaterials under the EU 2022 definition based on primary particle size (5–50 nm). This classification does not affect performance but requires updated REACH dossiers, SDS nano-labeling, and downstream exposure documentation — verify your supplier has completed the transition.
Yes, under the EU 2022/C 229/01 recommendation, fumed silica qualifies as a nanomaterial because its primary particles (5–50 nm) exceed the 50% number-based threshold in the 1–100 nm range. This applies regardless of aggregate or agglomerate size in the shipped powder.
Primary particles (5–50 nm) form during flame hydrolysis and immediately fuse into permanent aggregates (100–500 nm) via sintered Si–O–Si bonds. Aggregates cannot be broken by mechanical dispersion. Agglomerates are loose clusters of aggregates (1–100 μm) held by van der Waals forces.
No. The nano classification is purely regulatory. A 200 m²/g fumed silica grade provides the same thixotropy, anti-settling, and reinforcement performance regardless of whether it is labeled as a nanomaterial. Only documentation and compliance obligations change.
Nano-form registration requires updated Annexes I, III, and VI–XII with particle size distribution, surface chemistry, and shape data per Regulation 2018/1881. Safety Data Sheets must declare nano-form identity in Section 3, and exposure scenarios must address nano-specific inhalation endpoints.
Request the supplier’s nano-form characterization package: TEM-measured primary particle size distribution, DLS or disc centrifuge aggregate sizing, BET surface area certificate, and surface treatment specification. The SDS (Section 3) should explicitly reference nano-form status under 2022/C 229/01.
No. Both hydrophilic (untreated silanol) and hydrophobic (DDS, HMDS, or silicone-treated) fumed silica grades have primary particles in the 5–50 nm range. Surface treatment modifies wettability and dispersion behavior but does not change the particle size that determines nano classification.
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