Hydrophobic Silica

Updated: 2026-07-22

Overview

Hydrophobic fumed silica is a high-purity silicon dioxide powder with surface-treated non-polar groups (e.g., siloxane or dimethylsilyl) to repel water. Unlike hydrophilic silica, it exhibits excellent compatibility with organic polymers and resists moisture absorption. The material is produced through flame hydrolysis of silicon tetrachloride followed by chemical modification. First commercialized in the 1960s, hydrophobic grades address limitations of untreated silica in non-polar systems. Major producers include Evonik (Aerosil R-series), Cabot, and Wacker. The global market exceeds 300,000 tons annually, driven by demand from silicone rubber and specialty coatings.

Physical and Chemical Properties

The material features a three-dimensional branched chain structure with primary particles of 7–40 nm forming aggregates. Surface treatment reduces the hydroxyl group density from ~3/nm² (hydrophilic) to <1/nm², lowering surface energy to 20–30 mN/m. This results in a contact angle >90° with water. Key metrics include specific surface area (50–400 m²/g), tapped density (30–60 g/L), and pH (3.5–5.5 for most grades). Unlike hydrophilic silica, it maintains flowability even at high humidity (RH>80%). The hydrophobic layer typically withstands temperatures up to 200°C before degradation.

Main Applications

In silicone rubber, hydrophobic fumed silica provides reinforcement without viscosity drift, critical for liquid silicone rubber (LSR) and high-consistency rubber (HCR). It improves tensile strength (up to 12 MPa) while maintaining elongation (>800%). The coatings industry utilizes its anti-settling and anti-sag properties, especially in solvent-borne systems. At 0.5–2% loading, it prevents pigment sedimentation and enables vertical surface application without drips. Other uses include: thickening polishes (3–8% load), improving powder flow in cosmetics, and enhancing dielectric properties in cable compounds.

Safety and Storage

Although chemically inert, inhalation of airborne particles may cause temporary respiratory irritation. NIOSH recommends keeping airborne concentrations below 6 mg/m³ (total dust) with proper ventilation. Skin contact is generally benign but may cause dryness. Storage requires protection from moisture ingress even with hydrophobic properties. Bulk bags should be kept on pallets away from walls. Shelf life typically exceeds 2 years in original packaging. For small quantities, desiccant packs in resealed containers prevent caking. Never mix with strong acids/bases that could degrade the surface treatment.

B2B Procurement Guide

Technical specifications should include: surface area (±10%), carbon content (1–5% for most treated grades), loss on drying (<1.5% at 105°C), and sieve residue (<0.05% on 45μm). For rubber applications, request Mooney viscosity test data (typically 30–60 MU at 10% loading). Sample evaluation should test dispersion ease in your base material – poor dispersion indicates mismatch with the carrier medium. Large orders (10+ tons) often qualify for 8–12% discounts. Lead times range from 2 weeks (standard grades) to 8 weeks (custom treatments). Always verify REACH/SDS compliance for your region.