Overview
EN ISO 18473-3:2019 (ISO 18473-3:2018) defines requirements and test methods for fumed silica in powder form intended for silicone rubber applications. Adopted by CEN, this part of the ISO 18473 series covers both untreated (hydrophilic) and surface-treated (hydrophobic) fumed silica used as functional pigments and extenders. The standard helps manufacturers, suppliers and test laboratories ensure consistent quality and fitness for use in silicone rubber formulations.
Key topics and technical requirements
- Classification: fumed silica is classified as hydrophilic or hydrophobic depending on dispersibility in water.
- Designation: normally designated by specific surface area (m2/g) measured by the BET method (ISO 9277); typical designation values include 150, 200, 300, 380 m2/g (subject to agreement).
- Essential parameters and test methods:
- Specific surface area: designated value ±10% (ISO 9277).
- pH of aqueous suspension: ≥ 3.8 (ISO 787-9).
- Loss on drying (105 °C): hydrophilic ≤ 2.0%, hydrophobic ≤ 1.0% (ISO 787-2).
- Residue on 45 µm sieve: ≤ 150 mg/kg (ISO 787-18).
- Metal impurities (Fe, Al, Ti): limits given (e.g., Fe ≤ 11 mg/kg) using ISO 3262-20:2000 methods.
- Carbon content: hydrophilic ≤ 0.2%, hydrophobic ≥ 0.3% (ISO 3262-20:2000).
- Loss on ignition: ≤ 2.5% (ISO 3262-1).
- Conditional requirement: tamped density reported per ISO 787-11 if agreed between parties.
- Sampling, marking and reporting:
- Sampling per ISO 15528; packing must be labelled with manufacturer, product name/type, net weight, batch number and a “keep away from rain” mark.
- Test reports must identify the product, reference ISO 18473-3:2018 and list methods/results.
Practical applications
- Primary use: reinforcing filler in silicone rubbers - RTV, HTV, HCR and LSR - to improve strength, rheology and optical properties.
- Secondary uses: specialist formulations in paints, coatings and plastics where nano-structured fumed silica properties are required.
- The standard ensures material consistency for formulators aiming for predictable curing, mechanical performance and transparency in silicone-based products.
Who should use this standard
- Fumed silica manufacturers and surface-treaters
- Quality control and analytical laboratories
- Silicone rubber compounders and formulators
- Suppliers, purchasers and procurement teams specifying raw materials
- Accreditation and compliance professionals
Related standards
Keywords: EN ISO 18473-3:2019, ISO 18473-3, fumed silica, silicone rubber, specific surface area, BET, hydrophilic, hydrophobic, test methods.