ISO 18473-3:2018

Overview

ISO 18473-3:2018 specifies requirements and corresponding methods of test for fumed silica in powder form intended for silicone rubber applications. The standard applies to both untreated (hydrophilic) and surface‑treated (hydrophobic) fumed silica used as reinforcing functional fillers in RTV, HTV, HCR and LSR silicone formulations. Key objectives are to ensure consistent quality, appropriate physicochemical characteristics and suitable labeling and test reporting for supply-chain and manufacturing use.

Key Topics

• Definition and classification: Fumed silica is defined as amorphous silica produced by high‑temperature flame hydrolysis. It is classified as hydrophilic or hydrophobic depending on dispersibility in water.
• Designation: Products are commonly designated by specific surface area (BET method, ISO 9277) with typical values such as 150, 200, 300 and 380 m2/g and a ±10 % tolerance unless otherwise agreed.
• Essential requirements: The document lists technical limits and test methods, including:
  • Specific surface area - measured by ISO 9277 (BET).
  • pH - minimum value (tested per ISO 787‑9).
  • Loss on drying - limits differ for hydrophilic and hydrophobic grades (ISO 787‑2).
  • Residue on 45 µm sieve - ISO 787‑18.
  • Trace element limits (Fe, Al, Ti) and carbon content - methods referenced to ISO 3262‑20 and related parts.
• Conditional requirements: Tamped density is conditional and may be agreed between parties; measured according to ISO 787‑11.
• Sampling and test reporting: Sampling follows ISO 15528. Test reports must identify the product, reference ISO 18473‑3:2018, list methods and results, declare deviations and unusual observations, and include the test date.

Applications

Fumed silica specified to ISO 18473‑3 is used primarily as a reinforcing filler in silicone rubber formulations where it:

• Improves mechanical strength and rheology of cured polydimethylsiloxane (PDMS) systems.
• Influences transparency and surface properties depending on grade and surface treatment.
• Serves in a wide range of silicone rubber products including room temperature vulcanized (RTV) sealants, high‑consistency and liquid silicone rubbers (HCR, LSR), and high‑temperature vulcanized (HTV) compounds.

Practical benefits for manufacturers and material engineers include predictable performance, easier quality control, and clear acceptance criteria for incoming materials.

Related Standards

• ISO 9277 - Specific surface area (BET) method
• ISO 787 series - General methods of test for pigments and extenders (pH, loss on drying, tamped volume, residue on sieve)
• ISO 3262‑20:2000 - Extenders for paints: fumed silica test clauses referenced for trace elements and carbon content
• ISO 15528 - Sampling
• ISO 18451‑1 - Terminology for pigments, dyestuffs and extenders

Note: ISO 18473‑3:2018 provides test method references and limits to support specification, procurement and quality assurance of fumed silica for silicone rubber. For detailed test procedures and exact numeric limits consult the full standard.