ISO 4263-3:2015

Overview - ISO 4263-3:2015 (TOST test, anhydrous)

ISO 4263-3:2015 defines an anhydrous TOST test method to determine the ageing behaviour (oxidation stability) of synthetic hydraulic fluids - specifically categories HFDU, HEES, HEPG and HETG. The test accelerates ageing by exposing a test portion to oxygen and metal catalysts at elevated temperature (95 °C) and follows fluid degradation by measuring changes in acid number / TAN (total acid number). Results are reported as an oxidation lifetime when either TAN reaches 2.0 mg KOH/g or increases by 2.0 mg KOH/g. The procedure is intended for fluids not prone to water contamination and includes detailed apparatus, reagents, sampling and calculation requirements.

Key topics and technical requirements

• Test principle: Oxidation in an oxygen atmosphere with steel and copper catalyst coil; periodic aliquot removal and acid-number titration.
• Endpoint criteria: TAN = 2.0 mg KOH/g or increase of TAN by 2.0 mg KOH/g; optional fixed durations (e.g., 500 h or 1 000 h) where applicable.
• Temperature control: Test portions held at 95.0 °C ± 0.2 °C in a light-excluding bath.
• Oxygen supply: Minimum purity 99.5%; regulated flow (flowmeter example 3.0 l/h ± 0.1 l/h).
• Catalyst and hardware: Specified steel and copper wire diameters for catalyst coil (≈1.60 mm and 1.63 mm respectively); borosilicate oxidation cells and mushroom condensers.
• Reagents and cleaning: Heptane (≥99.75 %), acetone, propan-2-ol; strong oxidizing cleaning solutions (chromosulfuric acid alternatives noted); surfactant cleaning options.
• Safety and limitations: Use of hazardous oxidizing agents and oxygen-handling warnings; precision for synthetic fluids is not established (interlaboratory data lacking) - caution using results for contractual specifications or disputes.
• Additional observations: The standard notes other deterioration signs (insoluble sludge, catalyst coil corrosion, viscosity change) that may not be reflected in calculated oxidation lifetime.

Applications and who uses it

ISO 4263-3:2015 is primarily used by:

• Lubricant and hydraulic fluid formulators for oxidation-stability screening and product development.
• Independent test laboratories and QC teams for comparative stability testing of synthetic hydraulic fluids.
• OEMs and maintenance engineers evaluating fluid selection where water contamination is unlikely.
• R&D groups assessing antioxidant performance and expected service life trends under accelerated conditions.

Use cases include quality control, formulation comparison, and research. Note that correlation with in-service performance varies by application and is best judged with field experience.

Related standards

• ISO 4263 (other parts: Part 1 - mineral oils; Part 2 - HFC fluids; Part 4 - gear oils)
• ISO 12922, ISO 15380 (fluid category definitions)
• ISO 3170, ISO 3696, ISO 7537 (sampling, water quality, TAN titration)
• EN 10130, EN 13601 (material and wire standards)

Keywords: ISO 4263-3:2015, TOST test, oxidation stability, synthetic hydraulic fluids, ageing behaviour, anhydrous procedure, TAN, acid number.