Thermal Oxidative Stability Testing: Methods, Standards & Applications
Asphalt and bitumen chemical property testing per ASTM D36 and D92 at Infinita LabWhat Is Thermal Oxidative Stability?
Thermal oxidative stability (TOS) describes a material’s resistance to chemical degradation when exposed simultaneously to elevated temperature and oxygen (or air). It is a critical durability property for polymers, elastomers, lubricants, coatings, and engineering materials that operate at elevated temperatures in oxygen-containing environments throughout their service life.
Thermal oxidation initiates via free-radical chain reactions that break polymer backbone bonds, add oxygen functionality, generate volatile degradation products, and cause cross-linking or chain scission — ultimately reducing mechanical properties, optical performance, and service life.
Why Thermal Oxidative Stability Testing Is Essential
Materials that appear adequate at ambient temperature may suffer rapid degradation in elevated-temperature service. Without TOS characterisation, engineers cannot predict service life, select appropriate stabiliser packages, or set safe operating temperature limits for materials in demanding applications.
Thermal oxidative stability testing enables:
- Comparison of antioxidant and stabiliser effectiveness in polymer formulations
- Prediction of material service life at defined operating temperatures
- Setting safe use temperature limits (upper service temperature)
- Qualification of lubricants and thermal fluids for high-temperature service
- Compliance with material specifications requiring minimum TOS performance
Key Test Methods for Thermal Oxidative Stability
Oxidative Induction Time (OIT) by DSC — ASTM D3895 / ISO 11357-6
OIT is the most widely used TOS test for polymers. A small specimen is heated to a defined isothermal temperature (typically 180–220°C) in a DSC instrument under nitrogen purge, then switched to oxygen flow. The time elapsed until the onset of rapid exothermic oxidative degradation — measured as a sharp departure from the flat DSC baseline — is the OIT. Longer OIT values indicate better thermal oxidative stability and more effective antioxidant protection.
ASTM D3895 covers OIT testing for polyolefins; ISO 11357-6 provides the equivalent international method.
Pressure Differential Scanning Calorimetry (PDSC) — ASTM E1858
PDSC tests OIT under elevated oxygen pressure (typically 500–3500 kPa), greatly accelerating the oxidation reaction and enabling testing at lower temperatures or shorter durations than atmospheric OIT. Preferred for lubricants, greases, and highly stabilised polymer compounds where atmospheric OIT would exceed practical test durations.
Oven Ageing — ASTM D573 / ISO 188
Rubber and polymer specimens are exposed to hot air in a circulating air oven at defined temperatures for defined periods. Mechanical properties (tensile strength, elongation, and hardness) are measured before and after ageing, and the percentage retention of these properties characterises thermal oxidative stability. This method is closest to real-world thermal ageing in air.
Thermogravimetric Analysis (TGA) in Air — ASTM E1131
TGA in air (or oxygen) measures weight loss as a function of temperature, indicating the temperature at which oxidative degradation initiates and the rate of subsequent thermal-oxidative mass loss.
Rotating Bomb Oxidation Test (RBOT) — ASTM D2272
Used for lubricating oils and hydraulic fluids. The oil sample is placed in a sealed bomb with oxygen under pressure and rotated in a heated bath. The time for oxygen pressure to drop by a defined amount indicates the lubricant’s oxidative resistance.
Factors Affecting Thermal Oxidative Stability
Polymer base chemistry (polyolefins oxidise faster than fluoropolymers or silicones), antioxidant type and concentration (primary and secondary AOs), filler type (carbon black inhibits oxidation in rubber; some metal oxides catalyse it), processing history (residual catalyst metals from polymerisation can accelerate oxidation), and oxygen partial pressure all govern thermal oxidative stability.
Industrial Applications
In the automotive industry, rubber seals and hoses for under-hood applications must demonstrate adequate OIT and oven ageing retention at 120–150°C. In power cables, XLPE and EPR insulation TOS data ensures reliable performance over 30+ year service lifetimes. In lubricant formulation, RBOT data validate the effectiveness of the additive package for turbine, hydraulic, and compressor oil grades.
Conclusion
Thermal oxidative stability (TOS) testing — utilizing methods such as OIT by DSC (ASTM D3895), PDSC (ASTM E1858), oven ageing (ASTM D573), TGA (ASTM E1131), and RBOT (ASTM D2272) — provides critical insight into a material’s resistance to degradation under combined heat and oxygen exposure. These techniques enable the evaluation of antioxidant effectiveness, the prediction of service life, and the validation of material performance in demanding environments. Selecting the appropriate test method based on material type, operating conditions, and required sensitivity is essential for accurately assessing long-term durability, making the testing strategy as important as the performance results themselves.
Why Choose Infinita Lab for Thermal Oxidative Stability Testing?
Infinita Lab provides OIT by DSC, PDSC, oven ageing, TGA in oxidative atmosphere, and RBOT through our nationwide accredited thermal analysis and polymer testing laboratory network.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What is oxidative induction time (OIT) and why is it used for polyolefin quality control? OIT is the time elapsed at a defined temperature and oxygen exposure before the onset of rapid oxidative degradation, measured by DSC. It directly measures antioxidant effectiveness in polyolefin compounds — a longer OIT indicates more effective stabilisation. It is used as a routine incoming QC test for polyethylene pipe, geomembrane, and cable insulation compounds.
What is the difference between standard OIT (ASTM D3895) and high-pressure OIT (ASTM E1858)? Standard OIT uses atmospheric oxygen pressure — suitable for moderately stabilised polyolefins with OIT values of 10–60 minutes at test temperature. High-pressure OIT (PDSC) uses elevated oxygen pressure (3500 kPa) to accelerate oxidation, enabling testing of highly stabilised materials (OIT >100 minutes at standard conditions) in practical laboratory timeframes.
How does oven ageing testing (ASTM D573) differ from OIT testing? OIT measures the time to the onset of oxidative degradation under isothermal, oxygen-exposed DSC conditions — it detects the point at which antioxidants are consumed. Oven ageing measures the actual mechanical property change after defined exposure periods at elevated temperature in air, simulating real-world service degradation more directly.
Why do some metal fillers accelerate thermal oxidative degradation of polymers? Transition metal ions (copper, manganese, iron, cobalt) catalyse the decomposition of hydroperoxide intermediates formed during polymer oxidation, generating highly reactive radical species that dramatically accelerate the chain oxidation mechanism. Metal deactivators are incorporated in polymer formulations that contact copper wire or metal components to suppress this pro-oxidant effect.
What TGA heating rate is typically used for oxidative stability characterisation? For dynamic TGA in air/oxygen, heating rates of 5–20°C/min are typical. For comparative screening, 10°C/min is most commonly used. Onset temperatures for oxidative degradation are typically reported as the extrapolated onset temperature from the TGA derivative (DTG) peak. Isothermal TGA at a fixed elevated temperature can also quantify oxidative mass loss rate.
