Plastic oxygen aging test is an accelerated aging procedure that evaluates the resistance of polymeric materials to oxidative degradation by exposing them to elevated temperature and oxygen, often under increased pressure. This test is essential for assessing long-term durability, antioxidant efficiency, and compliance with international standards such as UL 62, ASTM D572, ISO 188, and ISO 11357-6. In this article, we explain the test principles, sample types, testing items, methods, and typical applications across industries.
Technical Overview
Oxygen aging, also known as oxidative aging or thermal-oxidative aging, refers to the gradual degradation of plastic materials caused by the reaction of polymer chains with oxygen. The process follows a free-radical chain mechanism: thermal energy initiates the formation of polymer radicals, which react rapidly with oxygen to form peroxide radicals and hydroperoxides. These unstable intermediates decompose and trigger chain branching, resulting in simultaneous chain scission and crosslinking. Macroscopically, the material exhibits embrittlement, discoloration, surface cracking, loss of elongation, reduced tensile strength, and deteriorated dielectric performance.
A plastic oxygen aging test accelerates this natural degradation process in a controlled laboratory environment, typically by raising the temperature and increasing the oxygen partial pressure. Because the oxidation rate roughly doubles for every 10°C increase in temperature, and increases further under elevated oxygen pressure, weeks of laboratory exposure can simulate years of real-world service. The test is widely used to evaluate the effectiveness of antioxidant packages, verify material formulations, qualify products for safety certification, and estimate service life.
Testing Samples
Oxygen aging tests are applicable to a broad range of polymeric materials supplied as molded plaques, dumbbell specimens, sheets, films, granules, or finished product sections. Typical sample categories include:
Wire and Cable Insulation and Sheathing Compounds
PVC, cross-linked polyethylene (XLPE), polyethylene, TPE, and TPU insulation and jacket compounds represent the largest application area. Flexible cord and cable standards such as UL 62 require oxygen pressure aging (commonly called the oxygen bomb test) as a mandatory qualification item.
Polyolefin Resins, Compounds, and Films
PE and PP resins, masterbatches, blown and cast films, and rotational molding powders are tested to verify stabilizer performance and processing stability.
Engineering Plastics
PA (nylon), PC, POM, PBT, ABS, and modified compounds are evaluated for oxidative stability, particularly when intended for long-term heat exposure in electrical and automotive applications.
Pipe, Fitting, and Geosynthetic Materials
PE and PP pipe grades, geomembranes, and geotextiles are frequently qualified by oxidation induction time (OIT) requirements defined in piping and geosynthetic product standards.
Rubber-Plastic Blends and Recycled Materials
Thermoplastic elastomers, sealants, and recycled polymer feedstocks are tested to assess degradation levels and the adequacy of restabilization.
Testing Items
Depending on the product specification, the following properties are measured before and after oxygen aging exposure:
- Tensile strength and elongation at break, expressed as retention percentage or variation rate
- Oxidation induction time (OIT) and oxidation onset temperature (OOT)
- Change in mass after conditioning
- Color change and yellowness index
- Hardness, impact strength, and flexural properties
- Melt flow rate (MFR) variation indicating chain scission or crosslinking
- Dielectric strength, volume resistivity, and insulation resistance for cable materials
- Visual assessment of surface cracking, crazing, blooming, and embrittlement
Testing Methods
Several complementary methods are used, depending on the material and the governing product standard. All tests should be performed in calibrated environmental chambers or pressure vessels at accredited third-party laboratories.
Oxygen Pressure Vessel Method (Oxygen Bomb Test)
Specimens are suspended in a sealed stainless steel pressure vessel charged with commercial-grade oxygen at elevated pressure and temperature. A widely used condition for cable compounds, referenced in UL 62 and UL 2556 test protocols, is 70°C with oxygen pressure of approximately 2.1 MPa (300 psi) for 96 to 168 hours. After exposure, tensile strength and elongation at break are measured, and retention values are compared against the pass criteria of the applicable specification. Similar oxygen pressure procedures are described in ASTM D572 and ASTM D865 for elastomeric materials and in GB/T 13939 for vulcanized rubber.
Hot Air Oven Aging Method
Specimens are exposed to circulating hot air in a ventilated oven under atmospheric oxygen content. Representative standards include ASTM D573, ISO 188, GB/T 3512 for rubber, and GB/T 7141 for plastics. Exposure temperatures and durations are selected according to the material class and intended service condition, typically ranging from 80°C to 150°C for 168 to 1000 hours or longer.
Oxidation Induction Time by DSC
OIT is determined by differential scanning calorimetry under an oxygen atmosphere, following ISO 11357-6, ASTM D3895, ASTM D5885 (high-pressure OIT for geosynthetics), and EN 728 for polyolefin pipe materials. In a typical procedure, the specimen is heated under nitrogen to a set isothermal temperature, the gas is switched to oxygen, and the time elapsed until the onset of exothermic oxidation is recorded. A longer OIT indicates stronger antioxidant protection. High-pressure OIT methods apply oxygen at 3.4 to 5.0 MPa to extend measurable induction times for highly stabilized materials.
Result Evaluation
Property retention is calculated as the ratio of the post-aging property value to the original value, expressed as a percentage. Product specifications typically require minimum retention values, for example 75% to 85% retention of tensile strength and elongation after the prescribed oxygen aging period. Statistical comparison of before-and-after data and visual failure analysis are included in the final test report.
Applications
Plastic oxygen aging tests support material development, quality control, and certification across many industries:
- Wire and cable manufacturing: qualification of insulation and sheathing compounds for flexible cords, power cables, and appliance wiring under UL, IEC, and EN standards
- Polymer and additive development: comparative evaluation of antioxidant systems, stabilizer packages, and formulations
- Piping and infrastructure: service life prediction for PE gas and water pipes, PP fittings, and PVC systems using OIT requirements
- Automotive and aerospace: durability validation of under-hood plastics, connectors, seals, and interior components subjected to long-term heat
- Packaging and agriculture: shelf life and outdoor durability assessment for films, woven fabrics, and greenhouse coverings
- Recycling and sustainability: verification of property restoration in mechanically recycled polymers
- Regulatory compliance: generation of test data for product certification submissions to certification bodies and end users
FAQ
What is the difference between oxygen aging and hot air aging?
Oxygen aging exposes specimens to pure oxygen at elevated pressure, dramatically accelerating oxidation kinetics, while hot air aging uses atmospheric air in a ventilated oven. Oxygen pressure aging is more aggressive and is required for safety-critical applications such as flexible cord insulation, whereas hot air aging better simulates general long-term thermal service.
What are the typical test conditions for the oxygen bomb test?
The most common condition for cable compounds is 70°C at an oxygen pressure of approximately 2.1 MPa for 96 to 168 hours. Specific temperature, pressure, and duration are always defined by the governing product standard.
What does a short OIT value indicate?
A short oxidation induction time indicates weak antioxidant protection, meaning the material will oxidize quickly at elevated temperatures. Pipe and geosynthetic standards commonly set minimum OIT limits, such as 20 minutes or more at the specified test temperature.
How is pass or failure determined?
The specimen passes if the post-aging tensile strength and elongation at break retention meets or exceeds the minimum percentages specified in the product standard, and no visible cracking or embrittlement is observed.
How many specimens are required?
Generally, a minimum of five dumbbell specimens per condition for tensile testing, plus reference specimens for original property determination, are recommended. Additional specimens may be needed for OIT, mass change, and electrical testing.
Can finished products be tested directly?
Yes. Sections of insulation, sheath, film, or molded parts can be aged and evaluated directly, provided specimens of the required dimensions can be prepared and the material is identified.