Determination of mechanical properties of aged plastics is a critical evaluation process that quantifies how thermal, UV, and environmental aging affect the tensile strength, elongation, flexural performance, and impact resistance of polymer materials. By comparing specimens before and after accelerated aging exposure in accordance with ISO, ASTM, and GB standards, manufacturers can assess material durability, calculate property retention rates, and validate long-term product reliability.
Technical Overview
Plastics undergo progressive degradation when exposed to heat, ultraviolet radiation, oxygen, moisture, and mechanical stress over their service life. This aging process triggers molecular chain scission, oxidation, crosslinking, and changes in morphology at the microscopic level, which manifest as measurable changes in macroscopic mechanical behavior. Determination of the mechanical properties of aged plastics therefore serves as the fundamental method for quantifying material durability and predicting service life.
The core concept of this evaluation is the comparison of mechanical property values measured on unaged reference specimens against specimens subjected to controlled aging exposure. The resulting property retention rate, expressed as a percentage, indicates the degree of degradation. A retention rate close to 100% signifies excellent aging resistance, while significant reduction signals embrittlement, loss of toughness, or structural weakening.
Typical degradation patterns include reduced elongation at break and impact strength due to embrittlement, decreased tensile strength from chain scission, increased or decreased modulus depending on whether crosslinking or chain breaking dominates, and surface cracking or chalking caused by photo-oxidation. Accredited third-party laboratories perform these evaluations under standardized aging and mechanical testing conditions to generate reproducible, comparable data for material qualification, quality control, and research purposes.
Testing Samples
Standard Molded Specimens
Injection-molded or compression-molded test pieces produced according to ISO 20753 or ASTM D638 Type I dimensions are the preferred sample format. These include dumbbell-shaped tensile bars, rectangular flexural bars, and impact plaques. Parallel batches are prepared so that one group serves as the unaged control while others undergo aging exposure.
Machined or Cut Specimens
When evaluating finished products, specimens may be machined or die-cut from actual components such as pipes, profiles, sheets, and housings. This approach reflects the real processing history, residual stress state, and additive distribution of the commercial article.
Films and Sheets
Thin polymer films used in packaging, agriculture, and lamination are tested using dedicated specimen geometries specified in ISO 527-3, with strip or dumbbell specimens cut from both machine and transverse directions to capture anisotropic aging effects.
Pipe and Tubing Segments
For pressure pipes and cable insulation, aging evaluation may combine specimen-level mechanical testing with whole-product assessments, including tensile testing of strip specimens cut from pipe walls per ISO 6259.
Testing Items
The evaluation covers the full range of mechanical properties most sensitive to polymer degradation:
- Tensile strength and tensile stress at yield and at break
- Elongation at break and elongation at yield
- Tensile modulus of elasticity
- Flexural strength and flexural modulus
- Charpy and Izod impact strength, notched and unnotched
- Shore hardness and ball indentation hardness
- Compression strength and compressive modulus
- Property retention rate and percentage change after each aging interval
- Tear strength and puncture resistance for films
Among these, elongation at break and notched impact strength are widely recognized as the most sensitive early indicators of oxidative embrittlement, often declining substantially before tensile strength shows measurable change.
Testing Methods
Aging Exposure Conditioning
Prior to mechanical testing, specimens are subjected to controlled aging in laboratory equipment. Common exposure regimes include:
- Thermal aging in air-circulated ovens per ISO 188, ASTM D573, and GB/T 3512, typically at elevated temperatures for defined durations
- Xenon-arc lamp weathering per ISO 4892-2, ASTM G155, and GB/T 16422-2 to simulate full-spectrum sunlight
- Fluorescent UV condensation weathering per ISO 4892-3, ASTM G154, and GB/T 16422-3 for moisture and UV cycling
- Carbon-arc lamp exposure per ISO 4892-4 and ASTM G152 where specified
- Humidity and hydrolytic aging in water or humid environments for moisture-sensitive polymers such as polyesters and polyamides
Mechanical Test Procedures
After each aging interval, specimens are conditioned at 23°C and 50% relative humidity per ISO 291 before mechanical evaluation:
- Tensile testing per ISO 527, ASTM D638, and GB/T 1040 using universal testing machines with appropriate extensometry
- Flexural testing per ISO 178, ASTM D790, and GB/T 9341 using the three-point bending configuration
- Charpy impact per ISO 179 and GB/T 1043, and Izod impact per ASTM D256, ISO 180, and GB/T 1843 using pendulum impact testers
- Shore hardness per ISO 868, ASTM D2240, and GB/T 2411; ball indentation hardness per ISO 2039
- Compression testing per ISO 604 and ASTM D695 for rigid materials under compressive loading
Data Analysis and Retention Rate Calculation
The property retention rate is calculated as the measured value after aging divided by the corresponding value of the unaged control, multiplied by 100%. Testing at multiple exposure intervals allows construction of degradation curves, enabling estimation of the time required for a property to fall to a critical threshold. Statistical analysis of replicate specimens ensures the reported changes reflect genuine material degradation rather than test scatter.
Applications
Determination of aged mechanical properties supports decision-making across many industries:
- Automotive engineering: validation of interior and exterior components resisting heat and sunlight exposure over vehicle service life
- Construction and infrastructure: qualification of pipes, window profiles, roofing membranes, and geosynthetics for long-term outdoor durability
- Electrical and electronics: assessment of enclosure materials, connectors, and cable insulation for thermal endurance classes under IEC and UL frameworks
- Packaging: verification of film performance after storage and sterilization or pasteurization processes
- Material research and development: screening of stabilizer packages, antioxidants, and UV absorbers in new formulations
- Failure analysis: comparison of degraded field returns against reference data to diagnose premature failure causes
- Regulatory compliance and certification: generation of test reports supporting product declarations and customer specifications
FAQ
What is a typical acceptance criterion for property retention after aging?
Requirements vary by application and specification, but a common benchmark for many qualification programs is that elongation at break and impact strength retain at least 50% of their initial values after the specified exposure. Critical safety parts may require retention of 70% or higher, while commodity applications may accept lower thresholds.
How should the appropriate aging method be selected?
The method should mirror the dominant degradation mechanism in the intended service environment. Thermal oven aging suits heat-dominated applications such as engine compartments and electrical enclosures, while xenon-arc or fluorescent UV weathering suits outdoor sunlight exposure. Hydrolytic conditioning applies to polymers used in hot, humid, or aqueous environments.
How long should specimens be aged?
Exposure duration depends on the material and objective. Screening studies often apply several intervals such as 168, 500, and 1000 hours, whereas durability validation may extend to several thousand hours. Selecting multiple time points is recommended so that degradation trends can be established rather than relying on a single measurement.
Why do elongation and impact strength decrease before tensile strength during aging?
Oxidation and chain scission initially reduce molecular weight and ductility, causing embrittlement. Tensile strength depends partly on the load-bearing capacity of the polymer network and may remain nearly unchanged until degradation advances substantially, whereas ductility-dependent properties respond immediately to molecular weight loss and surface microcracking.
Can specimens be tested immediately after removal from the aging chamber?
No. Unless otherwise specified, aged specimens should be conditioned in a standard atmosphere of 23°C and 50% relative humidity before mechanical testing to eliminate transient effects of temperature and moisture, ensuring comparability with the unaged control results.
