Aging brittleness determination is a critical durability test that evaluates how plastic materials lose ductility and become embrittled after exposure to heat, UV radiation, humidity, and other environmental stressors. This article explains the mechanisms of polymer embrittlement, the internationally recognized test methods such as ISO 974, ASTM D746, and GB/T 5470, and the typical sample requirements, procedures, and industrial applications. Read on to learn how accredited third-party laboratories measure brittleness temperature and quantify the long-term reliability of plastic products.
Technical Overview
Plastic aging brittleness determination refers to the systematic evaluation of how polymeric materials transition from a ductile, tough state to a brittle, fracture-prone state as a result of thermal, photo-oxidative, or hydrolytic aging. During service, plastics are continuously exposed to oxygen, ultraviolet radiation, elevated temperatures, and mechanical stress, which trigger molecular-level degradation mechanisms including chain scission, oxidative crosslinking, crystallinity changes, and plasticizer or additive migration. These processes reduce elongation at break, increase modulus, and raise the temperature below which the material fails catastrophically under impact.
The most widely used quantitative indicator is the brittleness temperature (Tb), defined as the temperature at which 50% of the test specimens exhibit failure when struck by a defined impact energy under standardized conditions. A shift of brittleness temperature toward higher values after aging is a direct, measurable sign of embrittlement. Complementary indicators include the retention percentage of tensile elongation and impact strength before and after aging exposure. Accredited third-party laboratories routinely perform these tests to support material qualification, supplier control, and failure analysis.
- Primary impact-brittleness standards: ISO 974, ASTM D746, GB/T 5470
- Film and sheeting brittleness: ASTM D1790
- Aging preconditioning standards: ISO 2578, ASTM D3045 (thermal aging); ISO 4892-2, ISO 4892-3, ASTM G155, ASTM G154 (artificial weathering); ISO 877, ASTM D1435 (natural weathering)
- Post-aging mechanical verification: ISO 527, ASTM D638 (tensile), ISO 179, ASTM D256 (impact)
Testing Samples
Molded Rigid Plastics
Injection-molded or compression-molded plaques of polypropylene (PP), polyethylene (PE), polystyrene (PS), ABS, polyvinyl chloride (PVC), polycarbonate (PC), and engineering thermoplastics can be machined or die-cut into strip specimens. For the impact brittleness method, specimens are typically 20 mm in length and 2.5 mm in width with a preferred thickness of 1.6 mm, in accordance with ISO 974, ASTM D746, and GB/T 5470.
Films and Sheeting
Thin flexible materials such as packaging films, agricultural films, geomembranes, and protective sheeting are tested per ASTM D1790 or via fold and bend embrittlement assessment. Film samples are cut into narrow strips with clean, defect-free edges, since nicks and burrs can cause premature failure and misleading results.
Finished Products and Components
Samples cut from wire and cable insulation, pipes, fittings, gaskets, automotive trim, and appliance housings may be evaluated directly. When component geometry prevents full-size specimens, reduced-scale strips or comparative pass/fail testing at a specified service temperature is performed, and the deviation from the standard specimen is documented in the test report.
Sample Quantity and Conditioning
A minimum of 30 specimens per temperature point is generally recommended for statistical determination of the brittleness temperature, and more than one temperature level is tested to construct the failure-probability curve. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C and 50% relative humidity in accordance with ISO 291 or ASTM D618, unless otherwise specified.
Testing Items
- Brittleness temperature (Tb) by impact, determined as the 50% failure temperature
- Pass/fail verification at a specified low service temperature (for example, no failure at −40 °C)
- Embrittlement assessment after thermal-oxidative aging in air-circulating ovens
- Embrittlement assessment after xenon-arc or fluorescent-UV artificial weathering
- Retention of tensile elongation at break and tensile strength after aging
- Retention of Charpy or Izod impact strength after aging
- Visual and microscopic evaluation of surface cracking, chalking, and crazing induced by aging
Testing Methods
Impact Brittleness Method (ISO 974 / ASTM D746 / GB/T 5470)
Specimens are clamped as cantilever beams in a specimen holder immersed in a temperature-controlled heat-transfer medium, typically silicone oil or ethanol chilled with dry ice to temperatures from ambient down to approximately −70 °C or lower. After a defined equilibration period (usually 3 minutes or longer, depending on specimen thickness), a striker with a defined radius travels at a controlled velocity and delivers a single bending impact to each specimen. The specimen is then removed, allowed to recover briefly, flexed, and examined visually for cracks or complete fracture. A specimen that cracks or breaks at any point is recorded as a failure.
Determination of the Brittleness Temperature
Testing is repeated at several temperature levels spanning the transition from fully ductile to fully brittle behavior. The failure fraction at each temperature is plotted, and the temperature corresponding to 50% failure probability is calculated using the statistical method prescribed by the standard, such as the geometric-mean procedure in ASTM D746 or probit analysis in ISO 974. The resulting brittleness temperature is reported together with the confidence interval and the number of specimens tested.
Aging Preconditioning Protocols
To determine aging-induced embrittlement, sets of specimens are exposed before testing using one or more of the following protocols, after which brittleness temperature or mechanical property retention is measured against unaged control specimens.
- Thermal aging: long-term exposure in ventilated ovens at elevated temperatures per ISO 2578 or ASTM D3045, with property retention plotted against exposure time
- Artificial weathering: xenon-arc lamp exposure per ISO 4892-2 or ASTM G155, or fluorescent-UV condensation exposure per ISO 4892-3 or ASTM G154, with controlled irradiance, temperature, and humidity cycles
- Natural weathering: direct outdoor exposure per ISO 877 or ASTM D1435 for correlation with accelerated results
Complementary Mechanical Evaluation
Because brittleness temperature alone does not capture all aspects of embrittlement, accredited third-party laboratories typically supplement the impact test with tensile testing per ISO 527 or ASTM D638 to determine elongation retention, and with impact testing per ISO 179 or ASTM D256. A common acceptance criterion is that elongation at break after aging must remain at or above 50% of the initial value, or that the brittleness temperature must not rise above the minimum design service temperature.
Applications
- Wire and cable: verifying that insulation and jacketing compounds remain flexible at rated low-temperature service conditions and after thermal aging
- Automotive: qualification of under-hood plastics, exterior trim, battery housings, and elastomeric seals exposed to heat cycling and sunlight
- Construction: durability assessment of PVC profiles, pipes, fittings, geomembranes, and roofing materials subjected to outdoor weathering
- Packaging: confirming that films retain fold resistance and puncture behavior after processing and storage
- Consumer goods and appliances: shelf-life studies and material comparison for housings and functional components
- Failure analysis: identifying whether field failures of plastic parts are attributable to aging embrittlement, processing defects, or formulation issues
- Regulatory compliance: supporting product certification where low-temperature flexibility or aging resistance is mandated by specification
FAQ
What is the brittleness temperature of a plastic?
The brittleness temperature is the temperature at which 50% of specimens fail when subjected to a standardized impact in a chilled medium. Below this temperature, the material behaves in a predominantly glassy, brittle manner; above it, the material exhibits ductile behavior. It is determined statistically from failure data at multiple temperatures, not measured as a single inherent material constant.
How does aging cause plastics to become brittle?
Aging mechanisms such as oxidative chain scission, UV-induced radical reactions, post-crystallization, loss of plasticizers, and secondary crosslinking reduce molecular mobility and the ability of the polymer to deform plastically. Macroscopically, this appears as reduced elongation at break, surface cracking and chalking, diminished impact strength, and an upward shift of the brittleness temperature.
What is the difference between brittleness temperature and glass transition temperature?
The glass transition temperature (Tg) is an intrinsic thermal transition measured by techniques such as DSC or DMA, whereas the brittleness temperature is an engineering performance parameter determined by an impact test. For a given material, brittleness temperature depends on specimen geometry, impact velocity, notch or edge quality, and the applied failure criterion, so results are only comparable when test conditions are identical.
How many specimens are required for a reliable result?
Most standards require a minimum of several dozen specimens in total, distributed across at least two to four temperature levels, to establish the 50% failure temperature with acceptable confidence. For simple pass/fail verification at one temperature, a smaller defined number of specimens is tested, and any single failure may constitute rejection depending on the specification.
What acceptance criteria are typically applied?
Common criteria include no specimen failure at the rated minimum service temperature (for example, −25 °C or −40 °C), a brittleness temperature below the design temperature with a defined safety margin, or retention of at least 50% elongation at break after a specified aging exposure. The applicable criterion is normally defined by the product standard, the end customer specification, or the relevant certification scheme.
Can finished products be tested without preparing standard specimens?
Yes, comparative or pass/fail impact brittleness testing can often be performed on strips cut from finished parts. However, because results are sensitive to specimen dimensions, edge condition, and processing history, such data are treated as qualitative comparisons rather than absolute material properties, and this must be clearly stated in the test report.