Impact Brittleness Aging Test: Evaluating Low-Temperature Embrittlement of Polymers After Aging

The impact brittleness aging test is a critical durability evaluation that measures how thermal-oxidative and environmental aging shift the low-temperature impact brittleness of plastics and elastomers. By combining accelerated aging exposure with impact brittleness temperature determination in accordance with standards such as ASTM D746, ISO 974, and GB/T 5470, this test helps manufacturers predict long-term cold-service performance and material reliability. This article explains the test principle, sample requirements, methods, applications, and frequently asked questions.

Technical Overview

The impact brittleness aging test is a combined performance evaluation that determines the brittleness temperature of polymeric materials by impact, both before and after an accelerated aging exposure. The brittleness temperature is defined as the temperature at which 50% of the specimens fail (crack, shatter, or rupture) when struck by a standardized striker traveling at a specified velocity. As plastics and rubbers undergo thermal-oxidative degradation during service, chain scission, crosslinking, and loss of plasticizers can dramatically raise the brittleness temperature, turning a material that was flexible at -40 °C into one that cracks on impact at -10 °C.

The impact portion of the test is typically performed in accordance with ASTM D746 (Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact), ISO 974 (Plastics — Determination of the brittleness temperature by impact), or GB/T 5470. For flexible polymers and coated fabrics, ASTM D2137 and ISO 812 are often applied, while GB/T 1682 covers the single-specimen method for vulcanized rubber. The aging preconditioning is usually conducted per ISO 188, ASTM D573, or GB/T 3512 for hot-air oven aging, and may be supplemented by weathering exposure per ISO 4892 or ASTM G154 when outdoor service is simulated. By comparing the brittleness temperature before and after aging, engineers obtain a quantitative measure of aging-induced embrittlement, expressed as a shift in brittleness temperature.

Testing Samples

Plastic Specimens

Rigid and semi-rigid plastics are tested using die-cut strips, typically 6.35 mm wide and approximately 25 mm long, taken from compression-molded or injection-molded sheets with a thickness of about 2.0 mm. Specimens may be unnotched or include a defined nick at the midpoint, depending on the governing standard. Materials commonly evaluated include polyethylene, polypropylene, PVC, ABS, polycarbonate blends, and thermoplastic elastomers.

Rubber and Elastomer Specimens

Vulcanized rubber compounds are sampled as thin strips, typically 25 to 40 mm long, 6 mm wide, and 2 mm thick, cut from standard cured slabs. Flexible elastomers such as EPDM, NBR, silicone rubber, and thermoplastic polyurethanes are frequently tested because cold-weather flexibility is a primary functional requirement for seals, gaskets, and hoses.

Finished Products and Components

When permitted by the specification, specimens may be taken directly from finished articles such as cable insulation and sheathing, footwear soling units, automotive weatherstrips, protective coatings, geomembranes, and medical tubing. Sampling from actual production parts provides the most realistic assessment of processing effects such as crystallinity, orientation, and cure state on long-term brittleness behavior.

Testing Items

The scope of an impact brittleness aging test program typically includes the following measurable items:

• Initial brittleness temperature (Tb) of the unaged material, defined at 50% failure
• Brittleness temperature after hot-air oven aging at specified temperature and duration
• Shift in brittleness temperature (ΔTb) caused by aging
• Pass/fail impact assessment at a specified low service temperature
• Failure rate statistics at multiple test temperatures for probit or bracketing analysis
• Comparison of aged versus unaged specimen appearance at the fracture surface
• Optional combined weathering (UV, moisture) followed by low-temperature impact evaluation

Testing Methods

Aging Exposure

Specimens are first conditioned at standard laboratory atmosphere and then divided into an unaged control group and one or more aged groups. Aging is most commonly performed in a forced-ventilation air oven per ISO 188, ASTM D573, or GB/T 3512, with typical exposure conditions such as 70 °C, 100 °C, or 125 °C for durations of 168 hours, 336 hours, or 1000 hours, depending on the material specification. After exposure, specimens are allowed to cool and recondition before impact testing. For materials intended for outdoor use, xenon-arc or fluorescent UV weathering per ISO 4892 or ASTM G154 may precede the brittleness determination.

Low-Temperature Impact Procedure

The impact brittleness determination follows the apparatus and procedure of ASTM D746 or ISO 974. Specimens are clamped in a specimen holder immersed in a temperature-controlled bath using a heat-transfer medium such as silicone oil, ethanol with dry ice, or liquid nitrogen. The bath is held at the target temperature, typically within ±0.5 °C, and specimens are allowed to equilibrate for a defined soak period, commonly 3 minutes. A striker pivoting at approximately 2000 mm/s then delivers a single impact to each specimen with fixed energy. Each specimen is removed, flexed, and examined visually for cracks, fissures, or complete rupture, and the result is recorded as pass (no visible damage) or fail.

Data Analysis and Reporting

To determine the brittleness temperature, batches of typically ten specimens are tested at a series of temperatures that bracket the expected failure region. The failure data are analyzed using graphical bracketing or statistical probit methods to calculate the temperature at which 50% of specimens fail, together with confidence intervals. The test report includes the material identification, aging conditions, specimen geometry, test temperatures, number of failures at each temperature, calculated brittleness temperature, and the shift in brittleness temperature relative to the unaged control.

Applications

Impact brittleness aging testing is widely applied across industries where polymeric components must retain flexibility after prolonged exposure to heat, oxygen, and cold service environments. Typical applications include:

• Wire and cable insulation and sheathing required to remain flexible and crack-resistant in cold-climate installations
• Automotive seals, gaskets, boots, bellows, and coolant hoses exposed to engine-bay heat followed by winter service
• Footwear soling and outsole compounds that must survive repeated flexing and impact in sub-zero conditions
• Geomembranes, pond liners, and flexible sheets for civil and environmental engineering in cold regions
• Medical tubing, bags, and device components requiring low-temperature handling integrity after sterilization-temperature exposure
• Consumer appliances, power tools, and sporting goods housings used outdoors
• Material qualification, incoming inspection, and formulation optimization by compound designers
• Service-life prediction and accelerated durability studies supporting product warranties

Product designers and quality engineers use the results to set material specifications, compare candidate formulations, verify supplier claims, and demonstrate compliance with end-user or regulatory requirements for cold-service performance. Accredited third-party laboratories routinely perform these evaluations as part of broader polymer aging and reliability test programs.

FAQ

What is the brittleness temperature and how is it defined?

The brittleness temperature is the temperature, determined statistically, at which 50% of the specimens fail when subjected to a specified impact. It is not an absolute physical constant of the material; it depends on specimen geometry, notch condition, impact velocity, and test method, so results are only comparable when the test conditions are identical.

Why is aging combined with impact brittleness testing?

Many polymers pass low-temperature impact tests when new but embrittle prematurely as thermal-oxidative degradation, crosslinking, crystallization, or plasticizer loss progresses. Aging the specimens before the impact test reveals this degradation, allowing prediction of cold-service behavior after months or years of exposure and enabling material ranking on a like-for-like basis.

What causes the brittleness temperature to increase after aging?

Common mechanisms include oxidative chain scission or additional crosslinking in rubbers, increased crystallinity in semi-crystalline plastics, volatilization or migration of plasticizers in flexible PVC, and surface oxidation from UV or ozone exposure. Each of these reduces molecular mobility and raises the temperature below which the material behaves in a glassy, brittle manner.

How is the brittleness temperature different from the glass transition temperature?

The glass transition temperature (Tg), measured by DSC or DMA, is an intrinsic thermal transition of the polymer. The brittleness temperature is an engineering performance value obtained under dynamic impact conditions and reflects defects, morphology, and thickness effects. The brittleness temperature typically lies above Tg for many elastomers because it incorporates the severity of the impact event.

What cooling media and equipment are used?

A dedicated brittleness temperature apparatus uses a well-insulated bath containing a stirred heat-transfer medium, commonly silicone oil for moderate temperatures or ethanol chilled with dry ice, with liquid nitrogen used for very low temperatures. Precise temperature control within ±0.5 °C and a striker mechanism with calibrated velocity and energy are essential for reproducible results.

What acceptance criteria are typically specified?

Specifications usually require that no specimen fails when impacted at a designated service temperature, such as -40 °C for cold-climate products, or that the brittleness temperature after a defined aging exposure remains below a specified limit, for example not more than 10 °C above the unaged value. The exact limits are set by the product standard, the end customer, or the material datasheet.