Brittleness temperature determination is a key low-temperature performance test that reveals how thermal, oxidative, and photo-oxidative aging degrades the impact resistance of plastic materials. This technical guide explains the measurement principle, the governing international standards such as ISO 974, ASTM D746, and GB/T 5470, specimen preparation, test procedures, and typical applications across the automotive, wire and cable, packaging, and polymer processing industries.
Technical Overview
The brittleness temperature (also referred to as the brittle point) of a plastic is defined as the temperature at which 50% of the test specimens crack or completely fracture when struck by a specified impact while immersed in a temperature-controlled heat transfer medium. Above this temperature the material deforms in a ductile manner; below it, molecular chain mobility is insufficient to dissipate impact energy and the material fails in a brittle fashion. The brittleness temperature therefore represents a practical lower limit for the safe service temperature of a plastic product.
Plastic aging — whether caused by thermo-oxidative degradation, ultraviolet (photo-oxidative) exposure, hydrolysis, plasticizer migration, or post-crystallization — progressively reduces the toughness of polymeric materials. Chain scission, crosslinking, and changes in crystallinity and morphology all restrict segmental chain mobility, which shifts the ductile-to-brittle transition toward higher temperatures. Measuring the brittleness temperature of a material before and after an aging treatment quantifies the degree of degradation and provides an objective basis for assessing long-term, low-temperature serviceability.
The most widely used determination method is the impact (cantilever) method, standardized internationally in ISO 974, ASTM D746, and the Chinese national standard GB/T 5470. A clamped specimen is struck at a controlled velocity by a calibrated striker while fully immersed in a chilled medium, and the fraction of failed specimens is determined at a series of temperatures. The result is evaluated statistically to obtain the temperature corresponding to a 50% failure rate.
Testing Samples
Molded Plaques and Compound Slabs
For material qualification and comparative aging studies, specimens are typically die-cut or machined from compression-molded or injection-molded plaques. In accordance with ASTM D746, the standard specimen is a strip approximately 31.75 mm in length and 6.35 mm in width, with a preferred thickness of 1.91 mm for rigid plastics. ISO 974 and GB/T 5470 use comparable cantilever strip specimens of similar geometry. Specimen edges must be smooth and free of nicks and burrs, because stress concentrators artificially raise the measured failure rate.
Films, Sheets, and Coatings
Thin flexible materials such as packaging films, geomembranes, and protective coatings may be tested as single plies or as stacked layers built up to the required thickness, subject to prior agreement between the interested parties. Because films are highly sensitive to surface oxidation, brittleness temperature measurement is a particularly sensitive indicator of photo-degradation in this product category.
Finished Parts and Coupons
Specimens may also be taken directly from finished components — gaskets, seals, cable insulation, pipe fittings, or trim pieces — to evaluate the combined effect of processing history and in-service aging. Sampling location and orientation should be documented, as molded-in stresses and weld lines influence the result.
Aged Specimens
To study the effect of aging, specimens are divided into a control group and aged groups exposed to defined degradation conditions before testing. Common pretreatments include forced-ventilation air-oven heat aging (for example, per ISO 188 or ASTM D3045), accelerated artificial weathering using xenon-arc or fluorescent-UV sources (per ISO 4892, ASTM G155, or ASTM G154), natural outdoor weathering exposure, and immersion in compatible fluids. Testing both unaged and aged specimens allows the brittleness temperature shift to be calculated as a quantitative measure of embrittlement.
Testing Items
- Brittleness temperature of virgin (unaged) plastic materials
- Brittleness temperature after air-oven thermal aging at specified temperature and duration
- Brittleness temperature after artificial weathering (xenon-arc or fluorescent-UV exposure)
- Brittleness temperature after natural weathering or in-service exposure
- Comparative embrittlement assessment expressed as the brittleness temperature shift relative to the unaged control
- Pass/fail verification at a specified low temperature called up in a product specification
Testing Methods
Principle and Apparatus
The test instrument consists of a specimen holder, a rotating or traveling striker with a defined striking-edge radius, and a thermally insulated bath containing a heat transfer medium such as ethanol, methanol, silicone fluid, or another liquid that remains fluid and inert at the test temperature. The medium is cooled with solid carbon dioxide (dry ice) or liquid nitrogen and continuously stirred to maintain temperature uniformity within ±0.5 °C. Temperature is measured with a calibrated low-temperature thermometer or thermocouple positioned at the level of the specimens, and the striker velocity is verified against the value specified in the standard.
Conditioning and Test Sequence
Specimens are first conditioned under standard laboratory atmosphere (23 °C / 50% relative humidity per ISO 291) unless otherwise specified. A batch of specimens — typically ten per temperature point — is then immersed in the medium at the selected temperature and held for at least three minutes to reach thermal equilibrium. The striker is immediately actuated so that each specimen is struck once with the specified impact energy and velocity. The specimens are removed, bent or flexed for inspection, and examined — with magnification if necessary — for any visible crack or complete fracture. A cracked or broken specimen is recorded as a failure.
Temperature Bracketing and Calculation
The test begins at a temperature estimated to be near the transition region. Based on the observed failure fraction, the temperature is adjusted in steps (commonly 5 °C, refined to 2–3 °C near the transition) until the failure rate brackets the 50% level — that is, temperatures at both higher and lower failure percentages are obtained. The brittleness temperature is then calculated from the failure data using the statistical formula or graphical interpolation specified in the standard, yielding the temperature at which 50% of the specimens fail. Where a product specification defines a mandatory test temperature, the result may instead be reported simply as the failure count or failure percentage at that temperature.
Aging Pretreatment Protocols
For aged-material evaluations, specimens are exposed in calibrated aging ovens or weathering devices under documented conditions of temperature, duration, irradiance, and humidity. Parallel control specimens are retained in standard conditions. Both groups are tested under identical brittleness temperature conditions, and the embrittlement trend is plotted as brittleness temperature versus aging time, allowing lifetime extrapolation and material comparison.
Factors Influencing Results
- Specimen edge quality: nicks, notches, or die-cutting defects act as crack initiation sites and bias results toward higher brittleness temperatures
- Thickness control: deviations from the specified thickness alter the bending stress at impact
- Thermal equilibrium: insufficient immersion time leads to specimens that are warmer than the recorded bath temperature
- Temperature uniformity: poor bath circulation creates gradients between the thermocouple and the specimen bank
- Mechanical calibration: striker velocity, striking-edge radius, and clamping torque must be verified periodically
- Failure interpretation: consistent criteria for what constitutes a crack should be agreed before testing begins
Applications
- Automotive engineering: verifying the cold-impact performance of bumpers, body trim, seals, batteries housings, and under-hood components after prolonged heat exposure
- Wire and cable: qualifying insulation and sheathing compounds for low-temperature flexibility and impact resistance in outdoor and buried installations, in coordination with IEC 60811 series and UL-based requirements
- Pipes, fittings, and geomembranes: assessing embrittlement of polyolefin materials used in gas and water distribution and environmental containment liners
- Packaging: confirming that films for frozen-food applications remain flexible at freezer temperatures after storage and sterilization
- Material research and quality control: comparing stabilizer packages, evaluating recycled content, and supporting durability lifetime predictions in accredited third-party laboratories
FAQ
What does a higher brittleness temperature mean?
A higher brittleness temperature indicates that the material becomes brittle at a warmer condition, meaning its low-temperature impact resistance has deteriorated. In aging studies, a rise in brittleness temperature relative to the unaged control is direct evidence of embrittlement caused by degradation.
Which standards are used for the determination?
The principal standards are ISO 974 and ASTM D746 for plastics by the impact method, GB/T 5470 as the corresponding Chinese national standard, and ISO 812 / ASTM D2137 for vulcanized rubber and elastomeric compounds. Cable products additionally reference the low-temperature test methods of the IEC 60811 series.
How many specimens are required?
A complete statistical determination generally requires testing at least ten specimens at each of several temperatures until the 50% failure point is bracketed, which typically consumes 30 to 60 specimens in total. For a simple pass/fail check at a specified temperature, a smaller batch defined by the specification is sufficient.
Can thin films be tested by this method?
Yes. Films and thin sheets may be tested as single plies or as stacked layers reaching the required effective thickness, provided the stacking arrangement is documented and agreed upon in advance. Stacked-ply testing is a common practice for evaluating weathered packaging films.
What safety precautions are necessary?
The test involves cryogenic or sub-zero media and flammable liquids such as ethanol or methanol. Operators should wear cryogenic gloves and face protection, ensure adequate ventilation, keep ignition sources away from the bath, and follow the equipment manufacturer's operating procedures and the laboratory's chemical safety rules.