Impact brittleness temperature determination is a critical low-temperature performance test used to evaluate how plastics and elastomers behave under impact loading at sub-zero temperatures. This article explains the testing principles, applicable standards such as ISO 812, ASTM D746, and GB/T 5470, specimen requirements, test procedures, and typical industrial applications. Understanding brittleness temperature data helps manufacturers select materials that remain flexible and impact-resistant in cold service environments.
Technical Overview
Impact brittleness temperature determination is a standardized laboratory method used to identify the lowest temperature at which a plastic or elastomeric material does not exhibit brittle failure when subjected to a specified impact. As temperature decreases, polymeric materials undergo a transition from ductile, rubbery behavior to glassy, brittle behavior. The temperature at which this transition becomes critical directly determines whether a product can safely perform in cold climates, cryogenic handling, refrigeration, or winter outdoor service.
In this test, specimens are immersed in a temperature-controlled heat transfer medium, typically cooled with liquid nitrogen or dry ice, and struck by a traveling striker arm at a defined velocity. After impact, specimens are examined for cracks, breaks, or complete fracture. By testing groups of specimens at progressively lower temperatures, the brittleness temperature is established, commonly defined as the temperature at which 50% of the specimens fail (Tb50) or the lowest temperature at which no failure occurs.
Brittleness temperature data are essential for material qualification, quality control, and cold-environment design verification in industries such as automotive, wire and cable, aerospace, construction sealing, footwear, and consumer goods. The test is typically performed by accredited third-party laboratories or in-house materials laboratories equipped with dedicated low-temperature impact apparatus.
Testing Samples
Rigid and Semi-Rigid Plastic Specimens
For thermoplastics such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and engineering resins, specimens are typically cut from compression-molded or injection-molded sheets. Specimens are usually 6.35 mm wide, approximately 1.91 mm or 2.0 mm thick, and long enough (typically 20 to 32 mm) to be clamped as a cantilever beam with a defined free length. Edges must be smooth and free of nicks or stress raisers, as surface defects can artificially shift the brittleness point.
Elastomer and Rubber Specimens
Vulcanized rubbers, thermoplastic elastomers (TPE), and flexible compounds are tested using strip specimens, typically 25 mm to 40 mm long, 6 mm wide, and approximately 2 mm thick. Specimens may be die-cut directly from standard cured sheets prepared according to ISO 23529 or ASTM D3182 practice.
Product Samples and Cut Sections
In many cases, specimens can be taken directly from finished products such as cable insulation, hose walls, gaskets, seals, or film materials, provided the geometry can be adapted to the specimen holder. Testing actual product sections provides the most representative assessment of in-service low-temperature performance.
Sample Quantity and Conditioning
Because the method is statistical in nature, multiple specimens (typically at least 5 per temperature point, and often 10 or more) are required to bracket the failure transition region. Specimens should be conditioned at standard laboratory atmosphere (23°C ± 2°C, 50% ± 5% relative humidity) for a minimum of 16 hours before testing unless otherwise specified.
Testing Items
The following parameters are typically evaluated and reported:
- Brittleness temperature at 50% failure (Tb50): the temperature at which 50% of specimens fail by cracking or fracture, statistically derived from failure data across the transition region.
- No-failure brittleness point: the lowest temperature at which no specimen exhibits cracking or breaking under the specified impact energy and velocity.
- Pass/fail verification at a specified temperature: for quality control purposes, a defined set of specimens is tested at a single contract temperature (for example, minus 40°C) to confirm compliance.
- Failure mode classification: ductile deformation, partial cracking, or complete brittle fracture of each specimen.
- Impact velocity and striking energy verification of the apparatus.
- Temperature accuracy of the cooling medium at the moment of impact, typically controlled within ±0.5°C.
Testing Methods
Governing Standards
Several international and national standards define the apparatus, specimen geometry, and calculation methods:
- ISO 812 — Rubber, vulcanized or thermoplastic — Determination of low-temperature brittleness.
- ASTM D746 — Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact.
- ASTM D2137 — Standard Test Methods for Rubber Property — Brittleness Point of Flexible Polymers and Coated Fabrics.
- GB/T 15256 — Rubber, vulcanized or thermoplastic — Determination of low-temperature brittleness (Chinese national standard, technically aligned with ISO 812).
- GB/T 5470 — Plastics — Determination of brittleness temperature by impact (Chinese national standard, technically aligned with ASTM D746).
- JIS K 6261 — Japanese standard covering low-temperature testing of rubber, including brittleness determination.
Test Apparatus
The test rig consists of a specimen holder that clamps specimens as cantilever beams, a motor-driven striker arm rotating at a controlled peripheral speed (typically 1.8 to 2.2 m/s for plastics; 2.0 ± 0.2 m/s for rubber), and a temperature-controlled bath containing a non-freezing heat transfer medium such as methanol, ethanol, silicone oil, or a suitable halogenated hydrocarbon mixture. Cooling is achieved by incremental addition of liquid nitrogen or dry ice, and temperature is measured with a calibrated low-temperature thermometer or thermocouple positioned near the specimens.
Test Procedure
The general procedure is as follows:
- Prepare and condition the specimens, then mount them vertically in the holder with the specified free projecting length.
- Precisely position the striker arm at its starting position and verify the impact geometry and velocity.
- Bring the bath to the selected starting temperature, typically estimated slightly below the expected brittleness point.
- Immerse the specimen holder for a defined equilibrium period, commonly 3 minutes (plastics) or 5 minutes (rubber) plus a time allowance based on holder mass, ensuring specimens reach bath temperature.
- Release the striker within a defined time window (typically within 5 seconds of reaching the equilibrium period) so that each specimen is struck once while fully submerged.
- Remove the specimens, allow them to reach room temperature, and inspect both surfaces for visible cracks or fracture under good lighting, with magnification where necessary.
- Repeat the test at fresh temperature increments (usually 1°C to 5°C steps, finer near the transition) using new specimens until the failure pattern brackets the 50% failure point.
- Calculate Tb50 by graphical interpolation or linear regression of failure percentage versus temperature.
Key Factors Affecting Accuracy
- Temperature uniformity and stability of the bath; agitation is required to eliminate thermal gradients.
- Impact velocity consistency; worn or misaligned striker arms must be re-calibrated.
- Specimen preparation quality; die-cut burrs, rough edges, or surface scratches lower the measured brittleness point artificially.
- Immersion time and time delay before striking, since premature striking under-cools or over-cools specimens relative to the recorded temperature.
- Correct interpretation of partial cracks, which count as failures even if the specimen does not separate completely.
Applications
Impact brittleness temperature data support material selection and product qualification across many industries:
- Automotive: weatherstrips, seals, gaskets, air brake hoses, underbody components, and battery packs that must withstand winter temperatures as low as minus 40°C or minus 55°C in arctic regions.
- Wire and cable: insulation and jacketing compounds for outdoor power cables, automotive wiring harnesses, and submarine cables requiring verified flexibility at low service temperatures.
- Construction: window and door gaskets, expansion joint seals, roofing membranes, and pipe systems installed in cold climates.
- Footwear and consumer goods: outsole compounds, winter sports equipment, and flexible polymer components.
- Packaging and films: cold-chain packaging, freezer bags, and flexible laminates used in refrigerated logistics.
- Aerospace and defense: elastomeric seals, O-rings, and dampers exposed to high-altitude or cryogenic-adjacent thermal environments.
- Regulatory compliance: demonstration of conformance to product standards, purchase specifications, and material datasheets where a maximum allowable brittleness temperature is stipulated.
FAQ
What is the difference between brittleness temperature and glass transition temperature (Tg)?
The glass transition temperature is a fundamental thermal property measured by DSC or DMA, marking the onset of segmental mobility loss. The brittleness temperature is an engineering performance value determined by an actual impact test under specified geometry and velocity. Brittleness temperature is generally close to, but not identical with, Tg, and it directly reflects whether a part survives a mechanical shock at low temperature.
Does a crack count as a failure even if the specimen does not break apart?
Yes. Under ISO 812, ASTM D746, and GB/T 5470, any visible crack in the tested region constitutes failure, whether or not the specimen separates into two pieces. Both surfaces of each specimen must be inspected after recovery to room temperature.
Why are so many specimens required?
The transition from ductile to brittle failure is statistical rather than sharp. Individual specimens fail at slightly different temperatures, so a group of specimens must be tested at each temperature and the 50% failure point must be interpolated from the failure distribution across several temperature levels.
What heat transfer media are used in the bath?
Common media include methanol, ethanol, silicone oil, and other low-freezing-point liquids that remain fluid and thermally stable at the test temperature and do not affect the specimen material. Cooling is achieved by adding dry ice or liquid nitrogen incrementally while monitoring the bath temperature.
How long must specimens soak before impact?
Typical standards specify 3 minutes for plastic specimens and 5 minutes for rubber specimens at temperature, plus additional time proportional to the mass of the specimen holder, ensuring complete thermal equilibrium. The impact must be delivered within a short, specified interval after the soak period ends.
Can the test be performed as a simple pass/fail check?
Yes. For routine quality control, a defined number of specimens can be tested at a single contract temperature, for example minus 35°C or minus 40°C. If no specimen cracks or breaks, the batch passes the specified brittleness requirement. This single-point approach is faster but does not provide the full transition curve.
Which standard should be specified on a datasheet: ISO 812 or ASTM D746?
Both are widely accepted. ISO 812 and GB/T 15256 are generally specified for vulcanized and thermoplastic rubbers, while ASTM D746 and GB/T 5470 cover plastics and elastomers by impact. Because specimen geometry and calculation details differ slightly between standards, results from one method should not be directly compared with the other; the same standard should be used when comparing materials or verifying datasheet values.