Rubber brittleness temperature determination is a critical low-temperature performance test that identifies the point at which elastomeric materials transition from flexible to brittle behavior under impact loading. This comprehensive guide covers internationally recognized test methods, including ISO 812, ASTM D2137, and GB/T 1682, along with specimen requirements, testing procedures, and industry applications for quality control and material qualification.
Technical Overview
Rubber brittleness temperature determination is a fundamental low-temperature test used to evaluate the behavior of vulcanized or thermoplastic rubber when subjected to impact at reduced temperatures. As temperature decreases, rubber materials lose their elastic properties and eventually become glass-like and brittle. The brittleness temperature is defined as the lowest temperature at which rubber specimens do not crack or fracture when struck by a specified impact device under controlled conditions.
Below the brittleness temperature, a rubber component can no longer absorb mechanical energy through elastic deformation, making it vulnerable to catastrophic cracking or shattering even under minor mechanical stress. This property is essential for evaluating whether a rubber material is suitable for service in cold environments, such as outdoor equipment, aerospace systems, automotive components operating in winter conditions, and cryogenic applications.
The test is typically performed by accredited third-party laboratories using specialized brittleness temperature apparatus consisting of a temperature-controlled bath, a specimen holder, and a spring-actuated or motorized impact striker. The measurement provides a single-point characterization of low-temperature flexibility and is widely specified in material datasheets, procurement specifications, and regulatory requirements.
Testing Samples
Standard Vulcanized Rubber Specimens
For testing in accordance with ISO 812, standard specimens are strips measuring 40 mm in length, 6 mm in width, and 2 mm in thickness. These specimens are die-cut from flat cured sheets that are free from surface defects, blisters, porosity, or non-uniform areas. Specimens must be conditioned at standard laboratory temperature and humidity for at least 16 hours prior to testing.
Finished Product Samples
When testing actual products such as O-rings, seals, gaskets, hoses, or molded parts, specimens may be taken directly from the finished article, provided that appropriate dimensions can be obtained. Complete small components may also be tested as whole articles. Test results from finished products reflect the combined effects of the compound formulation, the curing process, and any post-processing treatments such as post-curing or surface coating.
Thermoplastic Elastomers
Thermoplastic elastomer (TPE) and thermoplastic vulcanizate (TPV) materials can also be evaluated using this method. Specimens are typically injection-molded or compression-molded plaques from which strips are cut. Care must be taken to avoid molecular orientation effects that may arise from processing, as these can influence brittleness behavior.
Coated Fabrics and Flexible Polymers
Flexible polymeric films and rubber-coated fabrics may be evaluated under ASTM D2137, which provides specific specimen geometry and conditioning requirements for these material forms. Specimens are cut with clean, sharp dies to avoid edge defects that could initiate premature cracking.
Testing Items
The rubber brittleness temperature test can be conducted in several modes depending on the purpose of the evaluation:
- Brittleness temperature (single specified temperature pass/fail test): determines whether specimens remain uncracked at a specified temperature, commonly used for acceptance testing
- Brittleness point (multi-temperature determination): identifies the exact temperature transition by testing replicate specimens at progressively lower temperatures
- Low-temperature impact resistance: evaluates the ability of specimens to withstand impact energy at reduced temperatures without visible cracking
- Comparative compound evaluation: ranks different rubber formulations or suppliers based on their low-temperature flexibility limits
- Verification after aging or fluid exposure: determines the effect of thermal aging, oil, or chemical exposure on low-temperature performance
The standard reporting outputs include the test temperature, the number of specimens tested, the number of specimens that failed, the observed brittleness temperature or brittleness point, and a full description of the test method and specimen geometry used.
Testing Methods
ISO 812 Method
ISO 812, titled "Rubber, vulcanized or thermoplastic — Determination of low-temperature brittleness," is the internationally harmonized procedure. In this method, strip specimens are secured in a specimen holder and immersed in a heat-transfer medium, typically ethanol or silicone oil, cooled by mechanical refrigeration or dry ice to the target temperature. The specimens are soaked for a minimum of 5 minutes to ensure thermal equilibrium. A striker then impacts the specimens with a linear velocity of approximately 2.0 m/s at the moment of contact. Specimens are immediately removed, bent to an inspection angle, and examined visually for cracks, fissures, or complete fracture. Any visible crack constitutes failure. Testing is repeated at temperature intervals, typically 1 °C or 5 °C, until the temperature at which no failures occur is bracketed.
ASTM D2137 Method
ASTM D2137, "Standard Test Methods for Rubber Property — Brittleness Point of Flexible Polymers and Coated Fabrics," is the North American equivalent. Specimens are immersed in a bath at a selected temperature, held for a specified conditioning period of 3 minutes, and then impacted by a striker traveling at a controlled velocity. The brittleness point is defined as the temperature at which no specimen fails when tested, using a minimum of 10 specimens per temperature for statistical confidence. The method permits both pass/fail testing at a single temperature and full brittleness point determination across a temperature range.
GB/T 1682 Method
GB/T 1682 is the Chinese national standard for the determination of low-temperature brittleness of vulcanized rubber using the single-specimen method. In this procedure, individual specimens are clamped and immersed in the cooling medium, then subjected to impact at the designated temperature. The test is conducted at successively lower temperatures to establish the temperature at which the specimen first exhibits brittle fracture, or alternatively as a pass/fail determination at a contractually specified temperature. This method is commonly referenced in Chinese domestic specifications and supply chains.
Apparatus and Test Conditions
Regardless of the standard applied, the test system consists of the following key elements:
- A thermally insulated bath containing a heat-transfer medium such as methanol, ethanol, or silicone oil, or alternatively an air-cooled chamber for specimens that must remain dry
- A cooling system based on mechanical refrigeration or the introduction of solid carbon dioxide to achieve temperatures as low as approximately -70 °C, with specialized systems reaching -80 °C or below
- A calibrated temperature measurement device, such as a platinum resistance thermometer or thermocouple, positioned adjacent to the specimens with an accuracy of ±0.5 °C
- A spring-driven or solenoid-actuated impact device delivering a controlled impact velocity, typically around 2 m/s, with a defined striking edge radius
- A specimen holder that firmly grips the specimen at a precise overhang length to ensure repeatable stress conditions at the point of impact
Critical procedural controls include verifying thermal soak time, preventing frost formation on specimens, performing the impact within a defined short interval after removal from the bath where required by the standard, and conducting visual inspection under adequate lighting, often supplemented by magnification.
Applications
Rubber brittleness temperature data support design, material selection, and quality assurance across numerous industries:
- Automotive: qualification of door and window seals, fuel system components, coolant hoses, bushings, and weatherstrips intended for cold-climate service, often referenced against OEM material specifications
- Aerospace: verification of O-rings, gaskets, and vibration isolators used in aircraft and spacecraft, where low-temperature embrittlement has historically been a critical failure mode
- Oil and gas: evaluation of sealing compounds, blowout preventer elastomers, and flexible hose linings exposed to sub-zero ambient and process conditions
- Railway and infrastructure: assessment of rubber pads, suspension components, and expansion joint bearings operating in winter environments
- Consumer and industrial products: cold-storage door seals, refrigeration gaskets, protective gloves for cryogenic handling, and outdoor electrical enclosures
- Research and development: screening of polymer blends, plasticizer systems, and novel elastomers such as silicone and fluorocarbon rubbers engineered for extreme low-temperature flexibility
Material selection engineers frequently use brittleness temperature as a screening threshold, specifying that the brittleness point must be at least 10 °C to 20 °C below the lowest anticipated service temperature to provide an adequate safety margin.
FAQ
What is the difference between brittleness temperature and glass transition temperature?
The brittleness temperature is an empirical, impact-based single-point measurement that indicates the temperature below which the material fails under a specific mechanical shock condition. The glass transition temperature, typically measured by DSC or DMA, is a fundamental thermal property describing the temperature range over which the polymer changes from a rubbery to a glassy state. Brittleness temperature is generally higher than the glass transition temperature measured by DSC, because impact loading requires a measurable level of segmental mobility, and the two values are related but not interchangeable.
What factors influence the brittleness temperature of rubber?
Key influencing factors include the base polymer chemistry, with silicone and polyacrylate rubbers offering excellent low-temperature flexibility and conventional NBR being more limited; the type and concentration of plasticizers, which lower the brittleness point; the crosslink density, where excessive curing raises brittleness; the filler type and loading; and the presence of crystallization effects in polymers such as natural rubber and EPDM during prolonged cold exposure.
How many specimens are required for a valid determination?
Under ISO 812, at least four specimens are tested at each temperature for the brittleness temperature determination, while ASTM D2137 typically calls for a minimum of 10 specimens per temperature for brittleness point determination. Testing at additional temperatures is required to confirm the transition. Individual product specifications may impose stricter requirements.
Can rubber recover its flexibility after being cooled below its brittleness temperature?
Yes, in most cases the embrittlement is a reversible physical phenomenon. When the material is warmed above the brittleness point, its elastic properties are restored, provided no cracks or structural damage were generated during cold exposure. However, any cracks formed while brittle typically remain as permanent defects that compromise sealing integrity.
What does a failed brittleness test mean for product qualification?
A failure indicates that the material does not meet the specified low-temperature impact requirement and may crack or fracture in service at or near the test temperature. Corrective actions typically include reformulating with a more flexible base polymer, increasing plasticizer content, adjusting the cure system, or selecting an alternative grade. Retesting by an accredited third-party laboratory after corrective changes is standard practice to confirm compliance.