Rubber Low-Temperature Testing: TR Test (ASTM D1329), Brittleness (ASTM D2137), and Gehman (ASTM D1053) — With Material TR10 Values

1. Why Low-Temperature Performance Matters

Rubber stiffens, loses elasticity, and eventually becomes brittle as temperature decreases. This transition is not abrupt — it is a progressive loss of molecular mobility as the polymer segments have insufficient thermal energy to undergo conformational changes. Three failure modes emerge:

1. Stiffening: The rubber becomes too hard to deform under normal load — seals leak because they cannot conform to the mating surface

1. Loss of elasticity: The rubber cannot recover from deformation — a deformed seal remains deformed, losing sealing force

1. Brittleness: The rubber fractures under impact or rapid loading — catastrophic failure with no ductility

Each failure mode has a dedicated test method. Together they define the low-temperature operating envelope of a rubber compound.

2. TR Test (Temperature Retraction): ASTM D1329 / ISO 2921

2.1 Principle

The TR test is the most industrially relevant low-temperature test for rubber seals and dynamic applications. It measures the elastic recovery of a stretched rubber specimen as it is warmed from a low temperature.

Procedure:

1. A dumbbell specimen is stretched to 250% elongation at room temperature and frozen in the stretched state (at approximately -70°C, well below any rubber's Tg).

1. While frozen, the specimen is released from the grips — it remains elongated because molecular mobility is frozen.

1. The specimen is warmed at a controlled rate (1°C/min).

1. As molecular mobility returns, the specimen retracts (recovers) toward its original length.

1. The retraction percentage is recorded vs. temperature.

2.2 TR Values Defined

TR10 is the most commonly specified value. Below TR10, the rubber has lost so much elasticity that it cannot reliably maintain a seal. For dynamic applications (reciprocating seals), TR30 is the more relevant limit.

2.3 Material TR10 Values

Key observations:

• • NR has the lowest TR10 of any general-purpose rubber, making it the preferred choice for low-temperature dynamic applications without oil exposure.
• • Silicone (VMQ) is Tg-limited to -115°C but crystallization-limited to approximately -55°C. Below -55°C, PDMS crystallizes, stiffening the material even though the glass transition is far lower.
• • NBR's TR10 is directly controlled by ACN content. Every 1% increase in ACN raises TR10 by approximately 0.8–1.0°C.
• • FKM has the highest TR10 of common elastomers and is unsuitable for low-temperature applications below -15°C (standard grades).

3. Brittleness Test: ASTM D2137 / ISO 812

3.1 Principle

The brittleness test measures the temperature at which rubber fractures under a specified impact — it tests the "impact toughness at low temperature" failure mode.

Procedure:

1. A specimen (40 × 6 × 2 mm) is conditioned at the test temperature for 3 minutes.

1. A striker moving at 2.0 ±0.2 m/s impacts the specimen.

1. The specimen either fractures (brittle failure) or does not (ductile).

1. The brittleness temperature (Tb) is reported as the temperature at which 50% of specimens fail.

3.2 Brittleness Temperature vs. TR10

Why the difference? TR10 measures the return of elasticity (a molecular motion phenomenon starting at Tg + 5–15°C). Brittleness measures catastrophic fracture (requiring sufficient molecular mobility for energy dissipation). Typically, the brittleness temperature is 5–10°C below TR10 — you lose sealing function (TR10) before the part shatters on impact (Tb).

4. Gehman Torsional Stiffness Test: ASTM D1053 / ISO 1432

4.1 Principle

The Gehman test measures the torsional stiffness (apparent shear modulus) of rubber as a function of temperature. It quantifies the progressive stiffening before failure.

Procedure:

1. A strip specimen is connected in series with a torsion wire of known stiffness.

1. The specimen is cooled and the angular deflection at a given torque is measured.

1. The relative modulus (ratio of modulus at temperature T to modulus at 23°C) is plotted vs. temperature.

4.2 Gehman Values

For most seal applications, T2 is the conservative low-temperature design limit. For vibration isolators, even a doubling of stiffness (T2) can shift the natural frequency by √2 (41%), potentially moving it into resonance with the excitation frequency.

5. Selecting the Right Low-Temperature Test

6. Design Rules for Low-Temperature Applications

1. Static seals: TR10 is the design limit. Below TR10, the seal cannot recover from compression set and will leak. Add a 10°C margin: design minimum temperature = TR10 + 10°C.

1. Dynamic seals (reciprocating): TR30 is the design limit. Add 15°C margin for reliable dynamic performance.

1. Vibration isolators: T2 (Gehman) is the limit. Stiffness doubling shifts the isolation frequency; verify fn remains below f_excitation / √2.

1. Impact-loaded parts: Tb (brittleness) at the minimum service temperature must be below the minimum service temperature.

1. Crystallization-aware design: For NR and CR, prolonged exposure at 0 to -25°C can cause crystallization stiffening (a time-dependent phenomenon, not captured by short-duration TR/Gehman tests). If the part sees extended cold soak, include crystallization testing (24–168 h at 0 to -25°C) in your specification.

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Nanjing Yuhang Rubber Co., Ltd. operates a full low-temperature testing laboratory including TR test equipment (ASTM D1329/ISO 2921, to -70°C), brittleness impact testers (ASTM D2137), Gehman torsional stiffness apparatus (ASTM D1053), and DSC for Tg measurement. We provide complete low-temperature characterization reports for every compound we formulate. When ordering seals, gaskets, or molded components for cold-climate or cryogenic applications, specify your minimum service temperature and we will recommend the correct polymer and formulation. Serving over 75 countries from Nanjing, China.

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