NBR Low-Temperature Brittleness Testing: Standards, Methods, and Applications

The NBR low-temperature brittleness test determines the critical temperature at which nitrile butadiene rubber loses its elasticity and cracks or fractures under impact in cold environments. This technical guide explains the test principle, specimen requirements, applicable standards such as GB/T 1682, ISO 812, and ASTM D2137, and typical brittleness temperature values for NBR compounds used in sealing, automotive, and oil and gas applications.

Technical Overview

Nitrile butadiene rubber (NBR) is a copolymer of acrylonitrile and butadiene that is widely specified for seals, O-rings, hoses, and gaskets because of its excellent resistance to mineral oils, fuels, and hydraulic fluids. Like all elastomers, however, NBR progressively stiffens as the ambient temperature drops, and below a critical point it transitions from a flexible rubbery state to a glassy, brittle state in which it can crack or shatter under mechanical shock. The low-temperature brittleness test quantifies this critical point, known as the brittleness temperature (Tb), by impacting cooled specimens under strictly controlled conditions and examining them for cracks or complete fracture.

Because the acrylonitrile (ACN) content of NBR typically ranges from 18% to 50%, low-temperature performance varies significantly between grades: higher ACN content improves oil and fuel resistance but raises the glass transition temperature and therefore worsens cold flexibility. The brittleness test is consequently a key acceptance criterion for NBR compounds intended for outdoor, arctic, high-altitude, or refrigerated service, and it is routinely included in material datasheets, purchase specifications, and quality control programs.

• GB/T 1682 — Vulcanized rubber — Determination of low-temperature brittleness (single-specimen method)
• ISO 812 — Rubber, vulcanized or thermoplastic — Determination of low-temperature brittleness
• ASTM D2137 — Standard Test Methods for Rubber Property: Brittleness Point of Flexible Polymers and Elastomers

Testing Samples

Strip Specimens

The standard specimen is a flat strip cut from a fully vulcanized sheet, with typical dimensions of 40 mm ± 6 mm in length, 6 mm ± 1 mm in width, and 2 mm ± 0.3 mm in thickness as specified in GB/T 1682 and ASTM D2137; ISO 812 uses similar strip geometry in accordance with the current edition of the standard. Specimens must be cut cleanly with smooth edges, free of nicks, voids, flow marks, or surface defects, since any flaw can act as a crack initiator and artificially raise the measured brittleness temperature. Thickness should be verified at several points along the strip, and specimens outside the tolerance band must be discarded.

Preparation and Conditioning

Specimens should be taken from material cured under production conditions and allowed to rest after vulcanization before testing. Unless otherwise specified, conditioning is performed in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 16 hours in accordance with GB/T 2941 / ISO 23529. For the single-specimen method, a sufficient quantity of strips (often 15 or more) is required to bracket the expected brittleness temperature through successive temperature steps; for the multi-specimen method, a minimum of four to five specimens is tested at each temperature level.

Testing Items

The test program can be structured to deliver several distinct results depending on the purpose of the evaluation:

• Brittleness temperature (Tb): the highest temperature, determined by successive trials, at which the specimen exhibits cracks or fracture under the specified impact, or equivalently the lowest temperature at which no failure occurs, depending on the governing standard.
• Pass/fail verification at a specified temperature: for qualification purposes, specimens are tested at a contract temperature (for example −40 °C) and must show no cracking after impact.
• Statistical brittleness temperature: in the multi-specimen approach, the failure percentage is plotted against temperature to determine the temperature at which 50% of specimens fail.
• Comparative cold-resistance ranking: evaluation of compound formulations, plasticizer systems, or supplier batches against a reference material.

Testing Methods

Apparatus and Cooling Medium

The test is performed in a dedicated low-temperature brittleness apparatus consisting of a specimen holder, a pendulum or spring-driven striker, and an insulated cooling bath. The bath is typically charged with ethanol or silicone oil cooled by dry ice for temperatures down to approximately −70 °C, or with liquid nitrogen for lower set points, and the medium is stirred continuously to ensure uniformity. Temperature is monitored with a calibrated low-temperature thermometer, thermocouple, or platinum resistance sensor positioned adjacent to the specimens, with control accuracy of ±0.5 °C or better.

Single-Specimen Impact Method (GB/T 1682)

One specimen at a time is clamped in the holder and immersed in the cooling medium. After the bath returns to the set point, the specimen is held at temperature for a defined dwell period, commonly 3 minutes, to ensure thermal equilibrium throughout the strip. The striker is then released, striking the specimen at an impact velocity of 2.0 m/s ± 0.2 m/s. Each specimen may be impacted only once. Testing begins near the expected brittleness temperature and proceeds in increments of about 1 °C; the brittleness temperature is reported as the highest temperature at which impact produces visible cracking or breakage, confirmed by adjacent retests. The specimen is examined immediately after impact while still cold, using magnification where necessary.

Multi-Specimen Method (ISO 812 / ASTM D2137)

In the multi-specimen configuration, a group of four to five strips is mounted on a common rack and conditioned simultaneously at the test temperature. All specimens are impacted in rapid succession at the specified striking velocity, removed, and inspected for cracks or fracture. The test is repeated at a series of temperatures spaced at regular intervals across the failure transition, and the results are plotted as percentage of failed specimens versus temperature. The brittleness point is then reported either as the lowest temperature at which no specimen fails or as the statistically derived temperature corresponding to 50% failure, in accordance with the specific standard invoked.

Factors Influencing Results

Several material and procedural variables must be controlled to obtain reproducible data:

• Acrylonitrile content: higher ACN levels shift both the glass transition and brittleness temperatures upward.
• Plasticizer type and loading: ester plasticizers such as DOA or DOS can lower Tb by 5 °C to 15 °C or more.
• Filler type and loading: high-loadings of fine reinforcing blacks restrict chain mobility and raise Tb.
• Crosslink density: over-cured compounds with dense networks become glassy at higher temperatures.
• Procedural variables: bath uniformity, dwell time, impact velocity, and specimen edge quality all affect the outcome.

Applications

Nitrile rubber components are frequently deployed in equipment that must start up and seal reliably after prolonged cold exposure, making brittleness testing an essential screening and acceptance tool across multiple industries.

Automotive and Fuel Systems

Fuel hoses, filler neck gaskets, injector seals, and diaphragms must survive cold starts at ambient temperatures down to −40 °C in northern markets. Brittle fracture of an NBR fuel system seal during a winter cold start can cause immediate leakage, so qualification testing at or below the specified service temperature is mandatory.

Aerospace and Defense

Hydraulic couplings, actuator seals, and ground support equipment experience deep cold soak at high altitude or in polar operations. Brittleness testing verifies that NBR sealing elements will tolerate the shock loads of valve actuation and landing impact after thermal equilibrium at extreme low temperatures.

Oil, Gas, and Petrochemical

Valve seats, pipeline gaskets, tank seals, and pumping equipment operating in arctic fields and LNG-adjacent service must retain sufficient toughness. The brittleness temperature defines the absolute lower bound for NBR use and supports material selection decisions, including substitution with low-temperature elastomers such as silicone or fluorosilicone when required.

Industrial Sealing and Refrigeration

O-rings, hydraulic wipers, gloves, and diaphragms used in cold storage warehouses, outdoor machinery, and refrigeration plants are batch-tested for brittleness to confirm consistent compounding and plasticizer effectiveness in production quality control.

FAQ

What is the typical brittleness temperature of NBR?

Standard commercial NBR compounds with 33% to 34% acrylonitrile content typically show brittleness temperatures in the range of −35 °C to −45 °C. Low-ACN grades (around 18%) with suitable ester plasticizers can reach −50 °C to −60 °C, while high-ACN fuel-resistant grades above 45% may be limited to approximately −20 °C to −25 °C. The exact value depends on the complete formulation and must always be verified by test.

How does acrylonitrile content affect the result?

Acrylonitrile increases the polarity and intermolecular forces within the polymer chain, raising the glass transition temperature. Consequently, oil resistance and low-temperature flexibility are in direct competition in NBR design, and the brittleness test documents where a given compound sits on this trade-off curve.

How is brittleness temperature different from glass transition temperature and TR10?

The glass transition temperature (Tg) is a thermal property measured by DSC or DMA, TR10 is obtained from the low-temperature retraction test (ISO 2921 / ASTM D1329) and reflects sealing recovery, while the brittleness temperature is a dynamic mechanical pass/fail criterion determined under impact. Tb is generally the most conservative indicator, defining the temperature below which the material will fail catastrophically rather than merely stiffen.

Can NBR parts be used below their brittleness temperature?

No. Below Tb the material behaves as a rigid glass and will crack under even modest shock or flexure. Practical service limits are set comfortably above the brittleness temperature, and for sustained low-temperature sealing, alternative elastomers such as silicone (VMQ), fluorosilicone (FVMQ), or specially formulated low-temperature compounds should be considered.

How can the brittleness temperature of an NBR compound be lowered?

Effective formulation strategies include selecting a lower-ACN NBR grade, incorporating cold-resistant ester plasticizers such as DOA, DOS, or TP-95, reducing reinforcing filler loading, using larger-particle carbon blacks, and optimizing the cure system to avoid excessive crosslink density. Each adjustment must be balanced against the oil swelling and compression set requirements of the application.