Determination of Elongation at Break of Nitrile Butadiene Rubber (NBR)

Elongation at break is one of the most critical mechanical parameters for evaluating the flexibility and service performance of nitrile butadiene rubber (NBR). This article provides a comprehensive technical guide to the determination of elongation at break of NBR, covering test standards, specimen preparation, instrumentation, procedures, and practical applications. Whether you are involved in rubber compounding, quality control, or product certification, understanding this test method is essential for ensuring reliable and comparable results.

Technical Overview

Nitrile butadiene rubber (NBR) is a synthetic elastomer produced by the copolymerization of acrylonitrile and butadiene. It is widely valued for its excellent resistance to oils, fuels, and greases, which makes it indispensable in sealing, hose, and gasket applications. Because NBR components frequently undergo repeated stretching, flexing, and deformation during service, elongation at break serves as a key indicator of the material's ability to deform without failure.

Elongation at break is defined as the extension of a test specimen at the moment of rupture, expressed as a percentage of the original distance between gauge marks. It is calculated using the following relationship:

• Elongation at Break (Eb) = (L − L0) / L0 × 100%
• L0 = initial gauge length between benchmarks
• L = gauge length at the moment of rupture

The determination is typically performed as part of a complete tensile stress-strain test, conducted in accordance with internationally recognized standards such as ISO 37, ASTM D412, and GB/T 528. These standards specify specimen geometry, testing speed, conditioning requirements, and data treatment to ensure reproducibility between laboratories.

Testing Samples

Vulcanized Standard Test Sheets

Compression-molded NBR sheets with a preferred thickness of 2 mm ± 0.2 mm are the most common source of test specimens. The sheets should be vulcanized under controlled conditions of temperature, pressure, and time, and allowed to rest after curing before specimens are cut. A minimum conditioning period of 16 hours between vulcanization and testing is generally required.

Dumbbell (Dog-Bone) Specimens

Dumbbell specimens are die-cut from the cured sheets using a sharp steel cutting die. ISO 37 defines several specimen types, including Type 1 (25 mm gauge length), Type 2 (20 mm gauge length), and smaller Types 3 and 4 for limited material quantities. The die must be kept sharp and regularly maintained, as dull dies can introduce edge defects that cause premature failure and artificially low elongation values.

Ring Specimens

Ring-shaped specimens may be used when specified, particularly for materials where die cutting is impractical. Ring specimens are stretched between two pulleys or spindles on the tensile machine, and elongation is calculated on the basis of the internal circumference of the ring.

Finished Product Samples

Specimens cut from finished NBR products such as seals, hoses, gloves, or diaphragms may also be tested. In these cases, the actual wall thickness and any surface texture of the product must be recorded, as they can influence the measured elongation values compared with standard laboratory sheets.

Testing Items

Elongation at break is usually measured together with other tensile properties in a single test run. The complete test program for NBR typically includes:

• Elongation at break (Eb) — the primary parameter indicating ultimate deformability
• Tensile strength (TS) — the maximum tensile stress recorded before rupture
• Tensile stress at given elongation (modulus at 100% and 300%) — indicating stiffness and reinforcement level
• Yield behavior, where applicable, for thermoplastic or lightly crosslinked compounds
• Break type and break location, which help identify specimen preparation defects

Testing Methods

Specimen Conditioning

All test pieces must be conditioned in a controlled atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity prior to testing, in accordance with ISO 23529 or the equivalent clauses of ASTM D412. Proper conditioning stabilizes the rubber and minimizes variability caused by ambient temperature and moisture.

Instrumentation

The test is performed on a calibrated universal tensile testing machine equipped with pneumatic or mechanical grips, a calibrated load cell appropriate to the expected force range, and a system for measuring extension. Extension may be measured with a contact extensometer, a non-contact video extensometer, or grip separation, although direct gauge-mark tracking provides the highest accuracy. Thickness is measured with a dial gauge or digital micrometer fitted with a flat foot, applying low contact pressure as specified for soft elastomers.

Test Procedure

• Measure the thickness of each specimen at three points within the test section and record the median value; calculate the cross-sectional area using the die width.
• Mark the gauge length on the narrow section of the dumbbell using a bench marker or ink stamp, taking care not to nick the rubber.
• Mount the specimen symmetrically in the grips, ensuring the gauge marks are aligned with the direction of extension.
• Set the crosshead speed as specified by the standard, typically 500 mm/min for standard dumbbells and ring specimens, with reduced speeds for small dumbbells.
• Start the test and continue until the specimen ruptures; record the extension at the moment of break.
• Calculate elongation at break for each specimen and report the median of at least three valid results, in accordance with ISO 37.

Data Treatment and Reporting

The test report should include the median and individual elongation values, specimen type and dimensions, crosshead speed, conditioning atmosphere, test temperature, die identification, and any observations regarding break type. Specimens that break outside the gauge section or at a defect are typically discarded and replaced, since they do not represent the true material behavior.

Applications

Determination of elongation at break supports a wide range of engineering and commercial activities involving NBR materials:

• Quality control of NBR compounds during mixing and vulcanization to verify batch-to-batch consistency
• Material qualification and incoming inspection for seals, O-rings, and gaskets used in automotive and industrial fluid systems
• Research and development of fuel hoses, hydraulic hoses, oil-resistant conveyor belts, and rolled goods
• Certification of disposable and industrial NBR gloves, where stretch performance is a safety-critical property
• Failure analysis and aging studies, including the assessment of degradation after heat, oil, or ozone exposure
• Comparison of formulation variables such as acrylonitrile content, filler loading, plasticizer level, and cure system

FAQ

What is a typical elongation at break value for NBR?

Standard sulfur-cured NBR compounds generally exhibit elongation at break values between 300% and 600%. Soft, lightly filled, or plasticized compounds can exceed 600%, while highly filled, high-acrylonitrile, or peroxide-cured compounds may show values closer to 200%. The exact value depends strongly on the formulation.

Which standards govern the test method?

The most widely used standards are ISO 37 (Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties) and ASTM D412 (Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension). GB/T 528 is the Chinese national standard, which is technically aligned with ISO 37.

How does acrylonitrile content affect elongation at break?

Increasing the acrylonitrile content generally raises oil resistance, hardness, and tensile strength, but reduces elongation at break and low-temperature flexibility. Compounds with lower acrylonitrile content are more elastic and exhibit higher elongation values.

What factors can cause unexpectedly low elongation results?

Common causes include dull cutting dies that create edge nicks, excessive crosshead speed, testing outside the specified temperature range, over-cured or aged material, incorrect specimen thickness, and grip slippage. Systematic verification of die condition, machine calibration, and laboratory conditions helps eliminate these errors.

How many specimens should be tested?

A minimum of three valid specimens is required for a routine determination, and the median value is reported as the result. For referee or certification testing, five or more specimens are recommended to improve statistical confidence.