NBR Cork Rubber Physical Properties Testing: Methods, Standards, and Technical Guide

NBR cork rubber is a high-performance composite sealing material that combines the oil and fuel resistance of nitrile-butadiene rubber with the compressibility, resilience, and recovery of natural cork granules. Physical properties testing of NBR cork rubber—including hardness, tensile strength, compression set, compressibility and recovery, and fluid resistance—is essential to verify material quality and conformity for demanding gasket and sealing applications. This technical guide covers the key testing items, applicable international standards, sample preparation requirements, and evaluation criteria for NBR cork rubber materials.

Technical Overview

NBR cork rubber (nitrile-butadiene cork rubber) is an engineered composite in which selected cork granules—typically ranging from 0.5 mm to 3 mm in particle size—are uniformly dispersed and vulcanized within an acrylonitrile-butadiene rubber (NBR) matrix. The material merges two complementary property profiles: the excellent resistance to oil, fuel, and heat contributed by the polar nitrile rubber phase, and the high compressibility, elastic recovery, and low density contributed by the closed-cell structure of cork. The result is a lightweight, conformable gasket material that maintains sealing stress under bolt load while resisting swelling and chemical degradation in petroleum-based fluids.

Depending on the formulation, cork loading typically ranges from approximately 60 to 200 parts per hundred of rubber, and the acrylonitrile (ACN) content of the NBR may vary from roughly 18% to 50% in order to balance oil resistance against low-temperature flexibility. Finished products are commonly supplied as sheets, rolls, die-cut gaskets, or molded components, with material densities generally between 0.65 g/cm³ and 0.95 g/cm³. Because NBR cork rubber is used predominantly in leak-critical sealing applications—such as oil pans, valve covers, oil-immersed transformers, and fuel systems—systematic physical properties testing is required to confirm that every production batch meets the specified mechanical, compression, aging, and fluid-resistance limits before release.

Testing Samples

Sheets and Rolls

Standard sheet stock is supplied in thicknesses from approximately 0.8 mm to 10 mm. Test specimens are die-cut from the sheet at the supplied thickness and taken at least 25 mm away from the sheet edges. Where a test method requires a minimum specimen thickness greater than the supplied thickness, specimens may be plied only where the applicable standard explicitly permits it, and this must be noted in the test report.

Die-Cut Gaskets and Finished Parts

Finished gaskets, ring seals, and die-cut parts may be tested as complete components for functional properties such as compressibility, sealability, and blow-out resistance. For destructive material tests such as tensile strength, specimens are cut from the trim web or from the flat land area of the part, provided the surface is free of embossing, coating, or adhesive.

Molded and Reinforced Products

Molded plates and reinforced grades containing fabric inserts or metal wire mesh require dedicated specimen preparation. The orientation of the reinforcement relative to the loading direction must be recorded, since it significantly influences tensile and compression results.

Specimen Conditioning

Unless otherwise specified, all specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours before testing, and no less than 24 hours should elapse between vulcanization and testing to allow full stabilization of the material.

Testing Items

A complete physical properties evaluation program for NBR cork rubber typically includes the following test items, selected according to the applicable material specification, customer drawing, or end-use requirements:

• Hardness (Shore A durometer)
• Density and specific gravity
• Tensile strength
• Elongation at break
• Compression set under constant deflection
• Compressibility and recovery (gasket characteristics)
• Creep relaxation (retention of bolt load)
• Oil resistance—volume change and mass change
• Fuel resistance—volume swell in reference fuels
• Heat air aging—retention of hardness, tensile strength, and elongation
• Low-temperature flexibility and brittleness
• Water absorption and swelling
• Sealability and blow-out pressure (functional gasket tests)
• Dimensional tolerances, thickness, and visual surface quality

A representative acceptance specification for a commercial oil-resistant cork-rubber gasket grade may include: hardness 65 ± 5 Shore A; tensile strength not less than 2.5 MPa; elongation at break not less than 80%; density 0.75 to 0.90 g/cm³; compression set not more than 35% after 22 h at 100 °C; and volume change in IRM 903 oil within −5% to +15% after 70 h at 100 °C. Actual limits are always defined by the governing material specification, such as a classification under ASTM F104 for nonmetallic gasket materials, or by the customer's technical drawing.

Testing Methods

Hardness Testing (ASTM D2240 / ISO 48 / GB/T 531.1)

Shore A hardness is measured with a calibrated durometer on a specimen at least 6 mm thick, with a minimum test area of 12 mm in each direction. Readings are taken with the presser foot in firm contact, at points spaced at least 6 mm apart, and the median of five readings is reported. Elevated temperatures are avoided, as hardness increases as temperature drops.

Tensile Strength and Elongation (ASTM D412 / ISO 37 / GB/T 528, ASTM F152)

Dumbbell-type specimens are pulled at a crosshead speed of 500 mm/min using a universal testing machine with pneumatic or mechanical grips. Tensile strength is calculated at break, and elongation at break is determined from the extension between benchmark gauge marks. For sheet gasket materials, ASTM F152 provides the dedicated tension test procedure. The median of at least three specimens is reported for each direction of the sheet.

Density (ASTM D297 / ISO 2781 / GB/T 533)

Density is determined by hydrostatic weighing (Archimedes principle). The specimen is weighed in air and then in water, and density is calculated from the mass difference. Because cork composites can absorb water during immersion, immersion times are kept short or the specimen is coated as permitted by the standard. Density is a valuable quality control check for batch-to-batch consistency of cork loading.

Compression Set (ASTM D395 Method B / ISO 815-1 / GB/T 7759.1)

Cylindrical specimens are compressed to 25% constant deflection between parallel plates for 22 hours at 70 °C or 100 °C. After release, the specimen recovers for 30 minutes at room temperature, and the compression set is calculated as the percentage of the applied deflection that is not recovered. This test predicts the long-term ability of a gasket to maintain sealing pressure.

Compressibility and Recovery (ASTM F36)

Gasket compressibility is measured by loading a specimen between polished platens under a prescribed standard load and recording the thickness reduction, while recovery is the thickness rebound after the load is removed. Cork-rubber grades are typically required to show compressibility in the range of 10% to 30% with recovery of not less than 50% to 75%, reflecting the characteristic conformability of the cork phase.

Creep Relaxation (ASTM F38)

A ring specimen is clamped in a calibrated bolted fixture at a defined initial bolt load and exposed to elevated temperature (commonly 100 °C for 22 hours). The residual load is measured and expressed as a percentage of initial load or as retained sealing stress. High stress retention confirms that the gasket will not relax to a leak-inducing level in service.

Oil and Fuel Resistance (ASTM D471 / ISO 1817 / GB/T 1690)

Square or rectangular specimens are immersed in standard reference fluids—IRM 901, IRM 902, or IRM 903 oils, or ASTM Reference Fuels A, B, or C—for 70 hours at the specified temperature (commonly 100 °C for oil, room temperature to 40 °C for fuel). Volume change is determined by submerged weighing, and the test also records changes in hardness, tensile strength, and mass. Well-compounded NBR cork grades exhibit limited swell, confirming the nitrile rubber matrix is functioning correctly.

Heat Air Aging (ASTM D573 / ISO 188 / GB/T 3512)

Sets of specimens are exposed in a forced-air oven, typically for 70 hours at 70 °C, 100 °C, or 125 °C depending on the service class. Hardness, tensile strength, and elongation are retested after aging and compared with original values. Typical acceptance limits include hardness change within ±10 Shore A and tensile strength retention not less than 75% of the original value.

Low-Temperature Flexibility (ASTM D2137 / GB/T 15256)

The low-temperature performance is assessed by brittleness or stiffness testing after conditioning at the specified cold temperature, commonly down to −30 °C or −40 °C depending on the ACN content of the NBR phase. The specimen is bent through a defined angle and examined for cracking.

Functional Sealability Testing

For finished gaskets, application-level tests may be performed in a flange fixture: leakage rate under a defined bolt torque and internal media pressure, and blow-out resistance at increasing pressure until failure. These tests correlate laboratory material data with real-world sealing performance.

Applications

Physical properties test data for NBR cork rubber support qualification and quality release across a wide range of industries:

• Automotive: oil pan gaskets, valve cover gaskets, timing cover seals, transmission and differential housing gaskets
• Electrical power: flange, radiator, conservator, and manhole gaskets for oil-immersed transformers and switchgear
• General industrial: pipe flange gaskets for lubricating oil, fuel, water, and low-pressure steam lines
• Marine and stationary engines: crankcase, gearbox, and inspection cover seals
• Anti-vibration and cushioning: machinery mount pads, dust seals, and vibration-damping sheets

In each of these applications, the certified physical properties—particularly compression set, compressibility and recovery, and oil swell—directly determine the seal's service life and reliability, making rigorous batch testing a critical element of supply quality assurance.

FAQ

What is NBR cork rubber?

NBR cork rubber is a vulcanized composite of cork granules dispersed in a nitrile-butadiene rubber binder. It combines the oil and fuel resistance of NBR with the compressibility and resilience of cork, making it a preferred material for soft-face gaskets subjected to oil exposure and moderate bolt loads.

Why is compression set testing so important for cork rubber gaskets?

Compression set measures how much of the applied deformation becomes permanent. A gasket with excessive set loses sealing stress over time, leading to leakage. Compression set testing after heat aging at 70 °C to 100 °C is therefore the key predictor of long-term sealing performance.

Which standards govern NBR cork rubber physical properties testing?

The most commonly applied standards include ASTM F104 (classification of nonmetallic gasket materials), ASTM D2240 (hardness), ASTM D412 and ASTM F152 (tension), ASTM D395 (compression set), ASTM F36 (compressibility and recovery), ASTM F38 (creep relaxation), ASTM D471 (fluid resistance), and their ISO and GB/T equivalents, including GB/T 531.1, GB/T 528, GB/T 7759.1, GB/T 1690, and GB/T 3512.

How long does oil resistance testing take?

A standard immersion test runs for 70 hours at 100 °C in IRM 903 oil, although customer specifications may require shorter or longer exposures and different reference fluids. Volume swell, hardness change, and mass change are reported after conditioning the specimens for the standard recovery period.

Can finished gaskets be tested instead of sheet samples?

Yes. Finished gaskets can be tested directly for compressibility, creep relaxation, sealability, and blow-out resistance, while destructive material tests such as tensile and aging are performed on specimens cut from the part or from companion sheet stock of the same batch, subject to agreement between supplier and customer.