NBR Cork Rubber Compression Set Testing: Standards, Specimens, and Methods

NBR cork rubber compression set testing evaluates how well nitrile cork rubber sealing materials recover their thickness after prolonged compression at elevated temperatures. This technical guide explains the applicable standards, specimen requirements, test procedures, calculation formulas, and typical performance criteria for this widely used gasket material in automotive and industrial sealing applications.

Technical Overview

Nitrile cork rubber, commonly designated NBR cork rubber, is an engineered composite material produced by binding granulated cork particles within a nitrile butadiene rubber (NBR) matrix, then curing the compound into sheet form. The material combines the excellent oil, fuel, and grease resistance of nitrile rubber with the high compressibility, conformability, and elastic recovery of cork. This unique balance of properties makes it a preferred gasket material for stamped and flanged sealing applications where flange surfaces are uneven or bolt loads are limited.

Compression set testing is one of the most important quality control and qualification tests for this material. The test measures the permanent, non-recoverable deformation that remains after a specimen has been compressed to a fixed deflection and held at an elevated temperature for a defined period. A low compression set indicates that the gasket will maintain its thickness and sealing pressure over long-term service, while a high compression set predicts leakage, bolt load loss, and seal failure.

Testing is typically performed in accordance with the following reference standards:

• ASTM D395 – Standard Test Methods for Rubber Properties in Compression Set (Method A and Method B)
• ISO 815-1 – Rubber, vulcanized or thermoplastic – Determination of compression set (ambient and elevated temperatures)
• GB/T 7759.1 – Chinese national standard, technically aligned with ISO 815-1
• ASTM F36 and ASTM F104 – Supplementary gasket-material methods for compressibility, recovery, and material classification

Testing Samples

Standard Cylindrical Buttons

For classical compression set testing per ASTM D395 Method B, standard cylindrical specimens are used: Type 1 buttons measure 29.0 mm in diameter and 12.5 mm in thickness, while Type 2 buttons measure 13.0 mm in diameter and 6.3 mm in thickness. Because NBR cork rubber is manufactured in sheet form with typical thicknesses of 1 mm to 6 mm, standard-thickness specimens may be either molded directly from the production compound or built up by laminating multiple plies of sheet, provided the plies are free of adhesive and tested consistently.

Sheet and Disc Specimens

For gasket-material qualification, specimens are frequently tested at the natural thickness of the production sheet. Discs or square pads are die-cut from representative sheet stock and compressed in a fixture with spacers sized to achieve the specified deflection. This approach reflects real-world performance better than laminated buttons and is common in supplier data sheets for cork rubber gasket materials.

Finished Gasket Parts

Die-cut or molded production gaskets, such as oil pan gaskets, valve cover gaskets, and transformer sealing rings, can be tested as finished parts. Spacer bars are calculated from the actual compressed thickness zone of the part, and the reported compression set characterizes the sealing performance of the real component rather than a laboratory coupon.

All specimens should satisfy the following requirements before testing:

• Free of visible defects such as cracks, delamination, crumbling, or surface contamination
• Conditioned for at least 24 hours at 23 ± 2 °C and 50 ± 5% relative humidity
• Thickness measured at a minimum of three points, with results averaged and recorded to 0.01 mm
• Sampled from material representative of normal production cure and cork particle distribution

Testing Items

The compression deformation test program for NBR cork rubber typically includes the following evaluations:

• Compression set under constant deflection (ASTM D395 Method B / ISO 815-1 Type B) – the primary test, normally at 25% compression
• Compression set under constant force (ASTM D395 Method A) – used for special load-defined applications
• Compression set at multiple temperatures – commonly 23 °C, 70 °C, 100 °C, and 125 °C, reflecting the thermal service limits of nitrile rubber
• Multiple exposure durations – standard cycles of 22 h, 70 h, and 168 h to assess aging behavior
• Short-time compressibility and recovery (ASTM F36) – frequently reported together with compression set on gasket data sheets
• Post-test visual examination – checking for cracking, cork particle fallout, blistering, or delamination after thermal exposure

Reported results include the compression set percentage, original and recovered thickness values, test temperature and duration, compression ratio, and any visual defects observed after testing.

Testing Methods

Test Procedure (Constant Deflection, ASTM D395 Method B)

The standard procedure is performed as follows:

• Measure and record the original specimen thickness t0 after conditioning
• Place the specimen between clean, parallel polished steel plates and insert spacer bars sized to produce the required deflection, normally 25% (spacer thickness ts = 0.75 × t0)
• Assemble the compression fixture and tighten the bolts uniformly until the plates contact the spacers, ensuring the specimen is not twisted or overloaded
• Place the entire fixture in a forced-air oven at the specified test temperature within ±2 °C, typically 100 °C for 22 h for NBR cork gasket grades
• Remove the fixture from the oven and allow it to cool at standard laboratory temperature for 30 to 60 minutes before releasing the compression
• After a recovery period of 30 minutes at room temperature, measure the final thickness t1 of the specimen

Calculation of Compression Set

The compression set under constant deflection is calculated using the standard formula:

CB (%) = [(t0 − t1) / (t0 − ts)] × 100, where t0 is the original thickness, t1 is the thickness after recovery, and ts is the spacer bar thickness. A lower percentage indicates better elastic recovery and long-term sealing capability.

Typical Results and Influencing Factors

General-purpose commercial NBR cork rubber gasket sheet is commonly specified with a compression set of 35% to 45% maximum after 22 hours at 100 °C, while premium grades can achieve values of 15% to 25% under the same conditions. Room-temperature compression set values are typically well below 20%. The following factors significantly influence the test result:

• Cork particle size and loading: finer particles and optimized cork-to-rubber ratio generally reduce permanent set
• NBR polymer content and acrylonitrile level: higher rubber content improves elastic recovery
• State of cure: under-cured material shows elevated set and softening after heat aging
• Test temperature and duration: set increases sharply as temperature approaches the thermal limit of nitrile rubber
• Specimen construction: laminated plies may show slightly higher set than solid molded specimens

Applications

NBR cork rubber materials whose compression set behavior is verified by this test are used across a wide range of demanding sealing applications:

• Automotive oil pan gaskets, valve cover gaskets, timing cover seals, and transmission pan gaskets
• Oil-immersed transformer flange gaskets, radiator seals, and bushing sealing rings
• Fuel and lubricant flange gaskets for pumps, gearboxes, compressors, and hydraulic systems
• Marine engine and outdrive sealing where fuel and oil resistance are required
• Agricultural and construction equipment access covers subjected to vibration and thermal cycling
• General industrial gaskets conforming to flange irregularities under moderate bolt loads

In all of these applications, verified compression set performance ensures that the gasket retains sufficient thickness and contact pressure to prevent leakage throughout the equipment service life, even after prolonged heat exposure.

FAQ

Which standard is most commonly used for NBR cork rubber compression set testing?

ASTM D395 Method B (constant deflection) is the most widely applied method, along with its international equivalents ISO 815-1 and GB/T 7759.1. Gasket-specific specifications frequently combine compression set testing with ASTM F36 compressibility and recovery testing to fully characterize the material.

Why is 25% compression typically selected?

The 25% deflection level is the standardized condition defined in ASTM D395 Method B and represents a realistic gasket compression in flanged joints. It provides sufficient deformation to reveal the viscoelastic and cellular recovery behavior of the cork rubber composite without mechanically crushing the material.

Why does cork rubber show a higher compression set than solid NBR rubber?

The granulated cork phase contains closed and open cell structures that undergo partially irreversible deformation under sustained compression and heat. While solid nitrile rubber may achieve compression set values below 15%, the cork phase trades some permanent set for superior conformability and recovery over rough flange surfaces.

What test temperature should be chosen?

The test temperature should match the intended service temperature. Standard nitrile compounds are generally rated for continuous service up to approximately 120 °C, so common test points are 70 °C, 100 °C, and 125 °C. Testing above 125 °C is not recommended for standard NBR grades due to oxidative hardening and polymer degradation.

What is the difference between compression set and creep relaxation?

Compression set (ASTM D395) measures the permanent thickness loss after the load is removed, while creep relaxation (ASTM F38) measures the loss of sealing stress while the gasket remains compressed. Both properties are relevant to long-term joint integrity and are often evaluated together.

How can compression set performance be improved?

Improvement methods include increasing the nitrile rubber content, using finer and more uniform cork granules, optimizing the curing system for complete crosslinking, applying post-curing to stabilize the compound, and ensuring proper flange design so that actual gasket stress remains within the material's recommended compression range.