Determination of Permanent Deformation (Compression Set) of Fluorosilicone Rubber: Standards, Method

Permanent deformation, most commonly evaluated as compression set, is one of the most critical performance indicators for fluorosilicone rubber (FVMQ) sealing materials, directly reflecting long-term sealing reliability under sustained compression and thermal exposure. This article provides a comprehensive technical overview of permanent deformation testing for fluorosilicone rubber, covering specimen types, test items, measurement methods in accordance with ISO 815-1, ASTM D395, and GB/T 7759.1, typical test conditions, calculation formulas, and industry applications. Whether you are qualifying FVMQ compounds, validating O-ring performance, or conducting failure analysis, this guide explains how accredited third-party laboratories determine and interpret permanent deformation values.

Technical Overview

Fluorosilicone rubber (FVMQ) is a specialty elastomer based on a siloxane backbone modified with trifluoropropyl groups, combining the broad operating temperature range and excellent low-temperature flexibility of silicone rubber with substantially improved resistance to fuels, oils, and non-polar solvents. These properties make FVMQ the material of choice for demanding sealing applications in aerospace, automotive fuel systems, and petrochemical equipment, where seals must maintain contact force over years of service under compression.

Permanent deformation refers to the residual deformation that remains after a rubber specimen has been compressed under defined conditions of deflection, temperature, and time, and then allowed to recover. In practical sealing terms, a compound with high permanent deformation will take a permanent "set" and lose its ability to maintain sealing pressure, resulting in leakage. For this reason, permanent deformation—most frequently quantified as compression set—is one of the key acceptance criteria in material specifications for fluorosilicone rubber.

Compression set is determined by international and national standards including ISO 815-1 (vulcanized or thermoplastic rubber, constant deflection or constant force at ambient or elevated temperatures), ISO 815-2 (low-temperature compression set), ASTM D395 (Methods A and B), ASTM D1414 (O-ring testing), and the Chinese national standards GB/T 7759.1 and GB/T 7759.2. The lower the compression set value, the better the elastomer retains its elastic recovery capability and sealing performance.

Testing Samples

Molded Cylindrical Button Specimens

The standard test piece for compression set determination is a cylindrical button. Type A specimens measure 29 mm in diameter and 12.5 mm in height, while Type B specimens measure 13 mm in diameter and 6.3 mm in height. Type A specimens may be stacked to reach greater heights when required by the test fixture. Specimens are typically compression or injection molded from the FVMQ compound, cured, and post-cured under controlled conditions representative of production material.

O-Rings and Seal Cross-Sections

Finished O-rings and other seal profiles can be tested directly using groove-type compression fixtures in accordance with ASTM D1414 or comparable procedures. The cord diameter, actual deflection percentage, and cross-sectional geometry must be reported, since set values obtained on O-rings are not directly interchangeable with those obtained on solid buttons.

Specimens Cut from Finished Parts

When molded buttons are unavailable, specimens may be cut from finished FVMQ components by agreement between the parties. Cut specimens must have parallel faces, a minimum thickness as specified by the applicable standard (thin pieces may be stacked), and must be free of surface defects that could bias thickness measurement or recovery behavior.

Testing Items

Permanent deformation testing for fluorosilicone rubber is typically performed as a coordinated set of measurements addressing different service scenarios:

• Compression set at elevated temperature under constant deflection (typically 25%), the most widely specified test item for FVMQ seals
• Compression set under constant load, relevant for cushioning and load-bearing applications
• Low-temperature compression set and recovery, evaluating sealing capability after cold exposure
• Compression set after heat aging, assessing the combined effect of thermal degradation and sustained strain
• Compression set after immersion in fuel, oil, or reference fluids, evaluating set behavior in a swollen state
• Thickness recovery measurement and calculation of set percentage after defined recovery intervals
• Tensile permanent set (extension set) per ISO 2285, evaluated when the application involves sustained tensile strain

Testing Methods

Specimen Preparation and Conditioning

Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 16 hours prior to testing, following the general requirements of ISO 23529. The original thickness t0 of each specimen is measured at a minimum of three points to the nearest 0.01 mm using a calibrated thickness gauge or optical device, and the median value is recorded. Only specimens free of porosity, flash, and surface damage are accepted.

Constant Deflection Method

In the constant deflection method (Method A of ISO 815-1 and GB/T 7759.1, corresponding to Method B of ASTM D395), the specimen is placed between clean, polished parallel plates and compressed to a defined deflection, most commonly 25%, using fixed spacers or a standardized compression fixture. The compressed assembly is then placed in a circulating air oven at the specified test temperature for the specified duration. Because fluorosilicone rubber is normally used in high-temperature sealing environments, typical test temperatures range from 150 °C to 200 °C, with common durations of 22 h, 70 h, 168 h, 336 h, or 1000 h.

Constant Load Method

In the constant load method (Method B of ISO 815-1, corresponding to Method A of ASTM D395), the specimen is subjected to a constant compressive force applied through a spring-loaded fixture or dead-weight system rather than a fixed deflection. This method better simulates applications where the elastomer supports a defined load. Care should be taken when comparing results, since the letter designations of the methods are reversed between the ISO and ASTM standards.

Recovery and Measurement

Immediately after the aging period, the assembly is removed from the oven and the specimen is released. The specimen is then allowed to recover at 23 °C for 30 minutes (30 ± 3 minutes per ISO 815-1) before the recovered thickness t1 is measured at the same locations as the original measurement. For low-temperature testing per ISO 815-2 and GB/T 7759.2, the specimen is released at the test temperature, and recovery is measured at defined intervals while warming, providing insight into cold-sealing capability.

Calculation of Permanent Deformation

For the constant deflection method, compression set CB is calculated as: CB = [(t0 − t1) / (t0 − ts)] × 100%, where t0 is the original specimen thickness, t1 is the recovered thickness after the recovery period, and ts is the thickness of the spacer defining the compressed height. A compression set of 0% indicates complete recovery, while a value approaching 100% indicates complete loss of elastic recovery. For the constant load method, the set is calculated as the percentage reduction in thickness relative to the original thickness.

Typical Performance of FVMQ

Well-formulated, post-cured fluorosilicone compounds typically achieve compression set values of approximately 10% to 25% after 22 h at 175 °C under 25% deflection, with premium grades remaining below 15% after 70 h at 175 °C. At 200 °C or after extended 1000 h exposure, set values increase and must be evaluated against the specific material specification. FVMQ also exhibits excellent low-temperature recovery, with usable set behavior at temperatures down to −50 °C or lower, owing to its very low glass transition temperature.

Applications

Permanent deformation testing of fluorosilicone rubber supports material qualification, quality control, and design verification across multiple industries:

• Aerospace fuel system seals, O-rings, and grommets, where long-term set resistance is required under thermal cycling and fuel exposure
• Automotive fuel injection systems, tank seals, and emission control components exposed to gasoline, ethanol blends, and oils
• Seals for electric and hybrid vehicle thermal management and battery systems requiring reliable compression retention
• Petrochemical and industrial equipment seals in contact with fuels, lubricants, and solvents at elevated temperatures
• Semiconductor and analytical instrumentation components requiring both chemical resistance and stable mechanical recovery
• Material research, including formulation comparison, post-cure optimization, and accelerated aging or service-life studies based on set retention

FAQ

What is the difference between permanent deformation and compression set?

Permanent deformation is the general phenomenon of residual deformation after strain is removed. Compression set is its standardized quantitative expression for compressive loading, calculated by the formula described above and expressed as a percentage. In elastomer testing practice, the two terms are largely used interchangeably for compressive loading conditions.

Which standard should be selected: ISO 815-1 or ASTM D395?

The choice depends on the governing material specification and target market. ISO 815-1 and GB/T 7759.1 are widely used in international and Chinese specifications, while ASTM D395 dominates North American specifications. Note that Method A of ISO 815-1 (constant deflection) corresponds to Method B of ASTM D395; accredited third-party laboratories can advise on equivalence and comparability of data.

Why does fluorosilicone rubber exhibit low permanent deformation?

The highly flexible silicon–oxygen backbone of FVMQ retains chain mobility across a wide temperature range, and properly engineered peroxide or platinum-catalyzed cure systems form stable crosslink networks that resist stress relaxation. A controlled post-cure, commonly several hours at elevated temperature, further reduces volatile content and residual cure by-products, measurably lowering compression set.

How do temperature and time affect test results?

Higher temperatures and longer exposure times accelerate chain rearrangement and crosslink degradation, increasing the measured set value. For this reason, test conditions must always be reported together with the result, for example "compression set after 70 h at 175 °C, 25% deflection." Data obtained under different conditions are not directly comparable.

Can finished seals be tested instead of molded buttons?

Yes. O-rings and other finished seal cross-sections can be evaluated in groove fixtures per ASTM D1414 or agreed procedures. The results reflect the actual product geometry and deflection, which is valuable for design verification, but the values should not be compared directly with button specimen data from material datasheets.

How are failing or out-of-specification results investigated?

Common root causes include insufficient or excessive cure, missing or inadequate post-cure, formulation deviations, contamination, or excessive deflection during testing. Re-testing with controlled specimens, verification of cure state, and comparison against retained reference compound batches are standard investigation steps, and accredited third-party laboratories can provide the necessary instrumentation and technical analysis.