Compression Set Testing of Elastomeric Seals After Transformer Oil Immersion: Methods, Standards, an

Compression set testing under transformer oil immersion is a critical accelerated aging evaluation used to determine whether elastomeric sealing materials can maintain reliable long-term sealing performance inside oil-filled electrical equipment. This article explains the test principle, applicable international standards, specimen requirements, test procedures, result calculation, and typical acceptance criteria, providing a practical technical reference for transformer manufacturers, seal suppliers, and material engineers.

Technical Overview

Compression set is the permanent deformation retained by an elastomer after it has been compressed under a fixed deflection for a defined period and then allowed to recover. When rubber seals installed in transformers, bushings, tap changers, radiator valves, and flange joints operate for decades under sustained compression, elevated temperature, and continuous contact with mineral insulating oil, the combined thermal, chemical, and mechanical stress can break down the polymer network. The result is a progressive loss of contact stress at the sealing interface, which ultimately manifests as oil leakage and potential ingress of moisture and contaminants.

A compression set test conducted with the specimen immersed in transformer oil reproduces this service environment far more realistically than a test performed in air. The insulating oil can cause swelling, plasticizer extraction, crosslink density changes, and post-vulcanization reactions, all of which accelerate or alter the set behavior of the elastomer. Therefore, oil-immersed compression set data is frequently specified in purchase specifications for transformer gaskets, O-rings, and custom-molded sealing components.

The test is typically performed by accredited third-party laboratories or in-house material laboratories using internationally recognized methods, including:

• ASTM D395 (Standard Test Methods for Rubber Properties — Compression Set), Method B, constant deflection
• ISO 815-1 (Rubber, vulcanized or thermoplastic — Determination of compression set — Part 1: At ambient or elevated temperatures)
• ISO 815-2 (Part 2: Compression set at low temperatures, when applicable)
• ASTM D1414 (Standard Test Methods for Rubber O-Rings), which covers compression set of finished O-rings
• GB/T 7759.1, the Chinese national standard technically aligned with ISO 815-1

Supporting immersion and fluid-resistance methods include ASTM D471, ISO 1817, and GB/T 1690, while the reference insulating liquids are defined in IEC 60296, ASTM D3487, and GB 2536 for unused mineral transformer oils.

Testing Samples

Standard Molded Compression Set Buttons

The primary specimen geometry is a cylindrical button molded directly from the sealing compound. Two standard sizes are widely used:

• Type 1 / Type A specimen: 29.0 mm diameter × 12.5 mm height (ASTM D395 Type 1; ISO 815-1 Type A)
• Type 2 / Type B specimen: 13.0 mm diameter × 6.3 mm height, requiring less material and preferred when compound availability is limited

Specimens should be molded as close as possible to finished part thickness, free of porosity, blisters, and surface defects. Unless otherwise specified, vulcanized samples are conditioned at standard laboratory temperature and humidity for at least 16 hours, and preferably 24 to 72 hours after vulcanization, before testing. A minimum of three specimens per test condition is recommended for statistical validity.

Finished O-Rings and Molded Seals

When the objective is to evaluate the actual production part rather than the base compound, finished O-rings and sealing gaskets can be tested in accordance with ISO 815-2 or ASTM D1414. The ring is compressed between flat plates to a defined squeeze percentage, and the cross-section recovery is measured with a calibrated optical or contact measurement system. Testing finished hardware captures the effects of molding quality, part geometry, splice quality, and post-curing, which may differ significantly from molded button performance.

Laminated or Die-Cut Coupons

For sheet stock or when only limited material is available, specimens may be prepared by laminating plies or die-cutting coupons to the required thickness, provided the material supplier and the customer agree on this approach and the ply orientation is documented, since laminated specimens can show artificially high or variable set values.

Testing Items

A comprehensive oil-immersed compression set program usually evaluates a combination of the following parameters to characterize both dimensional recovery and chemical degradation:

• Compression set percentage in transformer oil (constant deflection, Method B), reported at specified temperature and duration
• Hardness change before and after immersion, measured in Shore A (IRHD) per ISO 48, ASTM D2240, or GB/T 531
• Tensile strength and elongation at break retention, per ISO 37, ASTM D412, or GB/T 528
• Volume change and mass change (swelling or shrinkage), per ASTM D471, ISO 1817, or GB/T 1690
• Visual assessment after exposure: cracking, blistering, tackiness, surface glazing, or physical disintegration
• Sealing-force or stress-relaxation measurements, where instrumented fixtures allow continuous load decay monitoring

The compression set value itself is the headline result; the supplementary items help interpret whether the observed set originates from chemical crosslink breakdown, oil absorption and swelling, or additive extraction.

Testing Methods

Specimen Preparation and Initial Measurement

After conditioning, the thickness of each specimen is measured at the center (and at multiple points where required) using a gauge with an adequately sized foot and controlled pressure. The original thickness is recorded as the baseline value. Specimens are then inspected for defects, and identification marks are applied on the side surface so that measurements can be traced to individual samples.

Compression and Immersion Procedure

The standard procedure follows a constant-deflection scheme:

• Each specimen is placed between polished steel plates with a spacer bar or fixed-limit stops that limit compression to a defined percentage of the original thickness, typically 25% for rubber buttons and 15% to 30% squeeze for O-rings, depending on cross-section
• The assembled compression fixtures are fully immersed in unused mineral transformer oil conforming to IEC 60296, ASTM D3487, or GB 2536, held in a temperature-controlled oil bath or aging oven
• Common test temperatures correspond to the thermal class of the application: 70 °C, 100 °C, 105 °C, 120 °C, 125 °C, or 150 °C
• Common exposure durations include 24 h, 70 h, 168 h, 336 h, and 1000 h; long-term qualification programs may extend to several thousand hours for service-life extrapolation
• The oil volume-to-specimen surface area ratio and oil renewal requirements follow ASTM D471 or ISO 1817 to ensure the fluid is not depleted of reactive components during extended aging

Recovery and Final Measurement

At the end of the exposure period, the fixtures are removed from the bath, disassembled rapidly, and the specimens are allowed to recover. Specimens are typically rested at standard laboratory temperature for 30 minutes before the final thickness measurement is taken, in accordance with the specified recovery time in the governing standard. Immediate and delayed measurements may both be recorded when low-temperature or slow-recovery behavior is of interest.

Calculation of Results

The compression set under constant deflection is calculated as:

CB = [(t0 − t1) / (t0 − ts)] × 100 %

where t0 is the original specimen thickness, ts is the thickness of the spacer (the compressed height), and t1 is the recovered thickness after the specified recovery period. A result of 0% indicates complete elastic recovery, while a value approaching 100% indicates essentially complete permanent deformation. Reported results include the median (or individual values), the test temperature, duration, deflection percentage, the identity of the immersion oil, and the recovery interval.

Test Report

A complete test report should identify the material and part designation, the standard and method used, specimen type and dimensions, deflection percentage, oil specification and batch, bath temperature, exposure time, recovery conditions, individual and median compression set values, and any supplementary hardness, tensile, volume-change, or visual observations.

Applications

Oil-immersed compression set testing is applied throughout the life cycle of transformer sealing technology:

• Qualification of gasket materials for transformer tank flanges, cooler and radiator connections, conservator systems, and bushing turrets
• Validation of O-rings and seals used in on-load tap changers, off-circuit tap changers, valves, drain plugs, and gas relay interfaces
• Material selection studies comparing NBR, FKM (fluoroelastomer), silicone (VMQ), and specialty compounds for mineral oil and ester-based insulating liquid service
• Incoming quality control and batch release testing of sealing compounds by seal manufacturers and transformer OEMs
• Accelerated aging and service-life estimation of gaskets in distribution, power, and instrument transformers using time-temperature extrapolation
• Failure analysis of leaking transformer joints, where elevated residual set values confirm stress-relaxation-induced loss of sealing force
• Research and development of improved formulations, including peroxide-cured and low-compression-set compounds for high-temperature transformer operation

FAQ

What compression set value is considered acceptable for transformer seals?

Acceptance limits depend on the equipment specification and the elastomer family. As a general industry practice, values of 25% or less after 168 hours at 100 °C in transformer oil are commonly specified for general-purpose nitrile gaskets, while higher-grade applications and fluoroelastomers are frequently required to achieve 15% or lower under comparable or more severe conditions. The end user and supplier should always agree on the pass/fail threshold in the purchase specification.

Why is the test performed in transformer oil instead of air?

Air-aging compression set results can substantially overestimate seal performance in oil-filled equipment. Mineral insulating oil penetrates the elastomer, extracts plasticizers and antidegradants, and alters the crosslink network, producing swelling, softening, or secondary crosslinking that changes the set behavior. Testing in the actual service fluid provides a realistic, conservative assessment of long-term sealing capability.

Which test temperature and duration should be selected?

The conditions should simulate or accelerate the intended service environment. For distribution transformers with top-oil temperatures around 60 °C to 90 °C, 100 °C × 70 h or 168 h is a common screening condition. For high-temperature or high-load applications, 120 °C to 150 °C exposures, extended to 1000 hours or longer, are used for qualification and life estimation.

Can finished O-rings be tested instead of molded buttons?

Yes. ISO 815-2 and ASTM D1414 define compression set procedures specifically for O-rings using cross-section squeeze between parallel plates. Testing finished parts is often preferable for qualification because it includes the influence of molding and production processing on sealing performance.

Which elastomer families perform best in transformer oil?

Nitrile rubber (NBR) offers a good balance of oil resistance and cost and is the most widely used material for transformer gaskets. Fluoroelastomers (FKM) provide superior oil and heat resistance with very low compression set for demanding locations. Silicone rubber (VMQ) offers wide temperature capability and is used selectively, while EPDM, although excellent in many environments, has poor resistance to mineral oils and is generally unsuitable for direct immersion in hydrocarbon-based transformer oil.

Should new or in-service aged oil be used for the test?

Standard qualification testing uses unused mineral transformer oil conforming to IEC 60296, ASTM D3487, or GB 2536 to ensure repeatability and comparability. Where the objective is to study seals operating in equipment with degraded oil, in-service aged oil can be used as an additional investigative condition, provided the oil characteristics, including oxidation products and acid number, are documented.

What causes a high compression set result?

Elevated set values typically stem from oxidative or thermal crosslink breakdown, oil-induced swelling followed by plasticizer or additive extraction, insufficient or over-vulcanization, poor compound design, or physical creep of low-molecular-weight components. Cross-checking compression set results with hardness, volume change, and tensile retention data allows the laboratory to identify the dominant degradation mechanism.