Testing the Effect of Transformer Oil on Rubber Compression Set Deformation

Rubber sealing components in oil-immersed transformers are permanently exposed to hot transformer oil, and their long-term sealing reliability depends on resistance to compression set deformation. This article explains how accredited third-party laboratories evaluate the effect of transformer oil on rubber compression set, covering test specimens, methods, standards, evaluation criteria, and practical applications in the power equipment industry.

Technical Overview

Oil-immersed transformers rely extensively on elastomeric sealing components, including flange gaskets, O-rings, valve seals, bushing seals, and corrugated expansion joint membranes, to prevent oil leakage and moisture ingress. Throughout their service life, these rubber parts remain in permanent contact with mineral transformer oil at operating temperatures that typically range from 60 °C to 105 °C, with hotspots occasionally exceeding these values. The combination of sustained mechanical compression, elevated temperature, and continuous oil exposure causes physicochemical changes in the elastomer, including oil absorption, swelling, extraction of plasticizers and antioxidants, crosslink density changes, and chain scission.

Compression set is one of the most critical indicators of whether a rubber seal can maintain its sealing force over decades of service. When a compressed elastomer loses its ability to recover after the compressive load is released or relaxed, leakage paths form even though the seal appears visually intact. Testing the effect of transformer oil on rubber compression set therefore evaluates the combined action of thermal aging and oil-mediated chemical degradation under constant deflection, providing essential data for material selection, quality control, and remaining service life estimation of transformer sealing systems.

The test is typically performed by compressing standard rubber specimens to a fixed deflection between parallel plates, immersing the compressed assembly in transformer oil at an elevated temperature for a specified duration, releasing the load, and measuring the residual deformation after a defined recovery period. Results are compared against values obtained on unaged specimens and against acceptance limits specified in product standards or purchase specifications.

Testing Samples

Nitrile Butadiene Rubber (NBR)

NBR is the most widely used elastomer for transformer sealing applications due to its excellent resistance to mineral oils and cost-effectiveness. High-acrylonitrile grades (typically 34–40 % ACN content) offer the best oil resistance and are the primary material for gaskets in transformer flanges, radiators, and conservator systems.

Fluoroelastomer (FKM)

FKM provides superior resistance to heat, oil, and chemical attack, and is used in high-temperature zones, high-voltage bushings, and premium sealing designs where extended service life is required. Its compression set performance in hot oil is generally superior to NBR, though at higher material cost.

Silicone Rubber (VMQ) and Fluorosilicone (FVMQ)

Silicone rubbers offer a wide service temperature range and good recovery characteristics. They are used in applications requiring low-temperature flexibility and are evaluated for oil resistance when specified by the equipment designer.

EPDM and Other Elastomers

EPDM exhibits poor resistance to mineral oils and is normally evaluated only when used in air-side sealing positions or when special oil formulations are involved. Other materials, such as polychloroprene (CR), may also be tested upon request.

Specimen Forms

• Standard cylindrical buttons, Type A (29 mm diameter × 12.5 mm height) or Type B (13 mm diameter × 6.3 mm height) per ISO 815-1 / GB/T 7759.1.
• Ring specimens (O-rings) of specified cross-section per ISO 815-2 when the actual product geometry must be evaluated.
• Finished components such as die-cut gaskets, molded seals, and vulcanized joints cut from actual transformer sealing assemblies.
• Reference transformer oil meeting GB 2536 or IEC 60296 (unused mineral insulating oil), or in-service oil supplied by the client to simulate real operating conditions.

Testing Items

A comprehensive evaluation of the oil–rubber interaction combines compression set measurement with complementary physical property tests, allowing the degradation mechanism to be fully characterized. Typical test items include:

• Compression set under constant deflection after immersion in transformer oil (CB or CA value, expressed as a percentage).
• Compression set in air as a baseline reference for comparison with oil-immersed results.
• Volume change and mass change after oil immersion, indicating swelling or extraction.
• Hardness change (Shore A, IRHD) before and after oil exposure.
• Tensile strength and elongation at break retention after immersion.
• Visual and microscopic examination for cracking, blistering, surface tackiness, or embrittlement.
• Oil-side analysis, where required, including acid value and dissipation factor changes of the oil after contact with the elastomer, to assess whether seal degradation contaminates the insulation system.

Testing Methods

Standards and References

• ISO 815-1 / GB/T 7759.1 — Rubber, vulcanized or thermoplastic; determination of compression set at ambient or elevated temperatures.
• ASTM D395 — Rubber properties; compression set (Method B, constant deflection).
• ISO 1817 / GB/T 1690 / ASTM D471 — Determination of the resistance of rubber to liquids, governing immersion procedures and change-in-property calculations.
• ISO 815-2 — Compression set at low temperatures, applicable to cold-climate installations.
• IEC 60296 / GB 2536 — Specifications for unused mineral insulating oils used in transformers and switchgear.
• ISO 48 / GB/T 531.1 — Determination of hardness for supplementary characterization.

Test Procedure

The standard test sequence consists of the following steps, performed in an accredited third-party laboratory under controlled environmental conditions:

• Conditioning: specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours, and their initial thickness (t0) is measured at multiple points using a calibrated dial gauge or non-contact device.
• Assembly: each specimen is placed between polished steel plates with spacers limiting the compression to the specified deflection, commonly 25 % of the original thickness.
• Oil immersion under compression: the assembled fixtures are fully immersed in transformer oil inside a temperature-controlled oil bath or aging oven with oil vessels. Typical test temperatures are 100 °C or 125 °C, with common durations of 24 h, 70 h, 168 h, or extended aging up to 1000 h for life assessment studies.
• Recovery: at the end of the aging period, the specimens are removed, released from the fixtures within a short defined time, and allowed to recover at 23 °C for 30 minutes (short-term recovery) and 24 hours (final recovery) before the final thickness (t1) is measured.
• Calculation: the compression set CB is calculated as CB = (t0 − t1) / (t0 − ts) × 100 %, where t0 is the original thickness, t1 is the thickness after recovery, and ts is the spacer thickness. A lower value indicates better elastic recovery.

Evaluation Criteria

Acceptance limits depend on the seal design, material class, and equipment specification. In general industrial practice for transformer sealing elastomers, a compression set of no more than 30 % to 50 % after oil immersion at the specified temperature and duration is commonly required. The comparison between oil-immersed and air-aged results reveals the specific contribution of the oil medium: a significantly higher compression set in oil indicates plasticizer extraction or network degradation driven by oil contact, whereas similar values suggest that thermal oxidation is the dominant mechanism. Concurrent volume and hardness measurements support this diagnosis and guide corrective actions such as compound reformulation or material substitution.

Applications

Testing of the transformer oil effect on rubber compression set is applied throughout the power equipment value chain:

• Material qualification for seal manufacturers supplying gaskets and O-rings to power transformer, distribution transformer, and instrument transformer producers.
• Incoming quality control of rubber sealing lots before assembly into transformers, reactors, tap changers, and radiator valve systems.
• Failure analysis of leaking transformer flanges, Buchholz relay interfaces, bushing gaskets, and conservater diaphragms, distinguishing material degradation from installation defects.
• Accelerated life assessment programs in which long-term oil immersion compression set data are extrapolated to estimate seal service life under actual operating temperatures.
• Type testing and periodic requalification of sealing systems for GIS bushings, cable boxes, oil-immersed reactors, and on-load tap changers in accordance with purchaser technical specifications.
• Research and development support for new elastomer compounds, surface treatments, and alternative insulating liquids, including natural and synthetic ester fluids.

FAQ

Why is compression set more important than tensile strength for transformer seals?

Seals function under permanent compression, so their ability to sustain contact pressure over time governs leak-tightness. Tensile strength may remain acceptable while recovery capability is already lost; compression set therefore correlates far better with real sealing performance.

What test temperature and duration should be selected?

Common screening conditions are 100 °C for 70 h or 168 h, matching typical NBR service limits. For high-temperature designs or FKM materials, 125 °C or 150 °C conditions are used. Extended durations such as 1000 h provide data suitable for lifetime estimation.

Should the specimen be compressed during oil immersion or immersed first and compressed afterward?

Compressing the specimen while immersed best simulates real service conditions, because oil penetration, stress relaxation, and chemical aging act simultaneously. Some specifications also require sequential testing to separate the individual effects; the choice depends on the applicable product standard.

How does transformer oil cause compression set to increase?

Mineral oil penetrates the elastomer and swells the network, while aromatic fractions extract plasticizers and antioxidants. Combined with thermal oxidation and hydrolysis, these mechanisms reduce crosslink integrity and elastic recovery, producing permanent deformation.

What additional tests are recommended alongside compression set?

Volume and mass change, hardness change, and tensile retention tests should be run in parallel to fully characterize oil compatibility. Analysis of the contacted oil for acid value and dielectric dissipation factor is advisable when seal–oil interaction with the insulation system is a concern.

Can finished seals from an operating transformer be tested?

Yes. Laboratory aging of newly manufactured seals against oil sampled from the specific transformer provides a realistic compatibility assessment, and recovered aged seals can be measured directly to support condition assessment and maintenance planning.