Compression Set Testing of Rubber Materials in Transformer Oil

Compression set testing of rubber materials in transformer oil is a critical laboratory evaluation used to determine the permanent deformation and long-term sealing reliability of elastomeric gaskets, O-rings, and seals used in oil-filled electrical equipment. By combining compression set test principles from ISO 815-1, ASTM D395, and GB/T 7759.1 with immersion aging in mineral insulating oil, this test simulates real service conditions and helps manufacturers select qualified sealing materials, verify supplier quality, and predict seal service life in transformers and other oil-immersed apparatus.

Technical Overview

Elastomeric seals in oil-immersed transformers operate under sustained compressive load while in continuous contact with hot mineral insulating oil. Over time, the combined action of heat, mechanical stress, and oil immersion causes chemical swelling, extraction of plasticizers and additives, and stress relaxation of the rubber compound. The result is permanent deformation, known as compression set, which reduces contact pressure at sealing interfaces and eventually leads to oil leakage and moisture ingress—two of the most common failure modes in power transformers.

Compression set testing in transformer oil quantifies this permanent deformation under accelerated laboratory conditions. The test applies principles from international and national standards for compression set measurement (ISO 815-1, ASTM D395, GB/T 7759.1) together with liquid immersion aging procedures (ISO 1817, ASTM D471, GB/T 1690). Unused mineral insulating oil conforming to IEC 60296, ASTM D3487, or GB 2536 is typically used as the immersion medium. The test is an essential tool for material qualification, quality control, failure analysis, and service-life estimation of sealing components in transformers, on-load tap changers, bushings, and oil-filled switchgear.

Testing Samples

Standard Cylindrical Specimens

Preferred test pieces are molded cylindrical buttons, such as Type A (29 mm diameter × 12.5 mm height) or Type B (13 mm diameter × 6.3 mm height) as defined in ISO 815-1 and GB/T 7759.1. Cylindrical specimens provide uniform stress distribution and the most reproducible compression set data. Laminated or cut specimens from sheet stock may also be used when molded buttons are not available, although surface effects may increase data scatter.

O-Ring Specimens

O-rings used in transformer valves, flanges, and bushing assemblies can be tested either as complete rings or as segments. The cross-sectional thickness is measured at several marked positions using a low-force optical or dial gauge to avoid artificial deformation during measurement.

Finished Gaskets and Seals

Actual production components—flat flange gaskets, corrugated sealing rings, rubber-coated cork or fiber gaskets, and custom-molded seals—can be evaluated in purpose-built fixtures that reproduce the compression ratio and geometry of the real joint. Testing finished parts verifies both material quality and part design.

Specimen Conditioning

All specimens should be vulcanized and conditioned prior to testing. A minimum interval between vulcanization and testing (typically 16 hours) and conditioning at standard laboratory atmosphere (23 ± 2 °C, 50 ± 5% relative humidity for at least 3 hours) are required to ensure dimensional and mechanical stability before the original thickness is recorded.

Testing Items

The core test item is compression set in transformer oil at elevated temperature. A complete evaluation program typically includes the following supporting items:

• Compression set in transformer oil at specified temperature and duration (primary item)
• Compression set in air under identical conditions, used as a baseline for comparison
• Volume change and mass change of the rubber after oil immersion
• Hardness change (Shore A) before and after immersion and compression
• Tensile strength retention and elongation at break retention after oil aging
• Compatibility assessment of the oil after contact with the rubber, including breakdown voltage and acid value of the contacted oil
• Visual inspection for swelling, cracking, blistering, or surface tackiness

Testing Methods

Apparatus and Fixtures

The test uses a compression fixture consisting of two parallel steel plates with rigid spacers (limit stops) that control the compressed height. A precision thickness gauge with 0.01 mm resolution and a low-force measuring foot is used for dimensional measurement. A forced-air oven or oil bath capable of maintaining temperature within ± 2 °C holds the fixtures during the aging period.

Specimen Preparation

The original thickness of each specimen is measured at three points and the median value recorded as the original thickness. The deflection is calculated and spacers of the corresponding height are selected. A compression of 25% of the original thickness is the standard condition, although 15% or other values may be specified for particular seal designs.

Immersion and Aging Conditions

The assembled fixture, with the specimen compressed between the plates, is fully immersed in transformer oil conforming to IEC 60296, ASTM D3487, or GB 2536. The oil volume should be generous relative to the rubber volume, typically at least 15 to 20 times, to ensure the chemical environment remains representative. Commonly specified test conditions include:

• 100 °C × 24 h or 100 °C × 70 h for general-purpose NBR sealing compounds
• 125 °C × 24 h or 135 °C × 24 h for high-temperature and heat-shrinkable sealing materials
• 168 h and extended 1000 h exposures for accelerated life assessment

Recovery and Measurement

After the aging period, the fixture is removed from the oven and the specimen is released. Two measurement approaches are defined in ISO 815-1 and ASTM D395: measurement immediately at test temperature (Method A) or, more commonly, measurement after recovery for 30 minutes at standard laboratory temperature of 23 °C (Method B). The recovered thickness is measured at the same marked positions as the original measurement.

Calculation of Compression Set

Compression set expressed as a percentage of the applied compression is calculated as: CS (%) = (t0 − t1) / (t0 − ts) × 100, where t0 is the original thickness, t1 is the recovered thickness after the test, and ts is the spacer thickness defining the compressed height. Some specifications alternatively express compression set as a percentage of the original thickness, calculated as CS (%) = (t0 − t1) / t0 × 100; the calculation basis must always be stated in the report.

Acceptance Criteria and Reporting

Acceptance limits depend on the material class and product specification. For oil-resistant NBR gaskets in transformer applications, compression set values of 50% or less after 24 h at 100 °C in transformer oil are commonly required, while premium compounds and fluoroelastomers may be specified with tighter limits or tested at higher temperatures. A minimum of three specimens per condition is recommended, and the median value is reported. The test report should identify the material and specimen type, the oil specification, the deflection, the temperature and duration, the measurement method, and all individual and median results.

Applications

Compression set testing of rubber in transformer oil supports a wide range of engineering and quality activities across the electrical equipment industry:

• Qualification of sealing compounds for power transformer flanges, radiator joints, butterfly valves, and butterfly-valve sealing plates
• Verification of O-rings and gaskets for transformer bushings, cable boxes, and oil-sampling valves
• Material evaluation for on-load tap changers, oil-immersed reactors, and switchgear enclosures
• Incoming quality control and supplier comparison for rubber gasket manufacturers serving transformer OEMs
• Failure analysis of leaking joints and root-cause investigation of seal degradation in service-aged transformers
• Accelerated aging studies and extrapolation of long-term sealing performance for life assessment
• Supporting data for product certification against IEC, EN, GB, and ASTM-based technical specifications

FAQ

Why is the test performed in transformer oil instead of only in air? Transformer oil penetrates the rubber network and causes swelling, extraction of plasticizers and antidegradants, and chemical changes that do not occur in dry heat aging. Air-only compression set data can substantially overestimate the sealing capability of a compound in actual transformer service, so immersion testing provides far more realistic results.

Which oil should be used for the test? Unused mineral insulating oil conforming to IEC 60296, ASTM D3487, or GB 2536 is used for standard qualification. In-service aged oil may be used when the purpose of the test is to study compatibility with a specific operating environment.

What are the most common test conditions? Typical conditions are 100 °C for 24 h or 70 h for nitrile rubber, 125 °C to 135 °C for 24 h for high-temperature compounds, and 168 h or 1000 h for long-term aging evaluation, always with 25% compression unless the product standard specifies otherwise.

What compression set value is considered acceptable? For most transformer sealing applications, a compression set of 50% or less of the applied deflection after standard aging in oil is considered acceptable, with values below 35% indicating excellent long-term sealing reliability. The specific limit must always be agreed against the relevant product or purchase specification.

How many specimens are required? At least three specimens per test condition are recommended to ensure statistical validity, with the median value reported as the result in accordance with ISO 815-1 and ASTM D395.

Can finished gaskets be tested instead of standard buttons? Yes. Testing actual production seals in a fixture that reproduces the joint geometry and compression ratio is strongly recommended during design validation, because part geometry, surface finish, and cure state all influence the compression set performance of the finished seal.