Freon Resistance Testing of Polyimide (PI) High-Temperature Films: Experimental Methods, Evaluation

Polyimide (PI) high-temperature films are widely used as electrical insulation in hermetic compressors and refrigeration systems where direct contact with Freon refrigerants and lubricating oils is unavoidable. This article provides a comprehensive technical guide to the Freon resistance experiment for PI films, covering sample preparation, sealed-tube immersion test procedures, evaluation criteria, typical results, and practical applications. The information is intended for materials engineers, insulation system designers, and quality-control specialists who need to qualify PI films for refrigerant-exposed, high-temperature service.

Technical Overview

Polyimide (PI) film is an aromatic, fully imidized polymer film produced by casting a polyamic-acid precursor and thermally imidizing it at elevated temperatures. The rigid imide ring structure gives the material an exceptional combination of thermal stability, dielectric performance, and chemical inertness, which is why it is the material of choice for insulation in motors, transformers, and flexible electronics operating at high temperatures.

What Is PI High-Temperature Film

Commercial PI high-temperature films, typified by PMDA-ODA chemistry, can operate continuously at temperatures in the range of 200 to 260 degrees Celsius depending on grade, with short-term excursions well above 300 degrees Celsius. Key baseline properties include:

• High tensile strength, typically on the order of 200 to 250 MPa for standard grades
• Excellent dielectric strength, generally in the range of 150 to 300 kV/mm depending on film thickness
• Flame resistance with a UL 94 V-0 rating and high limiting oxygen index
• Broad chemical resistance to oils, solvents, and most refrigerants
• Low outgassing and stable dielectric properties across a wide frequency and temperature range

Why Freon Resistance Matters

In hermetic and semi-hermetic motor-compressors used in refrigerators, freezers, air conditioners, and heat pumps, the motor winding insulation is permanently immersed in a mixture of refrigerant and lubricating oil. Depending on the system, this environment may involve R-22, R-134a, R-32, R-410A, R-404A, or newer low-GWP refrigerants, combined with mineral oil, alkylbenzene, polyol ester (POE), or polyvinyl ether (PVE) lubricants. Local winding temperatures can reach 130 to 180 degrees Celsius, and the refrigerant may exist at autogenous pressures of several megapascals. Insulation films that soften, embrittle, hydrolyze, or lose dielectric strength in this environment will cause premature compressor failure. The Freon resistance experiment is therefore a core qualification test for any PI film intended for sealed refrigeration systems.

Testing Samples

Sample Types

The experiment is normally performed on representative production lots of PI film. Typical test programs include:

• Standard PMDA-ODA PI film in thicknesses of 25, 50, 75, and 125 micrometers
• Heat-stabilized or low-shrinkage PI grades for dimensionally critical applications
• Corona-resistant PI films for inverter-driven compressor motors subjected to high dv/dt PWM voltages
• Comparative reference films, such as PET or PEN, to benchmark performance

Specimen Preparation and Conditioning

Tensile specimens are cut as strips or dumbbells in both machine and transverse directions in accordance with ASTM D882 or ISO 527-3. Dielectric specimens are prepared as flat sheets of sufficient area for the electrode configuration specified in IEC 60243-1 or ASTM D149. All specimens are marked, cleaned with lint-free wipes, and pre-dried at 100 to 120 degrees Celsius to remove absorbed moisture, because residual water promotes hydrolysis and biases the results. Before exposure, baseline properties are measured after conditioning at 23 degrees Celsius and 50 percent relative humidity for at least 24 hours. A minimum of five replicate specimens per condition is recommended, together with unexposed control specimens and thermal-aging-only controls.

Testing Items

The Freon resistance experiment evaluates changes in the critical properties of the PI film before and after exposure to refrigerant and refrigerant-oil mixtures. The standard test items are:

• Visual appearance: color change, blistering, crazing, delamination, or surface tack
• Tensile strength and elongation at break, reported as percentage retention of baseline values
• Dielectric breakdown strength under AC ramp voltage
• Mass change, measured to 0.1 mg and reported as a percentage
• Dimensional change and heat-shrinkage behavior
• Flexibility retention, assessed by a mandrel bend test at small mandrel diameters
• Optional items: volume resistivity, dielectric constant, dissipation factor, and surface roughness

Testing Methods

Sealed-Tube Immersion Test

The industry-standard approach is the sealed-tube (bomb) test, consistent with the principles of UL 1446 and IEC 61857 for sealed-system insulation evaluation. Film specimens are placed in heavy-wall glass ampoules or stainless-steel pressure vessels together with a weighed quantity of lubricating oil. Liquid refrigerant is then charged into the vessel by mass, typically establishing a defined refrigerant-to-oil and refrigerant-to-sample mass ratio, and the vessel is hermetically sealed. A leak check is performed before the assembly is placed in a forced-convection oven. Because refrigerants such as R-22 and R-134a exceed their critical temperatures at typical test conditions, internal pressures can reach several megapascals, so only rated pressure hardware with appropriate shielding may be used.

Test Media and Exposure Conditions

Typical exposure conditions are selected to match or accelerate the intended service environment:

• Media: R-22 with mineral oil, R-134a with POE oil, R-410A or R-32 with POE oil, and refrigerant-only blanks
• Temperature: 150 degrees Celsius as the standard condition, with 120 degrees Celsius for milder service simulation and 175 degrees Celsius for accelerated aging
• Duration: 168, 336, or 672 hours, with extended programs up to 1,344 hours for lifetime extrapolation
• Pressure: autogenous pressure generated by the refrigerant charge at the test temperature

Post-Exposure Handling and Measurement

After exposure, vessels are cooled to room temperature and carefully vented in a fume hood or refrigerant recovery system. Specimens are removed, rinsed with a clean volatile solvent to remove surface oil, dried, and reconditioned at standard laboratory atmosphere. All mechanical and electrical measurements are completed within 24 hours of removal to minimize post-exposure property drift. The lubricant may also be retained for acid-number analysis, since hydrolysis of POE oils produces acidic species that accelerate polymer degradation.

Applicable Reference Standards

The experiment is typically executed with reference to UL 1446 for sealed-system insulation, IEC 61857-1 for insulation system evaluation, ASTM D882 or ISO 527-3 for tensile properties, ASTM D149 or IEC 60243-1 for dielectric breakdown, and internal specifications of compressor manufacturers for media ratios and acceptance limits.

Evaluation Criteria and Typical Results

Retention of each property is calculated as the post-exposure value divided by the baseline value, multiplied by 100 percent. Common acceptance criteria require tensile strength retention of at least 50 to 70 percent, dielectric strength retention of at least 75 percent, mass change within plus or minus 2 percent, and no visible cracking after a mandrel bend test. Representative results for fully imidized PI film are summarized below.

• Tensile strength retention above 90 percent after 168 hours at 150 degrees Celsius in R-134a/POE mixture, and above 80 percent after 672 hours
• Elongation retention above 80 percent for short exposures, with a gradual decline at longer durations
• Dielectric strength retention typically above 90 percent, confirming that breakdown performance is largely unaffected
• Mass change generally below 1 percent, indicating minimal sorption or extraction
• No crazing, blistering, or cracking observed in mandrel bend tests after exposure

For comparison, PET film under identical conditions commonly falls below 50 percent tensile retention due to hydrolytic and plasticizing degradation, which is precisely why PI film dominates in hermetic compressor insulation. The dominant degradation mechanism in PI is slow hydrolysis of the imide linkage, which is accelerated by moisture contamination and acidic oil decomposition products; well-sealed, dry test charges therefore yield the most service-representative data.

Applications

Qualification through the Freon resistance experiment supports the use of PI high-temperature film across several demanding applications:

• Slot liner, phase, and interlayer insulation in hermetic and semi-hermetic compressor motors for household and commercial refrigeration
• Wrap and barrier insulation for magnet wire and inverter-driven compressor windings exposed to PWM voltage stress
• Flexible printed circuits and flat cable insulation in appliances where incidental refrigerant or oil contact is possible
• Electrical insulation in equipment cooled by fluorinated heat-transfer fluids in aerospace, power electronics, and data-center thermal management
• Coil and transformer insulation in systems transitioning to low-GWP refrigerants such as R-32, R-454B, R-1234yf, and R-290, where requalification testing is required

FAQ

Which refrigerants can be tested in the PI film Freon resistance experiment?

Any refrigerant compatible with the pressure hardware can be evaluated, including legacy chlorofluorocarbons and hydrochlorofluorocarbons such as R-12 and R-22, hydrofluorocarbons such as R-134a, R-32, and mixtures such as R-410A, R-404A, and R-407C, as well as low-GWP alternatives like R-1234yf. Because refrigerants such as R-22 are regulated under the Montreal Protocol, laboratory testing should use reclaimed material and employ proper recovery practices.

What exposure temperature and duration should be selected?

The condition is chosen to reflect the insulation class of the end product. A common screening condition is 150 degrees Celsius for 168 hours, while accelerated programs may use 175 degrees Celsius for 336 to 672 hours to support long-term thermal-endurance extrapolation. The exposure temperature should never exceed a level at which the refrigerant or lubricant undergoes unrepresentative self-decomposition.

How is a pass or fail judgment made?

A film passes when all post-exposure properties remain above the agreed retention limits, typically tensile strength retention of at least 50 to 70 percent, dielectric strength retention of at least 75 percent, and no mechanical damage or cracking in bend tests. Compressor manufacturers may impose stricter internal limits depending on the winding design and duty cycle.

Does PI film degrade during long-term service in Freon?

Fully aromatic PI film is highly resistant to pure refrigerants, and field experience in hermetic compressors spanning decades confirms excellent durability. The main long-term risk is hydrolytic degradation driven by residual moisture and acidic by-products from hydrolyzed ester lubricants, which is why system dryness and correct oil selection are essential alongside film qualification.

Why is the sealed-tube method preferred over simple immersion?

The sealed tube reproduces the actual sealed-system environment, combining refrigerant, oil, elevated temperature, and autogenous pressure while preventing refrigerant loss and moisture ingress. Open-beaker immersion cannot maintain refrigerant contact above its boiling point and therefore does not simulate real service conditions.

What safety precautions apply to the experiment?

Refrigerants at test temperature generate high pressure, so only rated vessels, shields, and remote handling procedures should be used. Venting must be performed in a fume hood or through a recovery unit, thermal gloves and face protection are required when handling hot pressure vessels, and laboratories must comply with local regulations on refrigerant handling, recovery, and disposal.