PET High-Temperature Insulation Film Performance Testing: A Comprehensive Technical Guide

PET high-temperature insulation film performance testing is essential for verifying the dielectric, thermal, and mechanical reliability of biaxially oriented polyester films used in motors, transformers, batteries, and flexible electronics. This guide explains the key testing items, internationally recognized standards such as IEC 60674 and ASTM D149, and the equipment and procedures required to qualify PET insulation films for demanding high-temperature applications.

Technical Overview

PET high-temperature insulation film is a biaxially oriented polyethylene terephthalate (BOPET) film engineered for electrical insulation applications where sustained elevated temperatures, high dielectric stress, and mechanical stability are required. Thanks to its excellent dielectric strength, high tensile properties, strong chemical resistance, and relatively low cost compared with polyimide films, PET film has become one of the most widely used flexible insulation materials in the electrical and electronics industries.

Standard PET insulation film typically offers a continuous operating temperature range of approximately -60°C to 120°C, while specially heat-stabilized grades can endure long-term exposure at up to 130-155°C depending on the thermal class assigned by thermal endurance testing. Because film performance degrades progressively at elevated temperatures through oxidation, hydrolysis, and physical shrinkage, systematic laboratory testing is indispensable for confirming whether a given film lot meets the requirements of the intended application. Performance testing evaluates the film under both ambient and accelerated aging conditions to simulate years of service in a compressed time frame.

Testing Samples

Standard Thickness Grades

Test specimens are normally drawn from commercially standard thickness grades, including 23 µm, 25 µm, 36 µm, 50 µm, 75 µm, 100 µm, 125 µm, 188 µm, and 250 µm. Because dielectric strength, heat shrinkage, and tensile behavior all vary with thickness, each tested lot must be clearly identified by nominal thickness and measured actual thickness according to ASTM D374 or IEC 60674-2. Samples are cut from the machine direction (MD) and transverse direction (TD) separately whenever anisotropic properties are being evaluated.

Conditioning Before Testing

Prior to testing, specimens must be conditioned in a standard atmosphere of 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 24 hours, following ASTM D618 or ISO 291. Conditioning is critical because moisture content measurably influences dielectric loss, volume resistivity, and elongation at break. Specimens intended for high-temperature tests are additionally pre-dried at 105°C for 1 to 2 hours when the applicable specification requires moisture-free reference values.

Visual and Dimensional Inspection

Each specimen batch undergoes incoming inspection for surface defects such as gels, scratches, pinholes, corrugations, and contamination, since even microscopic flaws can initiate dielectric breakdown. Thickness is measured at multiple points across the web width, and width, length, and roll uniformity are documented. Only film that passes dimensional tolerance, typically ±5% on thickness for insulation-grade BOPET, proceeds to the electrical and thermal test program.

Testing Items

A complete performance test program for PET high-temperature insulation film covers electrical, thermal, mechanical, and physicochemical properties:

• Dielectric breakdown voltage and dielectric strength
• Volume resistivity and surface resistivity
• Dielectric constant (relative permittivity) and dissipation factor
• Tensile strength, yield strength, and elongation at break in MD and TD
• Heat shrinkage at 150°C and 180°C
• Long-term thermal endurance and temperature index
• Melting point and thermal transitions by differential scanning calorimetry (DSC)
• Flammability rating for thin films
• Water absorption and moisture resistance
• Chemical resistance to insulating oils, electrolytes, and cleaning solvents
• Corona resistance and partial discharge characteristics for inverter-duty applications

Testing Methods

Dielectric Breakdown Testing

Dielectric strength is measured in accordance with ASTM D149 or IEC 60243-1 using a rapid-rise, slow-rise, or step-by-step voltage application. Specimens are placed between cylindrical or spherical electrodes immersed in an insulating medium to prevent flashover, and the breakdown voltage is recorded at the moment of failure. Breakdown values are converted to dielectric strength in kV/mm and statistically analyzed using Weibull distribution methods. Well-manufactured 25 µm PET insulation film typically achieves a dielectric strength above 250 kV/mm, with the value decreasing as thickness increases due to volume effects.

Resistivity and Dielectric Measurements

Volume resistivity and surface resistivity are determined by ASTM D257 or IEC 60167 using a guarded three-terminal electrode assembly and a sensitive electrometer applying 100 V or 500 V DC. Insulation-grade PET film generally exhibits volume resistivity exceeding 10 to the 14th power ohm·cm at ambient conditions, with measurements repeated at elevated temperatures such as 130°C to characterize decay at operating temperature. Dielectric constant and dissipation factor are measured at 1 kHz and 1 MHz per ASTM D150 or IEC 60250, with typical dissipation factor values below 0.5% at room temperature.

Mechanical Property Testing

Tensile strength and elongation are measured per ASTM D882 or ISO 527-3 using strip specimens of 10 mm or 15 mm width with a gauge length of 100 mm and a crosshead speed of 100 mm/min or 50 mm/min as specified. Biaxially oriented PET film commonly shows tensile strength of 200-260 MPa in both directions. Retained tensile strength after thermal aging is a key acceptance criterion, with most specifications requiring at least 50% retention after prolonged aging at the rated temperature.

Thermal Aging and Heat Shrinkage

Heat shrinkage is determined by measuring dimensional change of 100 mm square specimens after 30 minutes in a forced-air oven at 150°C or 180°C; thermally stabilized insulation grades should exhibit shrinkage below 1.5-3%. Long-term thermal endurance is evaluated according to IEC 60216 using an aging oven program at multiple elevated temperatures, with breakdown voltage or tensile strength serving as the diagnostic criterion. The resulting Arrhenius extrapolation establishes the temperature index, the temperature at which the film retains 50% of its critical property after 20,000 hours, which determines whether the film qualifies as Class 120 (E), Class 130 (B), or Class 155 (F) insulation material.

Flammability and Supplementary Tests

Flammability Rating

Thin PET films are classified under UL 94 using the VTM (vertical thin material) test methods VTM-0, VTM-1, or VTM-2, involving ignition by a small flame applied to vertically suspended specimens. Flame extinguinguishing time, afterglow, and dripping of cotton-igniting particles are recorded. Flame-retardant grades achieve VTM-0 rating.

Chemical and Environmental Testing

Chemical resistance is assessed by immersing specimens in transformer oil, electrolyte solvents, or weak acids and bases at specified temperatures, followed by retesting of tensile and dielectric properties. Water absorption is measured per ASTM D570, and typical values for PET film remain below 0.8% after 24-hour immersion. For battery and flexible circuit applications, additional tests for electrolyte corrosion resistance, dimension stability after moisture exposure, and surface energy or corona treatment level may be required.

Applications

Qualified PET high-temperature insulation film serves a broad range of electrical insulation and electronic applications:

• Slot, phase, and layer insulation in motors, generators, and transformers
• Insulation wrapping for wires, cables, and busbars
• Barrier and insulation layers in lithium-ion and sodium-ion battery cells and modules
• Base film and coverlay substrates for flexible printed circuits
• Dielectric layers in film capacitors and metallized film components
• Insulation for heating elements, thermistors, and thermal cutoff devices
• Masking and release films in high-temperature lamination and curing processes

In each application, the test results define safe design margins. For example, dielectric breakdown data establish the minimum creepage and insulation thickness in motor slot liners, while temperature index data verify compatibility with Class B or Class F insulation systems in combination with varnishes and resins.

FAQ

What is the maximum operating temperature of PET insulation film?

Standard PET film is generally rated for continuous use at about 105-120°C, while heat-stabilized insulation grades qualified through IEC 60216 thermal endurance testing can be assigned a temperature index up to 130-155°C. The exact rating depends on the measured aging behavior and must not be inferred from short-term oven exposure alone.

What dielectric strength value should PET film deliver?

Insulation-grade BOPET film typically provides dielectric strength in the range of 200-320 kV/mm, decreasing with increasing thickness. A 100 µm film commonly withstands more than 10 kV before breakdown, though acceptance limits should always be defined by the applicable specification such as IEC 60674-3-2.

Which standards govern PET film testing?

The principal standards are IEC 60674-1/-2/-3 for plastic films for electrical purposes, ASTM D149 and IEC 60243-1 for dielectric breakdown, ASTM D257 and IEC 60167 for resistivity, ASTM D882 and ISO 527-3 for tensile properties, IEC 60216 for thermal endurance, and UL 94 for flammability of thin materials.

How is PET film different from polyimide film in high-temperature testing?

Polyimide film withstands continuous temperatures of 200-260°C and offers superior corona resistance, whereas PET film is limited to about 155°C at most but provides a far more economical dielectric solution. Test protocols are similar, but polyimide films are aged and evaluated at significantly higher oven temperatures and are often additionally tested for corona endurance under partial discharge conditions.

Why is heat shrinkage testing important for PET insulation film?

Excessive shrinkage at operating temperature can unwind cable wraps, displace slot liners, and stress joints in wound components. Measuring dimensional change at 150°C and 180°C confirms that the film has been adequately heat stabilized for the intended service temperature and lamination processes.