PET high-temperature insulation film heat resistance performance testing evaluates how biaxially oriented polyester films withstand prolonged thermal stress in motors, transformers, and electronic devices. This technical guide explains the thermal aging methods, heat shrinkage measurements, dielectric strength evaluations, and international standards used to determine the temperature index and long-term service limits of PET insulation films.
Technical Overview
PET (polyethylene terephthalate) insulation film is a biaxially oriented polyester film widely used as electrical insulation in motors, transformers, coils, flexible printed circuits, cables, and lithium-ion battery assemblies. Its combination of high dielectric strength, excellent mechanical toughness, chemical resistance, and relatively low cost makes it one of the most consumed insulating materials in the electrical industry.
However, PET is a thermoplastic material with a glass transition temperature of approximately 78-80°C and a melting point of approximately 255°C. When operated continuously at elevated temperatures, the film undergoes gradual physical and chemical degradation, including molecular chain scission, oxidation, crystallinity changes, and dimensional shrinkage. Heat resistance performance testing is therefore essential to verify whether a specific PET film grade can reliably serve at its rated thermal class, which typically ranges from Class E (120°C) for standard film to Class B (130°C) or higher for heat-stabilized grades under IEC 60085.
The objectives of heat resistance testing are threefold: to confirm short-term dimensional stability under thermal stress, to quantify the retention of mechanical and electrical properties after long-term thermal aging, and to establish the temperature index (TI) that defines the maximum continuous operating temperature for a service life of approximately 20,000 hours.
Testing Samples
Standard BOPET Insulation Film
Conventional biaxially oriented PET film in common electrical grades, with thicknesses typically ranging from 23 μm to 350 μm. Representative thicknesses such as 50 μm, 100 μm, and 188 μm are selected to cover thin, medium, and heavy insulation applications.
Heat-Stabilized PET Film
Annealed or chemically stabilized grades with reduced internal stress, designed to minimize thermal shrinkage in applications such as flexible printed circuits and high-temperature slot liners.
Functional Coated or Modified PET Film
Films with corona treatment, flame-retardant coatings, antistatic layers, or inorganic fillers, whose thermal endurance may differ from unmodified base film and therefore require independent verification.
Prior to testing, all specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 24 hours in accordance with ISO 291 or ASTM D618. Specimens are cut in both machine direction (MD) and transverse direction (TD) to capture anisotropic behavior introduced by the biaxial orientation process.
Testing Items
A complete heat resistance evaluation of PET insulation film typically includes the following test items:
- Long-term accelerated thermal aging and temperature index (TI) determination
- Heat shrinkage at elevated temperature (MD and TD)
- Tensile strength and elongation at break retention after thermal exposure
- Dielectric strength (AC breakdown voltage) before and after aging
- Insulation resistance and volume resistivity after thermal exposure
- Weight loss and thermal decomposition behavior via thermogravimetric analysis (TGA)
- Glass transition temperature, melting point, and crystallinity changes via differential scanning calorimetry (DSC)
- Dimensional stability and visual assessment for embrittlement, discoloration, curling, or delamination
Testing Methods
Accelerated Thermal Aging Test (IEC 60216 Series)
The thermal endurance of PET film is determined according to IEC 60216-1 and IEC 60216-2. Specimens are exposed in forced-air aging ovens at a minimum of three elevated temperatures, for example 180°C, 200°C, 220°C, and 240°C, selected so that the diagnostic property degrades to its endpoint criterion over exposure times ranging from roughly 100 hours to several thousand hours. The diagnostic property is typically tensile strength, elongation at break, or dielectric breakdown voltage, with the endpoint defined as 50% retention of the initial value.
The logarithm of the time-to-endpoint is plotted against reciprocal absolute temperature (1/T) in an Arrhenius diagram. Linear regression yields the thermal endurance curve, from which the temperature index (TI) and the halving interval (HIC) are derived. The TI represents the temperature at which the film survives approximately 20,000 hours before reaching the endpoint criterion.
Heat Shrinkage Test (ASTM D1204 / IEC 60674-2)
Square specimens of 100 mm × 100 mm are marked with reference lines, then suspended freely in a circulating air oven at a specified temperature, commonly 150°C or 180°C, for 30 minutes. After conditioning back to room temperature, dimensional changes in MD and TD are calculated. Standard grades are generally required to exhibit shrinkage below 3-5% at 150°C, while heat-stabilized grades should remain below 1-2% at the same condition. Excessive shrinkage indicates residual orientation stress and risks insulation displacement in wound or laminated structures.
Dielectric Strength Testing (IEC 60243-1 / ASTM D149)
AC breakdown voltage is measured using a short-time (rapid rise) or 20-second step-by-step method with cylindrical electrodes or sphere-plane electrode configurations immersed in insulating oil to prevent surface flashover. Testing is performed on both aged and unaged specimens. A well-performing PET film should retain at least 75-80% of its initial dielectric strength after extended thermal exposure, with typical unaged values of 150-300 kV/mm depending on thickness.
Tensile Property Retention (ISO 527-3 / ASTM D882)
Tensile strength and elongation at break are measured on strip specimens before aging and after defined aging intervals. Elongation at break is the most sensitive aging indicator for PET film because embrittlement from chain scission and increased crystallinity reduces ductility long before strength declines significantly. Retention of elongation above 50% of the initial value is a common acceptance criterion.
Thermal Analysis (DSC, TGA, TMA)
DSC verifies glass transition temperature, melting point, and cold crystallization behavior, and detects post-aging crystallinity growth that correlates with embrittlement. TGA measures the onset temperature of thermal decomposition, typically above 380°C for PET, and quantifies isothermal weight loss during long-term exposure. TMA evaluates dimensional change and softening behavior under load. These analytical results supplement the aging data by revealing the physical mechanisms behind degradation.
Evaluation Criteria
A PET insulation film is considered to pass heat resistance evaluation when the temperature index meets or exceeds its declared thermal class, heat shrinkage remains within specification at the rated temperature, dielectric strength and elongation retention satisfy the acceptance thresholds after long-term aging, and no visible embrittlement, cracking, or delamination is observed.
Applications
Verified heat-resistant PET insulation film is used across many demanding electrical and electronic applications:
- Slot liner, phase insulation, and interlayer insulation in motors and generators
- Layer and wrap insulation in transformers, choke coils, and relays
- Base film for flexible printed circuits and flexible copper-clad laminates
- Barrier and insulation layers in lithium-ion battery cells and packs
- Wire and cable wrapping tape for thermal and dielectric protection
- Insulating substrates for heating elements, power supplies, and household appliances
- Masking and release applications requiring dimensional stability during heat processing
FAQ
What is the maximum continuous operating temperature of PET insulation film?
Standard PET film is generally rated for continuous operation at approximately 105-120°C (up to Class E), while heat-stabilized grades can achieve Class B rating at 130°C. Actual limits must be confirmed through thermal endurance testing to IEC 60216, since the rating depends on film grade, thickness, and the diagnostic property selected.
How is the temperature index (TI) of PET film determined?
The TI is determined by aging specimens at multiple elevated temperatures until a diagnostic property, usually elongation at break or dielectric strength, falls to 50% of its initial value. An Arrhenius plot of exposure time versus reciprocal temperature is extrapolated to 20,000 hours, and the corresponding temperature is reported as the TI.
Which standards govern heat resistance testing of PET insulation film?
The key standards include IEC 60216 for thermal endurance determination, IEC 60674 for plastic films for electrical purposes, IEC 60243 and ASTM D149 for dielectric strength, ASTM D1204 for heat shrinkage, ISO 527-3 and ASTM D882 for tensile properties, and IEC 60085 for electrical insulation thermal classification.
Why does elongation at break decrease faster than tensile strength during aging?
Thermal aging causes molecular chain scission and increased crystallinity in PET, which embrittles the film and reduces ductility first. Tensile strength often remains close to its initial value even after elongation has dropped significantly, making elongation retention the more conservative and sensitive indicator of thermal degradation.
How does PET compare with PEN or polyimide film for high-temperature insulation?
PET offers the best cost-performance balance up to approximately 120-130°C. PEN film extends reliable service toward 155-160°C, while polyimide film serves continuous temperatures of 180°C and above. For applications exceeding the validated thermal endurance of PET, upgrading to PEN or polyimide film is recommended.