PET high-temperature insulation film is a critical dielectric material used in motors, transformers, flexible electronics, and battery systems, where long-term reliability directly depends on accurate service life prediction. This article provides a comprehensive guide to PET high-temperature insulation film service life evaluation, covering testing standards, sample preparation, evaluation items, accelerated aging methodologies, and industrial applications.
Technical Overview
PET (polyethylene terephthalate) insulation film is a biaxially oriented polyester film widely used as a dielectric and barrier material in electrical and electronic applications. Because PET is a thermoplastic polymer with a glass transition temperature of approximately 78–80°C and a melting point near 255°C, its mechanical and dielectric properties degrade gradually when exposed to elevated temperatures over long periods. Service life evaluation is therefore essential to determine how long the film can reliably perform its insulating function under specific thermal stress conditions.
The service life evaluation of PET high-temperature insulation film is primarily governed by thermal endurance standards such as IEC 60216 (electrical insulating materials – determination of thermal endurance properties) and UL 746B (relative temperature index for polymeric materials). These frameworks use accelerated thermal aging at multiple elevated temperatures, followed by extrapolation using the Arrhenius model, to determine key endurance parameters.
- Temperature Index (TI): the temperature, expressed in degrees Celsius, at which the material reaches a defined end-of-life criterion after 20,000 hours (approximately 2.3 years) of exposure.
- Relative Temperature Index (RTI): the UL-certified equivalent, assigned by comparison with a reference material of known performance.
- Halving Interval (HIC): the temperature interval corresponding to a 50% reduction in lifetime, which reflects the slope of the thermal endurance curve.
- Thermal Class: plain PET film is typically rated Class E or B (120–130°C); stabilized or coated PET products can reach Class F (155°C) or higher.
A scientifically sound service life evaluation allows manufacturers and end users to select the correct film grade, define safe operating envelopes, support insulation system certification, and satisfy reliability requirements in demanding industries such as automotive electrification, renewable energy, and aerospace.
Testing Samples
Sample selection and preparation significantly influence the accuracy of service life evaluation. Representative material grades and thicknesses must be tested under controlled conditioning before aging begins.
Plain Biaxially Oriented PET Film
Standard electrical-grade BOPET films in common thicknesses of 23 µm, 25 µm, 36 µm, 50 µm, 75 µm, 100 µm, 125 µm, 188 µm, and 250 µm are tested in both machine direction (MD) and transverse direction (TD) because orientation creates anisotropic mechanical behavior. Specimens are die-cut into standardized strips and electrical test coupons.
Heat-Stabilized PET Film
Thermally stabilized grades that have undergone heat setting above 200°C exhibit reduced residual shrinkage and improved dimensional stability at operating temperature. These are evaluated separately because pre-stabilization changes the degradation kinetics and typically extends thermal endurance compared with unstabilized film.
Functional Coated PET Film
PET films coated with acrylic, epoxy, PVDF, or corona-resistant layers are tested as complete systems, since the coating–substrate interface can dominate failure modes. Coated samples must retain their original surface structure during specimen preparation to avoid introducing artificial weak points.
Sample Conditioning
- Condition all specimens at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 24 hours prior to baseline testing, in accordance with ISO 291.
- Record production metadata including lot number, thickness tolerance, and surface treatment type to ensure traceability.
- Prepare statistically valid sample quantities—at least 10 specimens per temperature per aging interval for mechanical tests and 5 for dielectric tests—to support confidence interval calculation.
Testing Items
Service life endpoints are defined by measurable property degradation. The following items constitute the core evaluation portfolio for PET insulation film.
Dielectric Strength
Breakdown voltage is measured per IEC 60243-2 using a short-time or step-by-step method. Thermal aging causes chain scission, oxidation, and micro-void formation that reduce dielectric strength. A typical end-of-life criterion is retention of 50% of the initial dielectric strength value.
Tensile Strength and Elongation at Break
Measured per ISO 527-3, elongation at break is the most sensitive mechanical indicator of embrittlement in PET film. Because elongation declines sharply as molecular weight decreases through hydrolytic and oxidative chain scission, a 50% retention of initial elongation is the most commonly used endpoint criterion in IEC 60216 evaluations.
Electrical Resistance and Dielectric Response
Volume resistivity, surface resistivity, dissipation factor (tan δ), and relative permittivity are measured per IEC 62631-2-1 and IEC 60250 to track the development of ionic impurities and polar degradation products within the polymer matrix.
Thermal Shrinkage
Dimensional change after exposure at 150°C and 180°C for 30 minutes quantifies residual stress relaxation and is critical for applications requiring tight dimensional control such as flexible printed circuits and battery insulation.
Chemical and Structural Characterization
- Intrinsic viscosity / molecular weight: detects chain scission resulting from hydrolysis and thermooxidation.
- DSC analysis: monitors crystallinity changes, cold crystallization shifts, and melting point depression.
- TGA weight loss: tracks volatile degradation products and provides inputs for rapid lifetime estimation models.
- FTIR carbonyl index: quantifies oxidative degradation by tracking the growth of carbonyl absorption bands near 1715 cm⁻¹.
Testing Methods
The core of PET high-temperature insulation film service life evaluation is multi-temperature accelerated aging combined with statistical extrapolation to operating conditions.
Conventional Multi-Temperature Aging Procedure
- Select at least three, preferably four, aging temperatures spaced 10–20°C apart, chosen so predicted failure times range from approximately 100 hours at the highest temperature to about 5,000–20,000 hours at the lowest.
- Expose specimens in forced-ventilation ovens with temperature uniformity of ±2°C, with fresh air exchange maintained to provide consistent oxygen supply for thermooxidative conditions.
- Withdraw aged specimen sets at predetermined intervals and perform the selected diagnostic tests within 24 hours of removal.
- Fit property-versus-time curves at each temperature to determine the time to reach the endpoint criterion, typically using a second-order (curvilinear) regression when early rapid change is followed by a plateau.
Arrhenius Extrapolation and Endurance Graph
The logarithm of failure time is plotted against reciprocal absolute temperature (1/T in Kelvin). The resulting thermal endurance graph yields the regression line from which the Temperature Index and Halving Interval are derived with 95% confidence limits, per IEC 60216-1. Extrapolation should not exceed 2.5–3 times the ratio of lowest test temperature to use temperature to preserve statistical validity.
Rapid Screening Methods
- TGA-based estimation (Toop method): uses the temperature at 5% weight loss at multiple heating rates to approximate long-term thermal endurance for early-stage screening.
- DSC oxidation onset temperature: provides comparative rankings of antioxidant effectiveness between film grades.
- Time-temperature superposition: constructs master curves from short-term data to estimate property evolution over extended horizons.
Hydrolytic Aging and Combined Stress Testing
PET is particularly susceptible to hydrolysis, in which water molecules attack ester linkages and accelerate molecular weight reduction. Because many applications involve simultaneous heat and humidity, evaluations should include exposure at 85°C/85% RH or 121°C/100% RH (pressure cooker) conditions, and where relevant, combined thermal-cycling and electrical-stress protocols that replicate actual operating environments more faithfully than dry heat alone.
Applications
Service life evaluation results directly guide material selection across the major application fields for PET high-temperature insulation film.
- Motor and generator insulation: slot liners, phase insulation, and wedge materials in Class B and Class F rotating machines require verified thermal endurance to guarantee 20,000-hour minimum life at rated temperature.
- Transformer and coil insulation: interlayer and wrap insulation benefits from TI data to coordinate film life with varnish and wire enamel endurance in complete insulation systems.
- Flexible printed circuits: PET base films and coverlayers rely on dimensional stability and mechanical retention data for reflow survival and long-term flexing reliability.
- Lithium-ion battery insulation: barrier films and cell-level insulating layers require validated endurance under combined thermal, humidity, and electrical stress to support automotive safety standards.
- Capacitors and cable wrapping: metallized PET dielectric layers depend on dielectric strength retention data for voltage rating and self-healing performance over design life.
- Aerospace and rail transit: weight-sensitive insulation applications demand lightweight films with certified long-term thermal performance in harsh environments.
FAQ
What is the typical service life of PET high-temperature insulation film?
At its rated temperature index, PET film is expected to retain critical properties beyond 20,000 hours. Operating 8–10°C below the rated index roughly doubles the expected life according to typical Halving Interval values of 8–12°C, while exceeding the rating dramatically shortens life.
How is the Temperature Index of PET film determined?
The TI is determined by aging specimens at multiple elevated temperatures, measuring the time for a key property (usually elongation at break or dielectric strength) to fall to 50% of its initial value, and extrapolating the Arrhenius plot to the temperature corresponding to 20,000 hours, following IEC 60216.
Does humidity significantly reduce PET film lifetime?
Yes. PET undergoes hydrolytic degradation at ester bonds, and high humidity at elevated temperature can reduce lifetime by an order of magnitude compared with dry thermal exposure. Humidity aging should be included whenever the application involves a humid environment.
Can coated PET films be evaluated using the same methods?
The aging framework is identical, but coated films must be tested as complete systems with appropriate endpoint properties, because coating degradation, adhesion loss, or interfacial phenomena may control failure rather than the PET substrate itself.
How reliable are accelerated aging predictions?
When test temperatures are properly selected, endpoint criteria are well-defined, and extrapolation ratios are respected, Arrhenius-based predictions provide good engineering accuracy. Deviation from simple Arrhenius behavior can occur at very high test temperatures where degradation mechanisms change, which is why at least three aging temperatures and statistical confidence intervals are required.
Which standards govern PET insulation film life evaluation?
The principal standards are IEC 60216 series for thermal endurance determination, UL 746B for relative temperature index assignment, IEC 60243 for dielectric strength, ISO 527-3 for tensile testing, and IEC 62631 for resistivity measurements.