PET High-Temperature Insulation Film Thermal Aging Test: Standards, Methods, and Evaluation

PET high-temperature insulation film thermal aging tests are essential for verifying the long-term thermal endurance and electrical reliability of polyethylene terephthalate films used in motors, transformers, and electronic devices. This technical guide explains the testing standards, sample preparation, evaluation parameters, and accelerated aging methods used to determine the temperature index of PET insulation films. Understanding these procedures helps manufacturers select the right film grade and certify products for demanding high-temperature applications.

Technical Overview

PET (polyethylene terephthalate) film is one of the most widely used organic insulation materials in the electrical and electronics industry due to its excellent dielectric strength, high mechanical toughness, good dimensional stability, and relatively low cost. In practical service, PET films are frequently exposed to elevated temperatures inside motors, transformers, coils, and flexible circuits, where thermal stress gradually degrades the polymer chains and reduces both mechanical and electrical performance.

Thermal aging testing simulates this long-term degradation process in a compressed timeframe by exposing film specimens to controlled elevated temperatures in precision ovens. At predetermined intervals, samples are removed and measured for key properties such as tensile strength, elongation at break, and dielectric strength. By analyzing how quickly each property declines at several aging temperatures, engineers can extrapolate the thermal endurance profile of the material and establish its temperature index (TI) and thermal class rating.

The testing framework is governed primarily by the IEC 60216 series for determination of thermal endurance properties of electrical insulating materials, along with IEC 60674 for plastic films used in electrical applications and UL 746B for long-term property evaluation of polymeric materials. These standards define aging procedures, endpoint criteria, and statistical data treatment methods that ensure results are comparable between laboratories.

Testing Samples

Standard Film Specimens

Specimens are typically cut from production-grade PET insulation film rolls with common thicknesses ranging from 23 μm to 350 μm. The most frequently evaluated thicknesses are 25 μm, 50 μm, 75 μm, 100 μm, and 188 μm, which cover the majority of slot liner, layer insulation, and flexible laminate applications. Film must be visually inspected before testing to exclude specimens with creases, pinholes, gel particles, or edge damage that could distort results.

Tensile Test Coupons

For mechanical property evaluation, rectangular strips are die-cut in both machine direction (MD) and transverse direction (TD). Typical dimensions are 15 mm width by 150 mm to 200 mm length, with a gauge length of 100 mm as specified in ASTM D882 or ISO 527-3. Both orientations must be tested because PET film is biaxially oriented and exhibits anisotropic behavior during thermal degradation.

Dielectric Specimens

For breakdown voltage and dielectric measurements, specimens are cut into squares or circles large enough to prevent flashover at the electrode edges, typically 100 mm by 100 mm or larger depending on the electrode configuration. Specimens must be conditioned at standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours before initial measurement.

Sample Quantity and Grouping

Thermal aging programs require statistically sufficient sample populations. For each aging temperature, a minimum of five specimens per property per measurement interval is recommended, and enough specimen sets must be prepared to cover all planned sampling checkpoints, typically eight to twelve intervals per temperature point.

Testing Items

A comprehensive thermal aging evaluation of PET insulation film includes the following measurement categories:

• Tensile strength (machine and transverse direction), measured before and after aging to track embrittlement and loss of load-bearing capability
• Elongation at break, which is the most sensitive indicator of thermal degradation in PET film; a 50% retention of initial elongation is the most common endpoint criterion
• Dielectric strength (breakdown voltage), determined by short-time or step-by-step methods under AC conditions
• Volume resistivity and surface resistivity, evaluated as indicators of insulation integrity at elevated temperature
• Dielectric constant (relative permittivity) and dissipation factor, monitored at reference frequencies such as 50 Hz, 1 kHz, and 1 MHz
• Dimensional shrinkage, measured as heat-induced contraction that affects registration in laminated and wound structures
• Visual assessment of discoloration, brittleness, curling, and crystallization haze after each aging cycle
• Weight loss, determined gravimetrically to quantify volatile evolution and oxidative degradation of the polymer

Testing Methods

Selection of Aging Temperatures

According to IEC 60216-1, at least three aging temperatures are required, with four or more recommended for improved confidence. Aging points are selected so that the lowest temperature produces end-of-life in approximately 5,000 hours and the highest temperature produces failure within approximately 100 to 500 hours. For standard PET film with an expected temperature index near 120 to 130, aging temperatures typically span from 150 °C to 200 °C, while heat-stabilized grades intended for Class F (155) service may be aged at higher points such as 180 °C to 220 °C. Temperature spacing is usually set at 10 °C to 20 °C intervals.

Aging Oven Procedure

Specimens are suspended or supported on inert racks in forced-ventilation ovens with temperature uniformity within ± 2 °C of the setpoint and continuous fresh air exchange. The air exchange rate should be at least 3 to 10 volume changes per hour, and data loggers are used to record actual temperature profiles throughout the exposure. Care is taken to prevent specimens from touching each other or the oven walls, and metal contact is avoided to eliminate contamination effects.

Cyclic Sampling and Measurement

At each scheduled checkpoint, one specimen group is withdrawn from the oven, conditioned for at least 16 hours at standard atmosphere, and tested for the selected diagnostic properties. The time intervals are commonly arranged in a geometric progression, such as doubling intervals, so that early rapid degradation and later slow degradation are both captured. Aging continues at each temperature until the median property value of the specimen group falls below the defined endpoint criterion.

Endpoint Criteria

The most widely used endpoint for PET film is 50% retention of the initial elongation at break, because elongation decreases sharply and reproducibly as chain scission and embrittlement progress. Alternative or supplementary criteria include 50% retention of dielectric strength or a specified absolute breakdown voltage threshold, chosen according to the requirements of the end application and the relevant product specification.

Arrhenius Analysis and Temperature Index Determination

For each aging temperature, the median time to reach the endpoint is determined from the property-versus-time curves. The logarithm of the median time to endpoint is then plotted against the reciprocal of absolute temperature (1/T) in accordance with the Arrhenius relationship. The resulting thermal endurance graph yields a straight line from which the time-to-endpoint at any service temperature can be extrapolated. The temperature index is reported as the temperature at which the extrapolated lifetime equals 20,000 hours, and the halving interval (HIC) describes the temperature increase that halves the lifetime. Confidence limits are calculated statistically to express the reliability of the extrapolation.

Quality Control and Verification

Interlaboratory comparison, retention of reference control specimens at room temperature, and periodic calibration of ovens and test instruments are required to validate the aging program. Any anomaly such as oxidation catalyzed by residual catalysts, humidity effects, or thickness-related degradation kinetics should be documented and addressed in the final report.

Applications

Thermal aging test data for PET insulation film directly supports material qualification and design decisions across multiple industries:

• Motor and generator insulation, where PET film serves as slot liner, phase insulation, and turn-to-turn insulation, and the verified thermal class determines the machine temperature rise rating
• Transformer and coil manufacturing, including layer insulation in wound components for power supplies, ballasts, and magnetic components
• Flexible printed circuits and flexible copper-clad laminates, in which heat-stabilized PET film provides the base dielectric layer
• Film capacitors, where long-term dielectric endurance at elevated temperature governs component lifetime predictions
• Lithium battery insulation tapes and barrier films, which require validated performance in sustained heat environments
• Cable wrap and wire insulation systems used in appliance and automotive harness applications
• Certification programs under UL, IEC, and regional mark schemes, where measured temperature indices substantiate product safety file claims

FAQ

What is the typical temperature index of standard PET insulation film?

Standard biaxially oriented PET insulation film typically exhibits a temperature index of approximately 120 to 130, corresponding roughly to Class E or Class B insulation systems. Specially heat-stabilized grades can achieve a temperature index of 155 or higher, qualifying them for Class F applications.

Why is elongation at break used as the main endpoint criterion?

Elongation at break decreases rapidly and consistently as thermal degradation progresses through chain scission and loss of molecular orientation in PET film, making it far more sensitive to early aging than tensile strength or dielectric strength. This sensitivity provides a conservative and statistically stable life prediction basis.

How long does a complete thermal aging test take?

A full multi-temperature aging program conducted per IEC 60216 can take from several months up to one year, because the lowest aging temperature alone may require 5,000 hours or more of exposure. Accelerated screening assessments at a single high temperature can be completed in weeks but do not yield a certified temperature index.

Do moisture and oxygen affect the aging results?

Yes. Oxidative degradation plays a major role in PET thermal aging, so the oven air exchange rate must be controlled and documented. High humidity can accelerate hydrolytic degradation, which is why conditioning atmosphere and oven environment are tightly specified in the test protocols.

What standards should be referenced for PET film thermal aging tests?

The primary references are the IEC 60216 series for thermal endurance determination, IEC 60674-2 for test methods applicable to electrical plastic films, ASTM D882 for tensile testing of thin sheeting, ASTM D149 for dielectric breakdown, and UL 746B for long-term thermal endurance evaluation of polymeric materials.

Can thermal aging results from film be directly applied to finished devices?

Film-level data establish the material thermal endurance foundation, but complete insulation systems should additionally be verified through system-level thermal aging tests that account for interactions between the film, adhesives, varnishes, conductors, and other adjacent materials in the actual device construction.