Long-Term Thermal Aging Evaluation of PET Insulation Films

Long-term thermal aging evaluation is essential for determining the reliable service life, temperature index, and thermal class of PET insulation films used in motors, transformers, cables, and flexible electronics. This article provides a comprehensive technical overview of the degradation mechanisms, testing items, aging protocols, and Arrhenius-based lifetime extrapolation methods specified in IEC 60216, IEC 60674, and UL 746B, helping engineers and quality professionals qualify PET films for demanding electrical insulation applications.

Technical Overview

Polyethylene terephthalate (PET) insulation film is a biaxially oriented polyester film widely used as a dielectric barrier, slot liner, interlayer insulation, and flexible substrate in electrical and electronic products. It offers an excellent combination of dielectric strength, mechanical toughness, chemical resistance, and dimensional stability at a competitive cost. However, when PET film operates continuously at elevated temperatures, it undergoes irreversible physical and chemical changes that progressively reduce its insulating capability.

Thermal degradation of PET film proceeds through several concurrent mechanisms. Thermo-oxidative chain scission breaks the polymer backbone and reduces molecular weight, causing embrittlement and loss of elongation. Hydrolysis of ester linkages accelerates degradation when moisture is present, particularly in the 100 to 160 degrees Celsius range. Post-crystallization and relaxation of the oriented molecular structure cause film shrinkage and dimensional change, while oxidation generates carbonyl byproducts, discoloration, and increased dielectric loss. The combined effect is a steady decline in tensile elongation, dielectric strength, and flexibility until the film can no longer withstand mechanical and electrical stresses in service.

Long-term thermal aging evaluation quantifies this deterioration under controlled laboratory conditions and extrapolates the results to predict service life at operating temperature. The key deliverable is the temperature index (TI) or relative thermal endurance (RTE), together with the halving interval in temperature (HIC), which together define the thermal class of the material. The evaluation framework follows the IEC 60216 series for thermal endurance determination of electrical insulating materials, IEC 60674 for specification of plastic films for electrical purposes, and UL 746B for long-term property ratings of polymeric materials.

Testing Samples

Standard Biaxially Oriented PET Films

Commercial-grade biaxially oriented PET insulation films in thicknesses from 23 micrometers to 350 micrometers form the baseline sample group. Specimens are cut in both machine direction (MD) and transverse direction (TD) to account for anisotropy introduced by the stretching process. Prior to aging, all samples are conditioned at standard laboratory atmosphere, typically 23 degrees Celsius and 50 percent relative humidity, to ensure consistent initial property measurements.

Heat-Stabilized PET Films

Thermally stabilized grades, which have undergone heat setting during manufacture to suppress shrinkage, are tested separately because their aging behavior differs significantly from standard films. These grades are commonly used where dimensional stability above 130 degrees Celsius is required, and their thermal endurance data support extended temperature ratings.

Coated, Treated, and Composite Samples

Samples with corona treatment, acrylic or FEP coatings, printable surfaces, or laminated structures must be evaluated in their finished form, since coatings and interfaces can alter oxygen diffusion, moisture uptake, and degradation kinetics. When the film is qualified as part of an insulation system, such as in combination with impregnating varnish, system-level aging tests supplement the film-only evaluation.

Sample Preparation Requirements

Sufficient specimen quantity must be prepared so that destructive tests can be performed at every planned aging interval without reusing samples. Edges must be clean and free of nicks or cracks that would act as stress concentrators during tensile testing. Each specimen is labeled with a durable coding system that withstands oven exposure, enabling full traceability of exposure temperature, duration, and test direction.

Testing Items

The evaluation tracks a defined set of diagnostic properties, each sampled periodically throughout the aging program.

• Tensile strength and elongation at break retention, the primary end-point criterion for films
• Dielectric strength (breakdown voltage) measured on aged specimens
• Volume resistivity and surface resistivity
• Dissipation factor (dielectric loss tangent) and relative permittivity
• Dimensional stability and heat shrinkage in both MD and TD
• Visual condition assessment, including discoloration, brittleness, curling, and delamination
• Supporting characterization, including crystallinity by DSC, carbonyl index by FTIR, molecular weight distribution by GPC, and thermal stability by TGA

Testing Methods

Aging Protocol per IEC 60216

Aging is conducted in forced-ventilation ovens with temperature uniformity and stability compliant with IEC 60216-2, typically within plus or minus 2 degrees Celsius of the set point. Specimens are exposed simultaneously at a minimum of three, and preferably four or more, discrete aging temperatures selected to produce end-point failure in a suitable spread of times. For PET film, aging temperatures commonly fall between 160 and 220 degrees Celsius. The highest temperature should remain well below the melting range of approximately 255 to 265 degrees Celsius to avoid changing the dominant degradation mechanism. Sampling intervals at each temperature follow a geometric progression, so that each successive interval is a fixed multiple, typically two, of the previous one.

Property Measurement and End-Point Criteria

Tensile testing is performed on aged specimens according to ISO 527-3 or ASTM D882, using strip specimens with appropriate gauge length and crosshead speed. For PET insulation film, the conventional end-point is retention of 50 percent of the initial elongation at break, because elongation is the most sensitive indicator of embrittlement. Alternative end-points, such as 50 percent retention of tensile strength or a specified dielectric strength value, may be applied when justified by the application. Dielectric breakdown is measured using IEC 60243-2 or ASTM D149 with the film immersed in a suitable insulating medium between opposing electrodes. The chosen diagnostic property must be measured identically at every interval to generate valid aging curves.

Data Analysis and Arrhenius Extrapolation

For each aging temperature, property values are plotted against logarithmic exposure time to construct an aging curve, from which the time to reach the end-point criterion is determined. The resulting time-to-end-point values are then plotted against reciprocal absolute temperature on an Arrhenius diagram. Linear regression of the data, performed in accordance with the statistical procedures of IEC 60216-3, yields the thermal endurance line. From this line the temperature index is derived, defined as the temperature at which the estimated life equals 20,000 hours, together with the halving interval in temperature (HIC), which expresses the temperature increase that halves the lifetime. Confidence limits and prediction intervals are reported so the reliability of the extrapolation is transparent.

Supplementary Characterization

To strengthen the mechanistic interpretation, DSC tracks crystallinity evolution and glass transition shifts, FTIR monitors carbonyl growth as an oxidation marker, and GPC quantifies the decline in molecular weight. TGA under nitrogen and air differentiates thermal decomposition from oxidative degradation. These techniques help confirm that the aging mechanism remains consistent across the temperature range, which is a fundamental validity requirement for Arrhenius extrapolation to lower operating temperatures.

Applications

Long-term thermal aging data for PET insulation film directly support design and qualification decisions across many industries.

• Slot liner, phase insulation, and wedge insulation in small and medium electric motors and generators
• Layer and interwinding insulation in distribution transformers, current transformers, and magnetic components
• Base dielectric substrate and coverlay in flexible printed circuits and membrane switches
• Dielectric layers in film capacitors and barrier insulation in battery packs and power electronics modules
• Wrapping and binding tape for wires, cables, and busbar insulation
• Insulation components in household appliances, power tools, lighting ballasts, and automotive electrical systems

FAQ

What is the temperature index of PET insulation film?

Standard biaxially oriented PET film typically carries a continuous-use rating of about 105 to 120 degrees Celsius depending on grade, while heat-stabilized grades and varnish-treated systems can be qualified up to 130 degrees Celsius or higher. The exact value must be established through a formal IEC 60216 thermal endurance evaluation rather than assumed from supplier literature.

Why is elongation at break used as the main end-point criterion?

Elongation at break declines earlier and more sharply than tensile strength as PET embrittles through chain scission and post-crystallization, making it the most conservative and sensitive marker of insulation integrity. A retention of 50 percent of the initial elongation is the widely accepted end-point for flexible film insulation.

How long does a complete aging evaluation take?

A full multi-temperature aging program typically requires 6 to 18 months, depending on the selected temperatures, the number of diagnostic properties, and the statistical confidence required. Higher aging temperatures shorten the program, but the range must still overlap realistic extrapolation limits for the Arrhenius analysis to remain valid.

Which standards govern the evaluation?

The IEC 60216 series governs thermal endurance determination, IEC 60674 specifies requirements for plastic films for electrical purposes, and UL 746B defines relative temperature index programs for polymeric materials. Mechanical test methods follow ISO 527-3 or ASTM D882, and dielectric breakdown follows IEC 60243-2 or ASTM D149.

Does humidity affect the thermal aging result?

Yes. Moisture accelerates hydrolytic scission of PET ester bonds, which can significantly shorten apparent lifetime, especially between 100 and 160 degrees Celsius. For humid service environments, combined thermal-humidity aging or evaluation under IEC 60068 environmental conditioning should be considered in addition to dry-heat aging.

Can aging data be extrapolated to any service temperature?

Extrapolation is only valid within the range where the degradation mechanism is unchanged. Generally, the extrapolated service temperature should not lie more than 25 to 30 Kelvin below the lowest aging temperature used, unless supporting evidence confirms mechanism stability, and confidence limits must always accompany the reported temperature index.