PET insulation film creep performance analysis is essential for verifying the long-term dimensional stability and dielectric reliability of polyester films used in motors, transformers, capacitors, and flexible electronics. This technical article explains the viscoelastic creep behavior of biaxially oriented PET film, describes standard test samples and testing items, and outlines internationally recognized creep test methods such as ASTM D2990 and ISO 899-1. Engineers and quality managers will also find practical guidance on data interpretation, acceptance criteria, and creep mitigation strategies for demanding electrical insulation applications.
Technical Overview
PET (polyethylene terephthalate) insulation film, in most cases biaxially oriented (BOPET), is one of the most widely used organic insulation materials in electrical and electronic engineering. It combines high dielectric strength, typically above 150 kV/mm for thin gauges, with excellent tensile strength, good chemical resistance, low moisture absorption, and a continuous operating temperature generally rated between 105°C and 120°C, extendable to higher classes with heat-stabilized grades. Because PET is a viscoelastic polymer, its mechanical response depends not only on the applied stress but also on time and temperature.
Creep is defined as the time-dependent, progressive deformation of a material under a sustained load, even when the stress is well below the short-term yield strength. In a typical creep curve of PET film, three stages can be identified: primary creep, where the strain rate decreases as molecular chains rearrange; secondary creep, characterized by an approximately steady-state strain rate; and tertiary creep, where voiding, crazing, or necking accelerates deformation and eventually leads to rupture. For thin films under moderate stress at room temperature, tertiary creep may never be reached, but at elevated temperatures the transition can occur quickly.
Creep performance is critical for PET insulation films because the film is rarely installed stress-free. In wound motors, transformers, and capacitors, the film experiences sustained winding tension, compressive pressure from adjacent layers, and thermal loads simultaneously. Creep-induced thinning or elongation can reduce insulation thickness, degrade partial discharge inception voltage, cause wrinkling or loosening of wound structures, and ultimately shorten service life. Since electrical equipment is often expected to operate for 20 to 30 years, laboratory creep data must be carefully extrapolated using validated viscoelastic models, which is precisely the purpose of a systematic creep performance analysis.
Temperature is the dominant external factor in PET creep because the glass transition temperature of PET lies at approximately 75°C to 80°C. Below the glass transition, molecular segment mobility is restricted and creep strains remain small; above it, chain segments gain mobility, physical aging effects become significant, and creep rates can increase by orders of magnitude. This is why heat-stabilized PET grades, which are annealed to reduce residual stress and shrinkage, are specified for rotating machines and dry-type transformers operating at Class B or Class F temperatures.
Testing Samples
Standard Biaxially Oriented PET Insulation Film
The baseline sample set consists of commercial-grade biaxially oriented PET insulation film in common thicknesses of 23, 36, 50, 75, 100, 125, 190, and 250 micrometers. Standard BOPET exhibits a tensile strength of roughly 170 to 250 MPa in the machine direction and a tensile modulus of 3.4 to 4.4 GPa. Because biaxial orientation produces anisotropy, specimens are prepared in both the machine direction (MD) and transverse direction (TD) so that directional creep differences can be quantified.
Heat-Stabilized PET Film
Heat-stabilized, or thermofixed, PET film is produced by annealing the oriented film under tension at elevated temperature, which relaxes internal stresses and locks in dimensional stability. These grades show markedly lower shrinkage and slower creep at 120°C to 155°C and are the preferred material for motor slot insulation, transformer layer insulation, and other thermally demanding uses. Creep comparison between standard and heat-stabilized grades is a standard part of material qualification.
Coated and Filled PET Film Grades
Additional sample types include corona-resistant coated PET film, acrylic or epoxy coated films used in flexible printed circuits, matte or surface-treated films for adhesive bonding, and flame-retardant filled grades. Coatings and fillers alter the viscoelastic response, so each formulation must be evaluated on its own creep data rather than relying on base-film values.
Specimen Preparation and Conditioning
Specimens are die-cut as rectangular strips per ISO 527-3 or as miniature dumbbells per ASTM D638 Type V, with gauge lengths typically of 50 mm or 30 mm for film testing. Edges must be free of nicks and tears, because stress concentrations dominate thin-film creep rupture. All specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for at least 40 hours in accordance with ASTM D618 or ISO 291 before testing. Thickness is measured at a minimum of five positions along the gauge length using a low-pressure dead-weight micrometer, and the average value is used for stress calculations.
Testing Items
A complete creep performance analysis of PET insulation film covers the following measured and derived quantities:
- Tensile creep strain as a function of time under constant engineering stress
- Instantaneous elastic strain and delayed elastic strain components
- Creep compliance and the decay of apparent creep modulus over time
- Steady-state creep rate in the secondary creep region
- Creep rupture time, or time-to-failure, at each stress and temperature combination
- Recovery strain after load removal, quantifying the permanent set
- Stress relaxation behavior at fixed strain, relevant to clamped insulation structures
- Isochronous stress-strain curves generated at fixed times such as 1 h, 100 h, and 1000 h
- Compressive creep of stacked film laminations, simulating layer pressure in wound assemblies
- Anisotropy ratio between machine-direction and transverse-direction creep
- Temperature dependence of creep at 23°C, 70°C, 100°C, 120°C, and 155°C
- Post-creep dielectric properties, including breakdown voltage and partial discharge inception voltage, to link mechanical deformation with electrical reliability
Testing Methods
Test Standards and Equipment
Tensile creep testing of PET film is performed in accordance with ASTM D2990, which governs tensile, compressive, and flexural creep and creep-rupture of plastics, and ISO 899-1, which governs tensile creep of plastics. Short-term tensile reference properties are determined first using ASTM D882, the standard test method for tensile properties of thin plastic sheeting, so that creep stress levels can be expressed as defined fractions of the yield strength. Test equipment includes dead-weight lever-arm creep frames equipped with calibrated masses, environmental chambers, and precision extensometry such as video extensometers or low-mass contact transducers. For smaller specimens and faster screening, dynamic mechanical analyzers operated in static creep mode and thermomechanical analyzers can be used, provided force and strain resolution are adequate for thin films.
Tensile Creep Test Procedure
The recommended procedure is as follows. First, mount the conditioned specimen in the grips with a small preload, typically below 0.01 MPa, to remove slack without inducing measurable strain. Second, apply the dead load smoothly within 1 to 5 seconds to avoid dynamic overshoot. Third, record strain continuously or at logarithmically spaced intervals for the planned duration, with a minimum test time of 1000 hours for qualification programs and shorter durations permitted for comparative screening. Fourth, at the end of the loading period, remove the load and record the recovery strain for a period at least equal to the loading time. Recommended stress levels are 10%, 20%, 30%, and 40% of the short-term yield strength at the test temperature, and each condition should include at least three replicate specimens.
Compressive and Multi-Stress Creep Evaluation
Because PET film in transformers, capacitors, and battery packs is loaded mainly in compression between wound layers, compressive creep is evaluated by stacking film plies between polished rigid platens under defined pressures, commonly 0.5 to 10 MPa, and monitoring thickness reduction with time using ASTM D2990 compressive provisions or dedicated laminated-film methods. Where the application involves simultaneous electrical stress, creep specimens can be placed under DC or AC field while mechanically loaded, allowing detection of any electro-mechanical coupling that accelerates deformation.
Data Analysis and Lifetime Prediction
Creep data are fitted with standard viscoelastic models. The four-element Burgers model expresses total strain as the sum of an instantaneous elastic term, a delayed elastic term, and a viscous flow term, and works well for primary and early secondary creep of PET film. The Findley power law, in which strain equals the initial strain plus a coefficient multiplied by time raised to an exponent typically between 0.1 and 0.3 for oriented PET, is widely used for engineering extrapolation. For long-term prediction, time-temperature superposition constructs a master curve by horizontally shifting isothermal creep curves obtained at several elevated temperatures, using Arrhenius-type shift factors when the material is below the glass transition and WLF-type factors above it. Creep rupture life is extrapolated with the Monkman-Grant relationship, which links time-to-failure to the steady-state creep rate, providing a conservative estimate of the stress needed to survive design lifetimes of 100,000 hours or more.
Acceptance Criteria and Typical Results
Typical acceptance criteria for PET insulation film in electrical machine applications specify that total tensile creep strain after 1000 hours at 20% of yield strength and 105°C shall not exceed 2%, and that no tertiary creep initiation may appear within the qualification window. Standard BOPET generally meets this requirement at or below 100°C, while heat-stabilized grades maintain low creep rates up to 130°C to 155°C. Compressive creep of stacked laminations is usually limited to a thickness reduction of 1% to 3% over the design life, ensuring that insulation clearances and partial discharge margins are preserved. Any condition under which the apparent creep modulus falls below 50% of its initial value within the intended service life is treated as a design limit.
Applications
Creep performance analysis directly supports the reliability of PET insulation film in the following applications:
- Slot liners, phase insulation, and wedge insulation in electric motors and generators, where sustained electromagnetic and thermal loading demands stable film geometry
- Interlayer and layer insulation in distribution and dry-type transformers, where compressive creep controls long-term clearances
- Dielectric material in metallized film capacitors, where creep and shrinkage affect capacitance stability and self-healing behavior
- Base films and coverlays in flexible printed circuits, where dimensional stability under soldering temperatures and clamping loads is essential
- Wrapping and binding tapes for cable insulation, where sustained winding tension must not relax over decades
- Cell-level insulation barriers and adhesive tape backings in lithium-ion battery packs, where compressive stack pressure is permanent
- Photovoltaic backsheets and electrical insulation components exposed to continuous thermal aging in outdoor environments
In each of these applications, the creep analysis results feed directly into design decisions such as winding tension settings, insulation overlap margins, thermal class selection, and preventive maintenance intervals. Selecting a heat-stabilized PET grade, limiting sustained stress to a small fraction of yield strength, avoiding operation above the material temperature rating, and verifying performance through standardized creep testing are the four most effective measures for ensuring long-term reliability.
FAQ
What is creep in PET insulation film and why does it matter?
Creep is the slow, time-dependent deformation of PET film under a constant load. It matters because insulation films in motors, transformers, and capacitors carry sustained mechanical stresses for decades. Progressive creep can thin the insulation, reduce breakdown and partial discharge margins, and loosen wound structures, so creep analysis is a core element of insulation system qualification.
At what temperature does creep of PET film become significant?
Creep accelerates sharply as temperature approaches the glass transition region of PET, which lies around 75°C to 80°C. At room temperature creep strains are small, while above approximately 100°C, and certainly above 120°C, creep rates increase substantially unless a heat-stabilized grade is used.
Which standards govern creep testing of PET insulation film?
The principal standards are ASTM D2990 for creep and creep-rupture of plastics, ISO 899-1 for tensile creep of plastics, and ASTM D882 for the short-term tensile reference properties of thin film. Product-level requirements for PET film are additionally covered by the IEC 60674 series for plastic films used in electrical insulation.
How is long-term creep behavior predicted from short-term tests?
Short-term creep curves obtained at several elevated temperatures are combined into a master curve using time-temperature superposition, and the data are fitted with the Burgers model or the Findley power law. Creep rupture life is extrapolated using the Monkman-Grant relationship, allowing conservative estimates of allowable stress for service lives of 20 years or more.
What stress levels are used in a typical creep test program?
Stress levels are set as defined fractions of the short-term yield strength measured at the same temperature, commonly 10%, 20%, 30%, and 40%. Testing at multiple stress levels and temperatures allows the full creep response surface to be mapped and safe operating envelopes to be established.
How can creep deformation in PET insulation film be minimized?
Effective measures include specifying heat-stabilized or annealed film grades for elevated temperature service, limiting sustained tensile and compressive stresses to well below the creep-rupture threshold, avoiding operating temperatures above the material class rating, controlling winding tension during installation, and verifying each design through standardized creep testing before release.