PET High-Temperature Insulation Film Thermal Shrinkage Rate Testing: Standards, Methods, and Quality

Thermal shrinkage rate testing is a critical quality control procedure for PET high-temperature insulation films used in motors, transformers, flexible printed circuits, and other electrical insulation systems. Because PET film is biaxially oriented during manufacturing, exposure to elevated temperatures causes the polymer chains to relax and the film to contract, which can compromise insulation coverage, layer registration, and long-term device reliability. This article explains the testing standards, sample requirements, test procedures, calculation methods, and acceptance criteria used to evaluate the thermal shrinkage performance of PET insulation films.

Technical Overview

PET (polyethylene terephthalate) film is a biaxially oriented polyester film widely used as electrical insulation in motors, transformers, cables, capacitors, and flexible printed circuits. Its combination of high dielectric strength, good mechanical toughness, chemical resistance, and thermal endurance makes it one of the most common insulating materials for mid-range thermal classes. However, because the film is stretched in both the machine direction (MD) and transverse direction (TD) during production, internal orientation stresses remain locked within the material.

When PET film is heated above its glass transition temperature of approximately 80 degrees Celsius, these oriented polymer chains begin to relax and the film contracts. The magnitude of this contraction, expressed as a percentage of the original dimension, is the thermal shrinkage rate. If shrinkage is excessive, slot liners may pull back from motor slot edges, insulation layers in transformers may shift, and circuit patterns on flexible substrates may lose dimensional registration. Thermal shrinkage rate testing therefore serves as a key indicator of film quality, heat-setting adequacy, and suitability for a given thermal class.

• Quantifies dimensional stability at elevated service and processing temperatures
• Distinguishes standard films from heat-stabilized grades
• Supports incoming material inspection and lot acceptance decisions
• Helps prevent wrinkling, edge retraction, and layer misalignment in finished products
• Provides data for thermal class verification and supplier qualification

Testing Samples

Standard Biaxially Oriented PET Insulation Film

Standard grade PET film, typically available in thicknesses from 19 to 350 micrometers, is the most commonly tested material. It is used for general-purpose electrical insulation, slot insulation in small motors, and layer insulation in transformers. Typical shrinkage limits for standard grades are evaluated at 150 degrees Celsius for 30 minutes, with acceptance thresholds commonly set at 3.0 percent or less in the machine direction and 1.5 percent or less in the transverse direction, depending on the purchasing specification.

Heat-Stabilized PET Film

Heat-stabilized (heat-set) grades undergo an additional annealing process at the manufacturing line, which relieves most of the residual orientation stress. These films are intended for high-temperature insulation applications and flexible printed circuit substrates, where dimensional stability is critical. Heat-stabilized films are often tested at more severe conditions such as 180 or 200 degrees Celsius, with typical requirements of 1.0 percent or less, and premium grades may achieve shrinkage below 0.5 percent.

Coated and Surface-Treated PET Films

Corona-treated, silicone-coated, fluoropolymer-coated, and metalized PET films used in specialized insulation systems also require shrinkage verification. Coatings and surface treatments can affect heat transfer and stress relaxation behavior, so these materials are tested in their finished, as-delivered condition rather than as base film. Specimens should represent the full coating structure that will be exposed to heat in service.

For all sample types, test specimens are typically cut as 100 mm by 100 mm squares or as strips with marked gauge lengths of at least 100 mm in each principal direction. The machine direction must be clearly identified and marked on every specimen before testing.

Testing Items

Thermal shrinkage testing evaluates the dimensional change of PET insulation film after controlled exposure to elevated temperature. The core measured and derived items include the following:

• Machine direction (MD) free shrinkage rate after heat exposure
• Transverse direction (TD) free shrinkage rate after heat exposure
• Shrinkage behavior at multiple temperature setpoints, for example 120, 150, 180, and 200 degrees Celsius
• Shrinkage after different exposure durations, commonly 15, 30, and 60 minutes
• Directional anisotropy ratio, comparing MD shrinkage to TD shrinkage
• Visual assessment of the specimen after exposure, including curling, wrinkling, blistering, delamination of coatings, and color change

Some specifications additionally require constrained or loaded shrinkage testing, in which the specimen is held under a defined tension during heating, to simulate conditions in continuous processing such as tape wrapping or lamination.

Testing Methods

Specimen Preparation and Conditioning

A minimum of three specimens per direction per lot is recommended. Specimens are cut cleanly without edge defects, and the machine direction is marked with an arrow. Reference marks or gauge lines defining the initial measurement length are applied with a fine marker or by scribing, taking care not to stress the film. Before testing, specimens are conditioned in a standard laboratory atmosphere of 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for at least 24 hours to ensure dimensional equilibrium.

Oven Exposure Procedure

The test follows the general principle of ASTM D1204 for dimensional changes of nonrigid thermoplastic film at elevated temperature. A forced-air laboratory oven with verified temperature uniformity of plus or minus 2 degrees Celsius is preheated to the specified temperature, commonly 150 degrees Celsius for 30 minutes for standard grades. Each specimen is placed flat and unconstrained in the oven, ideally supported on a bed of inert powder such as talc or suspended freely so that shrinkage is not restricted by contact friction. Specimens must not touch each other or the oven walls. After the specified exposure time, specimens are removed and allowed to cool freely on a flat surface at room temperature before reconditioning and final measurement.

Measurement and Calculation

Initial and final dimensions are measured with a calibrated steel rule, optical comparator, or non-contact video measuring system, with a resolution of 0.5 mm or better for 100 mm gauge lengths. The thermal shrinkage rate in each direction is calculated as S equals L0 minus L1, divided by L0, multiplied by 100 percent, where L0 is the initial gauge length before exposure and L1 is the gauge length after exposure. A positive result indicates contraction. The reported value for each direction is the average of all valid specimens, together with the individual values and any visual observations.

Referenced Standards

• ASTM D1204, Standard Test Method for Linear Dimensional Changes of Nonrigid Thermoplastic Sheeting or Film at Elevated Temperature
• IEC 60674-2, Specification for plastic films for electrical purposes, Part 2, Methods of test
• GB/T 13542 series, Chinese national standards for electrical insulating films and their test methods
• JIS C 2318, Polyester film for electrical insulation, which includes shrinkage requirements
• ISO 11501, Plastics film and sheeting, determination of dimensional change on heating

Applications

Thermal shrinkage rate testing directly supports material selection, incoming inspection, and process control across the electrical and electronics industries. Applications that depend on verified shrinkage performance include:

• Slot liners, phase insulation, and wedge insulation in motors and generators
• Interlayer and wrap insulation in distribution and power transformers
• Base film for flexible printed circuits, where shrinkage affects circuit registration
• Insulating tapes and pressure-sensitive adhesive tape backings
• Dielectric layers and wrap films in film capacitor manufacturing
• Barrier and protective insulation components in lithium-ion battery packs
• Polyester core films in photovoltaic backsheets exposed to sustained field temperatures
• Insulation for heating elements, household appliances, and power electronics modules

In each of these applications, a verified low shrinkage rate ensures that the film retains its designed coverage area, maintains dielectric barriers between conductive parts, and preserves dimensional integrity through both manufacturing processes and long-term thermal aging.

FAQ

What test temperature and duration are typically specified for PET insulation film?

The most common condition is 150 degrees Celsius for 30 minutes, which aligns with typical specification limits for standard grades. Heat-stabilized films intended for higher thermal classes are frequently tested at 180 or 200 degrees Celsius. The exact condition must always follow the material specification agreed between supplier and user.

Why is machine direction shrinkage usually higher than transverse direction shrinkage?

Biaxial orientation draws the film sequentially, and the degree of molecular orientation and residual stress generally differ between the two axes. Because the machine direction typically retains more residual stress after sequential stretching, relaxation upon reheating produces greater contraction along that axis. Balanced or simultaneous stretching processes reduce this anisotropy.

What is the difference between free shrinkage and constrained shrinkage testing?

Free shrinkage testing, as described in ASTM D1204, allows the specimen to contract without restriction and measures the total dimensional change. Constrained shrinkage testing holds the specimen under tension or fixes one dimension during heating, measuring the force or stress developed, which better simulates conditions in continuous web processing.

How many specimens should be tested per lot?

A minimum of three specimens per direction is standard practice, and five specimens provide better statistical confidence. Specimens should be taken from different positions across the web width to capture transverse variation in film properties.

What does an unexpectedly high shrinkage result indicate?

A high shrinkage value usually indicates insufficient heat setting during film manufacture, exposure to excessive temperature during processing or storage, or use of a film grade unsuited to the intended thermal class. It signals a risk of dimensional instability in the finished insulation system and warrants investigation before the material is released to production.

Can shrinkage testing be performed on coated or laminated film structures?

Yes, and it is recommended that testing be performed on the complete structure as it will be used, since coatings and adhesive layers can influence heat distribution and stress relaxation. Any interlayer delamination or coating defects observed after exposure should be recorded as part of the test result.