Elongation at break is one of the most critical mechanical performance indicators for PET insulation film, directly reflecting the material's ductility, formability, and reliability in electrical insulation applications. This article provides a comprehensive guide to the determination of elongation at break of PET insulation film, covering applicable standards such as ASTM D882, IEC 60674-2, and GB/T 13542, specimen preparation, testing procedures, calculation methods, and result evaluation. Whether you are a quality engineer, materials scientist, or procurement specialist, this guide will help you understand how accurate tensile testing ensures the performance and safety of PET films used in motors, capacitors, cables, and flexible electronics.
Technical Overview
PET insulation film is a biaxially oriented polyethylene terephthalate (BOPET) film widely used as a dielectric and insulating material in the electrical and electronics industry. It is characterized by high dielectric strength, excellent tensile strength, good thermal stability (continuous use temperature up to approximately 120°C), strong chemical resistance, and superior dimensional stability. These properties make PET film a preferred insulation material for motors, transformers, capacitors, flexible printed circuits, wire and cable wrapping, and lithium battery applications.
Elongation at break refers to the percentage increase in the gauge length of a film specimen at the moment of rupture during a tensile test. It quantifies the ability of the material to undergo plastic deformation before failure. For PET insulation film, elongation at break is a key quality control parameter because it indicates how well the film can withstand stretching, winding, embossing, and forming operations during manufacturing and service without cracking or tearing. Insufficient elongation may lead to film breakage during high-speed winding of capacitors or splitting during motor slot liner insertion, while excessively low elongation combined with uneven orientation can signal processing defects.
The determination of elongation at break is typically performed in accordance with internationally recognized standards, including ASTM D882 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting), IEC 60674-2 (Specification for Plastic Films for Electrical Purposes), GB/T 13542.2 (Chinese national standard for electrical insulation PET film), and ISO 527-3. Because biaxially oriented PET film exhibits pronounced anisotropy, elongation at break must be measured separately in the machine direction (MD) and transverse direction (TD).
Testing Samples
Biaxially Oriented PET Insulation Film (Standard Grade)
Standard BOPET insulation films are supplied in thicknesses ranging from 6 μm to 350 μm. Common commercial thicknesses include 12 μm, 19 μm, 23 μm, 36 μm, 50 μm, 75 μm, 100 μm, 125 μm, 188 μm, and 250 μm. Specimens are cut from the film in both the machine direction and transverse direction to evaluate directional mechanical properties. Typical elongation at break values for balanced BOPET film range from approximately 60% to 160%, depending on thickness, orientation ratio, and manufacturing process.
PET Film for Capacitor Applications
Thin-gauge PET films used as capacitor dielectric material (typically 0.9 μm to 12 μm) require stringent elongation control to ensure smooth high-speed winding. Higher elongation at break in the transverse direction improves the film's ability to conform to the winding tension profile, reducing the risk of web breaks and wrinkling during metallization and capacitor element winding.
Heat-Resistant and Modified PET Films
PET films modified for enhanced thermal performance, flame retardancy, or slip properties may exhibit elongation at break values that differ from standard grades. Testing of these specialty films verifies that property modifications have not compromised the ductility required for downstream processing.
Specimen Dimensions and Preparation
- Specimen geometry: rectangular strips cut with clean, sharp edges free from nicks, notches, and burrs that could cause premature stress concentration and failure.
- Width: typically 10 mm to 15 mm, depending on the selected standard; IEC 60674-2 specifies strips 15 mm in width.
- Initial gauge length (distance between grips): typically 100 mm to 250 mm for ASTM D882, or 100 mm for IEC 60674-2.
- Specimen quantity: a minimum of five valid specimens per direction (MD and TD) to ensure statistical reliability of the mean value.
- Conditioning: specimens shall be conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 24 hours prior to testing, unless otherwise specified.
Testing Items
The core testing item is the elongation at break of PET insulation film in both the machine direction and transverse direction. Depending on client requirements and product specifications, the test program may also include related tensile properties measured simultaneously on the same specimens:
- Elongation at break (%) — the primary parameter, calculated from the extension at rupture relative to the initial gauge length.
- Tensile strength at break (MPa) — the maximum tensile stress carried by the specimen before failure.
- Tensile strength at yield (MPa) — the stress at the yield point, relevant for forming operations.
- Elongation at yield (%) — the strain at which plastic deformation begins.
- Tensile modulus (Young's modulus) — the elastic stiffness of the film, typically 3.5–4.5 GPa for BOPET.
- MD/TD property balance assessment — evaluation of orientation uniformity across the web.
Testing Methods
Test Equipment
Testing is performed using a constant-rate-of-extension (CRE) universal tensile testing machine equipped with a calibrated load cell of appropriate capacity, pneumatic or mechanical grips with flat faces, and a displacement or extensometer measurement system capable of recording the extension at break. The crosshead speed is set according to the governing standard: IEC 60674-2 specifies 100 mm/min for PET film specimens with a 100 mm gauge length, while ASTM D882 assigns speeds of 50 mm/min to 500 mm/min based on film thickness and elongation characteristics.
Test Procedure
- Condition the specimens in a standard laboratory atmosphere of 23 ± 2°C and 50 ± 5% relative humidity for at least 24 hours.
- Measure and record the thickness of each specimen at multiple points using a precision micrometer or low-pressure thickness gauge, and verify the specimen width.
- Set the initial grip separation (gauge length) to the value specified by the standard, typically 100 mm.
- Mount the specimen centrally in the grips, ensuring the longitudinal axis of the specimen aligns with the line joining the grips to avoid uneven loading; apply sufficient clamping pressure to prevent slippage without initiating jaw breaks.
- Set the crosshead speed as required (100 mm/min per IEC 60674-2, or the applicable speed per ASTM D882).
- Start the test and continuously record the force-extension curve until the specimen ruptures.
- Record the extension at the moment of break, defined as the point where the force drops substantially or reaches zero.
- Repeat the test for a minimum of five valid specimens in each direction; discard specimens that fail within the grips (jaw breaks) and replace them with new specimens.
Calculation
The elongation at break is calculated using the following formula:
Elongation at break (%) = (ΔL / L0) × 100, where ΔL is the extension at break (the increase in gauge length at rupture) and L0 is the initial gauge length between the grips. If grip separation is used to monitor extension, the initial grip separation is taken as L0. If an extensometer is used, the extensometer gauge length is taken as L0. The result is reported as the arithmetic mean of all valid specimens, rounded according to standard practice, together with the individual values, standard deviation, test direction, specimen dimensions, thickness, conditioning parameters, and crosshead speed.
Typical Reference Values
For standard biaxially oriented PET insulation film, elongation at break typically falls within the following ranges: machine direction 60% to 130%, and transverse direction 60% to 160%, depending on film grade and orientation balance. Product specifications such as those in IEC 60674-3 and GB/T 13542.2 define minimum acceptance limits, commonly on the order of 60% or higher for each direction. Results significantly below specification indicate over-orientation, brittleness, aging, or processing defects, while excessive variability between MD and TD suggests non-uniform biaxial stretching during film manufacture.
Factors Affecting Test Results
- Specimen edge quality: nicks or rough cut edges cause stress concentration and artificially low elongation values.
- Specimen alignment: misalignment in the grips produces skewed loading and premature failure.
- Clamping pressure: insufficient pressure causes slippage and overestimated extension; excessive pressure causes jaw breaks.
- Test speed: higher crosshead speeds generally reduce measured elongation due to the viscoelastic nature of PET.
- Environmental conditions: temperature and humidity variations during conditioning and testing influence the mechanical response of the film.
- Testing direction: MD and TD results differ due to molecular orientation introduced during biaxial stretching.
- Film aging or prior heat treatment: thermal exposure can embrittle the film and reduce elongation at break.
Applications
Accurate determination of elongation at break supports quality control and product qualification across a wide range of electrical insulation applications:
- Motor and generator insulation: slot liners, phase insulation, and ground insulation require adequate ductility to conform to slot geometry without cracking during insertion and operation.
- Capacitor manufacturing: metallized and plain PET dielectric films rely on controlled elongation for stable, high-speed winding of capacitor elements.
- Transformer and coil insulation: PET films used as layer and interlayer insulation must tolerate winding tension and thermal cycling.
- Wire and cable wrapping: tensile elongation performance ensures the film survives tight wrapping and bending during cabling operations.
- Flexible printed circuits and electronics: base films for FPC, adhesive tapes, and barrier layers demand consistent mechanical ductility for lamination, punching, and bending processes.
- Lithium battery insulation: PET films used for cell wrapping and barrier applications require verified mechanical integrity after assembly stresses.
- Photovoltaic backsheets: PET-based core and outer layers are qualified through tensile testing to ensure long-term durability of solar modules.
- Incoming inspection and supplier qualification: manufacturers of electrical equipment use elongation at break data to verify film conformity against purchase specifications and international standards.
FAQ
What is elongation at break and why is it important for PET insulation film?
Elongation at break is the percentage increase in specimen length at the moment of rupture during tensile testing. It measures the ductility of PET insulation film and predicts how well the film will withstand stretching, forming, winding, and mechanical stresses during processing and service without cracking or tearing.
Which standards govern the determination of elongation at break for PET insulation film?
The most commonly applied standards are ASTM D882 for tensile properties of thin plastic sheeting, IEC 60674-2 for plastic films for electrical purposes, GB/T 13542.2 for electrical insulation PET film in China, and ISO 527-3 for tensile testing of plastic films. Product acceptance limits are specified in standards such as IEC 60674-3.
Why must elongation at break be tested in both MD and TD directions?
Biaxially oriented PET film is anisotropic: molecular chains are preferentially aligned in the machine and transverse directions during stretching. As a result, elongation at break, tensile strength, and modulus differ significantly between MD and TD. Testing both directions provides a complete picture of the film's mechanical balance and suitability for the intended application.
What are typical elongation at break values for PET insulation film?
For standard BOPET insulation film, elongation at break typically ranges from 60% to 130% in the machine direction and 60% to 160% in the transverse direction. Most product specifications, including IEC 60674-3 and GB/T 13542.2, set minimum acceptance limits on the order of 60% for each direction, depending on film thickness and grade.
What causes low or highly variable elongation at break results?
Common causes include poor specimen edge quality, jaw breaks from excessive clamping pressure, specimen slippage, misalignment in the grips, incorrect crosshead speed, inadequate conditioning, film aging or thermal embrittlement, and non-uniform biaxial orientation introduced during film manufacturing. Systematic investigation of these factors is essential before rejecting a material lot.
What test report information should accompany the results?
A compliant test report should include the test standard, film identification and thickness, specimen dimensions and cutting direction, number of specimens, conditioning atmosphere, gauge length, crosshead speed, individual and mean elongation at break values with standard deviation, any discarded results and their justification, and the testing laboratory's accreditation status, for example ISO/IEC 17025.