PET Film Breaking Strain Determination: Standards, Methods and Technical Guidelines

PET film breaking strain determination measures the elongation capacity of polyethylene terephthalate film before rupture under uniaxial tensile loading, and it is one of the most important quality indicators for packaging, optical, and electronic film products. This technical guide explains the applicable standards such as ASTM D882 and ISO 527-3, specimen preparation, equipment configuration, test procedures, calculation methods, and typical result ranges for oriented and amorphous PET films.

Technical Overview

Polyethylene terephthalate (PET) film is a biaxially oriented or cast thermoplastic polyester film widely used in flexible packaging, photovoltaic modules, optical displays, electrical insulation, and release liners. The breaking strain, also referred to as elongation at break or strain at break, is defined as the tensile strain of the film at the moment of rupture and is calculated as the ratio of the increase in gauge length at break to the original gauge length, expressed as a percentage.

Determination of breaking strain provides direct insight into the ductility, toughness, and formability of PET film. The measured value reflects the degree of molecular orientation introduced during biaxial stretching, the effectiveness of heat stabilization and annealing, the influence of coating or metallization processes, and the extent of degradation caused by aging, UV exposure, hydrolysis, or sterilization. A significant reduction in breaking strain is often the earliest measurable sign of embrittlement, which makes this test a sensitive and cost-effective quality control tool.

The test is performed by pulling a rectangular strip of film in a universal testing machine until it fractures, while the extension at break is recorded using either grip separation (crosshead displacement) or an extensometer. Oriented PET films typically exhibit breaking strains in the range of approximately 50% to 180%, while amorphous cast PET films can exceed 300%, and heat-stabilized low-shrink grades generally fall at the lower end of the oriented range.

Testing Samples

Biaxially Oriented PET (BOPET) Films

Biaxially oriented PET film, typically 4.5 to 100 micrometers in thickness, is the most frequently tested sample type. Because the stretching process aligns polymer chains in both the machine direction (MD) and transverse direction (TD), the breaking strain is strongly direction-dependent, and specimens must be cut and tested separately in each direction. Surface-treated, slip-modified, and antistatic grades are included in this category.

Amorphous and Cast PET Films

Cast or amorphous PET (APET/CPET) films used in thermoforming and window materials show highly ductile behavior with breaking strains frequently above 300%. These films are less anisotropic than oriented grades, but consistent specimen preparation remains critical because large deformations magnify any misalignment in gripping.

Heat-Stabilized and Low-Shrink PET Films

Annealed or heat-stabilized films used in optical and display applications are partially crystallized to suppress thermal shrinkage, which reduces chain extensibility and lowers the breaking strain. Testing these films before and after heat treatment quantifies the effect of the stabilization process.

Coated, Metallized, and Laminated PET Films

Samples with hard coatings, silicon oxide or aluminum oxide barrier layers, vacuum-metallized surfaces, or adhesive laminations are tested to evaluate how the additional layers affect film ductility. Brittle inorganic coatings often initiate cracks that localize deformation and reduce the measured breaking strain compared with the base film.

Specialty and Recycled PET Films

This category includes ultra-thin capacitor-grade films below 3 micrometers, photovoltaic backsheet core films of 175 to 250 micrometers, release liner films, and films containing recycled PET (rPET). For ultra-thin films, handling and specimen cutting require exceptional care, and thickness measurement uncertainty becomes a dominant factor in data quality.

Testing Items

A complete tensile characterization of PET film typically includes the breaking strain together with the following parameters obtained from the same force-extension curve:

• Breaking strain (elongation at break) in the machine direction, transverse direction, and optionally the 45-degree diagonal direction
• Tensile strength at break and tensile stress at yield
• Strain at yield, indicating the onset of permanent deformation
• Tensile modulus, usually determined as the 1% secant modulus in accordance with ISO 527-1
• Tensile energy to break, representing the total energy absorbed up to rupture
• Nominal breaking strain based on crosshead displacement compared with extensometer-based strain where required

Testing Methods

Applicable Standards

The primary standards are ASTM D882 for tensile properties of thin plastic sheeting with thickness below 1 millimeter, and ISO 527-1 together with ISO 527-3, which specify test conditions for films and sheets. Equivalent national standards include GB/T 1040.3 and JIS K 7127. Supporting standards cover conditioning (ISO 291 or ASTM D618) and thickness measurement of plastic films (ISO 4593 or ASTM D6988). When comparing historical data, note that ASTM and ISO methods differ in recommended specimen width, gauge length, and test speed, so results should not be mixed without correlation.

Specimen Preparation

Rectangular strips are cut with a sharp die cutter or a razor-blade cutting jig to the required width, commonly 15 millimeters for ISO 527-3 specimens and 10 to 25 millimeters for ASTM D882 practice. Edges must be straight, parallel, and free of nicks or burrows, because edge defects act as stress concentrators and cause premature brittle fracture. Specimens are taken at least 50 millimeters away from the roll edge, clearly labeled by direction, and conditioned before testing. At least five valid specimens per direction are required, although ten specimens are recommended when scatter is significant. Thickness is measured at several points along each specimen using a low-pressure micrometer and the mean value is recorded.

Conditioning and Test Environment

Specimens are conditioned in the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 40 hours in accordance with ASTM D618. Although PET is only mildly moisture-sensitive, standardized conditioning ensures reproducible comparison between laboratories. Testing is performed in the same atmosphere, and any deviation must be reported with the results.

Equipment Configuration

A universal testing machine fitted with a low-capacity precision load cell, typically in the range of 50 to 500 newtons and calibrated to at least class 0.5 according to ISO 7500-1, is used. Pneumatic side-action grips with rubber or paper-lined faces provide uniform clamping pressure and prevent both specimen slippage and grip-induced stress concentration. The initial grip separation serves as the gauge length for nominal strain and is typically set at 100 millimeters, with 50 millimeters as a practical minimum. The crosshead speed is selected according to the expected elongation at break: slower speeds such as 25 or 50 millimeters per minute are used for films expected to break below about 100% strain, while speeds of 100 to 500 millimeters per minute are applied to highly extensible films. For accurate strain measurement, a lightweight auto-contacting extensometer or a non-contact video extensometer is preferred, since conventional clip-on contact extensometer knives can damage thin PET films and initiate early rupture.

Test Procedure

• Verify the calibration status of the load cell, grip separation, and extensometer before starting the test series.
• Measure and record the width and mean thickness of each conditioned specimen.
• Mount the specimen in the grips with careful axial alignment, apply a small preload of approximately 0.1 to 0.5 newtons to remove slack, and zero the extension reading.
• Select the gauge length and crosshead speed according to the governing standard and the expected elongation level.
• Start the test and record the complete force-extension curve until the specimen ruptures completely.
• Identify the extension at break from the last data point before load drop or from the specified failure criterion for films that tear gradually.
• Inspect the fracture location: specimens breaking inside or at the grip edges are normally rejected and retested according to the standard acceptance rules.
• Repeat the test until at least five valid results are obtained for each material direction.

Calculation and Reporting

The breaking strain is calculated as the extension at break divided by the initial gauge length multiplied by 100 percent. When strain is derived from crosshead travel, the initial grip separation is used as the gauge length, and system compliance correction is applied whenever modulus is also reported. The test report should include the material identification, thickness, specimen width, gauge length, test speed, conditioning details, specimen orientation, individual and mean breaking strain values, standard deviation, number of valid specimens, and any observations on fracture appearance or invalid breaks.

Key Factors Affecting Accuracy

• Edge quality of the cut strip: tiny nicks reduce the measured breaking strain by triggering early crack initiation
• Grip slippage: produces overestimated extension and artificially high breaking strain values
• Molecular orientation: MD and TD results differ substantially and must never be averaged together
• Strain rate: higher crosshead speeds generally shift the observed breaking strain and must match the standard requirement
• Thermal history: heat setting, annealing, and prior shrinkage treatment lower chain extensibility
• Aging and environmental exposure: UV radiation, hydrolysis, and certain sterilization processes progressively embrittle PET film
• Instrument factors: load cell resolution, grip alignment, and extensometer technique influence both stress and strain readings

Applications

Breaking strain data for PET film support decision-making across the entire product life cycle:

• Incoming material inspection and supplier qualification for packaging, optical, and electronic film converters
• Process optimization of biaxial stretching lines by relating draw ratios and heat-setting conditions to film ductility
• Formability assessment for thermoforming, embossing, and deep-draw packaging applications
• Reliability evaluation of photovoltaic backsheet core films before and after damp-heat aging
• Shelf-life and sterilization validation for medical packaging films based on elongation retention
• Quality monitoring of coated, metallized, and laminated structures to detect process-induced embrittlement
• Recycled content qualification by comparing rPET-containing films with virgin resin benchmarks
• Failure analysis of field-returned films where reduced breaking strain indicates degradation or processing faults

FAQ

What is the difference between breaking strain and elongation at break?

The two terms describe the same quantity. Strain is a dimensionless ratio of extension to original gauge length, and elongation at break is simply that ratio expressed as a percentage. Test reports should always state the gauge length basis used for the calculation.

Which standard should I choose, ASTM D882 or ISO 527-3?

Both are technically valid for PET films below 1 millimeter in thickness. ASTM D882 is common in North American supply chains, while ISO 527-3 dominates in Europe and Asia. Because the recommended specimen dimensions and speeds differ, results from the two methods should be correlated before they are compared or combined.

How many specimens are required for a valid determination?

A minimum of five valid specimens per direction is required by ISO 527, and ten specimens are recommended when the material is known to scatter or when the result will be used for certification. Machine direction and transverse direction must always be treated as separate data sets.

Can I use crosshead displacement instead of an extensometer?

For breaking strain of relatively extensible films, crosshead displacement with the grip separation as gauge length is acceptable and widely practiced, because the extension at break is large compared with machine compliance. However, an extensometer or video strain measurement is strongly recommended whenever modulus is reported or whenever films break at low strains.

Why do MD and TD breaking strains differ?

Biaxial stretching aligns polymer chains preferentially along the stretching directions, and the balance between MD and TD orientation depends on whether the film was produced by sequential or simultaneous stretching and on the heat-setting conditions. This structural anisotropy is directly reflected in direction-dependent breaking strain values.

What are typical breaking strain values for PET films?

Standard biaxially oriented PET films generally show breaking strains of roughly 50% to 180%, amorphous cast films can exceed 300%, and heat-stabilized low-shrink grades typically fall toward the lower end of the oriented range. Exact values depend on thickness, orientation balance, and thermal history, so manufacturer data sheets should be consulted for grade-specific limits.

What should I do if specimens keep breaking in or at the grips?

Reject those results, review the grip pressure and face material, check specimen alignment, and verify that the cut edges are defect-free. Persistent grip failures usually indicate excessive clamping pressure, hard grip faces, or edge damage from a dull cutting die, and corrective action must be taken before results are accepted.