PET Film Tensile Strength Testing: Standards, Methods, and Key Parameters

PET film tensile strength testing is a critical mechanical evaluation used to determine how polyethylene terephthalate films perform under stretching forces in packaging, electronics, and industrial applications. This guide explains the applicable standards, sample preparation, test procedures, calculation methods, and typical results, helping manufacturers and quality engineers ensure consistent film quality and process control.

Technical Overview

Polyethylene terephthalate (PET) film, particularly biaxially oriented PET (BOPET), is one of the most widely used flexible materials in packaging, electronics, photovoltaics, and graphic applications. Its mechanical reliability depends directly on tensile properties such as tensile strength, elongation at break, and elastic modulus. A tensile force test measures the force required to pull a film specimen to failure and the amount of stretching it can withstand before breaking.

PET film tensile testing is typically performed on a universal testing machine (UTM) equipped with pneumatic or mechanical grips and a suitable load cell. Because PET film is anisotropic due to biaxial orientation during manufacturing, specimens must be tested in both the machine direction (MD) and the transverse direction (TD). Commonly applied standards include ASTM D882 (Tensile Properties of Thin Plastic Sheeting), ISO 527-3 (Plastics – Determination of Tensile Properties – Part 3: Test Conditions for Films and Sheets), and GB/T 1040.3, which is technically aligned with ISO 527-3.

The test results are essential for incoming material inspection, supplier qualification, new product development, lamination process control, and failure analysis of film-based products.

Testing Samples

Plain BOPET Film

Uncoated, biaxially oriented PET film is the baseline sample type, available in thicknesses typically ranging from 6 μm to 250 μm. Common commercial grades include 12 μm, 19 μm, 23 μm, 36 μm, and 50 μm films used in flexible packaging and release liners. Plain film results establish the reference mechanical profile of the material.

Metallized and Coated PET Films

Metallized PET (VMPET), silicon oxide coated PET (SiOx), aluminum oxide coated PET (AlOx), and heat-sealable or slip-coated films are tested to evaluate how surface treatments affect tensile performance. Coating layers are generally thin enough that tensile values remain close to the base film, but testing verifies that coating processes have not induced brittleness or micro-cracking.

PET-Based Laminates and Composites

Multilayer structures such as PET/PE, PET/AL/PE, and PET used in photovoltaic backsheets are tested as finished laminates. The results reflect the combined contribution of all layers and the interlayer bonding quality, and are often compared against the theoretical sum of individual layer strengths.

Testing Items

The core parameters measured in a PET film tensile force test include:

• Tensile strength (MPa): The maximum tensile stress the specimen withstands before failure, calculated from the peak force divided by the original cross-sectional area.
• Yield strength (MPa): The stress at which the material begins to deform plastically, visible as the first plateau or inflection on the stress-strain curve.
• Elongation at break (%): The percentage increase in gauge length at the moment of rupture.
• Elongation at yield (%): The strain corresponding to the yield point, useful for assessing formability.
• Tensile modulus / Young's modulus (MPa): The stiffness of the film in the initial elastic region, determined as a secant modulus between specified strain limits.
• Break strength (MPa): The stress at the actual breaking point, which for PET may be lower than the maximum stress due to necking behavior.

Testing Methods

Sample Preparation and Conditioning

Rectangular strip specimens are cut cleanly with a sharp blade or dedicated sample cutter, free of nicks, burrs, or edge defects that could cause premature failure. A typical specimen configuration is a strip 15 mm in width with sufficient length to allow an initial grip separation (gauge length) of 100 mm, in accordance with ASTM D882 practice. Specimens are cut in both the machine direction and the transverse direction, and the film thickness is measured at multiple points using a precision thickness gauge per ASTM D6988 or ISO 4593 to calculate the cross-sectional area accurately.

Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 24 hours, as humidity and temperature can measurably influence the ductility of PET film.

Test Procedure

The specimen is mounted vertically in the grips of a universal testing machine, ensuring that the long axis is aligned with the applied force axis. Key procedural parameters include:

• Load cell capacity selected so that the breaking force falls within 10% to 90% of the capacity range; for most PET films, a 100 N to 1 kN load cell is appropriate.
• Pneumatic side-action grips with rubber or smooth faces and moderate pressure to prevent slippage without crushing the film.
• Initial grip separation of 100 mm for films with expected elongation above 100%.
• Crosshead speed commonly set between 100 and 500 mm/min, selected according to the expected elongation as directed by the applicable standard; 200 to 300 mm/min is frequently used for PET film.
• A minimum of five valid specimens tested per direction, with breaks occurring within the gauge region; breaks at the grips are rejected and retested.

Calculation of Results

Tensile strength is calculated by dividing the maximum force by the original cross-sectional area (width multiplied by average thickness), and elongation at break is calculated from the extension at rupture relative to the initial gauge length. The elastic modulus is determined from the slope of the initial linear portion of the stress-strain curve, using the secant method between the strain limits defined in the standard, such as 0.05% to 0.25% strain per ISO 527-1. Results are reported as the arithmetic mean, standard deviation, and coefficient of variation for each direction.

Applications

• Flexible packaging: verifying that PET films and laminates survive high-speed filling, sealing, and distribution stresses without tearing or pinholing.
• Electronics: confirming the mechanical durability of PET substrates in flexible printed circuits, insulation films, and adhesive tapes.
• Photovoltaic modules: qualifying PET-based backsheets for long-term mechanical integrity under thermal cycling and wind loads.
• Labeling and graphic arts: ensuring films withstand die-cutting, dispensing, and rewinding tensions.
• Quality control and R&D: comparing raw material lots, optimizing orientation and annealing processes, and supporting failure investigations.

Typical reference values for 12 μm plain BOPET film are approximately 160–240 MPa tensile strength and 120–200% elongation in the machine direction, and 200–280 MPa tensile strength and 40–90% elongation in the transverse direction, although exact values vary with resin grade, orientation ratio, and thickness.

FAQ

Which standard should be used for PET film tensile testing? For films thinner than 1 mm, ASTM D882 is the most widely used method in North America, while ISO 527-3 (and the equivalent GB/T 1040.3) is common in international and Chinese markets. The chosen standard should be stated in test reports because specimen dimensions and speeds differ between methods, which affects comparability.

Why do MD and TD results differ so much? Biaxial orientation stretches the polymer chains predominantly in the machine direction during casting and then in the transverse direction during the tenter frame process. The resulting chain alignment and residual stress distribution make the film significantly stronger but less extensible in the transverse direction.

What causes invalid or scattered results? Common causes include poor edge cutting quality, specimen slippage in the grips, breaks occurring at the grip faces, incorrect thickness measurement, and inadequate conditioning. Systematic rejection criteria and proper grip pressure settings minimize scatter.

How many specimens are required? A minimum of five valid specimens per direction is standard practice, though ten specimens per direction are recommended when statistical confidence is required, for example during supplier qualification.

Does thickness affect the reported strength? Tensile strength is normalized to cross-sectional area, so it is largely thickness independent for the same film grade; however, very thin films may show slightly lower apparent strength due to edge defect sensitivity and thickness measurement uncertainty.