Tensile strength determination is the most fundamental mechanical test for plastic films, providing essential data on material performance, durability, and processability for packaging, agriculture, and industrial applications. This technical article explains the testing standards, specimen preparation, test procedures, calculation methods, and typical applications involved in measuring the tensile properties of plastic films in accredited third-party laboratories.
Technical Overview
Tensile strength determination quantifies the ability of a plastic film to withstand uniaxial tensile loading until yield or rupture. Because films are produced by blown extrusion, cast extrusion, or biaxial orientation, their mechanical behavior is strongly anisotropic, and testing in both the machine direction (MD) and the transverse direction (TD) is essential for a complete evaluation. The test is performed on a universal testing machine in which a rectangular or dumbbell-shaped specimen is gripped at both ends and pulled at a constant crosshead speed while force and extension are continuously recorded, generating a stress-strain curve from which all key tensile parameters are derived.
The results of tensile testing directly support packaging structural design, film down-gauging decisions, lamination and printing process control, incoming material inspection, and verification of compliance with product specifications and regulatory requirements.
Testing Samples
Polyolefin Films
Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE) films, together with cast polypropylene (CPP) and biaxially oriented polypropylene (BOPP) films. These materials account for the largest share of the flexible packaging market and exhibit pronounced orientation-dependent tensile behavior.
Polyester and Polyamide Films
Biaxially oriented polyethylene terephthalate (BOPET) and polyamide (nylon) films, which are specified where high tensile strength, stiffness, and puncture resistance are required, typically as reinforcement or barrier layers in laminated structures.
Multilayer and Laminated Films
Composite structures produced by lamination or coextrusion, such as PET/AL/PE, BOPP/VMPET/PE, or PA/EVOH/PE barrier laminates. Testing is normally performed on the complete laminate, and the reported results reflect the combined mechanical contribution of every layer.
Functional and Industrial Films
Agricultural mulch and greenhouse films, stretch wrap and shrink films, surface protection films, sterile medical packaging films, and battery separator films, where tensile properties govern fitness for purpose and conformity with industry specifications.
Testing Items
Depending on the specified standard and the customer's product specification, the following parameters are typically determined and reported for each test direction:
- Tensile strength (MPa): the maximum force sustained by the specimen divided by its original cross-sectional area
- Tensile stress at yield (MPa): the stress at the point where the material continues to elongate without a further increase in stress
- Tensile stress at break (MPa): the stress at the moment of rupture
- Tensile strain at break, or elongation at break (%): a measure of the ductility and extensibility of the film
- Tensile modulus (MPa): for films, commonly determined as the secant modulus between two defined strain points, for example 0.25% and 1%, due to the difficulty of capturing the true initial slope
- Force at break (N) and force at yield (N), frequently used for specification purposes in converting and sealing operations
Testing Methods
Applicable Standards
- ISO 527-3: Determination of tensile properties of plastics, Part 3, test conditions for films and sheets
- ASTM D882: Standard test method for tensile properties of thin plastic sheeting with thickness up to 0.25 mm
- GB/T 1040.3: Chinese national standard, technically aligned with ISO 527-3
- JIS K 7127: Japanese standard for tensile properties of plastic film and sheeting
Specimen Preparation
Two principal specimen geometries are used. ISO 527-3 permits rectangular strip specimens, typically 10 mm to 25 mm wide with a free length of at least 150 mm, or dumbbell-shaped specimens such as types 2 and 5A. ASTM D882 uses rectangular strips, commonly 10 mm to 25.4 mm wide and 150 mm to 250 mm long. Specimens must be cut cleanly with a sharp die or blade and must be free of nicks, tears, wrinkles, and edge defects, since any flaw causes premature stress concentration and invalidates the result. For each material, at least five valid specimens are tested in the machine direction and five in the transverse direction.
Conditioning and Test Atmosphere
Specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618, extended to at least 88 hours for hygroscopic materials such as polyamide films. The test itself is carried out in the same controlled atmosphere to ensure reproducibility.
Equipment and Test Procedure
A calibrated universal testing machine verified to ISO 7500-1 or an equivalent accuracy class is used, fitted with a load cell matched to the expected force range so that the measured forces remain within the calibrated portion of the range. Gripping of thin films is critical: pneumatic side-action grips with resilient faces, or capstan-type wrap grips, are preferred to prevent slippage and jaw breaks. The thickness of each specimen is measured at several points using a low-pressure micrometer in accordance with ISO 4593 or ASTM D6988, and the average is used in the cross-sectional area calculation.
A typical initial grip separation (gauge length) is 50 mm to 100 mm depending on the standard and specimen type. The crosshead speed is selected according to the material and specification: ISO 527-3 commonly uses 100 mm/min for relatively stiff films and up to 500 mm/min for highly extensible films, while ASTM D882 specifies 50 mm/min for films with a break elongation below 100 % and 500 mm/min for more extensible materials. Elongation may be measured from crosshead displacement or, for modulus determination, with a contact or video extensometer.
Data Reduction and Calculation
Tensile strength is calculated as the maximum force divided by the original cross-sectional area, that is, the product of the measured width and the mean thickness, and is expressed in megapascals. Elongation at break is calculated as the increase in gauge length at rupture divided by the initial gauge length, expressed as a percentage. Specimens that fail within the grips or at an obvious flaw are discarded and replaced. The final report states the standard used, the test direction, the specimen type, the crosshead speed, and the individual values, arithmetic mean, and standard deviation for each reported parameter.
Applications
Tensile strength data for plastic films are used throughout the value chain, from resin and film producers to converters, brand owners, and regulators:
- Quality control of film extrusion lines, including blown and cast film processes, and verification against internal or customer specifications
- Incoming material inspection by packaging converters and end users before lamination, printing, or filling operations
- Research and development, including formulation comparison, additive evaluation, and down-gauging studies for source reduction and cost optimization
- Aging and durability assessment, where tensile retention is measured after thermal aging, UV exposure, humid conditioning, or retort processing
- Compliance demonstration for food contact packaging, pharmaceutical packaging, and industry-specific requirements verified in accredited third-party laboratories
FAQ
Which standard should be selected: ISO 527-3 or ASTM D882?
The choice depends on the market and the governing specification. ISO 527-3 and GB/T 1040.3 are widely used in regions following ISO-based systems, while ASTM D882 dominates in North America. The two methods differ in specimen geometry, gauge length, and test speed, so results are not directly interchangeable, and the method must be stated explicitly in any comparative report.
Why must films be tested in both the machine and transverse directions?
Biaxially oriented films such as BOPP and BOPET can show tensile strength differences of several hundred percent between the machine and transverse directions. Directional data are essential for predicting performance on packaging machinery, controlling orientation balance, and diagnosing process drift.
How many specimens are required?
A minimum of five valid specimens per direction is standard practice for average values, and additional specimens are recommended when the scatter is high or when the data will be used for certification purposes.
What are the most common causes of invalid results?
Specimen slippage in the grips, fracture inside or immediately adjacent to the grips, edge damage introduced during cutting, incorrect thickness measurement, and mismatched load cell capacity are the most frequent causes of anomalous or non-repeatable results.
What are typical tensile strength values for common films?
Representative ranges are approximately 10 MPa to 25 MPa for LDPE films, 20 MPa to 45 MPa for LLDPE and HDPE films, 130 MPa to 200 MPa for BOPP, and 170 MPa to 250 MPa for BOPET in the machine direction. Actual values depend on thickness, orientation, formulation, and processing conditions, and should always be confirmed by testing against the applicable specification.