Plastic Film Tensile Breaking Strain Testing: Standards, Methods, and Applications

Tensile breaking strain, commonly known as elongation at break, is one of the most critical mechanical parameters for evaluating the ductility, formability, and reliability of plastic films used in packaging, agriculture, and industrial applications. This test is typically performed in accordance with ISO 527-3, ASTM D882, and GB/T 1040.3 using universal testing machines in accredited third-party laboratories. This article provides a comprehensive technical overview of plastic film tensile breaking strain testing, covering test principles, sample requirements, procedures, and typical applications.

Technical Overview

Tensile breaking strain, also referred to as elongation at break, is defined as the increase in length of a plastic film specimen at the moment of rupture, expressed as a percentage of its initial gauge length. It quantifies the capacity of a film to undergo plastic deformation before failure and is a direct indicator of ductility, extensibility, and toughness. The parameter is calculated using the following relationship:

Breaking strain (%) = (Lb − L0) / L0 × 100, where L0 is the initial gauge length (or grip separation) and Lb is the length at break expressed as extension.

The test is conducted on a universal testing machine equipped with pneumatic or manual grips and a suitable load cell. The specimen is stretched at a constant crosshead speed until it ruptures, while the force-extension curve is recorded continuously. From this curve, the strain at break is determined together with other tensile properties such as tensile strength and yield behavior. For thin and highly extensible films, elongation is normally derived from grip separation or crosshead displacement rather than a contact extensometer, in line with the provisions of ASTM D882 and ISO 527-3. Testing in accredited third-party laboratories ensures traceable, reproducible results suitable for quality control, supplier qualification, and product certification.

Testing Samples

Polyolefin Films

This category includes blown and cast films of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), metallocene polyethylene (mPE), cast polypropylene (CPP), biaxially oriented polypropylene (BOPP), and biaxially oriented polyethylene (BOPE). These materials range from highly extensible lamination and heavy-duty films to stiff, high-clarity orientation films.

Polyester and Polyamide Films

Biaxially oriented polyester (BOPET), unoriented PET films, and biaxially oriented polyamide (BOPA) films are widely tested. These films exhibit relatively low breaking strain but high tensile strength, and accurate strain measurement is essential for assessing orientation balance and process consistency.

Barrier and Specialty Films

Samples also include high-barrier structures such as EVOH and PVDC-coated films, aluminum-metallized films, silicon oxide and aluminum oxide vapor-deposited barrier films, retort-grade films, heat-shrink films, and stretch films used in pallet wrapping and protective packaging.

Multilayer and Composite Films

Laminated flexible packaging structures combining layers such as PET/AL/CPP, BOPP/VMPET/PE, and PA/PE are routinely evaluated. Breaking strain data for each layer and for the finished laminate support structure design, lamination process control, and overall package performance assessment.

Other Film Materials

Additional testable materials include PVC films, polystyrene films, PLA and other biodegradable films, cellulose-based films, and films for labels, tapes, medical packaging, and photovoltaic encapsulation.

Testing Items

The primary test item is the tensile breaking strain (elongation at break) of the plastic film. During the same tensile test, the following related properties can also be determined to provide a complete mechanical profile:

• Tensile breaking strain (elongation at break) in machine direction (MD) and transverse direction (TD)
• Tensile strength at break
• Tensile stress at yield and strain at yield
• Tensile modulus of elasticity or 1% secant modulus
• Force-extension and stress-strain curves for engineering analysis
• Statistical evaluation including mean values and standard deviation

Testing Methods

Applicable Standards

Testing is generally performed according to internationally and nationally recognized standards, including:

• ISO 527-3: Plastics — Determination of tensile properties — Part 3: Test conditions for films and sheets
• ASTM D882: Standard Test Method for Tensile Properties of Thin Plastic Sheeting
• GB/T 1040.3: Chinese national standard, technically aligned with ISO 527-3
• JIS K 7127: Japanese standard for tensile properties of plastics films and sheets
• ISO 291 / ASTM D618 / ASTM D618 conditioning standards for specimen conditioning atmospheres

Specimen Preparation

Rectangular strip specimens are cut from the film in both the machine direction and the transverse direction. Typical specimen widths range from 10 mm to 25 mm, with specimen lengths of at least 150 mm to allow adequate clamping beyond the gauge length. Specimen thickness is measured at multiple points using methods consistent with ISO 4593 or ASTM D6988. Specimens must be free from wrinkles, nicks, edge defects, and visible flaws, as such imperfections cause premature failure and distort the strain results.

Conditioning and Test Environment

Specimens are conditioned prior to testing in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 40 hours, unless otherwise specified by the material standard or the client. Testing is performed under the same controlled conditions to minimize the influence of temperature and moisture on the mechanical response of hygroscopic films such as polyamide.

Test Procedure

The test is executed on a calibrated universal testing machine using appropriately rated grips, often with soft faces or pneumatic pressure to prevent slipping and jaw breaks. A specified initial grip separation, commonly 100 mm, serves as the gauge length. After mounting the specimen and applying a small straightening preload, the crosshead is driven at a constant speed until the specimen ruptures. Crosshead speeds typically range from 10 mm/min to 500 mm/min depending on the expected elongation and the selected standard; highly extensible films such as LDPE are commonly tested at higher speeds, while less extensible oriented films are tested at lower speeds. The force-extension data are recorded continuously throughout the test.

Data Processing and Reporting

The breaking strain is calculated from the extension at break divided by the initial gauge length, expressed as a percentage. At least five valid specimens are tested for each direction, and specimens that fail within or immediately adjacent to the grips are typically discarded and retested. The final report includes individual and mean breaking strain values with standard deviation, specimen orientation, dimensions and thickness, test speed, conditioning details, and the applicable standard. This ensures full traceability and comparability of results between laboratories.

Applications

Tensile breaking strain testing supports quality control, materials development, and compliance verification across the entire plastic film value chain. Typical applications include:

• Incoming inspection and supplier qualification for packaging converters and film buyers
• Quality control of blown film, cast film, and biaxial orientation production lines
• Verification of mechanical performance for food, pharmaceutical, and medical packaging materials
• Evaluation of agricultural films, mulch films, greenhouse films, and silage films
• Assessment of stretch films, shrink films, tapes, labels, and protective films
• Research and development of new resin formulations, multilayer structures, and sustainable or biodegradable films
• Failure analysis of film breakage during printing, lamination, bag making, filling, or transportation
• Compliance testing and third-party verification in accredited laboratories for regulatory or customer requirements

FAQ

What is tensile breaking strain and how is it calculated?

Tensile breaking strain is the percentage elongation of a film specimen at the moment it ruptures. It is calculated by dividing the extension at break by the initial gauge length and multiplying by 100. A higher value indicates greater ductility and the ability to withstand greater deformation before failure.

What is the difference between breaking strain and tensile strength?

Tensile strength measures the maximum stress a film can withstand before breaking, while breaking strain measures the maximum deformation it can sustain before breaking. A film can be strong but brittle, or weaker but highly extensible, so both parameters together provide a complete picture of mechanical performance.

Why are specimens tested in both machine and transverse directions?

Manufacturing processes such as blown extrusion and biaxial orientation introduce molecular orientation, causing mechanical properties to differ between the machine direction and the transverse direction. Testing both directions is necessary to characterize anisotropy, evaluate orientation balance, and predict real-world performance during downstream processing.

How many specimens are required for a reliable result?

Most standards require at least five valid specimens per test direction. Specimens that break in or immediately adjacent to the grips are excluded from the calculation, and additional specimens are tested until the required number of valid results is obtained.

What factors can affect the accuracy of breaking strain results?

Key influencing factors include specimen preparation quality, edge defects, accurate thickness measurement, correct gauge length setting, crosshead speed selection, conditioning temperature and humidity, and the prevention of grip slippage or jaw breaks. Strict adherence to standard procedures minimizes variability and ensures reproducible results.

Can the test be performed on multilayer composite films?

Yes. The method is fully applicable to laminated and coextruded structures. In these cases, the measured breaking strain reflects the overall behavior of the laminate, and the test is valuable for comparing adhesive lamination quality, sealant film selection, and structural design changes.