Plastic Film Tensile Testing and Determination: Standards, Methods, and Key Parameters

Plastic film tensile determination measures the tensile strength, elongation, and elastic modulus of thin polymer films using a universal testing machine in accordance with ISO 527-3, ASTM D882, GB/T 1040.3, and related standards. The resulting stress–strain data are essential for quality control, material specification, and packaging design in the plastics converting industry. This article explains the applicable sample types, test items, equipment, procedures, calculations, and typical applications of plastic film tensile testing.

Technical Overview

Plastic film tensile testing is the standardized method for quantifying how a thin polymeric film responds to uniaxial tensile loading. A rectangular strip specimen is clamped between the grips of a universal testing machine and pulled at a constant crosshead speed while force and extension are recorded continuously. The resulting stress–strain curve provides the tensile strength, yield behavior, elongation at break, and tensile modulus values that determine whether a film can withstand the stresses of converting, printing, lamination, filling, sealing, transport, and end use.

Because films are produced by blown extrusion, cast extrusion, or biaxial orientation, their mechanical properties are strongly directional. Testing is therefore performed in both the machine direction (MD) and the transverse direction (TD) to characterize the anisotropy introduced during processing. The method applies to films and sheeting with a thickness of 1 mm or less; thicker materials are instead evaluated as molded or machined specimens under ISO 527-2 or ASTM D638.

• Lot release and routine quality control against material specifications
• Comparison of resin formulations, additives, and process settings during development
• Verification of incoming film lots at converters and accredited third-party laboratories
• Input data for packaging design, machine setup, and finite-element simulation

Testing Samples

Representative sample types routinely submitted for plastic film tensile determination include the following categories.

Polyolefin Films

Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE) blown films, cast polypropylene (CPP), in-line polypropylene (IPP), and biaxially oriented polypropylene (BOPP). These represent the largest volume of packaging films for bags, pouches, shrink applications, and lamination webs.

Polyester and Polyamide Films

Biaxially oriented polyester (BOPET/PET) and polyamide (nylon, PA) films, which exhibit high tensile strength and dimensional stability and are widely used as barrier layers in laminated structures and as substrates for metallization and coating.

Multilayer, Laminated, and Coated Films

Coextruded structures, metallized films, and adhesive or extrusion laminates. Tensile data confirm that the finished laminate behaves as intended and help identify weak or delamination-prone interfaces between plies.

Specialty and Sustainable Films

Compostable films based on PLA, PBAT, PBS, and starch blends; medical-grade and pharmaceutical packaging films; battery separator and capacitor-grade films; and technical films for flexible electronics that require tight strength tolerances.

Testing Items

The following parameters are determined for each test direction and reported as arithmetic means with standard deviations.

• Tensile strength (maximum tensile stress, MPa)
• Tensile stress at break (MPa)
• Tensile stress at yield (MPa)
• Elongation at break (%)
• Elongation at maximum force (%)
• Tensile modulus (secant or chord modulus, MPa)
• Tensile energy per unit volume (toughness, MJ/m³)
• Failure mode description, such as ductile tearing, brittle fracture, or necking

Tensile strength is calculated as the maximum force sustained by the specimen divided by its original cross-sectional area. Yield stress identifies the point at which the film begins to deform plastically, which is critical for stretch films and thermoforming webs. Elongation at break reflects the ductility of the film, while the tensile modulus characterizes stiffness in the low-strain region and is defined in ISO 527-1 as the secant between 0.05% and 0.25% strain.

Testing Methods

Applicable Standards

• ISO 527-1 and ISO 527-3: determination of tensile properties of plastics, Part 3 covering film and sheeting up to 1 mm
• ASTM D882: tensile properties of thin plastic sheeting less than 1 mm in thickness
• GB/T 1040.3: Chinese national standard corresponding to ISO 527-3
• JIS K 7127: Japanese standard for tensile properties of plastic film and sheeting
• ISO 291 or ASTM D618: conditioning atmospheres prior to testing
• ISO 4593, ISO 4591, or ASTM F2251: determination of film thickness

Specimen Preparation

Rectangular strips are cut with a sharp die cutter or razor slitter to avoid nicks and edge defects that cause premature failure. The preferred specimen width is 10 mm, with overall lengths of at least 150 mm sufficient for a grip separation of 100 mm. Specimens are prepared in both the machine direction and the transverse direction, and at least five valid specimens per direction are required. Width is measured with a calibrated microscope or gauge, and thickness is measured at several points along the test length using a dead-weight micrometer with 1 µm resolution; for very thin films, gravimetric thickness determination may be used.

Conditioning

Specimens are conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 24 hours in accordance with ISO 291, or 40 hours per ASTM D618 Procedure A, and tested in the same atmosphere unless otherwise specified. Because polyamide and other hygroscopic films are moisture sensitive, conditioning is especially important for reproducible results.

Equipment Setup

A universal testing machine fitted with a load cell of suitable capacity, typically 50 N to 500 N for films, ensures that measured forces fall within the calibrated working range. Pneumatic side-action grips with rubber-faced inserts are preferred because they maintain constant clamping pressure without crushing or slipping the specimen. Elongation may be determined from grip separation for ductile films with large strains, but an extensometer, either a clip-on mechanical device or a non-contact video system with gauge-length accuracy in the micron range, is required for reliable modulus measurement because machine compliance distorts small-strain data.

Procedure and Calculation

The specimen is mounted axially between the grips, typically at an initial grip separation of 100 mm, a small preload is applied to remove slack, and the crosshead is driven at a constant speed selected according to the governing standard and the expected elongation of the material, commonly between 50 mm/min and 500 mm/min. Force and extension are recorded until specimen break. Specimens that fail inside the grips or slip are invalid and must be retested.

Tensile strength is calculated as the maximum force divided by the original cross-sectional area, stress at break as the force at break divided by the same area, and elongation as the extension at break expressed as a percentage of the initial gauge length. The tensile modulus is the slope of the stress–strain curve between 0.05% and 0.25% strain per ISO 527-1, or the chord modulus between the designated strain points defined by ASTM D882. The test report should identify the material and thickness, specimen direction, standard used, test speed, conditioning, and the mean and standard deviation of each reported parameter.

Applications

Tensile data from plastic film testing support decisions across the entire value chain, from resin producers to end users.

• Food packaging: qualifying webs for form-fill-seal operations, verifying lamination strength after printing, coating, and adhesive bonding
• Stretch and shrink films: confirming yield strength and elongation at break required for pallet load containment and heat-shrink performance
• Agricultural films: evaluating mulch films and greenhouse covers against installation loads and wind-induced stresses
• Pharmaceutical and medical packaging: supporting material qualification for blister lidding and sterile barrier systems within medical packaging frameworks
• Industrial and consumer products: bags, liners, adhesive tapes, battery separators, and flexible electronic substrates
• Manufacturing and research: statistical process control, incoming inspection by accredited third-party laboratories, aging and shelf-life studies, and comparative evaluation of recyclate-containing or bio-based formulations

FAQ

Should I test to ASTM D882 or ISO 527-3?

Both standards cover films up to 1 mm in thickness and produce comparable parameters, but they differ in preferred specimen geometry, recommended test speeds, and modulus definition, so results may not be directly interchangeable. The choice is normally dictated by the contractual specification, the target market, or the referencing regulatory document.

Why must both machine direction and transverse direction be tested?

Blown, cast, and biaxially oriented films develop molecular and morphological orientation during production, so tensile strength and elongation can differ substantially between MD and TD. Biaxially oriented films such as BOPP and BOPET are designed specifically for balanced or tailored directional properties, and single-direction testing would not represent real-world loading.

How many specimens are required for a valid result?

A minimum of five valid specimens per direction is required by the standards. Specimens that break within the grips, slip, or fail at an edge defect are excluded and replaced. Means and standard deviations are reported for each direction, and coefficients of variation above expected levels typically trigger investigation of specimen preparation or material uniformity.

What can be done if specimens break in the grips or slip during testing?

Use pneumatic grips with rubber inserts, reduce clamping pressure to the minimum that prevents slip, verify axial alignment, and ensure specimens are long enough for adequate grip engagement. Gentle preconditioning of the grips and recutting specimens with a sharp die to eliminate edge nicks also reduce invalid failures.

Can elongation be measured from crosshead displacement instead of an extensometer?

For ductile films with large elongations, grip-separation-based strain is widely accepted and practical. However, for modulus determination, an extensometer is necessary because frame and grip compliance introduces errors that are significant at strains below 0.25%. When an extensometer is used, it should be removed or disengaged before break for highly extensible films to avoid damage.

How does film thickness influence the test configuration?

Thinner films require lower-capacity load cells, gentler clamping, and more precise thickness measurement to keep cross-sectional area errors within acceptable limits. Films above 1 mm fall outside the scope of the film standards and should be tested as rigid specimens under ISO 527-2, ASTM D638, or GB/T 1040.2.