Tensile Strength Evaluation Methods for Packaging Materials: Standards, Procedures and Applications

Tensile strength evaluation is one of the most fundamental quality control procedures for packaging materials, quantifying how well plastic films, laminates, paper, tapes and woven fabrics withstand the mechanical stresses of converting, filling, sealing and distribution. This technical article explains the internationally recognized evaluation methods for packaging material tensile strength, covering applicable ISO, ASTM, EN, GB and JIS standards, specimen preparation, instrumentation, calculation of key parameters and typical industry applications. Whether you operate an in-house laboratory or rely on accredited third-party laboratories, a clear understanding of these test methods helps ensure packaging reliability, process control and regulatory compliance.

Technical Overview

Tensile strength is defined as the maximum engineering stress a material can sustain before rupturing under a uniaxial pulling load. For packaging materials, it is a primary indicator of mechanical robustness: films must survive orientation, printing, lamination and form-fill-seal operations; sacks and bags must carry product weight without splitting; seals must resist peeling during handling and transport. Tensile strength evaluation therefore underpins material qualification, supplier approval and packaging design.

The evaluation is performed on a universal testing machine (UTM), in which a conditioned specimen of standardized geometry is gripped at both ends and extended at a constant crosshead speed until failure. The resulting force-versus-elongation curve is used to derive tensile strength, yield stress, elongation at break, elastic modulus and other characteristic values. For thin films, results are frequently expressed as force per unit width (N/15 mm or N/25 mm), while for thicker sheets and rigid specimens the stress (MPa) is calculated from the cross-sectional area.

Because packaging materials are typically anisotropic, evaluation is conducted in both the machine direction (MD) and the transverse direction (TD), and results are reported separately for each orientation.

Testing Samples

Plastic Films and Sheets

Monolayer and coextruded films of PE, PP, PET, PA, PVC, EVOH-containing barrier structures, shrink films, stretch films and lidding materials with thickness typically below 1 mm. These are the most commonly tested packaging materials and are evaluated using thin-film tensile methods such as ISO 527-3 and ASTM D882.

Multilayer Laminates and Foil Structures

Flexible laminates combining PET, PA, aluminum foil, metallized films and sealant layers used in retort pouches, coffee bags and pharmaceutical blister base foils. Tensile testing of the finished laminate verifies that the composite structure delivers the strength predicted from its individual layers.

Paper and Paperboard

Kraft sack paper, corrugated linerboard and fluting, wrapping paper and coated board. In addition to tensile strength, the tensile energy absorption (TEA) is an important parameter for sack papers that must absorb shock energy during filling and transport.

Woven and Nonwoven Packaging Fabrics

Polypropylene woven sacks, FIBC bulk bags, leno bags and nonwoven shopping bags. These materials are tested using textile strip or grab test methods to reflect their yarn-based construction.

Pressure-Sensitive Tapes and Adhesive Materials

Packaging sealing tapes, labels and adhesive transfer materials, where the tensile strength of the backing determines elongation behavior and machine performance on automatic taping equipment.

Seals and Joints from Finished Packages

Heat seal and weld joints cut from finished pouches and bags are evaluated in tensile mode to determine seal strength, using methods such as ASTM F88. While distinct from bulk tensile strength, this evaluation is closely related and often performed within the same test program.

Testing Items

A comprehensive tensile strength evaluation of packaging materials typically includes the following measured and calculated items:

• Tensile strength (maximum stress sustained, in MPa, or maximum force per unit width, in N/mm)
• Tensile stress and force at break
• Yield stress and yield strain
• Elongation at break (percent)
• Tensile modulus (Young's modulus) and secant modulus (for example, 1% secant modulus for films)
• Breaking force (N) for tapes, yarns and seal strips
• Tensile energy absorption (TEA) for paper and sack materials
• Stress-strain curve characterization and failure-mode description
• Heat seal strength of seals cut from finished packages (related test)

Testing Methods

Applicable Standards

The evaluation method is selected according to the material category and the market region where compliance is required:

• ISO 527-1 and ISO 527-3: determination of tensile properties of plastics, including films and sheeting below 1 mm thickness
• ASTM D882: tensile properties of thin plastic sheeting by strip specimens
• ASTM D638: tensile properties of plastics using dumbbell-shaped (dogbone) specimens for thicker or rigid materials
• ISO 1924-2 and ISO 1924-3, ASTM D828 and GB/T 12914: tensile properties of paper and board, including TEA determination
• GB/T 1040 series and GB/T 13022: Chinese national standards for plastics and plastic film tensile testing, technically aligned with ISO 527
• JIS K 7127: Japanese standard for tensile properties of plastic films and sheeting
• ISO 29864 and ASTM D3330: tensile strength of pressure-sensitive adhesive tapes
• ISO 13934-1, ASTM D5035 and ASTM D5034: strip and grab methods for woven and textile-based packaging fabrics
• ASTM F88: seal strength of flexible barrier materials

Specimen Preparation and Conditioning

Specimens are cut with sharp dies or precision cutters to avoid edge defects that cause premature failure. Thin films are tested as rectangular strips of uniform width, commonly 15 mm or 25 mm, with sufficient length for gripping; rigid plastics use machined dogbone specimens; paper strips are typically 15 mm wide; tape specimens follow the width specified in the applicable tape standard. Directionality is controlled by cutting parallel sets in the machine and transverse directions.

Unless otherwise specified, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity, following ISO 291 for plastics and ISO 187 for paper, generally for a minimum of 40 hours before testing. Proper conditioning is essential because temperature and humidity strongly affect the modulus and ductility of polymer films and cellulose-based materials.

Instrumentation and Setup

Testing is conducted on a calibrated universal testing machine fitted with a load cell appropriate to the expected force range, verified in accordance with ISO 7500-1 or ASTM E4. Pneumatic, rubber-faced, line-contact or capstan grips are selected to prevent specimen slippage and grip-end fracture. Elongation is measured either by grip separation, suitable for extensible films, or by a contact or optical extensometer meeting ISO 9513 or ASTM E83 when accurate modulus determination is required.

Test Procedure

The general procedure consists of the following steps:

• Measure specimen width and, where stress reporting is required, thickness at multiple points along the gauge region and calculate the cross-sectional area
• Set the gauge length according to the standard, for example 100 mm for film strip tests
• Mount the specimen symmetrically in the grips and apply a small straightening preload to remove slack
• Extend the specimen at the standard-specified crosshead speed, which is selected based on the expected elongation at break and typically falls within 20 to 500 mm/min
• Record force and elongation continuously until specimen rupture
• Discard specimens that fail in or at the grips and repeat the test until a valid number of results is obtained

Data Reduction and Reporting

Tensile strength is calculated as the maximum force divided by the initial cross-sectional area, or, for films, as maximum force per unit width. Elongation at break is the increase in gauge length at rupture expressed as a percentage of the initial gauge length. The tensile modulus is determined from the slope of the stress-strain curve in its initial linear region, defined in ISO 527 between strain limits of 0.05% and 0.25%, or as a secant modulus when the curve exhibits no clear linear portion.

The test report identifies the material and construction, specimen geometry, conditioning, testing standard, crosshead speed, test direction, individual and mean values with standard deviation, and the observed failure mode. A minimum of five valid specimens per direction is generally required, with ten recommended for materials with high variability. When results are used for certification purposes, testing performed by accredited third-party laboratories to ISO/IEC 17025 provides independent assurance of validity.

Applications

Tensile strength evaluation methods for packaging materials support a wide range of industrial and regulatory activities:

• Incoming inspection of films, laminates, papers and tapes, and formal supplier qualification programs
• Material selection and packaging design, including downgauging studies that reduce material usage while maintaining protective performance
• Process validation for film orientation, lamination, curing, annealing and heat-sealing parameters
• Aging, accelerated storage and post-sterilization studies to confirm that mechanical performance is retained throughout shelf life
• Failure analysis when packages split, stretch or delaminate during filling, transport or field use
• Technical documentation and compliance files for food-contact and pharmaceutical packaging submissions
• Correlation with distribution simulation testing, where measured film strength supports drop, vibration and compression performance predictions

FAQ

What is tensile strength and why is it important for packaging materials?

Tensile strength is the maximum stress or force per width a packaging material withstands before breaking under a stretching load. It directly determines whether films, laminates, sacks and seals can survive converting, filling, stacking and transport without failure, making it a core acceptance criterion in packaging specifications.

Which standard should be used for flexible plastic film testing?

ISO 527-3 and ASTM D882 are the primary methods for thin plastic films. GB/T 13022 and JIS K 7127 are the corresponding Chinese and Japanese film standards. Because specimen geometry, gauge length and test speeds differ between these standards, results are not directly interchangeable and the specification should always name the exact method.

Why must packaging films be tested in both machine and transverse directions?

Blown and cast films are molecularly oriented during extrusion, so strength and elongation differ significantly along and across the web. Testing both MD and TD specimens characterizes this anisotropy, which is critical for predicting performance on vertical form-fill-seal machines and during pallet stretch wrapping.

How are results reported for very thin films?

For films where thickness measurement carries high relative uncertainty, tensile strength is commonly expressed as force per unit width, such as N/15 mm or N/25 mm. When stress in MPa is required, thickness must be measured at several points with a precision micrometer and the measurement method stated in the report.

How many specimens are required for a valid tensile test?

Most standards require at least five valid specimens per test direction, and ten are recommended when variability is high. Specimens that slip in the grips or break at the grip edges are discarded and replaced, as they do not represent the true material strength.

What is the difference between tensile strength and heat seal strength?

Tensile strength characterizes the bulk material, whereas heat seal strength, evaluated by methods such as ASTM F88, measures the force needed to separate a sealed joint. Both evaluations are performed in tensile mode and together they provide a complete picture of flexible package mechanical integrity.