Determination of Tensile Properties of Packaging Materials: Standards, Methods, and Applications

Tensile property testing is one of the most fundamental mechanical evaluations for packaging materials, quantifying how films, foils, papers, and laminates respond to stretching forces during manufacturing, filling, sealing, and distribution. This article provides a comprehensive technical overview of tensile strength determination for packaging materials, covering key standards such as ISO 527 and ASTM D882, specimen preparation, test procedures, measured parameters, and typical industry applications.

Technical Overview

Tensile testing, also referred to as tension testing, is the most widely performed mechanical test for characterizing the strength and extensibility of packaging materials. During the test, a precisely prepared specimen is clamped between two grips of a universal testing machine (UTM) and stretched at a controlled constant speed until it yields, necks, or fractures. The instrument continuously records the force applied and the corresponding extension, generating a stress-strain curve from which the essential mechanical parameters are derived.

For packaging materials, tensile performance directly determines whether a package can survive the mechanical stresses of real-world handling. Films must withstand the tension of form-fill-seal machines, laminates must resist stretching during lamination and printing, paper sacks must carry static loads, and strapping must secure palletized goods during vibration and shock in transit. Poor tensile performance leads to web breaks, seal failures, package rupture, and product loss, which is why manufacturers, converters, and brand owners routinely verify tensile properties as part of material qualification and routine quality control.

Testing is typically conducted by accredited third-party laboratories or in-house quality departments using electromechanical universal testing machines equipped with appropriate load cells, pneumatic or manual grips, and extensometers. Because results are sensitive to specimen geometry, gripping pressure, and environmental conditions, strict adherence to recognized international standards such as ISO 527, ASTM D882, ASTM D638, and ISO 1924 is essential for obtaining reproducible and comparable data.

Testing Samples

Plastic Films and Sheets

Single-layer and multilayer plastic films are the most commonly tested packaging materials, including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), polyvinyl chloride (PVC), and ethylene-vinyl alcohol (EVOH) barrier films. Thin films below 1 mm in thickness are usually tested as rectangular strips in accordance with ASTM D882 or ISO 527-3, both in the machine direction (MD) and transverse direction (TD), since film properties are frequently anisotropic due to orientation during extrusion or blowing.

Paper and Paperboard

Paper sacks, kraft paper, corrugated liners, and carton board are tested as strips cut in both principal directions following ISO 1924-2, ASTM D828, or the Chinese standard GB/T 12914. Paper testing requires specific attention to humidity conditioning because cellulose-based materials are highly hygroscopic and their tensile strength can vary significantly with moisture content.

Foils and Flexible Laminates

Aluminum foil, metallized films, and multi-layer flexible laminates used for food, pharmaceuticals, and coffee packaging are tested to evaluate the contribution of each layer and the integrity of the composite structure. For these materials, both free tensile strength and seal or laminate bond strength are commonly evaluated, using methods such as ASTM F88 for seal strength of flexible barrier materials.

Woven Fabrics, Strapping, and Tapes

Woven polypropylene bags, jute and textile-based wraps, PET and steel strapping, and pressure-sensitive adhesive tapes used in case sealing are tested using modified strip methods with reinforced or capstan grips to prevent premature failure at the jaw faces. Breaking force and elongation at a specified load are the primary reported parameters for these product categories.

Testing Items

A full tensile characterization of a packaging material typically includes the following measured and calculated parameters, each of which describes a distinct aspect of mechanical behavior:

• Tensile strength: the maximum tensile stress sustained by the specimen before rupture, expressed in MPa for rigid materials or N per unit width (for example, N/15 mm) for films and papers.
• Yield strength: the stress at which the material exhibits the first departure from linear elastic behavior, indicating the onset of permanent deformation.
• Elongation at break: the increase in gauge length at the moment of rupture, expressed as a percentage of the initial gauge length, which reflects material ductility and formability.
• Elastic modulus (Young's modulus): the ratio of stress to strain within the initial linear region of the stress-strain curve, describing material stiffness and resistance to stretching.
• Secant modulus: the slope between two predefined strain points, used when the stress-strain curve lacks a distinct linear region, as is common for many soft polyolefin films.
• Breaking force: the maximum force recorded during the test, reported in newtons (N), which is the standard result format for strapping, tapes, and seals.
• Tensile energy absorption (TEA): the total energy absorbed by the specimen up to failure, equal to the area under the force-extension curve, particularly important for sack kraft papers.
• Stress-strain curve: the complete graphical record of the test, which reveals strain hardening, necking behavior, and failure mode.
• Seal and bond strength: evaluated on sealed or laminated specimens to quantify the force required to separate joined layers under peel or tensile loading.

Testing Methods

Applicable Standards

The selection of the test method depends on the material type, thickness, and end use. The most widely applied standards include:

• ISO 527-1 and ISO 527-3: determination of tensile properties of plastics, with Part 3 covering films and sheets less than 1 mm thick.
• ASTM D882: standard test method for tensile properties of thin plastic sheeting (film thickness below 1 mm).
• ASTM D638: tensile properties of plastics using dumbbell-shaped specimens for thicker sheets and molded materials.
• ISO 1924-2 and ASTM D828: tensile properties of paper and paperboard using constant rate of elongation apparatus.
• ASTM F88/F88M: seal strength of flexible barrier materials.
• ASTM D5034 and ASTM D5035: breaking strength and elongation of textile fabrics by the grab and strip methods, applicable to woven packaging fabrics.
• GB/T 1040, GB/T 12914, and GB/T 21302: Chinese national standards corresponding to plastics, paper, and flexible packaging laminate testing.

Specimen Preparation and Conditioning

Specimens are cut from representative samples free from visible defects, wrinkles, and edge damage. For film testing, rectangular strips are typically 15 mm or 25 mm wide with a gauge length of 100 mm (ASTM D882) or 50 mm to 150 mm depending on elongation class (ISO 527-3). At least five valid specimens are tested in each direction, and more when results are scattered. All specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity in accordance with ISO 291 or ASTM D618 for a minimum of 40 hours (88 hours for paper and board per ISO 187) before testing.

Test Procedure

The conditioned specimen is mounted centrally in the grips with minimal slack and consistent preload, typically 0.1 MPa to 1 MPa for films. Care is taken to avoid excessive grip pressure that could cause jaw breaks, which are invalid results. The crosshead speed is selected according to the material and standard: common speeds include 50 mm/min, 100 mm/min, 200 mm/min, or 300 mm/min for highly extensible films, and 20 mm/min for rigid specimens. The test continues until specimen fracture, during which the force-extension data are acquired at a sufficiently high sampling rate to capture yield and break points accurately.

Data Evaluation and Reporting

Tensile strength is calculated as the maximum force divided by the original cross-sectional area (or by specimen width for films), while elongation at break is calculated as the extension at break divided by the initial gauge length multiplied by 100. The modulus is determined from the initial linear slope of the stress-strain curve, and the tensile energy absorption is computed by numerical integration of the force-extension curve. The test report should identify the material and its construction, specimen dimensions, conditioning atmosphere, test speed, grip type, the standard followed, individual and mean results for each direction, and the failure mode of each specimen. Results from specimens that fail within the grips are excluded and replaced with additional tests.

Applications

Tensile property determination supports quality assurance, material development, and regulatory compliance across the entire packaging supply chain. Typical application scenarios include:

• Flexible food packaging: verifying the machinability and durability of films used on vertical form-fill-seal and horizontal flow-wrap lines, where consistent tensile strength and modulus prevent web breaks and wrinkling.
• Pharmaceutical packaging: qualifying blister foil laminates, sachet films, and cold-formable aluminum structures for mechanical integrity in accordance with pharmacopeial and regulatory expectations.
• Heavy-duty and industrial packaging: evaluating sack kraft paper, woven PP bags, and FIBC materials for safe filling weights and stacking resistance during storage and transport.
• Sealing and lamination validation: measuring seal strength and interlayer bond performance to optimize heat-sealing parameters and adhesive systems.
• Logistics and unitization: confirming the breaking force and elongation of stretch films and strapping to secure palletized loads against vibration and shock.
• Material comparison and supplier qualification: benchmarking incoming raw materials against agreed specifications, supporting root-cause analysis of field failures, and validating reformulated or recycled-content materials in research and development programs.

FAQ

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

Tensile strength testing measures the force required to break the base material itself, whereas seal strength testing measures the force required to separate two sealed layers at the seal interface. Seal strength is commonly determined by methods such as ASTM F88, and both tests are frequently performed together to fully characterize a package's mechanical performance.

Why must films be tested in both machine direction and transverse direction?

Film manufacturing processes such as blown extrusion and cast orientation impart directionality to the polymer chains, causing the tensile strength, modulus, and elongation to differ significantly between the machine direction and transverse direction. Testing both directions provides a complete picture of anisotropic behavior relevant to package forming and performance.

What causes jaw breaks and how are they avoided?

Jaw breaks are premature failures occurring at or inside the grips due to stress concentration, excessive clamping pressure, or grip serration biting into soft films. They can be minimized by using pneumatic grips with rubber-faced inserts, capstan or wrap-around grips for slippery materials, reinforcing specimen ends, and applying only the clamping pressure needed to prevent slippage.

How many specimens are required for a valid test?

Most standards require a minimum of five valid specimens per direction for films and plastics, and ten determinations per direction are recommended for paper and board due to its inherent variability. Specimens failing in the grips or outside the gauge section are discarded and retested until the required number of valid results is obtained.

How do temperature and humidity affect tensile results?

Polymeric films become softer and more extensible at elevated temperatures, while cellulose-based papers lose strength as humidity increases. Standard conditioning at 23 °C and 50% relative humidity, as specified in ISO 291 and ISO 187, is therefore mandatory before testing, and any results obtained under non-standard conditions must be clearly reported with the actual test environment.

Can finished packages be tested instead of flat material?

Yes. Cut strips taken from finished packages, side seams, and seal areas can be tested to verify real-world performance after conversion. In addition, complementary whole-package tests such as internal pressure resistance and drop impact are often combined with tensile data to assess overall package integrity in accredited third-party laboratories.