Tensile Strength Testing of Packaging Materials: Key Indicators, Standards, and Test Methods

Tensile strength testing is one of the most fundamental quality control procedures for packaging materials, measuring how much pulling force films, papers, laminates, and seals can withstand before failure. This technical guide explains the key tensile indicators for packaging materials, the applicable ISO, ASTM, EN, and GB standards, specimen preparation, test procedures, and typical industry applications. Whether you perform in-house quality control or rely on accredited third-party laboratories, this article will help you interpret tensile test data and design an appropriate testing program.

Technical Overview

Packaging materials are subjected to repeated mechanical stress throughout their life cycle: tension during film converting and printing, pulling forces on high-speed filling and sealing lines, compressive and vibration loads during stacking and transport, and finally the forces applied by the end user when opening the package. The tensile force index, therefore, serves as a primary indicator of whether a material can survive its intended processing and service environment without breaking, tearing, or delaminating.

Tensile testing is performed on a universal testing machine (UTM), which stretches a prepared specimen at a controlled rate while continuously recording force and elongation. From the resulting force-elongation curve, engineers extract tensile strength, elongation at break, yield strength, elastic modulus, and energy absorption values. For packaging structures, related tensile-mode tests such as heat seal strength and interlayer peel strength are equally important, because package integrity is often limited by the weakest seam or interface rather than by the bulk film itself.

Testing is governed by internationally recognized standards, including ISO 1924-2 and ASTM D828 for paper and paperboard, ISO 527-3 and ASTM D882 for thin plastic films, ASTM D638 for molded plastics, ASTM F88 for heat seal strength, and the corresponding GB/T standards such as GB/T 12914 and GB/T 1040.3 used in the Chinese market. Selecting the correct standard ensures that results are comparable, reproducible, and accepted by customers and regulators worldwide.

Testing Samples

Plastic Films and Sheets

Mono-layer and co-extruded films such as PE, PP, PET, PA, and EVOH-based structures with a thickness below 1 mm are tested as rectangular strips per ISO 527-3 or ASTM D882. Because blown and cast films are molecularly oriented during production, specimens must be cut in both the machine direction (MD) and transverse direction (TD) to characterize the anisotropy of the material.

Paper and Paperboard

Kraft paper, art paper, release liners, and the liner and fluting components of corrugated board are tested as narrow strips under ISO 1924-2, ASTM D828, or GB/T 12914. Paper properties are highly sensitive to moisture, so humidity conditioning is mandatory before measurement. Results may additionally be expressed as breaking length in kilometers, which normalizes strength against basis weight and allows comparison between different grades.

Composite Laminates, Metallized Films, and Foils

Multilayer flexible structures containing aluminum foil, metallized PET, or adhesive-bonded layers require both overall tensile testing and interlayer bond (peel) strength evaluation per ASTM F904. The lamination adhesive is frequently the weakest element, and tensile-mode delamination data are essential for validating retort, vacuum, and modified-atmosphere packaging designs.

Seals, Tapes, Strapping, and Nonwoven Materials

Heat-sealed seams of pouches and sachets are tested per ASTM F88 using 25.4 mm wide strips peeled at 200 mm/min. Pressure-sensitive tapes, plastic and steel strapping used for palletizing, and nonwoven wrapping materials are evaluated with their own dedicated tensile procedures, such as ISO 9073-3 for nonwovens.

Testing Items

A comprehensive tensile testing program for packaging materials typically covers the following indicators:

• Tensile strength (Rm / σt): the maximum force sustained by the specimen divided by its cross-sectional area, expressed in MPa, or by its width for thin films, expressed in N/15 mm or N/25 mm.
• Force at break: the force recorded at the moment of specimen separation, which may differ from maximum force for materials that yield before failing.
• Elongation at break (εb): the increase in gauge length at failure relative to the initial gauge length, expressed as a percentage; a key indicator of ductility and formability.
• Yield strength: the stress at which the material begins to deform plastically, critical for predicting permanent deformation of packaging during filling and handling.
• Elastic (Young's) modulus: the slope of the stress-strain curve in the elastic region, describing stiffness and web-handling behavior on printing and converting lines.
• Tensile energy absorption (TEA): the area under the force-elongation curve, expressed in J/m², representing toughness and resistance to shock loads during distribution.
• Breaking length: a paper-specific index, expressed in km, that relates tensile force per width to grammage and enables fair comparison between papers of different weights.
• Heat seal strength: the force required to peel or separate a sealed seam, expressed in N/25 mm per ASTM F88.
• Interlayer peel / bond strength: the tensile force needed to delaminate a composite structure, expressed in N/15 mm or N/25 mm per ASTM F904.

Testing Methods

Specimen Preparation and Conditioning

Specimens are die-cut or razor-slit to the width specified by the applicable standard, with clean, parallel, and defect-free edges, since nicks and burrs act as stress concentrators that cause premature failure. Film thickness is measured in accordance with ISO 4593 or an equivalent method at multiple points to calculate cross-sectional area. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 1 °C and 50 ± 5% relative humidity per ISO 187 for paper or ISO 291 / ASTM D618 for plastics, typically for a minimum of 24 to 88 hours depending on the material.

Instrumentation and Setup

A universal testing machine fitted with an appropriately sized load cell is used so that the measured forces fall comfortably within the calibrated range, generally between 20% and 80% of cell capacity. Pneumatic side-action grips with rubber-faced jaws are preferred for films, while capstan (wrap-type) grips minimize slippage and jaw breaks for strong, thin materials. Grips are aligned carefully to eliminate bending stresses, and the gauge length is set as specified, commonly 100 mm for films and paper strips.

Test Procedure

The specimen is mounted in the grips, preload is applied to remove slack, and the crosshead is driven at the standard-specified speed: 20 mm/min is typical for paper under ISO 1924-2, while film speeds commonly range from 100 to 500 mm/min depending on material elongation and the chosen standard. The machine records force and extension continuously until the specimen breaks. A minimum of five valid specimens per direction is tested, and ten or more are recommended for statistical confidence; breaks occurring at or in the jaws are evaluated against the acceptance criteria and, where permitted, discarded and replaced.

Data Processing and Expression of Results

Tensile strength is calculated as the maximum force divided by the original cross-sectional area for molded plastics, or divided by specimen width for thin films and paper. Elongation at break is computed from crosshead displacement or, where higher accuracy is required, from an extensometer. Modulus is determined from the initial linear segment, and TEA is integrated from the force-elongation curve. The report should present the mean, standard deviation, and coefficient of variation for each direction, together with the standard used, conditioning history, specimen dimensions, and test speed, so that results are traceable and repeatable.

Applications

Tensile force index testing supports decision-making across the entire packaging value chain:

• Incoming material inspection: verifying that film, paper, and laminate deliveries conform to purchase specifications before committing them to production.
• Food and beverage packaging: validating film suitability for form-fill-seal lines, retort sterilization, and vacuum or modified-atmosphere applications.
• Pharmaceutical packaging: demonstrating that blister foils, lidding materials, and pouch laminates meet stringent seal and strength requirements under GMP quality systems.
• Transport and e-commerce packaging: confirming that tapes, strapping, courier bags, and cushioning materials withstand automated sorting and distribution hazards.
• Process and formulation development: comparing resin blends, lamination adhesives, annealing conditions, or recycled-content levels during R&D and supplier qualification.
• Compliance and certification: generating standardized data packages for customer audits, regulatory submissions, and reports issued by accredited third-party laboratories operating under ISO/IEC 17025.

FAQ

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

Tensile strength characterizes the bulk material itself when stretched to failure, whereas heat seal strength measures the force needed to peel apart a sealed seam. A film may have excellent tensile strength yet exhibit poor seal performance if sealing parameters are incorrect, so both tests are typically required for flexible packaging qualification.

Which tensile standard applies to my packaging material?

Thin plastic films are tested under ISO 527-3 or ASTM D882, paper and board under ISO 1924-2, ASTM D828, or GB/T 12914, thicker plastic sheets and molded parts under ISO 527-1 or ASTM D638, and heat seals under ASTM F88. Composite laminates should also be evaluated for interlayer bond strength per ASTM F904. The choice often depends on the target market and customer specification.

Why must specimens be tested in both machine and transverse directions?

Manufacturing processes orient polymer chains in films and fibers in paper, producing pronounced anisotropy. Machine-direction strength can substantially exceed transverse-direction strength, and packages fail along the weaker axis, so both directions must be characterized to predict real-world performance.

How does conditioning affect tensile results?

Paper tensile strength decreases markedly as relative humidity rises, while plastic films become measurably weaker and more extensible at elevated temperatures. Testing without standard conditioning produces non-comparable data, which is why ISO 187 and ISO 291 conditioning is a mandatory element of every valid test report.

What causes specimens to break in the grips, and how is it handled?

Jaw-edge stress concentration, insufficient gripping pressure, or misalignment can trigger premature failure near the grips. Solutions include pneumatic or capstan grips, protective tabbing, and careful alignment; breaks within the jaws are assessed against the standard's validity criteria and replaced with additional specimens when required.

Can test reports be used for customer submissions and regulatory purposes?

Yes, provided the testing is performed strictly to the cited standard with calibrated equipment and traceable procedures. Many companies engage accredited third-party laboratories with ISO/IEC 17025 accreditation to obtain independently recognized reports for customer approval, supplier qualification, and export documentation.