Maximum Tensile Force Testing of Packaging Materials: Standards, Methods, and Applications

Maximum tensile force testing is a fundamental mechanical evaluation that measures the highest pulling load a packaging material can withstand before failure, providing essential data for material qualification, quality control, and package design. Performed with universal testing machines in accordance with international standards such as ISO 527, ASTM D882, and ASTM D638, this test verifies that films, foils, papers, laminates, and straps meet the strength requirements of production, filling, sealing, and distribution.

Technical Overview

Maximum tensile force testing, also known as peak load or ultimate tensile force testing, determines the greatest force a packaging material can resist when stretched along its axis until it breaks. The result, typically expressed in newtons (N) or kilonewtons (kN), represents the highest point on the force-extension curve recorded during the test. For packaging engineering, this value is a primary indicator of whether a material can survive web handling on converting lines, the mechanical stresses of filling and sealing, and the loads encountered during storage and transport.

During the test, a specimen of defined dimensions is clamped between two grips of a universal testing machine (UTM) and extended at a constant crosshead speed. A calibrated load cell continuously measures the resisting force while the machine records the force-extension curve. From this curve, the maximum tensile force, tensile strength, elongation at maximum force, and elongation at break can all be derived. Because most packaging materials are anisotropic, testing is usually performed in both the machine direction (MD) and the transverse direction (TD).

The test is governed by a family of internationally recognized methods, including ISO 527-1 and ISO 527-3 for plastics and plastic films, ASTM D882 for thin plastic sheeting, ASTM D638 for rigid and semi-rigid plastics, ASTM F88 for seal strength of flexible barrier materials, ASTM D828 and ISO 1924-2 for paper and paperboard, and corresponding national standards such as GB/T 1040.3 and GB/T 12914. Accredited third-party laboratories routinely offer these tests as part of packaging material qualification programs.

Testing Samples

Plastic Films and Sheets

Single-layer and co-extruded films such as PE, CPP, BOPP, PET, and PA are the most common samples. Thin, flexible films below 1 mm thickness are tested as rectangular strip specimens, while thicker sheets may use dumbbell-shaped specimens. Both machine and transverse directions are evaluated.

Laminated and Metallized Structures

Multilayer laminates used for food, pharmaceutical, and cosmetic packaging—such as PET/AL/PE, BOPP/VMCPP, or paper/foil/polymer combinations—are tested to verify overall laminate strength. When delamination or ply strength is of concern, additional adhesion testing may complement the tensile test.

Paper and Paperboard

Kraft paper, sack paper, corrugated liners, and folding carton board are tested to determine breaking force and breaking length, which are critical for bag-making, printing, and converting performance.

Woven and Textile-Based Packaging

PP woven sacks, jute bags, and fibrous wrapping materials require tensile evaluation of the woven tape or fabric itself, often in warp, weft, and seam orientations.

Straps, Tapes, and Seals

PET and PP strapping, pressure-sensitive tapes, tear strips, and heat-sealed fin or lap seams are tested to confirm they can carry expected loads. Seal specimens are frequently evaluated using the techniques described in ASTM F88.

Testing Items

A maximum tensile force test program typically reports the following parameters:

• Maximum tensile force (Fmax): the peak load sustained by the specimen before break, in N or kN
• Tensile strength: the maximum force divided by the original cross-sectional area, in MPa
• Elongation at maximum force: the extension of the specimen at the peak load, expressed as a percentage of the initial gauge length
• Elongation at break: the total extension at the moment of failure
• Modulus of elasticity: the slope of the initial linear region of the stress-strain curve, where applicable
• Seal or seam strength: the force required to separate a sealed joint, for sealable packaging structures
• Directional comparison: results in machine direction versus transverse direction to characterize anisotropy

Testing Methods

The general test procedure follows a consistent sequence regardless of the material type. First, specimens are die-cut or precisely slit to the dimensions required by the applicable standard. For thin films, ASTM D882 and ISO 527-3 specify rectangular strips with a typical width of 10 to 15 mm and a sufficient length for an appropriate gauge length, usually 100 mm. For thicker plastics, dumbbell specimens per ISO 527-2 (specimen types 1A/1B) or ASTM D638 (types I through V) are used. Paper specimens follow the strip geometry of ISO 1924-2 or ASTM D828.

Before testing, specimens are conditioned in a standard atmosphere—commonly 23 ± 2 °C and 50 ± 5% relative humidity per ISO 291 or ASTM D618, or ISO 187 for paper—until moisture equilibrium is reached, typically for at least 40 hours. Conditioning is essential because temperature and humidity significantly influence the tensile behavior of polymers and cellulosic materials.

Testing is performed on a calibrated universal testing machine fitted with grips appropriate to the material: pneumatic side-action grips for films, wedge or mechanical grips for rigid plastics, and capstan or wrap grips for high-elongation or high-slip materials. The crosshead speed is selected per the standard and the material's stiffness—films are commonly tested at speeds between 100 and 500 mm/min, while rigid plastics use slower rates. A small preload is applied to remove slack, and alignment is checked to avoid bending stresses. The specimen is then extended to break, and the force-extension data are recorded.

At least five valid specimens per direction are tested, and breaks that occur within the grip faces are rejected and repeated. The maximum tensile force is read directly as the peak of the curve, and tensile strength is calculated as the maximum force divided by the original width and thickness of the specimen. The final report includes individual and mean values, standard deviations, specimen dimensions, conditioning details, test speed, and the failure mode of each specimen.

Applications

Maximum tensile force testing supports packaging development and quality assurance across many industries:

• Incoming inspection: verifying that purchased films, papers, and straps conform to purchase specifications and supplier certificates
• Material selection and package design: comparing candidate structures for form-fill-seal, blister, pouch, and heavy-duty bag applications
• Supplier qualification: benchmarking materials from different sources and monitoring lot-to-lot consistency
• Food and pharmaceutical packaging validation: confirming film and laminate strength as part of overall package integrity programs
• Distribution performance support: providing material strength inputs for packages subsequently evaluated under transport simulation regimes such as ASTM D4169 or ISTA procedures
• Aging and shelf-life studies: tracking tensile properties after heat, humidity, or sterilization exposure to assess long-term durability
• Failure analysis: determining whether field failures such as splitting, tearing, or seam rupture are attributable to insufficient material strength

FAQ

What is the difference between maximum tensile force and tensile strength?

Maximum tensile force is the absolute peak load, in newtons, that the specimen withstands. Tensile strength is that peak load normalized by the original cross-sectional area of the specimen, expressed in MPa. Tensile strength allows comparison between materials of different thicknesses, while maximum force reflects the performance of the actual specimen or package component.

Which standard should be used for a given packaging material?

Thin flexible films under 1 mm are usually tested to ASTM D882 or ISO 527-3, thicker plastic sheets and molded materials to ASTM D638 or ISO 527-1/-2, paper and board to ISO 1924-2, ASTM D828, or TAPPI T 494, and heat seals to ASTM F88. National standards such as GB/T 1040.3 and GB/T 12914 define equivalent methods for the Chinese market.

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

Films, papers, and woven materials are processed in one primary direction during manufacturing, which aligns polymer chains or fibers and produces different strength in each direction. Reporting both MD and TD values ensures the package is not overstressed in either orientation during converting or use.

How many specimens are required for a valid result?

Most standards require a minimum of five valid specimens per direction. Specimens that slip in the grips or break at the grip edges are excluded, and replacement specimens are tested until the required number of valid failures is obtained.

What causes unexpectedly low maximum tensile force values?

Common causes include inadequate conditioning, incorrect specimen dimensions, grip-induced stress concentration, incorrect crosshead speed, poor load cell calibration for the force range, and genuine material degradation. Reviewing the failure mode and repeating the test with adjusted grips usually distinguishes a test artifact from a true material weakness.