Tensile strength analysis is one of the most fundamental mechanical tests for evaluating the performance, durability, and safety of packaging materials such as plastic films, laminates, foils, and paperboard. This technical article explains the key testing standards, sample types, measured parameters, laboratory methods, and typical industrial applications of tensile strength analysis for packaging materials. Understanding these principles helps manufacturers ensure product quality, comply with international standards such as ISO 527 and ASTM D882, and prevent costly packaging failures during production, filling, and distribution.
Technical Overview
Tensile strength analysis measures the maximum tensile stress a packaging material can withstand before failure when it is stretched under controlled conditions. During the test, a specimen of standardized dimensions is clamped between two grips of a universal testing machine and pulled at a constant crosshead speed until it breaks. The instrument continuously records the force–extension curve, from which key mechanical properties such as tensile strength, elongation at break, yield strength, and elastic modulus are calculated.
For packaging materials, tensile performance directly affects processability and end-use reliability. Films that are too weak may rupture during high-speed form-fill-seal operations, while materials with insufficient elongation may crack under impact or vacuum load. Tensile data also supports material qualification, supplier comparison, thickness optimization, and failure investigation. Because packaging materials are often anisotropic, testing is typically performed in both the machine direction (MD) and transverse direction (TD) of the web.
- Core property evaluated: maximum tensile stress at break, expressed in MPa or N/15 mm for strip specimens
- Primary international standards: ISO 527 series, ASTM D882, ASTM D638, ASTM D828, and ISO 1924-2
- Typical equipment: universal testing machine with appropriate load cell, pneumatic or mechanical grips, and extension measurement system
- Directional testing: separate MD and TD results are reported because film extrusion and lamination induce orientation
Testing Samples
Plastic Monolayer Films
Single-layer films such as polyethylene (PE), polypropylene (PP), cast polypropylene (CPP), polyethylene terephthalate (PET), polyamide (PA), and polyvinyl chloride (PVC) are tested as rectangular strips per ASTM D882 or ISO 527-3. These materials are widely used in bags, pouches, liners, and overwraps, and their tensile profile determines suitability for sealing, shrink, and load-bearing applications.
Multilayer Laminated Films
Composite structures such as PET/AL/PE, BOPP/CPP, PA/PE, and paper/foil/polyethylene laminates are tested both as complete laminates and, where required, for interlayer adhesion via peel strength testing. Laminates combine the stiffness of outer layers with the sealability of inner layers, and tensile analysis verifies that lamination has achieved the designed strength.
Aluminum Foil and Metallized Substrates
Bare and coated aluminum foils used in blister packs, cold-formable foils, and high-barrier wrappers are evaluated for tensile strength and elongation. Because foil has limited elongation, accurate measurement of breaking force and fracture behavior is essential for converting and deep-drawing operations.
Paper and Paperboard
Tensile testing of paper substrates follows ISO 1924-2, ASTM D828, or TAPPI-based methods. Parameters such as tensile strength, tensile energy absorption, and breaking length are critical for sack paper, carton board, and corrugated components.
Seals, Joints, and Converted Products
Heat seals, lap joints, welded seams, and tapered closures cut from finished packages can be tested in tension to evaluate seal integrity and consistency, complementing dedicated seal strength methods such as ASTM F88.
Testing Items
Depending on the material and the applicable specification, tensile strength analysis for packaging materials may include the following measurable parameters:
- Tensile strength: maximum force per unit cross-sectional area sustained before rupture
- Breaking force: the peak load at failure, often reported in Newtons or N per unit width
- Yield strength and yield point: the stress at which plastic deformation begins
- Elongation at break: the percentage increase in gauge length at the moment of rupture
- Tensile modulus (Young's modulus) or secant modulus: the stiffness of the material in the elastic region
- Heat seal strength: tensile force required to separate a sealed area, per ASTM F88 or related methods
- Peel strength of laminated layers: interlayer adhesion force, per ASTM F904 or comparable techniques
- Tensile energy absorption: the total energy absorbed to failure, particularly relevant for paper products
- Statistical characterization: mean values, standard deviation, and coefficient of variation across replicate specimens
Testing Methods
Testing is performed by accredited third-party laboratories or in-house quality laboratories using calibrated universal testing machines. The general procedure follows recognized international methodologies and consists of the stages described below.
- Conditioning: specimens are conditioned in a standard atmosphere, typically 23 °C ± 2 °C and 50% ± 5% relative humidity, in accordance with ISO 291 or ASTM D618, to ensure reproducible results
- Specimen preparation: plastic sheeting below 1 mm is cut into rectangular strips (for example, 15 mm or 25 mm wide) per ASTM D882; thicker plastics are die-cut into dumbbell-shaped specimens per ASTM D638 or ISO 527-2; paper strips are prepared per ISO 1924-2
- Grip setup: specimens are mounted in pneumatic or mechanical grips with defined gauge length, care being taken to avoid slippage, pre-stress, and misalignment
- Test speed: crosshead speed is selected according to the standard and material type, commonly 200–500 mm/min for thin films per ASTM D882, or lower speeds per ISO 527-3 depending on modulus range
- Data acquisition: force and extension are recorded continuously; extension is measured by grip separation, contact extensometer, or non-contact video extensometry as required by the standard
- Directional testing: at least five valid replicates are tested in both machine direction and transverse direction, and failed specimens (breaks within grips, slips) are excluded and re-tested
- Calculation and reporting: tensile strength, elongation, modulus, and related values are calculated from the force–extension curve and reported with specimen dimensions, test conditions, and standard references
Commonly referenced method standards include ASTM D882 for thin plastic sheeting, ASTM D638 and ISO 527 for rigid and semi-rigid plastics, ISO 527-3 for plastic films, ASTM D882 and ISO 1924-2 for paper substrates, ASTM F88 for flexible seal materials, and corresponding national specifications in the GB standard series for laboratories operating under Chinese regulatory frameworks.
Applications
Tensile strength analysis is applied throughout the packaging value chain, from raw material qualification to finished package validation:
- Incoming material inspection: verifying that purchased films, foils, and laminates meet contractual mechanical specifications before converting
- Food and beverage packaging: ensuring pouches, sachets, shrink films, and lidding withstand filling, sealing, pasteurization, and distribution stresses
- Pharmaceutical packaging: qualifying blister foils, tropical laminates, and barrier films for mechanical integrity in compliance with pharmacopeial and customer requirements
- Process optimization: adjusting sealing temperature, pressure, and dwell time based on heat seal tensile data to maximize line speed without compromising seal quality
- Material development: comparing new resin grades, recycled content levels, downgauging trials, and biodegradable film formulations
- Failure analysis: diagnosing package rupture, tearing, delamination, or seal leakage complaints through comparative mechanical testing
- Regulatory and customer compliance: providing test reports referenced to ISO, ASTM, EN, JIS, UL, or GB standards for audits, tenders, and export documentation
FAQ
What is tensile strength in packaging materials?
Tensile strength is the maximum stress a material can resist while being stretched before it breaks. For films and laminates it indicates how well the package will survive filling, handling, and transport loads.
Which standards are most commonly used?
Thin plastic films are typically tested to ASTM D882 or ISO 527-3, thicker plastics to ASTM D638 or ISO 527, paper and board to ISO 1924-2 or ASTM D828, and heat seals to ASTM F88. Equivalent methods also exist in the GB, JIS, and EN standard series.
Why test in both machine direction and transverse direction?
Film manufacturing processes orient polymer chains along the web, so strength and elongation differ significantly between MD and TD. Both directions must be characterized to predict real-world performance.
How many specimens are required?
Most standards require a minimum of five valid replicates per direction. Specimens that slip in the grips or break near the grips are discarded and replaced, as required by the relevant method.
What factors can affect the results?
Specimen preparation quality, conditioning atmosphere, crosshead speed, grip pressure, load cell accuracy, and specimen alignment all influence measured values, which is why testing should be performed under standardized, calibrated conditions in accredited laboratories.
Can finished packages be tested directly?
Yes. Strips cut from finished pouches, seals, and converted products can be tested to evaluate seal strength, seam integrity, and overall package mechanical quality in addition to raw film properties.