Tensile Strength Determination of Novel Packaging Materials: Testing Standards, Methods, and Applica

Tensile strength determination is one of the most fundamental mechanical tests for evaluating the durability, processability, and safety of novel packaging materials such as biodegradable films, multilayer barrier laminates, and nano-reinforced polymers. This technical article explains the sample types, key testing parameters, applicable international standards, and laboratory procedures used by accredited third-party laboratories and in-house quality control teams. By understanding these methods, manufacturers can optimize material formulations, verify regulatory compliance, and ensure reliable packaging performance throughout the supply chain.

Technical Overview

Tensile strength determination measures the maximum stress a packaging material can withstand before failure when subjected to a uniaxial stretching force. For conventional polymers, this property is well characterized; however, novel packaging materials—including bio-based polymers, high-barrier coated films, recycled-content structures, and nanocomposites—often exhibit mechanical behavior that deviates significantly from traditional materials. Systematic tensile testing is therefore essential to validate their suitability for forming, filling, sealing, and distribution operations.

During a tensile test, a specimen is clamped between grips and stretched at a constant crosshead speed until rupture. The resulting stress-strain curve reveals critical parameters such as tensile strength, yield stress, elongation at break, and Young's modulus. These values directly influence packaging performance metrics, including drop resistance, puncture resistance, seal integrity, and down-gauging potential. Accurate determination also supports sustainability objectives, as emerging compostability and recyclability standards require documented mechanical performance for new material claims.

Testing Samples

Biodegradable and Bio-based Films

Films produced from polylactic acid (PLA), polybutylene adipate terephthalate (PBAT), polyhydroxyalkanoates (PHA), and starch blends are widely adopted as sustainable alternatives to conventional polyolefins. These materials are typically moisture-sensitive and anisotropic, requiring careful conditioning and directional testing (machine direction and transverse direction).

Multilayer Composite Laminates

Structures such as PET/PE, PET/AL/PE, OPP/CPP, and films incorporating EVOH or transparent oxide barrier coatings (SiOx, AlOx) are tested to assess interlayer adhesion and overall laminate strength. Delamination behavior and layer contribution to load bearing are important observations during testing.

Nanocomposite and Coated Packaging Materials

Films reinforced with nano-clay, cellulose nanocrystals, or graphene additives, as well as bio-nanocomposite coatings, are evaluated to quantify mechanical enhancement and confirm uniform dispersion of the reinforcing phase.

Recycled-Content and Mono-Material Films

As the packaging industry transitions toward circular economy targets, films containing post-consumer recycled (PCR) resin and recyclable mono-material structures are tested to verify that mechanical properties remain within specification after reprocessing.

Edible Films and Functional Coatings

Protein-based, polysaccharide-based, and lipid-based edible films used for food surface protection require tensile evaluation under strictly controlled humidity, since their properties are strongly plasticized by moisture.

Rigid and Semi-Rigid Packaging Components

Sheets, thermoformed trays, blister materials, and molded bio-based components are tested using rigid-plastic specimen geometries rather than thin-film strip methods.

Testing Items

A comprehensive tensile characterization of novel packaging materials typically includes the following parameters:

• Tensile strength (MPa), defined as the maximum force divided by the original cross-sectional area
• Tensile stress at yield (MPa), indicating the onset of plastic deformation
• Elongation at break (%), reflecting ductility and formability
• Young's modulus (MPa), quantifying stiffness in the elastic region
• Tensile energy to break (J), representing toughness
• Breaking force (N) and tensile strength per unit width for ultra-thin films
• Heat seal strength (N/15 mm) for sealable structures
• Directional comparison between machine direction (MD) and transverse direction (TD)

Testing Methods

Tensile strength determination is performed using a universal testing machine (UTM) equipped with appropriate load cells, grips, and deformation measurement devices. The principal applicable 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
• ASTM D638: Standard test method for tensile properties of plastics, applied to rigid and semi-rigid packaging components
• GB/T 1040.1 and GB/T 1040.3: Chinese national standards technically aligned with the ISO 527 series
• JIS K 7127: Japanese industrial standard for tensile properties of plastic films and sheets
• ASTM F88: Standard test method for seal strength of flexible barrier materials
• ISO 291 and ASTM D618: Standard atmospheres for conditioning and testing

Specimen preparation depends on material rigidity. Flexible films are cut into rectangular strips, commonly 15 mm in width with gauge lengths of 50 mm to 100 mm, avoiding edge defects that cause premature failure. Rigid materials are machined or die-cut into dumbbell-shaped specimens such as ISO Type 1A or ASTM Type I. Thickness is measured at multiple points along the gauge length according to ISO 4593 or ASTM D6988, since thickness variation is a major source of measurement uncertainty for thin films.

Conditioning is performed at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours, or longer for hygroscopic bio-based materials. Crosshead speed is selected based on the expected elongation of the material, typically ranging from 50 mm/min to 500 mm/min for flexible films as specified in ISO 527-3 and ASTM D882. A minimum of five valid specimens per direction is tested, and results are reported as the mean value with standard deviation. The fundamental calculations are tensile strength equal to maximum force divided by original cross-sectional area, elongation at break equal to extension at rupture divided by original gauge length, and Young's modulus determined from the slope of the initial linear region of the stress-strain curve.

For accurate results on novel materials, laboratories should verify grip slippage, use pneumatic or capstan grips with appropriate pressure for delicate films, and employ non-contact video extensometers when contact blades risk initiating cracks. Ruptures occurring within the grip region are discarded and retested.

Applications

Tensile strength data for novel packaging materials support decision-making across the entire packaging value chain:

• Food and beverage flexible packaging: validating film suitability for high-speed form-fill-seal lines and retort or pasteurization processes
• Pharmaceutical packaging: confirming mechanical integrity of blister foils, sachet laminates, and barrier films under transport and storage stresses
• E-commerce and logistics: assessing mailers, stretch films, and protective wraps against tearing during automated sorting and handling
• Sustainability compliance: documenting mechanical performance required by compostability standards such as EN 13432 and ASTM D6400, and supporting recyclability claims for mono-material designs
• Research and development: comparing bio-based formulations, nanofiller loadings, and recycling processes to guide material optimization
• Supplier qualification and incoming inspection: establishing acceptance criteria and statistical process control limits for purchased films

FAQ

Which standard should be used for flexible novel packaging films?

ISO 527-3 and ASTM D882 are the primary standards for films and sheets below 1 mm in thickness. Rigid or semi-rigid packaging components are tested according to ISO 527-1, ISO 527-2, or ASTM D638 using dumbbell specimens.

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

Film extrusion and orientation processes create anisotropy, meaning strength and elongation can differ substantially between directions. Reporting both MD and TD values provides a complete picture of in-service performance.

How does humidity affect the tensile results of bio-based films?

Materials such as PLA, starch blends, and cellulose-based films absorb moisture, which acts as a plasticizer and lowers tensile strength while increasing elongation. Strict conditioning and reporting of the test atmosphere are therefore essential.

How many specimens are required for a valid test?

Both ISO and ASTM standards require a minimum of five valid specimens per condition. Testing ten specimens improves statistical reliability, particularly for novel materials with higher variability.

Can seal strength be evaluated together with tensile strength?

Yes. Heat-sealed specimens can be tested in tensile mode following ASTM F88, providing seal strength values that complement the base film tensile characterization.