Peel strength and tensile strength testing of packaging composite materials is a critical quality control process that evaluates the bonding integrity between laminate layers and the mechanical durability of multilayer flexible packaging. This technical article explains the testing standards, sample types, test methods, and equipment parameters used by accredited third-party laboratories to verify that composite packaging structures such as PET/AL/PE and BOPP/CPP laminates meet international performance requirements.
Technical Overview
Packaging composite materials, commonly known as flexible laminated films, are multi-layer structures formed by bonding together substrates such as polyethylene terephthalate (PET), biaxially oriented polypropylene (BOPP), nylon (PA), aluminum foil (AL), cast polypropylene (CPP), and polyethylene (PE) using adhesives, extrusion coating, or co-extrusion processes. The overall performance of the finished package—its barrier properties, sealability, puncture resistance, and shelf-life performance—depends directly on two fundamental mechanical characteristics: the peel strength between adjacent laminate layers and the tensile strength of the composite structure itself.
Peel strength testing measures the force required to separate two bonded layers of a laminate along their interface, typically expressed in newtons per unit width (N/15 mm or N/mm). Insufficient interlayer adhesion can lead to delamination during filling, transportation, or storage, resulting in loss of barrier function and product spoilage. Tensile strength testing, on the other hand, determines the maximum tensile stress the laminate can withstand before fracture, together with elongation at break and modulus values, which describe the material's ability to survive mechanical stresses during converting, handling, and distribution.
These tests are routinely performed by accredited third-party laboratories and in-house quality control departments in accordance with internationally recognized standards published by ISO, ASTM, and GB (Chinese national standards), providing objective data for material qualification, supplier comparison, adhesive system optimization, and failure analysis.
Testing Samples
Flexible Laminated Films
The most common test articles are two-layer and multi-layer laminates used for food, beverage, and consumer goods packaging. Typical structures include BOPP/CPP, PET/PE, PA/PE, PET/AL/PE, and BOPP/VMPET/CPP. Specimens are usually die-cut into strips with a width of 15.0 mm and a length of at least 150 mm, ensuring the bond line is uniform and free from visible defects, bubbles, or wrinkles.
Aluminum Foil Laminates
Laminates incorporating aluminum foil, such as paper/AL/PE and PA/AL/CPP structures used for coffee, pharmaceuticals, and retort pouches, require special attention during specimen preparation because foil layers are prone to tearing. Pre-separation of the interface by mechanical means or by selective chemical activation may be required to initiate the peel process.
Heat-Sealed and Finished Package Specimens
Seal strength testing is performed on strips cut from heat-sealed pouches, sachets, lids, and blister foils. Finished packages such as stand-up pouches, three-side-seal bags, and lidded cups may be tested directly to evaluate seal integrity at the pouch level in addition to flat-film laminate testing.
Coated and Metallized Substrates
Metallized films (VMPET, VMCPP), coated papers, and extrusion-coated structures are tested to verify coating adhesion and anchor layer performance, since delamination of a metallized layer dramatically reduces oxygen and light barrier properties.
Testing Items
- Peel strength (T-peel and 180-degree peel) of laminate interfaces, reported as average and peak peel force per unit width
- Tensile strength and tensile stress at break of the composite laminate in machine direction (MD) and transverse direction (TD)
- Elongation at break and percentage strain at maximum force
- Heat seal strength of sealed joints, including hot tack evaluation where applicable
- Bond strength between aluminum foil and polymer or paper layers
- Seal strength of finished packages measured on complete pouches
- Yield strength and elastic modulus for engineering analysis purposes
- Failure mode classification (adhesive failure, cohesive failure, or substrate tear)
Testing Methods
Peel Strength Testing (T-Peel Method)
The T-peel method is the primary technique for flexible laminate laminated film structures and follows ASTM D1876 and GB/T 8808. An initial separation is created at one end of the specimen between the two layers to be evaluated, typically over a length of approximately 25 to 50 mm. The two separated legs are clamped in the grips of a universal testing machine so that the peel front travels along a plane perpendicular to the applied force, forming a T configuration. The test runs at a constant crosshead speed, commonly 300 mm/min as specified in GB/T 8808, and the peel force is recorded continuously. The average peel force over the central portion of the force-displacement curve, excluding initiation and termination regions, is divided by the specimen width to obtain the peel strength result.
Peel Strength Testing (180-Degree Method)
For laminates where one layer is flexible and the other is a relatively rigid backing, the 180-degree peel configuration per ASTM D3330 and ISO 8510-2 is applied. The flexible layer is peeled back over itself at 180 degrees while the rigid substrate is supported on a rigid carrier. This method is widely used for coated papers, label stocks, and laminates containing stiff barrier layers.
Tensile Strength Testing
Tensile properties of composite films are determined using a universal testing machine equipped with appropriate grips and a constant crosshead speed, in accordance with ISO 527-3, ASTM D882, and GB/T 1040.3. Specimens are longitudinal strips, commonly 15 mm wide with a gauge length of 100 mm (or 50 mm gauge length for very extensible films), tested in both machine and transverse directions. The crosshead speed is selected according to the expected elongation of the material, typically 100 to 500 mm/min. The machine records the maximum force and the extension at break, from which tensile strength and elongation at break are calculated.
Heat Seal Strength Testing
Seal strength is measured per ASTM F88 and GB/T 2358 using specimens cut from laboratory-prepared seals or from finished packages. Sealed strips are pulled apart in a tensile tester at a defined speed, commonly 200 to 300 mm/min, and the maximum force divided by the specimen width is reported as seal strength. The failure mode—adhesive separation, cohesive failure within the sealant layer, or film tearing—must also be documented because it provides diagnostic information about the sealing process window.
Conditioning and Environmental Considerations
All specimens must be conditioned prior to testing, typically at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours according to ISO 291 and ASTM D618, unless the product standard specifies otherwise. For retort or sterilizable packaging, testing after simulated processing (autoclave exposure) is frequently specified to evaluate residual bond strength. A minimum of five to ten replicate specimens per direction and per interface is recommended to ensure statistical validity.
Instrumentation
Testing is performed on universal testing machines (tensile testers) with load cells selected so that measured forces fall between 10% and 90% of the cell capacity for optimal accuracy. Accessories include pneumatic or mechanical film grips with smooth or rubber-faced faces, 180-degree peel fixtures, and software capable of calculating average peel force, peak force, statistical parameters, and force-displacement curves.
Applications
- Incoming material inspection and supplier qualification for laminated film roll stock in food, beverage, and pharmaceutical packaging production
- Adhesive formulation comparison and lamination process optimization, including curing time and coating weight studies
- Heat seal process window development for form-fill-seal (FFS) and pouch-making operations
- Verification of performance after retort, pasteurization, sterilization, or deep-freeze exposure
- Failure analysis of delaminated or burst packages during transport simulation and shelf-life studies
- Compliance demonstration for regulatory submissions and customer specifications requiring certified test data
- Research and development of new structures such as recyclable mono-material laminates with functional barrier coatings
FAQ
What is the difference between peel strength and seal strength?
Peel strength measures the adhesion between two pre-bonded layers of a laminate created during the lamination process, while seal strength measures the force needed to separate two layers joined by a heat-sealing operation at the converting or filling stage. Both are expressed as force per unit width but evaluate different interfaces and different process variables.
Which standards are most commonly referenced for these tests?
Peel strength of flexible laminates is typically tested to GB/T 8808, ASTM D1876, and ISO 8510 series; 180-degree peel follows ASTM D3330; tensile properties follow GB/T 1040.3, ISO 527-3, and ASTM D882; and heat seal strength follows ASTM F88. The choice of standard should align with the customer specification and the intended market region.
Why do peel results sometimes show substrate tearing instead of clean separation?
When the adhesion between layers exceeds the tear resistance of one of the substrates, the weaker film tears before the interface separates. This is generally regarded as evidence that bond strength is at least equal to the strength of the failing layer, and the failure mode should be recorded in the test report as substrate tear rather than interfacial peel.
How should specimens be prepared to obtain reproducible peel data?
Specimens must be die-cut cleanly with parallel edges at the specified width, conditioned in a controlled atmosphere, and pre-separated carefully without stretching or contaminating the interface. Testing speed, gauge geometry, and the length of peel over which the average force is calculated must be held constant, as all of these parameters measurably influence the reported peel strength.
Can peel and tensile tests be performed on finished packages?
Yes. Seal strength of complete pouches can be measured directly by cutting strips containing intact seals, and whole-package testing provides a realistic assessment of conversion quality. However, laminate flat-film testing remains necessary to isolate the contribution of the lamination process from the heat-sealing process.
