Determination of Heat Seal Strength of Plastic Films: Test Methods, Standards, and Applications

Heat seal strength is one of the most critical quality parameters for plastic films used in flexible packaging, directly determining whether pouches, sachets, and bags can withstand filling, sterilization, transport, and handling without leaking or bursting. This article explains the testing principles, applicable standards, sample types, test procedures, and data evaluation methods for the determination of heat seal strength of plastic films. It serves as a practical reference for packaging engineers, quality control professionals, and material scientists.

Technical Overview

Heat seal strength refers to the maximum force required to separate a heat-sealed joint of a plastic film, normalized to the specimen width and typically expressed in N/15 mm or N/m. It quantifies the ability of a seal to resist opening under mechanical stress and is a decisive factor in the integrity of flexible packaging such as pouches, bags, sachets, and lidding films. A seal that is too weak may fail during filling, handling, or distribution, while an excessively aggressive seal condition can cause film distortion, burn-through, or loss of peel functionality.

The measurement principle is straightforward: two pieces of film are sealed together under precisely controlled temperature, pressure, and dwell time using a laboratory heat sealer, and the sealed specimen is then peeled apart in a tensile testing machine at a constant speed. The resulting force-displacement curve provides the peak force, average force, and fracture behavior of the seal. Internationally recognized test methods include ASTM F88/F88M (seal strength of flexible barrier materials), ASTM F2029 (preparation of laboratory heat seals for evaluation), QB/T 2358 (heat seal strength of plastic films for packaging), and JIS Z 0238 (testing of heat-sealed portions of flexible packages). Related methods such as GB/T 8808 evaluate the peel strength between layers of laminated composite films.

Beyond a single numerical value, a complete evaluation also classifies the failure mode: adhesive failure at the seal interface, cohesive failure within the sealant layer, film elongation or tear, delamination of laminate layers, or a mixed failure. The failure mode is often as important as the force value itself when judging whether a sealing process is robust.

Testing Samples

Single-Layer Heat-Sealable Films

Monolayer films whose sealing surfaces can be fused by heat, including LDPE, LLDPE, mPE, CPP, EVA, and ionomer films. These materials are frequently tested to establish the intrinsic sealability of a resin grade and to define an optimum sealing temperature window.

Laminated Composite Films

Multilayer structures in which an inner sealant layer provides sealing performance while outer layers contribute stiffness, barrier, or printability. Typical constructions include BOPP/CPP, PET/PE, PA/PE, PET/AL/PE, and paper/AL/PE laminates used for food, pharmaceutical, and industrial packaging.

Coated, Metallized, and Functional Films

Films carrying heat-sealable coatings, metallized layers, or peelable (easy-open) sealants. For these structures, heat seal strength testing verifies that the coating or functional layer delivers the intended peel force, such as a controlled easy-open behavior for lidding applications.

Finished Packages and Seal Coupons

Seal areas cut from production-made pouches, sachets, or bags. Testing finished-package seals allows verification of actual machine settings, evaluation of seal uniformity across the seal width, and investigation of field failures or consumer complaints.

Testing Items

Depending on the purpose of the evaluation, a heat seal strength test program may include the following items:

• Maximum heat seal strength (peak peel force, N/15 mm)
• Average and minimum peel force along the peel distance
• Heat seal strength distribution across the seal width and along the seal length
• Sealing temperature window determination (heat sealability curves generated by sealing at a series of temperatures)
• Optimization of sealing pressure and dwell time
• Failure mode classification (adhesive, cohesive, tear, delamination, or mixed)
• Verification of easy-open or peelable seal performance against specification limits

Testing Methods

Specimen Preparation and Heat Sealing

Test specimens are cut from the film with a width of 15.0 mm (or another width as specified by the applicable standard) and a sufficient length, commonly at least 100 mm, so that each specimen consists of two free tails joined by a controlled seal of approximately 10 mm width at the center. The seals are produced on a laboratory heat sealer equipped with temperature-controlled jaws, a calibrated pressure system, and an adjustable dwell timer. To map the sealing window, a matrix of specimens is prepared by varying sealing temperature while keeping pressure and dwell time constant, or by systematically varying all three parameters.

Conditioning

After sealing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a defined period, typically not less than 4 hours (or as specified by the product standard), so that the sealant layer fully solidifies and test results are reproducible. Testing immediately after sealing without conditioning yields "hot" values that do not represent the final package performance.

Tensile Peel Test Procedure

The conditioned specimen is mounted in a universal testing machine, with the two unsealed tails clamped in the grips and the sealed area unsupported between them. The grips separate at a constant crosshead speed, commonly 200-300 mm/min depending on the standard cited, for example 300 ± 20 mm/min per QB/T 2358. For thin or flexible films, alignment tabs or fixtures are used to maintain a consistent 90° T-peel geometry and to prevent film twisting during the test. The force-displacement curve is recorded until the seal is fully separated or the film breaks.

Data Processing and Reporting

From the force-displacement curve, the maximum force and, where required, the average force over the peel plateau are extracted and normalized by the specimen width to give results in N/15 mm. A statistically valid test normally uses at least five specimens per sealing condition, and many specifications require ten; the arithmetic mean, standard deviation, and coefficient of variation are reported together with the observed failure mode. The complete test report should identify the material, sealing parameters (temperature, pressure, dwell time), conditioning, specimen dimensions, test speed, and the standard applied.

Equipment Requirements

The essential instrumentation comprises a laboratory heat sealer with independently controlled sealing temperature, pressure, and dwell time; a universal testing machine with suitable pneumatic or manual grips and a force accuracy adequate for the expected load range; a specimen cutting device ensuring parallel, burr-free edges of accurate width; and a conditioning chamber maintaining standard temperature and humidity. Calibration of the sealer jaw temperature and the load cell of the testing machine at regular intervals is essential for reliable, comparable results in accredited third-party laboratories and in-house QC labs alike.

Applications

Heat seal strength determination is applied throughout the flexible packaging value chain:

• Film and resin manufacturers: quality release testing of sealant films, comparison of resin grades, and development of new sealant formulations.
• Converters and packaging manufacturers: optimization of converting and filling machine sealing parameters, incoming material inspection, and validation of laminate structures.
• Food packaging: verification of seals for snack pouches, frozen food bags, retort pouches, vacuum skin packs, and lidding films on cups and trays.
• Pharmaceutical and nutraceutical packaging: evaluation of sachets, stick packs, and lidding foils where seal integrity is directly linked to product stability and regulatory compliance.
• Medical device packaging: qualification of sterile barrier systems made from porous and non-porous flexible materials, where seal strength is a core requirement of package validation.
• Root cause analysis: laboratory reproduction of sealing defects, investigation of leaking packages, and comparison of before-and-after process changes.

FAQ

What is heat seal strength and in what units is it expressed?

Heat seal strength is the force required to peel apart a heat-sealed joint, normalized by the specimen width. The most common unit is N/15 mm because 15 mm is the standard specimen width in ASTM F88 and QB/T 2358; results may also be converted to N/m.

Which standards govern heat seal strength testing of plastic films?

The principal methods are ASTM F88/F88M for measuring seal strength, ASTM F2029 for preparing laboratory heat seals, QB/T 2358 for plastic films in packaging, and JIS Z 0238 for heat-sealed portions of flexible packages. Related standards such as GB/T 8808 cover interlayer peel strength of laminated films.

What are the standard specimen dimensions and test speed?

Specimens are typically 15.0 mm wide and at least 100 mm long with a seal of about 10 mm width centered between the two tails. The tensile peel test is performed at a constant speed of roughly 200-300 mm/min, with 300 ± 20 mm/min specified in QB/T 2358.

What process parameters most influence the result?

Sealing temperature, sealing pressure, and dwell time are the three primary variables, followed by sealant layer thickness, film surface treatment or contamination, jaw design, and conditioning time before testing. Systematic variation of these parameters is used to define a robust sealing window.

How does heat seal strength differ from hot tack strength?

Heat seal strength is measured after the seal has cooled and fully solidified, whereas hot tack strength is measured while the seal is still hot, simulating the loads applied immediately after sealing on vertical form-fill-seal machines. Both properties are relevant, but they describe different stages of package formation.

What failure modes should be recorded?

Reports should classify failure as adhesive failure at the seal interface, cohesive failure within the sealant, film tear or break, delamination between laminate layers, or a mixed mode. A high force value accompanied by film tear may not indicate a good seal interface, so the failure mode must always accompany the numerical result.