Determination of Heat Seal Strength of Packaging Bags: Principles, Standards, and Test Methods

Heat seal strength is one of the most critical quality indicators for flexible packaging bags, directly determining whether a package can protect its contents throughout filling, transport, and storage. This article explains the principles, sample types, test items, and standardized test methods—such as ASTM F88/F88M and QB/T 2358—used for the determination of heat seal strength of packaging bags. It also provides practical guidance on specimen preparation, test conditions, and result evaluation for quality control engineers and laboratory professionals.

Technical Overview

Heat seal strength, also referred to as seal strength or hot-seal peel strength, is the maximum tensile force per unit width required to separate a heat-sealed joint of thermoplastic packaging materials. It is expressed in units such as N/15 mm, N/25 mm, or N/m, and it quantifies the ability of a sealed bag to withstand internal pressure, mechanical shock, and handling loads without opening during distribution and storage.

The heat seal strength of a packaging bag is governed by four primary process variables: sealing temperature, sealing pressure, dwell time, and cooling conditions. It is also influenced by material factors, including the type and thickness of the sealant layer (for example, LDPE, LLDPE, CPP, or ionomer), the structure of the laminate, surface contamination, and additive migration. Because seal quality depends on the interaction of all these parameters, laboratory determination of heat seal strength is essential both for validating sealing processes and for verifying the quality of finished bags.

The most widely referenced standards for this test include ASTM F88/F88M (Standard Test Method for Seal Strength of Flexible Barrier Materials), ASTM F2029 (Standard Guide for Making Heatseals for Determination of Heatsealability of Flexible Webs), and the Chinese light industry standard QB/T 2358 (Test Method for Heat Seal Strength of Plastic Film Packaging Bags). For pharmaceutical packaging, the YBB standards series specifies corresponding heat seal strength requirements for composite sheets and bags used for drug packaging.

Testing Samples

Monolayer Plastic Film Bags

Bags made from single-layer polyethylene (PE), polypropylene (PP), or cast polypropylene (CPP) films are commonly tested to verify the sealing performance of the base film and the consistency of bag-making machine settings.

Laminated Flexible Packaging Bags

Multi-layer structures such as PET/AL/PE, BOPP/VMPET/PE, PET/NY/PE, and paper/AL/PE laminates are the most common samples. The test evaluates the cohesive and adhesive bonding of the inner sealant layer, and it can also reveal delamination issues within the laminate structure.

Pharmaceutical and Medical Packaging

Aluminum-plastic composite sheets, tropical blister lidding, infusion bags, and breathable porous-film sterile barrier pouches for medical devices require high and consistent seal strength to guarantee product sterility and shelf life. These samples are usually tested in accordance with pharmacopoeia or YBB requirements.

Heavy-Duty and Industrial Bags

Fertilizer bags, chemical bags, valve bags, and FIBC inner liners are tested at higher force ranges, because their seals must resist significant static and dynamic loads during palletizing and transport.

Testing Items

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

• Heat seal strength of finished bag seals (bottom seals, side seals, back seals, and zipper seals)
• Heat seal strength of laboratory-prepared specimens sealed under controlled parameters
• Heat seal strength profiles (heat seal curves) across a range of sealing temperatures, pressures, and dwell times
• Seal strength after aging, sterilization, or environmental conditioning
• Hot tack strength, which measures seal strength while the seal is still hot and molten, relevant to vertical form-fill-seal (VFFS) operations
• Failure mode analysis, including peel failure, cohesive failure, delamination, and substrate break

Testing Methods

Test Principle

The measurement is performed as a tensile peel test. A strip specimen containing the heat seal is clamped in the grips of a universal testing machine (UTM) or a dedicated heat seal strength tester, with the seal located midway between the grips. The two legs of the specimen are pulled apart at a constant speed, and the force required to separate or break the seal is recorded. The maximum force, or another agreed force characteristic, is divided by the specimen width to obtain the seal strength value.

Apparatus and Specimen Preparation

The main equipment includes a laboratory heat sealer with controllable temperature, pressure, and dwell time (for specimen preparation), a universal testing machine with suitable grips, and a calibrated force measurement system. Specimens are typically prepared and conditioned as follows:

• Cut specimens of 15 mm width (per QB/T 2358) or 25.4 mm width (per ASTM F88) from the sealed area, ensuring the seal runs perpendicular to the specimen axis.
• Specimen length is generally 100 mm or more so that at least 10 mm of each leg can be gripped and the seal remains unsupported.
• Condition specimens at 23 ± 2 °C and 50 ± 5% relative humidity for at least 4 hours, or as specified by the relevant standard.
• Prepare at least five valid specimens for each sealing condition, and ten specimens when statistical reliability is required.
• For laboratory sealing studies, prepare seals across a temperature ladder (for example, 100 °C to 180 °C in 5 °C or 10 °C increments) at fixed pressure and dwell time to construct a heat seal curve.

Test Procedure

Set the peel speed according to the governing standard: 300 ± 20 mm/min is specified by QB/T 2358, while ASTM F88 commonly uses 200 mm/min unless otherwise agreed. Place the specimen in the grips so that the seal line is equidistant from both grips and the two legs form the required peel geometry (180° peel or T-type configuration). Start the test and record the force-displacement curve until the seal has completely separated or the specimen has broken. Note the maximum force and observe the failure mode of each specimen.

Data Processing and Evaluation

Calculate the seal strength of each specimen by dividing the recorded force by the specimen width, and report the arithmetic mean, standard deviation, and minimum value for the specimen group. Classify the failure mode as peel failure at the sealant interface, cohesive failure within the sealant layer, delamination between laminate layers, or substrate break (film tearing). A substrate break indicates that the seal is stronger than the film itself, which is generally considered an acceptable and often desirable result. When acceptance criteria are specified in product standards or supply agreements, the mean value and the individual minimum value are compared against the specified limit, for example a commonly specified range of 8 to 15 N/15 mm for typical laminated food packaging bags.

Applications

Heat seal strength determination is applied across the entire packaging value chain, from film manufacturers and converters to brand owners, regulatory bodies, and accredited third-party laboratories. Typical applications include:

• Optimizing sealing parameters on bag-making, form-fill-seal, and blister packaging equipment by establishing the heat seal window
• Incoming inspection of films, laminates, and premade pouches against agreed specifications
• Quality control of finished packaging bags in food, pharmaceutical, cosmetic, and chemical industries
• Validation of packaging processes under GMP and quality management systems
• Shelf-life studies and transport simulation, including evaluation of seals after aging, sterilization, or low-temperature exposure
• Failure analysis of leaking or burst packages to identify root causes such as insufficient sealing temperature, contamination, or poor sealant compatibility

FAQ

What is the difference between heat seal strength and hot tack strength?

Heat seal strength is measured after the seal has fully cooled and solidified, and it reflects the final strength of the seal. Hot tack strength is measured while the seal is still hot, immediately after the sealing jaws open, and it is critical for vertical form-fill-seal lines where the package is filled before the seal has cooled.

Which specimen width and peel speed should be used?

For QB/T 2358 and most Chinese product standards, the specimen width is 15 mm and the peel speed is 300 ± 20 mm/min. For ASTM F88, a 25.4 mm (1 inch) wide specimen tested at 200 mm/min is common. The width and speed used should always be stated in the test report, because results from different configurations are not directly comparable.

How many specimens are required for a valid determination?

At least five valid specimens should be tested for each sealing condition, and ten specimens are recommended when the result is used for acceptance decisions, so that the mean value and scatter can be evaluated statistically.

Why does the specimen sometimes break outside the seal during testing?

When the film itself tears before the seal peels apart, the seal is stronger than the substrate. This failure mode, known as substrate break or film break, is usually recorded as a satisfactory result, with the force at break reported as the seal strength for that specimen.

What sealing parameters should be used to prepare laboratory specimens?

There is no universal set of parameters. Laboratory sealing should follow ASTM F2029 guidance, using the pressure, dwell time, and temperature range representative of the actual production equipment. Testing across a temperature ladder allows identification of the heat seal initiation temperature and the upper limit where the film distorts or the sealant degrades, thereby defining the usable heat seal window.

Are there universal acceptance criteria for heat seal strength?

No single criterion applies to all packages. Acceptance limits depend on the bag structure, contents, package size, distribution hazards, and regulatory expectations, and they are normally agreed between the packager and the customer or defined in the applicable product standard. Typical laminated food packaging bags are commonly specified in the range of 8 to 15 N/15 mm, while sterile medical packaging may have different requirements based on validated performance.