Neutral PE-coated paper heat seal strength testing evaluates the sealing performance of polyethylene-extrusion-coated paper widely used in food, medical, and industrial packaging. This technical guide explains the testing principle, relevant standards such as ASTM F88 and QB/T 2358, sample preparation, sealing parameters, and peel testing procedures. By following these methods, manufacturers can validate seal integrity, optimize sealing windows, and ensure consistent package quality.
Technical Overview
Neutral PE-coated paper is a composite packaging material produced by extrusion-coating a thin layer of polyethylene (PE) onto one or both sides of an unprinted, chemically neutral paper substrate. The extrusion coating process bonds molten PE resin directly to the paper surface, creating a material that combines the printability, stiffness, and renewability of paper with the moisture resistance, grease resistance, and heat-sealability of polyethylene. Because the paper base itself does not melt, the PE layer functions as the exclusive sealing medium, and the quality of the final package depends entirely on the integrity of the PE-to-PE bond formed during heat sealing.
Heat seal strength is defined as the maximum force per unit width required to separate a heat-sealed joint under controlled tensile testing conditions. It is typically expressed in N/15 mm or N/25 mm. For neutral PE-coated paper, this property determines whether packages such as food bags, sterilization pouches, and wrapping sheets can survive filling, transport, storage, and handling without seal failure or leakage. Insufficient seal strength leads to product contamination and shelf-life loss, while excessive sealing energy may cause PE squeeze-out, fiber tear, or reduced production speed. Systematic heat seal strength testing therefore serves two purposes: quality control of finished packaging and process optimization on packaging lines.
Why Heat Seal Strength Testing Is Essential
- Verifies that seals meet minimum strength specifications required by customers and regulatory bodies.
- Identifies the optimal combination of sealing temperature, pressure, and dwell time for production equipment.
- Detects raw material issues such as insufficient PE coating weight, poor resin distribution, or surface contamination.
- Supports validation protocols for medical and food packaging under ISO 11607 and HACCP-based quality systems.
- Reduces customer complaints, product recalls, and material waste caused by weak or inconsistent seals.
Testing Samples
Representative sampling is critical for meaningful test results. Samples should be taken from different positions across the paper web (left edge, center, right edge) to account for coating thickness variation, and from different production batches when evaluating overall material quality. All specimens must be conditioned in a standard laboratory atmosphere before testing.
Single-Side PE-Coated Paper
Single-side coated paper carries PE on one surface only and is commonly used for bread bags, hamburger wrappers, envelopes, and medical pouch bodies. For seal strength testing, specimens are folded so that the PE-coated surfaces face each other, or two strips are overlapped with the coated sides in contact. This configuration simulates the fin seals and lap seals used on actual packaging machines.
Double-Side PE-Coated Paper
Double-side coated paper has PE on both surfaces and is widely used for paper cups, liquid packaging, and moisture-sensitive applications. Testing can be performed on face-to-face seals between two coated surfaces or against other sealing layers such as PET, CPP, or PE films when the coated paper forms part of a laminate structure. Both coating layers should be evaluated because coating weight and melt flow index may differ between the two sides.
Specimen Dimensions and Conditioning
- Standard specimen width: 15 mm (a width of 25 mm is also acceptable when specified; report the width used).
- Specimen length: at least 100 mm, providing an unsealed grip section at both ends for clamping.
- Sealed area length: approximately 10 mm at one end of the overlapped strips.
- Number of specimens: a minimum of 5 to 10 seals per condition to obtain statistically valid averages.
- Conditioning: 23 ± 2 °C and 50 ± 5 % relative humidity for at least 24 hours prior to sealing and testing.
- Cut edges must be clean and parallel; burrs, wrinkles, or contamination in the seal zone will distort results.
Testing Items
A complete evaluation of neutral PE-coated paper sealing performance typically includes the following test items, each addressing a different aspect of seal quality.
- Heat seal strength (N/15 mm): the maximum peel force of a seal made under specified conditions.
- Sealing window determination: the range of sealing temperatures that produce acceptable seal strength.
- Sealing curve analysis: seal strength plotted against temperature, pressure, and dwell time.
- Hot tack strength (optional): seal strength measured while the seal is still hot, relevant to vertical form-fill-seal operations.
- Seal integrity and leak performance: verified through pressure decay or vacuum immersion methods.
- Failure mode classification: adhesive failure, cohesive failure, or substrate fiber tear.
Heat Seal Strength (N/15 mm)
This is the core test item. A laboratory heat sealer creates seals under precisely controlled temperature, pressure, and time, and a tensile tester then peels the seal apart at a constant speed. The average maximum force divided by specimen width yields the heat seal strength value, which is compared against specification limits agreed with the end user.
Sealing Window Determination
Seals are produced across a temperature gradient, for example from 100 °C to 170 °C in 10 °C increments, while pressure and dwell time remain constant. Plotting strength against temperature reveals the plateau region where strength is stable and maximum. A wide plateau indicates a forgiving material suitable for high-speed lines; a narrow plateau signals sensitivity that requires tight machine control.
Failure Mode Analysis
Recording how the seal fails is as important as the numerical result. Adhesive failure at the PE interface indicates incomplete fusion and weak sealing. Cohesive failure within the PE layer, where stretching and thinning of the resin occurs before separation, indicates a well-fused seal. Fiber tear of the paper substrate indicates that the seal is stronger than the paper itself, which is generally the target condition for maximum package security.
Testing Methods
The testing procedure consists of three stages: preparation of sealed specimens, tensile peel testing, and data evaluation. Strict control of each stage ensures repeatability and comparability between laboratories.
Relevant Standards
- ASTM F88/F88M: Standard test method for seal strength of flexible barrier materials, widely used internationally for coated papers and films.
- QB/T 2358: Chinese light industry standard for heat seal strength of flexible packaging materials, commonly applied to PE-coated paper in domestic supply chains.
- YY/T 0681.2: Seal strength test method for medical packaging within the Chinese medical device standard system.
- ISO 11339: T-peel determination of flexible-to-flexible bonded assemblies, applicable to certain laminate configurations.
- ASTM F2029: Standard practice for making heat seals for laboratory determination of heat sealability, governing specimen preparation.
Step 1: Heat Sealing the Specimens
Place two conditioned strips with their PE-coated surfaces in contact, and position them in a laboratory heat sealer equipped with flat, parallel jaws of controlled width. Set and verify the following parameters:
- Sealing temperature: typically 110 °C to 160 °C for LDPE coatings; verify jaw temperature with a calibrated surface thermometer or foil indicator.
- Sealing pressure: 0.2 to 0.5 MPa (commonly 0.3 MPa), applied normal to the seal area.
- Dwell time: 0.5 to 1.5 seconds (commonly 1.0 second), chosen to represent production line conditions.
- Jaw configuration: flat smooth jaws are standard; knurled jaws may be used to simulate specific machines but must be reported.
After sealing, allow specimens to cool to ambient temperature for at least 30 minutes before peel testing, unless hot tack is being measured. Cooling allows the PE to crystallize and reflects the condition of seals in real packages.
Step 2: Tensile Peel Testing
Mount the specimen in a universal tensile testing machine or dedicated heat seal strength tester, clamping the two free legs of the specimen so that the seal line lies equidistant between the grips. The test geometry should create peel separation rather than tensile rupture of the paper:
- Test speed: 200 to 300 mm/min, as specified by the applicable standard.
- Peel geometry: 180° peel or T-peel configuration depending on specimen stiffness and standard requirements.
- Gauge length: consistent across all specimens, typically 50 to 100 mm.
- Record the force-displacement curve and capture the maximum force and, where required, the average force over the peel region.
The instrument should be a Class 0.5 or better force system, calibrated regularly, with a load cell selected so that measured forces fall within 20 % to 80 % of its capacity for best accuracy.
Step 3: Data Processing and Acceptance Criteria
Calculate heat seal strength as the average maximum force of the valid specimens divided by the specimen width. Discard results from specimens that slipped in the grips, tore at the grip line, or contained visible seal defects, and retest with additional specimens. Report the following information:
- Individual values, mean value, standard deviation, and coefficient of variation for each sealing condition.
- Complete sealing parameters: temperature, pressure, dwell time, and jaw type.
- Observed failure mode for each specimen.
- Specimen width, coating weight, and conditioning environment.
Typical acceptance limits for neutral PE-coated paper in general food packaging range from 5 to 8 N/15 mm, while medical packaging and heavy-duty applications may require higher values or mandatory fiber tear. The exact limit must always be established by agreement between supplier and converter based on the end use of the package.
Applications
Neutral PE-coated paper heat seal strength testing supports quality assurance and product development across a broad range of packaging sectors. Wherever a PE-coated paper is sealed into a finished package, this test provides the quantitative evidence of seal reliability.
- Food packaging: bread bags, fast food wrappers, sugar and flour sachets, dried food pouches, and bakery liners where hermetic sealing preserves freshness and prevents grease penetration.
- Beverage and liquid packaging: paper cups, bowl lids, and carton components that must withstand hot filling and liquid pressure.
- Medical packaging: sterilization pouches and header bags made from medical-grade coated paper, where seal integrity is validated under ISO 11607 and microbial barrier requirements.
- Daily chemical and pharmaceutical packaging: sachets for powders, granules, and single-dose products requiring precise seal strength control.
- Industrial and logistics packaging: moisture-barrier wraps, interleaving papers, and protective bags for metal parts and electronic components.
- Material development: coating weight optimization, PE resin selection, and comparative evaluation of coated papers from different suppliers.
FAQ
What is a good heat seal strength value for neutral PE-coated paper?
For most food packaging applications, a seal strength of 5 to 8 N/15 mm is considered acceptable. Many specifications instead require substrate failure, meaning the seal is stronger than the paper itself and fibers tear during peeling. Acceptance criteria should always match the end-use requirements of the specific package.
What sealing temperature should I use for testing?
LDPE-coated paper is typically tested between 120 °C and 150 °C, using a fixed pressure of about 0.3 MPa and a dwell time near 1 second. Rather than a single temperature, best practice is to run a temperature ladder to identify the sealing plateau, then select a set point near the middle of that plateau for routine quality control.
Why is my measured seal strength lower than expected?
Common causes include insufficient sealing temperature or dwell time, contaminated or aged PE surfaces, low coating weight, uneven jaw pressure, and testing specimens before they have fully cooled and crystallized. Verify jaw temperature calibration, specimen conditioning, and coating uniformity before investigating material issues.
What is the difference between heat seal strength and hot tack strength?
Heat seal strength is measured after the seal has cooled to room temperature and represents the final package seal quality. Hot tack strength is measured immediately after sealing while the PE is still molten and determines whether seals can withstand product load on vertical form-fill-seal machines without opening. Both properties can be evaluated on the same material using appropriate instrumentation.
Which standards should govern the test report?
ASTM F88/F88M is the most widely referenced method for international trade, while QB/T 2358 is commonly required in the Chinese market. Medical device packaging should reference YY/T 0681.2 or the ISO 11607 series. The chosen standard, specimen geometry, and all sealing parameters must be clearly stated in the test report to ensure comparability.