Seal Strength of Flexible Barrier Materials: Testing Methods & Standards

What Are Flexible Barrier Materials?

Flexible barrier materials are multi-layer laminate structures designed to resist the transmission of moisture, oxygen, light, gases, or contaminants into a packaged product. They typically comprise combinations of films, foils, coatings, and laminates selected to provide the required barrier properties, seal performance, and mechanical durability.

Common flexible barrier material constructions include:

• Polyester/polyethylene (PET/PE) laminates — general-purpose packaging
• Aluminum foil laminates — high-barrier moisture and oxygen protection
• Nylon/polyethylene (PA/PE) laminates — puncture-resistant packaging
• Medical-grade Tyvek/film combinations — sterile barrier systems for medical devices
• Metallized films — intermediate-barrier applications

The integrity of the seal formed when two layers of flexible barrier material are heat-bonded together is the most critical performance parameter — and it must be reliably tested.

What Is Seal Strength and Why Does It Matter?

Seal strength is the force required to peel or separate a heat-sealed joint in a flexible barrier material. It is a direct measure of the bond quality formed during the heat-sealing process, which is controlled by seal temperature, dwell time, and pressure.

Adequate seal strength is critical because:

• Insufficient seal strength leads to premature seal failure during distribution, handling, or storage — compromising product protection and, in medical and sensitive applications, sterility or product contamination.
• Excessive or inconsistent seal strength may indicate over-processing that has damaged the packaging material — reducing puncture resistance or causing delamination of the laminate structure.
• Seal strength variability within a batch signals process instability — a quality control concern that must be investigated and corrected before product release.

ASTM F88: The Standard for Seal Strength Testing

ASTM F88, Standard Test Method for Seal Strength of Flexible Barrier Materials, covers the measurement of the strength of seals in flexible barrier materials — specifically seals between two flexible materials or between a flexible material and a rigid substrate.

Specimen Preparation and Conditioning

A sample is selected adequately to represent the production packaging units. Specimens are conditioned at 23°C and 50% relative humidity for a minimum of 40 hours before testing to normalize moisture effects on the flexible film properties.

Test Configurations

ASTM F88 defines three test configurations to accommodate different packaging constructions and peeling geometries:

• Method A — Unsupported: The seal specimen is clamped in opposing grips and pulled without supporting either tail. Used for packaging where both tails are flexible.
• Method B — Supported by Hand: One tail is supported manually at a 90° angle during testing. Applicable where one material tail tends to curl or deform.
• Method C — Supported by Fixture: The package is held in a support fixture during testing. Used for more rigid packaging constructions.

The tensile machine is calibrated per the manufacturer’s instructions, and tests are performed at a defined crosshead separation rate. Results are reported as peel force in Newtons per unit width (N/25mm or N/15mm).

Key Measurements

• Peak seal strength — maximum force recorded during the peel test
• Average seal strength — mean force over the peeling length
• Failure mode — seal peel (adhesive failure at the seal interface), film tear (cohesive failure in the substrate), or delamination

The failure mode is as informative as the force value: film tear at or near the seal indicates optimal seal strength (the seal is stronger than the film itself); seal peel indicates the seal itself is the weak link.

Related Standards and Complementary Tests

ASTM F88 is commonly used alongside:

• ASTM F1921 — Hot tack strength of flexible packaging materials (seal strength measured while the seal is still hot after formation)
• ASTM D882 — Tensile properties of thin plastic sheeting (characterizes the substrate properties)
• ASTM D1894 — Coefficient of friction of plastic film and sheeting
• ISO 11607 — Packaging for terminally sterilized medical devices (sealing and packaging integrity requirements)

Industry Applications of Seal Strength Testing

Electronics Packaging: Moisture-sensitive components (ICs, passive devices) are sealed in metallized foil pouches. Seal strength testing per ASTM F88 verifies barrier integrity before shipment and storage.

Medical Devices: Sterile medical devices must maintain packaging integrity through distribution and sterilization cycles. ISO 11607 requires documentation of seal strength as part of sterile barrier system validation.

Aerospace and Defense: High-value components shipped in flexible protective packaging require seal strength qualification as part of packaging design validation.

Consumer and Industrial Products: Protective packaging for sensitive instruments, electronic assemblies, and precision parts is validated through seal-strength testing to reduce field damage during distribution.

Conclusion

Seal strength testing per ASTM F88 is critical for ensuring the integrity and performance of flexible barrier packaging, directly impacting product protection, sterility, and shelf life. By evaluating seal quality, strength, and failure modes, manufacturers can validate packaging processes, maintain consistent quality, and prevent failures during storage and distribution, ensuring reliability across industries such as medical, electronics, and consumer products.

Infinita Lab’s Seal Strength Testing Services

Infinita Lab provides ASTM F88 seal-strength testing for flexible barrier materials through its nationwide accredited laboratory network. Testing covers all three ASTM F88 test configurations, with full documentation of failure modes and statistical analysis. Conditioning per standard requirements ensures representative test conditions. Comprehensive reports support packaging qualification, process control, and regulatory submissions.

Contact Infinita Lab: (888) 878-3090 | www.infinitalab.com

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