Testing the integrity of packaging seals - Is Your Seal Satisfactory?

When a food product is wrapped, the package’s overall integrity, particularly of the seal area, is crucial to avoid contamination and resultant spoilage or safety issues.

The role of a packaging seal varies between products, and the importance of several factors will have different weights depending on its use. Some require excellent mechanical strength or a perfect airtight seal, whereas some must be easy to open.

A good seal protects the product inside. It is critical for food safety reasons, and to maintain quality. ‘Seal integrity’ is a property tested regularly by packaging manufacturers, but particularly when a package design is adapted to suit lower budget materials, sustainable material alternatives, or when sealing conditions are altered on the production line. Additionally, this is important when distribution methods change (such as by introducing online shopping) or when a new product is introduced onto the market. When any of these factors are altered, it is important to check that seals are still sufficiently robust.

The smallest breaks in a food package seal will allow the admission of bacteria, whereas larger breaks can admit contaminated fluids or airborne contaminants.

The main measurements performed to assess a seal are of its strength and of its tendency to leak over a product’s shelf life. It is vital to know how seal strength varies over time, or at different stages in the distribution process.

Campden BRI presented some interesting points in a recent newsletter, that will be of use to manufacturers looking to perform package seal tests:

•    Not all test methods are relevant for different packs
•    Different test methods have different sensitivities for finding leaks in packs
•    Increased temperature during sealing does not always produce a stronger seal
•    Different test methods will give different seal strength results
•    There is a lot of variability between seal strength amongst different materials

Seal Measurement Methods

There are many ways a manufacturer may choose to measure the integrity of their packaging seals. They are a standard part of the process for HACCP (Hazard Analysis and Critical Control Point), food safety and hygiene testing and quality control procedures. When there is a change in the production procedure for any reason (e.g. new staff, new sealing equipment or a new batch of raw materials), it is advisable for quality control testing to be increased.

Some distribution methods put a greater stress on packaging. In particular, a product bought from an online shop is likely to go on a longer journey than one bought in a supermarket. It will be transported from the manufacturer to a distribution centre and to the online distributor, often loose in a large shipping crate. Following this, they are packaged in smaller boxes and shipped in crates with other products, onto other distribution centres, retailers and finally the customer. This is only one example of a distribution process that can add unpredictable stresses to food packaging, particularly in the stress concentration around its seals.

Road tests, both simulated and real, can be used to mimic the stresses a product may be subjected to. Seal integrity tests are carried out both before and after. Tests may be destructive or non-destructive, and they aim to make sure packages are intact after the stresses of shipping and consumer handling over the product’s life, and under different environmental conditions.

There are standards for different food distribution methods, particularly when food moves under extreme conditions. Packaging in these cases has to pass specific tests.

Most standards that put the seals, in particular, under scrutiny are based on the principle of leak detection. For example, in the bubble emission test (ASTM D3078 – 02) a submerged package is put under vacuum, and a leak will be shown by the appearance of bubbles. In the dye penetration test (ASTM F3039 – 15), dye is applied to one side of a seal. If there is a leak, it will migrate through the opening. The ultrasonic seal test (ASTM E1002 – 11) is used as a rapid assessment before investigating leaks in more detail. The gas leak test (ASTM F2391 – 05) is for entirely sealed samples – a mass spectrophotometer is used to measure the migration of gas from inside the package to the environment. Lastly, the vacuum decay test (ASTM F2338 – 09) shows a reduction in vacuum pressure when a sample package is put under vacuum and a leak is present.

A method to put the whole seal to the test mechanically is the ‘burst test’. This works by applying pressure internally to the package until it bursts, and the location of failure is recorded. This allows the manufacturer to ensure the package can meet a minimum force without failing, and to determine its weakest point. There are several burst types that can be carried out per the ASTM standards (ASTM F1140, ASTM F2054) including dynamic/static burst, creep test, and creep to fail. These burst tests can be carried out either on open or closed packages, using either unrestrained or restrained methods.

The Use of a Texture Analyser in Seal Testing

However, standard methods also exist that use mechanical testing on the seal itself, and a TA.XTplus or TA.HDplus Texture Analyser is ideal for performing these tests. The method most frequently used is the seal strength test (ASTM F88/F88M – 15) – the ‘Standard Test Method for Seal Strength of Flexible Barrier Materials’.

In this test, each side of a one-inch section of the packaging seal is placed into a pair of tensile grips. The grips are moved apart at a set speed, and the seal strength is defined as the maximum force reached before the seal fails. This technique is useful for products that are made up of both flexible and rigid films, for example a yoghurt lid and pot.

It is important that the section used in the test is representative of the whole seal, and that repeats are carried out. Compliant or ductile packaging materials are not suitable for this test, as they lead to a large strain at low stress, underestimating seal strength.

For packages that contain a product stored in an atmosphere of gas, such as crisps, a compression test on the whole package may be used, by means of a large compression platen. This is very similar to the burst test, as the failure force and weakest point are both determined. This helps the manufacturer to determine the package’s resilience while being stacked in a large crate during shipping.

Universal Peel Rig

For an imitative test that gives the actual force required for a user to peel the lid off a package, the Universal Peel Rig may be used. This consists of a multi-position platform which is adjustable to allow the container to be held at 0, 45 and 90 degrees. The platform is screwed securely into position on the base of the Texture Analyser, to suit the container. An adjustable rubber strap accommodates different sized containers and multiple shapes. When attached to the peel tab of the container, a low profile peel clip and post maximise the peel length for use on a standard height Texture Analyser.
 

To explore the wide range of materials and packaging test methods we can offer, contact Stable Micro Systems today.

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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