Putting packaging to the test

Materials testing has long been an established part of quality control procedures in laboratories throughout many sectors of industry. But sometimes efforts have focused mostly, or entirely, on the products themselves. Jo Smewing explains why manufacturers shouldn’t ignore their packaging, and looks into some of the common and most useful tests in this area.

MANUFACTURERS today, particularly in the fast moving consumer goods arena, are aware of the importance of packaging to the success of their businesses. No matter how desirable or valuable a product, if it’s packaging fails, it may become unfit for purpose, impossible to handle and store, or even completely unusable and unsellable. Poor packaging performance can not only damage the product it contains, but also the reputation and profitability of the company that supplied it.

As a result, materials testing for packaging materials and finished packs is just as important as assessments of the products themselves. There is now a wide range of testing equipment and methods which can help manufacturers establish which packaging types best meet their individual needs; from assessing substrates and adhesives right through to filled packs that are ready for shipping and sale. All the tests outlined below can be performed using various rigs and attachments in conjunction with a single desktop laboratory instrument, either the TA.XTPlus or TA.HDPlus, which are well-established in the measurement of properties of material products in the adhesive, food, pharmaceutical and cosmetic industries.

Tensile strength and tear characteristics can be measured using grips. Self-tightening roller grips are most effective for thin samples with a smooth surface, or materials whose characteristics change when tensile force is applied. Spring-loaded cross-hatched rollers function as self-tightening mechanisms that grip the sample at each end, applying the pull force to determine the rupture point. Tensile grips, on the other hand, are ideal for testing the mechanical properties of heavy-duty packaging materials such as heavier plastics. With a straightforward pulling action, the instrument measures the maximum force required to stretch the sample to breaking point. Alternatively, used in a cycling motion, tensile grips assess the flexibility of the substrate material. Assessing and comparing different materials allows manufacturers to select packaging materials that are resilient enough to withstand the rigours of handling and distribution but at the same time allow users simple access to the product inside.

As well as strength, friction can have a significant impact on the performance characteristics of packaging, particularly during shipping and handling. The coefficient of friction (COF) represents the resistance to sliding of two surfaces in contact with each other. Higher COF values indicate more resistance to sliding, i.e. more friction. Excessive friction can cause material deformation or damage, but insufficient friction would lead to slipping, which can be problematic during product storage and/or display. The coefficient of sliding friction rig enables manufacturers to determine static and dynamic coefficients of friction by fixing the reference material to the rig’s base, and the test material to the sled. The sled is pulled across the reference material at a constant speed, while the instrument measures the force. ASTM D 1894-90 is the recognised standard for plastic films and sheeting.

Peel lids and covers are one of the most popular packaging methods in applications like medical devices and fast moving consumer goods (FMCG) products, particularly personal care and parts of the food industry. As well as measuring the characteristics of the lid’s material when it is in sheet form, equally important is investigating its adhesion to the pack. The performance of the seal between lid and pack is not only critical to the quality and safety of products, but is also a vital factor in user-friendliness. The adhesive bonds must be strong enough to withstand handling, shipping and storage without rupture or breakage, while being easy to peel back when necessary.

A popular test involves the use of the “peel strength test rig”. The container is positioned in a carrier plate mounted on the base of the instrument. This carrier plate is positioned so that a pulley and captive wire are located precisely below the instrument’s loadcell. One end of the wire is connected to the loadcell and the other is attached to the pull-tab on the lid. Results of a typical test on a yoghurt pot are shown in graph 1. It shows around 650g of force were needed to break the first adhesive bonds, after which variations of force are shown as the lid is gradually peeled away from the circumference of the pot to a distance of around 90mm, when the lid detached completely.

The “universal peel rig” incorporates a multi-position platform, allowing containers to be held flat, or at 45° and 90° angles. Using this, manufacturers can more precisely simulate the action of consumers, or users, to assess accurately both ease of use and the stability of packaging seals.

Sachets and tubes have become increasingly popular as plastic qualities improve and manufacturers look for lighter and more flexible packaging formats. In sectors as diverse as personal care, food, healthcare, hardware and hobbies and crafts, handling transport and storage are optimised while ensuring ease of use by consumers.

In these applications, if we assume the aperture of the tube/sachet remains constant and the product’s consistency/flow properties are appropriate, success depends on the consumer’s ability to squeeze the packaging. A sachet/tube extrusion rig holds the product vertically between two rollers, clamping the closed end with a grip at the top. The grip then pulls the sachet/tube upwards through the rollers, forcing the contents out. The higher the force recorded during this test, the more difficulty a consumer would experience in squeezing out the contents. In the food industry, results might vary over time, if the product’s consistency changes throughout its shelf life. Equally, extrusion testing might help manufacturers decide whether a metallic or plastic tube is the most appropriate for their application.

Safety is the overriding requirement in most blister pack applications as they are common in healthcare and medicine. This is a particularly difficult balance to strike: while being childproof, blister packs must also be useable quickly by consumers that may lack strength in their hands and fingers, such as elderly people. A “blister pack rig” closely imitates users’ actions, with a vertical probe that resembles a human finger which descends downwards onto the sample pack, applying constant pressure on the blister pocket. The instrument records the force required for the probe to rupture the sealed surface of the pack, expelling the content. This test gives a good indication of materials best suited for use in packaging of consumable goods.

Uniaxial compression testing helps manufacturers and retailers evaluate the crush resistance of packaging containers, estimating their resistance to deformation when force is evenly applied across their surface. This is particularly important if the pack’s contents are fragile and/or valuable. The test involves a compression plate that descends vertically, with a constant speed and applying a set pressure, onto the sample positioned directly below. The force required to deform the sample container is measured, giving an indication of how well the pack will withstand transport, storage (especially stacking) and handling. Uniaxial compression tests of small boxes are common throughout the food and beverage, cosmetic, pharmaceutical, and FMCG industries.

It is clear that accurate, repeatable materials testing is as important in evaluating packaging as it is in perfecting the product itself. Using one instrument, manufacturers and brand owners can safeguard quality standards and ensure consistent performance during production processes, transport, storage and end use. Optimising packaging improves product safety, functionality and aesthetics and customer satisfaction. And in today’s competitive markets, every one of these factors counts.