Compression and Tension

Simple examples of tests on packaging materials include actions that consumers use everyday when opening items like yogurt containers with lids that must be peeled off. A peeling jig can be used with a texture analyzer to measure the adhesive strength needed to remove the lid from a sealed container. Tests like these are performed at different angles to simulate the approach taken by customers around the world. The preferred orientation for taking the lid off may be explained by the manufacturer in the information on the side of the container. Peel strength needed to open the container can be adjusted to accommodate the user group for which the product is intended, such as senior citizens and children.

Film materials are used extensively to package all kinds of food items. The ability of film to stretch without tearing is one of its important properties. A film support fixture makes it possible to evaluate the tear strength of the film. The texture probe has the shape of a punch that pushes down on the film sample, which is clamped in place during the test. The rate of travel for the probe can vary from 0.01millimeter (mm)/second to 10 mm/second and is usually selected according to the packaging process on which the film will be applied. R&D labs may conduct tests at different speeds to simulate multiple ways in which the film might be used. The test results not only qualify the film for use in packaging processes, but also provide guidelines for choosing films with specific tear strengths.

One of the more popular fixtures for general purpose testing using the tension mode is the dual grip assembly. This general-purpose device has two separate clamps that fasten the sample material and pull it apart. The bottom clamp is fixed to the base table while the top clamp attaches to the probe drive on the instrument. The top clamp moves upward at the start of the test and the instrument displays the measured force as the material stretches. The test objective is to measure the maximum force that a material can withstand when in tension mode.

A sliding friction test can also be performed on packaging materials in accordance with ASTM D1894. The objective is to measure how easily two materials slide over each other. The instrument measures the force needed to pull a weight robed in one material across the surface of a second material. The rate of travel for the weight is usually between 1 mm/second and 10 mm/second. When there is too much resistance between the materials, the friction may cause damage to one of the materials. This is an important test because it confirms, for example, that boxes of a specific food item, such as cereal packaged in a large container, will arrive at their destination with the printed information on the outside of the box in tact. Customers often judge a product by appearance on the shelf, so this test confirms that the surface of the package withstands the rigors of shipment.

The field of texture analysis has proliferated over recent years with the advent of many fixtures that can measure the firmness and robustness of packaging materials. The objective is to ensure survival of both the food item and its container from the production plant to the end user’s kitchen, as well as to insure that the customer use of the item is optimized for ease and acceptability.

McGregor is general manager, global marketing/high-end lab instrument sales, at Brookfield Engineering Laboratories, Inc. Reach him at [email protected].