Plastic Film Seal Strength
ASTM F88 specifies a method for testing the seal strength of a thin plastic sheet or similar flexible material. These types of seals are used extensively in the food and beverage sector as well as in bio-medical applications. Most one-time use items such as bandages, catheters, and needles are packaged inside a hermetically sealed bag.
The seal strength test is a combination of both an adhesive test and a tensile test on the thin plastic material. It follows then that there are a few failure modes – either the adhesive can fail, or the flexible barrier material can be ripped apart.
The test specimens for F88 consist of two pieces of 1 inch wide material that are bonded together. The overlay of the bond is 1 inch so the total area of interest is a 1x1 inch square that has been adhered together. An appropriate amount of space and material must be left on both ends so that the grips can attach and pull it apart. The specimen should be crafted in such a way that it opens like a clam or a book instead of being lapped such as in the ASTMD1002 test.
Vise grips are the most economical and simplest solution for this type of test. Care should be taken to specify the correct grip faces. Serrated grip faces are usually good. However, if the adhesive bond is strong the test procedure could result in a tear of the material at the grip face which is not an acceptable test result. Rubber faces or flat metal faces can also be used.
The calculations for ASTM F88 are relatively straightforward. The seal strength is given by the peak load per unit width. There are a couple failure modes so the type of failure must also be recorded. If there is a good amount of peeling happening between the sealed area, an average peel strenght calculation can be made.