Coefficient of Friction (COF) is measured using a friction tester. Friction may be measured film-to-film or film-to-metal depending on the situation. If a film is being processed on a packaging machine, especially an intermittent motion packaging machine such as a TFFS machine, then film to metal may be appropriate. If several filled bags are to be stacked on a shelf without sliding and falling off, film-to-film may be more appropriate. The test film specimen is attached to the bottom of a sled and dragged across a stainless steel plate (or film mounted on that plate if measuring film-to-film). The force required to drag the specimen is measured and recorded. The coefficient of friction is calculated by dividing the force required to pull the sample by the weight of the sled, thereby rendering the result a dimensionless number between 0 and 1. Typically, the initial and kinetic coefficient of friction is reported. COF is an important property of flexible medical packaging films. Films with a COF which is too high (say 0.5 or higher) may be difficult to process on a packaging machine causing sticking and stretching. Bags made from such film may be difficult to load by hand. Films with a COF that is too low (around 0.1) may produce bags which are too slippery and cannot be satisfactorily stacked. Migratory slip additives are used to make films more slippery, and an excess of these additives can contribute to other problems such as poor ink adhesion and poor heat sealing.
|ASTM D1894 Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting