Determination of the Coefficients of Friction (COF) of Films

Determination of the static and dynamic coefficients of friction is particularly interesting for films that are further processed on packaging and printing machines. Coefficients of frictionprovide information about the processability and the surface structure, which is important for determining the printability of the material.

The static and dynamic coefficient of friction for flexible films can be easily measured on a static testing machine with an additional fixture. This standard test is defined in many standards.

The usual measurement is film against film. For this purpose, film attached to the measurement carriage is moved against film attached to the measuring table with a sliding motion. Normally, the films used for this test are identical so that the sliding processes of a film stack or film roll can be evaluated.

Other material pairings can be evaluated with the same test method, such as film against metal, film against glass, or two different types of film.

The test device used for determination of the coefficients of friction consists of a level support table and a carriage with known mass. The top of the support table can either be metal or glass. For the test, it is irrelevant whether the carriage is moved across the table, or whether the movement stems from the table.
The film pair is mounted between the table and the carriage in such a way that there full-surface support with evenly distributed pressure on the entire known surface area. This is accomplished by using a layer of felt.

To determine static friction, the carriage is connected to the force sensor via a standardized tension spring with a fixed spring rate and a thread. This way, the tensile force on the carriage can be accurately measured during the movement. Stick-slip effects should not occur in the measurement of dynamic friction, which is why a tension spring in not used.

The coefficient of friction is represented as the quotient of the tensile force measured on the carriage and the contact force of the carriage that results from its own weight. The static coefficient of friction (static friction) is calculated from the first peak value of the tensile force on the carriage, while the dynamic coefficient of friction (sliding friction) is calculated from the average value of the tensile force on the carriage during a specified sliding distance.