The co-efficient of friction is a resistive force caused by the interaction of sliding matter. For our purposes, we will use two solid physical surfaces to measure the characteristics of friction. A sliding weight can be used to measure the surface properties of a thin plastic film and determine its kinetic and static coefficients of friction. These calculations give insight into the surface roughness, plowing effect, and any additional molecular adhesion beyond the normal force caused by the Pauli Exclusion Principle.
The shape of the sliding weight can either be rectangular or square. The main area of focus is the total surface area of contact. The standard 200g weight is about 1.5x1.5 inches, however other sizes are available. The surface area should be noted in the test report.
The friction test grip and fixture package will allow you to slide a 200g weight across a table giving about 10 inches of travel. Weights can be surfaced with alternate materials such as vulcanized rubber, felt, or a custom material. The weight is attached to a string and a bearing pulley is used to redirect the string pull. Be sure to secure the bottom surface material so that it remains taut throughout the test. Alternate specifications are available.
A custom sled is also available which has the ability to afix a flexible substrate. The sled makes sample prep much easier and reduces the time that is taken messing around with tapes.
There are two unit values which can be gleaned from this procedure. The first is the static coefficient of friction which usually appears as a spike on the force curve during the beginning of the test. The second piece of data is the kinetic COF which is an average and/or best fit line over a period of time, usually after removing the initial data spike. It is incredibly important that the machine is able to pull at a constant rate of extension.
ASTM D1894 is typically a low force test and therefore it requires extremely sensitive load cells. These cells are generally in the 1 - 5 lb. force range and can be easily damaged if they are mishandled. The operator must also take care to ensure that the weight of the pulling grip is accounted for. The COF fixture package includes a standard hook with a 5/8" female adapter and is less than .25 lbs.