Coefficient of friction, often abbreviated COF, is measured per ASTM D1894 by pulling one film surface across another (or across a metal or other reference surface) at a controlled speed while measuring the force required to start and sustain that movement. The result is reported as both static COF, the force needed to start the surfaces moving from rest, and kinetic COF, the force needed to keep them sliding once already in motion. Both are unitless ratios, with lower numbers indicating a slicker, lower-friction surface and higher numbers indicating a grippier, higher-friction surface.
COF is directly tied to how slip agents and anti-block additives are formulated into a film; too high a COF and the film will drag, jam, or fail to feed properly through packaging and converting equipment, while too low a COF can cause its own problems, like rolls of film slipping on conveyors or finished bags sliding off of stacks or pallets when they shouldn't. The right COF target is entirely application-specific, since a film destined for a high-speed automatic bagging machine has very different friction needs than a film meant to grip and hold a pallet load in place.
Film-to-film COF and film-to-metal COF are sometimes both specified, since a film can behave differently sliding against itself than it does sliding across the steel or stainless surfaces of packaging equipment, and converters often need both values to properly predict how a film will run.
Glossary
Coefficient of Friction
A numerical measure of how easily one film surface slides against another or against a machine part, tested using ASTM D1894.
Example
A film running through a vertical form-fill-seal machine needs a COF low enough to feed smoothly through the machine's guides and rollers, which is why slip agent dosing is tuned specifically against a target COF range rather than guessed at.
Nuance
It's sometimes assumed lower COF is always better because it sounds like less friction equals smoother performance, but an excessively low COF can cause stacked bags or rolls to slide unpredictably, creating its own handling and safety problems. The right COF is a target range matched to the specific equipment and handling conditions, not a number to minimize indefinitely.