Meaning
The ratio of the force required to sustain sliding motion between two contacting surfaces to the perpendicular force holding them together determines the ease of movement. This dimensionless number, called the kinetic coefficient of friction, governs the ease of sliding during high-speed film conversion and automated packaging processes. The property applies to dry solid contact between polymer films or molded parts and excludes hydrodynamic lubrication regimes where liquid lubricants are present.
Surface Resistance
Molecular interactions between the polymer chains and the opposing contact surface create resistance to sliding. The kinetic coefficient of friction determines how much pulling force is required once the initial static resistance is overcome. Rougher surfaces usually reduce the contact area and therefore decrease this value for non-tacky polymers.
Additive Lubrication
Slip agents like erucamide or synthetic silicas are blended into resins to reduce surface tack. They migrate to the surface to form a boundary layer that lowers the kinetic coefficient of friction. This migration is highly dependent on both curing time and storage temperatures.
Processing Consequence
High friction causes polymer sheets to wrinkle and stick in packing equipment, which slows down the production rate. Molded plastic parts with low friction coefficients eject easily from steel mold cores without deformation. Tooling layouts must account for these friction values to ensure automated parts handling.
