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Coefficient of Friction (COF)

Coefficient of Friction – “COF”

Working in virtually every facet of our lives, friction is the unseen force that allows, or hinders the movement of two surfaces that are in contact with one another.  A packaging film’s “coefficient of friction” (COF) provides a relative indication of frictional characteristics.  Controlling COF gives processors the ability to optimize performance and avoid problems in forming, transporting, and storing of packages.
  • In HFFS (horizontal form fill and seal) systems, too much friction of the sealant side of the film can lead to film dragging or jamming as it passes over metal plates.
  • In VFFS (vertical form fill and seal) systems, too much friction of the sealant side of the film can cause poor film feeding over metal forming collars, inconsistent package sizes, and squealing.
  • In either system, too much friction can result in lateral slipping that leads to poor seals (leakers).
  • Too little friction on the outside can cause packages to slip or fall-off-of inclined conveyor belts.
  • Too much friction on the outside can slow packages’ progress down delivery chutes.
  • Too little friction on the outside can result in packages sliding off of stacks or pallets.

COF is determined by dividing the force needed to move one surface across another by the force perpendicular to the surfaces.  COF values range from 0.00-1.00, with lower COF values indicating lesser resistance to sliding, or higher “slip.” 

  • COF < 0.25 is considered LOW COF, HIGH-SLIP
  • COF > 0.45 indicates HIGH COF, LOW/NON-SLIP

COF is reported in two values: static (µs) - the force needed to begin movement, and kinetic (µk) – the force required to maintain movement.  Generally, Kinetic COF is used for roll films.  Static COF is of greater concern for stacked or palletized items.

Customized COF’s are achieved by adding a “slip agent” to a film resin during production.  The traditional approach to reducing COF involves adding a compound that is incompatible with the film resin, and will migrate to the surface of the film over time.  Non-migratory slip agents offer benefits in the area of thermal stability and consistency, but can affect film clarity. 

COF can be affected by a number of factors including antiblock additives, corona treatment, antistats, inks, varnishes, adhesives.  Because laboratory testing cannot simulate every element of any packaging process, COF should not be considered an indication of system-specific performance.  It is, instead, a means of reproducing frictional properties shown to be successful under a given set of conditions. 

The first step to creating and optimizing the perfect packaging film for your product and process is talking with an expert.  To learn more about optimizing coefficient of friction for your product and process, to be sure you’re using the best packaging film for your application, contact Flair today.