COF in Printed Packaging

Published: June 30, 2005, By Dr. Richard M. Podhajny, Ph.D. Contributing Editor

Material Science

Automatic packaging operations require a specific range of lubricity to process packaging products. Printing inks contribute to the package lubricity and affect both the static as well as kinetic coefficient of friction (COF).

Static COF refers to the force necessary to move a printed surface in contact with another surface, whereas kinetic COF is the force necessary to maintain the motion of the printed surface while it is in contact with another surface.

The value of the static COF always will be higher than that of the kinetic COF value. In principle, if you have a perfect lubricated surface with no friction between surfaces, the static COF would equal the kinetic COF.

As a rule of thumb, inks have the following effect on the printed substrate COF:

If the substrate COF is lower than the printed ink, the printed substrate will show COF lower than the printed ink itself.
If the substrate COF is higher than the printed ink, the printed substrate will show COF higher than the printed ink itself.
If the ink has a higher COF than the substrate, the COF of the printed substrate will be lower than the printed ink itself.
If the ink has a lower COF than the substrate, the COF of the printed substrate will be higher than the ink itself.
If the ink is softer than the substrate, the COF of the printed substrate will be higher than that of the substrate itself.
If the ink is harder than the substrate, the COF of the printed substrate will be lower than that of the substrate itself.
If the COF changes with time, drying, lubricant, or solvent migration could be at play. These changes usually take about 24 hrs.
If the COF increases, migration of solvent or plasticizer to the ink surface is a possibility.
If the COF decreases, the ink could be drying or solvent/plasticizer migration could be occurring.
COF can vary under conditions of changing temperature and/or humidity.

Common printing ink lubricants are fatty acid amides, natural waxes, PE waxes, fluorocarbons such as tetrafluoroethylene (PTFE), as well as silicone materials.

Silicone products move quickly to the surface of printed products. PE and PTFE are micron-sized particles. Fatty acid amides depend on their unique chemistry. The migration of many lubricants is increased if there are high levels of retained solvent in the printed product.

The selection of the appropriate lubricants is determined by the ink formulator for the specific application.

Key to COF behavior over 24 hrs is setting appropriate green and final specifications. Significant changes to COF that may occur on prolonged storage usually are associated with chemical changes.

For example, auto-oxidation of some materials can occur during storage and lead to higher COF.

Many COF problems exist in packaging and printing plants. With all the different ink chemistries (UV, water-based, solvent-based), it is not surprising that understanding and control of COF can be a challenging event for even the best converter.