- June 30, 2004, Dr. Richard M. Podhajny, Ph.D. Contributing Editor
Packaging products require a specific range of coefficient of friction (COF) to allow automatic packaging operations to run unhindered and to prevent the package from sliding during storage.
Control of COF is essential, but it can be difficult to attain in printing and converting processes.
Films utilize various lubricants such as fatty acid amides to meet desired COF specifications. The types of lubricants used to meet the desired COF in printing inks can vary from micron-sized particles of PE and polyfluroethylene to silicone and fatty acid amides.
COF is defined as a ratio of the force necessary to move (static COF) or maintain movement (kinetic COF) compared to the force keeping two solids together.
Whereas automatic packaging operations are more concerned about the kinetic COF, the package's slippage tendency is controlled by the static COF. Even under the best of circumstances, packaging products run into problems with falling or rising COF.
When an ink or coating is applied to a film, the COF of the printed film usually is higher than the film itself. The printed film COF will drop over time as the slip agents distribute themselves within the film and the ink layer.
If the printed film has retained solvent, this will cause a higher COF. As the ink dries, the COF will drop. The presence of retained solvent also can increase the migration of slip agents. In some cases this can cause the COF of printing inks to increase as slip agent migration into the ink causes the ink to soften.
The COF of a printing ink is controlled by the lubricants added as well as by the resin and solvents used. These lubricants can be fine particles of PE, tetrafluroethylene, silicone, fatty acid amides, or other products that lower COF.
Of particular importance to a printing ink, COF is the binder's softening point, Tg. In general, the higher the Tg, the lower will be the COF. The presence of plasticizers and high boiling solvents will raise the COF.
COF increase of printed films is unusual, but it can and does occur. Increase of COF with time usually suggests some type of migration is occurring. Either lubricant levels are decreasing in the ink surface, or the ink is becoming softer.
High slip levels in the film can cause migration into the ink layer, which can raise the ink surface COF by “plasticizing” the surface layer.
Rise in COF also can occur due to chemical changes. For instance, passing the printed film through a corona treater will raise the COF of the printed film.
In this case the corona treated film raises the COF by chemically oxidizing the surface components and increasing the surface energy of the printed film. This can be useful if you need to raise the COF of printed films that are out of specification.
Increase of COF with time is a rare occurrence. However, should you encounter such a problem, begin a careful analysis of the components that can migrate, such as solvents and plasticizers.
Softening of the ink/film layer raises the static as well as the kinetic COF. When ink softening is involved due to plasticizer migration, the difference between static and kinetic COF values becomes wider and can be used to estimate the degree of this effect.
UV-curable inks can show differences in COF. For example, UV inks can “cure” to different degrees due to their spectral absorbance characteristics. Black UV inks may show the “lowest” UV cure and have higher COF while other colors may not be affected.
Water-based inks are made up of alkali-soluble resins as well as emulsions. Ink drying and film formation can cause uneven “slip migration of lubricants.” This situation can be aggravated when the inks are overcoated with water-based resins of different composition.
Most COF problems experienced by converters are related to variations within the converting process. Control of corona treatment, solvent retention, and ink formulation will provide a high degree of COF consistency.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 267/695-7717; firstname.lastname@example.org