Additive Movement
Transfer of slip agents from the bulk of a plastic film to its surface creates a thin lubricating layer that reduces the coefficient of friction. This chemical process, known as erucamide migration, is a standard feature of many polyethylene and polypropylene films used in high-speed packaging. Erucamide is a fatty acid amide that is added to the resin during the extrusion process.
Because it is not fully compatible with the polymer matrix, the molecules slowly move toward the exterior of the film. Once they reach the surface, they form a microscopic wax-like coating that allows the film to slide easily over metal rollers and other sheets of plastic. This prevents the film from sticking to itself or jamming the converting machinery.
Surface Friction
Control of the friction level is the primary reason for using this specific additive in film formulations. During erucamide migration, the concentration of the amide on the surface increases until it reaches a state of equilibrium. This equilibrium depends on the initial concentration in the resin, the thickness of the film and the storage temperature.
At higher temperatures, the molecules move more quickly, leading to a faster drop in the coefficient of friction. This is useful for packaging lines that operate at extreme speeds where heat is generated by friction. However, if the migration is too heavy, it can create a greasy feel on the film and interfere with the optical clarity.
The presence of this layer is often measured by the coefficient of friction, with target values typically falling between zero point one and zero point three. If the migration is insufficient, the film will be too tacky and may cause tracking issues on the press. Conversely, excessive migration can cause the film to slip out of alignment during winding or slitting operations.
Testing the friction at regular intervals helps to ensure that the additive is performing as expected across the entire roll.
Adhesion Interference
Presence of a lubricating wax on the surface of the film presents a significant challenge for printing and lamination. While erucamide migration is helpful for handling, the same molecules can act as a barrier that prevents inks and adhesives from bonding to the polymer. The slip agent covers the polar sites created by corona treatment, effectively lowering the usable surface energy.
This can lead to ink adhesion failure or low peel strength in laminated structures. Converters often have to balance the need for low friction with the requirement for high bond strength. If a film is stored for too long before printing, the amide layer may become too thick to be overcome by standard treatments.
In some cases, the heat of the printing process can cause more migration to occur, leading to problems that only appear after the package is finished. Selecting the right slip agent involves choosing between fast-blooming and slow-blooming amides to match the production schedule. Most high-performance laminations use low-slip or no-slip films to avoid these bonding issues entirely.
Accurate monitoring of the surface state is the only way to prevent migration-related failures.
