Question: We extrude and corona treat high-slip PE blown film. In a recent evaluation, the film went from a COF of .420 hot off the line to .188 five days later. The treat level went from over 50 to below 36. This seems like an excessive amount of treat loss. Any comments or suggestions?
Answer: That is an unusual amount of treatment loss, but on the other hand, trying to treat freshly blown PE to 50 dynes/cm in one step is pushing things – usually the film extruder will treat to about 38 to 44 dynes/cm, and the printer/converter will then bump treat as needed. The higher the treat level, the faster the treat loss, especially on high-slip films.
The degree to which the COF (and dyne level) dropped in five days suggests that you are using a traditional low molecular weight amide slip agent, such as erucamide or oleamide. These are intended to migrate quickly to the surface, especially on treated film, where the surface polarity attracts the slip molecules. Secondary amides such as oleyl palmitamide have approximately twice the molecular weight, and subsequently bloom less aggressively. They are also amenable to higher process temperatures without degradation, and tend to offer somewhat more stable results from corona treatment.
As a rule of thumb, films extruded from resin blends containing these compounds should condition for at least a day or two to allow the migration to take its course, with a corresponding COF reduction. After this conditioning period, blooming and COF reduction will continue, but at a much slower rate.
There is an alternative to these compounds: non-migratory slip agents which have a molecular weight 30 to 50 times that of the traditional amide formulations. These compounds are too massive to bloom to the surface, so their effect on COF is more or less immediate, and stable. As such, they will also have a vastly reduced effect on corona treatment loss. Another advantage is that they are stable at higher processing temperatures than are the amide-based slip agents.
Based on this information, my suggestion is twofold: Consider treating to a slightly lower dyne level initially, and investigate the feasibility of non-migratory slip agents, or at least of the secondary amide formulations.