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a.k.a. corona discharge treatment.
Treating is etching the surface of the film at a molecular level to raise the surface tension so the surface can accept inks and adhesives. It is called " corona " treatment because in the early days they shot a flame across the roll creating a "corona" effect. This didn't last long. All it took was a few fires when the web stopped. That's when somebody tried electricity. The first ones were a giant razor blade that made high pitched noises OSHA would fine for big time. Next was some threaded rod followed by present day anodes which are bars of metal. They get flipped up or down for each job.
The unit of measure is dynes.
The natural surface energy of PE and PP is in the low 30 range.
To get the film to accept solvent based inks, 38 to 40 is the default range. If you are using water-based inks, you need more tension in the 44 to 46 range. If the film is being used as a substrate to make pressure sensitive tape, it should be in the 42 to 44 range.