Polyolefin films — BOPP, PE, CPP — are chemically inert with low surface energy, around 29-31 dynes/cm. Almost nothing adheres to them in that state.
Corona treatment passes the film through a high-voltage electrical discharge, which oxidises the surface and creates polar functional groups. Surface energy typically rises to 38-44 dynes/cm, enough for inks and coatings to wet and bond.
The critical operational fact is that corona treatment decays. Treated surface energy falls over time as the oxidised groups reorient into the bulk and as additives migrate to the surface. Decay rate depends on the film, its additive package, and storage temperature. A film treated to 42 dynes at manufacture may be at 34 dynes after months in a warehouse — and it will fail on press.
This is why adhesion troubleshooting should always begin with a dyne pen on the actual roll, not the specification sheet.