The four films that dominate flexible packaging each behave differently under a coating.
**BOPP** (biaxially oriented polypropylene) is the highest-volume film — low cost, good moisture barrier, good clarity — but low surface energy, so it always requires corona treatment.
**PET** (polyester) has higher surface energy, excellent dimensional and thermal stability, and good strength. It is the easiest of the four to print, and the usual choice for the outer web of a laminate.
**PE** (polyethylene) is soft, has excellent sealability and very low surface energy, and often carries slip additives that migrate to the surface and destroy adhesion. It is usually the sealant layer.
**CPP** (cast polypropylene) offers better clarity and heat resistance than PE as a sealant web.
For any of them, the number that matters on press is the current dyne level of the actual roll, not the film specification.