Adhesion is achieved by a combination of mechanical interlocking into surface texture, chemical bonding between coating and substrate functional groups, and physical forces across the interface.
The controlling variable is usually not the coating — it is surface energy. A coating only wets and bonds to a substrate whose surface energy exceeds the coating's surface tension. Untreated polyolefins like polypropylene and polyethylene sit around 29-31 dynes/cm, well below what most coatings need, which is why corona treatment exists.
In practice, adhesion failures trace back to a short list of causes: inadequate or decayed surface treatment, surface contamination from slip additives or plasticiser migration, cure shrinkage pulling the film off, or simply a mismatch between coating and substrate chemistry.
It is tested by cross-hatch tape test, peel test or rub test, depending on the industry.