Two different problems under one heading
Plastic substrates present two distinct adhesion challenges that are often conflated. Polyolefins (PP and PE) have very low surface energy and essentially no polar functionality, so there is nothing for a coating to bond to chemically. Plasticised PVC has adequate surface energy but contains mobile plasticiser that migrates into the coating over time.
The first problem is solved at the interface, through surface treatment or a coupling chemistry that can interact with a non-polar surface. The second is a long-term compatibility problem that only appears after weeks or months, and no interfacial treatment will fix it.
Coupling chemistry for low-energy surfaces
For polyolefins, chlorinated polyolefin promoters remain the most reliable option. They are partially compatible with the substrate surface and carry polar functionality that the coating binder can interact with, forming a genuine transition layer. They perform best as a thin primer rather than as a topcoat additive.
Silane coupling agents work through a different mechanism, hydrolysing to silanols that condense with hydroxyl groups on inorganic surfaces. That makes them highly effective on glass, mineral fillers and treated metals, and largely ineffective on untreated polyolefins, which have no hydroxyl groups to react with. Selecting a silane for a PP substrate is a common and expensive mistake.
Designing around plasticiser migration
In plasticised PVC, migration is a certainty and the formulation has to tolerate it. A coating that becomes soft, tacky or loses adhesion after a month has usually not failed at the interface. It has absorbed plasticiser and changed its own properties.
The practical routes are a binder with low plasticiser compatibility, a barrier layer, or accepting the migration and formulating for the plasticised end state. Whichever route is chosen, evaluation has to include accelerated ageing at elevated temperature. Initial adhesion testing on fresh PVC tells you almost nothing about performance in service.
Surface control still matters
Low surface energy also means wetting problems, so adhesion promotion and substrate wetting have to be solved together. A coating that dewets before it cures has no interface to promote adhesion at, regardless of the coupling chemistry present.
Fluorine-modified and polyether siloxane additives provide the surface tension reduction needed on plastic substrates. Where a textile or synthetic leather finish is involved, slip and hand feel usually become explicit requirements too, which brings the same siloxane chemistry back for a second reason.
Apply this to your own formulation
Send us the binder system, substrate and the defect you are seeing. We will suggest a specific grade and dosage range.