Free-radical cure shrinks 5–10% by volume, and the thermal expansion mismatch with metal or glass is large, so interfacial stress appears the moment the coating cures. If only van der Waals forces hold the interface, that stress pulls weak spots apart — visible as edge lifting and flaking in cross-hatch testing.
Metal surfaces carry oxide layers and processing oils; glass is a highly polar but inert silica network. Neither offers reactive sites for the resin. Acrylate resins alone have limited ester polarity and cannot build a strong interface.
Water adds a third failure path: it migrates along the interface and disrupts hydrogen bonds, which is why many formulations pass initial adhesion but peel off after two hours in boiling water.
The polar phosphate head migrates to the interface and forms a self-assembled monolayer. P–OH condenses or coordinates with surface hydroxyls and metal ions, producing M–O–P bonds — chemical bonds, not physical adsorption.
The acrylate end copolymerizes with the resin during cure, stitching the interphase into the bulk coating. The molecule is locked into the network, so unlike silanes or titanates it does not migrate or bloom.