The phenomenon of adhesion between polymer films and metal has been a focal point of research for scholars. To date, four adhesive mechanisms for metal film coating, namely mechanical interlocking theory, diffusion theory, electrostatic theory, and adsorption theory, have been widely accepted.
Among them, adsorption theory is the most widely recognized theory to explain the phenomenon of adhesion. Adsorption theory suggests that the bonding force between two materials capable of closely binding primarily arises from the interaction forces between the materials. When sufficient molecular contact is achieved at the interface, the adhesive and adherend can adhere through the interaction forces acting on the interface area. Interaction forces between molecules include hydrogen bonds and van der Waals forces. The bond energy of hydrogen bonds ranges from 10 to 40 kJ/mol, while that of van der Waals forces is below 10 kJ/mol. However, they are much lower than the bond energies of ionic bonds (590-1050 kJ/mol), covalent bonds (63-710 kJ/mol), and metallic bonds (113-347 kJ/mol). Ionic bonds and metallic bonds are relatively non-directional because the ions involved in ionic bonds and metallic bonds are approximately spherical, whereas covalent bonds are highly directional because they usually involve non-spherical orbitals. This hybrid bonding situation results in both directional covalent bonds and certain characteristics of non-directional metallic or ionic bonds. Therefore, adsorbates bonded by chemical bonds tend to adhere to specific positions, and the bonding interactions they exhibit largely depend on their exact positions and orientations relative to the substrate. On the other hand, van der Waals forces and hydrogen bonds do not have such strong directional constraints, and the adhesion theory formed by them has broad applicability and is less dependent on surface morphology. Thus, they adhere more firmly to specific sites and form attractive interactions with the surface, making the entire surface more uniform. Hydrogen bond complex theory is a type of adsorption theory that can be used to explain the organic bonding between film-coated iron. The schematic illustration of the organic bonding between TFS substrate and PET film is shown in Figure . During the film coating process, a layer of hydrated oxide forms on the surface of the metal steel plate. When it is laminated with polyester film, it binds to the film in the form of hydrogen bonds, exhibiting relatively high bonding strength. However, this theory still requires substantial experimental data to support it.
