The reactive species contained in the plasma create radical sites along the molecular chains, and polar groups can attach themselves to the radical sites. Since the process generally works with air plasma, primarily oxygenated groups such as hydroxyl (-OH), carbonyl (-CO) and carboxyl (-COOH), are bonded to the upper molecular layer. This changes the material that was originally nonpolar to a polar substrate that can be properly wetted with ink, paint, adhesive, etc. The wettability of the surface does not guarantee that the applied material will bond securely though. The interaction between the active surface and the functional groups of the surface is ultimately decisive for the strength of bonding.
The energy input and functionalization of the surface cause the surface to roughen within the µm range. The benefit is that the coating is anchored to a "larger" area.
To be able to successfully bond, coat, print or paint polymers and metals, the surface must first meet a few requirements. It has to be very clean and has to be sufficiently active to form an adhesive bond with the coating material. Plasma pretreatment takes care of both of these requirements: The reactive plasma species transform the finest organic layers to the gas phase during ultra-fine cleaning. And with polymers, the functional groups described above are bonded to the outer-most molecular layer, the surface polarity increases and the prerequisite for adhesive bonding with the applied material is fulfilled.
Plasma pretreatment (ultra-fine cleaning / activation)
Increase of substrate surface energy
Good surface wettability
Interaction between substrate and application
Adhesive bonding of material to ink, paint, adhesive