Also designated "Flaking".
Peeling of a coating or paiting usually in form of flakes indicates that adhesion to the substrate is not sufficient. Reasons may be:
- The surface is continaminated by substances which prevent adhesion. Such substances may be silicons or hydrocarbons (grease, oil) which may come into contact with the surface by manual handling or by storage without special protection. Such contaminations also often result from treatment with release agents during the production process.
- On some materials, mainly metals, oxidation of the surface may cause peeling.
- The surface energy may be too low to be wetted by the coating substance.
- Scratches off the coating may be caused by aging (UV-irradiation, mechanical treatment, aggressiv chemicals) and water or chemicals may reach the contact layer between coating and substrate where it causes degeneration of one or both contact partners.
By Plasmatreatment of the substrate before coating, in some cases also of the substrate allready coated, processes which may cause peeling may be prevented:
- By Plasma cleaning, Plasma Activation and Plasma Etching the surface may be modified so that sufficient wetting by the coating substance may be achieved. Also see ⇒ Improving Adhesion
- By an additionla Plasma Coating on the surface of the coated substrate, the coating may be protected from aging processes. In particular HDMSO surface coating is resistant aggainst many aggressive chemicals and carbon coatings may be resistent against mechanical treatment.
- By Hydrophobing of the substrate before coating water solving processes of the surface may be prevented.
Also see ⇒ Pretreatment
Peeling of flakes resulting from the Cross-Cut Test indicate unsufficient adhesion of a coating or painting.