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Feb 12, 2014
When a liquid coating is applied on a substrate, it is expected, that it will wet that substrate easily and evenly. Good wetting is necessary for an optically perfect surface, good adhesion to the substrate and full functionality of the coating (substrate protection).
Most important for the substrate wetting are the surface tension values of the involved materials: the liquid coating and the solid substrate. In general, for good substrate wetting the surface tension of the coating material (p) should be lower than the surface tension of the substrate (s); If the surface tension of the coating material is higher than that of the substrate poor wetting may occur. The real reason for improper substrate wetting is the too high surface tension of the liquid coating, however, other factors will also have an influence how strongly this defect will show up. Film thickness, e.g., is quite important: wetting problems are easily seen in thin coating layers, high film build can eventually cover the defects. In a similar way also the viscosity/rheology of the coating plays an important role: low viscous systems with a more newtonian flow behaviour will show substrate wetting problems at once, whereas high viscous systems will not show the defects at all.