Paints are composed of four components; binders, solvent, pigment and additives. The composition of varnishes is the same, but they lack the pigment.
Additives, like wetting agents, are used to lower the surface tension of the liquid, thus allowing the better wetting of the substrate. Surface tension on contact angle measurements can be used to determine the best wetting agent as well as optimize the amount of wetting agent in the formulation.
Most coating evaluation tests are based on the visual appearance of the coating and are thus qualitative. Both surface tension and contact angle measurements offer quantitative assessments that can be used to evaluate the coating properties. This is especially useful when several coating formulations give a similar coating appearance.
The surface tension value gives an indications of how well the coating will spread on the substrate. A lower surface tension values usually leads to better coating, although a too low surface tension value has been shown to cause problems like levelling issues. Another aspect in measuring surface tension is its behavior as a function of time. Surface aging is a known phenomenon which causes surface tension to change as a function of time. If, for example, a pendant drop measurement is used for the surface tension measurement, immediately when the droplet is formed, the surfactant molecules will start to migrate to the air-liquid interface. This will cause a drop in surface tension until equilibrium is reached. The time it takes to reach equilibrium should generally be as short as possible since that will lead to better coating.
Contact angle measurements can be used to determine the best coating formulation for a given substrate in situations where visual inspection cannot ascertain a difference between different coatings. Low contact angles are usually desirable as they indicate better wettability. Although surface tension measurements are also able to give indications on the best wetting formulation, it is important also to evaluate contact angles as the interaction between formulation and substrate is more complex than it might seem at first glance.
The surface free energy of the substrate is the property of the solid which is equivalent to the surface tension of the liquid. Both surface free energy, or SFE for short, and surface tension are composed of polar and dispersive components. Depending on the how the polar and dispersive components are divided in solid and liquid, this will determine the contact angle. Thus, a higher surface tension formulation may yield lower contact angle values if the polar/dispersive balance is more similar to the solid than a lower surface tension liquid.