The wetting of the pigment agglomerates through the binder solution is influenced by many factors. The penetration speed of the liquid phase into the interstices of the agglomerates is – under considerably simplified conditions – mathematically expressed by the Washburn equation.
The capillaries between the pigment particles are assumed to be cylindrical tubes with radius r. The first factor on the right hand side of the equation shows that for fast wetting (high penetration speed) loosely packed agglomerates (r is large) and a low viscosity are favorable. However, the agglomerate structure cannot be influenced by the coating producer and the possibilities for viscosity reduction are fairly limited.
The second factor looks more promising: here we have the surface tension of the penetrating liquid and the contact angle. Both can be influenced by wetting additives but they are not independent from each other: it is not possible to have a high surface tension together with a low contact angle. In practice, wetting additives are used to lower the surface tension as far as necessary to bring the contact angle close to zero; lower surface tension values should be avoided.
Wetting additives can be defined as substances which are designed to reduce the contact angle between the pigment and binder solution and which, as a result, accelerate the penetration speed of the liquid into the agglomerate structure. Characteristic for such substances is their surfactant structure: polar, hydrophilic structural elements along with nonpolar, hydrophobic structural elements are combined in one molecule. Because of this very structure, such compounds are interface-active, i.e. the wetting additives migrate to the pigment/binder solution interface. From a chemical perspective, wetting additives can be classified as either ionic or non-ionic – according to how the polar segment is incorporated into the molecule. The non-polar segment is, as a rule, represented by hydrocarbon chains.
