PIGMENT surface exhibits certain surface tension. Surface active agent with corresponding surface tension will form low contact angle with the pigment surface or wet the pigment surface properly. HLB of the pigment surface is this corresponding HLB of the surface-active agent. This explains why pigments gets wet easily in the surfactant of equivalent HLB.
In order to achieve maximum wetting it is therefore necessary that the HLB of the surrounding medium or resin should match to the HLB of the pigment surface.
This can be achieved by adding suitable wetting agent, which alters the surface tension of the surrounding medium or binder. The concentration of wetting agent should be just adequate to achieve the desired surface tension of the resin binder.
There is a distinct relation between HLB of pigment and the hydrogen bonding properties of solvent-resin system. Lipophilic pigments preferably disperse easily in low hydrogen bonding system. Hydrophilic pigments on the other hand disperse in high hydrogen bond resin solvent system such as waterbase reisn systems.
For example, it is difficult to disperse carbon black in water because its lipophilic surface properties and low hydrogen bonding, where as TiO2 gets dispersed easily in water due to it's hydrophilic surface properties.
It is therefore obvious that a pigment requires both resin and solvent system to be in close proximity to its own hydrogen bonding and HLB, for developing optimum properties and to retain them in the pigment dispersion. While dispersing two or more pigments with wide variation in HLB values, many a times, a lot of compromise is necessary. Proper wetting agents are required to minimise the possibility of the flocculation.