How they work!
The Dispersion process can be split into 3 areas: • Wetting • Dispersing • Stabilisation
1) Wetting is the first step of the dispersion process in which air and moisture are displaced from the pigment surface by the dispersing agent (surfactant). This also causes the surface tension to be lowered.
2) Dispersing stage is where the pigment agglomerates are separated by energy (milling machines) to their primary particle size. As the particle size reduces the surface area increases thus bringing about an increase in gloss etc.
3) Stabilisation is the most critical stage of the process and the dispersant needs to have good wetting and dispersing properties to be able, over a period of time, to keep the particles separated. The dispersant which anchors onto the pigment surface helps to repel the attractive forces between the pigment particles and stops the formation of agglomerates.
To achieve the stabilisation there are two main mechanisms:
• Electrostatic stabilisation – this occurs when particles have the same electrical charge and hence cause repulsion between the particles. What generally happens is stabilisation through an electrical double layer, in which each layer has an equal charge and when two particles come together, their respective charged double layers overlap and repulsion occurs. However when this type of stabilization is the only mechanism, this is susceptible to changes in pH.
• Steric stabilization – this happens by adsorbed polymers on the surface of pigments. It is dependent on the type and structure of the polymer layer. One of the more typical ways of adsorbing is using anchoring groups which as the name suggests, anchors the polymeric dispersant onto the surface of the pigment. The rest of the polymer extends into the resin medium and helps to stabilise the system.