Dispersants are highly specialized additives used to wet, disperse and stabilize solid particles in various continuous phases such as solvents, water and plastics. Dispersants are designed to reduce viscosity and increase dispersion stability while enhancing the aesthetic properties of the coating.
The solid particles to be dispersed are usually pigments, but it can also be silica matting agents, waxes, conductive particles (carbon, graphene, carbon nanotubes), inorganic fillers (calcium carbonate, talc, barite), or even are noble metals (gold, silver and platinum) - any solid particles dispersed in a continuous phase (liquid or solid).
By reducing viscosity, dispersants make dispersions more workable, while also helping to improve productivity economics by increasing pigment loading and dispersion rates. This offers a shipping advantage: the more pigment in the dispersion, the less water there is, making it easier to handle and ship. Formulation flexibility is also enhanced through the ability to add dispersions to a wider range of base finishes.
Dispersant structure
A typical dispersant is a two-component structure consisting of an anchor group capable of strongly adsorbing to the particle surface and a polymer chain attached to the anchor group and providing stability. The anchor groups surround the particles and the chains sterically stabilize the particles to prevent the particles from flocculating or becoming gels in the dispersion.
There are many structures designed to achieve this stability, since all particles have different surface properties, and there are many media - water, solvents, UV monomers, resins - with more polarity changes. Different anchor groups are more receptive to different particle surfaces. It is the specific combination of anchor groups and polymer chains that is responsible for the effectiveness of the dispersant.
The decentralization process consists of three stages:
Wetting of granules (premixed)
Particle separation (by mechanical means)
Stabilization of particles (overcoming van der Waals attractions)
The first and third stages can be improved by using effective dispersants/stabilizers, the third stage being the critical stage for optimizing system performance. This is because the third stage controls the final quality and stability of the decentralized system.
Dispersants achieve stability through steric stabilization, which is based on the adsorption of polymeric materials to particle surfaces to overcome van der Waals attractions. Properties required for effective stabilization include:
Strong adsorption of stabilizing polymers to particle surfaces
Chain length (too long and it can fold back and compress the steric barrier; too short and it can't overcome van der Waals forces)
Good solubility in the medium used in the dispersion process
Compatibility with resin after solvent evaporation
How to add dispersant?
Most dispersants are pourable liquids and some are waxy or granular solids. Ideally, the dispersant should be added before the mechanical process of breaking everything down - before the mechanical energy is applied. A good time to add the dispersant is during grinding of the base phase (mainly resin, solvent or water) to ensure the dispersant dissolves before adding the pigment.
Good results come from the dispersant getting to the particle surface because the mechanical device exposes the surface. Dispersants surround the available particle area and prevent the particles from reaggling together, thereby reducing viscosity.
Having said that, in some cases dispersants can be post-added at the end of the process to improve stability or color, but they do not go through the original factory.
Improving the quality and stability of dispersions also leads to improved coating quality. A reduction in average pigment particle size increases color intensity. Similar benefits can be seen in gloss, clarity and brightness. However, if particle stability cannot be maintained in the dispersion, the resulting performance improvements may be reversed.
