The dispersion process is designed to get fine, solid particles distributed evenly throughout the liquid medium or binder. Although additives help with wetting and stabilization, it is critical to choose the proper dispersion equipment to achieve the desired result. The dispersion process is composed of three main parts: wetting, mechanical breakdown of agglomerates and aggregates, and preventing flocculation.
The solid particles must be sufficiently mixed in during the wetting phase so that the particle surfaces are completely saturated by the liquid millbase. The next phase is the physical breakdown of particles into the smallest components to achieve a smooth, uniform dispersion. Ultimately, the particles must be so fine that they remain uniformly dispersed throughout the suspension and do not settle out. The final phase is stabilization where additives are incorporated so the finely dispersed particles remain that way and do not flocculate. It is important to understand the relationship between the liquid and solid components and know the how the solids will perform during the wetting and stabilizing phases.
The Dispersion Doughnut
What do doughnuts have to do with dispersion? A lot actually. In the dispersion process, it indicates that everything is in the right proportion for the best dispersion. Once the disperser is powered up to the right speed, a flow pattern forms that resembles a doughnut. This means that the millbase is being disturbed with enough power to break up all aggregates and agglomerates. The flow pattern is such that a channel forms around the shaft and part of the dissolver disc is visib
The revolving dissolver disc tip projects the millbase toward the vessel wall where the fluid stream divides with a portion going down to the bottom and the other going up the wall of the vessel. The upward portion then flows back toward the center of the dissolver disc. This repetitive pattern is known as the doughnut effect and indicates the application of maximum power and that the millbase is being thoroughly agitated to ensure it all reaches the dissolver disc. The flow pattern observed is also dependent on the pigment concentration and other ingredients in the millbase.
The stream of fluid that is directed downward gets caught in a circular rotation between the bottom of the vessel and the bottom of the dissolver disc as the flow is directed back toward the middle. This happens simultaneously with the doughnut flow pattern in the upper portion of the vessel.
So, what happens if the doughnut effect is not seen? Most likely it means that the solid content is either to high or too low. If it’s too high, the viscosity will also be higher, and the dissolver disc may not even contact the millbase at times thus preventing the formation of the doughnut flow pattern. On the other hand, if the solid content is too low, the corresponding viscosity will be low which leads to splashing and bubble formation rather than development of the optimal flow pattern. The ideal power input will also not be realized which affects the deagglomerating ability of the dissolver disc.
The Dissolver Disc Effect on Dispersion
A large portion of dispersion takes place at the surface of the dissolver disc due to the high amount of shear generated from the blade speed. The shear stress between the underside of the blade and the bottom of the vessel is dependent of the distance between them. Decreasing this distance can improve the shear gradient since the shear rate within the gap is increased if a higher rotational speed is used. Higher speed means more power is introduced to the millbase and can change from laminar to turbulent flow. It’s important to remember that ideal dispersion results are realized with laminar flow (doughnut effect).
The rotating dissolver disc creates areas of high and low pressure ahead of and behind the teeth as they move through the millbase that aids the dispersion of agglomerates. Larger agglomerates will also be impacted by shattering that occurs on the edges and surfaces of the vanes.
Optimizing the Millbase & Operating A DISPERMAT®
Premixing the millbase is done by adding the base liquid component to the dispersion vessel then slowly adding pigments and fillers while running the dissolver disc at a moderate speed. The speed can then be increased until the doughnut effect is achieved. After premixing, the vessel walls and the shaft of the disperser should be cleaned to remove all traces of fluid.
The dispersion is then executed at high speed to ensure that enough mechanical power is applied to achieve the doughnut effect. High speed is critical at this point for optimal results. For example, a 25 mm diameter dissolver disc should be operated at 15,000 RPM to reach outer velocities of 20m/s.
The end result is typically reached in 10-15 minutes and adding more time does not usually provide more deagglomeration. If further particle size reduction is desired, use of a DISPERMAT® bead or basket mill is recommended.