Fullbrook Systems - © 2011 #1
Fullbrook Systems
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The Doughnut Effect
The best dispersion results with a DISPERMAT® are obtained when the geometry of the dispersion container,the diameter,the peripheral velocity and the height of the dissolver disc above the bottom of the vessel as well as rheological millbase properties are matched to one another.
After adding pigments and fillers to the resin solution,the millbase is brought into a laminar rolling flow pattern by increasing the speed of the shaft until no standing material can be seen at the wall of the container.
At the correct speed,a channel begins to form around the shaft and a part of the dissolver disc becomes visible.At this point, the millbase will form a doughnut-like flow pattern. The doughnut-like flow pattern is a signal that the maximum mechanical power possible is being transferred into the millbase and furthermore that the millbase is being agitated so that all the agglomerates will eventually reach the dissolver disc.
The doughnut effect develops because the millbase is accelerated outwards from the tip of the dissolver disc.When it hits the wall of the vessel,the stream is divided into two parts. The one going downwards flows back to the middle of the dissolver disc along the bottom of the dispersion vessel and rises up to hit the disc once again.
The second part flowing upwards has the same circular path,which is limited in by the force of gravity and the rheological properties of the millbase.
The flow pattern of the doughnut effect is greatly influenced by the amount of pigment and filler in the millbase.When the solids content is not high enough,the viscosity tends to be too low. This leads to splashing and generation of bubbles during dispersion.
In addition,the mechanical power input is limited and the deagglomerating capability of the dissolver disc is diminished. Adversely,if the solids content is too high,then the viscosity will be too high for the doughnut flow pattern to develop.
The flow of the millbase may also be hindered by a yield value of viscosity.This will result in a tearing action of the dissolver disc,which may at times even turn without having contact with the millbase.