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Introduction to Dispersion Technology with the DISPERMAT® Dissolver
1. The Dispersion Process
The most frequent application of high speed dispersion is to incorporate extremely fine solid particles into fluids, to produce colloidal suspensions.
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Colloidal suspensions are characterised by their behaviour that the finely divided small particles do not settle under the force of gravity. A sequence of related steps take place during the dispersing process.
These are:
- the wetting of the surface of the solid particles by the fluid components of the millbase
- the mechanical breakdown of associated particles leading to smaller particles (agglomerates and aggregates)
- the smaller particles generated during the dispersion are stabilised, preventing renewed association (flocculation).
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| Special interaction between the solid particles and the fluid components of the millbase determine their wetting and resistance to flocculation. |
2. 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.
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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. |
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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. Conversely, 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. |
3. The Dispersing Effect of the Dissolver Disc on Agglomerates
When the vanes of the disc are moved through the millbase at a high velocity, areas of higher and lower pressure are generated in front of and behind the vane. The alternating stress acting on the agglomerates in these areas facilitate their dispersion. In addition to this, a smashing impact should be considered for larger agglomerates being hit by the edges and the surfaces of the vanes.
However, a considerable share of the total dispersion work takes place at the surface of the dissolver disc. Due to the fast movement of the blade, a gradient of shear builds up on these surfaces in which the dispersion takes place. |
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The shear stress which acts particularly between the lower disc surface and the bottom of the container largely depends upon the distance between the two. The efficiency of the shear gradient may be enhanced by decreasing this separation since the shear rate within the gap is increased and since a higher rotational speed may be chosen due to the fact, that the change from laminar to turbulent flow takes place at higher rotational speeds.
When higher speeds are used, more mechanical power is introduced into the millbase. The best dispersion results are obtained with the highest possible mechanical power input, as long as the doughnut flow pattern (laminar flow) is maintained.
The mechanical power is a product of rotational speed and momentum (torque) of the shaft.
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Peripheral Speed of Dissolver disc:
18-25 m/s. (3500 to 5000 ft./min) | |
4. Running the DISPERMAT® and Optimising Millbase Formulations
In practice, a simple procedure has proven to yield satisfactory results:
- First the liquid component is put into the dispersion container. Then, under moderate agitation by the dissolver disc, pigments and fillers are added slowly the dissolver disc speed can then be increased until the doughnut effect is detected at a higher rpm (circumference velocities of approximately 18-25 m/s).
- After premixing, the walls of the dispersion container and the shaft should be cleaned removed adhering millbase.
- Then the dispersion is carried out at high peripheral velocities that guarantee the formation of the doughnut effect.
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At this stage, the capability of the DISPERMAT® to transfer high mechanical power into the millbase should be exploited. One must not be afraid to use high rotational speeds. If e.g. an dissolver disc of 25 mm diameter is used, the DISPERMAT® must be run at a rotational speed of 15.000 rpm in order to obtain peripheral velocities of 20 m/s. The final dispersion result is normally reached after 10 to 15 minutes.
Use of the dissolver for a longer period of time is not likely to improve the result. Sample analysis shows that further deagglomeration does not take place. The particle size of demanding or difficult products can be reduced further using a DISPERMAT® bead mill or basket mill. |
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| Recommended dissolver disc-Ø in relation to viscosity and vessel size |
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5. Scaling up Laboratory Results to Production
An important fact is that the dispersion results obtained with a DISPERMAT® can be scaled up to a production size dissolver. It was mentioned earlier that the dispersion depends upon the rate with which the agglomerates are transported into the zones of shear and on the mechanical power which is transferred into the millbase. The mechanical power is the parameter that limits the maximum degree of dispersion which can be achieved. The rate at which the transportation of the agglomerates into the vicinity of the dissolver disc takes place, determines the time necessary to reach the optimum dispersion result.
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DISPERMAT ® and TORUSMILL ® | |
The deagglomeration process mainly takes place within the area of shear which surrounds the dissolver disc. The most effective shearing conditions are found at the tip of the dissolver disc, as this part is moved through the millbase at the highest speed. It is for this reason, that the tip speed (peripheral velocity) is to be considered as the key parameter for scaling of laboratory results to production. This statement refers to the maximum achievable degree of dispersion and not to the time necessary to obtain it. The DISPERMAT® will normally be faster in dispersing than a production scale machine, as the distance of the agglomerates must cover to reach the disc are shorter than in larger equipment.
Exact correlation between the dispersion result with a DISPERMAT® and a larger dissolver will naturally also depend upon comparable temperature conditions. For temperature control, the use of a double wall temperature control container is recommended.
For a laboratory dissolver to reach the peripheral velocities necessary for dispersion, it must be able to run at high speeds with utmost accuracy and reproducibility. When using the dissolver discs of different diameters, the circumference velocities may easily be calculated by the following formula:
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v = circumference velocity
π = 3.141...
d = diameter of the dissolver disc in m
n = revolutions of shaft in rpm |
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Example for a millbase volume of 100 ml:
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container capacity: |
250 ml |
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inner diameter of container: |
65 mm |
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Container height: |
85 mm |
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dissolver disc diameter: |
30 mm |
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revolutions of shaft: |
11500 -16000 rpm |
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peripheral velocity of disc: |
18 - 25 m/sec | |
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Example for a millbase volume of 2500 ml:
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container capacity: |
5000 ml |
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inner diameter of container: |
180 mm |
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Container height: |
200 mm |
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dissolver disc diameter: |
80 mm |
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revolutions of shaft: |
4300 - 6000 rpm |
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peripheral velocity of disc: |
18 - 25 m/sec | |
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Example for a millbase volume of 30 l:
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container capacity: |
65 ml |
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inner diameter of container: |
440 mm |
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Container height: |
440 mm |
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dissolver disc diameter: |
200 mm |
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revolutions of shaft: |
1700 -2400 rpm |
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peripheral velocity of disc: |
18 - 25 m/sec | |
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7. Steps to Improve Dispersion Results
In cases where the quality of dispersion does not meet the required standard, the following parameters should be checked:
- Duration of the Dispersion Operation
The quality of formulations dispersed using a DISPERMAT® generally reaches its final value after a short period of time (approximately 10 -15 minutes). Increasing the dispersion time to more than 20 minutes does not normally lead to improved results.
- Doughnut Effect
The doughnut flow pattern should be maintained during the course of the whole dispersion.
- Shaft Speed
The mechanical power input should be optimised by using the highest possible rotational speed and thereby the greatest peripheral velocity, without destroying the doughnut flow pattern.
- Geometrical Considerations
The distance between the dissolver disc and the bottom of the vessel can be changed to obtain better results and to make higher rotational speeds possible.
- Dissolver Disc
The use of smaller or larger dissolver disc may lead to better results.
- Amount of Millbase
Better flow characteristics may be achieved by using more or less millbase in the container.
- Pigment and Filler Concentration
A High viscosity millbase of tacky consistency with dilatant flow is recommended this may be obtained by increasing the percentage of solids, but without destroying the doughnut flow pattern.
- Flocculation
Does flocculation take place after dispersion? If so, check additives.
- Temperature
When dispersing, high energy transfer into the millbase will lead to an increase in temperature. In many cases this destroys the flow characteristics of the formulation. In addition, thermally sensitive paint ingredients may be harmed. Using a water cooled vessel will solve the problem.
- Raw materials
Partial re-formulation of the paint using more suitable resins, pigments, fillers or additives: It should be kept in mind, that the DISPERMAT® is a dispersion device and not a piece of milling machinery. Therefore it is incapable of grinding primary particles down to a smaller size. |
8. The Rheological Behaviour of Millbases
To obtain excellent dispersion results, the millbase must exhibit certain rheological properties. Unfortunately, the flow behaviour of a millbase may not be expressed by one single parameter, such as the apparent viscosity.
Viscosity is the measure of the internal friction of a fluid, and is a product specific Constant Value which is defined as the quotient of shear stress (t ) and shear rate (D).
Only Newtonian fluids retain a constant viscosity and are independent of variations in shear rate (that is i.e. water, mineral oil, etc.). All other substances which have a viscosity which is dependent on shear rate are classified as Non-Newtonian and are more commonly found than Newtonian liquids.
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newtonian fluid |
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pseudoplastic substance |
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Is a fluid whose viscoity is independent of the shear rate at which it is measured. |
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Viscosity decreases when the shear rate is increased. |
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plastic substance |
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dilatant substance |
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Newtonian: Is a fluid whose viscoity is independent of the shear rate at which it is measured. |
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Viscosity increases when the shear rate is increased |
Millbase formulations are complex rheological systems, for whose characterisation information concerning apparent viscosity, plastic behaviour, yield stress, thixotropy, rheopexy, dilantancy is needed.
The millbase should exhibit mild dilatancy without having a noticeable yield stress value that may hinder the free circulation of the millbase during dispersion. The rheological properties should not alter too much during the course of the dispersion, although the viscosity does not tend to increase whereas dilatancy decreases in most cases. |
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