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Three in one go: more efficient, faster, cheaper

For manufacturers of paints and coatings, more efficient production is becoming an important factor in securing competitiveness against the backdrop of sharply rising energy and raw material prices. Compared to the dissolver, which is widely used in paint and varnish production, modern technologies achieve enormous improvements.

ystral Conti-TDS The aspirated powder is dispersed immediately and with a high shearing effect

Compared to the dissolver widely used in paint and varnish production, modern technologies achieve enormous improvements: Depending on the application, energy requirements, production times and production costs can be reduced by up to 90% or more.

In process engineering, very different process steps take place in one container - simultaneously and often with the same tool. This is also the case with the dissolver technology commonly used in paint and varnish production, which dates back to the 1930s and has changed very little in recent decades. The dissolver disk is used simultaneously for mixing, dispersing and adding powdered raw materials to the liquid, whereby the machine's output is distributed over the entire contents of the container. The disk generates only a very low shearing effect and therefore requires high viscosities for dispersion, which counteracts effective powder introduction. The principle of the disk agitator hinders vertical mixing. The powder application by means of a drum brings a lot of air into the product, which not only reduces the dispersing effect, but also requires additives that later have a negative effect in the finished product. The dispersion results vary greatly. All this makes production based on dissolver technology energy-intensive, slow and inefficient.

The decisive lever for increasing efficiency in the production of paints and coatings, lowering energy requirements and reducing production costs and production times is a massive increase in the intensity of the key dispersion and powder wetting processes by several orders of magnitude. However, this intensive dispersion no longer takes place in the entire container, but outside, in a very small space and in the shortest possible time in the circulation process. At the same time, permanent homogeneous mixing takes place in the container. The inline disperser is connected to the process container, in which an effective mixer is installed, via pipes in the circuit.

Avoidance of agglomerates
Agglomerates are a central problem when using conventional technologies in paint and varnish production. In the vast majority of cases, powders are agglomerated - and the finer a powder is, the more it tends to agglomerate. If these agglomerates are not broken up when the powder is added or if further agglomerates are formed when it is added to the liquid, then these agglomerates must be broken down afterwards by long and laborious post-dispersion or grinding.

In the dissolver, the powder particles do not come into contact with the liquid individually, but as a compact bulk. The liquid surface available to the powder for wetting is orders of magnitude smaller than the particle surface to be wetted. Powders have specific surface areas of between one thousand and several hundred thousand square meters per kilogram. With the dissolver, only around 100 square meters per minute are available for wetting. The particles are therefore not completely wetted immediately and agglomerates are formed. Post-dispersion to break down the agglomerates not only makes production processes energy- and time-intensive, but also reduces product quality: for example, it destroys polymers and overheats resins or binders. The required fineness can often only be achieved in a dissolver process with subsequent grinding.

In contrast, modern inline dispersing technologies based on vacuum expansion during powder application achieve complete disagglomeration and wetting of the powder particles within microseconds. Mills are only required in exceptional cases. With the vacuum expansion method, the air contained in the powder is expanded many times over by the suction vacuum directly in the wetting and dispersing zone, which increases the distances between the particles enormously. The particles are separated and fluidized. The machine generates a specific liquid surface area of around one million square meters per minute. This is more than the powder surface to be wetted and around 10,000 times as much as a dissolver. Powder and liquid only come into contact with each other in the wetting chamber - under maximum vacuum and maximum turbulence. In the dispersion zone, the powder particles have the greatest possible distance from each other and can therefore be completely wetted and dispersed individually.

In a newly developed inline dispersing machine, the wetting and dispersing processes are concentrated in a dispersing zone with an effective volume of only around a quarter of a liter. Compared to a dissolver operated in a container, such an inline disperser generates a volume-specific output that is around 30,000 times higher. This concentrated output is crucial for the success of the disperser. The inline disperser also generates 1,000 times higher shear forces via a rotor-stator system. The dwell time is extremely short, so that only a fraction of the energy is required compared to the dissolver.

No tumbling, no additional air input
Another problem with dissolver technology is air entrainment. On the one hand, this is caused by the powder materials themselves, as powders contain a lot of air. Even heavy powders, such as titanium dioxide, have an air content of over 75% by volume. With light powders, the proportion is over 90 %. If this air is not completely substituted by liquid and separated, but dispersed together with the powder particles, this leads to microfoam - this is the case in a dissolver process.

If the powder is added to an open container from above, this also creates air bubbles, through which large quantities of additional air are introduced. Air is elastic and therefore hinders effective dispersion. The power of a machine used for compressing, expanding and breaking up air bubbles is not available for dispersing and mixing.

When using modern technologies in paint and varnish production, the powder is therefore sucked directly into the liquid externally in the circuit. There are no droplets and no additional air is introduced during the entire process. During powder wetting in a vacuum expansion process with a rotor-stator system, the air contained in the powder is separated from the significantly heavier dispersion by the centrifugal effect of the fast-running rotor and coalesces into large air bubbles. These are then conveyed together with the liquid flow to the process container, where they can easily escape.

Turbulent micro-mixing and virtually turbulence-free macro-mixing
A conventional agitator can move the entire container liquid with minimal power requirements due to an almost turbulence-free laminar flow in the container with low electrical power and a small motor. However, it is not only the electrical power generated by a machine that is relevant to the energy requirements of a production process, but also the process time required - and this is very long with conventional technology. Modern jet mixers, on the other hand, rely on process intensification and local concentration of machine power by combining a turbulent micro-mixing zone in their mixing head with virtually turbulence-free vertical macro-mixing of the entire container contents. Due to the turbulence generated in this micromixing zone, such mixers initially require more power than a simple conventional agitator that does not generate turbulence. However, because the mixing times are reduced by up to 90 % with a jet mixer, depending on the product, the energy requirement is less than a third despite the two to three times higher output. Unlike when using a conventional agitator, the product is actually completely homogeneously mixed at the end of the mixing process - without unmixed zones, without sediment - and consistent results are achieved regardless of the batch size and the fill level in the container. Jet mixers can be installed in a container from above, below or from the side.

On average, processes with inline dispersers for powder application and jet mixers save around two thirds of the energy previously required. In the production of pigment pastes, where there is no need for a mill, the savings are even higher. For white pigment paste, for example, the energy saving compared to a conventional process is 85%, while the energy requirement for black pigment paste is even reduced by 90%.

More efficient use of raw materials
In addition to significantly reduced energy requirements, modern technologies open up further potential for reducing costs - particularly with regard to more efficient utilization of raw materials. In the vacuum expansion process, for example, powder materials can be processed without dust or losses, whereas some of the solids are always lost when the powder is added via a chute with an extraction system. In addition, the quantity of raw materials used can be reduced due to better particle disintegration. In the case of expensive raw materials such as titanium dioxide, considerable cost savings are possible in this way: for wall paints, the amount of titanium dioxide can be reduced by up to 8 % while maintaining the same color strength and hiding power; for printing inks, the savings are even higher.

In addition, when using modern technologies, powders can be wetted and dispersed in the optimum sequence for the product. In a dissolver process, the thickening agent must first be introduced due to the high viscosity required. This not only hinders the wetting of very fine powders. Because thickeners are often shear-sensitive in the paint and varnish sector, thickeners introduced at the beginning of the process are degraded in an uncontrolled manner during the process, which is why the thickener must be over-concentrated. With inline dispersion by vacuum expansion, on the other hand, powder can be introduced into liquids at both high and low viscosities - whereby low viscosities are advantageous here because they significantly accelerate the process. In an inline dispersion process, the thickening agent is only added at the end.

Saving on additives, reduction of biocides, simplified cleaning
Wetting agents, which are used in the dissolver process to reduce surface tension, can be completely omitted in an inline dispersion process under vacuum. With deaerators and defoamers, two further additives that have to be used in a conventional dissolver process can also be reduced.

Because the germ load in the product is drastically reduced in a closed, clean process with powder application below the liquid level, biocides can also be saved to a considerable extent in the production of paints and varnishes. Further cost savings are achieved during cleaning. Modern machine concepts follow the rules of hygienic design and therefore enable easy cleaning with a low cleaning agent requirement.

Reduction in production costs and batch times
All in all, modern technologies in paint and lacquer production lead to enormous economic advantages. Production times are drastically reduced with an inline disperser, which is operated in a circuit on a process tank with a built-in jet mixer, compared to conventional technologies: resins can be dissolved in 1/50 of the time and production times can be reduced by more than 80 % overall. In the production of pigment pastes, where a mill can be dispensed with, production times are even significantly shorter: For yellow pigment pastes, time savings of 88% are achieved, and for white and black pigment pastes, a batch time reduction of 94% is even possible. On the cost side, production costs can be reduced by 90% and more. In the production of automotive coatings, the new technologies reduce costs to less than 8%, and for solvent-based flexographic printing inks and primers and fillers for furniture production even to less than 5%.

Against the backdrop of a sharp rise in energy and raw material prices and intense competition in the coatings and ink industry, manufacturers are coming under increasing pressure to replace traditional dissolver technology with modern technologies - not least because further efficiency gains are possible here beyond a simple closed-loop process: Twin-tank concepts, for example, in which an inline disperser is operated alternately on two identical process tanks instead of one, can increase system effectiveness by 60-100%, while slurry production and inline processes can also tap into enormous rationalization potential.

Article preview "Drei auf einen Streich"

Magazine: Paint and varnish
Issue: 09/2022
Author: Dr. Hans-Joachim Jacob

About the author

Dr. Jacob is Senior Expert Process and Applications at ystral. Dr. Jacob, who studied mechanical engineering, joined the company in 1990 as a process engineer and has since been responsible for our key accounts worldwide. His professional passion is the mixing and dispersion of powders in liquids. During his long career, he has gained experience in handling thousands of powders from a wide range of industries and is happy to share his expertise in various technical articles, online seminars and lectures.

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