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08.11.2023 | TECHNICAL ARTICLE

Highly efficient mixing and dispersing - Modern technologies for the production of paints and coatings

Optimized processes in paint and lacquer production not only reduce energy consumption and use raw materials more efficiently, they also open up significant potential for reducing costs. Compared to the widely used dissolver, modern mixing and dispersing technologies can reduce energy requirements, production times and production costs by up to 90 % or more, depending on the application.

Animation of the circulation process with inline disperser and power jet mixer installed in the process tank Circulation process with inline disperser and process container with integrated jetstream mixer

The dissolver technology commonly used in paint and coating production dates back to the 1930s and has changed little over the past 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. As the disk only generates a very low shearing effect, it requires high viscosities for dispersing, which counteracts effective powder introduction. The principle of the disk agitator also hinders vertical mixing. The powder application by means of a Trombe brings a lot of air into the product, which not only reduces the dispersing effect but also makes additives necessary. These factors and the fluctuating dispersion results make production energy-intensive, slow and inefficient. In order to increase production efficiency, the intensity of the dispersion and powder wetting processes must be significantly increased. In modern processes, 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.

 Conti-TDS view of the mixing chamber The Conti-TDS inline powder wetting and dispersing machine

Avoid agglomerates

Inline dispersing technologies based on vacuum expansion during powder application achieve complete deagglomeration and wetting of the particles within microseconds. Mills are only required in exceptional cases. 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 1 million m2/min. 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 in the wetting chamber - under maximum vacuum and maximum turbulence. In the dispersing zone, the powder particles have the greatest possible distance from each other and can therefore be completely wetted and dispersed individually. In the Conti-TDS inline dispersing machine from ystral, the wetting and dispersing processes are concentrated in a dispersing zone with an effective volume of only around 1/4 l. Compared to a dissolver operated in a container, such an inline disperser generates a volume-specific output that is around 30,000 times higher. At the same time, the inline disperser uses a rotor-stator system to generate a thousand times higher shear forces. The dwell time is extremely short, so that only a fraction of the energy is required.

No additional air input

Another problem with dissolver technology is the introduction of air. On the one hand, this is caused by the powder materials themselves. Light powders have a volume proportion of over 90 % air. If this air is dispersed together with the powder particles, microfoam is created. If the powder is fed into an open container from above, air bubbles are also formed, through which additional air is introduced. In the newer processes, the powder is therefore sucked directly into the liquid externally in a circuit. There is neither the formation of bubbles nor is additional air introduced. 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.

Efficient mixing technology

Modern jet mixers focus on process intensification and local concentration of machine performance 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. However, as 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 and the product is actually completely homogeneously mixed at the end of the mixing process - without unmixed zones and sediments. 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: 85% for white pigment paste and 90% for black pigment paste.

Better utilization of raw materials

In the vacuum expansion process, powder materials can be processed without dust and loss, whereas 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. 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 greater. In addition, powder materials can be wetted and dispersed in an optimum sequence for the product. In a dissolver process, the thickening agent must first be added due to the high viscosity required. This not only hinders the wetting of very fine powders. As thickeners are often shear-sensitive in the paint and varnish sector, thickeners introduced at the start of the process are degraded in an uncontrolled manner during the process, which is why they have to be added in an over-concentrated form. With inline dispersion using vacuum expansion, powder can be added to liquids at both high and low viscosities - low viscosities significantly accelerate the process here. The thickening agent is only added at the end. Wetting agents, which are used in the dissolver process to reduce the surface tension, can be completely dispensed with during inline dispersion under vacuum. With deaerators and defoamers, two further additives that have to be used in a conventional dissolver process can also be reduced. In a closed, clean process with a powder feed below the liquid level, the bacterial load in the product can also be drastically reduced, which reduces the use of biocides. Further savings are made during cleaning due to the implementation in accordance with the rules of hygienic design.

The advantages pay off

Production times are drastically reduced with an inline disperser that is operated in a circuit on a process tank with a built-in jet mixer: resins can be dissolved in a fiftieth of the time and production times can be reduced by more than 80 % overall. Time savings of 88% are achieved in the production of yellow pigment pastes, and batch times can even be reduced by 94% for white and black pigment pastes. In addition, production costs can be reduced by 90 % and more. In the production of automotive coatings, costs are reduced to less than 8%, and for solvent-based flexographic printing inks as well as primers and fillers for furniture production to less than 5%. In addition to simple circulation processes, system efficiency can be increased by 60 to 100 % with twin-tank concepts, in which an inline disperser is operated alternately on two process tanks instead of one.


[Translate to Englisch:] Titelseite cav Artikel Hocheffizient mischen und Dispergieren

Magazine: cav
Issue: 11/2023 
Author: Dr. Hans-Joachim Jacob
Publication date: 08.11.2023

 


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.

About ystral

With our vast knowledge and many years of experience in Process- and Application Engineering we offer targeted, customer-oriented solutions across industries - from lab equipment to production machines or plants. Together with you, we develop concepts and implementations for your individual applications, which mean mmediately realisable and quantifiable added value for you..

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