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Disperse powder correctly

Complete wetting and optimum dispersion of powders is essential in the production of plastics. Traditional processes based on dissolvers or agitators are usually unable to achieve this. Transport and dispersing systems (TDS) provide a remedy here. Conti-TDS machines are particularly important for powder application in plastics production.

ystral Conti-TDS In the Conti-TDS, liquid and powder enter the wetting zone via separate paths

Many properties of plastics are achieved using powders. For this purpose, these are introduced together with fillers into the still liquid phase, i.e. into the resin, solvent or resin-solvent mixture. Dispersion takes place before polymerization or cross-linking. The problems associated with this process range from health risks during powder handling to the risk of explosion when added to solvent-containing systems and problems with temperature limits during dispersion.

The problem
Incompletely wetted powders, fillers, fibers or pigments not only mean that their effect cannot be fully utilized; in many cases, wetting errors cause structural damage, defects and weakening in the end product. Traditional agitators or dissolvers never wet powdered fillers singularly, but always agglomerate them first - this cannot be avoided with these systems.

As a result, powders inside the agglomerates are not completely wetted. Later, these agglomerates can usually still be moistened internally due to capillary action, but only selectively. The agglomerate inside is preferably wetted with only one component of the liquid mixture; this is often the solvent. This phenomenon is also known as pseudo-wetting. During curing, the solvent component evaporates and pseudo-wetted fillers are again incompletely wetted, i.e. partially dry.

This problem is common in the production of casting resins, adhesives or molded parts made from polyester resins. If the dispersion has not been sufficient, the reinforcing ribs on the back of the part can be clearly seen in molded parts on flat surfaces. The surface has shrunk more here and small indentations appear. This is not acceptable for high-quality housings or automotive components.

The cause of the pseudo-wetting is the way in which the powder is added. The problem can be traced back to the fact that the powder particles touch each other during wetting and are not separated. This is the case when powder is added to a container from above, but also when it is mixed from below into a vacuum container as a compact stream or inline into a liquid.

The safety risk
Incomplete wetting is not the only problem. It becomes particularly dangerous when a powder is transported or poured from above into a process container with solvent-containing media. As the process temperature is usually above the flash point, there is a risk of explosion. The liquid itself is not flammable or explosive; it is the flammable gases and vapors above the liquid that are critical. If their concentration in the air is between the upper and lower explosion limit, this is referred to as an ignitable mixture. The powder is poured through this critical area. The only thing missing to trigger an explosion is an ignition source with sufficient ignition energy. This could be the flowing powder itself or the stirred liquid. However, the risks associated with adding the powder to the solvent-containing liquid from above can be avoided very easily by not adding the powder from above and not adding it inside the container.

The solution
The solution is a system that can completely wet and optimally disperse powder particles in both liquid and viscous media and does not transport them through the solvent vapors above the liquid. TDS machines are systems with which powder is sucked in, wetted and dispersed agglomerate-free using a vacuum generated directly in the liquid. The term TDS therefore means: transport and dispersion system. The machines are available in different versions. However, the inline version of this machine, the Conti-TDS, is particularly important for powder application in plastics production. With this system, powders can be sucked in directly from the bag, big bag, powder hopper or silo without dust or loss and wetted in the liquid. The Conti-TDS is installed outside the container and is connected to it via pipes in the circuit. No powder is poured onto the liquid surface; no bag has to be lifted onto the container; no dust sticks to the container wall; no powder is emptied onto the liquid surface in the presence of solvent vapors. Powder and liquid enter the machine separately and only come into contact with each other in the dispersion zone. There, dispersion takes place under massive shearing action and vacuum. This means that the powder does not come into contact with solvent vapors above the liquid.

The vacuum has an extraordinary effect: air expands under vacuum. The powder contains up to 98 percent air. All particles are in flight in the powder, which is sucked in by vacuum conveying and flows at high speed. During vacuum conveying, the vacuum in the powder increases steadily from the point of addition to the zone of maximum vacuum. The distances between the individual particles therefore also increase accordingly along this path.

The Conti-TDS generates its suction effect directly in the liquid. The maximum vacuum is exactly in the wetting and dispersing zone. The distances between the individual particles are greatest when they enter this zone. No additional conveying or fluidizing air is required for this effect. Only the air present in the powder expands and later compresses again after dispersion.

The particles are separated and thus completely wetted; pseudo-wetting is excluded. After powder application is complete, the powder inlet is closed. The Conti-TDS can then be used as a normal inline disperser. It can be installed on existing tanks or in complete systems.

Advantages of the Conti-TDS
The use of the Conti-TDS accelerates all processes. The greatest time savings are achieved when dissolving resin powders or resin granulates. When using the YSTRAL Conti-TDS, each individual particle is completely wetted separately on first contact with the solvent in the shear field of the dispersion zone and begins to dissolve immediately. Dissolving times of a few seconds are not uncommon. Whereas a dissolver needs 6 to 8 hours, the Conti-TDS only needs around 3 minutes.

Another aspect is the minimal energy input and the associated minimal temperature rise. When dissolving resins, often only one percent of the energy is required. But even with more demanding tasks, even with intensive dispersion of the finest or even liquid-repellent fillers, only a maximum of 30 percent of the energy is required compared to previous stirring or dissolver processes. The temperature increase is correspondingly low. This also saves time and energy for cooling.

ystral Fachartikel PA

Magazine: P&A
Issue: 10/2019
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.

About ystral

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