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Cleanly vacuumed

Effectively adding and dispersing powders in plastics

Whether as a pigment or as a flame retardant, whether to increase conductivity, to adjust rheology or as an antiblocking agent in fibers and films, powders are used everywhere in plastics. Simple agitators or dissolvers are usually not sufficient to introduce them as dust-free as possible, to wet them completely and to disperse them optimally. This is shown by application examples such as the production of filled polyurethane systems or glass fiber reinforced polyester.

ystral Conti-TDS Powder and liquid enter the machine via completely separate paths and only come into contact with each other in the dispersion zone.

Many functional properties of plastics are adjusted using powders. Their color is achieved using pigments or dyes in powder form; physical properties, such as strength or density, are influenced by the addition of fillers. Specific fillers allow properties and also price-performance ratios that cannot be achieved with traditional materials.

These powders and fillers must be introduced into the plastic before it solidifies or cross-links: by dispersing them into the liquid phase. Particularly challenging are tasks where powders have to be dispersed intensively, but certain maximum temperatures must not be exceeded, otherwise cross-linking will start in the production container (plastisols, organosols). Another interesting aspect is the introduction of conductive carbon blacks, where a single kilogram of powder has a specific surface area of almost one million square meters. This surface must be completely wetted in a fraction of a second.
All these applications require an effective process for dispersing the powders. Simple stirring using an agitator or dissolver does not provide sufficient quality. In the case of molded parts made of highly filled polyester resins, the end product shows how well the filler was dispersed. If it was incompletely wetted and not sufficiently dispersed, the course of reinforcing ribs can be clearly seen on flat surfaces, but these are located on the back of the component.

Powders are not only used in the finished plastic, but also in upstream process stages. In PVC production, for example, protective colloids are used during suspension polymerization. These are aqueous solutions made from methyl cellulose and polyvinyl alcohol powder. Both types of powder tend to stick, clump or adhere to the container wall when introduced into the water. The process described in the following section reliably prevents these problems and dissolves the powders mentioned in the shortest possible time without agglomerates. A machine system is used for this purpose, which can suck powder into liquid and viscous media and disperse it optimally under vacuum.

Transporting and dispersing
TDS machines are systems with which powder is sucked into liquids, wetted and dispersed without agglomerates (TDS transport and dispersing system). The Conti-TDS design is particularly important for powder feeding in plastics production. The Conti-TDS is installed outside the container and is connected to it via pipes or hoses. In this way, it works completely independently of the container size and fill level. It is often connected not just to one, but to two or more containers.

With this machine, powders can be sucked in directly from the bag, big bag, powder hopper or silo without dust or loss and wetted in the liquid. No powder is poured onto the surface of the liquid. No bag has to be lifted onto the container or into a filling chute. No dust sticks to the container wall, no powder is emptied in the presence of solvent vapors. Nevertheless, TDS machines are not comparable with powder transport systems, as they do not require any transport air, filters, infeed or outfeed, or vacuum generators. They generate the suction vacuum directly in the liquid and thus suck the powder directly into the liquid with 100% no loss.

Similar to a pump, the Conti-TDS pumps the liquid independently in a circle. For this reason, it can also be used to pump out the finished products at the end of the process. Additional pumps are not required. A positive displacement pump is only connected downstream of the machine for powder feed in very high-viscosity media.

Powder and liquid enter the machine via completely separate paths and only come into contact with each other in the dispersion zone. Dispersion takes place there under high shear and vacuum. Even the finest powder particles are completely wetted immediately. Dispersion qualities and solid concentrations can be produced that could never be achieved by stirring in via the liquid surface or with dissolvers.

Better wetting with resin dispersions
For resin dispersions with a high filler content, a decisive advantage of Conti-TDS comes into play. Powders usually have irregular shapes as well as outer and inner surfaces and are often agglomerated. Tough resin-solvent mixtures are not exactly the medium that penetrates all these structures and wets them completely. Agitators and dissolvers only wet incompletely. Inner structures of the particles are not reached.

The solvent contained in the resin has a higher adsorption speed and penetrates deeper into capillaries and pores. This is known as competitive adsorption or pseudo-wetting. The consequences are clear: if the solvent escapes in the subsequent process, the powder particles are dry again. Even if it does not escape, homogeneity is not guaranteed. The quality of the end product is inadequate and fluctuates.
This is completely different when using the Conti-TDS. Wetting takes place under massive shear and under vacuum. This vacuum is omnipresent, even within unreachable air-filled pores and capillaries. Under vacuum, air expands, including of course the air that fills the pore and capillary volume.

The excess air escapes from the pores. The powder is then completely wetted from the outside, leaves the vacuum zone and enters the overpressure zone of the machine. The air that has just expanded is strongly compressed and thus draws the surrounding resin-solvent mixture into every inner pore, every inner capillary of the powder. In this way, complete wetting with resin and solvent actually takes place, not pseudo-wetting. The product quality is demonstrably much better, it is reproducible and consistent.

The use of Conti-TDS naturally also speeds up all processes. The greatest time savings are achieved when dissolving resin powders or resin granulates. In the stirring or dissolver tank, where the powder or granulate is poured onto the solvent surface, wetted powder lumps and agglomerates form on the outside, which only dissolve after a very long stirring and dispersing time. The highly viscous resin layer on the surface prevents the solvent from penetrating deeper into the powder lump. The outer layer is smooth, and the powder chunks take a correspondingly long time to dissolve.

Extremely accelerated dissolving and wetting
When using the ystral Conti-TDS, the powder or granulate is almost completely wetted on first contact with the solvent in the shear field of the dispersion zone. In this way, extreme time savings can be realized. Wetting and dissolving times of a few seconds per bag are not uncommon (example: polystyrene powder from hopper 6 s per bag). The entire dissolving process is accelerated. When dissolving PMMA in MMA, for example, the same result was achieved in three minutes as in the dissolver tank after six to eight hours.

Flexible installation and retrofitting
The Conti-TDS can be operated both vertically and horizontally and its connections can be easily adapted to the local conditions. It can be retrofitted very easily in existing systems. Structural changes, the installation of new flanges, renewed TÜV inspections of the containers or the relocation of other machines are not necessary.
Once powder feeding is complete, the powder inlet is closed. The Conti-TDS can then continue to be used as a normal disperser until the required particle size distribution, homogeneity or consistency is achieved. In this phase, the machine operates as an inline dispersing machine with a high throughput and many passages via the high-shear zone.

Application 1: Introduction of silica into unsaturated polyester resins
Unsaturated polyester resins (UP resins) are among the most commonly used thermosets. They are characterized by easy processability and universal adaptability. Excellent properties can be achieved with unsaturated polyester resins, particularly in combination with fiber reinforcements. For this reason, 70 percent of all unsaturated polyester resins are used in glass fiber reinforced plastics. A typical application is the rotor blades of wind turbines. The glass fibers are laminated with unsaturated polyester resins in the form of normal fibers, cut rovings, fabrics or mats. From manual lamination to fiber spraying, winding and continuous rolling, there are numerous more or less automated production methods in which the fibers must be completely impregnated with the reaction resin. It is of crucial importance for the quality of the end product that the resins are distributed perfectly on the fiber, but do not run off again afterwards. For this purpose, the flow behavior of the resin must be thixotropic. Under mechanical stress, i.e. during application, it flows very well. As soon as it is applied, the viscosity increases many times over. The resin does not flow off and hardens in place.

Highly dispersed fumed silicas (Aerosil, Cab-O-Sil, HDK or similar) are mainly used to adjust the thixotropy. Fumed silicas have bulk densities of around 50 kg/m³ and are therefore extremely light, fine and extremely dusty. Different silica concentrations are used depending on the application (laminating, coating, filling ...).

In the past, highly dispersed silica was processed openly, i.e. poured into an agitator or dissolver container made of sacks. This resulted in unpleasant dust formation. The operator was exposed to a mixture of solvent vapor and dust. The highly electrostatically chargeable powder was poured directly into flammable solvent vapors. This wetting method should generally be rejected.

Pneumatic conveying systems have also not proved effective. These suck in the powder and transport it dust-free to the process container. Exactly the same volume that is fed into the container as powder and transport air is simultaneously displaced from it in the form of exhaust air containing dust and solvent. As the bulk density of highly dispersed silica is extremely low, the volume of dust produced is very large.
This does not solve the problem of powder layers sticking to the container wall and agitator shaft. Wetting and dispersing takes time because the light powder tends to float on the surface of the liquid. Dust extraction systems designed for solvent vapors with dust content must always be used. The powder loss that occurs in the extraction system is missing in the end product and is difficult to calculate.

The use of Conti-TDS has enormously rationalized the introduction of highly dispersed silica into resins. The powder is sucked directly into the liquid without dust or loss and, although it also has a specific surface area of 200 m² per gram, it is completely wetted immediately. The first two Conti-TDS machines were used for this application as early as 1993; they had very low outputs compared to current machines, but were already extremely successful. The machines have been continuously developed and today there is a specific design for the requirements of this special task. One machine can often handle several large containers and complete 30-ton batches in just a few minutes. A large proportion of the thixotropic polyester resins produced in Europe and the USA are already manufactured using Conti-TDS.

One qualitative aspect is certainly the fact that when using Conti-TDS, the powder is introduced directly into the resin and does not remain in the solvent vapors above the liquid, as is the case with stirring. The powder has a high affinity to bind the solvent vapors, which reduces its effect in the end product. This is another reason why higher qualities are achieved when using Conti-TDS.

Application 2: Filled polyurethanes
Polyurethanes modified with fillers can be found in a wide variety of applications. For example, in the production of flexible polyurethane foams, heavy fillers are added to the liquid polyol phase in order to achieve a higher density in the end product. On the one hand, the higher density is in the interests of the manufacturer, as flexible foams are sold by weight, and on the other hand it gives a seat cushion, for example, a more pleasant and high-quality feel. Lightweight fillers in rigid polyurethane foams, such as those used in cars, in model and mold making or in the manufacture of prostheses, have the exact opposite effect. What is important here is to reduce the weight. However, the pores should not be open as in a sponge, but closed-cell to prevent the accumulation of moisture. Microscopically small, closed hollow spheres, known as microspheres, are used as lightweight fillers. Their bulk density is less than 30 kg/m³. Such powders create a lot of dust, they float on the surface of the liquid and are difficult to wet.

Conti-TDS is an ideal solution for both heavy and light fillers. This process is by far the most effective method for light fillers. The microspheres are sucked in with a vacuum without dust and completely wetted without destroying them.

Application 3: Man-made fibers
Large quantities of titanium dioxide are used in the production of polyester, polyamide (nylon) or viscose fibers. The titanium dioxide pigment has the task of structuring the fiber surface so that individual fibers do not adhere to each other (antiblocking). It also influences the color, light stability and mechanical properties of the fibers.

High-quality fibers require optimum wetting and dispersion of the titanium dioxide. The spinnerets are very fine, so the requirements for the particle size distribution of the dispersed titanium dioxide powder are correspondingly high.

By using the YSTRAL Conti-TDS, the entire wetting and dispersing process can be realized in one machine. The titanium dioxide is sucked in directly from the big bag, completely wetted immediately and then dispersed extremely finely. The Conti-TDS is in widespread use for this task in Taiwan's textile fiber plants. In Europe and
this is not yet the case in Europe and the USA.

ystral Fachartikel P&A Dispergieren

Magazine: P&A
Issue: 10/2007
Author: Dr. Hans-Joachim Jacob

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