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Light as a feather on the road

Dust-free dispersion of lightweight fillers

Lightweight fillers such as expanded and expandable hollow microspheres can be easily and quickly vacuumed and dispersed without dust or loss using a powder wetting machine. This allows costs to be saved and products with improved properties to be manufactured.

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

Lightweight fillers are always used when the weight of a coating needs to be reduced. Certain lightweight fillers can also make application easier, shorten drying times, vary the feel of the coated surface and create matt effects. However, lightweight fillers usually create a lot of dust and are often difficult to wet and disperse. Their extremely low specific weight and high specific surface area, which in extreme cases can be over 100,000 m²/kg, make conventional dispersion using a dissolver or stirrer a real game of patience and luck.

A special powder wetting and dispersing machine can provide a remedy here by processing these lightweight fillers without dust or loss. The combination of powder wetting machine and lightweight filler opens up a high rationalization potential, but also the possibility of producing systems with improved technical properties.

Powder versus liquid

If fine powders are poured onto a liquid surface of only a few square meters, an enormous disproportion of liquid to powder surface is created, apart from the dust load. Depending on the properties of the powders, the liquid penetrates more or less into the fill and only wets it incompletely. The powders remain on the liquid surface and can only be incorporated by generating strong turbulence (e.g. formation of a tumbler) and the associated air entrainment. The product quality suffers and can vary from batch to batch.

In addition to the dust pollution that occurs with this method of working, more raw material is often used to produce the desired effect than would be necessary for complete wetting due to inefficient raw material utilization. Some of the raw materials "disappear" as dust in operating equipment, suction devices and filters. Powders that tend to stick encrust on the container wall and on built-in parts, such as the agitator shaft. Agglomerates form, which can only be destroyed with increased energy and time expenditure.

Dust and loss-free dispersion

The Conti-TDS powder wetting and dispersing machine differs from all other systems on the market. It allows lightweight fillers to be sucked into liquids directly from the bag, big bag or silo, wetted and dispersed in seconds without any dust or loss. The machine works according to the rotor/stator principle. It is not installed inside but outside the container and connected to it via pipes or hoses. Similar to a pump, the machine circulates the liquid in the container. The special geometry of the rotor creates a vacuum inside the dispersion chamber through which the liquid flows. With the help of this vacuum, powders can be sucked in directly from the bag, big bag or silo in a dense flow without dust or loss (Fig. 1).

The liquid flow is dispersed in the dispersion chamber with high shear energy, which greatly increases the surface area of the liquid into which the powder is sucked. The vacuum inside the dispersion chamber causes the air contained in the powder flow to expand, increasing the distance between the particles. The two material flows (liquid and powder) enter the dispersion chamber via completely separate paths and are only brought together in the dispersion zone at defined shear gradients. This creates ideal conditions for wetting the individual particles. This process enables fine distribution at the moment of wetting and in many cases ensures significantly better product properties compared to conventional powder application.

Also for highly viscous products

In addition to paints and varnishes, numerous low- and medium-viscosity products can be produced with just one machine. The product is conveyed by the powder wetting and dispersing machine itself, so that product transfer or discharge is also possible without an additional pump. An additional positive displacement pump is only used for the production of highly viscous products, such as sealants, insulating materials and adhesives, casting resins or modeling and filling compounds.

Depending on the recipe, several different raw materials can be dispersed one after the other, whereby the suction can take place from different containers. Until the recipe quantity of powder is reached, the liquid is conveyed in circulation and concentrated with solids (chalk, pigments, etc.). After the solids have been added, dispersion can continue in the circulation at a high flow rate with the powder inlet closed until the required particle size distribution is achieved or a dissolving process has been completed. Shear-sensitive or viscosity-increasing powders (e.g. thixotropic agents, matting agents, hollow microspheres) are usually sucked in at the end.

Explosion and health protection

Working with an open agitator or dissolver tank is associated with risks in the manufacture of solvent-based products, especially when adding powder. Solvent vapors above the liquid form an explosive mixture together with the oxygen from the powder spillage. If the powder tends to become electrostatically charged, the mixture can be ignited by sparks when the powder is poured in.
In contrast, the powder wetting and dispersing machine offers considerable advantages. It sucks the powders directly ' into a circulating liquid flow. The powders are wetted directly in the dispersing chamber and fed into the container under a mirror. This eliminates the risk of explosion.

Dispersing microspheres

Spherical microspheres, which can be expanded under the influence of temperature to form a flexible hollow sphere with a very low density, are establishing themselves as an alternative filler. They serve as a physical blowing agent and are used wherever low density and high compressibility are important. They also offer other advantages, such as matting properties. The products are also available as pre-expanded hollow microspheres.

This results in various possible uses for such microspheres, such as the expandable "Expancel" microspheres. When processing these extremely light powders, the advantages of the described machine for dust- and loss-free dispersion are fully utilized. Using a special suction device, the powder is sucked directly from the bag (see Fig. 2) or silo into the liquid provided (e.g. a finished coating system) and immediately dispersed without agglomerates and with a narrow particle size spectrum.

Moreover, during the dispersing and wetting process, the micro air bubbles adhering to the particle surface are separated so that the particles are already colloidally wetted in the dispersing chamber. The separated micro air bubbles coagulate and can thus also escape from products that are difficult to deaerate. This effect contributes massively to the deaeration of the product.

In contrast to powder dispersion using a dissolver, the powder input is constant and independent of the operator. Thanks to this defined process, the matting effect created by the hollow microspheres can be adjusted and repeated from batch to batch. The product quality is constant and the properties of the hollow microspheres are utilized effectively.

Lower density thanks to small balls

Even small quantities of hollow microspheres added significantly reduce the density of a system. This lightweight filler is generally much lower than conventional lightweight fillers with a density of 0.024 g/cm³ to 0.07 g/cm³. An increase in volume of up to 30 % in paint and varnish systems is not uncommon. This sharp increase in volume contributes to the fact that solvent-based paint systems automatically become VOC-compliant.
If approx. 2 % by weight of expandable microspheres are added to a standard formulated aqueous interior wall paint with a density of 1.55 g/cm³, it is possible to reduce the density to 1.22 g/cm³ (Fig. 3). This reduces the weight of a 12-liter container of interior dispersion paint from approx. 18 kg to 14 kg. This not only has advantages during transportation, but also increases the yield of the paint. Larger areas can be painted with the same amount of paint.

Reduction in material costs

Raw material costs have become a key issue in the field of paints and varnishes. In particular, the sharp rise in the cost of TiO2 and various binders has had a negative impact on margins in the past.

The microspheres contribute to the fact that less binder is required in the formulation, as the oil absorption by volume is very low. Here, an increase in the PVC (pigment volume concentration) can be achieved without compromising quality. The matting property of the hollow microspheres also makes it possible to reduce the proportion of matting agent in the formulation. The raw material costs per liter of paint are reduced. The cost saving can be between 3 and 5 % for interior wall paints and up to 7 % for facades (Fig. 4).

VOC reduction

VOCs are one of the causes of increased ozone levels. For this reason, the EU member states committed themselves to drastically reducing VOC emissions in the European VOC Directive of 1999. The latest Decopaint Directive (2004/42/EC) has further tightened these limits.

Expanded microspheres are already used in many solvent-based paint systems to reduce VOCs. Due to the low microsphere density of 0.036 g/cm³ to 0.070 g/cm³ (depending on quality), the volume can be increased by up to 30 % with an addition of just 1 % by weight. The required VOC limit values are thus significantly undercut.

Microspheres and water

Microspheres can do more than just reduce density. They help to optimize water vapour permeability in standard formulations (Fig. 5). The hydrophobic properties of the microspheres help to reduce water absorption in the formulation, thereby greatly delaying the degradation of the paint film (Fig. 6).

ystral Fachartikel Farbe und Lack

Magazine: Paint and varnish
Issue: 11/2012
Author: Oliver Dudak, Dominik Seeger

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