Comparison of Coating Powder and Powder SlurrySoon after the development of coating powders, the concept of powder slurry was tested for different purposes in industrial coatings, but it was not successfully implemented in production processes. Nevertheless, the slurry concept offers some important advantages over powder coatings (see Table 1).
- The film build is reduced. The film build of slurry clearcoats is about 45 (Greek Mu), and thus equivalent to solventborne coatings. In contrast, a powder clearcoat requires about 60–70 (Greek Mu) to ensure sufficient leveling. When considering the amount of organic substances used, a powder clearcoat that is not recycled has no significant advantages if compared with high-solids solventborne coatings. Slurry, however, saves raw materials.
- For slurries, conventional automotive application equipment can be used. This is important for four reasons. First, the customer can continue using his tried-and-tested application system; he would have to give up his application know-how and master a new technology if he switched to powder coatings. Second, slurries can be used in existing application lines when minor modifications are made. This technology can therefore be used by many customers and not only by those who are willing to build new coating lines and start using a completely new coating technology. The third advantage is that wet paint systems can be cleaned very fast when contamination occurs or when a product change is necessary. The last advantage is that electrostatic application offers a higher transfer efficiency than powder application, not taking direct recycling into account.
- Slurry allows higher flexibility in paint formulation. When a powder coating is milled, there is hardly any possibility to modify the formulation. With slurries, in contrast, the basic properties are also determined by the powder itself, but many parameters can be influenced through additives in the aqueous phase.
- Slurries are more homogeneous. Production in large batches and lower particle size result in more homogeneous coating material. The salt and pepper effect, which means that contamination or inhomogeneity is easily detected, is reduced significantly.
- Filtration is more efficient. For slurries, much lower pore sizes can be used in filtration than with powder coatings. During production and in the automotive circulation line, filter units with pore sizes of 25 or 50 (Greek Mu) can be used. They will reliably remove dirt or agglomerates, and reduce contamination.
- Leveling is influenced by many factors. Besides other factors, leveling depends on the particle size, the film build and the viscosity of the wet film or, with powder coatings, during the curing reaction. With slurries, the aqueous phase and the low particle size contribute strongly to the leveling, while the viscosity during the crosslinking reaction has only a minor influence. This again allows high flexibility in formulating the powder used in a slurry. With coating powders, however, both the resin and the formulation have to be designed very specifically for good leveling.
Production and Properties of the Clearcoat SlurryThe basis of the slurry production is an acid/epoxy-crosslinking clearcoat powder. This powder is produced by a conventional manufacturing process and milled down to 30 (Greek Mu).
After being suspended in water, this powder is wet milled down to the final particle size of 3 (Greek Mu). After completion with further additives and filtration, the slurry is obtained as a milky white suspension. The pH is about 6.0–6.5, and the solvent content is lower than 2%. The solvents are created by the additives used during slurry production. Only a small amount of additives are required to stabilize the slurry in the aqueous phase. These additives have hardly any influence on the coating’s properties.
ECOCLEAR has a solids content between 35% and 38%. The rheological behavior of powder slurry is pseudoplastic: At a shear rate of 1,000 s-1, the viscosity is 150–200 mPa•s at 23°C. The density of the powder itself is about 1.1 g/cm3 and thus similar to the density of water. With this combination of properties, the slurry has a good sedimentation stability. Sediment that has formed can be stirred up easily. Powder slurry is best used within one month if stored at 20°C, or within three months if stored at temperatures below 10°C. Table 2 indicates the wet paint properties.
Application and Clearcoat PropertiesA conventional circulation line system for waterborne paint in which some minor modifications have been made is required for application. The temperature has to be kept in the range from 18 to 26°C, and rinseable gear pumps are needed. The slurry is applied by electrostatic or air spraying. The standard application conditions are temperatures from 20 to 24°C and a relative humidity of 65–75%. The extended application temperature is up to 29°C, but this will slightly impair leveling. An application window is depicted in Figure 1.
In comparison with solventborne coatings, the viscosity is higher, which gives the slurry very good sag resistance. Overspray remelting is excellent and equivalent to solventborne coatings.
Zero-emission clear coatings generally require specially adapted basecoats to ensure compatibility for wet-on-wet application. In developing these basecoats, special care was taken to avoid mud cracking, the formation of small cracks when a wet slurry film is drying. If mud cracking occurs, it will still be visible after the bake of the clearcoat. With these basecoats, wet-on-wet application is possible up to dry clearcoat film builds of 70–80 (Greek Mu).
SummaryZero-emission clearcoat systems are always an issue for the automotive industry. ECOCLEAR powder slurry is a coating powder dispersed in water that combines the positive properties of wet paint and powder coatings, such as good leveling at low film builds and minimum solvent content.
With an average particle size of 3 (Greek Mu) and a solids content of 35–38%, ECOCLEAR has good sedimentation stability. The solvent content is close to zero.
Application is by conventional application equipment for solvent- or waterborne products. This is the most important advantage for car manufacturers when ECOCLEAR is applied. The circulation line requires only slight modification. During pre-drying, at least 95% of the water is evaporated. The baking is 20 minutes at above 140°C panel temperature. The resulting clearcoat has very good appearance and long-term stability.
The ECOCLEAR powder slurry clearcoat technology was developed in cooperation with Daimler Benz and Dürr as a new approach to zero-emission clearcoat technology. It is being used for the first time in a new coating process that was set up at Rastatt, Germany, for the Mercedes A class.
For more information on powder slurries, contact BASF Coatings AG, Postfach 6123, 48136 Münster; phone 00492501140; fax 00492501143373; visit www.basf-coatings.de.