Aluminium powder coating

Powder painting is an advanced method of applying protective and decorative coatings on the surface, as a finished aluminum products, and semi-finished products, such as aluminum profiles or sheets.

Powder painting - dry technology

Coating liquid paints require the use of solvents, to deposit organic compounds on the surface of paint. These liquid solvents necessitate use in the production of equipment for venting, filtration and capture environmentally hazardous volatile organic compounds. Powder painting is essentially the same dry technology, which does not use organic volatile substance nor at its one step.

powder coating technology is, that the fine particles of organic resins and coloring pigments are electrostatically charged and sprayed onto the surface of the article. The spraying process can be done manually or by automatic horizontal and vertical lines of paint. Hardware, for example, aluminum profiles, which is subjected to color, grounded, and is neutral. Therefore, the charged powder paint particles adhere to the surface and are retained thereon, until melted in the polymerization furnace to form a homogeneous, durable and high coating.

Powder paint is cheaper

Generally, powder coating lines include efficient recirculation system and returning the sprayed powder for reuse. Therefore, the overall efficiency of the use of powder 95-98 %. color change on the line powder coating takes place fairly quickly. Near 99 % powder, which was deposited, but I did not get into the finished powder coating, returned to the dyeing cycle, and again sprayed. Therefore, waste paint lines powder coating are minimal.

Modern line powder coating coating provide a wide range of colors, shine, textures and thicknesses.

Two types of powder coatings

The main types of powder paints are:

• thermoplastic and
• thermosetting.

Thermoplastic powder coatings

Under the influence of heating thermoplastic powder paint melts and flows over the surface in the form of a film. This film has the same chemical composition, which had a starting powder. The thermoplastic powder coatings based on thermoplastic resins with high molecular weight. These resins are difficult to grind into small particles and therefore they are usually used to create special thicker coatings. Powder coating on a product usually occurs during its immersion in a tank of liquefied powder. Liquefaction of the powder produced by means of compressed air.

To this species include polyethylene powders, polypropylene, nylon, PVC and some other.

Thermoplastic powder coatings are not used for mass dyeing of aluminum products, such as extruded aluminum profiles or rolled aluminum sheets.

The thermosetting powder paint

Thermosetting powder paints are significantly different from the thermoplastic. They are based on solid resins with low molecular weight. Under the influence of heat melt thermosetting powder, spreading over the surface and are connected with each other as a result of chemical reactions to form high molecular weight organic compounds. This process is called polymerization. Therefore, the finished powder coating has a chemical composition, which differs from the chemical composition of the starting resin in powder paint.

Material finished coating of thermosetting powder paints is resistant to heat and is not melted to a liquid state. Thermosetting powder paints are based on a solid resin systems, which can be pulverized into fine particles ranging in size from 10 to 40 um or even smaller. These resins can form thin layers of paint thickness from 25 to 75 m, characteristics which are equivalent to or even higher, than coatings based liquid paints.

When the powder coating of aluminum products, first profiles and sheets, used is thermosetting powder paint. The main types of thermosetting powder paints, which are used for coloring aluminum products and semi-finished products, are:

• epoxy,
• polyester,
• polyurethane,
• acrylic,
• hybrid (mixed).

Select the type of powder paint it depends on the type of aluminum product, his appointment, its operating conditions and other factors. The standard and most popular colors are polyester powder paint.

Chemical preparation of the aluminum surface

For, to obtain a good surface on the aluminum powder coating, this surface is subjected to chemical preparation of complex technology. The purpose of chemical preparation of the surface of any metal for powder painting is:

• Removing all contaminants from the surface, including dirt, oxides, greases and oils
• Creating a surface for optimal adhesion of the powder coating
• Achieving maximum uniformity over the entire surface to be coated products.

The technology of chemical preparation of the aluminum surface usually includes the following stages:

• Degreasing cleaning solution
• Thorough rinsing in clean water after working all chemical operations
• The etching solution based on caustic soda for a predetermined time at a predetermined temperature and
• Surface treatment in a solution of nitric or sulfuric acid (clarification)
• Treatment to create a chromate conversion coating (or technically equivalent coating) to ensure good adhesion of the powder coating
• Final rinsing in deionized water

It is no exaggeration to emphasize, that only full compliance with all the requirements for the preparation of aluminum surface makes it possible to realize the full potential of powder paint to create a strong and durable protective and decorative coatings.

On small powder coating lines, horizontal chemical treatment of profiles is usually used in a sequence of working and washing baths (Figure 1). In large vertical profiles used industries processing system, in which working solutions and flushing water are supplied from a sprayer system (Figure 2).

Figure 1 - line chemical treatment baths
surface of aluminum profiles [2]

Figure 2 - Vertical powder coating system [2]

Powder paint

Aluminum products to the prepared surface, for example, aluminum profiles, are supplied to the powder coating line (picture 3). The first step in the process of powder coating is a powder coating on the surface of the article, for example, aluminum profile. Aluminum profile enters the electrostatic spraying chamber, wherein the electrically charged particles "stick" to the airfoil surface.

Figure 3 - Powder coating on aluminum profiles [1]

The electric charge of a powder particle is obtained when passing through special pistols (Figure 4). all of the powder, who did not or could not "stick" to profile, misses the powder recycling system, and is returned for reuse.

Figure 4 - Principle of electrostatic powder coating [1]

Aluminum profiles with the deposited powder layer further tested on the processing chain to the furnace, termed polymerization oven. In this furnace are profiles during 15-20 minutes at a temperature of from 160 to 210 ° C depending on the type of powder paint. Under the influence of heating the powder particles soften and form a continuous uniform coating. When this polymeric compound, powder consisting, undergo polymerization with the formation of new chemical bonds.

After exiting the furnace the profiles given time to cool to room temperature to control, packing and shipping.

RAL color scale

Thermosetting powder coatings are available in a wide range of colors. When aluminum powder staining, including, aluminum profiles, apply the international RAL color scale (picture 5). RAL - an organization in Germany, which is engaged in standardization and certification of paint colors and coatings. Furthermore RAL colors on a scale typically also specify gloss coating - Matt, Honed, brilliant. Matte coatings are especially popular for powder coating of building products made of aluminum profiles, for example, windows and doors, so they hide the most pressing defects, such as risks and scratches.

Figure 5 - Example of the international RAL color scale

Aluminum for powder coating

The level of surface quality of extruded aluminum profiles and sheets rolled aluminum in the state, where they exit from the press line or mill, in principle, suitable for powder coating. However, precisely because, that the powder coatings have good fluidity, they only partially smooth the imperfections of the painted metal. It is therefore important, to the quality of surface profiles and sheets by the powder coating does not drop below a certain level of roughness and defect size, such as scratches, risks, bully or dents.

Typical aluminum alloys, which are often subjected to powder coating, are alloys 6060 and 6063, which are analogues of domestic alloy AD31. Of these alloys, the vast majority are manufactured aluminum profiles, including, for aluminum windows and doors. Moreover, product with enhanced strength properties of aluminum alloys used 6061 (AD33), 6082 (AD35) and 6005.

Aluminum sheets 1xxx series alloys, 35xxx xxx and also satisfactorily painted with powder paints.

Aluminum casting alloys may also be painted with powder paints, however, in this case, the quality of the painted surface largely depends on the quality of the aluminum casting. Powder painting castings with high porosity may lead to the formation of bubbles at the coating surface. These bubbles are formed when gases exit from the casting [1].

Design a profile for powder coating

If the aluminum profile will be subjected to powder coating, except the destination standard characteristics, such as the type of powder paint, coating thickness, color and gloss, it is also necessary to consider the following questions, which are associated with powder coating technology features.

Visible and significant surface

On the drawing of the profile (or other product) visible or significant surfaces must be indicated. All requirements for the quality of the powder coating are presented only to such surfaces.

The degree of corrosivity of the medium

Information about the extent of corrosiveness environment, which will work painted objects, It helps correct choice of the type of powder paint.

point hanging

To be sprayed on the aluminum profile powder coating, it must be properly hung on the conveyor line powder coating. To do this on the profile indicate the place, on which you can drill holes or install a pointed contact the conveyor so, so as not to damage the visible or significant surface.

Drainage chemical solutions

Usually chemical preparation of products (for example, profiles) requires immersing them in baths with working solutions. It is therefore important, profiles to immersion in a bath is not floated on the surface, as well immersed in the bath. After removal from the bath, they should not contain a solution or leaching residues. If the product cavity, for example, aluminum profile, remain such residual moisture, they may boil in the polymerization furnace. This will lead to damage or even a powder coating oven accident. In most cases diameter holes 6 mm is sufficient to ensure complete drainage of residual solutions and washes.

«Box Faraday»

The particles of powder paint adhere to the surface of metal products due to electrostatic forces. However, the phenomenon can occur in some cases,, which is called the "Faraday cage" (figure 6). This phenomenon prevents the penetration of powder into the narrow grooves, slit or interior acute angles. Generally for normal coloration width of such slits should be greater, than its depth.

Figure 6 - "Faraday's Cage" [1]

dissimilar metals

When a powder dyeing finished articles or assembly units need to avoid a combination of dissimilar metals. Dissimilar metals may be incompatible with the chemical preparation or to cause galvanic corrosion.

massive products

Solid products require a longer duration of the heating cycle in the curing oven. Appearance and color of products, stained with the same powder coating may be a little different for aluminum products, that differ significantly in their weight.

Protect powder coating

Although powder coating is very dense and strong, however mechanical stress when handling and transportation of painted objects can cause scratches on it or spots. Of course, to avoid such damage it is best to use gentle handling of painted objects, both in their machining, and when installed on a building site. Nonetheless, for the protection of finished painted products, most of the profiles, very widely used special protective polymer films. These films must have very low adhesive properties, so as solvents, which are part adhesives can chemically affect the powder coating. Prior to use one type or another protective film for production lot should be checked for its compatibility with the powder coating.

Sources:

1. Powder Coater’s Manual by Roger Talbert, 1998
2. Aluminium Extrusion Manual, 4th Edition, Aluminium Extrusion Council, 2014
3. User’s Guide to Powder Coating, 4th Edition / N. free, 2003