Technical | Coatings Directory | Powder Coating Explanation

What is powder coating, how does it compare to paint and why use it?

Powder coating is a type of dry coating, which is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is applied electrostatically and is then cured under heat to allow it to flow and form a "skin." The powder may be a thermoplastic or a thermoset polymer. It is used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as galvanized steel, "white goods", aluminium extrusions, and automobile and motorcycle parts. Newer technologies allow other materials, such as MDF (medium-density fiberboard), to be powder coated using different methods. The application of powder is very simple. Filtered, compressed air, usually at 20-30psi pushes the powder out of the gun past the electrode which gives the powder a positive charge. The part being coated is grounded so the positive powder particles are attracted to it. When the part is completely covered, the part is put into the curing oven.

Advantages and Disadvantages of Powder Coating

There are several advantages of powder coating over conventional liquid coatings:

  1. Powder coatings emit zero or near zero volatile organic compounds (VOC).
  2. Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.
  3. Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating.
  4. For steel, galvanizing prior to powder coating is the best pretreatment for external applications.
  5. With galvanizing as the primer coat, edge protection is increased, rather than decreased as in normal wet coat applications
  6. Powder coating production lines produce less hazardous waste than conventional liquid coatings.
  7. Powder Coating is available in an anti graffiti specification for external applications
  8. Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines.
  9. Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items.
  10. A wide range of special effects is easily accomplished which would be impossible to achieve with other coating processes.
  11. Powder coating is a barrier protection system, and like paints, perforation of the barrier (scratch, chip etc) removes the protection.

While powder coatings have many advantages over other coating processes, there are limitations to the technology. While it is relatively easy to apply thick coatings which have smooth, texture-free surfaces, it is not as easy to apply smooth very thin films.

For optimum material handling and ease of application, most powder coatings have a particle size in the range of 30 to 50 µm. For such powder coatings, film build-ups of greater than 50 µm may be required to obtain an acceptably smooth film. The surface texture which is considered desirable or acceptable depends on the end product. Many manufacturers actually prefer to have a certain degree of orange peel since it helps to hide metal defects that have occurred during manufacture, and the resulting coating is less prone to show fingerprints. Various textured coatings such as “leathergrain” are available.

Powder coatings have a major advantage in that the overspray can be recycled.

Types of powder coatings

There are two main categories of powder coatings: thermosetting and thermoplastic. The thermosetting variety incorporates a crosslinker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder polymer and increases the molecular weight and improves the performance properties. The thermoplastic variety does not undergo any additional reactions during the baking process, but rather only flows out into the final coating.

The most common polymers used are polyester, polyester-epoxy (known as hybrid), straight epoxy (Fusion bonded epoxy) and acrylics.

The powder coating process

The powder coating process involves three basic steps:

  1. Part preparation or the Pre treatment
  2. The powder application
  3. Curing

Part Preparation Processes & Equipment

Removal of oil, soil, lubrication greases, metal oxides, welding scales etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of soil to be removed and the performance requirement of the finished product.

Powder Application Processes

The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun, or Corona gun. The gun imparts a positive electric charge on the powder, which is then sprayed towards the object, which is Earthed. The object is then heated, and the powder melts into a uniform film, and is then cooled to form a hard coating. It is also common to heat the metal first and spray the powder onto the hot substrate. Preheating can help to achieve a more uniform finish but can also create other problems, such as runs caused by excess powder.

Powder can also be applied using specifically adapted electrostatic discs, and the alternative charging system called tribo guns.

Curing

When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process, called crosslinking, requires a certain degree of temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200° C (390° F) in 10 minutes. The curing schedule could vary according to the manufacturer's specifications.

The application of energy to the product to be cured can be accomplished by convection cure ovens or infrared cure ovens.

How does powder coating compare to liquid paints?

  • Powder is a harder and more resilient coating
  • Powder does not have VOC emissions
  • Powder cannot be site applied, but is a factory controlled process
  • Powder has more textured finishes available that liquid paint