Coatings Directory

Powder Coating Explanation

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:

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

  • Part preparation or the Pre treatment
  • The powder application
  • 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

Factory Controlled Application

Galvanizing and Powder Coating are both factory controlled applications in a controlled environment, applying coatings to the relevant British and European Standards. This results in a finished product, shrink wrapped, being delivered to site. This product is ready for erection in a completed state.

As a result there will be reduction in weather delays, scaffolding erection times, sub-contractor delays, and site congestion. These all represent savings in time and cost.

In the unlikely event that the coating is damaged on site, the full range of RAL colours, and varying gloss contents, are available in spray, wet coat and touch up brush formats for remedial work through Du Pont‘s CROMADEX division. These are produced from the powder source, and are therefore an exact match, to the British Standard, when applied on site.

Furthermore, on the Du Pont web-site approved remedial contractors are also named, who have undergone rigorous on site auditing.

In summary site applications can lead to:

  • Access systems, involving scaffolding or powered access being required for longer, restricting access to site traffic
  • Overspray or spillage onto surrounding areas can become an environmental issue, or a practical issue where other items get accidentally coated
  • Weather is a variable which has a huge effect on many coating systems, not just at application, but between coats. Many cannot be applied in wet conditions, with others often having restrictions on humidity and temperature levels
  • Congestion on sites is an issue particularly where many trades are working at the same time. Eliminating one trade from the site (painters) reduces risk
  • Timescale - being weather dependent, painting on site, (even touch up), will add delay to the activity when weather is adverse. This delay will have knock on effects if scaffolding is required, or where other activities are dependent on completion of the painting. (e.g. cladding waits on structural steel being painted.) Simpler surface preparation for the galvanized and powder coated solution:
  • Most painting systems rely on steelwork being shot-blasted prior to painting, and also that the steel work does not get wet (rain) between blasting and painting
  • Blasting on site is expensive and creates blast overspray (the blast media), which can damage surrounding infrastructure Lower environmental impact for the galvanized and powder coated solution:
  • Site application systems suffer from contamination. Sprayed, brushed, or roller applied systems have spillages which enter the surrounding environment. Spray borne systems can travel a long distance in mildly windy conditions, affecting plants, humans, or building surfaces at long distances from the site
  • Paint residues can enter the water system through ground contamination and subsequent rain fall carrying residues into watercourses. In marine sites, sea, river or loch contamination can be an issue

Standard Issues to Consider

Site applications

  • Typically access systems can involve scaffolding, or powered access, which can restrict others‘ access to the site.
  • Overspray or spillage onto surrounding areas can be an environmental issue, or a practical issue where other items get accidentally coated.
  • Weather is a variable that has a huge effect on many coating systems, not just at application, but between coats. Many systems cannot be done in wet conditions, some even have restricted humidity and temperature limits.
  • Congestion on construction sites is an issue particularly where many trades are working at the same time. Eliminating one trade from the site (painters) reduces risk.
  • Timescale: Being weather dependent, painting on site, (even touch up), will add delay to the activity when weather is adverse. This delay will have knock on effects if scaffolding is required, or where other activities are dependent on completing the painting. (e.g. cladding waits for structural steel to be painted).

Preparation

  • Most painting systems rely on steelwork being shotblasted prior to painting, and also that the steel does not get wet (rain) between blasting and painting.
  • Blasting on site is expensive and creates blast overspray ( the blast media), can damage surrounding infrastructure.

Environmental

  • Site application systems suffer from site contamination. Sprayed, brushed or roller applied systems have spillages which enter the surrounding environment. Spray borne paint systems can travel a long distance in mildly windy conditions, affecting plants, humans or building surfaces at long distances from the site.
  • Paint residues can enter the water system through ground contamination and subsequent rain carrying residues into watercourses.
  • In marine sites, sea, river or loch contamination can be an issue.

Pricing

These are indicative prices only. Clearly quantity can have a big impact on price as can location, size of items and other commercial factors.

Durability

Generally this is shown as time until the need for coating maintenance or re-coating. At this time some action must be taken otherwise the substrate is being attacked and compromised.