POWDER COATING
©Copyright 1999/2003 Mario S Pennisi

WHAT IS POWDER COATING?
Powder coating is by far the youngest of the surface finishing techniques in common use today. It was first used in Australia about 1967.

Powder coating is the technique of applying dry paint to a part. The final cured coating is the same as a 2-pack wet paint. In normal wet painting such as house paints, the solids are in suspension in a liquid carrier, which must evaporate before the solid paint coating is produced.

In powder coating, the powdered paint may be applied by either of two techniques.

  1. The item is lowered into a fluidised bed of the powder, which may or may not be electrostatically charged, or
  2. The powdered paint is electrostatically charged and sprayed onto the part.

The part is then placed in an oven and the powder particles melt and coalesce to form a continuous film.

There are two main types of powder available to the surface finisher:

  1. Thermoplastic powders that will remelt when heated, and
  2. Thermosetting powders that will not remelt upon reheating. During the curing process (in the oven) a chemical cross-linking reaction is triggered at the curing temperature and it is this chemical reaction which gives the powder coating many of its desirable properties.

PREPARATION
The basis of any good coating is preparation. The vast majority of powder coating failures can be traced to a lack of a suitable preparation.

The preparation treatment is different for different materials.

In general, for all applications the preparation treatment for aluminium is as follows:

Oils and greases are removed in weak alkali or neutral detergent solutions and the surface is etched to remove heavy oxides. After rinsing, the aluminium is dipped into a chromate or phosphate solution to form a conversion coating on the aluminium. This film is chemically attached to the aluminium. After rinsing the aluminium is finally rinsed in demineralised water. Some non-chrome, dried in place pretreatment is beginning to come onto the market; currently, these are not recommended for exterior applications.

The conversion coating has two functions:

  1. It presents a surface to the powder which favours adhesion more than the oxides that form very readily on aluminium surfaces, and
  2. It reduces the incidence of under film corrosion, which may occur at holidays in the coating.

The use of demineralised water reduces the presence of chemical salts on the aluminium surface. These salts have been found to cause filiform corrosion in humid conditions.

For steel the preparation for interior applications may be:

For exterior applications:

The grain refiner is used after acid cleaning of steel surfaces and before zinc phosphating, otherwise the zinc phosphate coatings produced will be very coarse with low adhesion. The powder coating applied to a coarse phosphate will produce rough coatings (a little like "sandpaper") and possess low adhesion.

For hot dipped galvanized coatings, which have been stored for more than about 4 hours before powder coating, the following process is necessary for exterior applications.

The etch is required to remove the zinc corrosion products which begin to form almost immediately the zinc is removed from the galvanizing kettle. The grain refiner ensures a fine phosphate is produced.

HOW IS IT DONE - DIP?
Graphic Powuip16b.tif powqip17a.tif

The powder is fluidised in a hopper and the part is lowered into the powder cloud. The part can be at room temperature when an electrostatic charge is applied to the powder or the part can be preheated to above the melting point of the powder. The powder particles melt onto and attach themselves to the hot surface. Control of coating thickness is very difficult with this process. Both thermoplastic and thermosetting powders may be used. When using thermosetting powders it is advisable to return the parts to the oven to complete the curing reactions.

HOW IS IT DONE ELECTROSTATIC SPRAY?

The powder is applied with an electrostatic spray gun to a part that is at earth (or ground) potential. Before the powder is sent to the gun it is fluidised: to separate the individual grains of powder and so improve the electrostatic charge that can be applied to the powder and so that the powder flows more easily to the gun. Because the powder particles are electrostatically charged, the powder wraps around to the back of the part as it passes by towards the air offtake system. By collecting the powder, which passes by the job, and filtering it, the efficiency of the process can be increased to 95% material usage.

Two types of electrostatic charging is used

  1. Corona, and
  2. Tribo

Corona charging can occur within the gun or outside and in front of the gun. A charged field is generated by electricity. The powder passes through this field and becomes charged.

In Tribo charging, the charge is always applied within the gun through the action of friction. The powder particles are charged when they rub against the wall of the gun (usually a hose).

By increasing the flow speed of the powder and constricting the spray gun bore an electrostatic charge is imparted to the powder particles and no powerful electric fields and lines of force develop between the spray gun and the object. This often makes it possible to achieve better powder penetration into otherwise inaccessible areas.

By being less dependent on gun to object distance and the geometry of the components, the Tribostatic method often produces a more uniform coating thickness.

The powder will remain attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated items are placed in an oven and heated to temperatures that range from 160 to 210C (depending on the powder). Under the influence of heat a thermosetting powder goes through 4 stages to full cure.

  1. MELT
  2. FLOW
  3. GEL
  4. CURE

The final coating is continuous and will vary from high gloss to flat matt depending on the design of the powder by the supplier.

POWDER COATING GUNS
There are at least three types of electrostatic guns in use:

  1. Corona charging guns where electric power is used to generate the electrostatic charge. Corona guns are either internal or external charging.
  2. Tribo charging guns where the electrostatic charge is generated by friction between the powder and the gun barrel.
  3. "Bell" charging guns where the powder is charged by being "flung" from the perimeter of the "bell"

Not all powder is applied using guns. One system makes use of electrostatic tunnels, while another uses flame spraying techniques.

HOW IS COLOUR INTRODUCED?
Colour is added to powder coatings during the powder manufacturing process, ie before the powder reaches the powder coater. There is little that can be done to change the colour consistently, once the powder leaves the manufacturing plant.

WHY POWDER COAT?
Powder coating produces a high specification coating which is relatively hard, abrasion resistant (depending on the specification) and tough. Thin powder coatings can be bent but this is not recommended for exterior applications.

The choice of colours and finishes is almost limitless, if you have the time and money to have the powder produced by the powder manufacturer.

Powder coatings can be applied over a wide range of thickness. The Australian Standard AS4506:1998 - Metal finishing - Thermoset powder coatings recommends 25 micron minimum for mild interior applications and up to 60 micron minimum for exterior applications. Care must be exercised when quoting minimum thickness because some powder will not give "coverage" below 60 or even 80 micron. "Coverage" is the ability to cover the colour of the metal with the powder. Some of the white colours require about 75 micron to give full "coverage". One of the orange colours must be applied at 80 micron. Colour matching is quite acceptable batch to batch.

APPLICATIONS
Powder coatings were introduced into Australia in the early 70s and have already settled into a diversity of applications.

  • Highly protective, thick powder coatings are applied to produce linings on the inside of oil drilling pipes where severe pressures, high temperatures and corrosive materials are common. Only a select few coatings are satisfactory in these conditions.
  • Reinforcing steel bars for highway and bridge decks are powder coated to greatly reduce the formation of corrosion products, which result in concrete spalling and cracking. The cost to repair damaged concrete is very high.
  • With the advances in powder application using flame spraying techniques, powder coatings can now also be applied in the field.
  • In automotive applications:
    • some primers used on bodywork have converted to powder. This has given improved coating quality and provided ecological advantages.
    • where a functional as well as a decorative finish is desirable powder coatings are being used increasingly eg
      • - wheels,
      • - bumper bars,
      • - mirror frames,
      • - oil filters,
      • - battery trays,
      • - coil springs and
      • - heads.
  • Appliances:
    • Specially formulated, hard and scratch resistant powder coatings are replacing the energy-intensive porcelain finishes on washer tops and lids.
    • Powder coatings are being used on range housings, freezer cabinets, drier drums, and microwave oven cavities.
    • Outdoor furniture, farm and garden implements and garden tractors also are being finished with powder coating materials. - Interior fluorescent light reflectors are coated using a highly reflective, very thin powder coating.
    • Exterior lamp housings on highway and parking deck fixtures are coated with specially formulated powder which give ultra violet light protection and added corrosion protection.
  • Building products:
    • Major applications include aluminium extrusions used for window and door frames
    • Kitchen appliances and fittings, bathroom fittings, interior furniture all can be powder coated.
  • Other:
    • Glass, wood products and plastic parts also can be powder coated.

In other words, everything from spaghetti drying racks to football stadium seats can be powder coated. More traditional applications are bicycle frames, air conditioner housings, small appliances, and office furniture components. The rough edges sometimes associated with stamped parts can be protected with powder coatings and the need to grind edges prior to finishing can be reduced.

INSTALLATIONS AND MAINTENANCE
During installations, the powder coating should be protected from damage due to abrasion and materials of construction such as mortar and brick cleaning chemicals.

Once installed, maintaining the initial appearance of a powder coating is a simple matter. The soot and grime, which builds up on surfaces from time to time, contains moisture and salts which will adversely affect the powder coating and must be removed. Powder coatings should be washed down regularly (at least once each 6 months in less severe applications and more often in marine and industrial environments). The coating should be washed down with soapy water use a neutral detergent - and rinsed off with clean water.

When powder coated items are installed without damage to the powder coating and they are maintained regularly, they should be relatively permanent. The correctly applied coating, although not metallurgically bonded to the metal will not crack, chip or peel as with conventional paint films.