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 typically 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 usually used to create a hard finish that is tougher than
conventional paint. Powder coating is mainly used for coating of metals,
such as "white goods", aluminium extrusions, and automobile and
motorcycle parts. Newer technologies allow other materials, such as MDF
(medium-density fibreboard), to be powder coated using different
* 1 Advantages and disadvantages of powder coating
* 2 Types of powder coatings
* 3 The powder coating process
* 4 See also
* 5 External links
Advantages and disadvantages of powder coating
There are several advantages of powder coating over conventional liquid
1. Powder coatings emit zero or near zero volatile organic compounds
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. Powder coating production lines produce less hazardous waste than
conventional liquid coatings.
5. Capital equipment and operating costs for a powder line are generally
less than for conventional liquid lines.
6. Powder coated items generally have fewer appearance differences
between horizontally coated surfaces and vertically coated surfaces than
liquid coated items.
7. A wide range of specialty effects is easily accomplished which would
be impossible to achieve with other coating processes.
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 thin films. As the film thickness is reduced,
the film becomes more and more orange peeled in texture due to the
particle size and TG (glass transition temperature) of the powder.
One variation of the dry powder coating process, the Powder Slurry
process, combines the advantages of powder coatings and liquid coatings
by dispersing very fine powders of 1–5 micon particle size into water,
which then allows very smooth, low film thickness coatings to be
Powder coatings have a major advantage in that the overspray can be
recycled. However, if multiple colors are being sprayed in a single
spray booth, this may limit the ability to recycle the overspray.
Types of powder coatings
There are two main categories of powder coatings: thermosets and
thermoplastics. 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.
1. The polymer granules are mixed with hardener, pigments and other
powder ingredients in a mixer
2. The mixture is heated in an extruder
3. The extruded mixture is rolled flat, cooled and broken into small
4. The chips are milled to make a fine powder
The powder coating process
The powder coating process involves three basic steps:
1. Part preparation or the Pre treatment
2. The powder application
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 negative electric charge on the powder, which is then sprayed
towards the object, which is grounded. 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. See the article "Fusion Bonded Epoxy Coatings"
Another type of gun is called a Tribo gun, which charges the powder by (triboelectric)
friction. In this case, the powder picks up a positive charge while
rubbing along the wall of a Teflon tube inside the barrel of the gun.
These charged powder particles then adhere to the grounded substrate.
Using a Tribo gun requires a different formulation of powder than the
more common Corona guns. Tribo guns are not subject to some of the
problems associated with Corona guns, however, such as back ionization
and the Faraday Cage Effect.
Powder can also be applied using specifically adapted electrostatic
Another method of applying powder coating, called the Fluidized Bed
method, is by heating the substrate and then dipping it into an aerated,
powder-filled bed. The powder sticks and melts to the hot object.
Further heating is usually required to finish curing the coating. This
method is generally used when the desired thickness of coating is to
exceed 300 micrometres. This is how most dishwasher racks are coated.
Electrostatic Fluidized Bed Coating: Electrostatic fluidized bed
application uses the same fluidizing technique and the conventional
fluidized bed dip process but with much less powder depth in the bed. An
electrostatic charging media is placed inside the bed so that the powder
material becomes charged as the fluidizing air lifts it up. Charged
particles of powder move upward and form a cloud of charged powder above
the fluid bed. When a grounded part is passed through the charged cloud
the particles will be attracted to its surface. The parts are not
preheated as they are for the conventional fluidized bed dip process.
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 (392° F) in 10 mins. 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.