 The
technology for coating products with dry powder rather than conventional
liquids has been available since the 1950's. The powder used for the process
is a mixture of finely ground particles of pigment and polymeric resin.
The powder is either sprayed electrostatically onto a surface to be coated,
or the substrate is dipped into a fluidized bed of suspended powder. The
powders adhere to a preheated substrate surface in the fluidized bed process,
or they adhere electrostatically in the spray process. When heated further
in a curing oven, the particles flow and fuse into a strong, adhering
coating. The result is a high quality coating with an attractive finish
and excellent durability.
The growth of powder coatings has been dramatic during
the last two decades with new applications continually being developed
for both industrial and consumer related markets. The growth can be attributed
to the powder coating industry meeting customers' demands with a two-pronged
attack:
-
The development of new formulations and
-
The development of advanced equipment and application
processes.
These developments have created many new market opportunities
and helped overcome finishing obstacles that were common in the early
days of the powder coating industry.
Powder coatings have been shown to possess significant
durability and resistance to abrasion, corrosion, scratching, and chemicals
when compared to liquid coatings. Powder coatings stay bright with less
fading, and color selection is virtually unlimited with high and low gloss,
metallic, and clear finishes available. Texture selections range from
smooth surfaces to wrinkled or matte finishes, and rough textures are
available for hiding surface imperfections. Thick coatings can be achieved
quickly and efficiently.
Although the final properties of the powder coatings
are often superior to liquid coating systems, the reason for the fast
growth of this technology has been more related to the evidence that powder
coatings maximize production, cut costs, improve efficiencies, and offer
maximum compliance with increasing stringent environmental regulations
- factors all related to the end-user's bottom line.
Environmental advantages have led the way for the conversion
of liquid coatings to powder coatings. Powder coating contains no solvent,
and thereby the process emits negligible, if any, polluting volatile organic
compounds (VOCs) into the atmosphere. Furthermore, the processes used
for powder coating do not require venting, filtering, or solvent recovery
as is necessary with liquid finishing. Costs are saved because there is
less need for heating outside air to supply oven exhaust air, and most
of the powder coating over-spray can be retrieved and re-used.
Table 1 summarizes the advantages and disadvantages of
powder coatings. Recent developments in materials and process technology
have greatly minimized the disadvantages that are listed here. These developments
are summarized in this article.
|
Advantages |
Disadvantages |
|
No solvents so that VOCs are nil |
Very thin coatings (less than 1.0
mil) are difficult because of pinholes |
|
Exhaust air from the coating booth can be returned to
the coating room, thus less oven air is exhausted to the
outside |
Frequent color changes
could entail extensive downtime |
|
Over-spray (up to 98%) can be retrieved and reused |
Storage and handling
of powder requires special climate controls |
|
No drying or flash time required so that parts can be
racked closer together |
Color matching and
color uniformity is somewhat more difficult than with
liquid coatings |
|
Easily adapted to continuous, automatic processes |
Uniformity of coating
thickness is sometimes difficult to maintain |
|
Coating does not run, drip, or sag, thereby lowering rejection
rates |
Cure temperatures required
for some powders are too high for temperature sensitive
parts |
|
Minimum operator training and supervision |
Powder coating is difficult
on sharp corners |
| Thick coatings are
easily possible |
Conversion from liquid
coating processes is expensive |
| High
throughput / output options |
Inside corners have
low film thickness owing to the Faraday cage effect |
| Simple clean-up and
maintenance |
|
Table 1: Advantages and Disadvantages of Powder
Coatings |
|
