Getting the most
out of
powder coatings
Makers of the largest, most complete line of fluoropolymer coatings in the world
The most common resins used in powder
coatings are polyester, polyester-epoxy,
straight epoxy, and acrylics. These materials
have a broad spectrum of uses and are available from various suppliers. In contrast, Whitford offers a number of specialized powder
coatings with outstanding properties — including excellent release and resistance to
high temperatures — for use in particularly
demanding applications.
What is powder coating?
Simply stated, powder coating is the process of coating a surface with a powdered
material (as opposed to a liquid).
Powder coating materials are comprised
of polymeric resins (and often additives and
pigments) that have been reduced to small
particles with an average diameter of 25 to 90
microns. The main difference between powder
and conventional liquid coatings is that powder coatings are applied in the form of freeflowing powders, whereas liquid coatings
require water or an organic solvent to keep
the resin in the form of a suspension.
Since their inception in the 1960s, powder
coatings have gained acceptance on a vast
array of metal parts, both consumer and industrial. Common applications include automotive components such as body panels and
oil filters, architectural items such as aluminum columns and window frames, and material handling products such as valves and
pipes in the chemical processing industry.
The great advantage of powder coatings
is that they can be applied with almost zero
environmental problems. The application
processes for powder coatings are essentially
mechanical and thermal — there are no VOCbearing effluents or wash-down solutions to
clean up. In addition, the application methods
are relatively inexpensive.
How to apply them
Powder coatings are normally applied in
one of two ways — either by fluidized bed or
by electrostatic spray.
Both thermoplastic and thermosetting
resins are available as powder coatings.
1. Fluidized bed: Preheated parts are
dipped into a bed of powder that is made
fluid-like by small streams of air arising from
beneath the powder. As the fluidized powder
comes into contact with the heated part, it
melts and fuses to the surface of the part. The
coated parts are placed back in an oven for
the final flow-out of the powder.
• A thermoplastic powder coating melts
when it is heated for a second (and subsequent) time. Examples of thermoplastics include polyethylene, polypropylene and nylon.
• A thermosetting powder coating does
not melt after curing. Examples include epoxy,
polyester, acrylic and
PPS. When these resins
are cured, a chemical
cross-linking reaction is
triggered, which gives
the coating many of its
desirable properties.
Many powder-coating
materials adhere to
metallic substrates without the use of primers.
But resins such as fluoropolymers require the
use of a separate primer
layer to ensure adhesion
to the substrate.
Air enters here
under pressure
Fluidized bed
Air passes through vents,
“liquifying” powder,
causing it to rise
-2-
Powder adheres,
fusing to heated part
This technique requires little equipment
other than the bed, a source of air, and a
means of heating the parts. A variation of this
technique is to apply a negative electrostatic
charge to the powder which draws the powder particles to the part via electrostatic attraction.
2. Electrostatic spray: An electrostatic
charge brings the particles and the part together. One benefit is that the parts are
coated uniformly — even on the back side of
the part — because the attraction between
the powder and the part is uniform, all around
the part. Then the part is heated so that the
powder melts, fusing to the part surface.
to run, drip, or sag.
Overall coating thickness can be built to
maximum levels allowed by the resin without
fear of blistering. In general, powder-coated
parts are more resistant to cracking, peeling,
and marring during handling and normal use.
Thicker coatings also tend to be tougher and
more durable and are particularly useful on
corrosion-prone applications where thicker
coatings provide added protection. They also
help cover small surface defects such as
sharp edges, dimples, and machining marks.
As a result, some secondary machining operations, such as polishing, grinding, or deburring may be eliminated.
2. Advantages in deep-well applications:
Powder coatings can be advantageous when
applied to parts that have recesses and depressions (see page 7). Using the fluidizedbed method of application,
powder coatings can be applied
Electrostatic spray
almost evenly to channels, keySpray gun
ways, holes and other recesses.
No other application method results in coatings as uniform and
blister-free in these areas. This is
because coating thickness deElectrostatic
pends primarily on the heat in the
charge
part, not on electrostatic attraction or how well volatile carriers
Air/powder input
are driven off, as is the case with
Powder surrounds
liquid
coatings.
part, attracted
This is the application method of choice
for coating many parts at once, or when the
coating thickness needs to be controlled.
Power supply
electrostatically
3. Easier to use: Cleanup is
greatly
simplified with powders.
Diagram of typical electrostatic powder application process.
Cleanup of liquid coatings often
requires stripping agents and solvents that
Advantages vs liquid coatings
must be discarded. In contrast, a powder
Powder coatings offer several advantages booth or spray area can be cleaned up by
over liquid coatings in terms of performance,
simply vacuuming the powder overspray.
ease and efficiency of use, cost, and environ4. More efficient: If the powder overspray
mental concerns.
is not contaminated, it can be collected and
1. Better performance from higher film
recycled (liquid coatings cannot), so it is posthickness: Powder coatings provide the same
sible to use nearly 100 percent of the coating
level of corrosion, wear protection, and rematerial.
lease as their liquid counterparts — if applied
5. Low capital investment: Many of the adat the same film thickness. However, in comvantages of powder coatings lie in what they
parison to liquid coatings, powders make
enable users to avoid. For instance, powder
thicker films possible because they tend not
-3-
coatings do not require investment in expensive cleanup and effluent monitoring systems.
In addition, far less wastewater is generated,
resulting in fewer problems and expenses associated with waste disposal.
ing, it is important to ensure that the fusing
temperature of the powder is compatible with
the temperature resistance of the substrate.
6. Cost savings in operations: The savings
in energy, labor, rework, material, line efficiency, waste disposal, air processing, and cleanup can be substantial compared to liquids.
Powder coatings are particularly well
suited to applications where thicker films are
desirable because of their increased durability and resistance to corrosion.
7. More environmentally friendly: Powders
enable the elimination of solvents and hazardous wastes. Unless the coatings are accidentally overheated during the fusing process, powder coatings emit zero or near- zero
VOCs during the coating process.
1. Thick-films: Powder coatings are ideal
for applications where thick liquid coatings of
similar properties would ordinarily have been
specified, but where VOCs are unacceptable.
Also, powder coatings should be considered
when the necessary buildup of film thickness
with liquids may result in blistering.
When to specify powder
Drawbacks vs liquid coatings
Not all advantages, however, lie on the
side of powder. Liquid coatings are often preferred because of their ability to form thin films
or to be cured at lower temperatures.
1. Liquids are well suited to thin coatings:
Thinner coatings are advantageous in many
applications, particularly on small mechanical
parts where assembly or operations would be
impeded by a coating that is thicker than
about 38 microns (1.5+ mils).
2. Higher fusing temperatures: Liquid coatings fuse through drying and curing, in which
the volatile carriers are driven off by heat,
leaving only the solids behind. Volatile components can be removed over a wide range of
temperatures, from about 65˚-370˚C (150˚700°F), in a time/temperature relationship. So
some liquid systems can be processed at low
temperatures and still incorporate high-melting components into the cured coating.
On the other hand, a powder coating such
as PFA must reach its specific melt temperature before it can flow and fuse. Yet some fusion temperatures are incompatible with
certain substrate materials. For instance, fusing a die-cast part at 370°C (700°F) is likely to
produce eruptions from subsurface voids.
Also, forged aluminum parts should not be
fused above 205˚C (400°F) because they may
soften. Thus, when selecting a powder coat-
2. Corrosion: Powder coatings are applied
to retard or eliminate corrosion in many industrial applications.
Corrosion is an electrochemical process
with three components: a cathode, an anode,
and an electrolyte, and there are many circumstances in which all are present. Common
examples include metal parts used in or near
sea water or acid solutions; parts made from
dissimilar metals that are joined together; and
vibrating parts that are tightly pressed together. Specific examples include components of CPI systems such as valves, pipes,
hangers, unions, joints, filters, and housings
for motors and pumps.
The key to protecting these parts is total
encapsulation of the surface, which can be
ensured by applying the coating in multiple
layers, if possible. The reason is that, in any
single coating layer, small voids or pin holes
can serve as an electrolytic path for corrosives. Multiple coating layers can eliminate
essentially all of these voids by overlapping
them (see diagram next page).
3. Durability: This is a common requirement in applications where frequent contact is
made with the coated surface. For example,
the surfaces of equipment used for materialshandling, packaging, sealing, molds, underbody parts of vehicles, stone crushing,
pulpwood, grain processing, and building ma-4-
heating “profile”. If an oven is set at a temperature of 205°C (400°F), the real oven
temperature may be 215°C (420°F) in the
Topcoat/
Prime/
Second coat
Pin holes
upper-left corner and 185°C (365°F) in the
Substrate
First coat
lower-right corner, which can damage the
quality of finished coatings if the profile is
not known. For instance, parts in one section of the oven might be withdrawn before
fusing is complete or, conversely, could become overheated and burned or blistered.
Overlapping layers of the topcoat fill in and cover
Therefore, prior to long production runs, a
any minute pin holes in the prime coat.
thermal profile of the oven should be obtained
terials can often be eroded by the materials
in order to enable adjustment of, or compenthat are being processed. Given enough time, sation for, temperature imbalances.
these surfaces can be completely destroyed
• Processing temperatures: The processby the materials passing over them. Thick
ing (curing or fusing) temperature in the oven
films of certain powder coatings can slow
depends on the resin used. Many polymers
down this process. Although a powder coating may not stop the erosion entirely, it is likely can be cured in the range of 160°-210°C
(320°-410°F); however, certain high-perforto prolong the life of parts that are used in
mance resins such as fluoropolymers require
these environments.
processing temperatures in the range of 300°4. Release: Liquid coatings are often pre400°C (600-750°F). How this heat is applied to
ferred for release applications, although fluothe part depends on the size of the part, its
roplastic powder coatings are frequently used
composition and shape, as well as the numwhen extra durability is required. For example,
ber of units to be coated. Large, thick items,
powder coatings are specified on a number of
such as castings, tend to be fused in convecpaper-handling and mold surfaces where
tion ovens where heat is supplied remotely,
long-lasting release is critical to the process.
via gas or electric heaters. Small parts, such
Other examples include slides and chutes for
as stampings and small wrought parts tend to
printing equipment and wear surfaces of xerobe cured by banks of IR heaters.
graphic equipment.
Two coats to avoid corrosion
Choosing the process
A word on fusing
Powder coatings are fused by heating the
coated parts in an oven. In some cases, the
parts are heated prior to powder application.
The choice of how to powder-coat a part
depends on (1) the mass and size of the part,
(2) its thermal conductivity, (3) its shape, and
(4) the number to be coated.
• Preheating parts: Preheating of parts,
often called “hot flocking” is, of course, necessary when coating via the (non-electrostatic) fluidized-bed method. However, it can also
be quite useful in the electrostatic process.
For example, in cases where unusually high
builds of powder are required, the parts can
be preheated to the fusion temperature of the
powder. This will cause the powder to fuse on
the part as soon as it is applied, enabling a
thicker-than-normal coating to be attained.
1. Mass and size: For a powder coating to
fuse on the surface, the part must be heated
to the melt (fusion) temperature of the polymer
and remain there for as long as is required for
the powder to melt and flow. This is a time/
temperature relationship (see next page). The
larger and thicker the part, the longer it takes
to reach the fusion temperature of the coating.
Extreme examples: a cast valve body and a
stamped shell of an automotive oil filter.
If a large, thick steel casting is to be
• Oven profiling: Every oven has a unique
-5-
Temperature
Time/temperature relationship
Thin
stamped
part
Thicker sheetmetal part
Fusion
temperature
Large casting
Time
As the graph demonstrates, the thicker the part,
the more time required to achieve full cure.
coated, it will require hours of “heat soak”
(oven dwell) in order to reach the fusion temperature. Such a part can be coated in a fluidized bed or sprayed electrostatically,
depending on its complexity and whether multiple coats are required.
If the shape is complex (e.g., including
fins, bosses, or holes), then the fluidized-bed
method may be the better way to coat the part
completely. However, if a complex part requires multiple coats for added protection, the
coating must be applied by electrostatic
spray because the conventional (non-electrostatic) fluidized-bed process is a one-step
system (only one coat can be applied).
2. Thermal conductivity: Parts with relatively low thermal conductivity (such as cast
ferrous metals) are often heated for long periods in a convection oven because they require a long time to reach the powder’s fusing
temperature. For example, large tanks or compressor bodies often soak overnight in a convection oven to affect a cure on the coating.
On the other hand, highly conductive parts
(such as aluminum and brass) may be cured
rapidly via IR heaters because the parts reach
fusion temperature quickly.
3. Shape of the part: Shape can influence
the choice of application method. For example, round, cylindrical, and cubic-shaped
items are usually coated more efficiently using
electrostatic spray. Because the electrostatic
charge enables the powder to wrap around all
sides of the part, the coating is uniform. But:
irregularly shaped parts — including those
with bosses, indents, keyways, slots and
holes — are often best coated using a fluidized bed because this may be the only way
to achieve uniform application of the powder
(see illustration below).
4. Number of parts to be coated: Fluidized
beds lend themselves to batch-coating
processes in which one or more parts are
dipped at one time. In this case, production
rates are typically measured in multiples of 10
parts per hour. On the other hand, electrostatic spraying lends itself to automated application, and application rates may reach
hundreds or even thousands of parts per hour.
Difficult applications
Parts comprised of components with different thicknesses, deep recesses, or subject
to high loads can be problematic for powder
coating.
• Thickness ratio: A rule of thumb is to
avoid powder coatings on parts where wall
thickness varies by a ratio of more than four to
one. Example: a part with a thick, solid base
and an array of thin fins or other projections. If
such a part remains in an oven long enough
to fuse the powder on the base, the powder
on the fins could over-bake and degrade. The
tendency for powder coatings to degrade with
exposure to heat is resin specific, and the effects vary from one type of coating to another.
• Recesses: Another problem when using
electrostatic systems is a deep recess. In
Simple shapes
Complex shape
Simple shapes are easy to coat with fluidized-bed
or electrostatic spray systems. Complex shapes
are usually easiest to coat with a fluidized bed.
-6-
cases where the width-to-depth ratio is small,
the coating may not be uniform. This problem
is caused by uneven application of powder
due to the “Faraday effect.” Because electrostatic attraction is inverse to distance (like
gravity), charged particles are attracted to the
substrate as they approach the opening of the
recess, leading to a buildup of powder at the
opening and less powder inside the recess.
coating performance on specific materials.
• Cast aluminum: Both die-cast and sand-
cast aluminum parts can be porous. Prior to
coating, it may be necessary to heat the part
to a temperature above the fusing temperature of the powder in order to allow any nearsurface cavities to erupt. If these cavities
erupt after the item is coated, the eruptions
will spoil the finished coating.
• Wrought aluminum: Most parts are
chromate conversion-coated prior to powder coating, which improves the corrosion
resistance. On a forged aluminum part, if
the oven processing temperature exceeds
the substrate’s annealing temperature of
218°C (425°F), metallurgical damage can
occur. Some parts that have been substantially work-hardened by forging may be
stress-relieved to the point that they warp
and become misshapen.
The Faraday effect
Conventional
Electrostatic
Fluidized bed
Powder coating
A representation of how the results of the Faraday effect depend upon the application method. In this
case, the fluidized bed results in the most uniform
application of the powder.
• High loads: When wear resistance is required, the “PV limit” expresses the maximum
PV (pressure times sliding velocity) value — or
limit of sliding severity — that a composition
can tolerate without catastrophic failure. The
physics of coatings is such that the thicker
they are, the more they act like plastics. Conversely, the thinner the coating, the more it
acts like the substrate on which it is applied,
usually a metal. For example, a thin, 12-micron (0.5 mil) coating may have a PV limit of
150,000; yet the same coating material may
have a PV limit of only 50,000 when applied at
25 microns (1 mil). Because powder coatings
are usually applied at thicknesses of at least
37 microns (1.5 mils), their use in high-load
applications is not usually recommended.
Preparation of substrates
The guidelines regarding surface preparation of parts prior to coating with powders are
similar to those with liquids. Application of
coatings over dirty, oily parts will result in poor
adhesion and/or reduced performance. Observing the following guidelines will improve
• Cold-rolled steel: This material provides a good surface for coating. It should
be cleaned and phosphated prior to the application of powder.
• Hot-rolled steel: This is usually covered
with mill scale that must be removed prior to
coating. Remove the scale by mechanical
means (blasting or tumbling) and then treat
the metal as you would cold-rolled steel.
• Cast iron and steel: Treat these as you
would hot-rolled steel.
Want more information?
Whitford was founded in 1969 to develop
and market high-performance fluoropolymer
and powder coatings. Today, Whitford makes
the largest, most complete line of fluoropolymer coatings in the world.
This brief booklet provides an overview of
ways to take full advantage of the benefits of
powder coatings. Additional information is
available in various publications and may be
obtained by contacting your Whitford representative or by going to our website: whitfordww.com. You can also email us at
sales@whitfordww.com.
-7-
Whitford Tests for Coating Quality
Impact Test (ASTM D-2794)
Coating on a 0.036 in. thick phosphated steel panel must withstand the impact of a 1/2” Gardner impact tester ball at 26 in.-lb., and reverser. No grazing or loss of adhesion is acceptable.
The coating shall not be able to be removed at the impact area with 3M Y-939 tape.
Adhesion Test (ASTM D-3359)
Scribe parallel lines through the coating to the substrate, spacing the scribe marks 1/4” apart,
over a distance of 1”. Scribe a second set of parallel lines 1/4” apart, perpendicular to the first
set. Apply 3M Y-9239 tape, then remove slowly. There should be no lifting of film between
scribe lines.
Chemical Resistance Test (ASTM D-1308)
Place approximately 10 drops of test solvent (95% by weight of toluene and 5% by weight of
MEK) on the surface of the coating. Allow to stand for 30 seconds. Wipe off with a soft, dry
cloth. The coating shall show no more than a slight circular mark.
Salt-Spray Corrosion Test (ASTM D-117)
Use a 5% salt solution at 33˚-36˚C (92-97°F) in a sealed weather cabinet. Scribe an “X” mark
to bare metal in the coated steel test panel, pretreated with zinc phosphate. Inspect every 24
hours. End test and total hours after 1/4” creepage from scribed area. Creepage shall not exceed 1/4” in either direction from scribe line after 500 hours of exposure.
Bend Test (ASTM D-522)
Coating on a 0.036” thick phosphated steel panel shall withstand 180° bend over 1/4” mandrel. There shall be no crazing or loss of adhesion and finish at the bend, and no lifting when
using 3M Y-9239 tape.
How to contact Whitford
Whitford manufactures and maintains sales offices
in many countries. For more information, please
contact your Whitford representative or the nearest Whitford office
(see our website: whitfordww.com) or sales@whitfordww.com.
Makers of the largest, most complete line of fluoropolymer coatings in the world
NON-WARRANTY: THE INFORMATION PRESENTED IN THIS PUBLICATION IS BASED UPON THE RESEARCH AND
EXPERIENCE OF WHITFORD CORPORATION. NO REPRESENTATION OR WARRANTY IS MADE, HOWEVER, CONCERNING THE ACCURACY OR COMPLETENESS OF THE INFORMATION PRESENTED IN THIS PUBLICATION. WHITFORD CORPORATION MAKES NO WARRANTY OR REPRESENTATION OF ANY KIND, EXPRESS OR IMPLIED,
INCLUDING WITHOUT LIMITATION ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR
PURPOSE, AND NO WARRANTY OR REPRESENTATION SHALL BE IMPLIED BY LAW OR OTHERWISE. ANY PRODUCTS SOLD BY WHITFORD CORPORATION ARE NOT WARRANTED AS SUITABLE FOR ANY PARTICULAR PURPOSE TO THE BUYER. THE SUITABILITY OF ANY PRODUCTS FOR ANY PURPOSE PARTICULAR TO THE BUYER IS
FOR THE BUYER TO DETERMINE. WHITFORD CORPORATION ASSUMES NO RESPONSIBILITY FOR THE SELECTION OF PRODUCTS SUITABLE TO THE PARTICULAR PURPOSES OF ANY PARTICULAR BUYER. WHITFORD CORPORATION SHALL IN NO EVENT BE LIABLE FOR ANY SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES.
©Whitford 2009 WC2/09