By: Kevin Biller
One of the more popular powder coating looks is metallics. Metallics range from a
bright sparkle finish to a muted reflective patina that approximates the surface of a
metal. Metallic effect powder coatings are found across a wide spectrum of decorative
finishes, including automotive wheels and trim, under hood components, architectural
facades, lawn mowers, office furniture, appliances and bicycles.
Let’s delve into the basics of metallic powder coatings based on aluminum pigments and
give you a fundamental understanding of how they are made and what performance to expect.
Manufacture of Aluminum Flakes
Aluminum pigments are manufactured using a ball mill
process in which high purity aluminum is combined with a
solvent and lubricant to facilitate the particle size reduction
of the alloy. After hours of processing the solventborne
aluminum flakes are collected, screened and passed through a
press to remove most of the solvent. From here the remaining
product can be processed into a paste and drummed off for
shipment. For powder coatings this paste undergoes further
processes to create discreet fine particles. These discreet
particles can be used in some lower performance powder
coatings however most products for powder coatings undergo
an encapsulation process to protect the flakes from oxidation
and to ease dispersion and application performance.
Leafing Vs. Non-leafing Aluminum Pigments
Aluminum pigments can be classified as “leafing” or
“non-leafing.” From a practical standpoint, leafing aluminum
pigments orient at the surface of a powder coating during
the melt/fusion phase in the powder coating cure oven. This
parallel orientation creates a relatively continuous metallic
looking surface sometimes referred to as a “chrome” look. A
unique orientation phenomenon occurs due to the lubricant
used in the aluminum flake manufacturing process. The
lubricant used with leafing aluminum pigments is stearic
acid which is incompatible with most powder coating resin
chemistries. This incompatibility forces the flakes away from
the powder binder to the surface of the film. Cross-sectional
analysis shows that most flakes reside at or near the top
surface of the powder coating finish.
On the other hand, non-leafing aluminum flakes are
randomly distributed throughout the powder coating film and
thus create more of a sparkle effect. Non-leafing aluminum
pigments typically use oleic acid as a lubricant during
manufacture. Oleic acid is a sufficiently compatible with most of
the polymers used in powder coatings and therefore facilitates
an even dispersion of flakes throughout the powder film.
One important note—leafing aluminum pigments
and the resultant “chrome” look surface effect they create
is susceptible to chemical attack from the environment.
Oxidation and attack by acids or alkalis will discolor exposed
leafing aluminum flake. Hence it is strongly recommended
to apply a clear topcoat over these types of powder coating
surfaces. The clearcoat effectively isolates the free aluminum
to provide a durable and decorative finish.
Aluminum pigments are available in a variety of particle
sizes and distributions. The smallest aluminum particles used
in powder coatings are approximately 6 microns in diameter
and produce a fine grain metallic pattern. Larger particles tend
to provide a higher degree of sparkle and usually less opacity.
Particle size approaching 100 microns have been used in
powder coating formulating to yield a “super sparkle” effect.
Conventional powder coatings typically have a particle size
distribution ranging from about 1.0 micron up to 100 microns
with a median of 35 to 40 microns. In addition decorative
powder coatings are applied at film thicknesses around 2.0 to
3.0 mils (50 to 75 microns). The incorporation of aluminum
pigments greater than 75 microns can be problematic in spray
application performance and can cause film surface defect.
Particles larger than the film thickness will undoubtedly
protrude through the surface and create unsightly bumps.
Particle size and morphology influences opacity. Smaller
and thinner particles provide more hiding than thicker, larger
particles. Consequently aluminum pigments with broad
particle size distribution provide better opacity than pigments
with a narrow particle size distribution. On the other hand,
narrow particle size produces more consistent and brighter
Aluminum pigments are available in a three different
particle shapes, “cornflake,” “silver-dollar” (lenticular), and
spherical. Cornflake geometry is supplied in a relatively broad
particle size distribution and finds use in many industrial
applications. These two dimensionally shaped pigments have
irregular edges and are offered in leafing and non-leafing
grades. Silver-dollar or lenticular geometry pigments are
brighter than their cornflake counterparts and possess narrow
particle distributions making them suitable for higher end
applications such as automotive and major appliance finishes.
This type of particle has smooth edges and creates the
brightest and whitest sparkle effects.
Spherical aluminum pigments are manufactured by
atomizing molten aluminum and are not commonly used in
powder coatings. This type of aluminum pigment does find
use in plastics as it creates a unique three dimensional effect
and is easier to process than flake aluminum grades.
Uncoated aluminum flake is susceptible to oxidation and
chemical attack. In addition, dry blended powder coatings
containing uncoated aluminum flake create a number of
application issues. The significantly different specific gravity
of powder coatings (1.2 to 1.7) compared to aluminum
flake (2.7) causes particle segregation in fluidization and
transport of a dry blended powder. Furthermore the higher
conductivity of aluminum compared to powder coating
causes a dry blended aluminum containing powder to behave
erratically with electrostatic spray technique. Aluminum
particles gravitate to part edges creating a “picture frame”
effect. Aluminum tends to accumulate on gun tips and particle
segregation occurs with overspray and reclaim.
Several encapsulation schemes are employed to protect the
aluminum from oxidation and chemical attack. Additionally,
these surface treatments improve the dispersion of aluminum
flake in dry powder blends and mitigate electrostatic
Encapsulants can be inorganic or organic in nature. Fumed
silica is a common surface treatment for aluminum flakes and
offers a cost-effective improvement in handling characteristics,
electrostatic application performance and resistance to
environmental effects. In addition to coating the aluminum
particle’s surface amorphous silica reduces the overall specific
gravity of the pigment particle which reduces segregation from
the base powder coating.
Organic polymers are also used to coat the surface
of aluminum pigments. This more expensive technique
thoroughly covers the surface of the aluminum flakes and
further facilitates improved handling and electrostatic
performance. In addition, higher concentrations can be
incorporated because silica coated flakes possess a higher oil
absorption and a corresponding reduction powder coating
flow and leveling (increased texture).
Aluminum pigment manufacturers have taken surface
treatment techniques to another level by combining both
inorganic and organic encapsulation. These obviously more
expensive products provide excellent brightness, dispersion,
outdoor durability and electrostatic performance.
Some grades of aluminum pigment are available as
predispersed pellets or granules. The aluminum pigment
producers blend polymers (olefinic, aldehyde or acrylic) with
about 70 percent to 80 percent aluminum and form pellets
that are easily incorporated in a premix of powder coating
formula. These pellets are added to the powder formula
pre-extrusion and are therefore dispersed in the powder
coating matrix through the extrusion process. Aluminum flake
dispersed in an extruder yields a darker, less sparkling metallic
During the powder coating extrusion process a
considerable amount of shear is delivered to the molten
mixture of resins, pigments and additives. Incorporating a
relatively delicate aluminum flake into
the extrusion process literally destroys
the flake and all of its brightness and
reflectivity. Consequently, aluminum
pigment can be added to a powder
coating material at this stage however
the resultant coating appearance is a
muted metallic look which is seldom a
Aluminum flake can be dry-blended
at low intensity into a base powder
coating (one that has already been
produced). Uncoated aluminum types
readily segregate from the powder
coating creating application issues and
inconsistent looking finishes. Surface
treated types offer an improvement in
dispersion and hence better handling
properties and application performance.
Regardless high speed finishing lines
make it difficult to use dry-blended
metallic powder coatings because of
the volume and rate of delivery of
powder used. In addition recycling and
reuse of dry-blended powder coatings
is usually impractical as it leads to
different concentrations of aluminum
pigment. Nonetheless dry-blended
metallic powder coatings are regularly used in “spray-towaste”
finishing processes where the recycling of overspray is
Powder coating innovators developed a technique called
“bonding” to create a metallic powder coating that fluidizes
and transports well, applies consistently and is reclaimable.
This unique process essentially attaches the aluminum flake to
the powder particles thus creating a unified particle consisting
of bright flake and powder coating particle. Some independent
companies specialize in bonding metallic flakes to powders
whereas a few powder coating manufacturers bond their
powder coatings in house. Any high volume metallic powder
coating finishing line should be using a bonded product.
Concentration Levels and “Flop”
Because of powder coating’s unique solid particulate
form there is a finite amount of aluminum flake that can be
incorporated into a formula. Dry-blends can use as much as
5% concentration of flake before handling and application
performance become unmanageable. Bonded powder coatings
can have concentrations at high as 9 percent to 10 percent
The limit on the practical concentration of aluminum flake
in a powder coating makes it different for powder formulators
to achieve the same brightness and sparkle of some metallic
look liquid paints. Liquid paints have the advantage of
solvents and/or co-solvents that allow for concentrations
of aluminum pigment as high as 20 percent. Consequently,
matching some very bright liquid paint standards is nearly
impossible for the powder chemist.
In addition to the difficulty of matching the brightness of a
liquid metallic paint a crucial appearance characteristic known
as “flop” is hard to equal as well. Flop refers to the reflection
of the aluminum particles as the viewing angle changes. Some
metallic finishes flop darker as the viewing angle changes
whereas others flop lighter. This behavior in metallic powder
coatings is quite different than that with liquid metallic paints
because the orientation of aluminum particles in liquid paint
is relatively orderly and the orientation in a powder coating
film is significantly more random.
The potential hazards associated with handling
aluminum pigments are mostly related to the powder coating
manufacturer; however, safe handling practices are always
wise regardless of your operation. There are four possible
hazard potentials when handling aluminum flakes.
- Aluminum powder is combustible and considered flammable.
- Mixtures of aluminum and oxygen are ignitable over a range of concentrations.
- A strong electrical charge on aluminum can discharge to a ground creating an ignition source.
- Aluminum reacts with water, acids and alkalis to create highly flammable hydrogen gas.
Hence, care must be exercised when handling aluminum
flake. Please consult your supplier’s Safety Data Sheet for
recommendations on the safe storage and handling of
aluminum pigment and/or aluminum containing powder
Metallic powder coatings provide a very attractive finish
for a wide array of products. Because of powder’s inherent
particulate form the incorporation of aluminum pigments
is unique process that can provide a high level of both
application and film performance. Effects from a soft metallic
patina to a bright sparkle are achievable with the right
concentration and type of aluminum pigment. Finally caution
should be exercised when handling all industrial materials
including metallic powder coatings.
Kevin Biller is technical editor of Powder Coated Tough
magazine and president of The Powder Coating Research Group.
He can be reached via email at email@example.com.