Posted in: Raw Materials

Technology Interchange: What's in a Flake? Metallic Look Powder Substrates

Posted on Wednesday, April 19, 2017

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.

Particle Size

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 sparkle effects.

Particle Geometry

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 application issues.

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 effect.

Incorporation Techniques

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 desired look.

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 uneconomical.

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 aluminum pigment.

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 coatings.

In Summary

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