Technology Interchange—Powders with Pizzazz: A Look at Special Effects
Posted on Monday, July 22, 2019
Special effect is a rather broad term with vast applications from Hollywood to
coatings. For our purposes, let’s define it as powder coatings that elicit a unique aesthetic which includes speckle finish, metallic, pearlescent, textures, wrinkles and the uniquely powder-based veins and hammertones.
One often overlooked characteristic of powder coatings that sets them apart from liquid paint is that they are comprised of discreet fine particles that remain individual particles as they are being sprayed. Unlike most liquid paints, mixing two colors doesn’t result in an intermediate color, but a blend of individual particles, commonly known as a speckle. For example, mixing a red will yield an orange paint. With powder Physically blending two or more powders will produce a variegated finish. For example, mixing an orange, medium brown and a dark brown simulates a rust look. Blends of different color large particle textured powder can give the effect seen on roofing shingles. Mixing a light teal green with a black powder will give a weathered copper patina.
Speckle finishes can provide a highly attractive look to an item; however, achieving consistency in appearance can be a challenge for the powder applicator. Fluidization of the blend of powders can sometimes cause segregation and stratification of particles. Reclamation and recycling of speckle powders is often problematic as the overspray can look significantly different than the original powder coating. It is therefore common to use speckle powders in a “spray-to-waste” process which does not utilize the over sprayed powder coating.
One of the more popular powder coating looks is metallics. Metallics range from a bright sparkle finish to a muted reflective veneer reminiscent of the surface of a buffed 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. Aluminum pigments are most commonly used to create metallics and 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. Cross-sectional analysis shows that the majority of 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 creating more of a sparkle effect. Non-leafing aluminum pigments are coated with oleic acid, which acts as a lubricant during manufacture. Oleic acid is sufficiently compatible with most of the polymers used in powder coatings and therefore creates an even dispersion of flakes throughout the powder film.
Aluminum flakes are most commonly used to create powder finishes with a metallic appearance; however, bronze and stainless-steel pigments can be used to create fascinating effects. Metallic flakes can be dry-blended at low intensity into a base powder coating (one that has already been produced). Concentrations of 0.2 to 5.0 percent are typical. Formulators use surface-treated types of aluminum that ensure even dispersion and reasonable 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, the reclamation and reuse of dry-blended powder coatings is usually impractical as it leads to different concentrations of aluminum pigment. Nevertheless, dry-blended metallic powder coatings are regularly used in “spray-to-waste” finishing processes where the recycling of overspray is deemed uneconomical. Long ago, 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, thereby 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 seriously consider using a bonded product.
Some metallic effect powder coatings are susceptible to chemical attack, fingerprinting and oxidation and may require a clear topcoat to enhance durability. It is wise to confer with your powder coating supplier regarding whether a clear topcoat is recommended to ensure expected
performance of your metallic finish.
This outlet cover is coated with a silver sparkle finish.
Pearlescent pigments are specially synthesized flakes that give a shimmering effect. Initial pearlescents were generated from ground up seashells. Nowadays, pearlescents are produced by applying a thin layer of colorant pigment to mica flakes. Mica, a natural mineral, has a platy morphology (particle shape) which results in flakes that can be used to create a characteristic glistening effect. Similar to metallic pigments, pearlescents come in a variety of particle sizes that provide a range of effects from sparkle to a fine veneer. These can be incorporated just like metallic pigments. Variations of pearls include color shifting flakes that appear as one color at a given viewing angle and another color at different angle. Often pearlescents are used in combination with aluminum pigments to achieve a desired appearance.
Glow-in-the-dark or photoluminescent powder coatings are created by blending a very specific pigment into a clear powder coating base. Photoluminescence is defined as light emission from a material that has absorbed photons. Consequently, the pigment used to create glow-in-the-dark absorbs incident light and remits photons that are visible in the absence of light. The most common pigment used is a doped strontium aluminate. The doping agent is typically comprised of europium and dysprosium oxides. (Look them up on the periodic table of elements – they really exist.) Like many special effect powder coatings, glow-in-the-dark types rely on the dry mixture of the photoluminescent pigment into an already manufactured powder coating. Care must be taken with the application of these products and the reclaim and reuse of overspray can create variable results. In addition, it is recommended to apply a bright white basecoat before applying the glow-in-the-dark powder to ensure optimal brightness and photoluminescent performance.
Fluorescent powder coatings are a cousin to the glow-in-the-dark powders. With fluorescence, a material absorbs relatively short wavelength light (e.g. UV) and reflects longer wavelength light in the visible range. Fluorescent powder coatings are typically bright yellow, orange or lime green and can be used for safety items or just to elicit more attention from the consumer. Like glow-in-the-dark powder coatings, fluorescents look brighter and cleaner when applied over a white basecoat.
Textured finishes represent a very common special effect in powder coatings. These can range from a very fine texture, sometimes referred to as a “mini-tex,” to intermediate textures and up to a “sand texture,” which is commonly used as an anti-skid coating for stair treads and truck steps. Textured powder coatings are typically formulated with large non-melting particles that disrupt the surface of the powder finish. The size and concentration of the non-melting particles dictates the degree of texture in the final film. Large, loopy textures that exhibit a characteristic “orange peel” appearance are usually generated by adjusting the melt viscosity of the powder coating binder to reduce flow and leveling. The particle size of textured powder coatings is critical in achieving the desired surface profile. Incorporating over sprayed powder can be done; however, caution should be exercised to ensure an even, consistent texture.
Veins and Hammertones
Veins and hammertone powder coatings exhibit a unique large-wavelength, deep texture unachievable with traditional liquid paints. Most common among these special effect powders are the antique vein look finishes which combine the large-wavelength surface with a metallic veneer atop the ridges of the pronounced orange peel. Hammertone powder finishes exhibit the same surface profile as a “vein” without the metallic veneer.
Creating veins and hammertones is tricky business for the powder formulator. This technique requires a precise blend of incompatible materials that cause macro-cratering, albeit without exposing the substrate. If too low a concentration is incorporated, objectionable fish-eye craters
result, whereas using too high a concentration creates an unacceptable shallow surface topography. Consequently, the powder producer must very carefully control the addition of the incompatible agent.
This Dutch barn door was coated by Stallworks, LLC with an
The unmistakable surface profile of a wrinkle finish powder can be found on a number of iconic consumer products including toolboxes, business equipment and aftermarket car parts. This effect is created through a rather complex chemical reaction. In the powder coating world there are two formulation types that produce a wrinkle finish. The most common relies on polyester chemistry that utilizes a unique crosslinker and a specialized catalyst to drive the curing reaction. Since the development of the wrinkle is chemistry driven, it is requisite to ensure adequate oven conditions and even the state of the substrate surface in order to obtain a consistent, even wrinkle profile.If high chemical resistance is needed in a wrinkle finish, an epoxy wrinkle powder is recommended. The epoxy binder system provides a tougher coating compared to its polyester counterpart. Please be advised that although this epoxy product is more resistant to chemical attack, it performs poorly when exposed to sunlight. The epoxy wrinkle is also generated by a unique chemical reaction; therefore, it is wise to ensure proper curing conditions to achieve best results.
One of the most stunning special effects is created by incorporating microscopic prismatic flakes to produce a holographic effect. Small particle size flakes comprised of multiple layers of film orient in a manner that yields a rainbow of colors as the observer alters the viewing angle of the surface. Most dramatic effects are realized with darker, more vibrant colors. These holographic flakes are rather expensive but produce an amazing and distinctive special effect at a fairly low concentration (about 1.0 percent).
A woodgrain, or really any other graphic image associated with powder coatings, is actually not an effect directly attributable to the powder coating itself. High resolution woodgrain powder finishes are developed by dye sublimation of a printed image into a powder coated film. The printed image resides in a paper-like medium and is transferred into the surface of a powder coating with the application of heat and pressure. The dye pattern in the medium passes out of the medium and directly into the powder coated surface.
Not only can ever-popular woodgrain effects be generated, but an almost endless array of images such as marbling, granite, herringbone,camouflage, leopard skin, denim and company logos can be created. Anything that can be printed into an image can be transferred via this process into a durable powder coating.
This fan was coated by a powder coating hobbyist with a brown
This dome is a textured multi-polyester with granite finish.
Special effect powder coatings can enhance the appearance of a myriad of consumer and industrial products. They can be as mundane as a simple black texture or as exotic as a color-shifting holographic finish. Innovative formulators continue to push the boundaries of the aesthetics possible in powder coating finishes so keep in touch with your powder supplier for emerging product lines.
Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.