March 9, 2018
by Ben Mitchell and Gabriel Morales-Sada
Irrespective of the type of coating (e.g. powder or liquid) or the surface of application (e.g. extrusions, louvers, or metal coil), architectural coatings can be formulated with similar chemistries and performance characteristics. Why, then, would considerations such as color selection and matching be dependent
upon which coating application method was specified?
The answer lies in the fact that finish color is dependent upon good coverage as well as good technique on the applicator’s part. Variations in equipment, or even different settings and adjustments on the same equipment, can influence the way a color appears. To ensure multiple architectural elements within a given building envelope have consistent appearances, specifiers should coordinate with the coating supplier to manage pigmentation and color standards across various batches and application methods. Differences in final finish color can be apparent at the time of installation, or they can affect a building’s appearance after a few years of weathering. If variables are controlled during coating production and application, fewer inconsistencies appear as a result of exposure.
Understanding electrostatic coating application
Most extrusion and panel coatings are applied in a factory setting using electrostatic guns or sprayers. These guns are based on a familiar principle of physics—like forces repel and unlike forces attract. Coating particles (in the case of liquids, atomized droplets) are given a charge within the gun. A
high-voltage power supply provides a controlled level of electrons (negative charge) in the form of DC current. These electrons charge the coating through direct contact with conductive components in the atomizer or ionization field. When they leave the gun, the charged coating particles are attracted by the grounded metal substrate. Optimal grounding is provided by a rod driven directly into the ground and connected to the part, or to the part’s holding fixture, with a heavy gauge wire (Figure 1).
An ionization field takes the shortest path between the source of the charge and the grounded part. This attraction is strong enough to overcome the force of gravity, as well as any velocity a slight overspray may impart to the coating. Therefore, most of the sprayed coating ends up on the intended surface, making the system very efficient.
Achieving high transfer efficiency—defined as the ratio between the amount of coating adhering to a part and the amount of coating sprayed—results in consistent point coverage with little waste or recycle volumes. It also increases facility throughput. However, many factors affect the performance of this basic system. They must be understood and addressed during the coating application process.
The needs of state-of-the-art coatings
Many of today’s popular coating colors are grays or silvers, which lend a natural metal appearance to the substrate. These coatings contain embedded mica or metallic flakes. Metallic flakes reflect light according to the physics of specular reflection (i.e. light striking the flake reflects at the same angle as the incident) and have been commonly used in the industry for many years. Since metallic coatings need a clear spread in order to protect the aluminum flakes from moisture and ultraviolet (UV) light, mica particles have become a more popular choice. Mica flakes bend light, refracting it like a prism. They do not degrade when exposed to moisture and therefore do not require a clear coat, resulting in savings in product and application costs.
The multitude of angles at which light is reflected from a surface that contains mica flakes results in a scintillating, sparkling effect. Thus, a coating’s percentage of mica flakes, as well as the orientation of those flakes, makes a difference in how the color and sparkle of a finished surface are perceived. This means matching mica coatings on panels, extrusions, and other components can prove challenging.
Favorable results can be achieved, however, by following the proper order of production. There are differences in feasibility of color adjustment between powder, liquid, and coil applications. Adjusting the color is easier when working with liquid coatings because they can hold a greater percentage of mica flakes. Therefore, architectural components requiring powder coating should be produced first, and then matched in liquid. Additional variables are introduced in the coil coating process, which involves high line speeds and a lower film thickness than that of extrusions. This makes coil-coated metal more limited when using translucent pigments requiring higher film thicknesses. Further, when using a coating with mica flakes, the velocity of the coil coating process imparts a horizontal orientation to the flakes, which makes the final composition more directional and less random than it is in other applications. Like the powder-coated components, coil components should be produced before liquid-coated ones to take advantage of liquid’s adaptability.
Mica flakes are susceptible to breakage in certain types of guns. While some spray guns have an electrode at the tip to generate negative ions, others use an electrostatic rotary bell, also called a rotary atomizer. A key component of the rotary bell gun is the bell cup (a funnel-shaped attachment located at the end of a turbine). Coating material is fed into the rear of the bell and centrifugal forces then pull it toward the bell’s wider, open end. A mechanical shearing action breaks the coating into atomized droplets.
Traditional liquid paints perform well when applied using a rotary bell; they break into droplets of similar size and produce uniform coverage on the coated surface. However, coatings containing mica or metallic flakes do not fare as well. The breakage—a normal part of a bell’s atomizing process—fragments the flakes and alters the way they are oriented in the final finish. This, in turn, disrupts the flakes’ ability to reflect light, resulting in a perceived lightening (or darkening) of the finished surface.
Intermediary steps of color matching between components is the solution to this issue, just as it is when matching powder-, liquid-, and coil-applied coatings. Coating manufacturers should avoid running large, single batches to fulfill an order. Instead, their batches should be coater-specific or coil manufacturer-specific.
While electrostatic coating processes rely on the laws of physics, certain forces can impede product transfer and evenness of coating coverage. For example, negatively charged particles are attracted to the closest grounded surface, which has the benefit of both attracting the coating to a part’s edges and reducing overspray. However, this same tendency can make it difficult to achieve even coverage in the deep, recessed areas commonly seen in aluminum extrusion profiles. Particles are attracted to the prominent edges of the profile, experiencing what is known as the Faraday cage effect, and can create a thick layer of coating prone to blisters.
Reducing the fan pattern of the spray can direct the ionization field toward the recessed area. If a part is being hand sprayed, the technician can move closer to the workpiece. Gun target distance should be kept within 203 to 305 mm (8 to 12 in.) from the part being coated. As distance decreases, the operator needs to reduce the fluid or air pressure—or in some cases, both—to avoid applying too much coating. Further reducing a gun’s voltage allows more of the coating to pass by the sharp edges of the extrusion and penetrate its recessed areas. To achieve optimum performance, gun voltage must be between 50 and 70 kV for powder coatings. Liquid coatings are often applied at 60 to 80 kV. For both liquid and powder formulations, manual adjustment during the coating process accomplishes better coverage in areas of extrusions experiencing the Faraday cage effect.
Another consideration is the size of a coating’s particles. The amount of charge a particle carries is directly related to its surface area. If a large droplet of liquid paint breaks into smaller droplets during the application process, those smaller droplets will have a combined surface area that is greater than the surface area of the original, larger drop. Having a greater total surface area for the same volume means the coating is carrying a greater electric charge and will be affected more by the attractive forces created between positive and negative charges. This means smaller droplets have a greater affinity for edges
and sharp corners and have less ability to penetrate recesses. Equipment operators can reduce fluid and air pressures to create larger droplets. Reducing the voltage on the coating gun also produces larger droplet sizes and enables the momentum generated by equipment to drive the coating into the recessed corners. It is not uncommon to adjust both voltage and atomization to provide the best coverage.
Every liquid coating needs a balance of true active solvents and diluents. Solvents commonly reduce the raw coating by 25 to 30 percent. Additionally, the solvents employed must be both polar and nonpolar and also have a range of evaporation rates. Varying solvents’ evaporation rates and their conductivity manipulates the way in which particles charge and discharge as they travel along the path to their target. (Typically, for proper transfer, the coating should have resistivity that ranges anywhere between 0.5 to 1.5 megaohms.) It is critical the tail solvent—the last solvent to evaporate out of the coating—be an active one for the resin system being used, because it must perform the function of dissolving the ‘skin’ (the partially crosslinked coating surface). If an inactive solvent is the last to evaporate out of a coating, blisters or ‘solvent pops’ will form. Temperature and humidity should also inform the ratio of fast- and slow-evaporating solvents being used. Additionally, volatile organic compound (VOC) restrictions or caps in the location where a material is being coated may influence solvent selection and volume.
Powder coatings must have the correct average of large and small particles for optimal transfer efficiency. If over-sprayed powder is being reclaimed and reused, virgin powder needs to be added to the hopper at a rate calculated to maintain the specified particle size average balance. Monitoring this balance during the application process is key.
Temperature and humidity can also affect powder particles. The optimal temperature range is 20 to 27 C (68 to 80 F)—too much heat causes particles to prematurely cure or fuse. Humidity should range between 47 and 55 percent. High humidity can cause clumping of particles. To correct this, airflow into the hopper can be increased or a prep stage can be added to the process to break up the clumps.
Grounding and maintenance
Proper maintenance of an extrusion coating line’s racking system is critical. Coating buildup on hangers, hooks, and carts has an insulating effect and prevents proper grounding of the parts. An ungrounded part builds a negative charge, repelling the coating being applied and thereby reducing transfer efficiency, as well as causing low or uneven film coverage. Having a buildup of coating on the rack system is also hazardous. A coating needs to ‘bleed’ its electrical charge as soon as it hits the part it is targeting. If the charge cannot be bled, sparks may occur and potentially result in a flash fire.
The pathway a coating particle follows toward a substrate is predictable, but the final surface finish on architectural elements may not be if careful oversight of the coating process is not observed. Coatings that are applied using different methods—whether on coil lines, by hand spraying at low shear, or via turbo bells at high shear—vary in their transfer properties. Further, mica or metallic particles orient in different directions within the finished coating layer.
Ionization fields, coating ingredients, and other factors can be adjusted to direct a coating to its intended target and to ensure even coverage. Applicators should work with their sales or tech service representatives to determine the best application techniques for a given substrate. Designers can support this process by ensuring the coating supplier is supervising color application across all building components and is performing color matching at strategic points during the entire process. For example, if a given project requires a coating to appear uniform between various building elements, the supplier should ensure coil-coated materials are produced first because the application is not flexible in terms of adjusting color. Liquid coatings can then be produced and have their color adjusted during the spray application process to accomplish the best match in the final finish.
Ben Mitchell is the architectural aluminum coatings manager for AkzoNobel, a global paints and coatings company. He has a bachelor’s degree in comprehensive science as well as an MBA from Urbana University in Ohio. Mitchell started at AkzoNobel in 1990 as a lab chemist formulating polyvinylidene fluoride (PVDF) coatings, then moved into product management. He can be reached at firstname.lastname@example.org.
Gabriel Morales-Sada resides in San Diego, California, and serves as architecture powder coatings marketing manager for AkzoNobel Paints and Coatings. Morales-Sada served as regional director for AkzoNobel and other global coatings companies from 1996 to 2014. He has gained strong market depth and technical expertise in ‘shop-applied’ architectural coatings. Morales-Sada earned his bachelor’s degree in business administration at the Universidad Regiomontana A.C. in Mexico. He can be reached at email@example.com.
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