Design considerations for metallic finishes on architectural aluminum

by sadia_badhon | March 12, 2020 3:48 pm

by Tammy Schroeder

[1]

Metallic architectural paint coatings offer a vibrant, visual appeal on buildings. When applied to architectural aluminum products, the resulting finish provides a lively shimmer or sparkle. This is produced by the way visible light reflects off the metallic flakes contained within the coating. Understanding metallic coatings’ optical dynamics, composition, application, as well as performance will allow designers and specifiers to achieve the desired appearance and intended durability for their projects.

Aesthetic appeal and typical applications

Architectural aluminum building products’ painted finishes protect the metal and enhance aesthetics. Similarly, when used on the exterior, these products enclose the building to shelter occupants from the weather and define the structure’s architecture.

Most often, metallic coatings are selected for exterior building products such as wall panel systems, canopies, and sun shades. Curtain walls, windows, doors, and entrances—also popular applications for metallic coatings—have both exterior and interior surfaces, which allow for dual finish possibilities when thermal strut is utilized. Different colors or finish formulations can be applied to each surface to match a room’s interior decor, while maintaining a consistent exterior palette and climate-focused performance.

[2]

Large-scale, high-profile projects featuring metallic coatings include healthcare facilities, entertainment and sports venues, university buildings, transit centers, and corporate campuses. Smaller projects can also make a memorable impression with metallic coatings applied on an aluminum feature wall, main entry, or decorative accent.

With the exception of bright and exotic colors, nearly any hues can be created in a metallic paint. However, darker shades do not provide as much sparkle and bling as the lighter ones. Aluminum flake is used to create the popular sparkle. These metallic flakes can generate different looks, but the results are largely dependent on the paint production process, the paint line application techniques, and the finished material’s final installation.

Optical dynamics

The light reflectance of the metallic flakes within the coating gives these finishes their lively appearance. The size and reflectivity of the flakes in the coating, their orientation, and the degree to which they are all oriented in the paint coating will affect the brightness of the color. This is sometimes referred to as the ‘flop of the flake.’

Like tiny mirrors, the flakes that lie horizontal in the paint will reflect the most light. In an optimally applied metallic coating, the flakes are randomly oriented. This will achieve the greatest level of sparkle and depth.

If all the flakes in the coating were to lie horizontally, the maximum amount of light would be reflected. The surface would look very bright to the point of appearing almost solid rather than sparkling. If the metallic flake content is too high, the shimmer is also diminished and again, it will appear to be solid color paint rather than a metallic.

Taken to another extreme, if all the metallic flakes were to lie vertically in the paint, very little light will be reflected. The surface would appear very dark and possibly eliminate the coating’s intended brilliance.

Finish composition and application

The paint composition and application play a key role in achieving the preferred optical dynamics of a consistently random flake orientation in metallic coatings.

Generally, a metallic paint consists of three layers: The first layer is a primer coat, necessary to assist with paint adhesion to the metal. The second layer is the resin, pigment, and metallic flake that provide the color and sparkle. The third and top layer is a clear coat, which is required with metallic finishes in order to impart the ultraviolet (UV) and weathering performance characteristics and to prevent the metallic flake from oxidizing.

KAISER PERMANENTE SAN DIEGO MEDICAL CENTER

[3] The Leadership in Energy and Environmental Design (LEED) Platinum-certified Kaiser Permanente San Diego Medical Center, California, features an exterior aluminum panel system with mica and metallic color finishes. Clean, modern, soothing—the visual impact of mica and metallic color finishes on the exterior of the Kaiser Permanente San Diego Medical Center in California reflects the hospital’s philosophy of high-tech healing in a patient-centered, sustainable environment. Opened in April 2017, it is one of few healthcare facilities in the world to achieve Leadership in Energy and Environmental Design (LEED) Platinum certification through the U.S. Green Building Council (USGBC). Kaiser Permanente worked had closely with the design team of CO Architects and general contractor (GC) Hensel Phelps to create a medical center focusing on innovative care. The interior and exterior designs of the 57,321-m2 (617,000-sf), seven-story building is reminiscent of San Diego’s coastal colors and climate. Bringing the design from concept to reality, the glazing contractor Architectural Glass & Aluminum selected products with durable metallic and mica coatings. For Level 1, the 3-mm (1/8-in.) aluminum plate panel system, balcony plate panels, interior curtain wall closer panels, and column covers were painted in a platinum mica sunstorm color. For all the other levels on the medical center, the curtain wall panels were finished in a light blue metallic color. These finishes add shimmer and sparkle under the California sun. The color can seem to change with the way the visible light is reflected off the surface, depending on the intensity and angle of the light. Protecting against ultraviolet (UV) damage and unwanted color change, both of the paint color formulations use 70 percent fluoropolymer resin-based coatings. These offer the highest performance finish for architectural aluminum products, providing outstanding resistance to humidity, chalk, gloss loss, chemicals, and salt spray. Supporting environmentally sound practices, the high-performance painted coatings were applied in a controlled facility, contributing to enhanced durability for long-lasting materials and improved indoor air quality (IAQ) for healthier buildings.

It is recommended that a professional finisher apply the metallic paints under climate-controlled conditions where the process and quality is closely monitored. There are two main application methods used: coil coating and spray coating.

Coil coating is used for painting high quantities of aluminum in coil form that can be later cut into sheet form. With this process, continuous rolls of sheet are passed through a rolled application of paint. The uniform geometry of the flat sheets and the spray distance of the paint remain constant, so there is very little opportunity for variation in the coating application.

Spray coating either by hand or by automatically controlled equipment is used for painting almost all aluminum parts other than rolled coil aluminum. The differing sizes and geometries of brake metal, extrusion profiles, and flat sheet parts are just some of the numerous variables that make it more difficult to apply a perfectly random flake orientation.

Other aspects that affect flake orientation include the following.

Paint lines

Different types of paint lines used by the applicator will affect the outcome of the painted finish. Paint application lines can be automated or manual, horizontal or vertical, continuous or batch. Any variance in the application can greatly affect the positioning of the metallic flakes.

Shapes of parts

Different shapes of parts mean different painting distances from a part’s surface. Most spray-applied, architectural aluminum does not have a totally flat surface.

Application equipment

Different application equipment will cause various types of flake distortion. A cup gun will minimally affect the flakes whereas a mini bell at a high rotation will cause the greatest distortion. The distorted or bent flakes will reflect light in a scattered ray for an inconsistent appearance.

Pressures, atomization, and human elements

Different pressures, atomization, and human elements of supplemental hand-spraying with manual guns often will produce finishes that look different than an area sprayed with the accuracy of automated equipment. Some amount of hand-spraying almost always is required to reach recessed or angled areas on extrusions or formed metal.

Electrostatic attraction

Different levels of electrostatic attraction within or between parts can occur. Even without electrostatic painting equipment, electrostatic/grounding effects are present.

Solvent types

Different solvent types are used by different applicators to blend the paint. Faster evaporating solvents orient flakes differently than slower evaporating ones.

Coverage

Different coverage should be anticipated for overlapping spray areas because these have twice the flake content as other areas. The initial layer of flakes will be disturbed and re-oriented by the re-application of paint on the overlapped area.

[4]

Managing visible variation

With all of the variables to control in spray-applied metallic coatings, the reality is some inconsistencies will result. The first step in managing and minimizing visible variation is to recognize and mitigate for the greatest risks.

Be aware the same qualities that create the vibrancy and reflectivity of metallic coatings can also cause variations in appearance within a project, an elevation, or even a single part of a metal building product. A project with lots of aluminum wall panels is more likely to show variation than one with extrusions, such as on window or curtain wall framing, or one with panels separated by masonry or other materials that disrupt the overall visual continuity of a building’s exterior. It is pertinent to always paint the same elevations and adjacent panels on the same application run.

It is recommended to specify all the paint be made in a single batch, especially when working on a large project or one with multiple phases. Similarly, when possible, it is important to indicate a single finisher should apply all the paint to ensure the greatest consistency and quality control. This will minimize the potential variation between different paint applicators and equipment.

Panels painted in a factory setting can measure within specification and appear visibly to match panel to panel on the paint line, but when installed, they can be perceived as non-matching under natural light conditions. The difference in actual versus perceived color is known as metamerism. Remember that sunlight and shade show color differently, and that metamerism will be experienced differently throughout the day and year. The reflective properties of the metallic flakes means that the same painted surface will look different depending on the viewing angle, the angle of the sun or other light source, and the intensity and type of the light source.

[5]

Metamerism can also present challenges when creating and viewing samples. A complete color range of sample variations is not possible with these paints since the ‘color space’ is a three-dimensional sphere and there are an infinite number of directions a color can range. The ‘L-scale’ is one of the three axis in the Lab color sphere and the one that best measures brightness. For most realistic metallic paint samples, it is advisable to review a sample in the L-scale. Also, it is recommended to request color chips from the applicator rather than the paint manufacturer. The finisher’s application equipment can result in a slightly different color or appearance relative to the paint manufacturer’s laboratory-produced samples.

Some metallic colors have a greater propensity for visible variation. This can be particularly noticeable with bright, semi-transparent colors such as silver metallics. When bright silver metallic coatings are specified, it is important to consult with the finisher to avoid unpleasant surprises and discuss alternatives such as clear anodize. When a uniform aluminum alloy is specified, an anodize finish can enhance the metallic aesthetic and may have less visible variation than the flop of the flake phenomenon seen with painted metallic coatings.

As another, economical alternative to metallic coatings, mica paint coatings provide a similar appearance with slightly less brilliance or depth. These two-coat systems are manufactured with a non-conductive mineral rather than with conductive metal. Due to their inert formulation, they eliminate the need for a clear coat, are less sensitive to flake orientation, and minimize visible variation improving the overall color consistency.

[6]

Performance specifications

The industry-leading specification guidelines for applying architectural coatings and anodize finishes are published by the American Architectural Manufacturers Association (AAMA).

For painted metallic coatings on exterior aluminum building products, AAMA 2605, Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels (with Coil Coating Appendix) and 2604, Voluntary Specification, Performance Requirements and Test Procedures for High Performance Organic Coatings on Aluminum Extrusions and Panels (with Coil Coating Appendix) are the appropriate performance specifications.

AAMA 2605

This is the high-performance exterior specification. A paint that meets this specification would be a 70-percent fluoropolymer resin-based coating. These finishes exhibit outstanding resistance to humidity, salt spray, color change, chalk, gloss loss, and chemicals. An application for this finish would include monumental architectural projects and ones that require long-lasting aluminum surfaces.

[7]

AAMA 2604

This is an ‘intermediate’ specification. A paint meeting this specification would be a 50-percent fluoropolymer resin-based coating. An application for this paint would be storefronts, doors, or other high-traffic areas. This finish will provide good color and gloss retention. It will also provide adequate hardness and abrasion resistance.

For color uniformity of solid paint colors, both performance specifications recommend a maximum deviation of 2 Delta E from an agreed upon color standard. Delta E is the standard unit of measure for color variation measured typically with a digital spectaphotometer. With metallic coatings, it can be challenging for this tool to register a proper color reading. For this reason, AAMA specifications note paints containing mica and metallic flake reflect and scatter light in random patterns, therefore, exact color uniformity should not be expected. Color shifting should be anticipated when viewing from varying angles and distances.

[8]

Per the AAMA-recommended guide specifications, coatings should be visibly free from flow lines, streaks, blisters, or any other surface imperfections in the dry-film state on exposed surfaces when observed at a distance of 3 m (10 ft) from the metal surface and inspected at an angle of 90 degrees to the surface. For applicators following AAMA guidelines, the quality assurance team inspects material as it exits the paint line at a distance of 3 m, looking for objectionable defects to the surface of the part, such as blisters, pops, paint sags, inconsistent coverage, etc.

Due to the metallic coatings’ translucent nature, having to rework panels will result in heavier paint thickness and will alter the appearance and color of the panel, causing it to look different than the other panels on the project. For small defects, it might be in the best interest of the building team to choose aesthetic uniformity over reworked, noticeable inconsistencies. These small defects seen at 3 m will not affect the performance of the coating.

Metallic painted aluminum panels can also appear to have a different color if they are not all installed on the same plane, or are allowed to billow or to oil can. Panels could measure an identical Delta-E color match, but will appear completely different due to the slight variance in installation.

[9]

To inform and reassure the entire building team about the expectations of these finished products, consider advising the owner, architect, general contractor (GC), and installer that some degree of variation is acceptable and should be expected.

Additional considerations

Supporting projects with sustainability goals, metallic coatings can be applied to recycled aluminum. At the end of its useful life on the building, the finished aluminum can be recycled or mechanically or chemically stripped and reused.

Factory-applied finishes also can contribute to the project’s indoor air quality (IAQ) and the installing contractors’ wellness because the volatile organic compounds (VOCs) associated with some liquid coatings’ solvents have dissipated prior to reaching the jobsite. Environmentally responsible finishers safely capture and destroy the VOCs in their facilities. Some of these finishers use a 100 percent air capture system and destroy the VOCs with a regenerative thermal oxidizer, so there is no adverse environmental impact.

Installers should also be advised that most metallic coatings are not intended for field touch-up. If the finished material is reworked in the field, a small blemish can become a larger issue. Touch-ups also add thickness to the paint, which can change the perceived color and reflection.

Architectural aluminum building products finished to AAMA’s performance guidelines with metallic coatings can remain durable and attractive for many years. Some finishers offer warranties up to 20 years.

[10]Tammy Schroeder, LEED GA, is the marketing manager at Linetec, an independent architectural metals finishing company. With 20 years of experience in the finishing industry, she serves as an industry educator on high-quality, high-performance architectural coatings and services. She enjoys sharing her knowledge with architects, specifiers, and architectural product manufacturers working in commercial and residential building markets. Schroeder can be reached at tammy.schroeder@linetec.com[11].

Source URL: https://www.constructionspecifier.com/design-considerations-for-metallic-finishes-on-architectural-aluminum/

---