A building’s exterior surfaces experience some of the most rigorous, day-to-day durability challenges. It can be a daunting task to determine which finishes for exterior architectural aluminum will be the best choice to maintain the longest lifespan. The finishing technologies and processes are extremely important to a building’s longevity.
Aluminum provides an ideal finish base for a variety of coatings. In commercial construction, the three types of factory-applied, high performance finishes for aluminum are:
- Polyvinylidene fluoride (PVDF) paint
- Powder coat
- Anodize
All three processes can be securely and permanently applied to buildings’ aluminum components with environmental protection inherent in the finish. The finish choice is most often a combination of personal taste and performance criteria.
PVDF Paint
High-performance 70 percent PVDF coatings, such as Arkema Inc.’s Kynar 500, allowthe building owner and construction team to select nearly any imaginable color, or combination of colors, while protecting the building against weathering, aging and pollution. The leading brands include PPG Duranar, Valspar Fluropon and Akzo Nobel Trinar.
Paints with 70 percent PVDF coatings are available in standard color and mica flake twocoat systems, as well as in three- and fourcoat metallic and exotic color systems. The three- and four-coat systems include a clear, top coat and sometimes a barrier coat.
The clear, top coat is essential to these systems, providing added protection against ultraviolet light, humidity and chemicals. A clear, topcoat is required by the paint manufacturer for bright, exotic colors and for all colors containing metallic flake.
Standard colors and mica two-coat paint systems usually do not require a clear, top coat as this generally will not extend performance or warranty periods. In geographic areas with highly abrasive dust or sand, the addition of a clear coat may be helpful to build additional film thickness.
Paints with 70 percent PVDF coatings are considered to offer the ultimate protection in building performance. Its high-performing resin chemistry promotes rich and vibrant palettes, without losing color and gloss, and while withstanding the rigors of nature and time. The resin system incorporated into the paint determines these specific characteristics and performance properties.
Other coating resin systems include polyester, acrylic, urethane and silicone polyester. With PVDF coatings, the bond created between fluorine and carbon is one of the strongest chemical bonds known and is vital to the fluoropolymer resin system’s effectiveness. Decades of testing demonstrate that PVDF offers the greatest durablity when it contains70 percent of the resin (of the total binder weight). Only 70 percent versions may be sold under the Kynar 500 PVDF trade name.
Fluoropolymer resin systems are used frequently on metal curtainwall systems, windows, skylights, panel systems, storefronts and doors. When selected for exterior building materials, it typically is chosen for construction projects where protection against UV light, fading and other environmental conditions is imperative. Resistance to chalk and fade are two key weatherability characteristics of paints used on buildings’ exteriors.
A building’s color delivers a first impression quickly and more effectively than almost any other architectural component. But a color’s beauty can be short-lived. Sunlight can turn red to pink, deep blue to sky blue and brown to tan. Kynar 500 PVDF resin is transparent to UV rays, and when combined with durable pigments, creates a coating system that protects and prevents color from fading.
Fade is the lightening in color of a pigmented paint due to the effects of light, heat, time, temperature and chemicals. Most coatings break down under the effects of sun, rain and pollutants. Kynar 500 PVDF resists the chemical breakdown that occurs as coatings age.
Paints also exhibit varying degrees of resistance to chalking. This is caused by the degradation of the resin system at the surface, due predominantly to UV exposure. As the resin system breaks down, resin particles along with embedded pigment particles, lose adhesion and take on a whitish appearance. Chalking is measured on a numerical scale—the higher the number, the better the chalk resistance.
Finishes with 70 percent PVDF coatings are shown to exhibit outstanding resistance to humidity, color change, chalk, gloss loss and chemicals. These ensure a long-lasting, durable finish considered to be unparalleled in the industry for painted, metal architectural products.
The performance strengths and limitation of PVDF paint coatings are summarized as follows:
Strengths of Paint:
- Long-life color retention and resistance to fading and chalking
- Exceptional resistance against weathering, aging, pollution and salt-spray
- Limitless color choices
- Field touch-up/repainting capabilities
- Cost-effective, quick-turn for small-batch and custom colors
Limitations of Paint:
- Fair hardness
- Cost of high-performance products
- Potential for inconsistent appearance of “metallic” paints
- Environmentally challenging if the solvent based paints’ applicator does not have a thermal oxidizer to capture and destroy the VOCs emitted during the painting process
Environmental Considerations
The primary environmental concern with liquid paints is the solvents used to deliver the paint to the material. Some of the solvents used are considered volatile organic compounds and this content must be destroyed.
When released directly into the atmosphere, VOC content has been identified as a contributor to ozone depletion. Environmentally-responsible finishers invest in equipment to safely address these concerns, such as a 100 percent air capture system and a regenerative thermal oxidizer to destroy the VOCs and mitigate environmental impact.
Powder Coat
High-performance, architectural-quality, powder coatings offer similar performance as a liquid PVDF coating. It is the same basic process as liquid paint, with the exception of using dry powder, rather than wet paint. A comparable finish can be found in both liquid and powder, whether its resin system is acrylic, polyester or fluoropolymer (PVDF).
The performance strengths and limitation of high-performance, powder coatings are summarized as follows:
Strengths of Powder Coat:
- Long-life protection, while maintaining the structural integrity of the aluminum
- Environmentally friendly
- Hardness and abrasion resistant
- Vast array of color, gloss and texture options
Limitations of Powder Coat:
- Limited and more restrictive warranty than most liquid paints
- Less metallic flake content in the coating
- Difficult touch-up and field repair
- Cost-prohibitive for small-batch and custom colors
- Longer lead times without in-house blending
Environmental Considerations
Because powder coatings lack solvents, they emit no VOCs.
Anodize
Anodize coatings offer one of the hardest, most durable surfaces produced. It is the second hardest substance on earth, second only to the diamond. This characteristic makes anodized aluminum an excellent choice for use in high-traffic areas such as entrances and storefronts, where resistance properties are important.
Unlike a painted coating, anodize highlights aluminum’s metallic appearance. Anodizing is a reacted finish integrated with the underlying aluminum for total bonding. Because it is an integral part of the aluminum, the anodic coating results in a durable surface that is tested and proven to deliver excellent wear with minimal maintenance. It is highly resistant to abrasion, temperature, humidity, warping and the ravages of time.
Architectural Class I anodize, with a minimum mil thickness of 0.7 mils (18 microns), can be a suitable choice for nearly any building in any location. It is considered to possess exceptional resistance to corrosion, discoloration and wear. However, its natural beauty can be marred by harsh chemicals, rough conditions or neglect. These conditions may only affect the surface finish and may not reduce the service life of the aluminum. In contrast, mortar, cement and other alkaline materials will quickly corrode an anodize finish if it is allowed to dry on the metal surface.
The colors seen on anodized, architectural products range from clear to champagne, light to dark bronze tones, and black. Linetec also offers a proprietary, copper-colored anodize that gives the look of rich, real copper without the side effects of salt run-off stains, galvanic corrosion and the formation of patina.
The performance strengths and limitation of Architectural Class I anodize are summarized as follows:
Strengths of Anodize:
- Unmatched adhesion and abrasion resistance
- Accentuated metallic appearance
- Good stability to UV rays; no chipping or peeling; easily repeatable
- Low-maintenance
Limitations of Anodize:
- Limited color selection
- Color variation, depending upon aluminum alloy, temper and shape
- Inability to conceal surface defects
Environmental Considerations
Because anodize lacks solvents, it emits no VOCs.
The U.S. Green Building Council’s LEED Green Rating System allows building teams to seek Innovation category credits for the use of low-VOC exterior coatings, including either powder coat or liquid paint applied with an air capture system. In addition, a few finishers may provide environmentally friendly anodize.
The improved chemistry of this anodize process creates an aesthetically appealing, frostier, matte finish that helps hide small defects, such as extrusion die lines, mild scratches and corrosion, that can occur on the aluminum surface of architectural products. This gives the material a better aesthetic finish on both primary aluminum and secondary(recycled) billet.
Recycled content is recognized within LEED’s credit categories. With the use of ecofriendly anodize, secondary billet can be utilized to its full potential and warranted with confidence that the finish will perform as intended.
At the end-of-life stage in a building,anodized aluminum is 100 percent recyclable, as it can be stripped back to raw aluminum, recycled and reused in building components without any loss in quality.
With so many options available, there is every opportunity to provide finished aluminum products that deliver the desired looks, performance and durability, as well as environmental sensitivity.
Tammy Schroeder is the national marketing specialist for Linetec in Wausau, Wis. With a decade of experience in paint and anodize finishing, Schroeder develops and maintains the company’s American Institute of Architects/Continuing Education System and other educational presentations. For more information, visit www.linetec.com.
