By Ben Mitchell, Manager, Extrusion Coatings, AkzoNobel
Perhaps no building element typifies contemporary architecture more than the curtain wall. Lightweight and flexible, curtain walls provide the practical benefit of allowing more usable square footage within a building. They also epitomize upscale, modern construction.
Aluminum, prized for its durability, recyclability and strength, is the most popular framing material for curtain walls. Aluminum and glass both have long service lives and can be expected to last throughout the life of the main (structural) building materials. However, the overall wall system could be compromised by suboptimal performance of its other constituents. This means that considerations such as the building finish become crucial. Designers should not gamble by choosing a low-quality finish. Coatings that meet the highest standards, as defined by third-party organizations, should be used on structures with large expanses of structural glazed systems. Additional considerations, such as environmental concerns, ease of use and color requirements, should be carefully weighed during the design process so that a client’s needs can be met and the long-term success of the building can be ensured.
Choosing a Finish
A traditional finish for aluminum is anodization. Anodizing is not a true coating—it is a process that increases the thickness of the naturally occurring oxide layer by electrochemical means. The anodic film of aluminum oxide acts as a protective layer. While anodized surfaces are cost-effective, offer a very hard surface that is mar- and scratch-resistant and have a popular metallic look, anodizing has several drawbacks:
- Color options are limited.
- Cannot be field-repaired.
- No cut edge protection.
- Poor mortar and chemical resistance.
- Color variations in the metal (as a result of processing) cause variation in the finished color and appearance.
- Environmental drawbacks
- Water-intensive, using millions of gallons a year.
- Production requires the use of corrosive acids and chemicals.
- The extremely high electricity consumption that is typically generated by carbon fuels results in a large carbon footprint.
What is Paint?
Architectural coatings offer a good way to protect an aluminum substrate. But a thorough understanding of available options is critical.
The rising popularity of powder coatings has introduced some confusion among specifiers. The first question typically asked is whether powder or liquid is “best,” and which is most cost-effective. The answer is not a simple one, as there are several factors to consider.
Cured paint film is comprised of two principal ingredients: resin and pigment. Regardless of whether a coating is applied as a liquid or powder, it’s the resin and pigment quality (and not the coating’s application method) that determines the weather resistance and durability properties desirable in an architectural finish. Resin gives the film its adhesion to the substrate, chemical resistance, gloss and other film properties. Pigment durability confers color stability. The difference in coating formulations is that solvent is added to liquid coatings to allow smooth application, while powder has no solvent. Powder coatings are typically applied as a single coat but at a higher-cured film thickness while liquid coatings are at a lower film and many times include a primer.
A common mistake is to specify merely a powder or a liquid coating. But this basic description does not denote a specific product or quality, because different coatings are comprised of different resin types and different chemistries. Similarly, specifying a coating by brand name can limit your options, as it restricts choices to whatever is offered by an individual company.
To achieve the desired performance level for a given application, coatings must be specified to meet third-party industry standards. The American Architectural Manufacturers Association (AAMA) is a national trade association that establishes voluntary standards for the fenestration industry, including specification, performance requirements and testing procedures for pigmented organic coatings on aluminum extrusions and panels. AAMA standards 2603, 2604 and 2605 are the most commonly referenced coating standards for painted fenestration products, with the highest-performance coatings meeting AAMA 2605 standards.
- To meet AAMA 2603, a coating is tested for South Florida weathering for only one year, whether in liquid or powder form. These coatings are normally acrylic or polyester systems and are used in residential or interior applications.
- AAMA 2604 performance testing requires a five-year South Florida exposure. Coatings meeting these requirements have a lower cost. An example of an AAMA 2604 liquid offering is the new-generation silicone-modified polyester, which has been the workhorse system in the metal building market with proven performance. Powder coatings meeting the AAMA 2604 specification are super durable polyesters.
- AAMA 2605 performance testing requires a 10-year South Florida exposure. Long-standing options meeting this performance level include the following standard products: polyvinylidene fluoride (PVDF), polyvinyl difluoride (PVF2) and fluoroethylene vinyl ether (FEVE).
When powder coatings were first introduced into the architectural market, they were heralded as a superior product and were expected to quickly replace liquid paint. The coatings were gaining visibility at a time when LEED certification was also in the spotlight and volatile organic compounds (VOCs) were often the only criterion being used to measure a coating’s success. Their biggest selling point was the fact that powder is VOC-free, whereas the solvent in liquid paint does contain VOCs. But an investment in pollution-control systems for liquid-applied coatings has helped even the playing field between liquid and powder coatings. Painted extrusions must be cured by baking, and it is during this process that solvents—and VOCs—in liquid paint escape into the environment. VOC-abatement systems capture more than 98 percent of VOC emissions from the liquid paint line and send them to a regenerative thermal oxidizer (RTO), where the VOCs are destroyed. An added benefit is that the heat from this process is captured and used for curing. This helps to lower the amount of carbon fuels required to cure the surface—a process that requires temperatures of 450 degrees F.
In order for paint to be in compliance with the AAMA 2605 standard, a pretreatment is required (neither liquid nor powder coatings will adhere to untreated aluminum). The type of pretreatment used, then, also becomes an environmental consideration. Traditionally, pretreatment consists of a chromate (hexavalent) conversion coating. Since chrome is a heavy metal that is toxic to people and the environment, its use is highly regulated and many manufacturers have turned to chrome-free pretreatments. These pretreatments meet the performance standard of AAMA 2605; however, they have limited field-performance history, so some still do not have full confidence in these systems.
Another key to the continued popularity of liquid paint is that it allows for mixing at the coater’s site, making it suitable for just-in-time production and providing the ability to remix excess paint into a new color, thereby avoiding waste. Additionally, in the commercial architectural market, bright mica or metallic colors are very popular. Liquid paints can use a higher level of mica/metallic to produce brighter colors. A clear coat containing some of these pigments can be added for additional sparkle, especially in sunlight.
Benefits associated with powder coatings include the fact that powders can be reclaimed and used, resulting in 95 percent transfer efficiency. Powders are very scratch- and mar-resistant, making them ideal for interior or railing type applications.
Formulas and application methods for architectural metal coatings continue to evolve. Most companies now offer both liquid and powder options that meet the same AAMA standards. The key, therefore, when specifying a coating, is to not jump too quickly to an assumption about which application method or brand is best. Specifiers should evaluate all of the options, balancing the costs and benefits of each, based on a project’s individual requirements.