Powder Coatings are becoming the product of choice among architects, manufacturers, custom coaters, and government agencies, as the benefits of powder coating compared to liquid coatings are better understood. Powder coatings have also evolved to include more stringent verification processes. They are governed by national and international specifications that are widely known and respected, and they offer users a broader range of looks.
For instance, TCI Powder Coatings, a powder coating manufacturer and subsidiary of RPM International, released its powder coating product line called TruAnodize. As part of the company’s TruDurance architectural coatings product line, these powder coatings are designed specifically for architectural applications where the anodized look is specified. The coatings are a smooth, low gloss powder coating applied in one coat with no clear coat required. It comes in six of the most popular anodized finishes and meets the AAMA 2604 performance specification.
A comparison of a spring that was applied with powder vs. liquid coating.
Performance specifications for architectural coatings are generally written with aluminum as the substrate of choice, with a few exceptions for steel. Over time, the long-term performance of properly pretreated and powder-coated aluminum has established a solid track record in terms of performance. These powders, formulated to established specifications, have proven to provide long-term appearance and corrosion resistance.
The use of powder coatings in the architectural market dates back to the 1970s and took hold in Europe when powder coatings were still in their infancy. The polyester powder coatings made it possible for powder to be considered as an alternative to liquid coatings. During this period, a number of architectural projects in France and Germany utilized powder coating on the final product.
The expansion of polyester resin technology, namely stable, low-gloss formulas, provided architectural market opportunities that promoted growth for powder coatings on both sides of the Atlantic. Additionally, the applied cost of powder vs. liquid saw significant decreases as vertical powder coating lines reduced the footprint (and cost) necessary to install powder finishing systems in Europe.
In Europe, the German organization, GSB International, began developing the first performance specifications for powder coating. Several other organizations, including Qualicoat and the BSI (British Standards Institute) quickly followed suit. These organizations developed construction specifications long before powder coatings entered the market.
In the U.S., AAMA (American Architectural Manufacturers Association) developed specifications for the North American market. During the 1980s, the AAMA 603 and 605 and Qualicoat Class 1 and Class 2 specifications gained recognition in North America. These specifications cover physical properties of the cured film as well as corrosion resistance and weathering properties. Updated performance specifications such as AAMA 2603, AAMA 2604, and AAMA 2605, were introduced in the 1990s. These upgraded standards reflected new raw material technologies and improved formulations available to the market such as:
-The introduction of formulas that are free of TGIC (triglycidyl isocyanurate crosslinkers), and their replacement combined with super durable polyester resins that enhanced the weathering performance of powder vs. liquid.
-The introduction of chrome-free pre- treatments improved the already impressive corrosion results seen in powder, and was a win for the environment.
Real world applications
How coatings perform under actual field conditions is the result of numerous factors that combine to either support or undermine the coating’s properties. This is true for both liquid and powder systems. The primary variables that should be considered in selecting a coating system include:
-Quality of the substrate to be coated
-Pretreatment of the substrate
-Application and cure of the coating material
-Maintenance of the finished product in the field
Aside from the coating and application processes, several other factors can impact a coating’s performance in the field. These factors are beyond the control of pretreat suppliers, coatings manufacturers, or applicators. Qualicoat, which is used as the standard for performance specifications on architectural aluminum projects in 39 countries, explains it this way on their website:
“The powder manufacturer guarantees the coating performance of the Qualicoat approved powder. They do not guarantee the pre- treatment or application of the powder. The pre-treatment and application is guaranteed by the applicator. Coatings need to be reasonably maintained to validate the warranty and maintain the finish.”
Factors that impact performance include:
-Part design and configuration
-Assembly processes before and after coatings are applied
-Transportation and storage of the finished product
-Installation of the finished product
-Field touch up
Users of powder coatings, or those considering using them, may have some misconceptions about some products based on their description. One common misconception in the industry is that a powder coating that is identified as “super durable” equates to enhanced corrosion properties. In reality, enhanced corrosion properties are primarily derived from the quality of the substrate and the type of pre-treatment used in the finishing process. What “super durable” coatings do offer is additional color and gloss retention over a longer period of time when compared to a standard durable polyester.
The pretreatment system is critical to the success of any coating system. The right size washer with correct number of stages—cleaning, rinsing, chemical application, final rinse, and seal—comprise the backbone of corrosion protection. Other factors to consider include the length of the stages, the number of headers and nozzles for each stage, and the PSI rating needed to achieve proper impingement for cleaning and rinsing parts.
Another common misconception is that a “super durable” polyester product automatically meets the AAMA 2604 performance specification. This is not necessarily the case. Many raw materials are required to develop a formula that meets the performance criteria of AAMA 2604. In addition to proper resin selection, specific color and filler pigments are necessary to meet the five-year gloss and color requirements of the AAMA 2604 specification.
AAMA has specific performance specifications for powder coatings. It also has a standard for cleaning and maintaining the finish after the project is completed. That specification is AAMA 609 & 610-09. AAMA uses South Florida as its test site for chalking, film integrity, color, and gloss retention performance. This is one of the harshest environments in the North American market. TCI Powder Coatings has an R&D Technology center in Jacksonville, Fla., for beta testing in addition to utilizing the Q-lab facility in Miami.
Application and cure of the coating
Application equipment should be selected after careful consideration is given to the size, configuration, and number of parts that will be processed in production. Booth size, number of guns, type of guns and nozzles, whether or not to reclaim certain colors, appropriately located touch-up stations, air supply, and temperature and humidity control are all important factors to be considered. (For more on equipment, application, tips and more, click here to read TCI’s troubleshooting guide.)
If all the variables related to cleaning and coating parts are considered, then users should expect to see a quality finish. The final piece of the finishing process is the cure of the film.
Parts can be cured by any of several available methods:
-Gas convection ovens, which is the most common method
-Combinations of infrared and convection cure
There are many ways to determine if a part is cured adequately and verify that the oven is operating as required. These test methods include:
-Solvent rub method – ASTM D5402- 06 gives specific instructions for this method, which indicates solvent resistance of a cured coating.
-Physical testing, such as cross-hatch adhesion, provides detailed adhesion results. This destructive test, ASTM D3359-5B, is used by most powder manufactures as the test standard.
-Infrared thermometers and heat tape confirm part temperatures within certain limitations and will give a general idea of the temperatures a part actually is exposed to during the cure process.
-Data loggers, such as Datapaq, provide numerical and graphical representations of the metal and air temperature throughout the curing process. A data logger paints a detailed picture of what the part undergoes in terms of temperature, and how the oven performs through a full cycle. The data can often be surprising. These profiles can show cold spots in the oven, help identify heat loss in entrance and exit areas, and show areas of a part that do not achieve the correct cure temperature.
Most thermoset powders fully develop their performance characteristics after 90% of cure is achieved. Because of this, it is imperative that parts are exposed to the recommended cure temperature for the necessary amount of time to reach full cure. If this is not done, it is possible that the finished product will not have the expected performance characteristics or appearance.
For example, a standard cure time for most polyester powders is ten minutes at 400°F metal temperature. A common misconception is that ten minutes at 400° F means the part itself needs to be at or above 400° F for ten minutes for the powder to fully cure. This is not quite accurate, because this time frame does not include the ramp-up time necessary for the part to go from ambient to cure temperature when entering the oven.
See the attached chart (Figure 1) that gives estimated ramp-up time for various substrate thicknesses.
Figure 1. Estimated ramp-up times for various substrate thicknesses.
Many elements go into the process to produce a powder-coated part that meets the aesthetic and performance requirements of the end customer. If all these steps are designed correctly, and the steps are followed as recommended, the user will produce one of the best-finished products on the market. Remember, the coating process, when done properly with the correct product, makes all the other manufacturing processes look good. If the finished product does not meet appearance requirements such as color and gloss, or does not have the necessary physical properties, the part will be rejected, no matter how well the part was built in the other manufacturing processes.