A Primer for Aluminum Spray Coatings
by Carl Wagus
Among the reasons why architects might choose aluminum windows or doors is the materialís flexibility. It can be finished in any long-lasting color to enhance aesthetics as well as provide protection against environmental damage.
The most significant and widely-used applied finishes for architectural aluminum are carbon-based organic coatings. These must be formulated, applied and tested to ensure that they provide adequate protection against their major enemies, which include ultraviolet radiation, moisture, temperature, atmospheric pollutants and physical damage.
Preparation and Pretreatment of Aluminum
To realize the full protective potential of any coating, the metal must be cleaned and deoxidized properly, then treated chemically to convert the surface to an inert film to which the subsequent coating will firmly bond.
The importance of proper pretreatment cannot be too strongly emphasized. It should take place immediately prior to application of the coating with no intermediate handling or time lag, subject to strict quality control.
Most architectural applications require a seven-stage cleaning and pretreatment process as depicted in the following table. When mild conditions of soil and oxide film are encountered, the processing may be reduced to five steps, eliminating steps three and four.
The Painting Process
The demand for faster production rates has led to the widespread adoption of electrostatic spraying. This process may use air pressure, hydraulic (airless) pressure or centrifugal force (turbo disc) to change the liquid stream into a fine spray or mist. Then, each particle of the spray is charged electrically by exposing it to a strong electrical potential of 60,000 to 120,000 volts. Because each particle of the coating is similarly charged, the particles repel each other strongly, enhancing atomization. Grounding the conveyor line bearing the object to be coated causes the charged particles to be attracted to the grounded object. Liquid that would be wasted in conventional spraying is pulled through the air and around corners to all exposed areas. Note, however, that when electrically charged particles approach a complex extruded shape, they deposit more heavily on projecting edges or beads. A deep channel or recess becomes coated on the three exterior sides but receives relatively little coating on the interior surfaces. In most cases manual touch-up spraying of such inaccessible areas is required. To ensure a uniform coating at minimal cost, profiles should be designed with such limitations in mind, avoiding deep recesses in exposed faces. Freshly-sprayed finishes are cured by baking them at a specified time and temperature (typically 450įF or higher for high-performance coatings) to ensure that the coating performance characteristics are achieved. This should be followed by an appropriate cool-down time before further processing.
Types of Coatings
The term organic coatings includes everything from an inexpensive brush-on form of bituminous coatings, paints and shellacs to the most sophisticated factory-applied baked-on synthetic coatings. For exterior finishes on
aluminum fenestration products, organic coatings typically are composed of some form of carbon-based polymer, formulated with an appropriate pigment to add color and certain additives to enhance performance in the job-site environment. Organic coatings for aluminum include acrylics, polyesters, and various types of fluoropolymers.
All of these coatings have the following qualifications in common: Good adhesion to properly prepared aluminum surfaces; and a wide range of colors, with reliable color uniformity.
Additionally, the 70 percent fluoropolymers offer excellent resistance to fading and chalking.
The fluorocarbon polymers (e.g., Kynarô and Hylarô, recognized as high-performance premium coatings, are among the most stable resins known and have excellent resistance to most forms of environmental stress. Fluoro-polymer coatings also have high resistance to chemicals, and their unique release properties make them easy to maintain. They are resistant to abrasion and impact, and have a very low rate of erosion.
AAMA specifications designate various performance levels of factory-applied painted finishes. It is recommended that the architect specify finishes based on the AAMA designation and call out a specific color, rather than to simply state that the color is to be selected by the architect.
Carl Wagus serves as technical director for the American Architectural Manufacturers Association in Schaumburg, Ill.
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