Strong on the Surface
Assuring the Durability of Aluminum Finishes
by Dean Lewis
Aluminum is well known for its strength-to-weight ratio, a property that makes it the material of choice for commercial fenestration systems such as curtainwall and storefronts. However, durability is as important an attribute as strength. It is here that coatings and finishes take center stage.
The hard aluminum oxide surface provided by an anodized finish possesses exceptional resistance to corrosion, discoloration and wear.
To ensure quality and durability under long-term outdoor exposure, the standard AAMA 611-14, Voluntary Specification for Anodized Architectural Aluminum, requires the anodized finish to pass tests for color and gloss uniformity, abrasion and corrosion resistance, and resistance to “crazing” (the formation of a network of fine cracks). It must also pass a weathering test, which requires minimal color fading or chalking (formation of a white film) due to sunlight after up to 10 years of exposure (depending on the specified performance level) in a Florida test location.
AAMA 611 defines two levels of quality. The highest, Class I, designates “high performance” finishes that are at least 0.7 mils thick and are used in exterior applications receiving periodic maintenance, such as curtainwalls. The second level, Class II, encompasses finishes between 0.4 and 0.7 mils thick that are used in interior applications or exterior applications receiving regularly scheduled cleaning and maintenance, such as storefronts.
Externally Applied Coatings
Coatings applied externally can be in the form of organic (hydrocarbon-based) paints such as acrylics, polyesters, siliconized polyesters, as well as various types of fluoropolymers.
AAMA standards describe laboratory test methods and performance criteria that address various parameters shown to be critical to coating performance. They are organized in a multi-level (a) basic performance, (b) high-performance and (c) superior performance—i.e., “good, better and best” —sequence of increasingly stringent criteria. This hierarchy allows an architect to choose the best and most cost-effective match for the type and location of the building, as well as its typical weather exposure.
For aluminum profiles, these standards are AAMA 2603, 2604 and 2605.
The entry-level AAMA 2603-compliant finish must be 0.8 mils (20 microns) thick, while those meeting 2604 and 2605 must be 1.2 mils (30 microns) thick.
All three standards test for color uniformity, specular gloss, dry film hardness, film adhesion, impact resistance, chemical resistance, resistance to corrosion caused by humidity and salt spray, and resistance to color fading or deterioration due to weathering.
Coatings for 2603 qualification are subjected to outdoor exposure for one year at a South Florida test site, and then evaluated for color retention, chalking, adhesive loss and change in appearance.
For 2604 “high performance” qualification, the exposure time is lengthened to five years, and then evaluated for the same aspects, plus erosion and at least 30-percent gloss retention.
For 2605 “superior performance” coatings, the weathering exposure time is doubled to 10 years, and evaluated for the same aspects as for 2604, except that at least 50-percent gloss retention is required.
Exposure time to the corrosive influences of high humidity and salt spray also increases across 2603, 2604 and 2605, either by exposure time or severity of the test method applied.
Solar Reflectivity Enhances Energy Efficiency
Advances in coatings technologies for architectural products have led to solar reflective coatings that keep the coated surfaces cooler than they would be with standard coatings.
A supplement to the 2603/2604/2605 series, AAMA 643-13, Voluntary Specification, Performance Requirements and Test Procedures for Solar Reflective Finishes, describes the test procedures and performance requirements for pigmented organic coatings. It prescribes a solar reflectance test per ASTM C1549 and either accelerated outdoor weathering using concentrated natural sunlight per ASTM G90 with nighttime wetting, or three years of actual outdoor weathering at a designated Florida test site.
AAMA 643 will assist the architect, owner and contractor to specify and obtain factory-applied finishes with higher-than-standard Total Solar Reflectance values and still maintain a high level of performance in terms of film integrity, exterior durability and general appearance over a period of many years.
Proper Care Preserves Longevity Aspects
Once the proper finish has been specified, reasonable care must be exercised from the manufacturing plant through installation to maintenance. The former have fairly standard guidelines, but the extent of the latter depends on the conditions at the building location.
Intended for use on anodized or painted architectural products, AAMA 609/610, Cleaning and Maintenance Guide for Architecturally Finished Aluminum, outlines methods, equipment and materials for cleaning finished architectural aluminum after installation and for subsequent periodic maintenance.
Any of the AAMA standards may be obtained by visiting the Publication Store at www.aamanet.org.
Dean Lewis is the educational and technical information manager for the American Architectural Manufacturers Association in Schaumburg, Ill.
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