Aluminum is an ideal substrate for the application of architectural finishes. There are many reasons to treat the surface.
Attributes that can be changed by surface treatment include aesthetics, corrosion resistance, wear resistance, emissivity and adhesion of other materials.
The Untreated Surface
Surfaces do not always need treatment after extruding. Load-bearing structures and machine parts are examples of products where the surface quality is satisfactory without any treatment.
Why Coat Aluminum Extrusions?
Aluminum is coated for two primary reasons:
- To control the appearance for color coordination, uniformity or visual appeal and to enhance the substrate.
- To protect the substrate from environmental damage (e.g., acid rain, sulfur pollution, salt corrosion and oxidation).
Aluminum Extrusion Coating Processes
Aluminum extrusions are coated on two types of lines, vertical and horizontal. Both processes offer quality coated products and can handle a variety of shapes and sizes. Both systems share the following basic stages for successful coating applications.
Liquid coatings are fluid materials that, when applied to a surface, form an adhering film to protect and beautify the substrate.
Liquid coatings are composed of three primary ingredients:
- Resin, referred to as binder, is the polymeric substance that forms the film. Binder is the backbone of the coatings system.
- Pigments are the particles that give the coating color, hiding power, gloss control and some corrosion protection.
- Solvents act to fluidize the coating and control application characteristics.
Some liquid coatings have additional ingredients generically referred to as additives. These additives are chemicals present in coatings to give a special effect. They may have either wet or dry film properties. Wet film properties include viscosity, forming, skinning and emulsion stability. Dry film properties include gloss, hiding power, color, strength and hardness. Pigment and binder are the “solids” components that make up the hardened film and provide the required performance characteristics. Solvents normally evaporate entirely after application of the coating. The solvent portions plus minor amounts of chemical reaction by-products, less any water or exempted solvent, comprise the volatile organic compounds (VOCs).
Powder coatings are applied electrostatically from an air fluidized hopper.
The primary powder ingredients are as follows:
- Binders consist of the resin, polymer and crosslinker. This ingredient provides the powder with its fundamental film properties.
- Prime color pigments can be either organic or inorganic.
- Additives serve numerous functions but generally affect fluidization and application properties.
For more information on the performance standards for liquid and powder coatings, please reference AAMA 2603, Voluntary Specification, Performance Requirements and Test Procedures for Pigmented Organic Coatings on Aluminum Extrusions and Panels, AAMA 2604, Voluntary Specification, Performance Requirements and Test Procedures for High Performance Organic Coatings on Aluminum Extrusions and Panels and AAMA 2605, Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels.
What is Anodizing?
Anodizing is an electro-chemical process. Extruded aluminum shapes (profiles) are immersed in an acid solution under controlled conditions. An electrical current passes through the solution to produce an aluminum oxide film on the profiles. This oxide film is extremely hard, durable and resistant to corrosion. It serves as a color anchor in the anodizing process.
Typically, the anodizing process is carried out in a series of tanks, filled with processing chemicals and rinse tanks. Extruded aluminum profiles are cleaned, etched to a desired finish and deoxidized prior to anodizing. In the anodizing step, the thickness of the aluminum oxide is closely controlled to comply with the AAMA 611 specification. After anodizing, many colors can be obtained. Sealing completes the process of developing this true metallic finish. Coloring methods fall into four categories: organic dyeing, inorganic dyeing, electrolytic coloring and electrolytic coloring in combination with organic dyeing.
For more information on the performance standards for anodizing, reference AAMA 611, "Voluntary Specification for Anodized Architectural Aluminum" and AAMA 612, "Voluntary Specification, Performance Requirements, and Test Procedures for Combined Coatings of Anodic Oxide and Transparent Organic Coatings on Architectural Aluminum."