Volume 33, Number 6, June 1998

Architectural Aluminum

Basic Primer of Architectural Finishes

by Penn M. McClatchey

The purpose of this article is to provide a basic overview of anodizing, painting and powder coatings for architectural aluminum. Architectural aluminum is typically either anodized or coated with what chemists call an “organic coating.” An organic coating is simply a painted coating, applied either in wet or dry (powder-coated) form. Anodized aluminum gained general acceptance first, so we’ll start there.

What is Anodizing?

Anodizing, a simple electrochemical process developed more than 60 years ago, produces a protective coating of aluminum oxide on the surface of aluminum. Anodizing remains popular because it is inexpensive, attractive and has excellent abrasion resistance. Anodizing involves immersing aluminum in a tank of acid and running electricity through it to produce a uniform layer of aluminum oxide.

Aluminum oxide is a hard, durable, weather-resistant substance. Aluminum oxidizes naturally upon exposure to air, but an anodized coating is about 2,000 times as thick as the natural oxide layer. The layer of aluminum oxide produced by anodizing is nevertheless very thin. Architectural anodized coatings typically range from 0.0001- to 0.001-inch thick.

An anodized coating may be colored by dye or may exhibit bronze tones through diffraction phenomena. The coating grows from the base aluminum metal by this electrochemical process. The Achilles heel of anodizing is its chemical resistance. As with other building components, anodized surfaces must be protected from acidic attack during construction.

Advantages of Anodizing

In general, anodizing is less expensive than painting with the exception of coil painted products.
Anodizing is harder than polyvinyledene fluoride (PVDF). Anodizing is better for aluminum in high-traffic areas where the coating is subject to physical abuse and abrasive cleaners.
Anodizing cannot peel off. The coating is actually part of the metal.
Anodizing gives aluminum a deeper, richer metallic appearance than is possible with organic coatings. Anodized coatings are translucent, so the base metal is visible underneath the coating. This translucence contributes to color variation problems, but computerized color matching with quantitative color data is now possible.
Anodizing is unaffected by sunlight. All organic coatings will eventually fail due to exposure to UV light.

The performance of any organic coating (paint or powder) depends on the pretreatment, resin and pigmentation. With aluminum the pretreatment is of utmost importance, which is why organic coatings for aluminum should be factory-applied. Resins are often the weak link in an organic coating system. Some resins have outstanding weatherability, while epoxy coatings are meant only for interior use. There are many resins available for architectural use, such as urethanes, polyurethanes, aliphatic urethanes, polyesters, silicon polyesters, polyester TGICs, PVDF, etc. Only a few of these coating systems will last for more than five years in exterior architectural applications.

Chemical resistance and resistance to UV light are the strengths of PVDF coatings. PVDF has come to dominate the curtainwall and metal roofing markets because of its weatherability and availability in a wide variety of colors.

Advantages of PVDF Coatings

PVDF is relatively chemically inert and will outlast anodizing in corrosive environments.
PVDF coatings are offered in a nearly unlimited selection of colors.
Coil painted sheet is less expensive than anodized sheet and does not craze as badly when fabricated.
Color consistency is usually better than with anodizing. It is surprising, however, how many people do not expect the slight color variations seen in painted products. Metallic coatings are especially prone to color variation. To improve color consistency, all the metal for a project should be painted by one applicator in a single setup.
Window washers can be less discriminating about the types of chemicals they use to clean a building. However, even PVDF can be damaged if corrosive cleaners are used.

Liquid paint is composed of pigment, resin and solvent. Powder paint is simply pigment encapsulated in a powdered resin and is thus often thought of as “paint without the solvent.” Powder coatings and liquid coatings made from the same resin and pigment will have practically the same performance characteristics.

A disadvantage of powder is the large batch sizes typically required. Solvent-borne colors will continue to maintain a market because of the ease with which small batches can be mixed. The ability to mix and match gives painters and their customers unequaled flexibility and ease of use. The powder manufacturers are aware of this problem and a few of them keep colors in stock that they distribute in small batches. Several companies manufacture exterior-grade powders using a polyester TGIC resin. Polyester TGIC powders are available in more stock colors than any other powder, with several companies stocking hundreds of hues.

Some resins are more easily manufactured in liquid coatings, while others are more easily manufactured in powder coatings. Some resins are identified with either powder or solvent-based coatings, but not both. Examples of this are epoxies, which are predominantly powder coatings, and PVDF, which historically has been manufactured as a liquid coating. Many of the perceived advantages of powders, such as hardness and gloss, are characteristics of the resin.

Uses in Storefront Materials

The parts of a storefront that receive the most abrasion from traffic should be anodized. Anodizing’s superior abrasion resistance means it will outlast paint on a door stile or push/pull bar. On the other hand, painting aluminum framing materials above the doors adds a nice accent to a storefront. Sometimes one will see aluminum doors installed in a mill finish to be painted on the job site. This is always a mistake.

The finish of choice depends on the application and is not merely a matter of personal preference. Anodized and polyester coatings are best for storefront and handrails. PVDF coatings are best suited for metal roofing and curtainwall components. The many options available for finishing aluminum make it a popular construction material. The question of which finish to apply is not always an easy decision because of all the options available. In conclusion, the reader should remember to always consult with your finisher.

Penn M. McClatchey is vice president and marketing manager for Southern Aluminum Finishing Co. in Charlotte, NC.

USG