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Liquid and Powder Coating of Aluminum Extrusions

There are two principle reasons to coat aluminum extrusions. The first one is to control the appearance for purposes of color coordination, uniformity, or visual appeal. Secondly, to protect the substrate from environmental damage due to acid rain, sulfur pollution, salt corrosion, or excess oxidation. The type of finish selected for aluminum extrusions depends upon the use of the product and the preferences of the market. In some instances no coating is required at all, but when there is a requirement liquid paint or powder coating can be applied.

Powder Coating

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 and provide the paint with its color.
  • Additives – serve numerous functions but generally affect fluidization and application properties. Having few volatile organic compounds (VOC), powder is an environmentally friendly coating that serves a growing market.

Powder coating is one of the most influential finishing methods introduced in the past several years. This finishing method differs from liquids in several ways, most notably in the application and cure of the film. Electrostatic application enables positively charged powder particles to adhere to a negatively charged (or grounded) aluminum profile. After the proper amount of powder is applied, the profile is baked in an oven where the powder particles are melted to a liquid state, fusing together to form a homogenous film. This fully functional film has final physical properties that meet many stringent performance criteria while also meeting environmental regulations limiting the use of volatile organic compounds (VOCs) inherent in many solvent-based liquid coatings.

Powder coating is one of the lowest cost finishing methods employed today. While material and equipment costs are similar to those incurred for liquid finishing systems, the savings achieved in energy, labor, production, waste disposal, and regulatory compliance make the total cost of powder coating more cost effective than other methods.

Liquid Coating

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.
  • 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.

Liquid coatings can be classified based on the amount of volume solids they contain. The solid components of liquid coatings are called pigments and binders. The remainder of the composition, the solvents, generally contains volatile organic compounds (VOCs), which are driven off during the curing or baking process. The volume solids form the actual film left on the aluminum extrusion when the VOCs are gone.

Classification of liquid coatings by percent solids is expressed as follows:

Classification      Percent Solids
Conventional Solids Up to 40%
Medium Solids 40 - 55%
High Solids 55 - 70%

Liquid coatings yield a uniform film thickness, resulting in a smooth finish that may be preferred for some applications.

Coating Properties

Although there are significant differences between liquid and powder coatings, both perform two essential functions – cosmetic and protective. It is important, therefore, to look beyond the superficial appearance of the finish and address the physical properties required by the product’s service environment. Consider the following:

  • Hardness
  • Flexibility
  • Mar resistance
  • Chemical resistance
  • Corrosion resistance
  • Chalk resistance
  • Color requirements
  • Initial gloss and gloss retention
  • Protection from color loss due to ultra-violet radiation (e.g., sunlight)
  • Other properties unique to the specific service environments

Coating Performance

Coating Performance Acrylic Polyester Silicon Polyester Fluoropolymer
Application 1 2-3 2 2
Solvent Resistance 2 2 2 1
Chemical Resistance 2 2 2 1
Corrosion Resistance 2 2 2 1
Exterior Durability 3 3 2-3 1
Hardness 2 1 2 2-3
Adheasion 2 2 2 2
Flexibility 2 2-3 2 1
Mar Resistance 1 2 2 3
Color/Glossy Retention 3 3 2-3 1

Ratings: 1=Excellent 2=Good 3=Fair 4=Poor

Cost Drivers

As for all processes and services of manufacturing, there are cost drivers that are included to complete the job. For liquid and powder coating these cost drivers are as follows:

  1. Plugging holes
  2. Masking
  3. Number of paint coatings
  4. Filling
  5. Packaging
  6. Racking
  7. Stripping

Whatever your coating needs, Alexandria Industries has integrated a process that can help you meet or exceed the necessary appearance or protection requirements of your aluminum extruded parts. We have liquid and powder paint capabilities (12’ max), chromate (12’ max), silkscreen, iron phosphate, zinc phosphate, pad printing and non-chrome pretreatments. Along with application capabilities we have resources to test customer specifications using methods such as: cross hatch, over bake, bend testing, impact testing, salt spray, pencil hardness, color, gloss and thickness.

Please contact your sales specialist for additional details on liquid and powder coatings or to talk specifically about other coating options offered by Alexandria Industries.