There are four basic powder coating processes: electrostatic spraying, fluidized bed, electrostatic fluidized bed, and flame spray. Electrostatic spraying is the most commonly used powder application method. For all application methods, surface preparation (i.e., cleaning and conversion coating) is required to develop a good coating adhesion substrate. Characteristics of the four different powder coating application techniques are described below.

In electrostatic spraying, an electrical charge is applied to the dry powder particles while the component to be painted is electrically grounded. The charged powder and grounded workpiece create an electrostatic field that pulls the paint particles to the workpiece. The coating deposited on the workpiece retains its charge, which holds the powder to the workpiece. The coated workpiece is placed in a curing oven, where the paint particles are melted onto the surface and the charge is dissipated.

In a fluidized bed, powder particles are kept in suspension by an air stream. A preheated workpiece is placed in the fluidized bed where the powder particles coming in contact with the workpiece melt and adhere to its surface. Coating thickness is dependent on the temperature and heat capacity of the workpiece and its residence time in the bed. Post heating is generally not required when applying thermoplastic powder coatings. However, post heating is required to cure thermoset powder coatings completely.

Electrostatic fluidized beds are similar in design to conventional fluidized beds, but the air stream is electrically charged as it enters the bed. The ionized air charges the powder particles as they move upward in the bed, forming a cloud of charged particles. The grounded workpiece is covered by the charged particles as it enters the chamber. No preheating of the workpiece is required. However, curing of the coating is necessary. This technology is most suitable for coating small objects with simple geometry.

The flame-spray technique was recently developed for application of thermoplastic powder coatings. The thermoplastic powder is fluidized by compressed air and fed into a flame gun where it is injected through a flame of propane, melting the powder. The molten coating particles are deposited on the workpiece, forming a film on solidification. Since no direct heating of the workpiece is required, this technique is suitable for applying coatings to most substrates. Metal, wood, rubber, and masonry can be coated successfully using this technique. This technology is also suitable for coating large or permanently-fixed objects.

The choice of powders is dependent on the end-use application and desired properties. Typically, powders are individually formulated to meet specific finishing needs. Nevertheless, powder coatings fall into two basic categories: thermoplastic and thermosetting. The choice is application dependent. Generally, thermoplastic powders are more suitable for thicker coatings, providing increased durability, while thermosetting powders are often used when comparatively thin coatings are desired, such as decorative coatings. The principal resins used in thermoplastic powders are polyethylene, polyvinyl, nylon, and fluoropolymer. Thermosetting powders use primarily epoxy, polyester, and acrylic resins.

Powder coating virtually eliminates waste streams associated with conventional painting techniques. These waste streams include air emissions, waste streams generated from air emission control equipment, and spent cleaning solvents. Powder coating also greatly reduces employee exposure, and liabilities associated with liquid coating (wet solvent) use. Because the powder is dry when sprayed, any overspray can be readily retrieved and recycled regardless of the complexity of the system This results in shorter cleanup times. In all cases, the dry powder is separated from the air stream by various vacuum and filtering methods and returned to a feed hopper for reuse. Powder efficiency (powder particles reaching the intended surface) approaches 100 percent. Other advantages over conventional spray painting include greater durability; improved corrosion resistance; and elimination of drips, runs, and bubbles.

Powder coatings are somewhat limited in their application to aerospace equipment. They typically are not used with primer systems that inhibit corrosion, but they can be successfully applied over many primed and pretreated metal substrates. If primers or pretreatments are not used, the powder coating provides protection as a barrier and prevents corrosion only as long as it is intact and undamaged. The temperatures required to cure the coating are too high for many materials used in aerospace structures (primarily aluminum); however, recently developed formulations allow baking as low as 250 degrees F which enables the use of powder coating on most materials. Powder coating can be implemented in high-production facilities with highly automated application systems or on low volume, manual batch applications. The Air Force installed the first DoD high production automated powder coating facility at Kelly AFB in 1997. In addition, there is currently an initiative to use powder coatings on aircraft landing gear at Hill AFB.

• Elimination of volatile organic compounds (VOCs) used as solvents in paints, which are associated with the formation of smog typically regulated by federal and state agencies as well as local air pollution control districts.
• Elimination of hazardous air pollutants (HAPs) used as solvents in paints, which are regulated by federal, state, and local regulations including the National Emissions Standards for Hazardous Air Pollutants (NESHAPs) (40 CFR 63).
• Elimination of all SARA Title III reporting substances from coating process (by the Navy) (40 CFR 300, 355, 370, and 372).
• Reduced generation of coating waste and cleaning solvents that may need to be handled and disposed as hazardous waste under 40 CFR 260 and related sections.
• Reduced hazardous materials usage as required of federal facilities by Executive Order (EO) 13148, Greening the Government Through Leadership in Environmental Management.
• Reduced occupational exposures that are regulated under 29 CFR 1910.

Compliance benefits include: 1) reduction or elimination of recordkeeping and reporting requirements under the Title V Operating Permits Program, NESHAP Program and SARA programs; 2) reduced administrative burden associated with hazardous waste (i.e., tracking, plans, reports, training); and 3) reduced administrative burden associated with OSHA (i.e., training and recordkeeping).

The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

As with all coatings, powders require a well prepared, clean surface for optimum adhesion and protection performance. Primers and pretreatments are not necessary, but additional surface preparation (for example, iron phosphate for steel, zinc phosphate for galvanized or steel substrates, and chrome phosphate for aluminum substrates) will enhance the performance of powder coatings.

Inhalation of the powders should be avoided, as this can cause irritation to the lungs and mucous membranes. Proper personal protective equipment should be used.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing this technology.

• Eliminates air emissions, waste streams generated from air emission control equipment and spent cleaning solvents
• Allows overspray to be easily recycled for reuse.
• Offers transfer efficiencies approaching 100%.
• Reduces energy requirements resulting from recirculation of powder coating spray booth air.
• Provides a superior finish, greater durability, improved corrosion resistance, and eliminates drips, runs and bubbles.
• Has lower raw material (powder) costs.
• Reduces clean up time (no extra solvent required for equipment maintenance).
• Lower turnaround time (powder to cure is considerably less than solvent paint system).

• Booth environment must be controlled to eliminate explosion hazards (accumulation of suspended particulate).
• System configurations are somewhat application specific, but not severely limited.
• Certain application limitations - such as intricate shapes and assembled components.

Assumptions:

• 2,500,000 ft² painted annually.
• Cost of convention coating: $11/gal.
• Cost of powder coating: $2.35/lb.
• Surface area covered by conventional coating: 250 ft²/gal.
• Surface area covered by powder coating: 96 ft²/lb.
• Labor required for conventional coating: 5,000 hrs/year (500 ft²). This labor includes preparation time and clean up costs.
• Powder coating system will provide a 20% saving in labor, as a result of no paint mixing requirements and reduced clean up costs.
• Labor rate: $45/hr.
• Waste generated: 33 x 55 gal drums of waste sludge generated by the conventional coating system. 3 x 55 gal drums of waste powder generated by the powder coating system.
• Disposal cost: $300/drum.

Annual Operating Cost Comparison for Powder Coating System and Conventional Coating System

Economic Analysis Summary:

• Annual Savings for Powder Coating System: $103,402
  
• Capital Cost for Diversion Equipment/Process: $145,000
  
• Payback Period for Investment in Equipment/Process: < 1.5 Years

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INNOTEK Powder Coatings
3400 West Seventh Street
Big Spring, TX 79720
Phone: (800) 753-5263
FAX: (915) 267-1318
Service: Coating Manufacturer

Diagram of Powder Coating Painting System - Navy Environmental Quality Initiative (EQI)