| POWDER COATING
PAINTING SYSTEM CH3(CH2)2CHOH; plumbum, Pb; ethyl methyl ketone, 2-butanone, MEK, C6H4(CH3)2 Revision: 9/96 Process Code: Navy and Marine Corps: ID-05-03, ID-05-04; Air Force: PA01; Army: PNT Usage List: Navy: Medium; Marine Corps: Medium; Army: Medium; Air Force: Low Substitute For: Conventional Solvent-based, Waterborne, or High Solids Painting Applicable EPCRA Targeted Constituents: Acetone (CAS: 67-64-1), n-Butanol (CAS: 71-36-3), Lead (CAS: 7439-92-1), Methyl Ethyl Ketone (CAS: 78-93-3), Toluene (CAS: 108-88-3), and Xylenes (CAS: 1330-20-7) Overview: Powder coating paint systems, also referred to as "dry painting",
eliminates volatile organic compounds (VOCs), hazardous air pollutants
(HAPs), and solvents, and produces superior surface finish. 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 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 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.
From Misev, Tosko A. 1991. Powder Coatings: Chemistry and
Technology, Table 6.4, p. 350. 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 vinyl, 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. Materials Compatibility: Only workpieces that can be oven heated are suitable for coating by
electrostatic, fluidized bed, and electrostatic fluidized bed application
methods. Hence, these technologies are most appropriate for relatively
small, metal objects. The flame-spray method allows powder coatings to be
applied to other substrates such as wood, rubber, and plastic, and to
large or stationary structures. Safety and Health: Powder and air mixtures can be a fire hazard when an ignition source is
introduced. The concentration of powder in air must be controlled to
maintain a safe working environment. Despite the absence of flammable
solvents, any finely divided organic material, such as dust or powder, can
form an explosive mixture in air. This is normally controlled by
maintaining proper air velocity across face openings in the spray booth.
In the dust collector, where it is difficult to maintain the powder
concentration below the lower explosive limit, either a suppression system
or a pressure relief device must be considered. Inhalation of the powders should be avoided, as this can cause
irritation to the lungs and mucous membranes. Proper personal protection
equipment (PPE) should be used. Consult your local Industrial Health specialist, your local health and
safety personnel, and the appropriate MSDS prior to implementing any of
this technology. Benefits:
Disadvantages:
Economic Analysis: The following economic data and basis was provided by the Powder
Coating Institute. It compares a convention solvent based coating system
to a powder coating system. The conventional system includes 2 water wash
spray booths. Assumptions:
Powder Coating System and Conventional Coating System
Economic Analysis Summary
Approval Authority: Navy: Contact NAVAIR Code 530 (Ref. R 182002Z) for further approval for
use on aircraft and aircraft components. Phone is (703) 692-6025; DSN
222-6025. This recommendation should be implemented only after engineering
approval has been granted by cognizant authority. Points of Contact: Powder Coating Institute Scott Mauro Vendors: The following is a list of powder coating system manufacturers. This is
not meant to be a complete list, as there may be other manufacturers of
this type of equipment. Coating Manufacturers: Cardinal Industrial Finishes EVTECH Farboil Company Plastic FlamecoatŪ Systems, Inc. Pratt & Lambert Inc. Equipment Manufacturers: Nordson Corp. Sames Electrostatic, Inc. Gema Source(s): Mr. Jeff Palmer, Powder Coating Institute,
Alexandria, VA 22314 |