The powder coating process applies dry, finely ground particles of pigment and resin to surfaces. Unlike liquid coatings, each individual powder particle contains the entire coating formulation (resin, curing agent, catalysts, fillers, colorants, and flow control agents). After the powder material is applied to a part, the part is heated to a temperature hot enough to melt and cure the resin. After cooling, the coating is complete; there is no cure time beyond the cool-down period.
Some powder coatings do not contain liquid solvents, and those that do are very low in volatile organic compounds (VOC). These coatings do not require thinning or mixing. The protective film formed by powder coating is uniform and very durable, which extends the service life of the component being treated. There are potential limiting factors when considering powder coating over liquid paints. Powder coatings are not practical for large components or ship structures. The component to be powder coated must be able to withstand oven temperatures, typically between 275°F and 500°F, in order to cure the coating. The component must be small enough to fit in the curing oven.
Thermoplastic Powder Coatings
Thermoplastics used in powder coating react physically rather than chemically when heated. Thermoplastics are high molecular weight polymers that tend to be applied in thick films. Typical thermoplastic coatings are: polypropylene, nylon, vinyl, polyvinyl chloride (PVC), and polyethylene. Thermoplastic powder coatings are well suited for high performance applications where durability is desired.
Parts to be coated with thermoplastic powder are generally preheated and then dipped in a bed of fluidized powder. Some types of thermoplastics can be sprayed on. In most thermoplastic applications, use of a primer is necessary to achieve sufficient bonding to the substrate. Primers are usually solvent-based and are applied by spraying or dipping. Primer coats must be completely cured before the powder can be applied.
Thermosetting Powder Coatings
When heated, thermoset powder coatings chemically cross-link or polymerize to form a protective film. Cross-links are chemical bonds that link polymer chains together and give the coating its strength. Resins used to make thermosetting powders include epoxy, triglycidyl isocyanurate (TGIC) polyester, polyurethane, or hybrids of these resins. Examples of thermoset powder coatings derived from those resins include epoxies, polyesters, and epoxy-polyester hybrids.
Thermosetting powders are usually applied to parts using an electrostatic spray gun. The powder is given an electrical charge that causes it to be attracted to the part, which is electrically grounded. Primers are generally not used with thermoset powders. Thermosetting powders can be applied in very thin film thicknesses and are used for both decorative and protective purposes
Powder Coating Application
Proper surface preparation is paramount. Inadequate surface preparation leads to poor adhesion, coating defects, and premature failure of the coating system. Per NAVSEA Standard Item (NSI) 009-32, a near white metal blast (NACE 2/SSPC-SP 10) is required. Surface preparation by chemical treatment requires approval by NAVSEA. Powder coatings are applied by fluidized bed, electrostatic spray, or flocking. Fluidized bed and electrostatic spray are the most common methods.
The fluidized bed method involves the use of a large tank or chamber that holds the powder. Dry air is circulated through the powder, giving it a fluid-like texture. The preheated component is dipped into the fluidized powder. The powder melts on contact with the heated surface to form a continuous coating. The fluidized bed is generally used for very thick applications (10–30 mils).
The electrostatic spray method uses a pump to force the powder and air through feed hoses to a spray gun. The spray gun provides a charge to the powder. The charged particles are sprayed onto the component, which is electrically grounded through metallic hangers or hooks. The component is then placed in an oven to cure the powder coating.
On Navy ships, powder coating is not authorized for use on components, covers, or any parts to be installed in potable, reserve feed water, or freshwater drain collecting tanks aboard nuclear powered ships. The use of powder coatings on submarines is restricted. Refer to NSI 009-32 for more information.
Exterior portable bolted louvers intakes and uptakes and watertight doors can be powder coated by the fluidized bed method. The louvers and doors are coated with a MIL-PRF-23236 Type VIIIa powder coat primer followed by a MIL-PRF-24712 TGIC polyester topcoat. NSI 009-32 also specifies single coat powder coating requirements for removable parts in non-immersion service found in both exterior and interior environments, as well as interior wet or immersion application areas. These performance specifications can be viewed or downloaded from the ASSIST Quick Search website.