Hi there, a little info on powder coating,

One of our generous sponsors for the Classics in Paradise Car Show is a local powder coating shop . So I thought some information on the powder coating process, advantages and disadvantages might be of interest.

What is powder coating?

Powder coating is by far the youngest of the surface finishing techniques in common use today. It was first used in Australia about 1967.

Powder coating is the technique of applying dry paint to a part. The final cured coating is the same as a 2-pack wet paint. In normal wet painting such as house paints, the solids are in suspension in a liquid carrier, which must evaporate before the solid paint coating is produced.

In powder coating, the powdered paint may be applied by either of two techniques.

• The item is lowered into a fluidised bed of the powder, which may or may not be electrostatically charged, or
• The powdered paint is electrostatically charged and sprayed onto the part.

The part is then placed in an oven and the powder particles melt and coalesce to form a continuous film.

There are two main types of powder available to the surface finisher:

• Thermoplastic powders that will remelt when heated, and
• Thermosetting powders that will not remelt upon reheating. During the curing process (in the oven) a chemical cross-linking reaction is triggered at the curing temperature and it is this chemical reaction which gives the powder coating many of its desirable properties.

General Description

Powder coating uses 100% resin in a dry, powdered form (MnTAP, p. 4). Powder coating works on the principle that opposite charges attract. The powder is pneumatically fed from a reservoir through a spray gun where the powder gains a low amperage, high-voltage positive charge. Parts to be painted are electrically grounded so that the positively charged powder particles are strongly attracted to the parts’ surfaces. The powder-coated part is then pulled through an oven where the powder melts and fuses into a smooth coating (IHWRICe). Substrates must generally be able to withstand temperatures of 260°F or higher (EPAd, p. 33).

Advantages and Disadvantages

Powder-coating materials can provide a high-quality, durable, corrosion-resistant coating. Powder coatings do not produce hazardous overspray wastes or wastewater sludges, and most do not release VOCs when cured (some powder coatings will release VOCs, such as caprolactam, a former HAP). With powder coating, users can collect the powder overspray and reuse it, resulting in transfer efficiencies of up to 99% (MnTAP, p. 4). However, powder coating systems require the complete conversion of a coating line, which can be costly. For more information on other advantages and disadvantages of powder coating, see table 23 at the end of this section.

Table 23. Advantages and Disadvantages of Powder Coatings (NCP2P, p. 3)

Advantages

Disadvantages

Reduces cost due to: no solvent flash required no coatings mix room needed minimal oven length required low ventilation required less floor space required, i.e., system requires two-thirds to three-quarters of wet paint systems VOC and HAP compliant, i.e., no solvents Improves finish quality Improves finish durability Has good corrosion resistance Has coating utilization efficiencies that reach 95-99% Saves energy Requires little operator expertise Has quick “packageability” Has a variety of resins available Has no hazardous overspray, waste sludge or contaminated water Reduces worker exposure to solvent vapros

Has heat requirements that restrict application of powder to metal finishing surfaces Has powder manufacturing limitations: difficult to make small amounts control of texture size and distributiaon limited metallic powder coatings not as attractive as wet metallic finishes Has recirculating system that creates negative pressure in booth Needs gentle air stream to apply powder Enhances Faraday cage effect (VT DEC) Is difficult to achieve thin films below 1.0-1.5 mils May cause powder clumping Is difficult to change colors Needs cool, dry storage area Must pretreat substrate

Types of Powder Coatings

Product manufacturers can specify the properties required in a finish (such as resistance to ultraviolet light, high durability, corrosion resistance and color) to a powder coating manufacturer who then formulates the appropriate powder (IHWRICe). Coating powders are frequently separated into decorative and functional grades; decorative grades generally have a finer particle size than functional grades. Powders are also divided between thermoset and thermoplastic resins (EPA, p. 163-164).

Thermoset resins crosslink to form a permanent film that withstands heat and cannot be remelted. They are used for decorative and protective coatings for architectural structures, on appliances and furniture, and elsewhere. Thermosetting resins are characterized by their excellent adhesion to metal; they are one-coat systems and do not require a primer (Farrell, p. 81). The five basic families of thermoset resins are epoxies, hybrids, urethane polyesters, acrylics and triglycidyl isocyanurate (TGIC) polyesters as described below:

• Epoxies are used for both functional and decorative coatings. Their functional properties include outstanding corrosion resistance and electrical insulation. Decorative epoxies offer attractive finishes that are flexible, tough, and have excellent corrosion resistance and high-impact strength. However, these coatings lack ultraviolet resistance and, therefore, are not recommended for outdoor use. In prolonged exposure to sunlight, they tend to chalk and discolor. Various types of hardeners are used with epoxy powder to optimize its properties.
• Epoxy polyester hybrid coatings are mainly used for decorative applications. They are more resistant to chalking and over-bake yellowing than pure epoxies, but have a lower surface hardness and are less resistant to solvents. They exhibit better transfer efficiency and a greater degree of penetration into recessed areas of a part than other resins.
• Urethane polyesters are formulated with polyester hydroxyl resin combined with blocked isocyanate hardeners. They exhibit outstanding thin film appearance and toughness as well as good weathering properties.
• Acrylic-urethane coatings are formulated with acrylic resins crosslinked with blocked isocyanates. They have excellent color, gloss, hardness, weatherability and chemical resistance, and have an excellent thin film appearance. However, they are less flexible than polyesters.
• TGIC polyesters contain a polyester resin crosslinked with TGIC as a curing agent. They offer very good mechanical properties, impact strength and weather resistance. They are resistant to chalking and are often used for outdoor parts, such as patio furniture, lawn mowers, as well as aluminum extrusions and panels for large commercial buildings. In Europe, reduced occupational-exposure limits were recommended for TGIC powders as a result of in vivo mutagenicity tests (EPAd, p. 28).

Thermoplastic resins form a coating, but do not undergo a change in molecular structure. These resins can be remelted after they have been applied. Thermoplastic powder coatings melt and flow when heat is applied, but retain the same chemical composition when they are cool and solidified (KSBEAP, p. 10). Although some thermoplastic materials provide adhesion to metal, most require a primer (Farrell, p. 81). Thermoplastic resins are mainly used in functional coatings, such as thick, protective coatings on dishwasher trays. Examples of thermoplastic resins used in powder coating are polyethylene, polypropylene, nylon, polyvinyl chloride (PVC), and thermoplastic polyester. These examples are described below:

• Polyethylene provides excellent chemical resistance and outstanding electrical insulation properties. These coatings are smooth, and have a medium gloss and good release properties that allow sticky materials to be cleaned from their surfaces. These are often used as coatings for laboratory equipment.
• Polypropylene produces a surface that is very inert and is often used in applications where the part that is powder coated might be exposed to chemicals.
• Nylon offers excellent abrasion, wear and impact resistance, and a low coefficient of friction. Nylon is commonly used as a mechanical coating for sliding and rotating bearing applications in appliances, farm equipment and textile machinery.
• PVC provides good durability as well as flexibility; dishwasher trays are an example of a product coated with PVC.
• Thermoplastic polyester offers good exterior durability and weatherability. The coating does not usually require a primer for good adhesion to most metals. These materials are often used on outdoor metal furniture (EPAd, p. 26 and PCI, p. 6-7).

Share on Facebook Sphere: Related Content