Powder Coatings
Chemistry and Technology

Powder coatings can be an attractive alternative to conventional coating methods. They are highly efficient, have a relatively low environmental impact and yield very durable and color-stable coatings that effectively resist chipping, scratching and fading. Since no solvents or liquid diluents are present in the process, the powders can be fully recycled and reused which reduces both emissions of volatile organic compounds (VOCs) and waste generated from conventional liquid coating processes.

A typical powder coating consists of a film-forming resin system, pigments, and a variety of additives. This mixture is fed into an extruder, where it is melted, uniformly mixed, extruded, solidified and broken up to small pieces. The resulting granulate is then milled to produce a powder with the desired particle size distribution which is typically in the range of 10 - 100 μ.

The pigmented powder can be applied either by immersing the articles in a fluidized bed of powder, which is kept mobile by passing air through it or, more commonly, by special electrostatic spray-guns. The latter method produces a more uniform coating and allows for thinner layers. To produce the film, the coated articles are placed in an oven where the powder particles are sintered and cross-linked.

Based on the type of resin employed, powder coatings can be classified as thermoplastic and thermosetting. The main components of a thermosetting powder coating are crosslinking resin and curing agent while thermoplastic powder coatings are made up of thermoplastic resin, plasticizer and stabilizer. To both pigments and fillers are typically added. To improve performance, appearance and application properties, small amounts of a number of functional additives are also added. This includes additives that increase or decrease electrostatic attraction, facilitate surface leveling, improve recoatability, create decorative effects, change conductivity, increase surface hardness, control flow, facilitate degassing, and improve UV resistance.

 

The most common thermoplastic powder coating resins are polyethylene, polyamide, polyester, polyvinyl chloride, and polyvinylidene fluoride. Polyethylene powder coatings have excellent corrosion and solvent resistance and good electrical insulation characteristics. They are of relative low cost and are easy to apply. Polyamide (Nylon 11) powder coatings have high mechanical strength and excellent abrasion, wear, and impact resistance and a low coefficient of friction. They also possess good chemical and solvent resistance. Polyvinyl chloride (PVC) powder coatings have good all-round physical and chemical properties including good to excellent solvent, corrosion and impact resistance. They also have high insulation strength of electrostatic spraying. Because of PVC’s rigidity it has to be formulated with plasticizers to achieve sufficient flexibility. Polyvinylidene fluoride (PVDF) has outstanding resistance to degradation and is an excellent choice for outdoor applications. It also has very good abrasion resistance, very low surface friction, and excellent chemical resistance to many corrosive chemicals. Polyester based coatings are known for their excellent adhesion to metal without primer. They provide a high degree of weathering resistance and, thus, are well suited to outdoor use.

The most common thermosetting resins employed in powder coatings are polyester, epoxy, polyurethane and acrylic which are cured with a multitude of hardeners (see table below). The general performance of polyester powders can be categorized between those of epoxy and acrylic powders; their weatherability and chemical resistance is often comparable to that of acrylic powder coatings which is typically very good, and its adhesion properties are similar to epoxy powders. Like acrylic powders, they have good flow properties and often yield coatings with high gloss. Corrosion and chemical resistance are typically good but not as good as those of epoxy powders.

 

 

Acrylic powders can be formulated to have high gloss and good color retention but adhesion to plastics is typically inferior to epoxy and polyester powders. They also possess good weather resistance and color retention, and adhere well to metals and thus are often a good choice for decorative powder coating applications. Epoxy powders have excellent adhesion, hardness and solvent and chemical resistance. They also have good to excellent flexibility and impact resistance. However, they have a tendency to yellow at elevated temperatures and weatherability can be poor. To improve their resistance to overbake yellowing, they are sometimes combined with polyester resins. These hybrid powders have properties similar to those of epoxy powders with somewhat lower chemical and heat resistance. Polyurethane powders provide good all-round physical and chemical properties and good exterior weatherability. However, they are typically softer than polyester and epoxy powder coatings.

The typical ultimate film properties of the four most common thermosetting powders are summarized in the table below. Good weatherability is an important property for outdoor applications. Powder coatings based on aromatic epoxy resins are usually not a good choice, while epoxy functional acrylates or methacrylates are suitable. Epoxy powder coatings, on the other hand, are often a good choice for industrial applications where high heat, corrosions and chemical resistance are required.

 

 

Powder coatings are used across almost all industries including automotive, furniture, household appliances, electrical and architectural. Important automotive applications include aluminum wheels, bumpers, interior and exterior trims, suspension components, running boards, and roof racks. Important outdoor applications for powder coatings include outdoor furniture, bicycle and vehicle frames, tractor components, building facades, as well as windows and door frames. Powder coatings also find increasing use in the furniture industry including kitchen and bathroom fixtures, filing cabinets, book cases, and desks. Important applications in the electrical industry include electrical cabinets, cable trays, and lamp housings. Powder coatings are also applied to many household appliances including microwaves, washing machines, ovens, coolers, and freezers.