Say What? Powder Coating Nomenclature- Application Terminology
Posted on Monday, June 1, 2015
Words, they’re just words; but if we don’t all agree, confusion and mistakes are inevitable. Today’s Technology Interchange is a primer on powder coating nomenclature.
We’ll start with the term “Powder Coating.” Ah, you’re probably chuckling and think, “I know that one.” However, these two words undoubtedly mean different things to different people.
Powder Coating—The term “powder coating” refers to a material as well as a process. As a material, it is defined as a mass of particulates that are intended to be applied to a substrate to form a coating. Powder coatings can be organic or inorganic in nature.
Some debate has been proffered that the term “powder coating” syntactically refers to a coating that has been applied to powder and the material should be labeled “coating powder” instead of “powder coating.” Perhaps English language purists would follow this convention, however the industry has adopted and used the term “powder coating,” and we should respect this traditional moniker.
Inorganic powder coatings are vitreous or ceramic in nature and can be applied electrostatically to substrates and subsequently fired to affect melt and film formation. For most of us we recognize inorganic powder coatings as a separate technology and hence are not part of our industry.
Organic powder coatings are particles comprised of a mixture of polymers, additives and optionally pigments, extenders, curing agents and other components. Organic powder coatings can be thermoplastic or thermosetting in nature. Thermoplastic powders contain polymers that melt and flow at elevated temperatures to form films. These polymers do not chemically react and will re-melt and flow when reheated.
Thermosetting powder coatings are based on relatively low molecular weight solid polymers that chemically react when exposed to sufficient heat. These resins possess chemical groups that either react with themselves (homopolymerization or self-cure) or with other chemical groups (crosslinking). Thermoset reactions occur during the bake or stoving cycle that the coating undergoes after it has been applied to a substrate and exposed to a heat source. The chemical reaction is essentially irreversible, that is, under normal conditions the polymerization remains intact. The thermosetting reaction produces a very high molecular weight polymeric matrix which provides the ultimate performance of the coating.
Powder coating also describes the process in which a powder coating is applied. This terminology can be used in all appropriate tenses, e.g., to powder coat a chair, powder coated ware, etc.
Now that we have established what a powder is, let’s review powder coating application terminology.
Application—The process of coating a substrate with a powder coating.
Substrate—The characteristic material of an item to be coated. Steel, aluminum alloys and some plastics are substrates commonly powder coated.
Electrostatic—Using ions in a process. Powder coatings are typically electrostatically applied to a substrate by introducing negative ions onto the powder particle surface by the corona charging of air. Alternately, powder particles can be positively charged by frictional means and is known as tribo-charging.
Corona Charging—Breaking down oxygen molecules with high voltage to create negative ions (free electrons). Corona charging is used to apply electrons to powder particle surfaces, which enables attraction to a grounded conductive substrate.
Tribo-Charging—Transferring electrons from one material to another through contact. Also known as frictional charging this process typically imparts a net positive charge on powder coating particles.
Handling Properties—Performance relating to the physical transport of powder coatings. This includes transportation of the bulk product and also the preparation and transport of powder through application equipment.
Fluidity—The ability of a mass of powder to incorporate air (or other gas) between particles to enhance handling properties. Fluidity allows a powder coating to be transported through application equipment (spray guns and hoses).
Fluidized Bed—A vessel that introduces air (or other gas) to a mass of powder coating. The air is typically introduced through a semi-porous plate at the bottom of the vessel and aerates the powder particles. Also known as a fluidized hopper.
Powder Pump—A device that pneumatically attracts powder from a hopper into a small chamber and redirects it to transport hose. Powder pumps typically use a venturi to retrieve powder from a fluidized hopper and direct it through a hose to a spray gun.
Transfer Efficiency—The ratio of powder coating applied to the part versus the amount sprayed.
Application System Efficiency—The sum total of powder applied to parts versus powder coating used. This takes into account loss from powder applied to hooks, waste powder from contamination or excessive recycling.
Reclaim—The process of capturing over spray powder coating and reusing it. Also known as recycling.
Sintering—When powder particles soften and stick together.
Storage Stability—The ability of a powder coating to remain intact during transport and storage. Physical storage refers to the ability to resist softening and fusing of particles. Chemical storage stability refers to the ability of thermosetting powder coatings to avoid premature chemical reaction during storage and transport.
Impact Fusion—Powder particles adhering and accumulating in application equipment due to contact of particles on interior surfaces (typically spray guns, hoses and powder pumps).
Pretreatment—A process to enhance the surface characteristics of a substrate to be powder coated. Pretreatment refers to a process as well as materials and can involve many process stages such as cleaning, rinsing and chemical conversion.
Blasting—Impinging a medium (metal oxide grit, plastic beads, etc.) onto a substrate to clean and roughen a substrate prior to powder coating.
Now that you’re armed with a new found appreciation for the jargon and technical terminology for powder coat and its application, you can rest assured that we’re all on the same page. Next time we’ll cover another fascinating aspect of powder coating nomenclature.
Kevin Biller is technical editor of Powder Coated Tough and the president of The Powder Coating Research Group. He can be reached at email@example.com