Although versions of the powder coating process were being experimented with prior to 1950, fluidized bed powder coating did not gain much traction until the 1960s. The popularity of spraying the powder coating with a gun that electrically charged the powder coating particles become popular in the 1960s and 1970s as well. Both methods are still in use and provide some excellent advantages over other coating processes for suitable applications. (Related reading: Coatings Advances: Nanoparticle Technology and Cold Sprays.)
How Powder Coatings are Applied
One of the most popular methods used to apply powder coatings is electrostatic gun spraying. During this process, the spray gun not only propels the electrostatic paint, but gives them a positive electrical charge as well. The base material is charged negatively, thus creating an attraction between it and the negatively charged powder coating particles. Once the attraction between the two is established, both are heated until the powder coating particles melt and adhere to the base material. The base material is then allowed to cool and a durable powder coating is created.
Another common method to apply a powder coating is the fluidized bed method. To perform a fluidized bed operation, a base material is heated. Once the base material reaches a specified temperature, it is immersed in or fed through an aerated bath or bed of powder coating particles. The heat from the base material causes the powder coating particles to melt and adhere to its surface. The base material is then allowed to cool and the coating solidifies.
Safety Advantages of Powder Coatings
One of the most important advantages of powder coating materials over liquid coating materials is that they can be much safer for the operator. Applying a powder coating typically requires an operator to wear a dust mask and safety goggles. However, many powder particle types do not emit any fumes. This means that as long as they are not being accelerated, they are relatively harmless if they are not inhaled or ingested. This is very different from paints that have volatile organic compounds (VOCs) or other solvents in them, because these chemicals can cause serious respiratory and neurological damage if inhaled excessively. (For more on safety considerations, read Writing Safety Into Your Coating Specification.)
Liquid coatings can cause irritation or chemical burns if they contact the skin. Furthermore, these solvents can be quite flammable, resulting in the risk of explosion or burns. Powders do not cause burns when placed in contact with skin, and powders almost always do not require the use of flammable solvents. (To learn more about what goes into a coating, read The Composition of a Paint Coating.)
Efficiency of Powder Coatings
Powder coating processes are usually more efficient than other coating methods for several different reasons. One reason is that there is less overspray. This is because the electrically charged substrate has a negative polarity and the electrically charged powder coating particles have a positive polarity, thus creating an attraction between the two. This reduction in overspray means that less coating material is required.
Another benefit is that it usually requires less time for the operator to spray the substrate. Adding to the efficiency of the powder coating process is the magnitude of the coating thickness that can be applied in one spraying. Coatings such as liquid paint spraying require the first coat to be applied, then waiting for the dry-to-recoat time and cure time, then another coat, and so on until the desired thickness is achieved. (Discover the details in the article When is a Paint Dry?) With powder coatings, greater coating thicknesses can be achieved in one application and curing cycle. The time that was spent waiting for curing and applying a subsequent coat can be spent coating another batch of parts or another portion of the structure.
Quality of Powder Coatings
Powder coatings have a high quality finish. During the powder coating application, the charged powder coating particles automatically level themselves to a certain degree. This reduces the risk of uneven coatings that can easily occur when applying a liquid coating. Furthermore, powder coatings are not susceptible to running or dripping. This also decreases the possibility of uneven coatings.
As mentioned previously, powder coatings require fewer layers than many liquid coating systems. This helps prevent the possibility of a powder coating having defects between layers.
Mechanical Properties of Powder Coatings
Powder coatings also have advantages with regard to their mechanical properties. Powder coatings are known for being ductile enough to bend and yield with many of the base materials to which they are adhered. Many powder coatings, such as epoxies, have excellent impact strength as well, which lessens the chance of chipping upon collision with other materials. Many other coating types are prone to this type of failure mode.
Powder Coatings are Better for the Environment
Many coating materials (other than powder coatings) contain harsh chemicals. Solvents, such as VOCs, can create lasting damage to the environment in which they are released and can cause ozone to form when exposed to nitrogen and oxygen. This also promotes the formation of particulates in the air, creating smog that can cause damage not only to humans but to the surrounding wildlife as well. Furthermore, because paint is applied wet, it is possible for these VOCs to run off, escape the job site, be absorbed by the ground, and eventually find their way into streams, lakes and other water sources thus creating a serious threat to the environment. Because powder coatings do not contain VOCs, this risk is not present when they are used.
Additionally, powder coatings are less likely to loosen or chip away from the substrate that they were applied to compared to other types of coatings. This in turn reduces the risk of solid particles contaminating the environment into which the base material and coating were placed.