Introduction
Industries are searching for coating systems that contain a lower amount of organic solvents. A variety of approaches are used to achieve this. The switch from solvent-based to water-based systems has been made in several applications. Also, liquid systems that contain less solvent (high-solids coatings) or no solvent at all (solvent-free coatings) are being used. Powder coatings are a winning technology for specific industrial applications.
Powder paint
A powder paint consists of solid particles. Each particle contains the key ingredients of the system.
The core of a powder paint is the binder system, that may consist of more than one component. Both resin(s) and crosslinker(s) are present in a thermosetting binder system. Most powder paints contain solid particles, being pigments and/or fillers. Finally, the system contains a low percentage of additives. These can be components that accelerate the crosslink reaction of resin and crosslinker, assure the escape of air or improve flow and levelling, see the Film Formation section of this article.
A powder paint is storage stable when the glass transition temperature (Tg) of the binder system is high enough. At temperatures below Tg , the powder particles are in the glass state, thus preventing that the particles melt together during storage and transport. Preferably, the Tg of a powder paint is above 50 °C.
Production
Powder paint is produced in a few steps. The raw materials, which are solid at room temperature, are mixed on macroscopic level in a mixer. This mixture is fed into a kneader, called extruder.
The core of a single-screw extruder is a screw that rotates in a heated shaft. The material is transported through the extruder because the screw rotates. The shaft of the extruder is heated in steps to a temperature between 70 and 120 °C. The binder system and the additives melt in the Melting zone. Mixing of the molten raw materials up to molecular level takes place in the last part of the extruder. Also, wetting and dispersion of the pigments and fillers takes place in the Mixing & Dispersion zone. The homogeneous melt that comes out of the extruder is cooled and becomes solid. The material is then flaked and ground into solid particles of the desired size.
Application
Most powder paints are applied on metal objects by spray application. An airflow transports the powder through the spray gun and the powder particles are electrostatically charged within the spray gun. The particles adhere to the metal surface because of the charge difference between the paint and the substrate.
Film formation
During film formation, the solid powder paint particles transform into a strong and smooth coating. Several phenomena play an important role during film formation.
The object, that is covered with powder, goes into an oven. The particles melt and merge to form a uniform and smooth layer. A critical aspect is that air, that is present between the solid particles, must be able to escape during film formation. De-aeration is assured by using a degassing agent as one of the additives in the powder paint formulation.
A chemical reaction of resin and crosslinker, called cure or crosslinking, takes place during film formation of a thermosetting system.
A thermoplastic powder coating, on the other hand, does not contain crosslinker. The resin of a thermoplastic system only melts, flows and levels during film formation. The film becomes hard upon cooling.
The future
Many powder paints are thermal curing systems, implying that they cure at elevated temperatures. A key challenge in powder coating technology is the development and improvement of systems that can be applied on heat-sensitive substrates like wood, Medium Density Fiberboard (MDF) and plastics1,2.
Radiation cure, by using ultraviolet (UV) or electron beam (EB) radiation is more and more used to arrange film formation of powder paints3,4.
A relative thin coating layer is sufficient for many applications. Regular powder paint, with particle diameter between 20 and 70 µm, results in a coating layer thickness of 50-100 µm. So-called thin film powder coatings, having a layer thickness that can be as low as 25 µm, require powder paints that consist of only small particles. Both production and application of fine powders is challenging, but it is already done in powder coating industry.
References
- Article Considerations for Developing Wood Coatings, Ron Lewarchik, 20 March 2015, Prospector Knowledge Center.
- Article Powder Coatings for Heat-Sensitive Substrates, Jochum Beetsma, 2 September 2016, Prospector Knowledge Center.
- Article Using UV/EB Resins in Coatings, Marc Hirsch, 21 October 2016, Prospector Knowledge Center.
- Article Radiation Cure Coatings – The Ultimate in Rapid Cure Technology, Ron Lewarchik, 2 April 2021, Prospector Knowledge Center.
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Its interesting when you said that most powder paints are applied on metal objects by spray application. My uncle wants to get some powder coating for his steel manufacturing company. Thanks for the article and I will forward your blog to my uncle so that he can decide on which type of power coating he wants.
Hey there! The moment you pointed out that powder coatings are applied using static electricity, it totally caught my attention. My son has been thinking of beautifying the entirety of his car, including the tire rim. I’ll forward this article to him so he’ll choose the right procedure so the end result will look smashing afterwards.
I found it interesting when you said the binder system, which can be made up of several components, is the heart of powder paint. I used to think differently about it not until you explained it briefly and it really got my attention. What you said about coating suppliers was really interesting to me.
Dear Mr. Carbine,
Thanks for your positive feedback!
Greetings, Jochum Beetsma.