Powder coating revolutionized architecture, furniture manufacturing, and outdoor metal maintenance. This versatile coating protects metals and other materials from corrosion, dents, and scratches while elevating the surface finish. How does it work, though?
- What is Powder Coating?
- How Does Powder Coating Work?
- Powder Coating Process
- Benefits of Powder Coating
What is Powder Coating?
Powder coating is a surface treatment used to protect metals and other materials against potentially harmful environmental factors. This coating is applied electrostatically as a free-flowing powder and cured under heat or UV light. It can be applied to any material, but it is commonly used on steel, aluminum, and plastic.
Powder coating differs from traditional liquid paint, which is generally applied using an evaporating solvent. Though it is not as durable as PVD coating due to its thickness, it provides excellent rust prevention, color uniformity and scratch protection.
Why Use Powder Coating?
Metals tend to corrode under normal usage conditions. Even stainless steel can rust under specific conditions. Here, it is helpful to apply a protective surface layer that is corrosion resistant and tough enough to withstand expected wear that the part in question may undergo.
Powder coating is a commonly used surface coating since it is tough, can vary in thickness, and comes in various colors and surface finishes. Areas where powder coating is advantageous include marine applications and roadways where de-icing salts are commonly applied.
Non-metallic materials sometimes require a protective surface layer too. These include plastics, carbon fiber, composites, and MDF (medium-density fiberboard). Here, the surface coating will usually protect the part in question from the sun or other harmful environmental factors.
How Does Powder Coating Work?
Powder coating is divided into three categories: thermoset, thermoplastic, and UV curable. While these categories differ in application and execution, they all have this in common: a powder is applied to a surface, which hardens under specific conditions. This creates an even, corrosion-resistant surface coat.
Types of Powder Coating
The three main types are,
- Thermoplastic and
- UV Curable Coatings
Thermoset powder coating is most commonly used since it is generally the least expensive option and easy to apply. Thermoplastic powder coating is usually more expensive, although recent technological advancements enabled enormous cost reduction in the process.
Thermoplastic coating offers higher performance than thermoset. It is also less dense than thermoset powders. Thus you can use far less powder to achieve the same finish. Thermoplastic powders are also thicker and more flexible than thermoset powders. Here, thermoset powders, which are harder than their thermoplastic counterpart, offer greater scratch resistance. Conversely, thermoplastic powders offer greater impact resistance due to their superior flexibility, although these are less resistant to scratches.
Thermoplastic powders can be recycled since they become liquid again when reheated. Unfortunately, thermoset powders cannot be recycled since their set is final after the initial curing process.
UV curable powder coating is similar to thermoplastic coating, except it cures under UV light instead of at high temperatures. The advantages here are legion. Since this process doesn’t require high temperatures, heat-sensitive materials such as wood and organic fibers can also be powder coated. The decreased heat requirements also translate into economic savings since you no longer require an oven on-site, reducing space and energy costs. UV powder coating also cures faster than the heat-sensitive thermoset and thermoplastic powders, decreasing turnaround times.
Each powder coating category has specific strengths and weaknesses. Thus the requirements of each application will determine the one best suited to that situation.
Powder Coating Process
The powder coating process requires three basic steps, regardless of the type of powder used: preparation, or pre-treatment, application, and curing. Here is an overview of each step.
Preparation is essential for successful powder coating. All dirt, oil, and debris get cleaned off the part in question since these could hinder powder adhesion in later steps. This preparation step could consist of several steps in itself, depending on the material in question and the initial state of the part. The first step is generally physical cleaning, where the part is scrubbed, sand-blasted, or washed using a high-pressure spray. This reliably removes most of the physical dirt that could be present on the part.
Often, metallic parts are chemically treated in processes called phosphating and chromating. Here, the metals are dipped in corrosive acids for a predetermined, concise time, after which they’re rinsed and dipped in phosphate or chromate solutions. This improves the subsequent bond between the metal and powder, improving the surface finish and product performance. The phosphate and chromate solutions used aren’t environmentally friendly. For this reason, many coating services have replaced them with titanium zirconium silanes, which offer similar corrosion resistance and powder adhesion properties.
Heat-sensitive plastics and composite materials are often hydrophobic, have a low degree of wettability, and have low-energy surfaces. These aspects all negatively affect powder adhesion during the powder coating process. These materials undergo plasma treatment to overcome this challenge, which creates chemically active binding sites on the material’s surface. Now, this surface is wettable and hydrophilic, providing ideal conditions for powder adhesion.
Powder application varies slightly depending on the material being coated. Generally, one of three application methods is used: electrostatic gun, electrostatic fluidized bed, or electrostatic, magnetic brush.
When powder coating metal objects, the powder is usually sprayed onto the object using an electrostatic gun, also called a corona gun, after the most common nozzle configuration. Here, the metal part is electrically grounded, while the spray nozzle imparts a negative electrostatic charge to the powder particles. The particles are shot towards the metal, either mechanically or through compressed air. Once airborne, they’re accelerated by the electrostatic attraction between the negatively charged particles and grounded metallic surface.
Various spray nozzle configurations are available, each suited to coating specific object shapes. This variety enables precise coating practices, minimizing waste and optimizing surface finish.
After coating, curing takes place – this is most commonly achieved through high heat. In some cases, the metallic part is heated before powder application. Here, the powder melts onto the surface, achieving a more uniform surface finish. When too much powder is applied in this manner, the powder could run down the surface, ironically creating a poor surface finish.
Electrostatic Fluidized Bed
A fluidized bed is essentially a tub of powder with air flowing through it. Aerated powder behaves like a fluid, which allows parts to be dipped into it. In a conventional fluidized bed, this part would be metallic and heated. When dipped into the bed, the powder melts and sticks to the part, after which conventional curing would take place.
An electrostatic fluidized bed is similar to a conventional fluidized bed, but it has more powder depth and an electrostatic charging medium added to the bed. This creates a cloud of electrostatically charged particles floating above the bed. The metallic part to be coated is grounded and passed through this cloud, causing the negatively charged particles to stick to its surface. This is followed by curing.
Electrostatic Magnetic Brush
Electrostatic magnetic brush, or EBM, coating is relatively new and works on similar principles as a copier machine. Here, the powder is applied to a flat surface using a roller. This application is highly accurate, allowing layer thicknesses between 5 and 100 micrometers, applied efficiently at great speed.
Curing differs depending on the type of powder used. Thermoplastic and thermoset powders both require heat to cure. Here, each powder requires exposure to a specific temperature for a set amount of time to attain the desired properties. Traditionally, thermoplastic powders were cured at 200֯C (390֯F) for ten minutes. Recently, a low-bake approach enabled adequate curing at 160֯C (320֯F) for ten minutes, decreasing production costs and rendering the process more environmentally friendly.
Since the low-bake approach necessitates the use of catalysts to ensure proper curing, they’re not as color-stable as their high-temperature cured counterparts.
UV, or ultraviolet, curing still requires heat for proper curing, but at far lower temperatures than thermoplastics and thermosets. UV powder coating curing times are far shorter than their traditional counterparts, requiring only one or two minutes at 110-130֯C (230-266֯F). UV powder coating applications use UV LED curing systems, which are more environmentally friendly than conventional curing ovens. Their high energy efficiency, paired with the low-temperature requirements and fast curing times of UV-cured powders, decrease production costs considerably compared to thermoplastic and thermoset applications.
Powder Coating Gun
Typically, powder coating applications use one of two powder coating guns: the tribo gun or the electrostatic gun, also known as the corona gun. The corona gun is most commonly used and works on imparting a negative electrostatic charge to the powder particles. After receiving the charge, these particles are shot at the part to be coated through compressed air or mechanical action.
The corona gun has various nozzle configurations, enabling the user to easily coat surfaces in multiple configurations in any orientation. A downside of using the corona gun is its affinity for back ionization and the Faraday cage effect. Here, the charged powder particle fails to find a ground point on the surface to be coated and thus builds onto particles already stuck to the surface. This causes uneven coverage in what’s called the “orange peel effect.”
The tribo gun imparts a positive electrostatic charge to the powder particles through triboelectric friction. Here, the powder particles move through a Teflon tube inside the gun. Friction with this surface imparts the charge, which then causes the particles to be attracted to the grounded metal object to be coated. While the tribo gun requires a different powder type to the corona gun, it doesn’t suffer from the same weaknesses (back ionization and the Faraday cage effect).
Benefits of Powder Coating
Powder coating provides better corrosion resistance, longer life, and better appearance than the traditional liquid-coating method. Here are some of the benefits of powder coating.
Powder coating is an excellent method to protect the steel pieces from rust. The coated layer prevents the atmosphere from coming into contact with the steel and eliminates the possibility of oxidization. You can also coat metals such as brass, copper, bronze etc. to prevent them from oxidation. You can even powder coat stainless steel to enhance the rust resistance of the metal alloy.
Tougher Than Paint
Powder coating offers a tougher layer than conventional paint, rendering it more hard-wearing and thus suitable for a greater range of applications. This hard-wearing aspect also prolongs the surface coating’s useful lifespan compared to conventional paint. Powder coating layers can be thicker than paint since they won’t sag or run, adding to their toughness.
High-Quality Surface Finish
Paint is liable to the formation of pinholes. These tiny air bubbles burst as the paint dries, leaving tiny holes in the surface finish. Here, corrosive substances gain access to the material underneath, causing long-term damage and negating the protection offered by the paint. Since powder coating isn’t applied using a brush, there are no pinholes or other surface blemishes. This enhanced quality in the surface finish offers greater protection to the finished product.
Additionally, parts can be powder coated in any orientation since there is no difference between parts coated horizontally or vertically. This is opposed to traditional paint, where the orientation plays a vital role in the subsequent surface finish.
Wide Aesthetic Range
Powder coating comes in various colors and surface finishes. It is thus suitable for a wide range of architectural and aesthetic applications. This allows for designs that incorporate the structural strength of metals, paired with the aesthetic finish of any color and texture.
Various powder colors can be applied in a single layer and cured together, allowing for color blending and bleeding. These special effects are often hard to achieve using traditional paints.
No Volatile Compounds
Powder coating is just that – powder. There are no carrier liquids and thus no volatile organic compounds (VOC) that will evaporate during curing. This renders powder coating safer than traditional paint.
Recycling the Overspray
When applying powder coating or traditional paint using a spray gun, a large amount of material is likely wasted as “overspray.” This refers to the portion of material dispensed from the spray gun that never reaches the intended surface. In the case of traditional paint, this is wasted. However, in powder coating applications, this overspray can be collected and recycled, saving material and minimizing waste.
Note that when various colors are mixed and applied to the same surface, the overspray may not be viable for recycling since the color balance will be incorrect.
Fast Curing Time
The curing time for powder coating is far shorter than that required for traditional paint. This is especially true in applications using UV curing or advanced low-bake thermosetting powders.
Limited by Heat Curing
Powder coating applications that require heat curing can only be used on certain materials, mainly metals. The high temperatures required in this process rule out most polymers, composites, and MDF (medium-density fiberboard), since these materials aren’t heat resistant.
Thin Layers Don’t Always Cure Well.
While thick layers of powder coating cure smoothly and are easily applied, thin layers can be challenging. Since the powder’s particle size is quite large, thin layers often show the orange peel effect. Many applications call for a smooth surface finish, although the orange peel effect is desirable in some applications. This effect hides metal defects and renders the finished surface less prone to showing fingerprints, both of which are desirable in specific applications.
Powder Coating Has A Large Capital Outlay
Purchasing and maintaining the equipment needed for powder coating is expensive and often prohibitive. This is far more expensive than traditional paint, for which you only need a paintbrush and cleaning products.