Metallurgical Aspects of Powder Coating Technology, Advantages, and Applications

By Dr Thoguluva Raghavan Vijayaram PhD

Senior Lecturer Faculty of Manufacturing Engineering, UTeM
Universiti Teknikal Malaysia Melaka, Ayer Keroh, 75450 Melaka Malaysia
Email: thoguluva@utem.edu.my

The history of powder coating begins in the late 1940s and early 1950s, at a time in which organic polymers were still being spray coated in a powder form on to metallic bases. Dr. Erwin Gemmer, a German scientist, developed in those days the fluidized-bed process for the processing of thermosetting powder coatings, and registered an appropriate process patent in May 1953. Between 1958 and 1965, literally all powder coatings, generally only functional applications with a film thickness of 150 µm to 500 µm, were processed by means of fluidized-bed application. Electric insulation, corrosion and abrasion resistance were in the foreground. The coating materials in those days comprised nylon 11, CAB, polyethylene, plasticized PVC, polyester and chlorinated polyether, among others. It was the firm of Bosch that developed the basic type of expoxy resin powder when searching for a suitable electric insulation material.

The high film thicknesses for numerous applications, and the technology of electrostatic processing of powder coating, which was developed shortly after in the U.S.A., and was used commercially between 1962 and 1964 in the U.S.A. With the electrostatic spray-guns made by the firm of Sams for electrostatic application and which gave rise to the term "Samesizing", this hurdle was also overcome. Between 1966 and 1973 the four basic types of thermosetting resins, which are still defining today, were developed and commercially marketed: epoxy, epoxy polyester hybrid, polyurethane and polyester. The number of powder-coating plants in Germany alone rose from four in 1966 to 51 in 1970.

From the early 1970s, powder coating then began its march of triumph worldwide, even though the growth of the powder coating market was until 1980 initially slight. The plants up to that time were expensive, the film thicknesses too high for commercial use, color-change problems and high curing temperatures greatly limited the color tone, effect and substrate diversity. From the early 1980s, powder coatings have developed worldwide through continuous growth, which, driven forward by continuous innovations in the raw materials available, improved formulation and advances in application technology.

Powder coating is a dry finishing process. Before coating, the parts to be coated are first pretreated similarly to conventional liquid coated parts. The pretreatment process is normally conducted in series with the coating and curing operations. Powder coating is a method by which electrically charged powder coating material is spray-applied to a grounded work piece. Electrostatic attraction holds the powder to the part to be coated until heat is added to flow the powder together and cure it. Since powder may adhere to the part for several hours, heat curing can be done at the user's convenience. Should the uncured powder coat become damaged or blemished during handling, the powder can be simply blown off with air or vacuumed, and a new coat applied.

An electrostatic powder coating system is comprised of a powder feeder, power unit or electrostatic voltage generator, electrostatic spray gun, and a powder recovery system. In the feed unit, powder is diffused by compressed air into a fluid-like state. It is then siphoned out by the movement of high velocity air flowing through a venturi and propelled through powder feed tubing to the spray gun. The feeder provides a controlled flow of powder to the spray guns. Since powder and air volume are independently controlled, dilution ratios can be adjusted to obtain the desired thickness coverage needed to meet specific production requirements. The feeder can provide sufficient discharge pressure and velocity to feed a gun as remote as 50 feet away. A high voltage, low amperage power unit supplies an electrode at the front of the gun. As the powder leaves the gun, the electrode emits a field charge that is imparted to the coating material as it is propelled toward the part. Once charged, the powder particles are drawn and attach themselves to the grounded work piece. The power unit has sufficient voltage to assure maximum wraparound. Voltage is operator adjustable up to 100 kV to minimize Faraday Cage effect when spraying into corners and deep recesses, and to ensure excellent wraparound and surface coverage on all surfaces being sprayed.

Electrostatic powder guns are designed to shape the spray pattern and impart an electrostatic charge to the powder. The spray pattern is easily configured to accommodate the type o coating used and the shape of the part being coated. There is essentially two common ways of applying powder coating: by electrostatic spray and by fluidized bed powder coating. There are several other processes that have been developed, but they are far less used. These include flame spraying, spraying with a plasma gun, airless hot spray, and coating by electophoretic deposition. A few powder coating techniques are explained here.

1. Electrostatic spray powder coating

Electrostatic spray powder coating uses a powder-air mixture from a small fluidized bed in a powder feed hopper. In some cases, the feed hoppers vibrate to help prevent clogging or clumping of powders prior to entry into the transport lines. The powder is supplied by a hose to the spray gun, which has a charged electrode in the nozzle fed by a high voltage dc power. The actual process is shown below in Figure-1

Figure-1 Electrostatic spray powder coating process

Electrostatic powder spray guns direct the flow of powder; control the deposition rate; control the pattern size, shape, and density of the spray; and charge the powder being sprayed .The spray guns can be manual (hand-held) or automatic, fixed or reciprocating, and mounted on one or both sides of a conveyorized spray booth. Electrostatic spray powder coating operations use collectors to reclaim over-spray. This reclaimed powder is then reused, adding significantly to the powder coating's high transfer efficiency. There are various gun designs that mainly differ in the method of applying electrostatic charge to the powder. In some cases, the powder is electrostatically charged by friction. The advantage is that the powder is free to deposit in an even layer over the entire surface of the part, and deposition into recesses is improved. The film thickness is dependent on the powder chemistry, preheat temperature, and dwell time. Film thicknesses of 1.5 - 5.0 mils (37.5 - 125 µm) can generally be applied on cold products. If the products are preheated slightly, 20 - 25 mils (500 - 625 µm) coatings can easily be applied in a single coat.

2. Fluidized bed powder coating The fluidized bed coating process is a simple dipping process that can be either conventional or electrostatic. In the convention fluidized bed process, the fluidized bed is a tank with a porous bottom plate. The plenum below the porous plate supplies low pressure air uniformly across the plate. The rising air surrounds and suspends the finely divided plastic powder particles, so the powder-air mixture resembles a boiling liquid as shown in Figure-2. Products that are preheated above the melt temperatures of the powder are dipped in the fluidized bed, where the powder melts and fuses into a continuous coating. A high transfer efficiency results from little drag out and no dripping.

Figure- 2 Illustration of fluidized bed process

The fluidized bed powder coating method is used to apply heavy coats in one dip, 3 - 10 mils (75 - 250 µm), uniformly to complex shaped products. It is possible to build a film thickness of 100 mils (2500 µm) using higher preheat temperatures and multiple dips.

3. Electrostatic fluidized bed powder coating

An electrostatic fluidized bed is essentially a fluidized bed with a high voltage dc grid installed above the porous plate to charge the finely divided particles. Once charged, the particles are repelled by the grid, and they repel each other, forming a cloud of powder above the grid. These electrostatically charged particles are attracted to and coat products that are at ground potential. Film thicknesses are similar to what can be achieved in the electrostatic spray process. The advantages of electrostatic fluidized bed coating is that preheating of parts is generally not necessary and small products, such as electrical components, can be coated uniformly and quickly. The disadvantages are that the product size is limited and inside corners have low film thickness owing to the well known Faraday cage effect.

4. Powder curing method

Thermoplastic powders require heat only to fuse the powder together into a continuous film. However, thermosetting powders often require additional heat to cure the film on the product. There are four basic methods normally used in the curing of powder coated parts: convection, infrared, a combination of the two, and ultraviolet (UV) curing. Convection ovens can be either gas or electric. Hot air is circulated around the powder coated parts, and the parts attain the temperature within the oven. UV curing is commonly used with heat sensitive substrates. Specifically formulated UV powders flow at very low temperatures (121°C) and can be cured via UV radiation in a matter of seconds. Infrared ovens, using either gas or electricity as their energy source, emit radiation in the IR wavelength. The radiated energy is absorbed by the powder and the substrate immediately below the powder, so the entire part need not be heated to the cure temperature. This allows a relatively rapid heat rise causing the powder to flow and cure when exposed for a sufficient time. Combination ovens generally use IR as the first zone to melt the powder quickly. This process is termed near infrared (NIR) cure, and powders are formulated specifically to take advantage of this process. The part then progresses into a second zone, which is a convection oven.

Benefits of powder coating technique

1. Economy

Up to 99 percent of powder overspray can be captured and recycled.
One-coat coverage without runs or sags.
Easy cleanup.
No solvents to mix or recover.
No viscosity balance to maintain.
No flash-off time requirements.
No need to provide heat booth make-up when air is returned to work area.
Reject rate can be kept low because damaged coating can be blown off and recoated before heat cure.

2. Excellence of Finish

Readily available resins can provide one or a combination of desirable characteristics including: durability, resilience, high gloss, electrical insulation, toughness; and resistance to wear, corrosion, impact, chemical action, and weather.
Powder manufacturers now offer a complete range of colors plus excellent color matching

3. Ease of Application

Consistent finish characteristics and electrostatic "wrap-around" reduce the need for highly skilled operators.
Process is easily automated and can include automatic gun movers and/or contouring mechanisms.
Production time can often be reduced, as one coat will do the job

Applications of powder coating technology

The following substrates are coated with powder coating technique.

Cold Rolled/Hot Rolled Steel
Galvanized Steel
Iron Castings
Zinc Castings
Copper and Brass
Magnesium
Aluminum Extrusions and Castings
Rare Earth Magnets
Nickel Zinc Plated Steel
Stainless Steel
Powder Metallurgy Parts
Non-Metallics

About the author
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

Microgravity Solidification Technology
Electroforming Technology
Advances In Diamond Coating Technology