How to Protect Against Corrosion - Powder Coatings
The Short History of Powder Coatings
The essential feature of powder coatings is that the paint is applied in a dry state onto the object. The paint powder is then melted onto the product by putting the product into an oven.
Powder coatings are a postwar invention, when plastics became more sophisti-cated. The powder coating process was patented in 1953 by Dr. Erwin Gemmer.
Until 1965 powder coatings have been only used for functional applications (not aesthetical), where it was applied a layer thickness of 150 to 500 microns. The used powders then were mainly Nylon 11, Cellulose Acetate Butyrate (CAB), Polyethylene, (chlorinated) polyester and thermo curing Epoxies.
Powder coatings process at that time were mostly used for coating boat accessories, metal furniture, dishwasher and washing machine baskets. The process technique to apply powder coatings at that time is called the fluidized bed technology.
The very high layer thicknesses that were the result of the fluidized bed process, were not desirable for many applications. In general counts that the higher the layer thickness, the less smooth the surface texture becomes.
In the US some companies started in the early 70's to experiment with applying powder coatings by using the now commonly used electrostatic powder gun.
Have you ever heard of Powder Coating?
Development of Powder Coatings
Powder coatings give better protection against corrosion than wetpaints. Wetpaints are mainly used when slickness (smoothness) of the surface structure is more important than corrosion and scratch resistance, like for example in cars. Powder coatings are technically superior to wetpaints except in surface smoothness.
Since the slickness of car bodies is one of the most important properties to be regarded as desirable, they are still wetpainted. Otherwise they also they would have been started to powder coat bodies long time ago.
In the late 60's and early 70's there were four basic types of thermal curing resins developed that are still used today. These are: Epoxy, Epoxy-Polyester hybrids, Polyurethane and Polyester (TGIC) resins. Since a few years also TGIC-free Polyester coatings are available.
The number of powder coating plants in the world grew fast since the early 70's. The electrostatic powder coating process took over the environmentally harmful and labor-intensive wetpaintings.
Only in the early 80's more commercial applications of powder coatings started to emerge, because the equipment began to drop in price and the initial limited choice of color and shine started to expand rapidly. Thereafter, the growth rate went explosively, and the technique began to displace the traditional spraying on most applications.
Applications of Powder Coatings
- Steel structures,
- Aluminum frames, windows and doors,
- Office furniture,
- Steel fencing,
- Light fictures,
- Car bodies and parts,
- Agricultural machinery,
Different Types of Powder Coatings
Epoxy coatings are often used in two-layer system to protect heavy duty steel beams like on the adjacent photograph.
As replacement of the environment unfriendly dip galvanizing, there are nowadays highly advanced Sink-Epoxy powder coatings available. When processed properly in a duplex system, these systems can give up at least 20 to 30 years protection against corrosion.
The Principle of Electrostatic Powder Coating
The essence of powder coating is that tiny solid paint parts are charged in a magnetic field.
By charging the solid powder particles, via the gun, and grounding the metal object, arises a magnetic field between the powder and the object. As soon as the powder particles have left the gun, they start moving towards the grounded product.
The powder particles follow during their travel between the gun and the object the so called (magnetic) field lines. After passing through the magnetic field, the powder particles end up on the product, and adhere at the place where the field lines end, like a magnet on a steel object.
At locations where field lines are weak, there will group also less powder particles.
Composition of Powder Coatings
By varying the type of raw material and the proportions, the properties of the coating can be influenced.
Main Properties of Powder Coatings
- Structure Powders
- Degassing powders
- Thin layer Powders
- Low bake powders (low temperature)
The Faraday Cage Effect
One of the most occurring problems during the application of powder coatings with Corona guns is caused by the Faraday cage effect, because this technique uses an active electrical technique. A problem that the so called Tribo guns don't have.
In the Corona technique not all the field lines are everywhere equal on the grounded product. Most field lines pinpoint on the outer corners of the product. On the flat surfaces are fewer field lines, and in cavities are the least field lines.
As a result most of the powder will gather on the corners and on sharp edges, and is it often difficult to inject the powder in small cavities.
Corona Versus Tribo
The adjacent picture shows the different results between the Corona and Tribo techniques. The left sample shows the result with the most commonly used Corona gun. And the right shows the result with the Tribo gun.
The Tribo technique has less problems with the Faraday cage effect, because the powder particles are statically charged in the gun itself, and doesn't use the active charge that the Corona technique uses. But due to a few major disadvantages of the Tribo guns, like lower spray capacity and longer cleaning time during color change, the Corona guns are most used.
By means of the correct settings of equipment, the Faraday cage effect can be reduced, but cannot be avoided completely. Hence, the operator must often squirt manually to fill cavities of objects. Because of this, the production process becomes more labor intensive, and thus more expensive. Therefore, it is now an often well practiced part during the design process, to avoid unnecessary cavities in products, in order to avoid insufficient coverage.
Pros and Cons of Powder Coating
The process is easy to automate
The investment for production line is high
Waste is reusable by recycling
It takes a lot of energy to cure out large heavy products
High layer thicknesses applied to be in one cycle
Color changes with active recovery requires more time
Excellent mechanical resistance
Powder coatings are slightly less smooth in appearance than wetpaints
No use of solvents
Impossible to mix powder coatings, in contrast to wetpaints
Product is immediately ready after baking
Special colors have to made by the suppliar
An important advantage of powder coating in contrast to liquid coatings, is that the recovered powder can be reused.
The powder particles leaving the cyclone (at the middle on the picture above) at the bottom side are returned to the storage tank. It is often assumed that all of the powder that ends up in the suction is fully recovered. In reality, it is only 80%, sometimes even less. This depends on the type of cyclone, and the condition of the filters in the extraction system.
If the filters are clogged, the air velocity in the cyclone will drop, and this diminishes the recovery power of the cyclone. As a result, larger amount of powder particles are also separated off as waste.
The cleaning of the cabin is a very significant efficacy. The quality of the cleaning has much influence on the quality of the final product. In modern installations, the color change procedure can be performed in less than 10 minutes, whereas with older systems sometimes takes up to an hour.
Insufficient cleaning of the booth, the suction tubes or the cyclone leads to color contaminations. Any color contamination leads to rejection at the quality control.
7 Main Process Steps
- Attaching products to conveyor
- Pretreatment (chemically or mechanically)
- Drying (in case of chemical pre-treatment)
- Powder Coating
- Quality control
- Collection and packaging
#1 - The Conveyor
An important part of the powder coating process is how the products are hung onto the conveyor. The more products, the higher the process efficiency.
Despite recovery, powder consumption will also drop when the products are hung closer to each other, since there will be less powder sprayed alongside the products.
Hanging systems are mostly made with the help of hooks. These are available in all shapes and sizes. In some cases, it pays to make special mounting tools. In particular, when there are large numbers of (approximately) the same goods to be coated, the design of mounting tools can become a skilled well paid occupation.
There are 3 types of overhead conveyors:
- Manual / Hand Pushed
- Power & Free
#2 - Pretreatment
Pretreatment of products to be coated, has the purpose of removing dirt and grease.
In addition, there may be a conversion layer applied by means of chemical pretreatment, which increases the corrosion resistance of the paint system, as well as the bond between products and coating.
#3 - Drying
Drying of the products takes place in an oven. The temperature at which this occurs varies from 110 to 160 °C (230 °F to 320 °F). Depending of the type of pretreatment here are limitations. The conversion layer can be damaged by drying at too high temperatures, which can result in several forms of corrosion.
An Example of a Powder Coating Line
#4 - Powder Coating
The powder coating takes place in the powder booth. The booth is a complex system that consists of several components:
- Control Unit
- Powder center / storage
- Extraction unit
Outline of a Powder Spray Booth
#5 - Curing
An important part of the powder coating process is heating in the oven - the actual curing of the coating. Powder coatings melt and undergo a chemical reaction when it is heated above a certain temperature. During this chemical reaction the molecules of the binder connects themselves with each other. Initially as a long wire, but later with branches. The branches of the paint is much stronger. This process is called cross-linking.
The cross-linking reaction is irreversible. This means that if the layer has not been liquefied, and once the reaction has taken place, it will not be re-liquefied if the product is once more heated.
At the moment that the product goes into the oven this process starts, wherein the powder undergoes a change of temperature for a certain period of time. The time between entering and leaving the oven, is called the baking cycle.
Not all powders have the same optimal conditions for curing. Some powders require more heat than others in order to establish a good cross-linking. Therefore, every powder coating comes with its own curing time and curing temperature.
Important thing to know about the baking cycle, is that this is not the oven temperature is referred to, but the temperature of the product, the so-called object temperature. The thicker the product is, the more heat is required in order to get it to the desired object temperature. This means that the oven temperature has to be set higher for thicker products, or the time in the oven should be longer.
To plan this well can be quite an logistic challenge.
#6 - Quality Control
The main cause of rejection in powder coating processes is pollution. Ranging from a few great spots to many small dots. Possible causes are:
- Pollution from the environment by hairs, dust and pollen. This is exacerbated during spraying on recovery.
- Contaminated material base: ingrained grease, collegiate or tape residues, etcetera.
- Pollution from dirty rinsing baths.
- Contamination with other powder by a poorly executed change of color.
- Poor quality of the supplied powder.
- Pollution in the oven.
- Pollution of the chain conveyor (rust particles falling down).
In addition, too high or too low layer thickness, degassing, poor bonding or other abnormalities in mechanical properties is reason to reject products.
#7 - Collection and Packaging
The final step in the powder coating process is the collection of the coated products.
Packing has two functions - the protection of the coating and to improve the transportability.
When the products are coated and are checked on quality, it is important to make sure that the products are delivered in the state to the customer.
The packaging of products ensures that damage is prevented. For this, to obtain a large number of packing containers. The powder coating industry is an extensive use of various types of foam, seal and plastic bubble wrap.
Well, that's all about powder coatings folks!
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