Understanding Metallic Effect Powder Coatings
Metallic effect powder coating is a broad term, and Axalta’s Mike Withers says it usually covers powder coatings that have aluminum or mica pigments.
Q: What is new in metallic effect powder coatings, and what are the advantages of using them?
A: Metallic effect powder coating is a broad term and usually covers powder coatings that have aluminum pigments, mica pigment and others. Many end-use products today have a metallic appearance, including appliances, office furniture, architectural and more. It is important to know how to apply metallic effect powder coatings.
Let’s begin by understanding how metallic effect powder coatings are produced. There are three ways that they can be produced: extrusion, blending and bonding
For extrusion, the metallic effect pigment is extruded with the base powder. This is a lower cost method of production, but the shear in the extruder will reduce the metallic appearance. There are now metallic effect pigments that are not aluminum and can be added at the extruder.
Blending is exactly what it sounds like — it’s a base powder coating that is produced and then blended with effect pigments. This process increases the metallic appearance. It does increase cost because you have a secondary production process. If you are spraying to waste, not reclaiming, it is okay to use blended effect products. However, it is not advisable when using a vibratory hopper. The pigment and powder are different weights and the vibration will cause them to separate and give an inconsistent appearance. It is best to use a fluidized hopper to apply effect coatings. Even a small one will give good results.
Bonding the effect pigment is the last method. It has the highest cost. Again, a base powder is produced and, subsequently, the pigment particles are attached to the powder particle. The process for doing this has been refined over the years to produce very good quality bonded products. The ‘state-of-the-art’ manufacturing processes give the brightest metallic appearance, best bonding performance and lowest cost.
What does that mean for you? Extrusion will have limited appearance capabilities. Blending will have limited application and reclaim capabilities. Blending can give a more metallic appearance, but has limited options for reclaim. Metallic pigment can collect on the electrode of the gun and cause a defect when it releases and attaches to the part surface.
In bonding, the better the bond quality, the more consistent the end appearance. Maybe that isn’t as important if you’re only coating small parts that aren’t close to each other on the finished goods. You might not see the variation. However, if you are coating 24-foot long extrusions for a building, then it will be obvious if the appearance is inconsistent or blotchy. The larger the surface area being coated, the higher the chance of seeing an inconsistent appearance.
Also, if you are applying powder and reclaiming through a cyclone, you can lose pigment in the cyclone if the bond quality is poor. The result can be changing effect appearance from part to part over time.
Effect pigments have changed over time as well. Now we can use encapsulated aluminum pigments. In previous years, powder manufacturers would recommend a clearcoat to maximize protection of the aluminum pigment in the coating. That meant you would have to coat the part twice, which significantly increases the cost.
Today, we can use effect coatings outdoors without a clearcoat and have very good weathering performance. However, you should always check with the manufacturer to be sure that the coating you are using does not require a clearcoat.
One concern with encapsulated pigments is that, in the manufacturing process, it is possible to shear the pigment and expose the aluminum. The ‘state-of-the-art’ manufacturing processes give greater control of the bonding process and, thus, nearly eliminate that concern.
An application recommendation is to lower the microamps on your gun. We would recommend lowering the kVs on older guns that have limited capabilities. Newer guns may have presets that will automatically lower kV and microamps.
If you haven’t tried using different nozzles with your powder application gun, then you should. You can change nozzles from flat spray to conical and you may see a change in effect appearance. Try it out to check if you see a difference.
Another application recommendation is to keep the gun further away from the parts you are coating. Ideally, 8-12 inches is a good distance. If you must go closer to coat a Faraday cage area (inside corner of a box, for example), then it is recommended that you coat the Faraday cage areas first before coating the rest of the part.
Ideally, you have the least amount of powder coming out of the gun to build the appropriate film build in the time you have available to coat the part. If you have high powder flow rates, then there is more powder in the cloud than you have ions available to charge the powder particles. If you are spraying to waste, then you are throwing away good powder. This is because you are hurting your first pass transfer efficiency. Remember, metallic effect powder coatings can be your highest cost per pound. You want to maximize the amount of powder you get on the part!
It is also important to understand how metallic effect pigments work in powder coatings differently than in liquid coating. In liquid, the film stays open (liquid state) longer than powder coating does. This allows the pigment to align perpendicular to the surface, meaning that you get a consistent appearance. In powder coatings, the time they can align is compressed and, therefore, there’s often no alignment at all. They are random in appearance.
Powder coating is also limited in what percentage of effect pigment can be safely added to a formula. Liquid coatings can have much more in their formula. In the past, the limits meant the highest metallic effect liquid formulas just couldn’t be replicated in powder. With the ‘state-of-the-art’ manufacturing techniques, powder manufacturers can expand their capabilities to match most liquid metallic appearances.
In closing, bonded metallics benefits include:
- Higher first pass transfer efficiency
- Consistent color, effect and gloss
- More batch-to-batch consistency
- No powder spits
- Better transfer on coating line
- No free floating pigment in application booth
As always, check with your powder supplier for their recommendations. They will tell you what is best for the coating you are using.
Mike Withers is the architectural segment leader and electrocoat segment leader in the Powder Group at Axalta Coating Systems.
Infrared cure is gaining increased attention from coaters as a result of shorter cure cycles and the possibility of smaller floor space requirements when compared to convection oven curing.
A review of available test methods, common applications and innovative instrumentation...
This alternative to TGIC-based polyester powder coatings offers similar performance and enhanced transfer efficiencies.