Powder Clinic: Outdoor Durable Powder

Outdoor Durable Powder

Q. We have a large job that requires protection from sunlight. What kind of powder we should use?

A. The long answer on outdoor applications would discuss the substrate, part preparation, possible use of primer and a lot of issues that affect a coating when exposed to rain, sun and other atmospheric considerations. I will keep that part simple by saying that you should know your substrate material and what it will take to get it clean and ready for good adhesion. You should apply enough powder or use a primer so that you can protect against moisture.

As far as the powder material goes, a polyester, such as polyester urethane or triglycidylisocyanurate (TGIC) polyester, is the most common resin system for outdoor because it has good sunlight resistance. For thicker films and good edge coverage, the TGIC is usually the preferred material.

You can also go with an acrylic if you need exceptional UV resistance, but there are some challenges. It is more expensive and can cause pinholes in other types of powders, so it is not often used in the same system as other powder.

Find out exactly what level of performance you need and then talk to your powder supplier. They can suggest the best material for a specific application. And remember, powder is not a commodity, it is an engineered product that is designed to meet certain conditions. Do not be fooled by low price. Make sure the performance is there.

Seeking Source of Pin-Holes

Q. We have had an ongoing problem we call micro pores. When the parts are inspected it looks like thousands of very small pin pricks. We are a batch coater that blasts 99 percent of our product with 80-grit aluminum oxide, creating a manageable profile without leaving ferrous residue. We usually add 50 pounds of fresh aluminum oxide to our blast system per day. Our air system has an air dryer that gets to at least a 38°F dew point and a three-filter arrangement to remove any aerosol contaminants, as well as a reclaim unit that filters the media of any dust and debris. The powder gun has a dedicated earth ground only a few feet away through the concrete (the one-use-only clean hangers also use this ground). We have used at least six different powders on aluminum, steel and stainless steel, as well as test panels, with no differences. We suspect the problem is back ionization.

A. Normally, micro-porosity in the coating is caused by one of three things: surface contamination, powder material issues or application problems or techniques. Most likely, the problem is not the substrate, powder, preparation or compressed air, although it is hard to rule anything out completely without a design of experiment, which will test issues to reveal the cause by process of elimination.

I would not rule out anything you have discussed until organizing a testing process, but there are still several potential issues: hopper contamination, powder feed system contamination, airborne contamination in shop, airborne contamination in oven, and/or excess film thickness or ionization.

Eliminate these items one at a time: use a different hopper; use a different feed system and gun; use a different location; use a different oven. Carefully organize the testing to be sure it has a big enough sample size, it is accurately documented and it generates reliable results. If the problem is back ionization, it is caused by excess negative ions in the applied film. To test this, reduce these items one at a time or in combination:

Reduce micro-amperage to around 20
Move gun a few inches farther from target
Reduce flow rate around 5 to 10 percent
Reduce film thickness to no more than 3 mils
Isolate one idea at a time and carefully measure results.

Film Build Control

Q. We added a powder coating in our plant three years ago with one booth and two hand sprayers. Though we have been fairly successful, we seem to have ongoing problems with inconsistent coverage and powder use. How can we control the film build and coverage over a variety of parts?

A. It can be challenging to maintain accurate film build on a part, especially if the part has a complex shape. It starts with the powder material, with particle sizes varying from fine (5 micron) to coarse (120 micron). Finer particles are more easily influenced by gravity and the movement of air from the gun and collector. The size affects the amount of charge taken on by a particle, so it is important to test powder materials for an application before selecting a vendor or a product. Reclaimed powder should be controlled to avoid excess accumulation of over-sprayed powder, which will be less efficient than fresh material. Another highly critical factor for film build control is the quality of the earth ground to the target part. Good ground (<1.0 mega-ohm resistance) improves the uniformity of film development and coverage in hard-to-coat regions. Poor earth ground produces uneven film thicknesses and light coverage in critical areas where resistance is high.

Another fundamental part of film-build control is the spray gun setup and technique. Application efficiency is affected by a number of variables, including the electrostatic settings, the aerodynamic influences of the gun and the surrounding environment. Generally, voltage should be higher for a high-charging level and micro-amps should be lower to avoid excess negative energy at the part surface. Adjust the setup based on your results.

Generally, the powder velocity should be low with a sufficient volume of powder to coat the part in a reasonable amount of time. If the powder volume is too low, it takes longer to coat a part. On the other hand, if the volume of powder is too high, it can lead to more film thickness and inconsistency. A good balance of air-to-powder will result in a smooth powder cloud with consistent distribution of the material and low velocity.

Gun position and movement is also important for consistency. Gun-to-target distance should be as consistent as possible. Stroke speed and pattern should be accurate and consistent depending on the part shape and size. Some testing to develop a good pattern and gun setup is recommended for optimum results.

Training is also a critical part of any successful operation. Without training, bad habits are developed and parts that are acceptable, but not optimum for good film-build control, are produced. A combination of knowledge about the
fundamentals of application, good earth ground, training and testing is the road to good film build control and low reject rates.