Removing Terephthalic Acid Dust in Powder Coating Ovens

Q: We have seen a powder oven with a “fuzz” buildup inside the vestibule and other areas, usually seen near the conveyor openings. The research I have done tells me this is terephthalic acid dust and is a byproduct of the curing process. It is usually associated with polyester powders. Does an oven curing polyester powder require more air changes than a typical oven? And how can this material be removed from the oven?

A: That material you see is called condensate. It is a combination of byproducts of the resin system and airborne dust. It occurs when the hot air of the oven meets the cooler air at the oven entrance/exit.

During the curing process of polyester powder coatings that are acid-functional systems, a resinous material is generated in the air from the terephthalic acid. This material will condense as the warm oven air meets cooler air near the oven opening and creates a resinous deposit on the oven walls. It attracts airborne dust and the resulting resinous substance builds over time, creating the fuzz that you have seen on some cure ovens.

Powder ovens that run a lot of polyester powders should have roughly double the normal exhaust rates specified for organic powders to limit this buildup. Oven vestibules should be longer so that the cooling of the oven area takes place over a longer distance and more of the resinous material stays inside the oven and goes up the exhaust, limiting the possibility of condensation occurring at the oven opening.

The removal of this buildup is not easy. It is not soluble in water or alcohol. It takes a hydrocarbon solvent and mechanical scraping to clean the material from oven walls. Scrape away as much of the resin as possible with a putty knife before soaking with any solution. If that is not sufficient, the material can be softened with a solvent and then scraped.

Another option is to blast the surface with Co₂. This can work if the material is hardened, but may be less effective if the material is too resinous. Co₂ is unique because the blast media melts and does not leave any additional debris in the oven. Care must be taken to avoid warping the steel panels, so the pressure cannot be too intense.

Question: Bubbling in the Powder Finish

Q: We use a 3-stage washer and our cure oven is around 25 minutes long. We are coating with a bronze metallic powder acoustical panels that are around 2 feet square and have perforations that are about 1 inch apart to provide the acoustical effect. We are having a lot of trouble maintaining a consistent appearance on the parts. They look splotchy and there are light/dark areas and more gloss and metal-flake appearance in some areas with less in other areas. How can we fix this?

A: What you are seeing is the electrostatic behavior of this powder on a perforated surface. Even on a flat surface with no holes, you can have trouble getting consistent appearance with some metallic powder coatings. When the manual gun is moved across the surface, the gun position and target distance are changing and that impacts the relative voltage and current draw from the spray gun. The changes in the charge affect the deposition of the powder and the orientation of the metal flakes at the surface. You may get more or less metal and you may see flakes standing on edge or lying flat. You may get more of the organic pigments in some areas than in others. Edges of the part are particularly hard because the charge is concentrated into a smaller area as the amount of metal in the gun spray pattern is smaller. You have just as much current as you had in the part center, but it is concentrated into a smaller space. The rapid buildup of free ions from the current can have a big impact on the orientation of the metal flakes and pigments in the powder, and produce an inconsistent look. With a heavy concentration of small holes in the surface, you essentially have a large number of small edges that are impacting the behavior of the powder as it deposits on the surface.

The method that would be recommended to achieve a more uniform look would be to apply with relatively low microamps in a Faraday Cage-type setting. Movement of the gun and gun-to-target distance must be as consistent as possible, with no fanning or arcing of the gun at all. Use steady strokes across the part with a consistent 50% overlap. The process must be as precise as possible.

I would also take a look at the powder. Is it a bonded metallic, dry blended or extruded metallic? It should definitely be bonded for the best possible electrostatic effect and consistency. How much metal-flake content does it have? Lower metal content can be easier to work with. Talk with the customer about the look. Is it possible that the powder color could be a standard organic powder without metal flake? That would make your job much easier. I have seen acoustical panels like this installed in areas such as subway tunnels where the metallic look is certainly not needed.

Question: Blasting and Chemical Options

Q: We are setting up a new line to powder coat our steel parts before assembly. We manufacture maintenance vehicles and we powder coat many of the parts that attach to the vehicle, such as blades, buckets and other gear. Our subassemblies use some heavy steel plate and various lighter gauges, ranging from 16-gauge to ¼ foot plate. We are looking at blasting and chemical options. Can you give us some pointers on what will work best for this type of product? Should we consider blast only, chemical treatment or both?

A: The heavier steel parts with welded joints will have a lot of inorganic soils that will need to be mechanically cleaned or pickled to provide the cleanliness needed for reliable adhesion and corrosion resistance. Start with the understanding that you will need to grind or blast welds and hot-rolled steel parts. Also, you are most likely cutting steel with a laser and will, therefore, have an oxide layer on the cut edges. This laser oxide edge has to be removed or it will lead to loss of powder adhesion at the edge. So that will also need to be ground or blasted. The 16-gauge should be able to take blasting without warpage, but, if you have anything lighter, you may not be able to put it through a blast process. Pickling is an option, but be aware that it could mean a strong, heated acid that you may not want to deal with in your plant.

A blast-only process can work well, provided you match the powder film to the desired quality level. If long-term corrosion resistance is required, you will need to add a primer layer before topcoat to ensure that you have a thick enough, moisture-resistant film. The two-coat process will provide good protection for outdoor exposure.

Chemical treatment can be used to remove organic soils and provide a conversion coating for added corrosion protection. You could consider iron phosphate, zinc phosphate or a transitional metal treatment such as zirconium oxide. You will have a cleaner part and the surface will provide good adhesion and promote corrosion resistance. You will still need to mechanically clean or pickle those inorganic soils. Overall, the best approach is probably to do both. I would recommend some trials of the different options to learn more about what is involved in their use and what they can provide in performance. Treat some parts with the different options mentioned and then run some corrosion testing, such as salt spray or cyclic corrosion testing, and see what you get. Look at the labor involved in grinding and see if that makes sense. Volume is a factor too. If the volume is small, you can take a more batch-size approach that may include substantial manual labor. If the volume is high, you are more likely to invest in an inline blast approach and consider a spray washer too. Just make sure that, whatever option you choose, it will meet your quality standards.