Q: I’m a manufacturer of powder coatings in Vietnam. We have a problem in product as follows:
The reclaimed powder is being sprayed on an object, but it has very poor transfer efficiency.
I know: The reclaimed powder should be added with fresh powder at ratio 70/30 or 60/40, but my customer, he’s spraying the reclaim alone. The particle size of fresh powder is 5 microns to 80 microns, D50=28 microns, percentage of particle size 5-10 microns about 10%.
Would you suggest for us how to enhance transfer efficiency with reclaimed powder?
A: Thank you for your question. Indeed, this is a challenge. As you know, the best solution is to correct the root cause, which can be delineated into two issues:
1. Improve first-pass transfer efficiency to minimize the creation of overspray that needs to be recycled. This can be accomplished by regular gun maintenance, always ensuring excellent earthing of the parts to be coated, keeping racks clean, and using powders with good particle size distribution (minimal fines and coarse particles).
2. Only introducing a low level of reclaim to the virgin powder. Less is always better, but it is wise to never exceed a 30% reclaim to virgin powder ratio.
Obviously, you need help now because your customer insists on spraying high levels of reclaim. There is hope, though the solution is imperfect. It is possible to improve the fluidization and application performance of reclaim powder. The use of a dry-blend additive is worth attempting as a solution to your customer’s application problem.
These materials are based on fumed silicas that have been surface treated to make them hydrophobic. They are supplied as agglomerates of very fine particles of silica, typically around 0.2 microns in diameter. Think of them as clusters of grapes. These agglomerates can be quite large and therefore will create defects in the finished powder if they have not been adequately de-agglomerated. Hence, the mixing process used to disperse the dry-blend additive is important.
Here are a few guidelines for incorporating a dry-blend additive to the powder. A good starting point is a concentration of 0.2% by powder coating weight. Mix the modified powder coating thoroughly. I suggest a V-blender or something similar to accomplish this. After mixing, I recommend sifting the powder through a relatively coarse screen (80 mesh or 175 microns). This can further de-agglomerate the dry-blend additive to help minimize the possibility of “seeds” due to large agglomerates.
The modified powder will fluidize and transport through the application system much better than unmodified reclaim powder. I would pay close attention to the film thickness of the coated parts. Thicknesses less than 50 microns (2.0 mils) may exhibit “seeds” or defects caused by the protrusion of silica agglomerates.