Ask Joe Powder Nov/Dec 2020

Posted on Tuesday, November 17, 2020

Pretty in Pink
Q: Dear Joe,
I work in R&D at a company where we produce powder coatings in Vietnam. We have followed you on the web for a long time and often find answers to the errors we have encountered. We also have a lot of questions we want to ask about problems with applying powder coatings. It is my pleasure when you reply to my question.
This is one of the projects we are doing: spraying powder coatings on bottles for compressed gas. It includes two layers: a high zinc primer and a pink topcoat. We encountered some specific issues like when spraying powder coatings at the customer’s powder application line, our powder does not adhere well to the edges. The picture shows the powder has poor adhesion. We have tried several ways to improve application: particle size solutions, anti-faraday additives, but it still does not work. Could you please advise us how to solve this issue?

Thanks, and best regards,
Thuong P. Vung Tau City, Vietnam

A: Dear Thuong,
I hope you are doing well today. It is Monday morning for me and I thought I would try my best to help you. First of all—it is great that you are manufacturing powder coatings. I have always found it to be interesting work.

Let’s talk about your customer’s application system. Here is what I would recommend they do. (They may be doing these things already, but I am not sure.)

  1. Always make sure that the hooks have a very good ground (earth). Application problems such as Faraday Cage penetration and poor coverage on edges can often be traced to a poor ground on the parts. Clean hooks are essential and the contact to earth can be checked with a Megohmeter. Connect it to the part and a good ground. It should read <500 ohms, preferably <250 ohms. If too high, contact points should be inspected carefully and corrected.
  2. Powder coating spray guns need to be maintained regularly (once a week is usually good). Make sure that they are charging correctly and that there is no excessive wear in internal parts.
  3. Be very careful with the percentage of reclaim mixed with virgin powder. Make sure that the customer never sprays 100 percent reclaim. A good rule is to keep the reclaim concentration at 20 percent or below.
  4. I would recommend that the gas tanks be sprayed when they are warm. This is especially helpful when applying the pink topcoat. Warm parts provide better transfer efficiency. It is also wise to spray the difficult-to-reach areas first in the application process. Otherwise the powder builds on the easy-to-spray areas and makes it more difficult to reach the Faradays and edges. If possible, have a manual touch-up at the beginning of the application process to ensure best coverage.
  5. Another idea—I would try a non-zinc primer to see if the electrostatics are better for the pink topcoat. Zinc-rich primers are difficult to handle and exhibit unpredictable electrostatics.
  6. Regarding the powder coating itself, particle size distribution is important, and it sounds like you have explored this already. I would target a median particle size of about 35 to 38 microns and keep the fines (<10 microns) percentage at <8 percent.
  7. In my experience, electrostatic agents (anti-Faraday) used to try to improve application performance usually do not correct the problem. It would be interesting, however, to compare a polyester-HAA powder versus a polyester TGIC powder for the pink topcoat. Polyester-HAA powders usually have better transfer efficiency.

I hope that these recommendations help you solve the problem. Please let me know if you have any further questions and how you solved your problems.

Kind regards,
– Joe Powder

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Editor’s Note: Letters to and responses from Joe Powder have been edited for space and style.