Pros or cons of powder coating with zinc primer vs galvanizing and then powder coating over top?

Hello Joe,

Do you have any articles that talk about the pros or cons of powder coating with zinc primer vs galvanizing and then powder coating over top? Thanks for sharing your wealth of knowledge :)

Sincerely,

Allison Lee

Boise, Idaho

Hi Allison,

It's nice to hear from you. How are things in Boise? This is a great question. Personally I like the idea of bare steel (non-galvanized) being properly cleaned and pretreated followed by a good epoxy primer (doesn't have to be zinc rich), then a high-quality polyester topcoat. The most critical step is cleaning/pretreating the steel. If it is hot-rolled pickled and oiled (HRPO), then the cleaning/pretreating step is difficult. The challenge here is to completely remove the “oiled” layer. This can sometimes require a strong acid etch stage in your pretreating scheme.

If it is cold rolled steel then the pretreatment process is much easier. A good alkaline cleaner followed by a rinse and then a chemical conversion like iron phosphate or a zirconate followed by a rinse and perhaps a sealer.

It’s a very good idea to gel, but not cure, the epoxy primer prior to applying the topcoat. This enhances intercoat adhesion by allowing the unreacted chemical groups in the epoxy to react with carboxyl groups in the polyester.

I don't like HDG (hot dipped galvanized) because it is variable, it needs degassing, and pretreatment can be tricky. A really good electrogalvanized surface is a different story, but a job coater like yourself probably doesn't see electrogalvanizing too often.

As for an article about this, I dunno. I've written about HDG and the challenges it brings but I don't think I've compared it to epoxy. Perhaps my next column?

Please let me know if you have any more questions or ideas.

Thanks,

Joe

Good day Mr. Powder,

My name is Alan and I am a production engineer in Columbus, Indiana. I have a few questions regarding the curing process with powder coat that I was told you may be able to assist me with. Currently we are having issues with leaving touch marks on the parts from where they contact our cart. It leaves a black line on the part, but we have been able to remove it with rubbing alcohol.

Our current oven setup is turning parts out of the oven at roughly 300-350 ⁰F with only 3-4 minutes of cooling before they are placed onto our carts. I believe that our parts have not had the proper amount of time to cool down and harden before they are placed on the carts. Most of our carts are lined with rubber edging, nylon or plastic tubing to keep from scratching the metal cart structure.

My first question is what is the general amount of time a part should rest before being handled or worked with? Additionally is there a material or handling method that you might recommend for handling them with our current setup.

Thank you,

Alan Young

Columbus, Indiana

...and Good Day to you, Mr. Young,

Your observations are good. The powder coating surface may be a little too soft to be contacted with the cart liners. Powder coatings, being thermosetting materials, harden or crosslink, when exposed to the proper curing conditions (heat and time). This thermosetting phenomenon causes the polymer and curing agent to chemically react, thereby creating a harder, more durable film. The hardness of this film will vary depending upon its temperature. Most thermosetting powder coatings reach a glass transition temperature (aka Tg) of somewhere between 185 and 250 ⁰F. Tg is the temperature in which the coating goes from a hard, glassy state to a softer, rubbery condition.

Therefore it is important to avoid handling a powder coated part if it is above the coating’s Tg. Most polyester and hybrid type powders have post-cure Tgs in the 185 to 200 ⁰F range. I suggest that you allow a part to cool to room temperature and see if you still observe marking. If that is the case, then you may need to change the composition of your cart liners. Alternately you can switch to a powder coating that has a higher Tg. One other thing to check is whether or not the powder is completely cured. Solvent rub testing is a simple means to ascertain degree of cure.

Another option is to request a lower-curing powder coating from your supplier. With a lower-curing product you can lower the temperature of your oven, and hence the parts will exit at a lower temperature and take less time to cool.

It is critical to accurately measure the temperature of the powder coating surface (you probably have already thought of this). A non-contact IR thermometer is a good way to do this.

I hope that this helps.

Thank you,

Mr. Powder

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