Ask Joe Powder – October 2023

PPCJ’s regular columnist gives advice to readers on all things powder coatings. Joe Powder is Kevin Biller’s alter ego and you can submit questions to [email protected]

Dear Joe,

I hope you are well.

While doing some research on thermoplastic powder coating an idea came to my mind which I don’t know if anyone tried it out, or if it is nonsense! One of the benefits of thermoplastic powders is that they eliminate or reduce coating porosity. This is often a problem especially on galvanised steel. We tend to apply the epoxy primer after preheating the metal structures to avoid pin holes.

Would it be commercially viable to apply the primer as thermoplastic in a fluidised bed and then apply the top coat with a gun? Also, I saw other methods such as flame spraying powder, like the metallising method of applying zinc wire.

I would appreciate your advice on this.

Thanks

Matthew, Malta

Hey Matthew,

Excellent question, as always. This is definitely something to explore. Here are some comments:

1. As you know, thermoplastics do not chemically react to form a tight, three-dimensional molecular network. Hence, they can reflow (move) when heat is reapplied. This may cause wrinkling of a subsequently applied coating, a topcoat, for instance.
2. Thermoplastic powders include a vast array of polymer types with varying molecular weight and rheological performance (melt point and viscosity). So, some are rather soft like PVC or hard like polyamide (nylon).
3. Examples of thermoplastic polymers used as powder coatings are PVC, nylon, polyethylene (high density and low density), polyester, co-polymer, PVDF and PTFE. The co-polymer types are evolving and some now offer the toughness of a soft, elastic polymer with a harder, more rigid one that provides improved hardness and scratch resistance.
4. Nevertheless, I would investigate commercially available thermoplastic powder coatings that are described as primers. In particular I would check out the polyamide powders that Evonik and Arkema offer. This could be a great idea. Please keep in mind that thermoplastics are typically applied at relatively high film builds, commonly 150 to 200 microns depending on the type and particle size.
5. Another thing – many thermoplastic powder coatings are offered in “electrostatic spray grades” so fluidized bed application or ES spray could be contemplated. As for flame spray – I’m not a big fan. It takes specialised equipment and film thickness control would be an issue. Furthermore, I don’t like the idea of introducing a cloud of combustible organic particles into a source of ignition either.

I hope that you and your loved ones are doing great and that your business is flourishing. I see your posts on LinkedIn and am always impressed with the quality of your work. You should be proud.

Please let me know your further thoughts on the TP primer concept.

Best regards,

Joe

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Hi Joe,

My name is Chad and I have attended two seminars with you and have a question about powder.

We currently are looking into spraying some plastic chips and some other possible designs in our lab for an upcoming coating show but have been unsuccessful in finding a conductive primer. I contacted a major coating supplier after finding a tech data sheet online about a conductive primer they offered, but they no longer make it and told me they have nothing at all to offer that can go under powder.

Do you happen to know of anything that can be sprayed onto plastic objects prior to the powder being sprayed? Any help would be great.

Chad, Cincinnati

Hi Chad,

Thanks for the question. Indeed, there are a couple options to make a plastic substrate conductive enough to electrostatically apply powder coating. First a few caveats:

1. Ensure that the plastic can take the powder bake cycle. Plastics are characterised by heat distortion temperature (HDT) which signifies the maximum temperature in which the plastic will maintain its integrity. At the HDT the plastic will begin to soften and distort. Make sure that the HDT of the plastic to be used is at least 10°C higher than the powder bake temperature.

2. Make sure that the plastic is clean. Mould release compounds are notorious for causing poor wetting and adhesion of powder coatings and any other coating for that matter. You may want to clean the surface with an appropriate solvent prior to attempting to apply the powder.

3. Applying the powder is a little tricky. Make sure that the substrate has a good earth. In addition, reduce the kV’s and amperage of the powder spray gun. I would start at 55kV’s and move higher or lower until you develop good, even deposition.

Now for the key component to make this work. I know of two approaches that provide sufficient conductivity on non-conductive substrates which will facilitate electrostatic deposition of powder coatings.  The first is Ransprep™ which is available from a company called Chemical Technology Inc. They can be reached at +1 (313) 893-4930.

The second approach involves applying an iodine-based solution. You can make your own solution by diluting tincture of iodine (alternately you can use a “teat dip” product). Use either isopropyl or ethyl alcohol for the dilution. I recommend a 1% concentration. The solution should be applied to the substrate by spray or dipping. Then thoroughly dry the substrate. This should provide the conductivity needed to apply the powder coating.

Good luck with your project Chad and let me know if you have any further questions.

Best Regards,

Joe Powder