Particle size of powder and role of DFT

A. Normally, particle size of powder is 5 micron. Bigger particle size gives High DFT & consumption; and also would be a reason of orange peel after application.

Thanks,
SURYA

A. DFT (Dry Film Thickness) is not applicable to powder coating, but it is important to know the coating thickness. (Powder coating isn't ever "wet", so it doesn't dry).

PC coating thickness is much more uniform than DFT in wet (solvent based, or liquid borne) paints.

To some extent, the thicker coatings are more protective, but there's a limit. Typically powder coating might be 50-80 microns, but its possible to get to about 120µ.
With wet paint you might achieve 200-300µ with multiple coats.

But too thick of anything leads to increased risk of delamination.

Dear Surya,
I was very interested in your reply and the new technology that you quote. I would be very interested in exploring the technology that produces the majority of the powder within the 5 micron bracket. I know that the Japanese attempted to apply powder at these microns and ran into problems with transporting the powder to the gun head due to the poor fluidity at these low microns.
Normally, with any coating, liquid or powder, opacity would be controlled by the choice of pigment to binder ratio depending upon the pigment used. Therefore a correctly formulated white liquid paint applied at 18 microns should result in achieving opacity as good as a powder at 75 microns. What is the P/B of your powder and what is its opacity like using a 5 microns powder?

A. To answer the original question "Dry Film Thickness" DFT and its effect on general powder coating:
1) Naturally, a film wt of 75-100 microns will weather longer than one at 45-50. However, the choice of resin would be considered to contribute more to the long term properties required as would the pre-treatment used at the application stage including paying attention to attaining a good final cure of the system.
2) Opacity will vary at various DFT, but a good formulator will try to achieve an acceptable opacity at what we believe to be a universal minimum DFT of 75 microns. However, it is believed that there are some patented powders achieving good opacity at 20 microns.
3) Coverage, by which I presume you mean "Mileage per Kilo"? DFT is one of the factors in determining coverage higher the DFT, lower the mileage. However, those powders containing higher proportions Barytes will have lower mileage per kilo due to increased SG's.
4) My understanding is that consumption and coverage is the same thing. However, see the mileage calculator by following the image to the left. As an exercise, try and alter the SG, film weight and application efficiency figures in the spreadsheet to see how these alterations affect the mileage per kilo.
Particle size of powder would take a whole lot longer to explain but suffice it to say that it has an influence upon the powder's ability to retain an electrical charge and therefore its transfer efficiency. A commercial PC product having some particles that are as low as one micron to a very small percent that are over one hundred microns appears to satisfy most users' requirements for a material that is cost effective, decorative and in many cases where it is required, protective.

A. The most common method employed is to use a metal comb device with varying length teeth.
These are commercially available.
You drag it over the uncured object and note the depth of the teeth which pick up powder.
Hope this helps.

A. There is such a device for measuring uncured powder thickness. It is called a dry film thickness gauge. Here is a link to one: www.elcometer.com/international%20index%20pages/international/Coatings/index.htm. They are quite expensive though. Last time I got a quote they were around $8000 US dollars.

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Ed. note Jan. 2014: That particular link is now broken, although the overall site is still live.

A. A cheap gauge ($25.00) supplied by a USA company, see www.farwestcorrosion.com/ccpcoatings/gardner01.htm
It consists of a disc with 0-100 mil along its outer rim. Just roll through the powder film to obtain DFW. I think it's call Hotgauge ... or similar. Let me know how it works.

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Ed. note Jan. 2014: That particular link is now broken, although the overall site is still live.

A. Hi there.

When the powder coating is dry do you use a DFT meter to record the dry film thickness? -- we do. If the results of the average readings are between 50-100µm this is an OK value. Anything less than 50 needs more coating. We do our powder coating on galvanized steel (you didn't mention your product). Hope this is relevant, Lisa