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Ask Joe Powder

"Ask Joe Powder"
"Ask Joe Powder" is a question and answer column authored by Kevin Biller of the Powder Coating Research Group. Mr. Biller has over 30 years experience formulating and manufacturing powder coatings. He welcomes your questions regarding powder coating technology. Please write to: askjoepowder@yahoo.com 

Kevin Biller
The Powder Coating Research Group
15 W. Cherry Street, 3rd Floor
Columbus, OH 43215
614-354-11988
www.powdercoatingresearch.com 



Dear Joe,
Is there any method, equation or software program to calculate powder density? Thanking you in advance,
Ali Baluli

Dear Ali,
I am aware of two methods used to determine powder density. Both are covered in detail in ASTM D5965 - 02(2007) Standard Test Methods for Specific Gravity of Coating Powders.

One uses the volume displacement of the powder into a fluid (kerosene or hexane) with a known density. The weight of the powder is known so the relationship between weight and volume can then be calculated.

Powder Specific Gravity = [Weight of Powder (grams)] /[ Final Volume – Original Volume (milliliters)]

This method involves introducing the fluid into a graduated cylinder. The volume and weight of the fluid is recorded. Next a given weight of powder is mixed into the fluid and the displaced volume is determined. It is essential that you eliminate all air pockets in the mixture to obtain a reasonably accurate measurement. Please be aware that this method doesn’t easily account for the surface porosity common with most powder coatings and typically results is a lower than true specific gravity. Nonetheless it can be used as a decent tool to compare powders.

A much more accurate method is based on the Ideal Gas Law and utilizes a gas pyncometer instrument which measures volume of a known weight of powder by gas displacement. These are relatively expensive instruments and are available from a number of commercial instrument suppliers. Each instrument is slightly different; some measure volume, others can measure volume and density. You would have to consult the specific procedure provided by the instrument manufacturer to successfully measure specific gravity of powders.

I recommend you use the simpler fluid method but always run a control sample of know specific gravity along with your samples to be evaluated.

I hope that this helps.

Best Regards,

Joe


Powder Coating Magnesium Alloys

Hi Joe,
I have a cleaner/phosphate that is supposed to treat Magnesium but how should it be handled as far as dry-off and cure temps?

I tried a couple of parts this morning, with a low gloss clear coat and they came out looking like desert storm Camouflage. This was cured for 12 minutes at 180°C.
Bryan

Dear Bryan,
Thanks for the question. Magnesium alloys are a tricky substrate to powder coat unless you know how to do it. Most magnesium fabricated products are cast resulting in a certain degree of porosity on its surface. Cleaning the substrate is a great idea however the cleaners/ pretreatment can remain harbored in the pores. Indeed, even without cleaning air resides in the pores. As the powder melts and flows the cleaners and air escape from the pores. Most powders are curing at this point and can't recover or reseal the holes caused by the volatiles. The result is pinholes, low gloss and unsightly surface disruptions.

My advice is to continue cleaning as you are but run the parts through a relatively high temperature dry-off before you apply the powder coating. It's preferable to coat the parts very soon after the dry-off, even while they are still warm so they don't re-absorb ambient moisture. As for dry-off temperature - 200°C for 10 minutes is a good place to start.

You should also be aware that many powder suppliers offer product lines that are better suited for porous substrates such as magnesium. It may be best to use one of these with a well- controlled dry-off process.

Good luck.

Joe