Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

April, 2005

Calculating Retained Solvents in Paints

Q. I need to estimate the amount of methyl ethyl ketone (MEK) remaining in dry paint. The following are my assumptions: Use of an epoxy paint with MEK as one of the solvents, and the coating of paint is from 0.8 mils to 4.5 mils thick. I also need to know the weight (mg/L) of the paint when wet (or when applied) and when dry. Can you tell me the percentage of MEK in the paint initially and also give me an estimate of the remaining MEK after drying?

A. You've asked an interesting question, but I'm not sure that I can answer it.

It is easy to determine the amount of MEK in the wet paint, since you can get the info from the MSDS.

Since film thickness is a major parameter in the amount of retained solvent in the coating film, your range of 0.8 - 4.5 mils is very wide. Other major parameters are the type of spray gun used, the atomizing air pressure, the velocity of air passing over the coating after it has been applied, the ambient temperature, and the length of time allowed for the coating to dry before you determine the amount of retained solvent.

My guess is that the amount of retained solvent at the low film thickness (<2 mils) is probably less than 2%, but I don't have any basis for saying this. It's only a GUESS! At 4.5 mils you might retain 2-5%, again only a guess.

If there were more retained solvent in the coating you would experience blisters and delamination; therefore it is always our hope that essentially all of the solvent evaporates.

I assume that you intend to use this data for environmental reporting purposes, in which case you can conduct experiments using your own painters, spray guns, spray booth, etc., to give you reasonable data. Also, if you do intend to use this in emissions calculations, you will have justifiable numbers to report.

Again assuming that this is for environmental purposes please go to where you will see that I'm running a 3 1.2 day class that covers all this and more.

I hope this gives you some direction.

Best wishes,

Ron Joseph

Q2. Thank you for your response. I estimated the concentration of MEK in dried paint to be 1%. I used an applied thickness of 4.5 mls. I assumed that 4.5 mils was the thickness of paint applied. However, when I calculated the amount of paint used to coat a 55 gallon drum (surface area of 2.3 square meters) it came out to be just 0.26 ml (.37 grams of paint per drum based on a 12 lb/gallon weight of paint). I have dissention in the ranks regarding these numbers. Some of the assumptions I made was that the paint was sprayed on and after the paint was applied, the paint was cured by baking. Do you concur with my assumptions and results?

A2. Paint coverage is calculated by the equation (using US units):

Coverage (gals/ft2) = Gals x 1604 x %Volume solids x %Transfer efficiency
                                                           Dry film thickness (mils)

Where 1604 = coverage (ft2) of a solid gallon of any liquid applied at a film thickness of 1 mil (= 0.001 inches)

Volume solids is available from the coating datasheet or MSDS. If you add solvents (thinners) to the paint, you must recalculate the volume solids.

You will need to estimate the transfer efficiency. Depending on the spray gun used, the technique of the painter and other parameters, the transfer efficiency will vary. With a paint brush the TE is almost 100%. Since I'm not there to see how the paint is being applied I suggest that as a first assumption you can use a conservative value of 30%.

Don't assume the film thickness. Measure it either with a dry film thickness gauge, or strip some paint from the drum and measure it with a micrometer gauge.

Here is another thought:

I'll let you worry about the units.

The volume of SOLID paint on the drum (after the solvents have evporated) (ft3) = Surface area of the drum (ft2) x Dry film thickness (ft).

The weight of the solid paint on the drum (lbs) = Volume of paint (gal) x Density of the SOLID paint (lbs/gal).
(Convert gals to ft3)

Density of SOLID paint = %weight solids of the coating (from the MSDS) x Density of the wet paint (lbs/gal) also obtained from the MSDS)

Obviously you will need to use coversion factors to convert the units to what you want.

Please get back to me with your findings.

Best wishes,

Ron Joseph

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