by Ron Joseph
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 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
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 www.ronjoseph.com/compliancetraining.htm
where you will see that I'm running a 3 1.2 day class that covers all this and
I hope this gives you some direction.
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
A2. Paint coverage is calculated by the equation (using US units):
Coverage (gals/ft2) = Gals x 1604 x %Volume solids x %Transfer efficiency
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
Please get back to me with your findings.