Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2003

Solvent Evaporation from Paint

Q. How do you calculate the Evaporation Rate in pints/min as is required for the dilution ventilation equation Q = (403X10^6 *SG * ER * K )/ (MW * C) We have the MSDS for the paint, but are not sure as to how to calculate in pints / min from the % by weight of each ingredient.

A. Can I ask you why you need to calculate the evaporation rate? Frankly, no calculation will give you an accurate or even moderately accurate answer. The evaporation rates of the various solvents in the coating are affected by the other solvents and also by the speed with which the coating starts to setup and cure. If you were to place all the solvents that are in the coating into a pint can and watch their evaporation rates, they would be very different from the evaporation rate of the same solvents, in the same concentration, within the coating.

Temperature and air movement cearly are major factors, and I don't see them referenced in the equation that you provided.

Another complication relates to the application. If you spray apply a coating at moderate pressure, a high percentage of the solvents will evaporate even before the coating has formed a film. The extent of evaporation will depend on the atomizing air pressure, fluid flow rate, type of spray gun being used, paint particle size, etc. This flash evaporation will have a huge influence on your theoretical calculation.

Therefore, I'm back to my original question. Why do you need this information? I hope you are not intending to use it for EPA purposes. Please fell free to get back to me.