- MSDS sheets: An evaporation rate is the rate at which a material will vaporize (evaporate, change from liquid to vapor) compared to the rate of vaporization of a specific known material. This quantity is a ratio, therefore it is unitless.
- General usage: The mass of material that evaporates from a surface per unit time (examples: 3 grams per square meter per hour, 1 inch per acre per month).
|Evaporation rate can be useful in evaluating the health and fire hazards of a material. For example, a substance with a high evaporation rate will readily form a vapor which can be inhaled or explode.Evaporation rates generally have an inverse relationship to boiling points; i.e. the higher the boiling point, the lower the rate of evaporation.
The general reference material for evaporation rates is n-butyl acetate (commonly abbreviated BuAc) which has the chemical structure shown below. Whenever a relative evaporation rate is given, the reference material must be stated.
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The relative evaporation rate of butyl acetate is 1.0. Other materials are then classified as:
(BuAc = 1.0)
|Fast||> 3.0||Methyl Ethyl Ketone = 3.8
Acetone = 5.6
Hexane = 8.3.
|Medium||0.8 to 3.0||95% Ethyl Alcohol = 1.4
Naphtha = 1.4
|Slow||< 0.8||Xylene = 0.6
Isobutyl Alcohol = 0.6
Water = 0.3
Mineral Spirits = 0.1
We are not aware of a specific number for the absolute evaporation rate (i.e. in mass/time units) of butyl acetate. Presumably, such a number would depend on myriad variables such as temperature, atmospheric pressure, humidity, air flow, viscosity etc. The ASTM has developed a standard test method, D3539-87(2004) Standard Test Methods for Evaporation Rates of Volatile Liquids by Shell Thin-Film Evaporometer. We don’t own a copy so we can’t give you a synopsis of the variables involved.
- Several formulas for predicting evaporation rates of liquid from a non-boiling pool are available in U.S. units and metric units at air-dispersion.com.
- Review of Glycol Ether and Glycol Ether Ester Solvents Used in the Coating Industry at Mindfully.org. Or download the original at http://ehp.niehs.nih.gov/members/1984/057/57001.PDF (158 KB PDF). This article touches on the concept of evaporation rate and how it has been used in the coatings industry.
- Additional suggestions welcome. We are unaware of any tabular source of evaporation rate data. If you need to find an evaporation rate, consult the MSDS for the material.