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Evaporation Rate

Definition

• 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).

MSDS Relevance

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:

Speed	Evaporation Rate (BuAc = 1.0)	Examples
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.

In the absence of evaporation rate data, you can roughly assess the volatility using the vapor pressure of the material.

Further Reading 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. See also: alcohol, boiling point, vapor pressure, volatility. Additional definitions from Google and OneLook.

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Disclaimer: The information contained herein is believed to be true and accurate, however ILPI makes no guarantees concerning the veracity of any statement. Use of any information on this page is at the reader's own risk. ILPI strongly encourages the reader to consult the appropriate local, state and federal agencies concerning the matters discussed herein.