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Feature Article - June 2006

MEK No Longer a HAP

Barbara Kanegsberg

It may seem as unlikely as water flowing up-hill. However, in terms of regulatory restrictions, Methyl ethyl ketone (MEK) is now a more accessible option for general and critical cleaning applications, albeit with definite provisos. MEK is an aggressive cleaning agent with many applications for removing adherent soils. However, at the Federal level, MEK had been listed as one of nearly 200 hazardous air pollutants (HAPs).

Background
Just to be sure we are on the same page: MEK or 2-butanone has the CAS number of 78-93-3; a CAS number is useful for unambiguously identifying chemicals that might have different names; it’s sort of like a social security number for chemicals.

The status of MEK as a HAP prompted a number of solvent substitution studies. Compounds such as methyl propyl ketone (MPK) and methyl iso-butyl ketone (MIBK) were tested, with partial success. This is not surprising; solvency properties are fairly specific. Further, other properties such as evaporation rate and odor limit the utility of proposed substitutes. “Drop-in” substitutes are not the norm; and some of the proposed substitutes have other regulatory constraints. In fact, MIBK appears on the “EPA 17” list.

MEK HAP status
The good news is that MEK has been delisted as a HAP (1). The delisting as a HAP occurred because the American Chemical Council (formerly the Chemical Manufacturers Association) petitioned the EPA to do so. MEK was delisted at the end of 2005.

Prop 65 & ODC status
MEK has other favorable regulatory properties. MEK is not listed under California Proposition 65. It is not an ozone depleting chemical (ODC). ODC’s destroy stratospheric ozone; or desirable ozone.

Low-level Ozone
Unfortunately, MEK is a volatile organic compound (VOC); and it is likely to remain a VOC. Very often, solvents (organic compounds) are either ODC’s or VOC’s; the chemical properties tend to favor an “either/or” situation. There are few that are neither. Those that react near the earth’s surface and produce ozone are VOC’s. Those that survive until they reach the upper atmosphere may destroy ozone and therefore are ODC’s. At the Federal level, all organic compounds are classed as VOC’s unless compelling evidence to the contrary is presented to the EPA. Further, interested parties have to gather this evidence and apply to the EPA for VOC-exemption.

Not all VOC’s are created equal. Classically, reactivity is judged relative to ethane, where ethane serves as a “line-in-the sand.” That is, compounds with a lower reactivity than ethane are more likely to be exempt as VOC’s. VOC-exempt chemicals may still make smog, but they are looked on more favorably by regulatory agencies.

MEK has a relatively high tropospheric reactivity (i.e. it can react with light and other air contaminants to produce ground-level ozone). Recent maximum incremental reactivity (MIR) numbers are:
MEK = 1.48
Ethane = 0.31
While there has been some impetus to look at the actual reactivity numbers rather than use ethane as an “off/on” switch, the bottom line is that it is not likely that the EPA would consider MEK for a VOC exemption (2). This means there will continue to be restrictions, particularly in areas of poor air quality.

Use MEK Responsibly
Delisting MEK as a HAP is NOT synonymous with “use as much as you want without worker safety or engineering controls.” As with all chemicals, MEK must be used thoughtfully and responsibly.

MEK is flammable, with a very low flashpoint (various MSDS indicate minus 3 to minus 9 Deg. C.) This means that ignition sources must be eliminated. Please keep in mind that ignition sources can come not only from your process but also from nearby processes. By contrast, acetone has a lower flashpoint of minus 20 degrees C; MEK is still definitively a flammable solvent.
Worker exposure should be controlled and minimized. MEK is an eye and skin irritant; and it can cause dermatitis. Even though it is no longer a HAP, and even though MEK has a relatively high inhalation level (200 ppm TWA, PEL), inhalation exposure should be minimized. Excess exposure to MEK can result in CNS (central nervous system) depression. Of course, no one should ingest MEK.

Consider the process
MEK can be an option for cleaning, if other regulatory, worker safety, and process-specific issues are addressed.

Like acetone, MEK is a ketone. This means it has a carbon portion (good for dissolving oils) and a double-bonded oxygen portion (more water-like). This double-punch gives MEK a high solvency range. It has a boiling point of 80 Deg C and it has an evaporation rate of 2.7; this means it sticks around long enough before evaporating to interact with the soil and the part being cleaned. In contrast, acetone has a lower boiling point (57 Deg. C); and a much higher evaporation rate; so some people consider acetone to be a less effective cleaning agent, because it may evaporate before it removes the soil.

As we indicated, it is also a rather aggressive solvent. Aggressive solvents can react with the substrate (the material being cleaned); you do not want to have materials compatibility issues. Therefore, even if you think you remember how MEK behaved, test it again with your current product line.

Note: Data are obtained from published MSDS and from regulatory sources and are believed to be accurate. Please check all physical and chemical properties prior to using in your process. Please confirm your local regulatory constraints, as well as any company or customer constraints.

Ref:
(1) http://www.access.gpo.gov/su_docs/fedreg/a051219c.html
(2) MIR numbers are from recent studies by W. Carter, U.C. Riverside; the information was kindly provided by William Johnson, U.S. EPA.

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