The low flammability of the chlorinated solvents is one important consideration for using them in surface cleaning and product formulations. Methylene chloride (MEC), perchloroethylene (PCE or perc), and trichloroethylene (TCE) are highly ignition resistant. This is one of the reasons why the U.S. EPA, in its Significant New Alternatives Program (SNAP) for ozone depleting substances (ODS), lists MEC, PCE and TCE as acceptable replacements for ODSs. The document also states that the three chlorinated solvents should be used in aerosols only when non-flammability is a critical factor.

These chlorinated solvents have no flashpoint or firepoint, as determined by standard test methods. The flashpoint is defined as the lowest temperature of a substance at which application of an ignition source causes the vapor of a specimen to ignite under specific conditions. The tests commonly used to determine this temperature, as specified by the American Society for Testing and Materials (ASTM), are the Tag Open Cup test (ASTM D 56), the Pensky-Martens test (ASTM D 93), and the Setaflash test (ASTM D 3278 or D 3828).

However, with the exception of PCE, the vapors of these chlorinated solvents do have flammable limits in air. These limits define the concentrations in air (in volume percent) of a homogeneous mixture of solvent vapors in which a flame will propagate. These limits for MEC are 14-22%. For TCE they are from 8% to saturation point, 9.2%, at 77 deg F (25 deg C), and increase with temperature. At 212 deg F (100 deg C), the upper flammable limit is 44.8%.

Such concentrations, which greatly exceed the safe and legally acceptable breathing concentrations, are rare, but they may occur in confined or poorly ventilated areas, such as drums, tanks, enclosed mixing vessels, driers, or enclosed areas where a solvent spill occurs.

Certain precautions must be taken wherever flammable concentrations of vapor may occur. All sources of ignition should be eliminated (including lighted cigarettes) and only electrical equipment approved for use in hazardous locations should be used. It is a good general practice to ground all tanks to prevent the introduction of a spark. In addition, welding or flame-cutting procedures should not be done on empty tanks, drums or other vessels, which previously contained chlorinated solvents, unless the vessel is first purged with air or filled with water.

In addition to flammable limits, the chlorinated solvents (again with the exception of PCE) have an auto-ignition temperature at which the solvent vapors, within the flammable limits and in an enclosed chamber, will spontaneously ignite without the presence of a flame. This temperature is, for MEC, 1,185 deg F (640 deg C), and for TCE, 788 deg F (420 deg C).

It should be noted that the lower flammable limits and the auto-ignition temperatures of the chlorinated solvents are quite high compared to those of hydrocarbons and other solvents; further, above the upper flammable limit, the ignition resistance of the solvents comes into play. This results in comparatively narrow flammable ranges for the chlorinated solvents - an added margin of operational safety.

As noted above, none of these warnings regarding flammable limits or auto-ignition temperatures apply to perchloroethylene. PCE has no flammable limits within which a flame will propagate, and no auto-ignition temperature at which vapors will spontaneously ignite.

For more information on flammability, contact Dow Chemical Co., phone 800/447.4369 or visit www.chlorinated