Solvents are used in many critical and precision cleaning processes because they are “sharp scalpels” that dissolve specific soils. Other factors include cost, employee safety, and environmental considerations. For example, acetone is widely utilized because it is an aggressive cleaner; it has a relatively favorable profile in terms of toxicity; and it is not considered a VOC.
However, acetone and other common cleaning agents such as isopropyl alcohol and the azeotropic blend of IPA and cyclohexane have very low flashpoints (see definitions in Table 1).
Many solvents that can be used in standard vapor degreasers with effective chilling coils do not have a flashpoint; but they do have flammability limits.1 If the concentration of solvent in air is below the LEL, it is too lean (not enough fuel) to burn; if it is above the UEL, it is too rich (not enough oxygen). For example, the range for gasoline is 1.2% to 7.1%. The role of a carburetor or fuel injector is to achieve the proper mix of gasoline and air (remember when a car engine could “flood” and fail to start from too much gasoline in the combustion chamber? The concentration of gasoline vapor was above 7%). The concern with chemicals having flammability limits is with open trays and spraying in areas with ignition sources.
HEAT
Elevated temperature increases solvency, but brings the chemical closer to the flashpoint. OSHA requirements for process fluids depend not only on the class but also on the temperature. For example under 29CFR- 1910.106(a)(18)(iii), when a combustible liquid is heated to within 30 °F of its flashpoint, it is to be handled in accordance with the requirements for the next lower(more flammable) class of liquids.
INERTING FLAMMABILITY
Replacing hydrogen with a halogen in an organic compound reduces its flammability. One use for halogenated solvents is to blend them with a flammable solvent that has the desired solvency properties to inert flammability.
Cleaning agents may change in composition or chemically. This is a concern with non-azeotropes or nearazeotropes; 2 chemicals can, over time, convert to flammable compounds; and soils have the potential to contribute to flammability.
EQUIPMENT
We emphasize that flammable liquids should be used only in equipment (e.g. vapor degreasers and ultrasonic systems) specifically designed for low flashpoint materials.
In point of use cleaning, the fuel (the flammable or combustible solvent) and the oxidizer (oxygen) are present. It is crucial to remove proximal ignition sources—electrical equipment, light switches, anything that might generate sparks—from the vicinity.
FLAMMABLE AND COMBUSTIBLE SOLVENTS HAVE VALUE
You do not need to avoid using flammable and combustible solvents; many such solvents are essential tools to achieve effective cleaning and contamination control. Be cognizant of the chemical, of the process, and of proximal activities that might impact flammability. In addition, with all cleaning chemistries, be aware of the potential for compositional or chemical changes that may affect flammability.
The authors wish to thank James Unmack, CIH for his helpful review and comments.
References:
- B. Kanegsberg and R. Shubkin, “Solvent Flammability Basics,” Clean- Tech Magazine, November/December, 2003.
- B. Kanegsberg and E. Kanegsberg, “Are the Cleaning Agents Clean Enough? Part III—Solvents”, Controlled Environments Magazine, June, 2008.
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of The Handbook for Critical Cleaning, CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; [email protected]; www.bfksolutions.com.
