Cleaning metal parts

Although perchloroethylene is best-known in North America as a solvent for textile finishing and dry cleaning garments, it also is suitable for surface cleaning metal parts. Because perchloroethylene (also called PCE, perc, and tetrachloroethylene) has the highest boiling point, weight, and vapor density of all the chlorinated solvents, it is used for several applications in the metal forming and fabricating industry.

PCE's high boiling point gives it a clear advantage in removing high-melting-point waxes and resins during vapor degreasing. The high temperature of the vapors also means there is more condensation on the workpiece compared to solvents with lower boiling points, washing the work with a larger volume of solvent.

Lightweight and light-gauge parts, which might reach the operating temperature of lower-boiling-point solvents before cleaning is complete, have longer vapor contact for more effective cleaning. Also, PCE's high boiling point is essential for obtaining good penetration in parts with fine orifices or spot-welded seams. Inherently more stable than the other chlorinated solvents, PCE also is more forgiving of moisture, which makes it particularly effective in cleaning parts with entrapped moisture.

While it can be used to degrease all common metals, PCE is especially suitable for cleaning those that stain or corrode easily, such as aluminum, magnesium, zinc, brass, and their alloys.

PCE's suitability for vapor degreasing has been recognized by the U.S. Environmental Protection Agency (EPA, www.epa.gov). The Significant New Alternatives Policy (SNAP) for ozone-depleting substances, issued by the EPA on March 18, 1994, gives industry permission to consider PCE (as well as certain other chlorinated solvents) as an alternative to 1,1,1-trichloroethane, which was phased out of production in 1995.

This rule applies to applications in surface cleaning, aerosols, adhesives, coatings, and inks. The EPA has stated, however, that these alternative solvents should be used in aerosols only where nonflammability is a critical factor.

Safety and Health Considerations

PCE is not known to present a health hazard to workers when used according to the manufacturer's instructions and government regulations. Many manufacturers and industry associations offer information on proper, safe handling of PCE.

The most significant potential hazard in industrial use is inhalation of vapors at concentrations higher than those recommended for safe operation. Strict adherence to all applicable exposure guidelines and regulations, using low-emission equipment or equipment retrofitted to reduce emissions, and proper ventilation and engineering controls in the work area minimize the potential for vapor inhalation.An effective education program for equipment operators also is essential.

Extensive tests and epidemiological studies have been undertaken to characterize PCE. In a recent report, the American Council on Science and Health (ACSH, www.acsh.org), an independent nonprofit consortium of physicians and scientists dedicated to consumer education, noted that studies of PCE done by government agencies differ widely.

"A careful examination of the science behind such differences," the report states, "reveals that the more data that are considered, the lower the estimated risk. Indeed, the agency that incorporates the most data, Health Canada, concludes that PCE poses little or no risk to the public."

ACSH's conclusion? There is "no credible evidence that adverse public health effects...are caused by environmental exposure to PCE."

Most industry experts believe that when all the data are considered, PCE can be used safely when users follow the recommended exposure limits, which were established by taking into account the relevant data on both hazard and risk. Trade associations such as the Halogenated Solvents Industry Association (HSIA, www.hsia.org) currently are undertaking additional research and independent reviews of existing and new laboratory and epidemiology studies.

Regulatory Standards

The Occupational Safety and Health Administration's (OSHA, www.osha.gov) 1971 permissible exposure limit (PEL) for PCE was 100 parts per million (PPM) on an eight-hour time-weighted average (TWA), with a short-term exposure limit (STEL, which is 15 minutes) of 200-PPM ceiling concentration and a 300-PPM peak. In 1989 OSHA reduced the eight-hour TWA to 25 PPM, with no ceiling or peak concentrations. However, in 1992 this standard was overturned, and the PEL for PCE has reverted to the limits set prior to 1989. Dow Chemical Co. continues to recommend following the 25-PPM standard.

The American Conference of Governmental Industrial Hygienists (ACGIH, www.acgih.org) recommends a threshold limit value (TLV) of 25 PPM for PCE, with a STEL of 100 PPM.

Emissions can be held below these exposure levels when engineering controls are implemented or fully enclosed degreasing equipment is used.

The EPA recognizes that PCE does not contribute appreciably to the formation of ground-level smog. Consequently, the agency exempted PCE from regulations governing volatile organic compounds (VOC) March 8, 1996.

Title III of the Clean Air Act (Hazardous Air Pollutants) lists PCE as one of the substances to be regulated, and major sources of PCE emissions are required to use maximum achievable control technology (MACT) to reduce emissions. PCE use also must be reported under the Superfund Amendment and Reauthorization Act (SARA) Title III, Section 313.

Emissions standards for chlorinated solvent degreasing operations now are governed by EPA's National Emissions Standards for Hazardous Air Pollutants (NESHAP) for organic solvent cleaning (Federal Register, Vol. 59, No. 231, 61801-61820). These standards cover both vapor degreasing and cold cleaning with PCE, as well as with several other solvents. The goal of the NESHAP regulation is an overall reduction in solvent emissions of 50 to 70 percent of current nationwide emissions.

Emissionless Degreasing

Many companies have found it cost-effective to adopt new degreasers that have no air-vapor interface. These sealed and almost emissionless units were first introduced in Europe to meet the stringent environmental regulations in some countries, and most of them originally were designed for use with PCE, which is the vapor degreasing solvent most commonly used in Germany.

Although these emissionless units can be costly, a number of plants in the U.S. have found them economical because they comply with safety and environmental regulations, they conserve solvent, they save floor space, and they provide good parts cleaning performance. Several brands of these degreasers are available in North America today.

With the application of engineering controls to minimize PCE vapor emissions and the use of emission-free equipment, users can achieve compliance with safety and environmental regulations.