NATIONAL METAL FINISHING ENVIRONMENTAL R&D PLAN

II. R&D NEEDS AND POTENTIAL PROJECTS

B. Chlorinated Solvents

1. Background

Traditionally, the metal finishing industry has used chlorinated hydrocarbon solvents to remove oils, fats, waxes, and other organics from metal surfaces prior to finishing. Chlorinated solvents have been widely used until recently because they are very effective cleaners and are safe in the sense that they are nonflammable. The solvents most commonly used are 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113); 1,1,1-trichloroethane (TCA; also called methyl chloroform); trichloroethylene (TCE); tetrachloroethylene (also called perchloroethylene, or PERC); and dichloromethane (also called methylene chloride).

In the 1970s it was realized that some chloroflurocarbons (CFCs) undergo chemical changes in the upper atmosphere that subsequently lead to the destruction of stratospheric ozone which filters out much of the sun's harmful ultraviolet radiation. For this reason, the international community has sought to eliminate production and use of these ozone depleting compounds (ODCs). In addition, other chlorinated solvents, although not ODCs, are potential or probable human carcinogens. The Montreal Protocol and its London Amendments (1990) led to future changes in the U.S. Clean Air Act. The Clean Air Act Amendments of 1990 (CAAA) established a time frame to eliminate all fully halogenated CFCs, certain chlorinated hydrocarbons, and hydrochlorofluorocarbons (HCFCs). Because most of the chlorinated solvents are either carcinogenic or are ODCs, the general trend is to replace these solvents with more environmentally friendly chemicals.

2. Uses

Chlorinated solvents have always been very effective cleaning chemicals for metal finishers because of their great solvency for oils and greases, their high volatility that results in rapid drying and no residual film, and their non-flammable nature. A number of alternatives to chlorinated solvents have been developed in the metal finishing industry including various types of aqueous cleaning solutions; organic compounds with varying level of water miscibility such as terpenes, esters, and glycol ethers; non-chlorinated petroleum hydrocarbons; alcohols; supercritical fluids; and carbon dioxide snow and pellets. Unfortunately each of these alternatives has its own drawbacks. For example, much attention has been focused on aqueous cleaners that contain surfactants and sequestering agents to enhance their solvency. With these materials, however, there is the possibility of flash rusting and the need for energy-intensive drying prior to subsequent operations. In addition, the aqueous cleaning processes always generate a wastewater stream which must be treated and/or disposed of in some way.

Terpene-based cleaners can be very effective in cleaning oil and grease, but many times they have very low flash points and, therefore, must be handled carefully. In addition, the odors produced by some of these compounds can be very irritating to workers. Supercritical fluids, with water and carbon dioxide being the most common in cleaning, are very effective in cleaning; their physical properties facilitate their separation from the soil after the cleaning process is complete. Because of the high pressures involved in supercritical processes, however, the cost of equipment is prohibitive except in special cases.

A problem with many of the substitutes for chlorinated solvents is that they are very dependent on the method and form of delivery. Therefore, the equipment that is used to apply the cleaning chemical is extremely important and must be designed on almost a case-specific basis.

3. Federal Survey

The Federal survey yielded seven projects in alternatives to chlorinated solvent cleaners and strippers, and the Pacific Northwest database added one funded by a State agency to that number. Total funding was approximately $3.3M. All projects looked at alternatives, including aqueous and semi-aqueous operations, ultrasonic, liquid carbon dioxide, various innovative mechanical equipment, and others. Surprisingly, there were no studies devoted to recovery or "closing the loop" of chlorinated solvents-based cleaning or stripping systems; although one project is studying recovery of aqueous wash waters. EPA and DOD Army and Air Force are major funders of the work, which is usually demonstration and technology transfer.

4. Projects

The following are possible R&D projects.