Chlorinated solvents metal cleaning

Commercial use of many chlorinated derivatives imposes stress on the stabHity of the solvent. Inhibitors classified as antioxidants (qv), acid acceptors, and metal stabilizers are added to minimize these stresses. AH the chloriaated derivatives hydrolyze at a slow but finite rate when dissolved ia water. Hydrolysis of chloriaated solvents typicaHy Hberates hydrogen chloride that can corrode storage containers and commercial metal-cleaning equipment. The Hberated hydrogen chloride can be neutralized by an appropriate epoxide to form noncorrosive chlorohydrins (qv). 

AH volatile organic solvents are toxic to some degree. Excessive vapor inhalation of the volatile chloriaated solveats, and the central nervous system depression that results, is the greatest hazard for iadustrial use of these solvents. Proper protective equipment and operating procedures permit safe use of solvents such as methylene chloride, 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene ia both cold and hot metal-cleaning operations. The toxicity of a solvent cannot be predicted from its chlorine content or chemical stmcture. For example, 1,1,1-trichloroethane is one of the least toxic metal-cleaning solvents and has a recommended threshold limit value (TLV) of 350 ppm. However, the 1,1,2-trichloroethane isomer is one of the more toxic chloriaated hydrocarboas, with a TLV of only 10 ppm. 

Trichloroethane and other chlorinated solvents are used for vapor degreasing (84—90). Other uses include cold metal cleaning, printed ckcuit board cleaning, and as a solvent for inks, coatings, adhesives, and aerosols. 1,1,1-Trichloroethane is an excellent solvent for development of photoresist polymers used in printed ckcuit board manufacture (see Integrated circuits Photoconductivepolymers). 

Tetrachloroethylene was first prepared ia 1821 by Faraday by thermal decomposition of hexachloroethane. Tetrachloroethylene is typically produced as a coproduct with either trichloroethylene or carbon tetrachloride from hydrocarbons, partially chloriaated hydrocarbons, and chlorine. Although production of tetrachloroethylene and trichloroethylene from acetylene was once the dominant process, it is now obsolete because of the high cost of acetylene. Demand for tetrachloroethylene peaked ia the 1980s. The decline ia demand can be attributed to use of tighter equipment and solvent recovery ia the dry-cleaning and metal cleaning iadustries and the phaseout of CFG 113 (trichlorotrifluoroethane) under the Montreal Protocol. 

Chemistry (Continued) polyimide, 287-300 polyurethane, 222-236, 546 transition metal coupling, 483-490 Chiral conjugated polymers, 479-480 Chlorinated solvents, 91 Chlorofluorocarbons (CFCs), 201, 205 Chloroformate endgroups, 87 Chloromethylation, 354 Church, A. Cameron, 431 Circular dichroism, 490 Classical catalysts, 433 Clean Air Act of 1990, 201, 205 Clearcoat, 240... 

Common gases such as oxides of carbon and nitrogen, hydrogen sulphide, and inert gases. Liquids which pose a health hazard due to volatilization, e.g. mercury and degreasing with chlorinated solvent, i.e. dry cleaning with perchloroethylene or metal cleaning with trichloroethylene. 

MC was a popular solvent for cleaning of semiconductor and degreasing of metal parts, but due to its ozone depletion potential, it has beer phased out and its use is strictly controlled. We have studied the photocatalytic decomposition of this as a typical chlorinated saturated hydrocarbon. 

Other Chlorinated Ethylenes. Trichloroethylene was a major solvent for degreasing in the late 1960s and early 1970s. Since that time, its production has decreased from 500 million lb to 100 million lb in 1993 because of environmental pressures on the solvent users and replacement by 1,1,1-trichloroethane. Recently, trichloroethylene has recovered market share in metal cleaning due to the phasing out of 1,1,1 -trichloroethane in 1996. Also, the use as precursor for HFC-134a synthesis continues to increase. The production volume in 1998 was 245 million lb. 

Chlorinated solvents and many other solvents used in industrial finishing and processing, dry-cleaning plants, metal degreasing, printing operations, and so forth, can be recycled and reused by the introduction of carbon adsorption equipment. To predict the size of the adsorber, you first need to know the vapor pressure of the compound being adsorbed at the process conditions. 

In addition to releases from the various components or activities that make up the production and distribution system for petroleum products (the oil system), many older waste sites show TPH-related site contamination. Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) site descriptions often mention petroleum, oil and grease, or petroleum, oil, and lubricants (POL) as present at a former waste disposal site. An example is given below for a waste oil recycling site, where TPH-type chemicals were obviously a common site contaminant. The CERCLA clean-up actions, however, focus on a range of specific hazardous or toxic chemicals. Some of the specific chemicals (e.g., toluene) would show up in a TPH test, but the chlorinated solvents and metals do not. Since a site cannot be prioritized for CERCLA attention if the only problem involves TPH site... 

Recent work indicates that metallic iron emplaced in a permeable barrier or wall can degrade chlorinated solvents in groundwater by a coupled redox reaction in which the solvent is the oxidant and the reduciant is Fe metal or product Fe or H2 (cf. Wilson 1995 Roush 1995). A similar Fe metal barrier has been used to reduce chromate concentrations to values below detection, and has been suggested as a means of cleaning up groundwaters high in nitrate (Wilson 1995). 

Phosgene is released during the welding of metals that have been cleaned up with chlorinated solvents, so welders may be exposed to this compound. 

Phosgene was originally manufactured as a war gas. It is now used in the manufacture of dyes, resins, and pesticides. It is also commonly produced when chlorinated compounds are burned, such as in a fire, or in the process of welding metal that has been cleaned with chlorinated solvents. 

Tetrachloroethylene is commercially important as a chlorinated hydrocarbon solvent and as a chemical intermediate. An estimate of the current end-use pattern for tetrachloroethylene is as follows 55% for chemical intermediates, 25% for metal cleaning and vapor degreasing, 15% for dry cleaning and textile processing, and 5% for other unspecified uses (Chemical Profile 1995). 

Aluminum should not be used as a material of construction for pumps, tanks, pipelines, valves, spray equipment and other handling equipment used for chlorinated solvents. Properly stabilized halogenated solvents are however commonly used in cleaning aluminum and other sensitive metals. 

Uses Stabilizer in chlorinated solvents for metal cleaning and degreasing prod, of butanediols, glycol esters and ethers prep, of suractants and gasoline additives intermediate acid scavenger for chlorinated compds. 

The second Montreal Protocol (1990) restricted the production and use of 1,1,1-TCA because of its high ODP, ultimately leading to its phase out by 2005. Metal cleaning is one of the important end uses of C2 chlorinated solvents, and emission of these solvents is a major issue. The metal industry has improved operations to recover these solvents and minimize emissions into the atmosphere. A similar trend is ongoing in dry cleaning operations to reduce the use of PCE. Phase-out of PCE is well underway. 

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