Environmental Engineering Reference
In-Depth Information
solvent. Cold cleaning in open tanks usually employed a layer of water atop the solvent to prevent
solvent losses from evaporation (Gregersen and Hansen, 1986). Baskets i lled with parts are
immersed in the solvent tank and allowed to soak long enough to loosen and dissolve oils, grease,
rosins, or l ux. In processing printed circuit boards, a photoresist stripper employs cold cleaning by
using an in-line design to dip parts on a conveyor rack (USEPA, 1989b).
Physical agitation of parts in a solvent bath aids cleaning. Operators use brushes or spray wands
i tted with nozzles to clean parts, or they may use mechanical agitators or l ow provided by circula-
tion pumps. Circulation pumps produce solvent l ow at the desired velocity. Agitation is achieved
with oscillators that move parts back and forth through the cleaning solution or with ultrasonic units
that use sound waves to induce high-frequency vibrations in and around the part (AFCEE, 1999;
PRO-ACT, 1999; Dow Chemical Company, 1999b).
In ultrasonic cleaning, the cleaning action of the solvent is supplemented by sonically induced
cavitation, which produces intense physical cleaning. Electricity at approximately 20 Hz powers
a crystal or magnetic transducer, which expands and contracts with each cycle to generate shock
waves in the solvent. Shock waves cause the rapid formation and collapse of low-pressure bubbles
throughout the solvent, which creates a vigorous scrubbing action. Ultrasonic cleaning is often
utilized in the manufacture of electronic components and printed circuits as part of a cold-cleaning
or vapor-degreasing operation. Methyl chloroform is a common ultrasonic cleaning agent
(Considine, 1974). Ultrasonic cleaning is especially useful for cleaning metal chips from blind
holes or removing small insoluble materials from i ne machined surfaces, such as particulate resi-
due from honing operations and some bufi ng compounds. Ultrasonic cleaning is most efi cient in
nonagitated liquids at temperatures 25-40°C below the solvent's boiling point (Dow Chemical
Company, 1999b).
Methyl chloroform was preferred for cold cleaning because its worker-exposure threshold was
signii cantly higher than that of other chlorinated solvents. Cold cleaning applications for methyl
chloroform included maintenance cleaning of electronic components and precision instruments,
small parts, aircraft parts, and other parts that require a high degree of cleanliness (USEPA, 1989b).
TCE, perchloroethylene, and dichloromethane, while being very effective as cold-cleaning solvents,
have lower worker-exposure thresholds and are regulated as hazardous air pollutants and as volatile
organic compounds (USEPA, 1994a). In 1987, 70 million pounds of chlorinated solvents were used
by cold cleaners in the United States. However, the amount of chlorinated solvents used for cold
cleaning decreased dramatically as methyl chloroform was phased out following the Montreal
Protocol in the 1990s. Methyl chloroform was replaced primarily with dichloromethane, but also
with 1-bromopropane, limonene, terpenes, and other “designer” solvents (Petroferm, Inc., 1997;
Dow Chemical Company, 1999b). Solvents used in immersion tanks were usually recycled on-site
by i ltering or distilling the solvent (AFCEE, 1999; PRO-ACT, 1999).
1.1.1.3 VaporDegreasing
Degreasers have played a vital role in industrial manufacturing, particularly in the automobile,
heavy equipment, aircraft, appliance, railroad, and electronics industries. Approximately 25,000
vapor degreasers operated in the United States in 1979 (Tarrer et al., 1989). Degreasing is favored
for the removal of oils imparted to the item being cleaned (“the work”) from cutting, molds, dies,
welding, soldering, stamping, and processing, as well as for the removal of soil carried in cutting
oils, such as metal i nes and l uxes. Parts that have been die cast, stamped, machined, welded,
soldered, or molded carry residues that can be cleaned by vapor degreasing. Vapor degreasers intro-
duce solvents to small inaccessible crevices or “blind holes,” allowing quick and efi cient cleaning
and drying (Mertens, 2000a). Vapor degreasing is used for removing mineral oil contamination
prior to electrocleaning and subsequent electroplating.
Vapor degreasing produces very clean, dry parts, suitable for subsequent i nishing work such as
the application of rust protection, primer, or paint. Clean dry parts are needed to facilitate quality
control inspection and for subsequent assembly of machines. Parts requiring further metal work,
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