Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 28

Solvents can rapidly deteriorate to the point where the work and the degreasing equipment are
damaged. For example, methyl chloroform and methylene chloride readily undergo a decomposition
reaction in the presence of aluminum chloride; to a lesser extent, TCE and perchloroethylene may
also react with aluminum chloride. Solvent degradation is accompanied by a rapid rise in tempera-
ture and a discoloration of the solvent. The reaction advances until a black tarry mass forms and
deposits on the work and the degreasing equipment (Starks and Kenmore, 1960a; Petering and
Aitchison, 1945). Unstabilized methyl chloroform exposed to aluminum chloride or other alkali
metal salt catalysts completely decomposes to tar in 20 min (Hardies, 1966). In addition to the tarry
by-product of solvent decomposition, acids formed in the reaction may corrode the metal surfaces
in a degreaser or dry cleaner. The i rst sign of acidity in a degreaser is often corrosion at the vapor-
atmosphere interface, where rust appears on the walls of the unit around the cooling coils and
condensate trough (Dow Chemical Company, 1999b).
The overall consequences of solvent decomposition—worker exposure to harmful or fatal gases,
damage to the products being cleaned, and damage to the degreasing equipment with corresponding
loss in production—provide a strong motivator to keep solvents in good condition. Operators dealing
with solvent that has become acidic owing to loss of stabilizers or other factors seek either to recover
the solvent through on-site distillation or to replace the solvent. Solvent replacement could involve
on-site recycling or collection and hauling to off-site solvent recyclers or to a hazardous waste incin-
erator or landi ll; more expedient on-site disposal would have been in burn trenches, by drum burial,
or directly disposing to the ground, before such practices were prohibited by actively enforced
regulation.
The consequences of solvent breakdown were also strong motivators for the solvent producers to
develop stabilizer packages capable of withstanding the numerous adverse conditions encountered
in the industrial uses of chlorinated solvents. Equipment manufacturers for degreasing and dry
cleaning were also challenged to design solutions to ensure that solvents were not subjected to
extremes of temperature and stabilizer loss. Examples include increasing the freeboard in the
degreaser to minimize vapor loss and the resulting imbalance of stabilizers in the solvent, as well as
improvements to water separators and carbon adsorption systems for vapor recovery.

1.2.3 S TABILITY OF THE M AJOR S OLVENTS
Among the four major chlorinated solvents, perchloroethylene is often listed as the most stable,
whereas methyl chloroform is considered the least stable. Mixtures of chlorinated solvents, such as
perchloroethylene and methyl chloroform, tend to decompose at an accelerated rate, compared to
perchloroethylene alone (Goodner et al., 1977). Table 1.13 summarizes the relative stability of the
major chlorinated solvents to aggressive agents encountered in operating environments.

TABLE 1.13
Solvent Stability in Harsh Operating Environments

Solvent Oxidation Hydrolysis Alkali Metals Pyrolysis UV Light

Dichloromethane Stable Stable Slightly unstable in

vapor phase

Slightly unstable Stable

Methyl chloroform Slightly unstable Slightly unstable Unstable Unstable Stable

Perchloroethylene Slightly unstable Stable Stable Stable Unstable

Trichloroethylene Unstable Stable Slightly unstable Slightly unstable Slightly unstable

Sources: Solvay, S.A., 2002a, Chlorinated solvents stabilisation. GBR-2900-0002-W-EN Issue 1—19.06.2002, Rue du
Prince Albert, 33 1050 Brussels, Belgium. http://www.solvaychemicals.com (accessed November 9, 2003) and
Irani, M.R., 1977, United States Patent 4,032,584: Stabilized methylene chloride. Assignee: Stauffer Chemical
Company, Westport, CT.