Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 31

ultrasonic cold cleaning, in vapor degreasing, and as a paint thinner. Perchloroethylene with a minor
complement of stabilizers may remain stable for multiple uses when degreasing large pieces of
stainless steel. The same formulation may become unstable when cleaning a more reactive metal or
the same metal in which i ne turnings, grindings, and other metal i nes create a substantially
increased surface area over which reactions may occur or which may cause hot spots near heating
coils and lead to pyrolysis of the solvent. Determination of the preferred stabilizer formulation
therefore depends on the severity of the environment to which the solvent is subjected.
Among the most demanding applications requiring stabilization of perchloroethylene is its use as
a dielectric l uid. A dielectric l uid must remain stable for up to 30 years without breaking down to
form electrically conductive or corrosive materials. Breakdown of perchloroethylene by dehydro-
chlorination forms hydrogen chloride, which is conductive and therefore deleterious to the dielectric
l uid and the electrical device used (EPO, 1989).
Another application in which perchloroethylene and TCE are subjected to severe stress is in
phosphatizing baths. Nonaqueous phosphatizing baths are used to apply phosphate coatings to
metallic surfaces, for example, automobile exteriors, to reduce corrosion and improve paint adhe-
sion. Besides orthophosphoric acid and an agent to solubilize the acid, the phosphatizing bath
is mostly TCE or perchloroethylene. These two solvents can undergo rapid decomposition by a
mechanism different from the type of decomposition for which most stabilizers were developed;
thus, an application-specii c group of stabilizers is required (Fullhart and Swalheim, 1962).
1.2.4 C AUSES OF S OLVENT B REAKDOWN
The primary causes of solvent breakdown—oxidation, hydrolysis, pyrolysis, ultraviolet light, and
reaction with alkali metals—have been introduced. This section examines the chemical reactions
that cause solvent deterioration. The structures of the chlorinated solvents ( Table 1.1 ) play a decisive
role in the nature of their decomposition.
1.2.4.1 Oxidation
TCE and perchloroethylene are susceptible to oxidative attack on the ethylene carbon-carbon double
bond. Oxidative attack is a problem for the ethylene compounds perchloroethylene and TCE, but is not
a problem for the alkane compounds methyl chloroform, dichloromethane, and carbon tetrachloride,
which have single carbon-carbon bonds or only a single carbon atom (Levine and Cass, 1939). The
initial deterioration of the ethylene bond in TCE is due to the oxidation initiated by light (photolysis,
in which light energy is absorbed by a molecule). The reaction is strongly accelerated by ultraviolet
rays from natural light, l uorescent lighting, and arc welding, by heat, and by the presence of catalysts,
including metals such as iron and aluminum, and metallic salts such as ferric chloride and aluminum
chloride. TCE is relatively immune to photolysis, except in the presence of oxygen (Shepherd, 1962).
The products resulting from the oxidation of TCE also serve to promote further degradation of TCE.
Humidity does not appear to affect the rate of oxidation of TCE (Solvay SA, 2002b). Air, moisture, and
acid favor further deterioration once oxidation has been initiated. Because acid is evolved, the deterio-
ration of TCE is autocatalytic (Pitman, 1933). As the breakdown of TCE progresses, the reaction takes
the form of a self-accelerating Friedel-Crafts reaction. Once started, the reaction will proceed rapidly
and, in some cases, explosively with the evolution of heat and large quantities of hydrogen chloride.
The acceleration of TCE degradation by ultraviolet light when oxygen is present suggests a free
radical mechanism. A variety of products are formed, including phosgene gas, carbon monoxide,
dichloroacetic acid, formic acid, glyoxylic acid, hydrochloric acid, chlorine gas, and a variety of
polymer compounds (Shepherd, 1962).
Most halogenated solvents are resistant to attack by oxygen until a chlorine atom is removed.
Removing a chlorine atom and an adjacent hydrogen atom leaves a double bond, which is more
susceptible to oxidation, particularly in the presence of metals such as aluminum, copper, and vana-
dium (Howell and Tarrer, 1994).