Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 33

To show the instability of unstabilized perchloroethylene to light, a typical test might proceed for
72 h, during which perchloroethylene with water and copper strips is heated to boiling and the vapor
is exposed to l uorescent light. A change in pH of 1 unit or more is taken as an indicator of light
instability. Without a light stabilizer, perchloroethylene can change from pH 6.8 to less than pH 2 in
72 h in the presence of light (Stevens, 1955). The decomposition of perchloroethylene is more pro-
nounced when it is in contact with iron or copper. Products of perchloroethylene decomposition
cause metal corrosion. Perchloroethylene will also degrade in the presence of light from a welder's
arc (Dow Chemical Company, 1999b).

1.2.4.3 Thermal Breakdown
The major chlorinated solvents are stable and resistant to thermal breakdown at their normal boiling
temperatures, but are vulnerable to thermal breakdown and acid formation if operating temperatures
exceed their stability range (Petering and Aitchison, 1945; Archer and Stevens, 1977). The order of
increasing thermal stability does not directly correspond to the order of the solvents' boiling points.
The order, from least to most thermally stable, is methyl chloroform (b.p. = 74°C) < dichloromethane

(b.p. 121.4°C) (Archer and Stevens, 1977).

TCE is stable up to 130°C; at higher temperatures, it is subject to pyrolysis. Perchloroethylene is
stable up to 150°C in the presence of air and moisture (Solvay SA, 2002b). In the absence of any
catalysts, such as in the sealed environment of an electrical transformer, perchloroethylene may be
stable up to 500°C (World Health Organization, 1984a). Decomposition in the presence of air and
moisture begins above 125°C for TCE and above 150°C for perchloroethylene. For safety reasons,
equipment operators are advised to avoid exceeding temperatures of 110°C and 140°C for TCE and

= 41°C) < TCE (b.p. = 86.7°C) < perchloroethylene (b.p. =

TABLE 1.15
Pyrolysis Test Results for the Major Chlorinated Solvents

Temperature at Which
Slow Increase in
Pyrolysis Begins (°F)

Temperature at Which
Rapid Increase in
Pyrolysis Begins (°F) Solvent Metal Vapor Condition

Dichloromethane Black iron Vapor only 525 700

Black iron With dry air <400 600

Aluminum Vapor only 376 525

Copper Vapor only 475 525

Methyl chloroform Black iron Vapor only 525 625

Black iron With dry air — 325

Aluminum Vapor only — 675

Copper Vapor only ~350 675

Trichloroethylene Black iron Vapor only — 825

Black iron With dry air — 550

Aluminum Vapor only ~550 725

Copper Vapor only — 575

Perchloroethylene Black iron Vapor only —

>900

Black iron With dry air — >900

Aluminum Vapor only — >750

Copper Vapor only —

>850

Source: Archer, W.L. and Stevens, V.L., 1977, Industrial & Engineering Chemistry, Product Research and Development
16(4): 319-326.
Note: Pyrolysis test consisted of passing chlorinated solvent vapor (0.2 mol/h), with or without air, through a heated metal
pipe whose internal surface area was 200 in. 2 at several elevated temperatures. Acidity of heated vapors was then
determined by collecting 40 mL of condensed solvent from a cold trap; acid generated was determined by titrating
with caustic soda.