Comparison of Chlorinated, Aliphatic, Aromatic, and Oxygenated Hydrocarbons as Solvents Wesley L. Archer* and Violete L. Stevens Dow Chemical U.S.A., Inorganic Product Department, Midland, Michigan 48640
Properties of chlorinated solvents are compared with those of hydrocarbon solvents. The comparison includes four chlorinated, two aliphatic, two aromatic, and four oxygenated solvents. In general, the physical and electrical properties, solvencies, and other parameters of the chlorinated solvents are intermediate between those of the hydrocarbon and the oxygenated solvents. Environmental impact and EPA regulations are considered. Emission of hydrocarbon solvents is restricted in many areas. Most chlorinated solvents have been classified as having low reactivity. Although toxicity varies among solvents, and there is considerable uncertainty concerning carcinogenic properties, there are no significant differences among OSHA regulations covering worker exposure to any of the hydrocarbon or chlorinated solvents.
Introduction More than four billion pounds (Chemical Economics Handbook, 1975) of solvents are used annually in the U S . to dissolve or disperse monomers, resins, and polymers for subsequent reactions, for extraction, or for the purpose of applying surface coatings and inks. Hydrocarbon solvents and oxygenated solvents dominate these applications. Here we discuss a class of solvents not widely known in these applications, whose properties may prove useful and timely-the chlorinated solvents. Chlorinated solvents are colorless, volatile liquids that exhibit high solvency for fats, greases, oils, waxes, and different resins and polymers. They are nonflammable by standard flash-point determinations and show low water solubility. Chlorinated solvents are miscible with alcohols, aromatic and aliphatic hydrocarbons, esters, ketones, and many other industrial solvents. The four chlorinated solvents listed in Table I represent a boiling range from 40 “C (104 O F ) for methylene chloride to 121 “C (250 O F ) for perchloroethylene. Solvent densities range from 1.32 to 1.62, and vapor densities (air = 1.0) range from 2.9 to 5.7. A detailed listing of physical properties is given. Chlorinated Solvent Applications As a class, chlorinated solvents have a variety of industrial applications, including use in adhesives, aerosol products, drycleaning, the electronic industry, extraction solvents, industrial solvent blends, metal cleaning, paint and coating solvents, pharmaceuticals, textile processing, and reaction media. The solvents most often used in these applications include methylene chloride, l,l,l-trichloroethane, trichloroethylene, and perchloroethylene. Metal cleaning, the largest single application, consumes more than 900 million pounds of chlorinated solvents yearly (Rekstad, 1974). Drycleaning, using perchloroethylene, is the second-largest solvent application. The estimated U S . demand (Bureau of Census) for the four solvents in 1976 was 1.965 billion pounds. Individual solvent demands included: methylene chloride, 500m lb; l,l,l-trichloroethane, 515m lb; trichloroethylene, 280m lb; and perchloroethylene, 670m lb. Initially, synthesis and formulation chemists often select the familiar classical solvents such as hydrocarbons and oxygenated hydrocarbons, because technologists are less aware of the properties of chlorinated solvents. This report cites specific comparisons of properties, toxicology, environmental impact and stability of these groups of solvents. The solvents in each of the groups were selected on the basis of boiling point
as well as use popularity. Table I lists the names and formulas of the related solvents. P r o p e r t y Comparison Basic physical properties useful in choosing a solvent for a specific application are listed in Tables I1 and 111. (Data listed in Tables I1 through VI are taken from references (Solvents Guide, 1963; Organic Solvents and Chemicals, 1967; and technical literature of Dow Chemicals Co., 1975)). Chlorinated solvents have the highest specific gravities (1.3 to 1.61, whereas oxygenated and hydrocarbon solvents have specific gravities that cover a range from 0.66 to 0.90. Surface tension and viscosity values for the chlorinated solvents are, on the average, greater than those for oxycarbons and hydrocarbons of similar boiling points. Chlorinated solvents have much lower specific heats and heats of vaporization than oxycarbons and hydrocarbons of similar boiling point and molecular weight. Chlorinated solvents, as a class, exhibit solvency properties that bridge those of oxycarbons and hydrocarbons. Table IV gives solubility parameters for the solvents. Chlorinated solvents have a solubility parameter range of 8.5-9.7, whereas oxycarbon and hydrocarbon types cover ranges from 9.1 to 14.5 and 7.3 to 8.9, respectively. The Kauri butanol values of chlorinated solvents are, on the average, higher than those of the hydrocarbons. Electrical property data are summarized in Table V. The values of chlorinated-solvent electrical properties fall between those of oxycarbon and hydrocarbon solvents. Products such as adhesives, coatings, and ink formulations use hundreds of millions of gallons of oxycarbon and hydrocarbon solvents. A principal disadvantage of these is their flammable nature, as shown by the flash-point data in Table VI. Perchloroethylene, on the other hand, has no flash point, no explosive limits in air, and no auto-ignition temperature. It might well be the solvent of choice where flammability or explosivity is a problem. Environmental Impact Environmental pressure on many industrial solvents began in the mid-1960’s with the adoption of Los Angeles County Rule 66. Experimental evidence indicated that smog formation was related to complex atmospheric chemical reactions between the oxides of nitrogen and certain reactive hydrocarbons in the presence of sunlight and oxygen. Rule 66 regulations limited the emission of certain reactive hydrocarbons to a maximum of 40 lb/day per emission source. “Reactive” Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
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Table I. Selected Chlorinated, Aliphatic, Aromatic, and Oxveenated Hvdrocarbons Solvent
Formula
Methylene chloride 1,l,I -Trichloroethane 1,1,2-Trichloroethylene Perchloroethylene n-Hexane Cyclohexane Toluene Ethylbenzene Methyl ethyl ketone Acetone Methanol Ethyl acetate hydrocarbons under Rule 66 included ethylbenzene, trichloroethylene, and toluene. However, solvent mixtures could contain as much as 20% of these three solvents in combination before control regulations applied. The original regulations permitted users who reduced solvent emissions by 85%to continue using controlled solvents. The specified 85% reduction could be accomplished by incinerating vent vapors or by carbon adsorption. The most expedient route of compliance under Rule 66, however, was use of an exempt solvent. Thus, during the first two years of controls in Los Angeles County, trichloroethylene used in vapor degreasing applications was almost entirely replaced by 1,1,1-trichloroethane or perchloroethylene. The Clean Air Act and its Amendments of 1970, which established the Environmental Protection Agency (EPA), extended many of the original Rule 66 limitations to other areas of the United States. A list of unreactive and, therefore, exempt solvents published by the EPA (Federal Register, Aug
14,1971) is as follows: . . . organic solvents which have been shown to be virtually unreactive in the formation of oxidants, e.g. saturated halogenated hydrocarbons (includes methylene chloride and l,l,l-trichloroethane), perchloroethylene, benzene, acetone, and CI-C5 n-paraffins also may be considered for exemption. Other compounds which have been shown to have low reactivity include cyclohexanone, ethyl acetate, diethylamine, isobutyl acetate, isopropyl alcohol, methyl benzoate, 2-nitropropane, phenyl acetate, and triethylamine. . . . Therefore, EPA-controlled solvents included here are trichloroethylene, n-hexane, toluene, ethylbenzene, cyclohexane, and methyl ethyl ketone. A summary of individual state air-quality-control regulations in association with EPA efforts has been published (Community Services Bulletin No. 61, 1974). Generally, the Rule 66 controls are followed, although some areas limit emission of photochemically reactive solvents to 15 lblday per emission source. Methyl ethyl ketone was not controlled under the original Rule 66 regulations. Under certain state regulations 85%reduction in emission of a controlled solvent is not an option. There are also more liberal controls on exempt solvents in some areas; for example, in Connecticut total emission for an exempt solvent is limited to 160 lblh or 800 lblday. During the past few years, use of a suitable substitute solvent has been the most practical means of complying with varied air emission standards. However, the EPA has recently indicated that it will develop new stationary source guidelines to reduce all organic emissions. The EPA is currently defining what reasonable control technology might be available t o reduce organic emissions. Rosen (1976) has commented on how the new EPA strategy might affect the paint and coating industry.
Table 11. General Physical Properties of Solvents
Solvent Methylene chloride 1,1,1-Trichloroethane Trichloroethylene Perchloroethylene n-Hexane Cyclohexane Toluene Ethylbenzene Methyl ethyl ketone Acetone Methanol Ethyl acetate
Specific gravity 1.336(10.5h5.5) 1.3249(26/4) 1.4655(20/70) 1.618(25/25) 0.6594(20/4) 0.7784(20) 0.868(20/70) 0.867(20/20) 0.8051(20) 0.7911(20/70) 0.7924(20/70) 0.9020( 20/70)
Refractive index
Surface tension, dyn/cm2
1.4237(25"C) 1.435(25"C) 1.478(20"C) 1.50547(20"C) 1.375(20"C) 1.429(15"C) 1.499(20"C) 1.493(25"C 1.37620(25"C) 1.3573(25"C) 1.3290(20 "C) 1.3725(20 "C)
26.52(18 "C) 25.56(20 "C) 26.36(20 "C) 32.32(20 "C) 17.91(25"C) 24.38(25 "C) 30(26.7 "C) 28.48(25 "C) 24.6(20 "C) 23.7(20 "C) 22.69(20 "C) 23.9(20 "C)
Viscosity (25 "C), CP 0.42 0.79 0.55 0.75
0.30 0.8 0.55 0.63 0.42(21.2 "C) 0.32 0.55 0.45(20 "C)
Table 111. Other Physical Properties of Solvents
Solvent
BP, "C
Methylene chloride l,l,l-Trichloroethane Trichloroethylene Perchloroethylene n-Hexane Cyclohexane Toluene Ethylbenzene Methyl ethyl ketone Acetone Methanol Ethyl acetate
40 74 87 121
68.7 80.7 110.6 136.2 79.6 56.2 64.8 77.2
FP, "C -97 -38 -87 -22
-95.3 6.5 -95 -94.9 -85.9 -94.3 -97.8 -83.6
320 Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
Sp heat, cal/g "C
Heat of vap., calk
0.24 0.24 0.22 0.21 0.53 0.47 0.39 0.41 0.490 0.528 0.597 0.478
78.7
56.7 56.4 50.1
82 86 86 145.7 106 125.3 262.8 87.63
Mol wt 84.93 133.42 131.4 165.85 86.16 84.16 92.13 106.16 72.10
Evap. rate (n- butyl acetate = 1) 14.5 6.0 4.46 2.1 -
1.5
58.08
7.7
32.04 88.10
4.1
3.5
Table IV. Solubilitv of Solvents
Solvent
Solubility parameter (6)"
Kauri butanol valueb
Water soluble in 100 g of solvent, 25 "C (g of HzO)
Solvent soluble in 100 g of water, 25 "C (g of solvent)
Solvent-HZ0 azeotrope, wt % HZOlbp "C
2.0 1.5138.1 9.7 136 0.20 Methylene chloride 0.07 4.3165 8.5 124 0.05 1,1,1-Trichloroethane 5.4173.3 0.10 9.3 129 0.04 Trichloroethylene 0.015 15.8187.8 90 0.01 9.4 Perchloroethylene 0.014 7.3 5.6161.6 30 0.01 n-Hexane 8.4168.95 65 0.01 8.2 Cyclohexane 0.047 19.6/84.1 105 0.05 8.9 Toluene 0.02 33/92 98.6 0.05 8.8 Ethylbenzene 26.8 11/73.45 11.8 9.3 Methyl ethyl ketone 10.0 Acetone 14.5 Methanol 9.1 3.3 8.7 Ethyl acetate a Solubility parameters ( 6 ) are numerical constants calculated from measurable physical properties, such as mol wt, density, and heat of evaporation. b Kauri butanol value (or KB number) is a measure of the relative solvent power of hydrocarbons or similar material. It is defined as the volume in milliliters at 25 "C of the solvent required to produce a defined degree of turbidity when added to 20 g of standard solutions of Kauri resin in n-butyl alcohol. Table V. Electrical ProDerties of Solvents
Solvent
Dielectric constant, 1 kHz, 20 "C
Electrical conductivity at 20 "C, mho
Methylene chloride l,l,l-Trichloroethane Trichloroethylene Perchloroethylene n-Hexane Cyclohexane Toluene Ethylbenzene Methyl ethyl ketone Acetone Methanol Ethyl acetate
9.14 7.53 3.27 2.20 1.91 2.10 2.38 2.30 15.45 21.45 31.20 6.40
4.3 x 10-1' 7.3 x 10-9 8 x 10-12 5.58 x 10-14
