EPA/600/A-97/095
High Performance Metal Cleaning Using Liquid CO, and Surfactants
by
Charles H. Darvin
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Research Triangle Park, NC 27711
Brad Lienhart
MiCELL Technologies Inc.
NCSU Centennial Campus
1017 Main Campus Dr., Suite 3500
Raleigh, NC 27606
Introduction
The landmark treaty, the Montreal Protocol signed in 1987, and the U.S. Clean Air Act
Amendments of 1990 mandated the end to production and use of substances that deplete the
Earth's ozone layer, sometimes called ozone depleting substances (ODSs). Since that time the
developed industrial world has been struggling to find suitable and effective alternatives for these
critical industrial chemicals. Two of the more widely used ODSs are the chlorinated solvents
1,1,1-trichloroethane (TCA) and trichloroethylene (TCE), both used for surface cleaning of metal
components and parts.
Many industrial operators have dealt with the challenge of surface cleaning by pushing
the problem off onto their suppliers, by utilizing "no-clean" options where possible, and to a
large extent by attempting to find workable aqueous cleaning processes. While much
improvement has been made to the aqueous cleaning technology, from improved detergents, to
new agitation techniques, it is clear that a growing number of users are still looking for solvents
that perform like the chlorinated solvents TCE and TCA. One potential option, still in the
developmental stage, is the use of carbon dioxide (C02) in either the liquid (LC02) or the
supercritical (scC02) state. These two states differ only in their containment pressures. CO, is a
plentiful, non-ozone-depleting compound, that is non-toxic, non-flammable, and easily
recyclable. Unfortunately, C02 alone in either state is not capable of precision cleaning for most
applications without some type of enhancement to improve its solvency capability.
Background
The solvency capability of C02 has been known and used in the food processing industry
and in wastewater treatment for a number of years. Since the early 1980s, research has been
underway to investigate the use of supercritical carbon dioxide (scC02) as a solvent substitute in
the polymerization of hydrocarbon monomers. This work also generated an interest in the
potential use of CO, as a surface cleaning agent. The use of scC02 as a surface cleaning agent,
however, has not been a totally satisfactory solution by itself. Since it is not a true liquid, scC02
docs not possess the capability that a liquid system might provide in washing the surface to allow
removal of particles from the surface that is being cleaned. For cleaning beyond removal of light
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oils, it has been found to be generally ineffective. Even LC02 does not provide a significantly
greater surface cleaning capability other than for general surface cleaning.
Beginning in 1990 and working under the leadership of Dr. Joseph DeSimone of the
chemistry department at the University of North Carolina at Chapel Hill, a group of 25 research
colleagues at major universities, made a major breakthrough in the development of a C02
compatible surfactant chemistry.1 The research to polymerize hydrocarbon monomers in scCO,
led to the development of a group of unique C02 compatible phillic surfactants. The key to both
polymerization and surface cleaning is the surfactant used which adds functionality and enhances
the efficiency of the solvent. This development of a class of C02 compatible surfactant results in
significant improvements in the potential of C02 as a solvent and thus as a cleaning agent. This
work was supported by the U.S. EPA and a consortium of eight chemical companies: DuPont,
Hoechst Celanese, Eastman Chemical, General Electric, Xerox, Air Products, BF Goodrich, and
Bayer.2
General and Precision Cleaning
The key to the eventual use of C02 for general cleaning' applications must be the
improvement of its cleaning efficiency for a broad range of contaminants and surfaces. In
chlorinated and aqueous based solvent systems, this is accomplished by the addition of
surfactants and other chemicals to the cleaning system. However, conventional solvents are not
compatible with C02. The newly developed C02 compatible surfactants will serve the same
purpose in the LC02 system as conventional surfactants in chlorinated or aqueous based surface
cleaning systems. They will make it possible to clean the parts of contaminants that up to now
have not been affected by C02 in any state including heavy oils, greases, and waxes. Being in a
liquid state, it will also allow for washing of the surface as in conventional liquid systems. In
addition, in the liquid state the process will operate at ambient temperatures and at pressures of
only 800 to 1200 psi (81 x 106 to 122 x 106 Pa), which most components can withstand without
any damage.
LC02 has been shown to be capable of cleaning some metal surfaces where precision
cleaning is not of great concern.3 However without the enhancement provided by the surfactants,
LC02 has limited application for precision cleaning. With a density close to that of water, and
a viscosity more like a gas, LC02 has the capability to remove particulate matter to a greater
degree than scC02. Laboratory testing at MiCELL Technologies Inc., and at North Carolina
State University's Chemical Engineering Department, have demonstrated significant cleaning
performance improvement when using LC02 and the new surfactants on various contaminants
and material surfaces. MiCELLTechnologies Inc. has been licensed to produce and market C02
surfactants for use in cleaning systems.
Potential of the LC02 Cleaning Technology
The application of LCO, with the surfactant additives technology is one of the most
promising surface cleaning options since the development of chlorinated solvent cleaners. A
LC02 system will function similar to most present liquid systems and preserve all of the
functional advantages of those systems. Since it is liquid, it can penetrate surface depressions,
holes, and surface imperfections, and evaporates from the surface without leaving a residue.
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I
Finally, with the proper surfactant package, it will remove the surface contaminants removed by
CFC and aqueous based solvents including oils, greases, and particulates.
CO, is not an ozone depleting compound and can be acquired as a by-product from
ammonia fermentation and petrochemical processes. This eliminates a requirement to
manufacture CO, which would add to the discharge of additional greenhouse gas into the
atmosphere. The material cost of C02 and the surfactant package is relatively low when
compared to typical chlorinated solvents or equivalent substitutes used for surface cleaning.
There are no applicable regulations for the LC02 process. The process residue stream
could be deemed hazardous depending on the nature of the contamination removed from the
surface. The Occupational Safety and Health Administration (OSHA) has guidelines for C02
exposure which encourage monitoring to ensure that C02 concentrations do not exceed 10
percent by volume of the average daily air intake.4
Conclusions
The development of a viable surfactant system for use with LC02 offers an
environmentally safe and efficient system. The development of a compatible surfactant allows
the potentials of CO, to be fully realized as a clean environmentally safe and non-polluting
surface cleaning process . Since C02 can be recycled and reused, the only waste product from
the process is the contaminant itself. With the development of viable surfactant additives, the
major remaining developmental activity for this technology is the design of a fully integrated and
economical system for industrial application.
References
1. McClain, J.B., Betts, D.E.,Canelas, D.A., Samulski, E.T., Desimone, J.M., Londono, J.D.,
Cohran, H.D., Wignall, G.D., Chillura-Martino, D., and Triolo, R., "Design of Nonionic
Surfactants for Supercritical Carbon Dioxide," Science 1996,274, 2049-2052.
2. Black, Harvey, "Supercritical Carbon Dioxide: The Greener Solvent." Environmental Science
and Technology/News, Vol 30, No 3, 1996:124-127.
3. Darvin, C. H., and Hill, E.A., "Demonstration of Liquid C02 as an Alternative for Metal Parts
Cleaning," Precision Cleaning, IV, 9: 25,1996.
4. Department of Labor, Occupational Safety and Health Administration, 29 CFR, 1910.1000,
Subpart Z, "Toxic and Hazardous Substances," 1987.
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moodt -rtp-p-'JR'? TECHNICAL REPORT DATA
IV ivinL n l JT l tSOI (Please read Instructions on the reverse before completing}
1. REPORT NO. 2.
EPA/600/A-97/095
3. REI
4. TITLE AND SUBTITLE
High Performance Metal Cleaning Using Liquid CO2
and Surfactants
5. REPORT OATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Charles H. Darvin (EPA) and Brad Lienhart (MiCELL)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME ANO ADDRESS
MiCELL Technologies, Inc.
North Carolina State University Centennial Campus
1017 Main Campus Drive, Suite 3500
Raleigh, North Carolina 27606
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
NA (Inhouse)
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper; 1990-1997
14. SPONSORING AGENCY CODE
EPA/600/13
IS.supplementary notes APPCD project officer is Charles H. Darvin, Mail Drop 61, 919/
541-7633, For presentation at International Conference on Ozone Protection Tech-
nologies, Baltimore, MD, 11/12-13/97.
16.abstract paper discusses high-performance metal cleaning using liquid carbon
dioxide (C02) amd novel surfactants. {NOTE: The chlorinated solvents 1,1,1-tri-
chloroethane (TCA) and trichloroethylene (TCE), both used for surface cleaning of
metal components and parts, are two of the more widely used ozone depleting sub-
stances.) While much improvement has been made to the aqueous cleaning technol-
ogy, from improved detergents to new agitation techniques, a growing number of
users are still looking for solvents that perform like TCA and TCE. A potential op-
tion, still in development, is the use of C02 in either the liquid (LC02) or super-
critical (scC02) state. There has been a major breakthrough in developing a C02-
compatible surfactant chemistry. Research to polymerize hydrocarbon monomers
in scC02 led to the development of a group of unique C02-compatible phillic surfac-
tants. The key to both polymerization and surface cleaning is the surfactant used
which adds functionality and enhances the efficiency of the solvent. This development
of a class of C02~ compatible surfactants results in significant improvements in the
potential of C02 as a solvent and thus as a cleaning agent. This work was supported
by the U. S. EPA and a consortium of eight ehcmical companies: DuPont, Hoechst
Celanese, Eastman, G. E., Xerox, Air Products, B. F. Goodrich, and Bayer.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COS ATI Field/Group
Pollution
Metal Cleaning
Carbon Dioxide
Surfactants
Solvents
Ozone
Pollution Prevention
Stationary Sources
13 B
13H
07B
UK
18. DISTRIBUTION statement
Release to Public
19. SECURITY CLASS (This Report J
Unclassified
21.NO. OF PAGES
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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