TITLE: Precision Cleaning with Supercritical Carbon

Dioxide for the Elimination of Organic

Solvents and Reduction of Hazardous Waste

AUTHOR(S): W. D. Spall, S. B. Williams, and K. E. Laintz

Los Alamos National Laboratory

Chemical Science and Technology Division

CST-12, MS E537

Los Alamos, New Mexico 87545

PRESENTED AT: AIChE 1995 Summer National Meeting

in Extraction in Environmental Applications

Paper 41d

**ABSTRACT**

Private and governmental industrial facilities are accustomed to using
chlorofluorocarbons and chlorocarbons for the cleaning of a variety of
items. The Montreal Protocol (1987) and amendments to this act will
phase out the use of chlorofluorocarbons, or CFCs, by the year 2000
because they are toxic, carcinogenic, and implicated in the depletion of
the Earth's ozone layer. The United States has pledged to eliminate
these substances by 1995. To stay competitive in the global market, US
industries require an economical replacement. Supercritical fluids,
which have been used in food, fragrance, and petroleum processes for
years, are attractive replacement solvents because of their low
environmental impact, high diffusivities, low viscosities, and
temperature-pressure dependence of solvent strengths. In the case of
nontoxic and nonflammable carbon dioxide (CO~2~), its critical
temperature and pressure are readily accessible with well established
process technology and equipment. In addition, applications using a
supercritical fluid such as CO~2~ are generally safer and
environmentally benign as compared to conventional organic solvents.
Extractions using supercritical CO~2~ use less energy than distillation
and incineration processes and are less expensive than liquid extraction
processes using toxic and costly organic solvents. Finally, CO~2~ has a
very high volatility compared to virtually any organic extractant which
facilitates its separation from extract solutions for extract recovery
and CO~2~ recycle. Data will be presented on the successful removal of
cutting and machine oils, silicone oils, body oils, and hydraulic fluids
from a variety of industrial substrates with supercritical CO~2~ to, at,
or below precision cleaning levels (less than 10 micrograms of
contaminant per square centimeter of surface). The applicability of this
technique to commercial operations was evaluated in the areas of
contaminant removal, surface interactions, operational costs, and waste
reduction and elimination.

**INTRODUCTION**

The importance of product cleaning in many industrial operations is
obvious. Current parts cleaning processes can be broadly divided into
aqueous and nonaqueous based cleaning systems. Aqueous based cleaning
systems can basically be considered a soap and water approach to parts
cleaning and are quite effective for many cleaning applications.
However, aqueous cleaning has some disadvantages. The most obvious is
that many parts are not amenable to water cleansing, and this can be
compounded by long drying times and flash rusting which are often
associated with aqueous cleaning. In addition, the user can be faced
with expensive water treatment processes both prior to actual use in the
cleaning system and prior to waste water disposal. These problems led to
nonaqueous or organic solvent based cleaning systems. Many industrial
facilities currently employing nonaqueous cleaning technology are
accustomed to using chlorocarbons and chlorofluorocarbons (CFCs) for the
cleaning of a variety of items. Unfortunately, the Montreal Protocol
(1987) and amendments to this act will phase out the use of CFCs by the
year 2000 because they are toxic, carcinogenic, and implicated in the
depletion of the Earth's ozone layer. For these reasons, the United
States has pledged to eliminate these substances by 1995. To stay
competitive in the global market, US industries require an economical
replacement. Any cleaning solvent replacement must be discussed in terms
of a definition of what type of item is being cleaned and what
cleanliness level is required.

Most cleaning specifications are based on the amount of specific or
characteristic compounds or contaminants remaining on the surface being
cleaned. Potential common contaminants can include machining oils and
greases, fingerprints, body oils, hydraulic and damping fluids,
adhesives, waxes, and particulates. Table 1 presents a simplified
contamination scale for oil and can be regarded as a reasonable estimate
of cleanliness levels (Spell, 1993). For the purposes of this paper,
precision cleaning is defined as an organic contaminant level of less
than 10 micrograms of contaminant per square centimeter (ANSI, 1990).
This 10 µg/cm2 level of cleanliness is either very desirable or required
by the function of parts such as electronic assemblies, optical and
laser components, electromechanical elements, hydraulic items, computer
parts, ceramics, plastics, and many cast or machined metals (Spall,
1993). It should be noted, however, that the goal for most precision
cleaning levels is less than 1 µg/cm2 for all types of soils, with the
most stringent military specification being 0.4 µg/cm2 (McHardy, et al.,
1993). Now, any CFC replacement solvent under consideration should be
able to remove commonly encountered soils to these levels from a variety
of surfaces, including printed circuit boards, plastics, metals,
rubbers, composites, and glasses. Supercritical fluids, which have been
used in food, fragrance, and petroleum processes for years, are
attractive replacement solvents since they can remove many of these
common contaminants from a variety of surfaces.

**CLEANING WITH SUPERCRITICAL FLUIDS**

In order to appreciate the unique properties of supercritical fluids
that make them ideal solvents for many cleaning applications, it is
helpful to define a supercritical fluid. The physical states of all pure
substances can be represented using a phase diagram, which is a
representation of the states of the material as a function of
temperature and pressure or of other properties of the material. In
order to define a supercritical fluid, it is helpful to use a phase
diagram based on temperature and pressure, and a temperature-pressure
phase diagram of a pure substance is shown in Figure 1. The lines in the
phase diagram depict the conditions of phase changes of the material
(e.g., from liquid to gas). The critical point is defined by both a
pressure (Pc) and a temperature (Tc). The region beyond the critical
point is called the supercritical fluid region. Once a substance attains
this state, the phase boundary between the gas and liquid states
disappears, hence the fluid exhibits properties of both a gas and a
liquid. Supercritical fluids have low viscosities and nearly zero
surface tensions. Diffusion coefficients of supercritical fluids are
between those of liquids and gases. These properties make supercritical
fluids ideal for cleaning parts having porous, intricate, or rough
surfaces or confined work areas because the solvent can penetrate into
these regions to remove contaminants. As a result, the removal process
is often more rapid than when using conventional liquid organic
solvents. Supercritical fluids can also possess \"liquid-like\"
densities, which leads to excellent solubilities for many compounds.
With supercritical fluids, density is a selective function of
temperature and pressure. Generally, density increases with increasing
pressure and decreases with increasing temperature. These combined
effects can provide unique sets of solvation parameters. In general, it
should be noted that a large increase in compound solubility occurs when
going from the gas to the supercritical state. This enhanced solubility
of organic compounds in the supercritical state combined with the low
viscosity, low surface tension, and high diffusion rates of
supercritical fluids form the basis for using supercritical fluids as
cleaning solvents.

The solvent of choice in supercritical fluid processing and cleaning
applications is often carbon dioxide for several reasons. One reason
CO~2~ is often used is because its critical parameters (critical
temperature and pressure) are easily attainable. The critical point of
CO~2~ occurs at 31.1 degrees Celsius and 1070 pounds-per-square-inch
(psi) of pressure. In addition, CO~2~ is generally safe to work with.
That is, it is environmentally benign and nontoxic. For this reason,
CO~2~ has been used in the food industry for many years.

Extraction or cleaning apparatuses using supercritical fluids are
conceptually simple, and a basic schematic diagram of such a system is
illustrated in Figure 2. Some source of liquid CO~2~ such as standard
gas cylinders or a large storage vessel provides fluid flow to a pump.
The pump is then used to pressurize the CO~2~ above its critical
pressure. At this point, the CO~2~ is usually in a high pressure liquid
state. This liquid can then be introduced in the extraction vessel and
then heated above the critical temperature or heated prior to
introduction to the extraction vessel. The cleaning action then occurs
in the vessel as the supercritical CO~2~ extracts the contaminants from
the substrate being cleaned. The supercritical CO~2~ containing
dissolved contaminants then flows through the extraction cell and is
then expanded into the separator. It is here that as CO~2~ becomes a gas
through the expansion process, the contaminants fall out of solution and
are collected. The gaseous CO~2~ is then passed back into the flow
stream to be used again in the cleaning process. The system shown in
Figure 2 and many cleaning systems run as a batch mode operation. While
batch cleaning may be slow when compared to a continuous cleaning
system, a batch CO~2~ cleaning process can be used to recycle the
contaminant. For example, expensive oils used in the processing of
beryllium metal are recovered and reused. In addition, with this type of
cleaning, the solvent CO~2~ is removed by releasing the pressure in the
extraction chamber resulting in cleaned components that are dry and
ready for immediate use.

Overall operational costs for CO~2~ cleaning tend to be lower than for
other cleaning processes. The low critical parameters of CO~2~ make it
quite cost efficient in terms of energy use during operation. It was
found that energy costs for our large scale cleaning operation for a
standard run at a temperature of 30°C and a pressure of 1500 psi cost
\$0.90 per hour while the highest energy costs occurred at 40°C and 3500
psi for a total electrical cost of about \$2.75 per hour (Barton, 1994).
While energy costs are low, the elevated pressures required to attain
the supercritical state can cause the cost of the cleaning vessel to be
relatively high. This initial high capital cost of supercritical CO~2~
cleaning equipment and has caused many potential users to opt for
aqueous and semi-aqueous based cleaning systems (Snowden-Swan, 1994).
However, with more companies beginning to offer supercritical CO~2~
cleaning systems and custom tailoring of the cleaning systems to actual
process needs, the capital cost of CO~2~ cleaning technology has
decreased substantially. Other operational costs for CO~2~ cleaning
systems remain low. For example, when compared to various organic
solvents, CO~2~ is much less expensive. For example we can purchase
liquid CO~2~ for about \$0.03 per pound, whereas we pay \$45.00 per
pound for Freon-113 (1,1,2-trichlorotrifluoroethane). With all of these
factors in consideration, supercritical CO~2~ cleaning is a viable
alternative for many cleaning applications.

Supercritical fluids are in no way an absolute solution to all cleaning
problems. Many substances requiring removal in cleaning operations,
inorganic or ionic contaminants, for example, are insoluble in
supercritical fluids with low polarities such as carbon dioxide. More
polar supercritical fluids such as ammonia or water are capable of
dissolving polar and ionic compounds, but these fluids are very reactive
and cause deterioration and corrosion of the materials to be cleaned as
well as the cleaning apparatus itself. It should be noted that
supercritical CO~2~ is best suited for the removal or organic compounds
with mid-to-low volatilities (Spell, 1993). These types of compounds
often occur as common contaminants encountered in precision cleaning,
and it is on these compounds that our experimental studies were focused.

**SUPERCRITICAL CO~2~ CLEANING RESULTS**

Preliminary cleaning studies were conducted on a small scale as single
point extraction data. Two separate studies were conducted, one on the
removal of common machine contaminants (machining, cutting, drawing,
lubricating, and body oils) and one on the removal of a wide variety of
chemical compounds, both from a wide variety of substrates. All in all,
the removal of 14 cueing oils, 17 machining oils, SiOX fluids, 5 water
soluble cutting fluids, and 120 individual chemicals from 18 metals,
glass, quartz, sapphire, and 24 polymers. A portion of the substrates
cleaned using supercritical CO~2~ are summarized in Table 2. For the
chemical compound removal studies only 340 stainless steel, electrolytic
grade topper sheet, glass fiber filled epoxy board, borosilicate glass,
and cast magnesium were used. The contaminant compounds were applied as
a dilute solution to the entire single 0.5\" by 2\" substrate surface
using a manual pipettor for a contamination level of 2 µg/cm2. While it
is noted that a contamination level of 2 µg/cm2 is below the precision
clean standard of 10 µg/cm2, 2 µg/cm2 of contamination was visible in
many cases and was required to provide a reasonable detector signal for
proper quantitation of the contaminant removal results After the
application solvent had evaporated, the extraction was begun. Each
substrate was placed in a 10 ml commercial extraction vessel and
dynamically extracted (solvent flow through the cell) using pure CO~2~
at 300 atm and 45°C for 15 minutes using a Suprex SFE/50 supercritical
fluid extraction instrument with a supercritical fluid flow rate of 2.8
ml/min. The extracted fluid was let down directly to the inlet of an HP
5971 GCMS operated in the split mode with a split ratio of 150 to 1. The
GC column was a 60m x 0.25 mm i.d. DB-5 (5% crosslinked Ph-Me silicone)
column programmed from 30 to 275°C with a temperature ramp of 7°C/min.
Areas and concentrations of the extracted compounds were calculated from
the total ion chromatogram by the HP software. The data were prepared as
percent of original material removed from the substrate, and have 4%
relative standard deviation as determined from three trials.

Typical results for the removal of oils from a variety of substrates is
illustrated in Figure 3. Oil removal rates from all of the smooth
surfaces such as epoxies, glasses, and plastics were near quantitative
as seen from the figure. The overall removal rates of oils from all
metal surfaces were quite good, averaging from about 85 to 90%. The
removal rates shown are not quantitative since the cast metals
investigated were highly porous. Cleaning efficiency using the described
conditions was not quite as high for porous substrates, and this tends
to lower the overall cleaning average depicted in the figure. For
example, cleaning ranged from removal rates as low as 75% for the
removal of 3-IN-ONE® oil from cast aluminum to as high as 98% from
stainless steel 340. Cleaning from cast materials has the potential to
be quantitative with a longer extraction time or the use of a static
extraction step (substrate immersed in supercritical CO~2~ with no flow
through the cell) followed by a dynamic extraction. Of particular
importance is the quantitative removal of fingerprints, which tends to
be a commonly encountered contaminant, from most all of the substrates
investigated. Again, however, using the test conditions as described,
cast metals showed lower extraction efficiencies. For example, cast
magnesium had a fingerprint removal rate of 56%, while stainless steel
306 had a removal rate of 97%.

A portion of the overall results for the removal of a variety of
chemicals from several substrates is shown in Figure 4. This study
investigated the removal of 120 different individual chemicals from 340
stainless steel, electrolytic grade copper sheet, glass fiber filled
epoxy board, borosilicate glass, and cast magnesium. The substrates also
range from polished smooth, 340 stainless steel, for example, to porous
cast magnesium. In this study, the removal rates were largely dependent
on the functionality of the different chemicals. For example, cleaning
ranged from removal rates as low as 35% for the removal of benzoic acid
from cast magnesium to as high as 99% for the removal of 2-nitroaniline
from epoxy board. In general, supercritical CO~2~ proved to be an
effective cleaning solvent for nonpolar or lipophilic chemical
contaminants, while it was not so effective in the removal of polar or
hydrophilic compounds. Of particular interest is the fact that removal
rates from all of the substrates tested in this case had no statistical
difference. Of course this is with the exception of cast magnesium for
the same reason as observed with oil removal from porous substrates. In
any event, this observation implies that the removal of contaminants
using supercritical CO~2~ is dependent more on the contaminant itself
rather than on the surface that is being cleaned.

While it appears from the results thus far presented that cleaning with
supercritical CO~2~ is quite effective, it is of interest to compare
CO~2~ cleaning with current degreasing techniques. In this case, the
removal of a typical drawing oil from a stainless steel surface using
supercritical CO~2~ was compared with a standard Freon-113 wash. In this
case, after the application of the drawing oil, the stainless steel
surface was cleaned using both supercritical CO~2~ at different
densities and a Freon wash. The CO~2~ density was varied using both
temperature and pressure. After cleaning with supercritical CO~2~ the
stainless steel surface was subjected to a Freon wash so that the amount
of oil remaining could be determined. All Freon washes were monitored
with a Buck IR total hydrocarbon analyzer to measure the amount of
drawing oil in the wash solution which is determined from hydrocarbon
absorption at a fixed wavelength of 2924 cm-1. The results of this study
are shown in Figure 5. As seen from the figure, once the CO~2~ density
reaches about 0.5 g/ml, the cleaning efficiency is equivalent to that
obtained using a Freon wash. As the density of the supercritical fluid
is increased, the cleaning efficiency also increases to a level below
that obtained using a Freon wash. This observation illustrates the
selective solvation capability of a supercritical fluid based on
temperature and pressure. As a result, a supercritical fluid has a
unique advantage in that it could be used to selectively remove one
contaminant from a surface while leaving a desired coating intact.

**CONCLUSION**

Supercritical carbon dioxide is an excellent solvent for precision
cleaning, particularly for porous, intricate parts or parts that are
relatively accessible by conventional solvents. This has been summarily
demonstrated by results presented in this paper. Furthermore, to a first
approximation, cleaning with supercritical CO~2~ appears to be
contaminant dependent while being surface independent. In addition, the
use of supercritical CO~2~ as a cleaning solvent can reduce the need for
washing in organic solvents, thus reducing their overall use in
manufacturing process. This in turn reduces hazardous waste by
minimizing the solvent required to dispose of collected contaminants.
Along with the effectiveness of cleaning with CO~2~, the economics of
the entire cleaning process may direct the use of CO~2~ in cleaning
applications other than precision cleaning.

**ACKNOWLEDGMENTS**

This work was performed under funding from the Industrial Waste Program
Office, Office of Industrial Technology, Department of Energy.

**References**

1\. Spall, W. D., Supercritical Carbon Dioxide Precision Cleaning for
Solvent and Waste Reduction, International Journal of Environmentally
Conscious Design & Manufacturing, 1993, 2(1), pp. 81-86.

2\. American National Standards Institute, ANSI IPC-CH-65, 1990, 7.2.3,
pp. 47-48.

3\. McHardy, J., Stanford, T.B., Benjamin, L. R., Whiting, T. E., and
Chao, S.C., Progress in Supercritical CO2 Cleaning, SAMPE Journal, 1993.
09(5). pp. 20-27

4\. Barton. J., The Los Alamos SUPERSCRUB™: Supercritical Carbon Dioxide
System Utilities and Consumables Study, LA-12786, 1994, L os Alamos
National Laboratory, Los Alamos, New Mexico.

5\. Snowden-Swan. L.J., Supercritical Carbon Dioxide Cleaning Market
Assessment and Commercialization/Deployment Plan, PNL-10044/UC-602,
1994, Pacific Northwest Laboratory, Richland, Washington.