TITLE: A Survey on the Use of Supercritical

Carbon Dioxide as a Cleaning Solvent

AUTHOR(S): W. Dale Spall and Kenneth E. Laintz

SUBMITTED TO: prepared as chapter in:

Supercritical Fluid Cleaning

J. McHardy and S. Sawan, Eds.

Noyes Publications

Park Ridge, NJ

**ABSTRACT**

Because the physiochemical properties of supercritical carbon dioxide
make it ideally suited for removing commonly encountered contaminant
found in the cleaning of a wide variety of components and assemblies, an
overall survey was conducted using a small scale supercritical fluid
extraction system to investigate removal efficiencies of a wide variety
of compounds from an assortment of surfaces using supercritical carbon
dioxide. Data is presented demonstrating the successful removal of
numerous oils, fluids, adhesives, and chemical compounds from a wide
variety of surfaces with supercritical carbon dioxide. In total, the
removal of 145 compounds from some 49 different substrates was
investigated. It was found that to a first approximation, cleaning with
supercritical CO~2~ appears to be contaminant dependent while being
surface independent, with an 85-95% removal rate for a wide variety of
the compounds investigated.

**INTRODUCTION**

Many industrial facilities currently using chlorocarbons and
chlorofluorocarbons (CFCs) for the cleaning of a variety of items are
facing a difficult situation because of the U.S. amendments to the
Montreal Protocol (1987) banning the use of CFCs at the end of 1995. For
this reason, these companies must implement economical replacement
technologies for cleaning applications. Of course, any solvent cleaning
replacement technology must take into account the type of items being
cleaned, the contaminant to be removed from these items, and the final
cleanliness level that the items must possess. Alternate technologies
such as aqueous and semi-aqueous based systems are currently being
implemented. While these systems have advantages over CFC cleaning
methods, these systems suffer from disadvantages that may not be
desirable to many cleaning operations. In the case of aqueous systems,
disadvantages include long drying times and flash rusting in addition to
many parts not being amenable to water cleansing. In addition, water
treatment costs may also be prohibitive. Many semi-aqueous cleaning
systems employ toxic terpenes or CFC replacements, and it is only a
matter of time before these compounds face regulation. A final
alternative technology involves the use of supercritical fluids, which
have been used in food, fragrance, and petroleum processes for years,
for the extraction of many common compounds.

Ultimately, most cleaning specifications are based on the amount of
specific or characteristic contaminants remaining on the surface being
cleaned. Common contaminants can include machining oils and greases,
hydraulic and cleaning fluids, adhesives, waxes, human contamination,
and particulates. In addition, a whole host of other chemical
contaminants from a variety of sources may soil a surface. Therefore,
any CFC replacement solvent under consideration should be able to remove
any of these commonly encountered soils to specified levels from a
variety of surfaces, including printed circuit boards, plastics, metals,
rubbers, composites, and glasses. For the purposes of this paper,
precision cleaning will be addressed as opposed to bulk cleaning. This
precision level can be defined as an organic contaminant level of Iess
than 10 micrograms of contaminant per square centimeter.^2^ This 10
µg/cm2 level of cleanliness is either very desirable or required by the
function of parts such as metal devices, machined parts, electronic
assemblies, optical and laser components, precision mechanical parts,
and computer parts.1

While supercritical carbon dioxide may be an excellent cleaning solvent
for many organic contaminants, many substances requiring removal in
cleaning operations, inorganic or ionic contaminants, for example, are
insoluble in carbon dioxide. In addition, many items requiring cleaning
are intolerant of pressures associated with supercritical CO~2~. for
cleaning considerations, it should be noted that supercritical CO~2~ is
best suited for the removal of organic compounds with mid-to-low
volatilities.1 These types of compounds are often encountered as
contaminants in precision cleaning, and it is on these compounds that
our experimental studies were focused. Since the goal for most precision
cleaning levels is less than 1 µg/cm2 for most soils,^3^ the 49
substrate materials used in this survey were initially contaminated with
2 µg/cm2 of the 145 contaminants investigated. It is the removal of this
amount of material to below the desired 1 µg/cm2 contamination level for
this survey to determine the general applicability of supercritical
fluid cleaning technology.

**EXPERIMENTAL SECTION**

The small scale supercritical CO~2~ charting survey was undertaken to
investigate the removal efficiency of a wide variety of contaminants and
compounds from a wide assortment of substrates which could be
encountered In a cleaning situation. The survey investigated the removal
of six human based organic contaminants, five adhesives, seven different
hydrocarbons, waxes, high molecular weight compounds, and thirteen
different machining oils, fluids, and lubricants, including water
miscible types, from fifteen different metal, nineteen polymeric, five
rubber, five cable, three glass, and two fabric substrates. The
different contaminants and substrates investigated are summarized in
Tables 1-16. In addition, the removal of 114 different miscellaneous
chemical compounds including polycyclic aromatic hydrocarbons (PAHs),
amines, substituted phenols, substituted benzenes, phosphates, acids,
and acid esters from 340 stainless steel, electrolytic grade copper
sheet, glass fiber filled epoxy board, borosilicate glass, and cast
magnesium. All of the different chemicals investigated in the survey are
listed in Tables 16-20.

The contaminant materials were applied as a dilute solutions to 0.5 in.
by 2 in. (12.9 cm2) coupons made from the different substrate materials
using a manual pipettor. The contaminant solutions were applied in such
a manner so that the entire surfaces of the coupons were coated with 2
µg/cm2 of each contaminant compound. 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. Once the application solvent had
evaporated to dryness, a contaminated coupon was placed in a 10 ml
extraction or cleaning vessel in a Suprex SFE/50 supercritical fluid
extractor (Suprex Corp., Pittsburgh, PA). All contaminated coupons were
cleaned or extracted dynamically, meaning that there was continuous
solvent flow through the cell for each survey. The extractions were
conducted using SFC/SFE grade CO~2~ (with siphon tube and 1500 psi He
head space, Scott Specialty Gases, Inc., Longmont, CO) at 300 atm and
45°C for 15 min. with a flow rate of 2.8 ml/min. After flowing through
the extraction cell, the supercritical CO~2~ containing dissolved
contaminant was depressurized directly into the inlet of a Hewlett
Packard (HP) 5971 gas chromatograph equipped with an HP 5972 series mass
selective detector (GC-MS). The GC-MS was operated in the split mode
with a split ratio of 150 to 1. The GC column was a 60 m 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. Chromatographic peak areas and
subsequent corresponding concentrations of the extracted compounds were
calculated from the total ion chromatograms by the HP software. The
concentrations obtained using this method were then compared to the
initial concentrations of contaminant placed on the substrate coupons
and prepared as percent of original material removed from the
substrates. The extraction surveys were run in triplicate which yielded
an overall average 7% relative standard deviation for all of the
compounds investigated.

**RESULTS AND DISCUSSION**

Of the wide variety of contaminants and compounds investigated in the
small scale supercritical CO~2~ cleaning survey, of particular
importance are compounds associated with human based contamination which
is often a significant component of organic contamination found in many
cleaning operations, especially those involved in precision cleaning.
Human based contaminants can be found in sweat, fingerprints, and other
human soils and can contain hundreds of different chemical compounds.
Generally, the major constituents of this type of organic contamination
are made up of fatty acids and oils found in the skin. For this study,
representatives of the chemical classes found in skin lipids were used
and consisted of squalene, triglycerol, diglycerol, cholesterol, and
palrnitylpalmitate. In addition to skin oils, fingerprints tend to be
commonly encountered contaminants on parts, components, and assemblies.
In order to investigate the removal of fingerprints from surfaces, a
fingerprint surrogate consisting of a mixture of skin lipids was
prepared based upon previous work.^4--6^ The components of the surrogate
fingerprints consisted of 30% triolein, 25% oleic acid, 25% cotyl
palmitate, 15% squalene, 2.5% cholesterol, and 2.5% cholesterol oleate
(components obtained from the Aldrich Chemical Company, Wl). While salts
are certainly components of fingerprints, these compounds were not added
to the mixture since they were incompatible with the experimental
detection system. In any event, the surrogate used for this study was
assumed to behave in an analogous manner to actual fingerprints.

The results for the removal of human based organic contamination from
the 49 different substrates investigated are summarized in Tables 1-3.
The results presented in Table 1 summarize the removal of squalene,
triglycerol, diglycerol, cholesterol, palmitylpalmitate, and synthetic
fingerprints from 15 metal and 3 glass surfaces. These results show near
quantitative removal of synthetic fingerprints, squalene, and
palmitylpalmitate from most all of the metal and glass surfaces.
However, using the test conditions as described, the cast metals, cast
aluminum, magnesium, and iron, showed lower extraction efficiencies. For
example, cast magnesium had a synthetic fingerprint removal rate of 56%,
while stainless steel 306 had a removal rate of 97%. The low removal
rate from the cast metals is believed to be due to the porosity of the
substrate surface. Because of their high diffusivities and low
viscosities, supercritical fluids are inherently capable of penetrating
porous surfaces and removing contaminants, and increased removal rates
from the cast metals were easily accomplished through parametric
changes. For example, longer extraction times of 30 to 45 min. resulted
in quantitative removal of the synthetic fingerprints from the cast
magnesium surface. The removal rates of the gIycerols and cholesterol
were lower than the other human contaminants due to their lower
solubilities in supercritical CO~2~.^7^ The removal of these compounds
can be improved with a longer extraction time as in the case of the cast
metals or through the use of a static extraction step where the
substrate is immersed in supercritical CO~2~ with no flow through the
cell and then followed by a dynamic extraction.

The results of the removal of the skin lipids from the 19 polymeric
materials used in this survey are summarized in Table 2. These results
compare similarly with those observed for the removal of the lipids from
the 3 glass surfaces shown in Table 1. Again, near quantitative removal
of synthetic fingerprints, squalene, and palmitylpalmitate was observed
with the same lower removal efficiencies for the glycerol and
cholesterol. In general, the same results were observed for the removal
of these compounds from the 5 rubber, 5 cable, and 2 fabric substrates
as seen in Table 3. Palmitylpalmitate was not as effectively removed
from the rubber surfaces, probably due to surface interactions with the
acid moiety of the compound. While the fabric samples can be thought of
as porous substrates, contaminant removal efficiencies from these
surfaces were much higher than the cast metals because unlike the
metals, supercritical CO~2~ can flow through the fabrics thus limiting
surface interactions between contaminant and substrate.

The results for the removal of common machining oils and fluids from the
selected substrates are summarized in Tables 4-6. Oil removal rates from
the 34 smooth surfaces investigated, metals, glasses, and plastics, were
near quantitative as seen from Tables 4 and 5. The overall removal rates
of the oils and fluids from all of these surfaces were quite good,
averaging from about 90 to 97%. Of particular note, as seen in Table 4,
is that supercritical CO~2~ was quite effective in the removal of the
various oils and fluids from all smooth metal surfaces, removing, for
example, from about 89 to 99% of the Tapmatic® cutting fluid. These
results show the applicability of supercritical CO~2~ cleaning to
machined end precision metal parts and components. Again, however,
observed cleaning efficiencies using the described conditions were not
as high for the porous metal substrates. On the other hand, quantitative
removal of the investigated machining oils and fluids from the rubber,
fabric, and cable substrates listed in Table 6 was observed to be near
quantitative, averaging from about 85 to 99%. The two compounds that did
not extract well from any of the 49 surfaces and not included in the
aforementioned average removal rates were Molykote lubricant and
silicone oil. Since Molykote consists primarily of inorganic particulate
matter in a high molecular weight grease, it was expected to have low
removal efficiencies with supercritical CO2. Silcone oil was also not as
efficiently removed as the other contaminants due to low solubility or
to fractionation of the oil with the higher molecular weight, less
soluble components remaining on the surface.

Other common contaminants associated with a machining environment can
include water miscible machining fluids and surfactants. For this
reason, the removal efficiencies of select compounds from these classes
of fluids were also investigated. For the survey, the removal
efficiencies of TRIM® SOL, Cimcool, Cimtap, which are water miscible
machining fluids, and the nonionic surfactant Triton X-100 from the 49
substrates were studied. The results of this set of experiments are
summarized in Tables 7-9. Surprisingly, these water soluble materials
had fairly high removal rates, generally averaging above 80% removal
from all but the porous metal substrates using the specified conditions.
This example suggests that while an aqueous cleaning process might under
consideration as a cleaning system replacement, supercritical CO~2~ may
be a viable cleaning option in cases where components are not ideally
suited to aqueous immersion.

Due to the physiochemical properties of a supercritical fluid, cleaning
with supercritical CO~2~ has a potential advantage over other cleaning
technologies due to its ability to rapidly clean completely assembled
components systems. In many instances, assembled components that are in
need of cleaning contain adhesives, epoxies, and/or sealants. While in
some cases it may be desirable to remove these substances from a
surface, in other cases it may be desirable to clean the surface and
leave these substances intact. With both of these strategies in mind, a
selection of adhesives, epoxies, and sealants were applied to each of
the 49 substrates and extracted with supercritical CO~2~. The results
from this portion of the cleaning survey are summarized in Tables 10-12.
As seen from these tables, it is clear that supercritical CO~2~ was
ineffective in removing the various adhesives from any of the
substrates. For the specified conditions, from about 23 to 52% of the
RTV-732 and 3110 Silastic Adhesive Sealant and the Loctite® 242
Threadlocker were removed from the surfaces. While, parametric
variations such as longer extraction times, increased temperatures
and/or pressures, or the inclusion of a static extraction step may
increase removal rates, it is unlikely that these compounds would be
quantitatively removed from the surfaces, thus precluding effective
cleaning with supercritical CO~2~. On the other hand, removal rates of
Iess than 10% were observed for Devcon F-Fast Setting Epoxy and Eastman
910 Super Glue. This low removal rate could conceivably correspond to
the extraction of residual solvents from the adhesives, thus
demonstrating relative inertness to CO~2~ exposure. Therefore, it is
conceivable that components assembled with these or similar products
could be cleaned with supercritical CO~2~ without damage to the adhesive
bonds.

The results from the last set of contaminants surveyed for removal
efficiencies from the 49 different substrates are listed in Tables
13-15. These contaminants are representative of larger classes of
contaminants which may be encountered in cleaning operations. For
example, hexadecane and tetracontane can be found in kerosene and
diesel. Waxes, such as paraffin wax, are used as lubricants and mold
releases. Carbowax and Microwax are chromatographic stationary phases,
but they are forms of polyethylene glycol which is also a lubricant.
Finally, methyl silicone gum and other methyl silicone resins are often
used in protective coatings. The lower molecular weight materials,
hexadecane, tetracontane, and paraffin, had fairly high removal rates,
generally averaging above 80% removal from all but the porous metal
substrates using the specified conditions. On the other hand, the high
molecular weight waxes had fairly low removal efficiencies in the 13 -
39% range. This was to be expected since supercritical fluid extraction
using CO~2~ is known not to do well in dissolving high molecular weight
compounds. Again, however, the removal of these compounds could probably
be improved with a longer extraction time or through the use of a static
extraction step followed by dynamic extraction, but it is unlikely that
CO~2~ cleaning alone would quantitatively remove such high molecular
weight contaminants.

Because a wide combination of chemical contaminants from a variety of
sources may soil a surface, the cleaning survey also included removal
studies of 114 different organic chemicals from a variety of classes of
compounds. These compounds include PAHs, amines, substituted phenols,
substituted benzenes, phosphates, acids, acid esters, as well as an
assortment of miscellaneous compounds. Using the aforementioned cleaning
or extraction conditions, the removal of the compounds listed in Tables
16-20 from 5 surface representatives from the larger, previously
investigated group was investigated. The surfaces that were contaminated
consisted of coupons made from 340 stainless steel, electrolytic grade
copper sheet, glass fiber filled epoxy board, borosilicate glass, and
cast magnesium.

The results for the removal of PAHs from the 5 surface substrates are
summarized in Table 16. In general, the 23 PAHs listed in the table
averaged removal rates around 90% from the smooth surfaces and over 80%
for the porous cast magnesium surface. In contrast, supercritical fluid
extraction studies using CO~2~ for the removal of PAHs from soils for
environmental applications have shown relatively poor removal
efficiencies for many of the compounds listed in the table often
requiring the addition of secondary solvents to the CO~2~.^8^ However,
it appears that from the results on the removal of the PAHs shown in
Table 10, surface contamination is much easier to extract and remove
than interstitial or sorbed contamination as in the case of soils where
a wide range of contaminant-substrate interactions are possible. Since
surface interactions with the contaminants are expected to be minimal
with the stainless steel, epoxy board, copper sheet, and borosilicate
glass, the observed high removal rates were intuitively expected. While
surface interactions may not be a dominant controlling factor in PAH
removal, the chemical nature of individual PAHs control removal
efficiencies. In this case, as substitution increased for various PAHs,
removal efficiencies decreased. For example, pyrene had a removal rate
of 97% from the glass surface whereas indeno(1,2,3-CD)pyrene had only an
86% removal rate.

Organic amines constitute a wide class of compounds ranging from
solvents such as aniline and pyridine to familiar chemicals such as
nicotine. Altogether, a selection of 23 organic amines, most of them
aromatic compounds, was investigated in the cleaning survey. The results
of the removal efficiencies of organic amines from stainless steel,
copper sheet, epoxy board, borosilicate glass, and cast magnesium are
summarized in Table 17. In this case, removal efficiencies were entirely
compound dependent, based predominantly on contaminant solubilities in
supercritical CO~2~. For example, compounds such as
N-nitrosodimethylamine, which is soluble in water, and
N-nitrosophenylaniline had low removal efficiencies ranging from 30 to
40% from the smooth surfaces and only 21% removal from cast magnesium.
On the other hand, 2-nitroaniline and 4-nitroaniline, which are soluble
in polar organic solvents had over 97% and 90% removal rates,
respectively. All in all, the organic amines had a general average
removal rate near 80% which still shows fairly effective supercritical
CO~2~ cleaning.

In general phenols are polar organic compounds primarily soluble in
polar organic solvents, and in some cases, water. For this reason, it
was expected that removal rates for these types of compounds from the 5
substrates in the chemical removal survey would be rather low. In fact,
for most of the 19 substituted phenols surveyed, the removal rates
averaged near 60%, and these results are summarized in Table 18.
However, several of the substituted phenols, the cresols, for example,
had very effective removal rates, averaging above 90% removal using
supercritical CO~2~ at 10°C and 350 atm despite the fact that CO~2~ is a
nonpolar solvent. It is possible that the high removal rates of these
compounds could be attributed to the fact that they are liquids at 40°C,
thus facilitating extraction due to faster kinetics. This implies that
higher removal efficiencies for the other phenols could be accomplished
through higher temperature extractions. AIso summarized in Table 18 are
the removals of substituted benzenes from the same surfaces. Again, in
this case the chemical nature of individual compounds was the
controlling factor governing removal efficiencies. On average, these
compounds had removal rates around 85%, again showing fairly effective
supercritical CO~2~ cleaning.

Summarized in Table 19 are the results for the removal of organic
phosphates, acids, and acid esters. Only one organic acid, benzoic acid,
was investigated. Since this compound is soluble in water, it was
expected to have a low removal efficiency from all 5 of the surfaces.
This was indeed the case with an average removal of 42% from the smooth
surfaces and 35% from cast magnesium. Once an organic acid is
esterified, it is generally less polar than the precursor thus
increasing lipophilicity. The acid esters investigated in this survey
were all phthalates, and they averaged around 90% removal efficiencies.
The three organic phosphates listed in the table show average removal
efficiencies around 77% for the smooth surfaces. While
trimethylphosphate is water soluble, it had the highest removal
efficiency of the phosphates once again suggesting that supercritical
CO~2~ cleaning may be an aqueous cleaning alternative is some limited
instances.

Finally, Table 20 lists the removal efficiencies of another 29
miscellaneous chemical compounds from stainless steel, copper sheet,
epoxy board, borosilicate glass, and cast magnesium. Again, the
important observation is that surface interactions appear not to be
controlling compound removals as the smooth surfaces tend to have the
same removal efficiencies. The chemical nature of individual compounds
controls the removal efficiencies.

**CONCLUSION**

While supercritical CO~2~ is not an absolute or drop-in solution to all
cleaning problems, it is noted for its solvation of organic compounds
having mid-to-low volatilities, and these types of compounds are common
contaminants requiring removal to precision clean levels. Based upon the
survey results presented in this chapter for the removal of 145
different compounds from 49 surfaces, it was shown that to a first
approximation, cleaning with supercritical CO~2~ is contaminant
dependent and surface independent. Furthermore, in the case of PAHs, it
was shown that surface contamination was much easier to extract and
remove than interstitial or sorbed contamination. In addition, it was
shown that supercritical CO~2~ is also capable of removing many
compounds traditionally removed by aqueous cleaning, thus expanding the
scope of cleaning applicability. Therefore, besides the effectiveness of
cleaning with CO~2~, the economics of the entire cleaning process may
direct the use of CO~2~ in cleaning applications where other replacement
technologies are under consideration as well as processes other than
precision cleaning. Finally, the use of supercritical CO~2~ as a
cleaning solvent can reduce the overall use of organic solvents in
manufacturing processes.

**ACKNOWLEDGMENTS**

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

**REFERENCES**

1.  American National Standards Institute, ANSI/IPC-CH-65, 7.2.3, pp.
    47-48 (1990)

2.  Spall, W.D., Supercritical Carbon Dioxide Precision Cleaning For
    Solvent And Waste Reduction, Intntl. J. Environ. Conscious Design &
    Manufact., 2(1): 81-86 (1993)

3.  McHardy, J., Stanford, T.B., Benjamin, L.R., Whiting, T.E., and
    Chao, S.C., Progress in Supercritical CO2 Cleaning, SAMPE Journal,
    29(5): 20-27 (1993)

4.  Downing, D.T., Strauss, J.S., and Pochi, P.E., Variability in the
    Chemical Composition of Human Skin Surface Lipids, J. Invest.
    Dermatol. 53(5):322-327 (1969)

5.  Downing, D.T. and Strauss, J.S., Synthesis and Composition of
    Surface Lipids of Human Skin, J. Invest. Dermatol. 62(3):228-244
    (1974)

6.  Strauss, J.S., Pochi, P.E., .and Downing, D.T., The Sebaceous
    Glands: Twenty-Five Years of Progress, J. Invest. Dermatol.
    67(1):90-97 (1976)

7.  Bartle, K.D., Clifford, A.A., Jafar, S.A., and Shilstone, G.F.,
    Solubilities of Solids and Liquids of Low Volatility in
    Supercritical Carbon Dioxide, J. Phys. Chem. Ref. Data 20(4):713-756
    (1991)

8.  Yang, Y., Gharaibeh, A., Hawthorne, S.B., and Miller, D.J., Combined
    Temperature/Modifier Effects on Supercritical CO2 Extraction
    Efficiencies of Polycyclic Aromatic Hydrocarbons from Environmental
    Samples. Anal. Chem. 67(3):641-646 (1995)