Applications of Supercritical Carbon Dioxide for Removal of Surface Contaminants

Chapter 6

Applications of Supercritical Carbon Dioxide for Removal of Surface Contaminants Rajiv Kohli The Aerospace Corporation, NASA Johnson Space Center, Houston, TX, USA

Chapter Outline 1 Introduction 2 Surface Contamination and Cleanliness Levels 3 Applications 3.1 Supercritical Carbon Dioxide 3.2 Application Examples 4 Other Considerations

1

209 210 211 211 223 230

4.1 Costs 230 4.2 Advantages and Disadvantages of Supercritical CO2 Cleaning 231 5 Summary 233 Acknowledgment 234 References 234

INTRODUCTION

For many well-known reasons, conventional precision cleaning techniques for removal of particle contaminants are largely centered on aqueous and solvent cleaning. These cleaning techniques have several serious limitations, including nonremoval of sub-μm particles, involved processing, and expensive disposal of toxic solvent–waste matrices. In the search for alternate cleaning techniques, carbon dioxide (CO2) in its different states has been found to be an effective medium for cleaning in a wide variety of industries for many years. It is inexpensive, naturally abundant, relatively inert towards reactive compounds, nontoxic, nonflammable, and it can be easily recycled, making CO2 highly desirable as an alternate substance to chlorinated solvents, hydrochlorofluorocarbons (HCFCs), trichloroethane and other ozone-depleting solvents for precision cleaning [1–24]. Supercritical CO2 (SCCO2) has both gas-like transport properties and liquid-like solvent properties which makes it more flexible than the other physical states of CO2 for contaminant removal, although the process must be operated at very high pressures. The application of SCCO2 for removal of surface

Developments in Surface Contamination and Cleaning, Volume 11. https://doi.org/10.1016/B978-0-12-815577-6.00006-2 Copyright © 2019 Elsevier Inc. All rights reserved.

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210 Developments in Surface Contamination and Cleaning

contaminants was reviewed recently [20,21]. The objective of this chapter is to update the previously published information on SCCO2 and its cleaning applications.

2 SURFACE CONTAMINATION AND CLEANLINESS LEVELS Surface contamination can be in many forms and may be present in a variety of states on the surface. The most common categories of contaminants are given below [25]: l

l

l l l

l

Particles such as dust, metals, ceramics, glass, and plastics in the sub-μm to macro size range Organic contaminants which may be present as hydrocarbon films or organic residue such as oil droplets, grease, resin additives, waxes, etc. Molecular contamination that can be organic or inorganic Metallic contaminants present as discrete particles Ionic contaminants including cations (such as Na+ and K+) and anions (such as Cl
, F
, SO3
, BO3 3
and PO4 3
). Microbiological contaminants such as bacteria, fungi, biofilms, etc.

Other contaminant categories include toxic and hazardous chemicals, radioactive materials, and biological substances, that occur on surfaces employed in specific industries, such as semiconductors, metals processing, chemical production, nuclear industry, pharmaceutical manufacture, and food processing, handling, and delivery. Common contamination sources include machining oils and greases, hydraulic and cleaning fluids, adhesives, waxes, human contamination, and particulates, as well as from manufacturing process operations. In addition, a whole host of other chemical contaminants from a variety of sources may also soil a surface. Typical cleaning specifications are based on the amount of specific or characteristic contaminant remaining on the surface after it has been cleaned. Cleanliness levels for space hardware and for other precision technology applications are typically specified for particles by size (in the micrometer (μm) size range) and number of particles, as well as for hydrocarbon contamination represented by nonvolatile residue (NVR) in mass per unit area for surfaces or mass per unit volume for liquids [26–28]. The surface cleanliness levels are based on contamination specifications established in industry standard IEST-STD-CC1246E for particles from Level 5 to Level 1000 and for NVR from Level R1E-5 (10 ng/ 0.1 m2) to Level R25 (25 mg/0.1 m2) [28]. The cleanliness levels have been revised or redesignated from the previous version D of this standard [29]. The maximum allowable number of particles for each particle size range has been rounded in revision E, while the NVR designation levels have been replaced with a single letter R followed by the maximum allowable mass of NVR. For example, former NVR level J has the new designation R25; level A/2 is now R5E-1; and level AA5 is now R1E-5. The changes from the previous

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revision of the standard are discussed in greater detail in Volume 7 of this series [25]. In many other commercial applications, the precision cleanliness level is defined as an organic contaminant level less than 10 μg of contaminant per cm2, although for many applications the requirement is set at less 1 μg/cm2 [28]. These cleanliness levels are either very desirable or are required by the function of parts such as machined parts, electronic assemblies, optical and laser components, precision mechanical parts, and computer parts. A new standard ISO 14644-13 has been published that gives guidelines for cleaning of surfaces in cleanrooms to achieve defined levels of cleanliness in terms of particles and chemical classifications [30]. Many of the products and manufacturing processes are also sensitive to, or they can even be destroyed by, airborne molecular contaminants (AMCs) that are present due to external, process or otherwise generated sources, making it essential to monitor and control AMCs [31]. AMC is a chemical contaminant in the form of vapors or aerosols that can be organic or inorganic, and it includes everything from acids and bases to organometallic compounds and dopants [32,33]. A new standard ISO 14644-10, “Cleanrooms and associated controlled environments – Part 10: Classification of surface cleanliness by chemical concentration” [34] is now available as an international standard that defines the classification system for cleanliness of surfaces in cleanrooms with respect to the presence of chemical compounds or elements (including molecules, ions, atoms, and particles).

3

APPLICATIONS

The basic characteristics of SCCO2 relevant to cleaning applications are discussed in the following sections.

3.1 Supercritical Carbon Dioxide Above a critical temperature, the distinction between the liquid phase and the gas phase disappears, resulting in a single supercritical fluid1 phase [35]. The attractiveness of supercritical fluids as solvents stems from their unique combination of liquid-like and gas-like properties. Table 6.1 compares the diffusivity, viscosity and density of a typical organic fluid in the liquid, gas, and supercritical fluid state. The properties of the supercritical phase are intermediate between the gas phase and the liquid phase, with the diffusivity and viscosity similar to the transport properties of gases, but the density is similar to that of a liquid. Of the many different supercritical fluids available for precision cleaning (Table 6.2), SCCO2 is the most widely preferred solvent [1,2]. When CO2 is 1. Throughout this chapter “fluid” refers to the liquid phase.

212 Developments in Surface Contamination and Cleaning

TABLE 6.1 Comparison of Physicochemical Properties of a Typical Organic Fluid in the Liquid, Gas, and Supercritical Fluid States State

Diffusivity (cm2/s)

Viscosity (mPas)

Density (kg/m3)

Liquid

<10
5

1

1000


2


3


2

Supercritical fluid

10 –10

10

300

Gas

0.1

10
2

1

compressed, it acquires increasingly liquid-like densities. Depending on the pressure and temperature, CO2 is obtained as the liquid or as a supercritical fluid. On the pressure-temperature phase diagram for CO2 (Fig. 6.1), the liquid phase exists above the triple point (0.52 MPa, 216.75 K) and the supercritical phase exists above the critical point (7.38 MPa, 304.2 K). CO2 has no permanent dipole moment, but it has a strong quadrupole moment which affects its physical properties, including the high critical pressure of CO2 [40–44].

3.1.1 Physical and Transport Properties Small changes in the temperature or pressure near the critical point result in larges changes in density, as seen in Fig. 6.2 for CO2 [45–53]. As the solvating power of a fluid is generally related to its density, this gives SCCO2 its strong solvating powers. Thus, the tunable density, and therefore the tunability of solvent power, which can be varied from gas to liquid via a simple change of pressure at constant temperature, and the solvation effects at the densities in the vicinity of the critical point provide the most attractive attributes of supercritical fluids for surface contaminant removal. However, as the temperature is raised, the change becomes more pronounced only at higher pressures. This makes it difficult to control the density near the critical temperature, and, consequently, control of processes in the critical region is difficult. The transport properties of CO2 are also important to its application in cleaning. The gas-like viscosity enables it to effectively penetrate fine scale structures such as high aspect ratio vias, through- holes, small pores and cervices, and to clean components with complex geometries and tight spaces. The viscosity rises with pressure similarly to the density, however, the effect is less pronounced (Fig. 6.3) [47,51–57]. In general, the viscosity is an order of magnitude lower than the viscosity of typical organic solvents [36]. The self-diffusivity of CO2 in the vicinity of the critical point is approximately 5  10
4 cm2/s, which is nearly two orders of magnitude larger than the diffusivity of solutes in normal liquids [1]. Many organic solutes too have significantly higher diffusion

39.948

44.01

30.07

46.069

28.054

16.043

32.04

44.096

42.081

Argon (Ar)

Carbon dioxide (CO2)

Ethane (C2H6)

Ethanol (C2H5OH)

Ethylene (C2H4)

Methane (CH4)

Methanol (CH3OH)

Propane (C3H8)

Propylene (C3H6)

471.1

137.7

Trichlorofluoromethane (R11)b

4.38

5.841

21.94

3.759

4.63

4.25

8.09

4.596

5.076

6.14

4.889

7.38

4.898

11.28

4.70

Pc MPa

247.8

0.1194

57.1

198.8

0.188

203.0

118.0

99.2

130.4

167.1

148.3

93.9

74.9

72.5

209

Vc cm3/ mole

554.24

1.11

0.322

6.27

0.232

0.217

0.272

0.162

0.215

0.276

0.203

0.469

537.7

235

0.278

ρc kg/m3

0.279

0.291

0.235

0.282

0.286

0.281

0.224

0.288

0.280

0.240

0.285

0.274

0.292

0.241

0.232

Zc

0.42

0.0211

18.0

0.0141

0.09

0.11

0.59

0.01028

0.00951

1.095

0.00855

0.0137

0.0211

0.0098

0.3311

η mPa.s

0.22

28.27

0.72

0.29

0.000189

0.26

22.50

17.78

18.1

22.39

21.16

0.05

0.0122

0.234

23.70

σ mN/m

6

Vc ¼ critical volume; ρc ¼ critical density; Zc ¼ critical compressibility ratio; η ¼ viscosity; σ ¼ surface tension. a The data in this table have been compiled from references 36–39, manufacturer data sheets, and data available from various websites. b Due to concerns about its ozone-depleting potential, the consumption of trichlorofluoromethane (R11) is stopped. It is replaced by alternative chlorinated solvents such as dichlorotrifluoroethane (R123).

289.77

131.293(6)

Xenon (Xe)

647.13

18.015

318.6

365.57

369.8

512.6

190.5

282.6

513.9

305.3

304.2

150.86

405.65

508.1

Tc K

Water (H2O)

146.054

17.031

Ammonia (NH3)

Sulfur hexafluoride (SF6)

58.08

Acetone (C3H6O)

Solvent

Mol Wt g/mol

TABLE 6.2 Critical Properties of Various Solventsa

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214 Developments in Surface Contamination and Cleaning

FIGURE 6.1 Pressure–temperature phase diagram for CO2. Pc is the critical pressure; Ptp is the pressure at the triple point; Tc is the critical temperature; Ttp is the temperature at the triple point [36–39].

FIGURE 6.2 Variation of density of CO2 as a function of temperature at different pressures.

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FIGURE 6.3 Variation of viscosity of CO2 as a function of temperature at different pressures.

coefficients in SCCO2, although the diffusion coefficients drop off in the critical region and are nearly zero at the critical point [58]. This has the advantage of faster transport and shorter process times during supercritical removal of contaminants. Finally, the very low surface tension of SCCO2 (Table 6.2) provides for excellent wettability of components with complex geometries that make it very attractive in the dense phase for commercial precision cleaning applications.

3.1.2 Solubility Considerations SCCO2 is an excellent solvent for nonpolar, low molecular weight organic compounds, such as greases, oils, lubricants, and fingerprints [1,2]. It is this ability to dissolve organic compounds that underlies its commercial applications. On the other hand, hydrophilic compounds, such as inorganic salts and metal ions, do not dissolve. This has been attributed to the large quadrupole moment and the weak van der Waals forces in CO2 [59,60]. Extensive compilations of the solubility of organic compounds in SCCO2 can be found in [61–64]. The solubility behavior in binary mixtures of CO2 and a co-solvent can be considered in terms of the contributions of the individual forces to the total energy of vaporization, E. Thus, E ¼ Ed + Ep + Eh

(6.1)

where Ed, Ep, and Eh are the energy contributions due to dispersion forces, polar interactions (dipole–dipole forces), and hydrogen bonding, respectively [65].

216 Developments in Surface Contamination and Cleaning

The Hildebrand solubility parameter, δ, is (E/V)½ and Eq. 6.1 becomes δ2 ¼ δ2d + δ2p + δ2h

(6.2)

where δd, δp and δh are the Hansen solubility parameters for the dispersion, polar and hydrogen bonding interactions, respectively [65,66], and V is the molar volume. This approach considers the solubility in terms of the intermolecular forces of the solvents which have unique molecular structures and exhibit unique solubility behavior. Thus, the Hansen solubility parameters are of significant value in predicting the solubility behavior of cosolvents. For cosolvents such as methanol, ethanol, propanol, acetone, ethylene glycol and water, the values for δd do not differ significantly from the values for SCCO2. By contrast, the polar and hydrogen bonding Hansen parameters, δp and δh, are considerably higher than the values for SCCO2 [65]. This would suggest that there is a lack of miscibility in these binary mixtures. In the liquid state, these cosolvents have high polarities and they form self-associates. Also, these compounds can hydrogen bond with their own molecules in CO2, leading to reduced dipole moment and polarity of the binary mixture. The reduced polarity of the cosolvent component due to self-association leads to better matching of the polarity with SCCO2 and high miscibility, as has been observed in the CO2alcohol binary and ternary systems [67–69]. In fact, it is possible to form and maintain a single phase at high pressures over the entire composition range [68]. The large quadrupole moment of CO2 affects the value of δ, which reduces the nonpolar δd for ionic compounds. For example, at pressures above 20 MPa, the value of δ for CO2 is higher than the value for ethane, yet CO2 is incapable of dissolving an ionic surfactant with a hydrocarbon tail, such as AOT (sodium bis (2-ethylhexyl) sulfosuccinate) [70], which readily dissolves in ethane. Nearly 20% of the δ value can be attributed to the quadrupole moment, which reduces δd considerably below the value for ethane. Several techniques have been proposed for correlations or predictions of the solubilities of solid solutes in SCCO2 using density-based empirical equations, equations of state (EOS), or solution models [64,71–92]. These models require parameter optimization by fitting available experimental data for the systems under consideration, and also critical properties and sublimation pressures for solutes for the EOS models. Thus, these models are applicable only to systems for which sufficient solubility data are available. A recently developed activity coefficient model employs is a more general approach that does not require data fitting [83]. In general, the percentage of predicted values versus experimental data for solutes containing only C, H, and O atoms is higher as compared with solutes containing other atoms such as N, S, Cl, and F. Until recently, the lack of solubility of hydrophilic solutes in CO2 has been a serious limitation to widespread use of SCCO2 for removing polar inorganic contaminants and particles in precision cleaning applications. This limitation

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is increasingly being overcome by new developments to incorporate cosolvents, such as low molecular weight alcohols and acetone in SCCO2 to increase the solvent strength. For example, the addition of only 2 mol percent of tri-n-butyl phosphate to CO2 increases the solubility of hydroquinone by a factor of 250 [37]. Surfactants and water too have been added to SCCO2 to form microemulsions and dendritic micelles, as well as chelating fluorinated ligands [37,39,93–103]. These molecules can dissolve inorganic solutes and ionic species. A novel application is to incorporate fluoropolymers in SCCO2 for cleaning and protection of historical stone buildings and monuments [104].

3.1.3 Thermodynamic Properties To understand the chemical interactions and design optimum CO2/cosolvent formulations for cleaning applications, reliable thermodynamic properties are needed at the temperatures and pressures of interest for the species under consideration for the cleaning application. As very few direct measurements have been reported on the molar thermodynamic properties of cosolvents in SCCO2, it becomes necessary to consider thermodynamic models to predict these properties. Recent advances in high temperature solution chemistry make it possible to accurately predict the properties of aqueous and nonaqueous species by extending the thermodynamic data base into the supercritical region for CO2 as well as for water, because the solution behavior of SCCO2 is analogous to nonpolar supercritical water. The presence of highly polar compounds in binary CO2 systems requires that a rigorous molecular thermodynamic model take into account all of the interactions in the system, including hydrogen bonding, electrostatic attraction and ion hydration, as well as hard sphere repulsion and dispersion-attraction forces [105–107]. The Helmholtz energy A of the system is composed of the sum of the individual contributions of these interactions: A ¼ ARef + AAssoc + ABorn + ACoul + ADis + ARep

(6.3)

where the superscripts denote, respectively, the reference state for an ideal fluid; the association term due to hydrogen bonding among polar molecules and electrostatic interactions between solvent–solute molecules; Born interaction due to ion formation; Coulombic ion–ion interaction; the dispersion term; and the repulsion among the solute–solvent molecules. In the case of SCCO2 systems, depending on polarity of the cosolvent and the composition of the system, not all of these interactions will take place. A substance-dependent EOS limits the applicability of the EOS to predict thermodynamic properties. A widely used approach for predicting thermodynamic properties is the use of the revised Helgeson–Kirkham–Flowers (HKF) equation [108,109]. Although this approach was developed strictly for ions in aqueous electrolyte

218 Developments in Surface Contamination and Cleaning

solutions, the basic principle is applicable to CO2 binary systems because of strong solvent clustering about the solute even in nonpolar systems [110]. The revised HKF model provides correlation algorithms which permit prediction of species-dependent parameters in revised equations-of-state to calculate the standard partial molal thermodynamic properties at pressures and temperatures to 500 MPa and 1273 K [108,109,111–115]. Recently, a group contribution model has been proposed for improved prediction of the thermodynamic properties of organic aqueous solutions based on the contributions of the functional groups of the compounds [116–120]. In this model, the thermodynamic functions are determined as the sum of the structural contributions of individual species based on the assumption of functional group additivity. An alternate approach has been to adjust the correlation algorithms of the revised HKF model based on new parameters generated from experimental high temperature data and a correlation developed for the Gibbs energy of hydration [121]. Both these approaches have improved the prediction accuracy and have extended the applicability of the revised HKF model to aqueous systems, nonelectrolytes, ionic and neutral species, and organic and inorganic species over the entire pressure and temperature range from ambient into the supercritical region. A new unified theoretical model has been developed that is based on correction to the Born equation for non-Born hydration effects [122]. This model has the advantage that the effective ionic radius is very accurately defined by the standard state Gibbs free energy of hydration. Only two parameters in the model are required that can be derived from the experimental standard state partial Gibbs free energies of the species. The constants are independent of temperature and pressure, which makes the model very useful for predicting the high temperature thermodynamic properties of the electrolyte species. The correlation algorithms in the revised HKF model can be modified for application to CO2 binary systems [7]. Regression analysis of available thermodynamic data for 150 polar and nonpolar fluids at 298 K and 0.1 MPa has been performed to obtain the revised EOS parameters. This information has been used to calculate the thermodynamic properties of several CO2 binary systems. One example of the calculated values for molar volume is shown in Table 6.3 for CO2 + 1-propanol system along with recent experimental data [62,123]. In general, the agreement is within 1-3% between the calculated values of the thermodynamic functions and the limited available experimental data for CO2 binary mixtures of the species considered here. Based on these results, the correlation algorithms of the revised HKF model with modified EOS parameters can be applied to predict the thermodynamic properties of CO2 binary mixtures. The revised HKF model has also been used to estimate the molar volume of saturated alcohol/SCCO2 mixtures at different temperatures and pressures (Table 6.4). The estimated values are in good agreement (within 5%) of

8.31 8.75 –

2.25

4.37

6.48

8.26

–

–

0.2

0.4

0.6

0.8

0.9

0.95

6.61

4.53

2.31

1.87

1.88

0.1

Calculated

Experimental

8.0

Mol fraction

Pressure (MPa)

2.44

2.89

2.91

2.43

1.84

0.87

0.45

Experimental

9.0

2.51

2.97

3.01

2.48

1.91

0.89

0.46

Calculated

1.08

1.32

1.46

1.32

1.01

0.53

0.25

Experimental

9.8

1.09

1.39

1.52

1.36

1.21

0.59

0.27

Calculated

TABLE 6.3 Comparison of Molar Volumes (1022 ×m3.kmol21) for CO2 + 1-propanol Mixtures at 313 K for Different mol Fractions of CO2

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219

220 Developments in Surface Contamination and Cleaning

TABLE 6.4 Calculated Values of the Molar Volume of 2% Alcohol/SCCO2 Mixtures as a Function of Pressure and Temperature Molar Volume, cm3.mol21 Alcohol

250 K

350 K

450 K

Methanol 100 MPa

36.75

41.12

43.04

200 MPa

35.68

38.03

40.59

250 MPa

34.97

37.61

39.27

100 MPa

53.08

59.35

62.36

200 MPa

51.60

55.74

58.59

250 MPa

50.69

54.93

56.66

Ethanol

the experimental data [112,124–126], thereby suggesting that the assumption of SCCO2 as a nonpolar solvent analogous to supercritical water is generally valid [3].2

3.1.4 Basic Cleaning Principles SCCO2 exhibits strong solvent power for dissolving nonpolar organic compounds. Organic contaminants most often encountered in industrial applications consist of nonpolar compounds as films or as particles. Thin films are removed easily by dissolution either in pure CO2 or in a mixture with a cosolvent. Organic particles too can be removed because CO2 will help loosen the particles on the surface. For highly polar contaminants, the solubility can be enhanced by adding a suitable cosolvent in an amount sufficient to maintain the supercritical phase at the system operating conditions (temperature and pressure). The discussion and calculations presented in the previous section can help define the solubility and phase equilibria and the process operating conditions for removal of contaminants. In many precision cleaning applications, the contaminants are present as nonorganic particles. These particles adhere to the surface by strong van der Waals, electrostatic, or capillary forces [7]. It is necessary to penetrate the boundary layer and also to overcome the adhesion force by an applied drag force 2. An independent check of the application of the updated revised HKF model was made by estimating the standard molar heat capacity of NaCl (aq) solution in the temperature range 573 to 673 K. The estimated values were in excellent agreement with recent experimentally measured values [127,128].

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FIGURE 6.4 Minimum particle size removed as a function of stream velocity by a) SCCO2 at 30 MPa; and b) air flow at 0.1 MPa (1 atmosphere) [2,129].

to remove these particles. In an SCCO2 system for cleaning applications, the critical drag force is achieved by turbulent flow. Based on theoretical considerations, in order to remove 0.1-μm size particles, a stream velocity of approximately 200 cm/s at 375 K is required with SCCO2, as compared with air which requires a velocity greater than 1000 cm/s (Fig. 6.4) [2,129]. Other considerations that are important to removal of contaminants are the chemistry of the system and the operating temperature and pressure. These parameters are optimized for the specific cleaning application being considered [3,130,131].

222 Developments in Surface Contamination and Cleaning

FIGURE 6.5 Typical SCCO2 cleaning process [20,23].

The basic technology of the SCCO2 cleaning process [20,21 and references therein] is shown schematically in Fig. 6.5. The component to be cleaned is placed in the cleaning chamber which is filled with CO2. As the pressure and temperature are raised above the critical point, the gas transforms into the supercritical phase. SCCO2 dissolves the contaminants and it flows to the separation chamber where the fluid is subjected to a pressure and temperature change (pressure is reduced and the carbon dioxide vaporizes). As that occurs, the solubility of the contaminant in the carbon dioxide decreases, causing the contaminant to separate from the bulk fluid. Once all the CO2 is evacuated from the separator, the concentrated contaminant is usually in form of an oily or tar-like liquid residue that is simply drained from the separator. The residue can be recovered, recycled, or reused, if suitable; or the residue can be disposed as a sole component of the waste stream. No solvents, wastewater, or other contaminants are present to increase the volume of waste disposed. Cosolvents or other additives (surfactants, dispersants, and chelating agents) are added as needed. In order to mix SCCO2 and cosolvent or additive, it is often necessary to inject the cosolvent or additive into a flowing stream of SCCO2 as it enters the chamber. Process temperatures generally range from 308 K to 440 K. The operating pressures are in the range of 10 MPa to 17 MPa, although the cleaning chamber is often designed to operate at pressures as high as 30 MPa for SCCO2 cleaning operations, resulting in significantly greater equipment weights and footprints. Agitation by mechanical or other means (ultrasonic transducers or magnetic stirring) enhances the effectiveness of the cleaning process [23,132]. The cleaning systems are designed as modular units in which additional modules

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can be added for increased capacity. Several commercial SCCO2 cleaning systems were discussed previously [20,21]. Many polymers absorb significant amounts of gases and vapors when exposed at high pressures [1,133]. In particular, CO2 absorption can induce plasticization, swelling, and a decrease in the glass transition temperature significantly below that observed at atmospheric pressure [134–140]. This effect can be utilized to achieve effective polymer-based photoresist removal. Lowering the glass transition temperature of the polymer softens it, thereby enhancing penetration and diffusion of SCCO2 to the polymer/substrate interface. By suddenly releasing the pressure, a rapid volume change occurs that ruptures the polymer from the substrate [141]. A small amount of cosolvent enhances the solubility of the polymer and the chemical interactions at the polymer/ substrate interface, which promotes photoresist removal.

3.2 Application Examples SCCO2 has been employed for a wide range of precision and commercial cleaning applications including metal surfaces, glass, optical elements, Si wafers, polymers, parts with complex geometries and tight spaces, engine valves, terminal sterilization of medical equipment and microbially contaminated surfaces, garment cleaning, pesticide mitigation in museum collections, soil and drilling mud decontamination, and decontamination of radioactively contaminated surfaces. SCCO2 has been successfully used with additives (cosolvents, surfactants, dispersants, and chelating agents) to remove a wide variety of contaminants [1–17,20–24,142–181]. The types of contaminants removed include greases, lubricants, silicone oils, machining oils, flux residue, trace metals, photoresist containing plasticizers, outgassing compounds, printing ink, adhesives, and small particles. Addition of a small amount of a polar cosolvent, such as ethanol or propanol, can significantly enhance the removal of small particles. As an example, the cleaning efficiency for fabrics contaminated with clay particles (<10 μm diameter) is two to three times greater with the addition of the cosolvent compared with cleaning without a cosolvent (20 to 30% vs approximately 10%) [160,161]. The cosolvent tends to adsorb on the surface to increase the particle-surface distance and decrease the attractive adhesion force. Even nanoparticles may be dislodged from the surface by use of appropriate cosolvents and surfactants. In the following sections we provide updated information on many of the applications reviewed previously [20,21].

3.2.1 Photoresist Cleaning Photoresist stripping and cleaning was one of the first applications of SCCO2 in integrated circuit processing. Fig. 6.6 shows a typical example of removal of post-etching contaminant residue removed with SCCO2 [157,171,182–188].

224 Developments in Surface Contamination and Cleaning

FIGURE 6.6 Example of removal of post-etch residue from a silicon wafer with SCCO2. The residue, shown in the left image, has been removed as shown in the right image. The scale bars are 100 nm [157].

The results of SCCO2 cleaning generally show high degree of cleanliness with contaminant removal efficiencies better than 90%. One advantage of SCCO2 for precision cleaning is that the process leaves no residues, as it evaporates completely when depressurized. The cleaned surfaces will usually meet the commonly established precision cleanliness level of 1 μg/cm2 of the contaminant on the surface.

3.2.2 Wafer Cleaning and Drying Particle removal from the wafer surface has been successfully demonstrated using SCCO2 [182,186,188]. The ability to remove particles on wafer surfaces using an appropriate chemical formulation in SCCO2 has also been demonstrated. The use of optimum chemical formulation in SCCO2, their molar ratios, and specific process conditions can profoundly affect the particle removal efficiency [185]. Conventional drying processes, such as solvent-assisted Marangoni drying and sublimation drying, frequently result in pattern collapse or leaning of high aspect ratio and low mechanical strength nanostructures [22]. Drying with SCCO2 was found to be superior in noncollapse performance to all conventional drying processes. Release of sporadic stiction and complete recovery from global pattern leaning were both achieved with this SCCO2 drying process [189,190]. This emerging technology is expected to play a prominent role as the ultimate sustainable solution to device structure collapse problems. 3.2.3 Cleaning of Spacecraft Components and Planetary Protection Traditional cleaning methods are not very effective in removing live and dead microorganisms and hydrophilic biomolecules from spacecraft piece parts of

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slightly complicated geometry, such as tubing and loosely fitted nuts and bolts. These microorganisms are relevant to life detection on outer planets, as well as to planetary protection. A SCCO2 precision cleaning system has been developed for removing organic and particulate contaminants from spacecraft components [178]. The parts to be cleaned are secured in an inner basket and can be rotated up to 1400 rpm by a magnetic drive. The fluid flows within the vessel and generates tangential forces on the surfaces of the parts, thereby enhancing the cleaning effectiveness and shortening the soaking time. During the flushing cycle, the pump subsystem delivers fresh CO2 into the cleaning vessel at a constant flow rate between 0.01 and 200 mL/min. Both temperature and pressure are strictly controlled during decompression to prevent bubbles from being generated in the cleaning vessel that could stir up the contaminants that sank to the bottom by gravity. The results using this new cleaning system demonstrated that both SCCO2 and SCCO2 with 5% water as a cosolvent can achieve cleanliness levels of 0.01 mg/cm2 or less for contaminants of a wide range of hydrophobicities. The latter composition is representative of the Martian environment which consists of 95% CO2.

3.2.4 Cleaning of Printing Rollers A recent application of SCCO2 has been to clean rollers used in the printing and packaging industry [191]. These engraved rollers contain large numbers of microscopic cells (5 to 100 μm diameter) that carry inks or adhesives to the film substrate and are very difficult to clean due to the size and the depth of the cells on the roller surface. Conventional cleaning methods include manual brush cleaning, ultrasonic chemical cleaning, high pressure washing, blasting with fluid or dry media, and vapor injection. These cleaning methods can be expensive and time consuming; require complex maintenance of the cleaning system; employ harsh caustic and toxic chemicals; and often do not provide adequate cleaning of the rollers. The proposed new cleaning technique is based on the use of N-methyl pyrrolidone (NMP), which has high solubility for polyurethane adhesives, and CO2. The presence of SCCO2 lowers the critical point of the cleaning solution to 10 MPa at 313 K, thus providing processing conditions that are technologically favorable. Nearly complete removal of the dried red ink (polychlorovinyl resin) residue from the microscopic cells has been obtained after cleaning in 80% (NMP)-CO2 mixture at 313 K and 15 MPa (Fig. 6.7). Cleaning is ineffective at low (10%) NMP concentrations, but increases with NMP concentration. 3.2.5 Cleaning of Carbon Nanotubes Carbon nanotubes (CNTs) are increasingly being used as electron field emitters for field emission displays. For this application, the CNTs must be free of contamination and adsorbed moisture that often result from the manufacturing process. This contamination degrades the field emission characteristics of the

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FIGURE 6.7 Optical microscope images of the cells of an engraved roller after cleaning for 60 minutes with a supercritical solution of 10% NMP + CO2 (left figure) and 80% NMP + CO2 (right figure) at 15 MPa and 313 K. The dried red ink residue is nearly completely removed with mixture composition of 80% NMP [191].

CNTs. A novel method has been developed to enhance the emission characteristics of CNTs by treating them for 5 minutes at 327 K with SCCO2 + 5 volume % isopropyl alcohol as cosolvent [192,193]. This treatment was sufficient to remove the adsorbed moisture as was evident from the stability of the emission current of the SCCO2 + cosolvent treated CNTs. By comparison, the emission performance of the CNTs treated with pure SCCO2 was significantly degraded. In another example, single-walled carbon nanotubes (SWNTs) were purified by removing metal and amorphous carbon contaminants from production of the nanotubes by an in situ chelation/ SCCO2 extraction method [194]. More than 98% of the impurities were removed without obvious damage to the nanotube structures.

3.2.6 Soil Cleaning with Ionic Liquids and SCCO2 A novel application of dense phase CO2 is to employ SCCO2 with ionic liquids (ILs) serially to clean contaminated soils [195–197]. The IL is used to dissolve soil contaminants under ambient conditions and SCCO2 is used to recover the contaminants from the IL extracts. The efficacy of the process has been demonstrated by extracting naphthalene from soil by 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6] ionic liquid. The amount of naphthalene remaining in the soil was below the allowable contamination limit. Subsequently, SCCO2 was used to recover the naphthalene dissolved in the IL. At 313 K and 14 MPa, naphthalene recovery was nearly 84% for an extraction time of 4 hours. A process flowsheet has been developed for IL extraction of contaminated soils and continuous SCCO2 extraction of the contaminants from IL extracts for recovery and reuse of the IL.

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3.2.7 Conservation of Historical Art Objects and Structures Several applications of dense phase CO2 have been developed for cleaning of historical art objects and structures for conservation, for cleaning and disinfecting historical textiles, and for radiocarbon dating of archeological artifacts [198–209]. Museum collections often use organic and inorganic pesticides containing toxic metals such as arsenic and mercury for conservation and protection of the art objects. Unfortunately, this often resulted in contamination of the indoor air from the dust settling on the treated surfaces of the objects. In other cases, fragile textiles of historic value that may have also been damaged must be cleaned to preserve their fiber structure and value. Other materials needing remediation include waterlogged paper and wood, fluoropolymer coated stone, and various fabrics such as silk, wool and leather. Surface cleaning will not remove the hazardous chemicals embedded in the matrix. The use of SCCO2 has been successfully demonstrated for cleaning various objects in museum collections. Unmodified SCCO2 has been shown to effectively remove 70–90% of mercury, 50% of arsenic, 80% of DDT (dichlorodiphenyltrichloroethane), 60% of Lindane (ɣ-hexachlorocyclohexane), and 50% of Phencyclidine (PCP) residues, dust, grease, linseed oil and water without causing additional damage. For more polar organic pesticides, such as Diazinon, small quantities (1%) of a polar co-solvent are added to SCCO2 to remove the contaminant. Treatment times are generally just a few minutes to extract the contaminants from artifacts without damaging fragile materials and without leaving a residue. In the case of silk textiles, the fibers and the textile structure were not physically damaged and there was no loss of material by SCCO2 + isopropanol + water -assisted cleaning (Fig. 6.8). Extraction effectiveness is improved with a cosolvent such as water, isopropanol, or acetone. Detailed knowledge about the properties of the different materials (contaminants and objects) is necessary in order to prevent any possible damage to objects sensitive to SCCO2.

FIGURE 6.8 Removal of contaminant particles on an antique silk textile (a) before and (b) after cleaning with SCCO2 + isopropanol + water [202].

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3.2.8 Cleaning of Banknotes and Other Secure Documents Soiling of banknotes is primarily a yellowing of the notes due to the accumulation of oxidized sebum from handling, rendering the soiled notes unfit for high speed sorting and for general circulation [210,211]. Other contaminants that may be present are oils, organic compounds, bacteria and viruses, pesticides, insecticides, and heavy metals (such as mercury). These contaminants can be toxic to processing staff on repeated exposure of large numbers of soiled banknotes. The cost for replacing unfit currency is approaching US$10 billion annually with the added challenge of environmentally-friendly means of disposal of 10 billion unfit notes (weighing nearly 150,000 tons) [212]. In a recent innovative process, SCCO2 was effectively utilized to remove sebum and other oils and contaminants, including common bacterial colonies, from both paper and polymer banknotes without destroying the security features employed by central banks to prevent counterfeiting of the notes [213,214]. SCCO2 cleaning at 323 K and 34.5 MPa for 3 to 8 hours was shown to be effective in cleaning conventional straps, bundles and bags of banknotes, extracting nearly 4% of the initial weight. In addition, a U.S. one dollar note having one colony of micrococcus luteus, a skin bacteria, and 234 colonies of yeast (fungus) was cleaned and disinfected, and none of the pathogens remained on the note. Measurements of SCCO2-cleaned notes on a high-speed banknote sorting machine running at 10 banknotes per second showed that significantly fewer cleaned notes were classified as unfit for recirculation due to soiling. Other gases in the supercritical phase can also be used on their own or as mixtures with CO2 [214]. For example, as a dipolar species, N2O results in enhanced solubility in a mixture with CO2. Similarly, CO or SF6 may be used as a mixture with CO2. SF6 is particularly useful in a cleaning system because of its highly electronegative properties. The method of cleaning banknotes may also be used to clean and restore other materials, including secure documents and artwork, such as paintings that may include images, paint textures, and print. 3.2.9 Cleaning of Narrow Tubes and Pipes An innovative commercial system has been developed to clean umbilicals and control lines by flushing with SCCO2 [215,216]. Conventional flushing with clean oil leads to laminar flow (Reynolds number <3000) through the tube. As a result, only the fluid is cleaned, but the contamination from the inner wall of the tube is not removed. By contrast, flushing a 13,000-meter line with a 0.635 cm diameter with SCCO2 results in a pressure drop of only 15 MPa with a Reynolds number of >19,000. Thus, turbulent flow is maintained over the entire length of the narrow tube system, which facilitates both cleaning of contaminants in the tube and removes contaminants from the walls of the tube. The system has been employed for cleaning control lines on oil platforms in various parts of the world.

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3.2.10 Sterilization In the health and food sectors, the risk of transmission of diseases due to contamination is a subject of growing concern. As an example, viral and bacterial contamination of implants and allograft tissue, widely used by transplant surgeons for orthopedic (joint replacement), trauma, and cancer (surgical reconstruction) procedures, can have disastrous consequences for the patient and are a major concern [217]. Similarly, safe contamination-free liquid foods and beverages are critical to human health [218]. Efforts to minimize disease transmission have generally included donor screening, bioburden assessment, and aseptic handling, as well as sterilization before, during and after processing [219–239]. Terminal sterilization refers to a sterility assurance level (SAL6) of 10
6 (SAL6 is considered the standard for medical devices [240] and describes the process that ensures that the medical devices and implants are sterile at the point of use). Common sterilization methods include: steam autoclaving; dry heat; sterilizing gaseous (for example, ethylene oxide, chlorine dioxide, hydrogen peroxide) or liquid (such as glutaraldehyde, hydrogen peroxide, formaldehyde) chemicals; gamma, X-ray or electron beam irradiation; and UV-ozone treatment. All of these methods have certain drawbacks and limitations and cannot be applied to many materials and substances that are temperature sensitive or reactive with other forms of sterilization. For example, achieving terminal sterilization with these methods frequently compromises the osteogenic and biomechanical properties of the allograft. A novel approach to sterilization, with emphasis on reducing the process temperature and minimizing contamination, is based on dense-phase CO2 technology [219–221,224–239]. The process involves exposure of the item to SCCO2 with additives such as hydrogen peroxide or peracetic acid, at temperatures in the range 313 K to 333 K and pressures from 5 to 30 MPa. Treatment times can range from 5 minutes to 6 hours to achieve SAL6, depending on the microorganism. Clinically relevant grampositive and gram-negative vegetative bacteria can be deactivated in single step, but the treatment efficacy increases rapidly with temperature. Rapid pressurization/depressurization cycles also have a very positive effect on sterilization efficacy due to membrane disruption and cell lysis. Bacterial spores can also be sterilized with CO2-based processes. There is some evidence, however, that microorganisms such as spores and biofilms may survive short-term exposure to SCCO2 (minutes to hours) [241,242]. SCCO2 can penetrate and sterilize delicate products and materials, such as allograft tissue and engineered tissues including skin, ligament, tendon and bone, without damage to the structural or chemical integrity of the products. The very low surface tension of SCCO2 facilitates penetration into the interior of the tissue, thereby allowing inactivation of embedded pathogens. Other biological sterilization applications include: inactivation of viruses; elimination of endotoxins and pyrogens; production of sterile immunogenic preparations;

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medical implants and devices; sterilization of active and inactive pharmaceutical ingredients; and pest control (killing of insect eggs and larva). Recently, attention has been focused on food pasteurization and sterilization using SCCO2 [218,233–236]. Microorganisms are destroyed and enzymes can be inactivated in liquids, such as juices for example, by continuously flowing SCCO2 along flow paths. Contact between the flows can be achieved with countercurrent columns, vessel agitation (stirring or mixing), or with a membrane containing minute pores at which the flows contact each other in a nondispersive manner. The process does not adversely affect properties of the liquid, such as taste, aroma, and nutritional content.

3.2.11 Monitoring of SCCO2 Precision Cleaning Processes with the Quartz Crystal Microbalance SCCO2 cleaning has mostly used ultraviolet fluorescence detection for in situ monitoring of the cleaning process, although other techniques such as ellipsometry and gravimetric techniques have also been used [142,243]. However, these techniques do not provide sufficient sensitivity required for precision cleaning requirements, or they are complicated to use, or they require long equilibrium times in high pressure environments. Quartz crystal microbalances (QCMs) have a sensitivity of 10
9 g and can be used for accurate in situ monitoring of SCCO2 cleaning processes [243–247]. One application of the QCM monitoring method involves partial or complete removal of a film or coating from the QCM surface by SCCO2 using precision cleanliness criteria defined in IEST-STD-CC1246E [28]. The thickness and the amount of CO2 dissolved in a surface polymer film can affect the QCM response, although the QCM is applicable for films up to 1 μm thickness. Another application is to sample the SCCO2 fluid following the cleaning cycle into a QCM measurement chamber at a pressure at which SCCO2 is converted to a gas. The contaminants in the sample stream deposit on the QCM and the mass change can be monitored during the cleaning process. The QCM is also capable of monitoring the mass changes in SCCO2 drying of MEMS (microelectromechanical systems) structures.

4 OTHER CONSIDERATIONS 4.1 Costs Commercial SCCO2 cleaning systems are expensive, but operating and waste disposal costs are usually low [10,130,248–258]. The installed costs can range from less than US$100 000 for small-capacity (1-liter volume) equipment to several hundred thousand dollars for large-capacity (30-liter volume) equipment. The cost of the equipment increases considerably with size, and also depends on the complexity of the controls and other components, and the degree of automation required. For large parts it may be more cost effective to install lower-pressure-rated equipment and operate the cleaning system in

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supercritical mode for longer times at lower pressure. The systems are operated in batch mode. Interlocks for continuous operation between high and ambient pressure are too expensive to be justified. Recovery and recycling of the CO2 will add 15 to 25%. If the contaminant is readily soluble in liquid CO2, the system can be rated for lower pressure (at 5–6 MPa) operation as an alternative to SCCO2 which can reduce the equipment cost by 10–15%. Operating costs are generally low. Power costs are minimal (2.5 kWh for a 120-liter system) because cleaning cycles are short and there is no heat input into the process. The cost of supplying beverage-grade liquid CO2 to the system is quite small (US$1.30–$1.50 per kg), although high purity supercritical grade CO2 is considerably more expensive [257,258]. Typically, CO2 consumption per cleaning cycle is approximately 0.8 kg for a 120-liter system. Maintenance costs for the equipment range between 5–7% per year. Waste disposal costs for SCCO2 cleaning are lower than competing cleaning technologies as the waste residue is 100% contaminant. If the contaminant can be recovered, recycled, or reclaimed, there is no cost associated with disposal of waste. For contaminants that are readily soluble in SCCO2, the costs for cleaning are competitive with aqueous or solvent cleaning technologies. In one example, SCCO2 processes developed for preparing polymeric materials have successfully replaced solvent and thermal vacuum extraction processes for military and commercial spacecraft applications. SCCO2 proved to be much faster, less costly, more effective, and cleaner than solvent processes [259]. For sterilization of heat-sensitive materials and devices, dense-phase CO2 has been shown to have significantly lower cost per unit (cubic foot or load) than conventional sterilization with ethylene oxide, and comparable costs with hydrogen peroxide gas plasma sterilization [255].

4.2 Advantages and Disadvantages of Supercritical CO2 Cleaning SCCO2 has become an established process for removal of surface contaminants. The advantages and disadvantages of the process are discussed in the following subsections.

4.2.1 Advantages l SCCO2 has very low surface tension with excellent wettability, which makes it very attractive for cleaning intricate parts, or parts with deep crevices, tiny holes, or very tight tolerances. l CO2 has a supercritical temperature near ambient temperatures which is a significant advantage for cleaning temperature-sensitive parts. l The low viscosity of SCCO2 results in very high Reynolds numbers for flowing CO2. Such turbulent flow is an advantage in particle removal applications.

232 Developments in Surface Contamination and Cleaning

l

l

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The liquid-like high density of CO2 in the dense phase gives it very high solvation power for many low molecular weight organic compounds and many common fluorinated solvents. High purity cosolvents, surfactants (anionic, cationic, and nonionic), dispersants, and chelating compounds are readily available to broaden the range of cleaning applications for dense phase CO2. CO2 is nontoxic with a high threshold limit value (TLV) of 5000 ppm as compared with common organic solvents such as acetone (750 ppm) and chloroform (10 ppm). CO2 is nonflammable, which is a significant safety advantage in cleaning. SCCO2 is compatible with virtually all metals. High-density cross-linked polymers are not affected by SCCO2, but low crystallinity amorphous polymers are susceptible to plasticization and resulting brittleness of the component. For most cleaning applications, the degree of cleanliness is equal to or better than conventional aqueous or solvent cleaning processes. A further advantage of SCCO2 cleaning in the medical industry is the ability to remediate bacterial contamination by sterilization. Sterilization by SCCO2 is a technologically and economically viable alternative to conventional processes. SCCO2 cleaning is typically performed in closed-loop systems, designed to maximize recycling of the carbon dioxide. For challenging applications such as particle removal, the addition of agitation can significantly enhance the cleaning effectiveness, as well as reduce the time required for cleaning. The cleaning process times are relatively short, typically 15 to 30 minutes per batch, which leads to reduced process operating costs. Completely dry and clean parts are obtained at room temperature. No supplemental drying is needed which reduces the amount of energy, water, and time required for processing. CO2 is plentiful, inexpensive and recyclable, making the solvent consumption cost an insignificant contributor to the overall cleaning process costs. The process operating costs are low. Energy consumption is generally low since there is no heat input to the process. Energy is required to operate the pumps to perform cleaning. Contaminants are the sole waste. Hence, the waste disposal costs are low. In fact, waste disposal costs may be eliminated if the contaminants can be recovered, reclaimed or recycled. This is a noncorrosive, environmentally-friendly process. No hazardous wastes and emissions are generated.

4.2.2 Disadvantages l The low dielectric constant of CO2 makes it difficult to dissolve polar compounds.

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The process is ineffective in removing hydrophilic (polar molecules) contaminants, inorganic contaminants, and large bulk particles and other debris. Particle removal can be enhanced with mechanical or sonic (ultrasonic or megasonic) agitation, but that increases the capital costs. Cosolvents and other additives are available to remove these kinds of contaminants; however, the process costs are correspondingly high. SCCO2 cleaning is a batch process. The high costs of interlocks between high pressure and ambient pressure required for continuous operation are not justified. The installed costs of SCCO2 cleaning equipment are very high due to the need for expensive high pressure equipment. The equipment costs can be reduced by operating at lower pressures and longer cycle times. The high operating pressures of SCCO2 cleaning requires robust heavy cleaning chambers that scale with the capacity. In addition, peripheral equipment for storage, distillation, and recovery of the CO2 necessitates equipment with a large footprint. The extremely high pressures at which SCCO2 cleaning takes place makes it unsuitable for cleaning components containing gas or evacuated spaces because they could implode or deform during the cleaning cycle. Delicate critical parts can also be damaged by the high operating pressures. Process complexity is high especially if the chemistry has to be tailored for unknown contaminants. This also requires high level of technical skill. A major concern with SCCO2 cleaning processes is the safety risk of high operating pressures. Equipment must be properly maintained to prevent overpressure or failure of the high-pressure components in the cleaning system. Although CO2 is nontoxic and nonflammable, it can displace oxygen and cause asphyxiation if leakage occurs in closed, occupied spaces. CO2 monitoring may be required.

SUMMARY

Dense-phase CO2 in its supercritical state is an established precision cleaning technique with application in many different industries. The gas-like viscosity and the liquid-like density of CO2 are key characteristics that allow the process to be tuned to the application. In addition, the very low surface tension of CO2 in the dense-phase ensures high wettability and makes it very attractive for precision cleaning applications, particularly for intricate parts with complex geometries. The cleaning process is operated at near-ambient temperatures. Although the operating pressures for SCCO2 cleaning are high, this can be compensated for by operating at lower pressures in the liquid phase and for longer cleaning cycles. SCCO2 is a batch process. Applications range from cleaning and drying of micro and nanostructures such as carbon nanotubes, terminal sterilization of microbial organisms and food pasteurization, cleaning of metal surfaces, glass,

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optical elements, silicon wafers, and polymers, precision cleaning and drying of narrow tubes and parts with complex geometries and tight spaces, sterilization of medical equipment, pesticide mitigation in museum collections, and soil and surface decontamination.

ACKNOWLEDGMENT The author would like to thank the staff at the STI Library at the Johnson Space Center for help with locating obscure reference articles.

DISCLAIMER Mention of commercial products in this chapter is for information only and does not imply recommendation or endorsement by The Aerospace Corporation. All trademarks, service marks, and trade names are the property of their respective owners.

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