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On the Value of C02 Cleaning Operations : An Engineeringand Chemical Analysis

John B . Durkee, Creative EnterpriZe sLaurie L . Williams, Los Alamos National Laboratory

Conference Proceedings : NINTH PARTICLES ONSURFACES SYMPOSIUM

NATIONAL LABORATOR YLos Alamos National Laboratory, an affirmative actioniequal opportunity employer, is operated by the University of California for the U .S .Department of Energy under contract h--7405-ENG-36 . By acceptance of this article, the publisher recognizes that the U .S . Governmentretains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U .S .Government purposes . Los Alamos National Laboratory requests that the publisher identify this article as work performed under theauspices of the U .S . Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right topublish ; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness .

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On The Value of CO2 in Cleaning OperationsAN ENGINEERING AND CHEMICAL ANALYSI S

byJohn B . Durkee, II, PhD, Creative EnterpriZes'

Laurie L. Williams, PhD, Los Alamos National Laboratory 2

Abstract

This study examines the chemical and engineering reasons why cleaning and particle-removal applications involving pressurized liquid and supercritical C02 have beensuccessful, and not .

It is possible to predict which applications involving CO2 cleaning are likely to besuccessful , and which are not. The two key issues , which apply to use of any solventfor cleaning work, are :

✓ How well the Hansen Solubility Parameters of the solvent match those ofthe soil - either chemical or polymer .

✓ How well the physical properties of the solvent augment the removalmethods intended to liberate soil and particles from the substrate to becleaned .

Introduction

The chronology about past work in immersion3 cleaning with CO2 would not becomplete without mention of two words : expensive and oversold4 . Another word wouldbe empirical - because most applications are proposed based on past work orexperimental trial, rather than technical analysis .

1 . Email to jdurkee(o)_precisoncleaning .com. Phone to (830)-968-5919 . Mail to 122 Ridge RoadWest, Hunt, TX 78024 .

2. Email to williamsl@lanl .gov . Phone to (505)-667-3706 . Mail to Los Alamos Natl . Lab., P.O. Box1663, Los Alamos, NM 87545 .

3 . Not to be confused with either non-immersion cleaning with C02 where fragments of solid CO2impinge upon soil at the part surface, or cosolvent CO2 cleaning where secondary chemicals areadded to the CO2 .

4 . A knowledgeable friend writes " . . . I have NOT found liquid C02 particularly good at removinganything but very small and loose particles . . ." A . Giilman to J . Durkee, personal emailcommunication, December 2003 .

Page 1 of 25

It is the purpose of this paper to change characterization of future work with CO2 byeliminating the dreaded word oversold . The description expensive must remain - butonly relative to the needed investment for facilities .

The Rationale for CO 2

Concern about environmental affairs is the foremost reason why many consider CO2 forcleaning operations . CO2 is seldom chosen for its effectiveness - though perhaps itshould be so in the future .

CO2, whether used as a pressurized liquid or a supercritical fluid, is an environmentallyneutrals cleaning agent. CO2 is a common, non-ozone-depleting compound that isnontoxic, nonflammable, and recyclable . It can be acquired as a by-product gas fromvarious production processes ; thus, its purchase price is low and it does not add to orencourage formation of greenhouse gases .

The Misunderstanding About Solvency

Even proponents of CO2 cleaning technology can mis-characterize6 it . C02 is not a pooror good solvent . There are none such . Solvents should be rated relative to soils not toother solvents7'8'9 . That will be shown in this paper .

Because of the belief that CO2 was a poor solvent, developers have tried to enhancethe solvency of fluid CO2 by adding surfactants or cosolvents to the pressurizedcleaning bath10. Of course, this affects process control, rinsing, and performance ; andincreases the already high investment . Particle removal has become an area of

5 . While environmentally neutral doesn't mean chemically neutral, the limitation on use of CO2 ineither the pressurized liquid or supercritical forms is the absence of intermolecular forces whichmatch those of soils .

6. Darvin, C. H ., and Lienhart, R . B ., "Surfactant Solutions Advance Liquid C02 CleaningPotentials," Precision Cleaning Magazine, February 1998, p 25 to 27 . They write " . . .However, C02alone is a poor solvent and will not dissolve a broad range of typical contaminants . . . "

7 . Williams, L . Ph .D. dissertation, Colorado State University, Fort Collins, CO, 2001 .

8 . Hansen, Charles M ., Hansen Solubility Parameters - a User's Handbook, CRC Press, New York(2000), page 2 .

9. Durkee, J. B ., On Solvent Cleaning, Elsevier, London, ISBN 185617 4328, to be published in2005 .

10 . Recent patents include: 6,613,157 Methods for removing particles from microelectronicstructures, and 6,200,943 Combination surfactant systems for use in carbon dioxide-basedcleaning formulations . Typically liquid organic solvents comprise less than 10% by volume of themixture .

Page 2 of 25

activity" as solvency isn't involved .

Past Use of CO2

immersion CO2 cleaning has been evaluated for many soils12'13 . The approach hasbeen to expose soiled substrate parts or coupons to either pressurized liquid CO2 orsupercritical 14 CO2 and examine the coupons . Often, to either simulate a productioncleaning system or improve stated soil removal, the soiled surface is continuallyexposed to fresh C02 .

Soils where excellent cleaning results were obtained were : human based organiccontamination, some common machining oils, and a few polymers . Intermediateperformance was observed with soils such as amines, substituted phenols, substitute dbenzenes, phosphates, acids, and acid esters . A selection of adhesives, epoxies, andsealants was not well-cleaned . Larger molecular-weight molecules such as heavy oils,greases, waxes, and polymers are generally not soluble in C0215 .

Operating conditions are a crucial issue in any solvent cleaning process . Work such asreported above was conducted at a common value of pressure and temperature (300atm and 45 °C) . However, this produces an incongruity . Cleaning performance can beranked by soil because all coupons were all treated identically . But, the cleaning of nosoil can be truly evaluated because the best conditions for its removal were notidentified and used . This is particularly in the case of pressurized liquid or supercriticalC02 where the solvency is a function of temperature and pressure .

In general, immersion cleaning has not been done, until recently, with mechanica l

11 . Patents of interest include : 6,596,093 - Methods for cleaning microelectronic structures withcyclical phase modulation, 6,506,259 - Carbon dioxide cleaning and separation systems, and5,344,493 - Cleaning process using microwave energy and centrifugation in combination withdense fluids and 5,377,705 - Precision cleaning system .

12. Spall, W. D . and Laintz, K . E ., "A Survey on the Use of Supercritical Carbon Dioxide as aCleaning Solvent," Supercritical Fluid Cleaning, Noyes Publications, NJ, 1998. This study wasrelated to critical cleaning operations where the initial soil level was -2 pg/cm2 and the goal finalsoil level was < 1 pg/cm2 .

13. Williams, S. B . , Laintz. K . E ., Barton, J . , and Spall, W . D ., Elimination of Solvents and Waste byUsing Supercritical Carbon Dioxide in Precision Cleaning, American Chemical Society EmergingTechnologies in Hazardous Waste Management VI Conference Atlanta, Georgia, September19-21, 1994 . Manuscript A-703

14 . The demarcation between pressurized liquid and supercritical fluid is the critical point . For C02,this is -304 .31 K and -74 .843 bar. In other units, these values are 31 °C and 1085 psig .

15. Beckman , E .J . "Carbon Dioxide Extraction of Biomolecules ," Science, Vol . 271, February 2, 1996 .

Page 3 of 25

assistance16, as is commonly done when cleaning with other solvents or aqueouscleaning agents . Basically, work has been done as would be done in solventextraction" .

Predictive work has not been a common event . Previous predictive work was based onan empirical approach - past observation of good and poor performance .

Needed Technical Capability

An improved method to evaluate solvent - soil interactions for C02 is needed . Thiscapability would allow selection of pressurized liquid or supercritical C02 as acandidate for testing for soil removal only when there was an excellent chance ofsuccess . Further, a major advantage could be obtained if this capability couldreasonably predict the temperature and pressure where CO2 and the candidate soilexhibited the greatest similarity in intermolecular forces . This would allow testing to bedone at optimum or near-optimum conditions .

That capability will be displayed in this paper .

Solubility Parameters

A One-dimensional Solubility Parameter

The logic of transition from solvents and soils to "cohesive energy density and tomultiple solubility parameters was provided by Dr . Joel Henry Hildebrand18 and thosewho have built on his work .

Hildebrand's basic idea was that dissolution (or solubility) occurs when there was anenergy match within a fluid . Specifically, the attractive interaction energy of the solventmolecules must approximate the attractive intermolecular forces in the solute (soil) .Solvent molecules therefore appear to the solute molecule to be other solutemolecules, and freely situate themselves between and around the solute molecules .

16. This includes hydraulic and centrifugal forces, and pressure fluctuations generated by ultrasonicor megasonic waves .

17 . In fact, two commercially successful applications of CO2 are extraction of nicotine and caffeinefrom substrates .

18 . Professor of Chemistry at the University of California Berkeley, awarded the A .C .S .'s highestrecognition, the Priestley Medal in 1962, elected President of the Sierra Club (1937-1040),appointed manager of the U .S . Olympic Ski Team in 1936, discovered that the solubility of heliumwas much less than that of nitrogen at a given pressure which led to use of helium / oxygenmixtures for deep underwater diving, authored classic books such as The Solubility ofNonelectrolytes and Regular and Related Solutions : the Solubility of Gases, Liquids, and Solids .

Page 4 of 25

The solute molecules, now separated and interacting with solvent molecules, arethereby soluble in the solvent .

Hildebrand's work has two tenets :

To overcome intermolecular forces, energy is involved . Hildebrand, and thosewho built on his ideas, showed that these energy requirements were met if thesolute (soil) and solvent exerted the same forces upon one another .

Said another way, "like dissolves like . "

2. This energy requirement for combining a solute (soil) into a solvent is the sameas the energy holding the molecules of the solvent together .

This energy, in normal experience, is the heat of vaporization! All intermolecularforces are overcome when a solvent is vaporized . As a result, the magnitude ofenergy involved with the solution of a solute (soil) into a solvent is usually theheat of vaporization of the solvent .

Said another way, the same intermolecular attractive forces have to be overcome tovaporize a liquid as to dissolve something in it .

In 1936 Hildebrand proposed the square root of the cohesive energy density as anumerical value indicating the solvency behavior of a specific solvent . The term"solubility parameter" was proposed for this value and the quantity represented by adelta (6) . Then:

(d H - RT)X

V m Equation (1 )

2 ; or calories" / ccThe units of this parameter are (Pressure ' in mega-pascals (MPa) Y3/2 ; or in honor of it's creator the Hildebrand . 1 Hildebrand = 1 MPa' = 2 .0455calories" / cc 3/2

.

A solvent can dissolve a soil when the S values for the solvent and the soil are thesame.

EXCELLENT SOLVENT CLEANING RESULTS WHEN :

S solvent = S soi l

The difference between 6 for a solvent and 6 for the soil is a measure of the difficultyof dissolving (cleaning) the soil with that solvent .

Page 5 of 25

POOR SOLVENT CLEANING RESULTS WHEN :

S solvent ( 8 soil OR 8 solvent soi l

One should choose C02 or another cleaning solvent to have the same, or similar,Hildebrand solubility parameter as does the soil .

This is an absolutely crucial point . Solubility parameter data for either solvents or soilsis worthless! It is only by comparison of both sets of data than inference can be madeabout intermolecular forces between soil and solvent molecules . Solution of a soilwithin a solvent, and good solvent cleaning, happens when those forces are equal, orsimilar.

A Three-Dimensional System of Solubility Parameters

Dr. Charles Medom Hansen's approach19 solves two fundamental problems with theHildebrand Solubility Parameter .

The Hildebrand parameter was originally derived for compounds that interacted only bydispersion force interactions (relatively simple molecules) . However, as will bediscussed later, many molecules interact through a variety of forces, making themdifficult to classify in terms of a single parameter .

Also, some solvents have equivalent values of Hildebrand Solubility Parameter despitedissimilar molecular structures . The inter-molecular forces between them and othermolecules aren't similar . Their solubility performance isn't the same . Some examples ofsolvents which have the same Hildebrand Solubility Parameter but don't have similarstructures are: chloroprene and di-isobutyl ketone, cyclopropane and dimethylCellosolve, xylene and ethyl acrylate, and toluene and 1,1-Dichloropropane, to namebut a few pairs .

It is these problems, with Hansen's solution, which makes Hildebrand SolubilityParameters of limited practical value20 and which makes Hansen Solubility Parametersof significant value in predicting solution behavior between soils and solvents .

Hansen Solubility Parameter technology was developed in the 1960s by Charle s

19. His initial work was trial and error . Today Hansen Solubility Parameters (HSP) are determinedsolely on experiments or calculations using the data one can find for latent heat, dipole moment,group contributions etc .

20. Though they are of immense theoretical value .

Page 6 of 25

Hansen and others2 1'22,23'24,25'26'27' Their aim was to evaluate solute - solventinteractions associated with the formulation of paints and coatings . The outcome was asystem of "three-dimensional" solubility parameters . They are called Hansen SolubilityParameters (HSP).

Hansen's idea was that the total energy of vaporization of a solvent consists of severalindividual components . Each arises because of an inter-molecular force . In otherwords, the Hansen methodology proposes matching both the strength as well as thetype of intermolecular force. Those forces are :

✓ Polar - dipole-dipole28 forces that exist on molecules that are slightly charged oneach side )

✓ Hydrogen-bonding - a hydrogen bond is a dipole-dipole interaction that occursbetween any molecule with a bond between a hydrogen atom and any of Oxygen/ Fluorine / Nitrogen . The dipole exists because Oxygen, Nitrogen, and Fluorineare extremely good at attracting electrons and Hydrogen is extremely good atlosing them.)

✓ Disperse - On average, the envelope containing a single molecule has aconstant condition of electrostatic charge . BUT, at any moment, every portion ofthat envelope will have a changing electrostatic environment . Forces of shortduration - temporary forces -- are produced . They are of considerably less

21 . Hildebrand, J ., and Scott, R.L ., The Solubility of Nonelectrolytes, 3r° Edition, Reinhold, New York(1950) .

22. Hildebrand, J ., Scott, R . L., Regular Solutions, Prentice-Hall, Englewood Cliffs NJ (1962) .

23. Hansen, C.M., "The three Dimensional Solubility Parameter - Key to Paint Component Affinities, :International Journal of Paint Technology, Vol 305, No . 511, 1967, pages 104 to 117, 505 to 510 .

24. Hansen, C. M ., The Three Dimensional Solubility Parameter - Key to Paint Component Affinities,Journal Paint Technology, Vol 39 (No . 505), pages 104 - 117, 1967 .

25. Blanks, R .F., Prausnitz, J .M ., "Thermodynamics of Polymer Solubility in Polar and NonpolarSystems," Industrial and Engineering Chemistry Fundamentals, Vol 3, No. 1,1964, pages 1 to 8 .

26. Hansen, C.M., Skaarup, K ., The three Dimensional Solubility Parameter - Key to Pain tComponent Affinities, : International Journal of Paint Technology, Vol 305, No . 511, 1967, pages511 to 514 .

27. Barton, Allan, CRC Handbook of Solubility Parameters and Other Cohesion Parameters, 2ndedition (1991) .

28 . A dipole is a pair of magnetic poles, each with opposite charge, separated by a short distance .

Page 7 of 25

magnitude than are dipole-based forces2 9

Hansen Solubility Parameters (HSP) derive from equating the total cohesive energy ofa solvent to the sum of three cohesive energy terms - each one representing one of thecomponent forces .

(A IIT~ap - RT) = E Poi, + E Hydrogen - Bonding + EDisperse Equation (2)

Dividing each term by the molar volume of the solvent, one gets the defining equation :

.52 = SZ Potar + 52 Dipole + (52 Disperse Equation (3 )

This "Pythagorean Theorem" of solubility parameters, Equation (3), allowsdecomposition of the solubility parameter developed by Hildebrand into threecomponents - each one representing one of the component forces .

Equivalence or similarity of all three HSP values among both soils and solvents wassubsequently found to be a superior methodology for predicting solution of the soilwithin the solvent, and good solvent cleaning .

Definition of HSP for Supercritical Fluids

Identification of the components in the left-hand term of Equation (2) is not intuitivelyobvious for supercritical fluids, which don't vaporize . This uncertainty preventedcalculation of HSP values for supercritical CO2 . Recent work, reported in reference 4,has broadened understanding of the definition of solubility parameters, and HSPs, forsupercritical fluids .

This work returned to Hildebrand's basic idea about energy relationships in solute -solvent interactions that was derived from an approximation of the internal pressure3° of

29 .

30 .

Durkee, J . B ., "Solubility Parameters II," Metal Finishing Magazine, May 2004 . These matters arediscussed in more detail in references 4, 5, 6, 17, 18, and 21 .

Internal pressure is a phrase from thermodynamics which quantifies the internal energy requiredto isothermally expand 1 mole of solvent a very small amount .

(continued . . . )

Page 8 of 25

a fluid . Internal pressure and the term "cohesive energy density" was found byHildebrand and others to be nearly equivalent for nonpolar liquids (liquids whichinteract solely by dispersion forces", 32, 33, 3 4

To those seeking to solve cleaning problems, internal pressure may not seemsignificant. But it is so because internal pressure is a quantity which can be calculatedfor any material at any operating conditions through a equation of state3 s

In other words, the three HSP values can be calculated for any pure substance36 over arange of pressure and temperature through an equation of state for the component, thethermodynamic equation of state 31-- Equation (5) ,, and Equation (3) .

30. (. . .continued )For nonpolar and also polar liquids where the dipole moment is less than 2D and where specificinteractions ( particularly hydrogen bonding ) is largely absent , the energy required to do this isanalogous to the heat of vaporization . The thermodynamic equation expressing this equivalenceis :

€9 E/ 9V} r = n A E t A V with A E = A H- R T Equation (4 )

For nearly all solvents, n is between 0 .9 and 1 . 1 with most values between 0 . 95 and 1 .05 .Consequently, the simplifying approximation is made that n = 1 . See Scott, , R .L ., 1948, A Criterionfor Normal Liquids , Journal of Chemistry and Physics , Vol . 16, pg . 256 .

The definition of solubility parameter stated in Equation ( 1) has achieved common usage becausethe heat of vaporization is a quantity which is familiar and easily available .

31 . Westwater , W ., Frantz, H .W., and Hildebrand, J .H ., 1928, The Internal Pressure of Pure andMixed Liquids, Physical Review, Vol . 31, pp . 135-144 .

32. Hildebrand , J .H ., 1929, Intermolecular Forces in Liquids , Physical Review , Vol . 34, pp . 984-993 .

33. Hildebrand , J .H ., and Carter, J .M ., 1932 , A Study of Van Der Waals Forces Between TetrahalideMolecules, Journal of American Chemical Society, Vol . 54, pp . 3592-3603 .

34. Hildebrand , J .H ., and Scott , R.L ., 1950, The Solubility of Nonelectrolytes , 3rd ed ., Reinhold, NewYork, pg . 402 .

35 . An equation of state ( EOS) generally describes relationships among the pressure, temperature,and volume of a material . An EOS allows one to know all thermodynamic properties of a fluid atany specified temperature and pressure . See Wark, K ., 1995, Advanced Thermodynamics ForEngineers , McGraw- Hill, Inc ., New York, NY, pg . 159 .

36 . Mixtures can be evaluated through a volumetric-based blend rule - see Equation (6) .

37 . The thermodynamic equation of state, for a single compressible material , forms the basis of thecohesive energy density , developed by Hildebrand and Scott . See reference 34, page 97 .

GCE / OV 'T = T x € OP 19T 1, - P Equation (5 )

These individual terms, from left to right , are pressures: internal, thermal , and external .(continued . . . )

Page 9 of 25

Evaluation of CO2 Cleanin g

Calculation of HSP for Supercritical Fluid s

The method described above was used tocalculate the three HSP values C02through a range of pressure andtemperature conditions spanning thecritical point (-304 °K and -74 bar) andextending into the supercritical region .

Hansen Solubility Parameters for 002Operation Spanning Critical Poin t

7

G 5

4U; 0= 2

n

0 200 400 600 800 1000

Liquir1303 -K PYOSSI,/):$~ rPriticnl 323 • K

-$upercrltico1343 • K ---- Supcr<riticp1363 •K

--- Supercritical 303 • K

Figure 2 Polar HS P

All three HSP values, although of differentmagnitude, show similar trends withchanges in pressure, and temperature .

lansen Solubility Parameters for C02Operation Spanning Critical Poin t

©s ~n I I IN s :i ~ _ _ I= 4

tl 3CO 2

0 200 400 600 800 1000

Limit 303'K PreSSLLM r. ritc.aI 323' K

- Supercrltlcal 343' K - Supercrltlcai 363' K

Supercritical 303 • K

Figure 1 H2 Bonding HS P

They are shown as Figure 1 (H2 bondingHSP), Figure 2 (Polar HSP), and Figure 3(Disperse HSP) .

Hansen Solubility Parameters for C02G ;Operetion Spanning Critical Poin t

20 (..~«~.i....... ..., ._...~ [

1 210

4o

0 200 400 GOO 800 1000

Liquid 303 "K Pressure AQr?ritic nl 323 • K

- Supercritical 343'x - ^-^SupererltIcal 363 - K

- 8u per<rlti<nl 303 K

Below the triple point of C02 (-216 °K and Figure 3 Disperse HSP- 5 .2 bar), C02 is a gas . C02 is also a gasbelow the vapor-liquid equilibrium line, which ends at the critical point .

Near the critical point of C02 (-304 °K and -74 bar), HSP values (or intermolecularinteractions) are practically zero . In this situation, C02 would .be worthless as a solventbecause its intermolecular forces would not be similar to those of any soil molecule38 .

HSP values markedly increase with increasing pressure, especially above the criticalpressure of C02. For example, liquid C02 (at 303 °K), disperse HSP values changefrom 2 .5 (at 70 bar) to 8 .5 (at 75 bar) . For supercritical CO2, (at 323 °K) .disperse HSPvalues change from 1 .5 (at 70 bar) to 7 (at 120 bar) . Further pressure increase ca n

37. (. . .continued )

38 . Unless the soil were also a completely neutral molecule such as solid CO2 . But using liquid orfluid CO2 to remove solid CO2 isn' t a common commercial cleaning activity .

Page 10 of 25

achieve disperse HSP values of - 15 .

Limitation of HSPs

Considerable data abound of HSP and pure chemicals and polymers . The authors haveassembled a database of HSP values for more than 900 chemicals, 400 polymers, and50 general soils . The best published sources are references 6, 25, and Tables 2 .6 .4.2and 2 .6 .4.3 for reference 7 .

Seldom is removal of pure components the object of a cleaning operation . Many soilsare multi-component blends . The correct approach to compute HSPs for multi-component blends is to use the volume fraction of each component to calculate theHSP values39,18 . This approach applies for all three HSP values41 ,4, The equation for atwo component blend is :

S Blend = (Dix Si + 0 2 .X (5 2

with: 5 = HSP (Disperse, Polar, or H2 - Bonding) Eq (6)

= Volume Fraction Either Component

Equation (6) is the method by which HSPs for CO2 - cosolvent blends would beestimated . Equation (6) is general - it can be extended to multiple components .

But, Equation (6) can't be used when the identity of the components or their proportionsin the blend aren't known . Unfortunately, that's the normal situation with commercialproducts (lubricants, coolants, abrasives, drugs or medications, etc .) which becomesoils after use . Most such formulations are proprietary . Hence, cleaning data describingwork with specific commercial soils, and most general multicomponent soils42, can't beanalyzed using HPSs .

39 . Scott, R. L ., and Magat , M ., 1954 , Journal of Chemistry and Physics , Vol . 13, p . 173-177

40. Breon, T .L ., Mauger, J .W., and Paruta, AN ., 1980, Determination of Solubility Parameter Valuesfor Pure Solvents and Binary Mixtures, Drug Development and Industrial Pharmacy, Vol . 6, No . 1,p . 87- 98 .

41 . Van Dyk, J.W., Frisch, H .L ., and Wu, D .T ., 1985, Solubility, Solvency, and Solubility Parameters,Industrial Engineering and Chemistry . Product Research and Development, Vol . 24, p . 473-478 .

42 . Such as brake fluid or fingerprints .

Page 11 of 25

Implementation of HSP s

RA is the distance in solubility parameter space between the soil and the solvent . That

distance should be as small as possible (RA = 0)a3,a a

The equation to be used for solvent selection is :

RA 2solution = (S Polar for Solvent - S Polar for Soil) 2

+ (5 H2 - Bonding for Solvent - 5 H2 -Bonding for Soil) 2 Equation (7)

2+ [ 4 (8 Disperse for Solvent - 5 Disperse for Soil) 2 1

The independent variable in equation (7) is the choice of solvent . The dependentvariable in that equation is how well the soil is dissolved - how close RA approacheszero. In *practice, RA is never zero . RA values below -5 and between -7, 8, or 9represent good opportunities for solvent cleaning . RA values of more than -10represent situations where another solvent might produce improved results .

The HSP approach was independently applied to two areas by the authors4-6 of thispaper . The areas are, respectively , general cleaning with solvents and removal of somephotoresist .

Cleaning of Chemicals with CO 2

Data from a single source offer the advantage of being produced in a single facility bysimilar procedures. Data in reference 12 meet this criteria . They were developed aspart of a survey in the mid-1990s about the capability of supercritical CO2 for cleaningof many specific substances . Thirty-one chemicals were used in the survey for whichHSP values were available . These chemicals were applied to and removed fromvarious solid substrates . Only those data from the most neutral substrate (stainlesssteel) were selected for this paper .

The cleaning agent was supercritical C02 at 45 °C (318 °K) and 300 atm ( 304 bar) .

43. Hansen, C.M., Skaarup, K ., The three Dimensional Solubility Parameter - Key to PaintComponent Affinities, : International Journal of Paint Technology, Vol 305, No . 511, 1967, pages511 to 514 .

44. Finding the number 4 in Equation (2 .6 .5 .1 .1) may be surprising . There is considerable discussion

in the literature about the need for it . It has been found empirically useful over several decades of

practical experience . There are two justifications for it : (1) it converts three-dimensional spheroidalplots of boispe,sion and 6 H2-8onding and 6 Pow, into spherical ones, and (2) theoretical considerations

associated with the Prigogine Theory of Corresponding States .

Page 12 of 25

HSP values for CO2 at this condition were : b Dispersion = 11 .81, Polar = 4 .65, and b Polar5 .21 .

HSP values for the chemical, the RA values from Equation (6) for the chemical andsupercritical C02, and the percentage of the chemical (soil) removed are shown inTable 1 .

Table I Comparison of HSP Valueswith Cleanin Results From Reference 1 0

Soil 6 Dispers ion 6 H2 Bonding 6 Polar RA

% Soi lRemove d

Cholesterol 0 .4 2 .8 9 .4 9 .81 52%Motor Oil SAE 20W 4.7 0 .4 0 .4 6 .94 92%Mineral Oil 14 .5 0 .0 0 .0 7 .38 95%Silicone DC Oil 6 .4 0 .0 7 .8 7 .09 86°R E ox Hot Curing 8.3 12 .3 9 .7 11 .06 6%Ethylene Glycol 7 .0 11 .0 2 .6 8 .56 26%Hexadecane 6.3 0 .0 0 .0 8 .21 92%Pyridine 9.0 8 .8 5 .9 8 .35 96%Aniline 9.4 5 .1 10 .2 9 .18 56%

chloro henol 0 .3 5 .5 13 .9 12 .29 65%2,5-dichlorophenol 0.0 6 .3 12 .1 10 .93 56%

4,6-trichloro henol 0 .3 5 .1 10 .8 10 .26 57%,4,5-trichlorothio henol 1 .0 4 .5 9 .1 10 .05 56%

Naphthalene 9.2 2 .0 5 .9 7 .90 97%hio henol 0 .0 4 .5 10 .3 9 .73 87°

M-cresol 18 .0 5 .1 12 .9 10 .01 910011,3-benzenediol (Resorcinol) 18 .0 8 .4 21 .0 17 .51 89%Benzoic Acid 18.2 6 .9 9 .8 8 .27 40%O-dichlorobenzene 9.2 6 .3 3 .3 7 .78 86%P-dichlorobenzene 19.7 5 .6 2 .7 8 .29 89%M-dichlorobenzene 9.7 5 .1 2 .7 8 .25 86%Nitrobenzene 0.0 8 .6 4 .1 9 .14 82%

rimeth l Phosphate 15.9 15 .5 12 .4 13 .70 79%Dibutyl Pthalate 5.2 10 .8 7 .5 7 .45 82%Dimeth l Pthalate 15.3 13 .6 8 .3 10 .15 88%Dioctyl Pthalate 14.3 9 .3 6 .3 5 .42 97%Benzyl Alcohol 18 .4 6 .3 13 .7 11 .00 47%Iso horone 16.6 8 .2 7 .4 6 .41 90%Diethyl Sulfate 15.7 14 .7 7 .1 10 .97 78%Phenol 8 .0 5 .9 14 .9 ' 11 .70 56%

Using the arbitrary hypothesis that if RA < 8 .36, then the % soil removed must be > 85%for the data in reference 12, then 25 of these 31 experiments produce resultsconsistent with that hypothesis .

Page 13 of 25

Six of these 31 experiments produce results inconsistent with that hypothesis . Fourchemicals are removed less well than expected from HSP values : thiophenol, m-cresol,1,3-benzenediol (resorcinol), and dimethyl Cleaning Performance Analysis via HSPpthalate . Two chemicals are removed to a Data From Reference 1 0

greater extent than expected from HSP C 1values: benzoic acid dibutyl pthalate . o .s

0 0.6

E 0 .4

Of these 31 experiments, results from only 0 .22 strongly conflict with the hypothesis : 1,3- obenzenediol and benzoic acid . This isshown in figure 4 .

Cleaning of Polymeric Coatings with CO2

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U`lycn l

1` epoxy a ; c. ,. 3

ReBor neI)

10 12 14 16 18

RA

Figure 4

The removal of polymer coatings can be considered to be a cleaning application, sinceit is the selective separation of the polymer from the surface of the substrate that isdesired .

However, with the notable exception of some siloxane and fluorinated polymers, fewpolymers are soluble in supercritical CO2 . Rather, it is the sorption of supercritical CO2into a wide variety of polymers and the resulting swelling of the polymer that can b eutilized to achieve the desired separation .

In fact , near its critical point, CO2 is as soluble in many polymers as are typical liquidorganic solvents, ranging from (approximately) 10% to more than 30% by mass4 5

Maximizing C02 solubility and the subsequent polymer swelling, or disruption of thepolymer/polymer interactions, followed by rapid depressurization, causes the polymercoating to fracture and/or debond from the substrate .

CO2 Sorption Behavio r

The ability of a polymer to solublize C02 depends on its chemical structure .Particularly, because CO2 is a potential electron acceptor compound (Lewis acid )41,11 ,

45. Berens, A .R ., Huvard, G .S ., Korsmeyer, R .W ., and Kunig, F .W., 1992, Application ofCompressed Carbon Dioxide in the Incorporation of Additives into Polymers, Journal of AppliedPolymer Science, Vol . 46, pp . 231-242 .

46. Meredith, J .C ., Johnston, K.P ., Seminario, J .M ., Kazarian, S .G ., and Eckert, C .A ., 1996,Quantitative Equilibrium Constants between CO2 and Lewis Bases from FTIR Spectroscopy ,

(continued . . . )

Page 14 of 25

48 the presence of a basic site in the polymer 's structure is favorable for the sorption ofC02

However, the presence of crystallinity in the polymer will inhibit the sorption of C02,since C02 sorption is believe to occur only in the amorphous regions49 . Highlycrosslinked polymers will expand less freely than those with less crosslinking50 .

Two distinct types of reversible swelling and sorption isotherms have been identified forpolymers in the presence of C02 at elevated pressures . The sorption isotherm behavioris found to depend on the glass transition (T9) of the amorphous or semi-crystallinepolymer in the presence of compressed C02 .

✓ One isotherm is characterized by swelling and sorption that begin to level offand reach limiting values at elevated pressures .

In the absence of a glass transition, the equilibrium solubility of C02 in apolymer will exhibit a maximum with increasing pressure owing to freevolume effects . Therefore, at a given temperature, if the pressurecorresponding to the maximum gas solubility is reached before the glasstransition, this type of isotherm characterizes the sorption and swelling .

✓ The other isotherm is characterized by sorption and swelling that continue toincrease with pressure5 1

However, if the glass transition occurs below the pressure correspondingto maximum C02 solubility, this second type of isotherm characterizessorption and swelling.

46. (. . .continued )Journal of Physical Chemistry, Vol . 100, pp . 10837-10848 .

47. Kazarian, S., Vincent, M ., Bright, F ., Liotta, C ., and Eckert, C ., 1996, Specific IntermolecularInteraction of Carbon Dioxide with Polymers, Journal of the American Chemical Society, Vol .,118, pp 1729-1736 .

48. Walsh, J .M ., Ikonomou, G .D ., and Donohue, M .D., 1987, Supercritical Phase Behavior : TheEntrainer Effect, Fluid Phase Equilibria, Vol . 33, pp . 295-314 .

49. Shieh, Y., Su, J ., Manivannan, G., Lee, P ., Sawan, P ., and Spall, W .D ., 1996, Interaction ofSupercriticl Carbon Dioxide with Polymers I . Crystalline Polymers, Journal of Applied PolymerScience, Vol . 59, pp . 695-705 .

50. Chang, S.H., Park, S .C., and Shim, J .J, 1998, Phase Equilibria of Supercritical Fluid-PolymerSystems, Journal of Supercritical Fluids, Vol . 13, pp . 113-119 .

51 . Wissinger, R., and Paulaitis, M ., 1987, Swelling and Sorption in Polymer -CO2 Mixtures atElevated Pressures, Journal of Polymer Science, Part B : Polymer Physics, Vol . 25, No . 12, pp .2497-2510 .

Page 15 of 25

It is expected that these two isotherms will also eventually level off at some highpressure due to free volume effects in the polymer melt52 .

SWELLING AND SORPTION IN POLYMER-CO ,

2 8

uE

a0m

U

E._0

e 32 .7 `C

• 42.0 ' C

•- 58.8 'C * i

i

24

2 0

16

12

20 40 60 80 100

Pressure (atm )

Figure 5

The contrasting isotherms characterizingpolymer sorption and swelling behaviorwith CO2 are shown in figure 5 .

Isotherms at 42 .0 °C and 58.8 °C, arecharacterized by swelling and sorptionwhich increase with pressure .

Isotherms at 32.7 °C , are characterized byswelling and sorption that levels off withincreasing pressure .

There have been many studies on thesorption . of CO2 in polymers, although thetemperatures and pressures where theseexperiments have been conducted are

generally below the critical point of C02 (31 °C, 73 . 8 bar ) . A comprehensive review ofthis data is not possible here , but interested readers are encouraged to see reference51 .

Nevertheless, some general observations from the numerous studies can be made :

1 . In the case of structurally similar polymers, polymers with lower T9 valuesfavor C02 solubility . Poly(vinyl acetate, PVAc), with a low T9, solubilize nearlythree times more C02 than poly(methyl methacrylate, PMMA) or poly(ethylmethacrylate, PEMA) at the same temperature and pressure . Stated anotherway, all other things being equal, a low T9 indicates low intra-molecularinteractions., and therefore lowers cohesive energy densities .

2 . Silicone containing (Lewis base) polymers, which exhibit weakpolymer-polymer interactions, as evidenced by the low HSP values and low T9 ofpoly(dimethyl siloxane), favor CO2 sorption, indicating that the presence of Lewisbase groups, in the absence of interfering intra-molecular interactions within thepolymer promote C02 sorption .

3 . Hydrocarbon polymers, such as polybutadiene and polypropylene, sho w

52. Wissinger, R., and Paulaitis, M ., 1987, Swelling and Sorption in Polymer-CO2 Mixtures atElevated Pressures, Journal of Polymer Science, Part B : Polymer Physics, Vol . 25, No . 12, pp.2497-2510 .

Page 16 of 25

lower uptake of CO2 per unit of polymer, despite relatively low polymer-polymerinteraction (as evidenced by the low HSP values and low T . for both polymers .This indicates that the absence of Lewis base groups, even without interferingintra-molecular interactions within the polymer, results in low CO2 solubilities .

4. When Lewis base functional groups are present, their location on aside-chain versus the main-chain favors higher C02 solubilities .

Swelling of Polymers Containing Dissolved CO2

It is important to understand the type of swelling behavior a polymer will have insupercritical C02 in order to optimize the cleaning condition (temperature andpressure) .

✓ For example, operating at too high of a pressure may result in compression ofthe polymer, inhibiting the C02 ability to swell and weaken it's polymer/polymerbonding .

✓ On the other hand, too low of a pressure will result in low C02 HSP values andsubsequently a lack in swelling .

Behavior with CO2 and PMMA

Hansen solubility parameter values for PMMA have been determined experimentally byHansen53, and Van Dyk et al .54, and calculated by Shaw55 and Koenhen andSmolders56 . As with many commercial polymers, the composition, and therefore theHSP's, will vary between particular manufacturers, and in the case of PMMA a range ofHSP values have been determined ,

6d = 19 .4 - 15.6 MPa112 ; Sp = 10 .5 - 5.7 MPa'12 ; Sh = 7 .8 - 4.7 MPa112

The following average values will be used for PMMA in this analysis ,

53. Hansen, C.M., 1967, The Three Dimensional Solubility Parameter - Key to Paint ComponentAffinities . I . Solvents, Plasticizers, Polymers, and Resins, J . Paint Technology, Vol . 39, No . 104 .

54. Van Dyk, J .W ., Frisch, H .L ., and Wu, D .T ., 1984, Solubility, Solvency, Solubility Parameters,Industrial Engineering and Chemical Product Research Development, Vol . 24, pp. 473 .

55. Shaw, M.T., 1974, Studies of Polymer-Polymer Solubility Using a Two-Dimensional SolubilityParameter Approach, Journal of Applied Polymer Science, Vol . 18, pp. 449-472 .

56. Koenhen, D .M ., and Smolder, C .A ., 1975, The Determination of Solubility Parameters of Solventsand Polymers by Means of Correlations with Other Physical Quantities, Journal of AppliedPolymer Science, Vol . 19, pp . 1163-1179 .

Page 17 of 25

Fd = 17 .6 MPa1 '2 ; bd = 7

However, as discussed above, the HSPvalues of the polymer are affected not onlyby the temperature and pressure of theexperimental conditions, but throughswelling caused by CO2 sorption .

Shown in figure 6 are the calculateddispersion HSP (a similar trend was alsocalculated for the polar and hydrogenbonding values) values for PMMA as afunction of temperature, pressure, and CO2sorption .

MPa112 ; Sd = 5.0 MPa1/ 2

18

1 7

,.16

;, 15

0-1a

_1 3

121110

ren'Ae,,,

rure (C

Figure 6

18-f1 7

16 -

'~14 ag

13 p

1 21110

17S15 0

125100

The general observation about figure 6 is the area of low HSP values between 40 and60 °C at 200 bar. The HSP values for CO2 at 60 C and 200 bar are did = 9.1 MPa12,8p= 4.2MPa", and 6h=4.5MPa112 .

Prediction of Cleaning of PMMA with CO 2

Using equation (7) to calculate the R,A value between CO2 and PMMA at theseconditions results in a value of 4 .2 MPa112 . The required interaction radius, determinedon the basis of PMMA dissolution behavior in a range of liquid solvents is Ro'Q = 8 .6MPa1/2 57 .

Therefore with an RA < R0 a favorable interaction is predicted .

Cleaning of PMMA with CO 2

This prediction was experimentally verified at Los Alamos National Laboratory, where aPMMA photoresist layer was removed (by debonding) from a silicon wafer .

57 Hansen, C .M., 1967, The Three Dimensional Solubility Parameter - Key to Paint ComponentAffinities . I . Solvents, Plasticizers, Polymers, and Resins, J . Paint Technology, Vol . 39, No . 104 .

Page 18 of 25

Figure 7 Wafer Before CO2 Cleaning Figure 8 Wafer After CO2 Cleanin g

Cleaning performance with supercritical C02 is shown in Figures 7 (before cleaningand 8 (after cleaning) .

Many operations such as this one are reported in reference 7 .

Engineering Aspects of Cleaning With CO2

More than solvency is involved with choosing and using CO2 as a cleaning solvent ofchoice. In addition to possessing a range of HSPs not found in other solvents, C02 alsodisplays unique physical properties which can aid in decontaminating surfaces .

It's the Process, Not the Solven t

As with all other applications, the selection of liquid vs supercritical C02 is based onthe details of the application - the soil and the parts .

Supercritical C02 and pressurized C02 are the same solvent . They differ by theprocess in which they are produced and used . It is the processes, not the solvents,which should be compared . Specific soils, specific parts, and other specific detailsshould dominate the choice among C02 cleaning processes .

Page 19 of 25

One virtue of eitherpressurized liquid orsupercritical C02 as acleaning solvent is that iseasy to separate the soil fromthe solvent . Reduce thepressure - the CO2 vaporizesand the soil remains in a formwhich often pure enough forrecycle .

Cleaning with supercriticalC02 brings the sameenvironmental advantages asdoes cleaning with liquid

'l.

P

•l,

CO2 . Investment andoperating cost are higherthan with liquid C02 becausethe increased pressurerequires more rigid vessel sand consumes more electrical power .

Supercritical vs Liquid CO2

Solid

I i :~uir1

1 L,; ;,,,, AllpCr .lrttcal H U0

r ~_aV 11w, J~

fit pf-uJ (Itas

? .i 1

ILr.lpcrali rt ( °C)Figure 9 - A Chemical, A solvent, A Proces s

Which is preferred? In a sense this question is a trick .

The choice among supercritical and pressurized liquid C02 is analogous to theselection of the solvent with the most "power ." Solvent "power" is relative, not absolute .A specific cleaning solvent has "power" only in relation to a specific soil .

The HSP system allows matching of soils to solvents based on similarity ofintermolecular forces . No polar HSP value of supercritical C02 is better than one ofliquid C02-

Figures 10 (supercritical phase) and 11(pressurized liquid phase) show how thepolar HSP values of supercritical C02 andpressurized liquid CO2 are differentlymatched to those of soils . Supercritical C02and pressurized liquid C02 cover differentregions of "HSP space ."

Hansen Solubility Parameters for C02Supercritical Phase

20Wate :-Glycol H yd, s Flui d

15 _ _. -hlnnPh Yll

1 0

C-I T Prtch

J L,ns ee ~Oi hole sterolsps" Spefm l

00 5 10 15 20 25

HSP Disperse

Figure 1 0

Page 20 of 25

Hansen Solubility Parameters for C02Liquid Phase

20 Wote, -Glycol Hydraulh Flu, d

1 5

10 . ., i.--.,E-hoaA~~><a"1iYtlraWtd ..Fluid . . . .,-.. . _.

NT~h

5Lnlsee~t~t~r buin

. `~ `~

.

P- Oil Chol ~terof

o _ --0 5 10 15 20 25

HSP Dispers e

Figure 11

A comparison among Figures 10 and 11shows how solubility properties of C02 varywith operating conditions between thepressurized liquid and supercriticalregimes" .

Furthermore, an exact match of HSP valuesamong solvent and soil is no guarantee forsuccess of a cleaning application .

Unique Physical Properties of CO 2

Selection of a C02 - based process can involve more than just solvency - thoughsolvency is usually given primacy .

Solvent cleaning is more than just solutionof soil . Typically, a solvent is chosen basedon at least four considerations : (1) solutionof a soil , (2) rinsing of the soiled solventfrom the pa rts , (3) drying of the rinsedparts , and (4 ) removal of insoluble residue(particles) .

HSP play little or no role in the 'ast threeitems. Physical properties of the solventare likely to be the dominant issues . Keyproperties are density (specific gravity),viscosity, an surface tension .

Liquid Surface TensionAffects Particle Remova l

Figure 1 2

Calculated data for these physicalproperties is plotted in Figures 12, 13, an d

58 . It must be noted that HSP values for soils computed and plotted in Figures 4, 5, and 6 have tobeen corrected for the effect of elevated pressure and temperature . Chiefly, in the case of Figures5 and 6, this is because the soils are multicomponent mixtures whose specific recipe is unknown .In the case of Figure 4, this is because the needed information (coefficient of thermal expansionand for each chemical and isothermal compressibility) is not readily available . Both authorsbelieve this defect does not compromise the integrity of this analysis because the effect ofpressure and temperature on HSP is typically not more than 5 to 10% .

Page 21 of 25

14 . The temperature range covers use between the normal freezing and boilingpoints59 .

Liquid Viscosity

A-fc,_.t' Ls unda-y Laves Thicknes s

The calculations are based on standard ~• - ,engineering correlations60 Results fo

r twenty-five solvents are included in eac hplot so that differences between solvents M Mnormally used in cleaning and C02 can b enoted. -

., ..- 7_,

. .. . : :. _ : : ..

In this case, the three issues are major Figure 14

changes in the physical properties of C02 :

✓ Surface tension of pressurized liquid CO2 declines to essentially zero, as seen inFigure 1261 .

✓ Density of pressurized liquid CO2 declines at a high rate with the temperaturechange, as seen in Figure 13 .

✓ Viscosity of pressurized liquid CO2 declines to nearly zero, as seen in Figure 14

Dimensionless Number s

As the collective interaction of intermolecular forces is evaluated by examining theireffects in parameters (Hildebrand and Hansen solubility parameters, for example), soalso is the collective interaction of physical properties evaluated by examining theireffects in parameters .

59. CO. is the exception to this statement . The temperature range is from the triple point, along thevapor-liquid equilibrium line, through the critical point . This is the so-called pressurized liquidregime . Naturally, pressure varies throughout this regime .

60 . Yaws, C . W., Chemical Properties Handbook: Physical, Thermodynamics, EngironmentalTransport, Safety & Health Related Properties for Organic & Inorganic Chemicals, McGraw-Hill,1998, ISBN : 0070734011 . These equations are programmed into software provided by the QualityMonitoring and Control (QMC) Corp . This excellent software was made available by Jane VanBuskirk of QMC, whose support is appreciated .

61 . Water is the solvent whose surface tension is given by the upper line . CO. is the solvent whosesurface tension is given by the lower line. The information in Figure 7 describes why water is apoor choice of cleaning solvent, no matter its HSP values, when surface forces play a crucial rolein a cleaning operation .

The surface tension of liquid CO2 is significantly less than that of CFC 113 - a solvent well usedfor penetration of tight clearances or small stand-offs . .

Page 22 of 25

In this case, the parameters are dimensionless numbers . Dimensionless numbers oftencorrelate with some performance parameter and greatly aid engineering analysis anddesign62 . Dimensionless numbers are often defined as ratios of processing rates orratios of the effects of physical mechanisms which are simultaneously occurring . Theseratios allow estimation of outcomes at different scales of operation . Dimensionlessnumbers allow the impact of competing or parallel factors to be evaluated .

Two dimensionless numbers are of particular interest to those doing cleaning work : theWeber Number (WN) and the Capillary Number (CN) .

Weber Dimensionless Number(Inertial (Surface Tension

✓ The WN relates to formation ofcavitation bubbles via ultrasonicpressure waves. WN is proportionalto the ratio of inertial forces tosurface tension forces . WN isdefined as DV2 - g ,

Higher values of WN reflect F igure 15dominance of inertial forces oversurface tension forces. Strong surface tension forces 64 are necessary for theformation of high-energy cavitation bubbles .

CO2 is not a useful solvent if ultrasonic cavitation is necessary for successfulcleaning work because it is not conducive to formation of cavitation bubbles .Note in Figure 15 how it is unique relative to the other twenty-four solvent s

62. Kota, P. R ., "Principles of Reactive Interfacial Chemical Cleaning," Proceedings of the 1995Precision Cleaning Conference, page 247 to 270 .

63 . D is a characteristic length, V is fluid velocity, g, is a dimensional constant used to from force tomass , and a is surface tension . All are in a compatible system of units so that WN has no units .

64 . This is a key value of aqueous cleaning . Water is an excellent fluid for cavitation because its highsurface tension allows growth of stable bubbles of significant size relative to organic solventswhich have lower surface tension . Bubble size is basically determined from a force balancebetween density difference between vapour and liquid which acts to increase bubble expansionand surface tension which acts to constrain bubble expansion .

Page 23 of 25

✓ The CN relates to removal ofparticles from surfaces via cleaningsolvents . CN is proportional to theratio of viscous forces in a liquid tosurface tension forces in the sameliquid . CN is defined as uV_ g, o7 6 5

Higher values of CN reflectdominance of viscous forces oversurface tension forces .

Capillary Dimensionless Number.Viscous i (Surface Tensio n

Figure 1 6

C02 is a useful solvent if it is necessary to avoid surface forces so as to liberateparticles from parts . Note in Figure 16 how it is unique relative to the othertwenty-four solvents .

Hiding Particles in Boundary Layers

Sub-micron particles can "hide" in theboundary layer along the part surface" .

This is because their size is small enoughthat they are enveloped in the boundarylayer .

The thickness of this layer can becomputed from conventional principles offluid mechanics . Boundary layer thicknessfor the twenty-five solvents is shown inFigure 17 .

The thinnest boundary shown in Figure 17 is that for C02 . Values can be in the rangeof 1 to 2 microns, or sightly lower . Thus, sub-micron particles probably cannot beremoved by C02, but particles several microns in size can be done so easily .

65. p is viscosity, V is fluid velocity, g, is a dimensional constant used to from force to mass , and pis density . All are in a compatible system of units, so that CN has no units . CN is also equal to theratio of Reynolds Number to Weber Number .

66 . Baker , J . A ., and Durkee , J . B ., "Hiding Particles in Fluid Boundary Layers , A'C2 Magazine, PartsI, II, III," December 2001 to Februa ry , 2002 . Viscous forces between fluid elements producevelocity gradients between fluid adjacent to a su rface and fluid in a free stream . At a surface the,fluid does not move - the velocity is zero . Fluid velocity is positive , can be large, and usuallyturbulent . The boundary layer is a region adjacent to a surface . Here high-velocity turbulent flowscan't penetrate . Consequently , particles at the su rf ace can ' t be dislodged .

Page 24 of 25

Because of the extremely low (approaching zero) viscosity, boundary layer thicknessfor CO2 can be - 1/5`h of that with water for the same rinsing velocity .

Removal Mechanism for Polymers

Supercritical COz, and other highly-pressurized solvents, offer an advantage notavailable with other processes. In the supercritical regime, sorption of the solvent intothe soil can be a significant factor in removal of the soil4 . Sorption allows the polymer tobecome swollen with solvent . The removal mechanism then becomes one involving twoprocesses : (1) sorption and swelling, followed by (2) solution of the swollen polymer inadditional solvent . The optimum temperature and pressure is not always the same forboth processes . This is often quite significant when removing polymeric soils .

Summary

There is no perfect cleaning solvent - because there is more than one cleaningapplication . Every solvent presents advantages and disadvantages when consideredfor every cleaning application . The advantages and disadvantages are not absolute,but relative to the details of the application .

C02, whether in the pressurized liquid or supercritical form, can be expected to bringtechnical67l value for applications where :

✓ Its HSP values closely match those of the soils native to the applicationExamples are paraffinic materials .

✓ A solvent with a low surface tension brings value - to penetrate crevices andflow through thin stand-offs .

C02 is not suitable, in either form, for applications where :

✓ Its HSP values don't closely match those of the soils native to the application .Examples are benzene derivatives and soils with a high level of hydrogenbonding .

✓ Collapsing bubbles, generated by ultrasonic-generated cavitation, are requiredto provide surface energy .

67. In addition to environmental value .

Page 25 of 25