'1 *, - .L**LIL. I,.. .. ." larernarional Journal of Environmentally Consciaus Design & Manufacturing. Vol. 2, No. 1. pp, 8 1-86. 1993 Printed in the U.S.A. .. mest- -. Pollution Prevention Research Center 1326 Ftfth Avenue, Sutre 650 Seattle, WA 98101 (206)223-1 151

SUPERCRITICALCARBONDIOXIDE PRECISIONCLEANING FOR ANDWASTE REDUCTION

W. DALESPALL Los ALAMOSNATIONAL LABORATORY Los ALAMOS,NM 87545

Murph?s Law of : You can't get thepart cleanerthan the dishwater, but is itpossible to get both the part and the dishwater dirty. Supercritical as a cleaning solvent offers many advantages for the cleaning of selected materials. This paper discusses the applicability ofsupercritical carbon dioxide to precision cleaning of a wide variety of parts. The economics involved and a description of the work in progress is included.

INTRODUCTION after cleaning. Specifications such as this lead to many Ascetics, performance, improved work life, product parts being overcleaned, while others are undercleaned. specifications, and marketing strategies are among the The rigidity of specifications often rests on habitual prac- many reasons to clean an object. Currently, cleaning tices rather than actual needs. Specifications should con- technologies can be divided into two broad categories, sider how clean a part needs to be to meet speicific aqueous and non-aqueous based. These technologies requirements on the whole. This should lead to more face an upheaval brought on by imposition of the Montreal cleaning for some parts, but to less cleaning for most parts. Protocol, which restricts or prevents the use of chlo- Because theentire cleaning process is now at question, rofluorocarbons (CFCs) for all uses, from refrigerants to manufacturing engineers should seize this oppofluniiy to dry cleaning. Some 20% of the total CFC production change the specifications of the cleaning process. The worldwide is used in cleaning, generally during the throughput rate of the process, acceptable surface con- manufacturing process. The loss of CFCs as cleaning tamination levels, and types of contaminants to be re- has led to a reevaluationnf the entire cleaning moved should be reevaluated. The overall cleaning pro- process from an environmental point of view. A sense of cess, which includes solvent preparation, waste disposal, urgency is associated with search for solvent replace- drying time, rinse operations, pre- and post- treatment ment because CFCs and many other solvents will be times, worker safety, and ease of operation, as well as phased out in the next few years. The problem facing total time spent in the process, must be reassessed. The industry is lo find acceptable replacement cleaning strat- need to clean is directly related to cost of a part or egies quickly. assembly. Increased cleaning during the manufacture Any discussion of cleaning should begin with a defi- process will alwaysdrive unit cost up. By the same token, nition of what is being cleaned and what level of clean- increased cleaning can lead to a higher-quality part while liness is expected. In other words, how clean is clean? reducing worker risk, improving throughput, and gener- The answer is often couched in terms of specifications ating less waste material in the production line. pertaining lo the amount of soil remaining on the part The simplified scale in Fig. 1 is a reasonable estimate of -- - cleanliness levels. Many cleaning specifications are based * Los Alamoc National Laboratory ID Number LA-UR-93-1279. on the level of specific or characteristic compounds (e.g.,

81 82 Supercritical Carbon Dioxide Precision Cleaning W. DALESPAU., a.ul.

approach. Supercritical carbon dioxide cleaning can Oil contamination scale remove many of these common contaminants.

SUPERCRITICAL FLUIDS CLEANING To appreciate the unique properties of supercritical fluids, particularly supercritical carbon dioxide, that make them ideal solvents for many cleaning applications, we must define what a is. All elements and compoundscan be described in terms of a diagram. which is a representation of the states of the material as a function of temperatureandpressure orof otherpropenies of the material. The phase diagram of carbon dioxide is

011 adsorbed interstitblly 1

TABLE 1 Common ,taminants and Substrates Encoun -d in Precision Cleaning Substrates _+ 100 Pure Metals aluminium beryllium magnesium copper gold iron V nickel 10wm2 oil wets surface -1 6 '0:- silver tantalum Fig. 1. How clean isclean? Above, isa schematic representation titanium of the levels of contamination and the desired degree of Alloys carbon steels cleanliness of parts. stainless steels brass chrome alloys inorganic or organic) remaining. It is generally assumed mOWl that contaminants are uniformly spread across a part's inconel hastelloy surface, which is obviously not true at the molecular or aluminium alloys ' ' - near-molecular level. Contamination concentrates in pock- Etas tomers viton ets on the surface, in surface irregularities, and in the least neoprene accessible locations. This clustering of contaminants pre- buna rubbers sents special difficulties to solvent cleaning at low con- silicon rubbers taminant levels because the contaminated surface area is polyimide polyester smaller than if it were spread uniformly across a perfectly nylon smooth surface. The greater the interaction between con- ethylene propylene taminant andsurface,themoredifficultit willbetoremove polyethylene (UHMW, LD, HD) teflon the contaminant. For this paper, precision cleaning is polystyrene defined as cleaning a part's surface to less than 10 micro- grams of contaminant per square centimeter, although the Contaminants machining oils (lubricants, cutting goal for most precision-cleaning levels is less than 1 fluids, engine oils) microgram per square centimeter. hydraulic fluids The 10-microgram level of cleanliness is either very damping fluids desirable or required by the function of parts such as fingerprints body oils electronic assemblies, optical and laser components, la no1 in electromechanical elements, hydraulic items, computer grease parts, ceramics, plastics, and many cast or machined waxes adhesives ' metals. A large number of potential contaminants must be sealants removed from these parts. Table 1 lists common sub- fluxes strates and contaminants. Some contaminants are ame- particulates (fibers, machining fines, dust, cotton fibers) nable to solvent cleaning; others require a different 'C .

Inrcrnoriortul .Iot~rrtulof En~dr~nnioirullyCoriscious Dcsipti & Mo~ir~ucrrotng.Vol. 2. No. I, pp, R 1-86. 1993 83 Printcd in the U.S.A. Phase Diagram of CO, penetrate into these regions to remove contaminants. Pressure (bar) A large increase in the solubilily oicom- pounds generally results when going from the gas to the supercritical state. Most ma- terials are nearly insoluble in the psphase. but many have quite high solubilities in the supercritical state. This enhanced solubil it y 350 of organic Compounds in the supercriticid state fomis the basis for using supercritical r/ 300 fluids as cleaning solvents. The low viscos- ity, low surface tension. and hizh d rates mean that supercritical fluids c;w 250 readily penetrate into small regions 10 re- move contaminnnts. As i~ result. ihc. rc- moval process is niore rapid thui \4,lic.n 200 using liquid solvents. Although this sounds as if supercrilical fluids are an absolute solution to the cle:rn- 150 ing problem, many substances (e.g. ionic solids) are insoluble in supercriIica1 I'luids with low polaritiessuch ascarbon ilioside. 100 Many of the more polar supercritical Ilu- ids such as water, which would he c:ipablt. of dissolving polar and ionic compounds. 50 are very reactive and cause deterioration of the materials to be cleaned. Volritile, 0 compounds (having high vapor pressures) -70 -50 -30 -10 10 30 50 70 90 are generally quite soluble in CO,, but I Temperature ("C) separating them from gaseous CO, is dif- Sublimation line TC 31.06"C ficult. Supercritical carbon dioxide-is best applied to the removal of organic com- PT diagram of COn with the density as third dimension. Densities pounds with mid-to-low volatilities. This given from 100 to 1200 gRer. LoS Alamos class of compound often occurs as CLS-91-1 138 common contaminants encountered in Fig. 2. Temperaturepressure phase diagram for carbon dioxide. precision cleaning. Supercritical fluids cleaning or extrac- shown in Fig. 2. A few salient points on the phase diagram tion apparatus is conceptually simple, as shown in Fig. 3. have special significance. The lines depict phase changes A source of CO, such as standard gas cylinders provides of the material (e.g., from liquid to gas). The critical point the fluid for the pump used to elevate the CO, to pressures (designated CP in the diagram) is defined by both a above the critical pressure. At this point, the physic,'I I \[ate: pressure and a temperature. Any fluid above the critical is usually liquid in the extraction vessel. The temperalure temperature cannot be tumed into a liquid no matter how is raised to thedesired point above thecriticnl lemperature, much pressure applied. For CO,, this point occurs at 3 1.1 and extraction begins.Thesupercritica1 CO, flows throuzh degrees centrigrade and 74.8 atmospheres (atm) of pres- the cell and reaches the throttle valve. The fluid is then sure. The region above the critical point is called the expanded intoa volume so the physical state is that of a gas. supercritical fluid region; this region exhibits some prop- The extracted compounds are collected in the separator. erties of a gas and some of a liquid. and the gaseous CO, is passed back into the flow stream to Supercritical fluids have low and nearly be used again in the cleaning process. zero surface tension. Diffusion coefficients of substances Although many compounds can easily attain their in the fluid are between those of liquids and those of critical points, making them potential supercritical fluids, gases. These properties make supercritical fluids ideal these substances are generally flammable or toxic. Car- for cleaning parts having porous, intricate, or rough bon dioxide is the most frequently used supercritical surfaces or confined work areas because the solvent can fluid, primarily because of its low-cost, non-toxic, envi- 84 Supercritical Carbon Dioxide Precision Cleaning .W. DALESPALL. el. ul

Generalked equipment schematic space utilization, and energy costs. Obviously, if existing equipmentcan be retrofit to accommodate an alter- native cleaning process, the capital costs of new unit installation can be avoided. This orothereconomic fac- tors may prove to be the driver for cleaning-method selection. It must be stressed, however, that the entire Extraction vessel CU cleaning process must be evaluated (agitation, reservoir to make an environmentally sound flow, Sonia, decision on method selection. spray) A final point in support of using supercritical CO, as a cleaning agent is that many of the alternate systems suggested for solvent cleaning have not been tested for toxicity and health LosA!anm w-2587 effects. Many of these solvents have known health and safety problems; Fig. 3. Generalized equipment schematic for supercritical fluids extraction. many are considered volatile organic compounds; and all are potentially ronmentally benign nature, as well as its relatively low subject to future regulation as the health and safety critical temperature (88 degrees Fahrenheit) and pres- factors are identified. Carbon dioxice has been used in a sure (1 100 pounds per square inch). These low critical variety of industrial situations for some time, and its parameters make the use of CO, relatively efficient in health effects and handling requirements are well under- terms of energy use during operation, although not as stood. It is unlikely that CO, as a cleaning solvent will be efficient as operations conducted at room temperature. regulated beyond existing standards, thus making it im- Theelevatedpressureneededforattainingthesupercritical probable that another cleaning methodology will replace state adds cost to the cleaning vessel and makes batch- CO, cleaning in the future. mode operation necessary. Many increased costs are Although CO, used for cleaning is environmentally offset by the low cost of CO, (about $0.03 perpound) and benign (CO, production distills carbon dioxide from air, the simplicity of the overall cleaning operation. so no new carbon dioxide is added to the atmospheric Operational costs for CO, tend to be lower than for burden), the materials extracted into supercritical CO, other cleaning processes. With CO,, the solvent is re- must be removed before it can be disposed of or recycled moved by releasing the pressure in the extraction cham- for additional cleaning. For compounds with low vapor ber. The solvent in the chamber is lost, but in today's pressures, this removal can usually be accomplished commercialunits, this loss of solvent amounts to approxi- simply by lowering the pressure of the stream using a mately 20 pounds of CO, per &hour operating day, simple throttle valve. The pressurized supercritical CO, making the cost of solvent loss during operationsabout $1 then expands into a large volume. When the pressure and per day. For alternate cleaning ystems, it is often neces- temperature drop below the critical point, the CO, will be sary to rinse a part in a separate unit, then dry the part in in the gas phase, and contaminants will form droplets or yet another unit. Any rinsing needed with CO, is per- aerosols in the gas. The concentration of contaminant is formed in the chamber used for cleaning; no drying is determined by its solubility in the CO, and by its vapor needed because the solvent dissipates upon pressure pressure at the let-down temperature. Allowing the pres- release. These factors make supercritical CO, cleaning a sure to fall too low is undesirable because energy will be viable alternative for many cleaning applications. consumed to bring the pressure back to the supercritical A single-site cost comparison of four cleaning pro- range. The expansion of the supercritical fluid is accom- cesses used on identical parts indicated supercritical CO, panied by a drop in temperature; energy must usually be was slightly less expensive. Results of this study are added at this point to keep the fluid from turning into CO, shown in Fig. 4. For trueeconomiccomparison, however, snow in the separator. Because the quantity of materiai theentire cleaningprocess must beconsidered, including remaining in the gas phase will, to a large degree, be costs arising from solvent handling and disposal, drying determined by the vapor pressure of the contaminant, equipment, total handling and processing time, dollar substances with high vapor pressures will be difficult to value of parts held in the production line for cleaning, remove from the CO,. Cleanliness is important because Inrcrtiational Journal of Dtvirunmenrally Conscious DesiRn & ManufacturinR. Vol. 2, No. 1, pp. 8 1-86, 1993 85 Printed in the U.S.A

you cun'i gei the dish cleaner than the dish wafer. as conditionsarecritical fortheamountofchangeobserved. Murphy's Law of Cleaning states. While feasible from a Not all polymersareequal1yaffected.anddifferent formu- solubility standpoint, removal of volatilecontaminants in lations of the same basic polymeric formulation react CO, is not feasible from an operational standpoint. differently. The best counsel is to check specific parts Our studies on supercritical CO, cleaning focused on containing polymeric materials on an individual basis. four areas: materials compatibilcy, contaminant and Millions of potential contaminants can be found on ' substrate removal efficiencies, mechanics of the con- thousands ofsubstrates. Obviously, it is impossible to test taminant removal, and physical parameters determining removal efficiency for all cases. The most common the effectiveness of the separator. Materials compatibil- contaminants tend to be body oils, cutting and machining ity is a critical issue because many of the substrates being fluids, oils and greases, adhesives and sealants, and cleaned are organic and the potential for the CO, attack- particulate matter. Common substrates are pure metals, ing the substrate cannot be ignored. One study centered alloys, elastomers, polymers, and ceramics. We began on components commonly found in the aerospace indus- the study by investigating the removal of contaminates try, but many of those materials are used in a wide variety that are mixtures of compounds from selected matrices. of manufacturing-processes. The study was designed to During the study we examined the removal process of determine the mechanical, electrical, and physical stabil- specific compounds from selected substrates. The goal ity of a large number of components and materials when was to define a set of conditions that could be used as a exposed to supercritical conditions. More than 100 ma- guide for the cleaning of substances from surfaces. terials have been investigated (Table 2), and with few Rather than determine all possible combinations, we exceptions, all were found to be stable to supercritical decided to use specific compounds as a guide to a wide CO,. The exceptions tend to be polymers, and exposure variety of mixtures. The principal components of a Prospects - operational economics

Direct labor Alternative cleaning operating costs Indirect labor Benefits Labor expenses Factory OH Factory costs Depreciation Cycle time costs Maintenance costs New tech changeover cost Facility space costs Consumables Consumables DisposalAiability

Energyhtilities Energy Process 1 Process 2 Process 3 Process 4 Taxeslfees Factory costs Energyhtilities Administration G&A 0Quality Finance charges Consumables 0Finances Cost of capital Finance

Los Alamos CLS-92-2585

Fig. 4. Economic comparison of alternativecleaning processes. Process 1 is an aqueous process, Processes 2 and 3 are non-aqueous cleaning processes, and Process 4 is supercriticalcarbon dioxide cleaning. All processes were used to clean identical parts to the same level of cleanliness. 86 Supe+tical Carbon Dioxide Precision Cleaning W. DALESPALL. n. ol.

surface. High-fired ceramics () have the same TABLE 2 cleaning properties as very smooth surfaces. Materials Tested for Compatibility The surface extraction of substances is governed by a with Supercritical Carbon Dioxide variety of physical and chemical factors. The density of Materials tested at 300 atm, 45"C, for the supercritical fluid relates most closely to the chemical 30 minutes factors, but parameters such as mass flux rates from the surface are governed to a large degree by the rate of fluid Clad integrated circuits, diodes, flow at the boundary layer, and the boundary layer is chokes, capacitors, resistors controlled by the rate of bulk flow past the surface to be cleaned. A recent study performed at Battelle Pacific Printed circuit boards including Northwest Laboratories has vividly demonstrated this multilayer versions, most effect. Cleaning rates can be significantly enhanced by common plastics, iron, increasing the mass flow rate of the fluid past the surface aluminum, magnesium, copper, to be cleaned. Other mechanical aspects of the cleaning stainless, cast aluminum and process such as the ratio of dead volume in the cleaning magnesium, silver, platinum, cell to part surface area need to be evaluated. Some nickel, chromium observers suggest that the removal rate for contaminants Common rubbers, composite from the surface is very rapid for smooth surfaces, and that the majority of the extraction time is spent clearing the extraction chamber of the extracted material. These Silicon adhesives, epoxies, composite topics are food for future research. sealants, most coax cables Research being performed in the Department of En- ergy program includes determiningthe best design for the separator portion of the equipment. In the case of preci- mixture can be easily determined by using modem ana- sion cleaning, it is probably necessary to use CO, that is lytical methods such as FTIR and GCMS or by consulting very clean; for the recycle of CO,, it must remain clean the material safety data sheet (MSDS) supplied with the during use. Current studies indicate that the concentra- product. Although some components in the mixture may tion of low vapor pressure compounds must be kept be proprietary, the major constituents are listed, along below 1 part per million to achieve cleanliness levels at with health, safety, and handling information. or below 1 microgram per square centimeter. Design of When coupled with the individual compound data a separator system that can accomplish this while using identified in the recent study, the MSDS information minimal energy is not a simple task. serves as a guide for selecting the initial set of conditions for removing compounds from surfaces. Using a selec- CONCLUSIONS tion of extraction conditions, we determined the removal Supercritical carbon dioxide is an excellent solvent for process for more than 150 compounds from a variety of precision cleaning, particularly for porous, intricate parts surfaces. This data will be available shortly and will serve or parts that are relatively accessible by conventional as a guide to customizing CO, extraction conditions for solvents. The economics of the entire cleaning process a variety of cleaning problems. may dictate the use of CO, in cleaning uses other than In general, the results indicate that, as expected, high- precision cleaning. Work in progress will clarify the polarity compounds extract more slowly from surfaces areas for the application, but current indications are that than do low-polarity compounds when pure CO, is used the applicability will be large. as the extractant. Modifying the polarity of the CO, forces a compromise on the amount and rate of extraction of ACKNOWLEDGMENTS mixtures having a range of polarities in the constituent My special thanks to all members of.the Joint Associa- compounds. As the solvent polarity increases, more- tion for the Advancement of Supercritical Fluids, and polar compounds become more readily extractable and especially to Joel Williams, Laura Silva, Carol Adkins, ' less-polar compounds become less readily extracted. The Tom Stanford, and Ron Stephenson for their stimulating extraction process becomes one of compromise in re- conversations and insight into the problems associated moval efficiency, with the polarity of the CO, the deter- with cleaning. mining factor. As expected again, porous surfaces clean This work was performed under funding from more slowly than smooth surfaces. The two most difficult the Industrial Waste Reduction Program Office, surfaces to clean appear to be cast metals, which tend to Department of Energy, Division of Conservation be porous, and ceramics, particularly those with rough and Renewable Energy.