'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 SOLVENT ANDWASTE REDUCTION W. DALESPALL Los ALAMOSNATIONAL LABORATORY Los ALAMOS,NM 87545 Murph?s Law of Cleaning: You can't get thepart cleanerthan the dishwater, but is itpossible to get both the part and the dishwater dirty. Supercritical carbon dioxide 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- solvents 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 supercritical fluid is. All elements and compoundscan be described in terms of a phase 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 Polymers 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- Ceramics 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;