METHOD OF CLEANING ARTICLES USING SUPER-CRITICAL GASES

Abstract: In the fabrication of a variety of articles, organic contaminants become affixed to the surfaces of these articles and must subsequently be removed. The disclosed method is effective for the removal of organic contaminants from a variety of articles, and is efficient to enable the rapid removal of such contaminants without damage to the article. The method has special utility in the cleaning of components used in aerospace applications. In the disclosed method, a structural component bearing the contaminant is contacted in a pressure vessel with a gas under super-critical conditions of temperature and pressure whereby the contaminant is absorbed by the gas. The gas, having the contaminant absorbed therein, is then purged from the vessel to obtain the cleaned component.

Description: METHOD OF CLEANING ARTICLES USING SUPER-CRITICAL GASES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates in general to a method of removing contaminants from articles and, in particular, to a simple, rapid and effective method of removing from the surface and interstices of a solid article a variety of contaminants with which the article may possibly have come in contact during its manufacture. More specifically, the present invention relates to a method of removing organic contaminants from such articles using gases in the super-critical state. 2. Description of the Prior Art Components and materials used in the manufacture of instruments for aerospace applications must be free from contaminants. The presence of trace amounts of contaminants in components of precision instruments used in space vehicles which ordinarily do not interfere with the operation of these devices on earth, manifest themselves under the conditions of outer space and interfere with the accurate, normal operation of these sensitive devices. Thus, it is critical that the components be free of any and all contaminants, particularly organic contaminants. The contamination of the component may consist of saponifiable- materials such as oils as well as non-saponifiable materials such as resins. Components formed from metal or synthetic plastic materials may contain gaseous or vaporizable contaminant residues from the manufacture and processing of the metal such as uncured prepolymers, release agents and unreacted monomers used in the processing of these materials. To effect the required level of cleaning of the materials used in the manufacture of components which meet government standards for cleanliness, the art has developed cleaning processes for these materials utilizing high vacuum, e.g., lO""**-* torr (millimeters of mercury or mmHg) and elevated temperatures up to 250°C to remove absorbed and adsorbed organic contaminants from the materials. This cleaning technique, referred to in the art as "thermal vacuum cleaning", is not completely satisfactory in that the cleaning process must be carried out in an expensive and complex high vacuum system which normally requires about fifteen hours to obtain the desired contaminant free surface. An alternative to thermal vacuum cleaning, used by the prior art to effect cleaning of metal components, is solvent extraction. The solvent extraction cleaning process, in addition to requiring protracted treatment times, has the drawback that when cleaning porous materials, trace amount of the solvent used for cleaning, e.g., chlorinated hydrocarbons, may be adsorbed on the part being cleaned thereby, contri- buting to the contamination problem. SUMMARY OF THE INVENTION In accordance with the present invention, the rapid removal of organic-based contaminants from articles, both porous and non-porous, without damage or contamination to the article is effected by contacting the article bearing the contaminant in a pressure vessel with a gas under super-critical conditions of temperature and pressure, whereby the contaminant on the surface and/or in the interstices of the article is absorbed by the gas and, thereafter, purging the gas from the pressure vessel to obtain the article having the contaminant removed therefrom. By surfaces is meant not only exterior surfaces but also interior surfaces which communicate therewith. By the practice of the present invention, organic contaminants are removed from articles in one hour or less to achieve a substantially contaminant-free article as compared to thermal vacuum cleaning processes which require fifteen hours or more to achieve an equivalent level of cleanliness and several days of treatment by solvent extraction. As will hereinafter be further demonstrated, by following the practice of the present invention, twice as much volatile contaminant was removed to temperatures near ambient from difficult-to-clean silicone rubber parts in one-fiftieth the time when compared to cleaning equivalent rubber parts using thermal vacuum cleaning. DETAILED DESCRIPTION OF THE INVENTION I is known that when the temperature of a gas is above a certain temperature, known as the critical temperature, it is not possible to liquefy the gas by application of pressure alone. It is necessary to reduce the temperature below the critical temperature in order to be able to liquefy the gas. At the critical temperature, as the gas is subjected to increasingly higher pressure, e.g., on the order of several thousand pounds per square inch (psi) (one psi equals 51.71493 mm of mercury), the density of the gas approaches that of a liquid and the gas acts as a solvent for a variety of different types of organic and organo-metallic materials, including aliphatic and aromatic hydrocarbon organo- metallics such as metal alkyIs and alcoholates, silicones and boroalkyls and organic esters or inorganic acids such as sulfuric and phosphoric acid. The critical temperatures and pressures for a variety of gases at which they exist in the super-critical condition may be found in U.S. Patent No. 4,124,528, the teachings of which are hereby incorporated by reference. In the practice of the present invention, the article of manufacture to be cleaned is placed in a suitable vessel such as a pressure chamber or autoclave and the gas which is to effect the cleaning of the article surface is admitted to the vessel in a super¬ critical condition. Cleaning of the article is accomplished in the pressurized vessel under conditions which maintain the super-critical condition of the gas used for cleaning. Normally, the cleaning is conducted at a temperature range of about 35°C to about 100°C at about 1200 psi (62,058 mmHg) to about 10,000 psi (517,149 mmHg) pressure and preferably about 40°C to about 50°C and about 3,000 psi (155,145 mmHg) to about 8,000 psi (413,719 mmHg) pressure. Inert gases having a critical temperature below about 200°C are considered most advantageous in the practice of the present invention. Examples of such gases are alkanes and especially lower alkanes such as ethane, propane and butane, alkenes and especially lower alkenes such as ethylene, propylene and butylene, dialkyl ethers such as dimethyl ether, SO2, CO2, halogenated alkanes such as CHF3, CCIF3, CFCI3, CF2=CH2r CF 3 , CF 3 -CF 2 -CF 3 , CF 4 , CH3-CF3, CHC1 2 F. CC1 2 F 2 , 2 0, noble gases such as argon, NH3 and N . Gases such as CO2 are preferred in the practice of the present invention as the super-critical temperature of such gases is near ambient temperature; the gases are inexpensive, non-toxic, and relatively inert to most solid substrates. C0 is especially preferred as this gas in the super-critical state has a very low viscosity, namely 0.05 centipoise, which is one-twentieth that of water. As a result, the gas in the super-critical state can penetrate very readily into the contaminant to effect its rapid removal from the article being cleaned. To promote the rapid cleaning of the article with the super-critical gas, it is further advantageous to the practice of the present invention that the article to be cleaned be preheated prior to its place¬ ment in the pressure vessel to a temperature above ambient, e.g., about 30°C to about 100°C, and preferably about 40° to 50°C. The absorptive capacity of the gases in the super¬ critical condition with respect to most contaminants, and particularly contaminants of basically organic origin, is raised with increased pressure. Thus, when practicing the cleaning procedure in accordance with the practice of the present invention, a pressure which is substantially higher than the critical pressure of the gas and a temperature only slightly above the critical temperature is selected for maintaining the gas in the super-critical condition. OMPI— It is still further advantageous to the practice of the present invention that the temperature and pressure conditions under which the gas is caused to contact the article to be cleaned be sufficiently above the critical temperature and pressure in order to have a single physical phase, i.e., the gaseous phase, of the gas present in the pressurized vessel during the cleaning operation. Thus, for CO2 which has a critical temperature of 32°C and a critical pressure of 1073 psi (55,490 mmHg), when such gas is used as the cleaning medium, the gas is maintained at a temperature of about 35°C to about 100°C and a pressure of 2,000 psi (103,430 mmHg) to 10,000 psi (517,149 mmHg) in the pressure vessel. In effecting cleaning of the surfaces of articles in accordance with the practice of the present invention, the article, when placed in the pressure vessel for cleaning, is contacted with the gas under super-critical conditions for a period of time ranging from about 0.25 hour to about four hours and preferably about 0.5 hour to about one hour to effect complete removal of contaminants. After sufficient time has elapsed in the pressure vessel for the contaminants to be absorbed by the gas and removed from the articles, the pressure in the vessel is released and the gas containing the absorbed contaminants are vented or purged from the vessel into the atmosphere. When ambient pressure is attained, the cleaned article is then removed from the vessel. If it is intended that the gas be recycled for reuse in removing contaminants, the gas in the super¬ critical condition is vented or purged from the pressurized vessel into a suitable collection vessel where the pressure is reduced or the temperature lowered at constant pressure, which conditions render the gas a non-solvent for the contaminant which then precipitates from the gas. The gas, freed of contaminants, can then be recompressed and recycled for use in the cleaning of contaminated articles. The following are examples showing the cleaning of various articles of manufacture using gases in the super-critical condition according to the method of the present invention. Example I A high pressure autoclave (10,000 psi or 517,149 mmHg maximum working pressure) of 300 milliliter (ml) capacity was equipped with a gas inlet, a gas outlet, pressure gauge, a thermocouple well, and heating means. Connected to the gas inlet was a CO2 supply bottle which delivered the C0 at 800 psi (41,372 mmHg) gauge. A gas booster pump operating on the 100 psi (5171 mmHg) shop air and having the capability to raise the bottle pressure to a maximum to 10,000 psi (517,149 mmHg) was connected to the C0 bottle. The autoclave was purged with CO2 and heated to 100°F (37.8°C). An O-ring formed of silicone rubber, weighing 0.460 grams, was placed in the autoclave. C0 gas was fed to the booster pump and the autoclave was pressurized to 8,000 psi (413,719 mmHg). OMPI After contact of the O-ring with the CO2 at 100°F (37.8°C) and 8,000 psi (413,719 mmHg) pressure for one hour, the pressure in the autoclave was released and the CO2 containing the absorbed contaminants was vented to the atmosphere. When ambient pressure was attained in the autoclave, the cleaned O-ring was removed from the autoclave and weighed to determine the extractable weight loss effected by the cleaning operation. The weight loss was determined to be 0.011 grams which represented the removal of 2.4 weight percent (wt. %) contaminants. By way of contrast, an identical silicone rubber O-ring weighing 0.494 grams, was cleaned using an all glass thermal vacuum cleaning unit wherein the O-ring was heated for 120 hours and 180°C under a vacuum of approximately 10~5 torr (mmHg). The extractable weight loss was determined to be 0.0014 grams, representing the removal of 0.4 wt. % contaminants. Subsequent contact of the thermal vacuum cleaned O-ring with isopropanol solvent at ambient laboratory temperature removed another 0.004 grams of contaminant or an additional 0.8 wt. % representing a total contaminant removal of only 1.1 wt. %. To determine the effect of the super-critical CO2 treatment on the physical properties of the silicone rubber material, O-rings which had been treated in accordance with the procedure of Example I were subjected to tensile and hardness tests used for the evaluation of rubber mechanical and physical properties. The results of these tests are recorded in Table I below. TABLE I Tensile and Hardness Properties Silicone Tensile Elongation* Hardness* Rubber Strength* (%) (Shore A) Treatment (psi) (mmHg) Super-critical C0 2 854 132 75 44,165 None 907 158 72 46,905 ♦ Average of six tests The results recorded in Table I show that the treat¬ ment of silicone rubber with CO2 under super-critical conditions produces only a minor change in the mechanical and physical properties of the rubber. Example II The procedure of Example I was repeated to clean a polyimide polymer containing contamination in the form of volatile solvents by exposure to CO2 for one hour under super-critical conditions of 8,000 psi (413,719 mmHg) pressure and a temperature of 45 β C. The amount of volatile contaminants remaining in the polymer after cleaning was determined by the American Society for Testing and Materials (ASTM) TEST E-595, described in the 1981 Annual Handbook of ASTM Standards, OMPI under conditions of 125°C temperature and a vacuum of 10~ 5 torr (mmHg) or thermogravimetry mass spectrometry (TGA-MS) under conditions of one atmosphere and temperatures of 210°C or 820 β C. The results of these tests are recorded in Table II below. For purposes of contrast, identical samples of the polyimide polymer were cleaned using thermal vacuum cleaning conditions wherein the polymer sample was heated for four hours at 80°C under a vacuum of approxi¬ mately 10""5 torr (mmHg). The results of these comparative tests are also recorded in Table II below. TABLE II Volatiles Remaining in Polyimide Polymer Sample Super-critical Thermal Vacuum Analysis No. CO2 Cleaning Cleaning Conditions 1 0.56% 8.31% ASTM E-595 2 0.76% 6.50% ASTM E-595 3 1.00% 6.00% TGA-MS, 210°C 4 37.00% 47.00% TGA-MS, 820°C The reduction in volatiles is important in polymer processing as it suppresses the formation of voids and areas of weakness in the finished product. By reference to Table II, it is immediately apparent that by the practice of the present invention the removal of volatiles from polymeric products can be achieved to a much greater degree in a shorter period of time as compared to the practice of the prior art as represented by thermal vacuum cleaning. Example III The procedure of Example I was repeated with the exception that thin-sectioned parts of less than 0.25 inch thickness of a diverse selection of organic and inorganic materials were cleaned by exposure to C0 2 under super-critical conditions with only minor changes in the mechanical and physical properties of the materials being observed thereafter. The materials exposed to the super-critical CO2 conditions were as follows: A. Laser casting alloy. B» Fluorosilicone sheet. C. Glass reinforced epoxy resin multilayer sheet. D. Fiberized carbon. E. Absorptive fabric containing activated carbon. F. Phenolic laminate cloth. G. Polyimide resin sheet. H. Quartz crystal assembly. I. Cryogenic cooler part. The laser casting alloy was subjected to a vacuum- pressure cycle in silicone oil (Dow-Corning DC-200) to saturate the metal with the silicone oil. The oil- saturated metal part was then cleaned according to Military Interim Specification (MIS) 23542D, a cleaning specification for these parts. According to MIS-23542D, the material to be cleaned is subjected to an exhaustive extraction in a Soxhlet apparatus using toluene as the solvent followed by evaporation of the solvent and an infrared (IR) spectra examination of the residue. In accordance with MIS-23542D, the IR examination must indicate the absence of silicone or other residues to establish removal of all traces of silicone oil contaminant. To achieve this result required four days of treatment with the Soxhlet extraction apparatus, whereas by using the procedure of Example I, removal of all traces of silicon oil contaminants from a similar laser casting alloy similarly saturated with silicone oil was achieved in two hours. While specific components of the present system are defined above, many other variables may be introduced which may affect, enhance or otherwise improve the present invention. These are intended to be covered herein. Further, while variations are given in the present application, many modifications and ramifications will occur to those skilled in the art upon reading the present disclosure. These, too, are intended to be included herein. O PI

Claims: CLAIMS What is Claimed is; 1. A method of removing from a solid article contaminants adhering to the surfaces of the article, which comprises: placing the article having contaminants affixed to the surfaces thereof in a pressure vessel; introducing a gas into the vessel and maintaining the gas at a temperature and pressure at which the gas is placed in a super-critical condition; maintaining the article in contact with the super-critical gas for a time sufficient to effect absorption of the contaminants by the gas; and removing the gas containing the contaminants from the vessel, whereby the contaminants are separated and removed from the article. 2. The method of Claim 1 wherein the gas is carbon dioxide. 3. The method of Claim 2 wherein the carbon dioxide is maintained in the vessel at a temperature in the range of about 35°C to about 100°C and a pressure of about 1,500 psi (77,572 mmHg) to about 10,000 psi (517,149 mmHg). 4. The method of Claim 1 wherein the contaminants are composed of organic-based materials. 5. The method of Claim 1 wherein the article is maintained in contact with the gas in the super-critical state for a period of time ranging from about 0.25 to about four hours. 6. The method of Claim 1 wherein the article comprises a rubber. 7. The method of Claim 1 wherein the article comprises a metal. 8. The method of Claim 1 wherein the article comprises a synthetic organic polymer. 9. The method of Claim 1 wherein the article comprises a polyimide polymer. 10. The method of Claim 1 wherein the article comprises carbon. 11. The method of Claim 1 wherein the article comprises a quartz crystal. 12. The method of Claim 1 wherein the article comprises an assembly of parts. 13. The method of Claim 1 wherein the contaminant is removed from the external surface of the article by the gas. 14. The method of Claim 1 wherein the contaminant is removed from the interstices of the article.

Inventors: MUELLER WILLIAM A, MUELLER, WILLIAM, A.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1983
Main classification: B08B7
IPC codes: B08B7