PERFLUOROCARBONS FOR SPECIALIZED CLEANING APPLICATIONS

Transcription

1 PERFLUOROCARBONS FOR SPECIALIZED CLEANING APPLICATIONS by Karl J. Warren and Mark W. Grenfell Abstract The January 1, 1996 termination of the manufacture of ozone depleting substances and proposed product labeling requirements have heightened the economic urgency for switching to acceptable alternatives. While aqueous and semi-aqueous processes are anticipated to have broad applicability, these have not been demonstrated to be viable alternatives for many demanding cleaning and drying applications. For demanding cleaning and drying applications where a non-aqueous cleaning method is required, perfluorochemicals are a unique choice as non-ozone-depleting, essentially non-toxic, non-flammable, and thermally and hydrolytically stable alternatives. In addition, they are now commercially available. This paper will cover the fundamental aspects, and application of these materials to non-aqueous cleaning and drying processes. Application examples, fluid containment, and environmental impact of the processes will be discussed. Introduction Time is running out in the scramble to find suitable replacements for ozone depleting substances in cleaning and drying processes. The accelerated phase-out of CFCs and the introduction of product labeling requirements have increased that urgency. In addition to performance, there are a number of other factors which should be considered in selecting a replacement material or process. Although advancements in manufacturing technologies have made it possible, in some applications, to modify or minimize the soil, thereby eliminating the need for subsequent cleaning, for many applications cleaning still is necessary. Advances in water-based cleaning technology have made it possible to switch to aqueous or semi-aqueous processes. However, there are a number of applications where contact with water, or acceptable drying after contact with water, make solvent-based cleaning or drying the only viable alternatives. Although there are many reports of drop-in solvent substitutes, replacing CFC-113 or l,l,l-trichloroethane is not as easy as the commercial literature would lead one to believe. Those who find themselves in this situation are faced with a confusing array of choices, and their most desirable alternative may vary, depending on their application, facilities, local regulatory requirements, and corporate philosophy. For demanding cleaning and drying applications where a viable non-aqueous cleaning method is required, perfluorocarbons (PFCs) offer a unique combination of properties. These materials have many of the desirable performance properties of CFCs, but do not contribute to the depletion of the ozone layer. The physical properties of PFCs will be described in the next section. Several cleaning processes will then be presented which take advantage of the unique physical properties of PFCs. Particulate and neat PFC cleaning, AVD TM cleaning and higher solvency cleaning, using PFC/hydrocarbon blends, will be explained. The environmental impact of PFCs will complete the discussion section. Perfluorinated Liquids Perfluorinated liquids are a class of completely fluorinated organic compounds where all of the carbon-bound hydrogen atoms on the hydrocarbon parent molecule have been replaced with fluorine atoms. The strength of the carbon-fluorine bond results in the high degree of thermal and chemical stability, and in the extremely low toxicity of many perfluorinated molecules. Due to their high chemical stability as well as low solvency for polar materials and most hydrocarbons, PFCs have excellent materials compatibility with most plastics and elastomers. The only exceptions to this are fluorinated plastics and elastomers, where some swelling can occur on prolonged exposure. 310 Precision Cleaning 95 Proceedings

2 TABLE 1 - TYPICAL PHYSICAL PROPERTY COMPARISON OF 3M PERFORMANCE FLUIDS * CFC-113 is not a 3M product and is include in this table only for comparative purposes. Particulate and Neat PFC Cleaning By themselves, PFCs are not usually adequate to replace CFCs in removing hydrocarbon soils or polar contaminants. However, there are several cleaning applications where the pure perfluorochemical may be well suited. The extremely low surface tension, more than 30% lower than CFC-113, low viscosity and high fluid density of perfluorochemicals makes them ideally suited to remove particulates from precision components where they have been demonstrated to be statistically better than CFC-113 [l]. An example of this sort of application is in cleaning instrument bearings, where very low particle levels are required to achieve long term reliability. High molecular weight fluorocarbon surfactant may be dissolved in the perfluorochemical liquid to increase the efficacy of particle removal, by as much as an order of magnitude over that obtained using CFC-113 alone [2]. After spraying or sonication in the surfactant solution, the surfactant is rinsed off the parts using only the perfluorocarbon solvent. Due to their good solvency for fluorinated materials, PFCs also can be employed to remove fluorinated grease, lubricants, or damping fluids, such as BTFE (polybromotrifluoroethylene) and CAFE (polychlorotrifluoroethylene), typically found in gyroscopes. A VD TM Cleaning Precision Cleaning 95 Proceedings 311

3 For dissolving organic soils, few materials give equivalent performance to that achievable with an organic solvent. However, if an organic solvent is employed which is volatile enough so that the part can be easily dried, flammability, worker exposure, and high VOC emissions become an issue. If a less volatile (higher molecular weight) organic solvent is used, drying the part following cleaning is lengthy and difficult. The AVD TM process, developed by Petroferm Inc. [3]. combines the beneficial features of organic solvents and perfluorochemicals to accomplish two separate processes, cleaning and drying. First a non-flammable organic solvating agent of relatively low volatility is used to dissolve and remove the soils from the contaminated parts. Then a more volatile inert perfluorochemical is used to rinse the organic solvating agent and dry the part. A number of properties combine to make perfluorochemicals excellent rinsing agents for removing an aqueous or organic solvent. The lower viscosity and surface tension permits greater penetration into narrow gaps and complex geometries. They are virtually immiscible with water and most hydrocarbons, and since many soils are soluble in either hydrocarbon or aqueous phases, the perfluorinated rinsing material is not easily contaminated. Once they have displaced the solvent, this low soil solvency combined with their low heat of vaporization allows them to evaporate quickly, leaving a dry and residue-free part. A diagram of a typical AVD TM unit is shown in Figure 1. The unit appears very similar to a typical vapor degreaser with some notable changes. Recirculation pumps and filters have been added to both the boiling and rinse chambers. To further improve containment, a refrigerated freeboard chiller and extended freeboard have been added. The soils are solvated and cleaned from the parts by immersion in the liquid in the boiling sump. The boiling sump contains approximately 50% organic solvating agent and 50% PFC rinsing agent by weight (approximately 30% PFC by volume). Although these two materials are virtually immiscible, they are mixed by the agitation due to the boiling PFC and the external recirculation pump. The temperature of the boiling sump is controlled by the boiling temperature of the PFC. Since the solvating agent is much less volatile than the PFC and has lower vapor pressure at the operating conditions, there is very little VOC emission from the process. Once the parts have been cleaned in the boil sump, they are transferred and submerged into the rinsing sump where the solvating agent is displaced from the parts and floats to the surface of the PFC rinsing sump. Macroscopic agitation of the rinsing sump is provided by submerged spray headers and microscopic agitation is typically provided by sonication. The material in the rinsing sump is virtually 100% PFC. Since the solvating agent has very little vapor pressure at the operating conditions, the vapor phase above the liquid and below the condensing coils is nearly all PFC. The condensate from the condensing coils is directed through a water separator (to remove any water condensed from the ambient air) to the rinsing sump. The PFC in the rinsing sump cascades back continuously over a weir into the boiling sump, sweeping any floating solvating agent back with it. After immersion in the rinse sump, a final vapor rinse can be accomplished by suspending the parts above the liquid interface and below the condenser coil. The parts are then raised into the freeboard zone while the remaining PFC liquid is vaporized and recovered within the unit. The high vapor density and low heat of vaporization of the PFC are great advantages in this step. Typically the overall AVD TM cleaning process takes 5 to 7 minutes. Higher Solvency PFC/Hydrocarbon Blends The low solvency of PFCs for most non-halogenated materials has traditionally precluded the applicability of PFCs in numerous cleaning functions. However, the formation of PFC/hydrocarbon (HC) azeotropes and PFC/HC mixtures has improved the solvency and subsequently the cleaning ability of PFCs. One azeotropic cleaning material is currently available for evaluation in experimental quantities. The azeotrope L is composed of 90 wt % perfluoro-n-ethyl morpholine (PNEM) and 10 wt % 2,2,4 trimethylpentane. The physical properties of this material can be found in Table 2. This azeotrope exhibits improved hydrocarbon soil loading characteristics, as compared to neat PFCs, and excellent halogenated soil loading characteristics. Because azeotropes maintain identical vapor and liquid compositions at their boiling point, they will act as a single substance, thus facilitating recovery and containment via condensation. Therefore, these azeotropes can be used as vapor phase cleaners for light soils in existing vapor degreasers; however, emissive losses from conventional vapor degreasers should be minimized. A discussion of fluid containment is available in the Environmental Impact section of this paper. 312 Precision Cleaning 95 Proceedings

4 TABLE 2 - AZEOTROPE PHYSICAL PROPERTIES Name Density Boiling Pt. Viscosity (cs) Surface Phase Split Flash Point (g/ml) (ºc) Tension Temp (ºC) (mn/m) L Cloud -1 Layer -4 None *Under acceptable range of viscometer Liquid phase cleaning is also available using azeotropic blends. The relatively high density of L allows it to be used in particulate cleaning applications. This azeotrope is ideally suited for applications where light soils are also present and a particulate-plus cleaning is required. For more aggressive cleaning requirements, where higher solvent power is needed, mixtures of PFCs and hydrocarbons are also available. These are not azeotropic, but they are formulated to take advantage of the inerting ability of the PFCs and therefore, these compositions do not have flash points. Although most hydrocarbons do not exhibit appreciable solubility in PFCs, numerous useful PFC/HC combinations do exist. Some PFC/HC mixtures exhibit complete miscibility and are thus limited only by flash points and flammability. The hydrocarbon solvents can be selected to provide the required cleaning and substrate compatibility, then a PFC inerting solvent can be determined. Table 3 shows a list of PFC/HC mixtures which have no flash point at the indicated concentrations. Some of these mixtures, or their recipes, are being offered on an experimental basis for evaluation purposes. TABLE 3 - NON-FLAMMABLE SOLVENT BLENDS PF-5050: Perfluoropentane PF-5060: Perfluorohexane PF-5052: Perfluoro-N-methyl-morpholine PF-5070: Perfluoroheptane The mixtures in Table 3 above are non-flammable solvent blends. Non-flammable indicates that no flash was observed by the ASTM test method D or D 56 below the boiling point of the solvent or below 100 F. whichever is smaller (this is the DOT, ANSI, and NFPA definition). Because of the inerting action of the PFCs, the azeotrope L and mixtures do not possess flash points and they do not aggressively attack the substrate to be cleaned Although these materials do not have flash points using standard ASTM test procedures, it is possible that they have flammable limits in air. For example, L has no Precision Cleaning 95 Proceedings 313

5 flash point, but it does become flammable in air between concentrations of 2.7 vol % and 11.5 vol %. Several common materials exhibit this type of behavior. 1,1,2 trichloroethylene does not have a flash point and is shipped as a non-flammable, however this material shows flammability limits between 12.5 vol % and 90 vol % in air.[4] Therefore, caution must be exercised when evaluating the aforementioned azeotropes and mixtures. In addition to explosion limits, the composition of the azeotrope liquids and the PFC/HC mixtures cannot be guaranteed during use, due to the differing vapor pressures of the individual constituents. Azeotropes used as saturated vapor phase cleaners will maintain their composition, as shipped. Environmental Impact A comprehensive assessment of the environmental impact of any CFC alternative should be taken into account, weighing its relative benefits and liabilities. The magnitude, impact, and disposition of all waste streams should be evaluated. Differences in safety and toxicity need to be considered. Energy requirements may vary substantially resulting in environmental as well as economic impact. If handled responsibly, PFCs are an excellent choice to replace ozone-depleting compounds. Perfluorinated liquids are colorless, odorless, essentially non-toxic and non-flammable. In addition, since they are not precursors to photochemical smog, PFCs are exempt from the US EPA s volatile organic compounds (VOC) regulations. Most importantly, these materials do not contain the carbon-bound chlorine or bromine which can result in ozone depletion. Minimizing emissions of PFCs is desirable. PFC s have long atmospheric lifetimes and high global warming potential, but should make no measurable impact on global warming due to the small volumes expected to be emitted in ozone-depleting substance replacement and other applications. Nevertheless, it is certainly prudent that equipment using PFCs be designed properly to contain the material for both economic as well as environmental reasons. A number of factors contribute to very low emission rates of PFCs. First, lower loss rates can be obtained because of several beneficial physical properties of PFCs compared to CFCs and other halogenated solvents. The PFCs have lower heats of vaporization, higher vapor densities, and lower diffusivities than CFCs. These properties interact to facilitate easier containment. Second, improvements in containment technology have led to significantly lower emissions rates with PFCs compared to CFCs, typically 5 to 10 times less in actual industrial practice. Data from commercial applications employing AVD equipment indicates emissive losses of 0.05 lb/(hr x ft 2 ) have been achieved using current technology. This should result in a net decrease in the relative global warming impact with PFCs, in terms of both the rate of rise and maximum, compared to the CFCs they are replacing. Finally, improvements in containment and recovery technology should make it possible to further reduce emissions. PFCs are readily adsorbed on carbon and the non-polarity of these molecules permits virtually quantitative thermal desorption and regeneration. Improvements in membrane technology may also be of use in recovering these compounds. US EPA Significant New Alternatives Policy (SNAP) Program: The US EPA s SNAP program regulates the use of CFC alternatives. Despite a concern about the atmospheric lifetime and global warming potential of PFCs, under SNAP the US. EPA has recognized that PFCs can play a useful role in many specialized applications where PFC s safety and/or performance requirements exceed those of other alternatives. References 1. Agopovich, J.W., Evaluation of ODC Alternatives in the Cleaning of Gyroscope Hardware, Presented at Rolling Element Bearing Workshop, Sponsored by Draper Laboratory and DoD/Instrument Bearing Working Group, Boston, MA, March l&2, Kaiser, Robert, Enhanced Particle Removal From Inertial Guidance Instrument Parts by Fluorocarbon Surfactant Solutions, Presented at Symposium on Particles on Surfaces, 23rd Annual Meeting of the Fine Particle Society, Las Vegas, NV, July 15, Hayes, Michael E., The AVD TM Process: Vapor Degreasing without Chlorinated Solvents, Presented at The 1992 International CFC and Halon Alternatives Conference, Sponsored by The Alliance for Responsible CFC 314 Precision Cleaning 95 Proceedings