COSOLVENT CLEANING: A NON-VOLATILE SOLVENT WASH-FLUOROCHEMICAL RINSE. Kevin R. Hrebenar and Christine S. Fouts

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1 COSOLVENT CLEANING: A NON-VOLATILE SOLVENT WASH-FLUOROCHEMICAL RINSE by Kevin R. Hrebenar and Christine S. Fouts ABSTRACT Solvent wash/fluorochemical rinse processes offer a high-performance vapor degreasing method of cleaning which is economical, easy, fast and efficient. They use no ozone-depleting solvents, toxic chemicals or water. Among the solvent wash/fluorochemical rinse processes, the Advanced Vapor Degreasing (AVD) process avoids the use of flammables, as well. It works in virtually all vapor degreasing applications in the electronics, electrical, precision and metalworking industries, providing highly effective degreasing of complex, delicate and difficult-to-clean parts. It is ideal for manufacturers who need to produce clean, dry parts without using water. The cleaning performance of the process is as good as, or better than, conventional degreasing systems that use conventional vapor degreasing solvents. INTRODUCTION Among the many thousands of sophisticated cleaning operations in the world which have used ozone-depleting solvents, a major fraction (perhaps even a majority) utilize vapor degreasing as the preferred process. Vapor degreasing gives clean, dry parts in short cycle times, and it does not subject the parts to contact with water. In fact, many or even most precision-engineered parts have been designed with vapor degreasing viewed as the preferred (and in some cases, the only) viable cleaning alternative. However, there are no chemical substances currently available which have properties making them suitable for use as the working fluid in vapor degreasers and which simultaneously offer zero ozone depletion potential, good solvency, low toxicity and nonflammability. While there are a few chlorinated solvents which are not ozone-depleting, provide good solvency and are nonflammable, all of them pose significant toxicity concerns. Their use as CFC alternatives simply trades an environmental problem for an employee exposure problem. Consequently, it seemed that the vapor degreasing process was doomed to disappear with ozone-depleting solvents. One can, however, combine a solvent wash with a fluorochemical rinse to offer a Precision Cleaning 95 Proceedings 41

2 high-performance vapor degreasing method of cleaning which is economical, easy, fast and efficient. This combination can be designed to use no ozone-depleting solvents, toxic chemicals or water. Among the solvent wash/fluorochemical rinse processes which are currently commercially available, the Advanced Vapor Degreasing (AVD TM *) process avoids the use of flammables, as well. PROCESS DESCRIPTION In this process, a combination of two appropriately chosen types of materials, each having particular characteristics, allows for replacing ozone-depleting solvents used in vapor degreasing cleaning applications with a process which is functionally indistinguishable from conventional vapor degreasing. These two types of substances, named according to function, are a solvating agent and a rinsing agent. A solvating agent for this process typically has low vapor pressure ( < 0.1 mm Hg at 25 C) and high boiling point ( > 200 C), along with good solvency properties. A rinsing agent for this process typically has high vapor pressure (> 200 mm Hg at 25 C) and low boiling point (< 100 C), but has no need for good solvency. The solvating agent and the rinsing agent may be either soluble or substantially insoluble in each other. Azeotropic behavior between the solvating agent and the rinsing agent is not necessary. Systems in which the solvating agent and rinsing agent are not miscible with each other are especially advantageous. A number of solvating agents and rinsing agents have been successfully used in this process. Because of their many desirable properties, such as nonflammability, low toxicity and zero ozone depletion potential, perfluorocarbons (PFCs) are the most commonly chosen rinsing agents, When a perfluorocarbon is the rinsing agent, the choice of solvating agent is especially important. PFCs are extremely weak solvents, and there are only a limited number of materials that they can adequately remove from the surfaces to be cleaned. For example, most semi-aqueous cleaning products are not suitable for use as AVD solvating agents because they are not well rinsed by perfluorocarbons. Fortunately, however, there exist materials which are both excellent solvents for the most common soils and which can be effectively rinsed from the parts by perfluorocarbons. The best choice of solvating agent for a given application depends on both the kinds of parts to be cleaned and the soils to be removed. PROCESS EQUIPMENT The process is designed to be operated in equipment similar to that used for conventional vapor degreasing, with only a few modifications. Schematics for standard vapor degreasing equipment (Figure 1) and AVD equipment (Figure 2) are given for reference. The essential difference between conventional vapor degreasing equipment and AVD process equipment is that for some applications, especially with perfluorocarbon rinsing agents, agitation (either 42 Precision Cleaning 95 Proceedings

3 mechanical or ultrasonic or both) should be provided in both the boil and rinse sumps. Further, some simple steps should be taken to minimize vapor emissions. Vapor containment modifications include extending the freeboard to a total of at least about 150%) adding a -20 F (-29 C) chiller coil above the standard condenser coils, and making provision for placing a layer of the solvating agent over the fluorocarbon rinsing agent when the machine is shut down. FIGURE 1 CONVENTIONAL VAPOR DEGREASING EQUIPMENT Extended FIGURE 2 AVD PROCESS EQUIPMENT I Vapor Vapor Condensate Heating Elements Filtration Pump and Ultrasonics Recirculating Pump, Spray Nozzles, and Ultrasonics Heating Elements Recirculating Pump, Spray Nozzles, and Ultrasonics PROCESS OPERATION Referring to Figure 2, the process operation can be summarized as follows. Dirty parts are immersed into the boil chamber, which contains an agitated mixture of solvating agent and rinsing agent. During normal process operation, the boil sump in a two-sump machine contains about 70% solvating agent and 30% rinsing agent by volume (approximately 50/50 by weight). Soils are dissolved by the solvating agent. After being cleaned in the boil sump, the parts are immersed in the rinse sump, which contains nearly 100% rinsing agent. Because the rinsing agent is a very weak solvent, essentially no cleaning takes place in the rinse sump. Instead, the primary function of the rinse sump is to remove the solvating agent and the soil which is dissolved in it. Solvating agent and dissolved soil float to the top of the rinse sump and are swept over the weir back into the boil sump. This helps maintain very clean fluid in the rinse sump. Eventually, when the soil loading in the solvating agent becomes too great, the rinsing agent will no longer completely remove the solvating agent and dissolved soil within a reasonable time period. The solvating agent must then be recycled or replaced. Precision Cleaning 95 Proceedings 43

4 Following immersion in the rinse liquid, the parts are moved into the vapor phase for a final rinse with rinsing agent which condenses from the vapor. There is essentially no solvating agent in the vapor, a fact which contributes considerably to good cleaning results in high-performance applications. Finally, the parts are raised into the freeboard zone, where any remaining rinsing agent evaporates from the parts and returns to the sump (as condensate). The cleaning cycle is now complete, and the parts are clean and dry. The cleaning results achieved with the AVD process in production use at customer factories have been excellent, generally equaling or exceeding those obtained with the solvent being replaced, usually CFC-113 or 1, 1, 1-trichloroethane. Typically, printed circuit boards can be defluxed to give ionic contamination results similar to those obtained with CFC-113/methanol cleaning. In fact, the new process has received a Better than than the benchmark rating in Phase 2 testing. Metal and other precision parts can be cleaned to meet or exceed the same standards met by conventional vapor degreasing processes. Tables 1, 2, 3 and 4 offer examples of the types of parts which are typically cleaned using this process. Note the sophisticated nature of these parts, and thus the rigorous demands placed on the process used for cleaning them. The process can meet the requirements of a wide variety of precision parts manufacturersthere are few cleaning problems not amenable to solution via this new technology. 44 Precision Cleaning 95 Proceedings

5 TABLE 1 Types of Parts Cleaned: Characteristics of Parts: Soils: Hybrid circuits, ceramic substrates, fluoropolymer PC boards, heat shrink sleeving, and polyethylene sleeving Parts require very low residue levels, are easily damaged and have complex geometric configurations and capillary spaces Alpha RMA 386EH solder paste, Indium RMAX26 flux vehicle, and Kester cored solder wire with 282 flux (62/36/2) Solvating Agent: 24 Cleaning Cycle Times: 10- to 15-minute wash; 10- to 15-minute rinse; 2-minute vapor TABLE 2 Types of Parts Cleaned: Characteristics of Parts: Soils: Solvating Agent: Cleaning Cycle Times: Braided stainless steel shielding, IFF cable assemblies, solder fixtures, and precision connectors with cavities and capillaries Parts with complex geometric configurations and capillary spaces Light oils, fingerprints, particulate and RMA flux residues 24 l2-minute wash; 12-minute rinse; 2-minute vapor TABLE 3 Types of Parts Cleaned: Characteristics of Parts: Soils: Solvating Agent: Cleaning Cycle Times: Temperature- and water-sensitive aircraft guidance systems Corrosion-sensitive and temperature-sensitive materials requiring very low residue levels Flux residues 19 Varied TABLE 4 Types of Parts Cleaned: Characteristics of Parts: Soils: Solvating Agent: Cleaning Cycle Times: Electrical connectors made of or plated with silver, brass, nickel and copper Parts requiring very low residue levels Light cutting and stamping oils 19 Parts are tumbled: 6-min wash, 6-min rinse, 2-min vapor Precision Cleaning 95 Proceedings 45

6 RETROFITTING EXISTING EQUIPMENT Some existing vapor degreasing equipment can be retrofitted to operate the new process. Retrofitted equipment is distinguished from conventional vapor degreasing equipment and from newly-built AVD equipment in several ways. Figure 3 presents a schematic of a typical vapor degreaser retrofitted to standards that enable it to satisfactorily operate the AVD process. Compare Figures 2 and 3 to see the principal differences between newly-built equipment and conventional equipment retrofitted to run the process. Suitably retrofitted equipment can produce clean, dry parts with idling mode rinsing agent emissions which are very economical. The modifications which are usually required to convert conventional vapor degreasing equipment to AVD equipment include providing, as a minimum, the following: 150 % freeboard a rinse sump sparger filtration system in the boil sump ultrasonics in the boil sump chiller coils above the primary condensing coils ultrasonics in the rinse sump FIGURE 3 RETROFITTED EQUIPMENT Vapor Filtration Pump Heating Filtration Pump and Ultrasonics Elements and Ultrasonics Features in italics are modifications to conventional vapor degreasing equipment. 46 Precision Cleaning 95 Proceedings

7 to late In the meantime, the use of presently-available PFCs will allow the fastest possible transition away from ozone-depleting solvents. SUMMARY A new, non-ozone-depleting cleaning process which is functionally identical to the vapor degreasing process has been identified and commercialized. This process uses commercially available ingredients which are nonflammable and which are not toxic in use. The equipment needed is very similar to that commonly used for vapor degreasing with chlorinated solvents, including CFC- 113 and methyl chloroform. Some of the existing vapor degreasing equipment base can be simply and economically retrofitted to use this new technology. The new process offers all of the advantages often associated with vapor degreasing, such as the ability to clean a wide variety of soils quickly and reliably, and parts that emerge completely dry. Just as importantly, it avoids the disadvantages of aqueous processes, such as difficult drying and water disposal. The cleaning performance obtained for a broad range of tasks, including defluxing and cleaning precision parts, is at least as good as is available with conventional vapor degreasing. Process economics are favorable, especially as applied to high-value, delicate, or difficult-to-clean parts such as are often found in the precision cleaning, optical, electrical and electronics industries. 48 Precision Cleaning 95 Proceedings

8 PROCESS ENVIRONMENTAL IMPACT In general, rinsing agent choice is not significantly constrained by choice of solvating agent, since rinsing agent solvency (for either soil or solvating agent) is not required. At present, the most suitable commercially available rinsing agents are known as perfluorocarbons. These materials have all of the advantages of CFCs (very low toxicity, nonflammability, convenient boiling points, excellent materials compatibility, etc.), but without the disadvantage of being ozone depleters. In fact, their ozone depletion potential is zero. Their only disadvantages are that they are relatively expensive and, like CFCs and 1, 1, 1-trichloroethane, they have the potential to contribute to global warming. However, emissions from AVD equipment have proven to be so low that even these expensive materials can be used economically. Further, low emissions help to minimize any possible concerns with respect to global warming potential. The United States EPA, in its most recent presentations relating to alternatives for ozone-depleting solvents, has taken the position that those manufacturers who need to use PFCs for solvent cleaning applications may do so. It is important to note that, unlike the hypothesis of stratospheric ozone depletion by chlorine- and bromine-containing compounds, which has been established beyond any reasonable doubt, global warming is not a scientifically demonstrated phenomenon. Moreover, the low level of rinsing agent emissions from the process mean that replacing a typical ozone-depleting vapor degreasing process with the new process (using a perfluorocarbon rinsing agent) significantly reduces the global warming impact relative to that from cleaning the same parts using ozone-depleting solvents. Careful analyses of the environmental impact of replacing CFC-113 and methyl chloroform vapor degreasing in an average factory with the new process (using PFCs as rinsing agents) show that not only is the emission of ozone-depleting substances completely eliminated, but the potential global warming impact is also substantially reduced, as a result of the much lower volumes of chemicals emitted from the process as compared to conventional vapor degreasing. Nonetheless, it is prudent to minimize any potential environmental impact of industrial emissions whenever possible. PFCs can serve as important interim materials, allowing the immediate replacement of ozone-depleting solvents in the most demanding applications. But PFCs are not perfect. While the unique properties of PFCs will continue to be required for some applications, PFCs may be replaced in other, less demanding applications. Intensive work on this next generation of AVD rinsing agents is already underway. The leading candidates for replacing PFCs as rinsing agents are hydrofluorocarbons (HFCs). HFCs have most of the advantages of the perfluorocarbons, such as nonflammability and zero ozone depletion potential, but they have short atmospheric lifetimes, and thus essentially eliminate any global warming concerns. HFCs which appear suitable for use as drop-in replacements for perfluorocarbons in AVD equipment have already been identified, and successful small-scale testing has been conducted. As this is being written, HFC rinsing agents are not yet commercially available, but the first solvent HFCs are expected to be ready by mid- Precision Cleaning 95 Proceedings 47