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Many approaches were taken to replace CFC-113 in solvent cleaning applications while this material was being phased out under the provisions of the Montreal Protocol, and a progressively increasing permit pound tax was being imposed. They included use of HCFC-141b, aqueous cleaning, semi-aqueous cleaning, no clean fluxes and flammable solvents. HCFC-141b was an interim solution since it already has been phased out for most solvent applications. To date, the replacement percentage of CFC-113 by HFC solvents is probably no more than 2%.

Competing solvents to the HFCs, and not in kind technologies include: HCFC-141b, (CH3CCl2F), HCFC-123 (CF3CHCl2), HFEs, volatile methyl siloxanes, n-propylbromide, flammable hydrocarbons, alcohols and ketones, aqueous cleaning, semi aqueous cleaning, no clean fluxes, and inert gas soldering. Diagrams of a vapor phase degreaser, an aqueous cleaning machine, and a semi-aqueous cleaning machine are shown in Figures 10-1 through 10-3, respectively.

Figure 10-1: Batch Solvent Cleaning System (Vapor Degreaser)

Figure 10-2: Batch Type Semi-Aqueous Cleaning System

Figure 10-3: In-Line Aqueous Cleaning System

HFC-43-10 is a mild solvent that is generally very compatible and stable in the presence of many metals, plastics and elastomers, and has also been blended with alcohols and other substances to increase solvency and reduce material cost. This material has a moderate boiling point, does not have a flash point and possesses a moderate level of toxicity where the recommended exposure level is 200 ppm . Since the cost for this solvent is high, it is generally used for defluxing and/or degreasing high value parts and printed wiring assemblies (PWAs, i.e., printed circuit boards). Vapor degreasing machines using this solvent would need secondary cooling along with an extended freeboard in order to minimize solvent losses. Open top vapor degreasers would not be suitable and would require retrofitting.

HFC-365mfc is a mild solvent that is generally very compatible and stable in the presence of many metals, plastics and elastomers, and has been blended with other substances to increase solvencyand produce nonflammable mixtures. This material has a moderate boiling point and low flash point. However, due to the distinctive properties of HFC-365mfc a relatively wide range of nonflammable mixtures can be created. This solvent possesses a low level of toxicity. Since the cost for this solvent is lower than for other highly fluorinated solvents, it is generally used for blends with these other materials for defluxing and/or degreasing parts and PWAs. Vapor degreasing machines using this solvent would need secondary cooling along with an extended freeboard in order to minimize solvent losses due to the lower boiling point/high volatility of these blends. Open top vapor degreasers would not be suitable and would require retrofitting.

HFC-245fa (1,1,1,3,3-pentafluoropropane) is a new HFC solvent which was developed to replace HCFC-141b and other halogenated solvents in specialty cleaning applications. HFC-245fa is nonflammable, and exhibits excellent stability and compatibility properties. It is a mild solvent that may be blended with various co-solvents to improve solvency. In this regard, numerous useful azeotropes have been identified. HFC-245fa has good toxicological properties, with a recommended exposure limit of 300 PPM. The cost of this solvent is low compared to other nonflammable fluorinated alternatives. HFC-245fa has a much lower boiling point than most traditional solvents. Boiling at 15 degrees C (59.5 degrees F), it is not well suited for use in open-top vapor degreasers unless they are designed for very low emissions. However, HFC-245fa is well suited for specialty cleaning applications such as flushing, spraying, etc.

Methyl perfluorobutyl ether is a mild solvent that is generally very compatible and stable in the presence of many metals, plastics and elastomers, and has also been blended with alcohols and other substances to increase solvency and reduce material cost. This material has a medium boiling point, does not have a flash point, and possesses a low level of toxicity where the recommended exposure level is 750-ppm. Since the cost for this solvent is high, it is generally used for defluxing and/or degreasing high value parts and PWAs. Vapor degreasing machines using this solvent would need secondary cooling along with an extended freeboard in order to minimize solvent losses. Open top vapor degreasers would not be suitable and would require retrofitting.

N-propylbromide is an aggressive solvent with a moderate boiling point that generally requires a stabilizer package and does not have a flash point. This solvent possesses a moderate- high level of toxicity. The EPA has tentatively suggested an exposure level of 50-100 ppm pending a review of the toxicity data. However, recently published study results addressing the male reproductive, neurotocicity and liver toxicity suggest that the exposure guideline for n-propylbromide will need to be much lower to be adequately protective of workers. The necessity to control exposures to far lower levels than originally thought may limit the utility of n-propylbromide in many traditional solvent applications. Studies to address the carcinogenicity of n-propylbromide will begin in 2002. [Atofina, 2001]. In addition n-propylbromide is a volatile organic compound (VOC) and is being considered by the Montreal Protocol process.

N-PropylBromide (Decisions XIII/7):

Parties are requested to inform industry and users about concerns surrounding nPB use and emissions and their potential threat to the ozone layer, and to urge them to consider limiting its use to applications where more economically feasible and environmentally friendly alternatives are not available. As well as to minimize exposure and emissions during use and disposal. The TEAP is requested to report annually on nPB use and emissions.

The cost of this solvent is moderate, and may function as a replacement for 1,1,1-trichloroethane in some applications provided that the cleaning equipment would be engineered to ensure that worker exposure levels do not exceed the recommended value.

The volatile methyl siloxanes are mild, VOC solvents, have a flash point, and possess a moderate level of toxicity with a recommended exposure level of 200 ppm. Since the cost for this solvent is high, it is generally used for defluxing and/or degreasing high value parts and PWAs, and is suited primarily for cleaning silicone and other light, nonpolar residues. In many instances this solvent is used for cold cleaning and wiping. Vapor degreasing machines using this solvent need secondary cooling along with an extended freeboard in order to minimize solvent losses. Open top vapor degreasers are not suitable and would require retrofitting.

The cleaning formulations consist primarily of water containing one or more additives, including detergents, surfactants, saponifiers, inhibitors, pH buffers, and others. The advantages of aqueous cleaning systems are as follows:

The formulations can be custom made to fit the cleaning task;
The formulations have a very low toxicity and are not flammable;
They are good at cleaning inorganic salts, polar soils, oils and greases;
The cost of chemicals is very low; and
Ultrasonics can be used more effectively in water-based than in organic-based cleaning systems.
The disadvantages of the aqueous cleaning system are as follows:

• It is difficult to clean parts with crevices;
• It is difficult to rinse some of the additives from the surface of the parts;
• Drying may be difficult and may require extra time and energy due to the high enthalpy of vaporization for water;
• More floor space is required for aqueous equipment compared to vapor phase degreasers using organic solvents;
• More energy is required to clean and dry parts than for organic-based systems;
• Additional cost and equipment is required for wastewater disposal and/or recycle;
• Corrosion may be greater with some metals than for an organic-based solvent system due to exposure to water; and
• Control and engineering of aqueous systems may be more complicated than with organic-based processes.

Semi-aqueous cleaning is carried out through the following steps. First the part is washed in a sump containing a hydrocarbon/surfactant mixture which may be followed by an aqueous wash containing a detergent. Then the part is rinsed in a deionized water sump, followed by forced air-drying.

The advantages of semi-aqueous processes are as follows:

• Lower water consumption compared to an aqueous process;
• Good cleaning for heavy oils, greases, and tars;
• Less solvent consumption compared to an organic solvent process; and
• Less attack on metals compared to an aqueous since alkaline additives are usually not used.

The disadvantages of the semi-aqueous system are as follow:

• System must be carefully engineered to counteract the flammability resulting from spraying of hydrocarbon/surfactant mixture;
• Odors from the terpene cleaners may be objectionable;
• Some of the compounds used are VOCs.
• Drying may be difficult and may require extra time and energy due to the high enthalpy of vaporization for water;
• More floor space is required for semi-aqueous equipment compared to vapor phase degreasers using organic solvents;
• More energy is required to clean and dry parts than for organic-based systems;
• Additional cost and equipment is required for waste water disposal and/or recycle; and
• Additional cost will result if a dionized water rinse is necessary

Alcohols are polar solvents that have flash points and are very effective cleaners. However, alcohols are VOCs and the equipment must be carefully engineered to avoid the flammability hazard, and there has been some reluctance to utilize this type of system even with the controls.

One approach to avoid the necessity of cleaning PWAs is to use low solid fluxes. In some cases activators are used that sublime at the soldering temperature. This approach saves money since cleaning equipment and solvent are not required. With no-clean fluxes, it is often necessary to specify higher cleanliness levels of the PC boards and components.

However, this process could not be applied universally since for some applications, the level of contaminants left on the board would be too high to meet cleanliness specifications. Also, use of the "no clean" fluxes might serve only to postpone the need to clean since additional contamination can occur due to subsequent processes on the PWA.

Another approach to avoid the necessity of cleaning PWAs is to use a continuous nitrogen purge to reduce the oxygen level on the wave soldering machine to about 5 ppm to minimize oxidation of the flux, which along with the use of activators that sublime at the soldering temperature, avoids the need for solvent cleaning. So far, this type of system has not made a lot of inroads. Potential drawbacks include high capital cost, high cost of nitrogen, and the level of contaminants left on the board may be too high in some instances to meet cleanliness specifications.

The Arthur D. Little TEWI II Solvent Report has been updated to reflect the 1995 IPCC GWP values, and has included TEWI calculations for three new solvents: n-propylbromide, a hydrofluoroether (CH3-OC4F9), and a volatile methyl siloxane [(CH3)3-Si-O-(CH3)3]. A diagram showing the sources of CO2 and other greenhouse gases generated during the cleaning is shown in Figure 10-4; a diagram showing the major material flows in the cleaning process, including both parts and solvent is shown in Figure 10-5 below.

Figure 10-4: Sources of CO2 and Other Greenhouse Gases in Cleaning Processes

Figure 10-5: Major Material Flows in Cleaning Processes

Graphs showing the indirect contribution, direct contribution, and total TEWI for batch cleaning of metal parts, batch cleaning of printed wiring assemblies (PWAs), and in line cleaning of PWAs are shown respectively, in Figures 10-6, 10-7 and 10-8, below.

Figure 10-6: TEWI Batch Metal Cleaning --- 100 Year ITH

Figure 10-7: TEWI Batch PWA Cleaning --- 100 Year ITH

Figure 10-8: TEWI In-Line PWA Cleaning --- 100 Year ITH

Major findings from these studies are as follows:

• HFC, HCFC, HFE and volatile siloxane solvents use less energy per unit work (indirect contribution) than aqueous or semi aqueous systems;
• Aqueous and semi aqueous systems have a lower TEWI than HFC, HFE or HCFC solvent cleaning systems
• N-propylbromide has an indirect contribution comparable to the aqueous and semi aqueous systems, and a lower TEWI than the HFC, HCFC, HFE and volatile siloxane systems;
• The TEWI for the HFC, HCFC and HFE systems can be lowered by improved control technology to reduce the escape of solvent vapors;
• Inert gas soldering and no-clean fluxes have a negligible TEWI; and
• TEWI should be one of the criteria for selecting a solvent system, along with other factors including cleaning performance, cost of solvent and equipment, throughput; size of equipment; system performance, ease of maintenance, toxicity, ease of recycling, waste disposal costs.
• The direct contributions to the TEWI are due to evaporative and drag out losses. These values were determined in the Arthur D. Little TEWI II study by determining these losses for HCFC-141b/HCFC-123, and calculating the drag out and evaporative losses for the other solvents.

The 3M company carried out a batch metal cleaning study to calculate the TEWI for HFC-7100, HCFC-141b, and compare the values to an aqueous and semi aqueous cleaning process. The vapor phase degreaser used extended freeboard and secondary cooling coils to minimize vapor loss. There was a 1-minute dwell time in the freeboard and a 1-minute dwell time in the vapor zone. The TEWI for HFC-43-10 was estimated based on the similarity of the drag out loss curve for HFC-43-10 compared to HFE-7100 and similarity of the boiling points.

The data are plotted in Figure 10-9 below.

Figure 10-9: TEWI Batch Metal Cleaning --- 3M Study

Unlike the TEWI data in Figures 10-6 through 10-8, where the drag out losses were calculated based on scaling experimental results on HCFC-141b/HCFC-123 systems (80/20) weight percent, the data in the 3M study are based on actual experimental data where care was taken to minimize solvent vapor loss. In this case the direct contribution from both HFC-7100 and HFC-43-10 were less than for the lower boiling HCFC-141b. Also, the TEWI for HFE-7100 was comparable to the aqueous system.

Since HFC-43-10 is a mild solvent and high priced and HFC-365mfc is being used in blends with higher priced solvents, they are being utilized only for the cleaning of high value added parts where good solvent compatibility and stability would be an issue. Furthermore, the use of higher end vapor phase degreasers with high freeboards, and secondary cooling systems are required and economically justified in order to minimize of loss of expensive solvent.

The total use of HFC-43-10 has been estimated to be less than 2 million pounds per year which translates to <3.2 × 10^-1 MMTCE. Use of HFC-365mfc as a solvent is under development, with an industrial-scale plant expected to come on stream at the end of 2002.

CFC-113 and CFC-113 azeotropes containing alcohol a provided a solvent cleaning alternative that was comparatively safe --- non-flammable, low toxicity --- and chemically stable, with good solvency, at a moderate price. No environmentally acceptable substitute has been identified that duplicates these characteristics. While many cleaning processes that formerly were solvent based have changed to water or dry-ice based processes, no-clean processes, or other non-organic solvent based alternatives, some cleaning and drying requirements can only be met with organic solvents. The available, environmentally acceptable alternatives all fall short of providing the combination of properties of CFC-113, particularly with respect to safety. The HFC solvent systems, though expensive, do approach the level of safe handling and chemical stability provided by CFC-113. The high cost of these materials inherently limits their use to applications where these characteristics are truly needed.