Solvent Cleaning is Alive and Well

A misunderstanding has been making the rounds among manufacturers involved in surface cleaning: a belief that solvents are on the way out. A major contributor to this misunderstanding has been the banning of 1,1,1-trichloroethane as a depleter of stratospheric ozone. It was the most popular of the chlorinated solvents.

Problems related to a few solvents should not reflect on all solvents. The other chlorinated solvents, methylene chloride (MEC), perchloroethylene (PCE) and trichloroethylene (TCE), have no effect on stratospheric ozone and therefore are not regulated for ozone depleting potential (ODP). These three solvents have been approved under EPA's Significant New Alternative Policy (SNAP) ruling as replacements for 1,1,1 trichloroethane. There are also other solvents, including hydrofluoroethers, blends of hydro-fluoroethers, N-methyl pyrrolidone and parachlorobenzotrifluoride that are safe, effective and SNAP approved for cleaning and do not deplete the ozone layer.

In spite of the vast array of surface cleaning products and processes, including aqueous, semi-aqueous and mechanical cleaning methods, many manufacturers who experimented with these alternatives to 1,1,1-trichloroethane have returned to solvent cleaning.

Chlorinated solvents manufactured by Dow Chemical have excellent solvency, are virtually non-flammable and have low toxicity when used in accordance with established safety procedures. Chlorinated solvents do an especially good job on small parts where racking would be troublesome or with complex designs and parts with numerous blind holes. Chlorinated solvents can dissolve a wide range of organic substances, have moderate to extremely rapid evaporation rates, low flammability and relative inertness to chemical reactivity.

Hydrofluoroethers manufactured by 3M are a replacement for CFCs. The cleaners are suitable for various industrial and commercial applications and particularly for precision cleaning of parts and assemblies. The structure of the proprietary HFE molecule, including the presence of oxygen in its chemical makeup, combined with the effect of segregating hydrogen and fluorine molecules around the oxygen, give these materials their special properties. 3M HFE-7100 and its azeotropes, 3M HFE-71DE and HFE-71DA are listed under SNAP as "acceptable without restrictions" for use in precision cleaning, electronics cleaning and metal cleaning.

High density, low viscosity and low surface tension combine to give the HFE's excellent penetrating and cleaning performance on components with complex geometries. These properties are important for effective solvent draining. A low heat of vaporization aids in rapid drying and minimizes liquid dragout from cleaning processes.

Vertrel XF from DuPont is designed to replace many 1,1,1, CFC, HCFC and PFC uses. It is ideal for use in vapor degreasing equipment. It is a non-VOC compound with relatively low direct warming potential. It has a higher boiling point and lower surface tension compared to CFC-113. Its solvency is more selective than CFC-113, but can be enhanced by use of azeotropes and blends with alcohols, hydrocarbons and esters.

N-Methyl Pyrrolidone (NMP), from BASF, is associated with high solvent activity, especially at elevated temperatures. This is reflected in the ability of NMP or its blends to solvate many soils and greases. Cleaning is accomplished by either immersing parts into hot liquid NMP or by allowing the parts to "hang" in heated air that is totally saturated with NMP. In an inert atmosphere, the NMP can be safely heated to the high temperatures needed to melt waxes/greases. Also, pulling a vacuum on the system, after rinsing, ensures 100% removal of the NMP during drying.

Methyl Soyate, from USB, is a methyl ester derived from soybean oil. It offers good potential as an industrial solvent for use in cold cleaning, carrier solvent and resin removal and cleanup. It is a biodegradable, low-toxicity and high-flash point (421F), high-boiling point (>400F) alternative to traditional solvents.

Ensolv, from Enviro Tech, is a non-flammable solvent designed as a direct replacement for 1,1,1-trichloroethane. The solvent's physical properties closely match 1,1,1.

It can be used in most vapor degreasers and ultrasonic cleaners that use best available control technology. It leaves behind no measurable residue after cleaning and has a moderately fast evaporation rate.

Oxsol 100, from Occidental Chemical Co., is a fluorinated toluene that can be used in vapor degreasing, precision wipe cleaning, cold cleaning and electronics cleaning. It is exempt from regulation and is not an air toxic (HAP), ozone depleter (ODP) or suspected carcinogen. Oxsol alternative solvents are manufactured in both pure and blended formulations to specific cleaning requirements. Approximately 80% of solvent cleaning operations can be converted to Oxsol cold cleaning, and the remainder can be converted vapor degreasing using new equipment or modified existing equipment.

There are other solvents on the market. This article covers some of the major products.

Reducing emissions. The National Emission Standard for Hazardous Air Pollutants (NESHAP) for solvent cleaners allows the user flexibility. When an operator has a vapor degreaser that is in compliance with the standard, it will have emissions at least 50 to 70% lower than they were prior to the implementation of controls. In some cases, this may reduce the emissions below the 10,000 lbs per year threshold. This is the threshold for permitting required in Title V of the federal regulations. Most solvents can be used with a variety of equipment upgrades and new equipment designs that control emissions and make it possible to meet the exposure levels required by the strictest regulations.

Chlorinated solvents, HFE's and their blends as well as Vertrel and Methyl Soyate, Ensolv, Oxsol and others are permitted by the government, and companies can continue to use a vapor degreasing process. It is fairly easy to adapt an old system to today's solvents, and new and improved equipment is also available.

Safety Considerations

Chlorinated solvents are nonflammable in most end-use conditions. They have no flash point as determined by standard test methods. With the exception of perchloroethylene, they have flammable ranges when high concentrations are mixed with air and exposed to a high-energy ignition source.

Lower and upper flammability limits for methylene chloride in air (77F) are 14 to 22%. The flammable range of trichloroethylene runs from 8% to saturation, with the saturation point increasing with temperature. At 100C, the upper flammability limit is 44%. Perchloroethylene has no flammability range.

Most HFC cleaning solvent spills present little threat to worker safety As long as they are carefully contained. These cleaning solvents are nonflammable and low in toxicity. Also, the evaporative loss of the HFE solvents in cleaning systems is measurably lower because of higher molecular weight and relatively low vapor pressure.

Vertrel exhibits no flash point and is not classified as a flammable liquid by NFPA or DOT. However, the product does have vapor flammability limits in air. It is thermally stable and does not oxidize or degrade during storage.

NMP vapor may have to be heated to as temperature higher than its flash point to generate a sufficient concentration in the cleaning chamber. This ensures that there is efficient/effective rinsing action. It is not recommended that NMP, or any other solvent, be heated to a temperature close to or above the flash point of the solvent in an oxygen-rich atmosphere.

Methyle soyate is a nearly colorless liquid with low water solubility. It is safe to use and transport because it has a high flash point and boiling point. It is not considered a VOC, HAP or ODP by the EPA and is not subject to air emission regulations under the Clean Air Act.

Ensolv is non-flammable. Its physical properties closely match 1,1,1-trichloroethane, and its boiling point is 156F.

Oxsol is highly stable and recyclable. It features moderate evaporation rates. It has a flash point of 109F and an auto-ignition temperature greater than 500C.

Health Considerations

Chlorinated solvents. Health effects from exposure to chlorinated solvents have been studied extensively. Exposure to vapor concentrations within recommended guideline levels will not result in any known adverse effects on most people. Acute overexposure may cause anesthetic or narcotic effects (solvent drunkenness) and death at high enough concentrations. Central nervous system effects and liver and/or kidney effects can result from chronic overexposure. Proper ventilation when using chlorinated solvents is essential. Because chlorinated solvent vapors are heavier than air, high concentrations can accumulate in poorly ventilated and low-lying areas, such as pits.

Small amounts ingested inadvertently are not likely to cause injury; however, larger amounts could result in serious injury or death.

Occasional brief skin contact with these solvents is not likely to produce any significant adverse effects. A single prolonged exposure is unlikely to result in absorption of harmful amounts through the skin. Chlorinated solvents can de-fat the skin. This may allow dermatitis to develop, especially when there is frequent gross daily contact.

HFEs. Under normal conditions, workers are exposed to a small amount of cleaning solvents during their shift. The acceptable exposure limit for HFEs is higher than for typical solvents. Based on extensive toxicity testing, the eight-hour worker exposure guideline for HFE solvents ranges from 200 to 600 ppm. Also, HFE's are not irritating to the skin or eyes and have tested negative in all mutagenicity screens.

NMP, just as other solvents, will de-fat/de-oil the skin. When working with NMP users must wear NMP-resistant butyl or neoprene rubber gloves. Also goggles should be worn to protect the eyes.

Oxsol should be used with adequate ventilation and is harmful to eyes and skin. Proper protective equipment should be worn. The vapors are heavier than air and will collect in low areas. Avoid use in confined spaces.

Standards for degreasing. Emission standards for surface cleaning are now governed by the new National Emission Standards for Hazardous Air Pollutants (NESHAP) for new and existing solvent cleaning operations. Issued by the U.S. EPA on December 2, 1994, the standards cover both vapor degreasing and cold cleaning with trichloroethylene, perchloroethylene and methylene chloride, as well as with chloroform and carbon tetrachloride. The goal of the NESHAP is an overall reduction in solvent emissions from 50 to 70%.

For both vapor degreasing and cold cleaning, procedures are now available that greatly decrease vapor losses from existing equipment. In addition, totally enclosed degreaser designs are available that can reduce emissions by more than 95%. Methods and equipment are also in the development stage for trapping remaining solvent emissions before they leave the plant. Some enclosed degreasers are able to contain and reuse essentially all the solvent vapors, providing a virtually emission-free operation.

According to information from Research Triangle Institute there are design requirements and work practices for vapor degreasers to help with the equipment standard. Vapor degreasing tanks should be covered and freeboard ratio should be increased from 50% to 75%. This can reduce emissions up to 46%. Units should have automated parts handling. Low-liquid-level switches should be installed to turn off the heating coils as well as vapor level controls for the same purpose. The system also needs a primary condenser and, if it has a lip exhaust, it needs a carbon adsorber.

Other recommendations include:

• Reducing room draft can reduce solvent loss 30%
• Baskets that cover less than 50% of the solvent surface unless the vertical velocity of the part handler is less than three fpm
• Install freeboard chillers in addition to cooling jackets to reduce solvent use by as much as 60%
• Spray only in the vapor zone
• Orient parts for good drainage or rotate if possible.
• Leave parts in machine until dripping stops and bring parts up to temperature before removing
• Turn on the primary condenser before the heater
• Shut off heater and allow the vapor blanket to collapse before turning off the condenser
• Consolidate cold cleaning operations into a centralized vapor degreasing operation.
• Drain or add solvent through a leakproof coupling below the liquid surface
• Collect and store waste solvent in a closed container

Research Triangle Institute also provides a quiz on solvent cleaning procedures that could be used to train workers on properly operating equipment.

1.What is the maximum allowable speed for parts entry and removal?

A. 28 fpm
B. 11 fpm
C. 36 fpm
D. No Limit

2. How do you ensure that parts enter and exit the solvent cleaning machine at the speed required in the regulation?

A. Program on computerized hoist monitors speed
B. Judge speed by looking at it
C. Measure the time it takes the parts to travel a measured distance

3. Identify sources of air disturbances.

A. Fans
B. Open doors
C. Open windows
D. Ventilation vents
E. All of the above

4. What are the three operating modes?

A. Idling, working and downtime
B. Precleaning, cleaning and drying
C. Startup, shutdown, off
D. None of the above

5. When can parts or part baskets be removed form the solvent cleaning machine?

A. When they are clean
B. Any time
C. When dripping stops
D. Either A or C is correct

6. How must parts be oriented during cleaning?

A. It doesn't matter as long as they fit in the basket
B. So that solvent pools in the cavities where dirt is concentrated
C. So that solvent drains freely

7. During startup, what must be turned on first, the primary condenser or the sump heater?

A. Primary condenser
B. Sump heater
C. Turn both on at the same time
D. Either A or B is correct

8. During shutdown, what must be turned off first, the primary condenser or the sump heater?

A. Primary condenser
B. Sump heater
C. Turn both off at the same time
D. Either A or B is correct

9. In what manner must solvent be added to and removed form the solvent cleaning machine?

A. With leakproof couplings
B. With the end of the pipe in the solvent sump below the liquid solvent surface
C. As long as solvent does not spill, the method does not matter
D. A and B

10. What must be done with waste solvent and still and sump bottoms?

A. Pour it down the drain
B. Store in closed container
C. Store in bucket
D. A or B

11. What types of materials are prohibited from being cleaned in solvent cleaning machines using halogenated HAP solvents?

A. Sponges
B. Fabrics
C. Paper
D. All of the above

12. What temperature must the freeboard refrigeration device achieve?

A. Below room temperature
B. 50F
C. Below the solvent boiling point
D. 30% below the solvent boiling point

13. When can a working cover be open?

A. While parts are in the cleaning machine
B. During part entry and removal
C. During maintenance
D. During measurements for compliance purposes
E. A and C
F. B, C and D

14. In what condition must covers be maintained?

A. Free of holes
B. Free of cracks
C. So that they completely seal the cleaner opening
D. All of the above

15. Where must the part be held for the appropriate dwell time?

A. In the vapor zone
B. In the freeboard area above the vapor zone
C. Above the cleaning machine
D. In the immersion sump

The answers to the questions are 1. B; 2. A or C; 3. E; 4. A; 5. C; 6. C; 7. A; 8. B; 9. D; 10. B; 11. D; 12. D; 13. F; 14. D; and 15. B. The quiz and other information can be found on the Web at http://clean.rti.org.