Parts cleaning 1996.

Aqueous-based cleaning approaches are succeeding in replacing "the silver bullet."

It's not hard to imagine that replacing chlorinated solvents ("the silver bullet") for parts cleaning would be difficult. The reasons are readily apparent. Substitute solvents and government regulations require virtually emission-free cleaning devices unlike open-top tanks or leaky vapor degreasers used in the past. The switch from solvents to aqueous-based cleaning systems and chemistries brings with it a whole new set of operating requirements.

Meanwhile, passing of the deadline for producing ozone-depleting substances (ODS) such as 1,1,1-Trichloroethane (TCA), a.k.a. methyl chloroform (MCF), and carbon tetrachloride has not gone unnoticed. They have joined other Class I ODS chlorinated fluorocarbons (CFC-113, HCFC-141b) in being phased out of production for emissive applications as of December 31, 1995.

Stockpiling of such cleaning agents by users and distributors has been brisk in spite of the fact that users will pay an onerous excise tax to continue using them for metal degreasing operations. In the interim, totally enclosed emission-free vapor degreasers are essential, whether bought new or retrofits of older systems. Open tank cold immersion cleaning with solvents may become a thing of the past.

Major metalworking companies responded quickly and responsibly to adopting an ODS-free stance in their cleaning processes. The requirement that warning labels be required on all products containing Class I ozone depleting substances, including TCA, and on all containers used for the transport and storage of TCA, as well as all products manufactured with TCA was reason enough. They have been followed by companies of all sizes that are beginning to find some competitive advantage in selecting acceptable alternative cleaning systems.

The difficulty of matching aqueous cleaning systems and chemistry to the cleaning task and solids at hand has been well documented in these and other pages. Most observers believe that anywhere from 75% to 95% of metalworking cleaning can be handled by the right aqueous-based cleaning system.

No slam dunk

Aqueous cleaning, however, is not a simple "dunk and dry" process, like solvent cleaning, explains James T McEachen, president, Hurricane Systems Inc, Jackson, MI. It is application-specific and requires commitment of resources and capital as well as the time to analyze cleaning needs, reconfigure manufacturing processes if necessary, and deal with waste water management issues.

"Five to ten years ago, parts cleaning was thought of as a second class operation in the plant. In the past, you may have found a bag of screws at the hardware store with metal shavings and oil on the threads. You rarely, if ever, find this situation anymore," says Mr McEachen. He cites the example of the automobile manufacturer that had experienced numerous transmission failures after the car had reached the 60,000-mile threshold.

The automaker evaluated eight different transmissions (including those of other auto manufacturers) with over 100,000 miles on them to determine the causes of failure. "Expecting to find metal shavings caused by transmission component wear, they instead found that over 80% of the particulates in the fluid were introduced in the manufacturing process. The company immediately began improvements in its cleaning process and transmission life improved dramatically."

Other factors in the drive toward more in-process cleaning are the requirements of cellular manufacturing systems that don't work well with centralized cleaning systems, the need for cleaner work-pieces for advanced processes like laser welding, and flexible manufacturing that produces custom "one -of- a-kind" assemblies.

The search for alternative cleaning systems is not an easy one, however. Often the solution is to totally re-think the parts cleaning process. At a recent SME conference, manufacturers explained how they are finding additional benefits to their processes when they adopt new cleaning systems.

At one Ford Motor Co plant, the cleaning system for transmission components went through an evolutionary process in which an old-style cleaning line that was 40 ft long and 15 ft high was reduced to a machine that was 5 ft high and 6x8 ft. In the process, issues addressed included visibility, safety, access, and maintainability. Various design configurations put doors on hinges and placed all systems at the back of the machine, away from the operator and easily accessed for maintenance. The final result was compact and cost-effective and one which meets the plant's own need to compete internally with other Ford plants for manufacturing business.

Going mobile

Steelcase Inc, Grand Rapids, MI, has adopted a portable electric-heated aqueous cleaning system to replace a large gas-fired centralized cleaning unit. The large centralized cleaning unit was used to clean parts for two departments and needed 17 people to operate (ten on days and seven on nights). When the central cleaning unit was down for maintenance and cleaning, production in both departments was curtailed. The new aqueous-based system uses seven small portable washers, with three used by the machining department and three by the welding department. The seventh unit is a floater, available to fill in when any one of the other units is down..

A Steelcase task force team made its recommendation for the mobile parts cleaning system based on cost, safety, ergonomics, and environmental considerations. Cost avoidance results from the elimination of 17 positions at the wash tank. Wash attendants were given training and one year to post on another Steelcase job. Floorspace was freed up for manufacturing, and maintenance and waste disposal were simplified.

The old wash tank took two days to clean, and production had to be scheduled around it. In the new system, tanks can be cleaned in one hour. With the floating washer available, there is no need to stop production for cleaning. The new system uses biodegradable chemicals that are approved by the EPA and the city to be dumped into the city sewer system, eliminating the need to haul hazardous waste chemicals used in the old system away.

Oil drips and slicks on the plant floor due to transporting oil soaked parts to the washer were minimized, if not entirely eliminated, increasing safety for personnel.

The solvent option

The chlorinated solvent option still exists. The chlorofluorocarbons (CFCs) and Halons were the first groups of substances subject to control and phaseout under both international and US regulations. TCA was originally proposed as a temporary replacement for CFCs in certain surface cleaning and electronic cleaning applications because its ozone depletion potential is one-tenth that of CFC-11. However, when the solvent's high volume usage and forecast for future use were considered, TCA itself was brought under scrutiny and regulatory control leading to phaseout.

The 1987 Montreal Protocol, an international agreement regulating substances that deplete the ozone layer, provided for freezing and phasing out the production and consumption of CFCs and Halons. Subsequently, new compounds were added to the list for phaseout, including TCA, which had its phaseout accelerated to Jan 1, 1996.

These regulations affect production of TCA for only emissive applications like parts cleaning. Production for use as chemical feedstock in the production of other chemical products will still be allowed, and TCA may continue to be recycled and re-used.

For a limited number of difficult-to-clean parts, the chlorinated solvent alternative most nearly like TCA involves methylene chloride, perchloroethylene, and trichloroethylene. These three solvents share most of the advantages of TCA, but are not scheduled for phaseout under the Clean Air Act or the international Montreal Protocol. Nor are they subject to the excise taxes levied on ODSs. In addition, they are listed by the US EPA in its Significant New Alternatives Program (SNAP) for ozone depleting substances (March 1994) as acceptable alternatives to TCA.

Methylene chloride (MEC, also called dichloromethane and methylene dichloride) is a powerful and versatile chlorinated solvent. Its high solvency and low boiling point make it especially effective for cold cleaning and vapor degreasing of metal parts. It is used extensively to remove grease, oil, tough paint residues and hard-to-dissolve resins from temperature-sensitive parts. MEC's ease of recoverability has led metal manufacturers to increase recycling efforts.

Perchloroethylene (PCE or perc, also called tetrachloroethylene) is a clear, colorless liquid with a distinctive, somewhat ether-like odor. It is a powerful and versatile solvent for oils, waxes, and greases, and is compatible with most plastics, coatings, resins, elastomers, and rubbers. Because of its relatively high boiling point, it can be used in vapor degreasing of products with high melting wax or paraffin soils. In addition, the solvent's resistance to degradation by water makes it usable in wet applications. Many industries, including aerospace, appliance manufacturing, and automotive, use PCE for vapor degreasing metal parts during various production stages.

Trichloroethylene (TCE) has been characterized as the ideal degreasing solvent because of its high solvency for oils, greases, waxes, tars, resins, lubricants, and coolants generally found in metal processing industry. In addition, TCE will not attack steel, copper, zinc, or other metals. It is highly stable in the presence of common chemical stabilizers, and its low boiling point permits vapor degreasing with low heat input and rapid handling of work following degreasing.

Generally, chlorinated solvents have been preferred by industry because they are practically non-flammable. They have no flash point as determined by standard test methods. With the exception of perchloroethylene, however, they have flammable ranges when high concentrations are mixed with air and exposed to a high energy ignition source. Consequently, all ignition sources must be eliminated in areas where relatively high vapor concentrations may occur and only electrical equipment approved for use in explosive atmospheres should be used in these areas.

Each of these three alternative chlorinated solvents, however, brings with it significant health concerns not associated with the ozone-depleting CFCs. Methylene chloride and perchloroethylene are carcinogens. Trichloroethylene is suspected as a carcinogen and a threat to the liver. All three are poisons and suspected causes of cardiac dysfunction.

Hydrofluoroether (HFE) compounds will be introduced by 3M Co, St Paul, MN, as CFC/ODS replacements for parts cleaning by mid-1996 for applications such as precision cleaning/degreasing in the medical device manufacturing, electronics, and metalworking.

HFEs are said to closely resemble, in performance, the ozone depleting substances they will replace, but unlike CFCs they have zero ozone depletion potential, very short atmospheric lifetimes, and low global warming potential. They are viewed as an acceptable replacement for such ODSs as CFC-113, HCFC-141b, and TCA. They have been tested in immersion cleaning, vapor cleaning, and as a co-solvent rinsing agent on variety of soils including light oils, heavy oils, and greases as well as for solder flux removal.

Obviously, careful containment in vapor degreasing devices is essential and mandated under standards for new and existing haologenated solvent cleaning operations, which are governed by the National Emission Standards for Hazardous Air Pollutants (NESHAP). Procedures are available for both vapor degreasing and cold cleaning that greatly decrease vapor losses from existing equipment. Totally enclosed degreaser designs can reduce emissions by more than 95% compared with open top degreasers, and methods and equipment are in the development stages for trapping remaining solvent emissions before they leave the plant. Some enclosed degreasers are available to contain and re-use most all the solvent vapors, thus providing virtually emission-free operation.