Department of Defense
Pollution Prevention Technical Library

ODS-Free Metal Cleaning Overview

Revision: 4/95
Product / Process: Cleaning and degreasing of metal equipment
Process Code: ID-02-06, ID-02-07, ID-02-08, ID-02-09
Substitute for: Vapor degreaser and cold cleaning (soaking, spraying, or wiping with ambient temperature cleaner)
Waste Stream: N/A
Applicable EPA Hazardous Waste Codes: N/A
Applicable EPCRA Targeted Constituents: 1,1,1-trichloroethane, also known as Methyl Chloroform (MCF), Trichlorotrifluoroethane (CFC-113)

Introduction: In many manufacturing processes, the chlorofluorocarbon (CFC) solvents CFC-113 and MCF have been used as the principal metal component cleaner. CFC-113 and MCF exhibit good solvency, are chemically stable, and have low toxicity when properly applied. In addition, the volatility of these solvents allows their use in vapor degreasing processes and allows cleaned parts to dry by evaporation. However, both CFC-113 and MCF are Class I ozone-depleting substances (ODSs) scheduled for a production ban on January 1, 1996. Fortunately, many ODS-free alternatives are now available to replace CFC-113 and MCF. This data sheet gives a brief list and description of some of the ODS-free alternatives to cleaning/degreasing with CFC-based solvents.

Description: Solvent metal cleaning falls into two general processes: cold cleaning and vapor degreasing. In cold cleaning processes, parts are cleaned by either immersion and soaking, spraying, or wiping with ambient temperature solvents. The vapor degreasing process uses a boiling solvent to effect cleaning. A volatile solvent, such as CFC-113 or MCF, is heated in a reservoir below a suspended part. Solvent vapor rises to the top of the cleaning vessel and condenses on the part, dissolving contaminants. As the droplets collect and fall, contaminants are carried off the part and into the solvent reservoir. Since the contaminants are generally low or non-volatile, solvent vapors remain essentially pure; effective cleaning is thus maintained, despite the increasing contamination of the solvent reservoir.

Several steps should be taken to eliminate CFC solvents for metal cleaning. First and foremost, the existing cleaning processes can be examined to identify and prioritize the most important operations. In some cases, solvent cleaning can be eliminated through a change in materials, a change in production processes, or even by realizing such high cleanliness standards are not required. Also, "good housekeeping" practices, such as proper spill control and containment, and elimination of open pouring of make-up solvent, will optimize solvent use.

Once required cleaning operations are identified, material balances should be evaluated to determine the locations and quantities of solvent used, recovered, and lost. By characterizing soils and substrates, cleaning processes can be refined, and the solvent requirements correspondingly reduced. Finally, CFC-free alternatives should be considered.

When solvent cleaning cannot be eliminated, alternative solvents processes are available to replace CFC-based cleaners. Chemical, mechanical, and specialty alternatives should be compared. Chemical alternatives comprise those replacement solvents for use in existing cold cleaning and vapor degreasing systems. In contrast, mechanical alternatives commonly require replacement of the entire process. Generally, higher initial costs are offset by a safer workplace and lower operating costs. Specialty alternatives include processes such as gas plasma systems and supercritical cleaning. The table below lists several methods to reduce chlorinated solvents.

Methods to reduce chlorinated solvent use:

Selection Methodology

Four areas should be considered to determine the best cleaning alternative:

1. Governmental regulations
2. Technical feasibility
3. Process economics
4. Environmental, health, and safety.

Obviously, solvent cleaning must comply with federal, state, and local governmental regulations. For example, alternatives using volatile organic compounds (VOCs) are restricted by emission control standards. Other specific federal regulations include sections 608, 611, and 612 of the Clean Air Act Amendments regarding stratospheric ozone protection.

Technical feasibility refers to interactions between substrate, contaminant, and cleaning agent or process. To determine the best alternative, answer the following questions:

• What are the cleanliness requirements of the part?
• Does the alternative meet all military specifications?
• Is the part material/coating compatible with the cleaning process?
• Can the new process meet required production volumes?
• Is the process easily installed, operated, and maintained?
• How does the new process affect subsequent production steps?

When comparing the economics of various cleaning alternatives, include costs associated with initial capital equipment; process operation; permit applications; and waste treatment, storage, and disposal. Process operation should include material, labor, maintenance, and utility costs.

The last step deals with environmental, health, and safety considerations. Several concerns need to be considered: compatibility with regulatory trends; public perception about cleaning chemicals and associated risks; global warming potential of alternatives; energy efficiency; toxicity and worker safety; flammability; and emissions, effluents, and wastes generated.

Chemical Alternatives

Many alternatives to MCF and CFC-113 are available for use in cold cleaning and vapor degreasing applications, including wipe cleaning, dip cleaning, immersion soaking, pressure washing, and vapor degreasing. Some solvents are recommended for a specific application, while others are useful for many applications. In general, the following properties are desirable when considering alternative solvents: low surface tension to penetrate small spaces, high density to remove small particles, high volatility to provide rapid drying, good solvency to readily remove organic soils, low cost, low toxicity, non-flammable, little residue, and easy to cleanup/recycle/dispose.

Drop-in solvent replacement of MCF or CFC-113 is not usually possible. However, since vapor degreasing is effective at cleaning delicate parts contaminated with organic materials (oil and grease), it may be desirable to maintain the process. Many substitutes will require system modifications. Vapor degreasing replacement solvents should be non-flammable, have a low boiling point, and leave no residue, among other characteristics. Some possible CFC-free alternative chemicals include the following

• Aliphatic Hydrocarbons: Aliphatic compounds comprise a wide range of solvents, such as mineral spirits and kerosene. These solvents have superior cleaning ability; are compatible with most plastics, rubbers, and metals; and are recyclable by distillation (see the Pollution Prevention Opportunity Data Sheet, "Solvent Recycling"). However, aliphatic hydrocarbons are flammable, slow to dry, and have low occupational exposure limits. As a result, vapor cleaning should not be attempted with aliphatic hydrocarbons.
• Other Chlorinated Solvents: The main advantage to using a chlorinated solvent such as trichloroethylene, perchloroethylene, or methylene chloride is their similarity in both physical properties and cleaning effectiveness to CFC solvents, especially in vapor degreasing. However, all three have been classified as Hazardous Air Pollutants (HAP) by the EPA, and are EPCRA-targeted chemicals as well. Furthermore, as spent solvents, they are listed hazardous wastes. These qualifications make their handling and disposal more involved and expensive than other cleaning alternatives. Future regulations may ban the manufacture of all chlorinated solvents.
• Other Organic Solvents: Organic solvents, such as ketones, alcohols, ethers, and esters, are effective but dangerous. Many are hazardous air pollutants, while others have very low flash points; for example, 0 degrees F for acetone. Extreme caution is, therefore, required when handling organic solvents. Additionally, organic solvents can be toxic and malodorous and, as a result, are generally incompatible with vapor degreasing.
• N-Methyl-2-Pyrrolidone: Also known as M-Pyrol or NMP, N-Methyl-2-Pyrrolidone has a high purity, high flash point, and low volatility, and is effective in ultrasonic applications. Miscible with water and most organic solvents.
• Volatile Methyl Siloxanes: VMS compounds are relative newcomers to the solvent cleaning scene. They are low molecular weight silicone fluids available in a variety of blends, exhibiting good compatibility with plastics and elastomers. However, all blends are either flammable or combustible and somewhat toxic. Advantages of VMSs include the following: good cleaning capabilities for a large variety of contaminants, dry without leaving a residue, spread rapidly, readily penetrate tight spaces, and can be used in existing cleaning equipment designed to safely handle flammable liquids including vapor degreasers (with some modification). Finally, VMSs are easily recycled by distillation and/or filtration, facilitating their reuse.
• Hydrochlorofluorocarbons: While HCFCs are similar to CFC-113 and MCF in solvency and cleaning effectiveness, the use of HCFCs is severely restricted, due to their ozone-depleting potential and health effects. A production ban on HCFCs is scheduled for the year 2010, but could be accelerated by the EPA at any time. Emission control equipment must be used to provide a safe working environment for HCFCs in vapor degreaser systems. HCFC-225 is the only long-term SNAP-approved HCFC, but it is strictly limited to precision cleaning operations. It also has relatively low occupational exposure limits.

For wipe cleaning and dip cleaning, X-Caliber or PF-Degreaser can usually be directly substituted for MCF, but every substitution should always first be tested for each individual application.

Mechanical Alternatives

Mechanical alternatives listed in this report include aqueous and semi-aqueous cleaning. Information on steam cleaning and blasting operations can be found in additional Pollution Prevention Opportunity Data Sheets under the solvents directory.

• Aqueous Cleaning: Aqueous cleaning, with three major equipment configurations listed below, has advantages and disadvantages when compared with alternative chemical solvents. Advantages include increased safety and flexibility, decreased material and waste disposal cost, and multiple cleaning mechanisms (chemical reaction, displacement, emulsification, dispersion, and others). Aqueous cleaning solutions can be tailored for specific parts and contaminants. Compared with chemical solvents, aqueous systems generally have higher capital costs, elaborate rinsing and drying procedures, and require tighter process control for optimum cleaning. Applied in an aqueous dip tank, Johnson Wax "Believe" has been used successfully at Puget Sound Naval Shipyard as a replacement process for vapor degreasing in removing grease and dirt from parts before plating. Equipment costs were low for adding heating coils and an air sparging unit to the aqueous dip tank.
• Immersion with Ultrasonic Agitation: Ultrasonic agitation is especially suited for high levels of cleanliness for complex parts. However, the high level of cleanliness is accompanied by high cost (see Pollution Prevention Opportunity Data Sheet, "Ultrasonic Cleaning as a CFC Solvent Alternative," in the solvent directory).
• Immersion with Mechanical Agitation: A major advantage of mechanical agitation is the use of existing vapor degreasing equipment. In addition to being somewhat difficult to automate, mechanical agitation requires orientation of parts in solution.
• Spray Washer: Inexpensive and easy to operate, the spray washer is the least effective method of cleaning complex parts. For parts which are not complex, a high degree of cleanliness is attainable (see Pollution Prevention Opportunity Data Sheet, "High Pressure Spray Washing," in the solvents directory).
• Semi-Aqueous Cleaning: Semi-aqueous cleaning, a combination of a non-aqueous-based cleaner with an aqueous rinse, is frequently used for metal cleaning, especially where aqueous methods alone are ineffectual on heavy grease, tar, and soils. Precision and electronics applications are limited. Primary concerns generally deal with the properties of the non-aqueous cleaner: volatility, flammability (especially in heated processes), exposure risks, residues requiring rinsing, and high costs of disposal. A nitrogen blanket can reduce the risk of combustion. Hydrocarbon and surfactant mixtures, alcohol blends, terpenes, and petroleum distillates are solutions available for semi-aqueous cleaning. Advantages include the following:
• Compatible with most metals and plastics.
• Non-alkalinity helps prevent metals from entering the waste stream.
• Compatible with rust inhibitors.
• Potential decrease in solvent purchase costs.
• Decrease in evaporative losses.

Immersion and spray equipment and rinse tanks can be used for aqueous or semi-aqueous cleaning. During semi-aqueous immersion, less mechanical agitation is needed due to increased solution solvency. Heat, agitation, and ultrasonics all improve the level of cleanliness. In general, semi-aqueous cleaning involves four stages: wash, emulsion rinse, rinse, and dry. The emulsion rinse cleans the part and reduces contamination of the second stage rinse. Two rinse stages removes residue left by low-volatility solvents. Finally, the drying step prepares the product for further processing and prevents rust.

• Wastewater Disposal: In many instances, due to local, state, or federal regulations, wastewater from aqueous or semi-aqueous processes has to be treated before discharging to the municipal wastewater treatment plant. Contaminants of concern include organic matter (oil and grease), metals (dissolved or in suspension), and alkaline cleaners, which raise pH to an unacceptable level. Pretreatment technologies include gravity separators, ultrafiltration, chemical treatment, precipitation, and carbon adsorption.

Regardless of disposal method, wastewater generated by cleaning processes should be minimized, since excess wastewater results in excess costs. Five steps are recommended for reducing wastewater volumes:

• Unnecessary loading increases water and cleaner use and maintenance.
• Promptly removing sludge and soils from the cleaning equipment reduces major maintenance.
• Monitor cleaning solution to effectively maintain solution strength.
• Practice preventive maintenance to minimize clogs, spills, and leaks.
• Add features such as fog nozzles, demineralized water, and counterflow systems.

Specialty Alternatives

• Gas Plasma: Used for attaining organic-free surfaces, gas plasma systems use electrically excited, non-toxic gases to remove thin layers of organic residue left behind by other cleaning processes. The gas plasma processes are non-toxic and not caustic, offering a high level of worker safety. High capital costs are offset by the low cost of process gases and the lack of disposal costs. Byproduct vapor can usually be vented without scrubbing.
• Supercritical Cleaning: Above the supercritical point, many compounds take on liquid and gas-like properties. The combination of properties allows supercritical fluids to penetrate complex assemblies and remove trace soils and particle contamination. Because operating pressures can reach 4,000 psig, supercritical fluid (SCF) equipment is very expensive, and only parts which can withstand the high pressures are eligible for cleaning. CO2 is the most widely used supercritical fluid due to its physical and cleaning properties, safety, and availability. In addition, CO2 is a byproduct of industrial processes and natural sources, and is not a new source of global warming CO2 (see Pollution Prevention Opportunity Data Sheet, "Supercritical Fluid Cleaning as a Solvent Alternative").

General Considerations

Although aqueous cleaning is generally perceived as the safest, most cost-effective cleaning process, it is not always the best option. Whatever alternative is chosen, some basic criteria should be evaluated:

• Have processes been evaluated for the least hazardous substitute?
• Are waste stream quantities treatable at peak demand?
• Is water use minimized using timers and control valves?
• Are raw and waste material containers labeled and segregated?
• Have personnel been trained in proper handling and spill prevention?
• Can cross-contamination of solvents and water be minimized?
• Is sludge regularly removed/dewatered? Is digestion advantageous?
• Is spent solvent recycled to the full extent possible? Recycling systems include centrifuges, filters, stills, ion exchange, and semi-permeable membranes. If solvent is depleted, can it be rejuvenated for reuse?
• Is there a vapor-recovery system to capture evaporated solvents?
• Is the equipment fitted with refrigerated freeboard to reduce emissions?
• Are spill prevention methods and equipment in place?
• Have flammability concerns been addressed?
• Will odors from the solvents cause workers discomfort?
• Are occupational exposure limits known and safeguards taken?

Materials Compatibility: Materials compatibility depends on the alternative solvent/procedure implemented. Considerations can include corrosion, damage to coatings and adhesives, and swelling and deformation (especially for organic substitutes: alcohols, ketones, ethers, chlorinated solvents, etc.). Testing will reveal damage to parts: stress, embrittlement, immersion corrosion.

Safety and Health: High pressure gases should be handled with great care. Always chain or secure high pressure cylinders to a stationary support before using. Organic solvents can be extremely flammable/combustible. Use only in areas with good ventilation. Aliphatic hydrocarbons are also flammable and have low occupational exposure limits. Consult the MSDS of particular solvents so that solvent is used properly and all necessary safety requirements (i.e., personal protective equipment, increased ventilation, fire fighting equipment) can be met. In addition, consult your local Industrial Health specialist, local health and safety personnel, and the SNAP comments prior to converting to any replacement product.

Benefits: Reduction of ODSs and EPCRA-listed chemicals entering the environment.

Economic Analysis: Substitute processes and chemicals need to be evaluated with regards to each specific application. Taxes on CFCs will continue to rise.

Major Assumptions: Aqueous cleaners may be subject to requirements of the Clean Water Act. Organic cleaners could require Maximum Achievable Control Technology (MACT), while others may be restricted by additional OSHA requirements.

Points of Contact:

(619) 545-9757
Materials Engineering Laboratory
Naval Aviation Depot, North Island, California

Hazardous Materials Department
Puget Sound Naval Shipyard
(360) 476-8448 Code 248
Puget Sound Naval Shipyard uses "Believe" as an aqueous cleaner.

Ms. Nina Bonnelycke, Solvent Specialist, SNAP Program
Office of Stratospheric Ozone Protection
US EPA
(202) 233-9079

The International Cooperative for Ozone Layer Protection (ICOLP), (202) 737-1419, is a non-profit organization made up of technical associations, corporations, and governmental agencies exchanging information on ozone layer protection, ODSs, and alternative technologies. ICOLP has developed seven different guidance manuals on ODS replacement products and processes, each including case studies and extensive vendor lists.

National Defense Center for Environmental Excellence - (800) 282-4392

Vendors: See Pollution Prevention Opportunity Data Sheet, "Alternative Cleaning Vendor List," for an extensive list of equipment and product vendors.

Approving Authority: Approving authority is controlled locally and is not required by the major claimant.

Note: This recommendation should be implemented only after engineering approval has been granted by cognizant authority.

Sources:

PA Technical Inquiry 3031.

Pollution Engineering, p. 55-58, 1 June 93.

ICOLP Manual, Alternatives for CFC-113 and Methyl Chloroform in Precision Cleaning.

Hume, Bob, "Ozone Depleting Substances," NAVAIR 4th Annual Pollution Prevention and Technology Exchange Conference, p. 415-446, May 26, 1994.