Cold Cleaning

Cold cleaning is a process for removing oil, grease, loose metal chips, and other contaminants from the surfaces of metal parts. Common organic solvents such as petroleum distillate fractions, chlorinated hydrocarbons, chlorofluorocarbons, hydrofluorocarbons, or blends of these classes of solvents are used. Cleaning is usually performed at, or slightly above, room temperature. Parts are cleaned by being immersed and soaked in the solvent, with or without agitation. Parts that are too large to be immersed are sprayed or wiped with the solvent. Ultrasonic agitation is sometimes used in conjunction with solvent cleaning to loosen and remove soils, such as abrasive compounds, from deep recesses or other difficult-to-reach areas. This reduces the time required for solvent cleaning of complex shapes.

Cold cleaning is chosen when one or more special conditions exist: water will not remove the soils, water would promote corrosion or rusting, or soil must be removed from temperature-sensitive parts. Equipment for cold cleaning can be as simple as a small tank or a pail with a cover. Thus, cold cleaning is a convenient choice for temporary operations, operations where each machinist must be able to clean parts, or operations where capital intensive equipment cannot be justified.

Solvents

Table 1 lists aliphatic petroleums, chlorinated hydrocarbons, chlorofluorocarbons, alcohols, and other solvents commonly used in cold cleaning. Stoddard solvent, mineral spirits, and VM&P naphtha are widely used because of their low cost and relatively high flash points. The chlorinated hydrocarbons and chlorofluorocarbons exhibit a wide range of solvency and are nonflammable, but most are far more expensive than the aliphatic petroleums. Blends of solvents are offered to provide improved solvency, reduce cost, reduce fire hazard, adjust evaporation rates, and so on. The alcohols are used alone, or in conjunction with chlorocarbons or chlorofluorocarbons, for special cold cleaning applications such as removing activated soldering fluxes. Acetone and other solvents having low flash points are used for special purposes only, such as cleaning the components of precision instruments, but may pose a serious fire hazard. Their storage and use require strict observance of all safety precautions.

Table 1 Properties of cold cleaning solvents

Solvent

Flash point(a)

OSHA TWA,

ppm

°C

°F

Aliphatic petroleums

Kerosene

63

145

Naphtha, hi-flash

43

110

Mineral spirits

14

57

500

Naphtha, VM&P

9

48

500

Stoddard solvent

41

105

100

Chlorinated hydrocarbons(c)

Methylene chloride

None

None

500

Perchloroethylene

None

None

100

Trichloroethane (1,1,1)

None

None

350

Trichloroethylene

None

None

100

Trichlorotrifluoroethane

None

None

1000

Alcohols

Ethanol, SD

14

57

1000

Isopropanol

10

50

400

Methanol

12

54

200

Other solvents

Acetone

-18

0

750

Benzol

-11

12

10

Cellosolve(d)

40

104

50

Toluol

4

40

100

(b) OSHA exposure values expressed as parts of vapor or gas per million parts of air by volume at 25 °C (77 °F) and 760 mm Hg pressure. These values should not be regarded as precise boundaries between safe and dangerous concentrations. They represent conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect. The values refer to time-weighted average concentrations for a normal workday.

(c) Also used for vapor degreasing.

(d) 2-ethoxyethanol

In choosing an organic solvent for a particular operation, the most important characteristics to consider are its:

• Solvency for soils, water, and salts

• Evaporation rate

• Biodegradability

• Ease of conservation/recovery/distillation

• Compatibility with part or assembly materials

• Ease of disposal

• Associated regulatory requirements

The importance of any specific characteristic is related to the cleaning required, the sophistication of the equipment engineering, and other properties of the candidate solvent. For example, a more toxic solvent might be acceptable if the equipment prevents overexposure of workers. Solvency for the soil to be removed is usually essential, but solubility of water may be preferred for drying parts. On the other hand, solubility of water could be a disadvantage if the discharge water contains excessive amounts of solvent. A low-vapor-pressure solvent is lost through evaporation more slowly, and may be more easily controlled below its acceptable worker exposure standard, than a solvent with greater volatility. However, slow evaporation causes prolonged drying time. Removal of one soil only to have it replaced by a different soil from the solvent is normally not desirable. Therefore, initial solvent purity is important, and a means is required (usually distillation) of maintaining a level of purity to prevent redeposition of soil from previously cleaned parts. Highly biodegradable solvents may be more acceptable in discharge to public water treatment plants, but even so they could cause fish kills due to oxygen depletion in ponds or lakes. Tight equipment may conserve solvent to the extent that a preferred higher-price solvent may be a practical choice. Greater conservation results in less addition of fresh solvent to the system and increases the need for purification by distillation. Identification markings, paint, or plastic components may require a solvent that is selective in dissolving the soil without damaging the parts. Critical factors in cost control may be the use of a minimum of labor, elimination of reject parts, and reduction of disposal costs, rather than the price of the solvent. Disposal costs are another factor in the overall operating costs. Regulations have become another major consideration in the solvent selection process. The best illustration of this is the production ban on 1,1,1-trichloroethane and trichlorotrifluoroethane, beginning January 1, 1996, because they deplete stratospheric ozone. Table 1 provides some information that can be used in choosing a solvent.

Process Control Variables

Cold cleaning is chosen for its simplicity and the low capital cost for the great majority of its uses. It is not surprising that most operations are conducted in a simple tank or pail with a cover at room temperature. A course spray, mechanical agitation (usually manual), brushing, and ultrasound are used to speed cleaning and assist in the removal of insoluble matter. Increasing the solvent temperature will increase its solvency, but this option is infrequently used. Elevated temperatures can significantly increase the fire hazard of flammable solvents, and control of worker vapor exposures becomes difficult as the solvent evaporates more rapidly.

Cleanness of Solvent. As contamination of the solvent increases, cleaning efficiency and the cleanness of processed parts decrease correspondingly. Cleanness requirements prescribe the time at which the solvent must be replaced. For example, a service business that has become quite popular, especially in automotive repair shops, provides the tank equipment and solvent, periodically removes the dirty solvent, and replaces it with clean solvent.

Solvent Reclamation. All solvents can be reclaimed by either a factory-operated still or a licensed reclamation service. In general, the reclamation process is one of simple distillation. However, explosion-proof equipment is essential for the distillation of flammable solvents.

Factory distillation equipment must be selected on the basis of the volume of solvent used, whether the solvent is flammable, the boiling point of the solvent, the nature of the contaminants, and the degree of purity required. A still may service multiple cold cleaning locations, or it may be incorporated into the large sizes of dip or soak equipment on a semiautomated basis.

Standards for recovered solvent usually relate to color, clarity, moisture content, and neutrality, although tests for specific contaminants may be included. Chlorinated hydrocarbons contain stabilizers, added during manufacture; many times, distillation necessitates supplemental inhibition.

The time to replace dirty solvent with clean solvent is determined by the degree of redeposition. Each part placed in a dip solvent comes out of the solvent with a thin film of soil redeposited on its surface. The permissible degree of redeposition determines the practical limit of usefulness of a solvent and the rate at which fresh solvent must be introduced. Alternatively, immersion in sequentially cleaner solvent baths can prolong the useful life of the solvent. In spray wipe applications in which the solvent is aided by strong mechanical action, there is a nearly continuous use of fresh solvent, which is seldom reused.

Each solvent typically has a temperature range where ultrasonic energy optimally agitates it. If the solvent bath is heated too close to the boiling point of the solvent by the sonic energy, the mechanical action diminishes. Control of the bath temperature is important to effective use of ultrasonic cleaning, which is often employed to remove insoluble matter that would need to be filtered from the solvent to maintain cleaning effectiveness.

Tests of cleanness made directly on parts generally are more practical for determining the reclamation point than are measurements of soil buildup in the solvent. Although checking the cleaned item for satisfactory performance in subsequent operations is a practical method for determining whether a required degree of cleanness has been obtained, various other methods of testing for cleanness are also available. In order of increasing degree of cleaning requirements, they are:

1. Visual observation of parts and solvent condition

2. Wiping parts with a clean dry white cloth and then examining the cloth for adhering soil

3. Applying tape to the cleaned surface, removing it, and examining it for adhering soil (Scotch tape test)

4. Tests for the adhesion of paints, ranging from special low-adhesion test paints to conventional paint

5. Microscopic examination of parts

6. Resoaking parts in fresh solvent and weighing the nonvolatile residue

7. Chemical analysis for specific soils

8. Electrical test (on combinations of conductors and nonconductors only)

9. Use of radioactive tracers

Methods from the above list generally are used for specific purposes according to the following table:

Method No.

Purpose of cleaning

1, 2, 3

Preclean only

4

Preparation for paint or adhesive

5, 6, 7, 8

Precision instrument parts

6, 7, 8, 9

Initial studies on precision parts

Drying the Work. Cold cleaning solvents are selected so that the evaporation of the solvent film on parts does not require an excessively long time. In all drying operations, solvent fumes must be exhausted to prevent the possibility of fire, explosion, or health hazards.

Equipment

Pails, tanks, and spray equipment are used in solvent cleaning. Pails with covers are the simplest containers and are often used to contain kerosene, mineral spirits, or chlorinated hydrocarbons for hand brush cleaning or wiping.

Soaking tanks of various designs and sizes are used, depending on the nature of the work. Such tanks may be heated by steam coils, but more often they are used at room temperature. Agitation is sometimes provided by mixer impellers or forced air. For in-process cleaning of small parts, such as those encountered on subassembly lines, a variety of specially made safety tanks are available. Some are designed to permit quick opening and closing by means of a foot pedal, minimizing evaporation and fire hazard. Some are equipped to supply fresh solvent quickly to the work zone and dispense contaminated solvent to another reservoir for subsequent discarding or reclamation.

Small bench sprayers, similar to the unit shown in Fig. 1, are used on assembly lines for cleaning delicate components.

Fig. 1 Spray cleaning equipment

Washing machines also are available for cleaning small precision parts. Some of these machines are similar to home laundry machines in design. Parts are placed on trays, and the agitated solvent provides a constant washing action. In many applications in which the removal of oil and grease is not the main purpose, the equipment is used to remove the residue of polishing or lapping compounds. A filtering system on the machine continuously removes solid particles from the solvent as they are washed from the workpieces.

Equipment requirements for solvent cleaning vary with the size, shape, and quantity of workpieces, as well as the amount of soil to be removed. No matter what equipment is selected, proper covers to minimize solvent loss should be used. Regulations controlling the emissions of smog producing volatile organic compounds require specific designs of cold cleaning equipment and operating procedures in most states. Permits may also be required for construction/installation and operation.

Specific Applications

Solvent cleaning has traditionally been regarded as a method for precleaning or as one reserved for special applications. However, with the rise in the manufacture of electronic components and other assemblies that comprise many small parts, the use of solvents as a final cleaner has increased. At present, most solvent cleaning applications fall within one of the following categories:

• Inexpensive precleaning of parts

• Hand cleaning of parts too large for immersion or spray machine cleaning

• Cleaning heat-sensitive, water-sensitive, or chemical-sensitive parts

• Removal of organic materials such as plating stopoffs, marking crayons, or soldering flux

• Cleaning of precision items in a succession of steps in which the work is first cleaned in nonpolar solvent to remove oil

• Temporary general cleaning where the cost of vapor degreasing equipment is not justified

• Cleaning electrical or electronic assemblies in which the presence of inorganic salt deposits may cause current leakage

Process Limitations

Virtually all common industrial metals can be cleaned in the commonly used cleaning grade solvents without harm to the metal, unless the solvent has become contaminated with acids or alkalis. Cleaning cycles should be adjusted to minimize the immersion time. Certain plastic materials can be affected by cleaning solvents, and tests must be conducted to determine compatibility.

Solvent degreasing is ineffective in removing such insoluble contaminants as metallic salts and oxides; sand; forging, heat treat or welding scale; carbonaceous deposits; and many of the inorganic soldering, brazing, and welding fluxes. Likewise, fingerprints can resist solvent removal.

Size and shape of the workpiece is seldom a limitation. Highly intricate parts have been solvent cleaned by devising techniques of handling that allow the solvent to reach and drain from all areas.

Quantity of Work. Although many high-production applications regularly use cold cleaning, it is more likely to be used for maintenance and intermittent cleaning of small quantities. Because cold cleaning is usually done at or near room temperature, the problem of heating, or otherwise preparing, equipment for a small quantity of work is eliminated. Unless there is some special requirement, other methods of cleaning, such as vapor, alkaline, emulsion, or acid, are usually cheaper and more satisfactory for cleaning large quantities in continuous production.

Lack of uniformity is often a severe limitation of cold cleaning. The process is basically one of dissolving a contaminant in a solvent; therefore, immersion cleaning causes resoiling as the solvent is reused. The work parts do not receive a final rinse in pure solvent as they do in vapor degreasing. The parts are seldom, if ever, perfectly clean. Therefore, except in special applications where spray techniques are used, solvent cleaning is more likely to be used as a preliminary, rather than as a final, cleaning method. The amount of soil that remains on the part depends on how much was there initially and on the quality of the solvent (how often the solvent was reclaimed). In some applications, the use of two or more consecutive solvent baths serves to provide more uniform cleaning results.

Applicability to Soils. The range of soils on which solvents are highly effective is greater than for vapor degreasing because: (a) lower temperatures permit a wider choice of solvents; and (b) lower drying temperatures usually used in solvent cleaning do not bake on insolubles, such as polishing or buffing compounds. Mechanical agitation, ultrasonics, and sometimes hand scrubbing are used in solvent cleaning to help loosen and float away insolubles.

Safety and Health Hazards

Fire and excessive exposure are the greatest hazards entailed in the use of solvents for cleaning. The flash points and permissible vapor concentrations of the solvents adopted for specific operations must be known (Table 1). All flammable solvents should be stored and used in metal containers, such as groundable safety cans.

Adequate ventilation should be provided to prevent accumulation of vapor or fumes. No solvents should be used close to an open flame or heaters with open coils.

Operators should be cautioned against repeated exposure of the skin to solvents. The use of basket, hangers, and other devices that prevent skin exposure is common practice and is recommended. Protective gloves or protective hand coatings should be used to prevent extraction of natural oils from the skin, which can cause cracking of the skin and dermatitis.

Common solvents vary in relative toxicity, and the vapors of these solvents are capable of exerting a potentially lethal anesthetic action when excesses are inhaled. Common solvents have a relatively slight toxic effect, but maintenance workers have lost their lives after working inside tanks containing very high concentrations of vapor, as a result of its strongly narcotic effect. When working in an enclosed space, such as tanks or pits, workers should follow confined space entry procedures.

• Drain and vent thoroughly.

• Check air for adequate oxygen and the absence of flammable or toxic vapor concentrations.

• Always use an air-supplying respirator and life belt.

Any person working with a solvent should be familiar with its material safety data sheet, which can be obtained from the supplier.

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