cleaning, pretreatment & surface preparationTHE DOW CHEMICAL CO., MIDLAND, MICH.Degreasing is an essential part of the modern production process, particularlyin industries fabricating or assembling metal parts including aircraft, appliance,automotive, electronics, and railroad manufacturers. The process is widely usedto remove oils and oil-borne soils, such as chips, metal fines, and fluxes, from

objects that have been stamped, machined, welded, soldered, molded, or diecast.Parts ranging from tiny printed-circuit assemblies to diesel motors canall be safely, completely, and quickly cleaned with modern vapor degreasingtechniques, in most cases more effectively than with aqueous or semiaqueousprocesses, particularly when a high level of cleanliness is required.Vapor degreasing is ideal for reaching into small crevices in parts with convolutedshapes to remove stubborn soils. Parts degreased in chlorinated solventvapors come out of the process dry; there is no need for an additional drying stage,as in aqueous and semiaqueous processes.The chlorinated solvents trichloroethylene, perchloroethylene, and methylenechloride are the solvents most commonly recommended for the vapor degreasing

process for the following reasons:

• The chlorinated solvents have high solvency for organic materials, aswell as good chemical compatibility with a wide range of materials (e.g.,metals, glass, plastics, elastomers, etc.), so that parts consisting of several

materials can be effectively cleaned in these solvents.

• They are virtually nonflammable in most end-use conditions (consultMSDS) because they have no flash point as determined by standard testmethods.

• They have low latent heat of vaporization, resulting in relatively lowenergy requirements.

• They have relatively high stability and are noncorrosive. Further, vapordegreasing grades of the chlorinated solvents contain stabilizing additivesthat prevent the buildup of corrosive material and inhibit the formation

of oxidation products.

• Because the vapors of the chlorinated solvents are heavier than air theycan be contained relatively easily in degreasing equipment.

• The chlorinated solvents can be readily recycled, thus reducing waste andpermitting high solvent “mileage” in a continuous degreasing operation.

• The latest generation of vapor degreasing equipment permits fullyenclosed operation, thus virtually eliminating the loss of solvent vaporsto the environment.

 

The traditional vapor degreasing process is carried out in either a batch or anin-line degreaser. The standard batch degreaser is an open-top tank into whichthe dirty parts (the “work”) are lowered (see Fig. 1). Solvent in the bottom of

 

the tank is heated to produce vapor, and since the vapor is heavier than air itremains in the tank. Cooling coils below the lip of the tank create a cool zone,which forms the upper boundary of the vapor zone.On contact with the cooler work the vapor condenses into pure liquid solvent,which dissolves the grease and carries off the soil as it drains from the parts intothe reservoir of solvent below. The cleaning process continues until the workreaches the temperature of the vapor, at which point condensation ceases andthe work is lifted out of the vapor, clean and dry.The degreasing process may be supplemented by adding a spray lance to theopen-top degreaser so that hard-to-remove soils can be flushed off by the operator.In addition many degreasers also contain one or several immersion tanksbelow the vapor zone so that parts can be lowered into liquid solvent — often ina tumbling basket — before being raised into the vapor for final rinsing.If scrubbing is required to remove heavy oil deposits and solid soils, ultrasoniccleaning can be added by installing transducers in the degreaser. Whenultrasonic energy is transmitted to a solution, it produces cavitation — the rapidbuildup and collapse of thousands of tiny bubbles, which impart a scrubbing

action to the surface of soiled parts.Although the vapor generally stays below the cool zone of an open-topdegreaser there is always some solvent loss. Drafts in the area around thedegreaser will cause solvent vapor to be pulled out. Parts loading causes lossesas work to be cleaned disturbs the solvent/air interface. In addition cleanedFig.

 

1. Typical batch open-top vapor degreaser.

107

parts may also carry solvent with them when removed from the degreaser. Upto 70% of the solvent in a traditional open-top degreaser can be lost throughthese factors over a year. Consequently, procedures are called for to minimizethis loss, in order to provide a working environment in which vapor exposure isbelow the levels permitted by Occupational Safety and Health Administration(OSHA) regulations and an industrial environment, which meets requirementsset by the EPA. These procedures are outlined below.In-line vapor degreasers include several types of conveyorized equipment — large,automatic units, which can handle a large volume of work and are enclosed toprovide minimal solvent loss. These units include the monorail, cross-rod, andvibratory degreasers.The monorail conveyorized degreaser uses a straight-line conveyor to carry

parts into the degreaser, lower them into the vapor zone, raise them into a coolingzone, and finally out of the degreaser. This process is ideal when productionrates are high and large parts to be cleaned can be suspended from hooks orhangers.The cross-rod conveyorized degreaser is generally used for processing smallparts in baskets, trays, or even mesh cylinders. In this equipment the parts are

placed in the degreaser and removed from it at the same opening, while the conveyorcarries the work through immersion dips, vapor zones, and drying zones.The vibratory degreaser is a patented unit. In this process the work is dipped insolvent then rises on a vibrating spiral elevator trough through a counterflowingrinse of clean solvent distillate, a vapor zone, and finally a drying section.Although these units are enclosed there is still some solvent loss through theopenings where work enters and leaves the equipment, and through the jointsand seams of the equipment.

 

For many years 1,1,1-trichloroethane (methyl chloroform or MCF) was the solventof choice for vapor degreasing because of its status as a non-VOC; it was notclassed as a VOC compound under federal regulations because it does not contributeto the generation of ground-level ozone and the consequent productionof urban smog. This situation changed drastically at the end of December 1995,

when MCF was phased out of production for all uses except chemical feedstock,under the provisions of the U.S. Clean Air Act and the international MontrealProtocol, due to its action as a depleter of stratospheric ozone.

This led to a misunderstanding in many quarters that all chlorinated solventswere coming under a ban. In fact corporate edicts in some companies requiredtheir manufacturing divisions to cease using chlorinated solvents for degreasingas soon as practicable. This unfortunate misunderstanding has led a numberof companies to experiment with a variety of alternate cleaning technologies.The fact is, chlorinated solvents are still very viable products and remain oneof the best choices for surface cleaning and vapor degreasing. Trichloroethylene,perchloroethylene, and methylene chloride are not regulated for ozone-depletingpotential (ODP). In fact these three solvents have been approved under the U.S.Environmental Protection Agency’s Significant New Alternatives Policy (SNAP)as replacements for 1,1,1-trichloroethane. When the EPA published its SNAP

ruling for ozone depleting substances on March 18, 1994 (see Federal Register 59FR 13044-13161), it gave industry the official go-ahead to consider the three

chlorinated solvents as acceptable alternatives to 1,1,1-trichloroethane in surface

cleaning as well as other applications.This policy also pointed out that worker exposure and environmental emissionsof these solvents should be controlled properly and in accordance withother workplace, environmental, and consumer regulations established by theEPA and other agencies. The policy is particularly applicable, however, in caseswhere nonflammability is a critical prerequisite for safety and where effects onpersonal health and the environment are reduced to a minimum by engineeringand operating design.Each of the three chlorinated solvents has its own advantages for specificapplications, based on its physical profile (see Table I for physical properties).Trichloroethylene (TCE) is a clear, heavy liquid (12.11 lb/gal) with excellentsolvency. Long recognized for its cleaning power, TCE boils at 189oF (87oC)and freezes at -124oF (-86.7oC). The high density of TCEs vapor (4.53 times thatof air) assures low vapor loss and easy recovery from vapor degreasing systems.TCEs aggressive solvent action works well on the oils, greases, waxes, tars,

lubricants, and coolants generally found in the metal processing industries. Itis especially effective in removing difficult soils such as semicured varnish orpaint films, heavy rosins, and buffing compounds.

Perchloroethylene (PCE or perc, also called tetrachloroethylene) is a clear, colorlessliquid with a distinctive, somewhat ether-like odor. It has the highest boilingpoint (250oF, 121.1oC) and freezing point (-9oF, -22.8oC), weight (13.47 lb/gal),and vapor density (5.76 times that of air) of the chlorinated solvents.The high boiling point of PCE makes it especially effective in removinghigh-melting pitches and waxes and for cleaning grossly contaminated parts.The high temperature of PCE vapors also permits complete and thoroughdrying of work by vaporizing moisture entrapped in porous metals, deeplyrecessed parts, and blind holes.

Methylene chloride (MEC, also called dichloromethane) is a powerful and ver-Table I: Physical Properties of Chlorinated Solventsa

Properties Trichloriethylene Perchloroethylene Methylene Chloride

Chemical formula C2HCI3 C2Cl3 CH2CI2

Molecular weight 131.4 165.8 84.9

Boiling point 189oF 250oF 103.5oF

87oC 121.1oC 39.7oC

Freezing point –124oF –9oF –139oF

–86.7oC –22.8oC –95oC

Specific gravity 1.456 1.619 1.32

Density (lb/gal) 12.11 13.47 10.98

Density (kg/L) 1.456 1.619 1.32

Vapor density 4.53 5.76 2.93

Viscosity 0.54 0.84 0.41

Flash point None None None

Flammable limits 8-9.2% None 14-22%

(volume of solvent in air) (saturation)

Kauri butanol value 129 90 136

Solubility (g/100g)

Water in solvent 0.04 0.0105 0.17

Solvent in water 0.10 0.015 1.70

satile chlorinated solvent known for its high solvency capabilities. MEC has thelowest boiling point (103.5oF, 39.7oC) and freezing point (-139oF, -95oC) of thechlorinated solvents, as well as the lightest vapor density (2.93 times that of air)and weight (10.98 lb/gal). Because of its low boiling point MEC is often used fordegreasing sensitive parts such as thermal switches and thermometers, whichwould be damaged by high temperatures. It is also chosen when parts must benear room temperature after cleaning for immediate handling or for tolerancetesting and measurements.

These three solvents are widely used in surface cleaning, particularly in thevapor degreasing process. They are also used in cold cleaning, both dip and wipemethods, but the need to keep workplace vapor levels and environmental vaporlosses low, in accordance with federal, state, and local regulations, limits theiruse in cold processes.

 

Health effects from exposure to chlorinated solvents have been studied extensively.Exposure to vapor concentrations within recommended guideline levelswill not result in any known adverse effects on most people. Acute overexposureto vapors, however, may cause anesthetic or narcotic effects (solvent drunkenness)and death at high enough concentrations. Central nervous system effectsand liver and/or kidney effects can result from chronic overexposure.Proper ventilation when using chlorinated solvents is essential. Because chlorinatedsolvent vapors are heavier than air, high concentrations can accumulatein poorly-ventilated and low-lying areas, such as pits, causing dizziness, unconsciousness,and eventually death.The chlorinated solvents have been subjected to a great many animal tests aswell as epidemiological studies on humans to determine their health profile andso far the available scientific data indicate that they are not human carcinogens.On the basis of animal tests, however, the chlorinated solvents, like manyother chemicals, have been given cancer classifications by different agencies.The U.S. EPA classes all three solvents as B2, “Probable Carcinogen,” while theAmerican Council of Governmental Industrial Hygienists (ACGIH) classes PCEand MEC in Category A3, “Animal Carcinogen,” and TCE in Category A5 “notsuspected as a human carcinogen.” All three solvents are listed under California’sProposition 65 as “Known to the State of California to cause cancer.”

On the international scene the International Agency for Research on Cancer(IARC) places TCE and PCE in Group 2A, “probably carcinogenic to humans”and MEC in Group 2B, “possibly carcinogenic to humans.” The German MAKcommission lists PCE and MEC in Category IIIB, “possible carcinogen” and TCEin Category IIIA, “human carcinogen.”

Users of chlorinated solvents for parts cleaning are aware of the regulationsaffecting the use, handling, transportation, and disposal of these solvents.They are regulated at the U.S. federal level under the Clean Air Act, the CleanWater Act, Occupational Safety and Health Administration (OSHA) rulings,the Resource Conservation and Recovery Act (RCRA), and the ComprehensiveEnvironmental Response Compensation and Liability Act (CERCLA), as well asunder state and local regulations aimed at controlling emissions.Fortunately, these regulations are manageable because a great deal of assis-

Regulation Trichloroethylene Perchloroethylene Methylene Chloride 1,1,1-Trichloroethanea

Ozone depletion (CAAA, Montreal Protocol)

VOC (Votatile Organic Compounds)

state-to-state differences

Permits

CAA Title V

HAP (Hazardous Air Pollutants NESHAP

Clean Water Act

OSHA

PEL (Permissible Exposure Limit)

RCRA (Resource Conservation and

Recovery Act Hazardous Waste)

CERCLA (Superfund reportable quantities)

No

Yes

<100 tpy marginal1

<50 tpy serious

<25 tpy severe

<10 tpy extreme

Yes

10 tpy or combination of

25 tons HAP

Yes

Yes

Yes

50 ppm5

Worker right to know

Training

Record keeping

Reporting

Yes

<100 kg: Cond. exempt

100-1,000 kg: Small

quantity generator

>1,000 kg: Large quantity

generator

Yes

100 lb Reportable quantity

spill

No

No2

Yes

10 tpy or combination of

25 tons HAP

Yes

Yes

Yes

25 ppm6

Worker right to know

Training

Record keeping

Reporting

Yes

<100 kg: Cond. exempt

100-1,000 kg: Small

quantity generator

>1,000 kg: Large quantity

generator

Yes

100 lb Reportable quantity

spill

No

No3

Yes

10 tpy or combination of

25 tons HAP

Yes

Yes

Yes

25 ppm7

Worker right to know

Training

Record keeping

Reporting

Yes

<100 kg: Cond. exempt

100-1,000 kg: Small

quantity generator

>1,000 kg: Large quantity

generator

Yes

100 lb Reportable quantity

spill

Yes

No4

Yes

10 tpy or combination of

25 tons HAP

Yes

Yes

Yes

350 ppm

Worker right to know

Training

Record keeping

Reporting

Yes

<100 kg: Cond. exempt

100-1,000 kg: Small

quantity generator

>1,000 kg: Large quantity

generator

Yes

100 lb Reportable quantity

spill

a1,1,1-Trichlorothane included for comparison.

1In areas of the country classed as having marginal smog problem up to 100 tpy emissions are allowed to a facility. Allowable emissions are lower in areas with more serious problems.

2Perchloroethylene listed as exempt February 7, 1996, Federal Register, 4588.

3Methylene chloride listed as exempt July 8, 1977, Federal Register, 35314

41,1,1-trichloroethane listed as exempt July 8, 1977, Federal Register 35314

5This limit in OSHA’s 1989 rule has been overturned. dow continues to use these 1989 PELs in MSDS and labels.

6This limit in OSHA’s 1989 rule has been overturned. dow continues to use these 1989 PELs in MSDS and labels.

7Currently challenged by industry ACGIH recommends 50 ppm, and Dow supports this limit.

tance is available to help companies reach full compliance.The following are the federal regulations governing the chlorinated solvents(see Table II). The Clean Air Act Amendments (CAAA) of 1990 legislated the phaseoutof 1,1,1-trichloroethane as an ODS as of December 31, 1995.VOC regulations under the Act apply to TCE and limit its emissions, particularlyin ozone-nonattainment areas, in order to reduce smog formation. Exactrequirements vary by state but generally include obtaining a permit allowing aspecific amount of VOC emission from all sources within a facility.The CAAA also calls for MEC, PCE, and TCE to be regulated as hazardousair pollutants (HAPs), but a complete set of regulations has not yet been issuedfor their control. EPA has, however, issued National Emission Standards forHazardous Air Pollutants (NESHAP) for solvent cleaning with chlorinated solvents(Federal Register, Vol. 59, No. 231, 61801-61820). Other NESHAPs have beenissued governing dry cleaning with PCE and the use of MEC in aerospace manufactureand rework, while NESHAPs governing the use of chlorinated solventsin wood furniture manufacture and asbestos brake cleaning are still awaited.Clean Water Act. The Federal Clean Water Act defines chlorinated solvents astoxic pollutants and regulates their discharge in to waterways.The Occupational Safety and Health Administration (OSHA) has set permissibleexposure limits (PELs) for chlorinated solvents based on an 8-hour time-weightedaverage (TWA). The PEL for MEC is 25 parts per million (ppm), for PCE 100ppm, and for TCE 100 ppm.OSHA also specifies a minimum element of training for people working withthe solvents. This includes how to detect the presence or release of a solvent, thehazards of the solvent, and what protective measures should be used.OSHA’s Hazard Communication (HAZCOM) standard regulates the labelingof all hazardous chemicals. Labels must contain a hazard warning, the identityof the chemical, and the name and address of the responsible party. Guidelinesare provided by an OSHA compliance document [OSHA Instruction No. CPL-2-2.38 C (1990)] and by the American Standards Institute (ANSI) publicationon precautionary labeling (ANSI Z129.1-1994).Under the federal Resource Conservation and Recovery Act (RCRA), wastes containingchlorinated solvents from solvent cleaning operations must be consideredhazardous waste. Generators, transporters, and disposers of such hazardouswaste must obtain an EPAID number.According to the Comprehensive Environmental Response, Compensation andLiability Act (CERCLA or Superfund), if are portable quantity of a chlorinatedsolvent is released into the environment in any 24-hr period, the federal, state,and local authorities must be notified immediately. Reportable quantities are1,000 lb of MEC and 100 lb of PCE or TCE.This is only a checklist of regulations. It is important to confer with yourenvironmental consultant or your legal counsel to determine just how theseregulations apply to your business.Other sources of help and advice are also available. For example the CAAA mandateda Small Business Ombudsman under EPA and a Small Business AssistanceProgram (SBAP) in each state. State SBAPs provide a Small Business Ombudsmanand a Technical Assistance Director to facilitate communications between the EPAand small businesses and to provide information on new and existing environmentalregulations and policies. To qualify as a small business, a company must havefewer than 100 employees and must not be “dominant in its field.” The federal

Small Business Ombudsman provides literature and a toll-free hot-line to answerquestions. Primary assistance for a business comes from the state SBAP office, anda small business can find out who to contact on a state level by calling the federalSmall Business Ombudsman’s hotline: 1-800/368-5888.The Halogenated Solvents Industry Alliance (HSIA), a trade association ofproducers of chlorinated solvents, provides legislative and regulatory news forthe solvent industry, sponsors research on chlorinated solvents, and presentsinformation from research to the EPA and other solvent regulators. Users ofchlorinated solvents can obtain information from this organization, as well asliterature on the use of the solvents and how to comply with regulations, by callingHSIA at 202/775-0232.Help is also available from the producers of chlorinated solvents. Producersand distributors are required by law to provide a MSDS, containing completeinformation on safety and handling, to all customers. In addition many producersalso supply other forms of assistance.

 

Emission standards for chlorinated solvent degreasing operations are nowgoverned by EPA’s NESHAP for new and existing halogenated solvent cleaningoperations. These standards cover both vapor degreasing and cold cleaningwith TCE, PCE, and MEC, as well as with chloroform. The goal of the NESHAPregulation is an overall reduction in solvent emissions of 50 to 70% of currentnationwide emissions.The NESHAP provides a number of control procedures for reducing solventemissions from equipment. Operators of batch vapor degreasers and in-linecleaning machines may choose from a series of combinations of two or three ofthe procedures, which include:

• Freeboard ratio of 1.0: The height of the freeboard above vapor level mustbe equal to the width (shorter dimension) of the degreaser.

• Freeboard refrigeration device: This is a refrigerated system that supplementsor replaces the traditional water cooling system and creates a coldair blanket above the vapor zone.

• Reduced room draft: Wind speed above the freeboard must not exceed50feet per minute (15.2 m/min).

• Working-mode cover: This is defined as any cover or machine design thatshields the cleaning machine from outside air disturbances during theparts cleaning cycle.

• Dwell: This refers to the time in which cleaned parts remain in the freeboardarea above the vapor zone after cleaning. The EPA defines properdwell time as 35% of the time required for the parts to cease dripping in

the vapor zone.

• Superheated vapor (vapor temperature maintained 10oF above the boilingtemperature of the solvent): This promotes more thorough drying ofthe work before it is removed from the degreaser.

• Carbon adsorption equipment in the ventilation system connected withthe degreaser.In addition vapor degreasing operators must employ an automated hoist orconveyor that carries parts at a controlled speed of 11 fpm or less through thecomplete cleaning cycle.Each of these procedures used alone will reduce solvent loss by a respectableamount while combining two or three procedures reduces loss even further. Forexample creating a freeboard ratio of 1.0 in an open-top degreaser will reduceloss by 30 to 40% over the traditional process. Combining an increased freeboardratio with superheated vapor or reduced room draft increases the control levelto 60%. And in most cases, if a user combines three of the procedures, this willbring the control up to 70%. Table III illustrates how these incremental improvementscan be brought about.Many companies have found it cost effective to adopt one of the new degreasers,which have no air/vapor interface. These sealed and virtually emissionlessunits were first introduced in Europe to meet the stringent environmental

regulations of some countries (see Fig. 2).Typically these degreasers perform the cleaning operation in a sealed chamberinto which solvent is introduced after the chamber in closed. Solvent vapor isintroduced as the final rinse and all vapors are exhausted after each cycle andpassed into a solvent recovery system. With the sealed chamber control of solventloss exceeds 90%; in other words virtually no solvent escapes.Programmed automated operation permits a variety of cleaning programsincluding cold or warm solvent dipping, as well as vapor degreasing. Solventrecovery cycles make use of advanced methods of carbon adsorption and hot air

desorption. The manufacturer of one such unit, a “closed open-top degreaser”with a large cleaning chamber, claims that solvent emission losses average less

Fig. 2. Totally enclosed vapor degreaser can meet stringent environmental regulations.than 100 lb/yr. Solvent concentrations in the work area of the “closed open-top”unit average between 5 and 10 ppm, well below the permissible exposure limitsset by OSHA for the chlorinated solvents.Although these “emissionless” units can be costly, a number of plants in theU.S. have found them economical because they provide excellent compliancewith safety and environmental regulations,conserve solvent, save floor space,and provide excellent parts cleaning performance. Several brands of these “emissionless”

degreasers are available in North America today.Most producers and distributors of chlorinated solvents provide support forsolvent users. The Responsible Care initiative of the Chemical Manufacturers

Association, to which all solvent producers subscribe, and the ResponsibleDistribution code of the National Association of Chemical Distributors require

members to share product stewardship information, safety training, and regulatorydata with customers. When selecting a supplier be sure to review what kind