United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-93/191 July 1994
EPA Project Summary
Minimizing Pollution in Cleaning
and Degreasing Operations
S. Garry Howell and A.R. Tarrer
As part of an effort to prevent or
minimize waste, the U.S. Air F:orce stud-
ied ways to extend the useful life of
cleaning solvents, particularly 1,1,1,-
trichloroethane (TCA). The use of addi-
tives such as acid acceptors, oxidation
inhibitors, and metal stabilizers are ex-
plained as solvent extenders. Because
knowing the condition of the solvent is
important and because visual inspec-
tion is unreliable, several tests (acid
acceptance value [AAV], viscosity, wa-
ter content, visible absorbance, non-
volatile matter [NVM], and electrical
conductivity) are suggested to analyze
for stabilizer content and to estimate
the remaining useful life.
When these tests indicate there is
some useful life left, filtration, replen-
ishing stabilizers, distillation, ultrafil-
tration, and more efficient operation are
explained as means to return an al-
most spent solvent to useful life.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
U.S. Air Force overhaul and mainte-
nance operations use large volumes of
solvents for vapor cleaning and degreasing
aircraft, including engine parts, gyroscopes,
guidance systems, and electronic compo-
nents. A major portion of these solvents
are chlorinated compounds such as TCA,
methylene chloride, and perchloroethylene
(perc). During normal use, a portion of
these materials are lost through evapora-
tion and are thought to contribute to strato-
spheric ozone depletion. Further, the
cleaning solvents are discarded after
reaching a point where grease and dirt
build up in cleaning baths. Reducing
evaporation losses and the amount of
spent solvent to be disposed would save
the value of the solvent, requiring fewer
purchases of fresh solvent, as well as
disposal costs.
As part of the implementation of its
waste minimization program, the U.S. Air
Force is trying to find the most efficient,
safe, and economical methods to extend
the useful life of, or to recycle, cleaning
solvents, particularly TCA. Other, though
less widely used, solvents considered for
recycling are methylene chloride, perc,
CFC-113 (trichlorotrifluoroethane) and PD-
680 (a petroleum fraction similar to the
well-known Stoddard solvent). As part of
this research project, Auburn University,
in cooperation with major solvent manu-
facturers, developed a program to develop
fairly simple methods based on key physi-
cal and chemical properties so that users
could get the longest useful life from their
cleaning baths. Then, after testing deter-
mines that the solvent is no longer useable,
two options are offered. Either critical ad-
ditives may be replenished or the solvent
may be recycled by distillation, filtration,
or other means.
Vapor degreasing is a common indus-
trial process in which the parts to be
cleaned are suspended in the vapor space
above a boiling bath of solvent. The most
common solvents for such operations are
Printed on Recycled Paper
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TCA and perc. Because the vapors are
much heavier than air, a properly designed
and operated bath will confine most of the
vapors in the "head space" and very little
will be lost to the atmosphere. During the
course of operation, however, solvents in
a Vapor degreaser undergo both chemical
and physical changes and solids (such as
metal chips, dirt, or paint particles) build
up in the boiling sump liquid. The oils and
greases in solution in the liquid phase
may build up to the point where the vis-
cosity is increased and heat transfer is
seriously impaired. In electrically heated
baths, heater sheath temperatures may
increase and cause failure. In addition, as
the concentration of lighter oils increases,
they will appear in the vapors and will
leave a noticeable residue on the "cleaned"
parts. A chemical analysis aimed only at
the composition of the solvent itself may
show that it is still useable, but its effi-
ciency has been severely impaired by dis-
solved material. At least part of this
efficiency may be restored by the physical
methods described in this report.
A number of previous studies have
shown that efficient operation of degreaser
baths can also reduce solvent usage and
minimize emissions. A separate, but re-
lated, part of this research study investi-
gated the operation of several vapor
degreasing units at Tinker AFB in Okla-
homa. A survey, made by a representa-
tive of the Dow Chemical Co., resulted in
recommendations for changes which would
result in reduced solvent loss.*
Additives to Maximize Solvent
Life
As partially chlorinated solvents, TCA,
methylene chloride, and perc have a ten-
dency to decompose after prolonged heat-
ing. This is manifested by a chlorine atom
breaking off the hydrocarbon backbone to
form hydrogen chloride (HCI). The HCI
then combines with any water present to
form hydrochloric acid which corrodes
metals. (Fluorine, as in CFC-113, is more
tightly bound, and less likely to be re-
leased). The HCI itself promotes further
dechlorination; this is inhibited by addi-
tives called acid acceptors, which are de-
signed to combine with and neutralize HCI.
In addition to dechlorination, relatively
high temperatures can lead to oxidation of
the solvents. Many halogenated solvents
• Momion of trade names or commercial products does
not constitute endorsement or recommendation for
use.
are relatively resistant to attack by oxygen
until a halogen (chlorine, fluorine, etc.) is
abstracted. Removal of the chlorine (or
fluorine as in the case of CFC-113) and
an adjacent hydrogen leaves a "double
bond," which is susceptible to oxidation,
particularly in the presence of metals such
as copper or vanadium. Perc and trichlo-
roethylene have these double bonds al-
ready in place and, consequently, are more
susceptible to oxidation. Another class of
stabilizers called oxidation inhibitors, or
antioxidants, may be added to prevent
this.
The interaction between acid and metal
can etch or corrode a metal surface. Con-
versely, some metals, such as copper and
vanadium, promote oxidation which re-
quires some way to protect the metal from
the solvent. Metal stabilizers can do this
by forming a protective film that retards
attack by acids, while deactivating the cata-
lytic properties of the surface.
The concentration of these metal stabi-
lizers in halogenated solvents is critical,
particularly the ratio of metal stabilizer to
acid acceptor. Although other stabilizers
decompose chemically or react with acids
and become depleted, metal stabilizers
are intentionally chosen to be somewhat
volatile so they maintain some presence
in the vapor phase. An almost monomo-
lecular layer is formed on the metal sur-
faces so, as the parts are withdrawn, the
adherent stabilizer is consumed.
Methods to Determine a
Solvent's Useful Life
The full report lists typical acid accep-
tors, oxidation inhibitors, and metal stabi-
lizers. Visual inspection alone cannot
determine the concentration of these ad-
ditives in the baths nor can it identify other
possible deleterious conditions. Visual in-
spection is unreliable as a means of tell-
ing the condition of degreasing solvents:
a perfectly clear bath could be deficient in
inhibitor and a badly discolored one can
still be well stabilized. Although the most
reliable method of determining the levels
and types of stabilizers in halogenated
solvents is by gas chromatography/mass
spectrometry (GC/MS), its expense makes
it impractical for most users. There are,
however, several reliable methods for ana-
lyzing for stabilizers and for estimating the
remaining useful life of a degreasing bath.
When in doubt, two or more of the tests
listed below should be used.
Acid Acceptance Value
The acid acceptance value (AAV), the
most important parameter, is an accurate
measure of stabilizer content. It is nor-
mally measured by a simple titration (see
Figure 1).
Water Content
Water in halogenated solvents is detri-
mental in two respects: it promotes corro-
sion at metal/water interfaces and it causes
hydrolysis of the halogen-carbon bond re-
leasing chlorine or fluorine. The corre-
sponding corrosive acids, HCI and HF,
are then formed.
Visible Absorbance
Color changes can be quantified with
the use of a spectrophotometer to mea-
sure light transmission at a set wavelength.
Non-Volatile Matter
Evaporating a solvent sample over a
steam bath and weighing the residue indi-
cates the content of oil, grease, and dirt.
Electrical Conductivity
Ionic materials and water decrease the
electrical conductivity of halogenated sol-
vents. Conductivity measurements are
relatively easy to make with inexpensive
meters.
Viscosity
Viscosity is a good indicator of dissolved
oil and grease content. Several methods
may be used for its determination (e.g.,
an inexpensive, simple, falling ball vis-
cometer).
Returning the Solvent to a
Useable Condition
As a degreasing or cleaning bath is
used and dissolved oil and grease, dirt,
metal particles, and water accumulate,
some steps can be taken to return it to a
useable condition. One or more of the
above tests might indicate that there is
some useful life left and that filtering the
solvent and monitoring changes in its prop-
erties may be a feasible alternative before
prematurely discarding the solvent. In an
experimental filtration setup, the solvent
was pumped from a stirred tank through a
series of three filters: the first, a bag to
remove very coarse (25 |o.m) particles, then
through 1.0 and 0.5 (xm filters (see Figure
2). Pumping continued until the tank's con-
tents were passed through the filters ap-
proximately five times, a single cycle was
about 10 min. Samples were taken after
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Solvent condition
borderline
- Check daily
Solvent condition
unacceptable
- Discard or distill for reuse
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Acid acceptance as weight percent sodium hydroxide
Figure 1. TCA calibration chart.
Feed
tank
Circulate for 5 cycle times
Bag
filter
1' \m
filter
0.5/MI
filter
Filtration system
Figure 2. Filtration scheme for reclaiming solvents.
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each cycle to monitor the condition of the
solvent. Filters were cleaned when the
differential pressure across them reached
15 psi. Some test results from these filtra-
tion experiments are given below.
Filtration of 1,1,1-TCA
Acid Acceptance Value
The presence of acid acceptors (as
measured by AAV) was not affected by
filtration; five passes through the system
left the AAV almost unchanged at three
times the minimum value. This is in con-
trast to distillation or adsorption on car-
bon, both of which decrease AAV.
Water Content
As stated before, the presence of water
Is deleterious to the solvent and to parts
being cleaned. If the starting content was
above 0.5%, repeated filtration cycles ac-
tually reduced it. When the initial concen-
trations were less than 0.5%, water content
remained almost unchanged despite the
number of passes through the filtration
system. The reason for this is unclear, but
may possibly be due to the fact that water
solubility in TCA is about 500-600 ppm
(0.05-0.06%), and above this level, the
water exists as a dispersion. At higher
water contents, filtration would cause some
coalescence of droplets, causing them to
float to the surface of the feed tank and
evaporate. Whatever the mechanism, fil-
tration done in this manner does remove
some of the water.
Visible Absorbance
New TCA is water white, and its clarity
is decreased by either dissolved materials
or suspended particles. A portable spec-
trophotometer was used to measure the
light (580 nm) absorbed or scattered by
the solvent samples; this is reported as
percent transmittance. Although water
droplets would always be expected to have
some effect on the light transmitted (or
absorbed), the presence of water soluble
or wettable impurities can affect absor-
bance. Dissolved colored waxes, oils, and
grease as well as suspended particles of
dirt, etc. also lower light transmission. Ab-
sorbance measurements should always be
used in tandem with at least one other
test, as it is possible that a clear, clean
looking solvent could be completely de-
void of acid acceptors or other stabilizers.
Non-Volatile Matter
Weighed samples of contaminated sol-
vent were evaporated under vacuum (un-
specified) at 105 +/- 5°C. The NVM
remained almost constant during all the
runs. Measuring the solids content by fil-
tering the samples, then drying the filter
paper at 60°C gave higher values than
the NVM; this was attributed to volatiliza-
tion of lighter oils in the vacuum oven.
Evaporating the samples on a steam table
(the preferred method) would probably give
higher results than the filter paper method,
particularly if dissolved solids were present.
Electrical Conductivity
Electrical conductivity tests made after
every pass through the filtration system
showed very little variation, however com-
parison to virgin solvent indicated con-
tamination by some conductive material.
The small amount of water dissolved in
the TCA (the literature values are 500-
600 ppm) could carry minute amounts of
HCI or salts, which would raise the con-
ductivity well above the 10'10 mhos/cm of
the new material. Filtration, as stated pre-
viously, will not remove dissolved materi-
als, so little or no effect on conductivity
would be expected.
CFC-113 Filtration Tests to
Remove Water and Solids
Tests were conducted on CFC-113 to
determine if simple filtration is effective in
removing water and solids, thereby ex-
tending the useful life of the solvent.
Samples of used CFC-113 (trichlorotrifl-
uoroethane) with high water and high sol-
ids contents were reclaimed by filtration at
Warner Robins Air Logistics Center (ALC).
Water content was reduced by about 90%,
to very nearly the solubility level of water
in CFC-113, which is 0.009%. Solids were
almost completely removed, indicating that
most solids were particles > 0.05 urn.
Since CFC-113 is primarily used for clean-
ing delicate precision parts, base person-
nel thought that the filtered solvent would
be usable for preliminary cleaning, if fol-
lowed by a rinse with virgin solvent.
Replenishing Stabilizers
If the AAV or GC/MS analysis indicates
that a solvent is deficient in stabilizer(s)
but still acceptable as a cleaner, two op-
tions can be used to restabilize it. The
first, easiest, and safest is to buy new
solvent containing higher-than-usual lev-
els of stabilizer and add this to the old
bath in sufficient quantity to raise the sta-
bilizer to a safe level. The second option,
which requires more skill and training, is
to buy stabilizers (often including acid ac-
ceptor, metal stabilizer, and oxidation in-
hibitor) and add them as needed. Careful
analysis is required to determine the
amount to be added, and safety precau-
tions must be strictly observed, as the
chemicals (see the table below for some
examples) are flammable, toxic, and pos-
sibly carcinogenic. For these reasons, only
those knowledgeable in and adept at han-
dling chemicals should attempt to make
up their own stabilizer packages.
Distillation
If, after filtration, the level of dissolved
oils and grease is too high, one of the
best options for reclaiming solvents of all
types is distillation. Properly done, distilla-
tion can yield the base solvent in almost
virgin condition. Petroleum solvents, such
as Stoddard Solvent, or PD 680, might
require vacuum to lower their boiling points,
but otherwise they are unaffected by the
process. TCA, methylene chloride, and
perc all boil at 250°F or below and may
easily be handled in either steam or elec-
trically heated stills at atmospheric pres-
sure. The major disadvantage of distilling
halogenated solvents is that the stabiliz-
ers could be partially retained in the still
bottoms and the condensate might be
understabilized. Therefore redistilled sol-
vents should be analyzed to confirm sta-
bilizer levels.
Table 1. Typical Inhibitor/Stabilizers and Their Functions
Function Compound
Acid acceptor
Acid acceptor
Metal stabilizer'*
Oxidation inhibitor
Butylene oxide
(1,2-epoxybutane)
Epichlorohydrin
(1,2-epoxy-3-chloropropane)
1,4-Dioxane
(1,4-dioxacyclohexane)
/one/™", BHT
(2,6-di-t-butyl-p-cresol)
* Flammable and classed as possible carcinogen.
+ Fatty acid amines are sometimes used; these give some rust inhibition after parts are
exposed to air.
++ tono/ and BHT (Shell Chemical Co.) are illustrative of a broad class of phenolic antioxidants;
they may not be suitable for all types of degreasing baths.
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Many stills have been installed, tried a
few times, and abandoned presumably
because they are difficult to operate. These
could quite likely be reactivated with more
thorough operator training and retrofitting
with modern automatic control systems.
An excellent example of this is the opera-
tion at Warner Robins ALC, where distilla-
tion has been successfully carried out
since 1982. In 1987, an estimated
$800,000 was saved on CFC-113 alone.
Personnel at the base found that redis-
tilled TCA usually retains enough acid ac-
ceptors to be safely used, but as a
precaution, the personnel keep some 1,2-
butylene oxide on hand for addition to the
reclaimed solvent.
Ultrafiltration
Conventional filtration will normally re-
move particles of > 0.5 u.m. When par-
ticles smaller than 1 urn are to be removed,
polymeric membranes may be used in-
stead of the solid media of ordinary filters.
Some newer membranes are resistant to
many solvents and are capable of retain-
ing molecules larger than 150 to 200
daltons (molecular weight). This means
that Stoddard solvent (ca. 150 to 160
daltons) or TCA (133.4 daltons) would pass
through, but dissolved oils and greases
would be retained. The higher molecular
weight inhibitors such as butoxy-
methyloxirane (130.2 daltons) would also
pass through the membrane. Membrane
filters should always be preceded by a
conventional filter to minimize fouling. De-
velopment of these solvent-resistant mem-
branes, as well as those resistant to acids
and bases, is continuing, and membrane
separations are expected to replace distil-
lation in many instances.
Minimizing Pollution by More
Efficient Operation
The period between rejuvenation or re-
cycling solvents can be greatly increased
by more careful operation of degreasing
operations. A technical and economic fea-
sibility investigation was made of several
methods of reducing solvent use and am-
bient vapor concentrations from a TCA
vapor degreaser at Wright Patterson AFB.
The recommendations resulted in $25,2007
yr savings in solvent costs with a payback
period of 0.6 yr. In addition, almost 4000
gallons of TCA did not escape to the at-
mosphere annually.
A similar survey of the various
degreasing operations at Tinker ALC was
made by a representative of Dow Chemi-
cal Co., a major solvent supplier; follow-
ing those recommendations should save
about $68,000/yr. Although the recommen-
dations referred to specific units at Tinker,
they are applicable to many operations.
The complete survey is too lengthy to be
included in this summary, but an edited
version is included in the report summa-
rized here.
Conclusion
The life of solvents employed to clean
and degrease aircraft engine parts can be
successfully extended by using additives
such as acid acceptors, oxidation inhibi-
tors, and metal stabilizers. When the sol-
vent is no longer useful, it can be
effectively rejuvenated by filtration, distil-
lation, and more efficient operation.
The full report was submitted in fulfill-
ment of Contract No. EPA CR816219 by
Auburn University under the partial spon-
sorship of the U.S. Environmental Protec-
tion Agency; the U.S. Air Force supplied
most of the funding.
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S.G. Howellis with the U.S. Environmental Protection Agency and A.R. Tarrer
Is with Auburn University, Auburn, AL 36849-5127
Joseph D. Wander was the Air Force Project Officer (AFCESA, TyndallAFB);
Mary Ann Curran is the EPA Project Officer (see below).
Ths complete report, entitled "Minimizing Pollution in Cleaning and Degreasing
Operations" (Order No. AD-A-277094; Cost: $17.50, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
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EPA/600/SR-93/191
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