Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a method for stripping organic
coatings from coated objects. More particularly, the present invention
concerns a method for strippinX a coating obtained from compositions based on
organic resins and/or prepared with organic vehicles, such as paint~ shellac,
varnish, lacquer and the like, as well as various oils and asphalts. Lhe
method of the invention is especially useful for removing such coatings Erom
objects having irregular surfaces and from large surfaces, including vertical
and inclined surfaces in the interior of large constructions, such as storage
bins and tanks on land and the holds and ballast tanks of ships. ~ -
Commonly, paint is stripped from painted objects by application of
an organic or inorganic solvent or mixture thereof. As discussed in Kirk-
Othmer's ENCYCI,OP~DIA OF CHEMICAL TECHNOLOGY, Vol. 14, pp. 485-493, 2nd
Edition, John Wiley and Sons, 1967, organic paint removers generally fall into
three classes: compositions ba6ed on chlorinated hydrocarbon solvents,
compositions consisting of mixtures of other solvents and removers based on
aqueous solutions or dispersions of phenols and/or organic acids and other
compounds. Inorganic strippers, such as an aqueous solution of caustic soda
and in some cases, mineral acids are also used, particularly for industrial
applications.
Among the chlorinated hydrocarbon solvents, methylene chloride (di-
chloromethane) has been found to be particularly efective and formulations of
methylene chloride suitable for application by dip, brush, spray and delivery
Prom aerosol cans are known. Such compositions also usually con~ain additives
including thickeners, evaporation retarders and detergents.
Organic solvent formulations for stripping paint and other coatings
may be of the "scrape off" type or "flush off" type. Generally, the stripping
composition is applied to the coated objeot by one of the foregoing methods
and allowed to stand for some time, after which, the coating which has become
swollen and/or softened is removed from the surface, by scraping, in the case
`30 of "scrape off" formulations or by flushing with water and/or by wiping with a
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damp rag in the case of "flush off" formulations.
The foregoing methods are relatively expensive, since the organic
solvent, except in the case of application by immersion, is not in a form
which can be recovered practically. Moreover, all of the foregoing procedures
are generally imp~actical and prohibitively expensive where large surfaces are
involved. In addition, extreme safety measures would be required to effective-
ly treat large surfaces by any of the foregoing methods; the measures necessary
to protect personnel from stripping chemicals, many of which are exceedingly
toxic, essentially prohibit their use for stripping large objects. Another
important problem with the foregoing procedures is the difficulty of completely
removing the additives from the stripped surface, particularly the waxes used
as evaporation retarders in formulation~ of organic chemical stripping composi-
tiOIIS; any residual wax interferes with the adhesion of subsequent coatings to
the surfaces.
Processes have also been described in U.S. patent Nos. 2,689,198 to
Judd; 3,794,52~ to Nogueira et a] and 3,832,235 to Cooper et al, wherein paint
18 stripped from a relatively small object by contact with the vapors from a
boiling solvent composition. In thcse processes the hot vapors condense to
- ~ liquîds on the painted surface. The resultant hot liquid not only provides a
high local concentration of the paint stripping composition, but it also
washes off any soluble components of the coating or breakdown products thereof.
Such methods are not applicable for removing organic coatings from
extensive s~rfaces because oE the cost of heating a sufficient amount of
solvent to reflux is~prohibitive and moreover, expensive equipment would be
needed to carry out such an operation on a large scale. Furthermore, in some
constructions, such as large metal tanks and ships, even a moderate temperature
differential from one part of the construction to another can be harmful.
It is the current industrial practice to remove paint and other
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protective ccatings from large tanks and other large constructions by the
slow, unpleasant and relat;vely expensive procedure of abrasive blasting. It
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is important that a ballast tank of a ship, which usually carries ballast
water, be rust-proof. To this end, ballast tanks are coated with a layer of
paint. If the paint coating blisters or fails in any way, it is necessary to
remove the paint from the interior of the ballast tank and repaint 9 to avoid
the possibility of rust and eventual holes. This is especially important for
ships which carry liquified natural gas. A ballast tank of a ship may have a
capacity as large as one million gallons or more and often has a complex
"honeycomb" configuration which makes it difficult and laborious for a blaster
to work through. Also, the removal of the large amount of blasting grit
needed is costly.
To date, even though abrasive-blasting has severe disadvantages, it
is practically the only procedure in use for removing paint from large surfaces;
hydroblasting and even pounding wi~h hammers are sometimes employed.
There is a ~remendous demand for more effective and less labor-ineen-
sive methods for cleaning fixed storage tanks, tank trucks, railroad tank
cars, and barge and ship holds of residual tar, pitch, asphalt, and petroleum
and vegetable oil residues of many kinds preparatory to a change in type of
cargo, structural repairs, or inspection by government agencies. Some of
these tanks and holds are very large, for example, 20,000,000 gallons or more
in capacity.
At present they are cleaned mainly with hand-held high pressure
streflms of water-based solutions or emulsions, oEten accompanied or Eollowed
by scraping with shovels and other hand tools. ~n large tanks, scaffolding
must be used. Labor costs, insurance charges, and long turn-around times run
cOses very high and the cleanliness achieved is often marginal or unacceptable,
especially in the case of asphalts and other pitches, high-paraffin deposi~s
from many crude oilsJ and various vegetable oil residue3.
;~ ~ The pr mclpal object of the present invention is the provision of a `~
method of s~tripping~an organic coating from a coated object by an economical
procedure which avolde the problems associated with known stripping procedures.
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Another important object of the present invention is the provision
of a method for stripping an organic co~ting from extensive surfaces by a
procedure which is more economical, and safer ~o workers, and less damaging to
the environment than present methods.
Another object, which becomes more important daily, is to provide a
method of removing unwanted coatings which requires far less energy
and material than presently used methods.
A further important object of the present invention is the provision
of a method for stripping protective organic coatings which avoids the use of
additives which may interfere with subsequent recoating of the SUL face.
A further object of the present invention is the provision of a
method for stripping an undesired organic coating from extensive surfaces in
the interior of a large construction, such as storage tanks, ballast tanks and
holds of ships.
Still another object of the prese.nt invention is the provision of an
economical method for stripping organic coatings from surfaces irrespective of
the shape, comple~ity or inclination thereof.
Yet another object of the invention is the provision of a method for
removing organic coatings and residues which remain after a tank has been
drained of crude oils, lncluding high paraffin crude oils (particularly high
paraffin crude oils which include sludge deposited from the oil), asphalts,
No. 6 fuel oil, vegetable oils and other cargos. The mater;al referred to in
this application as No. 6 Euel oil, sometimes iclentie;ed in the art as "Blmker
C" oil, is a heavy ~uel oil distillation residue. Asphalts encompass many sub-
generic classes, such as air blown asphalts, where air injection is believed
to cause a modi~ication of properties by dehydrogenation~ and vacuum tar
bottoms, a soft asphalt consisting of the distillation residue of high vacuum
distillation of crude petroleum.
`~ Other objects of the invention will in part be obvious and will in
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part appear hereinafter.
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With the above and other objects of the invention in view, our
invention involves the novel method for stripping an organic coating from a
coated surface by contacting the surface with a stripping composition in the
gas phase capable of destroying the adhesion between the coating and the
surface, substantially in the absence of liquid stripping composition conden-
sate on the surface.
We have discovered that organic coatings may be substantially
loosened and in many cases completely stripped fr~m surfaces, solely by the
action of the vapors of a stripping composition. By organic coatings is meant
any coating based on an organic resin or organic vehicle, such as paint,
shellac, varnish~ lacquer and the likej which is applied to a surface such as
metal or wood. The process can be used to remove protective organic coatings,
applied to a surface for the protection and/or enhancement thereof. In
addition, the process can be used to remove coatings not usually called
protective, which are included within the meaning of coatings in this disclo-
sllre. These incl~lde residual crude oil, Bunker C (No. 6) fuel oil, asphalt,
tars, vegetable oils, andi the like, which have to be completely removed from
the surfaces of holds or tanks when they are to be filled with a different
substance which would be contaminated by these residues, or when it is necessary
to clean them for repairs, Coast Guard inspection, or other reasons.
In accordance with our invention, a surEace which is to be stripped
is contacted with the vapors of a stripping composition until the adhesion
between the coating alld the surface i8 ~lestroyed or until the coat;ng forms a
solution which flows to the floor. The gaseous stripping composition is
introduc~d into contact with the coated surface at a ccncentration, pressure
and temperature such that substantially no condensation of the gaseous composi-
tion occurs on the coated surface and thus the process takes place substantial-
Iy in the absence of liquid condensate. Adsorption and/or absorption of
vapors occurs in the coating durlng the process. The vapors may generally be
recovered in high yield. Even comple~ and very large surfaices can be treated - ;
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readily by the procedure.
Depending on the particular coating, the particular stripping compo-
sition, the size of the surface to be stripped and the equipment used, the
time required to remove or destroy the adhesion of the coating ranges from
a few minutes to a few days. Preferably t~e process is carried out at about
aTnbient temperature for economic reasons. There is no theoretical lower
temperature limit for the process. As long as the stripping chemicals have a
little vapor pressure, the process works if the right formulation is used.
While it might be expected that stripping rates would always increase
10 with temperature due to the well-known increase of the rate of difEusion of '
the gas with temperature, an assumption made in all previous patents on
stripping, there are other factors which could make stripping rates increase
with a decrease in te~perature, as exemplified by the following statements in
the prior art:
"Increase of pressure and decrease of temperature increase the
extent of adsorption of a gas by a solid" (S. Glasstone, "Elements of Physical
Chemistry", Van Nostrand, New York, New York (19~6, p. 5~8), and "The solubility
of a gas in liquids is usually decreased by an increase in temperature" (F.
Daniels "Outline of Physical Chemistry" John Wiley & Sons, New York, New York
(1948), p. 20~). The gas laws state that a given volume holds more gas at
lower temperatures~
Unexpectedly and surprisingly, it was found ~hat asphalt was
stripped much faster at 72F (22C) than at 90F.(32C) or 97F. (36C) at
constant concentrations oE the vapors of a stripping solution, as well as
at constant concentrations of t'he vapors of a second very different stripping
liquid. In fact, in ot~e instance, 1~ times as much asphalt was removed at
7~F.(21C) than at 97F.(36C). No. 6 oil was removed faster when either the
substrate or the vapor was cooled than with warmer substrates or vapors. This
can be explained by the fact that at equilibrium the extent of adsorption of a
; 30 gas by a solid is increased by a decrease in temperature and the solubility of
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a gas in a liquid is usually increased by a decrease în temperature. At con-
stant vapor concentration at least one paint is stripped faster at 90F.(32C)
than at 72F.(22C). It appears that the competition between these different
effects may cause the over-all result to go either way, and, at present, the
effect of temperature must be determined empirically for each system~ However,
to date all hydrocarbon materials tested, including asphalts, strip faster at
lower temperatures. Generally5 we prefer ambient temperature because it is
convenient, economical, and safel but there will be situations where either
cooling or warming of the gases may be advantageous.
It is a particular advantage of our process that it is unnecessary
to heat the stripping composition to reflux and ;n most cases it is unnecessary
or undesirable to heat the gaseous stripping composition at all, since prefer-
ably the process is carried out essen~ially at or below the temperature of the
environment.
When carrying out the process at about ambient temperature, it is
preferred that the atnbient temperature be at least about 32F. ~0C.), other-
wise the~process may be inconveniently slow for some coatings; although even
at an ambient temperature below 32F. (0C.) the present process will usually
be preferable to other available processes~ such as abrasive blasting, hydro-
blasting, or scraping with hand tools. In some instances it is most pre~erableto contact the coating with gas phase s~ripping composition with neither the
stripping composition~nor coated surface above about ambient temperature and
with at lea8t one of either the surface or stripping composition cooled
signi~;cantly below ambient temperature. This is particularly true where the
coating is a petroleum product or an epoxy protective coating.
~When the surface which has been thu~ contacted is freed from the
~ gaseous stripping composition, by air drying or other convenient means, it is
; found that in many cases~the coating, particularly a paint coating, has -~-~
either fallen off completely or ca~l be brushed off readiIy leaving only small
specks of paint. In most cases, the surface which has been contacted with the
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gaseous stripping composition is about 75-9~% free of v;sible coating residlle
and sometimes no residue is visible. Usually the surEace can be recoated
without further treatment. However, even when an objectionable amount of
coating remains, the surface can be abrasive-blasted so as to be 100% clean,
i.e., a "white metal blast", in a substantially shorter time than that required
to obtain a surface which is 100% clean by abrasive-blasting alone. Often,
after the surface has been contacted with th~ gaseous stripplng composition in
accordance with our invention, sand blasting to achieve a 100~ clean sur~ace
can be achieved in about 5% to about 20% of the time normally required. In
almost all cases, the coated surface which has been vapor treated in accordance
with our invention can be abrasive-blasted 100% clean in no more than about
50% of the time normally required. In cleaning surfaces of oily or tarry
substances 95-100% removal is normally achieved.
During the process of our invention, it is believed that the vapor
is adsorbed on and/or absorbed into solid coatings causing the coating to
undergo physical changes and to brea~ loose from the substraee. Many epoxy,
alkyd, polyurethane and polyester coatings form dry flakes which can be read-
ily and economically disposed of or even sold. This is a particularly unex-
pected further advantage of treatment with a gas phase stripping composltion
~0 subatantially in ehe absence of liquid condensate in accordance with our
invention. When a surface is treated with li~lid str;pping composition, which
occurs ~hen the vapors of a refluxing composition condense to liquids on a
cooler painted surface, the liquid stripping composition may leach out soluble
components of the coating. With many coatings, the result can be a sticky
mess, the cleaning of which is difficult and substantialLy less economical
than the removal of dry ~lakes.
It has been found that some coatings of chlorinated rubber in
particular may be turned into a powder, rather than ~lakes by a treatment in
accordance with the invention; however, the powder is àlso readily disposed
of. In one caseS a chlorinated rubber paint was li~uified, solely by the
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vapors oE an organic stripping fluid. However, even in such a situation,
treatment in accordance with our invention is preferable to abrasive-blasting,
which is especially slow or impracticable with flexible coatings such as
rubbers. In the treatment of other chlorinated rubber coatings, it has been
found that the coatings are embrittled but are not significantly removed from
the surface; however, abrasive-blasting, hydroblasting or brushing removes the
treated coating substantially faster after treatment with a gaseous stripping
composition in accordance with our invention, than in the absence thereo~.
In removing oily materials and paints which flow of the surface as a solution,
the solution may be pumped out of the tank, subjected to distillation, and
both the stripping composition and the substrate recovered and used again.
The method of the present inventi.on is particularly advantageous for
removing coatings from sur~aces within a sealed or sealable container.
Moreover, the present method is as effective with irregular surfaces as with
regular surfaces and the surface may ha~e any inclination; relatively large
surfsces, such as the inner sur~aces of storage tanks may be treated readily
in accordance with the present process.
In another preerred embodiment of our invention, the surface to be
treated is substantially sealed from the atmosphere to form a paint stripplng
2~ zone. ~ stream of paint stripping composition in the gas phase preferably
close to or, in some cases, below ambient temperature is introduced into the
paint stripping zone into contact with the painted surface. In the case o~ a
stripping composition which is liquid at ambiellt temperature, the gas stream
can be generated conveniently by blowing air over the surface of the liquid
stripping composition in an evaporator, which is connecte~ to the paint
stripping zone by means of chemically resistant conduits. If a stripping
compound is used which i9 normally a gas at ambient temperature, the gas may -~
be i~troduced dirsctly into the paint stripping zone without use of an air
blower. The paint stripping zone is preferably prcvided with a return conduit
to the vacuum side o~ the air blower, which allows the air and the gaseous
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stripping composition to be recirculated. As the partial pressure of the
gaseous stripping composition increases in the paint stripping zone, air is
bled from the stripping zone; normally, the density of the gaseous stripping
composition is greater than that of air, so that the air can usually be bled
out near the top o~ the stripping zone. This allows higher concentrations o~
the gaseous stripping composition to be reached and can be used to prevent an
undesirable rise in pressure from occurring.
The evaporators must be heated to replace the heat of vaporization
of the liquid stripping composition to prevent the liquid stripping composition
]0 from cooling excessively. But, it is generally preferable that the gaseous
stripping composition be at or near ambient temperature in the conduit and
stripping zone. In cases where the coating strips faster at lower temperatures
the heat of evaporation may not be completely replaced; the vapors will be
cooler and energy will be saved.
It is also possible to evaporate the liquid stripping composition
inside the paint stripping zone in which case, a special evaporation zone may
be eliminated.
Means for circulation of the gaseous stripping composition are
desirably included in the paint stripping æone. The efficiency of the present
process is increased and the time required to destroy the adhesion of the
coating and the surEace is decreased when the gaseous stripping composition is
thoroughly circulated throughout the stripping æone. For this purpose,
eflicient gas pumps and/or blowers may be employed.
In the stripping zone the gaseous stripping composition is adsorbed
on and absorbed into the coating to be stripped, whereby the coating undergoes
physical changes and breaks loose from the substrate. The gaseous stripping
~.
composition is then pumped from the stripping zone and desorbed from the
coating~as the partial pressure of the stripping composition drops. The
gaseous strlpping composition may then be recovered by condensation or vented.
The cost of the chemical components of the stripping composltion iæ minute
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compared to the cost of abrasive-blasting. However, it is not difficult to
recover most of the stripping composition used in the present process and
recovery a~oids air pollution. Air is bled into the paint stripping zone
through a vacuum release valve during the removal of the gaseous stripping
composition to avoid creating a possibly dangerous vacuum. In many cases and
particularly in the case o~ most epoxy coatings, after the removal of the
gaseous stripping composition, tne coating is in the form of dry flakes,
mainly on the floor of the paint stripping zone; the dry flakes can be quickly
and economically removed, for example, by a vacuum cleaner.
l~ ~Le gaseous stripping composition may be continuously introduced
into the stripping zone and it is also preferable, especially when time is a
factor, to continuously remove air from the top o~ the stripping zone, which
may be accomplished through a pressure relief ~alve set at about 1-2 psi until
the air has been substantially removed and the highly concentrated vapors of
the more dense stripping compouncls are vented. The vented chemical vapors can
be condensed for reuse. The gaseous stripping composition may also be continu-
ously withdrawn from the stripping zone, condensed, or retained in the gaseous
state and recycled back to the paint stripping ~one or where two or more areas `
are being stripped, the ~aseous str;pping composition withdrawn from one
stripping zone may be circulated to another stripping zone. In large scale
operations~ blowers are used, in order to distribute the vapors throughout the
structure in a reasonable time.
In the event that it is impractical to visually observe the condition
of the coating itl all parts of a complex construction once it has been sealed,
properly located viewports and/or fiber optic devices usually cal1 be convenient-
ly employed to the extent necessary to determine when the process is complete.
~ o single gaseous compound or mixture thereof has yet been found
which is ideally suited to the many types of organic coatings in use today.
Normally, a few simple experiments will enable one of ordinary skill in the
art ~o determine an effective compound or mixture. Organic and inorganic
5~
compounds known to be useful for stripping paint, shellac, varnish, and the
like, which have a partial pressure of at least about 5 mm. ~g at ambient
temperature can be used in our process. In practice, we prefer ~o use mixtures
containing a relatively high percentage of lower chloroalkanes, particularly
chloroalkanes containing 1-3 carbon atoms and 1-3 chlorine atoms. Methylene
chloride is an especially useful stripping agent from the point of view of
effectiveness, as well as of safety and economy. However, other chloroalkanes,
such as 1,2-dichloroalkanes and chloroform are also advantageous. Not only
are such chloroalkane mixtures usually more effective and economical, but also
fire and e~plosion hazards may be reduced or eliminated. Stripping composi-
tions containing methylene chloride in an acount of about 25 to 100~ by volume,
more preferably compositions containing methylene chloride as the principal
ingredient and even more preferably, particularly or economy and safety, com-
positions containing about 70, 80 or 85 to 95% oE methylene chloride are used.
For most petroleum products and some paint6, 100% methylene chloride is pre-
ferred.
Compounds which we have found to increase the ef~ectiveness of methy-
lene chloride and other lower chloroalkanes with various coatings include
aliphatlc hydrocarbGns containing up to about 8 carbon atoms, water, lower
carboxylic acids, sach as formic acid, ammonia, lower-alkylamines, lower
alkanols, and lower alkyl ethers, esters, ketones, nitriles, amides, lower
alkanes, arenes, such as benzene and lower-alkyl and halogen substituted
benzene, and voLatile inorganic acids. The term "lower", refers to a compound
having one to four carbon atoms. In general~ vapor phase com~ositions, which
contain about 70 to 95% of methylene chloride, at least about l~ water and
about ~ to 29~ by volume of other compounds, such as those just listed are
most effective.
:
For exa~ple~, gaseous mixtures of methylene chloride and commercial
ormic acid (85-90% aqueous) in proportions of about 90-97% of methylene
3~ chloride to 3-10% of formic acid are very ef~ective for destroying the adhesion
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of a variety of epoxy coatings to sand blasted steel as well as wood. Several
other types of paint, including an alkyd, a polyurethane and a bituminous
aluminum paint have been 100% delaminated with a gaseous mixture of ~ethylene
chloride/formic acid/water in proportion by volume of about 95% of methylene
chloride and about 5% of 85% formic acid. Such a mixture removed a ketimine
epoxy coating substantially completely in about 48 hours; on ~he other hand, a
chlorinated rubber coating was converted into a soft powder and a thick
coating of glass flake polyester was only partially removed with this mixture
of vapors.
It has also been found that lower alkyl amines are powerful acti-
vators for methylene chloride in the gas phase compositions containing about
70 to 90% by volu~e of methylene chloride and lO to 30% of 33 to 75% aqueous
ethyl amine are particularly useful. A mixture of about 10% by volume of
monoethylamine (33% aqueous) and 90% by volume of methylene chloride is more
effective than a gaseous mixture containing about 95% of methylene chloride
and about 5% of formic acid (85% aqueous) for certain polyurethane, alkyd and
epoxy coatings. An epoxy coating which was almost unaffected by a formic acid-
methylene chloride mixture has been substantially completely delaminated by
means of a gaseous mixture containing about 70% of methylene chloride and 30%
of monoethylamine (33~ aqueous). Diethylamine has also been found to acti.vate
methylene chloride, but generally appears to act more slowly than ethylamine.
Small molecules with dipole moments and acidic or basic character
seem to be the most generalJy useful alone ancl in combination with methylene
chloride for stripping paint. Thus, compositions containing about 70 to 95%
of methylene chloride, about 1% of water and 4 to 29% of either methyl alcohol
or methyl ethyl ketone are also useful.
In some cases, compounds in the gas phase appear to have a synergis~
tic effect with respect~to methylene chloride. For e~ample, the gaseous mix-
ture of formic acid and methylene chloride and the gaseous mixture of methylene
chloride, diethylamine, methanol and water are surprisingly effective with
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respect to an arylamine epoxy coating on metal. It is surprising that the
latter four-component system has a substantially faster action than that o~
three-component mixtures wherein either methanol or the amine is omitted. The
rate of delamination of an epoxy coating using ~5% formic acid vapors was
greatly accelerated by the addition of methylene chloride vapors, yet this
epoxy was not stripped by methylene chloride vapor alone.
In the event that a gaseous stripping composition is chosen which
contains two or more components which do not form a homogeneous solution in
the liquid phase3 it is preferable to have separate evaporators for each of
such compo~mds.
The particular amount of stripping composition used varies widely,
depending upon the nature and thickness of the coating, the ambient temperature
and the particular stripping composition selected, as well as the volume of
the s~ripping zone and the area of the coated surface to be treated. Broadly
; speaking, the ratio of the weight of stripping composition used to that of the
coating removed may be from about 0.5 : 1 to as much as about ~ : 1. When the
coating is slow to strip and time is important it is advantageous to replace
most of the air (up to 100%) in the stripping zone with vapors of the chosen
composition 80 that the coating absorbs the maximum amount of the vflpors
~0 possible ~t the prevailing temperature or at the lowest ambient temperature
expected during the process. When most o the air is bled ofE, as in the
recycling variation of our process, the atmosphere in the stripping zone is
mainly strlpping gases. However, the combined partial pressure of the strip-
ping ga~es is such that substantially no liquid condenses during the treatment.
In many cases rapid stripping occurs with only 30% or less of the air replaced
by vapors of the stripping composition.
The method of our invention is particularly advantageous in reducing
the cost required to strip unsatisfactory coatings from very large surfaces;
the time and material required for the usual procedure of abrasive stripping
.
~ 30 can be eliminated or substantially reduced.
Providecl that the area to be stripped can be substantially sealed
from the atmosphere, there is no practical upper limit to the si~e or complex-
ity of painted structures which can be treated with gaseous stripping composi-
tions in accordance with our invention. rhe fact that the present procedure
neither endangers nor damages the structure by pressure or temperature change
is an important advantage of the present process. Moreover, we have observed
no corrosion problems whatsoever with respect to metal surfaces using the
preferred stripping compositions as disclosed above.
~ ur method is very economical, since the cost of the chemicals is
currently low and moreover, most of the chemicals can be recovered by condensa-
tion or distillation for reuse. The equipment needed is commercially available
at reasonable cost and the manpower requirements are low.
Another important advantage of our paint stripping procedure is that
personnel need not be exposed to the chemical stripping agents; the chemicals
can be transferred from shipping containers to the stripping system with
little or no exposure to the atmosphere and there is no need for the operators
to enter the stripping zone until the vapors have been replaced with air, and
then only for inspection.
The method of our invention is especially useful for removing paint
from interior surfaces of ballast tanks of ships and large tanks used for
stor-ing or processing water, beverages and chemicals. Removal of paint from
such large areas with liquid chemicals i9 clearly impractic~l; applying a
stripping fluid by any of the usual methods is ha~ardous, time consuming,
expensive and may leave undesirable residues. Removal of oily or tarry ~ ;
residues from tal~ks is a serious industrial problem of great magnitude to
which our proce~s is highly applicable. There is widespread dissatisfaction
with present methods and labor problems are enormous. Few people are willing
to work aC temperatures up to 140F shoveling residue out of a ship's hold or
; opera~ing a sand blasting~or~hydroblasting gun from a scaffold while wearing
30 heavy protective gear. ~te work is dirty and dangerous. -~
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The following examples further illustrate the present invention, but
must not be construed as limiting the invention in any manner whatsoever. In
the following examples, as well as in the disclosure as a whole, all propor-
tions of stripping components are by volume unless otherwise indicated;
relative proportions of solvents to palnt coating are by weight.
Example 1
A 16 sq. cm~ area of a steel plate which has been abrasive-blasted
and spray painted with two coats (12 mils) of an epoxy manufactured by
Carboline Co. was grit-blasted to a near white metal condition with a small
Speedaire "Sandblasting Gun~ (3/16 iod~ nozzle) using "Stanblast" grit (furnace
residue) and a pressure of 80 psi. The time required was 85 seconds.
Another portion of the painted surface was placed over a plastic
beaker containing methylene chloride (9 ml.) and 90% formic acid (1 ml.);
after 14 hrs. exposure to the vapors at 73F. most oE the exposed epoxy
coating had delaminated in fragments and fallen into the beaker. The plate
was allowed to stand in air (73F.) for four hours. A 16 sq. cm. area of the
treated surEace was then grit~blasted to white metal using the afore-described
equipment and conditlons. This took only a fast sweep of not over 5 seconds,
only 6% of the time needed for the untreated coating.
~ ~xample 2
A test panel coated with an arylamine epoxy made by Southern Imperial
Coating Corporation required 180 seconds to blast a 16 sq. cm. area to white
metal using t~he sflme equipment as in ~xflmple 1. ~hen exposed to the vapors oE
99% methylene chloride - 1% water in a thin layer chromatography (TLC) chamber
for 47 hrs. a~ 73F. the~coating appeared largely separated from the metal.
After standing in air 8 hours a 16 9q. cm. area was grit-blasted for 25
seconds; about 90-95% of the surface was free of visible paint residues.
1: ~
other~ 2;5: seconds blQsti~g took it to white metal (no visible paint residue).
A 72~ reductl in blaotlng time wa6 realiæed.
~ ~ The same plate~wQs next exposed to the vapors of 95% methylene
:
.. .. . . . , . ,,, . " .
chloride - 5% formic acid (88%) in a TLC (thin layer chromatography~ chamber
for several hours at 72F. and aired for 20 rninutes. The blasting time to
white metal for 16 sq. cm. was reduced to 7.9 sec., 4% of the time needed for
the untreated epoxy. Similar results were obtained when the preliminary
treatment with wet methylene chloride vapor was omitted. When a steel panel
coated with Bunker C fuel oil was treated similarly the oil ran off the panel
completely in 30 minutes.
Example 3
A sandblasted steel panel coated with an 8-9 mils of an aryl amine
epoxy (Southern Imperial Coating Corporation #1204) was placed in a 148 ml.
screw capped iar at 72F. with 1.32 g. of methylene chloride, 0.03 g. of tap
water, and 0.067 g. of 85% formic acid. The panel was supported above the
liquids on wire gau~e. Within an hour the coating was almo~t completely
delaminated. After 2 days the coat;ng fragments, which had completely fallen
off the panel were removed, and dried to constant weight at room temperature.
It was found that 1.51 g. of dry paint had been removed by the vapors from
1.44 g. of chemicals.
Example 4
A panel coated with a polyurethane paint (Southern Imperial Coating
Corp. ~4311j exposed at 7~F. to the vapors oE a mixture of methyl ethyl
ketone, water, and methylene chloride in the ratio oE 5:1:94 by volume was
100~ del~minated; the coa~ing fell off in one piece in 2 days~
Example 5
A steel panel with a heavy coating of a glass flake polyester (18
mils) was exposed to the vapor mixture of methanol, water, and methylene
chloride (5:1:94) for 48 hours at 72F. klost of the polyester fell off
leaving a thln ragged~soft coating adhering weakly to the ~etal. A similar
experimene with a 25 mil coating treaCed with 99% methylene chloride - 1
water gave a similar result except that 7-12 ml. of brittle residual polyester
remained. Blasting times were not measured but would obviously be reduced by
'.',
'7~S~
at least 50%.
Example 6
A panel coated on both sides with 6 mils of an alkyd paint on 4 mils
of a red primer was put on a wire screen in a desiccator containing 5 ml. of
33% aqueous ethylamine and 45 ml. of methylene chloride for 48 hours. The
alkyd coating was about 90% delaminated in 8.5 hrs. and 100% removed in 48
hrs. A Eaint color due to a little primer left in the anchor pattern was
observed.
Ex mple 7
A steel panel coated with a 2-coat arylamine epoxy paint (10 mils)
was exposed at 63F. to the vapors from a mixture of 95% methylene chloride
and 5% formic acid (90% aqueous) for 16 hours in a covered cylinder. When the
panel was lifted out of the cylinder, the coating slipped off leaving only a
few fragments of the primer coat on the metal.
Example 8
(A) Two steel panels:
~1) coated with an aromatic amine type epoxy (Carboline)
(2) coated with a polyamide type epoxy ~Southern Imperial
Coating Corp. ~1201)
are placed inside a one-gallon receptacle which has been fitted wi.th vapor
supply and vapor withdrawal lines (1/4 inch i.d~). The receptacle is then ~-
substantially sealed from the a~mosphere. A small squirrel cage blower
(Dayto ~ C 047) sealed in a container and connected to the vapor supply and
vapor withtrawal lines is used to evaporate a mixture of 93% methylene chloride,
2% water and 5% formic acid (85% aqueous) contained in a round bottom flask
and to force the resultant gaseous mixture through the vapor supply line into
the one-~allon receptacle, out of the container through the vapor withdrawal
.
pipes and back to the blower, so that ~he mixture of vapors is continuously
recirculated. ~fter ;six hours of contact with the circulating vapors at
69F., panel (1~ coated with aromatic type epoxy i~s completely delaminated;
-18-
however, panel (2) painted with the polyamide type epoxy is not visibly
a~fec-t~c1 by this stripping composition~
(B) A 3 x 5" steel p~mel was brush-painted completely with a
4.5 mil coatiny of Southern .Imp~rial Corp. ~ 01 polyamide epoxy, the
same paint used for panel (2) in part A above~ When curecl it was tound
that it took 60 sec. to grit blast a 16 sq. cm. area of the panel to
white metal with the blasting equipmen.t described before (80 psi).
The panel was hung in a jar ahove a mixture of 65% me-thylene
chloricle, 25% ethylamine, and 10% water by volume, After 30 hours e~posure
to the vapors of this mixh~re, the adhesion of the coating to the metal
was almost completely broken and the coatin~ had swollen considerably
; without tearing much. The panel thus treated appeared to be hangi.ng in a
loose plastic bag. When the absor~ed vapors were allowed to evaporate, most
of the coating shrc~nk back against the panel ~surface with only a few
wrinkles show.ing; however, very little adhesion was reestablished; one 16 sq.
cm. area was blasted to whi.te me-tal in 12 seconds, another in 5 sec" for an :: :
average 86~ xeduction in blasting time.
Example 8A shows the specificity of stripping ccmpositions and
Examples 8A and 8B show that a coating which is substantially unaffect~d by
one vapor phase stripping composition is subs-tantially ccmpletely delaminated
by a.different vapor phase composition in accordance with the invention.
~æ 9
A I~T~ steel test panel was grit-blasted to whi.te metal and brush-
paintecl with t~o coats o:E Cook "Ph~ni.con~ 980l' epo~Yy to an avera~ O.e lo ~ils
dry Ei.~ thic~ness. A photo of a Kl~A test panel is shown in Fig. 7,1-3 of
'tSt.eel Structures Painting Manual", J. D. Keene~ Ed., pub].ished by the Steel .:
Structure Paint~ cJ C~uncil, ~00 Fifth Ave., Plttsburgh, Pa., Vol, 1, 1966,
p. 381. It is a 4 by 6 inch steel plate to which has been roughly welded
a piece of~l~4 inch steel 4 in. by 1 in. w~ich had been f~rmed into a
. "
square-bottcmed "U" shape. An area 7 cm. by
: :
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- ~n~
.:
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~7~6
7 cm. which included the protuberance and the rough welding areas was very
difficult to bla.st, requiring 6.5 minutes to blast to 98% white metal using
the small gun described before at a pressure oE 90 psi.
The panel, similarly painted, was exposed to the vapors from a
mixture of methylerle chloride (90%) and 85% for~lic acid (10%) for four days at
72F. and air dried three days. Much of the coating fell off during drying.
A small vacuum cleaner (G.E. Model MV-l) pulled off all the remaining shattered
coating from the 7 cm. x 7 cm. area of comple~ structure except for one of the
inside corners of the "U". The 7 by 7 cm. area was 100% cleaned to white
metal by 8 seconds of blasting at 90 psi. The reduction in blasting time (and
grit used) was flbout 98%.
Example 10
A thin layer chromatography chamber containing 200 ml. of a mixture
of methylene chloride t90%) and 85% formic acid (10%), with blotting papers to
speed up the attainment of equilibrium between the liquid and the vapor
phases, was cooled to 0C. in crushed ice in an insulated box. A sand
blasted metal panel, coated 5 mils thick with "Phenicon 980", an epoxy paint
obtained from Cook Paint and Varnish Company, was suspended in the vapor
space. Within 6 hours9 the coating was mainly in the form of large, torn
20~ blisters with not more than 20% of the coating still adhering to the metal.
AEter another 16 hours (overnight - longer than was needed), the panel was
removed and most of the paint fell off. I`he rest, except for a few specks,
was easily removed by brush;ng after the absorbed cherllicals had evaporated.
In the same equipment at 72F. (22C.), a similar panel was 99%
~: .
delaminated in 3 hours. The amount of chemicals in the vapor phase is of
course much greater at 72F than at 32F; thus two variables were changed by
cooling.
Example 11
In a small desiccator containing 100 ml. of a mixture of 70~ aqueous
ethylamine (14%) and methylene chloride ~86%), the liquid and vapor phases
20-
were separated by a wire screen. Two pine wood panels covered with different
p~otective coatings, ~h;ch has been applied by dipping, were placed on the
screen and allowed to stand for two days at 72F. (22C.).
Panel (1) was coated with "Flow-Lac~Varnish Stain" (Sherwin-WiLliams);
this coating ran off the wood leaving it with only a faint tan stain.
Panel (2) was covered with bro~m "Rustoleu ~ (Rust-oleum Corp.) a
linseed-menhaden alkyd resin based paint which had been baked for 24 hours at
212F. (100C.). This coating blistered and ran off the wood~ leaving it
clean but slightly stained.
In both cases the wood remained smooth and in good condition, without
the "raised grain" effect observed when conventional hot caustic soda is used
for stripping.
Example 12
The process was scaled up over 1600-fold in an experiment conducted
on a hydrostatic paint test tank with interior dimensions 6 x 6 x 6 ft (volume
1640 gal. or 216 cu. ft.) made by welding six 6 x 6 steel plates together. Tt
had an exceptionally deep anchor pattern resulting from several abrasive
blastings. The interior of the tank was heavily brush-painted at the welds
and then spray-painted several months before with "Phenicor~980", an aromatic
amine type epoxy coating made by Cook Paint and Varnish Co. The average dry
Eilm thickness on the walls was about 9 mils, and the area painted was about
210 aq. ft. (Two 3.ll 8q. ft. hatches were not painted). Be~ides the hatches,
which were sealed with rubber gaskets and bolts, there were two 2 in. openings --
one in the ceiling in the front right-hand corner and one in the floor at the
rear-right corner. These were used to introduce and remove chemical vapors.
Using ordinary one-inch steel pipe, a few pieces of one-inch chemical-
ly resistant hose~for flexlbility, and a number of elbows and Ts an exterior
paint-stripping apparatus was built as follows: ~ -
From th~e top openlng steel Ts led to: (1) a mercury U-tube manometer,
(2) a vent to the~atmosphere connected to 9 Et. of downward sloping pipe
' ; :
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.
;6
surrounded by a 6 in. metal tube which could be filled with coolant and (3)
about 10 ft. of pipe leading to the suction side of a positive displacement
gas pump. The pump was a General Motors Model 3-53 from a Model 271 diesel
engine modified to accept one inch pipe. It was driven by a 5 H.P. electric
motor equipped for reversing the direction of shaft rotation and speed control.
The pressure side of the pump was connected to two steel vessels A and B in -
series used for evaporating and later for condensing the chemicals used. The
first, made from 8 in. pipe had a useful volume of 5 gal. while the second,
from from a 6 in. pipe would hold 2 gal. Each had two one inch pipes leading
into the caps and was equipped with a sight glass with neoprene seals. Vessel
B was connected to the bottom opening of the test tank. The vessels were ;
sitting in metal baths containing 17 gal. of water and equipped with immersion
heaters.
Methylene chloride (4.2 gal, 16 1.) was put into A and 90% formic
acid (0.79 gal., 3 1.) into B. The motor was started at about 425 rpm,
forcing about 10 cu. ft. / min (cfm) of air from the test tank over the
surface of the chemicals in A and B then into the bottom of the test tank,
carrying the vapors of the chemicals which evaporated into the air stream.
,
; The air-gas stream was recirculated through the system untiL the concentration
of gases was high enough to wreck the adhesion of the paint. The water baths
were heated to replace the heat of vapori~ation oE the liquids but the tempera-
ture of the gas stream entering the tank was closc to the ambient temperature
~34-70F~, 1-21C.) and no evidence oE the presence o any liquid stripping
composition condensate wae ever observed in the eank during the experiment.
; ~ Pumping was interrupted once to add more methylene chloride.
The gas pump was Fun for 8 hours at 10 cfm with the vent open. The
; condenser attached to the vent turned out to be underdesigned, and much of the
vapors~escaped with~the vented air; the a~ount of each chemical which stayed
in the teet Cank wa6~not determined unt11 later, when the vapors were withdrawn
and analyzed. Inspection of the interior of the tank was done at intervals by
-,
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:
removing the cap from a 3 in. opening in the top hatch cover and lowering a
light bulb and an adjustable angle mirror with a 2 ft. handle into the tank.
At 3 hours the paint was gone from a 3 sq. ft. area above the input opening in
the floor and damaged in an irregular pattern elsewhere. At 5 hours delamina-
tion was extensive and, from laboratory experience, it seemed probable that
enough chemicals were present to complete the stripping. To make certain the
pump was run another 3 hours, when the walls were almost completely bare and
pieces of paint were falling like snow from Lhe ceiling. The pump was stopped
and the tank was loosely closed with valves and cork stoppers. During most of
the experiment the open vent kept the pressure in the tank within 1 torr of
atmospheric pressure. -~
After standing 14 hours a close inspection showed that essentially
all the coating had fallen from the ceiling and walls with the exception of
some fragments hanging loosely from the metal and some strips in the welds.
This appearance was essentially unchanged during recovery experiments (32
hours) and ventilation with fresh air (4 days~. When the tank was opened it
was found that the loose E~agrnents could be easily removed with a compressed
air blast, a soft brush, or a vacuum except for a few sq. in. of paint trapped
in pockets along the welds, and tiny fragments trapped in some of the pits of
. .
~0 the anchor pattern all over the tank. The con3ensus of opinion of several
experts who e~amined the tank was that the condition of the surface was much
better than a "commercial blast", a~d for ordinary purposes, could probably be
repainted without any abrasive blastillg, but that for the most demanding jobs,
for which a near white metal blast is required, a fast "sweep" with an abrasive
blast was all that would be needed. This was variously estimated to require
5-15% of the time and material needed for untreated paint.
Recovery of the chemicals absorbed in the paint residue and in the
vapor space of the tank was accomplished without noticeable 1089 by cooling
Vessels A and B wiCh ice water and reversing the rotation direction of the
motor so that gases were sucked out of the bottom opening at 7 cfm and through
:
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, , , . . . .. - . . .
the cooled vessels. Air entered through the vent and no odor was detected
anywhere. In 6 hrs. 4.7 l. was collected after which the rate of recovery
dropped off rapidly. A total of 6 liters (1.6 gal) was collected in 32 hours.
The paint flakes were vacuumed out and found to weight 12.5 Kg (27.5 lb) of
which about 1.0 Kg (2.2 lb) was formic acid. Analysis of the collected
liquids showed that during the stripping the tank contained a total of 7.1 Kg.
(15.7 lb) of methylene chloride, 1.2 Kg (2.6 lb) of formic acid and 0.15 Kg
(0.3 lb) of water. Thus a total of 8.6 Kg of chemicals stripped 11.5 Kg of
paint and 7.4 Kg of the chemicals was recovered.
Example 13
Welghed test tubes of dimension 12.5 cm ~ 0.9 cm were coated with a
substrate and suspended in the vapor phase above the specified liquid which was
at ambient temperature in a glass chamber covered with metal foil. One oE the
tubes was allowed to remain at ambient temperature (72F. or 22C.), or was held
at 70F~ (21C.) by cooling with tap water. A second test tube was warmed by
passing a stream of warm water through it. The amount oE substrate removed
from the outside surface of each test tube, which was exposed to the vapor
phase in the glass chamber, was measured by either drying and weighing the
tubes ~fter a given time or by weighing the coating which had dropped from the
tube after evaporation of absorbed and/or adsorbecl gases. It should be noted
that the concentration of the vapor in these experiments was much less than
the maximum amo~mt possible, about 50% in the case oE methylene chloride.
A coating of air blown asphalt was applied to the test tubes as the
substrate in the amount of appro~imately 3.5 gm. The liquid stripping composi-
tion was 100% methylene chloride, and the coating temperatures of each test
tube were 70F. (21'C.) and 97F. (36C.). Each test tube was i~mersed in the
vapor rom the liquld for a period of 45 minutes. The amount of air blown
asphalt coating removed was then measured, showing that only 3.5% of the
warmer tube substrate coating was remo~ed, while 63.5% of the cooler tube
coating was removed.
-24-
~7~
Example 14
A test similar to that described Eor Example 13 was carried out with
approximately 4.5 gm of roofing grade air blown asphalt as the coating for each
tube, using n-hexane as the liquid forming the vapor phase. In one experiment,
the tubes were immersed in the vapor for 18 minutes, the warmer tube being
held at 89F. (32C.), with 0.5% measured removal Gf the substrate coating,
while the cooler tube was held at 72F. (22C.) with 11.3% removal of the
substrate coating.
In a second experiment using the same stripping vapor, the tubes
were immersed for 53 minutes (0.88 hours), the warmer tube being held at
90F. (32~C.~ and exhibiting a measured coating substrate removal of 33.5%,
while the cooler tube was held at 72F. (22DC.), exhibiting a removal of
coating substrate of 77%.
In a third experiment, the tubes were immersed in the vapor for 4
hours, the warmer tube being held at 90F. (32C.) and showing a 96% removal
of substrate, while the cooler tube was held at 72F. (22C.), showing a
~ measured substrate removal of 99.7%.
; Example 15
Followlng the procedure of Example 13, a coating of Rustoleum paint,
a rust-proofing primer coating composition obtained from Rust-oleum Corp., was
applied to the tubes in a quantity of approximately 0.55 gm. The liq~lid
composition forming the stripping vapors was prepared by mixing by volume 90%
methylene chloride, 7% ethylamine and 3% water.
~le warmer tube was maintained at 90F. (32C.) and the cooler
tube was maintained at 72F. (22C.). After 22 hours, the warmer tube measured
35% removal of the substrate coating and the cooler tube measured 16% removal.
In a similar experiment with an epoxy paint, removal was much faster
from the cooler tube.
Example 16
A similar procedure to that of Example 13 was carried out where each
. ."
-25-
::
,, , , , ;, . . ~ ' :
7~
tube was coated with No. 6 oil, sometimes referred to as "Bunker C" oil.
The liquid stripping composition was 100% chloroform. The tubes were immersed
in the stripping vapor for 5 minutes, the warmer tube being held at 92F.
(33C.), and ~he cooler tube at 72F. (22aC.). The warmer tube shGwed 70.2%
removal of the coating, while the cooler tube showed 87.5% removal of the
coating.
The 72F. tube was cleaner after 5 minutes than the 92F. tube
after 50 minutes, by visual observation. The 92 tube was approximately as
clean at 100 minutes as the 72~ tube at 5 minutes by visual observation of the
translucent film remaining after the stated periods of immers;on in chloroform
vapors.
Example 17
In Examples 13 to 16 the temperature and concentration of the vapors
was constant while the temperature of the coatings was varied. In the follow-
ing experiments the temperature of the coatings was maintained constant by
running tap water through the coated tubes and the temperature of the stripping
vapor was varied while its concentration in air was constant at about 30~ by
volume.
(A) Two~tubes coated with No. 6 oil were mounted in a TLC chamber or
tank loosely covered with aluminum foil. One tube was maintained at 74.5F by
running tap water throu~h it and one was allowed to warm to the tank tempera-
ture, which wa8 7~2F during the stripping process.
The vapor generator was made with a small air compressor which drove
fresh air through a tube ending in a fritted glass diffuser which extend 6 cm
be].ow the surface of a 3-necked flask half full of methylene chloride ~MC)
; maintained at constant level by means of a device known in the art as a
"chickea feeder". The air - MC vapor mixture exited through one neck of the
flask into a tube connect~ed to a coil oE copper tubing immersed in a water
bath heated at 78C and was then conducted to the middle of the test tank.
They were vented to the ~air in a hood. The hot gases (13 liters per min.)
.
~ ~ ~ -26- ~
s~
soon raised the temperature in the TLC chamber to S0+2F. It took 52 minutes
to remove about 73% of the oil from the 74.5F tube, at w~ich point the
coating was so thin that a red cross on a white background can be seen thrugh
the tube. The coating on the warmer tube was still opaque.
(B) The same set-up was used as in A except that the tank was partly
immersed in the same water bath as the coil of copper tubing and the water was
cooled at 12_1C (52-55F). The temperature in the TLC chamber was 16C
(62F~ during the run. Two tubes were mounted in the chamber one coated
with No. 6 oil (0.285g) and one with vacuum tar bottom (VTB) asphalt (1.68g).
Both were maintained at 78F (26C) with tap water. After blowing the 30% MC
vapor-air mixture t.hrough the chamber for 35 minutes the No. 6 oil had been ~-
rernoved to the same degree of transparency as in A and later weighing showed
that 73% of it had been removed. At 35 minutes 77% of the V~B asphalt had ~ ~`
been stripped.
It is evident that, with everything else constant cool vapors
removed No. 6 oil faster from a 78F tube than the warmer vapors of Exeriment
A did from a 74.5F tube. Again, stripping i9 Easter at lower temperatures.
From Examples 13 to 17, it ;s apparent that a high degree of unpre-
dictability exists iD the temperature dependence of removal of various protec
tive coatings in various stripying compositions, although the petroleum
derived products were all removed faster from cooler surEaces than from warmer
surfaces, and faster with cool vapors than with warm vapors~
_ample 18
Sand blasted, rusty metal panels were coated with linseed oil~ and
. .
other with cottonseed oil, and allowed to stand in air for over three months.
Another panel was freshly coated with "crude degum" grade soybean oil. When
these panels were placed In a tank saturated with the vapors of methylene
chloride at~72F (22C), the oils were stripped within one hour.
~ E~ _19
A test similar to that described for Example 13 was carried out with
, ' ' :
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..
56
approximately 4.5 gm of roofing grade air blo~n asphalt as the coating of each
tube, using wet methylene chloride as the liquid forming the vapor phase. In
one experiment the tubes were immersed in the vapor for the time necessary to
completely strip the asphalt coating, the warmer tube being held at 72F
(22C) and requiring 4 hours to strip the coating to 99.7% removal. The
cooler tube was held at about 8C and required 10 minutes to essentially
complete removal.
Example 20
A test similar to that of Example 19 was conducted with one similarly
asphalt coated tube held at -40C+5C by adding dry ice to acetone inside the
tube. Removal of the coating was very rapid for the first ten minutes, but a
continuous buildup of ice on the coating interferred with coating removal.
After 30 minutes most of the asphalt had been removed.
Example 21
An experiment similar to that of Example 13 was conducted. The tubes
were coated with 1.8 gm of epoxy tank lining (No. 343, made by Pan-American
Coatings, Inc.) and exposed to the ~apors from a mixture of liquid 90% MC,
10% aqueous form~c acid (90% formic acid, 10~ water). One coated tube was
left at ambient temperature (21C) while the second ~oated tube was cooled to
0~2C by filling the second tube with an ice-salt slush. ~le cold tube was
at least 99% delaminated in 16 min. The 21C tube required 70 min. for 99%
delamination.
Thus, in one embodimetlt of the present process it is particularly
advantageous to cool the stripping composition and/or the surface to be
str1pped about 10C to about 70C below ambient temperature, more preferably
abou~ 20C to abo~lt 50C below ambient temperature. ~lerefore, in a
preferred embodiment of the invention, the process is preferably carried out
. .
from about -40PC to about 8C, more preferably from about -20C to about O~C.
: ~ ~ ' '
