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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a method for stripping organic coatings
from coated objects. More particularly, the present invention concerns a
method for stripping 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. The
method of the invention is especially useful for removing such coatings
from 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 theeof. As discussed in
Kirk-Othmer's ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 14, pp. 485-493,
2nd Edition, John Wiley and Sons, 1967, organic paint removers generally
fall into three classes: compositions based 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
(dichloromethane) has been found to be particularly effective and
formulations of methylene chloride suitable for application by dip, brush,
spray and delivery from aerosol cans are known. Such compositions also
usually contain 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 object 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 of "scrape off" formulations or by flushing with
water and/or by wiping with a 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 impractical and prohibitively expensive where large surfaces are
involved. In addition, extreme safety measures would be required to
effectively 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
formulations of organic chemical stripping compositions; any residual wax
interferes with the adhesion of subsequent coatings to the surfaces.
Processes have also been described in U.S. Pat. Nos. 2,689,198 to Judd;
3,794,524 to Nogueira et al and 3,832,235 to Cooper et al, wherein paint
is stripped from a relatively small object by contact with the vapors from
a boiling solvent composition. In these processes the hot vapors condense
to liquids 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 surfaces because of 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 protective
coatings from large tanks and other large constructions by the slow,
unpleasant and relatively expensive procedure of abrasive blasting. It 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, 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 with hammers are sometimes
employed.
There is a tremendous demand for more effective and less labor-intensive
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 streams of
water-based solutions or emulsions, often accompanied or followed by
scraping with shovels and other hand tools. In large tanks, scaffolding
must be used. Labor costs, insurance charges, and long turn-around times
run costs very high and the cleanliness achieved is often marginal or
unacceptable, especially in the case of asphalts and other pitches,
high-paraffin deposits from many crude oils, and various vegetable oil
residues.
SUMMARY OF THE INVENTION
The principal object of the present invention is the provision of a method
of stripping an organic coating from a coated object by an economical
procedure which avoids the problems associated with known stripping
procedures.
Another important object of the present invention is the provision of a
method of stripping an organic coating from extensive surfaces by a
procedure which is more economical, and safer to 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 surface.
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 present invention is the provision of an
economical method for stripping organic coatings from surfaces
irrespective of the shape, complexity 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, including 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 material
referred to in this application as No. 6 fuel oil, sometimes identified in
the art as "Bunker C" oil, is a heavy fuel oil distillation residue.
Asphalts encompass many subgeneric classes, such as air blown asphalts,
where air injection is believed to cause a modification of properties by
dehydrogenation, and vacuum tar bottom, 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 part
appear hereinafter.
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
condensate on the surface.
DESCRIPTION OF THE INVENTION
We have discovered that organic coatings may be substantially loosened and
in many cases completely stripped from 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 like, 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 disclosure. These include residual crude oil, Bunker C
(No. 6) fuel oil, asphalt, tars, vegetable oils, and 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 surface which is to be stripped is
contacted with the vapors of a stripping composition until the adhesion
between the coating and the surface is destroyed or until the coating
forms a solution which flows to the floor. The gaseous stripping
composition is introduced into contact with the coated surface at a
concentration, pressure and temperature such that substantially no
condensation of the gaseous composition occurs on the coated surface and
thus the process takes place substantially in the absence of liquid
condensate. Adsorption and/or absorption of vapors occurs in the coating
during the process. The vapors may generally be recovered in high yield.
Even complex and very large surfaces can be treated readily by the
procedure.
Depending on the particular coating, the particular stripping composition,
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 the process is carried out at about
ambient 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 with
temperature due to the well-known increase of the rate of diffusion of the
gas with temperature, an assumption made in all previous stripping
processes, there are other factors which could make stripping rates
increase with a decrease in temperature, 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 (1946, p. 548), 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. 204). The gas laws state that a given
volume holds more gas at lower temperatures.
Unexpectedly and surprisingly, it was found that asphalt was stripped much
faster at 72.degree. F. (22.degree. C.) than at 90.degree. F. (32.degree.
C.) or 97.degree. F. (36.degree. C.) at constant concentrations of the
vapors of a stripping solution, as well as at constant concentrations of
the vapors of a second very different stripping liquid. In fact, in one
instance, 18 times as much asphalt was removed at 70.degree. F.
(21.degree. C.) than at 97.degree. F. (36.degree. C.). 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 gas by a solid is increased by a
decrease in temperature and the solubility of a gas in a liquid is usually
increased by a decrease in temperature. At constant vapor concentration at
least one paint is stripped faster at 90.degree. F. (32.degree. C.) than
at 72.degree. F. (22.degree. C.). 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. Generally, we
prefer ambient temperature because it is convenient, economical, and safe,
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 in most cases it is unnecessary or
undesirable to heat the gaseous stripping composition at all, since
preferably the process is carried out essentially at or below the
temperature of the environment.
When carrying out the process at about ambient temperature, it is preferred
that the ambient temperature be at least about 32.degree. F. (0.degree.
C.), otherwise the process may be inconveniently slow for some coatings;
although even at an ambient temperature below 32.degree. F. (0.degree. C.)
the present process will usually be preferable to other available
processes, such as abrasive blasting, hydroblasting, or scraping with hand
tools. In some instances it is most preferable to contact the coating with
gas phase stripping composition with neither the stripping composition nor
coated surface above about ambient temperature and with at least one of
either the surface or stripping composition cooled significantly 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 thus 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 can be brushed off readily leaving only
small specks of paint. In most cases, the surface which has been contacted
with the gaseous stripping composition is about 75-95% free of visible
coating residue and sometimes no residue is visible. Usually the surface
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 the gaseous stripping composition in accordance with our
invention, sand blasting to achieve a 100% clean surface 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 break loose from the substrate. Many
epoxy, alkyd, polyurethane and polyester coatings form dry flakes which
can be readily and economically disposed of or even sold. This is a
particularly unexpected further advantage of treatment with a gas phase
stripping composition substantially in the absence of liquid condensate in
accordance with our invention. When a surface is treated with liquid
stripping composition, which occurs when 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
flakes.
It has been found that some coatings of chlorinated rubber in particular
may be turned into a powder, rather than flakes by a treatment in
accordance with the invention; however, the powder is also readily
disposed of. In one case, a chlorinated rubber paint was liquified, solely
by the vapors of 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 thereof. In removing oily materials and
paints which flow off 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 invention is particularly advantageous for
removing coatings from surfaces within a sealed or sealable container.
Moreover, the present method is as effective with irregular surfaces as
with regular surfaces and the surface may have any inclination; relatively
large surfaces, such as the inner surfaces of storage tanks may be treated
readily in accordance with the present process.
In another preferred embodiment of our invention, the surface to be treated
is substantially sealed from the atmosphere to form a paint stripping
zone. A 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 of a stripping composition which is liquid at ambient temperature,
the gas stream can be generated conveniently by blowing air over the
surface of the liquid stripping composition in an evaporator, which is
connected to the paint stripping zone by means of chemically resistant
conduits. If a stripping compound is used which is normally a gas at
ambient temperature, the gas may be introduced directly into the paint
stripping zone without use of an air blower. The paint stripping zone is
preferably provided with a return conduit to the vacuum side of the air
blower, which allows the air and the gaseous 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 of the stripping zone. This allows higher concentrations of 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
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 zone. The efficiency of the present
process is increased and the time required to destroy the adhesion of the
coating and the surface is decreased when the gaseous stripping
composition is thoroughly circulated throughout the stripping zone. For
this purpose, efficient gas pumps and/or blowers may be employed.
In the stripping zone the gaseous 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 stripping composition may then be recovered by condensation or
vented. The cost of the chemical components of the stripping composition
is minute 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 avoids 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 of most epoxy coatings,
after the removal of the gaseous stripping composition, the 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.
The 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 of the stripping zone,
which may be accomplished through a pressure relief valve set at about 1-2
psi until the air has been substantially removed and the highly
concentrated vapors of the more dense stripping compounds are vented. The
vented chemical vapors can be condensed for reuse. The gaseous stripping
composition may also be continuously withdrawn from the stripping zone,
condensed, or retained in the gaseous state and recycled back to the paint
stripping zone or where two or more areas are being stripped, the gaseous
stripping 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 in all parts of a complex construction once it has been
sealed, properly located viewports and/or fiber optic devices usually can
be conveniently employed to the extent necessary to determine when the
process is complete.
No 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 to determine an effective compound or mixture. Organic and
inorganic compounds known to be useful for stripping paint, shellac,
varnish, and the like, which have a partial pressure of at least about 5
mm. Hg at ambient temperature can be used in our process. In practice, we
prefer to 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 explosion hazards
may be reduced or eliminated. Stripping compositions 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 for economy and safety, compositions
containing about 70, 80 or 85 to 95% of methylene chloride are used. For
most petroleum products and some paints, 100% methylene chloride is
preferred.
Compounds which we have found to increase the effectiveness of methylene
chloride and other lower chloroalkanes with various coatings include
aliphatic hydrocarbons containing up to about 8 carbon atoms, water, lower
carboxylic acids, such 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
compositions, which contain about 70 to 95% of methylene chloride, at
least about 1% water and about 4 to 29% by volume of other compounds, such
as those just listed are most effective.
For example, gaseous mixtures of methylene chloride and commercial formic
acid (85-90%) in proportions of about 90-97% of methylene chloride to
3-10% of formic acid are very effective for destroying the adhesion 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
methylene 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 the 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 activators for
methylene chloride in the gas phase; compositions containing about 70 to
90% by volume of methylene chloride and 10 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 activate 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 generally useful alone and 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 synergistic
effect with respect to methylene chloride. For example, the gaseous
mixture of formic acid and methylene chloride and the gaseous mixture of
methylene chloride, diethylamine, methanol and water are surprisingly
effective with respect to an arylamine epoxy coating on metal. It is
surprising that the latter four-component system has a substantially
faster action than that of three-component mixtures wherein either
methanol or the amine is omitted. The rate of delamination of an epoxy
coating using 85% 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 phase, it is preferable to have separate evaporators for each of
such compounds.
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 stripping 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 4:1. 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 so that the
coating absorbs the maximum amount of the vapors possible at the
prevailing temperature or at the lowest ambient temperature expected
during the process. When most of the air is bled off, as in the recycling
variation of our process, the atmosphere in the stripping zone is mainly
stripping gases. However, the combined partial pressure of the stripping
gases 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 can be eliminated or substantially reduced.
Provided that the area to be stripped can be substantially sealed from the
atmosphere, there is no practical upper limit to the size or complexity of
painted structures which can be treated with gaseous stripping
compositions in accordance with our invention. The 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.
Our method is very economical, since the cost of the chemicals is currently
low and moreover, most of the chemicals can be recovered by condensation
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
storing or processing water, beverages and chemicals. Removal of paint
from such large areas with liquid chemicals is clearly impractical;
applying a stripping fluid by any of the usual methods is hazardous, time
consuming, expensive and may leave undesirable residues. Removal of oily
or tarry residues from tanks is a serious industrial problem of great
magnitude to which our process is highly applicable. There is widespread
dissatisfaction with present methods and labor problems are enormous. Few
people are willing to work at temperatures up to 140.degree. F. shoveling
residue out of a ship's hold or operating a sand blasting or hydroblasting
gun from a scaffold while wearing heavy protective gear. The work is dirty
and dangerous.
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
proportions of stripping components are by volume unless otherwise
indicated; relative proportions of solvents to paint 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 i.d. 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 73.degree. F. most of the exposed epoxy
coating had delaminated in fragments and fallen into the beaker. The plate
was allowed to stand in air (73.degree. F.) for four hours. A 16 sq. cm.
area of the treated surface was then grit-blasted to white metal using the
afore-described equipment and conditions. This took only a fast sweep of
not over 5 seconds, only 6% of the time needed for the untreated coating.
EXAMPLE 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 the same equipment as in Example 1. When exposed to the
vapors of 99% methylene chloride--1% water in a thin layer chromatography
(TLC) chamber for 47 hrs. at 73.degree. F. the coating appeared largely
separated from the metal. After standing in air 8 hours a 16 sq. cm. area
was grit-blasted for 25 seconds; about 90-95% of the surface was free of
visible paint residues. Another 25 seconds blasting took it to white metal
(no visible paint residue). A 72% reduction in blasting time was realized.
The same plate was next exposed to the vapors of 95% methylene chloride--5%
formic acid (88%) in a TLC (thin layer chromatography) chamber for several
hours at 72.degree. F. and aired for 20 minutes. 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
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