CA1173725A - Vapor stripping process - Google Patents

Abstract

VAPOR STRIPPING PROCESS

ABSTRACT OF THE DISCLOSURE

Protective and residual organic coatings are stripped from a surface by substantially sealing the surface from the atmosphere to provide a stripping zone, forming a stream of gaseous stripping composition in an evaporation zone, contacting the surface with the stripping composition in the gaseous state at about ambient temperature and recirculating the gaseous mixture from the stripping zone to the evaporating zone and back to the stripping zone to rapidly increase the concentration of gaseous stripping composition in the stripping zone.
Optionally the surface may be rinsed by raising the temperature of the gaseous stripping composition slightly above ambient temperature, whereby the gaseous stripping composition condenses on the surface and flushes the surface free of adhering particles. This process is useful for objects which can be placed in a stripping zone and substantially sealed from the atmosphere, especially objects having an irregular surface, and also for interior surfaces which can be substantially sealed from the atmosphere, including extensive surfaces such as those of storage tanks, tank cars and the holds and ballast tanks of ships .

Description

-- ~ l ~'737 . . I
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Background of the Invention The present invention relates to a method for stripping organ~c coatings ~rom coated objectsO More particularly, the p.resent invention concerns a method for stripping protective coatinys obtained from compositions based on organic resins and~
or prepared with organic vehicles, such as palnt, shellac, varnish, lacquer and the like, as well as residual coatings9 including oils, asphalts, tars and synthetic polymers such as polystyrene. The method of the invent~on is especially useful for removing such coat~ngs from objects having irregular surfaces and from large surfaces, including vertical and inclined surfaces in the inter~or of large constructions, such as storage bins and tanks on land and holds and ballast tanks of ships.
~:: Gommonly, paint is stripped from painted objects on a small scale by appl~cation of an organic or inorganic solvent or .
: mixture thereof, as discussed in Kirk-Othmer's ENCYCLOPEDIA
.~ . OF CHEMICAL TECHNOL06Y, Yol. 14, pp. 485-493 7 2nd Edition, John Wiley and Sons, 1967.
~: Among solvents whlch inany formulations employ, hydrocarbons, halocarbons, and part~cularly methylene chloride ~dlchloro-:~ methane) have been found to be part~cularly effective. Liquid stripping compositions usually contain add~tlves including thickeners, evaporatlon retarders and detergents.
.

1 -73~5 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 tD stand for some time, after which, the ooating which has become swollen and/or softened is removed from the sur~ace, by scraping~ in the case of "scrape of~" formulations or by flushing with water and/or by ~iping with a damp rag 1n the case of "flush off"
formulations.
The foregoing methods are expens~ve9 since the organic solvent~ except in the case o~ application by immersion, is not reoovered. Moreover, all of the known procedures are generally dangerous and prohibitively expensive where large surfaces are involved. In addition, the waxes used as evaporation retarders in such formulations are difficult to remove completely and any residual wax interferes w~th the adhes~on of subsequent coatings to the surfaoesO
Processes have also been described in U.S. Patent Nos.

2,689,198 to Judd, 3,794,524 to Nogueira et al and 3,832,235 to Cooper et al, where~n paint is stripped from a relatively small ob~ect by contact w1th the vapors from a boiling solvent composi tion . In these processes the hot vapors condense to liquids on the pa~nted surface.
A method for cleaning tanks has been disclosed ~n U,S.
Patent NoO 3,042,553 to Kearney et al wherein a solvent ~s heated to the boil~ng point to produce sol~ent vapors which are fed to a tank where they condense and wash the tank walls.
Such methods are not applicable for removlng organic coatings from extens~ve surfaces because the cost of heating a '~ ~3~7~ ~

sufficient`amount of solvent to reflux for an extended period is proh~bitive and moreover, expensive solvent-proof equipment is needed to carry out such an operation on a large scale.
Furthermore, in some constructicns, such as large metal tanks and ships, a temperature different1al from one part of the construction to another can be harmful.
Systems where~n a liqu~d solvent m~st is sprayed on a workpiece have alsn been used to clean surfaces, as disclosed for example in U.S. Patent Nos. 4,101,340 to Rand and 2,756,168 to Ruggles. Spray wash~ng requires a large amount of energy and therefore i~ undeslrable in view of the expense and the increasing need to conserve energy.
It 1s the current ~ndustrial practice to remoYe paint and most other protect~ve coat~ngs from large tanks and other large construct~ons ~y the slow, unpleasant, expensjve and dangerous procedure of abrasive blasting. It is important that a ballast tank of a ship, wh~ch usually carries ballast ~ater, be rust-proof. To this end, ballast tanks are coated w~th a layer of paint. If the pa1nt coatin~ bl~sters or fa~ls in any .
way, ~t ~s necessary to remove the paint from the ~nterlor of the ballast tank and repaint, to avoid the possib~l~ty of rust and eventual holes. This is especia.l.ly important for ships wh~ch carry liqu~f~ed natural gas. A ballast tank of a ship may have a capaclty as 1 arge as one mi 11 i on gal 1 ons or more and often has a cornplex "honeycomb" configuration whieh makes it diff~cult and labor~ous for a blaster to work through. Removal and disposal o~ the large amount of blasting grit needed are castl y .

To date, even tho~ a~a~ ~e-blasting has severe disadvantages, it is practically the only procedure in use for removiny 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 as well as tank cars of railroads and trucks, barge.and ship holds of petroleum residues such as tar, pitch, asphalt, and oil, vegetable oil residues and residues of synethetic polymers such as polystyrene which result from storage of the monomer, preparatory to a ch.ange in type of cargo, structural repairs, or inspection by government agencies. Some of these tanks and holds h.ave a capacity of more than 20,000,000 gallons.
At present they are cleaned mainly with hand-held high pressure streams of water or water-based solutions or emulsions, oEten followed by scraping with shovels and other hand tools. Caustic soda solutions may be used too. Costs are very high and the cleanliness achieved is often marginal or unacceptable~ Residues are usually wasted and their disposal is a problem~
In our parent applications, Serial Nos. 870,103 and 1,053, now U.S. Patent Nos. 4,231,804 and 4,231,805 respectively, we have disclosed an economical process for stripping protective and residual coatings by means of a gaseous stripping composition wherein neither the gaseous stripping composition nor the surEace being stripped is substantially above ambient temperature.
While our process is surprisingly effective for removing almost all of pc,~

~ 3~25 such coati~gs, in some cases this process ~s slow for indus~r~al use. Also/ we have found that some coatings contain impurities or pigments wh~ch are insoluble in any practical stripping composition. For example, many oils and tarry substances, such as No. 6 Oil (Bunker C oil) and various tars and asphalts encountered in commerce contain small amounts of carbonized material and/or incrganic substances, such as rust pjarticles, some of which are left on the surface, sometimes together with a small amount of the initial petroleum coating which is trapped in and around these part~cles after our process has been carried out. This residue generally amounts to less than 1% of the orlginal coating and is usually unobjectionable;
but some~imes, where a change of cargo is to take place, even such a minor amount of res~due is undesirable.

Summary of the Invention The principal object of the present invention ls the provision of a method of stripping an organic protective or residual coating from a coated object by an economical procedure which avsids problems associated with known str~pping procedures. .
Other important objects of the invention include the provision of a method for stripping such organic coat~ngs from extensive surfacesg i.e. interiors of large constructions, such as stora~e tanks, ballast tanks and holds of ships and from surfaces of any shape, complexity or inclination, by a procedure which is more economi cal, safer to workers , l ess damaging to the environment and uses less energy than present methods and ~hich also avoids additives which interfere with subsequent recoating of the surface.

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Still another important object of the invention is the prov~sion of a process whereby protective and res~dual coat~ngs are stripped rap~dly from a surface.
A further important object o~ the invention is to prnvide a method for removing such organic coatings and residues substantially completely, so tha~ the resultant surfaces are clean enough for any change of cargo.
Other objects of the ~nvention will be obvious in part and w~ll ln part appear here~nafter.
With the above and other objects of the invention in view, our invent10n ~nvolves stripping an organic coating from a coated surface by contacting the sur~ace with a stripping compos~tion in the gas phase capable of destroying the adhes~on between the coating and the surface wherein: the surface to be stripped is substantially sealed from the atmosphere in a stripping zone; a gaseous stream of stripping composition normally liqu~d at ambient temperature is formed in an evaporation zone which is connected to the str~pping zone to provide a substantially closed syste~; the gaseous stream is circulated Into the str~pping zone and then the gaseous mixture in the strlpping zone is recircul~ted to the e~aporat10n zone and back to the strtpping zone, in order to ~ncrease the concentrat~on of gaseous str~pplng compos~tion rapidly in the stripp~ng zone, but whereln the gaseous pressure in the stripping zone ~s not above about 200 torr gauge pressure.
In a particular embod~ment of our process, the temperature of the gaseous mixture be1ng recirculated is increased to about 1 to 15 C. above the temperature of the surface being stripped.
~ue to the increase ~n temperature, the concentrati on of recircula~1ng gaseous stripp~ng composition ~ncreases to the saturation concentrat~on of the surface being ~173~Z5 stripped9 whereby stripping composition condenses on the surface being stripped so as to flush the surface and quickly remove any adhering particles. The r~nsing procedure is advantageous where a surface which is substantîally 100% free of coating is desiredu Description of the Invention We have discovered that organic protective and res~dual coatings may be stripped from surfaces very rapidly by the action of the vapors of a strippin~ compos~tion. By organic coatings is meant any coatlng based on an organlc resin or organic vehicle, such as paint, shellac, varnish, lacquer and the like, which is applied to a surface such as metal or wood .
Our process is useful for removing protective organic coatings applied to a surface for the protection and/or enhancement thereof and also for re~oving nesldual coat~ngs. Residual coatings include for example, petroleum prcducts such as crude oil, Bunker C (No. 6) fuel oil, high paraffin crude oil, asphalt~
such as air blown asphalts and vacuum tar bottoms, i.e. the hiyh vacuum distillation residue of certain crude petroleums, and synthetic polymeric materials such as polystyrene and .
mixtures of residual monomers and polymers which form when monomers are stored, and the like, which have to be remoYed from the surfaces of holds or tanks for a change of cargo or when it 7s necessary to clean them for repairs, Coast Guard ~nspection, and the like.
In accordance with our invention, the surface to be treated is substant1ally sealed from the atmosphere to form a stripping zone. An eYaporation zone containing a ~lower and a pool of stripping composition is connected to the stripping zune by 11737Z~

means of an input conduit suitable for the introduction of gaseous s~ripping composition into the str~pping zone and a return conduit for returning the gaseous mixture from the stripping zone into the vacuum side of the blower. ~he evapora-tion 70ne and the str~pping zone together form a closed system.
The evaporation zone is suppl~ed ~ith means to heat the liquid stripping composit~on and may also include means regulating and/or mainta~ning the liquid stripping composition at a particular temperature.
In operation a stream of ~aseous stripping composition is generated by the action of the blower on the liquid in the evaporation zone and is then forced into the stripping zsne by way of the imput conduit. The gaseous mixture in the stripping zone is then circulated back to the evaporation zone via the return condu~t where the concentration of the str~pping composition in the gaseous mixture is ~ncreased.
The gaseous mixture containing a higher concentration of gaseous stripping composition is then recirculated to the strip-ping zone. In this way, the concentration of the gaseous stripping composition is rapidly raised in t~e stripping zone.
We have found that the rate sf stripping seems to be directly related to the gaseous stripping composition conoentration in the stripping zone and that the time to strip most o~ls, asphalts and many polymers is greatly reduced by the use o~ a high concentration of gaseous stripping compos~t~on compared to a lower concentration at a given temperature.
A str~pping composition is used which is normally liquid at ambient temperature and pressure and has a vapor pressure _g_ ~17372S

at amb1ent temperature, of about 0.2 to 200 torr9 more preferably 10-100 torr. Hydrocarbons and chlorocarbons are preferred stripp~ng agents, particularly hydrocarbons and chloro-carbons wh~ch have a reflux temperature of at leas~ 40 C.
above ambient temperature and more preferably within the range of 80 - 200 ~.
In our process, h~gh blower speeds and high evaporation rates can be used to quickly raise the vapor concentration of the gaseous str~pping composition in the stripping zone. Most structures and the eonneGting parts of cur system are unaffected by a pressure rise of 100-200 torr ~nd in most cases a stripping composition is used which has a vapor pressure such that the maximum pressure of the system is not above 100-200 torr. If a stripping composit~on having a high vapor pressure ~s used, it may be necessary to reduce the blower speed and/or to bleed a~r through a vent to avoid excess~ve pressure. In the latter case a sol vent recovery system may also be necessary.
~ We have found surprisingly that the temperature of the : gaseous str~pping composit~on ~s not critical and~ moreover~ .
that usually strfpping is accomplished faster when either, or more preferably both the stripping composition and surlFace are at a relatively low temperature. It ~s generally convenient tc carry out our process at about ambient temperature, but a gaseous stripping compos~lon below amb~ent temperature may also be used.
It ~s a part1cular advantage of our process that it is unnecessary and i n fact undesi rabl e to heat the - .

1~737ZS

stripping composition to reflux. In some instances it ~s preferable to contact the coating with gaseous stripping composition wherein ne~ther the strippin~ eomposltion nor coated surface ~s above about ambient temperature and either the surface or stripp~ng romposition is cooled below ambient temperature.
We have found that organ~c coatings are generally stripped much faster by contact w~th a gaseous str~pping composit~on at ambient temperature than would be expected w~th reference to the time required a higher temperatures from considenations of the higher concentration of the gaseous str~pping composition and expected faster rate of any chemical reaction at higher temperature.
When a pa~nted surface wh~oh has been treated ~n ~ccordance with our vapor stripp~ng process ls freed from the gaseous strlpp1ng composition, by air drying or other conven~ent means, it ~s ~ound that ~n many cases, the paint coating has either fallen off completely or can be brushed off readilyleaY~ng only small specks of paint. In most cases, the surface which has been contacted w~th gaseous stripping composit~on is about 75-lOOX free of visible paint res~due. However~ even when an ob~eetionable amount of coat~ng rema~ns, the surface can be abrasive-blasted so as to be 100% clean, 1n a substantially shorter time than that required to obtain a 100% clean surface by abrasive-blasting alone. Alternately, our process can include a flushing or rins~ng operat~on.

~737Z5 In cleaning surfaces of oily or tarry substances normally more than 99X and in some cases lOOX removal is achieved.
During the process of our invention, it is believed that the vapor is adsorbed on and/or absorbed into the coatiny causing the coating to undergo physical and sometimes chemical changes 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, which is a further unexpected advantage of treatmen~ with a stripping composition substantially in the absence of liquid condensate in accordance ~ith our invention.
Means ~or circulation of the gaseous strip~ing compos~tion may also be supplied ins~de the stripping zone such as a gas pump or blower. The efficiency of the present process is increased and the time required to destroy the adhesion of the coating and the surfa~e is decreased when the gaseous stripping composition is thoroughly circulated throughout the strlpping zone~ Where two or more areas are being stripped, the gaseous stripping co~position withdrawn from one strippin~ zone may be circulated to another strippin~ zone. In large scale .
operations, blowers are used, in order to distribute the vapors throughout the structure ~n a reasonable time.
In the event that a ~aseous strlpping composition is chosen wh~ch contains two or more components which do not form a homogeneous solution in the liquid phase, it ~s preferable t have separate ev~por~tors for each of such compounds.

~:~7~

Some organlc coatings conta~n impurlties such as carbon and rust particles or pigments which are ~nsoluble ~n any practi cal gaseous s tri ppi ng composi ti on, and whi ch remai n on the walls of a tank or hold being cleaned-and o~ten trap other components of the or1 gi nal coat~ ng . In cases where the presence of th~ s smal 1 amount of res~ due, whi ch ~ s usual ly less than lX of the or~ginal coat~ng, is object~onable, the following modificat~on of our process generally quickly removes all or almost all of the res~due.
After the bulk o~ the coati ng has been remoYed at the most economical vapor temperature, which may be ambient temperature or below the temperature of the surface being cleaned, the temperature of the rec~rculat~ng gaseous mixture is ra~sed slightly above the temperature of the surface3 i.e. abou~ 1 to 15 C., more preferably 1 to 10 C.~ and ~cst preferably 1 to 5 C. above the temperature of the surface being cleaned~
by ~ncreasing the heat applied in the evaporation zone and/or by heat1ng the input conduit. In the stripping ~one the concen-~ration of the gaseous str~pp~ng compos~tion is rapidly raised .
to saturat~on. As a result, liquid stripp~ng composition quickly condenses on the cosler surface and r~rses the surface free of offending partic1es. Although in thls embod~ment of our process a larger amount of stripping oomposition is used than when condens~ng vapors are avoided, only a mlnimal lncrease in the overall cost of stripping occurs ~n most cases, sinee the resulting solution o~ str~pped coat~ng normally can be returned to the evaporating zone and added to the pool of l~quid stripping composit~on. Concentrated or substant~ally pure eYaporati4n res~dues are drawn from a tap in the bottom of the evaporating zone when enough has accumulated.

~7~7;~5 Depending upon the particular conditions, the time re~uired to remove a coating in accordance with our process may vary from a few minutes, or for a very large area to several hours.
A 3,000 gallon steel tank coated with 2.5 kg. of No. 6 oil has been comple~ely cleaned in about twenty minutes by carrying out our process with a gaseous stripping composition at about 28-29 C. The gaseous stripping composition was maintained at about 2 C. aboYe the temperature of the tank walls during the last seven minut~ of the operat~n.
The particular amount of stripping composition used varies widely, depending upon the nature and thickness of the coating, the particular stripping composition selected, as well as the volume of the stripping zone and the area of the coated surface to be treated, and whether the process includes a rinsing step. .
Broadly speaking9 the ratio of the weight of stripping composition used to that of the coating removed may be from about 0.5 : 1 to about 4 : 1; preferably the maximum amount of stripping composition used, i.e. absorbed and condensed wlth the coating is less than about 3.5 : 1 and more preferably less than 3.0 : 1.
Occasionally an extremely th~ck coating must he removed as when asphalt has been transported in an unheated vessel nr styrene has acciden ally polymerized. The inherent flexibility of our process 1s especially useful for such csndit~ons; the best conditions for removing the coating using a minimum amount o~ stripping composition can be balanced ayainst the importance of the rapidity or "turnaround time" for the operation.

11 7 3~ Z 5 Provided that the area to be stripped can be substant~ally sealed from the atmosphere, there is no practical upper limit to the size or complexity of coated struckures which can be treated with gaseous stripping compositions in accordance with our invention. We have observed no corrosion problems whatsoever with respect to metal surfaces uslng 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 for reuse. When stripping petroleum products such as olls, asphalts and also some paints, distillation of the resultant solution allows recoYery of both the stripping compos~tion and the coating material. The equipment needed is commercially available at reasonable cost and the manpower requirements are low; Ener~y requirements are also low.
Another important advantage of our stripp~ng procedure ~s that personnel need not be exposed to the chem~cal stripping agents or to the coatings, some of which, such as certa~n petroleum products, are dangerous; the chemicals can be transferred from shipping containers to the stripping system .
with littl e 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 w~th air.
: The following examp1es further ~llustrate 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 ind~oated; relative proportions of solvents to paint coating are by weight 1~ 7 3 Examples 1-7 In all examples a compos~te of 45 samples of No. 6 oil of different origins (3.5 9.) was smeared on the upper wa11s o~
a tank (a 500 ml. glass resin kettle) over an area of 190 cm2., capped with a rubber stopper equipped with a thermometer and input and return tubes (11 ~m. inside diameter). A vapor stream was gen~rated by a 1/125 HP squirrel cage blower ~hich blew air over the surface of liquid stripping composition contained in a 500 ml. 3-necked flask wh~ch could be heated electrically, and then the resultant gaseous mixture of air and stripping composition was circulated into the tank. The Yapor stream was then returned to the vacuum side of the blower to be again forced over the surface of the liquid so that the concentration of str~ppin~ co~positi~n in the circulating gaseous mixture was rapidly increased. All tub~ng was 11-12 mm.
i.d~ glass or ~ubber, and the tank input tube was wrapped with heating tape and insulation. The tank was partially immersed in a water bath which kept the temperature of the tank walls constant. The system was brought near equilibrium befo~e each run with the tank replaced by a glass tube fitted with a thermometer. The blower was then stopped, the fitting removed, the tank with No. 6 oil on its walls put ~n its place and the blower restarted. The specific conditions and results of each exa~ple are set forth in Table 1.
In Table 1 St. means stripping and St. Cp. means stripping , composition. The percent of stripping composition used includes 6 stripping composition which ls absorbed and/or condensed during the process.

11 ~3~ 25 As shown by the resul ts of Exampl e 1, when the gaseous stripping composition is recirculated at a temperature 10 C.
above that o~ the wall surfaces of the tank being str~pped, stripping and rinsing are complete and rap-id, and the amount of stripping camposition used is acceptable.
In Example 29 ~he gaseous stripping composition is main-tained at a temperature of 6 C.,below the sur~aces bein~
,stripped. In l2 minutes almost all of the oil (about 99~) is removed, however, no condensation takes place. There is no rinse and a thin spotted film of insoluble residue remains on the surface. Only 0.4 9. of toluene was used per gram of No. 6 oil.
In Example 3, the surface is cooled 8 C. below the rather low temperature of the gaseous stripp~ng composition which causes condensation and rinsing to oocur. Stripping ~s slower but at an acceptable rate; however, only a small amount of stripping composltion is used to provide a very clean surface.' In Example 4, wherein the temperatue of the gaseous strlpping composition is raised 8 C. absve the temperature of the walls, stripping and rinsing are fast and the amount of .
solven~ used is within acceptable limits.
The temperature of the gaseous stripping compos~tion in Example 5 ls increased to l5 C. above the temperature of the walls. The vapor conoentration is higher than in Examples 1-4.
The stripping and, rinsing t~mes are substant~ally reduced, how-ever9 the amount of solvent used, although still acceptable, is relatively high.
Example 6 illustrates a prooess which is fast, but wherein the amount of stripp~ng composit~on used ~s hi~h, although still within acceptable lim~ts. The amount of stripping composition used ~s due in part to the relatively high YOl atility of l,l,l-trichioroethene, ~73725 When a less volatile solvent is used as ir, Example 7, althou~h the strippin~ and rinsing times are increased, substantia11y 1 ess s tripping composition is used.

Examples 8 and 9 The rusted inside surfaces of a cylindrical steel tank of 11 .4 m3. in vol ume were coated with 2.5 kg. of a No. 6 oil cnmposite. A positive displacement blower driven at about 3,000 rpm forced air and recirculated stripping composition vapors over the surface of llquid stripping composition contained in a 2-necked steel evaporation vessel, heated with an immersion heater, and into the tank through insulated 2.5 cm. i.d. tubing, which was polyethylene except for a one meter length of copper tubing, wrapped with heating tape. The input line extended into the center of the bottom of the tank. A return line of the same size and length connected the top of the tank to the vacuum side of the blower. At the end of the cleaning cycle, the solution of No. 6 oit in stripping composition was removed from the tank, the vacuum side of the blower was opened to the air, and the return line connected to a solvent recovery system.
Running the blower for one hour replaced essentially all vapors in the tank with fresh air so that it could be entered for nspection~
The specific conditions of these Examples are set forth in Table 1.
At the conclusion of Example 8, in which no rinsing occurred, streaks of the somewhat oily carbonaceous particles which are present 7n suspension in the compos~te sample of No. 6 oil used were observed on the walls of the tank~

7 2 ~

At the end o~ Example 9, in which the vapor space had been saturated with vapors at-a temperature 2 C. above the tank wall temperature so that rinsing occurred during the last several minutes of the cleaning process, the walls were 100% free of oil and of dark specks of carbonized oil so that a cloth wiped on the surface picked up only a little brown rust.
All of the foregoing examp7es illustrate the excellent results of the present process and show that the process can be carried out rapidly, economically, and w~thout heating the stripping composition to reflux, and also that by varying the conditions, substantial reductions in either time and/or use of stripping compos1tlons can be effected.

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Tabl e 1 Temp. Temp. St Cp-Liquid Gas Temp. St. 9./9O
Ex. St. St.Cp. St.Cp. Surface Time Removed % Clean No . Cp . ** C . C . C . (mi n . ) Boatl n~ R1 nse Surfaoe 1 A 46 30 20 4 . 5 2 .1 yes100 2 A 20 30 36 12 . û O . 4 no99+*
l 3 A 25 25 17 20.0 0.9 yes100 ¦ 4 B 40 30 22 7.0 2.4 yes100 B 56 37 22 3 . 5 3 . 2 yes100 6 C 39 .30 21 5 . 0 3 . 5 yes100 7 D 67 30 21 18.0 1.9 yes100 8 B 53 31 37 43.0 2.1 no 99 9 B 85 29 27 ~0.0 ~.7 yes lOû

*spots of residue remained :::
**A - Tol uene B - Tetrachl oroethyl ene C - 1,1 ,l-Tr~chloroethane D z o-D; chl orobenzene

Claims (14)

What We Desire to Claim and Protect by Letters Patent Is:

1. In a process of stripping a protective or residual coating from a surface, wherein a gaseous stream of an organic stripping. composition normally liquid at ambient temperature and pressure is circulated into contact with the coated surface and wherein neither the coated surface nor the gaseous stream of organic stripping composition is substantially above ambient temperature, the improvement comprising substantially sealing said coated surface from the atmosphere to form a stripping zone, forming a gaseous stream of a stripping composition in an evaporation zone containing a body of liquid stripping composi-tion having a vapor pressure of at least 0.2 torr at ambient temperature and pressure, said evaporation zone being connected directly to said stripping zone so as to provide a substantially closed system, introducing said stream of gaseous stripping composition into said stripping zone into contact with said coated surface and recirculating the gaseous mixture from said stripping zone to said evaporation zone and then back to said stripping zone to rapidly increase the concentration of said stripping composition in said stripping zone but wherein the gaseous pressure in said stripping zone is not increased above about 200 torr.

2. A process according to Claim 1 wherein the gaseous stripping composition has a reflux temperature of at least about 40°C. above ambient temperature and the further step of raising the temperature of the gaseous s-tripping composition being recirculated to about 1° to 15°C. above the temperature of the surface being stripped but below the reflux temperature of said stripping composition, thereby quickly condensing gaseous stripping composition on said surface to rinse said surface so that said surface is substantially completely freed of said coating.

3. A process according to Claim 2 in which the temperature of the gaseous stripping composition is raised about 1° to 10°C. above the temperature of the surface being stripped.

4. A process according to Claim 2 in which the temperature of -the gaseous stripping composition is raised about 1° to 5°C. above the temperature of the surface being stripped.

5. A process according to one of Claims 2, 3 or 4 in which the temperature of the gaseous stripping composition is raised toward the end of the stripping process.

6. A process according to Claim 1 in which said stripping composition has a vapor pressure of about 0.2 to 200 torr at 20°C.

7. A process according to Claims 2, 3 or 4 in which said stripping composition has a vapor pressure of about 0.2 to 200 torr at 20°C.

8. A process according to Claim 1 in which said stripping composition has a vapor pressure of 10 to 100 torr at 20°C.

9. A process according to Claims 2, 3 or 4 in which said stripping composition has a vapor pressure of 10 to 100 torr at 20°C.

10. A process according to Claim 1 in which the stripping composition is a hydrocarbon or halocarbon which has a reflux temperature of about 80° to 200°C.

11. A process according to Claims 2, 3 or 4 in which the stripping composition is a hydrocarbon or halocarbon which has a reflux temperature of about 80°
to 200°C.

12. A process according to Claim 1 in which the gaseous pressure in the stripping zone is not increased above about 100 torr.

13. A process according to Claims 2, 3 or 4 in which the gaseous pressure in the stripping zone is not increased above about 100 torr.

14. A process according to one of Claims 1 or 2 wherein stripping and optionally rinsing is carried out without heating the stripping composition to reflux.