VAPOR STRIPPING PROCESS

Abstract: Organic coatings, such as paint, varnish, shellac, asphalt and oils are stripped from the surface of a coated object by contacting the surface with a stripping composition in the gaseous state. This process is particularly 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: -1- DESCRIPTION VAPOR STRIPPING PR0CE5S Technical Field The present invention relates to a method for strip- ping organic coatings from coated objects. More par¬ ticularly, 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 holds and ballast tanks of ships. Background Art ' Commonly, paint is stripped from painted objects on a small scale by application of an organic or inorganic solvent or mixture thereof, as discussed in Kirk-Othme r' s ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 14, pp. 485-493, 2nd Edition, John Wiley and Sons, 1967. Among chlorinated hydrocarbon solvents which many formulations employ, methylene chloride ( di chloromethane) has been found to be particularly effective. Liquid stripping compositions usually contain additives includ¬ ing 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 -2- 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 recovered. Moreover, all of the known procedures are generally dangerous and prohibi¬ tively 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 interfe res' with the adhesion of subsequent coatings to the surfaces. Processes have also been described in U.S. patent Nos. 2,689,198 to Judd; 3,794,524 to Nogueira ej _al and 3,832,235 to Cooper e_t _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, A method for cleaning tanks has been disclosed in U.S. patent No. 3,042,553 to Kearney e_t _al wherein a solvent is heated to the boiling point to produce solvent vapors which are fed to a tank where they condense and wash the tank walls. Such methods are not applicable for removing organic coatings from extensive surfaces because the cost of heating a sufficient amount of solvent to reflux- for an extended period is prohibitive and moreover, expensive solvent proof equipment is needed to carry out such an operation on a large scale. Furthermore, in some con- structions, 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 most other protective coatings from large tanks and other large constructions by the slow, unpleasant, expensive and dangerous procedure of abrasive blasting. -3- 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" con¬ figuration which makes it difficult and laborious for a blaster to work through. Removal and disposal of the large amount of blasting grit needed are costly. To date, even though abrasive-blasting has severe disadvantages, it is practically the only procedure in use for removing paint from large surfaces; hydrob lasting 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 tar, pitch, asphalt, petroleum and vegetable oil residues, preparatory to a change in type of cargo, structural repairs, or inspection by government- agencies. Some of these tanks and holds have 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, often 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. « Disclosure 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 -4- avoids problems associated with known stripping proce¬ dures . Important objects of the invention include the pro¬ vision of a method for stripping organic coatings from extensive surfaces, i.e. interiors of large construc¬ tions, such as storage tanks, ballast tanks and holds of ships and f-rom surfaces of any shape, complexity or inclination, by a procedure which is more economical, safer to workers, less damaging to the environment and uses less energy than present methods and which also avoids additives which interfere with subsequent recoat- ing of the surface. A further object of the invention is to provide a method for removing organic coatings and residues which remain after a tank has been drained of a petroleum product or vegetable- oil. Other objects of the invention will be obvious in part and will in part appear hereinafter. With the above and other objects of the invention in view, our invention involves stripping an organic coating from a coated surface by contacting the surface with a stripping composition in the gas phase capable of des¬ troying the adhesion between the coating and the surface, wherein neither the stripping composition nor the surface to be stripped is substantially above ambient temperature. In one preferred embodiment of the invention the gaseous stripping composition is contacted with the surface to be stripped at about ambient temperature, substantially in the absence of liquid stripping composition condensate on the surface being stripped. In another preferred embodi¬ ment of the invention either the gaseous stripping compo¬ sition or the surface to be stripped is below ambient temperature . -5- Best Modes for Carrying Out the Invention We have discovered that organic coatings may be sub¬ stantially 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 residual coatings not usually called protective, which are included within the meaning of coatings in this disclosure. Residual coatings include 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 high vacuum distillation resi¬ due of certain crude petroleums, tars, vegetable oils, and the like, which have to be removed from the surfaces of holds or tanks for a change of cargo or when it is nec- essary to clean them for repairs, Coast Guard inspection, .etc. 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. In one embodiment of our process the gaseous stripping composition is introduced into contact with the coated sur¬ face 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. There is no theoretical lower temperature limit for the process, as long as the stripping chemicals _6_ have a little vapor pressure. Preferably the process is carried out at about ambient temperature for economic reasons . Depending on the particular conditions, the time required to remove or destroy the adhesion of the coating ranges from a few minutes to a few days. We have found that organic coatings are generally stripped much faster by contact with a gaseous stripping composition at ambient temperature than would be expected with reference to the time required at higher temperatures from consider¬ ations of the higher concentration of the gaseous stripping composition and expected faster rate of any chemical reaction at higher temperature. Even more surprising is our discovery that petroleum products such as asphalt and oil and some paints are stripped faster when either the gaseous stripping compositio or the surface to be- stripped is at a lower temperature than at ' higher temperature. For example, asphalt is stripped much faster at 22 C. than at 32 C. or 36 C. at constant concentrations of the vapors of a stripping composition, and No. 6 oil was removed faster when either the substrate or the vapor was cooled. It is a particular advantage of our process that it is unnecessary to heat the stripping composition to reflux and in many cases it is unnecessary or undesirable to heat the gaseous stripping composition at all, since preferably the process is carried out substantially at or below the temperature of the environment. When carrying out the process at about ambient temper- ature, it is generally preferred that the ambient temperatur be at least about 0 C. , otherwise the process may be inconveniently slow for some coatings; although even at an ambient temperature below 0 C. the present process will usually be preferable to other available processes. In some instances it is preferable to contact the coating with gaseo - 7_ stripping composition wherein neither- the stripping composition nor coated surface is above about ambient temperature and either the surface or stripping composition is cooled below ambient temperature. When a painted surface which has been treated in accordance with our vapor stripping process is freed from the gaseous stripping composition, by air drying or other convenient means, it is found that in many cases, the 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 gaseous stripping composition is about 75-100?. free of visible paint residue. However, even when an objectionable amount of coating remains, the surface can be abrasive-blasted so as to be 100?. clean, in a substantially shorter time than that required to obtain a 100?ό clean surface by abrasive- blasting alone.. 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 the coating causing the coating to undergo physical and sometimes chemical 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, which is a further unexpected advantage of treatment with a stripping composition substantially in the absence of liquid condensate in accordance with our invention. When a surface is treated with a liquid or condensate from vapors of a refluxing compo¬ sition, the liquid stripping composition may leach out soluble components of the coating; with many coatings a sticky mess results, the cleaning of which is difficult and substantially less economical than the removal of dry flakes. With oils, asphalts and some paints, distillation of the resultant solution, allows recovery of both.the stripping composition and the coating material. In another preferred embodiment of our invention, the surface to be treated is substantially sealed from the atmosphere to form a stripping zone. A stream of stripping composition in the gaseous state close to or, in some cases, below ambient temperature is introduced into the stripping zone into contact with the coated surface. If the stripping composition is liquid at ambient temperature, the gas stream can be generated conveniently by blowing air over the surface of the liquid in an evaporator with a conduit to the ' stripping zone. The stripping zone is preferably provided with a return conduit to the vacuum side of the gas 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 stripping zone, air may be bled from the stripping zone; normally the density of the gaseous stripping composition is greater than that of air, 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 a substantial increase 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 below ambient temperature in the conduit and stripping zone. In cases -9- where the coating strips faster at lower temperatures, the heat of evaporation may not need to 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 desirable 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 surface is decreased when the gaseous stripping composition is thoroughly circulated throughout the stripping zone. In the stripping zone the coating adsorbs and/or absorbs the gaseous stripping composition and thereby undergoes physical and/or chemical changes and breaks loose from the substrate. The gaseous stripping composition is then pumped from the stripping zone and desorbed from the coating. 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 and to make he tanks safe to enter. The gaseous stripping composition may be continuously introduced into the stripping zone and it is also preferable with compositions of high vapor pressure 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 recovered easily by condensation and/or adsorption on charcoal for reuse and avoidance of air pollution. The -10- gaseous stripping composition may also be continuously .. withdrawn from the stripping zone and recovered or re¬ tained 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. 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. Organdie and inorganic compounds known to be useful for stripping paint, shellac, varnish, and the like, which have a par¬ tial pressure of at least about 2 mm. Hg at ambient temperature can be used in our process. In practice, we prefer to use ' mixtures containing a relatively high per- centage of lower chlo ' rocarbon, particularly chlorocarbons containing 1-3 carbon atoms and 1-4 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 chloroalk anes , such as 1 ,2-di chloroalkanes and chloroform are also advantageous. Not only are such chlorocarbon mixtures usually effective and economical, but also fire and explosion hazards may be reduced or eliminated. Stripping compositions containing methylene chloride in an amount of about 25 to 100?ό 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 100?ό of methylene chloride are used. 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 r lower carboxylic acids, such as formic acid, ammonia, lower- alkylamines, lower alkanols, and lower alkyl ethers, esters, ketones, nitriles, amides, 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?ό of methylene chloride, about l?ό water and about 4 to 29?. by volume of other compounds, such as those just lis.ted are very effective. It has also been found that lower alkyl and dialkyl 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?ά aqueous ethylamine are particularly useful. In general 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. 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. Provided that the area to be stripped, can be substan¬ tially sealed from the atmosphere, there is no practical upper limit to the size or complexity of coated 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 for reuse. The equipment needed is commercially available at reasonable cost and the manpower requirements are low. Another important advantage of our stripping procedure is that personnel need not be exposed to the chemical stripping agents or to the coatings, some of which, such as certain petroleum products, are dangerous; 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. 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. -13- EXAMPLE 1 STRIPPING PAINT A 16 cm. 2 area of a steel plate which has been abrasive-blasted and spray painted with two coats (0.3 mm.) of paint, o. 1 (See Table 1) was grit-blasted to a near white metal condition with a small Speedaire "Sandblasting Gun" (4.8 mm. i.d. nozzle) using "Stanblast" grit (furnace residue) and a pressure of 5,6 kg. /cm. 2 . 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?ό formic acid (1 ml.) i.e. a solvent ratio of MC : FA : H 2 0 of 9 : 0.9 : 0.1; af*ter 14 h rs . exposure to the vapors at 23 C. most of the exposed epoxy coating had delaminated in fragments and fallen into the beaker. The plate was allowed to stand in air 23°C. for four hours. A 16 cm. 2 ' area of the treate ' d surface was then grit-blasted to white metal using the afore- dεs cribed equipment and conditions. This took only a fast sweep of not over 5 seconds, only 6?_ of the time needed for the untreated coating. -14- EXAMPLES 2-15 STRIPPING VARIOUS PAINTS Test panels coated with different paints were exposed to a gaseous stripping composition by placing a coated test panel over the top of a container of a liquid stripping composition or inside a closed container such as a capped jar, desiccator or thin layer chromatography (TLC) chamber containing a pool of liquid stripping com¬ position except Example 15. In all cases the test panel was not in contact with liquid stripping composition, removal of the coating was due solely to the action of stripping composition in the gaseous state. In Example 15 a coated steel panel was placed inside a four-liter receptacle fitted with vapor supply and withdrawal lines and sealed from the atmosphere; a strip- ping composition in the gaseous state was continuously circulated therethrough. All test panels were metal except in Examples 8 and 9 wooden test panels were used. Also, all examples were carried out at the specified ambient temperature except Examples 7a and 7b wherein a TLC chamber containing a pool of stripping composition was placed in a bed of crushed ice and the chamber cooled to 0°C; blotting papers were used to speed up the equilibrium between the vapor and liquid phases in the chamber. The conditions and results of stripping in Examples 2-15 are set forth in Table 2. The paint reference no. in Table 2 is the number of the paint in Table 1, wherein the paints stripped in all of the Examples are identified. Where measured, the thickness of the paint is indicated in parenthesis below the paint reference no. in Table 2. The following abbreviations have been used for the components of the stripping compositions in all of the examples : MC methylene chloride PCE perchloroe thy lene FA formic acid DMF dimethyl formamide MEK methyl ethyl ketone CLF chloroform EA ethylamine HEX hexane • TABLE 1 PAINT COATINGS REMOVED BY GASEOUS STRIPPING COMPOSITIONS Paint Ref e-ren ce Paint Name and/or No. Paint- ype ' Manufacturer 1 Epoxy Carboline Co. 2 Arylamine Epoxy 1204 Southern Imperial Coatings, Inc. Polyurethane 4311 Southern Imperial Coatings, Inc. 4 Alkyd Unknown 5 Aromatic Amine Epoxy Carboline Co. 6 Polyamide- Epoxy 1201 Southern Imperial Coatings , Inc. Phenicon 980-Epoxy Cook Paint and Varnish Flow-Loc Varnish Sherwin-Williams Stain Rust oleum- Linseed- Rust-Oleum Corp. Menhaden Alkyd Resin 10 Epoxy Tank Liner 343 Pan-American Coatings 11 Polyamide Epoxy 313 Pan-American Coatings 12 Permachlor 890 , Southern Imperial Chlorin ate d Coatings , Inc. Rubber Base TABLE 2 STRIPPING PAINT COATINGS 1 Paint Ref. No. Stripping Exposure Ex. (thickness Composition Temp. Time ■ De lamination No. mm. ) ( ratio) °C. hrs . Amount Remarks 2 2 MC:H 2 0 23 47 subs tan ial 72?o' reduction in blasting time for 100?ό clean surfa (99:1) 3 2 MC:FA:H 2 0 22 1 almost complet ) (100:5:4) e (.20-.23 48 • 100* coating fell off panel in solid fragments 4 3 MC: EK:H 2 0 22 48 100* coating fell off in one pie . (94:5:1) 5 4 MC:EA:H_0 22 8.5 ' 90?. C15) (90:3:7) 48 100 6 . 2 MC:FA:H 2 0 17 16 substantially coating fell off leaving a (.25) (95:4.5:0. ) all few fragments of primer 7n 7 MC:FΛ :H 2 n 0 6 W\ no % coating mainly large torn (.13) (90:8.5:1.5) blisters 7b 7 MC:FA:H 2 0 0 22 almost 100* most of paint fell off, al ?>- (.13) (90:8.5:1.5) but a few specks TABLE 2 (CONT. ) Paint Ref. No. Stripping Exposure Exp . ( hickness Compositi on Temp . Time, Dela ination No. mm. )' ( ratio) °C. hrs. • ■ • ■ Amount • Remarks 7C 7 MC:FA:H-0 22 99?- (90:8.5:1.5) 8 8 MC:EA:H„0 22 48 100 coating ran off wood leav (86:9.8:4.2) brown stain - no damage to wood MC:EA:H 2 0 22 48 100* coating blistered and ran (86:9.8:4.2) wood leaving it clean a undamaged but slightly s aine d 10 10 PCE:DMF 22 24 99- 100* (.23-.25) (93:7) 1.1 10 MC:FA:ll_0 22 1/2 99-100 * (.25) (90:9:1) 12 11 MC:FA:H 0 22 1.75 99-100 (90:9:1) 13 12 MC 22 95 TABLE 2 (CONT.) Paint Ref. No. Stripping Exposure Exp. ( thickness Composition Te p . Time. Delamination No. mm. ). ■• ( ratio) C. hrs. Amount Remarks 14 6 MC.EA-H-O 22 30 subs anti al coating swollen, hanging f 65:25:10 panel; 86?ό reduction in blasting time to obtain 100?ό clean surface 15 MC:FA:H 2 0 20.5 100* 93:4:3 -19- EXAMPLE 16 STRIPPING PAINT FROM THE INTERIOR OF A TANK A 1.85 x 1.85 x 1.85 m. steel tank was sand blasted and the interior spray-painted with paint No. 7 to a dry film thickness of .23 mm. Several months later the paint was stripped as follows: To the output side of a GM 3-53 gas pump were connected two steel evaporators, Vessels A and B, in series using 2.5 cm. steel pipe and chemical resistant plastic hose. The evaporators were cylindrical steel vessels equipped with sight glasses, and fitted at the top with an inlet and exit of 2.5 cm. pipes and were placed in heated water baths. The exit pipe was connected to the bottom of the test tank. From the top of the tank, piping led to the input side of the blower. At the exit of the tank, T's led to a vent pipe. -Stripping reagents MC (16 1.) and FA (3 1.) were introduced into Vessels A and B respectively; the blower was started, forcing about 280 l./min. of air from the tank over the surfaces of the stripping reagents, which were heated enough to maintain the vapor stream at about ambient temperature, which varied from 1-21°C. during the run. The pump was run for 8 hours at which time the walls of the tank were nearly bare and paint flakes were falling from the ceiling. The tank was closed from the atmosphere and allowed to stand overnight, after which only a few loose fragments of paint were visible on the walls and ceiling. The vent was opened, Vessels A and B were cooled with a slush of ice and water and the direction of rotation of the gas pump reversed. This replaced the stripping vapors with air and allowed recovery of much of the reagents used. The tank hatches were opened and the dry p.aint flakes (11.5 kg.) removed quickly with a vacuum cleaner. Close inspection of the interior surfaces showed some small specks of paint remaining in the anchor pattern and in pits -20- in the welds at corners. Over 99?ό' of the paint had been removed and it was judged that the surface was clean enough to be repainted without abrasive blasting. EXAMPLES 17-24 AND COMPARATIVE EXAMPLES 17C-21C STRIPPING ASPHALT AND OIL COATINGS AT VARIED TEMPERATURES OF SUBSTRATE Weighed test tubes 12.5 cm. x 0.9 cm. were coated with a substrate and suspended in the vapor phase above the specified liquid stripping composition which was at ambient temperature in a chamber covered with metal foils. To exemplify the practice of the invention, one of the tubes was allowed to remain at ambient temperature: 22 C. or was held at about 21 C. by cooling with tap water. For purposes of comparison a second test tube coated and suspended in a manner identical with the first test tube was warmed by passing a stream of warm water through it. The amount of the substrate removed from the- outside surface of each test tube was measured after exposure to the vapor phase in the glass chamber, either by drying and weighing the tubes after a given time or by weighing the coating which had dropped from the tube after evaporation of absorbed and/or adsorbed gases. Blotters dipping into the liquid helped keep the vapor phase near saturation, however the concentration of the vapor in all of these examples and comparative examples was much less than the ma-ximum amount possible, about 50?ό in the case of methylene chloride. The coating, amount thereof, gaseous stripping composition and percent removal of the coating at different temperatures are shown in Table 3, wherein the letters RG and AB refer to roof ing .grade and air blown asphalt respecti ve ly . -21- TABLE 3 STRIPPING ASPHALT AND OIL COATINGS Gas Coating Stripping Agent ' Composi¬ Amount of Ex. tion Temp Composi¬ Temp. Period of Coating No. (A t. q.) •Q c. tion Treatment Removed ?ό 17 RG Asphalt 21 MC Amb. 45 min. 63.5 (3.5) 17C RG Asphalt 36 MC Amb. 45 min. 3.5 (3.5) 18 AB Asphalt 22 HEX Amb. min 11.3 (4.5) 18C AB Asphalt 32 HEX Amb. 18 min 0.5 (4.5) 19 AB Asphalt 22 HEX Amb, 53 min, 77 (4.5) 19C AB Asphalt 32 HEX Amb. 53 min 33.5 (4.5) 20 AB Asphalt 22 HEX Amb, 4 hrs, 99.7 (4.5) 0C AB Asphalt 32 HEX Amb. 4 hrs 96 (4.5) 21 No. 6 Oil 22 CHF Amb 5 min . 87.5 1C No. 6 Oil 33 CHF Amb 5 min . 70.2 2 Dried Lin¬ 22 MC Amb, 60 99-100 seed Oil 3 Dried 22 MC Amb 60 99-100 Cottonseed Oil 4 Fresh Soy¬ 22 MC Amb, 60 99-100 bean Oil __C ?I [ . '.-IPO " m -22- EXAMPLE5 25 AND 26 STRIPPING OIL AT VARIED TEMPERATURE ' S OF GASEOUS STRIPPING COMPOSITION The temperature of the coating was maintained constant in these examples by running water through the coated tubes and the temperature of the stripping vapor was varied whil its concentration in air was constant. Coated test tubes were suspended in a small tank loosely capped with aluminum foil. Tap water was run through the tubes of the . examples . A stream of air containing 30.0 MC was generated by bubbling 13 l./min. of air through liquid MC maintained at constant level and temperature and passed through a copper coil heater to 78 C. and then into the center of the tank. In Example 26 the tank was cooled in a water bath at 12 C. while in Example 25 the tank was fully exposed to the atmosphere. • When one of the coatings on the tube was so thin that a red cross on a white background could be seen there¬ through, the stripping was discontinued and the amount of coating remaining on the almost transparent test tube was determined. The results are set forth in Table 4. , TABLE 4 Gas Coating Stripping Agent Composi¬ Amount of Ex. tion Temp. Composi- Temp. Period of Coating No. ( Amt. g. ) °C. tion oC. Treatment Removed % 25 No. 6 Oil 23.6 MC 78 52 min 73 26 No. 6 Oil 26 MC 16 35 mi n 73 (0.285) -23- EXAMPLES 27_.3 Q STRIPPING ' WITH A GASEOUS STRIPPING ' ' COMPOSITION AT AND ' BELOW AMBIENT ' TEMPERATURE Tubes coated with a substrate were exposed to the vapors of a stripping composition in a chamber, one tube was left at ambient temperature (A) and one was cooled to a specified temperature. The conditions and results of each example are set forth in Table . ' TABLE 5. STRIPPING AT AMBIENT ' AND BELOW AMBIENT ' TEMPERATURE I — ' - t Period | Ex. Coating Gas Strip Temp . of '■ No. ( A t . q. ) Compositi on ' °C. ' Exposure Removal • 27 RG Asphalt MC (A)22 4 hrs. 99.7 (4.5) 28- RG Asphalt MC 8 10 min. 100 29 Paint ' MC.FA. ""' " (A)21 70 min. 99 No. 10 90:9:1 (1.8) , ' 30 Paint MC:FA:H 0 0-2 16 min. 99 No. 10 90:9:1 ! (1.8) Thus, in one embodiment of the present process it is particularly advantageous to cool the stripping composition or the surface to be stripped about 10 C, to about 70 C. below ambient temperature, more preferably about 20 C. to about 50 C. below ambient temperature. Therefore, in a preferred embodiment of the invention, the process is preferably carried out from about -40 C. to about 8 C. , more preferably from about -20 C. to about 0 C.

Claims: -24- CLAIMS A method for stripping an organic coating from the surface of a coated object characterized by contacting said surface with a stripping composition in the gaseous state capable of destroying the adhesion between said coating and said surface, wherein the temperature of said surface and said gaseous stripping composition during said contacting are at a tempera¬ ture and pressure not substantially above ambient temperature and pressure. A method according to Claim 1 wherein said contacting is carried out substantially in the absence of liquid condensate on the surface of said coated object. A method according to Claim 1 wherein said contacting is carried out while either said surface or said stripping composition is cooled below ambient temperature . A method according to Claim 1 wherein said stripping composition contains a chlorocarbon as the principal ingre dient . A method according to Claim 1 in which said coating stripping composition consists essentially of a mix¬ ture of a chlorocarbon as the principal ingredient and at least one additional component selected from the group consisting of water, ammonia, aliphatic hydrocarbon alkylamine, carboxylic acid, alkanol, alkyl ether, alkyl ester, alkyl nitrile , carboxylic acid amide, alkyl ketone , benzene, lower-alkyl and halogen substituted benzene, heteroaroΛiatic compounds containing up to 8 carbon atoms and volatile inorganic acids, wherein said component has a vapor pressure of at least about 2 mm. of Hg at ambient temperature. 6. A method according to Claim 1 in which said stripping composition is normally liquid at ambient temperature and a stream of stripping composition in the gas phase is formed by passing a stream of air over said stripping composition without heating said stripping composition to reflux temperature. 7. A method according to one of Claims 1-6 in which said organic coating comprises paint, asphalt or oil. 8. A method for stripping an organic coating according to one of Claims 1-6 and the further steps comprising sealing said surface from the atmosphere in a stripping zone , circulating said gas phase stripping composition within said stripping zone, and maintaining said gas phase stripping composition in contact with said surface, until the adhesion between said coating and said surface is substantially des¬ troyed. A method according to one of Claims 1-6 wherein an organic coating is stripped from an interior coated surface of a large construction, including in parti¬ cular the interior of a storage tank on land or the hold or ballast tank of a ship or barge and wherein said method comprises the further steps of sealing said interior surface from the atmosphere to form a stripping zone, forming a stream of stripping composition in the gas phase capable of destroying the adhesion between said coating and said interior surface not substan¬ tially above about ambient temperature, and pressure, introduction said stripping composition in the gas phase into said stripping zone into contact with said interior surface, -26- circulating said stripping composition within said stripping zone and, maintaining said stripping composition in the gas phase in contact with said coated surface not sub¬ stantially above about ambient temperature and pres¬ sure until the adhesion between said coating and said surface is substantially destroyed. fcn- C.\

Inventors: LOUBIER R, PETTERSON R
Applicants: LOUBIER R, PETTERSON R
Application year: 1980
Main classification: B08B5
IPC codes: B08B5, B44D3, C09D9