Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B08—CLEANING
  • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
  • B08B5/00—Cleaning by methods involving the use of air flow or gas flow

Definitions

• 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 holds and ballast tanks of ships.
• 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-Othmer's ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 14, pp. 485-493, 2nd Edition, John Wiley and Sons, 1967.
• Liquid stripping compositions 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.
• 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.
• ballast tank of a ship which usually carries ballast water, be rust-proof.
• 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. Removal and disposal of the large amount of blasting grit needed are costly.
• 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
• Important objects of the invention include the provision of a method for stripping organic coatings from extensive surfaces, i.e. interiors of large constructions, such as stor age tanks, ballast tanks and holds of ships and from 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 recoating 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.
• 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 destroying the adhesion between the coating and the surface, wherein neither the stripping composition nor the surface to be stripped is substantially above ambient temperature.
• 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.
• either the gaseous stripping composition or the surface to be stripped is below ambient temperature.
• 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.
• 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.
• 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 residue 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 necessary to clean them for repairs, Coast Guard inspection, etc.
• 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 solutionwhich 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. There is no theoretical lower temperature limit for the process, as long as the stripping chemicals have a little vapor pressure.
• the process is carried out at about ambient temperature for economic reasons.
• the time required to remove or destroy the adhesion of the coating ranges from a few minutes to a few days.
• 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 considerations of the higher concentration of the gaseous stripping composition and expected faster rate of any chemical reaction at higher temperature.
• the ambient temperatur be at least about 0oC. , otherwise the process may be inconveniently slow for some coatings; although even at an ambient temperature below 0o C. the present process will usually be preferable to other available processes. In some instances it is preferable to contact the coating with gaseous 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.
• 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.
• 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
• 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.
• the gaseous stripping composition be at or below ambient temperature in the conduit and stripping zone. In cases 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.
• 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 the 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 gaseous stripping composition may also be continuously withdrawn from the stripping zone and recovered 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.
• Methylene chloride is an especially useful stripping agent from the point of view of effectiveness, as well as of safety and economy.
• chloroalkanes such as 1,2-dichloroalkanes 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.
• 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, loweralkylamines, 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.
• lower refers to a compound having one to four carbon atoms.
• vapor phase compositions which contain about 70 to 95% of methylene chloride, about 1% water and about 4 to 29% by volume of other compounds, such as those just listed 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% 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.
• a gaseous stripping composition 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.
• 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 composition 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.
• TLC thin layer chromatography
• 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 stripping composition in the gaseous state was continuously circulated therethrough.
• 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
• 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 perchloroethylene FA formic acid DMF dimethyl formamide
• 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:
• a GM 3-53 gas pump 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 1./min.
• 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 in the welds at corners. Over 99% of the paint had been removed and it was judged that the surface was clean enough to be repainted without abrasive blasting.
• 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.
• one of the tubes was allowed to remain at ambient temperature: 22oC. or was held at about 21oC. by cooling with tap water.
• 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.
• 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.
• Example 26 the tank was cooled in a water bath at 12oC. while in Example 25 the tank was fully exposed to the atmosphere.
• 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.
• A ambient temperature
• Table 5 The conditions and results of each example are set forth in Table 5.
• the process is particularly advantageous to cool the stripping composition or the surface to be stripped about 10oC. to about 70oC. below ambient temperature, more preferably about 20oC. to about 50oC. below ambient temperature. Therefore, in a preferred embodiment of the invention, the process is preferably carried out from about -40oC. to about 8oC. more preferably from about -20oC. to about 0oC.

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• Paints Or Removers (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Air-Conditioning For Vehicles (AREA)

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Citations (13)

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• 1980
  • 1980-04-02 JP JP55501042A patent/JPH0416360B2/ja not_active Expired
  • 1980-04-02 WO PCT/US1980/000414 patent/WO1981002858A1/en active IP Right Grant
  • 1980-04-02 AU AU59959/80A patent/AU5995980A/en not_active Abandoned
  • 1980-04-02 EP EP19800900863 patent/EP0049240B1/en not_active Expired
  • 1980-04-02 DE DE8080900863T patent/DE3072068D1/de not_active Expired

Patent Citations (14)

Non-Patent Citations (1)

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