WO2000047370A1 - Method for stripping paint using hard shell pits - Google Patents

Abstract

A method of stripping paint from a painted surface is disclosed. In one preferred embodiment, the method comprises the step of blasting the painted surface with a media comprising hard shell pit particles sized between 12 mesh and 50 mesh.

Description

METHOD FOR STRIPPING PAINT USING HARD SHELL PITS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional application 60/119,879, filed February 12, 1999.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

Paint on ships, bridges, military vehicles and airplanes serves many purposes, including protection from moisture, heat, salt spray or other chemicals. Defense uses also include camouflage to make the painted surface less visible, as well as bright colors and insignias to make the surface more noticeable. In the Air Force and Marines, new coats of paint are often applied to change camouflage patterns or to change unit insignia patterns when aircraft or ships move from one squadron or region of the country to another. In addition, new coats of paint are often applied to spruce up the aircraft before an inspection or an open house. On bridges, new coats of paint are added to deter structural damage due to climatic changes. Eventually, the weight of this paint is enough to affect the operational characteristics of the painted surface.

Paint must be removed from the surfaces in order to allow detail surface in sections, to perform other rework and repair operations, and to keep the weight down to acceptable levels. In the past, chemical pa r.- strippers have been used for removing paints. Due to the development of tougher and tougher paint systems to meet the increasing demands of the industry, more aggressive chemical paint strippers have been developed. These aggressive paint strippers are very efficient in doing the job, but they are hazardous and toxic to the environment and generate large amounts of hazardous waste. Over the last ten to fifteen years there has been a move to reduce the use of hazardous chemicals and the amount of hazardous waste generated by chemical paint stripping. In the defense and aerospace industries, one of the major uses of hazardous chemicals has been for the removal of paint from commercial and military aircraft, ships, bridges and armored vehicles.

Highly toxic materials such as phenol are often used in paint strippers, but the key component is usually the primary solvent, methylene chloride. Because of its unique properties, it has been the backbone of chemical strippers for many years. It not only has the ability to penetrate paint layers, but it is also nonflammable and suppresses the flammability of flammable co-solvents. A number of years ago, methylene chloride was found to cause cancer in some laboratory animals and was labeled as a suspected human carcinogen. Even though recent findings have questioned this conclusion methylene chloride still poses a significant health risk. Metnylene chloride is also considered a toxic crgar. t material in water.

Approximately 72,000 tons of the methylene chloride sold in the United States in 1994 were used in paint stripping. Methylene chloride is listed as a Clean Air Act toxin, must be reported under the Superfund Amendment and Reauthorization Act Toxic Release Inventory, and is a suspected carcinogen. Additionally, methylene chloride is one of the seventeen targeted chemicals of the Toxics Release Inventory. Currently, the Occupational Safety and Health Administration (OSHA) is considering lowering the threshold limit value of methylene chloride from a time-weighted average of 500 ppm to 25 ppm. Further, the U.S. Environmental Protection Agency has promulgated National Emission Standards for Hazardous Air Pollutants (NESHAPs) that will all but eliminate the use of methylene chloride-based strippers for the depainting of aircraft .

Benzyl Alcohol-based Paint Strippers During the past few years, considerable progress has been made in the development of chemical strippers that are non-toxic, safe on the aircraft and not hazardous to the workers or the environment. Products have been developed based on benzyl alcohol. This solvent has a very low vapor pressure, which greatly reduces air pollution, and because of its very low toxicity, does not create hazardous wastewater during its use. These products are neutral to mildly alkaline or acid activated. These products have been tested by tr.e V .5. military for a few years. The performance αata show t.at these products require significantly longer periods of time to strip than the equivalent methylene chloride- based products, but the difference is acceptable in most cases. These products have low toxicity; therefore, it is not always necessary to totally evacuate the area being used for stripping.

Chemical Immersion Paint Stripping The two most common methods of chemical paint stripping are hot and cold stripping.

Hot strippers are normally associated with the use of hot alkali and the stripping of organic cpatings from steel, copper and magnesium. There are other hot strippers that are not highly alkaline but are of relatively neutral pH, which form an emulsion when brought into contact with water or are solubilized and form a clear solution. These hot strippers can strip coatings from aluminum, zinc and brass without etching or darkening the base metals. The hot alkaline paint strippers are formulated with sodium hydroxide as the preliminary element acting together with chelating agents, surface activating agents, and any other solvents that can be added to the mixture to enhance the ability to strip without causing the compounds to cake.

Most alkaline strippers have been used for coatings that are easily saponified such as alkyds, nitrocellulose, and ureas. Coatings, such as epoxy and the new polymers used in the high solids coatings, waterborne coatings, and powders show a resistance to strippers composed primarily of alkali.

The new school of stripping technology embraces the use of hot di-phase strippers. The lower phase generally consists of concentrated alkaline solution and the upper phase consists of odorless blends of non-phenolic water or alkaline-insoluble organic compounds. The compounds have certain characteristics such as high boiling point without being able to steam distill, odorless, and a composition of organic compounds such as glycols, polyglycols, ketones or amines. Currently, solvents are being sought that have high boiling properties, low toxicity, and good ability to strip paint. At the present time, the solvent being tested is not as efficient as the chlorinated or phenolic solvents. Cold strippers are the most universally used chemical strippers. Cold strippers are preliminarily based on the following key ingredients: methylene chloride, phenolics, ester alkaline and acid activators, and any other compounds that can be added to break the bond existing between the organic coating and the substrate, whether it is phosphated, chromated, anodized, or has any other surface treatment used to give organic coatings a better anchor to the metallic substrate.

Currently, a whole set of new paint strippers, bo.th hot and cold, are being developed to replace these chemicals. Unfortunately, this is not an easy task. Few, if any, chemicals perform with the speed and efficiency of methylene chloride.

Plastic Media Blasting for Paint Stripping Plastic media blasting, more environmentally friendly than solvent-based stripping, consists of blasting the surface of an airplane, bridge, ship or armored vehicle with small plastic pellets in a manner similar to sandblasting. This method has been studied in greater detail. This process is partly accepted for one -cycle only by the FAA and is widely used by the military. There are seven types of media, varying from soft to hard and mild to aggressive. The soft media requires a longer dwell-time, so it does not necessarily cause less^' damage then the harder types. Plastic media is quite aggressive on composites, causing an unacceptable amount of erosion and/or fiber damage, with the exception of graphite or boron epoxy. The primary weakness of the plastic media blasting method is the generation of a substantial hazardous waste stream.

Sodium Bicarbonate Blasting for Paint Stripping Sodium bicarbonate blasting, better known as the "Bicarbonate of Soda" stripping process, is similar to the plastic media process except that a small volume of water is injected into the blast stream at the nozzle to eliminate nuisance dust. The media is very effective without causing much damage to the substrate. Since rinsing is very difficult, there is a high potential for corrosion problems. The sodium bicarbonate breaks own in water, forming sodium sesquicarbonate, which has a pH of approximately 10. Leaving this on aluminum parts will be very detrimental to the part. The usual solution to this problem is to use a dilute acid rinse. The dilute acid rinse also creates more hazardous waste, driving up the cost of the total paint removal process. The media itself is not expensive. Unfortunately, it is not recyclable. Enormous amounts of water are required to dissolve and spent media before sewering. The positive aspect of non-recyclable media is that a dedicated facility is not required.

Carbon Dioxide Pellet Blasting for Paint Stripping This process is ideal in the sense that no toxic substrates are generated or released, a dedicated facility is not required, and pre-cleaning or surface preparation of the painted surface is not needed. However, here are some very serious areas of concern with this process, some of which limit the use of CO_: blasting to steel or very thick aluminum parts. Very high pressures are used to accelerate the particles. The particles impact the surface at extremely high velocities, high enough to leave dents in 0.020-inch thick aluminum. Unfortunately, this process is also not recyclable. BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention is a method of stripping paint from the painted surface comprising the step of blasting the painted surface with a media comprising hard shell pit particles sized between 12 mesh and 50 mesh. In a preferred embodiment of the present invention, the hard shell pits are cherry pits and the pits are between 20 mesh and 30 mesh.

It is an advantage of the present invention that the paint stripping method generates less toxic waste than most prior art methods.

It is another advantage of the present invention that the method is both effective and efficient.

Other advantages, features, and objects of the present invention will become apparent after one of skill in the art has reviewed the specification and claims.

DETAILED DESCRIPTION OF THE INVENTION

Work that led to the development of the present invention was begun with the goal of identifying and demonstrating the feasibility of a replacement for the

19% phenolic-methylene chloride stripper currently in use and to replace non-degradable abrasives that add to hazardous solid waste disposal. The ideal replacement stripper should meet some or all of the following criteria: not on the Toxics Release Inventory list; less hazardous to workers and the environment; produces less waste, or less hazardous waste; requires less personnel protective equipment for its use; can be used in the current spray delivery systems. Presumably, performance on stripping paint will be equivalent or superior to the methylene chloride stripper.

We have developed a new paint stripping process using either untreated or sulfide-reacted hard shell pit particles, preferably cherry particles. This efficacy of this method has been successfully demonstrated at a U.S. military base for paint stripping of airplanes and military equipment. This process is cost-effective and environmentally-friendly. It does not require special equipment or a dedicated facility. The sized cherry pit particles could be used for several paint stripping cycles, and the cherry pit waste is non-hazardous and biodegradable. Since these abrasive cherry pits could be used for several paint stripping cycles and they are biodegradable, this process reduces the amount of hazardous waste generated compared to the chemical stripping process. This product does not require any personnel protective equipment or additional personnel for paint stripping. This process is similar to plastic media blasting and the same equipment could be used for the paint stripping. This process consists of blasting the surface of a bridge, ship, airplane or armored vehicle with sized hard shell pit particles.

The pit particles are prepared by first removing any residual fruit from the pit and then drying. The pits are then cracked open, the seed removed, and then the cracked shell is ground until it passes a preset mesh screen based upon the end-use need.

The blasting is done using conventional blasting equipment. This typically consists of a compressor which forces the blasting medium (comprised primarily of the sized pit particles) through an orifice directed toward the surface to be stripped. The blasting medium may be recycled continuously using this equipment until the media reaches a size that passes a preset mesh screen defining a size at which the particles are considered to generally be non-functional for the desired purpose. The surface is judged as sufficiently stripped by visual observation, but the stripped surface may also be measured with an analytical tool for smoothness in some cases.

Four different sizes of ground cherry pits, varying from 12-20 mesh, 20-30 mesh, 40-50 mesh and 60-70 mesh have been tested. Sizes ranging from 12 to 50 mesh appear to be suitable, but we have listed 20 to 30 mesh as preferred because the U.S. Department of Defense specifies that the blasting media it purchases for stripping paint must be in a range of 20 to 30 mesh. Preferably, the cherry pit media is recycled until it lacks sufficient abrasiveness to effectively remove paints. The preferred number of cycles is at least 3. At some point, the cherry pit media becomes too small and passes a screen, along with the stripped paint, and goes to the waste.

We envision that pit material other than cherry pits may be used in the practice of the present invention. Other suitable hard shell (dense) pit material include, for example, almonds, olives, apricots, coconut shells, cocoa nuts, and peaches.

The ground cherry pit media has been tested against aluminum with polymer paints and steel with multiple layers of paints and compared to plastic media blasting using standard conditions typically used with paint stripping materials.

The results are summarized in Table 1. The 60-70 mesh had flow problems that prevented a good test. It may be possible to obtain satisfactory results with the 60-70 mesh pits by first making a slurry from the pits similar to vapor blasting.

The results show that the ground cherry pit media is very effective without causing much damage to the surface. However, this media is more dense than plastic media and tends to raise havoc on aluminum surfaces.

This does not mean that the cherry pit blasting medium is unsuitable for aluminum, only that it is not suitable when the use is for aircraft. This media demonstrates a very good strip rate and would probably be well suited to heavy metal type stripping such as stripping paints from bridges, armored vehicles and large ocean ships.

The cherry pit media is a recyclable paint stripping media, and the final hazardous waste generated by this process is much less than (about 10 to 20 percent) of the hazardous waste generated by the plastic media blasting process .

A generic cherry pit media is cherry pits only—no additives. Although not contemplated at this point, it may be possible that a combination of different types of pits might give a preferred stripping characteristic or that some other additive may be beneficial in some applications . The cherry pit particle media could be used for multiple cycles. It is a natural and inexpensive product and the generated waste is non-hazardous and easily biodegradable. The generated waste is expected to be compostable and to produce up to an 80% reduction in solid waste disposal. The cherry pit media blasting process is environmentally friendly and does not require any special equipment, a dedicated working area or any specialized personnel protective equipment.

In summary, this process is well suited for paint stripping from hard surfaces like armored vehicles, commercial bridges and large ocean ships. The cherry pits are biodegradable, and the pits could be reused to strip paint for several cycles until the pits lose their abrasiveness. The waste generated by this process is tremendously reduced compared to sand blasting or other blasting methods. The generated solid waste is biodegradable and further reduces the solid waste by another 30% .

Conclusions

1. It has been demonstrated that hard shell pits (both treated and untreated) media blasting is effective in stripping paints for airplanes and armored vehicles and could be used for commercial bridges and ocean ships. ("Treated" and "untreated" refer to processing that is done when the whole cherry is processed to remove the pits. Sweet cherries undergo no added treatments. However, maraschino cherries, for example, undergo a sulfite-type treatment which may confer an altered state on the pit. (We have not encountered a difference in paint stripping properties.)

2. The waste is mostly biodegradable. More than 80% reduction in solid waste could be achieved.

3. This cherry pit media is natural, derived from a renewable resource and not on the Toxics Release Inventory List.

4. This process does not require any special equipment or special working area.

5. This process is cost effective and the media could be used for multiple cycles. 6. This process does not require any personnel protective equipment.

7. The paint stripping process could be achieved using material made from any other hard shell pits such as almond, cocoa, and peach, with proper sizing.

EXAMPLE

Cherry pits were received from a cherry processor.

Any residual fruit remaining on the pits was removed, and the pits were dried in an oven. The dry pits were cracked open, and the outer shell was separation from the inner seeds by density. The pits were further reduced in size in a hammer-type mill.

The resulting ground pits were then sized using standard sieve trays. Four size ranges were selected based upon a known end-use requiring a blasting medium of between 20-30 mesh. The ranges selected were: 12-20,

20-30, 40-50 and 60-70 mesh.

A lab-scale paint stripping booth was used. Pieces of painted aircraft metal of about 6 x 9 inches were used and the blasting medium reservoir was loaded with one of the specified test size ranges listed above. The compressor was started, the control valve was opened and media began to come from the nozzle used to direct the blasting medium toward the surface to be stripped of paint.

Data were recorded as shown in Table 1. Sizes smaller than 60 mesh were not useful because they absorbed moisture generated by the compressor and clogged the nozzle.

Claims

CLAIMSWe claim:

1. A method of stripping paint from a painted surface comprising the step of blasting the painted surface with a media comprising hard shell pit particles.

2. The method of claim 1 wherein the hard shell pit particles are cherry pit particles.

3. The method of claim 1 wherein the hard shell particles are sized between 12 mesh and 50 mesh.

4. The method of claim 3 wherein the hard shell particles are sized between 12 mesh and 20 mesh.

5. The method of claim 3 wherein the hard shell particles are sized between 20 mesh and 30 mesh.

6. The method of claim 1 • wherein the paint is stripped from a metal surface.

7. The method of claim 1 wherein the hard shell pit particles are recycled for at least one more stripping cycle.