Coating processes – Removable protective coating applied
Reexamination Certificate
2002-10-15
2004-03-23
Chen, Bret (Department: 1762)
Coating processes
Removable protective coating applied
C427S407100
Reexamination Certificate
active
06709697
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to coatings and coatings removal and more specifically to a barrier coating applicable to aircraft and other structures where repetitive stripping and repainting is anticipated. Aircraft are generally painted with a primer, which can for example be based on an epoxy or urethane resin, and a topcoat, which is based on polyurethane. The subject coating system must resist water and certain organic solvents such as aviation fuel and hydraulic fluids based on phosphate esters such as tricresyl phosphate or tri-n-butyl phosphate. In addition, various coatings removal processes such as chemical, pressurized water and dry media blasting are utilized to remove the coating to the bare substrate. This involves a large amount of manpower, materials, and waste disposal and causes degradation to the substrate. The full coating removal and re-application process also represents a hazard to the environment and personnel.
A need remains for an improved barrier coating and selective paint stripping process that allows for the complete removal of the topcoat for structures where repetitive stripping is anticipated. One such example of this removal process is graffiti removal. The following U.S. Patents, which are incorporated herein by reference, give guidance for this application:
U.S. Pat. No. 5,970,993, Oct. 26, 1999, pulsed plasma jet paint removal, Witherspoon, F. Douglas,
U.S. Pat. No. 4,590,097, May 20, 1986, Method of coating dual protective layers, Booth, Roger G.,
U.S. Pat. No. 5,773,091, Jun. 30, 1998, Anti-graffiti coatings and method of graffiti removal, Perlman, Daniel,
U.S. Pat. No. 5,712,234, Jan. 27, 1998, Graffiti removers which comprise a dye bleaching agent, Pourreau, Daniel B.,
U.S. Pat. No. 5,030,290, Jul. 9, 1991, Paint stripping compositions and method of using same, Davis, Elbert,
U.S. Pat. No. 4,231,805, Nov. 4, 1980, Vapor stripping process, Petterson, Robert,
U.S. Pat. No. 4,231,804, Nov. 4, 1980, Vapor stripping method, Petterson, Robert.
“Blessings on all the kids who improve the signs in the subways . . . ”
So begins Edward Field's 1963 poem “Graffiti,” a sarcastic tribute to the artist-vandals who vanish with the daylight, but leave behind perverse, permanent reminders of their nocturnal antics. A well-known urban problem then, graffiti continues to plague our landscape. Public restrooms, schools, buses, subway cars, road signs, and bridges are just a few of the prime targets for the vandals.
In addition to a painting and paint stripping of aircraft, a need remains to remove graffiti paint, permanent ink and other coatings embedded in structural materials such as concrete block, limestone, metal and wood. Black provides a background to the graffiti problem, and complications involved in removing graffiti from painted and unpainted surfaces in the prior patent as follows:
In attempting to cope with the graffiti problem, much emphasis has been placed on cleaning techniques for use in treating surfaces to remove graffiti. A common technique is to apply a coating of paint remover, such as methylene chloride, benzene or toluene. The paint remover is applied, allowed to dwell on the material for a predetermined period, and then removed by a pressurized water washer. This process may be repeated two to four times to remove as much graffiti as possible. Any graffiti residue is then removed by sandblasting or sanding, which may require a refinishing of the surface to restore it to its former appearance. A second technique for coping with graffiti is to simply repaint the surface, which is timely, expensive, adds weight to the structure and makes the coating system more brittle. The use of a chemical paint remover also has the disadvantage of releasing harmful vapors to the environment and that are detrimental to the health of the workers. In addition, chemical paint strippers tend to soften and remove the underlying paint.
Specialized cleaning compositions have been developed for removing graffiti. Examples of such cleaning compositions are disclosed in U.S. Pat. No. 5,024,780 to Leys. These cleaning compositions are designed for graffiti removal and not as anti-graffiti coatings. The intent of this patent is to introduce a barrier coating and selective coating removal process that will safely remove any surface coating leaving the barrier and primer intact and thus protecting the substrate.
As was mentioned in the previous text, aircraft structures are generally painted with an epoxy or urethane-based primer, and a protective polyurethane topcoat. The entire coating system must be capable of resisting water and certain organic solvents that are used to remove aviation fuel and hydraulic fluids that are based on phosphate esters such as tricresyl phosphate or tri-n-butyl phosphate.
In addition to the conventional two coats of paint, prior art discloses an invention by Booth (U.S. Pat. No. #4,590,097) that describes a solvent-resistant barrier coating that is used beneath commercial aerospace polyurethane topcoat systems. The barrier coating comprises a film-forming addition of a polymer comprising units of a vinyl aromatic monomer and units of a monomer containing a hydroxyl alkyl group and a polyisocyantate containing at least two isocyanate groups per molecule. The addition polymer has hydroxyl content in the range 0.5 to 5 percent by weight due to the presence of the hydroxyl alkyl groups and preferably contains at least 30 percent by weight of the vinyl aromatic monomer. The ratio of isocyanate groups to hydroxyl groups is in the range 1:1 to 2:1. It has been demonstrated that the polyurethane topcoat can be selectively removed from the substrate through the use of a stripping solvent such a methylene chloride. For the subject process, the barrier coating remains undamaged on the surface of the coated parts.
The Booth invention relates to a coating composition that produces a protective coating that is resistant to organic stripping solvents such as methylene chloride. Claims within the invention describe a method for coating a primer coated substrate with a “barrier” coating that is then over-coated with a coating that can be easily stripped off by using commercial organic solvents. The subject coating compositions are particularly useful for coating aircraft structures.
Both Booth, and the present invention introduce the intermediate barrier coat concept as a means of providing a coating composition that is used below a polyurethane topcoat. For example, this intermediate barrier coating is applied between the primer and topcoat, and designed to resist different paint stripping processes that are commonly used to remove polyurethane coatings from aircraft structures. This intermediate barrier coating encapsulates the hazardous materials beneath it by providing protection to the primer coating during the referenced paint removal processes. This encapsulation process, and a subsequent elimination of primer coating removal eliminates the need to wash, etch, alodine and primer coat the stripped surfaces of metallic aircraft structures, which significantly reduces the amount of hazardous waste generated during the paint stripping and re-paint processes.
One specific type of intermediate coating that is disclosed in British Patent No. 1511935 is referenced as a non-cross-linked polyamide. This material is at least partially resistant to the solvents used to remove the topcoat; however, it is designed to be easily removed from the primer coating by using alcohol-based solvents. One opinion suggests that many aircraft systems engineers and maintenance facilities would prefer to use an intermediate barrier coat that is more highly insoluble with the primer coatings. This feature would ensure minimal contact between the solvents and the protective primer coating. An additional requirement for the referenced British intermediate coating is improved adhesion between the intermediate coat and the topcoat.
Polyurethane topcoats for aircraft are generally derived from hydroxyl-functional polyester and a p
Russell, III Robert E.
Slife Richard I.
Stropki John
Wolterman Rick
Auton William G.
Chen Bret
Fletcher, III William Phillip
The United States of America as represented by the Secretary of
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