Coating apparatus – Projection or spray type – With projector heating – cleaning or conditioning
Reexamination Certificate
2000-08-11
2002-12-03
Crispino, Richard (Department: 1734)
Coating apparatus
Projection or spray type
With projector heating, cleaning or conditioning
C239S135000, C427S422000
Reexamination Certificate
active
06488773
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a spray apparatus and methods of applying coatings of polymers to application surfaces. More particularly, this invention relates to a method and apparatus for transforming a solid polymer into its molten state and transporting the molten polymer to a spray head for subsequent delivery, in combination with a heated pressurized gas stream, in the form of molten droplets. When the molten polymer droplets strike the application surface, they adhere and combine to form a solid coat of polymer upon cooling.
2. Description of Related Art
It has long been appreciated that thermoplastic polymer coatings offer advantages over solvent-based coatings for providing protection afforded the substrate and to the process (in elimination of solvent vapors to the environment). The coated substrate enjoys enhancement in adhesion, chemical resistance, flex strength/modules, impact resistance, and repairability, as well as providing a broader range of material properties in the polymer coating. The substrate to be coated can be any material relatively resistant to heat, including wood, metals, glass, fibrous glass reinforced synthetic resin, or even cardboard without damaging the material surface. Employing the invention apparatus for applying a polymer coating instead of methods employed in applying a solvent-based coating material offers the environmental advantages of (1) safe and easy transportation, storage, and handling of non-hazardous raw materials; (2) no volatile organic compound (VOC) emissions during application; (3) no hazardous waste generated; and (4) no toxic organic chemical solvents or thinners, as well as the advantages of (5) no messy overspray (with attendant product loss) and (6) no shelf or pot life restrictions.
The earliest thermoplastic polymer coatings were electrostatic powder coatings, which involved electrostatic attraction/attachment of the thermoplastic polymer in powder form onto the metallic surface and heating to temperatures causing the polymer to melt and flow to form a continuous film. While effective, this process suffers practical limitations. The coating cannot be applied in the field. The size of the item to be coated is limited to the size of the curing/melting oven. Further, the thickness of the coating is limited by the electrical insulation (reducing or eliminating the electrostatic attraction force) as the powder thickness increases.
Alternatively, it is known to coat substrate surfaces using flame (or thermal) coating technology. Known thermal spray processes are characterized by chemical combustion heating including powder flame spraying, wire/rod flame spraying, and detonation/explosive flame spraying, and by electrical heating processes including plasma flame spraying. Plasma flame spraying involves the use of an ionized gas consisting of free electrons, positive ions, atoms, and molecules as a means of heating a material, such as metal powder, to a molten state at a high temperature and depositing the metal as a coating on a substrate, such as a chrome plate on an automobile part.
There are a number of known devices for spraying powders of high temperature thermoplastics or other high temperature polymer coatings to a variety of surfaces such as U.S. Pat. No. 3,676,638, which discloses a nozzle whereby powder is fed into the plasma stream downstream from the arc. U.S. Pat. No. 2,774,625 teaches an apparatus which uses detonation waves in spraying powders. U.S. Pat. No. 3,111,267 teaches a thermal spray gun apparatus for applying heat fusible coatings on solid objects wherein powder material is fed directly through a heating zone in the spray in which it reaches a molten or, at least, a hot plastic condition and is then propelled at a relatively high velocity onto the object to be coated. U.S. Pat. No. 3,627,204 discloses a spray nozzle arrangement for plasma gun wherein powder material is fed into a spray nozzle downstream of an arc chamber. U.S. Pat. Nos. 4,004,735 and 4,231,518 teach apparatuses for a detonating application of coating with powdered material. U.S. Pat. No. 4,290,555 teaches a method for introducing powder into a gas stream to be provided to a burner. U.S. Pat. No. 4,370,538 teaches an apparatus for spraying heated powder and the like wherein the apparatus includes a combustion chamber which is cooled by air flowing through an annular passage. U.S. Pat. No. 4,688,722 discloses a nozzle assembly for a plasma spray gun. Also, U.S. Pat. No. 4,911,363 teaches a flame spray apparatus including a combustion head provided with radially spaced longitudinal channels extending inwardly from the periphery thereof along which water passes to cool the combustion head. Finally, U.S. Pat. No. 5,520,334 discloses an air and fuel mixing chamber for a tuneable, high-velocity, thermal spray gun.
While overcoming some of the limitations of electrostatic polymer coating processes, flame coating is inefficient in that it creates new concerns and presents practical limitations of its own. These concerns and limitations relate to the common requirements of all conventional thermal spray systems: first, an open flame (or the equivalent thereof) to melt the thermoplastic polymer; and, second, the necessity that the polymer fed to the spray system be in powder form. In addition to its high-cost, plastic powder is difficult to handle and is conducive to material loss.
It is manifest that any open flame is dangerous and presents serious hazards, both to the applicator and to anyone in his general vicinity. The industrial use of flame spray coating processes essentially amounts to placing flame throwers into the hands of workers in a manufacturing facility. Another impediment to the efficiency of such processes is that plastic is a good insulator. Melting the plastic presents a heat transfer problem. Transferring heat energy into plastic by way of conduction is inefficient. Even a very hot flame is a slow, inefficient solution to the basic heat transfer problem. As a result, most flame systems can spray only about ten (10) pounds of plastic per hour or less. To compound the inefficiency of this slow delivery, most flame spray systems result in only a part of the delivered material being applied to the target substrate material. The application process is dangerous, expensive, and slow.
The velocity of a low velocity flame spray chemical process produces a coating of low bond strength and uneven particle melt; wherein some of the thermoplastic particles are amorphous, and overheated particles are crystalline. The plastic particle's exposure to heat energy is limited to its residence time in the flame. Each particle must reach its melt/sticky temperature during this residence time. Too short a residence time results in particles, that do not achieve this temperature, and thus do not stick to the target surface. The particles that do not stick to the surface fall off and become waste/scrap material. Too long a residence time results in particles that melt and then bum, or crystallize.
The problem of slow delivery has been addressed by one practitioner. Weidman, in U.S. Pat. Nos. 5,041,713 and 5,285,967, discloses high velocity thermal spray guns for spraying a melted powder of thermoplastic compounds onto a substrate to form a coating thereon. The latter patent, in particular, discloses a gun including a high velocity, oxygen fueled (HVOF) flame generator for providing an HVOF gas stream to a fluid cooled nozzle. The heat transfer problem is addressed by diverting a portion of the gas stream for preheating the powder, with the preheated powder being injected into the main gas stream at a downstream location within the nozzle. This method/apparatus approach to overcoming the heat transfer problem to produce a higher velocity spray still leaves concerns associated with the high temperature arc/flame exposure danger and the reliance on a thermoplastic polymer powder as the raw material.
The powder form of the thermoplastic polymer has continued as the material of choice
Ehrhardt Walter L.
Turocy Dennis L.
Crispino Richard
Needle & Rosenberg, PC.
Plastic Stuff, LLC
Tadesse Yewebdar T
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