WO2002036271A2

WO2002036271A2 - Method and apparatus for applying low-solids paint onto plastic parts using an electrostatic process and a supercritical fluid

Info

Publication number: WO2002036271A2
Application number: PCT/US2001/044786
Authority: WO
Inventors: Edward Joseph Boyden
Original assignee: Patent Holding Company
Priority date: 2000-11-06
Filing date: 2001-11-01
Publication date: 2002-05-10
Also published as: AU2002219938A1; WO2002036271A3

Abstract

A method and apparatus for applying a coating or paint such as a low-solids adhesion promoter onto a substrate such as a plastic part using an electrostatic process and a supercritical fluid such as CO2 is disclosed. Also disclosed is a spray gun that is desinged to have minimized coating leakages for use in connection with such a method. In one embodiment of the present invention, a paint spraying gun includes a gasket that seals the constituent parts of the gun that form a fluid flow path defined therein so that substantially no combustion or atomization of the fluid flowing therethrough occurs prior to exit from the guns spray tip. In this embodiment, the gun housing is designed to substantially reduce leaking of paint composition into the gun. It should be understood that the abstract of the disclosure is not intended to be and should not be found to be limiting to the scope of the claims contained in this application or in any patent issuing therefrom.

Description

TITLE OF THE INVENTION

METHOD AND APPARATUS FOR APPLYING LOW-SOLIDS

PAINT ONTO PLASTIC PARTS USING AN ELECTROSTATIC

PROCESS AND A SUPERCRITICAL FLUID

FIELD OF THE INVENTION The present invention generally relates to a method and apparatus for applying a coating onto a substrate and, more particularly, to a method and apparatus for applying paint such as a low-solids paint onto a substrate such as a plastic part using an electrostatic process and a supercritical fluid.

BACKGROUND OF THE INVENTION The instructional manual that describes Graco Inc.'s Precision Mix®/Unicarb® Proportioning System Kit (the "manual") contains a description of what is called the Unicarb® process. The manual states that the Unicarb® process was developed and patented by Union Carbide to reduce volatile organic compound (VOC) emissions and to cut production costs. The manual also states that the Unicarb® process is based on the concept that solvents of common paints, coatings and adhesives can be replaced with supercritical carbon dioxide (or CO₂) in many spray applications.

The manual states that typical paints, coatings and adhesives are reformulated by the suppliers, with the assistance of Union Carbide, to remove the majority of their common solvents. The manual further states that the resulting fluid is a high viscosity, low VOC coating. The manual further states that the material is then mixed with supercritical CO₂ just prior to application.

The manual states that the supercritical CO₂ reduces the viscosity of the material and aids in its atomization. The manual further states that the fluid is atomized by a combination of fluid pressure through a pre-orifice and spray tip and the rapid expansion of the CO₂. The manual further states that once the CO₂ and coating leaves the spray nozzle, the CO₂ returns to its gaseous state, which releases energy that breaks the coating into smaller droplets and improves the quality of the finish. The manual further states that the CO₂ flashes away immediately, leaving the solids of the coating behind.

The manual states that the most common applications for the Unicarb® process include the spray application of paints used in automotive components, adhesion promoters in decorative plastics manufacturing, and conversion coatings and other materials in paper processing. The manual further states that spray applications are typically performed with fluid pressures ranging from 1200 to 2500 psi (8 MPa, 83 bar to 17 MPa, 172 bar) and at temperatures ranging from ambient to 160° F (71° C). The manual further states that the spray gun is typically airless or air-assisted, without using the air-assist feature.

Graco Inc.'s air-assisted spray gun model Pro AA5500sc™/Unicarb® (the "Graco gun") is commercially available, and is recommended for use in the Unicarb® process. The Graco gun includes a pre-orifice and a pre-orifice gasket in the fluid flow path prior to the spray tip that causes at least a portion of the fluid flowing therethrough to combust or atomize.

The following patents apparently relate to various aspects of the Unicarb® process: 4,882,107, 4,923,720, 5,009,367, 5,027,742, 5,057,342, 5,066,522, 5,098,194, 5,105,843, 5,106,650, 5,108,799, 5,141,156, 5,170,727, 5,171,089, 5,171,613, 5,178,325, 5,190,373, 5,203,843, 5,211,342, 5,212,229, 5,214,925, 5,254,260, 5,290,602, 5,290,603, 5,290,604, 5,304,001, 5,304,390, 5,306,350, 5,308,648, 5,312,862, 5,318,225, 5,374,305, 5,387,619, 5,403,089, 5,419,487, 5,455,076, 5,464,154, 5,466,490, 5,505,539, 5,509,959, 5,716,558, 5,989,638, 6,106,742, and 6,106,896. The content of all of these patents are incorporated by reference into this application as if fully set forth herein.

Red Spot paint model no. XUBE, and a CO₂ level of 24% by volume are recommended for use in connection with the one application of the Unicarb® process. The Red Spot XUBE paint is a high solids paint that has 35.3% solids by weight, and 22.0% solids by volume.

Fluid Transfer Systems is a distributor of Graco Inc. products, including certain paint spraying equipment and the Graco gun referenced above. When such equipment was used in connection with the recommended high-solids XUBE paint referenced above together with a 24% CO₂ level, the paint spraying equipment and gun were not operative to produce commercially acceptable results for a number of different reasons as discussed in greater detail hereafter.

First, when the equipment was used under the above-referenced recommendations, it was observed that the coatings produced by the paint spraying equipment on substrate parts were extremely gritty and, therefore, were commercially unacceptable. Second, it was observed that the pre-orifice and the pre-orifice gasket in the Graco gun consistently would clog with the high solids XUBE paint, thereby requiring the paint equipment to be shut down to allow the gun to be cleaned. This is highly undesirable, especially in commercial scale assembly line manufacturing applications. Third, it was observed that paint leaks occurred in the Graco gun because, for example, the gun housing was not properly designed to ensure a sufficient seal around the paint flow path through the Graco gun. Fourth, it was observed that the air-assist portion of the Graco gun consistently became clogged with paint, thereby causing the gun to fail and become inoperative.

BRIEF SUMMARY OF THE INVENTION It is desirable to provide a method and apparatus for applying coatings such as a low-solids adhesion promoter onto a substrate such as a plastic part using an electrostatic process and a supercritical fluid such as CO₂. It also is desirable to provide a spray gun that is designed to have minimized coating leakage for use in connection with such a method. In one embodiment of the present invention, a paint spraying gun includes a gasket that seals the constituent parts of the gun that form a fluid flow path defined therein so that substantially no combustion or atomization of the fluid flowing therethrough occurs prior to exit from the gun's spray tip. In this embodiment, the gun housing is designed to substantially reduce leaking of paint composition into the gun due, for example, to tightened tolerances of the spray gun shroud.

Providing such a method and a paint spraying gun for use in connection therewith has a number of distinct advantages. First, coatings of a commercially acceptable quality are able to be produced in a continuous, in-line paint spraying process on a commercial scale. Second, shut-downs of the paint spraying process due to problems associated with the paint gun are substantially eliminated, thereby allowing the paint spraying equipment to be used for multiple shifts each day to produce large quantities of coated goods for sale. Third, the emission of VOC materials into the environment is substantially reduced over conventional electrostatic painting techniques. Fourth, production costs are substantially reduced.

Other features and advantages of the invention will become apparent from the description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein:

Fig. 1 is general, schematic diagram of an apparatus for applying a coating such as a low-solids adhesion promoter onto a substrate such as a plastic part using an electrostatic process and a supercritical fluid such as CO₂;

Fig. 2 is a general, schematic diagram of a paint booth 56 in which robot gun assemblies 28 and 30 shown in Fig. 1 are utilized;

Fig. 3 is schematic diagram of the CO₂ source 12 shown in Fig. 1; Fig. 4 is a schematic diagram of the resin source 14 shown in Fig. 1;

Fig. 5 is a schematic diagram of the mixing station 16 shown in Fig. 1;

Fig. 6 is a front perspective of an exploded view of the paint gun 78 shown in Fig. 2; and

Fig. 7 is an enlarged rear view of a portion of the paint gun 78 shown in Fig. 6.

DETAILED DESCRIPTION OF THE INVENTION While the present invention is susceptible of embodiment in various forms, there is shown in the drawings a number of presently preferred embodiments that are discussed in greater detail hereafter. It should be understood that the present disclosure is to be considered as an exemplification of the present invention, and is not intended to limit the invention to the specific embodiments illustrated. It should be further understood that the title of this section of this application ("Detailed Description Of The Invention") relates to a requirement of the United States Patent Office, and should not be found to be limiting to the subject matter disclosed herein.

Referring to Fig. 1, a general, schematic view of an apparatus 10 for applying a coating or paint to a substrate part using an electrostatic process and a supercritical fluid such as, for example, CO₂ is shown. In one example, apparatus 10 is utilized to apply a low-solids adhesion promoter or paint onto plastic substrate parts that are used, for example, in automotive applications. A preferred low-solids adhesion promoter for use in com ection with the present invention is, for example, Rohm and Hass paint model no. HP 210544G1-T (the "4G1T paint"). The 4G1T paint has 20.6% solids by weight, has 15.0% solids by volume, and includes a certain percentage of organic solvent that is added by Rohm and Hass for viscosity reduction purposes.

Apparatus 10 includes a CO₂ source 12 and a resin source 14, both of which are connected to mixing stations 16 and 18 via lines 20, 22, 24, and 26 as shown. The CO₂ from source 12 can be supplied in a number of ways such as, for example, in bulk tanks or cylinders from gas supply vendors. Resin source 14 conditions and delivers resin from a supply container (not shown) such as, for example, a 55 gallon drum at a predetermined pressure. In one embodiment of the present invention, resin source 14 provides, for example, a low-solids paint to mixing stations 16 and 18.

Mixing station 16 is connected to two robot/gun assemblies 28 and 30 via output lines 32, 34, and 36 as shown in Fig. 1. Similarly, mixing station 18 is connected to two robot/gun assemblies 38 and 40 via output lines 42, 44, and 46 as shown in Fig. 1. Control stations 48, 50, 52, and 54 are operatively electrically connected to the robot/gun assemblies 28, 30, 38, and 40 as shown in Fig. 1 so that a desired coating such as a low-solids paint may be applied to a substrate part as discussed in greater detail hereafter.

Fig. 2 is a general, schematic diagram of a paint booth 56 that incorporates the robot/gun assemblies 28 and 30 shown in Fig. 1. Control stations 48 and 50 are operatively electrically connected to the robot motor controls 66 and 68 that are positioned adjacent a trolley 58 and an index point 60 in the paint booth 56. Trolley 58 moves unpainted substrates such as, for example, plastic parts for use in automotive applications that are mounted on a movable support (not shown) to index point 60 from prior points in an assembly line denoted by location 62.

Paint operations take place at the index point 60 in paint booth 56. When the painting operations are completed, the coated parts are moved out of the paint booth 56 for further processing, noted as location 64. Examples of such further processing include, for example, baking the coated parts in a drying oven, or applying additional coats of paint in further paint booths, if desired.

Control stations 48 are operatively electrically connected to robot motor controllers 66 and 68 as shown. Robot motor controllers 66 and 68 include positioning arms 70, 72 and 74, 76, respectively, so that paint guns 78 and 80 are operatively positioned with respect to the index point 60 to allow paint spraying operations to take place. The path of movement of paint guns 78 and 80 with respect to the index point 60 is a function of, for example, the particular part that is being sprayed.

Referring to Fig. 3, a schematic diagram of the CO₂ source 12 shown in Fig. 1 is illustrated. CO₂ source 12 includes an inlet line 82 that is connected to a source of CO₂ such as, for example, a supply from an outside gas vendor or a CO₂ generator. CO₂ from inlet line 82 is supplied to two pumps 84 and 86 that are used to pressurize CO₂ to a desired pressure that is set at a user selected level as noted on the pump air regulators 88 and 90. In one embodiment of the present invention, the CO is pressurized to a level of 1500 psi. However, it should be understood that other pressure levels may be utilized as well without departing from the scope of the present invention disclosed herein.

Two pressure gauges 92 and 94 indicate the pressure of the CO₂ that is supplied to output line 96. CO₂ output line 96 is connected to lines 20 and 22 shown in Fig. 1 to supply pressurized CO₂ to the mixing stations 16 and 18. Ambient air is supplied to the pumps 84 and 86 via air line 98.

Fig. 4 is a schematic diagram of the resin source 14 shown in Fig. 1. Resin source 14 includes a main air line 100 that supplies air to pumps 102 and 104. Pumps 102 and 104 receive resin from input resin lines 106 and 108 that are connected to a source of a desired resin such as, for example, a 55 gallon drum of a low-solids adhesion promoter. Pumps 102 and 104 supply pressurized resin to an output resin line 110. Output resin line 110 is connected to lines 24 and 26 shown in Fig. 1 to supply resin to mixing stations 16 and 18. Resin source 14 also includes, if desired, a resin return line 112 that receives waste resin that is collected in the paint booths.

Fig. 5 is a schematic diagram of the mixing station 16 shown in Fig. 1. Mixing station 16 includes a proportioning unit 114 that ensures that an operator selected mixture of CO₂ and resin is supplied to paint gun 78 through mixture output line 116. A mixing station suitable for use in connection with the present invention is PrecisionMix® SC model 969-868 that is manufactured by Graco Inc., and that is available from Grace's distributor Fluid Transfer Systems Inc.

If desired, a resin output line 118 is connected to line 20 via heater 120 so that heated resin can be supplied to additional paint booths other than paint booth 56 shown in Fig. 1. Similarly, if desired, a CO₂ output line 122 is connected to line 24 so that CO₂ can be supplied to additional paint booths other than paint booth 56. Mixing station 16 includes a solvent input line 124 that is utilized to flush the mixing station with solvent for cleaning purposes, if and when desired.

Mixing station 16 includes a water input line 126 that feeds water to tank 128. Heater 130 heats water from tank 128 to a predetermined temperature. Such heated water is supplied to a jacketed output line (not shown) that surrounds the resin output line 116. This causes the resin/CO₂ mixture fed to paint gun 78 via line 116 to be heated to a desired temperature. In one embodiment of the present invention, the resin/CO₂ mixture is heated to a temperature of 150° F. However, it should be understood that the mixture could be heated to other temperatures as well without departing from the scope of the present invention disclosed herein.

Referring to Fig. 6, a front perspective of an exploded view of the paint gun 78 shown in Fig. 2 is illustrated. Paint gun 78 includes a gun barrel assembly 132 that is operatively connected to resin output line 116 (Fig. 5). Assembly 132 includes an electronics package (not shown) and an electrode 134 that is connected to air cap 136 to allow the paint/CO₂ mixture flowing through gun 78 to be charged with a high voltage. In one embodiment, the resin/CO₂ mixture is positively charged. Such gun barrel assemblies and electronics packages suitable for use in connection with the present invention include, for example, the gun barrel assembly and electronics package utilized in Graco Inc.'s Model PRO AA5500sc™/Unicarb® air-assisted spray gun. When a part 56 is electrically grounded at index point 60 in paint booth 56, this allows, for example, paint to be applied to a part by an electrostatic process as discussed in greater detail hereafter.

Paint gun 78 includes a seat housing 138 and a contact ring 140 that are received in a central portion of the threaded end 142 of the gun barrel assembly 132. Paint gun 78 also includes an annular sealing gasket 144 and a spray tip 146 that communicate with the resin/CO₂ output line 116. A rear perspective of cap 136, spray tip 146, and gasket 144 is shown in Fig. 7.

A feature of the present invention is that the aperture in the annular sealing gasket 144 is sized so that substantially none of the resin/CO₂ mixture passing therethrough combusts or atomizes by virtue of the aperture in gasket 144. This provides a number of advantages such as, for example, a significant reduction in plugging and/or fouling of the resin/CO₂ mixture supply path through the gasket 114. Additional reductions in clogging of the flow path of gun 78 are achieved by, for example, the utilization of a low-solids paint such as, for example, the 4G1T adhesion promoter referenced above.

An additional feature of the present invention is that gun 78 provides an airless spray, and does not include any air assist ports for providing an air-assisted spray. This is desirable because, for example, surplus or leaked paint is not able to pass through the air assist ports into the inside of the paint gun 78. If such leakage were to occur, then the gun could fail due, for example, to the presence of leaked paint in the gun's turbine. In any event, air-assisted guns can be utilized in accordance with the teachings of this aspect of the present invention by, for example, sealing the air assist ports by suitable techniques such as, for example, filling the ports with epoxy.

A tip guard 148 is included with gun 78 to protect spray tip 146 from damage. Gun 78 also includes an O-ring seal 150, a retaining nut 152, and a shroud 154, a partial view of which is shown in Fig. 6. Retaining nut 152 includes threads 156 that engage threads 158 on the end 142 of the gun barrel assembly 78 to secure the illustrated components of paint gun 78 together in an operative, substantially leakproof manner.

Yet another feature of the present invention is that the shroud 154 is designed to have a proper length to ensure that the components of gun 78 illustrated in Figs. 6 and 7 are joined together in a sufficiently tight manner to minimize leakage of resin/CO₂ mixture from the flow path through gun 78. This minimizes the chance that gun 78 will fail due to paint leaking problems. Additionally, the shroud 154 is made with generally high tolerances to further minimize mixture leakage into gun 78 due, for example, to the fact that retaining nut 152 is able to be tightened to a further degree than would otherwise be possible.

From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims when the claims are properly interpreted.

Claims

CLAIMS What is claimed is:

1. A method for coating an electrically grounded substrate part with a coating composition, said method comprising the steps of: providing an airless spray gun, said airless spray gun having a flow path defined therethrough and a spray tip at a distal end of said flow path; causing an electrically charged mixture at a predetermined pressure and temperature to flow through said flow path and exit said spray tip, said mixture including a portion of a supercritical fluid, a portion of organic solvent, and a portion of a coating composition, the passage of said electrically charged mixture through said spray tip causing said supercritical fluid to atomize into a gas thereby causing said coating composition and said solvent to break into droplets that are electrically attracted to said substrate part to coat said substrate part; and wherein said spray gun includes an annular gasket that forms a portion of said fluid flow path, said annular gasket preventing the atomization of said supercritical fluid as said mixture passes through said gasket.

2. The method of claim 1 wherein said coating composition comprises a low-solids coating composition.

3. The method of claim 2 wherein said low-solids coating composition is a low-solids adhesion promoter.

4. The method of claim 1 wherein said temperature is about 150° F.

5. The method of claim 1 wherein said pressure is about 1500 psi.

6. The method of claim 1 wherein said supercritical fluid comprises supercritical carbon dioxide.

7. The method of claim 1 wherein said gun includes a shroud, said shroud having a length that allows the gun to be assembled such that leakage of said mixture during use generally is minimized.

8. Apparatus for coating an electrically grounded substrate part with a coating composition, said apparatus comprising: an airless spray gun, said airless spray gun having a flow path defined therethrough and a spray tip at a distal end of said flow path; a spraying mechanism, said spraying mechanism causing an electrically charged mixture at a predetermined pressure and temperature to flow through said flow path and exit said spray tip, said electrically charged mixture including a portion of a supercritical fluid, a portion of organic solvent, and a portion of a coating composition, the passage of said electrically charged mixture through said spray tip causing said supercritical fluid to atomize into a gas thereby causing said coating composition and said solvent to break into droplets that are electrically attracted to said substrate part to coat said substrate part; and wherein said spray gun includes an annular gasket that forms a portion of said fluid flow path, said annular gasket preventing the atomization of said supercritical fluid as said mixture passes through said gasket.

9. The apparatus of claim 8 wherein said coating composition comprises a low-solids coating composition.

10. The apparatus of claim 9 wherein said low-solids coating composition is a low-solids adhesion promoter.

11. The apparatus of claim 8 wherein said temperature is about 150° F.

12. The apparatus of claim 8 wherein said pressure is about 1500 psi.

13. The apparatus of claim 8 wherein said supercritical fluid comprises supercritical carbon dioxide.

14. The apparatus of claim 8 wherein said gun includes a shroud, said shroud having a length that allows the gun to be assembled such that leakage of said mixture during use generally is minimized.