US3741793A

US3741793A - Method of electrostatically coating hollow articles

Info

Publication number: US3741793A
Application number: US00171396A
Authority: US
Inventors: C Simmons
Original assignee: ELECTRO COATING CORP
Current assignee: ELECTRO COATING CORP; ELECTRO-COATING CORP US
Priority date: 1971-08-12
Filing date: 1971-08-12
Publication date: 1973-06-26
Anticipated expiration: 1990-06-26

Abstract

HOLLOW ARTICLES OF INSULTING MATERIALS SUCH AS GLASS BOTTLES ARE ELECTROSTATICALLY COATED BY INTRODUCING A VAPOR INTO THEIR INTERIOR TO BE CONDENSED INTO A CONDUCTIVE COATING ON THEIR INSIDE SURFACE. THE COATING OF CONDENSATE IS MAINTAINED AT A POTENTIAL THAT ATTRACTS A SPRAY OF CHARGED COATING MATERIAL PARTICLES TO THE OUTSIDE SURFACE OF THE ARTICLE.

D R A W I N G

Description

7 Original Filed July 31. 1967 June 26, 1973 c. c. SIMMONS 3,741,793

METHOD OF BLECTROSTATICALLY COATING HOLLOW ARTICLES 2 Sheets-Sheet 1 y CHARLES puanggmhous BY f/ c. c. SIMMONS 3,741,793

METHOD OF ELECTROSTATICALLY COATING HOLLOW ARTICLES June 26, 1973 2 Sheets-P3110421. 2

Original Filed July 31, 1967 INVENTOR CHARLES CURTIS SIMMONS United States Patent 3,741,793 METHOD OF ELECTROSTATICALLY COATING HOLLOW ARTICLES Charles Curtis Simmons, Skokie, Ill., assignor to Ransburg Elech'o-Coating Corp., Indianapolis, Ind. Continuation of abandoned application Ser. No. 657,199, July 31, 1967. This application Aug. 12, 1971, Ser.

Int. Cl. B05b 5/02 US. Cl. 11793.4 NC 7 Claims ABSTRACT OF THE DISCLOSURE This is a continuation of Ser. No. 657,199 filed July 31, 1967, now abandoned.

This invention relates generally to an improved method of applying coatings to the exterior surface of hollow objects such as bottles, jars, glasses, or similar containers which are made of non-conducting materials such as glass or plastics. In particular, this invention relates to a method whereby such objects can have their surfaces coated using the well-known electrostatic deposition methods.

As has been known for a considerable period of time an objects surface can be coated with liquid or solid particulate material by electrostatically charging the coating material particles and introducing them adjacent the surface to be coated while the surface is maintained at particle attracting potenial with respect to the particles. Such coating can be done by several methods. For example in the method disclosed in Us. Pat. Nos. 2,247,963 and 2,334,648 the article is positioned adjacent an electrode and the electrode is raised to a high electrical voltage with respect to the article. The material to be applied is then distributed as a spray of discrete small particles into the space between the electrode and the article. Under the action of the field between the electrode and the articles, the particles become charged to the same sign as the electrode and are, therefore, repelled by it and attracted to the articles. In another method applicable to the coating with liquid materials, the coating material is positioned as a thin extended film supported on a surface which is placed opposite to the article surface to be coated. The film is charged to a high potential with the respect to the article surface and under the action of this high potential the film is formed into a series of cusps. From the end of each of these cusps a succession of small highly charged liquid particles are formed. These particles because of their charge move toward and are deposited on the article surface. This process has the marked advantage of a very high utilization efliciency of the coating material. Other techniques of electrostatic deposition are in existence and can be utilized with the proposed invention.

In each of the above described methods the surface being coated must be brought into particle collecting relationship to the charged material. So long as the surface has some electrical conductivity this condition can be satisfied by placing the surface at a spray attracting potential. Metallic surfaces naturally have this property. If, however, the surface is madeof aninsulating material such as glass or plastic, the necessary conductivity is not present, and if the process is to operate satisfactorily, it becomes a requirement of the application of the process that the conductivity of the surface must be created or that an electrode at a spray-attracting potential be" "ice placed behind the surface to be coated. US Patent 2,7 23,- 921 discloses a method of electrostatic coating in which the necessary conductivity is imparted to a porous insulating surface by spraying it with or dipping it in a conducting liquid such as water. This method, however, could not be used with insulating surfaces such as glass or glazed ceramics because the water would not form a film on the surface and impart the necessary uniform conductivity to the surface to be coated. US. Patent 3,236,679 discloses a method of electrostatic-ally coating an insulating surface by pretreating the surface to form an electrically conductive gel layer which is chemisorbed on the surface. Although this method can be used with insulating surfaces such as nonporous glass and glazed ceramic, its use is frequently undesirable because unless the gel layer is so formulated as to be assimilated into the applied coating it oft times will interfere with the adhesion of the coating to the surface. This method is particularly undesirable if the gel layer must be formed upon the inside surface of a hollow article such as a bottle because its residue may prove contaminating to the product eventually placed in the container. British patent specification 1,035,678 discloses the electrostatic application of coating material particles which are maintained in a vaporous atmosphere during coating to prevent a deposition inhibiting accumulation of electric charge on the article being coated and to facilitate the aggregation of the particles over the surface of the article. US. Patent 2,662,833 discloses a method of electrostatically coating hollow dielectric articles such as bottles by positioning within the bottle an ionizing discharge electrode to provide within the bottle an ionized cloud at spray attracting potential.

It is the object of this invention to provide a method of giving hollow articles, such as bottles, having non-conducting surfaces the necessary conductivity to allow them to be coated electrostatically. It is a further object of this invention to coat such articles without requiring a subsequent cleansing of their interiors. Another object of this invention is to establish the necessary conditions for electrostatic coating without contaminating the surface to be coated. Reference will be made to the following figures for further understanding of the invention,

FIG. 1 is an overall view of one form of the apparatus used in practicing the method.

FIG. 2 is a close-up view of the article and its support.

FIG. 3 is a portion of the-apparatus of FIG. 1 modified to provide a conductive coating on the inside of the article by different means.

Referring to FIG. 1, the hollow articles to be coated are shown as glass bottles of a normal type. They are made of any of the standard bottle glasses which are electrically non-conductors at room temperature. Each bottle is positioned in a special workholder 12 which is made of metal and attached to a chain type conveyor 13. The conveyor is arranged to transport the articles in succession past and in spaced relationship to an electrostatic atomizing bell 14 of the type described in more detail in US. Pat. No. 2,764,012.

In advance of the position at which the articles are loaded into the workholder, a box-type oven 15 may be located through which the top portion of the workholders 12 pass. Electrical element 16 within the oven when energized maintains the temperature within the oven well-above room temperature. The workholders passing through the oven as they travel along on the conveyor may be heated by this oven to a temperature sufficient to vaporize a liquid such as water.

Each workholder as shown in FIG. 2, has a cupshaped receptacle 17 into which the neck of the article loading station such as L there is located a liquid injection device 19. This injection device is so positioned with respect to the workholders that when activated it will inject into the receptacle 17 of the workholder a small quantity of liquid. The liquid supply for this device is indicated as 20. A microswitch activating means 21 is located adjacent the conveyor in such a position that it will be tripped by the passage of each workholder in turn. When the workholder is in position to trip the switch 21, the switch will activate injector 19 and cause a small quantity of liquid material to be injected into the receptacle of the workholder. Loading of the article in the workholder will take place at a position along the conveyor that is sufficiently forward of the spray head 14 to permit the liquid to vaporize and condense on the interior surface of the bottle, for example, a distance sufficient to permit 14 to 20 seconds time between the loading at L and the passage past head 14. A disk shaped porous member 18 capable of receiving and absorbing a liquid material such as water may be positioned at the bottom of the receptacle of each workholder. Such porous members are, however, unnecessary. The cup-shaped receptacle is desirably made of steel or other such material having a high specific heat. When the bottle 11 is placed on the workholder 12, the opening of the bottle is positioned in receptacle 17 of the workholder in such a manner that the internal portion of the bottle is exposed to vapors given off from the liquid which has been injected into the receptacle.

'Each of the workholders are so attached to the conveyor 13 as to be free to rotate about their axis. Each is likewise equipped with a gear or pulley 22 which contacts a stationary bar 23 positioned alongside the conveyor in such a Way as to cause the supports to rotate as they are carried along on the conveyor. Electrostatic atomizing bell 14 to produce a spray of charged coating material particles is positioned downstream from the loading position L. The atomizing bell is mounted on a support stand 24 which consists of a base 25 and an insulating column 26. The bell 14 of the atomizer is mounted on its rotating motor 27. The rotating motor and bell of the atomizer are connected by means of an insulation-covered conducting cable 28 to the outlet 29 of a DC high voltage supply 30. The other end of the high voltage supply is grounded as at 31. The high voltage supply 30 is connected to a voltage source of relatively low voltage by connections 32. In this manner the input voltage supplied at 32 is amplified and rectified by suitable electrical components within the supply 30 so that the output at terminal 29 is a high voltage of approximately 100 kilovolts DC. This voltage by way of conductor 28 is applied to the atomizing head 14 which is thus maintained at high voltage with respect to the objects on the conveyor. The conveyor itself is maintained at ground potential by connection as at 33.

The coating material to be applied to the surface of the bottles is contained in container 34 and is pumped by way of pump 35 through

hose

36 and 37 from the container to the back end of the drive motor 27 of the atomizing bell 14. This material is supplied at a controlled rate by means of varying the speed of the pump 35 through its regulatable drive arrangement 38. The coating material is supplied to the rear end of the hollow shaft of the atomizer and eventually appears as an extended thin film at the forwardly disposed edge of the cone shaped atomizer.

'In operation of the invention the conveyor is set into motion. The movement of the conveyor carries the spindle and its upper support receptacle 17 into oven which raises the temperature of the receptacle to a temperature sufficient to vaporize quickly the liquid injected into it by liquid injection device 19. As the spindle exits from the oven it contacts switch 21 which energizes the injector 19 which in turn injects a small amount of water, or similar liquid, into the receptacle 17 of the holder. If the oven 15 is not available for use, the liquid may be heated prior to its injection into the receptacle. For example, good results have been obtained when water at a temperature of 170 F. has been injected into a receptacle that is at a temperature of F. The spindle advances to loading station L where the bottle 11 is placed in the receptacle 17. Because the liquid is at a temperature sufficient to vaporize, the liquid vapors from the liquid will accumulate onto the inside surface of the bottle 11 which is maintained at a vapor condensing temperature such as room temperature. This condensation will be relatively uniform over the entire internal surface and will have served to establish on the inside surface of the container a coating 39 (FIG. 2) which will be electrically conducting and electrically connected to receptacle 17 at the workholder 12. As the object is transported along the conveyor it comes into spraying relation with the atomizing head 14. The conducting coating 39 on the inside of the container will be maintained at ground potential by virtue of being connected to the conveyor by way of the workholder 12. The electrostatic atomizer 14 will be maintained at high potential by virtue of being connected to the voltage supply 30. An electrostatic field is therefore established between the internal conducting layer 39 of bottles 11 and the atomizing head 14. Under the action of this field the coating material which appears at the forward edge of the atomizing bell as a thin extended film will be broken up into a number of small charged particles which will be attracted toward the conducting layers 39 on the inside surfaces of the bottles 12. These particles will be intercepted by and deposited on the outer surface of the bottles 12 which, therefore, will be coated in the desired fashion.

This invention allows the high efiiciency of electrostatic deposition methods to be applied to the coating of objects which are normally made of non-conducting materials. As the object after being coated passes into an oven which is responsible for the curing of the coating material, the conductive layer 39 will be further vaporized and will leave the internal surface of bottle 12 in exactly the same condition it was prior to the accumulation of the condensed liquid vapor.

It will be evident that if the interval between the coating material curing station and the coating station is short the oven 15 is not necessary to heat the receptacles because the receptacles of the workholder will return to the coating station sufiiciently hot to vaporize the liquid, having been heated during their passage through the coating material curing oven. Under these circumstances it will only be necessary to remove the bottle with its cured coating from the spindle, inject a new quantity of liquid into the receptacle in the workholder and load a new bottle onto the workholder before it passes into the coating station.

*In practicing the method of this invention, the liquid injection apparatus can be eliminated by immersing the neck of the bottle (that portion of the hollow article adjacent the opening) prior to placing it on a heated support.

Many liquids may be used to form a conductive coating inside the bottles. It is only necessary that the liquids chosen be conductive enough in its liquid state to form a conductive layer, such as tap water or most polar organic solvents. Tap water is a preferred liquid since it is inexpensive, readily available, can be vaporized during the curing of most coating materials, and will not leave any harmful residue.

FIG. 3 shows another embodiment of the invention. In this embodiment the workholder 40 advances to the loading station L where a bottle 11 is placed into the receptacle 41 of the workholder. As the conveyor advances the workholder and article toward the coating station, workholder 40 trips a switch 42 supported by conveyor 13. Switch 42 opens a valve 43 which permits the flow of a vapor, such as steam, through nozzle 44. Nozzle 44 directs the vapor through a passageway 45 in the workholder and into the interior of the bottle where it condenses to form the conductive film.

Whenever I use the term bottle in this application, it is understood that I mean any hollow article such as bottles, glasses, jars, bowls, cups, angle irons, concave articles and other such containers. The methods of this invention may be used with other types of material applicators. Electrostatic compressed air or hydraulic atomizers may be used instead of the bell illustrated to produce a spray of charge coating material particles. Likewise coating materials in forms other than liquids can also be used. Powders can likewise be applied electrostatically using these techniques. The described details of the operation are intended to be illustrative only and are in no sense to be considered as limiting on the disclosed invention.

I claim:

1. A method of forming a coating on the exterior surface of a bottle, comprising the steps of injecting water into a receptacle portion of a moving support of a conveyor, the receptacle portion being hot enough to vaporize the water, placing the bottle upside down in the receptacle portion of the support, maintaining the bottle at substantially room temperature to condense evaporated water on the inside surface of the bottle in the form of a coating while moving it to a spray coating zone, producing a spray of charged coating material particles adjacent the bottle while connecting the condensed water coating inside the bottle to ground potential to deposit the coating material on the exterior of the bottle, and heating the coated bottles to cure the coating on the exterior of the bottle and evaporate the water inside the bottle.

2. The method of claim 1 including the step of transporting the support of the conveyor between the point at which the exterior coating of the bottle is cured and the water inside the bottle is evaporated and the point at which the water is injected quickly enough so that the heat absorbed by the receptacle during curing of the coating material may be used to vaporize the Water.

3. The method as set forth in claim 1 wherein the projection of a spray of charged coating material at the bottle comprises the steps of supplying a liquid coating material to a surface of an atomizer, forming the coating material into a thin extended film supported on the surfface, and establishing an electrostatic field between the thin extended film of coating material and the condensed water coating on the inside of the bottle to atomize the coating material for deposition on the outside surface of the bottle.

4. A method of forming a coating on the exterior surface of a bottle, comprising dipping the neck of the bottle into a conductive liquid to be vaporized, placing the bottle upside down on a heated support, connecting the support to ground potential, condensing vaporized liquid to form a substantially uniform conductive film on the interior surface of the bottle, transporting the bottle past a source of electrically charged coating material particles while maintaining the film at a spray attracting potential to deposit charged coating material particles on the exterior surface of the bottle, and heating the coated bottle to cure the coating material on the exterior of the bottle and to evaporate the film formed on the interior surface of the bottle.

5. A method of forming a coating on the exterior surface of a bottle, comprising injecting an electrically conductive liquid to be vaporized into a heated receptacle, placing the neck of the bottle in the receptacle so that its interior is exposed to the surface of the liquid, connecting the receptacle to ground potential, condensing the vaporized liquid to form a substantially uniform conductive film on the interior surface of the bottle, transporting the bottle past a source of electrically charged coating material particles while maintaining the film at a spray attracting potential to deposit changed coating material particles on the exterior surface of the bottle, and heating the coated bottle to cure the coating material on the exterior of the bottle and to evaporate the film formed on the interior surface of the bottle.

6. An apparatus for electrostatically applying coating material particles to a first insulative surface of a hollow article, comprising means to introduce a vapor adjacent a second inside surface of the article, including a liquid injection device, a source of hot liquid to be vaporized, a support including a receptacle to hold the liquid and the article, means to operate the liquid injection device to inject a small amount of liquid into the receptacle, the vapor of said liquid condensing upon the second inside surface of the article to form a substantially uniform conductive film thereon,

means connected to the conductive film for maintaining the conductive film at a coating material particle attracting potential, and

means to provide a spray of charged coating material particles adjacent the first insulative surface of the article, the charged particles of coating material being drawn toward and deposited upon the first insulative surface of the article.

7. The apparatus of claim 6, including means to move the support, means to operate the liquid injection device as the receptacle of the support passes the liquid injection device and to inject a small amount of liquid into the receptacle, and heating means for the receptacle located to 'heat the receptacle prior to its passage past the liquid injection device.

References Cited UNITED STATES PATENTS 11/1955 Starkey 11793.4 NC 9/1967 Point et a1 117-93.4 NC

U.S. C1. X.R.