US3672927A - Electrostatic coating method - Google Patents

Abstract

A CONDUCTIVE SENSITIZED BINDER COMPRISING A POLYMERIZABLE BINDER MATERIAL SUCH AS POLYVINYL ALCOHOL AND A METALLIC SALT SUCH AS AMMONIUM DICHROMATE DISSOLVED IN A WATER SOLUBLE ALCOHOL AND WATER IS APPLIED TO A NONCONDUCTIVE SURFACE OF A TELEVISION PICTURE TUBE. ELECTROSTATICALLY CHARGED PHOSPHOR PARTICLES ARE PLACED ADJACENT THE TUBE SURFACE. THE CONDUCTIVE COATING OF SENSITIZED BINDER IS MAINTAINED AT PARTICLE-ATTRACTING POTENTIAL. PARTICLES ARE ELECTROSTATICALLY DEPOSITED ON THE CONDUCTIVE SENSITIZED BINDER, WHICH IS ACTIVATED, AFFIXING THE PARTICLES. ONE METHOD OF DEPOSITING PHOSPHOR PARTICLES INCLUDES PROJECTING THE PARTICLES FROM A FLUIDIZED BED TOWARD THE

CONDUCTIVELY COATED TUBE SURFACE AND ESTABLISHING AN ELECTROSTATIC FIELD TO THE CONDUCTIVE COATING FROM AN ELECTRODE TO ELECTROSTATICALLY CHARGE AND DEPOSIT THE PHOSPHOR PARTICLES. THE TELEVISION PICTURE TUBE AND THE ELECTRODE MAY BE MOVED RELATIVE TO EACH OTHER DURING DEPOSITION.

Description

June 27, 1972 L. SPILLER ETA!- 3,672,927

ELECTROSTATIC COATING METHOD Original Filed Feb. 28, 1966 INVENTOIZS if i z// 553;; J 52-1 United States Patent 01 ace 3,672,927 Patented June 27, 1972 3,672,927 ELECTROSTATIC COATING METHOD Lester L. Spiller and Stephen J. Smith, Indianapolis, Ind.,

assignors to Ransburg Electra-Coating Corp., Indianapolis, Ind.

Continuation of application Ser. No. 530,473, Feb. 28, 1966. This application Oct. 29, 1969, Ser. No. 873,761 Int. Cl. B05b 5/00; B44d I/095 U.S. Cl. 117-17 Claims ABSTRACT OF THE DISCLOSURE -A conductive sensitized binder comprising a polymerizable binder material such as polyvinyl alcohol and a metallic salt such as ammonium dichromate dissolved in a water soluble alcohol and water is applied to a nonconductive surface of a television picture tube. Electrostatically charged phosphor particles are placed adjacent the tube surface. The conductive coating of sensitized binder is maintained at particle-attracting potential. Particles are electrostatically deposited on the conductive sensitized binder, which is activated, aiiixing the particles. One method of depositing phosphor particles includes projecting the particles from a fluidized bed toward the conductively coated tube surface and establishing an electrostatic field to the conductive coating from an electrode to electrostatically charge and deposit the phosphor particles. The television picture tube and the electrode may be moved relative to each other during deposition.

This invention is concerned with methods for coating television picture tubes with phosphors by the use of electrostatic forces. This application is a continuation of prior application Ser. No. 530,473 filed Feb. 28, 1966, now abandoned.

It has heretofore been the practice in the production of picture tubes for television receivers to apply phosphors either as dry powders or as sediments from liquid suspensions to an adherent layer on the glass surface of the tube face. In the case of those tubes to be used in color television receivers, the process has involved the application of a binder containing a sensitizing agent to the inside surface of the tube face, and then dry particles of the phosphor are dusted thereover to produce a thin power layer. The thin powder layer over the sensitized binder layer is then exposed to a suitable image of an array of dots, and the material within the area of the dots is polymerized, oraffixed, onto the tube face. The remaining material, being water soluble, is then removed by a water rinse and a grid of isolated dots is thus formed over the face surface. The process is then repeated again using the sensitized binder and a different phosphor with the polymerization of dots of the material located in different areas than the first set of dots. The process is repeated for a third time using the sensitized binder and still a third different phosphor with the polymerization of the dots of the material being located in still different areas to thus produce a finished coating of the phosphors, preferably one particle deep, over the face surface being coated. Since the three specialized groups of dots are each of a phosphor designed to emit, when excited, a different primary color, the complete surface is thus adapted to reproduce the entire color spectrum.

Such methods are extremely expensive. The necessity for dusting the phosphors over the tube face is time consuming and thus yields low production rates. Further, such hand operations create a high incidence of tube faces which have to be rejected by reason of nonuniform coatings. The phosphors employed in the coating are extremely expensive, and with the high incidence of rejects, substantial quanitities of these expensive phosphors are thus wasted.

It is an object of this invention to provide a method for coating television receiver picture tubes which will overcome the difiiculties and disadvantages discussed above. More specifically, it is an object of this invention to provide a method which will electrostatically deposit a coating of phosphor powders on television picture tube faces, which will direct said phosphor powders to and deposit them on a tube face by electrostatic forces, which will provide a uniform coating on the surface to be coated, and which will be efficient in operation. A further object of the invention is to provide a method of coating such tube faces in which the sensitized binder for adhering the phosphors onto the tube face will serve as an agent for supplying surface conductivity to said face for reception of the electrostatically charged phosphor particles.

According to one form of the invention for coating color television tube faces, the uncoated face has a sensitized binder applied in a uniform layer to its surface to be coated. The layer of the sensitized binder is placed in contact with ground, and the tube face is disposed over a fluidized bed of the phosphor to be deposited. A charging electrode is interposed between the surface of the bed and the tube face, and a plurality of charging members are carried in said bed. Application of a potential difference between the electrode and charging members and the face electrostatically charges the phosphor particles and directs them onto the sensitized layer as said particles are projected upwardly from the bed. Conveniently, during deposition the electrode and the tube face are moved relative to each other.

After the phosphor particles have been deposited, light is projected onto the coated surface through a dotted screen to activate the sensitizing agent, polymerize the binder, and thus cause the deposited phosphor particles to be bound at the dots onto the tube surface. After such polymerization, the nonpolymerized binder and phosphor particles at the areas other than the dots are removed from the surface. The application of the sensitized binder, phosphor powder deposition, and polymerization are repeated twice again, each time using a different phosphor and different dot locations. This results in a finished coating consisting of a triple array of nonoverlapping dots uniformly disposed over the surface of the tube; each dot having uniform thickness of phosphor.

Other objects and features of the invention will become apparent from the more detailed description which follows and from the accompanying drawings in which:

FIG. 1 is a vertical section through an apparatus with which the invention can be used; and

FIG. 2 is a plan view of the electrode assembly shown in FIG. 1.

In the production of picture tubes for color television receivers, a coating of phosphors is applied to the tubes. Said coating, which is substantially only one particle thick, is formed of phosphors having a particle size of about three microns which are adapted to produce visible red, blue, and green phosphorus when bombarded by electrons.

To produce such a coating, we clean the tube face 10 and apply thereto, as by spraying, brushing, or the like, a polar binder solution containing a water soluble lightactivatable sensitizing agent. Desirably, said solution comprises about 3% of polyvinyl alcohol and about /2 of a chromic salt, such as ammonium dichromate, dissolved in isopropyl alcohol and water in a 1:1 ratio. The polyvinyl alochol serves as a binding agent for the phosphors to be deposited. The chromic salt, in addition to providing light sensitivity, acts in combination with the polar solvents, isopropyl alcohol and water, to increase the conductivity of the solution to thereby give the surface to be coated a surface resistivity in the range of from about 10 to 3 about ohms per square. After the solution has been applied to the inside surface of the tube face, it is allowed to dry to a tacky condition. The tube face is then ready to have the first phosphor applied thereto and is placed in an apparatus for coating, for example as illustrated in FIG. 1.

As shown, the apparatus for coating comprises a chamber 11 formed from an electricity insulating material and comprises a plurality of interconnected side walls 12 projecting upwardly from a bottom wall 13. Conveniently, a jacket 14 formed from electrically insulating material and supported on a plurality of feet 15 extends around the chamber 11A pair of arms 16 extend across the jacket 15 to support the chamber 11 in spaced relation thereto whereby said jacket catches any of the phosphor powder that flows over the edges of tht chamber. As shown, the upper end of the jacket 14 is spaced slightly below the upper ends of the chamber walls 12, and the lower end 17 of said jacket has a frustoconical configuration for collecting the powders in the jacket. Conveniently, a vacuum pump 18 is mounted at the bottom of the jacket for conveying the nondeposited powder from the jacket to a collector reservoir 19.

A pair of arms 20 project upwardly from the jacket 14 and extend transversely across the chamber 11. Said arms support a rotational driving motor 21 to which a hanger 22 is removably connected. Said hanger has outwardly projecting arms 23 joined at their outer ends to the upper face of a ring 24 having an upturned lip 25 at its inner circumference disposed in contact with the conductive binder layer on the tube face 10 for supporting said tube face in an elevated position about the upper end of the chamber 11. An annular shield 26 formed from an electrically insulating material is connected to the lower face of the ring 24 and projects laterally outwardly over the chamber 11 and jacket 14. The hanger 22, ring 24, and arms are formed from an electrically conductive mate rial and are connected to ground, as at 27, to thus ground the sensitized binder layer on the tube face 10. Thus, upon actuation of the motor 21, the grounded tube face 10 will be rotated with respect to the chamber 11.

An air permeable plate 28 formed from an electrically insulating material is mounted in the chamber 11 above the bottom wall 13 and supports the phosphor particles 30 to be deposited. Air is introduced into the compartment 32 between the bottom wall 13 and plate 28 through an air inlet 33 connected to a source of pressurized air 34. The air moves upwardly from the compartment 32 through the plate 28 to maintain the phosphor particles 30 in suspension immediately above said plate and thus provide a fluidized bed of said particles. By increasing the flow rate at the inlet 33, the particles will billow upwardly from the bed in the chamber toward the tube face 10 and thus form a cloud having a particle density substantially less than the bed; the degree of billowing being dependent upon the flow rate at the inlet 33.

An electrode assembly is mounted on the plate 28 and comprises a base 36 supporting an upwardly projecting spindle 37. The upper end of the spindle is connected to a charging head 38 conveniently having the same general shape and size as the surface to be coated and comprising a plurality of annuluses 39 interconnected by a wire grid 40. A plurality of electrode points 42 project upwardly from the head 38 toward the tube face with their ends substantially equally spaced from said face. Conveniently, a plurality of pins 43 also be mounted in the plate 28 to project thereabove into the fluidized powder bed. The electrode assembly and the pins 43, when the latter are employed, are formed from an electrically conductive material and are connected to a high voltage source 44 for electrostatically charging the phosphor particles.

When the cloud of phosphor particles has billowed around the electrode head above the points 42, the tube face 10 is rotated and the voltage is applied to the head; preferably in an interrupted manner in two successive applications. The voltage is negative and provides an average voltage gradient between the points 42 and the tube face of at least 5 kilovolts per inch. With the tube face being grounded and the electrode being charged, the phosphor particles projected upwardly from the bed around the head and in the field extending between the head and tube face acquire an electrostatic charge an are thus attracted to and deposited onto the tube face. When the pins 43 are employed, they charge the adjacent particles in the bed and help to direct the particles to the tube face to help insure a uniform coating thereon. With the head rotating, a uniform voltage gradient will be established between the head and tube face to insure a uniform deposition of the phosphor particles onto the tube face. Any of the upwardly projected phosphor particles which are not deposited will flow over into the space between the chamber 11 and jacket 14 and can be reclaimed in the reservoir 19. The shield 26, being nonconductive, will not have any of the particles deposited on it and will serve to direct the nondeposited particles into the space between the chamber and jacket.

An example of the deposition step of our invention may be described as follows: The inside of an 11 inch tube face was cleaned and then washed with the sensitized binder, after which it was allowed to dry for six minutes at a relative humidity of 40% and a room temperature of F. until it was in a tacky condition. The face was then mounted on the hanger 22 over the deposition chamber 11, which chamber had a length and width of 12 inches and an overall height of 15 inches. The plate 28 containing four equally spaced pins 43 was located 5 inches above the chamber bottom wall, and the electrode head 38 was 5 /2 inches above the plate 28. This provided a 6 inch spacing between the electrode head and tube face. The tube face was rotated at r.p.m., and air was introduced into the compartment 32 at the rate of 5 c.f.m. When the phosphor particles had billowed around the head 38, the head and pins were charged at 60 kv. in two successive intervals of one second each with about a one second interval between each charging interval. This produced a uniform coating over the insidesurface of the tube face, and said face was removed for subsequent processing.

After the phosphor particles have been deposited, the tube face 10 is removed from the coating chamber, and a beam of light is projected through a dotted image-producing screen and onto the coated surface. The light passing through said screen will polymerize the polyvinyl alcohol to thus retain or afiix the phosphor particles as a coating on the tube face within the dotted areas not masked by the screen. After polymerization, the coating is rinsed with water, and the nonpolymerized polyvinyl alcohol and the phosphor particles not within the dotted areas are rinsed away.

After the dotted coating has dried, a second layer of the binder solution is applied thereover. The deposition process is repeated in the manner perviously described using a phosphor having a different color phosphorescence. After deposition, the polyvinyl alcohol in the binder solution is polymerized by projecting a beam of light through a second dotted image-producing screen and onto the coated surface. Said screen has its dotted pattern offset from the dotted pattern of the first screen whereby the second layer of binder solution will be polymerized in dotted areas adjacent the polymerized dots of the first layer of solution. In this manner, a double array of nonoverlapping dots of the first and second phosphors will be aflixed to the tube face.

After this second coating has dried, the sequence is again repeated using a third layer of binder solution and depositing a third phosphor having still a different color phosphorescence. After deposition of the third phosphor, the polyvinyl alcohol in the third layer of binder solution is polymerized by projecting a beam of light through a third dotted image-producing screen and onto the coated surface. The dot pattern in the third screen is offset from the dot patterns in the first and second screens whereby the third layer of binder solution will be polymerized in dotted areas adjacent the polymerized dots of the first and second layers of solution. In this manner, a finished coating is produced having a triple array of nonoverlapping dots of the first, second, and third phosphors aflixed to the tube face.

As explained above, the production of picture tubes for color television receivers requires the use of three different phosphors with their particles afiixed to the tube face in a triple array of adjacent nonoverlapping dots. In the production of picture tubes for black and white television receivers only one phosphor need be employed, and it does not have to be deposited in any dotted pattern on the tube face, but only in a uniform thin coating over said tube face. Therefore, a single application of the binder solution and the electrostatic deposition of only one phosphor need be employed in the production of such picture tubes. Further, because such tubes do not require the phosphor to be in a dotted pattern, the entire deposited coating is subjected to light projected onto it directly, and no image-producing screens need be employed.

While we have described the tube face as being rotated with respect to the fixed electrode assembly, it is to be understood, of course, that the tube face may be fixed while the electrode assembly is rotated with respect to it, or in some applications it may even be desirable to retain both the tube face and electrode fixed during deposition.

For convenience, reference has been made herein to powder particles suspended in air. However, said particles may be suspended in gaseous medium other than air, and air is intended to include other suitable gases.

We claim:

1. A method of applying a coating of phosphor particles to a television picture tube face, comprising the steps of applying a conductive coating of a binder and a sensitizing agent to a nonconductive surface of the tube face to be coated, the coating formed from a solution of about 3% polyvinyl alcohol and about /2 of chromic salt dissolved in isopropyl alcohol and water in a 1:1 ratio, establishing an adjoining relationship between electrostatically charged phosphor particles and the surface to be coated, maintaining the conductive coating on the surface at a particle-attracting electrical potential to effect deposition of said particles on said coating, and activating said coating to affix the deposited particles onto the tube face surface.

2. The invention as set forth in claim 1 in which said coating is dried to a tacky condition prior to establishing an adjoining relationship between the particles and the surface to be coated.

3. The invention as set forth in claim 1 in which said coating of a binder and a sensitizing agent provides said tube face with a surface resistivity in the range of to about 10 ohms per square.

4. A method of applying a coating of phosphor particles to a nonconductive television picture tube face, comprising the steps of applying a conductive coating formed from a polymerizable binder material and a metallic salt dissolved in a water soluble alcohol and water to the surface of the tube face to be coated, projecting a cloud of said particles suspended in gas toward said tube face, establishing an electrostatic field to said coating, imparting an electrostatic charge to said particles and effecting deposition of said particles on said coating, polymerizing the binder of said coating afiixing the deposited particles onto the tube face surface, and removing the nonpolymerized binder and non-aflixed particles.

5. The invention as set forth in claim 4 in which said binder is light-activatable, and said activating step comprises subjecting the coated surface of the tube face to a source of light to polymerize said binder.

6. The invention as set forth in claim 4 in which the step of removing the nonactivated binder and nonaflixed particles comprises flushing the coated tube face surface with water.

7. The invention as set forth in claim 4 in which said electrostatic field is interrupted at least once while said particles are being projected toward the tube face.

8. The invention as set forth in claim 4 in which said coating provides said surface of the tube face with a surface resistivity in the range of from about 10 to about 10 ohms per square, and there is a voltage gradient between said electrode means and tube face of at least 5 kilovolts per inch.

9. The invention as set forth in claim 4 with the addition that electrode means used to establish the electrostatic field to the coating and tube face are moved relative to another while said particles are being deposited.

10. The method as set forth in claim 4 wherein the coating is dried to a tacky condition prior to projecting a cloud of particles toward the tube face.

References Cited UNITED STATES PATENTS Re. 22,419 1/1944 Smysen 11717 3,327,685 6/ 1967 Heyl et al. 117-17 3,513,011 5/1970 Miller 117-17 2,837,429 6/ 8 Whiting 11733.5 2,996,380 8/1961 Evans 11733.5 3,140,176 7/1964 Hoffman 11733.5 2,940,864 6/ 1960 Watson 11717 3,000,752 9/1961 Jackson 117-17 3,019,126 1/ 1962 Batholomew 1-1717 3,248,253 4/1966 Barford et al. 11721 3,269,838 8/1966 Saulnier 11733.5 3,317,319 5/ 1967 Maynud 1-1733.5 3,323,933 6/1967 Barford et al. 117-17 3,377,183 4/1968 Hurt et al. 117-17.5

FOREIGN PATENTS 592,860 10/ 1947 Great Britain.

WILLIAM D. MARTIN, Primary Examiner M. SOFOELEOUS, Assistant Examiner U.'S. Cl. X.R.

1-17--33.5 E, 33.5 C, 33.5 CM, Digest 6, 33

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