US3513011A - Electrostatic coating method - Google Patents

Description

E.- P. MILLER ELECTROSTATIC CAOTING METHOD May 19, 1970 2 Sheets-Sheet 1 Filed April 22, 1966 m ll\ 3 4 INVENTOR. EMERY P. MiLLER May 19, 1970 E. P. MILLER ELECTROSTATIC CAOTING METHOD 2 Sheets-Sheet 2 Filed April 22. 1966 INVENTOR. EMERY P MILLER United States Patent 3,513,011 ELECTROSTATIC COATING METHOD Emery P. Miller, Indianapolis, Ind., assignor to Ransburg Electro-Coating Corp., Indianapolis, Ind., a corporation of Indiana Filed Apr. 22, 1%6, Ser. No. 544,594 Int. Cl. B05]: 5/02; B44d 1/095, 1/50 US. Cl. 117-17 6 Claims ABSTRACT OF THE DISCLOSURE An adhesive binder is applied to a surface to be coated, such as a television picture tube. A cloud of powder particles is formed. The surface to be coated is introduced into the cloud with its binder-coated surface facing upwardly. An electrostatic field is established between the binder-coated surface and an electrode above the surface. The powder particles are electrostatically charged and deposited downwardly onto the surface. In coating picture tubes the binder can contain a sensitizing agent and be made conductive to serve as an electrode in the electrostatic field. The sensitizing agent can be activated to afiix deposited phosphor powders in a dotted array.

This invention is concerned with a method and apparatus for coating television picture tubes with phosphors with the aid of electrostatic forces. It is also concerned with the application of powders generally to any surface where extreme uniformity of coating is required such as in the formation of electroluminescent plates and the like.

It has heretofore been the practice in the production of picture tubes for television receivers to apply the phosphor coating to the tube face either as dry powders sprayed onto an adherent layer on the glass surface or as sediments from liquid suspensions which are substantially adhered by being sprayed with an overlay of suitable resin. In the case of those tubes to be used in color television receivers, the process has involved the application of a binder containing a light sensitizing agent to the inside surface of the tube face, and then the dusting of the dry particles of the phosphor thereover to produce a thin adherent powder layer. The thin powder layer over the sensitized binder layer is then exposed to a suitable light image of an array of dots, and the material within the. area of the dots is polymerized onto the tube face as insoluble resin areas to which the powder is adhered. The remaining material, being soluble, is then removed by a rinse and a grid of isolated dots in definite array and location is thus formed over the face surface. The process is then repeated again using the sensitized binder and a different phosphor. A new array of light dots is projected onto the surface to promote the polymerization 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, the dots of the polymerized material being located in still different areas. There is thus produced 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, in addition to being wasteful of powder, is time consuming and thus yields low production rates. Further, such operations create a high incidence of tube faces which have to be rejected by reason of nonuniform coatings. The phosphors 'ice employed in the coating are extremely expensive, and with the high incidence of rejects, substantial quantities of these expensive phosphors are thus wasted.

It is an object of this invention to provide a method and apparatus for coating television receiver picture tubes which will overcome the difficulties and disadvantages discussed above. More specifically, it is an object of this invention to provide a method and apparatus which will rapidly 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 with the aid of electrostatic forces, which will provide a uniform coating on the surface to be coated, which will produce such surfaces with consistent characteristics, and which will be eflicient in operation and in the use of powders. A

further object of the invention is to provide a method of coating such tube faces in which the sensitizing 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 powdered phosphor to be deposited is placed in a fluidized bed and air is passed through it so as to fiuidize the material and form a powder cloud above the bed. The fiuidizing air is turned off and the face inserted into the cloud above the powder bed with its surface to be coated facing upward and with the sensitized binder layer in electrical contact with ground. A charging electrode is positioned above and spaced from the tube face. Application of a potential difference between the electrode and the face electrostatically charges the phosphor particles and in an accelerated manner directs them onto the sensitized layer as said particles tend to settle gravitationally from the cloud. 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 under the illuminated dots, and thus cause the deposited phosphor particles at the dots to be bound 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 by rinsing the surface with a solvent liquid. The sensitized binder application, phosphor powder deposition, and polymerization are repeated twice again, each time using a different phosphor and dilferent dot locations. This results in a finished coating consisting of a triple array of nonoverlapping dots uniformly disposed over the surface to 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 embodying the invention with the tube face in place to be coated; 7

FIG. 2 is a partial section through the apparatus with the tube face withdrawn from the coating chamber;

FIG. 3 is a plan view of the assembly taken through A-A in FIG. 1; and

FIG. 4 is an end View of the carriage mounting arrange ment.

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 a visible red, blue, and green phosphorescence when bombarded by electrons.

To produce such a coating, we clean the tube face and apply thereto, as by spraying, brushing, slushing, or the like, a polar binder solution containing a water soluble light-activatable 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 alcohol 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 while in the wet or partially dried state to thereby give the surface to be coated a surface resistivity in the range of from about 10 to about 10 ohms per square. After the solution has been uniformly applied to the inside surface of the tube face, it is allowed to dry to a tacky condition. The tube face is then ready for the first phosphor application. It is placed in the coating apparatus illustrated in FIG. 1 in the position shown in FIG. 2.

As shown, the coating apparatus comprises a chamber 11 formed from four interconnecting side walls 12 of electrically insulating material projecting upwardly from a bottom wall 13. Conveniently, an outer jacket 14 formed from electrically insulating material and supported on a plurality of feet 15 extends around the chamber 11. A pair of rods'16 extend across the jacket 14 to support the chamber 11 in spaced relation thereto whereby said jacket catches any of the phosphor powder that flows over the upper edges of the chamber. As shown, the upper end of the jacket 14 is spaced slightly above 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 fan 18 is mounted at the bottom of the jacket for conveying the nondeposited powder from the jacket to a collector reservoir 19.

An arm 20 also of insulating material projects upwardly from the jacket 14 and extends transversely across the chamber 11. Adjustibly aflixed to said arm is a rod of conducting material 21 to which is attached an air driving motor 22. Attached to the rotatable shaft of air motor 22 is a charging electrode system 23 composed of a plurality of points extending outwardly of a plane frame member. The electrode is of the same planar shape as the tube face to be coated. In the modification shown this is essentially square. The rod 21 is connected electrically by way of fitting 24 and wire 25 to a high voltage supply 26 whose other end is grounded as at 27. Thus, upon actuation of the motor 22 and energization of the high voltage supply 26, the electrode 23 will be electrically charged with respect to ground and will be rotated with respect to the chamber 11.

One of the walls 12 of chamber 11 has an opening 28 in it located above the level of the fluidized powder bed which is large enough and so shaped to admit the tube face 10 when it is held in a horizontal position. A similar matching opening 29 of slightly larger dimension is located in the corresponding surrounding wall 14.

Attached to the side of the enclosure having openings 28 and 29, there is a bracket member 30 which forms a support frame. Attached to the side members of this frame there are two sets of rollers 31 and two sets of rollers 32. These rolls are arranged to carry the tube face support carriage 33. Rolls at 31 are arranged above and below the angle side bars of carriage 33 as shown in FIG. 4. Rolls at 32 are single rolls arranged to support the angle only from below in the manner of the lower roll of rollers 31. The carriage thus can be moved from the position shown in FIG. 2 to that shown in FIG. 1 without tipping.

The carriage carries at its one end a cross bar 35 and at its other end a flexible thin metal plate 36. Appropriately placed along the carriage is another simi ar plate 37. Plates 36 and 37 are sized so that they will completely close the opening 28 from opposite sides of the panel 14 when the carriage is all the way toward the right as in FIG. 1 or all the way to the left as in FIG. 2. A third flexible plate 38 is attached to the carriage 33 at a position such that it will close opening 29 in wall 14 when the carriage is all the way toward the right. It is therefore made slightly larger than opening 29.

An air permeable plate 41 is mounted on the chamber 11 above the bottom wall 13 and supports the phosphor particles 42 to be deposited. Air is introduced into the compartment 43 between the bottom wall 16 and the plate 41 through an air inlet 44 connected to a source of pressurized air 45. The air moves upwardly from the compartment 43 through the plate 41 to maintain the phosphor particles 42 in suspension immediately above said plate and thus provide a fluidized bed of said particles. By increasing the fiow rate at the inlet 44, the particles wi l billow upwardly from the bed in the chamber and thus form a cloud having a particle density substantially less than the bed extending from the bed to the top opening of enclosure 11; the degree of billowing being dependent upon the flow rate at the inlet 44. The cloud will essentially pour out over the side walls 12 and settle into the space between 12 and 14 to be recovered by fan 18 and collector 19.

With the carriage all the way to the left, the tube face to which the alcohol solution has been applied, is positioned 0n carriage 33 as shown in FIG. 2. A spring clip 39 is attached to the carriage and arranged so that when the face is slipped under it, electrical contact will be made between the coating on the face and the carriage. The carriage is electrically grounded as at 40.

When the cloud of phosphor particles has billowed to the top opening of enclosure 12 and has become reasonably uniform in the space above the bed, the fluidizing air is shut oif and the carriage with the tube face on it is moved to the right. Once the face is in place in the chamber 11, the high voltage is applied to the electrode 23. The voltage is negative and provides an average voltage gradient between the points on electrode 23 and the tube face of at least 5 kilovolts per inch. With the coating on the tube face being grounded and the electrode being charged, the phosphor particles in the cloud from the bed which are in the field extending between the electrode and tube face acquire an electrostatic charge and are thus attracted to and deposited onto the tube face. With the electrode 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.

Phosphor particles which were generated in the cloud before the introduction of the face will flow over into the space between the chamber 11 and jacket 14 and can be reclaimed in the reservoir 45.

An example of the deposition step of our invention may be described as follows: The inside of an eleven 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. It was then in a tacky condition. The face was then placed on the carriage 33 outside of the chamber 11, which chamber had a length and width of 12 inches and an overall height of 15 inches. The plate 42 was located 5 inches above the chamber bottom wall 16. The pins of electrode head 23 were one inch above the top of chamber 11. When the face was moved into the chamber, it was 4 inches above the porous plate 42. This provided a 12 inch spacing between the electrode and tube face. The electrode was rotated at r.p.m., and air was introduced into the compartment 43 at the rate of 5 c.f.m. When the phosphor particles had billowed to fill the chamber 11 and the face had been introduced, the electrode was charged at 60 kv. This produced a uniform coating over the inside surface of the tube face, and said face was removed. If a heavier coating is desired, the cyclecan be repeated. When the desired coating is obtained, the face can be passed on to further processing.

After the phosphor particles have been deposited, the tube face 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 affiX the phosphor particles as a coating on the tube face within the dotted illuminated areas. After polymerization, the coating is rinsed with a suitable solvent, and the nonpolymerized polyvinyl alcohol and the phosphor particles not Within the dotted areas are rinsedaway.

After the dotted coating has dried, a second layer of the binder solution is applied thereover. The deposition process is repeated in the manner previously described in another fiuidizing chamber charged with 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 application. In this manner, a double array of nonoverlapping dots of the first and second phosphors will be affixed 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 from a third chamber 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 imageproducing 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 application. In this manner, a finished coating is produced having a triple array of nonoverlapping dots of the first, second, and third phosphors affixed 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 aflixed 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. The surface need not be non-conducting, but can equally well be made of metal. In such case, the adhesive liquid need not supply the electrical conductivity. Where a uniform layer is required, the binding layer need not possess photo chemical properties, but can simply harden upon drying to bind the particles to the surface.

In case the coating applied by a single application is not adequate, the face or surface can be given further treatment by removing it from the chamber, re-establishing the powder cloud by again turning on and off the fluidizing air, and again inserting the surface into the chamber.

While we have described the electrode as being rotated 6 with respect to the fixed tube face, it is to be understood, of course, that the electrode may be fixed.

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.

I claim:

1. A method of applying a coating of phosphors to a television tube face, comprising the steps of applying a conductive coating of a solution of a polymerizable binder and a light-activated sensitizing agent in water to a surface of the tube face to be coated, drying said coating to a tacky condition, establishing said phosphors in a gas suspension formed by passing an air stream through a bed of said phosphors, removing the forming air stream, placing said tube face with said conductive coating facing upwardly in the gas suspension, electrostatically charging said particles and depositing the charged particles downwardly onto the coating, subjecting part of the coated tube face surface to a source of light to activate said sensitizing agent and polymerize the binder for affixing the deposited particles onto the tube face, and flushing the coated tube face surface with Water to remove the nonpolymerized binder and non-afiixed deposited particles.

2. The invention as set forth in claim 1 in which said binder solution 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 an electrode means is placed in spaced relation to said conductive coating to create an electrostatic field gradient of at least 5 kilovolts per inch between the electrode means and the conductive coating.

3. The invention as set forth in claim 2 with the addition that said electrode means and tube face are moved relative to one another while said particles are being deposited.

4. The invention as set forth in claim 2 in Which the electrostatic field is interrupted while the phosphor particles are being formed into a gas suspension.

5. An electrostatic method of coating television tube faces, comprising applying a conductive coating of a binder and a sensitizing agent to a surface of the tube face to be coated, placing a powdered phosphor to be deposited in a bed, passing air through the bed to fluidize the phosphor and form a phosphor cloud above the bed, connecting said conductive coating with ground, turning ofii the air and inserting the tube face into the cloud with its conductively coated surface facing upwardly, charging an electrode positioned above and spaced from the surface to electrostatically charge the powdered phosphor and in an accelerated manner direct it onto the conductive coating as said particles tend to settle gravitationally from the cloud.

6. The method of claim 5 wherein the electrode and the tube face are moved relative to each other during deposition.

References Cited UNITED STATES PATENTS 2,191,827 2/1940 Bonner et al 117-17 X 2,686,141 8/1954 Sawyer 117-17 X 2,837,429 6/1958 Whiting 117-33.5 X 2,996,380 8/1961 Evans 117-33.5 X 3,019,126 1/1962 Bartholomew 117-17 3,140,176 7/1964 Hoffman 117-33.5 X 3,248,253 4/1966 Bardford et al 117-21 X 3,309,217 3/1967 Nagel 117-21 X 3,323,933 6/1967 Bardford et al. 117-17 3,327,685 6/1967 Heyl et al 117-17 X 3,377,183 4/1968 Hurt et a1 117-17.5 X

WILLIAM D. MARTIN, Primary Examiner E. J. CABIC, Assistant Examiner US. Cl. X.R. 117-18, 33, 33.5

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