US3235938A

US3235938A - Method of manufacturing an electroluminescent panel

Info

Publication number: US3235938A
Application number: US162878A
Authority: US
Inventors: Lawrence E Greene
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1961-12-28
Filing date: 1961-12-28
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22

Description

Feb. 22, 1966 L. E. GREENE 3,235,938

METHOD OF MANUFACTURING AN ELECTROLUMINESCENT PANEL Filed Dec. 28. 1961 L l f) r m .v x =4:

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Lawrence E. Greene b 6 6 8 His A t einey 3,235,938 METHOD OF MANUFACTURING AN ELEQTROLUMINESCENT PANEL Lawrence E. Greene, (Ileveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed Dec. 28, 1% er. No. 152,878 11 (Ilairns. (Cl. 2925.l6)

This invention relates in general to a method of making an electroluminescent lamp or panel which may bear indicia if desired.

An electroluminescent lamp generally includes a layer of phosphor, capable of generating light under the action of an electric field, sandwiched between a pair of conductive layers, at least one of the conductive layers being capable of transmitting light. One form of electroluminescent lamp now well known is of the flexible type described and claimed in U.S. Patent No. 2,774,004, Jaffe, issued December 11, 1956 and assigned to the same assignee as the present invention, such a lamp comprising a flexible laminated assembly of electrically active layers or elements each of which is of flexible character. The electrically active elements contained in such a lamp generally consist of aluminum foil or a similar back electrode layer coated with a layer of high dielectric constant material which is overcoated with a layer of electroluminescent phosphor and a transparent conductive sheet placed over the coated side of the aluminum foil, the layers being laminated together into a unitary assembly. The aluminum foil and the transparent conductive sheet constitute the electrodes of the electroluminescent cell. The application of an alternating potential across the foil and the transparent conductive sheet results in the creation of an electric field across the phosphor layer which causes the phosphor to emit visible light. The visible light thus emitted by the phosphor layer may readily pass through the transparent conductive sheet.

The transparent conductive sheet heretofore employed in the production of an electroluminescent cell such as that described above is generally comprised of conductive glass such as, for example, commercially available microfiber glass paper 0.001 inch thick. Conductivity is imparted to such paper in any suitable manner, as by dipping the paper in a solution of a suitable metal salt and subsequently drying and baking the paper at elevated temperatures to provide a conductive coating on the surface portions of the constituent gass fibers. For a more complete description of the materials and processes that may be employed in providing such a conductive glass paper, reference may be made to Patent No. 2,849,339, Jaffe, assigned to the assignee of this invention.

Electroluminescent lamps as described above generally emit light of even intensity over their entire surface area. It may be desirable in practice, however, to provide electroluminescent panels of such general type which display discrete light and dark areas when subjected to an alternating electric field, the areas defining certain desired indicia. In order to provide the desired indicia on an electroluminescent panel of the above described type in accordance with the present invention, the layers of electroluminescent phosphor and high dielectric constant material are applied in continuous layers onto a temporary support or release sheet to form a coating thereon, and the back electrode may then be provided in the shape of the desired indicia on the said coating. Since such a back electrode is necessarily an array comprised of a plurality of discrete areas, it will be obvious that the layers of electroluminescent phosphor and high dielectric constant material may not be conveniently coated directly onto the back electrode. In such a situation, one alternative States Patent 3,235,938 Patented Feb. 22, 1966 would be to deposit the coated layers of electroluminescent phosphor and high dielectric constant material on the conductive glass paper, and then deposit the discrete areas of the back conductor over the layer of high dielectric constant material. This solution is not practical, however, in the practice of conventional coating processes since extreme care must be taken when handling or cutting the fragile glass paper because of tendencies of the glass paper to crumble when handled and shatter when out. Ordinary coating processes would result in the destruction of the glass paper if an effort were made to apply the electroluminescent phosphor and high dielectric constant layers directly onto the glass paper.

It is a primary object of my invention, therefore, to provide a new and improved method for making an electroluminescent panel.

Another object of my invention is to provide a new and improved method for making an indicia-bearing electroluminescent panel.

A further object is to provide a new and improved method for making an indicia-bearing electroluminescent panel which includes a conductive layer comprised of a plurality of discrete areas defining the indicia.

Briefly stated, in accordance with the invention, these and other objects may be attained by applying onto a temporary support or release sheet a coating comprised of a layer of electroluminescent phosphor preferably overcoated with a layer of high dielectric constant material, applying a layer of electrically conductivematerial over the said coating to form a back electrode, removing the temporary support sheet to expose the electroluminescent layer, and then laminating electrically conductive glass paper under heat and pressure to the said layer of elec troluminescent phosphor or otherwise applying thereonto a layer of transparent electrically conductive material either of continuous character or in the form of a pattern or indicia, the conductive glass paper or other conductive material forming a front electrode. Electric terminals may then be attached to the front and back electrodes of the sub-assembly as thus produced, and the entire electroluminescent sub-assembly may be then encapsulated, as desired, to form an electroluminescent panel assembly.

The method of this invention is particularly applicable for making an indicia-bearing electroluminescent assembly. When making such an assembly, the temporary support sheet is provided with a coating comprised of a layer of electroluminescent phosphor and preferably, in addition, a layer of high dielectric constant material as described above, after which an array of discrete electrically conductive areas in the shape of the desired indicia is then applied over the said coating to form the back electrode. The temporary support sheet may then be removed or peeled off, and electrically conductive glass paper laminated to, or other conductive material applied onto the layer of electroluminescent phosphor to form the front electrode. Electric terminals may then be attached to the conductive glass paper or other top electrode material and to each of the discrete electrically conductive areas of the back electrode, and the entire electroluminescent panel may be then encapsulated, as desired.

The subject matter is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a pictorial view, on a greatly exaggerated scale, illustrating an intermediate stage in the manufacture of an electroluminescent panel in accordance with the method of the present invention, certain of the constituent layers of the panel being broken away to better illustrate the construction of the panel.

FIG. 2 is an end view of the electroluminescent panel at a later stage in the manufacture thereof by the method comprising my invention.

FIG. 3 is a sectional view on an exaggerated scale of a completed encapsulated electroluminescent panel of the rigid type made in accordance with the invention; and

FIG. 4 is a sectional view likewise on an exaggerated scale of a completed encapsulated electroluminescent panel of the flexible type made in accordance with the invention.

Referring first to FIG. 1, in the production of an electroluminescent panel according to the method comprising my invention, a temporary support sheet 1 comprised of a thin flexible material such as, for example, either polyethylene terephthalate or polytetrafluoroethylene, which are commonly known as Mylar and Teflon, respectively, may be first coated with a layer 2 of a suitable electroluminescent phosphor, which layer 2 is then preferably overcoated with a thin insulating layer 3 of barium titanate or other high dielectric constant material. Materials such as Mylar and Teflon are chosen for the temporary support sheet 1 because of their ability to be easily removed from layers of organic polymeric materials of the type such as are commonly employed as matrix materials for the phosphor and insulating layers 2 and 3. The temporary support sheet 1 alternatively may be composed of other thermoplastic materials or treated papers having the desirable release properties of Mylar and Teflon.

Following the application of the coating 2 of phosphor to the temporary support sheet 1, and after the application of the overcoating 3 of insulating material where such an insulating coating 3 is employed, an electrically conductive layer, which may conveniently be in the form of an array of discrete electrically conductive areas 4, is then applied to the coating on the temporary support sheet to form a discontinuous back electrode as illustrated in FIG. 1. Alternatively, the back electrode 4 may be comprised of a continuous electrically conductive layer.

More particularly, the layer 2 of electroluminescent phosphor may consist of a phosphor such as, for example, zinc sulfide-zinc oxide combined with suitable activators such as copper, manganese, lead, or silver, dispersed in an organic polymeric matrix. Examples of suitable organic polymeric matrices are cellulose nitrate, polyacrylates, methacrylates, polyvinylchlorides, cellulose acetate, alkyd resins, epoxy cements, and polymers of triallylcyanurates, to which may be added modifying substances or plasticizers such as camphor, dioctylphthalate, tricresylphosphate and similar materials. However, plasticized cyanoethyl polyglucosides such as cyanoethyl cellulose plasticized with cyanoethylphthalate, as described and claimed in copending application Serial No. 701,907, Jafie, filed December 10, 1957 and US. Patent 2,951,865, Jatfe et al. dated September 6, 1960, both assigned to the same assignee as the present invention, are preferred organic matrices which form a dense tough film of high dielectric constant and good mechanical and thermal stability. In the practice of this invention, electroluminescent phosphor dispersed in the preferable cyanoethyl cellulose solution may be applied to the temporary support sheet 1 through the use of a doctor blade to form the electroluminescent phosphor layer 2. Alternatively, the layer 2 may be deposited by spraying the phosphor-carrying cyanoethyl cellulose solution onto the temporary support sheet 1. The coated temporary support sheet 1 is then dried, after which a high-dielectric constant material such as barium titanate, which is also dispersed in the preferred cyanoethyl cellulose solution or a similar organic polymeric matrix, may be applied over the phosphor layer 2 by spraying or through the use of a doctor blade to form the thin insulating layer 3. The thin insulating layer 3 functions in the completed electroluminescent cell to prevent electrical shorting between the conductive layers. Following the application of the insulating layer 3 over the phosphor layer 2, the entire assembly may be again dried.

Following the drying of the insulating layer 3, a discontinuous back electrode layer may be applied over the layer 3, the discontinuous back electrode layer desirably being comprised of an array of discrete electrically conductive areas such as the areas 4 shown in FIG. 1. The discrete electrically conductive areas 4 shown in FIG. 1 are deposited over the insulating layer 3 in the form of indicia corresponding to the form of the illumination desired when an alternating electric field is applied across the layer 2 of electroluminescent phosphor. The electrically conductive areas 4 shown in FIG. 1, for example, are provided in the shape of the block number 8, made up of seven bar-shaped areas 4 arranged in two side-byside square patterns. The desired pattern of discrete electrically conductive areas 4 may be applied over the insulating layer 3 by any suitable process, as by a slik screen printing process, preferably in a conducting silver silk screen ink. Such conducting silk screen ink may be obtained commercially. Alternatively, the back electrode layer may be comprised of some form of conductive paint, paste, or similar conductive material which may be sprayed, rolled, or applied by similar means so as to form an array of discrete electrically conductive areas 4. Also, aluminum foil or similar conducting foil in the shape of indicia corresponding to the form of the illumination desired may be secured to the insulating layer 3 by means of a suitable conducting cement. After the back electrode comprised of either the array of electrically conductive areas 4 or a continuous electrically conductive layer is deposited over the insulating layer 3, the entire assembly may be dried if necessary. The flexible temporary support sheet 1 may then be peeled from the remainder of the assembly, leaving the assembly of the phosphor and insulating layers 2 and 3 with the back electrode or electrodes 4 thereon. This assembly is sulfiicently self-supporting, because of the self-supporting character of the organic polymeric materials employed for the matrices of the phosphor and insulating layers 2 and 3, to permit the subsequent handling and processing of the assembly without any danger of its disintegrating or breaking apart. For such reason also, the removal of the temporary support or release sheet 1 from the assembly may, if desired, be performed before the application of the back electrode to the phosphor and insulating layers instead of after such electrode application.

Referring now to FIG. 2, the assembly of the phosphor and insulating layers 2 and 3 having the electrically conductive areas 4 deposited thereon is provided, after the peeling of the temporary support sheet 1 therefrom, with a transparent electrically conductive layer 5 on the phosphor layer 2 to serve as a front electrode for the electroluminescent panel. The conductive layer 5 may be in the form of a sheet of electrically conductive glass paper of the type described previously. The conductive glass paper 5 forming a front electrode may be laminated to the phosphor layer 2 under heat and pressure so as to form, with the other layers, an electroluminescent panel sub-=assembly 6. The laminating of the conductive glass paper 5 to the assembly of the phosphor and insulating layers 2 and 3 and back electrode layer or layers 4, may be carried out in any suitable laminating press. Instead of being in the form of conductive glass paper, the conductive layer 5 constituting the front electrode of the electroluminescent sub-assembly alternatively may consist of any other suitable transparent electrically conductive material applied to the phosphor layer either as a continuous layer thereover or in the form of separate discrete areas forming a pattern or design thereon, as by silk-screening the conductive material onto the phosphor layer. Suitable selection of the temperature and pressure of the laminating operation will result in a tight bond between the glass paper 5 and the electroluminescent phosphor layer 2 which will promote even and high-level lighting of the desired indicia.

The electroluminescent panel sub-assembly 6 described above may be provided with electrical leads or terminals for supplying alternating electric potential to the front and

back electrodes

5 and 4, respectively, after which the sub-assembly 6 with the leads attached may then be encapsulated to form a unitary electroluminescent panel assembly 7 as shown in FIG. 3. More particularly, electrically conductive wires or pins such as the leads 10 and 11 shown in FIG. 3 may be connected, respectively, to the electrically conductive areas 4 of the back electrode and to the conducting glass paper 5 of the front electrode. If desired, a common lead such as lead 10 may be electrically connected to a plurality or all of the discrete conductive areas 4, or a separate lead may be attached to each of the conductive areas 4 in which latter case the electroluminescent lamp, by the proper selection of the leads 10 to be connected to the source of electric power supply, may be made to light up in the form of any one of the digits from (zero) to 9. While the wires or pins may be connected to the electrically conductive areas 4 by a variety of means, they may preferably be attached thereto by the use of a commercially available silver-loaded electrically conducting epoxy cement. A similar electrically conductive lead 10 may be electrically connected to the electrically conducting area 4 when it is in the form of a single continuous conductive layer on the panel.

It will be obvious to those skilled in the art that an electric field will be created between the wires or pins such as lead 19 leading to the discrete electrically conductive areas 4 and the conducting glass paper when the complete electroluminescent panel is in operation. However, due to the relatively small size of the leads and the spacing of the leads from the surface of the thin insulating layer 2, the electroluminescence caused thereby will ordinarily be negligible and Will not be noticeable.

Since the life of the electroluminescent panel may be reduced substantially by exposure to water vapor, such as the moisture normally present in the amosphere, it may be desirable to encapsulate the complete electroluminescent sub assembly in a vapor tight enclosure. A suitable rigid encapsulating structure is shown in FIG. 3. As disclosed in FIG. 3, a glass plate 12 is provided in front of the conducting glass paper 5 and the remainder of the electroluminescent sub-assembly is covered with a vapor barrier potting compound 13 such as a commercially available metallically filled epoxy potting resin, which extends over the edges of the sub assembly and the glass plate 12, to form a vapor tight housing.

Where it is desired to provide a flexible type indiciabearing electroluminescent panel, the electroluminescent sub-assembly 20 comprised of the conducting glass paper front electrode 5 and the back electrode comprising either conductive areas 4 or a continuous conductive layer with the phosphor and insulating layers 2 and 3 sandwiched therebetween, along with the attached leads and 11 may be enclosed by

transparent thermoplastic sheets

21 and 22 as shown in FIG. 4. The thermoplastic sheets overreach the unitary structure at their

marginal edges

23, 24 and are sealed together therearound. To lami nate the sub-assembly and the

thermoplastic sheets

21 and 22 together, a stacked assembly thereof may be placed between the press platens of a conventional type hydrostatic press, beneath a conformable diaphragm suitably of aluminum foil which separates the platens. As is Well known in the art, compressed fluid is then applied over the diaphragm to exert hydrostatic pressure on the stacked components, vacuum is applied under the diaphragm to remove any trapped gases or moisture, and heat is supplied by suitable means to the stacked structure in order to cause the

plastic encapsulating sheets

21 and 22 to soften and seal together at their marginal edges to form a vapor tight envelope. For a fuller description of a suitable procedure and apparatus for carrying out such a laminating operation, reference may be made to US. Patent 2,945,976, Fridrioh et al., issued July 19, 1960.

The new and improved process of the present invention makes possible the manufacture of electroluminescent panels which will light up in the form of specific desired indicia by first applying a coating, comprised of a phosphor layer 2 and preferably an insulating layer 3 also, to one side of a temporary support or release sheet 1, and then depositing discrete electrically conductive areas 4 of a shape corresponding to the shape of the desired indicia on the said coating, either before or after removal of the release sheet from the coating, after which conductive glass paper 5 is laminated to, or other transparent electrically conductive material applied to, the electroluminescent phosphor layer 2. Alternatively, a continuous layer 4 of electrically conductive material may be deposited on the insulating layer 3. Furthermore, it will be apparent that the novel process described herein for making an electroluminescent panel does not increase the likelihood of the destruction of the fragile conducting glass paper, where such material is used for the front electrode 5, since the electroluminescent phosphor and insulating layers 2 and 3 are not directly deposited on the glass paper.

Although specific embodiments of the present invention have been described in detail, it will be understood that the present invention is not to be considered to be limited to those embodiments but may be used in other ways without a departure from the spirit of the invention and the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor on a temporary support, applying an electrically conductive layer over said layer of electroluminescent phosphor, removing said temporary support, and then applying an electrically conductive layer over the exposed side of said layer of electroluminescent phosphor.

2. The method of making an electroluminescent lamp which comprises applying a coating comprised of a layer of electroluminescent phosphor and a layer of high-dielectric constant material on a temporary support, applying an electrically conductive layer to the exposed side of said coating, removing said temporary support from the other side of said coating, and then applying an electrically conductive layer over the said other side of said coating.

3. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor on a temporary support, applying an array of discrete electrically conductive elements over said layer of electroluminescent phosphor, removing said temporary support, and then bonding a transparent electrically conductive sheet to said layer of electroluminescent phosphor.

4. The method of making an electroluminescent lamp which comprises coating 2. layer of electroluminescent phosphor on a temporary support, coating a layer of highdielectric constant material over said layer of electroluminescent phosphor, applying an array of discrete electrically conductive elements over said layer of high-dielectric constant material, removing said temporary support, and then bonding a transparent electrically conductive sheet to said layer of electroluminescent phosphor.

5. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor dispersed in a dielectric plastic matrix on a temporary support, coating a layer of high-dielectric constant material dispersed in a dielectric plastic matrix over said layer of electroluminescent phosphor, applying an electrically conductive layer over said layer of high-dielectric constant material, removing said temporary support, and then bonding a transparent electrically conductive sheet to said layer of electroluminescent phosphor.

6. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor dispersed in a dielectric plastic matrix on a temporary support, coating a layer of high-dielectric constant material dispersed in a dielectric plastic matrix over said layer of electroluminescent phosphor, applying an array of discrete electrically conductive elements over said layer of high-dielectric constant material, removing said temporary support, and then bonding a transparent electrically conductive sheet to said lay-er of electroluminescent phosphor.

7. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor dispersed in a dielectric plastic matrix on a flexible release sheet, depositing separate areas of conductive metal on said layer of electroluminescent phosphor, removing said flexible release sheet to expose the surface of said phosphor layer, and then laminating a transparent electrically conductive lamina to the said exposed surface of said phosphor-layer.

8. The method of making an electroluminescent lamp which comprises coating a layer or" electroluminescent phosphor dispersed in a dielectric plastic matrix on a flexible thermoplastic release sheet, coating a layer of high-dielectric constant material dispersed in a dielectric plastic matrix over said layer of electroluminescent phosphor, depositing separate areas of conductive metal on said layer of high-dielectric constant material, removing said flexible thermoplastic release sheet, and then laminating electrically conductive glass paper to said layer of electroluminescent phosphor under heat and pressure.

9. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor dispersed in a dielectric plastic matrix on a flexible thermoplastic release sheet, coating a layer of high-dielectric constant material dispersed in a dielectric plastic matrix over said layer of electroluminescent phosphor, silk-screening areas of conductive ink on said layer of high-dielectric constant material, removing said flexible thermoplastic release sheet, and then laminating electrically conductive glass paper to said layer of electroluminescent phosphor under heat and pressure.

lit. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor on a temporary release sheet, applying an electrically conductive back electrode over said layer of electroluminescent phosphor, removing said temporary release sheet to expose the surface of said phosphor layer, bonding a transparent electrically conductive front electrode to the said exposed surface of said phosphor layer to form an electroluminescent subassembly, securing electric terminals to said front and back electrodes, and then encapsulating said sub-assembly in a vapor-tight envelope.

11. The method of making an electroluminescent lamp which comprises coating a layer of electroluminescent phosphor dispersed in a dielectric plastic matrix on a temporary release sheet, coating a layer of high-dielectric constant material dispersed in a dielectric plastic matrix over said layer of electroluminescent phosphor, applying an array of discrete electrically conductive elements over said layer of high-dielectric constant material, removing said temporary release sheet, bonding electrically conductive glass paper to said layer of electroluminescent phosphor to form an electroluminescent sub-assembly, securing electric terminals to said front and back electrodes, placing a glass plate flatwise against the said conductive glass paper, and covering the remainder of said sub-assembly with a vapor barrier potting compound to encapsulate the same.

References Cited by the Examiner UNITED STATES PATENTS 2,945,976 7/1960 Fridrich et al 3l3-108.1 3,064,155 11/1962 Bell 313l08.1 3,068,440 12/1962 Mash 313l08.1

RICHARD H. EANES, 1a., Primary Examiner.

Claims

11. THE METHOD OF MAKING AN ELECTROLUMINESCENT LAMP WHICH COMPRISES COATING A LAYER OF ELECTROLUMINESCENT PHOSPHOR DISPERSED IN A DIELECTRIC PLASTIC MATRIX ON A TEMPORARY RELEASE SHEET, COATING A LAYER OF HIGH-DIELECTRIC CONSTANT MATERIAL DISPERSED IN A DIELECTRIC PLASTIC MATRIX OVER SAID LAYER OF ELECTROLUMINESCENT PHOSPHOR, APPLYING AN ARRAY OF DISCRETE ELECTRICALLY CONDUCTIVE ELEMENTS OVER SAID LAYER OF HIGH-DIELECTRIC CONSTANT MATERIAL, REMOVING SAID TEMPORARY RELEASE SHEET, BONDING ELECTRICALLY CONDUCTIVE GLASS PAPER TO SAID LAYER OF ELECTROLUMINESCENT PHOSPHOR TO FORM AN ELECTROLUMINESCENT SUB-ASSEMBLY, SECURING ELECTRIC TERMINALS TO SAID FRONT AND BACK ELECTRODES, PLACING A GLASS PLATE FLATWISE AGAINST THE SAID CONDUCTIVE GLASS PAPER, AND COVERING IT REMAINDER OF SAID SUB-ASSEMBLY WITH A VAPOR BARRIER POTTING COMPOUND TO ENCAPSULATE THE SAME.