US3226272A - Electroluminescent lamp manufacture - Google Patents

Description

Dec. 28, 1965 w. LONGFELLOW 3,226,272

ELECTROLUMINES CENT LAMP MANUFACTURE Filed Sept. 13, 1961 ITlVfiBTWTCDT". fievber t W. Lo cgFeLLow mfzr United States Patent 3,226,272 ELECTROLUMINESCENT LAMP MANUFACTURE Herbert W. Longfellow, tCincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed Sept. 13, 1961, Scr. No. 137,924 7 Claims. (Cl. 156-67) This invention relates in general to electroluminescent devices and to a light-transmitting electrically conductive plastic film material therefor, and is especially concerned with a new and improved method for making a flexible type laminated electroluminescent cell. The present application is a continuation-in-part of my prior copending application Serial No. 126,103, filed July 24, 1961, and assigned to the same assignee as the present invention.

As it is presently known in the art, an electroluminescent cell generally comprises a phosphor layer sandwiched between two conducting layers, one of which is transparent or is, at least, light transmitting. A well known form of such an electroluminescent cell is described in US. Patent 2,945,976 issued to Fridrich et al., and assigned to the same assignee as the present invention. The lamp therein disclosed comprises a flexible laminated assembly of electrically active layers encased in a thin envelope of thermoplastic material which has been evacuated and heat sealed around its edges. The electrically active elements contained in such a lamp generally consist of a flexible conductive layer composed of aluminum foil coated with a layer of a high dielectric constant material which, in turn, is overcoated with a layer of an electroluminescent phosphor, and finally overlayed with a light-transmitting conductive sheet. In addition, a thin flexible film of a suitable thermoplastic material such as low density polyethylene or nylon 6 may preferably be placed over the light-transmitting conducting sheet which thermoplastic film, following the laminating step, firmly holds the conducting sheet in place and cements it to both the underlying phosphor layer and the overlying layer of the encasing thermoplastic envelope.

In a structure such as described in the aforesaid Fridrich et al. patent, the aluminum foil and the electrically conductive sheet constitute the electrodes of the assembled electroluminescent unit. Accordingly, the application of an alternating potential across these electrodes results in the excitation of the phosphor with a consequent emission of light through the respective light-transmitting layers of the lamp assembly.

The light-transmitting conductive sheet employed in the production of an electroluminescent cell such as that described above heretofore has been generally comprised of conducting glass paper such as, for example, the commercially available micro-fiber glass paper around .002 inch thick. Conductivity is imparted to such paper 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 glass fibers. A suitable solution employed for rendering such glass paper conductive comprises indium basic trifluoroacetate with stannic chloride sncn dissolved in an organic solvent such as ethylene glycol monoethyl ether acetate. In this regard, reference may be made to US. Patent 2,849,339, Jaffe, assigned to the same assignee as the present invention for a more complete description of the materials and processes employed in providing such a conducting glass paper.

Although the prior art methods involving the use of conducting glass paper have resulted in the production of electroluminescent cells which have been entirely satis- 3,226,272 Patented Dec. 28, 1965 factory, the rather fragile nature of this glass paper severely handicaps the ease with which such electroluminescent cell assemblies can be produced. Specifically, the glass paper employed, due to its extremely delicate nature, crumbles easily, and therefore requires the utmost degree of care while cutting, and while handling during cell assembly. In addition, the fragile nature of the paper imposes severe difiiculty in cutting the paper into intricate shapes without simultaneously shattering the paper during the cutting process. Moreover, and perhaps most importantly, it should be noted that current techniques of producing electroluminescent cells having an area of 36 square inches or greater, which techniques necessarily require the dificult step of cutting a sheet of glass paper to the appropriate dimensions, results in high production rejects or so-called shrinkage due apparently to the electrical shorting of the lamps caused by the contact of the conductive glass fibers with the underlying metallic base foil. An explanation of this phenomenon may be that during the subsequent pressure laminating operation, portions of the conducting glass paper are forced through the underlying phosphor and insulating layers to the extent that they contact the conductive metallic film. Whatever the cause of such shorting may be, it remains apparent that a new and improved scheme of preparing electroluminescent cell assemblies which would obviate the problems attending the handling of extremely fragile conductive glass paper, and which would, if possible, reduce or eliminate the production rejects in the case of larger sized electroluminescent cells due to electrical shorting between the conducting glass paper and the underlying base foil, would indeed be highly desirable from the standpoint of simplicity and economy in manufacture.

It is accordingly the primary object of the present invention to provide a new and improved method of making an electroluminescent cell.

Another object of the present invention is to provide a new and improved method for making an electroluminescent cell of the type having a conductive glass paper electrode, which method is simpler and more economical than the methods heretofore employed for the production of such type cells.

A further object of the present invention is to provide a new and improved method for making a flexible electroluminescent cell assembly of the type employing a layer of fragile electrically conductive material, which method substantially eliminates the problem of handling and cutting such fragile material without breakage thereof such as has been prevalent with the methods heretofore employed in the art for the manufacture of such type cell assemblies.

Yet another object of the present invention is to provide a new and improved method of making an electroluminescent cell assembly of the type employing a conductive glass paper electrode, which method affords a substantial elimination of the problem of high production rejects heretofore encountered in such cell production as a result of the electrical shorting of the cells due to the Contact of the conductive fibers of the conducting glass paper layer with the underlying conducting base member of the cell assembly.

A still further object of the invention is to provide a flexible light-transmitting conductive plastic sheet material suitable for use as an electrode layer for an electroluminescent cell.

Briefly stated, these and other objects may be obtained in accordance with the invention by first prelaminating the conductive glass paper together with a suitable thermoplastic film material to form a light-transmitting electrically conductive plastic film material or laminate of tough and flexible character which can be handled with ease without fracturing, and then laminating a sheet of such conductive plastic film material together with, or otherwise affixing thereto, the other component laminar elements of an electroluminescent cell, to thereby form the completed electroluminescent cell assembly.

Still other objects and advantages of the invention will be apparent from the following description of a species thereof and from the accompanying drawing in which:

FIG. 1 is a pictorial view of a flexible, laminated electroluminescent cell made in accordance with the method of the present invention, the various constituent layers being delaminated or peeled open at one corner to show the internal construction thereof, and

FIG. 2 is a fragmentary sectional view, on a greatly enlarged scale, of the light-transmitting electrically conductive plastic sheet material comprising my invention and constituting the light-transmitting electrode of the electroluminescent cell.

Referring to FIG. 1, there is shown a flat rectangular electroluminescent cell 1 made up entirely of components or layers which have been laminated together and sealed in a plastic outer encapsulating envelope in accordance with the method 'of the present invention. The cell 1 may be energized by applying a suitable potential such as an alternating voltage, for example, 120 volts, 60 cycles AC. to the copper terminals 2 and 3 projecting laterally from the edge of the plastic outer envelope which is composed of sheets 4 and 5 of plastic material. The lowermost film or lamina 4 and the uppermost lamina 5 which form respectively the underside and the topside of the envelope in the completed lamp construction, consist of sheets of thermoplastic material which fiow under heat and pressure, and they are heat-sealed together along their margins. The materials selected for this use are preferably tough and stable in addition to exhibiting light transmitting qualities and high impermeability to moisture, and further they are preferably flexible in nature. Among the materials which may be successfully employed in this regard are polyethylene, polytetrafiuoroethylene, polychlorotrifluoroethylene, polystyrene, methyl methacrylate, polyvinylidene chloride, polyvinyl chloride, polycarbonate materials such as, for example, the reaction products of diphenylcarbonate and bisphenol A, and polyethylene terephthalate. A preferred material consists of polychlorotrifiuoroethylene film, known as Kel-F, of approximately 0.005 inch thickness.

The electrically active elements of the cell 1 comprise a rectangular sheet of thin metal foil 6 coated with an insulating layer 7 (indicated by cross-lining) of high dielectric constant material which is overcoated with a lightproducing layer 8 (indicated by speckling) of an electroluminescent phosphor. The metal foil sheet 6 is placed over the lowermost plastic sheet 4 leaving a clear margin all around. A light-transmitting and electrically conductive laminar sheet 9 comprised of a layer of conductive micro-fiber glass paper is placed over the coated side of the metal foil 6, leaving a narrow margin of the coated side of the foil uncovered all around. The conductive glass paper layer 9 and the metal foil 6 constitute the two electrodes of the electroluminescent cell. The conductive glass fibers of the glass paper layer 9 are preferably bound in place and cemented to the phosphor layer 8 by a layer 10 of a suitable thermoplastic material in which the glass fibers are partly embedded. As shown in the drawing, the juxtaposed margins of the bottom and top thermoplastic sheets 4 and 5 project beyond the edges of the metal foil 6 and are fused or sealed together to form a sealed outer envelope encapsulating the electrically active elements of the cell.

The coated metal foil 6 may consist of dead soft annealed aluminum of, for example, around 0.002 inch thickness coated with a thin insulating layer 7 of barium titanate dispersed in an organic polymeric matrix and overcoated with an electroluminescent phosphor layer 8 consisting of any known electroluminescent phosphor such as, for example zinc sulfide-zinc oxide with suitable activators such as copper, manganese, lead or silver, likewise dispersed in an organic polymeric matrix such as that used in connection with the insulating layer 7. Examples of suitable organic polymeric matrixes are cellulose nitrate, polyacrylates, methacrylates, polyvinyl chloride, cellulose acetate, alkyd resins, epoxy cements, and polymers of triallyl cyanurate, to which may be added modifying substances or plasticizers such as camphor, dioctyl phthalate, tricresyl phosphate and similar materials. A preferred organic polymeric matrix forming a dense tough film of high dielectric constant and good mechanical and thermal stability and consisting of cyanoethyl cellulose with suitable plasticizers such as cyanoethyl phthalate is described and claimed in US. Patent 2,951,865, Jaffe et al., assigned to the same assignee as the present invention. The barium titanate layer dispersed in a cyanoethyl cellulose solution may be applied to the aluminum foil by spraying, or preferably through the use of a doctor blade, and then drying; the phosphor layer, likewise dispersed in a cyanoethyl cellulose solution, may then be applied over the barium titanate layer in a similar manner.

The electroluminescent cell is energized by applying a suitable potential between the conductive layers, that is, between aluminum foil 6 and the conductive thermoplastic sheet 9. The projecting copper braids or ribbons 2, 3 provide convenient terminals for so doing and are connected respectively to the aluminum foil 6 and the electrically conductive glass paper layer 9. During the laminating process, the copper ribbons become embedded in the thermoplastic sheets 4 and 5 and are at the same time pressed against and into electrical contact with the aluminum foil 6 and the conductive glass paper 9, as the case may be.

The manufacture of an electroluminescent cell 1 such as described is customarily accomplished by stacking all the various cell components or layers together in their proper positional relationship, and then subjecting the stacked assembly to pressure and heat to laminate the cell components together. The laminating of the cell components may be performed in the manner, and by the use of a hydrostatic laminating press such as described and claimed in Us. Patent 2,945,976, Fridrich et al. As therein described, the stacked assembly of cell components is placed between the top and bottom platens of the hydrostatic press, beneath a conformable diaphragm separating the press platens, the conformable diaphragm being constituted of a flexible gas-impervious sheet material such as soft annealed aluminum foil or polyethylene terephthalate film such as Mylar. Pressurized gas is admitted into the closed chamber of the press over the diaphragm therein to exert hydrostatic pressure on the cell components, vacuum is applied under the diaphragm to remove any trapped gases or moisture from the space therebelow, and heat is then applied by suitable means to the stacked assembly of cell components to cause the plastic encapsulating sheets 4 and 5 to soften and seal together at their margins. For a more complete understanding of a suitable laminating technique, reference may be made to the aforementioned Fridrick et al. Patent 2,945,976.

According to the prior practice for making electroluminescent cells 1 as described above, the glass paper layer 9 of the cell and, where employed, the plastic binder layer 10 also, have consisted of separate component members at the time of their assembly together with the other components of the cell in readiness for the laminating together thereof. Because of the highly fragile and delicate character of the glass paper 9 by itself, considerable difficulty has therefore been experienced in the handling and cutting of the glass paper without breakage thereof. As a result, a high percentage of production rejects or shrinkage has been encountered in the manufacture of the cell assemblies 1 by prior cell making techniques, thus materially increasing the manufacturing cost of such devices.

In accordance with the invention, the conductive glass paper 9, prior to its lamination together with or the aflixation thereto of the other components of the electroluminescent cell, is pre-laminated to a suitable thermoplastic sheet material to form a light-transmitting electrically conductive plastic sheet material or prelaminate 11 (FIG. 2) having conductive glass fibers partially embedded in one face thereof but sufliciently exposed thereat to render the said face electrically conductive. The electrically conductive plastic prelaminate 11 may then be stacked together with the other components of the cell 1, with the conductive glass fiber side of the prelaminate 11 next to the phosphor layer 8, and the stacked assembly or lay-up then laminated together under pressure and heat in the same manner such as described previously to thereby complete the manufacture of the electroluminescent cell 1. The glass paper 9 and the plastic sheet material may be laminated together by placing the conducting glass paper 9 over the thermoplastic sheet material 10 and subjecting the stacked assembly to a pressure of about 400 p.s.i. and a temperature of approximately 200 C. suflicient to soften the plastic so as to flow between the glass fibers of the conductive glass paper and cause the fibers to become partially embedded in the face of the plastic sheet. The resulting product is a light-transmitting electrically conductive smooth surfaced plastic sheet material 11 of tough and flexible character which can be handled with ease and lends itself to cutting and bending without fracturing or breaking apart. Because of the smooth surfaced character of the glass fiber embedded face of such conductive plastic sheet material 11, the likelihood of any of the glass fibers being forced through the insulating and phosphor layers 7 and 8 of the cell and into contact with the underlying metal foil layer 6 thereof during the cell laminating operation, with resultant short circuiting of the cell and damage thereto, is greatly minimized.

In general, any thermoplastic material may be employed for the plastic sheet 10 of the conductive plastic sheet material 11 comprising my invention. Included among the materials which have been found suitable for such purpose, however, are nylon, cellulose acetate, cellulose acetate butyrate, polyvinyl alcohol, polyvinylpyrrolidone, polyvinylchloride, polyvinylidine chloride, copolymers of polyvinylchloride and polyvinylidine chloride, polyvinylacetate, polystyrene and polymers of methyl methacrylate. cell which has a substantially improved resistance to water depreciation, it may be advantageous to select for the plastic layer 10 a thermoplastic material which exhibits hydrophilic properties, i.e., has an aflinity for water. As disclosed in copending application Serial Number 80,613 of Devol et al., filed January 4, 1961, now Patent No. 3,148,299, and assigned to the same assignee as the present invention, polyamide condensation products such as nylon 66, or nylon 6 such as that known as Caplene, have been found to be particularly effective as hydrophilic materials for the plastic layer 10.

The pressure required to laminate together the contiguous layers of conducting glass paper 9 and plastic sheet material 10 to form the prelaminate 11 may be applied thereto in any suitable manner, as by compressing them between coperating flat pressure plates or pressure rolls, or by means of a gas impervious conformable diaphragm in a hydrostatic press such as described in the aforementioned Fridrich et al. Patent 2,945,976. The electrically conductive plastic prelaminate material according to the invention may be formed in large sheets or in continuous roll form, from which individual sheets 11 of proper size and shape may then be cut for lamination together with the other component elements of the electroluminescent cell 1.

In order to provide an electroluminescent Instead of laminating the conductive plastic prelaminate 11 together with the other components of the electroluminescent cell as described hereinabove to form the completed cell assembly, the prelaminate 11 may be coated with the light-producing layer 8 of electroluminescent phosphor and then overcoated with the insulating layer 7 of high dielectric constant material, following which the second or back electrode 6 is then applied over the insulating layer 7. This electrode layer 6 may be comprised of some form of electrically conductive paint or paste, or a similar conductive material which may be brushed, sprayed, rolled or silk screened onto the insulating layer 7. Alternatively, a layer of a suitable metal such as aluminum may be applied over the insulating layer 7 by evaporation under high vacuum according to techniques well known in the art, or metal foil such as aluminum foil may be either laminated to the insulating layer 7 or cemented thereto by conductive cement.

By utilizing the new and improved method of the present invention wherein the conductive glass paper is initially laminated to a thermoplastic sheet to form a flexible conductive plastic sheet material, the problem of handling and cutting the highly fragile conductive glass paper heretofore attending the production of flexible electroluminescent lamps is reduced to a minimum and is, in fact, never encountered during the course of the actual lamp assembly operation. Moreover, it has been discovered that, as a result of the new and improved method of the present invention, the electrical shorting which occurs during the manufacture of electroluminescent cells by the conventional methods of the prior art has been substantially eliminated.

Although specific embodiments of the present invention have been described in detail, it should be understood that the present invention is not to be considered limited to such embodiments, but may be used in other ways without 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. In the manufacture of an electroluminescent cell comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers at least one of which is light-transmitting, the method which comprises the steps of preliminarily forming a self-supporting, tough, flexible and light-transmitting electrically conductive sheet, which can be readily handled and cut to size without fracturing, by applying a sheet of extremely fragile electrically conductive glass paper comprised of electrically conductive glass fibers to a face of a flexible thermoplastic sheet and laminating the said sheets together under pressure and heat so that the said conductive glass paper is partly embedded in the said face of the plastic sheet but sufliciently exposed thereat to render the said face electrically conductive, and then affixing to the said conductive face of the plastic sheet a layer of electroluminescent phosphor and an overlayer of electrically conductive material.

2. In the manufacture of an electroluminescent cell comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers at least one of which is light-transmitting, the method which comprises the steps of preliminarily forming a self-supporting, tough, flexible and light-transmitting electrically conductive sheet, which can be readily handled and cut to size without fracturing, by applying a sheet of extremely fragile electrically conductive glass paper comprised of electrically conductive glass fibers to a face of a flexible thermoplastic sheet and laminating the said sheets together under pressure and heat so that the said conductive glass paper is partly embedded in the said face of the plastic sheet but sufliciently exposed thereat to render the said face electrically conductive, and then laminating the said conductive plastic sheet to the phosphor layer of an electroluminescent cell sub-assembly with the said electrically conductive face of 7 the conductive plastic sheet next to the said phosphor layer.

3. In the manufacture of an electroluminescent cell comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers at least one of which is light-transmitting, the method which comprises the steps of preliminarily forming a self-supporting, tough, flexible and light-transmitting electrically conductive sheet, which can be readily handled and cut to size without fracturing, by applying a sheet of extremely fragile electrically conductive glass paper comprised of electrically conductive glass fibers to a face of a flexible thermoplastic sheet and laminating the said sheets together under pressure and heat so that the said conductive glass paper is partly embedded in the said face of the plastic sheet but sufficiently exposed thereat to render the said face electrically conductive, coating the said electrically conductive face of the conductive plastic sheet with a layer of electroluminescent phosphor, and then applying an electrically conductive layer over said phosphor layer.

4. In the manufacture of an electroluminescent cell comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers at least one of which is light-transmitting, the method which comprises the steps of preliminarily forming a self-supporting, tough, flexible and light-transmittin g electrically conductive sheet, which can be readily handled and cut to size without fracturing, by applying a sheet of extremely fragile electrically conductive glass paper comprised of electrically conductive glass fibers to a face of a flexible thermoplastic sheet and laminating the said sheets together under pressure and heat so that the said conductive glass paper is partly embedded in the said face of the plastic sheet but sufiiciently exposed thereat to render the said face electrically conductive, coating the said electrically conductive face of the conductive plastic sheet with a layer of electroluminescent phosphor, overcoating said phosphor layer with an insulating layer of high-dielectric constant material, and then applying an electrically conductive layer over said insulating layer.

5. The method of making an electroluminescent cell as specified in claim 1 wherein the said thermoplastic sheet consists of a hydrophilic thermoplastic material.

6. The method of making an electroluminescent cell as specified in claim 1 wherein the said t-herornplastic sheet consists of a material selected from the group consisting of polyamide condensation polymers.

7. The method of making an electroluminescent cell as specified in claim 1 wherein the said thermoplastic sheet consists of nylon 6.

References ited by the Examiner UNITED STATES PATENTS 2,511,887 6/1950 Vinal. 2,699,415 1/1955 Nachtman. 2,781,287 2/1957 Gustus 154-46 2,827,414 3/1958 Bussard. 2,901,652 8/1959 Fridrich 313-1081 2,944,177 7/1960 Piper 313-1081 2,945,976 7/1960 Fridrich 313-1081 3,000,772 9/1961 Lunn 154-525 3,110,836 11/1963 Blazek et al 313-108.1

EARL M. BERGERT, Primary Examiner.

DOUGLAS J. DRUMMOND, Examiner.

Claims (1)

1. IN THE MANUFACTURE OF AN ELECTROLUMINESCENT CELL COMPRISING A LAYER OF ELECTROLUMINESCENT PHOSPHOR SANDWICHED BETWEEN A PAIR OF ELECTRODE LAYERS AT LEAST ONE OF WHICH IS LIGHT-TRANSMITTING, THE METHOD WHICH COMPRISES THE STEPS OF PRELIMINARILY FORMING A SELF-SUPPORTING, TOUGH, FLEXIBLE AND LIGHT-TRANSMITTING ELECTRICALLY CONDUCTIVE SHEET, WHICH CAN BE READILY HANDLED AND CUT TO SIZE WITHOUT FRACTURING, BY APPLYING A SHEET OF EXTREMELY FRAGILE ELECTRICALLY CONDUCTIVE GLASS PAPER COMPRISED OF ELECTRICALLY CONDUCTIVE GLASS FIBERS TO A FACE OF A FLEXIBLE THERMOPLASTIC SHEET AND LAMINATING THE SAID SHEETS TOGETHER UNDER PRESSURE AND HEAT SO THAT THE SAID CONDUCTIVE GLASS PAPER IS PARTLY EMBEDDED IN THE SAID FACE OF THE PLASTIC SHEET BUT SUFFICIENTLY EXPOSED THEREAT TO RENDER THE SAID FACE ELECTRICALLY CONDUCTIVE, AND THEN AFFIXING TO THE SAID CONDUCTIVE FACE OF THE PLASTIC SHEET A LAYER OF ELECTROLUMINESCENT PHOSPHOR AND AN OVERLAYER OF ELECTRICALLY CONDUCTIVE MATERIAL.

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