US2733367A - Electroluminescent lamp structures - Google Patents

Description

Jan. 31, 1956 ELECTROLUMINESCENT LAMP STRUCTURES Filed Oct; 7, 1952 I N VENTOR Joseph L. Gillson, J.

ATTORNEY J. L. GILLSON, JR 2,733,367

Patented Jan. 31, 1956 2,733,367 ELECTRQLUMINESCENT LAMP STRUCTURES Joseph L. Gillson, Jr., Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application October 7, 1952, Serial No. 313,526 1 Claim. (Cl. 313-108) This invention relates to structures for luminescence and, more particularly, to improved structures forelectroluminescence.

due provision bemg made for transmission of the light emitted by the phosphor. In the electroluminescent lamp structure disclosed in U. S. P. 2,566,349, for exlight-transmitting electrically-conductive layer and an electrically-conductive layer which may, but need not be, light-transmitting. The two electrically-conductive layers are connected to a source of alternating or pulsating current of the desired potential which serves to excite the phosphor material to luminescence. troluminescent lamp differs from the fluorescent lamp in that in the latter, the voltage or field is placed across a gas and the radiation from the gas is used to excite a phosphor; whereas, in the electroluminescent lamp, light is obtained by the direct application of a varying voltage across a phosphor or by placing the phosphor in a varying electric field.

In the operation and construction of electroluminescent lamp structures, it is known that a number of factors affect the intensity of the light emitted. Thus, the

thickness, the resistivity, and the dielectnc constant of luminosity may be nited States Pa -5C 2 luminescent structures in a great variety of lighting applications. Other objects will be apparent from the follow ing description of the invention.

I have found that theunique combination of chemia semi-transparent and flexible supelectrolumi- Polyethylene terephthalate may be prepared by the condensation of ethylene glycol and terephthalic acid, or, preferably, by carrying out an ester group containing from 1 to 7 carbon atoms. Owing to the availability of dimethyl terephthalate, it is preferred to carry out the ester interchange reaction between ethylene glycol and dimethyl terephthalate. Polyethylene terephthalate and analogous polymers are described.

terephthalic acid, the alkyl The invention will now be specifically described with reference to the accompanying drawing wherein! Figure 1 is a cross-sectional view of a preferred elecate film, each with the phosphor-bearing element thin coatings or films 4 and 5 of electrically-conductive material which provide the lamp electrodes and which are connected through lines 6 and 7 to a suitable source (not shown) of alternating or pulsating electric current. At least one of these coatings should be of a nature and thickness such as would not substantially impair the light-transmitting qualities of the base film upon which it is formed.

Either unstretched or stretched polyethylene terephthalate film may be used for purposes of this invention. However, inasmuch as the pertinent physical properties, e. g.,

impact strength, flex life, etc., of stretched film which has been stretched to substantially the same extent. 1. e., from 2.5 to 3.25 times (X) its original dimensions, in both directions, and heat-set under tension at a temperature within the range of 150 to 250 C., it is preferred to use suchfilm in the electroluminescent lamp structures of this invention. The-combination of chemicai, physical and electrical properties of biaxially oriented, balanced polyethylene terephthalate film, together with the production of such film, is fully disclosed in copending application U. S. Serial No. 287,345 filed May 12, 1952, in the name of B. W. Fuller. With respect to the use of polyethylene terephthalate film as a dielectric member in electroluminescent structures, its high dielectric strength (high voltage breakdown), dielectric constant, high resistivity,, moisture insensitivity, and the fact that the film may beproduced in very thin gauges substantially free from electrical faults, are a number of the essential properties which make it outstanding in the application of the present invention. Table 1 represents a comparison of a number of these properties with the same properties of various other dielectric materials.

TABLE 1 Poly. Tcre.

Film 0.002" in thickness Property (stretched 3X Mica Nylon biaxially and heat-set at 20 C.) Tensile Strength (lbs/sq. .in.)- 24,000 5,000 9,000 Percent Tensile Strength Loss 7 I Alter 24 Hours at 200 C 1 l 1 94 20 Dielectric Strength (Volts/0.001") 4, 500 3 3, 000 3, 000 Dielectric Fatigue (Voltsl0.001). 200-300 3 300 Moisture Absorption (24 Hours in Water) (Percent by weight). 0. 1 24 2 Chemical Resistance Good Good Fair 1 Fish paper/mica laminate .012 in. thick with approx. .002 in. of fish paper.

0.001 Polyethylene terephthalate film (stretched 3X biaxially and heat-set at 200 C.)

5 Pure mice .002 in. thick.

The following tables, Tables 2 and 3, illustrate certain of these electrical properties of biaxially oriented, polyethylene terephthalate film under various conditions of temperature and humidity. Table 2 shows measurements of dielectric strength at temperatures from 0 to 150 C. at 60 cycles per second. The average dielectric strengths (breakdown voltages), on the basis of ten samples, range from 3,150 to 4,500 volts per 0.001; and this indicates that this property is outstanding. Most dielectric materials presently in use have a breakdown voltage in the range of 500 to 1,000 volts per 0.001.

TABLE 2 Dielectric strength of polyethylene terephthalate film 1 in air Average Temperature, C. Kilovolts Gauge Volts/0.001

(Inches) 1 Stretched 3X biaxially and heat-set at 150 C.

Table 3 shows measurements of volume resistivity at temperatures from 0 C. to 150 C.

TABLE 3 Volume resistivity of polyethylene terephthalate film 1 Temperature, C.: Volume Resistivity (ohm cm.)

1 Stretched 3X biaxially and heat-set at 150 0.

The following table (Table 4) illustrates the surface resistivity of the film at 25 C. over the 0-l00% relative humidity range at 500 volts, direct current.

TABLE 4 Surface resistivity of polyethylene zerephthalate film 1 at 25 C. Relative humidity, percenti Surface resistivity (ohms) 12 100 4.8 l0

1 Stretched ax blaxially and heat-set at 150 0.

Table 5 clearly illustrates the outstanding improvement in most of the important physical properties by stretching the amorphous film biaxially. In actuality, if such pertinent physical properties as tensile strength, impact strength, fiex life, etc., of polyethylene terephthalate film could not be improved by stretching, the utility of the film as a dielectric material would be greatly curtailed.

TABLE 5 Polyethylene terephthalate film Stretched Stretched Stretched Property 3X Bi- 2.5x Bi- 2X Binxinlly axially axially Thickness (inches) 0. 001 0. 001 0. 001 Tensile (p. s.. 26,000 10, 500 16,000 Break Elongation 100 160 200 Impact (kg-em.) 76 63 40 Tear Strength (g 22 10 20 Flex Life (cycles). 20, 000 Water Vapor Permeabllityf g./100 sq. meters/hr 160 Tensile Modulus (p.s. 1.) 500, 000 105, 000 470, 000 Density, g./ce 1 1 At slow elongation rate.

The foregoing tables of pertinent electrical properties of polyethylene terephthalate film illustrate outstanding advantages of employing this film as the dielectric in electroluminescent structures, and particularly outstanding is the high breakdown voltage of the film at various temperatures. In essence, this indicates that higher potentials may be applied to excite the phosphors and, consequently, light of higher intensity is emitted. Moreover, because of its excellent strength properties and because it may be prepared substantially free of electrical faults-in thin sections, 'biaxially oriented, balanced polyethylene terephthalate film may be employed in thick nesses as-thin as-0;000l"'0.00025; and films up to 0.01 are useful in specific structures of this invention. In such thin sections, the film is highly flexible and may be employed to fabricate electroluminescent structures in roll form, e. g., to be applied to walls, ceiling, etc., or draped around objects to produce decorative illumination. Possible structures-of this type will be described hereinafter.

Where polyethylene terephthalate film is to serve as the phosphor-containing medium, any convenient expedient may be employed to embed or to disperse the phosphor particles in the film. For example, suitable electrical field-responsive phosphors, e. g., an activated a ises-ea directlyin molten polyethylene terephthalate, and thereafterthe molten material may be extruded directly into film form. Furthermore, depending upon the general physical requirements of the overall electroluminescent structure, the film containing the dispersed phosphors may or may not be subjected ,to biaxial stretching followed by heat-setting in order to obtain a film having physical, electrical and chemical properties hereinabove recited.

In addition to its outstanding electrical, chemical and physical properties, polyethylene terephthalate filmv is readily metallized by known techniques and, hence, is particularly suited for use as the electrode-supporting, light-transmitting elements of electroluminescent lamp structures. Metals such as aluminum, zinc, silver, etc., may be readily applied to one side of the film by evaporation at high vacuum according to techniques well known in the art, to form thereon continuous metal coatings which, though electrically conductive, may be so thin, e. g., in the neighborhood of 0.0001 mil, as not to substantially impair the light-transmitting properties of the film. On the other hand, instead of having a thin continuous coating of metal on the film, a printed pattern of a metal deposit may be employed to give a decorative efi'ect. That is, light of highest intensity would be emitted from areas immediately adjacent to the edges of the metallized portions. Alternative methods of rendering polyethylene terephthalate film electrically conductive include applying coatings of conductive sizes comprising various inorganic salts such as aqueous solutions containing zinc chloride, lithium chloride and other conductive materials which would not substantially impair the light-transmitting properties of the film. Other types of conductive coatings which might be employed include various polyelectrolytes, such as polyacids or poly-quaternary salts. Still another method of rendering the film conductive consists in dissolving various inorganic salts in molten polyethylene terephthalate, forming homogeneous solutions thereof, and thereafter extruding the molten polymer into film form. If desired, the phosphor-bearing element and at least one electrode of the lamp structure may be combined simply by applying metal, as above indicated, to phosphorbearing polyethylene terephthalate film.

The following example is illustrative of a preferred embodiment wherein each of the elements of the representative electroluminescent lamp structure shown in Figure 1 comprises polyethylene terephthalate film.

EXAMPLE 1 A specially prepared phosphor comprising zinc sulfide with copper and lead particles as activators was uniformly dispersed in molten polyethylene terephthalate at a temperature of 300 C. At this temperature, the molten polymer was extruded into the form of a film onto the surface of a rotating quench drum maintained at a temperature of 6080 C. The resulting film was 0.002 in thickness, and this film was employed as element 1 of the electroluminescent lamp structure illustrated in Figure l. Element 2 was a polyethylene terephthalate film 0.001" in thickness and stretched 3 times (3X) in both directions and heat-set at 200 C. Element 4 was an adherent coating of aluminum (0.0025 mil in thickness) on the polyethylene terephthalate film. Element 3 was the same type of film as that of element 2. Element 5 was also an adherent aluminum coating which was somewhat thicker (0.02 mil in thickness) than element 4 in order to provide a reflective backing for light emitted through elements 2 and 4. These elements were bonded together with an adhesive cement comprised of neoprene and phenol-formaldehyde resin in toluene. A source of alternating current at a frequency of kilocycles was employed to apply a potential of 400 volts between the electrodes of the ca- 6 paeitor construction. 5 a] result, a green glow was emitted 'through elements} and'4. I

From the foregoing, itis readily-apparent that inorder to take full advantage of the outstanding combination of physical, chemical and electrical properties offered by bi axially oriented, heat-set polyethylene terephthalate film in electroluminescent structures, the entire structure should be made up of components fabricated from oriented, heatset film. In essence, the resulting-structure for electroluminescence consists of individual films, i. e., biaxially oriented and heat-set, having substantially equivalent properties; and, consequently, the compositeof thesefilms is outstanding with respect to its combination of physical chemical and electrical. properties. However, polyethylene terephthalate film may also be used to great advantage in conjunction with conventional materials of construction employed heretofore in electroluminescent structures. For example, a conductive glass may be made by exposing a heated glass surface to vapors of silicon, tin, or titanium chloride, and placing the fresh coating in a reducing atmosphere. This coated glass may be used as the lighttransmitting and electrode elements 2 and4 in electroluminescent structure of Figure l. A selected phosphor, for example, a powdered mixture of zinc sulfide and zinc oxide, which has been activated at elevated temperatures in an inert gas, may be imbedded in such dielectric materials as suitable solidified oil, wax, plasticized cellulose nitrate, etc., this forming the phosphor-containing element 1. Further, the conducting layers 4 and 5 may be sheets of metal foil instead of being in the form of an adherent coating.

Further advantage of the high dielectric strength of polyethylene terephthalate film may be taken by employing the film as a dielectric protector. In other words, in cases where the dielectric strength of the phosphor-containing layer is considerably lower than that of polyethylene terephthalate film, the phosphor-containing layer may be inserted between adjacent layers of polyethylene terephthalate film; and the conductive layer is on the outside (see Figures 2 and 3), e. g., a metal-coated polyethylene terephthalate film in which the film is immediately adjacent the phosphor-containing layer and the metal coating is on the outside. The following examples illustrate the use of polyethylene terephthalate film in this capacity in addition to illustrating its use in conjunction with conventional materials of construction employed heretofore in electroluminescent structures.

EXAMPLE 2 Referring to Figure 2, an electroluminescent structure was fabricated by employing a polyethylene terephthalate film (element 9), i. e., 0.001 in thickness, and stretched 3 times (3X) in both directions and heat-set under tension at 200 C., coated on the outside with a layer of aluminum (element 10) 0.0025 mil in thickness. Element 8 of the structure was a thin film of beeswax (0.0005" in thickness) in which was dispersed a specially prepared phosphor comprising zinc sulfide with copper and lead as activators. Element 11 was a solid brass plate about 0.125 in thickness. A source of alternating current at a frequency of 10 kilocycles was employed to apply a potential of 400 volts between the electrodes of the capacitor construction. As a result, a green glow was emitted through the thin coating of aluminum on the polyethylene terephthalate film.

EXAMPLE 3 A structure corresponding to the arrangement of Figure 3 was fabricated and element 12 was a thin film (0.0005 in thickness) of beeswax containing a dispersion of the phosphor of Example 2. This phosphor-containing dielectric was protected with two adjacent films of polyethylene terephthalate (elements 13 and 14). These films had a coating of aluminum (0.0025 mil) on the outer surfaces, and the aluminum coatings were elements 15 and 16. A source of alternating current at a frequency of 10 7 kilocycles was employed to apply a potential of 400 volts across the electrodes. In order that the illumination could be observed, the aluminum coating (element 15) on the polyethylene terephthalate film was removed from the edge portions of three sides, and a green illumination was emitted from those portions of the structure. The light of highest intensity Was emitting from portions immediately adjacent to the edges of the metallized area.

As many widely difierent embodiments may be made without departing from the spirit and scope of this invention, it is to be understood that said invention is in no wise restricted save as set forth in the appended claim.

I claim:

A flexible electroluminescent lamp structure which comprises a'fiexible bottom layer of biaxially oriented, balanced, heat-set polyethylene terephthalate film having a fiexible coating of metal, a flexible intermediate layer of polyethylene terephthalate film containing a phosphor material which is excited to luminescence under the influence of an electrical field, and a flexible top light-transmitting layer of biaxially oriented, balanced, heat-set polyethylene terephthalate film having a coating of metal about 0.0001 of a mil in thickness.

Mager Sept. 4, 1951 Mager Jan. 6, 1953

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