WO2001010571A1

WO2001010571A1 - Printable electroluminescent lamps having efficient material usage and simplified manufacture technique

Info

Publication number: WO2001010571A1
Application number: PCT/US2000/022021
Authority: WO
Inventors: Bryan D. Haynes; Gerald Scripter; Matthew Wilkinson
Original assignee: Add-Vision, Inc.
Priority date: 1999-08-11
Filing date: 2000-08-10
Publication date: 2001-02-15
Also published as: AU6538000A

Abstract

EL lamps may be constructed on a variety of surfaces including optically transparent, translucent and opaque substrates (110). Lamps manufactured according to the present invention may use translucent conductive ITO ink selectively printed as a replacement electrode (120) instead of a sputtered ITO transparent conductor.

Description

PRINTABLE ELECTROLUMINESCENT LAMPS HAVING

EFFICIENT MATERIAL USAGE AND

SIMPLIFIED MANUFACTURE TECHNIQUE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electroluminescent lamps. In particular, the invention relates to thin electroluminescent lamps which can be manufactured using printing processes. 2. Background of the Related Art

Since the discovery of electroluminescent lamps (EL lamps) in 1936 by Destriau in Paris, engineers and scientists have held out the hope that these new lamps would be a commercially useful alternative to conventional incandescent and fluorescent lamps owing to their flatness, flexibility, low power dissipation, coolness, multicolor emission and lack of catastrophic failure. To date, however, these hopes have remained mainly unfulfilled due to inherent performance limitations and a variety of technical problems.

Most recently, engineers and scientists involved in EL lamp design have concentrated their efforts in a laminar EL lamp design based on indium tin oxide (ITO) sputtered onto various clear substrates such as a polyethylene terephthalate (PET) film substrate or glass. In this design, a phosphor material is typically sandwiched between a back electrode often made of a metal film such as aluminum foil, and a translucent sputtered ITO coated PET film front electrode. These lamps are operated by applying an AC voltage across the electrodes and this causes the phosphor material to luminesce. In order for the light produced in the phosphor material to escape from the lamp, one or more of the electrodes must be translucent or transparent. This translucent electrode was originally made of tin oxide thin film on glass or transparent plastic; currently, however, nearly all EL lamps use a sputtered ITO coated PET film as the translucent electrode. Sputtering is done by a non-printing process that starts with placing the PET on a coating drum in a high vacuum chamber. As the PET travels past cathodes, ultra-thin layers of target material such as ITO are deposited onto the substrate surface. The ITO layers are produced by argon ions hitting a negatively biased target loosening target atoms from a plate or tube. These atoms accelerate in the direction of the PET where they are stopped and bonded, forming a uniform, adherent coating.

Although the sputtered ITO-based EL lamps currently produced are brighter and more durable than the original lamps of fifty years ago, these lamps still possess a number of drawbacks that have precluded their use in many everyday applications.

The main drawbacks of sputtered ITO-based lamps are cost and processability. Although sputtered ITO-coated PET films are readily available, they are expensive and can account for the bulk of the cost of materials for EL lamps. In addition to this excessive cost, EL lamps based on sputtered ITO-coated PET films are not easily shaped. When using sputtered ITO-coated films in a typical lamp construction, the ITO layer will crack when the EL lamp is bent, creased or stretched. Sputtered, ITO/PET- based lamps cannot therefore easily be shaped, using well-known polymer processing techniques such as sheet forming techniques. For these reasons the sputtered ITO-based lamps cannot easily be shaped into anything other than the simplest geometric forms.

Additionally, the nature of sputtered ITO requires that any patterning of it be a subtractive process; that is, to form a desired design, a block of ITO must be sputtered en masse and then selectively etched away with hazardous etching solutions, leaving the final design. In addition to wasting a significant amount of expensive ITO material, this additional etching step unnecessarily complicates the fabrication process.

In summary then, the discovery and development of EL lamps held out the hope of inexpensive, long-lived, flat and flexible lamps that do not generate much heat and which consume very little power. Unfortunately, fifty years of development has failed to fulfill many of these initial promises. Present day lamps typically are of laminar construction based on a PET-mounted transparent sputtered ITO electrode. These lamps suffer from a variety of drawbacks that make their commercialization for many applications infeasible. Important among these drawbacks are cost, lack of easy processability, and an extra etching step.

SUMMARY OF THE INVENTION The present invention addresses the above problems in lamp construction and provides new approaches to EL lamp design. As described in detail below, these new approaches permit the construction of bright, durable, energy efficient, cool, flexible and moldable EL lamps which can be driven at high electrical frequencies for extended periods of time, thus increasing the brightness and lifetime of the EL lamps. In addition to these technical advantages, the lamps of the present invention are much simpler and much less expensive to manufacture than conventional lamps.

The present invention permits the construction of EL lamps on a variety of surfaces including optically transparent, translucent and opaque substrates. Lamps manufactured according to the present invention may use translucent conductive ITO ink selectively printed as a replacement electrode for a sputtered ITO transparent conductor. Producing lamps incorporating the translucent ink requires a precisely controlled step-by-step process. This process includes, but is not limited to, using a printing environment having particle contaminant reduction and control of ambient temperature, controlling the thickness of the sequentially printed ink layers, controlling residence cure times and temperatures, and selecting inks that are compatible with adjacent materials that are used in lamp laminar construction. If an incompatibility exists, the probability of defects is great. Such defects typically lead to lamp failure or malfunction. These defects may include swelling of print layers due to residual solvent attack on an underlying print layer and an irregular print surface or thickness caused by non-uniform solvent removal or irregular underlying print surfaces. In addition, an out of desirable, range resistivity may result when an ink is incompletely mixed, not thoroughly dried, or used after the expiration of its shelf life. Likewise, undesirable resistivity can result from poor or incomplete curing or non-uniform or incorrect print thickness or an unacceptable ink transparency. According to the present invention, proper control of these variables and parameters results in an operational electroluminescent lamp with desirable operating characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features, and advantages of the present invention are better understood by reading the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a cross sectional view depicting the relative positions of print layers according to a first preferred embodiment of the present invention.

FIGURE 2 is an elevation view of the lamp according to a first preferred embodiment of the present invention. FIGURE 3 is an elevation view of the lamp including circuitry, according to a first preferred embodiment of the present invention.

FIGURE 4 is a cross sectional view depicting the relative positions of print layers according to a second preferred embodiment of the present invention

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS By using new EL lamp designs and new methods of fabrication, it is possible to produce flexible, bright, durable and long-lasting EL lamps that consume little power and may be formed into, a variety of geometric shapes. The present invention provides for the manufacture of electroluminescent displays that are completely printable, and the following provides the required process documentation to support the volume manufacture of a product embodying the invention and to ensure the required levels of in- process control are applied to ensure satisfactory and improving yields. The product is printed in a number of registered layers. Process requirements are stringent as the overall product performance is very dependent on the print of specific layers, and as it is not possible to fully test the final result until the product is complete.

The principle of the product is that when a layer of phosphor-loaded ink is sandwiched between two conductive planes, and a high frequency alternating field is applied across the planes, the phosphor will emit light. A preferred embodiment of the product requires that all the layers of ink used be correctly applied and cured, or the light output and life of the product will be adversely affected.

There are two basic versions: single layer lamps and switchable segment lamps. For single layer lamps, the layers are printed as shown in FIGS - 1 and 2. On a polyester sheet 110 as will be defined below, a clear conductive ink layer 120 is printed. Then a silver bus layer & tracking 130, phosphor ink 140, first dielectric ink layer 150, second dielectric ink layer 155, third dielectric ink layer 160, silver back plane layer & tracking 170, and finally dielectric ink layer 180 are printed on top. Switchable segment lamps as shown in FIG. 3 are constructed as single layer lamps pictured in FIGS. 1 and 2 but with the addition of cross-over tracking and dielectric layers 180 and 185, respectively.

A method of manufacturing a first preferred embodiment of the present invention will be described below. In this embodiment shown in FIGS. 1 and 2, the electroluminescent lamp emits light forward through a clear substrate 110; thus, this embodiment is called a "forward build" electroluminescent lamp.

The manufacturing process begins with a substrate 110. The substrate 110 may be made from any suitable thin, transparent or semi-transparent material such as vellum, cellophane, or Mylar. Preferably, however, the substrate 110 is made from 2-7 mil thick heat-stabilized polyester.

Next, a mesh (not shown) is used to screen print a clear conductive ink 120 on the substrate 110. The ink used here preferably is chosen from a group which includes Acheson EL 020, Electrodag PF-427, and DB2320, as well as coatings from other manufacturers that have the same characteristics of being translucent and conductive. To print the ink, a photoreactive emulsion is applied to a screen mesh stretched across a 205N frame to 20-40 N using Newman roller frames or a 230 mesh in a diamond chase frame. A film positive consisting of the image to be reproduced is taped to the screen. The screen is exposed to ultraviolet light for a predetermined amount of time using an UV exposure unit and then developed with water, where the dark images on the film that were against the screen were not exposed to the UV light and wash out with the water, leaving the desired image on the screen. The screen is then allowed to dry and is clamped into a printing press. The squeegee and floodbar of the printing press are perpendicular to the screen. The floodbar consists of a solid flat piece of aluminum and is used to spread the ink across the screen in the up position. The squeegee is a piece of polyurethane clamped between two pieces of aluminum and is used to shear the ink through the screen in the down position. The substrate to be printed is registered to the screen and registration tabs are mounted on the print platen to insure repeatable registration. The image is applied as the squeegee passes over the top of the screen and the squeegee and screen come in contact with the substrate with sufficient force to cause the ink to pass through the screen.

Where a wider mesh opening is desirable, a coarser mesh is used. This is evident in the case of the phosphor layer 140 (discussed later), where the relatively coarse phosphor particles must pass through the mesh in an even and consistent manner. The total thickness of the mesh is also one of the largest variables in controlling the ink deposition thickness, as in the case of the barium titanate dielectric ink layers 150, 155 and 160 (also discussed below) because the space allowed by the mesh thickness directly correlates to the ink deposition thickness. Mesh tension, i.e., how tight the screen is pulled in all directions, also plays a major role in relation to the viscosity and rheology of the ink. This is because if the mesh is not sufficiently taut, the resulting image that is printed will be blurred when printed because the squeegee will drag the mesh; if the viscosity of the ink is too high it may cause the mesh to stick to the printed substrate after the squeegee has passed, causing more blurring of the image. For example, due to the viscous, plastic nature of the clear conductive layer 120, it is more desirable to have a high tension mesh since this gives a better snapoff (the ability and speed at which the screen releases from the substrate after the squeegee has passed and the printed image is applied) at the print point and therefore a more even deposition with less chance of the ink clinging to the mesh, creating irregularities in the printed layer.

Belt drying is preferably used in the production process. Belt drying is used in print production where the speed of drying is needed to be high, which is often. The advantage of the belt dryer over a batch dryer is production value; the belt draws the part down the production line where it is further processed by another employee, leaving the corresponding space open for the printer to place the next printed part down. Drying of the clear conductive layer 120 is at 100-120°C for 10-20 minutes in the batch drier or alternatively five minutes on the belt drier at similar temperatures. This layer 120 is preferably printed in two passes with a dry step between each. The double layer is used to reduce the resistivity associated with this material. This material seems to have a better success rate when printed with a higher tension (30 or higher) due to the stickiness of the ink, and benefits from a better snapoff for a more uniform coat. Consideration must be given to coating thickness due to light attenuation vs. resistivity; that is, a thicker deposit is more conductive but allows less light to pass through it. Care must also be given to the drying rate and time since they are known to greatly affect the resistivity of this layer. This is because if the drying rate is too short, the resistivity of the film increases dramatically. If the dryer is set too hot, bubbles of gas from evaporated liquid are caught when the surface of the ink dries more quickly than the rest of the layer. Since the clear conductor has some resistance to current flow, a highly conductive material such as silver (or other screen printable conductive material with suitably low resistance) is applied as a bus bar around the perimeter of the lamp to ensure the even distribution of current to the clear conductor. This bus bar layer may be applied at the same time as the rear electrode by using appropriate artwork. Next, a phosphor ink layer 140 is printed. The phosphor ink may be any ink employing a clear or translucent binder with the addition of phosphor particles (encapsulated or not), where the binder is used as the vehicle for the deposition of such particles. This layer 140 may include the addition of an internal or external filter for changing the color of the light produced by the phosphor and may be screen printed in the forward build fashion (alternatively, it may be printed on the outside of the lamp). The screen mesh used to print the phosphor layer 140 should be 160 mesh, but may vary according to the size of phosphor particles. Tension of the screen should be at 18-20 N, but also may vary according to ink rheology. Viscosity in well-mixed ink should- be 15,000 to 25,000 cp, with optimal results between 17,000-20,000 cp. Flood and squeegee stroke should be set slow, with moderate to maximum angle applied to the squeegee. The resultant print should exhibit dense particle deposition with as few gaps as possible when viewed under magnification. Optimal results should be a dense singular layer of particles, with a thickness of 1.25-1.5 mils. Optionally, this layer may be allowed to remain at room temperature for 5- 15 minutes prior to oven drying to allow the particles to settle in a more even formation, for the purpose of improving density and singularity of layer thickness. Oven drying should be at 125 135°C for 5-10 minutes.

Optionally, an additional layer of binder may be printed over the phosphor ink layer 140 to aid in the planarizing of this layer in order to achieve parallelism between the front and rear electrodes. This has been found to increase lamp efficiency and life.

Preferably, the squeegee material used in the printing processes is a composite polyurethane material consisting of three laminated layers. The outside layers are 70 durometer and the inside is 90 durometer. The composite is referred to as 70/90/70 triple durometer polyurethane. This is in no way a set limit to any of the above processes, and positive results have been exhibited from single durometer squeegees in the past at ranges of 60-90 durometer. The squeegee should be sharpened prior to production of each layer with a hot blade-style sharpener for optimal results.

Next, the dielectric ink layers 150, 155 and 160 are printed. Three layers, each made of the same material and having the same image, are used for several reasons. First, three layers help ensure that there are no shorts between the front and rear electrodes. Second, the multiple layers help to smooth over the rough surface of the phosphor (similar to spackling in drywall) in order to create a smooth surface on which to print the rear electrode so the two electrodes are as parallel as possible. Third, the barium titanate material has dielectric properties which allow the buildup of an electric field while preventing current from flowing through it.

A barium titanate compound such as Dupont 7153 or a similar dielectric insulator compound from another manufacturer should be used for these layers 150, 155 and 160 and printed through a 160 mesh. The viscosity may range between 30,000 and 45,000 cp, with optimal viscosity of 35,000-40,000 cp. The coating is preferably dried at 125-135°C for 5 minutes in the batch dryer or alternatively for 2-3 minutes on the belt drier at comparable temperatures. A dry step is preferably done after deposition of each layer of dielectric ink 150, 155 and 160. The individual coats should be about 0.2 mils thick, and the total thickness should be 0.5-0.6 mils. Other mesh counts may be used to achieve a thicker or thinner individual layer as deemed appropriate. Triple pass on this layer is used to provide the advantages described above. A double layer may be used when a brighter lamp is required; however, the double layer exhibits slightly more roughness than a triple layer. A single layer is unable to provide enough dielectric strength to keep the front and rear electrodes from shorting. After printing the dielectric ink layers 150, 155 and 160, the rear electrode or back plane layer 170 is printed. The parameters used for bus bar / front grid electrode 130 can be used here, and if a double print is done a brighter lamp can be provided. Also, the double print will add durability to any subsequent traces. This electrode 170 typically has a line width of 0.005-0.030". Line spacing is preferably between 0.020-3", but may be adjusted as necessary. Preferably, the material used to form the bus bar and grid electrode 170 is Dupont 5025 silver ink or other conductive ink with similar conductivity printed through 205N or 230 mesh and dried at 110-13O°C for 3-5 minutes on a belt dryer, or alternatively for 5-10 minutes in a batch drier at similar temperatures. Preferably, two coats are deposited with a dry step between. This is because the double coat has less electrical resistance and is more durable.

Preferably, indirect emulsion in rolls is used in printing the layer 130, the standard being cap 25 (relating to the thickness). Indirect emulsion is applied via capillary action with a fine water mist and standard window squeegee to remove excess water. The screen is placed in a dryer to evaporate any moisture prior to ultraviolet light exposure.

The emulsion is exposed at 105 light units, developed (or rinsed) with water and placed in a batch drier at 115°F for 2-4 minutes to increase the drying speed of the emulsion. Different types of emulsion react differently to various lengths of exposure. If the exposure is too short, the stencil is not strong enough and may fall off the screen, develop pinholes, or exhibit other problems. If the exposure is too long, open areas may not open up in the development stage due to light leakage through the black areas in the film positive.

Other thickness of capillary film may be used (in general, capillary film is used when an extremely even stencil thickness is necessary), but it has been noted that a thicker stencil will cause the coating at the stencil edge to be notably thicker, which can result in an undesirable effect in subsequent layers. Direct (liquid) emulsion may also be used when a specific emulsion thickness is not required. Direct emulsion is applied to the screen via a trough-like device called a scoop coater having a flat blade edge on one side. The liquid is placed inside the trough, with the blade edge placed against the screen.

Both are tipped back and the emulsion flows against the screen mesh and is immediately leveled even and thin by the blade edge. Usually this is done twice on each side of the mesh. The high viscosity of the emulsion allows it to cling to the mesh. This is then air or oven dried prior to exposure. The dielectric insulating layer 180 may be applied to avoid electric shock.

Alternatively, a film lamination may be applied for the same purpose.

A certain amount of static is built up during production, which can attract foreign material, especially when printing on polyester or similar substrates. Running these substrates through deionizing bars mounted over a parts cleaner above the press, above the front or back of the belt drier or anywhere the substrates are exposed to dust, removes any particles and static. The parts cleaner machine includes two parallel rollers with sticky contact paper and a deionizing bar which the lamps being manufactured roll between and under respectively.

Details for manufacturing a specific forward build example embodiment of the present invention are now described. Here, the printable ITO layer is designed to be 0.15" larger then rear electrode. Also, the bus bar width is a minimum of 0.005" and needs to run along the longest two sides of each cell. The maximum distance to closest bus bar = 3.0". Cell Construction: • Polyester - 0.005" or 0.007" thick heat stabilized Printable ITO -DB-2320,230 Mesh, Cap 25, 1/1 Phosphor - 7141, 714 1 w/rod, 160 Mesh, Cap 25 1 or 2 wet passes White Dielectric - Dupont 7153, 160 Mesh, Cap 25, 1/1/1 Rear Electrode / Bus Bar - Dupont 5025, 230 Mesh, Cap 25, 1/1 Safety Lam - 0.001 - 0.005" polyester PSA Total Cell Thickness = 0.002" Testing Parameters and Requirements:

Preferably, drying is performed at 280±5°F at 16 FPM on a 27' tunnel with 2' of infrared lamps (a total of 6 lamps) at the front of the dπer, with the conductive layers 120 and 170 travelling through the drier twice to decrease resistivity; layer 130 may be dried at half the normal belt speed (double the time in the tunnel) to ensure optimum cure (this may be done for layers 120 15 and 170 as well). Optionally, a flat substrate such as aluminum foil, polyester, or Teflon® may be placed under the substrate during the drying process to improve the drying rate.

In a second preferred embodiment of the present invention shown in

FIG. 4, the electroluminescent lamp emits light backward away from the substrate. This embodiment is called a "reverse build" electro luminescent lamp. Here, polyester sheet substrate 2 10 generally corresponds to substrate 110 in the first embodiment, silver bus electrode 220 generally corresponds to silver back plane layer and tracking 170, dielectric ink layers 225, 230 and 235 generally correspond to dielectric ink layers 150, 155 and 160, phosphor ink layer 240 generally corresponds to phosphor ink layer 140, front electrode ink layer 250 generally corresponds to clear conductive ink layer 120, and silver bus edge electrode 260 generally corresponds to silver bus layer with tracking 130. In the second embodiment, however, the silver bus plate electrode 220 is somewhat different from its counterpart in the first embodiment. Here, the line width is preferably 0.005-0.03 0" and line spacing is preferably between 0.010-1. 5", although it can be adjusted as circumstances dictate. Preferred material for the electrode 220 is Dupont 5025 silver printed through 205N or 230 mesh dried at 110-130°C for 5-10 minutes. Preferably two coats of the electrode material are laid down with a dry step in between to decrease resistance and increase durability.

The above reverse build structure of the second embodiment provides many of the same advantages as the forward build first embodiment. Additionally, in the reverse build design the substrate need not be clear in order to have a viewable lamp, and it need not be thin enough to be transparent or translucent since light is emitted through the clear conductor 250 printed in the reverse order.

The above description of the preferred embodiment of the present invention is provided for purposes of illustration only, and the invention is not so limited. Variations on the preferred embodiment will be readily apparent to those skilled in the art, and the invention encompasses these variations as well. Thus, the present invention should be understood to be limited only by the scope of the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A method of manufacturing an electroluminescent lamp comprising: printing a first conductor layer on a substrate using a conductive ink; printing a phosphor layer proximate to the first conductor layer using a phosphor ink; and printing a second conductor layer proximate to the phosphor layer using a conductive ink.

2. The method of claim 1 wherein the substrate is clear and the first conductor layer is clear.

3. The method of claim 1 further comprising printing a bus and tracking layer on the first conductor layer, the phosphor layer being printed on the bus and tracking layer.

4. The method of claim 1 further comprising printing at least one dielectric layer on the phosphor ink layer using dielectric ink.

5. The method of claim 4, wherein the at least one dielectric layer is three dielectric layers.

6. The method of claim 4, wherein the dielectric ink includes barium titanate.

7. The method of claim 1 , further comprising printing a dielectric layer on the second conductor layer.

8. The method of claim 1, further comprising printing a dielectric layer on the first conductor layer using dielectric ink, the phosphor layer being printed on the dielectric layer.

9. The method of claim 1, further comprising printing a front electrode ink layer on the phosphor layer using a clear conductive ink, the second conductor layer being printed on the front electrode ink layer.