Abstract:
An ink composition for use in a dielectric or electroluminescent (EL) layer of an EL device, as well as a method of preparation of such composition, are presented. The composition comprises a water-based dispersion of a hydrophobic polymer with a high dielectric constant, rheological additives and a dielectric powder or EL phosphor powder.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an electroluminescent (EL) light source and a method of manufacturing thereof.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional AC powder EL type devices include at least one layer containing EL phosphor and at least one layer containing a dielectric powder with a high dielectric constant. To obtain these layers, inks (pastes) are usually applied in the form of a solution of a polymer with high dielectric constant, in which particles of either a dielectric material (typically, barium titanate or titanium oxide) or a powder of EL phosphor are dispersed for obtaining, respectively, dielectric or EL layers. The deposition of inks (pastes) is carried out by screen printing or pulverization, or by dip coating. After the deposition process, the layers are dried resulting in the evaporation of the solvent and the hardening of the layers. The polymer solutions typically used in inks (pastes) include cyanoethylated compounds such as cyanoethylated ethylene-vinyl alcohol copolymer, cyanoethylated cellulose, cyanoethylated pullulan, cyanoethylated starch, cyanoethylated saccharose, cyanoethylated glycerine, etc. Fluoropolymers, such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene terpolymer, etc., can also be used in the inks. To prepare the inks, these polymers are dissolved in organic solvents, such as ketones, glycols, DMF, etc. All these solvents are more or less toxic, and some of them are flammable and explosive. When dealing with mass production, the vapors of these solvents pollute the environment. Moreover, the frequent use of these solvents presents potential danger for personal health.  
         SUMMARY OF THE INVENTION  
         [0003]    There is accordingly a need in the art to facilitate the manufacture of EL devices by providing novel ink compositions and a method of their manufacture suitable for mass production. The inks according to the invention for use in the fabrication of dielectric and luminescent layers of EL light sources do not contain any solvents harmful to the environment, are suitable for deposition and drying processes with standard equipment, and provide the desired quality of the layers.  
           [0004]    Thus, according to one aspect of the present invention, there is provided an ink composition comprising a water-based dispersion of a hydrophobic polymer with a dielectric constant of no less than 6 (at 1 kHz and 25° C.), rheological additives and a powder containing particles of a dielectric or EL material.  
           [0005]    Preferably, the aqueous dispersions of the hydrophobic polymer are based on fluoropolymer material. In this case, for one weight part of dry fluoropolymer, the following relative concentrations (in weight percentage) of the other components may be used: 0.25-4 parts of water, 0.001-0.1 parts of rheological additives, and 1.5-15.0 parts of EL phosphor or 1.5-15.0 parts of a dielectric powder. The fluoropolymer may be PVDF based elastomer (Latex) or PVDF based thermoplastic polymer.  
           [0006]    In order to improve the quality of inks, they may contain anti-foam agents, for example, an emulsion of modified polysiloxane and/or surface active agents, for example, fluoroorganic surfactants.  
           [0007]    Preferably, the dielectric particles or EL phosphor powder used in the water based ink of the invention have hydrophobic surface.  
           [0008]    The rheological additives may be thixotropic agents, thickeners and flow modifiers. Thixotropic agents are organic titanium chelates. Organic chelates include triethanolamine and/or 2,2′-oxydiethanol and/or diethylenglycol and/or ethylenglycol and/or triisopropanoamine. According to a preferred embodiment, bis(triethanolamine) titanium di(methyl diglycolate)—trade name Tyzor ETAM from Du Pont and or titanium glycol alkanolamine complexes—trade name Tyzor TA 300, TA 400, are used as thixotropic agents.  
           [0009]    Thickeners are rheological additives which at low concentrations can effectively change the viscosity of a coating system. In the case of water based coatings having usually low viscosity, thickeners provide rheological profiles needed for each application. According to a preferred embodiment, polyether urea polyuretanes—trade name Rheolate 210, Rheolate 216 by Rheox Inc. or trade name Nopco DSX 1550 by Henkel or polyether polyols—trade name Rheolate 350, Rheolate 300 by Rheox Inc.  
           [0010]    Flow modifiers are rheological additives which provide a homogeneous coating without flow fractures and drops appearing. Flow modifiers are water soluble polyacrylates. According to a preferred embodiment, a pre-neutralized aqueous solution of an acrylic copolymer—trade name Modaflow AQ-3000, by Solutia Inc. is used as a flow modifier.  
           [0011]    While the hydrophobic polymer is a fluoropolymer, the composition preferably also includes an anti-foam agent, for example, an emulsion of modified polysiloxane at a ratio of 0.002-0.05 weight parts for one part of the dry fluoropolymer. In addition to the anti-foam agent, the composition may also comprise other surfactants. If the fluoropolymer is a PVDF based elastomer, the ink composition also includes a curing agent, for example, piperazine dipropanamine at a ratio of 0.001-0.1 parts for one part of the PVDF based elastomer.  
           [0012]    The dielectric powder may be a barium titanate powder. The EL powder may be in encapsulated form. The surface of the particles of the EL phosphor powder or dielectric powder may be modified by hydrophobic agents e.g., fluorosilicons or perfluoropolyethers.  
           [0013]    According to another aspect of the present invention, there is provided a method of preparation of an ink composition of a dielectric powder comprising the steps of:  
           [0014]    (i) dispersing a dielectric powder in water together with a thixotropic agent;  
           [0015]    (ii) preparing a polymeric solution of a thickening agent;  
           [0016]    (iii) mixing a water dispersion of a hydrophobic polymer with at least one anti-foam agent;  
           [0017]    (iv) mixing said polymer solution obtained in step (ii) above with said water dispersion of polymer prepared in step (iii);  
           [0018]    (v) adding the dispersion prepared in step (i) into the mixture obtained in step (iv), thereby obtaining a mixture presenting the ink composition; and  
           [0019]    (vi) introducing a flow modifier into the composition of step (v).  
           [0020]    The hydrophobic polymer is preferably a fluoropolymer, for example PVDF based material. The PVDF based material may be a thermoplastic polymer, or an elastomer, in which case a curing agent is added into the mixture obtained in step (v).  
           [0021]    According to yet another aspect of the present invention, there is provided a method of preparation of an ink composition of EL phosphor powder comprising the steps of:  
           [0022]    (a) adding at least one anti-foam agent into a water dispersion of a hydrophobic polymer;  
           [0023]    (b) preparing a solution of a polymeric thickening agent;  
           [0024]    (c) mixing said polymer solution of thickening agent with the dispersion prepared in step (a);  
           [0025]    (d) mixing the mixture obtained in step (c) with an EL phosphor powder, thereby obtaining the ink composition.  
           [0026]    The hydrophobic polymer is preferably a fluoropolymer, for example PVDF. The PVDF based material may be a thermoplastic polymer, or an elastomer, in which case a curing agent is added into the mixture obtained in step (c). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawing:  
         [0028]    [0028]FIG. 1 illustrates an electroluminescent light source utilizing links according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Referring to FIG. 1, there is illustrated an EL light source  10  in the form of wire utilizing inks according to the invention. The light source  10  comprises a central wire-like electrode  12  sequentially coated by identical dielectric layers  14   a  and  14   b , and identical EL layers  16   a  and  16   b . In order to obtain the dielectric layer  14   a , an ink wet layer with the thickness of 100 μm is deposited by dip coating onto the surface of the central wire electrode  12  and then dried to form the layer  14   a  with the thickness of 15 μm. The layer  14   b  also has a thickness of 15 μm and is obtained in a similar manner. The EL layers  16   a  and  16   b  with typical thickness of 25 μm each, are obtained in a similar manner, namely, by ink deposition and drying procedures. A transparent electrode  18  is formed on the outer surface of the layer  16   b  by depositing an indium-tin oxide (ITO) layer with a thickness of 400-500 Å. A wire electrode  20  is clamped to the surface of the transparent electrode  18  by means of a polymer insulating layer  22 , and provides contact to the transparent electrode  18 .  
         [0030]    The dielectric and EL layers are accommodated concentrically with the central electrode  12 . The thickness and concentricity of layers are defined by using floating dies of a specific form. The EL light source  10  emits light in response to the application of AC voltage to the electrodes.  
         [0031]    In the present example, the drying of layers is carried out in a continuous process in a 6 m height vertical column formed by an array of furnaces. The temperature at the entrance of column is 60° C., and at the exit from the column is 190° C. The velocity of the wire path through the column is 9 cm/sec. The central electrode is a copper wire of the 0.5 mm diameter. The experimental results have shown that such EL wires are characterized by a brightness of 50 cd/m 2  and more and by a half-life time of brightness not less than 1400 hours at the applied voltages of VAC=120V and the frequency of 400 Hz.  
         [0032]    The following are two examples of the ink compositions for dielectric and EL phosphor layers, respectively, which were used in the fabrication of the above-described device  10 .  
       EXAMPLE1  
     Ink for Dielectric Layers  
       [0033]    [0033]                                                     Material   Type   %                                Fluoroelastomer based Latex   Fluorobase 100 commercially   28.2           available from Ausimont       Deionized Water       23.4       Curing Agent   Fluorobase T-520 commercially   0.2           available from Ausimont       Barium Titanate   HPB commercially available   46.6           from TAM Ceramics       Thixotropic agent - organic   TYZOR ETAM commercially   0.5       titanium chelate   available from Dupont       Rheological additive - flow   Modaflow AQ 3000 commercially   0.5       modifier   available from Solutia       Rheological additive -   Nopeo DSX 1550 commercially   0.3       thickener   available from Henkel       Anti-foam agent   Serdas GBR commercially   0.3           available from Servo Deldon BV                    
       EXAMPLE 2  
     Ink for EL Phosphor Layers  
       [0034]    [0034]                                                     Material   Type   %                                Water dispersion of fluoro   THV 340CD commercially available   49.3       thermoplastic polymer   from Dyneon       EL Phosphor   IPHS001 commercially available   49.3           from Durel       Thixotropic agent - organic   TYZOR ETAM commercially   0.5       titanium chelate   available from Dupont       Rheological additive - flow   Modaflow AQ 3000 commercially   0.5       modifier   available from Solutia       Rheological   Rheolate 300 commercially available   0.5       additive-thickener   from Rheox       Anti-foam agent   Serdas GBR commercially available   0.4           from Servo Delden BV                    
         [0035]    The following two examples of ink compositions are also suitable to be used for the fabrication of dielectric and phosphor layers, respectively.  
       EXAMPLE 3  
     Ink for Dielectric Layers  
       [0036]    [0036]                                                     Material   Type   %                                Water dispersion of   THV 220D commercially available   55       Fluorothermoplastic   from Dyneon       Polymer       Barium Titanate   HPB commercially available   43           from TAM Ceramics       Thixotropic agent - organic   TYZOR ETAM commercially   0.5       titanium chelate   available from Dupont       Rheological additive - flow   Modaflow AQ 3000 commercially   0.5       modifier   available from Solutia       Rheological   Rheolate 208 commercially available   0.5       additive-thickener   from Rheox       Anti-foam agent   Serdas GBR commercially available   0.5           from Servo Delden BV                    
       EXAMPLE 4  
     For Phosphor Layers  
       [0037]    [0037]                                                     Material   Type   %                                Fluoroelastomer based Latex   Fluorobase 100 commercially   37.4           available from Ausimont       Deionized Water       12       Curing Agent   Fluorobase T-520 commercially   0.3           available from Ausimont       EL Phosphor   ANE430 commercially available   49.4           from Sylvania       Rheological additive - flow   Modaflow AQ 3000 commercial   0.3       modifier   available from Solutia       Rheological   Rheolate 300 commercially   0.3       additive-thickener   available from Rheox       Anti-foam agent   Serdas GBR commercially   0.3           available from Servo Delden BV                    
         [0038]    Thus, the advantages of the present invention are self-evident. The environmentally friendly inks for EL light sources according to the invention are based on aqueous dispersions of hydrophobic polymers with high values of dielectric constant, rather than the conventionally used solutions of polymers in organic solvents. The EL light sources utilizing the inks of the present invention are characterized by high values of brightness and half-life.