Cathode ray tube contrast enhancement systems
RELATED APPLICATION 
     This application is a continuation-in-part of my U.S. application Ser. No. 894,984, filed 8/8/86, entitled &#34;Cathode Ray Tube Implosion Protection System.&#34; 
    
    
     FIELD OF THE INVENTION 
     The invention relates to methods and means for enhancing the image quality of a cathode ray tube. 
     BACKGROUND OF THE INVENTION 
     The implosion which occurs upon breakage of the envelope of an evacuated cathode ray tube (CRT) is quite dangerous. Impact on the glass faceplate of such a tube can cause the faceplate to shatter into many fragments, which may be violently driven into the interior of the tube by external air pressure. The glass fragments then rebound outwardly and are ejected with sufficient force to cause serious injury to a person standing in front of the tube. 
     Until recently, all color television tubes have consisted of CRT&#39;s with convexly curved faceplates. Such faceplates resist external air pressure in much the same manner as an arch supports an architectural load, and for that reason prior art methods of implosion protection have proved adequate. But curved faceplates require that the shadow mask employed in color TV systems must also be curved. Recently, a superior color CRT has been invented which employs a flat, tensioned shadow mask and a flat faceplate, and this has resulted in a major improvement in the brightness and/or contrast of the color image. 
     Unfortunately the implosion protection systems which have been used successfully with curved faceplate tubes have proven inadequate when used with flat faceplates. In particular, when prior art implosion protection systems are tested on the new flat tension mask tubes, they fail to meet UL1418, the relevant safety standard of Underwriters Laboratories, Inc. for television implosion hazards. 
     A new type of implosion protection, system however, has now been developed and is disclosed in the parent application cited above. That system employs a UV-curable resin formulation to bond an implosion panel to a CRT faceplate, the formulation being designed to achieve separation of the implosion panel from the faceplate upon impact. 
     The present invention is an improvement upon the above-described resin system, in that a contrast enhancement agent is added to the resin portion of the implosion protection system in order to improve the quality of the CRT image. 
     It is also an improvement upon contrast-enhancement systems of the type suggested in Robinder, U.S. Pat. No. 3,879,627; in which colloidal carbon or graphite is added as a neutral density filtration agent to an epoxy or polyester adhesive resin layer which bonds an implosion panel to a CRT faceplate. Column 3, lines 55-64 of that patent explain why neutral density filtration enhances CRT image contrast. 
     See also Ohkoshi, U.S. Pat. No., 3,909,524; in which a black &#34;paint&#34; such as carbon or silica is added as an optical filtering agent to a polyester adhesive resin layer which bonds an implosion panel to a CRT faceplate. 
     A contrast-enhancing neutral density filtration effect, combined with implosion protection, is also claimed by Barnes, U.S. Pat. No. 2,734,142; in which a sheet of cellulosic or other plastic material, treated with amino hydroquinone diethyl ether and a copper salt, is inserted between an external lens and a CRT faceplate. 
     Then there is British specification 889,457 of Darlaston et al.; which coats a CRT faceplate externally with layers of polymeric material for implosion protection, and adds an unspecified dye or pigment to the polymer for image enhancement purposes. 
     The above-described prior art, however, does not employ the type of neutral density filtration agent taught herein, nor does it disclose a method of preparing a UV-curable resin bonding system incorporating such a filtration agent. It also does not address the special case of neutral density filtration in the environment of the new flat tension mask type of CRT tube. 
     A preferred contrast-enhancing agent is one which will be uniformly distributed throughout the adhesive resin. When carbon particles and similar colloidal dispersions were used, it was not possible to obtain homogeneous distribution of the particles throughout the resin, and therefore the picture tube lacked the uniform appearance desired. The preferred contrast-enhancing agents are those which are organic and are soluble in an organic solvent, which in turn is soluble in and chemically reactive with the adhesive resin system. The best organic contrast-enhancing agents are generally the mono-azo metal complex dyestuffs. The specific material used here as an example is &#34;Orasol Black CN&#34; from Ciba-Geigy Corp., a material which has the following C.I. number in the publication &#34;Colour Index:&#34; C.I. Solvent Black 28. 
     Preferred embodiments demonstrating the various objectives and features of the invention will now be described in conjunction with the following drawings, which constitute a part of this specification: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial longitudinal cross-sectional view of a flat tension mask cathode ray tube having an implosion panel system with a contrast enhancement agent in accordance with this invention; 
     FIG. 2 is an enlarged cross-sectional detail view of the same tube illustrating one embodiment of a contrast-enhancing resin bonding system in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring to FIG. 1, an evacuated CRT tube 20 comprises a funnel 22, frame 16 and flat faceplate 10 all made of glass. A flat, tensioned color shadow mask 24 is mounted on the frame 16 within the evacuated envelope. Funnel 22 is sealed to frame 16 by means of glass frit in the circumferential sealing area 11 and in the registry grooves 18 which contain a plurality of registry balls 26. Faceplate 10 is sealed to the frame 16 in the identical fashion. A glass implosion panel 12 is bonded to the external surface of faceplate 10 by means of a resin system 14. Implosion panel 12 is substantially thinner and more flexible than the faceplate 10. The implosion panel is commercial double strength window glass with a thickness of an eighth of an inch. The window glass is coated with a thin layer of an anti-reflection material 25 on its outer surface. See FIG. 2. 
     The preferred embodiment of the resin system 14 has two resin layers 28 and 30 which are different compositions with different adhesive properties. The outer resin layer 28 adheres tightly to the implosion panel 12, and preferably has a thickness in the range from twenty to forty mils. The inner resin layer 30 adheres to the faceplate 10 and adheres weakly to the outer layer 28. The inner layer 30 has a thickness that may vary from 5-15 mils across the face of the tube 20, since the faceplate 10 generally has a slightly concave surface due to the internal vacuum of the CRT. 
     The resin layers must have a thermal stability sufficient to exceed U.L. standards (which require that laminated tubes withstand 149 degrees Celsius for 50 hours and 154 degrees Celsius for 40 hours). They must also exhibit ultra-violet stability and have an index of refraction that substantially matches the index of refraction of the glass faceplate and implosion panel. 
     The preferred composition of the outer layer 28 includes the following acrylates: 
     (a) 40 to 90% by weight multifunctional urethane acrylate oligomer, such as urethane polyester acrylate; 
     (b) 10 to 55% by weight monofunctional acrylic monomer, including 
     0 to 30% by weight caprolactone acrylate, 
     0 to 30% by weight isobornyl acrylate, and 
     0 to 30% by weight methoxy hexanediol acrylate; 
     (c) 0 to 20% by weight difunctional acrylic monomer; and 
     (d) 0 to 10% by weight trifunctional acrylic monomer. 
     The preferred composition of the inner layer 30 includes the following acrylates: 
     (a) 30 to 70% by weight multifunctional urethane acrylate oligomer, such as urethane polyester acrylate; 
     (b) 15 to 55% by weight monofunctional acrylic monomer, including 
     0 to 30% caprolactone acrylate, and 
     0 to 25% by weight isobornyl acrylate; and 
     (c) 0 to 50% by weight difunctional acrylic monomer, including 
     0 to 25% by weight hexanediol diacrylate, and 
     0 to 25% by weight triethylene gylcol diacrylate; 
     (d) 0 to 40% by weight trifunctional acrylic monomer; and 
     (e) 0.2 to 2% by weight of a releasing agent, such as a surfactant. 
     The above resin compositions also have added thereto various photo-initiators and neutral density filtering means as described below. 
     In accordance with this invention, a neutral density filtering agent in the form of about 1% by weight of a solution of an organic dye in a resin-reactive organic solvent is added to the outer layer 28 only. About 1% of the solution by weight is solute. &#34;ORASOL BLACK CN&#34; from Ciby-Geigy Corp. is a preferred organic dye, and &#34;VPRC&#34; brand of N-vinyl-2-pyrrolidone monomer from GAF Corp. is a preferred solvent. 
     While many combinations of materials can be used which exhibit the required properties, some actual examples are as follows: 
     The following Table I illustrates three examples of preferred compositions for the outer resin layer 28. The percentages are by weight. 
     
                       TABLE I                                                     
______________________________________                                    
Outer Resin Layer 28                                                      
Ingredient    Example 1  Example 2 Example 3                              
______________________________________                                    
893           67.5%      56%       60.05%                                 
PH8017        7.45%      none      none                                   
M-100         none       14%       18%                                    
IBA           22.5%      17.45%    20%                                    
QM920         none       10%       none                                   
907           0.5%       0.5%      0.5%                                   
ITX           0.05%      0.05%     0.05%                                  
1% Black CN in VPRC                                                       
              1%         1%        0.9%                                   
T328          1%         1%        0.5%                                   
______________________________________                                    
 
    
     The following Table II illustrates three examples of preferred compositions for the inner resin layer 30. The percentages are by weight. 
     
                       TABLE II                                                    
______________________________________                                    
Inner Resin Layer 30                                                      
Ingredient Example 4  Example 5  Example 6                                
______________________________________                                    
893        49%        60.75%     49%                                      
M-100      28.45%     15%        28.45%                                   
HDODA      20%        none       none                                     
SR272      none       none       20%                                      
DC193      1.5%       1%         1.5%                                     
IBA        none       22.5%      none                                     
907        1%         0.7%       1%                                       
ITX        0.05%      0.05%      0.05%                                    
______________________________________                                    
 
    
     In the above Tables I and II the ingredients are as follows: 
     893 is UVITHANE 893, a polyester urethane acrylate oligomer sold by Morton Thiokol, Inc. 
     PH8017 is PHOTOMER 8017, a methoxy hexanediol acrylate sold by Diamond Shamrock Chemical Company. 
     M-100 is Tone M-100, a caprolactone acrylate monomer sold by Union Carbide Corporation. 
     IBA is isobornyl acrylate sold by Alcolac, Inc. and also by Arco Chemical Corporation. 
     HDODA is 1,6 hexanediol diacrylate sold by Arco Chemical Company and also by Interez, Inc. 
     SR272 is triethylene glycol diacrylate sold by Arco Chemical Company. 
     QM920 is a trifunctional acrylic monomer sold by Rohm &amp; Haas Company. 
     DC193 is DOW CORNING 193, a urethane-compatible surfactant sold by Dow Corning, used as a releasing agent. 
     907 is Irgacure 907, a photo-initiator sold by Ciba-Geigy Corp. 
     ITX is 2-isopropyl thioxanthone from Aceto Chemical Co., Inc., a photo-initiator. 
     T328 is Tinuvin 328 from Ciba-Geigy Corp., an ultra-violet absorber which prevents fading of Black CN. 
     Black CN is Orasol Black CN, an organic dye from Ciba-Geigy Corp. 
     VPRC is N-vinyl-2-pyrrolidone monomer, a reactive organic solvent for Black CN, from GAF Corp. 
     The photo-initiators Irgacure 907 and ITX act synergistically to activate the curing of the resin at UV wavelengths above 400 nm. Upon exposure to UV wavelengths below 40 nm the dye is labile. Therefore, Tinuvin 328 is added to absorb those UV wavelengths and protect the dye, and curing is carried out entirely at longer wavelengths. 
     A preferred embodiment of the resin system 14 is an outer resin layer 28 with the formulation of Example 1 and an inner resin layer 30 having the formulation of either Example 4 or 5. All the formulations described herein work equally well, but they differ as to cost and viscosity. The less viscous formulations can be applied more easily in production. 
     Bonding of the implosion panel 12 to the faceplate 10 with the double layer resin bonding system of this invention can be achieved in several ways. One method begins with the application of a liquid release layer to a piece of &#34;dummy&#34; glass (a glass panel that will not become part of the CRT 20). The release layer may consist of 5% DC193 by weight dissolved in isopropyl alcohol. 
     Next, the resin layer 28 is applied in liquid form over the release layer. The implosion panel 12 is then placed on top of the dummy glass in contact with the resin layer 28, with the release layer between the resin layer and the dummy glass. The resin layer 28 is then cured by exposure to ultra-violet light from both sides using a Fusion Systems AEL-1B unit with a V type bulb at an exposure distance of about 13 inches for about 45 seconds from the implosion panel side. After curing, the resin layer 28 adheres strongly to the inner surface of the implosion panel 12. 
     Next, the dummy glass is removed with the aid of the DC193 release layer. This can be done by inserting a wedge, such as a razor blade, around the edges and then pulling the dummy glass away. 
     Then, the second resin layer 30 in liquid form is spread over the faceplate 10. The implosion panel with the cured resin layer 28 thereon is placed over the faceplate with the cured resin layer 28 in contact with the liquid resin layer 30. The resin layer 30 is then cured using the Fusion Systems AEL-1B unit with a V-type bulb at an exposure distance of about thirteen inches for about 45 seconds from the implosion panel side. The resin layer 30 then adheres to the resin layer 28, and also adheres relatively weakly to the faceplate 10. The bond with the faceplate is sufficient to retain the implosion panel on the faceplate through normal use, packaging and handling of the CRT, but not sufficient to maintain adhesion to the faceplate if the latter is deflected inwardly due to an impact. 
     Any UV exposures which are made of or through a tinted resin layer (such as, a resin layer containing Orasol Black CN in the above examples) should be made with Fusion Systems V-type bulbs instead of the D-type bulb employed in the parent application cited above, since the Tinuvin T328 UV absorber used herein will absorb too much of the short UV wavelengths emitted by the latter bulb. The V-type bulb has a longer wavelength spectral characteristic, and thus is more efficient when used in connection with the present tinted resin system. 
     A significant advantage of the present invention is that the tinted pigmented layer 28 can be made absolutely flat. Because the faceplate of a flat tension mask tube does not have a convex dome configuration as does a conventional faceplate, it yields slightly to external air pressure, which can generate forces of the order of 2000 pounds over a normal size tube face of less than 140 sq. inches. This has the effect of deflecting the nominally flat faceplate slightly inwardly, so that it is actually somewhat concave. As a result, if the tinted layer 28 were deposited on the faceplate 10 it would &#34;pool&#34; in the concavity and be of non-uniform thickness, i.e., thicker in the central region, and that non-uniformity will result in a neutral density gradient across the picture tube; i.e. the center of the display will be visibly darker than the edges. The faceplate 10 can also have various non-uniform irregularities and press marks if it is not polished, and this can result in a mottled effect. Both effects are undesirable. But when the tinted layer 28 is deposited on the flat, polished surface of the window glass implosion panel 12, the tint is distributed uniformly and there is no darkness gradient or mottling to mar the picture displayed on the CRT. 
     The UV-curable resins used in this invention cure in a matter of seconds, instead of several minutes or hours as in the case of prior art implosion panel bonding resin materials, which are all cured by heat or chemical curing agents. In particular, UV-curable resins do not require the admixture of chemical curing agents, as epoxy resins do. In addition UV-curable resin trapped inside the dispensing equipment does not need to be flushed out after a shut-down. Also, it is stable for many months at room temperature, which simplifies the storage of raw materials for production. UV-curable resins are also available in a wider range of viscosities, which offers more flexibility in choosing resin formulations to match production requirements. These resins also have the advantage of closely matching the index of refraction of glass, so as to minimize reflections from the glass-resin interfaces and thus avoid image-degrading reflection of ambient light and image light. 
     It will now be appreciated that such a system utilizing a dye-impregnated resin system to bond an implosion panel to a CRT faceplate darkens the faceplate and thus enhances the contrast of the CRT image displayed thereon. While the invention is of particular importance in connection with modern flat tension mask tubes of the kind described, it will also function in a conventional convex faceplate environment and therefore is not limited to use with flat-faceplate cathode ray tubes. 
     Still other embodiments of the principles of this invention are contemplated, and the appended claims are intended to cover such other embodiments as are within the spirit and scope of this invention.