Cathode ray tube with implosion protection and method for manufacturing it

Abstract

 A cathode-ray tube comprising an envelope including a glass faceplate panel (27) and an adjoining glass funnel (25) sealed to the panel, is provided with protection against the effects of implosion by means of an elastomeric coating (39) of polyurethane disposed around and adhered to external surfaces of the envelope. This mode of implosion protection is easier and cheaper to manufacture, especially in mass production, than is the prior art provision of an encircling rigid coating of plastic-impregnated fiber or fabric. The coating (39) is preferably produced by applying, drying and curing a coating of an aqueous emulsion of polyurethane on the desired external surfaces of the tube. The coating may extend from the panel (27) as far back on the adjoining funnel (25) as desired, and as far forward on the panel sidewall (29) and over the viewing window as desired. The polyurethane coating (39) may be used in combination with a metal tension band.

Description

[0001] This invention relates to providing cathode-ray tubes with implosion protection.

[0002] Cathode-ray tubes comprising evacuated glass bulbs are mass-produced articles of commerce. They usually include a glass faceplate panel hermetically sealed to the wide end of a glass funnel. A luminescent screen is carried on the inner surface of the panel, and one or more electron guns are housed in a neck attached to the narrow end of the funnel. Some adverse effects of implosion of the bulb can be reduced or eliminated by providing an implosion-protection system around the panel.

[0003] Examples of one family of such systems are described in U.S. patents Nos. 3,162,933, 3,206,056 and 3,220,593. In these systems, a rigid coating of a plastic-impregnated fiber or fabric encircles and adheres to the panel. An encircling steel band on or adjacent to the plastic impregnated coating may or may not also be provided. The plastic adhesive is usually a self-curing epoxy or polyester material. Such plastic-impregnated fiber or fabric coatings are relatively difficult and expensive to construct and are not well adapted to mass production. While these prior systems may provide the required degree of safety to the viewer of the tube, it is desirable to provide an implosion-protection system which is easier and cheaper to manufacture without sacrificing the degree of safety that is required for the viewer.

[0004] In accordance with the present inventive concept, a cathode-ray tube comprising an envelope including a glass faceplate panel and an adjoining glass funnel sealed to the panel, is characterized by the provision, for implosion protection, of an elastomeric film coating of polyurethane disposed around and adhered to external surfaces of the envelope. Also according to the inventive concept, the coating is preferably produced by coating the desired surfaces with an aqueous emulsion of polyurethane, and then drying and curing this coating to coalesce the polyurethane particles into a thin film coating that is well adhered to the adjacent glass surfaces. By employing this method, cheaper materials and simpler processes which are better adapted to mass production may be used in fabricating the tubes. Perhaps with the addition of tensioned metal bands, especially for larger tube sizes, adequate implosion protection can thereby be provided with lighter weight and at lower costs.

[0005] In the drawings:

FIGURES 1 to 6 are elevational views of six different embodiments of the invention.

FIGURE 7 is a graph showing the results of a series of tests for determining tensile strength of polyurethane coatings versus thickness of the coating.

[0006] The cathode-ray tube illustrated in FIGURE 1 includes an evacuated envelope designated generally by the numeral 21. The envelope 21 includes a glass neck 23 integral with a glass funnel 25, and a glass faceplate panel comprising a viewing window 27 having a peripheral sidewall 29. The rim of the sidewall 29 is sealed to the wide end of the funnel 25 by a seal 31, such as devitrified glass. The neck 23 is closed and sealed by a stem 35 having stem leads 37 extending therethrough. An anode button (terminal) 43 is sealed through the funnel wall. A luminescent screen (not shown) resides on the inside surface of the viewing window 27. The luminescent screen, when suitably scanned by an electron beam from a gun 33 housed in the neck 23, is capable of producing a luminescent image which may be viewed through the viewing window 27.

[0007] The interior of the envelope is evacuated to a high level of vacuum (low pressure) of the order of 10^-5 mm Hg. Considering a 19V 90 rectangular color television picture tube by way of example, atmospheric pressure pressing against the external surface of the viewing window exerts forces totaling about 1800 kilograms. Circumferential tensile stresses as high as ₇₀ kg/cm² are present in the sidewall 29 and the adjacent portions of the funnel. Should the viewing window fracture, atmospheric pressure would ordinarily drive window fragments inward against the funnel portion 25, from which they would then bounce outward. An implosion-protection system does not prevent such implosion but, instead, reduces the chance of injury to viewers near the tube face. Particularly, an implosion-protection system reduces the amount of glass fragments thrown and reduces the distances that they are thrown.

[0008] In accordance with one embodiment of the invention, a continuous peripheral film coating 39 of polyurethane about 0.125 mm (5 mils) thick is adhered to external surface portions of the sidewall 29 and the funnel 25 on each side of the seal 31. For the example considered, the film coating 39 is about 12.5 cm wide, extending from the seal about 5 cm toward the window 27 and 7.5 cm toward the neck 23. Should the window 27 fracture, the film coating 39 adherent to external envelope surfaces maintains the adjacent glass in place while permitting gas to rush into the tube, reducing the pressure differential on opposite sides of the window 27, thereby reducing the forces which drive glass fragments into flight. To determine the adequacy of implosion protection of tubes described herein, implosion tests specified in publication UL 1418 by Underwriters Laboratories, Inc., Chicago, Ill., U.S.A., were used.

[0009] The film coating 39 in the embodiment of FIGURE 1 is fabricated on the tube after the envelope 21 has been completely evacuated of gases and sealed, and the electrodes of the gun 33 have been electrically processed. In a preferred method of fabrication of the film, a quantity of an emulsion of polyurethane in a water base is diluted with water to the desired viscosity. One suitable polyurethane emulsion is RS 5302 marketed by PP_G Industries, Coatings and Resin Products Division, Springdale, Pa., U.S.A. The mixture is then brushed, flowed or preferably sprayed on the desired areas using a stencil to define such areas. When spraying on the emulsion, it has been found to be convenient to monitor the emulsion-coating thickness by including a water-soluble dye, such as HIdrocal Alpha Blue, marketed by Hercules, Inc., Glen Falls, N.Y., U.S.A., in the emulsion. The emulsion is applied to a depth of color corresponding to the desired thickness. In a preferred procedure, based on the spectral reflectivity of the dyed coating being a function of coating thickness, fluorescent light is used and reflectance measurements are taken with a blue and with a red filter. The thicker the coating, the higher the blue-to-red ratio of these reflectances. After the emulsion has been applied, the emulsion coating is dried and the solids therein coalesced to a substantially uniform film. This 'curing' of the film may be done by heating the tube in an oven in an ambient of air at about 20 to 120°C for about 30 to 5 minutes, preferably about 90⁰C for about 10 minutes, and then cooling the tube. Alternatively, or in addition, the tube may be preheated in an oven to about 20⁰ to 90°C, preferably about 5₀°C, prior to applying the emulsion coating. After the coating has been cured, the film is at least 0.075 mm (3 mils) thick and preferably about 0.125 mm (5 mils) thick. Greater thicknesses are not detrimental to implosion protection, although too thick a film results in excessive material costs. It is surprising that, without reinforcement by fabric or other fibrous material in the films, sufficient protection can be realized with such thin films and with the use of so little polymeric material.

[0010] The tubes of FIGURES 2, 3 and 4 are identical in structure to that of FIGURE 1 except for the extent of the film coating 39. Hence, similar reference numerals are used for similar structures. In FIGURE 2, a film coating 39a extends back on the funnel 25 almost to the neck 23. An open area 41 is left around the anode button 43 to permit the connection of a high-voltage lead thereto. In FIGURE 3, a film coating 39b extends forward over the entire viewing window and backward so that it lies only on the panel sidewall and not on the seal 31 or the funnel 25: In this case, it is preferred that the film coating over the window be colorless or gray-tinted, as thin as possible and of uniform thickness, so that there is a minimum degradation in the viewed images. In FIGURE 4, a film coating 39c is shortened so that it lies only on the panel sidewall and does not extend over the seal 31 or the window 27. Even though the coating 39c is narrow, it nevertheless provides implosion protection that is adequate for many tubes, particularly when used in combination with one or more tensioned steel bands.

[0011] The tubes of FIGURES 5 and 6 are identical in structure to that of FIGURE 1 except that one or more continuous steel bands are tensioned to about 450 to 675 kilograms around the sidewalls 29 of the panel; plastic coated bands are preferred. Hence, similar reference numerals are used for similar structures. In FIGURE 5, a band 45 and a metal clip 47 are on top of a film coating 39d. In FIGURE 6, a band 49 and a metal clip 51 are under the film coating 39c. Two tensioned bands, one on top of the other, may also be used over or under the film coating. These combinations of film coating and tension band are used on larger (above 19V) cathode-ray tubes. In one test on a 25V 100 tube, two bands each tensioned to about 450 to 625 kilograms over a film coating about 0.10 mm (4 mil) thick, as shown in FIGURE 5, provided adequate implosion protection, where one or the other alone was not adequate. In a further variation, the film coating 39d of FIGURE 5 was made discontinuous by leaving eight gaps of about 50mm round the periphery of the tube: here also, the film coating and tensioned-band combination provided adequate implosion protection.

[0012] Tensile tests were conducted on polyurethane films that were made with aqueous emulsions applied by draw-dawn blade or spray to mold-released glass plates. After being subjected to an appropriate cure schedule, and/or environmental test cycle, 1 x 2 inch (about 25 x 50 mm) sections of film coating were removed and pull tested. The applicable ASTM test was used to determine tensile strength at the breakpoint for specimens of 1 inch (about 25 mm) width. Results are plotted in the graph shown in FIGURE 7. It is concluded from this data, and confirmed by implosion experience with tubes, that the film coating should be at least 0.075 mm (3 mils) thick. During tensile tests, it was observed that the cured polyurethane films elongated about 400 to 500% in the direction of pull.

[0013] Adhesive strengths of polyurethane films to glass were determined by applying emulsions by drawdown blade or by spray to nonmold-released glass plates. After being outlined with a cutting tool, one end of a 2 inch (about 50 mm) strip was reinforced and attached to a spring scale and pulled off the plate at a 90° angle according to the ASTM method. This pull test was repeated on external funnel and sidewall surfaces of cathode-ray tubes. Pull test results averaged about 4.5 kilograms on funnel surfaces and about 6.4 kilograms on sidewall surfaces. These results are much higher than the minimum of about 1.4 kilograms considered necessary for adequate implosion protection.

[0014] For forming the protective films it is preferred to employ polyurethane latexes, that is, aqueous emulsions or sols in which each colloidal particle contains a number of macromolecules of polyurethane. The colloidal particles are about 0.05 to 1.0 micron, preferably less than 0.3 micron, in average size. The latexes are ones from which the water base can be removed and the macromolecules coalesced into an adherent film coating on a glass surface. Other aqueous emulsions of polymeric materials have been tried, but only polyurethane has been found to develop sufficient tensile strength and adherence in coalesced film coatings. The colloidal particles of the latexes should have a relatively low minimum film-forming temperature, or MFT preferably more than 209C below the temperatures at which curing is carried out. The latexes may include other constituents, such as a coloring dye, a defoaming agent and/or a stabilizing agent.

[0015] It is the practice to apply an electrically- insulating polymeric coating around the anode button of a cathode-ray tube and also an electrically-conducting coating, usually of graphite and a binder, on the outer surface of the funnel of the tube. From several tests, it was found that these coatings can be under, but preferably should be over, the polyurethane film coatings disclosed herein. When these other coatings are over the polyurethane film coatings, the latter have been found to have a negligible effect on the performance of the cathode-ray tube.

[0016] Reference has been made in the foregoing to 19V and 25V tubes. It is to be understood that these are tubes having viewing areas with nominal diagonal dimensions of 19 inches (about 48 cms) and 25 inches (about 63.5 cms) respectively.

Claims

1. A cathode-ray tube comprising an envelope (21) including a glass faceplate panel (27,29) and an adjoining glass funnel (25) sealed to said panel, characterized by an elastomeric film coating (39) consisting substantially of polyurethane disposed around said envelope (21) and adhered to external surfaces thereof.

2. A cathode-ray tube as claimed in Claim 1, characterized in that said coating is deposited in situ from an aqueous emulsion of polyurethane material.

3. A cathode-ray tube as claimed in Claim 1, characterized in that said coating is of a substantially uniform composition free from fibrous material.

4. A cathode-ray tube as claimed in any preceding Claim wherein said glass faceplate panel includes a viewing window (27) and an integral peripheral sidewall (29) to which said glass funnel is sealed, characterized in that said elastomeric coating (39) is a continuous band at least about 0.075 millimeter thick encircling said sidewall (29).

5. A cathode-ray tube as claimed in Claim 4, characterized in that said coating (39) extends over at least portions of said sidewall (29), over portions of said funnel (25) and over the seal (31) therebetween.

6. A cathode-ray tube as claimed in Claim 4, characterized in that said coating (39b) extends over all of said sidewall (29) and all of said viewing window (27).

7. A method for imparting implosion resistance to a cathode-ray tube (21), characterized by applying a coating (39) of a water-based emulsion of polyurethane in a band around and on the external surfaces of said tube, and then drying said coating until said water base has evaporated and said polyurethane has coalesced into a film adhering to said external surfaces.

8. A method as claimed in Claim 7, characterized in that said drying step includes heating said tube in an ambient of air and then cooling said tube.

9. A method as claimed in Claim 7 or 8, characterized by preheating said tube prior to applying the coating.

Drawing