CA1120990A - Crt with peripheral plastic coating and method of making same - Google Patents

Abstract

ABSTRACT
A CRT (cathode-ray tube) comprises an envelope including a glass faceplate panel and an adjoining glass funnel sealed to the panel, and an elastomeric coating of polyurethane around and adhered to external surfaces of the envelope. The coating may be 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 as far back on the adjoining funnel as desired,and as far forward on the panel sidewalls and viewing window as desired. Also, the polyurethane band may be used in combination with a metal tension band.

Description

RCA 71~882 1 This invention relates to a cathode-ray tube having an improved implosion-protection system.
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.
Examples of one family of such systems are des-cribed in U.S. Patents Nos. 3,162,933 issued 29 December 1964 to Trax et al., 3,206,056 issued 14 September 1965 to Stel 15 and 3,220,593 issued 30 November 1965 to Powell et al.
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 impreg-nated coating may or may not be present. The plastic adhe-sive is usually a self-curing epoxy or polyester material.
Such plastic-impregnated fiber or fabric coatings are rela~
tively 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 provicle an implosion-protection system which is easier and cheaper to manufacture without sacrificing the degree of safety that is required for the viewer.

In accordance with the present invention, a cathode-ray tube comprises an envelope

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RCA 71,882 1 including a glass faceplate panel and an adjoining glass funnel sealed to the panel. An elastomeric film coating of polyurethane is disposed around and adhered to external surfaces of the envelope. The coating is preferably pro-duced by a novel method comprising coating the desiredtube surfaces with an aqueous emulsion of polyurethane, and then drying and curing this coating to coalesce the polyure-thane 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 to fabricate the tubes. The tubes provide adequate implosion protection, with lighter weight and at lower costs.

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.

The cathode-ray tube illustratecl in FIGURE 1 includes an evacuated envelope designated generally by the numeral 21. The envelope 21 includes a glass neck 23 inteyral with a glass funnel 25,and a glass faceplate panel comprising a viewing window 27 having a peripheral sidewall 29. The extended end of the sidewall 29 is sealed to the wide end of the funnel 25 by a seal 31, such as devitrified glass. A luminescent screen (not shown3 resides on the 3o inside surface of the viewing window 27. The luminescent ~ 3 --~z~

RCA 71,882 1 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.
S The neck 23 is closed a~d sealed by a stem 35 having stem leads 37 extending therethrough. A continuous peripheral film coating 39 of polyurethane about 0.125mm (5 mils~ thick is adhered to external surface portions of the sidewall 29 and the funnel 25 on each side of the seal 31. The Eilm coating 39 is about 12.5cm (5 inches) wide, extending about 5cm toward the window 27 and 7.5cm -toward the neck 23.
The interior of the envelope is evacuated to a high level of vacuum (low pressure) of the order of 10 5mm Hg. In this example, with a l9V 90~C rectangular color tele~ision picture tube, atmospheric pressure pressing against the e~ternal surface of the viewing window exerts forces totaling about 1800 kilograms (4000 lbs.). Circumferential tensile stresses as high as 70 kg/cm2 (1000 lbs./in.2) 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 and then bounce them outward.
An implosion-protection system does not prevent such implo-sion 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. In tubes according to the invention, should the window 27 fracture, the film coating 39 which is adherent to external envelope surfaces maintains the adjacent glass in RCA 71,~82 1 place while permitting gas to rush into the tube, reducing the pressure differential on opposi-te sides of the window 27, thereby reducing the forces which drive gl~ss fragments into flight. To determine the adequacy of implosion protec-tion o~ tubes described herein, implosion tests specifiedin publication UL 1418 by Underwr ters Laboratories, Inc., Chicago, Ill., U.S.A., were used.
The film coating 39 for the novel tube of FIGURE 1 is abricated on the tube after the envelope 21 is completely evacuated of gases and sealed, and the electrodes of the gun 33 have been electrically processed. In a preferred method of fabrication, 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 PPG Industries, Coatings and Resin Products Division, Spring-dale, PA., U.S.A.
The mixture is then brushed, flowed or sprayed on the desired areas using a stencil to mask off such areas.
When spraying on the emulsion, the preferred application method, it has been found to be aonvenient to monitor the emulsion-coating thickness by including a water-soluable dye, such as HIdrocol Alpha Blue, marketed by Hercules, Inc., Glen Falls, N.Y., U.S.A., iIl the emulsion. The emulsion is applied to a depth of color corresponding to the desired thickness. In a preferred procedure, the spectral reflect-ivity of the dyed coating is a function of coating thick-ness. Using fluorescent light, reflectance measurements are taken with a blue and with a red filte~. The thicker the coating, the higher the blue-to-red ratio of these re-flectances. After the emulsion has been applied, the emulsion coating is dried and RCA 71,882 1 the solids therein coalesced to a fil~ whereby the coating is cured. This may be done by placing the tube in an oven at about 20 to 12~C for about 30 to S minutes, preferably about 90C for about 10 minutes. Alternatively, or in addition, the tube may be preheated in an oven to about 20 to 90C, preferably about 50C, 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 i6 surprising that sufficient protection can be realiæed with such thin films and with the use of so little polymeric material.
The tubes of FIGURES 2,3 and 4 are identical in structure to that of FIGURE 1 e~cept for the extent of the fllm coating 39. Hence, similar reference numerals are used for similar structures. In FIGURE 2, a film coating 39a ex-tends 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 for~ard over the entire viewing window and backward so that it lies only on the panel sidewalls and not on the seal 31 or the funnel 25. For this purpose, it is preferred that the film coating over the window be colorless or gray-tinted, as thin as possible and of unifo~m 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 no-t extend over the seal 31 or the window 27. Even though the coating 39 is narrow, it nevertheless provides implosion 1~2~

RCA 71,882 1 protection that is adequate for many tubes, par-ticularly when used in combination with one or more tensioned steel bands.
The tubes of FIG~RES 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 (1000 to 1500 lb.) around the sidewalls 29 of the panel; ~lastic 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 3g. In FIGURE 6, a band 49 and a metal clip 51 are under the film coating 39.
Two tensioned bands, one on top of the other, may also be used over or under the film coating 39. These combinations of film coating and tension band are used on the larger size , above 19V, cathode-ray tubes. In one test on a 25~ 100 tube, two bands each tensioned to about 450 to 625 kilograms o~er a film coating about 0.10 mm (4 mil) thick, as shown in FIG~RE 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 discon-tinuous by leaving eight 50.8-mm (2-inch)-wide gaps around the periphery of the tube. As in other embodiments, the film coating and tensioned-band combination provided ade-quate implosion protection.
Tensile tests were conducted on polyurethane films that were made with aqueous emulsions, draw-down blade or spray applied to mold-released glass platesO After being subjected to an appropriate cure schedule and/or environmen-tal test cycle, 25.4 x 50.8 mm (1 x 2 inch~ sections of film coating were removed and pull tested. The applicable ASTM

RCA 71,882 l test was used to determine tensile strength at the breakpoint for ~5.4-mm ( l-inch)-wide specimens. 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 ~n (3 mils) thick.
During tensile tests, it was observed that the cured polyurethane films elongated about 400 to 500% in the direc-tion of pull.
Adhesive strengths o~ polyurethane films to glass were determined by draw-down blade or by spray application of emulsions to nonmold-released glass plates. After being outlined with a cutting tool, one end of a 50.8~mm (2-inch) strip was reinforced and attached to a spring scale and pulled off the plate at a 90 angle accordiny 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 (10 pounds) on funnel surfaces and about 6.4 kilograms (14 pounds) on sidewall surfaces. These results are much higner than the minimum of about 1.4 ~ilograms (3 pounds) considered necessary for adequate implosion protection.
The novel method here employs 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, prefer-ably 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 materi-als have been tried, but only polyurethane has been found to RCA 71,882 1 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 20C 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.
It is the practice to apply an electrically-insulating polymeric coating around the anode button of a CRT 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 the coati~s are over the polyurethane film coatings, the latter have been found to have a negligible effect on the perform-ance of the CRT.

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Claims (3)

1. A cathode-ray tube comprising an evacuated envelope including a glass faceplate panel, said panel comprising a viewing window and an integral sidewall around said window, an adjoining glass funnel sealed to said sidewall, and an elastomeric film coating consisting essentially of polyurethane, said coating being a continuous band at least about 0.075 millimeter thick around portions of said envelope including said sidewall and adhering to external surfaces thereof, the external surface of said window being free of said coating.

2. The cathode-ray tube of claim 1, wherein the thickness of said coating is between about 0.075 millimeter and 0.125 millimeter.

3. A method for imparting implosion resistance to a cathode-ray tube comprising an evacuated envelope including a glass faceplate panel comprising a viewing window and an integral sidewall around said window, and an adjoining glass funnel sealed to said sidewall, the method comprising preheating said tube to between about 20°C and 90°C, brushing, flowing or spraying onto an external glass surface of said envelope excluding said window a water-based emulsion coating of polyurethane, and then heating said tube to between about 20°C
and 120°C for about 30 to 5 minutes to cure said coating.