Glass for cathode ray tube faceplate.

0001This invention relates to a glass for a cathode ray tube faceplate.

0002The faceplate for a cathode ray tube of the type used in television picture tubes is prepared from a transparent glass demonstrating a high degree of X-radiation absorption and excellent resistance to discoloration caused by the bombardment of high velocity electrons thereupon and exposure to X-radiation. The glass must also exhibit a number of physical properties mandated by the process for manufacturing cathode ray tubes. For example: <ul id="ul0001" list-style="none"><li>The faceplate glass must display a coefficient of thermal expansion approximating that of glasses customarily utilized in cathode ray tube manufacture (~99 X 10 °/C over the temperature range of 25°-300°C), in order to assure good compatibility with such glasses and the metal components that are sealed therein.</li></ul>

0003To minimize distortion of the glass components during manufacture of the tube, the glass will most preferably manifest an annealing point not lower than about 500°C and a strain point not lower than about 455°C.

0004On the other hand, for ease in melting and forming, the glass will evidence a liquidus temperature lower than 900°C and, preferably, no higher than about 880°C, and will exhibit a viscosity at the liquidus of at least 100,000 poises.

0005Further, in order to insure proper operation of the tube, the glass must exhibit an electrical resistivity, expressed in terms of Logp, greater than 9 at 250°C and greater than 7 at 350°C.

0006Where electric melting of the glass is contemplated, easily reducible metal oxides, particularly PbO and As₂0₃, will preferably be essentially absent from the composition. It has been conjectured in the glass art that the inclusion of readily reducible metal oxides in a faceplate composition renders the. glass susceptible to discoloration resulting from electron browning. Also, volatilization of arsenic creates an air pollution problem.

0007MgO will desirably be essentially absent from the compositions. Although the mechanism therefor is not fully understood, the presence of MgO appears to render the glass more susceptible to devitrification.

0008Finally, whereas fluorine has frequently been employed as a flux to assist the melting process because its presence does not substantially affect the electrical resistivity of the glass, its volatilization from the glass batch during melting creates a severe air pollution problem. Furthermore, the inclusion of fluorine exacerbates corrosion of the molds used in forming the faceplates. Consequently, means have been sought to eliminate the fluorine from the batch while compensating for its beneficial fluxing and softening effects on the glass without sacrifice of electrical properties.

Objective of the Invention

0009Therefore, the principal objective of the instant invention is to provide glass compositions essentially free from As₂0₃, MgO, fluorine, and PbO, which exhibit coefficients of thermal expansion between 97-100 X 10^-7/°C (25°-300°C), annealing points not lower than about 500°C, strain points not lower than about 455°C, liquidus temperatures lower than 900°C, electrical resistivities (Logp) greater than 9 and greater than 7 at 250°C and 350°C, respectively, and liquidus viscosities of at least 100,000 poises, which are suitable for use as faceplates for cathode ray tubes, and which are designed for melting in electrically-powered melting units (but which can also be melted in conventional fossil fuel fired melting units).

Summary of the Invention

0010That objective can be achieved through glass compositions essentially free from As₂O₃, MgO, PbO, and F consisting essentially, expressed in terms of weight percent on the oxide basis, of <tables id="tabl0001" num="0001"><img file="EP0131399A2_D0001.tif" /></tables>

0011Colorants conventionally utilized in television picture tube faceplate glasses, e.g., Co₃O₄, Cr₂O₃, and NiO, may also be included, if desired, in customary amounts. Such levels normally range up to about 20 ppm (parts/million) Co₃0₄, up to about 20 ppm Cr₂0₃, and up to 250 ppm NiO.

0012Although relatively readily reducible, Sb₂0₃ may be tolerated in amounts up to 0.5% to perform as a fining agent. Hence, where a fining agent is deemed necessary, Sb₂0₃ may be employed instead of As₂O₃ since it is not as easily reduced as As₂0₃. Even where electric melting of the glass is not contemplated such that reduction thereof is not as serious a problem, its absence is preferred and no more than about 0.25% can be tolerated.

0013The above ranges of components must be rigorously observed to insure the production of glasses suitable for use as cathode ray tube faceplates and exhibiting the desired physical properties. Thus, Ti0₂ and Ce0₂ provide protection against discoloration resulting from exposure to X-radiation. BaO, SrO, and ZrO₂ act to absorb X-radiation. And the levels of those components and the ranges of the other glass constituents must be carefully balanced to yield glasses exhibiting the required viscosity characteristics for melting and forming, along with the desired thermal expansion and liquidus value. To illustrate, an increase in the Na₂0 or K20 content lowers the viscosity of the glass at its liquidus. Li₂0 has a powerful effect in softening the glass, while increasing the electrical resistivity of the glass, and permits the elimination of fluorine as a fluxing agent in the glass. Nevertheless, because of this strong fluxing action, the Li₂0 content must be carefully controlled. A1₂0₃ imparts good chemical durability thereto but has the unwanted side effect of ardening the glass. CaO tends to soften the glass and increase its electrical resistivity, but has the undesirable effect of increasing the liquidus temperature. Higher levels of SrO, BaO, and ZrO₂ reduce the liquidus viscosity of the glass.

0014In summary, then, the several components of the inventive glasses will be manipulated within the delineated straitly-defined regimes in order to insure the demanded viscosity characteristics, liquidus values, and physical properties.

0015U.S. Patents 3,464,932; 3,805,107; 3,925,089; 3,987,330; 4,015,966; 4,065,697; 4,089,693; 4,331,770; 4,337,410; and Japanese application No. 53-113813 (1978) are considered to be representative of the state of the art; none of these teach or suggest this invention.

Description of the Preferred Embodiments

0016Table I recites a number of glass compositions, expressed in terms of parts by weight on the oxide basis, illustrating the parameters of the invention. Inasmuch as the sum of the individual components closely approximates 100, for all practical purposes the tabulated values may be deemed to represent weight percent. The actual batch ingredients may comprise any materials, either the oxides or other compounds, which when melted together, will be converted into the desired oxides in the proper proportions. Because it is not known with which cation(s) the fluorine is combined, it is merely reported in terms of fluoride ion, in accordance with conventional glass analysis practice.

0017The batch constituents were compounded, tumble mixed together to aid in obtaining a homogeneous melt, and then charged into platinum crucibles. Lids were placed upon the crucibles and the crucibles were introduced into a furnace operating at 1550°C. After melting for six hours, the molten batches were poured into steel molds to form glass slabs having dimensions of 6" x 6" x 0.5", (15.24 x 15.24 x 1.27 cm), and those slabs transferred immediately to an annealer operating at 500°C. Samples were thereafter cut from the slabs to be used for the determination of various physical properties. <tables id="tabl0002" num="0002"><img file="EP0131399A2_D0002.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0131399A2_D0003.tif" /></tables>

0018Table II sets forth measurements of various physical properties conducted on samples of Examples 1-8. Thus, values of the softening point (Soft.), the annealing point (Ann.), and the strain point (Str.), all in °C, the coefficient of thermal expansion between 25°-300°C (Exp.) in terms of X 10^-7/°C, the internal liquidus temperature in °C (Liq.) determined employing a platinum boat in a temperature gradient furnace, the electrical resistivity, expressed in terms of Logp, measured at 250°C and 350°C and the viscosity of the glass (Vis.) at the liquidus temperature in terms of poises, as derived from a standard curve generated by the glass, are included. <tables id="tabl0004" num="0004"><img file="EP0131399A2_D0004.tif" /></tables>

0019As₂O₃-free Example 8 represents the most preferred embodiment of the invention. Examples 1-5 clearly illustrate that minor variations in composition outside of the specified ranges yield glasses exhibiting properties and behavior outside of those desired.