WO2025127035A1 - Alkali-free glass plate - Google Patents

Abstract

Provided is an alkali-free glass plate which has a high strain point, is excellent in terms of productivity, and also has fewer internal defects and has excellent optical and physical characteristics. The alkali-free glass plate is particularly suitable for use in a front panel of a display.　This alkali-free glass plate has a glass composition that contains, in mass%, 59-62% of SiO2, 18-21% of Al2O3, 3-6% of B2O3, 1-5% of MgO, 4-7% CaO, 0.1-6% of SrO, 0.01-7% of BaO, 0.005-0.1% of ZrO2, 0.15-0.35% of SnO2, 0.005-0.03% of TiO2, and 0.00-0.02% of Fe2O3.

Description

Alkaline-free glass plate

The present invention relates to an alkali-free glass plate, and more specifically, to a glass plate for displays used mainly with oxide thin film transistors (TFTs) such as IGZO, or displays equipped with TFTs having LTPS (low temperature polysilicon), which is particularly suitable for the opposing substrate (also called the front panel, front plane, CF (color filter) substrate, or cap glass, etc.) that does not have TFTs.

In flat panel displays, a glass plate is generally used as the supporting substrate. Electrical circuit patterns such as TFTs are formed on the surface of this glass plate. For this reason, non-alkali glass plates that contain essentially no alkali metal components are used for this type of glass plate, so as not to adversely affect the TFTs and other circuits.

Glass plates are exposed to high-temperature atmospheres during processes for forming electrical circuit patterns, such as the thin film formation process and the thin film patterning process. When a glass plate is exposed to a high-temperature atmosphere, structural relaxation of the glass progresses, causing the volume of the glass plate to shrink (hereinafter referred to as "thermal shrinkage"). If thermal shrinkage occurs in the glass plate during the process for forming the electrical circuit pattern, the shape and dimensions of the electrical circuit pattern formed on the glass plate will deviate from the design values, making it difficult to obtain a flat panel display with the desired electrical performance. For this reason, it is desirable for glass plates, such as glass plates for flat panel displays, on whose surfaces thin film patterns such as electrical circuit patterns are formed, to have a small thermal shrinkage rate.

In particular, glass plates for high-definition displays equipped with TFTs having oxide films such as IGZO are exposed to a relatively high temperature atmosphere of 400°C to 500°C when forming the oxide film, which makes them susceptible to thermal shrinkage. Furthermore, because the electrical circuit patterns are highly precise, even the slightest thermal shrinkage makes it difficult to obtain the desired electrical performance. For this reason, it is highly desirable for glass plates used in such applications to have an extremely small thermal shrinkage rate.

　By the way, known methods for forming glass sheets used in flat panel displays and the like include the float method and the downdraw method, typified by the overflow downdraw method.

The float process is a method of forming glass sheets by pouring molten glass onto a float bath filled with molten tin, stretching it horizontally to form a glass ribbon, and then annealing the glass ribbon in an annealing furnace installed downstream of the float bath. In the float process, the glass ribbon is transported horizontally, so it is easy to make the annealing furnace long. This makes it easy to sufficiently reduce the cooling rate of the glass ribbon in the annealing furnace. Therefore, the float process has the advantage of easily obtaining glass sheets with a small thermal shrinkage rate.

On the other hand, the downdraw method is a method in which molten glass is stretched downward to form a sheet. The overflow downdraw method, which is one type of downdraw method, is a method in which molten glass that has overflowed from both sides of a forming body with a roughly wedge-shaped cross section is stretched downward to form a glass ribbon. The molten glass that has overflowed from both sides of the forming body flows down along both sides of the forming body and joins below the forming body. Therefore, with the overflow downdraw method, the surface of the glass ribbon does not come into contact with anything other than the air and is formed by surface tension, so that a glass sheet with a flat surface without any foreign matter adhering to the surface can be obtained without polishing the surface after forming. Another advantage of the overflow downdraw method is that it is easy to form thin glass sheets.

On the other hand, in the down-draw method, because molten glass flows downward from the forming body, if a long annealing furnace is to be placed below the forming body, the forming body must be placed at a high position. However, in practice, there are restrictions on the height at which the forming body can be placed due to restrictions on the ceiling height of the factory, etc. In other words, in the down-draw method, there are restrictions on the length of the annealing furnace, and it may be difficult to place a sufficiently long annealing furnace. If the length of the annealing furnace is short, the cooling rate of the glass ribbon increases, making it difficult to form a glass sheet with a small thermal shrinkage rate.

Therefore, it has been proposed to increase the strain point of glass to reduce its thermal shrinkage. For example, Patent Document 1 discloses a low-alkali glass composition with a high strain point. The same document also states that the lower the β-OH value, which indicates the amount of water in the glass, the higher the strain point.

JP 2013-151407 A

In general, there is a correlation between strain point and thermal shrinkage, and the higher the strain point, the smaller the thermal shrinkage. However, glass designed with a high strain point has the problem of poor productivity due to the large load it places on melting and molding during the manufacturing process.

The glass plates used in flat panel displays such as LCDs are broadly divided into rear panels (backplanes) on which TFTs are formed, and front panels (frontplanes) on which TFTs are not formed. Because front panels do not form TFTs, they do not require the same high heat resistance and low thermal shrinkage as rear panels. On the other hand, they do need to meet the optical properties, physical properties, dimensional accuracy, surface accuracy, etc. required of a display substrate.

Furthermore, because this glass plate for the front panel is placed on the front side (viewer side) of the display, there are very strict requirements for internal defects in the glass. Also, because the rear panel and front panel are placed very close to each other and are bonded with ultraviolet curing resin etc., compatibility between the glass composition and glass properties of both panels is required.

The present invention was made in consideration of these circumstances, and its purpose is to provide an alkali-free glass sheet that has a high strain point, excellent productivity, few internal defects, and excellent optical and physical properties, and is particularly suitable for use in the front panels of displays.

As a result of extensive research, the inventors have discovered that the above problems can be solved by using an alkali-free glass plate having a specific glass composition.

(1) That is, the alkali-free glass plate of the present invention is characterized by having a glass composition containing, by mass%, 59-62% SiO ₂ , 18-21% Al ₂ O ₃ , 3-6% B ₂ O ₃ , 1-5% MgO, 4-7% CaO, 0.1-6% SrO, 0.01-7% BaO, 0.005-0.1% ZrO ₂ , 0.15-0.35% SnO ₂ , 0.005-0.03% TiO ₂ , and 0.005-0.02% Fe ₂ O ₃ . Here, "alkali-free glass" refers to glass to which alkali metal oxide components are not added at levels equal to or greater than the levels of impurities mixed in from raw materials or the manufacturing process, and specifically refers to glass in which the content of alkali metal oxides ( _Li2O , _Na2O , and _K2O ) in the glass composition is 1000 ppm or less by mass. In particular, the _Na2O content in the glass composition is preferably 500 ppm or less, more preferably 300 ppm or less.

(2) The alkali-free glass plate of the present invention in the above (1) preferably contains, as a glass composition, by mass, 59-62% SiO ₂ , 18-21% Al ₂ O ₃ , 3-6% B ₂ O ₃ , 1-4% MgO, 4-7% CaO, 0.1-5% SrO, 0.01-7% BaO, 0.005-0.1% ZrO ₂ , 0.15-0.35% SnO ₂ , 0.005-0.03% TiO ₂ , and 0.005-0.02% Fe ₂ O ₃ .

(3) In the alkali-free glass plate of the present invention described above in (1) or (2), the glass composition preferably contains, by mass%, 4.5 to 6% CaO.

(4) In the alkali-free glass plate of the present invention, in any one of the above (1) to (3), the mass ratio of SnO ₂ /ZrO ₂ is preferably 5 to 30. In this specification, "x/y" means the value obtained by dividing the content of the x component by the content of the y component.

(5) In any one of the above (1) to (4), the alkali-free glass plate of the present invention preferably has a thermal expansion coefficient within the range of 37±2×10 ⁻⁷ /° C. This makes it easier to achieve affinity with the rear panel of a display, making it suitable for use as a front panel.

(6) In any of the above (1) to (5), the alkali-free glass plate of the present invention preferably has a thermal shrinkage of 13 to 40 ppm when heated from 25°C to 500°C at a rate of 5°C/min, held at 500°C for 1 hour, and then cooled to 25°C at a rate of 5°C/min. In this way, even if exposed to a high-temperature atmosphere in an electric circuit pattern forming process such as a thin film formation process or a thin film patterning process, the shape and dimensions of the electric circuit pattern are unlikely to deviate from the design values, making it easier to obtain a flat panel display with the desired electrical performance.

(7) In any of (1) to (6) above, the alkali-free glass plate of the present invention preferably has a transmittance of 89% or more at a wavelength of 360 nm and a transmittance of 90% or more at a wavelength of 400 nm when the plate is 0.5 mm thick. In a display, light emitted obliquely to the front panel passes through the front panel over a relatively long optical path. In this case, if the transmittance of the glass plate, particularly the transmittance of light at short wavelengths around 360 to 400 nm, is low, the color reproducibility of the image is likely to deteriorate. Specifically, there is a concern that even white light may appear blue-green, green, or brownish. Therefore, by restricting the transmittance at wavelengths of 360 nm and 400 nm as described above, it is possible to improve the color reproducibility of the display.

(8) In any of the above (1) to (7), the alkali-free glass plate of the present invention preferably has a Young's modulus of 75 to 83 GPa. This makes it possible to suppress deformation of the glass plate, such as warping, in electronic displays such as high-definition OLED displays, and to suppress deterioration of display performance, such as light leakage.

(9) In any of the above (1) to (8), the alkali-free glass sheet of the present invention preferably has a strain point of 680 to 720°C. In this way, the thermal shrinkage rate can be reduced to a required level.

(10) In any of the above (1) to (9), the alkali-free glass sheet of the present invention preferably has a β-OH value of less than 0.45/mm. This makes it possible to increase the strain point.

(11) In any one of the above (1) to (10), the alkali-free glass plate of the present invention preferably has a density of 2.46 to 2.6 g/cm ^3. This allows the display to be made lighter.

(12) In any one of the above (1) to (11), the alkali-free glass plate of the present invention preferably has a plate thickness of 0.51 mm or less and a substrate area of 4 ^m2 or more.

(13) In any of the above (1) to (12), the alkali-free glass plate of the present invention is preferably used for the front panel of a display.

(14) In any of the above (1) to (13), the alkali-free glass plate of the present invention is preferably for use in a high-definition liquid crystal display or an OLED display.

(15) In any of the above (1) to (12), the alkali-free glass plate of the present invention is preferably a dummy glass plate used in the manufacturing process of a liquid crystal display or an OLED display.

In the manufacturing process of liquid crystal displays or OLED displays, a wide variety of equipment is used, including various types of film formation equipment, equipment for exposure and etching in the photolithography process, and equipment for ion doping and heat treatment. In these manufacturing processes, at the start of daily production or when changing the type of product being manufactured, alkali-free glass plates of the same size as the products, known as dummy glass plates, are used to determine and adjust conditions. The alkali-free glass plate of the present invention is also suitable as such a dummy glass plate.

The present invention makes it possible to provide an alkali-free glass sheet that has a high strain point, excellent productivity, few internal defects, and excellent optical and physical properties, and is particularly suitable for use in the front panels of displays.

FIG. 2 is a plan view for explaining a procedure for measuring the thermal shrinkage rate of a glass plate.

The alkali-free glass plate of the present invention is characterized by containing, in mass %, SiO ₂ 59-62%, Al ₂ O ₃ 18-21%, B ₂ O ₃ 3-6%, MgO 1-5%, CaO 4-7%, SrO 0.1-6%, BaO 0.01-7%, ZrO ₂ 0.005-0.1%, SnO _{2 0.15-0.35%, TiO 2 0.005-0.03} %, and Fe ₂ O ₃ 0.005-0.02% as a glass composition. The reason for limiting the glass composition in this way will be explained below. In the following description of the content of each glass component, "%" means "mass %" unless otherwise specified. In addition, the alkali-free glass plate may be simply called "glass plate". In this specification, a numerical range indicated using "to" means a range that includes the numerical values before and after "to" as the minimum and maximum values, respectively.

SiO ₂ is a component that forms the skeleton of glass. The lower limit of the content of _{SiO 2} is preferably 59% or more, particularly 59.5% or more. On the other hand, the upper limit of the content of SiO ₂ is preferably 62% or less, particularly 61.5% or less. If the content of SiO ₂ is too small, the density of the glass becomes too high and the acid resistance is easily reduced. On the other hand, if the content of SiO ₂ is too large, the high-temperature viscosity becomes high and the melting property is easily reduced. In addition, devitrified crystals such as cristobalite are easily precipitated, and the liquidus temperature is easily increased.

Al ₂ O ₃ is a component that forms the skeleton of glass, increases the strain point and Young's modulus, and also has the effect of suppressing phase separation. The lower limit of the content of _{Al 2} O ₃ is preferably 18% or more, 19% or more, particularly 19.5% or more. On the other hand, the upper limit of the content of Al ₂ O ₃ is preferably 21% or less, particularly 20.5% or less, particularly 20% or less. If the content of Al ₂ O ₃ is too low, the strain point and Young's modulus are likely to decrease. On the other hand, if the content of Al ₂ O ₃ is too high, devitrified crystals such as mullite and anorthite are likely to precipitate, and the liquidus temperature is likely to increase.

B ₂ O ₃ is a component that enhances melting property and devitrification resistance. The lower limit of the content of B ₂ O ₃ is preferably 3%, particularly 3.5% or more. On the other hand, the upper limit of the content of B ₂ O ₃ is preferably 6% or less, 5.5% or less, particularly 5% or less. If the content of _{B 2} O ₃ is too small, melting property and devitrification resistance are likely to decrease. In addition, chemical durability is likely to decrease, and in particular durability against hydrofluoric acid-based chemicals such as buffered hydrofluoric acid is likely to decrease. On the other hand, if the content of B ₂ O ₃ is too large, the strain point and Young's modulus are likely to decrease. In addition, when the content of B ₂ O ₃ is relatively low as in the present invention, the viscosity is generally high and it is difficult to achieve high bubble quality. Therefore, if the β-OH value is kept low and SnO ₂ , which has a clarification effect at relatively high temperatures, is contained as an essential component, it becomes easier to achieve high bubble quality.

The above-mentioned three components, SiO ₂ , Al ₂ O ₃ and B ₂ O ₃ , are components that constitute the skeleton of the glass of the present invention and are very important in maintaining the physical properties, such as heat resistance, chemical durability, thermal expansion coefficient, density, and mechanical properties, of the glass. Therefore, the total amount of these three components is preferably in the range of 80% to 90%, 82 to 86%, and particularly preferably 83% to 85%.

MgO is a component that reduces high-temperature viscosity and increases melting properties, and among alkaline earth metal oxides, it is a component that significantly increases Young's modulus. The lower limit of the MgO content is preferably 1% or more, 2% or more, and especially 2.5% or more. On the other hand, the upper limit of the MgO content is preferably 5% or less, 4% or less, and especially 3.5% or less. If the MgO content is too low, Young's modulus is likely to decrease. On the other hand, if the MgO content is too high, devitrification resistance is likely to decrease and the strain point tends to decrease. In addition, the thermal expansion coefficient increases, which may reduce affinity with the rear glass when used as a front glass.

CaO is a component that reduces high-temperature viscosity and significantly improves meltability without lowering the strain point. In addition, among alkaline earth metal oxides, CaO is a component that reduces raw material costs because the raw materials used are relatively inexpensive. The lower limit of the CaO content is preferably 4% or more, 4.5% or more, 5% or more, and especially 5.1% or more. On the other hand, the upper limit of the CaO content is preferably 7% or less, 6.5% or less, and especially 6% or less. If the CaO content is too low, meltability is likely to decrease. On the other hand, if the CaO content is too high, the glass is likely to devitrify and the thermal expansion coefficient is large, which may reduce affinity with the rear glass when used as a front glass.

SrO is a component that suppresses phase separation and enhances devitrification resistance. Furthermore, it is a component that reduces high-temperature viscosity and enhances melting property without lowering the strain point. It is also a component that suppresses the rise in liquidus temperature. The lower limit of the SrO content is preferably 0.1% or more, 0.5% or more, 1% or more, 1.5% or more, and particularly 2% or more. On the other hand, the upper limit of the SrO content is preferably 6% or less, 5% or less, 4.5% or less, 4% or less, 3.5% or less, 3% or less, and particularly 2.5% or less. If the SrO content is too low, it becomes difficult to enjoy the above effects. On the other hand, if the SrO content is too high, reaction products are generated during HF etching, and the smoothness of the glass surface is easily lost.

BaO is a component that significantly improves devitrification resistance. The lower limit of the BaO content is preferably 0.01% or more, 0.1% or more, 1% or more, 2% or more, and particularly preferably 3% or more. On the other hand, the upper limit of the BaO content is preferably 7% or less, 6% or less, and particularly preferably 5.5% or less. If the BaO content is too high, the density becomes too high and the melting property is easily reduced.

The lower limit of the SrO+BaO content is preferably 0.1% or more, 1% or more, 3% or more, 5% or more, and particularly preferably 6% or more. On the other hand, the upper limit of the SrO+BaO content is preferably 9% or less, 8% or less, and particularly preferably 7% or less. If the SrO+BaO content is too low, the devitrification resistance of the glass becomes insufficient, and if it is too high, the density and thermal expansion coefficient of the glass tend to be high. In this specification, "x+y" means the combined amount of the x component and the y component.

To obtain the above-mentioned effects of SrO and BaO, one of the following combinations is preferable. That is, it is preferable to have 2-5% SrO and 0.01-2% BaO, or 0.1-3% SrO and 3-6% BaO.

The combined amount of the above-mentioned MgO, CaO, SrO, and BaO affects the physical properties, mechanical properties, liquidus temperature, bubble quality, etc., of the glass of the present invention. Therefore, it is preferable that this combined amount is in the range of 14 to 17%, and particularly 14.5 to 16%.

_ZrO2 is a component that improves chemical durability such as acid resistance. The lower limit of the content of _ZrO2 is preferably 0.005% or more, particularly 0.01% or more. On the other hand, the upper limit of the content of _ZrO2 is preferably 0.1% or less, 0.07% or less, particularly 0.05% or less. If the content of _ZrO2 is too small, it is difficult to enjoy the above effects. On the other hand, if the content of _ZrO2 is too large, the glass becomes easily inhomogeneous and devitrification particles are easily generated.

_SnO2 is a component that has a good clarification effect in the high temperature range. The lower limit of the content of _SnO2 is preferably 0.15% or more, particularly 0.17% or more. On the other hand, the upper limit of the content of _SnO2 is preferably 0.35% or less, 0.3% or less, particularly 0.28% or less. If the content of _SnO2 is too small, it becomes difficult to enjoy the above effects. On the other hand, if the content of _SnO2 is too large, devitrification crystals of _SnO2 tend to precipitate, and the precipitation of devitrification crystals of _ZrO2 tends to be promoted. In addition, the transmittance of the glass in the short wavelength range tends to decrease. _In addition, _SnO2 is a component that greatly affects the oxidation-reduction of glass, and as a result, it affects the coloring by transition metal oxides such as _TiO2 and _Fe2O3 . From such a viewpoint, it is preferable to regulate the content of _SnO2 within the above range.

In addition, the coexistence of _ZrO2 and _SnO2 in an appropriate ratio can suppress the precipitation of devitrification crystals. Therefore, it is preferable that the mass ratio of _SnO2 / _ZrO2 is 5 to 30, particularly 8 to 25.

_TiO2 is a component that absorbs near-ultraviolet rays, and when the glass plate of the present invention is used as, for example, the front panel of a display, it can suppress the deterioration of light-emitting materials such as OLEDs that constitute the display, liquid crystal materials, resin adhesives, and peripheral members due to ultraviolet rays. It also cuts ultraviolet rays emitted from the display, which has a favorable effect on users. The lower limit of the content of _TiO2 used is preferably 0.005% or more. On the other hand, the upper limit of the content of _TiO2 is preferably 0.03% or less, 0.025% or less, 0.02% or less, and particularly 0.018% or less. If the content of _TiO2 is too low, it becomes difficult to enjoy the above effects. On the other hand, if the content of _TiO2 is too high, the transmittance in the short wavelength range decreases, and the glass may be colored.

_Fe2O3 is also _a component that absorbs near ultraviolet rays like _TiO2 , and when used as a front panel of a display, for example, it can suppress deterioration of resin adhesives and peripheral components due to ultraviolet rays. The lower limit of _{the Fe2O3} content is preferably 0.005% or more. On the other hand, the upper limit of _{the Fe2O3} content is preferably 0.02% or less, 0.018% or less, and particularly preferably 0.015% or less.

In addition, _TiO2 and _Fe2O3 affect _each other and cause coloring. Therefore, it is preferable to regulate the total amount of both components. Specifically, the upper limit of the content of _TiO2 + _Fe2O3 is preferably 0.05% or less, 0.04% or _less , 0.035% or less, and particularly preferably 0.03% or less. In order to enjoy the above effects, the lower limit of the content _{of TiO2} + _Fe2O3 is preferably 0.01% or more, and particularly preferably 0.015% or more.

The alkali-free glass sheet of the present invention may contain the following components in addition to the components listed above.

Even a small amount _{of Cr2O3} and _CeO2 can cause coloring, so it is preferable to keep _the content as low as possible. Specifically, the _Cr2O3 content is preferably 5 ppm or less, particularly 3 ppm or less, and the _CeO2 content is preferably 10 ppm or less, 5 ppm or less, particularly 3 ppm or less.

As ₂ O ₃ and Sb ₂ O ₃ are environmentally undesirable components. In addition, when the alkali-free glass plate of the present invention is used for display applications, there is a risk of eroding the electrodes formed on the glass plate surface. Furthermore, if these components are contained in the raw materials, they erode the molybdenum electrodes during melting, making it difficult to perform stable electric melting over a long period of time. Therefore, it is preferable that As ₂ O ₃ and Sb ₂ O ₃ are not substantially contained. Here, "substantially not contained" means that glass raw materials or glass cullets containing these components are not intentionally added to the glass batch in amounts exceeding the level at which they are mixed in as impurities. More specifically, it means that the obtained glass contains 50 ppm or less of arsenic as As ₂ O ₃ and 50 ppm or less of antimony as Sb ₂ O ₃ .

It is also possible to incorporate Cl or F into the glass as a fining agent or the like. However, similarly to As ₂ O ₃ and Sb ₂ O ₃ , from the viewpoints of the environment and electrode corrosion, it is preferable that the content be less than 0.1%, particularly less than 0.05%.

Next, we will explain the characteristics of the alkali-free glass sheet of the present invention.

The strain point of the glass plate of the present invention is preferably 680°C or higher, particularly 690°C or higher. If the strain point is too low, the thermal shrinkage rate tends to be large. On the other hand, the upper limit of the strain point is preferably 720°C or lower, 715°C or lower, 710°C or lower, particularly 705°C or lower. If the strain point is too high, the temperatures during molding and melting become too high, which can easily lead to a rise in the manufacturing costs of the glass plate.

For example, in electronic displays such as high-definition OLED displays, deformation of the glass plate, such as warping, can cause light leakage and other degradation of display performance. For this reason, the rigidity of the glass plate is important. To ensure rigidity, it is desirable for the glass plate of the present invention to have a high Young's modulus. Specifically, it is preferable for the Young's modulus to be 75 to 83 GPa, and particularly 77 to 83 GPa.

The thermal shrinkage of the glass plate of the present invention is preferably 13 to 40 ppm, 15 to 35 ppm, and particularly preferably 17 to 30 ppm. Here, the "thermal shrinkage" refers to the value measured after heat treatment under conditions in which the glass is heated from room temperature to 500°C at a rate of 5°C/min, held at 500°C for 1 hour, and then cooled to room temperature at a rate of 5°C/min. If the thermal shrinkage is too large, when exposed to a high-temperature atmosphere in the electric circuit pattern forming process such as the thin film forming process or the thin film patterning process, the shape and dimensions of the electric circuit pattern are likely to deviate from the design values, making it difficult to obtain a flat panel display having the desired electrical performance. On the other hand, if the thermal shrinkage is too small, the manufacturing efficiency of the glass tends to decrease and the manufacturing cost tends to increase. When the glass plate of the present invention is used as the front panel of a display, it can be used even if the thermal shrinkage is relatively large, so the thermal shrinkage may be, for example, 20 ppm or more. If the thermal shrinkage is set in this manner, the manufacturing efficiency of the glass is high and the glass plate can be obtained at low cost.

The glass plate of the present invention is preferably colorless and highly transparent, particularly when used as the front panel of a display. In particular, when used in an OLED display, low transmittance of light emitted at an angle, particularly light with a short wavelength of around 400 nm, affects the color reproducibility of the display. Specifically, there is concern that even white light may appear blue-green, green, or brownish. Recently, new liquid crystal displays (LCDs), such as QD-LCDs, have been developed and put into practical use, using backlights that employ quantum dots. These quantum dots are also used as color filters for OLED displays. In such displays, light enters from various angles to increase the color conversion efficiency of the quantum dot layer, and light exits after color conversion.

Therefore, it is preferable that the glass plate of the present invention has a high light transmittance for short wavelengths around 360 to 400 nm. Specifically, it is preferable that the transmittance for a wavelength of 360 nm is 89% or more and the transmittance for a wavelength of 400 nm is 90% or more at a plate thickness of 0.5 mm.

When the glass plate of the present invention is used as a front panel, it is precisely laminated with a TFT substrate glass to form a display. At this time, if there is a large difference in thermal expansion coefficient between the glass plate and the TFT substrate, distortion occurs during lamination, resulting in problems such as pattern misalignment. Therefore, when the glass plate of the present invention is used as a front panel, it is preferable to strictly control the thermal expansion coefficient. Specifically, it is preferable that the thermal expansion coefficient measured in a temperature range of 30 to 380° C. is 37±2×10 ⁻⁷ /° C.

In order to reduce the weight of a display, the glass plate of the present invention is preferably lightweight. Specifically, the density thereof is preferably in the range of 2.46 to 2.6 g/cm ³ , and more preferably in the range of 2.48 to 2.56 g/cm ³ .

The glass plate of the present invention preferably has a β-OH value of less than 0.45/mm, 0.4/mm or less, and particularly preferably 0.3/mm or less. If the β-OH value is too large, the bubble quality of the glass may be reduced. The lower limit of the β-OH value is preferably 0.05/mm or more, 0.1/mm or more, 0.15/mm or more, and particularly preferably 0.2/mm or more. If the β-OH value is too small, the initial melting temperature of the glass base may increase, causing greater erosion of the refractory in contact with the molten glass and increasing the amount of foreign matter originating from the refractory in the glass. There may also be a risk of shortening the life of the melting equipment.

The alkali-free glass plate of the present invention preferably has a temperature at 10 ^2.5 dPa·s of 1610°C or less, 1600°C or less, 1590°C or less, 1580°C or less, 1570°C or less, particularly 1560°C or less. If the temperature at 10 ^2.5 dPa·s is too high, the glass becomes difficult to melt, the manufacturing cost of the glass plate rises, and defects such as bubbles are easily generated. On the other hand, if the temperature at 10 ^2.5 dPa·s is too low, it is difficult to design the viscosity at the liquidus temperature to be high. Therefore, the temperature corresponding to 10 ^2.5 dPa·s is preferably 1500°C or more, 1510°C or more, particularly 1520°C or more. The "temperature corresponding to 10 ^2.5 dPa·s" is a value measured by a platinum ball pulling method.

The alkali-free glass plate of the present invention preferably has a liquidus temperature of 1200°C or less, 1190°C or less, 1180°C or less, 1170°C or less, and particularly 1160°C or less. In this way, devitrification crystals are less likely to occur during glass production, and productivity can be improved. In addition, since it is easier to form the glass plate by the overflow downdraw method, it is easier to improve the surface quality of the glass plate and reduce the manufacturing cost of the glass plate. In addition, from the viewpoint of the recent increase in size of glass plates and the high resolution of displays, it is very important to improve devitrification resistance in order to minimize devitrification products that can become surface defects. The lower limit of the liquidus temperature is not particularly limited, but in reality it is 1050°C or more, or even 1100°C or more. The liquidus temperature is an index of devitrification resistance, and the lower the liquidus temperature, the better the devitrification resistance. "Liquid phase temperature" refers to the temperature at which devitrification (crystal inclusions) is observed in the glass after the glass powder that passes through a standard sieve of 30 mesh (500 μm) and remains on the 50 mesh (300 μm) is placed in a platinum boat and held in a temperature gradient furnace set at 1100-1350°C for 24 hours, and then the platinum boat is removed.

The alkali-free glass plate of the present invention preferably has a viscosity at the liquidus temperature of 10 ^5.0 dPa·s or more, 10 ^5.2 dPa·s or more, particularly 10 ^5.4 dPa·s or more. In this way, devitrification is unlikely to occur during forming, making it easier to form the glass plate by the overflow downdraw method, and as a result, it is possible to improve the surface quality of the glass plate. In addition, the manufacturing cost of the glass plate can be reduced. The viscosity at the liquidus temperature is an index of formability, and the higher the viscosity at the liquidus temperature, the better the formability. The viscosity at the liquidus temperature can be measured, for example, by the platinum ball pulling method.

The alkali-free glass plate of the present invention preferably has a substrate area of 4 ^m2 or more, particularly 5 ^m2 or more. If the substrate area is too small, it becomes difficult to efficiently manufacture a large LCD or OLED display driven by an oxide TFT such as IGZO. From the viewpoint of weight reduction, the thickness of the alkali-free glass plate of the present invention is preferably 0.51 mm or less, and desirably 0.5 mm or less.

In addition, when the glass plate of the present invention is used in mobile applications such as laptop computers, it is thinned by an etching process called slimming, which uses a hydrofluoric acid-based chemical. Therefore, it is important that the glass plate of the present invention has chemical durability that allows the slimming process to be carried out without problems.

The method for producing the alkali-free glass sheet of the present invention is described below.

The alkali-free glass plate of the present invention includes, for example, a batch preparation process for preparing a raw material batch, a melting process for melting the raw material batch, a fining process for fining the molten glass, and a forming process for forming the fined glass. Each process will be described in detail below.

(1) Batch Preparation Step First, glass raw materials are prepared so as to have the above-mentioned glass composition. The main components of the glass raw materials are as follows:

Silica sand, stone powder (SiO ₂ ), etc. can be used as the silicon source.

As the aluminum source, alumina (Al ₂ O ₃ ), aluminum hydroxide (Al(OH) ₃ ), etc. can be used.

As the boron source, orthoboric acid (H ₃ BO ₃ ) or boric anhydride (B ₂ O ₃ ) can be used. Since orthoboric acid contains water of crystallization, when the proportion of orthoboric acid used is large, the water content in the glass can be adjusted to be relatively high. For this reason, it is preferable to use both orthoboric acid and boric anhydride, and adjust their proportions according to the desired β-OH value.

Examples of the alkaline earth metal source that can be used include calcium carbonate ( _CaCO3 ), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) ₂ ), barium carbonate ( _BaCO3 ), barium nitrate (Ba( _NO3 ) ₂ ), strontium carbonate ( _SrCO3 ), and strontium nitrate (Sr( _NO3 ) ₂ ).

As the zirconia source, an oxide raw material such as _ZrO2 is used. In addition, when a Zr-containing refractory such as a zirconia electrocast refractory or dense zircon is used as a refractory material constituting a melting furnace, zirconia may be dissolved from the refractory as a zirconia component and contained in the glass.

Tin oxide (SnO ₂ ) and the like can be used as the tin source. When using tin oxide, it is preferable to use tin oxide having an average particle size D ₅₀ in the range of 0.3 to 50 μm, 2 to 50 μm, and particularly 5 to 50 μm. If the average particle size D ₅₀ of the tin oxide powder is too small, aggregation between particles occurs, which makes clogging in the blending plant more likely to occur, and there is a risk that undissolved particles of SnO ₂ crystals will remain in the glass plate. On the other hand, if the average particle size D ₅₀ of the tin oxide powder is too large, the dissolution reaction of the tin oxide powder into the molten glass is delayed, making it difficult to clarify the molten glass. As a result, oxygen gas cannot be sufficiently released at the appropriate time of glass melting, bubbles are more likely to remain in the glass product, and it is difficult to obtain a product with excellent bubble quality. There is also a risk that undissolved particles of SnO ₂ crystals will remain in the glass plate.

The weight loss batch may contain a nitrate raw material. The nitrate raw material can make the fining agent _SnO2 function efficiently. As the nitrate raw material, for example, barium nitrate (Ba( _NO3 ) ₂ ), strontium nitrate (Sr( _NO3 ) ₂ ), etc. can be used.

In addition to the above glass raw materials, glass cullet may be used. Here, "glass cullet" means defective glass generated during glass manufacturing, recycled glass collected from the market, etc. When glass cullet is used, the proportion of glass cullet used relative to the total amount of the raw material batch is preferably 1 mass% or more, 5 mass% or more, and particularly 10 mass% or more. There is no upper limit on the proportion of glass cullet used, but in order to accurately obtain the desired glass composition, it is preferable to make it 50 mass% or less, 40 mass% or less, and particularly 30 mass% or less. It is also desirable that at least a part of the glass cullet used is low-moisture glass cullet made of glass with a β-OH value of 0.45/mm or less, 0.4/mm or less, 0.3/mm or less, and particularly 0.25/mm or less. By doing so, it becomes easier to obtain a glass plate with a small β-OH value. The lower limit of the β-OH value of low-moisture glass cullet is not particularly limited, but in reality it is 0.05/mm or more.

(2) Melting Step Next, the prepared raw material batch is melted.

To melt the raw material batch, a melting furnace capable of heating with radiant heat from burner combustion or Joule heat generated by passing electricity between electrodes is used. It is particularly preferable to use a melting furnace capable of electric melting. By adopting this configuration, it is possible to suppress an increase in moisture in the atmosphere. As a result, it is possible to suppress the supply of moisture from the atmosphere to the glass, making it easier to manufacture glass with a high strain point. In addition, since the heat generated by the glass itself (Joule heat) is used to heat the molten glass, the glass can be heated efficiently. This makes it possible to melt the raw material batch at a relatively low temperature.

　A melting furnace capable of electric melting has multiple electrodes made of molybdenum, platinum, tin, etc., and by applying electricity between these electrodes, electricity flows through the molten glass, and the resulting Joule heat melts the glass continuously. Although radiant heating using a heater or burner may be used as an auxiliary, it is preferable to use electric melting as the main method from the viewpoint of lowering the β-OH value of the glass. When heating with a burner, moisture produced by combustion is absorbed into the glass, increasing the moisture content of the glass, so it is preferable to adjust the amount of combustion, temperature, number of burners used in the melting equipment, and even the raw materials to appropriately adjust the moisture content in the glass.

As the electrodes, it is preferable to use molybdenum electrodes. Molybdenum electrodes have a high degree of freedom in terms of placement location and electrode shape, making it easy to apply electrical heating to non-alkali glass, which is difficult to conduct electricity through. The electrode shape is preferably rod-shaped. If they are rod-shaped, it is possible to place the desired number of electrodes at any position on the side wall or bottom wall of the melting furnace, while maintaining the desired distance between the electrodes. It is preferable to arrange the electrodes in multiple pairs on the walls (side wall, bottom wall, etc.) of the melting furnace, especially on the bottom wall, with a short distance between the electrodes.

(3) Fining Step Next, the molten glass is heated and refined. The fining step may be carried out in an independent fining tank or in a downstream portion of a melting furnace.

　Polyvalent oxides such as tin compounds contained in the raw material batch dissolve in the molten glass and act as fining agents. For example, tin components release oxygen bubbles during the temperature rise process. The released oxygen bubbles expand the bubbles contained in the molten glass, causing them to rise to the surface and be removed from the glass. Furthermore, tin components absorb oxygen bubbles during the temperature drop process, eliminating any bubbles remaining in the glass. In this case, the greater the difference between the temperature during melting and the temperature during fining, the greater the fining effect. For this reason, it is desirable to keep the temperature during melting as low as possible.

(4) Forming step Next, the refined glass is supplied to a forming device and formed into a sheet. Note that a stirring tank, condition adjustment tank, etc. may be disposed between the refining tank and the forming device, and the glass may be passed through these before being supplied to the forming device. In addition, it is preferable that at least the surface of the communication flow path connecting the melting furnace, the refining tank, and the forming device (or each tank provided between them) that comes into contact with the glass is made of platinum or a platinum alloy to prevent contamination of the glass.

The forming method is not particularly limited, but if a downdraw method is adopted, which is limited by the length of the annealing furnace and makes it difficult to reduce the thermal shrinkage rate, the effects of the present invention can be easily obtained. As the downdraw method, it is preferable to adopt the overflow downdraw method. The overflow downdraw method is a method in which molten glass is made to overflow from both sides of a trough-shaped refractory with a wedge-shaped cross section, and the overflowing molten glass is stretched downward while joining at the bottom end of the trough-shaped refractory to form the glass into a plate. In the overflow downdraw method, the surface that will become the surface of the glass plate does not come into contact with the trough-shaped refractory, and is formed in a free surface state. For this reason, unpolished glass plates with good surface quality can be produced inexpensively, and it is also easy to make the glass larger and thinner.

The structure and material of the trough-shaped refractory material used in the overflow downdraw method are not particularly limited, so long as they can achieve the desired dimensions and surface precision. In addition, the method of applying force when performing downward stretching is also not particularly limited, and for example, a method of stretching the glass by contacting multiple pairs of heat-resistant rolls only near the edge of the glass can be used. Note that in addition to the overflow downdraw method, for example, the slot down method can also be used.

The glass formed into a sheet in this way is cut to a specified size and, if necessary, undergoes various chemical or mechanical processes to become a glass sheet.

The alkali-free glass plate of the present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples in any way.

Tables 1 to 3 show examples of the present invention (No. 1 to 15).

First, silica sand, aluminum oxide, orthoboric acid, boric anhydride, calcium carbonate, strontium carbonate, strontium nitrate, barium carbonate, stannic oxide, etc. were mixed and compounded to produce a raw material batch with the composition shown in Tables 1 to 3. Glass cullet (β-OH value = 0.4/mm) with the same composition as the target composition was used at 35 mass% of the total amount of the raw material batch.

The raw material batch was then fed into a melting furnace that combined burner combustion and electric melting to melt it, and the molten glass was then clarified and homogenized in a fining tank and an adjustment tank, while the viscosity was adjusted to a level suitable for molding. The maximum temperature in the fining tank was set to 1500-1610°C. The maximum temperature in the fining tank was confirmed by monitoring the temperature of the platinum or platinum alloy lined on the inner wall of the fining tank.

The molten glass was then fed into an overflow downdraw forming device, formed into a plate, and cut to obtain a 0.5 mm thick glass sample.

The obtained glass samples were evaluated for β-OH value, thermal expansion coefficient α, strain point Ps, annealing point Ta, softening point Ts, viscosity characteristics, liquidus temperature TL, density ρ, Young's modulus E, specific Young's modulus E/ρ, transmittance, bubble quality, and thermal shrinkage. The results are shown in Tables 1 to 3.

As is clear from Tables 1 to 3, the glasses of Examples 1 to 15 had low β-OH values of 0.43/mm or less, high strain points of 700°C or more, thermal shrinkage rates of 15 to 25 ppm, high T360 of 89% or more, high T400 of 90% or more, and excellent bubble quality with bubble counts per unit weight of 0.008 bubbles/kg or less.

The characteristics were measured as follows:

The β-OH value was calculated by measuring the transmittance of the glass using FT-IR (Fourier transform infrared spectroscopy) and using the following formula.

β-OH value = (1/X) log ₁₀ (T ₁ /T ₂ )
X: Glass thickness (mm)
T ₁ : Transmittance (%) at a reference wavelength of 3846 cm ⁻¹
T ₂ : Minimum transmittance (%) at a hydroxyl group absorption wavelength of about 3600 cm ⁻¹

The thermal expansion coefficient was measured using a dilatometer in the temperature range of 30 to 380°C.

The strain point Ps, annealing point Ta, and softening point Ts were measured based on ASTM C336 and C338.

The viscosity characteristics are values measured by a platinum ball pull-up method at temperatures where the high-temperature viscosity is 10 ⁴ dPa·s, 10 ³ dPa·s, and 10 ^2.5 dPa·s.

The liquidus temperature TL is the temperature at which crystals precipitate after the glass powder that passes through a standard sieve of 30 mesh (500 μm) and remains on the 50 mesh (300 μm) is placed in a platinum boat and held in a temperature gradient furnace for 24 hours.

The density ρ is a value measured using the well-known Archimedes method.

Young's modulus E refers to the value measured using the well-known resonance method.

The specific Young's modulus E/ρ is the Young's modulus divided by the density.

Transmittance was measured on glass samples that were mirror-polished on both sides using a Hitachi High-Tech UH-4150. The table shows the transmittance T360 at a wavelength of 360 nm and the transmittance T400 at a wavelength of 400 nm.

Bubble quality was evaluated by determining the number of bubbles with a diameter of 100 μm or more contained in a unit mass of glass sample.

The thermal shrinkage was measured by the following method. First, as shown in FIG. 1(a), a rectangular sample G of 160 mm×30 mm was prepared as a glass plate sample. Markings M were formed on both ends of the long side of the rectangular sample G at a position 20 to 40 mm away from the edge using #1000 waterproof abrasive paper. Then, as shown in FIG. 1(b), the rectangular sample G on which the markings M were formed was folded in two along the direction perpendicular to the markings M to prepare sample pieces Ga and Gb. Then, only one of the sample pieces Ga was subjected to a heat treatment in which the temperature was raised from room temperature (25° C.) to 500° C. at a rate of 5° C./min, and the sample was held at 500° C. for 1 hour, and then cooled to room temperature at a rate of 5° C./min. After the above heat treatment, as shown in Fig. 1(c), the sample piece Gb that was not subjected to heat treatment and the sample piece Ga that was subjected to heat treatment were arranged in parallel, and the positional deviations ( _ΔL1 and _ΔL2 ) of the markings M of the two sample pieces Ga and Gb were read by a laser microscope, and the thermal shrinkage rate was calculated by the following formula, where _l0 is the initial distance between the markings M.

Heat shrinkage rate (ppm) = [{ΔL ₁ (μm) + ΔL ₂ (μm)}×10 ³ ]/l ₀ (mm)

The alkali-free glass plate of the present invention is used in OLED displays, high-definition LCDs, etc., and is particularly suitable as a glass plate for a front panel that faces a rear panel on which oxide TFTs such as IGZO are formed. The alkali-free glass plate of the present invention is also suitable as a dummy glass plate used in the manufacturing process of liquid crystal displays or OLED displays.

Claims (15)

The alkali-free glass plate has a glass composition, in mass %, of 59-62% SiO ₂ , 18-21% Al ₂ O ₃ , 3-6% B ₂ O ₃ , 1-5% MgO, 4-7% CaO, 0.1-6% SrO, 0.01-7% BaO, 0.005-0.1% ZrO ₂ , 0.15-0.35% SnO ₂ , 0.005-0.03% TiO ₂ , and 0.005-0.02% Fe ₂ O ₃ . The alkali-free glass plate according to claim 1, comprising, as a glass composition, in mass %, 59-62% SiO _{2 ,} 18-21% Al ₂ O ₃ , 3-6% B ₂ O 3 , 1-4% MgO, 4-7% CaO, 0.1-5% SrO, 0.01-7% _BaO , 0.005-0.1% ZrO ₂ , 0.15-0.35% SnO ₂ , 0.005-0.03% TiO 2 , and 0.005-0.02% Fe ₂ O ₃ . The alkali-free glass sheet according to claim 1, which contains, by mass %, 4.5 to 6% CaO as a glass composition. The alkali-free glass plate according to claim 1, wherein the mass ratio of SnO ₂ /ZrO ₂ is 5 to 30. The alkali-free glass sheet according to claim 1, having a thermal expansion coefficient within the range of 37±2×10 ⁻⁷ /° C. An alkali-free glass sheet according to claim 1, which has a thermal shrinkage of 13 to 40 ppm when heated from 25°C to 500°C at a rate of 5°C/min, held at 500°C for 1 hour, and then cooled to 25°C at a rate of 5°C/min. The alkali-free glass sheet according to claim 1, which has a transmittance of 89% or more at a wavelength of 360 nm and a transmittance of 90% or more at a wavelength of 400 nm when the sheet has a thickness of 0.5 mm. The alkali-free glass sheet according to claim 1, having a Young's modulus of 75 to 83 GPa. The alkali-free glass sheet according to claim 1, having a strain point of 680 to 720°C. The alkali-free glass sheet according to claim 1, having a β-OH value of less than 0.45/mm. The alkali-free glass plate according to claim 1, having a density of 2.46 to 2.6 g/ ^cm3 . The alkali-free glass sheet according to claim 1, having a sheet thickness of 0.51 mm or less and a substrate area of 4 ^m2 or more. An alkali-free glass sheet according to any one of claims 1 to 12, which is used for a front panel of a display. The alkali-free glass sheet according to claim 13, which is for a high-definition liquid crystal display or an OLED display. The alkali-free glass plate according to any one of claims 1 to 12, which is a dummy glass plate used in the manufacturing process of a liquid crystal display or an OLED display.

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