Non-blooming tempered float glass sheet

Abstract

The present invention relates to a tempered float glass sheet having a glass composition comprising antimony expressed in total Sb2O3, at a level of 20 ppm to 500 ppm, based on the total weight of the glass composition. The tempered float glass sheet has a haze value equal to or less than 3.0%.

Description

Non-blooming tempered float glass sheetTechnical field

[0001] The present invention relates to float glass sheets that after thermal tempering, do not demonstrate bloom.Background art

[0002] In typical float glass production, the bottom surface of the glass ribbon remains several minutes in direct contact with molten tin in the tin batch. It results in tin ions migrating into the glass contact bottom surface and an enriched tin surface layer is formed - hereinafter referred to as ‘tin face’ versus the opposite surface being referred to as ‘air face’.

[0003] In the float glass process, tin species from the tin bath will diffuse into the produced glass sheet under the form of Sn2+and Sn4+. About 60% Sn2+is in the depth of 3pm and 80% Sn4+is in the depth greater than 3.5pm. Bloom is an undesired haze phenomenon appearing on the tin face after thermal tempering of a produced glass sheet. Indeed, during the step of tempering and the corresponding temperature increase, Sn2+form present at the surface of the glass is oxidized into the Sn4+form. Hence, a high concentration of Sn4+is formed at the surface of the glass with (almost) no Sn2+, inducing chemical gradient and diffusion of Sn2+ions towards the glass surface where the ions will be oxidized to Sn4+. Due to this oxidation and migration system, the thermally tempered glass exhibit Sn4+enriched extreme surface layer, then depletion of Sn species in the surface sublayer. This extreme layer (Sn4+rich) and the sublayer (lower Sn) does not exhibit same mechanical properties during the cooling down phase. It will result in a wrinkling of the surface on the final tempered glass product that will diffuse light and induce a haze veil well known from the state-of-the-art and called bloom. The bloom phenomenon limits the later application of float glass that requires the heat treatment such as the thermal tempering.

[0004] To avoid such bloom effect, the person skilled in the art focuses on reducing the tin migration from the tin bath to the glass sheet as much as possible. This route is complicated since the first levers to control this diffusion phenomenon are the temperature and residence time and the tin oxidation stage. Both are limited by the process parameters and the thicknessof the glass sheet. Another lever is to increase the hydrogen in the tin bath to capture as much as possible the oxygen present in the bath. Indeed, if less O2is present, then it decreases oxidation of tin and therefore prevents its diffusion. This lever is extremely costly but as well quite limited. Indeed, the H2level is limited in order to prevent other quality impact (for instance the bubbles created in the tin bath) or to prevent reaching the explosivity limit. Also H2increase in tin bath is limited in efficiency since the glass composition itself provides already a substantial amount of oxygen that will anyway induce a significant part of the oxidation and therefore the tin diffusion.

[0005] A technical solution known in the art is to control the iron content of the composition of the tin bath. Indeed, iron has a tendency to oxidize faster than tin and therefore to migrate more quickly to the tin surface. This reduces the surface concentration of tin at the surface of the glass sheet and therefore reduces the propensity to form bloom. Furthermore, iron is a polyvalent element which will be reduced by tin in glass composition. Hence, tin will be oxidized in the Sn4+form and iron will be reduced in the Fe2+form. Since Sn4+cannot be further oxidized, it will not diffuse during thermal tempering through the concentration gradient. Hence, no difference in composition and properties is created at the surface of the glass when tempered, and therefore, drape or wrinkling and bloom effect are prevented.

[0006] However, it is highly difficult to control the iron content inside the tin bath because iron diffuses both from the tin to the glass and from the glass to the tin bath. Therefore, the chemical composition and properties of the tin bath are not stable over time and depends greatly on the iron content in the glass produced. Also, the opening of the tin bath to eventually add iron introduces O2and causes tin bath atmosphere oxidation, leading to quality issues and further tin diffusion inside tin face, which is counter-productive.

[0007] Another technical solution known in the art is to control the iron content inside the glass composition itself. The mechanism is similar: iron is a polyvalent element which will be reduced by tin in glass composition, meaning that tin during its diffusion will be oxidized in the Sn4+form and iron will be reduced in the Fe2+form. Since tin is already under Sn4+oxidation state, it will not diffuse during thermal tempering through the concentration gradient. Hence, no difference in composition and properties is created at the surface of the glass when tempered, and therefore, drape or wrinkling and bloom effect are prevented. This solution could also be achieved by the addition of other polyvalent ions, such as chromium,... However, iron and other mentioned elements cannot be freely used because they negativelyaffect the color of the bulk of the glass sheet.

[0008] Among other elements, antimony, specifically in the form of antimony oxide (Sb2O3), is used in the glass manufacturing process. Antimony oxide serves as a fining agent or clarifying agent. Antimony oxide, when used in appropriate amounts, helps to remove bubbles from the glass melt, improving the clarity and quality of the final product. However, if used in excessive amounts, it could potentially lead to issues such as discoloration or other defects in the glass.

[0009] WO2013004470 discloses float glass compositions comprising by total weight of glass, of 0.02 to 0.07% in antimony (expressed as Sb2O3) for use in the field of solar energy. WO201 3 / 004470 selects the particular range of 0.02 to 0.07%wt of antimony associated with the other composition criteria, to allow a gain in energy transmission because, in this range, antimony causes an increase in said transmission due to its oxidizing power, which is greater than the transmission loss due to the phenomenon of coloring on the surface of float glass.

[0010] Antimony (Sb) and tin (Sn) can interact in the float glass process, particularly in the context of the molten tin bath where the glass ribbon is formed. Here are some potential interactions and considerations:• At the high temperatures of the float glass process, some antimony can volatilize from the glass melt. Volatile antimony compounds could potentially interact with the tin bath atmosphere. If these compounds recondense on cooler parts of the equipment or the glass ribbon, they might affect the local chemistry and the quality of the glass.• The interaction of antimony with tin can contribute to the formation of dross, which is a mixture of oxides and other impurities that can accumulate on the surface of the molten tin bath. Dross needs to be carefully managed and removed to prevent defects in the glass ribbon.

[0011] If antimony levels are not properly controlled, it could lead to defects in the glass. For example, if antimony oxide is reduced to metallic antimony, it could form particles or streaks in the glass, affecting its optical properties such as light transmission loss, color shift and haze.

[0012] It is important to note that the interactions, and therefore the coloration, between antimony and tin in the float glass process are complex and can be influenced by many factors, including the concentrations of these elements, the temperature and atmosphere of the tin bath, andthe composition of the glass melt. Given this, float glass manufacturers are considering Sb2O3addition inside the floated glass as incompatible with their process. Antimony is within the list of forbidden elements.

[0013] Therefore, there is still a need in that art, to find an easy and cost effective solution to avoid the appearance of bloom on the tin face of float glass sheet after thermal tempering while avoiding the coloring of the float glass sheet. The present invention teaches to incorporate antimony as an alternative oxidizing element to oxidize tin as the Sn4+specie at the tin face of the float glass sheet while avoiding the above described drawbacks.Figures

[0014] Figures 1 to 4 are schematic representations of the diffusion of the tin species at the tin face of the glass sheet during float process and tempering process.

[0015] Figure 5 is a photographic representation of the surface of a tempered float glass sheet that has bloom.

[0016] Figure 6 shows the bloom range.Summary of invention

[0017] The present invention relates to a tempered float glass sheet having a glass composition comprising antimony expressed in total Sb2O3, at a level of 20 ppm to 500 ppm, based on the total weight of the glass composition.

[0018] The tempered float glass sheet has a haze value equal to or less than 3.0%; preferably, having a haze value equal to or lesser than 2.5%, equal to or lower than 2.0%, equal to or lower than 1.5%; equal to or lower than 1.0%, more preferably equal to or lower than 0.5%. The present invention relates further to such tempered float glass sheet that preferably have a bloom value equal to or lower than 2.5, preferably equal to or lower than 2.0, preferably equal to or lower than 1 .5, more preferably equal to or lower than 1 .0.

[0019] In a preferred embodiment, the level of antimony is comprised at a level equal to orgreater than 25ppm, 30ppm, 35ppm, 40ppm, 45ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm, 125ppm and even, 150ppm by weight of total glass composition and / or, is comprised at a level equal to or lower than 450ppm, 400ppm, 350ppm, 300ppm, 250ppm, and even, 200ppm by weight of total glass composition. More preferably, the antimony comprised in the glass composition comes from glass cullet, even more preferably comes from glass cullet coming from recycling of photovoltaic panels.

[0020] In a preferred embodiment, the antimony present at the tin face at a depth of 0pm to 10pm is dissolved in the glass composition and / or in the form of Sb-containing particles having a diameter less than 5nm, preferably equal to or lower than 4nm, preferably equal to lower than 3nm, preferably equal to lower than 2nm, more preferably equal to lower than 1 nm and even better is in the dissolved form.

[0021] Typically, the glass composition is a soda-lime-silicate composition that comprises the following glass components at those levels in respect to the total weight of glass:

[0022] In a preferred embodiment, the glass composition used forthe tempered float glass sheet, comprises iron expressed as total Fe2O3, at a level of 20-2000ppm based on the total weight of the glass composition.

[0023] For clear glasses, the content of total iron expressed in total Fe2O3, is preferably comprised between 300ppm and OOppm (300ppm < Fe2O3< OOppm), preferably, it is comprised at a level equal to or greater than 400ppm, 450ppm, 500ppm, 550ppm, 600ppm and even, 650ppm by weight of total glass composition and / or preferably, total iron expressed in total Fe2O3, is comprised at a level equal to or lower than 900ppm, 850ppm, 800ppm, andeven, 750ppm by weight of total glass composition.

[0024] For extra clear glasses, the content of total iron expressed in total Fe2O3, is preferably comprised between 20ppm and less than 300ppm (20ppm < Fe2O3< 300ppm), preferably, it is comprised at a level equal to or greater than 20ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm and even, 100ppm by weight of total glass composition and / or preferably, total iron expressed in total Fe2O3, is comprised at a level equal to or lower than 250ppm, 200ppm, 175ppm, and even, 150ppm by weight of total glass composition.

[0025] It is preferred that the iron redox expressed in FeO / Fe2O3, is equal to or lower than 30% (FeO / Fe2O3< 30%), preferably equal to or lower than 28% (FeO / Fe2O3< 28%), preferably equal to or lower than 25% (FeO / Fe2O3< 25%), equal to or lower than 23% (FeO / Fe2O3< 23%), more preferably equal to or lower than 20% (FeO / Fe2O3< 20%).

[0026] In one embodiment, the tempered float glass sheet has visible light transmission LTD4 is equal to or greater than 85% (LTD4 > 85%), preferably equal to or greater than 88% (LTD4 > 88%), preferably equal to or greater than 89% (LTD4 > 89%), equal to or greater than 90% (LTD4 > 90%), equal to or greater than 91 % (LTD4 > 91%).

[0027] Typically, the tempered float glass sheet has a thickness comprised between 0.5mm and 25mm. Preferably, the tempered float glass sheet has a sheet thickness of 2.0 mm or more, preferably of 3.0 mm or more, preferably of 3.5 mm or more, preferably of 4.5 mm or more; preferably of 5.5 mm or more, preferably of 7.5 mm or more, preferably of 9.5 mm or more and more preferably of 11.5 mm or more; and / or has a sheet thickness of 20 mm or less, preferably 15 mm or less.Detailed description of the invention

[0028] The tempered float glass sheet of the present invention is a float glass sheet produced by a float glass process, the float glass sheet is then further subjected to a thermal tempering process. For a glass sheet obtained by a float method using molten tin, the concentration of tin in the surface layer of the bottom surface in contact with the molten metal of the glass sheet (herein after referred to as tin face) is relatively large. On the other hand, in the surface layer of top surface that was not in contact with the molten metal of the glass sheet (hereinafter referred to air face), the concentration of tin is relatively small.

[0029] It has been surprisingly found that adding antimony into the glass compositions at a concentration of 20ppm to 500ppm (expressed in total Sb2O3), does oxidize tin diffused on the tin face to its Sn4+form and therefore prevent the appearance of bloom after thermal tempering by maintaining an acceptable haze while avoiding coloration of the float glass sheet, both in the bulk and on the surfaces of the float glass sheet.

[0030] The present invention is based on the surprising finding that a delicate balance of the use of antimony in glass composition must be found: (1) enough antimony to oxidize the tin as Sn4+specie at the tin face of the float glass sheet (up to 10pm) to reduce and even avoid bloom for the tempered float glass sheet. (2) Limit the level of antimony to keep acceptable light transmission, acceptable haze and avoid coloration of the float glass sheet. Furthermore, if the antimony levels are not properly controlled, it could lead to defects in the float glass sheet. For example, if antimony oxide is reduced to metallic antimony, it could form particles or streaks in the float glass sheet, affecting its optical properties.

[0031] It has been found as well that the added oxidizing element will not color the float glass sheet, nor through the thickness of the float glass sheet, nor through the edge of the float glass sheet - long optical paths being the most revealing of the slightest colorations. This is further particularly desirable for thick float glass sheets that :• are often used for architectural applications where the edge is exposed,• are very often thermally toughened to ensure the customer's safety in the event of breakage, and• are the most sensitive to the bloom phenomenon. Indeed, because of their thickness, the residence time of the glass sheet in the tin bath is multiplied, and correspondingly the migration of Sn is increased into the tin face; reinforcing the propensity to bloom.

[0032] Hence, the present invention relates to glass compositions comprising antimony to provide tempered float glass sheet with an acceptable haze. The present invention teaches that the oxidizing power of the antimony pushes the oxidation of tin from the SnO form to the SnO2form.

[0033] It has Indeed been surprisingly found that the incorporation of antimony into the glasscomposition, as a versatile redox element, provides a glass float sheet that remains perfectly colorless through its edge. Furthermore, antimony is reduced by oxidizing SnO (Sn2+) into SnO2 (Sn4+) at least at the surface of the tin face through the whole diffusion profile of tin and thereby prevents bloom and therefore haze, from occurring when the float glass sheet is further thermal tempered.HAZE

[0034] The present invention is directed to a tempered float glass sheet having an acceptable haze meaning that the appearance of bloom after thermal tempering is avoided. Acceptable haze means that the tempered float glass sheet has a haze value equal to or lower than 3.0% (Haze(t) < 3.0%). In a preferred embodiment, the tempered float glass sheet has a haze value equal to or lower than 2.5%, equal to or lower than 2.0%, equal to or lower than 1 .5%; equal to or lower than 1.0%, more preferably equal to or lower than 0.5%.

[0035] Typically, classical float glass sheets with no antimony (except impurities) and not subjected to tempering, have a haze value about 0% to about 0.5%. The objective of the present invention is obtained via (1) maintaining the haze of the float glass sheet (Haze initial - no tempering) to a minimum despites the incorporation of antimony in the glass composition and (2) maintaining the haze of the tempered float glass sheet (Haze tempered) to a minimum via the maximization of the Sn4+form thanks to the oxidation power of antimony.

[0036] In a preferred embodiment, the absolute difference between Haze(i) before tempering and Haze(t) after tempering is equal to or less than 2.00% ( | Haze(i)-Haze(t) | < 2.00%).

[0037] Preferably the absolute difference between Haze(i) and Haze(t) is equal to or less than 1.50% (| Haze(i)- Haze(t) | <1.50%); is equal to or less than 1.25% ( | Haze(i)-Haze(t) | < 1.25%); is equal to or less than 1.00% (| Haze(i)- Haze(t) | < 1.00%); preferably is equal to or less than 0.75% (| Haze(i)-Haze(t) | < 0.75%); preferably is equal to or less than 0.60% (| Haze(i)-Haze(t) | < 0.60%); preferably is equal to or less than 0.50% ( | Haze(i)-Haze(t) | < 0.50%); preferably is equal to or less than 0.40% ( | Haze(i)-Haze(t) | < 0.40%); preferably is equal to or less than 0.30% (| Haze(i)-Haze(t) | < 0.30%); more preferably the absolute difference between Haze(i) and Haze(t) is equal to or less than 0.20% (| Haze(i)-Haze(t) | < 0.20%).

[0038] Transmission haze, also referred herein as “haze”, corresponds to the diffuse transmittance at wide angles scattering. Haze can be measured in accordance to ASTM standard D1003 with specific illuminant. Average value is obtained via at least 5 measurement points (each corner and center) on a 10cm*10cm sample. The specific tempering conditions after which the haze of the tempered float glass sheet (Haze(t)) is measured are described herein below within the description of the bloom test.ANTIMONY

[0039] Antimony expressed in total Sb2O3, is comprised into the glass composition at a level of 20 ppm to 500 ppm, based on the total weight of the glass composition.

[0040] In a preferred embodiment, the glass composition comprises antimony expressed in total Sb2O3by weight of total glass composition, at a level equal to or greater than 25ppm, 30ppm, 35ppm, 40ppm, 45ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm, 125ppm and even, 150ppm by weight of total glass composition. In a preferred embodiment, the glass composition comprises antimony expressed in total Sb2O3by weight of total glass composition, at a level equal to or lower than 450ppm, 400ppm, 350ppm, 300ppm, 250ppm, and even, 200ppm by weight of total glass composition.

[0041] The content of Sb2O3(expressed in total Sb2O3) in the glass sheet means that the average Sb content (consisting of Sb5+, Sb3+, and Sb metal) of the glass sheet is converted to the Sb2O3form. In the float glass sheet, although there is a possibility that the distribution of Sb is biased due to the generation of Sb-containing particles in the range from the surface layer to a depth of several tens of pm, it is considered that the average Sb content can be evaluated if measured in the range from the surface to a depth of 100 pm or more. Therefore, in the present specification, the content of Sb2O3(Sb content of the Sb2O3standard) is 100 pm of the surface layer of the air face and tin face of the float glass sheet is removed by grinding.Sb-particles

[0042] It is known that the glass composition comprising antimony, the Sb-element can be present in the form of particles containing Sb (hereinafter also referred to as Sb-containing particles) into the float glass sheet. This Sb-containing particle is considered to be a metalparticle containing Sb reduced under a reducing atmosphere during the float production. Sb- containing particles may also be referred to as Sb-containing colloids.

[0043] However, it has been further found that the appearance of the haze of the float glass sheet is substantially avoided when the antimony is in the dissolved form and / or in the form of Sb-containing particles having a diameter less than 5nm, at the tin face of the glass sheet at a depth of 0pm to 10pm. Preferably, the Sb-containing particles having a diameter equal to or less than 4nm, more preferably equal to or less than 3nm, even more preferably equal to or less than 2nm and even more preferably equal to or less than 1 nm. It is even more preferred that all antimony remains fully in the dissolved form at the tin face of the tempered float glass sheet at a depth of 0pm to 10pm.

[0044] Indeed, it has been found that if the amount of Sb-containing particles is excessive or the particle size is excessive, the haze increases and the glass sheet becomes cloudy, and the transparency and quality of the glass sheet may be greatly impaired. On the other hand, if the haze is less than 3.0 %, preferably equal to or lower than 2.5%, the amount and particle size of the Sb-containing particles are appropriate, and the float glass sheet is excellent in transparency and quality.

[0045] In the float process, Sb-containing particles can be contained in both tin face and air face of the glass sheet, but the number and diameter of Sb-containing particles on the tin face tend to be larger. Therefore, by having Sb-containing particles at an appropriate diameter on the tin face, it is possible to appropriately achieve acceptable haze and no coloration of the edges of the float glass sheet .

[0046] Sb-containing particles can be detected even when the size of the Sb-containing particles is relatively small. The presence or absence of Sb-containing particles and the diameter can be determined with a transmission electron microscope (TEM) by observing a cross-section processed perpendicular to the main face in a region at a depth of 0 pm to 10 pm.GLASS COMPOSITION

[0047] The glass composition comprises typically iron expressed as total Fe2O3, at a level of 20ppm - 2000ppm based on the total weight of the glass composition. It is indeed preferred that the content of Fe2O3is 2000 ppm or less in order to suppress coloring.

[0048] For clear glass compositions, it is preferred that the content of total iron expressed in total Fe2O3, is comprised between 300ppm and 1000ppm (300ppm < Fe2O3< OOppm). Preferably, it is comprised at a level equal to or greater than 400ppm, 450ppm, 500ppm, 550ppm, 600ppm and even, 650ppm by weight of total glass composition. Preferably, total iron expressed in total Fe2O3, is comprised at a level equal to or lower than 900ppm, 850ppm, 800ppm, and even, 750ppm by weight of total glass composition.

[0049] For extra clear glass compositions, it is preferred that the content of total iron expressed in total Fe2O3, is comprised between 20ppm and less than 300ppm (20ppm < Fe2O3< 300ppm). Preferably, it is comprised at a level equal to or greater than 20ppm, 50ppm, 60ppm, 70ppm, 80ppm, 900ppm and even, 100ppm by weight of total glass composition. Preferably, total iron expressed in total Fe2O3, is comprised at a level equal to or lower than 250ppm, 200ppm, 175ppm, and even, 150ppm by weight of total glass composition.

[0050] In a preferred embodiment, the iron redox expressed in FeO / Fe2O3, is equal to or lower than 30% (FeO / Fe2O3< 30%), preferably equal to or lower than 28% (FeO / Fe2O3< 28%), preferably equal to or lower than 25% (FeO / Fe2O3< 25%), equal to or lower than 23% (FeO / Fe2O3< 23%), more preferably equal to or lower than 20% (FeO / Fe2O3< 20%).

[0051] The type of glass composition is not particularly limited as long as it contains Sb2O3and is produced under a reducing atmosphere, for example, by a float method. The glass sheet may be a glass selected from the group consisting of, for example, soda-lime glass, aluminosilicate glass, alkali-free glass, and alkaline borosilicate glass. Preferably, the composition of the float glass sheet comprises the following in weight percentage, expressed with respect to the total weight of glass :SiO2 40 - 78%AI2O3 0 - 18%B2O3 0 - 18%Na2O 0 - 20%CaO 0 - 15%MgO 0 - 12%K2O 0 - 10%BaO 0 - 5%.

[0052] Advantageously, the expression soda-lime-silicate glass in the present invention is usedin a broad sense and relates to any glass which comprises the following components in weight percentage, expressed with respect to the total weight of glass (Comp. A). More preferably, the glass composition (Comp. B) is a soda-lime-silicate-type glass with a base glass matrix of the composition comprising the following components in weight percentage, expressed with respect to the total weight of glass.

[0053] Other advantageous glass compositions for the glass of the present invention, comprise the following components in weight percentage, expressed with respect to the total weight of glass :

[0054] The glass composition preferably contains less than 550ppm of water by weight of total glass composition (H2O < 550ppm). Water content is known to increase tin diffusion inside tinface as water will oxidize tin bath. Float glass produced with high water content is even more sensitive to bloom than others.

[0055] In a preferred embodiment, the glass composition comprises less than 450ppm of water by weight of total glass composition (H2O < 450ppm), less than 400ppm of water by weight of total glass composition (H2O < 400ppm), less than 350ppm of water by weight of total glass composition (H2O < 350ppm), less than 300ppm of water by weight of total glass composition (H2O < 300ppm), less than 250ppm of water by weight of total glass composition (H2O < 250ppm); more preferably less than 200ppm of water by weight of total glass composition (H2O < 200ppm). Typically minimal water level is equal to or greater than 50ppm (H2O > 50ppm), preferably equal to or greater than 100ppm (H2O > 100ppm).

[0056] The incorporation of antimony within the glass composition can be achieved via the addition of the antimony as a raw material and / or very much interestingly via glass cullet and preferably comes from glass cullet coming from the recycling of photovoltaic panels. Indeed, photovoltaic panels are typically soda lime silicate glass that do comprise antimony at levels such as 1000ppm to 3000ppm.

[0057] The introduction of glass cullet is already common to the trade. However, to address the challenges of the climate change, it is highly necessary to reduce the impact of glass manufacturing. Increased use of glass cullet, would lead to reduction in virgin raw materials, reduces the amount of melting energy and carbonated raw materials used during the production of glass plates, and as a result, reduces CO2emissions in the flat glass manufacturing process.

[0058] From the 2030s onward, a large number of solar panels will be discarded and represent a substantial source of glass cullet. However, because of their high content in antimony, it could not be used so far as a direct source of recycled glass materials. It has been found that the glass composition of the float glass sheet of the present invention permits that the glass cullet issued from the waste cover glass of a photovoltaic panels can be recovered and used as a raw material.TEMPERING OF THE FLOAT GLASS SHEET

[0059] The tempered float glass sheet of the present invention is a float glass sheet produced by a float glass process, using molten tin as a molten metal, the float glass sheet is further subjected to a thermal tempering process. The thermal tempering process comprises a high temperature step followed by a rapid cooling that can impact negatively the bloom of the float glass sheet.

[0060] Tempered float glass sheet is understood as a float glass sheet that is heat treated using a method of controlled heating (about 650°C) and accelerated cooling which puts glass surface under compression and the core under tension. It makes the glass sheet four to five times stronger than annealed glass sheet and therefore is called safety glass. Such stresses cause the glass, when impacted, to break into small granular particles instead of splintering into jagged shards. The granular particles are less likely to injure occupants or damage objects.

[0061] As indicated above, in the float glass process, tin species from the tin bath will diffuse into the produced glass sheet under the form of Sn2+and Sn4+. Indeed, during the step of tempering and the corresponding temperature increase, Sn2+form present at the surface of the glass is oxidized into the Sn4+form. Hence, a high concentration of Sn4+is formed at the surface of the glass with (almost) no Sn2+, inducing chemical gradient and further diffusion of Sn2+ions towards the glass surface. Due to this oxidation and migration system, the thermally tempered glass exhibit Sn4+enriched extreme surface layer, then depletion of Sn species in the surface sublayer. This extreme layer (Sn4+rich) and the sublayer (lower Sn) does not exhibit same mechanical properties during the cooling down phase. It will result in a wrinkling of the surface on the final tempered glass product that will diffuse light and induce a haze veil well known from the state-of-the-art and called bloom.BLOOM MEASUREMENT

[0062] In a preferred embodiment, the tempered float glass sheet of the present invention presents a bloom value equal to or lower than 2.5, preferably equal to or lower than 2.0, preferably equal to or lower than 1 .5, more preferably equal to or lower than 1.

[0063] The bloom test incorporates a 2 steps process via which the tested samples go through high temperature heating step followed by a cooling step. The bloom test therefore fixes the conditions of thermal tempering under which the bloom and haze of the tempered float glass sheet will be assessed.

[0064] The bloom test allows to quantify the tendency of the glass to be affected by heat treatment in oxidizing conditions like tempering. Samples (10cm x 10cm) will be subjected to a heat treatment in a Muffle furnace.

[0065] In the Muffle furnace, the maximum working temperature is 1200°C and the temperature to perform the test is 730°C. Refractory material is Si-AI or Si-Ca (Such as Monolux 800). The dimensions of the furnace are : length of 300-400mm, width of 150-200 mm and height of 125- 175mm. A sample support of 15mm - 25mm is used.

[0066] Procedure is as follows:• Measure the thickness of the glass sample and obtain the required residence furnace time, from the relationship illustrated in the graph below. The furnace time in second will be 77.056 x Thickness of the glass in mm + 104.74seconds.| Exposure time to bloom test depending on glass jI thickness |• Clean perfectly the glass sample and prevent grease and finger marks on the surface.• Place the sample into the furnace, with edges aligned to the refractory tile, in the centerand with tin face, upwards.• Shut furnace door quickly and start timer. Once the pre-determined time elapsed, open the furnace door, rake the sample onto a piece of insulating material or refractory tile and allow the sample to cool down to ambient temperature.

[0067] Attention must be paid to the handling of the samples to avoid fingers mark.

[0068] By comparison with standard samples described in representation hereunder define the bloom grade for the tested sample.

[0069] The bloom grade is dependent from the bloomed ( = exhibiting visible haze) area on the sample and the bloom grade ranges from 1 (no bloom) to 3 (max bloom grade) by grading 0,5 as indicated in Figure 6. If same area but more intense bloom, the bloom grade can be increased an additional 0,25 bloom grade.FLOAT PRODUCTION

[0070] The tempered float glass sheet of the present invention is a float glass sheet produced by float glass process, the float glass sheet is further subjected to a thermal tempering process. The glass sheet can be manufactured by using glass raw material containing Sb so that the content of Sb2O3in the obtained glass sheet is within the range described above, and can be manufactured under a reducing atmosphere. However, when the Sb-containing particles are formed excessively or the particle diameter becomes excessive, the haze, light transmission and color may deteriorate rapidly, so the amount of antimony and conditions for manufacturing the glass sheet are preferably appropriately adjusted.

[0071] As a method for producing a glass sheet under a reducing atmosphere, the float method is preferable because it is superior in terms of productivity and cost. The float method is a method where, for example, the glass raw material is adjusted and melted so as to have a desired glass composition, the molten glass is formed into a glass ribbon on the molten metal, the glass ribbon is slowly cooled, and the glass ribbon is cut into a predetermined shape to obtain glass sheet. In the float method, glass is generally produced in a reducing atmosphere (for example, in an atmosphere containing H2) in order to maintain a reduced state. The method for producing this glass sheet includes, for example, melting the glass raw materialand forming the molten glass into a glass ribbon on the molten metal. Molten tin is preferable since the glass can be floated thanks to the specific gravity of molten tin being larger than that of glass.

[0072] When this glass sheet is produced by the float method, the glass substrate temperature at which the molten glass starts to come into contact with the molten metal, that is, the molding start temperature, is preferably from 900°C to 1250°C. If the molding start temperature is 1250°C or less, the nucleation and growth of Sb-containing particles are limited in the glass sheet surface layer, so that the particle size of Sb-containing particles can be reduced. The molding start temperature is more preferably 1150°C or lower, and further preferably 1100° or lower.

[0073] When the molten glass separates from the molten metal, the glass substrate temperature, that is, the molding end temperature, is preferably from 500°C to 750°C. The molding end temperature is more preferably 660°C or lower, and further preferably 640°C or lower.

[0074] The time during which the molten glass is in contact with the molten metal, that is, the molding time is preferably from 3 minutes to 20 minutes. If the molding time is 20 minutes or less, the particle size of the Sb-containing particles can be reduced and the deterioration of the haze value can be suppressed. The molding time is more preferably 10 minutes or less, and even less than 8 minutes. The molding time is 3 minutes or more. The molding time is more preferably 5 minutes or more, and more preferably 6 minutes or more. Minimum molding time is requested to process the glass at reasonable process speed to ensure for example a good thickness profile.

[0075] In float production, the H2concentration in the atmosphere in the molten metal tank is preferably 0.5 to 15% by volume. In addition to the cost reason, if the H2concentration is 15% by volume or less, the nucleation and growth of Sb-containing particles can be decreased in the glass sheet surface layer, so that the particle size of Sb-containing particles can be reduced. The H2concentration is more preferably 7% by volume or less, and more preferably 5% by volume or less. If the H2concentration is 0.5% by volume or more, the tin defect (tin migration that will cause bloom) on the glass surface can be suppressed. The H2concentration is more preferably 1 % by volume or more, and more preferably 3% by volume or more.

[0076] When producing this glass sheet by the float method, the dew point in the atmosphere in the molten metal tank is preferably from -50°C to 20°C. When the dew point is -50°C or higher,the nucleation and growth of Sb-containing particles are suppressed in the surface layer of the glass sheet, so that the particle size of Sb-containing particles can be reduced. In order to reduce the particle size of the Sb-containing particles and suppress the deterioration of the haze value, the dew point is more preferably -30°C or higher, more preferably -20°C or higher, and particularly preferably -10°C or higher. If the dew point is below 20°C, the tin defect of the glass can be suppressed. The dew point is more preferably 10°C or lower, more preferably 0°C or lower, and particularly preferably -5°C or lower.

[0077] Preferably, the molding start temperature is from 900°C to 1250°C, the molding end temperature is from 500°C to 750°C, the molding time is from 3 minutes to 20 minutes, the H2 concentration in the atmosphere in the molten metal tank is from 0.5 % to 15% by volume, and the dew point is from -50°C to 20°C. More preferably, the molding start temperature is from 1000°C to 1150°C, the molding end temperature is from 530°C to 660°C, the molding time is from 5 minutes to 10 minutes, the H2concentration in the atmosphere in the molten metal tank is from 1 % to 7% by volume, and the dew point is from -30°C to 10°C.

[0078] The glass ribbon formed on molten metal may be held at a constant temperature after the tin bath and then slowly cooled in the annealing lehr, in order to reduce the particle size of the Sb-containing particles in the surface layer of the glass ribbon and reduce the number of Sb- containing particles. Further, the slower the slow cooling rate of the glass ribbon, the smaller the particle size of the Sb-containing particles in the surface layer of the glass ribbon, and the smaller the number of Sb-containing particles. Indeed, the glass cools down, and is hard enough to pass over the rollers of the cooling tunnel. The temperature of the glass drops steadily from molding end temperature at the start of the tunnel and completes its slow cooling to room temperature. Annealing the glass (controlled cooling) stabilizes the internal stresses.GLASS SHEET

[0079] In a preferred embodiment, the tempered float glass sheet of the present invention has a visible light transmission LTD4 is equal to or greater than 85% (LTD4 > 85%), preferably equal to or greater than 88% (LTD4 > 88%), preferably equal to or greater than 89% (LTD4 > 89%), equal to or greater than 90% (LTD4 > 90%), equal to or greater than 91% (LTD4 > 91 %).

[0080] As indicated above, the thickness of the flat glass sheet is an important parameter sincethicker glasses are often used for architectural applications wherein color is an important technical feature, where the edge is exposed, are very often tempered to ensure the customer's safety, and are the most sensitive to the bloom phenomenon. Typically, the thickness of the tempered float glass sheet ranges from 0.5 to 25 mm.

[0081] In a preferred embodiment, the float glass sheet thickness is preferably 2.0 mm or more, preferably 3.0 mm or more, preferably 3.5 mm or more, preferably 4.5 mm or more; preferably5.5 mm or more, preferably 7.5 mm or more, preferably 9.5 mm or more and further preferably11 .5 mm or more; to improve strength. On the other hand, the float glass sheet thickness is preferably 20 mm or less, is more preferably 15 mm or less, to manage productivity, light weight, reducing the particle size of the Sb-containing particles and reducing the haze.

[0082] The shape of the face of the tempered float glass sheet is not particularly limited, and may be any shape such as a rectangle, a polygon, a circle, or an oval. Further, the area of the face of the tempered float glass sheet is not particularly limited, and can be appropriately adjusted according to the application and the like.

[0083] The tempered float glass sheet may be flat or bent to include a more or less curvature, notably but not limited to the aid of the thermal tempering.

[0084] The tempered float glass sheet may have a functional layer on at least one of the faces. Examples of the functional layer include an ultraviolet cut layer, an infrared cut layer, a stainproof layer, a water-repellent layer, a hydrophilic layer, a conductive layer such as aluminum or silver, a decorative layer such as ceramic or glass, and a known functional layer such as a light functional layer such as a diffraction grating or a scattering layer. This tempered float glass sheet may include a plurality of functional layers. The method for forming the functional layer is not particularly limited, and a known film formation method may be used according to the type of functional layer.Description of drawings

[0085] Figure 1 is a schematic view of the tin diffusion profile, namely the tin concentration from the bottom tin face to a depth of several microns of a float glass sheet, excluding the extreme surface of the air face. During the float glass process, tin from the tin bath diffuses to the tinface under the form of Sn2+and Sn4+. There is a second tin concentration peak - also referred to as ‘satellite peak' in the diffusion profile. It corresponds to the transition region between the region wherein Sn2+is the most represented ion form and the region wherein the Sn4+is the most represented ion form.

[0086] Figure 2 is a schematic view of the tin species modification and migration during tempering of the float glass sheet. During the tempering temperature increase, Sn2+form present at the surface of the glass is oxidized into the Sn4+form. Hence, a high concentration of Sn4+is formed at the surface of the glass with (almost) no Sn2+, inducing chemical gradient and further diffusion of Sn2+ions towards the glass surface (tin face).

[0087] Figure 3 is a schematic view of the tin species modification during the thermal tempering of the float glass sheet after the oxidation and migration system as described in Figure 2. The thermally tempered float glass sheet exhibits Sn4+enriched extreme surface layer, then depletion of Sn species in the surface sublayer.

[0088] Figure 4 is a schematic view of tin species modification during the cooling step of the thermal tempering. This extreme layer (Sn4+rich) and the sublayer (lower Sn) does not exhibit same mechanical properties during the cooling down phase. It results in a wrinkling of the extreme surface on the final tempered glass product.

[0089] Figure 5 is a schematic over photographic siderepresentation of the wrinkling of the glass surface on the tempered float glass sheet occurring in steps represented in Figures 1 to 4. This wrinkling of the glass surface causes diffusion of the light and so, haze and bloom.

Claims

CLAIMS1 . A tempered float glass sheet having a glass composition comprising antimony expressed in total Sb2O3, at a level of 20 ppm to 500 ppm, based on the total weight of the glass composition; having a haze value equal to or less than 3.0%.

2. A tempered float glass sheet according to claim 1 wherein, having a haze value equal to or lower than 2.5%, equal to or lower than 2.0%, equal to or lower than 1 .5%; equal to or lower than 1.0%, more preferably equal to or lower than 0.5%.

3. A tempered float glass sheet according to any one of the preceding claims wherein antimony expressed in total Sb2O3by weight of total glass composition, is comprised at a level equal to or greater than 25ppm, 30ppm, 35ppm, 40ppm, 45ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm, 125ppm and even, 150ppm by weight of total glass composition.

4. A tempered float glass sheet according to any one of the preceding claims wherein antimony expressed in total Sb2O3by weight of total glass composition, is comprised at a level equal to or lower than 450ppm, 400ppm, 350ppm, 300ppm, 250ppm, and even, 200ppm by weight of total glass composition.

5. A tempered float glass sheet according to any one of the preceding claims wherein the tempered float glass sheet has a tin face and an air face and wherein the antimony present at the tin face at a depth of 0pm to 10pm is dissolved in the glass composition and / or in the form of Sb-containing particles having a diameter less than 5nm, preferably equal to or less than 4nm, preferably equal to or less than 3nm, preferably equal to or less than 2nm, more preferably equal to or less than 1 nm.

6. A tempered float glass sheet according to any one of the preceding claims wherein the antimony present at the tin face at a depth of 0pm to 10pm is dissolved in the glass composition.

7. A tempered float glass sheet according to any one of the preceding claims, having a bloom value of equal to or lower than 2.5, preferably equal to or lower than 2.0, preferably equal to or lower than 1 .5, more preferably equal to or lower than 1 .0.

8. A tempered float glass sheet according to any one of the preceding claims, wherein the glass composition comprises the following glass components at those levels in respect to the total weight of glass:

9. A tempered float glass sheet according to any one of the preceding claims wherein the glass composition comprises iron expressed as total Fe2O3, at a level of 20-2000ppm based on the total weight of the glass composition.

10. A tempered float glass sheet according to claim 9 wherein the content of total iron expressed in total Fe2O3, is comprised between 300ppm and WOOppm (300ppm < Fe2O3< WOOppm), preferably, it is comprised at a level equal to or greater than 400ppm, 450ppm, 500ppm, 550ppm, 600ppm and even, 650ppm by weight of total glass composition and / or preferably, total iron expressed in total Fe2O3, is comprised at a level equal to or lower than 900ppm, 850ppm, 800ppm, and even, 750ppm by weight of total glass composition.11 . A tempered float glass sheet according to claim 9 wherein the content of total iron expressed in total Fe2O3, is comprised between 20ppm and less than 300ppm (20ppm < Fe2O3< 300ppm), preferably, it is comprised at a level equal to or greater than 20ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm and even, 100ppm by weight of total glass composition and / or preferably, total iron expressed in total Fe2O3, is comprised at a level equal to or lower than 250ppm, 200ppm, 175ppm, and even, 150ppm by weight of total glass composition.

12. A tempered float glass sheet according to any one of the preceding claims 9 to 11 wherein the iron redox expressed in FeO / Fe2O3, is equal to or lower than 30% (FeO / Fe2O3< 30%), preferably equal to or lower than 28% (FeO / Fe2O3< 28%), preferably equal to or lower than25% (FeO / Fe2O3 < 25%), equal to or lower than 23% (FeO / Fe2O3 < 23%), more preferably equal to or lower than 20% (FeO / Fe2O3 < 20%).

13. A tempered float glass sheet according to any one of the preceding claims having a sheet thickness comprised between 0.5 mm and 25 mm, preferably a thickness of 2.0 mm or more, preferably of 3.0 mm or more, preferably of 3.5 mm or more, preferably of 4.5 mm or more; preferably of 5.5 mm or more, preferably of 7.5 mm or more, preferably of 9.5 mm or more and more preferably of 11 .5 mm or more; and / or having a sheet thickness of 20 mm or less, preferably 15 mm or less.

14. A tempered float glass sheet according to any one of the preceding claims wherein the antimony comprised in the glass composition comes from glass cullet, preferably from glass cullet coming from recycling of photovoltaic panels.

15. A tempered float glass sheet according to any one of the preceding claims having a visible light transmission LTD4 is equal to or greater than 85% (LTD4 > 85%), preferably equal to or greater than 88% (LTD4 > 88%), preferably equal to or greater than 89% (LTD4 > 89%), equal to or greater than 90% (LTD4 > 90%), equal to or greater than 91% (LTD4 > 91 %).

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