A neutral-tint, low-iron flat glass composition

Abstract

The invention relates to a flat-glass composition having a low iron content, a neutral color tone, and high reduction. The flat glass composition comprises, w / w 67-75% SiO2, 0,1-3, 5% Al2O3, 5-11% CaO, 2-6% MgO, 8-16% Na2O + K2O, Fe2O3 < 0,025%, and the ratio Fe2+ / Fe2O3 * 100 is greater than 25%.

Description

[0001] A NEUTRAL-TINT, LOW-IRON FLAT GLASS COMPOSITION

[0002] TECHNICAL FIELD

[0003] The present invention relates to a low-iron flat glass used in architectural and furniture products.

[0004] BACKGROUND ART

[0005] In glass used particularly in the architectural and furniture sectors, high visible-region light transmittance and a neutral color tone are desired. However, in these sectors it is not easy to achieve the high optical performance and neutral tone preferred, using standard flat-glass products. Even when high-purity raw materials are used in the batch, very small amounts of impurities such as iron oxide may enter from the raw materials. Iron oxide originating from the batch adversely affects both the color and the optical properties of the glass.

[0006] An example of an oxidizing material that may be present in the glass batch is CeO2. CeO2is used to oxidize iron in glass and also serves as a fining aid. However, certain undesirable properties of the cerium ion may arise under exposure to high UV radiation. Exposing low-iron glass products containing cerium oxide to sunlight has a solarizing effect on the glass resulting from photooxidation of Ce3+to Ce4+and photo-reduction of Fe3+to Fe2+. As a result, the glass may suffer a loss of transmittance and a shift toward a yellow-brown tone. This phenomenon is termed solarization.

[0007] Solarization may be defined as the tendency of glass to change color over time under the effect of sunlight. This occurs when exposure to light changes the valence state of ions. High-energy UV-region photons of sunlight play an important role in this process. It causes a decrease in glass transmittance, particularly at a wavelength of 380 nm and beyond. Because the drop in UV transmittance below 380 nm lies outside human visual sensitivity, it does not affect color; however, the decrease in transmittance may extend into the visible region and thereby degrade the color of the glass. For this reason, CeO2is an undesirable material especially in applications where low-iron glasses are used. In the production of low-iron glass, the aim is typically to produce the glass in an oxidizing form. Under the specified oxidizing conditions (redox ratio s 25%), situations may arise that are disadvantageous to glass quality. In oxidizing production, decreases in furnace performance due to high furnace temperatures and refractory corrosion may be observed, energy consumption may increase, and production costs may be high. Moreover, in oxidizing glass production the fining process occurs at higher temperatures, which significantly affects glass quality. Effective fining of glass under oxidizing conditions is more difficult than under reducing conditions. Reducing conditions generally allow operation at lower temperatures, which can reduce energy consumption and thereby lower production costs. Production under reducing conditions can result in less oxidation and fewer emissions, thereby reducing environmental impacts and providing a more sustainable manufacturing process.

[0008] EP2712849 A1 discloses a 2-12 mm flat glass comprising components in weight percentages as follows: to achieve a minimum daylight transmittance of 90% ± 1 , a minimum solar energy transmittance of 87% ± 1 , and a redox ratio for iron (s 0,25), the Fe2+ions in the flat-glass batch, originating from Fe2O3, must be present at a maximum of 0,006 wt%. In addition, this flat glass contains the following weight-percentage component ranges to oxidize and passivate the iron in the batch: SiO268-75%, Al2O30,7-1 ,8%, Fe2O3< 0,025%, CaO 5-11%, MgO 2-6%, Na2O + K2O 11-17%, S030,1-0,5%, CeO20,00-0,1 %, CoO 0-0,0003%.

[0009] BRIEF SUMMARY OF THE INVENTION

[0010] An object of the invention is to provide a glass having a low iron-oxide content and high reduction (reducing conditions), which achieves high transmittance without using decolorizing additives.

[0011] To achieve the stated object, the invention relates to a low-iron flat-glass composition having a neutral color tone and high reduction. The flat-glass composition comprises, in weight percent: 67-75% SiO2, 0,1-3, 5% Al2O3, 5-11% CaO, 2-6% MgO, 8-15% Na2O + K2O, Fe2O3< 0,025%, and the ratio Fe2+ / Fe2O3x 100 is greater than 25%. Thus, low-iron glasses having high transmittance allow light to be transmitted more efficiently and provide an advantage in achieving a view closest to that perceived by the human eye. In addition, refraining from the use of decolorizers and producing under reducing conditions reduces environmental harm, contributes to a sustainable manufacturing process, and lowers energy and production costs. In a preferred embodiment, the SiO2content is 69-72 wt%. In this way, the main glass former in which the other components dissolve is provided within the glass composition. If the SiO2content is below the stated percentages, its network-forming ability is reduced, making glass formation more difficult and lowering mechanical and chemical durability. If the SiO2content exceeds the stated value, the melting temperature rises and it becomes more difficult to obtain a homogeneous glass.

[0012] In a preferred embodiment, the Al2O3content is 0,1-2 wt%. Thus, the structural strength of the flat glass produced from the composition increases and its thermal and mechanical properties are improved. It contributes to improved chemical and thermal stability and strength.

[0013] In a preferred embodiment, the CaO content is 7-10 wt%. In this way, the melting and physical properties of the flat glass produced from the composition are improved. It improves high- temperature melting behavior while allowing easy forming. If the amount is below the stated percentages, high-temperature melting becomes difficult and mechanical and chemical durability decreases.

[0014] In a preferred embodiment, the MgO content is 3-5 wt%. Thus, the viscosity of the glass at high temperatures is reduced, facilitating melting.

[0015] In a preferred embodiment, the total K2O + Na2O content is 10-16 wt%. Alkali oxides such as Na2O and K2O are bound to the network by a single weak bond, so their mobility in the glass is high, and they lower the viscosity of the glass, facilitating melting behavior, forming, and processing. Glass containing both Na2O and K2O has greater chemical and thermal durability than glass containing only one of Na2O or K2O.

[0016] In a preferred embodiment, the SO3content is 0,15-0,35 wt%. In this way, the use of SO3assists fining during production of the glass.

[0017] In a preferred embodiment, the daylight transmittance is 89-91 %. Thus, the desired daylight transmittance values for low-iron glass are achieved.

[0018] In a preferred embodiment, the solar energy transmittance is 86-90%. Thus, the desired solar energy transmittance values for low-iron glass are achieved. In a preferred embodiment, the reduction ratio is greater than 30%.

[0019] In a preferred embodiment, no additional decolorizing material is present. Thus, producing without decolorizers reduces environmental harm, contributes to a sustainable manufacturing process, and lowers energy and production costs.

[0020] DETAILED DESCRIPTION OF THE INVENTION

[0021] In this detailed description, the development according to the invention is explained without limitation and only to better elucidate the subject, with reference to examples.

[0022] The present invention concerns a low-iron flat-glass composition having a neutral color tone and high reduction. The flat-glass composition comprises 67-75% SiO2, 0,1-3, 5% Al2O3, 5-11 % CaO, 2-6% MgO, 8-15% Na2O + K2O, Fe2O3< 0,025%, and other components. Production of low-iron glass plays an important role particularly for architectural and furniture applications. Glasses with high optical performance and a neutral color tone are also important for other fields of use (e.g., solar) and demand for such glasses is increasing day by day.

[0023] One of the most important parameters affecting the color and optical properties of glass is its chemical composition. In products required to have high transmittance, iron oxide, which enters as an impurity due to the raw materials used or the batch charging system, imparts a green color to the glass and changes the values of daylight transmittance and solar energy transmittance.

[0024] In soda-lime glasses, iron is present as Fe2+(ferrous) and Fe3+(ferric). Fe2+imparts a blue-green color to the glass, whereas Fe3+imparts a yellow-green color. The absorption coefficient of Fe2+is much higher than that of Fe3+. Under equal concentrations, the color imparted by Fe2+is approximately ten times stronger than that imparted by Fe3+. Fe2+exhibits strong absorption centered at a wavelength of 1050 nm in the infrared region. This absorption band extends into the visible region down to about 500 nm and blocks all wavelengths including sunlight. Fe3+, on the other hand, has an absorption band centered at 440-430 nm and 380 nm in the visible region and continuing into the ultraviolet. The color imparted by iron to the glass varies depending on the Fe2+ / Fe3+ratio.

[0025] In the production of low-iron glass, the aim is to produce the glass in an oxidizing form. However, when the glass becomes oxidizing, the amount of Fe3+increases and a yellowish-green tint occurs. To eliminate this tint and obtain a clearer, more neutral color, substances that provide complementary colors are added to the glass. After the iron entering from the batch is oxidized to Fe3+, the yellowish-green color provided by Fe3+is physically masked with colorants that provide complementary colors such as Mn, Se, Ni, Co, and Er. However, the absorption of these additives lies in the visible region (for Co2+at 530 nm, 590 nm, and 650 nm; for Ni2+between 400-600 nm; for Mn3+at 470 nm and 520 nm; for Er3+at 379 nm and 522 nm; and for Se at 490 nm). Therefore, depending on the amount of additive used, the percent light transmittance of the glass will decrease and the color will deviate from a neutral tone. To avoid this, in the preferred configuration production is carried out with raw materials having a low iron-oxide content and without using decolorizing agents (e.g., CoO or MnO).

[0026] If the glass to be produced is to have high transmittance and be used in architectural decoration products that are as colorless as possible, iron-oxide impurities that would change the color of the glass should be avoided as far as possible, and if feasible completely eliminated or minimized. Although this is the desired situation in industry, in practice the Fe2O3level does not fall below a certain limit value. Both in the literature and in the sector, flat-glass products with Fe2O3below 0,025 wt% are referred to as low-iron. These products may be used in many architectural applications, furniture products, and solar PV modules. To be suitable for these uses, the products must have high transmittance and a neutral color tone. In products containing Fe2O3, increasing transmittance and achieving a clearer / more neutral color tone can be achieved by adjusting oxidation conditions through auxiliary raw materials added to the glass batch (e.g., sodium sulfate, anthracite) and furnace operating parameters.

[0027] The chemical reactions of sulfate vary depending on the redox state of the glass, i.e., whether it is oxidizing or reducing. Reducing conditions are obtained by adding reducing raw materials to the batch, typically a carbon source. In reducing glasses, sulfate reactions develop at lower temperatures (~900-1300 °C) than in oxidizing glasses, i.e., earlier; thus, more time remains for fining. However, the solubility of SO3in reducing glasses is lower than in oxidizing glasses. Because the lower solubility causes these gases to be expelled from the glass more quickly, fining of reducing glasses not only begins at lower temperatures but also proceeds faster than in oxidizing glasses.

[0028] Under reducing conditions, the amount of Fe2+in the glass increases, and therefore daylight and solar transmittances decrease somewhat. Consequently, within the scope of the invention, production is targeted under redox conditions that provide efficiency in melting and fining without adversely affecting the optical performance.

[0029] Operation under reducing conditions also offers advantages for furnace and refractory life. Oxide and hydroxide species that arise under oxidizing conditions cause corrosive effects on refractories and shorten their service life. It is also known that oxide-containing species released in an oxidizing environment have a more erosive effect on regenerators. For all these reasons, by employing redox conditions advantageous for furnace and refractory life, an advantage is achieved in producing quality glass without compromising optical performance.

[0030] The weight percentages of the materials used for the glass composition are shown in the tables above. In the tables, the L-a-b values are calculated from the Tristimulus (Three-Stimulus) values. In this system, the position of the existing color is determined in a three-dimensional Cartesian coordinate system. While the a* axis moves from left to right, i.e., from green (-a) to red (+a), the b* axis moves from blue (-b) to yellow (+b). The L* value represents white at the top point with L = 100 and black at the bottom point with L = 0. %Tv indicates the daylight transmittance, and %Te indicates the solar energy transmittance, as percentages.

[0031] In Example 1 shown in Table 1 , the composition contains, by weight percentage, 71 ,5% SiO2, 1 % Al2O3, 0,024% Fe2O3, 8,7% CaO, 4,3% MgO, 14,2% Na2O + K2O, and 0,25% SO3. It was determined that the low-iron glass with the Example 1 composition has an L value of 96, an a- axis value of -0,8, and a b-axis value of -0,2. The daylight transmittance of the low-iron glass with the Example 1 composition was determined to be 89,9%, and the solar energy transmittance was determined to be 85,7%. The reduction value of the low-iron glass with the Example 1 composition was found to be 33,4%.

[0032] In Example 2 shown in Table 1 , the composition contains, by weight percentage, 71 ,5% SiO2, 1 % Al2O3, 0,022% Fe2O3, 8,7% CaO, 4,3% MgO, 14,3% Na2O + K2O, and 0,26% SO3. It was determined that the low-iron glass with the Example 2 composition has an L value of 96, an a- axis value of -0,7, and a b-axis value of -0,1. The daylight transmittance of the low-iron glass with the Example 2 composition was determined to be 90,0%, and the solar energy transmittance was determined to be 85,9%. The reduction value of the low-iron glass with the Example 2 composition was found to be 34,9%.

[0033] In Example 3 shown in Table 1 , the composition contains, by weight percentage, 71 ,4% SiO2, 1 % Al2O3, 0,02% Fe2O3, 8,7% CaO, 4,3% MgO, 14,3% Na2O + K2O, and 0,26% SO3. It was determined that the low-iron glass with the Example 3 composition has an L value of 96, an a-axis value of -0,7, and a b-axis value of -0,1. The daylight transmittance of the low-iron glass with the Example 3 composition was determined to be 90,1%, and the solar energy transmittance was determined to be 86,5%. The reduction value of the low-iron glass with the Example 3 composition was found to be 34,5%.

[0034] In Example 4 shown in Table 1 , the composition contains, by weight percentage, 71 ,4% SiO2, 1 % Al2O3, 0,019% Fe2O3, 8,7% CaO, 4,3% MgO, 14,2% Na2O + K2O, and 0,26% SO3. It was determined that the low-iron glass with the Example 4 composition has an L value of 96,1 , an a- axis value of -0,6, and a b-axis value of -0,1. The daylight transmittance of the low-iron glass with the Example 4 composition was determined to be 90,2%, and the solar energy transmittance was determined to be 86,7%. The reduction value of the low-iron glass with the Example 4 composition was found to be 34,7%.

[0035] In Example 5 shown in Table 1 , the composition contains, by weight percentage, 71 ,6% SiO2, 1 % Al2O3, 0,016% Fe2O3, 8,7% CaO, 4,4% MgO, 14,1% Na2O + K2O, and 0,27% SO3. It was determined that the low-iron glass with the Example 5 composition has an L value of 96,1 , an a- axis value of -0,5, and a b-axis value of -0,1. The daylight transmittance of the low-iron glass with the Example 5 composition was determined to be 90,3%, and the solar energy transmittance was determined to be 87,5%. The reduction value of the low-iron glass with the Example 5 composition was found to be 33,7%.

[0036] In Example 6 shown in Table 2, the composition contains, by weight percentage, 71 ,6% SiO2, 1 % Al2O3, 0,015% Fe2O3, 8,7% CaO, 4,4% MgO, 14,1% Na2O + K2O, and 0,22% SO3. It was determined that the low-iron glass with the Example 6 composition has an L value of 96,2, an a- axis value of -0,5, and a b-axis value of -0,1. The daylight transmittance of the low-iron glass with the Example 6 composition was determined to be 90,5%, and the solar energy transmittance was determined to be 87,7%. The reduction value of the low-iron glass with the Example 6 composition was found to be 34,0%. In Example 7 shown in Table 2, the composition contains, by weight percentage, 71 ,6% SiO2, 1 % Al2O3, 0,014% Fe2O3, 8,7% CaO, 4,4% MgO, 14,1% Na2O + K2O, and 0,22% SO3. It was determined that the low-iron glass with the Example 7 composition has an L value of 96,3, an a- axis value of -0,5, and a b-axis value of -0,1. The daylight transmittance of the low-iron glass with the Example 7 composition was determined to be 90,7%, and the solar energy transmittance was determined to be 87,9%. The reduction value of the low-iron glass with the Example 7 composition was found to be 34,3%.

Claims

CLAIMS1. A flat-glass composition having a low iron content, a neutral color tone, and high reduction, characterized by, the composition comprises in weight percent, 67-75% SiO2; 0, 1-3,5% Al2O3; 5-11% CaO; 2-6% MgO; 8-16% Na2O + K2O; Fe2O3< 0,025%, and by the ratio Fe2+ / Fe2O3* 100 (%) is greater than 25%.

2. The flat-glass composition according to claim 1 , wherein the SiO2content is 69-72 wt%.

3. The flat-glass composition according to any of the preceding claims, wherein the Al2O3content is 0,1-2 wt%.

4. The flat-glass composition according to any of the preceding claims, wherein the CaO content is 7-10 wt%.

5. The flat-glass composition according to any of the preceding claims, wherein the MgO content is 3-5 wt%.

6. The flat-glass composition according to any of the preceding claims, wherein the totalK2O + Na2O content is 10-16 wt%.

7. The flat-glass composition according to any of the preceding claims, wherein the SO3content is 0,15-0,35 wt%.

8. The flat-glass composition according to any of the preceding claims, wherein the daylight transmittance is 89-91%.

9. The flat-glass composition according to any of the preceding claims, wherein the solar energy transmittance is 86-90%.

10. The flat-glass composition according to any of the preceding claims, wherein the reduction ratio is greater than 30%.11 . The flat-glass composition according to any of the preceding claims, wherein it contains no additional decolorizing material.

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