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1. (WO2019064276) METHOD OF STRENGTHENING GLASS SUBSTRATES
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METHOD OF STRENGTHENING GLASS SUBSTRATES

Field of the invention

The present disclosed invention relates to a method of strengthening chemically glass substrates, and more particularly to a method of strengthening glass substrates by an ion exchange process using saturated solutions.

Background of the invention

Chemical strengthening is a process that toughens the surface of a glass by substituting smaller ions in the glass composition (e.g. sodium ions) with larger ions in a molten ionic salt (e.g. potassium ions), and thus the process is called "ion exchange" or "stuffing". The ion exchange process creates a thin layer of high compression on the surface which results in a layer of tension in the center. Conventional ion exchange technique is performed by submerging the glass into an ionic bath for several hours at temperatures usually below the strain point of the glass. This technique is widely used today to strengthen glass that are used in mobile phones, televisions and automobiles, among others. In order to make this process manufacturable, large quantities of salt are melted in large stainless-steel vessels, where the glass are ion exchanged following a specific temperature/time recipe, i.e. the ionic bath with the glass inside must be heated at a predetermined temperature T during a predetermined period of time t.

FIG. 1 shows an example of a production process of chemically strengthened glass substrates according to the conventional ion exchange technique, in which soda-lime glass substrates 1 are mounted on a support structure 2 with several slots 3 that are spaced apart, each slot 3 configured to receive and retain one glass substrate 1. An ionic bath composition 4 is prepared into a vessel 5 by melting ionic salt KN03 with heat. Next, the glass substrates 1 are preheated at 350°C inside a furnace 6 for two hours, and the temperature of the ionic bath composition 4 prepared is adjusted to 450°C for chemical strengthening, wherein the glass substrates preheating temperature depends on the temperature at which the glass substrates 1 will be immersed in the ionic bath 4. Next, the glass substrates 1 are immersed in the ionic bath composition 4 for eight to twelve hours to allow the ion exchange to take place. Finally, the glass substrates 1 are removed from the vessel 5 to cool them gradually to room temperature inside furnace 6 for three hours. The ion exchanged glass substrates 1 will have a compressive stress (CS) of about 650-700MPa and a depth of layer (DOL) of about 10-15μιη.

A problem with this conventional technique is that the process is potentially dangerous for the following reasons: (a) it produces large amounts of nitrogen oxides (NOx) because of the decomposition of the salt at the high temperature during the ion exchange process; (b) the salt could react violently with water at high temperature (e.g. a badly dried glass); and (c) the rate of vessel corrosion is elevated because of the high salt concentration. Another problem that arises with conventional technique is the salt cross contamination, i.e. as the salt is continuously used, the bath is progressively enriched with the original ions from the glass. The rate of ion exchange tends to decrease, and so does the compressive stress. At some point, the salt must be changed.

Finally, the conventional technique is a very time-consuming process that consumes a considerable amount of energy not only because of the preheating and heating steps, which last several hours, but also because of the bath preparation step. It is especially significant when different types of glasses are needed, each type of glass requiring different ionic bath compositions. Thus, between the processing of two different types of glasses, it is required to remove the current ionic bath composition from the vessel, clean the vessel and reload the vessel with the following ionic bath composition, which could take several days. Therefore, the ionic exchange process becomes the bottle neck of the entire production chain.

As a partial solution to those problems, some research has proposed methods of strengthening glass, wherein an aqueous solution containing at least one ionic salt is used to provide a film on the glass surface before heat treating. For example, U.S. Pat. No. 3,498,773 discloses a method of strengthening glass containers, wherein a glass container is coated with an aqueous solution that changes its salt to water concentration due to the vaporization of water to provide a solid film on the glass surface, followed by heating the glass container at an elevated temperature at or above the strain point for a period of time of about 5 to 30 minutes, so that the ion exchange can be carried out. In another example, U.S. Pat. No. 4,206, 253 discloses a method of strengthening soda-lime glass containers, which comprises forming a coating film of a metal oxide at an elevated temperature on the outer surface of the glass container and then applying to the outer and inner surface of the glass container an aqueous solution containing a surfactant, a potassium nitrate and others potassium salts, the temperature of the glass container being lower than that of the aqueous solution, thereby depositing out the potassium salts on the outer surface and inner surface of the glass, then holding the glass container at an elevated temperature below the strain point of the glass but near said strain point for a period of time sufficient to form a compressive stress layer on the outer surface and inner surface of the glass container.

However, both mentioned methods have some drawbacks. The former method requires preheating the glass at a temperature above the boiling point of water (temperatures between 150 - 485 °C are preferred) to allow water evaporation from the aqueous solution when applied to the preheated glass. Accordingly, as the conventional ion exchange technique, the preheating step is a time-consuming process that consume a considerable amount of energy. In addition, as ion exchange is carried out at an elevated temperature at or above the strain point of soda-lime glass, the resulting compressive stress would be attenuated by both viscoelasticity and structural relaxation of the glass. Likewise, the latter method has some limitations: (a) since the solution is an aqueous solution, solvents without water are not allowed; (b) the use of mixed potassium salts in the aqueous solution is mandatory, therefore only glass containing sodium metal ions could be used in this method; (c) glass containers are made with soda-lime glass (soda-lime glass having a strain point of about 510 °C is preferred), therefore this method is limited to soda-lime glass; (d) the aqueous solution contains a surfactant, which together with the elevated temperature used for ion exchange, affect the resulting compressive stress (max. CS obtained is 120MPa); and (e) the glass container requires a coating film of a metal oxide on the outer surface to protect the compressive stress layer formed by ion exchange treatment.

Consequently, there is a need in the art for methods of strengthening glass that be broad in application scope and optimize the use of resources without compromising the degree of chemical strengthening.

Summary of the invention

It is an object of the present invention to provide a method of chemically strengthening glass substrates, which decreases the process time, the energy consumption and the amount of salt used, while at the same time achieving similar or better degree of chemical strengthening than that obtained with conventional ion exchange technique.

Furthermore, it is an object of the invention to provide a method, which has no restriction on saturated solution and types of glasses being used. Moreover, it is an object of the present invention to improve extra glass surface properties, while performing the chemical strengthening. Lastly, it is an object of the invention to provide an environmentally friendly method.

These objects can be attained by a method of strengthening a glass substrate, which comprises the steps of applying a saturated solution at temperature Ti on the glass substrate at temperature T2, wherein the saturated solution contains an ionic salt and a liquid solvent, and wherein Ti > T2; allowing the solution on the glass substrate to cool, thereby precipitating the ionic salt as soon as the solution temperature decrease, leaving a crust of salt adhered to the surface of the glass substrate; and heating the glass substrate to a predetermined temperature T3 for a predetermined period of time Ϊ3, wherein the temperature T3 ranges from the melting point of the ionic salt to preferably temperatures below the strain point of the glass substrate, and the time t3 is enough to allow an ion exchange process.

As can be noted, in the case of a saturated solution as the method described above, not as much salt is required as in the case of the conventional ion exchange method where a molten salt is needed. Additionally, since the ion exchange does not occur inside a

container with the ionic solution, said solution does not become contaminated. Thus, the salt remaining in the saturated solution could be re-used efficiently for next ionic salt coatings and exchange processes. Furthermore, as the glass substrate does not require to be preheated prior to solution application and the heating step lasts less than conventional ion exchange method, time and energy consumed are improved.

Also, as the saturated solution is prepared as it is needed and the preheating/heating time is dismissed/reduced, not as much NOx is produced as in other methods. In addition, since the solution is a saturated solution, there is no probability of salt reacting violently with water as in the conventional method. Moreover, when different types of saturated solutions are needed, each of them could be prepared in a short time, even a previous saturated solution could be re-used to prepare new ones.

Another advantage of this method is that since the temperature for ion exchange ranges from the melting point of the ionic salt to preferably temperatures below the strain point of the glass substrate, there is minimal compressive stress attenuation by both viscoelasticity and structural relaxation. Additionally, as the resulting compressive stress is satisfactory, there is no necessity for applying an extra film on the glass. Finally, there is no particular limitation on the ionic salt, solvent and type of glass that could be used in the present invention and, therefore, it is possible to take advantage of the chemical strengthening process to change others glass surface properties.

Brief description of the drawings

These features and advantages of the present invention will become apparent from the detailed description of the following embodiments in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a schematic view of a production process of chemically strengthened glass substrates according to conventional method.

FIG. 2 shows a schematic view of a production process of chemically strengthened glass substrates according to one embodiment of the present invention.

FIG. 3 shows a saturated solution application process according to one embodiment of the present invention.

FIG. 4 shows the furnace temperature as a function of time according to the process of Example 1.

Detailed description of the invention

Referring now to the drawings, there are shown preferred embodiments of the method according to the present invention.

In FIG. 2, a saturated solution 7 is prepared into a vessel 8 by dissolving ionic salt in a liquid solvent (e.g. deionized water) at a temperature T1; wherein the solubility of the salt is higher as Tl increases according to the solubility curve of said salt which plots the changes of the solubility of a salt at different temperatures in a solvent. The ionic salt is a salt with the generic formula ANO3, or a mixed ionic salt (A, B)NC>3, or a mixture thereof; wherein both A and B are an alkali metal (e.g. NaNC>3, KNO3 and L1NO3, among others). Next, glass substrates 9 are immersed in the saturated solution 7, and immediately extracted from said saturated solution 7, allowing the solution 7 on the glass substrates 9 to cool at a cooling rate from l°C/min to 100°C/min, preferably from l°C/min to 50°C/min, thereby precipitating the ionic salt as soon as the solution temperature decrease along the solubility curve of said ionic salt, i.e. as temperature decreases it precipitates as much ionic salt as corresponds to the change of temperature at the curve.

As a result, each glass substrate 9 is evenly coated with a recrystallized salt, forming a crust of salt 10 on the surface of the glass substrate. Next, the glass substrates 9 are heating inside a heat source 11 at a predetermined temperature T3 for a predetermined period of time Ϊ3, wherein the temperature T3 ranges from the melting point of the ionic salt to preferably temperatures below the strain point of the glass substrates 9, and the time Ϊ3 is enough to allow an ion exchange process. Lastly, the glass substrates 9 are cooling inside the heat source 11, preventing the glass substrates 9 from shattering due to sudden temperature change. In this embodiment, the heat source 11 is a furnace. In all embodiments, the heat source is provided with at least one heat transfer mechanism selected from the group consisting of convection, radiation and conduction.

In this embodiment, the thickness of the crust 10 lies between 20 and 60 μιη. However, in other embodiments, the thickness of the crust lies between 16 and 600 um, preferably between 20 and 400 μιη, and even more preferably between 20 and 200 μιη.

Additionally, in the embodiment illustrated in FIG. 2, the glass substrates 9 are immersed in the saturated solution 7 (dipping). However, in other embodiments, the saturated solution is applied to the glass substrates by others means. FIG. 3 shows an embodiment wherein the application step is performed by atomizing a saturated solution 12 to a windshield glass 13 via spray means 14. In an alternative embodiment (not shown), the saturated solution is applied to glass substrates by painting a saturated solution via paint application means.

As can be noted, the present invention is not limited to a particular shape, geometry or size of the glass substrate. Furthermore, the invention can be used independently of the glass type and/or composition used, provided that the glass substrate contains alkalis or transition metals in its composition. Moreover, the present invention is able to take advantage of the chemical strengthening process to change others glass surface properties such as luminescence, index of refraction, antimicrobial properties and antibacterial properties, among others. Therefore, in the embodiments in which at least one of these properties are required, the ionic salt is a mixed ionic salt of the form (C, D)N03; wherein C is an alkali metal and D is selected from the group consisting of transition metals and rare-earth metals.

Alternatively, in some embodiments, the ionic salt contains at least one salt selected from the group consisting of sulfides, chlorides, halides or hydrates.

In several embodiments, the glass substrates are made of soda-lime, alkali aluminosilicate, lithium aluminosilicate, alkali alkaline earth aluminosilicate or another silicate.

In all embodiments, the liquid solvent is water or at least one organic solvent (e.g. ammonia and glycerol, among others). In some embodiments, wherein the liquid solvent is water, the liquid solvent is selected from the group consisting of deionized water, distilled water and potable water.

Subsequently, practical examples will be set forth to clarify the effects of the present invention.

Example 1

A mixture of 30g of KNO3 was mixed at 60°C with 30 cm3 of deionized water. The mixture was then applied to a 10cm2 soda-lime glass at room temperature with a spray bottle, forming a crust of salt adhered to the surface of the glass. Afterwards, the glass was heated in a furnace to 450°C for 6 hours and then cooled gradually inside the furnace for three hours. The result of the sprayed glass is reported as follow:

TABLE 1

Type of glass Compressive Stress - CS Depth of Layer - DOL (μιη)

(MPa)

SLG 700 10

Example 2

A mixture of lOOg of KNO3 was mixed at 60°C with 100 cm3 of deionized water. Three different types of glass substrates (100x100 mm2) of: soda lime glass (SLG), lithium aluminosilicate (LAS), and alkali aluminosilicate (AAS), were then immersed in the solution at 60°C and extracted rapidly from the solution, allowing the salt to precipitate on the glass substrates surfaces at room temperature. All three types of glasses were then heated in a furnace at different temperatures from 380 to 480°C for four hours for the ion exchange to take place and then cooled inside the furnace to room temperature. The results are reported as follow:

TABLE 2

Type of glass Compressive Stress - CS Depth of Layer - DOL (μιη)

(MPa)

SLG [140 - 700] [9 - 16]

LAS [140 - 480] [7 - 12]

AAS [80 - 410] [29 - 52]

In both examples, the compressive strength and depth of layer values obtained (TABLE 1, TABLE 2) are typically what have been reported in the literature for the conventional ion exchange method.

Regarding the Example 1, FIG. 4 shows that proposed method has a process time improvement of more than 40% over the conventional method by dismissing the preheating step and reducing by half the heating step time. As it was mentioned above, this process time improvement also leads to an improvement in energy consumption.

It must be understood that this invention is not limited to the embodiments described and illustrated above. A person skilled in the art will understand that numerous variations and/or modifications can be carried out that do not depart from the spirit of the invention, which is only defined by the following claims.