GB2326605A - Durable water repellent glass - Google Patents

Description

DURABLE WATER REPELLENT GLASS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to water repellent glass having excellent durability and its manufacturing process, in which various glass substrates, such as safety glass for automobile, building glass, and mirror, for the purposes of giving durability, are coated with silica solution which has been subjected to an aging process in the presence of basic and acidic catalysts. As a result therefrom, a coarse surface silica layer having micro-crystal silica particles is formed on the glass substrate. On top of such surface, fluoroalkylsilane (FAS) is applied, forming such water repellent layer. More particularly, this invention relates to such water repellent glass having superior durability and water repellent property, and the process for manufacturing the same, in which long-term use does not induce deformation in appearance and deterioration in water repellent property.

Background of the Invention In the case of various types of glasses exposed to extemal environment, such as safety glass for automobile, building glass, and mirror, the surface of such glass frequently becomes contaminated with rain or some other moisture, resulting in poor visual field. In order to solve such problem, water repellent property has been added to the conventional glass surface.

In general, to provide water-repellent property te the surface Dr glass, the surface energy at the glass surface should be lowered. To this end, a substance having low surface energy (hereinafter referred to as "water-repellent agent") should be placed at the glass surface. In line with this, a common type of soda lime glass plate shows a contact angle of about 20 ' to water, but a glass sample with excellent water-repellent treatment shows a contact angle of more than 100 As prior art, "RAIN-X" (Unelko Corp. USA, USP 3579540) the currently commercialized process for coating silicon based water repellent agent, is the most well known process vis-a-vis adding water repellent property to the glass via coating with the water repellent agent. In general, hydrocarbon-based, silicon-based, chlorine. or fluorine compounds are being used as a water-repellent agent. Among them, hydrocarbon- and silicon-based compounds have their critical surface tension of about 30 dyn/cm, and fluorated alkyl compound (hereinafter referred to as "Rf compound") containing CF3 and CF2 groups within the molecule has the critical surface tension of less than 20dyn/cm, thus effectively repelling water, polar solvent or even oil.

Hitherto, the fluoroalkylsilane-based water-repellent agent has been reported as having the best water-repellent property among the conventional water-repellent agents since it has an arnple amount of CF3 or CF2, a substance providing the lowest surface energy. Further, the corresponding linear structure is quite advantageous in increasing the density of water-repellent group.

Meanwhile, the water-repellent agent, providing the water repellent property to glass, consists of a molecular site with tendency to induce a strong siloxane bond (Si-O Si) by reacting with silanol (OH) group at the glass surface and the Rf group with fluorocarbon group providing hydrophobic property at the other side contacting air.

With such structural characteristics, various methods of providing water-repellent property on the glass surface using the Rf compounds have been developed as the most practical approach. However, in the case where these fluorine compounds are applied to soda lime glass, the dissolution of alkali substance within glass is responsible for degradating the water-repellent property with the time of usage.

Therefore, it is necessary that a fine and firm silica film should be coated prior to the water-repellent film in conjunction with inhibiting the dissolution of alkali substance within the glass. In particular, in the case of the automobile glass which is vulnerable to the external environment due to ceaseless surface friction and contamination, its water-repellent capability from the direct treatment of water-repellent agent is insufficient. Thus, additional factors for durability should also be considered.

Durability as applied here means the resistance from abrasion, scratch, rain, wind, chemical substances, sun rays, ultraviolet and temperature, etc. Namely, such durability allows the continuation of the said water-repellent property for a long period of time for the enjoyment of such functional benefits of the water-repellent glass.

In the past, many inventors have extended their intensive studies to improve the durability of water-repellent glass in various aspects. Japanese Unexamined Patent Hei No. 4-325446 and Hei No. 5-24885 disclose the methods of enhancing the durability in such a manner to form fine pores with a silica layer by mixing polyethylene glycol and triethylene glycol monoethylene ether when forming the silica layer. Further, the Japanese Unexamined Patent Hei No. 4-1 24047 and Hei No. 6-116430 disclose the method of forming at least one transparent metal oxide layer at the glass surface and the fine coarse surface thereafter via wet etching or plasma discharge etching, and then finally forming the water-repellent layer on the top of thereto.

However, the fine pores within the silica layer formed by the methods of the Japanese Unexamined Patent Hei No. 4-325446 and Hei No. 5-24885 are not stable during the thermal treatment process at temperature of more than 400 C , thereby unduly restricting the thermal treatment process. Furthermore, the methods disclosed by the Japanese Unexamined Patent Hei No. 4-124047 and Hei No. 6-116430 have the disadvantage of requiring an additional etching process after the formation of the silica layer.

Meanwhile, the Japanese Unexamined Patent Hei No. 5-147976 disclose the manufacturing method of the metal oxides, with an ample arnount of fine coarse surface via sol-gel method, which is prepared from and obtained by mixing two sols having different molecular weights, metalalkoxide or methylacetylacetonate.

Nevertheless, such methods have disadvantages and require improvement in that a) its durability effect is not satisfactory, and b) the processes are complicated so that there is little economical benefits.

SUMMARY OF THE INVENTION The objective of the invention herein is to provide water repellent glass with superior durability including abrasive resistance, which will solve the above mentioned problems. In the process of forming the silica layer on a glass substrate, the fine granular silica crystal particles are evenly distributed within such layer. Then, the water-repellent layer is formed on the top thereto. Henceforth, in a more economical and efficient manner, such film which is applied to the glass herein effectively prevents deterioration of functional aspects and deformation of the external appearance.

Brief Description of the Drawings Fig. 1 is the diagram showing the sectional structure of the durable waterrepellent glass according to this invention.

Fig. 2 is the photograph of transmissi" electron microscopy (1 x 1Cs times) showing the cross section of the silica layer formed after coating with silica solution on a glass substrate during the process of producing the durable water-repellent glass of this invention.

Detailed Description of Preferred Embodiments This invention is explained in more detail as set forth hereunder by way of accompanying drawings.

The durable water-repellent glass of this invention is characterized in that silica layer 2, by applying an aged solution of silane compound, is formed on a glass substrate 1. Particularly, an ample amount of fine silica crystal particles 4 are evenly distributed within the silica layer 2, in relationship to durability of the silica layer 2 and the water-repellent layer 3.

This invention may be explained in detail as set forth hereunder.

As illustrated in the cross-sectional diagram of Fig. 1, the invention herein relates to water-repellent glass, of which the silica layer 2 is formed on the glass substrate I for the purposes of providing durability to the same. On top of thereto, fluoroallcyisilane (FAS) is applied, forming the water-repellent layer 3. Then, in order to enhance the durability including abrasion resistance of the silica layer 2 and water-repellent layer 3, the fine granular silica crystal particles 4 are evenly distributed within the silica layer 2.

According to this invention, the formation of the silica crystal particle 4 is due to the fact that the aging process is conducted in the presence of basic and acidic catalysts during the preparing of the silica sol. Consequently, an ample amount of fine silica crystal particles 4 are generated within the said layer 2 so that the mechanical property and abrasion resistance of the glass are enhanced. Hence, the silica crystal particles, so formed are of granular type, and some granular particles whose inner part are hollow may be formed.

The silica layer 2 is formed by silica solution, being subjected to an aging process, and through the thermal treatment of the substrate, the surface coarseness is also formed on the silica layer 2.

The process for preparing the water-repellent glass of this invention is explained in more detail as set forth hereunder.

The process for preparing the water-repellent glass having the durable silica layer consisting of silica compound on the glass substrate is as follows: As for said silica compound, the metal alkoxide-based silane compound is subjected to the aging process for the manufacture of silica solution where two polymers are cross-linked. Then, the surface of the glass substrate is coated with this solution, after which the substrate undergoes thermal treatment for the formation of the silica layer having the silica crystal particle therein. Thereafter, the top surface of such substrate is coated with the water repellent agent for the formation of the water repellent layer.

According to this invention, in order that the silica sol solution for preparing the silica layer may have the cross-linking structure where granular colloid silica solution and linear polysiloxane solution are mixed, the silica sol solution is subjected to the first aging process in the presence of basic catalyst and then is subjected to the second aging process in the presence of acidic catalyst. Such procedure is conducted to provide surface coarseness to the silica layer and to distribute fine silica crystal particles in the film after the thermal treatment. Henceforth, the silica solution is formed by adding a solvent (e.g., ethanol) to tetraethoxysilane (TEOS). Further, in order to facilitate the hydrolysis, distilled water (H20) is used. Ammonia water (NH40H) may be selected as a basic catalyst used for the first aging process, and hydrochloric acid (HC1) may be selected as a acidic catalyst used for the second aging.

Further, according to this invention, a thermal curing process should be performed for the preparation of the silica layer using the present aging silica solution. It is preferred that a small amount of inorganic salt be added to the silica solution for the purposes of lowering the thermal curing temperature. It is advantageous to use the inorganic salt in aqueous solution in mixing. Also, it is preferred to use such inorganic salt in the amount of 0.1-3.0 vt% to water. The inorganic salts used for this reaction include Narc1, NH4Cl, KNO3, NaNO3, or CH3COONa, but it is particularly preferred to use KNOW.

According to this invention, the first aging process of the silica solution is performed in such a manner that tetraethoxysilane is added to the solvent for mixing and stirring at room temperature. With the addition of basic catalyst, the reacting solution is stirred until the pH is more than 9.0. Then, the first aging is carried out at room temperature-80 C for 12-24 hours. If the first-step aging time is longer than 24 hours, the poor water-repellent durability results therefrom due to the fact that the excessive development of granular particles (granular silica crystal particles) prevents the formation of the required structural density. Meanwhile, if the amount of basic catalyst used for the first aging is more than 1 wt% in proportion to the silica solution, opaqueness in coating may occur. Thus, it is preferred to use less than 1 wt% of basic catalyst. By hydrolysis and polycondensation associated with the first aging in the presence of basic catalyst, the colloidal silica which is granular silica polymer is formed by the nucleophilic condition of Si. In the process of such reaction, the present aging is performed with the addition of acidic catalyst for more than 2 hours until the pH becomes approximately 1.2-2.7. Then, unhydrolyed Si(OR)4 is subjected to hydrolysis and polymerization, thus effectuating the shape of polysiloxane which is linear polymer by the electrophilic attack. Ccnsequently, the sol formed via the present aging process will have the crosslinking form where both granular and linear polymers coexist. Based on such morphological characteristics, the aging silica solution is coated on the glass substrate and thermally treated. Then, the coarseness at the glass surface may be naturally formed without any separate process.

According to this invention, the coating with the silica solution may be made available by the methods of dipping and spray. In addition to these methods. some well-known coating methods may also be applied. If the dipping method is used. the soda lime glass of 2.5 x 7cm in proper size is immersed in silica sol solution, being subjected to the present aging process, so as to naturally form the coating film with slow raising.

Hence, the raising speed is in the range of 10-30 cm per minute. if the speed is fast, the increased thickness of the coating film results, which is responsible for inhomogeneous film and poor glass transparency. However, if such raising speed is too slow, the extremely thin film cannot have sufficient durability.

Further, according to the spray-coating method, the silica sol is injected into the soda lime glass substrate of 30 x 30 cm in size. Special caution in injection should be exercised. The thickness of the coating film increases if too much of a volume is injected and the distance between nozzle and specimen is too close. Under the spray method for coating the total specimen area, the nozzle is first moved from the left to right, and then the nozzle is lowered by approximately ; cm, after which the nozzle is moved from the right to left and so on. However, in the process of moving the nozzle, some area of the specimen are duplicated.

When the silica solution is applied at the glass surface under the procedure as above, it is preferred that the thickness of the coating film be in the range of 6O3-1500A, more preferably in the range of 600-1000A. If the coating film is extremely thin, there will be insufficient durability improvement. Further, if it is extremely thick, the film will be inhomogeneous and poor in quality.

Meanwhile, the glass coated with said silica sol solution undergoes thermal treatment at the temperature of 180-550 , preferably at the temperature of 1 803 50 'C for more than 30 minutes. If the thermal treatment temperature is higher than 550 'C.

this does not affect the water-repellent property regarding the change in the aging time in the presence of basic catalyst. However, in the case of the aging time in the presence of acidic catalyst, there will be a significant reduction in the water-repellent power after 3 days from such production. Also, if the thermal treatment temperature is lower than 180 'C, the density and hardness of the film will deteriorate. Meanwhile, according to this invention, when the silica sol, being subjected to the one-day aging process with the addition of NH4OH as basic catalyst and the 3-day aging process with the addition of HC1 as acidic catalyst, undergoes thermal treatment at approximately 200-300C, its water-repellent durability proves to be superior to that produced at a higher temperature.

Through the above thermal treatment, said silica forms the siloxane cross-linking layer of strong Si-O-Si bonds to the glass.

Further, Fig. 2 is a photograph showing the cross section of the silica layer by transmission electron microscopy, formed on the surface of the glass aforementioned.

Referring to Fig. 2, it is ascertained that the film thickness of the silica layer is in the range of 600-1500A depending on the solution and coating condition, and fine granular silica crystal particles are evenly distributed within the silica layer. It is ascertained that the particles of Fig. 2 are silica crystal particles when analyzed by X-ray diffraction pattern. The diameter of such particles is in the range of 100-;00A as granular type with the hollow or filled inner part. In general, it appears that they are in the silica matrix phase, amorphous and in chemically coherent state.

According to this invention, if such firm silica crystal particles, which are coherently bordered with the silica matrix, are diffused as shown in Fig. 2, the induced effect such as combined material thereto may enhance the mechanical property and durability of the film including the resistance on abrasion, in particular. Since the production of such particles is derived from the chemical and thermodynamic reaction.

this may be achieved via the manufacture of the silica sol solution based on the two-step aging process of this invention.

Meanwhile, some commonly available water-repellent solution may be used for coating the silica layer formed on the surface of the glass substrate according to this invention. For example, fluoromethoxysilane (CF;(CF2),CH2CH2Si(OCH3)3), alcohols, hydrochloric acid (HC1) and distilled water are blended in a certain chemical ratio and are under hydrolysis for a certain time prior to use. Hence, HC1 or similar acid is used for the catalyst to facilitate the hydrolysis, and ethyl alcohol is added as solvent. The reason why the water-repellent agent is under hydrolysis and polycondensation is to maximize the siloxane reaction (Si-O-Si) by facilitating the reaction between the Rf group and silanol group (OH) at the surface of the silica layer.

According to this invention, some Rf compounds used for the water-repellent agent solution include CF;(CF2)7CH2CH2SiCH;(C1)2, CF3CH2CH2SiCl and CF;CH2CH2Si(OC2H5)3) in addition to fluoromethylsilane (CF;(CF2)7CH2CH2Si(OCH3);). Further, nitric acid and acetic acid can be used as catalyst in addition to hydrochloric acid.

The water-repellent coating, based on the water-repellent agent solution, may be performed under the same condition as the coating for formation of the silica layer.

Then, if the glass substrate coated with water-repellent agent is heated and dried at 1 20l 70 C for more than 30 minutes, the water-repellent layer will thus be formed.

This invention is explained based on the following examples in more detail.

Example 1 (Preparation of Water-repellent Glass Based on the Present Aging Process of Silica Solution and Dip-coating Method) A. Synthesis of silica layer-forming solution A mixture of 52g of tetraethoxysilane (TEOS) and 448g of ethanol was stirred for 30 minutes, and with the addition of 2.5g of NH4OH, the reacting solution was further stirred for 1 hour. Then, 16g of 1 wt% KNO3 aqueous solution was added to the reacting solution, stirred for 2 hours, tightly closed and subjected to aging at a thermostatic bath of 30 'C for 24 hours.

Sg of hydrochloric acid was added to the solution, stirred for 2 hours. Then, the synthesis of silica layer-forming solution was completed.

B. Preparation of silica layer A soda lime glass was cut by 2.5 x 7 cm (thickness: 0.1cm) and was subjected to the first washing using a surfactant. After dipping the glass in distilled water, it was subjected to the second washing for 15 minutes using the ultrasonic cleaner. Then, the glass was subjected to the third washing using acetone and dried with a drver at 120 C.

The specimen was immersed in the silica layer-forming silica sol solution for 30 seconds using the motor driven dip coater and increasing the speed at a rate of 11 cm/min. The sample was under thermal treatment at 300 C with the increasing temperature rate of 7 "C /min.

C. Preparation of water-repellent layer The water-repellent coating was made on the glass substrate with the silica layer. Hence, fluoromethylsilane (CF3(CF2)7CH2CH2Si(OCH3)3), ethanol hydrochloric acid and distilled water as water-repellent solution were mixed in respective ratio of dug iSOg, ig and 0.3g. Then, the mixture was stirred at room temperature for 2 hours and subjected to aging for 1 day. The sample having the silica layer was coated with the water repellent agent under the same condition as above using the dip coater. The sample coated with said water-repellent agent was heated and dried at 150 C for 1 hour for the formation of the water-repellent layer.

D. Assessment and analysis of various physical properties The contact angle on the glass sample formed with the above silica layer and water-repellent layer was measured, together with abrasion/alkali resistance, anti-acidity and heat-resisting property. The contact angle was measured by the method of sessile drop using the contact angle instrument (model CA-X, Kyowa Interface Science Co.

Ltd.). After measuring the contact angle 5 times at different places, the mean value was calculated.

The anti-abrasiveness was measured in such a rnanner that the wiper blade of automobile was cut in 1 cm size in length. With a load of 300 g/cm given to the fragment, it was subjected to double-trip tests 5000 times at a double-trip rate of about 2 seconds and then, the contact angle was measured for assessment.

The alkali-resistance test was performed in such manner that the specimen was quenched in 1N NaOH solution for 6 hours and taken out for the measurement of the contact angle. The anti-acidity test was performed in such manner that the specimen was dipped in 1N HCl solution for 6 hours and taken out for the measurement of the contact angle.

The heat resistance property was assessed in such a manner that the specimen was dipped in boiling water for 2 hours and taken out for the measurement of the contact angle via the reduction of the contact angle.

With reference to the abrasion resistance and visual haze, the visual haze (KS L 2007 standard test method) was assessed by Hazemeter (BYK Gardner, Germany) after rotating the sample 100-500 times using the Taber abrasion device (5150 Taber Abraser, USA).

Meanwhile, with reference to the surface coarseness of the coated glass, its Ra (centerline average, centerline mean coarseness) value, a criterion of the surface shape and surface coarseness, was calculated using the Atomic Force Microscopy (AFM)(DI 3000, USA).

Further, the Transmission Electron Microscopy was used for the observation of the coated cross section.

The above measurements and observed results are shown in the following table 1 and 2.

Example 2 (Preparation of Water-repellent Glass Based on the Present Aging Process of Silica Solution and Spray-coating Method) The manufacture and assessment of silica sol solution, being subjected to the second-aging designed for the formation of a silica layer, was made available in the same procedure as described in the example 1.

In an effort to make a coating on the silica sol solution which has undergone the present aging process, the glass substrate having 30 x 30 cm in size was washed. The distance between the spray nozzle and specimen was 15 cm away, and the moving speed of nozzle was 40 cm/sec. The coating was made at room temperature, and the air pressure and solution pressure for injection were adjusted by 0.7 bar and 0.5 bar.

respectively. The spray coating for the total specimen area was performed in the following manner: After the nozzle was moved at 40 cm/sec from the upper left part of specimen to right. it was moved downward by 3 cm. Thereafter, the nozzle was moved from right to left at the same speed as above, and it was again moved doxnward by 3 cm, and so forth. Then, the spray-coated glass was under thermal treatment at 300 C for 30 minutes at the increasing temperature rate of 7'C/min. The thickness of the silica layer formed by the above process is 850 A.

By the same procedure as described in the example 1, the silica layer-formed sample was coated with the water-repellent agent for the formation of the waterrepellent layer.

The assessment of various physical properties relating to the water-repellent glass, so prepared, is shown in the following table 1 and 2.

Comparative Example 1-2: Preparation of Water-repellent Glass Using Polysiloxane-structure Silica Sol A mixture of TEOS, ethanol, distilled water and catalytic hydrochloric acid in respective blending ratio of 72.8g, 601.9g, 25.2g and 0.14g was added to the silicaforming solution and stirred at room temperature for 3 hours. The reacting solution was again subjected to the aging process at room temperature for 4 days to obtain polysiloxane-structure silica sol solution. The water-repellent glass, so prepared by such method, was assessed by the same procedure as the example 1. Hence, each silica layerformed specimen was prepared by dip coating method (c:jmparative example 1) and spray coating method (comparative example 2). The formation of the water-repellent layer was also made available by the same procedure as the example 1.

The assessment of various physical properties relating to the water-repellent glass so prepared, is shown in the following table 1 and 2.

Table 1. Comparison of Initial Contact Angle, Water-repellent Durability and Fundamental Properties

Classification Example Comparative Example 1 2 1 2 Appearance Excellent Excellent Excellent Excellent Fine Particles Presence Presence Absence Absence Surface Coarseness (Ra, A) 42 110 15 19 Initial Contact Angle (0) 122 123 106 105 Anti-abrasiveness (") 108 109 98 97 (Wiper Blade Method) Acid Resistance ( ) 118 119 102 102 Alkali Resistance (") 109 108 97 96 Heat Resistance (" ) 119 118 105 103 As revealed in the above table 1, no difflised crystal particles in silica layer was observed from the comparative example 1 and 2. Table 2. Assessment of Contact Angle and Visual Haze after TABER Abrasion Test

Classification Assessment of Contact angle and Haze after Taber Abrasion Test (100 - 500 times) 100 Times | 300 Times | 500 Times Contact Haze Contact Haze Contact Haze Angle ( ) (dH %) Angle ( ) (dH %) Angle The above table 2 shows the measurement results of the contact angle and visual haze value after the Taber abrasion test. Both example 1 and 2 maintained the waterrepellent power of more than 90 by 500 times, while the simple silica film had the contact angle of less than 90t by 300 times, but its contact angle was reduced at less than 80 by 500 times. Further, from the measurement of visual haze, all samples showed excellent water-repellent power having less than 2.0% of dH. In particular, less than 1.0% from both example 1 and 2 was observed in the change of haze value by 500 times. Further, since the change of haze on Taber abrasion was negligible, the durability of both example 1 and 2 were greatly improved.

As aforementioned, according to the invention herein, various glass substrates, such as safety glass for automobile, building glass, and mirror, for the purposes of giving durability, are coated in the presence of basic and acidic catalysts with silica solution which has been subjected to the present aging process. As a result therefrom, a coarse surface silica sub-layer having micro-crystal silica particles is formed on the glass substrate. On top of such surface, fluoroalkylsilane (FAS) is applied, forming such water repellent layer. The water repellent glass so prepared under the invention herein has excellent water-repellent property and better durability including abrasionresistance than that prepared under the conventional methods. Its long-term use does not induce deformation in appearance nor deterioration in water repellent property.

In particular, said process of manufacturing the water-repellent glass proves to be quite effective for providing superior durability to the glass in a simple and economic manner than the conventional method. The silica layer, formed by means of the silica solution which has undergone the present aging process in the presence of basic and acidic catalysts for the purpose of providing durability, has naturally an ample amount of micro crystal silica particles with the formation of the surface coarseness.

As set forth above, the invention herein relates to water repellent glass having superior durability and water repellent property, and the process for manufacturing the same, in which long-term use does not induce deformation in appearance and deterioration in water repellent property.

Claims (10)

1. A durable water-repellent glass comprising a silica layer and a water repellent layer, wherein fine silica crystal particles are evenly distributed within the silica layer.

2. A durable water-repellent glass according to claim 1, wherein the thickness of said silica layer is 600-1000 .

3. A durable water-repellent glass according to claim 1 or 2, wherein said silica layer is formed by applying of an aged solution of a metal alkoxidebased silane compound, which is subjected to a first aging process under basic condition, and then is further subjected to a second aging under acidic condition, the solution of which is coated onto a glass substrate, followed by thermal treatment.

4. A durable water-repellent glass according to any of claims 1 to 3, wherein the size of said silica crystal particle is 100-300A in diameter.

5. A process of producing a water repellent glass having an improved durability which comprises preparing a coating solution of a metal alkoxidebased silane compound aged in the presence of basic and acidic catalysts, applying the solution to a glass substrate so as to form thereon silica film, heating the glass substrate so as to transform the silica film into a silica layer having fine silica crystal particles evenly distributed therein, and applying water repellent agent to the silica layer so as to form water repellent layer.

6. A process according to claim 5, wherein the aging process of said metal alkoxide-based silane compound proceeds in such a manner that tetraethoxysilane is hydrolyzed under basic condition to form a granular polymer of colloidal silica, and then polycondensation is performed under acidic condition to form linear polymer of polysiloxane, thereby effecting cross-linking of the two polymers.

7. A process according to claim 5 or 6, wherein the aging process of said metal alkoxide-based silane compound is performed under basic condition of more than pH 9, and the polycondensation is performed under acidic condition in the range of pH 1.2-2.7.

8. A process according to any of claims 5 to 7, wherein said metal alkoxide-based silane compound has one or more inorganic salts selected from NaCI, NH4Cl, KNO3, NaNO3 and CH3COONa.

9. A durable water-repellent glass, substantially as described herein with reference to and as illustrated in Figure 1 of the accompanying drawings.

10. A process of producing a water-repellent glass, substantially as described herein in either non-comparative example.