Method and apparatus for structuring a vitreous surface

Field of the invention

The present invention generally relates to a method in accordance with claim 1 for structuring a vitreous surface and, more specifically, to a method for structuring a vitreous surface in a controlled manner so as to create a structure on said surface allowing the modification of the functionality of the vitreous surface.

Background of the invention

Surface structure affects the properties of many products. A well-known example is the Lotus surface wherein a hydrophobic surface changes into a superhydrophobic surface because the surface contains asperities spaced at 20 to 40 micrometres and, additionally, features wax crystals 200 to 2000 nanometres in size. More generally, an originally hydrophilic or hydrophobic surface can be converted into a superhydrophilic or superhydrophobic surface by modifying its micro/nanostructure. A superhydrophilic and/or superhydrophobic surface is of economic importance in the manufacture of easy-to-clean or self-cleaning surfaces.

Patent publication US 3,354,022, 21 November 1967, E.I. du Pont de Nemours and Company, discloses a water-repellent surface in which the ability to repel water is based on asperities present in the surface spaced at a maximum of 1000 micrometres with the minimum height of said asperities being at least half of the average distance between the individual asperities.

Patent Publication US 6,81 1 ,856 B2, 2 November 2004, Creavis Gesellschaft fur Technologie und Innovation GmbH, discloses a self-cleaning surface with a hydrophobic surface structure achieved through elevations and depressions in the surface by applying particles to it. The size of the particles is 20 to 500 nm.

Self-cleaning surfaces are of great economic importance, for example in windows. Patent Publication US 6,997,018 B2, 14 February 2006, Ferro Corporation, discloses a method for

structuring a vitreous surface on the micro- and nano-scale. The method is based on the application of 400 nm inorganic particles to the surface of a glass article at a temperature of 700 ⁰ C to 1200 ⁰ C.

Also of great economic importance is the impact of the micro/nanostructure on the refractive index of the surface; for example, the structure may be used for achieving a gradientally variable refractive index in order to create an antireflective surface. A description of a combined antireflective and hydrophobic surface is provided in Nanotechnology 18, 2007, Y.C. Chang et al., "Design and fabrication of a nanostructured surface combining antireflective and enhanced-hydrophobic effects", pp. 1 - 6.

The surface can be structured to provide a regular or irregular, or random, surface. For all practical purposes, randomly structured surfaces are of greater interest because it is easier to manufacture them. Micro- and nanostructured surfaces have been prepared using a number of methods, such as lithography, etching, microstamping, chemical vapour deposition (CVD) and physical vapour deposition (PVD).

Patent Publication US 6,309,798 Bl , 30 October 2001,Studiengesellschaft Kohle mbH, discloses a lithographic method for nanostructuring a surface. As the lithographic method involves multiple surface preparation phases, it is not an affordable method for structuring large surfaces.

Patent Publication US 6,468,916 B2, 22 October 2002, Samsung SDl Co., Ltd., discloses a method for creating a nanoscale surface structure. The method involves several phases: creation of the microstructure, deposition of a carbon-polymer layer on the microstructured substrate, the first plasma-etching process, deposition of a mask layer followed by a second reactive etching process. This method, too, involves multiple surface preparation phases and does not, therefore, provide an affordable method for structuring large surfaces.

The method for structuring a vitreous surface disclosed in patent publication US 6,997,018 is suitable for structuring large glass surfaces but its application is limited to structuring vitreous surfaces of glass articles within a temperature range of 700 ⁰ C to 1200 ⁰ C. Therefore, on a float glass

production line, the structuring of the glass surface can only be carried out in a tin bath. The integration of a coating apparatus with the tin bath is expensive and complicated.

A method known in prior art is to produce the hydrophobic coating on glass by depositing a layer of inorganic oxides on the glass surface (by means of silk screening, spraying or other similar method) after which said layer is heat-bonded to the glass surface at a temperature of 300 ⁰ C to 700 ⁰ C. The method is not suitable for the manufacture of glass (float process).

Patent publication WO 20071 10482 Al discloses a method in which nano-sized particles (1 to 1000 nm) are deposited on softened glass which may have a temperature below 700 ⁰ C. The particles applied to the surface are diffused inside the surface, at least partly. The glass surface can be heated with a flame, etc., and the nano-sized particles are produced in the flame via gas-to-particle conversion. The method disclosed in the publication is characterized by having the nano particles deposited on the surface diffused in the glass material as efficiently as possible. Typical of this method is that the cohesion energy of the nanoparticles is reduced after they have been produced and/or an energy flow is directed to the glass surface in order to intensify the diffusion process. The glass surface temperature may be between 500 ⁰ C and 1000 ⁰ C. The glass surface can be heated after the nanoparticles have been applied to the glass surface.

Patent publication US 6,221,175 Bl discloses a method for depositing a thick ceramic layer

(minimum thickness 100 micrometres) on a metallic substrate. The method is based on the spray- application of a powdery ceramic coating to a molten metallic material in a way that ensures that said coating material is also melted. Consequently, none of the coating particles are exposed in the coating so produced and do not, thus, affect the surface structure.

The problem with prior art technology is that it provides no method by which structuring of a vitreous surface could be integrated in the glass production or processing process, such as the float or casting process, where the temperature could be below 700 ⁰ C.

Summary of the invention

The object of the present invention is to provide a method for eliminating the problem associated with prior art.

This is achieved with the method in accordance with claim 1 in the characterizing part wherein a glass article or an article containing a vitreous surface is heated, at least at some point of its area, up to temperature of over 500 ⁰ C but not exceeding 700 ⁰ C. An essential feature of the invention is that it is the surface of the glass, not the body of glass article across its full thickness, that is heated. Particles with an average diameter of 10 to 1000 nanometres are applied to the heated site. The particles adhere to the heated, soft glass surface. Because only the surface layer of the glass is hot, the thermal energy contained in the glass is not high enough to cause the particles or their material adhering to the surface layer to be essentially diffused in the glass. As a result, the particles remain on the glass surface modifying the surface structure. Because only the surface layer of the glass is heated, deformation of the glass and optical errors can be avoided during heating. Preferably, the composition of the particles is such that the particles do not melt into the glass matrix.

Consequently, the particles may consist of aluminium oxide, titanium oxide, zirconium oxide or silicon oxide; the material of the particles may also be determined by the particles applied to the surface, so that the larger the particles are, the lower their melting point can be.

Microparticles, especially particles with an average size of less than 1000 nanometres, are difficult to apply to a glass surface because particles of this size tend to be carried by gas flows. The application of the particles to the surface can be improved by using an impactor with a Stokes number (Stk) high enough to ensure that the efficiency in collecting particles to the site involved is greater than 10%.

Generally, impaction nozzles operate such that the velocity of particles passing through the gap in the impaction nozzle increases according to the properties of the particles and the dimensions and properties of the nozzle gap, meaning that a certain amount of kinetic energy is imparted to the particles when they pass through the nozzle gap. When a substrate is placed downstream of the nozzle to receive the stream of particles from the impaction nozzle, the particles with sufficient

kinetic energy will strike the substrate surface whereas particles with insufficient kinetic energy will pass by the substrate surface without striking it. Accordingly, particles larger than the specific size D are imparted enough kinetic energy by the impaction nozzle to strike the substrate whereas particles smaller than said size D fail to do so.

The collection efficiency E of the impactor depends on the Stokes number (Stk) that is determined for a rectangular impactor nozzle gap as follows:

τU _ ptfUC.

H 12 9ηH

where ris relaxation time, f/ is gas velocity, H is the width of the impaction nozzle gap, p _p is particle density, d _p is the particle diameter, C _c is the Cunningham correction factor and η is viscosity. If the particles are not round, the particle size in formula 1 must be adjusted with the dynamic size factor χ that typically varies between 1 and 2. The impactor's collection efficiency is a highly sensitive function of the Stokes number, so that collection efficiency increases from almost zero to nearly 100% in response to a minute change in the Stokes number (William C. Hinds, Aerosol Technology, Properties, Behavior and Measurement of Airborne Particles, 2nd Edition, John Wiley & Sons, Inc., New York, 1999). A 10% collection efficiency represents a typical case of a threshold where, for example, a small increase in gas velocity through the impaction nozzle can increase the Stokes number in such a way that collection efficiency jumps from close to zero to almost 100%, and so impaction becomes the most important mechanism by which small particles are collected on a glass surface.

Thus, the method provides a way of producing a structured surface on a glass article with a temperature of less than 700 ⁰ C, and so structuring can be carried out on the float line outside of the tin bath, which essentially affects the exploitation of the method for economic purposes.

Brief description of the figures

Figure 1 is a schematic of the method and apparatus in accordance with the method integrated into a float glass production line.

Figure 2 illustrates a glass surface produced in accordance with the method in which the surface structure of the glass is achieved with titanium oxide (Figure B). A non-structured surface is also depicted for comparison (A).

A more detailed description of the invention is provided with reference to the drawings.

Detailed description of the invention

Structuring imparts the glass surface a number of preferred properties. A micro- and/or nanostructured surface improves the hydrophilicity or hydrophobicity of glass. An antireflective surface can be achieved by modifying the effective refractive index of the surface structure of glass. A structured surface improves the adhesion of surface layers, such as coatings applied by means of chemical vapour deposition (CVD) or physical vapour deposition (PVD), to a glass surface.

Figure 1 illustrates one embodiment of the method in accordance with the invention, i.e., an apparatus for structuring glass surfaces integrated into a float glass manufacturing process. Glass sheet 2, typically 1 to 4 metres wide and moving at a speed of 1 to 20 m/min, rises from the tin bath 3 on top of conveyor rollers 4. The temperature of the glass sheet 2 emerging from the tin bath 3 is typically 59O ⁰ C to 63O ⁰ C. The glass plate moves on to the heating section 5 of the apparatus in accordance with the invention. The heating section 5 may also be fully integrated into the apparatus 9 producing the structuring particles, but for the sake clarity it is here illustrated as a separate section. A flame 8 is generated in the heating section 5 by means of combustion gas 6 and oxidizing gas 7. Combustion gas 6 may consist of hydrogen, methane, ethane, propane, butane, carbon monoxide or other equivalent combustion gas. The oxidizing gas 7 may consist of air, oxygen, ozone or other equivalent gas providing the oxygen required for producing the flame 8. The purpose of the heating section 5 is to heat the topside 16 of the glass sheet 2 so that the underside 17 of the

glass does not reach a temperature in exceeding 700 ⁰ C. The flame 8 heats the glass mostly by convection. In order to ensure efficient heating by convection, it is preferable that the flame 8 is of the high-speed type, meaning that the combustion gas is preferably hydrogen, the oxidizing gas is oxygen, and the hydrogen and oxygen are supplied via lines 6 and 7 to the heating section 5 at a stoichiometric ratio. Typically, the speed of the hydrogen/oxygen flame is 20 to 80 m/s. The thermal conductivity of the glass sheet 2 is relatively poor, and typically, depending on the thickness of the glass, it takes 0.5 to 2 seconds to increase the temperature of the underside of the glass from 600 ⁰ C to 700 ⁰ C at a constant burner output. In the method in accordance with the invention, the length L of the heating section 5 is selected in such a way that the temperature of the underside 17 of the glass sheet always remains below 700 ⁰ C. For example, the temperature of the underside 17 of a 4 mm thick borosilicate glass sheet moving at a speed of 15 m/min rises to 700°C in 0.5 seconds if the temperature of the glass sheet emerging from the tin bath is 610 ⁰ C and the total calorific value of the gases in the heating section 5 is 3 MW/m ² . Then, the maximum length L of the heating section can be 0.8 metres. Because the temperature of the topside 16 of the glass sheet increases from 600 ⁰ C to over 1000°C more quickly than the temperature of the underside 17 of the glass sheet increases to 700 ⁰ C, the temperature of the topside 16 of the glass sheet can be successfully raised high enough in the heating section to ensure efficient adhesion of the micro- and nanoparticles.

The heating effect of the heating section 5 can be adjusted, for example by replacing hydrogen with methane as the combustion gas 6, which may be advisable for economic reasons as well, and/or by replacing oxygen with air as the oxidizing gas 7. Additionally, by adjusting the height of the heating unit 5 relative to the top side 16 of the glass sheet, it is possible to essentially change the convective heating power. Usually the highest convective heating power is achieved when the heating unit 5 is close to the topside 16 of the glass sheet, typically so that the flame impinges on the glass surface. Preferably, the flame height is 5 to 20 mm and the distance of the face 18 of the heating unit 5 from the topside 16 of the glass sheet is 10 to 50 mm.

Downstream of the heating section 5, the glass sheet 2, whose topside 16 is at a temperature of over 700 ⁰ C and underside 17 at a temperature of under 700 ⁰ C, moves on to the section 9 that produces the structured surface. The flame 13 is generated in the structuring section 9 by means of the combustion gas 10 and oxidizing gas 1 1. The combustion gas 10 can be the same or other than the

combustion gas 6 and the oxidizing gas 1 1 can be the same or other than the oxidizing gas 7. The flame 13 can be laminar or turbulent. Additionally, the precursor 12 for the micro- and nanoparticles, possibly consisting of more than one component, is supplied to the structuring section 9 in a gaseous, vaporised or liquid state or in a combination of these states. ■

The precursors 12 are selected and forced to react in the flame 13 in different ways depending on the size of the particles 14 that are to be produced. The nano-sized particles are produced via gas-to- particle conversion in such a way that the precursor 12 is vaporised and thermally decomposed in the flame 13. The molecules in the flame 13 collide with and adhere to one another. As a result of this, nuclei are generated around which material starts condensing, ultimately producing the nanoparticles 14. A similar mechanism of generation is at work with precursors 12 with a high boiling point because the vapour pressure of such precursors 12 is high due to the high temperature of the flame 13. One such precursor 12 is titanium.

Preferably, microparticles are generated in the flame 13 colder than the flame in which nanoparticles are generated and their precursor 12 is a liquid in which the salt of the preferred metal is dissolved, said liquid being atomized in the structuring section 9 to form the droplets that are introduced to the flame 13. In the flame 13 the solvent is vaporised while the remaining solids are thermally decomposed. As a result, the particles 14 remain large in size, being typically of micron size. Such particles are known as residual particles.

The precursor 12 may consist of a mixture containing simultaneously components generating both nanoparticles and microparticles. The apparatus 1 can also consist of several structuring units 9 placed one after the other of which the first unit is used for producing microparticles and the second nanoparticles.

The glass sheet 2 with the micro-/nanostructured surface 15 on the topside moves along the float line to the annealing kiln 19.

The embodiment shown in Figure 1 is just one of the potential embodiments of the invention. For instance, it is possible to create a micro- and/or nanostructure on a glazed ceramic tile in such a way

that the topside of the glazing is heated up to a temperature of over 700 ⁰ C using the method in accordance with the invention while the temperature of the ceramic tile itself remains under 700 ⁰ C, and micro- and nanoparticles are produced to structure the surface of the glazing using the method in accordance with the invention. Figure 2 shows the surface of a ceramic tile produced in this manner; in this surface, the nanostructure is produced by using nano-sized titanium oxide which, at the same time, produces a superhydrophilic surface on the tile.

If the objective is to have efficient adhesion of particles smaller than 1000 nm in diameter to the glass surface, the structuring section 9 can be accomplished by ensuring a high degree of particle impaction to the glass surface. Then, the structuring section is manufactured in such a way that the particles are accelerated in a narrow nozzle gap to achieve a Stokes number higher than 0.3. With such a configuration, the speed of the stream of particles in said narrow gap is typically 20 to 200 m/s, the width of the gap 0.5 to mm and the distance of the gap from the glass surface 2 to 10 mm.

It is obvious to a person skilled in the art that the invention can have several embodiments. Consequently, the invention and its embodiments are not to be limited in scope to the embodiment described herein but can be varied within the scope of protection defined by the attached claims.