METHOD FOR BENDING GLASS PANELS

Description

[0001] The present invention relates to the field of curved laminated glass, particularly for applications as automotive glazing. It relates more specifically to a process for the simultaneous curving of superimposed glass sheets, comprising the use of a separating agent that generates gas release under the curving conditions.

[0002] In the production of curved laminated glass, such as for automobile windshields, it is often necessary to simultaneously curve the glass sheets that will later be assembled into laminated glazing. Specifically, the glass sheets are stacked one on top of the other and heated to a suitable temperature to achieve simultaneous and uniform deformation through a gravity, pressing, and / or vacuum bending process. The curved glass sheets must be separated before the lamination process. After separation, the lamination is then carried out by inserting a lamination interlayer, typically made of a polymer sheet such as polyvinyl butyral.

[0003] However, glass sheets can sometimes adhere to each other through surface fusion during the bending process. To prevent this, it is known to apply a release agent between the glass sheets before bending (as described, for example, in FR 2 263 201). Commonly used release agents include gypsum, talc, calcium carbonate, diatomaceous earth (kieselguhr), and mica. However, it has been observed that these release agents, which are generally powders, can cause pitting on the glass sheets, resulting in aesthetically detrimental optical defects in the final glazing. This pitting is attributed to the deformation of a glass sheet around particles of the release agent during the bending process.When the upper glass sheet rests on the particles of the separating agent and the glass becomes deformable (typically above 600 °C), the support of each particle can cause deformation in the upper and / or lower sheet.

[0004] The objective of the present invention is therefore to provide a method for the simultaneous bending of superimposed glass sheets (generally two glass sheets) that overcomes the drawbacks mentioned above. More specifically, the Applicant has observed that it is possible to significantly reduce, or even eliminate, pitting by using a separating agent that generates gas release under the bending conditions. Thus, the present invention relates to a method for the simultaneous bending of superimposed glass sheets comprising: the supply of a first sheet of glass; the application of a separating agent to a surface of the first sheet of glass; the supply of a second sheet of glass superimposed on the first sheet of glass, the separating agent being between the first sheet of glass and the second sheet of glass; the heating of the sheets of glass to a temperature between 590 and 670°C allowing them to bend; and the simultaneous bending of the sheets of glass; characterized in that the separating agent generates a gas release under bulging conditions.

[0005] Without wanting to be linked to any theory, it is assumed that the creation of a gaseous cushion between the two sheets of glass resulting from the gaseous release of the separating agent at the time of the bending has the effect of reducing or even canceling the pressure exerted by the particles of the separating agent on the surface of the sheets of glass.

[0006] The gaseous release results from the decomposition of the separating agent under bulging conditions. The term "bulging conditions" in relation to the separating agent, as used in the present invention, obviously refers to the bulging temperature reached by the glass sheets, in particular the maximum temperature, but also to the temperature change kinetics, in particular the temperature ramp, and the immediate atmospheric environment of the separating agent, which is found in a highly confined atmosphere between the two glass sheets. Thus, the gaseous release of the separating agent occurs at temperatures above 450 °C, even above 500 °C, or even above 550 °C, and can continue up to temperatures of 600 °C, 620 °C, or even 640 °C, or even 670 °C.It is obviously possible that the decomposition of the separating agent may cause gas release outside these temperature ranges. A portion, generally a small amount, of the gas release may occur, for example, below 450 °C and / or above 640 °C without affecting the effectiveness of the separating agent. The separating agent preferably exhibits a mass loss of at least 10%, or even at least 20%, or at least 30% by weight between 450 and 640 °C. The volume of gas released by the separating agent between 450 and 640 °C, preferably between 500 and 620 °C, is advantageously at least 0.05 L / g, or even at least 0.1 L / g or at least 0.2 g / L, and may be as high as 1 L / g, or even 0.5 L / g.The temperature ranges of the gaseous release of the separating agent, as well as its mass loss and the volume of gas released, can be determined by thermogravimetric analysis, possibly combined with thermodifferential analysis, mass spectrometry and / or infrared spectrometry, with a heating rate of 50 °C / min and under a flux of the same chemical nature as that released by the separating agent during the bulging, for example a flux of CO2.

[0007] Known separating agents, such as calcium carbonate, talc, silica, alumina, or kaolin, do not meet these criteria. Therefore, the separating agent according to the invention is typically not chosen from among calcium carbonate, talc, silica, alumina, or kaolin. Indeed, although some of these agents are capable of releasing gas, this does not occur under the conditions of curvature. This gas release occurs either well before reaching the temperatures necessary for curvature (dehydration of calcium carbonate, for example) or above the curvature temperatures (decomposition of calcium carbonate or talc, for example), given the curvature conditions, which involve, in particular, a high rate of temperature rise and a confined atmosphere.

[0008] The gaseous release generated by the separating agent is preferably a release of carbon dioxide (from, for example, the decomposition of carbonate groups) and / or water (from, for example, the release of crystallized water and / or the decomposition of hydroxyl groups).

[0009] The identification of suitable separating agents can be achieved using thermal analyses (particularly thermogravimetric and / or differential thermogravimetric). The separating agent is advantageously chosen so that its decomposition residue is chemically inert with respect to the glass sheets. Furthermore, neither the separating agent nor the residue after decomposition should melt within the process temperature ranges. The separating agent is preferably chosen from the carbonate family, particularly magnesium and / or aluminum carbonates; hydroxides, particularly aluminum and / or magnesium hydroxides; hydrated mixed silicates, particularly hydrated aluminum and / or magnesium silicates; or a mixture thereof.The term "mixed silicate" refers to any silicate of natural or synthetic origin containing several (two or more) types of cations selected from alkali metals (e.g., Na, Li, K) or alkaline earth metals (e.g., Be, Mg, Ca), transition metals, and aluminum. The term "carbonate family" as used in the present invention refers to carbonates, acid carbonates (also called hydrogen carbonates or bicarbonates), basic carbonates, formates, acetates, and oxalates, each of which may optionally be hydrated. Varying the degree of hydration advantageously allows for fine-tuning the temperature of gas evolution. A separating agent particularly suited to the bulging conditions according to the invention comprises or consists of a magnesium acid carbonate (e.g., of formula MgHCO3·2). n H₂O, with 0 ≤ n ≤3), a basic magnesium carbonate (for example of formula x MgCO3 · Mg(OH)2· n H₂O, with 1 ≤ x ≤4 and 0≤ n ≤5 , such as 4 MgCO3 · Mg(OH)2 · 5 H2O or 3 MgCO3 · Mg(OH)2 · 3 H2O), possibly hydrated, or mixtures thereof.

[0010] The glass sheets constituting the glazing according to the present invention can be manufactured by various known processes, such as the float process in which molten glass is poured onto a bath of molten tin, and the fusion draw process in which molten glass overflows from a channel and forms a sheet by gravity, or the down-draw process in which molten glass flows downwards through a slit, before being drawn to the desired thickness and simultaneously cooled.

[0011] The first and second glass sheets may be of the same or different thicknesses. When they are of different thicknesses, the first glass sheet is generally the thicker one. The glass sheets have a thickness of at most 2.6 mm, preferably at most 2.1 mm, or even at most 1.6 mm. In a particular embodiment, the second glass sheet is thinner than the first glass sheet. The second glass sheet then has a thickness of at most 1.5 mm, or even at most 1.1 mm, or even less than or equal to 1 mm. Advantageously, the second glass sheet has a thickness less than or equal to 0.7 mm. The thickness of the first glass sheet is preferably at least 1.4 mm, or even at least 1 mm. The thickness of the second glass sheet is preferably at least 0.3 µm.Using thin sheets of glass makes it possible to lighten the laminated glazing and therefore meets the specifications currently required by manufacturers who are seeking to reduce the weight of vehicles.

[0012] The bending of the first and second sheets of glass is carried out simultaneously. The two sheets of glass are positioned one above the other on a bending support, if necessary, with the thinner sheet of glass being the one on top, furthest from the support.

[0013] The two sheets are separated by the separating agent according to the invention to prevent one sheet from sticking to the other. The separating agent is typically applied to the glass sheet as a dry powder, a suspension, or a solution in a liquid so as to obtain a homogeneous dispersion of the liquid on the surface of the glass sheet, for example, by spraying methods well known to those skilled in the art. The separating agent is preferably in powder form. It can be applied to the glass sheet at a rate of at least 0.1 g / m², or even 0.2 g / m², and generally up to 50 g / m², or even 40 g / m².The powder typically has a particle size of less than 150 µm, preferably less than 100 µm, typically ranging from 1 to 80 µm, or even from 5 to 60 µm, with the lower bounds corresponding to D5 (the diameter for which 5% of the particles are smaller) and the upper bounds to D95 (the diameter for which 95% of the particles are smaller). Particle size can be measured by laser diffraction.

[0014] The bending can be carried out by any method known to those skilled in the art, for example, gravity bending (or collapse bending), pressing, suction bending, or combinations thereof. In one particular embodiment, the bending can be carried out by gravity on a frame or skeleton-type support, in particular a double skeleton type (as described, for example, in EP 0448447, EP 0705798, and WO 2004 / 103922). In another particular embodiment, the bending can be carried out, in particular, by forming on a solid bending form using a clamping force. The clamping force of the glass against said form can be mechanical or pneumatic. If the force is mechanical, it can be applied by a solid or frame-shaped counter-form. In particular, it can be a frame as shown under reference (4) of the figure 1 of WO 95 / 01938 or the segmented frame referenced (9, 10, 11, 12) to figures 1 et 2 of US 5974836. If the force is pneumatic in nature, it can be applied by suction through the solid form via orifices in the contact surface of said solid form as shown on the figure 2 of WO 2006 / 072721. A pneumatic force can also be applied via a skirt surrounding the solid shape on the model of the skirt referenced 16 in the figure 2 of WO 04087590. The skirt provides a suction force, generating an airflow around the sheet by licking its edge. However, the pneumatic force exerted by a skirt is generally insufficient and is preferably supplemented by a mechanical or pneumatic force through the solid mold. In another particular embodiment, the curvature may also include forming against a solid mold preceded by curvature using another process, in particular and preferably by gravity curvature. The existence of such pre-curvature by gravity is even preferred because it ultimately allows for increased complexity of the glazing (greater curvature depths in all directions) without degrading the optical quality of the glazing.

[0015] During the bending stage, the glass sheets, at the point on the normal to their surface passing through their center of gravity, generally reach a temperature between 590 and 670 °C. For gravity bending, this temperature is preferably between 610 and 670 °C. For bending against a solid form, it is preferably between 590 and 630 °C.

[0016] The present invention also relates to a method for manufacturing laminated glazing comprising a process for simultaneously curving superimposed sheets of glass as described above, and a step of laminating the two sheets of glass with a polymer interlayer.

[0017] When the second glass sheet is thin, it is preferably chemically tempered to increase its mechanical strength. Chemical tempering is a process that involves ion exchange within the glass sheet: the surface substitution of an ion (usually an alkali ion such as sodium or lithium) with an ion of larger ionic radius (usually another alkali ion, such as potassium or sodium) from the glass surface creates residual compressive stresses on the surface of the glass sheet, thus achieving the desired strength. In this case, the process includes, before the lamination step, a chemical tempering step of the second glass sheet. Chemical tempering is generally carried out by placing the sheet in a bath filled with a molten salt of the desired alkali ion.This exchange usually takes place at a temperature lower than the glass transition temperature and the bath degradation temperature, advantageously below 490 °C. The chemical tempering time is preferably less than 24 hours. However, it may be desirable for the chemical tempering time to be shorter to be compatible with the productivity of manufacturing processes for laminated automotive glass. In this case, the tempering time is, for example, less than or equal to 4 hours, preferably less than or equal to 2 hours. The tempering temperatures and times should be adjusted according to the glass composition, the thickness of the glass sheet, as well as the compression thickness and the desired stress level. In particular, good tempering performance is obtained when it is carried out for a duration of 2 hours at a temperature of 460 °C.Ion exchange can be advantageously followed by a heat treatment step to reduce core tensile stress and increase compression depth.

[0018] The lamination stage is carried out in a manner known to those skilled in the art. It involves assembling the glass sheets with the thermoplastic interlayer by applying pressure in an autoclave and raising the temperature.

[0019] The polymer interlayer placed between the glass panes consists of one or more layers of thermoplastic material. It can be made of polyurethane, polycarbonate, polyvinyl butyral (PVB), polymethyl methacrylate (PMMA), ethylene vinyl acetate (EVA), or ionomer resin. The polymer interlayer can be a multilayer film with specific functionalities, such as improved acoustic or UV protection. Typically, the polymer interlayer includes at least one layer of PVB. The thickness of the polymer interlayer ranges from 50 µm to 4 mm, but is generally less than 1 mm. In automotive glazing, the polymer interlayer thickness is typically 0.76 mm.When the glass sheets that make up the glazing are very thin, it can be advantageous to use a polymer sheet with a thickness greater than 1 mm or even greater than 2 or 3 mm to give rigidity to the laminated glazing, without adding too much weight.

[0020] Curved laminated glass can be produced using the process described above. Such glass offers improved optical quality. This improvement is particularly significant when the curvature is achieved through pressing. The laminated glass thus obtained is advantageously suited for automotive applications, especially windshields. Once the laminated glass is installed in the vehicle, the second glass layer becomes the inner pane, meaning the one facing the interior of the passenger compartment. The first glass layer, therefore, is the one facing the exterior.

[0021] The following examples illustrate the invention without limiting its scope.

[0022] To evaluate the effectiveness of separation agents on a laboratory scale, a 30x30 cm sheet of glass, 1.6 mm thick, is placed flat on a sheet of glass-ceramic and heated in a furnace. With the furnace temperature stabilized at 620 °C, the heating time is 11 minutes. The maximum temperature is then maintained for 9 minutes. The maximum temperature reached by the glass sheet is 615 °C. This test replicates more extreme conditions than the industrial process of simultaneously bending two sheets of glass by pressing, as the glass-ceramic sheet does not deform at these temperatures.

[0023] Three series of tests (series 1, comparative series 2, and series 3 according to the invention) were carried out by placing a separating agent between the glass sheet and the glass-ceramic sheet. The separating agent used was, respectively, calcium carbonate with the formula CaCO3 for series 1, sodium bicarbonate with the formula NaHCO3 for series 2, and magnesium carbonate with the formula 4MgCO3·Mg(OH)2·5H2O for series 3. Optical defects (pitting) were then observed on the glass sheets. The glass sheets in series 1 and 2 exhibited a significant or very significant number of pits, respectively (see Fig.1 et Fig.2 ). On the contrary, the glass sheets of series 3 showed very few pits ( Fig.3 ).

[0024] Thermogravimetric and thermodifferential analyses of these three separating agents, coupled with mass spectrometry, were performed to understand their behavior. These analyses revealed that, under bulging conditions (rapid temperature rise of approximately 50 °C / min, and under a CO₂ flux), basic magnesium carbonate decomposes, releasing water and carbon dioxide primarily over a temperature range of 450 to 640 °C. The mass loss over this temperature range is approximately 36%, and the volume of gas released is approximately 0.18 L / g. In contrast, calcium carbonate does not decompose at temperatures below 640 °C. As for sodium bicarbonate, its decomposition occurs mainly at temperatures below 250 °C, which are well below the bulging temperatures.Furthermore, the decomposition residue of sodium bicarbonate, Na₂O, is highly reactive and can damage the surface of the glass sheet. This is not the case with the decomposition residue of magnesium carbonate, MgO, which is inert.

[0025] Two series of tests (comparative series 4 and series 5 according to the invention) were carried out on an industrial windshield manufacturing line. The curvature was achieved by pressing onto a solid mold at temperatures of 610-620 °C. For each series of tests, a powdered separating agent was homogeneously distributed over the surface of the glass sheet. The separating agent used was calcium carbonate with the formula CaCO3 for series 4, and a basic magnesium carbonate with the formula 4MgCO3·Mg(OH)2·5H2O for series 5. After curvature, the optical defects of the glass sheets were observed by shadowgraphy. The number of optical defects observed was significantly reduced on the series 5 glass sheets compared to those of series 4. This industrial test therefore confirms the interest of the separating agent according to the invention for reducing the number of optical defects in the manufacture of curved laminated glazing.

Claims

1. A process for simultaneous bending of superposed glass sheets comprising: - the provision of a first glass sheet; - the application of a parting agent to one surface of the first glass sheet; - the provision of a second glass sheet superposed on the first glass sheet, the parting agent being between the first glass sheet and the second glass sheet; - the heating of the glass sheets at a temperature between 590 and 670°C that allows the bending thereof; and - the simultaneous bending of the glass sheets; characterized in that the parting agent generates gas evolution under the bending conditions.

2. The process as claimed in claim 1, characterized in that the volume of gas released by the parting agent between 450°C and 640°C is at least 0.05 l / g.

3. The process as claimed in either one of claims 1 and 2, characterized in that the gas evolution is an evolution of water and / or carbon dioxide.

4. The process as claimed in any one of claims 1 to 3, characterized in that the parting agent is applied in powder form.

5. The process as claimed in claim 4, characterized in that the powder has a particle size of less than 150 µm, preferably of less than 100 µm, typically of from 1 to 80 µm, or from 5 to 60 µm.

6. The process as claimed in any one of claims 1 to 5, characterized in that the parting agent is applied to the first glass sheet in a proportion of at least 0.1 g / m2.

7. The process as claimed in any one of claims 1 to 5, characterized in that the parting agent is chosen from the family of carbonates, alumina hydroxides, hydrated mixed silicates or a mixture thereof.

8. The process as claimed in any one of claims 1 to 7, characterized in that the parting agent comprises an acid magnesium carbonate, a basic magnesium carbonate, each optionally being hydrated, or mixtures thereof.

9. The process as claimed in any one of claims 1 to 8, characterized in that the glass sheets have a thickness of at most 2.1 mm, preferably at most 1.6 mm.

10. The process as claimed in any one of claims 1 to 9, characterized in that the second glass sheet is thinner than the first glass sheet.

11. The process as claimed in claim 10, characterized in that the second glass sheet has a thickness of at most 1.5 mm, preferably of at most 1.1 mm, or less than 1 mm or even less than or equal to 0.7 mm.

12. The process as claimed in any one of claims 1 to 11, characterized in that the sheets of glass, at the point located on the normal to its surface passing through its barycenter, reach a temperature of between 590°C and 670°C.

13. The process as claimed in any one of claims 1 to 12, characterized in that the bending is press bending.

14. A process for manufacturing a laminated glazing comprising a process for simultaneous bending of superposed glass sheets according to any one of claims 1 to 13 and a step of laminating the two glass sheets with a polymer interlayer.

15. The process as claimed in claim 14, characterized in that it comprises, before the laminating step, a step of chemical tempering of the second glass sheet.