EP1765738A1

EP1765738A1 - Bending glass sheets comprising functional layers

Info

Publication number: EP1765738A1
Application number: EP04804884A
Authority: EP
Inventors: Joseph Dierckx; Philippe Hocks; Masashi Kudo
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 2003-12-23
Filing date: 2004-12-16
Publication date: 2007-03-28
Also published as: BE1015831A3; WO2005070841A1

Abstract

The invention concerns a method for bending glass sheets comprising layers modifying their thermal properties. The bending method particularly concerns glass sheets comprising at least one infrared reflecting layer, some parts of said sheets being free of said layer. The invention is characterized in that first the uniformly coated sheets are subjecting to bending, and, then the sheets bearing the uniform reflecting layer are subjected to local elimination of the layer in the parts which should be free thereof.

Description

GLASS SHEET BOMBING WITH FUNCTIONAL LAYERS

The present invention relates to the techniques used for bending glass sheets comprising functional layers which modify their behavior, particularly in the heat treatments to which they are subjected. The behavior of the glass sheets subjected to a heat treatment leading to the bending of the latter, is a function of their characteristics with respect to the thermal fluxes to which they are exposed. These flows are radiative, convective and conduction in very unequal proportions, and depend on the bending mode used. The fact remains that, in most cases, a very large part of the energy transmitted is by radiation. When these sheets are perfectly homogeneous over their entire surface, the only variations in conditions to which they are subjected come from their environment in the processing facilities. It is sometimes difficult to guarantee perfect uniformity of these conditions over the entire surface of the sheets, especially since they are of larger dimensions. Furthermore, it is not always desired to maintain perfect uniformity of treatment conditions over the entire surface of the sheets. Differences are thus often sought and favored, to lead to effects modulated according to the parts of the sheets concerned, for example for the parts having to undergo more accentuated curvatures. In this case, arrangements are made in the bending installations to locally provide the thermal inputs necessary for these differentiated treatments. These are in particular heating means located on the areas of the glass sheets which must undergo particularly accentuated curvatures, and which, for this reason, it is desirable to "overheat". They are also means intended to compensate for certain irregularities in the distribution of the heat fluxes on the sheets, due to the constraints notably linked to the structure of the oven or to the position of the sheets relative to the heating means of this oven. In gravity bending techniques in which the sheets are placed on a frame, for example, "thermal masses" are used, in other words elements associated with the frames, and whose role is to locally absorb part of the thermal fluxes, reducing thereby the energy absorbed by the glass. The presence of enamel coatings located on automotive glass is also a factor of thermal imbalance. Dark colored enamel absorbs much more than uncoated glass. The differences result in local overheating which can be compensated in particular by the use of the thermal masses indicated above. If, despite the means used, the glazing may have some forming defects due to the presence of these notched areas, they are located in locations, at the edge of the glazing, where they do not constitute excessive discomfort. The problem linked to glazings which do not offer uniform properties over their entire surface is much more troublesome when the parts of the glazing concerned are located in areas whose properties are regulated, in particular those which must be completely free of obstacle to the vision. For many applications, in particular for certain "automobile" glazing, the curved glass sheets comprise functional thin layers, in particular layers which modify the properties with respect to electromagnetic radiation, and in particular infrared. These are in particular the layers which reflect infrared radiation to limit the heating of vehicles exposed to the sun. The layers in question are composed of an assembly comprising at least one infrared reflecting layer, most often a metallic layer, in particular based on silver, a layer which is protected by thin dielectric layers. To obtain perfectly uniform functional layers over the entire surface of the sheets, the application is made to flat glass sheets. This application can be carried out either directly at the exit of the glass production lines, or more frequently, and in particular for all applications requiring the implementation of vacuum deposition installations, of the "magnetron" type, in recovery of the previously produced glass sheets. The various functionalities conferred on automobile glazing, however, lead to the provision of zones devoid of layers modifying the behavior vis-à-vis electromagnetic rays. On the windshield, this is the case, for example, of the location of the rain detector when this includes the use of infrared radiation emitted and received by the detector. This is also the case for all the electromagnetic wave receivers, radar detector, window for electronic toll collection device, receiver for telephone, antennas for radio frequency etc ... For all these applications. the presence of metallic layers on the glazing constitutes an obstacle to good transmission, and therefore requires the installation of zones devoid of the metallic coating. In the traditional manufacturing process, these zones are formed either by masking at the time of deposition, or by localized removal on the previously uniformly coated sheet. Masking is a relatively restrictive mode, and does not allow the production of very small patterns. In particular, the production of patterns formed by sets of very small width lines is not possible by masking. Removal after deposition, in particular by chemical, mechanical or laser ablation, allows diversified operations in terms of dimensions or shapes. The bending of glass sheets comprising layers reflecting infrared radiation poses various problems. The main difficulties are related to the relative fragility of the layers to the thermal conditions imposed by the bending. However, the inventors have also been confronted with the appearance of defects in the forming of glass sheets. The simultaneous presence on the sheet of large areas covered by the infrared reflecting layers, and other areas on which these layers are absent, appeared to the inventors as the cause of some of these forming defects. The inventors started from the observation that the heat input at temperatures corresponding to the softening of glass is. say at temperatures above 500 ° C, is essentially carried out by infrared radiation. Under these conditions, appreciable differences in temperature may appear locally between the zones coated with infrared reflecting layers, and those which are devoid of this coating. These differences, depending on the configuration, can reach, or even exceed several tens of ° C. It is therefore not surprising that these differences can translate into effects on sheet forming. It is true that these defects are in practice all the less easy to detect that the bending of the sheets is less accentuated and that the surfaces concerned by the removal of the layer are in more limited proportions. However, car manufacturers are always more demanding. The glazing, whether or not it has layers with or without uncoated parts, must have no defect, particularly in areas of restricted vision. Beyond regulatory requirements, the perfection of bending is also aesthetic. Finally, certain detectors or functional elements placed behind the glazing are of such sensitivity that their proper functioning requires a total absence of fault. The increasing proportion of glazing comprising very accentuated bending, leads to the appearance of differences in forming, differences which the inventors have been able to associate with the fact that the parts not coated with the reflecting layer do not behave in the bending ovens of the same way as the rest of the glazing. The inventors have set themselves the goal of ensuring that automotive glazing comprising in its final constitution zones offering non-uniform characteristics over their entire surface, with respect to radiation, does not have any defect resulting from this lack uniformity, even when they have very pronounced curvatures. The solution to this problem, proposed by the inventors, is, for glazing with an at least partially opaque layer (whether this results from reflection or absorption) to infrared radiation, which must have parts free of this layer, proceed first to complete bending on the sheets comprising a uniform layer, and once the bending is completed, secondly, to remove the layer on the parts which must be devoid of it. The means used for localized removal of the layer may vary depending in particular on the design of the zones which must be devoid of the layer in question. If mechanical abrasion means can be used to produce patterns having a certain extent, other types of means are necessary when the patterns are in the form of very small lines. Furthermore, one of the difficulties of proceeding according to the invention is to have to carry out the localized removal of the layer on a curved surface, the curvature of which can be relatively strong. Under these conditions, the elimination of the layer by mechanical or chemical means encounters obvious difficulties. On glazing with large curvature, removal by mechanical means is only possible insofar as the drawing of the zone devoid of layer is relatively simple. It is the same in a chemical removal mode, except to use photogravure techniques to form a pattern in a protective layer forming a screen, which is removed after the pattern has been produced by chemical attack. For simple patterns and not too curved glazing, mechanical means such as removal by brushing, if necessary with the help of masks can be envisaged. It is also possible to use wheels associated or not with abrasive pulverulent materials. Chemical removal using stencils or screens of the type used for screen printing is also possible as long as the curvatures remain moderate. The application of products corroding the layer can also be carried out by spraying, using techniques of the "inkjet" type. The inventors have shown that the operation of localized removal of the layer could advantageously be obtained by means of laser beams. In the following description, particular reference is made to removal by means of lasers, because of the convenience of adapting this technique to the different geometric configurations encountered in the implementation of the invention, whether it is 'is the complexity of the pattern, or that the shape of the glazing is particularly complex. The invention is not however limited to this type of means. On the contrary, it includes all the means capable of carrying out the localized removal of a layer covering a previously curved glazing. The choice of laser for localized elimination of the layer makes it possible to overcome certain difficulties linked to the curved shape treated. The means used to produce the laser ablation beam are not in contad with the surface, even if their location must take into account the shape of the treated sheet. The fact of ablating on non-flat surfaces requires some precautions. The laser beam, to present the maximum ef iciency, must be properly focused at the point of removal. The curvature of the treated sheet therefore requires that the laser source be at any point at practically constant distance from the point on which the impact takes place, or at least that the deviation from this distance remains within close limits although controlled. Consequently, as soon as the curvature becomes relatively accentuated, it is preferable to control the relative positions of the treated sheet and of the source of the laser radiation by means of one or more robots, which orient the beam and ensure a constant distance. Most usually, it is the laser source which is carried by the robot since its dimensions make its mobility relatively easy. Nevertheless, it is possible in the same way, to carry the sheet by the arm of a robot which orientates and moves it relative to a stationary laser source. The local removal of the layer is carried out point by point. The realization of zones devoid of layer is obtained by moving the point of impact step by step until covering the whole area in question. Preferably, the power of the laser and the extent of the beam at the point of convergence having the highest power are chosen so as to lead to treatment as fast as possible. The operation must be part of the production cycles without introducing additional time, and without requiring the multiplication of treatment stations. Processing speed is therefore an important factor in the economy of the process. The convenience of the treatment preferably results in a single sweep of the surface to be treated. The points of impact being of substantially drastic shape, a certain overlap is necessary for the removal of the layer on the surface of a pattern extending in two dimensions. The structure of the radiation means that the energy is normally higher in the center of it. The energy distribution in the section of this radiation is of Gaussian type, maximum in the center and decreasing rapidly on the edges of the radiation. This structure has the consequence that the size of the layer eliminated at the point of impact is not only a function of the cross-section of the radiation, but also of the power delivered in the plane of this impad. A wide but not very energetic ray can lead to a removal limited to a central zone of dimension much less than that of the section of this radiation. Conversely, a very energetic beam can produce a removal over the entire section at the point of impact. Given the usual commercialized devices, the focal point impad usually has "effective" dimensions which do not exceed 200 / x. The radiation characteristics are chosen so that the power released and its location eliminates the layer without appreciably modifying the characteristics and in particular the surface condition of the glass substrate. It is known that laser beams are used in various operations in the glass industry. For example, lasers are offered as tools for cutting sheets. The characteristics of these tools are obviously not suitable for the ablation of the thin layers envisaged according to the invention. Lasers have also been proposed to mark the glass. These modes of implementation are obviously excluded according to the invention. It is imperative that the treatment leaves the glass substrate intad. In particular the mechanical, but also the optical characteristics of these substrates must be kept practically unchanged. We know that the usual glasses are transparent at certain wavelengths. In other words, for these wavelengths, the radiation dissipates very little energy in the glass. It is therefore particularly desirable not to risk damaging the glass substrate to choose such lasers. The attached figures illustrate the characteristics of transmission, absorption and reflection of glass sheets. Figure 1 is typical of the optical characteristics of a sheet of clear "float" silica-soda-lime glass, as a function of the wavelength of the incident radiation. The sheet considered is 2.1mm thick. In this figure 1 we note the low proportion of reflected radiation (2) over the entire wavelength range. We also note that the glass absorbs (3) little near infrared (700 to 2500nm). The absorption in the infrared only becomes significant for wavelengths greater than 2700nm (outside the limits of the graph). Correlatively, the transmission (1) is high, of the order of 90%, over the whole range of wavelengths shown. Figure 2 illustrates in the same way the behavior of a sheet of 2.1mm thick, coated with a layer reflecting the infrared. The layer in question is based on silver. It is of the type described in patent EP 0336257. The transmission (4) of the coated sheet decreases very rapidly for wavelengths greater than 800 nm. Conversely, the reflection increases rapidly for infrared, whether the incident radiation is on the coated face (5), or on the opposite face (6). The absorption is, of course, very low when the radiation is directed onto the face carrying the layer (8). It is of the order of 15 to 20% when the radiation is directed on the other face (7). These characteristics are significant, for the conditions in which the bending of the sheets can be undertaken. They also play a role in the choice of conditions for removing the layer when it is produced by means of lasers. US Patent 5,728,994 illustrates in particular the use of this type of laser for the localized removal of an opaque layer disposed on a glass substrate. In this patent, the aim pursued is the formation of a transparent pattern, capable of forming bright images. This patent proposes in particular the use of an Nd: YAG type laser whose wavelength is 1064 nm. It is specified that for these wavelengths the radiation is little absorbed by the glass. Other lasers are also suitable, which have the same characteristics with respect to glass, in particular N <i: YLF with wavelength from 1047 to 1053nm, Nd: glass at 1060nm, Nd: ΥAP at 1080nm, Ho: YLF at 2060nm, Ho: YAG at 2100nm, Tm: YAG at 2020nrn. These lasers can be used for implementing the invention. Nd: vanadates lasers are preferably used. The latter make it possible to keep effective pulse powers at high frequency. In general, the lasers used must preferably have a wavelength such that the absorption in the glass remains limited so as not to modify the characteristics thereof. To obtain effective removal, the radiation advantageously has a wavelength between 500 and 2500 nm, and preferably from 700 to 1500 nm. The choice of a laser whose wavelength and physical characteristics ensure that the interaction with the glass is sufficiently weak to cause no significant modification. This has the advantage of being able to ablate the radiation: being directed either on the side of the layer or on the side of the sheet which does not carry the reflective layer. Preferably, the laser beam is directed through the glass sheet on the reflective layer. It is preferred insofar as this method of removal makes it possible to more completely eliminate the constituents of the layer. In the implementation of the invention, the power of the beam is maintained at a value which is not more than 10 ⁶ J / s and preferably not more than 10 ⁵ J / s. Advantageously, the power is not less than 10 ³ J / s. The frequency of the pulses is chosen so as to obtain the best compromise in removal efficiency. The power applied per unit area can still be adjusted by focusing the beam appropriately. The other consequence of choosing the dimensions of the impad point is to determine the processing speed for a given surface. A particularity of the use of lasers for the removal of the layer is to allow the formation of patterns which may be of dimensions as small as necessary. In particular, the use of the laser allows the ablation of the layer along lines undetectable to the eye, for example whose width can be of the order of a few micrometers. The focusing of the beams is advantageously carried out on the layer to be removed in order to concentrate the power at this point. A deviation between the focus point and the layer is, of course, possible. However, this results in a decrease in power per unit area. In practice, it is preferable to keep this difference less than 15mm, and more advantageously less than 5mm. Given the curvatures of the most commonly produced glazings, it is necessary to provide means guaranteeing that the focusing point is maintained within the limits indicated above. Large glazing often has arrows of ten or more ointimeters or even more. If the removal is to be performed for example on the periphery of such glazing, it is understood that it is necessary to adjust the distance substantially constantly. Likewise for glazing with certain curvatures that are particularly accentuated, such as so-called panoramic windshields, it is necessary to ensure that the angle of incidence of the beam is kept substantially constant. A deviation of a few degrees is also possible, but maintaining the incidence conditions both the focus and the area covered by the beam. Too large a gap must therefore be avoided so as not to compromise the effectiveness of the treatment. To maintain the most suitable geometrical positions of the glazing and of the laser source, a robot is advantageously used which allows adaptation to all the geometries of the glazing treated. Most often the laser source will be carried by the robot. The method of producing curved glazing comprising an infrared reflecting layer is described in more detail in the following examples. In a series of preliminary tests, laminated windscreens are curved in the traditional way by gravity on frames. The two superimposed sheets are passed simultaneously through the bending oven. The face of the lower sheet numbered H, according to the traditional name, is coated with an infrared reflecting layer. This silver-based layer is of the type described in patent EP 0336257. The final glazing has at the location intended to receive the rain sensor (200mm from the upper edge of the glazing, and in the median axis) 22mm. arrow. The curvature in the orthogonal direction is less pronounced. The established conditions lead to a glazing meeting the nominal characteristics, with no optical defect detectable according to the usual tests. For this, the optical distortions in reflection, at the location considered, measured in millidiopters, are of the order of 2000. For the same glazing another series of tests is carried out. This time the sheet coated with the infrared reflecting layer, has an area of 45mm in diameter, devoid of the layer and intended to receive the rain sensor. This zone is obtained by mechanical masking during the deposition of the layer. The glazings are curved under the previous conditions. The measurement in reflection this time shows a distortion corresponding to a value of 5000 millidiopters, clearly greater than that obtained previously. At this level the irregularity of shape at the location devoid of a layer relative to the rest of the glazing is significant. This local deformation of the curved shape is not accepted by the builders. The sheets obtained with a uniform coating of the infrared reflecting layer, and which do not contain any defects, are then subjected to laser removal under the following conditions. We use an Nd: vanadate 16w laser source. The pulse durations are short to have the highest peak power. In the test carried out, the pulse duration is of the order of 10 ns. The beam is focused on a point of 200μ in diameter. Given the limited dimensions of the area treated in the case of the example, the distance from the sheet to the source remains within the limits in question does not influence the treatment conditions. The local elimination of the layer in the latter case does not cause any modification of the bending. We therefore keep 2,000 millidiopters in reflection as on the parts of the glazing around the area thus devoid of layer. There are no faults related to the presence of this zone. Furthermore, under the conditions of these tests, the efficiency of the removal of the layer is determined by X-ray fluorescence. Before removal the layer present of the order of 198 mg / m ² of silver. After removal under the conditions indicated above, the same measurement shows a content which is less than ² mgm ² . In other words, the layer is completely eliminated. Ced is confirmed by the transmission measurement in the area thus treated. After removal, the light transmission in this area is approximately 86%, which corresponds to the transmission in the absence of a layer. Another series of tests is carried out on a windshield comprising the same type of layer as previously. This time the elimination of the layer corresponds to a network of lines forming a grid extending over a 180x180mm square. This pattern is intended to transmit radar waves to an on-board detector. The lines in this case are narrow enough that they are practically invisible to the naked eye. The width of the lines is less than 0.1mm. The horizontal and vertical spacing between the lines is 1mm. On the curved glass comprising the layer uniformly disposed on the glazing, at the location intended to receive this grid, the optical distortion is measured less than lOOmillidioptries in transmission. If one proceeds to the formation of removal before bending, there is a much higher distortion, of the order of 400 millidiopters, which once again constitutes a defect with respect to the rest of the glazing in the zone considered. If the removal is carried out on the curved glazing, no deformation is introduced. In this second case, the fineness of the lines is formed by means of the same laser but by focusing the beam so that the lines for removing the layer have a width of 90 micrometers. It is noted in this case that the laser is particularly well suited to the formation of such fine patterns. In this particular case, a masking operation is practically impossible.

Claims

1. A method of bending glass sheets comprising at least one layer modifying the characteristics with respect to the heat fluxes to which they are subjected, certain parts of these sheets being free from this layer, which is caradized in that initially the uniformly coated sheets are subjected to bending, and, in a second step, the sheets carrying the uniform layer are subjected to the elimination of the layer locally on the parts which must be devoid of it. 2 Method according to claim 1, wherein the layer concerned is an infrared reflecting layer. 3. Method according to one of the preceding claims, wherein the layer is removed locally by mechanical or chemical abrasion. 4. The method of claim 1 or claim 2, wherein the local removal of the layer is obtained by means of a laser beam. 5. The method of claim 4, wherein the removal by means of the laser beam, is carried out from a laser source located on the side of the sheet not comprising the layer subject to localized elimination. 6. Method according to one of claims 4 or 5, in which the laser beam is focused on the layer to be eliminated, the difference between the effective focusing point and the beam impad point being maintained at any point less than 15mm. 7. Method according to one of claims 4 to 6 wherein the distance positioning of the source, and incidence of the laser beam relative to the curved glass sheet, are kept constant by means of a robot carrying the laser source or leaf. 8. Method according to one of claims 2 to 6 wherein the infrared reflecting layer consists of a stack of layers including at least one layer of silver. 9. Method according to one of claims 3 to 8 wherein the local elimination of the layer is carried out with an Nd: vanadate type laser. 10. Method according to one of claims 3 to 8 wherein the impad area on the layer to be eliminated is not greater than 200μ in the largest dimension.