Title of Invention	TRANSPARENT GLAZING UNIT WITH A RESISTIVE HEATING COATING
Abstract	A transparent glazing unit (1) provided with a resistive heating coating (2) that extends over a substantial part of the glazing unit, in particular over a main viewing field (A), and is electrically connected to at least two busbars (4, 5) in such a way that, when an electrical supply voltage is applied between the busbars, a current flows, which heats a heating field in the coating, said heating field including at least one semiresistive region (6) in direct contact with at least one busbar (4, 5).

Title of Invention

TRANSPARENT GLAZING UNIT WITH A RESISTIVE HEATING COATING

Abstract

A transparent glazing unit (1) provided with a resistive heating coating (2) that extends over a substantial part of the glazing unit, in particular over a main viewing field (A), and is electrically connected to at least two busbars (4, 5) in such a way that, when an electrical supply voltage is applied between the busbars, a current flows, which heats a heating field in the coating, said heating field including at least one semiresistive region (6) in direct contact with at least one busbar (4, 5).

Full Text	TRANSPARENT GLAZING UNIT WITH A RESISTIVE HEATING COATING The invention relates to a transparent glazing unit 5 with a resistive heating coating that has the characteristics of the preamble of claim 1. The invention relates more particularly to a glazing unit whose resistive heating coating is a coating 10 placed on a substrate and having thermal insulation and/or solar protection capabilities. The glazing units incorporating this type of coating, when they are intended for equipping vehicles, make it possible in particular to reduce the air-conditioning load and/or 15 reduce excessive overheating ("solar control" glazing) and/or reduce the amount of energy dissipated to the outside ("low-e" or "low-emissivity" glazing) brought about by the ever growing use of glazed surfaces in vehicle passenger compartments. 20 One type of multilayer known for giving substrates such properties consists of at least two metal layers, such as a silver-based layer, each being placed between two coatings made of a dielectric. This multilayer is 25 generally obtained by a succession of deposition operations carried out using a vacuum technique, such as cathode sputtering, optionally magnetically enhanced or magnetron cathode sputtering. Two very thin metal layers, called "barrier layers" may also be provided, 30 these being placed beneath, on, or to each side of each silver layer, the underlayer as a tie, nucleation and/or protection layer, for protection during an optional heat treatment subsequent to deposition, and the overlayer as a protective or "sacrificial" layer so 3 5 as to prevent impairment of the silver if the oxide layer that surmounts it is deposited by sputtering in the presence of oxygen and/or if the multilayer undergoes a heat treatment subsequent to deposition. - 2 - In particular for vehicle windshields, there is high demand in the market for heating versions, in which the heating means must by nature be the least visible, or least obstructing for viewing, as possible. 5 Consequently, there is an increasing demand for a transparent heating coating for such glazing. A general problem with heating coatings having a low light absorption is their relatively high surface 10 resistance, which requires a high supply voltage, in any case for heated glazing of large dimensions or for long current paths, which voltage is in any case higher than the usual voltages on board vehicles. If it is desired to lower the surface resistance, this is 15 accompanied, in the multilayer systems known hitherto, with a reduction in visible light transmission because the conducting layers have to be thicker. For these technical reasons, glazing units heated by 2 0 wires, which may be supplied without any problem by the on board voltage, are still preferably being mounted at the present time. These laminated glazing units with integrated heating fields in the form of very fine wires are, however, not accepted by all purchasers. 25 Patent DE 1 256 812 Bl discloses a glass pane that can be heated by a coating made of metal or a metal oxide deposited continuously on one of its surfaces. This publication aims to solve problems due to the high 30 ohmic resistance of said coating, which is of the order of 200 Ω/. However, to be able to heat this coating using a relatively low voltage from two lateral busbars, narrow printed electrodes of low ohmic resistance (called "auxiliary electrodes") that extend 35 from said busbars over the heating field are provided. Said auxiliary electrodes terminate only a short distance in front of the opposite busbar, and they overlap each other with an alternating polarity. - 3 - However, said lines, which are optically perceivable as a hatching, obstruct vision and detract from the optical appearance of the main viewing field-of the pane thus produced. It is not possible to make use of 5 the optical advantage of a transparent heating coating. It is for this reason that such a pane is designed only for a rear window of automobiles. Another problem with heating coatings may arise owing 10 to the fact that they are sometimes not able to be deposited uniformly over the entire surface of the transparent glazing, but one or more interruptions, called "communication windows", have to be provided therein, which disturb the flow of the heating current 15 and may form "hot spots" (local overheating) along the edges of this or these communication windows.. Such communication windows serve to make the coating, which by nature is reflective for short-wave radiation, respectively infrared radiation, locally more permeable 20 to certain data streams or signals. To inject and extract the heating current in these coatings, at least one pair of electrodes (in the form of bands) or of busbars, which have to inject the 2 5 current into the heating coating and distribute it over a wide front as uniformly as possible, is provided. In vehicle glazing, which is substantially more wide than high, the busbars are usually found along the longer edges of the glazing (in the mounted position, the 30 upper and lower edges), so that the heating current can travel along the shortest path over the height of the glazing. At the same time, the aforementioned communication windows are most of the time located at the upper edge of the glazing and extend there over 35 several centimeters of width. Document WO 00/72635 Al discloses a transparent substrate with a coating that reflects IR rays and a communication window produced locally by removal or - 4 - omission of the coating. Obviously, each communication window which modifies the uniformity of the coating disturbs the current flow. 5 Local temperature spots ("hot spots") appear, which may result in damage to the substrate (thermal stresses) and to the coating itself. This is not only the case when the coating is defective over a large area, but also when the communication window is formed by a 10 relatively large number of slots that do not communicate with one another. These also result, in the surface part in question, in an appreciable increase in the layer resistance and also give rise to the abovementioned hot spots. 15 The last document mentioned proposes, as a means of reducing the problematic effect of an extensive communication window, to provide, along its edge, an electrically conducting band that has an" ohmic 20 resistance per square which is significantly lower than that of the heating layer. Said band purports to take the current around the cut. Preferably, a communication window is framed entirely by such a band. The band may be produced by printing a silver-containing conductive 25 screen-printing paste and by baking it. However, it may also be applied by the deposition of an electrically conducting lacquer or by depositing a metal strip. In all cases, a conducting electrical connection of the band to the coating is of course necessary in order for 30 it to operate. The band may be concealed from view by superposing an electrically nonconducting opaque masking strip, for example made of black enamel. As a general rule, such 35 masking strips are made up from black-colored nonconducting material (screen-printing paste) to be baked. Infrared radiation is not reflected by this material, but absorbed. - 5 - Document WO 03/024155 A2 discloses transparent glazing of this type with a heating coating, in which, on the one hand, a maximum nominal voltage of 42 V is indicated, which however aims also to solve the problem 5 of "hot spots" along the edges of a communication window. In general, various voltage levels are used, a lower voltage being applied to shortened current paths (for example because of the communication window) so as to avoid local overheating. Specifically, the 10 communication window region is cut out from the heating surface by placing a separate busbar between the communication window and the busbar located on the opposite side. 15 Also known, from document DE 36 44 297 Al, are many examples of heating coatings for a vehicle windshield that are divided. The divisions may thus be produced by parts that are not provided with surface layers and/or by notches produced mechanically or by a laser beam. 20 They are used for suitably adjusting and deflecting a current flow within the coated surface and have to ensure as uniform as possible a current density in the surfaces in question. 25 Document WO 2004/032569 A2 discloses another configuration of transparent glazing with a heating coating, which also aims to achieve uniformity of the heating power in the surface by separating lines traced in the coating. 30 Document DE 29 36 398 Al relates to measures intended for preventing current spikes in the transition between the busbars and the coating, in transparent glazing with a heating coating. In general, the aim is to 35 reduce the sudden difference in resistance between the coating and the busbars using materials or shapes with a higher resistance for the latter, or else with intermediate resistances. The above document indicates surface resistances of the coating of between 1 and 10 6 ohms per unit area. In one of the many embodiments described in that document, the edge of each busbar turn toward the opposite busbar is of corrugated form. The formation of sharp points turneded toward the 5 heating coating must thus be avoided. This approach aims to appreciably lengthen the transition line between the busbar and the coating and consequently to reduce the current density in this transition. However, all these measures seem poorly suited to be able to 10 supply the heating layer with a relatively low voltage. It is also known to provide, on the incident face of photovoltaic solar cells, grid or comb electrodes (see for example document WO 03/075351 Al) . They are often 15 produced by screen printing and made up of a busbar placed along the edge of the solar cell and of a plurality of small comb teeth that extend from the busbar over the surface of the solar cell. These electrodes allow surface connection for the 20 photovoltaic voltage, which is present on both faces of the absorber, between the front comb electrode and the rear metal electrode, respectively, over its entire surface without excessively reducing the penetration of light into the absorber. 25 Document DE 197 02 448 Al discloses a heated mirror, on the glass substrate of which two comb-shaped conducting tracks or electrodes are placed, these being indented one in the other, with a PTC coating (i.e. one having a 30 positive temperature coefficient of resistance) that covers them and fills the intermediate spaces between the comb teeth. However, that document does not consider the problem of making the heating invisible to the eye, because the conducting tracks and the heating 35 layer may be placed behind the mirror layer. Document DE 198 32 228 Al discloses vehicle glazing with an electrically conducting coating that is optically transparent and used as an antenna. Purely - 7 - capacitive high-frequency radio signals are picked up from the antenna layer using a coupling electrode, which is made up of several fine wires connected together and placed parallel to one another at a 5 certain distance apart that is large compared with their diameter, which wires extend from the edge into the viewing field of the glazing and terminate therein, without continuing. There is no galvanic coupling between the coating and these wires, because each time 10 they lie in different planes from the laminated glazing. The busbars already mentioned many times may be produced on the glass pane equally well by printing 15 (screen printing) before or after deposition of the coating, or by soldering thin strips of sheet metal, preferably made of tinned copper. Combinations of printed busbars and metal-strip busbars are also known (see for example document DE 198 29 151 Cl). 2 0 Admittedly, the busbars are usually narrow and in the form of strips, but they are not transparent. For optical reasons, they are therefore placed each time near the outer edge of the transparent glazing units in question. Most of the time, they may be masked by 25 opaque edge coatings (usually produced by screen printing). Likewise, the aforementioned communication windows may be masked by these edge coatings, provided that they are sufficiently permeable to the radiation to be transmitted via the communication window. 30 In standard vehicle windshields, these opaque coatings are in the form of a frame provided all around the glazing, which frame also has the function of protecting the bonded joint between the glazing and the 35 body from UV radiation. These frames surround the general viewing field of the glazing. In windshields, a distinction may also be made between the main viewing field A, approximately in the middle of the area of the glazing, in which there can be no perceptible 8 impairment of vision (for example by colorations or wires or other damage larger in size than 30 microns) , and the secondary viewing field B closer to the edges. 5 The problem at the basis of the invention therefore consists in how to provide a transparent glazing unit provided with a heating coating that can operate with relatively low nominal voltages, in particular around 12 to 14 volts, and which nevertheless produces a 10 uniform distribution of the heating, in particular without any hot spot, with viewing in the general viewing field of the glazing, and in particular in the main viewing field A of the glazing, which is impeded as little as possible. 15 This problem is solved according to the invention by the features of claim 1. The features of the dependent claims provide preferred embodiments of this invention. 20 According to the invention, the heating field, formed by the current flowing between the busbars when an electrical supply voltage is applied between these busbars, includes at least one semiresistive region in direct contact with at least one busbar. 25 The term "conducting" should be understood within the context of the present invention to mean that the element thus termed admittedly has an electrical resistance, as it is out of the question here to use 30 superconductors, but that this resistance is very low, in such a way that when the electric current used flows through this element, it does not heat up so as to be perceptible by touching it with one's hand within one minute of applying voltage, that is to say that this 35 element will be classed as constituting a cold region when the glazing unit is observed by thermography. The term "resistive" should be understood within the context of the present invention to mean that the - 9 - element thus termed has a high overall electrical resistance, in such a way that, when the electric current used passes through this element, it heats up so as to be perceptible by touching it with one's hand 5 within one minute after applying voltage, that is to say that this element will be classed as constituting a hot region when the glazing unit is observed by thermography. Within the technical field in question, the resistive regions have a surface resistance of 10 around 0.5 to 5 ohms per unit area and the hot regions formed at their places have power densities of at least 400 to 450 watts/m2. The term "semiresistive" should be understood within 15 the context of the present invention to mean that the element thus termed has a low overall electrical resistance, which is less than that of the resistive element (s), but greater than that of the conducting element(s). The expression "semiresistive region" used 20 in particular here denotes an area having a low overall resistance. However, the region may have a high resistance in certain places and a very low resistance in other places. For example, it may even include conducting elements and resistive elements, the 2 5 combination and configuration thereof make said region "semiresistive". The heating field is the direct result of the electric field when voltage is applied to the terminals of the 30 electric field. It also denotes the actual -heating region of said glazing, which extends between the two busbars. The object of the present invention is to create a 35 novel heating field through the use of a particular electric field. As in the electric fields of the prior art, the ends of the electric field are formed by conducting regions embodied by the busbars, and between these busbars an electric field is produced. However, - 10 - unlike the prior art, the surface resistance of this field is not uniform over its entire area - a semiconducting region is produced in contact with at least one busbar. This has the effect of promoting 5 electrical conduction in this region through which the current flows and has the effect as it were of favoring energy transport into the following resistive region. At least one part of the electric field according to 10 the invention thus has the following scheme, from one busbar to the other: conducting region/semiconducting region/resistive region/.../conducting region. 15 Thus over at least one path between two busbars, the current will firstly pass through a semiconducting region and then through a resistive region. The electric field between the busbars thus, has a 20 resistivity gradient formed from at least two separate (semiresistive/resistive) states. There may also be a gradual gradient formed from many states, thus passing gradually from the conducting state to the resistive state and then returning to the conducting state. 25 This may all be observed by thermography. The present invention is of most particular importance in the "panoramic windshield" technology. 30 In this technology, the aim is to produce windshields that are as wide and/or as tall as possible, which include portions extending laterally along the sides of the vehicle and/or on the roof of the vehicle. 35 Thanks to the invention, it is thus possible to produce heating panoramic windshields in which the heating power is concentrated in the essential part of the glazing, namely the main viewing field A. - 11 - With these features and arrangements, a relative shortening of the path of the current flow within the coating of relatively high resistance itself is 5 obtained because part of the distance between the actual busbars and the main central heating field, in the main viewing field A, is crossed by low-resistance auxiliary conductors or else by said semiresistive regions. 10 The main viewing field A of the glazing preferably has no semiresistive region and thus it remains optically free of any perturbations or obstructions. 15 In an alternative embodiment, at least one semiresistive region is preferably in direct contact with at least one busbar at the positive potential. At least one other semiresistive region is then 20 preferably in direct contact with at least one busbar at the negative potential and the main viewing field A then preferably lies between said two at least semiresistive regions. 25 In one particular version of the invention, the semiresistive region includes conducting strands formed from conducting printed lines, preferably printed on the heating coating 2 and/or from conducting wires, these conducting wires preferably being electrically 3 0 connected to the heating coating and at least to said semiresistive region by soldering at least at discrete contact points. The conducting strands cover only part of the heating 35 field (close to the edges of the glazing), especially a relatively wide strip along the busbars. They terminate blindly, preferably before the boundary of the central viewing field A. - 12 - Thus, a semiresistive region that does not greatly impair viewing, which in any case is the aim with any semiresistive region referred to here, is used. 5 Unlike in document DE 1 256 812 Bl, there are no overlapping elements of opposite polarity in the region of the main viewing field and the current flows, after activating the electrical power supply, approximately 10 in a direction normal to the busbars and therefore in a direction parallel to the overall longitudinal direction of the blindly terminating conducting strands. It should be understood that this "overall longitudinal direction" is the general direction or 15 extension along which said strands extend from the busbars toward the main viewing field. Furthermore, the resistance of the transition between the busbar and the coating is further reduced by 20 greatly increasing the areas of contact compared with the prior art. Consequently, the voltage needed to make the heating currents flow over the heating surface is lower. 25 Admittedly, this configuration is preferably used for windshields in which good visibility in the central viewing field suffices for safe driving, however heated glazing units according to the invention may also be fitted at other places on the vehicle, and also in 30 other machines and moving equipment and in buildings. Whereas in conventional solar cells with gate or comb electrodes the voltage is applied over the thickness of the layer of the absorber, a voltage is applied in the 35 application according to the invention for the purpose of making a current flow in the plane of the coating. The conducting strands and the semiresistive -regions according to the invention thus have the effect of bringing the busbars, customarily placed along the edge - 13 - of the glazing, electrically closer together without however appreciably degrading the general viewing field of the glazing and without degrading at all the main viewing field A. 5 Moreover, it should be recognized that in document WO 00/72635 cited above, the electric field, and consequently the heating field, does not include a semiresistive region since the strip that surrounds the 10 communication window is excluded from the electric field owing to the fact that a resistive region is interposed between the closest busbar and the communication window that it surrounds. The trace of the electric field lines in the last figure of the 15 above document shows that the lines go around the communication window while still remaining in the resistive coating, without passing through the strip that surrounds the communication window. 2 0 When used in a vehicle, the configuration according to the invention makes it possible in particular for the windshield to be supplied directly, in order to heat it, with the usual onboard voltage of 12 to 14 V DC, for which voltage a coating having as low as possible 25 an ohmic resistance is of course recommended. The extent of the semiresistive regions or else the length of the strands is dimensioned according to the effective surface resistance of the coating chosen the more conducting the coating itself, the narrower 30 the semiresistive regions may be or the shorter the strands may be. Measured from the busbars, the extent of the semiresistive regions or the length of the strands is 35 greater than the width of the respective busbar to which they are attached and the strands that extend into the heating field in contact with the heating coating. - 14 - Likewise, with this configuration it is possible to keep the entire coating on the surface of the transparent glazing - apart from the optionally provided communication windows - in such a way that 5 neither masking nor removal of coating is necessary. Thus, the positive properties of the coating, namely in particular its infrared reflection (thermal insulation) and its uniform color (in reflection and in transmission), are preserved over the entire surface. 10 The conducting strands have a width and/or a thickness of preferably 0.5 mm or less, and even more preferably 0.3 mm or less, measured in projection on the surface of the glazing unit. 15 The additional conducting strands, which are also as thin as possible, only imperceptibly impede vision through the glazing. 20 Given that the transparent glazing is in almost all cases a laminated glazing unit in which the coating itself is placed on a face lying on the inside of the composite glazing, the conducting strands could also, apart from printing, be produced in the form of thin 25 wires that are fixed, for example in a manner known per se, to a composite adhesive film and then deposited, with this film, on the coating, thus coming into electrical contact with the coating. This contact is stable over a long period after the final bonding of 3 0 the laminated glazing. In the embodiment in the form of screen-printed structures, the conducting strands are preferably deposited on a substrate (made of glass or plastic or a 35 plastic film) before the coating is applied. This may be carried out in a single operation together with the deposition of the actual busbars. It is also possible to straddle, with a region of low - 15 - resistance or with conducting strands, one or more communication windows produced along the edge of the glazing in the coating, without any risk of forming hot spots. The currents in the known problem areas along 5 the lateral edges of such communication windows are very greatly reduced by the strands. Other details and advantages of the subject of the invention will become apparent from the drawings of an 10 exemplary embodiment in the form of a vehicle windshield and from their detailed description that follows. In these schematic drawings, drawn to no particular 15 scale: figure 1 illustrates an embodiment of a transparent glazing unit with a resistive heating coating, in which embodiment busbars in the form of strips are connected to grid strands that extend in the 20 form of very fine fingers in the surface of the glazing; - figure 2 shows a second embodiment, in which the heating coating is divided into current paths using separating lines; 25 - figure 3 shows a partial cross section through a glazing unit according to the invention along the line III-III in figure 1; - figure 4 shows a detail taken from figure 3; figure 5 shows a detail similar to that of 30 figure 4 for another embodiment different from that of figure 4; figure 6 illustrates another embodiment of a transparent glazing unit with a resistive heating coating, in which the heating coating includes at least 35 one semiresistive region; and figure 7 illustrates another embodiment of a transparent glazing unit with a resistive heating coating, in which the adhesive layer has a semiresistive region, the figure being a partial cross - 16 - section, similar to that illustrated in figure 3, through another glazing unit. In the figures, an electrically resistive transparent 5 coating 2 is placed over the entire surface in a manner known per se in a heated laminated glazing unit 1 having an essentially trapezoidal (curvilinear) outline. The glazing unit 1 has been shown here only in one half - its other half is equivalent. 10 The coating 2 is deposited in a known manner on a main face of a substrate 11, this substrate then being integrated into the glazing unit 1. 15 A broken line denoted by 20 indicates that the outer edge of the continuously coated surface lies all around, but slightly set back toward the inside of, the peripheral outer edge of the laminated glazing unit 1, that is to say an edge band is provided in the coating 20 all around the surface. Thus, the coating is, "on the one hand, electrically isolated from the outside and, on the other hand, protected against any corrosion damage penetrating via the outer edge of the glazing. The outer edge 2 0 may be set back by removing the 25 coating along the edge of the glazing, by masking the outline of the substrate before the coating is deposited on this substrate, or else by tracing a separating line that passes through the coating and runs along the outer edge of the substrate, which may 30 be sufficient for meeting the isolation and corrosion protection objectives. The coating 2 itself is preferably made up, in a" manner known per se, from a multilayer solar-protection system 35 of high thermal resistance comprising at least one metal functional layer and preferably at least two metal functional layers, this system withstanding, without any damage, the temperatures above 650°C that are required for bending the glass panes, that is to - 17 - say without its optical, electrical and heat-reflecting properties being degraded. The multilayer system also includes, apart from the metal layers (which are preferably based on silver), other layers such as 5 antireflection layers and, optionally, barrier layers. However, in relation to the present invention, it is also possible to use other electrically conducting multilayer systems that have a low temperature 10 resistance, and in particular also multilayer systems that are not deposited directly on a rigid glass pane but on a plastic film, (preferably a PET film) . All these multilayer systems are preferably deposited by sputtering (magnetron cathode sputtering). 15 The surface resistivity of the current multilayer systems of the type mentioned above lies between about 0.5 and 5 Ω/. Vehicle windshields with such multilayer systems must achieve overall a light transmission of at 20 least 75% according to some standards, or 70% according to other standards. Of course, the composition and the production of the coating are of secondary importance here, so that there 25 is no need to dwell on details thereof. An opaque colored layer 3 in the form of a peripheral frame has been deposited along the edge of the laminated glazing unit 1, the inner edge 3 0 of which 30 layer, relative to the outer edge of the glazing unit, circumscribes the general viewing field of the transparent glazing unit 1. This layer may lie in a plane of the laminated glazing unit other than that of the coating 2 (being located on the inside or on the 35 outside of the composite glazing unit) . It serves as a layer for protecting a bead of adhesive, with which the finished glazing unit is bonded to a vehicle body, from UV radiation. Moreover, it can conceal from view connection elements for the main electrical heating - 18 - function and for the optional additional electrical functions of the glazing unit 1. Thus, the figure shows, along the upper edge of the 5 laminated glazing unit 1, in the region of the surface covered by the colored layer 3, a first busbar 4 and, along the lower edge, a second busbar 5. The two busbars 4 and 5 are in direct conducting electrical connection with the coating 2, in a manner known per 10 se. Figures 1, 2 and 5 show, in half in the middle of the glazing unit, a communication window 22 below the busbar 4, which communication window is also covered by 15 the colored layer 3 and is therefore concealed from view. It is also possible to provide several communication windows. Many vehicle windshields are provided, along their 20 upper edge, with a band (not shown here) that is bluish but transparent to light ("band filter"), which in particular reduces dazzling by sunshine. Likewise, such a band may also help to conceal the communication window from view. It may also replace part of the width 2 5 of the band of the colored layer 3 along the upper edge of the glazing unit, or it may be provided as a complement thereto. Since the general viewing field of the glazing is defined by the inner edge of the colored layer, it may consequently incorporate this bluish 3 0 band. As a general rule, the laminated glazing unit 1 is made up from two rigid glass and/or plastic panes 11 and 12 and from an adhesive layer 13 joining the panes at the 35 surface. The busbars 4 and 5 are placed on the adhesive layer 13 (for example a thermoplastic adhesive film made of polyvinyl butyral (PVB) , ethylene/vinyl -acetate (EVA) or polyurethane (PU) ) and are fastened to its surface before the adhesive layer is assembled and - 19 - bonded to the rigid panes. The busbars 4 and 5 may be made up from thin narrow strips of metal (copper or aluminum) film, which are 5 usually fixed beforehand to the adhesive film 13 and are applied together with an electrical contact to the coating 2 during assembly of the laminated layers. However, the electrical contact may also be provided by soldering the busbars 4 and 5. During the subsequent 10 autoclave process, a reliable contact is made between the busbars and the coating by the action of heat and pressure. The busbars 4 and 5 may, as indicated above, be 15 produced, as a variant or as a complement, by printing a conductive paste, which is baked while the glazing units are being bent. This is also appreciably less expensive than positioning parts of metal strips. In all cases, the printed busbars have, during continuous 2 0 manufacture, a higher ohmic resistance than that of the strips of metal film. The choice, between busbars made of metal film and busbars in screen-printed band form, therefore depends only on the type of glazing and possibly on the overall resistance of the multilayer 2 5 heating system. Compared with the coating 2, the busbars are always of negligible ohmic resistance and do not heat up appreciably during operation of the heating means, 30 Applying a voltage between the two busbars 4 and 5 in the heating coating generates an electric field and, through the resistive effect, a heating field. 35 Two (or more) heating fields may be provided, in a manner known per se, in the laminated glazing unit 1, which heating fields are to be supplied separately (for example with a vertical division down the middle of the glazing unit) and must also, of course, be connected to - 20 - their respective power supply via separate conducting connectors. In this case, a common ground conductor may be used for the two heating fields, in such a way that only the busbar 4 or the busbar 5 has to be divided 5 into two parts, whereas the other busbar is continuous. In the first version, four external connectors are required, whereas in the second only three are required. 10 The external connectors will not be discussed in detail here, because they have already been described many times in the art. The outer boundary of the main viewing field A of the 15 windshield, lying on the inside of the general "viewing field circumscribed by the edge 30 of the colored layer 3, is indicated schematically by a dotted line L. The line L does not constitute an actual edge or the like in the glazing or in the coating, rather it serves 20 merely to visually illustrate the approximate position of the main viewing field A. The latter is described in Annex 18 of the ECE R4 3 by means of certain parameters of an arbitrary vehicle environment. In this field, no perceptible degradation of vision having a size of 25 greater than 3 0 microns is permitted. In the secondary viewing field B, around the outside of the main viewing field A, slight limitations in vision, due to additions, etc., are permitted. 3 0 According to the invention, when an electrical supply voltage is applied between the busbars 4 and 5, a current flows, forming a heating field in the coating, this heating field including a semiresistive region 6 in direct contact with the upper busbar 4. 35 Thanks to the present invention, the heating current flows between the busbars via the coating 2 in a heating field that has a region of lower resistivity and then a region having a higher resistivity. - 21 - However, in a preferred version of the invention, the heating current flows between the busbars via the coating 2 in a heating field that has a region of lower 5 resistivity and then a region of higher resistivity and then once again a region of lower resistivity. As illustrated in figures 1 and 6 in particular, an upper semiresistive region 6 thus extends downward 10 beyond the area covered by the colored layer 3 in the general viewing field of the glazing toward the main viewing field A and a lower semiresistive region 6' extends upward beyond the area covered by the colored layer 3 in the general viewing field of the glazing 15 toward the main viewing field A. In a first version of the invention, illustrated in figures 1 to 5, starting from the upper busbar 4, a semiresistive region 6 formed by a set of lines extends 20 beneath the colored layer 3, then, in the general viewing field of the laminated glazing unit 1, from the edge region covered by the colored layer 3, into the secondary viewing field B. 25 These lines terminate blindly in the secondary viewing field B, more or less near the boundary of the main viewing field A. The length of the lines thus produced depends directly on the conductivity of the coating chosen. 30 These lines represent conducting strands 46, which are electrically connected to the busbar 4 and to the coating 2 and have a low ohmic resistance compared with the latter. A number of these lines also straddle the 35 communication window 22 in such a way as to guarantee that the coating 2 on either side of the communication window, seen from the busbar 4, is supplied directly with power. From the visual standpoint, these lines that straddle the communication window 22 are masked by - 22 - the colored layer 3. Another form of masking may, as already indicated, be optionally provided by a band of bluish color (band filter), but not shown here. 5 Conducting strands 56, similar to the conducting strands 46, also extend into the viewing field B of the laminated glazing unit 1 from the lower busbar 5. In each case, the combination of the conducting 10 strands, 46 or 56 respectively, forms with the coating 2 a semiresistive region according to the invention. It is unnecessary to provide such conducting strands 46 and 56 for both busbars, 4 and 5 respectively. However, 15 if conducting strands are provided on both sides of the main viewing field A, they in no case extend in such a way that conducting strands of opposite polarity overlap in a projection perpendicular to their overall directions. Thus, the central part of the general 20 viewing field and of the heating field, "and in particular the main viewing field A, remains undisturbed. Whereas in conventional glazing units with layer 25 heating without a semiresistive region the heating current has to flow only via the coating over the entire distance between the busbars, this distance may be reduced to values of between 2 5% and 80% by the semiresistive region(s) according to the invention and 3 0 in particular by the conducting strands according to the present invention, depending on the extent of the viewing field A, some of the current spanning the remaining distance in the semiresistive region'(s) and in particular in the conducting strands. 35 In figures 1 and 2, the strands 46 and 56 are placed uniformly apart and are all produced with the same length. - 23 - The inner end of the conducting strands is substantially at the same potential as the busbars to which they are connected. 5 In the general viewing field of the glazing unit, the current flows approximately perpendicular to the busbars 4 and 5 and parallel to the longitudinal general direction of the conducting strands 46, 56. 10 Thus, the current flows in the general viewing field in a direction approximately parallel to the longitudinal direction of the conducting strands. As previously, a flow of current, even though small, 15 remains over the entire surface of the coating, because the busbars are not separated from the coating in the parts located between the conducting strands. However, this current flow does not result in the formation of hot spots along the edges of the communication window 20 22. To meet the abovementioned objectives of the present invention, the conducting strands, 46 and 56 respectively, must also be in intimate galvanic contact 25 with the coating, in addition to their good conductivity (so as thereby to form, as mentioned above, a semiresistive region). Certainly, in principle it is conceivable to produce them in the form of wire portions. However, they are preferably printed, before 3 0 the coating is deposited, on that surface of the glazing unit that subsequently will have to bear the coating. Certainly it is also possible to print them on the finished coating, but this runs the risk of damaging the multilayer coating, which is mechanically 3 5 weak. Preferably, the conducting strands are printed using a very conductive screen-printing paste containing at least 80%, and preferably more than 85%, silver. - 24 - These conducting strands preferably have a dark color seen via an outer face of the glazing, so as to be not easily perceptible to the viewer's eyes when he is 5 looking from the outside into the vehicle and, also preferably, have a light color, seen via an inner face of the glazing, so as to be not easily perceptible to the eyes of the viewer when he is looking out from inside the vehicle. 10 If printed busbars 4 and 5 are used, they may then be produced in a single operation at the same time as the conducting strands 46/56 and using the same screen-printing paste. No separate operation is then 15 needed to bring the conducting strands into electrical contact with the busbars. However, if busbars in the form of strips of metal film are used, these then have to be connected with a low 2 0 ohmic resistance to the coating and to the conducting strands. The strips of film, which are preferably tinned, are therefore soldered to the conducting strands in a manner known per se. In principle, the screen-printing pastes with a high metal content that 25 are used here are easily soldered to the tinned metal strips. The lengths, separations and number of the conducting strands 4 6 and 56 and the dimensions of the busbars can 30 only be shown here schematically. However, their relative dimensions may be seen - whereas the busbars 4 and 5 are produced in the form of a usual band with a width of a few millimeters, the conducting strands 46 and 56 are as close together and as invisible as 3 5 possible, and however appreciably longer than the widths of the busbars. The individual configuration in a specific laminated glazing unit may certainly be broadly predetermined by - 25 - simulation. However, it will depend again very greatly on the respective magnitude of the dimensions of the specific glazing, on the type of busbars and on the actual electrical properties of the coating. It may for 5 example also be sufficient to add conducting strands only to one of the busbars. For a relatively short distance between the two busbars 4 and 5, the conducting strands themselves may also be shortened. 10 In the case of straight lines for a vehicle glazing unit, these lines are not parallel but converge toward the busbar to which they are attached, preferably in such a way that a longitudinal symmetry (along the longitudinal axis of the vehicle) may be observed in 15 the glazing. It has been determined, for one specific type of glazing unit, that spacing of 25 mm between the various conducting strands can be used. However, the surface 20 heating power levels available in the semiresistive regions can if necessary be adjusted, by varying the spacing, for a given resistance of the conducting strands. Furthermore, to simplify matters, only straight conducting strands have been shown here, but 25 * this does not exclude the possibility in practice of producing them in curved and/or corrugated " forms, and/or in the form of open or closed loops and/or in portions of arcs and/or in meanders, which could be less visible. 30 Figure 2 shows an alternative embodiment in which the coating 2 is divided by separating lines 24 in the general viewing field. The separating lines 24 may pass entirely through the coating down to the surface of the 35 substrate, or else they may penetrate only as far as the conducting partial layer close to the substrate. They have to divide the coating, which by nature is continuous, into current paths. Various techniques exist for producing such separating lines, among which - 26 - laser cutting is currently the most common, because it is the most economic regarding the result. In particular, the separating lines that can be produced are thus extremely narrow and only perceptible to the 5 naked eye with difficulty. If figure 2 is taken as representing the view seen by the driver of a left-hand drive vehicle, he will usually have to look through the surface part in which 10 the separating lines 24 are the closest together. The purpose of these lines is to collect the current flow through the coating 2 in the main viewing field A precisely in this region, and thus provide the highest heating power in this main viewing region when his 15 vision is obstructed by snow, ice or water dropl-ets and to provide clear vision as quickly and as effectively as possible. Here too, the arrangement of the separating lines 24 20 has been indicated only schematically and only few conclusions may be drawn as to actual configurations. It is also not always judicious to always trace continuous separating lines, rather it is possible to produce some or all of them as segmented separating 25 lines, so to speak dotted lines, or to provide, instead of the longer separating lines, a few short portions in order to deflect the current in certain predetermined paths. However, this is already also known from document DE 36 44 297 A1 mentioned above. 30 However, here again, it is obvious that the heating current in the general viewing field flows approximately in an overall direction perpendicular to the busbars and parallel to the longitudinal axes of 35 the conducting strands 46/56. Figure 3 shows a sectional view through the edge of the glazing unit 1 along the line III-III of figure 1. Two rigid individual panes 11, 12 (made of glass or - 27 - plastic) and an electrically insulating adhesive layer 13, which is optically transparent and joining said panes together by bonding in the usual manner, may be seen. This adhesive layer 13 is subdivided horizontally 5 by dot-dash lines in order to indicate that it is in fact considerably thicker than the transparent coating 2 deposited on the lower pane 12. This coating is shaded gray here for the sake of visibility. The adhesive layer may be formed in the usual manner by a 10 PVB film having a thickness of about 0.76 mm. The numerical references in figures 1 and 2 have been preserved. The coating 2, the outer edge region of which is separated by the separating lines 20, is, as 15 may be seen, located on the pane 12 above the busbar 5 and the conducting strands 56 connected to the latter, which strands have been deposited here, - before deposition of the coating 2, in the form of screen-printed structures. The opaque colored layer 3 2 0 here is printed on that face of the pane 11 located on the inside of the composite glazing unit and overlaps, in vertical projection (the viewing direction) the separating line 20, the busbar 5 and the conducting strands in their part which is connected directly to 25 the busbar. However, the conducting strands also extend beyond the edge 3 0 of the opaque colored layer 3 as far as the general viewing field of the glazing unit 1. The opaque colored layer 3 could also, unlike in the 30 representation, be located on the outer faces (not visible here) of one of the panes 11 or 12, or also on the same surface as the coating 2 and the busbars 4 and 5. 35 Figure 4 illustrates the arrangement of the conducting strands through another section, the viewing direction of which starts from the right in figure 3 in the plane of the surface of the pane 12. The view is therefore toward the front faces of the conducting strands 56, in - 28 - the background of which the busbar 5 extends transversely to the viewing direction. To improve viewing, in the region of the central 5 conducting strand in figure 4, the coating 2 on the surface of the pane 12 has been partly removed More precisely, it may be seen that both the busbar and the conducting strand lie beneath the coating 2 on the surface of the pane 12. 10 In another embodiment, illustrated in figure 5, with the busbar manufactured from strips of metal films, these would be applied to the coating 2 and assembled to the latter as continuously as possible and with as 15 low a resistance as possible by soldered assemblies or, where necessary, also by conductive adhesives (this also being known as an alternative). The soldered assemblies must of course be provided in particular at the conducting strands. 20 In a second version of the invention, illustrated in figure 6, starting from the upper busbar 4, a semiresistive region 6 formed by a particular region of the heating coating 2 and illustrated by crossed dotted 25 lines, extends beneath the colored layer 3 and then into the general viewing field of the laminated glazing unit 1 from the edge region covered by the colored layer 3 toward the inside of the secondary viewing field B. In this semiresistive region 6, the resistance 30 of the heating coating 2 is less than the resistance of the heating coating outside any semiresistive region. In this semiresistive region 6, the resistance of the heating coating 2 is two times, five times or even ten 35 times less, or even less, than the resistance of the resistive coating outside any semiresistive region. Starting from the lower busbar 5, a semiresistive region 6' formed by one particular region of the - 29 - heating coating 2, similar to the semiresistive region 6 and illustrated also by crossed dotted lines, extends into the viewing field B of the laminated glazing unit 1. 5 It is not necessary to provide such semiresistive regions of the coating for both busbars, 4 and 5 respectively. However, if semiresistive regions of the coating are provided on both sides of the main viewing 10 field A, they do not extend into the central part of the general viewing field and of the heating field, and in particular the main viewing field A remains undisturbed. 15 This second version may be obtained in industrial practice by depositing additional layers in semiresistive regions, either during deposition of the heating coating itself, by transverse variations in the thicknesses of the layers on the large plates coated 20 continuously with the heating coating, or after the substrates coated with the heating coating have been cut out, by an additional local deposition on these substrates. 25 In a third version of the invention, illustrated in figure 7, the glazing is a composite glazing unit that includes at least one adhesive layer 13' in electrical contact with the heating coating 2. This adhesive layer 13' includes at least one semiresistive region 30 illustrated here by a band 60 of semiresistive plastic. This band is included in that face of the adhesive layer that will be in contact with the heating coating 2 during manufacture of the layer so that, during manufacture of the glazing unit, the plastic band 60 is 3 5 in contact with the busbar 5. This third version is illustrated in the lower part of the glazing unit, but it may of course also, or instead, be used in the upper part of the glazing unit. - 30 - It is also possible for the plastic band or portion to be produced over the entire thickness of the adhesive layer 13' or else for it to be formed by a strip of 5 plastic that incorporates a conducting grid on its surface in contact with the heating coating. It is also conceivable to combine the plastic band 60, which is possibly conducting, with the coating*2 as a 10 semiresistive region, since in each case that part of the coating 2 which is covered by said band 6 0 conducts at least part of the heating current. To produce such a semiresistive or quite conductive 15 plastic band 60, it is for example possible to dope a "matrix" material of the thermoplastic film used (PVB, EVA or polyurethane) with conducting particles, especially metal particles, of such a density that the region or volume in question is at least semiresistive 20 or even conductive. Such doping is possible without excessively impeding the light transmission. The present invention has been described in the foregoing by way of example. Of course, a person 25 skilled in the art is capable of producing various alternative embodiments of the invention without thereby departing from the scope of the patent as defined by the claims. 30 A person skilled in the art is in particular capable of combining various versions and embodiments of the invention described above. - 31 - CLAIMS 1. A transparent glazing unit (1) provided with a resistive heating coating (2) that extends over a 5 substantial part of the glazing unit, in particular over a main viewing field (A) , and is electrically connected to at least two busbars (4, 5) in such a way that, when an electrical supply voltage is applied between the busbars, a current flows, which heats a 10 heating field in the coating, characterized in that said heating field includes at least one semiresistive region (6) in direct contact with at least one busbar (4, 5) . 15 2. The glazing unit as claimed in claim 1, characterized in that said main viewing field (A) does not include a semiresistive region. 3. The glazing unit as claimed in either of claims 1 20 and 2, characterized in that at least one semiresistive region (6) is in direct contact with at least one busbar at the positive potential. 4. The glazing unit as claimed in the preceding 25 claim, characterized in that at least one semiresistive region is in direct contact with at least one busbar at the negative potential. 5. The glazing unit as claimed in the preceding 30 claim, characterized in that the main viewing field (A) lies between said two at least semiresistive regions. 6. The glazing unit as claimed in any one of the preceding claims, characterized in that at least one 35 part of the peripheral edge of the glazing unit is concealed by an opaque colored layer (3), in particular in the region of the busbars (4, 5) , at least one semiresistive region (6) lying in the general viewing field of the glazing, beyond the area covered by the - 32 - colored layer. 7. The glazing unit as claimed in any one of the preceding claims, characterized in that the resistance 5 of the heating coating (2) in a semiresistive region (6) is at least less than the resistance of the heating coating outside any semiresistive region. 8. The glazing unit as claimed in the preceding 10 claim, characterized in that the resistance of the heating coating (2) in a semiresistive region (6) is at least two times, five times or even ten times less, or even less, than the resistance of the resistive coating outside any semiresistive region. 15 9. The glazing unit as claimed in any one of the preceding claims, characterized in that it is a composite glazing unit that includes at least one adhesive layer (13') in electrical contact with the 20 heating coating (2), said adhesive layer (13') having at least one semiresistive region (6). 10. The glazing unit as claimed in the preceding claim, characterized in that said semiresistive region 25 (6) of the adhesive layer (13') is produced by doping at least one band (60) of said adhesive layer (13') with conducting particles. 11. The glazing unit as claimed in any one of the 30 preceding claims, characterized in that a semiresistive region (6) comprises conducting strands (46, 56) formed from conducting printed lines and/or conducting wires. 12. The glazing unit as claimed in the preceding 35 claim, characterized in that said strands (46, 56) start from at least one of the busbars (4, 5), the length of said strands being greater than the width of the respective busbar, and said strands extending into the heating field in contact with the heating coating. - 33 - 13. The glazing unit as claimed in either of claims 11 and 12, characterized in that the strands (46, 56) have a width and/or a thickness of 0.5 mm or less, 5 preferably 0.3 mm or less, measured in projection on the surface of the glazing unit. 14. The glazing unit as claimed in any one of claims 11 to 13, characterized in that the strands (46, 56) 10 formed from conducting printed lines are printed on the heating coating (2). 15. The glazing unit as claimed in any one of claims 11 to 14, characterized in that the strands (46, 56) 15 formed from conducting wires are electrically connected to the heating coating and at least to said semiresistive region by soldering at least at discrete contact points. 20 16. The glazing unit as claimed in any one of claims 11 to 15, characterized in that the current flows in the general viewing field in a direction approximately parallel to the longitudinal direction of said conducting strands. 25 17. The glazing unit as claimed in any one of claims 11 to 16, characterized in that the strands (46, 56) are dark in color when viewed via an external -face of the glazing unit. 30 18. The glazing unit as claimed in any one of claims 11 to 17, characterized in that the strands (46, 56) are light in color when viewed via an internal face of the glazing unit. 35 19. The glazing unit as claimed in any one of claims 11 to 18, characterized in that the strands (46, 56) are placed uniformly apart. - 34 - 20. The glazing unit as claimed in any one of claims 11 to 19, characterized in that the strands (46, 56) are all produced with the same length. 5 21. The glazing unit as claimed in any one of claims 11 to 20, characterized in that the strands (46, 56) are produced in the form of straight lines, open or closed loops, portions of arcs and/or meanders. 10 22. The glazing unit as claimed in any one of the preceding claims, characterized in that the heating coating (2) is divided by separating lines (24) in the general viewing field, which lines divide up the heating coating (2) into current paths. 15 23. The glazing unit as claimed in the preceding claim, characterized in that said separating lines (24) collect the current in the main viewing field A. 20 24. The glazing unit as claimed in any one of the preceding claims, characterized in that the busbars (4, 5) are produced by printing and/or from metal strips. 25. The glazing unit as claimed in the preceding 25 claim, characterized in that the printed busbars are printed on the heating coating (2). 26. The glazing unit as claimed in claim 24, characterized in that the busbars (4, 5) produced from 30 metal strips are electrically connected to the heating coating (2) and to at least a semiresistive region (6) by soldering at least at discrete contact points. 27. The glazing unit as claimed in any one of the 35 preceding claims, characterized in that a semiresistive region is placed only over part of the longitudinal extent of one or each busbar (4, 5). 28. The glazing unit as claimed in any one of the - 35 - preceding claims in the form of a vehicle windshield, characterized in that the semiresistive region or regions (6) extends or extend at most as far as the boundary of the normalized viewing field B of this 5 windshield. 29. The glazing unit as claimed in any one of the preceding claims, characterized in that a bluish band extends at least along its upper edge and at least 10 partly conceals at least one semiresistive region (6) positioned at this point. A transparent glazing unit (1) provided with a resistive heating coating (2) that extends over a substantial part of the glazing unit, in particular over a main viewing field (A), and is electrically connected to at least two busbars (4, 5) in such a way that, when an electrical supply voltage is applied between the busbars, a current flows, which heats a heating field in the coating, said heating field including at least one semiresistive region (6) in direct contact with at least one busbar (4, 5).

Full Text

TRANSPARENT GLAZING UNIT WITH A RESISTIVE HEATING
COATING
The invention relates to a transparent glazing unit
5 with a resistive heating coating that has the
characteristics of the preamble of claim 1.
The invention relates more particularly to a glazing
unit whose resistive heating coating is a coating
10 placed on a substrate and having thermal insulation
and/or solar protection capabilities. The glazing units
incorporating this type of coating, when they are
intended for equipping vehicles, make it possible in
particular to reduce the air-conditioning load and/or
15 reduce excessive overheating ("solar control" glazing)
and/or reduce the amount of energy dissipated to the
outside ("low-e" or "low-emissivity" glazing) brought
about by the ever growing use of glazed surfaces in
vehicle passenger compartments.
20
One type of multilayer known for giving substrates such
properties consists of at least two metal layers, such
as a silver-based layer, each being placed between two
coatings made of a dielectric. This multilayer is
25 generally obtained by a succession of deposition
operations carried out using a vacuum technique, such
as cathode sputtering, optionally magnetically enhanced
or magnetron cathode sputtering. Two very thin metal
layers, called "barrier layers" may also be provided,
30 these being placed beneath, on, or to each side of each
silver layer, the underlayer as a tie, nucleation
and/or protection layer, for protection during an
optional heat treatment subsequent to deposition, and
the overlayer as a protective or "sacrificial" layer so
3 5 as to prevent impairment of the silver if the oxide
layer that surmounts it is deposited by sputtering in
the presence of oxygen and/or if the multilayer
undergoes a heat treatment subsequent to deposition.

- 2 -
In particular for vehicle windshields, there is high
demand in the market for heating versions, in which the
heating means must by nature be the least visible, or
least obstructing for viewing, as possible.
5 Consequently, there is an increasing demand for a
transparent heating coating for such glazing.
A general problem with heating coatings having a low
light absorption is their relatively high surface
10 resistance, which requires a high supply voltage, in
any case for heated glazing of large dimensions or for
long current paths, which voltage is in any case higher
than the usual voltages on board vehicles. If it is
desired to lower the surface resistance, this is
15 accompanied, in the multilayer systems known hitherto,
with a reduction in visible light transmission because
the conducting layers have to be thicker.
For these technical reasons, glazing units heated by
2 0 wires, which may be supplied without any problem by the
on board voltage, are still preferably being mounted at
the present time. These laminated glazing units with
integrated heating fields in the form of very fine
wires are, however, not accepted by all purchasers.
25
Patent DE 1 256 812 Bl discloses a glass pane that can
be heated by a coating made of metal or a metal oxide
deposited continuously on one of its surfaces. This
publication aims to solve problems due to the high
30 ohmic resistance of said coating, which is of the order
of 200 Ω/. However, to be able to heat this coating
using a relatively low voltage from two lateral
busbars, narrow printed electrodes of low ohmic
resistance (called "auxiliary electrodes") that extend
35 from said busbars over the heating field are provided.
Said auxiliary electrodes terminate only a short
distance in front of the opposite busbar, and they
overlap each other with an alternating polarity.

- 3 -
However, said lines, which are optically perceivable as
a hatching, obstruct vision and detract from the
optical appearance of the main viewing field-of the
pane thus produced. It is not possible to make use of
5 the optical advantage of a transparent heating coating.
It is for this reason that such a pane is designed only
for a rear window of automobiles.
Another problem with heating coatings may arise owing
10 to the fact that they are sometimes not able to be
deposited uniformly over the entire surface of the
transparent glazing, but one or more interruptions,
called "communication windows", have to be provided
therein, which disturb the flow of the heating current
15 and may form "hot spots" (local overheating) along the
edges of this or these communication windows.. Such
communication windows serve to make the coating, which
by nature is reflective for short-wave radiation,
respectively infrared radiation, locally more permeable
20 to certain data streams or signals.
To inject and extract the heating current in these
coatings, at least one pair of electrodes (in the form
of bands) or of busbars, which have to inject the
2 5 current into the heating coating and distribute it over
a wide front as uniformly as possible, is provided. In
vehicle glazing, which is substantially more wide than
high, the busbars are usually found along the longer
edges of the glazing (in the mounted position, the
30 upper and lower edges), so that the heating current can
travel along the shortest path over the height of the
glazing. At the same time, the aforementioned
communication windows are most of the time located at
the upper edge of the glazing and extend there over
35 several centimeters of width.
Document WO 00/72635 Al discloses a transparent
substrate with a coating that reflects IR rays and a
communication window produced locally by removal or

- 4 -
omission of the coating.
Obviously, each communication window which modifies the
uniformity of the coating disturbs the current flow.
5 Local temperature spots ("hot spots") appear, which may
result in damage to the substrate (thermal stresses)
and to the coating itself. This is not only the case
when the coating is defective over a large area, but
also when the communication window is formed by a
10 relatively large number of slots that do not
communicate with one another. These also result, in the
surface part in question, in an appreciable increase in
the layer resistance and also give rise to the
abovementioned hot spots.
15
The last document mentioned proposes, as a means of
reducing the problematic effect of an extensive
communication window, to provide, along its edge, an
electrically conducting band that has an" ohmic
20 resistance per square which is significantly lower than
that of the heating layer. Said band purports to take
the current around the cut. Preferably, a communication
window is framed entirely by such a band. The band may
be produced by printing a silver-containing conductive
25 screen-printing paste and by baking it. However, it may
also be applied by the deposition of an electrically
conducting lacquer or by depositing a metal strip. In
all cases, a conducting electrical connection of the
band to the coating is of course necessary in order for
30 it to operate.
The band may be concealed from view by superposing an
electrically nonconducting opaque masking strip, for
example made of black enamel. As a general rule, such
35 masking strips are made up from black-colored
nonconducting material (screen-printing paste) to be
baked. Infrared radiation is not reflected by this
material, but absorbed.

- 5 -
Document WO 03/024155 A2 discloses transparent glazing
of this type with a heating coating, in which, on the
one hand, a maximum nominal voltage of 42 V is
indicated, which however aims also to solve the problem
5 of "hot spots" along the edges of a communication
window. In general, various voltage levels are used, a
lower voltage being applied to shortened current paths
(for example because of the communication window) so as
to avoid local overheating. Specifically, the
10 communication window region is cut out from the heating
surface by placing a separate busbar between the
communication window and the busbar located on the
opposite side.
15 Also known, from document DE 36 44 297 Al, are many
examples of heating coatings for a vehicle windshield
that are divided. The divisions may thus be produced by
parts that are not provided with surface layers and/or
by notches produced mechanically or by a laser beam.
20 They are used for suitably adjusting and deflecting a
current flow within the coated surface and have to
ensure as uniform as possible a current density in the
surfaces in question.
25 Document WO 2004/032569 A2 discloses another
configuration of transparent glazing with a heating
coating, which also aims to achieve uniformity of the
heating power in the surface by separating lines traced
in the coating.
30
Document DE 29 36 398 Al relates to measures intended
for preventing current spikes in the transition between
the busbars and the coating, in transparent glazing
with a heating coating. In general, the aim is to
35 reduce the sudden difference in resistance between the
coating and the busbars using materials or shapes with
a higher resistance for the latter, or else with
intermediate resistances. The above document indicates
surface resistances of the coating of between 1 and 10

6
ohms per unit area. In one of the many embodiments
described in that document, the edge of each busbar
turn toward the opposite busbar is of corrugated form.
The formation of sharp points turneded toward the
5 heating coating must thus be avoided. This approach
aims to appreciably lengthen the transition line
between the busbar and the coating and consequently to
reduce the current density in this transition. However,
all these measures seem poorly suited to be able to
10 supply the heating layer with a relatively low voltage.
It is also known to provide, on the incident face of
photovoltaic solar cells, grid or comb electrodes (see
for example document WO 03/075351 Al) . They are often
15 produced by screen printing and made up of a busbar
placed along the edge of the solar cell and of a
plurality of small comb teeth that extend from the
busbar over the surface of the solar cell. These
electrodes allow surface connection for the
20 photovoltaic voltage, which is present on both faces of
the absorber, between the front comb electrode and the
rear metal electrode, respectively, over its entire
surface without excessively reducing the penetration of
light into the absorber.
25
Document DE 197 02 448 Al discloses a heated mirror, on
the glass substrate of which two comb-shaped conducting
tracks or electrodes are placed, these being indented
one in the other, with a PTC coating (i.e. one having a
30 positive temperature coefficient of resistance) that
covers them and fills the intermediate spaces between
the comb teeth. However, that document does not
consider the problem of making the heating invisible to
the eye, because the conducting tracks and the heating
35 layer may be placed behind the mirror layer.
Document DE 198 32 228 Al discloses vehicle glazing
with an electrically conducting coating that is
optically transparent and used as an antenna. Purely

- 7 -
capacitive high-frequency radio signals are picked up
from the antenna layer using a coupling electrode,
which is made up of several fine wires connected
together and placed parallel to one another at a
5 certain distance apart that is large compared with
their diameter, which wires extend from the edge into
the viewing field of the glazing and terminate therein,
without continuing. There is no galvanic coupling
between the coating and these wires, because each time
10 they lie in different planes from the laminated
glazing.
The busbars already mentioned many times may be
produced on the glass pane equally well by printing
15 (screen printing) before or after deposition of the
coating, or by soldering thin strips of sheet metal,
preferably made of tinned copper. Combinations of
printed busbars and metal-strip busbars are also known
(see for example document DE 198 29 151 Cl).
2 0 Admittedly, the busbars are usually narrow and in the
form of strips, but they are not transparent. For
optical reasons, they are therefore placed each time
near the outer edge of the transparent glazing units in
question. Most of the time, they may be masked by
25 opaque edge coatings (usually produced by screen
printing). Likewise, the aforementioned communication
windows may be masked by these edge coatings, provided
that they are sufficiently permeable to the radiation
to be transmitted via the communication window.
30
In standard vehicle windshields, these opaque coatings
are in the form of a frame provided all around the
glazing, which frame also has the function of
protecting the bonded joint between the glazing and the
35 body from UV radiation. These frames surround the
general viewing field of the glazing. In windshields, a
distinction may also be made between the main viewing
field A, approximately in the middle of the area of the
glazing, in which there can be no perceptible

8
impairment of vision (for example by colorations or
wires or other damage larger in size than 30 microns) ,
and the secondary viewing field B closer to the edges.
5 The problem at the basis of the invention therefore
consists in how to provide a transparent glazing unit
provided with a heating coating that can operate with
relatively low nominal voltages, in particular around
12 to 14 volts, and which nevertheless produces a
10 uniform distribution of the heating, in particular
without any hot spot, with viewing in the general
viewing field of the glazing, and in particular in the
main viewing field A of the glazing, which is impeded
as little as possible.
15
This problem is solved according to the invention by
the features of claim 1. The features of the dependent
claims provide preferred embodiments of this invention.
20 According to the invention, the heating field, formed
by the current flowing between the busbars when an
electrical supply voltage is applied between these
busbars, includes at least one semiresistive region in
direct contact with at least one busbar.
25
The term "conducting" should be understood within the
context of the present invention to mean that the
element thus termed admittedly has an electrical
resistance, as it is out of the question here to use
30 superconductors, but that this resistance is very low,
in such a way that when the electric current used flows
through this element, it does not heat up so as to be
perceptible by touching it with one's hand within one
minute of applying voltage, that is to say that this
35 element will be classed as constituting a cold region
when the glazing unit is observed by thermography.
The term "resistive" should be understood within the
context of the present invention to mean that the

- 9 -
element thus termed has a high overall electrical
resistance, in such a way that, when the electric
current used passes through this element, it heats up
so as to be perceptible by touching it with one's hand
5 within one minute after applying voltage, that is to
say that this element will be classed as constituting a
hot region when the glazing unit is observed by
thermography. Within the technical field in question,
the resistive regions have a surface resistance of
10 around 0.5 to 5 ohms per unit area and the hot regions
formed at their places have power densities of at least
400 to 450 watts/m2.
The term "semiresistive" should be understood within
15 the context of the present invention to mean that the
element thus termed has a low overall electrical
resistance, which is less than that of the resistive
element (s), but greater than that of the conducting
element(s). The expression "semiresistive region" used
20 in particular here denotes an area having a low overall
resistance. However, the region may have a high
resistance in certain places and a very low resistance
in other places. For example, it may even include
conducting elements and resistive elements, the
2 5 combination and configuration thereof make said region
"semiresistive".
The heating field is the direct result of the electric
field when voltage is applied to the terminals of the
30 electric field. It also denotes the actual -heating
region of said glazing, which extends between the two
busbars.
The object of the present invention is to create a
35 novel heating field through the use of a particular
electric field. As in the electric fields of the prior
art, the ends of the electric field are formed by
conducting regions embodied by the busbars, and between
these busbars an electric field is produced. However,

- 10 -
unlike the prior art, the surface resistance of this
field is not uniform over its entire area - a
semiconducting region is produced in contact with at
least one busbar. This has the effect of promoting
5 electrical conduction in this region through which the
current flows and has the effect as it were of favoring
energy transport into the following resistive region.
At least one part of the electric field according to
10 the invention thus has the following scheme, from one
busbar to the other:
conducting region/semiconducting region/resistive
region/.../conducting region.
15 Thus over at least one path between two busbars, the
current will firstly pass through a semiconducting
region and then through a resistive region.
The electric field between the busbars thus, has a
20 resistivity gradient formed from at least two separate
(semiresistive/resistive) states. There may also be a
gradual gradient formed from many states, thus passing
gradually from the conducting state to the resistive
state and then returning to the conducting state.
25
This may all be observed by thermography.
The present invention is of most particular importance
in the "panoramic windshield" technology.
30
In this technology, the aim is to produce windshields
that are as wide and/or as tall as possible, which
include portions extending laterally along the sides of
the vehicle and/or on the roof of the vehicle.
35
Thanks to the invention, it is thus possible to produce
heating panoramic windshields in which the heating
power is concentrated in the essential part of the
glazing, namely the main viewing field A.

- 11 -
With these features and arrangements, a relative
shortening of the path of the current flow within the
coating of relatively high resistance itself is
5 obtained because part of the distance between the
actual busbars and the main central heating field, in
the main viewing field A, is crossed by low-resistance
auxiliary conductors or else by said semiresistive
regions.
10
The main viewing field A of the glazing preferably has
no semiresistive region and thus it remains optically
free of any perturbations or obstructions.
15 In an alternative embodiment, at least one
semiresistive region is preferably in direct contact
with at least one busbar at the positive potential.
At least one other semiresistive region is then
20 preferably in direct contact with at least one busbar
at the negative potential and the main viewing field A
then preferably lies between said two at least
semiresistive regions.
25 In one particular version of the invention, the
semiresistive region includes conducting strands formed
from conducting printed lines, preferably printed on
the heating coating 2 and/or from conducting wires,
these conducting wires preferably being electrically
3 0 connected to the heating coating and at least to said
semiresistive region by soldering at least at discrete
contact points.
The conducting strands cover only part of the heating
35 field (close to the edges of the glazing), especially a
relatively wide strip along the busbars.
They terminate blindly, preferably before the boundary
of the central viewing field A.

- 12 -
Thus, a semiresistive region that does not greatly
impair viewing, which in any case is the aim with any
semiresistive region referred to here, is used.
5
Unlike in document DE 1 256 812 Bl, there are no
overlapping elements of opposite polarity in the region
of the main viewing field and the current flows, after
activating the electrical power supply, approximately
10 in a direction normal to the busbars and therefore in a
direction parallel to the overall longitudinal
direction of the blindly terminating conducting
strands. It should be understood that this "overall
longitudinal direction" is the general direction or
15 extension along which said strands extend from the
busbars toward the main viewing field.
Furthermore, the resistance of the transition between
the busbar and the coating is further reduced by
20 greatly increasing the areas of contact compared with
the prior art. Consequently, the voltage needed to make
the heating currents flow over the heating surface is
lower.
25 Admittedly, this configuration is preferably used for
windshields in which good visibility in the central
viewing field suffices for safe driving, however heated
glazing units according to the invention may also be
fitted at other places on the vehicle, and also in
30 other machines and moving equipment and in buildings.
Whereas in conventional solar cells with gate or comb
electrodes the voltage is applied over the thickness of
the layer of the absorber, a voltage is applied in the
35 application according to the invention for the purpose
of making a current flow in the plane of the coating.
The conducting strands and the semiresistive -regions
according to the invention thus have the effect of
bringing the busbars, customarily placed along the edge

- 13 -
of the glazing, electrically closer together without
however appreciably degrading the general viewing field
of the glazing and without degrading at all the main
viewing field A.
5
Moreover, it should be recognized that in document
WO 00/72635 cited above, the electric field, and
consequently the heating field, does not include a
semiresistive region since the strip that surrounds the
10 communication window is excluded from the electric
field owing to the fact that a resistive region is
interposed between the closest busbar and the
communication window that it surrounds. The trace of
the electric field lines in the last figure of the
15 above document shows that the lines go around the
communication window while still remaining in the
resistive coating, without passing through the strip
that surrounds the communication window.
2 0 When used in a vehicle, the configuration according to
the invention makes it possible in particular for the
windshield to be supplied directly, in order to heat
it, with the usual onboard voltage of 12 to 14 V DC,
for which voltage a coating having as low as possible
25 an ohmic resistance is of course recommended. The
extent of the semiresistive regions or else the length
of the strands is dimensioned according to the
effective surface resistance of the coating chosen
the more conducting the coating itself, the narrower
30 the semiresistive regions may be or the shorter the
strands may be.
Measured from the busbars, the extent of the
semiresistive regions or the length of the strands is
35 greater than the width of the respective busbar to
which they are attached and the strands that extend
into the heating field in contact with the heating
coating.

- 14 -
Likewise, with this configuration it is possible to
keep the entire coating on the surface of the
transparent glazing - apart from the optionally
provided communication windows - in such a way that
5 neither masking nor removal of coating is necessary.
Thus, the positive properties of the coating, namely in
particular its infrared reflection (thermal insulation)
and its uniform color (in reflection and in
transmission), are preserved over the entire surface.
10
The conducting strands have a width and/or a thickness
of preferably 0.5 mm or less, and even more preferably
0.3 mm or less, measured in projection on the surface
of the glazing unit.
15
The additional conducting strands, which are also as
thin as possible, only imperceptibly impede vision
through the glazing.
20 Given that the transparent glazing is in almost all
cases a laminated glazing unit in which the coating
itself is placed on a face lying on the inside of the
composite glazing, the conducting strands could also,
apart from printing, be produced in the form of thin
25 wires that are fixed, for example in a manner known per
se, to a composite adhesive film and then deposited,
with this film, on the coating, thus coming into
electrical contact with the coating. This contact is
stable over a long period after the final bonding of
3 0 the laminated glazing.
In the embodiment in the form of screen-printed
structures, the conducting strands are preferably
deposited on a substrate (made of glass or plastic or a
35 plastic film) before the coating is applied. This may
be carried out in a single operation together with the
deposition of the actual busbars.
It is also possible to straddle, with a region of low

- 15 -
resistance or with conducting strands, one or more
communication windows produced along the edge of the
glazing in the coating, without any risk of forming hot
spots. The currents in the known problem areas along
5 the lateral edges of such communication windows are
very greatly reduced by the strands.
Other details and advantages of the subject of the
invention will become apparent from the drawings of an
10 exemplary embodiment in the form of a vehicle
windshield and from their detailed description that
follows.
In these schematic drawings, drawn to no particular
15 scale:
figure 1 illustrates an embodiment of a
transparent glazing unit with a resistive heating
coating, in which embodiment busbars in the form of
strips are connected to grid strands that extend in the
20 form of very fine fingers in the surface of the
glazing;
- figure 2 shows a second embodiment, in which the
heating coating is divided into current paths using
separating lines;
25 - figure 3 shows a partial cross section through a
glazing unit according to the invention along the line
III-III in figure 1;
- figure 4 shows a detail taken from figure 3;
figure 5 shows a detail similar to that of
30 figure 4 for another embodiment different from that of
figure 4;
figure 6 illustrates another embodiment of a
transparent glazing unit with a resistive heating
coating, in which the heating coating includes at least
35 one semiresistive region; and
figure 7 illustrates another embodiment of a
transparent glazing unit with a resistive heating
coating, in which the adhesive layer has a
semiresistive region, the figure being a partial cross

- 16 -
section, similar to that illustrated in figure 3,
through another glazing unit.
In the figures, an electrically resistive transparent
5 coating 2 is placed over the entire surface in a manner
known per se in a heated laminated glazing unit 1
having an essentially trapezoidal (curvilinear)
outline. The glazing unit 1 has been shown here only in
one half - its other half is equivalent.
10
The coating 2 is deposited in a known manner on a main
face of a substrate 11, this substrate then being
integrated into the glazing unit 1.
15 A broken line denoted by 20 indicates that the outer
edge of the continuously coated surface lies all
around, but slightly set back toward the inside of, the
peripheral outer edge of the laminated glazing unit 1,
that is to say an edge band is provided in the coating
20 all around the surface. Thus, the coating is, "on the
one hand, electrically isolated from the outside and,
on the other hand, protected against any corrosion
damage penetrating via the outer edge of the glazing.
The outer edge 2 0 may be set back by removing the
25 coating along the edge of the glazing, by masking the
outline of the substrate before the coating is
deposited on this substrate, or else by tracing a
separating line that passes through the coating and
runs along the outer edge of the substrate, which may
30 be sufficient for meeting the isolation and corrosion
protection objectives.
The coating 2 itself is preferably made up, in a" manner
known per se, from a multilayer solar-protection system
35 of high thermal resistance comprising at least one
metal functional layer and preferably at least two
metal functional layers, this system withstanding,
without any damage, the temperatures above 650°C that
are required for bending the glass panes, that is to

- 17 -
say without its optical, electrical and heat-reflecting
properties being degraded. The multilayer system also
includes, apart from the metal layers (which are
preferably based on silver), other layers such as
5 antireflection layers and, optionally, barrier layers.
However, in relation to the present invention, it is
also possible to use other electrically conducting
multilayer systems that have a low temperature
10 resistance, and in particular also multilayer systems
that are not deposited directly on a rigid glass pane
but on a plastic film, (preferably a PET film) . All
these multilayer systems are preferably deposited by
sputtering (magnetron cathode sputtering).
15
The surface resistivity of the current multilayer
systems of the type mentioned above lies between about
0.5 and 5 Ω/. Vehicle windshields with such multilayer
systems must achieve overall a light transmission of at
20 least 75% according to some standards, or 70% according
to other standards.
Of course, the composition and the production of the
coating are of secondary importance here, so that there
25 is no need to dwell on details thereof.
An opaque colored layer 3 in the form of a peripheral
frame has been deposited along the edge of the
laminated glazing unit 1, the inner edge 3 0 of which
30 layer, relative to the outer edge of the glazing unit,
circumscribes the general viewing field of the
transparent glazing unit 1. This layer may lie in a
plane of the laminated glazing unit other than that of
the coating 2 (being located on the inside or on the
35 outside of the composite glazing unit) . It serves as a
layer for protecting a bead of adhesive, with which the
finished glazing unit is bonded to a vehicle body, from
UV radiation. Moreover, it can conceal from view
connection elements for the main electrical heating

- 18 -
function and for the optional additional electrical
functions of the glazing unit 1.
Thus, the figure shows, along the upper edge of the
5 laminated glazing unit 1, in the region of the surface
covered by the colored layer 3, a first busbar 4 and,
along the lower edge, a second busbar 5. The two
busbars 4 and 5 are in direct conducting electrical
connection with the coating 2, in a manner known per
10 se.
Figures 1, 2 and 5 show, in half in the middle of the
glazing unit, a communication window 22 below the
busbar 4, which communication window is also covered by
15 the colored layer 3 and is therefore concealed from
view. It is also possible to provide several
communication windows.
Many vehicle windshields are provided, along their
20 upper edge, with a band (not shown here) that is bluish
but transparent to light ("band filter"), which in
particular reduces dazzling by sunshine. Likewise, such
a band may also help to conceal the communication
window from view. It may also replace part of the width
2 5 of the band of the colored layer 3 along the upper edge
of the glazing unit, or it may be provided as a
complement thereto. Since the general viewing field of
the glazing is defined by the inner edge of the colored
layer, it may consequently incorporate this bluish
3 0 band.
As a general rule, the laminated glazing unit 1 is made
up from two rigid glass and/or plastic panes 11 and 12
and from an adhesive layer 13 joining the panes at the
35 surface. The busbars 4 and 5 are placed on the adhesive
layer 13 (for example a thermoplastic adhesive film
made of polyvinyl butyral (PVB) , ethylene/vinyl -acetate
(EVA) or polyurethane (PU) ) and are fastened to its
surface before the adhesive layer is assembled and

- 19 -
bonded to the rigid panes.
The busbars 4 and 5 may be made up from thin narrow
strips of metal (copper or aluminum) film, which are
5 usually fixed beforehand to the adhesive film 13 and
are applied together with an electrical contact to the
coating 2 during assembly of the laminated layers.
However, the electrical contact may also be provided by
soldering the busbars 4 and 5. During the subsequent
10 autoclave process, a reliable contact is made between
the busbars and the coating by the action of heat and
pressure.
The busbars 4 and 5 may, as indicated above, be
15 produced, as a variant or as a complement, by printing
a conductive paste, which is baked while the glazing
units are being bent. This is also appreciably less
expensive than positioning parts of metal strips. In
all cases, the printed busbars have, during continuous
2 0 manufacture, a higher ohmic resistance than that of the
strips of metal film. The choice, between busbars made
of metal film and busbars in screen-printed band form,
therefore depends only on the type of glazing and
possibly on the overall resistance of the multilayer
2 5 heating system.
Compared with the coating 2, the busbars are always of
negligible ohmic resistance and do not heat up
appreciably during operation of the heating means,
30
Applying a voltage between the two busbars 4 and 5 in
the heating coating generates an electric field and,
through the resistive effect, a heating field.
35 Two (or more) heating fields may be provided, in a
manner known per se, in the laminated glazing unit 1,
which heating fields are to be supplied separately (for
example with a vertical division down the middle of the
glazing unit) and must also, of course, be connected to

- 20 -
their respective power supply via separate conducting
connectors. In this case, a common ground conductor may
be used for the two heating fields, in such a way that
only the busbar 4 or the busbar 5 has to be divided
5 into two parts, whereas the other busbar is continuous.
In the first version, four external connectors are
required, whereas in the second only three are
required.
10 The external connectors will not be discussed in detail
here, because they have already been described many
times in the art.
The outer boundary of the main viewing field A of the
15 windshield, lying on the inside of the general "viewing
field circumscribed by the edge 30 of the colored layer
3, is indicated schematically by a dotted line L. The
line L does not constitute an actual edge or the like
in the glazing or in the coating, rather it serves
20 merely to visually illustrate the approximate position
of the main viewing field A. The latter is described in
Annex 18 of the ECE R4 3 by means of certain parameters
of an arbitrary vehicle environment. In this field, no
perceptible degradation of vision having a size of
25 greater than 3 0 microns is permitted. In the secondary
viewing field B, around the outside of the main viewing
field A, slight limitations in vision, due to
additions, etc., are permitted.
3 0 According to the invention, when an electrical supply
voltage is applied between the busbars 4 and 5, a
current flows, forming a heating field in the coating,
this heating field including a semiresistive region 6
in direct contact with the upper busbar 4.
35
Thanks to the present invention, the heating current
flows between the busbars via the coating 2 in a
heating field that has a region of lower resistivity
and then a region having a higher resistivity.

- 21 -
However, in a preferred version of the invention, the
heating current flows between the busbars via the
coating 2 in a heating field that has a region of lower
5 resistivity and then a region of higher resistivity and
then once again a region of lower resistivity.
As illustrated in figures 1 and 6 in particular, an
upper semiresistive region 6 thus extends downward
10 beyond the area covered by the colored layer 3 in the
general viewing field of the glazing toward the main
viewing field A and a lower semiresistive region 6'
extends upward beyond the area covered by the colored
layer 3 in the general viewing field of the glazing
15 toward the main viewing field A.
In a first version of the invention, illustrated in
figures 1 to 5, starting from the upper busbar 4, a
semiresistive region 6 formed by a set of lines extends
20 beneath the colored layer 3, then, in the general
viewing field of the laminated glazing unit 1, from the
edge region covered by the colored layer 3, into the
secondary viewing field B.
25 These lines terminate blindly in the secondary viewing
field B, more or less near the boundary of the main
viewing field A. The length of the lines thus produced
depends directly on the conductivity of the coating
chosen.
30
These lines represent conducting strands 46, which are
electrically connected to the busbar 4 and to the
coating 2 and have a low ohmic resistance compared with
the latter. A number of these lines also straddle the
35 communication window 22 in such a way as to guarantee
that the coating 2 on either side of the communication
window, seen from the busbar 4, is supplied directly
with power. From the visual standpoint, these lines
that straddle the communication window 22 are masked by

- 22 -
the colored layer 3. Another form of masking may, as
already indicated, be optionally provided by a band of
bluish color (band filter), but not shown here.
5 Conducting strands 56, similar to the conducting
strands 46, also extend into the viewing field B of the
laminated glazing unit 1 from the lower busbar 5.
In each case, the combination of the conducting
10 strands, 46 or 56 respectively, forms with the coating
2 a semiresistive region according to the invention.
It is unnecessary to provide such conducting strands 46
and 56 for both busbars, 4 and 5 respectively. However,
15 if conducting strands are provided on both sides of the
main viewing field A, they in no case extend in such a
way that conducting strands of opposite polarity
overlap in a projection perpendicular to their overall
directions. Thus, the central part of the general
20 viewing field and of the heating field, "and in
particular the main viewing field A, remains
undisturbed.
Whereas in conventional glazing units with layer
25 heating without a semiresistive region the heating
current has to flow only via the coating over the
entire distance between the busbars, this distance may
be reduced to values of between 2 5% and 80% by the
semiresistive region(s) according to the invention and
3 0 in particular by the conducting strands according to
the present invention, depending on the extent of the
viewing field A, some of the current spanning the
remaining distance in the semiresistive region'(s) and
in particular in the conducting strands.
35
In figures 1 and 2, the strands 46 and 56 are placed
uniformly apart and are all produced with the same
length.

- 23 -
The inner end of the conducting strands is
substantially at the same potential as the busbars to
which they are connected.
5 In the general viewing field of the glazing unit, the
current flows approximately perpendicular to the
busbars 4 and 5 and parallel to the longitudinal
general direction of the conducting strands 46, 56.
10 Thus, the current flows in the general viewing field in
a direction approximately parallel to the longitudinal
direction of the conducting strands.
As previously, a flow of current, even though small,
15 remains over the entire surface of the coating, because
the busbars are not separated from the coating in the
parts located between the conducting strands. However,
this current flow does not result in the formation of
hot spots along the edges of the communication window
20 22.
To meet the abovementioned objectives of the present
invention, the conducting strands, 46 and 56
respectively, must also be in intimate galvanic contact
25 with the coating, in addition to their good
conductivity (so as thereby to form, as mentioned
above, a semiresistive region). Certainly, in principle
it is conceivable to produce them in the form of wire
portions. However, they are preferably printed, before
3 0 the coating is deposited, on that surface of the
glazing unit that subsequently will have to bear the
coating. Certainly it is also possible to print them on
the finished coating, but this runs the risk of
damaging the multilayer coating, which is mechanically
3 5 weak.
Preferably, the conducting strands are printed using a
very conductive screen-printing paste containing at
least 80%, and preferably more than 85%, silver.

- 24 -
These conducting strands preferably have a dark color
seen via an outer face of the glazing, so as to be not
easily perceptible to the viewer's eyes when he is
5 looking from the outside into the vehicle and, also
preferably, have a light color, seen via an inner face
of the glazing, so as to be not easily perceptible to
the eyes of the viewer when he is looking out from
inside the vehicle.
10
If printed busbars 4 and 5 are used, they may then be
produced in a single operation at the same time as the
conducting strands 46/56 and using the same
screen-printing paste. No separate operation is then
15 needed to bring the conducting strands into electrical
contact with the busbars.
However, if busbars in the form of strips of metal film
are used, these then have to be connected with a low
2 0 ohmic resistance to the coating and to the conducting
strands. The strips of film, which are preferably
tinned, are therefore soldered to the conducting
strands in a manner known per se. In principle, the
screen-printing pastes with a high metal content that
25 are used here are easily soldered to the tinned metal
strips.
The lengths, separations and number of the conducting
strands 4 6 and 56 and the dimensions of the busbars can
30 only be shown here schematically. However, their
relative dimensions may be seen - whereas the busbars 4
and 5 are produced in the form of a usual band with a
width of a few millimeters, the conducting strands 46
and 56 are as close together and as invisible as
3 5 possible, and however appreciably longer than the
widths of the busbars.
The individual configuration in a specific laminated
glazing unit may certainly be broadly predetermined by

- 25 -
simulation. However, it will depend again very greatly
on the respective magnitude of the dimensions of the
specific glazing, on the type of busbars and on the
actual electrical properties of the coating. It may for
5 example also be sufficient to add conducting strands
only to one of the busbars. For a relatively short
distance between the two busbars 4 and 5, the
conducting strands themselves may also be shortened.
10 In the case of straight lines for a vehicle glazing
unit, these lines are not parallel but converge toward
the busbar to which they are attached, preferably in
such a way that a longitudinal symmetry (along the
longitudinal axis of the vehicle) may be observed in
15 the glazing.
It has been determined, for one specific type of
glazing unit, that spacing of 25 mm between the various
conducting strands can be used. However, the surface
20 heating power levels available in the semiresistive
regions can if necessary be adjusted, by varying the
spacing, for a given resistance of the conducting
strands. Furthermore, to simplify matters, only
straight conducting strands have been shown here, but
25 * this does not exclude the possibility in practice of
producing them in curved and/or corrugated " forms,
and/or in the form of open or closed loops and/or in
portions of arcs and/or in meanders, which could be
less visible.
30
Figure 2 shows an alternative embodiment in which the
coating 2 is divided by separating lines 24 in the
general viewing field. The separating lines 24 may pass
entirely through the coating down to the surface of the
35 substrate, or else they may penetrate only as far as
the conducting partial layer close to the substrate.
They have to divide the coating, which by nature is
continuous, into current paths. Various techniques
exist for producing such separating lines, among which

- 26 -
laser cutting is currently the most common, because it
is the most economic regarding the result. In
particular, the separating lines that can be produced
are thus extremely narrow and only perceptible to the
5 naked eye with difficulty.
If figure 2 is taken as representing the view seen by
the driver of a left-hand drive vehicle, he will
usually have to look through the surface part in which
10 the separating lines 24 are the closest together. The
purpose of these lines is to collect the current flow
through the coating 2 in the main viewing field A
precisely in this region, and thus provide the highest
heating power in this main viewing region when his
15 vision is obstructed by snow, ice or water dropl-ets and
to provide clear vision as quickly and as effectively
as possible.
Here too, the arrangement of the separating lines 24
20 has been indicated only schematically and only few
conclusions may be drawn as to actual configurations.
It is also not always judicious to always trace
continuous separating lines, rather it is possible to
produce some or all of them as segmented separating
25 lines, so to speak dotted lines, or to provide, instead
of the longer separating lines, a few short portions in
order to deflect the current in certain predetermined
paths. However, this is already also known from
document DE 36 44 297 A1 mentioned above.
30
However, here again, it is obvious that the heating
current in the general viewing field flows
approximately in an overall direction perpendicular to
the busbars and parallel to the longitudinal axes of
35 the conducting strands 46/56.
Figure 3 shows a sectional view through the edge of the
glazing unit 1 along the line III-III of figure 1. Two
rigid individual panes 11, 12 (made of glass or

- 27 -
plastic) and an electrically insulating adhesive layer
13, which is optically transparent and joining said
panes together by bonding in the usual manner, may be
seen. This adhesive layer 13 is subdivided horizontally
5 by dot-dash lines in order to indicate that it is in
fact considerably thicker than the transparent coating
2 deposited on the lower pane 12. This coating is
shaded gray here for the sake of visibility. The
adhesive layer may be formed in the usual manner by a
10 PVB film having a thickness of about 0.76 mm.
The numerical references in figures 1 and 2 have been
preserved. The coating 2, the outer edge region of
which is separated by the separating lines 20, is, as
15 may be seen, located on the pane 12 above the busbar 5
and the conducting strands 56 connected to the latter,
which strands have been deposited here, - before
deposition of the coating 2, in the form of
screen-printed structures. The opaque colored layer 3
2 0 here is printed on that face of the pane 11 located on
the inside of the composite glazing unit and overlaps,
in vertical projection (the viewing direction) the
separating line 20, the busbar 5 and the conducting
strands in their part which is connected directly to
25 the busbar. However, the conducting strands also extend
beyond the edge 3 0 of the opaque colored layer 3 as far
as the general viewing field of the glazing unit 1.
The opaque colored layer 3 could also, unlike in the
30 representation, be located on the outer faces (not
visible here) of one of the panes 11 or 12, or also on
the same surface as the coating 2 and the busbars 4 and
5.
35 Figure 4 illustrates the arrangement of the conducting
strands through another section, the viewing direction
of which starts from the right in figure 3 in the plane
of the surface of the pane 12. The view is therefore
toward the front faces of the conducting strands 56, in

- 28 -
the background of which the busbar 5 extends
transversely to the viewing direction.
To improve viewing, in the region of the central
5 conducting strand in figure 4, the coating 2 on the
surface of the pane 12 has been partly removed More
precisely, it may be seen that both the busbar and the
conducting strand lie beneath the coating 2 on the
surface of the pane 12.
10
In another embodiment, illustrated in figure 5, with
the busbar manufactured from strips of metal films,
these would be applied to the coating 2 and assembled
to the latter as continuously as possible and with as
15 low a resistance as possible by soldered assemblies or,
where necessary, also by conductive adhesives (this
also being known as an alternative). The soldered
assemblies must of course be provided in particular at
the conducting strands.
20
In a second version of the invention, illustrated in
figure 6, starting from the upper busbar 4, a
semiresistive region 6 formed by a particular region of
the heating coating 2 and illustrated by crossed dotted
25 lines, extends beneath the colored layer 3 and then
into the general viewing field of the laminated glazing
unit 1 from the edge region covered by the colored
layer 3 toward the inside of the secondary viewing
field B. In this semiresistive region 6, the resistance
30 of the heating coating 2 is less than the resistance of
the heating coating outside any semiresistive region.
In this semiresistive region 6, the resistance of the
heating coating 2 is two times, five times or even ten
35 times less, or even less, than the resistance of the
resistive coating outside any semiresistive region.
Starting from the lower busbar 5, a semiresistive
region 6' formed by one particular region of the

- 29 -
heating coating 2, similar to the semiresistive region
6 and illustrated also by crossed dotted lines, extends
into the viewing field B of the laminated glazing unit
1.
5
It is not necessary to provide such semiresistive
regions of the coating for both busbars, 4 and 5
respectively. However, if semiresistive regions of the
coating are provided on both sides of the main viewing
10 field A, they do not extend into the central part of
the general viewing field and of the heating field, and
in particular the main viewing field A remains
undisturbed.
15 This second version may be obtained in industrial
practice by depositing additional layers in
semiresistive regions, either during deposition of the
heating coating itself, by transverse variations in the
thicknesses of the layers on the large plates coated
20 continuously with the heating coating, or after the
substrates coated with the heating coating have been
cut out, by an additional local deposition on these
substrates.
25 In a third version of the invention, illustrated in
figure 7, the glazing is a composite glazing unit that
includes at least one adhesive layer 13' in electrical
contact with the heating coating 2. This adhesive layer
13' includes at least one semiresistive region
30 illustrated here by a band 60 of semiresistive plastic.
This band is included in that face of the adhesive
layer that will be in contact with the heating coating
2 during manufacture of the layer so that, during
manufacture of the glazing unit, the plastic band 60 is
3 5 in contact with the busbar 5.
This third version is illustrated in the lower part of
the glazing unit, but it may of course also, or
instead, be used in the upper part of the glazing unit.

- 30 -
It is also possible for the plastic band or portion to
be produced over the entire thickness of the adhesive
layer 13' or else for it to be formed by a strip of
5 plastic that incorporates a conducting grid on its
surface in contact with the heating coating.
It is also conceivable to combine the plastic band 60,
which is possibly conducting, with the coating*2 as a
10 semiresistive region, since in each case that part of
the coating 2 which is covered by said band 6 0 conducts
at least part of the heating current.
To produce such a semiresistive or quite conductive
15 plastic band 60, it is for example possible to dope a
"matrix" material of the thermoplastic film used (PVB,
EVA or polyurethane) with conducting particles,
especially metal particles, of such a density that the
region or volume in question is at least semiresistive
20 or even conductive. Such doping is possible without
excessively impeding the light transmission.
The present invention has been described in the
foregoing by way of example. Of course, a person
25 skilled in the art is capable of producing various
alternative embodiments of the invention without
thereby departing from the scope of the patent as
defined by the claims.
30 A person skilled in the art is in particular capable of
combining various versions and embodiments of the
invention described above.

- 31 -
CLAIMS
1. A transparent glazing unit (1) provided with a
resistive heating coating (2) that extends over a
5 substantial part of the glazing unit, in particular
over a main viewing field (A) , and is electrically
connected to at least two busbars (4, 5) in such a way
that, when an electrical supply voltage is applied
between the busbars, a current flows, which heats a
10 heating field in the coating, characterized in that
said heating field includes at least one semiresistive
region (6) in direct contact with at least one busbar
(4, 5) .
15 2. The glazing unit as claimed in claim 1,
characterized in that said main viewing field (A) does
not include a semiresistive region.
3. The glazing unit as claimed in either of claims 1
20 and 2, characterized in that at least one semiresistive
region (6) is in direct contact with at least one
busbar at the positive potential.
4. The glazing unit as claimed in the preceding
25 claim, characterized in that at least one semiresistive
region is in direct contact with at least one busbar at
the negative potential.
5. The glazing unit as claimed in the preceding
30 claim, characterized in that the main viewing field (A)
lies between said two at least semiresistive regions.
6. The glazing unit as claimed in any one of the
preceding claims, characterized in that at least one
35 part of the peripheral edge of the glazing unit is
concealed by an opaque colored layer (3), in particular
in the region of the busbars (4, 5) , at least one
semiresistive region (6) lying in the general viewing
field of the glazing, beyond the area covered by the

- 32 -
colored layer.
7. The glazing unit as claimed in any one of the
preceding claims, characterized in that the resistance
5 of the heating coating (2) in a semiresistive region
(6) is at least less than the resistance of the heating
coating outside any semiresistive region.
8. The glazing unit as claimed in the preceding
10 claim, characterized in that the resistance of the
heating coating (2) in a semiresistive region (6) is at
least two times, five times or even ten times less, or
even less, than the resistance of the resistive coating
outside any semiresistive region.
15
9. The glazing unit as claimed in any one of the
preceding claims, characterized in that it is a
composite glazing unit that includes at least one
adhesive layer (13') in electrical contact with the
20 heating coating (2), said adhesive layer (13') having
at least one semiresistive region (6).
10. The glazing unit as claimed in the preceding
claim, characterized in that said semiresistive region
25 (6) of the adhesive layer (13') is produced by doping
at least one band (60) of said adhesive layer (13')
with conducting particles.
11. The glazing unit as claimed in any one of the
30 preceding claims, characterized in that a semiresistive
region (6) comprises conducting strands (46, 56) formed
from conducting printed lines and/or conducting wires.
12. The glazing unit as claimed in the preceding
35 claim, characterized in that said strands (46, 56)
start from at least one of the busbars (4, 5), the
length of said strands being greater than the width of
the respective busbar, and said strands extending into
the heating field in contact with the heating coating.

- 33 -
13. The glazing unit as claimed in either of claims 11
and 12, characterized in that the strands (46, 56) have
a width and/or a thickness of 0.5 mm or less,
5 preferably 0.3 mm or less, measured in projection on
the surface of the glazing unit.
14. The glazing unit as claimed in any one of claims
11 to 13, characterized in that the strands (46, 56)
10 formed from conducting printed lines are printed on the
heating coating (2).
15. The glazing unit as claimed in any one of claims
11 to 14, characterized in that the strands (46, 56)
15 formed from conducting wires are electrically connected
to the heating coating and at least to said
semiresistive region by soldering at least at discrete
contact points.
20 16. The glazing unit as claimed in any one of claims
11 to 15, characterized in that the current flows in
the general viewing field in a direction approximately
parallel to the longitudinal direction of said
conducting strands.
25
17. The glazing unit as claimed in any one of claims
11 to 16, characterized in that the strands (46, 56)
are dark in color when viewed via an external -face of
the glazing unit.
30
18. The glazing unit as claimed in any one of claims
11 to 17, characterized in that the strands (46, 56)
are light in color when viewed via an internal face of
the glazing unit.
35
19. The glazing unit as claimed in any one of claims
11 to 18, characterized in that the strands (46, 56)
are placed uniformly apart.

- 34 -
20. The glazing unit as claimed in any one of claims
11 to 19, characterized in that the strands (46, 56)
are all produced with the same length.
5 21. The glazing unit as claimed in any one of claims
11 to 20, characterized in that the strands (46, 56)
are produced in the form of straight lines, open or
closed loops, portions of arcs and/or meanders.
10 22. The glazing unit as claimed in any one of the
preceding claims, characterized in that the heating
coating (2) is divided by separating lines (24) in the
general viewing field, which lines divide up the
heating coating (2) into current paths.
15
23. The glazing unit as claimed in the preceding
claim, characterized in that said separating lines (24)
collect the current in the main viewing field A.
20 24. The glazing unit as claimed in any one of the
preceding claims, characterized in that the busbars (4,
5) are produced by printing and/or from metal strips.
25. The glazing unit as claimed in the preceding
25 claim, characterized in that the printed busbars are
printed on the heating coating (2).
26. The glazing unit as claimed in claim 24,
characterized in that the busbars (4, 5) produced from
30 metal strips are electrically connected to the heating
coating (2) and to at least a semiresistive region (6)
by soldering at least at discrete contact points.
27. The glazing unit as claimed in any one of the
35 preceding claims, characterized in that a semiresistive
region is placed only over part of the longitudinal
extent of one or each busbar (4, 5).
28. The glazing unit as claimed in any one of the

- 35 -
preceding claims in the form of a vehicle windshield,
characterized in that the semiresistive region or
regions (6) extends or extend at most as far as the
boundary of the normalized viewing field B of this
5 windshield.
29. The glazing unit as claimed in any one of the
preceding claims, characterized in that a bluish band
extends at least along its upper edge and at least
10 partly conceals at least one semiresistive region (6)
positioned at this point.

A transparent glazing unit (1) provided with a
resistive heating coating (2) that extends over a
substantial part of the glazing unit, in particular
over a main viewing field (A), and is electrically
connected to at least two busbars (4, 5) in such a way
that, when an electrical supply voltage is applied
between the busbars, a current flows, which heats a
heating field in the coating, said heating field
including at least one semiresistive region (6) in
direct contact with at least one busbar (4, 5).

Documents:

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Patent Number

269605

Indian Patent Application Number

1261/KOLNP/2007

PG Journal Number

44/2015

Publication Date

30-Oct-2015

Grant Date

29-Oct-2015

Date of Filing

11-Apr-2007

Name of Patentee

SAINT-GOBAIN GLASS FRANCE

Applicant Address

18 AVENUE D'ALSACE F-92400 COURBEVOIE

Inventors:

#	Inventor's Name	Inventor's Address
1	BAUBET, CAROLE	BOXGRABEN 24-26 52064 AACHEN, ALLEMAGNE
2	SCHMIDT, LOTHAR	MARIA-THERESIA-ALLEE 267 52074, AACHEN, ALLEMAGNE
3	MAURER, MARC	44 RUE CARNOT 60200 COMPIEGNE

PCT International Classification Number

H05B 3/84

PCT International Application Number

PCT/FR2005/050843

PCT International Filing date

2005-10-12

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	102004050158.0	2004-10-15	Germany