REDUCTION OF CORROSION OF SILVER WIRES ON A GLASS SUBSTRATE

Description

[0001] The invention relates to measures for preventing or reducing the corrosive discoloration of silver-based wires on a glass substrate in vehicle windows, and in particular to protective coatings applied for this purpose. The invention further relates to vehicle windows with corresponding coatings, a method for their production, and the use of coating materials for the production of vehicle windows.

[0002] Rear windows of motor vehicles are now typically equipped with electric heating elements. A vehicle's rear window heater is used particularly during the colder months to prevent fogging or defrosting.

[0003] The standard windshield heater used today consists of several heating wires applied to the windshield, for example, printed on it. These wires are usually arranged horizontally and electrically connected at their ends via busbars. During operation, current flows through these busbars, generating heat through the resistance of the heating wires, which then heats the windshield. This type of windshield heater was developed by the Ford Motor Company in the late 1960s and was first installed in production vehicles in 1974. Elemental silver is frequently used as the conductive material in these heating systems, as this precious metal offers the best possible electrical conductivity and ensures minimal light obstruction.From the side where the silver is applied to the glass pane, the silver is perceived as black or grey, while from the other side of the glass pane an orange color impression may also occur; this is due to an interaction of the silver and a partial penetration of the silver into the glass surface.

[0004] In another version of the rear window heater, the wires are not printed on, but are located between the two layers of a laminated glass pane.

[0005] German patent DE 10018276 A1 discloses a composite windscreen with electrically conductive wires embedded in a thermoplastic adhesive layer, wherein the wires consist of at least two conductive material layers. In a heating system printed exclusively onto the windscreen, where the conductive traces contain elemental silver, the heating wires can develop a partial and superficial yellowing or browning over a longer period. This often uneven discoloration of the heating conductors detracts from the windscreen's aesthetics. Furthermore, it can give car users the impression that the wires' function is impaired. To avoid such concerns about the heating system's functionality, there was a need for measures to reduce or completely prevent discoloration of the heating wires in windscreen heaters.

[0006] US patent 2011 / 074796 A1 discloses a rear window with heating wires, which has a protective film that at least partially covers the heating wires.

[0007] WO 2005 / 117494 A1 discloses vehicle rear windows made of polycarbonate with a heating system and a protective coating made of a silicone hard coat.

[0008] The present invention relates to measures and a system with which undesirable discoloration of the heating wires in window heaters can be reduced or completely prevented.

[0009] Surprisingly, the investigations underlying this application revealed that the aforementioned discoloration of the silver in the heating wires of window heaters is caused by the surface oxidation of the silver. This oxidation process, in which Ag₂S is formed by the action of hydrogen sulfide, which can be present in very small quantities in the ambient air, is responsible for the observed yellow or brown discoloration of the wires. Based on these findings, this invention proposes a coating for the elemental silver in the heating wires and cables of a window heater. Such a coating protects the elemental silver in the heating wires and cables, in particular, from undesirable discoloration.

[0010] According to a first aspect, the present invention therefore relates to a vehicle windshield comprising at least a glass substrate with elemental silver applied thereon in a non-continuous form, wherein the elemental silver comprises a coating. The coating contains a thiol and / or a silicone resin and / or a silicate. The coating is a protective coating against discoloration.

[0011] The term "in non-continuous form" refers to the fact that the elemental silver is not applied to the entire surface of the glass pane as a uniform layer, but rather in such a way that uncoated areas exist between individual coated areas. The silver is applied to the glass substrate in the form of several strips or lines, between which are uncoated areas of the glass substrate. The strips or lines are connected at their ends, in particular by strips that are, for example, designed as busbars. The elemental silver is applied to the glass substrate as several lines or tracks. Within the scope of this application, the applied lines are also referred to as wires.

[0012] The statement that the glass substrate should "have elemental silver applied to it" is to be understood as meaning that the glass substrate has a coating in the relevant areas that contains elemental silver, but does not necessarily consist only of it.

[0013] In the invention described herein, the term "applied" encompasses the application of the silver using methods known to those skilled in the art, including printing methods such as screen printing, inkjet printing, aerosol printing, engraving printing or flexographic printing, but also methods by which the silver is printed onto the substrate as an extruded paste.

[0014] The statement "where the elemental silver has a coating" is to be understood as meaning that at least the elemental silver applied as several lines or tracks has a coating applied to it, i.e., that at least the elemental silver applied as several lines or tracks is covered with a coating.

[0015] The statement that the "vehicle windshield comprises at least one glass substrate" is to be understood in particular as meaning that the vehicle windshield is designed as a single pane made of a glass substrate. Alternatively, the vehicle windshield can be designed as laminated glass consisting of a glass substrate and another glass pane bonded together via a thermoplastic interlayer.

[0016] Preferably, the vehicle window has the shape of a rear window. Furthermore, it is preferred that the glass forming the glass substrate is heat-treated and preferably tempered glass. Such glass is available, for example, under the trade name Sekurit from Saint-Gobain and is produced by quenching a sheet heated to approximately 600°C with a cold air stream. This process creates strong compressive stresses in the surface, which significantly increase the flexural strength and, among other things, the thermal shock resistance of the glass.

[0017] The glass substrate preferably consists of soda-lime glass, as is common for window panes, but can also contain other types of glass, such as borosilicate glass, aluminosilicate glass, or quartz glass. The thickness of the glass substrate is typically from 0.5 mm to 10 mm, preferably from 1 mm to 5 mm.

[0018] The elemental silver applied to the glass substrate of the vehicle windshield is applied as a plurality of lines that are connected at their ends. Such a configuration of applied silver, as seen in Fig. 1 As depicted, this is a prerequisite for integrating the vehicle windshield into a heating device that allows it to be heated. The majority of the lines applied to the glass substrate of the vehicle windshield form part of a heating system. Such heating systems typically include, in addition to the lines themselves, connecting elements for the lines, allowing the individual lines to be connected to a power source, as well as a control element for activating or deactivating the connection to the power source.

[0019] The term "lines" is used synonymously with "paths" in the present invention and refers to a structure that extends significantly further in length than in width and thickness, i.e. the length of the structure is preferably at least 100 times and particularly preferably at least 250 times greater than the width and thickness of the structure.

[0020] The "majority" preferably consists of at least 3 and in particular at least 5 lines and / or preferably at most 20 and in particular at most 15 lines.

[0021] The glass substrate of the vehicle windscreen has a plurality of lines, and the area covered by the lines should be as small as possible; that is, preferably no more than 5% and particularly no more than 2% of the total area of ​​the glass substrate should be covered by the lines. On the other hand, the lines should be distributed as homogeneously as possible over the entire area of ​​the glass substrate, e.g., in such a way that the lines, including the area of ​​the glass substrate located between the lines, cover a total proportion of the glass substrate of at least 60%, preferably at least 70%, and particularly preferably at least 80%.

[0022] Regarding the thickness of the lines, the present invention is not subject to any relevant limitations. However, the thickness should be as thin as possible so that it is not perceived as disruptive by the user and does not restrict their field of vision. On the other hand, the lines should also be sufficiently thick so that their function as heating wires is not impaired. A range of 1 to 200 µm (micrometers) is suitable for the line thickness, particularly 5 to 100 µm, and most preferably in the range of approximately 20 to 50 µm. In particularly preferred cases, a thickness in the range of 10 to 20 µm can also be achieved.

[0023] The width of the lines is also not subject to any relevant restrictions, although here too a narrow width is preferred in order to avoid disturbing the field of vision and restricting it more than necessary. Preferably, the lines are 10 µm to 1 cm wide, and more preferably 100 µm to 2 mm wide. It is preferred within the scope of the invention that the coating itself does not alter the optical appearance of the elemental silver on the glass substrate. Consequently, colored coatings are preferably excluded within the scope of the present invention.

[0024] A suitable coating agent is one containing a thiol. Such thiols can effectively passivate silver surfaces. This can result in a color appearance that, while differing from the silver's color before thiol treatment, is uniform / homogeneous, stable, and does not change over the coating's service life. Thiols also have the advantage of being selectively adsorbed only onto the silver surface due to the Ag-S interaction.

[0025] A suitable class of thiols that are easy to process are, for example, thiocarboxylic acids, which are applied in aqueous emulsion and adhere well to the silver surface. Thiocarboxylic acids have the advantage that they can be formulated as aqueous emulsions without an emulsifier because they possess a hydrophilic carboxyl group, which makes them partially water-soluble. On the other hand, aqueous emulsions of thiocarboxylic acids have a short shelf life and are difficult to handle.

[0026] Another class of suitable thiols are longer-chain (i.e., with at least 6 carbon atoms) hydrophobic thiols or their esterification products without hydrophilic carboxyl groups. Such thiols offer the advantage not only of adequate protection against oxidative discoloration but can also be applied as aqueous emulsions. This application method is described in detail, for example, in EP 0492487 B1.

[0027] Particularly preferred are hydrophobic aliphatic thio compounds with at least 12 carbon atoms in the chain, and especially preferred are hydrophobic aliphatic thio compounds with no more than 20 carbon atoms in the chain. With fewer than 12 carbon atoms, a well-adhering protective coating may not be achievable in some cases, or a strong odor may occur. Hydrophobic aliphatic thio compounds with more than 20 carbon atoms, on the other hand, are so rigid in consistency that they cannot always be processed as an emulsion without organic solvents, which complicates the application of the thiols. Hexadecanethiol is a particularly preferred hydrophobic aliphatic thio compound in this context.

[0028] The thiols mentioned above are particularly advantageous because they can be emulsified very finely and with long-term stability and can be applied in a thin but dense layer to the surfaces to be protected. Emulsifiers, such as branched fatty alcohols with 9 to 20 carbon atoms, preferably with 10 to 15 carbon atoms, can optionally be added for this purpose. The branched fatty alcohols are preferably alkoxylated, particularly ethoxylated, and have a degree of alkoxylation of 2 to 10.

[0029] For thiol-based coatings, a pH value of 1–10 is suitable, a pH value of 1–8 is preferred, and a pH value of 2–4 is particularly preferred, since at this pH value the emulsifier can be almost completely washed out of the deposited thiol layer with water; this is important for reliable surface protection. If the pH value exceeds the specified range, less inhibitor is washed out, which increases the protective and lubricating effects, but can also lead to staining on the surface, which detracts from the appearance. At a pH value above 10, there is a risk of emulsion splitting.

[0030] Since thiols react selectively with silver, a thiol treatment typically results in the deposition of approximately one molecular layer of thiols on the silver surface. The thickness of the thiol coating is therefore generally determined by the chain length of the thiol and is usually less than 1 µm.

[0031] To fix the thiols to the silver, it is possible to overcoat the coating with another agent that, for example, provides better resistance to mechanical stress (scratching) and / or suppresses or prevents the thiols from being washed out.

[0032] Another suitable coating material, which is colorless and provides favorable protection for elemental silver against oxidative discoloration, is a coating material containing a silicone resin. Silicone resins also offer the advantage of providing favorable properties against abrasion, chemical agents, and UV radiation. Suitable silicone resins include, for example, alkylated silicone resins such as those available as BECKOLITE M-6652-60 or M-6650-60 (both from Dainippon Ink and Chemicals Co., Ltd.), and Baysilone Resin UD-460M or 180 (both from Bayer Co., Ltd.).

[0033] Another suitable class of silicone resins are silicone resins as described in JP 2012-056251 A.

[0034] Another suitable class of silicone resins for suppressing oxidative discoloration of elemental silver are siloxane-acrylic copolymers, as described in JPH-09 / 241532 A.

[0035] Another suitable class of silicone resins for suppressing oxidative discoloration of elemental silver is described, for example, in WO 1999 / 062646 A1. Such silicone resins comprise, as a main component, at least one resin type selected from the group consisting of a silicone acrylic resin, a silicone alkyd resin, and a multifunctional crosslinked silicone resin, wherein the multifunctional crosslinked silicone resin has an average composition formula of R n SiO (4-n) / 2 and a number-average molecular weight of 500–1000. In the formula, R represents a hydrogen atom, a low alkyl group (C1–C6), a phenyl group, or a substituted phenyl group; n is a number from 1.2 to 1.4.

[0036] Silicone resins can be applied to the glass substrate of the vehicle windshield in such a way that only the silver present on the glass substrate is covered by the silicone resin. However, it is also possible for the silicone resin to cover more than the silver on the glass substrate (i.e., an area that is 50% or 100% larger than the area covered by the silver; in the case of silver applied in a line on the glass substrate, the coating is, for example, 50% or 100% wider than the width of the line). It is also possible for the silicone resin to cover the entire surface of the glass substrate.

[0037] For a coating composition based on silicone resins, it can be advantageous if the coating composition contains, in addition to the silicone resin, a metal compound, preferably in a proportion of 0.01 to 5 parts by weight. A particularly suitable metal compound is, for example, a zinc salt or a zinc complex. Preferred zinc salts include, in particular, zinc salts of carboxylic acids with 3 to 20 carbon atoms and zinc phosphate salts. Particularly preferred zinc salts are zinc salts of aliphatic saturated or unsaturated carboxylic acids such as, in particular, 2-ethylhexyl carboxylic acid, neodecane carboxylic acid, lauric acid, ricinoleic acid, stearic acid, or undecylene carboxylic acid. Particularly suitable zinc phosphate salts are zinc phosphate ester salts. Particularly suitable zinc complexes are zinc complexes with carbonyl compounds such as acetylacetonate.

[0038] It is assumed that in the case of a coating composition containing such salts or complexes, any hydrogen sulfide penetrating the coating is absorbed by the salts or complexes, thereby converting them into zinc sulfide. As a result, the hydrogen sulfide can no longer penetrate to the elemental silver beneath the coating.

[0039] Alternatively or additionally, the coating composition can also contain zirconium salts or complexes, in particular zirconium complexes of carboxylic acids, preferably of lower carboxylic acids (C1 to C10, especially C2 to C6), which may be substituted with a benzoyl or methylbenzene group. Zirconium complexes of 4-methyl-γ-oxo-benzene-butanoic acid or p-toluoyl-propanoic acid are particularly suitable in this context.

[0040] The silicone composition may also contain common additives, such as in particular condensation catalysts for the silicone resin, solvents, smoothing agents, coupling agents, foam suppressants, matting agents, UV absorbers, antioxidants and the like, which are known to the skilled person for corresponding formulations.

[0041] A suitable thickness for a silicone coating can be specified as a range of about 1 to 150 µm, preferably 5 to 100 µm and particularly preferably 8 to 50 µm.

[0042] The coating compositions described above can be applied to the glass substrate using conventional coating methods, such as dip coating, spray coating, or vapor deposition. When coating with thiols, any unbound thiol can subsequently be washed off the substrate. Another suitable coating material is silicate (SiO₂ x with x ~ 2). Such a coating can be produced, for example, using a volatile silane precursor, which is burned in a flame to form silicate, with the silicate from the flame depositing directly onto the material to be coated. Devices for applying such silicate coatings are available, for example, from Arcotech under the name FTM. With a silicate coating, a very thin application is generally sufficient, with a layer thickness of approximately 10 to 50 nm, and particularly 15 to 30 nm, being considered suitable.

[0043] The invention relates to a vehicle windshield comprising a heating system formed from a plurality of wires comprising elemental silver, wherein the wires are covered with a coating containing a thiol and / or a silicone resin and / or silicate, as described in detail above. Coatings containing only one of these alternatives are particularly preferred.

[0044] The vehicle windshield can be designed such that the wires are located on an externally accessible surface of the glass substrate, which is designed as a glass pane. However, it is also possible for the wires to be mounted on a glass substrate as part of a laminated glass pane, with glass panes located above and below the wires. Since such a construction is generally considerably more complex to manufacture, and the problem of potential discoloration of the wires due to interaction with components of the ambient air is less pronounced in laminated glass panes, it is preferable for the vehicle windshield to have wires on an externally accessible surface of the glass pane, coated with the materials specified above. In this context, an externally accessible surface of the glass pane is understood to be a surface immediately adjacent to the vehicle interior or the external environment.

[0045] According to another aspect, the present invention relates to a method for manufacturing a vehicle windscreen, comprising the following steps: Providing a glass substrate having a heating system applied to it, which is formed from a plurality of wires containing elemental silver, applying a coating comprising a thiol and / or a silicone resin and / or silicate to the plurality of the wires, and optionally hardening the coating.

[0046] The coating can be advantageously applied by spraying or immersion in a solution containing the coating, by gas-phase coating, or by applying silicate by combustion of a suitable volatile precursor, as explained above. The coating composition applied comprises a thiol and / or a silicone resin and / or a silicate, and particularly preferably one of the thiol and / or a silicone resin and / or a silicate identified above as particularly suitable. Particularly preferably, the coating composition comprises only one of the aforementioned alternatives.

[0047] The glass substrate in the described process is advantageously designed as a vehicle windshield or as a precursor to a vehicle windshield, from which a vehicle windshield can be manufactured by finishing. The necessary steps for this, e.g., hardening, forming, or bending, can also be carried out on the coated glass substrate, provided this does not impair the applied coating. Silicate-coated glass substrates are particularly suitable for this purpose, as the coating exhibits the required thermal stability.

[0048] Finally, a further aspect of the present invention relates to the use of a composition comprising a thiol and / or a silicone resin for the manufacture of a vehicle windshield comprising a heating system formed from a plurality of wires comprising elemental silver, wherein the composition is applied to the wires as a protective coating. In this use, the coating is applied as a protective coating against discoloration. An analogous aspect relates to the use of a composition comprising a silicate precursor for the manufacture of a vehicle windshield comprising a heating system formed from a plurality of wires comprising elemental silver, wherein silicate is produced from the composition by combustion and applied to the wires as a protective coating.

[0049] The invention is explained in more detail with reference to figures and exemplary embodiments. The drawings are schematic representations and not to scale. The figures do not limit the invention in any way. They show: Fig. 1 shows a schematic view of a vehicle window designed as a rear window, comprising a glass substrate 1 with several elemental silver wires 2 applied to it, which are connected to each other at their ends via common busbars 3. According to the invention, at least the wires are provided with a coating. Fig. 2 shows a schematic view of a cross-section of an elemental silver wire 2 with a surface coating 4 of thiols on a glass substrate 1. The wire is, for example, a heating wire. Fig. 3 shows a schematic view of a cross-section of an elemental silver wire 2 with a surface coating 4 of silicone on a glass substrate 1, wherein the entire surface of the glass substrate 1 is covered with the silicone. The wire is a heating wire.

Claims

1. Vehicle pane at least comprising a glass substrate (1) with elemental silver (2) applied thereon discontinuously as a plurality of lines, wherein the plurality of lines are connected to one another at their ends, wherein the plurality of lines form part of a heating system, and wherein the elemental silver has a coating (4) that contains a thiol and / or a silicone resin and / or a silicate.

2. Vehicle pane according to claim 1, having the form of a rear window.

3. Vehicle pane according to one of claims 1 or 2, wherein the coating (4) contains a thiol, in particular an aliphatic C12 to C20 thiol.

4. Vehicle pane according to one of claims 1 through 3, wherein the coating (4) contains a silicone resin, in particular an alkylated silicone resin.

5. Vehicle pane according to claim 4, wherein the coating (4) additionally contains one or more of a zinc salt, a zinc complex, a zirconium salt, and a zirconium complex.

6. Vehicle pane according to one of claims 1 or 2, wherein the coating (4) contains silicate and preferably is made of silicate.

7. Vehicle pane according to one of claims 1 through 6, wherein the glass substrate (1) is a heat-treated and preferably a tempered glass.

8. Vehicle pane according to one of claims 1 through 7, wherein the coating (4) is applied only on the elemental silver.

9. Vehicle pane according to one of claims 1 through 7, wherein the coating (4) is applied substantially over the entire surface of the vehicle pane.

10. Method for producing a vehicle pane according to one of claims 1 through 9, comprising - providing a glass substrate (1) that has a heating system applied thereon formed from a plurality of wires that contain elemental silver (2), - applying a coating (4) that comprises a thiol and / or a silicone resin and / or silicate on the plurality of wires, and - optionally, curing the coating (4).

11. Use of a composition comprising a thiol and / or a silicone resin for the production of a vehicle pane that includes a heating system formed from a plurality of wires comprising elemental silver (2), wherein the composition is applied on the wires as a protective coating against discoloration (4).

12. Use of a composition comprising a silicate precursor for the production of a vehicle pane that includes a heating system formed from a plurality of wires comprising elemental silver (2), wherein silicate that is applied on the wires as a coating (4) is produced from the composition by combustion.