ENAMEL PASTE COMPOSITIONS, ENAMEL COATED PRODUCTS, AND METHODS OF MANUFACTURING THE SAME

Abstract

An enamel paste composition is provided for fabricating a coated glass article includes a glass substrate, an enamel layer disposed on the glass substrate, and a thin coating having a thickness of no greater than 0.5 micrometres disposed on the enamel layer. The enamel paste composition is formulated such that it provides a smooth surface and does not crystallize during processing of the coated glass article, thereby improving the quality of the thin coating disposed thereon.

Claims

1. An enamel paste comprising: 30 to 40% by weight of a bismuth silicate glass frit; 20 to 30% by weight of a zinc silicate glass frit; 15 to 25% by weight of a pigment; and 10 to 20% by weight of an organic carrier medium, wherein the bismuth silicate glass frit comprises at least silicon, bismuth, boron, and lithium, and is free of lead and cadmium, wherein the zinc silicate glass frit comprises at least silicon, zinc, sulphur, and lithium, and is free of lead, cadmium, and bismuth, wherein the glass frits and the pigment in the enamel paste each have a D90 particle size distribution of less than 15 micrometres, and wherein the enamel paste has a viscosity between 5 and 150 Pa.Math.s.sup.1 at a shear rate of 100 s.sup.1.

2. The enamel paste according to claim 1, wherein the bismuth silicate glass frit has the following composition: TABLE-US-00006 Component Mol % SiO.sub.2 45-55 Bi.sub.2O.sub.3 15-25 B.sub.2O.sub.3 10-20 Li.sub.2O 2-10 Al.sub.2O.sub.3 0-5 ZrO.sub.2 0-5 K.sub.2O 0-1 MgO 0-1 CaO 0-1

3. The enamel paste according to claim 1, wherein the zinc silicate glass frit has the following composition: TABLE-US-00007 Component Mol % SiO.sub.2 45-55 ZnO 20-30 Li.sub.2O 1-10 S 1-10 B.sub.2O.sub.3 0-10 Na2O 0-10 F 0-10 MnO 0-10 CaO 0-1 Fe.sub.2O.sub.3 0-1 MgO 0-1.

4. The enamel paste according to claim 1, wherein the pigment is a CuCr black pigment.

5. The enamel paste according to claim 1 for fabricating a coated glass article comprising a glass substrate, an enamel layer disposed on the glass substrate, and a thin coating having a thickness of no greater than 0.5 micrometres disposed on the enamel layer.

6. A method of fabricating a coated glass article, the method comprising: depositing an enamel paste according to claim 1 on a portion of a glass substrate; firing the enamel paste at a temperature T1 to form an enamel layer; depositing a thin coating over at least a portion of the enamel layer, the thin coating having a thickness of no greater than 0.5 micrometres; firing and bending the glass substrate with the enamel layer and the thin coating thereon at a temperature T2 which is greater than T1 to form the coated glass article, wherein the enamel paste is formulated to have a firing temperature lower than T1 and a crystallization temperature higher than T2.

7. The method according to claim 6, wherein the enamel layer has a roughness R.sub.a of no more than 1 micrometre and/or a roughness R.sub.z of no more than 5 micrometres.

8. The method according to claim 6, wherein the enamel layer has an L-value of no more than 5.

9. The method according to claim 6, wherein the thin coating is deposited over at least a portion of the glass substrate in addition to being deposited over at least a portion of the pre-fired enamel layer.

10. The method according to claim 6, wherein the thin coating is electrically conductive and at least one electrical connector is deposited on the thin coating over the pre-fired enamel layer.

11. The method according to claim 6, wherein the thin coating comprises a plurality of sub-layers.

12. The method according to claim 6, wherein the thin coating is deposited via a vacuum deposition process.

13. The method according to claim 6, wherein the thin coating is no more than 0.4, 0.3, 0.2 or 0.1 micrometres in thickness.

14. A coated glass article fabricated according to the method of claim 6.

15. The enamel paste according to claim 2, wherein the zinc silicate glass frit has the following composition: TABLE-US-00008 Component Mol % SiO.sub.2 45-55 ZnO 20-30 Li.sub.2O 1-10 S 1-10 B.sub.2O.sub.3 0-10 Na2O 0-10 F 0-10 MnO 0-10 CaO 0-1 Fe.sub.2O.sub.3 0-1 MgO 0-1.

16. The enamel paste according to claim 2, wherein the pigment is a CuCr black pigment.

17. The enamel paste according to claim 2 for fabricating a coated glass article comprising a glass substrate, an enamel layer disposed on the glass substrate, and a thin coating having a thickness of no greater than 0.5 micrometres disposed on the enamel layer.

18. A method of fabricating a coated glass article, the method comprising: depositing an enamel paste according to claim 2 on a portion of a glass substrate; firing the enamel paste at a temperature T1 to form an enamel layer; depositing a thin coating over at least a portion of the enamel layer, the thin coating having a thickness of no greater than 0.5 micrometres; firing and bending the glass substrate with the enamel layer and the thin coating thereon at a temperature T2 which is greater than T1 to form the coated glass article, wherein the enamel paste is formulated to have a firing temperature lower than T1 and a crystallization temperature higher than T2.

19. The method according to claim 18, wherein the enamel layer has a roughness R.sub.a of no more than 1 micrometre and/or a roughness R.sub.z of no more than 5 micrometres.

20. The method according to claim 7, wherein the enamel layer has an L-value of no more than 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0038] FIG. 1 shows an automotive glass panel configuration of the type described in US20170135160;

[0039] FIG. 2 shows a flow diagram of the steps involved in providing a coated glass sheet for use in the automotive glass panel configuration shown in FIG. 1;

[0040] FIGS. 3(a) and 3(b) show flow diagrams illustrating that following the method of FIG. 2 leads to damage of a thin film during processing unless the firing (melting) temperature of the enamel paste composition is lower than the firing temperature T.sub.1 and the crystallization temperature of the enamel paste composition is higher than the bending temperature T.sub.2;

[0041] FIG. 4 shows the colour development characteristics of an enamel layer formed using enamel paste composition according to this specification in terms of its L-value vs firing temperature, the enamel being formulated to have a low L-value at the final firing (bending) temperature T.sub.2 of the coated glass manufacturing method without crystallization; and

[0042] FIG. 5 shows the silver hiding characteristics of an enamel layer formed using an enamel paste composition according to this specification in terms of its dE value vs firing temperature, the enamel being formulated to have a low dE value (good silver hiding) at the final firing (bending) temperature T.sub.2 of the coated glass manufacturing method without crystallization.

DETAILED DESCRIPTION

[0043] As described in the background section, the glass panel configuration described in US20170135160 is shown in FIG. 1 and comprises two glass sheets 2, 4 united by an interlayer sheet 6. The outer side 8 of outer glass sheet 2 is conventionally designated side 1, the inner side 10 of outer glass sheet 2 is conventionally designated side 2, the side 12 of the inner glass sheet 4 which faces the outer glass sheet 2 is conventionally designated side 3, while the interior side 14 of inner glass sheet 4 is conventionally designated side 4. An enamel layer 16 is formed on side 2 of the outer glass sheet. A thin coating 18 is formed over the enamel layer 16 and over un-enamelled portions of side 2 of the outer glass sheet. An electrical bus bar 20 is formed on the thin coating 18 over the enamel layer 16. The glass sheets 2, 4 are fired and shaped and then laminated with the interlayer sheet 6 to form the glass panel.

[0044] It should be noted that while the configuration shown in FIG. 1 has the enamel layer 16 and thin coating 18 disposed on side 2 of a laminated glass panel comprising two glass sheets 2, 4, the present invention is not limited to this configuration. For example, the enamel layer and thin coating could be applied to a different side of such a laminated glass panel or may be applied to a glass panel comprising a single glass sheet.

[0045] FIG. 2 shows a flow diagram of the steps involved in providing the outer coated glass sheet 2 of the automotive glass panel configuration shown in FIG. 1. The method comprises: depositing enamel paste on side 2 of the outer glass sheet; firing at a temperature T.sub.1 C.; applying a thin layer coating over the enamel and over un-enamelled portions of the outer glass sheet; applying silver bus bars on the thin layer coating over the enamelled portions of the outer glass sheet; and bending the coated glass sheet at a temperature T.sub.2 C.

[0046] FIGS. 3(a) and 3(b) show flow diagrams illustrating that following the method of FIG. 2 leads to damage of the thin film coating during processing unless the firing (melting) temperature of the enamel paste composition is lower than the firing temperature T.sub.1 and the crystallization temperature of the enamel paste composition is higher than the bending temperature T.sub.2.

[0047] FIG. 3(a) shows that if paste is initially fired at a temperature T.sub.1 which is below the firing/melting temperature of the glass frit, which is the case in a standard pre-firing step designed to merely remove organic carrier medium rather than melt and fuse the glass frit, then a rough surface results which can lead to a damaged thin film coated when deposited onto such a surface using, for example, a vapour deposition technique.

[0048] FIG. 3(b) shows that even if the enamel paste is initially fired at a temperature T.sub.1 which is above the firing/melting temperature of the glass frit to provide a smooth surface on which to deposit a thin film coating, the thin film coating can be subsequently damaged during bending of the coated glass sheet if the bending temperature T.sub.2 is above the crystallization temperature of the glass enamel.

[0049] As such, FIGS. 3(a) and 3(b) illustrate that the glass frit in the enamel composition must have both a firing/fusing/melting temperature lower than T.sub.1 to produce a low surface roughness after firing at T.sub.1 and a crystallization temperature higher than T.sub.2 in order to ensure that the thin film coating is deposited in an undamaged form and remains in an undamaged form during processing (bending) at T.sub.2.

[0050] In addition to the above, the resultant enamel must meet the functional characteristics required for the application. For example, in the automotive glass panel application of FIG. 1, the enamel must have good optical characteristic and good properties to: (i) shield silver hiding glue/adhesive from UV decomposition; and (ii) mask the silver bus bars disposed behind the enamel. These characteristics are dependent on the chemical composition of the glass frit and also on the final firing/bending temperature T.sub.2 used to process the enamel coated glass sheet. As such, for this application the enamel paste composition must be tailored to meet a plurality of requirements including a firing/fusing/melting temperature lower than T.sub.1, a crystallization temperature higher than T.sub.2, and also meet the optical characteristics and silver hiding properties required for the end application after processing at T.sub.2. The enamel must also be chemically and structurally resistant to the thin film coating and vapour deposition method of fabricating the thin film coating over the enamel. Furthermore, for automotive applications it is required that the enamel is acid resistant (0.1N H.sub.2SO.sub.4 for 4 hours at 80 C.) and that the coated glass sheet meets a minimum mechanical strength specification.

[0051] The present specification provides s an enamel paste composition which meets all the aforementioned requirements for an enamel suitable to be used in combination with thin functional coatings, such as self-heated or IR reflective coatings.

[0052] The enamel paste comprises: (1) a bismuth silicate glass frit; (2) a zinc silicate glass frit; (3) a pigment; and (4) an organic carrier. Details of an example for the two frit compositions in terms of equivalent oxides are provided below.

Bismuth Silicate Frit:

[0053]

TABLE-US-00003 Component Mol % Mass % SiO.sub.2 51.18 21.70 Bi.sub.2O.sub.3 19.46 63.97 B.sub.2O.sub.3 16.12 7.92 Li.sub.2O 6.21 1.31 Al.sub.2O.sub.3 3.00 2.16 ZrO.sub.2 2.55 2.20 K.sub.2O 0.76 0.51 MgO 0.37 0.10 CaO 0.35 0.14

Zinc Silicate Frit:

[0054]

TABLE-US-00004 Component Mol % Mass % SiO.sub.2 49.09 47.11 ZnO 24.35 31.64 Li.sub.2O 7.82 3.73 B.sub.2O.sub.3 6.91 7.68 Na.sub.2O 4.82 4.77 F 2.34 0.71 MnO 2.13 2.41 S 1.71 0.87 CaO 0.59 0.53 Fe.sub.2O.sub.3 0.20 0.51 MgO 0.04 0.03

[0055] The pigment can be an automotive back pigment such as a CuCr black pigment.

[0056] The frits and pigment are milled to a desired particle size distribution of D90<15 micrometres and mixed with an organic carrier formulation to create a paste with a viscosity between 5 and 150 Pa.Math.s.sup.1 at a shear rate 100 s.sup.1. An example of the paste formulation is provided in the table below:

TABLE-US-00005 Ratio Intermediates Description (wt %) Frit 1 Pb/Cd/Bi free Zn/S/Li frit 25.6 Frit 2 Pb/Cd free Li/Bi/Si frit 38.4 Pigment Black pigment 21.4 Organic Media Solvent/Thinner/Thixotropic agent 14.6

[0057] The novelty of this paste formulation lies in the subtle mix of properties brought to the enamel to fit the process of producing a self-heated laminated motor vehicle glass panel. Self-heated windshields are relatively new products that have been brought to the market in the past 5 years at the time of writing, and the present specification enables an improvement in performance for such self-heated windshields. As previously indicated, key characteristics of the enamel paste formulation include: (i) low firing temperature; (ii) smooth surface after firing; (iii) good silver hiding properties; and (iv) high crystallization temperature. The zinc silicate frit brings silver hiding properties to the enamel. The bismuth silicate frit allows the enamel to be processed at low temperature and develops a smooth surface at a low firing temperature. Both frits have a high crystallization temperature. The combination of the two frits thus provides a low firing temperature smooth surface, silver hiding properties over a large range of temperatures, a high and crystallization temperature to avoid crystallization during bending of the coated glass sheet. This combination of features enables a nanometre scale silver coating to be applied over the enamel and subsequently subjected to a bending procedure without damaging the coating.

[0058] In addition to the above, the enamel paste composition provides an enamel layer after firing at T2 which has good optical and silver hiding characteristics. FIG. 4 shows the colour development characteristics of an enamel layer formed using the enamel paste composition in terms of its L-value vs firing temperature, the enamel being formulated to have a low L-value at the final firing (bending) temperature T.sub.2 of the coated glass manufacturing method without crystallization. FIG. 5 shows the silver hiding characteristics of an enamel layer formed using the enamel paste composition in terms of its dE value vs firing temperature, the enamel being formulated to have a low dE-value (good silver hiding) at the final firing (bending) temperature T.sub.2 of the coated glass manufacturing method without crystallization.

[0059] The resulting paste can thus be used to build an improved laminated motor vehicle glass panel. The enamel paste is deposited on a glass substrate, typically a soda-lime glass, dried at a given temperature and fired in order to obtain a smooth enamel surface. Subsequently, a thin multilayer silver coating is deposited over the glass sheet and the enamel, to form either a resistive heating element or an IR reflective barrier, after which bus bars are deposited over the enamel contacting the thin coating. The coated glass sheet is then heated and bent to form a shaped windshield component.

[0060] The specific paste composition as described above has been tailored for a specific windshield manufacturing process having specific T.sub.1 and T.sub.2 temperatures. However, it is also envisaged that for different applications the firing temperature T.sub.1 and the bending temperature T.sub.2 may differ. In this case, the formulation of the enamel paste can be modified to ensure that it has a firing temperature lower than T.sub.1 and a crystallization temperature higher than T.sub.2 in order to ensure that a thin film coating disposed on the enamel is not damaged during processing, while also achieving the colour characteristics required for an end application after processing at T.sub.2. As such, the method of this specification is not limited to the specific enamel paste composition tailored for an end application having a specifically defined T.sub.1 and T.sub.2. That is, a skilled person in the art of enamel paste formulation will be able to utilize the teachings of this specification and modify the composition of the paste described herein to be suitable for thin film coating applications having different T.sub.1 and T.sub.2 values. The key essential features of any modified paste are that the formulation of the enamel paste has a firing temperature lower than T.sub.1, a crystallization temperature higher than T.sub.2 in order to ensure that a thin film coating disposed on the enamel is not damaged during processing, and also that it achieves the desired optical characteristics required for an end application after processing at T.sub.2. Variants of the example described herein can achieve functional modifications for such end applications. As such, variants may fall within the ranges of frit compositions defined in the summary section, although it is also envisaged that a combination of different frit types could also meet the functional requirements of the method as described herein. Accordingly, while this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.