Classifications

• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C8/00—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions
  • C03C8/22—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions containing two or more distinct frits having different compositions
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
  • C03C17/02—Surface treatment of glass, not in the form of fibres or filaments, by coating with glass
  • C03C17/04—Surface treatment of glass, not in the form of fibres or filaments, by coating with glass by fritting glass powder
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C10/00—Devitrified glass ceramics, i.e. glass ceramics having a crystalline phase dispersed in a glassy phase and constituting at least 50% by weight of the total composition
  • C03C10/0054—Devitrified glass ceramics, i.e. glass ceramics having a crystalline phase dispersed in a glassy phase and constituting at least 50% by weight of the total composition containing PbO, SnO2, B2O3
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
  • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
  • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
  • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
  • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
  • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
  • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
  • C03C17/3668—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer the multilayer coating having electrical properties
  • C03C17/3673—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer the multilayer coating having electrical properties specially adapted for use in heating devices for rear window of vehicles
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
  • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
  • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
  • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
  • C03C17/3681—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer the multilayer coating being used in glazing, e.g. windows or windscreens
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C3/00—Glass compositions
  • C03C3/04—Glass compositions containing silica
  • C03C3/076—Glass compositions containing silica with 40% to 90% silica, by weight
  • C03C3/078—Glass compositions containing silica with 40% to 90% silica, by weight containing an oxide of a divalent metal, e.g. an oxide of zinc
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C3/00—Glass compositions
  • C03C3/04—Glass compositions containing silica
  • C03C3/076—Glass compositions containing silica with 40% to 90% silica, by weight
  • C03C3/089—Glass compositions containing silica with 40% to 90% silica, by weight containing boron
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C3/00—Glass compositions
  • C03C3/04—Glass compositions containing silica
  • C03C3/076—Glass compositions containing silica with 40% to 90% silica, by weight
  • C03C3/089—Glass compositions containing silica with 40% to 90% silica, by weight containing boron
  • C03C3/091—Glass compositions containing silica with 40% to 90% silica, by weight containing boron containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C8/00—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions
  • C03C8/14—Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/84—Heating arrangements specially adapted for transparent or reflecting areas, e.g. for demisting or de-icing windows, mirrors or vehicle windshields
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2217/00—Coatings on glass
  • C03C2217/40—Coatings comprising at least one inhomogeneous layer
  • C03C2217/43—Coatings comprising at least one inhomogeneous layer consisting of a dispersed phase in a continuous phase
  • C03C2217/44—Coatings comprising at least one inhomogeneous layer consisting of a dispersed phase in a continuous phase characterized by the composition of the continuous phase
  • C03C2217/45—Inorganic continuous phases
  • C03C2217/452—Glass
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2217/00—Coatings on glass
  • C03C2217/40—Coatings comprising at least one inhomogeneous layer
  • C03C2217/43—Coatings comprising at least one inhomogeneous layer consisting of a dispersed phase in a continuous phase
  • C03C2217/46—Coatings comprising at least one inhomogeneous layer consisting of a dispersed phase in a continuous phase characterized by the dispersed phase
  • C03C2217/48—Coatings comprising at least one inhomogeneous layer consisting of a dispersed phase in a continuous phase characterized by the dispersed phase having a specific function
  • C03C2217/485—Pigments
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B2203/00—Aspects relating to Ohmic resistive heating covered by group H05B3/00
  • H05B2203/013—Heaters using resistive films or coatings

Definitions

• the present specification relates to enamel paste compositions, enamel coated products, and methods of manufacturing the same.
• a primary function is to shield the glue that holds the glass components in place from ultraviolet radiation which would otherwise decompose the glue.
• a secondary function is to cover up electrical circuits, wires, and connectors that ensure functionality of electrical or electronic components attached to, or embedded into, the glass component and ensure a clean aesthetic appearance.
• Enamels typically comprise glass frit and pigment in an organic carrier medium.
• the enamels are applied as a paste or ink in a screen printing or ink jet process to a flat glass substrate and are subsequently fired at high temperatures, during which the organic carrier medium of the paste or ink burns off and the enamel fuses together and establishes a bond to the substrate.
• the firing process also softens the substrate which can be formed into the final shape by a bending process during the firing process.
• a two-step firing process to form an enamel layer, the process comprising: (i) a first "pre firing” heating step, at a temperature below the melting temperature of the glass frit in the enamel paste, in order to remove organic carrier medium components of the enamel paste, thus forming a dry, pre-fired layer; and (ii) a second "firing" heating step at a temperature above the melting point of the glass frit in the enamel paste to fuse the enamel.
• the substrate can be formed into the final shape by a bending process.
• the enamel formulation will typically comprise a metallic, electrically conductive powder (e.g. silver powder), glass frit, and an organic carrier medium.
• conductive enamel pastes can be applied in addition to the obscuration enamel pastes and fired in the same process to form a glass component which comprises both electrical components (e.g. antennae and/or heating elements) and obscuration enamel, with the obscuration enamel arranged to obscure electrical connections (e.g. bus bars) to the electrical components.
• Such functional thin films have also been applied to glass components using, for example, a vapour deposition process.
• Such functional thin films are of a nanometre scale thickness and can function, for example, as heating elements (substituting the more conventional screen or inkjet printed conductive silver-enamel heater lines) and/or IR reflective elements.
• US20170135160 (granted as US10455645B2) describes an example of a method for fabricating a motor vehicle glass panel comprising a system of thin, sputtered conductive layers for heating the glass panel.
• the glass panel configuration described in US20170135160 is shown in Figure 1 and comprises two glass sheets 2, 4 united by an interlayer sheet 6.
• glass sheet 2 forms the outer glass sheet of the glass panel while glass sheet 4 forms the inner glass sheet.
• the sides of the glass sheets in a motor vehicle glass panel are conventionally numbered from the outside towards the inside.
• 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
• 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 uncoated portions of side 2 of the outer glass sheet.
• the thin coating is a system of heated conductive layers formed by cathode sputtering and comprising an assembly of metal layers and of dielectric layers, the assembly exhibiting a thickness which may be 0.1 micrometres or less.
• An electrical bus bar 20 is then formed on the thin coating 18 over the enamel layer 16.
• US20170135160 discloses that the system can be fired in a two-step firing process including a first pre-firing step to remove organic carrier medium and a second firing step at higher temperature to melt and fuse the glass frit, bending being performed during the second firing step of the firing process to form the shaped glass article.
• US20170135160 discloses a problem with the configuration illustrated in Figure 1 and described above. Namely, US20170135160 identifies that the power delivered to the busbars is not completely transmitted to the conductive layers and this results in a loss of power, adversely affecting heating performance of the glass panel.
• US20170135160 identifies that this problem is at least partially caused by surface roughness of the enamel layer adversely affecting the overlying conductive coating and the connection of the bus bar to the conductive coating over the enamel layer.
• the surface roughness R a of the enamel layer should be no greater than 0.5 micrometres and preferably no greater than 0.1 micrometres.
• the roughness obtained was substantially reduced and a mean roughness of the order of 0.14 micrometres was obtained using an enamel composition produced by Johnson-Matthey (JM1L6027).
• the present invention is directed to an improved enamel paste composition specifically tailored for use in a method such as that described in US20170135160. Summary
• US20170135160 indicates that the composition of the enamel paste is not specific for use with thin functional coatings beyond the characteristics of the particle size of the glass frit and pigment and the viscosity of the enamel paste provided by the organic carrier medium.
• US20170135160 does not give any information about the types of glass frit used in the enamel beyond the use of a commercially available enamel composition from Johnson Matthey.
• the chemical composition of the glass frit will have any effect on the surface roughness of the enamel and the subsequent effect on the thin functional coating deposited thereon.
• US20170135160 indicates that a smoother enamel surface can be achieved by using small particle size pigment in the enamel composition and also suggests modifications to the printing process used to deposit the enamel.
• the present inventors have developed an improved enamel paste formulation for the application described in US20170135160 .
• the improved enamel paste formulation comprises a composition which has the following characteristics :
• a glass frit composition which has a low firing (melting) temperature such that the glass frit can soften and flow at a low firing temperature in order to provide a smooth enamel layer at low temperatures for application of a thin coating thereon;
• the chemical composition of the glass frit component of the enamel paste has an impact on the quality of a thin coating deposited thereon and the resultant coated glass article.
• the glass frit must be tailored to have a low firing temperature and a high crystallization temperature to yield a low roughness surface enamel layer and avoid damage of the thin functional coating, both during deposition of the thin functional coating and also during further processing of the coated glass article, i.e. heating and bending of the coated glass article.
• the enamel must also have good silver hiding and UV blocking properties, which are affected by the type of glass frit and the firing temperature as well as the pigment.
• the present specification thus provides an enamel paste composition tailored for application of thin functional coatings thereon.
• the enamel paste comprises:
• the bismuth silicate glass frit comprises at least silicon, bismuth, boron, and lithium, and is free of lead and cadmium
• the zinc silicate glass frit comprises at least silicon, zinc, sulphur, and lithium, and is free of lead, cadmium, and bismuth
• 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.s 1 at a shear rate of 100 s _1 .
• 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.
• the frits also have a high crystallization temperature.
• a combination of the two frits provides a low firing temperature smooth surface, silver hiding properties over a large range of temperature, and a high crystallization temperature.
• the bismuth silicate glass frit has the following composition (expressed as oxide components:
• the zinc silicate glass frit has the following composition:
• the pigment is optionally a CuCr black pigment.
• the present specification also provides for the use of the enamel paste to fabricate 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, 0.4, 0.3, 0.2 or 0.1 micrometres disposed on the enamel layer.
• the enamel paste is tailored for a method of fabricating a coated glass article, including, but not limited to, the type of coated glass articles described in US20170135160.
• the method comprises: depositing an enamel paste 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, 0.4, 0.3, 0.2 or 0.1 micrometres; firing and bending the glass substrate with the enamel layer and the thin coating thereon at a temperature T2 which is greater than Tl to form the coated glass article, wherein the enamel paste is formulated to have a firing temperature lower than Tl and a crystallization temperature higher than T2.
• the paste composition as previously described can be used in the method.
• the firing temperature Ti and the bending temperature T2 may differ.
• the formulation of the enamel paste can be modified to ensure that it has a firing temperature lower than Ti and a crystallization temperature higher than T2 in order to ensure that a thin film coating disposed on the enamel is not damaged during processing, while also achieving the colour characteristics and/or silver hiding characteristics required for an end application after processing at T2.
• the method of this specification is not limited to the specific enamel paste composition tailored for an end application having a specifically defined Ti and T2.
• enamel paste formulation will be able to utilize the teachings of this specification to modify the composition of the paste described herein so as to be suitable for thin film coating applications having different Ti and T2 values and utilizing different bending processes including, for example, sag bending, press bending, and variants thereof.
• the enamel layer has a roughness R a of no more than 1, 0.7, 0.5, or 0.3 micrometres and/or a roughness R z of no more than 5, 4, 3, 2, or 1 micrometre. Furthermore, for obscuration applications, the enamel layer advantageously has an L-value of no more than 5, 4.5, 4, or 3.5.
• the thin coating can be deposited via a vacuum deposition process such as a cathode sputtering process as is the case for the application described in US20170135160.
• the thin coating may be deposited over at least a portion of the glass substrate in addition to being deposited over at least a portion of the enamel layer.
• the thin coating can be electrically conductive, optionally comprising a plurality of sub-layers (e.g. 2 or 3 silver sub-layers), and at least one electrical connector can be deposited on the thin coating over the enamel layer.
• enamel pastes of the present invention have been developed for applications such as described in US20170135160, it is envisaged that enamel pastes according to the present specification will be useful for other functional thin film coatings on glass substrates including electrical, optical, and thermal functional coatings.
• Figure 1 shows an automotive glass panel configuration of the type described in US20170135160
• Figure 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 Figure 1;
• Figures 3(a) and 3(b) show flow diagrams illustrating that following the method of Figure 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 Ti and the crystallization temperature of the enamel paste composition is higher than the bending temperature T2;
• Figure 4 shows the colour development characteristics of an enamel layer formed using an 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 T2 of the coated glass manufacturing method without crystallization;
• Figure 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 T2 of the coated glass manufacturing method without crystallization.
• 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
• 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
• the glass sheets 2, 4 are fired and shaped and then laminated with the interlayer sheet 6 to form the glass panel.
• the present invention is not limited to this configuration.
• 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.
• Figure 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 Figure 1.
• the method comprises: depositing enamel paste on side 2 of the outer glass sheet; firing at a temperature Ti°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 2 °C.
• Figures 3(a) and 3(b) show flow diagrams illustrating that following the method of Figure 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 Ti and the crystallization temperature of the enamel paste composition is higher than the bending temperature T 2 .
• Figure 3(a) shows that if the enamel paste is initially fired at a temperature T 2 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.
• Figure 3(b) shows that even if the enamel paste is initially fired at a temperature T 2 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 2 is above the crystallization temperature of the glass enamel.
• Figures 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 2 to produce a low surface roughness after firing at T 2 and a crystallization temperature higher than T 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 2 .
• the resultant enamel must meet the functional characteristics required for the application.
• the enamel in the automotive glass panel application of Figure 1, the enamel must have good optical characteristic and good silver hiding properties to: (i) shield 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 2 used to process the enamel coated glass sheet.
• the enamel paste composition must be tailored to meet a plurality of requirements including a firing/fusing/melting temperature lower than T 2 , a crystallization temperature higher than T 2 , and also meet the optical characteristics and silver hiding properties required for the end application after processing at T 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 2 SC>4 for 4 hours at 80°C) and that the coated glass sheet meets a minimum mechanical strength specification.
• the present specification provides 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 .
• 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.
• Zinc Silicate Frit Component Mol% Mass% Si0 2 49.09 47.11 ZnO 24.35 31.64 Li 2 0 7.82 3.73
• the pigment can be an automotive back pigment such as a CuCr black pigment.
• 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.s -1 at a shear rate 100 s -1 .
• An example of the paste formulation is provided in the table below:
• 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.
• key characteristics of the enamel paste formulation include: (i) low firing temperature; (ii) smooth surface after firing; (ill) 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, and a high 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.
• the enamel paste composition provides an enamel layer after firing at T2 which has good optical and silver hiding characteristics.
• Figure 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 T2 of the coated glass manufacturing method without crystallization.
• Figure 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 T2 of the coated glass manufacturing method without crystallization.
• 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.
• 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.
• the specific paste composition as described above has been tailored for a specific windshield manufacturing process having specific Ti and T2 temperatures.
• the firing temperature Ti and the bending temperature T2 may differ.
• the formulation of the enamel paste can be modified to ensure that it has a firing temperature lower than Ti and a crystallization temperature higher than T2 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 T2.
• the method of this specification is not limited to the specific enamel paste composition tailored for an end application having a specifically defined Ti and T2.
• any modified paste has a firing temperature lower than Ti, a crystallization temperature higher than T2 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 T2. Variants of the example described herein can achieve functional modifications for such end applications.

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• 2021
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