Patent Application
20240368027
The present specification relates to enamel paste compositions, enamel coated products, and methods of manufacturing the same.
In the automotive glazing industry, it is common to decorate windshields, back and side lights, and other glass components with a black band of obscuration enamel extending around a peripheral region of the components. 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. It is also known to use 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. During the second firing step the substrate can be formed into the final shape by a bending process.
In addition to obscuration enamels, electrical components can also be printed onto a glass component in a similar manner. In this case, the enamel formulation will typically comprise a metallic, electrically conductive powder (e.g. silver powder), glass frit, and an organic carrier medium. Such 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.
More recently, 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 screen or inkjet printed conductive silver-enamel heater lines) and/or IR reflective elements.
US20170135160 (granted as U.S. Pat. No. 10,455,645B2) 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
The present invention is directed to an improved enamel paste composition specifically tailored for use in a method such as that described in US20170135160.
As described in the background 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. Furthermore, there is no indication that 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. Rather, 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. Specifically, the improved enamel paste formulation comprises a composition which has the following characteristics:
That is, 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. In particular, 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 35 coatings thereon. The enamel paste comprises:
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. As such, 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.
Optionally, the bismuth silicate glass frit has the following composition (expressed as oxide components:
Optionally, 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:
The paste composition as previously described can be used in the method. However, it is also envisaged that for different applications the firing temperature T1 and the bending temperature T2 may differ. In this case, the formulation of the enamel paste can be modified to ensure that it has a firing temperature lower than T1 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. 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 T1 and T2. That is, a skilled person in the art of 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 T1 and T2 values and utilizing different bending processes including, for example, sag bending, press bending, and variants thereof.
Advantageously, the enamel layer has a roughness Ra of no more than 1, 0.7, 0.5, or 0.3 micrometres and/or a roughness Rz 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. Furthermore, as also described in US20170135160, 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.
While the 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.
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:
As described in the background section, the glass panel configuration described in US20170135160 is shown in
It should be noted that while the configuration shown in
As such,
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
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.
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.
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. 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.
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.
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.
The specific paste composition as described above has been tailored for a specific windshield manufacturing process having specific T1 and T2 temperatures. However, it is also envisaged that for different applications the firing temperature T1 and the bending temperature T2 may differ. In this case, the formulation of the enamel paste can be modified to ensure that it has a firing temperature lower than T1 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. 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 T1 and T2. 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 T1 and T2 values. The key essential features of any modified paste are that the formulation of the enamel paste has a firing temperature lower than T1, 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. 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2105702.1 | Apr 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NL2022/050205 | 4/14/2022 | WO |
