WO2015071670A1

WO2015071670A1 - Glass composition

Info

Publication number: WO2015071670A1
Application number: PCT/GB2014/053369
Authority: WO
Inventors: Robin George ORMAN
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2013-11-15
Filing date: 2014-11-13
Publication date: 2015-05-21
Also published as: GB201320190D0

Abstract

The invention relates to glass compositions, in particular glass compositions for enamels such as for automotive applications. The invention also relates to inks comprising the glass compositions, a method for forming an enamel coating on a substrate and a substrate with an enamel coating formed thereon. The compositions comprising SiO₂, ZnO and Bi₂O₃ as major constituents, typically include no boron or a low level of boron, and are typically substantially free of lead.

Description

GLASS COMPOSITION

Field of the Invention

The present invention relates to glass compositions. In particular, it relates to glass compositions for enamels, particularly enamels suitable for automotive applications. The present invention also relates to inks comprising the glass compositions, a method for forming an enamel coating on a substrate, and a substrate with an enamel coating formed thereon. Background of the Invention

Glass enamels are widely used in automotive applications. For example, black enamels are typically applied to the edge of windscreens for aesthetic reasons and to protect glue from UV degradation due to exposure to light. Enamel compositions are typically applied to a surface (e.g. a windscreen surface) as an ink, e.g. by screen printing. The ink may comprise pigment and glass frit in an organic medium. After application to the surface, the enamel is fired to cause it to fuse.

Historically, PbO has been used as a primary ingredient in glass frits for automotive enamels, since it can provide a low firing temperature and a good surface finish. However, more recently the use of lead has been avoided, for regulatory, health and environmental reasons.

US5203902 (Johnson Matthey PLC) describes a glass flux composition which may optionally contain a low level of lead. The glass frit consists essentially by weight of

45-65% Bi₂0₃, 25-36% Si0₂, 4-6% Ti0₂, 1-3% Na₂0, 0.5-2% K₂0, 2-6% Li₂0, 0-3% Al₂0₃, 0-1 % Zr0₂, 0-1 % BaO, 0-1 % CaO, 0-1 % MgO, 0-1 % ZnO, 0-3% PbO and 0-0.5% CdO.

A range of different lead-free glass compositions have been suggested. US5252521 describes a lead-free glass composition comprising in weight percent from about 35% to about 77% of bismuth oxide, from about 10% to about 33% of boron oxide, and from about 10% to about 32% of zinc oxide.

US5616417 describes lead-free glass frits having the following composition in weight percent:

l

US6105394 describes lead and cadmium free glass enamel compositions. The glass frits include about 0.1 percent by weight to about 15 percent by weight Nb₂0₅, less than about 3 percent by weight alkali metal oxides, less than about 3 percent by weight ZnO, less than about 1 percent by weight B₂0₃, and precursors from which Bi₂Si0₅ can be crystallised upon firing.

US6624104 describes lead-free glass compositions with low boron content. The glass compositions have the following broad composition in weight %:

EP0955274 describes lead-free glass compositions which have a bismuth oxide content of 40-70 weight % and a Si0₂ content of 24-40 weight %. The glasses include less than 8 weight percent B₂0₃. JPH06234547 describes a crystallized glass consisting essentially of, by weight, 14-35% Si0₂, 50-75% Bi₂0₃, 3-10% Ti0₂, 0-3% B₂0₃ and 1-15%, in total of 0-15% Li₂0, 0-15% Na₂0 and 0-15% K₂0. In the experience of the present inventors, most modern lead-free glass frits in use commercially are based on borosilicate glasses, and contain significant levels of boron.

Summary of the Invention

There remains a need for glass compositions useful in enamels (e.g. automotive enamels) which are free of boron, or which contain a low level of boron. A reduction of boron content may be desirable for health and environmental considerations, and/or to comply with possible future regulatory requirements. However, the present inventors have found that reducing the boron content of some glasses can have an adverse effect on their properties, and in particular can lead to an increase in the temperature at which enamel comprising the glass can fuse. Accordingly, there remains a need for low boron and boron free glasses providing a useful combination of properties, such as a suitable fusing temperature, suitable acid resistance properties and/or a low coefficient of thermal expansion.

In a first preferred aspect the present invention provides a glass composition comprising:

60 to 75 weight % Bi₂0₃;

5 to 16 weight % ZnO;

10 to 18 weight % Si0₂; and

0 to 4 weight % of B₂0₃. The glass composition may be a glass powder, for example a glass powder suitable for use in a glass enamel composition. Accordingly, in a second preferred aspect, the present invention provides an ink for forming glass enamel, the ink comprising a solids portion and a liquid medium, the solids portion comprising glass powder having a composition according to the first preferred aspect.

In a third preferred aspect, the present invention provides a substrate having an enamel coating formed thereon, the enamel coating being obtained or obtainable by firing an ink according to the second aspect of the invention. The substrate may be a glass substrate, such as a glass panel suitable for use as a window or windshield, e.g. in a vehicle.

In a fourth preferred aspect, the present invention provides a method of forming an enamel coating on a substrate, the method comprising applying an ink according to the second aspect of the invention to the substrate and firing to form the enamel coating. The substrate may be a glass substrate, such as a glass panel suitable for use as a window or windshield, e.g. in a vehicle. Detailed Description

Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention, unless the context demands otherwise. Any of the preferred or optional features of any aspect may be combined, singly or in combination, with any aspect of the invention, unless the context demands otherwise.

The glass compositions of the present invention are low boron glass compositions. The compositions include 0 to 4 weight% B₂0₃. Preferably, the compositions include 3 weight % or less, 2.5 weight % or less, 2 weight % or less, 1.5 weight % or less, or 1 weight % or less of B2O₃. In some embodiments, the compositions may include at least 0.1 weight %, at least 0.5 weight %, or at least 1 weight % B₂0₃. Alternatively, the compositions may be substantially free of boron.

As used herein, where a glass composition is said to be substantially free of a component, it is intended that the composition includes a very low amount of the component, or is completely free of the component. For example, it may include less than 0.5 weight %, less than 0.1 weight % or less than 0.05 weight % of the component in question on an oxide basis. It may be preferred that there is no intentional addition of the component in question. The glass compositions of the present invention may be substantially free of lead. The compositions of the present invention may be substantially free of cadmium. The compositions of the present invention may be substantially free of niobium. The

compositions of the present invention may be substantially free of aluminium. The glass compositions of the present invention include 60 to 75 weight % Bi₂0₃. The glass compositions may include at least 63 weight %, at least 64 weight %, at least 65 weight %, at least 66 weight %, or at least 67 weight % Bi₂0_3. The glass compositions may include 74 weight % or less, 73 weight % or less, or 72.5 weight % or less of Bi₂0₃. The present inventors have found that where too high an amount of Bi₂0₃ is included, enamel compositions do not fully fuse at the preferred processing temperatures and/or tend to exhibit lower acid resistance. A particularly suitable amount of Bi₂0₃ is 64 to 73 weight %. In certain embodiments, it may be preferred that the glass composition includes at least 68 weight % or at least 69 weight % of Bi₂0₃, and 75 weight % or less, 74 weight % or less, or 73 weight % or less of Bi₂0₃. In certain embodiments, it may be preferred that the glass composition includes at least 63 weight %, at least 64 weight %, or at least 65 weight % on Bi₂0₃, and 73 weight % or less, 71 weight % or less, or 70 weight % or less of Bi₂0₃.

A suitable raw material for supplying bismuth to the glass composition is Bi₂0₃.

The glass compositions of the present invention include 5 to 16 weight % ZnO. The glass compositions may include at least 6 weight %, at least 7 weight % or at least 8 weight % ZnO. The compositions may include 15 weight % or less, 14 weight % or less, 13 weight % or less, 12 weight % or less or 1 1 weight % or less. A particularly suitable amount of ZnO is 7 to 13 weight %.

The present inventors have found that including higher levels of ZnO can have an adverse effect on acid resistance. However, where the compositions are substantially free of boron, acid resistance properties can be improved even where a relatively large amount of ZnO is included. Accordingly, where the composition is substantially free of boron, it may be preferred that the glass composition includes 16 weight % or less, 15 weight % or less, 14 weight % or less or 13 weight % or less. When the composition includes boron, it may be preferred that the composition includes 13 weight % or less, 12 weight % or less or 1 1 weight % or less ZnO.

A suitable raw material for supplying zinc to the glass composition is ZnO.

The glass compositions of the present invention include 10 to 18 weight % Si0₂. The glass compositions may include at least 1 1 weight %, or at least 12 weight % Si0₂. The glass compositions may include 17 weight % or less, or 16 weight % or less Si0₂. A particularly suitable amount of Si0₂ is 12 to 17 weight %.

Si0₂, ZnO and Bi₂0₃ are the major constituents of the glass compositions of the present invention. Typically, the glass composition includes a combined Si0₂, ZnO and Bi₂0₃ content of approximately 95 weight %. Accordingly, the sum of the Si0₂, ZnO and Bi₂0₃ content of the glass composition of present invention may be at least 90 weight %, at least 93 weight percent or at least 94 weight %. The sum of the Si0₂, ZnO and Bi₂0₃ content of the glass composition of the present invention may be, for example, 98 weight % or less, or 96 weight % or less.

The present inventors have found that it may be desirable to include F in the glass compositions of the present invention. They have found that the presence of F helps to reduce the melting point of the glass to compensate for low boron content. Accordingly, the compositions of the present invention preferably include 0.05 to 2 weight % F. The compositions of the present invention may include at least 0.1 weight %, or at least 0.15 weight % of F. The compositions may include 1.5 weight % or less, or 1 weight % or less of F. A particularly suitable amount of F is 0.1 to 1 weight %.

In certain embodiments, it may be preferable that the compositions include at least 0.2 weight %, or at least 0.3 weight % of F. In some embodiments, it may be preferable that the compositions include at least 0.4 weight %, at least 0.5 weight % or at least 0.6 weight % of F.

Note that the F weight percentages recited herein are not on an oxide basis, but instead are with respect to the fluoride anion. Suitable raw materials for supplying F to the glass compositions include BiF₃, ZnF₂, Na₂SiF₆, K₂SiF₆ and NaF. It is intended that fluoride is included as an ingredient in the glass compositions rather than being supplied as a separate additive in the enamel compositions.

The glass compositions of the present invention may include one or more alkali metal or alkaline earth metal oxides, typically 3 weight % or less of these oxides. The one or more alkali metal oxides or alkaline earth metal oxides may be selected from Li₂0, Na₂0, K₂0, MgO, CaO, SrO and BaO. Preferably, the oxides are one or more alkali metal oxides, e.g. selected from Li₂0, Na₂0, K₂0, in particular Li₂0 and/or Na₂0.

The glass composition may include at least 0.3 weight %, at least 0.4 weight %, at least 0.5 weight % alkali metal oxides or alkaline earth metal oxides. The glass composition may include at least 1 weight % of alkali metal oxides or alkaline earth metal oxides. The glass composition may include 2.5 weight % or less, 2 weight % or less, or 1.8 weight % or less alkali metal oxides or alkali earth metal oxides. In certain embodiments, it may be preferred that the glass composition includes 0.8 weight % or more, 1 weight % or more, or 1.2 weight % or more alkali metal oxides or alkali earth metal oxides. It may be preferred that the glass composition includes 1 weight % or less, or 0.8 weight % or less of Li₂0. The composition may include at least 0.5 weight % Na₂0. The composition may include at least 0.1 weight % K₂0.

The glass compositions of the present invention may include one or more additives, which are typically metal oxides or S. For example, the additives may be one or more selected from transition metal oxides, Al₂0₃, Ce0₂, La₂0₃, Sb₂0₃, S and Ge0₂. Particularly suitable transition metal oxides include MnO, CuO, Zr0₂, Ti0₂, V₂0₃, Mo0₃, NiO, W0₃ and Ta₂0₃. Particularly suitable additives are MnO, CuO, Fe₂0₃, Zr0₂ and Al₂0₃. Typically, the composition will include up to 10 weight %, up to 8 weight %, up to 6 weight %, up to 5 weight %, or up to 4 weight % of additives. The compositions may include at least 1 weight %, at least 2 weight %, or at least 3 weight % of additives. In particular, the glass compositions may include 0.01 to 1 weight % Fe₂0₃. Fe₂0₃ can reduce phase separation. It may be preferred that the compositions include at least 0.05 weight % or at least 0.1 weight % of Fe₂0₃. It may be preferred that the compositions include 0.5 weight % or less, 0.4 weight % or less, 0.3 weight % or less, or 0.2 weight % or less of Fe₂0₃. A particularly suitable amount of Fe₂0₃ is 0.1 to 0.3 weight %.

The glass compositions may include 0.5 to 2.5 weight % of MnO. For example, the compositions may include at least 0.8 weight %, at least 1 weight % or at least 1.5 weight % of MnO. The compositions may include 3 weight % or less, or 2 weight % or less of MnO. A particularly suitable amount of MnO is 1 to 2 weight %.

The glass compositions may include 0.1 to 2 weight % of CuO. For example, the compositions may include at least 0.3 weight %, at least 0.5 weight % or at least 0.7 weight % of CuO. The compositions may include 1.5 weight % or less, 1.3 weight % or less, or 1 weight % or less of CuO. A particularly suitable amount of CuO is 0.5 to 1.5 weight %. The glass composition of the present invention may comprise:

64 to 73 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

0 to 3 weight % B₂0₃; and

0.1 to 1 weight % F.

The glass composition may optionally include up to 3 weight % alkali metal oxides or alkaline earth metal oxides and up to 6 weight % additives. The glass composition of the present invention may consist essentially of:

64 to 73 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

0 to 3 weight % B₂0₃; and

0.1 to 1 weight % F,

the balance being alkali metal oxides or alkaline earth metal oxides (e.g. up to 3 weight %), additives (e.g. up to 6 weight %) and incidental impurities.

The glass composition of the present invention may comprise:

64 to 73 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

0 to 3 weight % B₂0₃;

0.1 to 1 weight % F;

0.3 to 3 weight % alkali metal oxides or alkaline earth metal oxides (e.g. one or more selected from Li₂0, Na₂0, K₂0, MgO, CaO, SrO and BaO);

1 to 10 weight % additives (e.g. one or more selected from transition metal oxides, Al₂0₃, Ce0₂, La₂0₃, Sb₂0₃, S and Ge0₂).

The glass composition of the present invention may consist essentially of:

64 to 73 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

0 to 3 weight % B₂0₃;

0.1 to 1 weight % F;

1 to 10 weight % additives (e.g. one or more selected from transition metal oxides, Al₂0₃, Ce0₂, La₂0₃, Sb₂0₃, S and Ge0₂). The skilled person will readily understand that the total weight % of the constituents will be 100 weight %, the balance being incidental impurities.

The glass composition of the present invention may comprise:

68 to 75 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

1 to 3 weight % B₂0₃;

0.1 to 1 weight % F;

The glass composition of the present invention may consist essentially of:

67 to 75 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

1 to 3 weight % B₂0₃;

0.1 to 1 weight % F;

1 to 10 weight % additives (e.g. one or more selected from transition metal oxides, Al₂0₃, Ce0₂, La₂0₃, Sb₂0₃, S and Ge0₂). The skilled person will readily understand that the total weight % of the constituents will be 100 weight %, the balance being incidental impurities. The glass composition of the present invention may comprise:

64 to 73 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

0 to 3 weight % B₂0₃;

0.1 to 1 weight % F;

0.5 to 1.5 weight % of alkali metal oxides, being one or more selected from the group consisting of Li₂0, Na₂0, and K₂0;

0.01 to 1 weight % of Fe₂0₃;

0.5 to 2.5 weight % of MnO; and

0.1 to 2 weight % of CuO.

The glass composition of the present invention may consist essentially of:

64 to 73 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

0 to 3 weight % B₂0₃;

0.1 to 1 weight % F;

0.5 to .5 weight % of alkali metal oxides, being one or more selected from the group consisting of Li₂0, Na₂0, and K₂0;

0.01 to 1 weight % of Fe₂0₃;

0.5 to 2.5 weight % of MnO; and

0.1 to 2 weight % of CuO.

The skilled person will readily understand that the total weight % of the constituents will be 100 weight %, the balance being incidental impurities.

The glass composition of the present invention may be substantially free of boron, and may comprise:

63 to 73 weight % Bi₂0₃;

7 to 15 weight % ZnO;

12 to 17 weight % Si0₂; and

0.4 to 1.5 weight % of F.

The glass composition may optionally include up to 3 weight % alkali metal oxides or alkaline earth metal oxides and up to 6 weight % of additives. The glass composition of the present invention may be substantially free of boron, and may consist essentially of:

63 to 73 weight % Bi₂0₃;

7 to 15 weight % ZnO;

12 to 17 weight % Si0₂; and

0.4 to 1.5 weight % of F;

the balance being alkali metal oxides or alkaline earth metal oxides (e.g. up to 3 weight %), additives (e.g. up to 6 weight %) and incidental impurities. The glass composition of the present invention may be substantially free of boron, and may comprise:

63 to 73 weight % Bi₂0₃;

7 to 15 weight % ZnO;

12 to 17 weight % Si0₂;

0.4 to 1.5 weight % of F;

0.8 to 3 weight % alkali metal oxides or alkaline earth metal oxides (e.g. one or more selected from Li₂0, Na₂0, K₂0, MgO, CaO, SrO and BaO); and

The glass composition of the present invention may be substantially free of boron, and may consist essentially of:

64 to 73 weight % Bi₂0₃;

7 to 15 weight % ZnO;

12 to 17 weight % Si0₂;

0.4 to 1.5 weight % of F;

0.8 to 3 weight % flux (e.g. one or more selected from Li₂0, Na₂0, K₂0, MgO, CaO, SrO and BaO); and

1 to 10 weight % additives (e.g. one or more selected from transition metal oxides, Al₂0₃, Ce0₂, La₂0₃, Sb₂0₃, S and Ge0₂). The skilled person will readily understand that the total weight % of the constituents will be 100 weight %, the balance being incidental impurities. The glass composition according to the present invention may be substantially free of boron and may comprise:

64 to 73 weight % Bi₂0₃;

7 to 15 weight % ZnO;

12 to 17 weight % Si0₂;

0.4 to 1.5 weight % of F;

0.8 to 2 weight % of alkali metal oxides or alkaline earth metal oxides, being one or more selected from the group consisting of Li₂0, Na₂0, K₂0, MgO, CaO, SrO and BaO;

0.01 to 1 weight % of Fe₂0₃;

0.5 to 2.5 weight % of MnO; and

0.1 to 2 weight % of CuO.

64 to 73 weight % Bi₂0₃;

7 to 15 weight % ZnO;

12 to 17 weight % Si0₂;

0.4 to 1.5 weight % of F;

0.8 to 2 weight % of alkali metal oxides or alkaline earth metal oxides being one or more selected from the group consisting of Li₂0, Na₂0, K₂0, MgO, CaO, SrO and BaO;

0.01 to 1 weight % of Fe₂0₃;

0.5 to 2.5 weight % of MnO; and

0.1 to 2 weight % of CuO.

The skilled person will readily understand that the total weight % of the constituents will be

100 weight %, the balance being incidental impurities.

The glass composition of the present invention is preferably in the form of a glass powder or frit. The powder is typically suitable for use in a glass enamel, such as an automotive glass enamel. Typically, glass powder has a d90 particle size of 5 to 15μηι, preferably 8 to 12μηι (by mass). Typically, glass powder has a d50 particle size of 3 to 6μηι, preferably 4 to 5μηι (by mass). The present invention also provides an ink for forming a glass enamel. The ink includes a solids portion and a liquid medium. The solids portion is the ingredients supplied to the ink in solid form. The solids portion may comprise one or more glass powders, optionally pigment, and optionally filler. The glass powder supplied to the ink includes a glass powder according to the present invention. 100 weight % of the glass powder may be glass powder according to the present invention. However, the skilled person will readily appreciate that a mixture of different glass powders may be provided in the ink. In that case, at least 10%, at least 20%, at least 50%, at least 70% or at least 90% by weight of the glass powder (with respect to the total weight of glass powder in the solids portion of the ink) may be glass powder according to the present invention. Alternatively, the glass powder supplied to the ink may include two or more glass powders which, taken together, provide a glass composition according to the present invention.

Typically, glass powder will make up between 10 and 95 weight % of the solids portion of the ink (with respect to the total weight of the solids portion). The solids portion may include at least 20 weight %, at least 40 weight %, at least 50 weight % or at least 60 weight % glass powder. The solids portion may include 90 weight % or less, 85 weight % or less, or 80 weight % or less glass powder.

Typically, the solids portion will include 0 to 75 weight % pigment (with respect to the total weight of the solids portion). The solids portion may include at least 5 weight %, at least 10 weight %, at least 20 weight % or at least 25 weight % pigment. The solids portion may include 65 weight % or less, 50 weight % or less, or 40 weight % or less pigment.

Typically, the solids portion will include 0 to 30 weight %, e.g. 5 to 25 weight %, filler.

The nature of the pigment is not particularly limited in the present invention. The skilled person will be familiar with the selection of suitable pigments for making different types and colours of enamels. Black pigments may be preferred for automotive applications. Suitable pigments include metal oxides and mixed metal oxides, e.g. of transition metals, such cobalt, chromium, manganese, iron, nickel and copper. Particularly suitable pigments include copper chromite black spinel CuCr₂0₄, iron cobalt chromite black spinel

(Co,Fe)(Fe,Cr)₂0₄, and chrome iron nickel black spinel (Ni,Fe)(Cr,Fe)₂0₄.

The nature of the filler is not particularly limited in the present invention, and includes a mixture of different fillers. As the skilled person will understand, filler is typically a particulate solid. The skilled person will be familiar with the selection of suitable fillers for making different types of enamels. Suitable fillers include alumina, zirconia, cordierite, transition metal oxides including CuO, and silica. The nature of the liquid medium is not particularly limited in the present invention.

Typically, it will be an organic medium. A suitable organic medium can be obtained by dissolving a polymer or a blend of polymers and resins such as acrylic resins, cellulose derivatives, phenolic resins, styrene resins or co-polymers and the like, in naturally occurring or synthetic solvents such as pine oil, a-terpineol, iso-tridecanol, butyl carbitol acetate, glycol ester and ether derivatives. The organic medium may contain further additives, e.g., in order to improve the wetting of the glass and pigment powders, in order to reduce the formation foam or bubbles during the printing operation, or additives in order to modify the rheology. A wide variety of organic medium suitable for many types of applications, including medium for producing UV-curable inks, are commercially available from e.g. Johnson Matthey Colour Technologies. Printing inks are usually prepared by dispersion of the solids portion in the vehicle by e.g. 3-roll milling, high speed mixing or other techniques known to those skilled in the art. In order to form an enamel coating on a substrate, the ink is applied to the substrate, typically by printing, e.g. screen printing. The ink is fired to cause it to fuse. The firing time and temperature are chosen to cause fusion of the enamel. Typically, the firing

temperature will be in the range from 550°C to 650°c, e.g. 560°C or 570°C to 640 °C. The firing step may be part of a processing step for the substrate. For example, where the substrate is a glass panel which is intended to be shaped (e.g. curved), the enamel firing and heating to enable shaping of the glass substrate may take place in the same process step, using a suitable firing temperature and firing time.

Preferably the ink of the present invention fuses at a temperature in the range from 550°C to 650°c, e.g. 560°C or 570°C to 640 °C. Preferably the glass composition of the present invention has a linear coefficient of thermal expansion measured in the temperature range from 20-320 °C which is 85·10^"7Κ^"1 or less, more preferably 84·10^"7Κ^"1 , 83·10^"7Κ^"1 , 82·10^"7Κ^"1 or 81 ^«10^"7K^"1 or less. The nature of the substrate to which the enamel is applied is not particularly limited in the present invention. However, the properties of the glass composition of the present invention render it particularly suitable for inclusion in glass enamels, e.g. automotive glass enamels. Accordingly, the substrate is typically a glass substrate, such as a window or windshield for a vehicle. The glass enamels are particularly suitable for use with glass substrates which are shaped after application of the enamel ink. Accordingly, the glass substrate having an enamel coating formed thereon may be a shaped substrate (i.e. a non- planar substrate). The substrate may be shaped after printing by press bending or gravity bending at temperatures >500°C. Where press bending is employed, crystallizing glass enamels are preferred in order to avoid sticking of the enamel to the mould.

Where ranges are specified herein, it is intended that each endpoint of the range is independent. Accordingly, it is expressly contemplated that each recited upper endpoint of a range is independently combinable with each recited lower endpoint for a range, and vice versa.

Where a composition is said to "consist essentially of" a list of components, the skilled person will readily understand that the total weight % of the recited constituents will be 100 weight %, any balance being incidental impurities.

Except where expressly indicated otherwise, weight percentages of components of the glass compositions described herein refer to the starting materials used to form the glass, on a nominal oxide basis, with respect to the total weight of starting materials, as is the custom in the glass making field.

Examples

Low boron and boron-free glass powders were prepared using conventional glass making techniques, to provide the compositions given in Tables 1 and 2 below (the compositions are given in percent by weight on an oxide basis, except F which is given as weight percent F^"). The compositions were heated to a temperature in the range 1000°C to 1400°C, before quenching to form a glass and milling to a particle size (d90) of about 8μηι.

Table 1

Table 2

The glass powders were formulated into inks comprising (i) a solids portion having 77wt% glass powder and 23wt% copper chromite pigment, and (ii) liquid organic vehicle (pine oil- based vehicle). The ratio by weight of solids to liquid medium was about 5.4. The inks were screen printed onto glass plates, dried at 140°C and fired in a tunnel kiln at temperatures ranging from 570°C to 640°C. The temperature at which the enamels became fully fused is recorded in Table 3 below. Fusion was confirmed using a wet through test. A drop of water was pressed against the surface of the enamel using an index finger, then viewing through the glass substrate to determine whether the water has passed through the enamel, which would be visible as a water spot or wetting mark. The enamel is determined to have fused if water is not absorbed through the enamel. Alternatively, a felt tip pen mark may be applied to the surface of the enamel, and viewed through the glass substrate to determine whether the pen ink has passed through the enamel.

The samples fired at 640°C were tested for acid resistance using 0.1 N H₂S0₄ at 80°C. The fired enamel was partially submerged in the acid solution within a sealed jar and kept at this temperature in an oven. Enamels were visually inspected after immersion for 4 and 24 hours. The time at which enamel failure was first observed is recorded in Table 3 below.

The coefficient of thermal expansion (CTE) of each enamel was measured using a BAHR dilatometer (20°C - 320°C), using a rod of glass poured at the same time as the frit. The results are given in Table 3 below.

Table 3

Claims

1. A glass composition comprising:

60 to 75 weight % Bi₂0₃;

5 to 16 weight % ZnO;

10 to 18 weight % Si0₂; and

0 to 4 weight % of B₂0₃.

2. A glass composition according to claim 1 further comprising 0.05 to 2 weight % F, preferably 0.4 to 1.5 weight % F.

3. A glass composition according to any one of the preceding claims, further comprising 64 to 73 weight % Bi₂0₃.

4. A glass composition according to any one of the preceding claims, further comprising 7 to 13 weight % ZnO.

5. A glass composition according to any one of the preceding claims, further comprising12 to 17 weight % Si0₂.

6. A glass composition according to any one of the preceding claims, further comprising 0.3 to 3 weight % alkali metal oxides or alkaline earth metal oxides.

7. A glass composition according to any one of the preceding claims, further comprising 1 to 10 weight % additives.

8. A glass composition according to claim 7 wherein the additives are selected from the group consisting of transition metal oxides, Al₂0₃, Ce0₂, La₂0₃, Sb₂0₃, S and Ge0₂.

9. A glass composition according to any one of the preceding claims, which is substantially free of boron.

10. A glass composition according to claim 9, further comprising 0.8 to 2 weight % F.

1 1. A glass composition according to any one claims 9 and 10, further comprising 0.8 to 3 weight % alkali metal oxides or alkaline earth metal oxides.

12. A glass composition according to claim 1 , comprising:

64 to 73 weight % Bi₂0₃;

7 to 13 weight % ZnO;

12 to 17 weight % Si0₂;

0 to 3 weight % B₂0₃;

0.1 to 1 weight % F;

0.5 to 1.5 weight % of alkali metal oxides or alkaline earth metal oxides, being one or more selected from the group consisting of Li₂0, Na₂0, K₂0, MgO, CaO, SrO and BaO;

0.01 to 1 weight % of Fe₂0₃;

0.5 to 2.5 weight % of MnO; and

0.1 to 2 weight % of CuO.

13. A glass composition according to claim 1 , which glass composition is substantially free of boron and comprises:

64 to 73 weight % Bi₂0₃;

7 to 15 weight % ZnO;

12 to 17 weight % Si0₂;

0.4 to 1.5 weight % of F;

0.8 to 2 weight % of alkali metal oxides or alkaline earth metal oxides, being one or more selected from the group consisting of Li₂0, Na₂0, K₂0, MgO,

CaO, SrO and BaO;

0.01 to 1 weight % of Fe₂0₃;

0.5 to 2.5 weight % of MnO; and

0.1 to 2 weight % of CuO.

14. An ink for forming an enamel, the ink comprising a solids portion and a liquid medium, the solids portion including a glass powder having a composition as defined in any one of the preceding claims.

15. An ink according to claim 14 wherein the solid portion comprises glass powder, pigment, and optionally filler.

16. An ink according to claim 14 or claim 15 wherein the liquid medium is an organic medium.

17. A method of forming an enamel coating on a substrate, the method comprising applying an ink as defined in any one of claims 14 to 16 to the substrate and firing to form the enamel coating.

18. A method according to claim 17 wherein the substrate is a glass panel.

19. A substrate having an enamel coating formed thereon, wherein the enamel coating is obtained or obtainable by firing an ink according to any one of claims 14 to 16.

20. A substrate according to claim 16 which is a glass panel.