WO2005073140A1

WO2005073140A1 - Improved glass compositions

Info

Publication number: WO2005073140A1
Application number: PCT/GB2005/000191
Authority: WO
Inventors: Geert Klass Bentem; Reinout Jozef Nikolaas Aloys Matthias Kicken; Detlef Rehorek
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2004-01-30
Filing date: 2005-01-18
Publication date: 2005-08-11
Anticipated expiration: 2006-07-30
Also published as: GB0401918D0

Abstract

A glass composition suitable for forming a frit comprises SiO2, B2O3, Zn0 and TiO2, together with minor amounts of other componente. The glass frit in combination with certain ceramic colours, can form inks for printing onto automotive glass which may be recycled at the end of the vehicle's life.

Description

IMPROVED GLASS COMPOSITIONS

The present invention provides an improved glass composition, more especially an improved glass frit composition, having uses in preparing enamels.

The formation of glasses derived from metal oxides is very well established and a wide variety of glass compositions are known in the art. Powdered glass, known as frit, can be prepared by rapid cooling of molten glass, usually by pouring a stream of molten glass into a bath of water, recovering the resulting glass granules and grinding to a desired particle size. Frits are used as a melting component in admixtures with ceramic pigments; such a mixture is termed an enamel. The enamel may be applied as artistic or functional decoration to substrates capable of withstanding firing or heat treatment to the temperatures at which the frit becomes molten. Such substrates include ceramics of most types, glass surfaces, metal surfaces, glazed ceramics or metals. The enamel is usually applied in the form of a fluid composition of enamel admixed with a liquid suspension medium, especially a fugitive organic medium, to form an ink by screen printing onto the substrate. Alternatives include manual brushing and applications by machine including offset printing. A variant comprises applying the enamel ink onto a carrier such as a release paper, for example gummed paper, to form a decal and the decal is then applied to the substrate. The skilled person recognises that a ceramic pigment is a pigment that is heat stable at the temperature at which the enamel is fired, that is at the temperature at which the glass frit component melts. Ceramic pigments are not necessarily heat stable under normal glass melting temperatures and under conventional conditions.

One particular use of enamel inks is to form, by screen printing, a black obscuration band on automotive glass. The automotive glass is held in position in the vehicle by an adhesive. The general requirements for such enamels is that they should be opaque in order to prevent degradation of the adhesive by sunlight and not cause sticking to tools used in the handling or forming of the glass sheets. There is a need for glass enamels which can be recycled together with the automotive glass on which they are applied. This brings several difficulties, however, and the requirements can be listed as including the following:

1. the enamel is free of toxic elements such as Pb and Cd;

2. after recycling by melting, under the chosen recycling conditions, in a glass smelter, the enamel should not form strong visible light absorbing or light scattering species. That is, the recycled glass should be transparent to visible light. Of course, this requirement is much more important for flat glass made from recycled components, than for bottle or other container glass. It should also be noted that if melting or recycling is carried out under a reducing atmosphere, there is a greater tendency for coloured species to form.;

3. the components of the enamel must not lead to excessive interaction with/destruction of the refractory material used to line the glass furnace or the glass handling components; and

4. the enamel must not contain Ni, since nickel sulphide stones may form in the glass melt, which can cause spontaneous glass breakage of tempered float glass.

In connection with requirement 3 above, attack on ceramics in glass furnaces includes the so-called "hole-drilling" by metals. This can happen, for instance, with easily reducible metal ions (eg: Bi³⁺) when reduced to their elemental form. Such metals react with the refractory ceramic lining of the furnace to form a hole that is impossible to repair without a furnace rebuild. Fluorides are other components that have a negative effect on the lifetime of the furnace.

Additionally, of course, the enamel has to meet the technical requirements set by the automotive manufacturer.

Accordingly, the present invention provides a glass composition that is particularly suitable for forming a component of recyclable enamels. Such a glass composition comprises 35-45 wt% SiO₂, 12-20 wt% B₂O₃, 10-16 wt% ZnO, 0-12 wt% Na₂O , 1-15 wt% K₂O, up to 2 wt% Li₂O, provided that Na₂O + K₂O + Li₂O is in the range 10-15 wt%, 8-18 wt% TiO₂, up to 2 wt% CaO, up to 1 wt% SrO and up to 5 wt% BaO, provided that CaO + SrO + BaO is in the range 0.4 to 5 wt%, and which composition is preferably essentially free of fluoride. Clearly, the total of components in all cases should be 100%. The glass composition may comprise additional components, perhaps by way of contamination, totalling no more than 2 wt% of each of ZrO₂, Fe₂O₃, MnO, MgO, SnO₂, La₂O₃, CeO₂, P₂O₅, N₂O₅ and sulphur. In order to improve the fusibility of the glass composition, small amounts of fluorides may be used as raw materials. Preferably, the glass composition should be fluoride-free as a deliberately added component. The glass composition is desirably in the form of a frit. A frit according to the invention may also be composed of a blend of frits having the glass composition according to the invention

The glass composition may comprise one or both of Al₂O₃ in an amount up to 3 wt% and Bi₂O₃ in an amount up to 5 wt%.

Preferably, the glass composition is essentially free of Bi₂O₃ . Preferably, the glass composition is essentially free of SnO₂.

The invention further provides a ceramic colour composition, or enamel, comprising at least one glass composition according to the invention and at least one ceramic pigment. Desirably, the enamel does not contain components that prevent recyclability.

Ceramic pigments are commercially available from a number of sources. For the preferred use of recyclable enamels, however, the choice of pigments is restricted. When decorated automotive glass, or possibly decorated architectural glass, is recycled, the pigment in the enamel is dissolved in the glass melt. The coloured transition metal ions that form the pigment are then incorporated into the glass structure. Depending upon the type of element, its oxidation state and its concentration in the pigment, the dissolved pigment contributes to the light absorption of the final glass, eg float glass. Elements such as Cr, Co and Νi - form light absorbing species regardless of their oxidation state. Cu can be reduced to colourless ions such as Cu(I) ions, but Cu(I) is unstable and disproportionates to give colloidal elemental Cu that reduces the light transmission of the glass. Therefore, the Cu content in the pigments should also be kept low.

Iron compounds are normally added to the glass batch for making float glass. No problems are therefore anticipated if the pigment contains iron compounds.

Manganese ions form coloured glasses if the oxidation state is >2, whereas Mn(II) ions hardly contribute to visible light absorption. Pigments containing Mn compounds do not cause problems for recycling.

Mixed valence iron oxides such as magnetite (see eg DE 44 01 657) or pigments based on iron-manganese mixed oxides are known to form black glass enamels. Pigments with this composition are commercially available, eg: Bayferrox 303 manufactured by Bayer Ag or Black 10G930, manufactured by Shepherd Color Company. The heat stability of magnetite pigments may be improved by coating with a glass composition (see DE 195 25 658).

Enamel powders are preferably made by dry blending of a frit powder with one or more pigment powders. Co- wet milling of the unground frit with the pigment may alternatively be used, or calcining a frit-pigment blend, followed by milling may also be used for making the enamel powder. The amount of pigment in the enamel powder is typically from 10 to 30 wt%, preferably from 15 to 25 wt%.

The invention also provides an ink suitable for application to glass, comprising an enamel according to the invention, suspended in a suitable fugitive medium, eg: a screen printing medium. The ink can be manufactured by mixing the frit and pigment powders and dispersing the enamel in the chosen medium using three-roll milling or by alternatives such as bead-milling or high-speed mixing. The resulting paste may be diluted, if necessary. Typically a screen-printing paste has a viscosity of from 5 to 25 Pas (at a shear rate of 10s^"1). The media used to make the ink are commercially available, eg: from the Johnson Matthey group and may be an IR-drying medium such as 650-63, 700-63 or 702-63, a UN-curable medium such as 660-70 or 724-70 or a thermoplastic medium such as 64-64.

Suitable particle sizes for the glass frit, expressed as d₉₀ are from 6 to 18μm. Suitable particle sizes for the pigment are from 1 to lOμm, preferably from 1 to 5μm.

The properties of the ink may be chosen according to the specific application method, firing schedule, etc, by routine experiment and optimisation. In general, the ink may be applied to automotive glass by screen printing, but alternative methods may be used. For example, an ink suitable for inkjet printing may be formulated and we incorporate the teaching of UK patent application No 0327340.6 (CBG1644) herein.

Suitable printing parameters and firing schedules are known in the art.

The present invention will now be described by reference to the following specific working examples. However, the skilled person will readily understand that the details may be modified without departing from the inventive concepts.

Glass samples Nos 1-11 The appropriate precursors in the form of oxides, borates, fluorides, phosphates or other suitable precursor, are mixed together, melted at approximately 1200°C for about 1 to 1.5 hours, then quenched by discharging into a flow of cold water to form a glass frit. Table 1 shows glass compositions in wt%. The frit may be ground into a powder, for example by ball milling or jet milling. Glass samples 3 and 11 are for comparative purposes, and are not according to the invention. Table 1

Table 1 (confl

The glass frits were tested according to conventional methods, with the results shown below in Table 2. Table 2: Physical and chemical data

Table 2 (conf)

Tg = glass transition temperature, measured by Differential Scanning Calorimetry. CTE = coefficient of thermal expansion

Acid resistance was measured according to ASTM C724-91 on a scale of 1 to 7, with 1 indicating no attack and 7 indicating total dissolution.

A variety of glass frits according to the invention, Nos 12 to 14, and a similar frit 15, were prepared with the compositions listed in Table 3 below.

Table 3

The frits Nos. 12 to 15 were formulated into enamels Examples Nos. 1 to 4 as set out in Table 4 below. Although frit 15 is not according to the invention, it may be blended together with one or more other frits to yield a frit according to the invention. Table 4 - Enamels

75 parts by weight of a glass frit or a blend of glass frits according to this invention (see Table above) is blended with 16 part by weight of Bayferrox 303 black pigment (Bayer AG) and 9 parts by weight of Black 10G930 pigment (Shepherd Color Company). To 100 parts by weight of this powder blend 25 parts by weight of screen printing medium 650-63 (Johnson Matthey Glass) are added. After intense mixing the final paste is formed by three- roll milling and adding more medium to give the desired viscosity of 15 Pas. This paste is screen printed onto a glass plate to give a wet layer of 25 μm thickness, dried at 120 °C and fired at 680 °C to give a well fused, pore-free ceramic coating.

The following data have been obtained by conventional testing methods for the glass enamel Examples, the compositions of which are shown in Table 4 above

Table 5 Characteristics of Fired Enamels Firing: 100 x 100 mm .2 clear glass (3 mm), roller kiln, 680 °C, speed 150 (4.16 cm/min)

Note + measured by a ball and ring compression test similar to the double ring compression test of DIN 52 292, but using a 1 cm stainless steel ball instead of the smaller ring specified in the standard test

Claims

CLAIMS 1. A glass composition comprising 35-45 wt% SiO₂, 12-20 wt% B₂O₃, 10-16 wt% ZnO, 0- 12 wt% Na₂O , 1-15 wt% K₂O, up to 2 wt% Li₂O, provided that Na₂O + K₂O + Li₂O is in the range 10-15 wt%, 8-18 wt% TiO₂, up to 2 wt% CaO, up to 1 wt% SrO and up to 5 wt% BaO, provided that CaO + SrO + BaO is in the range 0.4 to 5 wt%.

2. A glass composition according to claim 1, which is free of fluoride as a deliberately added component.

3. A glass composition according to claim 1 or 2, comprising also one or both of Al₂O₃ in an amount up to 3wt% and Bi₂O₃ in an amount up to 5wt%.

4. A glass composition according to any one of the preceding claims, wherein the amount of other components is not more than 2 wt% of each of ZrO₂, Fe₂O_3ι MnO, MgO, SnO₂, La₂O₃, CeO₂, P₂O₅, N₂O₅ and S.

5. A glass composition according to any one of the preceding claims, which is essentially

6. A glass composition according to any one of the preceding claims, which is essentially free of Bi₂O_3.

7. A glass composition according to any one of the preceding claims, in the form of a frit.

8. A blended frit having a composition according to any one of claims 1 to 6.

9. An enamel composition comprising at least one frit according to claim 7 or 8 in admixture with one or more ceramic pigments.

10. An enamel composition according to claim 9, wherein the amount of pigment is from 10 to 30 wt%, preferably from 15 to 25 wt%.

11. An enamel according to claim 9 or 10, wherein the pigment does not contain significant quantities of elements which yield light absorbing or light scattering species in a recycled glass material.

12. An enamel according to any one of claims 9 to 11, wherein the pigment is a black pigment.

13. An enamel according to claim 12, wherein the pigment is one or more of a mixed valance iron oxide and an iron-manganese mixed oxide.

14. An ink composition comprising one or more enamels according to any one of claims 9 to 13, suspended in a fugitive suspension medium.

15. An ink according to claim 14, formulated as a screen printing ink.