GB2104057A

GB2104057A - Enamels which are lustrous after firing

Info

Publication number: GB2104057A
Application number: GB08124902A
Authority: GB
Inventors: Stephen John Bennett
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1981-08-14
Filing date: 1981-08-14
Publication date: 1983-03-02

Abstract

The unfired enamel, suitable for firing onto a glass or ceramic substrate, comprising a dispersion of Al flakes, organic binder and glass flux, is characterised in that (a) it contains 3-25% Al, (b) the Al flakes are </=88 mu ; and (c) the glass flux has a glass transition temperature of 420- 490 DEG C and a softening temperature of 475-560 DEG C. Firing creates a fired enamel having better lustre, better scratch-resistance and better adhesion to the substrate. Also a process for making the unfired enamel in which the glass flux is finely ground and dispersed in binder oil before mixing with the aluminium flake.

Description

SPECIFICATION Enamels which are lustrous after firing This invention relates to enamels which are lustrous when fired onto a glass or a ceramic substrate.

Previous attempts to make such enamels have involved dispersing large aluminium flakes mixed with conventional organic binders for pigments into glass fluxes. On firing these enamels onto the substrate, it was found that the fired enamel had only a dull lustre. In an attempt to achieve a better lustre, coatings comprising aluminium flakes dispersed in epoxy resins were supplied commercially. However, such coatings were expensive and they lacked good scratch resistance and good adhesion to the substrate.

An object of this invention is to provide an enamel which can be fired onto a glass or ceramic substrate and which when fired produces an enamel having a bright lustre, good scratchresistance and good adhesion to the substrate combined with good chemical resistance to acids, alkalis and organic solvents.

Accordingly this invention provides an unfired enamel suitable for firing onto a glass or ceramic substrate to produce a lustrous fired enamel, the unfired enamel comprising a dispersion of aluminium flakes optionally pigment binder and glass flux characterised in that a) the unfired enamel comprises 3 to 25% (preferably 7 to 17%) by weight (based on the total weight of aluminium binder, glass flux and any pigment in the enamel of aluminium flakes, b) the aluminium flakes have a maximum dimension of less than 88 and c) the glass flux has a glass transition temperature (i.e. Tg) of from 420 to 4900C (preferably from 455 to 4800C) and a softening temperature (i.e.Ts) of from 475 to 5600C (preferably 490 to 5300C and especially 515 to 5250C), Tg and Ts are measured by conventional methods relying on changes of volume manifesting as changes in length as detected by a dilatometer.

The organic binder may be any organic oil dispersant which is easily burnt off during firing at over 5500 C.

Aluminium flakes having a maximum dimension of less than 88 4 are obtainable by screening using Sieve 1 70 as specified by ASTM designation El 1 of 1961. Large flakes produce a more sparkling lustre so for lustres of high sparkle it is preferred that at least 90% by weight of the flakes have maximum diameters larger than 63 y (i.e. flakes retainable by ASTM Sieve 230).

Smaller flakes create a matte effect and where this is required it is preferred to use flakes having a maximum dimension of less than 20 y and generally their weight average dimension is preferably from 5 to 1 5 y and especially from 8 to 12y Counter to conventional practice for making soft glasses, the glass fluxes having a Tg and Ts as specified for the purposes of this invention preferably contain at most only small amounts of lead, for example from 0 to 40% by weight (based on the weight of the flux) of lead. Frequently the flux will contain no lead.

This invention also provides a process for making an unfired enamel which is suitable for firing onto a glass or ceramic substrate to produce a lustrous fired enamel, the unfired enamel comprising a dispersion of aluminium flakes, optionally pigment, organic binder and glass flux characterised in that a) the glass flux is ground and then screened (for example by sieving using ASTM Sieve 170) to remove particles having a maximum dimension of greater than 88 y.

b) the screened glass flux is mixed with the organic binder (preferably ratio by weight of flux to oil is from 1:1 to 3:1) and c) the aluminium flakes are mixed into the mixture of flux and binder.

The unfired enamel may be appiied to the substrate by conventional coating techniques or by conventional printing techniques, especially by the socalled "silk screen techniques". The organic binder helps to create a printable enamel. Once the unfired enamel has been applied, the substrate is then fired at temperatures of preferably 550 to 6500C for periods of preferably from 5 to 30 minutes. In particular it is preferred not to fire at above 6690C in case the aluminium flakes suffer extreme oxidation. During firing the organic binder is burnt off and the unfired enamel is converted to a lustrous fired enamel.

Accordingly this invention also provides a glass or ceramic substrate bearing fired enamel comprising aluminium flakes dispersed in a glass flux characterised in that a) the fired enamel comprises 3 to 20% by weight (based on the weight of the aluminium and flux) of aluminium flakes b) the aluminium flakes have a maximum dimension of less than 88 y and c) the glass flux has a Tg of from 420 to 4900C and a Ts of from 475 to 5600C.

The glass fluxes used in the performance of this invention may be based on conventional soda glasses which may be modified by the inclusion of other components such as colouring agents (e.g.

pigments). Preferably the flux contains from 5 to 20% (by weight of the flux) of pigment. A fuller review of possible colouring agents is provided by the book "Ceramic Colours and Pottery Decoration" by K. Shaw published in 1962 by MacLaren s Sons Limited of London (the contents of which are herein incorporated by reference).

The lustrous fired enamels provided by this invention are useful in providing decoration and lettering on glass panels used in consumer durables such as cookers and washing machines and for providing heat and light reflecting surfaces generally.

The invention is illustrated by the following example.

Example 1 A mixture consisting of a) 100 parts by weight of a soda glass flux having a Tg of 4440C, a Ts of 4810C and containing 39.2% lead and b) 0.2 parts by weight of a soda glass flux having a Tg of 4330C and a Ts of 483"C was ground in the presence of water. 10 parts (by weight of the fluxes) of titanium dioxide were added to the fluxes and ground to achieve good mixing. The ground mixture was dried by evaporation of the water and sieved to remove particles having a maximum dimension of above 88 ,u.

The sieved mixture and an organic binder were charged to a mixing vessel in a weight ratio of 2 parts glass flux to 1 part binder. When the components had been thoroughly mixed, aluminium flakes having a maximum dimension of less than 88 u were added to the mixture until it comprised 10% by weight (based on the weight of the aluminium, titanium dioxide, binder and flux) of aluminium flakes. The vitreous unfired enamel so formed was then coated onto a flat strip of soda glass and fired at 6500C for 7 minutes.

The fired strip was found to bear a fired glass enamel having a bright lustrous sparkling appearance and possessing good scratchresistance and good adhesion to the surface of the soda glass strip. The fired enamel had good chemical resistance to alkalis and acids and to organic solvents, especially those used in cleaning printing apparatus.

Comparative Examples A to C The procedure of Example 1 was repeated except for the following modifications: In the case of Example A, the glass flux used had a Tg of 4970C and a Ts of 5670C. Use of a harder glass flux led to poor lustre and less adhesion between the fired enamel and the glass strip.

Example B was a repeat of Example A but using coarser aluminium flakes. Again the fired enamel lacked lustre and had poor adhesion to the glass strip.

Example C was a repeat of Example 1 except that the aluminium flakes were mixed with the ground glass flux before the flux had been dispersed in the organic binder. The fired enamel obtained lacked lustre.

Claims

1. An unfired enamel suitable for firing onto a glass or ceramic substrate to produce a lustrous fired enamel, the unfired enamel comprising a dispersion of aluminium flakes, organic binder and glass flux characterised in that a) the enamel comprises 3 to 25% by weight (based on the total weight of aluminium, binder and glass flux in the enamel of aluminium flakes, b) the aluminium flakes have a maximum dimension of less than 88 1 and c) the glass flux has a glass transition temperature of from 420 to 4900C and a softening temperature of from 475 to 5600C.

2. An unfired enamel as claimed in claim 1 wherein the glass flux contains no lead.

3. An unfired enamel as claimed in claim 1 wherein the glass flux contains lead in an amount less than 40% by weight of the flux.

4. An unfired enamel according to any one of the preceding claims wherein the unfired enamel comprises at least 90% by weight (based on the total weight of the aluminium in the enamel) of aluminium flakes having a maximum dimension of greater than 63,u.

5. An unfired enamel according to any one of the preceding claims wherein the unfired enamel contains 5 to 20% (by weight of the flux) of pigment.

6. A process for making unfired enamel which is suitable for firing onto a glass or ceramic substrate to produce a lustrous fired enamel, the unfired enamel comprising a dispersion of aluminium flakes, and glass flux in an organic binder characterised in that a) the glass flux is ground and then screened to remove particles having a maximum dimension of 88 u or greater, b) the screened glass flux is mixed with the organic binder and c) the aluminium flakes having a maximum dimension of less than 88 u are mixed into the mixture of flux and binder.

7. A glass or ceramic substrate bearing an enamel comprising aluminium flakes dispersed in a glass flux characterised in that a) the enamel comprises 3 to 25% by weight (based on the weight of the aluminium and flux) of aluminium flakes b) the aluminium flakes have a maximum dimension of less than 88 u and c) the glass flux has a Tg of from 420 to 4900C and a Ts of from 475 to 5600C.

8. A consumer durable comprising a substrate as claimed in claim 7.