WO2001009061A1

WO2001009061A1 - Improvements in the coating of ceramic substrates

Info

Publication number: WO2001009061A1
Application number: PCT/GB2000/002761
Authority: WO
Inventors: Andrew Mcdermott; Geoffrey Wallace Aston; Mark Dylan Francis
Original assignee: Lafarge Braas Research & Development Ltd.
Priority date: 1999-07-31
Filing date: 2000-07-18
Publication date: 2001-02-08
Also published as: GB9917953D0; GB2353485A; GB0016910D0

Abstract

A method of forming a coating of a lustre pigment on a fired or an unfired ceramic substrate, for example, a roofing tile, by applying to the substrate an engobe consisting of, or comprising, a clay based composition; applying a coating of a lustre pigment of between 0.55 grams to 16.7 grams per sqM and of 0.1 to 50νm thickness to overlie the clay based composition; and, heating the coated ceramic substrate sufficiently to effect the fusion together of the layers thereof. The clay based composition, which may be pre-fired on the ceramic substrate, may contain pigment and silica, boron oxide or phosphorous pentoxide based frit.

Description

Improvements in the Coating of Ceramic Substrates

This invention is concerned with improvements in or relating to the application of lustre pigments to ceramic substrates and is particularly, though not exclusively, concerned with the application of lustre pigments to clay roof tiles.

It is well known that clay roof tiles may be provided with a coloured surface which is different from the body colour of the roof tile. This is done to render the roof tiles more aesthetically pleasing and to provide a smooth upper surface to the tiles thereby reducing the incidence of detritus adhering to the upper surfaces of the tiles whereby the coloured surfaces retain a high degree of their original reflective characteristics throughout the lives of the roof tiles. The range of colours that may be utilised to improve the aesthetics of clay roof tiles is extensive; however, of late it has become fashionable to apply lustre pigments to appropriate products in order to benefit from the pearlescent appearances of those pigments .

Lustre pigments which have become widely used in the automative industry and for improving the appearance of artefacts intended for use in the home and for architectural purposes are, inter alia, mica or glass flakes coated with a metal oxide such as titanium dioxide and/or ferric oxide although other oxides such as Cr, Co or Ni may be used. In addition, the same or similar lustre pigments are used in the cosmetics, plastics, automative and printing industries.

These lustre pigments are normally applied to ceramic substrates by being admixed into a vitreous glaze applied to the substrate or by being applied to the vitreous surface of the glaze. In the automative industry, such lustre pigments are applied to automative paint lacquers.

In the manufacture of clay roof tiles colour is usually applied by admixing conventional pigments to an engobe comprised predominantly of a clay based composition.

The main difference between admixing pigment to a vitreous composition or admixing pigment to a clay based composition is apparent if one looks at products made by methods that include such steps. Because vitreous compositions are made from ground glass, glass forming minerals or mixtures of both, the end result is a clear or translucent body and/or a clear or translucent coating on such a body. Thus, if a pigment is admixed with the body and/or coating, the colour has apparent depth thereto. Admixing pigment into a clay based composition only gives a coloured surface effect because clay based compositions are opaque, i.e. they are not translucent.

The availability of inorganic lustre pigments at prices that are not too prohibitive has prompted our investigations into their use as coatings for clay roof tiles in order to obtain clay roof tiles with a high degree of colour reflectivity. In our investigations, which were carried out with a view to modifying the conventional manufacturing process for clay roof tiles rather than adopting the teachings of the prior art in relation to vitreous compositions admixed with or coated with lustre pigments, it was found that the application of lustre pigments to clay roof tiles pre- coated with an engobe comprised predominantly of a clay based composition resulted in a roof tile having a superior colour finish when compared to clay roof tiles coloured using engobes having admixed therein or coated thereon a conventional pigment.

Our continued investigations included the application of lustre pigments as a top coat to clay tiles pre- coated with an engobe of a predominantly clay based composition to which there was admixed a conventional pigment. The results of these further investigations gave a roof tile with superlative colour reflectivity of much greater aesthetic appeal than has been achieved hitherto in clay roof tile manufacture.

The present invention thus conveniently provides a method of forming a coating of lustre pigment on a ceramic substrate comprising the steps of :

a) applying an engobe consisting of, or comprising, a clay based composition to a surface of the ceramic substrate to form a layer of the clay based composition thereon; b) applying a coating of lustre pigment to overlie the layer of the clay based composition; and,

c) heating the coated ceramic substrate sufficiently to effect:

I) the firing of the ceramic substrate and the fusion thereto of the clay layer, and the fusion of the lustre pigment to the clay layer, where the ceramic substrate is in an unfired condition; or,

ii) the fusion of the clay layer to the ceramic substrate and the fusion of the lustre pigment to the clay layer, where the ceramic substrate is in a fired condition; or,

iii) the fusion of the lustre pigment to the clay layer , where the ceramic substrate and the clay layer are in a fired condition.

In a preferred method the clay based composition further comprises pigment admixed therewith.

Conveniently the clay based composition may comprise from 0 to 50% by weight of a silica, boron oxide or phosphorous pentoxide based frit .

Preferably the frit is provided as an inter layer between the clay substrate and the layer of the clay based composition.

In one embodiment provided by the present invention the ceramic substrate is an unfired or fired clay roof tile.

In a further embodiment provided by the present invention the ceramic substrate is an unfired or fired artefact other than a clay roof tile.

Conveniently, the lustre pigment may be applied at a rate which gives between 0.55 to 16.7 grams per sqM of lustre pigment on each tile; preferably there is between 4.47 to 11.18 grams per sqM; and, more preferably, there is approximately 7.82 grams per sqM of lustre pigment applied to each roof tile.

The application rates of the lustre pigment to the roof tiles conveniently results in thicknesses of lustre pigment on each tile of between 0.1 and 50μm.

The present invention is exemplified by the following examples .

Example 1

This Example illustrates a method of fixing lustre pigments to an unfired (green) , engobed tile substrate.

The substrate tile was an unfired, works formed tile. The chemical and mineralogical composition of the clay tile substrate (blend 1) and all other clay materials utilised in these examples are given in tables 1 and 2 below.

An engobe suspension was prepared by ball milling 200g of clay (blend 2) in 400g of water for 24 hours. No additional frit material was included for this particular formulation. Conventional absorption pigment, Bayer Bayferrox BF 330 T black, was then added to the engobe according to the following ratio: 12g Bayer black pigment to 90g of engobe suspension and thoroughly mixed for fifteen minutes. The pigmented engobe formulation was then applied to the clay tile surface by a gravity fed HVLP (High Velocity, Low Pressure) spray gun and allowed to dry under ambient conditions. Typically, between 6g and 12g of dried engobe was deposited per tile.

A lustre pigment coating was next applied. The pigment coating was a dispersion of lustre pigment based on mica flakes having an iron oxide coating. The lustre pigment, which was supplied by Merck KGaA/EM Industries, was Iriodin/Afflair 504. The application medium utilised was 5.00g of an acrylate copolymer solution mixed with 0.25g of lustre pigment. Prior to use, the formulation was agitated in order to counteract sedimentation.

The lustre pigment acrylate copolymer solution formulation was similarly applied to the engobed surface by a gravity fed HVLP spray gun. The lustre coated engobed tile was initially allowed to dry under ambient conditions, after which it was dried at 110°C to promote complete dryness. The amount of lustre pigment applied to the engobed tile surface, excluding residual acrylate copolymer solution was typically between 4.47 grams and 11.18 grams per sqM. It is believed that the acrylate copolymer solution is an effective application medium because of the film shrinkage associated with the copolymer constituent, which 'pulls' the pigment platelets close to the surface in a desired flat' orientation.

The coated tiles were fired in accordance with the normal firing schedule associated with clay blend 1, namely:

• 20 hours to 1005°C (at a uniform heating rate) • 2 hours at 1005°C

The excess lustre pigment which was not affixed during the firing process was subsequently removed using a combination of dry rubbing and rinsing with water. The fired layer of lustre pigment that adheres is typically between 0.1 and 50 μ in thickness and produces a pleasing appearance.

Example 2

This Example illustrates a method of fixing lustre pigments to a pre-fired engobed tile substrate.

The substrate tile was a works fired, engobed tile. The chemical and mineralogical composition for the substrate clay (blend 3) is given in tables 1 and 2.

Typically 18g of a commercially available dry red engobe is deposited per tile. The chemical and mineralogical composition for the commercially available red engobe is given in Tables 3 and 4. The initial firing schedule for the engobed tile substrate was :

• 19 hours to 1060°c (at a uniform heating rate) • 4 hours at 1060°C

A lustre pigment coating was next applied. The pigment coating was a dispersion of lustre pigment based on mica flakes having an iron oxide coating. The pigment, which was supplied by Merck KGaA/EM Industries, was Iriodin/Afflair 507. The lustre pigment acrylate copolymer formulation was applied to the pre- fired engobed surface by a gravity fed HVLP spray gun. The coated tile was initially allowed to dry under ambient conditions, afterwhich it was dried at 110°C to promote complete dryness . The amount of lustre pigment applied to the engobed tile surface, excluding residual acrylate copolymer solution, was typically between 4.47 grams and 11.18 grams per sqM.

The tile was fired in accordance with the normal works firing schedule for this particular clay, as described above . The excess lustre pigment which was not affixed during the firing process was subsequently removed using a combination of dry rubbing and rinsing with water. The fired layer that adheres is typically between 0.1 and 50μm in thickness and produces a pleasing appearance.

Further Examples

In direct accordance with the methodology described in Example 1 the following tiles have also been successfully produced:

In direct accordance with the methodology described in Example 2, tiles coated in the folowing lustre pigments have also been successfully produced:

Iriodin/Afflair 504

Iriodin/Afflair 505 Iriodin/Afflair 508

Based on the method described in Example 2, unfired tiles (clay blend 3) engobed in a commercially available black engobe have also been coated in lustre pigments. The chemical and mineralogical composition for the commercially available black engobe is given in Tables 5 and 6. The tiles were subsequently fired in accordance with the usual schedule for clay blend 3. During these investigations, coatings of the following lustre pigments have been demonstrated:

Iriodin/Afflair 505 Iriodin/Afflair 507 In this case, 17g of dry black engobe is deposited per tile .

Table 1

Typical chemical composition of clay materials by wt percent (oxide) .

Table 2

Mineralogical composition of clay materials - bulk analyses by wt percent .

Table 3

Typical chemical composition of the commercially available red engobe of example 2 by wt percent (oxide) .

Table 4

Mineralogical composition of the commercially available red engobe of example 2 - bulk analysis by wt percent.

Table 5

Typical chemical composition of the commercially available black engobe of the further examples by wt percent (oxide)

Table 6

Mineralogical compsition of the commercially available black engobe of the further examples - bulk analysis by wt percent .

Claims

1. A method of forming a coating of lustre pigment on a ceramic substrate comprising the steps of :

a) applying an engobe consisting of, or comprising, a clay based composition to a surface of the ceramic substrate to form a layer of the clay based composition thereon;

b) applying a coating of lustre pigment to overlie the layer of the clay based composition; and,

c) heating the coated ceramic substrate sufficiently to effect:

ii) the fusion of the clay layer to the ceramic substrate and the fusion of the lustre pigment to the clay layer , where the ceramic substrate is in a fired condition; or, iii) the fusion of the lustre pigment to the clay layer, where the ceramic substrate and the clay layer are in a fired condition.

2. A method according to Claim 1 wherein the clay based composition further comprises pigment.

3. A method according to either one of Claims 1 and 2 wherein the clay based composition further comprises from 0 to 50% by weight of a silica, boron oxide or phosphorous pentoxide based frit .

. A method according to any one of the preceding Claims wherein a layer of silica based frit is provided between the ceramic substrate and the layer of the clay based composition.

5. A method according to any one of the preceding Claims wherein the ceramic substrate is an unfired or a fired clay roof tile.

6. A method according to any one of Claims 1 to 4 wherein the ceramic substrate is an unfired or a fired artefact other than a clay roof tile.

7. A method according to any one of the preceding Claims wherein the amount of lustre pigment applied to the surface of each roof tile is between 0.55 grams to 16.7 grams per sqM.

8. A method according to any one of the preceding Claims wherein the amount of lustre pigment applied to the surface of each roof tile is between 4.47 grams and 11.18 grams per sqM.

9. A method according to any one of the preceding

Claims wherein the amount of lustre pigment applied to the surface of each roof tile is approximately 7.82 grams per sqM.

10. A method according to any one of the preceding

Claims wherein the layer of lustre pigment fused onto the layer of clay of the clay roof tile is between 0.1 and 50μm.