WO1996000195A1

WO1996000195A1 - Ceramic slurry glaze formulations

Info

Publication number: WO1996000195A1
Application number: PCT/GB1995/001455
Authority: WO
Inventors: John Francis Clifford; Jennifer Clare Watts
Original assignee: Cookson Matthey Ceramics & Materials Ltd.
Priority date: 1994-06-24
Filing date: 1995-06-21
Publication date: 1996-01-04
Also published as: GB9412695D0; AU2745795A

Abstract

A stable glaze slip formulation having a pH of above 7.5 which comprises particles of at least one glass forming material suspended in an aqueous medium containing a synthetic associative polymeric thickener which comprises a hydrophobically modified polyacrylate copolymer which forms aggregates or micelles in aqueous media, which is shear thinning in aqueous media but which does not break down under shear, the thickener being incorporated into the glaze slip formulation in an amount such that the suspension, prior to application, has a yield stress of greater than 2Pa, the glaze slip formulations, in use, exhibiting shear thinning behaviour on application to a substrate surface and on firing the thickener decomposing and/or burning out of the glaze slip formulation in order to leave substantially no residues or residual artefacts in the glaze so formed.

Description

CERAMIC SLURRY GLAZE FORMULATIONS

The present invention relates to glaze formulations and, in particular to glaze slip formulations in which additives are used to control the rheological behaviour, particle sedimentation, substrate wetting, drying time and green strength of the unfired glaze. Glaze slips are 'bespoke' systems in that they are tailored chemically and rheologically to the unique specifications of each user according to the substrate type, application method and firing regime.

Amongst the major requirements is minimization of the settling and sedimentation of particles such as frit or raw materials in glaze slurries since such sediments are often difficult to redisperse, leading to inhomogeneous systems. Several approaches to reducing the settling of particles have been employed, including the use of very fine particles, the use of polymeric thickeners or the use of fine particles and a thickener. There can be significant problems associated with each of these approaches.

A wide range of clay minerals (china clays, ball clays, bentonites) is used in glaze formulations, generally in an amount in the range of from 1 to 15% by weight. The presence of clays can cause problems, particularly on firing the glaze formulations for example by giving rise to gas bubbles during decomposition/dehydroxylation. This increases the likelihood of defects in glazes based on unleaded frits, the use of which is increasing; lead-based glazes have a lower viscosity and surface tension when molten and thus are less likely to show defects. High molecular weight polymeric thickening agents, such as cellulose derivatives, poly-saccharides, starch, natural gums or resins, polyacrylates or polyvinyl alcohols are typically added to the glaze formulations in amounts of from 0.5 to 2% by weight. Problems with these materials include for example reaction with soluble glaze components, microbial degradation, residues remaining on burnout, variability with temperature, breakdown under high shear, and limited workability or reworkability. Whilst not all thickeners are deficient in all of these respects, all have at least one drawback. Additional materials are often added to the glaze formulations to counteract the undesirable effects of other additives, resulting in complex glaze formulations. The disadvantage of the prior art systems are tabulated below for ease of reference. It will be noted from this Table that all of the prior art systems possess at least one and sometimes a significant number of disadvantages.

W093/12047 discloses a sanitary ceramic glaze formulation which is stated to have improved drying and levelling properties which comprises a thickening amount of a hydrophobically modified hydroxyethylcellulose. Such compositions suffer from the disadvantages that they are susceptible to microbial degradation, breakdown under shear and, being naturally derived, may not be batch consistent. Accordingly, there is still a requirement in the ceramics field for a glaze formulation which addresses the problems detailed above.

We have now developed glaze compositions which overcome the problems associated with the prior art compositions. They provide slurries of high viscosity at low shear, which prevents the settling of particles, yet with low viscosity at high shear, which provides ease of pumping and of spraying the slurries. The compositions are also reworkable and have substantially no residues on burnout at relatively low temperatures. Thus, the compositions of the present invention score as "excellent" in all of the columns of the above Table.

Accordingly, the present invention provides a stable glaze slip formulation having a pH of above 7.5 which comprises particles of at least one glass forming material suspended in an aqueous medium containing a synthetic associative polymeric thickener which comprises a hydrophobically modified polyacrylate copolymer which forms aggregates or micelles in aqueous media, which is shear thinning in aqueous media but which does not break down under shear, the thickener being incorporated into the glaze slip formulation in an amount such that the suspension, prior to application, has a yield stress greater than 2Pa, the glaze slip formulations, in use, exhibiting shear thinning behaviour on application to a substrate surface and on firing the thickener decomposing and/or burning out of the glaze slip formulation in order to leave substantially no residues or residual artefacts, such as bubbles, in the glaze so formed. The hydrophobically modified polyacrylate copolymers used in the present invention exhibit shear thinning behaviour in aqueous media, i.e. have a higher viscosity at low shear rates than the viscosity at high shear rates. The chemistry of associative thickeners is discussed in Polymers in Aqueous Media: Performance through Association, ed. J.E. Glass, Advances in Chemistry Series No. 223, American Chemical Society 1989.

The hydrophobically modified polyacrylate which is used in the invention is preferably a terpolymer which comprises a carboxylic monomer a hydrophobic monomer and an associative monomer. Currently available materials generally have a molecular weight in the range of from 1000 to 10,000. A particular example of such hydrophobically modified acrylate polymers are those sold under the Trade Name Rheovis CRX, CR, CR2 and CR3 (from Allied Colloids) . The associative thickener is generally included in the glaze slip formulations in an amount of from 1 ppm to 5.0% by weight based on the total weight of solids, preferably 0.1 to 2.0% -by weight based on the total weight of solids. Mixtures of associative thickeners may also be used, if desired, in order to tailor the rheology of the glaze slip formulations (magnitude of yield stress and degree of shear thinning) to match precise requirements. The glaze formulation is matched to the substrate and to the application method to achieve an optimized, finely balanced system.

The carboxylic functionality on the carboxylic monomer is provided by carboxylic acid or anhydride groups, examples of these groups being those derived from acrylic acid and methacrylic acid. The associative monomer normally comprises a long chain hydrophilic segment which is terminated with a hydrophobe. The hydrophilic segment preferably comprises a polyethoxy or poly(ethoxy/propoxy) chain situated between ethylenic unsaturation at one end of the molecule and a terminal hydrophobe at the other end of the molecule. Preferably there are at least 40 oxyethylene units or 40 oxyethylene/oxypropylene units between the surfactant hydrophobe and the ethylenic unsaturation. The associative monomers are generally made by coupling the hydroxyl end group of a conventional nonionic surfactant to a monomer containing ethylenic unsaturation. The hydrophobe on the associative monomer is the primary interactive component in the associative thickener. The hydrophobic monomer as such can be omitted from the polymer if the associative monomer is able to impart the hydrophilic-hydrophobic balance to the thickener necessary for pH-dependent solubility. The copolymer backbone of the associative thickener is predominantly hydrophilic since it contains sufficient carboxyl groups or anhydride groups to render the thickener water soluble at a high pH.

The mechanism by which the associative thickener acts is by the non-specific hydrophobic association of water-insoluble groups in water-soluble polymers. The terminal hydrophobes on the ethoxylate or ethoxylate/ propoxylate side chains are repelled from the hydrophilic portions of the molecule and are attracted to other terminal hydrophobes. This intermolecular association of terminal hydrophobes of associative thickeners in aqueous solution may be regarded as icellization in a continuous network and the driving force for association is the increase in entropy resulting from the loss of water around the hydrophobes and the minimization of water-hydrophobe contacts. The glass forming materials which may be used in the present invention include glass frits, clays and other beneficiated minerals.

Preferably the glaze slip formulation also includes therein at least one dispersant (as hereinafter defined) .

By the term "dispersant" as used herein is meant a compound or composition which acts to maintain the particles in a suspension in a dispersed state in which they do not associate with other particles to form aggregates. A dispersant acts by adsorbing on the particle surface, so altering the particle surface chemistry for minimum interparticle interaction. The chemistry of dispersants (surfactants) is discussed in Surfactants ed. Th F Tadros, Academic Press, London 1984 and Industrial Applications of Surfactants, ed. D.R. Karsa, Special Publication No. 59, Royal Society of Chemistry, London 1987.The dispersant maintains the particles in a dispersed state in the suspension when it is first formed, or if the suspension is recycled, or if the suspension is reconstituted after drying.

The glaze slip formulations of the present invention may be prepared from any essentially insoluble glass forming components, for example mixtures of raw materials and/or insoluble oxides which on exposure to high temperatures form a glassy layer. Suitable examples are borate, phosphate or silicate glass frits, such as a borosilicate glass frit or a lead-free frit, or a mixture of glass forming oxides and/or beneficiated minerals. Generally the insoluble glass forming materials are composed of particles 80% of which lie between 0.75 and 50μm, frequently with a d₉₉ of up to 80μm.

Examples of dispersants which may be used in the present invention are any appropriate cationic anionic, non-ionic or amphoteric surfactants, or mixtures thereof, which act under the prevailing conditions as a dispersant. The selection criteria and mode of use are as described in the literature. An example of a nonionic surfactant for use in the present invention is CT111, an acetylenic diol based material from Air Products. Examples of anionic surfactants for use in the present invention are naphthalene sulfonic acid, or a derivative or salt thereof, poly(naphthalene sulfonate-co-formaldehyde) , and Dispex N40, a poly(acrylic acid) -sodium salt from Allied Colloids. An example of a cationic surfactant for use in the present invention is cetyltrimethyl- ammonium bromide. An example of an amphoteric surfactant for use in the present invention is Mirataine JCHA, a sodium alkylamino propionate from Rhone Poulenc. The dispersant is preferably included in the glaze slip formulation in an amount of from lppm to 5% by weight based on the total weight of solids, more preferably 0.1 to 2% by weight based on the total weight of solids. The glaze slip formulations of the present invention may additionally include one or more clays therein, generally in an amount of up to 15% by weight based on the total weight of solids. It will be understood that in the glaze slip formulations of the present invention the amount of clay can be considerably reduced or even eliminated as compared to conventional formulations.

The present invention also included within its scope a method for the preparation of a glaze slip formulation, which method comprises adding the associative thickener to an aqueous suspension of at least one glass forming material, or adding a solution of the associative thickener to at least one glass forming material in powder form, or adding a dry blend of the associative thickener and at least one glass forming material to water. A compatible dispersant as previously described is preferably also added to the aqueous suspension of the glass forming material. Other compatible thickeners, binders, wetting agents, anti-foaming agents and defoaming agents may also be added as required for the specific end use. Such additives are well known in the art.

The glaze slip formulations of the present invention have a pH of above 7.5 and will generally have a pH in the range of from 8 to 10, preferably a pH of about 9 which is the natural pH of the frit suspension. At this pH the associative polymeric thickener is in a configuration which will provide optimum suspending power. The associative polymeric thickeners of the present invention are relatively low molecular weight polymers and thus are far more resistant to high shear mixing that conventional polymers. The thickeners thus do not break down under shear and this enables any residues or waste to be reslurried or reworked for re-use. For example, any residues may be dried and then redispersed in order to form fresh glaze slip suspensions. The associative polymeric thickeners are synthetic and are resistant to microbial degradation. The dispersants and the associative polymeric thickeners burn out completely at temperatures of below 500°C and so prevent bubble or residue problems in the glazes.

The glaze slip formulations of the present invention are generally of a gel-like consistency prior to application whilst exhibiting shear thinning. The surface active nature of the thickener lowers the surface tension of the formulation on application and hence promotes wetting. The glaze slip formulations thus can be applied onto a substrate surface without the need for any additional wetting agent. The glaze slip formulations of the present invention can be applied to a substrate surface without flowing off any steeply curved surface. The glaze slip formulations are also able to wet both hydrophobic decorated areas (e.g. wax based decals) and hydrophilic undecorated areas on the substrate. Prior to application, the glaze slip formulations have a yield stress of greater than 2Pa, preferably greater than 5Pa. The aim is to have a uniform over the entire surface (e.g. cups, tiles, toilets) prior to firing. The glaze coating frequently forms a layer of adequate green strength for handling under most conditions without the need for an additional binder. In some instances it may be desirable to alter the rheology of the glaze slip prior to use and this may be achieved by adjustment of the pH thereof. Accordingly, the glaze slips may be thinned to make them suitable for spray coating onto a substrate by the lowering of the pH, for example, by the addition of acid to the glaze slip formulation.

The glaze slip formulations of the present invention may optionally be dried before use and yield a material which may be readily redispersed in water to produce a glaze slip with the same rheological characteristics as the original material.

Alternatively, a glaze powder may be prepared as a dry blend of at least one glass forming material and an associative thickener (as hereinbefore defined) . The glaze powder may be dispersed in water to form a glaze slip.

The method of the present invention may be used in the preparation of glaze slip compositions from any glass frit or frits. The glaze compositions of the present invention may be applied to any substrate for example bone china, hotelware, earthenware, porcelain, sanitaryware or tiles, either for under glaze decorated substrates or for on glaze decorated substrates.

The present invention will be further described with reference to the following Examples. EXAMPLE 1

A glaze slip formulation was prepared by ball milling 2.8% (by weight) china clay,

53.1% (by weight) borosilicate tableware frit,

44.1% water to a particle size d₇₀ < 14μm. The resulting suspension had a pH of 9.5. To this suspension was added 0.6% (by weight, relative to water) Rheovis CR and the formulation thoroughly mixed to give a suspension with a yield stress of lOPa.

After four months there was no settling of the suspension and no hard cake formed on the bottom of the vessel containing the suspension.

The glaze slip was spray coated onto translucent china (non porous) and earthenware (porous) substrates. An even coating was deposited which had good green strength. The coating adhered fully to waxy underglaze - decorated areas on the substrate. On firing to 1100°C a glaze of acceptable quality was produced.

EXAMPLE 2

A glaze slip formulation was prepared by ball milling

55.9% (by weight) borosilicate tableware frit (70% < 14 m)

44.1% water

To samples A, B, C, D and E of this suspension were added 0.6% (by weight, relative to water) of Rheovis CRX, CR, CR2, CR3 and a 50/50 mix of CRX and CR2, respectively. The properties of the suspensions were as follows: Suspension EH Yield Stress (Pa)

A 9 . 1 12

B 9 . 2 10

C 9 . 1 7.5

D 9 . 1 8

E 9 . 2 9.5

After four months there was no settling of the suspensions and no hard cake formed on the bottom of the vessels containing the suspensions. The glaze slips were spray coated onto translucent china (non porous) and earthenware (porous) substrates. Even coatings were deposited which had good green strength. The coatings adhered fully to waxy underglaze - decorated areas on the substrates. On firing to 1100°C glazes of acceptable qualities were produced.

EXAMPLE 3

The following materials were added to water to give a slurry of solids content 71.5%

% by Weight

Alkali borosilicate frit 55.0 Aluminium and cobalt oxide 4.0

Ball clay 9.0

Silicon and molybdenum metal 23.0

Silicon carbide 9.0

Rheovis CR (0.4% by weight of solids) was added to the slurry using a stirrer.

No settling was observed over a period of several weeks. Alumina-graphite refractory samples were dipped into the glaze to produce an unfired glaze of thickness of from 0.3 to 0.5 mm. The unfired glaze had acceptable hardness for normal handling. On firing a glaze coating of acceptable quality was produced. EXAMPLE 4

A glaze slip formulation was prepared in accordance with the teaching of Example 2 with the addition to the suspension of 0.6% (by weight relative to water) of Rheovis CR3 and with the addition of 0.2% Surfynol 104 antifoam to the formulation. The glaze slip formulation had a viscosity of yield stress of 10.2Pa. After four months there was no settling of the suspension and no hard cake formed on the bottom of the vessel containing the suspension.

The glaze slip was sprayed onto translucent china (nonporous) and earthenware (porous) substrates. Even coatings were deposited. On firing to 1100°C, a glaze of acceptable quality was produced.

EXAMPLE 5

Glaze slip formulations were prepared in accordance with the teaching of Example 2 using wet ball milled borosilicate tableware frits having a particle size distribution of 60% < 14 μm (coarse) , 70% < 14μm (medium/standard) and 80% < 14 μm (fine) with the addition to each formulation of 0.6% (by weight relative to water of Rheovis CRX. The properties of the suspensions were as follows:

Suspension ES Yield Stress (Pa)

Coarse 9.2 8.4

Medium/Standard 9.1 10.2

Fine 9.3 11.1

After four months there was no settling of the suspensions and no hard cake formed on the bottom of the vessels containing the suspensions.

The glaze slips were spray coated onto translucent china (non porous) and earthenware (porous) substrates. Even coatings were deposited which had good green strength. On firing to 1100°C glazes of acceptable qualities were produced.

Claims

CLAIMS :

1. A stable glaze slip formulation having a pH of above 7.5 which comprises particles of at least one glass forming material suspended in an aqueous medium containing a synthetic associative polymeric thickener which comprises a hydrophobically modified polyacrylate copolymer which forms aggregates or micelles in aqueous media, which is shear thinning in aqueous media but which does not break down under shear, the thickener being incorporated into the glaze slip formulation in an amount such that the suspension, prior to application, has a yield stress of greater than 2Pa, the glaze slip formulations, in use, exhibiting shear thinning behaviour on application to a substrate surface and on firing the thickener decomposing and/or burning out of the glaze slip formulation in order to leave substantially no residues or residual artefacts in the glaze so formed.

2. A glaze slip formulation as claimed in claim 1 which additionally includes a dispersant which acts to maintain the particles of the formulation in a dispersed state in the suspension.

3. A glaze slip formulation as claimed in claim 1 or claim 2 wherein the hydrophobically modified polyacrylate copolymer is a terpolymer which comprises a carboxylic monomer, a hydrophobic monomer and an associative monomer.

4. A glaze slip formulation as claimed in claim 3 wherein the associative monomer comprises a polyethoxy or poly(ethoxy/propoxy) chain terminated at one end with a group containing ethylenic unsaturation and at the other end with a hydrophobe.

5. A glaze slip formulation as claimed in claim 4 wherein the hydrophobe is derived from a non-ionic surfactant.

6. A glaze slip formulation as claimed in any one of the preceding claims wherein the glass forming material comprises a borate, phosphate or silicate glass frit, a mixture of a glass forming oxides and/ or beneficiated minerals.

7. A glaze slip formulation as claimed in any one of the preceding claims wherein the glass forming material or materials are composed of particles 80% of which have a particle size in the range of from 0.75 to 50μm.

8. A glaze slip formulation as claimed in any one of the preceding claims which additionally contains one or more clays therein in an amount of up to 15% by weight.

9. A method for the preparation of a glaze slip formulation, which method comprises adding an associative thickener which comprises a hydrophobically modified polyacrylate copolymer which forms aggregates or micelles in aqueous media, which exhibits shear thinning behaviour in aqueous media but which does not breakdown under shear, to an aqueous suspension of at least one glass forming material, the hydrophobically modified polyacrylate copolymer being added in an amount such that the suspension, prior to application, has a yield stress of greater than 2Pa, the glaze slip formulations, in use, exhibiting shear thinning behaviour on application to a substrate surface and on firing the thickener decomposing and/or burning out of the glaze slip formulation in order to leave substantially no residues or residual artefacts in the glaze so formed.

10. A method as claimed in claim 9 wherein a dispersant which acts to maintain the particles of the formulation in a dispersed state is added to the aqueous suspension of the glass forming material.

11. A method as claimed in claim 9 or claim 10 wherein the dispersant is added in an amount of from lppm to 5.0% by weight based on the total weight of solids.

12. A method as claimed in any one of claims 9 to 11 wherein the hydrophobically modified polyacrylate copolymer is added in an amount of from lppm to 5.0% by weight based on the total weight of solids.

13. A method as claimed in any one of claims 9 to 12 wherein the rheology of the glaze slip is altered by adjusting the pH.

14. A dried glaze powder or a suspension concentrate formed by drying a glaze slip formulation as claimed in any one of claims 1 to 8.

15. A glaze powder which comprises a dry blend of at least one glass forming material and an associative polymeric thickener which comprises a hydrophobically modified polyacrylate copolymer which forms aggregates or micelles in aqueous media, which exhibits shear thinning behaviour in aqueous media but which does not break down under shear.

16. A glaze slip formed by adding water to a dried glaze powder or a suspension concentrate as claimed in claim 14, or a glaze powder as claimed in claim 15.

17. An article which has been glazed using a glaze slip formulation as claimed in any one of claims 1 to 8, or claim 16.

18. An article as claimed in claim 17 wherein during glazing thereof the associative thickener decomposes and/or burns out of the glaze in order to leave substantially no residues or residual artefacts in the glaze.