GB2360040A

GB2360040A - Ceramic pigments

Info

Publication number: GB2360040A
Application number: GB0005798A
Authority: GB
Inventors: Ishbel Sara Lily Gair; Anthony Charles Airey; Richard Hywel Jones
Original assignee: Keele University; Ceram Research Ltd
Current assignee: Keele University; Ceram Research Ltd
Priority date: 2000-03-11
Filing date: 2000-03-11
Publication date: 2001-09-12
Also published as: GB0005798D0

Abstract

A method of forming a ceramic pigment, the method comprising mixing two or more soluble pigment system components such as zirconia and silica, with a chromophore such as praseodymium to form a liquid form mixture, raising the pH, thereby causing the silica and zirconia to coagulate to yield a sol gel, and subsequently calcining the mixture.

Description

Ceramic Pigments This invention concerns a method of forming a ceramic pigment, and also a pigment made by such a method.

Inorganic pigments are used widely in the decoration and colouring of ceramic materials. The majority of pigments used in ceramics are manufactured using a mixed oxide method. The precursor materials consisting mainly of metal oxide powders are mixed and calcined to high temperatures (e.g. 1000 C to 1500 C). The calcinates are ground to specified particle sizes before being washed and dried. This process is inefficient in that intimate mixing of the powder precursors is seldom achieved with the consequent problem of incomplete reaction even at high calcination temperatures and hence reduced colour intensities. It is common practice for pigment manufacturers to blend pigment batches together to compensate for this variation in the colour intensities, thus achieving an acceptable colour but ultimately sacrificing optimum colour intensity.

The term "liquid form mixture" is to be hereinafter understood as including a liquid or a liquid incorporating a solid which flows as a liquid, such as a solution, suspension or a slurry.

According to the present invention there is provided a method of forming a ceramic pigment, the method comprising mixing two or more soluble pigment system components and a chromophore to form a liquid form mixture, and thereafter altering the pH of the mixture such that the pigment system components substantially simultaneously move to a solid phase, and subsequently calcining the mixture.

The mixture preferably remains in a liquid form subsequent to formation of the solid phase pigment system components. One or all of the pigment system components are preferably in solution, and desirably an aqueous solution, prior to mixing. The chromophore is preferably in a solution and desirably an aqueous solution, prior to mixing.

The pigment system components preferably form a sol gel upon moving to a solid phase.

Alternatively, the pigment system components may co-precipitate upon moving to a solid phase.

The mixture is preferably agitated at all times during the forming method.

The pH of the mixture is preferably increased to cause solid formation. The chromophore preferably forms a precipitate around the same time as the pigment system components move to a solid phase.

After the solid phase pigment system components start to be formed, the liquid form mixture maybe refluxed.

After completion of formation of the solid phase pigments system components, the solid phase is preferably dried.

The dried solid phase is preferably calcined, and desirably at a temperature in the range 800-1,200 C.

Following calcination, the pigment is preferably milled and may also be washed in any of acid, alkali or solvent.

A mineralises may be added to the mixture prior to solid formation.

The pigment system may be zircon based, comprising zirconia and silica as pigment system components, with the chromophore being any of praseodymium, vanadium, iron or cadmium. Alternatively the pigment systems may be spinel, garnet, or sphene based.

The invention also provides a ceramic pigment formed by a method according to any of the preceding fourteen paragraphs.

Examples of the present invention will now be described by way of example only.

Example 1.

A praseodymium zircon pigment was made by the process described below. The pigment system components, zirconia and silica, were in the form of zirconyl chloride and sodium silicate as metallic salt solutions dissolved in distilled water. The chromophore was present as praseodymium nitrate solution in distilled water. A mineraliser (ammonium chloride), and a fluorine compound (sodium fluoride) were provided to reduce required calcination temperature. The zirconia and silica were present in generally equimolar amounts.

The process is conducted at ambient temperature (-20 C) in a reaction vessel in which the contents are continuously agitated.

1. Zirconyl chloride (lmole) is dispersed in distilled water (ratio of 1 part zirconyl chloride to 10 parts water). At low concentrations and high acidity, the zirconium is present in solution as the Zr4+ ion.

2. 0.06 mole % of Pr(N03)3 solution is added.

3. Mineraliser and the fluorine compounds are dissolved in the mix, typically 4wt% ammonium chloride and 4 wt% sodium fluoride.

4. Sodium silicate (1.1 mole) dissolved in 10 parts water is added to the mix. The soluble precursors are mixed together, forming a solution at about pH 1 thus preventing any coagulation. The sodium silicate is added last to ensure intimate mixing of the other compounds beforehand.

S. A solution of ammonium hydroxide (2 mole) is added drop-wise into the mix. The alkali causes coagulation of the silica and zirconia to yield a sol gel at pH 7.

6. The sol gel is refluxed for 18 hours before being washed, filtered with fresh solvent and dried at 120 C.

7. The dried material is calcined in cordierite lidded crucibles to 850 C (150 C lower than the conventional mixed oxide route) at a ramp rate of 1000C hr-1 with a dwell of 2 hours.

8. The sintered powder pigments are wet ball milled to the desired particle size distribution.

9. The powder is acid washed to remove any unreacted material before being dried at 120 C.

Other zircon pigments such as vanadium zircon (blue), iron zircon (red/brown) and cadmium sulpho selenide zircon (reds) can be made by a similar route, and this route may be applicable to other chromophores. It has been established that zircon pigments are solid solutions formed as the result of the colouring ion entering the host lattice, or as inclusion colours where pigment particles are physically entrapped within the matrix, at the time of its formation and therefore cannot be produced from the dopant oxides and zircon.

Example 2.

The invention is also suitable for use with Spinel (Mg A1204) Pigment Systems. The following process was used to form an erbium spinel pigment. The process is conducted at ambient temperature (-20 C) in a reaction vessel in which the contents are continuously agitated.

1. (1 mole) and (lmole) are dispersed in distilled water (ratio of 1 part nitrate solution to 10 parts water).

2. 0.05 mole % or Er(N03)3 solution is added.

3. 3 wt % of Boric Acid H3 B04 mineraliser is dispersed in the mix.

4. The soluble precursors are mixed together, forming a solution at about pH 2.7.

S. A solution of ammonium hydroxide (2 mole) is added drop-wise into the mix until the coagulation of the mix to form a sol gel at pH 6.

6. The sol gel is refluxed for 18 hours before being washed, filtered with fresh solvent and dried at1200C.

7. The dried material is calcined in cordierite lidded crucibles to12500C, at a ramp rate of1000C hr-1 with a dwell of 2 hours.

Example 3.

The following process was used to produce a praseodymium garnet (3Ca0. 3Si02. A1203) pigment.

The process is conducted at ambient temperature(-200C) in a reaction vessel in which the contents are continuously agitated.

1. Ca(N03)3AH20 (3 moles) and Al(NOA.9Hz0 (imole) are dispersed in distilled water (ratio of 1 part nitrate solution to 10 parts water).

2. 0.05 mole % of Pr(N03)3 solution is added.

3. 2 wt% of Boric Acid mineralises is dispersed in the mix.

4. Sodium silicate (3 moles) dissolved in 10 parts water is added to the mix. The soluble precursors are mixed together. Typically the mix has a pH of 3.

5. A solution of ammonium hydroxide (2 mole) is added drop-wise into the mix. The alkali causes coagulation of the silica, calcia and alumina to yield a sol gel at pH 7.

7. The dried material is calcined in cordierite lidded crucibles to 1150 C, a ramp rate of 100 C hr-1 with a dwell of 2 hours.

There is thus described a pigment formation method usable for a wide range of different pigments. The system used is relatively dust free, involving mainly the handling of liquids. This provides considerable advantages over conventional routes, and can be used in continuous processing or batch processing, whilst conventional routes are only generally suitable for batch processing. The precursor materials are intimately mixed resulting in a high degree of chemical homogeneity. In sol gels very small particle sizes are involved which provide a very high surface area and hence a high reactivity, which enables calcination to take place at much lower temperatures than is conventional.

The reactions within the process are generally more efficient, resulting in the following benefits:- # The product can be sintered at lower calcination temperatures.

# The colouring mechanism(s) of the pigments are enhanced, as more of the chromophore is able to enter the host crystal, increasing the colour intensity of the pigment. Less chromophore is required to achieve the equivalent colour intensities obtained through the conventional mixed oxide route.

# There are major cost and environmental benefits in using less dopant as many of these materials are expensive and toxic.

# The development of new colour through the introduction of alternative chromophores via this versatile process.

# This process can be used for the synthesis of a range of pigment systems used by industry, and bring with it the opportunity to develop new pigment crystal system and therefore new pigments.

As noted above the invention is usable with a very wide range of materials, and may be usable with sphene pigment systems. Different conditions may be applied to those described above. Whilst the above examples relate to sol gel formation, in some instances rather than sol gel formation co- precipitation of the pigments system components can take place. In general for the system to work well the chromophore should precipitate around the time of the sol gel formation or co-precipitation of the pigment system components. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

Claims 1. A method of forming a ceramic pigment, the method comprising mixing two or more soluble pigment system components and a chromophore to form a liquid form mixture, and thereafter altering the pH of the mixture such that the pigment system components substantially simultaneously move to a solid phase, and subsequently calcining the mixture.
2. A method according to claim 1, wherein the mixture remains in a liquid form subsequent to formation of the solid phase pigment system components.
3. A method according to claims 1 or 2, wherein one or all of the pigment system components are in solution prior to mixing.
4. A method according to claim 3, wherein one or all of the pigment system components are in an aqueous solution prior to mixing.
5. A method according to any of the preceding claims, wherein the chromophore is in a solution prior to mixing.
6. A method according to claim 5, wherein the chromophore is in an aqueous solution prior to mixing.
7. A method according to any of the preceding claims, wherein the pigment system components form a sol gel upon moving to a solid phase.
8. A method according to any of claims 1 to 6, wherein the pigment system components co-precipitate upon moving to a solid phase.
9. A method according to any of the preceding claims, wherein the mixture is agitated at all times during the forming method.
10. A method according to any of the preceding claims, wherein the pH of the mixture is increased to cause solid formation.
11. A method according to any of claims 5 to 10, wherein the chromophore forms a precipitate around the same time as the pigment system components move to a solid phase.
12. A method according to any of the preceding claims, wherein after the solid phase pigment system components start to be formed, the liquid form mixture is refluxed.
13. A method according to any of the preceding claims, wherein after completion of formation of the solid phase pigments system components, the solid phase is dried.
14. A method according to claim 13, wherein the dried solid phase is calcined.
15. A method according to claim 14, wherein the dried solid phase is calcined at a temperature in the range 800 - 1200 C.
16. A method according to claims 14 or 15, wherein following calcination, the pigment is milled.
17. A method according to any of claims 14 to 16, wherein following calcination the pigment is washed in any of acid, alkali or solvent.
18. A method according to any of the preceding claims, wherein a mineraliser is added to the mixture prior to solid formation.
19. A method according to any of the preceding claims, wherein the pigment system is zircon based, comprising zirconia and silica as pigment system components, with the chromophore being any of praseodymium, vanadium, iron or cadmium.
20. A method according to any of claims I to 18, wherein the pigment systems may be spinel, garnet, or sphene based.
21. A ceramic pigment formed by a method according to any of the preceding claims.
22. A method of forming a ceramic pigment, the method being substantially as hereinbefore described.
23. A ceramic pigment substantially as hereinbefore described.
24. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.