THE RELATED ART

The invention relates to the use of cyclodextrin as a coloring aid for the coloring admixture of ceramics. The invention relates in particular to the coloring of dental ceramics especially for presintered and/or preformed CAD/CAM and manual milling zirconia and alumina based blocks by means of the coloring elements and cyclodextrines. Especially, the invention relates to the coloring of oxide ceramics by the aid of native or modified/ synthesized alpha, beta, and gamma cyclodextrines.

THE PRIOR ART

In prior art of the this invention some sulphate, chloride, nitrate, iodide, bromide etc. salts or organic ligands and or complexes of the coloring elements are used by means of dissolving in the solvent like water, alcohols, acetone etc. or the mixture thereof with the aid of several pH adjusting, viscosity modifiers, wetting agents, stabilizers etc.

One of the disadvantages of the prior art is the presence of high concentration of chloride, nitrate, iodide, bromide, and sulphate ions that reduce pH value of the solution below 1 that acts as a strong acid and requires hazardous substance labeling. Most of the viscosity modifiers used in the prior art reduce the solubility of the coloring elements and may cause the formation of precipitates that alter the coloring elements concentrations in the solutions during long term shelf life.

Addition of some ligands and / or organic complexing agents may alter the wetting behavior of the coloring solution that yields inhomogeneous coloring along with the ceramic microstructure. Another drawback of the prior art, the addition of several organic viscosity modifiers, wetting agents, stabilizers and pH modifiers may lead to formation of aggressive degradation gases that attack furnace linings and heating elements. Some of the organic viscosity modifiers and or stabilizers may be recrystaliized locally during drying that may lead to micro crack formation in the microstructure of ceramics due to retarded thermal degradation and sudden pressure increase along the pores.

In prior techniques, diffusion of coloring elements and/or their Ci-C ₃₀ ligands and complexes are not restricted along the pores during drying due to their high water solubility. For this reason drying originated fluid migration along the pores resulted in inhomogeneous coloring of the ceramics due to local abundance and/or deficiency of coloring elements.

PURPOSE OF THE INVENTION

The present invention is intended to improve the known coloring solutions by addressing the problems mentioned above.

A further object of the invention is to offer a coloring admixture that provides aesthetic appearance inconsequence of uniform diffusion of coloring elements.

Still a further object of the invention is to provide benign constituent to the coloring admixture. Still a further object of the invention is to overcome cracking and/or distortion problems arising from local concentration increase of coloring elements or modifiers during drying and or sintering.

Other aspects, embodiments and benefits of the current invention will be apparent from the following detailed descriptions. DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise specified, within the context of the text of the invention, the following terms have the following meanings:

The term "cyclodextrines" sometimes called cycloamyloses within the meaning of the invention is to be understood as family of compounds that made up of cyclic oligosaccharides. These cyclic oligosaccharides consist of (a- 1,4) linked a-D-glucopyranose units and contain a somewhat lipophilic central cavity and a hydrophilic outer surface. Due to the chair conformation of glucopyranose units, the cyclodextrins are shaped like a truncated cone rather than perfect cylinders. The natural α-,β- and γ- cyclodextrin consist of six, seven, and eight glucopyranose units, respectively. Alpha cyclodextrin is defined as C36H60O30, beta cyclodextrin is defined as C4.2H70O35 and gamma cyclodextrin is defined as C ₄ 8H8o0 ₄ o-

The term "coloring elements" within the meaning of the invention is to be understood as the doped atoms to the ceramics during heat treatment via solid phase diffusion that comprises the MB, IIIA, IVA, VA, VIA group metals of p-blocks of the periodic table, transition elements of the d-block of the periodic table and the lanthanide group elements of the f-block of periodic table.

The term "oxide ceramics" within the meaning of the invention is to be understood as all high strength oxides, which are in the presintered form with 10-200 nanometer pore sizes, of the elements of main group II, III, IV and the subgroups of III and IV and their mixtures.

The term "solvent" within the meaning of the invention is any solvent comprising water, primary, secondary, tertiary alcohols, acetone alone or a mixture thereof, that is able to at least partially dissolve one of the coloring elements and / or cyclodextrin, The "coloring admixture" within the meaning of the invention is the any type of mixture of coloring elements, cyclodextrin, solvent and other additives in the form of solid, liquid or paste. The "additives" within the meaning of the invention is the any type of organics removable during heat treatment. According to one embodiment, it would be desirable to provide coloring admixture having improved properties and more benign. Here, the present invention relates to the coloring admixture for oxide ceramics that is composed of;

• natural and or modified / synthesized cyclodextrines in the range of 0-90 % (volume/volume) of the solvent,

· coloring elements which are IIB, IIIA, IVA, VA, VIA group metals of p-blocks of the periodic table, transition elements of the d-block of the periodic table and the lanthanide group elements of the f-block of periodic table in an amount in the range of O.OOOOOlmolar to 8 molar,

• a solvent particularly deionized and or distilled water,

· preferably additives.

Cyclodextrines are defined as cyclic oligosaccharides and the most abundant native cyclodextrins, which are alpha, beta and gamma types, consist of six, seven and eight alpha- D-glucopyranose units respectively. They have truncated toroidal form with apolar cavity that the primary hydroxyl groups located outer, secondary hydroxyl groups located inner surfaces. Molecular formulas for the alpha, beta and gamma cyclodextrin are C ₃₅ H ₆₀ O3 ₀ , C42H7o0 _35/ C48H80O40 respectively. Unlike C1-C30 carbon and 1-10 heteroatom containing ligands and or complexing agents cyclodextrines have high molecular weights and still has appropriate water solubility. Solubilities of the pure native cyclodextrines are 14.5, 1.85, 23.2 g/lOOml water for alpha, beta and gamma types respectively at ambient conditions. Any additive except coloring elements in the coloring admixture must be thermally removable. Thermal degradation of cyclodextrines completed at the range of 250-350 °C and the subsequent oxidation reactions of residue completed below 700 °C under oxidative atmosphere. Those wide decomposition temperature ranges prevent colored ceramics from crack formation via sudden pressure increase inside the pores while heat treatment. Cyclodextrines with diameters respectively 1.37, 1.53, 1.69 nanometers for alpha, beta and gamma types, easily penetrates along the pores of oxide ceramics with the migration of carrying solvent. Surprisingly, because of their relatively low solubilities, cyclodextrines are trapped in the pores of ceramics with decreasing solvent concentration while drying. This phenomenon prevents coloring element migration along the pores from inside to the surface of ceramics and the homogeneous aesthetic coloring effect is achieved.

Fortunately addition of cyclodextrines to the coloring admixture has no detrimental effect on the viscosity and wetting behavior of solvent that ensures homogeneous distribution of coloring element resulted in desired aesthetic appearance. Preferably the ingredients of the coloring admixture mentioned above are dissolved in a proper container in an amount sufficient to attain desired coloring effect by preferentially proper mixing and the aid of heating below 100 °C. Any of the drying processes can be performed to obtain powdered coloring admixture of components. Higher temperatures over decomposition temperatures of cyclodextrins should be avoided. Any heating conditions that include pressure, vacuum, freezing that may initiate degradation of cyclodextrines should be avoided.

The powder attained from the solution of coloring admixture can be compacted in any form or encapsulated by the proper organic materials that can be dissolved by solvent and has no effect on rheological characteristics and coloring effect of the admixture. Any methods of dipping, spraying, brushing can be performed to achieve aesthetic coloring effects for oxide ceramic followed by post sintering at least 1200 °C.

The protection of the present application is defined in the claims and is not strictly limited with the explanation above for the purpose of exemplification. Obviously a person skilled in the art may introduce the novelty set forth in the present invention by using analogous engineering and/or may employ to the other themes are similar to mentioned in this text. Thereby, such an analogy will obviously be deprived of the novelty and will particularly be devoid of the criteria exceeding of the known art.