Method of preparing a glass ceramic material for use in dental restorations

Abstract

The present invention relates to a method of preparing a glass-ceramic material for dental restorations, said method comprising the steps of: a) Providing a silicon precursor solution containing a hydrolysed silicon precursor in a solvent and a zirconium precursor solution containing a hydrolysed zirconium precursor and a gelation additive in a solvent, b) Preparing a mixture containing the silicon precursor solution, the zirconium precursor solution and a base catalyst; thereby starting a gel formation process; c) ageing of the gel; d) drying of the gel by removing the solvent; e) calcination of the dried product; and f) heat treatment of the calcined product to obtain a glass ceramic material.

Description

[0001] METHOD OF PREPARING A GLASS CERAMIC MATERIAL FOR USE IN

[0002] DENTAL RESTORATIONS

[0003] The present invention relates to a method of preparing a glassceramic material for dental restorations , and more speci fically, to a method for large-scale production of a glass ceramic material using a sol-gel process . The present invention also relates to the use of the prepared material in the manufacturing of dental restorations .

[0004] Glass-ceramic materials comprise an amorphous ( glass ) phase and one or more crystalline ( ceramic ) phases embedded in the amorphous phase . As such, glass-ceramics share many properties with both glasses and ceramics . ZrO2-based ceramics are becoming more popular in the dental industry, because these ceramics not only have adequate mechanical properties for dental restorations , but also of fer great possibilities to mimic the optical properties of natural teeth .

[0005] Such zirconia based ceramic materials can be synthesi zed by powder route or sol-gel methods . In contrast thereto , the solgel method is a chemical synthesis technique that involves the conversion of a sol , which is a suspension of very small colloidal particles to a three-dimensional interconnected network termed gel . The fundamental principles are three- dimensional networks that are formed through controlled hydrolysis and condensation reactions of precursor molecules . The process was described by Nogami et al . in Journal of Noncrystalline Solids , 37 , pp . 191-201 ( 1980 ) . A process for producing translucent ZrO2-SiO2nanocrystalline glass ceramic with ultra-high flexural strength by sol-gel and pressure- assisted sintering or pressure-less sintering is disclosed in EP 3541760 .

[0006] Unlike conventional production methods for glass ceramics , the sol-gel route avoids the problem of high temperatures and allows tai loring reaction parameters such as pH, temperature , and precursor choice , to adj ust the chemical and physical properties of the resulting materials .

[0007] WO 2021165293 Al discloses a method of forming a glass ceramic by mixing a first sol containing a zirconium dioxide precursor with a second sol containing a silicon dioxide precursor, adding an acid catalyst and a precursor material for a hardness- enhancing additive to the mixture of the two sols ; drying and forming a xerogel ; calcination of the formed xerogel ; and sintering of the calcined xerogel . The method also includes a fractionation step reducing the particle si ze of the material .

[0008] As a maj or drawback, scaling up the sol-gel process from laboratory or small-scale production to industrial or large- scale manufacturing poses several challenges . For instance , ensuring ef ficient heat and mass trans fer becomes more complex in larger reactors . Inhomogeneous heating or cooling rates within larger volumes were found to af fect the gelation process , resulting in non-uni form gel structures or inconsistent properties . In addition, upon scaling up a process , topics related to process safety are becoming more and more important . Failure to scale up properly may lead to the loss of process control and accidents , such as unexpected exothermic reactions ( runaway reactions ) , health hazards while handling large amount of hazardous and / or toxic chemicals , damage of the equipment due to corrosive chemicals , or environmental hazards .

[0009] It is therefore an obj ect of the present invention to provide a method of preparing a glass ceramic material for dental restorations using the sol-gel technique that addresses the aforementioned challenges to ensure a reproducible , cost ef ficient , and high-quality production, while using less hazardous chemicals and / or reaction conditions to provide a process that is safer and easier to handle on a larger scale .

[0010] The obj ect is solved according to the subj ect-matter of the independent claims . Selected preferred embodiments are given in the dependent claims . Further details are given hereafter .

[0011] According to a first aspect , the present invention relates to a method of preparing a glass-ceramic material , in particular for dental restorations , said method comprising the steps of : a ) Providing a silicon precursor solution containing a silicon precursor in a solvent ; and a zirconium precursor solution containing a zirconium precursor and a gelation additive in a solvent , b ) Preparing a mixture containing the silicon precursor solution, the zirconium precursor solution and a base catalyst ; thereby starting a gel formation process ; c ) ageing of the gel ; d) drying of the gel by removing the solvent ; e ) calcination of the dried product ; and f ) Heat treatment of the calcined product to obtain a glass ceramic material . The method of the present invention includes a first step a) of providing an initial mixture of separately prepared solutions comprising a silicon precursor and a zirconium precursor in a respective solvent. The molar ratio of Si and Zr in the initial mixture are preferably in the range of 30:70 to 70:30, more preferably 60:40 to 40:60. The zirconium, and silicon precursors are preferably respective alkoxides, more preferably one or more zirconium ( IV) alkoxide (s) and one or more silicon(IV) alkoxide(s) . Preferred zirconium precursors include zirconium propoxide [Zr(OPr)4] , zirconium butoxide Zr(OBu)4, zirconium ethoxide [Zr(OEt)4] , zirconyl nitrate, ZrOCl2solution, and zirconium chloride. Preferred silicon precursors include tetra alkyl (e.g. Cl to C4) orthosilicates - including tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS) . Preferred solvents are ethanol, propanol, methanol and isopropanol, whereby ethanol is particularly preferred. In the context of up-scaling, cost considerations regarding the choice of solvent are particularly pertinent. Ethanol (EtOH) has proven to be a favourable option in this view. Ethanol is further beneficial as it aligns well with the afore-mentioned preferred precursor compounds, thereby mitigating the occurrence of solvent interchange reactions.

[0012] In line with the present invention, the zirconium precursor solution further contains a gelation additive. When synthesizing a multi component material, the reaction rates of the chosen precursors can vary significantly which complicates the procedure. In particular, the zirconium precursors are generally highly reactive with water. If hydrolysis reactions dominate over condensation reactions, unintended precipitation can be the result. Gelation additives or gelation retarding additives- also referred to as "chilling agents" - can be used to slow down the reaction and thus mitigate highly reactive precursors so they can be combined also with other, less reactive precursors to obtain co-polymerization and further a homogeneous multi-component material. It was found that without the additive, the rapid hydrolysis and condensation of metal alkoxides (like zirconium alkoxide) often led to premature precipitation and formation of inhomogeneous particles rather than a uniform gel.

[0013] Specifically, it is preferred that prior to preparing the mixture in step b) , the gelation retarding additive is mixed with the zirconium precursor before adding the solvent or it is added to a pre-mixture of the zirconium precursor solution and the solvent.

[0014] Upon addition of the base catalyst in step b) , the precursors, preferably the Zr / Si alkoxide precursors, undergo hydrolysis, where they react with water to break the metal-oxygen bonds and remove organic groups, which is a solvent group coming from the hydrolysis step, such as for example, but not limited to, ethyl or butyl groups, forming hydrolysed species, meaning having one or more -OH group (s) attached to the cation. This step is followed by condensation, where the hydrolysed species undergo polycondensation reactions to form a three-dimensional network. As the hydrolysis and condensation reactions progress, colloids continue to grow, leading to the formation of a gel network. The gelation or "gel-formation" process provides a gel. Hydrolysis and condensation reactions can be controlled by adjusting parameters such as temperature, pH, and the ratio of precursors to solvent. Preferably, the gelation step is conducted at 40°C to 90°C, preferably 50°C to 80°C, most preferably about 60°C. Surprisingly, FITR analysis showed that the quality of the product was not affected by using a base catalyst instead of an acidic catalyst. The effects of using alkaline conditions (base catalyst) versus using acidic conditions (acidic catalyst) is discussed in more detail further below in the experimental section and in connection with Figures 1 and 2.

[0015] Aging and Drying: In subsequent steps c) and d) , the gel is allowed to age to promote further polymerization and enhance its structural integrity. The ageing step strengthens the gel through continued polycondensation and helps in homogenizing the material. The ageing step is preferably performed at temperatures in the range of 20°C to 60°C. Subsequently, the wet gel is dried (drying step) by removing the solvent, resulting in the formation of a dry xerogel or an amorphous raw powder. Removal of the solvent is preferably achieved at higher temperatures and reduced pressure (in comparison to the ageing step) . In an exemplary embodiment the drying step is conducted at 80°C to 120°C and at a pressure of 20 to 100 mbar.

[0016] Calcination: The dried product is preferably subjected to a controlled heating process called calcination. During calcination, residual components and unwanted byproducts associated with the hydrolysis and condensation reactions are removed. Calcination is preferably conducted at a temperature range of about 600°C to about 1200°C, preferably at about 900°C.

[0017] After the calcination step, an amorphous raw powder is obtained, which needs to be modified to get a glass ceramic material. Said modification includes a heat treatment at elevated temperatures, which will generally lead to a densif ication of the calcined product. Depending on the heat treatment, the characteristics of the final glass ceramic material can be adjusted. In one embodiment, heat treatment of the calcined product is performed to obtain a glass ceramic material in form of a powder. Said glass ceramic powder can be used as it is or it can be further modified in view of using the glass ceramic material in the manufacturing of a dental article, in particular a dental restoration. If the latter is desired, the glass ceramic powder material can for example be compacted to a solid glass ceramic. Said compaction can be done by techniques know in the state of the art, for example, by sintering, which is a known process of compacting and forming powdered materials by heating them to a temperature below their melting point.

[0018] Thus, in a particularly preferred embodiment, the heat treatment in step f) involves sintering.

[0019] Preferred sintering techniques are for example, but not limited to, pressure assisted sintering, hot isostatic pressing (HIP) , or hot pressing. The sintering step preferably involves a pressure assisted densif ication process, such as e.g. "hot- press sintering" or "FAST / SPS sintering", which are known material processing techniques used to consolidate and densify powdered materials into solid objects or components. FAST (Field Assisted Sintering Technique) or SPS (Spark Plasma Sintering) is a specific variant or advanced version of hot- press sintering. It involves the application of pulsed direct current and high pressure to the powder material, which is held in a die or mold during the sintering process. The pulsed current facilitates rapid heating, allowing for quicker sintering times compared to traditional methods. FAST / SPS enables the consolidation and sintering of materials at relatively lower temperatures and shorter durations, preserving fine microstructures and achieving high-density products. It is therefore particularly preferred that the heat treatment in step f) involves FAST / SPS sintering.

[0020] The heat treatment as mentioned in step f) in general, and in particular if it involves sintering, is preferably performed at temperatures at or above 600°C, in particular from about 600°C to 1500°C, preferably from about 800°C to about 1400°C, more preferably about 900°C to about 1350°C, even more preferably at about 1100°C to about 1300°C.

[0021] With increasing sintering temperatures, the material undergoes structural changes, as can be seen in Figure 3, at approximately 900°C the crystallization of t-ZrO2is triggered leading to the formation of crystalline t-ZrO2spheres in a remaining SiO2based matrix. Thus, a heat treatment at about 900°C or above is beneficial. The sintered material shows strong mechanical properties along with remarkable aesthetics making it a suitable candidate for dental applications. Therefore, most preferably, step f) involves a pressure- assisted sintering process, between 1000°C and 1300°C.

[0022] Notably, while each of the ageing step, the calcination step and the heat treatment step involve thermal processing, they serve distinct purposes in the context of sol-gel synthesis and the preparation of glass ceramic materials. In view of using the glass ceramic material for dental applications, a highly dense glass ceramic material is desired. It was found that all three seps (ageing, calcination, and sintering) are necessary to achieve the required mechanical and thermal properties. On the other hand, other applications such as thin film coatings, for instance, may merely undergo a brief ageing step followed by drying or low-temperature annealing, without extensive calcination or sintering.

[0023] The final glass ceramic material or any of its intermediate products can be characterized using techniques like X-ray diffraction (XRD) , scanning electron microscopy (SEM) , and thermal analysis to assess its phase composition, structure, and thermal properties. Also FTIR and / or Raman spectroscopy can be employed to provide comprehensive information about the molecular structure, chemical composition, and properties of a material.

[0024] The inventive method allows for precise control over the composition, morphology, and crystalline phases of the resulting zirconia-silica glass ceramic, enabling the tailoring of its properties for specific applications, in particular dental applications, e.g. prosthetic parts, including, but not limited to, crowns, bridges, or full dentures .

[0025] The choice of the catalyst in sol-gel synthesis can greatly affect the process characteristics, e.g. reaction pathway or rate of gelation and as a consequence also affects the final material in terms of microstructure and final properties of the product .

[0026] In sol-gel processes for the production of glass ceramics, acid catalysts are usually used. Under acidic conditions hydrolysis is predominant, particularly the hydrolysis of silica species, and condensation reactions are determining the overall reaction rate. Upon scaling the process, safety is becoming more important due to possible exothermic reactions (runaway reactions) and health hazards while handling large amount of hazardous and / or toxic chemicals or environmental hazards .

[0027] In particular, it was found while scaling up that large quantities of acids were di f ficult to handle due to environmental hazards , and the highly corrosive nature of the acids . Surprisingly, it was found that the use of the base catalyst allowed scaling up the sol-gel process without compromising the reaction . In addition, it was found that formation of undesired Si / Zr precipitates , such as SiOH- species or ZrOH-species , which is a commonly encountered problem when using strong inorganic acids (HC1 , H2SO4) as catalysts , could be avoided . It was further found that compared to the use of an acid catalyst , the gelation and ageing time of the gel could be signi ficantly shortened . As such, the inventive finding of using a base catalyst not only mitigated the corrosive environment of acid catalysts but also provided a scalable framework for the synthesis .

[0028] The term " large scale" within the meaning of this application refers to a production of a final product in the kilogram or multi-kilogram range and involves handling of reactional volumes of several litres , such as 50 to 100 litres .

[0029] Apart from its scalability thanks to use of the base catalyst , the method of the present invention provides the further advantage that it allows for the initial mixing of reactants , gelation, and drying to all be conducted within a single device .

[0030] In a preferred embodiment , the catalyst is selected from the group consisting of aqueous ammonia solution, ethanolamine , sodium hydroxide (NaOH) , ammonium acetate , and mixtures thereof .

[0031] In a particularly preferred embodiment , the catalyst is an aqueous ammonia solution, preferably with an ammonia concentration of 1 % to 30% , more preferably 5% to 27 % , even more preferably 10% to 25% and in particularly with an ammonia concentration of about 25 % .

[0032] An ammonia solution refers to a solution where the active ingredient is ammonia (NH3) dissolved in water . The percentage indicates the concentration of ammonia by weight in the solution .

[0033] In this case , a 25% ammonia solution means that for every 100 parts (by weight ) of the solution, 25 parts consist of ammonia, while the remaining 75 parts are water . This concentration is often expressed as a weight / weight percentage ( % w / w) . An ammonia concentration of about 25% includes concentrations within the range of 23% to 27 % . The use of an ammonia solution in this concentration range was favourable in terms of reaction speed and mitigation of side reactions .

[0034] In a preferred embodiment , the gelation additive is preferably selected from the group consisting of acetylacetone , ethanolamine , diethanolamine , triethanolamine , and 2- methoxyethanol , and mixtures thereof .

[0035] In particular i f an aqueous catalyst is used, it is highly preferable to treat the zirconium precursor with the gelation additive before it comes into contact with the catalyst . For the sequence , it is important that the zirconium precursor or Zr-precursor is mixed or chelated first before to be added to the reaction mixture , said step can be referred as "chilling the precursor". Thus, the Zr-precursor is chilled before water is added (which is in form of the catalyst if the catalyst is ammonia) .

[0036] In a particularly preferred embodiment, the gelation additive is acetylacetone.

[0037] Acetylacetone, commonly known as "acac", is a p-diketone that acts as a chelating agent due to its ability to form stable complexes with metal ions.

[0038] When added to sol-gel precursor solutions, acetylacetone was found to slow down the hydrolysis and condensation reactions. This retards the formation of the three-dimensional network (gel) and delays the gelation process. This has been found very useful for large-scale zirconia-silica glass ceramic production, since it allowed addition of the base catalyst in one go. Without gelation additive, addition of the catalyst had to be performed gradually or dropwise to ensure formation of a homogeneous gel due to the highly reactive nature of the zirconium precursor (in particular zirconium alkoxides are highly reactive with water) .

[0039] In a preferred embodiment, the gelation additive is added in a concentration of 0.5 to 4, more preferably 0.5 to 1, most preferably about 0.5 mole per 1 mole of zirconium precursor. Thus, most preferably, 0.5 mole gelation additive is used per 1 mole of zirconium precursor. This ratio was found to slow down the reaction sufficiently to give adequate time for thorough mixing and integration of the zirconium precursor with other components in the sol. The formation of a homogeneous gel was found to be crucial for the desired formation of zirconia multi oxides instead of zirconia hydroxides that are prone to the formation of precipitates.

[0040] In a further embodiment, an aluminium precursor solution containing a hydrolysed aluminium precursor in a solvent is provided and added to the mixture of step b) . This embodiment is thus directed to the manufacturing of a multi-component oxide material, in particular from the system SiO2-ZrO2-Al2O3.

[0041] The aluminium precursor is preferably one or more aluminium ( I I I ) alkoxide (s) , such as e.g. aluminium- isopropoxide . Alternatively it is preferred that the aluminium precursor is aluminium ( I I I ) -acetylacetonate (Al (acac)3) . Al (acac)3has the advantage of being commercially available and easily dissolvable in organic solvents.

[0042] Preferably, the Zr-content in the glass ceramic material ranges between 40 to 60 wt.-% and the aluminium-content ranges from 0 to 5 wt.-%. In an exemplary embodiment, the glass ceramic material contains 60 wt.-% ZrO2, 36 wt.-% SiO2and 4 wt.-% A12O3.

[0043] It was found that the addition of an aluminium precursor, in particular A12O3, helps with the sintering step and provides a lower viscosity.

[0044] In a preferred embodiment, stirring is performed during drying in step d) to obtain a dry xerogel powder.

[0045] Stirring during drying not only significantly reduced the drying time but also had a micronization effect, meaning a reduction of particle size due to the stirring action. While it is common to micronize a dried xerogel by milling or grinding to yield a powdery product, the stirring action during the drying process directly yielded a xerogel powder . The stirring speed is preferably at 20 to 150rpm, more preferably at 25 to 120 , most preferred at 30 to 100 rpm .

[0046] It is particularly preferred that stirring is performed during each of steps b ) to d) .

[0047] It was found that stirring signi ficantly accelerated the reaction . In particular , continued stirring during the aging step allowed reducing the aging time signi ficantly . It was surprising that the stirring action, in particular during ageing and drying, accelerated these steps , without negatively af fecting the properties of the dried xerogel . In the prior art , stirring is generally only performed during initial hydrolysis , but not after the start of the gel formation reaction or even during ageing and / or drying .

[0048] It was surprisingly found that agitation during the ageing step signi ficantly accelerates the reaction and reduces the minimum necessary ageing time . Without stirring, the necessary aging times generally greatly increase for larger volumes . However, i f stirring was performed, the minimum ageing time was found to be largely independent of the volume . These findings were therefore highly important in view of using the inventive method for a large-scale production .

[0049] The method of the present invention not only provides a solution for preparing glass ceramic materials in a large scale , it also allows for a "one-pot process" , in which the initial mixing of reactants , gelation, and drying steps can all be conducted within a single device .

[0050] The term "one pot process" refers to the fact that the relevant reaction and drying steps are performed in sequence without isolating the intermediate product of each step . Thus , the process is simpli fied by omitting removal or exchange of any other component of the reaction mixture . Using a one pot sol gel process is a cost-ef fective method of synthesis , which in addition minimi zes the level of impurities while increasing the overall yield, thus rendering the synthesis commercially valuable and also viable .

[0051] In a particularly preferred embodiment , all steps b ) to d) are performed in a single reactor, said reactor including a stirring device , a distillation unit and a heating / cooling device . In accordance with this embodiment , hydrolysis , condensation reactions , gelation, aging, and drying is performed within a single reactor, in particular a reactor that allows for kilogram to multi kilogram scale synthesis . Thanks to the use of a single reactor, there is no need for material trans fer between multiple reactors , which saves material and time . The reactor preferably includes a helical stirrer element that employs an axial stirring motion that transports the product vertically from the reactor ' s base to the top . This allows agitation of the mixture or gel very ef ficiently .

[0052] It is further preferred that the reactor includes a helical stirrer element that can be heated and connected to a vacuum system .

[0053] In a further aspect , the present invention provides a glass ceramic material obtainable by the inventive method . It also relates to the use of said glass ceramic material for preparing a dental article . The method of the present invention was found to provide a glass ceramic material in which t-ZrO2spheres form a networklike structure embedded in a glassy SiO2matrix. In one example, t-ZrO2spheres with a diameter of approximately 30 nm were formed. This material showed mechanical properties along with remarkable aesthetics, making it a highly suitable candidate for dental applications.

[0054] The present invention is exemplified and illustrated by way of the following examples in combination with the attached figures .

[0055] EXAMPLES

[0056] Unless otherwise stated, starting materials are either commercially available or were prepared by known methods.

[0057] Materials

[0058] - Tetraethyl orthosilicate (TEOS) : CAS Number 78-10-4

[0059] - Zirconium ( IV) propoxide (Zr(OPr)4) : CAS Number 23519-77-9 ; 70% solution in 1-propanol

[0060] - Acetylacetonate (acac) : CAS Number 123-54-6

[0061] - Aluminium acetylacetonate (Al (acac)3) : CAS Number 13963-57- 0

[0062] Example 1: Multi gram lab-scale synthesis

[0063] Four different batches of glass ceramic material were prepared using different catalysts with a batch size of 60 g each. The synthesis only differed in the type of catalyst used for preparing the glass ceramic material and the chemical composition of the resulting glass ceramic material in all batches was as follows:

[0064] 60 mole- ZrO2, 36 mole-% SiO2, 4 mole-% A12O3The type of catalyst used for the synthesis was as follows:

[0065] For each batch, a silicon precursor solution was prepared by mixing and stirring at about 300 rpm tetraethyl orthosilicate (TEOS) and ethanol at room temperature (25°C) . Said silicon precursor solution was stirred at about 40 rpm (rpm = revolution per minute) .

[0066] Separately, a zirconium solution was prepared by mixing and stirring at about 300 rpm Zirconium ( IV) propoxide (Zr(OPr)4) and ethanol at room temperature (25°C) , followed by an addition of acetylacetonate (acac) in a concentration of 0.5 mole acac per 1 mole of zirconium precursor.

[0067] Aluminium acetylacetonate (Al (acac)3) was given into a glass beaker equipped with a magnetic stirrer. The silicon precursor solution was given into the glass beaker and stirred. Immediately, the separately prepared zirconium precursor solution containing Zirconium ( IV) propoxide (Zr(OPr)4) and acetylacetone in ethanol was added into the glass beaker.

[0068] For the ageing step, the reaction mass was then heated to 60°C, and continuously stirring, the formed gel was allowed to age for 24 hours at 60°C.

[0069] After ageing, drying was initiated by removal of the solvent through distillation by applying vacuum with the aid of a rotary evaporator (rotavap) . The drying was performed on rotavap at 120°C, at 40 mbar for 1 hour.

[0070] The dried xerogel powder was calcined at 600°C, at 10 K / min for one hour.

[0071] Finally, the raw xerogel powder was collected and analyzed using FTIR.

[0072] FTIR Analysis

[0073] All FTIR measurements were performed using a Bruker Alpha FTIR spectrometer with a Diamond Crystal ATR (Attenuated Total Internal Reflectance) accessory. The FTIR spectrum was measured in the wavenumber range of 400 to 4000 cm-1. The measurements were performed with 32 scans and a resolution of 4 cm-1, utilizing the Opus 7.0 software. Both liquid and solid samples can be analyzed directly without the need for a salt plate or specialized sample handling.

[0074] For any FTIR measurement, a sample of the material to be analyzed was prepared as follows: A solid sample of the material was dried at 120°C for 1 hour in an oven before analysis. The dried sample was ground into a fine powder to ensure uniformity. 2-5 g of the sample was placed directly on the diamond crystal plate, which was a measuring position with approximately 1 mm thickness of the sample. The pressure arm was positioned over the sample and pressure was applied to ensure tight contact with the crystal.

[0075] The results of the FTIR analysis of raw xerogel powder are shown in Figures 1 and 2, of which

[0076] Figure 1: shows a FTIR spectra of a glass ceramic material synthesized using low or high acidic conditions; and

[0077] Figure 2: shows a FTIR spectra of a glass ceramic material synthesized using low or high alkaline conditions.

[0078] Figures 1 and 2 show Fourier-transform infrared (FTIR) spectra of respective samples of glass ceramic material of batches 1 to 4 described above that were prepared using different reaction conditions. The FTIR spectrum shows the absorption of infrared light at different wavelengths by a sample and thus provides information about the functional groups, chemical bonds, and molecular structure present in the analyzed material .

[0079] Figure 1 shows an FTIR spectrum of material samples of batches 1 and 2 described above that were synthesized under low acidic conditions (Batch 1; full line of transmittance) and high acidic conditions (Batch 2; dotted line of normalized transmittance) . Figure 2 shows an FTIR spectrum of material samples of batches 3 and 4 described above that were synthesized under low alkaline conditions (Batch 3; full line of transmittance) and high alkaline conditions (Batch 4; dotted line of normalized transmittance) .

[0080] A comparison of the two FTIR spectra shown in Fig. 1 and Fig.

[0081] 2, respectively, reveals the following: glass ceramic materials synthesized using acidic conditions have more Si-O- Si bonds (1060 cm-1) and fewer mixed Si-O-Zr bonds (950 cm-1) , see Figure 1. On the other hand, the glass ceramic materials synthesized using alkaline conditions have increased mixed Si- O-Zr bonds and fewer Si-O-Si bonds, see Figure 2.

[0082] Example 2: Multi kilogram pilot scale synthesis

[0083] In the following, a multi kilogram pilot scale synthesis example is described. According to this example, a xerogel powder is obtained after drying.

[0084] The sol-gel reaction as well as solvent distillation and drying process were all performed in one production unit consisting of an IKA CD100 as the main reactor connected to a Btichi 100L, which served as a condenser unit for the distillation of the solvent and was further connected to two gas scrubbing towers for neutralization of the catalyst.

[0085] Notably, all steps were carried out sequentially in the IKA CD100, without isolating intermediates or transferring reaction mixtures between different containers. This approach allowed several reactions to occur in a continuous manner within the same "pot" or vessel. In a first step, a silicon precursor solution was prepared by mixing and stirring 4,58 kg of tetraethyl orthosilicate (TEOS) and 26,76 kg ethanol. Said silicon precursor solution was stirred at about 40rpm (rpm = revolution per minute) .

[0086] Separately, a zirconium solution was prepared by mixing and stirring at about 300 rpm 17,38 kg of Zirconium ( IV) propoxide (Zr(OPr)4) and 13,9 kg ethanol, followed by an addition of 1,77 kg of acetylacetonate (acac) in a concentration of 0.5 mole acac per 1 mole of zirconium precursor.

[0087] 1.59 kg of aluminium acetylacetonate (Al (acac)3) was given into the reactor (IKA CD100) . The silicon precursor solution was given into the reactor and heated to 30°C. Upon addition of 5.74 kg of aqueous ammonia (25%) as catalyst to start the solgel reaction, a reproducible exotherm of approximately 10°C was observed. Immediately, the separately prepared zirconium precursor solution containing Zirconium ( IV) propoxide (Zr(OPr)4) and acetylacetone in ethanol was added.

[0088] The reaction mass was then heated to 60°C, whereupon a slow increase in torque and formation of a gel was observed. Specifically, an opaque gel formed within 15 minutes with no oxides or hydroxides precipitates. Continuously stirring, the gel was allowed to age for 2.5 hours at 60°C.

[0089] It is currently believed that during aging, the material continuously evolves its structure. With increasing aging time, cross-linking intensifies and more covalent bonds are formed. The ageing process control included a careful evaluation of the basic appearance of the material (visual test) and further also the characterization of the material structure (FTIR) . For a visual test, small samples from the wet gel were collected at different stages of aging. These samples were then heated to 600°C for one hour. The optical appearance of the material was assessed after the heat treatment. Initially, the material presented a darker colour for short aging times, gradually transforming into a pristine white powder as the aging time extended. A completely white appearance of the material was associated with the minimum necessary aging time.

[0090] It was found that agitation during the ageing step, in particular with helical stirrer, significantly accelerated the reaction and reduced the minimum aging time from an overnight process to approximately 1 hour. These findings were highly surprising, since the necessary aging times generally greatly increase for larger volumes. If stirring was performed, the minimum ageing time was found to be largely independent of the volume .

[0091] After ageing, drying was initiated by removal of the solvent through distillation by applying vacuum. The distilled off solvent was collected in the Btichi 100L reactor. Once all the solvent was distilled off, the residual solid, i.e. the formed xerogel, was dried at higher vacuum (100 mbar) and evaluated temperature (ca. 80°C) while continuing the stirring action. Finally, 11,0 kg raw xerogel powder was collected and 56,87 kg of solvent was distilled. The glass ceramic powder (dried xerogel) was obtained in 93% yield.

[0092] Following this procedure, xerogels of different Si / Zr compositions were prepared. The ZrO2content ranged between 40 to 60 wt-% and the A12O3content ranged from 0 to 5 wt-%. The dried xerogel powder was then modified by a heat treatment. The temperature during the heat treatment had an effect on the characteristics of the produced glass ceramic material. This is shown in the FTIR spectrum shown in Figure 3:

[0093] Figure 3: shows an FTIR spectrum of two batches of glass ceramic material synthesized in line with the above described multi kilogram scale synthesis, after heat treatment of the raw xerogel at 120°C, 600°C or 950°C.

[0094] As can be seen in Figure 3, with increasing sintering temperatures, the material underwent structural changes, and at approximately 900°C the crystallization of t-ZrO2was triggered, leading to the formation of crystalline t-ZrO2spheres in a remaining SiO2matrix. Thus, heat treatment is preferably performed at about 900 °C or above. After sintering, the material showed strong mechanical properties along with remarkable aesthetics making it a suitable candidate for dental applications, i.e. for using the glass ceramic material for manufacturing a dental article, such as a dental restoration.

Claims

Claims1. A method of preparing a glass ceramic material, in particular for dental restorations, comprising the following steps: a) Providing a silicon precursor solution containing a hydrolysed silicon precursor in a solvent and a zirconium precursor solution containing a hydrolysed zirconium precursor and a gelation additive in a solvent; b) Preparing a mixture containing the silicon precursor solution, the zirconium precursor solution and a base catalyst; thereby starting a gel formation process; c) ageing of the gel; d) drying of the gel by removing the solvent; e) calcination of the dried product; and f) heat treatment of the calcined product to obtain a glass ceramic material.

2. Method according to Claim 1, wherein the heat treatment in step f) involves sintering, preferably at temperatures, preferably at temperatures at or above 600°C, in particular from about 600°C to 1500°C, preferably from about 800°C to about 1400°C, more preferably about 900°C to about 1350°C, even more preferably at about 1100°C to about 1300°C.

3. Method according to Claim 1 or 2, wherein the catalyst is selected from the group consisting of aqueous ammonia solution, ethanolamine, sodium hydroxide (NaOH) , ammonium acetate and mixtures thereof.

4. Method according to Claim 3, wherein the catalyst is anaqueous ammonia solution, preferably with an ammonia concentration of 1% to 30%, more preferably 5% to 27%, even more preferably 10% to 25% and in particularly with an ammonia concentration of about 25%.

5. Method according to any one of Claims 1 to 4, wherein the gelation additive is selected from the group consisting of acetylacetone, ethanolamine, diethanolamine and 2- methoxyethanol and mixtures thereof, whereby acetylacetone is particularly preferred.

6. Method according to any one of Claims 1 to 5, wherein the gelation additive is added in a concentration of 0.5 to 4 mole per mole, preferably 0.5 to 1 mole per mole, most preferably about 0.5 mole per 1 mole, of the zirconium precursor .

7. Method according to any one of Claims 1 to 6, wherein an aluminium precursor solution containing a hydrolysed aluminium ( I I I ) precursor, preferably aluminium ( I I I ) - isopropoxide or aluminium ( III ) -acetylacetonate, in a solvent is provided and added to the mixture of step b) .

8. Method according to any one of Claims 1 to 7, wherein stirring is performed during drying in step d) to obtain a dry xerogel powder.

9. Method according to any one of Claims 1 to 8, wherein stirring is performed during each of steps b) to d) .

10. Method according to any one of Claims 1 to 9, wherein the steps b) to d) are carried out in a one-pot process.

11. Method according to any one of Claims 1 to 10, wherein the steps b) to d) are performed in a single reactor, saidreactor including a stirring device , a distillation unit and a heating / cooling device .12 . Glass ceramic material obtainable by the method according to any one of the Claims 1 to 11 13 . Glass ceramic material according to Claim 12 , characteri zed by tetragonal ZrO2spheres with a diameter of 20 to 40 nm forming a network-like structure embedded in a SiO2matrix .14 . Use of the glass ceramic material according to Claim 12 for preparing a dental article , in particular a dental restoration .15 . Dental article prepared from the glass ceramic material according to Claim 12 or 13 .

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