Preparation of a metastable tetragonal zirconia aerogel

Abstract

The present application discloses a process for the preparation of metastable tetragonal zirconia in the form of an aerogel material, said material being capable of undergoing martensitic phase transformation to monoclinic zirconia. The application also discloses composite materials, such as dental filling materials, having included therein an aerogel material.

Claims

1. A metastable tetragonal zirconia aerogel material having a density of 1.0-4.5 g/cm.sup.3 and in particulate form having a particle size distribution wherein D10 value is from 30 m to 100 m.

2. The metastable tetragonal zirconia aerogel material according to claim 1 which has a proportion of tetragonal zirconia of at least 80% based on the total amount of zirconia in the aerogel material.

3. The metastable tetragonal zirconia aerogel material according to claim 1 which has a phase transformation capability of at least 60%.

4. The metastable tetragonal zirconia aerogel material according to claim 1 in particulate form having a particle size of less than 100 m.

Description

WORKING EXAMPLES

(1) The Proportion of Tetragonal Zirconia

(2) The proportion of tetragonal zirconia is determined by XRD analysis using a Bruker D8 advance, 2Theta analysis. The XRD analysis is performed by 2 analysis of a powder sample or a composite of the powder in a resin (bisGMA:UDMA:TEGDMA (40:40:20 by weight)+0.3 wt % campherquinone+0.6 wt % dimethylaminobenzoic acid ethyl ester (DABE)). Analysis of the ratio between the two phases is done by comparing the areas of the (1,1,1) reflection of the tetragonal and the monoclinic phases.

(3) The incorporation of the sample in a resin prevented premature phase transformation of the tetragonal crystals.

(4) Phase Transformation Capability

(5) The phase transformation capability of an aerogel material of tetragonal zirconia is determined according to the following method:

(6) A sample of the batch of a metastable tetragonal zirconia aerogel is analysed by XRD so as to determine proportion of tetragonal zirconia within the sample (T0).

(7) Another sample of the batch of an aerogel material is allowed to stand exposed to ambient atmosphere for 10 minutes at 25 C. at 70% relative humidity. The material is subsequently analysed by XRD so as to determine proportion of tetragonal zirconia within the sample (T10).

(8) The phase transformation capability, PTC, (%) is expressed as
PTC (%)=(T0T10)/(T0)*100%
Insofar that the tetragonal zirconia aerogel is present in e.g. a resin matrix, such material is first isolated from the resin matrix and are then analysed by XRD as described above.

(9) The Stability of the Tetragonal Zirconia Aerogel Material

(10) Correspondingly, the stability (i.e. the degree of phase transformation) is determined by storing a sample of the tetragonal zirconia aerogel material in a dry atmosphere at 25 C. for a predetermined period of time.

(11) Determination of Particle Size Distribution

(12) The particle size distribution is determined by Differential laser diffraction of a sample suspended in ethanol and assuming spherical particles using a Beckmann Coulter LS 13 320 fitted with an Universal Liquid module, with an assumed refractive index of particles of 2.2 and an imaginary refractive index of 0.8. The result is derived using Mie theory and using Polarization Intensity Differential Scattering (PIDS).

(13) Results are reported as a distribution of particles at D10, D25, D50, D75 and D90 (by volume).

(14) Determination of Translucency of Composite Materials

(15) Relative translucency is evaluated by measuring a transparency of a 2 mm thick sample with UV-VIS spectrophotometer over the wavelength range from 300 nm to 800 nm.

(16) The specimen for the investigation is prepared as a composite of 20 vol. % of the zirconia aerogel and 80 vol. % of a resin (bisGMA:UDMA:TEGDMA (40:40:20 by weight)+0.3 wt % campherquinone+0.6 wt % dimethylaminobenzoic acid ethyl ester (DABE)). The composite was curing with Demi Plus over 1 minute between two microscopic glasses being 2 mm apart. Wavelength of 450 nm to 470 nm and shifting output intensity from 1,100 mW/cm.sup.2 to a peak of 1,330 mW/cm.sup.2.

(17) Preparation of Zirconia Aerogels

(18) Materials

(19) ZBO1: Zirconium(IV) butoxide solution 80 wt. % in 1-butanol used as received from Aldrich chemistry

(20) ZBO2: Zirconium(IV) butoxide solution 80 wt. % in 1-butanol used as received from abcr GmbH & Co. KG

(21) HAc: Acetic acid, 98-100% used as received from Bie&Berntsen

(22) FAc: Formic acid, 98-100% used as received from Sigma-Aldrich

Example 1Preparation of Aerogel of Zirconia in Acetic Acid in scCO.SUB.2

(23) After a few minutes of mixing 20.06 g ZBO1 with 5.71 g HAc in a 37 mL pressure vessel, CO.sub.2 was applied until a pressure of 450 bars at a temperature of 40 C. was reached. The mixing was continued for 17 hours and the reaction mixture was then allowed to react for further 48 hours without mixing. The product was cleaned by flowing 3-5 g/minute CO.sub.2 through the vessel at 450 bars and 40 C. for 2 hours. While everything still was kept at 450 bars and 40 C., a second reaction with FAc was carried out. This was done by connecting to another 37 mL pressure vessel loaded with 5 g FAc pumping 1 g/minute CO.sub.2 through the series of reactors for 3 hours, until acid could be collected at the reactor vessel outlet. Subsequently the reaction was allowed to take place at static conditions for 18 hours. The remaining acid was thereafter removed by 5 hours flow with 1-2 g/minute of CO.sub.2 again at 450 bars and 40 C. Then the temperature was raised to 200 C. and the pressure at the same time reduced to 100 bars by controlled release of CO.sub.2 over a period of 1 hour followed by hours flow with 2-3 g of preheated CO.sub.2/minute at the new conditions. The product was allowed to mature for 16 hours at 200 C. and 100 bars. It was then cleaned by approx. 5 g/minute of CO.sub.2 flow for 3 hours. The pressure was released during 35 minutes and the sample was allowed to cool down. After opening the pressure vessel, 7 g of ZrO.sub.2 aerogel (ZrO.sub.2 Aerogel 1) was collected and transferred to an airtight container under nitrogen.

Example 2Preparation of Aerogel of Zirconia in Acetic Acid in scCO.SUB.2 .(Large Scale)

(24) After a few minutes of mixing 271.03 g ZBO2 with 77.41 g HAc in a 500 mL pressure vessel, CO.sub.2 was applied until a pressure of 450 bars at a temperature of 40 C. was reached. The mixing was continued for 65 hours while the mixture was allowed to form gel. The product was cleaned by flowing 6-7 g/minute CO.sub.2 through the vessel at 450 bars and 40 C. for 7 hours. While everything still was kept at 450 bars and 40 C., a second reaction with FAc was carried out. This was done by connecting to another 500 mL pressure vessel loaded with 67.5 g FAc pumping 5 g/minute CO.sub.2 through the series of reactors for 2 hours, until acid could be collected at the reactor vessel outlet. Subsequently the reaction was allowed to take place at static conditions for 14 hours. The remaining acid was thereafter removed by 8 hours flow with 6 g/minute of CO.sub.2, static for 15 hours and again 4 hours flow with 7 g/minute CO.sub.2 all at 450 bars and 40 C. Then the temperature was raised to 200 C. and the pressure at the same time reduced to 100 bars by controlled release of CO.sub.2 over a period of 2 hours followed by 2 hours flow with approx. 5 g of preheated CO.sub.2/minute at the new conditions. The product was allowed to mature for 16 hours at 200 C. and 100 bars. It was then cleaned by approx. 8-12 g/minute of CO.sub.2 flow for 4 hours. The pressure was released during 1 hour and the sample was allowed to cool dawn. After opening, 94.53 g of ZrO.sub.2 aerogel (ZrO.sub.2 Aerogel 2) was collected from the pressure vessel and transferred to an airtight container under nitrogen.

Example 3Calcination of ZrO.SUB.2 .Aerogel 1 and ZrO.SUB.2 .Aerogel 2

(25) An amorphous zirconia aerogel (ZrO.sub.2 Aerogel 1 and ZrO.sub.2 Aerogel 2) prepared according to Examples 1 and 2 were each milled to a particle size of up to 100 micrometer in a cutting mill. The particulate aerogel material was then placed in a ceramic tray and entered into a 650 C. hot zone of a tube furnace. The tube furnace had three zones, first a loading zone, secondly the hot zone and finally a cooling off zone. The latter was connected to a humid free glove box with a dew point of 45 C. or below. A flow of dry air was constantly flowing through the tube from the glove box, thereby keeping the tube humid free during the calcination.

(26) The calcination was conducted for 2 hours in the hot zone. Hereafter, the tray with the particulate metastable tetragonal zirconia aerogel was pushed into the cooling off zone and was kept for 0.5 hours. Thereafter, the tray and resulting material could be handled manually.

(27) The resulting metastable tetragonal zirconia aerogel showed a content of tetragonal phase zirconia of 90% for ZrO.sub.2 Aerogel 1 and above 85% for ZrO.sub.2 Aerogel 2 analysed as composites.

(28) The resulting metastable tetragonal zirconia aerogel showed a phase transformation capability of 82% for ZrO.sub.2 Aerogel 1 and above 85% for ZrO.sub.2 Aerogel 2 analysed as powder in air.

(29) The density of the aerogels were approx. 2.5 g/cm.sup.3.

Example 4Preparation of Aerogel in Acetic Acid

(30) 542.27 g of zirconium(IV)butoxide (ZBO2) and 144.33 g of Acetic acid were thoroughly mixed under heavy stirring for the first half an hour and heated to 80 C. for 21 hours. Subsequently slow addition of 10 g acetic acid was followed by heavy mixing for 2 hours at 80 C. After staying for additional 2 hours the gel was formed. The gel was matured additionally for 48 hours at 80 C. The matured gel was allowed to cool for half an hour before it was cut into 1-2 cm.sup.3 pieces. The pieces were kept under L of ethanol and a mixture of 130 g formic acid diluted up to L with ethanol were slowly added during gentle agitation. The liquid including small white particles was changed with L of fresh ethanol every second day.

(31) A Litre flow vessel was filled with the prepared gel pieces under ethanol. Additional L ethanol was flowed through at a flowrate of 5 mL/min. The temperature was raise from room temperature to 80 C. and the pressure was increased to 100 bars with a rate of 3 bars/min by pumping ethanol. CO.sub.2 was flown through at 100 bars in order to remove solvents and by-products. The flow was set to 2-3 g CO.sub.2/min for 16 hours followed by 5 g/min for additionally 6 hours. The pressure was released slowly over several hours and of big very transparent slightly yellowish aerogel pieces (ZrO.sub.2 Aerogel 3) were collected from the vessel and transferred to an airtight container under nitrogen.

(32) The resulting metastable tetragonal zirconia aerogel showed a content of tetragonal phase zirconia of 84% analysed as a composite.

(33) The resulting metastable tetragonal zirconia aerogel showed a phase transformation capability of 64% analysed as powder in air.

(34) The density of the aerogel was approx. 2.5 g/cm.sup.3.

Example 5Preparation of Aerogel of Zirconia in a Mixture of Acetic Acid and Formic Acid

(35) An experiment is started with 54.2 g of the tetrabutoxy-zirconate solution containing 80% of the zirconate and 20% butanol placed in a 100 ml beaker with lid. 20 gram 100% ethanol was added under stirring. To this is added a mixture of 9.95 g glacial acetic acid and 5.2 gram formic acid. The acids were mixed in a beaker and afterwards they were added using a pipette through a whole in a lid within 3 minutes. By this procedure fast stirring is possible without contact to air. The mixture got warm approximately 40 C. but was clear yellow. The total volume is now close to 100 ml. The reason for this choice of volume expansion is that thereby the gel volume is double of the zirconia butoxide. This is also the situation in the half liter scCO.sub.2 reactor in Example 1. The solution was placed in an ultrasonic bath at 30 C. During the US treatment the temperature was raising to 54 C. In the solution small bubbles are constantly formed but floating to the top due to the ultrasound. The gel formed with 45 min. After 3 hours, 100% ethanol was added to cover the gel and the beaker was left for 20 hours for the gel to mature. The gel is cut into pieces and placed in ethanol for 24 hours. This process is repeated 3 times. Hereafter a colourless transparent gel is present. The gel is transferred to the reactor and supercritically washed as in Example 1. The gel showed shrinking to a degree of 50% during the washing but the pieces remained transparent after washing (ZrO.sub.2 Aerogel 4). The density of the aerogel was approx. 2.5 g/cm.sup.3.

Example 6Preparation of Large Particles

(36) An amorphous zirconia aerogel (ZrO.sub.2 Aerogel 1 and ZrO.sub.2 Aerogel 2) prepared according to Examples 1 and 2 were each milled to a particle size of up to 150 micrometer in a cutting mill. A fraction of aerogel material from 90 to 150 m was collected with sieving, placed in a ceramic tray and entered into a 650 C. hot zone of a tube furnace. The tube furnace had three zones, first a loading zone, secondly the hot zone and finally a cooling off zone. The latter was connected to a humid free glove box with a dew point of 45 C. or below. A flow of dry air was constantly flowing through the tube from the glove box, thereby keeping the tube humid free during the calcination.

(37) The calcination was conducted for 2 hours in the hot zone. Hereafter, the tray with the particulate metastable tetragonal zirconia aerogel was pushed into the cooling off zone and was kept for 0.5 hours. Thereafter, the tray and resulting material could be handled manually.

(38) The resulting metastable tetragonal zirconia aerogel showed a content of tetragonal phase zirconia of 90% for ZrO.sub.2 Aerogel 1 and above 85% for ZrO.sub.2 Aerogel 2 analysed as a composite.

(39) The resulting metastable tetragonal zirconia aerogel showed a phase transformation capability of 85% for both ZrO.sub.2 Aerogel 1 and Aerogel 2 analysed as powder in air.

(40) The density of the aerogel was approx. 2.5 g/cm.sup.3.

Example 7Preparation of Prepolymerized Aerogel Fillers (PAF)

(41) An amorphous aerogel (ZrO.sub.2 Aerogel 2) was calcined as described in Example 3). The calcined tetragonal zirconia aerogel was placed in an excess of a monomer mixture (bisGMA:UDMA:TEGDMA:BPO-50,BHT (37.5:39:20:3:0.5 by weight)) and left for 10 days in order for the inner pores of the aerogel to become filled with the monomer mixture. The monomer mixture within the aerogel was then cured at 100 C. during 1 hour and crushed in a cryomill into required particle sizes (either 90-150 m or 150-180 m). After sieving, the powder could then be used for a composite fabrication such as mixing with monomers and fillers so as to obtain e.g. a dental material. All the listed steps were performed in humid free atmosphere with dew point below 45 C. The aerogel in the resulting PAF material showed a content of tetragonal phase zirconia of 60% and a phase transformation capability of 40%. The density of the prepolymerized aerogel filler was approx. 3.0 g/cm.sup.3

Example 8Preparation of a Composite Material (Model Example)

(42) In the humid free atmosphere in the glove box, 20 vol % the particulate metastable tetragonal zirconia aerogel prepared according to Example 3 was mixed with 80 vol % of a resin system (bisGMA:UDMA:TEGDMA (40:40:20 by weight)+0.3 wt % campherquinone+0.6 wt % dimethylaminobenzoic acid ethyl ester (DABE)). After formulation, the particulate metastable tetragonal zirconia aerogel was essentially protected from immature triggering of the phase transformation in that is showed excellent stability (0% phase transformation) after storing for 10 days.

(43) As desired, the mixture of resin and the particulate metastable tetragonal zirconia aerogel could be supplemented with glass fillers and colouring agents.

(44) After compounding the composite, it may be filled into capsules containing 0.2 to 0.5 grams or syringes with between 2 and 6 grams. The capsules/syringes may have a metal coating so as to obtain a shelf life of the composite of at least 2 years.

(45) Also, the refractive index of the individual composite components may be adjusted. In order to obtain a suitable transparency of a composite, the refractive index of its components should preferably be similar (typically with a difference not superseding 0.1). This is done by spiking the resin with nano-sized rutile (TiO.sub.2) before the particulate metastable tetragonal zirconia aerogel is added. As much rutile as possible is added without compromising handling qualities of the final composite, e.g. in the range of 5-30 wt. %.

Example 9Preparation of a Composite Material with Large Aerogel Particles

(46) In the humid free atmosphere in the glove box, 20 vol % of one of the particulate metastable tetragonal zirconia aerogels prepared according to Example 3 and Example 6 was mixed with 80 vol % of a resin system (bisGMA:UDMA:TEGDMA (40:40:20 by weight)+0.3 wt % campherquinone+0.6 wt % dimethylaminobenzoic acid ethyl ester (DABE)). After formulation, the particulate metastable tetragonal zirconia aerogel was essentially protected from immature triggering of the phase transformation in that is showed excellent stability (0% phase transformation) after storing for 10 days.

(47) As desired, the mixture of resin and the particulate metastable tetragonal zirconia aerogel could be supplemented with glass fillers and colouring agents.

(48) After compounding the composite, it could be filled into capsules containing 0.2 to 0.5 grams or syringes with between 2 and 6 grams. The capsules/syringes may have a metal coating so as to obtain a shelf life of the composite of at least 2 years.

(49) It has been shown an improved translucency of Example 9 compared with Example 8 such transparency increase as much as twice.