Sol containing nano zirconia particles for use in additive manufacturing processes for the production of 3-dimensional articles

1. A printing sol as construction material in an additive manufacturing process for producing a three-dimensional article, the printing sol comprising:

solvent(s);

nano-sized crystalline zirconia particles in an amount from 2 to 25 vol.-% with respect to the volume of the sol, the average primary particle size of the nano-sized crystalline zirconia particles being in a range up to 20 nm;

a first monomer being a polymerizable surface modification agent represented by formula A-B, with A being capable of attaching to a surface of the nano-sized crystalline zirconia particles and B being a radiation curable group;

optionally a second monomer, the second monomer comprising at least one radiation curable moiety but no acidic or silane group(s); and

photoinitiator(s),

wherein the solvent(s), the nano-sized crystalline zirconia particles, the first monomer, optionally the second monomer, and the photoinitiator(s) form a sol,

wherein the printing sol forms the three-dimensional article in a gel body state characterized by a Volume A,

wherein the printing sol forms the three-dimensional article in a sintered state characterized by a Volume F, and

wherein Volume A is more than 200% greater than Volume F.

2. The printing sol of claim 1 , the printing sol being characterized by at least one or all of the following features:

showing a transmission of at least 5% at 420 nm determined for a path length of 10 mm;

the sol having a viscosity of less than 500 mPa*s at 23° C.;

pH value: from 1 to 6 if brought in contact with water.

3. The printing sol of claim 1 , the polymerizable surface modification agent represented by formula A-B being characterized by at least one of the following features:

A comprising an acidic group or a silane group;

B comprising a vinyl group;

being present in the sol in an amount from 2 to 30 wt.-% with respect to the weight of the sol.

4. The printing sol of claim 3 , wherein B is selected from an acryl group and a methacryl group.

5. The printing sol of claim 1 , the nano-sized zirconia particles being characterized by at least one or all of the following features:

being essentially spherical, cuboidal or a mixture thereof;

being non-associated;

comprising ZrO 2 in an amount of 70 to 100 mol-%;

comprising HfO 2 in an amount of 0 to 4.5 mol-%;

comprising a stabilizer selected from Y 2 O 3 , CeO 2 , MgO, CaO, La 2 O 3 or a combination thereof in an amount of 0 to 30 mol-%;

comprising Al 2 O 3 in an amount of 0 to 1 mol-%.

6. The printing sol of claim 1 , the photoinitiator being characterized by at least one or all of the following features:

showing radiation absorbance in the range from 200 to 500 nm;

being combustible without residues at a temperature below 800° C.;

comprising a moiety selected from benzophenone, xanthone, quinone, benzoin ether, acetophenone, benzoyl oxime or acyl phosphine;

being present in the sol in an amount from 0.01 to 3 wt.-% with respect to the weight of the sol.

7. The printing sol of claim 1 , the solvent being characterized by at least one of the following features:

having a boiling point above 70° C.;

having a molecular weight from 25 to 300 g/mol;

having a viscosity from 0.1 to 50 mPa*s at 23° C.

8. The printing sol of claim 7 , the solvent being characterized as having a boiling point above 150° C.

9. The printing sol of claim 1 , the sol further comprising inhibitor(s) in an amount from 0.001 to 0.5 wt.-% with respect to the weight of the sol.

10. The printing sol of claim 1 , the printing sol being characterized as follows:

the solvent(s) in an amount from 25 to 70 wt.-%;

the nano-sized crystalline zirconia particles in an amount from amount from 2 to 25 vol.-%;

one or more of the first monomer and the second monomer present in an amount totaling from 2 to 30 wt.-%;

the photoinitiator in an amount from 0.001 to 3 wt.-%;

an inhibitor in an amount from 0 to 0.5 wt.-%;

wt.-% and vol.-% with respect to the weight or volume of the printing sol.

11. A process for producing a ceramic article, the process comprising:

providing a printing sol as described in claim 1 ;

processing the printing sol as construction material in an additive manufacturing process to obtain a 3-dim article being in a gel state, the 3-dim article in a gel state having a Volume A;

transferring the 3-dim article being in a gel state to a 3-dim article being in a dry state selected from an aerogel and a xerogel;

applying a heat treatment step to obtain a sintered 3-dim ceramic article, the sintered ceramic article having a Volume F

wherein Volume A is more than 200% greater than Volume F.

12. The process of claim 11 , wherein Volume A is at least 500% greater than Volume F.

13. The process of claim 11 , the process further comprising:

additive manufacturing desired geometries by sequential light curing of layers to obtain a 3-dim article being in a gel state;

optionally cleaning a surface of the 3-dim article being in a gel state;

optionally post-curing the 3-dim article being in a gel state to a temperature in the range of 35 to 80° C. or by additional light hardening to form a post-cured 3-dim article characterized by a Volume B;

optionally soaking the 3-dim article being in a gel state with a solvent;

applying a supercritical drying step to the 3-dim article being in a gel state to form a 3-dim article being in a dry state as an aerogel characterized by a Volume C;

optionally heating the 3-dim article being in a dry state as an aerogel to a temperature in the range of 400 to 800° C. to form a green body characterized by a Volume D;

optionally heating the green body to a temperature in the range of 800 to 1100° C. to form a pre-sintered body or a white body having a porous structure, the pre-sintered body or white body characterized by a Volume E;

optionally coloring at least a part of a surface of the pre-sintered body or white body;

applying the heat treatment step to obtain the sintered 3-dim ceramic article characterized by the Volume F.

14. The process of claim 11 , wherein the processing of the printing sol comprises one or more of the following parameters:

slice thickness of printing sol exposed to radiation: 0.001 to 0.500 mm;

energy dose per layer in the range of 5 mJ/cm 2 to 100 mJ/cm 2 .

15. The process of claim 11 , the process excluding one or more of the following:

heating the construction material during the processing step to a temperature above 70° C.;

applying pressure during the heat treatment step.

16. The process of claim 11 , the sintered 3-dim ceramic article being characterized by at least one of the following features:

density: more than 98.5% with respect to theoretical density;

translucency: more than 30% determined on a polished sample having a thickness of 1 mm;

flexural strength: at least 450 MPa according to ISO 6872;

phase content tetragonal phase: from 0 to 100 wt.-%;

phase content cubic phase: from 0 to 100 wt.-%;

size in either x, y or z direction: at least 0.25 mm.

17. The process of claim 16 , the sintered 3-dim ceramic article having a shape of a dental restoration or orthodontic bracket.