WO2014041244A1

WO2014041244A1 - Method and mixture for manufacturing ceramics

Info

Publication number: WO2014041244A1
Application number: PCT/FI2013/050877
Authority: WO
Inventors: Jan-Erik Teirfolk; Antti Laukkanen
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 2012-09-11
Filing date: 2013-09-11
Publication date: 2014-03-20
Anticipated expiration: 2015-03-11
Also published as: FI20125943A7

Description

Method and mixture for manufacturing ceramics Field of the Invention The present invention relates to a method for manufacturing of ceramics. The invention also relates to a mixture of ceramic particles and a processing aid, which can be used in a manufacturing process of ceramics.

Background of the Invention

Ceramics are regarded as non-metallic, inorganic substances that are manufactured through a process of mixing, shaping and exposure to high temperatures. The raw materials include inorganic solid particles with precisely controlled purity, size and size distribution. Typical materials are barium titanate, ferrite, aluminum oxide, zirconium oxide, aluminum nitride, silicon nitride and silicon carbide.

Achieving the highest level of homogenity is crucial for the properties of ceramics. In the initial stage of manufacturing, the raw materials are milled and mixed in a solvating media (such as water). Afterwards they are shaped and cut to precise requirements and fired at extreme heat in temperature controlled kilns. Typical for the raw materials is that they have very high densities, for example zirconium oxide 5,9 g/cm³ . Thus, the initial mix is prone to sedimentation with resulting inhomogenity.

Japanese published patent application JP 2010-265153 A describes the use of cellulose nanofibre as binder in a range of 0.5 to 3.0 % on the weight of inorganic ceramic particles in slip casting method to form porous ceramic bodies. The binder is added to the slurry obtained after ball mill mixing of silica particles in water.

Summary of the Invention

It is an object of the present invention to provide a method where the raw materials of the ceramic product can be kept in homogeneous mixture until the firing at high temperature to obtain the final product. The concept "homogeneous" means in this contect that the mixture is of uniform composition throughout the whole volume and no segregation or sedimentation of solids take place.

It is a further object of the present invention to provide a mixture comprising ceramic particles and mixing medium which is an intermediate product in the process of forming the ceramic product.

The objects are attained with a mixing medium that is liquid dispersion of fibril cellulose, especially aqueous fibril cellulose dispersion. Fibril cellulose forms a self-assembled hydrogel network even at low concentrations in water and has a surprisingly high stabilizing ability of ceramic particles of high density. It is also typical to these fibril cellulose gels that they are shear thinning and thixotrophic in nature. They also have good thermal resistance at elevated temperatures (for example at 100°C), which is useful if the product is dried at elevated temperature before firing.

The fibril cellulose concentration of only about 0.3 to 1 .5 wt-% in the dispersion of liquid and fibril cellulose is sufficient to accomplish an aqueous mixing medium that can stabilize even high density ceramic particles, even those exceeding the particle density of 4.0 g/cm³, such as zirconium oxide, in suspension in this mixing medium. The mixture comprising the mixing medium, the ceramic particles suspended uniformly in said mixing medium, and possible other constituents suspended or dissolved in said mixing medium, is shaped to a piece of desired shape and dimensions and dried to a green-ware, which is then fired in a kiln at hight temperature to a final ceramic body.

The fibril cellulose used in the mixing medium at the relatively low concentration is fibril cellulose where the cellulose molecules contain anionically charged groups. It can be for example chemically modified cellulose that contains carboxyl groups as a result of the modification. Cellulose obtained through N-oxyl mediated catalytic oxidation (e.g. through 2,2,6,6-tetramethyl-1 -piperidine N-oxide, known by abbreviation "TEMPO") or carboxymethylated cellulose are examples of anionically charged fibril cellulose where the anionic charge is due to a dissociated carboxylic acid moiety. The chemical modification in either of the above-mentioned ways makes the cellulose labile to such extent that the fibres can be disintegrated to fibrils with less energy. Another example of fibril cellulose where the cellulose is chemically modified is cationically charged fibril cellulose. One example of cationically charged fibril cellulose is the grade where the cellulose molecules contain quaternary ammonium groups. Other cationically charged groups may also be used for modification. The chemical modification by cationic groups also makes the cellulose labile so that fibres can be disintegrated to fibrils with less energy.

The anionically charged fibril cellulose obtained through the modification of cellulose acts particularly well at low concentrations (0.4 - 1 .0 wt-%) for keeping ceramic particles of high particle density in homogeneous mixture. The cationically charged fibril cellulose also acts well at low concentrations (0.4 - 1 .0 wt-%) for keeping ceramic particles of high particle density in homogeneous mixture.

The fibril cellulose may also be chemicaly unmodified, that is, containing unsubstituted cellulose. This grade of fibril cellulose is preferably used at concentrations of 0.5 - 1 .5 wt-% for keeping ceramic particles of high particle density in homogeneous mixture.

Further advantage in the use of fibril cellulose is that it can be used to adjust the nanoscale porosity of the final ceramic product by varying the quantity of the fibrils in relation to the ceramic particles (total pore volume) as well as the fibril diameter of the the fibril cellulose (size of the pores).

Description of the Drawings

Fig. 1 shows the concentration of the mixing medium needed for stabilization of different particles;

Fig. 2 shows sedimentation tests made with the novel mixing medium;

Fig. 3 shows sedimentation tests made with the mixing medium of prior art; Fig. 4 shows the yield stress vs. concentration curves of the novel mixing medium and the mixing medium of prior art,

Fig. 5 shows the diameter distribution of various fibril cellulose grades, and Figs 6 and 7 are SEM photographs of unmodified and anionically charged fibril cellulose, respectively. Detailed Description of the Invention

Ceramic particles are inorganic solid particles that have a predetermined size distribution and act as the major building elements of the ceramic product. The average size of the particles may vary according to the product made. Typical materials include barium titanate, ferrite, aluminum oxide, zirconium oxide, aluminum nitride, silicon nitride and silicon carbide. The materials used are characterized especially by high particle density (for example silicon nitride 3.1 , silicon carbide 3.2, aluminium nitride 3.3, cubic boron nitride or c- BN 3.45, aluminium oxide 3.9, ferrites above 4.0, barium titanate 6.0, in grams per cubic centimetre). The average particle size (D₅₀) of the material can be below 1 mm. The method can, however, be applied to other ceramic particulate materials than those listed above. Fibril cellulose refers to a collection of isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. Fibril cellulose has typically a high aspect ratio: the length might exceed one micrometer while the number-average diameter is typically below 200 nm. The diameter of nanofibril bundles can also be larger but generally less than 5 μιτι. The smallest nanofibrils are similar to so called elementary fibrils, which are typically 2-12 nm in diameter. The dimensions of the fibrils or fibril bundles are dependent on raw material and disintegration method. The fibril cellulose may also contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of fibril cellulose from cellulose raw material, cellulose pulp, or refined pulp is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. The fibril cellulose is preferably made of plant material. One alternative is to obtain the fibrils from non-parenchymal plant material where the fibrils are obtained from secondary cell walls. One abundant source of cellulose fibrils is wood fibres. The fibril cellulose is manufactured by homogenizing wood- derived fibrous raw material, which may be chemical pulp. The disintegration in some of the above-mentioned equipments produces fibrils which have the diameter of only some nanometers, which is 50 nm at the most and gives a dispersion of fibrils in water. The fibrils can be reduced to size where the diameter of most of the fibrils is in the range of only 2-20 nm only. The fibrils originating in secondary cell walls are essentially crystalline with degree of crystallinity of at least 55 %. Another alternative is to obtain the fibrils form parenchymal plant material where the fibrils are obtained from parenchyma cell walls of plants, sugar beet pulp and citrus pulp being sources that can be mentioned as one example. One example of fibril cellulose is fibril cellulose produced by certain bacteria, such as Giuconacetobacter xylinus, formerly known as Acetobacter xylinum.

The fibril cellulose used can be chemically modified. One example is is fibril cellulose containing anionically charged groups (anionically charged fibril cellulose). Such anionically charged fibril cellulose can be for example chemically modified cellulose that contains carboxyl groups as a result of the modification. Cellulose obtained through N-oxyl mediated catalytic oxidation (e.g. through 2,2,6,6-tetramethyl-1 -piperidine N-oxide) or carboxymethylated cellulose are examples of anionically charged fibril cellulose where the anionic charge is due to a dissociated carboxylic acid moiety. Anionically charged fibril cellulose is typically produced by modifying pulp chemically, whereafter the fibres of the pulp are disintegrated to fibril cellulose.

The chemically modified fibril cellulose can be fibril cellulose containing cationically charged groups. Such cationically charged fibril cellulose can be for example chemically modified cellulose that contains quaternary ammonium groups as a result of the modification. Cationically charged fibril cellulose is typically produced by modifying pulp chemically, whereafter the fibres of the pulp are disintegrated to fibril cellulose.

The fibril cellulose can also be chemically unmodified. The chemically unmodified cellulose has usually lower zero shear viscosity than the fibril cellulose grades prepared by similar methods from chemically modified pulps.

The ceramic particles are mixed in an aqueous mixing medium comprising fibril cellulose at a concentration of 0.3 - 1 .5 wt-%, preferably 0.4 - 1 .0 wt-% on the basis of weight of the aqueous dispersion of fibril cellulose and water. Other substances can be added to the mixing medium initially or during the milling. A mixture having the ceramic particles evenly distributed is obtained. This mixture is shaped to a predetrmined shape, allowed to dry, and then fired in a kiln at high temperature to obtain the final ceramic body. This ceramic body may be aftertreated to obtain the final product.

The steps of making the ceramic product comprises, as a rule

- mixing the ceramic particles in an aqueous mixing medium comprising the fibril cellulose to a homogeneous mixture

- shaping the mixture

- drying the shaped mixture to form a so-called green body, and

firing the green body at high temperature above 700°C, usually between 900 and 1400°C, to form the ceramic body with final strength properties.

In the firing step, all organic constituents including the cellulose are burnt away, leaving the ceramic particles and nanoscale porosity in the ceramic body which is determined by the grade, size distribution and amount of the fibril cellulose initially used in the mixture and burnt away during the firing.

Some finishing steps can still be performed to the ceramic body, such as surface treatment and machining.

The shaping of the mixture to a shape that approximates the final shape of the final ceramic product can be done by operations well known in ceramic technology, such as slip casting, powder pressing, hydroplastic forming (especially extrusion), or tape casting. The shaping operation may include also cutting a continuous shape into shorter lengths. The aqueous fibril cellulose dispersion, due to its shear-thinning behaviour, is particularly suitable as medium in mixing, where the medium is subjected to working, because the medium does not offer considerable resistance to mixing and other mechanical working because of the high shear applied. On the other hand, when the working (applied shear) has ceased, the medium recovers its initially high viscosity at zero shear, which helps in stabilizing the mixture (prevents the sedimentation or segregation of the solid ceramic particles and keeps them evenly distributed within the mixing medium). The particles will be evenly distributed in the volume of the medium or any preform made of it during later processing stages (shaping and drying), and eventually this leads to structurally homogeneous ceramic body after the firing. If some mechanical disintegration takes place during the mixing, the particles formed are kept well in dispersion.

In making the mixture before the shaping, the ceramic particles are mixed with a liquid medium that already comprises the fibril cellulose in dispersion at a desired concentration.

The ceramic particles and the aqueous mixing medium can be mixed in varying proportions depending on the manufacturing method. The mixing can be done in proportion of 10 - 100 weight parts of dispersion of water and fibril cellulose per 100 weight parts of ceramic particles. These figures shall not, however, be construed as limiting.

Fig. 1 shows the concentration of anionically charged fibril cellulose, produced by N-oxyl mediated catalytic oxidation, needed to stabilize different inorganic particles of different sizes and densities against the sedimentation. It can be seen that for example high-density zirconium oxide with particle size of 550 μιτι (density 5,9 g/cm³) can be stabilized at a fibril cellulose concentration of 0.6 wt-% in aqueous dispersion. Silver particles with particle size between 350-420 μιτι were stabilized at a concentration of 0.4 wt-%. Figs. 2 and 3 show the results of sedimentation tests made with the aqueous dispersions of anionically charged fibril cellulose and xanthan gum, a known suspension stabilizer for ceramics. Zirconium oxide particles had been sedimented in 5 days at 0.5 % concentration of xanthan gum (Fig. 3), whereas at the same concentration of fibril cellulose the suspension was stable even after 25 days (Fig. 2).

Fig. 4 illustrates the difference in yield-stress vs. concenration curves of the above-mentioned aqueous dispersions. The fibril cellulose has a clearly higher Yield stress (shear stress value at which the viscosity drops sharply) at all concentration values between 0.4 and 0.7 wt-%. The y-axis showing the yield stress values is logarithmic, which means that the difference is numerically larger than illustrated by the curves At the fibril cellulose concentration of 0.5 wt-% in aqueosu dispersion, the yield stress is above 8 Pa, the accurate value being 10 Pa in the curve.

Fig. 5 shows, as an example, some typical size distributions related to the fibril diameter for some fibril cellulose grades. The curve with the highest peak and smallest fibril width is related to anionically charged fibril cellulose which is obtained by initially oxidizing the starting pulp by N-oxyl mediated catalytic oxidation (TEMPO) prior to its disintegration into fibrils. Next to this curve is a curve which represents another anionically charged fibril cellulose grade obtained by initially carboxymethylating the starting pulp prior to its disintegration into fibrils. The two remaining curves with broader size distribution and larger average fibril width are chemically unmodified fibril cellulose grades obtained with different disintegration methods. The curves show how by choosing a fibril cellulose grade with a known size distribution the size and size distribution of the pores can be determined, and by adjusting the amount of fibril cellulose in relation to the amount of ceramic particles the total pore volume of the nanoscale porosity can be determined.

Figs. 6 and 7 illustrate in the form of SEM photographs the size differences of two grades of fibril cellulose, chemically unmodified (Fig. 6) and anionically charged (Fig. 7) fibril cellulose.

Claims

Claims:

1 . Method for manufacturing of ceramics, where ceramic particles are mixed with a liquid medium to make a mixture of ceramic particles and mixing medium where the ceramic particles are distributed, said mixture being formed in subsequent processing steps of shaping, drying and firing to a ceramic body, said method comprising

- mixing ceramic particles with liquid dispersion of fibril cellulose to form said mixture, and

- removing the liquid in the subsequent processing steps to form the ceramic body.

2. Method according to claim 1 , characterized in that the concentration of fibril cellulose in the liquid dispersion is 0.3 - 1 .5 wt-%, preferably 0.4 to 1 .0 wt-% on the basis of weight of the dispersion of liquid and fibril cellulose.

3. Method according to claim 1 or 2, characterized in that the fibril cellulose is anionically charged or cationally charged fibril cellulose.

4. Method according to claim 1 or 2, characterized in that the fibril cellulose is chemically unmodified fibril cellulose

5. Method according to any of the preceding claims, characterized in that the mixture is subjected to shearing action.

6. Method according to any of the preceding claims, characterized in that the ceramic particles are selected from barium titanate, ferrite, aluminum oxide, zirconium oxide, aluminum nitride, cubic boron nitride, silicon nitride and silicon carbide.

7. Method according to any of the preceding claims, characterized in that the density of the ceramic particles is above 3.0 g/cm³.

8. Method according to any of the preceding claims, characterized in that the density of the ceramic particles is above 4.0 g/cm³.

9. Method according to any of the preceding claims, characterized in that the liquid dispersion of fibril cellulose has a yield stress of at least 8 Pa when measured at 0.5 % concentration at ambient temperature.

10. Mixture of ceramic particles and a mixing medium, said mixing medium being liquid dispersion of fibril cellulose which acts as suspension stabilizer for the ceramic particles in the mixture.

1 1 . Mixture according to claim 10, characterized in that the concentration of fibril cellulose in the liquid dispersion is 0.3 - 1 .5 wt-%, preferably 0.4 to 1 .0 wt-%.

12. Mixture according to claim 10 or 1 1 , characterized in that the fibril cellulose is anionically charged or cationally charged fibril cellulose.

13. Mixture according to claim 10 or 1 1 , characterized in that the fibril cellulose is chemically unmodified fibril cellulose

14. Mixture according to any of the preceding claims 10 to 13, characterized in that the ceramic particles are selected from barium titanate, ferrite, aluminum oxide, zirconium oxide, aluminum nitride, cubic boron nitride, silicon nitride and silicon carbide.

15. Ceramic product made by a method according to any of the claims 1 to 9.

16. Ceramic product according to claim 15, characterized in that it has nanoscale porosity.