AU717033B2

AU717033B2 - Hollow microfiber of ceramic material, a method for its production and the use of such a fiber

Info

Publication number: AU717033B2
Application number: AU15435/97A
Authority: AU
Inventors: Klaus Rennebeck
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-01-21
Filing date: 1997-01-20
Publication date: 2000-03-16
Anticipated expiration: 2017-01-20
Also published as: EP0874788A1; NO983342L; CN1211966A; CA2243520A1; PL327974A1; DE59711649D1; KR100283551B1; EP1018495A2; AU1543597A; JP3061866B2; EP1018495B1; BR9707014A; JPH11502906A; WO1997026225A1; EP0874788B1; KR19990081864A; EP1018495A3; NO983342D0; ATE267149T1; ATE216356T1

Abstract

A novel hollow ceramic micro-fibre has a wall thickness of 0.01-15 (preferably 0.3-6, especially 0.5-3) microns and an outside diameter of 0.5-35 (preferably 1-25, especially 1-10) microns. Also claimed is a process for producing hollow micro-fibres, especially the above fibres, by (a) forming green hollow micro-fibres from a dispersion containing a ceramic precursor material and a thermally removable binder; and (b) removing the binder optionally using heat. Preferably, the ceramic is an oxide, silicate, nitride and/or carbide ceramic material.

Description

2198/0E505 1 HOLLOW MICROFIBER OF CERAMIC MATERIAL, A METHOD FOR ITS PRODUCTION AND THE USE OF SUCH A FIBER The invention relates to a hollow microfiber of a ceramic material, to a method for the production of such a fiber and to its use.

The production of compact ceramic fibers, that is, of fibers, which do not have a lumen or cavity in the center of the fibers in their longitudinal direction, is known. They generally consist largely or completely of a glass phase and are used, for example, as woven fabric, knitted fabric and nonwoven material in insulating materials and as heat protection shield for reinforcing metallic workpieces and in composite materials. In various area of application, these fibers are inadequate, for example, with respect to elasticity, bending strength and insulating effect. Moreover, it is desirable to lower the weight of the known fibers and to increase the spinning speed.

The disadvantages, addressed above, are not eliminated by known hollow fibers of polymeric synthetic materials. Because of their special features, these fibers are used in membrane woven fabrics, roof linings, tent canvas, membranes, etc.

Generally, however, the biological compatibility as well as the chemical and thermal stability are inadequate. In addition, they have the disadvantage that membranes, produced therefrom, have a comparatively low penetration speed and cannot be counter-washed or cleaned.

Ceramic hollow fibers are known, which have thick walls and large external diameters and comparatively large fluctuations in their dimensions and, in some cases, can be produced only in short lengths. The WO 94/23829 describes, for example, ceramic hollow fibers with an external diameter, in particular, 0.5 to 10mm and a wall thickness of 30 to 500 utm, which are produced by extruding a paste, containing a ceramic powder, removing the binder and sintering. However, such ceramic hollow fibers have a limited strength, a low elasticity, a small specific surface area and do not have semi-permeable properties. Moreover, thick fibers can be produced and wound up only at slow speeds. They are not suitable for woven and knitted fabrics and other textile formations, for which, in particular, a high flexibility is required.

The EP-A-O 195 353 discloses hollow microfibers with an external diameter of less than about 50 um and a wall thickness ranging from about 0.05 to 20 rlm. However, the external diameter as well as the wall thickness of these known hollow microfibers fluctuates appreciably. The fluctuations, related to the average, amount to 33% for the external diameter and even to 66% for the wall thickness. Because of their nonuniformity, these known hollow microfibers are unsuitable for numerous applications.

Moreover, hollow fibers with an external diameter up to 2000 pm are known from the US patent 4,268,278, an external diameter range of 100 to 550 ptm being particularly oo preferred. The wall thickness of these hollow fibers ranges from 20 to 300 .tm and especially from 50 to 200 .tm. Moreover, these fibers can also be produced and wound 20 up only at a low speed.

S.. It is therefore an object of the invention to make available hollow microfibers of a *ceramic material, which preferably do not have the deficiencies of the known fibers addressed above and, preferably in particular, have a high elasticity, a high bending 21170-OO.DOC/KLS strength, a good insulation effect and good biological compatibility and, moreover, preferably can be produced at high production speeds. Furthermore, it shall preferably be possible to use existing spinning equipment, intended for man-made fiber, for their production.

According to a broad aspect, the present invention provides a hollow microfiber of a ceramic material, having a wall thickness of about 0.01 to 15 jim and an external diameter of about 0.5 to 35 gm, wherein the fluctuation in wall thickness and external diameter does not exceed 6%.

The reference to variation of the wall thickness and of the outside diameter by not more than encompasses 6%.

Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to .be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

15 A wall thickness of less than about 0.01 pm is difficult to achieve due to manufacturing reasons and would, if it were possible to produce such a thickness, have the disadvantage that the inner or outer surface of the hollow fibers would non-uniform and any coatings, subsequently applied, would have defects, such as holes or a nonuniform thickness. If the wall thickness exceeds about 10 im (particularly about 20 jim), the use of the inventive hollow fibers for the separation of materials by permeation would merely lead to an inferior permeate flow, since the permeate would have to cover a larger distance, while the selectivity is not improved further. It is therefore preferred if S the wall thickness is about 0.3 to 6 pin and, more especially, about 0.5 to 3pm.

21170-00 DOC 3a If the external diameter drops below 0.5 lim, the hollow fiber has too small a lumen and the flow of liquids through the fibers is impeded. If the external diameter exceeds 35 pim, the flexibility of the hollow fibers is reduced and material separation modules, which are constructed from a plurality of inventive hollow fibers, become too voluminous in relation to the amount of permeate flowing. Preferably, the external diameter is about 1 to 25 pm and, more especially, 1 to 10 im and more particularly 5 to iLm.

Pursuant to the invention, hollow microfibers preferably are produced as endless fibers and are of particular value, if their wall thickness and their external diameter do not fluctuate by more than 6% and especially by not more than that is, the hollow microfibers advantageously are uniformly constructed hollow microfibers. In practical applications, this means that the inventive hollow fibers have uniform properties along their length. Short-cut fibers, which are produced by cutting endless fibers to length, have the advantage that they are free of needle fibers, that is, of fibers with a length less than 3 jim, which are regarded as a danger to health.

Typically, the external diameter of the inventive microfibers is of the order of i about 7 pm, the wall thickness is about 1 jm and the internal diameter, which corresponds to the external diameter of the lumen, is therefore of the order of 5 pm.

a Compared to compact fibers without a lumen, there is thus a savings of material and a 20 weight of about 10 to 95% and typically of about 40 to 60%. This is also associated with an appreciable reduction in material and manufacturing costs.

a..g.

go oa 21170-0ODOC 3b Wherever there is mention of "ceramic material" within the scope of the invention, this expression should be interpreted in the widest sense. It is intended to be a collective name for materials, which are built up from inorganic and

S.

S S

S.

S

S S

S

5.5.

S. S S S S *5 211 4 predominantly non-metallic compounds and represent, in particular, more than by volume of crystallized materials. In this connection, we refer to the R6mpp Chemie Lexikon, 9th edition, volume 3, 1990, pages 2193 to 2195. Preferably, the inventive hollow ceramic fibers consist of an oxide, silicate, nitride and/or carbide ceramic material. Especially preferred are those inventive hollow ceramic fibers, which are based on aluminum oxide, calcium phosphate (apatite) or related phosphates, porcelain-like or cordierite-like compositions, mullite, titanium oxide, titanates, zirconium oxide, zirconium silicate, zirconates, spinelles, emerald, sapphire, corundum, nitrides or carbides of silicon or other chemical elements or their mixtures. As doping agents, the materials known in ceramics, such as MgO, CaO, ZrO 2 ZrSiO 4

Y

2 0 3 etc.

or their precursors are optionally added to the main inorganic components.

The inventive hollow microfibers of ceramic materials can be made porous and semipermeable by comparatively mild baking. Such hollow microfibers can have, in particular, an internal specific surface area, measured by the BET method using nitrogen adsorption or by mercury porosimetry in the range of about 600 to 2,000 m 3 Because of their inorganic nature, they are usually hydrophilic and can be used for separating materials, the permeate preferably passing from outside through the semipermeable wall inwards to the lumen and leaving from the ends of the fibers, although materials can also be separated in the reverse direction. These hollow microfibers can exhibit a very good separation effect in the micro, ultra and nano range. The materials separated can be detected microscopically on the hollow microfibers. Such a material separation can be used, for example, for purifying or separating gases/gas mixtures, such as air, or liquids, blood or water, and also for hot gases or melts. For this purpose, it is sometimes advisable for the hollow microfibers to have a separation coating, the wall thickness of which preferably is 2.5 Um or less and particularly less than 0.5 [Lm and especially even about 0.1 j^m. The separation coating may consist essentially of inorganic or organic molecular sieves, such as a zeolite material, or of inorganic or organic separation layers, such as esters, silanes or siloxanes. Such layers are applied by known methods, such as the CVD (chemical vapor deposition) or PVD (physical vapor deposition) methods, galvanic methods and/or sedimentation methods.

Moreover, the ceramic hollow microfibers, coated with an inorganic separation layer, can be baked. In addition, there is also the possibility of providing the green hollow microfibers, which are described in the following, with this separation coating and to subject this composite to a baking process.

The hollow ceramic microfibers can be made tight and impermeable by a comparatively intensive baking. The consolidation of the material is attained especially by sintering or vitrifying. Because of the tightly baked wall, such inventive hollow microfibers, can, in particular, be vacuum tight, conduct light well and especially be capable of floating and flying.

Because of their inorganic nature, the baked, inventive hollow microfibers are corrosion resistant, incombustible, non-decayable, weather resistant, physiologically safe, biocompatible, transparent, heat insulating as well as usually electrically insulating and resistant to oxidation. They can be regarded as safety fibers and are usually permeable to electromagnetic radiation. They do not have any of the carcinogenic properties of needle minerals. They are frequently transparent and can be produced as colorless or colored fibers, depending on the composition and the manufacturing conditions. However, they may also be opaque. Their weight per fiber length (titer) is of the order of about 10 to 100 g/kmn (tex), and typically about g/km. The inventive hollow microfibers are usually resistant to heat and alternating temperatures up to 1000 0 C especially up to 1300 0 C and even much higher. This depends predominantly on the chemical composition.

For the production of the inventive hollow microfibers, preferably an emulsion, dispersion and/or suspension, which contains the precursors of a ceramic material and a binder, which can be removed by the action of heat, are extruded in a known manner to form green hollow microfibers and the binder is removed under the action of heat. Alternatively, the dispersion can be applied on a core of an organic compact fiber. Subsequently, the core as well as the binder are removed by the action of heat. The dispersion may contain varying amounts, for example up to 95% by weight and preferably 40 to 70% by weight of dispersion medium. A dispersion medium could also be omitted if the binder is, for example, thermoplastic and can be melted into a mass of low viscosity without any decomposition worth mentioning. In individual cases, it has turned out that the green hollow microfibers already make advantageous application possibilities accessible, so that the subsequent heat treatment is then omitted.

Within the scope of the invention, the following come into consideration as ceramic precursors: clay minerals, especially kaolin, illite, montmorillite, metal hydroxides, such as aluminum hydroxide, mixed metal hydroxides oxides, such as AlOOH, mixed metal oxides halides, metal oxides, such as BeO, MgO, A1 2 0 3 ZrO 2 and ThO 2 metal nitrates, such as Al(NO 3 3 metal alcoholates, especially aluminum alcoholates, such as Al(iPrO) 3 Al(sec-BuO) 3 magnesium aluminosilicate, felspar, zeolite, boehmite or mixtures of two or more of the materials named.

During the heat treatment ofAl 2 (OH)sCl 2-3H 2 0, the following reactions take place, for example, consecutively, ca-Al 2 0 3 finally being obtained: Al 2

(OH)

5 C1 2-3H 2 0 Al(OH) 3 -Gel y-Al 2 0 3 a-Al 2 0 3 The average particle size of the ceramic precursor material preferably is less than about 2 am and particularly less than 1 ptm, less than about 0.1 tm being particularly preferred. Preferably, the ceramic precursor material is colloidal, that is, it exists as a sol or a gel, or it is molecularly dissolved. Reversible conversions are possible between the sol and gel forms. In a preferred embodiment, the binder, used within the scope of the invention, such as polyvinyl alcohol, gelatins, or proteins, can act as a protective colloid for the colloidal ceramic precursor.

There are no critical limitations for selecting the binder, which can be removed within the scope of the inventive method by the action of heat. However, a film-forming binder is preferred. Such a binder may, for example, be a urea, a polyvinyl alcohol, wax, gelatins, agar, protein, saccharides. Optionally, additional organic auxiliary materials, such as binders, standardizing agents, defoamers and preservatives can be used. The mixture of the precursor of the ceramic material and the binder, which can be removed under the action of heat, exists in the form of a dispersion, this concept being defined broadly. Such a dispersion may be an emulsion or suspension, usually in the form of a paste. There is much latitude in selecting the dispersion medium. Generally, it will be water. Organic solvents, such as alcohols or acetone, optionally in admixture with water, are also conceivable. Particularly advantageous here are the so-called sol gel processes, for example, on the basis of the already addressed polyvinyl alcohol.

Preferred dispersions contain about 20 to 70% by weight of ceramic precursor material, about 10 to 40% by weight of binder, 0 to about 70% by weight of dispersion medium and about 30% by weight of optional components.

In order to mold the inventive hollow microfibers, any molding processes are suitable, especially the blow molding process, the extrusion process, the vacuum extrusion process or the spinning process, as well as the method addressed in the WO 94/23829, in which a paste is produced, which contains a polymeric binder.

However, for this process, explicitly a ceramic powder is used, which is consequently in crystallite form; this is usually not suitable for carrying out the inventive method.

Rather, the success aimed for pursuant to the invention is guaranteed if such crystallites are excluded.

The extruding can be carried out as wet, melt or dry extruding at about room temperature or at the temperature of the melt of a molten organic substance or substance mixture. Of the molding processes addressed, the spinning method is of particular advantage. This method is characterized in that the dispersion is added to a feeding container or a pressure vessel of spinning equipment, which causes the dispersion to flow through the spinning equipment at a temperature of about 200 to 400*C and to be pressed through the nozzle ring openings or nozzle profile openings, the diameter or width of which preferably are about 0.1 to 150 [4m, the partial streams, produced in the region of the nozzle openings, are divided centrally by cores and/or devices for blowing in a gas and consolidated by warming, by irradiation or by entry of a reactant into the green hollow microfibers and optionally the binder is removed by the action of heat. The spinning equipment addressed preferably is equipment for the production of man-made filaments which, however, need not be heated thermally.

Conventional spinning equipment can be used, which may have to be adapted with respect to the nozzles and the consolidation devices. Consolidation of the green hollow microfibers takes place, for example, by evaporation of the dispersion medium upon exiting the nozzle opening into an environment, which has a lower pressure than the spinning piston. The consolidation can also take place due to the entry of a reactant for the binder or the ceramic material. The reactant can be gaseous and flow in the opposite direction to the fibers that are being discharged, or be liquid and be present as a precipitating bath, through which the discharged fibers are conducted.

One advantage of the inventive method lies therein that heating or warming of the ceramic starting material for consolidating the starting material, with respect to the green hollow microfibers, generally is required, if at all, only after the nozzles. The spinning equipment preferably is designed so that it has a high number of nozzles. The duct, adjoining behind the extrusion head having the nozzles.

generally can be kept quite short and it is preferably about 0.1 to 0.3 m long. In comparison to the man-made fiber production, the spinning equipment, despite the frequently clearly low temperatures of the material flow, can nevertheless be operated at the same pressure level used for the production of man-made fibers. In the case of the inventive method, the temperatures of the material flow usually are only slightly above room temperature.

For the melt extrusion methods, temperatures are selected, which ensure good processability of the organic melt so that here, in the particular case, temperatures up to 400 0 C are selected. Care must be taken to ensure that the material flow in the spinning equipment is not interrupted.

The lumen of the hollow microfibers can be produced by cores or devices, introduced into the partial flows in the region of the nozzles for blowing in a fluid, such as oxygen, nitrogen, air or a different gas mixture. The openings of the nozzles can be so designed that the largest possible number of nozzles, such as several thousand in a very narrow space, are disposed in regular order. Ring openings or profile openings with a non-circular cross section can make do without cross membershaped holding devices if, for example, one core or several, optionally twisted cores, such as very thin fibers, are guided centrally in the flow direction in a nozzle. For a blow molding method, injection nozzles without or with one or several cores can be employed. The nozzle openings addressed for the respective manufacturing methods, preferably have a diameter or a largest width of about 150 Am, and especially of 120 A.m, 80 or even 50 Am being particularly preferred, especially when baking materials, which contract more heavily. Particularly for materials, which contract less, openings are preferably used, which have a diameter or a largest width of 90 Am and especially of 60 Am, 30 im being particularly preferred. Under some circumstances, the openings are several times larger than the diameters or profile width of the baked, inventive, hollow microfibers, since very high shrinkage, which frequently amounts to 50 to 95%, for a shrinkage from the final dimension to the initial dimension (technical overmeasure), usually occur during baking. If the shrinkage is less, it is more likely to be in the range from 10 to For the inventive spinning method, the spinning speeds advantageously are between 400 and 8000 m/min.

In contrast to the methods for spinning man-made fibers, the skin- and yam-forming spinning composition frequently leaves the nozzles at about room temperatures and not, as in the case of man-made fibers, at temperatures of about 2000 to 500'C. The partial flows of the spinning compositions, which represent the strands for forming the hollow microfibers, can be consolidated by heating, by irradiating with UV, visible or IR light or by the access or air to the green hollow microfibers and, at the same time, optionally be dried. The heating can be carried out, for instance, in hot air, in a hot convection current or by radiant heat and usually takes place at a temperature up to only 1001C. After the consolidation of the green hollow microfibers, the latter can still be drawn, in order to change wall thicknesses and external diameters and to improve or change the properties of the fibers, as well as the strength and optionally also the permeability. Before they are consolidated, the strands (partial flows), forming the hollow microfibers, have wall thicknesses preferably of about 0.5 to 50 jim, as well as external diameters of 1 to 160 Itm; after they are consolidated, they have wall thicknesses preferably of 0.4 to 45 jtm and especially of 1 to 25 jtm as well as external diameters of 0.8 to 155 pm and especially of 8 to a range of about 12 to 24 tm being particularly preferred. The fluctuation in the wall thickness and in the external diameter preferably are not greater than and, in particular, not greater than After leaving the nozzles and after being consolidated by heating, hollow microfibers, which are used without being baked as hollow textile fibers, can be wound up and optionally also cut without additional treatment and processed once again. They can be woven, knitted, felted, knotted and otherwise processed by textile means and, if required, even metallized.

Hollow microfibers, whether they have been baked or not, can be used as whiskers, short fibers, long fibers, staple fibers, chopped fibers, clutches of fibers, filaments, woven goods, nonwoven goods, knitted goods, felts, rovings, films, paper layers, yams, ropes, nets and the like and processed further into filament modules, laminates, preforms, prepregs and the like. For example, a filament can be produced as a layer carrier in the form of a circular disk or rectangular layer and optionally processed further by stacking several filaments into modules. Such modules can be used, for instance, as membrane modules.

In the course of producing the inventive hollow microfibers, further steps can be introduced during or after the process. The processing of the hollow microfibers as well as the further processing is accomplished in steps, which are known, and in known equipment. The ceramic hollow microfibers, which have not been baked, can be baked by methods known in industrial ceramics, by which means the ceramic material is finished. The baking processes involved are, for example, the following: gas baking methods, inert gas baking methods or electric baking methods.

The inventive, baked, hollow microfibers have the dimensions already dealt with above.

The unbaked or green hollow microfibers, as well as the inventive, baked, ceramic hollow microfibers have all the properties, which are typical of and required for textile fibers for being processed, for example, into clutches of fibers, filaments, woven materials, knitted materials, felts, nonwovens and films. Because of the fact that the inventive hollow microfibers are subjected to only very slight dimensional fluctuations, the scatter of the external diameters is very slight. For example, if the process is carried out in an appropriate manner, hollow microfibers, which have a diameter smaller than 3 Am and can be regarded as carcinogenic, are not produced. The inventive hollow microfibers can be used as so-called endless fibers and are, moreover, shot-free. They can be produced in a comparatively environmentally friendly manner, are not harmful to the environment and can, moreover, be recycled. They can replace known fibers without a lumen and hollow fibers, as well as wires and strands, particularly of polymers, carbon, etc.

It must be regarded as surprising that the consolidated, green, hollow microfibers can be woven and have a tensile strength, which corresponds to that of conventional polymeric fibers. Moreover, it was surprising that the inventive, ceramic, hollow microfibers can be provided with a separation layer, which does not have a noticeable gradient between the separation layer and the fiber wall during a fire. It was furthermore surprising that the inventive, ceramic hollow microfibers, which were produced by extrusion from a starting material of pure aluminum oxide and which have a wall thickness of about 0.9 Am and an external diameter of about 6 Am, have a tensile strength at break of 3,600 Mpa when measured in a tensile testing machine customarily used in industrial ceramics.

The inventive, semipermeable, hollow microfibers, in the form of whiskers, short fibers, long fibers, staple fibers, chopped fibers, clutches of fibers, filaments, woven goods, nonwoven goods, knitted goods, felts, rovings, films, paper layers, yarns, ropes, nets, etc. can also be used as membranes, for example, for dialysis as well as for microdialysis and electrodialysis, for osmometers for molecular weight determination, as molecular sieves for separating liquids and/or gases, as catalyst support, as filter for viruses, bacteria, fungi, spores, dust, hot gas, fly ash or carbon black, for heat insulation as piezo ceramic or as implant, such as a dialysis, bone, tooth or tissue support transplant. For refrigeration, the retentate flow can pass through an integral solid bed reactor with a packing of baked hollow microfibers; for this purpose, the permeate preferably passes out of the interior of the hollow microfibers through the openings of the lumen at the fiber ends or follows the reverse path. For the filtering precipitators of hot gas filtration, for example, of power plants, semipermeable or tightly baked filter cloth is suitable.

The inventive, tight, hollow microfibers can be used in the form of whiskers, short fibers, long fibers, staple fibers, chopped fibers, clutches of fibers, filaments, woven goods, nonwoven goods, knitted goods, felts, rovings, films, paper layers, yams, ropes, nets, etc., for thermal insulation, as a temperature-resistant conveyor belt, as piezo ceramics, as high-vacuum tight fibers for light transport, for example, for lasers, as a melted-on protective layer for space missiles, for seals and for lining electronic welding equipment, vacuum chambers, vacuum pumps and other vacuum facilities, as a metal-ceramic composite, for composite materials, as reinforcement instead of steel and as other reinforcement in construction, as elements in electro-rheology, for example, as liquid carriers and liquid conductors, in safety films for special papers and sheets, as gas-filled sheets, for example, for foods and stored blood, as a carrier material for forgery-proof means of payment, for incombustible, non-decaying paper qualities, as a matrix for reinforcing metal melts or as a matrix of thin-walled polymer components, such as bumper bars.

The invention is described in greater detail by means of the following examples.

Example 1 For producing a ceramic hollow microfiber, spinning equipment of Barnag, Germany was used, which had been adapted to the special requirements of the preferred inventive method with respect to the nozzle diameter, the placement of the cores and/or the directions at which the gas is blown in the opening region of the nozzles and with respect to the equipment for consolidating the strands. The ceramic starting material used was a ceramic composition from a sol-gel process, which was obtained by mixing AI 2

(OH)

5 C1 2.5HO 2 0 in water with polyvinyl alcohol in water.

These materials were used in the ratio of 20 kg of water to 40 kg of polyvinyl alcohol to 60 kg of A1 2

(OH)

5 C1 2.5H 2 0. The sol, thus produced, is converted into a gel or a sol-gel by stirring. The pasty composition obtained was filled into a feeding container of spinning equipment and forced bubble-free with an extrusion screw into the extruder head. The conveying temperature was about 25°C. The pasty composition was molded by more than 3,000 annular nozzle openings in the extruder head, coaxial baffle plates in the region of each nozzle being used as cores. The underside of the extruder head was thermally insulated from the drying shaft. The strands were heated in the drying shaft by IR radiation and convection heat to about 140 0 C and thereby dried and consolidated sufficiently, so that there were no unintended dimensional changes during the further textile handling. The take-off speed is about 1,200 m/min.

The green hollow microfibers have a wall thickness of about 5.5 /m and an external diameter of about 33 um. Figure 1 shows the unbaked or green hollow microfibers.

By drawing these green hollow microfibers, which still have a slight, set residual moisture content, in the ratio of 1 1.2 at room temperature, the wall thickness was changed to about 4.5 tm and the external diameter to about 28 jim. These hollow microfibers, after being heated slowly to about 1600'C, were kept for 1 hour at this temperature and then allowed to cool slowly. The baked hollow microfibers had a wall thickness of about 0.9 jim and an external diameter of about 6 tjm.

Examples 2 and 3 The inventive hollow microfibers were produced as in Example 1 from a composition based on porcelain or a zeolitic magnesium aluminosilicate. In Table i, the average dimensions of the inventive, baked hollow microfibers and their properties are compared with those of know fibers.

Table 1: 3 Porcelain Base Zeolite E-Glass Carbon Aramide (Mg-Al silicate) S-Glass (Example 1) (Example 2) (Example 3) Externaldiameter 6 7 7.5 12 6- 8 1 Lumen diameter 4.2 5 5.3 5 without lumen without lumnen Wall thickness 0.9 1 1.1 3.5 without lumen without lumen Theoreica e-nsity 3.9 2.5 2.4 2.0 1f.8 -2.0 1.45 glcc g/km 42 39 37 240 (tex) Tensile strength 3.6 1.7 3.5 1.86 5.6 GPa

Claims

1. A hollow microfiber of a ceramic material, having a wall thickness of about 0.01 to 15 pm and an external diameter of about 0.5 to 35 Lm, wherein the fluctuation in wall thickness and external diameter does not exceed 6%.

2. The hollow microfiber of claim 1, wherein the wall thickness is about 0.3 to 6 jtm.

3. The hollow microfiber according to claim 1 or 2, wherein the wall thickness is about 0.5 to 3 pim.

4. The hollow microfiber of any one of claims 1 to 3, wherein the external diameter is about 1 to 25 jtm.

5. The hollow microfiber according to claim 4, wherein the external diameter is about 1 to 10 im.

6. The hollow microfiber of any one of claims 1 to 5, wherein it comprises an oxide, silicate, nitride and/or carbide ceramic material.

7. The hollow microfiber of any one of claims 1 to 6, wherein it has a semipermeable 15 wall.

8. The hollow microfiber of any one of claims 1 to 6, wherein it has a tightly baked wall. a S"

9. The hollow microfiber of claim 7, wherein a separation coating is present on the outer sheath of the wall. S 20

10. The hollow microfiber of claim 9, wherein the separation coating has a wall thickness of 2.5 pim or less.

11. The hollow microfiber of any one of claims 1 to 10, wherein the cavity is constructed in the form ofa lumen. constructed in the form of a lumen.

21170-00 DOC 18

12. A method for the production of hollow microfibers, particularly according to any one of claims 1 to 11, wherein a dispersion, which contains the precursor of a ceramic material and a binder, which can be removed under the action of heat, is molded in a conventional manner into green hollow microfibers and the binder optionally is removed by the action of heat.

13. The method of claim 12, wherein the green hollow microfibers are produced by blow molding, extrusion, vacuum extrusion or spinning.

14. The method of claims 12 or 13, wherein a spinning method is employed, wherein the dispersion is added to a feeding container or a pressure vessel of spinning equipment, pumped at a temperature of about 200 to 400 0 C through the spinning equipment and pressed through nozzle ring openings or nozzle profile openings and the partial flows, generated in the region of the nozzle openings, are divided centrally by cores and/or devices for blowing in a gas and consolidated by heating, by radiation or by entry of a reactant to form green hollow microfibers.

15 15. The method of any one of claims 12 to 14, wherein the green hollow microfibers are consolidated into semipermeable hollow microfibers by baking.

16. The method of any one of the claims 12 to 14, wherein the green hollow Smicrofibers are consolidated into tight hollow microfibers by baking.

17. The method of any one of claims 12 to 16, wherein a separation layer is formed on 20 the green or ceramic hollow microfibers.

18. The method of claim 17, wherein the hollow microfibers, coated with an inorganic separation layer, are baked. 21170-00.DOC 19

19. The method of any one of claims 13 to 18, wherein endless hollow microfibers are produced. The method of any one of claims 13 to 19, wherein the hollow microfibers are processed into whiskers, short fibers, long fibers, staple fibers, chopped fibers, clutches of fibers, filaments, woven goods, nonwoven goods, knitted goods, felts, rovings, films, paper layers, yars, ropes or nets. 21. The method of any one of claims 13 to 19, wherein hollow microfibers are processed into filament modules, laminated, preforms or prepregs. 22. The use of ceramic hollow microfibers of any one of claims 1 to 11 for the production of membranes, molecular sieves, catalyst supports, filters, piezo ceramics, implants, high-temperature resistant conveyor belts, melt protection layers, metal- ceramic composites or other composites, reinforcement in construction, elements in electro-rheology, safety sheets, gas-filled sheets, support materials, incombustible and non-decaying paper qualities, the matrix of metal melts or the matrix of thin-walled 15 polymer components or electro-elements for refrigeration, for osmometers, for thermal insulation, for the transport of light or for seals and linings. 23. The use of green hollow microfibers, produced according to the method of any one of claims 12 to 22, for the production of membranes, molecular sieves, catalyst supports, filters, piezo ceramics, implants, high-temperature resistant conveyor belts, melt :t 20 protection layers, metal-ceramic composites or other composites, reinforcement in construction, elements in electro-rheology, safety sheets, gas-filled sheets, support materials, incombustible and non-decaying paper qualities, the matrix of metal melts or *o 21170-00.DOC the matrix of thin-walled polymer components or electro-elements for refrigeration, for osmometers, for thermal insulation, for the transport of light or for seals and linings. 24. A hollow microfiber, substantially as herein described with reference to any one of the Examples but excluding any comparative examples therein. 25. A method of preparing a hollow microfiber, substantially as herein described with reference to any one of the Examples but excluding any comparative examples therein DATED this 6th Day of January, 2000 KLAUS RENNEBECK Attorney: PAUL G. HARRISON Fellow Institute of Patent Attorneys of Australia of BALDWIN SHELSTON WATERS *oJo S S *555 S S S S S .5. 5555 21170-00.DOC