EP0540567A1

EP0540567A1 - Aluminium oxide fibres and process for producing them

Info

Publication number: EP0540567A1
Application number: EP19910912945
Authority: EP
Inventors: Arno Wegerhoff
Original assignee: Akzo NV; Akzo Nobel NV
Current assignee: Akzo Nobel NV
Priority date: 1990-07-23
Filing date: 1991-07-18
Publication date: 1993-05-12
Also published as: WO1992001644A1; JPH05509363A

Abstract

On fabrique des fibres d'alumine contenant jusqu'à environ 40 % en poids de SiO2 en faisant réagir de l'hydroxyde d'aluminium ou des hydrates d'alumine avec de l'acide oxalique dans un milieu aqueux, le cas échéant en présence d'un acide tel que l'acide formique, pour obtenir de l'oxalate d'aluminium, en ajoutant de l'acide silicique pyrogène et en filant à sec la solution à filer ainsi obtenue, le cas échéant en présence d'auxiliaires de filage, et en soumettant les fibres d'oxalate d'aluminium ainsi obtenues à un traitement thermique postérieur.Alumina fibers containing up to about 40% by weight of SiO2 are produced by reacting aluminum hydroxide or alumina hydrates with oxalic acid in an aqueous medium, where appropriate in the presence of an acid such as formic acid, to obtain aluminum oxalate, by adding pyrogenic silicic acid and by dry spinning the spinning solution thus obtained, if necessary in the presence of auxiliaries spinning, and subjecting the aluminum oxalate fibers thus obtained to a subsequent heat treatment.

Description

Aluminum oxide fibers and process for their production

The invention relates to aluminum oxide fibers which contain SiO 2 in amounts of up to about 40% by weight and to a process for their production by first preparing an aqueous solution of aluminum oxalate, adding silicic acid and optionally spinning aids and other additives to this solution and these The solution is then spun into a precursor fiber using the dry spinning process and the aluminum oxalate fibers obtained are thermally converted into aluminum oxide fibers.

Inorganic fibers with a high aluminum oxide content and a proportion of SiO 2 have been known for a long time. Their production directly from mixtures or compounds of corresponding composition by the melt spinning process encounters great technical difficulties because of the high melting points. Therefore, these fibers are obtained practically exclusively by first using an aluminum compound such as aluminum salts or

Organoaluminium produces compounds that can be spun in the form of solutions or dispersions to so-called precursor fibers (precursor). Thermal treatment then produces aluminum oxide from the respective aluminum compounds.

With regard to the raw materials used, two types of process can be distinguished. One of the possibilities is the partial hydrolysis and polycondensation of aluminum alkoxides to polyaluminoxane polymer, which is spun from organic solvents, e.g. described in DE-AS 2408 122. Apart from the fact that this type of production is expensive and complicated, the use of organic solvents in dry spinning requires a complex separation of the solvent from the exhaust gas. Another disadvantage is the comparatively low aluminum content of the precursor threads and the correspondingly high proportion of organic material to be expelled from the threads by pyrolysis.

SETBUVTT The second type of process is based on aqueous salt solutions which contain additives of water-soluble organic polymers to improve the spinnability. In a number of processes of this type, the raw material base is aluminum oxychloride (DE-OS 3447760, EP-OS 206634). The precursors accordingly contain aluminum chlorides and result in chlorine-containing, generally toxic pyrolysis products. To avoid environmental problems, these must be removed from the waste gas as quantitatively as possible.

When using e.g. Formats, acetates, etc. Carboxylic acid Al salts as the starting material contain no chlorine in the pyrolysis gases, which means that solving the environmental problems is associated with considerably less effort.

In contrast to the amorphous glass and silica threads, aluminum oxide threads are crystalline. The mechanical properties depend crucially on the crystal structure. Flexible, manageable threads - apart from the possibility of producing single-crystalline short fibers (whiskers) - are only obtained with a so-called polycrystalline structure, i.e. a structure made up of as small as possible crystallites with sizes below about 50 nm. Enlarging the crystallites is associated with embrittlement and a reduction in the level of the mechanical properties. So-called grain growth inhibitors must be used as an additive for the formation of a suitable polycrystalline structure. In the sintered threads, these form further amorphous or crystalline phases and limit this

Crystal growth of aluminum oxide. Most known processes use SiÜ as an additive, further grain growth inhibitors are Zrθ £ and MgO, in DE-OS 2054573 finely divided carbon takes over this task.

Process type 1 provides systems dissolved in organic solvents and has the advantage that SiO £ as a grain growth inhibitor in the form of, for example, polysiloxanes is distributed in a molecularly dispersed manner in the spinning mass can be. In process type 2, which is based on aqueous systems, SiO 2 can only be used in colloidal form. This takes place in the form of silica pebbles, as can be seen from US Pat. No. 4,047,965. The difficulty here is that the particle sizes of the silica particles are dependent on both the pH value and the treatment temperature of the system, are sensitive to changes in the production conditions and it is therefore difficult in practice to achieve reproducible results.

DE-OS 2054573, which has already been mentioned, describes the production of a wide variety of inorganic fibers which essentially consist of an oxidic phase serving as a matrix, in which another phase as a grain growth inhibitor is finely distributed. A whole series of oxides and their mixtures are suitable for the oxidic phase. As can be read on page 12 of the description and in examples 1 to 4 of this publication, aqueous aluminum oxalate solutions can be dry-spun and converted into aluminum oxide fibers which contain finely divided carbon by thermal treatment under a nitrogen atmosphere. The carbon content can be reduced by using formic acid and increased by adding tartaric acid. The fibers obtained are naturally black. If the thermal aftertreatment is carried out in air, carbon-free fibers are obtained which, however, no longer contain any grain growth inhibitor and are unusable because of their poor mechanical properties. The aluminum oxalate is produced by reacting oxalic acid with amalgamated aluminum.

DE-OS 2313002 teaches the production of aluminum oxide fibers, in which an aluminum compound which is decomposable in aluminum oxide, which according to claim 4 is, inter alia, chloride, sulfate, acetate, for iat, propionate, oxalate, phosphate, or nitrate or a mixture thereof can be deformed from a solution into a fiber. The fiber is subjected to a hydrothermal treatment, for example with water vapor at 250-500 ° C. It is then heated to temperatures of, for example, 600-1000 ° C. A similar process is described in US Pat. No. 4,047,965, according to which inorganic fibers with 67-77% by weight of aluminum oxide and 23-33% by weight of silicon dioxide are obtained. The silicon dioxide becomes the spinning solution in the form of silica! added. Aluminum oxalate as a precursor is not mentioned in this US patent.

Although a variety of processes are already known, inorganic fibers such as e.g. also aluminum oxide fibers using a variety of methods, including There is still a need for improved processes which can also be produced by the dry spinning process, which lead in a simple manner to such fibers with good properties and good usability.

It is therefore the object of the invention to provide a dry spinning process with which it is possible to produce aluminum oxide fibers with a content of up to about 40% silicon dioxide from starting materials which are easily and inexpensively accessible, which is more environmentally friendly, has fewer corrosion problems, and is high Enables productivity, the process parameters such as concentration, temperature, concentration, warpage and the like. can be varied to a considerable extent, so that the process can be made very variable and it is very insensitive to fluctuations in the production conditions during production, so that a qualitatively uniform production is possible, and this leads to fibers which are both staple and as a fila duck have good mechanical and thermal properties and can be used in a variety of ways.

This object is achieved by a process for the production of aluminum oxide fibers which contain up to 40% by weight of silicon dioxide by producing an aqueous aluminum oxalate solution, this silicon dioxide and, if appropriate, organic spinning aids! adds the spinning solution obtained to a dry spinning process

LAT Precursor fiber spun and this fiber thermally aftertreated, dissolved, which is characterized in that oxalic acid is reacted in an aqueous medium with aluminum hydroxide or aluminum oxide hydrate to form aluminum oxalate, the pyrogenic silica is added to the solution obtained, then the spinning solution is spun by the dry spinning process to form a precursor fiber and this is thermal Subjects treatment.

It is expedient to use a dry spinning shaft which has a temperature gradient or a graduated temperature control, the temperature preferably being between 20 and 180 ° C., as seen from top to bottom. A gas containing water vapor is advantageously used as the drying gas. The precursor fibers can be dried before the thermal treatment or pyrolysis, in particular under atmospheric pressure and at temperatures of at least 60 ° C. It is advisable to dry the precursor fibers if they are not thermally treated immediately after the dry spinning process.

The optionally dried precursor fibers are then subjected to a thermal aftertreatment at temperatures up to about 1,400 ° C. The aftertreatment is expediently carried out in several stages, preferably in at least three stages. The first stage has e.g. a temperature of about 120 to 400, the second a temperature of about 400 to 1000 and the third a temperature of 1000 to about 1400 ° C. The temperature in the individual stages can rise continuously or even be graduated again.

It is advantageous to use 10-30% by weight of silica; it is expedient to use pyrogenic silica in the context of the invention, the primary particles of which have an average size of about 7-16 nm.

Hydrargillite is advantageously used as aluminum hydroxide.

SHEET Polyethylene oxide and polyvinyl alcohol are particularly suitable as spinning aids, in particular in a mixture with glycerin. The polyethylene oxide is expediently used with molecular weights of approximately 100,000 to 600,000.

The reaction of oxalic acid with aluminum hydroxide can advantageously be carried out in the presence of formic acid.

The invention further relates to a process for the production of the precursor fiber formed as an intermediate stage, which largely consists of aluminum oxalate and which also contains SiO 2 and possibly aluminum formate and other constituents. This process is characterized in that an aqueous aluminum oxalate solution is prepared from water, oxalic acid and aluminum hydroxide or oxide hydrate, the solution obtained is admixed up to 40%, based on the total amount of AI2O3 + SiO, pyrogenic silica and the solution is optionally added spinning from spinning aids to threads using the dry spinning process and drying them.

When carrying out the method according to the invention, a spinning solution is first prepared. In general, oxalic acid is dissolved in water with gentle heating; then aluminum hydroxide, e.g. Hydrargillite, preferably added in portions with stirring. Finally, formic acid is added if necessary and the whole is kept at the boiling point until the aluminum hydroxide is largely dissolved.

At least the equivalent amount of acid should be used in the reaction of the aluminum hydroxide with the acid, but an excess of acid is preferably used. Part of the oxalic acid can be substituted by a water-soluble strong to medium-strong carboxylic acid with a pk below about 4.7. Up to about 0.4 equivalents are preferably substituted by an acid such as, in particular, formic acid and / or also acetic acid, tartaric acid, lactic acid, malonic acid, etc. Commercially available products can be used as aluminum hydroxide, for example the product sold by the company Vereinigte Aluminum-Werke AG Chemicals Schwandorf under the trade name APYRAL 25. Aluminum hydroxide is an easily accessible product that is obtained, for example, from bauxite digestion.

It is advisable to filter the solution, as there are usually undissolved residues.

After filtering off undissolved matter, fumed silica, e.g. Aerosil 200, 380 or 130 added and finely divided by intensive stirring. After adding glycerin and possibly polyethylene glycol, the solution is concentrated in vacuo.

By concentrating the solution, a viscosity suitable for spinning is set. This can vary within relatively wide limits and is advantageously around 70-500 Pa.s, measured at 30 ° C. using a Haake Viscometer type Rotovisko RV 2 System SV II, measuring head DMK 500 at 8 revolutions per minute.

By concentrating the solutions, highly concentrated spinning solutions with water contents of e.g. only about 20% can be obtained. Suitable spinning solutions contain e.g. 39-46% oxalic acid, 23-29% aluminum hydroxide, 1.5-8% formic acid, 0.6-3% polyethylene oxide, 1-4% glycerin and 0.8 to 8% SiO 2

During the concentration, some of the formic acid that may be present also distils off.

The spinning solution is spun into a dry spinning shaft by means of suitable nozzles. Nitrogen is preferably used as the drying gas, which advantageously contains a proportion of water vapor which corresponds to 10 to 50% of the water vapor saturation pressure given at room temperature. The proportions of water vapor can, for example. by passing the nitrogen flow through water one or more times from room temperature before introducing it into the drying shaft.

The spinning conditions such as exit speed, warpage and take-off speed can also be varied within fairly wide limits. They depend partly on the concentration of the spinning solution, but also on the desired titer and others. from. The suitable spinning conditions can be set favorably with just a few preliminary tests. Suitable spinning process parameters are e.g. Exit speed 5-30 m / min, delay 2.7-80, take-off speed 80-400 m / min.

The silicic acid used according to the invention is pyrogenic silicic acid, which means highly disperse silicic acid which is obtained from the gas phase at high temperatures by coagulation. The flame and high-temperature hydrolysis processes are particularly worth mentioning here. Further references to pyrogenic silicas can be found e.g. B. in Ullmann's Encyclopedia of Chemical Engineering 4th Edition Volume 2 page 464-465, Verlag Chemie Weinheim.

Fumed silicas are commercially available. For example, very suitable the products sold by Degussa under the trade name Aerosil, with the types 200, 380 and 130 being particularly noteworthy. The silica particles should have the smallest possible particle size distribution. Particle sizes with a particle size diameter of 7-16 nm are particularly advantageous.

The drying shaft expediently has temperatures rising from top to bottom. A continuous temperature gradient can be given, but a graduated temperature curve is also possible. It is beneficial if the temperature is just below the spinneret and over part of the spinning shaft Room temperature and then rises to 180 ° C by the end of the shaft.

The freshly spun precursor threads can be processed further to aluminum oxide fibers immediately after leaving the dry spinning shaft, i.e. be thermally treated accordingly. The thermal aftertreatment essentially involves three processes, namely drying, removing the water that is still present, which may be in the form of solvent residues or water of hydration, thermolysis, whereby the oxalate and any other carboxylic acid salts of aluminum such as Decompose aluminum formate to alumina, and a sintering process. It goes without saying that excessive heating can lead to uncontrolled thermolysis gas development and uneven porous structures, which affects the strength of the threads obtained. It is therefore advisable to heat the threads slowly, e.g. from room temperature to 400 ° C within 2 hours, then about 2 hours at 400 ° C and then slowly to e.g. Heat up to 1,000 ° C sintering temperature.

In a particularly advantageous embodiment of the method according to the invention, the thermal aftertreatment, with the exception of sintering, is carried out using a so-called coil package.

For this purpose, the aluminum oxalate threads are wound onto sleeves during dry spinning, on which a film, e.g., is better removed before winding for better separation of the package of bobbins. made of plastic. After e.g. After a spinning time of one hour, the package (spun) can be detached from the take-up spool. The coil packages must be sufficiently mechanically stable for further processing. This is e.g. the case with about 200 g of spun yarn, a spool circumference of 48 cm and a package length of 14 - 15 cm.

Of course, the amount of yarn spooled can be considerably higher. In this way, coil packages with 2 kg and more can be set up. The coil packages are then fed to the drying process and thermolysis. The thermal aftertreatment can e.g. B. done in such a way by heating slowly, for example within two hours at 360 ° C and then two hours at 360 ° C; the aluminum oxalate threads are first dried and then decomposed into aluminum oxide threads. Further possibilities are, for example, heating from room temperature to 250 ° C. within 75 minutes, and further treatment at 250 ° C. for one hour, heating to 360 ° C. within 50 minutes and further treatment at 360 ° C. for one hour. Another exemplary temperature program looks as follows: in 40 min to 150 ° C / 2 h 150 ° C / in 35 min to 250 ° C / 30 mi 250 ° C / in 50 min to 360 ° C / 2 h 360 ° C .

The threads are then unwound from the inside of the package and sent to the sintering process.

However, the freshly spun threads can also be stored, e.g. on spools. To do this, however, they must be stored under drying conditions, as they are hygroscopic. Drying conditions means that the water vapor pressure in the environment is lower than in the fiber or, when the fiber is dried, does not allow water to be absorbed. Suitable conditions are e.g. 100 ° C at atmospheric pressure or 65 ° C at a pressure of 100 mbar.

The stored fibers can then be processed to aluminum oxide fibers at a later point in time or can also be used directly or can be modified chemically or in some other way.

The fibers obtained can be processed in stacks, but they are also equally suitable for use as filaments.

Both filament yarns and fiber yarns can be produced in this way.

The fibers which are obtained according to the invention are notable for good mechanical properties, and in particular for excellent temperature resistance. The mechanical data remain practically unchanged for 2 hours at 1200 ° C. Due to the high modulus of elasticity, the fibers are excellent as reinforcing fibers for plastics and especially metals such as Al alloys. The excellent insulation properties of molded parts made from these fibers are based on the low material density of 3.0 g / ml and the very small fiber diameters below 14 μm.

It was particularly surprising that it is possible according to the invention to produce inorganic fibers from easily accessible and inexpensive starting materials in a simple manner. It is not necessary to start from metallic aluminum or to use an expensive, cleaned and specially made aluminum oxalate, rather you can directly use aluminum hydroxide e.g. use in the form of the industrially produced hydrargillite, it is not necessary to isolate the oxalate beforehand. The process works very environmentally friendly, special

There are no corrosion problems. The spinnability of the

Spinning masses are excellent, the premature crystallization which is frequently observed when spinning inorganic spinning solutions does not take place, so that there are far fewer spinning problems than with the conventional processes.

High winding speeds of 300 m / min. are essential for the economics of the process. In this context, the comparatively low water content of the spinning mass of approx. 20% is also favorable. The amount of water to be evaporated in the dry spinning process is correspondingly small. The advantage is particularly noticeable if the number of nozzle holes is to be increased in order to expand the capacity. Too large amounts of water vapor to be evaporated and removed are a limiting factor.

The invention is illustrated by the following examples. example 1

Preparation of aluminoxalate starting solution, type A, formic acid content <5%

33.6 kg (C00H) ₂ .2H ₂ 0 (266.7 mol) are dissolved in 48.0 kg water at 70-80 ° C in a 100 1 stirred kettle. In a period of about 15 minutes, 14.6 kg of AI (0H) 3 (187.2 mol) are added, and then 4.8 kg of 99 percent. Formic acid (103.3 mol) and heated to boiling temperature in about 75 minutes. Under reflux conditions, the dissolving process is complete after 14 hours. After cooling to 50-60 ° C, a small amount of undissolved Al (OH) 3 is filtered off, grown with water and dried at 150 ° C and about 150 mbar. This amount (e.g. 329 g) is taken into account when calculating the concentrations.

Composition of the starting solution:

24.3% (C00H) ₂ , 14.4% AI verb, calculated as A1 (0H) ₃ , 4.9% HCOOH, 56.4%

Water (all percentages by weight).

Example 2

Spinning compound production, additive lactic acid

A 35 l stirred tank equipped with a pressure gauge, inside temperature measurement and distillation bridge is used. The viscosity increase is controlled by continuous measurement of the stirring motor power consumption.

A dispersion is prepared from 31.0 kg of starting solution prepared according to Example 1, 430 g of Aerosil 200 and 1200 g of lactic acid (racemate) with the most complete possible possible dissolution of silica agglomerates. Distillation is carried out after adding 360 g of glycerol 4! Water at 100 mbar, adds a solution of 120 g polyethylene oxide 100000 in 1080 g water as a spinning aid and degasses for 16 h at room temperature and 100 mbar. At an internal boiler temperature of 70 ° C concentrated to a total amount of distillate of 16 630 ml, the viscosity of the mass is 288 Pa-s (30 ° C).

Measuring conditions: Haake viscometer Rotovisko RV 2, system SV II, measuring head DMK 500, rotating body 8 rpm.

In the distillate, formic acid is determined acidimetrically to determine the mass balance. Taking this amount into account, the spinning mass (percentages by weight) contains:

42.9% (C00H) ₂

25.5% AI as A1 (0H) 3 calculated 2.5% Si0 ₂ 6.8% lactic acid 2.0% glycerin 0.7% polyethylene oxide 100000 1.8% formic acid 17.8% water

Example 3

Lactic acid spinning mass, dry spinning and thermal precursor aftertreatment

The spinning mass container is connected to the head of a 5 m long dry spinning shaft via a bottom drain. The mass is extruded through 30 100 µm holes at a Te p. of 45 ° C and a pressure of 85 ° C bar, the delivery rate of the spinning pump is 3.3 ml / min. Carrier gas for drying is 3 N ih nitrogen, which are metered into the spinning head. The carrier gas contains water vapor, the amount corresponds to 30% of the saturation pressure at 25 ° C. The upper half of the shaft is heated to 20 and the lower half to 180 ° C. At the foot of the shaft, precursor threads are run at 200 m / min. wound up, removed from the bobbins, heated to 400 ° C. within 2 h and thermolysed at this temperature for 2 h. The Weight loss is 75%, ie the fibers contain less than 5% org. Material. They are colored light brown, resistant to moist air, easy to handle and can be used for fire protection purposes due to their non-flammability.

To produce polycrystalline fibers, the above, still X-ray amorphous "stabilized precursor" is heated to 1000 ° C. in 2 hours and sintered at 1000 ° C. for 15 minutes. The following mechanical individual thread data are recorded on the colorless fibers obtained, consisting of 87% Al2O3 and 13% SiO2:

Aluminum oxalate starting solution, type B, formic acid content 5-10%

An oxalate solution is prepared from:

32.13 kg (255 mol) (C00H) ₂ -2H 0 16.58 kg (212.5 mol) AI (0H) ₃

9.38 kg (204 mol) HCOOH

48 kg of water

Composition of the starting solution obtained:

22.4% (C00H) ₂ , 15.0% Al verb, calculated as A1 (0H) ₃ , 9.1% HCOOH, 53.5% water (all percentages by weight).

REPLACEMENT BLADE Example 5

Spinning mass production, no acid addition

31.0 kg of starting solution from Example 4 are processed into the spinning mass according to the information in Example 2. The lactic acid addition is not necessary. The following are added: 457 g of Aerosil 200, 464 g of glycerol and 116 g of polyethylene oxide 100000 in the form of a 10 percent. aqueous solution. A total of 16,430 ml of water are distilled off.

The viscosity of the resulting clear Sprinnmasse is 221 Pa ^'s (30 ° C) (measurement conditions s. Example 2). The mass is resistant to crystallization for at least one week and consists of the following components (% by weight):

41.6% (C00H) ₂ , 28.0% A1 (0H) ₃ , 2.7% Si0 ₂ , 0.7% polyethylene oxide, 2.8% glycerin, 7.1% HCOOH, 17.1% water.

Example 6

Dry spinning and thermal precursor after-treatment

After the example! 3 described process, the spinning mass of Example 5 is spun. At the nozzle, the spinning mass temperature is 40 ° C, the pressure is 62 bar.3 N πμ / h nitrogen as the carrier gas are moistened with water vapor, the amount corresponds to 20% of the saturation pressure at 25 ° C. After two-stage thermolysis and sintering (example 3) at 200 m / min. Colorless threads with the following mechanical single-fiber data are obtained with the speed-wound precursors:

Claims

1.) Process for the production of aluminum oxide fibers, which contain up to about 40 wt .-% Siθ by using an aqueous

Produces aluminum oxalate solution, adds this silicon dioxide and possibly organic spinning aids, spins the resulting spinning solution to a precursor fiber by the dry spinning process and thermally aftertreated, characterized in that oxalic acid in aqueous medium with aluminum hydroxide or

Alumina hydrates converted to aluminum oxalate, the obtained

Solution pyrogenic silica is mixed, and then the spinning solution is spun to aluminum oxalate fibers after the dry spinning process and these are subjected to a thermal aftertreatment.

2.) Method according to claim 1, d.g. that the spinning solution is spun in a drying chamber with a temperature gradient.

3.) Method according to claim 2, d. G. that the temperature gradient, seen from top to bottom, is between 20 and 180 ° C in the drying chamber.

4.) Method according to one of claims 1-3, ckg. that a drying gas containing water vapor is used in the drying shaft.

5.) Method according to one of claims 1-4 that the aluminum oxalate fiber is subjected to thermal aftertreatment in at least three stages, namely drying, thermolysis and a sintering process.

6.) Method according to any one of claims 1-5, dg that 10-40 wt .-% pyrogenic silica is used.

7.) Method according to one of claims 1-6, d.g. that one carries out the reaction before oxalic acid with the aluminum hydroxide or hydrated oxide in the presence of formic acid.

8.) Method according to one of claims 1-7, d.g. that you use polyethylene oxide and glycerin as spinning aids.

9.) Method according to one of claims 1-8, d.g. that pyrogenic silica is used, the primary particles of which have an average size of 7-16 nm.

10.) Method according to one of claims 1-9, d.g. that hydrargillite is used as aluminum hydroxide.

11.) Process for producing an aluminum oxalate and SiO containing fiber (precursor fiber) analogous to one of the processes according to one of claims 1-4 and 6-10, d.g. that the fibers obtained after dry spinning are dried and, if necessary, kept under drying conditions.

12.) Method according to one or more of claims 1-11, characterized in that one carries out the drying and thermolysis of the aluminum oxalate threads in a package of bobbins.

13.) Method according to claim 12, characterized in that after the thermolysis, the threads are withdrawn from the inside of the coil package and fed to the sintering process.

REPLACEMENT LEAF