EP2475812B1 - Stabilisation of polyacrylonitrile precursor yarn - Google Patents

Stabilisation of polyacrylonitrile precursor yarn Download PDF

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Publication number
EP2475812B1
EP2475812B1 EP10749843.8A EP10749843A EP2475812B1 EP 2475812 B1 EP2475812 B1 EP 2475812B1 EP 10749843 A EP10749843 A EP 10749843A EP 2475812 B1 EP2475812 B1 EP 2475812B1
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EP
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Prior art keywords
application space
yarn
precursor yarn
electromagnetic waves
temperature
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EP10749843.8A
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German (de)
French (fr)
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EP2475812A1 (en
Inventor
Bernd Wohlmann
Michael WÖLKI
Christian Hunyar
Rudolf Emmerich
Mathias Kaiser
Matthias Graf
Lukas Alberts
Klaus-Dieter Nauenburg
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Teijin Carbon Europe GmbH
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Toho Tenax Europe GmbH
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    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M10/00—Physical treatment of fibres, threads, yarns, fabrics, or fibrous goods made from such materials, e.g. ultrasonic, corona discharge, irradiation, electric currents, or magnetic fields; Physical treatment combined with treatment with chemical compounds or elements
    • D06M10/04—Physical treatment combined with treatment with chemical compounds or elements
    • D—TEXTILES; PAPER
    • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
    • D01F—CHEMICAL FEATURES IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS; APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OF CARBON FILAMENTS
    • D01F9/00—Artificial filaments or the like of other substances; Manufacture thereof; Apparatus specially adapted for the manufacture of carbon filaments
    • D01F9/08—Artificial filaments or the like of other substances; Manufacture thereof; Apparatus specially adapted for the manufacture of carbon filaments of inorganic material
    • D01F9/12—Carbon filaments; Apparatus specially adapted for the manufacture thereof
    • D01F9/14—Carbon filaments; Apparatus specially adapted for the manufacture thereof by decomposition of organic filaments
    • D01F9/20—Carbon filaments; Apparatus specially adapted for the manufacture thereof by decomposition of organic filaments from polyaddition, polycondensation or polymerisation products
    • D01F9/21—Carbon filaments; Apparatus specially adapted for the manufacture thereof by decomposition of organic filaments from polyaddition, polycondensation or polymerisation products from macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds
    • D01F9/22—Carbon filaments; Apparatus specially adapted for the manufacture thereof by decomposition of organic filaments from polyaddition, polycondensation or polymerisation products from macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds from polyacrylonitriles
    • D01F9/225—Carbon filaments; Apparatus specially adapted for the manufacture thereof by decomposition of organic filaments from polyaddition, polycondensation or polymerisation products from macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds from polyacrylonitriles from stabilised polyacrylonitriles
    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M10/00—Physical treatment of fibres, threads, yarns, fabrics, or fibrous goods made from such materials, e.g. ultrasonic, corona discharge, irradiation, electric currents, or magnetic fields; Physical treatment combined with treatment with chemical compounds or elements
    • D06M10/04—Physical treatment combined with treatment with chemical compounds or elements
    • D06M10/06—Inorganic compounds or elements
    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M2101/00—Chemical constitution of the fibres, threads, yarns, fabrics or fibrous goods made from such materials, to be treated
    • D06M2101/16—Synthetic fibres, other than mineral fibres
    • D06M2101/18—Synthetic fibres consisting of macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds
    • D06M2101/26—Polymers or copolymers of unsaturated carboxylic acids or derivatives thereof
    • D06M2101/28—Acrylonitrile; Methacrylonitrile

Definitions

  • the invention relates to a method for stabilizing polyacrylonitrile yarns.
  • Stabilized polyacrylonitrile multifilament yarns are needed in the production of carbon fibers.
  • Today's carbon fibers are predominantly made of polyacrylonitrile fibers, i. made of polyacrylonitrile precursor yarns.
  • the polyacrylonitrile precursor yarns are first subjected to stabilization by an oxidation treatment before the stabilized precursor yarns are subsequently carbonized at temperatures of at least 1200 ° C. in a nitrogen atmosphere and, if appropriate, graphitized in a further step at temperatures up to about 2800 ° C., thus carbon fibers to obtain.
  • the stabilization of polyacrylonitrile precursor yarns is generally understood to mean the conversion of the yarns via chemical stabilization reactions, in particular via cyclization reactions and dehydrogenation reactions, from a thermoplastic state into an oxidized, infusible and simultaneously flame-resistant state.
  • the stabilization is carried out today usually in conventional convection ovens at temperatures between 200 and 300 ° C and under an oxygen-containing atmosphere (see, eg F. Fourné: "Synthetic Fibers", Carl Hanser Verlag Kunststoff Vienna 1995, chapter 5.7 ).
  • a stepwise conversion of the Precursorgarns of a thermoplastic into an oxidized, infusible fiber instead of ( J.-B.
  • the yarn passes through differently tempered oven stages, by means of which a slow heating of the yarn can be set and thus sufficient removal of the exothermic heat from the yarn material can be achieved.
  • the stabilization can be carried out, for example, in a conventional convection oven with three oven stages, wherein in the first stage at temperatures in the range of 200 to 300 ° C usually a residence time of at least 20 minutes is required to perform the stabilization to the extent that the density of the precursor yarn is increased by about 0.03 g / cm 3 .
  • the precursor yarn based on a polyacrylonitrile polymer submitted in the process according to the invention is a yarn which contains at least 85% of polymerized acrylonitrile.
  • the polyacrylonitrile polymer may also contain portions of comonomers, such as e.g. of vinyl acetate, methyl acrylate, methyl methacrylate, vinyl chloride, vinylidene chloride, styrene or itaconic acid (ester).
  • thermoplastic polyacrylonitrile precursor yarn prepared may be a yarn which has not yet been subjected to any stabilization.
  • the precursor yarn presented can also be a polyacrylonitrile yarn which has already been subjected to partial stabilization, in which case the stabilization proceeds further in the process according to the invention.
  • the method according to the invention is not limited to stabilizing the precursor yarn completely by the method according to the invention, but it can also be carried out so that the yarn is only stabilized to a certain degree.
  • the method according to the invention is therefore suitable for partially or completely stabilizing an untreated polyacrylonitrile precursor yarn.
  • the inventive method comprises the further partial or complete stabilization of an already partially stabilized Precursorgarns. In this case, the previous partial stabilization and / or a subsequent completion of the stabilization can also be done under Use of the method according to the invention or by known methods in conventional convection ovens.
  • the applicator When carrying out the process according to the invention, e.g. generated in a magnetron high-frequency electromagnetic waves, which are guided via suitable means, preferably via a waveguide or a coaxial into the application space.
  • the applicator has a generally channel-shaped application space with a wall of a conductive material, which is traversed by the precursor yarn to be stabilized and into which the electromagnetic waves are fed.
  • the wall surrounding the application space can be, for example, a continuous metal wall. However, it is also possible to form the wall of a conductive grid-shaped material.
  • the application space preferably has a circular, oval or rectangular contour transversely to the feedthrough direction of the precursor yarn and thus transversely to the propagation direction of the electromagnetic waves.
  • the applicator is a rectangular waveguide.
  • the application space furthermore comprises, in its interior surrounded by the wall, a conductive element, which is preferably a metal rod.
  • a conductive element extends coaxially to the longitudinal axis of the application space, i. in the propagation direction of the electromagnetic waves, whereby a coaxial conductor is formed.
  • the conductive element is arranged in the center of the application space. In such coaxial conductors, it is advantageous if the application space has a circular contour transversely to the propagation direction of the electromagnetic waves.
  • the application space can at its inlet end, at which the Precursorgarn enters the applicator and / or at its outlet end, from which the Precursor yarn leaves the applicator, have diaphragms through which the precursor yarn is passed. These diaphragms hold the high-frequency electromagnetic waves in the application space.
  • the applicator e.g. a tube which is connected via an elbow with the application space, wherein the precursor yarn to be stabilized is guided in the region of the elbow by the wall into the application space.
  • the maximum electric field strength of the high-frequency electromagnetic waves in the application space is set to a level in the range from 3 to 150 kV / m.
  • the level of the field strength refers to the unladen state of the applicator, ie. to a state where the precursor yarn to be stabilized does not pass through the applicator.
  • microwaves High-frequency electromagnetic waves of a frequency of 300 MHz to 300 GHz, which are generally referred to as microwaves, are preferred for carrying out the method according to the invention. Particularly preferred are microwaves in the range of 300 MHz to 45 GHz and in a particular embodiment microwaves in the range of 900 MHz to 5.8 GHz. By default, microwaves are used with a frequency of 915 MHz and 2.45 GHz, which are suitable for carrying out the invention Method are best suited.
  • the process gas may be an inert gas, for example nitrogen, argon or helium.
  • nitrogen is used as the inert gas.
  • the process gas used in the process according to the invention is an oxygen-containing gas. It has been shown that can be achieved in the stabilization by means of an oxygen-containing gas higher carbon yields.
  • an oxygen-containing gas is understood as meaning a gas which contains molecular oxygen, the concentration of molecular oxygen in the oxygen-containing gas preferably being less than 80% by volume. Most preferably, the oxygen-containing gas is air.
  • the critical minimum temperature T crit is the temperature above which the high-frequency electromagnetic waves couple sufficiently into the precursor yarn passing through the application device, ie above which the electromagnetic waves are sufficiently absorbed by the yarn and the conversion reactions take place. It has been shown that the atmosphere surrounding the precursor yarn in the application space and thus the precursor yarn passing through the application space itself must exceed a certain threshold temperature, ie the critical minimum temperature, so that the high-frequency electromagnetic waves couple so strongly into the precursor yarn that the conversion reactions or chemical stabilization reactions, ie in particular cyclization reactions, dehydrogenation reactions and oxidation reactions can proceed to stabilize the yarn.
  • the high-frequency electromagnetic waves may already be coupled into the yarn below the critical minimum temperature, the coupled-in electromagnetic waves do not yet result in a temperature increase in the yarn sufficient for initiating the conversion reactions, since at the same time cooling takes place due to the process gas flowing relative to the yarn the yarn is done.
  • the critical minimum temperature T crit can be determined in a simple way for the precursor yarn guided in each case by the application device. As stated, above the critical minimum temperature, the electromagnetic waves are sufficiently absorbed by the precursor yarn and, as a result of the resulting temperature increase in the yarn, initiates the conversion reactions leading to the stabilization of the yarn. As a result, among other HCN gas is released.
  • the HCN gas can be measured by conventional analytical methods such as gas chromatography, mass spectrometry or electrochemical HCN sensors in the gas outlet, via which the process gas introduced into the applicator is removed from the applicator.
  • the minimum temperature is understood to be that temperature above which the electromagnetic waves couple so strongly or are absorbed so strongly by the yarn that the conversion reactions in the yarn, ie in particular the cyclization reaction, take place and as a result HCN gas is released becomes.
  • the conversion reactions may take place on the basis of the HCN cleavage associated cyclization can be detected by IR spectroscopy.
  • the maximum temperature T max is to be understood as meaning the temperature which is 20 ° C. below the decomposition temperature of the precursor yarn introduced into the application device.
  • the process gas in the application space has a temperature in the range between (T crit + 20 ° C) and (T max - 20 ° C).
  • the decomposition temperature can be easily determined by thermogravimetric measurements. In this case, the decomposition temperature is that temperature at which a sample of the precursor yarn presented in the method according to the invention loses 5% of its mass within a time of less than 5 minutes.
  • the respective critical minimum temperature T crit and the maximum temperature T max is dependent on the precursor material, ie, for example, of concrete polyacrylonitrile polymer.
  • the polyacrylonitrile Precursorgarne commonly used for the purpose of carbon fiber production can be used in the inventive method.
  • the critical minimum temperature and the maximum temperature can be influenced by additives which may be added to the polyacrylonitrile.
  • the precursor yarn may contain additives which bring about an improvement in the absorption capacity of the precursor yarn with respect to high-frequency electromagnetic waves.
  • these additives are polyethylene glycol, carbon black or carbon nanotubes.
  • the critical minimum temperature and the maximum temperature are also dependent on the degree of stabilization of the Precursorgarns submitted in the process according to the invention. This shows that with increasing degree of stabilization, the critical minimum temperature shifts to higher values. It is likewise evident that increasing stabilization has the effect of increasing thermal stability and, as a result, increasing decomposition temperatures, and thus also of increasing maximum temperatures in the context of the present invention.
  • the adjustment of the temperature of the process gas flowing through the application space can be effected for example by supplying a gas heated to the required temperature into a heat-insulated application space.
  • a process gas which is initially tempered to a lower temperature level may be present in the application space or in a heat exchanger upstream of the application space, e.g. be heated by means of suitable heating elements or by means of IR radiation to the required temperature.
  • a combination of different methods is possible to set the required temperature of the process gas in the application room.
  • the density of, for example, originally about 1.19 g / cm 3 by the stabilization ultimately increases to a value of up to about 1.40 g / cm 3 .
  • the degree of stabilization can thus also be determined on the basis of the density of the precursor material.
  • the process gas fed into the application space has, on the one hand, the task of ensuring a temperature level at the yarn, at which a sufficient coupling of the high-frequency electromagnetic waves into the yarn takes place.
  • the task of the process gas is to transport the released during the conversion reactions volatile degradation products such as HCN, NH 3 or H 2 O, on the other hand but also the resulting heat of reaction and to provide a temperature level in particular in the range Precursorgarns, the is below the maximum temperature T max .
  • an oxygen-containing gas is used as the process gas, this gas finally also has the task of providing the required amount of oxygen for the conversion or oxidation reactions leading to stabilization in the precursor yarn.
  • the process gas is guided through the application space such that it has a flow velocity of at least 0.1 m / s relative to the precursor yarn passing through the application space.
  • the flow rate is above 0.1 m / s relative to the Precursorgarn set so that the aforementioned requirements are met.
  • limits are set up to the extent that too high a flow velocity of the gas lead to instabilities in the yarn path of the precursor yarn and thus there is a risk of yarn breakage or tearing of the yarn.
  • the process gas is introduced into the application space and discharged therefrom so that it flows through the application space perpendicular to the precursor yarn, wherein the flow velocity is perpendicular to the precursor yarn in the range of 0.1 to 2 m / s.
  • the process gas is introduced into the application space and removed therefrom so that the process gas parallel to the Precursorgarn the application space or in countercurrent to the transport direction of Precursorgarns with a related to the free cross section of the application space average flow velocity of 0.1 to 20 m / s flows through relative to the application space passing through the precursor yarn.
  • the flow rate is particularly preferably in the range between 0.5 and 5 m / s.
  • the precursor yarn in the applicator is kept under a defined tension.
  • the precursor yarn is preferably passed through the applicator under a thread tension in the range from 0.125 to 5 cN / tex. Particularly preferred is a thread tension in the range of 0.5 to 3.5 cN / tex.
  • the residence time of the precursor yarn in the application space is at least 20 s.
  • An upper limit of the residence time results from e.g. the desired degree of stabilization, which is to be achieved after passing through the yarn by the applicator or from device-technical boundary conditions, for example with regard to the displayable length of the applicator.
  • the precursor yarn is passed through one after the other through a plurality of application devices, ie, through at least two application devices arranged one behind the other.
  • each of these application devices can be equipped with their own means for generating a field of high-frequency electromagnetic waves, but it is also possible that all the application devices have a common microwave generator, for example.
  • the series connection of several application devices offers the advantage that in each of the application devices taking into account, for example, the current degree of stabilization of the respective application device passing precursor yarn independent adjustment with respect to the optimal process parameters can be done, such as in terms of field strength, temperature, the flow velocity of the process gas, the oxygen content of the optionally used oxygen-containing gas, the residence time, the yarn tension, etc ..
  • the frequency is e.g.
  • the microwaves are technically determined by the availability of low-cost high-performance sources to specific areas.
  • the field distribution in the application space is determined by its geometry and by the frequency and the power of the supplied electromagnetic waves. In the application space, this results in the expression of field maxima, the distance of which is determined inter alia by the geometry of the application space.
  • the precursor yarn to be stabilized in the application space passes through the stationary field maxima in a rhythm predetermined by the yarn speed.
  • a pronounced heating or heating of the yarn takes place in the region of the maxima and cooling takes place in the region of the minima due to the process gas flowing in the fiber.
  • this can lead to the stabilization process gets into an unstable area.
  • the high intensity of the coupled-in electromagnetic waves can lead to a large extent to the described exothermic conversion reactions, which in turn lead to an increase in temperature in the yarn material.
  • This in turn results in an improved coupling of the electromagnetic waves and thus an intensification of the exothermic reactions, combined with a further increase in the temperature in the yarn.
  • the heat generated by the inflowing process gas can only be dissipated to a limited extent, so that the stabilization process becomes unstable. A stabilization of the process can be achieved in such cases, for example via a temporal change in field strength.
  • the field strength in the application space has a periodically varying intensity over time, the period being determined primarily by the yarn speed and by the distance of the stationary field maxima.
  • the intensity changes sinusoidally or in the form of pulses, wherein in the case of a pulsed change in intensity, the field strength can change, for example, between two non-zero levels or between zero and a non-zero level.
  • the polyacrylonitrile precursor yarn 8 to be stabilized is withdrawn from a bobbin 9, introduced into the applicator 2 after looping around a deflection roller 10 via an aperture 11 in the elbow 6 and passed through the application space 3.
  • the precursor yarn 8 treated in the applicator 2 leaves the application device 1 via an elbow 13 connected to the outlet end 12 of the applicator 2 through an aperture 14.
  • the yarn tension of the precursor yarn can be adjusted by the drive speeds of the deflection rollers 10, 15.
  • the process gas required in the process of the invention is introduced into the application space 3 and passes through the application space 3 in the illustrated case in direct current to Precursorgarn 8 via a attached to the elbow 13 outlet nozzle 19, the process gas together with the volatile degradation products due to in the application space 3 in the yarn 8 occurring conversion reactions have arisen, discharged from the applicator 2.
  • the elbow 13 at the outlet end 12 of the applicator 2 is connected in the illustrated case with a pipe section 20 which is closed at its free end with a metal plate 21. This ensures that the electromagnetic waves are reflected back into the application space 3.
  • An untreated polyacrylonitrile precursor yarn was prepared as it is suitable for the production of carbon fibers, the precursor yarn having 12,000 filaments with a filament diameter of about 8 ⁇ m.
  • the density of the precursor yarn was 1.18 g / cm 3 .
  • the application device used for microwave treatment corresponded in structure to the in FIG. 1 illustrated device.
  • a microwave generator microwaves having a wavelength of 2.45 GHz were generated and led via a rectangular waveguide connected to the microwave generator via an elbow in the application space (R 26 rectangular waveguide), which had a length of 120 cm.
  • R 26 rectangular waveguide In the rectangular waveguide hot air at a temperature of 190 ° C was supplied via a side-mounted nozzle and passed in direct current to the precursor yarn through the application space, the volume flow was so dimensioned that resulted in the application space, an average flow rate of 2 m / s.
  • the application space was tempered by heating elements arranged in the wall to a temperature of 170 ° C., so that an air temperature of 170 ° C. prevailed in the application space. In the application room, a maximum electric field strength of 30 kV / m was set.
  • the polyacrylonitrile precursor org was introduced into the application device and passed through the applicator continuously at a speed of 30 m / h and under a thread tension of 0.9 cN / tex. In the area of an elbow connected to the outlet of the applicator, the yarn was led out of the application device.
  • Example 2 The same application device as in Example 1 was used.
  • the process parameters were also the same as in Example 1.
  • a polyacrylonitrile Precursorgam was submitted, which had already been subjected in a conventional process in a convection oven of a partial stabilization.
  • the yarn presented in this example had a density of 1.19 g / cm 3 and had a yellow color.
  • the density of the yarn was increased to 1.20 g / cm 3 and the yarn had turned a dark brown color.
  • Example 2 The same application device was used as in Example 1, except that the applicator, unlike Example 1, had a length of 1.0 m.
  • a partially stabilized yarn was introduced, which had a density of 1.21 g / cm 3 and a dark brown to black color due to the partial stabilization.
  • the temperature of the supplied hot air and the temperature of the arranged in the wall of the applicator heating elements was set to 170 ° C, so that the hot air in the application room also had a temperature of 170 ° C.
  • the yarn speed was 10 m / h, the yarn tension 1.25 cN / tex.
  • a pulsating microwave field was set by switching the magnetron on / off, in which the maximum electric field strength was 25 kV / m (15 s) and zero kV / m (6 s).
  • the color of the yarn leaving the application device had changed in the direction of a black color.
  • the density had increased to 1.24 g / cm 3 .
  • Example 1 An application device was used as in Example 1, whereby the same process parameters as in Example 1 were set.
  • the yarn was used, which was also used in Example 1. Notwithstanding Example 1, however, the yarn was treated several times in succession in the application device by being passed a total of three times through the application device. The partially stabilized precursor yarn of the previous pass through the application device served as a template for the following pass.
  • the total residence time in the application device was about 7.5 min.
  • the precursor yarn thus treated three times had a density of 1.22 g / m 3 .
  • the originally white precursor yarn had a dark brown to black color after treatment.
  • Example 3 The procedure was as in Example 3, but the maximum electric field strength was set to a constant value of 30 kV / m.
  • yarn there was a partially stabilized polyacrylonitrile Precursorgarn having a density of 1.26 g / cm 3.
  • the treated yarn After passing through the application device, ie after a residence time of 6 min at a Yarn speed of 10 m / h, the treated yarn had a density of 1.40 g / cm 3 .
  • Example 2 In a conventional multi-stage convection oven for stabilizing polyacrylonitrile precursor yarns for the production of carbon fibers, stabilization was carried out on an unstabilized precursor yarn as presented in Example 1. Air was passed through the convection oven. In the first stage of the furnace, a temperature of about 230 ° C was set.
  • the partially stabilized precursor yarn After a residence time of 23 minutes, the partially stabilized precursor yarn left the first furnace stage.
  • the partially stabilized precursor yarn had a dark brown to black color and a density of 1.21 g / cm 3 .

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  • Engineering & Computer Science (AREA)
  • Textile Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Inorganic Fibers (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
  • Treatments Of Macromolecular Shaped Articles (AREA)

Description

Die Erfindung betrifft ein Verfahren zur Stabilisierung von Garnen aus Polyacrylnitril.The invention relates to a method for stabilizing polyacrylonitrile yarns.

Stabilisierte Multifilamentgarne aus Polyacrylnitril werden bei der Herstellung von Kohlenstofffasern benötigt. Heutige Kohlenstofffasern werden zum überwiegenden Teil aus Polyacrylnitrilfasern, d.h. aus Polyacrylnitril-Precursorgarnen hergestellt. Dabei werden die Polyacrylnitril-Precursorgarne zunächst einer Stabilisierung durch eine Oxidationsbehandlung unterzogen, bevor die stabilisierten Precursorgarne nachfolgend bei Temperaturen von mindestens 1200 °C in Stickstoffatmosphäre karbonisiert und gegebenenfalls in einem weiteren Schritt bei Temperaturen bis ca. 2800 °C graphitisiert werden, um so daraus Kohlenstofffasern zu erhalten.Stabilized polyacrylonitrile multifilament yarns are needed in the production of carbon fibers. Today's carbon fibers are predominantly made of polyacrylonitrile fibers, i. made of polyacrylonitrile precursor yarns. The polyacrylonitrile precursor yarns are first subjected to stabilization by an oxidation treatment before the stabilized precursor yarns are subsequently carbonized at temperatures of at least 1200 ° C. in a nitrogen atmosphere and, if appropriate, graphitized in a further step at temperatures up to about 2800 ° C., thus carbon fibers to obtain.

Als Stabilisierung von Polyacrylnitril-Precursorgarnen wird allgemein die Umwandlung der Garne über chemische Stabilisierungsreaktionen, insbesondere über Zyklisierungsreaktionen und Dehydrierungsreaktionen, von einem thermoplastischen Zustand in einen oxidierten, unschmelzbaren und gleichzeitig flammfesten Zustand verstanden. Die Stabilisierung erfolgt heute in der Regel in konventionellen Konvektionsöfen bei Temperaturen zwischen 200 und 300 °C und unter einer sauerstoffhaltigen Atmosphäre (siehe z.B. F. Fourné: "Synthetische Fasern", Carl Hanser Verlag München Wien 1995, Kapitel 5.7 ). Dabei findet über eine exotherme Reaktion eine schrittweise Umwandlung des Precursorgarns von einer thermoplastischen in eine oxidierte, unschmelzbare Faser statt ( J.-B. Donnet, R. C. Bansal: "Carbon Fibers", Marcel Dekker, Inc., New York and Basel 1984, Seiten 14-23 ). Visuell ist die Umwandlung anhand einer charakteristischen Verfärbung des zunächst weißen Garns über gelb nach braun und schließlich schwarz zu erkennen. Die Stabilisierung kann dabei auch in mehreren Schritten bzw. Stufen erfolgen, bei denen zunehmende Stabilisierungsgrade erreicht werden. Mit zunehmender Stabilisierung nimmt auch die Dichte des Garns zu, beispielsweise von 1,19 g/cm3 auf 1,40 g/cm3, wobei die Änderungen in der Dichte mit zunehmender Stabilisierung ausgeprägter werden.The stabilization of polyacrylonitrile precursor yarns is generally understood to mean the conversion of the yarns via chemical stabilization reactions, in particular via cyclization reactions and dehydrogenation reactions, from a thermoplastic state into an oxidized, infusible and simultaneously flame-resistant state. The stabilization is carried out today usually in conventional convection ovens at temperatures between 200 and 300 ° C and under an oxygen-containing atmosphere (see, eg F. Fourné: "Synthetic Fibers", Carl Hanser Verlag Munich Vienna 1995, chapter 5.7 ). In this case, via an exothermic reaction, a stepwise conversion of the Precursorgarns of a thermoplastic into an oxidized, infusible fiber instead of ( J.-B. Donnet, RC Bansal: "Carbon Fibers," Marcel Dekker, Inc., New York and Basel 1984, pages 14-23 ). Visually, the transformation is recognizable by a characteristic discoloration of the initially white yarn from yellow to brown and finally black. The stabilization can also take place in several steps or stages, in which increasing degrees of stabilization are achieved. With increasing stabilization, the density of the yarn also increases, for example, from 1.19 g / cm 3 to 1.40 g / cm 3 , the changes in density becoming more pronounced with increasing stabilization.

Bei den exothermen chemischen Reaktionen zur Umwandlung bzw. Stabilisierung des Polyacrylnitril-Precursors kann so viel Wärme entstehen, dass es zu einem Schmelzen oder einer thermischen Zersetzung des Garns kommt. Im konventionellen Stabilisierungsprozess durchläuft das Garn daher verschieden temperierte Ofenstufen, worüber eine langsame Erwärmung des Garns eingestellt und so eine ausreichende Abführung der exothermen Wärme aus dem Garnmaterial erreicht werden kann. So kann die Stabilisierung beispielsweise in einem konventionellen Konvektionsofen mit drei Ofenstufen erfolgen, wobei in der ersten Stufe bei Temperaturen im Bereich von 200 bis 300°C in der Regel eine Verweilzeit von mindestens 20 min erforderlich ist, um die Stabilisierung soweit auszuführen, dass die Dichte des Precursorgarns um ca. 0,03 g/cm3 erhöht wird. Ähnliche Verweilzeiten werden auch in den übrigen Ofenstufen benötigt, so dass im konventionellen Prozess insgesamt eine Verweilzeit von mindestens etwa einer Stunde für die Stabilisierung erforderlich ist. Die Stabilisierung erfordert gleichzeitig vergleichsweise langsame Prozessgeschwindigkeiten, wodurch bei der kontinuierlichen Herstellung von Kohlenstofffasern die Stabilisierung zum geschwindigkeitsbestimmenden Prozessschritt wird. Gleichzeitig sind wegen der niedrigen Prozessgeschwindigkeiten und wegen der notwendigen langen Verweilzeiten, die je nach Prozessführung auch bis zu ca. 2,5 Std. betragen können, große Stabilisierungsöfen erforderlich. Daher besteht der Wunsch nach Verfahren zur Stabilisierung von Polyacrylnitril-Precursorgarnen, die kürzere Verweilzeiten und/oder höhere Prozessgeschwindigkeiten erlauben.In the exothermic chemical reactions for the conversion or stabilization of the polyacrylonitrile precursor so much heat can arise that it comes to a melting or thermal decomposition of the yarn. In the conventional stabilization process, the yarn therefore passes through differently tempered oven stages, by means of which a slow heating of the yarn can be set and thus sufficient removal of the exothermic heat from the yarn material can be achieved. Thus, the stabilization can be carried out, for example, in a conventional convection oven with three oven stages, wherein in the first stage at temperatures in the range of 200 to 300 ° C usually a residence time of at least 20 minutes is required to perform the stabilization to the extent that the density of the precursor yarn is increased by about 0.03 g / cm 3 . Similar residence times are also required in the other furnace stages, so that a total retention time of at least about one hour is required for the stabilization in the conventional process. At the same time, the stabilization requires comparatively slow process speeds, which means that stabilization becomes the rate-determining process step in the continuous production of carbon fibers. At the same time, large stabilization furnaces are required due to the low process speeds and the necessary long residence times, which can be up to approx. 2.5 hours depending on the process control. Therefore, there is a desire for Process for stabilizing polyacrylonitrile precursor yarns which allow shorter residence times and / or higher process speeds.

Es ist daher Aufgabe der vorliegenden Erfindung, ein Verfahren zur Stabilisierung von Garnen aus Polyacrylnitril zur Verfügung zu stellen, bei dem die Nachteile der Verfahren des Stands der Technik zumindest reduziert sind und das die Stabilisierung von Polyacrylnitril-Precursorgarnen zur Herstellung von Kohlenstofffasern bei höheren Prozessgeschwindigkeiten und/oder geringeren Verweilzeiten erlaubt.It is therefore an object of the present invention to provide a process for the stabilization of polyacrylonitrile yarns, in which the disadvantages of the prior art processes are at least reduced and stabilizing polyacrylonitrile precursor yarns for producing carbon fibers at higher process speeds and / or lower residence times allowed.

Die erfindungsgemäße Aufgabe wird durch ein Verfahren zur Stabilisierung von Garnen aus Polyacrylnitril durch chemische Stabilisierungsreaktionen gelöst, welches die folgenden Schritte umfasst:

  • Vorlegen eines Precursorgarns auf Basis eines Polyacrylnitrilpolymers,
  • Bereitstellen einer Applikationsvorrichtung zur Behandlung des Precursorgarns mit hochfrequenten elektromagnetischen Wellen, umfassend einen Applikator mit einem Applikationsraum, Mittel zur Erzeugung der hochfrequenten elektromagnetischen Wellen sowie Mittel zur Einspeisung der hochfrequenten elektromagnetischen Wellen in den Applikationsraum,
  • Erzeugen eines Feldes der hochfrequenten elektromagnetischen Wellen im Applikationsraum, welches Bereiche mit minimaler elektrischer Feldstärke und Bereiche mit maximaler elektrischer Feldstärke aufweist und Einstellen der maximalen elektrischen Feldstärke im Applikationsraum im Bereich von 3 bis 150 kV/m,
  • kontinuierliches Einführen des Precusorgarns in und Hindurchführen des Precursorgarns durch den Applikationsraum und durch das Feld der hochfrequenten elektromagnetischen Wellen, dabei
  • Einleiten eines Prozessgases in den Applikationsraum und Hindurchleiten des Prozessgases durch den Applikationsraum mit einer Strömungsgeschwindigkeit relativ zu dem den Applikationsraum durchlaufenden Precursorgarn von mindestens 0,1 m/s, wobei die Temperatur des Prozessgases so im Bereich zwischen 150 und 300°C eingestellt wird, dass sie oberhalb der kritischen Minimaltemperatur Tkrit und unterhalb der Maximaltemperatur Tmax liegt und wobei die kritische Minimaltemperatur Tkrit diejenige Temperatur ist, oberhalb derer die hochfrequenten elektromagnetischen Wellen in das den Applikationsraum durchlaufende Precursorgarn einkoppeln und die chemischen Stabilisierungsreaktionen ablaufen, und die Maximaltemperatur Tmax diejenige Temperatur, die um 20°C unterhalb der Zersetzungstemperatur des in den Applikationsraum eingeführten Precursorgarns liegt.
The object according to the invention is achieved by a process for stabilizing polyacrylonitrile yarns by chemical stabilization reactions, which comprises the following steps:
  • Presentation of a precursor yarn based on a polyacrylonitrile polymer,
  • Providing an application device for treating the precursor yarn with high-frequency electromagnetic waves, comprising an applicator with an application space, means for generating the high-frequency electromagnetic waves and means for feeding the high-frequency electromagnetic waves into the application space,
  • Generating a field of the high-frequency electromagnetic waves in the application space, which has areas with minimum electric field strength and areas with maximum electric field strength and setting the maximum electric field strength in the application space in the range of 3 to 150 kV / m,
  • continuously introducing the precursor yarn into and passing the precursor yarn through the application space and through the field of the high-frequency electromagnetic waves, thereby
  • Introducing a process gas into the application space and passing the process gas through the application space at a flow velocity relative to the precursor yarn passing through the application space of at least 0.1 m / s, the temperature of the process gas being in the range is set between 150 and 300 ° C that it is above the critical minimum temperature T crit and below the maximum temperature T max and wherein the critical minimum temperature T crit is the temperature above which couple the high-frequency electromagnetic waves in the application space passing through precursor yarn and the run chemical stabilization reactions, and the maximum temperature T max that temperature which is 20 ° C below the decomposition temperature of the introduced into the application space Precursorgarns.

Im Rahmen der vorliegenden Erfindung handelt es sich bei dem im erfindungsgemäßen Verfahren vorgelegten Precursorgarn auf Basis eines Polyacrylnitrilpolymeren um ein Garn, welches mindestens 85 % polymerisiertes Acrylnitril enthält. Das Polyacrylnitrilpolymer kann auch Anteile von Comonomeren wie z.B. von Vinylacetat, Acrylsäuremethylester, Methacrylsäuremethylester, Vinylchlorid, Vinylidenchlorid, Styrol oder Itaconsäure (-ester) enthalten.In the context of the present invention, the precursor yarn based on a polyacrylonitrile polymer submitted in the process according to the invention is a yarn which contains at least 85% of polymerized acrylonitrile. The polyacrylonitrile polymer may also contain portions of comonomers, such as e.g. of vinyl acetate, methyl acrylate, methyl methacrylate, vinyl chloride, vinylidene chloride, styrene or itaconic acid (ester).

Das vorgelegte thermoplastische Polyacrylnitril-Precursorgarn kann ein Garn sein, das noch keinerlei Stabilisierung unterworfen wurde. Bei dem vorgelegten Precursorgarn kann es sich jedoch auch um ein Polyacrylnitrilgarn handeln, das bereits einer teilweisen Stabilisierung unterzogen wurde, wobei dann im erfindungsgemäßen Verfahren die Stabilisierung weiter fortschreitet. Andererseits ist das erfindungsgemäße Verfahren nicht darauf beschränkt, dass das vorgelegte Precursorgarn vollständig mittels des erfindungsgemäßen Verfahrens stabilisiert wird, sondern es kann auch so durchgeführt werden, dass das Garn nur bis zu einem bestimmten Grad stabilisiert wird. Das erfindungsgemäße Verfahren ist also geeignet, ein unbehandeltes Precursorgarn aus Polyacrylnitril teilweise oder auch vollständig zu stabilisieren. Ebenso umfasst das erfindungsgemäße Verfahren die weitere teilweise oder die vollständige Stabilisierung eines bereits teilweise stabilisierten Precursorgarns. Dabei kann die vorherige Teilstabilisierung und/oder eine nachgeschaltete Vervollständigung der Stabilisierung ebenfalls unter Anwendung des erfindungsgemäßen Verfahrens erfolgen oder auch nach bekannten Verfahren in konventionellen Konvektionsöfen.The thermoplastic polyacrylonitrile precursor yarn prepared may be a yarn which has not yet been subjected to any stabilization. However, the precursor yarn presented can also be a polyacrylonitrile yarn which has already been subjected to partial stabilization, in which case the stabilization proceeds further in the process according to the invention. On the other hand, the method according to the invention is not limited to stabilizing the precursor yarn completely by the method according to the invention, but it can also be carried out so that the yarn is only stabilized to a certain degree. The method according to the invention is therefore suitable for partially or completely stabilizing an untreated polyacrylonitrile precursor yarn. Likewise, the inventive method comprises the further partial or complete stabilization of an already partially stabilized Precursorgarns. In this case, the previous partial stabilization and / or a subsequent completion of the stabilization can also be done under Use of the method according to the invention or by known methods in conventional convection ovens.

Bei Durchführung des erfindungsgemäßen Verfahrens werden z.B. in einem Magnetron hochfrequente elektromagnetische Wellen erzeugt, die über geeignete Mittel, vorzugsweise über einen Hohlleiter oder einen Koaxialleiter in den Applikationsraum geführt werden. Der Applikator weist einen in der Regel kanalförmigen Applikationsraum mit einer Wandung aus einem leitfähigen Material auf, der von dem zu stabilisierenden Precursorgarn durchlaufen wird und in den die elektromagnetischen Wellen eingespeist werden. Die den Applikationsraum umgebende Wandung kann beispielsweise eine durchgehende Metallwandung sein. Es ist jedoch auch möglich, die Wandung aus einem leitfähigen gitterförmigen Material auszubilden. Vorzugsweise besitzt der Applikationsraum quer zur Durchführungsrichtung des Precursorgarns und damit quer zur Ausbreitungssrichtung der elektromagnetischen Wellen eine kreisförmige, ovale oder rechteckige Kontur. In einer besonders bevorzugten Ausführungsform handelt es sich bei dem Applikator um einen Rechteckhohlleiter.When carrying out the process according to the invention, e.g. generated in a magnetron high-frequency electromagnetic waves, which are guided via suitable means, preferably via a waveguide or a coaxial into the application space. The applicator has a generally channel-shaped application space with a wall of a conductive material, which is traversed by the precursor yarn to be stabilized and into which the electromagnetic waves are fed. The wall surrounding the application space can be, for example, a continuous metal wall. However, it is also possible to form the wall of a conductive grid-shaped material. The application space preferably has a circular, oval or rectangular contour transversely to the feedthrough direction of the precursor yarn and thus transversely to the propagation direction of the electromagnetic waves. In a particularly preferred embodiment, the applicator is a rectangular waveguide.

In einer ebenfalls bevorzugten Ausführungsform umfasst der Applikationsraum des Weiteren in seinem von der Wandung umgebenen Innenraum ein leitfähiges Element, bei dem es sich vorzugsweise um einen Metallstab handelt. Hierbei ist von Vorteil, wenn sich das leitfähige Element koaxial zur Längsachse des Applikationsraums erstreckt, d.h. in Ausbreitungssrichtung der elektromagnetischen Wellen, wodurch ein Koaxialleiter ausgebildet wird. Besonders bevorzugt ist das leitfähige Element dabei im Zentrum des Applikationsraums angeordnet. Bei derartigen Koaxialleitern ist es von Vorteil, wenn der Applikationsraum quer zur Ausbreitungssrichtung der elektromagnetischen Wellen eine kreisförmige Kontur aufweist.In a likewise preferred embodiment, the application space furthermore comprises, in its interior surrounded by the wall, a conductive element, which is preferably a metal rod. It is advantageous if the conductive element extends coaxially to the longitudinal axis of the application space, i. in the propagation direction of the electromagnetic waves, whereby a coaxial conductor is formed. Particularly preferably, the conductive element is arranged in the center of the application space. In such coaxial conductors, it is advantageous if the application space has a circular contour transversely to the propagation direction of the electromagnetic waves.

Der Applikationsraum kann an seinem Eintrittsende, an dem das Precursorgarn in den Applikator eintritt und/oder an seinem Austrittsende, aus dem das Precursorgarn den Applikator verlässt, Blenden aufweisen, durch die das Precursorgarn hindurchgeführt wird. Durch diese Blenden werden die hochfrequenten elektromagnetischen Wellen im Applikationsraum gehalten.The application space can at its inlet end, at which the Precursorgarn enters the applicator and / or at its outlet end, from which the Precursor yarn leaves the applicator, have diaphragms through which the precursor yarn is passed. These diaphragms hold the high-frequency electromagnetic waves in the application space.

Der Wellenleiter, über den die hochfrequenten elektromagnetischen Wellen von z.B. einem Magnetron in den Applikator geführt werden, kann z.B. ein Rohr sein, das über ein Kniestück mit dem Applikationsraum verbunden ist, wobei das zu stabilisierende Precursorgarn im Bereich des Kniestücks durch dessen Wand in den Applikationsraum geführt wird.The waveguide through which the high frequency electromagnetic waves of e.g. a magnetron can be guided into the applicator, e.g. a tube which is connected via an elbow with the application space, wherein the precursor yarn to be stabilized is guided in the region of the elbow by the wall into the application space.

Im Applikator, d.h. im Applikationsraum bilden die eingespeisten hochfrequenten elektromagnetischen Wellen eine durch die Geometrie des Applikationsraums definierte Feldstruktur mit Wellenmaxima und Wellenminima, d.h. mit Bereichen maximaler elektrischer Feldstärke und Bereichen minimaler elektrischer Feldstärke aus. Erfindungsgemäß wird im Applikationsraum die maximale elektrische Feldstärke der hochfrequenten elektromagnetischen Wellen auf ein Niveau im Bereich von 3 bis 150 kV/m eingestellt. Das Niveau der Feldstärke bezieht sich dabei auf den unbeschickten Zustand des Applikators, d:h. auf einen Zustand, bei dem das zu stabilisierende Precursorgarn den Applikator nicht durchläuft. In Versuchen hat es sich im Hinblick auf die im Precursorgarn bei der Stabilisierung ablaufenden Umwandlungsreaktionen als günstig herausgestellt, wenn im Applikationsraum eine maximale elektrische Feldstärke der hochfrequenten elektromagnetischen Wellen im Bereich von 5 bis 50 kV/m erzeugt wird. Hierbei zeigte sich gleichzeitig, dass bei Precursorgarnen, die bereits zu einem Teil stabilisiert sind, Feldstärken im oberen Bereich eingestellt werden können, wohingegen bei Garnen, die noch nicht einer (Teil-) Stabilisierung unterzogen wurden, eher geringere Feldstärken einzustellen sind, um zu heftige exotherme Umwandlungsreaktionen zu vermeiden, die zu einer Zerstörung des Precursorgarns führen können.In the applicator, ie in the application space, the high-frequency electromagnetic waves fed in form a field structure defined by the geometry of the application space with wave maxima and wave minima, ie with areas of maximum electric field strength and areas of minimal electric field strength. According to the invention, the maximum electric field strength of the high-frequency electromagnetic waves in the application space is set to a level in the range from 3 to 150 kV / m. The level of the field strength refers to the unladen state of the applicator, ie. to a state where the precursor yarn to be stabilized does not pass through the applicator. In experiments, it has turned out to be favorable with regard to the conversion reactions taking place in the precursor yarn during the stabilization when a maximum electric field strength of the high-frequency electromagnetic waves in the range of 5 to 50 kV / m is generated in the application space. At the same time it was shown that with precursor yarns, which are already partially stabilized, field strengths can be adjusted in the upper range, whereas yarns that have not undergone (partial) stabilization tend to set lower field strengths to violent To avoid exothermic conversion reactions that can lead to destruction of Precursorgarns.

Für die Durchführung des erfindungsgemäßen Verfahrens sind hochfrequente elektromagnetische Wellen einer Frequenz von 300 MHz bis 300 GHz bevorzugt, die allgemein als Mikrowellen bezeichnet werden. Besonders bevorzugt sind Mikrowellen im Bereich von 300 MHz bis 45 GHz und in einer besonderen Ausführungsform Mikrowellen im Bereich von 900 MHz bis 5,8 GHz Standardmäßig werden Mikrowellen mit einer Frequenz von 915 MHz und 2,45 GHz verwendet, die für die Durchführung des erfindungsgemäßen Verfahrens bestens geeignet sind.High-frequency electromagnetic waves of a frequency of 300 MHz to 300 GHz, which are generally referred to as microwaves, are preferred for carrying out the method according to the invention. Particularly preferred are microwaves in the range of 300 MHz to 45 GHz and in a particular embodiment microwaves in the range of 900 MHz to 5.8 GHz. By default, microwaves are used with a frequency of 915 MHz and 2.45 GHz, which are suitable for carrying out the invention Method are best suited.

Wesentlich bei der Durchführung des erfindungsgemäßen Verfahrens ist, dass ein Prozessgas in den Applikationsraum eingeführt wird und diesen durchströmt und dass die Temperatur des Prozessgases im Applikationsraum so im Bereich zwischen 150 und 300°C eingestellt wird, dass sie oberhalb der kritischen Minimaltemperatur Tkrit und unterhalb der Maximaltemperatur Tmax liegt. Bei dem Prozessgas kann es sich in einer Ausführungsform des erfindungsgemäßen Verfahrens um ein inertes Gas handeln, beispielsweise um Stickstoff, Argon oder Helium. Vorzugsweise wird Stickstoff als inertes Gas verwendet. In einer weiteren bevorzugten Ausführungsform handelt es sich bei dem im erfindungsgemäßen Verfahren eingesetzten Prozessgas um ein sauerstoffhaltiges Gas. Es hat sich gezeigt, dass sich bei der Stabilisierung mittels eines sauerstoffhaltigen Gases höhere Kohlenstoffausbeuten erzielen lassen. Dabei wird unter einem sauerstoffhaltigen Gas ein Gas verstanden, das molekularen Sauerstoff enthält, wobei die Konzentration des molekularen Sauerstoffs im sauerstoffhaltigen Gas bevorzugt weniger als 80 Vol.-% beträgt. Ganz besonders bevorzugt handelt es sich bei dem sauerstoffhaltigen Gas um Luft.Essential in carrying out the method according to the invention is that a process gas is introduced into the application space and flows through it and that the temperature of the process gas in the application space is set in the range between 150 and 300 ° C, above the critical minimum temperature T crit and below the maximum temperature T max is. In one embodiment of the process according to the invention, the process gas may be an inert gas, for example nitrogen, argon or helium. Preferably, nitrogen is used as the inert gas. In a further preferred embodiment, the process gas used in the process according to the invention is an oxygen-containing gas. It has been shown that can be achieved in the stabilization by means of an oxygen-containing gas higher carbon yields. In this case, an oxygen-containing gas is understood as meaning a gas which contains molecular oxygen, the concentration of molecular oxygen in the oxygen-containing gas preferably being less than 80% by volume. Most preferably, the oxygen-containing gas is air.

Im Zusammenhang mit der vorliegenden Erfindung ist unter der kritischen Minimaltemperatur Tkrit diejenige Temperatur zu verstehen, oberhalb derer die hochfrequenten elektromagnetischen Wellen in das die Applikationsvorrichtung durchlaufenden Precursorgarn in genügendem Maße einkoppeln, d.h. oberhalb derer die elektromagnetischen Wellen in genügendem Maße vom Garn absorbiert werden, und die Umwandlungsreaktionen stattfinden. Es hat sich nämlich gezeigt, dass die das Precursorgarn im Applikationsraum umgebende Atmosphäre und damit das den Applikationsraum durchlaufende Precursorgarn selbst eine bestimmte Schwellentemperatur, d.h. die kritische Minimaltemperatur überschreiten muss, damit die hochfrequenten elektromagnetischen Wellen in das Precursorgarn so stark einkoppeln, dass die Umwandlungsreaktionen bzw. chemischen Stabilisierungsreaktionen, d.h. insbesondere Zyklisierungsreaktionen, Dehhydrierungsreaktionen und Oxidationsreaktionen zur Stabilisierung des Garns ablaufen können. Unterhalb der kritischen Minimaltemperatur kann es zwar bereits zu einem Einkoppeln der hochfrequenten elektromagnetischen Wellen in das Garn kommen, jedoch führen die eingekoppelten elektromagnetischen Wellen noch nicht zu einer zur Initiierung der Umwandlungsreaktionen ausreichenden Temperaturerhöhung im Garn, da gleichzeitig durch das relativ zum Garn strömende Prozessgas eine Abkühlung des Garn erfolgt.In the context of the present invention, the critical minimum temperature T crit is the temperature above which the high-frequency electromagnetic waves couple sufficiently into the precursor yarn passing through the application device, ie above which the electromagnetic waves are sufficiently absorbed by the yarn and the conversion reactions take place. It has been shown that the atmosphere surrounding the precursor yarn in the application space and thus the precursor yarn passing through the application space itself must exceed a certain threshold temperature, ie the critical minimum temperature, so that the high-frequency electromagnetic waves couple so strongly into the precursor yarn that the conversion reactions or chemical stabilization reactions, ie in particular cyclization reactions, dehydrogenation reactions and oxidation reactions can proceed to stabilize the yarn. Although the high-frequency electromagnetic waves may already be coupled into the yarn below the critical minimum temperature, the coupled-in electromagnetic waves do not yet result in a temperature increase in the yarn sufficient for initiating the conversion reactions, since at the same time cooling takes place due to the process gas flowing relative to the yarn the yarn is done.

Die kritische Minimaltemperatur Tkrit kann dabei auf einfachem Wege für das jeweils durch die Applikationsvorrichtung geführte Precursorgarn ermittelt werden. Wie ausgeführt werden oberhalb der kritischen Minimaltemperatur die elektromagnetischen Wellen vom Precursorgarn in ausreichendem Maße absorbiert und infolge der daraus resultierenden Temperaturerhöhung im Garn die zur Stabilisierung des Garns führenden Umwandlungsreaktionen initiiert. Hierdurch wird unter anderem HCN-Gas frei. Das HCN-Gas kann mittels üblicher Analysenmethoden wie z.B. über Gaschromatographie, Massenspektroskopie oder mittels elektrochemischer HCN-Sensoren im Gasaustritt gemessen werden, über den das in den Applikator eingeführte Prozessgas aus dem Applikator abgeführt wird. Unter der minimalen Temperatur wird also im Rahmen der vorliegenden Erfindung diejenige Temperatur verstanden, oberhalb derer die elektromagnetischen Wellen so stark einkoppeln bzw. so stark vom Garn absorbiert werden, dass die Umwandlungsreaktionen im Garn, d.h. insbesondere die Zyklisierungsreaktion, stattfinden und infolgedessen HCN-Gas frei wird. Alternativ kann das Stattfinden der Umwandlungsreaktionen anhand der mit der HCN-Abspaltung einhergehenden Zyklisierung mittels IR-Spektroskopie detektiert werden.The critical minimum temperature T crit can be determined in a simple way for the precursor yarn guided in each case by the application device. As stated, above the critical minimum temperature, the electromagnetic waves are sufficiently absorbed by the precursor yarn and, as a result of the resulting temperature increase in the yarn, initiates the conversion reactions leading to the stabilization of the yarn. As a result, among other HCN gas is released. The HCN gas can be measured by conventional analytical methods such as gas chromatography, mass spectrometry or electrochemical HCN sensors in the gas outlet, via which the process gas introduced into the applicator is removed from the applicator. In the context of the present invention, the minimum temperature is understood to be that temperature above which the electromagnetic waves couple so strongly or are absorbed so strongly by the yarn that the conversion reactions in the yarn, ie in particular the cyclization reaction, take place and as a result HCN gas is released becomes. Alternatively, the conversion reactions may take place on the basis of the HCN cleavage associated cyclization can be detected by IR spectroscopy.

Unter der Maximaltemperatur Tmax ist im Rahmen der vorliegenden Erfindung diejenige Temperatur zu verstehen, die um 20°C unterhalb der Zersetzungstemperatur des in die Applikationsvorrichtung eingeführten Precursorgarns liegt. Für eine sichere kontinuierliche Prozessführung ist es erforderlich, dass die maximal im Applikationsraum vorherrschenden Temperaturen weit genug unterhalb der Zersetzungstemperatur des in die Applikationsvorrichtung eingeführten Garns liegen. Höhere Temperaturen würden zu einer Erhöhung der Gefahr des Zersetzens des Garns und des Fadenabrisses und damit zu einer Unterbrechung des Prozesses führen. In einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens weist das Prozessgas im Applikationsraum eine Temperatur im Bereich zwischen (Tkrit + 20°C) und (Tmax - 20°C) auf. Die Zersetzungstemperatur kann auf einfache Weise über thermogravimetrische Messungen ermittelt werden. Dabei ist die Zersetzungstemperatur diejenige Temperatur, bei der eine Probe des im erfindungsgemäßen Verfahrens vorgelegten Precursorgarns innerhalb einer Zeit von weniger als 5 Minuten 5 % seiner Masse verliert.In the context of the present invention, the maximum temperature T max is to be understood as meaning the temperature which is 20 ° C. below the decomposition temperature of the precursor yarn introduced into the application device. For a reliable continuous process control, it is necessary that the maximum temperatures prevailing in the application space are far enough below the decomposition temperature of the yarn introduced into the application device. Higher temperatures would increase the risk of yarn disintegration and thread breakage, thereby interrupting the process. In a preferred embodiment of the method according to the invention, the process gas in the application space has a temperature in the range between (T crit + 20 ° C) and (T max - 20 ° C). The decomposition temperature can be easily determined by thermogravimetric measurements. In this case, the decomposition temperature is that temperature at which a sample of the precursor yarn presented in the method according to the invention loses 5% of its mass within a time of less than 5 minutes.

Die jeweilige kritische Minimaltemperatur Tkrit sowie die Maximaltemperatur Tmax ist abhängig vom Precursormaterial, d.h. beispielsweise von konkreten Polyacrylnitrilpolymer. Dabei können bei dem erfindungsgemäßen Verfahren die üblicherweise für Zwecke der Kohlenstofffaserherstellung verwendeten Polyacrylnitril-Precursorgarne eingesetzt werden. Die kritische Minimaltemperatur sowie die Maximaltemperatur kann darüber hinaus durch gegebenenfalls dem Polyacrylnitril zugesetzte Additive beeinflusst werden. So kann das Precursorgarn in einer vorteilhaften Ausgestaltung Additive enthalten, die eine Verbesserung der Absorptionsfähigkeit des Precursorgarns gegenüber hochfrequenten elektromagnetischen Wellen bewirken. Besonders bevorzugt handelt es sich bei diesen Additiven um Polyethylenglykol, Ruß oder Kohlenstoffnanoröhrchen.The respective critical minimum temperature T crit and the maximum temperature T max is dependent on the precursor material, ie, for example, of concrete polyacrylonitrile polymer. In this case, the polyacrylonitrile Precursorgarne commonly used for the purpose of carbon fiber production can be used in the inventive method. In addition, the critical minimum temperature and the maximum temperature can be influenced by additives which may be added to the polyacrylonitrile. Thus, in an advantageous embodiment, the precursor yarn may contain additives which bring about an improvement in the absorption capacity of the precursor yarn with respect to high-frequency electromagnetic waves. With particular preference these additives are polyethylene glycol, carbon black or carbon nanotubes.

Die kritische Minimaltemperatur sowie die Maximaltemperatur sind darüber hinaus auch abhängig vom Stabilisierungsgrad des im erfindungsgemäßen Verfahren vorgelegten Precursorgarns. So zeigt sich, dass sich mit zunehmendem Stabilisierungsgrad die kritische Minimaltemperatur zu höheren Werten hin verschiebt. Ebenso zeigt sich, dass eine zunehmende Stabilisierung sich in Richtung einer zunehmenden thermischen Stabilität und daraus resultierend in zunehmenden Zersetzungstemperaturen auswirkt und damit auch in ansteigenden Maximaltemperaturen im Sinne der vorliegenden Erfindung.The critical minimum temperature and the maximum temperature are also dependent on the degree of stabilization of the Precursorgarns submitted in the process according to the invention. This shows that with increasing degree of stabilization, the critical minimum temperature shifts to higher values. It is likewise evident that increasing stabilization has the effect of increasing thermal stability and, as a result, increasing decomposition temperatures, and thus also of increasing maximum temperatures in the context of the present invention.

Die Einstellung der Temperatur des den Applikationsraum durchströmenden Prozessgases kann beispielsweise durch Zuführung eines auf die erforderliche Temperatur erhitzten Gases in einen wärmeisolierten Applikationsraum erfolgen. Ebenso kann ein zunächst auf ein niedrigeres Temperaturniveau temperiertes Prozessgas im Applikationsraum oder in einem dem Applikationsraum vorgeschalteten Wärmetauscher z.B. mittels geeigneter Heizelemente oder mittels IR-Strahlung auf die erforderliche Temperatur aufgeheizt werden. Natürlich ist auch eine Kombination verschiedener Methoden möglich, um die erforderliche Temperatur des Prozessgases im Applikationsraum einzustellen.The adjustment of the temperature of the process gas flowing through the application space can be effected for example by supplying a gas heated to the required temperature into a heat-insulated application space. Likewise, a process gas which is initially tempered to a lower temperature level may be present in the application space or in a heat exchanger upstream of the application space, e.g. be heated by means of suitable heating elements or by means of IR radiation to the required temperature. Of course, a combination of different methods is possible to set the required temperature of the process gas in the application room.

Bei der Stabilisierung von Precursorgarnen aus Polyacrylnitril finden Umwandlungsreaktionen wie z.B. Zyklisierungsreaktionen oder Dehydrierungsreaktionen statt, bei denen eine Umwandlung der Garne von einem thermoplastischen in letztlich ein thermisch vernetztes Garn und damit in einen unschmelzbaren und gleichzeitig flammfesten Zustand erfolgt. Dabei findet die bereits zuvor beschriebene charakteristische Verfärbung des Garns statt. Die ablaufenden Umwandlungsreaktionen zeigen eine stark exotherme Wärmetönung, und es kommt infolge der Stabilisierung zu einem Schrumpfen des Garns sowie zu einem Gewichtsverlust des Garns, verbunden mit der Bildung flüchtiger Abbauprodukte wie z.B. HCN, NH3 oder H2O. Gleichzeitig findet eine Erhöhung der Dichte des Precursorgarns statt. So ist z.B. für einen Precursor auf Basis eines Polyacrylnitrilpolymers festzustellen, dass die Dichte von beispielsweise ursprünglich ca. 1,19 g/cm3 durch die Stabilisierung letztlich auf einen Wert von bis zu ca. 1,40 g/cm3 ansteigt. Der Grad der Stabilisierung lässt sich somit auch anhand der Dichte des Precursormaterials bestimmen.In the stabilization of polyacrylonitrile precursor yarns, conversion reactions such as cyclization reactions or dehydrogenation reactions take place in which the yarns are converted from a thermoplastic into a finally thermally crosslinked yarn and thus into an infusible and simultaneously flame-resistant state. In this case, the already described characteristic discoloration of the yarn takes place. The ongoing conversion reactions show a highly exothermic heat of reaction and, as a result of stabilization, yarn shrinkage and weight loss of the yarn are associated with the formation of volatile degradation products such as HCN, NH 3 or H 2 O. At the same time there is an increase in density of the precursor yarn. For example, it is based on a precursor of a polyacrylonitrile polymer found that the density of, for example, originally about 1.19 g / cm 3 by the stabilization ultimately increases to a value of up to about 1.40 g / cm 3 . The degree of stabilization can thus also be determined on the basis of the density of the precursor material.

Im erfindungsgemäßen Verfahren hat das in den Applikationsraum eingespeiste Prozessgas zum einen die Aufgabe, am Garn ein Temperaturniveau zu gewährleisten, bei dem eine ausreichende Einkopplung der hochfrequenten elektromagnetischen Wellen in das Garn erfolgt. Darüber hinaus kommt dem Prozessgas die Aufgabe zu, zum einen die bei den Umwandlungsreaktionen freiwerdenden flüchtigen Abbauprodukte wie z.B. HCN, NH3 oder H2O, zum anderen aber auch die entstehende Reaktionswärme abzutransportieren und für ein Temperaturniveau insbesondere im Bereich des Precursorgarns zu sorgen, das unterhalb der Maximaltemperatur Tmax liegt. Im bevorzugten Fall, dass als Prozessgas ein sauerstoffhaltiges Gas eingesetzt wird, hat dieses Gas schließlich auch die Aufgabe, für die zur Stabilisierung führenden Umwandlungs- bzw. Oxidationsreaktionen im Precursorgarn die erforderliche Menge an Sauerstoff zur Verfügung zu stellen. Daher wird im erfindungsgemäßen Verfahren das Prozessgas so durch den Applikationsraum geführt, dass es relativ zu dem den Applikationsraum durchlaufenden Precursorgarn eine Strömungsgeschwindigkeit von mindestens 0,1 m/s aufweist. Die Strömungsgeschwindigkeit ist dabei oberhalb von 0,1 m/s relativ zum Precursorgarn so einzustellen, dass die zuvor genannten Anforderungen erfüllt werden. Andererseits sind hinsichtlich der Strömungsgeschwindigkeit insoweit nach oben hin Grenzen gesetzt, da eine zu hohe Strömungsgeschwindigkeit des Gases zu Instabilitäten im Fadenlauf des Precursorgarns führen und damit die Gefahr von Fadenbrüchen bzw. des Abreißen des Garns besteht.In the method according to the invention, the process gas fed into the application space has, on the one hand, the task of ensuring a temperature level at the yarn, at which a sufficient coupling of the high-frequency electromagnetic waves into the yarn takes place. In addition, the task of the process gas is to transport the released during the conversion reactions volatile degradation products such as HCN, NH 3 or H 2 O, on the other hand but also the resulting heat of reaction and to provide a temperature level in particular in the range Precursorgarns, the is below the maximum temperature T max . In the preferred case that an oxygen-containing gas is used as the process gas, this gas finally also has the task of providing the required amount of oxygen for the conversion or oxidation reactions leading to stabilization in the precursor yarn. Therefore, in the method according to the invention, the process gas is guided through the application space such that it has a flow velocity of at least 0.1 m / s relative to the precursor yarn passing through the application space. The flow rate is above 0.1 m / s relative to the Precursorgarn set so that the aforementioned requirements are met. On the other hand, with regard to the flow rate, limits are set up to the extent that too high a flow velocity of the gas lead to instabilities in the yarn path of the precursor yarn and thus there is a risk of yarn breakage or tearing of the yarn.

In einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens wird das Prozessgas so in den Applikationsraum eingeführt und aus diesem abgeführt, dass es den Applikationsraum senkrecht zum Precursorgarn durchströmt, wobei die Strömungsgeschwindigkeit senkrecht zum Precursorgarn im Bereich von 0,1 bis 2 m/s liegt. In einer weiteren bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens wird das Prozessgas so in den Applikationsraum eingeführt und aus diesem abgeführt, dass das Prozessgas den Applikationsraum parallel zum Precursorgarn im Gleichstrom oder im Gegenstrom zur Transportrichtung des Precursorgarns mit einer auf den freien Querschnitt des Applikationsraums bezogenen mittleren Strömungsgeschwindigkeit von 0,1 bis 20 m/s relativ zu dem den Applikationsraum durchlaufenden Precursorgarn durchströmt. Besonders bevorzugt liegt die Strömungsgeschwindigkeit im Bereich zwischen 0,5 und 5 m/s.In a preferred embodiment of the method according to the invention, the process gas is introduced into the application space and discharged therefrom so that it flows through the application space perpendicular to the precursor yarn, wherein the flow velocity is perpendicular to the precursor yarn in the range of 0.1 to 2 m / s. In a further preferred embodiment of the method according to the invention, the process gas is introduced into the application space and removed therefrom so that the process gas parallel to the Precursorgarn the application space or in countercurrent to the transport direction of Precursorgarns with a related to the free cross section of the application space average flow velocity of 0.1 to 20 m / s flows through relative to the application space passing through the precursor yarn. The flow rate is particularly preferably in the range between 0.5 and 5 m / s.

Um dem bei der Stabilisierung auftretenden Schrumpf entgegenzuwirken und um eine Orientierung der Polyacrylnitrilmoleküle zu erhalten bzw. zu erreichen, ist es erforderlich, dass das Precursorgarn im Applikator unter einer definierten Spannung gehalten wird. Vorzugsweise wird das Precursorgarn unter einer Fadenspannung im Bereich von 0,125 bis 5 cN/tex durch den Applikator geführt. Besonders bevorzugt ist eine Fadenspannung im Bereich von 0,5 bis 3,5 cN/tex.In order to counteract the shrinkage occurring in the stabilization and to obtain or achieve an orientation of the polyacrylonitrile molecules, it is necessary that the precursor yarn in the applicator is kept under a defined tension. The precursor yarn is preferably passed through the applicator under a thread tension in the range from 0.125 to 5 cN / tex. Particularly preferred is a thread tension in the range of 0.5 to 3.5 cN / tex.

Um einerseits eine ausreichende Stabilisierung bzw. Teilstabilisierung zu erreichen, andererseits aber Prozessbedingungen hinsichtlich z.B. der Feldstärke im Applikationsraum, der Temperatur des Prozessgases oder dessen Strömungsgeschwindigkeit einstellen zu können, die einen stabilen Fadenlauf und einen stabilen Prozess ermöglichen, beträgt die Verweilzeit des Precursorgarns im Applikationsraum mindestens 20 s. Eine Obergrenze der Verweilzeit resultiert dabei aus z.B. dem gewünschten Stabilisierungsgrad, der nach Durchlaufen des Garns durch den Applikator erreicht werden soll oder auch aus gerätetechnischen Randbedingungen etwa hinsichtlich der darstellbaren Länge des Applikators.On the one hand to achieve a sufficient stabilization or partial stabilization, but on the other hand process conditions with respect to e.g. The field strength in the application space to adjust the temperature of the process gas or its flow rate, which allow a stable yarn path and a stable process, the residence time of the precursor yarn in the application space is at least 20 s. An upper limit of the residence time results from e.g. the desired degree of stabilization, which is to be achieved after passing through the yarn by the applicator or from device-technical boundary conditions, for example with regard to the displayable length of the applicator.

Um genügend lange Verweilzeiten zum Erreichen hoher Stabilisierungsgrade zu realisieren, besteht zum einen die Möglichkeit, einen einzelnen Applikator entsprechen lang auszuführen. In einer bevorzugten Ausgestaltung des erfindungsgemäßen Verfahrens wird das Precursorgarn nacheinander durch mehrere, d.h. durch mindestens zwei hintereinander angeordnete Applikationsvorrichtungen kontinuierlich hindurchgeführt. Dabei kann jede dieser Applikationsvorrichtungen mit eigenen Mitteln zur Erzeugung eines Feldes hochfrequenter elektromagnetischer Wellen ausgestattet sein, es ist jedoch auch möglich, dass alle Applikationsvorrichtungen z.B. einen gemeinsamen Mikrowellengenerator aufweisen. Generell bietet die Hintereinanderschaltung mehrerer Applikationsvorrichtungen den Vorteil, dass in jeder der Applikationsvorrichtungen unter Berücksichtigung z.B. des aktuellen Stabilisierungsgrades des die jeweilige Applikationsvorrichtung durchlaufenden Precursorgarns eine unabhängige Anpassung hinsichtlich der optimalen Prozessparameter erfolgen kann, wie z.B. hinsichtlich der Feldstärke, der Temperatur, der Strömungsgeschwindigkeit des Prozessgases, des Sauerstoffanteils des gegebenenfalls eingesetzten sauerstoffhaltigen Gases, der Verweilzeit, der Fadenspannung usw..In order to realize sufficiently long residence times for achieving high degrees of stabilization, on the one hand, it is possible to design a single applicator to be long enough. In a preferred embodiment of According to the method of the invention, the precursor yarn is passed through one after the other through a plurality of application devices, ie, through at least two application devices arranged one behind the other. In this case, each of these application devices can be equipped with their own means for generating a field of high-frequency electromagnetic waves, but it is also possible that all the application devices have a common microwave generator, for example. In general, the series connection of several application devices offers the advantage that in each of the application devices taking into account, for example, the current degree of stabilization of the respective application device passing precursor yarn independent adjustment with respect to the optimal process parameters can be done, such as in terms of field strength, temperature, the flow velocity of the process gas, the oxygen content of the optionally used oxygen-containing gas, the residence time, the yarn tension, etc ..

In der Anwendung ist die Frequenz z.B. der Mikrowellen technisch durch die Verfügbarkeit günstiger leistungsstarker Quellen auf bestimmte Bereiche festgelegt. Gleichzeitig ist die Feldverteilung im Applikationsraum durch dessen Geometrie und durch die Frequenz und die Leistung der eingespeisten elektromagnetischen Wellen bestimmt. Hierbei kommt es im Applikationsraum zur Ausprägung von Feldmaxima, deren Abstand unter anderem von der Geometrie des Applikationsraums bestimmt ist.In the application, the frequency is e.g. The microwaves are technically determined by the availability of low-cost high-performance sources to specific areas. At the same time, the field distribution in the application space is determined by its geometry and by the frequency and the power of the supplied electromagnetic waves. In the application space, this results in the expression of field maxima, the distance of which is determined inter alia by the geometry of the application space.

In einem kontinuierlichen Prozess mit ausreichenden Verweilzeiten im Applikationsraum durchläuft das zu stabilisierende Precursorgarn im Applikationsraum in einem durch die Garngeschwindigkeit vorgegebenen Rhythmus die stehenden Feldmaxima. Dabei findet je nach mittlerer Feldstärke und Temperatur des Prozessgases im Bereich der Maxima eine ausgeprägte Erwärmung bzw. Erhitzung des Garns statt und im Bereich der Minima durch das die Faser anströmende Prozessgas eine Abkühlung. Bei relativ niedrigen Fasergeschwindigkeiten und insbesondere bei Precursorgarnen, an denen noch keine oder nur in geringem Maße eine Stabilisierung erfolgt ist, kann dies dazu führen, dass der Stabilisierungsprozess in einen instabilen Bereich gerät. Auf der einen Seite kann es im Bereich der Maxima durch die hohe Intensität der eingekoppelten elektromagnetischen Wellen in starkem Maße zu den beschriebenen exotherm ablaufenden Umwandlungsreaktionen kommen, die ihrerseits zu einer Temperaturerhöhung im Garnmaterial führen. Hieraus resultiert wiederum eine verbesserte Einkopplung der elektromagnetischen Wellen und damit eine Intensivierung der exothermen Reaktionen, verbunden mit einer weiteren Erhöhung der Temperatur im Garn. Auf der anderen Seite kann über das anströmende Prozessgas entstehende Wärme nur in begrenztem Umfang abgeführt werden, so dass der Stabilisierungsprozess instabil wird. Eine Stabilisierung des Prozesses kann in solchen Fällen beispielsweise über eine zeitliche Veränderung der Feldstärke erreicht werden.In a continuous process with sufficient residence times in the application space, the precursor yarn to be stabilized in the application space passes through the stationary field maxima in a rhythm predetermined by the yarn speed. Depending on the mean field strength and temperature of the process gas, a pronounced heating or heating of the yarn takes place in the region of the maxima and cooling takes place in the region of the minima due to the process gas flowing in the fiber. At relatively low Fiber speeds and especially in Precursorgarnen on which no or only a small degree of stabilization has occurred, this can lead to the stabilization process gets into an unstable area. On the one hand, in the region of the maxima, the high intensity of the coupled-in electromagnetic waves can lead to a large extent to the described exothermic conversion reactions, which in turn lead to an increase in temperature in the yarn material. This in turn results in an improved coupling of the electromagnetic waves and thus an intensification of the exothermic reactions, combined with a further increase in the temperature in the yarn. On the other hand, the heat generated by the inflowing process gas can only be dissipated to a limited extent, so that the stabilization process becomes unstable. A stabilization of the process can be achieved in such cases, for example via a temporal change in field strength.

In einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens weist daher die Feldstärke im Applikationsraum eine sich über der Zeit periodisch verändernde Intensität auf, wobei die Periodendauer vornehmlich durch die Garngeschwindigkeit und durch den Abstand der stehenden Feldmaxima bestimmt ist. Besonders bevorzugt ändert sich die Intensität sinusförmig oder in Form von Pulsen, wobei bei einer gepulsten Intensitätsänderung die Feldstärke sich beispielsweise zwischen zwei von Null verschiedenen Niveaus oder zwischen Null und einem von Null verschiedenen Niveau ändern kann.In a preferred embodiment of the method according to the invention, therefore, the field strength in the application space has a periodically varying intensity over time, the period being determined primarily by the yarn speed and by the distance of the stationary field maxima. Particularly preferably, the intensity changes sinusoidally or in the form of pulses, wherein in the case of a pulsed change in intensity, the field strength can change, for example, between two non-zero levels or between zero and a non-zero level.

Die Erfindung wird anhand der nachfolgenden Figur sowie anhand der nachfolgenden Beispiele näher erläutert:

  • In Figur 1 ist eine Applikationsvorrichtung 1 dargestellt, wie sie zur Durchführung des erfindungsgemäßen Verfahrens geeignet ist. Die Applikationsvorrichtung 1 weist einen Applikator 2 mit einem Applikationsraum 3 auf, der über einen Heizmantel 4 auf die erforderliche Temperatur temperiert werden kann. An seinem Eintrittsende 5 ist der Applikator 2 mit einem Kniestück oder Rohrkrümmer 6 verbunden, über den die in einem Magnetron 7 erzeugten hochfrequenten elektromagnetischen Wellen in den Applikationsraum 3 eingeleitet werden.
The invention will be explained in more detail with reference to the following figure and to the following examples:
  • In FIG. 1 an application device 1 is shown, as it is suitable for carrying out the method according to the invention. The application device 1 has an applicator 2 with an application space 3, which can be tempered via a heating jacket 4 to the required temperature. At his Inlet end 5, the applicator 2 is connected to an elbow or elbow 6, via which the high-frequency electromagnetic waves generated in a magnetron 7 are introduced into the application space 3.

Das zu stabilisierende Polyacrylnitril-Precursorgarn 8 wird von einer Spule 9 abgezogen, nach Umschlingung einer Umlenkrolle 10 über eine Blendenöffnung 11 im Kniestück 6 in den Applikator 2 eingeführt und durch den Applikationsraum 3 hindurchgeführt. Nach Durchlaufen des Applikationsraums 3 verlässt das im Applikator 2 behandelte Precursorgarn 8 über ein mit dem Austrittsende 12 des Applikators 2 verbundenes Kniestück 13 durch eine Blendenöffnung 14 die Applikationsvorrichtung 1. Nach Umschlingung einer weiteren Umlenkrolle 15 wird das behandelte, d.h. das zumindest teilweise stabilisierte Garn 16 auf einer Spule 17 aufgewickelt. Die Fadenspannung des Precursorgarns kann durch die Antriebsgeschwindigkeiten der Umlenkrollen 10, 15 eingestellt werden.The polyacrylonitrile precursor yarn 8 to be stabilized is withdrawn from a bobbin 9, introduced into the applicator 2 after looping around a deflection roller 10 via an aperture 11 in the elbow 6 and passed through the application space 3. After passing through the application space 3, the precursor yarn 8 treated in the applicator 2 leaves the application device 1 via an elbow 13 connected to the outlet end 12 of the applicator 2 through an aperture 14. After wrapping around another deflection roller 15, the treated, i. the at least partially stabilized yarn 16 wound on a spool 17. The yarn tension of the precursor yarn can be adjusted by the drive speeds of the deflection rollers 10, 15.

Über einen Eintrittsstutzen 18 wird das im erfindungsgemäßen Verfahren benötigte Prozessgas in den Applikationsraum 3 eingeführt und durchläuft im dargestellten Fall im Gleichstrom zum Precursorgarn 8 den Applikationsraum 3. Über einen am Kniestück 13 angebrachten Austrittsstutzen 19 wird das Prozessgas zusammen mit den flüchtigen Abbauprodukten, die infolge der im Applikationsraum 3 im Garn 8 ablaufenden Umwandlungsreaktionen entstanden sind, aus dem Applikator 2 abgeführt.About an inlet nozzle 18, the process gas required in the process of the invention is introduced into the application space 3 and passes through the application space 3 in the illustrated case in direct current to Precursorgarn 8 via a attached to the elbow 13 outlet nozzle 19, the process gas together with the volatile degradation products due to in the application space 3 in the yarn 8 occurring conversion reactions have arisen, discharged from the applicator 2.

Das Kniestück 13 am Austrittsende 12 des Applikators 2 ist im dargestellten Fall mit einem Rohrstück 20 verbunden, das an seinem freien Ende mit einer Metallplatte 21 verschlossen ist. Hierdurch wird erreicht, dass die elektromagnetischen Wellen in den Applikationsraum 3 zurückreflektiert werden.The elbow 13 at the outlet end 12 of the applicator 2 is connected in the illustrated case with a pipe section 20 which is closed at its free end with a metal plate 21. This ensures that the electromagnetic waves are reflected back into the application space 3.

Beispiel 1:Example 1:

Es wurde ein unbehandeltes Precursorgarn aus Polyacrylnitril vorgelegt, wie es zur Herstellung von Kohlenstofffasern geeignet ist, wobei das Precursorgarn 12000 Filamente mit einem Filamentdurchmesser von ca. 8 µm aufwies. Die Dichte des Precursorgarns betrug 1,18 g/cm3.An untreated polyacrylonitrile precursor yarn was prepared as it is suitable for the production of carbon fibers, the precursor yarn having 12,000 filaments with a filament diameter of about 8 μm. The density of the precursor yarn was 1.18 g / cm 3 .

Die verwendete Applikationsvorrichtung zur Mikrowellenbehandlung entsprach im Aufbau der in Figur 1 dargestellten Vorrichtung. In einem Mikrowellengenerator wurden Mikrowellen mit einer Wellenlänge von 2,45 GHz erzeugt und über einen mit dem Mikrowellengenerator verbundenen Rechteckhohlleiter über ein Kniestück in den Applikationsraum (Rechteckhohlleiter des Typs R 26) geführt, der eine Länge von 120 cm aufwies. In den Rechteckhohlleiter wurde über einen seitlich angebrachten Stutzen Heißluft mit einer Temperatur von 190°C zugeführt und im Gleichstrom zum Precursorgarn durch den Applikationsraum geführt, wobei der Volumenstrom so bemessen war, dass sich im Applikationsraum eine mittlere Strömungsgeschwindigkeit von 2 m/s ergab. Der Applikationsraum wurde durch in der Wand angeordnete Heizelemente auf eine Temperatur von 170 °C temperiert, so dass im Applikationsraum eine Lufttemperatur von 170 °C herrschte. Im Applikationsraum wurde eine maximale elektrische Feldstärke von 30 kV/m eingestellt.The application device used for microwave treatment corresponded in structure to the in FIG. 1 illustrated device. In a microwave generator, microwaves having a wavelength of 2.45 GHz were generated and led via a rectangular waveguide connected to the microwave generator via an elbow in the application space (R 26 rectangular waveguide), which had a length of 120 cm. In the rectangular waveguide hot air at a temperature of 190 ° C was supplied via a side-mounted nozzle and passed in direct current to the precursor yarn through the application space, the volume flow was so dimensioned that resulted in the application space, an average flow rate of 2 m / s. The application space was tempered by heating elements arranged in the wall to a temperature of 170 ° C., so that an air temperature of 170 ° C. prevailed in the application space. In the application room, a maximum electric field strength of 30 kV / m was set.

Im Bereich des Kniestücks wurde das Polyacrylnitril-Precursorgam in die Applikationsvorrichtung eingeführt und durchlief den Applikator kontinuierlich mit einer Geschwindigkeit von 30 m/h und unter einer Fadenspannung von 0,9 cN/tex. Im Bereich eines mit dem Auslass des Applikators verbundenen Kniestücks wurde das Garn aus der Applikationsvorrichtung herausgeführt.In the region of the elbow, the polyacrylonitrile precursor org was introduced into the application device and passed through the applicator continuously at a speed of 30 m / h and under a thread tension of 0.9 cN / tex. In the area of an elbow connected to the outlet of the applicator, the yarn was led out of the application device.

Bereits nach einer Verweilzeit von 2,4 min konnte anhand einer deutlich erkennbaren Gelbfärbung des Garns ein Fortschritt hinsichtlich der Garnstabilisierung festgestellt werden. Die Dichte des Garns war auf 1,19 g/cm3 angestiegen.Already after a residence time of 2.4 minutes, progress in terms of yarn stabilization could be determined on the basis of a clearly discernible yellowing of the yarn. The density of the yarn had increased to 1.19 g / cm 3 .

Beispiel 2:Example 2:

Es wurde dieselbe Applikationsvorrichtung wie in Beispiel 1 verwendet. Auch die Verfahrensparameter waren dieselben wie im Beispiel 1. Anstelle des unbehandelten Precursorgarns wurde jedoch ein Polyacrylnitril-Precursorgam vorgelegt, welches bereits in einem konventionellen Prozess in einem Konvektionsofen einer teilweisen Stabilisierung unterzogen worden war. Das in diesem Beispiel vorgelegte Garns hatte eine Dichte von 1,19 g/cm3 und wies eine gelbe Färbung auf.The same application device as in Example 1 was used. The process parameters were also the same as in Example 1. Instead of the untreated Precursorgarns but a polyacrylonitrile Precursorgam was submitted, which had already been subjected in a conventional process in a convection oven of a partial stabilization. The yarn presented in this example had a density of 1.19 g / cm 3 and had a yellow color.

Nach Durchlaufen der Applikationsvorrichtung war die Dichte des Garns auf 1,20 g/cm3 angestiegen und das Garn hatte eine dunkelbraune Farbe angenommen.After passing through the application device, the density of the yarn was increased to 1.20 g / cm 3 and the yarn had turned a dark brown color.

Beispiel 3:Example 3:

Es wurde dieselbe Applikationsvorrichtung wie in Beispiel 1 verwendet, wobei der Applikator jedoch abweichend von Beispiel 1 eine Länge von 1,0 m aufwies. Als Precursorgarn wurde ein teilweise stabilisiertes Garn vorgelegt, das aufgrund der Teilstabilisierung eine Dichte von 1,21 g/cm3 und eine dunkelbraune bis schwarze Farbe aufwies. Abweichend von den Verfahrensbedingungen des Beispiels 1 wurde die Temperatur der zugeführten Heißluft und die Temperatur der in der Wand des Applikators angeordneten Heizelemente auf 170°C eingestellt, so dass die Heißluft im Applikationsraum ebenfalls eine Temperatur von 170°C aufwies. Die Fadengeschwindigkeit betrug 10 m/h, die Fadenspannung 1,25 cN/tex.The same application device was used as in Example 1, except that the applicator, unlike Example 1, had a length of 1.0 m. As a precursor yarn, a partially stabilized yarn was introduced, which had a density of 1.21 g / cm 3 and a dark brown to black color due to the partial stabilization. Notwithstanding the process conditions of Example 1, the temperature of the supplied hot air and the temperature of the arranged in the wall of the applicator heating elements was set to 170 ° C, so that the hot air in the application room also had a temperature of 170 ° C. The yarn speed was 10 m / h, the yarn tension 1.25 cN / tex.

Es wurde im Applikationsraum ein pulsierendes Mikrowellenfeld durch Ein-/Ausschaltung des Magnetron eingestellt, bei dem die maximale elektrische Feldstärke 25 kV/m (15 s) und Null kV/m (6 s) pulste.In the application room, a pulsating microwave field was set by switching the magnetron on / off, in which the maximum electric field strength was 25 kV / m (15 s) and zero kV / m (6 s).

Bereits nach einem einfachen Durchlauf, d.h. nach einer Verweilzeit von ca. 6 min, hatte sich die Farbe des die Applikationsvorrichtung verlassenden Garns in Richtung einer schwarzen Färbung verändert. Die Dichte hatte sich 1,24 g/cm3 erhöht.After just one pass, ie after a residence time of about 6 minutes, the color of the yarn leaving the application device had changed in the direction of a black color. The density had increased to 1.24 g / cm 3 .

Beispiel 4:Example 4:

Es wurde eine Applikationsvorrichtung wie in Beispiel 1 verwendet, wobei auch dieselben Verfahrensparameter wie im Beispiel 1 eingestellt wurde. Als Precursorgarn wurde das Garn verwendet, das auch in Beispiel 1 eingesetzt wurde. Abweichend von Beispiel 1 wurde das Garn jedoch mehrfach hintereinander in der Applikationsvorrichtung behandelt, indem es insgesamt dreimal durch die Applikationsvorrichtung geführt wurde. Dabei diente das teilweise stabilisierte Precursorgarn des vorherigen Durchlaufs durch die Applikationsvorrichtung als Vorlage für den folgenden Durchlauf.An application device was used as in Example 1, whereby the same process parameters as in Example 1 were set. As a precursor yarn, the yarn was used, which was also used in Example 1. Notwithstanding Example 1, however, the yarn was treated several times in succession in the application device by being passed a total of three times through the application device. The partially stabilized precursor yarn of the previous pass through the application device served as a template for the following pass.

Die Gesamtverweilzeit in der Applikationsvorrichtung betrug ca. 7,5 min. Das so dreimal behandelte Precursorgarn hatte eine Dichte von 1,22 g/m3. Das ursprünglich weiße Precursorgarn hatte nach der Behandlung eine dunkelbraune bis schwarze Farbe.The total residence time in the application device was about 7.5 min. The precursor yarn thus treated three times had a density of 1.22 g / m 3 . The originally white precursor yarn had a dark brown to black color after treatment.

Beispiel 5:Example 5:

Es wurde wie in Beispiel 3 vorgegangen, jedoch wurde die maximale elektrische Feldstärke konstant auf einen Wert von 30 kV/m eingestellt. Bei dem in diesem Beispiel vorgelegten Garn handelte es sich um ein teilweise stabilisiertes Polyacrylnitril-Precursorgarn mit einer Dichte von 1,26 g/cm3. Nach Durchlaufen der Applikationsvorrichtung, d.h. nach einer Verweilzeit von 6 min bei einer Fadengeschwindigkeit von 10 m/h, hatte das behandelte Garn eine Dichte von 1,40 g/cm3.The procedure was as in Example 3, but the maximum electric field strength was set to a constant value of 30 kV / m. In the presented in this example, yarn there was a partially stabilized polyacrylonitrile Precursorgarn having a density of 1.26 g / cm 3. After passing through the application device, ie after a residence time of 6 min at a Yarn speed of 10 m / h, the treated yarn had a density of 1.40 g / cm 3 .

Vergileichsbeispiel 1:Comparative Example 1

In einem konventionellen mehrstufigen Konvektionsofen zur Stabilisierung von Polyacrylnitril-Precursorgarnen für die Herstellung von Kohlenstofffasern wurde an einem nicht stabilisierten Precursorgarn, wie es in Beispiel 1 vorgelegt worden war, eine Stabilisierung vorgenommen. Durch den Konvektionsofen wurde Luft hindurchgeleitet. In der ersten Stufe des Ofens wurde eine Temperatur von ca. 230°C eingestellt.In a conventional multi-stage convection oven for stabilizing polyacrylonitrile precursor yarns for the production of carbon fibers, stabilization was carried out on an unstabilized precursor yarn as presented in Example 1. Air was passed through the convection oven. In the first stage of the furnace, a temperature of about 230 ° C was set.

Nach einer Verweilzeit von 23 min verließ das teilweise stabilisierten Precursorgarn die erste Ofenstufe. Das teilweise stabilisierte Precursorgarn hatte eine dunkelbraune bis schwarze Farbe und eine Dichte von 1,21 g/cm3.After a residence time of 23 minutes, the partially stabilized precursor yarn left the first furnace stage. The partially stabilized precursor yarn had a dark brown to black color and a density of 1.21 g / cm 3 .

Claims (14)

  1. A method for stabilizing yarns made from polyacrylonitrile using chemical stabilization reactions, comprising the following steps:
    - Provision of a precursor yarn based on a polyacrylonitrile polymer,
    - Provision of an application device for treatment of the precursor yarn using high-frequency electromagnetic waves, comprising an applicator with an application space, means for generating the high-frequency electromagnetic waves, as well as means for supplying the high-frequency electromagnetic waves into the application space,
    - Generation of a field of high-frequency electromagnetic waves in the application space, which has areas with minimum electric field strength and areas with maximum electric field strength, and adjustment of the maximum electric field strength in the application space in the range from 3 to 150 kV/m,
    - Continuous supply of the precursor yarn into and conveying the precursor yarn through the application space and through the field of the high-frequency electromagnetic waves, thereby
    - Introducing a process gas into the application space and conveying the process gas through the application space with a flow rate of at least 0.1 m/s relative to the precursor yarn being conveyed through the application space, wherein the temperature of the process gas is set in the range between 150 and 300ºC, so that it is above the critical minimum temperature Tcrit and below the maximum temperature Tmax, and whereby the critical minimum temperature Tcrit is that temperature above which the high-frequency electromagnetic waves couple into the precursor yarn being conveyed through the application space and the chemical stabilization reactions proceed, and the maximum temperature Tmax is that temperature which lies 20ºC below the decomposition temperature of the precursor yarn being supplied into the application space.
  2. A method according to Claim 1, characterized in that a maximum electric field strength of the high-frequency electromagnetic waves from 5 to 50 kV/m is generated in the application space.
  3. A method according to Claim 1 or 2, characterized in that the precursor yarn is fed through the applicator at a thread tension in the range from 0.125 to 5 cN/tex.
  4. A method according to one or more of Claims 1 to 3, characterized in that the process gas flows through the application space vertically to the precursor yarn at a flow rate of 0.1 to 2 m/s.
  5. A method according to one or more of Claims 1 to 3, characterized in that the process gas flows through the application space parallel to the precursor yarn at an average flow rate, in relation to the open cross-section of the application space, of 0.1 to 20 m/s relative to the precursor yarn being conveyed through the application space.
  6. A method according to one or more of Claims 1 to 5, characterized in that the process gas is a gas containing oxygen.
  7. A method according to Claim 6, characterized in that the gas containing oxygen is air.
  8. A method according to one or more of Claims 1 to 7, characterized in that the precursor yarn contains additives to improve the absorption capability of the precursor yarn with regard to high-frequency electromagnetic waves.
  9. A method according to Claim 8, characterized in that the additives are polyethylene glycol, carbon black, or carbon nanotubes.
  10. A method according to one or more of Claims 1 to 9, characterized in that the high-frequency electromagnetic waves are microwaves with a frequency in the range of 0.3 to 45 GHz.
  11. A method according to one or more of Claims 1 to 10, characterized in that the residence time of the precursor yarn in the application space is at least 20 s.
  12. A method according to one or more of Claims 1 to 11, characterized in that the process gas in the application space has a temperature in the range between (Tcrit + 20ºC) and (Tmax - 20ºC).
  13. A method according to one or more of Claims 1 to 12, characterized in that the field strength in the application space has a periodically changing intensity over time.
  14. A method according to one or more of Claims 1 to 13, characterized in that the precursor yarn is fed through at least two application devices arranged in series.
EP10749843.8A 2009-09-11 2010-08-31 Stabilisation of polyacrylonitrile precursor yarn Active EP2475812B1 (en)

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EP2475812A1 (en) 2012-07-18

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