DE102012004118A1 - Carbon fibers, carbon fiber precursors and their production - Google Patents

Abstract

The present invention relates to a process for the continuous production of high-strength continuous filament yarns with a compact, particularly homogeneous structural morphology based on polyacrylonitrile for carbon fiber precursors, which are produced on the basis of this process, and carbon fibers, which are obtainable after carbonization of the carbon fiber precursors. The fiber formation takes place in the context of a wet or dry wet process from polymer solutions in ionic liquids. In the production of yarns and filaments, immediately after precipitation in an organic solvent or a dilute aqueous solution of the relevant ionic liquid or in a saturated steam zone, a re-stretching of the freshly spun moldings takes place. The resulting precursor yarns and filaments primarily have increased tensile strength at the same time sufficient elongation, a round cross-section and a compact, particularly homogeneous fiber morphology and are particularly well suited for further processing and production of high-strength carbon fibers for technical applications in composites. Such produced carbon fibers are further characterized by a tensile strength in the range of about 3.0 to 8.5 GPa, an elongation at break in the range of about 0.5 to 3.0%, a modulus of elasticity of between about 100 and 350 MPa, a titer from about 0.2 to 1.5 dtex and a density structure parameter of about 1700 to 1900 g / cm3.

Description

Technical area

The present invention relates to the field of carbon fibers and of precursors of such carbon fibers based on polyacrylonitrile polymers, which are prepared with the inclusion of ionic liquids and are converted into carbon fibers by subsequent carbonization and optionally graphitization.

State of the art

In view of rising raw material and energy costs, carbon fibers are increasingly in demand as substitutes for traditional materials due to their unique mechanical and functional properties. This is due to the energy-saving potential of the carbon fiber-reinforced structural component produced therefrom, which has a very high strength and at the same time low weight. In particular, aerospace and the automotive industry place high demands on the quality and processing characteristics of the carbon fibers used in structural components.

Carbon fibers are obtained commercially by pyrolysis of polyacrylonitrile copolymers and cellulose regenerates and pitch and have different properties depending on the manufacturing process parameters. For industrial use, polyacrylonitrile copolymers are particularly suitable as starting polymers for the production of carbon fibers.

Since carbon fibers are used in ever larger fields of application, they must have a higher tensile strength than before. To further extend the application of carbon fibers, they must be cheaper and more environmentally friendly can be made.

Polyacrylonitrile (PAN) decomposes at temperatures above 350 ° C without melting, so that fibers can only be produced via the solution state. However, the solubility of the polyacrylonitrile is very low due to the strong secondary valence forces between the macromolecules. Dissolution of polyacrylonitrile is only possible in high dipole moment solvents such as dimethylformamide, dimethylacetamide, dimethyl sulfoxide and the like. Most of these solvents allow the production of polyacrylonitrile fibers by both the wet and dry spinning processes.

The methods used in the prior art for the dissolution, storage and processing of PAN-based polymers from conventional solvents have technological or ecological disadvantages. For process-related reasons, these processes must be carried out at temperatures above 80 ° C., whereby the release of volatile solvents can not be completely prevented. Recovery of the solvents from both aqueous baths and from the exhaust air is associated with great expense. In most cases, in addition to the complicated technical and energy-consuming procedures required for this, environmental emissions as well as requirements for ensuring the health protection of the operating personnel of the fiber production exist.

Furthermore, in the wet spinning process relatively dilute polymer solutions are spun for procedural reasons. As a rule, porous fiber structures are produced which have a low strength. In addition, the mostly kidney-shaped or dumbbell-shaped fiber cross sections do not guarantee the necessary close and stable connection between fiber and matrix. The conventional techniques for improving the tensile strength of carbon fibers are aimed at reducing macro-defects, for example, reducing impurities contained in the individual filaments constituting the carbon fibers or inhibiting the formation of macro-voids contained in the carbon fibers individual filaments are formed and on an inhibition of the formation of defects, which are formed on the surfaces of the individual filaments from ( 59-88924 (Kokai) and the publication no .: 4-12882 (Kokoku) Japanese Patent, Publication No. 3-41561 (Kokoku)).

In addition to the above-mentioned macrodefects, the strength is also affected by microvoids or microdefects. A technique for increasing the density of unstretched fibers by optimizing the conditions of the coagulation bath is disclosed in Japanese Patent Laid-Open No .: 59-82420 (Kokai), and a technique for increasing the density of drawn fibers in which the stretching temperature in a bath is kept as high as possible is disclosed in Japanese Patent Laid-Open Publication No. 5-15722 (Kokoku). Although these techniques can increase the Fiber density can be achieved, however, this may cause the oxygen permeability in the fibers in the stabilization process is lowered, so that the improvement in the tensile strength of the carbon fibers obtained is associated with great disadvantages and has lost value. Therefore, these techniques are limited in their effect of improving the tensile strength.

In contrast, the dry-spun fibers, despite the compact morphology and round cross-sectional areas, have a less uniform structure and are therefore less suitable as carbon fiber precursors. In addition, the process of dry spinning is carried out at high temperatures, so that when using volatile solvents, the spinning process can be carried out only with great technical effort.

For industrial applications, ionic liquids (IL) have proven to be promising solvents for polymers. Ionic liquids are salts which are in liquid state at temperatures below 100 ° C. The most intensively investigated compounds today are based on imidazolium and pyridinium cations and anions, such as halides or more complex anions, such as acetates, amides, borates, phosphates, imides and sulfates.

Ionic liquids are characterized by a number of interesting properties: they are thermally stable, hardly inflammable, have a very low vapor pressure and have special highly selective dissolution properties. In addition, ionic liquids have excellent solubility for numerous fiber-forming polymers, so that the technically usable polymer concentrations in the solution can be significantly increased ( CN 1752302 to CN 1752305 . WO 2007/128268 A2 . WO 2007/128268 A3 ). In the CN 1752302 to CN 1752305 and DE 10 2009 019 120 A1 ionic liquids are listed for the processing of polyacrylonitrile to fibers from the solution state. However, the resulting fibers have unfavorable property profiles as a result of the wet-spinning processes and re-stretching processes used for the production of carbon fibers.

Object of the invention

In the preparation of PAN-based carbon fiber precursors, there is a need for solution systems and methods based thereon which provide high-strength, yet sufficiently high-elongation, fibers which are particularly well-suited for use in composites. Furthermore, the fibers obtained in this way should have optimum surface properties.

Surprisingly, this object is achieved by carbon fibers, which are characterized by a tensile strength (after DIN 53816 ) of about 3.0 to 8.5 GPa, b) an elongation at break (after DIN 53816 ) of about 0.5 to 3.0%, c) a modulus of elasticity (acc DIN 53816 ) of about 100 to 350 MPa, d) a titer (dtex) of about 0.2 to 1.5 dtex, and e) a density structure parameter of about 1700 to 1900 g / cm ³ .

Preferably, the tensile strength has a value (after DIN 53816 ) of about 3.5 to 7.0 GPa, the elongation at break is a value (after DIN 53816 ) of about 1.0 to 2.5%, the modulus of elasticity has a value of (after DIN 53816 ) about 200 to 300 MPa, and the titer (dtex) preferably has a value of about 0.3 to 1.2. The density structure parameter preferably assumes a value of about 1.750 to 1.850 g / cm ³ .

The carbon fibers following this technical instruction are particularly advantageous when their morphology has the least possible structural differences between the inner and outer layers of each individual filament than conventional carbon fibers obtained by AMF (Scanning Probe Microscopy) and RAMAN microscopy. In addition, these carbon fibers are particularly advantageous when their morphology has the lowest possible number of macrodefects of each individual filament than conventional carbon fiber precursors obtained by SEM images of filament cross sections exposed by means of a gallium ion beam (Focused Ion Beam) were. Please refer to the SEM images 1 directed. These go back to filaments taken directly after the coagulation bath, with the left-hand figure representing a cross-section. This results in a particularly advantageous, compact and homogeneous structure.

The carbon fibers described above are characterized by excellent roundness, as measured by the L / B ratio. The L / B ratio is the ratio of length to width of the fiber cross-section. The carbon fibers according to the invention preferably have an L / B ratio of 2.0 to 1.0, in particular 1.5 to 1.0.

The object of the present invention is achieved with particular advantage by carbon fibers in graphitized form, which are characterized by a) a tensile strength (according to DIN 53816 ) from 2.5 to 7.0 GPa, in particular from 3.0 to 6.0 GPa, b) an elongation at break (acc DIN 53816 ) from 0.2 to 3.0%, in particular from 0.5 to 2.5%, c) a modulus of elasticity (according to DIN 53816 ) from 200 to 1,000 MPa, in particular from 300 to 900 MPa, d) a titer (dtex) of from 0.3 to 1.2, in particular from 0.5 to 1.0, and / or e) a density of from 1.700 to 1,900 g / cm ³ , in particular from 1,750 to 1,850 g / cm ³ . For carbon fibers in graphitized form, it is preferred that they also meet the properties for smoothness, circularity and / or porosity described above for carbon fibers.

The above-described carbon fibers or carbon fibers in graphitized form can be prepared from carbon fiber precursors which have an increased tensile strength in the form of precursor yarns and filaments with sufficient elongation, a round cross section and a compact fiber morphology and for further processing and production of high-strength carbon fibers for technical applications in composites are particularly well suited. In particular, such carbon fiber precursors are suitable for the preparation of carbon fibers described above by stabilization, carbonation and optionally subsequent graphitization. The carbon fiber precursors described herein are based on polyacrylonitrile and are characterized by a) a tensile strength (after DIN 53816 ) from about 40 to 90 cN / tex, in particular from about 50 to 80 cN / tex, b) an elongation at break (acc DIN 53816 ) of about 8 to 20%, in particular of about 10 to 18%, c) a modulus of elasticity (acc DIN 53816 ) from about 800 to about 1600 cN / tex, in particular from about 900 to 1500 cN / tex, d) a titer of about 0.6 to 2.0, in particular from about 0.8 to 1.5 (dtex), and e) density of about 1.170 to 1490 g / cm ³ .

The polyacrylonitrile used can be prepared by cationic, anionic or free-radical polymerization, preferably by free-radical polymerization. For the polymerization, any conventional polymerization method such as solution polymerization, suspension polymerization or emulsion polymerization may be employed.

The polyacrylonitrile used may be both a homopolymer and a copolymer. If a copolymer is used as the polyacrylonitrile, the proportion of polymer copolymers in the polyacrylonitrile should be about 10.0% or less, in particular in the range of about 1 to 6 mol% and particularly preferably in the range of about 2 to 5 mol -%. As the copolymers to acrylonitrile, acrylamide, acrylic acid, butyl acrylate, ethyl acrylate and methyl methacrylate, methacrylic acid, itaconic acid, vinyl acetate, vinyl bromide, vinyl chloride or mixtures thereof can be suitably used.

With regard to the molecular weight, the polyacrylonitrile used for the carbon fiber precursor is not subject to any relevant restrictions. The average molecular weight of the polyacrylonitrile in the carbon fiber precursors is preferably about 50,000 to 300,000, more preferably about 80,000 to 250,000, and most preferably about 120,000 to 190,000. A higher degree of polymerization under the same spinning and stretching conditions is more effective in improving the tensile strength and elongation of the precursor fibers. On the other hand, the viscosity of the polymer increases with increasing degree of polymerization, which is associated with a deterioration of the processability in spinning and drawing. Thus, it is preferable to select the degree of polymerization taking into account a balance between these properties. When the degree of polymerization is low, the processability in spinning and drawing improves. However, as the heat resistance of the product decreases, merging between the individual filaments occurs more frequently in the spinning and stretching processes and in the carbonization process.

The polydispersity of the acrylonitrile polymer, as determined by gel permeation chromatography, is suitably in the range between about 1.5 and 3.5, preferably between about 1.8 and 3.0, and more preferably between about 1.8 and 2.8. A narrower molecular weight distribution enables improved stretchability in the spinning and stretching process and improves the strength of the resulting carbon fibers. It is therefore preferred to narrow the distribution of molecular weights.

The described carbon fiber precursors do not necessarily consist exclusively of polyacrylonitrile or copolymers thereof, as described above, but may contain, in addition to polyacrylonitrile, other polymers, in particular in the form of cellulose, aramides, lignin, polyetherester ketones and / or polyethersulphones. Such additional polymers are preferably contained in an amount of up to 50 wt .-%, based on the carbon fiber precursor in this.

In addition, the carbon fiber precursors also have the highest possible roundness, which is preferably characterized by an L / B ratio in the range from 2 to 1.0, in particular from 1.5 to 1.0. Alternatively or in addition to these properties, the carbon fiber precursors preferably have a porosity determined by the method of iodine absorption of 1.5 to 0.1%, in particular from 0.8 to 0.15%.

The above technical instruction following carbon fiber precursors are to be considered particularly advantageous if their morphology has the lowest possible porosity in each individual filament than conventional carbon fiber precursors, obtained by porosity measurements. Alternatively or in addition to these properties, the carbon fiber precursors preferably have a porosity, determined by the method of iodine absorption of about 1.5 to 0.1%, in particular from about 0.8 to 0.15%. The measuring method of iodine absorption takes place as described in the DE 698 28 417 T2 (there p. 10, [0066] and [0067]) has been described.

The carbon fiber precursors according to the invention can be prepared by a process described below. According to this method, a homo- or copolymer of polyacrylonitrile, optionally together with an additional other polymer as described above, dissolved in an ionic liquid, wherein the concentration of the polyacrylonitrile optionally together with another polymer, in the ionic liquid at least about 5 wt %, in particular about 10 to 25 wt .-%, wherein insoluble liquid or solid additives are largely excluded, is that this solution is wet-spun in a precipitation medium and the resulting fibers are stretched.

When conventional solvents are used, as the polymer concentration increases, the solubility decreases and the viscosity of the solution increases, so that the temperatures must be increased in order to avoid gel formation in the spinning solution or to make the process parameters continuous. As a result, the minimum temperatures required for the problem-free dissolution, storage and processing of the polymer solutions are increased or they must not fall below 80 to 90 ° C. Due to the high vapor pressure of conventional solvents, this leads to the formation of inhomogeneities at the boundary between the polymer solution and the environment. Ionic liquids have virtually no vapor pressure, which is why polymer solutions in ionic liquids can be produced and processed without problems even at higher temperatures.

In wet spinning the solution, the spinning process used may be wet spinning or dry wet spinning. Among them, dry-wet spinning is preferable since an increase in fiber density can be more easily ensured, and fibers having higher strength can be obtained more easily. In this process, the filaments exiting the die can pass through an air gap of up to 10 cm before they are precipitated in an underlying precipitation medium.

The processing of polymer solutions using the wet-spinning process is very much influenced by the flow and pressure conditions of the polymer solution at the nozzle. Therefore, the processing of highly structured polymer solutions is limited, among other things, by their high viscosity. In the above molecular weight range of the PAN-based polymers, polymer solutions in the concentration range between 10 and 18% by weight polymer in IL can be processed.

Spinning higher concentration solutions can be done after the dry-wet spinning process. In the above-mentioned molecular weight range of the PAN-based polymers, this method can be used to process polymer solutions in the concentration range between about 15 and 30% by weight of polymer in ionic liquids.

The viscosity of the spinning solution containing the polyacrylonitrile in the wet spinning process is preferably about 20 to 200 Pas, in particular about 50 to 120 Pas, in the dry-wet spinning process, however, preferably about 500 to 50,000 Pas, in particular about 1,000 to 20,000 Pas.

A solution of the polyacrylonitrile in the ionic liquid can be obtained either by directly dissolving the polyacrylonitrile in the ionic liquid or by suspending the polyacrylonitrile in a dilute aqueous solution of the ionic liquid, then removing the water or organic solvent at elevated temperature Temperature is removed with or without the action of a vacuum from the suspension. In this case, the suspension initially obtained is converted into a solution.

The polyacrylonitrile solution is preferably introduced by means of a spinneret into a precipitation medium. As the spinneret, a spinneret with circular holes is suitably used to obtain coagulated fibers having a circular or circular-like cross-sectional shape. Coagulated fibers with A cross-sectional shape different from the circular shape, such as a triangular, quadrangular or pentagonal cross-sectional shape, can be obtained by combining a plurality of filaments obtained from a slit set or a set of small circular holes ,

It is preferable that the spinneret plate has a diameter of about 160 μm or less. In particular, the diameter is preferably about 100 μm or less, and more preferably about 40 μm or less.

In the selection of the ionic liquid, the present invention is not subject to any relevant limitations. Thus, in general, such ionic liquids come into question as they do in the DE 10 2006 035 830 A9 (there p. 3, [0015] to p. 17, [0088]). It has been shown that the polyacrylonitrile is very soluble in certain ionic liquids.

darstellt. Bei den Resten R1, R2, R3, R4 bzw. R1 bis R8 in den Formeln (I) bis (VI) handelt es sich unabhängig voneinander um lineare, zyklische, verzweigte, gesättigte oder ungesättigte Alkylreste, mono- oder polycyclische, aromatische, heteroaromatische Reste oder mit weiteren funktionellen Gruppen substituierte Derivate dieser Reste, wobei R1, R2, R3 und R4 bzw. R1 bis R8 in den Formeln (I) bis (VI) untereinander verbunden sein können. Bei dem Anion [Z]^n– handelt es sich vorzugsweise um ein Halogenid, Pseudohalogenid, Amid, insbesondere Dicyanamid, Imid, eine Phosphorbindung oder eine Nitroverbindung.The preferred ionic liquids have the general form [Q +] _n [Z-] _n where the cation [Q +] _{n is} a quaternary ammonium [R1R2R3R4N +], phosphonium [R1R2R3R4P +] or tertiary sulfonium [R1R2R3S +] cation or an analogous quaternized nitrogen, phosphorus or a tertiary sulfur heteroaromatic of the following formulas (I), (II), (III), (IV), (V) and (VI)

represents. The radicals R1, R2, R3, R4 or R1 to R8 in the formulas (I) to (VI) are each independently linear, cyclic, branched, saturated or unsaturated alkyl radicals, mono- or polycyclic, aromatic, heteroaromatic Rests or substituted with further functional groups derivatives of these radicals, wherein R1, R2, R3 and R4 or R1 to R8 in the formulas (I) to (VI) may be interconnected. The anion [Z] ^n- is preferably a halide, pseudohalide, amide, in particular dicyanamide, imide, a phosphorus bond or a nitro compound.

The halides or pseudohalides may in particular be those having the formula F ^- , Cl ^- , Br ^- , I ^- , BF ₄ ^- , BF ₆ ^- , AlCl ₄ ^- , Al ₂ Cl ₇ ^- , Al ₃ Cl ₁₀ ^- , AlBr ₄ , FeCl ₄ ^- , BCl ₄ ^- , SbF ₆ ^- , AsF ₆ ^- , ZnCl ₃ ^- , SnCl ₃ ^- , CuCl ₂ ^- , CF ₃ SO ₃ ^- , (CF ₃ SO ₃ ) ₂ N ^- , CF ₃ CO ₂ ^-, CCl ₃ CO ₂ ^-, CN ^-, SCN ^-, OCN ^- act. Suitable phosphorus compounds are phosphates according to the chemical formula PO ₄ 3 ^- , HPO ₄ ^2- , H ₂ PO ^4- , R ¹ PO ₄ ^2- , HR ¹ PO ₄ ^- , R ¹ R ² PO ₄ ^- , phosphonates and phosphinates according to the formulas R ¹ HPO ₃ ^- , R ¹ R ² PO ₂ ^- , R ¹ R ² PO ₃ ^- , phosphates according to the formulas PO ₃ 3 ^- , HPO ₃ ^2- , H ₂ PO ₃ ^- , R ¹ PO ₃ ^2- , R ¹ HPO ₃ ^- , R ¹ R ² PO ₃ ^- , as well as phosphonites and phosphinites of the formulas R ¹ R ² PO ₂ ^- , R ¹ HPO ₂ ^- , R ¹ R ² PO ^- , R ¹ HPO ^- include.

The alkyl radicals optionally contained in the above-mentioned cations are preferably C _1-18 -alkyl radicals, in particular alkyl radicals having from 1 to 4 carbon atoms, such as, preferably Methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl radicals, cyclic alkyl radicals having 3 to 10 carbon atoms, in particular in the form of cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl radicals, unsaturated Alkyl radicals in the form of vinyl, 2-propenyl, 3-butenyl, cis-2-butenyl, trans-2-butenyl radicals, aromatic radicals in the form of phenyl or naphthyl radicals containing 1 to 3 halogen atoms, alkyl radicals having 1 to 4 carbon atoms or phenyl radicals, and heteroaromatic radicals in the form of O, S or Kn-containing heterocyclic radicals having 2 to 5 carbon atoms or mixtures thereof.

The molecular structure of the polymeric molecules of the homo- and copolymers of polyacrylonitrile leads to strong dipole interactions between the chains. Particularly suitable for disrupting the strong interactions between the polymer chains are ionic liquids consisting of cations and anions with high charge density (see HSAB concept).

The ionic liquids described below prove to be particularly suitable for the processing according to the invention.
1-ethyl-3-methylimidazolium chloride [EMIM] [Cl],
1-butyl-3-methylimidazolium chloride [BMIM] [Cl],
1-ethyl-3-methylimidazolium bromide [EMIM] [Br],
1-butyl-3-methylimidazolium bromide [BMIM] [Br],
1-ethyl-3-methylimidazolium thiocyanate [EMIM] [SCN],
1-butyl-3-methylimidazolium thiocyanate [BMIM] [SCN],
1-ethyl-3-methylimidazolium cyanate [EMIM] [OCN],
1-butyl-3-methylimidazolium cyanate [BMIM] [OCN],
1-ethyl-3-methylimidazolium dicyanamide [EMIM] [DCA],
1-butyl-3-methylimidazolium dicyanamide [BMIM] [DCA],
1,3-dimethylimidazolium dimethyl phosphate [MMIM] [DMP],
1-ethyl-3-methylimidazolium diethyl phosphate [EMIM] [DEP],
1-butyl-3-methylimidazolium dibutyl phosphate [BMIM] [DBP].

The following concrete compounds are of particular value for the effects sought with the invention: [EMIM] [Cl], [EMIM] [DCA] (DCA = dicyanamide = ΘN (CN) ₂ ), [MMIM] [DMP] and / or [ EMIM] [SCN].

Taking into account the specific details given above for the present invention, it is readily possible for the person skilled in the art to carry out suitable optimizations here, for example by varying the cations and anions of the ionic liquid, which favors the resulting structural parameters and the further properties of the precursor fibers. As a result, the invention can be achieved in an optimal form by the use of certain ionic liquid, in particular, the thread formation process at the nozzle or in the air gap is optimally controlled. Also, different desirable properties of the precursor fibers can be obtained by favorably controlling the diffusion processes in the regeneration / coagulation of the thread and the stretching conditions.

The conditions chosen during precipitation or coagulation of the polyacrylonitrile filaments have a great influence on the structure and tensile properties of the precursor fibers and the carbon fibers which can be produced therefrom. It is therefore preferable to set these conditions in consideration of both the tensile properties and the productivity. In order to obtain in particular dense precursor fibers with few cavities, a lower coagulation rate is preferred, which can be achieved by coagulation at low temperature and at high concentration.

In this case, preferably a temperature of the spinning solution of about 100 ° C or less preferred, more preferably about 60 ° C or less and in particular about 40 ° C or less. Further, it is preferable that the temperature of the coagulating medium is about 100 ° C or lower. More preferably about 60 ° C or lower, and even more preferably about 20 ° C or lower.

The use of ionic liquids, as described above, together with a vaporizable solvent has the advantage that the ionic liquid can be recovered from the coagulation medium. The process is therefore suitably carried out so that the ionic liquid is recovered from the coagulation medium used. The use of water as a coagulation medium is associated with particular advantages in this context. In particular, water with an included amount of another solvent can be used to optimize the coagulation rate. Preferably, an amount of ionic liquid adapted to optimize the coagulation rate remains in the coagulation medium.

When the concentration of the polymer used for spinning is higher, the amount to be replaced by a solvent and a precipitant during coagulation becomes lower, whereby the density of the precursor fibers can be increased. This leads to the formation of a compact morphology in the resulting precursor fibers, especially in dry-wet spinning. On the other hand, because of the higher polymer spinning solution viscosity, the processability in spinning and drawing decreases, making it more likely to gel in the processing of the polymer solution and impairing the spinning ability.

The production of fibers or the carbon fiber precursors takes place immediately after precipitation in an organic solvent or more or less dilute aqueous solutions of the relevant ionic liquid (non-solvent). It is preferred that the composition of the coagulation bath in the wet spinning process be the ratio of non-solvent / ionic liquid between 60:40 and 100: 0.

When the coagulation process is complete, the fibers obtained after wet spinning may be washed and stretched, preferably with water, both preferably simultaneously. Optionally, an additional acid treatment may be carried out. The temperature during stretching for the acceleration of the compression is of great importance. So it is useful if the maximum temperature when stretching in baths is about 60 to 100 ° C. A preferred range is about 70 to 100 ° C and a most preferred range is about 80 to 100 ° C. Optionally, the fibers withdrawn from the coagulation medium may also be dried, in particular in a convection oven, and stretching may then be carried out in a dry state.

It is preferable that the stretching is carried out in two or more baths because the strength of the obtained fibers can be improved. It is also preferable that a temperature profile from a low temperature to a higher temperature is established inside the baths and that the temperature difference between the adjacent baths is 20 ° C or lower. By this measure, a fusion between the individual fibers can be inhibited.

It is further preferred that the total stretch ratio when stretched in baths is 1.5 to 8 times, more preferably 2 to 5 times.

As the overall stretching ratio in the spinning and stretching method, including stretching in hot water baths, 7 times or more is preferred, and 10 times or more is particularly preferable for improving the orientation of the carbon fiber precursors and also for productivity in spinning and stretching To improve stretching. In terms of quality, such as fraying, a suitable upper limit of the total stretch ratio in the spinning and stretching method is 20 times or less. This allows use of high temperature heat carriers such as glycol and the like. If necessary, after the completion of stretching in steam under pressure or with the aid of the high-temperature heat carrier, a covering oil is applied to the precursor fibers.

From the viewpoint of productivity, it is preferable that the fineness of the individual fibers in the form of the carbon fiber precursors is about 0.6 dtex or more, more preferably about 1.1 dtex or more. If the fineness of the individual filaments is too large while the number of filaments is the same, the heat value in heat treatment processes, especially in the stabilization process, is too high and the stabilization temperature can not be increased, thereby lowering the productivity. Thus, it is preferable that the upper limit of fineness is about 2.2 dtex or less, more preferably about 1.9 dtex or less.

The number of individual filaments constituting the carbon fiber precursors is not limited. From the viewpoint of productivity, a number of about 1,000 filaments or more is preferable, particularly about 10,000 or more, and more preferably about 20,000 or more. The present invention can also be effectively applied to a thick strand of about 500,000 filaments or more. For the spinnerette, it is preferred that the number of spinning holes per spinnerette is about 3,000 or higher, more preferably about 6,000 or higher. The suitable upper limit of the number of holes is about 100,000 or less, because a very large spinnerette deteriorates the ease of handling.

Surprisingly, the fibers or filaments obtained in this way, in addition to an increased tensile strength and sufficient elongation, are characterized by a very compact fiber morphology in a round Cross section off. These properties are of immense advantage especially in a further processing of the PAN fiber or fiber cable obtained for the production of carbon or graphite fibers.

As the fiber density is increased as described above, dense carbon fiber precursors having no microvoids in the outer layer of each individual filament are obtained. However, when the density is higher, the oxygen permeability to the inner layer in the stabilization process becomes lower, whereby the inner layer is insufficiently stabilized; This increases the structural difference between the inner and outer layers of the obtained carbon fibers. As a result, problems arise such as a decrease in strength, a decrease in modulus and fiber breakage in the carbonization process.

When the density of the precursor fibers is larger, promotion of oxygen permeability into the carbon fiber precursors is important to improve the strength of the obtained carbon fibers.

The properties of the manufactured carbon fiber precursors in their stabilization, carbonization and graphitization have a decisive influence on the properties of the resulting carbon fibers. It is preferable that the stabilization temperature is about 200 to 300 ° C. The stabilization is carried out in an oxidizing atmosphere such as in air, but in view of higher productivity, stabilization in an inert gas atmosphere such as in nitrogen is partly effective at the beginning or later during the process. Since the stabilization of thermal cyclization and unsaturation is oxygen, the cyclization can be carried out to ensure higher productivity in an inert atmosphere at a higher temperature, free of any runaway reaction that would otherwise be caused by the presence of oxygen.

The stabilized carbon fiber precursors thus obtained are then carbonized and, if necessary, graphitized to obtain carbon fibers.

In carbonization, which occurs at gradually increasing temperatures between about 300 and 1500 ° C in the nitrogen atmosphere, the carbon content steadily increases and reaches about 95%. By the subsequent graphitization, the carbon content of the fibers can be increased to about 99%. The graphitization is carried out by a thermal treatment at about 2400 to 2600 ° C in an argon atmosphere. The graphitized fibers have a higher modulus than conventionally carbonized fibers.

The process for producing carbon fiber precursors described above has many advantages over the prior art. Thus, polyacrylonitrile polymers can be dissolved in ionic liquids in high concentrations and the spinning solution can be forced through a nozzle into a coagulation bath. After extrusion of the spinning solution into the coagulation medium, which preferably takes place via an air gap, and precipitation of the yarns and filaments, these can be washed, dried and then wound on godets. In addition, the properties of the spinning solution can be precisely adjusted with the help of different ionic liquids and offer innumerable possibilities for varying the thread formation process. These include different spinning processes, different spinning conditions and differences in coagulation behavior, which in turn offers many advantages for the selection of the coagulation medium and the coagulation bath composition. The present technology for producing polyacrylonitrile based carbon fiber precursors using ionic liquids is characterized by its wide variation in process control, and hence the ability to control precursor fiber properties, compared to prior art fiber production technologies with conventional solvents in a certain range to be able to change.

By means of the present invention, carbon fibers can be produced with the aid of ionic liquids, which have a particularly favorable fiber structure, wherein there are slight structural differences between the inner and outer layers of each individual filament. For example, by using bulky anions in the ionic liquid, the spinning process of the regeneration-coagulation processes can be delayed, which causes optimum diffusion of the solvent from the thread into the coagulation bath or the precipitation medium in the fibers and thus produces a particularly homogeneous fiber structure.

In the following, the invention described above will be illustrated in more detail by way of examples, which, however, are not intended to limit the scope of the present invention in any way.

example 1

A copolymer consisting of 98.0 mole% of acrylonitrile, 1.0 mole% of methacrylic acid and 1.0 mole% of itaconic acid was prepared by radical polymerization to obtain a spinning solution in [EMIM] [Cl] at a concentration of 22%. The spinning solution obtained was spun at a controlled 105 ° C using a spinneret with 32 holes, each with a diameter of 0.10 mm, with an air gap of 50 mm, including a 20 ° C tempered coagulation bath of an aqueous 20% [EMIM] [Cl] solution. The coagulated filaments were washed with water and stretched in 90 ° C hot water in three baths. Stretching in the baths was 700%. Subsequently, the filaments were used with the help of heated godets at 150 ° C for drying and stretching. Stretching on heated godets was 200%.

Precursor fibers were obtained from 32 filaments with a single filament fineness of 1.1 dtex and a circular cross-section (L / B = 1). The final process speed was 65 m / min.

The fracture surface of the precursor fibers was observed by REM. No microvoids were found in the fiber cross sections of each individual filament.

The porosity of the coagulated filaments was 29% and the iodine absorption measurements were 0.17%.

To obtain stabilized fibers, the precursor fibers were stabilized in an oven at atmospheric pressure at 250 ° C for 15 minutes. Further, it was stabilized at 270 ° C for 15 minutes.

The resulting fiber bundles were stabilized in air at gradually increasing temperatures between 230 and 250 ° C at a draw ratio of 0.90. The stabilized fibers were in a nitrogen atmosphere, at a temperature increase rate of 400 ° C / min, in a temperature range between 300 and 500 ° C and at a temperature rise rate of 500 ° C / min, in a temperature range between 1,000 and 1,200 ° C, to to 1,350 ° C, carbonized at a draw ratio of 0.90. The rate of carbonation was 9 m / min.

The carbon fibers thus obtained had a single filament diameter of 8.0 μm, a tensile strength of 6.0 GPa, a modulus of elasticity of 250 MPa, and an elongation at break of 2.50%. The resulting carbon fibers show little structural difference between the inner and outer layers of each individual filament, obtained by RAMAN and scanning probe spectroscopy. SEM images of filament cross sections exposed by means of a gallium ion beam show no macrodefects from each individual filament.

Example 2

Carbon fibers were obtained as described in Example 1 except that [EMIM] [SCN] was used as the solvent and the processing temperature of the spinning solution was increased to 110 ° C. The obtained carbon fibers had a tensile strength of 5.9 GPa, a modulus of elasticity of 240 GPa and an elongation at break of 2.6%.

Example 3

Carbon fibers were obtained as described in Example 1 except that a copolymer consisting of 98.0 mole percent acrylonitrile and 2.0 mole percent itaconic acid was prepared by free radical polymerization and a spinning solution in [MMIM [[DMP] at a concentration of 22% was used.

The carbon fibers thus obtained had a tensile strength of 5.2 GPa, a modulus of 230 MPa and an elongation of 2.5%.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• JP 59-88924 [0007]
• JP 4-12882 [0007]
• JP 3-41561 [0007]
• JP 59-82420 [0008]
• JP 5-15722 [0008]
• CN 1752302 [0011, 0011]
• CN 1752305 [0011, 0011]
• WO 2007/128268 A2 [0011]
• WO 2007/128268 A3 [0011]
• DE 102009019120 A1 [0011]
• DE 69828417 T2 [0025]
• DE 102006035830 A9 [0035]

Cited non-patent literature

• DIN 53816 [0013]
• DIN 53816 [0013]
• DIN 53816 [0013]
• DIN 53816 [0014]
• DIN 53816 [0014]
• DIN 53816 [0014]
• DIN 53816 [0017]
• DIN 53816 [0017]
• DIN 53816 [0017]
• DIN 53816 [0018]
• DIN 53816 [0018]
• DIN 53816 [0018]

Claims (24)

Carbon fibers, characterized by a) a tensile strength (according to DIN 53816) of about 3.0 to 8.5 GPa, b) an elongation at break (according to DIN 53816) of about 0.5 to 3.0%, c) a modulus of elasticity (according to DIN 53816) of about 100 to 350 MPa, d) a titer (dtex) of about 0.2 to 1.5 dtex, and e) a density structure parameter of about 1700 to 1900 g / cm ³ . Carbon fibers according to claim 1, characterized by a) a tensile strength (according to DIN 53816) of about 3.5 to 7.0 GPa, b) an elongation at break (according to DIN 53816) of about 1.0 to 2.5%, c) a Modulus of elasticity (according to DIN 53816) of about 200 to 300 MPa, d) a titer (dtex) of about 0.3 to 1.2, and / or e) a density structure parameter of about 1750 to 1850 g / cm ³ , Carbon fibers according to claim 1 or 2, characterized in that their roundness is given / characterized by the L / B ratio of about 2.0 to 1.0, in particular from about 1.5 to 1.0. Carbon fibers in graphitized form, characterized by a) a tensile strength (according to DIN 53816) of about 2.5 to 7.0 GPa, in particular from about 3.0 to 6.0 GPa, b) an elongation at break (according to DIN 53816) of about 0.2 to 3.0%, in particular from about 0.5 to 2.5%, c) a modulus of elasticity (according to DIN 53816) of about 200 to 1000 MPa, in particular of about 300 to 900 MPa, d) a Titer of about 0.3 to 1.2 dtex, in particular of about 0.5 to 1.0 dtex, and / or e) a density of about 1,700 to 1,900 g / cm ³ , in particular from about 1,750 to 1,850 g / cm ³ . Carbon fiber precursor, in particular for producing a carbon fiber according to at least one of the preceding claims by stabilization, carbonization and optionally graphitization, based on polyacrylonitrile, characterized by a) a tensile strength (according to DIN 53816) of about 40 to 90 cN / tex, in particular of about 50 to 80 cN / tex, b) an elongation at break (according to DIN 53816) of about 8 to 20%, in particular of about 10 to 18%, c) a modulus of elasticity (according to DIN 53816) of about 800 to about 1600 cN dtex, in particular from about 900 to 1500 cN / tex, d) a titer of about 0.6 to 2.0, in particular from about 0.8 to 1.5 (dtex), and e) density of about 1.170 to 1.190 g / cm ³ . Carbon fiber precursor according to claim 5, characterized in that it contains, in addition to the polyacrylonitrile, further polymer, in particular in the form of cellulose, aramides, lignin, polyetherester ketones and / or polyethersulphones, in particular in an amount of up to 50% by weight. Carbon fiber precursor according to at least one of claims 5 or 6, characterized in that its roundness given by L / B is from about 2.0 to 1.0, in particular from about 1.5 to 1.0. Carbon fiber precursor according to at least one of claims 5 to 7, characterized in that its porosity according to the method of iodine absorption of about 1.5 to 0.1%, in particular from about 0.8 to 0.15%. Carbon fiber precursor according to at least one of claims 5 to 8, characterized in that it is in the form of a copolymer, the comonomer thereof in particular acrylamide, acrylic acid, butyl acrylate, ethyl acrylate, methyl methacrylate, methacrylic acid, itaconic acid, vinyl acetate, vinyl bromide and / or vinyl chloride. Carbon fiber precursor according to claim 9, characterized in that its comonomer content is less than about 10 mol%, in particular about 1 to 6 mol%. Carbon fiber precursor according to at least one of claims 5 to 10, characterized in that the molecular weight of the polyacrylonitrile is about 50,000 to 300,000, in particular about 80,000 to 250,000. Carbon fiber precursor according to at least one of the preceding claims 5 to 12, characterized in that the polyacrylonitrile has a polydispersity of about 1.5 to 3.5, in particular from about 1.8 to 2.8. A process for producing a carbon fiber precursor, in particular for producing a carbon fiber precursor according to at least one of the preceding claims 5 to 12, characterized in that a homo- or copolymer of polyacrylonitrile, optionally together with an additional other polymer, dissolved in an ionic liquid is, wherein the concentration of the polyacrylonitrile, optionally together with another polymer, in the ionic liquid at least about 5 wt .-%, in particular about 10 to 25 wt .-%, wherein insoluble liquid or solid additives are largely excluded, this Solution is wet spun in a precipitation or coagulation medium and the resulting fibers are stretched. A method according to claim 13, characterized in that an air gap is arranged below the spinneret. Process according to Claim 14, characterized in that ionic liquids of the general formula [Q +] _n [Z] ^n- are used, where the cation [Q +] _{n is} a quaternized ammonium [R1R2R3R4N +], phosphonium [R1R2R3R4P +] or sulfonium [R1R2R3S +] cation or an analogous quaternized nitrogen, phosphorus or sulfur heteroaromatic compound of the following formulas (I), (II), (III), (IV), (V) and (VI)

represents, wherein the radicals R1, R2, R3, R4 or the radicals R1 to R8 in the formulas (I) to (VI) independently of one another linear, cyclic, branched, saturated or unsaturated alkyl radicals, mono- or polycyclic, aromatic or heteroaromatic Rests or substituted with further functional groups derivatives of these radicals, wherein R1, R2, R3 and R4 may be interconnected, wherein the anion [Z] ^n- in the form of a halide, pseudohalide, amide, in the form of phosphorus bonds or nitro compounds. A method according to claim 15, characterized in that the halides or pseudohalides the formula F ^- , Cl ^- , Br ^- , I ^- , BF ₄ ^- , PF ₆ ^- , AlCl ₄ ^- , Al ₂ Cl ₇ ^- , Al ₃ Cl ₁₀ ^- , AlBr ₄ , FeCl ₄ ^- , BCl ₄ ^- , SbF ₆ ^- , AsF ₆ ^- , ZnCl ₃ ^- , SnCl ₃ ^- , CuCl ₂ ^- , CF ₃ SO ₃ ^- , (CN) ₂ N ^- , (CF ₃ SO ₃ ) ₂ N ^- , CF ₃ CO ₂ ^- , CCl ₃ CO ₂ ^- , CN ^- , SCN ^- , OCN ^- , the phosphorus compounds are phosphates of the formula PO ₄ ^3- , HPO ₄ ^2- , H ₂ PO ₄ ^- , R ¹ PO ₄ ^2- , HR ¹ PO ₄ ^- , R ¹ R ² PO ₄ ^- ; Phosphonates and phosphinates of the formula: R ¹ HPO ₃ ^- , R ¹ R ² PO ₂ ^- , R ¹ R ² PO ₃ ^- ; Phosphites of the formula: PO ₃ ^3- , HPO ₃ ^2- , H ₂ PO ₃ ^- , R ¹ PO ₃ ^2- , R ¹ HPO ₃ ^- , R ¹ R ² PO ₃ ^- ; and phosphonites and phosphinites of the formula: R ¹ R ² PO ₂ ^- , R ₁ HPO ₂ ^- , R ¹ R ² PO ^- , R ¹ HPO ^- represent. Method according to one of claims 15 or 16, characterized in that the alkyl radical in the form of a C ₁ -C ₁₈ -alkyl radical, in particular an alkyl radical having 1 to 4 carbon atoms, preferably a methyl, ethyl, 1-propyl, 2- Propyl, 1-butyl, or 2-butyl radical is present, the cyclic alkyl radical in the form of a C _3-10 cycloalkyl radical, in particular in the form of a cyclopropyl, cyclobutyl, cyclopentyl or cyclohexyl radical is present, the unsaturated alkyl radical in the form a vinyl, 2-propenyl, 3-butenyl, cis-2-butenyl, trans-2-butenyl radical is present, the aromatic radical is present in the form of a phenyl or naphthyl radical containing 1 to 3 halogen atoms, alkyl radicals with 1 to 4 carbon atoms or phenyl radicals, and the heteroaromatic radical is in the form of an O, S or N-containing heterocyclic radical having 2 to 5 carbon atoms. Method according to at least one of claims 13 to 17, characterized in that as ionic liquid [EMIM] [DCA], [EMIM] [Cl], [EMIM] [SCN] and / or [MMIM] [DMP] are used. A method according to any one of claims 13 to 18, characterized in that the fibers obtained after the wet spinning are washed, in particular with simultaneous stretching. Method according to at least one of claims 13 to 19, characterized in that ionic liquid is recovered from the coagulation medium used. Method according to at least one of claims 13 to 20, characterized in that water is used as the coagulation medium, in particular with an included amount of a solvent which optimizes the coagulation rate. A method according to claim 21, characterized in that for optimizing the coagulation rate an adjusted amount of ionic liquid remains in the coagulation medium. A method according to any one of claims 13 to 22, characterized in that the fibers withdrawn from the coagulation medium are dried, in particular in a convection oven, and optionally a stretching in a dry state is performed. Method according to at least one of claims 13 to 23, characterized in that the viscosity of the spinning solution containing the polyacrylonitrile in the wet-spinning process to 20 to 200 Pas, in particular 50 to 120 Pas and in the dry-wet spinning process to 500 to 50,000 Pas, in particular 1,000-20,000 Pas is set.