FIBERS FOR PRODUCING COMPOSITE MATERIALS AND METHODS OF PRODUCING CARBON FIBER PRECURSOR FIBERS AND CARBON FIBERS

- SGL CARBON SE

A method for producing carbon-containing fibers, in particular carbon fibers and/or the precursor fibers thereof, contains the following steps: a) providing one or more starting material fibers; b) bringing the one or more starting material fibers in contact with at least one treatment fluid, wherein a treatment fluid has at least one silicon compound and has a content of 0-25 wt. % water, in relation to the total weight of the treatment fluid; c) treating the one or more starting material fibers with the treatment fluid during a treatment time having a duration of at least three minutes at a treatment temperature ranging from 126 C to 450 C.

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Description
CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application, under 35 U.S.C. §120, of copending international application No. PCT/EP2010/068880, filed Dec. 3, 2010, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2009 047 514.1, filed Dec. 4, 2009; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to fibers, which are particularly suitable for producing C/SiC composite material as well as methods for producing these materials and their use to produce composite materials.

Methods for producing composite materials and in particular C/SiC composite materials have been known for many years. Usually during the production of a C/SiC composite material a carbon body is infiltrated with a suitable infiltration agent, for example, liquid silicon and is thereby ceramicized by a chemical reaction to form silicon carbide. U.S. Pat. No. 4,725,635 describes an agent for the after-treatment of synthetic fibers.

For example, German patent DE 44 38 455 and U.S. Pat. No. 5,486,379 describe methods for the production of C/SiC composite materials, where however the fibrous material used to produce the composite material had hitherto received no or only little attention. Likewise numerous documents teach methods for producing carbon fibers and their precursor fibers but usually do not address the question as to how carbon fibers or their precursor fibers can be produced and optionally surface-treated in order to be able to obtain high-quality composite materials.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide access to carbon fibers and their precursor fibers which enables high-quality fibers to be provided which are particularly suitable for the production of composite materials, in particular C/SiC composite materials. Such a method should additionally enable a reliable, safe conversion of the starting materials into carbon fibers. Furthermore, such a method should fundamentally also be suitable for producing fibers from more problematical starting materials from process engineering viewpoints, such as for example, polyacrylonitrile homopolymers, modacryl or cellulose and enable a reaction to be carried out at comparatively low temperatures. For methods on an industrial scale it is also extremely important that the method can be carried out cost-effectively.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in fibers for producing composite materials and methods of producing carbon fiber precursor fibers and carbon fibers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a photograph showing an SEM image of a polyacrylonitrile fiber used as starting material in the method according to the invention;

FIG. 2 is a photograph showing an ESEM image of a prior art fiber initially oxidized in a gas phase and then carbonized at 1450° C.;

FIG. 3 is a photograph showing an ESEM image of a carbon fiber precursor fiber according to the invention from which the treatment fluid has not been removed by a washing process;

FIG. 4 is a photograph showing an ESEM image of a carbon fiber precursor fiber according to the invention from which the treatment fluid has been at least partially removed with methyl ethyl ketone;

FIG. 5 is a photograph showing an ESEM image of a carbon fiber according to the invention from which the treatment fluid has not been removed by a washing process;

FIG. 6 is a photograph showing an ESEM image of a carbon fiber according to the invention from which the treatment fluid has been at least partially removed with methyl ethyl ketone;

FIG. 7 is a graph showing a temperature-time profile of a treatment with treatment fluid; and

FIG. 8 is a graph showing a comparison of the chemical composition and density of precursor fibers according to the invention.

 DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is solved by providing a method for producing carbon-containing fibers, in particular carbon fibers and/or their precursor fibers which includes: a) providing one or more starting material fibers; b) bringing the one or more starting material fibers in contact with at least one treatment fluid wherein a treatment fluid contains at least one silicon compound and has a content of 0-25 wt. % of water, relative to the total weight of the treatment fluid; c) treating the one or more starting material fibers with a treatment fluid during a treatment time interval having a duration of at least three minutes at a treatment temperature in a range of 126° C. to 450° C.; d) optionally after-treating the fibers treated with treatment fluid, wherein the after-treatment comprises one or more mechanical after-treatment processes and/or one or more washing processes; e) optionally drying the treated and optionally after-treated fibers; f) optionally recovering the treated and optionally after-treated and/or optionally dried fibers.

During numerous experiments which led to the present invention, it was surprisingly established that not only carbon fibers and their precursor fibers having very advantageous combinations of material properties can be obtained by the method according to the invention but in addition, the method according to the invention makes it possible to transform the starting material with very high process safety. In particular, it is highly advantageous that the method according to the invention can optionally be carried out at comparatively low temperature and optionally a direct contact of the fibers with an oxygen-containing gas phase at elevated temperatures can be completely or at least partially avoided during the production of the carbon fiber precursor fibers. In addition, the carbon fibers produced according to the invention have a high carbon content, an advantageous density, a high silicon content, in particular on and/or at the surface and a good tensile strength and are exceptionally suitable for producing composite materials, in particular for producing C/SiC composite materials. Furthermore, the method according to the invention can be implemented on an industrial scale and can be controlled reliable and simply. Furthermore, the method according to the invention provides shorter process times and results in good yields of carbon fibers, where the carbon fibers additionally have a high carbon content and a small number of defects. Due to the treatment of the fibers with a treatment fluid, an advantageous heat transfer additionally takes place so that the method according to the invention exhibits good utilization of the input energy.

An important advantage of the method according to the invention is additionally that this makes it possible to produce carbon fibers starting from hitherto difficult starting material such as fibers based on homopolymeric polyacrylonitrile or modacryl and its hybrid fibers or fibers based on pitch, lignin, viscose or cellulose since the method can provide implementation of the initial oxidation reaction or the initial thermal stabilization or a section of the initial oxidation reaction or a section of the initial thermal stabilization at low temperatures.

The method according to the invention for producing carbon-containing fibers, in particular carbon fibers and/or their precursor fibers contains in a first step the provision of one or more starting material fibers.

The term “starting material fiber” as used in the present application contains any carbon-containing fiber which can be used in particular for producing carbon fibers and/or precursor fibers for producing carbon fibers. In addition to one or more carbon-containing fibers which can be used in particular for producing carbon fibers and/or precursor fibers for producing carbon fibers, the plurality of starting material fibers can additionally comprise one or more ceramic-based fibers.

Particularly good results are obtained in the method according to the invention when using one or more starting material fibers which comprise fibers or consist of these selected from the group consisting of polyacrylonitrile-based fibers, where this can comprise homopolymeric and/or copolymeric polyacrylonitrile, polyacetylene-based fibers, polyphenylene-based fibers, cellulose-based fibers, pitch-based fibers, lignin-based fibers, viscose-based fibers, polypropylene-based fibers, mixtures of carbon-containing fibers and ceramic-based fibers and mixtures thereof. Methods for producing these fibers are in each case familiar to a person skilled in the art and are additionally explained hereinafter. In the case of polymer-based starting material fibers, the fibers can be produced, for example, from the particular homopolymer and/or copolymer.

The method according to the invention further includes bringing the one or more starting material fibers in contact with at least one treatment fluid wherein a treatment fluid contains at least one silicon compound and has a content of 0-25 wt. % of water, relative to the total weight of the treatment fluid and treating the one or more starting material fibers with a treatment fluid during a treatment time interval having a duration of at least three minutes at a treatment temperature in a range of 126° C. to 450° C.

During the bringing in contact of the one or more starting material fibers with the treatment fluid, this can either comprise a selected treatment temperature from the range of about 126° C. to 450° C., in particular from the range of 185° C. to 282° C. or a lower temperature, in particular below the range of the treatment temperatures, for example a temperature from the range of 0° C. to below about 126° C., in particular from the range 0° C. to below about 185° C., and be heated from this temperature to the selected temperature(s) in the treatment temperature range. Optionally after the end of the treatment time interval, the one or more starting material fibers can remain in contact with the treatment fluid which then has a temperature below the treatment temperature range, for example, a temperature from the range of 0° C. to below about 126° C., in particular from the range of 0° C. to below about 185° C. and remain there for a further time interval, for example, of at least one minute.

Without the present invention being restricted to the correctness of the theory put forward previously, it is assumed that during this treatment of the starting material fibers, an at least partial cyclization and at least partial dehydrogenation or dehydration takes place in particular with the formation of H2O so that fibers are obtained which compared to the starting material fibers offer less possibilities for attack during an oxidative attack by oxygen. Within the framework of the present application, fibers which have undergone a treatment with the at least one treatment fluid and have thereby experienced an at least partial chemical change are also called stabilized fibers. Preferably the step of treatment with the treatment fluid is accomplished in such a manner that the fibers obtained after this step are no longer fusible.

In particular, in the method according to the invention it is advantageous that the step of treating the starting material fibers with at least one treatment fluid need not necessarily take place with the exclusion of oxygen or in an atmosphere having a reduced oxygen content but in an ambient air atmosphere (for example, an atmosphere comprising about 76 wt. % nitrogen, about 23 wt. % oxygen, about 1 wt. % noble gases in relation to the total weight of the atmosphere). A gas containing oxygen can optionally be introduced into the treatment fluid. However, no addition of oxygen donors to the mixture or introduction of inert gas and/or oxygen through the mixture is absolutely essential during the treatment step: optionally therefore such an addition and/or such an introduction can be dispensed with. Without the present invention being restricted to the correctness of the theory put forward previously, it is assumed that by using a treatment fluid the oxidation takes place uniformly and can be well controlled, where it is particularly advantageous that the transfer of heat to the fibers takes place via a liquid or at least partially liquid treatment fluid.

The term “treatment fluid” contains any fluid that contains at least one silicon compound and is in a liquid or at least flowable state at the selected treatment temperatures. The treatment fluid is preferably in a liquid or at least flowable state at 20° C., preferably at 125° C., further preferably at 150° C., in particular at 165° C. The term “silicon compound” as used within the framework of the present invention contains any chemical, in particular any organic chemical compound which contains silicon, carbon and hydrogen and optionally, for example, oxygen and nitrogen or is constructed from these. A person skilled in the art can select suitable silicon compounds on the basis of his technical knowledge and can select the treatment temperature depending on the ignition point and/or flame point and the decomposition effects which occur in the treatment fluid.

The term “treatment of the starting material fibers with at least one treatment fluid” not only includes a treatment with a treatment fluid of constant or substantially constant temperature and/or composition but also treatments in which the temperature or composition of the treatment fluid is varied during the treatment time interval. In particular, the water content or the content of one or more silicon compounds can be varied. This can be accomplished, for example, by adding water or one of several compounds to the mixture during the treatment time interval and/or by increasing or lowering the temperature during the treatment time interval. If desired, within the treatment time interval a transfer into at least one other spatially separate treatment fluid, for example, in one or more other containers can take place, which in each case contains at least one silicon compound (which can differ from the silicon compound(s) present in the first treatment fluid).

The term “treatment temperature” describes the temperature of the treatment fluid in which the fibers to be treated are located or that surrounds these fibers.

Fibers having very advantageous properties for use in producing composite materials can be obtained if the treatment temperature preferably lies in a range of 132° C. to 362° C., preferably in a range of 136° C. to 360° C., further preferably in a range of 165° C. to 320° C., in particular in a range of 185° C. to 282° C.

The treatment time interval can preferably have a duration of at least 5 minutes, preferably at least 9 minutes, preferably 5 to 750 minutes, further preferably 5 to 200 minutes, in particular 3 to 120 minutes, more particularly 17 to 100 minutes, even more particularly 21 to 65 minutes.

The treatment of the starting material fibers can be accomplished simply and advantageously in terms of process technology by bringing in contact, in particular, dipping the starting material fibers in a treatment fluid that has a temperature in the treatment temperature range or that after bringing in contact, in particular after dipping, is heated to a temperature in the treatment temperature range. Preferably the starting material fibers can be dipped in a treatment fluid in a container or bath. A treatment of the starting material fibers can be accomplished, for example, by an impregnation process, by spreading, by application by roller or other procedures known to a person skilled in the art.

The treatment fluid can contain one or more silicon compounds. Fibers having very good properties for producing composite material can be obtained, for example, if the treatment fluid contains at least one silicon compound selected from the group consisting of polydialkyl siloxanes, polydiaryl siloxanes and polymonoalkylmonoaryl siloxane. In particular, the treatment fluid can contain polydimethyl siloxanes, polyphenylmethyl siloxanes, polydiphenyl siloxanes and mixtures thereof. An example of a treatment fluid is available under the trade name UCOTHERM X-BF von FRAGOL Schmierstoff GmbH & Co. KG, D-45481 Mülheim an der Ruhr.

Fibers having very good properties for use in composite materials can be obtained, for example, if the treatment fluid has a water content of 0.001-22 wt. %, preferably 0.1-15 wt. %, preferably 0.5-12 wt. %, in particular 0.8-8 wt. % in each case relative to the total weight of the treatment fluid. The total content of silicon compounds in the treatment fluid can, for example, by at least 65 wt. %, preferably 70-100 wt. %, preferably 75-99 wt. %, in particular 88-98 wt. %, relative to the total weight of the treatment fluid.

Without the present invention being restricted to the correctness of the theory put forward previously, it is assumed that the presence of water in the treatment fluid promotes the desired conversion of carbon-containing starting material fibers into precursor fibers. At the present time it is assumed that this promotion of the conversion of the starting material fibers can be based on the capability of the water to form hydrogen bridging compounds and/or a stabilizing attachment of water, H3O+ and/or OH− to a carbon-containing fiber which allows an energetically more favorable transition state in the reactions taking place, for example, oxidations.

In addition, the treatment fluid can have one or more additives such as, for example, one or more lubricating agents, one or more stabilizers, one or more viscosity regulators, one or more oxygen donors such as for example manganese oxide (manganese dioxide) or permanganates such as potassium permanganate, one or more corrosion inhibitors, organic and inorganic acids, organic and inorganic bases, salts, buffer mixtures as well as formaldehyde. These additives can be selected by a person skilled in the art on the basis of his general technical knowledge and depending on the precursor fibers or carbon fibers to be produced and adapted quantitatively.

The production of the at least one treatment fluid can be accomplished by a person skilled in the art on the basis of his general technical knowledge. For example, the one or more silicon compound(s) and the water and optionally one or more additives can be mixed by stirring.

Very good quality fibers can be obtained, for example, if the step of treating the one or more starting material fibers with the treatment fluid includes the fact that the treatment of the one or more starting material fibers takes place in a first temperature range and at least one second temperature range, wherein the lowest temperature of the at least one second temperature range is higher than the highest temperature of the first temperature range and the treatment begins with a treatment of the one or more starting material fibers in the first temperature range. Without the present invention being restricted to the correctness of the theory put forward previously, it is assumed that such a reaction control leads to fibers having high loading capacity having a very small number of defects in the fibers.

The term “treatment in a first temperature range” and “treatment in at least one second temperature range” includes the fact that the treatment can take place at a specific temperature or in a range of temperatures.

In particular, the first temperature range can be a temperature range selected from the range of 126° C. to 260° C., preferably selected from the range of 152° C. to 250° C., preferably selected from the range of 190° C. to 240° C. and/or the second temperature range is a temperature range selected from the range of 140° C. to 450° C., preferably selected from the range of 180° C. to 290° C., preferably selected from the range of 195° C. to 282° C. In this case, the first and/or second temperature range can either contain the entire range of temperatures specified in each case or a partial region hereof, which for example can comprise 10° C., preferably 5° C., preferably 2° C., in particular 1° C. The term “the at least one second temperature range” can for example, contain a second to tenth temperature range, preferably a second to fifth temperature range, in particular a second to third temperature range. In this case, the lowest temperature of the temperature range with the respectively next higher numbering is in each case higher than the highest temperature of the temperature range having the numbering being considered, preferably in relation to a temperature range having a numbering being considered. The treatments preferably begin in the first temperature range, then take place in the second temperature range and are then optionally executed in the third temperature range, then in the fourth temperature range and in the further temperature ranges according to their numbering.

For example, a first temperature range can extend from 195 to 205° C., a second temperature range from 206 to 215° C., a third temperature range from 216 to 225° C. In addition, for example, a first temperature range can extend from 218 to 222° C., a second temperature range from 223 to 227° C., a third temperature range from 228 to 232° C., a fourth temperature range from 233 to 237° C., a fifth temperature range from 238 to 242° C., a sixth temperature range from 243 to 247° C., a seventh temperature range from 248 to 252° C. Very good results can be obtained in particular if the first temperature range extends from 200 to 245° C. and the at least one second temperature range extends from 246° C. to 260° C.

Numerous temperature profiles enable the treatment of the one or more starting material fibers to take place in a first temperature range and at least one second temperature range where the lowest temperature of the at least one second temperature range is higher than the highest temperature of the first temperature range. A restriction of the present invention to temperature profiles specified as an example within the framework of the present application is not intended.

The temperature of the treatment fluid can increase continuously or discontinuously during the entire treatment time interval or one or several time segments of the treatment time interval. For example, the temperature can increase during the entire treatment time interval. The temperature rise can take place in any manner, for example, linearly or exponentially. Preferably the entire duration of the one or more subsections of the treatment time interval with increasing temperature is at least 5%, preferably at least 30%, preferably at least 45%, in particular 5 to 20% of the total duration of the treatment time interval.

Fibers having a very advantageous property profile can be obtained, for example if time intervals having constant or substantially constant temperature and time intervals with increasing temperature alternate during the treatment time interval. With reference to a treatment with a treatment fluid, the term “having constant temperature” can contain, in addition to the temperature remaining constant, variation of the temperature by for example up to ±5° C., preferably up to ±3° C., preferably up to ±2° C. and the term “having substantially constant temperature” can contain variations in temperature by, for example, up to ±5° C., preferably up to ±3° C., preferably up to ±2° C. as well as slowly increasing or decreasing temperatures, for example, of up to less than 0.5° C. per minute, preferably of up to 0.3° C. per minute. Without the present invention being restricted to the correctness of the theory put forward previously, it is assumed that such a reaction control in which there are both time segments in which the temperature is kept constant or substantially constant and also time segments in which the temperature is varied, preferably increased, leads to fibers having a high loading capacity with a very small number of defects in the fibers.

Very high-quality fibers can be obtained in particular if the temperature of the treatment fluid is increased in steps at least during a section of the treatment time interval. In particular, the treatment time interval can contain at least one time segment during which the temperature is kept constant or substantially constant at one temperature and comprises at least one other time segment during which the temperature is varied, preferably increased. The treatment time interval preferably contains at least two, preferably at least three, further preferably at least five, even further preferably at least ten, in particular two to twelve time segments during which the temperature is varied. The variation in temperature can then take place in any manner, for example, linearly or exponentially. Furthermore, the treatment time interval can comprise at least two, preferably at least three, further preferably at least five, even further preferably at least ten, in particular two to twelve time segments during which the temperature is kept constant or substantially constant.

In particular a time segment during which the temperature is kept constant or substantially constant can have a duration of at least two minutes, preferably of at least 5 minutes, preferably of at least 15 minutes, further preferably at least 20 minutes, in particular of two to 40 minutes, more especially of three to five minutes. Furthermore a time segment during which the temperature is varied can have a duration of at least two minutes, preferably of at least 5 minutes, preferably of at least 15 minutes, further preferably at least 20 minutes, in particular of two to 115 minutes, more especially of three to five minutes.

Very good quality fibers can be obtained if the ratio of the total duration of the time segments in which the temperature is kept constant or substantially constant to the total duration of the time segments in which the temperature is varied is 1:10 to 15:1, preferably 1:5 to 10:1, particularly preferably 1:2 to 2:1.

Particularly advantageous results in relation to fiber quality can be obtained if during the entire treatment time interval or at least during 30% of the entire during of the treatment time interval, time segments in which the temperature is kept constant or substantially constant and also time segments in which the temperature is varied, preferably increased, alternate.

The variation of the temperature can be accomplished in such a manner, for example, that the temperature of the treatment fluid is increased or decreased at least during one minute by 0.5 to 15° C., preferably by 5 to 10° C. during one minute. The temperature variation during a temperature variation time interval can, for example, increase or decrease linearly or exponentially or can have any profile which can be selected by the person skilled in the art.

Fibers having particularly advantageous properties can be obtained, for example if the fibers are present in a clamped state before and/or at least partly during the step of treatment with the treatment fluid. In terms of process technology it is advantageous if, for example, the fibers present in a clamped state are drawn through a mixture containing at least one silicon compound and water, that is located for example in a container.

The transfer of the starting material fibers into a clamped state can be achieved in any manner known to the person skilled in the art. It is advantageous in terms of process technology if this is accomplished by clamping between one or several devices or by tensile loading. Procedures by which fibers can be transferred into a clamped state are familiar to a person skilled in the art.

The fibers treated with the treatment fluid are subjected to an optional after-treatment in another process step in which treatment fluid remaining on the treated fibers, in particular remaining silicon compounds and/or remaining water is at least partially or completely removed from the treated fibers. The after-treatment can contain one or more mechanical after-treatment processes and/or one or more washing processes.

This can be accomplished for example by subjecting the fibers to a pressing process, for example, by passing between rollers or pressing with absorbent material, for example, paper, where the fluid present or remaining on the fibers is at least partially removed. From the process technology viewpoint it is advantageous if these separated silicon compounds and/or this water is collected and optionally re-used in the method according to the invention after purification or filtering. Furthermore, the after-treatment can include allowing the fibers to drip, for example, for at least 1 minute. Extraction of the treatment fluid can also take place as after-treatment.

In addition, the after-treatment can contain a washing process being carried out with a solvent or a solvent mixture. Such a washing process can, for example, be carried out by spraying a solvent or solvent mixture. The washing process can also be carried out by dipping the treated fibers in a solvent or solvent mixture one or several times or by drawing or guiding the treated fibers through a container containing a solvent or solvent mixture. The solvent or solvent mixture during a washing process can have a temperature, for example, in a range of 0° C. up to the boiling point of the solvent or solvent mixture. In order to obtain high-quality fibers, the washing process can be executed, for example, in the form of an extraction, in particular an extraction of the Soxhlet type or a through-flow extraction. Numerous embodiments for extractions of the Soxhlet type are familiar to the person skilled in the art and can be adapted to the problems occurring in specific applications. A structure of a Soxhlet extraction apparatus is shown, for example in H. G. O. Becker et al., Organikum, VEB Deutscher Verlag der Wissenschaften, 1988, ISBN 3-326-00076-6, p. 55-56. Other embodiments are possible and can be selected by a person skilled in the art and adapted to the method according to the invention. For example, the fibers can be arranged on a solvent-permeable base and come in contact there with the particular solvent or solvent mixture. In this case a part of the treatment fluid dissolves in the solvent or solvent mixture or is entrained by this and passes through the solvent-permeable base. Optionally, the mixture thus separated, after passing through, enters into a container in which it is heated to boiling. In this case, the component having the lowest boiling point, the solvent or solvent mixture, is heated to boiling, separated and again brought in contact with the fibers to be extracted. In addition, the fibers can be treated with ultrasound in a bath containing solvent or solvent mixture. An after-treatment can also contain an application of silicon removers to the fibers where such an after-treatment is considered as a washing process within the framework of the present invention.

Preferably, fluid still remains on the fibers treated previously with treatment fluid after an after-treatment.

Both organic and inorganic solvents and mixtures thereof can be used as a solvent or solvent mixture for carrying out the washing process. Particularly good-quality fibers can be obtained if a solvent or solvent mixture is used for a washing process, selected from the group consisting of water, isopropanol, methyl ethyl ketone, ethanol, hexane, pentane, toluene, xylene, benzene, acetone, silicon removers and mixtures thereof. Optionally, one or more additives can be added to the solvent or solvent mixture such as for example, one or more lubricating agents, one or more stabilizers, one or more viscosity regulators, one or more anti-oxidants, one or more corrosion inhibitors. These additives can be selected by a person skilled in the art on the basis of his general technical knowledge and depending on the precursor fibers or carbon fibers to be produced and can be adapted quantitatively.

The washing process, which is also designated as de-sizing within the framework of the present invention, is preferably carried out by bringing the fibers in contact with the selected solvent, preferably by extracting, for example during at least 40 minutes, preferably during 230 to 250 minutes. Preferably methyl ethyl ketone or silicon removers and aqueous mixtures thereof are used. The quantity of silicon-containing deposits at and/or on the pre-oxidized fibers present after the treatment with the treatment fluid can be controlled by selecting a solvent or the duration and number of washing processes. The precise duration of the washing process or processes can be determined experimentally by a person skilled in the art. Likewise, it is possible to use fibers which have been subjected to a washing process, optionally after a preceding pressing, dripping or extraction of the treatment fluid.

Optionally, the washing process can also be repeated. In particular, a combination of one or more washing processes is possible. Such a combination contains two or more of the washing processes selected from the group consisting of washing process by spraying, washing process by dipping, particularly under the action of ultrasound, and washing process by extraction.

After-treatments can be carried out once or several times and optionally combined with one another in any sequence by a person skilled in the art on the basis of his general technical knowledge.

Furthermore, the after-treated, stabilized fibers, i.e., which have been subjected, for example, to a washing process and/or a pressing process and/or a dripping process and/or an extraction process, can be optionally dried at a temperature in a range of 20° C. to 170° C. A drying can take place in a gas mixture in the presence of oxygen, for example, with a volume content of 18-35 vol. % of oxygen, relative to the total volume of the gas mixture, further, for example, in an ambient air mixture or free from or substantially free from oxygen, for example with a volume content of less than 1 vol. %, preferably less than 0.08 vol. % of oxygen, in each case relative to the total volume of the gas mixture.

Following the steps explained hereinbefore, which include a treatment of the starting material fibers and an optional after-treatment of the fibers thus treated, as well as an optional drying, fibers can be obtained which are designated as precursor fibers within the framework of the present application.

If a polyacrylonitrile-based fiber is used as the starting material fiber, a pre-oxidized polyacrylonitrile-based fiber, which is usually designated as PAN-Ox fiber can be obtained as precursor fiber.

Fibers of exceptional quality can be obtained, for example if a polyacrylonitrile-based fiber is used which, in addition to acrylonitrile, contains 1.5 wt. % of itaconic acid and 4.5 wt. % of methyl acrylate.

The starting material fibers according to the invention preferably contain so-called endless fibers, that is, fibers which, for example, can have a length of at least 5 cm, preferably at least 10 cm. Endless fibers, e.g. in the form of a fiber spool, can preferably be several kilometers long.

A very advantageous carbon fiber precursor fiber can be obtained, for example, according to a method which includes the previously specified and explained process steps a) to c) and further the following steps: d) optionally after-treating the fiber treated with treatment fluid, wherein the after-treatment consists of one or more mechanical after-treatment process and no washing process takes place between step c) of the treatment and step e) of the drying; e) drying the treated and optionally after-treated fiber. The carbon fiber precursor fiber is also designated in everyday language as “non-sized” carbon fiber precursor fiber. A carbon fiber precursor fiber according to the invention can contain silicon-containing deposits on its surface and can have a silicon content, for example in a range of at least 19 wt. %, preferably of 19.3 to 20.6 wt. % and/or a carbon content of, for example, 49.9 to 63.5 wt. %, preferably of 50.0 to 63.4 wt. % and/or an oxygen content of, for example, 12.4 to 14.0 wt. %, preferably of 12.5 to 13.9 wt. % and/or a nitrogen content of, for example 3.3 to 16.2 wt. %, preferably of 3.4 to 16.1 wt. %, in each case relative to the total weight of the carbon fiber precursor fiber comprising silicon-containing deposits on the surface thereof. The density of the fiber is preferably 1.22 to 1.44 g/cm3. The fiber can preferably be obtained starting from a polyacrylonitrile-based fiber. The nitrogen content can be reduced and the carbon content increased depending on the duration of the treatment with the treatment fluid and the treatment temperature.

Without the present invention being restricted to the correctness of the theory put forward previously, it is assumed that in the case of carbon fiber precursor fibers according to the invention, the silicon is present at least partially, in particular predominantly, in the form of silicate compounds. This is indicated by the position of the Si peak in x-ray photoelectron spectroscopy (XPS) measurements.

FIG. 3 shows fibers which can be obtained starting from polyacrylonitrile-based fibers which after the step of treatment with the treatment fluid, have only been pressed between paper and then dried at 105° C. Silicon-containing deposits on or at the surface are clearly identifiable. Such a fiber which was not subjected to any washing process is designated within the framework of the present invention as non-sized, stabilized fiber.

The term “fiber having deposits on its surface”, as used within the framework of the present application, can contain fibers which have deposits on and/or partially or completely in and/or partially or completely under their surface.

Black washes, finish, preparation layers and/or coatings applied to a carbon fiber after completion of the carbonization (in particular after completion of a temperature treatment according to the method according to the invention) as well as black washes, finish, preparation layers and/or coatings applied to a carbon fiber precursor fiber after completion of the stabilization treatment (in particular after completion of the treatment with treatment fluid according to the method according to the invention) preferably do not come under the term deposits within the framework of the present application. Optionally however, black washes and/or one or more coatings can be additionally applied to the fibers according to the invention which can have silicon-containing deposits on their surface. Preferably, the deposits are completely or at least partially in the form of particles which adhere to the fiber surface and/or tend not to penetrate since otherwise the fiber is destroyed or no longer present as fiber. The particles can have any shape and can, for example, be leaf-shaped or substantially spherical. For example, the greatest length extension of the particles can be less than 40 μm, preferably less than 8 μm, preferably less than 2 μm. In particular the greatest length extension of the particles can be less than the greatest fiber diameter, preferably less than one third of the greatest fiber diameter, preferably less than one fifth of the greatest fiber diameter.

For comparison purposes, FIG. 1 shows a polyacrylonitrile-based fiber which was used as starting material fiber and FIG. 2 shows a fiber initially oxidized in the gas phase and then carbonized at 1450° C. in an inert atmosphere according to a method of the prior art.

In addition, the method according to the invention can be optionally combined with other treatment steps which take place before or after the treatment step with the treatment fluid, in which, before obtaining the precursor fiber, one or more further treatments take place which involve heating the treated fibers or the starting material fibers to a temperature of more than 126° C., in particular more than 200° C. For example, the method according to the invention can contain a further treatment step in which the treated fibers or the starting material fibers are brought into contact with an oxygen-containing gas, for example, in a furnace, at a temperature of more than 126° C., in particular more than 200° C.

With a view to a saving of working time, energy and process costs, as well as desired fiber properties and improvement of process safety, the method according to the invention preferably however merely includes the treatment step with the treatment fluid as a single process step at temperatures of more than 126° C., in particular more than 200° C. before an optional recovery of pre-oxidized carbon fiber precursor fibers and/or a final conversion to carbon fibers by carrying out a temperature treatment.

In order to obtain fibers which have advantageous properties in particular for the production of composite materials, the method according to the invention can be carried out in such a manner that after step c) of the treatment and before step g) of the temperature treatment, the one or the plurality of fibers does not come in contact with an atmosphere having a content of more than 5 wt. % of oxygen and a temperature of 70° C. or more, preferably of 50° C. or more, preferably of 38° C. or more or comes in contact for not longer than a time interval of, for example, one minute.

Fibers which have undergone a washing process are shown, for example, in FIG. 4. These fibers which can be obtained starting from a polyacrylonitrile-based fiber, were pressed with paper, extracted for 240 minutes with methyl ethyl ketone in a Soxhlet apparatus and then dried in an ambient air atmosphere at 105° C. Silicon-containing deposits on or at the surface are clearly identifiable. Fibers which have undergone one or more washing process are designed within the framework of the present invention as “de-sized” fibers. The term ambient air atmosphere can describe any air mixtures which are present in human working spaces and/or in the open air, in particular so-called atmospheric air. Compositions of these atmospheres are known to a person skilled in the art. In particular, the term ambient air atmosphere can describe atmospheres containing about 75-76 wt. % of nitrogen, about 23-24 wt. % of oxygen, about 1-2 wt. % of inert gases and optionally other gases having a content of less than 1 wt. % in each case relative to the total weight of the atmosphere, as well as mixtures thereof with water vapor.

A very advantageous carbon fiber precursor fiber can be obtained, for example, according to a method which comprises the previously explained process steps a) to c) and further the following steps: d) after-treatment of the fiber treated with the treatment fluid by one or more washing processes and optionally after-treatment by one or more mechanical after-treatment processes; e) optionally drying the after-treated fiber. This fiber is also designated in everyday language as “non-sized” carbon fiber precursor fiber. A carbon fiber precursor fiber according to the invention can contain silicon-containing deposits on its surface and can have a silicon content, for example in a range of at least 0.5 wt. %, preferably of 0.55 to 4.6 wt. %, preferably of 2.0 to 4.5 wt. % and/or a carbon content of, for example, 52.9 to 81.6 wt. %, preferably of 62.6 to 66.1 wt. % and/or an oxygen content of, for example, 3.3 to 9.7 wt. %, preferably of 5.6 to 9.6 wt. % and/or a nitrogen content of, for example 7.3 to 34.4 wt. %, preferably of 21.3 to 23.9 wt. %, in each case relative to the total weight of the carbon fiber precursor fiber comprising silicon-containing deposits on the surface thereof. The density of the fiber is preferably 1.30 to 1.50 g/cm3. The nitrogen content can be reduced and the carbon content increased depending on the duration of the treatment with the treatment fluid and the treatment temperature. The fiber can preferably be obtained starting from a polyacrylonitrile-based fiber.

Preferably during a treatment with a treatment fluid the treatment time and the treatment temperature are selected in such a manner that carbon fiber precursor fibers are obtained for which during a DSC measurement an amount of heat Q in the range of 175 to 310 J is released, determined in accordance with DIN ISO 11357-3.

After an optional after-treatment of the fiber treated with the treatment fluid and an optional drying of the after-treated fiber, the treated, and optionally after-treated and/or optionally dried fiber can optionally be recovered. This fiber can then be converted into a carbon fiber in one or more further steps. If desired, however, no separate step of extraction need take place but the fiber can be subjected directly to one or more steps for further conversion as far as the carbon fiber after treatment with the treatment fluid.

In a further process step, which can result in the carbon fiber, a temperature treatment takes place. The temperature treatment can be carried out with the treated and optionally after-treated and/or optionally dried fiber which was obtained after the preceding process steps. During the temperature treatment the fiber is exposed to one or more temperatures in the range of 450° C. to 1800° C., preferably 450° C. to 1620° C. in an inert atmosphere during a temperature treatment time interval. The temperature treatment can additionally contain a heating-up phase in which heating up takes place, for example, starting from 20° C. up to 450° C., and a cooling phase in which cooling takes place from 450° C. to lower temperatures, for example, down to 20° C. The temperature treatment time interval can, for example, lie in a range of 1 to 10 hours, preferably in a range of 3 to 8 hours.

Alternatively in a continuous process the treatment time at a temperature of 450°-1000° C. can be 1-5 min and a temperature of 1000° C.-1450° C., 1-5 min.

Continuous process means that the fiber is drawn continuously through two successive furnaces. In the first furnace the fibers are thermally treated from 450°-1000° C. in an inert gas atmosphere. In the second furnace then from 1000° C.-1450° C. Dwell times in the furnaces are in each case between 1-5 min, while the fibers are held under tension by driven rolling works.

The term “inert atmosphere” contains within the framework of the present invention any atmosphere which is free from oxygen or has an oxygen content of less than 5 wt. %, preferably of less than 1 wt. %, preferably of less than 0.2 wt. %, further preferably of less than 0.1 wt. %, in particular of less than 0.001 wt. % relative to the total weight of the atmosphere. A preferred inert atmosphere for example, has a content of at least 99.9 wt. % nitrogen and a remainder of other gases in relation to the total weight of the atmosphere. Preferably the gases of the remainder consist of noble gases, oxygen and carbon oxides, preferably of noble gases. In addition, an atmosphere can be used as “inert atmosphere” which has a pressure of less than 1 atm, i.e. an atmosphere which was obtained after a partial or extensive evacuation of the container containing the atmosphere. The inert atmosphere can advantageously contain nitrogen and/or noble gases.

The temperature treatment is preferably accomplished in such a manner that the fibers subjected to this treatment are converted into a carbon fiber. After the temperature treatment a recovery of carbon fibers can optionally take place.

Optionally further process steps can take place after a temperature treatment such as, for example, a coating of the carbon fibers or a surface treatment of the carbon fibers.

In addition, it has proved advantageous to achieve good-quality fibers if the temperature treatment is accomplished in such a manner that the temperature treatment takes in a first temperature treatment temperature range and at least one second temperature treatment temperature range where the lowest temperature of the at least one second temperature treatment temperature range is higher than the highest temperature of the first temperature treatment temperature range and the temperature treatment begins in the first temperature treatment temperature range. In particular, the first temperature treatment temperature range can, for example, be a temperature range selected from the range of 450° C. to 1000° C. (prior to which optionally heating from lower temperatures, for example, 20° C. can take place) and/or the at least one second temperature range can, for example, be a temperature range selected from the range of 1001° C. to 1450° C., preferably selected from the range of 1001° C. to 1375° C., preferably selected from the range of 1001° C. to 1320° C. In this case, the first and/or second temperature range can either comprise the entire range of temperatures specified in each case or a partial region hereof, which for example can contain 10° C., preferably 5° C., preferably 2° C., in particular 1° C.

In particular, the temperature treatment can contain an increase in temperature at least during one section, in particular a stepwise increase in temperature.

In addition, the temperature treatment can take place during at least one time interval having constant or substantially constant temperature and/or during at least one time interval with increasing temperature. With reference to the temperature treatment, the term “having constant temperature” can contain, in addition to the temperature remaining constant, variations of the temperature by for example up to ±10° C., preferably up to ±3° C., preferably up to ±2° C. and the term “having substantially constant temperature” can contain variations in temperature by, for example, up to ±10° C., preferably up to ±3° C., preferably up to ±2° C. as well as slowly increasing or decreasing temperatures, for example, of up to less than 0.5° C. per minute, preferably of up to 0.3° C. per minute.

A very advantageous carbon fiber can be obtained, for example, according to a method which comprises the previously process steps a) to c) and further the following steps: d) optionally after-treating the fiber treated with treatment fluid, wherein the after-treatment consists of one or more mechanical after-treatment process and no washing process takes place between step c) of the treatment and step f) of the temperature treatment; e) optionally drying the treated and optionally after-treated fiber; f) carrying out a temperature treatment of the treated and optionally after-treated and/or optionally dried fibers, wherein the fiber is exposed to a temperature of at least 450° C. in an inert atmosphere during a temperature treatment time interval. A carbon fiber according to the invention can contain silicon-containing deposits on its surface and can have a carbon content, for example of 94.0 wt. % to 94.6 wt. %, preferably of 94.2 wt. % to 94.4 wt. % and/or a silicon content in a range of more than 0.45 wt. %, preferably of 0.48 wt. % to 1.0 wt. %, preferably of 0.5 wt. % to 0.6 wt. % and/or a nitrogen content of 2.4 wt. % to 2.9 wt. %, preferably of 2.6 wt. % to 2.7 wt. % and/or an oxygen content of 1.5 wt. % to 2.1 wt. %, preferably of 1.7 wt. % to 1.9 wt. %, in each case relative to the total weight of the carbon fiber comprising silicon-containing deposits on the surface thereof.

Such a fiber has a density which is preferably more than 1.20 g/cm3 and preferably a density in a range of 1.42 g/cm3 to 1.82 g/cm3. The fiber can preferably be obtained starting from a polyacrylonitrile-based fiber.

Without the present invention being restricted to the correctness of the theory put forward previously, it is assumed that in the case of carbon fiber according to the invention, the silicon is present at least partially, in particular predominantly, in the form of silicon dioxide. This is indicated by the position of the Si peak in x-ray photoelectron spectroscopy (XPS) measurements.

Furthermore, the present invention also contains carbon fibers which have been subjected to one or more washing processes before the temperature treatment and which, for example, can be obtained by a method which contains the previously explained steps a) to c) and further the following steps: d) after-treatment of the fiber treated with the treatment fluid by one or more washing processes and optionally after-treatment by one or more mechanical after-treatment processes; e) optionally drying the treated after-treated fiber; f) carrying out a temperature treatment of the after-treated and optionally dried fibers, wherein the fiber is exposed to a temperature of at least 450° C. in an inert atmosphere during a temperature treatment time interval. Such a carbon fiber is also designated in everyday language as “non-sized” carbon fiber. A carbon fiber according to the invention can contain silicon-containing deposits on its surface.

A carbon fiber according to the invention has a density which is preferably more than 1.20 g/cm3, preferably a density in a range of 1.42 g/cm3 to 1.82 g/cm3.

This carbon fiber can preferably be obtained starting from a polyacrylonitrile-based fiber.

A surface treatment can optionally be carried out on the fiber obtained after the temperature treatment, in particular if the fiber is to be further processed to form a composite material in order to improve the adhesion of the fiber in the matrix.

Carbon fibers which have been produced by the method according to the invention, where these can be subjected to an optional after-treatment, in particular a washing process, can have tensile strengths of more than 800 MPa, preferably a tensile strength of more than 1300 MPa, preferably a tensile strength of more than 1900 MPa, further preferably a tensile strength of more than 2800 MPa, in particular a tensile strength in the range of 1300 to 6560 MPa, determined according to DIN 65382.

Furthermore, carbon fibers which have been produced by the method according to the invention, where these can be subjected to an optional after-treatment, in particular a washing process, can have elastic moduli of more than 60 GPa, preferably an elastic modulus of more than 150 GPa, preferably an elastic modulus of more than 210 GPa, in particular an elastic modulus in the range of 150 to 275 GPa, determined according to DIN 65382.

An advantageous property profile, in particular for producing composite material is exhibited by carbon fibers according to the invention which in combination have a density of 1.24 to 1.81 g/cm3 and a tensile strength of 250 to 6560 MPa, preferably in combination have a density of 1.65 to 1.81 g/cm3 and a tensile strength of 1900 to 6560 MPa.

An advantageous property profile, in particular for producing composite material is additionally exhibited by carbon fibers which in combination have a density of 1.64 to 1.82 g/cm3 and a fiber shrinkage of less than 28%.

The carbon fibers according to the invention which can be obtained, for example, by the method according to the invention, can have a diameter in the range of 5 to 13 μm, preferably of 6 to 8.6 μm.

A fiber suitable as starting material fiber is a polyacrylonitrile-based fiber, where such a fiber is also briefly designated within the framework of the present application as “polyacrylonitrile” fiber. Homopolymeric polyacrylonitrile and/or copolymeric polyacrylonitrile can be used as polyacrylonitrile. An arbitrary prior art method can be selected by the person skilled in the art to produce homopolymeric polyacrylonitrile or copolymeric polyacrylonitrile. A fiber fineness of a starting material fiber can, for example, lie in a range of 0.9 to 2.4 dtex.

According to one embodiment, it is preferred that the polyacrylonitrile contains a polymer that comprises at least 50 wt. %, preferably at least 95 wt, %, in particular 96-99 wt. % of acrylonitrile units (as monomeric units) relative to the total weight of the polymer. According to a further embodiment, the polyacrylonitrile can comprise so-called modacryl, that is a polymer that consists of more than 50 wt. % and less than 85 wt. % of acrylonitrile units relative to the total weight of the polymer. Optionally a polyacrylonitrile-based polymer can comprise further components and additives known to the person skilled in the art. The polymer can comprise an organometallic polymer, that is a polymer, that contains metal atoms in the polymer chain or complexly bound, but preferably no such polymer is used to produce the starting material fibers. A starting material fiber preferably contains a polymer that contains at least 80 wt. % acrylonitrile units (as monomeric units) and 1 to 3 wt. % itaconic acid units (as monomeric units) as well as 3 to 5 wt. % methylmethacrylate units (as monomeric units), in each case relative to the total weight of the polymer.

Methods of manufacture for polyacrylonitrile fibers are described, for example, in the publication by E. Fitzer, L. M. Manocha, “Carbon Reinforcements and Carbon/Carbon Composites”, Springer Verlag, Berlin, 1998, ISBN 3-540-62933-5, p. 10-17, as well as in the patent specifications U.S. Pat. No. 4,001,382 and U.S. Pat. No. 6,054,214. Co-polymeric polyacrylonitrile can be obtained, for example, by polymerization of acrylonitrile with a (co-)monomer which can be copolymerised thereby, for example, by a solution polymerization, by a redox polymerization or by a slurry polymerization.

The following can be used, for example, as a (co-)monomer which can be used for copolymerisation during production of co-polymeric polyacrylonitrile: a vinyl carboxylic acid derivative such as methacrylic acid, itaconic acid; a halogenated vinyl compound such as, inter alia, vinyl chloride; a (meth)acrylate derivative, such as inter alia a methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate, pentyl(meth)acrylate, hexyl(meth)acrylate, dimethylaminoethyl(meth)acrylate; as well as further, for example, methacrylamide, styrene. Also more than one co-monomer can be used within the framework of the present invention. Preferably a starting material fiber contains co-polymeric polyacrylonitrile.

Starting material fibers for producing pitch-based precursor fibers can be both isotropic and anisotropic pitch fibers. To produce these starting material fibers, mesophase pitch can be subjected to a melt spin process and stretched as long as it is plastically deformable where pitch fibers can be produced with a preferred orientation. Suitable methods of manufacture are known in the prior art and are explained, for example, in the publication by E. Fitzer, L. M. Manocha, “Carbon Reinforcements and Carbon/Carbon Composites”, Springer Verlag, Berlin, 1998, ISBN 3-540-62933-5, p. 24-34, and in the patent specification DE 697 32 825 T2.

Methods of manufacture for polyacetylene-based fibers, polyphenylene-based fibers, lignin-based fibers, cellulose-based fibers are known to a person skilled in the art. For example, the following cellulose fibers can be used: in particular from regenerated cellulose: viscose, Cupro, Modal, from cellulose residues: acetate and triacetate.

Methods of manufacture for ceramic-based fibers are known to a person skilled in the art. Ceramic fibers can, for example, comprise at least one compound selected from aluminum oxide, zirconium oxide, SiNC, SiBNC, SiC, B4C, BN, Si3N4, TiC, WC and mixtures thereof and consist completely thereof or at least 90 wt. %, preferably at least 93 wt. % relative to the total weight of the fibers. In particular, ceramic fibers can comprise basalt fibers and/or glass fibers or a mixture thereof with other ceramic fibers. For example, fibers, in particular highly heat-resistant fibers, based on Si, C, B, N, Al or compounds thereof are described in German patent DE 197 11 829 01, corresponding to U.S. Pat. No. 6,261,981.

Methods

a) Density

The density of fibers can be determined in particular in accordance with DIN 65569, Part 1.

b) Tensile Strength and Elastic Modulus

The tensile strength and elastic modulus can be determined in particular in accordance with DIN 65382.

c) Fineness of Fibers

The fineness of fibers can be determined in particular in accordance with DIN 60905, sheet 1.

d) Determination of Content of C, H, N, O

The content of C, H, N, O can be determined in particular by an elementary analytical apparatus to determine the content of C, H, N and O, where this can in particular be based on the principle of dynamic spontaneous combustion. Samples can be prepared in particular by drying at 105° C. as far as constant weight.

e) EDX Method

In particular an EDX detection can be made to determine the silicon content. Samples can be prepared preferably by drying to constant weight at 105° C.

EDX detections of carbon fibers can be made in particular in an SEM (scanning electron microscope). The model GEMINI DSM 982 made by LEO was used as equipment. All detections on the SEM to determine the silicon content are made at an acceleration voltage of 5 kV.

EDX detections of carbon fiber precursor fibers can be made in particular on an ESEM (Environmental Scanning Electron Microscope). The model XL 30 ESEM FEG made by the equipment manufacturer FEI Company was used as equipment. All detections on the ESEM to determine the silicon content are made at an acceleration voltage of 10 kV.

f) Fineness of Fibers

The fineness of fibers can preferably be determined in accordance with DIN 60905, sheet 1.

g) Fiber Shrinkage

The fiber shrinkage S is determined by the following formula: S=[1−(I1/I2)]×100, where I1=length of the fiber before temperature treatment (carbonization) and I2=length of the fiber after temperature treatment.

The invention is now explained in detail with reference to the following, non-restrictive examples:

Example 1

Treatment with the treatment fluid is now described.

In each experiment 300 ml of silicon oil (UCOTHERM X-BF from FRAGOL Schmierstoff GmbH & Co. KG, D-45481 Mülheim an der Ruhr) was placed in a 1000 ml beaker (SIMAZ®). The starting material fibers “SAF”, polyacrylonitrile-based fibers containing 1.5 wt. % of itaconic acid and 4.5 wt. % of methylacrylate) were heated in this oil bath. In order to ensure a uniform heating rate, the MSC basic C combined with the TC 3 temperature regulator was used as hot plate which could achieve oil heating independent of ambient air (manufacturer of the plate and the regulator: Yellow line). As a result of the open construction, it was possible to dip the PAN (polyacrylonitrile) fibers in the oil bath at a defined oil temperature. In order to ensure a homogeneous temperature distribution in the oil, the oil was mixed continuously during the experiments with the integrated magnetic agitator. As a result of the strong heat loss to the sides, the oil bath was insulated with ceramic wool so that the temperature range of 200° C. to 280° C. can be attained. In order to be able to lower starting material fibers of defined length into the oil bath, frames of copper tube were bent into shape and the fibers placed thereon. The frames could thus be lowered on a stand by means of a wire into the oil bath. Here care was taken to ensure that the fibers had no contact with the beaker wall and complete coverage with oil was ensured. The temperature/time profile was recorded in each experiment by the thermocouple connected to the Thermologger K204 (VOLTCRAFT®).

For a sample series A the oil temperature was kept constant during the heat treatment and only the stabilization time was varied. To this end the PAN fiber was dipped at oil temperatures of 210° C., 220° C. and 230° C. The fibers were removed from the oil after respectively 55 min, 200 min, 300 min, 420 min and 720 min.

In sample series B a constant heating rate was approximated with a step-shaped profile. Each sample in this series was dipped at an oil bath temperature of 200° C. After 10 min holding time, the temperature was increased in steps by 10 K and held for 10 min after reaching the new target temperature. In this way the treatment temperature was successively increased to 280° C.

In order to track the effects of the stepwise increase in temperature on the fibers, as shown in Table 2, nine pre-oxidized polyacrylonitrile fibers “PanOx 2” to “PanOx 10” were obtained. Fiber “PanOx 2” was removed after holding for 10 minutes at 200° C., fiber “PanOx 3” was removed after reaching a temperature of 210° C. and holding at this temperature for 10 minutes, fiber “PanOx 4” was removed after reaching a temperature of 220° C. and holding at this temperature for 10 minutes, fiber “PanOx 5” was removed after reaching a temperature of 230° C. and holding at this temperature for 10 minutes, etc. Fiber “PanOx 10” was finally removed after reaching a temperature of 280° C. and holding at this temperature for 10 minutes. FIG. 7 illustrates the temperature behavior during a fiber treatment with stepwise increase in temperature.

Example 2

After-treatment methods are now described.

a) Extraction with Methyl Ethyl Ketone

After removal from the oil bath, the fibers were dabbed with paper and then underwent a Soxhlet extraction with methyl ethyl ketone for 240 minutes. In this case, only the pure solvent reaches the fiber to be de-sized in each extraction cycle. The fibers were then dried in an ambient air atmosphere at 100° C.

b) Alternative Methods of after-Treatment

Some fiber samples were only dabbed with paper after the treatment with the treatment fluid to remove the oil remaining on the fibers. Such fibers are also designated as “non-de-sized” fibers within the framework of the present application.

Furthermore, in some series of experiments, after dripping with paper, samples underwent a Soxhlet extraction with toluene or isopropanol or acetone for 240 minutes. The extent of removal of the treatment fluid was controlled depending on the solvent selected in each case and the extraction time.

In addition, in some series of experiments, purification experiments were conducted in the ultrasound bath with the solvents acetone, isopropanol and toluene.

In addition, the purification was tested using a commercially available silicon remover. For this purpose the viscous mass was applied to the fibers with a brush.

c) Comparison of Different after-Treatments

Various after-treatment process of PAN fibers (“SAF” fibers as described previously) were compared following identical treatment with treatment fluid: (UCOTHERM X-BF from FRAGOL Schmierstoff GmbH & Co. KG): dabbing with paper with subsequent drying; dabbing with paper following by an extraction with methyl ethyl ketone or isopropanol or acetone or toluene, in each case for 240 minutes, with subsequent drying to constant weight at 105° C. as well as cleaning with a commercially available silicon remover as described in Example 2b, with subsequent drying to constant weight at 105° C.

EDX measurements yielded following silicon contents, in each case relative to the total weight of pre-oxidized carbon fiber precursor fibers containing silicon-containing deposits on the surface thereof:

TABLE 1 Type of treatment Fraction of Si (wt. %) Dabbing with paper 19.40 Purification with silicon remover 2.19 Soxhlet extraction with methyl ethyl ketone 0.58 Soxhlet extraction with isopropanol 4.42 Soxhlet extraction with acetone 4.52 Soxhlet extraction with toluene 2.43

Example 3

Elementary analytical date from pre-oxidized PAN fibers is now described. The pre-oxidized polyacrylonitrile fibers “PanOx 2” to “PanOx 10”, whose production is described in Example 1, were subjected to an extraction and drying according to Example 2a. The abbreviation SAF-PAN designates the starting material fiber described in Example 1.

Depending on the final temperature of a stepwise production, the following elementary analytical values were obtained by combustion analysis using a C,H,N,O analyzer for the pre-oxidized polyacrylonitrile fibers “PanOx 2” to “PanOx 10” (in the following table Nos. 2 to 10):

TABLE 2 F(X) Density (Y) C(Y) O(Y) N(Y) H(Y) 1 SAF-PAN 1.188 66 3.20 23.9 5.77 2 200 1.198 66.1 3.3 23.9 5.66 3 210 1.201 65.4 4.03 23.7 5.56 4 220 1.239 64.8 4.84 23.4 5.39 5 230 1.261 64.2 5.57 22.7 5.14 6 240 1.281 63.8 8.2 22.5 5.13 7 250 1.336 63.2 9.6 22 4.85 8 260 1.336 62.6 9.4 21.8 4.67 9 270 1.355 62.6 9.1 22.1 4.54 10 280 1.372 63.2 9.1 21.4 4.68

The second column gives values F(X) which give the final temperature after reaching which and holding for 10 minutes the fibers were each removed from the oil bath.

FIG. 8 shows a graphical plot of these values.

Example 4

A carbonization method (temperature treatment) is now described.

The tube furnace LORA 1800-32-600-1 (HTM Reetz GmbH) was used for the temperature treatment (also designated as “carbonization” within the framework of the present application) of the fibers described previously, which had been treated with the treatment fluid. During the carbonization the same chamber was flushed with nitrogen in order to ensure the lowest possible oxygen concentration in the furnace during the carbonization and to remove the gases produced during conversions of the sample. In order to obtain an oxygen fraction of less than 1 wt. % relative to the total weight of the atmosphere in the tube furnace, this was measured with a lambda probe (Bosch, Model 0258 104 004) in the exhaust gas path. Both the voltage signal of the lambda probe and the temperatures measured with the thermocouple integrated in the furnace were recorded in each carbonization process. In each carbonization passage the samples were placed as far as possible in the middle of the working tube and therefore in immediate proximity to the thermocouple. The fibers were thereby placed either on a piece of graphite or in a 10 cm long zirconium shuttle in order not to damage the working tube of the furnace unnecessarily. Various temperature profiles were investigated in series of experiments.

Temperature profile I was executed as follows: heating to 500° C. took place over 1.5 hours, during the next 30 minutes the temperature was held constant at 500±25° C., then the temperature was increased to 1450° C. during the next 2.75 hours in 8 stages and finally allowed to cool to 550° C. for 2.5 hours.

Temperature profile II was executed as follows: firstly heating to 1000° C. took place at a heating rate of 250° C./h, then the temperature was held for 2 minutes. This was then followed by heating to 1375° C. at a heating rate of 250° C./h, then holding the temperature of 1375° C. for 2 minutes. This is followed by cooling to room temperature.

Temperature profile III was executed as follows: first heating to 1000° C. took place at a heating rate of 250° C./h, then the temperature was held for 2 minutes. This was then followed by heating to 1320° C. at a heating rate of 250° C./h, then holding the temperature of 1320° C. for 2 minutes. This is followed by cooling to room temperature.

Example 5

Material properties of carbon fibers are now described.

The pre-oxidized polyacrylonitrile fiber “PanOx 10”, whose production is described in Example 1, was subjected to an extraction and drying according to Example 2a and then a temperature treatment (carbonization) according to temperature profile I. Since the elastic modulus for the fiber “C fiber 10” is low, the fiber was only carbonized at 1000° C.

Example 6

Manufacture of a carbon fiber having high loading capacity is now described.

SAF fibers (polyacrylonitrile-based fibers, containing 1.5 wt. % itaconic acid and 4.5 wt. % methylacrylate) were dipped in a silicon bath (UCOTHERM X-BF from FRAGOL Schmierstoff GmbH & Co. KG, D-45481 Mülheim an der Ruhr) having a temperature of 230° C. The oil bath temperature was held for 20 minutes, then the temperature was increased over 10 minutes to 250° C. and held for 30 minutes. After removal from the oil bath, the fibers were dabbed with paper and then subjected to a Soxhlet extraction with methyl ethyl ketone for 240 minutes. The fibers were then dried in an ambient air atmosphere at 105° C. The carbon fiber thus produced has the following characteristic: density 1.753 g/cm3, tensile strength 2844 MPa, elastic modulus 218 GPa, diameter 7.0 μm.

It should be stressed that using a step profile makes it possible to produce fibers having high tensile strength and high elastic modulus with a comparatively short treatment time interval of 60 minutes or less. Comparable experiments at constant temperature surprisingly yield carbon fibers of significantly inferior quality, in particular tensile strength.

Claims

1. A method for producing carbon-containing fibers, which comprises the steps of:

providing at least one starting material fiber;
bringing the at least one starting material fiber in contact with at least one treatment fluid having at least one silicon compound and a content of 0-25 wt. % of water, relative to a total weight of the treatment fluid; and
treating the at least one starting material fiber with the treatment fluid during a treatment time interval having a duration of at least three minutes at a treatment temperature in a range of 126° C. to 450° C. resulting in treated fibers.

2. The method according to claim 1, which further comprises carrying out a further temperature treatment on at least one of the treated fibers, after-treated fibers or dried fibers, wherein the fibers are exposed to a temperature of at least 450° C. in an inert atmosphere during a further temperature treatment time interval.

3. The method according to claim 1, wherein the at least one starting material fiber contains fibers selected from the group consisting of polyacrylonitrile-based fibers, polyacetylene-based fibers, polyphenylene-based fibers, cellulose-based fibers, pitch-based fibers, lignin-based fibers, viscose-based fibers, polypropylene-based fibers, mixtures of carbon-containing fibers and ceramic-based fibers, and mixtures thereof.

4. The method according to claim 1, wherein the treatment temperature lies in a range of 136° C. to 360° C.

5. The method according to claim 1, which further comprises performing the treating step at a first temperature range and at least one second temperature range, wherein a lowest temperature of the at least one second temperature range is higher than a highest temperature of the first temperature range and the treating step begins with a treatment of the at least one starting material fiber in the first temperature range.

6. The method according to claim 5, which further comprises:

setting the first temperature range to be 126° C. to 260° C.; and
setting the at least one second temperature range to be 140° C. to 450° C.

7. The method according to claim 1, wherein a temperature of the treatment fluid increases one of continuously or discontinuously during the treatment time interval entirely or at least one time segments of the treatment time interval.

8. The method according to claim 1, wherein the treatment time interval includes at least one time segment during which a temperature is kept constant or substantially constant at one temperature and includes at least one time segment during which the temperature is varied.

9. The method according to claim 1, wherein a temperature of the treatment fluid is one of increased or decreased at least during one time segment of the treatment time interval by 0.5 to 15° C./minute.

10. The method according to claim 1, wherein the treatment fluid comprises at least one silicon compound selected from the group consisting of polydialkyl siloxanes, polydiaryl siloxanes and polymonoalkylmonoaryl siloxanes.

11. The method according to claim 1, wherein the treatment fluid has a water content of 0.001-22 wt. % relative to the total weight of the treatment fluid.

12. The method according to claim 1, which further comprises providing the at least one starting material fiber before and/or at least partly during the treating step in a clamped state.

13. The method according to claim 1, which further comprises:

subjecting the treated fibers to at least one mechanical after-treatment process, the mechanical after-treatment process being at least one of a pressing process, an extraction process, a dropping process or a washing process having an extraction and/or a treatment in a bath under an action of ultrasound.

14. The method according to claim 2, wherein the at least one starting material fiber after the treating step and before the further temperature treatment step does not come in contact with an atmosphere having a content of more than 5 wt. % of oxygen and a temperature of 70° C. or more.

15-25. (canceled)

26. A carbon fiber precursor fiber, comprising:

a fiber having a surface and a carbon content of 49.9 wt. % to 63.5 wt. % relative to a total weight of the carbon fiber precursor fiber; and
silicon-containing deposits disposed on said surface of said fiber.

27. The carbon fiber precursor fiber according to claim 26, wherein the carbon fiber precursor fiber has a density of 1.30-1.50 g/cm3.

28. The carbon fiber precursor fiber according to claim 26, wherein said fiber has at least one of a silicon content of at least 19 wt. %, a nitrogen content of 3.3 wt. % to 16.2 wt. %, or an oxygen content of 12.4 wt. % to 14.0 wt. %, in each case relative to a total weight of the carbon fiber precursor fiber.

29. (canceled)

30. A method of producing a carbon fiber precursor fiber having a surface, a carbon content of 49.9 wt. % to 63.5 wt. % relative to a total weight of the carbon fiber precursor fiber, and silicon-containing deposits on the surface, which comprises the steps of:

a) providing at least one starting material fiber;
b) bringing the at least one starting material fiber in contact with at least one treatment fluid, wherein the treatment fluid contains at least one silicon compound and has a content of 0-25 wt. % of water, relative to a total weight of the treatment fluid; and
c) treating the at least one starting material fiber with the treatment fluid during a treatment time interval having a duration of at least three minutes at a treatment temperature in a range of 126° C. to 450° C. resulting in a treated fiber.

31. The method according to claim 30, which further comprises:

d) after-treating the treated fiber with at least one mechanical after-treatment process and no washing process takes place between step c) and step e) resulting in an after-treated fiber; and
e) drying the after-treated fiber.

32. A carbon fiber precursor fiber, comprising:

a fiber having a surface and a carbon content of 52.9 wt. % to 81.6 wt. % relative to a total weight of the carbon fiber precursor fiber; and
silicon-containing deposits disposed on said surface of said fiber.

33. The carbon fiber precursor fiber according to claim 32, wherein the carbon fiber precursor fiber which has a density which is more than 1.20 g/cm3.

34. (canceled)

35. The carbon fiber precursor fiber according to claim 32, wherein said fiber has at least one of a silicon content of at least 0.5 wt. %, a nitrogen content of 7.3 wt. % to 34.4 wt. %, or an oxygen content of 3.3 wt. % to 9.7 wt. %., in each case relative to the total weight of the carbon fiber precursor fiber.

36. (canceled)

37. A method of producing a carbon fiber precursor fiber having a surface, a carbon content of 52.9 wt. % to 81.6 wt. % relative to a total weight of the carbon fiber precursor fiber, and silicon-containing deposits disposed on the surface, which comprises the steps of:

a) providing at least one starting material fiber;
b) bringing the at least one starting material fiber in contact with at least one treatment fluid, wherein the treatment fluid contains at least one silicon compound and has a content of 0-25 wt. % of water, relative to a total weight of the treatment fluid; and
c) treating the at least one starting material fiber with the treatment fluid during a treatment time interval having a duration of at least three minutes at a treatment temperature in a range of 126° C. to 450° C. resulting in treated fibers.

38. The method according to claim 37, which further comprises:

d) after-treating the treated fibers by at least one of at least one washing process or by at least one mechanical after-treatment processes resulting in after treated fibers; and
e) drying the after-treated fibers.

39. A carbon fiber, comprising:

a fiber having a surface and a carbon content of 94.0 wt. % to 94.6 wt. % relative to a total weight of the carbon fiber; and
silicon-containing deposits disposed on said surface of said fiber.

40. The carbon fiber according to claim 39, wherein the carbon fiber has a density which is more than 1.20 g/cm3.

41. (canceled)

42. The carbon fiber according to claim 39, wherein said fiber has at least one of a silicon content in a range of more than 0.45 wt. %, a nitrogen content of 2.4 wt. % to 2.9 wt. %, or an oxygen content of 2.6 wt. % to 2.7 wt. %., in each case relative to the total weight of the carbon fiber.

43. (canceled)

44. A method of making a carbon fiber having a surface, a carbon content of 94.0 wt. % to 94.6 wt. % relative to a total weight of the carbon fiber, and silicon-containing deposits on the surface, which comprises the steps of:

a) providing at least one starting material fiber;
b) bringing the at least one starting material fiber in contact with at least one treatment fluid, wherein the treatment fluid contains at least one silicon compound and having a content of 0-25 wt. % of water, relative to a total weight of the treatment fluid;
c) treating the at least one starting material fiber with the treatment fluid during a treatment time interval having a duration of at least three minutes at a treatment temperature in a range of 126° C. to 450° C. resulting in treated fibers; and
d) after-treating the treated fibers by at least one washing process resulting in after-treated fibers.

45. The method according to claim 44, which further comprises:

e) performing at least one mechanical after-treatment process on the after-treated fibers;
f) drying the after-treated fibers resulting in dried fibers; and
g) carrying out a further temperature treatment on the dried fibers, wherein the dried fibers are exposed to a temperature of at least 450° C. in an inert atmosphere during a temperature treatment time interval.

46. A carbon fiber, comprising:

a fiber having a surface and a carbon content of 94.0 wt. % to 94.6 wt. % relative to a total weight of the carbon fiber; and
silicon-containing deposits disposed on said surface of said fibers.

47. The carbon fiber according to claim 46, wherein the carbon fiber has a density which is greater than 1.20 g/cm3.

48. (canceled)

49. The carbon fiber according to claim 46, wherein said fiber has at least one of a silicon content in a range of more than 0.45 wt. %, a nitrogen content of 2.4 wt. % to 2.9 wt. %, or an oxygen content of 1.5 wt. % to 2.1 wt. %, in each case relative to the total weight of the carbon fiber.

50. (canceled)

51. A method of making a carbon fiber having a surface, a carbon content of 94.0 wt. % to 94.6 wt. % relative to a total weight of the carbon fiber, and silicon-containing deposits disposed on the surface, which comprises the steps of:

a) providing at least one starting material fiber;
b) bringing the at least one starting material fiber in contact with at least one treatment fluid, wherein the treatment fluid contains at least one silicon compound and having a content of 0-25 wt. % of water, relative to a total weight of the treatment fluid; and
c) treating the at least one starting material fiber with the treatment fluid during a treatment time interval having a duration of at least three minutes at a treatment temperature in a range of 126° C. to 450° C. resulting in treated fibers.

52. The method according to claim 51, which further comprises:

d) after-treating the treated fibers with at least one mechanical after-treatment process and no washing process takes place between the step c) and step f) resulting in after-treated fibers;
e) drying the after-treated fibers resulting in dried fibers; and
f) carrying out a temperature treatment of the dried fibers, wherein the dried fibers are exposed to a temperature of at least 450° C. in an inert atmosphere during a temperature treatment time interval.

53. The carbon fiber precursor fiber according to claim 26, wherein said silicon-containing deposits contain particle-like deposits having a greatest length extension of less than 40 μm.

54-58. (canceled)

59. The carbon fiber according to claim 39, wherein said silicon-containing deposits contain particle-like deposits having a greatest length extension of less than 40 μm.

60-64. (canceled)

65. A method of using the carbon fiber precursor fiber according to claim 26, which comprises the steps of:

producing an item selected from the group consisting of a woven fabric, a fiber scrim, a composite material, and a C/SiC composite material from the carbon fiber precursor fiber.

66. A method of using the carbon fiber precursor fiber according to claim 32, which comprises the steps of:

producing an item selected from the group consisting of a woven fabric, a fiber scrim, a composite material, and a C/SiC composite material from the carbon fiber precursor fiber.

67. A method of using the carbon fiber according to claim 39, which comprises the steps of:

producing an item selected from the group consisting of a woven fabric, a fiber scrim, a composite material, and a C/SiC composite material from the carbon fiber.

68. A method of using the carbon fiber according to claim 46, which comprises the steps of:

producing an item selected from the group consisting of a woven fabric, a fiber scrim, a composite material, and a C/SiC composite material from the carbon fiber.
Patent History
Publication number: 20120280412
Type: Application
Filed: Jun 4, 2012
Publication Date: Nov 8, 2012
Applicant: SGL CARBON SE (WIESBADEN)
Inventor: SANDRA SITTER (MUNCHEN)
Application Number: 13/487,389
Classifications
Current U.S. Class: Filaments (264/29.2); With Twining, Plying, Braiding, Or Textile Fabric Formation (264/103); Including Free Carbon Or Carbide Or Therewith (not As Steel) (428/367)
International Classification: D01F 9/12 (20060101); D03D 15/00 (20060101);