EP3931381B1 - Method of ionizing irradiation of textile polyacrylonitrile fibres and use thereof as carbon fibre precursor - Google Patents

Description

The invention relates to a method for electron beam irradiation of textile polyacrylonitrile fibers and their use for the production of carbon moldings, in particular carbon fibers.

Flexible, elongate molded bodies containing at least 92% by weight of carbon and produced from organic polymeric precursors are referred to as carbon fibers.

According to the current state of the art, the primary precursor used in carbon fiber production is polyacrylonitrile, the polymer of acrylonitrile. To produce carbon fibers, polyacrylonitrile fibers are first stabilized by oxidation and then carbonized. If necessary, the carbon fiber is then graphitized. In the case of oxidative stabilization, the polyacrylonitrile is cyclized and dehydrated, ie converted into a polyaromatic structure at temperatures of 200 to 300° C. in air. This temperature range is referred to below as the cycling temperature. The resulting polyaromatic structure is called Ox-PAN in the following. The polyaromatic structure of the Ox-PAN enables the high carbon yield in the subsequent carbonization. At this will convert the Ox-PAN into a turbostratic modification of the carbon by pyrolysis, with the elimination of CO2 and HCN.

Not only the homopolymer of acrylonitrile is usually referred to as polyacrylonitrile, but also co- and terpolymers consisting of acrylonitrile and comonomers such as vinyl acetate, methyl acrylate, methyl methacrylate, itaconic acid, acrylic acid, acrylamide and others. According to the current state of the art, high-molecular terpolymers consisting of mostly more than 95% by weight of acrylonitrile and up to 5% by weight of other comonomers, primarily methyl acrylate and itaconic acid, with a number-average molecular weight of about 120,000 to 1,500,000 have proven suitable for carbon fibers g/mol as particularly suitable precursor polymers, which are referred to below as CF-PAN. The special suitability for carbon fibers results primarily from the thermal properties of such a terpolymer. In particular, comonomers with one or more carboxyl groups lead to a reduction in the cyclization temperature and a broader temperature window for the cyclization reactions that take place. Itaconic acid, with its two carboxyl groups, is therefore a frequently used comonomer for CF-PAN. The thermal properties can be verified using differential scanning calorimetry (DSC) in accordance with DIN EN ISO 11357-5:2014-07. The key parameter here is the onset temperature of the cyclization reaction, measured in air, referred to below as T _onset-Z , analogous to T _ei,r in DIN EN ISO 11357-5:2014-7. Typical values for Tonset _-Z for a CF-PAN are 200 to 240°C, depending on the composition of the CF-PAN. Another parameter is the temperature of the highest exotherm, measured under air, referred to below as T _{peak Z} , analogous to T _p,r in DIN EN ISO 11357-5:2014-7. T _{peak Z} is typically 280-300°C for CF-PANs. The distance between Tonset _-Z and T _peak-Z should be as large as possible to avoid overheating or even burning of the fiber during the oxidative stabilization. For CF-PAN this is typically >60°C.

Polyacrylonitrile is mostly converted into fiber form by wet or dry spinning. The productivity of the spinning plant correlates with molecular weight and comonomer content in such a way that a higher molecular weight or a lower comonomer content reduces productivity.

Polyacrylonitrile fibers are currently used not only for carbon fiber production, but also to a significant extent for textiles, especially for home and Outdoor textiles as well as work and sportswear. For these textile applications, polyacrylonitrile with a lower number-average molecular weight of about 30,000 to 250,000 g/mol and a comonomer content of up to 15% by weight is usually used in favor of productivity, referred to below as textile PAN. Vinyl acetate, methyl acrylate and others are usually used as comonomers in textile PAN. Comonomers with carboxyl groups are rarely used because, as already mentioned, they lead to a lower Tonset _Z. A low T _{onset Z} is usually not desired for textile PAN, since the cyclization reactions are accompanied by a color change. In the case of textile PAN, comonomers containing carboxyl groups would therefore lead to undesirable discolouration at elevated temperatures, for example when ironing. Textile PAN's low molecular weight and high comonomer content enables higher dope concentrations and thus higher productivity. Textile PAN fibers are therefore about 50% cheaper per kilogram than CF PAN fibers. Since the precursor, i.e. CF-PAN fibers, accounts for about half the cost of the resulting carbon fiber in state-of-the-art carbon fiber production, the use of textile PAN fibers would potentially reduce the cost of carbon fiber production by about 25%.

Compared to CF-PAN, however, the thermal properties of textile PAN are disadvantageous for carbon fiber production. Tonset _-Z is typically 240 to 300°C. As a result, the use of textile PAN leads to higher energy costs when tempering the oxidative stabilization. In addition, the distance between Tonset _-Z and T _peak-Z is smaller. The difference is usually between 10°C and 50°C. This narrower temperature window of the cyclization reaction leads to problems when handling the stabilization step, since even small temperature fluctuations in the stabilization furnace lead to a significantly higher exothermic nature of the cyclization reaction. Overheating of a multifilament with a large overall diameter, such as a "heavy tow" (50,000 filaments) customary in industry, is also conceivable due to precisely that exothermicity.

In the worst case, this leads to the ignition and burning of the fiber in the stabilization furnace. In addition, the mechanical properties of the carbon fibers made of textile-PAN are usually significantly worse than those of the carbon fibers made of CF-PAN. Because of this, despite the low price, the use of a textile PAN for carbon fiber production was avoided for a long time.

One way of changing the thermal properties of PAN is to irradiate the PAN with high-energy radiation, such as gamma rays or electron beams. Radicals are generated in the backbone of the polymer by this radiation. The amount of radicals generated correlates with the dose of irradiation. This enables shorter stabilization times. This was already 1996 in the JPH0827619A described. In this document, fibers irradiated with electrons are stabilized and carbonized in air, with the stabilization time being shortened compared to stabilization without prior irradiation. A CF-PAN with 0.1-10% by weight of comonomer containing carboxylic acid was used. The resulting carbon fibers achieve the usual tensile strengths of 3.0-3.5 GPa and 220 to 250 GPa modulus of elasticity for CF-PAN. According to the technical teaching disclosed in this document, costs are saved in the stabilization step, but the much more decisive precursor costs are unchanged because of the use of CF-PAN instead of textile PAN.

Recently also in KR 20160140268A a possibility is shown how textile PAN can be modified in its thermal properties by electron beam irradiation in such a way that it is suitable for carbon fiber production. Electron irradiation resulted in a lower T _{onset Z} and a wider temperature window of the cycling reaction, i.e. a larger distance between T _{onset Z} and T _{peak Z} of >50°C. Accelerating voltages of >1 MV were chosen for the irradiation. The radiation dose in the examples of the known teaching was between 200 and 1500 kGy, 50-3000 kGy were claimed. The atmosphere at the irradiation was air. The stabilization and the carbonization were carried out discontinuously. This document describes the great potential for savings when using a textile precursor, but the achieved mechanical properties of maximum 1.9 GPa tensile strength and 150 GPa modulus of elasticity are well below the typical mechanical properties of carbon fibers made from CF-PAN fibers. This outweighs the cost advantage of the inexpensive textile PAN precursor. Further relevant information can be found in the documents US2016/348283 , WO 2017/194103 and WO 2017/194103 .

Proceeding from the above prior art, the invention is therefore based on the object of proposing a method which is characterized in that it can be used in a practicable and economical manner, in particular for the production of carbon fibers from textile PAN. The mechanical properties of the carbon fibers produced from textile PAN should clearly surpass those of the prior art and be comparable with carbon fibers made from CF-PAN, so that the savings potential of around 25% of the carbon fiber production costs can be exploited.

The object on which the invention is based is achieved by a method for the irradiation and oxidative stabilization of PAN fibers for the production of a precursor fiber of carbon fibers, characterized in that (1) the PAN fibers are based on a homopolymer or copolymer of PAN, wherein the PAN homopolymer or copolymer has a Tonset _-z temperature of at least 245° C., measured in air (according to DIN EN ISO 11357-5:2014-07), a number-average molecular weight of 20,000 to 250,000 g/mol polymethyl methacrylate -Molar mass equivalents (determined according to DIN 55672-2:2016-03) and a comonomer content of not more than 15.0% by weight, (2) the PAN fibers are subjected to ionizing radiation with electron beams in an inert gas atmosphere with a radiation dose from 10 to 5000 kGy, (3) the reduced Tonset _-z temperature of the E-PAN fibers obtained by irradiation is determined in air ( T _{onset-Z E-PAN} ) (according to DIN EN ISO 11357-5 :2014-07), then the oxidative thermal stabilization is initiated at a starting temperature of Tonset _{-Z E-PAN} ± 30°C and the oxidative thermal stabilization with increasing temperature up to a minimum density of the oxidatively stabilized PAN fiber (Ox-PAN) of 1.30 g/cm ³ is carried out.

The invention shows a variety of advantageous configurations, which are identified below:
It is preferred that the PAN has a number average molecular weight of from 30,000 to 150,000 g/mol, in particular from 50,000 to 120,000 g/mol. It is also considered an advantage that the comonomer content of PAN is 0.0 to 15.0% by weight, in particular 0.0 to 12.0% by weight and particularly preferably 0.0 to 7.5% by weight. -% amounts to. In addition, it is expedient that the comonomer of PAN is a vinyl compound, in particular vinyl acetate, vinyl propionate, methyl acrylate, methyl methacrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, tert-butyl acrylate, sodium methallyl sulfonate, sodium vinyl sulfonate, acrylamide, methacrylamide and/or vinyl acetamide. It is particularly preferred if the PAN fibers are textile PAN fibers.

In the process according to the invention, the ionizing irradiation takes place with electron beams in an inert gas atmosphere, in particular in a nitrogen atmosphere. Surprisingly, various tests showed that an inert gas atmosphere compared to air leads to better mechanical properties of the resulting carbon fibers under otherwise identical process conditions. In this case, it is preferred that the radiation dose for the ionizing radiation is 70 to 2500 kGy, in particular 300 to 1000 kGy. In the case of ionizing irradiation with electron beams, the acceleration voltage is preferably 100 to 900 kV, in particular 160 to 600 kV, with the range from 180 to 400 kV being particularly preferred. Furthermore, it is preferred that the current intensity during the ionizing irradiation with electron beams is 0.1 to 100 mA, in particular 1 to 50 mA and particularly preferably 2 to 10 mA.

The oxidative heat stabilization to be used in the invention is accessible to a variety of advantageous configurations: It is preferred that the starting temperature in the oxidative heat stabilization Tonset _{-Z E-PAN} is ±20°C, in particular ±10°C. The end temperature of the oxidative thermal stabilization T _{Peak-Z E-PAN} is preferably ±30°C, in particular ±20°C. Furthermore, it is preferred that the oxidative thermal stabilization is carried out up to a density of the oxidatively thermally stabilized PAN (Ox-PAN) of 1.30 to 1.5 g/cm ³ , in particular of 1.35 to 1.39 g/cm ³ becomes. It has been shown that the end temperature of the oxidative heat stabilization is preferably between 250 and 300.degree. C., in particular between 260 and 290.degree.

In addition, it is preferred that as little time as possible elapses between the oxidative thermal stabilization and the ionizing irradiation, in particular electron irradiation; the oxidative thermal stabilization particularly preferably takes place immediately after the ionizing irradiation. It is preferred that the storage time between ionizing radiation and carbonization is less than one day, preferably less than one hour. In particular, it is preferred that the ionizing radiation occurs immediately before the oxidative thermal stabilization in the case of a continuous thread run.

The product obtained according to the invention can be put to advantageous uses. However, the use of those obtained according to the invention is preferred oxidatively thermally stabilized PAN fibers (Ox-PAN) for the production of carbon fibers by carbonization, optionally with subsequent graphitization.

The object on which the invention is based is therefore achieved by an advantageous method for stabilizing shaped bodies, in particular fibers, consisting in particular of textile PAN. The shaped bodies, in particular the fibers, are irradiated with ionizing radiation, preferably with electron beams, preferably in an inert gas atmosphere, particularly preferably in a nitrogen atmosphere, followed by oxidative thermal stabilization, which preferably takes place immediately after the irradiation and which has a temperature profile , which is tailored to the thermal properties of the particularly irradiated textile PAN. The shaped bodies or fibers thus stabilized according to the invention can be carbonized and optionally graphitized by conventional methods. The properties of the carbon fibers produced according to the invention correspond to typical carbon fibers made from CF-PAN.

The invention is to be presented in detail below for further explanation:

1. Output fiber

According to the invention, in particular, a fiber made of "textile PAN" is used as the starting fiber. The properties of this PAN are explained below in connection with the term "textile PAN". Even if a fiber is not expressly used for the production of textiles, it is referred to as "textile PAN fibre" within the meaning of the invention. The textile PAN preferably has a number-average molecular weight of from 20,000 to 250,000 g/mol, in particular from 30,000 to 150,000 g/mol of polymethyl methacrylate molar mass equivalents according to DIN 55672-2:2016-03. Textile PAN with a number-average molecular weight of 50,000 to 120,000 g/mol polymethyl methacrylate molar mass equivalents has proven to be particularly advantageous.

The textile PAN used according to the invention is also characterized in that it has a Tonset _Z measured in air of greater than 245°C, more preferably greater than 250°C and most preferably from 250 to 300°C. According to the invention, the Tonset _Z temperature is preferably up to 320°C, in particular up to 300°C. The Tonset _-Z temperature corresponds to T _ei,r in DIN EN ISO 11357-5:2014-07 and represents the extrapolated initial temperature of the cyclization reaction of the PAN in air to the Ox-PAN. In addition, the textile PAN used according to the invention has a T _{peak Z} , measured in air, corresponding to T _p,r in DIN EN ISO 11357-5:2014-7, from 260 to 360°C, in particular from 290 to 320°C. T _{on-set Z} and T _{peak Z} are determined via DSC under air using the temperature sweep method, the heating rate used for the purpose of comparability is 10 K/min. A TA-Instruments Q2000 differential scanning calorimeter with an autosampler unit was used for the measurements, and the "TZero" aluminum pans from TA-Instruments were used as measuring crucibles.

Various comonomers with a vinyl group, such as are typically used in textile PAN, can be used as comonomers. The use of a carboxylic acid-containing comonomer is not necessary because of the thermal properties that can be controlled by radiation. Vinyl acetate, vinyl propionate, methyl acrylate, methyl methacrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, tert-butyl acrylate, sodium methallyl sulfonate, sodium vinyl sulfonate, acrylamide, methacrylamide and vinyl acetamide are particularly suitable as comonomers. The comonomers vinyl acetate and methyl acrylate are particularly preferred.

The comonomer content in the polymer is between 0 and 15% by weight, in particular between 0 and 7.5% by weight, in view of the productivity when spinning the polymer.

Typically, but not explicitly limited thereto, the textile PAN of the invention is converted into fiber form via solution spinning processes, in particular via wet, dry, or airgap spinning. According to the invention, use is generally made here of industrial-scale spinning processes for polyacrylonitrile, which have been optimized over several decades in terms of productivity and economy.

According to the invention, the fibers used, preferably made of textile PAN, expediently have an individual filament diameter of 5 to 30 μm, in particular 8 to 18 μm and very particularly preferably 8 to 13 μm. It is also advantageous that the tensile strength of the fiber is between 25 and 80 cN/tex, in particular between 35 and 60 cN/tex. The modulus of elasticity of the fibers is preferably from 500 to 2500 cN/tex, moduli of elasticity from 900 to 1500 cN/tex and in particular from 950 to 1250 cN/tex are particularly preferred. The elongation of the textile PAN fibers is preferably from 5 to 25%, in particular from 8 to 16%.

For the successful implementation of the invention, it is also advantageous if the textile fibers are arranged in a multifilament, ie the fibers are arranged in a potentially endless fiber bundle consisting of several filaments. The multifilament preferably consists of 1000 to 10,000,000 filaments, in particular 3000 to 300,000 filaments.

2. Irradiation

An essential feature of the invention is the ionizing irradiation of the fibers, preferably using an electron beam. The resulting irradiated textile PAN fibers are referred to herein as E-PAN fibers.

Electron irradiation can be carried out under various atmospheric conditions. In the prior art, mostly air is used. However, it has surprisingly been shown that an inert gas atmosphere according to the invention, such as nitrogen, leads to better mechanical properties of the carbon fibers leads. In a preferred embodiment of the invention, the use of an inert gas atmosphere in the irradiation of the textile PAN fibers resulted in carbon fibers with 31% better tensile strength compared to the use of air. Various gases such as helium, neon, argon, krypton, xenon, nitrogen and carbon dioxide can be used as the inert gas. Nitrogen is particularly suitable according to the invention. The use of an inert gas during the irradiation also prevents the formation of ozone, which means that it does not have to be removed from the exhaust air.

The decisive parameter of electron irradiation for controlling the thermal properties of the E-PAN is the irradiation dose. Depending on this, T _onset-Z and T _peak-Z are shifted towards lower temperatures in such a way that a higher radiation dose results in lower temperatures T _onset-Z and T _peak-Z . These reduced T _{onset Z} and T _{peak Z} in the E-PAN compared to the non-irradiated textile PAN are called T _{onset Z E-PAN} and T _{peak Z E-PAN} in the following. According to the invention, the distance between Tonset _{-Z E-PAN} and T _{peak-Z E-PAN} is preferably adjusted to about 40 to 110°C, in particular to 60 to 90°C. In the case of non-irradiated textile PAN, the distance between Tonset _-Z and T _peak-Z is usually 10 to 60°C. The distance between Tonset _-E-PAN and T _peak-ZE-PAN is thus greater than that between Tonset _-Z and T _peak-Z . This minimizes the risk of a textile PAN multifilament overheating in an uncontrolled manner or even burning off during exothermic oxidative thermal stabilization.

In terms of the successful realization of the invention, the irradiation dose can preferably be 10 to 10000 kGy, in particular 10 to 5000 kGy. Irradiation doses of from 70 to 1500 kGy, in particular from 300 to 1000 kGy, are particularly preferred.

Another essential parameter of electron irradiation is the acceleration voltage. The higher this is, the deeper a fiber bundle is penetrated by the electron beam. However, radiation protection and the necessary financial outlay for plant construction are all the more complex. It was therefore an aim of the inventors to use the lowest possible acceleration voltage. High acceleration voltages of >1 MV, as used in the prior art, unnecessarily increase the required radiation protection precautions. At 900 kV, electron beam penetration depths of > 3 mm are already achieved in the multifilament, which corresponds to > 250 individual filaments lying one on top of the other. In view of the fact that the multifilaments are typically spread out to > 10 centimeters for better gas diffusion, a multifilament of > 2,000,000 individual filaments could be irradiated with an acceleration voltage of 900 kV, which is already a factor of 40 over the current state of the art typically used 50K multifilaments. According to the invention, therefore, acceleration voltages of 100 to 900 kV, in particular 160 to 600 kV and particularly preferably 180 to 400 kV have proven particularly advantageous.

In addition, the current is another important parameter of the irradiation. The radiation dose results from the current strength, acceleration voltage and time. The current of irradiation with electron beams is preferably 0.1 to 100 mA, with the preferred range being 1 to 15 mA, particularly 2 to 10 mA.

The irradiation can be carried out discontinuously or continuously, with regard to a technical, cost-efficient process in which the shortest possible time should also elapse between irradiation and oxidative thermal stabilization, continuous irradiation appears to be more advantageous. The process speed at which the fibers are continuously irradiated is preferably 0.5 to 100 m/min, in particular 5 to 50 m/min.

Furthermore, during the continuous irradiation it is advantageous if the fiber to be irradiated is subjected to a tensile force which prevents the fiber from shrinking. This tensile force is preferably between 0.001 and 1 cN/filament, in particular between 0.03 and 0.3 cN/filament.

3. Oxidative thermal stabilization

According to the invention, the irradiation of the fiber made of textile PAN is followed by the oxidative heat stabilization. The E-PAN fibers can be stored for several weeks. It would therefore be conceivable to carry out continuous irradiation "online" immediately before the oxidative thermal stabilization or to carry out the irradiation continuously but then lay down or wind up the fibers and store them for any desired period of time. Experiments were therefore carried out in which E-PAN fibers were stabilized into Ox-PAN fibers one hour, one day, one week and 6 weeks after irradiation. The Ox-PAN fibers were then carbonized. It was shown that the mechanical properties of the resulting carbon fibers correlated with the storage time between irradiation and stabilization in such a way that a longer storage time deteriorated the mechanical properties - especially the tensile strength. A storage time of the E-PAN fibers of 6 weeks reduced the tensile strength of resulting carbon fibers by an average of 18% compared to carbon fibers where the storage time was about one hour.

It is therefore advantageous for the fibers to be oxidatively thermally stabilized immediately after irradiation. "Immediately" can also mean that stabilization and irradiation are connected by continuous thread guidance and their process speeds are coordinated with one another.

The atmosphere during stabilization should have an oxidizing character, so it is particularly advisable to use air.

It has been shown that it is advantageous if the temperature is not constant during the oxidative thermal stabilization. A continuously increasing temperature during the oxidative thermal stabilization with a defined start and end temperature and a defined stabilization time is advantageous. Preferably, but not limited to, this is accomplished through the use of a multi-heat zone stabilization oven. The temperature profile over the entirety of the heating zones is characterized in principle by the fact that, starting with the second heating zone, each of the heating zones has a higher temperature than the previous one.

In order to match the temperature profile to the E-PAN according to the invention, Tonset _{-Z E-PAN} and T _{peak-Z E-PAN} must first be determined by DSC under air according to DIN EN ISO 11357-5:2014-07 to find out , in which temperature range the cyclization reactions of the E-PAN occur, which are necessary for the oxidative thermal stabilization. Heating rates of 10 K/min should be used. The starting temperature of the oxidative thermal stabilization should deviate from Tonset _{-Z E-PAN} by a maximum of 30° C., in particular preferably a maximum of 20° C., preferably a maximum of 10° C. The end temperature of the oxidative thermal stabilization should deviate from T _{Peak-Z E-PAN} by a maximum of 30°C, preferably by a maximum of 20°C.

Successful stabilization within the meaning of the invention is also distinguished by the fact that the density of the Ox-PAN fiber is at least 1.30 g/cm ³ . The density of the Ox-PAN fiber is preferably in the range from 1.35 to 1.5 g/cm ³ , particularly preferably between 1.35 and 1.39 g/cm ³ . If the density of the resulting Ox-PAN fibers falls outside of this density range, either the temperature in the heating zones of the stabilization furnace or the process speed must be changed. Increasing the temperature in the heating zones or slowing down the process speed leads to a higher density of the Ox-PAN fiber. Decreasing the temperature or increasing the process speed leads to a lower density of the Ox-PAN fiber. This iterative parameter change is carried out until the Ox-PAN fiber reaches the corresponding density range.

The oxidative thermal stabilization lasts preferably 10 minutes to 4 hours, in particular 1 hour to 3 hours, particularly preferably 1 hour to 2 hours. However, it has also been shown that the range from 1.5 to 2.5 hours is advantageous.

The oxygen content of the Ox-PAN fiber is preferably 5 to 25% by weight, in particular 10 to 15% by weight and very particularly preferably from 11 to 13% by weight.

Furthermore, it is important for the success of the invention that the fibers are preferably subjected to a tensile force during stabilization. A high tensile force during stabilization usually leads to better mechanical properties of the resulting carbon fibers. The magnitude of the tensile force also gives stretching or shrinkage of the fibers during stabilization. It has been shown that it is advantageous, especially at the beginning of the oxidative thermal stabilization, to select the tensile force in such a way that a stretching of 0 to 50%, preferably 0 to 10%, results. The tensile forces that occur are preferably 0.03 to 1 cN per filament, in particular 0.05 to 0.5 cN per filament, with the range from 0.1 to 0.3 cN per filament being particularly preferred. Surprisingly, it was also found that an increase in the radiation dose is accompanied by an increase in the tensile forces for the same stretching.

4. Carbonation

In order to obtain a carbon fiber, after stabilization, the Ox-PAN fibers are preferably carbonized under an inert gas atmosphere. Possible inert gases are helium, neon, argon, krypton, xenon and nitrogen, with the use of nitrogen being preferred.

The final temperature during carbonization is decisive for the degree of carbonization and for the mechanical properties of the resulting carbon fibers. The final temperature of the carbonization can be up to 1800°C.

The carbonization can be carried out continuously and discontinuously. In batchwise carbonization, the Ox-PAN fiber is heated under an inert gas atmosphere from any desired temperature, usually room temperature, to the final carbonization temperature. The heating rate during carbonization is preferably between 1 and 100 K/min, in particular between 5 and 20 K/min. The Ox-PAN fiber should experience a tensile force in the fiber axis during batchwise carbonization.

In addition, and preferably, continuous carbonization is possible. In the case of continuous carbonization, the Ox-PAN fibers are guided through a carbonization furnace via godets, which in an advantageous embodiment has several heating zones. The use of several carbonization furnaces is particularly preferred. When using two consecutive carbonization furnaces, these become LT (low temperature) and HT (high temperature) furnaces called. The temperature in the LT furnace can be between 200 and 1000°C, preferably between 300 and 750°C. This temperature range should also be completely covered in the furnace over several heating zones. In the HT furnace, the temperature can be between 800 and 1800°C, preferably between 1000 and 1400°C.

It is also advantageous during the continuous carbonization if the fibers are subjected to a tensile force. This should result in an overall stretching of the fibers of -10 to +10%. In particular, it is advantageous if positive stretching of 0.1 to 15% is achieved in the LT oven and negative stretching or shrinkage of -0.1 to -15% in the HT oven. The winding speed for continuous carbonization should be between 0.5 and 50 m/min and essentially depends on the size of the carbonization system.

The density of the resulting carbon fibers is preferably between 1.65 and 1.9 g/cm ³ , in particular between 1.7 and 1.8 g/cm ³ . A higher density often goes hand in hand with an improvement in mechanical properties. Surprisingly, the irradiation of the multifilaments made of textile PAN leads to higher densities of the carbon fibers with a comparable density of the Ox-PAN fiber.

5. graphitization

Optionally, a graphitization can also be carried out after the carbonization. As with carbonization, an inert gas atmosphere is used here. Helium, neon, argon, krypton and xenon can be used as inert gases. The use of argon is preferred.

The graphitization is preferably carried out between 1800 and 3000°C. This is achieved using one or more graphitization furnaces, each of which is preferably equipped with multiple heating zones. The starting temperature of graphitization can be between 1800 and 2200°C. The final temperature can be between 2200 and 3000°C.

The graphitization is advantageously carried out continuously. During the graphitization, the multifilament should experience a tensile force. This is preferably between 0.01 and 0.5 cN per filament. The stretching resulting therefrom is preferably between -5 and +5%, in particular between -2 and +2%.

In summary, it can be stated that the invention, as shown above, is associated with a wide range of advantages:
The invention relates to a simple, inexpensive method for electron beam irradiation of multifilaments consisting of textile PAN and their use as precursors for the production of carbon fibers.

The irradiation of the textile PAN multifilaments should preferably be carried out under nitrogen. Surprisingly, this inert gas atmosphere leads to better mechanical properties of the resulting carbon fibers. In addition, it has proven to be advantageous to carry out the oxidative thermal stabilization immediately after the irradiation. The irradiated multifilament can be converted into an Ox-PAN multifilament by means of a defined oxidative thermal stabilization which is advantageous with regard to the thermal properties of the precursor. This can be converted into carbon fibers under an inert gas atmosphere. The resulting carbon fibers have top values for maximum tensile strength (according to DIN EN ISO 5079:1995) of up to 3.1 ± 0.6 GPa on average. The modulus of elasticity is up to 212 ± 9 GPa on average.

Accordingly, the core of the present invention relates to the process control during electron irradiation and the oxidative thermal stabilization.

The present invention is to be explained in more detail below by means of various examples:

examples Example 1 (irradiation, stabilization and carbonization of textile PAN 1)

500 m of a wet-spun 3k multifilament of textile PAN 1 (93.5 wt.% acrylonitrile, 6 wt.% methyl acrylate, 0.5 wt.% methallylsulfonate, M _n = 72,000 g/mol, PDI= 3, 0, T _{onset Z} = 249°C, T _{peak Z} = 299°C) was irradiated continuously (winding speed = 6.6 m/min) with 1000 kGy electron beams at 200 kV acceleration voltage and 3.5 mA current under nitrogen. The mechanical properties of the multifilament before and after irradiation are given in Table 1 . <u>Table 1</u> fiber Tensile strength [MPa] Modulus of elasticity [GPa] Strain [%] diameter [µm] Density [g/cm ³ ] 0 kGy 540±35 11.3±0.9 12.3±0.8 12.1±0.8 1:18 E-PAN 1000 kGy 520±35 12.1±0.8 11.1±0.6 12.8±0.5 1:18 Ox-PAN 330±40 7.9 ± 1.0 19.3 ± 4.0 11.4±0.9 1.36 carbon fiber 2650±600 206±9 1.24 ± 0.25 7.0 ± 0.6 1.77

Note : Mechanical properties of a 3K wet-spun multifilament composed of textile PAN 1, unirradiated and electron irradiated under nitrogen with a dose of 1000 kGy, the resulting Ox-PAN fiber and the resulting carbon fiber. The values are based on 20 individual fiber measurements, for the carbon fiber on 30 individual fiber measurements.

A DSC measurement of the irradiated fiber was then used to determine its Tonset _{Z E-PAN} at 204°C and a T _{Peak Z E-PAN} at 282°C. The multifilament was then stabilized in a stabilization oven with 4 heating chambers. Based on the determined Tonset _{-Z E-PAN,} a temperature of 210°C in heating chamber 1 was selected. In the following heating chambers 2 to 4, 225°C, 245°C and 265°C were set. In heating chambers 1 and 2, the fiber was drawn 5% in each case. The tensile force occurring was 426 cN in heating chamber 1, 527 cN in heating chamber 2, 428 cN in heating chamber 3 and 460 cN in heating chamber 4. The density of the resulting Ox-PAN multifilament was 1.36 g/cm ³ . The mechanical properties and the density of the Ox-PAN fibers can be seen in Table 1.

Subsequent carbonization was carried out continuously with the aid of an LT and an HT carbonization furnace in a nitrogen atmosphere. The temperature profile and stretching in the LT oven can be seen in Table 2, the Temperature profile and stretching in the HT oven in Table 3. The resulting mechanical properties and the density of the carbon fibers can be seen in Table 1. <u>Table 2</u> heating zone 1 2 3 4 5 6 Temperature [°C] 300 390 480 570 660 750 stretch [%] +5

Note: Temperature profile and stretching in the LT oven during carbonization in Example 1. <u>Table 3</u> heating zone 1 2 3 Temperature [°C] 1000 1175 1350 stretch [%] -3.5

Note: Temperature profile and stretching in the HT oven during carbonization in example 1. <u>Table 4</u> Tensile strength [MPa] Modulus of elasticity [GPa] Strain [%] diameter [µm] 3250 211.23 1.47 6.84 2800 188.85 1.39 7.88 3470 225.68 1.47 6:18 2330 204.28 1:11 7.53 2390 207.99 1:11 7.33 2930 213.84 1.31 6.57 2870 204.10 1.35 7.59 3340 207.88 1.54 6.34 2680 212.53 1.22 8:11 3960 218.22 1.73 6.35 2360 207.74 1:11 6.73 2640 209.00 1.22 7.98 1670 203.82 0.82 7.40 1790 200.58 0.88 6.74 2120 213.07 0.99 6.49 2540 200.40 1.23 6.58 2710 196.56 1.32 5.65 2640 197.62 1.30 6.69 1660 218.81 0.76 6.55 2140 204.31 1.02 7.26 3370 193.09 1.60 7.41 3420 201.74 1.62 7.77 2560 215.99 1.14 6:14 2920 214.81 1.32 7.44 3080 186.63 1.56 7:14 2290 205.56 1.09 6.94 2190 205.65 1.04 7.36 1940 208.91 0.92 6.95 3040 211.56 1.39 6.61 2310 202.89 1:11 7.38

Note: Individual fiber values of the mechanical properties of the carbon fibers from example 1.

Comparative example 1: (Stabilization and carbonization of textile PAN 1, without irradiation)

The multifilament from Example 1 was continuously stabilized and carbonized without irradiation. Tonset _-Z was determined to be 249°C and T _peak-Z to be 299°C by means of DSC measurement. The temperature in heating chamber 1 of the stabilization furnace was 240°C, in heating chambers 2 to 4 250, 265 and 275°C were set. Analogously to example 1, 5% was stretched in heating chambers 1 and 2 in each case. The tensile force occurring was 171 cN in heating chamber 1, 203 cN in heating chamber 2, 255 cN in heating chamber 3, and 370 cN in heating chamber 4. The density of the Ox-PAN multifilament was 1.39 g/cm ³ , the mechanical properties can be seen in Table 5. The carbonization was also carried out analogously to Example 1. The mechanical properties and density of the resulting carbon fibers can be seen in Table 5. <u>Table 5</u> fiber Tensile strength [MPa] Modulus of elasticity [GPa] Strain [%] diameter [µm] Density [g/ml] Ox-PAN 280±40 8.2 ± 1.0 16.8±3.4 10.7±1.2 1.39 carbon fiber 2250±400 196±6 1.12±0.2 6.8±0.4 1.73

Note: Mechanical properties of Ox-PAN fibers and carbon fibers resulting from a 3K wet-spun multifilament of Textile-PAN 1, without electron beam irradiation.

Comparative example 2 : (irradiation in air, stabilization and carbonization analogous to KR 20160140268A)

The multifilament from example 1 was irradiated analogously to example 1, but under air instead of nitrogen, so that the irradiation dose and atmosphere correspond to those in KR 101755267 same. Thereafter, an approximately 15 cm long piece of the air-blasted fibers was fixed in graphite boats. The fibers were then oxidatively stabilized in air in a muffle furnace. The heating was from 200 to 240°C within 150 minutes and from 240 to 260°C within 90 minutes. The fibers were then carbonized at a heating rate of 5 K/min up to 1200° C. under nitrogen. The resulting mechanical properties can be seen in Table 6. The tensile strength of these fibers roughly corresponds to the tensile strength of carbon fibers KR 20160140268A , the E modulus is about 50 GPa higher than that of carbon fibers KR 20160140268A . <u>Table 6</u> dose Tensile strength [MPa] Modulus of elasticity [GPa] Strain [%] diameter [µm] Density [g/ml] 1000kGy 1740 ± 500 178±15 0.98 ± 0.28 8.4±0.5 1.75

Note : Mechanical properties of the carbon fibers resulting from Textile PAN 1, irradiated in air, stabilized and carbonized analogously to KR 101755267 .

Example 2: (irradiation, stabilization and carbonization of textile PAN 2)

500 m of a wet-spun 3k multifilament consisting of textile PAN 2 (6.5% by weight of vinyl acetate; M _n = 51,000 g/mol, PDI = 4.9, T _{onset Z} = 256°C, T _{peak Z} =314° C.) were irradiated continuously (6.6 m/min) with 1000 kGy electron beams at an acceleration voltage of 200 kV and a current intensity of 3.5 mA under nitrogen. The Tonset _{Z E-PAN} of the irradiated fiber was then determined at 210° C. and the T _{Peak Z E-PAN} at 291° C. by means of a DSC measurement. The multifilament was then stabilized in a stabilization oven with 4 heating chambers. Based on the determined Tonset _{-Z E-PAN,} a temperature of 210°C in heating chamber 1 was chosen.

In the following heating chambers 2 to 4, 225°C, 245°C and 265°C were set. In heating chambers 1 and 2, the fiber was drawn 5% in each case. The density of the resulting Ox-PAN multifilament was 1.36 g/cm ³ . Subsequent carbonization was carried out continuously with the aid of an LT and an HT carbonization furnace in a nitrogen atmosphere. The temperature profiles in LT and HT correspond to those in Example 1, the stretch in LT was +2% and in HT -3.5%. The mechanical properties of the carbon fibers can be seen in Table 7. <u>Table 7</u> Dose of EB irradiation [kGy] Tensile strength [MPa] Modulus of elasticity [GPa] Strain [%] diameter [µm] Density [g/ml] 1000 2600±600 184±6 1.38 ± 0.34 7.0 ± 0.5 1.71

Note: Properties of the carbon fibers resulting from a wet-spun 3-component multifilament of textile PAN 2, electron beam irradiated under nitrogen with a dose of 1000 kGy.

Example 3: (irradiation, stabilization and carbonization of textile PAN 1 with different time intervals between irradiation and stabilization)

800 m of a dry-spun 3-component multifilament made of textile PAN 1 (93.5% by weight of acrylonitrile, 6% by weight of methyl acrylate, 0.5% by weight of methallyl sulfonate, M _n = 72,000 g/mol, PDI = 3 ,0, T _{onset Z} = 249°C, T _{peak Z} = 299°C) were irradiated continuously (winding speed = 6.6 m/min) with 1000 kGy electron beams at 200 kV acceleration voltage and 3.5 mA current under nitrogen . The T _{onset Z E-PAN} and T _{Peak Z E-PAN} corresponded to that of Example 1. The multifilament was then tested in four experiments, each after a break of one hour, one day, one week and 6 weeks in one Stabilization oven stabilized with 4 heating chambers. The stretching and temperatures in the stabilization oven correspond to those of example 1. The tensile forces occurring in the stabilization oven can be seen in Table 8. <u>Table 8</u> Interval irradiation - stabilization Oven 1 2 3 4 Temperature [°C] 210 225 245 265 stretch [%] 5 5 0 0 1 hour Tensile force [cN] 196 268 272 274 1d Tensile force [cN] 206 267 268 260 1w Tensile force [cN] 182 252 257 266 6 w Tensile force [cN] 188 251 261 240

Note: Temperature profile, tensile forces and stretching in the stabilization oven for the oxidative stabilization of the dry-spun and 1000 kGy irradiated Dralon X fiber after different time intervals between irradiation and stabilization.

The density and mechanical properties of the resulting Ox-PAN multifilament are shown in Table 9. <u>Table 9</u> Interval irradiation - stabilization Tensile strength [cN/tex] Modulus of elasticity [cN/tex] Strain [%] Diameter [dtex] Density [g/ml] 1 hour 20.8±1.8 550±30 23±5 1.1±0.3 1.36 1d 18.4 ± 2.0 530±20 23±5 1.2±0.3 1.36 1w 19.9±2.2 560±30 20±6 1.1±0.3 1.36 6 w 16.2±1.1 500±30 23±5 1.2±0.3 1.37

Note: Properties of the stabilized fibers resulting from 1000 kGy irradiated dry-spun Dralon X fibers. The time between irradiation and stabilization was varied between about one hour and 6 weeks.

The carbonization was then carried out as in Example 1. Table 10 shows the mechanical properties and the densities of the resulting carbon fibers. In the LT oven, the stretching was varied between 2 and 7% in the tests. <u>Table 10</u> Interval Irradiation Stabilization LT stretch [%] Tensile strength [GPa] Modulus of elasticity [GPa] Strain [%] Diameter [dtex] 1 hour 2 2.49 ± 0.89 188±7 1.29±0.4 0.55 ± 0.09 5 3.08 ± 0.64 193±9 1.54 ± 0.29 0.58 ± 0.09 7 2.89 ± 0.89 193±7 1.46±0.43 0.61 ± 0.10 1d 2 2.52 ± 0.66 192±9 1.29 ± 0.31 0.61 ± 0.19 5 2.85 ± 0.60 196±9 1.42±0.29 0.66 ± 0.18 7 2.60 ± 0.62 195±9 1.30 ± 0.29 0.53 ± 0.08 1w 2 2.16 ± 0.63 185±11 1.15 ± 0.32 0.67 ± 0.25 5 2.67±0.52 192±10 1.35 ± 0.23 0.58 ± 0.09 7 2.46 ± 0.53 197±9 1.23 ± 0.25 0.61 ± 0.16 6 w 2 2.72 ± 0.64 184±8 1.44 ± 0.33 0.59 ± 0.07 5 2.17 ± 0.59 191±14 1.12±0.27 0.63 ± 0.16 7 2.07 ± 0.38 192±11 1.07±0.21 0.58 ± 0.13

Note : Mechanical properties of the carbon fibers from a dry-spun, 1000 kGy irradiated 3K textile PAN-1 multifilament. The time between irradiation and stabilization was varied between 1 h and 6 W. The density of all carbon fibers was 1.77 g/ml.

Example 4: (irradiation, stabilization and carbonization of textile PAN 3)

500 m of a wet-spun 3k multifilament consisting of textile PAN 3 (100% acrylonitrile, M _n = 84,000 g/mol, PDI = 5.6, T _{onset Z} = 298°C, T _{peak Z} = 313°C ), were irradiated continuously (6.6 m/min) with 1000 kGy electron beams at 200 kV acceleration voltage and 3.5 mA current under nitrogen. Subsequently, the Tonset _{Z E-PAN} of the irradiated fiber was determined as 217° C. and the T _{peak Z E-PAN} as 279° C. via a DSC measurement of the irradiated fiber. The multifilament was then stabilized in a stabilization oven with 4 heating chambers. Based on the determined Tonset _{-Z E-PAN,} a temperature of 210°C in heating chamber 1 was selected. In the following heating chambers 2 to 4 were 225°C, 245°C and 265°C set. In heating chambers 1 and 2, the fibers were each drawn 2%, in heating chambers 3 & 4 the drawing was -0.5%. The density of the resulting Ox-PAN multifilament was 1.37 g/cm ³ . Subsequent carbonization was carried out continuously with the aid of an LT and an HT carbonization furnace in a nitrogen atmosphere. The temperature profiles in LT and HT correspond to those in Example 1, the stretch in LT was +5% and in HT -3.5%. The mechanical properties of the carbon fibers can be seen in Table 11. <u>Table 11</u> Dose of EB irradiation [kGy] Tensile strength [MPa] Modulus of elasticity [GPa] Strain [%] diameter [µm] Density [g/ml] 1000 2980±660 212±9 1.38 ± 0.30 7.0 ± 0.5 1.78

Note: Properties of the carbon fibers resulting from a wet-spun 3K multifilament of textile PAN 3, electron beam irradiated under nitrogen with a dose of 1000 kGy.

Claims (15)

1. A method for the irradiation and oxidative stabilisation of PAN fibres for the production of a precursor fibre of carbon fibres, characterised in that

   (1) the PAN fibres are based on a homopolymer or copolymer of PAN, the homopolymer or copolymer of PAN having a T _onset-Z temperature of at least 245°C, measured under air (according to DIN EN ISO 11357-5:2014-07), a number-average molecular weight of 20,000 to 250,000 g/mol polymethyl methacrylate molar mass equivalents (determined according to DIN 55672-2:2016-03), and a content of comonomers of not more than 15.0 wt.%,

   (2) the PAN fibres are subjected to ionising irradiation with electron beams in an inert gas atmosphere with an irradiation dose of 10 to 5,000 kGy,

   (3) the reduced T _onset-Z temperature under air is determined from the E-PAN fibres obtained by irradiation (T _{onset-Z E-PAN}) (according to DIN EN ISO 11357-5: 2014-07), thereupon the oxidative thermostabilisation is initiated at a starting temperature of T _{onset-Z E-PAN} ± 30°C, and the oxidative thermostabilisation is carried out at increasing temperature up to a minimum density of the oxidatively stabilised PAN fibre (ox-PAN) of 1.30 g/cm³.
2. The method according to claim 1, characterised in that the PAN has a number-average molecular weight of from 30,000 to 150,000 g/mol, especially from 50,000 to 120,000 g/mol.
3. The method according to claim 1 or claim 2, characterised in that the comonomer content of PAN is 0.0 to 12.0 wt.%, especially 0.0 to 9.0 wt.%, and especially preferably 0.0 to 7.5 wt.%.
4. The method according to at least one of the preceding claims, characterised in that the comonomer of the PAN is a vinyl compound, especially vinyl acetate, propionic acid vinyl ester, methyl acrylate, methyl methacrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, tert-butyl acrylate, sodium methallyl sulphonate, sodium vinyl sulphonate, acrylamide, methacrylamide and/or vinyl acetamide.
5. The method according to at least one of the preceding claims, characterised in that the irradiation dose of the ionising irradiation is 70 to 2,500 kGy, especially 300 to 1,000 kGy.
6. The method according to at least one of the preceding claims, characterised in that the ionising irradiation is carried out with electron beams in a nitrogen atmosphere.
7. The method according to claim 6, characterised in that the acceleration voltage during the ionising irradiation with electron beams is 100 to 900 kV, especially 160 to 600 kV and especially preferably 180 to 400 kV.
8. The method according to claim 6 or claim 7, characterised in that the current intensity during ionising irradiation with electron beams is 0.1 to 100 mA, especially 1 to 50 mA, and especially preferably 2 to 10 mA.
9. The method according to at least one of claims 6 to 8, characterised in that the storage time between irradiation and oxidative thermostabilisation is less than one day, preferably less than one hour.
10. The method according to at least one of claims 6 to 9, characterised in that the ionising irradiation is carried out immediately upstream of the oxidative thermostabilisation in the case of continuous filament running.
11. The method according to at least one of the preceding claims, characterised in that the starting temperature in the oxidative thermostabilisation T _{onset-Z E-PAN} is ± 20°C,
    especially ± 10°C.
12. The method according to at least one of the preceding claims, characterised in that the end temperature of the oxidative thermostabilisation T _{Peak-Z E-PAN} is ± 30°C,
    especially ± 20°C.
13. The method according to at least one of the preceding claims, characterised in that the oxidative thermostabilisation is carried out up to a density of the oxidatively thermostabilised PAN (ox-PAN) of 1.30 to 1.5 g/cm³, especially of 1.35 to 1.39 g/cm³.
14. The method according to at least one of the preceding claims, characterised in that the T _onset-Z temperature is up to 320°C, especially up to 300°C.
15. Use of the ox-PAN fibres obtained by a method according to at least one of the preceding claims for the production of carbon fibres by carbonisation, optionally with subsequent graphitisation.