Method for producing carbon fiber precursor in one step

Abstract

A method of integrated and improved one-step production of carbon fiber precursors is described, in particular from comonomer start up until the spinning step to obtain the final fiber precursor.

Description

[0001] The present invention relates to an integrated and improved process for the production of carbon fiber precursors, in particular a process from the co-monomer starting up to the spinning step to obtain the final fiber precursor.

[0002] More specifically, the present invention constitutes part of the field of fiber precursor production, which provides a process for the preparation of polymers starting from acrylonitrile or mainly acrylonitrile (95-99.5 wt% of the total weight of the polymer) with one or more other co-monomers in a constant range of 0.5-5 wt% of the total weight of the polymer.

[0003] Preferred co-monomers are molecules with one or more acid groups such as acrylic acid, itaconic acid, sulfonated styrene and the like and optionally neutral vinyl molecules such as methyl acrylate, methyl methacrylate, vinyl acetate, acrylamide and the like.

[0004] The polymers and copolymers thus produced are then subjected to spinning to produce fiber precursors in the form of a tow, which are collected in bobbins or boxes.

[0005] These polyacrylonitrile-based fiber "precursors" are then subjected to a suitable thermal treatment to obtain carbon fibers.

[0006] There are various industrial processes for the preparation of acrylonitrile fibers, which use different polymerization and spinning methods.

[0007] The state of the art can be divided and summarized as follows:

[0008] A. Batch process (two steps) .

[0009] In the two-step batch process, the polymer is usually produced in aqueous suspension, separated and subsequently dissolved in a suitable solvent for spinning and conversion into fibers or fiber precursors in the case of carbon fibers. The most commonly used solvents for the preparation of the spinning solution are: dimethylacetamide (DMAc), dimethylformamide (DMF), aqueous sodium thiocyanate (NaSCN) and finally, as recently described in patent EP 2894243 B1, a mixture of dimethyl sulfoxide (DMSO) with different amounts of water.

[0010] B. Continuous process (one step) .

[0011] On the other hand, in the continuous process, the polymerization reaction is carried out in a solvent, the solution thus obtained being used directly for spinning, without an intermediate separation step of the polymer. The most commonly used solvents in these processes are: dimethylformamide (DMF), dimethyl sulfoxide (DMSO), aqueous zinc chloride (ZnCl2) and aqueous sodium thiocyanate (NaSCN).

[0012] Although from a management point of view the batch process has advantages, mainly because the polymerization and spinning steps are independent for large plants, when high performance carbon fibers are to be obtained, the one-step process is generally preferred.

[0013] The one-step process can be further divided into continuous polymerization processes and batch polymerization processes.

[0014] In the case of continuous polymerization processes, a solution of the comonomer in a solvent is continuously fed with the appropriate catalyst to one or more reactors in series. After a predetermined residence time, the solution containing the high molecular weight copolymer produced during the reaction, unreacted monomer or comonomer and un-decomposed catalyst is collected from the single reactor or from the last reactor of the series.

[0015] Historically the solvents used in these processes were mainly DMF or aqueous sodium thiocyanate, more recently the preferred solvent is DMSO, which has the characteristics of low environmental impact and very low toxicity. The catalysts used in these processes are generally peroxides or azo compounds, which initiate the chain reaction of the radical type by thermal decomposition.

[0016] These continuous one-step processes are very efficient and allow the production of fiber precursors and carbon fibers with high quality and high performance, but they are not without problems and contraindications.

[0017] In particular, the continuous process, by its very nature, is difficult to manage at an industrial level, because every problem that occurs at a certain point in the process has immediate and direct consequences on the other steps of the process, causing consequent damage to the entire production process (for example, a polymerization problem has immediate consequences on the spinning, and vice versa).

[0018] In addition, any change in operating conditions requires a long transition time before reaching the steady state, resulting in the production of off-spec material.

[0019] Another limitation of the continuous process is the long residence time and the possible presence of stagnant zones in the entire plant, inside the reactor or reactors in series, which tend to form gels. These gels hinder the correct heat exchange, thus hindering the necessary removal of the reaction heat. These formed gels require frequent shutdowns of the plant to provide mechanical removal of the gels themselves, with the associated operating costs and production losses.

[0020] The limitations of the continuous process described above can be overcome by using a polymerization process in batch mode, in which at the beginning of the reaction the solvent, preferably DMSO, the comonomer and the catalyst are fed into a stirred reactor equipped with a stirrer, the system is cooled and stirred at controlled temperature for a predetermined time, as described in JP 2018084002A. At the end of the reaction, the contents of the reactor, i.e. the polymer, unreacted comonomer and undecomposed catalyst in solution, are discharged and then fed to the subsequent steps of the process, while the reactor is ready for a new cycle. In this way, the contents of the reactor are renewed at each cycle, without creating stagnant areas of the polymer solution over time with a tendency to form gels.

[0021] In addition, by operating in batch mode, it is easier to manage the plant, for example in the event of malfunctions or operating errors, the contents of the reactor can be diverted to a waste treatment section, without involving the downstream spinning plants. In fact, these can continue to be fed with the dope (i.e. the homogeneous solution of the fiber precursor) stored in the intermediate tanks, or they can be placed in standby, without producing off-spec fibers.

[0022] However, overall, the fiber production process has various shortcomings to be improved in order to optimize the performance of the product obtained and the production costs.

[0023] A first disadvantage of the process according to the prior art is the difficulty of adding ammonia, primary or secondary amines, which are known to contribute significantly to improving the spinning process of the production precursor and to obtaining carbon fibers with high performance, due to the aggressiveness of the reactants, such as gaseous ammonia, and the difficulty of obtaining a homogeneous dispersion of them in high viscosity media: in fact, it is known, as described in EP3783132A1 and the references contained therein, that the acid end groups of the possible comonomers, such as acrylic acid or itaconic acid, can be converted into the corresponding ammonium salts, which are particularly advantageous in the subsequent treatments for the production of carbon fibers, in particular in the solidification and oxidation phases of the spinning. The conventional technique tries to solve the problem by treating the dope or the DMSO solvent by adding gaseous ammonia to the dope before feeding it into the spinning machine, as described in JP 2017186682.

[0024] To avoid the use of gaseous ammonia and in any case to obtain polymers with better hydrophilicity, the use of ammonium itaconate as comonomer has been claimed (CN105624819A).

[0025] In view of the poor solubility of this salt in common organic solvents, the use of aqueous solutions thereof is also taught by the patent applications CN104558397A, CN104558395A and CN106589223A. In this case, the ammonium itaconate salt is added to the polymerization reactor as an aqueous solution at the highest possible concentration, in order to maintain the amount of water in the system as low as possible (<0.5% by weight).

[0026] For the same reason, acrylonitrile is also distilled before use, in order to remove the water contained therein, generally about 0.5%.

[0027] Therefore, there is a technical prejudice against the presence of water in the reactant mixture, in order to obtain a water-free spinning solution at the end of the process.

[0028] The Applicant has surprisingly found that the presence of a significant amount of water (1-5% by weight with respect to the weight of the reaction mixture) does not negatively affect the polymerization reaction. Moreover, as taught by EP 2894243 (CN 104775174 B), DMSO slurries containing these amounts of water can be advantageously used for spinning to produce fiber precursors with excellent results.

[0029] Another drawback of the conventional process is the difficulty of removing the unreacted volatile monomer or comonomer from the slurry before sending it to the subsequent filtration and spinning steps. The existing process uses, for example, thin-film evaporators (TFE) or stripping columns to effectively remove most of the unreacted acrylonitrile. However, complete removal is only possible by distilling large amounts of solvent. In fact, it is problematic to produce a good spinning slurry with a residual acrylonitrile content lower than 1000 ppm (0.1%). In view of the carcinogenicity of acrylonitrile, acrylonitrile contents higher than this limit value require special precautions in the subsequent processing steps.

[0030] Therefore, the object of the present application relates to a process for the preparation of a fiber precursor which overcomes the above-mentioned limits and drawbacks of the known art and which allows significant advantages in terms of product quality and production costs. DETAILED DESCRIPTION

[0031] The present application relates to a process for the production of a carbon fiber precursor, said process comprising the following steps:

[0032] i) preparing an acid comonomer solution selected from aqueous solutions of itaconic acid or acrylic acid, said aqueous solution containing at least a stoichiometric amount of ammonia with respect to the acid groups present, the concentration of the acid comonomer in water varying in the range 3-50% by weight, the total amount of water entering the reactor being comprised between 1 and 5% by weight with respect to the total weight of the material fed to the reactor in step ii);

[0033] ii) feeding into a reactor acrylonitrile or a mixture of acrylonitrile and neutral vinyl comonomer, the aqueous solution of ammonium salt of the acid comonomer prepared in step i), DMSO, 2-2'-azobisisobutyronitrile AIBN and dodecyl mercaptan or octyl mercaptan, wherein the two comonomers acrylonitrile comonomer / neutral vinyl comonomer are present in a weight ratio ranging from 95:5 to 99.5:0.5;

[0034] iii) keeping the mixture thus obtained under stirring at a temperature ranging from 50 to 80 °C for a time ranging from 10 to 20 hours, preferably ranging from 12 to 15 hours;

[0035] iv) discharging the content of the reactor into a tank maintained at a temperature ranging from 35 to 40 °C to slow down or interrupt the reaction;

[0036] v) feeding the mixture thus obtained into a thin film evaporation system (TFE) operating at a temperature ranging from 40 to 80 °C and at a pressure ranging from 5 to 30 bar absolute;

[0037] vi) collecting from the top of the TFE a mixture of acrylonitrile, water and DMSO, said mixture being reused in the preparation process of a new reaction batch;

[0038] vii) collecting from the bottom of the TFE the polymer DMSO solution, diluting said solution with fresh DMSO to reach a concentration of 15 to 25 wt%, preferably 18 to 22 wt%, on the total weight of the solution;

[0039] viii) feeding the homogeneous spinning solution obtained at the end of step vii) into a spinning step or into a storage tank.

[0040] In particular, the presence of a greater amount of water in the polymerization phase simplifies the process and allows a reduction in the production costs with respect to the known art for various reasons:

[0041] - the initial distillation step of the raw material acrylonitrile to eliminate traces of water can be avoided;

[0042] - the use of gaseous ammonia is eliminated;

[0043] - the use of the more expensive ammonium salt can be replaced by the use of the common itaconic acid or acrylic acid;

[0044] - but the hydrophilicity of the polymer is improved by the presence of the itaconic acid or acrylic acid in the form of a salt with ammonia, thus obtaining a homogeneous and reproducible slurry;

[0045] - the removal of the unreacted acrylonitrile is more efficient since it can be removed as a mixture with water and DMSO, thus making it possible to recycle all the products recovered in the polymerization reactor.

[0046] The spinning step is carried out using a wet spinning process or a dry-jet wet spinning process, in which, after the coagulation phase carried out in a coagulation bath consisting of a mixture of water and solvent, the fibre bundle thus obtained is continuously stretched and washed to a length of about 10 times the initial length, and then subjected to a final washing step with water to remove the last traces of solvent.

[0047] Dodecyl mercaptan or octyl mercaptan as molecular weight regulator.

[0048] In step i) of preparation of the acid comonomer solution, the concentration of the acid comonomer in water preferably varies in the range from 3 to 50% by weight, more preferably from 5 to 30% by weight, the total amount of water fed to the reactor being in the range from 1 to 5% by weight of the total weight of the material fed to the reactor in step ii) of the process according to the application.

[0049] The total weight of the material fed to the reactor actually means the total weight of the components fed to the reactor in step ii) of the process according to the application.

[0050] In step i), the neutral vinyl comonomer can be selected from methyl acrylate or vinyl acetate.

[0051] In step ii) of feeding to the reactor, 90 to 150 parts by weight, more preferably 100 parts by weight, of acrylonitrile or a mixture of acrylonitrile and neutral vinyl comonomer are fed, in which the two comonomers acrylonitrile comonomer / neutral vinyl comonomer are present in a weight ratio of from 95:5 to 99.5:0.5; 2 to 15 parts by weight, more preferably 10 parts by weight, of the aqueous solution of the ammonium salt of the acid comonomer prepared in step i); 300 to 500 parts by weight, more preferably 400 parts by weight, of DMSO; 0.2 to 0.5 parts by weight, more preferably 0.3 parts by weight, of 2-2'-azobisisobutyronitrile AIBN and 0.05 to 0.15 parts by weight, more preferably 0.1 parts by weight, of dodecyl mercaptan or octyl mercaptan.

[0052] In step iii), the mixture obtained in step ii) is kept under stirring, the temperature preferably varying in the range from 65 to 75°C, for a time in the range from 10 to 20 hours, preferably from 12 to 15 hours.

[0053] The present application makes it possible to obtain a solution of acrylonitrile copolymer free from gels and not forming insoluble aggregates, improving the advantages associated with solution polymerization, but eliminating the dangerous and expensive step of treating the slurry or the solvent with gaseous ammonia, which is necessary to obtain a polymer solution with improved spinnability and improved performance in the subsequent stages of oxidation and carbonization.

[0054] Furthermore, the presence of water in the polymer solution and in the unreacted comonomer does not represent a risk for the recovery and recycling process of the polymerization reagents, as steps vi) and vii) are not dangerous. It is also possible to achieve a low level of residual comonomer, in particular acrylonitrile, of about 200-300 ppm, thus increasing the safety of the environmental conditions in the subsequent processing steps.

[0055] Another advantage of the process according to the present application is determined by the specific amount of water contained in the spinning solution or dope fed to the spinning step: the percentage of water left in the homogeneous solution obtained according to the process of the present application for the production of acrylic fibers is in fact fully compatible with the spinning techniques of acrylonitrile fibers, whether according to dry or wet spinning techniques or according to DJWS techniques (dry jet wet spinning or air gap): thus there is no need to completely remove the water from the solution fed to the spinning.

[0056] Furthermore, as claimed in US 3932577, the presence of a small percentage of water in the spinning solution of acrylic fibers is advantageous for the compatibility of the solution itself with the coagulation bath, obtaining fibers free from air bubbles and cracks.

[0057] Another advantage of the presence of water in the polymerization process is that it is possible to use fresh acrylonitrile, which usually contains small amounts of water (about 0.5 wt%) without purification, and also to use acrylonitrile recovered from the monomer removal process, which also contains small amounts of water. The presence of a small and variable amount of water in the main comonomer does not represent a problem, since the total amount of water in the reaction system is adjusted to the desired value and is completely reproducible from batch to batch by adding the aqueous solution of the acid comonomer salified with ammonia in an amount always guaranteeing the same water content in the system.

[0058] In the present description, the term polymer means a copolymer obtained from acrylonitrile and one or more other comonomers (the amount of acrylonitrile ranging from 95 to 99.5 wt% of the total weight of the polymer and the amount of one or more other comonomers usually ranging from 0.5 to 5 wt% of the total weight of the polymer).

[0059] The preferred comonomers are molecules with one or more acid groups such as acrylic acid, itaconic acid, sulfonated styrene and the like and optionally neutral vinyl molecules such as methyl acrylate, methyl methacrylate, vinyl acetate, acrylamide and the like.

[0060] In particular, the polymer is a high molecular weight polymer, ranging from 100000 to 300000 Da.

[0061] The dimethyl sulfoxide (DMSO) solvent is chosen because of its low environmental impact and limited toxicity.

[0062] The spinning solution or dope obtained at the end of step vii) can be immediately used to feed a suitable spinning line or can be stored in a heated tank.

[0063] The solution is sent to a set of filter presses with 40-5 μιη selective filter cloths to remove any particles, and then to the spinning line.

[0064] The spinning line used can be of the wet spinning type, with the spinneret immersed in a coagulation bath consisting of a mixture of water and solvent. After coagulation, the fiber bundle is continuously stretched and washed to produce a tow according to known techniques, which is collected on bobbins or boxes and then sent to the carbonization line to produce carbon fibers.

[0065] Alternatively, the spinning line used can be of the dry-jet wet spinning type (air-gap spinning), with the spinneret kept in air, at a small distance from the surface of a coagulation bath consisting of a mixture of water and solvent. After coagulation, the fiber bundle is continuously stretched and washed to produce a tow according to known techniques, which is collected on bobbins or boxes and then sent to the carbonization line to produce carbon fibers. Example

[0066] As a non-limiting example of the application, below some examples of implementation of the process according to the application are provided.

[0067] Example 1

[0068] At room temperature, 99 kg of acrylonitrile, 400 kg of DMSO, 0.1 kg of n-octyl mercaptan and 15 kg of an aqueous solution containing 1 kg of itaconic acid, 0.25 kg of ammonia and 13.75 kg of water are introduced into a stainless steel reactor equipped with a stirrer and a cooling jacket. The solution thus obtained is then heated to a temperature of 65°C and 0.3 kg of 2-2'-azobisisobutyronitrile (AIBN) is added. The heat of reaction is removed by circulating cold water in the cooling jacket of the reactor and the solution is stirred at 65°C for 7 hours, then the temperature is raised to 72°C and the system is left to stir for a further 7 hours. Under the conditions indicated, the conversion of acrylonitrile obtained is equal to 90.4%.

[0069] At the end of the phase described above, the contents of the reactor are discharged into a tank kept at a temperature of 35°C, which is then sent to a thin-film evaporator (TFE) kept at a temperature of 80°C and a residual pressure of 25 mbar.

[0070] The mixture containing acrylonitrile, water and DMSO is collected from the top of the TFE and incorporated into the feed mixture to the reactor.

[0071] A homogeneous solution of copolymer is collected from the bottom of the TFE, having a viscosity of 450 poise at 60°C and containing a residual amount of acrylonitrile equal to 0.03% by weight.

[0072] The so produced slurry is sent to a wet spinning production line in which 24000 hole spinnerets are immersed in a coagulation bath containing 60% DMSO and 40% water and kept at 55°C. The so obtained fiber bundle is continuously stretched to 10 times its initial length and washed. At the end of the stretching and washing section the filament bundle is collected on bobbins at a speed of 70 m / min obtaining a 24K precursor bobbin having the following characteristics:

[0073] • denier: 1.22 dtex;

[0074] • strength: 59.5 cN / tex:

[0075] • elongation: 14.5%;

[0076] Suitable for the production of carbon fibers.

[0077] Example 2

[0078] At room temperature, 97 kg of acrylonitrile, 2 kg of methyl acrylate, 400 kg of DMSO, 0.1 kg of dodecyl mercaptan and 15 kg of an aqueous solution containing 1 kg of itaconic acid, 0.250 kg of ammonia and 13.75 kg of water are introduced into a stainless steel reactor equipped with a stirrer and a cooling jacket. The so obtained solution is then heated to a temperature of 65°C and 0.3 kg of 2-2'-azobisisobutyronitrile (AIBN) is added. The heat of reaction is removed by circulating cold water in the reactor cooling jacket and the solution is stirred at 65°C for 10 hours, then the temperature is raised to 70°C and the system is continued to stir for 10 hours. Under the conditions indicated, the conversion of acrylonitrile obtained is equal to 91.8%.

[0079] At the end of the above phase, the contents of the reactor are discharged into a tank kept at a temperature of 35°C, which is then sent to a thin film evaporator (TFE) kept at a temperature of 80°C and a residual pressure of 25 mbar.

[0080] The mixture containing acrylonitrile, water and DMSO is collected from the top of the TFE and sent to the reactor feed.

[0081] A homogeneous solution of copolymer containing a residual amount of acrylonitrile equal to 0.03% by weight and having a viscosity of 420 poise at 60°C is collected from the bottom of the TFE.

[0082] The so produced slurry is sent to a wet spinning production line in which 48000 hole spinnerets are immersed in a coagulation bath containing 60% DMSO and 40% water and kept at 55°C. The so obtained fiber bundle is continuously stretched to 10 times its initial length and washed. At the end of the stretching and washing section the filament bundle is collected on bobbins at a speed of 60 m / min obtaining a 48K precursor bobbin having the following characteristics:

[0083] • Titer: 1.25 dtex;

[0084] • Strength: 56.2 cN / tex:

[0085] • Elongation: 13.6%;

[0086] Suitable for the production of carbon fibers.

[0087] Example 3

[0088] A spinning solution in DMSO was produced following the procedure described in Example 1.

[0089] The thus produced spinning dope was fed into a dry-jet wet spinning line, the spinneret having 3000 holes, located at a distance of 4 mm from the surface of the coagulation bath, the coagulation bath containing 35% DMSO and 65% water, at a temperature of 5°C. The fiber bundle obtained after coagulation was stretched in water, followed by a steam stretch (steam stretching) of nine times its initial length, and finally washed to remove residual solvent. At the end of the stretching and washing section, a 12K precursor bobbin was obtained by piling four 3K fiber bundles from individual spinnerets. The obtained fiber was collected on the bobbin at a speed of 240 m / min, had a perfect circular cross-section, was dense, crack-free and had the following properties:

[0090] • Titer: 1.0 dtex;

[0091] • Strength: 65.3 cN / tex:

[0092] • Elongation: 14.1%;

[0093] Suitable for the production of carbon fibers.

Claims

1. Process for the production of carbon fiber precursors, said process comprising the following steps: i) preparation of an acid comonomer solution selected from the group consisting of aqueous itaconic acid or acrylic acid, said aqueous solution comprising at least a stoichiometric amount of ammonia relative to the acid groups present, wherein the concentration of the acid comonomer in water varies in the range of 3 to 50 wt%, the total amount of water fed into the reactor being in the range of 1 to 5 wt% of the total weight of the material fed into the reactor in step ii); ii) feeding into a reactor acrylonitrile or a mixture of acrylonitrile and a neutral vinyl comonomer, the aqueous ammonium salt of the acid comonomer prepared in step i), dimethyl sulfoxide DMSO, 2-2'-azobisisobutyronitrile AIBN and dodecyl mercaptan or octyl mercaptan, wherein the acrylonitrile / neutral vinyl comonomer is present in a weight ratio of 95:5 to 99.5:0.5; iii) keeping the mixture thus obtained under stirring at a temperature in the range of 50 to 80 °C for a time in the range of 10 to 20 hours; iv) discharging the content of the reactor into a tank maintained at a temperature of 35 to 40 °C to slow down or interrupt the reaction; v) feeding the mixture thus obtained into a thin film evaporation system TFE operated at a temperature in the range of 40 to 80 °C and a pressure in the range of 5 to 30 mbar abs; vi) collecting from the top of the TFE a mixture of acrylonitrile, water and DMSO, said mixture being reused in the preparation process of a new reaction batch; vii) collecting from the bottom of the TFE the polymer DMSO solution, diluting said solution with fresh DMSO to reach a concentration of 15 to 25 wt% of the total weight of the solution; viii) feeding the homogeneous spinning solution obtained at the end of step vii) into a spinning step or into a storage tank.

2. Process according to claim 1, wherein in step vii) the solution is diluted with fresh DMSO to reach a concentration of 18 to 22 wt% of the total weight of the solution.

3. Process according to claim 1, wherein the spinning step is carried out by means of a wet spinning process or a dry-jet wet spinning process.

4. Process according to any one of claims 1 to 3, wherein the homogeneous spinning solution obtained in step vii) is fed into a coagulation step in a coagulation bath consisting of a mixture of water and solvent, the obtained fiber bundle being continuously stretched and washed to reach a length of about 10 times the initial length, then subjected to a final water washing step to remove the solvent before being fed into step viii).

5. Process according to claim 4, wherein the obtained fiber bundle is collected on bobbins or in boxes.

6. Process according to any one of claims 1 to 3, wherein in step i) of preparing the acid comonomer solution, the concentration of the acid comonomer in water varies in the range of 5 to 30 wt%, the total amount of water fed into the reactor being in the range of 1 to 5 wt% of the total weight of the material fed into the reactor in step ii) according to the process of the present application.

7. Process according to any one of claims 1 to 3, wherein in step i) the neutral vinyl comonomer is selected from the group consisting of methyl acrylate or vinyl acetate.

8. The process according to any one of claims 1 to 3, wherein in step ii) 90 to 150 parts by weight of acrylonitrile or a mixture of acrylonitrile and a neutral vinyl comonomer, 2 to 15 parts by weight of the aqueous solution of the ammonium salt of the acid comonomer prepared in step i), 300 to 500 parts by weight of DMSO, 0.2 to 0.5 parts by weight of 2-2'-azobisisobutyronitrile AIBN and 0.05 to 0.15 parts by weight of dodecyl mercaptan or octyl mercaptan are fed, wherein the acrylonitrile / neutral vinyl comonomer is present in a weight ratio of 95:5 to 99.5:0.

5.

9. The process according to claim 8, wherein in step ii) 100 parts by weight of acrylonitrile or a mixture of acrylonitrile and a neutral vinyl comonomer, 10 parts by weight of the aqueous solution of the ammonium salt of the acid comonomer prepared in step i), 400 parts by weight of DMSO, 0.3 parts by weight of 2-2'-azobisisobutyronitrile AIBN and 0.1 parts by weight of dodecyl mercaptan or octyl mercaptan are fed, wherein the acrylonitrile / neutral vinyl comonomer is present in a weight ratio of 95:5 to 99.5:0.

5.

10. The process according to any one of claims 1 to 3, wherein in step iii) the mixture obtained in step ii) is kept under stirring at a temperature in the range of 65 to 75 °C for a time in the range of 10 to 20 hours.