Spinning solutions for preparing acrylic fiber precursors for production of carbon fibers and improved processes for preparing related carbon fibers

Abstract

An optimized method for preparing a spinning solution for producing an acrylic fiber precursor (PAN) for producing carbon fibers and an optimized method for producing carbon fibers from said acrylic fiber precursor (PAN) are described.

Description

Technical Field

[0001] The present invention relates to an improved method for preparing a spinning solution for an acrylic fiber precursor (PAN) for the production of carbon fibers, and an improved method for producing carbon fibers from said acrylic precursor (PAN).

[0002] More specifically, the present invention falls within the field related to the production of carbon fibers, the preparation method of which has been known for many years and in most cases is based on the heat treatment of a suitable acrylic precursor (PAN) having a chemical composition suitable for allowing the gradual elimination of heteroatoms in a controlled manner. Background Art

[0003] This gradual elimination of heteroatoms is obtained due to the presence in the polymer chain of the precursor of specific comonomers having groups that allow the heat generated during the oxidation / stabilization treatment to be distributed over a relatively long time, avoiding sudden exothermic peaks that, in addition to providing poor-quality carbon fibers, also cause a risk of uncontrolled combustion during the heating phase. The most commonly used comonomers for this purpose are vinyl acids, mono- or dicarboxylic acids. In particular, acrylic acid, methacrylic acid or itaconic acid are used in an amount generally ranging from 0.5% to 5% by weight relative to the total weight of the monomers fed to the polymerization reactor. Other reagents are mainly acrylonitrile (95% to 99.5% by weight) and optionally a third component (0% to 3.0% by weight) usually selected from methyl acrylate, vinyl acetate and acrylamide.

[0004] The PAN precursor can be prepared starting from the selected comonomers by different methods. The state of the art can be divided and shown as follows:

[0005] A. Discontinuous method (two steps).

[0006] In the two-step discontinuous method, the polymer is usually produced, separated in an aqueous suspension and subsequently dissolved in a suitable solvent to be spun and converted into precursor fibers of carbon fibers. The most commonly used solvents for preparing the spinning solution are: dimethylacetamide (DMAC), dimethylformamide (DMF), aqueous solutions of sodium thiocyanate (NaSCN). A method using DMSO as a solvent has also been described (US 9,296,889).

[0007] B. Continuous method (one step)

[0008] On the other hand, in the continuous method, the polymerization is carried out in a solvent and the solution thus obtained is directly used for spinning without intermediate separation of the polymer. The most commonly used solvents in these methods are: dimethylformamide (DMF), dimethyl sulfoxide (DMSO), zinc chloride (ZnCl 2) aqueous solution and an aqueous solution of sodium thiocyanate (NaSCN).

[0009] Through these methods known to those skilled in the art, various types of carbon fibers with standard toughness and elastic modulus characteristics can be easily obtained. However, in order to improve the properties of the produced fibers, some modifications to these methods have been proposed.

[0010] One of the most significant modifications involves the use of nitrogen-containing compounds during the production process of PAN precursors, especially the use of low molecular weight primary and secondary amines, and especially ammonia.

[0011] In particular, U.S. Patent 5,804,108, US 6,054,214, and patent application US 2009 / 0224420A1 describe methods for increasing the elastic modulus of the produced carbon fibers by up to 2.5 times.

[0012] In this method, during the spinning step, the PAN precursor containing itaconic acid is treated with an amine or ammonia. In particular, the PAN fibers immediately after the coagulation stage at the exit of the spinneret are treated in a water bath containing an amine or ammonia, and then after spinning is completed and before undergoing carbonization, the fibers are treated at a temperature of 240°C to 260°C for 12 minutes to 15 minutes.

[0013] In other patents such as JPH1112856A, US 8,137,810, and US 8,674,045, the direct use of gaseous ammonia in the spinning solution (spinning dope) is claimed. In these cases, the use of ammonia is mainly promoted by the neutralization of the acid groups carried by the comonomers used (mainly itaconic acid and acrylic acid). The improvement in the production method of PAN precursors can be achieved by salifying the acid groups with ammonia to form ammonium carboxylates, which is beneficial to the coagulation stage due to the greater hydrophilicity of the polymer chain after salification at the acid chain ends and its conversion to ammonium salts.

[0014] Furthermore, in EP 2,894,243 (US 9,296,889), a method for preparing a carbon fiber precursor is described, in which an acrylic polymer containing itaconic acid or acrylic acid is dissolved under specific conditions, namely dissolved in a DMSO / water mixture in a ratio of 94.5% / 5.5% weight / weight to 97% / 3% weight / weight. As described in US11,313,053, the use of this method allows ammonia or a primary or secondary amine to be directly introduced into the aqueous solution during the formation stage of the spinning solution. Summary of the Invention

[0015] Compared with carbon fibers that can be obtained without ammonia or amines, the use of ammonia or primary or secondary amines helps in the production of carbon fibers with improved quality. Another advantage lies in reducing the heat treatment time required to achieve a fiber density of at least 1.35 g / cc to 1.43 g / cc, which is the density required to feed the oxidized product into the carbonization furnace. This reduction in stabilization time provides significant advantages both in terms of energy consumption and investment in building the stabilization furnace.

[0016] The reaction mechanism in which itaconic acid forms a salt with an amine or ammonia can be assumed. The ammonium salts after these heat treatments are converted to amides and ultimately cause crosslinking of PAN fibers by attacking the nitrile groups present in the polymer chains.

[0017] Reducing the stabilization time is also an object of CN 111910291A. In this case, the treatment of the fiber precursor in the form of a tow is provided by immersion in a bath containing an aqueous solution of ammonium polyphosphate or hydrazine hydrate, followed by a pre-oxidation step at a temperature of 190 °C to 320 °C. Although a reduction in the total oxidation / stabilization time is required, there are various taboos, such as difficulty in obtaining homogeneous impregnation of the entire tow, relatively poor repeatability of performance, and different behaviors related to the treatment itself. The author himself preferably recommends using tows up to 24K to minimize this phenomenon. It can also be assumed that foreign substances (residues of phosphorus compounds) remain in the final carbon fibers, which gives them poor toughness characteristics. In fact, a toughness of approximately 2.8 GPa is shown, which is not sufficient for most applications.

[0018] Therefore, the object of the present invention is to overcome the limitations of the known art and to determine an improved method for preparing a spinning solution of an acrylic fiber precursor (PAN) for the production of carbon fibers, and in particular, an improved method for producing carbon fibers from the said acrylic precursor (PAN), which allows for a reduction in production costs and the obtaining of carbon fibers with particularly high toughness and elastic modulus characteristics. Detailed Description

[0019] Therefore, the present invention relates to a method for preparing a homogeneous spinning solution of an acrylic fiber precursor (PAN) for the production of carbon fibers, wherein the solvent used is a mixture of DMSO or DMAC and an aqueous solution of a primary or secondary diamine selected from the group represented by the general formula (1):

[0020] RHN–(CHR”-CH 2 ) n -NHR’(1)

[0021] wherein R, R’ and R”, which may be the same or different from each other, may be hydrogen or an alkyl group having 1 to 10 carbon atoms, and n is equal to 0 or an integer from 1 to 6; when n = 0, at least one of R and R’ must be different from hydrogen.

[0022] Due to homogeneous impregnation, the diamine promotes the start of the crosslinking reaction and is thought to accelerate the subsequent heat stabilization step (also known as oxidation), which is the preparation for the carbonization step in the production of carbon fibers from PAN precursors.

[0023] The method objectives of the present invention include the following steps:

[0024] i) Preparing a homogeneous suspension by mixing an acrylonitrile copolymer in powder form with a solvent consisting of:

[0025] - A mixture of an aqueous solution containing 90% to 99% by weight, preferably 93% to 98% by weight, of DMSO and a primary or secondary diamine selected from the group represented by the general formula (1):

[0026] RHN–(CHR”-CH 2 ) n -NHR’(1)

[0027] wherein R, R’ and R”, which may be the same or different from each other, may be hydrogen or an alkyl group having 1 to 10 carbon atoms, and n is equal to 0 or an integer from 1 to 6; when n = 0, at least one of R and R’ is different from hydrogen; the amount of the diamine by weight is 0.1% to 5.0% by weight, preferably 0.5% to 2.5% by weight, relative to the total weight of the solvent; or

[0028] - A mixture of an aqueous solution containing 95% to 99% by weight, preferably 96% to 98% by weight, of DMAC and 0.1% to 5% by weight, preferably 0.5% to 3% by weight, of the diamine having the general formula (1) relative to the total weight of the solvent,

[0029] The mixing is carried out by spraying a stream of the solvent DMSO or an aqueous solution of DMAC / diamine having the general formula (1) onto a stream of the pulverized and pre-mixed acrylonitrile copolymer powder for a time of 5 minutes to 30 minutes;

[0030] ii) Heating the homogeneous suspension from step i) to a temperature of 70°C to 150°C for a time of 0.5 minutes to 30 minutes until the copolymer is completely dissolved and a homogeneous solution is formed.

[0031] Thus, the solvent consists of a mixture of DMSO or DMAC and an aqueous solution of a diamine having the general formula (1).

[0032] The aqueous solution of the diamine having the general formula (1) contains 0.1% to 5% by weight, preferably 0.5% to 3% by weight, of nitrogen-containing substances relative to the total weight of the solution.

[0033] The aqueous solution is preferably an aqueous solution of ethylenediamine (where R, R', and R'' = H and n = 1 in formula (1)), an aqueous solution of hexamethylenediamine (where R, R', and R'' = H and n = 3 in formula (1)), or an aqueous solution of 1,2-diaminopropane - DAP (where R = R' = H, R'' = CH 3 and n = 1), and even more preferably it is an aqueous solution of 1,2-diaminopropane.

[0034] The polymer for the purposes of the method of the present invention is a high molecular weight copolymer in the range of 100,000 Da to 300,000 Da (number average molecular weight), mainly composed of 90 wt% to 99 wt% of acrylonitrile relative to the total weight of the polymer and 1 wt% to 10 wt% of one or more comonomers relative to the total weight of the polymer.

[0035] Preferred comonomers are vinyl molecules with one or more acid groups, such as acrylic acid, methacrylic acid, itaconic acid, etc., preferably itaconic acid. The copolymer may also optionally contain a third comonomer selected from neutral vinyl molecules such as methyl acrylate, methyl methacrylate, vinyl acetate, acrylamide, etc.

[0036] The homogeneous spinning solution obtained at the end of the method according to the present invention is free of gels and undissolved residues and can be fed directly to a spinning line (equipment) or to a storage tank.

[0037] Thus, the formation of a good spinning solution free of gels and undissolved substances is achieved by step i) for preparing a homogeneous slurry under conditions of polymer insolubility and step ii) for subsequently rapidly heating the slurry so obtained.

[0038] The conditions of polymer insolubility in the case of DMSO are obtained by adding water to DMSO present in an aqueous solution of a diamine having the general formula (1). In the case of DMAC, the conditions of insolubility are achieved by maintaining the temperature at a value of 0 °C to -10 °C.

[0039] The main advantage of the method for preparing a PAN precursor according to the present invention is that it allows the preparation of a homogeneous suspension of the polymer in an aqueous DMSO solvent or in DMAC. Under these conditions, the solvent can actually penetrate inside each polymer particle without causing its partial dissolution, and subsequently form a surface film of the spinning dope, which would prevent the homogeneous uptake of all polymer materials.

[0040] According to conventional production techniques, a high-quality spinning dope suitable for the preparation of high-quality and high-performance carbon fibers is obtained by rapidly heating the suspension uniformly impregnated with the solvent.

[0041] The effectiveness of the presence of a diamine of general formula (1), especially in allowing a reduction in the oxidation time, in promoting the oxidation process is evident by comparing the heat release curves obtained by means of DSC (Differential Scanning Calorimetry) technology on samples of polymer solutions obtained by the method according to the invention, with polymer solutions having the same chemical composition, unmodified, and with polymer solutions having a chemical composition modified by the addition of ammonia.

[0042] As shown in Table 1 below, the peak temperature (peak T) and onset temperature (onset T) of the exothermic curves are significantly affected by the modification made to the polymer. In the polymer modified with ammonia (row 2), a decrease of about 3 °C is observed in both the peak temperature and the onset temperature. Similar results are observed in the case of the use of phenylhydrazine. On the other hand, the use of hydrazine causes a significant increase in temperature, but especially, the freshly prepared solution completely gels, thus making hydrazine unsuitable for the method according to the invention and preventing the advantages of the invention from being obtained.

[0043] The peak temperature (peak T) and onset temperature (onset T) of the exothermic curves simply indicate an increased reactivity in the stabilization / oxidation step, which allows the same results to be obtained in a shorter time or at a lower temperature (as shown by measuring the density of the oxidized fibers).

[0044] There is provided the use of a diamine of general formula (1) according to the invention, preferably an aliphatic diamine of general formula (1) in which R, R' and R'' are equal to H and n = 1 (ethylenediamine - row 5) or n = 3 (hexamethylenediamine - row 4) or R and R' = H, R'' = CH 3 and n = 1 (1,2-diaminopropane - row 6)) of the solution according to the invention unexpectedly does not show any tendency to solution gelation, and the values of both the peak temperature and the onset temperature are lower than those of the samples modified with ammonia or phenylhydrazine.

[0045] Table 1 - DSC curves of fiber samples

[0046]

[0047] The viscosity of the spinning dope was measured using a Haake iQ rotational viscometer, HAAKE VTiQ-T controller, and CC25DIN / Ti rotor.

[0048] The increased stabilization / oxidation rate indicated by the comparison of the thermal curves of samples treated with diamines of general formula (1) is further confirmed by the experimental data on the stabilization / oxidation treatment exerted on fibers obtained from samples of polymers with different chemical modifications as described in the present invention. The results of the stabilization / oxidation tests are shown in Examples 4 to 6 below.

[0049] The present invention also relates to a method for producing carbon fibers, in which the homogeneous solution obtained as described above is subjected to the following further steps:

[0050] iii) spinning the homogeneous solution from step ii) and obtaining a tow comprising from 500 (0.5K) to 400,000 (400K) filaments, preferably from 1,000 (1K) to 50,000 (50K) filaments;

[0051] iv) feeding the tow from step iii) to a stabilization or oxidation step, the oxidation being carried out at a temperature of 240 °C to 260 °C for a time of 40 minutes to 55 minutes;

[0052] v) feeding the oxidized tow from step iv) to a carbonization step with a maximum temperature of 1,600 °C.

[0053] The stabilization process (also called oxidation) provides treatment of the PAN precursor in the form of a tow comprising different numbers of filaments depending on the type of carbon fiber desired. Tows comprising from 500 (0.5K) to 400,000 (400K) filaments can be used, preferably those comprising from 1,000 (1K) to 50,000 (50K) filaments. The tow from spinning can be collected on a bobbin or in a cassette or crate, from which it can then be easily removed and fed to the stabilization section.

[0054] Another advantage of the method according to the invention is that the acrylic or PAN precursor thus obtained can be stabilized more quickly and at a lower temperature in the stabilization / oxidation step before the final carbonization step for the production of carbon fibers.

[0055] Examples

[0056] For illustrative and not limiting purposes of the present invention, some examples and some comparative examples for implementing the method according to the invention are provided below.

[0057] Example 1 (comparison - no addition)

[0058] Dissolution of a high molecular weight acrylic copolymer (MW = 150,000 to 180,000) composed of acrylonitrile (96% by weight relative to the total weight of the polymer), itaconic acid (1% by weight relative to the total weight of the polymer) and methyl acrylate (3% by weight relative to the total weight of the polymer). Example 2 (comparison with ammonia)

[0059] The polymer was dispersed in a 95 / 5 DMSO / aqueous solution maintained at a temperature of 5 °C until the polymer concentration in the solvent was equal to 18.0 wt% relative to the total weight of the mixture.

[0060] The dissolution of the polymer in the solvent solution was carried out in an industrial production line for producing an acrylic polymer spinning solution. After heating the dispersion to a temperature of 88 °C for 90 seconds through a tube bundle exchanger, a homogeneous spinning dope with a viscosity of 449 poises at 60 °C was obtained.

[0061] The polymer-solvent solution thus obtained was fed to a carbon fiber precursor spinning production line.

[0062] During the spinning process, the spinneret immersed in a coagulation bath composed of a mixture of water and DMSO produced perfect round, dense, crack-free fibers. The fibers thus obtained were subjected to washing with deionized water to remove the residual solvent, stretched in boiling water in several stages to about 10 times their initial length, dried on a hot roll and collected on a bobbin. The obtained tow was composed of fibers with a diameter of about 12 μm, an average tenacity of 56 cN / Tex and an ultimate elongation of about 17% measured on an Instron 5542 dynamometer with a 10 N cell according to method ASTM D-3822-2007.

[0063] The precursor tow thus obtained was treated in an oxidation furnace for 90 minutes with a temperature gradient of 240 °C to 270 °C, providing oxidized fibers with a density of 1.39 g / cc at the end. Subsequently, the oxidized fibers were fed to a carbonization section with a maximum temperature of 1,600 °C, providing carbon fibers with a tenacity equal to 4.60 GPa and an elastic modulus equal to 245 GPa.

[0064] Dissolution of a high molecular weight acrylic copolymer (MW = 150,000 to 180,000) composed of acrylonitrile (96% by weight relative to the total weight of the polymer), itaconic acid (1% by weight relative to the total weight of the polymer)

[0065] and methyl acrylate (3% by weight relative to the total weight of the polymer). Example 3 – (comparison with phenylhydrazine) Dissolution of a high molecular weight acrylic copolymer (MW = 150,000 to 180,000) composed of acrylonitrile (96% by weight relative to the total weight of the polymer), itaconic acid (1% by weight relative to the total weight of the polymer)

[0066] The polymer was converted into a spinning dope as described in Example 1, but using a mixture of DMSO (95 wt%) and an aqueous solution of 5 wt% of 0.6 wt% ammonia as the solvent medium until the polymer concentration in the solvent was equal to 18.0 wt% relative to the weight of the mixture. The slurry was prepared at a temperature of 5 °C and the spinning dope was obtained by subsequently heating to 88 °C for 90 seconds, obtaining a homogeneous spinning dope with a viscosity of 535 poises at 60 °C.

[0067] The polymer-solvent solution thus obtained was fed to a carbon fiber precursor spinning production line.

[0068] During the spinning process, the spinneret immersed in a coagulation bath composed of a mixture of water and DMSO produces perfectly round, dense, crack-free fibers. The fibers thus obtained are subjected to washing with deionized water to remove the residual solvent, stretched in boiling water in several stages to about 10 times their initial length, dried on hot rollers and collected on bobbins. The obtained tow is composed of fibers with a diameter of about 12 microns, an average tenacity of 58 cN / Tex and an ultimate elongation of about 18% measured according to method ASTM D-3822-2007 on an Instron 5542 dynamometer with a 10N cell.

[0069] The precursor tow thus obtained is treated in an oxidation furnace for 60 minutes with a temperature gradient of 240 °C to 260 °C, providing oxidized fibers with a density of 1.43 g / cc at the end. Subsequently, the oxidized fibers are fed to a carbonization section with a maximum temperature of 1,600 °C, providing carbon fibers with a tenacity equal to 5.20 GPa and an elastic modulus equal to 288 GPa.

[0070] and methyl acrylate (3% by weight relative to the total weight of the polymer).

[0071] Example 4 – Hexamethylenediamine Dissolution of a high molecular weight acrylic copolymer (MW = 180,000 to 200,000) composed of acrylonitrile (97% by weight relative to the total weight of the polymer) and itaconic acid (3% by weight relative to the total weight of the polymer). Example 5 - Ethylenediamine

[0072] The polymer is converted into a spinning dope as described in Example 1, however using a mixture composed of DMSO (95 wt%) and an aqueous solution of 5 wt% of 1.8 wt% phenylhydrazine as the solvent medium until a polymer concentration in the solvent equal to 18 wt% relative to the weight of the mixture is reached. The slurry is prepared at a temperature of 5 °C and the spinning dope is obtained by subsequent heating to 88 °C for a time of 90 seconds, obtaining a homogeneous spinning dope with a viscosity of 450 poises at 60 °C.

[0073] The polymer-solvent solution thus obtained is fed to a carbon fiber precursor spinning production line.

[0074] During the spinning process, the spinneret immersed in a coagulation bath composed of a mixture of water and DMSO produces perfectly round, dense, crack-free fibers. The fibers thus obtained are subjected to washing with deionized water to remove the residual solvent, stretched in boiling water in several stages to about 10 times their initial length, dried on hot rollers and collected on bobbins. The obtained tow is composed of fibers with a diameter of about 12 microns, an average tenacity of 61 cN / Tex and an ultimate elongation of about 18% measured according to method ASTM D-3822-2007 on an Instron 5542 dynamometer with a 10N cell.

[0075] The precursor tow thus obtained is treated in an oxidation furnace for 60 minutes with a temperature gradient from 240 °C to 260 °C, providing at the end oxidized fibers with a density of 1.43 g / cc. Subsequently, the oxidized fibers are fed to a carbonization section with a maximum temperature of 1,600 °C, providing carbon fibers with a toughness equal to 5.05 GPa and an elastic modulus equal to 278 GPa.

[0076] Dissolution of a high molecular weight acrylic copolymer (MW = 150,000 to 180,000) composed of acrylonitrile (96% by weight relative to the total weight of the polymer), itaconic acid (1% by weight relative to the total weight of the polymer)

[0077] and methyl acrylate (3% by weight relative to the total weight of the polymer). Example 6 - – 1,2 - Diaminopropane

[0078] The polymer is converted to a spinning dope as described in Example 1, but using a mixture of DMSO (95 wt%) and an aqueous solution of 5 wt% of 2% hexamethylenediamine as the solvent medium until a concentration of the polymer in the solvent of 18 wt% relative to the weight of the mixture is reached. The slurry is prepared at a temperature of 4 °C and the spinning dope is obtained by heating for 90 seconds to 88 °C, obtaining a homogeneous spinning dope with a viscosity of 515 poise at 60 °C.

[0079] The polymer-solvent solution thus obtained is fed to a carbon fiber precursor spinning line.

[0080] During the spinning process, the spinneret immersed in a coagulation bath consisting of a mixture of water and DMSO produces fibers that are perfectly round, dense, and crack-free. The fibers thus obtained are subjected to washing with deionized water to remove the residual solvent, stretched in boiling water in several stages to approximately 10 times their initial length, dried on hot rolls, and collected on bobbins. The tow obtained consists of fibers with a diameter of approximately 12 microns, an average tenacity of 62 cN / Tex, and an ultimate elongation of approximately 16% measured on an Instron 5542 dynamometer with a 10 N cell according to method ASTM D-3822-2007.

[0081] The precursor tow thus obtained is treated in an oxidation furnace for 40 minutes with a temperature gradient from 240 °C to 260 °C, providing at the end oxidized fibers with a density of 1.40 g / cc. Subsequently, the oxidized fibers are fed to a carbonization section with a maximum temperature of 1600 °C, providing carbon fibers with a toughness equal to 5.14 GPa and an elastic modulus equal to 265 GPa.

[0082] Dissolution of a high molecular weight acrylic copolymer (MW = 150,000 to 180,000) composed of acrylonitrile (96% by weight relative to the total weight of the polymer), itaconic acid (1% by weight relative to the total weight of the polymer)

[0083] and methyl acrylate (3% by weight relative to the total weight of the polymer). Table 2 ​

[0084] The polymer is converted to a spinning dope as described in Example 1, however using a mixture consisting of DMSO (95 wt%) and an aqueous solution of 5 wt% of 1 wt% ethylenediamine as the solvent medium until the polymer concentration in the solvent is equal to 18 wt% relative to the weight of the mixture. The slurry is prepared at a temperature of 5 °C and the spinning dope is obtained by subsequent heating to 88 °C for a time of 90 seconds, obtaining a homogeneous spinning dope with a viscosity of 514 poise at 60 °C.

[0085] The polymer-solvent solution thus obtained is fed to a carbon fiber precursor spinning line.

[0086] During the spinning process, the spinneret immersed in a coagulation bath consisting of a mixture of water and DMSO produces perfectly round, dense, crack-free fibers. The fibers thus obtained are subjected to washing with deionized water to remove the residual solvent, stretched in boiling water in several stages to about 10 times their initial length, dried on hot rollers and collected on bobbins. The obtained tow consists of fibers with a diameter of about 12 microns, an average tenacity of 67 cN / Tex and an ultimate elongation of about 17% measured on an Instron 5542 dynamometer with a 10 N cell according to method ASTM D-3822-2007.

[0087] The precursor tow thus obtained is treated in an oxidation furnace for 40 minutes with a temperature gradient of 240 °C to 260 °C, providing oxidized fibers with a density of 1.41 g / cc at the end. Subsequently, the oxidized fibers are fed to a carbonization section with a maximum temperature of 1,600 °C, providing carbon fibers with a tenacity equal to 5.02 GPa and an elastic modulus equal to 257 GPa.

[0088] ​

[0089] ​ ​ ​

[0090] The polymer is converted to a spinning dope as described in Example 1, but using a mixture of DMSO (95 wt%) and an aqueous solution of 5 wt% of 1.25 wt% 1,2-diaminopropane as the solvent medium until the polymer concentration in the solvent is equal to 18 wt% relative to the total weight of the mixture. The slurry is prepared at a temperature of 5 °C and the spinning dope is obtained by subsequent heating to 88 °C for a time of 90 seconds, obtaining a homogeneous spinning dope with a viscosity of 401 poise at 60 °C.

[0091] The polymer-solvent solution thus obtained is fed to a carbon fiber precursor spinning line.

[0092] During the spinning process, the spinneret immersed in a coagulation bath consisting of a mixture of water and DMSO produces fibers that are perfectly round, dense, and crack-free. The fibers thus obtained are subjected to washing with deionized water to remove the residual solvent, stretched in boiling water in several stages to approximately 10 times their initial length, dried on hot rollers, and collected on bobbins. The resulting tow is composed of fibers with a diameter of approximately 12 microns, an average tenacity of 64 cN / Tex, and an ultimate elongation of approximately 16% measured on an Instron 5542 dynamometer with a 10 N cell according to method ASTM D-3822-2007.

[0093] The precursor tow thus obtained is treated in an oxidation furnace for 40 minutes with a temperature gradient from 240 °C to 260 °C, providing oxidized fibers with a density of 1.43 g / cc at the end. Subsequently, the oxidized fibers are fed to a carbonization section with a maximum temperature of 1,600 °C, providing carbon fibers with a tenacity equal to 5.12 GPa and an elastic modulus equal to 265 GPa.

[0094] In Table 2 below, the tenacity and elastic modulus values of the carbon fibers obtained in Examples 1 to 6 are shown.

[0095] ​

[0096]

[0097] Compared to the samples of Comparative Example 1 in which the polymer was unmodified, the carbon fibers obtained in Examples 4 to 6 according to the present invention show particularly high tenacity and elastic modulus values. These values are substantially and unexpectedly comparable to those obtained in Comparative Example 2 (ammonia-modified polymer) and Comparative Example 3 (phenylhydrazine-modified polymer). However, there is a significant improvement in the stabilization / oxidation rate with respect to Comparative Example 2 and Comparative Example 3, as shown in Table 1.

Claims

1. A method for preparing a homogeneous spinning solution of an acrylic fiber precursor (PAN) for producing carbon fiber, wherein the solvent used is a mixture of DMSO or DMAC and an aqueous solution of a primary diamine or a secondary diamine selected from the group represented by the general formula (1): RHN-–(CHR”-CH2) n -NHR'(1) wherein R, R' and R'' are the same or different from each other, and may be hydrogen or an alkyl group having 1 to 10 carbon atoms, and n is equal to 0 or an integer from 1 to 6; when n=0, at least one of R and R' is different from hydrogen.

2. The method according to claim 1, characterized in that the method comprises the following steps: i) preparing a homogeneous suspension by mixing the acrylonitrile copolymer in powder form with a solvent consisting of: - a mixture comprising 90% to 99%, preferably 93% to 98% by weight of DMSO and an aqueous solution of a primary or secondary diamine selected from the group represented by the above general formula (1), wherein the amount of the diamine is 0.1% to 5.0%, preferably 0.5% to 2.5% by weight relative to the total weight of the solvent; or a mixture comprising an amount of 95% to 99%, preferably 96% to 98%, by weight of DMAC and an amount of 0.1% to 5%, preferably 0.5% to 3% by weight of an aqueous solution of said diamine of general formula (1); The mixing is performed by spraying a stream of a solvent DMSO or DMAC / an aqueous solution of a diamine having the general formula (1) onto a stream of the pulverized and premixed acrylonitrile copolymer powder for a period of 5 to 30 minutes; ii) heating the homogenous suspension from step i) to a temperature of 70°C to 150°C for a period of 0.5 to 30 minutes until the copolymer is completely dissolved and a homogenous solution is formed.

3. The process according to one or more of the preceding claims, wherein the aqueous solution of a diamine of formula (1) comprises from 0.1 to 5% by weight, preferably from 0.5 to 3% by weight, of nitrogen-containing substances relative to the total weight of the solution.

4. The process according to one or more of the preceding claims, wherein the aqueous solution of a diamine of the general formula (1) is an aqueous solution of ethylenediamine (in which R, R' and R" = H and n = 1 in the formula (1)), hexamethylenediamine (in which R, R' and R" = H and n = 3 in the formula (1)), or 1,2-diaminopropane-DAP (in which R = R' = H, R" = CH3 and n = 1 in the formula (1)).

5. The process according to one or more of the preceding claims, wherein the diamine is 1,2-diaminopropane.

6. The process according to one or more of the preceding claims, wherein the copolymer is a high number molecular weight copolymer ranging from 100,000 Da to 300,000 Da, composed of 90% to 99% by weight of acrylonitrile relative to the total weight of the polymer and 1% to 10% by weight of one or more comonomers relative to the total weight of the polymer.

7. The method according to claim 5, wherein the comonomer is a vinyl molecule with one or more acid groups, wherein the vinyl molecule with one or more acid groups is selected from acrylic acid, methacrylic acid, itaconic acid, preferably itaconic acid.

8. The method according to one or more of the preceding claims, wherein the copolymer further comprises a third comonomer selected from neutral vinyl molecules such as methyl acrylate, methyl methacrylate, vinyl acetate, acrylamide, etc.

9. A homogeneous spinning solution of an acrylic fiber precursor for producing carbon fiber obtainable by the method according to any one of claims 1 to 8.

10. A process for producing carbon fibers, wherein the homogenous solution obtained at the end of step ii) of the process according to claims 2 to 9 is subjected to the following further steps: iii) spinning the homogenous solution from step ii) and obtaining a tow comprising 500 (0.5K) to 400,000 (400K) filaments, preferably 1,000 (1K) to 50,000 (50K) filaments; iv) feeding the tow from step iii) to a stabilization or oxidation step, the oxidation being carried out at a temperature of 240° C. to 260° C. for a time of 40 minutes to 120 minutes; v) The oxidized tow from step iv) is fed to a carbonization step at a maximum temperature of 1,600°C.

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