Additive for reducing the stabilization time in the preparation of carbon fibers, and related processes for the preparation of acrylic fiber precursors and the related carbon fibers

Abstract

An additive is described, to be introduced in the production of acrylic fiber precursors of carbon fibers for reducing the stabilization time in the preparation of carbon fibers, together with the relative processes for the preparation of acrylic fiber precursors and the relative carbon fibers.

Description

[0001] The present invention relates to an additive for reducing the stabilization time in the preparation of carbon fibers, and the relative processes for the preparation of acrylic fiber precursors and the relative carbon fibers.

[0002] The present invention falls within the sector relating to the production of carbon fibers whose preparation process has been known for many years and which, in most cases, is based on the heat treatment of a suitable acrylic precursor (PAN), having a chemical composition suitable for allowing a controlled gradual elimination of the heteroatoms.

[0003] This gradual elimination of heteroatoms is obtained thanks to the presence in the polymeric chain of the precursor of specific co-monomers having groups that allow the heat developed during the oxidation / stabilization treatment to be distributed over relatively lengthy times, avoiding sudden exothermic peaks which, in addition to providing poor-quality carbon fibers, would create the risk of uncontrolled combustion during the heating phase.

[0004] The most commonly used co-monomers for this purpose are vinyl acids, mono or dicarboxylic. In particular, acrylic acid, methacrylic acid or itaconic acid are used, in quantities generally ranging from 0.5 to 5% by weight with respect to the total weight of the monomers fed to the polymerization reactor. The other reagents are mainly acrylonitrile (95 - 99.5% by weight) and, optionally, a third component generally selected from methyl acrylate, vinyl acetate and acrylamide (0 - 3.0% by weight).

[0005] The PAN precursors can be prepared by different processes starting from the co-monomers selected. The state of the art can be divided and schematized as follows:A. Batch processes (two-step).

[0006] In two-step batch processes, the polymer is generally produced in aqueous suspension, isolated and subsequently dissolved in a suitable solvent to be spun and transformed into fiber precursor of carbon fiber. The solvents most commonly used for the preparation of the spinning solution are: dimethylacetamide (DMAC), dimethylformamide (DMF), an aqueous solution of sodium thiocyanate (NaSCN). A process using DMSO as solvent has also been described (US 9,296,889).B. Continuous Processes (one-step)

[0007] In continuous processes, on the other hand, the polymerization takes place in a solvent and the solution thus obtained is used directly in spinning without the intermediate isolation of the polymer. The solvents most commonly used in these processes are: dimethylformamide (DMF), dimethylsulfoxide (DMSO), an aqueous solution of zinc chloride (ZnCl 2 ) and an aqueous solution of sodium thiocyanate (NaSCN).

[0008] By means of these processes, well-known to skilled persons in the field, various types of carbon fiber with standard characteristics of toughness and elastic modulus can be easily obtained. In order to improve the performance of the fibers produced, however, some modifications to these methods have been proposed.

[0009] One of the most significant modifications relates to the use of nitrogen compounds in the production process of the PAN precursor, in particular the use of ammonia, low-molecular-weight primary and secondary amines and primary and secondary diamines.

[0010] In particular, US patents 5,804,108, US 6,054,214 and patent application US 2009 / 0224420 A1 describe a method for increasing the elastic modulus of the carbon fibers produced.

[0011] In this process the PAN precursor containing itaconic acid is treated with amines or ammonia during the spinning step. In particular, the PAN fiber, immediately after the coagulation phase at the outlet of the spinneret, is treated in an aqueous bath containing amines or ammonia and subsequently, after the spinning has been completed, the fiber is treated at temperatures ranging from 240°C to 260°C for 12-15 minutes, before being subjected to carbonization.

[0012] In other patents such as JPH1112856A, US 8,137,810 and US 8,674,045 the use of gaseous ammonia directly in the spinning solution (dope) is claimed. In these cases the use of ammonia is mainly motivated by the neutralization of the acid groups carried by the co-monomers used (mainly itaconic acid, acrylic acid and methacrylic acid). The salification of the acid groups with ammonia to give ammonium carboxylates can allow an improved production process of the PAN precursor, facilitating the coagulation phase thanks to the greater hydrophilicity of the polymer chain after salification of the acid chain-ends and their transformation into ammonium salts.

[0013] Furthermore, in EP 2,894,243 (US 9,296,889) a process for the preparation of carbon fiber precursors is described, wherein an acrylic polymer containing itaconic acid or acrylic acid is dissolved under particular conditions, namely in a DMSO / water mixture in a ratio ranging from 94.5 / 5.5 to 97 / 3% w / w. This process has allowed the introduction of ammonia, primary or secondary amines as chemical modifiers which are added directly in aqueous solution during the formation step of the spinning solution as described in US 11,313,053. In Italian patent application nr. 102023000025554, the use of primary and secondary diamines allows a cross-linking reaction that favours the subsequent thermal stabilization step (also called oxidation), preparatory to the carbonization step in the production of carbon fibers from PAN precursors. The use of ammonia, primary or secondary amines or diamines, contributes to the production of carbon fibers having an improved quality compared to those obtained in the absence of said nitrogen compounds.

[0014] The main advantage lies in the reduction of the heat treatment times required for reaching a fiber density of at least 1.35-1.43 g / cc, a density required for feeding the oxidized product to the carbonization furnaces. This reduction in the stabilization time provides significant advantages both in terms of energy consumption and in terms of investment in the construction of stabilization furnaces.

[0015] The effect of additives in reducing the oxidation times required for producing high-quality carbon fiber can be conveniently evaluated by studying the thermal behaviour of the modified precursor. The methodology used is the so-called DSC analysis (Differential Scanning Calorimetry). DSC analysis is based on the measurement of the difference in heat flow between the sample under examination and a reference sample, while the two are constrained to a variable temperature defined by a pre-established program (usually a linear temperature ramp).

[0016] In the case of PAN, by applying a linear temperature ramp in an atmosphere of air, it is possible to observe the exotherm of the PAN cyclization and evaluate the onset and peak temperatures and the enthalpy of the transition. These temperatures can be used as guidelines in the operation of oxidation furnaces during the thermal conversion of the PAN fiber precursor into oxidized fiber.

[0017] In the use of nitrogen additives, a reaction mechanism is assumed wherein at least one carboxylic group of the acid comonomer is salified with amines, diamines or ammonia. These ammonium salts, after heat treatment, are transformed into amides and finally cause the cross-linking of the PAN fiber by attacking the nitrile groups present in the polymer chain.

[0018] Reducing the stabilization times is also the objective of CN 111910291A. In this case, a treatment of the fiber precursor in the form of a tow is provided by immersion in a bath containing an aqueous solutions of ammonium polyphosphate or hydrazine hydrate followed by a pre-oxidation step at temperatures ranging from 190 to 320°C. Although a reduction in the overall oxidation / stabilization time is claimed, there are various contraindications, such as the difficulty in obtaining a homogeneous impregnation of the entire tow with a relative poor repeatability of the performances and different behaviour in relation to the treatment itself. The authors themselves recommend using tows preferably up to 24K in order to minimize this phenomenon. It can also be assumed that foreign material (residues of phosphorous compounds) remain in the final carbon fiber, giving it poor toughness characteristics. A toughness in the order of 2.8 GPa is in fact indicated, which is insufficient for most applications.

[0019] The objective of the present invention is to overcome the disadvantages of the state of the art previously indicated.

[0020] A further objective of the present invention is to identify additives that allow the peak temperature in the DSC analysis to be reduced, also reducing the stabilization times in the preparation of acrylic fiber precursors of carbon fibers and in the preparation of the relative carbon fibers.

[0021] The objective of the present invention is therefore also to overcome the limitations of the known art and to identify an improved process for the preparation of a spinning solution for the production of acrylic fiber precursors (PAN) of carbon fibers and, in particular, an improved process for the production of carbon fibers from said acrylic precursor which allows the production costs to be reduced and carbon fibers with particularly high toughness and elastic modulus characteristics, to be obtained.

[0022] The Applicant has in fact surprisingly found that a very significant effect on the reduction of the peak temperature in the DSC analysis in air is also promoted by the addition, during the preparation of the spinning solution, of additives with no functionality containing nitrogen and therefore not reactive towards itaconic acid or other acid comonomers present in the polymer precursor.Detailed description of the invention

[0023] The present invention therefore relates to the use of 4-methoxyphenol having formula (1) as an additive in the production of acrylic fiber precursors of carbon fibers for reducing the stabilization time in the preparation of carbon fibers.

[0024] The present invention also relates to a process for the preparation of a homogeneous spinning solution for the production of acrylic fiber (PAN) precursors of carbon fibers, wherein the solvent used in the preparation step of a homogeneous suspension of acrylonitrile copolymer is a mixture of DMSO or DMAC and an aqueous solution of 4-methoxyphenol (MEHQ) having formula (1)

[0025] 4-Methoxyphenol (MEHQ), also called 4-hydroxyanisole, p-guaiacol or hydroquinone monomethyl ether, is normally used in the production of acrylonitrile homo-polymers or co-polymers, regardless of the production process, as a stabilizer of the acrylonitrile itself to prevent self-polymerization phenomena, induced for example by exposure to temperature during the transportation and storage of acrylonitrile and other vinyl co-monomers such as vinyl acetate, acrylic acid, methyl acrylate and the like.

[0026] The use of MEHQ in the production of modified precursors for an improvement in the production process and the performance of the carbon fiber thus obtained, does not therefore introduce new chemical products into the processing cycle, consequently not creating environmental risks or additional production and disposal costs compared to the industrial processes normally used.

[0027] As specified, in fact, 4-methoxyphenol has the advantage of being already present in the production process of the PAN precursor, as an inhibitor of acrylonitrile and other vinyl comonomers.

[0028] It has been surprisingly found that, when used in the process according to the present invention, 4-methoxyphenol, probably by promoting a radical reaction, accelerates the thermal stabilization step, decreasing the activation energy required in the oxidation step, preparatory to the carbonization step in the production of carbon fiber from PAN precursors. The consequent reduction in the stabilization times brings significant benefits from an industrial point of view, with a strong impact on productivity, as oxidation is the slowest step in the entire carbon fiber production process starting from the precursor fiber.

[0029] It has also been found that the introduction of larger quantities of 4-methoxyphenol (in excess of the range according to the present invention) directly in aqueous solution during the formation step of the spinning solution does not cause significant variations during the same thermal stabilization step, thus allowing the quantity of chemical modifier agent to be limited, i.e. 4-methoxyphenol, industrially necessary for obtaining the technical effect according to the present invention. The possibility of reducing the heat treatment times in the oxidation step and the limited consumption of the modifier to obtain the above-mentioned effect, thus contribute to improving the industrial performance, preserving the economy of the process.

[0030] The process, object of the present invention, comprises the following steps: i) preparing a homogeneous suspension by mixing an acrylonitrile copolymer in powder form with a solvent consisting of a mixture containing DMSO in a quantity ranging from 90 to 99% by weight, preferably from 93 to 98% by weight, and an aqueous solution of 4-methoxyphenol in a quantity ranging from 2 to 7% by weight, preferably from 3 to 6% by weight based on the total weight of the solvent; or a mixture containing DMAC in a quantity ranging from 95 to 99.5% by weight, preferably from 96 to 98% by weight, and an aqueous solution of 4-methoxyphenol in a quantity ranging from 0.5 to 5% by weight, preferably from 1 to 3% by weight, based on the total weight of the solvent, wherein said mixing is carried out over a time ranging from 5 to 30 minutes, by spraying a stream of the solvent onto a stream of the pre-mixed, disintegrated acrylonitrile copolymer powder; ii) heating the homogeneous suspension from step i) to a temperature ranging from 70 to 150°C over a time ranging from 0.5 to 30 minutes, until the copolymer has completely dissolved and a homogeneous solution is formed.

[0031] The solvent is therefore a mixture containing DMSO or DMAC and an aqueous solution of MEHQ.

[0032] The aqueous solution of MEHQ comprises at least 0.5% by weight of MEHQ, preferably from 0.5 to 15% by weight of MEHQ, more preferably from 0.5 to 9% by weight, even more preferably from 0.5 to 5% by weight, and in particular from 1 to 3% by weight, based on the total weight of the aqueous solution.

[0033] The polymer that can be used in the process of the present invention is a high-molecular-weight copolymer, ranging from 70,000-300,000 Da (number MW), prevalently composed of acrylonitrile, in a quantity ranging from 90 to 99% by weight with respect to the total weight of the polymer, and one or more co-monomers in a quantity ranging from 1 to 10% by weight with respect to the total weight of the polymer.

[0034] The preferred co-monomers are vinyl molecules bearing one or more acid groups such as acrylic acid, methacrylic acid, itaconic acid and the like, preferably itaconic acid. The copolymer can also optionally contain a third co-monomer selected from neutral vinyl molecules, such as methyl acrylate, methyl methyl acrylate, vinyl acetate, acrylamide and the like.

[0035] The homogeneous spinning solution obtained at the end of the process according to the present invention is free from gel and undissolved residues and can be fed directly to the spinning line (apparatus) or to a storage tank.

[0036] The formation of a good spinning solution free from gels and undissolved material is therefore achieved by means of a step i) for the preparation of a homogeneous slurry or homogeneous suspension under conditions of non-solubility of the polymer and a step ii) for the subsequent rapid heating of the slurry thus obtained to provide a homogeneous solution (dope).

[0037] The condition of non-solubility of the polymer in step i) of the process according to the present invention is obtained in the case of the solvent comprising DMSO by adding, to the DMSO, the water present in the aqueous solution of MEHQ. In the case of the solvent comprising DMAC, the condition of non-solubility is obtained by maintaining in step i) a temperature ranging from 0 to - 10°C.

[0038] One of the main advantages of the process for the preparation of PAN precursors according to the present invention is also that it allows the preparation of a homogeneous suspension of the polymer in the aqueous DMSO solvent or in the DMAC solvent. Under these conditions, the solvent is in fact able to penetrate inside each polymer granule without causing its partial dissolution with the consequent formation of a surface film of dope that would prevent a homogeneous imbibition of the whole polymeric material.

[0039] By rapidly heating the suspension homogeneously impregnated by the solvent thus obtained, a high-quality dope is obtained, suitable for the preparation of a high-quality and high-performance carbon fiber, according to a conventional production technique.

[0040] The present invention also relates to a process for the production of carbon fibers wherein the homogeneous solution obtained at the end of step ii) of the process for preparing the spinning solution according to the present invention, as described above, is subjected to the following further steps: iii) spinning the homogeneous solution from step ii) and obtaining a tow containing from 500 (0.5 K) to 400,000 (400 K) single filaments, preferably from 1,000 (1K) to 50,000 (50K) single filaments; iv) feeding the tow coming from step iii) to the stabilization or oxidation step, said oxidation being carried out for a time ranging from 35 to 120 minutes, at a temperature ranging from 230 to 260°C; v) feeding the oxidized tow coming from step iv) to a carbonization step with a maximum temperature of 1600°C.

[0041] The present invention further relates to a mixture of DMSO or DMAC and an aqueous solution of 4-methoxyphenol (MEHQ) having formula (1) wherein the aqueous solution of MEHQ comprises at least 0.5% by weight of MEHQ, preferably from 0.5 to 15% by weight of MEHQ, more preferably from 0.5 to 9% by weight, even more preferably from 0.5 to 5% by weight, and in particular from 1 to 3% by weight, based on the total weight of the aqueous solution.

[0042] The stabilization process, also called oxidation, involves the treatment of the PAN precursor, in the form of a tow containing a variable quantity of single filaments depending on the type of carbon fiber desired. It is possible to use tows containing from 500 (0.5 K) to 400,000 (400 K) single filaments, preferably using tows containing from 1,000 (1K) to 50,000 (50K) single filaments. The tow coming from the spinning can be collected on reels or in boxes or crates from which it can then be easily removed and fed to the stabilization section.

[0043] A further advantage of the process according to the present invention is that the acrylic precursors or PAN precursors thus obtained can be stabilized more rapidly and at a lower temperature in the stabilization / oxidation step preceding the final carbonization step with the production of carbon fiber.

[0044] The effectiveness of the presence of MEHQ in promoting the oxidation process is evident by comparing the thermal release curves using the DSC (Differential Scanning Calorimetry) technique conducted on a film sample obtained from the polymer solution obtained with the process according to the present invention (see Example 4), compared with a film sample obtained from a polymer solution having the same chemical composition as in Example 4 and with the same process, without the presence of MEHQ in the solvent, i.e. without any modification in the polymer (see Example 1) and with two film samples obtained from a polymer solution having a chemical composition modified by the addition of ammonia or 1,2-diaminopropane (1,2-DAP) to the solvent (see Examples 2 and 3).

[0045] As indicated in Table 1 hereunder, the peak temperatures (Peak T) and the onset temperatures (Onset T) are significantly affected by the modifications applied to the polymer. These temperatures can be used as guidelines in the operation of oxidation furnaces during the thermal conversion of the precursor PAN fiber to oxidized fiber.

[0046] In the polymer modified with ammonia (row 2) or with 1,2-diaminopropane (1,2-DAP) (row 3) a decrease of about 5 and 6°C, respectively, in the peak temperature is observed without any tendency towards gelation.

[0047] The sample modified by the addition of MEHQ, (row 4) in addition to not causing gelation of the sample, allows a decrease in the peak temperature of 9°C to be reached compared to the unmodified sample and of 3°C compared to the best results obtained with amines or diamines.

[0048] The effectiveness of MeHQ in the thermal stabilization step has been confirmed by studies conducted, varying the concentration of MeOH in the aqueous solution from 1% to 11% by weight of MeHQ.

[0049] As can be observed from Table 1, the introduction of greater quantities of 4-methoxyphenol (in excess of the range according to the present invention) directly in aqueous solution during the formation step of the spinning solution does not cause significant changes in the peak temperatures (Peak T), thus allowing the quantity of chemical modifying agent to be limited, i.e. 4-methoxyphenol, industrially necessary for obtaining the technical effect according to the present invention.

[0050] The possibility of reducing the heat treatment times in the oxidation step and the limited consumption of the modifier to obtain the above-mentioned effect, thus contribute to an improvement in the industrial performance, preserving the economy of the process.

[0051] From Table 1 hereunder, the effect of the MeHQ concentration on the peak temperature value (Peak T) is evident: it is also indicated that the variation is almost negligible when passing from a MeHQ concentration in the aqueous solution of 1% by weight (Example 7), to a concentration of 2% by weight (Example 4), to a concentration of 8% by weight (Example 5) and a concentration of 11% by weight (Example 6). Table 1 - DSC curves of fiber samplesExample Additive in the solvent for the preparation of the spinning solution (dope) w / w % MeHQ in aqueous solution DSC Peak T (°C) (Air, 20°C / min)DSC Onset T (°C) (Air, 20°C / min)Viscosity of the dope at 60 °C (P)Viscosity of the dope after 24h 60 °C (P)1. Comp.No additive297.72274.104014402. Comp.NH 3 292.77277.115105043. Comp.1,2-DAP291.10275.304104104.MEHQ2%288.77279.184464485.MEHQ8%286.81240.545315146.MEHQ11%288.54243.784834757.MEHQ1%288.23270.86605645

[0052] The viscosity of the dope is measured with a Haake Viscotester iQ rotational viscometer, HAAKE VTiQ-T controller, CC25 DIN / Ti rotor.

[0053] The DSC analysis was effected in a Perkin Elmer DSC 6000 instrument, applying a linear temperature ramp ranging from 40 to 400 °C at 20°C / min in an air atmosphere.

[0054] The improvement in the stabilization / oxidation rate is further confirmed by the experimental data obtained from the stabilization / oxidation tests carried out on the fibers obtained using different chemically modified polymer samples as described in the following examples.Examples

[0055] Some examples for the implementation of the process according to the present invention and some comparative examples are provided hereunder for illustrative but non-limiting purposes of the present invention.Example 1 (Comparative)

[0056] Dissolution of a high-molecular-weight acrylic copolymer (PMn=100,000-150,000) consisting of acrylonitrile (96% by weight with respect to the total weight of the polymer), itaconic acid (1% by weight with respect to the total weight of the polymer) and methyl acrylate (3% by weight with respect to the total weight of the polymer).

[0057] The above polymer was dispersed in a solvent consisting of a DMSO / Water 95 / 5 solution maintained at a temperature of 5°C until reaching a polymer concentration in the solvent equal to 18.0% by weight with respect to the total weight of the polymer / solvent mixture.

[0058] The dissolution of the polymer in the solvent was carried out in an industrial line for the production of acrylic polymer spinning solution. After heating the dispersion or slurry thus obtained by means of a tube-bundle exchanger to a temperature of 85°C for a time of 90 seconds, a homogeneous dope was obtained having a viscosity of 401 poises at 60°C.

[0059] The resulting polymer-solvent solution was fed to a spinning line for carbon fiber precursors.

[0060] During the spinning process, the spinnerets, immersed in a coagulation bath composed of a mixture of water and DMSO, generated a perfectly round, compact, crack-free fiber. The resulting fiber was washed with deionized water to remove the residual solvent, stretched in several steps in boiling water up to about 10 times its initial length, dried on hot rollers and collected on reels. The resulting tows are composed of fibers with a diameter of about 12 microns, an average toughness of 56 cN / tex and an ultimate elongation of about 17%, measured on an Instron 5542 dynamometer with a 10 N cell according to the method ASTM D-3822-2007.

[0061] The precursor tow thus obtained was treated in an oxidation oven for 90 minutes with a temperature gradient from 240 to 270°C, providing at the end an oxidized fiber having a density of 1.39 g / cc. The oxidized fiber was subsequently fed to a carbonization section with a maximum temperature of 1,600°C, providing a carbon fiber having a toughness equal to 4.65 GPa and an elastic modulus equal to 247 GPa.Example 2 (Comparative)

[0062] Dissolution of a high-molecular-weight acrylic copolymer (MW=100,000-150,000) consisting of acrylonitrile (96% by weight of the total weight of the polymer), itaconic acid (1% by weight of the total weight of the polymer) and methyl acrylate (3% by weight of the total weight of the polymer).

[0063] Using the same procedure described in Example 1, a homogeneous spinning solution or dope of the above-mentioned polymer was prepared, using however as solvent a mixture consisting of DMSO (95% by weight) and an aqueous solution containing 0.6% by weight of ammonia (5% by weight with respect to the solvent), until a polymer concentration in the solvent equal to 18.0% by weight of the weight of the polymer / solvent mixture was reached.

[0064] The slurry was prepared at a temperature of 5°C and the spinning dope was obtained by subsequent heating to 85°C, for a time of 90 seconds, thus obtaining a homogeneous dope having a viscosity of 510 poises at 60°C.

[0065] The polymer solution in solvent thus obtained was fed to a spinning line for carbon fiber precursors.

[0066] During the spinning process, the spinnerets, immersed in a coagulation bath composed of a mixture of water and DMSO, generated a perfectly round, compact, crack-free fiber. The fiber thus obtained was subjected to washing with deionized water to remove the residual solvent, stretched in several steps in boiling water up to about 10 times its initial length, dried on hot rollers and collected on reels. The tows obtained are composed of fibers with a diameter of about 12 microns, an average toughness of 58 cN / tex and an ultimate elongation of about 18%, measured on an Instron 5542 dynamometer with a 10N cell according to the method ASTM D-3 822-2007.

[0067] The precursor tow thus obtained was treated in an oxidation oven for 60 minutes with a temperature gradient from 240 to 260°C, providing at the end an oxidized fiber having a density of 1.43 g / cc. The oxidized fiber was subsequently fed to a carbonization section with a maximum temperature of 1,600°C, providing a carbon fiber having a toughness equal to 5.20 GPa and an elastic modulus equal to 288 GPa.Example 3 (Comparative)

[0068] Dissolution of a high-molecular-weight acrylic copolymer (MW=100,000-150,000) consisting of acrylonitrile (96% by weight with respect to the total weight of the polymer), itaconic acid (1% by weight with respect to the total weight of the polymer) and methyl acrylate (3% by weight with respect to the total weight of the polymer).

[0069] Using the same procedure described in Example 1, a homogeneous spinning solution or dope of the above-mentioned polymer was prepared, using however as solvent a mixture consisting of DMSO (95% by weight) and an aqueous solution containing 1.25% by weight of 1,2-diaminopropane (5% by weight with respect to the solvent), until reaching a polymer concentration in the solvent equal to 18.0% by weight with respect to the weight of the polymer / solvent mixture.

[0070] The slurry was prepared at a temperature of 5°C and the spinning dope was obtained by subsequent heating to 85°C, for a time of 90 seconds, thus obtaining a homogeneous dope having a viscosity of 401 poises at 60°C.

[0071] The resulting polymer-solvent solution was fed to a spinning line for carbon fiber precursors.

[0072] During the spinning process, the spinnerets, immersed in a coagulation bath composed of a mixture of water and DMSO, generated a perfectly round, compact, crack-free fiber. The resulting fiber was washed with deionized water to remove the residual solvent, stretched in several steps in boiling water up to about 10 times its initial length, dried on hot rollers and collected on reels. The resulting tows are composed of fibers with a diameter of about 12 microns, an average toughness of 64 cN / Tex and an ultimate elongation of about 16%, measured on an Instron 5542 dynamometer with a 10N cell according to the method ASTM D-3822-2007.

[0073] The precursor tow thus obtained was treated in an oxidation oven for 38 minutes with a temperature gradient from 240 to 260°C, providing at the end an oxidized fiber having a density of 1.43 g / cc. The oxidized fiber was subsequently fed to a carbonization section with a maximum temperature of 1,600°C, providing a carbon fiber having a toughness equal to 5.10 GPa and an elastic modulus equal to 265 GPa.Example 4

[0074] Dissolution of a high-molecular-weight acrylic copolymer (MW=100,000-150,000) consisting of acrylonitrile (96% by weight of the total weight of the polymer), itaconic acid (1% by weight of the total weight of the polymer) and methyl acrylate (3% by weight of the total weight of the polymer).

[0075] Using the same procedure described in Example 1, a homogeneous spinning solution or dope of the above-mentioned polymer was prepared, using however as solvent a mixture consisting of DMSO (95% by weight) and an aqueous solution comprising 2% by weight of MEHQ (5% by weight with respect to the solvent), until reaching a polymer concentration in the solvent equal to 18.0% by weight of the polymer / solvent mixture.

[0076] The slurry was prepared at a temperature of 5°C and the spinning dope was obtained by subsequent heating to 85°C, for a time of 90 seconds, thus obtaining a homogeneous dope having a viscosity of 446 poises at 60°C.

[0077] The polymer solution in solvent thus obtained was fed to a spinning line for carbon fiber precursors.

[0078] During the spinning process, the spinnerets, immersed in a coagulation bath composed of a mixture of water and DMSO, generated a perfectly round, compact, crack-free fiber. The fiber thus obtained was subjected to washing with deionized water to remove the residual solvent, stretched in several steps in boiling water up to about 10 times its initial length, dried on hot rollers and collected on reels. The tows obtained are composed of fibers with a diameter of about 12 microns, an average toughness of 61 cN / tex and an ultimate elongation of about 18%, measured on an Instron 5542 dynamometer with a 10N cell according to the method ASTM D-3 822-2007.

[0079] The precursor tow thus obtained was treated in an oxidation oven for 38 minutes with a temperature gradient from 230 to 250°C, providing at the end an oxidized fiber having a density of 1.43 g / cc. The oxidized fiber was subsequently fed to a carbonization section with a maximum temperature of 1,600°C, providing a carbon fiber having a toughness equal to 5.02 GPa and an elastic modulus equal to 273 GPa.Example 5

[0080] Dissolution of a high-molecular-weight acrylic copolymer (MW=100,000-150,000) consisting of acrylonitrile (96% by weight of the total weight of the polymer), itaconic acid (1% by weight of the total weight of the polymer) and methyl acrylate (3% by weight of the total weight of the polymer).

[0081] Using the same procedure described in Example 1, a homogeneous spinning solution or dope of the above-mentioned polymer was prepared, using however as solvent a mixture consisting of DMSO (95% by weight) and an aqueous solution comprising 8% by weight of MEHQ (5% by weight with respect to the solvent), until reaching a polymer concentration in the solvent equal to 18.0% by weight of the polymer / solvent mixture.

[0082] The slurry was prepared at a temperature of 5°C and the spinning dope was obtained by subsequent heating to 85°C, for a time of 90 seconds, thus obtaining a homogeneous dope having a viscosity of 525 poises at 60°C.

[0083] The polymer solution in solvent thus obtained was fed to a spinning line for carbon fiber precursors.

[0084] During the spinning process, the spinnerets, immersed in a coagulation bath composed of a mixture of water and DMSO, generated a perfectly round, compact, crack-free fiber. The fiber thus obtained was subjected to washing with deionized water to remove the residual solvent, stretched in several steps in boiling water up to about 10 times its initial length, dried on hot rollers and collected on reels. The tows obtained are composed of fibers with a diameter of about 12 microns, an average toughness of 62 cN / tex and an ultimate elongation of about 17%, measured on an Instron 5542 dynamometer with a 10N cell according to the method ASTM D-3 822-2007.

[0085] The precursor tow thus obtained was treated in an oxidation oven for 38 minutes with a temperature gradient from 230 to 250°C, providing at the end an oxidized fiber having a density of 1.45 g / cc. The oxidized fiber was subsequently fed to a carbonization section with a maximum temperature of 1,600°C, providing a carbon fiber having a toughness equal to 5.14 GPa and an elastic modulus equal to 284 GPa.Example 6

[0086] Dissolution of a high-molecular-weight acrylic copolymer (MW=100,000-150,000) consisting of acrylonitrile (96% by weight of the total weight of the polymer), itaconic acid (1% by weight of the total weight of the polymer) and methyl acrylate (3% by weight of the total weight of the polymer).

[0087] Using the same procedure described in Example 1, a homogeneous spinning solution or dope of the above-mentioned polymer was prepared, using however as solvent a mixture consisting of DMSO (95% by weight) and an aqueous solution comprising 11% by weight of MEHQ (5% by weight with respect to the solvent), until reaching a polymer concentration in the solvent equal to 18.0% by weight of the polymer / solvent mixture.

[0088] The slurry was prepared at a temperature of 5°C and the spinning dope was obtained by subsequent heating to 85°C, for a time of 90 seconds, thus obtaining a homogeneous dope having a viscosity of 483 poises at 60°C.

[0089] The polymer-solvent solution thus obtained was fed to a spinning line for carbon fiber precursors.

[0090] During the spinning process the spinnerets, immersed in a coagulation bath composed of a mixture of water and DMSO, generated a perfectly round, compact, crack-free fiber. The fiber thus obtained was subjected to washing with deionized water to remove the residual solvent, stretched in several steps in boiling water up to about 10 times its initial length, dried on hot rollers and collected on reels. The tows obtained are composed of fibers with a diameter of approximately 12 microns, an average toughness of 59 cN / tex and an ultimate elongation of approximately 18%, measured on an Instron 5542 dynamometer with a 10N cell according to the method ASTM D-3822-2007.

[0091] The precursor tow thus obtained was treated in an oxidation oven for 38 minutes with a temperature gradient from 230 to 250°C, providing at the end an oxidized fiber having a density of 1.42 g / cc. The oxidized fiber was subsequently fed to a carbonization section with a maximum temperature of 1,600°C, providing a carbon fiber having a toughness equal to 5.09 GPa and an elastic modulus equal to 269 GPa.Example 7

[0092] Dissolution of a high-molecular-weight acrylic copolymer (MW=100,000-150,000) consisting of acrylonitrile (96% by weight of the total weight of the polymer), itaconic acid (1% by weight of the total weight of the polymer) and methyl acrylate (3% by weight of the total weight of the polymer).

[0093] Using the same procedure described in Example 1, a homogeneous spinning solution or dope of the above-mentioned polymer was prepared, using however as solvent a mixture consisting of DMSO (95% by weight) and an aqueous solution comprising 1% by weight of MEHQ (5% by weight with respect to the solvent), until reaching a polymer concentration in the solvent equal to 18.0% by weight of the polymer / solvent mixture.

[0094] The slurry was prepared at a temperature of 5°C and the spinning dope was obtained by subsequent heating to 85°C, for a time of 90 seconds, thus obtaining a homogeneous dope having a viscosity of 605 poises at 60°C.

[0095] The polymer-solvent solution thus obtained was fed to a spinning line for carbon fiber precursors.

[0096] During the spinning process the spinnerets, immersed in a coagulation bath composed of a mixture of water and DMSO, generated a perfectly round, compact, crack-free fiber. The fiber thus obtained was subjected to washing with deionized water to remove the residual solvent, stretched in several steps in boiling water up to about 10 times its initial length, dried on hot rollers and collected on reels. The tows obtained are composed of fibers with a diameter of approximately 12 microns, an average toughness of 63 cN / tex and an ultimate elongation of approximately 17%, measured on an Instron 5542 dynamometer with a 10N cell according to the method ASTM D-3822-2007.

[0097] The precursor tow thus obtained was treated in an oxidation oven for 50 minutes with a temperature gradient from 240 to 280°C, providing at the end an oxidized fiber with a density of 1.42 g / cc. The oxidized fiber was subsequently fed to a carbonization section with a maximum temperature of 1,600°C, providing a carbon fiber with a toughness equal to 5.07 GPa and an elastic modulus equal to 279 GPa.

[0098] Table 2 hereunder indicates the toughness and elastic modulus values of the carbon fibers obtained in Examples 1-7. Table 2 - Toughness and elastic modulus of Examples 1-7Example Additive in the solvent for the preparation of the spinning solution (dope) w / w % MeHQ in aqueous solution Toughness (GPa) ASTM D-4018-99Elastic modulus (GPa) ASTM D-4018-99Viscosity of the dope at 60 °C (P) 1. ComparativeNo additive4.652474012. ComparativeNH 3 5.202885103. Comparative1,2-DAP5.102654104.MEHQ2%5.022734465.MEHQ8%5.142845316.MEHQ11%5.092694837.MEHQ1%5.07279605

[0099] The carbon fibers obtained in Example 4 according to the present invention show particularly high toughness and elastic modulus values compared to the sample of comparative Example 1, in which the polymer has not been modified. These values are substantially and surprisingly comparable with those obtained in comparative Example 2 (polymer modified by the addition of ammonia to the solvent) and 3 (polymer modified by the addition of 1,2-diaminopropane to the solvent) with respect to which, however, improvements in the stabilization / oxidation rate are evident, as demonstrated by the lower temperature used in the oxidation step, by the shorter residence time and as also confirmed by the data relating to the peak temperature at the DSC indicated in Table 1.

[0100] Finally, the carbon fibers obtained in Examples 5, 6 and 7 prepared by adding variable quantities of MeHQ directly into the aqueous solution of the solvent system confirm the toughness and elastic modulus values obtained in Example 4, while also being able to guarantee the improvements in the stabilization / oxidation rate mentioned above.

[0101] The effect of the chemical modification can be observed in the attached Figures 1 and 2: Figure 1 is a DSC thermogram relating to a polymer film obtained according to Example 1, comparative; Figure 2 is a DSC thermogram relating to a polymer film obtained according to Example 4 of the invention; the temperature (°C) is indicated on the abscissa, the heat flow is indicated on the ordinate, following the convention of presenting the endothermic phenomena facing upwards (mW).

[0102] In the DSC thermogram of figure 2 a reduction in the peak temperature and in the cyclization enthalpy is detected, thus showing the positive effect obtained by the addition of MEHQ in the solvent of the process according to the present invention and of the consequent chemical modification of the polymer.

Claims

1. Use of 4-methoxyphenol having formula (1) as an additive in the production of acrylic fiber precursors of carbon fibers for reducing the stabilization time in the preparation of carbon fibers.

2. A process for preparing a homogeneous spinning solution for the production of acrylic fiber (PAN) precursors of carbon fibers, wherein the solvent used in the preparation step of a homogeneous suspension of acrylonitrile copolymer is a mixture of DMSO or DMAC and an aqueous solution of 4-methoxyphenol (MEHQ) having formula (1) 3. The process according to claim 2, comprising the following steps: i) preparing a homogeneous suspension by mixing an acrylonitrile copolymer in powder form with a solvent consisting of - a mixture containing DMSO in a quantity ranging from 90 to 99% by weight, preferably from 93 to 98% by weight, and an aqueous solution of 4-methoxyphenol in a quantity ranging from 2 to 7% by weight, preferably from 3 to 6% by weight based on the total weight of the solvent; or - a mixture containing DMAC in a quantity ranging from 95 to 99.5% by weight, preferably from 96 to 98% by weight, and an aqueous solution of 4-methoxyphenol in a quantity ranging from 0.5 to 5% by weight, preferably from 1 to 3% by weight, based on the total weight of the solvent, wherein said mixing is carried out over a time ranging from 5 to 30 minutes, by spraying a stream of the solvent onto a stream of the pre-mixed, disintegrated acrylonitrile copolymer powder; ii) heating the homogeneous suspension from step i) to a temperature ranging from 70 to 150°C over a time ranging from 0.5 to 30 minutes, until the copolymer has completely dissolved and a homogeneous solution is formed.

4. The process according to one or more of claims 2 and 3, wherein the aqueous solution of MEHQ comprises at least 0.5% by weight of MEHQ, preferably from 0.5 to 15% by weight of MEHQ, more preferably from 0.5 to 9% by weight, even more preferably from 0.5 to 5% by weight, and in particular from 1 to 3% by weight, based on the total weight of the aqueous solution.

5. The process according to one or more of claims 2 - 4, wherein the polymer is a high-molecular-weight copolymer, ranging from 70,000-300,000 Da (number MW), of acrylonitrile, in a quantity ranging from 90 to 99% by weight with respect to the total weight of the polymer, and one or more co-monomers in a quantity ranging from 1 to 10% by weight with respect to the total weight of the polymer.

6. The process according to claim 5, wherein the co-monomers are vinyl molecules bearing one or more acid groups, such as acrylic acid, methacrylic acid, itaconic acid and the like, preferably itaconic acid.

7. The process according to claims 5 or 6, wherein the copolymer comprises a third co-monomer selected from neutral vinyl molecules, such as methyl acrylate, methyl methyl acrylate, vinyl acetate, acrylamide and the like.

8. The process according to one or more of claims 2-7, wherein step i) is carried out at a temperature ranging from 0 to -10°C when the solvent comprises DMAC.

9. A process for the production of carbon fibers wherein the homogeneous solution obtained at the end of step ii) of the process according to one or more of claims 2 to 8, is subjected to the following further steps: iii) spinning the homogeneous solution from step ii) and obtaining a tow containing from 500 (0.5 K) to 400,000 (400 K) single filaments, preferably from 1,000 (1K) to 50,000 (50K) single filaments; iv) feeding the tow coming from step iii) to the stabilization or oxidation step, said oxidation being carried out for a time ranging from 35 to 120 minutes, at a temperature ranging from 230 to 260°C; v) feeding the oxidized tow coming from step iv) to a carbonization step with a maximum temperature of 1,600°C.

10. A mixture of DMSO or DMAC and an aqueous solution of 4-methoxyphenol (MEHQ) having formula (1) wherein the aqueous solution of MEHQ comprises at least 0.5% by weight of MEHQ, preferably from 0.5 to 15% by weight of MEHQ, more preferably from 0.5 to 9% by weight, even more preferably from 0.5 to 5% by weight, and in particular from 1 to 3% by weight, based on the total weight of the aqueous solution.

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