INTEGRATED AND IMPROVED PROCESS FOR THE PRODUCTION OF ACRYLIC FIBERS

Abstract

An integrated and improved process for the production of acrylic fibers is described, specifically a process that starts from the comonomers and reaches the spinning step obtaining the final fiber.

Claims

1. An integrated process for the production of acrylic fibers comprising the following steps: i) polymerizing comonomers in aqueous suspension in the presence of a quantity of solvent ranging from 3 to 25% by weight, with respect to the total weight of the aqueous suspension, wherein the solvent is the same solvent used in the following steps ii)-vi) and wherein the water that constitutes the aqueous suspension comprises recycled water coming from the last washing step of the spinning step, in an amount ranging from 10 to 40% by weight, with respect to the total weight of the water of the aqueous suspension, said polymerization reaction being catalyzed by a redox ammonium persulfate/ammonium bisulphite pair in the presence of catalytic quantities of iron sulfate, with the subsequent removal of the unreacted monomers, filtration of the aqueous suspension and washing of the same, obtaining a filtration cake comprising polymer and water with traces of solvent in a ratio ranging from 40/60 to 60/40 by weight; ii) dispersing said first filtration cake obtained in step i) in a solvent/water mixture or in a pure solvent in a weight ratio ranging from 2 to 10 times the weight of the cake and for a time ranging from 5 to 15 minutes, under stirring; iii) separating the solid/liquid phases of said polymeric dispersion by filtration or centrifugation, obtaining a second cake comprising polymer and solvent/water mixture, in a ratio between solvent and water ranging from 60/40 to 85/15 by weight; iv) redispersing the second cake obtained in step iii) in the same solvent or water/solvent mixture of step ii) in a weight ratio ranging from 2 to 10 times the weight of the polymer in the cake, at room temperature or at a temperature lower than room temperature for a time ranging from 5 to 20 minutes, obtaining a homogeneous dispersion or “slurry” wherein the weight ratio between solvent/polymer ranges from 90/10 to 70/30; v) heating the homogeneous dispersion or slurry obtained in step iv) until the complete dissolution of the polymer and obtaining a homogeneous spinning solution; vi) feeding the homogeneous spinning solution obtained at the end of step v) to the spinning step.

2. The process according to claim 1, wherein the spinning step is carried out by means of a wet spinning process or dry-jet wet spinning process wherein, after the coagulation step in a coagulation bath consisting of a mixture of water and solvent, the bundle of filaments thus obtained is stretched and washed in succession up to a length of about 8-12 times the initial length, and then subjected to a final washing phase with water to remove the last traces of solvent, the washing water then being fed again as recycled water in step i), comprising a quantity of solvent lower than 0.5% by weight.

3. The process according to claim 1, wherein the polymerization step i) is terminated by adding an iron sequestering agent and the aqueous suspension is subjected to the subsequent steps for removing the unreacted monomers, filtering the aqueous suspension and washing it to obtain a filtration cake comprising polymer and water with traces of solvent in a ratio ranging from 45/55 to 55/45 by weight.

4. The process according to claim 1, wherein in steps i)-vi) and in the spinning step, the solvent is the same.

5. The process according to claim 1, wherein in step ii), the temperature is lower than 0° C.

6. The process according to claim 1, wherein the quantity of water in the solvent/water mixture of step ii) varies from 0 to 20% by weight with respect to the total weight of the mixture.

7. The process according to claim 1, wherein in step iv), the temperature is equal to 7° C.

8. The process according to claim 1, wherein the homogeneous spinnable polymeric solution has a polymer content ranging from 12 to 22% by weight with respect to the total weight of the spinning solution.

9. The process according to claim 1, wherein in steps iii) and iv), water-soluble additives are added.

10. The process according to claim 1, wherein the polymers are high-molecular-weight polymers, said high molecular weight ranging from 80,000 to 200,000 Da, or they are medium-molecular-weight polymers, said medium molecular weight ranging from 40,000 to 55,000 Da.

11. The process according to claim 1, wherein the acrylonitrile copolymer is composed of acrylonitrile in an amount ranging from 90 to 99% by weight with respect to the total weight of the copolymer and one or more comonomers in an amount ranging from 1 to 10% by weight with respect to the total weight of the copolymer.

12. The process according to claim 10, wherein the comonomers are selected from neutral vinyl compounds; compounds bearing one or more acid groups; and compounds capable of imparting different chemical-physical characteristics to the polymer.

13. The process according to claim 1, wherein polymerizing comonomers in aqueous suspension is in the presence of a quantity of solvent ranging from 5 to 10% by weight, with respect to the total weight of the aqueous suspension.

14. The process according to claim 1, wherein the water that constitutes the aqueous suspension comprises recycled water coming from the last washing step of the spinning step, in an amount ranging from 20 to 30% by weight, with respect to the total weight of the water of the aqueous suspension.

15. The process according to claim 1, wherein dispersing said first filtration cake obtained in step i) is at a temperature lower than or equal to 7° C.

16. The process according to claim 1, wherein heating the homogeneous dispersion or slurry obtained in step iv) is achieved by passing the slurry into heat exchangers.

17. The process according to claim 2, wherein the bundle of filaments obtained is stretched and washed in succession up to a length of about 9-10 times the initial length.

18. The process according to claim 3, wherein the iron sequestering agent is EDTA in the form of sodium or ammonium salt.

19. The process according to claim 3, wherein the filtration cake comprises polymer and water with traces of solvent in a ratio ranging from 1:1 by weight.

20. The process according to claim 1, wherein the solvent is selected from the group consisting of dimethylacetamide (DMAc), dimethylformamide (DMF) and dimethylsulfoxide (DMSO)

Description

[0050] The specification comprises two figures:

[0051] FIG. 1, previously disclosed, is a schematic representation of a process according to the state of the art;

[0052] FIG. 2 is a schematic representation of an embodiment of the process according to the present invention.

[0053] An embodiment of the process object of the present invention is thus schematically described in FIG. 2.

[0054] A further advantage of the process according to the present invention is determined by the specific quantity of water contained in the spinning or dope solution which is then fed to the spinning step: the percentage of water that remains in the homogeneous solution for the production of acrylic fibers obtained with the process according to the present invention is in fact absolutely compatible with the acrylic fiber spinning technologies both according to the dry or wet spinning technology and according to the DJWS (dry jet wet spinning or air gap) technology: it is therefore not necessary to completely remove the water from the solution intended for spinning.

[0055] Furthermore, the presence of small percentages of water in the acrylic fiber spinning solutions as claimed in U.S. Pat. No. 3,932,577, facilitates the compatibility of the solution itself with the coagulation bath, leading to a fiber free from vacuoles and cracks; these characteristics are particularly advantageous in the production of precursors for carbon fibers or textile fibers with a good gloss and compact structure.

[0056] A further advantage linked to the presence of such small quantities of water in the dope is the possibility of conveying, both in the redispersion step ii) and in the preparation step of the slurry iv), water-soluble additives capable of providing the polymer and therefore the final fiber with particular performances.

[0057] Non-limiting examples of additives capable of providing the polymer and therefore the final fiber with particular performances are ammonia, primary amines, secondary amines, quaternary ammonium salts, salts of metal ions capable of salifying the ionic end groups of the polymer such as copper or silver, water-soluble polymers for modifying the rheology of the polymer solution, etc.

[0058] In particular, for carbon fiber precursors, it should be remembered that the addition to the polymeric solution of suitable quantities of ammonia or amines, as described in European patent application EP 3783132, further improves the extrusion of the fiber in the coagulation bath, providing even more compact and vacuole-free fibers, and increases the reaction kinetics in the oxidation/carbonization phase.

[0059] The spinning or dope solution thus obtained can be used immediately for feeding a suitable spinning line or it can be stored in heated tanks.

[0060] As previously indicated, in order to illustrate the process according to the state of the art, reference is made to the plant scheme described in FIG. 1 where acrylonitrile (21), the possible liquid comonomer (22), the aqueous solution of the catalytic system (23), the aqueous solution of the possible solid comonomer (24) and water (25), are fed to the polymerization reactor 1 in continuous. The polymer coming from the polymerization reactor 1 in the form of slurry in water, after treatment in the stripping column 2 for the removal of the unreacted monomers, is washed and filtered on a rotary filter 3 under vacuum. The powdered polymer is conveyed to the storage silos 14 through a drying unit 12, generally operating with hot air or nitrogen, and subsequently through the line 13, the polymer is fed by means of a screw or other conveyor means to a mixer element 15, where the fresh solvent also arrives through line 16 coming from the storage tanks.

[0061] In the mixer, 15 the powder polymer is dispersed in the solvent and the polymer slurry thus obtained is fed to a storage tank 17 and transformed into a spinning solution by means of the exchanger 18. The solution is then sent to a battery of filter presses 19, with selectivity cloths of 40 μm to 5 μm for the removal of possible particles and, through the line 20, to the spinning line or to a storage tank (not shown in FIG. 1).

[0062] In order to illustrate an embodiment of the process according to the present invention, reference is made hereunder to the plant diagram shown in FIG. 2, wherein the process is preferably carried out in continuous.

[0063] Acrylonitrile (21), the possible liquid comonomer (22), the aqueous solution of the catalytic system (23), the possible solid comonomer in solution of the same solvent used in spinning (24), the solvent (25) and water (26) also comprising the required amount of recycled water coming from the last washing section of the spinning line, are continuously fed to the polymerization reactor 1.

[0064] The polymer coming from the polymerization reactor 1 in the form of slurry in water, after treatment in the stripping column 2 for the removal of unreacted monomers, is washed and filtered on a rotary filter 3 under vacuum, resulting in a cake consisting of polymer and water which passes from step i) to step ii) of the process according to the present invention. The cake coming from the rotary filter 3 is then redispersed in a stirred tank 4 where a solvent/water mixture, possibly containing ammonia, or pure solvent, is fed through the line 27 at a low temperature. In the stirred tank 4, the suspension is kept at a temperature equal to or lower than 7° C. The resulting suspension is kept under stirring for a few minutes and is then fed to a second rotary filter or to a centrifugal separator 5, where step iii) of the process according to the present invention is effected. The mass soaked in water/solvent is fed, by means of a cochlea or another conveyor instrument 6, to a stirred tank 17, where the fresh solvent coming from the storage tanks also arrives through line 16 and the whole mixture is transformed into a spinning solution by means of the exchanger 18.

[0065] The solution is then sent to a battery of filter presses 19, with selectivity cloths of 40 μm to 5 μm for the removal of any undissolved particles and through line 20, to the spinning line or to a dope storage tank (not shown in FIG. 2).

[0066] The spinning line used can be of the wet-spinning type with spinnerets immersed in a coagulation bath consisting of a mixture of water and solvent. After coagulation, the bundle of filaments is stretched and washed in succession according to the known art to produce tows which are collected on bobbins or in boxes and then sent to the carbonization line for the production of carbon fiber. The solvent in the spinning step described herein is the same solvent already used in steps ii)-vi).

[0067] Alternatively, the spinning line used can be of the dry-jet wet spinning type (air-gap spinning) with spinnerets kept in the air at a small distance from the surface of the coagulum bath consisting of a mixture of water and solvent. After coagulation, the bundle of filaments is stretched and washed in succession according to the known art to produce tows which are collected on bobbins and then sent to the carbonization line for the production of carbon fiber. The solvent in the spinning step described herein is the same solvent already used in steps ii)-vi).

[0068] In both spinning techniques, demineralized water is used in the final washing steps of the fiber for removing the last traces of solvent. This water, after washing, contains traces of solvent and instead of being sent to the solvent recovery plant (operation not economically convenient) or to the waste water treatment (waste of water) it is fed back to the polymerization reactor 1, thus contributing to a decrease in the overall consumption. of water and to an energy saving.

EXAMPLES

[0069] By way of non-limiting example of the present invention, some embodiment examples of the process according to the present invention are provided hereunder.

Comparative Example 1 According to the Prior Art. Reference to FIG. 1

[0070] 100 kg/h of acrylonitrile, 1 kg/h of methyl acrylate, 2 kg/h of itaconic acid dissolved in water at 5% by weight, 0.4 kg/h of ammonium persulfate dissolved in water, 0.5 kg/h of ammonium bisulfite dissolved in water, 2 g/h of iron sulfate dissolved in water and 250 kg/h of water with the addition of sulfuric acid sufficient for keeping the reaction pH at a value ranging from 2.0 to 3.5, were added continuously at a temperature of 62° C., to an aluminium reactor equipped with stirrer and overflow. The ingredients were fed at such a rate as to allow a residence time of 90 minutes. The reaction was stopped after 90 minutes by adding an aqueous solution of EDTA in the overflow and the slurry was fed to a stripping column where unreacted acrylonitrile and methyl acrylate were removed, obtaining a slurry of polymer in water at the bottom. A conversion of acrylonitrile to copolymer equal to 78% by weight of the feed to the reactor was obtained. The polymer was filtered, washed and dried as shown in FIG. 1 obtaining a powder stored in the silos 14. This polymer was subsequently dissolved in DMAc at a temperature of −10° C. by means of the static mixer 15 and the heat exchanger 18 shown. in FIG. 1. The solution thus obtained was filtered by means of a battery of filter presses with selectivity cloths progressively variable from 40 μm to 5 μm and fed to a wet spinning line with 24,000 hole spinnerets. At the end of the stretching and washing section with recovery water coming from the solvent recovery plant, a final washing step was carried out with demineralized water which was subsequently sent to waste water treatment.

[0071] At the end of the spinning process, 24 K precursor bobbins are obtained with the following characteristics:

[0072] Titer: 1.25 dtex;

[0073] Tenacity: 61.1 cN/tex;

[0074] Elongation: 15.2%

suitable for the production of carbon fiber.

Example 2. Reference to FIG. 2

[0075] 100 kg/h of acrylonitrile, 1 kg/h of methyl acrylate, 2 kg/h of itaconic acid dissolved in DMAc, 0.4 kg/h of ammonium persulfate dissolved in water, 0.5 kg/h of ammonium bisulfite dissolved in water, 2 g/h of iron sulfate dissolved in water and 200 kg/h of water with the addition of sulfuric acid sufficient for keeping the reaction pH at a value ranging from 2.0 to 3.5, 40 kg/h of water coming from the last washing step of the spinning phase and 15 kg/h of DMAc, were added continuously at a temperature of 62° C. to an aluminium reactor equipped with stirrer and overflow. The ingredients were fed at such a rate as to allow a residence time of 90 minutes. The reaction was stopped after 90 minutes by adding an aqueous solution of EDTA in the overflow and the slurry was fed to a stripping column where unreacted acrylonitrile and methyl acrylate were removed and a slurry of polymer in water was obtained at the bottom. A conversion of acrylonitrile to copolymer equal to 84% by weight of the feed to the reactor was obtained.

[0076] The polymer coming from the polymerization reactor in the form of slurry in water, after treatment in a stripping column for the removal of unreacted monomers, was washed and filtered on a rotary filter under vacuum resulting in a cake consisting of polymer (53% by weight) and water (47% by weight).

[0077] 100 kg of this cake were transferred to a stirred tank and 255 kg of pure dimethylacetamide, kept at a temperature of −10° C., were added. The resulting suspension was kept under stirring in the tank cooled at this temperature for 5 minutes and then fed to a rotary vacuum filter which, after filtration, allowed a mass containing 40% by weight of DMAc, 9% by weight of water and 51% by weight of polymer, to be obtained.

[0078] The cake discharged from the filter was transferred to a stirred tank containing 148 kg of DMAc maintained at a temperature of −5° C. and kept under stirring for 10 minutes, producing a slurry containing 21% by weight of polymer, 75% by weight of DMAC and 4% by weight of water.

[0079] This slurry was then transferred through a gear pump to the transformation step into dope, which was carried out using:

[0080] a tube-bundle heat exchanger;

[0081] a static mixer for homogenization;

[0082] a battery of filter presses with selectivity cloths progressively variable from 40 μm to 5 μm.

[0083] The dope thus produced was fed to a wet spinning line with 24,000-hole spinnerets immersed in a coagulation bath containing 60% of DMAc and 40% of water and kept at 55° C. The bundle of filaments thus obtained was stretched, in succession, 10 times its initial length and washed. At the end of the stretching and washing section with recovery water coming from the solvent recovery plant, a final washing step was carried out with demineralized water which was subsequently sent to the polymerization reactor 1 of FIG. 2. At the end of the spinning process the tow was collected on bobbins at a rate of 70 m/min, obtaining 24 K precursor bobbins with the following characteristics:

[0084] Titer: 1.22 dtex;

[0085] Tenacity: 59.5 cN/tex:

[0086] Elongation: 14.5%;

suitable for the production of carbon fiber.

Example 3. Reference to FIG. 2

[0087] 100 kg/h of acrylonitrile, 2 kg/h of methyl acrylate, 2 kg/h of itaconic acid dissolved in DMSO, 0.4 kg/h of ammonium persulfate dissolved in water, 0.5 kg/h of ammonium bisulfite dissolved in water, 2 g/h of iron sulfate dissolved in water and 200 kg/h of water with the addition of sulfuric acid sufficient for keeping the reaction pH at a value ranging from 2.0 to 3.5, 40 kg/h of water coming from the last washing step of the spinning phase and 25 kg/h of DMSO, were added continuously at a temperature of 62° C. to an aluminium reactor equipped with stirrer and overflow.

[0088] The ingredients were fed at such a rate as to allow a residence time of 90 minutes. The reaction was stopped after 90 minutes by adding an aqueous solution of EDTA in the overflow and the slurry was fed to a stripping column where unreacted acrylonitrile and methyl acrylate were removed and a slurry of polymer in water was obtained at the bottom. A conversion of the acrylonitrile to copolymer equal to 86% by weight of the feed to the reactor was obtained.

[0089] The polymer coming from the polymerization reactor in the form of slurry in water, after treatment in a stripping column for the removal of unreacted monomers, was washed and filtered on a rotary vacuum filter resulting in a cake consisting of polymer (55% by weight) and water (45% by weight).

[0090] 100 kg of this cake were transferred to a stirred tank and 570 kg of a mixture consisting of DMSO (80%) and water (20%), kept at a temperature of 7° C. were added. The resulting suspension was kept under stirring in the tank cooled at this temperature for 5 minutes and then fed to a rotary filter which, after filtration, allowed a mass containing 42% by weight of DMSO, 10% by weight of water, and 48% by weight of polymer, to be obtained.

[0091] The cake discharged from the filter was transferred to a stirred tank containing 180 kg of DMSO, producing a slurry containing 19% by weight of polymer, 77% by weight of DMSO and 4% by weight of water. This slurry is then transferred through a gear pump to the transformation step into dope, which was carried out using:

[0092] a tube-bundle heat exchanger;

[0093] a static mixer for homogenization;

[0094] a battery of filter presses with selectivity cloths progressively variable from 40 μm to 5 μm.

[0095] The dope thus produced was fed to a dry-jet wet spinning line with 3,000 hole spinnerets positioned at a distance of 4 mm from the surface of the coagulation bath containing 35% of DMSO and 65% of water at a temperature of 5° C. The bundle of filaments obtained after coagulation was stretched in water and subsequently in steam (steam stretching) nine times its initial length and finally washed to remove the solvent still present. At the end of the stretching and washing section with recovery water coming from the solvent recovery plant, a final washing step was carried out with demineralized water which was then sent to the polymerization reactor 1 of FIG. 2. At the end of the spinning process, 12 K precursor bobbins were obtained by superimposing four 3K tows coming from the single spinneret. The fiber obtained, collected on bobbins at a rate of 240 m/min, has a perfectly round section, is compact, free of cracks and has the following characteristics:

[0096] Titer: 1.0 dtex;

[0097] Tenacity: 65.3 cN/tex;

[0098] Elongation: 14.1%

suitable for the production of carbon fiber.