HIGH-SPEED PROCESS FOR PRODUCING ACRYLIC FIBERS AND RELATIVE APPARATUS

Abstract

A process for the production of acrylic fibers, in particular a spinning process for obtaining precursor fibers of carbon fiber by the wet spinning of a polymer solution in an organic solvent and the relative apparatus.

Claims

1. A method of spinning a homogeneous solution of acrylic copolymer in organic solvent for the production of precursor fibers including a wet-spinning step comprising spinning homogeneous solution at a speed ranging from 150 to 400 m/min, wherein the homogeneous solution of acrylic copolymer in organic solvent or dope is fed to one or more spinnerets with a hole diameter ranging from 150 to 300 microns, said spinnerets being immersed in a coagulation bath consisting of a mixture of organic solvent and non-solvent solvent with a concentration of organic solvent ranging from 78 to 85% by weight, with respect to the total weight of the mixture, at a temperature ranging from 5 to 40° C.; the dope at the outlet of said one or more spinnerets comes into contact with the coagulation bath where it coagulates forming a tow or bundle of filaments, with a jet stretch ranging from 5 to 15, said jet stretch being the ratio between the outlet speed of the dope from the spinneret into the coagulation bath and the collection speed after coagulation; said tow or bundle of filaments then being fed to a series of washing or washing and stretching steps, wherein each washing or washing and stretching step is carried out in co-current, the movement direction of the washing solution coinciding with the movement direction of the tow or bundle of filaments, the feeding and discharging of the washing solution in each washing or washing and stretching step being carried out in countercurrent with respect to the movement direction of the tow or bundle of filaments.

2. The method according to claim 1, wherein the non-solvent solvent of the coagulation bath is water.

3. The method according to claim 1, wherein the washing solution is water.

4. The method according to claim 1, wherein the organic solvent of the homogeneous solution of acrylic copolymer is the same organic solvent used in the coagulation bath.

5. The method according to claim 1, wherein the coagulation bath consists of a water/dimethylacetamide mixture or a water/dimethylsulfoxide mixture.

6. The method according to claim 1, wherein the coagulation bath consists of a water/dimethylsulfoxide mixture and the temperature of the bath is in the range of from 5 to 15° C.

7. The method according to claim 1, wherein the acrylic polymer is a copolymer of acrylonitrile with one or more monomers selected from the group consisting of methyl acrylate, methyl methylacrylate, vinyl acetate, acrylamide, acrylic acid, itaconic acid and sulfonated styrenes, wherein the acrylonitrile is present in a quantity ranging from 90 to 99% by weight and the comonomer in a quantity ranging from 1 to 10% by weight with respect to the total weight of the polymer.

8. A wet-spinning apparatus, comprising at least one washing or washing and stretching unit U, said unit U comprising: a washing tank configured for containing a washing solution; wherein said solution is fed to a first end of said tank at a first temperature T1 and wherein said solution is discharged at a second end of said tank at a second temperature T2, said temperature T1 being higher than said temperature T2; mechanical means configured for moving a tow or bundle of filaments from said first end to said second end of the tank; wherein in the washing tank, the movement direction of the washing solution is in co-current with respect to the movement direction of the tow or bundle of filaments.

9. The apparatus according to claim 8, wherein the solution being discharged at the second end of said washing tank at the second temperature T2, is fed through an overflow and a filter to an auxiliary recycling tank where a first portion of said solution is fed by means of a pump to an auxiliary heating tank, equipped with a heat exchanger, said auxiliary heating tank being configured for feeding said washing solution to the first end of said washing tank at the first temperature T1.

10. The apparatus according to claim 8, wherein said apparatus comprises two or more washing or washing and stretching units (U), arranged in sequence and fluidly connected to each other.

11. The apparatus according to claim 10, wherein the auxiliary recycling tank of each washing or washing and stretching unit U is configured for being fed with the remaining portion of the washing solution coming from the auxiliary recycling tank of the washing or washing and stretching unit U arranged downstream of said washing tank with respect to the movement of the tow or bundle of filaments, said auxiliary recycling tank also being configured for feeding the remaining portion of washing solution extracted at the second end of said washing tank at the second temperature T2, to the auxiliary recycling tank of the washing or washing and stretching unit U arranged upstream of said washing tank with respect to the movement of the tow or bundle of filaments.

12. The apparatus according to claim 10, wherein the feeding of the washing solution is in countercurrent with respect to the movement direction of the tow or bundle of filaments with respect to the sequential arrangement of the washing or washing and stretching units U.

13. The method according to claim 1 wherein the coagulation bath consisting of a mixture of organic solvent and non-solvent solvent has a concentration of organic solvent ranging from 78 to 84% by weight, with respect to the total weight of the mixture.

14. The method according to claim 1, wherein the organic solvent of the homogeneous solution of acrylic copolymer and the organic solvent used in the coagulation bath is taken from the group consisting of dimethylacetamide (DMAC) and dimethyl sulfoxide (DMSO).

15. The apparatus according to claim 8, wherein the mechanical means comprises rollers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be better understood when considered in view of the attached drawings, in which like reference characters indicate like parts. The drawings, however, are presented merely to illustrate the preferred embodiment of the invention without limiting the invention in any manner whatsoever.

[0035] FIG. 1 is a schematic view of the apparatus according to the state of the art.

[0036] FIG. 2 is a schematic view of an apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] The present invention therefore relates to a spinning process of a homogeneous solution of acrylic copolymer in organic solvent, preferably DMAC or DMSO, for the production of precursor fibers, said process comprising a wet-spinning step with a spinning speed ranging from 150 to 400 m/min, wherein [0038] the homogeneous solution of acrylic copolymer in organic solvent or dope is fed to one or more spinnerets with a hole diameter ranging from 150 to 300 microns, said spinnerets being immersed in a coagulation bath consisting of a mixture of organic solvent and non-solvent solvent with a concentration of organic solvent ranging from 78 to 85% by weight, preferably from 78 to 84% by weight, with respect to the total weight of the mixture, at a temperature ranging from 5 to 40° C.; [0039] the dope at the outlet of said one or more spinnerets comes into contact with the coagulation bath where it coagulates forming a tow or bundle of filaments, with a jet stretch ranging from 5 to 15, said jet stretch being the ratio between the outlet speed of the dope from the spinneret into the coagulation bath and the collection speed after coagulation; [0040] said tow or bundle of filaments then being fed to a series of washing or washing and stretching steps, wherein each washing or washing and stretching step is carried out in co-current, the movement direction of the washing solution coinciding with the movement direction of the tow or bundle of filaments, the feeding and discharging of the washing solution in each washing or washing and stretching step being carried out in countercurrent with respect to the movement direction of the tow or bundle of filaments.

[0041] The non-solvent solvent of the coagulation bath is preferably water.

[0042] The washing solution is preferably water.

[0043] The organic solvent of the homogeneous acrylic copolymer solution is the same organic solvent used in the coagulation bath, preferably the organic solvent is dimethylacetamide (DMAC) or dimethyl sulfoxide (DMSO).

[0044] The coagulation bath preferably consists of a water/dimethylacetamide mixture or a water/dimethylsulfoxide mixture.

[0045] When the coagulation bath consists of the water/dimethylsulfoxide mixture, the temperature of the bath is preferably in the range of from 5 to 15° C.

[0046] The acrylic polymer is a copolymer of acrylonitrile with one or more monomers selected from the group comprising methyl acrylate, methyl methylacrylate, vinyl acetate, acrylamide, acrylic acid, itaconic acid or sulfonated styrenes, wherein the acrylonitrile is present in a quantity ranging from 90 to 99% by weight and the comonomer in a quantity ranging from 1 to 10% by weight with respect to the total weight of the polymer.

[0047] The process according to the present invention allows a precursor fiber to be obtained with excellent mechanical characteristics and surface compactness, the absence of cracks, the mechanical characteristics being similar to those obtained by air-gap spinning.

[0048] A further advantage of the process according to the present invention is the possibility of reaching spinning speeds (150 - 400 m/min) similar to those obtained with the air-gap process.

[0049] As already mentioned, spinning speed refers to the collection speed of the fiber on the bobbin, whereas jet stretch refers to the ratio between the exiting speed of the dope from the spinneret into the coagulation bath and the collection speed after coagulation.

[0050] The objective of the present invention is to also define a spinning apparatus capable of allowing the implementation of the process described above.

[0051] The present invention therefore further relates to an apparatus for wet spinning, characterized in that it comprises at least one washing or washing and stretching unit U, said unit U comprising: [0052] a washing tank (6) suitable for containing a washing solution (8); [0053] wherein said solution (8) is suitable for being fed to a first end of said tank (6) at a first temperature T1 and wherein said solution (8) is suitable for being discharged at a second end of said tank (6) at a second temperature T2, said temperature T1 being higher than said temperature T2; [0054] mechanical means, preferably rollers (5, 2′), suitable for moving a tow or bundle of filaments (7), from said first end to said second end of the tank (6); [0055] wherein in the washing tank (6), the movement direction of the washing solution (8) is in co-current with respect to the movement direction of the tow or bundle of filaments (7).

[0056] Said washing or washing and stretching unit U is preferably characterized in that the solution (8) suitable for being discharged at the second end of said washing tank (6) at the second temperature T2, is fed through an overflow (11) and a filter (12) to an auxiliary recycling tank (9) where a first portion of said solution (8) is fed by means of a pump (13) to an auxiliary heating tank (10), equipped with a heat exchanger (14), said auxiliary heating tank (10) being suitable for feeding said washing solution (8) to the first end of said washing tank (6) at the first temperature T1.

[0057] The wet-spinning apparatus according to the present invention preferably comprises two or more washing or washing and stretching units (U), arranged in sequence and fluidly connected to each other.

[0058] The auxiliary recycling tank (9) of each washing or washing and stretching unit U is suitable for being fed with the remaining portion of the washing solution (8) coming from the auxiliary recycling tank (9′) of the washing or washing and stretching unit U arranged downstream of said washing tank (6) with respect to the movement of the tow or bundle of filaments (7), said auxiliary recycling tank (9) also being suitable for feeding the remaining portion of washing solution (8) extracted at the second end of said washing tank (6) at the second temperature T2, to the auxiliary recycling tank of the washing or washing and stretching unit U arranged upstream of said washing tank (6) with respect to the movement of the tow or bundle of filaments (7).

[0059] The apparatus for wet spinning comprising two or more washing or washing and stretching units U, is therefore characterized in that, with reference to the sequential arrangement of the washing or washing and stretching units U, the feeding of the washing solution is countercurrent with respect to the movement direction of the tow or bundle of filaments.

[0060] The apparatus according to the present invention allows the potential of the new spinning process to be fully exploited in terms of speed, which could not be achieved in devices according to the state of the art for wet spinning in organic solvent which do not allow smooth running at speeds higher than 100 m/min for fluid-dynamic reasons.

[0061] In the attached FIGS. 1 and 2, FIG. 1, as already mentioned, is representative of an apparatus according to the state of the art, whereas FIG. 2 is representative of an apparatus according to the present invention.

[0062] In the present description, for the illustration of the figures, identical reference numbers or letters are used for indicating construction elements with the same function. Furthermore, for clarity of illustration, some references may not have been repeated in all of the figures.

[0063] The apparatus according to the present invention is represented in the diagram of the attached FIG. 2, wherein the tows (7), coming from the roller container positioned at the inlet, leave the roller (5) and proceed towards the roller (2′) immersed in the solution contained in the tank (6). The roller (2′) extracts the tows from the washing solution (8) and accompanies them to the following rollers (3′, 4′ and 5′, not shown in FIG. 2), which feed the next tank (6′, not shown in the figure).

[0064] The tank 6 is equipped with auxiliary tanks (9, 10). The washing solution is collected in the auxiliary tank (9) or recycling tank and exits from the tank (6) through an overflow (11), filtered through the filter (12), which can be removed for cleaning. A pump (13) takes the filtered washing solution from the auxiliary tank (9) or recycling tank and feeds it to the auxiliary tank (10) or heating tank, in which a heat exchanger (14) is generally installed, powered with steam.

[0065] The washing solution, filtered and heated, returns to the main tank (6) through special distribution slots (15) and flows inside the same in co-current with respect to the movement direction of the tows.

[0066] As indicated above, therefore, the flow of the washing solution and of the tow or bundle of filaments is in co-current inside the tank of each washing unit, whereas, considering the series of different washing units present in the spinning line, the overall configuration is in countercurrent, an arrangement that allows a better efficiency of the overall washing process.

[0067] The configuration of the apparatus according to the present invention is characterized by the following advantages: [0068] 1. it allows the rise in the level of the washing solution near the second end or outlet of the tank (6) to be compensated, by acting on the adjustable overflow (11) in order to avoid overflows and leaks through the seals of the rollers; [0069] 2. it allows the liquid trail of the washing solution that accompanies the tows (7) to proceed towards the overflow (11) at the outlet without reversing the direction, causing a lower level of the washing solution; [0070] 3. it reduces the amount of splashes of washing solution released by the roller (2′) due to a lower immersion of the roller itself, with a more horizontal starting angle of the splashes

[0071] The tows or bundles of filament exiting from the washing and stretching phases are then rinsed with jets of demineralized water and squeezed by pressure rollers.

EXAMPLES

[0072] By way of non-limiting example of the present invention, some embodiment examples of the process according to the present invention and some comparative examples are provided hereunder.

Example 1

[0073] 100 kg/h of acrylonitrile, 1 kg/h of methyl acrylate, 2 kg/h of itaconic acid dissolved in 5% by weight of water; 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 containing sulfuric acid sufficient for keeping the reaction pH at a value ranging from 2.0 to 3.5, were added in continuous at a temperature of 62° C. to an aluminum reactor equipped with stirrer and overflow . The ingredients were fed at such a flow-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 polymer slurry in water at the bottom. The polymer was filtered, washed, dried and subsequently dissolved in DMAC. The solution thus obtained, containing 20% by weight of polymer was filtered by means of a battery of filter presses with selectivity cloths progressively varying from 40 .Math.m to 5 .Math.m and fed to a wet-spinning line with 12,000-hole spinnerets, having a capillary diameter equal to 250 microns.

[0074] The spinning solution, kept at a temperature of 80° C., was fed to the spinneret immersed in a coagulation bath by means of a 50 cc/rev spinning pump, i.e. at each revolution the pump doses 50 cc. The coagulation bath is composed of a mixture of water and DMAC containing 82% by weight of DMAC at a temperature of 20° C. The bundle of nascent fibers was subsequently passed into a post-coagulation bath, consisting of a mixture of water and DMAC containing 32% by weight of DMAC at a temperature of 40° C.

[0075] The speed of the first collection roller was equal to 26.42 m/min, corresponding to a jet stretch of 8.9.

[0076] The bundle of fibers was subsequently fed to a series of washing and stretching steps. The total stretching, divided into three steps, was equal to 10.06x, i.e. the initial length increased by 10.06 times. The washing operations were carried out using the tanks as shown in FIG. 2.

[0077] The tow thus produced was finally collected on bobbins at a speed of 250.7 m/min.

[0078] At the end of the spinning process, 12 K precursor bobbins were obtained with the following characteristics: [0079] Titer: 1.1 dtex; [0080] Breaking strength: 68.1 cN/tex; [0081] Ultimate elongation: 15.2%suitable for the production of carbon fiber.

Example 2

[0082] A spinning solution in DMAC prepared as described in Example 1, was fed to a wet-spinning line with 24,000-hole spinnerets having a capillary diameter of 300 microns.

[0083] The spinning solution maintained at a temperature of 80° C. was fed to the spinneret immersed in a coagulation bath by means of a 100 cc/rev spinning pump. The coagulation bath is composed of a mixture of water and DMAC containing 82% by weight of DMAC, at a temperature of 20° C. The bundle of nascent fibers was subsequently passed into a post-coagulation bath consisting of a mixture of water and DMAC containing 33% by weight of DMAC at a temperature of 40° C.

[0084] The speed of the first collection roller after the coagulation bath was equal to 31.6 m/min, corresponding to a jet stretch of 12.8.

[0085] The bundle of fibers was subsequently fed to a series of stretching and washing steps. The total stretching, divided into three steps, was equal to 10.06x. The washing operations were carried out using the tanks as shown in FIG. 2.

[0086] The tow thus produced was finally collected on bobbins at a speed of 300.1 m/min.

[0087] At the end of the spinning process, 24 K precursor bobbins were obtained with the following characteristics: [0088] Titer1.1 dtex; [0089] Breaking strength: 66.8 cN/tex; [0090] Ultimate elongation: 14.9% suitable for the production of carbon fiber.

Example 3

[0091] A spinning solution prepared as described in Example 1, using DMSO instead of DMAC as solvent and a polymer concentration in the solution of 19% by weight, was fed to a wet-spinning line with 12,000-hole spinnerets having a capillary diameter of 250 microns.

[0092] The spinning solution maintained at a temperature of 80° C. was fed to the spinneret immersed in a coagulation bath by means of a 50 cc/rev spinning pump. The coagulation bath is composed of a mixture of water and DMSO containing 81% by weight of DMSO, at a temperature of 5° C. The bundle of nascent fibers was subsequently passed into a post-coagulation bath consisting of a mixture of water and DMSO containing 31% by weight of DMSO at a temperature of 35° C.

[0093] The speed of the first collection roller after the coagulation bath was equal to 31.7 m/min, corresponding to a jet stretch of 8.6.

[0094] The bundle of fibers was subsequently fed to a series of stretching and washing steps. The total stretching, divided into three steps, was equal to 10.06x. The washing operations were carried out using the tanks as shown in FIG. 2.

[0095] The tow thus produced was finally collected on bobbins at a speed of 300.8 m/min.

[0096] At the end of the spinning process, 12 K precursor bobbins were obtained with the following characteristics: [0097] Titer: 1.25 dtex; [0098] Breaking strength: 70.1 cN/tex; [0099] Ultimate elongation: 14.2% suitable for the production of carbon fiber.

Example 4

[0100] A spinning solution prepared as described in Example 1, using DMSO instead of DMAC as solvent and a polymer concentration in the solution of 19% by weight, was fed to a wet-spinning line with 24,000-hole spinnerets having a capillary diameter of 300 microns.

[0101] The spinning solution, maintained at a temperature of 80° C., was fed to the spinneret immersed in a coagulation bath by means of a 100 cc/rev spinning pump. The coagulation bath is composed of a mixture of water and DMSO containing 81% by weight of DMSO at a temperature of 5° C. The bundle of nascent fibers was subsequently passed through a post-coagulation bath consisting of a mixture of water and DMSO containing 32% by weight of DMSO at a temperature of 35° C.

[0102] The speed of the first collection roller after the coagulation bath was equal to 31.6 m/min, corresponding to a jet stretch of 12.5.

[0103] The bundle of fibers was subsequently fed to a series of stretching and washing steps. The total stretching, divided into three steps, was equal to 10.06x. The washing operations were carried out using the modified tanks as shown in FIG. 2. The tow thus produced was finally collected on bobbins at a speed of 300.2 m/min.

[0104] At the end of the spinning process, 24 K precursor bobbins were obtained with the following characteristics: [0105] Titer: 1.24 dtex; [0106] Breaking strength: 71.2 cN/tex; [0107] Ultimate elongation: 14.0% suitable for the production of carbon fiber.

Example 5 Comparative

[0108] A spinning solution prepared as described in example 3, maintained at a temperature of 80° C., was fed to the spinneret having a capillary diameter of 250 microns immersed in a coagulation bath by means of a 50 cc/rev spinning pump. The coagulation bath is composed of a mixture of water and DMSO containing 75% by weight of DMSO at a temperature of 5° C. The bundle of nascent fibers was subsequently passed through a post-coagulation bath consisting of a mixture of water and DMSO containing 32% by weight of DMSO at a temperature of 35° C.

[0109] The speed of the first collection roller after the coagulation bath was equal to 31.6 m/min, corresponding to a jet stretch of 8.5. Under these conditions the spinning was impossible due to frequent breakages of the bundle of nascent fibers in the coagulation bath.

Example 6 Comparative

[0110] A spinning solution prepared as described in example 3, maintained at a temperature of 80° C., was fed to the spinneret having a capillary diameter of 250 microns immersed in a coagulation bath by means of a 50 cc/rev spinning pump. The coagulation bath is composed of a mixture of water and DMSO containing 86% by weight of DMSO at a temperature of 20° C. Under these conditions it was not possible collecting a bundle of fibers because the nascent fibers are dissolved in the coagulation bath immediately after exiting the spinnerets.

[0111] The speed of the first collection roller after the coagulation bath was equal to 31.6 m/min, corresponding to a jet stretch of 8.5. Under these conditions the spinning was impossible due to frequent breakages of the bundle of nascent fibers in the coagulation bath.