A method for producing a low-grain-size polyacrylonitrile precursor fiber

Abstract

The application discloses a preparation method of low-grain-size polyacrylonitrile precursor. The method comprises the following steps: drying and densifying the fiber after hot water drawing and oiling through 10-25 hot rollers, wherein the hot roller pressure is 0.45-0.55 MPa, the temperature interval is 146-167 DEG C, the temperature gradient between the hot rollers is 3-5 DEG C, and then the fiber after the drying and densifying treatment is subjected to steam drawing under the steam pressure of 0.30-0.60 MPa, and the drawing ratio is 4.0-4.5 times, so as to obtain the low-grain-size polyacrylonitrile precursor. The application uses steam as the heating medium, and realizes the effective regulation of the grain size of the precursor by the low-temperature long-time drying and densifying technology and the high-multiple steam drawing technology, the regulation of the drying temperature and time of the precursor bundle, the gradual removal of the water molecules in the bundle by the outward diffusion, and the crystal rupture of the grains under the high stress by the high-multiple steam drawing.

Description

Technical Field

[0001] This invention belongs to the field of polyacrylonitrile fiber preparation, and relates to a method for preparing low-grain-size polyacrylonitrile precursor fibers. Background Technology

[0002] Polyacrylonitrile-based carbon fibers possess advantages such as high strength, high elastic modulus, and strong heat resistance, and are widely used in cutting-edge fields such as wind power generation, high-pressure vessels, aerospace, and national defense. The condensed-state structure in polyacrylonitrile precursor fibers determines their properties, which in turn affects the pre-oxidation reaction process and the low-temperature and high-temperature carbonization processes. Therefore, preparing low-grain-size carbon fiber precursors is an important means to achieve carbon fiber fine-graining and thus prepare high-strength, high-modulus carbon fiber precursors.

[0003] The drying and densification process is a heat treatment of water-containing swollen filaments (in a swollen or porous state) under certain tension and temperature conditions. Drying and densification can close pores, making the network structure of the protofibrils more compact. It can also eliminate structural inhomogeneities caused by the interdiffusion of solvents and precipitants during solidification, as well as the numerous pores and cracks of varying sizes resulting from this. Furthermore, higher temperatures give the macromolecular chains higher energy to overcome potential barriers and rotate, while sufficient time allows the macromolecular chains to fully move, fold, and shape under heat. By rationally controlling the drying and densification temperature and time, higher crystallinity and finer grains can be achieved in the protofibrils, improving their quality.

[0004] Chinese patent CN 113846386 B discloses a method for simultaneous densification and orientation of carbon fiber precursor. This method involves heating and drying the fiber bundle in sections at a high temperature of 160-220℃ to achieve densification, and then loading it with an overall draw ratio of 1.0-4.0 to obtain precursors with high orientation and high crystallinity. However, excessively high crystallinity is not conducive to the concentrated exothermic reactions such as cyclization and dehydrogenation during the pre-oxidation and carbonization processes, and is not conducive to the oxygen carbonization reaction and the preparation of high-strength and high-modulus carbon fiber products.

[0005] The literature (Wang Hetuan, Shen Zhigang, Li Lei, et al. Effects of drying and densification conditions on the structure and properties of PAN fibers [J]. High Technology Fibers and Applications, 2021, 46(5):37-42.) studied the influence of drying and densification conditions on the structure of PAN fibers, and explained that increasing the temperature and extending the drying and densification time can reduce the crystallinity of the fibers and increase the grain size. However, the key factors of drying temperature and drying time on the aggregate state of the precursor fibers were not clearly analyzed, and the simple drying and densification process is difficult to control the precursor fibers with low grain size. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing low-grain-size polyacrylonitrile precursor fibers. This method achieves the preparation of low-grain-size precursor fibers by controlling the number of drying rollers, pressure, and evaporation draw ratio. After oxidative carbonization, the fine-grained precursor fibers can achieve carbon fiber reinforcement and modulus enhancement.

[0007] The technical solution for achieving the objective of this invention is as follows:

[0008] A method for preparing low-grain-size polyacrylonitrile precursor fiber includes the following steps:

[0009] (1) Copolymerization: Acrylonitrile and itaconic acid are used as comonomers, and one of acrylic acid, methyl acrylate, methyl methacrylate and isobutyl methacrylate is used as the third monomer. Azobisisobutyronitrile is used as the initiator. Homogeneous free radical solution polymerization is carried out in dimethyl sulfoxide (DMSO) to prepare the spinning solution. The temperature of the spinning solution is controlled at 40-70℃ and the solid content is 14-25%.

[0010] (2) Degassing: The polymerized raw solution is degassed under a pressure of -60 to -78 kPa, and then filtered through a 3 μm primary filter. The raw solution after primary filtration is degassed under a pressure of -78 to -100 kPa, and then filtered through a 1 μm secondary filter before being sent to the spinning unit.

[0011] (3) Coagulation bath: The refined spinning solution is extruded from the spinneret into the coagulation bath after being metered by a metering pump. The concentration of DMSO in the coagulation bath is 55-70% and the temperature is 5-25℃. At the same time, it is stretched by 1-2 times to obtain nascent fibers.

[0012] (4) Washing and stretching: The nascent fibers formed by the coagulation bath are washed with multi-stage desalination water at 20-35℃ to remove the solvent, and then washed with water and then passed through hot water at 60-80℃, while being stretched 1-2 times.

[0013] (5) Drying and densification: After being stretched by hot water, the fiber is coated with 1-3% silicone oil agent and then dried and densified by 10-25 hot rollers. The pressure of the hot rollers is 0.45-0.55 MPa, the temperature range is 146-167℃, and the temperature gradient between the hot rollers is 3-5℃.

[0014] (6) Steam drawing: After drying and densification, the fiber is steam drawn and drawn 4.0 to 4.5 times under a steam pressure of 0.30 to 0.60 MPa to obtain low crystal size polyacrylonitrile precursor.

[0015] Further, in step (1), with the total mass of monomers being 100%, the proportions of each monomer are: 92-95 wt.% acrylonitrile, 3-4 wt.% itaconic acid, and 2-5 wt.% a third monomer. In a specific embodiment of the present invention, the proportions of each monomer are: 92 wt.% acrylonitrile, 4 wt.% itaconic acid, and 4 wt.% methyl methacrylate.

[0016] Furthermore, in step (4), the DMSO content in the fiber after washing is ≤0.1%.

[0017] Furthermore, in step (5), the silicone oil agent is a silicone oil agent commonly used in the oiling section of the polyacrylonitrile precursor fiber preparation process, such as a silicone oil agent composed of amino-modified silicone oil, polyether-modified silicone oil, antistatic agent and emulsifier and other additives.

[0018] Furthermore, in step (5), the number of hot rollers is 20 to 25, the pressure of the hot rollers is 0.45 to 0.55 MPa, and the temperature range is 146 to 167°C.

[0019] Furthermore, in step (6), the stretching is 4.3 to 4.5 times.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] (1) This invention achieves the preparation of low grain size precursor by controlling the number of hot rollers, pressure and temperature range during the drying and densification stage and increasing the steam drawing ratio. The control method is simple and efficient.

[0022] (2) Low grain size precursors can be used to prepare fine-grained carbon fibers to improve the strength and modulus of carbon fibers. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to specific embodiments.

[0024] The third monomer used in the copolymerization stage of this invention is a monomer commonly used in the preparation of polyacrylonitrile fibers by dry-jet wet spinning. In the following examples, methyl methacrylate is used as an example. In the copolymerization process, acrylonitrile and itaconic acid are used as comonomers, and one of acrylic acid, methyl acrylate, methyl methacrylate, and isobutyl methacrylate is used as the third monomer. Azobisisobutyronitrile (AIB) is used as the initiator, and homogeneous free radical solution polymerization is carried out in DMSO to obtain the spinning solution. The proportions of each monomer are all conventionally used ratios, specifically 92–95 wt.% acrylonitrile, 3–4 wt.% itaconic acid, and 2–5 wt.% of the third monomer. This ratio can achieve the preparation of polyacrylonitrile polymers. In the following examples, 92 wt.% acrylonitrile, 4 wt.% itaconic acid, and 4 wt.% methyl methacrylate are representative examples. The spinning solution temperature and solid content are also conventionally used ratios; in the following examples, a solid content of 18.5% is representative.

[0025] Similarly, the defoaming, coagulation bath, and water washing and stretching processes described in this invention are all conventional preparation conditions. The silicone oil agent used in this invention is a silicone oil agent conventionally used in the oiling section of the polyacrylonitrile precursor fiber preparation process, which plays a role in protecting the nascent fibers in the drying and densification section. In the following examples, the silicone oil agent used is a silicone oil agent composed of amino-modified silicone oil, polyether-modified silicone oil, antistatic agent, and emulsifier.

[0026] Example 1

[0027] (1) Copolymerization: A polyacrylonitrile polymer with a solid content of 18.5% was obtained by free radical solution polymerization of 92 wt.% acrylonitrile, 4 wt.% itaconic acid and 4 wt.% methyl methacrylate in a mixed solution of dimethyl sulfoxide.

[0028] (2) Demonization and degassing: The polymer is first demonized in a demonization kettle at a pressure of -76 kPa, then filtered through a 3 μm primary filter, and then degassed in a degassing kettle at a pressure of -97 kPa before being sent to a 1 μm secondary filter for further processing.

[0029] (3) Coagulation bath: The refined spinning solution is pressurized and transported to the spinning unit, and the spinning solution temperature is maintained at 45°C. The spinning solution passes through an air layer and then goes to the coagulation bath for coagulation and molding. The air layer is 2mm, the coagulation bath concentration is 45wt%, the coagulation bath temperature is 5°C, and a 1.1 times draw ratio is applied to obtain nascent fibers.

[0030] (4) Washing and stretching: The nascent fibers are desalinated in multiple stages at 33-36°C to remove the solvent, and after washing, they are placed in hot water at 60-80°C and stretched by 1.3 times.

[0031] (5) Drying and densification: After being drawn by hot water, the fibers are passed through a 1.1% silicone oil bath. The oiled raw fibers are then dried and densified by hot rollers with a certain temperature gradient. The drying roller pressure is maintained at 0.45 MPa, there are 25 drying rollers, the temperature gradient of the drying rollers is 4℃, and the temperature range is 146-165℃.

[0032] (6) Steam drawing: The dried fiber bundles are steam drawn with a saturated steam pressure of 0.45 MPa and a steam drawing ratio of 4.5 to obtain low crystal size polyacrylonitrile precursor.

[0033] The test results showed that the crystal size of the precursor fiber prepared in this embodiment was 5.38 nm.

[0034] Example 2

[0035] This embodiment is basically the same as embodiment 1, except that (5) drying and densification: the fibers after hot water stretching are passed through a 1.1% silicone oil bath, and the oiled raw fibers are dried and densified by hot rollers with a certain temperature gradient. The drying roller pressure is maintained at 0.55 MPa, there are 20 drying rollers, the temperature gradient of the drying rollers is 4℃, and the temperature range is 150-167℃. (6) steam stretching: the dried fiber bundles are steam stretched, the saturated steam pressure is 0.45 MPa, and the steam stretching ratio is 4.3.

[0036] The test results showed that the crystal size of the precursor fiber prepared in this embodiment was 5.47 nm.

[0037] Example 3

[0038] This embodiment is basically the same as embodiment 1, except that (5) drying and densification: the fibers after hot water stretching are passed through a 1.1% silicone oil bath, and the oiled raw fibers are dried and densified by hot rollers with a certain temperature gradient. The drying roller pressure is maintained at 0.50 MPa, there are 20 drying rollers, the temperature gradient of the drying rollers is 4℃, and the temperature range is 148-167℃. (6) steam stretching: the dried fiber bundles are steam stretched, the saturated steam pressure is 0.45 MPa, and the steam stretching ratio is 4.0.

[0039] The test results showed that the crystal size of the precursor fiber prepared in this embodiment was 5.86 nm.

[0040] Comparative Example 1

[0041] This comparative example is basically the same as Example 1, except that (5) drying and densification: the fibers after hot water stretching are passed through a 1.1% silicone oil bath, and the oiled raw fibers are dried and densified by hot rollers with a certain temperature gradient. The drying roller pressure is maintained at 0.50 MPa, there are 10 drying rollers, the temperature gradient of the drying rollers is 4℃, and the temperature range is 148-161℃. (6) steam stretching: the dried fiber bundles are steam stretched, the saturated steam pressure is 0.45 MPa, and the steam stretching ratio is 3.8.

[0042] Tests showed that the crystal size of the precursor fiber prepared in this embodiment was 6.65 nm.

[0043] Comparative Example 2

[0044] This comparative example is basically the same as Example 1, except that (5) drying and densification: the fibers after hot water stretching are passed through a 1.1% silicone oil bath, and the oiled raw fibers are dried and densified by hot rollers with a certain temperature gradient. The drying roller pressure is maintained at 0.80 MPa, there are 10 drying rollers, the temperature gradient of the drying rollers is 4℃, and the temperature range is 164-177℃. (6) steam stretching: the dried fiber bundles are steam stretched, the saturated steam pressure is 0.45 MPa, and the steam stretching ratio is 3.0.

[0045] The test results showed that the crystal size of the precursor fiber prepared in this comparative example was 7.52 nm.

[0046] Comparative Example 3

[0047] This comparative example is basically the same as Example 1, except that (5) drying and densification: the fibers after hot water stretching are passed through a 1.1% silicone oil bath, and the oiled raw fibers are dried and densified by hot rollers with a certain temperature gradient. The drying roller pressure is maintained at 0.30 MPa, there are 25 drying rollers, the temperature gradient of the drying rollers is 4℃, and the temperature range is 144-167℃. (6) steam stretching: the dried fiber bundles are steam stretched. The dried fibers have a high water content, which causes breakage and cannot be steam stretched. The grain size of the dried fibers is tested to be 5.89 nm.

[0048] Comparative Example 4

[0049] This comparative example is basically the same as Example 1, except that (5) drying and densification: the fibers after hot water stretching are passed through a 1.1% silicone oil bath, and the oiled raw fibers are dried and densified by hot rollers with a certain temperature gradient. The drying roller pressure is maintained at 1.10 MPa, there are 20 drying rollers, the temperature gradient of the drying rollers is 4℃, and the temperature range is 171-188℃. (6) steam stretching: the dried fiber bundles are steam stretched, the saturated steam pressure is 0.45 MPa, and the maximum steam stretching ratio is 2.8.

[0050] The test results showed that the crystal size of the precursor fiber prepared in this comparative example was 8.22 nm.

[0051] Table 1. Experimental parameters and precursor fiber grain size for each experimental example.

[0052]

Claims

1. A process for the production of low grain size polyacrylonitrile precursor, characterized in that, Includes the following steps: (1) Copolymerization: Acrylonitrile and itaconic acid are used as comonomers, and one of acrylic acid, methyl acrylate, methyl methacrylate and isobutyl methacrylate is used as a third monomer. Azobisisobutyronitrile is used as an initiator. Homogeneous free radical solution polymerization is carried out in dimethyl sulfoxide to prepare the spinning solution. The temperature of the spinning solution is controlled at 40~70℃ and the solid content is 14~25%. (2) Degassing: The polymerized raw solution is degassed under a pressure of -60~-78KPa, and then filtered through a 3μm primary filter. The raw solution after primary filtration is degassed under a pressure of -78~-100KPa, and then filtered through a 1μm secondary filter before being sent to the spinning unit. (3) Coagulation bath: The refined spinning solution is extruded from the spinneret into the coagulation bath after being metered by a pump. The concentration of DMSO in the coagulation bath is 55~70% and the temperature is 5~25℃. At the same time, it is stretched by 1~2 times to obtain nascent fibers. (4) Washing and stretching: The nascent fibers formed by the coagulation bath are washed with desalination water at 20~35℃ to remove the solvent, and then washed with water and then subjected to hot water at 60~80℃, while being stretched 1~2 times. (5) Drying and densification: After being stretched by hot water, the fiber is coated with 1-3% of silicone oil agent and then dried and densified by 20-25 hot rollers. The pressure of the hot rollers is 0.45-0.55 MPa, the temperature range is 146-167℃, and the temperature gradient between the hot rollers is 3-5℃. (6) Steam drawing: After drying and densification, the fiber is steam drawn and drawn 4.0 to 4.5 times under a steam pressure of 0.30 to 0.60 MPa to obtain low crystal size polyacrylonitrile precursor.

2. The production method according to claim 1, characterized by, In step (1), with the total mass of monomers being 100%, the percentages of each monomer are: 92~95 wt.% acrylonitrile, 3~4 wt.% itaconic acid and 2~5 wt.% third monomer.

3. The preparation method according to claim 1, characterized in that, In step (1), the total mass of monomers is 100%, and the proportion of each monomer is: 92 wt.% acrylonitrile, 4 wt.% itaconic acid and 4 wt.% methyl methacrylate.

4. The preparation method according to claim 1, characterized in that, In step (4), the DMSO content in the fiber after washing is ≤0.1%.

5. The preparation method according to claim 1, characterized in that, In step (5), the silicone oil agent is a silicone oil agent composed of amino-modified silicone oil, polyether-modified silicone oil, antistatic agent and emulsifier.

6. The preparation method according to claim 1, characterized in that, In step (6), the stretch is 4.3 to 4.5 times.

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