EP4700164A1

EP4700164A1 - Polyacrylonitrile carbon fiber, polyacrylonitrile precursor, and preparation methods therefor

Info

Publication number: EP4700164A1
Application number: EP23933920.3A
Authority: EP
Inventors: Zhigang Shen; Jianning Wang; Lei Li; Hetuan WANG
Original assignee: Sinopec Shangai Research Institute Of Petrochemical Technology Co Ltd; China Petroleum and Chemical Corp
Current assignee: Sinopec Shangai Research Institute Of Petrochemical Technology Co Ltd; China Petroleum and Chemical Corp
Priority date: 2023-04-18
Filing date: 2023-12-29
Publication date: 2026-02-25
Also published as: KR20260002581A; CN118814317A; WO2024217058A1; TW202442961A

Abstract

The invention relates to polyacrylonitrile carbon fiber and precursor fiber and producing methods thereof. The polyacrylonitrile carbon fiber provided by the invention has groove structures on the surface, and the Si/C ratio of the carbon fiber of the region X between the top of the protrusion of the fiber surface and the bottom of the groove is in the range of 1% to 10%, the Si/C ratio of the region Y between the bottom of the groove and the inner 100nm is in the range of 0.1% to 1%, and the Si/C ratio of the region Z of exceeding 100nm is not higher than 0.06%. The invention also provides a method for producing polyacrylonitrile carbon fiber, polyacrylonitrile precursor fiber and a method for producing polyacrylonitrile precursor fiber. The carbon fiber of the present invention has a low ash content and high performances.

Description

Technical Field

The invention relates to polyacrylonitrile carbon fiber and a producing method thereof. The invention further relates to a precursor fiber for polyacrylonitrile carbon fiber for producing the carbon fiber and a producing method thereof.

Backgrounds

Carbon fiber is an inorganic polymer fiber material with a carbon content of equal to or more than 90%, has a stable chemical structure, has the characteristics of high strength, high modulus and low density, and has a plurality of excellent characteristics of good high temperature resistance, good corrosion resistance, low thermal expansion coefficient, excellent heat conductivity, good chemical stability, electric conductive and the like. Carbon fiber can be used as a reinforcing material of a high-performance composite material, and is widely used in the fields of aerospace, building reinforcement, sports goods, automobile structures, wind power blades, photovoltaic industries, medical devices and the like. However, the continuous development of aerospace and high-end equipments also puts new performance requirements on carbon fiber materials, wherein the requirements on carbon fibers are not limited to basic indexes such as strength and modulus. Especially in the applications of the aerospace field, the requirement of ash content is put forward on carbon fiber.
The properties of carbon fiber are mainly affected by defects. A good-quality oiling agent and reasonable oiling procedures are effective means for preventing generation of defects. After PAN precursor fiber is treated by using a specialized oiling agent, a layer of protective film with good heat resistance is formed on the surface layer of the fiber, so that the monofilaments are separated from one another and are prevented from being adhered and doubled in pre-oxidation process. At present, silicon-containing oil agents are mainly used in the production process of high-performance carbon fiber precursor fibers. Part of silicon-containing components in the silicon-containing oiling agent have poor decomposition performance at high temperature, so that the silicon components are not completely decomposed in the pre-oxidation and carbonization processes; and impurities generated such as silicon carbide, silicon nitride and the like are remained in the interior of carbon fibers and can become weak points of the fibers during drawing or bending, thereby seriously affecting the quality of the carbon fibers. And the silicon oxides generated during pre-oxidation and carbonization processes are the main source of ash in these processes. The ash not only pollutes the equipments, resulting in the shortening of the production cycle and service life of the equipments, but also shortens the on-stream time, resulting in the need of frequent shutdown for cleaning the equipments.
There are many precursor fibers that can be used to make carbon fibers. About 90% or more of the carbon fibers currently on the market are made from PAN fibers. PAN-based carbon fibers have the characteristics of high carbon yield, excellent mechanical properties, mature process and the like, and have become the main products of carbon fibers. Currently, PAN fibers are mainly prepared by wet spinning and dry-jet wet spinning. In the process of dry-jet wet spinning, after the solution is extruded from a spinneret, it firstly passes through an air section and then enters a coagulating bath for coagulating and forming; this operation causes great differences between dry-jet wet spinning and wet spinning from solution systems, spinning processes to spinning equipments and the like, and the morphologies and structures of the resulting precursor fibers and carbon fibers are also different. The dry-jet wet-spun carbon fiber has a smooth surface and a more compact structure, while the surface of the wet-spun carbon fiber has obvious groove structures, wherein the height difference between the top of the protrusion and the bottom of the groove can be up to dozens of nanometers, and the concave-protrusion groove structures can act as defects to cause the carbon fibers to break. The main structural features affecting the break in wet-spun and dry-jet wet-spun carbon fibers are therefore also different. In the production processes of PAN fibers, compared with dry-jet wet spinning, the wet spinning process is more mature, the spinning process is stable and easy to control, the residual solvent in the fiber is easy to remove, and the prepared carbon fiber is easier to combine with a composite material, so that the wet spinning process is an important method for producing the high-performance carbon fiber precursor fiber.
A high-quality carbon fiber precursor fiber requires characteristics such as minimal surface defects, minimal pore structures, dense structure, good stretchability, high thermal resistance and the like. To prepare high-quality carbon fibers with excellent properties and controllable silicon content and silicon penetrability, precursor fibers thereof need to have the characteristics of low silicon content, low silicon penetrability and good uniform coating property of oil film on the surfaces of the fibers. In the production process of carbon fiber precursor fibers, the coagulation process is a double diffusion process, so that the nascent fiber contains a plurality of holes. Part of the holes will gradually decrease or even close during the subsequent drawing and water washing processes. The unclosed holes can be infiltrated with silicon-containing oiling agent in the oiling procedure. In the subsequent drying densification procedure, the holes may be closed, making it difficult to completely remove the silicon-containing oiling agent in the holes and leaving it inside the fiber; and in turn more ash is generated in the carbonization procedure, and the penetrated Si is difficult to completely remove and remains in the carbon fiber, thereby affecting the performance of the final carbon fiber.
Therefore, the selection of a high-quality oiling agent and a reasonable oiling procedure is an important operation for producing low-ash high-performance polyacrylonitrile carbon fiber and carbon fiber precursor fiber.
CN113597484A discloses a method for producing carbon fibers in which the penetration of an oiling agent into the surface layer of the fiber and surface voids are suppressed, wherein the Si/C ratio at a certain depth from the fiber surface is calculated using SIMS (secondary ion mass spectrometry). It is explicitly stated in this application that this invention cannot be used to improve the strength of wet spun carbon fibers. In addition, wet spinning generally employs a multistage coagulation process, and the concentration, temperature and dipping time of the coagulation bathes are completely different from those of dry-jet wet spinning, so that the inventive points of dry-jet wet spinning coagulation conditions and residence time in air of CN113597484A are not applicable to the production of wet-spun carbon fiber precursor fibers, and accordingly cannot inhibit the invasion of the oiling agent into the fiber surface layer in the wet spinning process, and cannot inhibit the inter-fiber adhesion and the pores in the carbon fiber surface layer. In addition, compared with dry-jet wet spinning, the wet spun nascent fiber has a looser structure and larger pores in the fiber, and the oiling agent is easier to infiltrate into the interior of the fiber during the oiling process, so that a new method is needed to achieve the purposes of inhibiting Si penetration and improving the performance of the carbon fiber.
CN111088559A discloses a method for oiling wherein the first oiling procedure uses an ultra-low silicon oiling agent or a silicon-free oiling agent, and after a first drying densification, the second oiling procedure uses a general silicon-containing oiling agent, which is used for solving the problems of excessive carbon fiber broken filaments, poor mechanical properties and large coke discharge of low-carbon furnace.
CN114622417A discloses a silicon-containing oiling agent for carbon fibers. The oiling agent can reduce friction damage between precursor fibers and rollers, ensure homogeneous pre-oxidation of fibers, and inhibit adhesion and doubling of carbonized monofilaments, but is silent about the problems of ash of carbonization and impurities in carbon fiber.
CN112424418A discloses a silicon-containing oiling agent for carbon fiber. This application solves the fuzzing problem during the spinning process of precursor fiber, and can effectively protect the carbon fiber, but the resulting carbon fiber has a low strength, and is silent about the problems of ash of carbonization and impurities in carbon fiber either.
CN112726207A discloses a silicon-free oiling agent for carbon fiber precursor fiber production, which causes very little ash, and greatly reduces the damage to equipments such as carbonization furnaces and the like, but the resulting carbon fiber has a low strength and modulus, and is not suitable for the production process of high-strength and high-modulus carbon fiber.
CN110863270A discloses a method for reducing ash of high-strength polyacrylonitrile-based carbon fibers, which comprises the steps of subjecting the polyacrylonitrile-based carbon fibers to an organic solvent impregnation treatment and a hydrofluoric acid treatment, but this method adversely affects the properties of the carbon fibers, such as tensile modulus.
CN103290527A discloses a method for reducing ash of polyacrylonitrile-based carbon fibers. This application prepares a PAN spinning solution with a higher hydrophilicity on the basis of a quaternary ammoniation modified copolymerization system. The oil content of the strand is then controlled by controlling the degree of swelling before oiling of the precursor fibers and by introducing a low silicon oiling agent. However, the modified copolymerization system of this application is not suitable for preparing high performance carbon fiber.
In the prior art, in the wet spinning preparation of high-performance carbon fibers, the reduction of ash of the carbon fibers often leads to the reduction of the performance of carbon fibers.

Disclosure of Invention

The inventors of present invention have found through in-depth research that a large-amount of penetration and distribution of oiling agent in the fibers is the main cause for the excessive ash in the subsequent pre-oxidation and carbonization processes, and it would also lead to a high impurity content in the final carbon fibers, impairing the performance of the carbon fibers. On the other hand, if the Si content in the surface layer of the precursor fibers is too low, problems such as doubling, adhesion, excessive broken filaments and the like will easily occur during the pre-oxidation process, finally decreasing the performance of the carbon fibers. The inventors of present invention have found through further in-depth research that the controlling of the penetration depth and gradient of silicon in the carbon fibers can balance well the high performance and low ash of the carbon fibers, thereby obtaining carbon fibers with both low ash and high performance, thereby completing the present invention.
The first technical problem solved by the invention is the problem that the reducing of ash of carbon fibers in wet spinning in prior art results in low mechanical properties of the finally produced carbon fibers. The invention accordingly provides a low-ash high-performance polyacrylonitrile carbon fiber, and in the carbon fiber, silicon has a specific penetration gradient in the fiber surface and has a specific silicon content, so that the mechanical properties of the carbon fiber can be improved while the ash of the carbon fiber can be reduced.
The second technical problem solved by the present invention is to provide a method for preparing low-ash high-performance carbon fiber corresponding to the solving of above first technical problem. The method has the characteristic of low ash (requiring shutdown for cleaning ash in the equipments only once for half a year or longer) in the carbonization process, and the prepared carbon fiber has a low silicon impurity content and the carbon fiber has good strength, modulus and elongation.
The third technical problem solved by the present invention is to provide a polyacrylonitrile precursor fiber for preparing the low-ash high-performance carbon fiber corresponding to the solving of above first technical problem.
The fourth technical problem solved by the present invention is to provide a producing method for polyacrylonitrile precursor fiber corresponding to the solving of above third technical problem.
The invention provides a polyacrylonitrile carbon fiber having groove structures on the surface thereof, characterized in that in the carbon fiber, the Si/C ratio of a region X between the top of the protrusion of the fiber surface and the bottom of the groove is in the range of 1% to 10%, the Si/C ratio of a region Y between the bottom of the groove and the inner 100nm is in the range of 0.1% to 1%, and the Si/C ratio of a region Z of exceeding 100nm is not higher than 0.06%.
The invention also provides a method for producing polyacrylonitrile carbon fiber, preferably the polyacrylonitrile carbon fiber of the present invention, including a step of carbonizing a polyacrylonitrile precursor fiber to obtain the polyacrylonitrile carbon fiber; characterized in that in the polyacrylonitrile precursor fiber, the Si/C ratio of a region between the top of the protrusion of fiber surface and the inner 2 µm is in the range of from 1.0% to 20%, and the Si/C ratio of a region of exceeding 2 µm is not higher than 0.15%.
The invention provides a polyacrylonitrile precursor fiber, which is characterized in that in the polyacrylonitrile precursor fiber, the Si/C ratio of a region between the top of the protrusion of fiber surface and the inner 2 µm is in the range of from 1.0% to 20%, and the Si/C ratio of a region of exceeding 2 µm is not higher than 0.15%.
The invention also provides a method for producing polyacrylonitrile precursor fiber, preferably the polyacrylonitrile precursor fiber of present invention, which includes wet spinning and includes an oiling step, wherein the oiling step comprises two oiling procedures in succession and no drying densification stage is arranged between the two oiling procedures; wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the second oiling procedure uses a silicon-containing oiling agent.

Brief Description of Drawings

In order to more clearly illustrate the technical solutions of the present invention, the accompanying drawing is provided.
FIG. 1 is a schematic cross-sectional view of an individual fiber of the carbon fiber of the present invention.
In Fig. 1, 1 is a protrusion structure on the surface of the carbon fiber; 2 is a groove structure on the surface of the carbon fiber; a and b are scanning lines from the center of the fiber to adjacent protrusion structure and groove structure on the surface of the fiber respectively; X represents the region between the top of the protrusion of the carbon fiber surface and the bottom of the groove; Y represents the region between the bottom of the groove and the inner 100 nm of the carbon fiber (i.e., the depth of this region is 100 nm); and Z represents the region of exceeding 100 nm inside the carbon fiber.
In the present invention, the top of a protrusion refers to the highest point of the protrusion structure; and the bottom of a groove refers to the lowest point of the groove structure.

Detailed Description

In one aspect, the present invention provides a polyacrylonitrile carbon fiber, which has groove structures on its surface, wherein the Si/C ratio of the carbon fiber of a region X between the top of the protrusion of the fiber surface and the bottom of the groove is in the range of 1% to 10%, the Si/C ratio of a region Y between the bottom of the groove and the inner 100nm is in the range of 0.1% to 1%, and the Si/C ratio of a region Z of exceeding 100nm (from the bottom of the groove on the surface) is not higher than 0.06%.
In the invention, the Si/C ratio of the carbon fiber of the region X between the top of the protrusion of the fiber surface and the bottom of the groove is in the range of 1% to 10%. For example, said Si/C ratio can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 9.5%, 10%, or a range formed by any two of the foregoing values. In some embodiments, the Si/C ratio can range from 2% to 10%, from 3% to 10%, from 4% to 10%, or from 4% to 9.5%.
In the invention, the Si/C ratio of the carbon fiber of the region Y between the bottom of the groove and the inner 100nm is in the range of 0.1% to 1%. For example, said Si/C ratio can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 0.95%, 1.0%, or a range formed by any two of the foregoing values. In some embodiments, the Si/C ratio can range from 0.2% to 1%, from 0.3% to 1%, from 0.4% to 1%, or from 0.4% to 0.95%.
In the present invention, the Si/C ratio of the carbon fiber of the region Z of exceeding 100nm (from the bottom of the groove on the surface) is not higher than 0.06%. For example, said Si/C ratio can be 0%, 0.005%, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, or a range formed by any two of the foregoing values, such as 0% to 0.06%. In some embodiments, the Si/C ratio can range from 0.005% to 0.06%, or from 0.01% to 0.06%.
In the present invention, the Si/C ratio is the ratio of the number of Si atoms to the number of C atoms. In the present invention, the Si/C ratio is determined by means of TEM-EDS; see below for details.
In the present invention, the carbon fiber can have a total silicon content of 500 to 1500 ppm (by weight). For example, the carbon fiber can have a total silicon content of 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, or a range formed by any two of the foregoing values, by weight.
In some embodiments of the present invention, in the carbon fiber, the Si/C ratio of the region X between the top of the protrusion of the fiber surface and the bottom of the groove is not less than 8.5 times the Si/C ratio of the region Y between the bottom of the groove and the inner 100 nm. In some embodiments of the present invention, in the carbon fiber, the Si/C ratio of the region X between the top of the protrusion of the fiber surface and the bottom of the groove is 8.5 times to 12 times the Si/C ratio of the region Y between the bottom of the groove and the inner 100 nm.
In some embodiments of the present invention, for the carbon fiber of the present invention, the number of pores present in the regions X and Y, i.e. between the top of the protrusion of the fiber surface and said inner 100 nm, is less than 20, preferably less than 16; wherein the pores have an average length of 10 to 50nm and an average width of 5 to 25 nm. In the present invention, 10 carbon fibers are examined by gallium focused ion beam cutting and transmission electron microscopy test, and the above number of pores as well as average length and average width of pores are the average of the 10 carbon fibers.
In some embodiments of the invention, the orientation deviation angle along the axial direction of the fiber of the pores obtained by small-angle X-ray scattering method is less than or equal to 4°, preferably less than or equal to 3°.
For the carbon fiber, the tensile strength can be 5.6-5.8 GPa, the tensile modulus can be 360-390 GPa, and the elongation at break can be 1.45-1.6%.
In the invention, the polyacrylonitrile carbon fiber is produced from polyacrylonitrile precursor fiber prepared by wet spinning.
In another aspect, the present invention provides a method for producing the polyacrylonitrile carbon fiber of the present invention, which includes a step of carbonizing a polyacrylonitrile precursor fiber to obtain the polyacrylonitrile carbon fiber; wherein the Si/C ratio of a region between the top of the protrusion of fiber surface and the inner 2 µm of the polyacrylonitrile precursor fiber is in the range of from 1.0% to 20%, and the Si/C ratio of a region of exceeding 2 µm is not higher than 0.15%.
In some embodiments, the Si/C ratio of the polyacrylonitrile precursor fiber of the region between the top of the protrusion of fiber surface and the inner 2 µm can range from 2.0% to 20%, range from 3.0% to 20%, range from 4.0% to 20%, or range from 5.0% to 19%. In some embodiments, the Si/C ratio of the polyacrylonitrile precursor fiber of the region, from the top of the protrusion of the fiber surface, of the inner exceeding 2 µm can range from 0.01% to 0.15%, range from 0.02% to 0.15%, range from 0.03% to 0.15%, range from 0.04% to 0.15%, or range from 0.05% to 0.15%.
In some embodiments, the oil content of the precursor fiber can be from 0.5% to 2.5%, preferably from 0.8% to 2.5%, preferably from 0.8% to 2.4%, more preferably from 0.9% to 2.0%. For example, the oil content of the precursor fiber can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, or a range formed by any two of the foregoing values.
In some embodiments, the silicon content of the precursor fiber can be from 0.01% to 0.5%, preferably from 0.05% to 0.5%. For example, the silicon content of the precursor fiber can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, or a range formed by any two of the foregoing values.
In some embodiments, the polyacrylonitrile precursor fiber has a monofilament linear density of 0.7 to 1.0 dtex. For example, the polyacrylonitrile precursor fiber can have a monofilament linear density of 0.7dtex, 0.8dtex, 0.9dtex, 1.0dtex, or a range formed by any two of the above-mentioned values.
In the invention, the polyacrylonitrile precursor fiber is prepared by wet spinning.
In the invention, the polyacrylonitrile precursor fiber is prepared by wet spinning and includes an oiling step, wherein the oiling step comprises two oiling procedures in succession, wherein no drying densification stage is arranged between the two oiling procedures; wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the second oiling procedure uses a silicon-containing oiling agent.
In the invention, no drying densification stage being arranged between the two oiling procedures means that after the first oiling procedure is completed, the fiber resulting from the first oiling treatment is not subjected to a drying densification treatment, but is continuously subjected to the second oiling procedure.
In the preparation of carbon fiber precursor fiber, it is known in the art to apply an oiling agent onto the fiber by an oiling step. As is known in the art, the oiling agents used in the oiling step can include silicon-free oiling agents, low-silicon oiling agents, and silicon-containing oiling agents.
As for the silicon-free oiling agent, the invention has no particular limitations on the silicon-free oiling agent, and various silicon-free oiling agents generally used in the preparation of polyacrylonitrile precursor fiber in the art can be used.
In some embodiments, the silicon-free oiling agent comprises a non-silicone ingredient as the major ingredient of the oiling agent. For example, one or more of the following substances can be used as the main ingredient(s) of the silicon-free oiling agent: polyesters of long-chain fatty acids and polyhydric alcohols, ethylene oxide adducts of long-chain fatty amides, polybutenes, polyoxyethylene ethers, neopentyl alcohol derivatives, fatty acid esters, amide compounds, ammonium salts of fatty acid esters, aromatic esters, amide compounds, and the like.
As for the silicon-containing oiling agent, the invention has no particular limitations on the silicon-containing oiling agent, and various silicon-containing oiling agents used for the preparation of polyacrylonitrile precursor fiber in the art can be used. In some embodiments, the silicon-containing oiling agent comprises an organopolysiloxane and/or a derivative thereof. As examples, the following can be mentioned: polydimethylsiloxane, polyphenylmethylsiloxane, polymethylhydrosiloxane, alkylarylalkyl-modified polysiloxane, amino-modified polysiloxane, polyether-modified polysiloxane, epoxy-modified polysiloxane, amide-modified polysiloxane, and the like.
As for the low-silicon oiling agent, the invention has no particular limitations on the low-silicon oiling agent, and various low-silicon oiling agents used in the preparation of polyacrylonitrile precursor fiber in the art can be used.
In the present invention, the silicon content of the silicon-containing oiling agent can be 0.01% to 0.9% by weight, and the silicon content of the low-silicon oiling agent can be 0.001% to less than 0.01% by weight. In some embodiments, the silicon-containing ingredient(s) in the low-silicon oiling agent comprises less than or equal to 1 wt% of the total ingredients in the oiling agent.
In the present invention, the form of the oiling agent is not particularly limited. In the present invention, preferably, the oiling agent used is an oil-in-water emulsion.
In some embodiments, the silicon-free oiling agent can be an oil-in-water emulsion comprising oiling agent ingredients and water, wherein the content of the oiling agent ingredients is 1% to 40% by weight; the oiling agent ingredients comprise one or more of an aromatic ester compound, an aromatic polyoxyethylene ether, an amine compound, an alcohol compound and an acid compound, and further comprise a nonionic surfactant, an emulsifier, an antioxidant and an antistatic agent; wherein, calculated by the mass sum of the oiling agent ingredients being 100%, the content of the aromatic ester compound(s) is 40%-90%, the content of the aromatic polyoxyethylene ether(s) is 10%-30%, the content of the amine compound(s) is 0%-7%, the content of the alcohol compound(s) is 0%-7%, the content of the acid compound(s) is 0%-7%, the content of the nonionic surfactant(s) is 10%-65%, the content of the emulsifier(s) is 10%-30%, the content of the antioxidant(s) is 1%-10%, and the content of the antistatic agent(s) is 1%-10%.
In some embodiments, the silicon-containing oiling agent can be an oil-in-water emulsion comprising oiling agent ingredients and water, wherein the content of the oiling agent ingredients is 1% to 40% by weight; the oiling agent ingredients comprise a dimethyl siloxane, a modified siloxane, a nonionic surfactant, an emulsifier, an antioxidant and an antistatic agent; wherein, calculated by the mass sum of the oiling agent ingredients being 100%, the content of the dimethyl siloxane(s) is 10-50%, the content of the modified siloxane(s) is 10-75%, the content of the nonionic surfactant(s) is 10-65%, the content of the emulsifier(s) is 10-30%, the content of the antioxidant(s) is 1-10%, and the content of the antistatic agent(s) is 1-10%.
In some embodiments, the low-silicon oiling agent is an oil-in-water emulsion comprising oiling agent ingredients and water, wherein the content of the oiling agent ingredients is 1% to 40% by weight; the oiling agent ingredients comprise a heat-resistant ester compound, a modified siloxane, a nonionic surfactant, an emulsifier, an antioxidant and an antistatic agent; wherein, calculated by the mass sum of the oiling agent ingredients being 100%, the content of the heat-resistant ester compound(s) is 10-70%, the content of the modified siloxane(s) is 0.1-1%, the content of the nonionic surfactant(s) is 10-65%, the content of the emulsifier(s) is 10-30%, the content of the antioxidant(s) is 1-10%, and the content of the antistatic agent(s) is 1-10%.
In the present invention, the oiling agent ingredients includes all substances except water in the oiling agent.
The concentration of the oiling agent in the present invention is not particularly limited. In the invention, the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the weight concentration thereof can be 0.1-5.0%. For example, the concentration by weight of the silicon-free oiling agent, the low-silicon oiling agent or a combination thereof used in the first oiling procedure can be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, or a range formed by any two of the above-mentioned values. In the invention, the second oiling procedure uses a silicon-containing oiling agent, and the concentration by weight of the silicon-containing oiling agent can be 0.1-5.0%. For example, the concentration by weight of the silicon-containing oiling agent used in the second oiling procedure can be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, or a range formed by any two of the above-mentioned values. In the present invention, the concentration by weight of the oiling agent means the ratio of the weight of all oiling agent ingredients in the oiling agent to the weight of the oiling agent.
In some embodiments, preferably, the average particle sizes of the silicon-free oiling agent, the low-silicon oiling agent and the silicon-containing oiling agent can be each independently 50 nm-500 nm. For example, the average particle size of the silicon-free oiling agent, the low-silicon oiling agent and the silicon-containing oiling agent can be 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, or a range formed by any two of the above-mentioned values, such as 100nm to 400nm, and the like.
In the present invention, the silicon content of the silicon-containing oiling agent used in the second oiling procedure can be 0.01-0.9% by weight. For example, the silicon-containing oiling agent can have a silicon content by weight of 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or a range formed by any two of the foregoing values.
In the present invention, there is no particular limitations to the method for applying the oiling agent to the fiber, and various methods generally known in the art can be used. For example, a dipping method, a roll dipping method, a spraying method, or the like can be used.
In the present invention, the pH values of the silicon-free oiling agent, the low-silicon oiling agent and the silicon-containing oiling agent are not particularly limited, and those skilled in the art can appropriately determine and select the pH values of the silicon-free oiling agent, the low-silicon oiling agent and the silicon-containing oiling agent. In some embodiments, the pH difference between the silicon-free oiling agent or low-silicon oiling agent and the silicon-containing oiling agent can be not more than 1. In some embodiments, the pH difference between the silicon-free oiling agent or low-silicon oiling agent and the silicon-containing oiling agent can be greater than or equal to 0 and less than or equal to 1.
In the present invention, in some embodiments, the silicon-free oiling agent, the low-silicon oiling agent and the silicon-containing oiling agent each comprise a surfactant, wherein the polarity of the surfactant in the silicon-free oiling agent or the low-silicon oiling agent is not opposite to the polarity of the surfactant in the silicon-containing oiling agent. In the present invention, surfactants that can be used in the oiling agent include cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants. In the present invention, the opposite polarity means that the surfactant in the silicon-free oiling agent or the low-silicon oiling agent comprises a cationic surfactant and the surfactant in the silicon-containing oiling agent comprises an anionic surfactant, or the surfactant in the silicon-free oiling agent or the low-silicon oiling agent comprises an anionic surfactant and the surfactant in the silicon-containing oiling agent comprises a cationic surfactant; any situation other than the above two situations is considered to be a situation wherein the polarities are not opposite. Various surfactants known in the art can be used as long as the above requirements for polarity are met. In some embodiments, the silicon-containing oiling agent preferably comprises a nonionic surfactant.
In the present invention, there is no particular limitations on the temperature of the oiling agent during oiling, and the oiling agent temperature commonly used in the art can be used. In some embodiments, the temperature of the oiling agent during oiling can be 20-30°C.
The time required for oiling is not particularly limited in the present invention, and a time generally used in the art can be used. In some embodiments, the time for first oiling procedure, e.g., the residence time in the oiling tank, can be 0.05 to 0.5 seconds, preferably 0.06 to 0.4 seconds, more preferably 0.07 to 0.3 seconds. In some embodiments, the time for second oiling procedure, e.g., the residence time in the oiling tank, can be from 0.03 to 0.3 seconds, preferably from 0.04 to 0.3 seconds, more preferably from 0.04 to 0.2 seconds.
In some embodiments, the residence time in the second oiling procedure is less than the residence time in the first oiling procedure.
As is known in the art, a drying densification treatment is performed after oiling. In the present invention, there is no particular limitations on the drying densification treatment, and drying densification treatments known in the art can be used. In some embodiments, the drying densification treatment can include a multi-stage drying densification treatment. In some embodiments, the drying densification treatment can include a four-stage drying densification treatment at temperatures of 90 °C, 100 °C, 115 °C and 130 °C, respectively.
In some embodiments, before oiling, the fiber swelling degree of the polyacrylonitrile precursor fiber after water washing can be from 80% to 150%. For example, the fiber swelling degree of the water-washed polyacrylonitrile precursor fiber before oiling can be 80%, 90%, 100%, 110%, 120%, 130%, 140%, or a range formed by any two of the above-mentioned values, for example, the swelling degree can be 90% to 140%.
In some embodiments, the producing method can include the steps of wet coagulation forming of polyacrylonitrile stock solution, coagulation drawing, hot water drawing, water washing, oiling, drying densification, steam drawing and steam heat setting to obtain the polyacrylonitrile precursor fiber.
The preparation of precursor fibers from a polyacrylonitrile stock solution is generally known in the art. In the present invention, the precursor fibers are prepared by using a wet spinning process, which is known in the art.
As for the preparation of polyacrylonitrile precursor fiber from a polyacrylonitrile stock solution, the preparation process can include the steps of coagulation forming, coagulation drawing, drawing such as hot water drawing, washing such as water washing, oiling, drying densification, drawing such as steam drawing and heat setting such as steam heat setting. These steps are known in the art, and those skilled in the art can appropriately select and carry out these steps.
As mentioned above, in the method of the present invention, the oiling step comprises two oiling procedures in succession and no drying densification stage is arranged between the two oiling procedures; wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the second oiling procedure uses a silicon-containing oiling agent.
In some embodiments, in the methods of the present invention, as for the coagulation forming step, the nascent fiber exits the spinneret orifice and enters into the first stage coagulation bath to coagulate the nascent fiber. The composition and temperature of the first stage coagulation bath can be readily selected by one skilled in the art. In some embodiments, the temperature of the first stage coagulation bath is 28°C and the first stage coagulation bath is 51.5% dimethyl sulfoxide aqueous solution. After exiting the first stage coagulation bath, the strand can be further passed through one or more subsequent coagulation baths for further coagulation and drawing. In some embodiments, after leaving the first stage coagulation bath, the strand is further subjected to three stages of coagulation drawing, at concentrations of 30%, 20%, 10%, at temperatures of 30 °C, 40 °C and 50 °C and at drawing ratios of 1.0, 1.1 and 1.2, respectively. In some embodiments, the coagulated strand can be drawn after exiting the last stage coagulation bath. The drawing is carried out, for example, in steam or hot water. The drawing ratios can be appropriately selected by those skilled in the art. In some embodiments, four stages of hot water drawing can be performed at temperatures of 95 °C, 96 °C, 97 °C and 99 °C and drawing ratios of 1.35, 1.50, 1.70 and 2.00, respectively. After drawing, the strand can be washed, for example with water, to remove residual solvent. In some embodiments, six stages of water-washing process can be used, at temperatures of 70 °C, 70 °C, 80 °C, 80 °C, 90 °C and 90 °C, respectively. The swelling degree of the fiber after water washing can be 80-140%. In the method of the present invention, after washing, the strand can be oiled and drying densified. In the present invention, as mentioned above, the oiling comprises two oiling procedures in succession and no drying densification stage is arranged between the two oiling procedures; wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the second oiling procedure uses a silicon-containing oiling agent. The oiling can be performed using oiling agents generally used in the art and the drying densification can be performed using drying densification operations generally used in the art. In some embodiments, the drying densification treatment includes four stages of drying densification at temperatures of 90 °C, 100 °C, 115 °C and 130 °C, respectively. In some embodiments, after drying densification, the resulting strand can be subjected to a further drawing, such as steam drawing. The drawing ratios and the drawing conditions can be appropriately selected by those skilled in the art. In some embodiments, steam drawing is used, wherein the drawing ratio is 3.0 times and the steam pressure is 0.35 MPa. In some embodiments, after drawing, the resulting strand can be heat-set. For example, the heat-setting can be performed by passing the strand through multi-stage hot rolls, or steam heat-setting can be employed. After heat setting, precursor fibers are obtained and the precursor fibers can be rolled up for subsequent use.
As is well known to those skilled in the art, one or more of the above steps can be omitted or added or the order of these steps can be adjusted.
In the present invention, the monofilament linear density of the polyacrylonitrile precursor fiber is not particularly limited, and one skilled in the art can suitably select and determine the monofilament linear density. In some embodiments, the polyacrylonitrile precursor fibers can have a monofilament linear density of 0.7 to 1.0 dtex. For example, the polyacrylonitrile precursor fiber can have a monofilament linear density of 0.7dtex, 0.8dtex, 0.9dtex, 1.0dtex, or a range formed by any two of the foregoing values.
After the precursor fiber is obtained, the precursor fiber can be prepared into polyacrylonitrile carbon fiber. Processes for making polyacrylonitrile carbon fibers from polyacrylonitrile precursor fibers are known in the art. The present invention can use processes known in the art to prepare polyacrylonitrile carbon fibers from precursor fibers.
In the present invention, the polyacrylonitrile precursor fiber can be subjected to pre-oxidation treatment and carbonization treatment to obtain the polyacrylonitrile carbon fiber.
In some embodiments, the pre-oxidation treatment is carried out in air atmosphere, at a temperature of 170 to 300 °C and at a total drawing ratio of not more than 5%.
In some embodiments, the pre-oxidation treatment can be performed in air atmosphere using a gradient temperature ramp method in multiple temperature zones, for example, 2 to 6 temperature zones. The initial temperature of the pre-oxidation treatment can be 170-200 °C, and the final temperature of the pre-oxidation treatment can be 260-300 °C. During the pre-oxidation treatment, the fibers can be stretched to a certain extent, for example with a drawing ratio of 0-5%. The pre-oxidation treatment time can be 30 to 120 minutes.
In some embodiments, the carbonization treatment comprises a low temperature carbonization treatment at a temperature of 300-750 °C and a total drawing ratio of 0-4% in an inert atmosphere and a high temperature carbonization treatment at a temperature of 800-1500 °C and a total drawing ratio of -4% to -2% in an inert atmosphere. The inert atmosphere can be achieved, for example, by using nitrogen, helium, argon or xenon.
After the carbonization treatment, optionally, a graphitization treatment can be performed. In the present invention, the graphitization treatment is not particularly limited, and graphitization treatments known in the art can be used. In some embodiments, the graphitization treatment can be performed at 2800 °C.
In some embodiments, the producing method can further include a surface treatment step, a water washing step, and a sizing step.
As known to those skilled in the art, the carbon fiber of the present invention can be surface-treated before the sizing treatment. For example, an oxidation treatment can be performed to improve the affinity and adhesion of the carbon fibers and the matrix resin in a composite.
As for the sizing treatment, various sizing treatments known in the art can be employed. The sizing treatment processes, sizing agents and sizing conditions in the present invention are not particularly limited as long as a desired sizing agent can be applied to carbon fibers. After the sizing treatment, a drying treatment can be performed to remove the solvent or dispersion medium used during the sizing treatment. For example, drying can be carried out at 120 °C. Finally, the fibers can be rolled up to obtain carbon fibers.
In still another aspect, the present invention provides a polyacrylonitrile precursor fiber, wherein the Si/C ratio of a region between the top of the protrusion of fiber surface and the inner 2 µm of the polyacrylonitrile precursor fiber is in the range of from 1.0% to 20%, and the Si/C ratio of a region of exceeding 2 µm is not higher than 0.15%.
In some embodiments, the Si/C ratio of the polyacrylonitrile precursor fiber of the region between the top of the protrusion of fiber surface and the inner 2 µm can be in the range of 2.0% to 20%, in the range of 3.0% to 20%, in the range of 4.0% to 20%, or in the range of 5.0% to 19%. In some embodiments, the Si/C ratio of the region, from the top of the protrusion of fiber surface, of the inside exceeding 2 µm of the polyacrylonitrile precursor fiber can be in the range of 0.01% to 0.15%, in the range of 0.02% to 0.15%, in the range of 0.03% to 0.15%, in the range of 0.04% to 0.15%, or in the range of 0.05% to 0.15%.
In some embodiments, the oil content of the precursor fiber can be 0.5%-2.5%, preferably 0.8%-2.5%, preferably 0.8%-2.4%, more preferably 0.9%-2.0%. For example, the precursor fiber can have an oil content of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, or a range formed by any two of the foregoing values.
In some embodiments, the silicon content of the precursor fiber can be from 0.01% to 0.5%, preferably from 0.05% to 0.5%. For example, the silicon content of the precursor fiber can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, or a range formed by any two of the foregoing values.
In some embodiments, the polyacrylonitrile precursor fiber has a monofilament linear density of 0.7 to 1.0 dtex. For example, the polyacrylonitrile precursor fiber can have a monofilament linear density of 0.7dtex, 0.8dtex, 0.9dtex, 1.0dtex, or a range formed by any two of the foregoing values.
In the present invention, the polyacrylonitrile precursor fiber of present invention is prepared by wet spinning.
In an aspect, the invention provides a method for producing polyacrylonitrile precursor fiber of the present invention, which includes wet spinning and includes two oiling procedures in succession with no drying densification stage(s) arranged between the two oiling procedures; wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the second oiling procedure uses a silicon-containing oiling agent. In the present invention, the absence of drying densification stage(s) between the two oiling procedures means that after the first oiling procedure is completed, the precursor fiber treated by the first oiling procedure is not subjected to drying densification treatment(s), but is continuously treated by the second oiling procedure.
Various aspects and features of the method for preparing the polyacrylonitrile precursor fibers of the present invention are the same as said various aspects and features related to the preparation of the precursor fibers in the above section regarding the method for producing polyacrylonitrile carbon fibers, and therefore are not repeated here.
In the present invention, the polyacrylonitrile precursor fiber is prepared from a polyacrylonitrile stock solution. In the present invention, the polyacrylonitrile stock solution comprises a polyacrylonitrile polymer and a solvent. The concentration of polyacrylonitrile polymer in the polyacrylonitrile stock solution is not particularly limited, and any suitable concentrations commonly known in the art for polyacrylonitrile spinning stock solutions can be used. In the present invention, for example, the concentration of the polyacrylonitrile polymer in the polyacrylonitrile spinning stock solution can be 15 to 30% by weight. In the present invention, various solvents for the polyacrylonitrile stock solution can be used. For example, the solvent can be at least one of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAC), and a mixture thereof.
In the present invention, the polyacrylonitrile stock solution can be obtained by dissolving a polyacrylonitrile polymer in a solvent, or can be obtained by polymerizing a monomer in a solvent.
Both the polyacrylonitrile precursor fiber of present invention or the polyacrylonitrile precursor fiber prepared by the precursor fiber preparation method of present invention can be used for preparing the low-ash high-performance polyacrylonitrile carbon fiber of present invention.
In the present invention, except for the limitations on oiling and densification given above in this application, there are no special limitations on other aspects of the preparation of the polyacrylonitrile-based precursor fiber, and the precursor fiber can be prepared by using a polyacrylonitrile spinning stock solution commonly used or known in the art through a spinning process commonly used or known in the art, and there are no particular requirements for the polyacrylonitrile spinning stock solution and the spinning process. In the present invention, except for the limitations on oiling and densification given above in present application, the precursor fiber can be prepared from a polyacrylonitrile spinning stock solution using commonly known methods and process parameters for preparing precursor fiber. Compared with the production of polyacrylonitrile carbon fibers in prior art, the present invention can achieve the aims of improving the mechanical properties of the fibers and reducing ash.
In the present invention, the polyacrylonitrile polymer in the polyacrylonitrile stock solution can include polyacrylonitrile homopolymer, polyacrylonitrile copolymer or a mixture thereof. In the present invention, various polyacrylonitrile homopolymers, polyacrylonitrile copolymers or mixtures thereof for preparing carbon fibers can be used. For example, when a polyacrylonitrile copolymer is used, the comonomer can be a vinyl-containing monomer. In some embodiments, the comonomer is preferably one or more of acrylates, vinyl esters, acrylamides, sulfonates, and ammonium salt monomers. In some embodiments, the polyacrylonitrile polymer can contain 90 to 100 % by mass of monomer unit derived from acrylonitrile and 0 to 10 % by mass of structural unit(s) derived from monomer(s) copolymerizable with acrylonitrile. As the monomer copolymerizable with acrylonitrile, various comonomers generally known in the art can be used, including, for example, acrylic acid, methacrylic acid, itaconic acid, and alkali metal salts, ammonium salts and lower alkyl esters thereof; acrylamide and derivatives thereof; allyl sulfonic acid, methallyl sulfonic acid, and salts or alkyl esters thereof; and the like.
The inventors of present invention surprisingly found that by controlling the penetration gradient of silicon in the surface of the fiber and the content of silicon, ash of the carbon fiber can be reduced and mechanical properties of the carbon fiber can be improved. In the present invention, the problems in the prior art can be solved by controlling, in the carbon fiber, the Si/C ratio of the region X between the top of the protrusion of the fiber surface and the bottom of the groove to be in the range of 1% to 10%, the Si/C ratio of the region Y between the bottom of the groove and the inner 100nm to be in the range of 0.1% to 1%, and the Si/C ratio of the region Z of exceeding 100nm to be not higher than 0.06%.
On the other hand, the method of present invention, comprising two oiling procedures in succession, wherein no drying densification stage is arranged between the two oiling procedures, and wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof and the second oiling procedure uses a silicon-containing oiling agent, can achieve the controlling the penetration gradient of silicon in the surface of the fiber and the content of silicon, so that the precursor fiber for carbon fiber and the carbon fiber with low silicon content and good silicon penetration gradient can be produced. The present invention can achieve a long operation period and can produce carbon fibers having extremely little ash and excellent properties such as strength, modulus, etc.
In the present invention, the degree of swelling is determined in the following way:
taking about 10 g of wet water-washed fibers and washing in flowing deionized water for 30min, putting the fibers uniformly into a 50 ml centrifuge tube, and then putting the centrifuge tube into a centrifugal dehydrator (3000r/min, 15min) to dehydrate the fibers to remove water attached to the surfaces of the fibers and water trapped between the fibers. After the centrifugation, the dewatered fibers are obtained and the mass thereof is weighed and recorded as W. The dewatered fibers are then dried in a hot air drying cabinet at 110 °C for 2h, then cooled to room temperature in a desiccator; and the mass thereof is weighed and recorded as W₀. Swelling degree (wt%) = (W-W₀)/W₀×100%.
In the present invention, the silicon contents of the precursor fiber and the carbon fiber are determined in the following way:
putting 5 g of a sample into a quartz crucible, putting the quartz crucible into a muffle furnace, heating to 400 °C for 2 hours, and then heating to 960 °C for 3 hours; transferring the ash content in the crucible to a polytetrafluoroethylene test tube, adding hydrochloric acid, nitric acid and hydrofluoric acid at concentrations of 37%, 68% and 50%, respectively, in volumes of 3 ml, 1ml and 1ml, respectively, and heating at 120 °C for 5 hours, followed by quantitative determination using ICP-OES inductively coupled plasma emission spectrometry to determine the Si content in the sample.
In the present invention, the silicon content in oiling agent is determined in the following way:
adding 0.5 g of oiling agent into a quartz crucible, putting the quartz crucible into a muffle furnace, heating to 90 °C for 2 hours, then heating to 150 °C for 0.5 hours, continuing to heat to 300 °C for 0.5 hour, and finally heating to 400 °C for 2 hours; transferring the ash content in the crucible to a polytetrafluoroethylene test tube, adding hydrochloric acid, nitric acid and hydrofluoric acid at concentrations of 37%, 68% and 50%, respectively, in volume of 3 ml, 1ml and 1ml, respectively, heating at 120 °C for 5 hours, then diluting by 100 times with water, and quantitatively determining using ICP-OES inductively coupled plasma emission spectrometry to determine the Si content in the sample.
In the present invention, the linear density is determined in the following way:
a hand-cranked yarn linear density tester being used, winding precursor fibers with K number of N straightly onto hand-cranked roller, wherein the circumference of the hand-cranked roller is 1 meter, and the precursor fibers being wound for 10 circles; weighing the weight (unit g) of the fibers and recorded as W; and the linear density (dtex) = W/N; measurement is carried out for 10 times, and the average value is taken.
In the present invention, the mechanical properties of the carbon fiber precursor fibers, including tensile strength, initial modulus and elongation at break, are measured according to GB/T 14337-2008; the mechanical properties of the carbon fibers, including tensile strength, tensile modulus and elongation at break, are measured according to GB/T 3362-2017.
In the present invention, the particle size of the oiling agent (emulsion) is measured by laser particle size method. The particle size is determined by using laser particle size analyzer - Mastersizer 2000 Malvern, UK - in the present invention.
In the present invention, the Si/C ratio and the number and size of pores in the fiber are determined in the following way:
the fiber is cut into a slice of thickness of 100nm by low-temperature FIB (gallium ion focused ion beam) and the colorless, transparent pores are observed in regions X and Y between the top of the protrusion of the fiber surface and the inner 100 nm with TEM (transmission electron microscope), wherein the length being the longer side and the width being the shorter side. By using a TEM-EDS module in combination, 50 scanning lines are taken from the center of the fiber to the outer surface of the fiber along the circumferential direction, wherein 25 lines are taken from the center of the fiber to the tops of the protrusions on the surface of the fiber and 25 lines are taken from the center of the fiber to the bottoms of the adjacent grooves on the surface of the fiber; and by taking one point every 20 nm, the Si/C atomic number ratios at different depths from the center of the fiber to the surface are obtained by scanning. The average of the Si/C atomic number ratios of the region X between the top of the protrusion of the fiber surface and the bottom of the groove is calculated by using the data of the X regions of the scanning lines from the center of the fiber to the tops of the protrusions on the fiber surface (as indicated by the scanning line a in FIG. 1), and the averages of the Si/C atomic number ratios of the regions Y between the bottom of the groove and the inner 100nm and the regions Z of exceeding 100nm are calculated, respectively, by using the data of the Y and Z regions from the center of the fiber to the bottoms of the grooves on the fiber surface (as indicated by the scanning line b in FIG. 1). A total of 10 carbon fibers are measured. The Si/C ratios, the number of pores, the average length of pores, and the average width of pores in the present invention are average values obtained from 10 carbon fibers.
In the present invention, the oil content of the precursor fiber is determined in the following way:
taking 3g of precursor fiber sample, drying the precursor fiber sample for 2 hours at 105 °C in air atmosphere, and then measuring the mass of the precursor fiber sample and recorded as W1; placing the sample into a Soxhlet extractor and adding in cyclohexane, raising the temperature to 100 °C and extracting for 4 hours. The sample is taken out, dried at 105 °C in air atmosphere for 2 hours, and the mass thereof is measured and recorded as W2. oil content (wt%)=(W1-W2)/W1 × 100%.
In the present invention, the orientation deviation angle of pores in the carbon fiber along fiber axial direction is determined according to the following document [1] in the following way:
taking a carbon fiber bundle, fixing the carbon fiber bundle on a metal frame with the size of 4 cm×3cm×0.2cm and a central opening of the size of 3cm×2cm by using an adhesive tape, evenly spreading the carbon fiber bundle to uniform width, testing the sample by using the small-angle X-ray scattering (SAXS) device of the Shanghai Synchrotron Radiation Facility beamline station (SSRF, BL16B), calibrating the distance between the sample and the detector by using a standard beef tendon which being 1980 mm; and incident X-ray with a wavelength of 0.12398 nm being used. SAXS data is collected using a Mar CCD 165 imaging plate. The obtained data is processed using xPolar(Precision works NY, Inc., USA) software, and the average length (L_f ) and the orientation deviation angle (Bφ) of the micropores along the fiber axial direction are calculated according to the Cauchy-Cauchy formula: $s^{2} B_{obs}^{2} = \frac{1}{L_{f}^{2}} + s^{2} B_{φ}^{2}$ in the formula, s(s=2sinθ/λ) is the scattering vector corresponding to the azimuthal scaning curve, and B_obs is the angular width at half maximum width of the azimuthal scaning curve. The azimuth scanning curve is obtained by processing the SAXS pattern, wherein in the SAXS pattern with air scattering being properly subtracted, scanning is performed along the tangent line parallel to the meridian direction, and then integration is performed to obtain the azimuth scanning curve. The data is fitted using Gaussian distribution, then a linear fit is performed between $B_{obs}^{2}$ and the scattering vector s corresponding to each azimuthal angle, and the values of L_f and B_φ are calculated according to the intercept and slope obtained after the linear fitting, respectively.
Document [1] Andreas F. Thünemann et al. Microvoids in Polyacrylonitrile Fibers: A Small-Angle X-ray Scattering Study [J]. Macromolecules, 2000, 33(5):1848-1852.
During the carbon fiber production process, the equipments need to be shut down for cleaning and maintenance due to the decrease in carbon fiber strength or the occurrence of problems with the equipments. The methods of the present invention can continue production for a longer time without shutting down the equipments for cleaning and maintenance.
In the present invention, the precursor fiber of present invention has a good silicon penetration gradient, resulting in significantly less ash during the carbonization process, and the silicon content in the obtained carbon fiber is lower and the silicon penetration depth is effectively inhibited. After continuous production for half a year or longer, the produced carbon fiber can still have a tensile strength of greater than or equal to 5.6 GPa, a tensile modulus of greater than or equal to 360 GPa, and an elongation at break of greater than or equal to 1.5%.

Detailed Description

The invention is further illustrated by the following examples.
The raw materials and reagents used in Examples and Comparative Examples can be purchased directly or can be prepared according to the preparation methods disclosed in the prior art. The raw materials or reagents are treated as necessary prior to use using means known in the art to meet the needs of the reactions. For example, acrylonitrile is distilled to remove the polymerization inhibitor before use.
In the silicon content test of the precursor fibers, carbon fibers and oiling agents, ICP-OES inductively coupled plasma emission spectroscopy test was performed using Varian 725-ES of VARIAN, and the sample was dissolved using Digiblock ST36 electrothermal digestor of LabTech, Inc..
During the Si/C ratio tests, the low temperature FIB (gallium ion focused ion beam) cutting was performed using ZEISS Crossbeam Laser, and TEM and EDS tests were performed using FEI Titan Cubed Themis G2 300 and Aztec X-Max 100 TLE Oxford Instruments.
In the Examples and Comparative Examples, the silicon-free oiling agent used was TFA-1208 from Tianjin Gongda Textile Auxiliary Co., Ltd.; the silicon-containing oiling agent used was JH88 from Matsumoto Yushi Seiyaku Co., Ltd. The silicon-free oiling agents of different particle sizes in Examples 7, 8 and 12 and Comparative Examples 10 and 11 were TFA-1208 series synthesized by Tianjin Gongda Textile Auxiliary Co., Ltd., and the silicon-containing oiling agents of different particle sizes were JSYJ series from SINOPEC Shanghai Research Institute of Petrochemical Technology Co., Ltd.. In Comparative Example 4, the anionic silicon-free oiling agent was TFA-1208 and the cationic silicon-containing oiling agent was JLQR oiling agent from Jilin Qianren New Materials Co., Ltd.. The pH of the oiling agent in Example 16 was adjusted by adding an appropriate amount of dilute ammonia aqueous solution. The pH of the silicon-free oiling agent of Comparative Example 3 was adjusted by adding an appropriate amount of dilute ammonia aqueous solution and the acidic silicon-containing oiling agent was ADVALON^® CF 3295 of Wacker Chemicals (China) Co., Ltd..
In Examples and Comparative Examples, in the preparation of the spinning stock solution, the molar ratio of acrylonitrile to itaconic acid was 99:1, the concentration of monomers was 20% by weight, polymerization was carried out at 60°C for 20 hours under nitrogen atmosphere, and the conversion was 91.5%; and then the spinning stock solution was obtained after removing monomers and defoaming.
In the Examples and Comparative Examples, the spinning stock solution was precisely metered by a metering pump and was filtered using a 10 µm candle filter before being extruded through a spinneret.

[Example 1]

1. preparation of precursor fibers: wet spinning method was used, and the spinning stock solution used was prepared by copolymerizing acrylonitrile and itaconic acid in dimethyl sulfoxide, the intrinsic viscosity was 2.5dL/g, and the solid content was 18%. The spinning stock solution was precisely metered by a metering pump and was filtered, and was extruded through a spinneret with 6000 holes and 60 µm hole diameter into first coagulation bath for coagulation wherein the first coagulation bath was dimethyl sulfoxide aqueous solution, the coagulation temperature was 28 °C and the concentration was 51.5%; then the fibers were subjected to three-stage coagulation drawing, the concentrations were respectively 30%, 20% and 10%, the temperatures were respectively 30 °C, 40 °C and 50 °C, and the drawing ratios were respectively 1.0, 1.1 and 1.2; four-stage hot water drawing, wherein the temperatures were respectively 95 °C, 96 °C, 97 °C and 99 °C, and the drawing ratios were respectively 1.35, 1.50, 1.70 and 2.00; and six-stage water washing process, the temperatures were respectively 70 °C, 70 °C, 80 °C, 80 °C, 90 °C and 90 °C, and the swelling degree of the washed fiber was 110%.

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.2%, and the silicon content was 0.15%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 10%, and the Si/C ratio of the region of exceeding 2 µm was 0.11%; the tensile strength, initial modulus and elongation at break were 7.5 cN/dtex, 125 cN/dtex and 12.0%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized in air at 180-260°C with a pre-oxidation total drawing ratio of 0.95 times to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 800 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 5.8%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.5%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers (regions X and Y between the top of the protrusion of the fiber surface and the inner 100 nm) was 10, the average length of the pores was 20nm and the average width of the pores was 15 nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.3°. The tensile strength of the carbon fiber was 5.74GPa, the tensile modulus was 370GPa, and the elongation at break was 1.52%. After 7 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 2]

1. preparation of precursor fibers: wet spinning method was used, and the spinning stock solution used was prepared by copolymerizing acrylonitrile and itaconic acid in dimethyl sulfoxide, the intrinsic viscosity was 2.5dL/g, and the solid content was 18%. The spinning stock solution was precisely metered by a metering pump and was filtered, and was extruded through a spinneret with 6000 holes and 60 µm hole diameter into first coagulation bath for coagulation wherein the first coagulation bath was dimethyl sulfoxide aqueous solution, the coagulation temperature was 28 °C and the concentration was 51.5%; then the fibers were subjected to three-stage coagulation drawing, the concentrations were respectively 30%, 20% and 10%, the temperatures were respectively 30 °C, 40 °C and 50 °C, and the drawing ratios were respectively 1.0, 1.1 and 1.2; four-stage hot water drawing, wherein the temperatures were respectively 95 °C, 96 °C, 97 °C and 99 °C, and the drawing ratios were respectively 1.35, 1.50, 1.70 and 2.00; and six-stage water washing process, the temperatures were respectively 70 °C, 70 °C, 80 °C, 80 °C, 90 °C and 90 °C, and the swelling degree of the washed fibers was 110%.

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 3.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 3%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.35%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.2%, and the silicon content was 0.47%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 19%, and the Si/C ratio of the region of exceeding 2 µm was 0.14%; the tensile strength, initial modulus and elongation at break were 7.2 cN/dtex, 120 cN/dtex and 12.3%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 1400 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 9%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.9%, and the Si/C ratio of region Z of exceeding 100nm was 0.05%. The number of pores present in the surface layer of carbon fibers was 15, the average length of the pores was 42nm and the average width of the pores was 22nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 3.5°. The tensile strength of the carbon fiber was 5.63GPa, the tensile modulus was 360GPa, and the elongation at break was 1.56%. After 6 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 3]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.1%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 0.8%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.1%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 0.85%, and the silicon content was 0.09%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 6%, and the Si/C ratio of the region of exceeding 2 µm was 0.07%; the tensile strength, initial modulus and elongation at break were 8.1 cN/dtex, 137 cN/dtex and 11.8%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 620 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 4.5%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.47%, and the Si/C ratio of region Z of exceeding 100nm was 0.03%. The number of pores present in the surface layer of carbon fibers was 7, the average length of the pores was 15nm and the average width of the pores was 10nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.2°. The tensile strength of the carbon fiber was 5.75GPa, the tensile modulus was 374GPa, and the elongation at break was 1.51%. After 8 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 4]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.1%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.2%, and the residence time was 0.05 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 0.9%, and the silicon content was 0.13%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 9%, and the Si/C ratio of the region of exceeding 2 µm was 0.1%; the tensile strength, initial modulus and elongation at break were 7.8 cN/dtex, 121 cN/dtex and 11.8%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 690 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 4.8%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.54%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers was 8, the average length of the pores was 17nm and the average width of the pores was 13nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.2°. The tensile strength of the carbon fiber was 5.75GPa, the tensile modulus was 374GPa, and the elongation at break was 1.55%. After 8 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 5]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.25 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.13 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.8%, and the silicon content was 0.39%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 14%, and the Si/C ratio of the region of exceeding 2 µm was 0.12%; the tensile strength, initial modulus and elongation at break were 7.2 cN/dtex, 127 cN/dtex and 11.8%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 1340 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 8%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.85%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers was 15, the average length of the pores was 35nm and the average width of the pores was 20nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 3.0°. The tensile strength of the carbon fiber was 5.66GPa, the tensile modulus was 361GPa, and the elongation at break was 1.58%. After 6 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 6]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.08 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.05 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 0.9%, and the silicon content was 0.08%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 5%, and the Si/C ratio of the region of exceeding 2 µm was 0.05%; the tensile strength, initial modulus and elongation at break were 7.4 cN/dtex, 126 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 560 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 4.3%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.41%, and the Si/C ratio of region Z of exceeding 100nm was 0.02%. The number of pores present in the surface layer of carbon fibers was 10, the average length of the pores was 23nm and the average width of the pores was 18nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.8°. The tensile strength of the carbon fiber was 5.68GPa, the tensile modulus was 364GPa, and the elongation at break was 1.55%. After 8 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 7]

The fibers after water washing were firstly passed through the first oiling tank, wherein the nonionic silicon-free oiling agent TFA-1208-10 synthesized by Tianjin Gongda Textile Auxiliary Co., Ltd. was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 100 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein the nonionic silicon-containing oiling agent JSYJ-20 synthesized by SINOPEC Shanghai Research Institute of Petrochemical Technology Co., Ltd. was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 200 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.4%, and the silicon content was 0.14%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 9%, and the Si/C ratio of the region of exceeding 2 µm was 0.1%; the tensile strength, initial modulus and elongation at break were 7.7 cN/dtex, 130 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 700 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 5%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.55%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers was 8, the average length of the pores was 17nm and the average width of the pores was 13nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.2°. The tensile strength of the carbon fiber was 5.75GPa, the tensile modulus was 374GPa, and the elongation at break was 1.51%. After 8 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 8]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein the nonionic silicon-containing oiling agent JSYJ-20 synthesized by SINOPEC Shanghai Research Institute of Petrochemical Technology Co., Ltd. was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 200 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.04 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.1%, and the silicon content was 0.07%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 5%, and the Si/C ratio of the region of exceeding 2 µm was 0.06%; the tensile strength, initial modulus and elongation at break were 8.4 cN/dtex, 130 cN/dtex and 12.0%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 550 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 4.3%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.4%, and the Si/C ratio of region Z of exceeding 100nm was 0.02%. The number of pores present in the surface layer of carbon fibers was 7, the average length of the pores was 15nm and the average width of the pores was 9nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.1°. The tensile strength of the carbon fiber was 5.8GPa, the tensile modulus was 390GPa, and the elongation at break was 1.45%. After 8 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 9]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.3%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.3%, and the silicon content was 0.34%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 13%, and the Si/C ratio of the region of exceeding 2 µm was 0.12%; the tensile strength, initial modulus and elongation at break were 7.5 cN/dtex, 126 cN/dtex and 11.7%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 1200 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 6.7%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.7%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers was 14, the average length of the pores was 30nm and the average width of the pores was 19nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.8°. The tensile strength of the carbon fiber was 5.69GPa, the tensile modulus was 365GPa, and the elongation at break was 1.54%. After 6 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 10]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.2%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.1%, and the silicon content was 0.08%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 7%, and the Si/C ratio of the region of exceeding 2 µm was 0.07%; the tensile strength, initial modulus and elongation at break were 8.1 cN/dtex, 137 cN/dtex and 11.8%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 600 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 4.5%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.45%, and the Si/C ratio of region Z of exceeding 100nm was 0.03%. The number of pores present in the surface layer of carbon fibers was 7, the average length of the pores was 15nm and the average width of the pores was 10nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.2°. The tensile strength of the carbon fiber was 5.76GPa, the tensile modulus was 379GPa, and the elongation at break was 1.5%. After 8 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 11]

1. preparation of precursor fibers: wet spinning method was used, and the spinning stock solution used was prepared by copolymerizing acrylonitrile and itaconic acid in dimethyl sulfoxide, the intrinsic viscosity was 2.5dL/g, and the solid content was 18%. The spinning stock solution was precisely metered by a metering pump and was filtered, and was extruded through a spinneret with 6000 holes and 60 µm hole diameter into first coagulation bath for coagulation wherein the first coagulation bath was dimethyl sulfoxide aqueous solution, the coagulation temperature was 28 °C and the concentration was 51.5%; then the fibers were subjected to three-stage coagulation drawing, the concentrations were respectively 30%, 20% and 10%, the temperatures were respectively 30 °C, 40 °C and 50 °C, and the drawing ratios were respectively 1.0, 1.1 and 1.2; four-stage hot water drawing, wherein the temperatures were respectively 95 °C, 96 °C, 97 °C and 99 °C, and the drawing ratios were respectively 1.35, 1.50, 1.70 and 2.00; and six-stage water washing process, the temperatures were respectively 70 °C, 70 °C, 80 °C, 80 °C, 90 °C and 90 °C, and the swelling degree of the washed fibers was 140%.

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.11%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.3%, and the silicon content was 0.4%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 16%, and the Si/C ratio of the region of exceeding 2 µm was 0.14%; the tensile strength, initial modulus and elongation at break were 7.2 cN/dtex, 119 cN/dtex and 12.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 1350 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 8%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.85%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers was 15, the average length of the pores was 38nm and the average width of the pores was 20nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 3.2°. The tensile strength of the carbon fiber was 5.66GPa, the tensile modulus was 362GPa, and the elongation at break was 1.55%. After 6 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 12]

1. preparation of precursor fibers: wet spinning method was used, and the spinning stock solution used was prepared by copolymerizing acrylonitrile and itaconic acid in dimethyl sulfoxide, the intrinsic viscosity was 2.5dL/g, and the solid content was 18%. The spinning stock solution was precisely metered by a metering pump and was filtered, and was extruded through a spinneret with 6000 holes and 60 µm hole diameter into first coagulation bath for coagulation wherein the first coagulation bath was dimethyl sulfoxide aqueous solution, the coagulation temperature was 28 °C and the concentration was 51.5%; then the fibers were subjected to three-stage coagulation drawing, the concentrations were respectively 30%, 20% and 10%, the temperatures were respectively 30 °C, 40 °C and 50 °C, and the drawing ratios were respectively 1.0, 1.1 and 1.2; four-stage hot water drawing, wherein the temperatures were respectively 95 °C, 96 °C, 97 °C and 99 °C, and the drawing ratios were respectively 1.35, 1.50, 1.70 and 2.00; and six-stage water washing process, the temperatures were respectively 70 °C, 70 °C, 80 °C, 80 °C, 90 °C and 90 °C, and the swelling degree of the washed fibers was 90%.

The fibers after water washing were firstly passed through the first oiling tank, wherein the nonionic silicon-free oiling agent TFA-1208-10 synthesized by Tianjin Gongda Textile Auxiliary Co., Ltd. was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 100 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein the nonionic silicon-containing oiling agent JSYJ-10 synthesized by SINOPEC Shanghai Research Institute of Petrochemical Technology Co., Ltd. was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 100 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.4%, and the silicon content was 0.16%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 12%, and the Si/C ratio of the region of exceeding 2 µm was 0.12%; the tensile strength, initial modulus and elongation at break were 7.5 cN/dtex, 134 cN/dtex and 11.7%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 900 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 6%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.63%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers was 12, the average length of the pores was 26nm and the average width of the pores was 16nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.5°. The tensile strength of the carbon fiber was 5.7GPa, the tensile modulus was 377GPa, and the elongation at break was 1.52%. After 7 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Example 13]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 8, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 9, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1.2%, and the silicon content was 0.14%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 9%, and the Si/C ratio of the region of exceeding 2 µm was 0.11%; the tensile strength, initial modulus and elongation at break were 7.7 cN/dtex, 135 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 700 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 5%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.55%, and the Si/C ratio of region Z of exceeding 100nm was 0.04%. The number of pores present in the surface layer of carbon fibers was 8, the average length of the pores was 17nm and the average width of the pores was 13nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 2.2°. The tensile strength of the carbon fiber was 5.75GPa, the tensile modulus was 375GPa, and the elongation at break was 1.51%. After 8 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 1]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 3%, and the silicon content was 0.67%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 34%, and the Si/C ratio of the region of exceeding 2 µm was 0.5%; the tensile strength, initial modulus and elongation at break were 8.1 cN/dtex, 131 cN/dtex and 11.9%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 3000 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 77%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 20%, and the Si/C ratio of region Z of exceeding 100nm was 2%. The number of pores present in the surface layer of carbon fibers was 40, the average length of the pores was 70nm and the average width of the pores was 45nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.1°. The tensile strength of the carbon fiber was 5.23GPa, the tensile modulus was 355GPa, and the elongation at break was 1.46%. After 2.5 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 2]

The fibers after water washing were firstly passed through the first oiling tank, wherein water was arranged in the tank, the pH was 7, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.8%, and the silicon content was 0.55%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 24%, and the Si/C ratio of the region of exceeding 2 µm was 0.19%; the tensile strength, initial modulus and elongation at break were 7.9 cN/dtex, 131 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 2600 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 72%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 18%, and the Si/C ratio of region Z of exceeding 100nm was 1.8%. The number of pores present in the surface layer of carbon fibers was 39, the average length of the pores was 66nm and the average width of the pores was 45nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 3.8°. The tensile strength of the carbon fiber was 5.35GPa, the tensile modulus was 357GPa, and the elongation at break was 1.48%. After 3 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 3]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 8, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 6, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 3.1%, and the silicon content was 0.68%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 35%, and the Si/C ratio of the region of exceeding 2 µm was 0.5%; the tensile strength, initial modulus and elongation at break were 7.5 cN/dtex, 128 cN/dtex and 11.3%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 3200 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 80%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 13%, and the Si/C ratio of region Z of exceeding 100nm was 1.5%. The number of pores present in the surface layer of carbon fibers was 50, the average length of the pores was 77nm and the average width of the pores was 52nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.3°. The tensile strength of the carbon fiber was 5.05GPa, the tensile modulus was 361GPa, and the elongation at break was 1.39%. After 3 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 4]

The fibers after water washing were firstly passed through the first oiling tank, wherein an anionic silicon-free oiling agent TFA-1208-Y synthesized by Tianjin Gongda Textile Auxiliary Co., Ltd. was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a cationic silicon-containing oiling agent JLQR-Y synthesized by Jilin Qianren New Materials Co., Ltd. was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 3.1%, and the silicon content was 0.69%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 35%, and the Si/C ratio of the region of exceeding 2 µm was 0.5%; the tensile strength, initial modulus and elongation at break were 7.5 cN/dtex, 129 cN/dtex and 11.3%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 3300 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 82%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 15%, and the Si/C ratio of region Z of exceeding 100nm was 2%. The number of pores present in the surface layer of carbon fibers was 56, the average length of the pores was 80nm and the average width of the pores was 60nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.3°. The tensile strength of the carbon fiber was 5GPa, the tensile modulus was 358GPa, and the elongation at break was 1.37%. After 3 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 5]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.6%, and the silicon content was 0%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 0%, and the Si/C ratio of the region of exceeding 2 µm was 0%; the tensile strength, initial modulus and elongation at break were 7.2 cN/dtex, 120 cN/dtex and 11.6%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 0 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 0%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0%, and the Si/C ratio of region Z of exceeding 100nm was 0%. The number of pores present in the surface layer of carbon fibers was 64, the average length of the pores was 89nm and the average width of the pores was 71nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.8°. The tensile strength of the carbon fiber was 4.6GPa, the tensile modulus was 340GPa, and the elongation at break was 1.35%. After 7 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 6]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 6%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.3%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.6%, and the silicon content was 0.01%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 0.8%, and the Si/C ratio of the region of exceeding 2 µm was 0.09%; the tensile strength, initial modulus and elongation at break were 7.4 cN/dtex, 127 cN/dtex and 11.6%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 400 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 0.8%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.07%, and the Si/C ratio of region Z of exceeding 100nm was 0.01%. The number of pores present in the surface layer of carbon fibers was 58, the average length of the pores was 75nm and the average width of the pores was 50nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.5°. The tensile strength of the carbon fiber was 4.92GPa, the tensile modulus was 367GPa, and the elongation at break was 1.32%. After 6 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 7]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 3.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 6%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 1%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.7%, and the silicon content was 0.53%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 23%, and the Si/C ratio of the region of exceeding 2 µm was 0.19%; the tensile strength, initial modulus and elongation at break were 7.7 cN/dtex, 136 cN/dtex and 11.3%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 2400 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 48%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 15%, and the Si/C ratio of region Z of exceeding 100nm was 1%. The number of pores present in the surface layer of carbon fibers was 38, the average length of the pores was 65nm and the average width of the pores was 44nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 3.9°. The tensile strength of the carbon fiber was 5.3GPa, the tensile modulus was 354GPa, and the elongation at break was 1.47%. After 3 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 8]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.25 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.3 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.7%, and the silicon content was 0.55%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 26%, and the Si/C ratio of the region of exceeding 2 µm was 0.25%; the tensile strength, initial modulus and elongation at break were 7.6 cN/dtex, 132 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 2100 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 48%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 9%, and the Si/C ratio of region Z of exceeding 100nm was 1%. The number of pores present in the surface layer of carbon fibers was 39, the average length of the pores was 66nm and the average width of the pores was 44nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.1°. The tensile strength of the carbon fiber was 5.28GPa, the tensile modulus was 360GPa, and the elongation at break was 1.41%. After 3 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 9]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.02 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 1%, and the silicon content was 0.01%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 0.8%, and the Si/C ratio of the region of exceeding 2 µm was 0.05%; the tensile strength, initial modulus and elongation at break were 7.4 cN/dtex, 130 cN/dtex and 11.4%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 400 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 0.9%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.07%, and the Si/C ratio of region Z of exceeding 100nm was 0.01%. The number of pores present in the surface layer of carbon fibers was 60, the average length of the pores was 87nm and the average width of the pores was 69nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.6°. The tensile strength of the carbon fiber was 4.9GPa, the tensile modulus was 366GPa, and the elongation at break was 1.31%. After 7 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 10]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent TFA-1208-80 synthesized by Tianjin Gongda Textile Auxiliary Co., Ltd. was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 800 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.6%, and the silicon content was 0.52%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 22%, and the Si/C ratio of the region of exceeding 2 µm was 0.16%; the tensile strength, initial modulus and elongation at break were 7.3 cN/dtex, 128 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 2000 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 45%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 13%, and the Si/C ratio of region Z of exceeding 100nm was 0.08%. The number of pores present in the surface layer of carbon fibers was 45, the average length of the pores was 60nm and the average width of the pores was 40nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 5°. The tensile strength of the carbon fiber was 4.8GPa, the tensile modulus was 350GPa, and the elongation at break was 1.3%. After 4 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 11]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent TFA-1208-04 synthesized by Tianjin Gongda Textile Auxiliary Co., Ltd. was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 40 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent JSYJ-04 synthesized by SINOPEC Shanghai Research Institute of Petrochemical Technology Co., Ltd. was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 40 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.7%, and the silicon content was 0.63%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 31%, and the Si/C ratio of the region of exceeding 2 µm was 0.28%; the tensile strength, initial modulus and elongation at break were 7.5 cN/dtex, 129 cN/dtex and 11.3%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 2500 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 50%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 17%, and the Si/C ratio of region Z of exceeding 100nm was 1.3%. The number of pores present in the surface layer of carbon fibers was 48, the average length of the pores was 69nm and the average width of the pores was 50nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.2°. The tensile strength of the carbon fiber was 5.1GPa, the tensile modulus was 351GPa, and the elongation at break was 1.4%. After 3 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 12]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 6%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 1%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2.7%, and the silicon content was 0.16%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 12%, and the Si/C ratio of the region of exceeding 2 µm was 0.12%; the tensile strength, initial modulus and elongation at break were 7.8 cN/dtex, 130 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 1750 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 30%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 2.8%, and the Si/C ratio of region Z of exceeding 100nm was 0.07%. The number of pores present in the surface layer of carbon fibers was 36, the average length of the pores was 64nm and the average width of the pores was 30nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 3.6°. The tensile strength of the carbon fiber was 5.35GPa, the tensile modulus was 356GPa, and the elongation at break was 1.47%. After 3 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 13]

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s; then a four-stage drying densification treatment was conducted, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C; then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.85dtex, the oil content was 2%, and the silicon content was 0.15%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 0.5%, and the Si/C ratio of the region of exceeding 2 µm was 0.05%; the tensile strength, initial modulus and elongation at break were 7.6 cN/dtex, 132 cN/dtex and 11.5%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.
The carbon fiber had a total silicon content of 450 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 0.9%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 0.03%, and the Si/C ratio of region Z of exceeding 100nm was 0.01%. The number of pores present in the surface layer of carbon fibers was 43, the average length of the pores was 67nm and the average width of the pores was 41nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 4.1°. The tensile strength of the carbon fiber was 5.16GPa, the tensile modulus was 359GPa, and the elongation at break was 1.42%. After 6 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

[Comparative Example 14]

1. preparation of precursor fibers: wet spinning method was used, and the spinning stock solution used was prepared by copolymerizing acrylonitrile and itaconic acid in dimethyl sulfoxide, the intrinsic viscosity was 2.5dL/g, and the solid content was 18%. The spinning stock solution was precisely metered by a metering pump and was filtered, and was extruded through a spinneret with 6000 holes and 60 µm hole diameter into first coagulation bath for coagulation wherein the first coagulation bath was dimethyl sulfoxide aqueous solution, the coagulation temperature was 28 °C and the concentration was 51.5%; then the fibers were subjected to three-stage coagulation drawing, the concentrations were respectively 30%, 20% and 10%, the temperatures were respectively 30 °C, 40 °C and 50 °C, and the drawing ratios were respectively 1.0, 1.1 and 1.2; four-stage hot water drawing, wherein the temperatures were respectively 95 °C, 96 °C, 97 °C and 99 °C, and the drawing ratios were respectively 1.10, 1.15, 1.15 and 1.2; and six-stage water washing process, the temperatures were respectively 70 °C, 70 °C, 80 °C, 80 °C, 90 °C and 90 °C, and the swelling degree of the washed fibers was 170%.

The fibers after water washing were firstly passed through the first oiling tank, wherein a nonionic silicon-free oiling agent was arranged in the tank, the pH was 7.2, the concentration of the oiling agent was 1.5%, the particle size of the oiling agent was 300 nm, and the residence time was 0.15 s. Then the fibers passed through the second oiling tank, wherein a nonionic silicon-containing oiling agent was arranged in the tank, the pH was 7.8, the concentration of the oiling agent was 1.2%, the particle size of the oiling agent was 300 nm, the silicon content of the oiling agent was 0.15%, and the residence time was 0.1 s.
A four-stage drying densification treatment was then carried out, wherein the temperatures were respectively 90 °C, 100 °C, 115 °C and 130 °C, and then steam drawing was carried out, wherein the drawing ratio was 3.0 times, and the steam pressure was 0.35 MPa; finally, after heat setting, drawing ratio of 0.95 and steam pressure of 120 MPa, and after winding, high performance polyacrylonitrile carbon fiber precursor fibers were obtained.
The linear density of the precursor fiber was 0.9dtex, the oil content was 3.3%, and the silicon content was 0.74%. The Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm was 46%, and the Si/C ratio of the region of exceeding 2 µm was 0.69%; the tensile strength, initial modulus and elongation at break were 7.0 cN/dtex, 117 cN/dtex and 12.7%, respectively.
2. preparation of carbon fibers: the precursor fibers obtained in step 1 were pre-oxidized at 180-260°C with a pre-oxidation total drawing ratio of 0.95 to obtain the pre-oxidized fibers; then low-temperature carbonization and high-temperature carbonization were carried out at 300-750°C and 800-1500°C with a drawing ratio of 0.96 times; thereafter, graphitization treatment was carried out at 2800°C, and finally, carbon fibers were obtained after surface treatment, water washing, sizing, drying at 120°C and winding.

The carbon fiber had a total silicon content of 3500 ppm, and the Si/C ratio of the carbon fiber of region X between the top of the protrusion of the fiber surface and the bottom of the groove was 81%, the Si/C ratio of region Y between the bottom of the groove and the inner 100nm was 20%, and the Si/C ratio of region Z of exceeding 100nm was 3.7%. The number of pores present in the surface layer of carbon fibers was 70, the average length of the pores was 84nm and the average width of the pores was 77nm. The orientation deviation angle of the pores along the axial direction of the fiber obtained by small-angle X-ray scattering method was 5.5°. The tensile strength of the carbon fiber was 4.58GPa, the tensile modulus was 339GPa, and the elongation at break was 1.34%. After 2 months of continuous production, the strength of the carbon fibers began to decline, and the operation was shut down once to clean ash in the equipments.

Table 1

	swelling degree of washed fibers/%	pH of silicon-free oiling agent	concentra tion/%	particle size of oiling agent/nm	residence time/s	pH of silicon-containin g oiling agent	concentra tion/%	particle size of oiling agent/nm	silicon content of oiling agent/%	residence time/s
Example 1	110	7.2	1.5	300	0.15	7.8	1.2	300	0.15	0.1
Example 2	110	7.2	3.5	300	0.15	7.8	3	300	0.35	0.1
Example 3	110	7.2	1.1	300	0.15	7.8	0.8	300	0.1	0.1
Example 4	110	7.2	1.1	300	0.15	7.8	1.5	300	0.2	0.05
Example 5	110	7.2	1.5	300	0.25	7.8	1.2	300	0.15	0.13
Example 6	110	7.2	1.5	300	0.08	7.8	1.2	300	0.15	0.05
Example 7	110	7.2	1.5	100	0.15	7.8	1.2	200	0.15	0.1
Example 8	110	7.2	1.5	300	0.15	7.8	1.2	200	0.15	0.04
Example 9	110	7.2	1.5	300	0.15	7.8	2	300	0.3	0.1
Example 10	110	7.2	1.5	300	0.15	7.8	1.5	300	0.2	0.1
Example 11	140	7.2	1.5	300	0.15	7.8	1	300	0.11	0.1
Example 12	90	7.2	1.5	100	0.15	7.8	1.2	100	0.15	0.1
Example 13	110	8	1.5	300	0.15	9	1.2	300	0.15	0.1

Table 2

	total silicon content /ppm	Si/C ratio of region X between top of protrusion and bottom of groove/%	Si/C ratio of region Y between bottom of groove and inner 100nm/%	Si/C ratio of region Z of exceed ing 100nm /%	numbe r of pores in surface X and Y regions	averag e length of pores/n m	averag e width of pores/ nm	orienta tion deviati on angle/°	tensile strengt h of carbon fiber/G Pa	tensile modul us of carbon fiber/G Pa	elongat ion at break/ %	time elapse d when shut down/ months
Example 1	800	5.8	0.5	0.04	10	20	15	2.3	5.74	370	1.52	7
Example 2	1400	9	0.9	0.05	15	42	22	3.5	5.63	360	1.56	6
Example 3	620	4.5	0.47	0.03	7	15	10	2.2	5.75	374	1.51	8
Example 4	690	4.8	0.54	0.04	8	17	13	2.2	5.75	374	1.55	8
Example 5	1340	8	0.85	0.04	15	35	20	3.0	5.66	361	1.58	6
Example 6	560	4.3	0.41	0.02	10	23	18	2.8	5.68	364	1.55	8
Example 7	700	5	0.55	0.04	8	17	13	2.2	5.75	374	1.51	8
Example 8	550	4.3	0.4	0.02	7	15	9	2.1	5.8	390	1.45	8
Example 9	1200	6.7	0.7	0.04	14	30	19	2.8	5.69	365	1.54	6
Example 10	600	4.5	0.45	0.03	7	15	10	2.2	5.76	379	1.5	8
Example 11	1350	8	0.85	0.04	15	38	20	3.2	5.66	362	1.55	6
Example 12	900	6	0.63	0.04	12	26	16	2.5	5.7	377	1.52	7
Example 13	700	5	0.55	0.04	8	17	13	2.2	5.75	375	1.51	8

Table 3

	swelling degree of washed fibers/%	pH of oiling agent	concentra tion/%	particle size of oiling agent/nm	residence time/s	pH of silicon-containin g oiling agent	concentra tion/%	particle size of oiling agen/nm	silicon content of oiling agent/%	residence time/s
Comparative Example 1	110	silicon-containin g, 7.8	1.5	300	0.15	7.8	1.2	300	0.15	0.1
Comparative Example 2	110	water, 7.0	/	/	0.15	7.8	1.2	300	0.15	0.1
Comparative Example 3	110	8	1.5	300	0.15	6	1.2	300	0.15	0.1
Comparative Example 4	110	anionic, 7.2	1.5	300	0.15	cationic, 7.8	1.2	300	0.15	0.1
Comparative Example 5	110	7.2	1.5	300	0.15	silicon-free, 7.2	1.2	300	0.15	0.1
Comparative Example 6	110	7.2	6	300	0.15	7.8	2	300	0.3	0.1
Comparative Example 7	110	7.2	3.5	300	0.15	7.8	6	300	1	0.1
Comparative Example 8	110	7.2	1.5	300	0.25	7.8	1.2	300	0.15	0.3
Comparative Example 9	110	7.2	1.5	300	0.15	7.8	1.2	300	0.15	0.02
Comparative Example 10	110	7.2	1.5	800	0.15	7.8	1.2	300	0.15	0.1
Comparative Example 11	110	7.2	1.5	40	0.15	7.8	1.2	40	0.15	0.1
Comparative Example 12	110	7.2	1.5	300	0.15	7.8	6	300	1	0.1
Comparative Example 13	110	7.2	1.5	300	0.15	7.8	1.2	300	0.15	0.1
Comparative Example 14	170	7.2	1.5	300	0.15	7.8	1.2	300	0.15	0.1

Table 4

	total silicon content /ppm	Si/C ratio of region X between top of protrusion and bottom of groove/%	Si/C ratio of region Y between bottom of groove and inner 100nm/%	Si/C ratio of region Z of exceed ing 100nm /%	number of pores in surface X and Y regions	aver age lengt h of pore s/nm	averag e width of pores/ nm	orienta tion deviati on angle/°	tensile strengt h of carbon fiber/ GPa	tensile modul us of carbon fiber/ GPa	elonga tion at break/ %	time elapse d when shut down/ month s
Comparative Example 1	3000	77	20	2	40	70	45	4.1	5.23	355	1.46	2.5
Comparative Example 2	2600	72	18	1.8	39	66	45	3.8	5.35	357	1.48	3
Comparative Example 3	3200	80	13	1.5	50	77	52	4.3	5.05	361	1.39	3
Comparative Example 4	3300	82	15	2	56	80	60	4.3	5	358	1.37	3
Comparative Example 5	0	0	0	0	64	89	71	4.8	4.6	340	1.35	7
Comparative Example 6	400	0.8	0.07	0.01	58	75	50	4.5	4.92	367	1.32	6
Comparative Example 7	2400	48	15	I	38	65	44	3.9	5.3	354	1.47	3
Comparative Example 8	2100	48	9	I	39	66	44	4.1	5.28	360	1.41	3
Comparative Example 9	400	0.9	0.07	0.01	60	87	69	4.6	4.9	366	1.31	7
Comparative Example 10	2000	45	13	0.08	45	60	40	5	4.8	350	1.3	4
Comparative Example 11	2500	50	17	1.3	48	69	50	4.2	5.1	351	1.4	3
Comparative Example 12	1750	30	2.8	0.07	36	64	30	3.6	5.35	356	1.47	3
Comparative Example 13	450	0.9	0.03	0.01	43	67	41	4.1	5.16	359	1.42	6
Comparative Example 14	3500	81	20	3.7	70	84	77	5.5	4.58	339	1.34	2

Claims

A polyacrylonitrile carbon fiber having groove structures on the surface thereof, characterized in that the Si/C ratio of the carbon fiber of the region X between the top of the protrusion of the fiber surface and the bottom of the groove is in the range of 1% to 10%, the Si/C ratio of the region Y between the bottom of the groove and the inner 100nm is in the range of 0.1% to 1%, and the Si/C ratio of the region Z of exceeding 100nm is not higher than 0.06%.
The polyacrylonitrile carbon fiber according to claim 1, characterized in that the polyacrylonitrile carbon fiber is obtained from polyacrylonitrile precursor fiber prepared by wet spinning.
The polyacrylonitrile carbon fiber according to claim 1 or 2, characterized in that the total silicon content in the carbon fiber is 500 to 1500 ppm, and/or
the Si/C ratio of the region X between the top of the protrusion of the fiber surface and the bottom of the groove is not less than 8.5 times the Si/C ratio of the region Y between the bottom of the groove and the inner 100 nm.
The polyacrylonitrile carbon fiber according to any one of claims 1 to 3, characterized in that the number of pores present in the regions X and Y between the top of the protrusion of the fiber surface and the inner 100 nm is less than 20, preferably less than 16; wherein the pores have an average length of 10 to 50nm and an average width of 5 to 25 nm.
The polyacrylonitrile carbon fiber according to any one of claims 1 to 4, characterized in that the orientation deviation angle along the axial direction of the fiber of the pores obtained by small-angle X-ray scattering method is less than or equal to 4°; and/or
the carbon fiber has a tensile strength of 5.6-5.8 GPa, a tensile modulus of 360-390 GPa, and an elongation at break of 1.45%-1.6%.
A method for producing polyacrylonitrile carbon fiber, preferably the polyacrylonitrile carbon fiber according to any one of claims 1 to 5, including a step of carbonizing a polyacrylonitrile precursor fiber to obtain the polyacrylonitrile carbon fiber; characterized in that the Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm of the polyacrylonitrile precursor fiber is in the range of from 1.0% to 20%, and the Si/C ratio of the region of exceeding 2 µm is not higher than 0.15%.
The method for producing polyacrylonitrile carbon fiber according to claim 6, characterized in that the oil content of the precursor fiber is 0.5%-2.5%, preferably 0.8%-2.5%, preferably 0.8%-2.4%, more preferably 0.9%-2.0%; and/or
the silicon content of the precursor fiber is 0.01%-0.5%; and/or

the monofilament linear density of the polyacrylonitrile precursor fiber is 0.7-1.0 dtex; and/or

the polyacrylonitrile precursor fiber is prepared by wet spinning.
The method for producing polyacrylonitrile carbon fiber according to claim 6 or 7, characterized in that the polyacrylonitrile precursor fiber is prepared by wet spinning and includes an oiling step, wherein the oiling step comprises two oiling procedures in succession and no drying densification stage is arranged between the two oiling procedures; wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the second oiling procedure uses a silicon-containing oiling agent.
The method for producing polyacrylonitrile carbon fiber according to claim 8, characterized in that
the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof with a weight concentration of 0.1%-5.0%; and the second oiling procedure uses a silicon-containing oiling agent with a weight concentration of 0.1%-5.0%; and/or

the average particle sizes of the silicon-free oiling agent, the low-silicon oiling agent and the silicon-containing oiling agent are each independently 50 nm-500 nm; and/or

the silicon content of the silicon-containing oiling agent used for the second oiling procedure is 0.01wt%-0.9 wt%; and/or

the residence time of the second oiling procedure is less than the residence time of the first oiling procedure.
The method for producing polyacrylonitrile carbon fiber according to any one of claims 6 to 9, characterized in that the pH difference between the silicon-free oiling agent or low-silicon oiling agent and the silicon-containing oiling agent is not more than 1; and/or
the polarity of the surfactant in the silicon-free oiling agent or low-silicon oiling agent is not opposite to that of the surfactant in the silicon-containing oiling agent, and preferably the silicon-containing oiling agent is an oiling agent comprising a nonionic surfactant.
The method for producing polyacrylonitrile carbon fiber according to any one of claims 6 to 10, characterized in that before oiling, the fiber swelling degree of the polyacrylonitrile precursor fiber after water washing is 80% to 150%.
The method for producing polyacrylonitrile carbon fiber according to any one of claims 6 to 11, characterized in that the producing method includes the steps of wet coagulation forming of polyacrylonitrile stock solution, coagulation drawing, hot water drawing, water washing, oiling, drying densification, steam drawing and steam heat setting to obtain the polyacrylonitrile precursor fiber.
The method for producing polyacrylonitrile carbon fiber according to any one of claims 6 to 12, characterized in that the polyacrylonitrile carbon fiber is obtained by subjecting the polyacrylonitrile precursor fiber to pre-oxidation treatment and carbonization.
The method for producing polyacrylonitrile carbon fiber according to claim 13, characterized in that the pre-oxidation treatment is performed in an air atmosphere, the temperature is 170-300°C, and the total drawing ratio is not higher than 5%; and/or
the carbonization comprises a low-temperature carbonization treatment at a temperature of 300-750°C and at a drawing ratio of total drawing of 0-4% in an inert atmosphere and a high-temperature carbonization treatment at a temperature of 800-1500 °C and at a drawing ratio of total drawing of -4% to -2% in an inert atmosphere.
A polyacrylonitrile precursor fiber, which is characterized in that the Si/C ratio of the region between the top of the protrusion of fiber surface and the inner 2 µm of the polyacrylonitrile precursor fiber is in the range of from 1.0% to 20%, and the Si/C ratio of the region of exceeding 2 µm is not higher than 0.15%.
The polyacrylonitrile carbon fiber precursor fiber according to claim 15, characterized in that the oil content of the precursor fiber is 0.5%-2.5%, preferably 0.8%-2.5%, preferably 0.8%-2.4%, more preferably 0.9%-2.0%; and/or
the silicon content of the precursor fiber is 0.01%-0.5%; and/or

the monofilament linear density of the polyacrylonitrile precursor fiber is 0.7-1.0 dtex; and/or

the polyacrylonitrile precursor fiber is prepared by wet spinning.
A method for producing polyacrylonitrile precursor fiber, preferably the polyacrylonitrile precursor fiber according to claim 15 or 16, including wet spinning and including an oiling step, wherein the oiling step comprises two oiling procedures in succession and no drying densification stage is arranged between the two oiling procedures; wherein the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof, and the second oiling procedure uses a silicon-containing oiling agent.
The method according to claim 17, characterized in that
the first oiling procedure uses a silicon-free oiling agent, a low-silicon oiling agent or a combination thereof with a weight concentration of 0.1%-5.0%; and the second oiling procedure uses a silicon-containing oiling agent with a weight concentration of 0.1%-5.0%; and/or

the residence time of the second oiling procedure is less than the residence time of the first oiling procedure; and/or

the average particle sizes of the silicon-free oiling agent, the low-silicon oiling agent and the silicon-containing oiling agent are each independently 50 nm-500 nm; and/or

the silicon content of the silicon-containing oiling agent used for the second oiling procedure is 0.01wt%-0.9 wt%; and/or

the pH difference between the silicon-free oiling agent or low-silicon oiling agent and the silicon-containing oiling agent is not more than 1; and/or

the polarity of the surfactant in the silicon-free oiling agent or low-silicon oiling agent is not opposite to that of the surfactant in the silicon-containing oiling agent, and preferably the silicon-containing oiling agent is an oiling agent comprising a nonionic surfactant; and/or

before oiling, the fiber swelling degree of the polyacrylonitrile precursor fiber after water washing is 80% to 150%; and/or

the producing method includes the steps of wet coagulation forming of polyacrylonitrile stock solution, coagulation drawing, hot water drawing, water washing, oiling, drying densification, steam drawing and steam heat setting, so as to obtain the polyacrylonitrile precursor fiber.