Cyclomeric homogeneous pre-oxidized fibers, their preparation methods and applications
By embedding vinyl silane coupling agents and transition metal complexes into the polyacrylonitrile molecular chain, combined with gradient oxygen concentration and temperature control, the problem of uneven fiber cyclization during pre-oxidation was solved, enabling the preparation of high-cyclization pre-oxidized fibers and improving carbon fiber performance and production stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- ZHONGFU SHENYING CARBON FIBER
- Filing Date
- 2026-05-25
- Publication Date
- 2026-06-30
AI Technical Summary
In traditional pre-oxidation processes, the uneven cyclization between the fiber surface and core limits the improvement of carbon fiber performance, and existing improvement methods are either too complex or offer limited improvement.
The copolymer containing vinyl silane coupling agent and transition metal complex is used as a cyclization accelerator. It is embedded into the polyacrylonitrile molecular chain through covalent bonds. Combined with gradient-controlled changes in oxygen concentration and temperature, the radial homogenization of the fiber is achieved.
The pre-oxidized fibers prepared have a cyclization degree of over 90%, and the shrinkage rate, density, and heat resistance of each part of the fiber are consistent. After carbonization, the mechanical properties are significantly improved, and the dependence on the precision of temperature and oxygen control of the equipment is reduced, thus improving production stability.
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Figure CN122304051A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of carbon fiber preparation technology, and more specifically, to cyclized homogeneous pre-oxidized fibers, their preparation methods, and applications. Background Technology
[0002] Polyacrylonitrile (PAN)-based carbon fibers are widely used in aerospace, defense, and high-end civilian applications due to their excellent properties such as high specific strength and high specific modulus. Pre-oxidation treatment is a key step in carbon fiber preparation, aiming to transform linear PAN macromolecules into heat-resistant trapezoidal cyclic structures, laying the foundation for subsequent carbonization processes.
[0003] However, traditional pre-oxidation processes have long suffered from a persistent problem – the "core-skin structure." Because the pre-oxidation reaction is exothermic, and oxygen diffusion from the surface inwards occurs with a concentration gradient, the fiber surface undergoes cyclization, dehydrogenation, and oxidation first, forming a dense structure that hinders further oxygen diffusion into the interior. The resulting pre-oxidized fibers exhibit a radially non-uniform structure with high surface cyclization and oxygen content, while the core shows incomplete cyclization. This structural difference is inherited by the carbon fibers during subsequent carbonization, forming microscopic defects that severely restrict the improvement of the carbon fiber's mechanical properties.
[0004] To address this issue, various improvement schemes have been proposed in existing technologies. For example, combining infrared heating with hot air convection attempts to ensure oxygen penetration into the fiber interior; or humidifying the fiber before pre-oxidation; or introducing vinyl phenolic compounds through copolymerization modification to promote cyclization reactions. However, these methods either involve complex equipment modifications or offer limited improvement to the core-sheath structure, making it difficult to achieve truly radial homogeneous oxidation.
[0005] In view of this, the present invention is proposed. Summary of the Invention
[0006] The purpose of this invention is to provide cyclized homogeneous pre-oxidized fibers, their preparation methods, and applications, aiming to improve at least one of the problems mentioned in the background art.
[0007] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing cyclized homogeneous pre-oxidized fibers, comprising: Acrylonitrile, comonomer and cyclization accelerator are subjected to free radical polymerization in a first solvent to obtain modified polyacrylonitrile spinning solution; Modified polyacrylonitrile precursor fibers are obtained by spinning the modified polyacrylonitrile spinning solution. The modified polyacrylonitrile precursor fiber was pre-oxidized under an inert atmosphere to obtain the initial product. The temperature of the pre-oxidation process was gradually increased from 180~200℃ to 260~270℃, and the oxygen volume fraction was gradually decreased in multiple stages from 18~21% to 0.5~2%. The initial product was heat-set in an inert atmosphere to obtain cyclized homogeneous pre-oxidized fiber; The comonomer is an olefinic unsaturated dicarboxylic acid; the cyclization accelerator is a copolymer of a vinyl-containing silane coupling agent and a transition metal complex; The mass ratio of comonomer to acrylonitrile is 5.0~8.0:100, and the mass ratio of cyclization accelerator to acrylonitrile is 0.5~3.5:100.
[0008] In an optional implementation, at least one of the following features (1) and (2) is included: (1) The first solvent is selected from at least one of dimethyl sulfoxide, dimethylformamide and dimethylacetamide; (2) The olefinic unsaturated dicarboxylic acid is selected from at least one of itaconic acid, citraconic acid and mesocarboxylic acid; In an optional embodiment, the transition metal complex is a metal acetylacetonate; Optionally, the metal acetylacetone salt is selected from at least one of iron acetylacetone, cobalt acetylacetone, and nickel acetylacetone.
[0009] In an optional embodiment, the vinyl-containing silane coupling agent is selected from at least one of vinyltriethoxysilane, vinyltrimethoxysilane, and vinyltri(β-methoxyethoxy)silane.
[0010] In an optional embodiment, the preparation method of the cyclization accelerator includes: reacting a vinyl-containing silane coupling agent, a transition metal complex and an initiator in a second solvent at 70-90°C under an inert atmosphere, removing the second solvent, and obtaining the cyclization accelerator. Optionally, the initiator is selected from at least one of benzoyl peroxide, azobisisobutyronitrile, and tert-butyl peroxide; Optionally, the molar ratio of the initiator to the transition metal complex is 0.01 to 0.1:1; Optionally, the molar ratio of the vinyl-containing silane coupling agent to the transition metal complex is 1 to 3:1.
[0011] In an optional implementation, the pre-oxidation treatment may include: Treat at 180–200℃ with an oxygen volume fraction of 18–21% for 15–25 min; Treat at 210–230℃ with an oxygen volume fraction of 10–15% for 20–30 min; Treat at 240–260℃ with an oxygen volume fraction of 3–8% for 25–35 minutes; Treat at 260–270°C with an oxygen volume fraction of 0.5–2% for 10–20 minutes.
[0012] In an optional embodiment, the heat setting temperature is 220–250°C, and the time is 10–30 min.
[0013] In an optional implementation, the inert atmosphere is a nitrogen or argon atmosphere.
[0014] Secondly, the present invention provides a cyclized homogeneous pre-oxidized fiber, which is prepared by any of the preparation methods described in the foregoing embodiments; Optionally, its degree of cyclization is greater than 90%.
[0015] Thirdly, the present invention provides the application of the cyclic homogeneous pre-oxidized filament as described in the foregoing embodiments in the preparation of carbon fibers.
[0016] The present invention has the following beneficial effects: The pre-oxidized fiber prepared by the method provided by this invention has the following excellent properties: Radial homogeneity: The pre-oxidized filaments prepared by this invention have a cyclization degree higher than 90%. This means that the shrinkage rate, density, and heat resistance of each part of the fiber tend to be consistent; High carbonization conversion rate: Since the core of the pre-oxidized fiber is fully circumscribed, the fiber shrinks synchronously inside and outside during the subsequent high-temperature carbonization treatment, resulting in minimal thermal stress. This avoids fatal defects such as voids and cracks caused by severe core pyrolysis, thereby significantly improving the mechanical properties of the final carbon fiber.
[0017] Expanded process window: Due to the presence of endogenous catalysts, the extreme dependence on the temperature and oxygen control precision of pre-oxidation equipment is reduced, enabling the production of high-quality pre-oxidized fibers to be obtained even with certain furnace temperature fluctuations in large-scale industrial production, thus significantly improving production yield. Attached Figure Description
[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.
[0019] Figure 1 This is a flowchart of the preparation process for Example 1; Figure 2 Microscopic images of the pre-oxidized fiber sample prepared and sliced in Example 1; Figure 3This is a microscopic image of the pre-oxidized fiber sample prepared and sliced in Example 2; Figure 4 Microscopic images of the pre-oxidized fiber sample prepared and sliced in Example 3; Figure 5 Microscopic images of the pre-oxidized fiber sample prepared and sliced according to Comparative Example 1; Figure 6 Microscopic images of the pre-oxidized fiber sample prepared and sliced according to Comparative Example 2; Figure 7 Microscopic images of the pre-oxidized fiber sample prepared and sliced according to Comparative Example 3; Figure 8 Microscopic images of the pre-oxidized fiber sample prepared in Comparative Example 4 after sectioning; Figure 9 This is a microscopic image of the pre-oxidized fiber sample prepared and sliced according to Comparative Example 5. Detailed Implementation
[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.
[0021] The features and performance of the present invention will be further described in detail below with reference to embodiments.
[0022] The method for preparing cyclized homogeneous pre-oxidized fibers provided in this embodiment of the invention includes: Acrylonitrile, comonomer and cyclization accelerator are subjected to free radical polymerization in a first solvent to obtain modified polyacrylonitrile spinning solution; Modified polyacrylonitrile precursor fibers are obtained by spinning the modified polyacrylonitrile spinning solution. The modified polyacrylonitrile precursor fiber was pre-oxidized under an inert atmosphere to obtain the initial product. The temperature of the pre-oxidation process was gradually increased from 180~200℃ to 260~270℃, and the oxygen volume fraction was gradually decreased in multiple stages from 18~21% to 0.5~2%. The initial product was heat-set in an inert atmosphere to obtain cyclized homogeneous pre-oxidized fiber; The comonomer is an olefinic unsaturated dicarboxylic acid, and the cyclization accelerator is a copolymer of a vinyl-containing silane coupling agent and a transition metal complex. The mass ratio of comonomer to acrylonitrile is 5.0~8.0:100, and the mass ratio of cyclization accelerator to acrylonitrile is 0.5~3.5:100.
[0023] The method for preparing cyclized homogeneous pre-oxidized fibers provided in this invention uses a copolymer of a vinyl-containing silane coupling agent and a transition metal complex as a cyclizing agent. After polymerization, the transition metal ions are dispersed at the molecular level and covalently bonded to the polyacrylonitrile molecular chain. The transition metal ions act as Lewis acid catalysts inside the polyacrylonitrile macromolecules, playing a catalytic role in the pre-oxidation stage. This can significantly reduce the activation energy of the cyclization reaction of nitrile groups (-C≡N). This means that even in the fiber core where no oxygen has penetrated, the cyclization reaction can be initiated first, forming a "cyclization core" and building a heat-resistant skeleton in advance, providing a structural basis for the subsequent entry of oxygen.
[0024] The distribution of transition metal ions in the radial direction of the fiber is completely consistent with the distribution of polyacrylonitrile molecular chains, ensuring the uniformity of catalysis; chemical bonding ensures that the catalyst will not precipitate or migrate during subsequent complex thermal treatment processes (pre-oxidation, and the carbonization process after obtaining pre-oxidized fibers), guaranteeing the effective utilization of catalytic activity throughout the entire process.
[0025] In the preparation of high-performance carbon fibers using the pre-oxidized filaments obtained in the embodiments of this invention, when entering the high-temperature carbonization process, metal ions are uniformly dispersed in a network composed of thousands of carbon atoms. Although the content of metal ions is small, the average spacing between them is very large. Under the inert atmosphere exceeding 1000°C during the carbonization process, the transition metal ions react with the surrounding carbon and nitrogen to form extremely fine carbides or nitrides. When these particles are smaller than nanometers (<5nm) and extremely uniformly distributed, they are not defects but can instead become "nucleation centers" for the growth of graphite microcrystals, similar to the catalytic effect in the growth of carbon nanotubes, which helps to form a more perfect graphite structure.
[0026] In the low-temperature range (180~200℃), moderate surface activation is achieved using a high oxygen concentration; however, in the high-temperature range (240~270℃) where the cyclization reaction is highly exothermic, the oxygen concentration is significantly reduced to near an inert atmosphere. This brings two benefits: first, it suppresses the surface reaction rate, preventing the cortex from forming a dense shell due to overheating and explosive polymerization; second, it utilizes the intensified molecular chain movement at high temperatures to fully cyclize the unreacted molecular chains in the core under the influence of heat and micro-oxygen. This two-way advancement mode, "initiated from the inside and then proceeding from the outside in," achieves uniformity in the radial reaction degree of the fiber.
[0027] The pre-oxidized fiber prepared by the method provided by this invention has the following excellent properties: Radial homogeneity: The pre-oxidized filaments prepared by this invention have a cyclization degree higher than 90%. This means that the shrinkage rate, density, and heat resistance of each part of the fiber tend to be consistent; High carbonization conversion rate: Since the core of the pre-oxidized fiber is fully circumscribed, the fiber shrinks synchronously inside and outside during the subsequent high-temperature carbonization treatment, resulting in minimal thermal stress and avoiding fatal defects such as voids and cracks caused by severe pyrolysis of the core.
[0028] Expanded process window: Due to the presence of endogenous catalysts, the extreme dependence on the temperature and oxygen control precision of pre-oxidation equipment is reduced, enabling the production of high-quality pre-oxidized fibers to be obtained even with certain furnace temperature fluctuations in large-scale industrial production, thus significantly improving production yield.
[0029] It should be noted that the ratio of comonomer to acrylonitrile during the preparation process must be within the range required by this invention. Excessive comonomer usage will lead to decreased regularity of the polyacrylonitrile molecular chain, reduced crystallinity, and poorer mechanical properties of the precursor fiber, even affecting spinning continuity. Insufficient comonomer usage will result in inadequate cyclization reaction activation energy, insufficient cyclization during pre-oxidation, and minimal improvement in the core-sheath structure. The ratio of cyclization accelerator to acrylonitrile must also be within the range required by this invention. Excessive cyclization accelerator usage will lead to excessively high metal ion content in the fiber, resulting in excessively high density and small interparticle spacing of the nano-carbide particles formed after carbonization, causing stress concentration and reducing the mechanical properties of the carbon fiber. Insufficient cyclization accelerator usage will result in insufficient catalytic effect, failing to effectively initiate the cyclization reaction in the fiber core, leaving the pre-oxidized fiber with a distinct core-sheath structure, making homogenization difficult to achieve. Therefore, only when the ratio of each raw material in the preparation process meets the requirements of this invention can a pre-oxidized fiber with high cyclization degree be obtained.
[0030] Specifically, the preparation method includes: S1. Preparation of cyclization accelerator A vinyl-containing silane coupling agent, a transition metal complex, and an initiator are reacted in a second solvent at 70–90 °C for 5–7 h under an inert atmosphere. The second solvent is then removed by rotary evaporation to obtain a cyclization accelerator.
[0031] Optionally, the vinyl-containing silane coupling agent is selected from at least one of vinyltriethoxysilane, vinyltrimethoxysilane, and vinyltri(β-methoxyethoxy)silane.
[0032] Optionally, the transition metal complex is a metal acetylacetonate. Preferably, the metal acetylacetonate is selected from at least one of iron acetylacetonate, cobalt acetylacetonate, and nickel acetylacetonate.
[0033] Optionally, the initiator is selected from at least one of benzoyl peroxide, azobisisobutyronitrile, and tert-butyl peroxide.
[0034] Optionally, the molar ratio of the initiator to the transition metal complex is 0.01 to 0.1:1; Optionally, the molar ratio of the vinyl-containing silane coupling agent to the transition metal complex is 1 to 3:1.
[0035] Optionally, to ensure a more suitable concentration in the reaction system and to guarantee a stable reaction, the ratio of the transition metal complex to the second solvent is 1 mol: 500~1500 mL.
[0036] Optionally, the second solvent is toluene, tetrahydrofuran, or N,N-dimethylformamide.
[0037] S2. Preparation of modified polyacrylonitrile spinning solution Acrylonitrile, comonomer, and cyclization accelerator are subjected to free radical polymerization in a first solvent to obtain a modified polyacrylonitrile spinning solution.
[0038] The comonomer is an olefinic unsaturated dicarboxylic acid, which reduces the activation energy of the cyclization reaction of the nitrile group in the polyacrylonitrile molecular chain, promotes the formation of the ladder structure during the pre-oxidation process, and improves the hydrophilicity and spinnability of the fiber; it can be selected from at least one of itaconic acid, citraconic acid and mesocarboxylic acid.
[0039] The mass ratio of comonomer to acrylonitrile is 5.0~8.0:100, and the mass ratio of cyclization accelerator to acrylonitrile is 0.5~3.5:100.
[0040] Optionally, the first solvent is selected from at least one of dimethyl sulfoxide, dimethylformamide, and dimethylacetamide; Optionally, in order to ensure that the modified polyacrylonitrile spinning solution has a suitable concentration and that the fibers obtained by electrospinning have suitable size uniformity and mechanical properties, the mass ratio of acrylonitrile to the first solvent is 100:300~400.
[0041] S3, spinning The modified polyacrylonitrile spinning solution was spun using conventional spinning methods to obtain modified polyacrylonitrile precursor fibers with a single filament fineness of 0.75~1.00 dtex.
[0042] S4, Pre-oxidation treatment The modified polyacrylonitrile precursor fiber was pre-oxidized under an inert atmosphere to obtain the initial product. The temperature of the pre-oxidation process was gradually increased from 180~200℃ to 260~270℃, and the oxygen volume fraction was gradually decreased from 18~21% to 0.5~2%.
[0043] The pre-oxidation treatment specifically includes four gradient treatment stages: Treating at 180–200 °C with an oxygen volume fraction of 18–21% for 15–25 min constitutes the cyclization induction period (low temperature), providing sufficient oxygen to form the necessary oxygen-containing groups (such as -OH, C=O), which are the active sites for subsequent intermolecular crosslinking.
[0044] Treat at 210–230℃ with an oxygen volume fraction of 10–15% for 20–30 min; Treat at 240–260℃ with an oxygen volume fraction of 3–8% for 25–35 minutes; These two stages are the period of rapid increase in cyclization reaction (medium temperature). Simultaneously reducing the oxygen concentration allows the heat of reaction to be released steadily, avoiding fiber melting, breakage, or filament fusion caused by "thermal runaway".
[0045] Treat at 260–270℃ with an oxygen volume fraction of 0.5–2% for 10–20 minutes. This stage is the completion period of the cyclization reaction (high temperature), which almost cuts off the oxygen source, inhibits the surface reaction rate, and avoids the formation of a dense shell due to overheating and explosive polymerization of the skin. In addition, the increased molecular chain movement at high temperature can be used to fully cyclize the unreacted molecular chains in the core under the action of heat and micro-oxygen.
[0046] S5, Heat setting Under an inert atmosphere, the initial product is heat-treated at 220–250°C for 10–30 minutes.
[0047] The existing structure is further rearranged and densified by using appropriate temperatures.
[0048] The cyclized homogeneous pre-oxidized fiber provided in this embodiment of the invention is prepared by the preparation method provided in this embodiment of the invention, and its degree of cyclization is greater than 90%.
[0049] Optionally, the inert gas involved in the embodiments of the present invention is nitrogen or argon.
[0050] The cyclized homogeneous pre-oxidized fiber provided in this invention can be applied to the preparation of carbon fibers, and can produce carbon fibers with good performance.
[0051] Example 1 like Figure 1 As shown, this embodiment provides a method for preparing cyclized homogeneous pre-oxidized fibers, specifically as follows: S1: In a three-necked flask equipped with a stirrer, a condenser and a nitrogen inlet, add 1 mol of ferric acetylacetone and 1.2 mol of vinyltriethoxysilane. Use 1000 mL of toluene as solvent and add 0.05 mol of benzoyl peroxide as an initiator. React at 80 °C for 6 h. Remove the solvent by rotary evaporation to obtain a reddish-brown viscous liquid, which is the cyclization promoter.
[0052] S2: By weight, 80 parts acrylonitrile, 20 parts itaconic acid, and 2.5 parts of the above cyclization accelerator are added to 300 parts dimethyl sulfoxide to initiate polymerization and obtain a PAN spinning solution containing the cyclization accelerator.
[0053] S3: Modified polyacrylonitrile precursor fibers were prepared using a conventional dry-jet wet spinning process, with a single filament fineness of 1.2 dtex.
[0054] S4: Place the raw yarn in a five-zone pre-oxidation furnace for treatment: Zone 1: 190℃, 21% oxygen, 20 min; Zone 2: 220℃, 12% oxygen, 25 min; Zone 3: 250℃, 5% oxygen, 30 min; Zone 4: 265℃, 1% oxygen, 15 min; Zone 5 is a transition zone, under nitrogen protection.
[0055] S5: Heat-set at 240℃ for 20 min under nitrogen protection; the degree of cyclization of the obtained pre-oxidized fiber is 92%.
[0056] Example 2 This embodiment provides a method for preparing cyclized homogeneous pre-oxidized fibers, specifically as follows: S1: In a three-necked flask equipped with a stirrer, a condenser and a nitrogen inlet, add 1 mol of nickel acetylacetonate and 1.0 mol of vinyltriethoxysilane, use 500 mL of toluene as solvent, add 0.01 mol of benzoyl peroxide as an initiator, react at 80 °C for 6 h, remove the solvent by rotary evaporation, and obtain a reddish-brown viscous liquid, which is the cyclization promoter.
[0057] S2: By weight, 100 parts acrylonitrile, 5.0 parts itaconic acid, and 0.5 parts of the above cyclization accelerator are added to 300 parts dimethyl sulfoxide to initiate polymerization and obtain spinning solution.
[0058] S3: Modified polyacrylonitrile precursor fibers were prepared using a conventional dry-jet wet spinning process, with a single filament fineness of 0.8 dtex.
[0059] S4: Place the raw yarn in a five-zone pre-oxidation furnace for treatment: Zone 1: 180℃, 20% oxygen, 15 min; Zone 2: 210℃, 15% oxygen, 20 min; Zone 3: 240℃, 8% oxygen, 25 min; Zone 4: 260℃, 2% oxygen, 10 min; Zone 5 is a transition zone, under nitrogen protection.
[0060] S5: Heat-set at 220℃ for 30 min under nitrogen protection; the degree of cyclization of the resulting pre-oxidized fiber is 90%.
[0061] Example 3 This embodiment provides a method for preparing cyclized homogeneous pre-oxidized fibers, specifically as follows: S1: In a three-necked flask equipped with a stirrer, a condenser and a nitrogen inlet, add 1 mol of cobalt acetylacetonate and 3.0 mol of vinyltriethoxysilane, use 1500 mL of toluene as solvent, add 0.1 mol of benzoyl peroxide as an initiator, react at 80 °C for 6 h, remove the solvent by rotary evaporation, and obtain a reddish-brown viscous liquid, which is the cyclization promoter.
[0062] S2: By weight, 100 parts acrylonitrile, 8 parts itaconic acid, and 3.5 parts of the above cyclization accelerator are added to 400 parts dimethyl sulfoxide to initiate polymerization and obtain spinning solution.
[0063] S3: Modified polyacrylonitrile precursor fibers were prepared using a conventional dry-jet wet spinning process, with a single filament fineness of 1.5 dtex.
[0064] S4: Place the raw yarn in a five-zone pre-oxidation furnace for treatment: Zone 1: 200℃, oxygen 18%, hold for 25 min; Zone 2: 230℃, oxygen 10%, hold for 30 min; Zone 3: 260℃, oxygen 3%, hold for 35 min; Zone 4: 270℃, oxygen 0.5%, hold for 20 min; Zone 5 is a transition zone, under nitrogen protection.
[0065] S5: Heat-set at 250℃ for 10 min under nitrogen protection; the degree of cyclization of the resulting pre-oxidized fiber is 95%.
[0066] Comparative Example 1 This comparative example provides a method for preparing polyacrylonitrile pre-oxidized fibers, which is basically the same as that in Example 1, except that no cyclization accelerator is added in step S2.
[0067] Comparative Example 2 This comparative example provides a method for preparing polyacrylonitrile pre-oxidized fibers, which is basically the same as that in Example 1, except that the pre-oxidation treatment in step S4 adopts a conventional constant oxygen concentration process (oxygen concentration of 21% throughout the entire process) and the total treatment time is the same as that in Example 1.
[0068] Comparative Example 3 This comparative example provides a method for preparing polyacrylonitrile pre-oxidized fibers, which is basically the same as that in Example 1, except that: no cyclization accelerator is added in step S2; the pre-oxidation treatment in S4 adopts a conventional constant oxygen concentration process (oxygen concentration of 21% throughout the entire stage) and the total treatment time is the same as that in Example 1.
[0069] Comparative Example 4 This comparative example provides a method for preparing polyacrylonitrile pre-oxidized fiber, which is basically the same as that in Example 1, except that in step S1, iron acetylacetone is replaced with an equimolar amount of acetylacetone.
[0070] Comparative Example 5 This comparative example provides a method for preparing polyacrylonitrile pre-oxidized fibers, which is basically the same as that in Example 1, except that the cyclization accelerator is not introduced into the PAN molecular chain through copolymerization reaction, but is introduced by physical impregnation after the preparation of the precursor fiber.
[0071] The specific steps are as follows: (1) Preparation of unmodified PAN precursor yarn: According to the formulation of Example 1, but without adding cyclization accelerator, that is, using only 80 parts by weight of acrylonitrile and 20 parts by weight of itaconic acid as monomers, free radical polymerization was carried out in 300 parts by weight of dimethyl sulfoxide to obtain spinning solution. Then, unmodified PAN precursor yarn with a single filament fineness of 1.2 dtex was obtained by using the same wet spinning process as in Example 1.
[0072] (2) Preparation of impregnation solution: Take the same amount (2.5 parts by weight, relative to the mass of acrylonitrile) of cyclization accelerator (vinyltriethoxysilane-grafted acetylacetone iron complex) as in Example 1, dissolve it in 100 parts by weight of dimethyl sulfoxide, stir at room temperature until completely dissolved, and obtain an impregnation solution with a mass fraction of about 2.4%.
[0073] (3) Impregnation treatment: The unmodified PAN precursor obtained in step (1) is passed through the above impregnation bath under constant tension. The impregnation temperature is 60°C and the soaking time is 30 min, so that the catalyst is fully adsorbed on the fiber surface and near the surface layer. During the process, ultrasonic assistance (frequency 40kHz, power 100W) is used to promote penetration, but avoid damaging the fiber structure.
[0074] (4) Rinsing and drying: Rinse the surface of the impregnated fiber with deionized water for 30 seconds to remove excess, unadsorbed catalyst droplets. Then dry in a vacuum drying oven at 80°C for 2 hours to remove residual solvent and moisture.
[0075] (5) Pre-oxidation and heat setting: The subsequent pre-oxidation process (segmented gradient oxygen control) and heat setting process are exactly the same as in Example 1 (zone 1 190℃ / 21% / 20min, zone 2 220℃ / 12% / 25min, zone 3 250℃ / 5% / 30min, zone 4 265℃ / 1% / 15min, zone 5 transition nitrogen, and then heat setting at 240℃ for 20min).
[0076] Experimental Example (1) Cyclocyclization degree test of polyacrylonitrile pre-oxidized fiber Test method: The polyacrylonitrile pre-oxidized fibers prepared in the examples and comparative examples were used as samples. The degree of cyclization reaction of each sample was then tested, and the test results are summarized in Table 1. The specific test steps were as follows: the dried polyacrylonitrile pre-oxidized fibers were cut into small pieces, and KBr was mixed with the polyacrylonitrile pre-oxidized fibers at a mass ratio of 200:5 and ground to prepare KBr pellets for infrared analysis; where IC≡N is the characteristic absorption peak intensity of the cyano group, corresponding to a wavenumber of 2240 cm⁻¹. -1 IC=N represents the characteristic absorption peak intensity of C=N, corresponding to a wavenumber of 1580 cm⁻¹. -1 .
[0077] Table 1
[0078] Referring to Table 1, the test results of Example 1 and the comparative examples show that the polyacrylonitrile pre-oxidized fibers prepared according to the preparation process provided in the examples of this application have a higher degree of cyclization, indicating a higher degree of pre-oxidation. Comparative Example 1 did not add a cyclization accelerator, and its degree of cyclization was significantly lower; Comparative Example 2 used conventional constant oxygen concentration for pre-oxidation, and its degree of cyclization was significantly lower; Comparative Example 3 did not add a cyclization accelerator, nor did it use the pre-oxidation method of this invention, and the degree of cyclization of the pre-oxidized fiber was significantly reduced; Comparative Example 4, by using acetylacetone to replace iron acetylacetone, had a significantly lower degree of cyclization of the pre-oxidized fiber, indicating that the presence of iron ions can play a certain role in improving the degree of cyclization; Comparative Example 5 used a conventional physical impregnation method to add a catalyst (not grafted onto the PAN molecular chain through covalent bonds). The catalyst was mainly distributed on the fiber surface and was easily agglomerated and detached. Although the degree of cyclization was slightly higher than that of Comparative Example 1 (81%), it was still significantly lower than that of Example 1 (92%), and the core-sheath structure was still obvious, indicating that chemical bonding is the key to achieving uniform catalysis and radial homogenization.
[0079] (2) Core-sheath structure test of polyacrylonitrile pre-oxidized fiber Test method: The polyacrylonitrile pre-oxidized fibers prepared in Examples 1-3 and Comparative Examples 1-5 were used as samples. Each PAN pre-oxidized fiber sample was then embedded in a mixture of epoxy resin and curing agent at a mass ratio of 15:2. After curing at 60°C for 2 h, the samples were sectioned using an ultramicrotome with a thickness of approximately 400 nm. The sections were then observed under a high-power optical microscope with a 20× objective lens, and cross-sectional micrographs of the pre-oxidized fibers were taken.
[0080] See Figures 2-9 It can be seen that the polyacrylonitrile pre-oxidized fiber prepared according to the process provided in the embodiments of this application has significantly fewer core-sheath delamination defects compared to the other pairs.
[0081] In summary, the preparation method provided by this invention has the following main features compared with the prior art: It pioneered a dual homogenization mechanism of "endogenous catalysis + oxygen-limiting gradient change," fundamentally eliminating the core-skin structure: Chemical level (endogenous catalysis): This invention is not a simple physical blend, but rather uses molecular design to graft acetylacetone iron (or other transition metal complexes) with a vinylsilane coupling agent as a comonomer, which is then chemically bonded into the PAN molecular chain. Transition metal ions (Fe²⁺) + / Fe³ + As a Lewis acid catalyst within the PAN macromolecule, it can significantly reduce the activation energy of the nitrile (-C≡N) cyclization reaction. This means that even in the fiber core where no oxygen permeates, the cyclization reaction can be initiated first, forming a "cyclization core" and building a heat-resistant skeleton in advance, providing a structural basis for the subsequent entry of oxygen.
[0082] At the physical level (reverse gradient change): Traditional processes often increase oxygen concentration at high temperatures to increase the oxygen content in the core, but this only leads to excessive oxidation and shrinkage of the outer layer, thus blocking oxygen channels. This invention employs a reverse approach of "high temperature, low oxygen": moderate surface activation is achieved at a high oxygen concentration (18-21%) in the low-temperature range (180-200℃); then, in the high-temperature range (240-270℃) where the cyclization reaction is intensely exothermic, the oxygen concentration is significantly reduced to near-inert atmosphere (0.5-2%). This brings two benefits: first, it suppresses the surface reaction rate, preventing the outer layer from forming a dense shell due to overheating and explosive polymerization; second, it utilizes the intensified molecular chain movement at high temperatures to fully cyclize the unreacted molecular chains in the core under the influence of heat and micro-oxygen. This bidirectional progression, "initiated from the inside and then from the outside in," achieves uniformity in the radial reaction degree of the fiber.
[0083] 2. The "anchoring" effect of the cyclization accelerator avoids catalyst migration and residual defects. Existing technologies have also attempted to add catalyst solutions to the impregnation tank, but the catalyst is only adsorbed on the fiber surface and has difficulty penetrating into the core. Furthermore, it is easy to fall off during subsequent water washing or high-temperature treatment, which not only greatly reduces the effectiveness but also forms metal impurity voids after carbonization, thus becoming a weakness in strength.
[0084] This invention "anchors" cyclization accelerators to the PAN molecular chain via copolymerization using covalent bonds: Uniformity of distribution: The accelerator achieves molecular-level dispersion in the spinning solution, and its radial distribution in the fiber is completely consistent with the distribution of PAN molecular chains, ensuring the uniformity of catalysis.
[0085] Durability: Chemical bonding ensures that the catalyst will not precipitate or migrate during the complex heat treatment process (including water washing, stretching, and pre-oxidation) in the subsequent pre-oxidation of carbon fibers, thus guaranteeing the effective utilization of catalytic activity throughout the process.
[0086] High efficiency with trace amounts: The amount of metal ions used is precisely controllable (the ratio of cyclization accelerator to acrylonitrile is 0.5%~3.5%). Taking iron ions as an example, during the high-temperature carbonization process, a very small amount of iron will exist in the form of iron carbide or complex, and some can even be transformed into active centers for catalytic graphitization, which helps to perfect the final carbon fiber microcrystalline structure.
[0087] 3. The coupled design of process parameters enabled precise control of reaction kinetics. This invention employs a nonlinear coupling design between temperature and oxygen concentration (e.g., 220℃ / 12% oxygen in the second segment, and 265℃ / 1% oxygen in the fourth segment), which is not a simple parameter superposition but rather based on an in-depth analysis of the kinetic model of the PAN oxidation reaction. During the cyclization induction phase (low temperature region), sufficient oxygen is provided to form the necessary oxygen-containing groups (such as -OH, C=O), which are the active sites for subsequent intermolecular crosslinking.
[0088] During the period of rapid increase in cyclization reaction (medium temperature zone), the oxygen concentration is reduced simultaneously to ensure that the heat of reaction is released steadily, thus avoiding fiber melting, breakage, or filament fusion caused by "thermal runaway".
[0089] During the cyclization reaction completion period (high temperature zone), the oxygen source is almost cut off, and the already formed trapezoidal structure is further rearranged and densified by the heat setting effect.
[0090] This "temperature rises, oxygen concentration falls" scissor-like control ensures that the fiber remains in a mild and uniform reaction environment throughout the entire pre-oxidation process.
[0091] 4. Significantly improved product performance and superior subsequent processing capabilities Radial homogeneity: Through the above synergistic effect, the pre-oxidized filament prepared by this invention has a cyclization degree of over 90%. This means that the shrinkage rate, density, and heat resistance of each part of the fiber tend to be consistent.
[0092] High carbonization conversion rate: Since the core has formed a perfect trapezoidal structure, the fiber shrinks synchronously inside and outside during the subsequent high-temperature carbonization process, resulting in minimal thermal stress and avoiding fatal defects such as voids and cracks caused by intense pyrolysis of the core.
[0093] Expanded process window: Due to the presence of endogenous catalysts, the extreme dependence on the temperature and oxygen control precision of pre-oxidation equipment is reduced, enabling the production of high-quality pre-oxidized fibers to be obtained even with certain furnace temperature fluctuations in large-scale industrial production, thus significantly improving production yield.
[0094] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for preparing cyclic homogeneous pre-oxidized fibers, characterized in that, include: Acrylonitrile, comonomer and cyclization accelerator are subjected to free radical polymerization in a first solvent to obtain modified polyacrylonitrile spinning solution; The modified polyacrylonitrile spinning solution is spun into modified polyacrylonitrile precursor fibers. The modified polyacrylonitrile precursor fiber was pre-oxidized under an inert atmosphere to obtain the initial product. The temperature of the pre-oxidation process was gradually increased from 180~200℃ to 260~270℃, and the oxygen volume fraction was gradually decreased in multiple stages from 18~21% to 0.5~2%. The initial product was heat-set under an inert atmosphere to obtain cyclized homogeneous pre-oxidized fiber; The comonomer is an olefinic unsaturated dicarboxylic acid; the cyclization accelerator is a copolymer of a vinyl-containing silane coupling agent and a transition metal complex; The mass ratio of the comonomer to the acrylonitrile is 5.0~8.0:100, and the mass ratio of the cyclization accelerator to the acrylonitrile is 0.5~3.5:
100.
2. The preparation method according to claim 1, characterized in that, Includes at least one of the following features (1) and (2): (1) The first solvent is selected from at least one of dimethyl sulfoxide, dimethylformamide and dimethylacetamide; (2) The olefinic unsaturated dicarboxylic acid is selected from at least one of itaconic acid, citraconic acid and mesocarboxylic acid.
3. The preparation method according to claim 1, characterized in that, The transition metal complex is a metal acetylacetonate; Optionally, the metal acetylacetone salt is selected from at least one of iron acetylacetone, cobalt acetylacetone, and nickel acetylacetone.
4. The preparation method according to claim 1, characterized in that, The vinyl-containing silane coupling agent is selected from at least one of vinyltriethoxysilane, vinyltrimethoxysilane, and vinyltri(β-methoxyethoxy)silane.
5. The preparation method according to claim 1, characterized in that, The preparation method of the cyclization accelerator includes: reacting the vinyl-containing silane coupling agent, the transition metal complex and the initiator in a second solvent at 70-90°C under an inert atmosphere, removing the second solvent, and obtaining the cyclization accelerator; Optionally, the initiator is selected from at least one of benzoyl peroxide, azobisisobutyronitrile, and tert-butyl peroxide; Optionally, the molar ratio of the initiator to the transition metal complex is 0.01 to 0.1:1; Optionally, the molar ratio of the vinyl-containing silane coupling agent to the transition metal complex is 1 to 3:
1.
6. The preparation method according to claim 1, characterized in that, Pre-oxidation treatment methods include: Treat at 180–200℃ with an oxygen volume fraction of 18–21% for 15–25 min; Treat at 210–230℃ with an oxygen volume fraction of 10–15% for 20–30 min; Treat at 240–260℃ with an oxygen volume fraction of 3–8% for 25–35 minutes; Treat at 260–270°C with an oxygen volume fraction of 0.5–2% for 10–20 minutes.
7. The preparation method according to claim 1, characterized in that, The heat setting temperature is 220–250℃, and the time is 10–30 min.
8. The preparation method according to claim 1, characterized in that, The inert atmosphere is a nitrogen or argon atmosphere.
9. A cyclic homogeneous pre-oxidized fiber, characterized in that, Prepared by the preparation method according to any one of claims 1 to 8; Optionally, its degree of cyclization is greater than 90%.
10. The application of the cyclized homogeneous pre-oxidized fiber as described in claim 9 in the preparation of carbon fibers.