Method for determining pre-oxidation process parameters of polyacrylonitrile-based carbon fibers

Abstract

The application relates to a method for determining pre-oxidation process parameters of polyacrylonitrile-based carbon fibers, and belongs to the technical field of carbon fibers. The method for determining pre-oxidation process parameters of polyacrylonitrile-based carbon fibers comprises first-stage pre-oxidation, the first-stage pre-oxidation temperature is determined based on DSC of the original fiber, and the first-stage pre-oxidation time is determined based on a cyclization index of the fiber at the first-stage pre-oxidation temperature. More reasonable pre-oxidation parameters can be determined, and polyacrylonitrile-based carbon fibers with high strength and high modulus can be prepared.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon fibers, and particularly relates to a method for determining pre-oxidation process parameters of polyacrylonitrile-based carbon fibers. BACKGROUND

[0002] Carbon fibers have become a key strategic material for promoting modern industrial innovation due to their superior specific strength and high modulus characteristics, and have shown extremely broad and far-reaching application prospects in the fields of aerospace, new energy, infrastructure, and transportation. Carbon fibers with ultra-high mechanical properties are not only the core foundation for realizing the extreme lightweight and ultra-high structural rigidity requirements of new-generation aerospace cutting-edge equipment (such as hypersonic aircraft, next-generation fighter jets, large civil aircraft, advanced satellites, etc.), but also an indispensable "material engine" for equipment performance breakthroughs and iterative upgrades, having a decisive influence on aerospace power and cutting-edge equipment competitiveness.

[0003] At present, most high-performance carbon fibers are prepared by copolymerization of acrylonitrile (AN) and itaconic acid (ITA). The carboxyl group in ITA initiates intramolecular cyclization, converting the free radical type cyclization of acrylonitrile into ionic type cyclization, thereby reducing the initial exothermic temperature and improving the pre-oxidation degree of the fiber. However, as a comonomer, the accelerated pre-oxidation rate of ITA more easily leads to a skin-core structure with excessive surface cyclization and insufficient internal cyclization. For example, the Chinese patent with application number CN202410617014.1 controls the temperature of the precursor fiber to gradually increase from 225 ℃ to 242 ℃ in an air atmosphere, and the precursor fiber is pre-oxidized in a pre-oxidation furnace for 70-90 min at a draw ratio of 0.92-0.94. The slow temperature increase in the single zone easily exacerbates the differentiation of the skin-core structure, and the cyclization degree of the core layer is easily less than 85%, resulting in insufficient stability of the fiber modulus after carbonization.

[0004] Therefore, the carbon fibers prepared by the prior art cannot simultaneously achieve high strength and high modulus. SUMMARY

[0005] In view of the deficiencies of the prior art, the purpose of the embodiments of the present application includes providing a method for determining pre-oxidation process parameters of polyacrylonitrile-based carbon fibers. More reasonable pre-oxidation parameters can be determined, which is beneficial to preparing polyacrylonitrile-based carbon fibers with high strength and high modulus.

[0006] In a first aspect, the embodiments of the present application provide a method for determining pre-oxidation process parameters of polyacrylonitrile-based carbon fibers. The pre-oxidation includes first-stage pre-oxidation. The first-stage pre-oxidation temperature is determined based on the DSC of the precursor fiber, and the first-stage pre-oxidation time is determined based on the cyclization index of the fiber at the first-stage pre-oxidation temperature.

[0007] The application determines the first-stage pre-oxidation temperature based on the DSC of the raw fiber, and determines the first-stage pre-oxidation time based on the cyclization index of the fiber at the first-stage pre-oxidation temperature, so that the first-stage pre-oxidation temperature and the first-stage pre-oxidation time are more reasonable, the problems of high-temperature intense reaction heat release and low-temperature pre-oxidation inefficiency are effectively improved, and efficient and uniform pre-oxidation is promoted, thereby facilitating the preparation of polyacrylonitrile-based carbon fibers with high strength and high modulus.

[0008] In some embodiments of the application, the method for determining the first-stage pre-oxidation temperature comprises: determining the DSC curve of the raw fiber by differential scanning calorimetry, and taking the temperature corresponding to the intersection point of the tangent line of the peak curve before the first peak and the bottom line of the peak in the DSC curve as the first-stage pre-oxidation temperature.

[0009] The application determines the DSC curve of the raw fiber by differential scanning calorimetry, and takes the temperature corresponding to the intersection point of the tangent line of the peak curve before the first peak and the bottom line of the peak in the DSC curve as the first-stage pre-oxidation temperature, so that the first-stage pre-oxidation temperature is more reasonable, and the problems of high-temperature intense reaction heat release and low-temperature pre-oxidation inefficiency are effectively improved.

[0010] In some embodiments of the application, the method for determining the first-stage pre-oxidation time comprises: pre-oxidizing the raw fiber at the first-stage pre-oxidation temperature, and determining the curve of the cyclization index of the raw fiber and the pre-oxidation time by in-situ infrared spectroscopy, and taking the time corresponding to the intersection point of the tangent lines of the two curves at the inflection point of the curve as the first-stage pre-oxidation time.

[0011] The application determines the cyclization index of the raw fiber at the first-stage pre-oxidation temperature by in-situ infrared spectroscopy, obtains the curve of the cyclization index of the fiber and the pre-oxidation time, and takes the time corresponding to the intersection point of the tangent lines of the two curves at the inflection point of the curve as the first-stage pre-oxidation time, so that efficient and uniform pre-oxidation is promoted.

[0012] In some embodiments of the application, the pre-oxidation further comprises second-stage pre-oxidation, the second-stage pre-oxidation temperature is determined based on the DSC of the fiber after the first-stage pre-oxidation, and the second-stage pre-oxidation time is determined based on the cyclization index of the fiber at the second-stage pre-oxidation temperature.

[0013] The application determines the second-stage pre-oxidation temperature based on the DSC of the first-stage pre-oxidized fiber, and determines the second-stage pre-oxidation time based on the cyclization index of the fiber at the second-stage pre-oxidation temperature, so that the second-stage pre-oxidation temperature and the second-stage pre-oxidation time are more reasonable, the problems of high-temperature intense reaction heat release and low pre-oxidation efficiency at low temperature are effectively improved, and the pre-oxidation is promoted to be efficient and uniform, thereby further facilitating the preparation of polyacrylonitrile-based carbon fibers with high strength and high modulus.

[0014] In some embodiments of the application, the draw ratio of the pre-oxidation treatment is adjusted according to the comparison of the crystal region orientation degree of the original fiber and the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment.

[0015] The application adjusts the draw ratio of the pre-oxidation treatment by comparing the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment and the crystal region orientation degree of the original fiber, so as to reduce the large decrease of the crystal region orientation degree of the fiber during the pre-oxidation treatment, thereby facilitating the preparation of polyacrylonitrile-based carbon fibers with high strength and high modulus.

[0016] In some embodiments of the application, the initial draw ratio of the pre-oxidation treatment is set, the crystal region orientation degree of the original fiber is L1, the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment is L2, when (L1-L2) / L1≤0.1, the initial draw ratio is the target draw ratio; when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased, the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment is L2', until (L1-L2') / L1≤0.1, the target draw ratio is obtained.

[0017] The application determines the target draw ratio by measuring the crystal region orientation degree of the original fiber and the fiber after the last-stage pre-oxidation treatment, and calculating the relative decrease of the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment compared with the crystal region orientation degree of the original fiber, so as to maintain a high crystal region orientation degree of the fiber, thereby facilitating the preparation of carbon fibers with high strength and high modulus.

[0018] In some embodiments of the application, when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased by 0.01-0.05.

[0019] When the relative decrease of the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment compared with the crystal region orientation degree of the original fiber is large, the application increases the draw ratio of the pre-oxidation treatment by a suitable amount, thereby reducing the large decrease of the crystal region orientation degree of the fiber during the pre-oxidation treatment.

[0020] In some embodiments of the present application, the method for determining the degree of orientation of the crystal region comprises: obtaining a curve graph of the diffraction intensity and azimuth angle of the fiber by using two-dimensional wide-angle X-ray diffraction method, and calculating the degree of orientation of the crystal region of the fiber as (360-FWHM1-FWHM2) / 360; wherein FWHM1 is the half-height width of the first peak, and FWHM2 is the half-height width of the second peak.

[0021] By using the two-dimensional wide-angle X-ray diffraction method to obtain the curve graph of the diffraction intensity and azimuth angle of the fiber, the degree of orientation of the crystal region of the fiber is calculated, and the relative reduction of the degree of orientation of the crystal region of the fiber after the last stage of pre-oxidation compared with the degree of orientation of the crystal region of the original fiber is calculated, so as to determine the target draft ratio, so as to maintain a high degree of orientation of the crystal region of the fiber, thereby facilitating the preparation of carbon fibers with high strength and high modulus.

[0022] In some embodiments of the present application, the pre-oxidation adopts four-stage pre-oxidation treatment.

[0023] By using four-stage pre-oxidation treatment, better pre-oxidation and suitable cost are taken into account.

[0024] In some embodiments of the present application, the atmosphere of the first-stage pre-oxidation to the third-stage pre-oxidation is air, and the atmosphere of the fourth-stage pre-oxidation is nitrogen.

[0025] By using air atmosphere for the first three stages of pre-oxidation and nitrogen atmosphere for the fourth stage of pre-oxidation, the first three stages of pre-oxidation can moderately achieve the oxidation degree and high cyclization degree in the air atmosphere, and the last stage of pre-oxidation can stop increasing the oxidation degree and continue to increase the cyclization degree in the nitrogen atmosphere. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The DSC curve of the fiber in the pre-oxidation in Example 1 of the present application.

[0027] Figure 2 The curve graph of the cyclization index and pre-oxidation time of the fiber in the first-stage pre-oxidation in Example 1 of the present application.

[0028] Figure 3 The curve graph of the cyclization index and pre-oxidation time of the fiber in the second-stage pre-oxidation in Example 1 of the present application.

[0029] Figure 4 The curve graph of the cyclization index and pre-oxidation time of the fiber in the third-stage pre-oxidation in Example 1 of the present application.

[0030] Figure 5 The curve graph of the cyclization index and pre-oxidation time of the fiber in the fourth-stage pre-oxidation in Example 1 of the present application.

[0031] Figure 6A graph of the diffraction intensity versus azimuth angle of the precursor fiber in Example 1 of the present application.

[0032] Figure 7 An XRD graph of the carbon fiber prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below. If the specific conditions are not specified in the embodiments, the conventional conditions or the conditions suggested by the manufacturer are adopted. If the manufacturers of the reagents or instruments are not specified, they are all the conventional products that can be purchased in the market.

[0034] The embodiments of the present application provide a method for determining the pre-oxidation process parameters of polyacrylonitrile-based carbon fiber, the pre-oxidation includes first-stage pre-oxidation, the first-stage pre-oxidation temperature is determined based on the DSC of the precursor fiber, and the first-stage pre-oxidation time is determined based on the cyclization index of the fiber at the first-stage pre-oxidation temperature.

[0035] The temperature of the first-stage pre-oxidation is determined based on the DSC of the precursor fiber, after the temperature of the first-stage pre-oxidation is determined, the precursor fiber is pre-oxidized at the temperature of the first-stage pre-oxidation, and then the cyclization index of the fiber at the first-stage pre-oxidation temperature is determined, the first-stage pre-oxidation time is determined based on the cyclization index of the fiber, the first-stage pre-oxidation temperature and the first-stage pre-oxidation time determined by the method are more reasonable, which can effectively improve the problems of high-temperature intense reaction heat release and low-temperature pre-oxidation efficiency, and promote the pre-oxidation to be efficient and uniform, thereby facilitating the preparation of polyacrylonitrile-based carbon fiber with high strength and high modulus.

[0036] In some embodiments of the present application, the method for determining the first-stage pre-oxidation temperature includes: determining the DSC curve of the precursor fiber by differential scanning calorimetry, and taking the temperature corresponding to the intersection point of the tangent line of the peak front curve and the peak bottom line at the top point of the first peak in the DSC curve as the first-stage pre-oxidation temperature. The first-stage pre-oxidation temperature can be more reasonable, which can effectively improve the problems of high-temperature intense reaction heat release and low-temperature pre-oxidation efficiency. It should be noted that when the peak front baseline and the peak rear baseline of the peak are on a straight line, the connecting line of the peak front baseline and the peak rear baseline is taken as the peak bottom line; when the peak front baseline and the peak rear baseline of the peak are not on a straight line, the connecting line of the intersection points of the peak front baseline and the peak rear baseline extension lines and the peak front curve and the peak rear curve respectively is taken as the peak bottom line. The baseline refers to the heat flow curve with temperature when the sample cell and the reference cell are empty.

[0037] In some embodiments of the present application, the method for determining the first-stage pre-oxidation time comprises: pre-oxidizing the raw fiber at the first-stage pre-oxidation temperature, determining the cyclization index of the raw fiber and the pre-oxidation time by in-situ infrared spectroscopy, and taking the time corresponding to the intersection of the tangent lines of the two curves at the inflection point of the curve in the graph as the first-stage pre-oxidation time. This can promote efficient and uniform pre-oxidation reaction.

[0038] In some embodiments of the present application, the pre-oxidation further comprises a second-stage pre-oxidation, the second-stage pre-oxidation temperature is determined based on the DSC of the fiber after the first-stage pre-oxidation, and the second-stage pre-oxidation time is determined based on the cyclization index of the fiber at the second-stage pre-oxidation temperature. This can also make the second-stage pre-oxidation temperature and the second-stage pre-oxidation time more reasonable, effectively improve the problems of high-temperature intense reaction heat release and low pre-oxidation efficiency at low temperature, and promote efficient and uniform pre-oxidation reaction, thereby further facilitating the preparation of polyacrylonitrile-based carbon fibers with high strength and high modulus.

[0039] In some embodiments of the present application, the draw ratio of the pre-oxidation treatment is adjusted according to the comparison of the crystal region orientation degree of the raw fiber and the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment. Specifically, the crystal region orientation degree of the raw fiber and the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment are compared to determine the degree of reduction of the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment compared to the crystal region orientation degree of the raw fiber. This can reduce the large reduction of the crystal region orientation degree of the fiber during the pre-oxidation treatment, thereby facilitating the preparation of polyacrylonitrile-based carbon fibers with high strength and high modulus.

[0040] Further, the initial draw ratio of the pre-oxidation treatment is set, the crystal region orientation degree of the raw fiber is L1, the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment is L2, when (L1-L2) / L1≤0.1, the initial draw ratio is the target draw ratio; when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased to obtain the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment as L2', until (L1-L2') / L1≤0.1, the target draw ratio is obtained. The crystal region orientation degrees of the raw fiber and the fiber after the last-stage pre-oxidation treatment are determined, and the relative reduction of the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment compared to the crystal region orientation degree of the raw fiber is calculated to determine the target draw ratio, thereby facilitating the maintenance of a high crystal region orientation degree of the fiber, and thereby facilitating the preparation of carbon fibers with high strength and high modulus.

[0041] Specifically, the initial draw ratio of the pre-oxidation treatment is set to be 0.86-0.92. The crystal region orientation degree of the precursor fiber is L1, the crystal region orientation degree of the fiber after the last stage of pre-oxidation is L2, and the value is calculated again by (L1-L2) / L1. If (L1-L2) / L1≤0.1, the initial draw ratio is the target draw ratio; if (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased, and the crystal region orientation degree of the fiber after the last stage of pre-oxidation is measured again as L2', and the value is calculated again by (L1-L2') / L1. If (L1-L2') / L1≤0.1, the draw ratio is not adjusted any more; if (L1-L2') / L1>0.1, the draw ratio of the pre-oxidation treatment is continuously increased, and the crystal region orientation degree of the fiber after the last stage of pre-oxidation is measured again, and the above steps are repeated until (L1-L2') / L1≤0.1, and the target draw ratio is obtained.

[0042] In some embodiments of the present application, when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased by 0.01-0.05. Specifically, the draw ratio of the pre-oxidation treatment can be but is not limited to increased by 0.01, 0.02, 0.03, 0.04, 0.05. This is conducive to reducing the large decrease in the crystal region orientation degree of the fiber during the pre-oxidation treatment.

[0043] In some embodiments of the present application, the method for determining the crystal region orientation degree comprises: obtaining a curve graph of the diffraction intensity and azimuth angle of the fiber by using two-dimensional wide-angle X-ray diffraction method, and calculating the crystal region orientation degree by (360-FWHM1-FWHM2) / 360; wherein FWHM1 is the half-height width of the first peak, and FWHM2 is the half-height width of the second peak. It should be noted that when the crystal region orientation degree of the precursor fiber is determined, the curve graph of the diffraction intensity and azimuth angle of the precursor fiber is obtained; when the crystal region orientation degree of the fiber after the pre-oxidation treatment is determined, the curve graph of the diffraction intensity and azimuth angle of the fiber after the pre-oxidation treatment is obtained. In addition, the peak positions of the two peaks are the standard peak positions of polyacrylonitrile. This is conducive to maintaining a high crystal region orientation degree of the fiber, thereby facilitating the preparation of carbon fibers with high strength and high modulus.

[0044] In some embodiments of the present application, the pre-oxidation adopts four-stage pre-oxidation treatment. Both good pre-oxidation and suitable cost are taken into account.

[0045] Further, the third-stage pre-oxidation temperature is determined based on the DSC of the fiber after the second-stage pre-oxidation, and the third-stage pre-oxidation time is determined based on the cyclization index of the fiber at the third-stage pre-oxidation temperature. The fourth-stage pre-oxidation temperature is determined based on the DSC of the fiber after the third-stage pre-oxidation, and the fourth-stage pre-oxidation time is determined based on the cyclization index of the fiber at the fourth-stage pre-oxidation temperature.

[0046] Further, the initial draw ratio of the pre-oxidation treatment is set, the crystal region orientation degree of the original fiber is L1, the crystal region orientation degree of the fiber after the fourth stage pre-oxidation treatment is L2, when (L1-L2) / L1≤0.1, the initial draw ratio is the target draw ratio; when (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment is increased, the crystal region orientation degree of the fiber after the fourth stage pre-oxidation treatment is L2', until (L1-L2') / L1≤0.1, the target draw ratio is obtained.

[0047] In some embodiments of the present application, the atmosphere of the first stage to the third stage pre-oxidation is air, and the atmosphere of the fourth stage pre-oxidation is nitrogen. The first three stages of pre-oxidation can moderately reach the oxidation degree and the higher cyclization degree in the air atmosphere, and the last stage of pre-oxidation stops increasing the oxidation degree and continues to increase the cyclization degree in the nitrogen atmosphere.

[0048] It should be noted that the first stage to the third stage pre-oxidation is also carried out in the air atmosphere when the DSC of the fiber and the cyclization index of the fiber based on the pre-oxidation temperature are determined to determine the pre-oxidation temperature and the pre-oxidation time; the fourth stage pre-oxidation is also carried out in the nitrogen atmosphere when the DSC of the fiber and the cyclization index of the fiber based on the pre-oxidation temperature are determined to determine the pre-oxidation temperature and the pre-oxidation time.

[0049] Specifically, in air atmosphere, the DSC curve of the precursor fiber (PF) is determined by differential scanning calorimetry, the temperature corresponding to the intersection of the tangent line of the curve before the first peak and the bottom line of the peak in the DSC curve is taken as the first-stage pre-oxidation temperature, the PF is pre-oxidized at the first-stage pre-oxidation temperature in air atmosphere, in air atmosphere, the curve of the cyclization index of the fiber and the pre-oxidation time of the PF at the first-stage pre-oxidation temperature is determined by in-situ infrared spectroscopy, the time corresponding to the intersection of the tangent lines of the curves on both sides at the inflection point of the curve is taken as the first-stage pre-oxidation time, and the first-stage oxidation fiber OF1 is prepared; similarly in air atmosphere, the DSC curve of OF1 is determined by differential scanning calorimetry, the temperature corresponding to the intersection of the tangent line of the curve before the first peak and the bottom line of the peak in the DSC curve is taken as the second-stage pre-oxidation temperature, OF1 is pre-oxidized at the second-stage pre-oxidation temperature in air atmosphere, in air atmosphere, the curve of the cyclization index of the fiber and the pre-oxidation time of OF1 at the second-stage pre-oxidation temperature is determined by in-situ infrared spectroscopy, the time corresponding to the intersection of the tangent lines of the curves on both sides at the inflection point of the curve is taken as the second-stage pre-oxidation time, and the second-stage oxidation fiber OF2 is prepared; similarly in air atmosphere, the DSC curve of OF2 is determined by differential scanning calorimetry, the temperature corresponding to the intersection of the tangent line of the curve before the first peak and the bottom line of the peak in the DSC curve is taken as the third-stage pre-oxidation temperature, OF2 is pre-oxidized at the third-stage pre-oxidation temperature in air atmosphere, in air atmosphere, the curve of the cyclization index of the fiber and the pre-oxidation time of OF2 at the third-stage pre-oxidation temperature is determined by in-situ infrared spectroscopy, the time corresponding to the intersection of the tangent lines of the curves on both sides at the inflection point of the curve is taken as the third-stage pre-oxidation time, and the third-stage oxidation fiber OF3 is prepared; in nitrogen atmosphere, the DSC curve of OF3 is determined by differential scanning calorimetry, the temperature corresponding to the intersection of the tangent line of the curve before the first peak and the bottom line of the peak in the DSC curve is taken as the fourth-stage pre-oxidation temperature, OF3 is pre-oxidized at the fourth-stage pre-oxidation temperature in nitrogen atmosphere, in nitrogen atmosphere, the curve of the cyclization index of the fiber and the pre-oxidation time of OF3 at the fourth-stage pre-oxidation temperature is determined by in-situ infrared spectroscopy, the time corresponding to the intersection of the tangent lines of the curves on both sides at the inflection point of the curve is taken as the fourth-stage pre-oxidation time, and the fourth-stage oxidation fiber OF4 is prepared.

[0050] Further, the diffraction intensity and azimuth angle curve of the precursor fiber (PF) is obtained by two-dimensional wide-angle X-ray diffraction method, the crystal orientation degree is calculated as L1 by (360-FWHM1-FWHM2) / 360, the crystal orientation degree of the fiber OF4 after the fourth-stage pre-oxidation is measured and calculated as L2 according to the foregoing method, then the value is calculated by (L1-L2) / L1, if (L1-L2) / L1≤0.1, the initial draft ratio is the target draft ratio; if (L1-L2) / L1>0.1, the draft ratio of the pre-oxidation treatment is increased, the crystal orientation degree of the fiber OF4 after the fourth-stage pre-oxidation is measured and calculated as L2' according to the foregoing method, the value is calculated by (L1-L2') / L1 again, if (L1-L2') / L1≤0.1, the draft ratio is not adjusted any more; if (L1-L2') / L1>0.1, the draft ratio of the pre-oxidation treatment is continuously increased, the crystal orientation degree of the fiber OF4 after the fourth-stage pre-oxidation is measured and calculated, the above steps are repeated until (L1-L2') / L1≤0.1, and the target draft ratio is obtained.

[0051] The present application determines the first-stage pre-oxidation temperature based on the DSC curve of the precursor fiber, determines the first-stage pre-oxidation time based on the cyclization index of the fiber at the first-stage pre-oxidation temperature, determines the pre-oxidation temperature of the next-stage fiber based on the DSC of the fiber after the pre-oxidation of the previous stage, determines the pre-oxidation time of the stage based on the cyclization index of the fiber at the pre-oxidation temperature of the stage, adjusts the draft ratio of the pre-oxidation treatment according to the comparison of the crystal orientation degree of the precursor fiber and the crystal orientation degree of the fiber after the pre-oxidation treatment of the last stage, and simultaneously uses the fourth-stage pre-oxidation treatment, so that the pre-oxidation temperature and the pre-oxidation time of each stage of the fourth-stage pre-oxidation treatment are more reasonable, and the draft ratio is also more appropriate, thereby effectively improving the problems of high-temperature intense reaction heat release, low pre-oxidation efficiency at low temperature, and large reduction of the crystal orientation degree of the fiber, and promoting the efficient and uniform pre-oxidation reaction, thereby facilitating the preparation of the polyacrylonitrile-based carbon fiber with high strength and high modulus.

[0052] The test method used in the present application is as follows:

[0053] 1. DSC curve of the fiber: differential scanning calorimetry (DSC) test is performed on the fiber sample by using a DSC instrument (NETZSCH), the sample mass is 4.0 mg, the air atmosphere is 60 mL / min, the heating rate is 10 ℃ / min from 30 ℃ to 450 ℃, and the DSC curve is recorded by recording the change of heat flow with temperature.

[0054] 2. In-situ infrared spectroscopy test: The fiber samples were tested using a Fourier transform infrared spectrometer (Summit X) in variable temperature ATR diamond crystal mode; the detector was DTGS and the software was OMNIC Series to collect data and obtain the curve of fiber cyclization index versus pre-oxidation time.

[0055] 3. Curve of diffraction intensity versus azimuth angle of the precursor fiber: A 360° azimuth angle scan (-90°~270°) was performed using two-dimensional wide-angle X-rays with a fixed diffraction angle (the measured diffraction angle of the peak) to obtain the curve of diffraction intensity versus azimuth angle, and the crystal orientation degree (φ) was calculated: φ(%) = [360-(FWHM1+FWHM2)] / 360×100%, where FMWH is the half width at half maximum (WWH). The crystal orientation degree of the pre-oxidized fiber was also measured and calculated using the same method.

[0056] 4. XRD curve of carbon fiber: such as Figure 6 As shown, an X-ray diffractometer (model Bruker D2 PHASER) was used to perform diffraction angle scanning (2 q: 10~80) under Cu Kα radiation (λ=0.15406 nm) to obtain the XRD curves; (1) Interlayer spacing (d 002 The position of peak 002 is calculated using the Bragg equation: d 002 =λ / (2sinθ); d 002 The smaller the value, the higher the orderliness; (2) Calculation of grain size (Lc, La): Scherrer Equation is used: L=Kλ / (βcosθ); Lc (along the c-axis) is calculated using the half-width at half maximum (WHM) of the (002) peak, and K is taken as 0.89; La (along the a-axis) is calculated using the half-width at half maximum (WHM) of the (100) peak, and K is taken as 1.84; β is the instrument broadening that needs to be deducted (using standard silicon standard for calibration). (3) Estimation of the number of stacked layers (N): N≈L C / d 002 (N is an integer); N reflects the number of quasi-graphite crystal sheets stacked along the longitudinal direction. The larger the value, the higher the degree of ordered stacking of the crystals.

[0057] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0058] Example 1

[0059] This embodiment provides a method for determining the pre-oxidation process parameters of polyacrylonitrile-based carbon fibers, including:

[0060] Four oxidation furnaces are set for pre-oxidation treatment, which are OX1, OX2, OX3 and OX4, the crystal orientation degree of the raw fiber used for pre-oxidation is 93%, the crystallinity is 56%, and the grain size is 8.1 nm; as shown in Figures 1-5 ;

[0061] The DSC curve of the raw fiber (PF) is determined by differential scanning calorimetry in an air atmosphere, as shown in Figure 1 , the extension line of the baseline before the first peak in the DSC curve of PF is dotted line 1, and forms a front intersection point with the curve before the peak, the extension line of the baseline after the peak is dotted line 2, and forms a rear intersection point with the curve after the peak, the front intersection point and the rear intersection point are connected to form a peak bottom line 3, and then a tangent line 4 of the curve before the peak is drawn through the vertex of the first peak, and the intersection point A of the tangent line 4 and the peak bottom line 3 is the first-stage pre-oxidation temperature, the temperature of OX1 is set to 232 ℃, and PF is pre-oxidized at 232 ℃, the cyclization index and pre-oxidation time curve of PF at 232 ℃ is determined by in-situ infrared spectroscopy in an air atmosphere, the time corresponding to the intersection point of the tangent lines of the two curves at the inflection point of the curve in the curve graph is the first-stage pre-oxidation time, that is, PF is pre-oxidized at 232 ℃ in the air atmosphere of OX1 for 27 min to prepare a first-stage oxidation fiber OF1;

[0062] The DSC curve of OF1 is determined by the foregoing method in an air atmosphere, the second-stage oxidation temperature is determined to be 241 ℃, the temperature of OX2 is set to 241 ℃, the cyclization index and pre-oxidation time curve of OF1 at 241 ℃ is determined by the foregoing method in an air atmosphere, the second-stage pre-oxidation time is determined to be 34 min, that is, OF1 is pre-oxidized at 241 ℃ in the air atmosphere of OX2 for 34 min to prepare a second-stage oxidation fiber OF2;

[0063] The DSC curve of OF2 is determined by the foregoing method in an air atmosphere, the third-stage oxidation temperature is determined to be 256 ℃, the temperature of OX3 is set to 256 ℃, the cyclization index and pre-oxidation time curve of OF2 at 256 ℃ is determined by the foregoing method in an air atmosphere, the third-stage pre-oxidation time is determined to be 46 min, that is, OF2 is pre-oxidized at 256 ℃ in the air atmosphere of OX3 for 46 min to prepare a third-stage oxidation fiber OF3;

[0064] The DSC curve of OF3 was determined in the previous method under nitrogen atmosphere, and the fourth stage oxidation temperature was determined as 278 ℃. The temperature of OX4 was set as 278 ℃. The OF3 was pre-oxidized at 278 ℃ for 28 min in the nitrogen atmosphere of OX4 to prepare the fourth stage pre-oxidized fiber OF4.

[0065] The initial draw ratio of the pre-oxidation treatment was set as 0.90 times. The crystal region orientation degree of PF was determined as 93%, as shown in Figure 6 The crystal region orientation degree of OF4 was determined as 85.67%, i.e. L1=93%, L2=85.67%, which met the requirement of (L1-L2) / L1≤0.1. The initial draw ratio of 0.90 times was used as the target draw ratio.

[0066] The XRD curve of the polyacrylonitrile-based carbon fiber prepared in the example was determined, as shown in Figure 7 The stacking number was 5.2, the interlayer spacing (d 002 ) was 0.3568 nm, the grain thickness (Lc) was 1.76 nm, the grain length (La ∥ ) was 5.31 nm, the grain width (La ⊥ ) was 4.27 nm, the K number was 12K, the tensile strength was 7579 MPa, the tensile modulus was 336 GPa, the tensile strength Cv value was 1.1%, the tensile modulus Cv value was 0.6%, and the interlayer shear strength was 107 MPa.

[0067] Example 2

[0068] The example provided a method for determining the pre-oxidation process parameters of polyacrylonitrile-based carbon fiber. The difference from example 1 was that the initial draw ratio of the pre-oxidation treatment was set as 0.90 times. The crystal region orientation degree of PF was determined as 93.75%, and the crystal region orientation degree of OF4 was determined as 81.51%, i.e. L1=93.75%, L2=81.51%, which did not meet the requirement of (L1-L2) / L1≤0.1. Therefore, the pre-oxidation draw ratio was adjusted to 0.92 times. The crystal region orientation degree of OF4 was determined as 83.03% after pre-oxidation for 30 min, i.e. L1=93.75%, L2'=83.03%, which still did not meet the requirement of (L1-L2') / L1≤0.1. Therefore, the pre-oxidation draw ratio was further adjusted to 0.94 times. The crystal region orientation degree of OF4 was determined as 86.67% after pre-oxidation for 30 min, i.e. L1=93.75%, L2'=86.67%, which met the requirement of (L1-L2') / L1≤0.1. Therefore, the draw ratio of 0.94 times was used as the target draw ratio.

[0069] The polyacrylonitrile-based carbon fiber prepared in the example was detected to have a tensile strength of 7735 MPa, a tensile modulus of 334 GPa, a tensile strength Cv value of 0.8%, a tensile modulus Cv value of 0.6%, and an interlaminar shear strength of 112 MPa.

[0070] Comparative Example 1

[0071] The present comparative example provides a pre-oxidation method of polyacrylonitrile-based carbon fiber, which is different from that of Example 1 in that the pre-oxidation temperature and time are not determined according to the method of Example 1, but directly pre-oxidized at 230℃, 240℃, 250℃ and 260℃ respectively by using a conventional procedure for 30 minutes for each pre-oxidation.

[0072] Test Example 1

[0073] In the present test example, the carbon fibers prepared by the preparation methods of Examples 1-2 and Comparative Example 1 were detected for tensile strength and tensile modulus by GB / T 3362-2017 method, and the results are shown in Table 1.

[0074] Table 1 Carbon fiber performance

[0075]

[0076] As can be seen from the results in Table 1, the carbon fiber prepared by Example 1 has higher tensile strength and tensile modulus, and higher interlaminar shear strength of the multifilament, and lower tensile strength Cv value and tensile modulus Cv value, i.e. the pre-oxidation process parameters of the polyacrylonitrile-based carbon fiber are determined by the determination method of the present application, and the polyacrylonitrile-based carbon fiber prepared by the pre-oxidation has high strength and modulus and interlaminar shear strength.

[0077] Comparing Examples 1-2 and Comparative Example 1, the tensile strength and tensile modulus of the carbon fiber prepared by Comparative Example 1 are greatly reduced; since the pre-oxidation temperature and time are not dynamically adjusted in Comparative Example 1, it is easy to have high-temperature intense reaction heat release and non-uniform speed reaction, which is not conducive to the preparation of carbon fiber with high strength and modulus.

[0078] The above-described examples are part of the embodiments of the present application, rather than all the embodiments. The detailed description of the embodiments of the present application is not intended to limit the scope of the claimed application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present application.

Claims

1. A method for determining pre-oxidation process parameters of polyacrylonitrile-based carbon fibers, characterized in that, The pre-oxidation comprises a first-stage pre-oxidation, the first-stage pre-oxidation temperature is determined based on DSC of the raw fiber, and the first-stage pre-oxidation time is determined based on a cyclization index of the fiber at the first-stage pre-oxidation temperature; The determination method of the first-stage pre-oxidation temperature comprises: determining a DSC curve of the raw fiber by using differential scanning calorimetry, and taking a temperature corresponding to an intersection of a tangent line of a peak front curve and a peak bottom line at a top point of a first peak in the DSC curve as the first-stage pre-oxidation temperature; The determination method of the first-stage pre-oxidation time comprises: pre-oxidizing the raw fiber at the first-stage pre-oxidation temperature, and determining a curve graph of a cyclization index and a pre-oxidation time of the raw fiber by using in-situ infrared spectroscopy, and taking a time corresponding to an intersection of tangent lines of two sides of a curve at an inflection point of the curve in the curve graph as the first-stage pre-oxidation time.

2. The method of determining pre-oxidation process parameters according to claim 1, wherein, The pre-oxidation further comprises a second-stage pre-oxidation, the second-stage pre-oxidation temperature is determined based on DSC of the fiber after the first-stage pre-oxidation, and the second-stage pre-oxidation time is determined based on a cyclization index of the fiber at the second-stage pre-oxidation temperature.

3. The method of determining pre-oxidation process parameters according to claim 1, wherein, According to a comparison of a crystal region orientation degree of the raw fiber and a crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment, a draft ratio of the pre-oxidation treatment is adjusted.

4. The method of determining pre-oxidation process parameters according to claim 3, wherein, An initial draft ratio of the pre-oxidation treatment is set, the crystal region orientation degree of the raw fiber is L1, the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment is L2, when (L1-L2) / L1≤0.1, the initial draft ratio is a target draft ratio; When (L1-L2) / L1>0.1, the draft ratio of the pre-oxidation treatment is increased, the crystal region orientation degree of the fiber after the last-stage pre-oxidation treatment is L2', until (L1-L2') / L1≤0.1, and a target draft ratio is obtained.

5. The method of determining pre-oxidation process parameters according to claim 4, wherein, When (L1-L2) / L1>0.1, the draft ratio of the pre-oxidation treatment is increased by 0.01-0.

05.

6. The method of determining pre-oxidation process parameters according to claim 4, wherein, The determination method of the crystal region orientation degree comprises: A curve graph of diffraction intensity and azimuth angle of the fiber is determined by using two-dimensional wide-angle X-ray diffraction, and the crystal region orientation degree is calculated by (360-FWHM1-FWHM2) / 360; wherein, FWHM1 is a half-height width of the first peak, and FWHM2 is a half-height width of the second peak.

7. The method of determining pre-oxidation process parameters according to any one of claims 1-6, wherein, The pre-oxidation adopts four-stage pre-oxidation treatment.

8. The method of determining pre-oxidation process parameters according to claim 7, wherein, An atmosphere of the first-stage pre-oxidation to the third-stage pre-oxidation is air, and an atmosphere of the fourth-stage pre-oxidation is nitrogen.

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