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

Abstract

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

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 considered.

[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 solutions and advantages of the embodiments of the present application more clear, the technical solutions 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 manufacturers are adopted. If the manufacturers of the reagents or instruments are not specified, the conventional products that can be purchased in the market are adopted.

[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, the problems of high-temperature intense reaction heat release and low-temperature pre-oxidation efficiency are effectively improved, and the pre-oxidation is promoted to be efficient and uniform, thereby benefiting 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, and the problems of high-temperature intense reaction heat release and low-temperature pre-oxidation efficiency can be effectively improved. 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 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 again 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 again by (L1-L2') / L1, 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, promoting the efficient and uniform pre-oxidation reaction, and 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: 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 obtained by recording the heat flow change with temperature.

[0053] 2. In-situ infrared spectrum test: the fiber sample is tested by using a Fourier transform infrared spectrometer (Summit X), the test mode is temperature-variable ATR diamond crystal (Diamond), the detector is DTGS, the software is OMNIC Series, the data is collected, and the cyclization index and pre-oxidation time curve of the fiber is obtained.

[0054] 3. The curve of diffraction intensity and azimuth angle of the precursor fiber: the curve of diffraction intensity and azimuth angle is obtained by using two-dimensional wide-angle X-ray to carry out 360° azimuth angle scanning (-90°~270°) at a fixed diffraction angle (using the diffraction angle measured by the peak), and the crystal orientation degree (φ) is calculated: φ (%) = [360-(FWHM1+FWHM2)] / 360x100%, wherein, FWHM is the half peak width of the peak; the pre-oxidized fiber is also measured and calculated by using the same method to calculate the crystal orientation degree.

[0055] 4. The XRD curve of the carbon fiber: as shown in Figure 6 , the XRD curve is obtained by using an X-ray diffractometer (model: Bruker D2 PHASER) to carry out diffraction angle scanning (2 q: 10~80) under the test condition of Cu Kα radiation (λ=0.15406 nm); (1) the interlayer spacing (d 002 ) is calculated by the Bragg equation from the 002 peak position: d 002 =λ / (2sinθ); d 002 reflects the distance between the crystal layers, and the smaller the value, the higher the order; (2) the grain size (Lc, La) is calculated: the Scherrer equation is used: L=Kλ / (βcosθ); Lc (in the direction along the c axis) is calculated by the half width of the (002) peak, and K is 0.89; La (in the direction along the a axis) is calculated by the half width of the (100) peak, and K is 1.84; β is the instrument spread which needs to be deducted (calibrated by using a standard silicon sample); (3) the stacking number (N) is estimated: N≈L C / d 002 (N is an integer); N reflects the stacking number of the quasi-graphite crystal layer in the longitudinal direction, and the larger the value, the higher the degree of ordered accumulation of the crystal.

[0056] The features and performances of the present application are further described in detail in combination with the following examples.

[0057] Example 1 The present embodiment provides a method for determining the pre-oxidation process parameters of polyacrylonitrile-based carbon fiber, comprising: Four oxidation furnaces are set for pre-oxidation treatment, which are OX1, OX2, OX3 and OX4, and the crystal orientation degree of the precursor 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 ; In an air atmosphere, the DSC curve of the precursor fiber (PF) is obtained by using differential scanning calorimetry, as shown in Figure 1As shown, the extension of the baseline before the first peak in the DSC curve of PF is dashed line 1, which forms the front intersection point with the curve before the peak. The extension of the baseline after the peak is dashed line 2, which forms the back intersection point with the curve after the peak. Connecting the front intersection point and the back intersection point forms the peak line 3. Then, the tangent line 4 of the curve before the peak is drawn through the vertex of the first peak. The intersection point with the peak line 3 is A. The temperature corresponding to the intersection point A, 232℃, is the first-stage pre-oxidation temperature. The temperature of OX1 is set to 232℃. PF is pre-oxidized at 232℃. In an air atmosphere, the cyclization index of PF at 232℃ and the pre-oxidation time curve are measured by in-situ infrared spectroscopy. The time corresponding to the intersection point of the tangent lines on both sides of the curve at the inflection point of the curve is the first-stage pre-oxidation time of 31 min. That is, PF is pre-oxidized at 232℃ for 27 min in an air atmosphere of OX1 to prepare the first-stage oxidized fiber OF1. In an air atmosphere, the DSC curve of OF1 was determined according to the aforementioned method, and the second-stage oxidation temperature was determined to be 241 ℃. The temperature of OX2 was set to 241 ℃. In an air atmosphere, the cyclization index of OF1 at 241 ℃ and the pre-oxidation time were determined according to the aforementioned method, and the second-stage pre-oxidation time was determined to be 34 min. That is, OF1 was pre-oxidized in an air atmosphere of OX2 at 241 ℃ for 34 min to prepare the second-stage oxidized fiber OF2. In an air atmosphere, the DSC curve of OF2 was determined according to the aforementioned method, and the third-stage oxidation temperature was determined to be 256 ℃. The temperature of OX3 was set to 256 ℃. In an air atmosphere, the cyclization index of OF2 at 256 ℃ and the pre-oxidation time were determined according to the aforementioned method, and the third-stage pre-oxidation time was determined to be 46 min. That is, OF2 was pre-oxidized in an air atmosphere of OX3 at 256 ℃ for 46 min to prepare the third-stage oxidized fiber OF3. In a nitrogen atmosphere, the DSC curve of OF3 was determined according to the aforementioned method, and the fourth-stage oxidation temperature was determined to be 278 ℃. The temperature of OX4 was set to 278 ℃. In a nitrogen atmosphere, the cyclization index of OF3 at 278 ℃ and the pre-oxidation time were determined according to the aforementioned method, and the fourth-stage pre-oxidation time was determined to be 28 min. That is, OF3 was pre-oxidized at 278 ℃ for 28 min in a nitrogen atmosphere of OX4 to prepare the fourth-stage oxidized fiber OF4. The initial draw ratio for pre-oxidation treatment was set to 0.90, and the crystal orientation degree of PF was measured to be 93%. Figure 6 As shown, the crystal orientation degree of OF4 was measured to be 85.67%, i.e., L1=93% and L2=85.67%, which satisfies the requirement of (L1-L2) / L1≤0.1. The initial draw ratio of 0.90 was adopted as the target draw ratio.

[0058] The polyacrylonitrile-based carbon fiber prepared in this example was detected, and the XRD curve thereof is shown in Figure 7 The stacking layer number thereof was calculated as 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 interlaminar shear strength was 107 MPa. Example 2 The difference between this example and Example 1 is that the initial draw ratio of the pre-oxidation treatment is set to 0.90 times, the crystal region orientation degree of PF is 93.75%, and the crystal region orientation degree of OF4 is 81.51%, i.e. L1=93.75%, L2=81.51%, which does not meet the requirement of (L1-L2) / L1≤0.1, so the pre-oxidation draw ratio is adjusted to 0.92 times; the crystal region orientation degree of OF4 is measured to be 83.03% after pre-oxidation for 30 min, i.e. L1=93.75%, L2'=83.03%, which still does not meet the requirement of (L1-L2') / L1≤0.1, so the pre-oxidation draw ratio is continuously adjusted to 0.94 times; the crystal region orientation degree of OF4 is measured to be 86.67% after pre-oxidation for 30 min, i.e. L1=93.75%, L2'=86.67%, which meets the requirement of (L1-L2') / L1≤0.1, so the draw ratio of 0.94 times is used as the target draw ratio.

[0059] The polyacrylonitrile-based carbon fiber prepared in this example was detected, and the tensile strength thereof was 7735 MPa, the tensile modulus was 334 GPa, the tensile strength Cv value was 0.8%, the tensile modulus Cv value was 0.6%, and the interlaminar shear strength was 112 MPa.

[0060] Comparative Example 1 The difference between this comparative example and Example 1 is that the pre-oxidation temperature and time are not determined by the method of Example 1, but are directly determined by a conventional procedure, i.e. pre-oxidation is carried out at 230℃, 240℃, 250℃ and 260℃, respectively, for 30 min each time.

[0061] Test Example 1 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 the method of GB / T 3362-2017, and the results are shown in Table 1.

[0062] Table 1 Carbon fiber properties

[0063] From the results of Table 1, it can be seen that the carbon fiber prepared by Example 1 has high tensile strength and tensile modulus, and high interlaminar shear strength, and low tensile strength Cv value and tensile modulus Cv value, i.e., the polyacrylonitrile-based carbon fiber prepared by using the pre-oxidation process parameters determined by the method of the present application has high strength, modulus and interlaminar shear strength.

[0064] Comparative 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 temperature and time are not dynamically adjusted in the pre-oxidation of 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.

[0065] The above-described embodiments 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 creative labor fall within the scope of protection of the present application.

Claims

1. A method for determining the pre-oxidation process parameters of polyacrylonitrile-based carbon fibers, characterized in that, The pre-oxidation includes a 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.

2. The method for determining pre-oxidation process parameters according to claim 1, characterized in that, The method for determining the first-stage pre-oxidation temperature includes: The DSC curve of the precursor fiber was obtained by differential scanning calorimetry. The temperature corresponding to the intersection of the tangent line to the curve before the peak and the bottom line of the peak was taken as the first-stage pre-oxidation temperature.

3. The method for determining the pre-oxidation process parameters according to claim 1, characterized in that, The method for determining the first-stage pre-oxidation time includes: The precursor fiber was pre-oxidized at the first-stage pre-oxidation temperature. The cyclization index of the precursor fiber was measured as a curve of pre-oxidation time using in-situ infrared spectroscopy. The time corresponding to the intersection of the tangents on both sides of the curve at the inflection point of the curve was taken as the first-stage pre-oxidation time.

4. The method for determining the pre-oxidation process parameters according to claim 1, characterized in that, The pre-oxidation also includes a second-stage pre-oxidation, the temperature of which is determined based on the DSC of the fiber after the first-stage pre-oxidation, and the time of which is determined based on the cyclization index of the fiber at the second-stage pre-oxidation temperature.

5. The method for determining the pre-oxidation process parameters according to claim 1, characterized in that, The draw ratio of the pre-oxidation treatment is adjusted based on a comparison between the crystal orientation of the precursor fiber and the crystal orientation of the fiber after the final pre-oxidation treatment.

6. The method for determining pre-oxidation process parameters according to claim 5, characterized in that, The initial draw ratio for the pre-oxidation treatment is set, the crystal orientation degree of the precursor fiber is L1, and the crystal orientation degree of the fiber after the last stage of 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 orientation degree of the fiber after the last stage of pre-oxidation treatment as L2', until (L1-L2') / L1≤0.1, and the target draw ratio is obtained.

7. The method for determining pre-oxidation process parameters according to claim 6, characterized in that, When (L1-L2) / L1>0.1, the draw ratio of the pre-oxidation treatment increases by 0.01~0.

05.

8. The method for determining pre-oxidation process parameters according to claim 6, characterized in that, The method for determining the orientation degree of the crystal region includes: The diffraction intensity versus azimuth curve of the fiber was obtained by two-dimensional wide-angle X-ray diffraction. The orientation degree of the crystal region was calculated as (360-FWHM1-FWHM2) / 360, where FWHM1 is the full width at half maximum (FWHM1) of the first peak and FWHM2 is the full width at half maximum (FWHM2) of the second peak.

9. The method for determining the pre-oxidation process parameters according to any one of claims 1-8, characterized in that, The pre-oxidation process employs a four-stage pre-oxidation treatment.

10. The method for determining pre-oxidation process parameters according to claim 9, characterized in that, The atmosphere for the first to third stages of pre-oxidation is air, and the atmosphere for the fourth stage of pre-oxidation is nitrogen.

Images