High compression strength carbon fiber and method for producing the same

Abstract

The application provides a high-strength middle-film carbon fiber with high compression strength and a preparation method thereof. The high-strength middle-film carbon fiber is prepared by the following steps: preparing a raw yarn by wet spinning, pre-oxidizing and low-temperature carbonizing, and then performing middle-temperature carbonization under the protection of high-purity nitrogen containing CH4 gas at a temperature of 1000-1200 DEG C, and performing high-temperature carbonization at a temperature of 1300-1800 DEG C with gradient heating, so that the high-strength middle-film carbon fiber with the tensile strength of 5.5-6.0 GPa, the tensile modulus of 280-300 GPa, the average diameter of 5.5-7.5 mu m, the compression strength of 1.5-2.3 GPa, the circular or nearly circular cross section, and the cross section skin-core ratio of 1.11

Description

Technical Field

[0001] The invention relates to a high-compression-strength polyacrylonitrile (PAN)-based carbon fiber and a preparation method thereof, belonging to the technical field of fibers. Background Art

[0002] Polyacrylonitrile-based carbon fibers, with their high specific strength, high specific modulus, corrosion resistance, and excellent electrical and thermal conductivity, have become one of the most influential new materials of the 21st century. Due to their outstanding mechanical properties, designability, and ease of integral molding, polyacrylonitrile-based carbon fibers are widely used as reinforcements in resin-based composites and are favored by numerous industries. Typical applications include bicycles, aircraft, and rockets.

[0003] Carbon fiber is divided into high-strength type (strength 2000MPa, modulus 250GPa), high-strength medium model (strength 5300MPa or more, modulus 280GPa), high model (modulus 310GPa or more), ultra-high-strength type (strength greater than 6000MPa), ultra-high model (modulus greater than 450GPa), etc. according to its mechanical properties such as strength and modulus. High-strength medium model carbon fiber has obvious reinforcing effect, represented by T800 grade carbon fiber, and IM6, IM7, IMS, etc. as examples. Their fiber diameters are between 5 and 6 microns, and they are mainly used in the preparation of structural materials. The prepared composite materials have become important materials in the aerospace field. The tensile properties of high-strength medium model carbon fiber composites are very good, which has won the favor of many application fields. However, as an important main load-bearing structural component, it will be subject to compressive stress in the parallel fiber direction that cannot be ignored. Just like when an aircraft or a launch vehicle is turning, the main material of the aircraft or rocket is subjected to tensile stress on one side and compressive stress on the other side. This puts higher and higher demands on the strength and compressive resistance of the composite material. However, the compressive strength and tensile strength of the high-strength medium-model carbon fiber currently used are seriously unbalanced. Taking Japan's Toray's T800-grade carbon fiber epoxy resin system composite material as an example, the T800-grade carbon fiber composite material has a compressive strength of 1490MPa, a tensile strength of 3290MPa, and a compression-to-tensile ratio of only 0.45.

[0004] It is generally believed that increasing the diameter of the cylinder can prevent the occurrence of thin diameters buckling and breaking during compression. Carbon fiber monofilaments are essentially cylinders with a diameter of microns, so intuitively, increasing the diameter of the carbon fiber will also help improve its compressive strength. In actual applications, it is found that the collimation of carbon fiber is an important factor affecting the compression performance of its composite materials. The larger the diameter of the carbon fiber, the better its collimation. When coarse diameter carbon fiber is subjected to a compressive load, it can disperse the load to the fiber sheath and better transfer the stress to the fiber surface, which can effectively alleviate the compression deformation and reduce the degree of micro-bending. Therefore, as the fiber diameter increases, the stress that the monofilament can bear gradually increases, and the compressive resistance will gradually increase.

[0005] Some experts and scholars have conducted compressive strength tests on 5 kinds of domestic T800-grade high-strength medium-model carbon fiber monofilaments by stretching and rebounding method, analyzed their graphite microstructure and fracture morphology, explored the relationship between their structure and their mechanical properties, and found that the axial compressive strength of high-strength medium-modulus carbon fiber increases with the increase of its diameter. In order to increase the diameter of high-strength medium-model carbon fiber, patent CN109252251A announced large-diameter dry-wet polyacrylonitrile-based carbon fiber and its preparation method, which is a dry-jet wet spinning technology and controls the regulation of the precursor diameter to obtain carbon fiber performance of 7-20μm in diameter, 3.8-5.9GPa in tensile strength, and 230-300GPa in tensile modulus. Patent: CN109082730A announced large-diameter polyacrylonitrile-based carbon fiber and its preparation method, which controls the spinning process conditions to prepare carbon fiber with a diameter of 10-20μm, a tensile strength of 3.8-4.6GPa, and a tensile modulus of 230-260GPa. The surface of the carbon fiber obtained by dry and wet spinning is relatively smooth and has fewer defects, making it easy to prepare high-performance carbon fibers. However, due to its smooth surface, the interface performance of its composite material is poor, resulting in a decrease in the mechanical properties of the composite material. The surface of the raw silk obtained by wet spinning has a surface groove structure. After pre-oxidation carbonization or pre-oxidation carbonization graphitization, the groove structure on the fiber surface can be inherited. Therefore, the carbon fiber prepared by wet spinning can improve the bonding strength of the resin in the preparation of composite materials and ultimately improve the mechanical properties of the composite material. However, due to the presence of surface defects, it is not easy to prepare high-performance carbon fibers using conventional pre-oxidation methods when the radial structural differences of large-diameter fibers are difficult to weaken. Therefore, there are very few research reports on large-diameter high-performance carbon fibers with surface grooves.

[0006] Large diameter carbon fibers are beneficial to improving the resin impregnation of carbon fibers, the alignment of carbon fibers, the preparation efficiency and quality reliability of carbon fiber composites, and are beneficial to improving the compressive strength of carbon fiber composites. The design and control technology of radial structural differences is one of the important technical bottlenecks in the preparation of high compressive strength and high-strength medium-model carbon fibers. Many scholars have conducted research on the radial structural differences of polyacrylonitrile-based pre-oxidized fibers and graphite fibers, including the establishment and application of optical density method to characterize the radial structural differences of pre-oxidized fibers, the method of regulating the radial structural differences of pre-oxidized fibers by impregnation method, and Raman spectroscopy to study the formation and evolution of radial structural differences of polyacrylonitrile graphite fibers. Most of them studied the mechanism or evolution of the radial structure of pre-oxidized fibers or graphite fibers, and did not study the specific structural distribution and the influence of the distribution of graphitization degree inside and outside the carbon fibers on their compressive resistance.

[0007] From the intrinsic mechanical properties of carbon fiber, people mostly think that the carbon fiber with homogeneous skin-core is more excellent, but the influence of the rigidity of carbon fiber on its compression performance is ignored. For polyacrylonitrile-based carbon fiber, the difference between its compression capacity and tensile capacity is large, and the compression failure mechanism is not very clear, there are many contradictions, and an effective method to regulate the internal structure of polyacrylonitrile-based carbon fiber to improve its compression capacity has not been given. Generally, people think that the increase of the diameter of a cylinder is beneficial to improve the compression capacity of the object, and the increase of the macroscopic diameter size of carbon fiber is beneficial to improve its compression capacity, but the relationship between the internal structure distribution and the compression performance of carbon fiber and the design and regulation of the internal structure have been a difficult problem in the industry. It is found through experiments that, under the condition that the carbon fiber is in the high-strength medium modulus range, the rigidity of the skin of the polyacrylonitrile-based carbon fiber is improved by appropriately increasing the macroscopic diameter size of the carbon fiber, increasing the difference in the radial skin-core graphitization degree of the carbon fiber, and increasing the crystalline structure size of the skin of the polyacrylonitrile-based carbon fiber, that is, the rigidity of the skin structure of the fiber, which is beneficial to the improvement of the compression strength of the carbon fiber, which provides a structural regulation method for preparing high-strength medium modulus carbon fiber with high compression strength, but how to improve the compression strength while ensuring that the performance of the carbon fiber meets the requirements of high-strength medium modulus is the key and difficulty of the present application. SUMMARY

[0008] The present application provides a high-strength medium modulus carbon fiber with high compression strength and a preparation method thereof. The original yarn is prepared by wet spinning, and after pre-oxidation and low-temperature carbonization, the high-purity nitrogen gas containing CH4 gas is used for protection, and the medium-temperature carbonization is carried out at a temperature of 1000-1200 DEG C, and then the high-temperature carbonization is carried out at a temperature of 1300-1800 DEG C with gradient heating, so that the high compression strength high-strength medium modulus carbon fiber with a tensile strength of 5.5-6.0 GPa, a tensile modulus of 280-300 GPa, an average diameter of 5.5-7.5 mu m, a carbon fiber bundle compression strength of 1.5 GPa-2.3 GPa, a circular or nearly circular cross section, and a cross section skin-core ratio of 1.11

[0009] The present application provides a high-strength medium modulus carbon fiber with high compression strength and a preparation method thereof. The original yarn is prepared by wet spinning, and after pre-oxidation and low-temperature carbonization, the high-purity nitrogen gas containing CH4 gas is used for protection, and the medium-temperature carbonization is carried out at a temperature of 1000-1200 DEG C, and then the high-temperature carbonization is carried out at a temperature of 1300-1800 DEG C with gradient heating, so that the high compression strength high-strength medium modulus carbon fiber with a tensile strength of 5.5-6.0 GPa, a tensile modulus of 280-300 GPa, an average diameter of 5.5-7.5 mu m, a carbon fiber bundle compression strength of 1.5 GPa-2.3 GPa, a circular or nearly circular cross section, and a cross section skin-core ratio of 1.11

[0010] The cross section of the carbon fiber is circular or nearly circular, and the cross section skin-core ratio is 1.11<f≤1.30. The radial graphitization degree skin-core ratio f of the carbon fiber is the ratio of the g value of the skin 2 site to the g value of the core 0 site (g2 / g0, such as Figure 1 ) represents the ratio of the graphitization degree of the carbon fiber sheath to the core, and g is the peak area of ​​the G peak divided by the peak area of ​​the D peak in the Raman spectrum of the carbon fiber.

[0011] A method for preparing high-compression strength and high-strength medium-modulus carbon fiber, comprising the following steps: preparing large-diameter precursor by wet spinning, pre-oxidizing the precursor, low-temperature carbonization and high-temperature carbonization, performing medium-temperature carbonization before the high-temperature carbonization, and adopting two-stage gradient heating for the high-temperature carbonization so that the width of the microcrystalline of the skin L is a，r The carbon fiber cross section skin-core ratio is 1.11<f≤1.30. According to the literature, the crystallite size L in different parts of the fiber skin and core is a，r , through the relationship between the degree of graphitization g and the crystallite size L a，r =4.4*g calculated.

[0012] The medium-temperature carbonization adopts high-purity nitrogen protection, the oxygen content in the nitrogen is less than 1ppm, the CH4 gas volume content in the nitrogen is 0.2-5%, the medium-temperature carbonization temperature is 1000°C-1200°C, the time is 0.5-5min, and the drawing ratio is 0.96-1.05 times.

[0013] During the medium-temperature carbonization process, the carbon structure is optimized, reorganized, and microcrystals grow, accompanied by the removal of nitrogen elements. The nitrogen elements are removed from the inside to the outside along the fiber diameter. If the treatment temperature is too high, the nitrogen elements in the fiber core have not yet been removed, and the carbon structure in the skin has grown and perfected to form a dense crystal structure, which closes the channel for the internal nitrogen atoms to be discharged, seriously affecting the reorganization and growth of the carbon structure in the fiber core. At the same time, if the nitrogen element removal rate is too fast and the carbon structure reorganization rate is low, a void defect structure will be formed in the carbon structure. Therefore, in order to ensure the reorganization of the fiber core structure to enhance its supporting role in the compression process, we adopt a medium-temperature carbonization temperature in the optimal temperature range of 1000-1200℃ for the removal of non-carbon element nitrogen, and mix CH4 into the medium-temperature carbonization protective gas nitrogen to generate C atoms containing free radicals during the medium-temperature carbonization process. They are deposited into the vacancies through gas diffusion to fill the vacancies and utilize microcrystal growth to make the microcrystal width L in the skin. a，r Between 2.0 and 2.6 nm, the skin-core ratio of the carbon fiber section is 1.11<f≤1.30.

[0014] The high-temperature carbonization is protected by high-purity nitrogen, the high-temperature carbonization temperature is 1300-1800℃, the initial temperature is 1300-1500℃, the final temperature is 1500-1800℃, the total carbonization time is 2-5min, and the total relative draft ratio is 0.88-0.98 times; the high-temperature carbonization temperature is preferably 1400-1700℃, the total carbonization time is preferably 3-4min, and the total relative draft ratio is preferably 0.90-0.95 times. The high-temperature carbonization is treated by two-stage variable-temperature and variable-time processing between 1300-1800℃, and the processing time and the draft ratio are controlled to prepare carbon fibers with different macroscopic diameter sizes.

[0015] The pre-oxidation is performed by a four-stage gradient temperature raising method, the temperature ranges are 210-225℃, 230-245℃, 240-255℃ and 250-280℃ respectively, the total residence time is 60-90min, and the draft ratio is 1.0-1.2 times, preferably 1.1-1.2 times. By matching the pre-oxidation temperature gradient and the time tension, the coarse-diameter pre-oxidized fibers with small radial structure difference are prepared.

[0016] The low-temperature carbonization is protected by high-purity nitrogen, the low-temperature carbonization temperature is 350-850℃, the time is 0.5-5min, and the draft ratio is 1.05-1.15 times, preferably 1.08-1.12.

[0017] The wet spinning preparation of the raw yarn includes the preparation of the spinning dope, multi-stage coagulation molding, primary steam stretching, multi-stage water washing, oiling, drying densification, secondary superheated steam stretching and heat setting, and the raw yarn diameter is controlled to be 11-14μm, and the specific steps are as follows:

[0018] (1) Preparation of the spinning dope

[0019] azo-diisobutyronitrile is used as the initiator, dimethyl sulfoxide is used as the solvent, acrylonitrile and itaconic acid are mixed and added into a polymerization reaction container according to a molar ratio of (95-99):(1.0-5.0), the total parts of the above-mentioned molar ratio is 100, the polymerization reaction is performed at a temperature of 50-70℃ for 10-24 hours to prepare the spinning dope, and the spinning dope is obtained after single removal and degassing, wherein the mass percentage concentration of dimethyl sulfoxide in the weight of the monomer and dimethyl sulfoxide is 75%-80%, the molar fraction of azo-diisobutyronitrile in acrylonitrile is 0.1%-0.3%, and the viscosity of the spinning dope is controlled to be 2000-8000poise at 25℃, preferably 3000-5000poise;

[0020] (2) Multi-stage coagulation molding of the spinning dope

[0021] The wet spinning method is adopted. After the fiber leaves the spinneret, it enters the first coagulation bath. The temperature of the first coagulation bath is 35℃~70℃. The coagulation bath adopts a dimethyl sulfoxide aqueous solution, wherein the volume content of the dimethyl sulfoxide solvent is 50~80%. The coagulation time is 0.5~3 minutes, and the coagulation draft ratio is -0.5~3.0. After the coagulated filaments leave the first coagulation bath, they enter the second coagulation bath. The temperature of the second coagulation bath is 10℃~50℃. The coagulation bath adopts a dimethyl sulfoxide aqueous solution, wherein the volume content of the dimethyl sulfoxide solvent is 50~80%. The coagulation process comprises the following steps: the coagulation filaments are subjected to a second coagulation bath and then enter a third coagulation bath after exiting the second coagulation bath; the temperature of the third coagulation bath is 10° C. to 50° C., the coagulation bath adopts an aqueous solution of dimethyl sulfoxide, wherein the volume content of the dimethyl sulfoxide solvent is 0% to 30%, the coagulation time is 0.5% to 3 minutes, and the coagulation draft ratio is 1.0% to 3.0; preferably, the temperature of the first coagulation bath is 50° C. to 65° C., and the volume content of the dimethyl sulfoxide solvent is 50% to 70%.

[0022] (3) Preparation of raw silk

[0023] The coagulated fiber undergoes primary drawing, washing, oiling, drying and densification in a drawing medium of 100°C to 110°C steam, and secondary drawing and heat setting in superheated steam to produce raw silk. The primary drawing ratio is controlled at 3 to 10 times. The fiber after primary drawing undergoes multi-stage washing with a controlled drawing ratio of 0.95 to 1.05 during washing. After washing, the fiber is oiled and then subjected to multi-stage drying and densification treatment at 100 to 150°C with a controlled drawing ratio of 0.95 to 1.05 during drying and densification. The fiber after drying and densification enters secondary drawing in superheated steam with a controlled superheated steam temperature of 120°C to 160°C and a drawing ratio of 1.5 to 3.5 times. The fiber obtained after secondary drawing is heat set at a temperature of 140 to 180°C and a drawing ratio of 0.9 to 1.1 times to produce raw silk.

[0024] Advantages and effects of the present invention

[0025] The present invention prepares high-strength medium-modulus carbon fibers with high compressive properties by first performing low-temperature carbonization before high-temperature carbonization, and filling the nitrogen protective gas during the low-temperature carbonization with CH4 gas. The high-temperature carbonization adopts a gradient heating method to prepare high-strength medium-modulus carbon fibers with high compressive properties. The obtained carbon fibers have a diameter of 5.5 to 7.5 μm, a tensile strength of 5.5 to 6.0 GPa, a tensile modulus of 280 to 300 GPa, an average crystallite width La of 5.5 to 5.9 nm (the average grain size of the carbon fibers measured by XRD), and a compressive strength of the carbon fiber bundle of 1.50 to 2.30 GPa. Compared with the prior art, the structural control method of the present invention realizes a microcrystalline structure with a uniform gradient distribution that can be controlled from the core to the skin of the carbon fibers. While maintaining the basic mechanical performance indicators of the carbon fibers (maintaining the mechanical performance characteristics of the high-strength medium-modulus carbon fibers), the diameter of the carbon fibers is increased. In order to further improve the microcrystalline structure of the skin to improve its compressive strength, low-temperature carbonization is carried out before high-temperature carbonization, and CH4 gas is filled in the nitrogen protective gas during the low-temperature carbonization. The C free radicals generated by CH4 during the medium-temperature carbonization process can make up for the holes generated by the removal of N, which is conducive to the orderly growth of microcrystals and forms a relatively complete microcrystalline structure of the skin. When the fiber is subjected to an external compressive load, the large microcrystalline grain size of the skin and the large graphitization ratio of the skin to the core cause the load to be dispersed to the skin. The relatively complete microcrystalline structure of the skin improves its compressive strength, and ultimately improves the compressive strength of the carbon fiber and its carbon fiber composite material. The carbon fiber prepared by the present invention can increase the compressive strength of the carbon fiber composite unidirectional plate from 1490-1550MPa (the compressive strength of the T800 grade carbon fiber composite unidirectional plate reported on the official website of Toray Industries, Ltd. of Japan) to 1850MPa and above, an increase of more than 19%. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the test site of the radial graphitized carbon fiber structure skin-core

[0027] Figure 2 Example 1 PAN carbon fiber radial graphitization degree test g value at each point

[0028] Figure 3 Comparative Example 1 PAN carbon fiber radial graphitization degree test g value at each point

[0029] Figure 4 Example 2 PAN carbon fiber radial graphitization degree test g value at each point

[0030] Figure 5 Example 3 PAN carbon fiber radial graphitization degree test g value at each point

[0031] Figure 6 Example 4 PAN carbon fiber radial graphitization degree test g value at each point DETAILED DESCRIPTION

[0032] The present invention will be described in detail below with reference to the following embodiments, but the present invention is not limited to the following embodiments.

[0033] Example 1

[0034] (1) Preparation of spinning solution

[0035] Azobisisobutyronitrile is used as an initiator and dimethyl sulfoxide is used as a solvent. Acrylonitrile and itaconic acid are mixed in a molar ratio of 98.5:1.5 and added into a polymerization reaction container. The polymerization reaction temperature is 64°C, the polymerization reaction time is 19 hours, and the stirring rate is 55 rpm to obtain a spinning solution. The spinning solution is subjected to desingling and degassing to obtain a spinning solution. An acrylonitrile copolymer with a polymer viscosity-average molecular weight of 255,000 is obtained, wherein dimethyl sulfoxide accounts for 78% of the weight percentage concentration of acrylonitrile and dimethyl sulfoxide, azobisisobutyronitrile accounts for 0.25% of the molar fraction of acrylonitrile, and the viscosity of the spinning solution measured by a rotational viscometer at 25°C is 4500 poise.

[0036] (2) Coagulation and molding of spinning solution

[0037] A spinneret with an aperture of 0.10 mm is used. After leaving the spinneret hole, the fiber enters the first coagulation bath, the temperature of the first coagulation bath is 65° C., the coagulation bath uses an aqueous solution of dimethyl sulfoxide, wherein the volume content of dimethyl sulfoxide is 60%, the coagulation time is 2 minutes, and the coagulation draft ratio is -0.15; after exiting the first coagulation bath, the coagulated filaments enter the second coagulation bath, the temperature of the second coagulation bath is 30° C., the coagulation bath uses an aqueous solution of dimethyl sulfoxide, wherein the volume content of dimethyl sulfoxide solvent is 50%, the coagulation time is 2 minutes, and the coagulation draft ratio is 1.01; after exiting the second coagulation bath, the coagulation filaments enter the third coagulation bath, the temperature of the third coagulation bath is 25° C., the coagulation bath uses an aqueous solution of dimethyl sulfoxide, wherein the volume content of dimethyl sulfoxide solvent is 20%, the coagulation time is 2 minutes, and the coagulation draft ratio is 1.2.

[0038] (3) Preparation of raw silk

[0039] The coagulated fibers were subjected to primary steam drawing, washing, oiling, drying and densification, secondary superheated steam drawing, and heat setting to produce precursor fibers. The fibers were first drawn using steam at 100°C and a draw ratio of 5. The fibers then underwent multi-stage steam washing at a controlled draw ratio of 0.99. Residual dimethyl sulfoxide (DMSO) was removed by washing, and silicone oil was applied. The fibers were then dried and densified using heated rollers at 120°C and a draw ratio of 0.98. The fibers were then secondarily drawn using superheated steam at 140°C at a draw ratio of 2.5. The resulting fibers were heat-set at 155°C and a draw ratio of 1.0. After heat setting, the fibers were wound on a winder, with a total draw ratio of 10. The result was a dense and uniform PAN precursor with a surface groove structure.

[0040] (4) Pre-oxidation

[0041] The PAN precursor obtained in step (3) was thermally stabilized and pre-oxidized in a pre-oxidation furnace. The initial wire speed was 20 m / h. The temperature was gradually increased in four temperature zones, with temperatures of 220°C, 235°C, 248°C, and 265°C, respectively. The total drafting ratio was 1.15 times, the total pre-oxidation time was 90 minutes, and the time distribution of the four temperature zones was 1:3:3:1.

[0042] (5) Low temperature carbonization

[0043] The PAN pre-oxidized fiber obtained in step (4) is put into a low-carbon furnace under a high-purity nitrogen atmosphere for low-temperature carbonization treatment, with an oxygen content of 5 ppm in the nitrogen, a temperature of 650°C, a residence time of 2 minutes, and a draft ratio of 1.08 times.

[0044] (6) Medium temperature carbonization

[0045] The PAN low-carbon fiber obtained in step (5) is placed in a medium-carbon furnace under a high-purity nitrogen atmosphere for medium-temperature carbonization treatment. The oxygen content in the nitrogen is less than 1 ppm, and nitrogen with a CH4 gas volume fraction of 1.5% is introduced. The treatment temperature is 1200°C, the residence time is 2 minutes, and the drawing ratio is 0.985.

[0046] (7) High temperature carbonization

[0047] The PAN low carbon fiber obtained in step (6) is put into a high carbon furnace under a high-purity nitrogen environment atmosphere for high-temperature carbonization treatment, the oxygen content in the nitrogen is less than 1PPm, the first-stage treatment temperature is 1450°C, the residence time is 1 min, the second-stage treatment temperature is 1800°C, the residence time is 0.5 min, and the draft ratios are 0.97 and 0.96 times, respectively, to obtain high-strength medium modulus carbon fibers with high compression resistance and a skin-core graphitization degree ratio of 1.25, and the radial structure gradient distribution of the carbon fibers is uniform, and the graphitization structure distribution g value curve of the fiber cross section at each test site is as shown in Figure 2 .

[0048] The carbon fiber composite material prepared is stretched by using GB T 3362-1982 and GB T3354-1982, and the compression performance of the carbon fiber composite material is tested, to obtain high-strength medium modulus carbon fibers with high compression resistance and a skin-core graphitization degree ratio of 1.25, and the performance is: the fiber cross section is circular or nearly circular, the strength is 5.65GPa, the modulus is 300GPa, the fiber diameter is 6.9μm, the crystallite width L a,r of the carbon fiber skin layer is 2.47nm, the compression strength of the carbon fiber bundle is 2.24GPa, and the carbon fiber bundle compression tension ratio is 39.65%.

[0049] Skin-core graphitization degree test method:

[0050] The obtained carbon fiber is embedded and treated by using a mixture of room temperature curing epoxy resin: curing agent = 25:3 at 60°C for two hours, and then the resin block embedded with the carbon fiber is polished by using a polishing machine until the radial surface of the carbon fiber is exposed above the numerical block, whether the radial surface of the carbon fiber is clean and tidy without impurities is observed by using an optical microscope method, the magnification is 50 times, the radial surface of the carbon fiber is observed and photographed. Then the radial surface of the carbon fiber is tested by using a Raman spectrum tester. Select one carbon fiber, test 5 points on the diameter of its radial cross section from the skin to the core to the skin, the single scanning time is 10s, each scanning is 5 times, use a 514nm laser, and the scanning spectrum range is 1000-1800cm -1 .

[0051] Comparative Example 1

[0052] (1) Spinning solution preparation

[0053] The same as Example 1

[0054] (2) Coagulation molding of the spinning solution

[0055] A spinneret with an aperture of 0.05 mm was used. After leaving the spinneret hole, the fiber entered the first coagulation bath. The temperature of the first coagulation bath was 25° C. The coagulation bath used an aqueous solution of dimethyl sulfoxide with a volume content of 74%. The coagulation time was 2 minutes and the coagulation draft ratio was -0.3. The rest was the same as in Example 1.

[0056] (3) Preparation of raw silk

[0057] The superheated steam temperature is 145° C., the draft ratio is 2.15 times, and the rest is the same as in Example 1.

[0058] (4) Pre-oxidation

[0059] The PAN precursor obtained in step (3) was thermally stabilized and pre-oxidized in a pre-oxidation furnace. The initial wire speed was 20 m / h. The temperature was gradually increased in three zones, with temperatures of 230°C, 245°C, and 265°C, respectively. The drafting ratio was 1.05 times. The total pre-oxidation time was 120 minutes, and the time distribution among the three temperature zones was 1:6:1.

[0060] (5) Low temperature carbonization

[0061] The PAN pre-oxidized fiber obtained in step (4) is put into a low-carbon furnace under a high-purity nitrogen atmosphere for low-temperature carbonization treatment, with an oxygen content of 5 ppm in the nitrogen, a temperature of 650°C, a residence time of 2 minutes, and a draft ratio of 1.05 times.

[0062] (6) High temperature carbonization

[0063] The PAN-based carbon fiber obtained in step (5) is put into a high-carbon furnace under a high-purity nitrogen atmosphere for high-temperature carbonization treatment, with an oxygen content of 1 ppm in the nitrogen, a treatment temperature of 1550°C, a residence time of 2.5 min, and a draft ratio of 0.9 times to obtain high-compression-resistant high-strength medium-modulus carbon fibers with different degrees of graphitization of the skin and core. A high-compression-resistant high-strength medium-modulus carbon fiber with a skin-core graphitization ratio of 1.04 is obtained. The graphitization structure distribution g value curve of each test site on the fiber cross section is as follows: Figure 3 shown.

[0064] The properties of the obtained carbon fiber are as follows: the fiber cross section is nearly circular, the tensile strength is 5.50 GPa, the tensile modulus is 288 GPa, the fiber diameter is 5.5 μm, and the crystallite width L of the carbon fiber cortex is a,r The carbon fiber bundle compressive strength is 1.50GPa, and the carbon fiber bundle tensile ratio is 27.27%.

[0065] Compared with Comparative Example 1, the tensile strength and tensile modulus of the carbon fiber in Example 1 were substantially unchanged, the diameter of the carbon fiber was increased by 22.81%, and the compressive strength of the carbon fiber bundle was increased by 49.33%.

[0066] Example 2

[0067] (1) Preparation of spinning solution

[0068] Acrylonitrile and itaconic acid were mixed in a molar ratio of 97:3 and added to a polymerization reaction vessel. The polymerization temperature was 65°C. After polymerization, the viscosity of the spinning solution measured by a rotary viscometer at 25°C was 3500 poise. The rest was the same as in Example 1.

[0069] (2) Coagulation and molding of spinning solution

[0070] The temperature of the first coagulation bath is 60° C., the coagulation bath uses an aqueous solution of dimethyl sulfoxide, wherein the volume content of dimethyl sulfoxide is 58%, the coagulation time is 2 minutes, and the coagulation draft ratio is -0.10; the rest is the same as in Example 1.

[0071] (3) Preparation of raw silk

[0072] After drying and densification, the fiber was subjected to secondary drawing in superheated water vapor at 140° C. with a drawing ratio of 2.8 times. The rest was the same as in Example 1.

[0073] (4) Pre-oxidation

[0074] The PAN precursor obtained in step (3) was thermally stabilized and pre-oxidized in a pre-oxidation furnace. The initial wire speed was 20 m / h. The temperature was gradually increased in four temperature zones, with temperatures of 220°C, 233°C, 244°C, and 260°C, respectively. The drafting ratio was 1.15 times. The total pre-oxidation time was 90 minutes, and the time distribution of the four temperature zones was 2:2:2:2.

[0075] (5) Low temperature carbonization

[0076] The PAN pre-oxidized fiber obtained in step (4) is put into a low-carbon furnace under a high-purity nitrogen atmosphere for low-temperature carbonization treatment, with an oxygen content of 5 ppm in the nitrogen, a temperature of 650°C, a residence time of 2 minutes, and a draft ratio of 1.1 times.

[0077] (6) Medium temperature carbonization

[0078] The PAN low-carbon fiber obtained in step (5) is placed in a medium-carbon furnace under a high-purity nitrogen atmosphere for medium-temperature carbonization treatment. The oxygen content in the nitrogen is less than 1 ppm, and nitrogen with a CH4 gas volume fraction of 0.5% is introduced. The treatment temperature is 1000°C, the residence time is 2 minutes, and the drawing ratio is 0.99.

[0079] (7) High temperature carbonization

[0080] The PAN low carbon fiber obtained in step (6) is put into a high carbon furnace under a high-purity nitrogen atmosphere for high-temperature carbonization treatment, the oxygen content in the nitrogen is 1PPm, the first-stage treatment temperature is 1450°C, the residence time is 1.5 min, the second-stage treatment temperature is 1700°C, the residence time is 0.5 min, and the draft ratios are 0.96 and 0.94 times, respectively, to obtain high-compression-resistant high-strength medium-model carbon fibers with different skin-core graphitization degrees, and obtain high-compression-resistant high-strength medium-model carbon fibers with a skin-core graphitization degree ratio of 1.11, and the graphitization structure distribution g value curve of each test site of the fiber cross section is as shown in Figure 4 .

[0081] The obtained carbon fiber has the following properties: the fiber cross section is nearly circular, the tensile strength is 5.98 GPa, the tensile modulus is 290 GPa, the fiber diameter is 6.40 μm, the crystallite width L a,r of the carbon fiber skin layer is 2.18 nm, the carbon fiber bundle filament compression strength is 1.93 GPa, and the carbon fiber bundle filament compression-tension ratio is 32.27%.

[0082] Example 3

[0083] (1) Preparation of the spinning dope

[0084] (2) Coagulation and molding of the spinning dope

[0085] The same as in Example 1

[0086] (3) Preparation process of the precursor fiber

[0087] After drying and densification, the fiber is subjected to secondary drawing in superheated steam at 140°C, and the drawing ratio is 2.7 times, and the rest is the same as in Example 1.

[0088] (4) Pre-oxidation

[0089] The PAN precursor fiber obtained in step (3) is subjected to heat stabilization and pre-oxidation treatment in a pre-oxidation furnace, the wire running starting speed is 20 m / h, the step-by-step heating method is adopted, there are four temperature zones, the temperatures are 230°C, 240°C, 250°C and 265°C, respectively, the drawing ratio is 1.10 times, and the total pre-oxidation treatment time is 80 minutes, and the time proportion of the four temperature zones is 3:2:2:1.

[0090] (5) Low-temperature carbonization

[0091] The same as in Example 1

[0092] (6) Medium-temperature carbonization

[0093] The PAN low-carbon fiber obtained in step (5) is placed in a medium-carbon furnace under a high-purity nitrogen atmosphere for medium-temperature carbonization treatment. The oxygen content in the nitrogen is less than 1 ppm, and nitrogen with a CH4 gas volume fraction of 1% is introduced. The treatment temperature is 1150°C, the residence time is 2 minutes, and the drawing ratio is 0.995.

[0094] (7) High temperature carbonization

[0095] The PAN low carbon fiber obtained in step (6) is put into a high carbon furnace under a high-purity nitrogen atmosphere for high-temperature carbonization treatment, the oxygen content in the nitrogen is 1PPm, the first stage treatment temperature is 1400°C, the residence time is 1min, the second stage treatment temperature is 1750°C, the residence time is 0.5min, and the drafting ratios are 0.96 and 0.95 times respectively, to obtain high-compression-resistant high-strength medium-modulus carbon fibers with different degrees of graphitization of the skin and core, and obtain high-compression-resistant high-strength medium-modulus carbon fibers with a skin-core graphitization ratio of 1.18. The graphitization structure distribution g value curve of each test site on the fiber cross section is as follows: Figure 5 shown.

[0096] The properties of the obtained carbon fiber are as follows: the fiber cross section is nearly circular, the tensile strength is 5.89 GPa, the tensile modulus is 298 GPa, the fiber diameter is 6.66 μm, and the crystallite width L of the carbon fiber cortex is a,r The carbon fiber bundle compressive strength is 2.03 GPa, and the carbon fiber bundle compressive tensile ratio is 34.47%.

[0097] Example 4

[0098] (1) Preparation of spinning solution is the same as in Example 1

[0099] (2) Coagulation and molding of spinning solution

[0100] Same as Example 1

[0101] (3) Preparation process of raw silk

[0102] After drying and densification, the fiber was subjected to secondary drawing in superheated water vapor at 140° C. with a drawing ratio of 2.3 times. The rest was the same as in Example 1.

[0103] (4) Pre-oxidation

[0104] The PAN precursor obtained in step (3) was thermally stabilized and pre-oxidized in a pre-oxidation furnace. The initial wire speed was 20 m / h. The temperature was gradually increased in four temperature zones, with temperatures of 220°C, 240°C, 255°C, and 270°C, respectively. The total drafting ratio was 1.15 times, the total pre-oxidation time was 90 minutes, and the time distribution of the four temperature zones was 1:3:3:2.

[0105] (5) Low temperature carbonization

[0106] Same as Example 1

[0107] (6) Medium temperature carbonization

[0108] The PAN low-carbon fiber obtained in step (5) is placed in a medium-carbon furnace under a high-purity nitrogen atmosphere for medium-temperature carbonization treatment. The oxygen content in the nitrogen is less than 1 ppm, and nitrogen with a CH4 gas volume fraction of 2% is introduced. The treatment temperature is 1200°C, the residence time is 2 minutes, and the drawing ratio is 0.98.

[0109] (7) High temperature carbonization

[0110] The PAN low carbon fiber obtained in step (6) is put into a high carbon furnace under a high-purity nitrogen atmosphere for high-temperature carbonization treatment. The oxygen content in the nitrogen is less than 1PPm. The first treatment temperature is 1500°C, the residence time is 1.5min, the temperature is 1800°C, the residence time is 0.5min, and the drafting ratios are 0.98 and 0.97 times respectively. A high-strength medium-modulus carbon fiber with a skin-core graphitization ratio of 1.3 and high compression resistance is obtained, and the radial structure gradient of the carbon fiber is uniformly distributed. The graphitization structure distribution g value curve of each test site on the fiber cross section is as follows: Figure 6 shown.

[0111] The properties of the obtained carbon fiber are as follows: the fiber cross section is nearly circular, the tensile strength is 5.70 GPa, the tensile modulus is 295 GPa, the fiber diameter is 7.20 μm, and the crystallite width L of the carbon fiber cortex is a,r The carbon fiber bundle has a compressive strength of 2.28 GPa and a tensile strength ratio of 40%.

[0112] Table 1 Values ​​of each point in the g-value curve of radial graphitization degree of PAN carbon fiber

[0113]

[0114]

[0115] Table 2 Mechanical properties of PAN carbon fiber

[0116]

Claims

1. A high compressive strength and high strength medium modulus carbon fiber, characterized by: The average diameter of the carbon fiber is 5.5μm to 7.5μm, the tensile strength is 5.5GPa to 6.0GPa, the tensile modulus is 280GPa to 300GPa, and the compressive strength of the carbon fiber bundle is 1.5GPa to 2.3GPa. The carbon fiber preparation includes the following steps: preparing the precursor by wet spinning, pre-oxidizing the precursor, low-temperature carbonization and high-temperature carbonization, medium-temperature carbonization is performed before the high-temperature carbonization, and the high-temperature carbonization adopts two-stage gradient heating to make the width of the microcrystalline of the skin L a，r Between 2.1 and 2.7 nm, the skin-core ratio of the carbon fiber section is 1.11≤ f ≤1.30; the medium-temperature carbonization adopts high-purity nitrogen protection, the volume content of CH4 gas in the nitrogen is 0.2-5%, the medium-temperature carbonization temperature is 1000℃~1200℃, the time is 0.5-5 min, and the drawing ratio is 0.96-1.05 times.

2. The carbon fiber according to claim 1, characterized in that: The cross section of the carbon fiber is circular or nearly circular, and the cross section skin-core ratio is 1.11≤ f ≤1.

30.

3. The carbon fiber according to claim 1 or 2, characterized in that: The high-temperature carbonization adopts high-purity nitrogen protection, the starting temperature is 1300-1500°C, the ending temperature is 1500-1800°C, the total residence time is 2-5 minutes, and the relative total draft ratio is 0.88-0.98 times.

4. The carbon fiber according to claim 1 or 2, characterized in that: The pre-oxidation adopts a four-stage gradient temperature rising method, the temperature ranges are 210-225° C., 230-245° C., 240-255° C. and 250-280° C., the total residence time is 60-90 minutes, and the draft ratio is 1.0-1.2 times.

5. The carbon fiber according to claim 1 or 2, characterized in that: The low-temperature carbonization temperature is 350-850° C., the time is 0.5-5 minutes, and the draft ratio is 1.05-1.15 times.

6. The carbon fiber according to claim 1 or 2, characterized in that: The wet spinning process for preparing the precursor includes the preparation of spinning solution, multi-stage coagulation molding, primary drawing, multi-stage washing, oiling, drying and densification, secondary air drawing and heat setting. The diameter of the precursor is controlled at 11μm to 14μm.

7. The carbon fiber according to claim 6, characterized in that: The temperature of the first coagulation bath of the multi-stage coagulation molding is 35-70°C, the concentration of the first coagulation bath is 50-80%, the temperature of the superheated steam of the secondary gas drawing is 120-160°C, and the drawing ratio is 1.5-3.5.

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