Large-tow carbon fiber surface modification method based on microcosmic-microcosmic-macroscopic multi-scale simulation

Abstract

The invention discloses a large-tow carbon fiber surface modification method based on microcosmic-microcosmic-macroscopic multi-scale simulation, relates to a carbon fiber surface modification method, and aims to solve the technical problem of large workload of an existing large-tow carbon fiber composite material interface performance research method. The method comprises the following steps: 1, microcosmic interface simulation; 2, microcosmic interface testing; 3, testing a macroscopic interface; and 4, microcosmic interface simulation. On the basis of research on surface modification of large-tow carbon fibers based on microscopic-microscopic-macroscopic multi-scale simulation, a composite material is divided into three materials including fibers, resin and an interface for simulation by adopting microscopic simulation, and the problem that interface damage is difficult to analyze under the macroscopic scale is solved. The calculation method is clear in logic, simple in step and easy to operate. The method can be used in the field of carbon fiber composite research.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for surface modification of carbon fibers. BACKGROUND

[0002] Large tow carbon fibers are carbon fibers with a number of single fibers ≥ 48,000. Large tow carbon fibers have a wide range of applications in industry and are widely used in carbon fiber composites in the paper industry, metallurgical industry, chemical industry, textile industry, and medical machinery, including various parts that require high strength, high stiffness, light weight, high temperature resistance, corrosion resistance, and other environments, such as various shafts, containers, pipelines, and platforms. Large tow carbon fiber reinforced composites have become the preferred alternative material for automotive lightweighting due to their lightweight, high strength, and strong designability, and have attracted widespread attention in the field of rail transportation.

[0003] In the preparation process of carbon fibers, sizing process is crucial. The sizing agent directly contacts the surface of carbon fibers, and has a significant impact on the surface properties of carbon fibers, such as state, functional groups, and wettability. An appropriate sizing agent can enhance the surface activity of carbon fibers, improve the compatibility and wettability of fibers and resin matrix, and reduce the problem of hair in the processing process, thereby improving the overall quality and performance of carbon fibers. In actual production, in order to prevent fiber dispersion, carbon fibers are usually coated with sizing agent before leaving the factory. Small adjustments in the formula and ingredient content of the sizing agent can cause large changes in performance, which in turn affects the surface properties of carbon fibers. In the research of improving the interface performance of carbon fibers, if the interface performance is adjusted by replacing the sizing agent, the original sizing agent needs to be removed first, which may reduce the interface performance of carbon fibers. Direct surface modification of the sizing agent can optimize the bonding performance of the interface without affecting the original performance of the carbon fibers.

[0004] The modifier plays a "bridge" role in connecting carbon fibers and resin matrix in the composite material. A good interface phase can ensure uniform transmission of load between carbon fibers and resin matrix, reducing the risk of stress concentration, so the interface performance is crucial to the performance of carbon fiber reinforced resin matrix composites. In order to deeply understand the interface mechanical properties of large tow carbon fiber composites, the sizing agent of large tow carbon fibers needs to be modified to realize the regulation of its interface performance. However, due to the variety of modifiers and the complexity of process parameters, if traditional methods are used to prepare samples and test one by one, it will consume a lot of time and resources, resulting in waste of manpower and material resources. SUMMARY

[0005] The present application is to solve the technical problem of large tow carbon fiber composite material interface performance research method existing large workload, and provide a method for surface modification of large tow carbon fiber based on micro-meso-macro multi-scale simulation. Based on molecular dynamics simulation and finite element simulation, the micro-meso-macro multi-scale simulation of the surface modification of large tow carbon fiber is combined with micro and macro interface test, so that the interface performance regulation and optimization of large tow carbon fiber composite material can be realized in micro, meso and macro scale.

[0006] The flow chart of the method for surface modification of large tow carbon fiber based on micro-meso-macro multi-scale simulation of the present application is shown as Figure 1 The specific steps are as follows:

[0007] I. Micro-interface simulation:

[0008] (1) In the LAMMPS software, graphene unit cell is formed by cell expansion to form a periodic structure in x and y directions, and oxygen-containing functional groups are introduced according to the actual C / O ratio of the carbon fiber surface. The cross-linked structure of the sizing agent is established by the polymer modeling program, and relaxation is performed to make it tightly combined with the graphene oxide surface;

[0009] (2) Establish the structure of the modifier molecular thin layer to be screened above the sizing agent, and make the modifier thin layer react with the sizing agent by relaxation;

[0010] (3) The cross-linked structure of the matrix resin is established by the polymer modeling program and placed above the modifier, and the matrix resin reacts with the modifier thin layer by relaxation;

[0011] (4) Calculate the interface bonding energy of the established structure, and calculate the tensile properties of the interface at the same time. The modifier with high interface bonding energy and good tensile properties is screened out by the calculation results;

[0012] II. Micro-interface test:

[0013] (1) Using the modifier screened out in step one, prepare different concentration gradient modifier solutions to modify the carbon fiber, and obtain the modified carbon fiber;

[0014] (2) Observe the surface element distribution of the modified carbon fiber, test the surface morphology of the modified carbon fiber, and conduct surface energy and wettability test on the modified carbon fiber; Ensure that the modifier has reacted with the sizing agent on the surface of the carbon fiber;

[0015] (3) Perform single fiber tensile test and single fiber pull-out test on unmodified carbon fiber and modified carbon fiber, and obtain the interface strength of the modified carbon fiber prepared by different concentrations of modifier, wherein the interface strength is the tensile strength and shear strength; Then draw the relationship curve between interface strength and modifier concentration;

[0016] III. Macroscopic interface test:

[0017] (1) The unmodified carbon fiber and the modified carbon fiber prepared in step two (1) are cured with resin respectively to obtain carbon fiber composite materials;

[0018] (2) The carbon fiber composite material samples are subjected to tensile test, bending test and shear test to obtain the mechanical strength of the carbon fiber composite material samples, wherein the mechanical strength is tensile strength, bending strength and shear strength, and a relationship curve between the mechanical strength of the carbon fiber composite material and the concentration of the modifier is drawn;

[0019] IV. Meso-interface simulation:

[0020] (1) A composite material model is established in ABAQUS, the size of the composite material model is the same as that of the composite material sample in step three, and the resin and fiber are divided into regions, and the material properties of the resin material are assigned to the resin region, and the material properties of the fiber are assigned to the fiber region; wherein the material properties of the fiber are the tensile modulus, tensile strength, shear modulus and shear strength of the unmodified carbon fiber measured in step two;

[0021] (2) The same tensile load, bending load and shear load as the test process in step three are applied;

[0022] (3) cohesive elements are added between the resin region and the fiber region, the parameters of the cohesive elements are adjusted to make the strength value of the unmodified carbon fiber composite material model consistent with the strength value of the unmodified carbon fiber composite material tested in step three; then the parameters of the cohesive elements are modified to make the relationship curve between the strength value of the composite material model and the concentration of the modifier consistent with the relationship curve between the interface strength and the concentration of the modifier in step two; simulation is performed, the strength value of the simulated composite material model is compared with the strength value of the macroscopic interface test, if the simulated strength value is consistent with the strength value of the macroscopic interface test, then the larger range of the concentration of the modifier is simulated in the meso-interface, and the best concentration of the modifier is screened according to the strength of the composite material obtained by simulation, to obtain the surface modification scheme of the large tow carbon fiber; if the simulated strength value is not consistent with the strength value of the macroscopic interface test, then the parameters of the cohesive elements are adjusted again in step four (3).

[0023] Further, the modifier in step one is polyamide-imide (PAI), octadecyltrimethoxysilane (OTE), carbon black, acrylamide or phenylacrylamide.

[0024] Further, when the molecular thin layer structure of the modifier is established in step one, the intermolecular spacing is controlled at 0.3 nm to 0.8 nm; and the number of molecules of different modifier molecular thin layers differs by no more than 5%, facilitating comparison of the influence of different modifiers on the interface.

[0025] Further, in step one, when adding a new molecular layer to the structure, the new molecular layer should be kept at least 0.3 nm away from the original structure.

[0026] Further, in steps one (1), (2) and (3), the relaxation time is 18-22 ps, so as to reach the stable state of the structure under the time scale of molecular dynamics simulation.

[0027] Further, in steps one (1), (2) and (3), the relaxation, during the relaxation process, the coordinates of each atom in the output evolution region, the total energy and the stress value are set, which are used for subsequent calculation of binding energy and drawing of stress-strain curve; after LAMMPS calculates the binding energy, the unit atomic binding energy is calculated to avoid the influence of different number of atoms on the binding energy, and the calculation method of unit atomic binding energy is as follows:

[0028]

[0029] wherein, E inter is the binding energy of the interface, and the unit is kcal / mol;

[0030] N modifier is the number of atoms in the modifier thin layer;

[0031] is the unit atomic binding energy of the interface, and the unit is kcal / mol;

[0032] After LAMMPS calculates the tensile properties, the atomic stress value is further processed to obtain the stress-strain curve of the system, and the stress-strain calculation in the real unit system of LAMMPS is as follows:

[0033] ,

[0034] wherein, is the stress value, and the unit is GPa;

[0035] is the stress value output by LAMMPS, and the unit is atmospheres;

[0036] , , respectively are the lengths of the model xyz directions, and the unit is Å;

[0037] is the strain value; the unit is GPa;

[0038] is the displacement along the loading direction, and the unit is Å.

[0039] Further, in step two (2), the surface element distribution of the carbon fiber is obtained by surface element test. If the surface element distribution contains the characteristic element of the modifier, it indicates that the modifier has reacted with the sizing agent on the surface of the carbon fiber.

[0040] Further, in step two (2), the surface morphology is observed by scanning electron microscope. If the surface of the unmodified carbon fiber is smooth, and if the surface of the carbon fiber is rough, it indicates that the modifier has reacted with the sizing agent on the surface of the carbon fiber.

[0041] Further, in step two (2), the surface energy and wettability of the unmodified carbon fiber and the modified carbon fiber are obtained by surface energy and wettability test. If the surface energy of the modified carbon fiber is increased by more than 10% than that of the unmodified carbon fiber, it indicates that the modifier has reacted with the sizing agent on the surface of the carbon fiber. If the contact angle of the modified carbon fiber is decreased by more than 3° than that of the unmodified carbon fiber, it indicates that the modifier has reacted with the sizing agent on the surface of the carbon fiber.

[0042] Further, the specific method of the surface energy and wettability test of the modified carbon fiber in step two is as follows: the dynamic contact angle of the fiber is tested by surface / interface tension instrument, and the surface energy is calculated. The test conditions are as follows: the insertion length of the fiber is 3 mm, and the forward and backward speeds are both 0.05 mm / s. The contact angle is calculated according to the following formula:

[0043]

[0044] In the formula, θ is the contact angle, unit: °; m is the mass, unit: kg; g is the acceleration of gravity, 9.8 m / s; π is the circular constant, π = 3.14; d is the diameter of the carbon fiber, unit: μm; γ is the surface energy, unit: mN·m -1 .

[0045] The polar component, the dispersion component and the surface energy of the carbon fiber are calculated according to the following formula by the contact angle of the fiber in the polar and non-polar liquid:

[0046]

[0047]

[0048] In the formula, is the surface energy of the test liquid, unit: mN·m -1 ; is the polar component of the test liquid, unit: mN·m -1 ; is the dispersion component of the test liquid, unit: mN·m -1 ; is the surface energy of carbon fiber, in mN∙m -1 ; is the polar component of carbon fiber, unit is mN∙m -1 ; is the dispersion component of carbon fiber, in mN∙m -1 .

[0049] Furthermore, the tensile test of the modified carbon fiber monofilament described in step 2 was conducted in accordance with ASTM D3379-75 (1989) e1, "Test Method for Tensile Strength and Young's Modulus of Elastic Modulus Monofilament Materials." Specifically, an electronic universal testing machine was used to test the tensile strength (TS) of the carbon fiber monofilament. The carbon fiber monofilament was securely fixed to the centerline of a paper frame (40 mm × 20 mm outer frame, 20 mm × 10 mm inner frame) using 502 glue. The test rate was 10 mm / min, and the maximum load (F) at fiber breakage was recorded. At least 20 valid data points were collected for each fiber sample, and the average was calculated. Finally, the data results were statistically analyzed using the Weibull distribution function.

[0050] Furthermore, the modified carbon fiber single filament pull-out test described in step 2 is to test the interfacial shear strength (IFSS) of carbon fiber reinforced resin matrix composites by using a composite material interface evaluation device and a droplet debonding method. The test schematic is shown in the attached figure. Figure 4 As shown. First, the modified carbon fiber monofilament is fixed to the frame with double-sided tape, and the resin-curing agent system is prepared. A small amount of resin is dipped with a fine needle and evenly applied to the surface of the carbon fiber monofilament. At this time, resin droplets will be formed, and then placed in an oven for curing according to the resin curing procedure to finally obtain a carbon fiber / resin droplet composite material. The test specimen should try to select resin balls with a diameter of about 60~80μm, and close the upper and lower blades to clamp the front end of the ball. Set the load to move slowly to the left at a rate of 0.5μm / s until the ball is debonded. 50 valid data are selected for each group of specimens and their average value is taken. The IFSS value is calculated according to the following formula:

[0051]

[0052] Where F max is the maximum load when the resin is debonded, in N; d is the diameter of the carbon fiber monofilament, in m; l is the length of the carbon fiber coated in the epoxy resin droplet, in m.

[0053] Further, the tensile test of the carbon fiber composite material sample in step three is performed according to the national standard GB / T 1447-2005 "Fiber Reinforced Plastics Tensile Properties Test Method", and the tensile strength of the test sample is obtained. Specifically, in the test process, the maximum tensile stress of the sample in the tensile fracture process is taken as the tensile strength by using a loading speed of 2 mm / min. Three samples are selected for each test, and the average value is calculated.

[0054] The calculation formula of the tensile strength is:

[0055]

[0056] In the formula, σ is the tensile stress strength, the unit is MPa; F is the maximum load, the unit is N; b is the sample width, the unit is mm; d is the sample thickness, the unit is mm.

[0057] Further, the bending test of the carbon fiber composite material sample in step three is performed according to GB / T 1449-2005 "Fiber Reinforced Plastics Bending Properties Test Method", and the tensile strength of the test sample is obtained.

[0058] In the test, the loading speed is 2 mm / min. Three samples are selected for each test, and the average value is calculated.

[0059] The calculation formula of the bending strength is:

[0060]

[0061] In the formula, σ is the bending strength, the unit is MPa; P is the breaking load, the unit is N; l is the span, the unit is mm; h is the sample thickness, the unit is mm; b is the sample width, the unit is mm.

[0062] Further, the shear test of the carbon fiber composite material sample in step three is performed according to GB / T 3355-2005 "Fiber Reinforced Plastics Longitudinal and Transverse Shear Test Method", and the shear strength of the test sample is obtained. Specifically, the test is performed by using an Instron electronic universal material testing machine, the sample loading speed is 2 mm / min, three samples are selected for each test, and the average value is calculated as the interlaminar shear mechanical property of CF / PEEK.

[0063] Interlaminar shear strength S 12 The calculation method is as follows:

[0064]

[0065] In the formula, S 12 is the interlaminar shear strength, the unit is MPa; P is the maximum load when the sample is broken, the unit is N, b is the sample width, the unit is mm; h is the sample thickness, the unit is mm.

[0066] Furthermore, the composite material model described in step 4 is a transversely isotropic material model.

[0067] Furthermore, the strength value of the unmodified carbon fiber composite material model described in step 4 (3) is consistent with the strength value of the unmodified carbon fiber composite material tested in step 3, which means that the strength value of the unmodified carbon fiber composite material model and the strength value of the unmodified carbon fiber composite material tested in step 3 differ by 10%.

[0068] Furthermore, the relationship curve between the composite material model strength value and the modifier concentration in step 4 (3) is consistent with the relationship curve between the interface strength and the modifier concentration in step 2, which means that the difference between the composite material model strength value and the interface strength in step 2 at each modifier concentration is within 10%.

[0069] Furthermore, the consistency between the strength value after simulation and the strength value of the macro interface test in step 4 (3) means that the difference between the strength value after simulation and the strength value of the macro interface test is within 30%.

[0070] Furthermore, the modifier concentration range described in step 2 and the larger range of modifier concentration described in step 4 refer to the maximum and minimum values ​​of the modifier concentration being 2 to 10 times the maximum and minimum values ​​of the modifier concentration range in step 2, respectively.

[0071] Furthermore, a cohesive unit is added between the resin area and the fiber area, and the parameters of the cohesive unit are adjusted so that the strength value of the unmodified carbon fiber composite material model is consistent with the strength value of the unmodified carbon fiber composite material tested in step three; then the cohesive unit parameters are modified so that the relationship curve between the strength value of the composite material model and the modifier concentration is consistent with the relationship curve between the interface strength and the modifier concentration in step two; simulation is performed, and the strength value of the composite material model after simulation is compared with the strength value of the macro interface test. If the strength value after simulation is consistent with the strength value of the macro interface test, a micro-interface simulation is performed on a larger range of concentration modifiers. Based on the composite material strength calculated by simulation, the best modifier concentration is screened out to obtain a large-tow carbon fiber surface modification scheme.

[0072] The application realizes the research on the surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation, adopts meso simulation to simulate the composite material into three materials of fiber, resin and interface, and solves the defect that the interface damage is difficult to analyze under the macro scale. Among them, the material properties of the resin and the fiber can be obtained from the macro-micro test, and the interface properties are usually difficult to obtain by the test means. The interface properties can be obtained through the micro test and the micro simulation. In addition, in the micro simulation, a variety of modifiers can be simulated, and the modifiers can be preliminarily screened, and in the meso simulation, a larger range of modifier concentration can be simulated, so that a large range of tests on different modifier types and modifier concentrations can be avoided.

[0073] During the whole simulation process, the modification effect of the modifier on the interface is verified through the micro interface test and the macro interface test, and the modification effects of different modifiers under different scales are basically consistent. The calculation method has clear logic, simple steps and is easy to operate. The method is consistent with the actual situation in the whole simulation process, and the simulation results can be compared with the test data, and has obvious advantages and wide prospect in the research on the surface modification of large-tow carbon fibers.

[0074] The method of the application can conveniently design and control the interface modification of large-tow carbon fiber composite material, analyze the interaction mechanism of different interfaces from the macro, meso and micro levels, improve the interface performance and mechanical properties of the large-tow carbon fiber composite material, and better support the application of the carbon fiber composite material. BRIEF DESCRIPTION OF DRAWINGS

[0075] Figure 1 is a flowchart of the surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation of the application;

[0076] Figure 2 is a PAI modified epoxy sizing agent carbon fiber-epoxy resin system established based on step one in example 1;

[0077] Figure 3 is the microscopic scanning result after the carbon fiber surface is coated with different concentrations of modifiers.

[0078] Figure 4 is a single fiber pull-out test schematic diagram in example 1; 1 is a modified carbon fiber single fiber, 2 is double-sided adhesive tape, 3 is a frame, 4 is a resin droplet, and 5 is an upper and lower knife head;

[0079] Figure 5 is a comparison diagram of the bending simulation result and the test value of the composite material in example 1. DETAILED DESCRIPTION

[0080] The beneficial effects of the application are verified by the following examples.

[0081] Example 1: The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation of this embodiment is carried out in the following steps:

[0082] I. Micro-interface simulation:

[0083] (1) In the LAMMPS software, graphene unit cells are formed into x, y direction periodic structures by cell expansion, and oxygen-containing functional groups are introduced according to the actual C / O ratio measured in the Fourier infrared spectrum of the carbon fiber surface; the cross-linked structure of the sizing agent is established by the polymer modeling program, which is an epoxy sizing agent, and the epoxy sizing agent is tightly combined with the surface of the graphene oxide by relaxing for 20 ps;

[0084] (2) Establish the structure of the modifier molecular thin layer to be screened above the epoxy sizing agent, and make the modifier thin layer and the epoxy sizing agent fully react by relaxing for 20 ps; wherein the modifier to be screened is polyamide-imide (PAI), carbon black, OTE; the intermolecular distance in the modifier thin layer is controlled at 0.4 nm~0.5 nm; the distance between the modifier thin layer and the sizing agent layer is 0.3 nm;

[0085] (3) The cross-linked structure of the epoxy resin matrix is established by the polymer modeling program and placed above the modifier, and the epoxy resin and the modifier thin layer are fully reacted by relaxing for 20 ps; the distance between the epoxy resin layer and the modifier layer is 0.3 nm;

[0086] In steps (1), (2) and (3), the relaxation, the coordinates of each atom in the output evolution region, the total energy and the stress value during the relaxation process are set for subsequent calculation of binding energy and drawing of stress-strain curve; after the LAMMPS relaxation calculates the binding energy, the unit atomic binding energy is calculated to avoid the influence of different number of atoms on the binding energy, and the calculation method of the unit atomic binding energy is as follows:

[0087]

[0088] Wherein, is the binding energy of the interface (kcal / mol),

[0089] is the number of atoms in the modifier thin layer,

[0090] is the unit atomic binding energy of the interface (kcal / mol).

[0091] After the LAMMPS calculates the tensile properties, the atomic stress value is further processed to obtain the stress-strain curve of the system, and the stress-strain calculation in the real unit system of LAMMPS is as follows:

[0092] ,

[0093] wherein, is the stress value (GPa),

[0094] is the stress value (atmospheres) output by LAMMPS,

[0095] , , are the lengths of the model xyz directions (Å), respectively;

[0096] is the strain value,

[0097] is the displacement along the loading direction (Å).

[0098] (4) The interfacial bonding energy and tensile strength of the established unmodified and each modifier treated carbon fiber structure are shown in Table 1.

[0099] Table 1 Interfacial bonding energy and tensile strength of unmodified and each modifier treated carbon fiber

[0100]

[0101] It can be seen from Table 1 that the performance of the material using polyamide-imide as the modifier is better than the modification effect of other modifiers, so polyamide-imide is selected as the modifier; a system using polyamide-imide as the modifier is established, as shown in the accompanying drawings. Figure 2

[0102] II. Micro-interface test:

[0103] (1) Using the modifier polyamide-imide (PAI) screened in step one, polyamide-imide solutions with concentrations of 0.00wt%, 0.03wt%, 0.05wt%, 0.08wt% and 0.10wt% are prepared, which are coated on the surface of carbon fibers to modify the carbon fibers, and the modified carbon fibers are obtained, and the microscopic scanning results are shown in the accompanying drawings. Figure 3

[0104] (2) The surface element distribution of the modified carbon fiber is observed, the surface morphology and surface roughness of the modified carbon fiber are tested, and the surface energy and wettability test of the modified carbon fiber is carried out, and the specific method of the surface energy and wettability test is: the dynamic contact angle of the fiber is tested by using a surface / interface tension meter, and the surface energy is calculated, the test conditions are: the fiber insertion length is 3mm, the forward and backward speeds are both 0.05mm / s; the contact angle is obtained from the mass change during the process of inserting the carbon fiber into the test liquid, and the following calculation is made:​​

[0105]

[0106] wherein θ is the contact angle (°); m is the mass (kg); g is the gravity acceleration, 9.8 m / s; π = 3.14; d is the carbon fiber diameter (μm); γ is the surface energy (mN·m -1 ) of the test liquid.

[0107] The polar component, the dispersive component and the surface energy of the carbon fiber are calculated according to the following formulae, based on the contact angle of the fiber in polar and non-polar liquid:

[0108]

[0109]

[0110] wherein: is the surface energy (mN·m -1 ) of the test liquid, is the polar component (mN·m -1 ) of the test liquid, is the dispersive component (mN·m -1 ) of the test liquid, is the surface energy (mN·m -1 ) of the carbon fiber, is the polar component (mN·m -1 ) of the carbon fiber, is the dispersive component (mN·m -1 ) of the carbon fiber.

[0111] The surface element distribution of the carbon fiber is obtained by surface element test. The results show that the surface of the unmodified carbon fiber is mainly composed of C element, the characteristic element N of the polyamide-imide is found on the surface of the carbon fiber modified by polyamide-imide, the purity of the characteristic element C of the carbon black is improved on the surface of the carbon fiber modified by carbon black, and the characteristic element Si of the carbon black is found on the surface of the carbon fiber modified by OTE, which indicates that the modifier has reacted with the sizing agent on the surface of the carbon fiber.

[0112] The surface morphology is observed by scanning electron microscope. The results show that the surface of the unmodified carbon fiber is smooth, while the surfaces of the carbon fibers modified by polyamide-imide, carbon black and OTE are rough, which indicates that the modifier has reacted with the sizing agent on the surface of the carbon fiber.

[0113] The surface energy and contact angle of unmodified carbon fiber and modified carbon fiber were obtained through surface energy and wettability tests. It was found that the surface energy of modified carbon fiber increased by more than 10% than that of unmodified carbon fiber, indicating that the modifier reacted with the sizing agent on the carbon fiber surface. It was also found that the surface energy of modified carbon fiber decreased by more than 3° than that of unmodified carbon fiber, indicating that the carbon fiber surfaces modified by modifier polyamide-imide, carbon black and OTE all reacted with the sizing agent on the carbon fiber surface.

[0114] (3) The modified carbon fibers obtained under the conditions of 0.00wt%~0.10wt% were subjected to single-filament tensile test and single-filament pull-out test. The modified carbon fiber single-filament tensile test was carried out in accordance with ASTM D3379-75(1989)e1 "Test Method for Tensile Strength and Young's Modulus of Elastic Modulus Monofilament Materials". Specifically, an electronic universal testing machine was used to test the tensile strength (TS) of the carbon fiber single-filament. The carbon fiber single-filament was firmly fixed on the center line of the paper frame (outer frame 40 mm×20 mm, inner frame 20 mm×10 mm) with 502 glue. The test rate was 10 mm / min, and the maximum load (F) when the fiber broke was recorded. At least 20 valid data were taken for each fiber sample to calculate the average value. Finally, the data results were statistically analyzed using the Weibull distribution function.

[0115] The modified carbon fiber single filament pull-out test is to test the interfacial shear strength (IFSS) of carbon fiber reinforced resin matrix composites by using a composite material interface evaluation device and a microdroplet debonding method. The test diagram is shown in the attached figure. Figure 4 As shown. First, the modified carbon fiber monofilament 1 is fixed to the frame 3 with double-sided tape 2, and a resin-curing agent system is prepared. A small amount of resin is dipped with a fine needle and evenly applied to the surface of the carbon fiber monofilament. At this time, resin droplets 4 are formed, and then placed in an oven for curing according to the resin curing procedure, and finally a carbon fiber / resin droplet composite material is obtained. The test specimen should try to select resin balls with a diameter of about 60~80μm, and close the upper and lower blades 5 to clamp the front end of the ball. Set the load to move slowly to the left at a rate of 0.5μm / s until the ball is debonded. 50 valid data are selected for each group of specimens and their average value is taken. The IFSS value is calculated according to the following formula:

[0116]

[0117] Where F max is the maximum load when the resin is debonded (N), d is the diameter of the carbon fiber filament (m), and l is the length of the carbon fiber wrapped in the epoxy resin droplet (m).

[0118] The tensile modulus, tensile strength, shear modulus and shear strength of the modified carbon fibers prepared with different concentrations of modifiers are shown in Table 2.

[0119] Table 2 Mechanical strength of modified carbon fibers prepared by different concentrations of modifier

[0120]

[0121] From Table 2, a curve of the interfacial performance and the concentration of the modifier can be drawn, and the influence of the concentration of the modifier on the interfacial performance is obtained.

[0122] III. Macroscopic interfacial test:

[0123] (1) The modified carbon fibers modified by the different concentration gradient modifiers prepared in step two (1) are compounded with the resin to prepare carbon fiber composite materials.

[0124] (2) The carbon fiber composite material samples are subjected to tensile test, bending test and shear test,

[0125] The tensile test of the carbon fiber composite material sample is carried out according to the national standard GB / T 1447-2005 "Fiber Reinforced Plastics Tensile Property Test Method", and the tensile strength of the test sample is obtained. Specifically, in the test process, the maximum tensile stress of the sample in the tensile fracture process is taken as the tensile strength by using a loading speed of 2 mm / min. Three samples are selected for each test group, and the average value is calculated.

[0126] The calculation formula of the tensile strength is:

[0127]

[0128] In the formula, σ is the tensile stress strength (MPa), F is the maximum load (N), b is the sample width (mm), and d is the sample thickness (mm).

[0129] The bending test of the carbon fiber composite material sample is carried out according to GB / T 1449-2005 "Fiber Reinforced Plastics Bending Property Test Method", and the tensile strength of the test sample is obtained. In the test, the loading speed is 2 mm / min. Three samples are selected for each test group, and the average value is calculated.

[0130] The calculation formula of the bending strength is:

[0131]

[0132] In the formula, σ is the bending strength (MPa), P is the breaking load (N), l is the span (mm), h is the sample thickness (mm), and b is the sample width (mm).

[0133] The shear test of the carbon fiber composite sample is carried out according to GB / T 3355-2005 "Longitudinal and Transverse Shear Test Method for Fiber Reinforced Plastics", and the shear strength of the test sample is obtained. Specifically, an Instron electronic universal material testing machine is used for testing, the sample loading speed is 2mm / min, three samples are tested in each group, and the average value is calculated as the interlaminar shear mechanical property of CF / PEEK.

[0134] Interlaminar shear strength S 12 The calculation method is as follows:

[0135]

[0136] In the formula, S 12 is the interlaminar shear strength (MPa), P is the maximum load (N) when the sample is broken, B is the sample width (mm), and h is the sample thickness (mm).

[0137] Interlaminar shear modulus G 12 The calculation method is as follows:

[0138]

[0139] In the formula, G 12 is the interlaminar shear modulus (MPa), ΔS 12 is the shear stress increment (MPa) selected on the straight line segment of the interlaminar shear stress-strain curve, and Δε is the corresponding shear strain increment. 12

[0140] The tensile strength, bending strength and shear strength of the carbon fiber composite sample modified by each modifier at different concentrations are shown in Table 3;

[0141] Table 3 Influence of different concentrations of modifiers on the mechanical strength of carbon fiber composite samples

[0142]

[0143] From the data in Table 3, the relationship curve between the mechanical strength of the carbon fiber composite sample and the concentration of the modifier is drawn, and the influence law of the concentration of the modifier on the mechanical strength of the carbon fiber composite sample is obtained.

[0144] Four, meso-interface simulation:

[0145] (1) A composite material model is established in ABAQUS, the size of the composite material model is the same as the size of the composite material sample in step three, and the resin and fiber are divided into regions, and the material properties of the resin material are assigned to the resin region, and the material properties of the fiber are assigned to the fiber region; The material properties of the fiber are the tensile modulus, tensile strength, shear modulus and shear strength of the unmodified carbon fiber measured in step two.​

[0146] (2) Apply the same tensile load, bending load and shear load as the test process of step three;

[0147] (3) Add cohesive element between the resin region and the fiber region, adjust the parameters of the cohesive element, so that the strength value of the unmodified carbon fiber composite material model is within 10% of the strength value of the unmodified carbon fiber composite material tested in step three;

[0148] Then modify the cohesive element parameters so that the relationship curve between the strength value of the composite material model and the modifier concentration is within 10% of the relationship curve between the interfacial strength and the modifier concentration in step two at each concentration;

[0149] Perform simulation, compare the strength value of the simulated composite material model with the strength value of the macroscopic interface test, and find that the strength value of the simulated composite material model is within 10% of the strength value of the macroscopic interface test, indicating that the cohesive element parameter adjustment is qualified;

[0150] Microscopic interface simulation is performed on a large range of modifier concentrations from 0.10wt% to 1.00wt%, and based on the strength of the composite material obtained by simulation calculation, the best concentration of polyamide-imide as modifier is screened out as 1.00wt%, and the surface modification scheme of large-tow carbon fiber is obtained, that is, the carbon fiber is modified with polyamide-imide as modifier with a concentration of 1.00wt%, and the carbon fiber composite material with the best performance can be obtained.

[0151] Figure 5 The comparison chart of the bending simulation result of the composite material and the test value.

[0152] This embodiment first explores the influence of various modifiers on the interfacial bonding performance from the micro level, and completes the preliminary screening. Then the influence on the interfacial performance is verified through micro interface test and macro interface test, and the multi-parameter optimization of the interfacial performance is carried out. Further, the microcosmic simulation is adopted to realize the regulation and control of the interfacial performance, so as to realize the regulation and optimization of the interfacial performance of the large-tow carbon fiber composite material at the micro, meso and macro scales.

Claims

1. A method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation, characterized in that: The method proceeds as follows:

1. Micro-interface simulation: (1) In the LAMMPS software, the graphene unit cell is expanded to form a periodic structure in the x and y directions, and oxygen-containing functional groups are introduced according to the actual C / O ratio of the carbon fiber surface. The cross-linked structure of the sizing agent is established through the polymer modeling program, and it is tightly bonded to the graphene oxide surface through relaxation. (2) Establishing a thin layer structure of the modifier molecules to be screened, placing it on top of the sizing agent, and allowing the modifier layer to react with the sizing agent through relaxation; (3) The cross-linked structure of the matrix resin is established through a polymer modeling program, placed on top of the modifier, and the matrix resin and the modifier thin layer are reacted through relaxation; (4) Calculate the interface binding energy of the established structure and the tensile properties of the interface at the same time, and comprehensively screen out modifiers with high interface binding energy and good tensile properties through the calculation results; 2. Micro interface test: (1) Using the modifier screened in step 1, preparing modifier solutions with different concentration gradients to modify the carbon fibers to obtain modified carbon fibers; (2) Observe the surface element distribution of the modified carbon fiber, test the surface morphology of the modified carbon fiber, and conduct surface energy and wettability tests on the modified carbon fiber; ensure that the modifier reacts with the sizing agent on the carbon fiber surface; (3) Performing single-filament tensile tests and single-filament pull-out tests on unmodified carbon fibers and modified carbon fibers to obtain the interfacial strength of modified carbon fibers prepared with different concentrations of modifiers, where the interfacial strength is the tensile strength and the shear strength; and then drawing a curve showing the relationship between the interfacial strength and the modifier concentration; 3. Macro interface test: (1) curing the unmodified carbon fiber and the modified carbon fiber prepared in step 2 (1) with resin to obtain a carbon fiber composite material; (2) Conduct tensile tests, bending tests, and shear tests on carbon fiber composite material samples to obtain the mechanical strength of the carbon fiber composite material samples, where the mechanical strength is the tensile strength, bending strength, and shear strength, and draw a curve showing the relationship between the mechanical strength of the carbon fiber composite material and the concentration of the modifier; 4. Micro-interface simulation: (1) Establish a composite material model in ABAQUS. The size of the composite material model is the same as that of the composite material specimen in step 3. The model is divided into regions according to resin and fiber. The material properties of the resin material are assigned to the resin region, and the material properties of the fiber are assigned to the fiber region. The material properties of the fiber are the tensile modulus, tensile strength, shear modulus and shear strength of the unmodified carbon fiber measured in step 2; (2) Apply the same tensile load, bending load and shear load as in step 3; (3) Add a cohesive unit between the resin region and the fiber region, and adjust the parameters of the cohesive unit so that the strength value of the unmodified carbon fiber composite material model is consistent with the strength value of the unmodified carbon fiber composite material tested in step 3; then modify the cohesive unit parameters so that the relationship curve between the strength value of the composite material model and the modifier concentration is consistent with the relationship curve between the interface strength and the modifier concentration in step 2; Perform simulation and compare the strength value of the composite material model after simulation with the strength value of the macro interface test. If the strength value after simulation is consistent with the strength value of the macro interface test, perform micro-interface simulation on a larger range of concentration modifiers. Based on the composite material strength calculated by simulation, screen out the best modifier concentration and obtain a large-tow carbon fiber surface modification scheme. If the strength value after simulation is inconsistent with the strength value of the macro interface test, return to step four (3) to readjust the parameters of the cohesive unit.

2. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1, characterized in that: The modifier described in step 1 is polyamide-imide, octadecyltrimethoxysilane, carbon black, acrylamide or phenylacrylamide.

3. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: When establishing the modifier molecular thin layer structure in step 1, the distance between molecules is controlled to be 0.3nm~0.8nm, and the difference in the number of molecules of different modifier molecular thin layers does not exceed 5%.

4. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: When adding a new molecular layer to the structure in step 1, the new molecular layer should maintain a distance of at least 0.3 nm from the original structure.

5. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: The relaxation time described in steps 1 (1), (2), and (3) is 18 to 22 ps.

6. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: During the relaxation described in steps 1 (1), (2), and (3), the coordinates, total energy, and stress values ​​of each atom in the output evolution region must be set for subsequent calculation of binding energy and drawing of stress-strain curves. After LAMMPS relaxation calculates the binding energy, the unit atom binding energy is calculated to avoid the influence of different atomic numbers on the binding energy. The unit atom binding energy is calculated as follows: ， Among them, E inter is the binding energy of the interface, in kcal / mol; N modifier is the number of atoms in the modifier thin layer; is the unit atomic binding energy of the interface, in kcal / mol; After LAMMPS calculates the tensile properties, the atomic stress values ​​are further processed to obtain the stress-strain curve of the system. The stress-strain curve is calculated in LAMMPS real units as follows: ， ， in, is the stress value, in GPa; The stress value output by LAMMPS is in atmospheres. , , are the lengths of the model in x, y, and z directions, respectively, in Å; is the strain value; the unit is GPa; is the displacement along the loading direction, in Å.

7. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: The composite material model described in step 4 has transversely isotropic material properties.

8. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: The consistency between the strength value of the unmodified carbon fiber composite material model described in step 4 (3) and the strength value of the unmodified carbon fiber composite material tested in step 3 means that the strength value of the unmodified carbon fiber composite material model and the strength value of the unmodified carbon fiber composite material tested in step 3 differ by 10%.

9. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: The relationship curve between the composite material model strength value and the modifier concentration in step 4 (3) is consistent with the relationship curve between the interface strength and the modifier concentration in step 2, which means that the difference between the composite material model strength value and the interface strength in step 2 at each modifier concentration is within 10%.

10. The method for surface modification of large-tow carbon fibers based on micro-meso-macro multi-scale simulation according to claim 1 or 2, characterized in that: The consistency between the strength value after simulation and the strength value of the macro interface test in step 4 (3) means that the difference between the strength value after simulation and the strength value of the macro interface test is within 30%.

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