Long-short carbon nanotube reinforced toughened fiber composite material and preparation method thereof

Abstract

The present invention provides a long-short carbon nanotube reinforced and toughened fiber composite material and a preparation method thereof. The preparation method provided by the present invention comprises: a) mixing short carbon nanotubes, a thermosetting resin and an additive to obtain a resin-based slurry; b) pouring the resin-based slurry into a fiber preform and curing and forming it to obtain a long-short carbon nanotube reinforced and toughened fiber composite material; wherein: the fiber preform comprises: an upper fiber cloth layer, a long carbon nanotube fiber yarn layer, and a lower fiber cloth layer stacked and contacted in sequence; the long carbon nanotube fiber yarn layer is a yarn-like structure formed by long carbon nanotubes; the length of the short carbon nanotubes is 0.5 to 3 μm, and the average length is ≤2 μm; the length of the long carbon nanotubes is 50 to 1000 μm, and the average length is >100 μm. The present invention optimizes the spatial layout of long and short carbon nanotubes in the composite material, simultaneously achieving the dual purposes of intra-layer reinforcement and inter-layer toughening of the fiber composite material.

Description

Technical Field

[0001] The present invention relates to the field of composite materials, and in particular to a long-short carbon nanotube reinforced and toughened fiber composite material and a preparation method thereof. Background Art

[0002] The past half century has witnessed the rapid development of resin-based composites. Their higher specific stiffness, strength, excellent fatigue performance, and corrosion resistance make them attractive for applications in aircraft, automobiles, civil engineering, ships, and offshore platforms. The current international composites market is enormous, with numerous end-use products based on composites already entering the market. By enhancing the performance of nano-reinforced composites, reducing matrix weight, and replacing expensive carbon fibers and synthetic fibers in composites, they provide a sustainable competitive advantage in sectors such as aerospace, automotive, and energy. The development of high-performance nano-reinforced composites has become a crucial application area in this field. Carbon nanotubes are among the strongest materials discovered on Earth to date, boasting a tensile strength 20 times that of high-strength steel and a Young's modulus an order of magnitude higher than carbon fibers, approximately 100 times that of steel. Their exceptional mechanical properties, combined with excellent electrical, chemical, and thermal stability, make them versatile in the research and development of ultra-strong composites. However, there are still many technical bottlenecks. For example, the large aspect ratio and high specific surface area make it difficult for CNTs to be evenly dispersed in the resin matrix, and a small amount of CNTs will significantly increase the viscosity of the resin, making it difficult to introduce the resin and infiltrate the fiber preform.

[0003] When it comes to carbon nanotube-reinforced continuous fiber composites, the challenge is how to successfully penetrate the narrow gaps between the continuous fibers and evenly disperse them throughout the composite. Carbon fibers typically have a diameter of 5-10 microns, but composites with a carbon fiber content of 55% or more achieve optimal performance and maximize the carbon fiber's effectiveness. Figure 1 This is a schematic diagram of a cross-section of a carbon fiber epoxy composite. Assuming the diameter of the carbon fiber is 7 microns (such as T700 carbon fiber produced by Toray in Japan), its volume fraction in the composite is 55%. Assuming that the carbon fiber is evenly distributed in the epoxy resin matrix, the distance between adjacent carbon fibers is less than 2 microns. Since the length of commercial carbon nanotubes is typically between 20 and 100 microns, the carbon nanotubes cannot pass through the gaps between the carbon fibers, causing a "fiber filtering effect." If this problem is not solved, it will be difficult to significantly improve the mechanical properties of fiber composites.

[0004] Furthermore, fiber composites are typically used in the form of laminated products. Due to their hierarchical structure and the inherent brittleness of the resin matrix, their through-thickness load-bearing capacity is low. These materials are susceptible to delamination damage under loads such as in-plane compression, bending, fatigue, and lateral impact. Once delamination begins and propagates within the laminate, the stiffness of the entire structure gradually decreases, ultimately leading to catastrophic failure. Therefore, improving interlaminar fracture toughness is crucial in many engineering applications. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a long-short carbon nanotube reinforced toughening fiber composite material and a preparation method thereof. The long-short carbon nanotube reinforced toughening fiber composite material provided by the present invention can simultaneously achieve matrix reinforcement and interlayer toughening of the composite material.

[0006] The present invention provides a method for preparing a long-short carbon nanotube reinforced and toughened fiber composite material, comprising the following steps:

[0007] a) mixing short carbon nanotubes, a thermosetting resin, and an additive to obtain a resin-based slurry;

[0008] b) pouring the resin-based slurry into a fiber preform and curing the preform to obtain a long-short carbon nanotube reinforced and toughened fiber composite material;

[0009] in:

[0010] The fiber preform comprises: an upper fiber cloth layer, a long carbon nanotube fiber yarn layer, and a lower fiber cloth layer stacked and contacted in sequence;

[0011] The long carbon nanotube fiber yarn layer is a yarn-like structure formed by long carbon nanotubes;

[0012] The length of the short carbon nanotubes is 0.5 to 3 μm, and the average length is ≤ 2 μm;

[0013] The length of the long carbon nanotubes is 50 to 1000 μm, with an average length greater than 100 μm.

[0014] Preferably, the short carbon nanotubes are short carbon nanotubes without surface modification or short carbon nanotubes with surface modification;

[0015] The long carbon nanotubes are long carbon nanotubes without surface modification or long carbon nanotubes with surface modification;

[0016] In the surface-modified short carbon nanotubes, the surface-modified functional groups are selected from one or more of amino, carboxyl and carbonyl groups;

[0017] In the surface-modified long carbon nanotubes, the surface-modified functional groups are selected from one or more of amino, carboxyl and carbonyl groups.

[0018] Preferably, the thermosetting resin includes one or more of epoxy resin, polyester resin, phenolic resin, vinyl resin and bismaleimide resin.

[0019] Preferably, the mass of the short carbon nanotubes is 0.1% to 5% of the mass of the thermosetting resin;

[0020] The mass of the long carbon nanotubes is 0.1% to 5% of the mass of the thermosetting resin;

[0021] The total mass of the upper fiber cloth and the lower fiber cloth accounts for 40% to 80% of the mass of the composite material.

[0022] Preferably, the fiber cloth in the upper fiber cloth layer is unidirectional fiber cloth or multidirectional fiber cloth;

[0023] The fiber cloth in the lower fiber cloth layer is unidirectional fiber cloth or multidirectional fiber cloth;

[0024] The fiber cloth in the upper fiber cloth layer is continuous carbon fiber cloth, continuous glass fiber cloth or continuous aramid fiber cloth;

[0025] The fiber cloth in the lower fiber cloth layer is continuous carbon fiber cloth, continuous glass fiber cloth or continuous aramid fiber cloth;

[0026] The fiber cloth in the upper fiber cloth layer is a fiber cloth without surface modification or a fiber cloth with surface modification;

[0027] The fiber cloth in the lower fiber cloth layer is a fiber cloth without surface modification or a fiber cloth with surface modification.

[0028] Preferably, the number of fiber cloth layers in the upper fiber cloth layer is one or more layers;

[0029] The number of fiber cloth layers in the lower fiber cloth layer is one or more.

[0030] Preferably, the additive is a curing agent and / or an accelerator;

[0031] The amount of the curing agent is 1% to 50% of the mass of the thermosetting resin;

[0032] The amount of the accelerator is 0.1% to 5% of the mass of the thermosetting resin.

[0033] Preferably, in the step b), the resin-based slurry is infused into the fiber preform using a vacuum assisted resin transfer molding process.

[0034] Preferably, in the step b), the curing temperature is 25-500° C. and the pressure is ≤10 MPa.

[0035] The present invention also provides a long-short carbon nanotube reinforced and toughened fiber composite material prepared by the preparation method described in the above technical solution.

[0036] In the preparation method provided by the present invention, during the mutual infiltration of resin and fiber, short carbon nanotubes can easily penetrate the narrow gaps between fibers and be evenly dispersed in the composite material; while long carbon nanotubes are laid out on the surface of the fiber cloth in the form of fiber yarns before the resin-based slurry is poured. In this way, the problem of the difficulty in dispersing long carbon nanotubes is effectively solved, and the "fiber filtering effect" of carbon nanotubes that often occurs in the VARTM process is effectively alleviated. The spatial layout of long and short carbon nanotubes in the composite material is optimized, and short carbon nanotubes effectively pass through the narrow gaps between fibers and are evenly distributed in the entire fiber composite material board. Long carbon nanotubes are enriched in the interlayer area of ​​the composite material board, so that their excellent mechanical properties can be fully utilized, and the dual purposes of intra-layer reinforcement and inter-layer toughening of the fiber composite material are simultaneously achieved. In summary, the present invention effectively avoids the "fiber filtering effect" commonly found in carbon nanotubes in the VARTM process, allowing short carbon nanotubes to be evenly dispersed in the fiber composite material, while effectively filling the gaps or resin-rich areas between the fiber composite material layers. The long carbon nanotubes strengthen the connection between the composite material layers, effectively improving the interfacial bonding force between the layers, so that the prepared composite material has excellent mechanical properties such as strength and toughness, as well as physical properties such as thermal and electrical properties.

[0037] The test results show that the composite material prepared by the present invention has a flexural strength of more than 600 MPa, which is 6% higher than that of the reference sample, and a type I interlaminar fracture toughness of 1500 J / m 2 The above is an improvement of more than 150% compared to the baseline sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0039] Figure 1 Schematic diagram of the cross section of carbon fiber epoxy resin composite material;

[0040] Figure 2 A schematic diagram of a first method for preparing long carbon nanotube fiber yarn;

[0041] Figure 3 A schematic diagram of a second method for preparing long carbon nanotube fiber yarn;

[0042] Figure 4 Schematic diagram of infusing a fiber preform with a resin-based slurry using a vacuum-assisted resin transfer molding process;

[0043] Figure 5 Schematic diagram of the laying structure of the fiber preform in Example 1;

[0044] Figure 6 This is a SEM image of the cured sample of the resin-based slurry obtained in step S3 of Example 1;

[0045] Figure 7 Schematic diagram of the preparation process of Example 1;

[0046] Figure 8 Graph showing the three-point bending test results of the samples of Example 1 and Comparative Example 1;

[0047] Figure 9 Graph showing the double cantilever beam test results of the samples of Example 1 and Comparative Example 1;

[0048] Figure 10 Graphs showing R curves (curves showing crack propagation resistance versus crack propagation) of the samples of Example 1 and Comparative Example 1;

[0049] Figure 11 This is the SEM image of the interlayer structure of the fractured sample in Example 1. DETAILED DESCRIPTION

[0050] The present invention provides a method for preparing a long-short carbon nanotube reinforced and toughened fiber composite material, comprising the following steps:

[0051] a) mixing short carbon nanotubes, a thermosetting resin, and an additive to obtain a resin-based slurry;

[0052] b) pouring the resin-based slurry into a fiber preform and curing the preform to obtain a long-short carbon nanotube reinforced and toughened fiber composite material;

[0053] in:

[0054] The fiber preform comprises: an upper fiber cloth layer, a long carbon nanotube fiber yarn layer, and a lower fiber cloth layer stacked and contacted in sequence;

[0055] The long carbon nanotube fiber yarn layer is a yarn-like structure formed by long carbon nanotubes;

[0056] The length of the short carbon nanotubes is 0.5 to 3 μm, and the average length is ≤ 2 μm;

[0057] The length of the long carbon nanotubes is 50 to 1000 μm, with an average length greater than 100 μm.

[0058]

About step a

[0059] The short carbon nanotubes, thermosetting resin and additives are mixed to obtain a resin-based slurry.

[0060] According to the present invention, the raw materials for forming the resin-based slurry include short carbon nanotubes.

[0061] In the present invention, the length distribution of the short carbon nanotubes is 0.5 to 3 μm, and the average length is ≤ 2 μm. In the present invention, the diameter of the short carbon nanofiber tubes is preferably 1 to 50 nm. In the present invention, the short carbon nanotubes are obtained by shortening carbon nanotubes. There is no special restriction on the source of the carbon nanotubes, and they can be commercial products or prepared according to conventional preparation methods in the field. The shortening methods include: mechanical ball milling or chemical wet etching. The chemical wet etching preferably includes: placing the carbon nanotubes in a corrosive solution and ultrasonically treating them, thereby obtaining shortened carbon nanotubes with controllable length; specifically, the length of the carbon nanotubes can be controlled by controlling the conditions of the ultrasonic treatment. In some embodiments of the present invention, the carbon nanotubes are placed in aqua regia and ultrasonically treated at 70°C to obtain short carbon nanotubes with a length distribution of 0.5 to 3 μm and an average length of ≤ 2 μm.

[0062] In the present invention, the purity of the short carbon nanotubes is preferably above 95%.

[0063] In the present invention, the short carbon nanotubes are either unsurface-modified or surface-modified. The surface-modified short carbon nanotubes are short carbon nanotubes with functional groups grafted onto their surfaces, i.e., surface-modified with functional groups. In the surface-modified short carbon nanotubes, the surface-modified functional groups are selected from one or more of amino, carboxyl, and carbonyl groups.

[0064] In the present invention, the surface modification can be achieved by surface treatment (e.g., impregnation) with a surface modifier containing the corresponding surface modification functional group. For example, carboxyl groups can be obtained by placing carbon nanotubes in aqua regia and ultrasonically treating them. While shortening the carbon nanotubes to obtain short carbon nanotubes, the surface of the carbon nanotubes is oxidized, thereby grafting carboxyl groups onto them, resulting in carbon nanotubes with carboxyl groups. Aminated carbon nanotubes can be obtained by placing carboxyl carbon nanotubes in ethylenediamine and a coupling agent and ultrasonically dispersing them, thereby obtaining amino-grafted carbon nanotubes. The coupling agent is preferably O-(7-azabenzotriazole-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate (i.e., HATU). The mass ratio of ethylenediamine to coupling agent is preferably 1:(1-5). Carbonylated carbon nanotubes can be obtained by placing carbon nanotubes in a potassium hydroxide solution and ultrasonically dispersing them, thereby obtaining carbon nanotubes with carbonyl groups grafted onto them. In the above surface modification process, after ultrasonic treatment, washing and drying are preferably performed to obtain carbon nanotubes with functional groups grafted on the surface.

[0065] According to the present invention, the raw materials for forming the resin-based slurry include: thermosetting resin.

[0066] In the present invention, the thermosetting resin preferably includes one or more of epoxy resin, polyester resin, phenolic resin, vinyl resin and bismaleimide resin. Among them, the epoxy resin is preferably bisphenol A epoxy resin. In some embodiments of the present invention, the epoxy resin is bisphenol A epoxy resin Epon862.

[0067] In the present invention, different chemically modified short carbon nanotubes can be selected for use with different resin matrices. Carbon nanotubes grafted with appropriate functional groups can form covalent or non-covalent bonds with the resin matrix during the curing reaction, achieving the desired performance enhancement of the fiber composite material. In the present invention, the preferred combination of resin and surface-modified short carbon nanotubes is as follows: the resin is bisphenol A epoxy resin, and the short carbon nanotubes are amino-modified short carbon nanotubes. During the curing reaction, the two form covalent bonds, creating a cross-linked structure that significantly improves the mechanical properties of the matrix.

[0068] According to the present invention, the raw materials for forming the resin-based slurry include: additives.

[0069] In the present invention, the additive is a curing agent and / or an accelerator. The curing agent is preferably an amine curing agent, including but not limited to one or more of D230 and dicyandiamide. The accelerator is preferably an amine accelerator, including but not limited to one or more of DMP-30 and triethylamine.

[0070] According to the present invention, short carbon nanotubes, thermosetting resin and additives are mixed to obtain a resin-based slurry.

[0071] In the present invention, the usage ratio of the above three raw materials is preferably as follows: the mass of the short carbon nanotubes is 0.1% to 5% of the mass of the thermosetting resin, specifically 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0%. The mass of the additive is 0.1% to 50% of the mass of the thermosetting resin, wherein the amount of the curing agent is 1% to 50% of the mass of the thermosetting resin, specifically 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, and 50%; the amount of the accelerator is 0.1% to 5% of the mass of the thermosetting resin, specifically 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0%.

[0072] In the present invention, the mixing sequence preferably specifically includes: first dispersing the short carbon nanotubes in the thermosetting resin, and then mixing with the additives.

[0073] In the present invention, the method of dispersing short carbon nanotubes in thermosetting resin includes but is not limited to ultrasound, ball milling, grinding, mechanical stirring or microfluidics. Among them, when mixing short carbon nanotubes and thermosetting resin: (1) If the viscosity of thermosetting resin is low (viscosity ≤ 1000cps), the short carbon nanotubes are directly dispersed in the thermosetting resin. (2) If the viscosity of thermosetting resin is high (viscosity > 1000cps), the short carbon nanotubes are first dispersed in an organic solvent, then mixed with thermosetting resin, and then the organic solvent is removed. Among them, the organic solvent is preferably alcohol or acetone. The method of removing the organic solvent is preferably heating and stirring.

[0074] In the present invention, when mixing with additives, the mixing method is not particularly limited, and the materials can be mixed uniformly using conventional mixing methods in the art. After the mixing, degassing is preferably performed. After the above treatment, a resin-based slurry is obtained.

[0075]

About step b

[0076] The resin-based slurry is poured into a fiber preform and cured to form a long-short carbon nanotube reinforced and toughened fiber composite material.

[0077] According to the present invention, a fiber preform is used as a matrix.

[0078] In the present invention, the fiber preform comprises, in sequentially stacked contact, an upper fiber cloth layer, a long carbon nanotube fiber yarn layer, and a lower fiber cloth layer. Specifically, the fiber preform has a sandwich structure, with fiber cloth layers on both sides and a long carbon nanotube fiber yarn layer sandwiched between them. The terms "upper" and "lower" are not specifically limited in orientation and are used to refer to the areas on either side of the long carbon nanotube fiber yarn layer. If the fiber cloth layer on either side is the upper fiber cloth layer, then the fiber cloth layer on the other side is naturally the lower fiber cloth layer.

[0079] In the present invention, the fiber cloth in the upper fiber cloth layer is a unidirectional fiber woven fabric or a multidirectional fiber woven fabric. In the present invention, the fiber cloth in the upper fiber cloth layer is preferably a continuous carbon fiber cloth, a continuous glass fiber cloth, or a continuous aramid fiber cloth. In the present invention, the fiber cloth in the upper fiber cloth layer can be an unsurface-modified fiber cloth or a surface-modified fiber cloth.

[0080] In the present invention, the fiber cloth in the lower fiber cloth layer is a unidirectional fiber woven fabric or a multidirectional fiber woven fabric. In the present invention, the fiber cloth in the lower fiber cloth layer is preferably a continuous carbon fiber cloth, a continuous glass fiber cloth, or a continuous aramid fiber cloth. In the present invention, the fiber cloth in the lower fiber cloth layer can be an unsurface-modified fiber cloth or a surface-modified fiber cloth.

[0081] In the present invention, the number of fiber cloth layers in the upper fiber cloth layer is one or more layers; the number of fiber cloth layers in the lower fiber cloth layer is one or more layers. Wherein, the "multiple layers" include two or more layers. In the present invention, preferably, the number of fiber cloth layers in the upper fiber cloth layer is the same as the number of fiber cloth layers in the lower fiber cloth layer. In the present invention, preferably, the types of fiber cloth in each layer of the upper fiber cloth layer are the same. In the present invention, preferably, the types of fiber cloth in each layer of the lower fiber cloth layer are the same. In the present invention, preferably, the upper fiber cloth layer is the same as the lower fiber cloth layer, specifically, the fiber cloth in corresponding positions on both sides with the long carbon nanotube fiber yarn layer as the center is the same. In some embodiments of the present invention, the upper fiber cloth layer is 6 layers of carbon fiber cloth, and the lower fiber cloth layer is 6 layers of carbon fiber cloth.

[0082] In the present invention, the fiber yarn of the long carbon nanotube fiber yarn layer located in the central interlayer of the fiber preform is a yarn-like structure formed by long carbon nanotubes, that is, a thin yarn (or thin yarn net) formed by long carbon nanotubes. Wherein, the length distribution of the long carbon nanotubes is 50 to 1000 μm, and the average length is >100 μm. In the present invention, the diameter of the long carbon nanofiber tube is preferably 1 to 50 nm. In the present invention, the long carbon nanotubes are long carbon nanotubes that have not been surface-modified or long carbon nanotubes that have been surface-modified. Wherein, the surface-modified long carbon nanotubes are long carbon nanotubes with functional groups grafted on the surface, that is, surface functional group modification has been performed. In the present invention, in the surface-modified long carbon nanotubes, the surface-modified functional groups are selected from one or more of amino, carboxyl and carbonyl groups. The surface modification method and the combination of the surface-modified carbon nanotubes and the resin matrix are consistent with the case of the short carbon nanotubes mentioned above, and will not be repeated here.

[0083] In the present invention, the number of layers of long carbon nanotube fiber yarn in the long carbon nanotube fiber yarn layer is one or more. In the present invention, the thickness of the long carbon nanotube fiber yarn layer can be adjusted by controlling the deposition time of a single layer of long carbon nanotube fiber yarn or the number of layers of long carbon nanotube fiber yarn laid. In the present invention, the total thickness of the long carbon nanotube fiber yarn layer is preferably 1 μm.

[0084] In the present invention, the long carbon nanotube fiber yarn can be prepared by the following two methods: (1) by floating catalytic chemical vapor deposition (FCCVD). The specific process flow is as follows: Figure 2 As shown, the carbon source is cracked in the high-temperature reactor and carbon nanotubes grow on the catalyst surface. A large number of carbon nanotubes gather and entangle with each other to form carbon nanotube aerogel. The carbon nanotube aerogel can be continuously pulled out from the other end of the high-temperature reactor and deposited in situ on the surface of the fiber cloth substrate to form a fluffy yarn-like structure, namely long carbon nanotube fiber yarn. (2) Prepared by array spinning method. The process flow is as follows Figure 3 As shown, a carbon nanotube array of a certain length is vertically elongated on a silicon substrate via chemical vapor deposition (CVD). This array is then attached to the surface of a fiber cloth substrate in the form of a veil through continuous spinning. In the present invention, after the carbon nanotubes are obtained and before the veil is prepared, an acid treatment is preferably performed to remove impurities such as metal catalysts to improve the purity of the carbon nanotubes. The present invention preferably controls the purity of the carbon nanotubes to above 95%, which is beneficial for enhancing the material's strengthening and toughening effects.

[0085] In the above preparation process, the fiber cloth in the upper fiber cloth layer or the lower fiber cloth layer can be directly used as a substrate. After the long carbon nanotube fiber yarn layer is deposited on the substrate, other fiber cloth layers are stacked to form a fiber preform.

[0086] In the present invention, the mass of the long carbon nanotubes is preferably 0.1% to 5% of the mass of the thermosetting resin in step a), specifically 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0%; the long carbon nanotubes constitute a long carbon nanotube fiber yarn layer, and therefore, the mass of the long carbon nanotubes also represents the mass of the long carbon nanotube fiber yarn layer.

[0087] In the present invention, the combined mass of the upper and lower fiber cloths preferably accounts for 40% to 80% by mass of the composite material. Specifically, the mass ratio of the combined mass of the upper and lower fiber cloths to the mass of the resin-based slurry obtained in step a) is (0.67-4):1, specifically 0.67:1, 0.70:1, 1:1, 2:1, 3:1, or 4:1. Controlling the above-mentioned ratios allows for smooth preparation and enhanced strengthening and toughening effects of the composite material. If the fiber cloth content is too low, effective strengthening and toughening cannot be achieved. If the fiber cloth content is too high, smooth preparation and obtaining a uniform composite are difficult, and material properties are also affected.

[0088] In the present invention, the resin-based slurry obtained in step a) is poured into the fiber preform for curing and molding. In the present invention, the methods used include but are not limited to vacuum assisted resin transfer molding (VARTM), RTM molding, or hand lay-up molding, etc.; the present invention preferably adopts the vacuum assisted resin transfer molding (VARTM) method. The operation of pouring the resin-based slurry into the fiber preform using vacuum assisted resin transfer molding (VARTM) is as follows: Figure 4 As shown, the resin-based slurry is uniformly introduced into the fiber preform through the negative pressure of the vacuum pump. At this time, due to factors such as pressure difference and viscosity, resin enrichment will occur at the inlet end, which can easily lead to uneven thickness of the composite material board. One way to effectively alleviate this situation is: after the front end of the resin-based slurry flow reaches the outlet, first close the resin inlet, and then close the outlet after the excess resin is sucked out. A double-layer guide net is used in the process to promote the bidirectional diffusion of the resin in and out of the surface of the fiber preform. The guide net and the fiber cloth are separated by a peeling cover cloth to facilitate demoulding. Finally, it is sealed with a vacuum bag and set aside.

[0089] In the present invention, after the above-mentioned infusion operation, curing and molding are carried out. In the present invention, the temperature of the curing and molding is preferably 25 to 500°C, and the pressure is preferably ≤10MPa; different resins and curing agents can be adjusted within the above-mentioned conditions; for example, for the Epon862 epoxy resin and D-230 curing agent system, the curing conditions are: first cure at 80°C for 2 hours, and then cure at 120°C for 2 hours. Taking the vacuum assisted resin transfer molding (VARTM) infusion sample as an example, after the infusion is completed, the entire VARTM platform can be moved into an oven for curing, or pressurized and cured on a flat vulcanizer. After the curing is completed, the mold is cooled and demolded to obtain a long-short carbon nanotube reinforced and toughened fiber composite material product.

[0090] The present invention also provides a long-short carbon nanotube reinforced and toughened fiber composite material product prepared by the preparation method described in the above technical solution.

[0091] In the preparation method provided by the present invention, during the mutual infiltration of resin and fiber, short carbon nanotubes can easily penetrate the narrow gaps between fibers and be evenly dispersed in the composite material; while long carbon nanotubes are laid out on the surface of the fiber cloth in the form of fiber yarns before the resin-based slurry is poured. In this way, the problem of the difficulty in dispersing long carbon nanotubes is effectively solved, and the "fiber filtering effect" of carbon nanotubes that often occurs in the VARTM process is effectively alleviated. The spatial layout of long and short carbon nanotubes in the composite material is optimized, and short carbon nanotubes effectively pass through the narrow gaps between fibers and are evenly distributed in the entire fiber composite material board. Long carbon nanotubes are enriched in the interlayer area of ​​the composite material board, so that their excellent mechanical properties can be fully utilized, and the dual purposes of intra-layer reinforcement and inter-layer toughening of the fiber composite material are simultaneously achieved. In summary, the present invention effectively avoids the "fiber filtering effect" commonly found in carbon nanotubes in the VARTM process, allowing short carbon nanotubes to be evenly dispersed in the fiber composite material, while effectively filling the gaps or resin-rich areas between the fiber composite material layers. The long carbon nanotubes strengthen the connection between the composite material layers, effectively improving the interfacial bonding force between the layers, so that the prepared composite material has excellent mechanical properties such as strength and toughness, as well as physical properties such as thermal and electrical properties.

[0092] The test results show that the composite material prepared by the present invention has a flexural strength of more than 600 MPa, which is 6% higher than that of the reference sample, and a type I interlaminar fracture toughness of 1500 J / m 2 The above is an improvement of more than 150% compared to the baseline sample.

[0093] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0094] Example 1

[0095] S1. Preparation of amino-modified short carbon nanotubes

[0096] 10g of carbon nanotubes were poured into a beaker containing 300mL of aqua regia and ultrasonically treated at 70°C and 2.5kW for 12h to obtain short carbon nanotubes with a length distribution of 0.5-3μm and an average length of 2μm. During the carbon nanotube shortening process, the surface of the carbon nanotubes was oxidized and carboxyl groups were grafted onto them. The short carbon nanotubes with carboxyl groups were placed in a mixture of ethylenediamine and HATU coupling agent (the mass ratio of ethylenediamine to coupling agent was 1:1-5) and ultrasonically dispersed at 2.5kW for 12h at room temperature. The mixture was then washed several times with anhydrous ethanol and deionized water and dried to obtain approximately 10g of short carbon nanotubes with amino groups grafted onto their surfaces.

[0097] S2. Preparation of fiber preform

[0098] Take carbon fiber unidirectional cloth (Toray T300-3000, density 1.76g / cm 3 ), cut into 30×30cm pieces, and then follow [0] 6s The fiber preform is manually laid in a stacking manner, see Figure 5 Specifically, six layers of cut cloth were stacked as the lower fiber cloth layer, which served as the base. A carbon nanotube array of a certain length was vertically stretched on a silicon substrate by chemical vapor deposition (CVD). This was then attached to the fiber cloth base in the form of a thin yarn by continuous spinning, forming a long carbon nanotube fiber yarn layer (three layers in total, with a total thickness of 300 nm, where the length of the long carbon nanotubes ranged from 50 to 1000 μm, with an average length >100 μm). To prepare a double cantilever beam specimen (according to the requirements of the ASTM D5528 test standard), a PTFE film (25 μm thick) was covered on the long carbon nanotube fiber yarn layer and inserted at the end of the middle layer, covering an area approximately 60 mm wide, to form a pre-crack of a certain length (in order to prepare the double cantilever beam specimen). Then, six more layers of cut cloth were stacked on top of the above sample as the upper fiber cloth layer. At this point, a fiber preform consisting of 6 layers of carbon fiber unidirectional cloth + long carbon nanotube fiber yarn layer + 6 layers of carbon fiber unidirectional cloth is obtained, with a structure as shown below: Figure 5 Note: In the above preparation process, the PTFE film is laid only for the preparation of double cantilever beam specimens for subsequent performance testing. In the actual production process of composite materials, the PTFE film is not laid, that is, the actual composite material product does not contain PTFE film.

[0099] S3. Preparation of resin-based slurry

[0100] Using a microfluidic device, 2.4 g of the amino-modified short carbon nanotubes obtained in step S1 were uniformly dispersed in acetone, and then poured into a beaker containing 355 g of bisphenol A epoxy resin Epon862. The mixture was mechanically stirred at 1000 rpm and a water bath temperature of 60°C for 6 h. After the acetone was completely evaporated, 125 g of curing agent D-230 was added, and the mixture was mechanically stirred at 500 rpm for 10 min at room temperature. Finally, the mixture was degassed in a vacuum oven at 25°C for 10 min to obtain 482.4 g of resin-based slurry.

[0101] The cured sample of the obtained resin-based slurry was brittle fractured at low temperature and then SEM observation was performed. The results are as follows Figure 6 As shown, it can be seen that the short carbon nanotubes are uniformly dispersed in the resin.

[0102] S4. Preparation of composite materials

[0103] The VARTM platform built is as follows Figure 4 As shown, a double-layer guide net is used for the fiber preform, and the guide net and the fiber preform are separated by a glass cover cloth. The resin-based slurry obtained in step S3 is evenly introduced into the fiber preform through the negative pressure of the vacuum pump. At this time, due to factors such as pressure difference and viscosity, resin enrichment will occur at the inlet end, which can easily lead to uneven thickness of the composite material board. To alleviate this situation, after the front end of the resin-based slurry flow reaches the outlet, the resin inlet is first closed, and after the excess resin is sucked out, the outlet is closed. After the resin-based slurry is completely poured into the fiber preform, the VARTM platform is moved into the oven as a whole, first cured at 80°C for 2h, and then cured at 120°C for 2h. Afterwards, it is cooled and demoulded to obtain a composite material board.

[0104] The above-mentioned whole preparation process of Example 1 is as follows Figure 7 As shown, Figure 7 Schematic diagram of the preparation process of Example 1.

[0105] Comparative Example 1

[0106] Preparation of baseline sample (without carbon nanotubes):

[0107] The method is implemented in accordance with Example 1, except that, in the process of preparing the fiber preform in step S2, no long carbon nanotube fiber yarn layer is placed, and in the process of preparing the resin-based slurry in step S3, no short carbon nanotubes are added.

[0108] Example 2: Testing

[0109] (1) Strength test

[0110] The three-point bending strength tests were performed on the samples of Example 1 and Comparative Example 1, respectively. The results showed that the bending strength of the sample of Example 1 was 624 MPa, and the bending strength of the sample of Comparative Example 1 was 589 MPa.

[0111] (2) Fracture toughness test

[0112] The fracture performance test of the samples of Example 1 and Comparative Example 1 was carried out according to ASTM D5528. The results are shown in Table 1. Figure 8-10 As shown, Figure 8-9 The load-opening displacement curves of the samples of Example 1 and Comparative Example 1 are shown in FIG. Figure 8 The three-point bending test results of the samples of Example 1 and Comparative Example 1 are shown in FIG. Figure 9 The double cantilever beam test results of the samples of Example 1 and Comparative Example 1 are shown in FIG. Figure 10 Graph showing the R curve (curve of crack propagation resistance versus crack propagation) of the samples of Example 1 and Comparative Example 1.

[0113] It can be seen that compared with the reference sample of comparative example 1, the mode I interlaminar fracture toughness of the composite material plate of embodiment 1 is increased from 607 J / m 2 Increased to 1536J / m 2 , an increase of more than 150%, thus proving the high efficiency of the long / short carbon nanotube synergistic toughening mechanism proposed in the present invention.

[0114] (3) Characterization of interlayer structure

[0115] The fractured sample of Example 1 after the fracture performance test of item (2) was characterized, and the results were as follows: Figure 11 As shown, Figure 11 This is the SEM image of the interlayer structure of the fractured sample of Example 1. It can be seen that the long carbon nanotubes are enriched in the interlayer region of the composite material.

[0116] Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core ideas, including the best mode, and also enable any technician in the field to practice the present invention, including making and using any device or system, and implementing any combined method. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention. The scope of patent protection of the present invention is defined by the claims and may include other embodiments that can be thought of by those skilled in the art. If these other embodiments have structural elements similar to the literal description of the claims, or if they include equivalent structural elements that are not substantially different from the literal description of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. A method for preparing a long-short carbon nanotube reinforced and toughened fiber composite material, characterized in that: The following steps are involved: a) mixing short carbon nanotubes, a thermosetting resin, and an additive to obtain a resin-based slurry; b) pouring the resin-based slurry into a fiber preform and curing the preform to obtain a long-short carbon nanotube reinforced and toughened fiber composite material; in: The fiber preform comprises: an upper fiber cloth layer, a long carbon nanotube fiber yarn layer, and a lower fiber cloth layer stacked and contacted in sequence; The long carbon nanotube fiber yarn layer is a yarn-like structure formed by long carbon nanotubes; The length of the short carbon nanotubes is 0.5 to 3 μm, and the average length is ≤ 2 μm; The length of the long carbon nanotubes is 50 to 1000 μm, with an average length of >100 μm; The total mass of the upper fiber cloth and the lower fiber cloth accounts for 40% to 80% of the mass of the composite material.

2. The preparation method according to claim 1, characterized in that The short carbon nanotubes are short carbon nanotubes that have not been surface-modified or short carbon nanotubes that have been surface-modified; The long carbon nanotubes are long carbon nanotubes without surface modification or long carbon nanotubes with surface modification; In the surface-modified short carbon nanotubes, the surface-modified functional groups are selected from one or more of amino, carboxyl and carbonyl groups; In the surface-modified long carbon nanotubes, the surface-modified functional groups are selected from one or more of amino, carboxyl and carbonyl groups.

3. The preparation method according to claim 1, characterized in that The thermosetting resin includes one or more of epoxy resin, polyester resin, phenolic resin, vinyl resin and bismaleimide resin.

4. The preparation method according to claim 1, characterized in that The mass of the short carbon nanotubes is 0.1% to 5% of the mass of the thermosetting resin; The mass of the long carbon nanotubes is 0.1% to 5% of the mass of the thermosetting resin.

5. The preparation method according to claim 1, characterized in that The fiber cloth in the upper fiber cloth layer is unidirectional fiber cloth or multidirectional fiber cloth; The fiber cloth in the lower fiber cloth layer is unidirectional fiber cloth or multidirectional fiber cloth; The fiber cloth in the upper fiber cloth layer is continuous carbon fiber cloth, continuous glass fiber cloth or continuous aramid fiber cloth; The fiber cloth in the lower fiber cloth layer is continuous carbon fiber cloth, continuous glass fiber cloth or continuous aramid fiber cloth; The fiber cloth in the upper fiber cloth layer is a fiber cloth without surface modification or a fiber cloth with surface modification; The fiber cloth in the lower fiber cloth layer is a fiber cloth without surface modification or a fiber cloth with surface modification.

6. The preparation method according to claim 1, characterized in that The number of layers of fiber cloth in the upper fiber cloth layer is one or more; The number of fiber cloth layers in the lower fiber cloth layer is one or more.

7. The preparation method according to claim 1, characterized in that The additive is a curing agent and / or an accelerator; The amount of the curing agent is 1% to 50% of the mass of the thermosetting resin; The amount of the accelerator is 0.1% to 5% of the mass of the thermosetting resin.

8. The preparation method according to claim 1, characterized in that In the step b), the resin-based slurry is infused into the fiber preform using a vacuum assisted resin transfer molding process.

9. The preparation method according to claim 1, characterized in that In the step b), the curing temperature is 25-500° C. and the pressure is ≤10 MPa.

10. A long-short carbon nanotube reinforced and toughened fiber composite material prepared by the preparation method according to any one of claims 1 to 9.

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