Flexible epoxy resin, flexible wave-absorbing resin, prepreg and composite material

Abstract

The invention belongs to the technical field of high-performance polymer materials and composite materials, and particularly relates to a flexible epoxy resin, a flexible wave-absorbing resin, a prepreg and a composite material. The flexible epoxy resin comprises the following components in percentage by mass: 30-55% of a component A, 15-30% of a component B and 30-50% of a component C, the component A is an active group-terminated thermoplastic resin oligomer; the component B is high-molecular-weight thermoplastic resin with the same main chain structure as the component A; the component C is pure epoxy resin and a curing agent. The flexible epoxy-resistant resin has good temperature resistance and relatively high elongation at break. The wave-transparent prepreg and the composite material are prepared, the thermocuring deformation is small after co-curing, the wave-absorbing prepreg with high wave-absorbing filler content can be prepared, the bonding with a fiber interface is good, cracking is not prone to occurring, and high temperature resistance is achieved.

Description

Technical Field

[0001] This invention belongs to the field of high-performance polymer materials and composite materials technology, specifically relating to a flexible epoxy resin, and also to a flexible microwave absorbing resin, prepreg, and composite material prepared from the flexible epoxy resin. Background Technology

[0002] Stealth materials, especially radar-absorbing materials, can effectively reduce the radar cross-section of equipment, making it more difficult for radar to detect and significantly increasing its survivability on the battlefield, thus possessing broad military application prospects. Radar-absorbing functional layers are an important type of stealth structure, typically distributed on the surface of carbon fiber composite structural components to increase the surface's electromagnetic wave absorption capacity, thereby reducing electromagnetic wave reflection. In recent years, with the increasing demand for structurally integrated materials, the development of radar-absorbing functional layers with superior performance and good co-curing capabilities has become an urgent need, a typical example being radar-absorbing resin prepregs that can be co-cured with the resin matrix.

[0003] Existing microwave absorbing resin prepregs typically use co-cured resins with properties similar to the resin matrix. In order to achieve effective electromagnetic wave absorption while reducing weight gain, microwave absorbing resins are often filled with a large number of magnetic nanoparticles, which can be more than three times the weight of the resin. The presence of a large number of rigid magnetic nanoparticles causes a significant decrease in the elongation at break of the resin and changes its curing thermal expansion properties, resulting in large thermosetting deformation of the cured parts. Increased residual stress can also lead to cracking of the functional layer or delamination between layers.

[0004] Improving the flexibility of resin to increase its elongation at break, while simultaneously enhancing its toughness, can reduce thermosetting deformation and prevent cracking. Existing flexible resins typically achieve this by adding a large amount of long-chain, low-epoxy-value epoxy resin or using flexible curing agents such as polyamide oligomers. However, this method of reducing the crosslinking density significantly decreases the material's heat resistance. In practical applications, microwave absorbing resins often require heat resistance properties that match the matrix resin to avoid compromising the overall heat resistance of the material.

[0005] In summary, existing technologies have obvious shortcomings. Developing a flexible microwave absorbing resin and its prepreg with high elongation at break, good temperature resistance, and good toughness is of great significance to effectively reduce curing deformation and material cracking or delamination failure during the curing process. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a flexible epoxy resin, a flexible microwave-absorbing resin, a prepreg, and a composite material. This flexible epoxy resin is obtained by modifying an epoxy resin using a thermoplastic resin oligomer with active group-terminated ends and a high-molecular-weight thermoplastic resin with the same backbone. This resin exhibits good temperature resistance and high elongation at break. The microwave-transparent prepreg and composite material prepared by co-curing show minimal thermosetting deformation, allowing for the production of microwave-absorbing prepregs with high absorbent filler content. These prepregs exhibit good interfacial bonding with fibers, are not prone to cracking, and possess high temperature resistance.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A flexible epoxy resin, which, according to mass content, comprises 30-55% component A, 15-30% component B and 30-50% component C; component A is a thermoplastic resin oligomer with active group end capping; component B is a high molecular weight thermoplastic resin with the same main chain structure as component A; component C is pure epoxy resin and curing agent, wherein components A and B are soluble in C.

[0008] Furthermore, the mass ratio of component A to component C is 1:(0.5~2).

[0009] Furthermore, the mass ratio of component A to component B is (1.2~2.8):1.

[0010] Furthermore, the weight-average molecular weight of component A is 700-4000; the weight-average molecular weight of component B is 10000-80000.

[0011] Furthermore, component B is mixed in the resin in the form of particles with a particle size of 3~25μm.

[0012] Furthermore, in component C, the viscosity of the pure epoxy resin at 25°C is less than 15 Pa·s; the curing agent is selected based on the curing start temperature T in the epoxy resin, wherein 120°C ≤ T ≤ 180°C.

[0013] Furthermore, the pure epoxy resin is at least one of low-viscosity bisphenol A type epoxy resin, low-viscosity bisphenol F type epoxy resin, TDE85, and AG80; when the epoxy resin is a mixture, the total viscosity of the epoxy resin is less than 5 Pa·s.

[0014] Furthermore, the curing agent is at least one of 4,4'-diaminodiphenyl sulfone, 4,4'-dihydroxydiphenyl sulfone, and dicyandiamide.

[0015] Furthermore, the thermoplastic resin oligomer with active group end-capping is at least one of polyaryletherketone or polyarylethersulfone with active group end-capping; the active group is one of hydroxyl, amino, carboxyl, or epoxy groups.

[0016] Furthermore, component A is at least one of the following oligomers: hydroxyl-terminated phenolphthalein-modified polyethersulfone (DH-OES), with the structural formula: ; The hydroxyl-terminated phenolphthalein-modified polyarylether ketone (DH-OEKC) has the following structural formula: ; Hydroxyl-terminated phenolphthalein-modified polyarylene ether sulfone (DH-OESC), with the following structural formula: ; The amino-terminated phenolphthalein-modified polyarylene ether sulfone (DA-OESC) has the following structural formula: ; Carboxyl-terminated phenolphthalein-modified polyarylene ether sulfone (DC-OESC), with the following structural formula: ; The polyepoxy-terminated phenolphthalein-modified polyarylether ketone (DAG80-OEKC) has the following structural formula: .

[0017] The present invention also provides a flexible microwave absorbing resin, which includes microwave absorbing filler and flexible epoxy resin; wherein the mass of the microwave absorbing filler is 50-90% of the total mass of the flexible microwave absorbing resin.

[0018] Furthermore, in the flexible epoxy resin, the pure epoxy resin in component C has a viscosity of less than 8 Pa·s at 25°C.

[0019] The present invention also provides a prepreg comprising a flexible microwave absorbing resin.

[0020] The present invention also provides a composite material comprising a prepreg.

[0021] The beneficial effects of this invention are: (1) The present invention provides a flexible epoxy resin and a flexible microwave absorbing resin, a prepreg and a composite material. The flexible epoxy resin has a high elongation at break and good toughness, as well as high heat resistance. When combined with microwave absorbing filler and microwave transparent fiber, the microwave transparent prepreg and microwave absorbing prepreg prepared have low co-curing deformation and are not prone to problems such as interface debonding and cracking of the microwave absorbing layer.

[0022] (2) By introducing high content of high heat-resistant thermoplastic resin segments, and further reducing the crosslinking density and forming semi-soluble tough points by secondary dissolution of high molecular weight thermoplastic resin, the crosslinking density of the resin is greatly reduced and the heat resistance of the cured resin is improved; the compatible and continuous tough points greatly improve the surface roughness of the resin; under the combined effect of the two, the impact performance and elongation at break of the resin are greatly improved, and when applied to the wave-transparent layer and the wave-absorbing layer, the curing deformation caused by the co-curing of the wave-absorbing functional layer is significantly reduced, and there is no cracking or delamination. Attached Figure Description

[0023] Figure 1 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Example 1; Figure 2 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Example 3; Figure 3 This is a microstructure diagram of the cross-section of the composite material in Example 3; Figure 4 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Comparative Example 1. Figure 5 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Comparative Example 2. Figure 6 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Comparative Example 3. Figure 7 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Comparative Example 4. Figure 8 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Comparative Example 5. Figure 9 This is a microstructure diagram of the cross-section of the flexible epoxy resin in Comparative Example 6. Detailed Implementation

[0024] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0026] To prepare microwave-absorbing and microwave-transparent prepregs with low co-curing deformation, good temperature resistance, and good toughness, it is necessary to first prepare a flexible epoxy resin with low crosslinking density, high elongation at break, high temperature resistance, and good interlayer bonding properties.T g High-temperature resistant thermosetting resins include high-temperature curing epoxy resins, bismaleimide resins, etc. Their resin networks have high crosslinking density, resulting in good temperature resistance and shape stability. However, they have poor chain segment mobility and large curing shrinkage, leading to problems such as high brittleness, large co-curing deformation, and low elongation at break. In order to improve the flexibility of epoxy resins, the traditional methods are mainly as follows: (1) Modifying epoxy resins with toughening agents. Toughening agents include thermoplastic resins and rubbers, which can make epoxy resins maintain temperature resistance while having better toughness and elongation at break. However, the amount of toughening agent added is small. For example, thermoplastic resins greatly increase the viscosity of the system, and the large amount of rubber added reduces its mechanical and thermal properties. Therefore, the resin system is still mainly composed of thermosetting resins, and the toughening agent is distributed as a secondary phase, which limits the overall structural deformation ability of the material. Therefore, the improvement in flexibility is extremely limited, and the elongation at break of the resin is usually no more than 5%. (2) Using chain extension toughening methods to reduce the crosslinking density of the matrix resin. This includes modifying flexible long-chain epoxy resins and extending the chain of epoxy resins. The former will significantly reduce the material T g The latter will lead to an increase in the viscosity of the material.

[0027] To address the above problems, this invention takes the opposite approach. The cured product is not primarily composed of a high-crosslink density, heat-resistant epoxy resin, but rather a thermoplastic resin with higher heat resistance. This results in greater flexibility and toughness while maintaining its... T g Its main characteristics are: (1) Low-viscosity epoxy resin provides crosslinking and high dilution properties, epoxy resin's T g It can be lower than that of thermoplastic resins and has weak cross-linking, which can significantly reduce the viscosity of the system and increase the amount of thermoplastic resin oligomers added.

[0028] (2) Using active oligomers of thermoplastic resins as chain extenders, the oligomers used have high rigidity, thereby improving flexibility and heat resistance.

[0029] (3) Adding the same thermoplastic resin particles as a secondary enhancement of thermoplastic resin has a high molecular weight. It dissolves slowly at room temperature, but can quickly partially dissolve and diffuse in the same system at high curing temperatures. It works with cross-linked oligomers to form a cured product with thermoplastic resin structure as the main body, and at the same time forms tough anchor points that are firmly bonded to the resin.

[0030] Based on the above design principles, this invention provides a flexible epoxy resin, comprising, by mass content, 30-55% component A, 15-30% component B, and 30-50% component C; component A is a thermoplastic resin oligomer capped with active groups; component B is a high molecular weight thermoplastic resin with the same main chain structure as component A; component C is pure epoxy resin and a curing agent, wherein the pure epoxy resin is an epoxy resin without curing agent and other modifiers, and is composed of one or more pure epoxy resins; wherein components A and B are both soluble in C.

[0031] Specifically, the mass ratio of component A to component C is 1:(0.5~2).

[0032] Specifically, the weight-average molecular weight of component A is 700-4000; the weight-average molecular weight of component B is 10000-80000.

[0033] Specifically, component B is mixed in the resin in the form of particles with a particle size of 3~25μm.

[0034] Specifically, in component C, the viscosity of pure epoxy resin at 25°C is less than 15 Pa·s; the curing agent is selected based on the curing start temperature T in the epoxy resin, where 120°C ≤ T ≤ 180°C.

[0035] More specifically, the pure epoxy resin is at least one of low-viscosity bisphenol A type epoxy resin, low-viscosity bisphenol F type epoxy resin, TDE85, and AG80; when the epoxy resin is a mixture, the total viscosity of the epoxy resin is less than 5 Pa·s.

[0036] In order to achieve the objectives of this application and realize the application of materials, the material design needs to meet the following requirements: (1) Viscosity that simultaneously meets the requirements of material processing and structural design.

[0037] (2) In the material composition of this application, performance and structure complement each other. This application forms the required material structure by controlling the parameters of each component to achieve good temperature resistance, impact resistance and high elongation at break.

[0038] (1) Viscosity that simultaneously meets the requirements of material processing and structural design: Viscosity is an important control factor in the material scheme of this application. First, it affects the processability of epoxy resin, and second, it also affects the solubility characteristics of thermoplastic resin, thus affecting the final performance of the material. In this application: 1) Component B is added to the resin in the form of particles, which significantly reduces its influence on viscosity; 2) Maintaining a low viscosity is conducive to the dissolution of component B during the curing process, thereby achieving the material design objective. In this application, by controlling the molecular weight of component A (700~4000), the viscosity of pure epoxy resin at 25℃ ≤15 Pa.s, and the mass ratio of component A to component B (1.2~2.8):1, the system can basically meet the requirements of resin processability (viscosity ≤400 Pa.s at 50℃), and the viscosity near the curing start temperature is ≤20 Pa.s, which meets the basic viscosity condition that component B particles can be partially dissolved.

[0039] (2) In the material composition of this application, performance and structure complement each other.

[0040] The temperature resistance, impact resistance, and elongation at break of the material system in this application are significantly related to the total content of component A and the final form of component B. By controlling the content of component A to 30-55 wt%, component B to 15-30 wt%, and component C to 30-50 wt%, the material design objectives of this application were achieved. The contents of these three components are interdependent. Based on mass content, when the content of component C in the flexible epoxy resin is below 30% or above 50%, the glass transition temperature and unnotched impact strength of the resin are both relatively low. When the total content of components A and B is below 70 wt%, and component A is controlled to be above 30 wt%, it can effectively promote the solubility of component B in the system. However, the dissolution of component B significantly increases the solution viscosity, causing its dissolution rate to decrease significantly with increasing dissolution amount. Therefore, the proportion of spontaneous dissolution at high temperatures is limited, and its content should be controlled below 30 wt%. However, to ensure that component B is mostly dissolved without becoming completely homogeneous, the content of component B should not be less than 15 wt%.

[0041] The type, particle size, and molecular weight of component B significantly affect the mechanical properties of the system and are key to phase control in this application. Preferably, component B has a molecular weight of 10,000–80,000, is mixed in the resin in particulate form, and has a particle size of 3–25 μm. The curing agent is selected based on the curing initiation temperature T in the epoxy resin, where 120℃ ≤ T ≤ 180℃. Controlling the dosage of components A and B aims to ensure that component B, in conjunction with component A, forms a distribution system with a high content of tough polymer, and that component B forms certain toughness anchor points. Controlling the curing initiation temperature of the curing agent to ≥120℃ ensures that component B has good solubility at higher temperatures. Under these conditions, when the particle size and molecular weight of component B are too low, its dissolution is too complete, making it difficult to form anchor points; conversely, when the particles are too large, the solubility is low, resulting in fewer anchor points and less high-temperature thermoplastic resin segments dissolved in the matrix. When the molecular weight of component B is below 10,000, the anchor point toughness is low; when the molecular weight is too high, the solubility also becomes relatively limited. In this application, the higher the molecular weight of component B, the smaller the particle size should be.

[0042] In this application, phase control is achieved through material composition control. After the components A and C react fully, they form an interpenetrating network structure (IPN). After the addition of component B, B dissolves in the blend of A and C, increasing the content of this long-chain thermoplastic structure and forming a continuous thermoplastic resin phase, thereby improving the impact strength and temperature resistance. On the other hand, the component B particles, whose solubility is controlled, act as tough points, further improving the impact resistance. The formation of tough points significantly increases the surface roughness of the resin. Furthermore, together with the system designed in this application, a special resin system with a high content of thermoplastic resin segments and containing the same thermoplastic resin tough points is formed, exhibiting outstanding performance.

[0043] Specifically, the curing agent is at least one of 4,4'-diaminodiphenyl sulfone, 4,4'-dihydroxydiphenyl sulfone, and dicyandiamide.

[0044] Specifically, the thermoplastic resin oligomers with active group-terminated are at least one of polyaryletherketone and polyarylethersulfone with active group-terminated; the active group is one of hydroxyl, amino, carboxyl, and epoxy groups.

[0045] To better explain the present invention, component A involved in the embodiments and comparative examples of the present invention is at least one of the following thermoplastic resin oligomers. The weight-average molecular weight data of different thermoplastic resin oligomers are shown in Table 1. Table 1. Weight-average molecular weight data of different thermoplastic resin oligomers The oligomers, their structural formulas, and preparation processes are shown below: (1) Hydroxyl-terminated phenolphthalein-modified polyether sulfone (DH-OES), with the following structural formula: ; The preparation process includes the following steps: 1) Weigh 114.9g (0.4mol) 4,4'-dichlorodiphenyl sulfone, 132.5g (0.49mol) bisphenol A, and 193g (1.4mol) anhydrous potassium carbonate, add them to a glass reactor, and then add 500g dimethyl sulfoxide (DMSO). Turn on the stirrer and stir until homogeneous.

[0046] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0047] 3) Add 10g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 195℃ and react for 3 hours. Then stop heating and gradually cool down to below 60℃.

[0048] 4) Pour the reaction solution into a large amount of water, add acetic acid to adjust the pH to about 6.0, then filter, wash repeatedly, and vacuum dry to obtain DH-OES with a yield of 92%. The weight-average molecular weight of DH-OES was tested to be 2340.

[0049] (2) The hydroxyl-terminated phenolphthalein-modified polyarylether ketone (DH-OEKC) has the following structural formula: ; The preparation process includes the following steps: 1) Weigh 218.2g (1mol) of 4,4'-difluorobenzophenone, 493g (1.55mol) of phenolphthalein, and 483g (3.5mol) of anhydrous potassium carbonate, add them to a glass reactor, and then add 1000g of N,N-dimethylformamide (DMF). Turn on the stirrer and stir until homogeneous.

[0050] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0051] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 170℃ and react for 3 hours. Then stop heating and gradually cool down to below 60℃.

[0052] 4) Pour the reaction solution into a large amount of water, add acetic acid to adjust the pH to about 6.0, then filter, wash repeatedly, and vacuum dry to obtain DH-OEKC with a yield of 96%. The weight-average molecular weight of DH-OEKC was tested to be 1220.

[0053] (3) The hydroxyl-terminated phenolphthalein-modified polyarylene ether sulfone (DH-OESC) has the following structural formula: ; The preparation process includes the following steps: 1) Weigh 287g (1mol) of 4,4'-dichlorodiphenyl sulfone, 440g (1.38mol) of phenolphthalein, and 414g (3mol) of anhydrous potassium carbonate, add them to a glass reactor, and then add 1000g of DMSO. Turn on the stirrer and stir until homogeneous.

[0054] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0055] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 190℃ and react for 3 hours. Then stop heating and gradually cool down to below 60℃.

[0056] 4) Pour the reaction solution into a large amount of water, add acetic acid to adjust the pH to about 6.0, then filter, wash repeatedly, and vacuum dry to obtain DH-OESC with a yield of 95%. The weight-average molecular weight of DH-OESC was tested to be 1750.

[0057] (4) The amino-terminated phenolphthalein-modified polyarylene ether sulfone (DA-OESC) has the following structural formula: ; The preparation process includes the following steps: 1) Weigh 355g (1.24mol) of 4,4'-dichlorodiphenyl sulfone, 318g (1mol) of bisphenol A, and 304g (2.2mol) of anhydrous potassium carbonate, add them to a glass reactor, and then add 1200g of dimethyl sulfoxide (DMSO). Turn on the stirrer and stir until homogeneous.

[0058] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0059] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 195℃ and react for 3 hours, then lower the temperature to 160℃.

[0060] 4) Add 72.5g (0.48mol) of acetaminophen to the reaction solution, stir to dissolve and continue the reaction for 2h, then cool to 50℃.

[0061] 5) Add 50g of 37% concentrated hydrochloric acid to the reaction solution and stir for 30min; 6) Pour the reaction solution into a large amount of water, then filter, wash repeatedly, and vacuum dry to obtain DA-OESC with a yield of 92%. The weight-average molecular weight of DA-OESC was tested to be 2740.

[0062] (5) Carboxyl-terminated phenolphthalein-modified polyarylether sulfone (DC-OESC), with the following structural formula: ; The preparation process includes the following steps: 1) Weigh 439g (1.53mol) of 4,4'-dichlorodiphenyl sulfone, 318g (1mol) of bisphenol A, and 304g (2.2mol) of anhydrous potassium carbonate, add them to a glass reactor, and then add 1200g of dimethyl sulfoxide (DMSO). Turn on the stirrer and stir until homogeneous.

[0063] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0064] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 195℃ and react for 3 hours, then cool down to 150℃.

[0065] 4) Add 182g (1.2mol) of methyl p-hydroxybenzoate to the reaction solution, stir to dissolve and continue the reaction for 2h, then cool to 40℃.

[0066] 5) Add 52g (1.3mol) sodium hydroxide to the reaction solution and stir for 30min.

[0067] 6) Pour the reaction solution into a large amount of water, add dilute hydrochloric acid to adjust the pH to about 1-2, then filter, wash repeatedly, and vacuum dry to obtain DC-OESC with a yield of 93%. The weight-average molecular weight of DC-OESC was tested to be 1510.

[0068] (6) Phenolphthalein-modified polyarylether ketone (DAG80-OEKC) with polyepoxy groups capped, the structural formula is: ; The preparation process includes the following steps: 1) Weigh 218.2g (1mol) of 4,4'-difluorobenzophenone, 404g (1.27mol) of phenolphthalein, and 386g (2.8mol) of anhydrous potassium carbonate, add them to a glass reactor, and then add 1000g of N,N-dimethylformamide (DMF). Turn on the stirrer and stir until homogeneous.

[0069] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0070] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 170℃ and react for 3 hours. Then stop heating and gradually cool down to below 60℃.

[0071] 4) Pour the reaction solution into a large amount of water, add acetic acid to adjust the pH to about 6.0, then filter, wash repeatedly, and vacuum dry to obtain DH-OEKC with a yield of 97%.

[0072] 5) The obtained DH-OEKC was heated to 100℃ under continuous stirring and dissolved in 1500g of AG80 epoxy resin. The mixture was then heated to 130℃ and reacted for 2 hours. The solution was then poured into ethanol to precipitate the resin. The precipitate was then filtered, washed repeatedly, and dried under vacuum to obtain DAG80-OEKC with a yield of 95%. Excess AG80 resin was recovered from the ethanol. The weight-average molecular weight of DAG80-OEKC was determined to be 3180.

[0073] To better explain the present invention, component B involved in the embodiments and comparative examples of the present invention is at least one of the following high molecular weight thermoplastic resins. The resins, their structural formulas, and preparation processes are shown below: (1) Detailed introduction of several high molecular weight polyethersulfones (PES), with the following structural formulas: ; The preparation process of PES with a weight-average molecular weight of 8030 includes the following steps: 1) Weigh 287g (1mol) 4,4'-dichlorodiphenyl sulfone, 228g (1.065mol) bisphenol A, and 331g (2.4mol) anhydrous potassium carbonate, add them to a glass reactor, and then add 800g dimethyl sulfoxide (DMSO). Turn on the stirrer and stir until homogeneous.

[0074] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0075] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 195℃ and react for 3 hours. Then stop heating and gradually cool down to below 60℃.

[0076] 4) The reaction solution was poured into a large amount of water, then filtered, washed repeatedly, and dried under vacuum to obtain PES with a yield of 97%. The weight-average molecular weight of PES was tested to be 8030.

[0077] In addition, PES with a molecular weight of 44,100 was prepared by adjusting the amounts of dichlorodiphenyl sulfone and bisphenol A to equivalence, and Solvay 3600RP with a molecular weight of 13,000 was purchased commercially.

[0078] (2) Detailed introduction of several high molecular weight phenolphthalein-modified polyarylether sulfones (PESCs), with the following structural formulas: ; The preparation process of PESC with a weight-average molecular weight of 37,300 includes the following steps: 1) Weigh 287g (1mol) 4,4'-dichlorodiphenyl sulfone, 321g (1.01mol) phenolphthalein, and 304g (2.2mol) anhydrous potassium carbonate and add them to a glass reactor. Then add 900g dimethyl sulfoxide (DMSO), turn on the stirrer, and stir until homogeneous.

[0079] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0080] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 195℃ and react for 3 hours. Then stop heating and gradually cool down to below 60℃.

[0081] 4) The reaction solution was poured into a large amount of water, then filtered, washed repeatedly, and dried under vacuum to obtain PESC with a yield of 99%. The weight-average molecular weight of PESC was tested to be 37,300.

[0082] In addition, PESC with a molecular weight of 73,000 was purchased from Xuzhou Engineering Plastics Co., Ltd. through commercial channels.

[0083] (3) Detailed introduction of several high molecular weight phenolphthalein-modified polyarylether ketones, with the following structural formula: ; The preparation process of PEKC with a weight-average molecular weight of 21800 includes the following steps: 1) Weigh 218g (1mol) of 4,4'-difluorobenzophenone, 323g (1.015mol) of bisphenol A, and 304g (2.2mol) of anhydrous potassium carbonate, add them to a glass reactor, and then add 900g of dimethyl sulfoxide (DMSO). Turn on the stirrer and stir until homogeneous.

[0084] 2) Evacuate the reactor and then introduce nitrogen gas, repeating this process three times.

[0085] 3) Add 20g of xylene under sealed conditions, and with stirring on, raise the temperature of the reactor to 175℃ and react for 3 hours. Then stop heating and gradually cool down to below 60℃.

[0086] 4) The reaction solution was poured into a large amount of water, then filtered, washed repeatedly, and dried under vacuum to obtain PEKC with a yield of 92%. The weight-average molecular weight of PEKC was tested to be 21,800.

[0087] In addition, PEKC with a molecular weight of 49,100 was purchased from Xuzhou Engineering Plastics Company through commercial channels.

[0088] The present invention also provides a flexible microwave absorbing resin, which includes microwave absorbing filler and flexible epoxy resin; wherein the mass of the microwave absorbing filler is 50-90% of the total mass of the flexible microwave absorbing resin.

[0089] Specifically, in the flexible epoxy resin, the pure epoxy resin in component C has a viscosity of less than 8 Pa·s at 25°C. The system exhibits certain adhesion upon slight heating at room temperature, making it suitable for film application and prepreg preparation. The microwave-transparent prepregs and composites prepared from this flexible epoxy resin exhibit low curing deformation.

[0090] The present invention also provides a prepreg comprising a flexible microwave absorbing resin.

[0091] The present invention also provides a composite material comprising a prepreg.

[0092] Example 1 S1. Preparation of Flexible Epoxy Resin 1#: Take 350g of DH-OES (self-made, weight-average molecular weight 2340), 384g of epoxy resin E54, and 16g of dicyandiamide. Mix the three ingredients evenly, then heat to 120℃ under stirring to dissolve evenly. Cool to room temperature to obtain a homogeneous transparent solid. Weigh 250g of PES powder (purchased from Solvay, grade 3600RP, weight-average molecular weight 13000, pulverized to a particle size of 13μm), and mix it with the above-obtained transparent solid in a three-roll disperser at 70℃ for 6 times to obtain a total of 1000g of flexible epoxy resin 1#.

[0093] S2. Preparation of flexible microwave absorbing resin: Take 2500g of magnetite nanoparticles (particle size 150nm) and 15g of coupling agent KH550, add them to the above flexible epoxy resin, and mix them evenly at 80℃ using a mixer to obtain flexible microwave absorbing resin.

[0094] S3. Preparation of flexible microwave absorbing resin prepreg: The flexible microwave absorbing resin is coated into a uniform resin film with a surface density of 200 g / m³. 2 Then it is combined with aramid fiber plain weave fabric (area density of 120g / m²). 2 This process yields the corresponding flexible microwave-absorbing resin prepreg for aramid fabrics.

[0095] S4. Preparation of Composite Material: Flexible microwave absorbing prepreg and carbon fiber prepreg are co-laid to prepare a composite material containing a microwave absorbing layer. The carbon fiber epoxy resin prepreg used is CCF800 / BA9916 (fiber areal density of 135 g / m²). 2 Surface density 200g / m³ 2 The layup sequence is [45, 0, -45, 90]. 2sThe board measures 300mm x 300mm, with a flexible microwave-absorbing prepreg applied to the top layer. It is then cured using the same process as the carbon fiber prepreg. After curing, it is allowed to cool naturally to below 60°C before being removed from the mold, yielding a composite material board with a co-cured microwave-absorbing functional layer on the surface.

[0096] Example 2: Preparation of S1, Flexible Epoxy Resin 2#: Take 500g of DH-OEKC (self-made, weight average molecular weight 1220), epoxy resin is a mixture of TDE85 and E54 in a mass ratio of 1:3, with a total mass of 307g, and curing agent is dicyandiamide 13g. Mix the above ingredients evenly, then heat to 100℃ under stirring to dissolve evenly, and cool to room temperature to obtain a homogeneous transparent solid. Weigh 180g of PEKC powder (purchased from Xuzhou Engineering Plastics Company, weight average molecular weight 49100, pulverized to a particle size of 6μm), and mix it with the above-obtained transparent solid in a three-roll disperser at 70℃ for 5 times to obtain a total of 1000g of flexible epoxy resin 2#.

[0097] S2. Preparation of flexible microwave absorbing resin: Take 3000g of magnetic sheet carbonyl iron powder (sheet diameter 4~6µm) and 21.2g of coupling agent KH550, add them to the above flexible epoxy resin, and mix them evenly at 75℃ using a three-roll mill to obtain flexible microwave absorbing resin.

[0098] S3. Preparation of flexible microwave absorbing resin prepreg: The flexible microwave absorbing resin is coated into a uniform resin film with a surface density of 200 g / m³. 2 Then it is combined with quartz fiber plain weave fabric (area density of 100g / m²). 2 This yields a flexible microwave-absorbing resin prepreg reinforced with quartz fiber fabric.

[0099] S4. Preparation of composite materials: The same method as in Example 1 was used.

[0100] Example 3 Preparation of S1, Flexible Epoxy Resin 3#: Take 400g of DH-OESC (self-made, weight average molecular weight 1750), epoxy resin is a mixture of AG80 and E54 in a mass ratio of 1:3, where AG80 is 78.5g, E54 is 235.5g, and curing agent is DDS, 126g. Mix the above ingredients evenly, then heat to 100℃ under stirring to dissolve evenly, and cool to room temperature to obtain a homogeneous transparent solid. Weigh 160g of PESC powder (self-made, weight average molecular weight 37300, pulverized to a particle size of 18μm), and mix it with the above-obtained transparent solid in a three-roll disperser at 70℃ for 5 times to obtain a total of 1000g of flexible epoxy resin 3#. S2. Preparation of flexible microwave-transparent resin prepreg: The above-mentioned flexible resin is coated into a uniform resin film with a film surface density of 220 g / m³. 2 Then it is combined with quartz fiber plain weave fabric (area density of 220g / m³). 2 This process yields the corresponding quartz fiber plain weave fabric flexible microwave-transparent resin prepreg.

[0101] S3. Preparation of composite material: The same method as in Example 1 was used, except that the carbon fiber epoxy resin prepreg used was ZT7H / BA9916, and it was cured according to the curing process of the carbon fiber prepreg.

[0102] Example 4: Preparation of S1, Flexible Epoxy Resin 4#: Take 350g of DA-OESC (self-made, weight average molecular weight 2740), epoxy resin is a mixture of TDE85 and E54 in a mass ratio of 1:4, where TDE85 is 70g, E54 is 280g, and curing agent is DDS, 140g. Mix the above ingredients evenly, then heat to 100℃ under stirring to dissolve evenly, and cool to room temperature to obtain a homogeneous transparent solid. Weigh 160g of PESC powder (purchased from Xuzhou Engineering Plastics Company, weight average molecular weight 73000, pulverized to a particle size of 5µm), and mix it with the above-obtained transparent solid in a three-roll disperser at 70℃ for 5 times to obtain a total of 1000g of flexible epoxy resin 4#. S2. Preparation of flexible microwave absorbing resin: Take 4000g of magnetic spherical carbonyl iron powder particles (particle size 1~3µm), take 20g of coupling agent KH550, add them to the above flexible epoxy resin, heat to 65℃ and mix evenly using a three-roll mill to obtain flexible microwave absorbing resin. S3. Preparation of flexible microwave absorbing resin prepreg: The flexible microwave absorbing resin is coated into a uniform resin film with a surface density of 315 g / m³. 2 Then it is compounded with unidirectional aramid continuous fibers (area density of 100 g / m²). 2 This process yields a unidirectional aramid continuous fiber reinforced flexible microwave absorbing resin prepreg tape.

[0103] S4. Preparation of composite material: The same method as in Example 1 was used, except that the carbon fiber bismaleimide resin prepreg used was T800 / QY9611, and it was cured according to the curing process of the carbon fiber prepreg.

[0104] Example 5: Preparation of S1, Flexible Epoxy Resin 5#: Take 330g of DC-OESC (self-made, weight-average molecular weight 1510), epoxy resin is a mixture of TDE85 and E54 in a mass ratio of 1:4, where TDE85 is 48g, E54 is 192g, and curing agent is 4,4'-dihydroxydiphenyl sulfone, 170g. Mix the above ingredients evenly, then heat to 100℃ under stirring to dissolve evenly, and cool to room temperature to obtain a homogeneous transparent solid. Weigh 260g of PESC powder (self-made, weight-average molecular weight 37300, pulverized to a particle size of 18μm), and mix it with the above-obtained transparent solid in a three-roll disperser at 70℃ for 5 times to obtain a total of 1000g of flexible epoxy resin 5#. S2. Preparation of flexible microwave absorbing resin: Take 2200g of magnetic sheet carbonyl iron powder (sheet diameter 4~6µm), take 15g of coupling agent KH550, add them to the above flexible epoxy resin, and mix them evenly at 75℃ using a three-roll mill to obtain flexible microwave absorbing resin. S3. Preparation of flexible microwave absorbing resin prepreg: The flexible microwave absorbing resin is coated into a uniform resin film with a surface density of 150 g / m³. 2 Then it is combined with quartz fiber plain weave fabric (area density of 100g / m²). 2 This process yields a flexible microwave-absorbing resin prepreg reinforced with quartz fiber plain weave fabric. S4. Preparation of composite material: The same method as in Example 1 was used, except that the carbon fiber epoxy resin prepreg used was T800 / 5228A, and it was cured according to the curing process of the carbon fiber prepreg.

[0105] Example 6: Preparation of S1, Flexible Epoxy Resin 6#: Take 350g of DAG80-OEKC (self-made, weight average molecular weight 3180), 355g of epoxy resin CYDF70 (Yueyang Petrochemical), and 25g of curing agent dicyandiamide. Mix the above ingredients evenly, then heat to 100℃ under stirring to dissolve evenly. Cool to room temperature to obtain a homogeneous transparent solid. Weigh 270g of PEKC powder (self-made, weight average molecular weight 21800, pulverized to a particle size of 6μm), and mix it with the above-obtained transparent solid five times at 70℃ using a three-roll disperser to obtain a total of 1000g of flexible epoxy resin 6#. S2. Preparation of flexible microwave absorbing resin: Take 2100g of magnetic iron oxide nanoparticles (particle size 150nm) and 15g of coupling agent KH550, add them to the above flexible epoxy resin, and mix them evenly at 80℃ using a three-roll mill to obtain flexible microwave absorbing resin. S3. Preparation of flexible microwave absorbing resin prepreg: The flexible microwave absorbing resin is coated into a uniform resin film with a surface density of 123 g / m³. 2Then it is combined with aramid fiber nonwoven fabric (area density of 22g / m²). 2 The corresponding flexible microwave absorbing resin prepreg with aramid fiber nonwoven fabric as carrier was obtained, with a fiber volume fraction of 15.6%.

[0106] S4. Preparation of composite material: The same method as in Example 1 was used, except that the carbon fiber epoxy resin prepreg used was ZT7H / BA9916, and it was cured according to the curing process of the carbon fiber prepreg.

[0107] Example 7: Preparation of S1, Flexible Epoxy Resin 7#: Take 400g of DH-OESC (self-made, weight average molecular weight 1750), 415g of epoxy resin CYDF70 (Yueyang Petrochemical), and 25g of curing agent dicyandiamide. Mix the above ingredients evenly, then heat to 100℃ under stirring to dissolve evenly. Cool to room temperature to obtain a homogeneous transparent solid. Weigh 160g of PES powder (self-made, weight average molecular weight 44100, pulverized to a particle size of 5μm), and mix it with the above-obtained transparent solid five times at 70℃ using a three-roll disperser to obtain 1000g of flexible epoxy resin 6#. S2. Preparation of flexible microwave-transparent resin prepreg: The flexible resin is coated into a uniform resin film with a surface density of 55 g / m³. 2 Then it is combined with aramid fiber nonwoven fabric (area density of 22g / m²). 2 This process yields a flexible, wave-transparent resin prepreg with aramid fiber nonwoven fabric as the carrier. S3. Preparation of composite materials: The same method as in Example 3 was used.

[0108] Comparative Example 1: This embodiment is a typical example of existing microwave absorbing resin prepreg: S1. Preparation of epoxy resin: Take 960g of epoxy resin E54 and 40g of dicyandiamide, stir and mix evenly to obtain conventional epoxy resin D1.

[0109] S2. Preparation of microwave absorbing resin: Take 2500 g of magnetic iron oxide nanoparticles and 15 g of coupling agent KH550, add them to the above epoxy resin D1, and mix them evenly using a three-roll mill to obtain microwave absorbing resin DA1. S3. Preparation of microwave absorbing resin prepreg: DA1 microwave absorbing resin is coated into a uniform resin film with a surface density of 200 g / m³. 2 Then it is combined with aramid fiber plain weave fabric (area density of 100g / m²). 2 This process yields a microwave-absorbing resin prepreg reinforced with aramid fiber plain weave fabric.

[0110] S4. Preparation of composite materials: The same method as in Example 1 was used.

[0111] The unnotched impact strength of the epoxy resin obtained in this comparative example is 21 kJ / m. 2 The elongation at room temperature was 3.5%, and the bending deformation of the prepared composite material part was 10.1 mm. When it was made into a large part, the functional layer delamination occurred.

[0112] Comparative Example 2: This comparative example uses the same method as Example 1, except that in step S1, component B (high molecular weight thermoplastic resin PES powder 3600RP) was not added.

[0113] Steps S2, S3, and S4 all use the same method as in Example 1.

[0114] Comparative Example 3: This comparative example uses the same method as Example 1, except that in step S1, nylon 66 powder (molecular weight 110,000, particle size 12µm) is used instead of PES powder.

[0115] Steps S2, S3, and S4 all use the same method as in Example 1.

[0116] Comparative Example 4: This comparative example uses the same method as Example 1, except that in step S1, PEKC powder (molecular weight 49100, particle size 6µm) is used instead of PES powder.

[0117] Steps S2, S3, and S4 all use the same method as in Example 1.

[0118] Comparative Example 5: This comparative example uses the same method as Example 1, except that in step S1, PES powder with a lower molecular weight (molecular weight 8030, particle size 11µm) is used instead of PES powder.

[0119] Steps S2, S3, and S4 all use the same method as in Example 1.

[0120] Comparative Example 6: This comparative example uses the same method as Example 1, except that in step S1, large-particle PES powder (molecular weight 44100, particle size 45 µm) is used instead of PES powder.

[0121] Steps S2, S3, and S4 all use the same method as in Example 1.

[0122] Detailed information on each component in the examples and comparative examples is shown in Table 2.

[0123] Table 2. Detailed information on each component in the examples and comparative examples. The flexible epoxy resin and composite material in the examples and comparative examples underwent relevant performance tests. The performance test data are shown in Table 3, and the tests involved are as follows: Elongation at break of flexible epoxy resin: Refer to GB / T 2567-2008 standard.

[0124] Unnotched impact strength of flexible epoxy resin: Refer to GB / T 1043 standard.

[0125] Glass transition temperature of flexible epoxy resin ( T g ): Referring to the DMA method, the Tanδ value is used as its T g .

[0126] Tensile strength of composite materials: Refer to GB / T 5210 standard.

[0127] Composite material curing deformation: To ensure consistency during the curing deformation test, the corresponding microwave-transparent prepreg was prepared using the method in step S2 of Example 3, and then the test sample was prepared using the method in step S3 of Example 3. The difference in length between the two pairs of diagonal lines connecting the composite material plates was the amount of curing deformation.

[0128] Table 3 Performance test data of materials in the examples and comparative examples Based on the data in Table 3, the main properties of the materials in each embodiment and comparative example were compared. Compared with the conventional epoxy resin in Example 1, all modified epoxy resins in each embodiment exhibited better performance. The elongation at break of the resins was significantly improved, approaching or even exceeding the levels of the corresponding thermoplastic resins (PES 7~11%, PEKC 6~9%, PESC 6~9%). This is because the design of the tough thermoplastic resin anchor points greatly increases the energy dissipation of resin chain slip and the fracture surface area, reducing the influence of microcracks. The impact resistance of various resins was improved, and they also exhibited good temperature resistance. The microwave absorbing / transparent films and composite materials had good tensile strength and low curing deformation, which is due to the combined effect of high-content thermoplastic resin and tough thermoplastic resin anchor points.

[0129] Based on the data in Table 2, and Figure 1 , Figures 4-9The microstructure diagrams of the resin cross-sections show that Example 1 exhibits significantly better performance. Comparative Example 1, without toughening, has poorer properties; its higher crosslinking density results in greater curing deformation, and it lacks a large number of high-temperature-resistant thermoplastic resin segments. T g The results were also relatively low. Comparative Example 2 used oligomers for toughening, but its toughness and impact resistance were low. Comparative Example 3 used oligomers and nylon 66 particles for co-toughening. As shown in the figure, the nylon and resin interface bonded well, and the excellent insect-resistant properties of nylon significantly improved its impact resistance. However, the elongation at break and temperature resistance were not ideal because nylon is insoluble in resin, and the low amount of high-temperature resistant segments in the polymer network had little effect on improving these two properties. The composite material also had greater deformability. Comparative Example 4 used dissimilar high-molecular-weight thermoplastic resins as toughening agents. However, the two thermoplastic segment structures formed obvious phase separation during polymerization, and the interface between the two phases was weak, which actually worsened its performance. Comparative Example 5 used excessively low molecular weight PES, which has high solubility and is difficult to form effective toughness anchors. Furthermore, the toughness of the liquid was poor, resulting in a lower surface roughness than Example 1, and the performance improvement compared to Comparative Example 2 was minimal. Comparative Example 6 used large-particle PES, but there were many unswollen particles during polymerization, resulting in a distinct core-skin structure. The unswollen cores within the particles appeared prone to breakage, and the content of high-temperature resistant segments in the resin was reduced. Regarding the interlayer properties of the composite material, the interfaces between each microwave-absorbing / microwave-transmitting prepreg layer and the composite material layer showed good bonding. Figure 2 and Figure 3 Observations showed that the resin in the interlaminar fracture surface of the composite material in Example 3 exhibited rough toughness failure characteristics, and some cracks penetrated into the fiber layers of the composite material, demonstrating excellent interlaminar bonding performance. All these results indicate that the material design scheme of the present invention is highly effective, the resin performance is significantly improved, and the curing deformation of the composite material is significantly reduced.

[0130] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0131] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these modifications and improvements are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A flexible epoxy resin, characterized in that, The flexible epoxy resin comprises, by weight, 30-55% component A, 15-30% component B and 30-50% component C; Component A is a thermoplastic resin oligomer with active groups at the end; Component B is a high molecular weight thermoplastic resin with the same main chain structure as component A; Component C is pure epoxy resin and curing agent; Both components A and B are soluble in C.

2. The flexible epoxy resin according to claim 1, characterized in that, The mass ratio of component A to component C is 1:(0.5~2). The mass ratio of component A to component B is (1.2~2.8):

1.

3. The flexible epoxy resin according to claim 1, characterized in that, The weight-average molecular weight of component A is 700-4000; The weight-average molecular weight of component B is 10,000 to 80,000.

4. The flexible epoxy resin according to claim 1, characterized in that, Component B is mixed in the resin in the form of particles with a particle size of 3~25μm.

5. The flexible epoxy resin according to claim 1, characterized in that, In component C, the viscosity of the pure epoxy resin at 25°C is less than 15 Pa·s; The curing agent is selected based on the curing start temperature T in the epoxy resin, wherein 120℃≤T≤180℃.

6. The flexible epoxy resin according to claim 1, characterized in that, The active group-terminated thermoplastic resin oligomer is at least one of active group-terminated polyaryletherketone and active group-terminated polyarylethersulfone. The active group is one of hydroxyl, amino, carboxyl, or epoxy groups.

7. The flexible epoxy resin according to claim 6, characterized in that, Component A is at least one of the following oligomers: The hydroxyl-terminated phenolphthalein-modified polyethersulfone has the following structural formula: ； The hydroxyl-terminated phenolphthalein-modified polyarylether ketone has the following structural formula: ； The hydroxyl-terminated phenolphthalein-modified polyarylether sulfone has the following structural formula: ； The amino-terminated phenolphthalein-modified polyarylether sulfone has the following structural formula: ； The carboxyl-terminated phenolphthalein-modified polyarylene sulfone has the following structural formula: ； The polyepoxy-terminated phenolphthalein-modified polyarylether ketone has the following structural formula: 。 8. A flexible microwave absorbing resin, characterized in that, The flexible microwave absorbing resin includes microwave absorbing filler and the flexible epoxy resin according to any one of claims 1-7; The mass of the microwave absorbing filler is 50-90% of the total mass of the flexible microwave absorbing resin. In flexible epoxy resin, the viscosity of pure epoxy resin in component C is less than 8 Pa·s at 25°C.

9. A prepreg, characterized in that, The prepreg comprises the flexible microwave absorbing resin as described in claim 8.

10. A composite material, characterized in that, The composite material includes the prepreg as described in claim 9.

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