Modified aramid fiber and preparation method thereof

Abstract

The invention discloses a modified aramid fiber and a preparation method thereof, and the modified aramid fiber is characterized in that a bifunctional molecular layer is grafted on the surface of the aramid fiber through chemical bonds. The molecular layer is provided by an organosilane bifunctional coupling agent with a structural general formula of X-L-Y, X is a group (such as chlorosilane, alkoxy silane and the like) capable of forming a stable first covalent bond with the fiber surface, Y is a group (such as furyl, maleimide and the like) capable of forming a dynamic covalent bond interface with a resin matrix, and L is a connecting chain. The preparation method comprises the following steps: carrying out surface activation treatment on the aramid fiber, and then carrying out grafting reaction on the aramid fiber and a difunctional coupling agent under the action of a catalyst. According to the method, a molecular layer with stable anchoring and dynamic reversible reaction characteristics is introduced to the surface of the fiber, so that the obtained composite material obtains high interfacial shear strength (ILSS), the mode I interlayer fracture toughness (GIC) is remarkably improved, and the technical problem that the interfacial strength and toughness are difficult to synergistically improve in a traditional modification method is effectively solved.

Description

Technical Field

[0001] This invention belongs to the field of polymer fiber technology, specifically, it relates to a modified aramid fiber and its preparation method. Background Technology

[0002] Aramid fibers are widely used as key reinforcements in composite materials for aerospace, defense equipment, and high-end sports equipment due to their excellent specific strength, specific modulus, heat resistance, and chemical stability. However, the low surface energy, high chemical inertness, and relatively smooth nature of aramid fibers often result in insufficient interfacial bond strength between them and the resin matrix. This weak interfacial structure directly restricts further improvements in key mechanical properties of composite materials, such as interlaminar shear strength, impact resistance, and fatigue life.

[0003] To improve the interfacial bonding between aramid fibers and resins, common surface modification methods include physical and chemical approaches. Physical methods, such as low-temperature plasma treatment, mainly improve resin wettability through surface etching and the introduction of polar functional groups. However, their modification effect is not long-lasting, and improper treatment can easily damage the fiber matrix. Chemical methods, represented by silane coupling agent grafting, typically involve first activating the fiber with plasma or other methods, and then reacting it with a single-functional-group coupling agent such as aminosilane or epoxysilane to establish a chemical "bridge" between the fiber and resin. While these methods can improve interfacial bonding strength by forming stable covalent bonds (such as urethane bonds and ether bonds), the resulting interfacial phase is usually quite rigid. When the composite material is subjected to impact or cyclic stress, such rigid interfaces lack effective energy dissipation pathways, and stress concentration can easily lead to brittle debonding at the interface. This makes it difficult to achieve high interfacial strength while simultaneously maintaining excellent toughness and damage resistance.

[0004] The development of dynamic covalent chemistry has provided new possibilities for interfacial toughening. For example, dynamic covalent bonds such as the Diels-Alder reaction and disulfide exchange can undergo reversible breakage and recombination under thermal stimuli, making them a potential means to dissipate energy and improve material toughness. However, successfully introducing such dynamic bonding mechanisms into the microscopic interface of fiber-resin heterogeneous phases and achieving synergy with traditional high-strength bonding remains a challenge. Specifically for aramid fibers, how to construct a functional molecular layer on the surface that can be firmly anchored to the fiber and possess dynamic reversible reactivity, thereby preparing high-performance composite materials with simultaneous improvement in interfacial strength and toughness, has yet to yield a mature and effective solution in existing publicly available technologies.

[0005] Therefore, developing a novel aramid surface modification technology that can significantly improve the fracture toughness and damage tolerance of composite materials through innovative interface structure design while maintaining high-strength bonding with resin has clear application value for promoting the development of high-performance composite materials in related fields. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a modified aramid fiber and its preparation method.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows: A modified aramid fiber has a bifunctional molecular layer grafted onto its surface via chemical bonds. This bifunctional molecular layer is provided by an organosilane-based bifunctional coupling agent with the general structural formula XLY: X is a first active group selected from chlorosilyl, alkoxysilyl, isocyanate, or epoxy groups, used to form a stable first covalent bond with the fiber surface; Y is a second active group selected from furanyl, maleimide, or dithioester groups, used to form a dynamic covalent bond interface with the resin matrix; and L is a C2-C12 straight-chain or branched alkylene chain. This invention utilizes an organosilane-based bifunctional coupling agent with the general structural formula XLY to construct a molecular layer on the surface of aramid fibers possessing both functional groups. The first active group (X) forms a stable covalent anchor with the fiber surface, fundamentally solving the problem of strong inertness and weak bonding force on the aramid surface; while the second active group (Y) provides active sites for dynamic covalent bond reactions with the resin matrix. This design not only provides the modified fibers with a strong interfacial bond, but more importantly, it introduces the potential for reversible reactions, creating unique chemical conditions for energy dissipation at the composite material interface, thereby synergistically enhancing strength and toughness.

[0008] Furthermore, the bifunctional coupling agent is 3-(2-furanmethoxy)propyltrimethoxysilane or N-(4-maleimide-phenyl)-3-(trimethoxysilyl)propylamine. This invention further specifies the use of two specific bifunctional coupling agents, such as 3-(2-furanmethoxy)propyltrimethoxysilane or N-(4-maleimide-phenyl)-3-(trimethoxysilyl)propylamine. These coupling agents with specific structures have an X-terminal (methoxysilane) that, after hydrolysis, can efficiently bond to the fiber surface, while the Y-terminal (furanyl or maleimide group) is a classic functional group participating in the Diels-Alder reaction. This limitation ensures that the bifunctional molecular layer is not only conceptually feasible but can also be concretely realized through well-defined and readily available chemicals. Furthermore, the dynamic reaction mechanisms of the selected Y-terminal groups with various resin systems are mature, and the interface construction exhibits high predictability and reliability.

[0009] Furthermore, the thickness of the bifunctional molecular layer is 5–100 nanometers. Controlling the thickness of the bifunctional molecular layer within the range of 5–100 nanometers is a key structural parameter in this invention. This thickness range ensures the formation of a complete and dense surface functional layer, providing sufficient reaction sites, while avoiding interfacial stress concentration or negative impacts on the fiber's mechanical properties that might result from an excessively thick coating. It achieves a balance between surface modification and maintaining the fiber's inherent excellent properties.

[0010] Furthermore, the grafting density of the bifunctional molecular layer is 0.05~1.5 mmol / g fiber. This invention limits the grafting density to 0.05~1.5 mmol / g fiber, providing quantitative control over the degree of surface modification. A suitable grafting density ensures sufficient dynamic reaction sites on the fiber surface to significantly influence interfacial properties, while preventing excessively dense grafting that could lead to molecular chain crowding, limited reactivity of functional groups, or unnecessary cost increases, thus contributing to obtaining products with both excellent performance and economic benefits.

[0011] The present invention also provides a method for preparing the above-mentioned modified aramid, comprising the following steps: (1) Surface activation treatment is performed on the aramid fiber to introduce or expose hydroxyl or amino groups that react with the first active group (X) on its surface; (2) The activated fiber is reacted with the bifunctional coupling agent in the presence of a catalyst to bond the first active group (X) to the fiber surface, thereby grafting the bifunctional molecular layer.

[0012] The preparation method has clear steps, involving activation followed by grafting. The activation step specifically introduces or exposes hydroxyl and amino groups, providing a reaction basis for subsequent chemical bonding with the coupling agent; the grafting step, under the action of a catalyst, achieves a strong connection between the X-end of the coupling agent and the fiber. This method has a reasonable process flow, strong operability, and is a reliable and repeatable process route for achieving the structurally modified aramid defined in the aforementioned product claims.

[0013] Furthermore, in step (1), the surface activation treatment is either low-temperature plasma treatment or alkaline treatment. Low-temperature plasma treatment is a dry and environmentally friendly process that can effectively clean and activate the fiber surface, introducing oxygen-containing polar groups; alkaline treatment increases the number of hydroxyl groups by gently hydrolyzing the fiber surface. Both of these are mature and controllable surface treatment technologies in the field, which can be flexibly selected according to production conditions, ensuring the stability and effectiveness of the pretreatment steps.

[0014] Further, in step (2), the catalyst is at least one of an organotin compound, a titanate, or an organic amine. The catalyst can significantly promote the hydrolysis of the alkoxy group of the silane coupling agent and its condensation reaction with the hydroxyl group on the fiber surface, thereby improving the efficiency and extent of the grafting reaction, and ensuring that the bifunctional molecular layer is uniformly and firmly grafted onto the fiber surface.

[0015] Furthermore, the reaction in step (2) is carried out in an anhydrous organic solvent at a temperature of 40-80°C for 2-6 hours. These conditions effectively inhibit the premature hydrolysis and self-polymerization of the alkoxy group of the silane coupling agent, ensuring that it preferentially reacts with the fiber surface. The selected temperature and time windows ensure that the reaction proceeds fully while avoiding the adverse effects that prolonged high temperatures may have on the fiber or solvent system. The process window is wide and easy to implement and control.

[0016] Furthermore, after the reaction in step (2) is completed, a post-processing step is included: the reaction product is washed sequentially with organic solvents and alcohols, and then dried. Sequential washing with organic solvents and alcohols thoroughly removes physically adsorbed unreacted coupling agents, catalysts, and byproducts. The subsequent drying process ensures the product is pure and dry. This step guarantees the consistency of the final modified aramid product's performance and its storage stability.

[0017] Furthermore, in step (2), the mass ratio of the bifunctional coupling agent to the fiber is (0.01~0.20):1. This ratio range is the result of experimental optimization, which can effectively control the raw material cost while ensuring the surface grafting coverage and functionalization, and avoid waste caused by excessive use of coupling agent or interface defects that may be caused by its residue, making the entire preparation method more economical for industrial applications.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The core effect of this invention lies in resolving the contradiction between interfacial strength and toughness in traditional aramid modification technology through a bifunctional molecular layer design of "stable anchoring + dynamic reaction". After the modified fiber is combined with the resin, it can achieve high interfacial shear strength through stable first covalent bonds, and effectively dissipate impact energy through the reversible fracture and recombination mechanism of dynamic covalent bonds. This simultaneously and significantly improves the interlaminar shear strength (ILSS) and mode I interlaminar fracture toughness (GIC) of the composite material, achieving synergistic optimization of interfacial properties.

[0019] Second, the technical solution provided by this invention is clear and explicit, forming a complete and closed technical system from molecular structure design (general formula XLY) to specific compound embodiments, and then to quantifiable process parameters (thickness, grafting density, material ratio, temperature, time, etc.). The selected bifunctional coupling agent has a mature chemical reaction mechanism, and the activation, grafting, and post-treatment processes used in the preparation method are all reliable technologies in the field, ensuring that this solution is not only innovative in principle, but also repeatable and producible in practice, with good process scale-up constraints and controllable product quality.

[0020] Third, the composite materials prepared based on the modified aramid of this invention exhibit improved macroscopic properties due to the strengthening and toughening of the interfacial phase. It is expected to effectively enhance the anti-delamination ability, impact resistance, and fatigue life of composite parts, thereby extending their service life and reliability under harsh conditions such as aerospace and high-end equipment. This provides an effective and promising new technological approach to solving the interfacial bottleneck problem that has long constrained the application of high-performance aramid composite materials. Detailed Implementation

[0021] The specific embodiments are described in detail below, but it should be understood that the scope of protection of this invention is not limited to the specific embodiments. Unless otherwise specified, the raw materials and reagents used in the examples are commercially available. This invention has been verified through numerous experiments, and its objectives can be achieved within the stated parameter range. Those skilled in the art can adjust the process parameters within the scope of the claims according to the specific raw materials and equipment available.

[0022] Example 1 This embodiment provides a method for preparing modified aramid fibers. As the preferred embodiment of the present invention, the specific steps are as follows: (1) Pretreatment and degreasing cleaning: Weigh 10.0 kg of unfolded para-aramid woven fabric and immerse it in a mixed solvent consisting of 100.0 kg acetone and 100.0 kg anhydrous ethanol for 24 hours at room temperature (25±3℃). After removal, dry it in a forced-air drying oven at 80℃ for 10 hours, and then transfer it to another oven for heat treatment at 200℃ for 3 hours.

[0023] (2) Plasma activation treatment: The pretreated aramid fibers were placed in a radio frequency low-temperature plasma treatment device. After evacuating to a pressure of 50 Pa, argon gas was introduced as the working gas, and the gas flow rate was maintained at 50 sccm. The radio frequency power was set to 100 W and the treatment time was 10 minutes. After the treatment, the fibers were cooled to below 60°C in an inert atmosphere in the chamber and then removed and placed in a standard laboratory environment with a temperature of 23±2°C and a relative humidity of 50±10% for 2 hours to equilibrate.

[0024] (3) Silanization grafting reaction: In a 1000 L reactor equipped with an organic olive stirrer, reflux condenser and thermometer, 800.0 kg of xylene dried through a 4A molecular sieve was added. Stirring was started, and 15.0 kg of 3-(2-furanmethoxy)propyltrimethoxysilane and 0.30 kg (2.0% of the silane mass) of dibutyltin dilaurate catalyst were added sequentially. The mixture was stirred for 15 minutes to ensure homogeneity. The aramid fabric activated and balanced in step (2) was completely immersed in the solution. The oil bath heating was turned on, and the temperature of the reaction system was raised to 85°C. The reflux timing was started, and the reaction was kept at this temperature for 4 hours. During the reaction, the stirring was kept at a medium speed, and the aramid fabric was turned over once every 1 hour using clean tools.

[0025] (4) Post-treatment and drying: After the reaction is complete, turn off the heating and cool the reactor to room temperature (approximately 25°C). Remove the aramid cloth and immerse it in a cleaning tank containing 400.0 kg of fresh xylene for 10 minutes in an ultrasonic cleaner. Repeat this xylene ultrasonic cleaning step once. Then, transfer the aramid cloth to another cleaning tank containing 400.0 kg of anhydrous ethanol and perform ultrasonic cleaning twice, 10 minutes each time. After cleaning, place the aramid cloth in a vacuum drying oven and dry it for 6 hours at 80°C and a vacuum of -0.098 MPa to obtain the modified aramid product of this embodiment.

[0026] Example 2 This embodiment provides another method for preparing modified aramid fibers, the specific steps of which are as follows: (1) Pretreatment and degreasing cleaning: Weigh 10.0 kg of unfolded para-aramid woven fabric and immerse it in a corrosion-resistant container containing 150.0 kg of methanol for 24 hours at room temperature (25±3℃). After removal, dry it in a forced-air drying oven at 80℃ for 10 hours, and then transfer it to another oven for heat treatment at 180℃ for 2 hours.

[0027] (2) Alkali activation treatment: Prepare 1000.0 kg of 2% sodium hydroxide aqueous solution in a 2000 L alkali-resistant reactor. Turn on the water bath circulation system, heat the alkali solution to 50℃ and maintain a constant temperature. Immerse the pretreated aramid fabric completely in the alkali solution and maintain the temperature for 30 minutes. After treatment, immediately remove the aramid fabric and rinse it repeatedly with a large amount of deionized water under running conditions 6 times until the pH value of the final rinse water is 7 as measured by precision pH test paper. Place the washed aramid fabric in an 80℃ forced-air drying oven to dry for 2 hours, and then place it in a standard laboratory environment (23℃, 50% RH) for equilibration for 2 hours.

[0028] (3) Silanization grafting reaction: In a 1000 L reactor, add 850.0 kg of xylene dried through a 4A molecular sieve. Start stirring, and add 5.0 kg of 3-(2-furanmethoxy)propyltrimethoxysilane and 0.10 kg (2.0% of the silane mass) of dibutyltin dilaurate catalyst, and stir for 15 minutes to mix evenly. Completely immerse the aramid fabric activated and balanced in step (2) into the solution. Turn on the oil bath heating, raise the temperature of the reaction system to 80℃ and start the reflux timing. Maintain the reaction at this temperature for 5 hours, turning the aramid fabric once every 1 hour during the process.

[0029] (4) Post-treatment and drying: After the reaction was completed, the heating was turned off and the mixture was cooled to room temperature. The aramid cloth was removed and first immersed in a cleaning tank containing 400.0 kg of fresh xylene, and then cleaned in an ultrasonic cleaner for 10 minutes. After removal, the xylene ultrasonic cleaning step was repeated once. Subsequently, the aramid cloth was transferred to another cleaning tank containing 400.0 kg of anhydrous ethanol, and ultrasonically cleaned twice, 10 minutes each time. After cleaning, the aramid was placed in a vacuum drying oven and dried for 6 hours at 80°C and a vacuum of -0.098 MPa to obtain modified aramid product 2.

[0030] Example 3 This embodiment provides another method for preparing modified aramid fibers, the specific steps of which are as follows: (1) Pretreatment and degreasing cleaning: Weigh 10.0 kg of unfolded para-aramid woven fabric and immerse it in a mixed solvent consisting of 100.0 kg acetone and 100.0 kg anhydrous ethanol for 24 hours at room temperature (25±3℃). After removal, dry it in a forced-air drying oven at 80℃ for 10 hours, and then transfer it to another oven for heat treatment at 200℃ for 3 hours.

[0031] (2) Plasma activation treatment: The pretreated aramid fibers were placed in a radio frequency low-temperature plasma treatment device. After evacuating to a pressure of 40 Pa, nitrogen was introduced as the working gas, and the gas flow rate was maintained at 60 sccm. The radio frequency power was set to 150 W and the treatment time was 8 minutes. After the treatment, the fibers were cooled to below 60°C in an inert atmosphere in the chamber and then removed and placed in a standard laboratory environment with a temperature of 23±2°C and a relative humidity of 50±10% for 2 hours to equilibrate.

[0032] (3) Silanization grafting reaction: In a 1000 L reactor equipped with an organic olive stirrer, reflux condenser and thermometer, 900.0 kg of tetrahydrofuran (THF) dried through 4A molecular sieve was added. Stirring was started, and 8.0 kg of N-(4-maleimide-phenyl)-3-(trimethoxysilyl)propylamine and 0.08 kg (1.0% of the silane mass) of dibutyltin dilaurate catalyst were added sequentially. The mixture was stirred for 15 minutes to ensure homogeneity. The aramid fabric activated and balanced in step (2) was completely immersed in the solution. The oil bath heating was turned on, and the temperature of the reaction system was strictly controlled at 75°C. The reflux timer was started, and the reaction was kept constant at this temperature for 5 hours. During the reaction, gentle stirring was maintained, and the aramid fabric was turned over once every 1 hour using clean tools.

[0033] (4) Post-treatment and drying: After the reaction was completed, the heating was turned off, and the reactor was cooled to room temperature (approximately 25°C). The aramid cloth was removed and first immersed in a cleaning tank containing 400.0 kg of fresh tetrahydrofuran, and cleaned in an ultrasonic cleaner for 10 minutes. After removal, the tetrahydrofuran ultrasonic cleaning step was repeated once. Subsequently, the aramid cloth was transferred to another cleaning tank containing 400.0 kg of anhydrous ethanol, and ultrasonically cleaned twice, 10 minutes each time. After cleaning, the aramid was arranged in a vacuum drying oven and dried for 6 hours at 80°C and a vacuum of -0.098 MPa to obtain the modified aramid product of this embodiment.

[0034] Example 4 This embodiment provides another method for preparing modified aramid fibers, the specific steps of which are as follows: (1) Pretreatment and degreasing cleaning: Weigh 10.0 kg of unfolded para-aramid woven fabric and immerse it in a mixed solvent consisting of 100.0 kg acetone and 100.0 kg anhydrous ethanol for 24 hours at room temperature (25±3℃). After removal, dry it in a forced-air drying oven at 80℃ for 10 hours, and then transfer it to another oven for heat treatment at 200℃ for 3 hours.

[0035] (2) Weak acid activation treatment: Prepare 1000.0 kg of 5% acetic acid aqueous solution in a 2000 L acid-resistant reactor. Turn on the water bath circulation system, heat the acid solution to 60℃ and maintain a constant temperature. Completely immerse the pretreated aramid fabric in the acid solution and maintain the temperature for 1 hour. After the treatment, immediately remove the aramid fabric and rinse it repeatedly with a large amount of deionized water under running conditions until neutral (the pH of the last rinse water was measured to be 7 using precision pH test paper). Place the washed aramid fabric in an 80℃ forced-air drying oven to dry for 2 hours, and then place it in a standard laboratory environment for equilibration for 2 hours.

[0036] (3) Silanization grafting reaction: In a 1000 L reactor, add 850.0 kg of toluene dried through a 4A molecular sieve. Start stirring, and add 10.0 kg of 3-(triethoxysilyl)propylmethyl dithioester and 0.20 kg (2.0% of the mass of silane) of bismuth neodecanoate catalyst sequentially. Stir for 15 minutes to mix evenly. Completely immerse the aramid fabric, which has been activated and balanced in step (2), into the solution. Turn on the oil bath heating, raise the temperature of the reaction system to 70°C and start the reflux timing. Maintain the reaction at this temperature for 6 hours, and keep stirring gently during the process, turning the aramid fabric once every 1 hour.

[0037] (4) Post-treatment and drying: After the reaction is complete, turn off the heating and cool to room temperature. Take out the aramid cloth and first immerse it in a cleaning tank containing 400.0 kg of fresh toluene, and clean it in an ultrasonic cleaner for 10 minutes. After taking it out, repeat the toluene ultrasonic cleaning step once. Then, transfer the aramid cloth to another cleaning tank containing 400.0 kg of anhydrous ethanol, and perform ultrasonic cleaning twice, 10 minutes each time. After cleaning, arrange the aramid in a vacuum drying oven and dry it for 6 hours at 80℃ and a vacuum degree of -0.098 MPa to obtain the modified aramid product.

[0038] Comparative Example 1 This comparative example aims to illustrate that when surface modification is performed using traditional monofunctional silane coupling agents, the interfacial properties of the composite material, especially in terms of toughness, are significantly different from the bifunctional design of this invention.

[0039] (1) Pretreatment and degreasing cleaning: Same as step (1) in Example 1.

[0040] (2) Plasma activation treatment: Same as step (2) in Example 1.

[0041] (3) Silanization grafting reaction: In a 1000 L reactor, 800.0 kg of dried xylene was added. Stirring was started, and 15.0 kg of the traditional aminosilane coupling agent γ-aminopropyltriethoxysilane (KH-550) and 0.30 kg (2.0% of the silane mass) of the catalyst dibutyltin dilaurate were added sequentially and stirred until homogeneous. The activated aramid fabric was immersed in the solution and refluxed at 85 °C for 4 hours, with periodic stirring during the reaction.

[0042] (4) Post-processing and drying: Same as step (4) in Example 1, to obtain the comparative sample.

[0043] Comparative Example 2 This comparative example aims to illustrate that simply physical / chemically activating aramid fibers without introducing any chemical grafting layer has a very limited effect on improving the interfacial properties of the final composite material.

[0044] (1) Pretreatment and degreasing cleaning: Same as step (1) in Example 2.

[0045] (2) Alkali activation treatment: Same as step (2) in Example 2.

[0046] (3) Post-treatment: The activated and dried aramid fabric was not subjected to silanization grafting reaction. Instead, it was directly ultrasonically cleaned twice with 400.0 kg xylene and 400.0 kg anhydrous ethanol, each time for 10 minutes.

[0047] (4) Drying: Same as step (4) in Example 1, to obtain the comparative sample.

[0048] Comparative Example 3 This comparative example aims to illustrate that if the Y-terminal group of the grafted molecular layer is an inert group and cannot react specifically with the resin, it cannot effectively improve the interfacial properties, thus proving the necessity of the chemical reactivity of the Y-terminal group.

[0049] (1) Pretreatment and degreasing cleaning: Same as step (1) in Example 3.

[0050] (2) Plasma activation treatment: Same as step (2) in Example 3.

[0051] (3) Silanization grafting reaction: In a 1000 L reactor, 900.0 kg of dried tetrahydrofuran was added. Stirring was started, and 8.0 kg of inert silanepropyltrimethoxysilane and 0.08 kg of catalyst dibutyltin dilaurate were added sequentially and mixed evenly. The activated aramid fabric was immersed in the mixture and reacted at a constant temperature of 75 °C for 5 hours.

[0052] (4) Post-processing and drying: Same as step (4) in Example 3, to obtain the comparative sample.

[0053] Comparative Example 4 This comparative example aims to illustrate that in the bifunctional grafting system described in this invention, the choice of catalyst has a significant impact on the final effect, and not any catalyst can achieve the same effect as the preferred embodiment of this invention.

[0054] (1) Pretreatment and degreasing cleaning: Same as step (1) in Example 4.

[0055] (2) Weak acid activation treatment: Same as step (2) in Example 4.

[0056] (3) Silanization grafting reaction: In a 1000 L reactor, 850.0 kg of dried toluene was added. Stirring was started, and 10.0 kg of 3-(triethoxysilyl)propylmethyl dithioester and 0.20 kg of catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were added sequentially and mixed evenly. The activated aramid fabric was immersed in the mixture and refluxed at 70 °C for 6 hours.

[0057] (4) Post-processing and drying: Same as step (4) in Example 4, to obtain the comparative sample.

[0058] Comparison Example 5 (Core Mechanism Comparison) This comparative example aims to make the most crucial mechanistic comparison. By using a coupling agent capable of forming conventional high-strength, irreversible covalent bonds, it is demonstrated that the high toughness (GIC) obtained by this invention specifically originates from the unique reversible energy dissipation mechanism of "dynamic covalent bonds," rather than simply an increase in interfacial bonding strength.

[0059] (1) Pretreatment and degreasing cleaning: Same as step (1) in Example 1.

[0060] (2) Plasma activation treatment: Same as step (2) in Example 1.

[0061] (3) Silanization grafting reaction: In a 1000 L reactor, 800.0 kg of dried xylene was added. Stirring was started, and 14.2 kg of γ-glycidyl etheroxypropyltrimethoxysilane (KH-560) and 0.284 kg of catalyst dibutyltin dilaurate were added sequentially and mixed evenly. The activated aramid fabric was immersed in the mixture and refluxed at 85 °C for 4 hours.

[0062] (4) Post-processing and drying: Same as step (4) in Example 1, to obtain the comparative sample.

[0063] Performance comparison tests were conducted on Examples 1-4 and Comparative Examples 1-5, and the test methods are as follows: I. Interlaminar Shear Strength Test 1. Testing Standards The determination of interlaminar shear strength (ILSS) in this invention is performed in accordance with the People's Republic of China National Standard GB / T 30969-2014 "Test Method for Shear Strength of Short Beams of Polymer-Based Composite Materials".

[0064] 2. Sample preparation Aramid prepreg (using high-toughness epoxy thermosetting resin CYCOM® 977-2 as the resin matrix) is laminated according to a predetermined layup sequence and angle, and cured into a unidirectional composite laminate using a hot-pressing process. Standard specimens are then machined from the cured laminate using a water-cooled diamond wheel cutter. The specimens are cuboids with recommended dimensions of 20 mm length, 10 mm width, and 2 mm thickness. Each set should contain at least five specimens. After machining, the edges of the specimens must be ground to eliminate any machining defects.

[0065] 3. Testing equipment and conditions A universal testing machine equipped with a three-point bending fixture was used. The main test parameters are as follows: Span (L): Determined based on the specimen thickness (h), with a span-to-thickness ratio of L / h of 5:1. For example, for a specimen with a thickness of 2 mm, the span is 10 mm.

[0066] The radius of the loading head and support roller is 2 mm.

[0067] Loading rate: 1 mm / min.

[0068] Test environment: The test shall be conducted in a standard laboratory environment with a temperature of 23±2℃ and a relative humidity of 50±10%. The test sample shall be placed in this environment for no less than 24 hours before the test.

[0069] 4. Test Procedures and Calculations (1) Place the sample symmetrically at the center of the two supports, ensuring that the length direction is perpendicular to the axis of the support and the indenter; (2) Start the testing machine and apply a uniform load at the specified rate until the specimen fails. Record the load-displacement curve and the maximum failure load P_max (unit: N) throughout the process; (3) Interlaminar shear strength (ILSS) is calculated using the following formula: τ = 0.75 × (P_max / (b × h)) In the formula: τ is the interlaminar shear strength, in megapascals (MPa); P_max is the maximum load at which the specimen fails, in newtons (N); b is the specimen width, in millimeters (mm); h is the specimen thickness, in millimeters (mm).

[0070] (4) Take the arithmetic mean of the test results of a group of samples as the interlaminar shear strength value of the material.

[0071] II. Mode I Interlaminar Fracture Toughness Test 1. Testing Standards In this invention, the determination of Mode I Interlaminar Fracture Toughness (G_IC) is carried out in accordance with the People's Republic of China National Standard GB / T 41738-2022 "Fiber Reinforced Plastic Composites - Test Method for Mode I Interlaminar Fracture Toughness G_IC" (equivalent to International Standard ISO 15024).

[0072] 2. Sample preparation (1) Preparation of double cantilever beam specimens. During the process of laying unidirectional composite prepreg (the resin matrix used in the prepreg is high-toughness epoxy thermosetting resin CYCOM® 977-2), a polyimide film with a thickness of no more than 20 μm is inserted at one end of the specimen along the length direction at the centerline position to pre-create an initial crack with a length of a0 (usually 30~50 mm); (2) After curing, process into standard specimens. The recommended specimen dimensions are: length 150 mm, width 20~25 mm, thickness 3~5 mm; (3) On both sides of the sample, perpendicular to the crack propagation direction, use a high-precision ruler or a length ruler coated with a brittle coating to observe the position of the crack tip. (4) Use high-strength adhesive to attach the loading blocks in the hinge mounting area at both ends of the sample to ensure that the loading axis is coplanar with the middle surface of the laminate.

[0073] 3. Testing equipment and conditions A universal testing machine was used, equipped with a pair of clamps for holding the specimen loading block. The main test parameters are as follows: Loading rate: Displacement control is used, with a rate of 1-2 mm / min.

[0074] Data acquisition: Simultaneously record load-displacement curves.

[0075] Crack observation: An optical magnifying glass or camera is required to observe and record the position of the crack tip in real time (corresponding to the side scale).

[0076] 4. Test Procedures and Calculations (1) Mount the specimen onto the testing machine using the loading block, ensuring alignment; (2) Apply tensile load at a constant rate to allow the initial crack to propagate slowly along the interlaminar space (mode I, open type); (3) After the crack begins to propagate, the load-displacement curve recording method or the compliance calibration method shall be used. During the test, the loading shall be paused at intervals of about 1 to 5 mm when the crack propagates, and the load P, displacement δ and the corresponding crack length a shall be recorded at this time; (4) The calculation of interlaminar fracture toughness G_IC in Mode I is usually performed using the modified beam theory method. For each valid data pair (load, displacement, crack length), the following formula is used for calculation: G_IC = (3 P δ) / (2 ba) × F In the formula: G_IC represents the interlaminar fracture toughness of Mode I, in joules per square meter (J / m²). 2 P is the load, in Newtons (N); δ is the displacement of the loading point, in meters (m); b is the specimen width, in meters (m); a is the effective crack length from the loading line to the crack tip, in meters (m); F is the correction factor, used to consider effects such as large deformation and loading block rotation, and its calculation method is performed in accordance with Appendix of GB / T 41738-2022.

[0077] (5) Take the average value of the G_IC value calculated from multiple data points during the stable crack propagation stage (usually excluding the initial nonlinear and rapid instability stages) as the test result of the specimen. The number of effective specimens for each group of materials shall not be less than 3.

[0078] The test data is shown in the table below: Sample type Interlaminar shear strength (ILSS) / MPa <![CDATA[Mode I Interlaminar Fracture Toughness (GIC) / J / m 2 > Example 1 68.3 ± 2.5 338 ± 20 Example 2 58.5 ± 2.8 285 ± 18 Example 3 60.5 ± 2.2 290 ± 18 Example 4 53.8 ± 2.0 280 ± 16 Comparative Example 1 52.1 ± 3.0 192 ± 15 Comparative Example 2 41.2 ± 2.5 135 ± 12 Comparative Example 3 36.8 ± 2.8 118 ± 10 Comparative Example 4 46.5 ± 3.2 205 ± 20 Comparative Example 5 66.8 ± 2.7 209 ± 18 By comparing and analyzing the systematic performance test data of the examples and comparative examples, the following conclusions can be drawn.

[0079] The various embodiments covered by the claims of this invention have verified the feasibility and effectiveness of its technical solutions. Specifically, Example 1, using argon plasma in combination with furanylsilane, achieved the best overall performance (ILSS 68.3 MPa, GIC 338 J / m). 2 Examples 2 to 4, which employed alkaline solution and nitrogen plasma activation and used different combinations of maleimide and dithioester groups, showed ILSS data (58.5, 60.5, 53.8 MPa) and GIC data (285, 290, 280 J / m³). 2Although there are reasonable fluctuations, they are all significantly higher than all comparative benchmarks. This indicates that by varying the specific processes and components within the stated parameter range, the core objective of this invention—interface strengthening and toughening—can be achieved, and the technical solution possesses feasibility and necessary redundancy.

[0080] The series of comparative examples constitutes a rigorous chain of argumentation. Through specific numerical comparisons, other competing explanations are gradually eliminated, confirming the unique mechanism of the invention's effect. Comparative Example 1 (using conventional aminosilane, ILSS 52.1 MPa, GIC 192 J / m³) 2 Comparative Example 2 (activated only, ungrafted, ILSS 41.2 MPa, GIC 135 J / m) 2 The data show that its ILSS and GIC are only 76.3% and 56.8% and 60.3% and 40.0% of the best embodiment, respectively, proving that simply introducing ordinary chemical bonds or relying solely on physical modification is far less effective than the bifunctional grafting strategy of this invention. Comparative Example 3 (grafted inert alkyl chain, ILSS 36.8 MPa, GIC 118 J / m) 2 This further rules out the possibility that surface energy changes or physical anchoring alone are the main contributors. Comparative Example 4 (using the non-preferred catalyst DBU, ILSS 46.5 MPa, GIC 205 J / m) 2 The data show that even with the same bifunctional coupling agent, an inappropriate catalytic system can lead to a decrease in grafting efficiency and interfacial properties. Its ILSS and GIC were only 86.4% and 73.2% of those in Example 4, respectively, highlighting the importance of the catalytic system selected in this invention. The most diagnostically significant example is Comparative Example 5, which, using a highly active epoxy silane under the same process, achieved an ILSS almost equivalent to that of Example 1 (66.8 MPa, 97.8% of Example 1), but its GIC (209 J / m³) was significantly lower. 2 However, the performance of the original model was only 61.8% of that of Example 1, which is statistically significantly inferior. This key numerical comparison directly pinpoints the source of the performance gain to the essential difference in interfacial chemical bonds: traditional high-strength irreversible covalent bonds can effectively transfer stress (thus achieving close ILSS), but they are difficult to dissipate energy (leading to low GIC); while the dynamic covalent bonds introduced in this invention, through reversible breakage and recombination under stress, specifically contribute additional energy dissipation channels, thereby achieving a breakthrough improvement in toughness while maintaining high strength.

[0081] In summary, the experimental data fully demonstrate that the technical solution provided by this invention—"constructing a bifunctional molecular layer with both stable anchoring groups and dynamically reversible reactive groups on the surface of aramid fibers"—is not a simple optimization or combination of existing technologies. By introducing the novel interfacial interaction mechanism of dynamic covalent chemistry, it successfully solves the fundamental technical contradiction that has long existed in high-performance aramid composite materials: the inability to synergistically improve interfacial strength and toughness. Its technological advancement lies in providing a novel, experimentally verified, and effective approach that differs from all traditional interfacial modification ideas.

[0082] It should be understood that the above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. It should not be considered that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A modified aramid fiber, characterized in that: A bifunctional molecular layer is chemically grafted onto the surface of aramid fibers; the bifunctional molecular layer is provided by an organosilane bifunctional coupling agent with the general structural formula XLY, wherein: X is a first active group selected from chlorosilyl, alkoxysilyl, isocyanate or epoxy groups, used to form a stable first covalent bond with the fiber surface; Y is a second active group selected from furanyl, maleimide, or dithioester, used to form a dynamic covalent bond interface with the resin matrix; L is a C2~C12 straight-chain or branched alkylene chain.

2. The modified aramid according to claim 1, characterized in that: The bifunctional coupling agent is 3-(2-furanmethoxy)propyltrimethoxysilane or N-(4-maleimidephenyl)-3-(trimethoxysilyl)propylamine.

3. The modified aramid according to claim 2, characterized in that: The thickness of the bifunctional molecular layer is 5-100 nanometers.

4. The modified aramid according to any one of claims 1 to 3, characterized in that: The grafting density of the bifunctional molecular layer is 0.05~1.5 mmol / g fiber.

5. A method for preparing the modified aramid fiber as described in any one of claims 1 to 4, characterized in that, Includes the following steps: (1) Surface activation treatment is performed on the aramid fiber to introduce or expose hydroxyl or amino groups that react with the first active group (X) on its surface; (2) The activated fiber is reacted with the bifunctional coupling agent in the presence of a catalyst to bond the first active group (X) to the fiber surface, thereby grafting the bifunctional molecular layer.

6. The method according to claim 5, characterized in that: In step (1), the surface activation treatment is either low-temperature plasma treatment or alkaline solution treatment.

7. The method according to claim 5, characterized in that: In step (2), the catalyst is at least one of an organotin compound, a titanate, or an organic amine.

8. The method according to claim 5, characterized in that: The reaction in step (2) is carried out in an anhydrous organic solvent at a temperature of 40-80°C for 2-6 hours.

9. The method according to any one of claims 5 to 8, characterized in that, After the reaction in step (2) is completed, a post-processing step is also included: the reaction product is washed with organic solvent and alcohol in sequence and then dried.

10. The method according to claim 5, characterized in that: In step (2), the mass ratio of the bifunctional coupling agent to the fiber is (0.01~0.20):1.