Modified polyimide fiber as well as preparation method and application thereof

Abstract

The invention provides a modified polyimide fiber and a preparation method and application thereof, and the preparation method comprises the following steps: S1, adding a polyimide fiber into an alkali solution for reaction, cleaning a reactant with acid and alkaline alcohol, and drying to obtain an activated polyimide fiber; s2, adding the activated polyimide fiber into an alkaline alcohol solution, adding into a first silane coupling agent, carrying out modification reaction, adding modified epoxy resin, and mixing to obtain a bottom layer component; s3, adding the activated polyimide fiber into an alkaline alcohol solution, adding a second silane coupling agent and a hyperbranched polymer, and carrying out a modification reaction to obtain a surface layer component; and S4, covering the bottom layer component with the surface layer component, drying to obtain the modified polyimide fiber, and optimizing the coating to obtain the modified polyimide fiber with high mechanical property, excellent atomic oxygen resistance and ultraviolet resistance and environmental adaptability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new materials, in particular to a modified polyimide fiber and a preparation method and application thereof. BACKGROUND

[0002] In special environment operations such as aerospace low earth orbit (LEO) environment, the strong oxidation under the action of special environment will gradually etch the surface of the material, resulting in mass loss and structural failure; the extremely high dose of ultraviolet (UV) radiation is also easy to cause high molecular chain rupture, aging and embrittlement; at the same time, accompanied by sudden high-low temperature cycle, and strong friction in equipment operation, multiple severe conditions put forward high requirements on the material performance of individual protective clothing and space tether materials.

[0003] Polyimide (PI) fiber has become a core candidate material for coping with the above-mentioned environment due to its high strength, high temperature resistance and other excellent properties, and is widely used in aerospace, nuclear industry and other high-end fields. However, the inherent defects of PI fiber limit its service life in special environment: on the one hand, the fiber surface is smooth, the surface energy is low, and the chemical inertness is strong, which cannot form a stable interfacial bonding phase with the reinforcing material (such as resin matrix), which leads to the interfacial peeling of PI fiber under repeated friction or external impact; on the other hand, when PI fiber is exposed to atomic oxygen and ultraviolet radiation environment for a long time, the imine ring in the PI molecular chain is easy to be oxidized and broken, and etching pits and cracks will appear on the surface, resulting in a sharp decrease in mechanical properties.

[0004] At present, a large number of research works on PI material modification mainly focus on molecular design and processing modification of material structure, but lack of research on surface activity improvement. The methods to solve the inertness of PI material surface include PI alkali etching pretreatment, high-energy ray treatment and plasma treatment, but these methods will reduce the mechanical properties and high and low temperature resistance of PI material.

[0005] Therefore, it is of great practical significance to realize the synergistic improvement of PI fiber in "anti-erosion-strong interface-high wear resistance" through material modification, to improve the environmental adaptability and service life of PI material, and to develop polyimide fiber material with high mechanical properties, excellent atomic oxygen resistance and ultraviolet resistance functional environmental adaptability. SUMMARY

[0006] The purpose of the present application is to provide a modified polyimide fiber and a preparation method and application thereof, which can obtain a modified polyimide fiber with high mechanical properties, excellent atomic oxygen resistance and ultraviolet resistance functional environmental adaptability by optimizing the coating.

[0007] To achieve the above purpose, the technical scheme provides a preparation method of a modified polyimide fiber, comprising the following steps: S1: adding polyimide fibers into an alkali solution for reaction, and drying the reactant after cleaning with acid and basic alcohol to obtain activated polyimide fibers; S2: adding the activated polyimide fibers into a basic alcohol solution and adding silane coupling agent 1 for modification reaction, and then adding modified epoxy resin to obtain the bottom component; S3: adding the activated polyimide fibers into a basic alcohol solution and adding silane coupling agent 2 and hyperbranched polymer for modification reaction to obtain the surface component; S4: covering the surface component to the bottom component and drying to obtain the modified polyimide fibers. The polyimide fibers are washed with alkali in the present scheme to introduce carboxyl active sites on the surface of the polyimide fibers, and the bottom component and the surface component are respectively resisted, the bottom component of the polyimide fibers comprises polyimide fibers, a first silane coupling agent and a modified epoxy resin to enhance the interface component through chemical bonding, and the surface component of the polyimide fibers comprises polyimide fibers, a second silane coupling agent and a hyperbranched polymer, the surface utilizes phosphorus passivation and a siloxane network physical barrier to resist atomic oxygen and ultraviolet erosion, so that the modified polyimide fibers of the present scheme can realize multiple protection of the polyimide fibers through the synergistic design of the bottom and the surface.

[0008] In step S1 of the present scheme, the polyimide fibers after ultrasonic cleaning are subjected to surface alkali treatment to selectively hydrolyze the imine ring on the surface of the polyimide fibers by an alkali solution to introduce active groups, providing "anchor points" for subsequent chemical bonding with silane coupling agent 1, solving the problem of chemical inertness of the PI fiber surface, and avoiding the defect that the subsequent coating is easily detached by physical adsorption.

[0009] In step S1, the alkali solution is selected as a 1-10.0 mol / L sodium hydroxide solution, and the polyimide fibers after ultrasonic cleaning are added into the sodium hydroxide solution and maintained at 20-80°C for 2-60 min. Preferably, in some embodiments, the polyimide fibers after ultrasonic cleaning are added into a 4.0 mol / L sodium hydroxide solution and maintained at 30°C for 5 min.

[0010] In some embodiments, the activated polyimide fibers are obtained by cleaning the reactant with acid and ethanol and drying.

[0011] In some embodiments, the activated polyimide material is obtained by cleaning the reactant with 0.1 mol / L dilute hydrochloric acid and anhydrous ethanol and drying at 75-85°C for 10-14 h.

[0012] In some embodiments, the polyimide material after ultrasonic cleaning is added into a 4.0 mol / L sodium hydroxide solution and maintained at 30°C for 5 min, the reactant is cleaned with 0.1 mol / L dilute hydrochloric acid and anhydrous ethanol, and then dried at 80°C for 12 h to obtain the activated polyimide material.

[0013] Step S2 of the present scheme is to use the amino group (-NH2) at one end of the first silane coupling agent to react with the carboxyl group on the surface of the PI fiber to form a stable amide bond (-CONH-), so that the first silane coupling agent is "anchored" on the fiber surface through a chemical bond; the other end of the silane group remains active, and the silane group reacts with the modified epoxy resin to obtain a bottom component that enhances the interface component through chemical bonding.

[0014] In step S2, the first silane coupling agent is selected as aminotriethoxysilane. Preferably, the first silane coupling agent is selected as 3-aminopropyl triethoxysilane.

[0015] In step S2, the ratio of the activated polyimide fiber, the first silane coupling agent, and the modified epoxy resin is 1-2 g: 3-5 mL: 5-8 mL, which has the advantage of controlling the number of reactive sites to avoid the complete occupation of the amide functional group after the ring-opening reaction of the imide ring in the polyimide by the siloxane.

[0016] In step S2, the activated polyimide fiber is added to the basic alcohol solution and the silane coupling agent to perform the modification reaction, wherein the solid content of the activated polyimide fiber in the basic alcohol solution is 0.2-2wt%; the reaction conditions of the modification reaction are: stirring reaction in a closed environment at 20-80℃ for 4-20h.

[0017] In step S2, the modified epoxy resin is an acrylate-modified epoxy resin.

[0018] The preparation method of the acrylate-modified epoxy resin is as follows: (1) high-purity acrylate is prepared by reacting bisphenol A diglycidyl ether with triphenylphosphine and acrylic acid at a temperature of 50-100℃ for 1-10h; (2) 0.01%-1% of the first silane coupling agent based on the mass of the high-purity acrylate, and 0.01%-1wt% of dibutyltin dilaurate based on the mass of the high-purity acrylate are added to the high-purity acrylate, and the mixture is reacted at 20-80℃ for 1-8h. After the reaction is completed, the first silane coupling agent, anhydrous ethanol, and distilled water are added to the above reaction mixture and mixed uniformly.

[0019] In some embodiments, the mixture of bisphenol A diglycidyl ether and triphenylphosphine is heated to 70°C under an inert environment of inert gas, and then the acrylic acid is added dropwise, and the reaction is carried out at 80°C for 4h to obtain a transparent viscous liquid. Further, the transparent viscous liquid is subjected to five water washing in sequence to separate the unreacted acrylic acid (upper clear liquid), and then three gradient cleaning is carried out using a mixed solvent of acetone and cyclohexane to effectively remove the catalyst triphenylphosphine (TPP) and residual esterification by-products. Finally, the purified product is transferred to a vacuum drying oven, and dried at 30°C for 8h to completely remove trace amounts of water and organic solvents, thereby obtaining a high-purity acrylic ester (EA) product.

[0020] In some embodiments, the volume ratio of the first silane coupling agent, anhydrous ethanol, and distilled water is 1-5: 2-12: 1-4.

[0021] In step S3 of the present scheme, diethylphosphoethyl triethoxysilane is used as the phosphorus-containing silane coupling agent 3, and the phosphorus element can form a phosphate passivation layer on the surface layer of the coating to resist atomic oxygen erosion.

[0022] In step S3, the hyperbranched polymer is a hyperbranched polysiloxane.

[0023] In step S3, the second silane coupling agent is a phosphorus-containing silane coupling agent, preferably diethylphosphoethyl triethoxysilane.

[0024] In some embodiments, the ratio of the activated polyimide fiber, the second silane coupling agent, and the hyperbranched polymer is 1-2 (g): 4-6 (mL): 6-10 (mL), which has the advantage of controlling the amount of substance of the reaction, and too much siloxane and hyperbranched substance on the polyimide will cause the material mass to increase significantly, which is not conducive to application.

[0025] Preferably, the molar ratio of the hyperbranched polymer and the second silane coupling agent is 1:0.5. The hyperbranched polymer refers to the silane-modified acrylic ester, which itself forms a hyperbranched polymer between the reaction products during the S3 step reaction.

[0026] In step S4 of the present scheme, the bottom layer component is dried to a semi-dry state, the surface layer component is covered on the bottom layer component, and the modified polyimide fiber is obtained after drying.

[0027] In some embodiments, the drying temperature is 80-150°C, and the drying time is 10-600 min.

[0028] In the second aspect, the modified polyimide fiber prepared by the scheme has a bottom layer component and a surface layer component, wherein the bottom layer component comprises polyimide fiber, silane coupling agent and modified epoxy resin, and the surface layer component comprises polyimide fiber, diethylphosphoryl ethyl triethoxysilane and hyperbranched polymer.

[0029] In some embodiments, the absolute dry basis of the bottom layer component is 1-4 g / m 2 , and the absolute dry basis of the surface layer component is 0.5-2 g / m 2 .

[0030] In the third aspect, the scheme provides an application of the modified polyimide fiber, which is applied to individual protection and aerospace field.

[0031] In some embodiments, when the modified polyamide material is used as a satellite tether material, the stress layer and the outer protective sleeve of the tether are braided by the modified polyimide fiber, the outer surface of the protective sleeve prepared by the modified polyimide fiber has a coating, the erosion rate of the modified polyimide fiber to atomic oxygen is ≤0.07×10 -24 cm 3 / atom, and the fiber mechanical property retention rate is ≥97% after 120h of ultraviolet irradiation.

[0032] Compared with the prior art, the technical scheme has the following characteristics and beneficial effects: The modified polyamide material provided by the scheme effectively solves the protection problem of PI fiber in space environment through the synergistic design of bottom layer chemical bonding and surface layer double barrier. The bottom layer forms stable chemical bonding between PI fiber and modified epoxy resin with the help of the "bridge" effect of silane coupling agent, avoiding physical peeling of the coating; the phosphorus element introduced by the phosphorus-containing silane coupling agent forms a phosphate passivation layer, which actively consumes atomic oxygen energy, and the siloxane network constructed by the hyperbranched polymer forms a dense physical barrier to block the penetration of ultraviolet radiation and atomic oxygen impact. The ground simulation experiment verifies that the erosion rate of the modified PI fiber to atomic oxygen is ≤0.07×10⁻² 4 cm³ / atom, and the mechanical property retention rate is ≥97% after 120h of ultraviolet irradiation, which is several times higher than that of the unmodified PI fiber, and can meet the long-term service requirements in extreme space environments such as low earth orbit.

[0033] In addition, the present scheme realizes the synchronous improvement of multiple performances by accurately controlling process parameters: the alkali treatment only selectively activates the surface of the PI fiber to introduce carboxyl groups without damaging the molecular chains of the core layer, which, in combination with the chemical bonding coating structure, maximally retains the original high-strength characteristics of the PI fiber; the modification of the epoxy resin and the hyperbranched polymer enhances the interface bonding while further optimizing the wear resistance and thermal stability of the material, and the final product not only has excellent atomic oxygen resistance and ultraviolet resistance functions, but also maintains the mechanical strength and high-temperature resistance advantages of the PI fiber, breaking the bottleneck of the existing modification technology "trade-off".

[0034] Moreover, the modified polyimide fiber provided by the present scheme has wide applicability. On the one hand, it is suitable for PI fibers of different specifications (such as filaments, short fibers, and fibers for ropes), and does not need to adjust the core process for specific forms, and can cover multiple application scenarios such as individual protective clothing and satellite ropes. On the other hand, by adjusting the ratio of the surface phosphorus-containing silane coupling agent and the hyperbranched polymer, the protection strength can be flexibly controlled to adapt to different space environment requirements such as low earth orbit and deep space exploration. At the same time, the raw materials used are all industrialized mass production products, and the reaction conditions are mild (room temperature / medium temperature, normal pressure), which can be compatible with the existing PI fiber processing production line without the need for large-scale equipment modification, providing strong support for technology landing and large-scale application. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is the infrared spectrum of the PI fiber rope modified by different DEPTES concentrations.

[0036] Figure 2 is the XPS spectrum of the HPPS-EA-PI#2 fiber rope.

[0037] Figure 3 is the SEM morphology of the PI rope modified by different DEPTES concentrations.

[0038] Figure 4 is the AFM morphology of the PI fiber rope modified by different DEPTES concentrations.

[0039] Figure 5 is the surface roughness change graph of the PI fiber rope modified by different DEPTES concentrations.

[0040] Figure 6 is the thermal performance curve of the PI fiber rope modified by different DEPTES concentrations.

[0041] Figure 7 is the DSC curve of the PI fiber rope modified by different DEPTES concentrations.

[0042] Figure 8 is the mechanical property graph of the PI fiber rope treated by different DEPTES contents.

[0043] Figure 9 is the tether before and after AO / UV irradiation: a. after irradiation, b. before irradiation.

[0044] Figure 10 is the SEM morphology of the PI fiber tether after the protection force test of different concentrations of diethylphosphoryl ethyl triethoxysilane (DEPTES) modification.

[0045] Figure 11 is the mechanical property and mass loss graph of the PI tether before and after irradiation with different DEPTES contents. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art belong to the scope of protection of the present application.

[0047] The ultrasonically cleaned PI fiber was placed in a 4.0 mol / L sodium hydroxide (NaOH) solution, maintained at 30°C for 5 min, then washed with 0.1 mol / L dilute HCl and anhydrous ethanol for several times, and dried in an 80°C oven for 12 h. Then it was placed in a 3-aminopropyl triethoxysilane (APTES) solution and maintained at 60°C for 5 h.

[0048] 0.029 mol of bisphenol A diglycidyl ether and 0.2 g of triphenylphosphine (TPP) were added to a four-necked round-bottom flask, stirred thoroughly for 30 min, then slowly heated to 70°C under nitrogen atmosphere, and 4 mL of acrylic acid was added dropwise. After continuous reaction at 80°C for 4 hours, the system gradually changed into a transparent viscous liquid. After the reaction was completely terminated, the product was washed with distilled water for five times to separate the unreacted acrylic acid (upper clear liquid), then washed with a mixture of acetone and cyclohexane for three times to effectively remove the catalyst triphenylphosphine (TPP) and residual esterification by-products. Finally, the purified product was transferred to a vacuum drying oven, dried at 30°C for 8 hours to completely remove trace amounts of water and organic solvents, and a high-purity acrylic ester (EA) product was obtained.

[0049] Diethylphosphatoethyl triethoxysilane (DEPTES) was used as a typical phosphorus-containing compound. The EA molar ratio was 0, 0.25, 0.5, 0.75, and 1. The mixture solution containing EA and a small amount of catalyst DBTDL was added at 40°C and kept for 3 hours. After the reaction, anhydrous ethanol and distilled water were added to the above reaction mixture in a volume ratio of DEPTES: anhydrous ethanol: distilled water 1:5:2.2, and mixed uniformly to obtain a transparent and uniform liquid, The PI fibers treated with NaOH were placed in the above-mentioned mixed solution, and hydrochloric acid was added dropwise until the pH value was 4-5. The reactor was heated to 60°C and kept for 5 hours. The bottom component was coated on the surface component by dipping, soaking, spraying, etc. and placed at 80°C for 12 hours. Then the PI fibers were taken out and washed with anhydrous ethanol and distilled water, and dried at 80°C for 12 hours to obtain modified PI fibers.

[0050] The modified fiber samples were denoted as HPPS-EA-PI#0, HPPS-EA-PI#1, HPPS-EA-PI#2, HPPS-EA-PI#3, and HPPS-EA-PI#4, with DEPTES molar concentration from 0 to 100%.

[0051] Example one: Effect of different diethylphosphatoethyl triethoxysilane (DEPTES) concentration modification treatment Figure 1 The FTIR analysis results of PI tether modified by different diethylphosphatoethyl triethoxysilane (DEPTES) concentration modification treatment were shown. According to the analysis results, with the increase of DEPTES concentration, in addition to the characteristic peaks of PI matrix at 1 773 cm -1 (imide C=O asymmetric stretching vibration), 1 360 cm -1 (C-N stretching vibration), and 820 cm -1 (imide ring deformation vibration), the following new characteristic peaks were observed in the modified samples: a strong C-P bond stretching vibration peak at 750 cm -1 , a significant Si-O-Si bond anti-symmetric stretching vibration peak at 1095 cm -1 , a Si-C bond symmetric stretching vibration peak at 690 cm -1 , and a Si-O bond bending vibration peak at 460 cm -1 . The intensity of these characteristic peaks increased significantly with the increase of diethylphosphatoethyl triethoxysilane (DEPTES) concentration gradient (most prominent in HPPS-EA-PI#2 sample), indicating that the modified coating formed a dense coating layer on the PI surface through chemical reaction, rather than simple physical adsorption.

[0052] To analyze the surface chemical state, XPS was used to characterize the modified PI fiber ropes. As shown in Figure 2 Figure 2 The survey spectrum of (a) in FIG. 5 shows that the coating contains five elements: C, N, O, Si and P. Figure 2 The C1s fine spectrum of (b) in FIG. 5 is decomposed into five characteristic peaks: 284.7 eV (C-H), 285.3 eV (C-P), 285.4 eV (C-N), 286.4 eV (C-O) and 288.7 eV (C=O), which confirms the chemical bonding of phosphorus element with the carbon skeleton. Figure 2 In the Si2p spectrum of (c) in FIG. 5, the peak at 102.2 eV (Si-OH) and the main peak at 103.2 eV (Si-O-Si) indicate that the siloxane network forms a high cross-linked structure through hydrolysis and condensation reaction, Figure 2 The P2p of (d) in FIG. 5 is further decomposed into double peaks at 133.2 eV (P-R) and 134.1 eV (P-O), which verifies that the phosphorus element participates in the construction of the passivation layer through P-O bond. The above results systematically prove the chemical grafting reaction of HPPS-EA coating on the PI surface and its anti-erosion mechanism.

[0053] Example Two: Surface Structure of HPPS-EA Modified PI Fiber The surface morphology and appearance of the modified PI rope fiber samples were intuitively analyzed by SEM, Figure 3 (a) in FIG. 6 is PI#1, the surface is smooth; from Figure 3 (b) to Figure 3 (f) in FIG. 6, with the increase of the concentration of diethylphosphoryl ethyl triethoxysilane (DEPTES) in the coating, a small amount of particulate matter adheres to the surface of HPPS-EA-PI#0 rope, making it uneven; the deposition on the surface of HPPS-EA-PI#1 starts to increase, but it still does not form a complete coating layer; a relatively uniform and complete coating layer is formed on the surface of HPPS-EA-PI#2; on the basis of complete coating, some particles with slight protrusions appear on the surface of HPPS-EA-PI#3; a large amount of accumulation is formed on the surface of HPPS-EA-PI#4, and the flatness of the surface layer is poor, which may cause instability of the interface and affect the performance of the rope.

[0054] The evolution of surface roughness was further quantitatively analyzed by AFM as shown in Figure 4 and Figure 5 ​As shown, the results show that as the diethylphosphatoethyl triethoxysilane (DEPTES) concentration increases from 0 to 100%, the surface root mean square roughness (Rq) gradually increases from 203 nm to 536 nm. In the low concentration stage (HPPS-EA-PI#0, HPPS-EA-PI#1), the slow increase in roughness is caused by the random distribution of nanoscale particles; in the medium concentration stage (HPPS-EA-PI#2), the uniform coating formation stabilizes the roughness at 384 nm, representing the best coating effect; as shown in Figure 5 the high concentration stage (HPPS-EA-PI#3, HPPS-EA-PI#4), particle agglomeration and accumulation cause a sharp increase in roughness, consistent with the SEM observation of the morphology deterioration trend. In summary, when the molar concentration of DEPTES is 50% (HPPS-EA-PI#2), the coating on the PI surface achieves a balance between nanoscale dense coverage and roughness optimization, which is the optimal parameter for the modification process.

[0055] Thermal stability of modified PI fibers Figure 6 TG curves of PI ropes modified with different diethylphosphatoethyl triethoxysilane (DEPTES) contents, Figure 6 DTG curves of PI ropes modified with different DEPTES contents. Compared with unmodified PI (PI#1), the initial thermal decomposition temperature of all samples containing DEPTES (0-100%) shows a downward trend, indicating that the introduction of silane coupling agent reduces the thermal stability of the material. At around 400°C, all samples show a significant weight loss step (weight loss rate > 30%), which is due to the thermal cracking of organic components (such as ether bonds and ester groups) in the bisphenol A epoxy acrylate resin backbone. Notably, the high-temperature carbon residue rate at 700°C shows a fluctuating growth trend with the DEPTES content, specifically: 61.2% (no DEPTES), 62.1% (25%), 64.4% (50%), 63.5% (75%), and 61.9% (100%). This growth is due to the different contents of the silane coupling agent diethylphosphatoethyl triethoxysilane (DEPTES) in the system. The increased decomposition temperature and residual weight with increasing silane coupling agent content indicate that the thermal stability of the DEPTES-modified PI rope is slightly lower than that of the original PI rope, but will improve with increasing silane coupling agent content. From the DTG curve, it can also be seen that the modified PI rope degrades partially at 400°C, the degradation rate is fastest at 600°C, and the mass loss rate of the modified rope is smallest at a DEPTES concentration of 75%.

[0056] Further analysis of the segment motion characteristics of the modified coating by differential scanning calorimetry (DSC), as shown in Figure 7As shown, the results show that the glass transition temperature (T g ) increases monotonously with the increase of phosphorus content: the T g of the sample without phosphorus (HPPS-EA-PI#0) is 267.43 °C, while the T g of the sample with high phosphorus content (HPPS-EA-PI#4) increases to 308.71 °C, which is 41.28 °C higher than that of the unmodified PI (PI#1). This significant increase is attributed to the strong polar effect of P=O bond in the coating: the induced intermolecular dipole-dipole interaction increases the main chain rigidity, while the p-π conjugation effect of phosphorus atom lone pair electrons and PI aromatic ring further restricts the segment motion. Although the introduction of DEPTES slightly reduces the thermal decomposition temperature, the modified PI tether still maintains excellent thermal stability (char yield > 60%, T g > 300 °C), which meets the requirements of space applications. Overall, the thermal stability of the modified PI fiber tether remains good.

[0057] Example Four Mechanical properties of modified PI fiber tether Figure 8 The mechanical properties of modified PI tether with different DEPTES content are shown. With the increase of diethylphosphoryl ethyl triethoxysilane (DEPTES) molar concentration from 0 to 100%, the tensile strength of the modified tether shows a non-monotonic change trend of first increasing and then decreasing. In the low concentration stage (HPPS-EA-PI#0, HPPS-EA-PI#1 and HPPS-EA-PI#2), a uniform interface is formed between the coating and the PI matrix through chemical bonding, which promotes the increase of tensile strength. However, when the DEPTES concentration exceeds 50% (HPPS-EA-PI#, HPPS-EA-PI#4), the tensile strength decreases significantly due to the interface failure mechanism caused by excessive coating: coating agglomeration effect: high concentration of DEPTES leads to too fast hydrolysis and condensation rate of siloxane precursor, local agglomeration of nanoparticles forms micron-level protrusions (confirmed by SEM observation); stress concentration factor: uneven coating distribution causes distortion of load transfer path, local stress concentration factor increases, accelerating crack generation; interface weakening: the mismatch of thermal expansion coefficient of the thick coating induces interface micro-cracks, reducing the interface shear strength (quantitatively characterized by AFM phase diagram).

[0058] The above results show that the concentration of diethylphosphoryl ethyl triethoxysilane (DEPTES) needs to be controlled within 50% (HPPS-EA-PI#2) to achieve an optimal balance between coating uniformity, interface bonding strength and mechanical bearing performance.

[0059] Example Five Coating protection force test: To evaluate the space environment adaptability of the modified coating, this study designed an accelerated aging experiment (test parameters are shown in Table 1 below; the modified PI tether was subjected to an equivalent 120-hour space environment simulation (atomic oxygen flux: 4.32 × 10⁻⁶)). 20 atoms / cm 2 After ultraviolet irradiation dose of 120 ESH, its protective efficacy was verified by morphological and mechanical property analysis.

[0060] Table 1. Coating Protective Strength Test Data .

[0061] The modified PI tether was placed on a tray and put into a vacuum chamber. After atomic oxygen and near-ultraviolet irradiation, the tether was removed and compared with the tether before and after AO / UV irradiation. Figure 9 As shown, after irradiation for an equivalent time of 120 hours in space, the surface of the tether becomes noticeably rough and dull.

[0062] To investigate the surface changes of the tether, the SEM morphology of the irradiated tether protective sleeve was observed, and the results are as follows: Figure 10 As shown, Figure 10 Image (a) shows that the surface of the unmodified PI tether has been eroded into a planar shape, exposing the internal fiber structure; with increasing DEPTES concentration in the modified coating, surface erosion begins from HPPS-EA-PI#0, see [image / description]. Figure 10 In (b), because only a small amount of polysiloxane particles are generated on the surface of the rope at this time, a large area of ​​damage occurs on the surface, but the original shape of the fiber can still be seen; under HPPS-EA-PI#1 conditions, see Figure 10 In (c), the cracks on the surface of the tether have been significantly reduced, and it can be seen that the coating still covers most of the tether area; by HPPS-EA-PI#2, see Figure 10 In (d), it can be seen that atomic oxygen and ultraviolet irradiation only eroded micron-sized pores on the surface of the tethering fiber, while the main fiber structure remained intact; Figure 10 In (e), the HPPS-EA-PI#3 sample shows dense but minute cracks and pores on its surface, yet the coating still completely covers the tether; under HPPS-EA-PI#4 conditions, see... Figure 10 In (f), due to the large accumulation of coating particles on the surface of the tether, the tether is subjected to concentrated stress during use and when exposed to radiation. The thinnest areas are eroded first, resulting in large-area damage.

[0063] Carefully separate the rope core from the rope sheath, and test the maximum tensile load and mass loss of the stressed layer at this point. The test results are as follows: Figure 11It can be seen that after the equivalent space time of 120 h of atomic oxygen and ultraviolet double irradiation erosion, the modified tether presents a unique performance evolution law in the presence of the modified outer protective sleeve: the retention rate of mechanical properties: with the increase of DEPTES concentration, the mechanical properties of each tether before and after irradiation appear a downward trend, among them, the maximum tensile load of HPPS-EA-PI#2 is 2 025 N, which is only decreased by 2.36% compared with the PI tether without irradiation, and the performance retention rate is increased by 76.91% compared with the sharp attenuation of PI#1 (1 862 N, 10.22% reduction). The mass loss rate: with the increase of DEPTES concentration, it presents a "V" type change, and HPPS-EA-PI#2 reaches the minimum value of 0.07%, which is increased by 97.1% compared with the unmodified sample (PI#1: 2.39%), verifying the significant inhibition effect of the coating on AO / UV erosion.

[0064] The research team of this study explored the anti-atomic oxygen mechanism and anti-ultraviolet mechanism of the modified polyimide fiber, and believed that the anti-atomic oxygen mechanism of the modified polyimide fiber was mainly due to the synergistic effect of the formation of phosphate passivation layer and siloxane network. Among them, the phosphorus element (such as P-O bond in diethyl phosphonylethyl triethoxysilane (DEPTES)) in the coating reacts with AO under AO exposure to generate a stable phosphate layer (such as POx). The passivation layer covers the surface of the PI fiber, forming a chemical barrier that effectively prevents further erosion of the PI matrix. XPS analysis shows the presence of P-O bond (134.1 eV) in the coating, confirming the chemical passivation effect of phosphorus; while the polysiloxane (Si-O-Si) coating forms a dense network structure (SEM shows uniform coating) on the surface of PI through chemical bonding, directly blocking the physical impact of AO. The high thermal stability and mechanical strength of siloxane further enhance the durability of the coating, i.e. the siloxane layer provides physical isolation, and the phosphorus element consumes AO energy through chemical passivation, both of which synergistically reduce the AO erosion rate. Ground simulation experiments show that the mass loss rate of HPPS-EA-PI#2 is only 0.07%, verifying the effectiveness of the synergistic mechanism.

[0065] The anti-ultraviolet mechanism of the modified polyimide fiber is mainly as follows: 1. Absorption and scattering of ultraviolet energy. The phosphorus structure (such as P=O bond) and siloxane network in the coating can absorb or scatter ultraviolet light, reducing the direct damage of UV to the PI molecular chain. DSC results show that the glass transition temperature (T g) a significant increase (from 267 °C to 308 °C), indicating that the molecular chain movement is limited, the material heat resistance is improved, and the anti-UV ability is indirectly enhanced. 2. Coating integrity is maintained. Uniform and dense coating (AFM shows that the roughness is optimized to 391 nm) reduces the expansion of UV-induced surface cracks. SEM shows that only a few holes appear on the surface of HPPS-EA-PI#2 after irradiation, while the surface of the unmodified PI fiber has been severely eroded. 3. Chemical bond stability. The high bond energy of Si-O-Si and P-O bonds (Si-O bond energy ~ 452 kJ / mol, P-O bond energy ~ 335 kJ / mol) enhances the chemical stability of the coating, delaying UV-induced oxidative degradation. 4. The phosphorus-containing polysiloxane coating significantly improves the AO / UV resistance of PI fibers through the dual mechanisms of chemical passivation (phosphorus element) and physical barrier (siloxane). The introduction of phosphorus elements forms a stable phosphate layer, consuming AO energy; the siloxane network provides a dense protective layer, reducing UV penetration. The synergistic effect of the two makes the coating maintain high durability in extreme space environments, providing a reliable solution for the application of PI fibers in the aerospace field.

[0066] In summary, with diethylphosphoryl ethyl triethoxysilane (DEPTES) as the coupling agent, an hyperbranched organic / inorganic composite coating was constructed, and phosphorus elements were introduced to form a phosphate passivation layer to resist atomic oxygen erosion. Characterization by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed that the surface of the modified tether fiber was successfully grafted with -Si-O- and P=O functional groups. And with the increase of DEPTES modified PI fiber tether concentration, the grafting rate and surface coverage of the coating showed a positive correlation, and the surface roughness showed a non-monotonic change of first increasing and then decreasing, among which the DEPTES molar ratio 50% hyperbranched-modified epoxy resin-polyimide (HPPS-EA-PI) #2 sample showed the optimal surface integrity; the glass transition temperature was significantly increased by 41 °C, and the thermal stability was effectively improved; the tensile strength showed a parabolic change with the coating integrity, reaching a peak at HPPS-EA-PI#2, and excessive coating caused stress concentration effect leading to performance decline.

[0067] (1) The molecular structure and surface morphology of the modified material are clear. PI fiber is treated with 4 mol / L NaOH solution for 5 min to increase the content of carboxyl groups on the surface of PI, providing active sites for subsequent grafting and coupling agent reaction. DEPTES is used as a functional coupling agent to construct a Si-O-Si network and phosphonyl group (P=O) through hydrolysis and condensation reaction. FTIR shows 1095 cm -1The Si-O-Si characteristic peak was significantly enhanced, and the P-O bond characteristic peak (binding energy 134.1 eV) was detected by XPS, confirming that the phosphorus element was successfully introduced. When the DEPTES molar concentration was 50% (HPPS-EA-PI#2 sample), the coating coverage was more than 95% (observed by SEM), the surface roughness reached the optimal value (Ra=328 nm), and a continuous and dense protective layer was formed.

[0068] (2) The thermal stability and mechanical properties were effectively maintained. The HPS-EA-PI#3 fiber friction cycle number increased by 190% (6095 times), indicating that the hyperbranched structure effectively alleviated stress concentration and enhanced wear resistance. TG showed that the residual amount of the modified HPPS-EA-PI#2 tether at 700 ℃ increased to 64.4% (61.2% for the original PI tether), and the T g from 267 ℃ to 308 ℃, attributed to the synergistic effect of the phosphoryl group and the Si‒O‒Si network. Mechanical testing showed that the tensile strength of the HPPS‒EA‒PI#2 tether reached 3 694 N, only 1.89% lower than that of the unmodified PI (3 765 N).

[0069] (3) The atomic oxygen and ultraviolet irradiation resistance performance was improved. Through ground simulation experiments (atomic oxygen flux 4.32×10 20 atoms / cm 2 , ultraviolet irradiation 57.6 mW / cm 2 ), the HPPS-EA-PI#2 tether after equivalent 120 hours of LEO exposure: the mass loss rate was only 0.07%, which was 97.10% higher than that of the unmodified PI (2.39%); the mechanical property retention rate reached 97.64% (tensile strength decreased from 2 074 N to 2 025 N); the interface stability was excellent, and the fiber-coating bonding strength remained 92%, attributed to the synergistic protection mechanism of Si-O covalent bond and phosphate layer.

[0070] Those skilled in the art should understand that the technical features of the above embodiments can be combined in any way. In order to make the description concise, all possible combinations of the technical features in the above embodiments are not described, but as long as the combinations of the technical features do not exist, they should be considered as the scope of the description.

[0071] The above embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the present application. It should be noted that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A method for producing a modified polyimide fiber, characterized by, The method comprises the following steps: S1: adding the polyimide fiber cleaned by ultrasonic to an alkali solution for reaction, drying the reactant after cleaning with acid and basic alcohol to obtain activated polyimide fiber; S2: adding the activated polyimide fiber to a basic alcohol solution and adding a first silane coupling agent for modification reaction, and then adding modified epoxy resin to obtain a bottom component; S3: adding the activated polyimide fiber to a basic alcohol solution and adding a second silane coupling agent and hyperbranched polymer for modification reaction to obtain a surface component; S4: covering the surface component to the bottom component and drying to obtain modified polyimide fiber.

2. The method for producing a modified polyimide fiber according to claim 1, characterized by, The ratio of the activated polyimide fiber, the first silane coupling agent and the modified epoxy resin is 1-2 g: 3-5 mL: 5-8 mL.

3. The method of producing a modified polyimide fiber according to claim 1, characterized by, The first silane coupling agent is selected to be aminotriethoxysilane, and the modified epoxy resin is an acrylate modified epoxy resin.

4. The method of producing a modified polyimide fiber according to claim 1, characterized by, The preparation method of the acrylate modified epoxy resin is as follows: (1) high-purity acrylate is prepared by reacting bisphenol A diglycidyl ether with triphenylphosphine and acrylic acid at a temperature of 50-100℃ for 1-10 h; (2) 0.01%-1% of the silane coupling agent based on the mass of the high-purity acrylate and 0.01%-1wt% of dibutyltin dilaurate based on the mass of the high-purity acrylate are added to the high-purity acrylate, and the mixture is reacted at 20-80℃ for 1-8 h; after the reaction is completed, the first silane coupling agent, anhydrous ethanol and distilled water are added to the above reaction mixture and mixed uniformly.

5. The method of producing a modified polyimide fiber according to claim 1, characterized by, The ratio of the activated polyimide fiber, the second silane coupling agent and the hyperbranched polymer is 1-2 g: 4-6 mL: 6-10 mL.

6. The method of producing a modified polyimide fiber according to claim 1, characterized by, The second silane coupling agent is a phosphorus-containing silane coupling agent.

7. The method of producing a modified polyimide fiber according to claim 1, characterized by, The hyperbranched polymer is hyperbranched polysiloxane.

8. A modified polyimide fiber, characterized by, The modified polyimide fiber is prepared by the method according to any one of claims 1-7, and has a bottom component and a surface component, wherein the bottom component comprises polyimide fiber, a first silane coupling agent and a modified epoxy resin, and the surface component comprises polyimide fiber, a second silane coupling agent and a hyperbranched polymer.

9. The modified polyimide fiber according to claim 8, characterized by, The underlayer component is 1-4 g / m2 in absolute dry basis 2 The surface layer component is 0.5-2 g / m2 in absolute dry basis 2 .

10. A method of using the modified polyimide fiber according to claim 8, characterized by, It is applied to individual protection and aerospace field.

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