A sizing agent for reinforcing the interface of PBO fiber composite material and its preparation method and application

Abstract

The application belongs to the field of surface and interface modification of composite materials, and particularly relates to a sizing agent for reinforcing the interface of PBO fiber composite material and a preparation method and application thereof. The sizing agent comprises polybenzoxazole nanofiber containing an amide bond and water. The benzoxazole structure nanofiber containing an amide bond has a similar structure to the PBO fiber, and when attached to the surface of the PBO fiber, exists π-π interaction between the PBO fiber. The sizing agent has high compatibility with the PBO fiber, and the preparation process of the nanofiber is simple, easy to operate and has high safety.

Description

Technical Field

[0001] This invention belongs to the field of surface and interface modification of composite materials, and particularly relates to a sizing agent for reinforcing the interface of poly(p-phenylenebenzobisoxazole) (PBO) fiber composite materials, its preparation method and application. Background Technology

[0002] PBO (Poly-p-phenylene benzobisoxazazole) fibers possess extremely high strength, excellent heat resistance, chemical corrosion resistance, and flame retardancy, making them one of the most ideal fiber reinforcements for resin-based composites. However, the smooth and chemically inert surface of PBO fibers results in extremely weak interfacial bonding in PBO fiber / resin composites, severely limiting their practical applications. Currently, various surface modification methods (chemical treatment, surface coating, plasma treatment, etc.) have been used to improve the surface activity of PBO fibers and have achieved certain results. Among these, surface coating, due to its advantages of simple operation, mild conditions, and no damage to the fibers, has become one of the most popular and easily industrialized surface treatment methods.

[0003] In recent years, with the continuous advancement of composite material technology and the expansion of its application fields, the method of introducing nanofibers into the interface of composite materials to improve interfacial properties has been widely studied. By constructing a nanofacial layer between the fiber and the matrix, the surface area can be significantly increased, mechanical interlocking can be improved, stress concentration at the interface can be reduced, and the interfacial properties of the composite material can be enhanced. For example, Song Bo et al. improved the interfacial shear strength of PBO fiber composites by 54.4% and the interlaminar shear strength by 48.1% by introducing carbon nanotubes at the interface (Song B, Liu Z, Chen L, et al. Poly(p-phenylene benzobisoxazole) Fiber / Epoxy Composites Reinforced with Carbon Nanotubes and Graphene Oxide for Enhanced Interfacial Adhesion and Mechanical Strength[J].ACS Applied Nano Materials, 2021, 4(11): 12158-12169.); Steinke K et al. improved the interfacial shear strength of UHMPE / epoxy resin composites by grafting zinc oxide nanowires onto the surface of ultra-high molecular weight polyethylene fiber (UHMPE) by 135% (Steinke K, Sodano H A. Enhanced interfacial shearstrength in ultra-high molecular weight polyethylene epoxy composites through a zinc oxide nanowire interphase[J]. Composites Science and Technology, 2022, 219: 109-218.; Jalal Nasser et al. improved the interfacial shear strength of the composite material by coating carbon fiber with aramid nanofiber solution by 33.7% (Nasser J, Zhang L, Sodano H. Aramid nanofiber interlayer for improved interlaminar properties of carbon fiber / epoxycomposites[J]. Composites Part B: Engineering, 2020, 197: 108-130.).

[0004] However, in these surface treatment processes, the nanocrystals are mostly inorganic or polymeric nanocrystals. The structure of these nanocrystals differs significantly from the bulk structure of PBO fibers, resulting in poor compatibility, weak bonding, and limited improvement in efficacy. Furthermore, the preparation conditions for these nanocrystals are demanding, with long reaction times, hindering industrial scale-up and large-scale production. Moreover, these surface treatment processes often use solvents such as N-methylpyrrolidone, dimethyl sulfoxide, acetone, and ethyl acetate, which pose flammable and explosive hazards when used in online processing, causing serious environmental pollution and threatening the health of production operators. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention starts from small molecules and uses a simple and efficient bottom-up method to synthesize a sizing agent containing a benzoxazole structure that can be dispersed in water. This agent can improve the compatibility between the interfacial layer and the PBO fiber matrix and prepare PBO fiber composite materials.

[0006] In a first aspect, the present invention provides a sizing agent comprising polybenzoxazole nanofibers containing amide bonds and water.

[0007] According to an embodiment of the present invention, the diameter of the polybenzoxazole nanofiber containing amide bonds is 15-30 nm, preferably 20-25 nm, for example 17 nm, 22 nm, 24 nm, 26 nm, or 28 nm.

[0008] According to an embodiment of the present invention, the intrinsic viscosity of the polybenzoxazole nanofibers containing amide bonds, when dissolved in concentrated sulfuric acid, is 2.78–3.24 dL / g.

[0009] According to an embodiment of the present invention, the mass concentration of polybenzoxazole nanofibers containing amide bonds in the sizing agent is 0.01 to 2 wt%, preferably 0.05 to 1.5 wt%, for example, any value from 0.01 wt%, 0.03 wt%, 0.04 wt%, 0.06 wt%, 0.08 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, or any value within a range formed by any two points.

[0010] Secondly, the present invention provides a method for preparing the above-mentioned polybenzoxazole nanofibers containing amide bonds, comprising the following steps:

[0011] S1. Preparation of polybenzoxazole containing amide bonds;

[0012] S2. The polybenzoxazole containing amide bonds is emulsified and sheared to form polybenzoxazole nanofibers containing amide bonds.

[0013] According to an embodiment of the present invention, step S1 includes the following steps: subjecting an acyl chloride compound, benzoxazole diamine or its derivative to a polycondensation reaction to obtain polybenzoxazole containing amide bonds.

[0014] According to an embodiment of the present invention, the molar ratio of the benzoxazole diamine or its derivative to the acyl chloride compound is 1:

[0015] (0.95~1.05), preferably the molar ratio of the benzoxazole diamine or its derivative to the acyl chloride compound is 1:(1~1.03).

[0016] According to an embodiment of the present invention, the polycondensation reaction in step S1 is carried out in an organic solvent, wherein the organic solvent is selected from one or more of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone, for example, N-methylpyrrolidone.

[0017] According to embodiments of the present invention, the benzoxazole diamine or its derivatives include 6-amino-2-(3-aminophenyl)benzoxazole, 2,1,3-benzoxadiazole-4,5-diamine, 2-(4-aminophenyl)-6-aminobenzoxazole, 2-(4-aminophenyl)-5-aminobenzoxazole, 5-amino-2-(3-aminophenyl)benzoxazole, 2,6-diaminobenzoxazole, 2,4-diaminobenzoxazole, 2,5-diaminobenzoxazole, 4-aminobenzoxazole-2-methylamine, 5-aminobenzoxazole-2-methylamine, 6-aminobenzoxazole-2-methylamine, 5-amino-2- One or more of (2'-chloro-4'-aminophenyl)-benzoxazole, 2,2'-p-phenyl-bis(6-aminobenzoxazole), and 2-(3,5-diaminophenyl)benzoxazole, such as 2-(4-aminophenyl)-5-aminobenzoxazole, 2,1,3-benzoxadiazole-4,5-diamine, 2-(4-aminophenyl)-6-aminobenzoxazole, 5-amino-2-(3-aminophenyl)benzoxazole, 6-amino-2-(3-aminophenyl)benzoxazole, 2,6-diaminobenzoxazole, 2,4-diaminobenzoxazole, or 2,2'-p-phenyl-bis(6-aminobenzoxazole).

[0018] According to embodiments of the present invention, the acyl chloride-containing compound includes one or more of terephthaloyl chloride, isophthaloyl chloride, 4,4'-diacyl chloride diphenyl ether, phthaloyl chloride, 2,6-naphthalenedilicate chloride, 5-amino-2,4,6-triiodoisophthaloyl chloride, 2,5-furandicarboxylate chloride, 2,6-pyridinedicarboxylate chloride, trans-3,6-methylene-1,2,3,6-tetrahydrophthaloyl chloride, and 2,3,5,6-tetrachloroterephthaloyl chloride, for example, terephthaloyl chloride, isophthaloyl chloride, 4,4'-diacyl chloride diphenyl ether, phthaloyl chloride, 2,6-naphthalenedilicate chloride, 5-amino-2,4,6-triiodoisophthaloyl chloride, 2,5-furandicarboxylate chloride, or 2,6-pyridinedicarboxylate chloride.

[0019] According to an embodiment of the present invention, the temperature of the polycondensation reaction is 20-60°C, preferably 30-50°C, for example 25°C, 35°C, 40°C, 45°C, 50°C, or 55°C.

[0020] According to an embodiment of the present invention, the polycondensation reaction time is 0.5 to 5 hours, preferably 1 to 4 hours, for example 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, or 4.5 hours.

[0021] According to an embodiment of the present invention, step S1 includes the following steps: in an inert atmosphere, a metal chloride is dissolved in an organic solvent, and a polycondensation reaction is carried out by sequentially adding a benzoxazole diamine or its derivative and an acyl chloride compound, and the polycondensation is stopped when the Weissenberg effect occurs.

[0022] According to an embodiment of the present invention, the inert atmosphere refers to a nitrogen, helium, or argon environment.

[0023] According to an embodiment of the present invention, the metal chloride is dissolved in an organic solvent at a temperature of 80-120°C, preferably at a temperature of 100-110°C, for example, 85°C, 95°C, 100°C, 105°C, or 115°C.

[0024] According to an embodiment of the present invention, before adding benzoxazole diamine or its derivative in step S1, the following steps are further included: cooling the organic solvent containing the metal chloride to below 5°C, preferably subjecting the organic solvent containing the metal chloride to an ice-water bath until the temperature is 0°C.

[0025] According to an embodiment of the present invention, the metal chloride functions by combining its free chloride ions with protons in the amide bond during the polymerization of acyl chloride and diamine, thereby weakening the hydrogen bonding forces between molecular chains and hindering the aggregation of molecular chains, thus increasing solubility. The metal chloride is selected from at least one of CaCl2, KCl, LiCl, NaCl, FeCl2, FeCl3, CuCl2, MgCl2, AlCl3, and ZnCl2.

[0026] According to an embodiment of the present invention, the concentration of the metal chloride in the organic solvent is 0.5 to 1 wt%, preferably 0.6 to 0.8 wt%.

[0027] According to an embodiment of the present invention, the step between S1 and S2 includes the following step: adding an organic solvent to dilute the reaction product of step S1.

[0028] Preferably, the organic solvent used for dilution has the same definition as the organic solvent used in the polymerization reaction, and more preferably, the organic solvent used for dilution is the same as the organic solvent used in the polymerization reaction.

[0029] According to an embodiment of the present invention, after diluting the polybenzoxazole containing amide bonds obtained from the reaction with an organic solvent, the mass concentration of the polybenzoxazole containing amide bonds is 2 to 20 wt%, preferably 5 to 15 wt%, for example 3 wt%, 6 wt%, 8 wt%, 9 wt%, 11 wt%, 13 wt%, and 14 wt%.

[0030] According to an embodiment of the present invention, step S2 includes the following steps: emulsifying and shearing the diluted polybenzoxazole containing amide bonds, while adding water, to obtain a polybenzoxazole nanofiber solution containing amide bonds.

[0031] According to an embodiment of the present invention, the shearing speed for emulsifying and shearing the polybenzoxazole containing amide bonds is 800-2500 rpm, preferably 1000-2000 rpm, for example 900 rpm, 1200 rpm, 1500 rpm, 1700 rpm, 1800 rpm, 2100 rpm, or 2400 rpm.

[0032] According to an embodiment of the present invention, the emulsification shear dispersion time is 20-100 min, preferably 40-80 min, for example 30 min, 50 min, 60 min, 70 min, 80 min, or 90 min.

[0033] According to an embodiment of the present invention, after step S2, the following steps are further included: filtering the polybenzoxazole nanofiber solution containing amide bonds, rinsing to remove organic solvent, dispersing in water to obtain an aqueous solution of nanofibers.

[0034] According to an embodiment of the present invention, the mass concentration of nanofibers in the nanofiber aqueous solution is 2-20 wt%, preferably 5-15 wt%, and more preferably 8-10 wt%.

[0035] Thirdly, the present invention also provides a method for preparing the above-mentioned sizing agent, comprising the following steps: diluting the nanofiber aqueous solution with water to a nanofiber mass concentration of 0.01-5 wt%.

[0036] Fourthly, the present invention provides a sizing agent for surface modification of PBO fibers to improve the interfacial bonding strength between PBO fibers and the matrix.

[0037] Fifthly, the present invention provides a nanofiber-reinforced PBO fiber, wherein at least one surface of the PBO fiber is attached with nanofibers as described above, and there is a π-π interaction between the nanofibers and the PBO fiber.

[0038] In a sixth aspect, the present invention provides a method for preparing a nanofiber-reinforced PBO fiber composite material, comprising the following steps: contacting the surface of a PBO fiber material with the above-mentioned sizing agent to form a PBO fiber material with attached nanofibers, and combining the PBO fiber material with attached nanofibers with a matrix.

[0039] Preferably, the contact is, for example, wetting or coating.

[0040] According to an embodiment of the present invention, the soaking time is 30 to 100 seconds, preferably 40 to 80 seconds.

[0041] According to an embodiment of the present invention, the PBO fiber material with attached nanofibers is composited with a matrix, comprising the following steps: drying the PBO fiber material after contact to obtain the PBO fiber material with attached nanofibers, coating the surface of the PBO fiber material with attached nanofibers with a matrix resin solution and drying it, and then hot pressing it in a hot press.

[0042] According to an embodiment of the present invention, the drying temperature is 40-120°C and the drying time is 60-100 min.

[0043] According to an embodiment of the present invention, the PBO fiber material is, for example, PBO fiber cloth, which is plain weave, satin weave, twill weave, square plain weave, or unidirectional weave.

[0044] According to an embodiment of the present invention, the surface density of the PBO fiber cloth is 100-400 g / m². 2 Preferably, the surface density of the PBO fiber cloth is 100-200 g / m². 2 .

[0045] According to an embodiment of the present invention, the matrix resin is one of epoxy resin, cyanate ester resin, phenolic resin, bismaleimide resin, benzoxazine, and polyimide resin.

[0046] According to an embodiment of the present invention, the drying temperature after coating the base resin adhesive is 60-120°C, and the drying time is 20-60 min.

[0047] According to an embodiment of the present invention, the hot pressing temperature for hot pressing after drying is 70-370°C, the pressure is 5-20 MPa, the total hot pressing time is 70-230 min, and the venting time during the hot pressing process is 1-20 min.

[0048] Beneficial effects

[0049] (1) The benzoxazole nanofibers containing amide bonds in this invention have a structure similar to PBO fibers. When attached to the surface of PBO fibers, there is a π-π interaction between them. The sizing agent has high compatibility with PBO fibers. Furthermore, the preparation process of the nanofibers is simple, easy to operate, and has high safety.

[0050] (2) The solvent of the sizing agent in this invention is water, which overcomes the defects of existing sizing agents that use a large amount of flammable, explosive and toxic organic solvents. The sizing agent of this invention uses water as a solvent, which is simple to operate, mild in conditions, has little pollution to the environment, and is easy to process in batches.

[0051] (3) The sizing agent used in this invention is polyamide nanofiber containing benzoxazole structure. The PBO fiber after interface coating still has excellent heat resistance, which helps PBO fiber composite material to achieve large-scale application in the field of high-performance high-temperature thermal protection materials such as manned aircraft and rocket engine propulsion.

[0052] (4) The PBO fiber composite material modified with the sizing agent provided by this invention exhibits significantly improved interfacial shear strength and interlaminar shear strength while maintaining good mechanical properties. The sizing agent of this invention can improve the interfacial properties of PBO fiber reinforced composite materials while maintaining their excellent bulk properties. Detailed Implementation

[0053] The following detailed description, in conjunction with specific embodiments, will further illustrate the composite fibers of the present invention, their preparation methods, and applications. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0054] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0055] In this invention, the aqueous solution of nanofibers refers to a dispersion solution formed by nanofibers being dispersed in water in the form of particles.

[0056] In the following examples, the hot press used for hot pressing of PBO fiber composite materials was manufactured by IDM, model L003.

[0057] Example 1

[0058] S101. Synthesis of benzoxazole-containing nanofibers

[0059] In a nitrogen-sealed reactor, 5 g of CaCl2 was added and dissolved in 50 mL of N-methylpyrrolidone at 100 °C. The reactor was then cooled to 0 °C using an ice-water bath. 2.25 g of 2-(4-aminophenyl)-5-aminobenzoxazole was added at 0 °C and stirred until dissolved. 2.09 g of terephthaloyl chloride was added, and the mixture was stirred at 800 rpm. Polycondensation was carried out at 20 °C for 30 min, stopping when the Weissenberg effect occurred, yielding the reaction solution.

[0060] S102. Preparation of nanofiber aqueous dispersion

[0061] N-methylpyrrolidone was added to the reaction solution prepared in step S101 and diluted to a solid content of 2 wt%. Deionized water was added under the strong shearing action of a high-speed emulsifier and dispersed under high-speed shearing for 20 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and rinsed with deionized water to remove N-methylpyrrolidone. The solids were dissolved in pure water (solid content 2 wt%) to obtain an aqueous solution of nanofibers.

[0062] Hot pressing preparation of S103.PBO fiber composite materials

[0063] Plain PBO fiber fabric was impregnated and coated with the nanofiber aqueous solution (solid content 2wt%) prepared in step S102. Each piece of PBO fiber fabric was treated for 30 seconds, and then dried in an oven at 100℃ for 40 minutes to obtain plain PBO fiber fabric. A resin solution was prepared by mixing 50g of epoxy resin and 10g of 4,4'-methylenebis(2-ethyl)aniline curing agent. The resin solution was then uniformly coated onto the completely dried plain fabric (106g / m²). 2 The PBO fiber cloth surface was heated and dried at 100℃ for 20 minutes to remove the solvent from the adhesive. The dried PBO fiber cloth was then stacked neatly and placed in a hot press. First, it was held at 80℃ and 5MPa for 30 minutes; then the temperature was increased to 90℃ at 5℃ / min and held for 5 minutes, followed by venting for 5 minutes; finally, the temperature was increased to 120℃ at 5℃ / min and the pressure was 10MPa, and the hot press was held for 30 minutes to obtain a nanofiber interface-reinforced PBO fiber composite material.

[0064] Example 2

[0065] S201. Synthesis of benzoxazole-containing nanofibers

[0066] In a nitrogen-sealed reactor, 0.25 g of CaCl2 was added and dissolved in 50 mL of N,N-dimethylacetamide at 80 °C. The reactor was then cooled to 0 °C using an ice-water bath. At 0 °C, 0.75 g of 2,1,3-benzoxadiazole-4,5-diamine was added and stirred until dissolved. 1.02 g of isophthaloyl chloride was added, and the mixture was stirred at 2500 rpm. Polycondensation was carried out at 40 °C for 120 min, stopping when the Weissenberg effect occurred, yielding the reaction solution.

[0067] Preparation of S202. Aqueous Dispersion of Nanofibers

[0068] The reaction solution obtained in step S201 was diluted with N-methylpyrrolidone to a solid content of 10 wt%. Deionized water was added under the strong shearing action of a high-speed emulsifier, and the mixture was dispersed under high-speed shearing for 60 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and washed with deionized water to remove N,N-dimethylacetamide. The solids were dissolved in pure water (solid content 0.01 wt%) to obtain an aqueous solution of nanofibers.

[0069] Hot pressing preparation of S203.PBO fiber composite materials

[0070] The nanofiber aqueous solution (solid content 0.01 wt%) from step S202 was used to impregnate and coat the twill PBO fiber fabric. Each piece of PBO fiber fabric was treated for 60 seconds, and then dried in an oven at 80°C for 100 minutes. A phenolic resin (containing 25 wt% ethanol) solution was then brushed onto the completely dried twill fabric (106 g / m²).2 The PBO fiber cloth surface was heated and dried at 80℃ for 60 min to remove the solvent from the adhesive. The dried PBO fiber cloth was then stacked neatly and placed in a hot press. First, it was held at 130℃ and 10MPa for 60 min; then the temperature was increased to 150℃ at 5℃ / min and held for 20 min, followed by venting for 10 min; finally, the temperature was increased to 170℃ at 5℃ / min, and the pressure was increased to 20MPa, and the hot press was held for 60 min to obtain the nanofiber interface-reinforced PBO fiber composite material.

[0071] Example 3

[0072] S301. Synthesis of benzoxazole-containing nanofibers

[0073] In a nitrogen-sealed reactor, 0.35 g of CaCl2 was added and dissolved in 50 mL of N,N-dimethylformamide at 120 °C. The reactor was then cooled to 0 °C using an ice-water bath. At 0 °C, 2.25 g of 2-(4-aminophenyl)-6-aminobenzoxazole was added and stirred until dissolved. Then, 3.0 g of 4,4'-diphenyl chloride ether (in a 1:1 molar ratio with 2-(4-aminophenyl)-6-aminobenzoxazole) was added, and the mixture was stirred at 1500 rpm. Polycondensation was carried out at 60 °C for 200 min, stopping when the Weissenberg effect occurred, yielding the reaction solution.

[0074] Preparation of S302. Aqueous Dispersion of Nanofibers

[0075] N,N-dimethylformamide was added to the reaction solution prepared in step S301 until the solid content reached 20 wt%. Deionized water was added under the strong shearing action of a high-speed emulsifier, and the mixture was dispersed under high-speed shearing for 100 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and rinsed with deionized water to remove N,N-dimethylformamide. The solids were dissolved in pure water (solid content 0.05 wt%) to obtain an aqueous solution of nanofibers.

[0076] Hot pressing preparation of S303.PBO fiber composite material

[0077] The nanofiber aqueous solution (solid content 0.05 wt%) prepared in step S302 was used to impregnate and coat the satin-textured PBO fiber fabric. Each piece of PBO fiber fabric was treated for 40 seconds, and then dried in an oven at 70°C for 100 minutes. A mixture of 50 g of bismaleimide resin and 5 g of diallyl bisphenol A curing agent was brushed onto the completely dried satin fabric (113 g / m²). 2The PBO fiber cloth surface was heated and dried at 120℃ for 30 minutes to remove the solvent from the adhesive. The dried PBO fiber cloth was then stacked neatly and placed in a hot press. First, it was held at 120℃ and 5MPa for 20 minutes; then the temperature was increased to 135℃ at 5℃ / min and held for 20 minutes, followed by venting for 30 minutes; then the temperature was increased to 160℃ at 5℃ / min and pressurized to 15MPa for 60 minutes; finally, the temperature was increased to 180℃ at 5℃ / min and held for 60 minutes; and finally, the temperature was increased to 240℃ at 5℃ / min and hot-pressed for 60 minutes to obtain a nanofiber interface-reinforced PBO fiber composite material.

[0078] Example 4

[0079] S401. Synthesis of benzoxazole-containing nanofibers

[0080] In a nitrogen-sealed reactor, 0.25 g of CaCl2 was added and dissolved in 50 mL of N,N-dimethylacetamide at 100 °C. The reactor was then cooled to 0 °C using an ice-water bath. At 0 °C, 2.25 g of 5-amino-2-(3-aminophenyl)benzoxazole was added and stirred until dissolved. Then, 2.04 g of phthaloyl chloride was added, and the mixture was stirred at 1000 rpm. Polycondensation was carried out at 60 °C for 120 min, stopping when the Weissenberg effect occurred, yielding the reaction solution.

[0081] Preparation of S402 nanofiber aqueous dispersion

[0082] The reaction solution prepared in step S401 was diluted with N,N-dimethylacetamide to a solid content of 10 wt%. Deionized water was added under the strong shearing action of a high-speed emulsifier, and the mixture was dispersed under high-speed shearing for 100 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and rinsed with deionized water to remove N,N-dimethylacetamide. The solids were dissolved in pure water (solid content 1 wt%) to obtain an aqueous solution of nanofibers.

[0083] Hot pressing preparation of S403.PBO fiber composite materials

[0084] The unidirectional PBO fiber cloth was impregnated and coated with the nanofiber aqueous solution (1 wt% solid content) prepared in step S402. Each piece of PBO fiber cloth was treated for 100 s, and then dried in an oven at 120°C for 100 min. Allyl benzoxazine resin solution (containing 40 wt% acetone) was brushed onto the completely dried unidirectional (115 g / m²) nanofiber cloth. 2The PBO fiber cloth surface was heated and dried at 60℃ for 40 minutes to remove the solvent from the adhesive. The dried PBO fiber cloth was then stacked neatly and placed in a hot press. First, it was hot-pressed at 150℃ and 5MPa for 30 minutes, followed by degassing for 20 minutes. Then, the temperature was increased to 190℃ at a rate of 5℃ / min, and the pressure was increased to 10MPa, and held for 60 minutes. Next, the temperature was increased to 210℃ at a rate of 5℃ / min, and held for 60 minutes. Finally, the temperature was increased to 230℃ at a rate of 5℃ / min, and the hot press was held for 60 minutes to obtain a nanofiber interface-reinforced PBO fiber composite material.

[0085] Example 5

[0086] S501. Synthesis of benzoxazole-containing nanofibers

[0087] In a nitrogen-sealed reactor, 3 g of CaCl2 was added and dissolved in 50 mL of N,N-dimethylformamide at 80 °C. The reactor was then cooled to 0 °C using an ice-water bath. 2.25 g of 6-amino-2-(3-aminophenyl)benzoxazole was added at 0 °C and stirred until dissolved. 2.58 g of 2,6-naphthalenedicarboxylate chloride was added, and the mixture was stirred at 800 rpm. Polycondensation was carried out at 60 °C for 60 min, stopping when the Weissenberg effect occurred, yielding the reaction solution.

[0088] Preparation of S502 nanofiber aqueous dispersion

[0089] N,N-dimethylformamide was added to the reaction solution prepared in step S501 to dilute it to a solid content of 20 wt%. Deionized water was added under the strong shearing action of a high-speed emulsifier, and the mixture was dispersed under high-speed shearing for 100 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and rinsed with deionized water to remove N,N-dimethylformamide. The solids were dissolved in pure water (solid content 0.04 wt%) to obtain an aqueous solution of nanofibers.

[0090] Hot pressing preparation of S503.PBO fiber composite materials

[0091] The nanofiber aqueous solution (solid content 0.04 wt%) prepared in step S502 was used to impregnate and coat the plain-weave PBO fiber cloth. Each piece of PBO fiber cloth was treated for 80 seconds, and then dried in an oven at 80°C for 60 minutes. Polyimide resin solution (containing 40 wt% ethanol) was then brushed onto the completely dried plain-weave (115 g / m²) fabric. 2The PBO fiber surface was heated and dried at 80℃ for 40 minutes to remove the solvent from the adhesive. The PBO fiber cloth was stacked neatly and placed in a hot press. First, it was held at 200℃ and 5MPa for 30 minutes; then the temperature was increased to 260℃ at 5℃ / min and held for 30 minutes; then the temperature was increased to 300℃ at 5℃ / min and held for 30 minutes; next, the temperature was increased to 350℃ at 5℃ / min, the air was vented for 20 minutes, and it was held at 10MPa for 10 minutes; finally, the temperature was increased to 370℃ at 5℃ / min and hot-pressed for 60 minutes to obtain the PBO fiber composite material with nanofiber interface reinforcement.

[0092] Example 6

[0093] S601. Synthesis of benzoxazole-containing nanofibers

[0094] In a nitrogen-sealed reactor, 2g of CaCl2 was added and dissolved in 50ml of N-methylpyrrolidone at 80℃. The reactor was then cooled to 0℃ using an ice-water bath. At 0℃, 1.49g of 2,6-diaminobenzoxazole was added and stirred until dissolved. Then, 5.96g of 5-amino-2,4,6-triiodophthaloyl chloride was added, and the mixture was stirred at 2500rpm. Polycondensation was carried out at 40℃ for 120min. Polycondensation was stopped when the Weissenberg effect occurred, yielding the reaction solution.

[0095] Preparation of S602 nanofiber aqueous dispersion

[0096] In step S601, the reaction solution was diluted with N-methylpyrrolidone to a solid content of 5 wt%. Deionized water was added under the strong shearing action of a high-speed emulsifier, and the mixture was dispersed under high-speed shearing for 60 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and rinsed with deionized water to remove N-methylpyrrolidone. The solids were then dissolved in pure water (solid content 1 wt%) to obtain an aqueous solution of nanofibers.

[0097] Hot pressing preparation of S603.PBO fiber composite materials

[0098] The nanofiber aqueous solution (1 wt% solid content) from step S602 was used to impregnate and coat the twill PBO fiber fabric. Each piece of PBO fiber fabric was treated for 70 seconds, and then dried in an oven at 40°C for 120 minutes. A cyanate ester resin solution (containing 50 wt% acetone) was then brushed onto the completely dried twill fabric (163 g / m²). 2The PBO fiber cloth surface was heated and dried at 60℃ for 30 minutes to remove the solvent from the adhesive. The dried PBO fiber cloth was then stacked neatly and placed in a hot press. It was first held at 70℃ and 12MPa for 60 minutes, then heated to 80℃ at a rate of 5℃ / min and held for 30 minutes, followed by venting for 15 minutes. Finally, it was heated to 100℃ at a rate of 5℃ / min and pressurized to 15MPa for 60 minutes to obtain a nanofiber interface-reinforced PBO fiber composite material.

[0099] Example 7

[0100] S701. Synthesis of benzoxazole-containing nanofibers

[0101] In a nitrogen-sealed reactor, 1 g of CaCl2 was added and dissolved in 50 ml of N,N-dimethylacetamide at 120 °C. The reactor was then cooled to 0 °C using an ice-water bath. At 0 °C, 1.49 g of 2,4-diaminobenzoxazole was added and stirred until dissolved. Then, 1.93 g of 2,5-furandicarboxylic acid chloride was added, and the mixture was stirred at 1000 rpm. Polycondensation was carried out at 20 °C for 30 min, stopping when the Weissenberg effect occurred, yielding the reaction solution.

[0102] S702. Preparation of nanofiber aqueous dispersion

[0103] In step S701, the reaction solution was diluted with N,N-dimethylacetamide to a solid content of 15 wt%. Deionized water was then added under the strong shearing action of a high-speed emulsifier, and the mixture was dispersed under high-speed shearing for 60 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and rinsed with deionized water to remove N,N-dimethylacetamide. The solids were then dissolved in pure water (solid content 0.08 wt%) to obtain an aqueous solution of nanofibers.

[0104] Hot pressing preparation of S703.PBO fiber composite materials

[0105] The plain-weave PBO fiber fabric was impregnated and coated with an aqueous solution (solid content 0.08 wt%) of the nanofibers prepared in step S702. Each piece of PBO fiber fabric was treated for 30 seconds, and then dried in an oven at 60°C for 100 minutes. A phenolic resin (containing 25 wt% ethanol) solution was then brushed onto the completely dried plain-weave fabric (300 g / m²). 2The PBO fiber cloth surface was heated and dried at 80℃ for 30 minutes to remove the solvent from the adhesive. The dried PBO fiber cloth was then stacked neatly and placed in a hot press. First, it was held at 130℃ and 10MPa for 60 minutes; then the temperature was increased to 150℃ at 5℃ / min and held for 20 minutes, followed by venting for 10 minutes; finally, the temperature was increased to 170℃ at 5℃ / min, and the pressure was increased to 20MPa, and the hot press was held for 60 minutes to obtain a nanofiber interface-reinforced PBO fiber composite material.

[0106] Example 8

[0107] S801. Synthesis of benzoxazole-containing nanofibers

[0108] In a nitrogen-sealed reactor, 1.5 g of CaCl2 was added and dissolved in 50 mL of N,N-dimethylformamide at 100 °C. The reactor was then cooled to 0 °C using an ice-water bath. At 0 °C, 3.42 g of 2,2'-p-phenyl-bis(6-aminobenzoxazole) was added and stirred until dissolved. Then, 2.04 g of 2,6-pyridinedicarboxylic acid chloride was added, and the mixture was stirred at 900 rpm. Polycondensation was carried out at 50 °C for 90 min, stopping when the Weissenberg effect occurred, yielding the reaction solution.

[0109] Preparation of S802 nanofiber aqueous dispersion

[0110] In step S801, the reaction solution was diluted with N,N-dimethylformamide to a solid content of 3 wt%. Deionized water was then added under the strong shearing action of a high-speed emulsifier, and the mixture was dispersed under high-speed shearing for 30 min to obtain a nanofiber solution. The obtained nanofiber solution was filtered and rinsed with deionized water to remove N,N-dimethylformamide. The solids were then dissolved in pure water (solid content 1 wt%) to obtain an aqueous solution of nanofibers.

[0111] Hot pressing preparation of S803.PBO fiber composite materials

[0112] The nanofiber aqueous solution (solid content 1 wt%) prepared in step 802 was used to impregnate and coat the plain-weave PBO fiber cloth. Each piece of PBO fiber cloth was treated for 90 seconds, and then dried in an oven at 80°C for 60 minutes. A mixed adhesive solution of 50 g of bismaleimide resin and 5 g of diallyl bisphenol A curing agent was brushed onto the completely dried plain-weave (113 g / m²) fabric. 2The PBO fiber cloth surface was heated and dried at 90℃ for 40 minutes to remove the solvent from the adhesive. The dried PBO fiber cloth was then stacked neatly and placed in a hot press. First, it was held at 120℃ and 5MPa for 20 minutes; then the temperature was increased to 135℃ at 5℃ / min and held for 20 minutes, followed by venting for 30 minutes; then the temperature was increased to 160℃ at 5℃ / min and pressurized to 15MPa for 60 minutes; next, the temperature was increased to 180℃ at 5℃ / min and held for 60 minutes; finally, the temperature was increased to 240℃ at 5℃ / min and hot-pressed for 60 minutes to obtain a PBO fiber composite material with nanofiber interface reinforcement.

[0113] Test Example 1

[0114] a) In accordance with ASTM-D3379 standard "Test Method for Tensile Strength and Young's Modulus of High Modulus Monofilament Materials", the tensile strength of PBO fibers and PBO fiber composite materials before and after coating treatment in Examples 1-8 was tested using a microcomputer-controlled electronic tensile testing machine.

[0115] b) The maximum thermal decomposition temperature of PBO fiber refers to the temperature at which the surface-modified PBO fiber reaches the highest weight loss rate under heating conditions. The maximum thermal decomposition temperature of the PBO fibers prepared in Examples 1-8 was determined by a thermogravimetric analyzer. The heating temperature during the test was 100-800℃, and the heating rate was 20℃ / min.

[0116] c) Using a composite material interface performance evaluation device (HM-410, Toei Sangyo Co., Ltd., Japan), the interfacial shear strength between the PBO fiber composite material and the matrix resin before and after coating treatment was tested by the microsphere debonding method. The fiber migration rate during the test was 0.1 mm / min, and 20 valid data were collected for each sample group.

[0117] d) In accordance with GB / T 35100-2018 standard "General Rules for Test Methods of Fiber Reinforced Plastics", the interlaminar shear strength of the PBO fiber composites before and after coating treatment in Examples 1-8 was tested using a universal tensile testing machine.

[0118] Table 1. Tensile properties, thermal properties of PBO fibers before and after interface reinforcement, and related properties of PBO fiber / resin composites.

[0119]

[0120] In Examples 1-8, the PBO fiber monofilaments of the PBO fiber cloths are all the same, with only the surface texture being different, which does not affect the performance of the fiber monofilaments.

[0121] As shown in Table 1, compared with untreated PBO fiber composites, PBO fiber composites treated with nanofiber aqueous solutions exhibited improved IFSS (interfacial tensile strength) by 76.5%–96.4% and ILSS (interfacial tensile strength) by 105%–114%, while maintaining almost unchanged monofilament tensile strength and thermogravimetric temperature, thus preserving excellent mechanical and heat resistance properties. The nanofibers prepared in this invention increase the surface roughness of PBO fibers, enhancing the mechanical bonding between the fiber surface and the resin matrix. Furthermore, the amide groups inherent in the nanofibers themselves increase the surface energy of the PBO fibers, facilitating the chemical reaction between the resin matrix and the PBO fibers and increasing the bonding strength between the fibers. Moreover, this nanofiber preparation method is simple, mild, and cost-effective; the interface reinforcement method is also mild and does not damage the bulk properties of the PBO fibers. It significantly improves the interfacial properties of PBO fiber-reinforced composites while maintaining their excellent bulk properties, which is of great significance for promoting their large-scale application in high-tech fields such as national defense and aerospace, as well as in extreme environments such as ultra-high temperatures.

[0122] The specific embodiments of the present invention have been described above by way of example. However, the scope of protection of the present invention is not limited to the above exemplary embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A sizing agent, characterized in that, Including polybenzoxazole nanofibers containing amide bonds and water; The method for preparing the amide-containing polybenzoxazole nanofibers includes the following steps: S1. Preparation of polybenzoxazole containing amide bonds: In an inert atmosphere, metal chloride is dissolved in an organic solvent, and benzoxazole diamine or its derivatives and acyl chloride compounds are added sequentially to carry out a polycondensation reaction. The polycondensation is stopped when the Weissenberg effect occurs. The benzoxazole diamine or its derivatives include one or more of the following: 6-amino-2-(3-aminophenyl)benzoxazole, 2,1,3-benzoxadiazole-4,5-diamine, 2-(4-aminophenyl)-6-aminobenzoxazole, 2-(4-aminophenyl)-5-aminobenzoxazole, 5-amino-2-(3-aminophenyl)benzoxazole, 2,6-diaminobenzoxazole, 2,4-diaminobenzoxazole, 2,5-diaminobenzoxazole, 4-aminobenzoxazole-2-methylamine, 5-aminobenzoxazole-2-methylamine, 6-aminobenzoxazole-2-methylamine, 5-amino-2-(2'-chloro-4'-aminophenyl)-benzoxazole, 2,2'-p-phenyl-bis(6-aminobenzoxazole), and 2-(3,5-diaminophenyl)benzoxazole. S2. Add an organic solvent to the reaction product of step S1 for dilution, emulsify and shear disperse the diluted polybenzoxazole containing amide bonds, and add water at the same time to obtain a polybenzoxazole nanofiber solution containing amide bonds.

2. The sizing agent according to claim 1, characterized in that, The diameter of the polybenzoxazole nanofibers containing amide bonds is 15~30 nm.

3. The sizing agent according to claim 1, characterized in that, The intrinsic viscosity of the polybenzoxazole nanofibers containing amide bonds is 3.06 dL / g.

4. The sizing agent according to any one of claims 1-3, characterized in that, The mass concentration of polybenzoxazole nanofibers containing amide bonds in the sizing agent is 0.01~2 wt%.

5. The sizing agent according to any one of claims 1-3, characterized in that, The metal chloride is dissolved in an organic solvent at a temperature of 80~120 °C.

6. The sizing agent according to any one of claims 1-3, characterized in that, Before adding benzoxazole diamine or its derivative in step S1, the following step is also included: cooling the organic solvent containing the metal chloride to below 5°C.

7. The sizing agent according to claim 6, characterized in that, The organic solvent containing dissolved metal chlorides was placed in an ice-water bath until the temperature reached 0 °C.

8. The sizing agent according to any one of claims 1-3, characterized in that, The metal chloride is selected from at least one of CaCl2, KCl, LiCl, NaCl, FeCl2, FeCl3, CuCl2, MgCl2, AlCl3, and ZnCl2.

9. The sizing agent according to any one of claims 1-3, characterized in that, The concentration of the metal chloride in the organic solvent is 0.5~1 wt%.

10. The sizing agent according to any one of claims 1-3, characterized in that, The emulsification shear dispersion time is 20-100 min.

11. The sizing agent according to any one of claims 1-3, characterized in that, Step S2 is followed by the following steps: filtering the polybenzoxazole nanofiber solution containing amide bonds, rinsing to remove organic solvents, dispersing in water to obtain an aqueous nanofiber solution.

12. A nanofiber-reinforced PBO fiber, characterized in that, At least one surface of the PBO fiber is attached with the nanofibers according to any one of claims 1-11, and there is a π-π interaction between the nanofibers and the PBO fiber.

13. A method for preparing a nanofiber-reinforced PBO fiber composite material, characterized in that, The method includes the following steps: contacting the surface of the PBO fiber material with the sizing agent described in any one of claims 1-11 to form a PBO fiber material with attached nanofibers, and then combining the PBO fiber material with attached nanofibers with a matrix.

14. The method for preparing the nanofiber-reinforced PBO fiber composite material according to claim 13, characterized in that, The contact time is 30~100 s.

15. The method for preparing the nanofiber-reinforced PBO fiber composite material according to claim 13, characterized in that, The composite process of PBO fiber material with attached nanofibers and matrix includes the following steps: drying the PBO fiber material after contact to obtain PBO fiber material with attached nanofibers, coating the surface of the PBO fiber material with attached nanofibers with matrix resin and drying it, and then hot pressing it in a hot press.

16. The method for preparing the nanofiber-reinforced PBO fiber composite material according to claim 15, characterized in that, The drying temperature is 40-120 ℃, and the drying time is 60-100 min.

17. The method for preparing the nanofiber-reinforced PBO fiber composite material according to claim 15, characterized in that, The matrix resin is one of epoxy resin, cyanate ester resin, phenolic resin, bismaleimide resin, benzoxazine, and polyimide resin.

18. The method for preparing the nanofiber-reinforced PBO fiber composite material according to claim 15, characterized in that, The drying temperature after coating the base resin is 60~120 ℃, and the drying time is 20~60 min.

19. The method for preparing the nanofiber-reinforced PBO fiber composite material according to claim 15, characterized in that, After drying, the hot pressing temperature in the hot press is 70~370 ℃, the pressure is 5~20 MPa, the total hot pressing time is 70 min~230 min, and the venting time during the hot pressing process is 1~20 min.

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