Flame-retardant aramid fiber insulating film containing hexaphenoxy cyclotriphosphazene and preparation method of flame-retardant aramid fiber insulating film

Abstract

The invention relates to the technical field of electrical insulation and flame-retardant composite materials, in particular to a flame-retardant aramid fiber insulating film containing hexaphenoxy cyclotriphosphazene and a preparation method of the flame-retardant aramid fiber insulating film. Comprising the following steps: dissociating para-aramid fibers in an alkali-containing polar system to form para-aramid nanofiber dispersion liquid, dispersing HPCTP in a polar organic solvent, dropwise adding and blending the para-aramid fibers and the HPCTP under a stirring condition to form a composite dispersion system, performing vacuum filtration and deposition to form a film, performing hot-pressing densification treatment and the like. According to the invention, a specific synergistic strategy of nanofiber network construction, flame retardant stable dispersion-interface synergy and suction filtration deposition-hot pressing densification is adopted, so that HPCTP and p ANFs frameworks are uniformly compounded to form a continuous and compact film layer structure, and good dielectric strength and mechanical properties are still maintained while the flame retardant property is improved; the method is clear in technological process, mild in condition, good in repeatability and easy to implement on a large scale, and can be applied to electrical insulation fields such as motor turn insulation, slot insulation and transformer interlayer insulation and other insulation application scenes needing heat resistance and flame retardance.

Description

Technical Field

[0001] This invention belongs to the technical field of electrical insulation and flame retardant composite materials, specifically relating to a flame retardant aramid insulating film containing hexaphenoxycyclotriphosphazene and its preparation method. Background Technology

[0002] It is a key substrate for motor slot insulation, transformer interlayer / turn insulation, and electrical insulation structures. As electrical equipment develops towards higher voltage, higher power density, and miniaturization, insulating films / papers not only withstand the coupling of electric and thermal fields during service, but may also encounter extreme conditions such as partial discharge and transient overheating. Therefore, in addition to maintaining high dielectric strength and mechanical strength, the materials must also simultaneously meet higher requirements for flame retardancy and long-term stability. Otherwise, the risk of breakdown and life reduction may be caused by porosity defects, interface discontinuities, or migration and precipitation of flame retardant components introduced by flame retardant modification.

[0003] Improving the flame retardancy of existing aramid-based insulating materials typically involves inorganic filler composites, acidic phosphorus-based modification, or high-temperature melt bonding. However, these methods generally suffer from difficulties in achieving a synergistic effect between flame retardancy, insulation, and mechanical properties. Firstly, inorganic filler systems often rely on a high proportion of inorganic phase to achieve flame retardancy / heat resistance. However, the large number of interfaces between the inorganic and organic fiber phases and the difficulty in controlling interface continuity easily introduce micro-defects and affect the material's densification, leading to poor electric field uniformity and increased dielectric strength fluctuations. For example, patent CN110485195A proposes an aramid nanofiber-based insulating paper, which uses aramid nanofibers composited with insulating inorganic materials such as muscovite / phlogopite / synthetic mica / boron nitride, with the inorganic materials accounting for 30%–70% of the paper mass. The insulating paper is prepared by mixing a dispersion into a sol, solvent exchange to obtain a hydrogel, and drying into paper. This type of solution has problems such as a high proportion of inorganic phase and the tendency for the solvent exchange and drying process to introduce structural pores / interface non-uniformity. Therefore, there is still room for further optimization when it comes to applications of thinner and denser high field strength insulating films.

[0004] Secondly, while acidic phosphorus-based chemical modification can improve flame retardancy, it often involves treatment with highly corrosive media, process control during water washing to neutrality, and pressure treatment of waste liquid. The process window is narrow, and repeatability is affected by raw material batches and washing levels, leading to fluctuations in material properties. For example, patent CN117758547A discloses esterification modification of fibers using a phosphoric acid aqueous solution: 40wt%~45wt% of phosphoric acid is used to modify meta-aramid short-cut fibers, and 30wt%~35wt% of phosphoric acid is used to modify bacterial fibers, followed by washing to neutrality. The fibers are then compounded into pulp and hot-pressed into paper at a temperature of 200~260℃, a pressure of 8~10MPa, and a time of 10~30min. This route suffers from high dependence on acid treatment and high-temperature hot-pressing conditions, easily leading to increased process complexity, energy consumption, and equipment requirements. Furthermore, in long-term electrical insulation applications, further solutions are needed to address the interfacial stability and moisture absorption / aging risks associated with chemical modification. Thirdly, the high-temperature melt bonding route uses heat treatment or hot rolling to melt thermoplastic components to form bonding points, thereby improving strength and flame retardancy. However, this type of solution usually requires a higher temperature window, consumes more energy, and is not conducive to achieving film-level densification and low-defect control under mild conditions. For example, patent CN114808538A proposes mixing aramid chopped fibers and polyphenylene sulfide ultrafine fibers to form a web, then heat-treating at 300-340℃ for 1-10 minutes, followed by hot rolling (e.g., 200-300℃, linear pressure 50-150 N / mm) to obtain highly flame-retardant aramid composite paper. Although the highly flame-retardant aramid composite paper obtained by this method can achieve a high LOI, it suffers from dependence on heat treatment above 300℃ and hot rolling processes, resulting in high process thresholds and energy consumption. Moreover, the obtained structure is still more like a fiber felt system of composite paper, making it difficult to directly meet the film-level densification and interface continuity control required for higher dielectric strength. In summary, existing technologies generally suffer from the following problems: (1) high filler / multiphase interface leads to limited densification and increased defects, which in turn affects dielectric strength and consistency; (2) acidic chemical modification leads to complex processes and increased pressure on environmental protection and stability; (3) high-temperature melting / heat treatment leads to increased energy consumption and equipment threshold, and is not conducive to the construction of thin film-level low-defect structures. Therefore, a new method for preparing flame-retardant aramid insulating films is urgently needed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a flame-retardant aramid insulating film containing hexaphenoxycyclotriphosphazene, so as to solve the problem that flame-retardant aramid materials in the prior art are difficult to synergistically combine flame retardant, insulating and mechanical properties.

[0006] This invention is achieved through the following technical solution: A method for preparing a flame-retardant aramid insulating film containing hexaphenoxycyclotriphosphazene, the specific steps of which are as follows: S1, para-aramid fibers are added to an alkaline polar solvent, followed by the addition of isopropanol solution. The mixture is then magnetically stirred at 20-35°C to induce swelling, followed by ultrasonic dispersion and dissociation to obtain a nanofiber dispersion system. The obtained nanofiber dispersion system is subjected to aqueous phase replacement, and after standing and filtration, a para-aramid nanofiber dispersion is obtained. Hexaphenoxycyclotriphosphazene was added to a polar organic solvent under stirring conditions and then dispersed by ultrasonication to obtain a hexaphenoxycyclotriphosphazene dispersion. S2, under stirring conditions, hexaphenoxycyclotriphosphazene dispersion is added dropwise to para-aramid nanofiber dispersion to form p-ANFs / HPCTP composite dispersion system; S3, the p-ANFs / HPCTP composite dispersion system was vacuum filtered to obtain a composite wet membrane; S4, the composite wet film is densified by hot pressing to obtain p-ANFs / hexaphenoxycyclotriphosphazene flame-retardant aramid insulating film.

[0007] Preferably, in S1, the alkaline polar solvent is a potassium hydroxide / dimethyl sulfoxide system, wherein the amount of potassium hydroxide is 2~3 g, the amount of dimethyl sulfoxide is 140~160 g, the amount of para-aramid fiber added is 0.2~0.3 g, the volume of isopropanol added is 80~120 mL, and the solid content of the para-aramid nanofiber dispersion is controlled in the range of 0.2 wt%~2.0 wt%.

[0008] Preferably, in S1, during swelling, the temperature is 20~35℃, the time is 10~12h, and the magnetic stirring speed is 600~800rpm; During water phase replacement, the aramid dispersion system is mixed with deionized water at a volume ratio of (80-100) mL: (800-1000) mL.

[0009] Preferably, in S1, the polar organic solvent is dimethyl sulfoxide, the concentration of the hexaphenoxycyclotriphosphazene dispersion is 0.5 wt%~5 wt%, and the ultrasonication frequency is 20~40 Hz for 20~40 min.

[0010] Preferably, in S2, the dropping method is continuous dropping, the dropping rate is 10 mL / min, stirring is maintained during the dropping process, and stirring continues for 20~40 min after the dropping is completed, with the stirring speed being 600~800 rpm.

[0011] Preferably, in S2, the mass fraction of HPCTP relative to p-ANFs is 0~20 wt%.

[0012] Preferably, in S3, during vacuum filtration, the filter membrane is a microporous polytetrafluoroethylene membrane with a pore size of 0.22~0.45μm, the vacuum filtration pressure is 0.08~0.10MPa, the time is 3~4h, and the thickness of the composite wet membrane is 30~80μm.

[0013] Preferably, in S4, during hot pressing, the temperature is 95~105℃, the pressure is 5~15MPa, and the time is 8~12min.

[0014] A p-ANFs / hexaphenoxycyclotriphosphazene flame-retardant aramid insulating film prepared by the method according to any one of claims 1 to 8, wherein the flame-retardant aramid insulating film has the following properties: limiting oxygen index ≥27%, dielectric strength ≥30kV / mm, and maximum tensile stress not less than 119MPa.

[0015] Preferably, the flame-retardant aramid insulating film comprises a p-ANFs matrix and HPCTP dispersed in the matrix.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for preparing a flame-retardant aramid insulating film containing hexaphenoxycyclotriphosphazene. Using para-aramid nanofibers as a matrix, hexaphenoxycyclotriphosphazene is introduced to achieve flame-retardant modification. A stable dispersion of HPCTP is prepared using DMSO as the dispersion medium, and then continuously added dropwise to an aqueous p-ANFs dispersion under stirring conditions. Solvent phase transfer induces in-situ deposition and interfacial anchoring of HPCTP on the p-ANFs surface, forming a stable composite dispersion system. Subsequently, during vacuum filtration deposition, HPCTP is synchronously embedded along with the p-ANFs network and locked by the nanofiber network. Finally, hot pressing densification yields a flame-retardant insulating film with continuous interfaces and controlled defects.

[0017] This invention employs a p-ANFs / HPCTP synergistic design. While maintaining the high strength, high modulus, and excellent electrical insulation properties of para-aramid nanofibers, HPCTP is introduced to construct a phosphorus-nitrogen synergistic flame-retardant system. HPCTP is ultrasonically dispersed in a polar organic solvent, and then continuously added dropwise to the p-ANFs dispersion to form a composite dispersion system. This allows the flame-retardant component to synergistically combine with the nanofiber framework at the microscale. The results of the examples show that the limiting oxygen index of the composite film increased from 27.3% to 35.1%, indicating an effective improvement in flame-retardant performance. Simultaneously, the film maintains high dielectric strength and mechanical properties, achieving a synergistic balance between flame-retardant and insulation performance.

[0018] Meanwhile, the present invention employs a combination of polar solvent dispersion, continuous dropwise addition under stirring conditions, and continued stirring after dropwise addition during the blending stage. This allows HPCTP to enter the construction process of the p-ANF nanofiber network, thereby obtaining a more stable distribution state. At the same time, the high specific surface area and network structure characteristics of p-ANFs provide conditions for the interfacial bonding of HPCTP, making it more inclined to be distributed in the nanofiber crosslinking / interfacial region, reducing the risk of local defects caused by the random aggregation of large particles, thus helping to improve the continuity of membrane structure and the consistency of performance.

[0019] Furthermore, this invention employs a vacuum filtration membrane deposition method, allowing p-ANFs to deposit on the filter membrane surface to form a continuous nanofiber network. HPCTP is deposited along with the network and embedded within it. The resulting membrane is then subjected to a densification treatment at 95–105°C and 5–15 MPa for 8–12 minutes, further compacting the network and enhancing interfacial contact, thereby improving the membrane's density and structural stability, providing support for obtaining higher dielectric strength and mechanical properties. In the embodiments, the dielectric strength can reach a maximum of 77.88 kV / mm, and the maximum tensile stress is not less than 119 MPa, demonstrating the supporting role of the process of this invention in achieving comprehensive performance.

[0020] Finally, the process route of this invention is centered on physical dispersion and structural construction, mainly including steps such as p-ANFs dispersion preparation, HPCTP dispersion preparation, continuous dropwise addition and blending, vacuum filtration for film formation, and hot pressing densification. The conditions are mild, the equipment requirements are low, and the process has good repeatability, without relying on complex chemical modification or high-temperature sintering processes. At the same time, it adopts a halogen-free flame retardant system and the raw materials are readily available, which has the advantages of easy scale-up and controllable cost, providing a reliable technical foundation for the engineering application of high-performance flame retardant aramid insulating film materials. Attached Figure Description

[0021] Figure 1 This is a flowchart of a method for preparing a flame-retardant aramid insulating film containing hexaphenoxycyclotriphosphazene according to the present invention; Figure 2 This is a schematic diagram showing the structural changes of the hexaphenoxycyclotriphosphazene / p-ANFs flame-retardant aramid insulating membrane under vacuum filtration and hot pressing processes according to the present invention. Figure 3 This is a schematic diagram of the structure of the hexaphenoxycyclotriphosphazene / p-ANFs flame-retardant aramid insulating film of the present invention; Figure 4 A comparison chart of the limiting oxygen index of flame-retardant aramid insulating films with different hexaphenoxycyclotriphosphazene contents; Figure 5 A comparison of the dielectric strength of flame-retardant aramid insulating films with different hexaphenoxycyclotriphosphazene contents; Figure 6 A comparison of tensile strength graphs of flame-retardant aramid insulating films with different hexaphenoxycyclotriphosphazene contents. Detailed Implementation

[0022] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0023] Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the drawings is not intended to limit the invention. In the drawings, the same units / elements are referred to by the same reference numerals.

[0024] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0025] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0026] A method for preparing a flame-retardant aramid insulating film containing hexaphenoxycyclotriphosphazene, referring to... Figures 1-3 The specific steps are as follows: S1. 0.2–0.3 g of para-aramid fibers were added to an alkaline polar solvent, followed by 80–120 mL of isopropanol solution. The mixture was magnetically stirred at 600–800 rpm for 10–12 h at 20–35 °C to induce swelling, and then ultrasonically dispersed at 20–40 Hz for 40–50 min to obtain a nanofiber dispersion system. The obtained nanofiber dispersion system was subjected to aqueous phase replacement, and after standing and filtration, a para-aramid nanofiber dispersion with a solid content of 0.2 wt%–2.0 wt% was obtained. The alkaline polar solvent was a potassium hydroxide / dimethyl sulfoxide system, with 2–3 g of potassium hydroxide and 140–160 g of dimethyl sulfoxide. During aqueous phase replacement, the aramid dispersion system is mixed with deionized water at a volume ratio of (80-100) mL: (800-1000) mL. It can be further washed with deionized water until the pH of the washing solution is 6-8 to reduce the impact of residual solvent and alkali on subsequent performance.

[0027] Hexaphenoxycyclotriphosphazene (in this invention, Maclean H835383, CAS No.: 1184-10-7, lot: C12799111) was added to a polar organic solvent under stirring conditions, and the mixture was ultrasonically dispersed for 20-40 min to obtain a hexaphenoxycyclotriphosphazene dispersion with a concentration of 0.5wt%-5wt%. The polar organic solvent was dimethyl sulfoxide (DMSO).

[0028] S2, under stirring conditions, a hexaphenoxycyclotriphosphazene dispersion was added dropwise to a para-aramid nanofiber dispersion to form a p-ANFs / HPCTP composite dispersion system. The addition was continuous at a rate of 10 mL / min, with stirring maintained throughout the addition process. Stirring continued for 20–40 min after the addition was completed. The mass fraction of HPCTP relative to p-ANFs was 0–20 wt%.

[0029] S3, the p-ANFs / HPCTP composite dispersion system was vacuum filtered to obtain a composite wet membrane with a thickness of 30-80 μm. During filtration, a microporous polytetrafluoroethylene membrane with a pore size of 0.22-0.45 μm was used, the filtration pressure was 0.08-0.10 MPa, and the filtration time was 3-4 h.

[0030] S4, the composite wet film is densified by hot pressing to obtain a p-ANFs / hexaphenoxycyclotriphosphazene flame-retardant aramid insulating film. The hot pressing process involves a temperature of 95~105℃, a pressure of 5~15MPa, and a time of 8~12min.

[0031] The present invention also discloses a p-ANFs / hexaphenoxycyclotriphosphazene flame-retardant aramid insulating film prepared according to the method, the flame-retardant aramid insulating film comprising a p-ANFs matrix and HPCTP dispersed in the matrix, and the flame-retardant aramid insulating film having the following properties: limiting oxygen index ≥27%, dielectric strength ≥30kV / mm, and maximum tensile stress not less than 119MPa.

[0032] This invention discloses a flame-retardant aramid insulating film and its preparation method. First, para-aramid fibers are dissociated in an alkaline polar solvent system to form a stable p-ANF dispersion and complete the aqueous phase replacement. Then, hexaphenoxycyclotriphosphazene is ultrasonically dispersed in a polar organic solvent to form a stable dispersion. Next, the dispersion is dropwise added and blended, and then vacuum filtered to form a film. Finally, it is densified by mild hot pressing to achieve flame retardancy improvement while minimizing agglomeration, migration and interface defects, thereby obtaining a synergistic improvement in flame retardancy, dielectric strength and mechanical properties.

[0033] Example 1 S1, 2.50 g of KOH was weighed and completely dissolved in 150.0 g of DMSO to obtain a basic polar solvent; 90.0 mL of this basic polar solvent was measured into a beaker, 0.20 g of PPTA was added, followed by 15.0 g of isopropanol. The mixture was then magnetically stirred at 30℃ and 700 rpm for 11 h until the system was dark red and free of fibrous residue; finally, it was ultrasonically dispersed for 45 min to obtain an aramid nanofiber dispersion system.

[0034] Pour 90.0 mL of the obtained dispersion into 900 mL of deionized water, allow to stand, filter, and wash with deionized water until the pH of the filtrate is 7.0; the filtered p The wet ANFs sample was redispersed in 40.0 g of deionized water, stirred and ultrasonically dispersed for 10 min to obtain a p-ANFs dispersion with a solid content of 0.50 wt%.

[0035] 0.010 g of hexaphenoxycyclotriphosphazene (HPCTP) was weighed and added to 1.990 g of DMSO. After stirring magnetically for 5 min, the mixture was ultrasonically dispersed for 30 min to obtain a 0.50 wt% HPCTP dispersion.

[0036] S2, under stirring at 700 rpm, the HPCTP dispersion was added dropwise to the p-ANFs dispersion over a period of 20 min. After the addition was complete, stirring was continued for another 30 min to obtain the p-ANFs / HPCTP composite dispersion system. At this point, the dry basis mass fraction of HPCTP relative to p-ANFs was 5 wt%. S3, the above composite dispersion system is placed in a vacuum filtration device, using a PTFE filter membrane with a pore size of 0.45 μm; the filtration vacuum degree is 0.09 MPa, the filtration time is 3.5 h, and a composite wet membrane with a thickness of 50 μm is obtained.

[0037] S4, peel off the composite wet film; perform hot pressing densification treatment on the composite wet film. During hot pressing, the temperature is 95℃, the pressure is 10MPa, and the time is 10min to obtain p-ANFs / HPCTP flame-retardant aramid insulating film.

[0038] The flame-retardant aramid insulating film obtained in Example 1 had a mass of 0.22 g, a thickness of 0.034 mm, an LOI of 29.6%, a dielectric strength of 54.71 kV / mm, and a maximum tensile stress of 135.47 MPa.

[0039] Example 2 S1. Weigh 2.50 g of KOH and dissolve it completely in 150.0 g of DMSO to obtain a basic polar solvent. Measure 90.0 mL of this basic polar solvent into a beaker, add 0.20 g of PPTA, and then add 15.0 g of isopropanol. Stir magnetically at 30 ℃ and 700 rpm for 11 h until the system is dark red and there are no fibrous residues. Then, ultrasonically disperse for 45 min to obtain an aramid nanofiber dispersion system. Pour 90.0 mL of the obtained dispersion system into 900 mL of deionized water, let it stand, filter, and wash with deionized water until the pH of the filtrate is 7.0. Redisperse the filtered p-ANFs wet sample in 40.0 g of deionized water, stir, and ultrasonically disperse for 10 min to obtain a p-ANFs dispersion with a solid content of 0.50 wt%.

[0040] 0.020 g of hexaphenoxycyclotriphosphazene (HPCTP) was weighed and added to 3.980 g of DMSO. After stirring magnetically for 5 min, the mixture was ultrasonically dispersed for 30 min to obtain a 0.50 wt% HPCTP dispersion.

[0041] S2, under stirring at 700 rpm, the HPCTP dispersion was added dropwise to the p-ANFs dispersion over a period of 20 min. After the addition was complete, stirring was continued for another 30 min to obtain the p-ANFs / HPCTP composite dispersion system. At this point, the dry basis mass fraction of HPCTP relative to p-ANFs was 5 wt%. S3, the above composite dispersion system is placed in a vacuum filtration device, using a PTFE filter membrane with a pore size of 0.45 μm; the filtration vacuum degree is 0.09 MPa, the filtration time is 3.8 h, and a composite wet membrane with a thickness of 50 μm is obtained.

[0042] S4, peel off the composite wet film; perform hot pressing densification treatment on the film, with a hot pressing temperature of 100℃, a hot pressing pressure of 10MPa, and a hot pressing time of 10min, to obtain p-ANFs / HPCTP flame-retardant aramid insulating film.

[0043] The flame-retardant aramid insulating film obtained in Example 2 had a mass of 0.23 g, a thickness of 0.042 mm, an LOI of 32.1%, a dielectric strength of 77.88 kV / mm, and a maximum tensile stress of 126.71 MPa.

[0044] Example 3 S1. Weigh 2.50 g of KOH and dissolve it completely in 150.0 g of DMSO to obtain a basic polar solvent. Measure 90.0 mL of this basic polar solvent into a beaker, add 0.20 g of PPTA, and then add 15.0 g of isopropanol. Stir magnetically at 30 ℃ and 700 rpm for 11 h until the system is dark red and there are no fibrous residues. Then, ultrasonically disperse for 45 min to obtain an aramid nanofiber dispersion system. Pour 90.0 mL of the obtained dispersion system into 900 mL of deionized water, let it stand, filter, and wash with deionized water until the pH of the filtrate is 7.0. Redisperse the filtered p-ANFs wet sample in 40.0 g of deionized water, stir, and ultrasonically disperse for 10 min to obtain a p-ANFs dispersion with a solid content of 0.50 wt%.

[0045] 0.030 g of hexaphenoxycyclotriphosphazene (HPCTP) was weighed and added to 5.970 g of DMSO. After stirring magnetically for 5 min, the mixture was ultrasonically dispersed for 30 min to obtain a 0.50 wt% HPCTP dispersion.

[0046] S2, under stirring at 700 rpm, the HPCTP dispersion was added dropwise to the p-ANFs dispersion over a period of 20 min. After the addition was complete, stirring was continued for another 30 min to obtain the p-ANFs / HPCTP composite dispersion system. At this point, the dry basis mass fraction of HPCTP relative to p-ANFs was 5 wt%. S3, the above composite dispersion system is placed in a vacuum filtration device, using a PTFE filter membrane with a pore size of 0.45 μm; the filtration vacuum degree is 0.09 MPa, the filtration time is 3.7 h, and a composite wet membrane with a thickness of 51 μm is obtained.

[0047] S4, peel off the composite wet film; perform hot pressing densification treatment on the film, with a hot pressing temperature of 105℃, a hot pressing pressure of 10MPa, and a hot pressing time of 10min, to obtain p-ANFs / HPCTP flame-retardant aramid insulating film.

[0048] The flame-retardant aramid insulating film obtained in Example 3 had a mass of 0.23 g, a thickness of 0.044 mm, an LOI of 33.2%, a dielectric strength of 56.54 kV / mm, and a maximum tensile stress of 123.43 MPa.

[0049] Example 4 S1. Weigh 2.50 g of KOH and dissolve it completely in 150.0 g of DMSO to obtain a basic polar solvent. Measure 90.0 mL of this basic polar solvent into a beaker, add 0.20 g of PPTA, and then add 15.0 g of isopropanol. Stir magnetically at 30 ℃ and 700 rpm for 11 h until the system is dark red and there are no fibrous residues. Then, ultrasonically disperse for 45 min to obtain an aramid nanofiber dispersion system. Pour 90.0 mL of the obtained dispersion system into 900 mL of deionized water, let it stand, filter, and wash with deionized water until the pH of the filtrate is 7.0. Redisperse the filtered p-ANFs wet sample in 40.0 g of deionized water, stir, and ultrasonically disperse for 10 min to obtain a p-ANFs dispersion with a solid content of 0.50 wt%.

[0050] 0.040 g of hexaphenoxycyclotriphosphazene (HPCTP) was weighed and added to 7.960 g of DMSO. After magnetic stirring for 5 min, the mixture was ultrasonically dispersed for 30 min to obtain a 0.50 wt% HPCTP dispersion.

[0051] S2, under stirring at 700 rpm, the HPCTP dispersion was added dropwise to the p-ANFs dispersion over a period of 20 min. After the addition was complete, stirring was continued for another 30 min to obtain the p-ANFs / HPCTP composite dispersion system. At this point, the dry basis mass fraction of HPCTP relative to p-ANFs was 5 wt%. S3, the above composite dispersion system is placed in a vacuum filtration device, using a PTFE filter membrane with a pore size of 0.45 μm; the filtration vacuum degree is 0.09 MPa, the filtration time is 3.9 h, and a composite wet membrane with a thickness of 52 μm is obtained.

[0052] S4, peel off the composite wet film; perform hot pressing densification treatment on the film, with a hot pressing temperature of 100℃, a hot pressing pressure of 10MPa, and a hot pressing time of 10 min, to obtain p-ANFs / HPCTP flame-retardant aramid insulating film.

[0053] The flame-retardant aramid insulating film obtained in Example 4 had a mass of 0.24 g, a thickness of 0.048 mm, an LOI of 35.1%, a dielectric strength of 36.02 kV / mm, and a maximum tensile stress of 119.23 MPa.

[0054] Among them, the limiting oxygen index (LOI) is tested according to ASTM D2863. The samples are cut according to the standard requirements, and each group of samples is tested no less than 6 times and the average value is taken.

[0055] Dielectric strength: Breakdown test shall be performed in accordance with GB / T 1408.1. The average value of multiple measurements shall be taken for the sample thickness. The voltage rise rate shall be 200V / s. The breakdown voltage shall be recorded and the dielectric strength (kV / mm) shall be calculated according to the sample thickness. Each group of samples shall be tested no less than 10 times and the average value shall be taken.

[0056] Tensile properties: Tensile tests were conducted according to GB / T 1040.3, with a tensile rate of 5 mm / min and a clamp spacing of 10 mm. The maximum tensile stress (MPa) was recorded. Each sample group underwent at least 5 tests, and the average value was taken. Comparative Example 1 S1. Weigh 2.50 g of KOH and dissolve it completely in 150.0 g of DMSO to obtain a basic polar solvent. Measure 90.0 mL of this basic polar solvent into a beaker, add 0.20 g of PPTA, and then add 15.0 g of isopropanol. Stir magnetically at 30 ℃ and 700 rpm for 11 h until the system is dark red and there are no fibrous residues. Then, ultrasonically disperse for 45 min to obtain an aramid nanofiber dispersion system. Pour 90.0 mL of the obtained dispersion system into 900 mL of deionized water, let it stand, filter, and wash with deionized water until the pH of the filtrate is 7.0. Redisperse the filtered p-ANFs wet sample in 40.0 g of deionized water, stir, and ultrasonically disperse for 10 min to obtain a p-ANFs dispersion with a solid content of 0.50 wt%.

[0057] S2, the above p-ANFs dispersion was placed in a vacuum filtration device, and a micro PTFE filter membrane with a pore size of 0.45 μm was selected as the base membrane; the filtration vacuum degree was set to 0.09 MPa, and the filtration was carried out continuously for 3.5 h to obtain a wet membrane with a thickness of about 50 μm.

[0058] S3, the obtained wet membrane was peeled off from the filter membrane and dried to remove moisture and residual solvent from the membrane; subsequently, the dried membrane was subjected to hot-pressing densification treatment at a temperature of 95℃, a pressure of 10MPa, and a time of 10min to obtain a p-ANFs insulating membrane free of HPCTP, denoted as Comparative Example 1. Samples were cut and tested according to the corresponding standards: sample mass was 0.21 g, thickness was 0.030 mm; limiting oxygen index (LOI) was 27.3%; dielectric strength was 36.39 kV / mm; maximum tensile stress was 145.50 MPa.

[0059] Table 1 Performance comparison of Examples 1-4 and Comparative Example 1

[0060] As shown in Table 1, the limiting oxygen index of the composite film generally increases with the increase of HPCTP content, indicating that the introduction of HPCTP helps improve the flame retardant properties of the material. Meanwhile, the composite film maintains a relatively high dielectric strength and maximum tensile stress. In Example 2 (10 wt% HPCTP), the dielectric strength reaches 77.88 kV / mm, demonstrating good retention of insulation performance within an appropriate content range. It should be noted that when the HPCTP content is high, the rheological state and microscopic dispersion uniformity of the composite dispersion system are more sensitive to the film compactness; local agglomeration or micropore defects may lead to fluctuations in dielectric strength.

[0061] Figure 4 This is a comparison chart of the limiting oxygen index (LOI) of flame-retardant aramid insulating films with different HPCTP contents; the horizontal axis represents the mass fraction of HPCTP relative to p-ANFs (wt%), and the vertical axis represents the LOI (%). Comparative Example 1 (0 wt%), Example 1 (5 wt%), Example 2 (10 wt%), Example 3 (15 wt%), and Example 4 (20 wt%) correspond to the data in each group of the chart. Figure 2 As can be seen, with the increase of HPCTP content, the LOI of the composite membrane gradually increased from 27.3% to 35.1%, indicating that the introduction of HPCTP can effectively improve the flame retardancy level of the material. This trend is consistent with the test results in Table 1, reflecting the synergistic effect of phosphorus-nitrogen flame retardant components in the p-ANFs network.

[0062] Figure 5 This is a comparison graph of the dielectric strength of flame-retardant aramid insulating films with different HPCTP contents; the horizontal axis represents the mass fraction of HPCTP relative to p-ANFs (wt%), and the vertical axis represents the dielectric strength (kV / mm). Comparative Example 1 (0 wt%) and Examples 1-4 (5 wt%, 10 wt%, 15 wt%, 20 wt%) correspond to the data sets in the graph. Figure 3It is evident that the dielectric strength of the composite film exhibits a "first increase, then decrease with fluctuations" characteristic with the HPCTP content: the dielectric strength reaches its highest value of 77.88 kV / mm when the HPCTP content is 10 wt%; as the content further increases, the dielectric strength decreases (e.g., 56.54 kV / mm at 15 wt% and 36.02 kV / mm at 20 wt%). This result indicates that within a certain addition range, HPCTP, after ultrasonic dispersion in step S2 and continuous dropwise addition and blending in step S3, can enter the p-ANF nanofiber network deposition process relatively uniformly. This may help reduce micropore defects or improve local compaction during the filtration deposition and hot-pressing densification processes, thereby enhancing the dielectric strength. However, when the HPCTP content is high, the composite dispersion system becomes more sensitive to dispersion stability and interfacial continuity, easily leading to local agglomeration, interfacial discontinuities, or an increase in micropore defects, resulting in electric field concentration and a decrease in breakdown strength. Therefore, in order to obtain a high and stable dielectric strength, it is still necessary to control the dispersion conditions (such as ultrasonic time and dispersion concentration) in step S2 and the dropping / stirring process in step S3, as well as to optimize the parameters of vacuum filtration and hot pressing densification in a coordinated manner, so as to suppress the formation of structural defects under high content conditions.

[0063] Figure 6 This is a comparison chart of the maximum tensile stress of flame-retardant aramid insulating films with different HPCTP contents; the horizontal axis represents the mass fraction of HPCTP relative to p-ANFs (wt%), and the vertical axis represents the maximum tensile stress (MPa). Comparative Example 1 and Examples 1-4 correspond to those in Table 1. Figure 4 As can be seen, the maximum tensile stress of the composite membrane generally decreases with increasing HPCTP content, gradually decreasing from 145.50 MPa in Comparative Example 1 to 119.23 MPa in Example 4. This trend indicates that the introduction of HPCTP as a second phase may weaken the effective connection of the continuous load-bearing network of p-ANFs to some extent, thereby reducing the macroscopic tensile strength. Especially at higher contents, insufficient dispersion or inadequate interfacial bonding can easily lead to a decrease in stress transfer efficiency and induce local stress concentration, further reducing the maximum stress level. However, it can also be seen that under the combined process control of ultrasonic dispersion, continuous dripping blending, vacuum filtration deposition, and hot pressing densification of this invention, the composite membrane still maintains a high mechanical strength (not less than 119 MPa), indicating that this process route may help maintain the structural integrity of the fiber network and mitigate the mechanical attenuation caused by the introduction of the second phase. Therefore, while satisfying the improvement of flame retardant performance, a controllable balance between flame retardancy and mechanical properties can be achieved by optimizing the amount of HPCTP added and its dispersion / dripping / densification conditions.

[0064] This invention employs a combined process of ultrasonic dispersion with polar solvent, continuous dropwise addition under stirring, vacuum filtration to form a film, and hot pressing to densify. By controlling the dispersion and deposition processes, the probability of through-hole defects is reduced, thereby improving the LOI while maintaining high dielectric strength and mechanical properties.

[0065] The above description is merely a preferred embodiment of the present invention and is not intended to limit the technical solution of the present invention in any way. Those skilled in the art should understand that, without departing from the spirit and principles of the present invention, the technical solution can be modified and replaced in several simple ways, and these modifications and replacements are all within the scope of protection covered by the claims.

Claims

1. A method for preparing a flame-retardant aramid insulating film containing hexaphenoxycyclotriphosphazene, characterized in that, The specific steps are as follows: S1, para-aramid fibers are added to an alkaline polar solvent, followed by the addition of isopropanol solution. The mixture is then swollen under magnetic stirring, followed by ultrasonic dispersion and dissociation treatment to obtain a nanofiber dispersion system. The obtained nanofiber dispersion system is subjected to aqueous phase replacement, and after standing and filtration, a para-aramid nanofiber dispersion is obtained. Hexaphenoxycyclotriphosphazene was added to a polar organic solvent under stirring conditions and then dispersed by ultrasonication to obtain a hexaphenoxycyclotriphosphazene dispersion. S2, under stirring conditions, hexaphenoxycyclotriphosphazene dispersion is added dropwise to para-aramid nanofiber dispersion to form p-ANFs / HPCTP composite dispersion system; S3, the p-ANFs / HPCTP composite dispersion system was vacuum filtered to obtain a composite wet membrane; S4, the composite wet film is subjected to hot pressing densification treatment to obtain p-ANFs / hexaphenoxycyclotriphosphazene flame-retardant aramid insulating film.

2. The preparation method according to claim 1, characterized in that, In S1, the alkaline polar solvent is a potassium hydroxide / dimethyl sulfoxide system, wherein the amount of potassium hydroxide is 2~3 g and the amount of dimethyl sulfoxide is 140~160 g; the amount of para-aramid fiber added is 0.2~0.3 g, the volume of isopropanol added is 80~120 mL, and the solid content of the para-aramid nanofiber dispersion is controlled in the range of 0.2 wt%~2.0 wt%.

3. The preparation method according to claim 1, characterized in that, In S1, during swelling, the temperature is 20~35℃, the time is 10~12h, and the magnetic stirring speed is 600~800rpm; During aqueous phase replacement, the aramid dispersion system is mixed with deionized water at a volume ratio of (80-100) mL: (800-1000) mL.

4. The preparation method according to claim 1, characterized in that, In S1, the polar organic solvent is dimethyl sulfoxide, the concentration of the hexaphenoxycyclotriphosphazene dispersion is 0.5 wt%~5 wt%, and the ultrasonication frequency is 20~40 Hz for 20~40 min.

5. The preparation method according to claim 1, characterized in that, In S2, the dropping method is continuous dropping at a rate of 10 mL / min. Stirring is maintained during the dropping process, and stirring is continued for 20-40 min after the dropping is completed. The stirring speed is 600-800 rpm.

6. The preparation method according to claim 1, characterized in that, In S2, the mass fraction of HPCTP relative to p-ANFs is 0~20 wt%.

7. The preparation method according to claim 1, characterized in that, In S3, during vacuum filtration, a microporous polytetrafluoroethylene membrane is used as the filter membrane, with a pore size of 0.22~0.45μm, a vacuum filtration pressure of 0.08~0.10MPa, a time of 3~4h, and a composite wet membrane thickness of 30~80μm.

8. The preparation method according to claim 1, characterized in that, In S4, during hot pressing, the temperature is 95~105℃, the pressure is 5~15MPa, and the time is 8~12min.

9. The p-ANFs / hexaphenoxycyclotriphosphazene flame-retardant aramid insulating film prepared by the method according to any one of claims 1 to 8, characterized in that, The flame-retardant aramid insulating film has the following properties: limiting oxygen index ≥27%, dielectric strength ≥30kV / mm, and maximum tensile stress not less than 119MPa.

10. The flame-retardant aramid insulating film according to claim 9, characterized in that: The flame-retardant aramid insulating film comprises a p-ANFs matrix and HPCTP dispersed in the matrix.

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