Motor insulation paper and preparation method thereof

Abstract

The invention relates to the technical field of materials. The invention provides motor insulation paper and a preparation method thereof. The method comprises the following steps: carrying out chemical grafting on carboxyl functionalized PBO fibers, carboxymethyl chitosan, ethylenediamine and dimethylformamide to obtain PBO fibers grafted with carboxymethyl chitosan; preparing a shell solution by using the PBO fiber grafted with carboxymethyl chitosan; preparing a core layer solution by using boron nitride nanotubes and carboxymethyl chitosan; the shell layer solution and the core layer solution are subjected to coaxial electrostatic spinning, and shell-core structure nanofibers are obtained; the method comprises the following steps: adding deionized water into shell-core structure nanofibers, shearing, uniformly mixing and pulping to obtain nanofiber paper pulp; carrying out suction filtration on the nanofiber paper pulp, and drying to obtain coarse paper; and carrying out hot pressing treatment on the rough paper to obtain the motor insulation paper. The motor insulation paper has the advantages of high heat-conducting property, excellent electrical property and excellent mechanical property.

Description

Technical Field

[0001] The present invention relates to the technical field of materials, and in particular to motor insulating paper and a preparation method thereof. Background Art

[0002] As a common electrical insulation material, insulation paper should have good electrical insulation performance, high temperature resistance, thermal conductivity and mechanical properties. PBO fiber has become a new material in the field of insulation paper due to its excellent performance, especially outstanding mechanical properties and dielectric properties. However, PBO fiber has poor thermal conductivity. When the machine wrapped by the insulation paper works at high frequency and high voltage, the heat generated cannot be discharged in time, which can easily cause overheating.

[0003] In order to improve the thermal conductivity of PBO fibers, researchers currently dope boron nitride nanotubes BNNS into PBO fibers to improve their thermal conductivity. However, due to the smooth surface and chemical inertness of PBO fibers, the interface bonding strength between BNNS and PBO fibers is low, which makes the heat dissipation efficiency of the prepared composite insulation paper low, so the improvement of its thermal conductivity is very limited. Using ultraviolet light treatment and other means to increase the surface bonding force of PBO will cause damage to PBO fibers and reduce the mechanical strength of the composite insulation paper. Summary of the invention

[0004] The present invention provides a motor insulation paper and a preparation method thereof, which are used to solve the problem that the thermal conductivity of PBO insulation paper in the prior art cannot meet the current development demand of motor insulation materials.

[0005] A method for preparing motor insulating paper, comprising:

[0006] Step 1: Carboxyl functionalizing the PBO fiber to obtain carboxyl functionalized PBO fiber;

[0007] Step 2: chemically grafting the carboxyl functionalized PBO fiber, carboxymethyl chitosan, ethylenediamine, and dimethylformamide to obtain PBO fiber grafted with carboxymethyl chitosan;

[0008] Step 3: adding the PBO fiber grafted with carboxymethyl chitosan to deionized water, and slurrying by shear mixing to obtain a coaxial electrospinning shell layer solution; adding boron nitride nanotubes and carboxymethyl chitosan to deionized water, and slurrying by shear mixing to obtain a coaxial electrospinning core layer solution;

[0009] Step 4: transferring the shell layer solution and the core layer solution into a coaxial electrospinning syringe, and performing coaxial electrospinning to obtain shell-core structure nanofibers;

[0010] Step 5: adding the shell-core structure nanofibers to deionized water, and pulping by shear mixing to obtain nanofiber pulp; and filtering the nanofiber pulp, and drying to obtain rough paper;

[0011] Step 6: subjecting the rough paper to heat pressing treatment to obtain motor insulation paper.

[0012] Furthermore, in the method for preparing motor insulation paper as described above, the core diameter of the shell-core structured nanofiber is 100-200 nm, and the shell diameter is 300-500 nm.

[0013] Further, in the method for preparing the motor insulation paper as described above, step 3 comprises: mixing the PBO fiber grafted with carboxymethyl chitosan with deionized water at a mass ratio of 1:300 to 350, and mixing and slurrying at a shear rate of 10000 to 25000 rpm to obtain a coaxial electrospinning shell solution;

[0014] The step 4 comprises: mixing boron nitride nanotubes, carboxymethyl chitosan and deionized water in a mass ratio of 1:1-1.5:350-400, and mixing and slurrying at a shear rate of 10000-15000 rpm to obtain a coaxial electrospinning core layer solution.

[0015] Furthermore, in the method for preparing the motor insulation paper as described above, in step 2, the conditions for chemical grafting are: heating at 180° C. in an autoclave for 12 hours.

[0016] Furthermore, the method for preparing motor insulation paper as described above, before step 3, further comprises: washing the PBO fiber grafted with carboxymethyl chitosan with an excess of tetrahydrofuran, and then vacuum drying the washed PBO fiber grafted with carboxymethyl chitosan at 180° C. for 24 hours.

[0017] Furthermore, in the method for preparing the motor insulating paper as described above, step 1 comprises the following steps:

[0018] Step a: washing the PBO fiber with an acetone solvent, and then drying it in a vacuum oven at 80-85° C. to obtain dried PBO fiber;

[0019] Step b: adding the dried PBO fibers into deionized water and subjecting them to ultrasonic shearing treatment to prepare PBO nanofibers;

[0020] Step c: reflux oxidizing the PBO nanofibers in nitric acid at 80° C. for 0.5 to 1 h, and washing with deionized water for several times to finally obtain the carboxyl functionalized PBO fibers.

[0021] Furthermore, in the method for preparing motor insulation paper as described above, the concentration of nitric acid is 15.3-17.2 mol / L.

[0022] Furthermore, in the method for preparing the motor insulation paper as described above, the preparation of the carboxymethyl chitosan comprises:

[0023] Step a: dissolving chitosan in dilute acetic acid and precipitating with excess acetone to obtain chitosan acetate;

[0024] Step b: the chitosan acetate is transferred into a reaction bottle with stirring, a certain amount of sodium hydroxide solution and isopropanol are added, and the isopropanol solution of chloroacetic acid is added dropwise while stirring, the reaction temperature is controlled to 70° C., the reaction is carried out for several hours, cooled to room temperature, the pH value is adjusted to neutral with dilute acid, washed with 85% methanol, and dried to obtain carboxymethyl chitosan.

[0025] Furthermore, in the method for preparing motor insulation paper as described above, the diameter of the PBO fiber is 40-80 nm, and the length is 0.2 mm-0.3 mm; the diameter of the boron nitride nanotube is 5-60 nm, and the length is 100 nm-2 μm.

[0026] The motor insulation paper is prepared according to any of the above methods.

[0027] The motor insulation paper prepared by the present invention is formed by interlacing and connecting composite fibers with a leaf main vein structure as basic units. It uses PBO fiber as a skeleton, boron nitride nanotubes as a heat-conducting functional unit, and carboxymethyl chitosan as a chemical grafting agent of PBO fiber to construct an efficient heat-conducting channel, thereby forming a motor insulation paper with a leaf vein structure. The motor insulation paper has high thermal conductivity, excellent electrical properties and mechanical properties.

[0028] The preparation method of the motor insulation paper provided by the present invention increases the contact area between the PBO fiber and the boron nitride nanotube by introducing carboxylated chitosan and ethylenediamine as the grafting agent of the PBO fiber matrix; at the same time, the present invention integrates the high thermal conductivity boron nitride nanotubes and the carboxymethyl chitosan with excellent adhesion function into the PBO fiber, and makes the prepared insulation paper have the characteristics of imitating the leaf vein epidermis and vascular bundle structure through the coaxial electrospinning technology, realizes the construction of the directional heat conduction channel of the boron nitride nanotube, greatly improves the thermal conductivity of the composite insulation paper, and avoids the problem of overheating of the insulation paper when using the insulation paper, which causes the insulation paper to age too quickly and has a short life. The method provided by the present invention is simple and easy to operate, low in cost, high in quality, and suitable for industrial large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 It is a schematic diagram of the present invention using coaxial electrospinning to prepare shell-core structure nanofibers.

[0031] Figure 2 This is a comparison chart of thermal conductivity of motor insulation paper prepared in Examples 1 to 5;

[0032] Figure 3 A comparison diagram of the breakdown field strength of the motor insulation paper prepared in Examples 1 to 5;

[0033] Figure 4 This is a comparison chart of thermal conductivity of motor insulation paper prepared in Example 1, Comparative Example 1 and Comparative Example 2;

[0034] Figure 5 This is a comparison chart of the elastic modulus of the motor insulation paper prepared in Example 1, Comparative Example 1 and Comparative Example 2;

[0035] Figure 6 The relative dielectric constant comparison diagram of the motor insulation paper prepared in Example 1, Comparative Example 1 and Comparative Example 2;

[0036] Reference numerals:

[0037] 1-first syringe; 2-second syringe; 3-shell-core structure nanofiber. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The present invention provides a method for preparing motor insulating paper, comprising the following steps:

[0040] Step 1: Carboxyl functionalizing the PBO fiber to obtain carboxyl functionalized PBO fiber;

[0041] Step a: The PBO fiber is continuously washed with acetone solvent using a Soxhlet extractor for 24 hours to remove surface contaminants, and then dried in a vacuum oven at 80° C., and deionized water is added to disperse the decontaminated PBO fiber in the deionized water;

[0042] Step b: subjecting the PBO fiber raw material dispersed in water to high-speed ultrasonic shearing treatment to prepare PBO nanofibers.

[0043] In this step, the present invention adopts high-speed ultrasonic shearing to prepare PBO nanofibers, and the parameter setting is simple, convenient and fast; it can improve the preparation efficiency of PBO nanofibers. Moreover, the PBO nanofibers obtained by high-speed ultrasonic shearing have better dispersion effect, and after carboxyl functionalization, the surface binding force between them and BNNT can be stronger, and the stability of the coaxial wire structure can be ensured when shearing to make pulp.

[0044] Step c: reflux oxidize the PBO nanofibers in nitric acid (15.3-17.2 mol / L) at 80°C for 0.5-1h, then take out the carboxyl functionalized PBO fibers, wash them several times with deionized water with a neutral pH, and vacuum dry to obtain carboxyl functionalized PBO fibers.

[0045] In this step, the present invention introduces a certain number of carboxyl groups on the surface of the PBO nanofibers by subjecting them to acid reflux treatment, thereby obtaining carboxyl-functionalized PBO fibers with multiple active sites; and the carboxyl groups can be connected with the amino groups of ethylenediamine, which is beneficial for combining with boron nitride nanotubes in the subsequent coaxial electrospinning, thereby constructing a directional heat conduction channel of the boron nitride nanotubes and ultimately improving the thermal conductivity of the insulating paper.

[0046] Step 2: Dissolve chitosan in dilute acetic acid, precipitate with excess acetone to obtain chitosan acetate, transfer to a reaction bottle with stirring, add a certain amount of sodium hydroxide solution and isopropanol, add isopropanol solution of chloroacetic acid dropwise while stirring, control the reaction temperature to 70-75°C, react for several hours, cool to room temperature, adjust the pH to neutral with dilute acid, wash with 85% methanol, and dry to obtain carboxymethyl chitosan.

[0047] Step 3: The carboxyl functionalized PBO fiber obtained in step 1, the carboxymethyl chitosan obtained in step 2, and ethylenediamine and dimethylformamide are transferred to a polytetrafluoroethylene-lined autoclave, and then heated at 180°C for 12 hours to obtain PBO fiber grafted with carboxymethyl chitosan with ethylenediamine. The polytetrafluoroethylene-lined autoclave is cooled to room temperature, the obtained PBO fiber grafted with carboxymethyl chitosan is taken out, and rinsed with excess tetrahydrofuran under ultrasound to remove dimethylformamide and unreacted ethylenediamine. Finally, the prepared PBO fiber grafted with carboxymethyl chitosan is vacuum dried at 180°C for 24 hours.

[0048] In this step, ethylenediamine is used as a connector to connect carboxymethyl chitosan and carboxyl functionalized PBO fiber. Dimethylformamide is used as a solvent to increase the contact area of ​​several reactants. The amino group of ethylenediamine and the carboxyl group of carboxymethyl chitosan and the carboxyl group of carboxyl functionalized PBO fiber can form a peptide reaction, and ethylenediamine plays a role in connecting carboxymethyl chitosan and PBO. That is, the objects connected by ethylenediamine are the carboxyl functionalized PBO fiber and carboxymethyl chitosan. Its structure is similar to: carboxyl functionalized PBO-ethylenediamine-carboxymethyl chitosan. The present invention uses carboxymethyl chitosan as a grafting agent, which increases the surface area of ​​PBO, which is beneficial to the subsequent coaxial electrospinning.

[0049] Step 4: The PBO fiber grafted with carboxymethyl chitosan obtained in step 3 is mixed with deionized water in a mass ratio of 1:300-350, and pulped by high-speed shear mixing at 10000-25000rpm to obtain a coaxial electrospinning shell solution; the boron nitride nanotubes and the carboxymethyl chitosan obtained in step 2 and deionized water are mixed in a mass ratio of 1:1-1.5:350-400, and pulped by high-speed shear mixing at 10000-15000rpm to obtain a coaxial electrospinning core solution.

[0050] In this step, although carboxymethyl chitosan is used as a grafting agent to increase the surface area of ​​PBO, active sites will still remain on the surface of PBO, so the present invention can further increase the contact area of ​​PBO by using boron nitride nanotubes and carboxymethyl chitosan to prepare a coaxial electrostatic spinning core solution, so as to be more conducive to combining with boron nitride nanotubes during coaxial electrostatic spinning. On the other hand, due to the certain gap between the dielectric constants of PBO and boron nitride nanotubes, using boron nitride nanotubes alone may cause the accumulated charge between the core shells, resulting in an increase in the electric field between the core shells, affecting the breakdown performance, so the present invention uses carboxymethyl chitosan to play the role of electric field transition, so as to improve the breakdown field strength of motor insulation paper.

[0051] Step 5: Transfer the obtained coaxial electrospinning shell layer solution to the first coaxial electrospinning syringe 1, and transfer the coaxial electrospinning core layer solution to the second coaxial electrospinning syringe 2. At the same time, ensure that the needle is firmly connected to the electrospinning cylinder equipment, and reliably ground the spinning equipment to perform coaxial electrospinning to form a shell-core structure nanofiber 3 with a leaf vein-mimicking structure. The core of the shell-core structure nanofiber 3 imitates the vascular bundle of the leaf vein structure, and its diameter is 100-200nm. The shell of the shell-core structure nanofiber 3 imitates the epidermis of the leaf vein structure, and its diameter is 300-500nm.

[0052] The present invention directly contacts the PBO fiber grafted with carboxymethyl chitosan with the boron nitride nanotubes by coaxial electrospinning. On the one hand, the probability of directional construction of the heat conduction channel is increased, thereby effectively improving the thermal conductivity of the motor insulation paper. On the other hand, since the present invention uses only one coaxial wire to prepare the insulation paper, the interior of the insulation paper is uniform, thereby greatly avoiding the situation of charge accumulation or even local discharge under a strong field due to different dielectric constants of multiple materials. Therefore, the present invention adopts the coaxial electrospinning method to make it difficult for the electric field charge to accumulate at the junction between the core and the shell, thereby effectively improving the breakdown performance of the motor insulation paper.

[0053] like Figure 1 As shown, in this step, the vascular bundle (boron nitride nanosheet tube) of the bionic leaf vein structure and the mechanical tissue (PBO fiber grafted with carboxymethyl chitosan) of the leafless epidermis are connected to each other through coaxial electrospinning; during electrospinning, the thermal conduction channel of the boron nitride nanosheet is better constructed, the transmission speed of the phonon inside the material is increased, and the thermal conductivity of the motor insulation paper is more superior.

[0054] In addition, this step adopts a coaxial electrospinning process, so that the insulating paper has a uniform fiber distribution and a compact structure, which helps to disperse and resist the external electric field, reduce the local electric field concentration phenomenon, and reduce the risk of breakdown. Secondly, the fibers produced by coaxial electrospinning have a high degree of orientation, which makes them have higher mechanical stability and thermal stability, and can effectively resist the breakage of the molecular chain caused by the external electric field. If PBO and boron nitride nanotubes are simply combined, the orientation of the boron nitride nanotubes will be disordered, thereby significantly reducing the thermal conductivity of the prepared motor insulation paper.

[0055] Step 6: Add the shell-core structure nanofibers into deionized water, and pulp them by shear mixing to obtain nanofiber pulp; and filter the nanofiber pulp and dry it to obtain rough paper.

[0056] In this step, the present invention adopts a vacuum filtration method to prepare rough paper. On the one hand, the vacuum filtration method is suitable for the preparation of large-area insulating paper. By increasing the filter membrane area, a larger area of ​​insulating paper can be prepared. On the other hand, the preparation process of the vacuum filtration method is relatively stable, and the result has good repeatability.

[0057] Step 7: subjecting the rough paper to heat pressing treatment to obtain motor insulation paper.

[0058] The diameter of the PBO fiber is 40-80 nm, and the length is 0.2 mm-0.3 mm; the diameter of the boron nitride nanotube is 5-60 nm, and the length is 100 nm-2 μm.

[0059] The boron nitride nanotubes are commercially purchased. The theoretical thermal conductivity of boron nitride nanotubes can reach 1700-2000 W / mK. In addition to excellent thermal conductivity, the dielectric strength of boron nitride is as high as 35 kV / mm, which enables it to take into account both heat dissipation and insulation properties.

[0060] Embodiment 1:

[0061] Step 1: The PBO fiber is continuously washed with acetone solvent using a Soxhlet extractor for 24 hours to remove surface pollutants, and then dried in a vacuum oven at 80°C, and deionized water is added to disperse the decontaminated PBO fiber in deionized water; the PBO fiber raw material dispersed in water is subjected to high-speed ultrasonic shearing treatment to prepare PBO nanofibers; the PBO nanofibers are refluxed and oxidized in nitric acid (16 mol / L) at 80°C for 0.8 h, and then the carboxyl functionalized PBO fiber is taken out, washed several times with deionized water, the pH of the washing water is neutral, and vacuum dried to obtain PBO nanofibers.

[0062] Step 2: Dissolve chitosan in dilute acetic acid, precipitate with excess acetone to obtain chitosan acetate, transfer to a reaction bottle with stirring, add a certain amount of sodium hydroxide solution and isopropanol, add isopropanol solution of chloroacetic acid dropwise while stirring, control the reaction temperature to 70-75°C, react for several hours, cool to room temperature, adjust the pH to neutral with dilute acid, wash with 85% methanol, and dry to obtain carboxymethyl chitosan.

[0063] Step 3: The carboxyl functionalized PBO fiber obtained in step 1, the carboxymethyl chitosan obtained in step 2, and ethylenediamine and dimethylformamide are transferred to a polytetrafluoroethylene-lined autoclave, and then heated at 180°C for 12 hours to obtain PBO fiber grafted with carboxymethyl chitosan with ethylenediamine. The polytetrafluoroethylene-lined autoclave is cooled to room temperature, the obtained PBO fiber grafted with carboxymethyl chitosan is taken out, and rinsed with excess tetrahydrofuran under ultrasound to remove dimethylformamide and unreacted ethylenediamine. Finally, the prepared PBO fiber grafted with carboxymethyl chitosan is vacuum dried at 180°C for 24 hours.

[0064] Step 4: the PBO fiber grafted with carboxymethyl chitosan obtained in step 3 is mixed with deionized water at a mass ratio of 1:300, and slurried by high-speed shear mixing at 25000 rpm to obtain a coaxial electrospinning shell solution;

[0065] The boron nitride nanotubes, the carboxymethyl chitosan obtained in step 2, and deionized water were mixed in a mass ratio of 1:1:350, and slurried by high-speed shear mixing at 15000 rpm to obtain a coaxial electrospinning core layer solution.

[0066] Step 5: Transfer the obtained coaxial electrospinning shell layer solution and coaxial electrospinning core layer solution to the coaxial electrospinning syringe, ensure that the needle is firmly connected to the electrospinning cylinder equipment, reliably ground the spinning equipment, and perform coaxial electrospinning to form a shell-core structure nanofiber with a leaf vein-like core-shell structure, wherein the diameter of the core is 100 to 200 nm, and the diameter of the shell is 300 to 500 nm;

[0067] Step 6: adding the shell-core structure nanofibers to deionized water, and pulping by shear mixing to obtain nanofiber pulp; and filtering the nanofiber pulp, and drying to obtain rough paper;

[0068] Step 7: subjecting the rough paper to heat pressing treatment to obtain motor insulation paper.

[0069] Embodiment 2:

[0070] This embodiment is based on Embodiment 1, and differs from Embodiment 1 in that the mass ratio of the PBO fiber grafted with carboxymethyl chitosan to the boron nitride nanotube is 1:2.

[0071] Embodiment 3:

[0072] This embodiment is based on Embodiment 1, and differs from Embodiment 1 in that the mass ratio of the PBO fiber grafted with carboxymethyl chitosan to the boron nitride nanotube is 1:3.

[0073] Embodiment 4:

[0074] This embodiment is based on Embodiment 1, and differs from Embodiment 1 in that the mass ratio of the PBO fiber grafted with carboxymethyl chitosan to the boron nitride nanotube is 3:1.

[0075] Embodiment 5:

[0076] This embodiment is based on Embodiment 1, and differs from Embodiment 1 in that the mass ratio of the PBO fiber grafted with carboxymethyl chitosan to the boron nitride nanotube is 2:1.

[0077] Embodiment 6:

[0078] This embodiment is based on embodiment 1, and the difference from embodiment 1 is that: the concentration of nitric acid is 17.2 mol / L; the reflux oxidation time is 1 h; in step 4, the mass ratio of PBO fiber grafted with carboxymethyl chitosan to deionized water is 1:350; the shear rate is 10000 rpm; the mass ratio of boron nitride nanotubes, carboxymethyl chitosan and deionized water is 1:1.5:400; the shear rate of the three is 10000 rpm;

[0079] Embodiment 7:

[0080] This embodiment is based on Example 1, and differs from Example 1 in that: the concentration of nitric acid is 15.3 mol / L; the reflux oxidation time is 0.5 h; in step 4, the mass ratio of PBO fiber grafted with carboxymethyl chitosan to deionized water is 1:320; the shear rate is 20000 rpm; the mass ratio of boron nitride nanotubes, carboxymethyl chitosan and deionized water is 1:1.2:380; and the shear rate of the three is 12000 rpm.

[0081] Figure 2 This is a comparison chart of the thermal conductivity of the motor insulation paper prepared in Example 1 to Example 5. Figure 2 It can be seen that the higher the BNNT content, the higher the thermal conductivity of the motor insulation paper; Figure 3 The breakdown field strength comparison diagram of the motor insulation paper prepared in Example 1 to Example 5 is shown in FIG. Figure 3 It can be seen that the higher the content of PBO fiber, the higher the breakdown field strength of the motor insulation paper.

[0082] Comparative Example 1:

[0083] The difference between this embodiment and embodiment 1 is that PBO and deionized water are directly mixed at a mass ratio of 1:300 to prepare the coaxial electrospinning shell solution.

[0084] Comparative Example 2:

[0085] The difference between this embodiment and embodiment 1 is that the shell layer solution and the core layer solution are directly filtered to form motor insulation paper instead of being combined by coaxial electrospinning.

[0086] Figure 4 The thermal conductivity comparison diagram of the motor insulation paper prepared in Example 1, Comparative Example 1 and Comparative Example 2 is shown in FIG. Figure 4 It can be seen that if only pure PBO is used to prepare motor insulation paper, its thermal conductivity is much lower than that of motor insulation paper prepared by PBO grafted with carboxymethyl chitosan. Moreover, if the shell solution and the core solution are directly filtered to form motor insulation paper, the thermal conductivity of the motor insulation paper prepared is lower than that of the motor insulation paper prepared by coaxial electrospinning.

[0087] Figure 5 The elastic modulus comparison diagram of the motor insulation paper prepared in Example 1, Comparative Example 1 and Comparative Example 2; Figure 5 It can be seen that the elastic modulus of the insulating paper prepared by coaxial electrospinning of PBO fibers grafted with carboxymethyl chitosan and boron nitride nanotubes is comparable. Therefore, coaxial electrospinning does not affect the good mechanical properties of PBO. Figure 6 The relative dielectric constant comparison diagram of the motor insulation paper prepared in Example 1, Comparative Example 1 and Comparative Example 2; Figure 6It can be seen that the insulating paper prepared by coaxial electrospinning of PBO fibers grafted with carboxymethyl chitosan and boron nitride nanotubes does not increase the dielectric loss angle of the insulating paper, but reduces the dielectric loss angle of the insulating paper. Therefore, the insulating paper prepared by the method of the present invention has better electrical properties.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing motor insulating paper, characterized in that: include: Step 1: Carboxyl functionalizing the PBO fiber to obtain carboxyl functionalized PBO fiber; Step 2: chemically grafting the carboxyl functionalized PBO fiber, carboxymethyl chitosan, ethylenediamine, and dimethylformamide to obtain PBO fiber grafted with carboxymethyl chitosan; Step 3: adding the PBO fiber grafted with carboxymethyl chitosan to deionized water, and slurrying by shear mixing to obtain a coaxial electrospinning shell layer solution; adding boron nitride nanotubes and carboxymethyl chitosan to deionized water, and slurrying by shear mixing to obtain a coaxial electrospinning core layer solution; Step 4: transferring the shell layer solution and the core layer solution into a coaxial electrospinning syringe, and performing coaxial electrospinning to obtain shell-core structure nanofibers; Step 5: adding the shell-core structure nanofibers to deionized water, and pulping by shear mixing to obtain nanofiber pulp; The nanofiber pulp is filtered and dried to obtain rough paper; Step 6: subjecting the rough paper to heat pressing treatment to obtain motor insulation paper.

2. The method for preparing motor insulating paper according to claim 1, characterized in that: The core diameter of the shell-core structured nanofiber is 100-200 nm, and the shell diameter is 300-500 nm.

3. The method for preparing motor insulating paper according to claim 1, characterized in that: Step 3 comprises: mixing the PBO fiber grafted with carboxymethyl chitosan with deionized water at a mass ratio of 1:300-350, and mixing and slurrying at a shear rate of 10000-25000 rpm to obtain a coaxial electrospinning shell solution; The step 4 comprises: mixing boron nitride nanotubes, carboxymethyl chitosan and deionized water in a mass ratio of 1:1-1.5:350-400, and mixing and slurrying at a shear rate of 10000-15000 rpm to obtain a coaxial electrospinning core layer solution.

4. The method for preparing motor insulating paper according to claim 1, characterized in that: In step 2, the chemical grafting conditions are: heating at 180° C. in an autoclave for 12 h.

5. The method for preparing motor insulating paper according to claim 1, characterized in that: Before step 3, the method further includes: washing the PBO fiber grafted with carboxymethyl chitosan with an excess of tetrahydrofuran, and then vacuum drying the washed PBO fiber grafted with carboxymethyl chitosan at 180° C. for 24 hours.

6. The method for preparing motor insulation paper according to any one of claims 1 to 5, characterized in that: The step 1 comprises the following steps: Step a: washing the PBO fiber with an acetone solvent, and then drying it in a vacuum oven at 80-85° C. to obtain dried PBO fiber; Step b: adding the dried PBO fibers into deionized water and subjecting them to ultrasonic shearing treatment to prepare PBO nanofibers; Step c: reflux oxidizing the PBO nanofibers in nitric acid at 80° C. for 0.5 to 1 h, and washing with deionized water for several times to finally obtain the carboxyl functionalized PBO fibers.

7. The method for preparing motor insulation paper according to any one of claims 1 to 5, characterized in that: The concentration of the nitric acid is 15.3-17.2 mol / L.

8. The method for preparing motor insulating paper according to any one of claims 1 to 5, characterized in that: The preparation of the carboxymethyl chitosan comprises: Step a: dissolving chitosan in dilute acetic acid and precipitating with excess acetone to obtain chitosan acetate; Step b: the chitosan acetate is transferred into a reaction bottle with stirring, a certain amount of sodium hydroxide solution and isopropanol are added, and the isopropanol solution of chloroacetic acid is added dropwise while stirring, the reaction temperature is controlled to 70° C., the reaction is carried out for several hours, cooled to room temperature, the pH value is adjusted to neutral with dilute acid, washed with 85% methanol, and dried to obtain carboxymethyl chitosan.

9. The method for preparing motor insulation paper according to any one of claims 1 to 5, characterized in that: The diameter of the PBO fiber is 40-80nm, and the length is 0.2mm-0.3mm; the diameter of the boron nitride nanotube is 5-60nm, and the length is 100nm-2μm.

10. Motor insulation paper prepared according to any one of claims 1 to 9.

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