Motor insulation paper and preparation method thereof

Abstract

The present invention relates to the field of materials technology. A motor insulation paper and a preparation method thereof are provided. The method comprises: chemically grafting carboxyl-functionalized PBO fibers, carboxymethyl chitosan, ethylenediamine, and dimethylformamide to obtain PBO fibers grafted with carboxymethyl chitosan; preparing a shell solution using the PBO fibers grafted with carboxymethyl chitosan; preparing a core solution using boron nitride nanotubes and carboxymethyl chitosan; coaxially electrospinning the shell solution and the core solution to obtain shell-core structured nanofibers; adding deionized water to the shell-core structured nanofibers, pulping by shear mixing, and obtaining nanofiber pulp; filtering the nanofiber pulp and drying it to obtain rough paper; and hot-pressing the rough paper to obtain motor insulation paper. The motor insulation paper has high thermal conductivity and excellent electrical and mechanical properties.

Description

Technical Field

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

[0002] Insulation paper, a common electrical insulation material, should possess excellent electrical insulation, high-temperature resistance, thermal conductivity, and mechanical properties. PBO fiber, due to its superior properties, particularly its outstanding mechanical and dielectric properties, has become a new material in the insulation paper market. However, PBO fiber has poor thermal conductivity. When the machinery wrapped in the insulation paper operates at high frequencies and high voltages, the heat generated cannot be dissipated promptly, potentially leading to overheating.

[0003] To improve the thermal conductivity of PBO fibers, researchers are currently doping them with boron nitride nanotubes (BNNS). However, due to the smooth surface and chemical inertness of PBO fibers, the interfacial bonding strength between BNNS and PBO fibers is low, resulting in low heat dissipation efficiency through the thermal conduction channels of the resulting composite insulating paper, thus limiting the improvement in thermal conductivity. Furthermore, increasing the surface bonding strength of PBO, such as with UV treatment, can damage the PBO fibers and reduce the mechanical strength of the composite insulating 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 in the prior art that the thermal conductivity of PBO insulation paper cannot meet the current development requirements 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 a 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 solution; adding boron nitride nanotubes and carboxymethyl chitosan to deionized water, and slurrying by shear mixing to obtain a coaxial electrospinning core 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 structured 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 a heat pressing process 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] Furthermore, 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 in a mass ratio of 1:300 to 350, and slurrying the mixture at a shear rate of 10,000 to 25,000 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 speed of 10,000-15,000 rpm to obtain a coaxial electrospinning core layer solution.

[0015] Furthermore, in the above-mentioned method for preparing motor insulation paper, 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 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.

[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 to 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 hour, 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 stirred reaction flask, a certain amount of sodium hydroxide solution and isopropanol are added, and a chloroacetic acid isopropanol solution is added dropwise while stirring. The reaction temperature is controlled at 70° C., the reaction is carried out for several hours, cooled to room temperature, and the pH value is adjusted to neutral with dilute acid. The mixture is washed with 85% methanol and dried to obtain carboxymethyl chitosan.

[0025] Furthermore, in the above-mentioned method for preparing motor insulation paper, 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 produced by the present invention is composed of interlaced composite fibers with a leaf-like main vein structure as basic units. Using PBO fibers as the backbone, boron nitride nanotubes as the thermally conductive functional elements, and carboxymethyl chitosan as a chemical grafting agent for the PBO fibers, it creates efficient thermal conduction channels, resulting in a motor insulation paper with a leaf-like vein structure. This motor insulation paper combines high thermal conductivity with excellent electrical and mechanical properties.

[0028] The present invention provides a method for preparing motor insulating paper. By introducing carboxylated chitosan and ethylenediamine as grafting agents for the PBO fiber matrix, the contact area between the PBO fiber and the boron nitride nanotubes is increased. Furthermore, by incorporating highly thermally conductive boron nitride nanotubes and carboxymethyl chitosan, which has excellent adhesion properties, into the PBO fibers, the present invention uses coaxial electrospinning technology to impart to the prepared insulating paper characteristics that mimic the structure of leaf veins, epidermis, and vascular bundles. This allows the boron nitride nanotubes to construct directional heat conduction channels, significantly improving the thermal conductivity of the composite insulating paper and avoiding the problem of rapid aging and short lifespan of the insulating paper caused by overheating during use. The method provided by the present invention is simple and easy to operate, low in cost, and high in quality, making it 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, a brief introduction is given below to 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 any 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 the thermal conductivity of motor insulation paper prepared in Examples 1 to 5;

[0032] Figure 3 This is a comparison chart 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 the 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 This is a comparison chart of the relative dielectric constants of the motor insulation papers prepared in Example 1, Comparative Examples 1 and 2;

[0036] Reference numerals:

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

[0038] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall 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 was 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 was 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 uses high-speed ultrasonic shearing to prepare PBO nanofibers. This method simplifies parameter setting, is convenient, and improves the efficiency of PBO nanofiber preparation. Furthermore, the PBO nanofibers obtained using high-speed ultrasonic shearing exhibit better dispersion. Furthermore, carboxyl functionalization enhances surface adhesion with BNNTs, ensuring a stable coaxial filament structure during shearing to produce pulp.

[0044] Step c: oxidizing the PBO nanofibers in nitric acid (15.3-17.2 mol / L) at 80° C. for 0.5-1 h, then taking out the carboxyl-functionalized PBO fibers, washing them several times with deionized water to a neutral pH, and vacuum drying 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. Moreover, the carboxyl groups can be linked to 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 and precipitate with excess acetone to obtain chitosan acetate, transfer it to a stirred reaction flask, add a certain amount of sodium hydroxide solution and isopropanol, add chloroacetic acid isopropanol solution 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: Transfer the carboxyl-functionalized PBO fibers obtained in Step 1, the carboxymethyl chitosan obtained in Step 2, and ethylenediamine and dimethylformamide to a polytetrafluoroethylene-lined autoclave and heat at 180°C for 12 hours to obtain PBO fibers grafted with carboxymethyl chitosan. The polytetrafluoroethylene-lined autoclave is cooled to room temperature, and the resulting PBO fibers grafted with carboxymethyl chitosan are removed and rinsed with excess tetrahydrofuran under ultrasonication to remove dimethylformamide and unreacted ethylenediamine. Finally, the prepared PBO fibers grafted with carboxymethyl chitosan are dried in a vacuum at 180°C for 24 hours.

[0048] In this step, ethylenediamine acts as a connector to connect carboxymethyl chitosan and carboxyl-functionalized PBO fibers. Dimethylformamide acts as a solvent to increase the contact area of ​​the reactants. The amino groups of ethylenediamine and the carboxyl groups of carboxymethyl chitosan and the carboxyl groups of carboxyl-functionalized PBO fibers can react to form peptides, and ethylenediamine serves to connect carboxymethyl chitosan and PBO. That is, the objects connected by ethylenediamine are the carboxyl-functionalized PBO fibers and carboxymethyl chitosan. The structure is similar to: carboxyl-functionalized PBO-ethylenediamine-carboxymethyl chitosan. The present invention uses carboxymethyl chitosan as a grafting agent to increase the surface area of ​​PBO, which is beneficial for 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 10,000-25,000 rpm 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 10,000-15,000 rpm to obtain a coaxial electrospinning core solution.

[0050] In this step, although by carboxymethyl chitosan as grafting agent, increase the surface area of ​​PBO, but PBO surface still can residual active site, therefore the present invention prepares coaxial electrostatic spinning core solution by using boron nitride nanotubes and carboxymethyl chitosan, can further increase the contact area of ​​PBO, thus more conducive to in the process of coaxial electrostatic spinning, combined with boron nitride nanotubes.On the other hand, due to the dielectric constant of PBO and boron nitride nanotubes has a certain gap, using boron nitride nanotubes alone may make cumulative charge between core shell, cause the electric field between core shell to increase, affect breakdown performance, therefore the present invention uses carboxymethyl chitosan can play the effect of electric field transition, thus improve the breakdown field strength of motor insulating 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 head is firmly connected to the electrospinning cylinder equipment, reliably ground the spinning equipment, and perform coaxial electrospinning to form a shell-core structure nanofiber 3 with a leaf vein structure imitating the vascular bundle of the leaf vein structure, the diameter of which is 100 to 200 nm, and the shell of the shell-core structure nanofiber 3 imitates the epidermis of the leaf vein structure, the diameter of which is 300 to 500 nm.

[0052] The present invention uses coaxial electrospinning to bring PBO fibers grafted with carboxymethyl chitosan into direct contact with boron nitride nanotubes. On the one hand, this increases the probability of directional construction of heat conduction channels, thereby effectively improving the thermal conductivity of the motor insulation paper. On the other hand, because 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 accumulation of charges or even partial discharge under strong fields caused by the different dielectric constants of multiple materials. Therefore, the present invention uses coaxial electrospinning to prevent the accumulation of electric field charges at the junction between the core and the shell, effectively improving the breakdown performance of the motor insulation paper.

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

[0054] In addition, this step adopts the coaxial electrospinning process, which makes the insulating paper have 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 filtration method to prepare rough paper. On the one hand, the 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 filtration method is relatively stable, and the results have good repeatability.

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

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

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

[0060] Example 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. 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 0.8h, and then the carboxyl functionalized PBO fiber is taken out and 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 and precipitate with excess acetone to obtain chitosan acetate, transfer it to a stirred reaction flask, add a certain amount of sodium hydroxide solution and isopropanol, add chloroacetic acid isopropanol solution 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: Transfer the carboxyl-functionalized PBO fibers obtained in Step 1, the carboxymethyl chitosan obtained in Step 2, and ethylenediamine and dimethylformamide to a polytetrafluoroethylene-lined autoclave and heat at 180°C for 12 hours to obtain PBO fibers grafted with carboxymethyl chitosan. The polytetrafluoroethylene-lined autoclave is cooled to room temperature, and the resulting PBO fibers grafted with carboxymethyl chitosan are removed and rinsed with excess tetrahydrofuran under ultrasonication to remove dimethylformamide and unreacted ethylenediamine. Finally, the prepared PBO fibers grafted with carboxymethyl chitosan are dried in a vacuum at 180°C for 24 hours.

[0064] Step 4: The PBO fiber grafted with carboxymethyl chitosan obtained in step 3 was mixed with deionized water at a mass ratio of 1:300, and slurried by high-speed shear mixing at 25,000 rpm to obtain a coaxial electrospinning shell solution;

[0065] Boron nitride nanotubes, 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 15,000 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 into 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 core diameter is 100-200 nm and the shell diameter is 300-500 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 a heat pressing process to obtain motor insulation paper.

[0069] Example 2:

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

[0071] Example 3:

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

[0073] Example 4:

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

[0075] Example 5:

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

[0077] Example 6:

[0078] This embodiment is based on Example 1, and differs from Example 1 in 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 the PBO fiber grafted with carboxymethyl chitosan to deionized water is 1:350; the shear rate is 10,000 rpm; the mass ratio of boron nitride nanotubes, carboxymethyl chitosan, and deionized water is 1:1.5:400; and the shear rate of the three is 10,000 rpm;

[0079] Example 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 the PBO fiber grafted with carboxymethyl chitosan to deionized water is 1:320; the shear rate is 20,000 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 12,000 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 in 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 the thermal conductivity of motor insulation paper prepared using only pure PBO is much lower than that of motor insulation paper prepared using PBO grafted with carboxymethyl chitosan. Furthermore, if the shell and core solutions are directly filtered to produce motor insulation paper, the thermal conductivity of the resulting motor insulation paper is lower than that of motor insulation paper produced using 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 is shown in FIG. 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 it. 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 various 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 a 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 solution; adding boron nitride nanotubes and carboxymethyl chitosan to deionized water, and slurrying by shear mixing to obtain a coaxial electrospinning core 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 structured 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: hot pressing the rough paper to obtain motor insulation paper; In step 2, the chemical grafting conditions were as follows: heating at 180°C in an autoclave for 12 h; Step 3 comprises: mixing the PBO fiber grafted with carboxymethyl chitosan with deionized water in a mass ratio of 1:300-350, and mixing and slurrying at a shear rate of 10,000-25,000 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 speed of 10000-15000 rpm to obtain a coaxial electrospinning core layer solution.

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: 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 h.

4. The method for preparing motor insulating paper according to any one of claims 1 to 3, 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 a dried PBO fiber; Step b: adding the dried PBO fibers to 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 hour, and washing with deionized water for several times to finally obtain the carboxyl-functionalized PBO fibers.

5. The method for preparing motor insulating paper according to claim 4, characterized in that: The concentration of the nitric acid is 15.3-17.2 mol / L.

6. The method for preparing motor insulation paper according to any one of claims 1 to 3, 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 flask with stirring, a certain amount of sodium hydroxide solution and isopropanol are added, and a chloroacetic acid isopropanol solution is added dropwise while stirring. The reaction temperature is controlled at 70° C., and the reaction is carried out for several hours. The mixture is cooled to room temperature, the pH value is adjusted to neutral with dilute acid, and the mixture is washed with 85% methanol and dried to obtain carboxymethyl chitosan.

7. The method for preparing motor insulating paper according to any one of claims 1 to 3, characterized in that: 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.

8. Motor insulation paper prepared according to the method according to any one of claims 1 to 7.

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