Synthetic fiber blended with natural fiber electrical insulating paper and its preparation process

Abstract

This invention relates to the field of electrical insulation material preparation technology, specifically to an electrical insulation paper made from a blend of synthetic and natural fibers and its preparation process. The synthetic fiber is polyimide fiber prepared by electrospinning wet spinning, and the natural fiber is selected from one or more of cotton, flax, and sisal fibers. The electrical insulation paper has a dielectric strength ≥20kV / mm, a volume resistivity ≥10¹⁴Ω·m, a long-term operating temperature ≥180℃, a fiber distribution uniformity ≥90%, and a basis weight deviation ≤±3%. This invention completely avoids fiber agglomeration and delamination problems, ensuring the stability of the insulation paper's performance. The product exhibits superior high-temperature resistance and tensile strength compared to pure natural fiber insulation paper, while significantly reducing costs compared to pure polyimide insulation paper, achieving a balance between high performance and low cost.

Description

Technical Field

[0001] This invention relates to the field of electrical insulation material preparation technology, specifically to an electrical insulation paper made by blending synthetic fibers and natural fibers and its preparation process. Background Technology

[0002] Electrical insulating paper is an indispensable insulating material in electrical equipment, and its performance directly affects the safety, stability, and service life of electrical equipment. With the development of the power industry towards higher voltage, larger capacity, and smaller sizes, higher requirements are being placed on the high-temperature resistance, mechanical properties, and insulation performance of electrical insulating paper.

[0003] Polyimide (PI) fiber, as a high-performance synthetic fiber, possesses excellent high-temperature resistance (long-term operating temperature can reach 200-260℃), good dielectric properties, and mechanical strength, making it an ideal raw material for preparing high-end electrical insulating paper. Natural fibers (such as cotton, flax, and sisal fibers) are widely available, inexpensive, and possess good hygroscopicity and flexibility. Blending them with polyimide fibers to prepare insulating paper can reduce production costs and improve paper processing performance while ensuring insulation performance. However, significant differences exist between polyimide fibers and natural fibers in terms of surface properties, density, and morphology, leading to a high likelihood of agglomeration and delamination during blending, resulting in extremely poor blend uniformity and severely affecting the performance stability of the insulating paper. Existing blending processes often employ a single slurry tank to mix the two fiber slurries, using a stirring device for mixing. However, this method struggles to overcome the compatibility differences between fibers, making it difficult to guarantee mixing uniformity. Some technical solutions attempt to improve the blending effect by using stepwise feeding and adding dispersants. However, the introduction of dispersants may affect the dielectric properties of the insulating paper and still cannot fundamentally solve the problem of uneven fiber distribution. Another solution proposes to use felt to bind the fibers for transfer, but the traditional felt binding method has drawbacks such as unstable binding amount, easy fiber detachment during transfer, and inability to achieve precise ratio and uniform mixing of the two types of fibers, which limits its industrial application.

[0004] Therefore, developing a process for preparing polyimide fiber and natural fiber blended electrical insulation paper that can precisely control the ratio of the two fibers, significantly improve the uniformity of blending, and not affect the core performance of the insulation paper has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to address the above-mentioned shortcomings by providing an electrical insulating paper made from a blend of synthetic and natural fibers and its preparation process. This completely avoids the problems of fiber agglomeration and delamination, ensures the stability of the insulating paper's performance, and provides products with superior high-temperature resistance and tensile strength compared to pure natural fiber insulating paper. Furthermore, the cost is significantly lower than that of pure polyimide insulating paper, achieving a balance between high performance and low cost.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] An electrical insulating paper is a blend of synthetic and natural fibers. The synthetic fiber is polyimide fiber prepared by electrospinning wet spinning, and the natural fiber is selected from one or more of cotton fiber, flax fiber, and sisal fiber. The insulating paper has a dielectric strength ≥20kV / mm and a volume resistivity ≥10¹. 4 Ω·m, long-term operating temperature ≥180℃, fiber distribution uniformity ≥90%, paper basis weight deviation ≤±3%.

[0008] According to the above-described preparation process of an electrical insulating paper made from a blend of synthetic and natural fibers, the preparation process includes the following steps:

[0009] S1, preparation of polyimide chopped fibers;

[0010] S2, using a dual-slurry pool to prepare polyimide fiber slurry and natural fiber slurry respectively;

[0011] S3 uses a double felt roller adhesive mechanism to precisely adhesive polyimide fiber slurry and natural fiber slurry, and controls the ratio, and then premixes them with high-pressure airflow to obtain mixed fiber bundles;

[0012] S4, the mixed fiber bundles obtained in S3 are mixed a second time and then formed into a structurally stable wet paper sheet;

[0013] S5 involves post-processing the shaped wet paper sheet obtained in S4 to obtain blended electrical insulating paper.

[0014] Preferably, the method for preparing polyimide chopped fibers in step S1 is electrospinning wet spinning. The specific process is as follows: dissolve polyimide resin in N,N-dimethylacetamide or N-methylpyrrolidone solvent, add a dispersant, and stir at 50-70℃ for 2-4 hours to obtain a spinning solution with a mass fraction of 10-15% and allow it to stand to remove bubbles; inject the spinning solution into an electrospinning device, adjust the spinning voltage to 20-30kV, the distance between the spinning needle and the receiving device to 15-25cm, and the spinning solution flow rate to 0.5-1.5mL / h, solidify and shape it in a wet receiving tank containing a coagulation bath, and dry it at 120-150℃ for 2-3 hours after traction to obtain polyimide chopped fibers with a diameter of 500-1500nm and a length of 2-5mm.

[0015] Among them, polyimide fibers are prepared by electrospinning wet spinning technology, and after traction drying, they are chopped to obtain polyimide short fibers of specific specifications that are suitable for subsequent pulp preparation, providing excellent high temperature resistance and insulation basis for blended insulating paper.

[0016] Preferably, the dispersant in the spinning solution is polyethylene glycol, with a mass fraction of 1-3%; the coagulation bath is composed of N,N-dimethylacetamide and deionized water in a volume ratio of 3:7-5:5, and the temperature of the coagulation bath is controlled at 20-30℃.

[0017] Preferably, the preparation process of the polyimide fiber slurry in step S2 is as follows: short-cut polyimide fibers are added to the first slurry tank, deionized water is added to adjust the fiber mass fraction to 0.5-2%, and a cationic dispersant with a mass fraction of 0.2-0.5% is added. The ultrasonic stirring device is then turned on and stirred for 30-60 minutes. The preparation process of the natural fiber slurry is as follows: natural fibers are combed and chopped to 2-5 mm and added to the second slurry tank, deionized water is added to adjust the fiber mass fraction to 1-3%, and anionic dispersant with a mass fraction of 0.3-0.6% is added. Mechanical stirring is turned on and ozone treatment is introduced for 15-20 minutes, followed by stirring for 40-80 minutes.

[0018] The system includes two separate slurry tanks, one for polyimide chopped fibers and the other for natural fibers. By adding a suitable dispersant and using a dedicated stirring process, the system ensures that the two types of fibers are evenly dispersed in the slurry, thus avoiding fiber agglomeration caused by mixing in a single tank.

[0019] Preferably, the cationic dispersant is polyethyleneimine, the ultrasonic frequency of the ultrasonic stirring device is 20-40kHz, and the stirring speed is 300-500r / min; the anionic dispersant is sodium lignosulfonate, the mechanical stirring speed is 200-400r / min, and the ozone concentration of the ozone treatment is 50-100mg / L.

[0020] Preferably, the dual felt roller adhesion mechanism in step S3 includes a first felt roller corresponding to the first slurry pool and a second felt roller corresponding to the second slurry pool. The surface of the felt roller is covered with felt modified by silane coupling agent KH-550, and the pore size of the felt is 50-100μm. The rotation speed of the felt roller is adjusted to 10-30r / min and the contact depth with the slurry surface is 2-5mm. The internal air pressure of the felt roller is controlled to -0.02-0.05MPa by an air pressure regulating device.

[0021] In this process, the adsorption characteristics of the modified felt roller are utilized, and the fiber feeding amount is precisely controlled by adjusting the rotation speed and air pressure. Then, the two fibers are initially interwoven and mixed by the impact of high-pressure airflow, laying the foundation for subsequent uniform blending.

[0022] Preferably, in step S3, the mass ratio of polyimide fiber to natural fiber is controlled to be 3:7-7:3 by adjusting the rotation speed and air pressure of the two felt rollers; during premixing, the airflow velocity of the high-pressure airflow nozzle in the premixing chamber is 10-20m / s, and the airflow direction is 45° to the tangent direction of the felt roller.

[0023] Preferably, the specific process of secondary mixing in step S4 is as follows: the mixed fiber bundle falls into the secondary mixing tank, which is equipped with a spiral stirring paddle and an ultrasonic vibration device, and a fiber dispersion plate with uniform through holes of 1-3 mm; the speed of the spiral stirring paddle is adjusted to 200-300 r / min and the frequency of the ultrasonic vibration device is 30-50 kHz, and the stirring and ultrasonic mixing is carried out for 20-30 min; during papermaking, the fiber pulp is fed into the wire section of the papermaking machine, spread into a wet paper sheet by a vacuum suction filter device with a vacuum degree of 0.04-0.06 MPa, and pressed to a moisture content of 40-60% by a pressure of 0.5-1.0 MPa.

[0024] In this process, the premixed fiber bundles are fed into a secondary mixing tank equipped with a multi-dispersion device to further break up fiber agglomerations and ensure a uniform microscopic distribution of the two types of fibers. Then, the fibers are filtered and pressed through the wire section of a paper machine to form a structurally stable wet paper sheet.

[0025] Preferably, the post-processing in step S5 includes segmented drying, hot pressing modification, and cutting inspection; segmented drying is as follows: first drying at 80-100℃ for 1-2 hours, then drying at 120-140℃ for 0.5-1 hour, until the paper moisture content is ≤8%; hot pressing modification is as follows: hot pressing at 180-220℃ and 1.5-2.5MPa for 20-40 minutes.

[0026] In response to the characteristics of high moisture content and loose structure of wet paper sheets, moisture is removed by segmented drying, and the fiber bonding force is strengthened by coupling agent interface modification, the structural compactness is optimized by gradient hot pressing-in-situ crosslinking, and the insulation stability is improved by low-temperature plasma densification. Finally, qualified blended electrical insulation paper is obtained after cutting and inspection.

[0027] The beneficial effects of this invention are:

[0028] 1. This invention adopts a synergistic process of dual pulp tanks - precise bonding of modified felt + multi-stage mixing. Through the high adsorption of modified felt and the dual control mechanism of air pressure and speed, the two fibers are precisely mixed (3:7-7:3). Combined with the high-pressure airflow initial mixing in the premixing chamber and the "spiral stirring + ultrasonic vibration" compound mixing in the secondary mixing tank, compared with the traditional single pulp tank stirring method, the fiber dispersion uniformity is improved by more than 60%, completely avoiding the problems of fiber agglomeration and delamination, and ensuring the stability of the insulating paper performance.

[0029] 2. This invention employs a composite modification strategy of "targeted dispersant selection + coupling agent atomization interface modification," which maximizes the retention of core properties while improving the blending effect. Cationic and anionic dispersants are selected respectively to address the surface charge differences between the two fibers, avoiding the interaction of dispersants that could affect insulation performance. Subsequent solvent-free atomization coupling agent treatment increases the inter-fiber bonding force by 30-50%. Combined with hot-pressing modification, the final product exhibits superior high-temperature resistance (long-term operating temperature above 200℃) and tensile strength compared to pure natural fiber insulation paper, while reducing costs by 40-50% compared to pure polyimide insulation paper, achieving a balance between high performance and low cost.

[0030] 3. This invention employs a modular and quantifiable process design, possessing strong industrial adaptability. From precise diameter control (500-1500nm) in the electrospinning wet forming of polyimide fibers, to the clear quantification of parameters at each stage, and the standardized preparation of modified felt, all processes break free from the reliance on experience in traditional methods. Simultaneously, the process has no particularly demanding equipment requirements; existing paper production lines can be adapted with minor modifications. This solves the defects of unstable adhesion and difficulty in large-scale production in traditional felt bonding processes, providing a feasible solution for the mass production of high-end electrical insulation paper. Attached Figure Description

[0031] Figure 1 This is a simplified diagram of the manufacturing process of the electrical insulating paper of this invention. Detailed Implementation

[0032] Example 1

[0033] Step 1: Preparation of polyimide fibers (electrospinning and wet spinning)

[0034] 1.1 Preparation of polyimide spinning solution: Dissolve 10g of polyimide resin in 90g of N,N-dimethylacetamide (DMAc) solvent, add 1g of polyethylene glycol (1% by mass) dispersant, stir at 50℃ for 2h until completely dissolved, and obtain a spinning solution with a mass fraction of 10%. Let stand for 12h to remove bubbles before use.

[0035] 1.2 Electrospinning Wet Spinning Formation: The spinning solution is injected into the storage tank of the electrospinning equipment, and the spinning parameters are adjusted as follows: spinning voltage 20kV, distance between spinning needle and receiving device 15cm, spinning solution flow rate 0.5mL / h; the receiving device adopts a wet receiving tank, which contains a coagulation bath (made of DMAc and deionized water in a volume ratio of 3:7), and the temperature of the coagulation bath is controlled at 20℃; during the spinning process, the fiber is solidified and formed in the coagulation bath, and after being drawn by the traction roller, it is sent to the drying oven and dried at 120℃ for 2h to obtain polyimide short-cut fibers with a diameter of 500nm and a length of 2mm.

[0036] Step 2: Preparation of two types of fiber pulp

[0037] 2.1 Preparation of polyimide fiber slurry: The polyimide chopped fibers prepared in step 1 were added to the first slurry tank, and deionized water was added to adjust the fiber mass fraction to 0.5% (5g fiber + 995g deionized water). At the same time, 0.2% cationic dispersant polyethyleneimine was added. The ultrasonic stirring device in the first slurry tank was turned on (ultrasonic frequency 20kHz, stirring speed 300r / min), and stirred for 30min to make the polyimide fibers uniformly dispersed in the slurry, thus obtaining polyimide fiber slurry. The ultrasonic stirring device adopts a composite structure of ultrasonic and mechanical stirring, and the ultrasonic transducers are evenly distributed at the bottom of the first slurry tank to achieve uniform dispersion of the slurry throughout the entire area and avoid polyimide fiber agglomeration.

[0038] 2.2 Preparation of natural fiber pulp: Flax fibers (cut to 2mm) were added to the second pulping tank, deionized water was added, and the fiber mass fraction was adjusted to 1% (10g fiber + 990g deionized water). Sodium lignosulfonate anionic dispersant with a mass fraction of 0.3% was added. The mechanical stirring device in the second pulping tank was turned on (stirring speed 200r / min), and ozone (ozone concentration 50mg / L) was introduced for treatment for 15min to improve the surface activity of natural fibers. After stirring for 40min, natural fiber pulp was obtained.

[0039] Step 3: Dual Slurry Tank - Precise Adhesion and Premixing of Felt

[0040] 3.1 Adhesion Mechanism Debugging: A dual-felt roller adhesion mechanism is adopted, including a first felt roller (corresponding to the first slurry pool) and a second felt roller (corresponding to the second slurry pool). The surface of the felt rollers is covered with modified felt (felt pore size 50-100μm, modified with silane coupling agent KH-550). The rotation speed of the felt rollers is adjusted to 10-30r / min, and the contact depth between the felt rollers and the slurry surface is 2-5mm. At the same time, the internal air pressure of the felt rollers is controlled at -0.02-0.05MPa by an air pressure regulating device to achieve stable fiber adsorption. The modified felt is prepared by immersing the felt in a 5-8% (w / w) ethanol solution of silane coupling agent KH-550 for 2-3 hours, then removing it and drying it at 100-120℃ for 1 hour. The modified felt increases the fiber adsorption capacity by 30-50%, and the fiber shedding rate is ≤2%.

[0041] 3.2 Fiber Adhesion and Proportion Control: The first felt roller adheses polyimide fibers from the first slurry pool, and the second felt roller adheses natural fibers from the second slurry pool. By adjusting the rotation speed and air pressure of the two felt rollers, the mass ratio of polyimide fibers to natural fibers is controlled to be 3:7.

[0042] 3.3 Premixing: The two felt rollers with the fibers attached are rotated synchronously into the premixing chamber. The premixing chamber is equipped with a high-pressure airflow nozzle (airflow speed 10-20m / s, airflow direction at 45° to the tangent of the felt rollers). The high-pressure airflow blows the fibers off the surface of the two felt rollers and mixes them in the airflow to obtain a mixed fiber bundle.

[0043] Step 4: Multi-stage mixing and molding

[0044] 4.1 Secondary Mixing: The mixed fiber bundles fall into a secondary mixing tank, which is equipped with a spiral stirring paddle (rotation speed 200 r / min) and an ultrasonic vibration device (ultrasonic frequency 30 kHz). The mixture is stirred and ultrasonically mixed for 20 min to further improve the uniformity of fiber blending. The secondary mixing tank is equipped with a fiber dispersion plate with uniform through holes with a diameter of 1-3 mm. After passing through the dispersion plate, the mixed fiber bundles form finer fiber flocs, which are then stirred and ultrasonically mixed to further improve uniformity.

[0045] 4.2 Paper Forming: The fiber pulp after secondary mixing is fed into the wire section of the paper machine. A vacuum filter (vacuum degree 0.04-0.06MPa) is used to evenly spread the fibers on the wire to form a wet paper sheet. The wet paper sheet is then pressed by press rolls (pressure 0.5-1.0MPa) to remove water. The moisture content of the wet paper sheet after pressing is controlled at 40-60%.

[0046] Step 5: Post-processing and finished product

[0047] 5.1 Drying: The pressed wet paper sheet is sent into a segmented drying oven. The first stage of drying is at a temperature of 80-100℃ for 1-2 hours; the second stage of drying is at a temperature of 120-140℃ for 0.5-1 hour, until the moisture content of the paper sheet is ≤8%.

[0048] 5.2 Coupling agent atomization interface modification: After drying, the paper sheet is fed into a closed spraying chamber and sprayed with a 2.5% mass fraction silane coupling agent KH-560 ethanol solution (spraying amount 6g / m²) using an ultrasonic atomization device. After standing at room temperature for 12 minutes, the coupling agent penetrates into the fiber interface to strengthen the bonding force between different fibers.

[0049] 5.3 Hot pressing modification: The dried paper sheet is fed into a hot press and hot-pressed at a temperature of 180-220℃ and a pressure of 1.5-2.5MPa for 20-40 minutes to enhance the bonding force between fibers; specifically: gradient hot pressing: 180℃ / 1.5MPa (10min) → 200℃ / 2.0MPa (15min) → 220℃ / 2.5MPa (5min).

[0050] 5.4 Cutting Inspection: Cut the hot-pressed paper sheets into specified sizes according to requirements. After passing the performance test, polyimide fiber-natural fiber blended electrical insulation paper is obtained.

[0051] Example 2

[0052] Step 1: Preparation of polyimide fibers (electrospinning and wet spinning)

[0053] 1.1 Preparation of polyimide spinning solution: Dissolve 15g of polyimide resin in 85g of N-methylpyrrolidone (NMP) solvent, add 3g of polyethylene glycol (3% by mass) dispersant, stir at 70℃ for 4h until completely dissolved, and obtain a spinning solution with a mass fraction of 15%. Let stand for 12h to remove bubbles before use.

[0054] 1.2 Electrospinning Wet Spinning Formation: The spinning solution is injected into the storage tank of the electrospinning equipment, and the spinning parameters are adjusted as follows: spinning voltage 30kV, distance between spinning needle and receiving device 25cm, spinning solution flow rate 1.5mL / h; the receiving device adopts a wet receiving tank, which contains a coagulation bath (made of DMAc and deionized water in a volume ratio of 5:5), and the temperature of the coagulation bath is controlled at 30℃; during the spinning process, the fiber is solidified and formed in the coagulation bath, and after being drawn by the traction roller, it is sent to the drying oven and dried at 150℃ for 3h to obtain polyimide short fibers with a diameter of 1500nm and a length of 5mm.

[0055] Step 2: Preparation of two types of fiber pulp

[0056] 2.1 Preparation of polyimide fiber slurry: The polyimide chopped fibers prepared in step 1 were added to the first slurry tank, and deionized water was added to adjust the fiber mass fraction to 2% (20g fiber + 980g deionized water). At the same time, 0.5% cationic dispersant polyethyleneimine was added. The ultrasonic stirring device in the first slurry tank was turned on (ultrasonic frequency 40kHz, stirring speed 500r / min), and stirred for 60min to make the polyimide fibers uniformly dispersed in the slurry, thus obtaining polyimide fiber slurry. The ultrasonic stirring device adopts a composite structure of ultrasonic and mechanical stirring, and the ultrasonic transducers are evenly distributed at the bottom of the first slurry tank to achieve uniform dispersion of the slurry throughout the entire area and avoid polyimide fiber agglomeration.

[0057] 2.2 Preparation of natural fiber pulp: Cotton fiber + sisal fiber (1:1, chopped to 5mm) were added to the second pulp tank, deionized water was added, and the fiber mass fraction was adjusted to 3% (30g fiber + 970g deionized water). Sodium lignosulfonate anionic dispersant with a mass fraction of 0.6% was added. The mechanical stirring device in the second pulp tank was turned on (stirring speed 400r / min), and ozone (ozone concentration 100mg / L) was introduced for 20min to improve the surface activity of natural fibers. After stirring for 80min, natural fiber pulp was obtained.

[0058] Step 3: Dual Slurry Tank - Precise Adhesion and Premixing of Felt

[0059] 3.1 Adhesion Mechanism Debugging: A dual-felt roller adhesion mechanism is adopted, including a first felt roller (corresponding to the first slurry pool) and a second felt roller (corresponding to the second slurry pool). The surface of the felt rollers is covered with modified felt (felt pore size 50-100μm, modified with silane coupling agent KH-550). The rotation speed of the felt rollers is adjusted to 10-30r / min, and the contact depth between the felt rollers and the slurry surface is 2-5mm. At the same time, the internal air pressure of the felt rollers is controlled at -0.02-0.05MPa by an air pressure regulating device to achieve stable fiber adsorption. The modified felt is prepared by immersing the felt in a 5-8% (w / w) ethanol solution of silane coupling agent KH-550 for 2-3 hours, then removing it and drying it at 100-120℃ for 1 hour. The modified felt increases the fiber adsorption capacity by 30-50%, and the fiber shedding rate is ≤2%.

[0060] 3.2 Fiber Adhesion and Proportion Control: The first felt roller adheses polyimide fibers from the first slurry pool, and the second felt roller adheses natural fibers from the second slurry pool. By adjusting the rotation speed and air pressure of the two felt rollers, the mass ratio of polyimide fibers to natural fibers is controlled to be 7:3.

[0061] 3.3 Premixing: The two felt rollers with the fibers attached are rotated synchronously into the premixing chamber. The premixing chamber is equipped with a high-pressure airflow nozzle (airflow speed 10-20m / s, airflow direction at 45° to the tangent of the felt rollers). The high-pressure airflow blows the fibers off the surface of the two felt rollers and mixes them in the airflow to obtain a mixed fiber bundle.

[0062] Step 4: Multi-stage mixing and molding

[0063] 4.1 Secondary Mixing: The mixed fiber bundles fall into a secondary mixing tank, which is equipped with a spiral stirring paddle (300 r / min) and an ultrasonic vibration device (50 kHz ultrasonic frequency). The mixture is stirred and ultrasonically mixed for 30 min to further improve the uniformity of the fiber blend. The secondary mixing tank is equipped with a fiber dispersion plate with uniform through holes of 1-3 mm in diameter. After passing through the dispersion plate, the mixed fiber bundles form finer fiber flocs, which are then stirred and ultrasonically mixed to further improve the uniformity.

[0064] 4.2 Paper Forming: The fiber pulp after secondary mixing is fed into the wire section of the paper machine. A vacuum filter (vacuum degree 0.04-0.06MPa) is used to evenly spread the fibers on the wire to form a wet paper sheet. The wet paper sheet is then pressed by press rolls (pressure 0.5-1.0MPa) to remove water. The moisture content of the wet paper sheet after pressing is controlled at 40-60%.

[0065] Step 5: Post-processing and finished product

[0066] 5.1 Drying: The pressed wet paper sheet is sent into a segmented drying box. The first stage of drying is at a temperature of 80-100℃ and a drying time of 1-2 hours. The second stage of drying is at a temperature of 120-140℃ and a drying time of 0.5-1 hours, until the moisture content of the paper sheet is ≤8%.

[0067] 5.2 Coupling agent atomization interface modification: After drying, the paper sheet is fed into a closed spraying chamber and sprayed with a 2.5% mass fraction silane coupling agent KH-560 ethanol solution (spraying amount 6g / m²) using an ultrasonic atomization device. After standing at room temperature for 12 minutes, the coupling agent penetrates into the fiber interface to strengthen the bonding force between different fibers.

[0068] 5.3 Hot pressing modification: The dried paper sheet is fed into a hot press and hot-pressed at a temperature of 180-220℃ and a pressure of 1.5-2.5MPa for 20-40 minutes to enhance the bonding force between fibers; specifically: gradient hot pressing: 180℃ / 1.5MPa (10min) → 200℃ / 2.0MPa (15min) → 220℃ / 2.5MPa (5min).

[0069] 5.4 Cutting Inspection: Cut the hot-pressed paper sheets into specified sizes according to requirements. After passing the performance test, polyimide fiber-natural fiber blended electrical insulation paper is obtained.

[0070] Example 3

[0071] Step 1: Preparation of polyimide fibers (electrospinning and wet spinning)

[0072] 1.1 Preparation of polyimide spinning solution: Dissolve 12g of polyimide resin in 88g of N,N-dimethylacetamide (DMAc) solvent, add 2g of polyethylene glycol (23% by mass) dispersant, stir at 60℃ for 3h until completely dissolved, and obtain a spinning solution with a mass fraction of 12%. Let stand for 12h to remove bubbles before use.

[0073] 1.2 Electrospinning Wet Spinning Formation: The spinning solution is injected into the storage tank of the electrospinning equipment, and the spinning parameters are adjusted as follows: spinning voltage 25kV, distance between spinning needle and receiving device 20cm, spinning solution flow rate 1.0mL / h; the receiving device adopts a wet receiving tank, which contains a coagulation bath (made of DMAc and deionized water in a volume ratio of 4:6), and the temperature of the coagulation bath is controlled at 25℃; during the spinning process, the fiber is solidified and formed in the coagulation bath, and after being drawn by the traction roller, it is sent to the drying oven and dried at 135℃ for 2.5h to obtain polyimide short-cut fibers with a diameter of 1000nm and a length of 3.5mm.

[0074] Step 2: Preparation of two types of fiber pulp

[0075] 2.1 Preparation of polyimide fiber slurry: The polyimide chopped fibers prepared in step 1 were added to the first slurry tank, and deionized water was added to adjust the fiber mass fraction to 1.2% (12g fiber + 988g deionized water). At the same time, 0.35% cationic dispersant polyethyleneimine was added. The ultrasonic stirring device in the first slurry tank was turned on (ultrasonic frequency 30kHz, stirring speed 400r / min), and stirred for 45min to make the polyimide fibers uniformly dispersed in the slurry, thus obtaining polyimide fiber slurry. The ultrasonic stirring device adopts a composite structure of ultrasonic and mechanical stirring, and the ultrasonic transducers are evenly distributed at the bottom of the first slurry tank to achieve uniform dispersion of the slurry throughout the entire area and avoid polyimide fiber agglomeration.

[0076] 2.2 Preparation of natural fiber pulp: Flax fiber (chopped to 3.5 mm) was added to the second pulping tank, deionized water was added, and the fiber mass fraction was adjusted to 2% (20 g fiber + 980 g deionized water). An anionic dispersant (such as sodium lignosulfonate) with a mass fraction of 0.345% was added. The mechanical stirring device in the second pulping tank was turned on (stirring speed 300 r / min), and ozone (ozone concentration 75 mg / L) was introduced for treatment for 17 min to improve the surface activity of natural fiber. After stirring for 60 min, natural fiber pulp was obtained.

[0077] Step 3: Dual Slurry Tank - Precise Adhesion and Premixing of Felt

[0078] 3.1 Adhesion Mechanism Debugging: A dual-felt roller adhesion mechanism is adopted, including a first felt roller (corresponding to the first slurry pool) and a second felt roller (corresponding to the second slurry pool). The surface of the felt rollers is covered with modified felt (felt pore size 50-100μm, modified with silane coupling agent KH-550). The rotation speed of the felt rollers is adjusted to 10-30r / min, and the contact depth between the felt rollers and the slurry surface is 2-5mm. At the same time, the internal air pressure of the felt rollers is controlled at -0.02-0.05MPa by an air pressure regulating device to achieve stable fiber adsorption. The modified felt is prepared by immersing the felt in a 5-8% (w / w) ethanol solution of silane coupling agent KH-550 for 2-3 hours, then removing it and drying it at 100-120℃ for 1 hour. The modified felt increases the fiber adsorption capacity by 30-50%, and the fiber shedding rate is ≤2%.

[0079] 3.2 Fiber Adhesion and Proportion Control: The first felt roller adheses polyimide fibers from the first slurry pool, and the second felt roller adheses natural fibers from the second slurry pool. By adjusting the rotation speed and air pressure of the two felt rollers, the mass ratio of polyimide fibers to natural fibers is controlled to be 5:5.

[0080] 3.3 Premixing: The two felt rollers with the fibers attached are rotated synchronously into the premixing chamber. The premixing chamber is equipped with a high-pressure airflow nozzle (airflow speed 10-20m / s, airflow direction at 45° to the tangent of the felt rollers). The high-pressure airflow blows the fibers off the surface of the two felt rollers and mixes them in the airflow to obtain a mixed fiber bundle.

[0081] Step 4: Multi-stage mixing and molding

[0082] 4.1 Secondary Mixing: The mixed fiber bundles fall into a secondary mixing tank, which is equipped with a spiral stirring paddle (rotation speed 250 r / min) and an ultrasonic vibration device (ultrasonic frequency 40 kHz). The mixture is stirred and ultrasonically mixed for 25 min to further improve the uniformity of fiber blending. The secondary mixing tank is equipped with a fiber dispersion plate with uniform through holes with a diameter of 1-3 mm. After passing through the dispersion plate, the mixed fiber bundles form finer fiber flocs, which are then stirred and ultrasonically mixed to further improve uniformity.

[0083] 4.2 Paper Forming: The fiber pulp after secondary mixing is fed into the wire section of the paper machine. A vacuum filter (vacuum degree 0.04-0.06MPa) is used to evenly spread the fibers on the wire to form a wet paper sheet. The wet paper sheet is then pressed by press rolls (pressure 0.5-1.0MPa) to remove water. The moisture content of the wet paper sheet after pressing is controlled at 40-60%.

[0084] Step 5: Post-processing and finished product

[0085] 5.1 Drying: The pressed wet paper sheet is sent into a segmented drying oven. The first stage of drying is at a temperature of 80-100℃ for 1-2 hours; the second stage of drying is at a temperature of 120-140℃ for 0.5-1 hour, until the moisture content of the paper sheet is ≤8%.

[0086] 5.2 Coupling agent atomization interface modification: After drying, the paper sheet is fed into a closed spraying chamber and sprayed with a 2.5% mass fraction silane coupling agent KH-560 ethanol solution (spraying amount 6g / m²) using an ultrasonic atomization device. After standing at room temperature for 12 minutes, the coupling agent penetrates into the fiber interface to strengthen the bonding force between different fibers.

[0087] 5.3 Hot pressing modification: The dried paper sheet is fed into a hot press and hot-pressed at a temperature of 180-220℃ and a pressure of 1.5-2.5MPa for 20-40 minutes to enhance the bonding force between fibers; specifically: gradient hot pressing: 180℃ / 1.5MPa (10min) → 200℃ / 2.0MPa (15min) → 220℃ / 2.5MPa (5min).

[0088] 5.4 Cutting Inspection: Cut the hot-pressed paper sheets into specified sizes according to requirements. After passing the performance test, polyimide fiber-natural fiber blended electrical insulation paper is obtained.

[0089] Test case

[0090] The performance of the polyimide fiber-natural fiber blend electrical insulating paper prepared in Examples 1-3 was tested. The test results are shown in Table 1 below.

[0091] Table 1 Test results of the insulating paper prepared in Examples 1-3

[0092] Example 1 Example 2 Example 3 Long-term operating temperature (°C) 180 220 200 Dielectric strength (kV / mm) 20 22 21 Volume resistivity (Ω·m) <![CDATA[1.2×10¹ 4 ]]> <![CDATA[4.8×10¹ 4 ]]> <![CDATA[3.0×10¹ 4 ]]> Tensile index (N·m / g) 56 61 59 High temperature resistance (strength retention rate after 24 hours at 300℃) 94% 96% 95% Fiber distribution uniformity 91% 93% 92% Paper basis weight deviation ±2.5% ±2.2% ±2.3%

[0093] Comparative Example 1

[0094] Traditional single-slurry tank mixing process

[0095] Polyimide and flax fiber were added to the same slurry tank at a mass ratio of 5:5. Sodium lignosulfonate was used as a dispersant. After mechanical stirring, the mixture was formed into a paper. The average fiber distribution uniformity of the control sample C1 was only 55%.

[0096] The uniformity of sample E3 prepared according to Example 3 reached 92%.

[0097] Calculations show that the uniformity is improved by about 67%, exceeding 60%, which confirms the effectiveness of the "dual slurry pool - modified felt bonding + multi-stage mixing" process of this invention in solving the problems of fiber agglomeration and stratification.

[0098] Comparative Example 2

[0099] The results are basically the same as in Example 3, except that the comparative sample C2 was obtained by omitting the coupling agent atomization interface modification step.

[0100] The average tensile index was found to be 42 N·m / g.

[0101] The E3 tensile index of the complete process sample in Example 3 was 59 N·m / g, and the bonding strength was increased by about 40.5%.

[0102] Comparative Example 3

[0103] Based on the production of 1 ton of insulating paper, the raw material cost of pure polyimide insulating paper is approximately 200,000 yuan.

[0104] Using the preparation process of Example 3, 0.5 tons of polyimide fiber (cost 100,000 yuan) and 0.5 tons of natural fiber (cost 5,000 yuan) are required, with a total raw material cost of 105,000 yuan, which reduces the cost by about 47.5%.

[0105] The specific embodiments described herein are merely illustrative examples of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the scope defined by this invention.

Claims

1. An electrical insulating paper made from a blend of synthetic and natural fibers, characterized in that: The synthetic fiber is a polyimide fiber prepared by electrospinning wet spinning, and the natural fiber is selected from one or more of cotton fiber, flax fiber, and sisal fiber.

2. The dielectric strength of this electrical insulation paper is ≥20kV / mm, and the volume resistivity is ≥10¹. 4 Ω·m, long-term operating temperature ≥180℃, fiber distribution uniformity ≥90%, paper basis weight deviation ≤±3%.

3. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 1, characterized in that: The manufacturing process of this electrical insulating paper includes the following steps: S1, preparation of polyimide chopped fibers; S2, using a dual-slurry pool to prepare polyimide fiber slurry and natural fiber slurry respectively; S3 uses a double felt roller adhesive mechanism to precisely adhesive polyimide fiber slurry and natural fiber slurry, and controls the ratio, and then premixes them with high-pressure airflow to obtain mixed fiber bundles; S4, the mixed fiber bundles obtained in S3 are mixed a second time and then formed into a structurally stable wet paper sheet; S5 involves post-processing the shaped wet paper sheet obtained in S4 to obtain blended electrical insulating paper.

4. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 2, characterized in that, The method for preparing polyimide chopped fibers in step S1 is electrospinning wet spinning. The specific process is as follows: polyimide resin is dissolved in N,N-dimethylacetamide or N-methylpyrrolidone solvent, a dispersant is added, and the mixture is stirred at 50-70℃ for 2-4 hours to obtain a spinning solution with a mass fraction of 10-15%, which is then allowed to stand to remove bubbles. The spinning solution is injected into an electrospinning device, and the spinning voltage is adjusted to 20-30kV, the distance between the spinning needle and the receiving device is 15-25cm, and the spinning solution flow rate is 0.5-1.5mL / h. The solution is solidified and shaped in a wet receiving tank containing a coagulation bath, and then dried at 120-150℃ for 2-3 hours after traction to obtain polyimide chopped fibers with a diameter of 500-1500nm and a length of 2-5mm.

5. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 3, characterized in that, The dispersant in the spinning solution is polyethylene glycol, with a mass fraction of 1-3%; the coagulation bath is composed of N,N-dimethylacetamide and deionized water in a volume ratio of 3:7-5:5, and the temperature of the coagulation bath is controlled at 20-30℃.

6. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 2, characterized in that, The preparation process of polyimide fiber slurry in step S2 is as follows: add chopped polyimide fibers to the first slurry tank, add deionized water to adjust the fiber mass fraction to 0.5-2%, add cationic dispersant with a mass fraction of 0.2-0.5%, and turn on the ultrasonic stirring device to stir for 30-60 minutes; the preparation process of natural fiber slurry is as follows: comb and chop natural fibers to 2-5 mm and add them to the second slurry tank, add deionized water to adjust the fiber mass fraction to 1-3%, add anionic dispersant with a mass fraction of 0.3-0.6%, turn on mechanical stirring and introduce ozone treatment for 15-20 minutes, and continue stirring for 40-80 minutes.

7. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 5, characterized in that, The cationic dispersant is polyethyleneimine, the ultrasonic frequency of the ultrasonic stirring device is 20-40kHz, and the stirring speed is 300-500r / min; the anionic dispersant is sodium lignosulfonate, the mechanical stirring speed is 200-400r / min, and the ozone concentration of the ozone treatment is 50-100mg / L.

8. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 2, characterized in that, In step S3, the dual felt roller adhesion mechanism includes a first felt roller corresponding to the first slurry pool and a second felt roller corresponding to the second slurry pool. The surface of the felt roller is covered with felt modified by silane coupling agent KH-550, and the pore size of the felt is 50-100μm. The rotation speed of the felt roller is adjusted to 10-30r / min and the contact depth with the slurry surface is 2-5mm. The internal air pressure of the felt roller is controlled to -0.02-0.05MPa by the air pressure regulating device.

9. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 7, characterized in that, In step S3, the mass ratio of polyimide fiber to natural fiber is controlled to be 3:7-7:3 by adjusting the rotation speed and air pressure of the two felt rollers. During premixing, the airflow velocity of the high-pressure airflow nozzle in the premixing chamber is 10-20 m / s, and the airflow direction is 45° to the tangent direction of the felt roller.

10. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 2, characterized in that, The specific process of secondary mixing in step S4 is as follows: the mixed fiber bundles fall into the secondary mixing tank, which is equipped with a spiral stirring paddle and an ultrasonic vibration device, and a fiber dispersion plate with uniform through holes of 1-3mm; the speed of the spiral stirring paddle is adjusted to 200-300r / min and the frequency of the ultrasonic vibration device is 30-50kHz, and the stirring and ultrasonic mixing is carried out for 20-30min; during papermaking, the fiber pulp is fed into the wire section of the papermaking machine, spread into a wet paper sheet by a vacuum suction filter device with a vacuum degree of 0.04-0.06MPa, and pressed to a moisture content of 40-60% by a pressure of 0.5-1.0MPa.

11. The preparation process of an electrical insulating paper blended with synthetic fibers and natural fibers according to claim 2, characterized in that, The post-processing in step S5 includes segmented drying, hot pressing modification, and cutting inspection; The staged drying process involves first drying at 80-100℃ for 1-2 hours, then drying at 120-140℃ for 0.5-1 hour, until the paper moisture content is ≤8%. Hot pressing modification involves hot pressing at 180-220℃ and 1.5-2.5MPa for 20-40 minutes.

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