A kind of ultra-high voltage mica paper and its preparation process

Abstract

The invention discloses a kind of ultra-high voltage mica paper and its preparation process, belonging to the technical field of mica paper, the ultra-high voltage mica paper comprises the following raw materials by weight: 110-130 parts of mica, 25-40 parts of aramid fiber, 10-20 parts of hydroxylated hyperbranched polyethylene copolymer and 5-8 parts of boron nitride nanosheets. Boron nitride nanosheets can form a dense physical barrier in the mica paper matrix, block the formation of conductive paths, and can fill the micron-scale pores inside the mica paper material, reduce the electric field distortion point, and significantly improve the dielectric constant and mechanical strength of the mica paper. The added hydroxylated hyperbranched polyethylene copolymer can significantly improve the interfacial force between aramid fiber and mica, help penetrate into the gap of mica paper, make up for the interface defects, and at the same time improve the dispersibility of boron nitride nanosheets in the system, and cooperate with boron nitride nanosheets to further improve the dielectric strength and mechanical properties of the material.

Description

Technical Field

[0001] The invention belongs to the technical field of mica paper, and in particular relates to ultra-high voltage mica paper and a preparation process thereof. Background Art

[0002] With the development of process technology, UHV electrical equipment, such as ultra-high voltage transformers and traction motors, has increasingly stringent requirements on the performance of insulating materials. Mica paper, as a high-performance inorganic insulating material, is widely used in UHV power equipment due to its excellent high temperature resistance, dielectric strength and chemical stability.

[0003] Traditional mica paper is made from natural mica flakes by wet papermaking. The interlayer bonding relies on hydrogen bonds and mechanical interlocking, resulting in low mechanical strength and poor flexibility. In order to improve the mechanical properties of mica paper, the prior art has a method of compounding aramid fiber with mica paper to reinforce the mica paper. However, the surface inertia of aramid fiber is high and the interfacial bonding between aramid fiber and mica flake is weak, which easily leads to delamination and chipping of the obtained mica paper. The problem of interfacial debonding under dynamic mechanical load is prominent. The uneven dispersion of aramid fiber can also easily lead to local electric field distortion of the obtained mica paper. The structural defects of mica enriched on the mesh surface further weaken the electrical breakdown resistance of the mica composite aramid fiber material.

[0004] At the same time, there are a large number of micron-sized pores inside the existing mica paper-based materials, which are prone to cause partial discharge under high-voltage electric fields, accelerating the carbonization failure of the material. The high porosity also leads to an increase in the moisture absorption rate of the material, further deteriorating the insulation stability, resulting in the increasingly prominent performance shortcomings of existing mica-based materials under extreme electric fields, high temperatures and mechanical stress. The prior art (CN115341410A) records that by adding nano-inorganic substances such as aluminum oxide, aluminum nitride, silicon nitride, and boron nitride and adding polyethylene oxide, the physical indicators such as tensile strength and elongation of mica paper can be improved while ensuring electrical performance. However, on the basis of the composite of aramid fiber and mica paper, nano-inorganic matter is added, and the benzene ring structure contained in the aramid fiber will produce π-π interaction with the nano-inorganic matter, further aggravating the agglomeration of the nano-inorganic matter in the mica paper. The micron-sized particles formed after agglomeration may undergo interfacial debonding under dynamic loads, forming micron-sized cracks and pores, which become the preferential path of the breakdown channel, resulting in the mica composite aramid fiber material still being difficult to have good electrical breakdown resistance.

[0005] Therefore, there is an urgent need to develop an ultra-high voltage mica paper with high strength, low porosity and high corona resistance life. Summary of the invention

[0006] The purpose of the present invention is to provide an ultra-high voltage mica paper and a preparation process thereof, so as to solve the problem that the mica paper has low strength and low voltage resistance.

[0007] The purpose of the present invention can be achieved through the following technical solutions:

[0008] In a first aspect, the present invention provides an ultra-high voltage mica paper, comprising the following raw materials in parts by weight:

[0009] Mica 110-130 parts;

[0010] Aramid fiber 25-40 parts;

[0011] 10-20 parts of hydroxylated hyperbranched polyethylene copolymer;

[0012] 5 to 8 parts of boron nitride nanosheets.

[0013] Preferably, the aramid fiber is meta-aramid chopped fiber; the length of the meta-aramid chopped fiber is 4 to 10 mm.

[0014] Preferably, the boron nitride nanosheets have a thickness of 50 to 400 nm and a sheet diameter of 0.5 to 5 μm.

[0015] By adopting the above technical scheme, the ultra-high voltage mica paper of the present invention is obtained by compounding aramid fiber and mica, combining the excellent mechanical properties of aramid fiber and the outstanding electrical properties of mica. The obtained composite material can not only retain the excellent electrical properties of mica paper but also obtain better mechanical properties, further expanding the application scope of mica paper.

[0016] The ultra-high voltage mica paper of the present invention is further added with boron nitride nanosheets, which have a unique two-dimensional structure. The two-dimensional sheet structure can form a dense physical barrier in the mica paper matrix, thereby blocking the formation of a conductive path and uniformly dispersing the electric field stress. At the same time, the nanometer size of the boron nitride nanosheet can better fill the micrometer-level pores inside the mica paper, thereby greatly reducing the electric field distortion point, thereby inhibiting local discharge, and thus significantly improving the dielectric strength of the obtained mica paper, and obtaining ultra-high voltage mica paper. In addition, the two-dimensional sheet structure of the boron nitride nanosheet can also hinder the crack extension path and absorb the fracture energy, thereby improving the flexibility and anti-delamination ability of the mica paper.

[0017] However, during the composite process of aramid fiber and mica, since aramid fiber is a polymer organic material and mica is an inorganic material, the two lack active groups that can react to form a strong bond. Therefore, the interface between the two is poor and cannot form an effective interface bonding strength. In addition, due to the poor interface bonding between the two, the internal structure of the obtained composite material is loose and there are a large number of interface defects, including a large number of irregularly shaped pores inside the mica paper material. These pores are randomly distributed inside the material to form "weak zones". Even if the electric field is not high, the field strength at these defective parts will be very high, and local discharge is very likely to occur, eventually causing the mica paper to lose its insulating properties. In addition, after defects exist inside the mica paper, it is easy to cause local electric field concentration, and eventually electrical breakdown occurs. At the same time, the structural characteristics of boron nitride nanosheets also make it easy for them to agglomerate into micron-sized particles in the solution during processing. In particular, the benzene ring structure contained in the aramid fiber will produce π-π interaction with the boron nitride nanosheets, further aggravating the agglomeration. The agglomerated micron-sized particles are prone to interfacial debonding under dynamic loads, forming micron-sized cracks and pores, which become the preferred path of the breakdown channel. It will also cause the electric field to concentrate at the edge of the agglomeration, providing a channel for carrier migration and inducing local discharge, thereby limiting the improvement in the dielectric strength performance of the resulting composite material.

[0018] In order to solve the above problems, the present invention further adds a hydroxylated hyperbranched polyethylene copolymer to the mica paper. The mica and aramid fiber will expose polar groups in the aqueous phase system. A large number of terminal hydroxyl groups in the hydroxylated hyperbranched polyethylene copolymer can combine with the hydroxyl groups in the mica and the amino groups in the aramid fiber to form hydrogen bonding or dipole-dipole interaction, thereby building a bridge for interconnection between the aramid fiber and the mica interface. Moreover, the hyperbranched structure of the hydroxylated hyperbranched polyethylene copolymer can penetrate into the pores of the mica paper, and enhance the mechanical interlocking effect with the aramid fiber through physical entanglement, thereby compensating for a small amount of defects in the interface and forming a better interface effect, which is beneficial to stress transmission and dispersion, eliminates stress concentration, improves the mechanical properties of the mica paper, and can help fill the structural gap formed between the aramid fiber and the mica, play a role in interface modification, and form a multi-layer internal structure. The obtained mica paper can increase the moving path of electrons before breakdown during the breakdown process, thereby improving the dielectric breakdown strength of the mica paper and reducing the risk of local electric field concentration.

[0019] In addition, the three-dimensional branched structure of the hydroxylated hyperbranched polyethylene copolymer can form an adsorption layer on the surface of the boron nitride nanosheets, preventing the reagglomeration of the boron nitride nanosheets through the steric hindrance effect. Moreover, due to the certain binding effect between the hydroxylated hyperbranched polyethylene copolymer and the aramid fiber and mica, the interfacial energy difference between the boron nitride nanosheets and the mica paper substrate will be reduced, thereby promoting the dispersibility of the boron nitride nanosheets in the system, and improving the performance modification of the boron nitride nanosheets on the mica paper.

[0020] At the same time, the three-dimensional hyperbranched structure of the hydroxylated hyperbranched polyethylene copolymer can form a continuous insulating network in the mica paper, hindering the migration path of free charges and reducing leakage current. It can also form hydrogen bonds with the boron nitride nanosheets to obtain a double inhibition network, further improving the dielectric strength of the material. Moreover, the thermal conductive network of the boron nitride nanosheets can also complement the thermomechanical properties of the hydroxylated hyperbranched polyethylene copolymer, so that the breakdown field strength of the mica paper can still be maintained at a high level at high temperatures.

[0021] Preferably, the raw materials of the hydroxylated hyperbranched polyethylene copolymer include ethylene monomer and hydroxyethyl acrylate in a mass ratio of 1:(0.08-0.14).

[0022] Preferably, the hydroxylated hyperbranched polyethylene copolymer is prepared according to the following method:

[0023] After the ethylene monomer and hydroxyethyl acrylate are evenly mixed, a catalyst is added, the temperature is raised to 30-60°C in an inert gas atmosphere, and the reaction is stirred for 2-6 hours. Then, a sodium hydroxide / ethanol solution with a mass fraction of 20-25% is added, and stirring is continued for 0.5-1 hour. Finally, a hydroxylated hyperbranched polyethylene copolymer is obtained after washing and drying.

[0024] More preferably, the catalyst is a nickel / palladium catalyst.

[0025] By adopting the above technical scheme, under the action of the catalyst, the ethylene monomer will undergo frequent branching during the polymerization process, thereby obtaining polyethylene with a hyperbranched structure, and the ethylene monomer will further form a coordination copolymer with hydroxyethyl acrylate, thereby obtaining a hydroxyl-functionalized hyperbranched polyethylene copolymer.

[0026] Hydroxylated hyperbranched polyethylene copolymer can serve as a bridge between aramid fiber and mica, greatly improving the interfacial bonding between the two, and can also fill the pores of mica paper, thereby improving the mechanical properties of mica paper, and improve the electrical properties of mica paper by compensating for interface defects. At the same time, hydroxylated hyperbranched polyethylene copolymer can also greatly improve the dispersibility of boron nitride nanosheets in mica paper, and can cooperate with boron nitride nanosheets to improve the dielectric strength of mica paper.

[0027] Preferably, the boron nitride nanosheets are composite-coated with titanium dioxide.

[0028] Preferably, the raw materials for composite coating of titanium dioxide with boron nitride nanosheets include boron nitride nanosheets and titanium tetrachloride in a mass ratio of 1:(0.6-0.8).

[0029] Preferably, the boron nitride nanosheet composite coated with titanium dioxide is prepared according to the following method:

[0030] The boron nitride nanosheets are dispersed in water, and a titanium tetrachloride aqueous solution with a mass fraction of 3-5% is added, stirred and mixed, and the pH value of the solution is adjusted to 5.5-6, and the reaction is stirred at room temperature for 0.5-1h, and then the temperature is increased to 180-185°C in a hydrothermal reactor to react for 10-12h, and finally washed, dried and ground to obtain the product.

[0031] By adopting the above technical scheme, although the hydroxylated hyperbranched polyethylene copolymer can improve the dispersibility of boron nitride nanosheets in mica paper to a certain extent, due to the lack of active functional groups on the surface of boron nitride nanosheets, the binding between the boron nitride nanosheets and the hydroxylated hyperbranched polyethylene copolymer is mostly based on physical adsorption, and the interfacial bonding force is relatively weak, which will cause the stress to be unable to be effectively transferred from the hydroxylated hyperbranched polyethylene copolymer to the boron nitride nanosheets, and the performance improvement of the obtained mica paper material is limited.

[0032] Therefore, the present invention adopts a coating modification method to improve the surface polarity of the boron nitride nanosheets. Specifically, the surface of titanium dioxide is rich in hydroxyl groups, which can form strong hydrogen bonds with the hydroxyl groups in the hydroxylated hyperbranched polyethylene copolymer, thereby improving the interface bonding force with the boron nitride nanosheets. In addition, the titanium dioxide-coated modified boron nitride nanosheets themselves can achieve a certain dispersion stability by adjusting the surface charge, and can also form a physical barrier on the surface of the boron nitride nanosheets to reduce the agglomeration of the boron nitride nanosheets caused by the interlayer van der Waals force.

[0033] At the same time, after the boron nitride nanosheets are composite-coated with titanium dioxide, the heterogeneous interface formed can capture free charges and inhibit carrier migration. It can also regulate the dielectric constant of mica paper and reduce the dielectric mismatch problem between mica paper and boron nitride nanosheets, thereby homogenizing the electric field distribution and reducing the local field intensity peak. The titanium dioxide coating layer can also serve as a high dielectric barrier to disperse the electric field stress, inhibit the expansion of the breakdown path, and form a three-dimensional thermal conductive network with mica to add heat diffusion and avoid insulation aging caused by local temperature changes. At the same time, the hydroxyl group of titanium dioxide can participate in the interface connection between the hydroxylated hyperbranched polyethylene copolymer and the aramid fiber and mica, enhance the interface bonding force between the boron nitride nanosheets and the matrix material, reduce the initiation of microcracks, and improve the mechanical strength and dielectric strength.

[0034] Preferably, the mica is a composition of 20-mesh mica powder and 200-mesh mica powder; the mass ratio of the 20-mesh mica powder to the 200-mesh mica powder is (2-4): (6-8).

[0035] By adopting the above technical solution, 20-mesh mica powder is used as the skeleton support to bear the main mechanical load, and 200-mesh mica powder fills its gaps, which can greatly reduce the porosity inside the mica paper material and improve the density, thereby enhancing the dielectric strength and mechanical strength of the mica paper. The fine particles can also transfer stress through the interface to form a multi-level toughening mechanism and improve the tear resistance of the mica paper.

[0036] In a second aspect, the present invention provides a process for preparing ultra-high voltage mica paper, comprising the following process steps:

[0037] S1. Add water to mica to make mica slurry and send it to the papermaking slurry tank;

[0038] S2. The aramid fiber is dispersed in water, the mass fraction of the aramid fiber is controlled to be 30-40%, and then the hydroxylated hyperbranched polyethylene copolymer and the boron nitride nanosheets are added, the solution temperature is increased to 80-90°C, and the mixture is stirred for 3-5h to obtain a mixed solution;

[0039] S3. The mixed solution is sent to a papermaking pool, mixed evenly with mica pulp, and then formed through a rotary screen and dried to obtain ultra-high voltage mica paper.

[0040] Beneficial effects of the present invention:

[0041] 1. The ultra-high voltage mica paper of the present invention contains boron nitride nanosheets, which can form a dense physical barrier in the mica paper matrix, thereby blocking the formation of a conductive path. At the same time, it can fill the micron-level pores inside the mica paper material, reduce the electric field distortion points, and significantly improve the dielectric constant and mechanical strength of the mica paper to obtain an ultra-high voltage mica paper.

[0042] 2. The ultra-high voltage mica paper of the present invention is also added with hydroxylated hyperbranched polyethylene copolymer, which can improve the interfacial force between aramid fiber and mica. The hyperbranched structure can also help penetrate into the gaps in the mica paper, make up for the interface defects, improve the mechanical properties and dielectric breakdown strength of the mica paper, and can also improve the dispersibility of boron nitride nanosheets in the matrix material, improve the performance modification of mica paper by boron nitride nanosheets, and cooperate with boron nitride nanosheets to further improve the dielectric strength and high temperature stability of the material.

[0043] 3. The boron nitride nanosheets in the ultra-high voltage mica paper of the present invention are also treated with titanium dioxide coating modification, which can not only enhance the interfacial bonding force between the boron nitride nanosheets and the matrix material, but also regulate the dielectric constant of the mica paper, homogenize the electric field distribution, disperse the electric field stress, and improve the mechanical strength and dielectric strength of the mica paper. DETAILED DESCRIPTION

[0044] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only 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.

[0045] Preparation Example

[0046] Preparation Example 1 Hydroxylated hyperbranched polyethylene copolymer

[0047] Preparation Example 1-1, a hydroxylated hyperbranched polyethylene copolymer, prepared according to the following method:

[0048] After 100 g of ethylene monomer and 12 g of hydroxyethyl acrylate are evenly mixed, 1 g of nickel / palladium catalyst is added, the temperature is raised to 45°C in a nitrogen atmosphere, and the reaction is stirred for 4 hours. Then, a sodium hydroxide / ethanol solution with a mass fraction of 25% is added, and stirring is continued for 1 hour. Finally, a hydroxylated hyperbranched polyethylene copolymer is obtained after washing and drying.

[0049] Preparation Example 1-2, a hydroxylated hyperbranched polyethylene copolymer, is different from Preparation Example 1-1 only in that the added amount of hydroxyethyl acrylate is 8 g.

[0050] Preparation Example 1-3, a hydroxylated hyperbranched polyethylene copolymer, is different from Preparation Example 1-1 only in that the added amount of hydroxyethyl acrylate is 14 g.

[0051] Preparation Example 1-4, a hydroxylated hyperbranched polyethylene copolymer, is different from Preparation Example 1-1 only in that the added amount of hydroxyethyl acrylate is 4 g.

[0052] Preparation Example 1-5, a hydroxylated hyperbranched polyethylene copolymer, is different from Preparation Example 1-1 only in that the added amount of hydroxyethyl acrylate is 18 g.

[0053] Preparation Example 1-6, a hydroxylated hyperbranched polyethylene copolymer, is different from Preparation Example 1-1 only in that hydroxyethyl acrylate is not added.

[0054] Preparation Example 2 Boron nitride nanosheets composite coated with titanium dioxide

[0055] Preparation Example 2-1, a boron nitride nanosheet composite coated with titanium dioxide, prepared according to the following method:

[0056] 10g of boron nitride nanosheets (average thickness of 120nm, average sheet diameter of 2μm) were dispersed in water, and a 4% mass fraction of titanium tetrachloride aqueous solution (7g of titanium tetrachloride) was added, stirred and mixed, and the pH value of the solution was adjusted to 6. The reaction was stirred at room temperature for 0.5h, and then the temperature was increased to 180°C in a hydrothermal reactor to react for 12h. Finally, it was washed, dried and ground to obtain the product.

[0057] Preparation Example 2-2, a boron nitride nanosheet composite coated with titanium dioxide, is different from Preparation Example 2-1 only in that the amount of titanium tetrachloride added is 6 g.

[0058] Preparation Example 2-3, a boron nitride nanosheet composite coated with titanium dioxide, is different from Preparation Example 2-1 only in that the amount of titanium tetrachloride added is 8 g.

[0059] Preparation Example 2-4, a boron nitride nanosheet composite coated with titanium dioxide, is different from Preparation Example 2-1 only in that the amount of titanium tetrachloride added is 4 g.

[0060] Preparation Example 2-5, a boron nitride nanosheet composite coated with titanium dioxide, is different from Preparation Example 2-1 only in that the amount of titanium tetrachloride added is 10 g.

[0061] Example

[0062] Example 1, a super high voltage mica paper is prepared according to the following process steps:

[0063] S1. 120 parts of mica were added with water to form mica slurry and fed into the papermaking slurry pool, wherein the mica was a composition of 20 mesh mica powder and 200 mesh mica powder in a mass ratio of 3:7;

[0064] S2. 32 parts of meta-aramid short fibers (average length of 6 mm) were dispersed in water, the mass fraction of aramid fibers was controlled to be 35%, and then 15 parts of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 and 7 parts of boron nitride nanosheets (average thickness of 120 nm, average sheet diameter of 2 μm) were added, the solution temperature was increased to 90°C, and the mixture was stirred for 4 hours to obtain a mixed solution;

[0065] S3. The mixed solution is sent to a papermaking pool, mixed evenly with mica pulp, and then formed through a rotary screen and dried to obtain ultra-high voltage mica paper.

[0066] Embodiment 2 and embodiment 3 are ultra-high voltage mica papers, which differ from embodiment 1 only in that the raw material ratio is adjusted, as shown in Table 1:

[0067] .

[0068] Example 4, a super high voltage mica paper, differs from Example 1 only in that the mica is a composition of 20 mesh mica powder and 200 mesh mica powder in a mass ratio of 2:8; the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is replaced by an equal amount of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-2.

[0069] Example 5, a super high voltage mica paper, differs from Example 1 only in that the mica is a composition of 20 mesh mica powder and 200 mesh mica powder in a mass ratio of 4:6; the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is replaced by an equal amount of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-3.

[0070] Example 6, a super high voltage mica paper, is different from Example 1 only in that the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is replaced by an equal amount of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-4.

[0071] Example 7, a super high voltage mica paper, is different from Example 1 only in that the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is replaced by an equal amount of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-5.

[0072] Example 8, a super high voltage mica paper, is different from Example 1 only in that an equal amount of 20 mesh mica powder is used to replace the mica obtained by combining 20 mesh mica powder and 200 mesh mica powder in a mass ratio of 3:7.

[0073] Example 9, a super high voltage mica paper, is different from Example 1 only in that an equal amount of 200 mesh mica powder is used to replace the mica obtained by combining 20 mesh mica powder and 200 mesh mica powder in a mass ratio of 3:7.

[0074] Example 10, a super high voltage mica paper, is different from Example 1 only in that the boron nitride nanosheets are replaced by an equal amount of boron nitride nanosheets composite-coated titanium dioxide prepared in Preparation Example 2-1.

[0075] Example 11, a super high voltage mica paper, is different from Example 10 only in that the boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-1 is replaced by an equal amount of boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-2.

[0076] Example 12, a super high voltage mica paper, differs from Example 10 only in that the boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-1 is replaced by an equal amount of boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-3.

[0077] Example 13, a super high voltage mica paper, is different from Example 10 only in that the boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-1 is replaced by an equal amount of boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-4.

[0078] Example 14, a super high voltage mica paper, is different from Example 10 only in that the boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-1 is replaced by an equal amount of boron nitride nanosheet composite coated titanium dioxide prepared in Preparation Example 2-5.

[0079] Comparative Example

[0080] Comparative Example 1 is a kind of ultra-high voltage mica paper, which is different from Example 1 only in that the addition amount of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is 5 parts.

[0081] Comparative Example 2 is a kind of ultra-high voltage mica paper, which is different from Example 1 only in that the addition amount of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is 25 parts.

[0082] Comparative Example 3, a super high voltage mica paper, is different from Example 1 only in that the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is replaced by an equal amount of the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-6.

[0083] Comparative Example 4, a super high voltage mica paper, is different from Example 1 only in that an equal amount of hydroxyl-polyethylene (OH-PE) is used to replace the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1.

[0084] Comparative Example 5 is a kind of ultra-high voltage mica paper, which is different from Example 1 only in that the hydroxylated hyperbranched polyethylene copolymer prepared in Preparation Example 1-1 is not added.

[0085] Comparative Example 6 is a kind of ultra-high voltage mica paper, which is different from Example 1 only in that no boron nitride nanosheets are added.

[0086] Performance testing

[0087] 1. Mechanical property test: According to the relevant records in GB / T 5019.4-2009 "Mica-based insulating materials Part 4: Mica paper", the tensile strength of the ultra-high voltage mica paper obtained in the examples and comparative examples was tested.

[0088] 2. Electrical strength test: According to the relevant records in GB / T 5019.4-2009 "Mica-based insulating materials Part 4: Mica paper", the dielectric breakdown strength of the ultra-high voltage mica paper obtained in the examples and comparative examples was tested.

[0089] The above test results are shown in Table 2:

[0090] .

[0091] According to Table 2, in combination with Example 1, Example 6, Example 7 and Comparative Example 3, it can be seen that the tensile strength and dielectric breakdown strength of Example 6, Example 7 and Comparative Example 3 are reduced compared with Example 1, indicating that the mechanical properties and electrical properties of Example 6, Example 7 and Comparative Example 3 are reduced compared with Example 1. The reason is that the difference between Example 6, Example 7 and Comparative Example 3 and Example 1 is only that the hydroxyl content in the hydroxylated hyperbranched polyethylene copolymer is adjusted, wherein the hydroxyl content is reduced in Example 6, resulting in the hydroxylated hyperbranched polyethylene copolymer having a reduced direct bridging property to aramid fiber and mica and a reduced dispersibility to boron nitride nanosheets, the interface bonding force between aramid fiber and mica becomes poor, the porosity increases, and the agglomeration of boron nitride nanosheets increases, which will cause changes in the mechanical properties and dielectric strength of the mica paper material. In Comparative Example 3, the hyperbranched polyethylene is not hydroxylated, so the performance degradation is more obvious. In Example 7, the amount of hydroxyl added is increased. Too high a hydroxyl content will lead to excessive hydrogen bonds between polymer segments, resulting in increased rigidity of the material and reduced insulation performance, thereby causing a decrease in various properties.

[0092] In combination with Example 1, Example 8 and Example 9, it can be seen that the tensile strength and dielectric breakdown strength of Example 8 and Example 9 are reduced compared with Example 1. The reason is that the difference between Example 8 and Example 9 compared with Example 1 is only that the mica used is not obtained by combining two mica powders with different particle sizes. Specifically, only large-particle-size mica powder is used in Example 8, which will increase the porosity of the internal structure of the obtained mica paper, and it is difficult to achieve good results under the coordination effect of other components. Only small-particle-size mica powder is used in Example 8, and there is no skeleton structure for support, which will cause the mechanical properties of the obtained mica paper to decrease.

[0093] Combining Example 1, Example 10, Example 13 and Example 14, it can be seen that the tensile strength and dielectric breakdown strength of Example 10 are increased compared to those of Example 1. The reason is that the boron nitride nanosheets in Example 10 are modified by titanium dioxide coating, and their dispersibility in the system and the interfacial bonding force with the hydroxylated hyperbranched polyethylene copolymer are enhanced, which effectively improves the mechanical properties of the material, fills the internal pores, and cooperates with the effect of titanium dioxide itself to improve the overall dielectric strength. Compared with Example 10, the tensile strength and dielectric breakdown strength of Example 13 and Example 14 are reduced. The reason is that the only difference between Example 13 and Example 14 and Example 10 is that the content of titanium dioxide coated on the surface of the boron nitride nanosheets is adjusted, wherein Example 13 reduces the coating amount of titanium dioxide, which easily leads to incomplete coverage and failure to effectively disperse the electric field, thereby causing a decrease in the performance of the mica paper; Example 14 increases the coating amount of titanium dioxide. Too thick a coating layer will hinder the improvement of the mechanical and electrical properties of the mica paper by the boron nitride nanosheets themselves, and will also affect the thermal conductivity path of the boron nitride nanosheets, thereby affecting the actual use effect.

[0094] In combination with Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 5, it can be seen that the tensile strength and dielectric breakdown strength of Comparative Example 1, Comparative Example 2 and Comparative Example 5 are all reduced compared with those of Example 1. The reason is that the difference between Comparative Example 1, Comparative Example 2 and Comparative Example 5 and Comparative Example 1 is only that the addition amount of the hydroxylated hyperbranched polyethylene copolymer is adjusted. Specifically, the addition amount of the hydroxylated hyperbranched polyethylene copolymer is reduced in Comparative Example 1, the interface bridging effect between the aramid fiber and the mica is reduced, the penetration effect on the pores of the mica paper is reduced, the reinforcement effect on the interface defects is reduced, and the improvement effect on the dispersibility of the boron nitride nanosheets is reduced, thereby affecting the stress transfer and electron movement path of the mica paper, thereby resulting in the mechanical properties and electrical properties of the mica paper. In Comparative Example 5, no hydroxylated hyperbranched polyethylene copolymer is added, and accordingly, the performance degradation is more obvious. In Comparative Example 2, the amount of hydroxylated hyperbranched polyethylene copolymer added is increased, which will lead to excessive cross-linking between the polymer molecular segments in the system, the rigidity of the material is enhanced but the tensile strength is significantly reduced. At the same time, it will also lead to uneven distribution of molecular segments inside the mica paper, which will become stress concentration points, and the mechanical properties of the material will be reduced. In addition, excessive addition will increase the polarity of the material, increase the dielectric constant, and affect the electrical properties of the material.

[0095] Combining Example 1 and Comparative Example 4, it can be seen that the tensile strength and dielectric breakdown strength of Comparative Example 4 are lower than those of Example 1. The reason is that the hydroxylated polyethylene in Comparative Example 4 does not have a hyperbranched structure. On the one hand, the terminal hydroxyl groups of the hydroxylated polyethylene copolymer with a hyperbranched structure are densely distributed in large quantities, which can better bridge between the aramid fiber and the mica. On the other hand, the branched structure of the hydroxylated polyethylene copolymer can significantly enhance the dispersibility, reduce intermolecular entanglement, and improve the penetration and filling of pores. The hyperbranched structure can help the hydroxylated polyethylene copolymer to be better dispersed in the system and play a linking role, thereby significantly improving the mechanical properties and electrical properties of the material.

[0096] Combining Example 1 and Comparative Example 6, it can be seen that the tensile strength and dielectric breakdown strength of Comparative Example 6 are lower than those of Example 1. The reason is that no boron nitride nanosheets are added in Comparative Example 6, and the lack of a two-dimensional layer structure of boron nitride nanosheets to form a dense physical barrier in the mica paper directly affects the formation of a conductive path. Moreover, the filling effect of the micron-sized pores inside the mica paper is also reduced, thereby reducing the mechanical and electrical properties of the obtained mica paper.

[0097] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0098] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A super high voltage mica paper, characterized in that: Including the following raw materials by mass: Mica 110-130 parts; Aramid fiber 25-40 parts; 10-20 parts of hydroxylated hyperbranched polyethylene copolymer; 5-8 parts of boron nitride nanosheets; The boron nitride nanosheet has a thickness of 50 to 400 nm and a sheet diameter of 0.5 to 5 μm; The mica is a combination of 20-mesh mica powder and 200-mesh mica powder; The aramid fiber is meta-aramid chopped fiber; The raw materials of the hydroxylated hyperbranched polyethylene copolymer include ethylene monomer and hydroxyethyl acrylate in a mass ratio of 1: (0.08-0.14); The hydroxylated hyperbranched polyethylene copolymer is prepared according to the following method: After the ethylene monomer and hydroxyethyl acrylate are evenly mixed, a catalyst is added, the temperature is raised to 30-60°C in an inert gas atmosphere, and the reaction is stirred for 2-6 hours. Then, a sodium hydroxide / ethanol solution with a mass fraction of 20-25% is added, and stirring is continued for 0.5-1 hour. Finally, a hydroxylated hyperbranched polyethylene copolymer is obtained after washing and drying.

2. The ultra-high voltage mica paper according to claim 1, characterized in that: The boron nitride nanosheets are composite-coated with titanium dioxide.

3. The ultra-high voltage mica paper according to claim 2, characterized in that: The raw materials for the boron nitride nanosheet composite-coated titanium dioxide include boron nitride nanosheet and titanium tetrachloride in a mass ratio of 1: (0.6-0.8).

4. The ultra-high voltage mica paper according to claim 3, characterized in that: The boron nitride nanosheet composite coated titanium dioxide is prepared according to the following method: The boron nitride nanosheets are dispersed in water, and a titanium tetrachloride aqueous solution with a mass fraction of 3-5% is added, stirred and mixed, and the pH value of the solution is adjusted to 5.5-6, and the reaction is stirred at room temperature for 0.5-1h, and then the temperature is increased to 180-185°C in a hydrothermal reactor to react for 10-12h, and finally washed, dried and ground to obtain the product.

5. The ultra-high voltage mica paper according to claim 1, characterized in that: The mass ratio of the 20-mesh mica powder to the 200-mesh mica powder is (2-4): (6-8).

6. The ultra-high voltage mica paper according to claim 1, characterized in that: The length of the meta-aramid chopped fibers is 4 to 10 mm.

7. A process for preparing ultra-high voltage mica paper according to any one of claims 1 to 6, characterized in that: The process steps include: S1. Add water to mica to make mica slurry and send it to the papermaking slurry tank; S2. The aramid fiber is dispersed in water, the mass fraction of the aramid fiber is controlled to be 30-40%, and then the hydroxylated hyperbranched polyethylene copolymer and the boron nitride nanosheets are added, the solution temperature is increased to 80-90°C, and the mixture is stirred for 3-5h to obtain a mixed solution; S3. The mixed solution is sent to a papermaking pool, mixed evenly with mica pulp, and then formed through a rotary screen and dried to obtain ultra-high voltage mica paper.