Electromagnetic brake friction material for super-long time working condition and preparation method thereof

Abstract

The invention relates to the field of friction materials, and discloses an electromagnetic brake friction material used for an ultra-long time working condition and a preparation method of the electromagnetic brake friction material. The modified polyimide is a polyimide-silicon rubber interpenetrating complex, and the preparation method of the modified polyimide comprises the following steps: blending a silicon rubber prepolymer with a polyimide precursor and a silicon rubber prepolymer curing agent, and curing to obtain the interpenetrating complex; the reinforcing material comprises aramid pulp, copper fibers, potassium magnesium titanate lamellas, tetrapod-like zinc oxide whiskers and modified aluminum titanate fibers; the friction increasing material comprises nano aluminum hydroxide, fluororubber powder, barite, silicon carbide, feldspar powder, modified white carbon black and cashew nut shell oil friction powder; the antifriction material comprises halloysite nanotubes coated with redox graphene, molybdenum carbon phosphate and graphite fluoride. The friction material provided by the invention can meet the requirements of strength, heat resistance and wear resistance under an ultra-long working condition, and meanwhile, the thermal expansion at high temperature can also be reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of friction materials, and specifically, to an electromagnetic brake friction material for ultra-long working conditions and a preparation method thereof. Background Art

[0002] Electromagnetic brakes are widely used in various industries, such as servo motors, robots, aerial work platforms, forklifts, elevators, lifts, elevator stages, fans, etc., and belong to the key components of control systems. With the continuous expansion of the application fields of electromagnetic brakes, electromagnetic brakes have appeared in more and more fields, which requires electromagnetic brakes to continuously adapt to various harsh operating conditions. Currently, robots are widely used in 24-hour continuous production on production lines, and the operating efficiency is constantly increasing. This requires that the electromagnetic brake can adapt to the working conditions of ultra-long operation, and at the same time, the torque can be maintained for a long time without slipping. This poses higher requirements for the electromagnetic brake friction material, such as high impact strength, high temperature wear resistance, and small high temperature fade. Currently, there are few friction materials that can be applied to the above-mentioned working conditions. Therefore, developing electromagnetic brake friction materials for long-term working conditions is of great significance for expanding the application fields of electromagnetic brake friction materials. Summary of the Invention

[0003] Technical problems to be solved by the present invention: To solve the technical problems such as insufficient strength of the friction material, poor high temperature wear resistance, and large high temperature heat fade under ultra-long working conditions.

[0004] Technical solutions adopted by the present invention: In view of the above technical problems, the object of the present invention is to provide an electromagnetic brake friction material for ultra-long working conditions and a preparation method thereof.

[0005] The specific content is as follows: First, the present invention provides an electromagnetic brake friction material for ultra-long working conditions, and the raw material components include a binder, a reinforcing material, a friction increasing filler, and a friction reducing filler; The binder includes polyarylether nitrile and modified polyimide; the modified polyimide is a polyimide-silicone rubber interpenetrating complex; The reinforcing material includes aramid pulp, copper fiber, potassium magnesium titanate whisker, tetrapod zinc oxide whisker, and modified aluminum titanate fiber; The friction increasing materials include nano-aluminum hydroxide, fluororubber powder, barite, silicon carbide, feldspar powder, modified silica, and cashew shell oil friction powder; The friction reducing materials include halloysite nanotubes coated with reduced graphene oxide, molybdenum carbon phosphate, and fluorinated graphite.

[0006] According to some preferred embodiments, the components of the raw material are counted by weight parts, including 45 - 65 parts of binder, 30 - 40 parts of reinforcing material, 45 - 65 parts of friction increasing filler, and 10 - 20 parts of friction reducing filler.

[0007] According to some preferred embodiments, the components of the raw material are counted by weight parts, including 35 - 50 parts of polyarylether nitrile, 10 - 30 parts of modified polyimide, 1 - 3 parts of aramid pulp, 3 - 5 parts of copper fiber, 6 - 10 parts of potassium magnesium titanate whisker, 3 - 8 parts of tetrapod zinc oxide whisker, 8 - 14 parts of modified aluminum titanate fiber, 5 - 10 parts of nano aluminum hydroxide, 10 - 15 parts of fluororubber powder, 8 - 15 parts of barite, 1 - 3 parts of silicon carbide, 3 - 8 parts of feldspar powder, 3 - 8 parts of modified silica, 8 - 12 parts of cashew shell oil friction powder, 5 - 8 parts of halloysite nanotubes coated with redox graphene, 2 - 5 parts of molybdenum carbon phosphate, and 3 - 6 parts of fluorinated graphite.

[0008] According to some more preferred embodiments, the components of the raw material are counted by weight parts, including any one of combination A, combination B, and combination C: Combination A: 45 parts of polyarylether nitrile, 15 parts of modified polyimide, 2 parts of aramid pulp, 3 parts of copper fiber, 8 parts of potassium magnesium titanate whisker, 5 parts of tetrapod zinc oxide whisker, 12 parts of modified aluminum titanate fiber, 8 parts of nano aluminum hydroxide, 10 parts of fluororubber powder, 12 parts of barite, 1 part of silicon carbide, 7 parts of feldspar powder, 5 parts of modified silica, 10 parts of cashew shell oil friction powder, 8 parts of halloysite nanotubes coated with redox graphene, 4 parts of molybdenum carbon phosphate, and 4 parts of fluorinated graphite.

[0009] Combination B: 40 parts of polyarylether nitrile, 20 parts of modified polyimide, 1 part of aramid pulp, 4 parts of copper fiber, 10 parts of potassium magnesium titanate whisker, 8 parts of tetrapod zinc oxide whisker, 10 parts of modified aluminum titanate fiber, 6 parts of nano aluminum hydroxide, 12 parts of fluororubber powder, 10 parts of barite, 2 parts of silicon carbide, 6 parts of feldspar powder, 4 parts of modified silica, 9 parts of cashew shell oil friction powder, 6 parts of halloysite nanotubes coated with redox graphene, 2 parts of molybdenum carbon phosphate, and 5 parts of fluorinated graphite.

[0010] Combination C: 35 parts of polyarylether nitrile, 25 parts of modified polyimide, 3 parts of aramid pulp, 5 parts of copper fiber, 6 parts of potassium magnesium titanate whisker, 6 parts of tetrapod zinc oxide whisker, 14 parts of modified aluminum titanate fiber, 10 parts of nano aluminum hydroxide, 14 parts of fluororubber powder, 14 parts of barite, 3 parts of silicon carbide, 4 parts of feldspar powder, 8 parts of modified silica, 8 parts of cashew shell oil friction powder, 5 parts of halloysite nanotubes coated with redox graphene, 3 parts of molybdenum carbon phosphate, and 6 parts of fluorinated graphite.

[0011] According to some preferred embodiments, in the adhesive, the preparation method of the modified polyimide is to obtain an interpenetrating complex by blending and curing a silicone rubber prepolymer, a polyimide precursor, and a silicone rubber prepolymer curing agent.

[0012] Specifically: (1) Prepare a silicone rubber prepolymer solution: Dissolve the silicone rubber prepolymer in N-methylpyrrolidone, and then add KH550 (the addition amount accounts for 0.03-0.1 of the mass of the silicone rubber prepolymer), and obtain a 10-15 wt% silicone rubber prepolymer solution through ultrasonic blending.

[0013] (2) Prepare a polyimide precursor solution: Blend pyromellitic dianhydride and 4,4'-diaminodiphenyl ether in a molar ratio of 1:1, blend at 500 rpm for 2 h, and then add them to N-methylpyrrolidone to obtain a 15-20 wt% polyimide precursor solution.

[0014] (3) Solution blending: Gradually add the silicone rubber prepolymer solution to the polyimide precursor solution, control the mass ratio of the silicone rubber prepolymer to the polyimide precursor to be 7-8:2-3, then perform mechanical stirring, blend at 800 rpm for 1 h, and simultaneously add the silicone rubber prepolymer curing agent, and then obtain a blend through vacuum degassing. Vacuum degassing parameters: 40-70 °C, 1-3 h, -0.1 MPa.

[0015] (4) Curing of the blend: Heat the obtained blend to 70-100 °C in a nitrogen atmosphere and keep it warm for 1-3 h; then, continue to heat it to 120-180 °C, keep it warm for 0.5-2 h, and cool it to room temperature.

[0016] According to some preferred embodiments, in the reinforcing material, the preparation method of the modified aluminum titanate fiber is to dry the aluminum titanate fiber, then perform acid etching treatment, mix it with an activator emulsion, and finally filter and dry it to obtain.

[0017] Specifically: (1) Drying: Dry the aluminum titanate fiber at 300-400 °C for 1-2 h to remove surface impurities; (2) Etching: Put the dried aluminum titanate fiber into a phosphoric acid or oxalic acid solution with a concentration of 5-10% for acid etching to remove surface amorphous substances and impurities. The etching time is 60-120 min, and the temperature is 50-80 °C; (3) Post-treatment: After etching, filter to obtain aluminum titanate fiber, wash it with distilled water 3-5 times, and dry it at 80-100 °C for 30 min-60 min after washing.

[0018] (4) Activation: Put the dried aluminum titanate fibers into a hydroxyethyl cellulose solution with a solution concentration of 3 - 5%, soak at room temperature for 1 - 2 h, filter out the aluminum titanate fibers, and dry them at 80 - 100 °C for 1 - 2 h to obtain modified aluminum titanate fibers.

[0019] According to some preferred embodiments, in the friction-increasing filler, the modified silica is obtained by blending and ball-milling silica with bis-[γ-(triethoxysilyl)propyl]tetrasulfide (Si-69C) and stearic acid. The specific treatment method is as follows: Add 5 - 7% of Si-69C by mass fraction and 1 - 3% of stearic acid by mass fraction to the silica, put it into a high-speed ball mill for ball-milling, the rotational speed of the ball mill is 1000 - 1500 rpm, and the ball-milling time is 45 - 60 min to obtain the modified silica.

[0020] According to some preferred embodiments, in the friction-reducing filler, the preparation method of the halloysite nanotubes coated with reduced graphene oxide is as follows: (1) Add reduced graphene oxide and tannic acid to deionized water and disperse them by ultrasonic to obtain a suspension, and react the suspension under a nitrogen atmosphere to obtain modified reduced graphene oxide. The ratio of reduced graphene oxide to tannic acid is 5 - 10:1, and the reaction parameters are: 75 - 90 °C, 10 - 16 h.

[0021] (2) Weigh the halloysite nanotubes, pre-dry them in an oven at 100 - 200 °C for 1 h, add an ethanol aqueous solution of dimethyl selenoxide with a volume ratio of 1:1:0.5 - 1:2:0.5. The volume of the ethanol aqueous solution of dimethyl selenoxide should be sufficient to submerge the halloysite nanotubes, and perform ultrasonic oscillation at 30 - 50 °C for 1 - 2 h. After filtration, rinse with distilled water, and place the treated halloysite nanotubes in an oven at 50 - 60 °C to dry for 1 - 2 h. Then put the treated halloysite nanotubes into an aqueous solution of glycerol with a mass fraction of glycerol of 10% - 20%, and perform ultrasonic oscillation at 30 - 50 °C for 1 - 2 h. After filtration, rinse with distilled water, and dry the treated halloysite nanotubes in an oven at 50 - 60 °C for 1 - 2 h to finally obtain modified halloysite nanotubes.

[0022] (3) Disperse the modified reduced graphene oxide in deionized water, add the modified halloysite nanotubes and disperse them by ultrasonic to obtain a suspension, react the suspension at 35 - 50 °C for 0.5 - 2 h, and after the reaction, filter and dry to obtain halloysite nanotubes coated with graphene oxide. The mass ratio of the modified reduced graphene oxide to the halloysite nanotubes is 1:7 - 10.

[0023] According to some preferred embodiments, the polyarylether nitrile is 200 mesh, the polyimide is 800 mesh, the fluororubber powder is 40 - 60 mesh, the length of the aramid pulp is 1.5 - 2 mm, the copper fiber is 1 - 3 mm, the potassium titanate magnesium platelet has a length and width of 0.5 - 5 μm and a thickness of 0.2 - 1.5 μm, the tetrapod-like zinc oxide whisker is 300 - 400 mesh, the aluminum titanate fiber is 1 - 3 mm, the nano aluminum hydroxide is 20 - 40 nm, the barite is 325 mesh, the silicon carbide is 600 mesh, the feldspar powder is 325 mesh, the silica white is 400 mesh, the cashew shell oil friction powder is 100 mesh, the molybdenum carbon phosphate is 325 mesh, the reduced graphene oxide is 400 mesh, the halloysite nanotube is 1250 mesh, and the fluorinated graphite is 100 mesh.

[0024] Second, the present invention provides a preparation method of the electromagnetic brake friction material for ultra-long working conditions mentioned above, including the following steps: S1 Weigh each raw material according to the proportion and perform a blending treatment to obtain a blended material; S2 Dry the molded material after blending; S3 After drying, perform cold pressing and forming; through die pressing treatment, obtain a semi-finished product; S4 Then perform hot pressing treatment to obtain a semi-finished product; S5 Then perform heat treatment to obtain the friction material; S6 Perform surface grinding treatment on the friction material.

[0025] Specifically, that is to say: S1 Blending: Weigh each component raw material according to the proportion, and put each component raw material into a plowshare mixer for blending. The main shaft speed of the mixer is 250 - 300 rpm, and the reamer speed is 2500 - 3000 rpm.

[0026] S2 Drying: Dry the mixed molded material in an 80°C oven for 1 - 2 h.

[0027] S3 Pre-pressing and forming: Put the molded material into a cold pressing die and level it, and apply pressure to press the mixed material into a blank. The cold pressing pressure is 50 - 60 MPa, and press it repeatedly 2 times. Stop when the pressure of the press reaches the required value.

[0028] S4 Hot pressing: Use a 160T four-column hydraulic press for hot pressing. Put the cold blanks into the hot pressing die for pressing. The pressing temperature is 350 ± 10°C, the pressing pressure is 20 - 30 MPa, and the pressure holding time is 30 - 60 min / mm. Exhaust gas before pressure holding. Press for 15 s, exhaust gas for 3 s, and perform 4 times of exhaust gas before entering the pressure holding stage. After the pressure holding is completed, obtain a semi-finished friction material.

[0029] S5 Heat treatment: Heat from room temperature to 180 °C in 1 h, hold for 1 - 2 h, heat from 180 °C to 280 °C in 1 h, hold for 1 h, heat from 240 °C to 350 °C in 1 h, hold for 6 - 8 h, and then cool in the furnace to 50 °C and take out.

[0030] S6 Grinding: Grind the heat-treated friction material by a double-sided belt grinding machine to remove surface oxide scale, burrs, and flash.

[0031] Beneficial effects achieved by the present invention: (1) The binder provided by the present invention includes polyarylether nitrile and modified polyimide; Polyarylether nitrile has good heat resistance, good creep resistance, high strength and rigidity, and a small thermal expansion coefficient, and can be used as a binder for friction materials. As an organic filler, fluororubber powder is uniformly dispersed in the matrix, has the function of shock absorption and noise reduction, can effectively prevent the propagation of crack tips, and can effectively improve the impact strength of the material. Fluororubber can still maintain its use performance at -20 °C, can improve the toughness of the friction material in a low-temperature environment, and can improve the toughness of the resin matrix when blended with polyarylether nitrile, increase the fit between the friction material and the counterpart, and reduce braking noise.

[0032] Polyimide has excellent heat resistance and wear resistance. However, polyimide has high thermal expansion, which leads to the problem of interface stress accumulation in thermal cycling. Based on this, we form a polyimide-silicone rubber interpenetrating complex to solve the aforementioned defects. By utilizing the negative thermal expansion effect generated by silicone rubber and the ability of silicone rubber to dissipate thermal stress through its own molecular chains, the purpose of low thermal expansion and stress buffering is achieved. In addition, the added silicone rubber can also form a transfer film at the friction interface, thereby reducing adhesion loss.

[0033] (2) The reinforcing materials provided by the present invention include aramid pulp, copper fiber, potassium magnesium titanate whisker, tetrapod-shaped zinc oxide whisker, and modified aluminum titanate fiber; Tetrapod-shaped zinc oxide whiskers are the only kind of whiskers with a three-dimensional structure. The whiskers are small in size, almost like single crystals, without various defects in crystals, making the strength of the whiskers close to the theoretical value of a perfect crystal, and having excellent wear resistance, reinforcement, mildew and antibacterial resistance and other properties. Adding tetrapod-shaped zinc oxide whiskers to friction materials can make the materials isotropic, thus improving the overall strength of the materials. Due to its unique three-dimensional structure, it can pin long fiber materials and matrix materials, reduce the pull-out of fibers, and improve the overall strength and wear resistance of the matrix. At the same time, tetrapod-shaped zinc oxide whiskers have excellent thermal conductivity. Acting together with copper fibers, they can quickly transfer the heat on the friction surface out of the friction surface, reduce the friction surface temperature, and reduce the thermal wear of the friction material. In addition, the long fibers in aramid pulp can be intertwined with the three-dimensional network structure of tetrapod-shaped zinc oxide whiskers, thus forming a multi-scale reinforcement system, so as to prevent crack propagation. The rigidity of copper fibers and tetrapod-shaped zinc oxide whiskers is complementary, thus balancing the toughness and rigidity of the material.

[0034] The modified aluminum titanate fiber removes the amorphous substance on the fiber surface through acid etching, increases the surface area of the fiber, and then reacts with hydroxyethyl cellulose to improve the bonding strength between the aluminum titanate fiber and the matrix. Hydroxyethyl cellulose is a natural polymer compound. Its molecular structure contains hydroxyethyl units, making hydroxyethyl cellulose have good solubility and thickening property in water. At the same time, hydroxyethyl cellulose also has good surface activity, can form a stable complex with the surface of aluminum titanate fiber, prevent the aggregation of aluminum titanate fibers, and play a role in uniform dispersion. Aluminum titanate is a high-temperature resistant inorganic substance with a very small thermal expansion coefficient, and even may be negative, which can effectively reduce the thermal expansion of the friction material at high temperature and ensure that the working gap of the brake does not decrease due to high-temperature expansion.

[0035] As a high-temperature resistant inorganic substance, potassium magnesium titanate platelets can fill the micro-pores of the material, improve the hardness and wear resistance; its platelet structure can reduce the direct contact of the friction surface and reduce the wear rate.

[0036] (3)The friction-increasing material provided by the present invention includes nano aluminum hydroxide, fluororubber powder, barite, silicon carbide, feldspar powder, modified silica, cashew shell oil friction powder; Nano aluminum hydroxide can absorb the heat on the friction surface when heated, thus reducing the temperature rise.

[0037] In the modified silica, silica is blended and ball-milled with bis-[γ-(triethoxysilyl)propyl]tetrasulfide (Si-69C) and stearic acid, improving the dispersibility of silica and the wettability with the bonding system. The modified silica enhances the bonding performance with the matrix material and improves the wear resistance of the friction material.

[0038] Fluororubber powder can deform under stress to absorb energy and reduce the propagation of interface micro-cracks.

[0039] Barite is a high-density filler that can increase the volume stability of materials, thereby inhibiting interfacial peeling caused by thermal expansion.

[0040] The Mohs hardness of silicon carbide is 9, which can stabilize the friction coefficient of friction materials at high temperatures. Silicon carbide can keep the high-temperature friction coefficient of friction materials stable and avoid high-temperature decline of the friction coefficient.

[0041] Feldspar has a relatively high specific heat capacity, can absorb more heat, and can effectively reduce the temperature rise of the friction surface.

[0042] The friction powder of cashew shell oil decomposes and carbonizes under the action of friction heat to form a homogeneous carbonaceous lubricating film, which can balance the hard friction characteristics of silicon carbide and prevent excessive wear.

[0043] (4) The friction-reducing material provided by the present invention includes halloysite nanotubes coated with reduced graphene oxide, molybdenum carbon phosphate, and fluorinated graphite; Halloysite nanotubes are often multi-walled tubular structures, which are formed by the dislocation and curling of aluminum octahedra and silicon oxygen tetrahedra lattices. The inner wall is an aluminum octahedron, and the outer wall is a silicon oxygen tetrahedron, with a large number of hydroxyl groups and silicon oxygen groups. Through the reaction of dimethyl selenoxide and glycerol, the layer spacing of halloysite nanotubes is increased, making the interlayer slip of halloysite nanotubes easier and improving the lubrication performance. At the same time, through the modification treatment, the dispersibility of halloysite nanotubes in the system and the binding performance with the matrix are improved, which can greatly improve the strength, rigidity, wear resistance, and heat resistance of friction materials. Further, the combined action of modified halloysite titanium nanotubes and reduced graphene oxide is to form a nano-ball - two-dimensional sheet composite structure, which first exfoliates to form a similar "rolling bearing" during the friction process and gradually fills the wear pits formed by wear and spalling together with materials such as nano-aluminum oxide in the subsequent friction process, preventing further spalling of the matrix and gradually spreading to form a friction lubricating film in the subsequent friction process, greatly reducing the wear rate of the friction material.

[0044] Molybdenum carbon phosphate is easily dispersed on the friction surface and has excellent lubricating and friction-reducing effects in high-temperature environments. During the friction process, molybdenum carbon phosphate contacts the metal surface to form a wear-resistant layer and a surface adhesion layer. These two layers of films are resistant to extreme pressure and wear, can effectively isolate the friction material from the matrix of the counterpart, and can greatly reduce the high-temperature wear of the friction material. Specific Embodiments

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified in the manufacturer are all conventional products that can be obtained through commercial purchase.

[0046] Examples 1 - 3 The examples provided by the present invention specifically disclose a friction material for an electromagnetic brake under an ultra - long - time working condition, including the following steps: (1) Mixing: Weigh each component of the raw materials in proportion and put them into a plow - harrow mixer for co - mixing. The main shaft speed of the mixer is 250 - 300 rpm, and the reamer speed is 2500 - 3000 rpm.

[0047] (2) Drying: Dry the mixed molding compound in an oven at 80 °C for 1 - 2 h.

[0048] (3) Pre - pressing: Put the molding compound into a cold - pressing mold and spread it flat, and apply pressure to press the mixture into a blank. The cold - pressing pressure is 50 - 60 MPa, and press it repeatedly 2 times. Stop when the pressure of the press reaches the required value.

[0049] (4) Hot - pressing: Use a 160T four - column hydraulic press for hot - pressing. Put the cold blanks into the hot - pressing mold for pressing. The pressing temperature is 350 ± 10 °C, the pressing pressure is 20 - 30 MPa, and the pressure - holding time is 30 - 60 min / mm. Exhaust gas before pressure - holding. Press for 15 s, exhaust gas for 3 s, and perform 4 times of exhaust gas before entering the pressure - holding stage. After the pressure - holding is completed, a semi - finished friction material is obtained.

[0050] (5) Heat treatment: Heat from room temperature to 180 °C in 1 h, hold for 1 - 2 h, heat from 180 °C to 280 °C in 1 h, hold for 1 h, heat from 240 °C to 350 °C in 1 h, hold for 6 - 8 h, and then cool in the furnace to 50 °C and take out.

[0051] (5) Grinding: Grind the heat - treated friction material through a double - end surface belt grinder to remove the surface oxide scale, burrs, and flash.

[0052] In Examples 1 - 3, the ratio of each component of the raw materials is shown in Table 1.

[0053] Comparative Examples 1 - 10 Comparative Examples 1 - 5 In Comparative Examples 1 - 5, the ratio of each component of the raw materials is shown in Table 1, and the preparation method is the same as that of the examples.

[0054] Comparative Example 6 The difference between this comparative example and Example 1 is that polyimide is used to replace the modified polyimide.

[0055] Comparative Example 7 The difference between this comparative example and Example 1 is that white carbon black is used to replace the modified white carbon black.

[0056] Comparative Example 8 The difference between this comparative example and Example 1 is that graphene oxide is used to replace the halloysite nanotubes coated with reduced graphene oxide.

[0057] Comparative Example 9 The difference between this comparative example and Example 1 is that halloysite nanotubes are used to replace the halloysite nanotubes coated with reduced graphene oxide.

[0058] Comparative Example 10 The difference between this comparative example and Example 1 is that the halloysite nanotubes are not subjected to modification treatment.

[0059] Table 1 Component ratio table of raw materials (Example - A, Comparative Example - B)

[0060] In Table 1: Modified polyimide: Dissolve Sylgard184 prepolymer in N - methylpyrrolidone, and simultaneously add KH550 which accounts for 0.08 times the mass of Sylgard184 prepolymer. After regular ultrasonic blending, a silicone rubber prepolymer solution with a concentration of 10wt% is obtained. Mix pyromellitic dianhydride and 4,4'-diaminodiphenyl ether in a molar ratio of 1:1, and then stir at 500rpm for 2h. Then add it to N - methylpyrrolidone to obtain a 15wt% polyimide precursor solution. Gradually add the previously obtained silicone rubber prepolymer solution to the polyimide precursor solution, while controlling the mass ratio of the silicone rubber prepolymer to the polyimide precursor to be 7:3. Then mechanically blend at 800rpm for 1h. Next, add Sylgard184 curing agent (the addition amount accounts for 1 / 10 of Sylgard184), and then stir for 0.5h. Perform vacuum degassing treatment at - 0.1MPa, control the temperature at 50℃ and keep it for 1.5h. Then, heat the obtained blend to 80℃ under a nitrogen atmosphere and keep it for 2h. After the heat preservation is completed, continue to heat to 135℃ and keep it for 0.5h.

[0061] Modified aluminum titanate fiber: Dry the aluminum titanate fiber at 350℃ for 1h; put the dried aluminum titanate fiber into a 10% phosphoric acid solution for acid etching, the etching time is 100min, and the temperature is 65℃. After etching, filter to obtain aluminum titanate fiber, wash it 5 times with distilled water, and then dry it at 85℃ for 60min. Put the dried aluminum titanate fiber into a 5% hydroxyethyl cellulose solution, soak it at room temperature for 2h, filter out the aluminum titanate fiber and then dry it at 85℃ for 2h to obtain modified aluminum titanate fiber.

[0062] Modified silica: Add 5% of Si - 69C and 2% of stearic acid based on the mass fraction of silica, and then put it into a high - speed ball mill for ball milling. The rotation speed of the ball mill is 1500rpm, and the ball milling time is 45min to obtain modified silica.

[0063] Reduced graphene oxide-coated halloysite nanotubes: Reduced graphene oxide and tannic acid were added to deionized water at a mass ratio of 10:1 and ultrasonically dispersed to obtain a suspension. The suspension was reacted under a nitrogen atmosphere to obtain modified reduced graphene oxide, with reaction parameters: 90 °C, 12 h. Weigh halloysite nanotubes and place them in an oven at 150 °C for pre-drying for 1 h for drying. Add an aqueous solution of dimethyl selenoxide ethanol with a volume ratio of 1:2:0.5. The volume of the dimethyl selenoxide ethanol aqueous solution should be sufficient to submerge the halloysite nanotubes, and ultrasonically oscillate at 45 °C for 1 h. After filtration, rinse with distilled water, and place the treated halloysite nanotubes in an oven at 50 °C for drying for 2 h. Then, put the treated halloysite nanotubes into a glycerol solution with a glycerol concentration of 15%, and ultrasonically oscillate at 50 °C for 2 h. After filtration, rinse with distilled water, and dry the treated halloysite nanotubes in an oven at 50 °C for 2 h to finally obtain modified halloysite nanotubes. Disperse the modified reduced graphene oxide in deionized water, add the modified halloysite nanotubes and ultrasonically disperse to obtain a suspension. React the suspension at 45 °C for 1 h. After the reaction, filter and dry to obtain graphene oxide-coated halloysite nanotubes. The mass ratio of the modified reduced graphene oxide to the halloysite nanotubes is 1:10.

[0064] Test examples Using the friction materials prepared in Examples 1-3 and Comparative Examples 1-10 as samples, test according to the test method of GB / T 5764. The test results are shown in Table 2.

[0065] Table 2 Determination results of friction coefficient stability and wear rate of friction materials (Example - A, Comparative Example - B)

[0066] Using the friction materials prepared in Examples 1-3 and Comparative Examples 1-10 as samples, test according to the test method of GB / T 33835. The test results are shown in Table 3.

[0067] Table 3 Determination results of impact strength of friction materials (Example - A, Comparative Example - B)

[0068] Using the friction materials prepared in Examples 1-3 and Comparative Examples 1-10 as samples, conduct a high-temperature no-load test to verify the ultra-long-time operation of the friction materials. The test model is: CDC-S130B(24V)16.5G25, rated torque 16.5 N·m, clearance requirement 0.15 - 0.3 mm. After operating at room temperature for 14 days, wrap the brake with a heating belt to heat it until the temperature of the heating belt reaches 100 °C. The test results are shown in Table 4.

[0069]

[0070] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A friction material for electromagnetic brakes used in ultra-long working conditions, characterized in that: Including adhesives, reinforcing materials, friction-increasing fillers, and friction-reducing fillers; The adhesive comprises polyarylether nitrile and modified polyimide; the modified polyimide is a polyimide-silicone rubber interpenetrating composite; the preparation method of the modified polyimide is to blend and cure a silicone rubber prepolymer, a polyimide precursor and a silicone rubber prepolymer curing agent to obtain an interpenetrating composite; The reinforcing materials include aramid pulp, copper fiber, potassium magnesium titanate platelets, tetrapod-shaped zinc oxide whiskers, and modified aluminum titanate fiber; Friction-increasing materials include nano aluminum hydroxide, fluororubber powder, barite, silicon carbide, feldspar powder, modified white carbon black, and cashew nut shell oil friction powder; The friction-reducing materials include redox graphene-coated halloysite nanotubes, molybdenum carbon phosphate, and graphite fluoride.

2. The electromagnetic brake friction material for ultra-long working conditions according to claim 1, characterized in that: The raw material components are calculated by weight and include 35-50 parts of polyarylether nitrile, 10-30 parts of modified polyimide, 1-3 parts of aramid pulp, 3-5 parts of copper fiber, 6-10 parts of potassium magnesium titanate platelets, 3-8 parts of tetrapod-shaped zinc oxide whiskers, 8-14 parts of modified aluminum titanate fibers, 5-10 parts of nano aluminum hydroxide, 10-15 parts of fluororubber powder, 8-15 parts of barite, 1-3 parts of silicon carbide, 3-8 parts of feldspar powder, 3-8 parts of modified white carbon black, 8-12 parts of cashew nut shell oil friction powder, 5-8 parts of redox graphene-coated halloysite nanotubes, 2-5 parts of molybdenum carbon phosphate, and 3-6 parts of graphite fluoride.

3. The electromagnetic brake friction material for ultra-long working conditions according to claim 2, characterized in that: The components of the raw materials are calculated by weight, including any one of combination A, combination B, and combination C: Combination A: 45 parts of polyarylether nitrile, 15 parts of modified polyimide, 2 parts of aramid pulp, 3 parts of copper fiber, 8 parts of potassium magnesium titanate platelets, 5 parts of tetrapod-shaped zinc oxide whiskers, 12 parts of modified aluminum titanate fiber, 8 parts of nano aluminum hydroxide, 10 parts of fluororubber powder, 12 parts of barite, 1 part of silicon carbide, 7 parts of feldspar powder, 5 parts of modified white carbon black, 10 parts of cashew nut shell oil friction powder, 8 parts of redox graphene-coated halloysite nanotubes, 4 parts of molybdenum carbon phosphate, and 4 parts of graphite fluoride; Combination B: 40 parts of polyarylether nitrile, 20 parts of modified polyimide, 1 part of aramid pulp, 4 parts of copper fiber, 10 parts of potassium magnesium titanate platelets, 8 parts of tetrapod-shaped zinc oxide whiskers, 10 parts of modified aluminum titanate fiber, 6 parts of nano aluminum hydroxide, 12 parts of fluororubber powder, 10 parts of barite, 2 parts of silicon carbide, 6 parts of feldspar powder, 4 parts of modified white carbon black, 9 parts of cashew nut shell oil friction powder, 6 parts of redox graphene-coated halloysite nanotubes, 2 parts of molybdenum carbon phosphate, and 5 parts of graphite fluoride; Combination C: 35 parts of polyarylether nitrile, 25 parts of modified polyimide, 3 parts of aramid pulp, 5 parts of copper fiber, 6 parts of potassium magnesium titanate platelets, 6 parts of tetrapod-shaped zinc oxide whiskers, 14 parts of modified aluminum titanate fiber, 10 parts of nano-aluminum hydroxide, 14 parts of fluororubber powder, 14 parts of barite, 3 parts of silicon carbide, 4 parts of feldspar powder, 8 parts of modified white carbon black, 8 parts of cashew nut shell oil friction powder, 5 parts of redox graphene-coated halloysite nanotubes, 3 parts of molybdenum carbon phosphate, and 6 parts of graphite fluoride.

4. The electromagnetic brake friction material for ultra-long working conditions according to claim 1, characterized in that: Polyarylether nitrile 200 mesh, polyimide 800 mesh, fluororubber powder 40~60 mesh, aramid pulp length 1.5~2mm, copper fiber 1~3mm, potassium magnesium titanate platelets with length and width of 0.5~5μm and thickness of 0.2~1.5μm, four-needle zinc oxide whiskers 300~400 mesh, aluminum titanate fiber 1~3mm, nano aluminum hydroxide 20~40nm, barite 325 mesh, silicon carbide 600 mesh, feldspar powder 325 mesh, white carbon black 400 mesh, cashew nut shell oil friction powder 100 mesh, molybdenum carbon phosphate 325 mesh, redox graphene 400 mesh, halloysite nanotubes 1250 mesh, and graphite fluoride 100 mesh.

5. The electromagnetic brake friction material for ultra-long working conditions according to claim 1, characterized in that: The friction material includes at least one of the features (1) to (2): (1) The preparation method of the modified aluminum titanate fiber is to dry the aluminum titanate fiber, then etch it with an acid solution, mix it with a hydroxyethyl cellulose solution, and finally filter and dry it; (2) The modified silica is obtained by mixing silica with bis-[γ-(triethoxysilyl)propyl]tetrasulfide and stearic acid and ball milling.

6. The electromagnetic brake friction material for ultra-long working conditions according to claim 5, characterized in that: The modified aluminum titanate fiber comprises at least one of the features (1-1) to (1-4): (1-1) Drying: Dry the aluminum titanate fiber at 300-400°C for 1-2 hours to remove surface impurities; (1-2) Etching: The dried aluminum titanate fiber is put into a 5-10% phosphoric acid or oxalic acid solution for acid etching. The etching time is 60-120 minutes and the temperature is 50-80°C. (1-3) Post-treatment: After etching, the aluminum titanate fibers are filtered out and washed with distilled water for 3 to 5 times. After washing, they are dried at 80 to 100 °C for 30 to 60 min. (1-4) Activation: The dried aluminum titanate fibers are placed in a 3-5% hydroxyethyl cellulose solution for 1-2 h at room temperature, and the aluminum titanate fibers are filtered out and dried at 80-100°C for 1-2 h to obtain modified aluminum titanate fibers. Modified silica includes the following features (2-1): (2-1) Add 5-7% by mass of bis-[γ-(triethoxysilyl)propyl]tetrasulfide and 1-3% by mass of stearic acid to white carbon black, and place the mixture in a high-speed ball mill for ball milling at a speed of 1000-1500 rpm for 45-60 min to obtain modified white carbon black.

7. The electromagnetic brake friction material for ultra-long working conditions according to any one of claims 1 to 6, characterized in that: In the friction material, the modified polyimide includes at least one of the features (3-1) to (3-3): (3-1) dissolving the silicone rubber prepolymer in N-methylpyrrolidone, then adding KH550, and ultrasonically blending to obtain the silicone rubber prepolymer; (3-2) Blending pyromellitic anhydride and 4'4-diaminodiphenyl ether, and then adding them to N-methylpyrrolidone, a polyimide precursor is obtained; (3-3) The silicone rubber prepolymer is gradually added to the polyimide precursor, and then subjected to mechanical stirring. At the same time, a silicone rubber prepolymer curing agent is added, and then vacuum degassing is performed to obtain a blend; the blend is cured to obtain a modified polyimide.

8. The electromagnetic brake friction material for ultra-long working conditions according to claim 7, characterized in that: Feature (3-1) includes Feature (3-1-1): (3-1-1) The amount of KH550 added accounts for 0.03~0.1 of the mass of silicone rubber prepolymer; Feature (3-2) includes at least one of features (3-2-1) to (3-2-2): (3-2-1) The molar ratio of pyromellitic anhydride to 4'4-diaminodiphenyl ether is 1:1; (3-2-2) Blending: 500 rpm, 2 h; Feature (3-3) includes at least one of features (3-3-1) to (3-3-4): (3-3-1) The mass ratio of silicone rubber prepolymer to polyimide precursor is 7~8:2~3; (3-3-2) Mechanical stirring: 800 rpm, 1 h; (3-3-3) Vacuum degassing: 40~70℃, 1~3h, -0.1MPa; (3-3-4) Curing: The mixture is heated to 70-100°C in a nitrogen atmosphere and kept at this temperature for 1-3 hours. Then, the mixture is heated to 120-180°C and kept at this temperature for 0.5-2 hours, and then cooled to room temperature.

9. The electromagnetic brake friction material for ultra-long-duration operation according to any one of claims 1 to 6, characterized in that: In the friction material, the redox graphene-coated halloysite nanotubes include at least one of the features (4-1) to (4-3): (4-1) adding redox graphene and tannic acid into deionized water to obtain a suspension by ultrasonic dispersion, and reacting the suspension under a nitrogen atmosphere to obtain modified redox graphene; (4-2) drying the halloysite nanotubes, adding a dimethyl selenide sulfoxide ethanol aqueous solution, and then subjecting the treated halloysite nanotubes to ultrasonic oscillation, rinsing, and drying, and then placing the treated halloysite nanotubes in a glycerol solution, subjecting the treated halloysite nanotubes to ultrasonic oscillation, and then rinsing and drying to obtain modified halloysite nanotubes; (4-3) The modified redox graphene is dispersed in deionized water, and the modified halloysite nanotubes are added and dispersed by ultrasonic to obtain a suspension. The suspension is subjected to a reaction treatment, and after the reaction is completed, the suspension is filtered and dried to obtain graphene oxide-coated halloysite nanotubes.

10. A method for preparing a friction material for an electromagnetic brake used in ultra-long-duration working conditions according to any one of claims 1 to 9, characterized in that: The steps include: S1 Weigh the raw materials in proportion and blend them to obtain a mixed material; S2 Drying of the molded material after mixing; S3 is dried and then cold pressed to form; and pressed by a mold to obtain a semi-finished product; S4 is then subjected to heat pressing to obtain a semi-finished product; S5 is then heat treated to obtain the friction material; S6 friction material has a ground surface.

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