A composite material for railway freight car brake lever bushing, its preparation method and application

Abstract

This invention discloses a composition for a railway freight car brake lever bushing, a composite material thereof, and a railway freight car brake lever bushing. The composition comprises the following components in parts by weight: 100 parts by weight of polyimide resin, 50-60 parts by weight of styrene, 180-220 parts by weight of ultrafine heavy calcium carbonate, 45-60 parts by weight of solid lubricant, 25-30 parts by weight of thickener, 40-60 parts by weight of chopped fibers, 0.5-2 parts by weight of initiator, 0.1-0.2 parts by weight of anti-settling agent, and 0.1-0.2 parts by weight of defoamer; wherein the polyimide resin is selected from polyimide resin, polyamide-imide resin, and polyester-imide resin. The composite material of this invention has the characteristics of wide temperature range, high strength, long service life, self-lubrication, friction reduction, and wear resistance; the brake lever bushing prepared thereby solves the problems of corrosion, jamming, and noise of traditional metal bushings, ensuring driving safety.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a composite material for a brake lever bushing of railway freight cars, its preparation method, and its application. Background Technology

[0002] The basic braking system of railway freight cars mainly includes a brake cylinder, a brake lever (or piston push rod), a front lever of the brake cylinder, an upper pull rod, a fixed lever, a floating lever, a lower pull rod, a brake beam, and brake shoes. Its function is to transmit braking force to the brake shoes, achieving vehicle braking and brake release ("release") through the friction between the brake shoes and the wheels. It is the core component for realizing mechanical transmission. Each brake lever's main body is designed with pre-drilled circular through holes. Bushings are fixed into these holes by press-fitting, and finally, connecting pins pass through the bushings to connect the levers to the corresponding rods or components. A single basic braking system for freight cars can contain more than 10 bushings. Traditional bushings are mostly made of metal materials such as 45 steel isothermally hardened ductile steel (see...). Figure 1 However, metals are prone to corrosion. Figure 2 The image shows the corrosion of the metal bushings and pins of a freight car brake lever after approximately two years of service. Corrosion causes a sharp reduction or even complete disappearance of the clearance between the bushing and the pin, resulting in brake lever "sticking." On the one hand, during braking, the vehicle's braking response time is prolonged, braking force is uneven and fluctuates greatly, the train's braking distance increases, and braking noise exceeds standards. On the other hand, when releasing the brake, poor release occurs, the brake shoes cannot disengage from the wheels in time, resulting in "brake-holding" operation, which in severe cases can lead to wheel abrasion, axle overheating, and even derailment. In short, corrosion of the metal bushings seriously affects driving safety and transportation order.

[0003] Replacing metal materials with high-performance non-metallic materials, such as polymer materials, can not only solve the problem of metal corrosion, but also help to achieve lightweighting. For example, resin-based composite materials, such as polyimide resin-based composite bushings, have been used to replace metal bushings in aero engines (Wang Yunfei, et al. Research progress on polyimide resin-based composite bushings for aero engines [J]. Materials Engineering. September 2016. 44(9): 121-128). However, due to the complex operating conditions of railways, there are no reports to date of applying non-metallic polymer materials to the brake lever bushings of railway freight cars. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a fiber-reinforced calcium carbonate-polyimide resin composite material for railway freight car brake lever bushings and its preparation method. This composite material possesses advantages such as a wide temperature range, high strength, long service life, self-lubrication, and wear resistance. Freight car brake lever bushings prepared using the composite material of this invention can solve problems such as jamming and noise caused by corrosion in traditional metal bushings, and eliminate malfunctions such as poor braking or release of freight cars caused by corrosion between the metal bushing and the pin.

[0005] To achieve the above-mentioned technical effects, the present invention adopts the following technical solution:

[0006] A composition for a brake lever bushing of railway freight cars, comprising the following components in parts by weight:

[0007] 100 parts by weight of polyimide resin, 50-60 parts by weight of styrene, 180-220 parts by weight of ultrafine heavy calcium carbonate, 45-60 parts by weight of solid lubricant, 25-30 parts by weight of thickener, 40-60 parts by weight of chopped fiber, 0.5-2 parts by weight of initiator, 0.1-0.2 parts by weight of anti-settling agent and 0.1-0.2 parts by weight of defoamer;

[0008] The polyimide resin is selected from one of polyimide resin, polyamide-imide resin, and polyester-imide resin.

[0009] The polyimide resins mentioned are all commercially available products in liquid form, such as YBPI-1006 polyimide resin produced by DuPont, HPC-5012 polyamide-imide resin produced by Resonac, and JF-9956 polyester-imide resin produced by Suzhou Jufeng New Material Technology Co., Ltd.

[0010] As a preferred embodiment, the present invention provides a composition for a brake lever bushing of a railway freight car, comprising the following components in parts by weight:

[0011] The composition includes 100 parts by weight of polyimide resin, 55 parts by weight of styrene, 200 parts by weight of calcium carbonate, 48 parts by weight of solid lubricant, 28 parts by weight of thickener, 50 parts by weight of reinforcing fiber, 1 part by weight of initiator, 0.1 parts by weight of anti-settling agent and 0.1 parts by weight of defoamer.

[0012] Preferably, the particle size of the ultrafine heavy calcium carbonate is 1000-1200 mesh.

[0013] Preferably, the solid lubricant is selected from one or more of graphite, polytetrafluoroethylene, ultra-high molecular weight polyethylene, and magnesium hydroxysilicate.

[0014] Preferably, the polytetrafluoroethylene is a molded powder with a particle size of 10-15 μm.

[0015] Preferably, the ultra-high molecular weight polyethylene is a molded powder with a viscosity-average molecular weight > 2 million g / mol and a particle size of 500-1500 mesh.

[0016] More preferably, the solid lubricant is a combination of graphite and polytetrafluoroethylene in a mass ratio of:

[0017] Graphite: Polytetrafluoroethylene = 1:3-5.

[0018] Preferably, the thickener is selected from one or more of hydrophilic fumed silica, calcium oxide, calcium hydroxide, talc, and bentonite.

[0019] Preferably, the talc powder and bentonite each have a particle size of not less than 800 mesh.

[0020] More preferably, the thickener is a combination of hydrophilic fumed silica and one or two selected from calcium oxide and calcium hydroxide in the following mass ratio:

[0021] Fumed silica : (calcium oxide and / or calcium hydroxide) = 1:2-4.

[0022] Preferably, the reinforcing fiber is selected from one or more of nylon fiber, ultra-high molecular weight polyethylene fiber, aramid fiber, polyester fiber and carbon fiber.

[0023] Preferably, the reinforcing fiber has a diameter of 5-20 μm and a length of 6-12 mm.

[0024] Preferably, the initiator is selected from one or more of benzoyl peroxide (BPO), di-tert-butyl peroxide (DTBP), dicumyl peroxide (DCP), and diisophorone peroxide (DIPC).

[0025] Preferably, the anti-settling agent is selected from one or both of organobentonite and polyamide wax.

[0026] Preferably, the defoamer is selected from one or more of silicone-based defoamers, organic defoamers, polyether defoamers, and polydimethylsiloxane defoamers.

[0027] The second objective of this invention is to provide a composite material for a brake lever bushing for railway freight cars, which is prepared by pressure molding using the above-mentioned composition as raw material.

[0028] A third objective of this invention is to provide a method for preparing the composite material for railway freight car brake lever bushings, using the composition of this invention as raw material, comprising the following steps:

[0029] I. Material preparation

[0030] Prepare each component of the composition according to the stated mass fractions;

[0031] II. Ball milling dispersion

[0032] Mix the polyimide resin and styrene evenly, transfer them to a ball mill, add ultrafine heavy calcium carbonate, the solid lubricant and grinding media, disperse for 15-20 minutes, add the anti-settling agent and defoamer, continue to disperse for more than 30 minutes, sieve to remove the grinding media, obtain the matrix mixture, seal it and set it aside.

[0033] III. Fiber kneading

[0034] Add the thickener and initiator to the matrix mixture prepared in step II, mix evenly, let stand at room temperature for 15-20 minutes, then transfer to a kneader, add the reinforcing fiber, knead thoroughly for at least 20 minutes to obtain the fiber kneaded mixture, seal, store at low temperature for later use.

[0035] IV. Molding and Curing

[0036] The fiber kneading mixture obtained in step III is filled into a mold, pressurized, heated to 140-180°C, kept at the temperature and pressure, and reacted for more than 2 hours. Then it is demolded and cured at 140-180°C for more than 8 hours to obtain the final product.

[0037] The fourth objective of this invention is to provide the application of the above-mentioned composite material in the preparation of brake lever bushings for railway freight cars.

[0038] When the composite material is used to prepare the brake lever bushing of a railway freight car, the mold in step IV above is a mold for the brake lever bushing of a railway freight car.

[0039] Those skilled in the art should understand that the compositions described in this invention and the composite materials prepared therefrom are not limited to use in railway freight car brake lever bushings, but can replace metal bushings in fields such as aviation, shipbuilding, and engineering machinery.

[0040] The fifth objective of this invention is to provide a method for preparing a brake lever bushing for railway freight cars, comprising the following steps:

[0041] I. Material preparation

[0042] Prepare each raw material according to the stated mass proportions;

[0043] II. Ball milling dispersion

[0044] Mix the polyimide resin and styrene evenly, transfer them to a ball mill, add ultrafine heavy calcium carbonate, the solid lubricant and grinding media, disperse for 15-20 minutes, add the anti-settling agent and defoamer, continue to disperse for more than 30 minutes, sieve to remove the grinding media, obtain the matrix mixture, seal it and set it aside.

[0045] III. Fiber kneading

[0046] Add the thickener and initiator to the matrix mixture prepared in step II, mix evenly, let stand at room temperature for 15-20 minutes, then transfer to a kneader, add the reinforcing fiber, knead thoroughly for at least 20 minutes to obtain the fiber kneaded mixture, seal and set aside.

[0047] IV. Molding and Curing

[0048] The fiber kneading mixture obtained in step III is filled into a railway freight car brake lever bushing mold, pressurized to 7 MPa-17 MPa, heated to 140-180°C, and kept at temperature and pressure for more than 2 hours. Then it is demolded and cured at 140-180°C for more than 8 hours.

[0049] V. Oil absorption and friction reduction

[0050] Immerse the product obtained in step IV in base lubricating oil or synthetic lubricating oil at 30℃-50℃ for more than 6 hours, then remove and wipe dry to obtain the final product.

[0051] Preferably, in step II, the grinding medium is a zirconia grinding bead.

[0052] Preferably, in step II, the polyimide resin, styrene, ultrafine heavy calcium carbonate, solid lubricant, and zirconium oxide grinding beads are dispersed in a dispersion tank by ball milling with an H-type grinding wheel.

[0053] The matrix mixture obtained in step II can be stored for a long time at room temperature after being sealed. The fiber kneaded mixture obtained in step III needs to be sealed and stored at a low temperature (≤10℃), but the storage time should not exceed 15 days.

[0054] Preferably, in step IV, the pressure is increased to 12-14 MPa.

[0055] Therefore, the present invention also provides a railway freight car brake lever bushing, which is prepared by the above-described preparation method.

[0056] The composite material described in this invention has the characteristics of wide temperature range, high strength, long service life, self-lubrication, friction reduction and wear resistance. The brake rod bushing prepared by this material not only solves the problems of corrosion, jamming and noise of traditional metal bushings, but also can adapt to the complex working conditions of railway freight cars. Replacing traditional metal bushings with this material can eliminate or alleviate faults such as poor braking of freight cars and ensure train operation safety.

[0057] In this article, the "parts by mass" of each component refers to the relative proportions of the corresponding components, not the actual mass of the components. Depending on the actual situation, 1 part by mass can be any mass, such as 1g, 10g, 500g, 1kg, or even 1 ton. Attached Figure Description

[0058] The present application will be further described below with reference to the accompanying drawings.

[0059] Figure 1 The photo shows the metal bushing before assembly. The metal bushing in the photo has a smooth surface and is free of rust.

[0060] Figure 2 The photos show metal bushings and pins that have been installed on the brake lever for approximately two years. The metal bushings and pins in the photos are covered in rust and have an uneven surface.

[0061] Figure 3 The photograph shows the bushing prepared in Example 1. Detailed Implementation

[0062] The present application will be described below with reference to specific embodiments. Those skilled in the art will understand that these embodiments are for illustrative purposes only and do not limit the scope of the invention in any way.

[0063] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, all raw materials and reagents used in the following examples are commercially available products. The suppliers or manufacturers of some of the raw materials are as follows:

[0064] Polyesterimide impregnation resin JF-9956: Suzhou Jufeng New Material Technology Co., Ltd.;

[0065] Polyimide resin YBPI-1006: DuPont, USA;

[0066] Polyamide-imide resin HPC-5012: Resonac Corporation, Japan;

[0067] Styrene: Sinopharm Chemical Reagent Co., Ltd.;

[0068] Ultrafine heavy calcium carbonate: 1200 mesh, Xuancheng Chengxu New Materials Co., Ltd.

[0069] Graphite: 1200 mesh, ≥99.95% metals basis; Aladdin reagent;

[0070] Polytetrafluoroethylene (powder): 10-15μm, Zhejiang Juhua Co., Ltd.;

[0071] Organic bentonite: 1000 mesh, Shanghai Chenqi Chemical Technology Co., Ltd.

[0072] Dimethyl silicone oil: Shanghai McLean Biochemical Technology Co., Ltd.;

[0073] Dicumyl peroxide: Merck, Inc.

[0074] Benzoyl peroxide: Aladdin reagent;

[0075] Di-tert-butyl peroxide: Aladdin reagent;

[0076] Hydrophilic fumed silica: AEROSIL ® 200, Evonik Industries AG;

[0077] Calcium oxide: analytical grade, Tianjin Zhiyuan Chemical Reagent Co., Ltd.

[0078] Nylon 6: Diameter 10-15μm, length 6mm, Hubei Xianyuan New Materials Co., Ltd.

[0079] Polyethylene fiber: 10-15μm in diameter, 6mm in length, Guangdong Tevilon New Material Application Co., Ltd.

[0080] Zirconia grinding beads: φ2.0-3.0, Nanjing Juntian Fine Ceramics Co., Ltd.

[0081] Example 1: A matrix mixture A1

[0082] Add 550g of styrene to 1000g of polyesterimide impregnating resin JF-9956, stir for 10 minutes, then transfer to a dispersion tank. Add 2000g of ultrafine heavy calcium carbonate, 90g of graphite, 390g of polytetrafluoroethylene powder, and 1000g of zirconia grinding beads sequentially. Then, ball mill the multiphase material using an H-type grinding wheel for 20 minutes, adding 1g of organobentonite and 1g of silicone-based defoamer. Finally, continue ball milling for another 45 minutes. After filtering to remove the grinding beads, obtain matrix mixture A1, seal, and store for later use.

[0083] Example 2 A matrix mixture A2

[0084] Add 500g of styrene to 1000g of polyimide resin YBPI-1006, stir for 10 minutes, then transfer to a dispersion tank. Add 1800g of ultrafine heavy calcium carbonate, 80g of graphite, 370g of polytetrafluoroethylene powder, and 1000g of zirconium oxide grinding beads sequentially. Then, ball mill the multiphase material using an H-type grinding wheel for 20 minutes, adding 1g of organobentonite and 1g of silicone-based defoamer. Finally, continue ball milling for another 45 minutes. After filtering to remove the grinding beads, obtain matrix mixture A2, seal, and store for later use.

[0085] Example 3 A matrix mixture A3

[0086] Add 600g of styrene to 1000g of polyamide-imide resin HPC-5012, stir for 10 minutes, then transfer to a dispersion tank. Add 2200g of ultrafine heavy calcium carbonate, 120g of graphite, 480g of polytetrafluoroethylene powder, and 1200g of zirconia grinding beads sequentially. Then, ball mill the multiphase material using an H-type grinding wheel for 20 minutes, adding 1g of organobentonite and 1g of silicone-based defoamer. Finally, continue ball milling for another 60 minutes. After filtering to remove the grinding beads, obtain matrix mixture A3, seal, and store for later use.

[0087] Example 4 A fiber kneading mixture B1

[0088] Add 10g of dicumyl peroxide, 80g of hydrophilic fumed silica, and 200g of calcium oxide to the matrix mixture A1 prepared in Example 1, stir until homogeneous (about 10 min), and let stand at room temperature for 20 min. Then, transfer to a kneader, add 500g of nylon 6 while kneading, and continue kneading for 45 min after adding the nylon 6 to obtain fiber kneaded mixture B1, which is then sealed. Store at low temperature for later use.

[0089] Example 5: A fiber kneading mixture B2

[0090] Add 5g benzoyl peroxide, 60g hydrophilic fumed silica, and 190g calcium oxide to the matrix mixture A2 prepared in Example 2, stir until homogeneous (about 10 min), and let stand at room temperature for 20 min. Then, transfer to a kneader, add 400g ultra-high molecular weight polyethylene fiber while kneading, and continue kneading for 45 min after adding the fiber to obtain fiber kneaded mixture B2, which is then sealed. Store at low temperature for later use.

[0091] Example 6 A fiber kneading mixture B3

[0092] Add 20g of di-tert-butyl peroxide, 90g of hydrophilic fumed silica, and 210g of calcium oxide to the matrix mixture A3 prepared in Example 3, stir until homogeneous (about 10 min), and let stand at room temperature for 20 min. Then, transfer to a kneader, add 600g of carbon fiber while kneading, and continue kneading for 45 min after adding the carbon fiber to obtain fiber kneaded mixture B3, which is then sealed. Store at low temperature for later use.

[0093] Example 7

[0094] The measured fiber kneading mixtures B1, B2, and B3 are filled into the bushing and sheet molds, respectively. After the molds are assembled, they are pressurized to 12-14 MPa to compress the mixtures to the specified dimensions and structure. The mixtures are then placed in a 160°C oven for at least 2 hours to react and cure. After demolding, the mixtures are cured in a 160°C oven for 10 hours. After machining, composite material sheets C1, C2, and C3, as well as bushings D1, D2, and D3, are obtained.

[0095] Take a certain number of plates C1, C2 and C3, and bushings D1, D2 and D3 respectively, immerse them in 40℃ base lubricating oil (ExxonMobil, SN-500) for more than 8 hours, take them out and wipe the surface dry.

[0096] Photograph of the oil-soaked bushing D1 as shown Figure 1 As shown.

[0097] Comparative Example 1

[0098] Add 10g of dicumyl peroxide, 80g of hydrophilic fumed silica and 200g of calcium oxide to the matrix mixture A1 prepared in Example 1, stir evenly (about 10 min), let stand at room temperature for 20 min to obtain the mixture, seal it, store it at low temperature for later use.

[0099] The measured mixture was filled into the bushing and plate molds respectively. After the molds were assembled, plate E1 and bushing F1 were obtained according to the same process as in Example 7.

[0100] Take a certain number of plates E1 and bushings F1 and immerse them in 40℃ base lubricating oil (ExxonMobil, SN-500) for more than 8 hours. Take them out and wipe the surface dry.

[0101] Comparative Example 2

[0102] Add 550g of styrene to 1000g of polyesterimide impregnating resin JF-9956, stir for 10 minutes, then transfer to a dispersion tank. Add 90g of graphite, 390g of polytetrafluoroethylene powder, and 1000g of zirconia grinding beads sequentially. Then, ball mill the multiphase material using an H-type grinding wheel for 20 minutes, adding 1g of organobentonite and 1g of silicone-based defoamer. Finally, continue ball milling for another 45 minutes. After filtering to remove the grinding beads, obtain matrix mixture A5, seal, and store for later use.

[0103] Add 10g of dicumyl peroxide, 80g of hydrophilic fumed silica, and 200g of calcium oxide to the above matrix mixture A5, stir evenly (about 10 minutes), and let stand at room temperature for 20 minutes. Then, transfer it to a kneader, add 500g of nylon 6 while kneading, and continue kneading for 45 minutes after adding the nylon 6 to obtain fiber kneaded mixture B5, which is then sealed. Store at low temperature for later use.

[0104] The measured mixture B5 was filled into the bushing and plate molds respectively. After the molds were assembled, plate E2 and bushing F2 were obtained according to the same process as in Example 7.

[0105] Take a certain number of plates E2 and bushings F2 and immerse them in 40℃ base lubricating oil (ExxonMobil, SN-500) for more than 8 hours. Take them out and wipe the surface dry.

[0106] Test case

[0107] The properties of the oil-impregnated boards prepared in Example 7, Comparative Example 1, and Comparative Example 2 were measured, and the results are shown in Tables 1, 2, and 4, respectively. The methods used for each property measurement are as follows:

[0108] Hardness: Determined according to standard GB / T 2411-2008;

[0109] Tensile strength: determined in accordance with standard GB / T 1447-2005;

[0110] Slight bending: Measured according to standard GB / T 1449;

[0111] Compressive strength: determined according to standard GB / T 1448;

[0112] Notched impact strength: determined in accordance with standard GB / T 1451-2005;

[0113] Coefficient of linear expansion: determined in accordance with standard GB / T 2572;

[0114] Friction properties: determined according to standard GB / T 3960-2016;

[0115] Melting temperature (DSC method): determined according to standard GB / T 19466.3-2004;

[0116] Thermal decomposition temperature (TG method): determined in accordance with standard GB / T 33047.1-2016.

[0117] The friction properties of the unoiled sheets prepared in Example 7, Comparative Example 1 and Comparative Example 2 were measured in accordance with the standard GB / T 3960-2016. The results are shown in Tables 1 and 4.

[0118] The performance of the oil-impregnated bushings prepared in Example 7, Comparative Example 1 and Comparative Example 2 was measured, and the results are shown in Tables 3 and 5.

[0119]

[0120] The data in Table 1 show that the composite material of the present invention has high strength, good thermal stability, and is friction-reducing and wear-resistant. Furthermore, after soaking in lubricating oil, the coefficient of friction can be further reduced, enhancing its wear resistance.

[0121]

[0122] Comparing the data in Table 2 and Table 1, it can be seen that the composite material of the present invention has good resistance to acid, alkali, oil, and high-temperature aging, and can adapt to the service environment and conditions of railways.

[0123]

[0124] The data in Table 3 show that the bushing made of the composite material of the present invention has the characteristics of wide temperature range, high strength and long service life, and can adapt to the complex working conditions of railway freight cars.

[0125]

[0126]

[0127] Compared to the plates and bushings of the embodiments, plate E1 and the corresponding bushing F1 do not contain reinforcing fibers, and plate E2 and the corresponding bushing F2 do not contain ultrafine heavy calcium carbonate. A comparison of the data in Tables 1 and 4, and Tables 3 and 5, shows that the mechanical properties and wear resistance of E1 and E2 are significantly inferior to those of the plates of the embodiments of the present invention; the dimensional stability and compressive strength of F1 and F2 are also significantly inferior to those of the bushings of the embodiments of the present invention.

[0128] In summary, this invention provides a composition for a railway freight car brake lever bushing, a composite material prepared therefrom, and a railway freight car brake lever bushing. The composite material of this invention possesses outstanding advantages such as a wide temperature range, high strength, long service life, self-lubrication, and wear resistance. The bushing of this invention can fully adapt to the complex operating conditions of railway freight cars, thereby replacing traditional metal bushings and solving problems such as jamming and noise caused by corrosion in traditional metal bushings. It also eliminates malfunctions such as poor braking or relief of freight cars caused by corrosion between the metal bushing and the pin.

Claims

1. A composition for a brake lever bushing of railway freight cars, comprising the following components in parts by weight: 100 parts by weight of polyimide resin, 50-60 parts by weight of styrene, 180-220 parts by weight of ultrafine heavy calcium carbonate, 45-60 parts by weight of solid lubricant, 25-30 parts by weight of thickener, 40-60 parts by weight of chopped fiber, 0.5-2 parts by weight of initiator, 0.1-0.2 parts by weight of anti-settling agent and 0.1-0.2 parts by weight of defoamer; The polyimide resin is selected from one of polyimide resin, polyamide-imide resin, and polyester-imide resin.

2. The composition according to claim 1, characterized in that, The composition comprises the following components in parts by weight: The composition includes 100 parts by weight of polyimide resin, 55 parts by weight of styrene, 200 parts by weight of calcium carbonate, 48 parts by weight of solid lubricant, 28 parts by weight of thickener, 50 parts by weight of reinforcing fiber, 1 part by weight of initiator, 0.1 parts by weight of anti-settling agent and 0.1 parts by weight of defoamer.

3. The composition according to claim 1 or 2, characterized in that, The particle size of the ultrafine heavy calcium carbonate is 1000-1200 mesh; Preferably, the solid lubricant is selected from one or more of graphite, polytetrafluoroethylene, ultra-high molecular weight polyethylene, and magnesium hydroxysilicate; Preferably, the polytetrafluoroethylene is a molded powder with a particle size of 10-15 μm; Preferably, the ultra-high molecular weight polyethylene is a molded powder with a viscosity-average molecular weight > 2 million g / mol and a particle size of 500-1500 mesh; More preferably, the solid lubricant is a combination of graphite and polytetrafluoroethylene in a mass ratio of: Graphite:polytetrafluoroethylene = 1:3-5; Preferably, the thickener is selected from one or more of hydrophilic fumed silica, calcium oxide, calcium hydroxide, talc, and bentonite; Preferably, the talc powder and bentonite each have an independent particle size of not less than 800 mesh; More preferably, the thickener is a combination of hydrophilic fumed silica and one or two selected from calcium oxide and calcium hydroxide in the following mass ratio: Fumed silica : (calcium oxide and / or calcium hydroxide) = 1:2-4. Preferably, the reinforcing fiber is selected from one or more of nylon fiber, ultra-high molecular weight polyethylene fiber, aramid fiber, polyester fiber, and carbon fiber; Preferably, the reinforcing fiber has a diameter of 5-20 μm and a length of 6-12 mm.

4. The composition according to claim 1 or 2, characterized in that, The initiator is selected from one or more of benzoyl peroxide, di-tert-butyl peroxide, dicumyl peroxide, and diisophorone peroxide; Preferably, the anti-settling agent is selected from one or two of organobentonite and polyamide wax; Preferably, the defoamer is selected from one or more of silicone-based defoamers, organic defoamers, polyether defoamers, and polydimethylsiloxane defoamers.

5. A composite material for a brake lever bushing for railway freight cars, prepared by pressure molding using the composition of any one of claims 1 to 4 as raw material.

6. A method for preparing a composite material for a brake lever bushing of a railway freight car, using the composition of any one of claims 1 to 4 as raw material, comprising the following steps: I. Material preparation Prepare each component of the composition according to the stated mass fractions; II. Ball milling dispersion Mix the polyimide resin and styrene evenly, transfer them to a ball mill, add ultrafine heavy calcium carbonate, the solid lubricant and grinding media, disperse for 15-20 minutes, add the anti-settling agent and defoamer, continue to disperse for more than 30 minutes, sieve to remove the grinding media, obtain the matrix mixture, seal it and set it aside. III. Fiber kneading Add the thickener and initiator to the matrix mixture prepared in step II, mix evenly, let stand at room temperature for 15-20 minutes, then transfer to a kneader, add the reinforcing fiber, knead thoroughly for at least 20 minutes to obtain the fiber kneaded mixture, seal, store at low temperature for later use. IV. Molding and Curing The fiber kneading mixture obtained in step III is filled into a mold, pressurized, heated to 140-180°C, kept at the temperature and pressure, and reacted for more than 2 hours. Then it is demolded and cured at 140-180°C for more than 8 hours to obtain the final product.

7. The application of the composite material of claim 5 or the composite material directly obtained by the preparation method of claim 6 in the preparation of brake lever bushings for railway freight cars.

8. A method for preparing a brake lever bushing for railway freight cars, comprising the following steps: I. Material preparation Prepare each raw material according to the mass fractions described in claim 1 or 2; II. Ball milling dispersion Mix the polyimide resin and styrene evenly, transfer them to a ball mill, add ultrafine heavy calcium carbonate, the solid lubricant and grinding media, disperse for 15-20 minutes, add the anti-settling agent and defoamer, continue to disperse for more than 30 minutes, sieve to remove the grinding media, obtain the matrix mixture, seal it and set it aside. III. Fiber kneading Add the thickener and initiator to the matrix mixture prepared in step II, mix evenly, let stand at room temperature for 15-20 minutes, then transfer to a kneader, add the reinforcing fiber, knead thoroughly for at least 20 minutes to obtain the fiber kneaded mixture, seal and set aside. IV. Molding and Curing The fiber kneading mixture obtained in step III is filled into a railway freight car brake lever bushing mold, pressurized to 7 MPa-17 MPa, heated to 140-180°C, and kept at temperature and pressure for more than 2 hours. Then it is demolded and cured at 140-180°C for more than 8 hours. V. Oil absorption and friction reduction Immerse the product obtained in step IV in base lubricating oil or synthetic lubricating oil at 30℃-50℃ for more than 6 hours, then remove and wipe dry to obtain the final product.

9. The preparation method according to claim 8, characterized in that, In step II, the grinding medium is a zirconium oxide grinding bead; Preferably, in step II, the polyimide resin, styrene, ultrafine heavy calcium carbonate, solid lubricant, and zirconium oxide grinding beads are dispersed in a dispersion tank by ball milling with an H-type grinding wheel. Preferably, in step IV, the pressure is increased to 12-14 MPa.

10. A railway freight car brake lever bushing, prepared by the preparation method described in claim 8 or 9.

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