A method for preparing a solid self-lubricating material with high temperature resistance

Abstract

The application discloses a preparation method of solid self-lubricating material with high temperature resistance. First, a high-molecular matrix resin and a nano-lubricating filler are combined to form a uniform raw material powder; then, the raw material powder is placed in a hot-pressing mold cavity, and gradually heated to a first pre-pressing temperature under a first pre-pressing pressure; the temperature is kept for 90-120 minutes under the first pre-pressing temperature, and a hot-pressing pressure is controlled to a second pre-pressing pressure; the hot-pressing pressure is controlled to a third pre-pressing pressure, and gradually heated to a material solid-phase forming temperature; after the hot-pressing temperature reaches the material solid-phase forming temperature, the temperature is kept for 60-180 minutes under a forming pressure to perform solid-phase forming; after the forming, the temperature is gradually reduced to the first pre-pressing temperature, then the pressure is removed, and then the material is naturally cooled to room temperature to be demolded, so that the solid self-lubricating material is obtained. According to the application, the hot-pressing forming temperature is adjusted according to a glass transition temperature drift value, so that the problem of the decline of tribological and mechanical properties caused by uneven plasticization of the composite material and overheating in the compression molding process can be avoided.

Description

Technical Field

[0001] This invention belongs to the field of materials and relates to a self-lubricating material, specifically a method for preparing a solid self-lubricating material with high temperature resistance. Background Technology

[0002] Polyimide possesses excellent mechanical properties due to the rigid benzene ring structure and strongly polar amide bonds in its main and side chains. In particular, polyimide materials with high glass transition temperatures can be used for extended periods in high-temperature environments of 200℃-280℃. Based on the exceptional mechanical properties of polyimide, composite materials are obtained by adding friction-reducing and wear-resistant fillers, giving them self-lubricating properties. These composites can be widely used in friction-reducing and wear-resistant structural components under heavy-load, high-speed conditions, such as bearing cages, piston rings, and sliding bearings. Commonly used layered structural materials to improve the tribological properties of polyimide include graphite-based fillers, molybdenum disulfide and tungsten disulfide, soft metal fillers such as silver and copper, and self-lubricating polymer fillers such as polytetrafluoroethylene (PTFE). These lubricating fillers enable the composite material to reduce the coefficient of friction and wear rate at the friction interface through its own lubrication when interacting with frictional components. However, the addition of these macroscopic fillers inevitably causes microphase physical isolation between macromolecular chains in polyimide matrix resins composed of rigid benzene rings and strongly polar imide skeletons, weakens the interaction forces between organic groups, and thus reduces the mechanical properties of composite materials.

[0003] After years of theoretical analysis and applied development, nanomaterials have been widely used in various industries. They have also received considerable attention and importance in the field of solid lubricating materials. Especially as lubricating fillers, nanomaterials, due to their nanoscale size effect, produce composite materials when mixed with polymer matrix resins. Besides improved tribological properties, their mechanical properties are not only not reduced, but are further enhanced. However, the small size effect of nanomaterial lubricating fillers inevitably causes a shift in the glass transition temperature of the composite material after being added to polyimide matrix resins. For example, the inorganic filler graphene, when added to polymer resins, its two-dimensional nanoscale thin-layer structure dispersed in the resin matrix acts as a "physical cross-linking" effect of the inorganic microphase, restricting the rotation of molecular chain segments and thus increasing the glass transition temperature. For chemically modified inorganic nanofillers, the organic groups on the inorganic filler readily form strong polar bonds with polymer groups, further restricting chain segment movement and thus increasing the glass transition temperature.

[0004] Solid-state molding of polyimide involves applying high constraint stress to the polymer in a non-flowing, solid-state state at a molding temperature above the polymer's glass transition temperature and below its theoretical melting point to obtain a molded product. In solid-state molding, controlling the molding temperature parameters is particularly important. If the molding temperature is too low, the thermal energy provided by the temperature field is insufficient to allow the polyimide chains to move sufficiently, resulting in a product with insufficient mechanical strength. Conversely, if the molding temperature is too high, overplasticization will occur, making the product brittle and hard. Poor control of the molding process leading to a decrease in mechanical properties will significantly affect the tribological properties. Summary of the Invention

[0005] To overcome the above-mentioned defects, the present invention provides a method for preparing a solid self-lubricating material with high temperature resistance. By appropriately adjusting the solid-like phase molding temperature, the uneven molding and plasticizing phenomenon caused by glass transition temperature drift can be avoided.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a solid self-lubricating material with high temperature resistance includes the following steps: Step 1: Prepare a raw material powder composed of a polymer matrix resin and nano lubricating fillers and mix them uniformly; Step 2: Place the raw material powder in the hot press mold cavity, and gradually increase the first pre-press temperature under the first pre-press pressure; Step 3: Hold at the first pre-compression temperature for 90-120 minutes, while controlling the hot-compression pressure to the second pre-compression pressure; Step 4: Control the hot pressing pressure under the third pre-pressing pressure, and gradually increase the temperature to the solid-state molding temperature of the material, wherein the solid-state molding temperature of the material is 15-20℃ higher than the glass transition temperature of the material. Step 5: After the hot pressing temperature reaches the solid-state molding temperature of the material, hold the temperature and pressure under molding pressure for 60-180 minutes to perform solid-state molding. Step 6: After the solid-like phase molding is completed, gradually cool down to the first pre-compression temperature, then release the pressure, and then let it cool naturally to room temperature to demold, thus obtaining a solid self-lubricating material with high temperature resistance.

[0007] Furthermore, the molding parameters are as follows: In step 1, the first pre-compression pressure is 8-10 MPa, the first pre-compression temperature is 240-260 ℃, and the heating rate is 2-5 ℃ / min; In step 2, the second pre-compression pressure is 13-15 MPa; In step 3, the third pre-compression pressure is 25-35 MPa, and the heating rate is 1-3 ℃ / min; In step 4, the molding pressure is 35-45 MPa; In step 5, the cooling rate is 1 – 2℃ / min.

[0008] Furthermore, in the raw material powder of step 1, the weight ratio of polymer matrix resin to nano lubricating filler is 100:(1-10).

[0009] Further, in step 1, the polymer matrix resin is a polyimide block copolymer with a glass transition temperature of 300-360℃ and an average powder particle size of 10μm.

[0010] Furthermore, the nano-lubricating filler has at least one dimension that is nanoscale, that is, its size is less than 100 nm.

[0011] Furthermore, the nano-lubricating filler is an inorganic nano-filler or an organically grafted inorganic nano-filler. Through organic grafting modification, it contains polar groups, which further enhances the interaction force with the polymer matrix resin.

[0012] Furthermore, the inorganic nanofiller is any one or more of graphene, carbon nanotubes, fullerene, nano-graphene, and nano-molybdenum disulfide.

[0013] Furthermore, after the nano-lubricating filler is combined with polyimide, the glass transition temperature of the composite material drifts by 5-30°C compared to pure polyimide. Therefore, it is necessary to adjust the solid-phase molding temperature of the material to be 15-20°C higher than the glass transition temperature of the material.

[0014] Preferably, in step 1, the polyimide hard-soft segment block copolymer is obtained by copolymerization of polyimide hard segment I and polyimide soft segment II, and the reaction equation is as follows:

[0015] Where x and y are 5 – 15, and n is 1 – 200; The polyimide hard segment I is obtained by copolymerizing pyromellitic dianhydride (PMDA) and diaminodiphenyl ether (ODA) and then end-capping with diamine, as shown in the following reaction equation:

[0016] The polyimide soft segment II is obtained by copolymerizing benzophenone tetracarboxylic dianhydride and diphenylmethane diisocyanate and then capping with dianhydride, as shown in the following reaction equation:

[0017] Furthermore, there are two ways to mix the polyimide and the nano-lubricating filler: The first method involves introducing nano-lubricating fillers into the reaction system after polyimide hard segment I reacts with polyimide soft segment II to generate polyamic acid, before the imidization reaction. The second method involves copolymerizing polyimide hard segment I and polyimide soft segment II to obtain polyimide powder. Then, an ultrasonic dispersion-ball milling synergistic mixing process is used to mix the polyimide powder, nano lubricant filler, and solvent. The mixture is ultrasonically dispersed at room temperature for 60-120 minutes, and then heated and refluxed to extract the solvent to obtain a blend. The blend is then mixed in a planetary ball mill for 100-140 minutes, and then dried in an oven at 110-130℃ for 1-3 hours to remove residual volatiles, thus obtaining a uniformly mixed raw material powder.

[0018] Furthermore, the solvent is one or more of ethanol and acetone.

[0019] Furthermore, the polyimide soft and hard segment block copolymer is prepared by the following method: Step 1.1: Preparation of polyimide soft segment II. Under nitrogen atmosphere protection, benzophenone tetracarboxylic acid dianhydride and N-methylpyrrolidone were placed in a three-necked flask equipped with a stirrer and condenser, and heated until the solid reactants were completely dissolved. First, 1,4-diazabicyclo[2.2.2]octane was added, and then diphenylmethane diisocyanate was added in batches for copolymerization. The CO2 generated during polymerization was discharged through a condenser. Finally, a BTDA-MDI soft segment copolymer solution with dianhydride end caps was obtained and cooled for later use. Step 1.2: Preparation of polyimide hard segment I: Diaminodiphenyl ether is dissolved in N-methylpyrrolidone, and the temperature is controlled below 18-22℃. Then, pyromellitic dianhydride is added in batches to obtain a PMDA-ODA hard segment copolymer solution with diamine end-capping. Step 1.3: Add the cooled and prepared BTDA-MDI soft segment copolymer solution to the PMDA-ODA hard segment copolymer solution at a constant rate, and maintain the reaction temperature below 18-22℃ to carry out the block copolymerization reaction to obtain polyimide soft and hard segment block copolymers.

[0020] Further, in step 1.3, after obtaining the polyimide soft and hard segment block copolymer, it is spray-dried to obtain polyimide block copolymer powder that can be used for hot pressing in a hot pressing mold cavity.

[0021] Furthermore, the hot press mold cavity is designed with the appropriate shape according to the desired shape of the product to be produced.

[0022] The glass transition temperature of polyimide block copolymers is 300-360℃. The addition of nano-lubricating fillers causes a drift in the glass transition temperature of the composite material, affecting its mechanical and tribological properties, particularly its wear resistance, without changing the molding parameters. Based on this glass transition temperature drift, a high-temperature self-lubricating material with excellent overall performance can be obtained by adjusting the solid-phase molding temperature. The solid-phase molding process provided by this invention, by adjusting the molding process according to the glass transition temperature drift, can avoid the problems of uneven plasticization and overheating of the composite material during compression molding, which can lead to a decrease in tribological and mechanical properties.

[0023] The nanocomposite polyimide provided by this invention is a solid self-lubricating material with high temperature resistance, capable of long-term application in high-temperature environments of 200-280℃, and exhibiting excellent friction reduction and wear resistance properties. Furthermore, as a nanocomposite material, although inorganic fillers are added, its mechanical strength is not reduced; on the contrary, it is improved to a certain extent.

[0024] The nanocomposite polyimide provided by this invention is based on a solid-state molding process. By adjusting the solid-state molding temperature appropriately, the problem of poor plasticization or over-plasticization of the composite material can be avoided, thereby reducing the impact on its tribological properties. Attached Figure Description

[0025] Figure 1 This is a flowchart illustrating the preparation method of the high-temperature resistant solid self-lubricating material of the present invention.

[0026] Figure 2 These are stress-strain curves of the products in Embodiments 1-3 of the present invention.

[0027] Figure 3 This is a schematic diagram showing the tensile strength, elastic modulus, and elongation at break of the products in Examples 1-3 of the present invention.

[0028] Figure 4 This is a schematic diagram of the friction curves of the products in Embodiments 1-3 of the present invention.

[0029] Figure 5 This is a schematic diagram showing the average friction coefficient and wear rate of the products in Examples 1-3 of the present invention.

[0030] Figure 6 This is a low-magnification fracture cross-sectional view of the polyimide block copolymer material with high temperature resistance in Example 1 of the present invention.

[0031] Figure 7 This is a low-magnification fracture cross-section of the solid self-lubricating material with high-temperature resistance in Embodiment 2 of the present invention.

[0032] Figure 8This is a low-magnification fracture cross-section of the solid self-lubricating material with high-temperature resistance in Embodiment 3 of the present invention.

[0033] Figure 9 This is a high-magnification fracture cross-section of the solid self-lubricating material with high-temperature resistance in Embodiment 2 of the present invention.

[0034] Figure 10 This is a high-magnification fracture cross-section of the solid self-lubricating material with high-temperature resistance in Embodiment 3 of the present invention. Detailed Implementation

[0035] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0036] Example 1 Under a nitrogen atmosphere, 210.06 g (0.65 mol) of BTDA and 1000 g of NMP were placed in a 5 L three-necked flask equipped with a stirrer and condenser. The mixture was heated to 80 °C to completely dissolve the solid reactants. 1 g of TED was added, followed by the addition of 130.47 g (0.52 mol) of MDI in batches for copolymerization. CO2 generated during polymerization was vented through a condenser, resulting in a dianhydride-terminated BTDA-MDI soft-segment copolymer solution, which was cooled and stored. 113.23 g (0.57 mol) of ODA was dissolved in 2000 g of NMP, with the temperature controlled below 20 °C. Then, 89.97 g (0.41 mol) of PMDA was added in batches to obtain a diamine-terminated PMDA-ODA hard-segment copolymer solution. A cooled, pre-prepared BTDA-MDI soft-segment copolymer solution was added at a constant rate to a PMDA-ODA hard-segment copolymer solution, maintaining the reaction temperature below 20°C to carry out the block copolymerization reaction, yielding a polyimide soft-hard-segment block copolymer. Finally, spray drying was performed to obtain a polyimide block copolymer powder with an average particle size of 8-15 μm. Differential scanning calorimetry (DSC) analysis showed that the glass transition temperature of the obtained polyimide product was 330°C.

[0037] A solid-state process was used to prepare the molded body. Polyimide block copolymer powder without nano-lubricant filler was placed in a mold cavity and heated to 250°C at a rate of 5°C / min, while maintaining a pressure of 10 MPa. After reaching 250°C, the temperature was held for 120 min, while maintaining a pressure of 15 MPa. The temperature was then increased to 350°C at a rate of 1°C / min, while maintaining a pressure of 30 MPa. At 350°C, the pressure was reduced to 40 MPa, and the temperature was held for 60 min. After holding, the mixture was allowed to cool naturally to 250°C, the pressure was released, and then the mixture was cooled to room temperature before demolding, yielding a polyimide block copolymer material with high-temperature resistance.

[0038] Example 2 The preparation method of polyimide block copolymer powder is the same as in Example 1, except that after reacting PMDA-ODA block and BTDA-MDI block to obtain polyimide solution, 15.23g of carbon nanotubes are added to the reaction system, and then after subsequent reaction and spray granulation, carbon nanotube modified polyimide composite powder is obtained.

[0039] Carbon nanotube-modified polyimide composite powder was subjected to solid-phase molding, and the molding process was the same as that in Example 1, to obtain a solid self-lubricating material with high temperature resistance.

[0040] Example 3 The preparation of polyimide block copolymer powder was the same as in Example 1, resulting in carbon nanotube-modified polyimide composite powder.

[0041] Carbon nanotube-modified polyimide composite powder was subjected to solid-phase molding. The molding process was basically the same as that in Example 1, except that the molding temperature was changed from 350°C to 375°C when the material was in a solid-phase state, resulting in a solid self-lubricating material with high temperature resistance.

[0042] Example 4 The polyimide powder obtained in Example 1 was subjected to an ultrasonic dispersion-ball milling synergistic mixing process. 30g of polyimide powder, 0.9g of graphene oxide, and acetone were mixed and ultrasonically dispersed at room temperature for 60min. Then, the acetone was extracted by heating and reflux to obtain a blend. The blend was then mixed in a planetary ball mill for 120min and dried in an oven at 120℃ for 2h to remove residual volatiles, resulting in a polyimide composite powder modified with graphene oxide.

[0043] The graphene oxide-modified polyimide composite powder was subjected to solid-phase molding, and the molding process was the same as that in Example 1, to obtain a solid self-lubricating material with high temperature resistance.

[0044] Example 5 The preparation method of the graphene oxide-modified polyimide composite powder is the same as that in Example 4.

[0045] The graphene oxide-modified polyimide composite powder was subjected to solid-state molding. The molding process was basically the same as in Example 4, except that the molding temperature was changed from 350°C to 360°C when the material was in a solid-state state.

[0046] Table 1 shows the key process parameters and material properties of Examples 1-5.

[0047] In Table 1, the friction and wear test is a reciprocating friction test of an Rtec ball-and-disc: GCr15 bearing ball mating parts, diameter 6.4mm, reciprocating frequency 8Hz, reciprocating stroke 8mm, test time 90min, normal pressure 100N. Examples 1, 2, and 3 were conducted in a pure water environment, while Examples 4 and 5 were conducted in a dry friction environment.

[0048] Unlike melt processing, solid-state molding processes have a narrow processing temperature window due to their characteristics. An optimal processing temperature exists when the polymer is in a solid-state, highly elastic state. Too low a temperature results in insufficient free volume, restricting molecular chain movement and directly affecting the adhesion between polymer particles. Too high a temperature not only wastes energy but also causes oxidation and discoloration of the finished product. As shown in the table above, the addition of nano-lubricating fillers causes a shift in the glass transition temperature of the composite self-lubricating material. If produced using the same process as pure polyimide, this leads to a decrease in mechanical and tribological properties, particularly an exponential reduction in wear resistance. Figure 6 As shown, Example 1 exhibits a good plasticized fracture morphology and undergoes typical brittle fracture under tensile stress. With the addition of carbon nanotubes, and assuming the molding process remains unchanged, Example 2 shows a large number of manufacturing defects, such as porous structures, on the fracture surface. Figure 7 When magnified 5000 times, as... Figure 9 As shown, carbon nanotubes exhibit severe aggregation in localized locations. This is because insufficient thermal energy to soften the polymer particles hinders the entry of the nanofiller into the polymer matrix. Only after increasing the processing temperature based on the glass transition temperature drift value does the amplitude of molecular chain segment motion become significant, allowing the thermal energy to overcome the potential barriers of the translational and rotational motions of polymer molecules. Therefore, as... Figure 8 and Figure 10 As shown, void defects and agglomeration no longer occurred in Example 3. Therefore, for composite materials modified with nano-lubricating fillers, the solid-like molding process should be adjusted accordingly based on their glass transition temperature drift value.

[0049] The above embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Although the invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of the invention do not depart from the spirit and scope of the invention and should be covered within the scope of the claims of the invention.

Claims

1. A method for preparing a solid self-lubricating material with high-temperature resistance, characterized in that, Includes the following steps: Step 1: Prepare a raw material powder composed of a polymer matrix resin and nano lubricating fillers and mix them uniformly; Step 2: Place the raw material powder in the hot press mold cavity, and gradually increase the first pre-press temperature under the first pre-press pressure; Step 3: Hold at the first pre-compression temperature for 90-120 minutes, while controlling the hot-compression pressure to the second pre-compression pressure; Step 4: Control the hot pressing pressure under the third pre-pressing pressure and gradually increase the temperature to the solid-phase molding temperature of the material. The solid-phase molding temperature of the material is 15-20°C higher than the glass transition temperature of the material. Due to the glass transition temperature drift caused by the nano-lubricating filler, the glass transition temperature of the solid self-lubricating material is 5-30°C higher than the glass transition temperature of the polymer matrix resin. Step 5: After the hot pressing temperature reaches the solid-state molding temperature of the material, hold the temperature and pressure under molding pressure for 60-180 minutes to perform solid-state molding. Step 6: After the solid-like phase molding is completed, gradually cool down to the first pre-compression temperature, then release the pressure, and then let it cool naturally to room temperature to demold, thus obtaining a solid self-lubricating material with high temperature resistance. In step 1, the polymer matrix resin is a polyimide block copolymer with a glass transition temperature of 300-360℃ and an average powder particle size of 10μm. The nano-lubricating filler is an inorganic nanofiller or an organically grafted modified inorganic nanofiller.

2. The method for preparing the solid self-lubricating material according to claim 1, characterized in that, The molding parameters are as follows: In step 1, the first pre-compression pressure is 8-10 MPa, the first pre-compression temperature is 240-260 ℃, and the heating rate is 2-5 ℃ / min; In step 2, the second pre-compression pressure is 13-15 MPa; In step 3, the third pre-compression pressure is 25-35 MPa, and the heating rate is 1-3 ℃ / min; In step 4, the molding pressure is 35-45 MPa; In step 5, the cooling rate is 1 – 2℃ / min.

3. The method for preparing the solid self-lubricating material according to claim 1, characterized in that: In the raw material powder of step 1, the weight ratio of polymer matrix resin to nano lubricating filler is 100:(1-10).

4. The method for preparing the solid self-lubricating material according to claim 1, characterized in that: The nano lubricating filler has at least one dimension that is nanoscale, i.e., its size is less than 100 nm.

5. The method for preparing the solid self-lubricating material according to claim 4, characterized in that: The inorganic nanofiller is any one or more of graphene, carbon nanotubes, fullerene, nano-graphene, and nano-molybdenum disulfide.

6. The method for preparing the solid self-lubricating material according to claim 1, characterized in that: In step 1, the polyimide is a polyimide hard-segment block copolymer, obtained by copolymerizing polyimide hard segment I and polyimide soft segment II. The polyimide hard segment I is obtained by copolymerizing pyromellitic dianhydride and diaminodiphenyl ether and then end-capping with diamine. The polyimide soft segment II is obtained by copolymerizing benzophenone tetracarboxylic dianhydride and diphenylmethane diisocyanate and then end-capping with dianhydride.

7. The method for preparing the solid self-lubricating material according to claim 6, characterized in that: There are two ways to mix the polyimide and the nano-lubricating filler: The first method involves introducing nano-lubricating fillers into the reaction system after polyimide hard segment I reacts with polyimide soft segment II to generate polyamic acid, before the imidization reaction. The second method involves copolymerizing polyimide hard segment I and polyimide soft segment II to obtain polyimide powder. Then, an ultrasonic dispersion-ball milling synergistic mixing process is used to mix the polyimide powder, nano lubricant filler, and solvent. The mixture is ultrasonically dispersed at room temperature for 60-120 minutes, and then heated and refluxed to extract the solvent to obtain a blend. The blend is then mixed in a planetary ball mill for 100-140 minutes, and then dried in an oven at 110-130℃ for 1-3 hours to remove residual volatiles, thus obtaining a uniformly mixed raw material powder.

8. The method for preparing the solid self-lubricating material according to claim 6, characterized in that: The polyimide soft and hard segment block copolymer is prepared by the following method: Step 1.1: Preparation of polyimide soft segment II. Under nitrogen atmosphere protection, benzophenone tetracarboxylic acid dianhydride and N-methylpyrrolidone were placed in a three-necked flask equipped with a stirrer and condenser, and heated until the solid reactants were completely dissolved. First, 1,4-diazabicyclo[2.2.2]octane was added, and then diphenylmethane diisocyanate was added in batches for copolymerization. The CO2 generated during polymerization was discharged through a condenser. Finally, a BTDA-MDI soft segment copolymer solution with dianhydride end caps was obtained and cooled for later use. Step 1.2: Preparation of polyimide hard segment I: Diaminodiphenyl ether is dissolved in N-methylpyrrolidone, and the temperature is controlled below 18-22℃. Then, pyromellitic dianhydride is added in batches to obtain a PMDA-ODA hard segment copolymer solution with diamine end-capping. Step 1.3: Add the cooled and prepared BTDA-MDI soft segment copolymer solution to the PMDA-ODA hard segment copolymer solution at a constant rate, and maintain the reaction temperature below 18-22℃ to carry out the block copolymerization reaction to obtain polyimide soft and hard segment block copolymers.

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