Integrated Preparation Method of Nano-Aluminum Oxide Modified Polyimide Composite Material

Abstract

The present invention discloses an integrated preparation method of nano-aluminum oxide modified polyimide composite materials, belonging to the technical field of composite materials, including: according to the mass ratio of (50-90):(10-50), mixing polyimide powder with nano-aluminum oxide powder whose surface is modified by silane coupling agent evenly to obtain a mixed material; then carrying out high-speed stirring on the mixed material to obtain a powder material with uniform and dispersed distribution; and obtaining the nano-aluminum oxide modified polyimide composite material by hot pressing and secondary hot pressing curing. In the present invention, the interaction between nano-aluminum oxide in the nano-aluminum oxide powder modified by silane coupling agent on the surface and the polyimide molecular chain forms a three-dimensional network cross-linked structure. The steric hindrance effect of nano-aluminum oxide hinders the movement of macromolecular chains, effectively pins cracks, increases the crack propagation resistance, and improves the mechanical properties and thermal stability of the composite material.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite materials and relates to an integrated preparation method of nano-aluminum oxide modified polyimide composites. Background Art

[0002] Polyimide (PI) has excellent mechanical properties, high thermal stability, solvent and corrosion resistance, and good electrical insulation. As a resin with an extremely high temperature resistance grade in advanced composite resin matrices, the long-term service temperature of polyimide exceeds 300 °C, and the short-term service temperature exceeds 500 °C. It can be used as a special engineering plastic or the matrix resin of high-temperature composite materials and is widely used in high-tech fields such as aviation, aerospace, machinery, petrochemical industry, and microelectronics. Whether as a structural material or a functional material, its huge application prospects have been fully recognized. Polyimide can be used to replace metals and ceramics as structural components in the aerospace field. However, the high-temperature resistance of polyimide itself is limited. When exposed to an environment with sufficient heat and oxygen, it will undergo thermal oxidation damage, decompose and carbonize, resulting in a sharp drop in strength, which limits its further application.

[0003] To make up for the low strength and low temperature resistance of polymers, polymers are mixed with various nano-fillers, which can not only ensure the high specific strength and high toughness of the composite materials but also improve their modulus and high-temperature performance. Nano-fillers have unique properties that enhance the electrical conductivity, thermal conductivity, flame retardancy, optical and mechanical properties, thermal expansion, wear resistance, thermal deformation, energetics, and other properties of polymer nanocomposites. Common fillers are divided into metal materials, carbon-based materials (nanotubes, graphene, graphite flakes), and oxide fillers. However, introducing metal particles into polymers usually reduces the electrical insulation and dielectric properties of the composite materials; although carbon materials have high thermal conductivity and low weight, the high cost of the materials and the reduced electrical insulation performance hinder their practical application in the industry. Among oxide fillers, although MgO is inexpensive, it increases the viscosity of the system, has poor corrosion resistance, and has a narrow application range in insulating polymer composites. Due to its low inherent thermal conductivity, the application of ZnO in high-thermal-conductivity products is also limited. Al2O3 has good thermal conductivity, high strength and stiffness, mechanical strength, inertness to most acids and bases, high adsorption capacity, wear resistance, oxidation resistance, thermal stability, and electrical insulation. In addition, it is inexpensive, non-toxic, and highly abrasive. Al2O3 nanoparticles have great application potential in fields such as catalysts or catalyst carriers, electrical insulators, high-voltage insulators, furnace lining tubes, bulletproof armor, wear-resistant tubes, and temperature measurement sensors.

[0004] In the prior art, due to their small particle size and large surface energy, nanoparticles are in a thermodynamically unstable state. Due to the strong van der Waals force and electrostatic activity, nano-fillers show an affinity for aggregation in solvents. Coupled with the poor compatibility between hydrophilic nanoparticles and hydrophobic polymers, nanoparticles often disperse in the resin matrix in the form of large aggregates. The properties of polyimide-based composites are affected by the filler-polymer interfacial interaction, and the concentration of inorganic nano-fillers is usually kept below 10 vol%. To solve the problem of aggregation, special preparation methods such as intercalation compounding and in-situ compounding can be used. However, the intercalation compounding method is only applicable to a few inorganic substances with a layered structure such as clay, lacking universality for nano-materials; the in-situ compounding method has a cumbersome process, harsh synthesis conditions, high cost, and is difficult to mass-produce. The properties of composites depend on the interfacial interaction between the filler and the polymer. When the bonding strength between the nanoparticles and the resin is weak, the nanoparticles are prone to peeling off first due to stress concentration. The pores serve as the positions for crack propagation and failure, resulting in a decrease in the properties of the composites. When the dispersion of the particles is good and the interfacial bonding with the resin is high, the external force can be transmitted to the reinforcing phase, and the nanoparticles can also pin the cracks, increasing the crack propagation resistance and improving the strength of the composites. Summary of the Invention

[0005] In order to improve the mechanical properties of polyimide-based composites and make them better applicable to working conditions with harsh service environments, the purpose of the present invention is to provide an integrated preparation method for nano-alumina modified polyimide composites.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] An integrated preparation method for nano-alumina modified polyimide composites, comprising the following steps: According to the mass ratio of (50 - 90):(10 - 50), mix polyimide powder and nano-alumina powder whose surface is modified by a silane coupling agent evenly to obtain a mixed material; wherein, the total mass of the nano-alumina powder whose surface is modified by a silane coupling agent and the polyimide powder is 100.

[0008] Then, subject the mixed material to high-speed stirring to obtain a uniformly dispersed powder.

[0009] Subject the uniformly dispersed powder to hot pressing and secondary hot pressing curing to obtain nano-alumina modified polyimide composites.

[0010] Compared with the prior art, the beneficial effects of the present invention include:

[0011] In the present invention, the interaction between nano-aluminum oxide in the nano-aluminum oxide powder with its surface modified by a silane coupling agent and the polyimide molecular chains forms a three-dimensional network crosslinked structure. The steric hindrance effect of the nano-aluminum oxide hinders the movement of the macromolecular chains, effectively pins the cracks, increases the crack propagation resistance, and improves the strength of the composite material. In the present invention, by adjusting the mass fraction ratio of polyimide powder to nano-aluminum oxide powder with its surface modified by a silane coupling agent (50 - 90):(10 - 50), a composite material with excellent comprehensive performance and a three-dimensional crosslinked network structure and high mechanical properties can be obtained. The nano-aluminum oxide particles in the nano-aluminum oxide powder with its surface modified by a silane coupling agent can delay the start of thermal decomposition in the polymer and slow down the mass loss rate of the pyrolyzed polymer after reaching the thermal decomposition temperature, improving the thermal stability of the composite material. The uniformly distributed nano-fillers (aluminum oxide) result in enhanced nano-composite behavior. The high interfacial adhesion can effectively transfer the load from the polymer to the filler, and the nano-aluminum oxide particles increase the crack propagation path, improving the mechanical properties. The present invention has the characteristics of simple operation process, good repeatability, significant economic benefits, accurate and reliable results. The prepared filler has good interfacial compatibility between polyimide and alumina, low porosity, and high compressive strength. In order to improve the dispersibility of the nano-particles, in the present invention, secondary curing of the composite material can fully crosslink the polymer network, promoting the improvement of the strength and stiffness of the composite material. In the present invention, the use of nano-aluminum oxide powder with its surface modified by a silane coupling agent can improve the compatibility between the particles and the polymer matrix and enhance the mechanical properties of the composite material.

[0012] Furthermore, in the present invention, a silane coupling agent is used to chemically modify the surface of nano-aluminum oxide to improve the compatibility between the particles and the polymer matrix and enhance the properties of the composite material. One end of the silane coupling agent bonds with the surface of the nano-particles, which can reduce the agglomeration of the particles and ensure the dispersibility of the nano-particles in the polymer; the other end overlaps with the polymer, improving the interfacial interaction. Through compressive strength and phase analysis, a polyimide-based composite material with excellent mechanical properties is obtained by comparison, and it is expected to be actually used in the field of harsh service environments. Brief Description of the Drawings

[0013] The following further elaborates on the present invention in detail in conjunction with the drawings and specific embodiments.

[0014] Figure 1 This is a micrograph of the original powder used in the PI / Al2O3 composite material of the present invention. Among them, (a) is the SEM image of PI; (b) is the TEM image of Al2O3.

[0015] Figure 2 This is the infrared spectrum of the nano-Al2O3 particles before and after modification in the present invention. Figure 3 This is the compressive strength of the PI / Al2O3 composite material of the present invention under different compositions.

[0016] Figure 3 For Figure 2 The enlarged view at the square box.

[0017] Figure 4 This is the microstructure of the modified 30% nano-Al2O3 / PI composite material in Example 3 of the present invention.

[0018] Figure 5 This is the compressive strength curve of the nano-Al2O3 / PI composite material before and after modification of the present invention. Detailed implementation manners

[0019] The present invention will be described in detail below with reference to the accompanying drawings.

[0020] The present invention uses polyimide as the matrix, which has excellent mechanical properties, high thermal stability, solvent corrosion resistance, and good electrical insulation. To improve the mechanical properties and heat resistance of polyimide, nano-aluminum oxide particles with good thermal conductivity, high strength and stiffness, and mechanical strength are filled. Through the surface modification of nano-aluminum oxide particles with a silane coupling agent, the agglomeration of nano-Al2O3 particles is improved, and the interfacial compatibility between PI and Al2O3 is enhanced; a uniformly dispersed composite powder is obtained through high-speed mechanical stirring, ensuring that the nano-Al2O3 powder is uniformly distributed in the PI matrix during hot pressing, which is conducive to the formation of a network cross-linked structure. The uniformly distributed nano-filler system leads to enhanced nano-composite behavior, and high interfacial adhesion can effectively transfer the load from the polymer to the filler. The nano-Al2O3 particles increase the crack propagation path and improve the mechanical properties.

[0021] The integrated preparation method of the nano-aluminum oxide modified polyimide composite material of the present invention includes the following steps:

[0022] (1) Refer to Figure 1 in (b), dry the alumina particles (particle size 20 - 30 nm) at 100 °C for 12 h to remove surface adsorbed substances, and then add them to an ethanol solution, and ultrasonically disperse for 30 min to obtain an ethanol solution of nano-aluminum oxide;

[0023] Add a silane coupling agent (KH550, KH560, KH570, A-171 or A-172) to a mixed solution of ethanol and water (the mass ratio of the silane coupling agent to ethanol and water is 20:72:8), add acetic acid to adjust the pH = 4 - 5, ultrasonically hydrolyze for 30 min, and then add it to the ethanol solution of nano-aluminum oxide, stir and react at a temperature of 75 - 85 °C for 3 - 5 h, filter by suction, wash three times with ethanol, and dry to obtain nano-aluminum oxide powder with its surface modified by a silane coupling agent.

[0024] Among them, the mass of the silane coupling agent is 5-8% of the mass of the alumina powder.

[0025] There are many hydroxyl groups on the surface of nano-alumina, showing hydrophilicity, while polyimide is hydrophobic. The difference in properties between the two leads to weak interfacial bonding. After hydrolysis, the silane coupling agent bonds with the surface of nano-alumina, which can promote the interfacial compatibility with polyimide, reduce the agglomeration of nanoparticles, and improve the performance of the composite material.

[0026] (2) Refer to Figure 1 In (a) and (b) of , according to the mass ratio of (50-90):(10-50), polyimide powder (particle size of 2-10 μm) and nano-alumina powder modified by silane coupling agent on the surface are mixed to obtain a mixture. The total mass fraction of nano-alumina powder modified by silane coupling agent on the surface and polyimide powder is 100. The mixture is put into a mixer for high-speed stirring to make the flocculent powder refined to the greatest extent and evenly dispersed in the polyimide (PI) matrix, obtaining a uniformly dispersed powder; among them, the conditions for high-speed stirring are: first stir at a stirring speed of 1000-3000 r / min for 60-70 min, and then stir at a stirring speed of 3000-5000 r / min for 30-40 min.

[0027] (3) Weigh a certain mass of the uniformly dispersed powder and obtain a nano-alumina modified polyimide composite material through hot pressing. The molding pressure is 100-150 MPa, the molding temperature is 220-240 °C, keep the temperature and pressure for 30-120 min, then cool down and demold, and then perform secondary curing to obtain a nano-alumina modified polyimide composite material. The specific process of secondary curing is: first heat up from room temperature to 180-200 °C and keep it for 1-2 h, then keep it at 200-220 °C for 1-2 h, and finally keep it at 230-250 °C for 4-6 h.

[0028] In the present invention, secondary curing allows the polymer to have sufficient time to react and form a three-dimensional cross-linked network, which can greatly improve the strength of the composite material. Without secondary curing, the performance of the composite material will be affected.

[0029] With the increase of the nano-alumina content, the composite material with 30 wt.% alumina has the highest compressive strength, reaching 313.0 MPa. This is attributed to the three-dimensional network cross-linked structure with dispersed alumina inside.

[0030] Comparative Example 1

[0031] Put pure PI into a mixer and stir at high speed to refine the powder to the greatest extent, obtaining PI with a particle size of 2 - 10 μm. First, stir at a stirring speed of 2000 revolutions / min for 60 min, and then stir at a stirring speed of 4000 revolutions / min for 30 min.

[0032] Weigh a certain mass of PI powder and obtain a pure PI block through hot pressing. The forming pressure is 100 MPa, the forming temperature is 220 °C, keep the temperature and pressure for 30 min, then cool down and demold, and then perform secondary curing. The specific process of secondary curing is as follows: first, heat up from room temperature to 180 °C and keep for 1 h, then keep at 200 °C for 1 h, and finally keep at 230 °C for 4 h.

[0033] Example 1

[0034] Dry alumina particles (with a particle size of 20 - 30 nm) at 100 °C for 12 h to remove surface adsorbed substances, and then add them to an ethanol solution and ultrasonically disperse for 30 min to obtain an ethanol solution of nano-alumina; add a silane coupling agent (KH550) to a mixed solution of ethanol and water (the mass ratio of the silane coupling agent to ethanol and water is 20:72:8), add acetic acid to adjust the pH = 4, ultrasonically hydrolyze for 30 min, and then add it to the ethanol solution of nano-alumina and stir and react at 75 °C for 5 h, filter by suction, wash three times with ethanol, and dry to obtain nano-alumina powder modified with a silane coupling agent on the surface. Among them, the mass of the silane coupling agent is 5% of the mass of the alumina powder.

[0035] Mix polyimide powder and nano-alumina powder modified with a silane coupling agent on the surface according to a mass fraction ratio of 90:10 to obtain a composite powder. The total mass fraction of the modified nano-alumina powder and polyimide powder is 100. Put the composite powder into a mixer and stir at high speed to refine the flocculent powder to the greatest extent and make it evenly dispersed in the PI matrix, obtaining evenly dispersed powder; the conditions for high-speed stirring are: first, stir at a stirring speed of 1000 revolutions / min for 70 min, and then stir at a stirring speed of 4000 revolutions / min for 30 min.

[0036] Weigh a certain mass of the evenly dispersed powder and obtain a nano-alumina modified polyimide composite through hot pressing. The forming pressure is 100 MPa, the forming temperature is 220 °C, keep the temperature and pressure for 30 min, then cool down and demold, and then perform secondary curing. The specific process of secondary curing is as follows: first, heat up from room temperature to 180 °C and keep for 1 h, then keep at 200 °C for 1 h, and finally keep at 230 °C for 4 h.

[0037] Example 2

[0038] The alumina particles (with a particle size of 20 - 30 nm) are dried at 100 °C for 12 h to remove surface adsorbed substances, and then added to an ethanol solution and ultrasonically dispersed for 30 min to obtain an ethanol solution of nano-alumina; the silane coupling agent (KH560) is added to a mixed solution of ethanol and water (the mass ratio of the silane coupling agent to ethanol and water is 20:72:8), acetic acid is added to adjust the pH = 5, and it is ultrasonically hydrolyzed for 30 min, and then added to the ethanol solution of nano-alumina, and stirred and reacted at 80 °C for 4 h, filtered by suction, washed three times with ethanol, and dried to obtain nano-alumina powder modified with a silane coupling agent on the surface. Among them, the mass of the silane coupling agent is 6% of the mass of the alumina powder.

[0039] According to the mass fraction ratio of 80:20, the polyimide powder material is mixed with the nano-alumina powder modified with a silane coupling agent on the surface, and the total mass fraction of the modified nano-alumina powder and the polyimide powder material is 100. The composite powder is put into a mixer and stirred at high speed to make the flocculent powder refined to the greatest extent and uniformly dispersed in the PI matrix with each other, obtaining a uniformly dispersed powder material; the conditions for high-speed stirring are: first stir at a stirring speed of 3000 r / min for 40 min, and then stir at a stirring speed of 4000 r / min for 40 min.

[0040] A certain mass of the uniformly dispersed powder material is weighed and obtained as a nano-alumina modified polyimide composite material through hot pressing. The forming pressure is 100 MPa, the forming temperature is 220 °C, keep the temperature and pressure for 30 min, then cool down and demold, and then carry out secondary curing. The specific process of secondary curing is: first heat up from room temperature to 180 °C and keep for 1 h, then keep at 200 °C for 1 h, and finally keep at 230 °C for 4 h.

[0041] Example 3

[0042] The alumina particles (with a particle size of 20 - 30 nm) are dried at 100 °C for 12 h to remove surface adsorbed substances, and then added to an ethanol solution and ultrasonically dispersed for 30 min to obtain an ethanol solution of nano-alumina; the silane coupling agent (KH570) is added to a mixed solution of ethanol and water (the mass ratio of the silane coupling agent to ethanol and water is 20:72:8), acetic acid is added to adjust the pH = 4, and it is ultrasonically hydrolyzed for 30 min, and then added to the ethanol solution of nano-alumina, and stirred and reacted at 85 °C for 3 h, filtered by suction, washed three times with ethanol, and dried to obtain nano-alumina powder modified with a silane coupling agent on the surface. Among them, the mass of the silane coupling agent is 7% of the mass of the alumina powder.

[0043] Mix polyimide powder and nano-alumina powder with its surface modified by a silane coupling agent in a mass ratio of 70:30. The total mass fraction of the modified nano-alumina powder and polyimide powder is 100. Put the composite powder into a mixer and stir at high speed to maximize the refinement of the flocculent powder and make it evenly dispersed in the PI matrix, obtaining a powder with uniform dispersion; the conditions for high-speed stirring are: first stir at a stirring speed of 2000 rpm for 60 min, and then stir at a stirring speed of 5000 rpm for 30 min.

[0044] Weigh a certain mass of the powder with uniform dispersion and obtain a nano-alumina modified polyimide composite through hot pressing. The forming pressure is 100 MPa, the forming temperature is 220 °C, keep the temperature and pressure for 30 min, then cool down and demold, and then perform secondary curing. The specific process of secondary curing is: first heat up from room temperature to 180 °C and keep it for 1 h, then keep it at 200 °C for 1 h, and finally keep it at 230 °C for 4 h.

[0045] Example 4

[0046] Dry alumina particles (particle size 20 - 30 nm) at 100 °C for 12 h to remove surface adsorbates, then add them to an ethanol solution and ultrasonically disperse for 30 min to obtain an ethanol solution of nano-alumina; add a silane coupling agent (A-171) to a mixed solution of ethanol and water (the mass ratio of the silane coupling agent to ethanol and water is 20:72:8), add acetic acid to adjust the pH = 5, ultrasonically hydrolyze for 30 min, then add it to the ethanol solution of nano-alumina, stir and react at 75 °C for 5 h, filter by suction, wash three times with ethanol, and obtain nano-alumina powder with its surface modified by a silane coupling agent after drying. Among them, the mass of the silane coupling agent is 8% of the mass of the alumina powder.

[0047] Mix polyimide powder and nano-alumina powder with its surface modified by a silane coupling agent in a mass ratio of 60:40. The total mass fraction of the modified nano-alumina powder and polyimide powder is 100. Put the composite powder into a mixer and stir at high speed to maximize the refinement of the flocculent powder and make it evenly dispersed in the PI matrix, obtaining a powder with uniform dispersion; the conditions for high-speed stirring are: first stir at a stirring speed of 2000 rpm for 60 min, and then stir at a stirring speed of 4000 rpm for 30 min.

[0048] Weigh a certain mass of the uniformly dispersed powder, and obtain the nano-aluminum oxide modified polyimide composite material through hot pressing. The forming pressure is 100 MPa, the forming temperature is 220 °C, keep the temperature and pressure for 30 min, then cool down and demold, and then perform secondary curing. The specific process of secondary curing is as follows: First, heat up from room temperature to 180 °C and keep it for 1 h, then keep it at 200 °C for 1 h, and finally keep it at 230 °C for 4 h.

[0049] Example 5

[0050] Dry the alumina particles (particle size 20 - 30 nm) at 100 °C for 12 h to remove the surface adsorbed substances, then add them to the ethanol solution and ultrasonically disperse for 30 min to obtain the ethanol solution of nano-aluminum oxide; Add the silane coupling agent (A-172) to the mixed solution of ethanol and water (the mass ratio of the silane coupling agent to ethanol and water is 20:72:8), add acetic acid to adjust the pH = 4, ultrasonically hydrolyze for 30 min, then add it to the ethanol solution of nano-aluminum oxide, stir and react at 80 °C for 3 h, filter by suction, wash three times with ethanol, and dry to obtain the nano-aluminum oxide powder modified by the silane coupling agent on the surface. Among them, the mass of the silane coupling agent is 5% of the mass of the alumina powder.

[0051] Mix the polyimide powder and the nano-aluminum oxide powder modified by the silane coupling agent on the surface according to the mass fraction ratio of 50:50, and the total mass fraction of the modified nano-aluminum oxide powder and the polyimide powder is 100. Put the composite powder into a mixer and stir at high speed to make the flocculent powder refined to the greatest extent and uniformly dispersed in the PI matrix, and obtain the uniformly dispersed powder; The conditions for high-speed stirring are: First, stir at a stirring speed of 2000 r / min for 60 min, and then stir at a stirring speed of 4000 r / min for 30 min.

[0052] Weigh a certain mass of the uniformly dispersed powder, and obtain the nano-aluminum oxide modified polyimide composite material through hot pressing. The forming pressure is 100 MPa, the forming temperature is 220 °C, keep the temperature and pressure for 30 min, then cool down and demold, and then perform secondary curing. The specific process of secondary curing is as follows: First, heat up from room temperature to 180 °C and keep it for 1 h, then keep it at 200 °C for 1 h, and finally keep it at 230 °C for 4 h.

[0053] Example 6

[0054] Steps (1) and (2) are the same as those in Example 1.

[0055] (3) Weigh a certain mass of the uniformly dispersed powder and obtain a nano-aluminum oxide modified polyimide composite material through hot pressing. The forming pressure is 100 MPa, the forming temperature is 220 °C, keep the temperature and pressure for 120 min, then cool down and demold, and then perform secondary curing to obtain a nano-aluminum oxide modified polyimide composite material. The specific process of secondary curing is as follows: first, heat up from room temperature to 180 °C and keep for 2 h, then keep at 220 °C for 1 h, and finally keep at 250 °C for 4 h.

[0056] Example 7

[0057] Steps (1) and (2) are the same as those in Example 1.

[0058] (3) Weigh a certain mass of the uniformly dispersed powder and obtain a nano-aluminum oxide modified polyimide composite material through hot pressing. The forming pressure is 120 MPa, the forming temperature is 240 °C, keep the temperature and pressure for 30 min, then cool down and demold, and then perform secondary curing to obtain a nano-aluminum oxide modified polyimide composite material. The specific process of secondary curing is as follows: first, heat up from room temperature to 190 °C and keep for 1 h, then keep at 210 °C for 1.5 h, and finally keep at 230 °C for 6 h.

[0059] Example 8

[0060] Steps (1) and (2) are the same as those in Example 1.

[0061] (3) Weigh a certain mass of the uniformly dispersed powder and obtain a nano-aluminum oxide modified polyimide composite material through hot pressing. The forming pressure is 130 MPa, the forming temperature is 230 °C, keep the temperature and pressure for 70 min, then cool down and demold, and then perform secondary curing to obtain a nano-aluminum oxide modified polyimide composite material. The specific process of secondary curing is as follows: first, heat up from room temperature to 190 °C and keep for 1.5 h, then keep at 200 °C for 2 h, and finally keep at 240 °C for 5 h.

[0062] Example 9

[0063] Steps (1) and (2) are the same as those in Example 1.

[0064] (3) Weigh a certain mass of the uniformly dispersed powder and obtain a nano-aluminum oxide modified polyimide composite material through hot pressing. The forming pressure is 150 MPa, the forming temperature is 230 °C, keep the temperature and pressure for 100 min, then cool down and demold, and then perform secondary curing to obtain a nano-aluminum oxide modified polyimide composite material. The specific process of secondary curing is as follows: first, heat up from room temperature to 200 °C and keep for 1 h, then keep at 205 °C for 2 h, and finally keep at 245 °C for 5 h.

[0065] Example 10

[0066] Steps (1) and (2) are the same as those in Example 1.

[0067] (3) Weigh a certain mass of the uniformly dispersed powder, and obtain the nano-alumina modified polyimide composite material through hot pressing. The forming pressure is 140 MPa, the forming temperature is 235 °C, keep the temperature and pressure for 50 min, then cool down and demold, and then perform secondary curing to obtain the nano-alumina modified polyimide composite material. The specific process of secondary curing is as follows: first heat up from room temperature to 185 °C and keep it for 2 h, then keep it at 200 °C for 2 h, and finally keep it at 230 °C for 6 h.

[0068] See Figure 2 and Figure 3 From Figure 2 and Figure 3 it can be seen that in the infrared spectrum, 1645 cm -1 corresponds to the bending vibration of H-O-H, indicating that there are hydroxyl groups on the surface of alumina before modification; 1431 cm -1 corresponds to the Si-C-H vibration of the coupling agent, indicating that the coupling agent has successfully formed a bond with the surface of alumina after modification; 816 cm -1 , 746 cm -1 correspond to the vibration of Al-O. The decrease in the intensity of the two peaks after modification indicates that the surface of alumina is coated and the vibration intensity of Al-O is weakened.

[0069] See Figure 4 From Figure 4 it can be seen that the compression fracture surface of the 30 wt% nano-Al2O3 / PI composite material is relatively rough, with pores generated by the exfoliation of alumina, the deflection of the crack propagation path, the addition of alumina absorbing the energy of crack propagation, and the increase in compressive strength.

[0070] See Figure 5 From Figure 5 it can be seen that with the increase of the alumina content, the compression strength of the composite material first increases and then decreases. The compressive strengths of the composite materials with alumina contents of 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%, and 50 wt% are 247.5 MPa, 276.6 MPa, 305.2 MPa, 313.0 MPa, 188.2 MPa, and 135.9 MPa respectively. The composite material with an alumina content of 30 wt% has the highest strength, which is 26.5% higher than that of pure PI. The addition of an appropriate amount of nano-alumina consumes the energy of crack propagation, slows down the crack propagation rate, and increases the strength and toughness of polyimide; however, when the alumina content is too high, the filler cannot be uniformly dispersed in the matrix, the pores increase, and cracks are easily generated under the action of load, affecting the strength of the composite material.

[0071] Based on the excellent properties of nanoparticles, the present invention enhances the strength of polymers. Meanwhile, the surface of nanoparticles is modified using silane coupling agents to improve the compatibility between the particles and the polymer matrix and enhance the properties of the composite material. The nano-aluminum oxide modified PI-based composite material prepared by the present invention, through compressive strength and phase analysis, is a polyimide-based composite material with excellent thermal stability and mechanical properties, and is expected to be actually used in the field of high-temperature service environments. The specific advantages are as follows: Through the surface modification of nano-aluminum oxide particles with silane coupling agents, the agglomeration of nano-Al2O3 particles is improved, and the interfacial compatibility between PI and Al2O3 is enhanced; Uniformly dispersed composite powder is obtained through high-speed mechanical stirring, ensuring that nano-Al2O3 powder is evenly distributed in the PI matrix during hot pressing, which is conducive to the formation of a network cross-linked structure. The uniformly distributed nano-filler system leads to enhanced nano-composite behavior. High interfacial adhesion can effectively transfer the load from the polymer to the filler. Nano-Al2O3 particles increase the crack propagation path and improve the mechanical properties; In addition, nano-Al2O3 particles delay the onset of thermal decomposition in the polymer and slow down the mass loss of the pyrolyzed polymer after reaching the thermal decomposition temperature, improving the thermal stability of the composite material.

[0072] The present invention regulates the ratio of nano-A l2 O3 fillers to obtain a filling ratio with excellent comprehensive properties, and explores the internal relationship between the microstructure, mechanical properties, and thermal stability. It has the characteristics of simple operation process, good repeatability, significant economic benefits, and accurate and reliable results. The prepared filler has good interfacial compatibility between PI and Al2O3, high compressive strength, and excellent thermal stability.

[0073] The surface of nano-aluminum oxide particles is modified using silane coupling agents to reduce agglomeration, enhance the interfacial interaction of nano-Al2O3-PI, and improve the mechanical properties and thermal stability of polyimide composite materials. According to the examples, multi-stage hot pressing and sintering promote the formation of a three-dimensional network cross-linked structure with dispersed alumina in the composite material. As the content of nano-aluminum oxide increases, the compressive strength of the modified Al2O3 / PI composite material first increases and then decreases. The compressive strength of the composite material with 30wt% Al2O3 is the highest, reaching 313.0 MPa, which is 26.5% higher than that of pure PI.

[0074] The technical effects of the present invention include the following three points: First, the ceramic surface improves the filler-polymer interface interaction. The nano-alumina and polyimide molecular chains interact with each other to form a three-dimensional network cross-linked structure. The steric hindrance effect of the nano-alumina hinders the movement of the macromolecular chain, effectively pins the cracks, increases the crack propagation resistance, and improves the strength of the composite material. In order to improve the dispersibility of the nanoparticles, the surface of the nano-alumina is chemically modified using a silane coupling agent. To improve the compatibility of the particles with the polymer matrix and enhance the performance of the composite material. One end of the silane coupling agent forms a bond with the surface of the nanoparticles, which can reduce the agglomeration of the particles and ensure the dispersibility of the nanoparticles in the polymer; the other end overlaps with the polymer to improve the interface interaction. Through compression strength and phase analysis, a polyimide-based composite material with excellent mechanical properties is obtained, which is expected to be used in harsh service environments. Second, secondary curing of the composite material can fully cross-link the polymer network, thereby improving the strength and stiffness of the composite material. The third is to adjust the ratio of nano-Al2O3 fillers to obtain a filling ratio with excellent comprehensive performance, optimize the composite material with a three-dimensional cross-linked network structure, and explore the intrinsic relationship between the microstructure and high mechanical properties. Nano-Al2O3 particles can delay the onset of thermal decomposition in the polymer, and slow down the mass loss rate of the pyrolyzed polymer after reaching the thermal decomposition temperature, thereby improving the thermal stability of the composite material. The uniformly distributed nano-filler system leads to enhanced nano-composite behavior, and high interfacial adhesion can effectively transfer the load from the polymer to the filler. Nano-Al2O3 particles increase the crack propagation path and improve the mechanical properties. The present invention has the characteristics of simple operation process, good repeatability, significant economic benefits, and accurate and reliable results. The prepared filler has good interface compatibility between PI and Al2O3, low porosity, and high compressive strength.

Claims

1. An integrated preparation method of a nano-aluminum oxide modified polyimide composite material, characterized in that It includes the following steps: According to the mass ratio of (50 - 90) : (10 - 50), mix the polyimide powder and the nano-alumina powder with its surface modified by a silane coupling agent evenly to obtain a mixture; wherein, the total mass fraction of the nano-alumina powder with its surface modified by a silane coupling agent and the polyimide powder is 100; Then, perform high-speed stirring on the mixture to obtain a powder with uniform and dispersed distribution; Obtain the nano-alumina modified polyimide composite material by hot pressing and secondary hot pressing curing the powder with uniform and dispersed distribution.

2. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 1, characterized in that, The particle size of the polyimide is 2 - 10 μm.

3. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 1, characterized in that, The particle size of the nano-alumina is 20 - 30 nm.

4. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 1, characterized in that The silane coupling agent is KH550, KH560, KH570, A-171 or A-172.

5. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 1, characterized in that, The nano-alumina powder with its surface modified by a silane coupling agent is prepared through the following process: dry the alumina particles and add them to ethanol, perform ultrasonic dispersion for 30 min to obtain an ethanol solution of nano-alumina; add the silane coupling agent to a mixed solution of ethanol and water, adjust the pH = 4 - 5, perform ultrasonic hydrolysis, and then add it to the ethanol solution of nano-alumina, stir and react at a temperature of 75 - 85 °C for 3 - 5 h to obtain the nano-alumina powder with its surface modified by a silane coupling agent.

6. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 5, characterized in that, The volume ratio of ethanol to water is 9:

1.

7. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 4, characterized in that, The mass of the silane coupling agent is 5 - 8% of the mass of the alumina powder.

8. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 1, characterized in that, The specific conditions for high-speed stirring are: first stir at a stirring speed of 1000 - 3000 rpm for 60 - 70 min, and then stir at a stirring speed of 3000 - 5000 rpm for 30 - 40 min.

9. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 1, characterized in that, The conditions for hot pressing are: keep the powder with uniform and dispersed distribution under a pressure of 100 - 150 MPa and a temperature of 220 - 240 °C for heat preservation and pressure holding for 30 - 120 min.

10. The integrated preparation method of the nano-aluminum oxide modified polyimide composite material according to claim 1, characterized in that, The specific process for secondary curing is: first heat up from room temperature to 180 - 200 °C and keep for 1 - 2 h, then keep at 200 - 220 °C for 1 - 2 h, and finally keep at 230 - 250 °C for 4 - 6 h.

Images