High-temperature-resistant magnesium oxide electrothermal insulating material and preparation method thereof

Abstract

The invention relates to the field of insulating materials, in particular to a high-temperature-resistant magnesium oxide electric heating insulating material and a preparation method thereof, and the high-temperature-resistant magnesium oxide electric heating insulating material comprises high-purity nanometer magnesium oxide, modified silicon dioxide, modified cellulose nanofiber, nanometer aluminum oxide and zinc borate. A boric acid group on the surface of the modified silicon dioxide and an ortho-dihydroxyl group of the modified cellulose nanofiber are subjected to reversible reaction to form a dynamic boric acid ester bond, and cracks are automatically repaired. The boric acid ester performance agent is grafted on the surface of the silicon dioxide, the shell is triggered by stress, the boric acid ester repairing agent is released, and cyclic crack repairing is achieved. The modified cellulose nanofibers are excellent in toughness at low temperature, so that the fracture toughness of the insulating material is improved. The added nanometer aluminum oxide improves the thermal conductivity of the insulating material, and the nanometer aluminum oxide and the zinc borate prevent the modified cellulose nanofiber from being decomposed at high temperature. The magnesium oxide insulating material can be automatically repaired when cracking at low temperature, and the service life is prolonged.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of insulating materials, and particularly relates to a high-temperature-resistant magnesium oxide electric heating insulating material and a preparation method thereof. BACKGROUND

[0002] Magnesium oxide (MgO) has excellent high-temperature insulation, thermal conductivity, chemical stability, mechanical strength and voltage breakdown resistance, and is often used as an insulating material to prepare high-temperature-resistant fireproof cables. Compared with ordinary plastic insulated cables, it has many advantages such as fireproof, smokeless, non-toxic, high-temperature-resistant, corrosion-resistant, high mechanical strength and long service life.

[0003] However, although the cable prepared by using MgO as a traditional insulating material has stable performance, it has great brittleness, and the fracture toughness sharply decreases with the decrease of temperature, and the fracture toughness at-20 DEG C is only 60% of that at normal temperature, so there is a problem of low-temperature cracking when the cable is laid in a low-temperature environment in winter. In a low-temperature environment in winter, if the cable is laid at a corner, once the stress exceeds the critical value, brittle fracture is easily caused, cracks are formed, and because the fracture toughness of MgO sharply decreases at low temperature, once micro-cracks are formed, the cracks will rapidly expand under the action of bending stress, so that the whole material is cracked or even broken, and then the insulation resistance and dielectric strength of the insulating material sharply decrease, and the insulation fails. The cracks also interrupt the heat conduction path inside the MgO powder, and the local thermal conductivity decreases, and an overheating risk occurs. SUMMARY

[0004] (1) Technical problem to be solved The purpose of the present application is to provide a high-temperature-resistant magnesium oxide electric heating insulating material and a preparation method thereof, which can significantly improve the toughness of the magnesium oxide insulating material, so that the cracks caused by brittle fracture can be automatically repaired, insulation failure is avoided, and the service life of the cable is improved.

[0005] (2) Technical scheme To achieve the above purpose, on one hand, the present application provides a high-temperature-resistant magnesium oxide electric heating insulating material, which is characterized in that the raw materials include, in terms of mass fraction, high-purity nano magnesium oxide 150-200 parts, modified silicon dioxide 2-5 parts, modified cellulose nanofiber 2-5 parts, nano alumina 1-3 parts, and zinc borate 1-3 parts. The modified silicon dioxide includes borate performance agent 4-10 parts.

[0006] Further, the preparation method of the borate performance agent includes the following steps: S11. Catechol borate, pentaerythritol tetraacrylate are heated in a water bath, and then magnetically stirred until uniform. Then, perfluoropolyether oil is added, and ultrasonic dispersion is performed. Then, Pt@ZIF-8 catalyst is added, and centrifugal mixing is performed under nitrogen protection. Then, chloroform is added, and ultrasonic mixing is performed, to obtain an oil phase. S12. Polyvinyl alcohol is dissolved in deionized water, sodium dodecyl sulfate is added, magnetic stirring, deionized water is added to obtain an aqueous phase; S13. The oil phase is added dropwise to the aqueous phase, sheared and emulsified, and then transferred to a high-pressure homogenizer for circulation treatment to obtain a first compound; S14. Urea and formaldehyde are added to a reaction kettle, heated and stirred in a water bath, and NaOH solution is added to adjust the pH to obtain a second compound; S15. The second compound is added to the first compound, citric acid is added dropwise, and after sufficient stirring and reaction, melamine is added, and the heating and stirring reaction is continued. The solid precipitate is collected, washed with a mixture of ethanol and water, and freeze-dried to obtain a borate performance agent.

[0007] Further, the preparation method of the modified silica includes the following steps: S21. The silica is placed in a muffle furnace and activated for 4 hours, then cooled to room temperature and removed. A mixed solution of γ-aminopropyl triethoxysilane and toluene is added, and the reaction is carried out under reflux in a nitrogen atmosphere. The product is washed with toluene, ethanol and water in turn, and dried in a vacuum drying box to obtain a third compound; S22. The third compound is mixed and dispersed with 4-(6-aldehyde hexyl) phenyl boronic acid in anhydrous ethanol, and the reaction is carried out under reflux in the dark. The product is placed in a Soxhlet extractor and extracted with ethanol to obtain a fourth compound; S23. The borate performance agent is treated with NaOH to generate carboxyl groups on the surface, mixed with the fourth compound, ultrasonically dispersed, and catalyzed with carbodiimide to react at 4°C to obtain a fifth compound; S24. The fifth compound is placed in a fluidized bed CVD reactor, and a stream of perfluorooctyl triethoxysilane vapor is introduced to react at 120°C to obtain modified silica.

[0008] Further, the preparation method of the modified cellulose nanofiber includes the following steps: S31. The rice husk powder is sieved through an 80-mesh screen, heated and stirred with HCl, and washed with deionized water until neutral. NaOH and H2O2 are added, and the reaction is continued with heating and stirring. The product is washed with deionized water until neutral to obtain a first mixture; S32. Tetramethylpiperidinium oxide, NaBr, NaClO and deionized water are added to the first mixture, and the reaction is carried out with magnetic stirring in a NaOH buffer solution at pH=10. The reaction is terminated by adding HCl dropwise until the pH reaches 7. The precipitate is collected by centrifugation and filtration, and the precipitate is suspended in deionized water. The suspension is treated three times with a high-pressure homogenizer, ultrasonically broken, centrifuged, washed with ethanol, and vacuum dried to obtain a second mixture; S33. The second mixture is dispersed in water, ultrasonic mixing, slowly adding NaIO4, adjusting pH with acetic acid, stirring reaction at 4℃ in the dark, adding excess ethylene glycol, stirring reaction until deionized water dialysis to conductivity <5 μS / cm, to obtain a third mixture; S34. The third mixture is diluted and cooled with ice bath, NaBH4 is added in batches, continuous stirring and maintaining pH=9~10, after the reaction is completed, the temperature is raised to room temperature, continue to stir, neutralized to neutral with dilute hydrochloric acid, dialysis to remove borate, centrifugal washing, freeze-drying to obtain modified cellulose nanofiber.

[0009] Further, in the tetramethylpiperidine oxide oxidation reaction, sample every 30 minutes to measure pH, when pH is stable more than 9.5 and NaOH consumption rate <0.1 mL / min, the reaction is terminated.

[0010] Further, the first time treatment pressure of the high-pressure homogenizer is 50 MPa, and the second and third time treatment pressure is 100 MPa.

[0011] In another aspect, based on the same inventive concept, the application also provides a preparation method of high-temperature-resistant magnesium oxide electric heating insulation material, which is applied to the high-temperature-resistant magnesium oxide electric heating insulation material and comprises the following steps: S1. The modified cellulose nanofiber is dispersed in an ethanol solution, ultrasonic dispersion, magnetic stirring, and the pH is adjusted to neutral with ammonia water to obtain a modified cellulose nanofiber dispersion; S2. The modified silica is dispersed in anhydrous ethanol, defoaming, and placed in a high-pressure homogenizer for three times, and the pH is adjusted to neutral with acetic acid to obtain a modified silica dispersion; S3. The modified cellulose nanofiber dispersion and the modified silica dispersion are mixed at equal proportions, magnetic stirring, high-speed shearing machine shearing, and then treated with a high-pressure homogenizer, NaCl, 1-ethyl-(3-dimethylaminopropyl) carbonyl diimide / N-hydroxy succinimide mixed solution is added, after the reaction is completed, glycine is added to consume residual 1-ethyl-(3-dimethylaminopropyl) carbonyl diimide, hydroxyethyl cellulose is added, stirring until the viscosity is 480~500 mPa·s, and then filtered with a PVDF filter membrane to obtain a fourth mixture; S4. High-purity nano magnesium oxide is added to deionized water and sodium hexametaphosphate, ball-milled and mixed, the pH is adjusted with ammonia water, the fourth mixture, zinc borate and nano alumina pre-dispersed in an ethanol solution are added dropwise, placed in a high-speed shearing machine for shearing, and the product is placed in a centrifugal atomizer, dried under nitrogen protection at an inlet temperature of 160℃ and an outlet temperature of 85℃ to obtain a fifth mixture; S5. The fifth mixture is placed in an isostatic press, pressurized to 200 MPa, pressure maintained for 10 minutes, and then demolded with liquid nitrogen cooling to obtain a sixth mixture. S6. The sixth mixture is placed in a high-temperature atmosphere furnace, argon and H2 mixed gas are introduced, and programmed temperature sintering is performed, and the high-temperature resistant magnesium oxide electric heating insulation material is obtained after cooling to room temperature.

[0012] The mechanism of the above raw material components is as follows: High-purity nano magnesium oxide: The high-purity nano magnesium oxide is the main part of the insulation material, and has excellent high-temperature insulation, thermal conductivity, chemical stability, mechanical strength and voltage breakdown resistance.

[0013] Modified silicon dioxide: The silicon dioxide fills the MgO grain boundary, limits the growth of MgO grains, can inhibit brittle fracture at low temperature, and the surface of the silicon dioxide is treated by boric acid, introduces the boric acid group and the adjacent dihydroxy group provided by the modified cellulose nanofiber to form a dynamic borate ester bond. When external force causes local fracture of the magnesium oxide insulation material to form microcracks, the borate ester bond breaks to absorb energy, and restores to the boric acid group and the adjacent dihydroxy group. Because of the microcracks, water intrusion occurs, causing the fractured borate ester bond to spontaneously re-esterify, the molecular chain recombines, and the dynamic bond network is rebuilt to repair the cracks. However, due to the cracking of the magnesium oxide insulation material, water intrusion along the cracks forms an alkaline environment, and the boric acid group is easily hydrolyzed therein, resulting in the loss of the boric acid group in repeated freezing and thawing, and the repair function fails. Therefore, a borate ester performance agent needs to be grafted on the surface of the silicon dioxide. When the microcapsule shell is triggered by stress, the borate ester repair agent is released to infiltrate the crack interface and recombine the dynamic bond with the adjacent dihydroxy group, forming a new borate ester bond to repair the crack. The aldehyde component in the repair agent can react with the residual amino group to regenerate the boric acid group in situ.

[0014] Modified cellulose nanofiber: The adjacent dihydroxy group is introduced on the surface of the cellulose nanofiber to provide a reaction site for the dynamic borate ester bond, and the modified cellulose nanofiber is a natural toughening agent with excellent toughness at low temperature, which can significantly improve the fracture toughness of the magnesium oxide insulation material at low temperature.

[0015] Nano alumina: The modified cellulose nanofiber reduces the thermal conductivity of the magnesium oxide insulation material, so nano alumina is added to improve the thermal conductivity of the insulation material, and the nano alumina fills the interface between the modified cellulose nanofiber and the magnesium oxide, inhibits grain growth, and prevents the structure of the modified cellulose nanofiber from collapsing at high temperature.

[0016] Zinc borate: The modified cellulose nanofiber is easily decomposed at high temperature, and the addition of zinc borate melts to wrap the modified cellulose nanofiber at high temperature, preventing its decomposition at high temperature.

[0017] (3) Beneficial effects Compared with the prior art, the beneficial effects of the present application are: 1. The modified silica surface introduces boronic acid groups and the adjacent dihydroxy groups on the surface of the modified cellulose nanofiber to form a dynamic borate ester bond, which repairs the brittle cracks generated by the magnesium oxide insulation material at low temperature, and grafts borate ester performance agent on the surface of the silica to prevent the hydrolysis of the boronic acid groups in the repeated freeze-thaw, realize the multiple repair of the cracks, and prolong the service life of the magnesium oxide insulation material.

[0018] 2. The modified cellulose nanofiber is used as a toughening agent to improve the fracture toughness of the magnesium oxide insulation material, but it will reduce the thermal conductivity of the magnesium oxide insulation material, the addition of nano-alumina can improve the thermal conductivity of the material, and the addition of zinc borate can alleviate the decomposition of the modified cellulose nanofiber at high temperature. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 SEM image of the magnesium oxide insulation material in Example 1 of the present application at -20℃; Figure 2 SEM image of the magnesium oxide insulation material in Example 1 of the present application at 25℃. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0021] The test equipment and preparations of the following described embodiments are as follows: Electronic balance (Germany Sartorius), electric heating air constant temperature dryer (Shanghai Fuma Experimental Equipment), pH meter (Shanghai Precision Scientific Instruments), magnetic stirrer (Shanghai Meiyinqiu), Soxhelt extractor (Qingdao Juyuan), high pressure homogenizer (Kai Baisi), muffle furnace (Tianjin Zhonghuan), vacuum drying oven (Shanghai Pt), fluidized bed CVD reactor (Hangzhou Qianjiang), ball mill (Xuxinshengke), isostatic press (Tianjin Zhongtuo Technology), centrifugal atomizer (Hangzhou Xin Hai), high temperature atmosphere furnace (Bongsida), universal material testing machine (Tian Shisousen); chemical drugs and reagents were purchased from Sigma-Aldrich Company.

[0022] Example 1 The present embodiment discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises raw materials in parts by mass: high-purity nano-magnesium oxide 150 parts, modified silica 2 parts, modified cellulose nanofiber 2 parts, nano-alumina 1 part, and zinc borate 1 part. The modified silica comprises borate ester performance agent 4 parts.

[0023] The preparation method of the borate performance agent comprises the following steps: S11. The catechol borate, pentaerythritol tetraacrylate and water are mixed in a water bath, and then magnetically stirred until uniform. Then, perfluoropolyether oil is added and ultrasonically dispersed. Then, Pt@ZIF-8 catalyst is added, and the mixture is centrifuged under nitrogen protection. Then, chloroform is added and ultrasonically mixed to obtain an oil phase; S12. Polyvinyl alcohol is dissolved in deionized water, and then sodium dodecyl sulfate is added and magnetically stirred. Then, deionized water is added to obtain an aqueous phase; S13. The oil phase is added dropwise into the aqueous phase, and then sheared and emulsified. Then, the mixture is transferred into a high-pressure homogenizer for circulation treatment to obtain a first compound; S14. Urea and formaldehyde are added into a reaction kettle, and then heated and stirred in a water bath. Then, NaOH solution is added to adjust the pH to obtain a second compound; S15. The second compound is added into the first compound, and then citric acid is added dropwise. After sufficient stirring and reaction, melamine is added, and then the mixture is continuously heated and stirred. Then, the solid precipitate is collected and washed with a mixed solution of ethanol and water. Finally, the solid is freeze-dried to obtain a borate performance agent.

[0024] It should be noted that the Pt@ZIF-8 catalyst is a catalyst in which Pt nanoparticles are encapsulated in a ZIF-8 metal-organic framework.

[0025] It should be noted that the preparation of the aqueous phase is to supplement deionized water to 4 times the volume of the oil phase.

[0026] The preparation method of the modified silica comprises the following steps: S21. Silica is placed in a muffle furnace and activated for 4 hours. After cooling to room temperature, a mixed solution of γ-aminopropyltriethoxysilane and toluene is added. The mixture is refluxed under nitrogen protection, and then washed with toluene, ethanol and water in turn. Finally, the mixture is dried in a vacuum drying box to obtain a third compound; S22. The third compound and 4-(6-formylhexyl)phenylboronic acid are mixed and dispersed in anhydrous ethanol, and then subjected to dark reflux reaction. The product is placed in a Soxhlet extractor, and then extracted with ethanol to obtain a fourth compound; S23. The borate performance agent is treated with NaOH to generate carboxyl groups on the surface. Then, the fourth compound is mixed and ultrasonically dispersed. Then, carbodiimide is added for catalysis, and the mixture is reacted at 4℃ to obtain a fifth compound; S24. The fifth compound is placed in a fluidized bed CVD reactor, and then steam of perfluorooctyltriethoxysilane is introduced. The mixture is reacted at 120℃ to obtain modified silica.

[0027] It should be noted that after the magnesium oxide insulation material cracks, water enters along the cracks, forming an alkaline environment, and the boronic acid group is easily hydrolyzed in the alkaline environment, which leads to the hydrolysis and loss of the boronic acid group on the surface of the silicon dioxide in repeated freezing and thawing, so that the borate dynamic bond repair function fails. Therefore, it is necessary to re-graft a borate performance agent on the surface of the silicon dioxide, which provides a repair agent. When the magnesium oxide insulation material cracks and the crack tip stress exceeds 15 MPa, the outer layer of the borate performance agent is broken, the repair agent and the catalyst are released, a new borate bond is formed with the ortho-dihydroxy, and the aldehyde component in the repair agent can react with the residual amino group, so that the boronic acid group can be regenerated in situ, thereby realizing the multiple repair of the dynamic borate bond. The third compound is placed in a fluidized bed CVD reactor, and perfluorooctyltriethoxysilane vapor is introduced to react, so as to form a super-hydrophobic layer on the surface of the modified silicon dioxide, alleviate the hydrolysis of the borate bond, and prevent water from invading the magnesium oxide insulation material to cause insulation failure.

[0028] The preparation method of the modified cellulose nanofiber comprises the following steps: S31. The rice husk powder is passed through an 80-mesh sieve, heated and stirred with HCl, washed with centrifugation until neutral, and then NaOH and H2O2 are added for further heating and stirring reaction, and washed with deionized water until neutral to obtain a first mixture; S32. In the first mixture, tetramethylpiperidinium oxide, NaBr, NaClO, and deionized water are added, and the reaction is carried out under magnetic stirring in a NaOH buffer solution with pH=10, and the reaction is terminated by adding HCl to pH=7, and then centrifuged and filtered, and the precipitate is collected and suspended in deionized water, and then treated with a high-pressure homogenizer for three times, ultrasonically broken, centrifuged, washed with ethanol, and vacuum dried to obtain a second mixture; S33. The second mixture is dispersed in water and ultrasonically mixed, NaIO4 is slowly added, the pH is adjusted with acetic acid, and the reaction is carried out under stirring at 4℃ in the dark, an excess of ethylene glycol is added, and the reaction is carried out under stirring until the deionized water is dialyzed to a conductivity of <5 μS / cm to obtain a third mixture; S34. The third mixture is diluted and cooled with an ice bath, NaBH4 is added in batches, the pH is maintained at 9-10 during continuous stirring, the reaction is terminated after the temperature is raised to room temperature, the stirring is continued, and the neutralization is carried out with dilute hydrochloric acid, the borate salt is removed by dialysis, and the modified cellulose nanofiber is obtained by centrifugation and freeze-drying.

[0029] In the tetramethylpiperidinium oxide oxidation reaction, the pH is measured every 30 minutes, and the reaction is terminated when the pH is stable and greater than 9.5 and the NaOH consumption rate is <0.1 mL / min.

[0030] The high-pressure homogenizer is treated at a pressure of 50 MPa for the first time, and at a pressure of 100 MPa for the second and third times.

[0031] The preparation method of the high-temperature-resistant magnesium oxide electric heating insulation material comprises the following steps: S1. Disperse the modified cellulose nanofiber in an ethanol solution, ultrasonically disperse, magnetically stir, and adjust the pH to neutral with ammonia water to obtain a modified cellulose nanofiber dispersion; S2. Disperse the modified silicon dioxide in anhydrous ethanol, stand to defoam, place in a high-pressure homogenizer for three times of treatment, and adjust the pH to neutral with acetic acid to obtain a modified silicon dioxide dispersion; S3. Mix the modified cellulose nanofiber dispersion and the modified silicon dioxide dispersion at an equal ratio, magnetically stir, shear with a high-speed shearing machine, and then treat with a high-pressure homogenizer. Add NaCl and a 1-ethyl-(3-dimethylaminopropyl) carbodiimide / N-hydroxysuccinimide mixed solution. After the reaction is complete, add glycine to consume residual 1-ethyl-(3-dimethylaminopropyl) carbodiimide. Add hydroxyethyl cellulose, stir until the viscosity is 480-500 mPa·s, filter with a PVDF filter membrane, and obtain a fourth mixture; S4. Add high-purity nanometer magnesium oxide to deionized water and sodium hexametaphosphate, ball mill and mix, adjust the pH with ammonia water, drop the fourth mixture, zinc borate pre-dispersed in an ethanol solution, and nanometer alumina into the mixture, shear with a high-speed shearing machine, place the product in a centrifugal atomizer, dry under nitrogen protection at an inlet temperature of 160 DEG C and an outlet temperature of 85 DEG C to obtain a fifth mixture; S5. Place the fifth mixture in an isostatic press, pressurize to 200 MPa, keep pressure for 10 minutes, and cold shrink demold with liquid nitrogen to obtain a sixth mixture; S6. Place the sixth mixture in a high-temperature atmosphere furnace, pass in argon and H2 mixed gas, program temperature sintering, and cool to room temperature to obtain the high-temperature-resistant magnesium oxide electric heating insulation material.

[0032] It should be noted that the modified silicon dioxide and the modified cellulose nanofiber chemically react through premixing to form amide bonds, the interfacial bonding capacity is enhanced, and the bending strength and fracture toughness of the material are greatly improved.

[0033] Example 2 The example discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises raw materials in parts by mass: high-purity nanometer magnesium oxide 170 parts, modified silicon dioxide 3 parts, modified cellulose nanofiber 3 parts, nanometer alumina 2 parts, and zinc borate 2 parts. The modified silicon dioxide comprises borate ester performance agent 6 parts.

[0034] The preparation method of the modified silicon dioxide and the modified cellulose nanofiber of the example is consistent with that of example 1. The preparation method of the high-temperature-resistant magnesium oxide electric heating insulation material of the example is consistent with that of example 1.

[0035] Example 3 The embodiment discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises the following raw materials in parts by mass: high-purity nano magnesium oxide 190 parts, modified silicon dioxide 4 parts, modified cellulose nanofiber 4 parts, nano aluminum oxide 2 parts, and zinc borate 2 parts. The modified silicon dioxide comprises borate performance agent 8 parts.

[0036] The preparation method of the modified silicon dioxide and the modified cellulose nanofiber in the embodiment is consistent with that in Embodiment 1. The preparation method of the high-temperature-resistant magnesium oxide electric heating insulation material in the embodiment is consistent with that in Embodiment 1.

[0037] Embodiment 4 The embodiment discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises the following raw materials in parts by mass: high-purity nano magnesium oxide 200 parts, modified silicon dioxide 5 parts, modified cellulose nanofiber 5 parts, nano aluminum oxide 3 parts, and zinc borate 3 parts. The modified silicon dioxide comprises borate performance agent 10 parts.

[0038] The preparation method of the modified silicon dioxide and the modified cellulose nanofiber in the embodiment is consistent with that in Embodiment 1. The preparation method of the high-temperature-resistant magnesium oxide electric heating insulation material in the embodiment is consistent with that in Embodiment 1.

[0039] Embodiment 5 The embodiment is different from Embodiment 1 in that the modified silicon dioxide is not grafted with borate performance agent.

[0040] The embodiment discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises the following raw materials in parts by mass: high-purity nano magnesium oxide 150 parts, modified silicon dioxide 2 parts, modified cellulose nanofiber 2 parts, nano aluminum oxide 1 part, and zinc borate 1 part.

[0041] The preparation method of the modified silicon dioxide comprises the following steps: S21. The silicon dioxide is placed in a muffle furnace and activated for 4 hours, and then taken out after cooling to room temperature, and then a mixed solution of γ-aminopropyl triethoxysilane / toluene is added, and the mixture is refluxed under the protection of nitrogen, and then washed by centrifugation with toluene, ethanol and water in turn, and dried in a vacuum drying box to obtain a third compound; S22. The third compound is mixed and dispersed with 4-(6-aldehyde hexyl) phenyl boronic acid in anhydrous ethanol, and the mixture is refluxed in the dark, and then the product is placed in a Soxhlet extractor and extracted with ethanol to obtain a fourth compound; S23. The fourth compound is placed in a fluidized bed CVD reactor, and perfluorooctyl triethoxysilane vapor is introduced, and the mixture is reacted at 120 DEG C to obtain modified silicon dioxide.

[0042] The preparation method of the modified cellulose nanofiber of the present embodiment is consistent with that of Embodiment 1. The present embodiment is a preparation method of a high-temperature-resistant magnesium oxide electric heating insulation material, which is consistent with that of Embodiment 1.

[0043] Control group 1 The difference between the present control group and Embodiment 1 is that the present control group does not contain modified silica.

[0044] The present control group discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises raw materials in parts by mass: 150 parts of high-purity nanometer magnesium oxide, 2 parts of modified cellulose nanofiber, 1 part of nanometer aluminum oxide, and 1 part of zinc borate.

[0045] The preparation method of the modified cellulose nanofiber of the present control group is consistent with that of Embodiment 1. The present control group is a preparation method of a high-temperature-resistant magnesium oxide electric heating insulation material, which is consistent with that of Embodiment 1.

[0046] Control group 2 The difference between the present control group and Embodiment 1 is that the present control group does not contain modified cellulose nanofiber.

[0047] The present control group discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises raw materials in parts by mass: 150 parts of high-purity nanometer magnesium oxide, 2 parts of modified silica, 1 part of nanometer aluminum oxide, and 1 part of zinc borate. The modified silica comprises 4 parts of borate ester performance agent.

[0048] The preparation method of the modified silica of the present control group is consistent with that of Embodiment 1. The present control group is a preparation method of a high-temperature-resistant magnesium oxide electric heating insulation material, which is consistent with that of Embodiment 1.

[0049] Control group 3 The difference between the present control group and Embodiment 1 is that the present control group does not contain nanometer aluminum oxide and zinc borate.

[0050] The present control group discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises raw materials in parts by mass: 150 parts of high-purity nanometer magnesium oxide, 2 parts of modified silica, and 2 parts of modified cellulose nanofiber. The modified silica comprises 4 parts of borate ester performance agent.

[0051] The preparation methods of the modified silica and the modified cellulose nanofiber of the present control group are consistent with those of Embodiment 1. The present control group is a preparation method of a high-temperature-resistant magnesium oxide electric heating insulation material, which is consistent with that of Embodiment 1.

[0052] Control group 4 The present control group is a blank control group.

[0053] The present control group discloses a high-temperature-resistant magnesium oxide electric heating insulation material, which comprises raw materials in parts by mass: 150 parts of high-purity nanometer magnesium oxide.

[0054] Test verification: Specimen Preparation: The magnesium oxide insulation materials from the experimental and control groups were prepared into rectangular strip specimens measuring 50 mm x 10 mm x 1 mm in length, width, and thickness. Five specimens were prepared for each group. The fracture toughness of the materials was tested at room temperature, and the average value for each group was calculated and recorded as K0.

[0055] Low-temperature fracture toughness test: Prepare a pre-notched specimen by cutting an initial notch in the middle of the specimen with a blade. Lightly press the blade tip to create a sharp crack. Place the pre-notched specimen at -20°C for 12 hours. Observe the surface with a scanning electron microscope. Test the low-temperature fracture toughness of the material using a universal testing machine. Calculate the average value for each group, recording it as K1.

[0056] Crack repair performance test: The pre-notched specimens were placed at -20°C for 12 hours, then heated to 25°C and left to stand for 72 hours. The surface of the specimens was observed using a scanning electron microscope, and the fracture toughness of the material was tested using a universal testing machine. The average value of each group was calculated and recorded as K2.

[0057] Crack repair performance test: Place the pre-notched specimen at -20°C for 12 hours, then heat it to 25°C and let it stand for 72 hours. After the crack of the specimen is repaired, pre-make a new notch near the original crack, cool it down and heat it up again, repeat the operation 5 times, and use a universal testing machine to test the fracture toughness of the material. Calculate the average value of each group and record it as K3.

[0058] The specimen with prefabricated notch was placed at -20°C for 12 hours, then heated to 25°C and left to stand for 72 hours. After the crack of the specimen was repaired, a new notch was prefabricated near the original crack. The operation was repeated 10 times. The fracture toughness of the material was tested using a universal testing machine, and the average value of each group was calculated and recorded as K4.

[0059] Table 1. Fracture toughness data of magnesium oxide insulation materials in experimental and control groups

[0060] The fracture toughness of the magnesium oxide insulating materials in the embodiment group and the control group are shown in Table 1. It can be seen from the table that the fracture toughness of the materials with modified silica and modified cellulose nanofibers added is higher, indicating that modified silica and modified cellulose nanofibers can significantly improve the fracture toughness of magnesium oxide insulating materials. When the magnesium oxide insulating material is treated at low temperature and then heated to 25°C, the boric acid groups on the surface of the modified silica react reversibly with the ortho-dihydroxy groups of the modified cellulose nanofibers to form dynamic borate ester bonds, repairing the cracks and restoring the fracture toughness of the magnesium oxide insulating material. Figure 1 As shown in FIG, the pre-notched sample prepared in Example 1 was left at -20°C for 12 hours, and cracks were evident in the sample under the electron microscope. Figure 2As shown, when the temperature rises to 25℃, the borate ester bond plays a role, the crack is repaired, and the sample under the electron microscope has no crack. Moreover, it can be seen from the data that the borate ester performance agent plays an obvious role. After the sample is repeatedly treated by rising and falling temperature for 10 times, the comparison between Example 1 and Example 5 shows that the crack of the sample of Example 1 grafted with the borate ester performance agent is still well recovered, and the fracture toughness is obviously recovered. It is shown that the borate ester performance agent grafted on the surface of the silica releases the borate ester repair agent in its shell when the shell is triggered by stress, and the borate ester repair agent is captured by the borate groups on the surface of the silica, and a new borate ester bond is recombined, thereby realizing the cycle of “release-capture-repair” to repair the crack for multiple times.

[0061] Finally, it should be noted that although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or equivalently replace part of the technical features, and any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A high temperature resistant magnesium oxide electric thermal insulation material, characterized in that: The raw materials include, by mass: 150-200 parts of high-purity nano-magnesium oxide, 2-5 parts of modified silicon dioxide, 2-5 parts of modified cellulose nanofiber, 1-3 parts of nano-aluminum oxide, and 1-3 parts of zinc borate; The modified silica includes 4 to 10 parts of a borate performance agent.

2. The high temperature resistant magnesium oxide electric thermal insulation material according to claim 1, characterized in that: The preparation method of the borate ester performance agent comprises the following steps: S11. Catechol boric acid and pentaerythritol tetraacrylate were heated and mixed in a water bath, magnetically stirred, and then perfluoropolyether oil was added. Ultrasonic dispersion was performed, and a Pt@ZIF-8 catalyst was added. The mixture was centrifuged under nitrogen, and chloroform was added and ultrasonic mixing was performed to obtain an oil phase. S12. Dissolve polyvinyl alcohol in deionized water, add sodium lauryl sulfate, stir magnetically, and add deionized water to obtain an aqueous phase; S13. The oil phase is added dropwise to the aqueous phase, sheared and emulsified, and transferred to a high-pressure homogenizer for circulation to obtain a first compound; S14. Urea and formaldehyde were added to the reactor, heated in a water bath with stirring, and NaOH solution was added to adjust the pH to obtain a second compound; S15. Add the second compound to the first compound, add citric acid dropwise, stir and react fully, add melamine, continue heating and stirring to react, wash and collect the solid precipitate, wash with a mixed solution of ethanol and water, and freeze-dry to obtain a borate performance agent.

3. The high temperature resistant magnesium oxide electric thermal insulation material according to claim 1, characterized in that: The preparation method of the modified silicon dioxide comprises the following steps: S21. The silica was placed in a muffle furnace and activated for 4 hours, cooled to room temperature, removed, and a γ-aminopropyltriethoxysilane / toluene mixed solution was added. Under nitrogen protection, the reaction was refluxed, and the mixture was washed alternately with toluene, ethanol, and water by centrifugation and dried in a vacuum drying oven to obtain a third compound; S22. The third compound and 4-(6-formylhexyl)phenylboronic acid were mixed and dispersed in anhydrous ethanol, and the mixture was refluxed in the dark. The product was placed in a Soxhlet extractor and extracted with ethanol to obtain a fourth compound. S23. The borate ester performance agent is treated with NaOH to generate carboxyl groups on its surface, mixed with the fourth compound, ultrasonically dispersed, and reacted at 4°C with a carbodiimide catalyst to obtain a fifth compound; S24. Place the fifth compound in a fluidized bed CVD reactor, introduce perfluorooctyltriethoxysilane vapor, and react at 120° C. to obtain modified silica.

4. The high temperature resistant magnesium oxide electric thermal insulation material according to claim 1, characterized in that: The preparation method of the modified cellulose nanofiber comprises the following steps: S31. The rice husk powder was passed through an 80-mesh sieve, HCl was added, heated and stirred, centrifuged and washed until neutral, NaOH and H2O2 were added, the reaction was continued with heating and stirring, and washed with deionized water until neutral to obtain a first mixture; S32. Tetramethylpiperidinyl oxide, NaBr, NaClO, and deionized water were added to the first mixture, and the reaction was stirred magnetically in a NaOH buffer solution at pH = 10. HCl was added dropwise to terminate the reaction at pH = 7, and the reaction was centrifuged and filtered to collect the precipitate. The precipitate was suspended in deionized water and treated three times with a high-pressure homogenizer, ultrasonically broken, centrifuged, washed with ethanol, and dried in vacuo to obtain a second mixture. S33. The second mixture was dispersed in water, ultrasonically mixed, and NaIO4 was slowly added. The pH was adjusted with acetic acid. The reaction was stirred at 4°C in the dark. Excess ethylene glycol was added and the reaction was stirred until the conductivity was <5 μS / cm after dialysis with deionized water to obtain a third mixture. S34. The third mixture was diluted and cooled in an ice bath. NaBH4 was added in batches while stirring continuously and maintaining the pH at 9-10. After the reaction was completed, the mixture was warmed to room temperature while continuing to stir. The mixture was neutralized with dilute hydrochloric acid until neutral. The borate was removed by dialysis, and the mixture was washed by centrifugation and freeze-dried to obtain modified cellulose nanofibers.

5. The high temperature resistant magnesium oxide electric thermal insulation material according to claim 4, characterized in that: During the tetramethylpiperidinium oxide oxidation reaction, samples were taken every 30 minutes to measure the pH, and the reaction was terminated when the pH was stably greater than 9.5 and the NaOH consumption rate was less than 0.1 mL / min.

6. The high temperature resistant magnesium oxide electric thermal insulation material according to claim 4, characterized in that: The high-pressure homogenizer has a first treatment pressure of 50 MPa, and a second and third treatment pressure of 100 MPa.

7. A method for preparing a high-temperature resistant magnesium oxide electric thermal insulation material, which is used to prepare a high-temperature resistant magnesium oxide electric thermal insulation material according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: S1. Dispersing the modified cellulose nanofibers in an ethanol solution, ultrasonically dispersing, magnetically stirring, and adjusting the pH to neutral with aqueous ammonia to obtain a modified cellulose nanofiber dispersion; S2. The modified silica was dispersed in anhydrous ethanol, allowed to stand for defoaming, placed in a high-pressure homogenizer and treated three times, and the pH was adjusted to neutral with acetic acid to obtain a modified silica dispersion; S3. The modified cellulose nanofiber dispersion and the modified silica dispersion were mixed in equal proportions, magnetically stirred, sheared with a high-speed shear, and then treated with a high-pressure homogenizer. NaCl and a mixed solution of 1-ethyl-(3-dimethylaminopropyl)carbodiimide / N-hydroxysuccinimide were added. After the reaction was complete, glycine was added to consume the residual 1-ethyl-(3-dimethylaminopropyl)carbodiimide. Hydroxyethyl cellulose was added and stirred until the viscosity reached 480-500 mPa·s. The mixture was then filtered through a PVDF filter membrane to obtain a fourth mixture. S4. High-purity nano-magnesium oxide was added to deionized water and sodium hexametaphosphate, ball-milled, and the pH was adjusted with aqueous ammonia. The fourth mixture, zinc borate and nano-alumina pre-dispersed in an ethanol solution were added dropwise thereto. The mixture was sheared on a high-speed shearing machine. The product was placed in a centrifugal atomizer and dried under nitrogen at an inlet temperature of 160°C and an outlet temperature of 85°C to obtain a fifth mixture. S5. The fifth mixture was placed in an isostatic press, pressurized to 200 MPa, held for 10 minutes, and cooled with liquid nitrogen to give a sixth mixture; S6. Place the sixth mixture in a high-temperature atmosphere furnace, introduce a mixture of argon and H2, perform programmed temperature sintering, and cool to room temperature to obtain a high-temperature resistant magnesium oxide electric thermal insulation material.

Images