Polyvinyl chloride composite film for liquid oxygen fracturing and preparation method thereof

Abstract

The invention relates to the technical field of high polymer material compounding, in particular to a polyvinyl chloride composite film for liquid oxygen fracturing and a preparation method of the polyvinyl chloride composite film. The polyvinyl chloride composite film for liquid oxygen fracturing is prepared from the following raw materials in parts by mass: 90-95 parts of polyvinyl chloride, 5-7 parts of an antistatic agent, 8-10 parts of modified polyurethane, 6-8 parts of a temperature-resistant filler and 3-5 parts of a low-temperature plasticizer, and the antistatic agent is a modified carbon nanotube. The polyvinyl chloride composite film provided by the invention has excellent antistatic property, low-temperature property and mechanical property while meeting the application of liquid oxygen fracturing engineering, can effectively prevent the problem of oxygen leakage, and is a material solution with excellent performance.

Description

Technical Field

[0001] The present application relates to the technical field of polymer material composites, and in particular to a polyvinyl chloride composite membrane for liquid oxygen fracturing and a preparation method thereof. Background Art

[0002] The air-energy fracturing and rock-breaking process based on liquid oxygen takes advantage of the easy evaporation of liquid oxygen (1L of liquid oxygen can be gasified into 800L of oxygen) and transports the liquid oxygen to the fracturing tube in the borehole through a pipeline. The fracturing tube has a three-layer structure: the inner layer is a porous aluminum tube, the middle layer is a paper pulp adsorption layer, and the outer layer is an anti-static film. After the liquid oxygen overflows through the holes on the aluminum tube and is fully absorbed by the paper pulp, a pulse igniter is used to excite the heating wire to instantly ignite the mixture of liquid oxygen and paper pulp. The temperature in the borehole rises suddenly and the volume expands rapidly, achieving the purpose of fracturing and rock-breaking. The current outer layer of the anti-static film is easy to become brittle at low temperatures, resulting in the risk of air leakage, insufficient anti-static performance, easy to cause static sparks, low safety, and insufficient oxygen permeability. Summary of the invention

[0003] The purpose of the present application is to provide a polyvinyl chloride composite membrane for liquid oxygen fracturing and a preparation method thereof in view of the deficiencies in the current technology. The polyvinyl chloride composite membrane for liquid oxygen fracturing prepared in the present application has excellent antistatic properties, low oxygen permeability, low temperature resistance and mechanical properties, and can meet the application requirements of liquid oxygen air-energy fracturing and rock-breaking engineering.

[0004] In the first aspect, the present application provides a polyvinyl chloride composite membrane for liquid oxygen fracturing, adopting the following technical scheme: a polyvinyl chloride composite membrane for liquid oxygen fracturing, comprising the following preparation raw materials, calculated by mass: 90-95 parts of polyvinyl chloride, 5-7 parts of antistatic agent, 8-10 parts of modified polyurethane, 6-8 parts of temperature-resistant filler, and 3-5 parts of low-temperature plasticizer, wherein the antistatic agent is modified carbon nanotubes.

[0005] By adopting the above technical solution, polyvinyl chloride: as the main component of the composite film, provides the basic physical structure and chemical stability. Antistatic agent (modified carbon nanotubes): By utilizing the high conductivity of carbon nanotubes and the charge transport characteristics of polydopamine, the antistatic performance of the composite film is significantly improved. At the same time, the introduction of carbon nanotubes also enhances the mechanical properties of the material. Modified polyurethane: The hyperbranched molecular chain of modified polyurethane forms physical chain entanglements with the polyvinyl chloride molecular chain, and forms a hydrogen bond crosslinking network in polyvinyl chloride through urethane groups, playing a toughening role. At the same time, the modified polyurethane improves the denseness of the composite film and reduces the oxygen transmission rate. Temperature-resistant filler: The boron nitride nanosheets are vinyl-modified by vinyltriethoxysilane, and then in-situ polymerized with acrylic acid and glycidyl methacrylate under the initiation of potassium persulfate, so as to form a polyacrylate layer on the surface of boron nitride, significantly enhancing its compatibility and dispersibility with the polyvinyl chloride matrix. At the same time, the compatibility of polyacrylate and polyvinyl chloride enhances the interfacial bonding force between boron nitride nanosheets and polyvinyl chloride, thus playing a role in strengthening and toughening and improving the low-temperature performance. Low-temperature plasticizer: Helps to improve the low-temperature performance and processing performance of the composite film. The synergistic effect of these components in the composite film is reflected in: the antistatic agent and the modified polyurethane act together to improve the antistatic performance and mechanical properties of the composite film. The modified polyurethane and the temperature-resistant filler improve the toughness and impact strength of the material through physical chain entanglements and hydrogen bond crosslinking networks. The modified polyurethane reduces the oxygen transmission rate, while the temperature-resistant filler improves the denseness of the material. In summary, through their respective roles and the synergistic effects between them, these components jointly improve the comprehensive performance of the composite film, enabling it to meet the application requirements of the liquid oxygen fracturing rock engineering.

[0006] Preferably, the preparation method of the modified carbon nanotubes includes the following steps: S21. According to the mass parts, 100 parts of carbon nanotubes are placed in a mixed acid composed of 800 parts of nitric acid with a mass concentration of 75% and 400 parts of sulfuric acid with a mass concentration of 98%. After ultrasonic treatment for 8 - 10 h, it is cooled, diluted with deionized water and then filtered by suction. The filter cake is washed with distilled water to obtain hydroxylated carbon nanotubes; S22. According to the mass parts, 30 parts of N-aminoethyl-γ-aminopropyltriethoxysilane and 3 parts of distilled water are added to a container and mixed evenly at room temperature. After sufficient stirring at room temperature, it is slowly heated to 40 - 50 °C and reacted for 5 - 8 h to obtain a colorless transparent liquid, which is dried in vacuum to obtain amino-terminated hyperbranched polysiloxane; S23. According to the mass parts, under stirring, polydopamine, amino-terminated hyperbranched polysiloxane, and hydroxylated carbon nanotubes are successively added to isopropanol, stirred and mixed. Finally, ammonia water is added to adjust the pH of the system to 7.5, and stirring is continued for 2 - 3 h, and then left standing for 4 - 5 h to obtain modified carbon nanotubes.

[0007] By adopting the above technical solutions, the modified carbon nanotubes can effectively improve the antistatic performance of the polyvinyl chloride composite film by utilizing the high conductivity of the carbon nanotubes and the charge transport characteristics of polydopamine. The carbon nanotubes themselves have extremely high strength and modulus. After being grafted into the polyvinyl chloride composite film, they can significantly improve the mechanical properties of the material, especially the tensile properties and impact strength. Improving compatibility and dispersibility: The amino-terminated hyperbranched polysiloxane as a coupling agent helps to improve the dispersibility of the carbon nanotubes in the polyvinyl chloride matrix, reduce the agglomeration phenomenon, and at the same time increase the interfacial interaction between the carbon nanotubes and polyvinyl chloride, thereby improving the overall performance of the material.

[0008] Preferably, in step S23, the mass ratio of isopropanol, polydopamine, amino-terminated hyperbranched polysiloxane and hydroxylated carbon nanotubes is 60:17:26:(80 - 100).

[0009] Preferably, the preparation method of the modified polyurethane comprises the following steps: S41. According to the molar ratio, add 1600 parts of methanol, 36 parts of tolyl glycidyl ether, and 15 parts of 4,4'-diaminodiphenyl sulfone into a flask, heat to 54 °C, stir and react for 5 - 6 h, concentrate under reduced pressure, separate by silica gel column chromatography, and elute with a gradient of dichloromethane and methanol solution to obtain the diphenyl sulfone chain extender; S42. According to the molar ratio, add 20 parts of polycaprolactone diol into a flask, heat to 125 °C, and dehydrate under vacuum for 2 h. Cool to 70 °C, then add 48 parts of hexamethylene diisocyanate. In a nitrogen atmosphere, after reacting for 3 h, the temperature is lowered to 55 °C, add 100 parts of acetone, 10 parts of diphenyl sulfone chain extender, and 0.13 parts of dibutyltin dilaurate, react for 2 h, and concentrate under reduced pressure to obtain the modified polyurethane.

[0010] By adopting the above technical solutions, the prepared modified polyurethane has a hyperbranched three-dimensional molecular chain, forms physical chain entanglements with the polyvinyl chloride molecular chain, and the urethane groups contained in the modified polyurethane form a hydrogen bond cross-linked network in the polyvinyl chloride material, which plays a very good toughening role for polyvinyl chloride. When the material is subjected to stress and impact, the hyperbranched modified polyurethane can dissipate part of the impact energy through stress transfer, thereby improving the toughness and impact strength of the material. In addition, it also improves the denseness of the polyvinyl chloride composite film, significantly reduces the oxygen transmission rate, and prevents oxygen leakage.

[0011] Preferably, the preparation method of the temperature-resistant filler comprises the following steps: S51. According to the mass parts, disperse 10 parts of boron nitride nanosheets in 10 parts of ethanol aqueous solution with a concentration of 75% and obtain a boron nitride suspension after ultrasonic dispersion. Add 10 parts of tannic acid and 0.5 part of vinyltriethoxysilane to the boron nitride suspension, mix and stir to react. After the reaction is completed, filter, wash and dry to obtain modified boron nitride nanosheets for standby; S52. According to the mass parts, add the modified boron nitride nanosheets into N,N-dimethylformamide. Under nitrogen protection, add acrylic acid and glycidyl methacrylate, heat up to 75-80 °C, add potassium persulfate, stir and react for 24 h. After the reaction is completed, filter, wash the filter cake with absolute ethanol and vacuum dry at 50 °C to obtain a temperature-resistant filler.

[0012] By adopting the above technical solution, the temperature-resistant filler first carries out vinyl modification on the boron nitride nanosheets through vinyltriethoxysilane, introducing unsaturated double bonds on the surface of the boron nitride nanosheets. Then, under the initiation of potassium persulfide, acrylic acid and glycidyl methacrylate are used as polymerization monomers for in-situ polymerization to obtain polymethacrylate-grafted boron nitride nanosheets. A compatibilizer polyacrylate is grafted in-situ on the surface of the boron nitride nanosheets. Since polyacrylate has good compatibility with polyvinyl chloride, the dispersibility of the boron nitride nanosheets in the polyvinyl chloride matrix is increased, and the interfacial bonding force between the boron nitride nanosheets and polyvinyl chloride is greatly improved, thus playing a role in enhancing toughness and improving low-temperature performance.

[0013] Preferably, the mass parts ratio of the modified boron nitride nanosheets, N,N-dimethylformamide, acrylic acid, glycidyl methacrylate and potassium persulfate is 10:150:0.6-0.8:0.9-1.1:0.012.

[0014] Preferably, the low-temperature plasticizer is composed of dioctyl phthalate and diisobutyl adipate in a mass parts ratio of 3:2.

[0015] By adopting the above technical solution, dioctyl phthalate (DOP): DOP has excellent flexibility and cold resistance. Under low-temperature conditions, DOP can effectively reduce the glass transition temperature (Tg) of polyvinyl chloride, so that the material can maintain a certain flexibility at low temperatures and prevent cracking caused by hardening. Diisobutyl adipate (DOA): DOA also has good flexibility and can keep the material soft at lower temperatures. In addition, DOA can also improve the thermal stability of the material and prevent decomposition during high-temperature processing. The synergistic effect of DOP and DOA enables the polyvinyl chloride composite film to exhibit better flexibility and anti-cracking ability under low-temperature conditions. The main contribution of DOP lies in its low-temperature flexibility, while DOA provides additional thermal stability and processing performance. The addition of plasticizers can improve the fluidity of polyvinyl chloride, making it easier to process and form. The addition of plasticizers can also optimize the mechanical properties of polyvinyl chloride, such as tensile strength and impact toughness. The synergistic effect of DOP and DOA enables the material to maintain a certain mechanical strength while keeping flexibility. In summary, through the synergistic effect of dioctyl phthalate (DOP) and diisobutyl adipate (DOA), the low-temperature plasticizer significantly improves the flexibility, thermal stability and chemical stability of the polyvinyl chloride composite film under low-temperature conditions, thus better meeting the requirements of the liquid oxygen fracturing project.

[0016] In a second aspect, the present application provides a method for preparing a polyvinyl chloride composite film for liquid oxygen fracturing, adopting the following technical solution: As a general technical concept, the present application also provides the above method for preparing a polyvinyl chloride composite film for liquid oxygen fracturing, including the following steps: S81. According to the mass parts, add polyvinyl chloride, antistatic agent, modified polyurethane, temperature-resistant filler and low-temperature plasticizer into a high-speed mixer, and mix evenly at 100 - 120 °C to obtain a mixture; S82. Feed the mixture into a twin-screw extruder through an automatic feeding system for melt extrusion, and then adopt an extrusion blow molding process to obtain a polyvinyl chloride composite film for liquid oxygen fracturing with a thickness of 1 - 1.5 mm.

[0017] Preferably, the working temperatures of each zone of the twin-screw extruder are successively the temperature of the first zone 170 - 180 °C, the temperature of the second zone 170 - 180 °C, the temperature of the third zone 180 - 190 °C, the temperature of the fourth zone 180 - 190 °C, the temperature of the fifth zone 190 - 200 °C, and the temperature of the sixth zone 180 - 190 °C.

[0018] Preferably, the temperature control of the extrusion blow molding process is 180 - 190 °C.

[0019] In summary, the beneficial technical effects of the present application: 1. Improvement in antistatic performance: By using modified carbon nanotubes as antistatic agents, the composite film has excellent antistatic performance. The conductivity of carbon nanotubes and the charge transport characteristics of polydopamine work together, enabling the composite film to have good antistatic ability while maintaining mechanical properties.

[0020] 2. Optimization of low-temperature performance: By introducing temperature-resistant fillers and modified polyurethane, the composite film has good low-temperature performance. The hyperbranched three-dimensional molecular chains and urethane groups of modified polyurethane can effectively dissipate impact energy when the material is subjected to stress and impact, improving the toughness and impact strength of the material. At the same time, the use of temperature-resistant fillers also enhances the low-temperature resistance of the material.

[0021] 3. Enhancement of mechanical properties: The introduction of modified polyurethane and carbon nanotubes significantly improves the mechanical properties of the composite film, especially the tensile properties and impact strength. At the same time, by using amino-terminated hyperbranched polysiloxane as a coupling agent, the dispersion of carbon nanotubes in the polyvinyl chloride matrix is improved, reducing the agglomeration phenomenon and enhancing the overall performance of the material.

[0022] 4. Reduction of oxygen permeability: The hyperbranched structure of modified polyurethane improves the denseness of the material, significantly reducing the oxygen permeability and effectively preventing the problem of oxygen leakage. This is crucial for the application of liquid oxygen fracturing because oxygen leakage will affect the fracturing effect.

[0023] 5. Enhancement of interfacial interaction: By vinyl-modifying boron nitride nanosheets with vinyltriethoxysilane and carrying out in-situ polymerization under the initiation of potassium persulfide, the interfacial bonding force between boron nitride nanosheets and polyvinyl chloride is increased. This enhanced interfacial interaction helps to improve the reinforcement and toughening effect and low-temperature resistance of the material. Detailed implementation manners

[0024] The following will describe the implementation schemes of the present application in detail in combination with examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified in the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0025] In the following examples and preparation examples, 1 part by mass means 100 g; 1 part by mole means 1 mmol.

[0026] Preparation Example 1 Preparation of modified carbon nanotubes The preparation method of modified carbon nanotubes includes the following steps: S21, placing 100 parts of carbon nanotubes in a mixed acid consisting of 800 parts of 75% nitric acid and 400 parts of 98% sulfuric acid according to their mass fractions, cooling after ultrasonic treatment for 9 hours, diluting with deionized water and filtering, washing the filter cake with distilled water to obtain hydroxylated carbon nanotubes; S22, according to the mass fractions, add 30 parts of N-aminoethyl-γ-aminopropyltriethoxysilane and 3 parts of distilled water into a container and mix them evenly at room temperature, stir them thoroughly at room temperature, then slowly heat up to 45°C, react for 7 hours to obtain a colorless transparent liquid, and vacuum dry to obtain amino-terminated hyperbranched polysiloxane; S23. According to the mass parts, 17 parts of polydopamine, 26 parts of amino-terminated hyperbranched polysiloxane and 90 parts of hydroxylated carbon nanotubes were added to 60 parts of isopropanol in a stirring state, and the mixture was stirred. Finally, ammonia water was added to adjust the pH value of the system to 7.5. The mixture was stirred for 2.3 hours and then allowed to stand for 4.5 hours to obtain modified carbon nanotubes.

[0027] Preparation Example 2 The preparation method of modified polyurethane comprises the following steps: S41, according to the molar fractions, add 1600 parts of methanol, 36 parts of cresyl glycidyl ether, and 15 parts of 4,4'-diaminodiphenyl sulfone into a flask, heat to 54°C, stir and react for 5.6 hours, concentrate under reduced pressure, separate by silica gel column chromatography, and gradient elute with dichloromethane and methanol solution to obtain a diphenyl sulfone chain extender; S42. According to the molar fraction, 20 parts of polycaprolactone diol were added into the flask, heated to 125°C, and vacuum dehydrated for 2 hours, then cooled to 70°C, and 48 parts of hexamethylene diisocyanate were added. After reacting for 3 hours in a nitrogen atmosphere, the temperature was lowered to 55°C, 100 parts of acetone, 10 parts of diphenyl sulfone chain extender, and 0.13 parts of dibutyltin dilaurate were added, and the reaction was continued for 2 hours. The mixture was concentrated under reduced pressure to obtain a modified polyurethane.

[0028] Preparation Example 3 Preparation of temperature-resistant filler The preparation method of the heat-resistant filler comprises the following steps: S51. By mass parts, disperse 10 parts of boron nitride nanosheets in 10 parts of an ethanol aqueous solution with a concentration of 75% to obtain a boron nitride suspension after ultrasonic dispersion. Add 10 parts of tannic acid and 0.5 part of vinyltriethoxysilane to the boron nitride suspension, and mix and stir to cause a reaction. After the reaction is completed, perform suction filtration, washing, and drying to obtain modified boron nitride nanosheets for standby; S52. By mass parts, add 10 parts of the modified boron nitride nanosheets to 150 parts of N,N-dimethylformamide. Under nitrogen protection, add 0.7 part of acrylic acid and 1 part of glycidyl methacrylate, heat up to 78 °C, add 0.012 part of potassium persulfate, and stir and react for 24 h. After the reaction is completed, filter, wash the filter cake with absolute ethanol, and then vacuum dry at 50 °C to obtain a temperature-resistant filler.

[0029] Example 1 A polyvinyl chloride composite film for liquid oxygen fracturing, by mass parts, comprises the following preparation raw materials: 90 parts of polyvinyl chloride, 5 parts of antistatic agent, 8 parts of modified polyurethane, 6 parts of temperature-resistant filler, and 3 parts of low-temperature plasticizer. Among them, the antistatic agent is modified carbon nanotubes, and the low-temperature plasticizer is composed of dioctyl phthalate and diisobutyl adipate in a mass ratio of 3:2.

[0030] The preparation method of the above polyvinyl chloride composite film for liquid oxygen fracturing comprises the following steps: S81. By mass parts, add polyvinyl chloride, antistatic agent, modified polyurethane, temperature-resistant filler, and low-temperature plasticizer into a high-speed mixer, and mix evenly at 100 °C to obtain a mixture; S82. Feed the mixture into a twin-screw extruder through an automatic feeding system for melt extrusion, and then adopt an extrusion blow molding process to obtain a polyvinyl chloride composite film for liquid oxygen fracturing with a thickness of 1 mm. The working temperatures of each zone of the twin-screw extruder are 170 °C for the first zone, 170 °C for the second zone, 180 °C for the third zone, 180 °C for the fourth zone, 190 °C for the fifth zone, and 180 °C for the sixth zone; the temperature control of the extrusion blow molding process is 180 °C.

[0031] Example 2 A polyvinyl chloride composite film for liquid oxygen fracturing, by mass parts, comprises the following preparation raw materials: 95 parts of polyvinyl chloride, 7 parts of antistatic agent, 10 parts of modified polyurethane, [X] parts of temperature-resistant filler, and 5 parts of low-temperature plasticizer. Among them, the antistatic agent is modified carbon nanotubes, and the low-temperature plasticizer is composed of dioctyl phthalate and diisobutyl adipate in a mass ratio of 3:2.

[0032] The preparation method of the above polyvinyl chloride composite film for liquid oxygen fracturing comprises the following steps: S81. According to the parts by mass, add polyvinyl chloride, antistatic agent, modified polyurethane, temperature-resistant filler and low-temperature plasticizer into a high-speed mixer, and mix evenly at 120 °C to obtain a mixture; S82. Feed the mixture into a twin-screw extruder through an automatic feeding system for melt extrusion, and then adopt an extrusion blow molding process to obtain a polyvinyl chloride composite film with a thickness of 1.5 mm for liquid oxygen fracturing. The working temperatures of each zone of the twin-screw extruder are as follows: the temperature of the first zone is 180 °C, the temperature of the second zone is 180 °C, the temperature of the third zone is 190 °C, the temperature of the fourth zone is 190 °C, the temperature of the fifth zone is 200 °C, and the temperature of the sixth zone is 190 °C; the temperature control of the extrusion blow molding process is 190 °C.

[0033] Example 3 A polyvinyl chloride composite film for liquid oxygen fracturing, by mass, comprises the following preparation raw materials: 93 parts of polyvinyl chloride, 6 parts of antistatic agent, 7 parts of modified polyurethane, 7 parts of temperature-resistant filler, 4 parts of low-temperature plasticizer. Among them, the antistatic agent is modified carbon nanotubes, and the low-temperature plasticizer is composed of dioctyl phthalate and diisobutyl adipate in a mass ratio of 3:2.

[0034] The preparation method of the above polyvinyl chloride composite film for liquid oxygen fracturing comprises the following steps: S81. According to the parts by mass, add polyvinyl chloride, antistatic agent, modified polyurethane, temperature-resistant filler and low-temperature plasticizer into a high-speed mixer, and mix evenly at 110 °C to obtain a mixture; S82. Feed the mixture into a twin-screw extruder through an automatic feeding system for melt extrusion, and then adopt an extrusion blow molding process to obtain a polyvinyl chloride composite film with a thickness of 1.3 mm for liquid oxygen fracturing. The working temperatures of each zone of the twin-screw extruder are as follows: the temperature of the first zone is 175 °C, the temperature of the second zone is 175 °C, the temperature of the third zone is 185 °C, the temperature of the fourth zone is 190 °C, the temperature of the fifth zone is 195 °C, and the temperature of the sixth zone is 185 °C; the temperature control of the extrusion blow molding process is 185 °C.

[0035] Comparative Example 1 Same as Example 3, except that carbon nanotubes with equal mass parts are used instead of modified carbon nanotubes.

[0036] Comparative Example 2 Same as Example 3, except that boron nitride nanosheets with equal mass parts are used instead of the temperature-resistant filler.

[0037] Comparative Example 3 Same as Example 3, except that there are 100 parts of polyvinyl chloride and 0 part of modified polyurethane.

[0038] Comparative Example 4 Same as Example 3, except that the low-temperature plasticizer is dioctyl phthalate.

[0039] Comparative Example 5 Same as Example 3, except that the low-temperature plasticizer is diisobutyl adipate.

[0040] Performance Test Samples of the polyvinyl chloride composite films for liquid oxygen fracturing prepared in Examples 1 - 3 and Comparative Examples 1 - 5 were taken for the following performance tests. Three parallel samples were taken for each group, and the results were averaged. The test results are shown in Table 1. Surface resistance: Tested according to GB / T1410-2006 "Test Methods for Volume Resistivity and Surface Resistivity of Plastics". Notched impact strength: Tested in accordance with GB / T1843-1996. Tensile strength and elongation at break: Tested according to the standard GB / T10470-1992. Low-temperature resistance: The test strips were cooled to -196 °C with liquid nitrogen, and the tensile strength and elongation at break were immediately tested after maintaining a constant temperature for 24 hours. Oxygen transmission rate: Determined according to the standard GB / T 1038-2022 "Test Methods for Gas Permeability of Plastic Films and Sheets".

[0041] Table 1 Performance Test Analyzing the data in Table 1, it can be seen that: 1) The polyvinyl chloride composite films for liquid oxygen fracturing prepared in Examples 1 - 3 have excellent antistatic performance, low oxygen permeability, low-temperature resistance and mechanical properties, and can meet the application requirements of the air energy fracturing rock engineering with liquid oxygen.

[0042] 2) Through the comparative analysis of the performance of the polyvinyl chloride composite films for liquid oxygen fracturing prepared in Example 3 and Comparative Example 1, it shows that the modified carbon nanotubes prepared in this application can effectively improve the antistatic performance of the polyvinyl chloride composite film by utilizing the high conductivity of carbon nanotubes and the charge transport characteristics of polydopamine. Carbon nanotubes themselves have extremely high strength and modulus. After grafting into the polyvinyl chloride composite film, they can significantly improve the mechanical properties of the material, especially the tensile properties and impact strength. Improving compatibility and dispersibility: As a coupling agent, amino-terminated hyperbranched polysiloxane helps to improve the dispersibility of carbon nanotubes in the polyvinyl chloride matrix, reduce the agglomeration phenomenon, and at the same time increase the interfacial interaction between carbon nanotubes and polyvinyl chloride, thereby improving the overall performance of the material.

[0043] 3) Comparative analysis of the properties of the polyvinyl chloride composite film prepared by combining Example 3 and Comparative Example 2 shows that the temperature-resistant filler prepared in this application first modifies the surface of boron nitride nanosheets with vinyltriethoxysilane to introduce unsaturated double bonds on the surface of boron nitride nanosheets. Then, under the initiation of potassium persulfide, acrylic acid and glycidyl methacrylate are used as polymerization monomers for in-situ polymerization to obtain polymethacrylate-grafted boron nitride nanosheets. A compatibilizer polyacrylate is grafted in-situ on the surface of boron nitride nanosheets. Since polyacrylate has good compatibility with polyvinyl chloride, the dispersibility of boron nitride nanosheets in the polyvinyl chloride matrix is increased, and the interfacial bonding force between boron nitride nanosheets and polyvinyl chloride is greatly improved, thus playing a role in enhancing toughness and improving low-temperature performance.

[0044] 4) Comparative analysis of the properties of the polyvinyl chloride composite film prepared by combining Example 3 and Comparative Example 3 shows that the modified polyurethane prepared in this application has a hyperbranched three-dimensional molecular chain, which forms physical chain entanglement with the polyvinyl chloride molecular chain. In addition, the urethane groups contained in the modified polyurethane form a hydrogen bond crosslinking network in the polyvinyl chloride material, which plays a very good toughening role for polyvinyl chloride. When the material is subjected to stress and impact, the hyperbranched modified polyurethane can dissipate part of the impact energy through stress transfer, thereby improving the toughness and impact strength of the material. In addition, it also improves the compactness of the polyvinyl chloride composite film, significantly reduces the oxygen permeability rate, and prevents oxygen leakage.

[0045] 5) Comparative analysis of the properties of the polyvinyl chloride composite film prepared by combining Example 3 and Comparative Examples 4-5 shows that the low-temperature plasticizer is composed of dioctyl phthalate and diisobutyl adipate in a mass ratio of 3:2. By utilizing the synergistic effect between dioctyl phthalate (DOP) and diisobutyl adipate (DOA), the flexibility, thermal stability and chemical stability of the polyvinyl chloride composite film at low temperatures are significantly improved, thus better meeting the requirements of the liquid oxygen fracturing project.

[0046] The above embodiments are only used to explain the technical solutions of this application and are not intended to limit them. Although the above embodiments have specifically described this application, those skilled in the art should understand that the specific implementation manners of the present invention can still be modified or equivalently replaced. Any modification and equivalent replacement without departing from the spirit and scope of this application shall be covered by the protection scope of this application.

Claims

1. A polyvinyl chloride composite membrane for liquid oxygen fracturing, characterized in that: The preparation raw materials include the following by weight: 90-95 parts of polyvinyl chloride, 5-7 parts of antistatic agent, 8-10 parts of modified polyurethane, 6-8 parts of heat-resistant filler and 3-5 parts of low-temperature plasticizer, wherein the antistatic agent is modified carbon nanotubes.

2. A polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 1, characterized in that: The method for preparing the modified carbon nanotubes comprises the following steps: S21, placing 100 parts of carbon nanotubes in a mixed acid consisting of 800 parts of 75% nitric acid and 400 parts of 98% sulfuric acid according to their mass fractions, ultrasonicating for 8-10 hours, cooling, diluting with deionized water, and filtering, washing the filter cake with distilled water to obtain hydroxylated carbon nanotubes; S22, according to the mass fractions, add 30 parts of N-aminoethyl-γ-aminopropyltriethoxysilane and 3 parts of distilled water into a container and mix them evenly at room temperature, stir them thoroughly at room temperature, then slowly heat up to 40-50°C, react for 5-8h to obtain a colorless transparent liquid, and vacuum dry to obtain amino-terminated hyperbranched polysiloxane; S23. According to the mass fractions, add polydopamine, amino-terminated hyperbranched polysiloxane and hydroxylated carbon nanotubes to isopropanol in sequence under stirring, stir and mix, and finally add ammonia water to adjust the pH of the system to 7.5, continue stirring for 2-3 hours, and then let it stand for 4-5 hours to obtain modified carbon nanotubes.

3. A polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 2, characterized in that: In step S23, the mass ratio of the isopropanol, polydopamine, amino-terminated hyperbranched polysiloxane and hydroxylated carbon nanotubes is 60:17:26:(80-100).

4. A polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 1, characterized in that: The preparation method of the modified polyurethane comprises the following steps: S41, according to the molar fractions, add 1600 parts of methanol, 36 parts of cresyl glycidyl ether, and 15 parts of 4,4'-diaminodiphenyl sulfone to a flask, heat to 54°C, stir to react for 5-6 hours, concentrate under reduced pressure, separate by silica gel column chromatography, and gradient elute with dichloromethane and methanol solution to obtain a diphenyl sulfone chain extender; S42. According to the molar fraction, 20 parts of polycaprolactone diol were added into the flask, heated to 125°C, and vacuum dehydrated for 2 hours, then cooled to 70°C, and then 48 parts of hexamethylene diisocyanate were added. After reacting for 3 hours in a nitrogen atmosphere, the temperature was lowered to 55°C, 100 parts of acetone, 10 parts of diphenyl sulfone chain extender, and 0.13 parts of dibutyltin dilaurate were added, and the reaction was continued for 2 hours. The mixture was concentrated under reduced pressure to obtain a modified polyurethane.

5. A polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 1, characterized in that: The method for preparing the temperature-resistant filler comprises the following steps: S51, dispersing 10 parts of boron nitride nanosheets in 10 parts of 75% ethanol aqueous solution by weight, obtaining a boron nitride suspension after ultrasonic dispersion, adding 10 parts of tannic acid and 0.5 parts of vinyltriethoxysilane to the boron nitride suspension, mixing and stirring to react, and after the reaction is completed, filtering, washing and drying to obtain modified boron nitride nanosheets for standby use; S52. Add modified boron nitride nanosheets to N,N-dimethylformamide according to their mass fractions, add acrylic acid and glycidyl methacrylate under nitrogen protection, raise the temperature to 75-80°C, add potassium persulfate, stir and react for 24 hours, filter after the reaction is completed, wash the filter cake with anhydrous ethanol and vacuum dry it at 50°C to obtain a temperature-resistant filler.

6. A polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 5, characterized in that: The mass ratio of the modified boron nitride nanosheets, N,N-dimethylformamide, acrylic acid, glycidyl methacrylate and potassium persulfate is 10:150:0.6-0.8:0.9-1.1:0.

012.

7. A polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 1, characterized in that: The low-temperature plasticizer is composed of dioctyl phthalate and diisobutyl adipate in a mass ratio of 3:

2.

8. A method for preparing a polyvinyl chloride composite membrane for liquid oxygen fracturing according to any one of claims 1 to 7, characterized in that: The following steps are involved: S81. Add polyvinyl chloride, antistatic agent, modified polyurethane, temperature-resistant filler and low-temperature plasticizer into a high-speed mixer according to their weight proportions, and mix them at 100-120° C. to obtain a mixture; S82. The mixed material is fed into a twin-screw extruder through an automatic feeding system for melt extrusion, and then an extrusion blow molding process is adopted to obtain a polyvinyl chloride composite film with a thickness of 1-1.5 mm for liquid oxygen cracking.

9. The method for preparing a polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 8, characterized in that: The working temperatures of the zones of the twin-screw extruder are 170-180°C for the first zone, 170-180°C for the second zone, 180-190°C for the third zone, 180-190°C for the fourth zone, 190-200°C for the fifth zone and 180-190°C for the sixth zone.

10. The method for preparing a polyvinyl chloride composite membrane for liquid oxygen fracturing according to claim 8, characterized in that: The temperature of the extrusion blow molding process is controlled at 180-190°C.

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