PVC (polyvinyl chloride) casing material for buried power cable and preparation method of PVC casing material

Abstract

The invention discloses a PVC (polyvinyl chloride) casing material for a buried power cable and a preparation method of the PVC casing material, and relates to the technical field of cable protection pipes. The PVC sleeve material for the buried power cable at least comprises the following raw materials in parts by weight: 100 parts of polyvinyl chloride resin, 8-12 parts of chlorinated polyethylene, 5-12 parts of dodecanedioic acid dioctyl ester, 1-3 parts of epoxidized soybean oil, 3-5 parts of a calcium-zinc composite stabilizer, 1-2 parts of calcium stearate, 8-10 parts of nitrile rubber, 5-8 parts of ethylene-vinyl acetate and 10-20 parts of a composite flame retardant. The material is prepared from the following components in parts by weight: 5-15 parts of polymer coated modified kaolin, 10-20 parts of modified nano calcium carbonate, 2-3 parts of modified boron nitride, 1-2 parts of an antioxidant and 1-2 parts of an anti-aging agent 4010. The prepared PVC casing material for the buried power cable has high temperature resistance and cold resistance, can be used in a wide temperature range, and has the advantages of good aging resistance, high insulating strength and volume resistivity, good flexibility, large compressive strength and long product life.

Description

Technical Field

[0001] The present invention relates to the technical field of cable protection tubes, and in particular to a PVC casing material for buried power cables and a preparation method thereof. Background Art

[0002] Cable protection conduit is a new type of conduit material being promoted for use in power engineering. With the rapid development of underground power cable installations, higher requirements are being placed on cable conduits. Polyvinyl chloride (PVC) is one of the most widely used general-purpose plastics in the world. PVC possesses excellent toughness, thermal stability, and electrical insulation properties, making it widely used as the insulation layer and outer sheath material for wires and cables.

[0003] PVC must play an insulating and protective role within a larger operating temperature range and have greater strength. Conventional PVC sheathing can withstand higher temperatures and has good strength, but in the process of being used as an actual product in actual scenarios, it will inevitably be affected by the special external environment. When used in saline-alkali areas, the buried cable sheath is prone to corrosion and aging. When used in high-altitude cold areas, when the temperature drops, it will cause the PVC to harden and become brittle rapidly, with poor impact resistance and prone to brittle cracking, making it difficult to adapt to extreme weather changes in high-altitude cold areas. Conventional PVC-C sheathing also has good insulation properties, but when used on high-voltage power cables, in order to meet the insulation requirements, the thickness has to be increased, which will increase the difficulty of installation and transportation. Therefore, the existing PVC sheathing materials have defects such as poor low-temperature impact resistance, poor corrosion resistance, and insufficient insulation performance, which makes the use of this technology very limited. Summary of the Invention

[0004] The purpose of the present invention is to provide a PVC sheath material for buried power cables and a preparation method thereof, to solve the following technical problems: Existing PVC sheath materials used in buried power cables have problems such as poor low-temperature impact resistance, poor corrosion resistance, and insufficient insulation performance.

[0005] The purpose of the present invention can be achieved through the following technical solutions: A PVC casing material for buried power cables, comprising at least the following raw materials in parts by weight: The invention comprises 100 parts of polyvinyl chloride resin, 8-12 parts of chlorinated polyethylene, 5-12 parts of dioctyl dodecane dicarboxylate, 1-3 parts of epoxy soybean oil, 3-5 parts of a calcium-zinc composite stabilizer, 1-2 parts of calcium stearate, 8-10 parts of nitrile rubber, 5-8 parts of ethylene-vinyl acetate, 10-20 parts of a composite flame retardant, 5-15 parts of polymer-coated modified kaolin, 10-20 parts of modified nano-calcium carbonate, 2-3 parts of modified boron nitride, 1-2 parts of an antioxidant, and 1-2 parts of an antioxidant 4010. The polymer-coated modified kaolin is prepared by surface-pretreated kaolin, adjusting the pH value with a sodium lauryl sulfate solution, mixing the kaolin with potassium persulfate, butyl acrylate, and methyl methacrylate, and performing a post-treatment process. The surface-pretreated kaolin is prepared by mixing kaolin with a silane coupling agent and performing a post-treatment process.

[0006] As a further embodiment of the present invention, the preparation method of the polymer-coated modified kaolin comprises the following steps: adding kaolin to the hydrolyzed solution of the silane coupling agent, reacting, filtering, drying, and grinding to obtain surface pretreated kaolin; The pH of the sodium lauryl sulfate solution is adjusted to 8-9, and the surface pretreated kaolin is added. After dispersion, potassium persulfate, butyl acrylate and methyl methacrylate are added. After reaction, filtration and drying, the polymer-coated modified kaolin is obtained.

[0007] As a further solution of the present invention: the mass ratio of the surface pretreated kaolin, the potassium persulfate, the butyl acrylate and the methyl methacrylate is 100:0.8-1.5:10-30:10-30.

[0008] As a further solution of the present invention: the mass ratio of the kaolin to the silane coupling agent is 100:0.5-1.5.

[0009] As a further embodiment of the present invention, the concentration of the sodium lauryl sulfate solution is 2-3 g / L.

[0010] As a further solution of the present invention: the composite flame retardant is composed of 1-3 parts of microcapsule red phosphorus, 3-5 parts of zinc borate and 10-15 parts of magnesium hydroxide.

[0011] As a further solution of the present invention: the polyvinyl chloride resin is composed of 40-60 parts of a polyvinyl chloride resin with a degree of polymerization of 2000-2500 and 40-60 parts of a polyvinyl chloride resin with a degree of polymerization of 1000-1500.

[0012] As a further embodiment of the present invention: the antioxidant is composed of 0.4-0.8 parts of antioxidant 1010 and 0.2-0.4 parts of antioxidant 168.

[0013] As a further solution of the present invention: the modified nano-calcium carbonate is titanate-modified nano-calcium carbonate, and the mass ratio of the nano-calcium carbonate to the titanate coupling agent is 100:1-2.

[0014] As a further solution of the present invention: the modified boron nitride is boron nitride modified with a titanate coupling agent, and the mass ratio of the boron nitride to the titanate coupling agent is 100:1-2.

[0015] A method for preparing PVC sheathing material for buried power cables, comprising at least the following preparation steps: Add polyvinyl chloride resin, chlorinated polyethylene, dioctyl dodecane dicarboxylate, epoxy soybean oil, calcium zinc composite stabilizer and calcium stearate into a high-speed mixer, and then add polymer-coated modified kaolin, modified nano calcium carbonate and modified boron nitride to obtain a premix; Nitrile-butadiene rubber, ethylene-vinyl acetate, composite flame retardant, antioxidant and antioxidant 4010 are added to the premix, mixed and then extruded into granules, granulated and dried to obtain PVC sheath material for buried power cables.

[0016] Beneficial effects of the present invention: The PVC sheathing material for the buried power cable prepared in the present invention is prepared by selecting polyvinyl chloride with two polymerization degrees, compounding chlorinated polyethylene, a low-temperature plasticizer, a calcium-zinc composite stabilizer, calcium stearate, a composite flame retardant, an antioxidant and an antioxidant 4010, and adding a plurality of modifiers such as modified nano calcium carbonate, a cold-resistant modifier, polymer-coated modified kaolin and modified boron nitride. The polyvinyl chlorides with different polymerization degrees are melt-blended to balance the mechanical strength and processing performance of the sheathing material. The addition of the plurality of modifiers ensures that the obtained PVC sheathing material for the buried power cable has both high-temperature resistance and cold-resistant performance, can be used in a wide temperature range, has good anti-aging performance, has high insulation strength and volume resistivity, can be used at a low temperature of -50°C, has good low-temperature impact resistance and alkali corrosion resistance, and has a long product life. The PVC sheath material for buried power cables obtained by the present invention has excellent insulation, high strength, corrosion resistance and cold resistance, can be used in saline-alkali areas and severely cold areas, effectively ensures the operational reliability of cable lines, has huge economic and social benefits, and has broad application prospects.

[0017] The cold-resistant modifiers of the PVC sheathing material for buried power cables of the present invention are nitrile rubber and ethylene vinyl acetate. The ethylene vinyl acetate contains polar vinyl acetate chains that form hydrogen bonds with the cyanide groups of the nitrile rubber, improving interfacial bonding and enhancing compatibility with the PVC resin. The ethylene vinyl acetate can be considered a solid plasticizer, which can both mutually fill the gaps between PVC molecules and form a network-like distribution within the PVC matrix, effectively absorbing and fixing the plasticizer, thereby preventing or slowing its extraction or migration. The long, flexible chains of the ethylene vinyl acetate form a low-temperature elastic network with the nitrile rubber. When the material is impacted and cracks are generated, the impact stress is dispersed, thereby improving the low-temperature impact resistance of the PVC sheathing material. The low-temperature plasticizer of the present invention includes dioctyl dodecane dicarboxylate and epoxy soybean oil. The linear long-chain ester group of dioctyl dodecane dicarboxylate can be inserted between PVC molecular chains, increasing the chain spacing, significantly reducing the glass transition temperature, improving low-temperature flexibility, reducing plasticizer migration through polar action, and maintaining the durability of low-temperature performance. At the same time, the epoxy group of the epoxy soybean oil reacts with the chlorine atoms of PVC and the cyano group of the nitrile rubber to form a cross-linked network, further improving the compatibility between the nitrile rubber and the PVC substrate, achieving permanent plasticization, no precipitation, anti-migration, and significantly reducing the intermolecular force of polyvinyl chloride. The low-temperature plasticizer used in the present invention can significantly reduce the glass transition temperature, overcoming the defects of high glass transition temperature of chlorinated polyethylene and the easy generation of reverse plasticizing effect by plasticizers. The chlorine atoms in the chlorinated polyethylene further added in the present invention are compatible with the polar groups of PVC and nitrile rubber, forming a homogeneous dispersion, further reducing the problem of cracking of the nitrile rubber when added to the PVC substrate.

[0018] The present invention also adds polymer-coated modified kaolin, modified nano-calcium carbonate, and modified boron nitride. The addition of multiple modified fillers greatly improves the mechanical properties of the PVC casing material. Kaolin composite particles modified with methyl methacrylate and butyl acrylate are prepared by an emulsion method. The dispersing effect of sodium lauryl sulfate and the steric hindrance effect of the polymer chain form a uniform polymer shell on the surface of the kaolin, namely, butyl acrylate / methyl methacrylate copolymer, which changes its surface from hydrophilic to lipophilic. The polymer shell reduces the surface polarity of the kaolin, improves its compatibility with PVC, and improves the dispersibility of the kaolin in the PVC material. The polar groups (ester groups) of the polymer shell interact with the Cl in the PVC molecular chain. -The formation of hydrogen bonds or dipole effects results in high interfacial bonding strength. Kaolin particles act as stress concentrators, inducing the matrix around the particles to produce a large number of silver streaks and shear bands, absorbing a large amount of energy while preventing the silver streaks from further developing into cracks, thereby improving the impact resistance of the material. The flexible and rigid segments of the copolymer synergistically enhance the stress transfer efficiency, delay crack propagation, and further improve the impact resistance of the composite material. In addition, kaolin itself has excellent electrical insulation properties. After high-temperature calcination and loss of crystal water, many cavities are formed inside, generating a large number of adsorption centers to adsorb ionic impurities in the system, reducing the concentration of free ions in the PVC matrix, and greatly improving the electrical insulation properties of the PVC casing material. Nano-calcium carbonate and boron nitride are modified by titanate coupling agents, which improve the dispersibility of nano-calcium carbonate and boron nitride in the polyvinyl chloride matrix. Using a chemical modification method, isopropyl tri(dioctyl pyrophosphate) titanate is grafted onto the surface of boron nitride, thereby improving the dispersion and compatibility of boron nitride in the PVC matrix. Simultaneously, the titanate coupling agent acts as a "bridge" between boron nitride and the PVC matrix, increasing the interfacial bonding between boron nitride and the PVC substrate, effectively enhancing its corrosion resistance. Furthermore, the isopropyl tri(dioctyl pyrophosphate) titanate coupling agent effectively improves the surface properties of nano-calcium carbonate, transforming its hydrophilic and oleophobic properties to oleophilic and hydrophobic properties, improving its compatibility with the PVC matrix. The nano-calcium carbonate particles serve as the skeleton of the composite material. The surface modification prevents calcium carbonate from agglomerating and improves its compatibility with the PVC matrix, thereby enhancing the dimensional stability of the composite material and improving the high-temperature resistance of the PVC casing material. The flame retardant in the PVC sheathing material for buried power cables in the present invention includes microencapsulated red phosphorus, zinc borate, and magnesium hydroxide. The microencapsulated red phosphorus carbonizes at high temperatures to form a thermal insulation layer, inhibiting flame spread. The zinc borate reacts with PVC decomposition products at high temperatures to form a zinc borate glass layer, isolating oxygen and capturing free radicals to terminate the combustion chain reaction. The magnesium hydroxide decomposes and absorbs heat, lowering the material surface temperature and releasing water vapor to dilute oxygen and combustible gases. The composite flame retardant in the present invention can produce synergistic flame retardancy in the gas phase and condensed phase. Since all three are insulating materials, they do not affect the insulation performance of the cable sheathing. DETAILED DESCRIPTION

[0019] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0020] Example 1 The preparation method of polymer-coated modified kaolin includes the following steps: 100 g of kaolin was placed in an oven at 105° C. and dried for 3 h. 1.2 g of silane coupling agent KH570 was added to 95% ethanol and hydrolyzed for 1.5 h until clear to obtain a hydrolyzate of the silane coupling agent. The hydrolyzate of the silane coupling agent was added to kaolin and reacted in a water bath at 65° C. for 3 h. Surface pretreated kaolin was obtained through post-treatment steps such as filtration, drying, and grinding. Prepare 250 mL of sodium dodecyl sulfate solution with a concentration of 2.6 g / L, add ammonia water dropwise to adjust the pH to 8-9, add 100 g of the above-mentioned surface pretreated kaolin, stir electromagnetically for 30 minutes, ultrasonically disperse for 40 minutes, heat and stir, add 1 g of potassium persulfate initiator and react for a certain time, then slowly add 15 g of butyl acrylate monomer and 30 g of methyl methacrylate monomer, react at 75°C for 5 hours, cool, filter and dry to obtain polymer-coated modified kaolin.

[0021] Example 2 The preparation method of modified boron nitride comprises the following steps: 100 g of boron nitride was dispersed in a 20% sodium hydroxide solution, ultrasonicated for 1 hour, and reacted at 100°C for 16 hours. After the reaction, the solution was centrifuged at 8000 r / min for 10 minutes, washed three times, and dried to obtain activated boron nitride.

[0022] 500 g of titanate coupling agent NDZ201 was dispersed in 4500 g of isopropanol, 10 g of the above-mentioned activated boron nitride was added, ultrasonicated for 10 min, and then reacted at 70° C. for 1 h. After the reaction, centrifuged and washed 3 times, and dried to obtain modified boron nitride.

[0023] Example 3 The preparation method of modified nano calcium carbonate comprises the following steps: Add 100g of nano calcium carbonate to deionized water to prepare a slurry with a calcium carbonate mass fraction of 10%, add it to a three-necked flask, heat it to 85℃ and stir evenly; Keeping the temperature constant at 85°C, 1.5 g of titanate coupling agent NDZ201 was added into the three-necked flask, stirred for 1.5 h, filtered, dried, crushed, and sieved to obtain modified nano-calcium carbonate.

[0024] Example 4 A method for preparing a PVC sheath material for an underground power cable comprises the following steps: 40 parts by mass of a polyvinyl chloride resin having a degree of polymerization of 2000-2500, 60 parts by mass of a polyvinyl chloride resin having a degree of polymerization of 1000-1500, 8 parts by mass of chlorinated polyethylene, 3 parts by mass of a calcium-zinc composite stabilizer, 8 parts by mass of dioctyl dodecanedicarboxylate, and 2 parts by mass of epoxy soybean oil were added into a high-speed mixer and mixed at 110° C. for 5 min. 10 parts by mass of the polymer-coated modified kaolin prepared in Example 1, 15 parts by mass of the modified nano-calcium carbonate prepared in Example 3, and 2 parts by mass of modified boron nitride were then added and mixed at 120° C. for 8 min to obtain a mixture. The temperature was lowered to 90°C, and 8 parts by mass of nitrile rubber P-83, 8 parts by mass of ethylene vinyl acetate, 2 parts by mass of microcapsule red phosphorus (MRP, average particle size 19 μm), 4 parts by mass of zinc borate, 12 parts by mass of magnesium hydroxide, 0.4 parts by mass of antioxidant 1010, 0.2 parts by mass of antioxidant 168 and 1 part by mass of antioxidant 4010 were added to the above mixture. The mixture was mixed for 5 minutes, and the mixture was added to a twin-screw extruder for extrusion and granulation. The set temperatures were: 150°C for zone 1, 165°C for zone 2, 170°C for die head, 40 rpm for screw speed, and pelletized and dried to obtain PVC casing material for buried power cable.

[0025] Example 5 A method for preparing a PVC sheath material for an underground power cable comprises the following steps: 50 parts by mass of a polyvinyl chloride resin having a degree of polymerization of 2000-2500, 50 parts by mass of a polyvinyl chloride resin having a degree of polymerization of 1000-1500, 10 parts by mass of chlorinated polyethylene, 4 parts by mass of a calcium-zinc composite stabilizer, 10 parts by mass of dioctyl dodecanedicarboxylate, and 2 parts by mass of epoxy soybean oil were added into a high-speed mixer and mixed at 110° C. for 5 min. 10 parts by mass of the polymer-coated modified kaolin prepared in Example 1, 15 parts by mass of the modified nano-calcium carbonate prepared in Example 3, and 2 parts by mass of modified boron nitride were then added and mixed at 120° C. for 8 min to obtain a mixture. The temperature was lowered to 90°C, and 10 parts by mass of nitrile rubber P-83, 6 parts by mass of ethylene vinyl acetate, 2 parts by mass of microcapsule red phosphorus (MRP, average particle size 19 μm), 4 parts by mass of zinc borate, 15 parts by mass of magnesium hydroxide, 0.4 parts by mass of antioxidant 1010, 0.2 parts by mass of antioxidant 168 and 1 part by mass of antioxidant 4010 were added to the above mixture. The mixture was mixed for 5 minutes, and the mixture was added to a twin-screw extruder for extrusion and granulation. The set temperatures were: 150°C for zone one, 165°C for zone two, 170°C for die head, 40 rpm for screw speed, and pelletized and dried to obtain PVC casing material for buried power cable.

[0026] Example 6 A method for preparing a PVC sheath material for an underground power cable comprises the following steps: 60 parts by mass of a polyvinyl chloride resin having a degree of polymerization of 2000-2500, 40 parts by mass of a polyvinyl chloride resin having a degree of polymerization of 1000-1500, 12 parts by mass of chlorinated polyethylene, 3 parts by mass of a calcium-zinc composite stabilizer, 12 parts by mass of dioctyl dodecanedicarboxylate, and 3 parts by mass of epoxy soybean oil were added into a high-speed mixer and mixed at 110° C. for 5 min. 12 parts by mass of the polymer-coated modified kaolin prepared in Example 1, 15 parts by mass of the modified nano-calcium carbonate prepared in Example 3, and 3 parts by mass of modified boron nitride were then added and mixed at 120° C. for 8 min to obtain a mixture. The temperature was lowered to 90°C, and 10 parts by mass of nitrile rubber P-83, 5 parts by mass of ethylene vinyl acetate, 2 parts by mass of microcapsule red phosphorus (MRP, average particle size 19 μm), 4 parts by mass of zinc borate, 12 parts by mass of magnesium hydroxide, 0.4 parts by mass of antioxidant 1010, 0.2 parts by mass of antioxidant 168 and 1 part by mass of antioxidant 4010 were added to the above mixture. The mixture was mixed for 5 minutes, and the mixture was added to a twin-screw extruder for extrusion and granulation. The set temperatures were: 150°C for zone one, 165°C for zone two, 170°C for die head, 40 rpm for screw speed, and pelletized and dried to obtain PVC casing material for buried power cable.

[0027] Comparative Example 1 The preparation method of kaolin modified with a titanate coupling agent comprises the following steps: Place kaolin in a 105°C oven for 2 hours to dry. Weigh a certain amount of titanate coupling agent NDZ201 and hydrolyze it in 100g of 90% isopropanol at room temperature for 2 hours until the solution becomes clear, obtaining a hydrolyzed solution of titanate coupling agent NDZ201. Weigh 50g of the dried kaolin into a three-necked flask, then add 100g of the titanate coupling agent hydrolyzed solution. Place the flask in a water bath at 65°C, stirring continuously, and react for 3 hours. Then, filter and dry the solution to obtain titanate coupling agent-modified kaolin.

[0028] Comparative Example 2 Compared with Example 4, Comparative Example 2 only replaces the polymer-coated modified kaolin prepared in Example 1 in Example 4 with the surface pretreated kaolin prepared in Example 1. The other components and preparation methods are exactly the same as those in Example 4.

[0029] Comparative Example 3 Compared with Example 4, Comparative Example 3 only replaces the polymer-coated modified kaolin prepared in Example 1 added in Example 4 with the titanate coupling agent-modified kaolin prepared in Comparative Example 1. The other components and preparation methods are exactly the same as those in Example 4.

[0030] Comparative Example 4 Compared with Example 4, no modified boron nitride was added in Comparative Example 4, and the remaining components and preparation method were completely consistent with those of Example 4.

[0031] Comparative Example 5 Compared with Example 4, in Comparative Example 5, no epoxidized soybean oil was added, and the remaining components and preparation method were completely consistent with those of Example 4.

[0032] Comparative Example 6 Compared with Example 4, ethylene vinyl acetate was not added in Comparative Example 6, and the remaining components and preparation method were completely consistent with those of Example 4.

[0033] Performance testing Tensile Properties Test: Based on the GB / T1040.3-2006 standard, the casing materials obtained in Examples 4-6 and Comparative Examples 2-6 were fed into a twin-screw extruder and extruded at 200°C to form PVC casing. Dumbbell-shaped specimens with a thickness of 1±0.1 mm were cut according to the standard. The tensile speed was 250 mm / min, and each test set consisted of at least five specimens. The test results are shown in Table 1. Low-temperature impact resistance test: The tubing materials obtained in Examples 4-6 and Comparative Examples 2-6 were fed into a twin-screw extruder and co-extruded at 200°C to form PVC tubing. The tubing was placed in a -55°C low-temperature test chamber for 1 hour, then immediately removed and bent 180° at a 280mm bending diameter to observe whether the tubing had broken. Thirty specimens were tested for each formulation, with a value below 15 / 30 indicating that the tubing had passed the low-temperature brittle impact test. The test results are shown in Table 1. Hardness test: The national standard GB / T1040.3-2006 was used for testing. The casing materials obtained in Examples 4-6 and Comparative Examples 2-6 were added to a twin-screw extruder and co-extruded at 200°C to form PVC casing products. The products were cut into standard specimens. Five specimens were tested for each formulation, and the average value was calculated. The test results are shown in Table 1. Corrosion resistance test: According to GB / T 11547-2008, the sleeve materials obtained in Examples 4-6 and Comparative Examples 2-6 were added to a twin-screw extruder and co-extruded at 200°C to form PVC sleeves, which were cut into 60mm×60mm squares. Different samples were placed in an oven and taken out after 24 hours as samples before the alkali resistance test. The samples were weighed and the mass was recorded as m1. They were then placed in a 40% sodium hydroxide solution and placed at 70°C for 24 hours. They were rinsed with deionized water and dried, weighed, and the mass was recorded as m2 as the sample after the alkali resistance test. The test results are shown in Table 1. The calculation formulas for weight loss rate and dimensional change rate are as follows: Weight loss rate = (m1-m2) / m1×100% Resistivity test: The volume resistivity of the material was tested in accordance with GB / T 1410-2006. The sleeve materials obtained in Examples 4-6 and Comparative Examples 2-6 were added to a twin-screw extruder and extruded at 200°C to form a PVC sleeve. The sleeve was then cut into two 10 mm diameter specimens and measured using a high resistance meter. The test results are shown in Table 1. Thermal aging test: Referring to the standard GB / T8815-2008, the casing materials obtained in Examples 4-6 and Comparative Examples 2-6 were added to a twin-screw extruder and co-extruded at 200°C to form a PVC casing. The extruded pieces were then cut into 1*1 mm pieces and placed in a test tube, with the total volume required to be less than 1 / 3 of the test tube volume. A pH test paper was placed in the top 1 / 3 of the test tube. The test tube was sealed to ensure that it was leak-proof and then placed in a thermal stability tester. The time it took for the pH test paper to turn red was the thermal stability time. The test results are shown in Table 1. Table 1: Statistical table of performance test data of test pieces of Examples 4-6 and Comparative Examples 2-6 Tensile strength (MPa) -50℃(≤15 / 30) Shore hardness (A) Weight loss rate (%) Resistance (Ω˙m) Thermal stability time (min) Example 4 22.1 8 / 30 86 0.3 <![CDATA[6.1×10 13 ]]> 136 Example 5 20.3 7 / 30 84 0.3 <![CDATA[6.2×10 13 ]]> 134 Example 6 21.2 6 / 30 82 0.2 <![CDATA[6.5×10 13 ]]> 135 Comparative Example 2 14.1 11 / 30 80 0.6 <![CDATA[4.2×10 13 ]]> 124 Comparative Example 3 15.2 12 / 30 80 0.5 <![CDATA[4.5×10 13 ]]> 120 Comparative Example 4 19.7 9 / 30 78 1.6 <![CDATA[5.2×10 13 ]]> 133 Comparative Example 5 16.2 15 / 30 76 0.4 <![CDATA[5.8×10 13 ]]> 130 Comparative Example 6 15.4 18 / 30 78 0.4 <![CDATA[5.9×10 13 ]]> 132 As shown in Table 1, the volume resistivity of the PVC sheathing material for buried power cables prepared in Examples 4-6 of the present invention is greater than 6×10 13 Ω˙m, a low-temperature brittle temperature of -50°C, excellent mechanical properties, a good extrusion process, and good corrosion resistance, meeting the requirements for low-temperature cable production. In Comparative Examples 2 and 3, where kaolin was modified with a coupling agent alone, the resulting PVC sheathing material exhibited reduced insulation and mechanical properties, indicating that the polymer-modified kaolin in the present invention significantly enhanced its dispersion in PVC, improving the material's mechanical properties and electrical insulation. In Comparative Example 4, where no modified boron nitride was added, the resulting PVC sheathing material exhibited reduced corrosion resistance, demonstrating that the addition of modified boron nitride can significantly improve the material's alkali corrosion resistance. In Comparative Example 5, where no epoxy soybean oil was added, and Comparative Example 6, where no ethylene-vinyl acetate was added, the resulting PVC sheathing material exhibited reduced low-temperature impact resistance, demonstrating that the low-temperature plasticizer and cold-resistance modifier provided by the present invention can synergistically enhance low-temperature toughening.

[0034] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0035] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the invention as claimed.

Claims

1. A PVC sheath material for buried power cable, characterized in that: At least include the following raw materials in parts by weight: The invention comprises 100 parts of polyvinyl chloride resin, 8-12 parts of chlorinated polyethylene, 5-12 parts of dioctyl dodecane dicarboxylate, 1-3 parts of epoxy soybean oil, 3-5 parts of a calcium-zinc composite stabilizer, 1-2 parts of calcium stearate, 8-10 parts of nitrile rubber, 5-8 parts of ethylene-vinyl acetate, 10-20 parts of a composite flame retardant, 5-15 parts of polymer-coated modified kaolin, 10-20 parts of modified nano-calcium carbonate, 2-3 parts of modified boron nitride, 1-2 parts of an antioxidant, and 1-2 parts of an antioxidant 4010. The polymer-coated modified kaolin is prepared by surface-pretreated kaolin, adjusting the pH value with a sodium lauryl sulfate solution, mixing the kaolin with potassium persulfate, butyl acrylate, and methyl methacrylate, and performing a post-treatment process. The surface-pretreated kaolin is prepared by mixing kaolin with a silane coupling agent and performing a post-treatment process.

2. The PVC sheath material for buried power cable according to claim 1, characterized in that: The mass ratio of the surface pretreated kaolin, the potassium persulfate, the butyl acrylate and the methyl methacrylate is 100:0.8-1.5:10-30:10-30.

3. The PVC sheath material for buried power cable according to claim 1, characterized in that: The mass ratio of the kaolin to the silane coupling agent is 100:0.5-1.

5.

4. The PVC sheath material for buried power cable according to claim 1, characterized in that: The concentration of the sodium lauryl sulfate solution is 2-3 g / L.

5. The PVC sheath material for buried power cable according to claim 1, characterized in that: The composite flame retardant is composed of 1-3 parts of microcapsule red phosphorus, 3-5 parts of zinc borate and 10-15 parts of magnesium hydroxide.

6. The PVC sheath material for buried power cable according to claim 1, characterized in that: The polyvinyl chloride resin is composed of 40-60 parts of a polyvinyl chloride resin with a polymerization degree of 2000-2500 and 40-60 parts of a polyvinyl chloride resin with a polymerization degree of 1000-1500.

7. The PVC sheath material for buried power cable according to claim 1, characterized in that: The antioxidant is composed of 0.4-0.8 parts of antioxidant 1010 and 0.2-0.4 parts of antioxidant 168.

8. The PVC sheath material for buried power cable according to claim 1, characterized in that: The modified nano-calcium carbonate is titanate-modified nano-calcium carbonate, and the mass ratio of the nano-calcium carbonate to the titanate coupling agent is 100:1-2.

9. The PVC sheath material for buried power cable according to claim 1, characterized in that: The modified boron nitride is boron nitride modified with a titanate coupling agent, and the mass ratio of the boron nitride to the titanate coupling agent is 100:1-2.

10. A method for preparing PVC sheath material for buried power cable, characterized in that: The method comprises at least the following preparation steps: Add polyvinyl chloride resin, chlorinated polyethylene, dioctyl dodecane dicarboxylate, epoxy soybean oil, calcium zinc composite stabilizer and calcium stearate into a high-speed mixer, and then add polymer-coated modified kaolin, modified nano calcium carbonate and modified boron nitride to obtain a premix; Nitrile-butadiene rubber, ethylene-vinyl acetate, composite flame retardant, antioxidant and antioxidant 4010 are added to the premix, mixed and then extruded into granules, granulated and dried to obtain PVC sheath material for buried power cables.