High-flame-retardant PVC (polyvinyl chloride) pipe and preparation method thereof

Abstract

The invention relates to the technical field of high polymer materials, and particularly discloses a high-flame-retardant PVC pipe and a preparation method thereof. The high-flame-retardant PVC pipe comprises the following components in parts by weight: 100 parts of PVC resin, 10-15 parts of an inorganic flame retardant, 0.5-10 parts of a maleic anhydride grafted alpha olefin oligomer, 3-8 parts of ACR resin, 1-5 parts of a heat stabilizer, 0.5-2 parts of a lubricant and 10-30 parts of a filler. According to the present invention, through the bridge effect of the maleic anhydride grafted alpha olefin oligomer, the inorganic flame retardant is easily and uniformly dispersed in the PVC matrix, the flame retardant efficiency is completely performed, the high mechanical property is maintained, and the addition of the maleic anhydride grafted alpha olefin oligomer can significantly improve the toughness of the PVC pipe.

Description

Technical Field

[0001] This application relates to the field of polymer materials technology, and more specifically, it relates to a highly flame-retardant PVC pipe and its preparation method. Background Technology

[0002] Polyvinyl chloride (PVC) pipes are widely used in building drainage, electrical cable protection, water supply and drainage systems, and industrial pipelines due to their excellent corrosion resistance, high mechanical strength, low cost, and good plasticity. However, the limiting oxygen index (LOI) of ordinary PVC pipes is usually around 20%, classifying them as flammable materials. They are highly susceptible to combustion when near a fire source or in high-temperature environments, producing large amounts of dense smoke and toxic gases. This significantly limits their application in situations with high fire safety requirements.

[0003] To improve the flame retardant properties of PVC pipes, the industry typically uses the addition of flame retardants. Traditional flame retardants mainly include halogenated flame retardants (such as chlorinated paraffin and tetrabromobisphenol A), phosphorus-based flame retardants (such as phosphate esters and ammonium polyphosphate APP), nitrogen-based flame retardants (such as melamine), and inorganic flame retardants (such as aluminum hydroxide ATH and magnesium hydroxide MDH). Although halogenated flame retardants are highly efficient, they release large amounts of toxic and corrosive fumes during combustion, posing potential hazards to the environment and human health, and their application is increasingly restricted by stringent environmental regulations. Inorganic flame retardants are favored due to their environmental friendliness, low toxicity, and smoke-suppressing properties, but they have poor compatibility with PVC matrix resins and are prone to agglomeration in the system, resulting in uneven dispersion. To achieve the ideal flame retardant effect, large amounts of inorganic flame retardants are often added, which severely deteriorates the mechanical properties of PVC materials, especially leading to a significant decrease in impact toughness and tensile strength, making the pipes brittle and prone to brittle fracture during actual installation and use. In addition, poor compatibility between flame retardants and the matrix may cause the flame retardants to migrate and precipitate during processing or use, which not only affects the appearance of the product, but also causes the flame retardant performance to degrade over time.

[0004] To address compatibility issues, existing technologies have attempted to use compatibilizers, such as maleic anhydride-grafted polyethylene (MAH-g-PE) or maleic anhydride-grafted polypropylene (MAH-g-PP). While these compatibilizers improve the dispersibility of inorganic fillers to some extent, their high molecular weight and long molecular chains limit their migration and dispersion capabilities within the PVC matrix. Consequently, their toughening effects, particularly in improving low-temperature toughness, are often less than ideal, and they can sometimes adversely affect other mechanical properties or processing flowability.

[0005] In addition, some patented technologies (such as CN202420896740.7 and CN202422446419.1) propose multi-layer structures or surface-coated flame-retardant layers. While these methods can improve flame retardancy to some extent, they suffer from complex processes and increased costs. Furthermore, the surface coating is prone to peeling off after physical damage such as scratches or impacts, rendering its flame-retardant protective function ineffective and exposing the flammable PVC substrate inside, thus maintaining the fire risk. Other studies (such as CN202510464374.7) have explored the use of intumescent flame retardants or nanocomposite materials (such as nano-silica and carbon nanotubes) to improve flame retardancy and smoke suppression, but these methods either face cost pressures or struggle to simultaneously achieve a balance between high flame retardancy and high mechanical properties.

[0006] Therefore, developing a highly flame-retardant PVC pipe that can significantly improve flame retardant efficiency, ensure excellent mechanical properties, avoid flame retardant release, and has a simple process and controllable cost has become an urgent technical problem to be solved in this field. Summary of the Invention

[0007] To address the aforementioned technical problems, this application provides a highly flame-retardant PVC pipe and its preparation method.

[0008] In a first aspect, this application provides a high flame-retardant PVC pipe, which adopts the following technical solution: A high flame-retardant PVC pipe comprises the following components in parts by weight: 100 parts PVC resin, 10-15 parts inorganic flame retardant, 0.5-10 parts maleic anhydride grafted α-olefin oligomer, 3-8 parts ACR resin, 1-5 parts heat stabilizer, 0.5-2 parts lubricant, and 10-30 parts filler.

[0009] The addition of maleic anhydride-grafted α-olefin oligomers improves the compatibility between inorganic flame retardants and the matrix resin. While enhancing the flame retardant effect, it prevents the precipitation of inorganic flame retardants, allowing the product to maintain high strength. Furthermore, compared to maleic anhydride-grafted polyethylene, the organic molecular chains of maleic anhydride-grafted α-olefin oligomers are relatively shorter and more regular, allowing them to penetrate the PVC resin molecular backbone. This not only improves the dispersion of inorganic flame retardants but also significantly enhances the toughness of PVC resin materials through internal plasticizing.

[0010] More preferably, the maleic anhydride-grafted α-olefin oligomer is obtained by reacting maleic anhydride with α-olefin oligomer under heating conditions in the presence of a polymerization inhibitor.

[0011] Preferably, the α-olefin oligomer is a 1-hexene dimer, a 1-hexene trimer, a 1-hexene tetramer, a 1-heptene dimer, a 1-heptene trimer, a 1-octene dimer, a 1-octene trimer, or a 1-decene dimer.

[0012] Preferably, the heating temperature is 120-160℃.

[0013] Preferably, the polymerization inhibitor is at least one selected from p-hydroxyanisole, 2,5-di-tert-butylhydroquinone, and p-benzoquinone.

[0014] More preferably, the inorganic flame retardant undergoes surface treatment.

[0015] Preferably, the specific steps of the surface treatment are as follows: dispersing an inorganic flame retardant in a solvent to form a slurry, adding an aluminum source at 60-100°C, adjusting the pH to 10.5, and then performing post-treatment after the reaction is complete.

[0016] At the aforementioned temperature and pH, the aluminum source generates γ-AlOOH (boehmite) on the surface of the inorganic flame retardant, resulting in an inorganic flame retardant coated with γ-AlOOH. On one hand, the temperature gradient decomposition of magnesium hydroxide and boehmite achieves sustained flame retardancy over a wide temperature range. On the other hand, the hydroxyl groups on the surface of boehmite can react with maleic anhydride-grafted α-olefin oligomers, further improving the dispersion performance of the inorganic flame retardant, thereby achieving high content addition and excellent mechanical property retention of the inorganic flame retardant in PVC resin.

[0017] Preferably, the inorganic flame retardant is selected from at least one of aluminum hydroxide, magnesium hydroxide, antimony trioxide, and zinc borate.

[0018] Preferably, the heat stabilizer is an organotin stabilizer or a calcium-zinc composite stabilizer.

[0019] Preferably, the lubricant is selected from at least one of stearic acid, paraffin wax, and polyethylene wax.

[0020] Preferably, the filler is selected from at least one of calcium carbonate and talc.

[0021] Secondly, this application provides a method for preparing a highly flame-retardant PVC pipe, which adopts the following technical solution: A method for preparing a high flame-retardant PVC pipe includes the following preparation steps: S1. Add PVC resin, inorganic flame retardant, maleic anhydride grafted α-olefin oligomer, ACR resin, heat stabilizer, lubricant and filler to a high-speed mixer according to the formula, and heat mix at 90-120℃ for 5-15 minutes. S2, after cooling the mixed material to 40-60℃, it is fed into a twin-screw extruder; S3 controls the temperature of each section of the twin-screw extruder to 165-185℃, and obtains high flame-retardant PVC pipes through melt extrusion, granulation, shaping, cooling and cutting.

[0022] Compared to ordinary PVC resin formulations, this application has better processing performance, better melt flow, and is easier to process and mold due to the addition of maleic anhydride-grafted α-olefin oligomers.

[0023] In summary, this application has the following beneficial effects: (1) Excellent flame retardant performance: Through the bridging effect of maleic anhydride grafted α-olefin oligomer, the inorganic flame retardant is evenly dispersed in the PVC matrix, giving full play to the flame retardant performance, and the oxygen index of the pipe can reach more than 42%. (2) High mechanical properties and toughness: The addition of maleic anhydride-grafted α-olefin oligomers not only improves the dispersion of flame retardants, but its short-chain structure can also effectively penetrate the PVC molecular chain, playing an internal plasticizing role and significantly improving the material's toughness, especially its low-temperature impact resistance. The room-temperature notched impact strength of the pipe can be maintained at 42 kJ / m. 2 above; (3) Good processability: Maleic anhydride grafted α-olefin oligomers improve the rheological properties of the system, making the melt flow better and easier to process and mold. Detailed Implementation

[0024] The following specific examples illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification. This application can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this application.

[0025] Furthermore, it should be understood that the one or more method steps mentioned in this application do not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and is not intended to limit the order of the method steps or limit the scope of implementation of this application. Changes or adjustments to their relative relationships, without substantially altering the technical content, shall also be considered as within the scope of implementation of this application.

[0026] Unless otherwise specified, the experimental conditions used in the examples are generally in accordance with conventional conditions in the art or the conditions recommended by the reagent company. Unless otherwise specified, the materials and reagents used in the examples can be purchased commercially.

[0027] Example Example 1: Preparation of maleic anhydride-grafted α-olefin oligomers 52.1g of 1-hexene dimer (obtained from 1-hexene oligomerization, purchased from Apex Technology) was mixed with 40g of maleic anhydride, and 0.55g of 2,5-di-tert-butylhydroquinone polymerization inhibitor was added. The mixture was stirred continuously at 120°C for 12 hours. After the reactants were cooled, they were subjected to conventional post-treatment with bleaching clay to remove unreacted raw materials and impurities, yielding the product, namely maleic anhydride-grafted α-olefin oligomer.

[0028] The product was characterized by nuclear magnetic resonance (NMR). The NMR instrument was manufactured by Bruker GmbH, Germany, and the model was AVANCE III HD400.

[0029] 1 H NMR (400 MHz, CDCl3): δ=6.83 (dt, J = 15.3, 6.8 Hz, 1H), 5.52 (d, J= 15.3 Hz, 1H), 3.35 (dd, J = 9.5, 4.2 Hz, 1H), 2.95 (dd, J = 9.5, 4.2 Hz,1H), 2.48 (td, J = 7.3, 1.5 Hz, 2H), 2.05 (quin, J = 7.3 Hz, 2H), 1.42–1.25(m, 12H), 0.88 (t, J = 6.8 Hz, 3H).

[0030] LC-MS (ESI+): 266.2 [M+H] + .

[0031] This application uses the maleic anhydride-grafted α-olefin oligomer obtained in Example 1 as a compatibility and toughening additive to prepare PVC resin pipes.

[0032] Example 2 Preparation of maleic anhydride-grafted α-olefin oligomers 69.5g of 1-octene dimer (obtained from 1-octene oligomerization, purchased from Apex Technology) was mixed with 40g of maleic anhydride, and 0.55g of 2,5-di-tert-butylhydroquinone polymerization inhibitor was added. The mixture was stirred continuously at 140℃ for 12h. After the reactants were cooled, they were subjected to conventional post-treatment with bleaching clay to remove unreacted raw materials and impurities, yielding the product, namely maleic anhydride-grafted α-olefin oligomer.

[0033] Example 3 Preparation of maleic anhydride-grafted α-olefin oligomers 86.8g of 1-decene dimer (obtained from 1-decene oligomerization, purchased from Apex Technology) was mixed with 40g of maleic anhydride, and 0.55g of 2,5-di-tert-butylhydroquinone polymerization inhibitor was added. The mixture was stirred continuously at 160℃ for 12h. After the reactants were cooled, they were subjected to conventional post-treatment with bleaching clay to remove unreacted raw materials and impurities, yielding the product, namely maleic anhydride-grafted α-olefin oligomer.

[0034] Example 4 Inorganic flame retardant surface treatment Magnesium hydroxide with a D50 of 10 μm was used as an inorganic flame retardant for surface treatment. 200 g of magnesium hydroxide was dispersed in 1000 mL of deionized water, heated to 80 °C, and sodium aluminate solution (0.5 mol sodium aluminate content) was added dropwise at a rate of 1 mL / min with slow stirring. The pH was adjusted to 10.5, and the reaction was maintained at this temperature for 3 h. After the reaction was complete, the sample was centrifuged, washed, and dried at 100 °C for 12 h to obtain the final product.

[0035] Example 5: Preparation of PVC Pipes Formula: 100kg of Sanyou SG3 PVC resin, 10kg of magnesium hydroxide with D50=10μm, 5kg of maleic anhydride grafted α-olefin oligomer prepared in Example 1, 3kg of Braun LP-23 ACR resin, 1kg of Wandao New Materials 181 organotin heat stabilizer, 0.5kg of stearic acid, and 10kg of calcium carbonate with D50=500 mesh.

[0036] All the above raw materials are added to a high-speed hot mixer and mixed at 120°C and 1200 rpm. The mixed material is then fed into a twin-screw extruder, with the temperatures of each section controlled sequentially from the feeding section to the die at 165°C, 175°C, 185°C, 190°C, and 185°C, and the speed at 11 rpm. After melt extrusion, shaping, cooling, and cutting, PVC pipes are obtained.

[0037] Example 6: Preparation of PVC Pipes Formula: 100kg of Sanyou SG3 PVC resin, 12kg of magnesium hydroxide with D50=10μm, 5kg of maleic anhydride grafted α-olefin oligomer prepared in Example 1, 3kg of Braun LP-23 ACR resin, 1kg of Wandao New Materials 181 organotin heat stabilizer, 0.5kg of stearic acid, and 10kg of calcium carbonate with D50=500 mesh.

[0038] All the above raw materials are added to a high-speed hot mixer and mixed at 120°C and 1200 rpm. The mixed material is then fed into a twin-screw extruder, with the temperatures of each section controlled sequentially from the feeding section to the die at 165°C, 175°C, 185°C, 190°C, and 185°C, and the speed at 11 rpm. After melt extrusion, shaping, cooling, and cutting, PVC pipes are obtained.

[0039] Example 7: Preparation of PVC Pipes Formula: 100kg of Sanyou SG3 PVC resin, 15kg of magnesium hydroxide with D50=10μm, 5kg of maleic anhydride grafted α-olefin oligomer prepared in Example 1, 3kg of Braun LP-23 ACR resin, 1kg of Wandao New Materials 181 organotin heat stabilizer, 0.5kg of stearic acid, and 10kg of calcium carbonate with D50=500 mesh.

[0040] All the above raw materials are added to a high-speed hot mixer and mixed at 120°C and 1200 rpm. The mixed material is then fed into a twin-screw extruder, with the temperatures of each section controlled sequentially from the feeding section to the die at 165°C, 175°C, 185°C, 190°C, and 185°C, and the speed at 11 rpm. After melt extrusion, shaping, cooling, and cutting, PVC pipes are obtained.

[0041] Example 8: Preparation of PVC Pipes Formula: 100kg of Sanyou SG3 PVC resin, 10kg of magnesium hydroxide with D50=10μm, 5kg of maleic anhydride grafted α-olefin oligomer prepared in Example 2, 3kg of Braun LP-23 ACR resin, 1kg of Wandao New Materials 181 organotin heat stabilizer, 0.5kg of stearic acid, and 10kg of calcium carbonate with D50=500 mesh.

[0042] All the above raw materials are added to a high-speed hot mixer and mixed at 120°C and 1200 rpm. The mixed material is then fed into a twin-screw extruder, with the temperatures of each section controlled sequentially from the feeding section to the die at 165°C, 175°C, 185°C, 190°C, and 185°C, and the speed at 11 rpm. After melt extrusion, shaping, cooling, and cutting, PVC pipes are obtained.

[0043] Example 9: Preparation of PVC Pipes Formula: 100kg of Sanyou SG3 PVC resin, 10kg of magnesium hydroxide with D50=10μm, 5kg of maleic anhydride grafted α-olefin oligomer prepared in Example 3, 3kg of Braun LP-23 ACR resin, 1kg of Wandao New Materials 181 organotin heat stabilizer, 0.5kg of stearic acid, and 10kg of calcium carbonate with D50=500 mesh.

[0044] All the above raw materials are added to a high-speed hot mixer and mixed at 120°C and 1200 rpm. The mixed material is then fed into a twin-screw extruder, with the temperatures of each section controlled sequentially from the feeding section to the die at 165°C, 175°C, 185°C, 190°C, and 185°C, and the speed at 11 rpm. After melt extrusion, shaping, cooling, and cutting, PVC pipes are obtained.

[0045] Example 10: Preparation of PVC Pipes Formula: 100kg of Sanyou SG3 PVC resin, 10kg of magnesium hydroxide treated in Example 4, 5kg of maleic anhydride grafted α-olefin oligomer prepared in Example 1, 3kg of Braun LP-23 ACR resin, 1kg of Wandao New Materials 181 organotin heat stabilizer, 0.5kg of stearic acid, and 10kg of calcium carbonate with a D50 of 500 mesh.

[0046] All the above raw materials are added to a high-speed hot mixer and mixed at 120°C and 1200 rpm. The mixed material is then fed into a twin-screw extruder, with the temperatures of each section controlled sequentially from the feeding section to the die at 165°C, 175°C, 185°C, 190°C, and 185°C, and the speed at 11 rpm. After melt extrusion, shaping, cooling, and cutting, PVC pipes are obtained.

[0047] Comparative Example Comparative Example 1: Preparation of PVC Pipes The preparation method is the same as in Example 5. The difference is that magnesium hydroxide flame retardant was not added to this comparative formulation.

[0048] Comparative Example 2: Preparation of PVC Pipes The preparation method is the same as in Example 5. The difference from Example 5 is that maleic anhydride-grafted α-olefin oligomers were not added in this comparative formulation.

[0049] Comparative Example 3: Preparation of PVC Pipes The preparation method is the same as in Example 5. The difference from Example 5 is that the maleic anhydride-grafted α-olefin oligomer in this comparative formulation is replaced with an equal mass of maleic anhydride-grafted polyethylene (Ningbo Haoxin Yulong New Materials).

[0050] Performance testing Oxygen index testing: The pipes prepared in each example and comparative example are cut and pressed into samples. The marking line of the sample is drawn 50 mm away from the ignition end. The sample surface is clean and free of defects that affect combustion behavior, such as bubbles, cracks, flash, burrs, etc. Then, the test is carried out in accordance with GB / T 2406.2-2009.

[0051] Tensile strength: Tested in accordance with GB / T 8804.2-2016.

[0052] Impact strength shall be tested in accordance with GB / T 13525-1992.

[0053] Elongation at break: Tested in accordance with GB / T5836-1996.

[0054] The test results are recorded in Table 1.

[0055] Table 1 Test Results

[0056] As can be seen from the test results in Table 1, the PVC pipes prepared in this application have excellent flame retardant properties, with limiting oxygen indexes all above 40%, as well as good mechanical properties such as high strength and high toughness.

[0057] Compared to Example 5, Comparative Example 1, which did not contain magnesium hydroxide flame retardant, had a significantly lower oxygen index of only 23.5%, indicating a substantial decrease in the flame retardant properties of the material, as well as a slight decrease in its strength and impact resistance.

[0058] Compared to Example 5, Comparative Example 2, which did not contain maleic anhydride-grafted α-olefin oligomers, had an oxygen index that decreased to 38.1%. The tensile strength, impact strength, and elongation at break of the material were all significantly reduced, indicating that the overall mechanical properties of the material were significantly weakened, especially the mechanical properties of the material.

[0059] Compared to Example 5, Comparative Example 3 used an equal mass of maleic anhydride-grafted polyethylene instead of maleic anhydride-grafted α-olefin oligomers. Its oxygen index was 40.8%, and its tensile strength was 53.9 MPa, which was lower than that of Example 3, while its impact strength was 36.1 kJ·m. 2 The elongation at break was 112%, and the toughness was significantly worse, indicating that maleic anhydride-grafted α-olefin oligomers have a significant effect on improving the toughness of the material. Compared with maleic anhydride-grafted polyethylene, α-olefin oligomers have a lower molecular weight and better migration in the PVC matrix. They are more likely to physically entangle and diffuse with PVC molecular chains, thus dispersing more uniformly in the matrix and reducing the risk of performance degradation due to the aggregation of the compatibilizer itself.

[0060] Combining the test results of Examples 5, 6, and 7, it can be found that: keeping other components constant, as the amount of magnesium hydroxide flame retardant added increases, the limiting oxygen index of the material gradually increases, indicating that the flame retardant performance is continuously enhanced. At the same time, the tensile strength and elongation at break show small fluctuations, showing that the flame retardancy of the material is further optimized while maintaining good mechanical properties. This indicates that in the presence of maleic anhydride-grafted α-olefin oligomers, increasing the amount of magnesium hydroxide flame retardant added helps to further improve the flame retardant performance of the material without having a significant negative impact on the mechanical properties of the material.

[0061] Based on the test results of Examples 5, 8, and 9, it can be found that the maleic anhydride-grafted α-olefin oligomers prepared using 1-octene dimer and 1-decene dimer as raw materials, respectively, all have certain compatibility-promoting and toughening effects on inorganic flame retardants in PVC resin. However, it can be found that as the molecular chain of the oligomer increases, the toughening and compatibility-promoting effects weaken slightly.

[0062] Combining the test results of Examples 5 and 10, it can be found that the flame retardant, mechanical and impact resistance properties of Example 10 are significantly improved. This indicates that surface modification of inorganic flame retardants, by depositing and coating a layer of γ-AlOOH on their surface, can improve their flame retardant effect, while also improving the dispersibility of inorganic flame retardants in PVC resin and enhancing the mechanical and impact resistance properties of the resin.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any form or substance. It should be noted that those skilled in the art can make various improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention. Any modifications, alterations, and equivalent changes made by those skilled in the art based on the above-disclosed technical content without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A high flame-retardant PVC pipe, characterized in that, It includes the following components in parts by weight: 100 parts PVC resin, 10-15 parts inorganic flame retardant, 0.5-10 parts maleic anhydride grafted α-olefin oligomer, 3-8 parts ACR resin, 1-5 parts heat stabilizer, 0.5-2 parts lubricant, and 10-30 parts filler.

2. The high flame-retardant PVC pipe according to claim 1, characterized in that... The maleic anhydride-grafted α-olefin oligomer is obtained by reacting maleic anhydride with α-olefin oligomer under heating conditions in the presence of a polymerization inhibitor.

3. The high flame-retardant PVC pipe according to claim 2, characterized in that, The α-olefin oligomers are 1-hexene dimer, 1-hexene trimer, 1-hexene tetramer, 1-heptene dimer, 1-heptene trimer, 1-octene dimer, 1-octene trimer, and 1-decene dimer.

4. The high flame-retardant PVC pipe according to claim 2, characterized in that, The heating temperature is 120-160℃.

5. The high flame-retardant PVC pipe according to claim 2, characterized in that, The polymerization inhibitor is at least one of p-hydroxyanisole, 2,5-di-tert-butylhydroquinone, and p-benzoquinone.

6. The high flame-retardant PVC pipe according to claim 1, characterized in that, The inorganic flame retardant also undergoes surface treatment. The specific steps of the surface treatment are as follows: dispersing the inorganic flame retardant in a solvent to form a slurry, adding an aluminum source at 60-100℃, adjusting the pH to 10.5, and then performing post-treatment after the reaction is complete.

7. The high flame-retardant PVC pipe according to claim 6, characterized in that, The inorganic flame retardant is selected from at least one of aluminum hydroxide, magnesium hydroxide, antimony trioxide, and zinc borate.

8. The high flame-retardant PVC pipe according to claim 1, characterized in that, The heat stabilizer is an organotin stabilizer or a calcium-zinc composite stabilizer.

9. The high flame-retardant PVC pipe according to claim 1, characterized in that, The lubricant is selected from at least one of stearic acid, paraffin wax, and polyethylene wax, and the filler is selected from at least one of calcium carbonate and talc.

10. A method for preparing a high flame-retardant PVC pipe according to any one of claims 1-9, characterized in that, The preparation steps include the following: S1. Add PVC resin, inorganic flame retardant, maleic anhydride grafted α-olefin oligomer, ACR resin, heat stabilizer, lubricant and filler to a high-speed mixer according to the formula, and heat mix at 90-120℃ for 5-15 minutes. S2, after cooling the mixed material to 40-60℃, it is fed into a twin-screw extruder; S3 controls the temperature of each section of the twin-screw extruder to 160-200℃, and obtains high flame-retardant PVC pipes through melt extrusion, granulation, shaping, cooling and cutting.

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