Biaxially-oriented polyolefin pipe and preparation method thereof

Abstract

The invention provides a biaxially oriented polyolefin pipe and a preparation method thereof, and belongs to the technical field of polyolefin pipes. By adding a certain amount of random copolymerized polyolefin, homopolymerized polyolefin and block copolymerized polyolefin and utilizing a two-step forming two-way stretching process, molecular chain arrangement in the pipe is changed, so that the pressure bearing capacity of the prepared two-way stretching polyolefin pipe is improved, the capability of resisting fracture, leakage or excessive deformation is remarkably enhanced, and the service life of the two-way stretching polyolefin pipe is prolonged. The biaxial stretching expansion treatment is utilized to form a layered structure of the pipe, so that the brittleness of the biaxial stretching polyolefin pipe is reduced, and the use safety and durability of the biaxial stretching polyolefin pipe under the scenes of low temperature, stress impact and the like are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of polyolefin pipe technology, and in particular to a biaxially oriented polyolefin pipe and its preparation method. Background Technology

[0002] Plastic pipes are widely used in agricultural irrigation, municipal construction, and water conservancy projects. With the rapid development of the plastics industry, metal and cement pipes are increasingly being replaced by plastic pipes in various fields. Compared to metal and cement pipes, plastic pipes have advantages such as lower density, corrosion resistance, good toughness, smooth inner walls, easy recycling of raw materials, lower production costs, longer service life, easier installation and connection, and convenient transportation. Therefore, the annual demand for plastic pipes has seen a significant increase. There are many types of plastic pipes, among which polyolefin pipes such as polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are the most widely used. Compared with other plastic pipes, polyolefin pipes are environmentally friendly, corrosion-resistant, wear-resistant, have a long service life, and are easy to install. They are widely used in various sectors of the national economy and are the fastest-growing type of plastic pipe in recent years, second only to PVC pipes in terms of production volume. With the advancement of science and technology, more and more engineering applications are placing higher demands on the mechanical properties of plastic pipes.

[0003] The "brittleness" of polyolefin pipes essentially refers to their ability to fracture or rupture directly without significant plastic deformation under external forces (such as impact, tension, and bending). It is a core indicator for measuring the pipe's "fracture resistance" and "degree of toughness deficiency," directly determining its safety and durability in low-temperature and impact-exposed environments. Conversely, the "pressure-bearing capacity" of polyolefin pipes essentially refers to their ability to resist rupture, leakage, or excessive deformation under long-term or short-term internal and external pressure. It is a core mechanical property for assessing the pipe's ability to safely transport fluids (such as water, gas, and chemical media), directly determining its applicable pressure rating, operating temperature range, and design service life. Despite the numerous advantages of polyolefin pipes, current technologies generally suffer from insufficient pressure resistance, necessitating further improvements in resistance to external and internal pressure. For example, in applications requiring high-pressure fluid transport, ordinary polyolefin pipes may not meet the requirements. Furthermore, the molecular chain segment mobility of some polyolefin pipes decreases under low temperature conditions, causing them to change from a tough state to a brittle state. In addition, if the cooling process is not properly handled during the extrusion of polyolefin pipes, imperfect crystallization and stress concentration may occur, thereby increasing the brittleness of the pipes. Summary of the Invention

[0004] The purpose of this invention is to provide a biaxially oriented polyolefin pipe and its preparation method. The biaxially oriented polyolefin pipe provided by this invention has high pressure resistance, strong resistance to cracking, leakage or excessive deformation, and low brittleness. It has excellent safety and durability in low temperature and impact scenarios.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a biaxially oriented polyolefin pipe, characterized in that, by weight, the biaxially oriented polyolefin pipe is prepared from the following raw materials:

[0007] 20-45 parts of random copolymer polyolefin, 25-50 parts of homopolymer polyolefin, 23-53 parts of block copolymer polyolefin, and 1-4 parts of color masterbatch;

[0008] The monomer olefins used in the preparation of the random copolymer polyolefins, homopolymer polyolefins and block copolymer polyolefins are at least one of propylene, ethylene and butene;

[0009] The biaxially stretched polyolefin pipes were subjected to biaxial stretching expansion treatment during the manufacturing process.

[0010] Preferably, the biaxially oriented polyolefin pipe is prepared from the following raw materials in parts by weight:

[0011] 22-40 parts of random copolymer polyolefin, 27-42 parts of homopolymer polyolefin, 25-43 parts of block copolymer polyolefin, and 1.5-3 parts of color masterbatch.

[0012] Preferably, the random copolymer polyolefin has a molecular weight of 250,000 to 350,000, the homopolymer polyolefin has a molecular weight of 350,000 to 500,000, and the block copolymer polyolefin has a molecular weight of 250,000 to 400,000.

[0013] The present invention also provides a method for preparing the biaxially oriented polyolefin pipe described in the above technical solution, comprising the following steps:

[0014] (1) After mixing random copolymer polyolefin, homopolymer polyolefin and block copolymer polyolefin, the mixture is subjected to stirring hot mixing, first cooling, stirring cold mixing, second cooling and extrusion cutting in sequence to obtain a premix;

[0015] (2) The premix obtained in step (1) is added to an extruder and extruded and cooled in sequence to obtain a tube blank;

[0016] (3) The tube blank obtained in step (2) is subjected to preheating and biaxial stretching expansion treatment in sequence to obtain biaxially stretched polyolefin tube;

[0017] The biaxial stretching and expansion process includes: inserting one end of the preheated tube blank into a conical expander, and axially stretching the preheated tube blank to pass through the conical expander and the cylinder in sequence, followed by cooling, traction and discharge to obtain a biaxially stretched polyolefin pipe.

[0018] Preferably, the temperature of the stirring and hot mixing in step (1) is 210~250 ℃, and the stirring and hot mixing time is 15~30 min;

[0019] The stirring and cold mixing temperature is 100~150 ℃, and the stirring and cold mixing time is 5~15 min.

[0020] Preferably, in step (2), the crystallinity of the tube blank is 25-35%, the wall thickness of the tube blank is 15-25 mm, and the inner diameter of the tube blank is 80-100 mm.

[0021] Preferably, in step (3), the internal air temperature of the preheated tube blank rises to 210~240 ℃, and the external surface temperature of the preheated tube blank rises to 200~230 ℃.

[0022] Preferably, the axial stretching force of the bidirectional stretching expansion treatment in step (3) is 3500~5000N; the speed at which the preheated tube blank passes through the conical expander is 25~35cm / min.

[0023] Preferably, in step (4), the holes on the surface of the conical expander allow air at 60-80°C to be supplied to the inner surface of the preheated tube blank; the lower bottom surface of the conical expander and the cylinder are connected as one unit; the upper bottom surface diameter of the conical expander is 6-10cm, and the lower bottom surface diameter of the conical expander is 18-24cm.

[0024] Preferably, in step (4), the wall thickness of the biaxially stretched polyolefin pipe is 10-15 mm, and the inner diameter of the biaxially stretched polyolefin pipe is 140-180 mm.

[0025] This invention provides a biaxially oriented polyolefin pipe, which, by weight, is prepared from the following raw materials: 20-45 parts of random copolymer polyolefin, 25-50 parts of homopolymer polyolefin, 23-53 parts of block copolymer polyolefin, and 1-4 parts of color masterbatch; the monomer olefins used in preparing the random copolymer polyolefin, homopolymer polyolefin, and block copolymer polyolefin are at least one of propylene, ethylene, and butene. This invention, by adding a certain amount of random copolymer polyolefin, homopolymer polyolefin, and block copolymer polyolefin, and utilizing a two-stage biaxially oriented process, alters the molecular chain arrangement in the pipe, thereby improving the pressure-bearing capacity of the prepared biaxially oriented polyolefin pipe. This significantly enhances its resistance to rupture, leakage, or excessive deformation. Furthermore, by performing biaxial stretching and expansion treatment during the preparation process, a layered structure is formed in the pipe wall, reducing the brittleness of the biaxially oriented polyolefin pipe and significantly improving its safety and durability in low-temperature and impact-induced scenarios. Attached Figure Description

[0026] Figure 1 The above is a statistical bar chart of the hydrostatic ring stress and tensile strength of the pipes prepared in Examples 1-11 and Comparative Examples 1-10 of the present invention.

[0027] Figure 2 This is a statistical bar chart showing the elongation at break of the pipes prepared in Examples 1-11 and Comparative Examples 1-10 of the present invention. Detailed Implementation

[0028] This invention provides a biaxially oriented polyolefin pipe, which is prepared from the following raw materials in parts by weight:

[0029] 20-45 parts of random copolymer polyolefin, 25-50 parts of homopolymer polyolefin, 23-53 parts of block copolymer polyolefin, and 1-4 parts of color masterbatch;

[0030] The monomer olefins used in the preparation of the random copolymer polyolefins, homopolymer polyolefins and block copolymer polyolefins are at least one of propylene, ethylene and butene;

[0031] The biaxially stretched polyolefin pipes were subjected to biaxial stretching expansion treatment during the manufacturing process.

[0032] Unless otherwise specified, all raw materials used in this invention are commercially available products in the art.

[0033] In this invention, the biaxially oriented polyolefin pipe is preferably prepared from the following raw materials in parts by weight:

[0034] 22-40 parts of random copolymer polyolefin, 27-42 parts of homopolymer polyolefin, 25-43 parts of block copolymer polyolefin, and 1.5-3 parts of color masterbatch.

[0035] In this invention, the molecular weight of the random copolymer polyolefin is preferably 250,000 to 350,000, the molecular weight of the homopolymer polyolefin is preferably 350,000 to 500,000, and the molecular weight of the block copolymer polyolefin is preferably 250,000 to 400,000. This invention controls the molecular weight of each polymer within the above ranges to fully utilize the different physical and mechanical properties resulting from the varying molecular weight distributions and chain lengths of the various raw materials. Through full integration, complementary properties are achieved, effectively improving the overall performance of the pipe products.

[0036] In this invention, the color masterbatch is preferably blue or gray.

[0037] The present invention also provides a method for preparing the biaxially oriented polyolefin pipe described in the above technical solution, comprising the following steps:

[0038] (1) After mixing random copolymer polyolefin, homopolymer polyolefin and block copolymer polyolefin, the mixture is subjected to stirring hot mixing, first cooling, stirring cold mixing, second cooling and extrusion cutting in sequence to obtain a premix;

[0039] (2) The premix obtained in step (1) is added to an extruder and extruded and cooled in sequence to obtain a tube blank;

[0040] (3) The tube blank obtained in step (2) is subjected to preheating and biaxial stretching expansion treatment in sequence to obtain biaxially stretched polyolefin tube;

[0041] The biaxial stretching and expansion process includes: inserting one end of the preheated tube blank into a conical expander, and axially stretching the preheated tube blank to pass through the conical expander and the cylinder in sequence, followed by cooling, traction and discharge to obtain a biaxially stretched polyolefin pipe.

[0042] The present invention involves mixing random copolymer polyolefin, homopolymer polyolefin and block copolymer polyolefin, and then sequentially performing hot mixing with stirring, first cooling, cold mixing with stirring, second cooling and extrusion cutting to obtain a premix.

[0043] In this invention, the preferred temperature for the stirring and hot mixing is 210~250 °C, more preferably 220~240 °C; the preferred time for the stirring and hot mixing is 15~30 min, more preferably 18~25 min. This invention achieves the full melting and effective fusion of all materials through stirring and hot mixing.

[0044] In this invention, the first cooling is preferably performed to cool the stirred and mixed product to a temperature of 140-160°C, more preferably 150°C. This invention utilizes the first cooling to achieve the effect of cooling the material and causing partial crystallization.

[0045] In this invention, the stirring and cold mixing temperature is preferably 100~150℃, more preferably 110~140℃; the stirring and cold mixing time is preferably 5~15 min, more preferably 8~13 min. This invention ensures the uniformity of the materials through stirring and cold mixing.

[0046] In this invention, the second cooling is preferably performed to cool the product of the stirred and cold-mixed mixture to a temperature of 110~130°C, more preferably 120°C; the second cooling time is preferably 8~12 minutes. This invention utilizes the second cooling to ensure that the materials are fully mixed.

[0047] The present invention does not impose any special restrictions on the extrusion cutting method; any technical solution known in the art can be used.

[0048] After obtaining the premix, the present invention adds the premix to an extruder and performs extrusion and cooling shaping in sequence to obtain a tube blank.

[0049] In this invention, the extrusion speed is preferably 12~18 r / min.

[0050] In this invention, the crystallinity of the tube blank is preferably 25-35%, more preferably 26-30%. This invention controls the crystallinity of the tube blank within the above range to ensure uniform and effective extrusion of the tube. In this invention, the wall thickness of the tube blank is preferably 15-25 mm, and the inner diameter of the tube blank is preferably 80-100 mm.

[0051] After obtaining the tube blank, the present invention performs preheating and biaxial stretching expansion treatment on the tube blank in sequence to obtain biaxially stretched polyolefin tube.

[0052] In this invention, the internal air temperature of the preheated tube blank is preferably raised to 210~240℃, more preferably 220~230℃; the outer surface temperature of the preheated tube blank is raised to 200~230℃, more preferably 210~225℃. This invention, through preheating, fully softens the tube blank, unwinds the molecular chains, and facilitates subsequent biaxial tensile expansion, thereby improving the product's strength, pressure resistance, and toughness.

[0053] In this invention, the biaxial stretching and expansion process preferably includes: inserting one end of the preheated tube blank into a conical expander, and axially stretching the preheated tube blank to pass through the conical expander and the cylinder in sequence, followed by cooling, traction and discharge to obtain a biaxially stretched polyolefin tube.

[0054] In this invention, the axial tensile force of the biaxial stretching expansion treatment is preferably 3500~5000N, more preferably 3800~4500N. By controlling the axial tensile force of the biaxial stretching expansion treatment within the above range, this invention effectively controls the axial orientation factor of the pipe within a reasonable range, thereby improving the tensile strength. In this invention, the speed at which the preheated tube blank passes through the conical expander is preferably 25~35cm / min, more preferably 27~33cm / min. By controlling the speed of the preheated tube blank passing through the conical expander within the above range, this invention effectively controls the circumferential orientation factor of the pipe and solidifies it within a certain range, resulting in optimal circumferential pressure-bearing capacity of the pipe.

[0055] In this invention, the perforations on the surface of the conical expander allow air at 60-80°C to circulate through to the inner surface of the preheated tube blank. In this invention, the lower base of the conical expander and the cylinder are integrally connected; the height of the conical expander is preferably 15cm, and the diameter of the lower base is preferably 18-22cm. In this invention, the height of the cylinder is preferably 30cm, and the diameter of the cylinder's base is preferably 18-22cm. This invention controls the height and lower base diameter of the conical expander, within the aforementioned ranges, to ensure smooth biaxial stretching and maintain optimal performance.

[0056] In this invention, the wall thickness of the biaxially oriented polyolefin pipe is preferably 10-15 mm, and the inner diameter of the biaxially oriented polyolefin pipe is preferably 150-180 mm. This invention controls the wall thickness and inner diameter of the biaxially oriented polyolefin pipe within the above ranges to achieve the optimal overall performance of the pipe.

[0057] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0058] Unless otherwise specified, all experiments were repeated three times, and the results are expressed as averages.

[0059] Example 1

[0060] A biaxially oriented polyolefin pipe, prepared from the following raw materials in parts by mass:

[0061] 30 parts random copolymer polyolefin, 35 parts homopolymer polyolefin, 33 parts block copolymer polyolefin, and 2 parts blue masterbatch.

[0062] The monomeric olefins used to prepare the random copolymer polyolefin, homopolymer polyolefin and block copolymer polyolefin are propylene, butene and pentene, respectively.

[0063] The biaxially stretched polyolefin pipe undergoes biaxial stretching expansion treatment during the manufacturing process.

[0064] The preparation method of the above-mentioned biaxially oriented polyolefin pipe includes the following steps:

[0065] (1) After mixing random copolymer polyolefin, homopolymer polyolefin and block copolymer polyolefin, the mixture is stirred and hot-mixed at 230°C for 20 min, then cooled to 150°C for 30 min, and then cooled to 120°C for 10 min. The mixture is then extruded and cut to obtain the premix.

[0066] (2) The premix obtained in step (1) is added to an extruder and extruded at a speed of 15 r / min. After cooling and shaping, a tube blank is obtained.

[0067] The tube blank has a crystallinity of 30%, a wall thickness of 20 mm, and an inner diameter of 90 mm.

[0068] (3) The tube blank obtained in step (2) is preheated so that the internal air temperature of the tube blank rises to 230°C and the external surface temperature of the tube blank rises to 220°C, so as to obtain the preheated tube blank.

[0069] The preheated tube blank is subjected to biaxial stretching and expansion treatment to obtain biaxially stretched polyolefin pipe.

[0070] The biaxial stretching and expansion process includes: inserting one end of the preheated tube blank into a conical expander, and axially stretching the preheated tube blank to pass through the conical expander and the cylinder in sequence, then cooling it to 180°C, and then drawing and unloading it to obtain a biaxially stretched polyolefin tube.

[0071] The axial tensile force of the biaxial stretching expansion treatment is 4000N; the speed at which the preheated tube blank passes through the conical expander is 30cm / min;

[0072] The conical expander has holes on its surface that allow air at 70°C to pass through to the inner surface of the preheated tube blank; the lower bottom surface of the conical expander is connected to the cylinder as one piece; the height of the conical expander is 15cm and the diameter of the upper bottom surface is 8cm; the height of the conical expander is 30cm and the diameter of the lower bottom surface is 20cm.

[0073] The biaxially oriented polyolefin pipe has a wall thickness of 12 mm and an inner diameter of 160 mm.

[0074] Example 2

[0075] The difference from Example 1 is that 22 parts of random copolymer polyolefin were used.

[0076] Example 3

[0077] The difference from Example 1 is that 40 parts of random copolymer polyolefin were used.

[0078] Comparative Example 1

[0079] The difference from Example 1 is that 15 parts of random copolymer polyolefin were used.

[0080] Comparative Example 2

[0081] The difference from Example 1 is that no bidirectional stretching expansion treatment was performed.

[0082] Example 4

[0083] The difference from Example 1 is that 27 parts of homopolymer polyolefin were used.

[0084] Comparative Example 3

[0085] The difference from Example 1 is that 15 parts of homopolymer polyolefin were used.

[0086] Example 5

[0087] The difference from Example 1 is that 25 parts of block copolymer polyolefin were used.

[0088] Comparative Example 4

[0089] The difference from Example 1 is that 15 parts of block copolymer polyolefin were used.

[0090] Example 6

[0091] The difference from Example 1 is that the crystallinity of the tube blank obtained in step (2) is 28%.

[0092] Example 7

[0093] The difference from Example 1 is that the crystallinity of the tube blank obtained in step (2) is 32%.

[0094] Comparative Example 5

[0095] The difference from Example 1 is that the crystallinity of the tube blank obtained in step (2) is 20%.

[0096] Comparative Example 6

[0097] The difference from Example 1 is that the crystallinity of the tube blank obtained in step (2) is 40%.

[0098] Example 8

[0099] The difference from Example 1 is that the axial stretching force in the bidirectional stretching expansion treatment in step (3) is 3800N.

[0100] Example 9

[0101] The difference from Example 1 is that the axial stretching force in the bidirectional stretching expansion treatment in step (3) is 4500N.

[0102] Comparative Example 7

[0103] The difference from Example 1 is that the axial stretching force in the bidirectional stretching expansion treatment in step (3) is 2000N.

[0104] Comparative Example 8

[0105] The difference from Example 1 is that the axial stretching force in the bidirectional stretching expansion treatment in step (3) is 6000N.

[0106] Example 10

[0107] The difference from Example 1 is that the preheated tube blank passes through the conical expander at a speed of 25 cm / min.

[0108] Example 11

[0109] The difference from Example 1 is that the preheated tube blank passes through the conical expander at a speed of 35 cm / min.

[0110] Comparative Example 9

[0111] The difference from Example 1 is that the preheated tube blank passes through the conical expander at a speed of 15 cm / min.

[0112] Comparative Example 10

[0113] The difference from Example 1 is that the preheated tube blank passes through the conical expander at a speed of 40 cm / min.

[0114] The performance of the pipes prepared in Examples 1-11 and Comparative Examples 1-10 was tested using the following methods:

[0115] The compressive strength was tested according to GB / T6111 (hydrostatic test), the tensile strength according to GB / T8804, the brittleness was tested according to GB / T18743 (impact test of simply supported beam), and the elongation at break according to GB / T8804. The test results are shown in Table 1 and... Figure 1 , 2 .

[0116] Table 1 Performance test results of the pipes prepared in Examples 1-11 and Comparative Examples 1-10

[0117]

[0118] In summary, the biaxially stretched pipe provided by this invention can effectively improve its pressure-bearing capacity and tensile strength, and on this basis, enhance the toughness of the pipe. Moreover, by adjusting the proportions and process parameters during the production process, various properties can be adjusted to achieve the optimal combination.

[0119] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A biaxially oriented polyolefin pipe, characterized in that, The biaxially oriented polyolefin pipe is prepared from the following raw materials in parts by weight: 20-45 parts of random copolymer polyolefin, 25-50 parts of homopolymer polyolefin, 23-53 parts of block copolymer polyolefin, and 1-4 parts of color masterbatch; The monomer olefins used in the preparation of the random copolymer polyolefins, homopolymer polyolefins and block copolymer polyolefins are at least one of propylene, ethylene and butene; The biaxially stretched polyolefin pipes were subjected to biaxial stretching expansion treatment during the manufacturing process.

2. The biaxially oriented polyolefin pipe according to claim 1, characterized in that, The biaxially oriented polyolefin pipe is prepared from the following raw materials in parts by weight: 22-40 parts of random copolymer polyolefin, 27-42 parts of homopolymer polyolefin, 25-43 parts of block copolymer polyolefin, and 1.5-3 parts of color masterbatch.

3. The biaxially oriented polyolefin pipe according to claim 1 or 2, characterized in that, The random copolymer polyolefin has a molecular weight of 250,000 to 350,000, the homopolymer polyolefin has a molecular weight of 350,000 to 500,000, and the block copolymer polyolefin has a molecular weight of 250,000 to 400,000.

4. A method for preparing a biaxially oriented polyolefin pipe according to any one of claims 1 to 3, characterized in that, Includes the following steps: (1) After mixing random copolymer polyolefin, homopolymer polyolefin and block copolymer polyolefin, the mixture is subjected to stirring hot mixing, first cooling, stirring cold mixing, second cooling and extrusion cutting in sequence to obtain a premix; (2) The premix obtained in step (1) is added to an extruder and extruded and cooled in sequence to obtain a tube blank; (3) The tube blank obtained in step (2) is subjected to preheating and biaxial stretching expansion treatment in sequence to obtain biaxially stretched polyolefin tube; The biaxial stretching and expansion process includes: inserting one end of the preheated tube blank into a conical expander, and axially stretching the preheated tube blank to pass through the conical expander and the cylinder in sequence, followed by cooling, traction and discharge to obtain a biaxially stretched polyolefin pipe.

5. The method for preparing biaxially oriented polyolefin pipe according to claim 1, characterized in that, The temperature of the stirring and hot mixing in step (1) is 210~250 ℃, and the stirring and hot mixing time is 15~30 min; The stirring and cold mixing temperature is 100~150 ℃, and the stirring and cold mixing time is 5~15 min.

6. The method for preparing biaxially oriented polyolefin pipe according to claim 1, characterized in that, In step (2), the crystallinity of the tube blank is 25-35%, the wall thickness of the tube blank is 15-25 mm, and the inner diameter of the tube blank is 80-100 mm.

7. The method for preparing biaxially oriented polyolefin pipe according to claim 1, characterized in that, In step (3), the internal air temperature of the preheated tube blank rises to 210~240 ℃, and the external surface temperature of the preheated tube blank rises to 200~230 ℃.

8. The method for preparing biaxially oriented polyolefin pipe according to claim 1, characterized in that, The axial stretching force of the bidirectional stretching expansion treatment in step (3) is 3500~5000N; the speed at which the preheated tube blank passes through the conical expander is 25~35cm / min.

9. The method for preparing biaxially oriented polyolefin pipe according to claim 1, characterized in that, In step (4), the holes on the surface of the conical expander allow air at 60-80°C to be supplied to the inner surface of the preheated tube blank; the lower bottom surface of the conical expander and the cylinder are connected as one unit; the upper bottom surface diameter of the conical expander is 6-10cm, and the lower bottom surface diameter of the conical expander is 18-24cm.

10. The method for preparing biaxially oriented polyolefin pipe according to claim 1, characterized in that, In step (4), the wall thickness of the biaxially stretched polyolefin pipe is 10-15 mm, and the inner diameter of the biaxially stretched polyolefin pipe is 140-180 mm.

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