Three-layer composite pipe and manufacturing method therefor

Abstract

Disclosed in the present invention is a three-layer composite pipe, comprising an inner layer, a middle layer and an outer layer. The outer layer is prepared from the following raw materials in percentage by mass: 50-95% of a high-viscosity low-melting-point polyamide resin, 4.5-30% of a toughening agent, 0-20% of a plasticizer, 0.1-2% of a chain extender, and 0.1-1% of a heat-resistant agent. The inner layer is prepared from the following raw materials in percentage by mass: 40-90% of high-viscosity block copolymer polypropylene, 5-40% of high-viscosity homopolymer polypropylene, and 0-20% of a toughening agent. The middle layer is a bonding layer. Also disclosed in the present invention is a manufacturing method for the three-layer composite pipe. The three-layer composite pipe of the present invention can be used for straight pipes and corrugated pipes, has excellent mechanical properties, low-temperature toughness, and heat aging resistance, and good formability, and is easy to shape, and suitable for use in cooling pipeline systems for new energy vehicle batteries and energy storage batteries.

Description

Three-layer composite pipe and preparation method thereof Technical Field

[0001] The present invention relates to the technical field of polymer material processing, in particular to a three-layer composite pipe and a preparation method thereof. Background Art

[0002] With the rapid development of new energy vehicles and the energy storage industry, battery module cooling is becoming increasingly important. Due to the heavy weight, vulcanization requirements, and environmental concerns of rubber tubing, polyamide tubing is gradually being replaced. In particular, the PA12 / adhesive layer / PP three-layer cooling tubing, with its low cost and excellent hydrolysis resistance, is becoming the mainstream cooling tubing development trend.

[0003] CN114213750A discloses a multilayer pipe with a polyamide PA / adhesive layer / PP structure; the polyamide is selected from one or more of PA1012, PA12, PA612, PA610, PA614, PA12, PA1212, PA616, and PA618. However, the melting point of materials such as P610 is relatively high, significantly different from that of the inner layer PP. During thermoforming, the pipe experiences significant rebound after cooling, making it difficult to control the post-thermoforming pipe shape and causing difficulties in subsequent installation. Furthermore, the inner layer's matrix resin is homopolymer PP, requiring the addition of large amounts of expensive toughening agents to improve low-temperature toughness. This significantly increases the pipe's manufacturing cost, hindering widespread use.

[0004] Summary of the Invention

[0005] Based on the technical problems existing in the background technology, the present invention proposes a three-layer composite pipe and a preparation method thereof.

[0006] The present invention provides a three-layer composite pipe comprising an inner layer, a middle layer and an outer layer;

[0007] The raw materials of the outer layer include by mass percentage: 50-95% high-viscosity low-melting point polyamide resin, 4.5-30% toughening agent, 0-20% plasticizer, 0.1-2% chain extender, 0.1-1% heat resistant agent; the sum of the mass percentages of the raw materials of the outer layer is 100%; the melting point of the high-viscosity low-melting point polyamide resin is 170-214.5 ℃, and the melt index under the test conditions of 235 ℃ and 2.16kg is 0.1-10g / 10min;

[0008] The raw materials of the inner layer include, by mass percentage, 40-90% of high-viscosity block copolymer polypropylene, 5-40% of high-viscosity homopolypropylene, and 0-20% of a toughening agent; the sum of the mass percentages of the raw materials of the inner layer is 100%; the melt index of the high-viscosity block copolymer polypropylene under test conditions of 230°C and 2.16kg is 0.1-6g / 10min, and the melt index of the high-viscosity homopolypropylene under test conditions of 230°C and 2.16kg is 0.1-6g / 10min;

[0009] The middle layer is a bonding layer.

[0010] Preferably, the high-viscosity, low-melting-point polyamide resin is obtained by copolymerization and solid-phase viscosity enhancement of three or more monomers, wherein the monomers are selected from dibasic acids, diamines or lactams, and the monomers of the high-viscosity, low-melting-point polyamide resin include one or more dibasic acids and one or more diamines;

[0011] In the monomers of the high-viscosity, low-melting-point polyamide resin, the molar ratio of the diamine monomer to the dibasic acid monomer is 1:1; the lactam monomer accounts for 0-20% of the total molar number of the monomers of the high-viscosity, low-melting-point polyamide resin;

[0012] The dicarboxylic acid is selected from adipic acid, suberic acid, azelaic acid, sebacic acid, undecanedicarboxylic acid, dodecanedioic acid, tridecanedioic acid, tetradecanedioic acid, pentadecanedicarboxylic acid or hexadecanedioic acid;

[0013] The diamine is selected from hexamethylenediamine, octanediamine, nonanediamine, decanediamine, undecanediamine, dodecanediamine, tridecanediamine, tetradecanediamine, pentadecanediamine or hexadecanediamine;

[0014] The lactam is selected from caprolactam, undecanoic acid or laurolactam.

[0015] Preferably, the high-viscosity, low-melting-point polyamide resin is copolymerized with a diamine and two dibasic acids; or the high-viscosity, low-melting-point polyamide resin is copolymerized with two diamines and a dibasic acid; or the high-viscosity, low-melting-point polyamide resin is copolymerized with a diamine, a dibasic acid and a lactam.

[0016] Preferably, the preparation method of the high-viscosity, low-melting-point polyamide resin is as follows: the monomer is subjected to a salt-forming reaction in deionized water, the obtained salt solution is added to a polymerization kettle, first, the reaction is carried out at 240-260°C under nitrogen protection for 2-5 hours, and then the reaction is carried out under vacuum for 1-3 hours, the viscosity is adjusted during the reaction, and after the reaction is completed, the low-melting-point copolymerized polyamide is obtained by underwater pelletizing and drying, and the low-melting-point copolymerized polyamide is added to a vacuum rotary kettle, and the viscosity is increased at 140-160°C under vacuum conditions for 2-24 hours to obtain a high-viscosity, low-melting-point polyamide resin.

[0017] Preferably, among the raw materials of the outer layer, the toughening agent is at least one of maleic anhydride grafted POE, maleic anhydride grafted SEBS, and maleic anhydride grafted EPDM, the plasticizer is N-butylbenzenesulfonamide, the chain extender is BRUGGOLEN M1251, and the heat resistant agent is BRUGGOLEN H3336, BRUGGOLEN H3346, or a combination thereof.

[0018] Preferably, the high-viscosity block copolymer polypropylene is high-viscosity propylene-ethylene block copolymer PP, high-viscosity propylene-butene block copolymer PP or a combination thereof, wherein the ethylene content of the high-viscosity propylene-ethylene block copolymer PP is 1-8%, and the butene content of the high-viscosity propylene-butene block copolymer PP is 1-8%.

[0019] Preferably, in the raw materials of the inner layer, the toughening agent is POE, SEBS or a combination thereof.

[0020] Preferably, the raw materials of the outer layer further include a PA antioxidant, the mass of the PA antioxidant accounting for 0.1-2% of the total mass of the raw materials of the outer layer; the PA antioxidant is at least one of BASF Irganox 1098, BASF Irganox 245, and BASF Irganox 168; the raw materials of the inner layer further include a PP antioxidant, the mass of the PP antioxidant accounting for 0.1-2% of the total mass of the raw materials of the inner layer; the PP antioxidant is BASF Irganox 1010, BASF Irganox 168, or a combination thereof.

[0021] Preferably, the raw materials of the outer layer further include PA masterbatch, the mass of which accounts for 0-2% of the total mass of the raw materials of the outer layer; the raw materials of the inner layer further include PP masterbatch, the mass of which accounts for 0-2% of the total mass of the raw materials of the inner layer.

[0022] Preferably, the raw material of the intermediate layer is maleic anhydride grafted PP. Preferably, the maleic anhydride grafted PP has a maleic anhydride grafting rate of 0.1-3%.

[0023] Preferably, the maleic anhydride grafted PP is Mitsui ADMER QB520, Mitsui ADMER QB510 or a combination thereof.

[0024] Preferably, the outer diameter of the three-layer composite pipe is 4-50 mm, and the total wall thickness is 0.6-4 mm, wherein the thickness of the outer layer is ≤ 80% of the total wall thickness of the composite pipe, the thickness of the inner layer is ≤ 60% of the total wall thickness of the composite pipe, and the thickness of the middle layer is ≤ 30% of the total wall thickness of the composite pipe.

[0025] Preferably, the outer diameter of the three-layer composite pipe is 8-36 mm, and the total wall thickness is 1-2 mm.

[0026] Preferably, the thickness of the outer layer of the three-layer composite pipe accounts for 50-70% of the total wall thickness of the composite pipe.

[0027] Preferably, the thickness of the inner layer of the three-layer composite pipe accounts for 20-50% of the total wall thickness of the composite pipe.

[0028] Preferably, the thickness of the middle layer of the three-layer composite pipe accounts for 10-20% of the total wall thickness of the composite pipe.

[0029] In the present invention, the three-layer composite pipe can be a round straight pipe or a corrugated pipe.

[0030] A method for preparing the three-layer composite pipe comprises the following steps:

[0031] S1. After the raw materials of the outer layer are mixed evenly, melt-extruded and granulated to obtain outer layer material particles; after the raw materials of the inner layer are mixed evenly, melt-extruded and granulated to obtain inner layer material particles;

[0032] S2. Add the outer layer material particles, the middle layer raw material and the inner layer material particles into a three-layer tube extruder respectively, and perform melt extrusion to obtain.

[0033] Preferably, in S1, the components of the raw materials of the outer layer except the plasticizer are mixed evenly, added to the main feeding port of the twin-screw extruder, and the plasticizer is added to the side feeding port, and melt extrusion granulation is carried out to obtain outer layer material particles; wherein the temperature of the twin-screw extruder barrel is: zone 1 temperature 150-250°C, zone 2 temperature 190-270°C, zone 3 temperature 190-270°C, zone 4 temperature 190-270°C, zone 5 temperature 190-270°C, zone 6 temperature 190-270°C, zone 7 temperature 190-270°C, zone 8 temperature 190-270°C, zone 9 temperature 190-270°C, zone 10 temperature 190-270°C; head temperature 200-280°C; twin-screw extruder main engine speed is 300-900r / min.

[0034] Preferably, in S1, after the raw materials for the inner layer are mixed evenly, they are added to a twin-screw extruder for melt extrusion and granulation to obtain inner layer material particles; wherein the temperature of the twin-screw extruder barrel is: zone 1 temperature 150-220°C, zone 2 temperature 200-260°C, zone 3 temperature 200-260°C, zone 4 temperature 200-260°C, zone 5 temperature 200-260°C, zone 6 temperature 200-260°C, zone 7 temperature 200-260°C, zone 8 temperature 200-260°C, zone 9 temperature 200-260°C, zone 10 temperature 200-260°C; head temperature 210-270°C; twin-screw extruder main engine speed is 150-800r / min.

[0035] Preferably, the temperature of the outer heating section of the three-layer tube extruder is: 180-200°C in zone 1, 190-260°C in zone 2, 190-260°C in zone 3, 190-260°C in zone 4, and 190-260°C in zone 5; the temperature of the inner heating section is: 140-160°C in zone 1, 190-250°C in zone 2, 190-250°C in zone 3, 190-250°C in zone 4, and 190-260°C in zone 5. 0-250℃; the temperature of the middle layer heating section is: zone 1 160-180℃, zone 2 190-250℃, zone 3 190-250℃, zone 4 190-250℃, zone 5 190-250℃; the temperature of the die head is: zone 1 200-270℃, zone 2 200-270℃, zone 3 200-270℃, zone 4 200-270℃, zone 5 210-270℃.

[0036] The beneficial effects of the present invention are as follows:

[0037] 1. The outer layer copolymerized polyamide resin is prepared by using three or more dibasic acids, diamines and lactams as monomer raw materials. After polymerization, the melting point is controlled to be 170-214.5°C by adjusting the components. Compared with the common PA12 / adhesive layer / PP cooling tubes on the market, the raw material sources of the present invention are more extensive, all raw materials can be obtained through procurement, and the polymerization process is mature. The melting point of the copolymerized polyamide can be adjusted to be closer to the melting point of PP. Since the thermoforming temperature of the tube is lower than the melting point of PP by 165°C, the melting of the PP inner layer is prevented. When the melting point of the outer layer modified copolymerized polyamide material is adjusted to be close to the melting point of PP, the three-layer tube has little rebound after thermoforming and cooling, and has a stable shape, which is easy to control and convenient to install. The addition of heat-resistant additives improves the long-term heat resistance of the material. The addition of chain extenders increases the molecular weight length and improves the long-term weather resistance of the material.

[0038] 2. For the first time, the inner layer PP material proposed the use of copolymer PP and homopolymer PP blended modification. Copolymer PP provides good low-temperature toughness, but due to its low crystallinity, ethylene glycol aqueous solution will penetrate between its molecules, seriously affecting its mechanical strength; the presence of homopolymer PP, due to its high crystallinity, can prevent ethylene glycol aqueous solution from penetrating into the interior of the PP material, ensuring its good hydrolysis resistance. The inner layer made by blending copolymer PP and homopolymer PP can have excellent low-temperature toughness and hydrolysis resistance without using or using less toughening agents.

[0039] Compared with the existing PA / adhesive layer / PP three-layer tube, the present invention selects a high-viscosity, low-melting-point polyamide resin obtained by copolymerizing a variety of different polyamide resin monomers as the material for the outer layer, and copolymerized PP and homopolymerized PP are blended and modified as the material for the inner layer. The raw material source is abundant, providing more diverse choices for pipes used in the cooling systems of new energy vehicles and energy storage battery modules. At the same time, the performance and cost of the pipes are more advantageous. DETAILED DESCRIPTION

[0040] The technical solution of the present invention is described in detail below through specific embodiments.

[0041] In the following examples and comparative examples:

[0042] High-viscosity polyamide resin G1 (melting point 190°C): obtained by copolymerization of PA612 salt and PA614 salt in a weight ratio of 1:3 and solid phase viscosity enhancement. The preparation method is as follows: hexamethylenediamine and dodecanedioic acid are added to deionized water to react to obtain a polyamide 612 salt solution with a solid content of 50%, hexamethylenediamine and tetradecanedioic acid are added to deionized water to react to obtain a polyamide 614 salt solution with a solid content of 50%, the PA612 salt solution and the PA614 salt solution prepared above are added to a polymerization kettle, firstly, the mixture is kept warm and reacted under nitrogen protection at 250°C for 3 hours, then vacuumed and reacted under the condition of controlling <500 Pa for 1 hour, the viscosity is adjusted during the reaction, and after the reaction is completed, the mixture is pelletized underwater and dried to obtain a low-melting-point copolyamide, and the low-melting-point copolyamide is added to a vacuum rotary kettle and viscosity-increased at 160°C for 15 hours under vacuum conditions (<500 Pa) to obtain a high-viscosity polyamide resin G1 with a melt index of 3.2 g / 10 min (GB / T 3682, 235°C, 2.16 kg).

[0043] Medium-viscosity polyamide resin g1 (melting point 190°C): obtained by copolymerizing PA612 salt and PA614 salt in a weight ratio of 1:3. The preparation method is as follows: Hexamethylenediamine and dodecanedioic acid are added to deionized water to react to obtain a polyamide 612 salt solution with a 50% solid content. Hexamethylenediamine and tetradecanedioic acid are then added to deionized water to react to obtain a polyamide 614 salt solution with a 50% solid content. The PA612 and PA614 salt solutions prepared above are added to a polymerization kettle and reacted at 250°C under nitrogen for 3 hours. The reaction is then carried out under vacuum with the pressure controlled at <500 Pa for 2 hours. The viscosity is adjusted during the reaction to obtain a medium-viscosity polyamide resin g1 with a melt index of 22.8 g / 10 min (GB / T 3682, 235°C, 2.16 kg).

[0044] High-viscosity polyamide resin G2 (melting point 203°C): Produced by copolymerization of PA612 salt and PA610 salt in a weight ratio of 3:1, followed by solid-phase tackification. Preparation method is the same as for high-viscosity polyamide resin G1. High-viscosity polyamide resin G2 has a melt index of 2.2 g / 10 min (GB / T 3682, 235°C, 2.16 kg).

[0045] High-viscosity polyamide resin G3 (melting point 214.5°C): Produced by copolymerization of PA610 salt and PA66 salt in a weight ratio of 9:1, followed by solid-phase tackification. Preparation method is the same as for high-viscosity polyamide resin G1. High-viscosity polyamide resin G3 has a melt index of 1.6 g / 10 min (GB / T 3682, 235°C, 2.16 kg).

[0046] High-viscosity polyamide resin G4 (melting point 170°C): Produced by copolymerization of PA610 salt and caprolactam salt in a weight ratio of 3:2, followed by solid-phase tackification. Preparation method is the same as for high-viscosity polyamide resin G1. High-viscosity polyamide resin G4 has a melt index of 0.8 g / 10 min (GB / T 3682, 235°C, 2.16 kg).

[0047] The outer layer material composition and mechanical properties test results of the composite pipe are shown in Table 1:

[0048] Table 1

[0049] As can be seen from Table 1:

[0050] Comparison of Example A1 with Example A1 shows that POE-MA is not added, and the low temperature impact performance is 12.5KJ / m 2 Reduced to 2.6KJ / m 2 , poor low temperature resistance and toughness;

[0051] Compared with Example A1, Comparative Example a2 did not add a chain extender. After heat aging at 150°C for 1000 hours, the elongation at break decreased from 121.3% of Example A1 to 69.2%, indicating poor heat aging resistance.

[0052] Compared with Example A1, Comparative Example a3 did not add heat-resistant copper salt. After heat aging at 150°C for 1000 hours, the elongation at break decreased from 121.3% of Example A1 to 56.2%, indicating poor heat aging resistance.

[0053] Comparing Example a4 with Example A1, when a medium-viscosity copolymerized polyamide resin is selected, the initial melt index increases from 0.9 in A1 to 12.8 g / 10 min, which leads to unstable tube extrusion; after heat aging at 150°C for 1000 hours, the elongation at break decreases from 121.3% in Example A1 to 76.1%, indicating poor heat aging resistance.

[0054] Comparing Example a5 with Example A1, when the polyamide resin PA610 with a high melting point is selected, the melting point of the modified material is 222°C, which is significantly higher than 190°C. After the subsequent three-layer tube is thermoformed, the tube rebounds severely and the shape of the tube is difficult to control.

[0055] The extrusion-grade block copolymers and homopolymers used in the following examples and comparative examples were commercially purchased. Copolymer PP1 was Shanghai SECCO T3401, copolymer PP2 was Shanghai SECCO K7002, and homopolymer PP3 was Sinopec Maoming T30S. All examples and comparative examples were produced by blending and modification using the aforementioned method; the raw materials were proportioned by weight, totaling 100%. After pelletization, the materials were dried and injection-molded into test strips. Hydrolysis testing conditions were as follows: tensile test strips were placed in a 50% ethylene glycol aqueous solution at 130°C for 1000 hours, and mechanical properties were measured.

[0056] The test results of the inner layer material composition and mechanical properties of the composite pipe are shown in Table 2:

[0057] Table 2

[0058] Compared with Example B1, Comparative Example b1 only uses copolymerized PP without adding homopolymerized PP, and the tensile strength decreases from 23.2MPa of B1 to 20.1MPa of b1; at the same time, the elongation at break after heat aging decreases from 252.1% of B1 to 121.2% of b1; Compared with Example B1, Comparative Example b2 only uses homopolymerized PP without adding copolymerized PP, and the low-temperature notched impact strength decreases from 6.1KJ / m 2 Down to 2.1KJ / m 2After heat aging at 150°C for 1000 hours, the elongation at break decreased from 252.1% to 72.1%, indicating that the material has poor heat aging resistance. For comparative example b3, no antioxidant was added. After heat aging at 150°C for 1000 hours, the elongation at break decreased from 252.1% to 20.1%, indicating that the material has poor heat aging resistance.

[0059] Using the aforementioned three-layer composite pipe extrusion method, a round straight pipe with an outer diameter of 16 mm and a wall thickness of 1.5 mm was produced. The pipe was then tested for room-temperature burst pressure. A 10 mm section of the pipe was longitudinally cut and peeled to test the adhesion between the three layers. Low-temperature impact testing was performed at -40°C using a 7.5J pendulum according to QC / T80-2011. A 50 cm section of the pipe was filled with a coolant solution of 50% ethylene glycol in water, sealed at both ends, and heat aged for 1000 hours in a 130°C oven. After 1000 hours, the burst pressure and elongation at break were measured. The pipe performance tests are shown in Table 3:

[0060] Table 3

[0061] From Table 3 we can see that:

[0062] Compared with Example 1, the outer layer of the pipe of Comparative Example 1 ruptures upon low-temperature impact at -40°C, indicating that its low-temperature resistance is poor.

[0063] Compared with Example 1, the inner layer of the pipe of Comparative Example 2 ruptures upon low-temperature impact at -40°C, indicating that its low-temperature resistance is poor.

[0064] Compared with Example 1, after 1000 hours of heat aging, the burst pressure of the pipe of Comparative Example 3 decreased from 4.1 MPa in Example 1 to 2.1 MPa; the elongation at break of the pipe decreased from 132.1% to 35.1%, indicating that its heat aging resistance is poor.

[0065] Compared with Example 1, when the combined tube of Comparative Example 4 was extruded, the tube extrusion was unstable and could not be formed due to the large melt index of the outer layer.

[0066] Compared with Example 1, in Comparative Example 5, after being thermoformed at 160° C., the pipe rebounded severely and could not meet the installation requirements.

[0067] In summary, the overall performance of Example 1, Example 2, Example 3, and Example 4 is good and can meet the cooling pipe applications of new energy vehicles, energy storage, etc.

[0068] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A three-layer composite pipe, characterized in that: It includes inner layer, middle layer and outer layer; The raw materials of the outer layer include, by mass percentage, 50-95% of high-viscosity and low-melting-point polyamide resin, 4.5-30% of toughening agent, 0-20% of plasticizer, 0.1-2% of chain extender, and 0.1-1% of heat-resistant agent; the melting point of the high-viscosity and low-melting-point polyamide resin is 170-214.5° C., and the melt index under the test conditions of 235° C. and 2.16 kg is 0.1-10 g / 10 min; The raw materials of the inner layer include, by mass percentage: 40-90% of high-viscosity block copolymer polypropylene, 5-40% of high-viscosity homopolymer polypropylene, and 0-20% of toughening agent; the melting index of the high-viscosity block copolymer polypropylene under the test conditions of 230°C and 2.16kg is 0.1-6g / 10min, and the melting index of the high-viscosity homopolymer polypropylene under the test conditions of 230°C and 2.16kg is 0.1-6g / 10min; The middle layer is a bonding layer.

2. The three-layer composite pipe according to claim 1, characterized in that: The high-viscosity, low-melting-point polyamide resin is obtained by copolymerization and solid-phase viscosity enhancement of three or more monomers, wherein the monomers are selected from dibasic acids, diamines or lactams, and the monomers of the high-viscosity, low-melting-point polyamide resin include one or more dibasic acids and one or more diamines; In the monomers of the high-viscosity, low-melting-point polyamide resin, the molar ratio of the diamine monomer to the dibasic acid monomer is 1:1; the lactam monomer accounts for 0-20% of the total molar number of the monomers of the high-viscosity, low-melting-point polyamide resin; The dibasic acid is selected from adipic acid, suberic acid, azelaic acid, sebacic acid, undecanedicarboxylic acid, dodecanedioic acid, tridecanedioic acid, tetradecanedioic acid, pentadecanedicarboxylic acid or hexadecanedioic acid; The diamine is selected from hexamethylenediamine, octanediamine, nonanediamine, decanediamine, undecanediamine, dodecanediamine, tridecanediamine, tetradecanediamine, pentadecanediamine or hexadecanediamine; The lactam is selected from caprolactam, undecanoic acid or laurolactam.

3. The three-layer composite pipe according to claim 1, characterized in that: Among the raw materials of the outer layer, the toughening agent is at least one of maleic anhydride grafted POE, maleic anhydride grafted SEBS, and maleic anhydride grafted EPDM, the plasticizer is N-butylbenzenesulfonamide, the chain extender is BRUGGOLEN M1251, and the heat resistant agent is BRUGGOLEN H3336, BRUGGOLEN H3346, or a combination thereof.

4. The three-layer composite pipe according to claim 1, characterized in that: Among the raw materials of the inner layer, the toughening agent is POE, SEBS or a combination thereof.

5. The three-layer composite pipe according to claim 1, characterized in that: The raw materials of the outer layer also include a PA antioxidant, the mass of which accounts for 0.1-2% of the total mass of the raw materials of the outer layer; the PA antioxidant is at least one of BASF Irganox 1098, BASF Irganox 245, and BASF Irganox 168; the raw materials of the inner layer also include a PP antioxidant, the mass of which accounts for 0.1-2% of the total mass of the raw materials of the inner layer; the PP antioxidant is BASF Irganox 1010, BASF Irganox 168, or a combination thereof.

6. The three-layer composite pipe according to claim 1, characterized in that: The raw materials of the outer layer also include PA masterbatch, the mass of which accounts for 0-2% of the total mass of the raw materials of the outer layer; the raw materials of the inner layer also include PP masterbatch, the mass of which accounts for 0-2% of the total mass of the raw materials of the inner layer.

7. The three-layer composite pipe according to claim 1, characterized in that: The raw material of the middle layer is maleic anhydride grafted PP.

8. The three-layer composite pipe according to claim 1, characterized in that: The outer diameter of the three-layer composite pipe is 4-50mm, and the total wall thickness is 0.6-4mm, wherein the thickness of the outer layer is ≤80% of the total wall thickness of the composite pipe, the thickness of the inner layer is ≤60% of the total wall thickness of the composite pipe, and the thickness of the middle layer is ≤30% of the total wall thickness of the composite pipe.

9. A method for preparing a three-layer composite pipe according to any one of claims 1 to 8, characterized in that: The steps include: S1. After the raw materials of the outer layer are mixed evenly, they are melt-extruded and granulated to obtain outer layer material particles; after the raw materials of the inner layer are mixed evenly, they are melt-extruded and granulated to obtain inner layer material particles; S2, respectively adding the outer layer material particles, the middle layer raw material and the inner layer material particles into a three-layer tube extruder, and performing melt extrusion to obtain.

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