A low-emission fuel composite pipe and its preparation method

Abstract

This invention discloses a low-emission fuel composite pipe and its preparation method. The composite pipe includes an outer layer, a middle layer, and an inner layer. The outer layer is composed of the following components by mass percentage: 46-97.9% first polyamide resin and the balance of a first additive. The middle layer is composed of ethylene-vinyl alcohol copolymer (EVOH). The inner layer is composed of the following components by mass percentage: 69-97.9% second polyamide resin and the balance of a second additive. The composite pipe of this invention adopts a three-layer co-extrusion structure, eliminating the need for a separate adhesive or tackifying layer. It can be directly formed in one step using a conventional three-layer extruder, significantly reducing the complexity of extrusion equipment and manufacturing costs. Furthermore, both the inner and outer layer materials are prepared using conventional blending and granulation processes, requiring no high-temperature reaction or special post-treatment. This results in a wide process window, good production stability, and ease of industrial-scale mass production.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, specifically to a low-emission fuel composite pipe and its preparation method. Background Technology

[0002] With increasingly stringent vehicle emission regulations, fuel line systems not only need to possess excellent fuel resistance and mechanical strength, but also face higher requirements regarding fuel leaching. Soluble and insoluble leaching substances in the fuel can clog high-pressure injection valves, affecting normal engine operation. Therefore, developing fuel composite pipes with low leaching and high barrier properties has become an urgent industry need.

[0003] To reduce fuel release, existing technologies often use ethylene-vinyl alcohol copolymer (EVOH) as a barrier layer. For example, CN119974719A discloses a five-layer hollow structure, including an inner layer, a thickening layer, an EVOH layer, another thickening layer, and an outer layer. The thickening layer requires a high-temperature (>280℃) reaction to form a block copolymer to achieve reliable adhesion to the EVOH. However, this five-layer structure requires five layers of co-extrusion equipment, resulting in a complex process, high manufacturing costs, and stringent preparation conditions for the thickening layer, limiting its widespread application. Summary of the Invention

[0004] In view of this, the present invention provides a low-emission fuel composite pipe and its preparation method, providing a three-layer composite pipe structure (inner layer / EVOH / outer layer). By compounding at least three different polyamide resins in the polyamide resin of the inner and / or outer layers, direct and high-strength bonding with the EVOH layer is achieved without any separate adhesive or tackifying layers. This structure can be formed in one step using a conventional three-layer extrusion machine, significantly simplifying the production process, reducing costs, and simultaneously meeting the stringent requirements for low fuel emission.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention discloses a low-emission fuel composite pipe, comprising an outer layer, a middle layer, and an inner layer; wherein: The outer layer is composed of the following components in mass percentage: 46-97.9% first polyamide resin, balance first auxiliary agent; The raw material for the intermediate layer is ethylene-vinyl alcohol copolymer EVOH; The raw material of the inner layer is composed of the following components in mass percentage: the raw material of the inner layer includes: 69-97.9% of the second polyamide resin and the balance of the second auxiliary agent.

[0006] As a further aspect of the present invention: the first polyamide resin is at least three of PA610, PA612, PA613, PA614, PA615, PA616, PA1010, PA1012, PA1013, PA1014, PA1015, PA1016, PA6, PA11, PA12, and MXD6.

[0007] As a further aspect of the present invention: the second polyamide resin is at least three of PA610, PA612, PA613, PA614, PA615, PA616, PA1010, PA1012, PA1013, PA1014, PA1015, PA1016, and MXD6.

[0008] As a further aspect of the present invention: the first additive is composed of a first toughening agent, a plasticizer, a first antioxidant, a first chain extender, and a first color masterbatch in a mass ratio of 2-25:0-20:0.1-3:0-3:0-3.

[0009] As a further aspect of the present invention: the second auxiliary agent is composed of a second toughening agent, a second antioxidant, a second chain extender, and a second color masterbatch in a mass ratio of 2-25:0.1-3:0-3:0-3.

[0010] As a further aspect of the present invention: the first toughening agent is selected from at least one of POE-g-MAH, SEBS-g-MAH, EPDM-g-MAH, and LLDPE-g-MAH; and / or, The first chain extender is at least one of ethylene-maleic anhydride copolymer, low-viscosity polycarbonate masterbatch in acid-terminated PA6, and copolymer containing epoxy functional groups; and / or, The first antioxidant is at least one of antioxidant 1098, antioxidant 245, and antioxidant 168; and / or, The first color masterbatch is PA color masterbatch.

[0011] As a further aspect of the present invention: the second toughening agent is at least one selected from POE-g-MAH, SEBS-g-MAH, EPDM-g-MAH, and LLDPE-g-MAH; and / or, The second chain extender is at least one of the following: ethylene-maleic anhydride copolymer, low-viscosity polycarbonate masterbatch in acid-terminated PA6, and copolymer containing epoxy functional groups; and / or, The second antioxidant is at least one of Irganox 1010, Irganox 1098, Irganox 245, and Irganox 168; The second color masterbatch is PA color masterbatch.

[0012] As a further aspect of the present invention: the total wall thickness of the composite pipe is 0.3-5mm, the outer layer thickness is no more than 70% of the total wall thickness; the inner layer thickness is no more than 60% of the total wall thickness; and the intermediate layer thickness is no more than 60% of the total wall thickness.

[0013] Preferably, the outer diameter of the low-emission fuel composite pipe is 3-60 mm, and the total wall thickness is 0.3-5 mm, wherein the thickness of the outer layer is ≤ 60% 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 ≤ 60% of the total wall thickness of the composite pipe.

[0014] More preferably, the outer diameter of the low-emission fuel composite pipe is 4-20 mm, and the total wall thickness is 1-2 mm.

[0015] More preferably, the thickness of the outer layer of the low-emission fuel composite pipe accounts for 20-40% of the total wall thickness of the composite pipe.

[0016] More preferably, the thickness of the inner layer of the low-emission fuel composite pipe accounts for 20-40% of the total wall thickness of the composite pipe.

[0017] More preferably, the thickness of the intermediate layer of the low-emission fuel composite pipe accounts for 20-50% of the total wall thickness of the composite pipe.

[0018] In this invention, the low-emission fuel composite pipe can be a circular straight pipe or a corrugated pipe.

[0019] Secondly, the present invention discloses a method for preparing a low-emission fuel composite pipe as described above, comprising the following steps: After the raw materials for the outer layer are mixed evenly, they are melted, extruded, and granulated to obtain outer layer material particles. After the raw materials of the inner layer are mixed evenly, they are melted, extruded, and granulated to obtain inner layer material particles. The outer layer material particles, the middle layer raw material, and the inner layer material particles are added to a three-layer extrusion machine and melt-extruded to obtain the low-emission fuel composite pipe.

[0020] As a further aspect of the present invention: the temperatures of the outer heating sections of the three-layer extruder are sequentially: Zone 1 120-250℃, Zone 2 230-290℃, Zone 3 230-290℃, Zone 4 230-290℃, Zone 5 230-290℃; the temperatures of the inner heating sections are sequentially: Zone 1 120-250℃, Zone 2 230-290℃, Zone 3 230-290℃, Zone 4 230-290℃, Zone 5 230-290℃, Zone 5 230-290℃, Zone 6 230-290℃, Zone 7 230-290℃, Zone 8 230-290℃, Zone 9 230-290℃, Zone 1 ... Zone 230-290℃; the intermediate layer heating section temperatures are as follows: Zone 1 160-220℃, Zone 2 190-270℃, Zone 3 190-270℃, Zone 4 190-270℃, Zone 5 190-270℃; the common mold head temperatures are as follows: Zone 1 230-290℃, Zone 2 230-290℃, Zone 3 230-290℃, Zone 4 230-290℃, Zone 5 230-290℃.

[0021] Compared with the prior art, the beneficial effects of the present invention are: The composite tube of this invention adopts a three-layer co-extrusion structure, eliminating the need for a separate adhesive layer or tackifying layer. It can be directly formed in one step using a conventional three-layer extrusion machine, significantly reducing the complexity of extrusion equipment and manufacturing costs. Furthermore, both the inner and outer layer materials are prepared using conventional blending and granulation processes, requiring no high-temperature reaction or special post-treatment. This results in a wide process window, good production stability, and ease of industrial-scale mass production.

[0022] The three-layer composite pipe of this invention provides excellent fuel barrier properties through the EVOH intermediate layer, effectively inhibiting the permeation and precipitation of fuel components from the pipe wall, significantly reducing the amount of soluble and insoluble substances precipitated, and meeting the stringent low precipitation requirements of fuel pipelines. Furthermore, the inner and outer layers achieve direct and firm adhesion to the EVOH layer through the synergistic blending of various polyamide resins, avoiding the risk of interlayer separation, while also giving the pipe good flexibility and low-temperature impact resistance. It can be used in both straight and corrugated pipe configurations, meeting the diverse installation and usage requirements of fuel systems. Detailed Implementation

[0023] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0025] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail; unless otherwise specified, the parts in the following embodiments refer to parts by weight.

[0026] The reagents used in the following examples and comparative examples, along with their manufacturers and suppliers, are as follows: Polyamide 610 (PA610), polyamide 612 (PA612), polyamide 613 (PA613), polyamide 614 (PA614), polyamide 615 (PA615), polyamide 616 (PA616), polyamide 1010 (PA1010), polyamide 1012 (PA1012), polyamide 1013 (PA1013), polyamide 1014 (PA1014), polyamide 1015 (PA1015), polyamide 1016 (PA1016), poly(m-phenylene adipamide) (MXD6), polyamide 6 (PA6), polyamide 11 (PA11), and polyamide 12 (PA12) are all produced by Huitong New Materials Co., Ltd., with a relative viscosity of 2.0-5.0. The manufacturer of maleic anhydride-grafted ethylene-octene copolymer (POE-g-MAH) is Dow Chemical Company, and its brand name is FUSABONDN216. The manufacturer of maleic anhydride-grafted hydrogenated styrene-butadiene-styrene block copolymer (SEBS-g-MAH) is KETEN Polymers Co., Ltd., and the grade is FG1901. The manufacturer of maleic anhydride-grafted ethylene propylene diene monomer (EPDM-g-MAH) is ExxonMobil, and the grade is Vistamaxx VA1803. The manufacturer of maleic anhydride-grafted linear low-density polyethylene (LLDPE-g-MAH) is Arkema, and the brand name is OREVAC. ® 18362; The manufacturer of BBSA plasticizer is Proviplast, and the brand name is Proviplast 024. The antioxidants used are Irganox 245, Irganox 1098, Irganox 1010, and Irganox 168, all manufactured by BASF. Chain extender, ethylene maleic anhydride copolymer, using ZeMac E60P from the Vantrus brand; Chain extender, low viscosity polycarbonate masterbatch in acid-terminated PA6, using BrugGOLEN M1251 from the BrugGOLEN brand; The chain extender is a copolymer containing epoxy functional groups, specifically Joncryl ADR-4468 manufactured by BASF.

[0027] The PA black masterbatch is PA1012 black masterbatch purchased from the market; The intermediate barrier layer EVOH uses EVAL from Kuraray Corporation of Japan. TM LA170B.

[0028] The reagents described above are only for illustrating the source and composition of the reagents used in the experiments of this invention, so as to provide full disclosure, and do not imply that the invention cannot be achieved by using other similar reagents or reagents provided by other suppliers.

[0029] The following examples and comparative examples were prepared according to the following method: (1) Preparation of outer layer material According to the formulations shown in the examples and comparative examples in Table 1, polyamide resin, toughening agent, antioxidant, chain extender, and color masterbatch were added to a high-speed mixer and mixed evenly to obtain a premix. The premix was added to the main feed port of a twin-screw extruder in proportion using a metering scale, while the plasticizer was added to the fifth feed port of the twin-screw extruder in proportion using a liquid scale. The mixture was then melt-extruded, drawn into strands, granulated, and dried to obtain outer layer material particles. The temperatures of the twin-screw extruder were set as follows: Zone 1: 180℃; Zone 2: 240℃; Zone 3: 240℃; Zone 4: 240℃; Zone 5: 240℃; Zone 6: 240℃; Zone 7: 240℃; Zone 8: 240℃; Zone 9: 240℃; Zone 10: 240℃; Die head temperature: 250℃; Main extruder speed: 600 r / min.

[0030] (2) Preparation of inner layer material According to the formula shown in Table 2, long-chain polyamide, toughening agent, antioxidant, chain extender, and color masterbatch are added to a high-speed mixer and mixed evenly to obtain a premix. The premix is ​​then added to the main feed port of a twin-screw extruder for melt extrusion, stranding, pelletizing, and drying to obtain inner layer material granules.

[0031] The temperature settings for the twin-screw extruder are as follows: Zone 1: 190℃, Zone 2: 250℃, Zone 3: 250℃, Zone 4: 250℃, Zone 5: 250℃, Zone 6: 250℃, Zone 7: 250℃, Zone 8: 250℃, Zone 9: 250℃, Zone 10: 250℃; Die head temperature: 260℃; Main extruder speed: 400 r / min.

[0032] (3) Preparation of composite pipe The outer layer material particles, inner layer material particles, and middle layer EVOH material particles obtained above are respectively added to a three-layer extruder for melt co-extrusion. After cooling, shaping, traction, and cutting, a three-layer composite pipe is obtained. The three-layer composite pipe is a circular straight pipe, and its specific specifications are shown in Table 3.

[0033] The temperature settings for the three-layer tube extruder are as follows: outer layer heating section: zone 1 190℃, zone 2 240℃, zone 3 250℃, zone 4 250℃, zone 5 250℃; middle layer heating section: zone 1 250℃, zone 2 250℃, zone 3 250℃, zone 4 250℃, zone 5 250℃; inner layer heating section: zone 1 190℃, zone 2 240℃, zone 3 250℃, zone 4 250℃, zone 5 250℃; die head: zone 1 250℃, zone 2 250℃, zone 3 250℃, zone 4 250℃, zone 5 250℃.

[0034] The performance testing items and standards for the inner and outer layer materials are as follows: Tensile properties were tested according to ISO 527, using type 1A injection-molded specimens at a tensile speed of 50 mm / min.

[0035] Low-temperature notched impact strength was tested according to ISO 179, using a 1eA type injection molded specimen, at a test temperature of -30℃, using a simply supported beam impact method.

[0036] The melt mass flow rate was tested according to ISO 1133 at 235°C and 2.16 kg.

[0037] Fuel release performance was tested according to Volkswagen standard VW TL52712.

[0038] Table 1 shows the component ratios of the outer layer materials and the performance test data of the resulting outer layer particles in each embodiment and comparative example.

[0039] Table 1

[0040] As can be seen from Table 1: Compared with Example A1, Comparative Example a1 differs in that PA1010 was not added, and the outer polyamide resin component only contains two long-chain nylon resins. When using this outer layer material to prepare a three-layer composite pipe (see Comparative Example t1 in Table 3), delamination occurs between the outer layer of the pipe and the EVOH intermediate layer, resulting in poor interlayer adhesion.

[0041] Compared to Example A1, Comparative Example a2 differs in that PA12 was not added, and the outer polyamide resin component also only contains two types of long-chain nylon resins. When using this outer layer material to prepare a three-layer composite pipe (see Comparative Example t2 in Table 3), the adhesion between the outer layer of the pipe and the EVOH intermediate layer was also poor.

[0042] Comparative Example a3 differs from Example A1 in that it did not contain a toughening agent, resulting in a decrease in the notched impact strength of the outer layer material in a simply supported beam at -30°C from 12.9 kJ / m in Example A1. 2 Significantly decreased to 3.5 kJ / m 2The low-temperature toughness deteriorates significantly. When this outer layer material was used to prepare the three-layer composite pipe (see Comparative Example t3 in Table 3), the outer layer of the pipe cracked during the low-temperature impact test.

[0043] Compared to Example A1, Comparative Example a4 differs in that no chain extender was added, resulting in a sharp increase in the melt flow rate of the outer layer material from 0.3 g / 10min (235℃ / 2.16kg) in Example A1 to 9.9 g / 10min. When using this outer layer material for three-layer composite pipe extrusion (see Comparative Example t4 in Table 3), the extrusion process is unstable, the thickness distribution between pipe layers is uneven, and continuous and stable production cannot be achieved.

[0044] Comparative Example a5 differs from Example A1 in that the outer layer uses only PA612 single polyamide resin. When a three-layer composite pipe was prepared using this outer layer material (see Comparative Example t5 in Table 3), obvious delamination occurred between the outer layer of the pipe and the EVOH intermediate layer, resulting in poor adhesion.

[0045] Table 2 shows the component ratios of the inner layer materials in each embodiment and comparative example, as well as the performance test data of the obtained inner layer particles.

[0046] Table 2

[0047] As can be seen from Table 2: Compared to Example B1, Comparative Example B1 differs in that PA612 was not added, and the inner polyamide resin component only contains two types of long-chain nylon resins. When using this inner layer material to prepare a three-layer composite pipe (see Comparative Example t6 in Table 3), delamination occurs between the inner layer of the pipe and the EVOH intermediate layer, resulting in poor interlayer adhesion.

[0048] Compared to Example B1, Comparative Example B2 differs in that PA614 was not added, and the inner polyamide resin component also contains only two long-chain nylon resins. When using this inner layer material to prepare a three-layer composite pipe (see Comparative Example t7 in Table 3), the adhesion between the inner layer of the pipe and the EVOH intermediate layer is also poor.

[0049] Compared to Example B1, Comparative Example B3 differs in that the toughening agent SEBS-g-MAH was not added. When this inner layer material was used to prepare a three-layer composite pipe (see Comparative Example t8 in Table 3), the overall stiffness of the pipe was too high, and the flexibility was significantly insufficient.

[0050] Compared to Example B1, Comparative Example b4 differs in that the chain extender ZeMac E60P was not added. When using this inner layer material to prepare a three-layer composite pipe (see Comparative Example t9 in Table 3), the melt flow rate of the inner layer material was too high, the extrusion process was unstable, and the thickness distribution between pipe layers was uneven.

[0051] Compared to Example B1, the inner polyamide resin component of Comparative Example b5 contains only one long-chain nylon resin. When this inner layer material was used to prepare a three-layer composite pipe (see Comparative Example t10 in Table 3), significant delamination occurred between the inner layer of the pipe and the EVOH intermediate layer, resulting in poor adhesion.

[0052] The three-layer composite pipes of each embodiment and comparative example were prepared according to the above method, and their performance was tested. A 200mm long pipe sample was taken and longitudinally cut according to the method specified in Section 7.11 of QC / T 798-2008 standard for interlayer peel testing to evaluate the adhesion performance between the inner, middle, and outer layers. A low-temperature impact test was conducted on the pipe at -40℃ using a pendulum impact device with an energy of 7.5J, according to the method specified in QC / T 80-2011 standard, to observe whether the pipe cracked. Referring to Volkswagen standard VW TL52712, a 200cm long pipe sample was taken, FAMB fuel was sealed inside the pipe, and the content of soluble and insoluble precipitates was tested.

[0053] The results of all the above tests are listed in Table 3.

[0054] Table 3

[0055] As can be seen from Table 3: As shown in Table 3, the three-layer composite pipes of Examples T1 to T4 all exhibited good interlayer adhesion, with a strong bond between the inner layer, outer layer, and middle EVOH layer, and no delamination. Under the low-temperature impact condition of -40℃, none of the pipes cracked, demonstrating excellent low-temperature toughness. The fuel release test results met the requirements of Volkswagen VW TL52712 standard, with the release of both soluble and insoluble substances controlled at a low level.

[0056] In comparison, the pipes in each comparative example have significant shortcomings in various aspects, as detailed below: Comparative examples t1, t2, t5, t6, t7, and t10 exhibit poor interlayer adhesion or complete delamination. This is because the outer or inner polyamide resin composition contains only one or two types of polyamide resin, failing to meet the required blending ratio of three or more types as specified in this invention, thus failing to form sufficient adhesion with the EVOH layer. Comparative example t3's outer layer cracked during the low-temperature impact test due to the absence of a toughening agent in the outer layer material, resulting in a significant decrease in low-temperature toughness. Comparative examples t4 and t9 exhibited instability during extrusion, with uneven interlayer thickness distribution. This was caused by excessively high melt flow rates, failing to meet the process requirements for stable extrusion. Comparative example t8's overall stiffness was excessively high, and its flexibility was poor, due to the absence of the toughening agent SEBS-g-MAH in the inner layer material.

[0057] The results of the above comparative examples further confirm that in the three-layer composite pipe structure proposed in this invention, the inner and outer polyamide resins must simultaneously meet the technical requirements of "component of at least three types of polyamide resins" and "appropriate amount of toughening agents, chain extenders and other additives" in order to achieve good interlayer adhesion, stable processing performance and excellent low-temperature toughness.

[0058] Although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0059] Therefore, the above description is only a preferred embodiment of this application and is not intended to limit the scope of this application; that is, all equivalent modifications made in accordance with the scope of the claims of this application shall be within the protection scope of the claims of this application.

Claims

1. A low-extraction fuel oil composite pipe, characterized by comprising: It includes an outer layer, a middle layer, and an inner layer; among which: The outer layer is composed of the following components in mass percentage: 46-97.9% first polyamide resin, balance first auxiliary agent; The raw material for the intermediate layer is ethylene-vinyl alcohol copolymer EVOH; The raw material of the inner layer is composed of the following components in mass percentage: the raw material of the inner layer includes: 69-97.9% of the second polyamide resin and the balance of the second auxiliary agent.

2. The low-extraction fuel composite tube according to claim 1, characterized by The first polyamide resin is at least three of PA610, PA612, PA613, PA614, PA615, PA616, PA1010, PA1012, PA1013, PA1014, PA1015, PA1016, PA6, PA11, PA12, and MXD6.

3. The low-emission fuel composite pipe according to claim 2, characterized in that, The second polyamide resin is at least three of PA610, PA612, PA613, PA614, PA615, PA616, PA1010, PA1012, PA1013, PA1014, PA1015, PA1016, and MXD6.

4. The low-emission fuel composite pipe according to claim 1, characterized in that, The first additive is composed of a first toughening agent, a plasticizer, a first antioxidant, a first chain extender, and a first color masterbatch in a mass ratio of 2-25:0-20:0.1-3:0-3:0-3.

5. The low-emission fuel composite pipe according to claim 1, characterized in that, The second auxiliary agent is composed of a second toughening agent, a second antioxidant, a second chain extender, and a second color masterbatch in a mass ratio of 2-25:0.1-3:0-3:0-3.

6. The low-emission fuel composite pipe according to claim 4, characterized in that, The first toughening agent is selected from at least one of POE-g-MAH, SEBS-g-MAH, EPDM-g-MAH, and LLDPE-g-MAH; and / or, The first chain extender is at least one of ethylene-maleic anhydride copolymer, low-viscosity polycarbonate masterbatch in acid-terminated PA6, and copolymer containing epoxy functional groups; and / or, The first antioxidant is at least one of antioxidant 1098, antioxidant 245, and antioxidant 168; and / or, The first color masterbatch is PA color masterbatch.

7. The low-emission fuel composite pipe according to claim 5, characterized in that, The second toughening agent is at least one of POE-g-MAH, SEBS-g-MAH, EPDM-g-MAH, and LLDPE-g-MAH; and / or, The second chain extender is at least one of the following: ethylene-maleic anhydride copolymer, low-viscosity polycarbonate masterbatch in acid-terminated PA6, and copolymer containing epoxy functional groups; and / or, The second antioxidant is at least one of Irganox 1010, Irganox 1098, Irganox 245, and Irganox 168; The second color masterbatch is PA color masterbatch.

8. The low-emission fuel composite pipe according to claim 5, characterized in that, The total wall thickness of the composite pipe is 0.3-5mm, the outer layer thickness is no more than 70% of the total wall thickness, the inner layer thickness is no more than 60% of the total wall thickness, and the intermediate layer thickness is no more than 60% of the total wall thickness.

9. A method for preparing a low-emission fuel composite pipe as described in any one of claims 1-8, characterized in that, Includes the following steps: After the raw materials for the outer layer are mixed evenly, they are melted, extruded, and granulated to obtain outer layer material particles. After the raw materials of the inner layer are mixed evenly, they are melted, extruded, and granulated to obtain inner layer material particles. The outer layer material particles, the middle layer raw material, and the inner layer material particles are added to a three-layer extrusion machine and melt-extruded to obtain the low-emission fuel composite pipe.

10. The preparation method according to claim 9, characterized in that, The outer heating section temperatures of the three-layer extruder are sequentially: Zone 1 120-250℃, Zone 2 230-290℃, Zone 3 230-290℃, Zone 4 230-290℃, Zone 5 230-290℃; the inner heating section temperatures are sequentially: Zone 1 120-250℃, Zone 2 230-290℃, Zone 3 230-290℃, Zone 4 230-290℃, Zone 5 230-290℃. 90℃; the heating temperatures of the intermediate layers are as follows: Zone 1 160-220℃, Zone 2 190-270℃, Zone 3 190-270℃, Zone 4 190-270℃, Zone 5 190-270℃; the temperatures of the common mold head are as follows: Zone 1 230-290℃, Zone 2 230-290℃, Zone 3 230-290℃, Zone 4 230-290℃, Zone 5 230-290℃.

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