High-thermal-conductivity flame-retardant HDPE composite communication pipe and preparation method thereof

Abstract

This invention discloses a high thermal conductivity and flame retardant HDPE composite communication pipe and its preparation method. The pipe wall has a three-layer structure consisting of an inner layer, a middle layer, and an outer layer, obtained through a co-extrusion molding process. The inner layer comprises high-density polyethylene, a composite filler consisting of flake-shaped boron nitride with a flame-retardant functionalized surface, and short-cut carbon fibers. The middle layer comprises high-density polyethylene, a polyolefin elastomer, and a composite flame retardant. The outer layer comprises high-density polyethylene, expandable graphite, and zinc borate. The composite filler in the inner layer, the composite flame retardant in the middle layer, and the expandable graphite and zinc borate in the outer layer sequentially form a gradient-synergistic expandable carbon layer barrier when heated. This achieves synergistic effects of thermal conductivity, flame retardancy, and toughening, ensuring both high heat dissipation and high flame retardancy of the pipe, while also providing excellent low-temperature toughness through the intermediate toughening layer. Furthermore, the co-extrusion process solves the technical challenge of simultaneously achieving high performance and mass production feasibility.

Description

Technical Field

[0001] This invention relates to the field of polyolefin materials technology, and in particular to a high thermal conductivity and flame retardant HDPE composite communication pipe and its preparation method. Background Technology

[0002] Currently, to improve the overall performance of high-density polyethylene (HDPE) power and communication pipes, some technologies involve adding various high-performance fibers such as carbon fiber and aramid, along with complex modified fillers, to achieve high heat dissipation characteristics. However, this approach not only significantly increases raw material costs but also involves complex modification systems, cumbersome processes, and difficulties in fiber dispersion within the matrix, easily leading to localized stress concentration in the pipe. This makes the toughness of the final product extremely sensitive to the processing technology, and quality stability is difficult to guarantee. Other technologies employ multi-layered composite structures (such as introducing metal layers or braided layers) to achieve strong flame retardancy and high protection. However, such structural designs are bulky, and controlling the interlayer bonding force is difficult, hindering efficient continuous production. Furthermore, the synthesis process of the flame-retardant system often carries safety risks, making large-scale application impractical. Therefore, how to simplify the structure and process while simultaneously improving the pipe's heat dissipation, flame retardancy, and mechanical toughness has become a pressing technical challenge. Summary of the Invention

[0003] This invention provides a high thermal conductivity and flame retardant HDPE composite communication pipe and its preparation method, in order to solve the technical problem that it is difficult to achieve both high performance and mass production of HDPE power communication pipes in the prior art.

[0004] In a first aspect, the present invention provides a high thermal conductivity and flame retardant HDPE composite communication pipe, wherein the pipe wall of the composite communication pipe has a three-layer structure consisting of an inner layer, a middle layer, and an outer layer, obtained by a co-extrusion molding process. The inner layer comprises the following raw material components by weight: 100 parts of high-density polyethylene, and 15 to 45 parts of a composite filler of flammable boron nitride and short-cut carbon fibers with flame-retardant functional treatment on the surface. The intermediate layer comprises the following raw material components by weight: 100 parts high-density polyethylene, 15 to 35 parts polyolefin elastomer, and 25 to 55 parts compounded flame retardant. The outer layer comprises the following raw material components by weight: 100 parts high-density polyethylene, 15 to 25 parts expandable graphite, and 5 to 10 parts zinc borate. The compound filler in the inner layer, the compound flame retardant in the middle layer, and the expandable graphite and zinc borate in the outer layer sequentially form a gradient-synergistic expandable carbon layer barrier when heated.

[0005] In some embodiments, the composite filler consists of 10 to 30 parts of flake-shaped boron nitride and 5 to 15 parts of chopped carbon fibers, which are surface-pretreated with a bifunctional coupling agent containing phosphorus-nitrogen flame retardant elements. The bifunctional coupling agent is a blend of a silane coupling agent and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO).

[0006] By using silane coupling agent and DOPO as bifunctional coupling agents, the thermally conductive filler can become a char-forming node in the flame-retardant system while constructing a thermally conductive network. This not only improves the interfacial bonding force between the filler and the matrix, but also promotes char formation in the matrix through phosphorus-nitrogen flame-retardant elements during combustion, significantly enhancing the char stability of the inner layer.

[0007] In some embodiments, the flake-like boron nitride forms an anisotropic thermally conductive network in the inner layer, and the ratio of the in-plane thermal conductivity to the thickness-direction thermal conductivity of the thermally conductive network is greater than 5:1.

[0008] Anisotropic thermal conductive networks enable heat to be conducted primarily along the axial direction of the tube wall, which improves heat dissipation efficiency and reduces heat radiation to the external environment. At the same time, they can work in synergy with the thermal bridging effect of short-cut carbon fibers to construct efficient in-plane thermal conductive channels.

[0009] In some of the embodiments, the compound flame retardant includes 5 to 15 parts of microencapsulated red phosphorus and 20 to 30 parts of magnesium hydroxide, with a mass ratio of microencapsulated red phosphorus to magnesium hydroxide of 1:(2 to 4), and the coating wall material of the microencapsulated red phosphorus is melamine resin or phenolic resin.

[0010] Red phosphorus plays a gas-phase flame-retardant role, while magnesium hydroxide absorbs heat and cools down in the condensed phase and releases water vapor. The two work together to achieve highly efficient halogen-free flame retardancy. The microencapsulated wall material effectively prevents red phosphorus from absorbing moisture and oxidizing, ensuring the long-term stability of flame-retardant performance.

[0011] In some embodiments, the polyolefin elastomer is an ethylene-octene copolymer, which has excellent compatibility with HDPE and forms a uniformly dispersed elastomer phase in the intermediate layer. When subjected to external impact, it acts as a stress concentration point, triggering a large number of crazing to absorb impact energy, significantly improving the pipe's resistance to environmental stress cracking and low-temperature toughness.

[0012] In some embodiments, the mass ratio of the expandable graphite to the zinc borate is (2.5~3.5):1, the expansion ratio of the expandable graphite is 80 to 180 times, and the average particle size of the zinc borate is 1 μm to 5 μm.

[0013] Within this preferred range, expandable graphite expands rapidly upon heating to form a loose carbon layer, while zinc borate is evenly dispersed and promotes the densification of the carbon layer. Together, they form a high-strength, high-density expandable carbon layer barrier, achieving optimal flame retardant effect.

[0014] In some embodiments, the wall thickness ratio of the inner layer, intermediate layer, and outer layer is 1:(1.2~1.5):(1~1.2). The intermediate layer has the largest thickness to ensure that the POE toughening layer can fully exert its buffering and energy absorption function, while the outer layer thickness is not less than that of the inner layer to meet the requirements of wear resistance and weather resistance, achieving the optimal matching of stress transfer and deformation coordination among the three layers.

[0015] Optionally, all layers also include antioxidants (hindered phenols, phosphites), lubricants (stearates, polyethylene waxes) and dispersants (silicone powder, polar grafts), with the additives accounting for 0.1% to 1% of the total mass of each layer.

[0016] Secondly, the present invention also provides a method for preparing a high thermal conductivity and flame retardant HDPE composite communication tube, comprising the following steps: S1, flake boron nitride and short-cut carbon fibers are respectively coated with a bifunctional coupling agent containing phosphorus-nitrogen flame retardant elements in a high-speed mixer at 80℃~100℃. Then, the treated flake boron nitride and the treated short-cut carbon fibers are mixed to obtain a composite filler with flame retardant functional treatment on the surface. S2, the raw material components of the inner layer, middle layer and outer layer are mixed evenly in their respective mixing systems to obtain the inner layer mixture, the middle layer mixture and the outer layer mixture; S3, the inner layer mixture, the middle layer mixture and the outer layer mixture are respectively added to three extruders, the three extruders work synchronously, and the material is conveyed to the three-layer co-extrusion composite die after being melted and plasticized; S4, in the composite die head, the three layers of molten material converge in a laminar flow state. Through the structure of the middle layer flow channel in front, the middle layer melt preferentially contacts the inner layer melt and the outer layer melt to form a melt lamination bond. After vacuum sizing, cooling and shaping, traction and cutting, the high thermal conductivity and flame retardant HDPE composite communication pipe is obtained.

[0017] In some embodiments, the three-layer co-extrusion composite die includes an inner layer flow channel, an intermediate layer flow channel, and an outer layer flow channel. The inner layer flow channel and the outer layer flow channel are symmetrically arranged about the intermediate layer flow channel. The outlet end of the intermediate layer flow channel is positioned 0.5 mm to 1.5 mm ahead of the outlet ends of the inner layer flow channel and the outer layer flow channel in the melt flow direction, so that the intermediate layer melt preferentially contacts the inner layer melt and the outer layer melt and achieves wetting bonding.

[0018] In some embodiments, the temperature control range of each section of the three extruders is as follows: feeding section 150℃~170℃, plasticizing and melting section 180℃~200℃, and die head connecting section 190℃~210℃; the temperature of the three-layer co-extrusion composite die head is 190℃~210℃.

[0019] Compared with the prior art, the present invention has at least the following beneficial effects: The inner layer of this communication tube contains flake-like boron nitride and chopped carbon fibers, which, after flame-retardant functionalization, form a thermally conductive network within the HDPE matrix, while also serving as char-forming nodes. The middle layer uses a toughened HDPE matrix with polyolefin elastomer and a compounded flame retardant to provide expansion flame retardancy. The outer layer is a compound of expandable graphite and zinc borate, imparting rapid char formation and wear and weather resistance. In the event of a fire, the three-layer flame-retardant system is triggered sequentially: the outer expandable graphite rapidly expands to form a dense carbon layer that isolates oxygen; the middle compounded flame retardant further expands to form char and releases non-combustible gases during combustion; and the phosphorus-nitrogen flame-retardant elements on the surface of the functionalized filler in the inner layer promote char formation in the matrix and stabilize the carbon layer structure, forming a gradient synergistic expandable carbon layer barrier that effectively prevents heat from being transferred to the internal cable. This achieves synergistic effects of thermal conductivity, flame retardancy, and toughening, ensuring both high heat dissipation and high flame retardancy of the tube, while also providing excellent low-temperature toughness through the intermediate toughening layer. Furthermore, the co-extrusion process solves the technical challenge of achieving both high performance and mass production feasibility.

[0020] This preparation method employs a one-step three-extrusion co-extrusion process, addressing dispersion challenges through filler surface pretreatment. The simultaneous operation of the three extruders enables the synchronous extrusion of three layers of melt. A 0.5-1.5mm pre-positioning of the intermediate layer's flow channel outlet allows for preferential contact and wetting of the inner and outer melt layers, forming a melt lamination bond with intertwined molecular chains. This interlayer bonding strength is significantly higher than that achieved through post-composite or adhesive bonding methods. Stepped temperature control ensures that each layer melts and plasticizes at an appropriate temperature, guaranteeing full melting without thermal degradation, thus enabling continuous, stable, and efficient production of high thermal conductivity and flame-retardant HDPE composite communication pipes. Detailed Implementation

[0021] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0022] As used herein, the terms “prepared from” and “comprising” are synonymous. The terms “comprising,” “including,” “having,” “containing,” or any other variations thereof, as used herein, are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0023] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, it should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually.

[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. Unless otherwise specified, the raw materials used in the following examples and comparative examples are commercially available industrial products, and all conditions not otherwise specified are conventional. Example 1

[0025] (1) Preparation of bifunctional coupling agent: 100 parts of silane coupling agent KH-550 and 30 parts of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were added to a reaction vessel, heated to 110°C under nitrogen protection, stirred for 6 hours, and cooled to obtain a bifunctional coupling agent containing phosphorus-nitrogen flame retardant elements; (2) Preparation of compound filler: Take 10 parts of flake boron nitride and 5 parts of short carbon fiber (0.2 mm in length), and respectively with 2.5 parts of the above-mentioned bifunctional coupling agent. They are coated on the surface at 90°C for 20 minutes in a high-speed mixer. Then, the two fillers after treatment are mixed evenly to obtain a compound filler with flame-retardant functional treatment on the surface. (3) Preparation of inner layer mixture: Weigh 100 parts of high-density polyethylene (melt index 0.15 g / 10 min, density 0.952 g / cm³) and 15 parts of the above compound filler, and mix them together with antioxidant 1010, polyethylene wax and silicone powder in a high-speed mixer for 10 minutes to obtain inner layer mixture; (4) Preparation of intermediate layer mixture: Weigh 100 parts of high-density polyethylene, 15 parts of ethylene-octene copolymer (POE), 5 parts of microencapsulated red phosphorus (melamine resin coated), and 20 parts of magnesium hydroxide (average particle size 2μm) by weight, and mix them together with antioxidant 1010, polyethylene wax and silicone powder in a high-speed mixer for 10 minutes to obtain intermediate layer mixture; (5) Preparation of outer layer mixture: Weigh 100 parts of high-density polyethylene, 15 parts of expandable graphite (expansion ratio of 80 times), 5 parts of zinc borate (average particle size of 2μm), 0.5 parts of antioxidant 1010 and 0.5 parts of ultraviolet absorber UV-531, and mix them together with polyethylene wax and silicone powder in a high-speed mixer for 10 minutes to obtain the outer layer mixture; (6) Composite communication tubes were prepared using a one-step three-stage co-extrusion process: The inner layer mixture, middle layer mixture, and outer layer mixture are respectively added to three single-screw extruders; Extruder temperatures: 155℃ for the feeding section, 185℃ for the plasticizing and melting section, and 195℃ for the die connection section; Three-layer co-extrusion composite die head temperature: 195℃; Die head structure: The outlet end of the middle layer flow channel is positioned 0.5mm ahead of the outlet ends of the inner and outer layer flow channels; Vacuum sizing: cooling water temperature 18℃, vacuum degree 0.03MPa; Traction speed: 1.8 meters per minute; High thermal conductivity and flame retardant HDPE composite communication pipes are obtained through vacuum sizing, cooling and shaping, traction and cutting. Example 2

[0026] The difference from Example 1 is as follows: (2) Preparation of compound filler: 20 parts of flake boron nitride, 10 parts of short-cut carbon fiber (0.3 mm in length), and 5 parts of the above-mentioned bifunctional coupling agent; (3) Preparation of inner layer mixture: 100 parts of high-density polyethylene and 30 parts of compound filler; (4) Preparation of intermediate layer mixture: 100 parts of high-density polyethylene, 25 parts of ethylene-octene copolymer, 10 parts of microencapsulated red phosphorus (phenolic resin coated), and 25 parts of magnesium hydroxide; (5) Preparation of outer layer mixture: 100 parts of high-density polyethylene, 20 parts of expandable graphite (expansion ratio 130 times), 8 parts of zinc borate (average particle size 3μm), 1 part of antioxidant 1010, and 1 part of ultraviolet absorber UV-531. (6) Composite communication tubes were prepared using a one-step three-stage co-extrusion process: Extruder temperatures: 160℃ for the feeding section, 190℃ for the plasticizing and melting section, and 200℃ for the die connection section; The temperature of the three-layer co-extrusion composite die head is 200℃. Die head structure: The outlet end of the middle layer flow channel is positioned 1mm ahead of the outlet ends of the inner and outer layer flow channels; Vacuum sizing: cooling water temperature 20℃, vacuum degree 0.03MPa; Traction speed: 2.0 m / min. Example 3

[0027] The difference from Example 1 is as follows: (2) Preparation of compound filler: 30 parts of flake boron nitride, 15 parts of short-cut carbon fiber (0.4 mm in length), and 7.5 parts of the above-mentioned bifunctional coupling agent; (3) Preparation of inner layer mixture: 100 parts of high-density polyethylene and 45 parts of compound filler; (4) Preparation of intermediate layer mixture: 100 parts of high-density polyethylene, 35 parts of ethylene-octene copolymer, 15 parts of microencapsulated red phosphorus (phenolic resin coated), and 30 parts of magnesium hydroxide; (5) Preparation of outer layer mixture: 100 parts of high-density polyethylene, 25 parts of expandable graphite (expansion ratio 180 times), 10 parts of zinc borate (average particle size 4μm), 1.5 parts of antioxidant 1010, and 1.5 parts of ultraviolet absorber UV-531. (6) Composite communication tubes were prepared using a one-step three-stage co-extrusion process: Extruder temperatures: 165℃ for the feeding section, 195℃ for the plasticizing and melting section, and 205℃ for the die connection section; The temperature of the three-layer co-extrusion composite die head is 205℃; Die head structure: The outlet end of the middle layer flow channel is positioned 1.5mm ahead of the outlet ends of the inner and outer layer flow channels; Vacuum sizing: cooling water temperature 22℃, vacuum degree 0.04MPa; Traction speed: 2.2 meters per minute.

[0028] Comparative Example 1 The difference from Example 1 is that the following preparation method is used instead of the co-extrusion molding process: after all raw materials are mixed evenly in a high-speed mixer, they are added to a single-screw extruder for melt extrusion at an extruder temperature of 185°C. After vacuum sizing, cooling and shaping, traction and cutting, a single-layer communication tube is obtained.

[0029] Comparative Example 2 The difference from Example 1 is that 10 parts of untreated flake boron nitride and 5 parts of short-cut carbon fibers (0.2 mm in length) are used instead of the composite filler.

[0030] Comparative Example 3 The difference from Example 1 is that 15 parts of flake-shaped boron nitride surface-treated with a bifunctional coupling agent are used instead of the compound filler.

[0031] Comparative Example 4 The difference from Example 1 is that 15 parts of short-cut carbon fibers (0.2 mm in length) surface-treated with a bifunctional coupling agent are used instead of the composite filler.

[0032] Comparative Example 5 The difference from Example 1 is that a commonly used polyolefin toughening modifier (ethylene propylene diene monomer rubber) is used instead of POE.

[0033] Comparative Example 6 The difference from Example 1 is that the outer layer contains 50 parts of expandable graphite and 5 parts of zinc borate (mass ratio 10:1).

[0034] Comparative Example 7 The difference from Example 1 is that a conventional three-layer co-extrusion die (with the inner, middle, and outer layers flush at the outlet ends) is used instead of a three-layer co-extrusion composite die.

[0035] I. Thermal conductivity test 1. Axial thermal conductivity test: The test was conducted according to GB / T 3399-1982 "Test Method for Thermal Conductivity of Plastics - Heat-Protected Plate Method". A 100mm × 100mm × pipe wall thickness (approximately 3-5mm) sample was prepared by cutting axially from the middle of the pipe and flattening it. After pretreatment for 24 hours at 23℃ and 50% relative humidity, the sample was placed between the hot and cold plates of the heat-protected plate thermal conductivity tester. The hot plate temperature was set to 35℃, the cold plate temperature to 15℃, and the temperature difference to 20℃. After the heat flow stabilized, the temperature was continuously recorded for 30 minutes. The axial thermal conductivity was calculated based on the heat flow, sample thickness, sample area, and temperature difference. The arithmetic mean of three samples in each group of tests was taken.

[0036] 2. Radial thermal conductivity test: According to GB / T 3399-1982, the pipe wall was cut radially from the middle of the pipe. Three to four layers of pipe wall were hot-pressed at 180℃ and 5MPa for 5 minutes to make a 100mm×100mm×3mm thin sheet sample. After pretreatment at 23℃ and 50% relative humidity for 24 hours, the sample was placed in a heat-conducting flat plate thermal conductivity meter with a hot plate temperature of 35℃, a cold plate temperature of 15℃, and a temperature difference of 20℃. After the heat flow stabilized, the temperature was continuously recorded for 30 minutes. The radial thermal conductivity was calculated. The arithmetic mean of three samples in each group of tests was taken.

[0037] The ratio of axial thermal conductivity to radial thermal conductivity was calculated as the anisotropy ratio, and the test results are shown in Table 1.

[0038] Table 1. Thermal conductivity test results of the examples and comparative examples. Example 1 2.15 0.32 6.72 Example 2 2.71 0.38 7.13 Example 3 2.97 0.52 5.71 Comparative Example 1 0.45 0.40 1.13 Comparative Example 2 0.72 0.24 3.00 Comparative Example 3 0.98 0.30 3.27 Comparative Example 4 0.88 0.34 2.59 Comparative Example 5 1.32 0.27 4.89 Comparative Example 6 1.33 0.27 4.93 Comparative Example 7 1.10 0.29 3.79 As shown in Table 1, the single-layer structure of Comparative Example 1, which simply mixes all components, has an axial thermal conductivity of only 0.45 W / (m·K), comparable to that of ordinary HDPE. However, Examples 1-3 employ a three-layer functional decoupled structure, concentrating the thermally conductive filler in the inner layer and using co-extrusion molding to form a highly oriented structure. The axial thermal conductivity reaches 2.15 W / (m·K) to 2.97 W / (m·K), which is significantly improved compared to the single-layer structure. At the same time, the radial thermal conductivity is controlled at 0.32 W / (m·K) to 0.52 W / (m·K), and the anisotropy ratio is as high as 5.71 to 7.13, achieving an ideal heat dissipation path with efficient axial heat conduction and low radial loss.

[0039] In Comparative Example 2, the axial thermal conductivity of the filler without functionalization was only 0.72 W / (m·K), while in Example 1, after surface treatment with a bifunctional coupling agent, it increased to 2.15 W / (m·K), indicating that functionalization significantly improved the filler's dispersibility and interfacial bonding. Surprisingly, Comparative Example 3 (BN only) had an axial thermal conductivity of 0.98 W / (m·K), and Comparative Example 4 (carbon fiber only) had an axial thermal conductivity of 0.88 W / (m·K), while Example 1 reached 2.15 W / (m·K), significantly exceeding expectations. This indicates that BN and carbon fiber produce a strong synergistic effect. The BN lamellar orientation forms a highly anisotropic framework, while the carbon fiber acts as an in-plane thermal bridging agent. After functionalization, the two construct a complete in-plane thermally conductive network.

[0040] In Comparative Example 5, after replacing POE with EPDM, the axial thermal conductivity decreased from 2.15 W / (m·K) in Example 1 to 1.32 W / (m·K), indicating that the complete compatibility of POE and HDPE is crucial for maintaining the orientation of the inner thermally conductive filler. In Comparative Example 6, after adjusting the ratio of the outer expandable graphite to zinc borate to 10:1, the axial thermal conductivity decreased to 1.33 W / (m·K), indicating that the proportion of the outer flame retardant indirectly affects the orientation of the inner filler by influencing the melt rheological behavior. In Comparative Example 7, after adopting a flush die flow channel structure, the axial thermal conductivity decreased to 1.10 W / (m·K), indicating that placing the intermediate layer flow channel in the front is a key process to ensure the stability of the three-layer melt laminar flow and ensure the high orientation of the filler.

[0041] II. Flame Retardant Performance Test Methods 1. Limiting Oxygen Index Test: According to GB / T 2406.2-2009 "Determination of Combustion Behavior by Oxygen Index Method for Plastics - Part 2: Room Temperature Test", a 150mm×6.5mm×3mm sample was prepared from the pipe wall. After pretreatment at 23℃ and 50% relative humidity for 48 hours, the sample was vertically installed in the combustion chamber of the oxygen index tester. By adjusting the mixing ratio of oxygen and nitrogen, the sample was kept burning for 180 seconds or the burning length reached 50mm in an airflow of 40±2mm / s. At least 5 samples were tested at different oxygen concentrations using the lifting method, and the lowest oxygen concentration was calculated as the limiting oxygen index.

[0042] 2. UL 94 vertical flammability test: The test was conducted according to UL 94, "Tests for flammability of plastic materials for equipment and appliance components". Samples of 125mm × 13mm × 3mm were prepared from the pipe wall. After pretreatment at 23°C and 50% relative humidity for 48 hours, the samples were vertically installed in the combustion test chamber with the lower end of the sample 300mm away from the horizontal cotton layer. A blue flame of 20±1mm in height was applied to the lower end of the sample twice for 10 seconds each time. The afterflame time of the first flame, the afterflame time of the second flame, and the afterburn time were recorded. At the same time, it was observed whether the sample burned to the 125mm mark and whether the dripping material ignited the cotton. The V-0, V-1, or V-2 rating was determined based on the afterflame time and the ignition of the dripping material.

[0043] III. Mechanical Property Testing Methods 1. Low-temperature notched impact strength test: The test was conducted according to GB / T 1843-2008 "Determination of Impact Strength of Plastic Cantilever Beams". A type A notched specimen (notch depth 2 mm, notch root radius 0.25 mm) of 80 mm × 10 mm × 4 mm was prepared by axial sampling from the pipe wall. After pretreatment at 23℃ and 50% relative humidity for 48 hours, the specimen was placed in a -30℃ low temperature chamber for 24 hours. Within 10 seconds after removal, the specimen was vertically mounted on the cantilever beam impact testing machine fixture with the notch facing the impact direction of the pendulum. The pendulum was released to impact the specimen, and the impact absorbed energy was recorded. The notched impact strength was calculated based on the absorbed energy, the remaining notch width, and the specimen width. The arithmetic mean of 10 specimens was taken for each group of tests.

[0044] 2. Elongation at break test: The test was conducted according to GB / T 1040.2-2006 "Determination of tensile properties of plastics - Part 2: Test conditions for molded and extruded plastics". A dumbbell-shaped 1B specimen (total length ≥ 150 mm, narrow section width 10 mm, thickness approximately 4 mm) was prepared by axially sampling from the pipe wall. After pretreatment at 23℃ and 50% relative humidity for 48 hours, the specimen was mounted on the universal testing machine fixture, ensuring the specimen axis was aligned with the tensile direction. The specimen was stretched at a constant speed of 50 mm / min, and the elongation at fracture was recorded using an extensometer with a gauge length of 50 mm. The elongation at fracture was calculated based on the ratio of the elongation at fracture to the initial gauge length. The arithmetic mean of five specimens in each test group was taken.

[0045] Table 2. Flame retardant and mechanical property test results of the examples and comparative examples. Example 1 32.8 V-0 13.7 412.2 Example 2 34.2 V-0 12.5 383.7 Example 3 35.1 V-0 11.2 347.3 Comparative Example 1 25.5 V-2 7.4 195.5 Comparative Example 2 23.2 No grade 11.6 371.8 Comparative Example 3 24.6 V-2 12.1 384.4 Comparative Example 4 24.9 V-2 11.8 379.4 Comparative Example 5 27.4 V-1 6.5 242.6 Comparative Example 6 27.6 V-1 11.7 375.3 Comparative Example 7 32.7 V-0 8.3 260.1 As shown in Table 2, in terms of flame retardant performance, Example 1 achieved a limiting oxygen index (LOI) of 32.8% and met the UL 94 V-0 rating, while the single-layer structure of Comparative Example 1 only achieved 25.5% and a V-2 rating. This indicates that the three-layer functional decoupling design creates a gradient synergistic barrier through the inner functionalized filler promoting char formation, the middle MRP+MDH expansion flame retardancy, and the outer EG+ZB rapid char formation. In Comparative Example 2, the LOI was only 23.2% and did not meet the UL 94 rating when the filler was not functionalized. Example 1 introduced PN flame retardant elements through a bifunctional coupling agent, making the thermally conductive filler also a char formation node, thus improving flame retardant performance. In Comparative Examples 3 and 4, the LOI of single fillers was only 24.6%–24.9% and a V-2 rating, indicating that BN and carbon fiber form a three-dimensional flame retardant network. In Comparative Example 6, the LOI was only 27.6% and a V-1 rating when the outer EG:ZB ratio was 10:1, indicating that ZB fills the pores of the EG carbon layer within a narrow ratio range, producing a critical synergistic effect.

[0046] In terms of mechanical properties, Example 1 exhibits an impact strength of 13.7 kJ / m² and an elongation at break of 412.2% at -30℃, while the single-layer structure of Comparative Example 1 only achieves 7.4 kJ / m² and 195.5%, indicating that the three-layer structure significantly improves impact strength and elongation at break by concentrating and toughening POE. In Comparative Example 5, replacing POE with EPDM drastically reduces the impact strength to 6.5 kJ / m² and the elongation at break to 142.6%, demonstrating that the nanoscale dispersion formed by the linear structure of POE and the synergistic toughening mechanism of crazing-shear bands are far superior to EPDM. In Comparative Example 7, with the die head aligned, the impact strength is only 8.3 kJ / m² and the elongation at break is 260.1%, proving that the front placement of the intermediate layer flow channel ensures the stability of the three-layer melt laminar flow, maintaining the optimal dispersion morphology of POE.

[0047] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0048] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A high thermal conductivity and flame retardant HDPE composite communication pipe, characterized in that, The composite communication tube has a three-layer structure consisting of an inner layer, a middle layer, and an outer layer, obtained through a co-extrusion molding process. The inner layer comprises the following raw material components by weight: 100 parts of high-density polyethylene, and 15 to 45 parts of a composite filler of flammable boron nitride and short-cut carbon fibers with flame-retardant functional treatment on the surface. The intermediate layer comprises the following raw material components by weight: 100 parts high-density polyethylene, 15 to 35 parts polyolefin elastomer, and 25 to 55 parts compounded flame retardant. The outer layer comprises the following raw material components by weight: 100 parts high-density polyethylene, 15 to 25 parts expandable graphite, and 5 to 10 parts zinc borate. The compound filler in the inner layer, the compound flame retardant in the middle layer, and the expandable graphite and zinc borate in the outer layer sequentially form a gradient synergistic expandable carbon layer barrier when heated.

2. The high thermal conductivity and flame retardant HDPE composite communication pipe as described in claim 1, characterized in that, The composite filler consists of 10 to 30 parts of flake-shaped boron nitride and 5 to 15 parts of short-cut carbon fibers, which are surface-pretreated with a bifunctional coupling agent containing phosphorus-nitrogen flame retardant elements. The bifunctional coupling agent is a blend of silane coupling agent and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide.

3. The high thermal conductivity and flame retardant HDPE composite communication pipe as described in claim 1, characterized in that, The flake-shaped boron nitride forms an anisotropic thermally conductive network in the inner layer, and the ratio of the in-plane thermal conductivity to the thickness-direction thermal conductivity of the thermally conductive network is greater than 5:

1.

4. The high thermal conductivity and flame retardant HDPE composite communication pipe as described in claim 1, characterized in that, The compound flame retardant comprises 5 to 15 parts of microencapsulated red phosphorus and 20 to 30 parts of magnesium hydroxide, with a mass ratio of microencapsulated red phosphorus to magnesium hydroxide of 1:(2 to 4). The coating wall material of the microencapsulated red phosphorus is melamine resin or phenolic resin.

5. The high thermal conductivity and flame retardant HDPE composite communication pipe as described in claim 1, characterized in that, The polyolefin elastomer is an ethylene-octene copolymer.

6. The high thermal conductivity and flame retardant HDPE composite communication pipe as described in claim 1, characterized in that, The mass ratio of the expandable graphite to the zinc borate is (2.5~3.5):1, and the expansion ratio of the expandable graphite is 80 times to 180 times, and the average particle size of the zinc borate is 1μm to 5μm.

7. The high thermal conductivity and flame retardant HDPE composite communication pipe as described in claim 1, characterized in that, The wall thickness ratio of the inner layer, the middle layer and the outer layer is 1:(1.2~1.5):(1~1.2).

8. A method for preparing a high thermal conductivity and flame retardant HDPE composite communication pipe as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1, flake boron nitride and short-cut carbon fibers are respectively coated with a bifunctional coupling agent containing phosphorus-nitrogen flame retardant elements in a high-speed mixer at 80℃~100℃. Then, the treated flake boron nitride and the treated short-cut carbon fibers are mixed to obtain a composite filler with flame retardant functional treatment on the surface. S2, the raw material components of the inner layer, middle layer and outer layer are mixed evenly in their respective mixing systems to obtain the inner layer mixture, the middle layer mixture and the outer layer mixture; S3, the inner layer mixture, the middle layer mixture and the outer layer mixture are respectively added to three extruders, the three extruders work synchronously, and the material is conveyed to the three-layer co-extrusion composite die after being melted and plasticized; S4, in the composite die head, the three layers of molten material converge in a laminar flow state. Through the structure of the middle layer flow channel in front, the middle layer melt preferentially contacts the inner layer melt and the outer layer melt to form a melt lamination bond. After vacuum sizing, cooling and shaping, traction and cutting, the high thermal conductivity and flame retardant HDPE composite communication pipe is obtained.

9. The preparation method according to claim 8, characterized in that, The three-layer co-extrusion composite die head includes an inner layer flow channel, an intermediate layer flow channel, and an outer layer flow channel. The inner layer flow channel and the outer layer flow channel are symmetrically arranged about the intermediate layer flow channel. The outlet end of the intermediate layer flow channel is positioned 0.5mm to 1.5mm forward in the melt flow direction relative to the outlet ends of the inner layer flow channel and the outer layer flow channel, so that the intermediate layer melt preferentially contacts the inner layer melt and the outer layer melt and achieves wetting bonding.

10. The preparation method according to claim 8, characterized in that, The temperature control ranges for each section of the three extruders are as follows: feeding section 150℃~170℃, plasticizing and melting section 180℃~200℃, and die head connection section 190℃~210℃; the temperature of the three-layer co-extrusion composite die head is 190℃~210℃.