Halogen-free flame-retardant double-walled pipe and method for producing the same

Abstract

This invention relates to the field of cable conduit technology, specifically disclosing a halogen-free flame-retardant double-walled pipe and its preparation method. The halogen-free flame-retardant double-walled pipe consists of an outer wall and an inner wall. The outer wall material includes polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole-based phosphorus-nitrogen-silicon flame retardant, aminated halloysite nanotubes, zinc-based synergists, and plasticizers. The imidazole-based phosphorus-nitrogen-silicon flame retardant, aminated halloysite nanotubes, and zinc-based synergists can form a multi-element flame-retardant system, effectively improving the flame-retardant performance of the double-walled pipe, achieving an oxygen index of over 35%, passing the UL-94 vertical burning test V-0 rating, achieving post-ignition expansion and smoke suppression, and producing no dripping or significant smoke during combustion. This allows the double-walled pipe to be widely used in various fields such as insulation flame retardancy, joint sealing, and pipe corrosion protection for wires and cables.

Description

Technical Field

[0001] This invention relates to the field of cable conduit technology, and more specifically, to a halogen-free flame-retardant double-walled conduit and its preparation method. Background Technology

[0002] With the rapid development of power, communication, and urban underground pipeline construction, the performance requirements for cable protection conduits are increasing. Single-layer cable conduits have limitations in terms of mechanical strength, compressive strength, corrosion resistance, and insulation performance. They are prone to deformation and cracking, especially in complex environments, affecting the safety and service life of cables. Double-walled conduits, on the other hand, typically consist of two layers. The outer layer provides insulation, corrosion resistance, and wear resistance, while the inner layer, with its low melting point, achieves waterproof sealing and strong adhesion. They can be widely used for waterproofing and leak-proofing wiring in electronic equipment; corrosion protection at wire branches; repair of wires and cables; and waterproofing wiring in water pumps and submersible pumps.

[0003] Currently, invention patent CN109096573A discloses a PE double-wall pipe, whose wall is composed of a double-layer structure. The inner layer consists of high-density polyethylene, chlorinated polyethylene, polymethyl methacrylate, light calcium carbonate, antioxidants, and plasticizers; the outer layer consists of low-density polyethylene, chlorinated polyethylene, poly-1-butyl ester, butadiene-methyl methacrylate copolymer, fluorescent agents, plasticizers, antioxidants, and ultraviolet absorbers. This double-wall pipe improves the wall strength, impact resistance, and toughness, as well as the mechanical properties of the double-wall pipe, through the differentiated design of the inner and outer layer formulations. However, the main components of the inner and outer layers... Both high-density polyethylene and low-density polyethylene are flammable materials, and their dust can burn and explode in the air. They do not have flame-retardant properties, and they generate a lot of heat and smoke during combustion, and can also produce molten droplets, which may cause secondary disasters. Chlorinated polyethylene contains chlorine and has certain flame-retardant properties, but it releases a large amount of toxic and harmful hydrogen halide gas and dense smoke during combustion, which can not only cause serious harm to the human body, but also corrode equipment, hinder escape and rescue operations, and pose a great threat to life and property safety. Therefore, the flame retardancy of this double-walled pipe is poor and cannot meet the stability and safety requirements in extreme environments.

[0004] Based on the above statements, the present invention provides a halogen-free flame-retardant double-walled tube and its preparation method. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a halogen-free flame-retardant double-walled tube and its preparation method.

[0006] In a first aspect, the present invention provides a halogen-free flame-retardant double-walled tube, which adopts the following technical solution:

[0007] A halogen-free flame-retardant double-walled pipe is composed of an outer wall and an inner wall; the outer wall material comprises the following components by weight: 65-85 parts of polyethylene, 10-20 parts of ethylene-vinyl acetate copolymer, 5-15 parts of polyethylene grafted maleic anhydride, 25-35 parts of ethylene propylene diene monomer (EPDM) rubber, 8-19 parts of imidazole-based phosphorus-nitrogen-silicon flame retardant, 5-10 parts of aminated halloysite nanotubes, 1-5 parts of zinc-based synergist, and 0.5-2.5 parts of plasticizer.

[0008] Preferably, the imidazole-based phosphorus nitrogen silicon flame retardant has the following structural formula:

[0009] .

[0010] Preferably, the imidazole-based phosphorus nitrogen silicon flame retardant is prepared by the following method:

[0011] S1. Under nitrogen protection, ethanolamine phosphate is dissolved in anhydrous dimethyl sulfoxide, and 2-phenylethyl silane trichloride is added dropwise through a constant pressure funnel. After the addition is completed, the temperature is raised to 50-80℃ and the reaction is continued for 8-12 hours. Then, triethylamine is added, the pH is adjusted to 7.5-8, the mixture is distilled under reduced pressure, recrystallized, filtered, and dried to obtain phosphorus-nitrogen-silicon flame retardant.

[0012] S2. Add 5-aminobenzimidazole to an aqueous ethanol solution, stir for 10-20 min, then add phosphorus nitrogen silicon flame retardant, heat to 60-75℃ and continue stirring for 0.5-1 h, cool to room temperature, filter, wash and dry to obtain imidazole-based phosphorus nitrogen silicon flame retardant.

[0013] Preferably, in step S1, the ratio of 2-phenylethyl silane trichloride, ethanolamine phosphate, triethylamine, and anhydrous dimethyl sulfoxide is 10 mmol: (33-35) mmol: (33-35) mmol: (40-60) mL;

[0014] In step S2, the ratio of phosphorus nitrogen silicon flame retardant, 5-aminobenzimidazole, and ethanol aqueous solution is 10 mmol: (33-35) mmol: (50-80) mL; the concentration of the ethanol aqueous solution is 60-80 wt%.

[0015] The chemical reaction equations involved in the preparation of imidazole-based phosphorus nitrogen silicon flame retardants are as follows:

[0016]

[0017] Preferably, the aminated halloysite nanotubes are obtained by modifying halloysite nanotubes with an aminosilane coupling agent.

[0018] Further, preferably, the aminated halloysite nanotubes are prepared by the following method:

[0019] The aminosilane coupling agent was added to an aqueous ethanol solution and stirred at room temperature for 0.5-1 h. The pH was adjusted to 3-4 with glacial acetic acid, halloysite nanotubes were added, stirred evenly, heated to 60-70℃, and stirred for 2-3 h. After filtration, washing, and drying, amino-modified halloysite nanotubes were obtained.

[0020] Preferably, the ratio of halloysite nanotubes, aminosilane coupling agent, and ethanol aqueous solution is 10g:(4-9)g:(50-100)mL; and the concentration of the ethanol aqueous solution is 60-80wt%.

[0021] Preferably, the aminosilane coupling agent is one or more of KH-550 (3-aminopropyltriethoxysilane), KH-540 (3-aminopropyltrimethoxysilane), KH-791 ((N-(2-aminoethyl)-3-aminopropyl)tri-(2-ethoxy)silane), KH-792 (N-β-aminoethyl-γ-aminopropyltrimethoxysilane), and KH-902 (γ-aminopropylmethyldiethoxysilane).

[0022] Preferably, the zinc-based synergist is two or more of zinc stearate, zinc citrate, and zinc acetate.

[0023] Preferably, the plasticizer is one or more of epoxidized soybean oil, dioctyl adipate, and phthalate.

[0024] Preferably, the inner wall material comprises the following components by weight: 80-120 parts of ethylene-vinyl acetate copolymer, 6-10 parts of polyethylene, 20-60 parts of tackifier, and 0.1-1 parts of antioxidant.

[0025] Preferably, the tackifier is one or more of C5 petroleum resin, C9 petroleum resin, hydrogenated rosin, rosin glycerol ester, and terpene resin; and the antioxidant is one or more of antioxidant 1010, antioxidant 1076, antioxidant 1098, and antioxidant 1135.

[0026] Secondly, the present invention provides a method for preparing a halogen-free flame-retardant double-walled tube, which adopts the following technical solution:

[0027] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0028] Step (1): Weigh out the outer wall material and the inner wall material according to the formula;

[0029] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated by a twin-screw extruder.

[0030] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system and extruded into pipes to obtain the initial product;

[0031] Step (4): After the initial product is cross-linked and expanded by irradiation, it is cooled and shaped to obtain the finished product.

[0032] Preferably, the expansion is infrared heating expansion or oil heating expansion.

[0033] In summary, the present invention has the following beneficial effects:

[0034] 1. The halogen-free flame-retardant double-walled tube of the present invention is composed of an outer wall and an inner wall; the outer wall material includes polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole-based phosphorus nitrogen silicon flame retardant, aminated halloysite nanotubes, zinc synergist, and plasticizer; the inner wall material includes ethylene-vinyl acetate copolymer, polyethylene, tackifier, and antioxidant; the combination of the outer wall and the inner wall gives the double-walled tube the advantages of halogen-free flame retardancy, flexibility and impact resistance, aging resistance, and insulation, and it is suitable for fields such as insulation and flame retardancy of wires and cables, joint sealing, and pipeline corrosion protection.

[0035] 2. The imidazole-based phosphorus-nitrogen-silicon flame retardant of the present invention has a unique stereostructure. It undergoes a nucleophilic substitution reaction by attacking the electrophilic center of the silicon atom in 2-phenylethyltrichlorosilane with the lone pair electrons on the amino nitrogen atom in ethanolamine phosphate, generating a phosphorus-nitrogen-silicon flame retardant containing Si-N bonds. Under heating conditions, the phosphorus-nitrogen-silicon flame retardant undergoes a salt-forming reaction with 5-aminobenzimidazole to form an imidazole-based phosphorus-nitrogen-silicon flame retardant. The phosphorus element in this flame retardant molecule promotes char formation, effectively isolating oxygen and heat; nitrogen is released... The inert gas significantly reduces the concentration of combustibles; silicon enhances the density and high-temperature resistance of the char layer. The combination of these three elements achieves both gas-phase flame retardancy and enhanced condensed-phase flame retardancy. The synergistic effect between the gas and condensed phases gives the flame retardant molecules significant flame-retardant properties. At the same time, the flame retardant molecule incorporates a benzimidazole structure, which has rigidity and a conjugated system. During the char layer formation process, it can be embedded into the structure of the char layer, enhancing the density and mechanical strength of the char layer, and further improving the flame-retardant efficiency of the imidazole-based phosphorus nitrogen silicon flame retardant.

[0036] 3. The aminated halloysite nanotubes of the present invention are obtained by modifying halloysite nanotubes with an aminosilane coupling agent, which increases the number of active groups on the surface, improves the compatibility and interfacial bonding force with other components, and reduces agglomeration. The tubular structure of the aminated halloysite nanotubes can form a physical barrier during combustion, hindering the transfer of heat and the diffusion of combustible gases during combustion. Moreover, the amino groups on the surface are converted into nitrogen-containing free radicals during pyrolysis, which react with polymer decomposition products to promote cross-linking into char and form a more stable expanded char layer, thereby enhancing the flame retardant performance of the double-walled tubes.

[0037] 4. The zinc-based synergist of the present invention is two or more of zinc stearate, zinc citrate, and zinc acetate. During the initial decomposition of combustion, the zinc-based synergist releases zinc ions. On the one hand, zinc ions, as Lewis acids, can catalyze the dehydrogenation reaction of polymers and promote the rearrangement of the carbon skeleton to form an aromatic fused ring structure. On the other hand, the benzimidazole structure of the imidazole-based phosphorus nitrogen silicon flame retardant forms a more stable chelate with zinc ions through multidentate coordination. These two aspects help to delay the thermal degradation initiation temperature of the double-walled tube, enhance the stability of the char layer, and further improve the flame retardant performance of the double-walled tube.

[0038] 5. The imidazole-based phosphorus-nitrogen-silicon flame retardant, aminated halloysite nanotubes, and zinc-based synergist of the present invention can form a multi-element flame retardant system, which not only enables the oxygen index of the double-walled tube to reach more than 35%, pass the UL-94 vertical burning test V-0 rating, achieve post-ignition expansion and smoke suppression, and produce no dripping or obvious smoke during the combustion process, but also helps to improve the mechanical and insulation properties of the double-walled tube, making its application range wider. Attached Figure Description

[0039] Figure 1 This is the 1H NMR spectrum of the phosphorus-nitrogen-silicon flame retardant provided in Example 1 of this invention;

[0040] Figure 2 This is the 1H NMR spectrum of the imidazole-based phosphorus nitrogen silicon flame retardant provided in Preparation Example 1 of this invention. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the embodiments.

[0042] Unless otherwise specified, all materials and reagents used in this invention are commercially available. Specifically, polyethylene (CAS No. 9002-88-4) was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. (L790453 low-density polyethylene powder, melt index: 70g / 10min, particle size: 600 mesh); ethylene-vinyl acetate copolymer (CAS No. 24937-78-8) was purchased from Shanghai Yuanye Biotechnology Co., Ltd. (vinyl acetate 12wt%, melt index 8g / 10min (190℃ / 2.16kg)); polyethylene grafted with maleic anhydride (CAS No. 9006-26-2) was purchased from Shanghai Yuanye Biotechnology Co., Ltd. (grafting rate: 8%, average molecular weight: 3000-8000); ternary... Ethylene propylene rubber, CAS No. 25038-36-2, was purchased from Shanghai Yuanye Biotechnology Co., Ltd. (ethylene content: 55%; ENB: 9.5%); C5 petroleum resin was purchased from Puyang Ruike Chemical Co., Ltd. (brand name: RK-A1100); terpene resin, CAS No. 9003-74-1, was purchased from Wuhan Jixin Yibang Biotechnology Co., Ltd.; hydrogenated rosin, CAS No. 65997-06-0, was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. (tetrahydroresin acid content ≥30%).

[0043] Preparation Examples 1-2 provide a method for preparing a flame retardant.

[0044] Preparation Example 1

[0045] Imidazole-based phosphorus-nitrogen-silicon flame retardants are prepared by the following method:

[0046] S1. Under nitrogen protection, 33 mmol (4.653 g) of ethanolamine phosphate was dissolved in 40 mL of anhydrous dimethyl sulfoxide. 10 mmol (2.396 g) of 2-phenylethyltrichlorosilane was added dropwise using a constant pressure funnel over 5 minutes. After the addition was completed, the temperature was raised to 50 °C, and the reaction was continued for 12 hours. Then, 33 mmol (3.333 g) of triethylamine was added, the pH was adjusted to 7.5, and the mixture was distilled under reduced pressure. Recrystallization was performed using 100 mL of toluene, filtered, and dried to obtain 5.041 g of the product. HPLC-MS analysis of the product showed m / z = 553, confirming the product as the phosphorus-nitrogen-silicon flame retardant of this invention. Based on this, the yield of the reaction was calculated to be 91.2%. The proton NMR spectrum of the phosphorus-nitrogen-silicon flame retardant was determined as follows: Figure 1 As shown, the peak at 3.99 is the characteristic absorption peak of hydrogen on the imino group linked to silicon in the phosphorus-nitrogen-silicon flame retardant, indicating that ethanolamine phosphate undergoes a nucleophilic substitution reaction with 2-phenylethyltrichlorosilane to generate a phosphorus-nitrogen-silicon flame retardant containing Si-N bonds.

[0047] S2. 26.4 mmol (3.511 g) of 5-aminobenzimidazole was added to 50 mL of ethanol aqueous solution (70 wt%), stirred for 10 min, and then 8 mmol (4.424 g) of phosphorus-nitrogen-silicon flame retardant was added. The mixture was heated to 60 °C and stirred for 1 h. After cooling to room temperature, the mixture was filtered, washed, and dried to obtain 6.366 g of the product. HPLC-MS analysis of the product showed m / z = 952, confirming the product as imidazole-based phosphorus-nitrogen-silicon flame retardant. Based on this, the yield of the reaction was calculated to be 83.6%. The proton NMR spectrum of the imidazole-based phosphorus-nitrogen-silicon flame retardant was determined as follows: Figure 2 As shown; where 6.23 is the absorption peak of the amino proton on the benzimidazole group in the imidazole-based phosphorus nitrogen silicon flame retardant, and 5.12 is the characteristic absorption peak of the hydrogen on the phosphate group in the imidazole-based phosphorus nitrogen silicon flame retardant, indicating that the imidazole-based phosphorus nitrogen silicon flame retardant of the present invention was successfully synthesized and is consistent with the expected chemical structure.

[0048] Preparation Example 2

[0049] Imidazole-based phosphorus-nitrogen flame retardants are prepared by the following method:

[0050] 10 mmol (1.33 g) of 5-aminobenzimidazole was added to 40 mL of ethanol aqueous solution (70 wt%), stirred for 10 min, and then 11 mmol (1.551 g) of phosphate ethanolamine was added. The mixture was heated to 60 °C and stirred for 1 h. After cooling to room temperature, the mixture was filtered, washed, and dried to obtain imidazole-based phosphorus nitrogen flame retardant.

[0051] Preparation Examples 3-5 provide a method for preparing aminated halloysite nanotubes.

[0052] Preparation Example 3

[0053] Aminated halloysite nanotubes were prepared by the following method:

[0054] 4 g of aminosilane coupling agent (KH550) was added to 50 mL of ethanol aqueous solution (concentration of 80 wt%). After stirring at room temperature for 0.5 h, the pH was adjusted to 3 with glacial acetic acid. 10 g of halloysite nanotubes (length of 1-2 μm, outer diameter of 50-100 nm, inner diameter of 10-50 nm) were added and stirred evenly. The mixture was heated to 60 °C and stirred for 2 h. After filtration, washing, and drying, amino-modified halloysite nanotubes were obtained.

[0055] Preparation Example 4

[0056] Aminated halloysite nanotubes were prepared by the following method:

[0057] 7 g of aminosilane coupling agent (KH540) was added to 80 mL of ethanol aqueous solution (concentration 70 wt%). After stirring at room temperature for 0.75 h, the pH was adjusted to 3.5 with glacial acetic acid. 10 g of halloysite nanotubes (length 1-2 μm, outer diameter 50-100 nm, inner diameter 10-50 nm) were added and stirred evenly. The mixture was heated to 65 °C and stirred for 2.5 h. After filtration, washing, and drying, amino-modified halloysite nanotubes were obtained.

[0058] Preparation Example 5

[0059] Aminated halloysite nanotubes were prepared by the following method:

[0060] 9 g of aminosilane coupling agent (KH-792) was added to 100 mL of ethanol aqueous solution (concentration of 60 wt%). After stirring at room temperature for 1 h, the pH was adjusted to 4 with glacial acetic acid. 10 g of halloysite nanotubes (length of 1-2 μm, outer diameter of 50-100 nm, inner diameter of 10-50 nm) were added and stirred evenly. The mixture was heated to 70 °C and stirred for 3 h. After filtration, washing, and drying, amino-modified halloysite nanotubes were obtained.

[0061] Examples 1-3 provide a halogen-free flame-retardant double-walled tube and its preparation method.

[0062] Example 1

[0063] A halogen-free flame-retardant double-walled pipe, consisting of an outer wall and an inner wall;

[0064] The outer wall material comprises the following components by weight: 65 parts polyethylene, 20 parts ethylene-vinyl acetate copolymer, 5 parts polyethylene grafted maleic anhydride, 25 parts ethylene propylene diene monomer (EPDM) rubber, 8 parts imidazole-based phosphorus-nitrogen-silicon flame retardant, 5 parts aminated halloysite nanotubes, 1 part zinc-based synergist, and 0.5 parts plasticizer; wherein, the imidazole-based phosphorus-nitrogen-silicon flame retardant was prepared in Preparation Example 1; the aminated halloysite nanotubes were prepared in Preparation Example 3; the zinc-based synergist was obtained by mixing zinc citrate and zinc stearate in a mass ratio of 1:1; and the plasticizer was epoxidized soybean oil;

[0065] The inner wall material comprises the following components by weight: 80 parts ethylene-vinyl acetate copolymer, 10 parts polyethylene, 20 parts tackifier, and 0.1 parts antioxidant; wherein the tackifier is C5 petroleum resin and the antioxidant is antioxidant 1010;

[0066] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0067] Step (1): Weigh out the outer wall raw materials (polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole phosphorus nitrogen silicon flame retardant, aminated halloysite nanotubes, zinc synergist, plasticizer) and the inner wall raw materials (ethylene-vinyl acetate copolymer, polyethylene, tackifier, antioxidant) according to the formula.

[0068] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated at 110°C using a twin-screw extruder.

[0069] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system. The extrusion temperature of the outer extruder is 120°C, and the extrusion temperature of the inner extruder is set to 110°C. The material is extruded into a pipe to obtain the initial product.

[0070] Step (4): After the initial product is cross-linked by irradiation and expanded by oil heating, it is cooled and shaped to obtain the finished product.

[0071] Example 2

[0072] A halogen-free flame-retardant double-walled pipe, consisting of an outer wall and an inner wall;

[0073] The outer wall material comprises the following components by weight: 70 parts polyethylene, 15 parts ethylene-vinyl acetate copolymer, 10 parts polyethylene grafted maleic anhydride, 30 parts ethylene propylene diene monomer (EPDM) rubber, 15 parts imidazole-based phosphorus-nitrogen-silicon flame retardant, 8 parts aminated halloysite nanotubes, 3 parts zinc-based synergist, and 1.5 parts plasticizer; wherein, the imidazole-based phosphorus-nitrogen-silicon flame retardant was prepared by Preparation Example 1; the aminated halloysite nanotubes were prepared by Preparation Example 4; the zinc-based synergist was obtained by mixing zinc stearate, zinc citrate, and zinc acetate in a mass ratio of 1:3:1; and the plasticizer was dioctyl adipate.

[0074] The inner wall material comprises the following components by weight: 100 parts ethylene-vinyl acetate copolymer, 8 parts polyethylene, 40 parts tackifier, and 0.5 parts antioxidant; wherein the tackifier is hydrogenated rosin; and the antioxidant is antioxidant 1098.

[0075] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0076] Step (1): Weigh out the outer wall raw materials (polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole phosphorus nitrogen silicon flame retardant, aminated halloysite nanotubes, zinc synergist, plasticizer) and the inner wall raw materials (ethylene-vinyl acetate copolymer, polyethylene, tackifier, antioxidant) according to the formula.

[0077] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated at 130°C using a twin-screw extruder.

[0078] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system. The extrusion temperature of the outer extruder is 140℃, and the extrusion temperature of the inner extruder is set to 125℃. The material is extruded into a pipe to obtain the initial product.

[0079] Step (4): After the initial product is cross-linked by irradiation and expanded by infrared heating, it is cooled and shaped to obtain the finished product.

[0080] Example 3

[0081] A halogen-free flame-retardant double-walled pipe, consisting of an outer wall and an inner wall;

[0082] The outer wall material comprises the following components by weight: 85 parts polyethylene, 10 parts ethylene-vinyl acetate copolymer, 15 parts polyethylene grafted maleic anhydride, 35 parts ethylene propylene diene monomer (EPDM) rubber, 19 parts imidazole-based phosphorus-nitrogen-silicon flame retardant, 10 parts aminated halloysite nanotubes, 5 parts zinc-based synergist, and 5 parts plasticizer; wherein, the imidazole-based phosphorus-nitrogen-silicon flame retardant was prepared in Preparation Example 1; the aminated halloysite nanotubes were prepared in Preparation Example 5; the zinc-based synergist was obtained by mixing zinc acetate and zinc stearate in a mass ratio of 1:1; and the plasticizer was obtained by mixing epoxidized soybean oil, dioctyl adipate, and phthalate in a mass ratio of 1:1:1.

[0083] The inner wall material comprises the following components by weight: 120 parts ethylene-vinyl acetate copolymer, 10 parts polyethylene, 60 parts tackifier, and 1 part antioxidant; wherein the tackifier is a terpene resin; and the antioxidant is antioxidant 1135.

[0084] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0085] Step (1): Weigh out the outer wall raw materials (polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole phosphorus nitrogen silicon flame retardant, aminated halloysite nanotubes, zinc synergist, plasticizer) and the inner wall raw materials (ethylene-vinyl acetate copolymer, polyethylene, tackifier, antioxidant) according to the formula.

[0086] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated at 150°C using a twin-screw extruder.

[0087] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system. The extrusion temperature of the outer extruder is 160°C, and the extrusion temperature of the inner extruder is set to 140°C. The material is extruded into a pipe to obtain the initial product.

[0088] Step (4): After the initial product is cross-linked by irradiation and expanded by oil heating, it is cooled and shaped to obtain the finished product.

[0089] To verify the performance of the halogen-free flame-retardant double-walled tubes obtained in Examples 1-3 of the present invention, comparative examples 1-4 were set up in the present invention.

[0090] Comparative Example 1

[0091] Comparative Example 1 differs from Example 1 in that an imidazole-based phosphorus-nitrogen-silicon flame retardant was used instead of an equal mass of phosphorus-nitrogen-silicon flame retardant. The phosphorus-nitrogen-silicon flame retardant was prepared from S1 of Preparation Example 1, as follows:

[0092] A halogen-free flame-retardant double-walled pipe, consisting of an outer wall and an inner wall;

[0093] The outer wall material comprises the following components by weight: 65 parts polyethylene, 20 parts ethylene-vinyl acetate copolymer, 5 parts polyethylene grafted maleic anhydride, 25 parts ethylene propylene diene monomer (EPDM) rubber, 8 parts phosphorus-nitrogen-silicon flame retardant, 5 parts aminated halloysite nanotubes, 1 part zinc-based synergist, and 0.5 parts plasticizer; wherein, the phosphorus-nitrogen-silicon flame retardant is prepared from S1 of Preparation Example 1; the aminated halloysite nanotubes are prepared from Preparation Example 3; the zinc-based synergist is obtained by mixing zinc citrate and zinc stearate in a mass ratio of 1:1; and the plasticizer is epoxidized soybean oil;

[0094] The inner wall material comprises the following components by weight: 80 parts ethylene-vinyl acetate copolymer, 10 parts polyethylene, 20 parts tackifier, and 0.1 parts antioxidant; wherein the tackifier is C5 petroleum resin and the antioxidant is antioxidant 1010;

[0095] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0096] Step (1): Weigh out the outer wall raw materials (polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, phosphorus nitrogen silicon flame retardant, aminated halloysite nanotubes, zinc synergist, plasticizer) and the inner wall raw materials (ethylene-vinyl acetate copolymer, polyethylene, tackifier, antioxidant) according to the formula.

[0097] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated at 110°C using a twin-screw extruder.

[0098] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system. The extrusion temperature of the outer extruder is 120°C, and the extrusion temperature of the inner extruder is set to 110°C. The material is extruded into a pipe to obtain the initial product.

[0099] Step (4): After the initial product is cross-linked by irradiation and expanded by oil heating, it is cooled and shaped to obtain the finished product.

[0100] Comparative Example 2

[0101] Comparative Example 2 differs from Example 1 in that an imidazole-based phosphorus-nitrogen flame retardant of equal mass is used instead of the imidazole-based phosphorus-nitrogen silicon flame retardant. The imidazole-based phosphorus-nitrogen flame retardant was prepared in Preparation Example 2, as detailed below:

[0102] A halogen-free flame-retardant double-walled pipe, consisting of an outer wall and an inner wall;

[0103] The outer wall material comprises the following components by weight: 65 parts polyethylene, 20 parts ethylene-vinyl acetate copolymer, 5 parts polyethylene grafted maleic anhydride, 25 parts ethylene propylene diene monomer (EPDM) rubber, 8 parts imidazole-based phosphorus-nitrogen flame retardant, 5 parts aminated halloysite nanotubes, 1 part zinc-based synergist, and 0.5 parts plasticizer; wherein, the imidazole-based phosphorus-nitrogen flame retardant was prepared in Preparation Example 2; the aminated halloysite nanotubes were prepared in Preparation Example 3; the zinc-based synergist was obtained by mixing zinc citrate and zinc stearate in a mass ratio of 1:1; and the plasticizer was epoxidized soybean oil;

[0104] The inner wall material comprises the following components by weight: 80 parts ethylene-vinyl acetate copolymer, 10 parts polyethylene, 20 parts tackifier, and 0.1 parts antioxidant; wherein the tackifier is C5 petroleum resin and the antioxidant is antioxidant 1010;

[0105] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0106] Step (1): Weigh out the outer wall raw materials (polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole phosphorus nitrogen flame retardant, aminated halloysite nanotubes, zinc synergist, plasticizer) and the inner wall raw materials (ethylene-vinyl acetate copolymer, polyethylene, tackifier, antioxidant) according to the formula.

[0107] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated at 110°C using a twin-screw extruder.

[0108] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system. The extrusion temperature of the outer extruder is 120°C, and the extrusion temperature of the inner extruder is set to 110°C. The material is extruded into a pipe to obtain the initial product.

[0109] Step (4): After the initial product is cross-linked by irradiation and expanded by oil heating, it is cooled and shaped to obtain the finished product.

[0110] Comparative Example 3

[0111] Comparative Example 3 differs from Example 1 in that it uses halloysite nanotubes of equal mass instead of aminated halloysite nanotubes, as detailed below:

[0112] A halogen-free flame-retardant double-walled pipe, consisting of an outer wall and an inner wall;

[0113] The outer wall material comprises the following components by weight: 65 parts polyethylene, 20 parts ethylene-vinyl acetate copolymer, 5 parts polyethylene grafted maleic anhydride, 25 parts ethylene propylene diene monomer (EPDM) rubber, 8 parts imidazole-based phosphorus-nitrogen-silicon flame retardant, 5 parts halloysite nanotubes, 1 part zinc-based synergist, and 0.5 parts plasticizer; wherein, the imidazole-based phosphorus-nitrogen-silicon flame retardant is prepared by Preparation Example 1; the zinc-based synergist is obtained by mixing zinc citrate and zinc stearate in a mass ratio of 1:1; and the plasticizer is epoxidized soybean oil;

[0114] The inner wall material comprises the following components by weight: 80 parts ethylene-vinyl acetate copolymer, 10 parts polyethylene, 20 parts tackifier, and 0.1 parts antioxidant; wherein the tackifier is C5 petroleum resin and the antioxidant is antioxidant 1010;

[0115] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0116] Step (1): Weigh out the outer wall raw materials (polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole phosphorus nitrogen silicon flame retardant, zinc synergist, plasticizer) and the inner wall raw materials (ethylene-vinyl acetate copolymer, polyethylene, tackifier, antioxidant) according to the formula.

[0117] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated at 110°C using a twin-screw extruder.

[0118] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system. The extrusion temperature of the outer extruder is 120°C, and the extrusion temperature of the inner extruder is set to 110°C. The material is extruded into a pipe to obtain the initial product.

[0119] Step (4): After the initial product is cross-linked by irradiation and expanded by oil heating, it is cooled and shaped to obtain the finished product.

[0120] Comparative Example 4

[0121] Comparative Example 4 differs from Example 1 in that an equal mass of magnesium hydroxide is used to replace the zinc-based synergist, as detailed below:

[0122] A halogen-free flame-retardant double-walled pipe, consisting of an outer wall and an inner wall;

[0123] The outer wall material comprises the following components by weight: 65 parts polyethylene, 20 parts ethylene-vinyl acetate copolymer, 5 parts polyethylene grafted maleic anhydride, 25 parts ethylene propylene diene monomer (EPDM) rubber, 8 parts imidazole-based phosphorus-nitrogen-silicon flame retardant, 5 parts aminated halloysite nanotubes, 1 part magnesium hydroxide, and 0.5 parts plasticizer; wherein, the imidazole-based phosphorus-nitrogen-silicon flame retardant was prepared in Preparation Example 1; the aminated halloysite nanotubes were prepared in Preparation Example 6; and the plasticizer was epoxidized soybean oil;

[0124] The inner wall material comprises the following components by weight: 80 parts ethylene-vinyl acetate copolymer, 10 parts polyethylene, 20 parts tackifier, and 0.1 parts antioxidant; wherein the tackifier is C5 petroleum resin and the antioxidant is antioxidant 1010;

[0125] A method for preparing a halogen-free flame-retardant double-walled tube includes the following steps:

[0126] Step (1): Weigh out the outer wall raw materials (polyethylene, ethylene-vinyl acetate copolymer, polyethylene grafted maleic anhydride, EPDM rubber, imidazole phosphorus nitrogen silicon flame retardant, aminated halloysite nanotubes, magnesium hydroxide, plasticizer) and the inner wall raw materials (ethylene-vinyl acetate copolymer, polyethylene, tackifier, antioxidant) according to the formula.

[0127] Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated at 110°C using a twin-screw extruder.

[0128] Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system. The extrusion temperature of the outer extruder is 120°C, and the extrusion temperature of the inner extruder is set to 110°C. The material is extruded into a pipe to obtain the initial product.

[0129] Step (4): After the initial product is cross-linked by irradiation and expanded by oil heating, it is cooled and shaped to obtain the finished product.

[0130] The performance of the halogen-free flame-retardant double-walled tubes obtained in Examples 1-3 and Comparative Examples 1-4 of the present invention was tested, and the results are shown in Table 1:

[0131] (1) Flame retardant properties of double-walled pipes

[0132] The combustion performance of double-walled tubes was determined with reference to standard GB / T 2408-2021; the oxygen index and droplet characteristics of double-walled tubes were determined with reference to standard GB / T 2406.1-2008; and the smoke density of double-walled tubes was determined with reference to standard GB / T 8323.2-2008. The specific results are shown in Table 1.

[0133] Table 1:

[0134] UL-94 Oxygen Index Does it burn? Smoke density Example 1 V-0 36.1 No dripping 143 Example 2 V-0 37.5 No dripping 139 Example 3 V-0 36.9 No dripping 151 Comparative Example 1 V-2 29.6 A small amount of dripping 219 Comparative Example 2 V-2 28.3 A small amount of dripping 233 Comparative Example 3 V-1 30.7 A small amount of dripping 184 Comparative Example 4 V-1 32.9 A small amount of dripping 178

[0135] As shown in Table 1 above, the double-walled tubes obtained in Examples 1-3 of the present invention have a UL-94 vertical combustion test rating of V-0, an oxygen index of over 35%, and no dripping during combustion, with a smoke density of no more than 160, which is significantly better than comparative examples 1-4. This fully demonstrates that the double-walled tubes obtained in the present invention have good flame retardant effect, anti-dripping, low smoke and other characteristics, and are halogen-free and environmentally friendly, with broad application prospects.

[0136] Comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the imidazole-based phosphorus-nitrogen-silicon flame retardant of the present invention promotes char formation, improves the char quality of the polymer, and reduces the combustion rate, thus achieving a synergistic flame retardant effect of benzimidazole, phosphorus, nitrogen, and silicon, thereby improving the thermal stability and flame retardant effect of the double-walled tube. Comparing Example 1 with Comparative Example 3, it can be seen that the aminated halloysite nanotubes of the present invention, compared with halloysite nanotubes, help the polymer form a stable expanded char layer, thereby enhancing the flame retardant performance of the double-walled tube. Comparing Example 1 with Comparative Example 4, it can be seen that the zinc-based synergist used in the present invention, compared with magnesium hydroxide, can form a flame retardant system with the imidazole-based phosphorus-nitrogen-silicon flame retardant and the aminated halloysite nanotube, strengthening the synergistic effect and jointly improving the flame retardant performance of the double-walled tube.

[0137] (2) Mechanical and insulation properties of double-walled tubes

[0138] The tensile properties of the double-walled tube were determined in accordance with the standard GB / T 1040.3-2006; the volume resistivity of the double-walled tube was determined in accordance with the standard GB / T1410-2006; as shown in Table 2.

[0139] Table 2

[0140] Tensile strength Elongation at break Volume resistivity / ×10¹⁵ Ω·m Example 1 13.03 320.7 3.85 Example 2 14.54 348.3 4.78 Example 3 13.29 331.8 5.23 Comparative Example 1 10.94 291.2 3.06 Comparative Example 2 11.18 305.7 3.27 Comparative Example 3 9.96 288.1 3.13 Comparative Example 4 12.01 319.0 3.25

[0141] As shown in Table 2 above, the mechanical and insulation properties of the double-walled tubes obtained in Examples 1-3 of the present invention are better than those in Comparative Examples 1-4. This fully demonstrates that the addition of the imidazole-based phosphorus nitrogen silicon flame retardant, amino-modified halloysite nanotubes and zinc-based synergist of the present invention can not only enhance the flame retardant properties of the double-walled tubes, but also help improve the mechanical and insulation properties of the double-walled tubes, thereby helping to expand the application range of the double-walled tubes.

[0142] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A halogen-free flame-retardant double-walled pipe, characterized in that, It consists of an outer wall and an inner wall; the outer wall material includes the following components by weight: 65-85 parts polyethylene, 10-20 parts ethylene-vinyl acetate copolymer, 5-15 parts polyethylene grafted maleic anhydride, 25-35 parts ethylene propylene diene monomer (EPDM) rubber, 8-19 parts imidazole-based phosphorus-nitrogen-silicon flame retardant, 5-10 parts aminated halloysite nanotubes, 1-5 parts zinc-based synergist, and 0.5-2.5 parts plasticizer; The structural formula of the imidazole-based phosphorus nitrogen silicon flame retardant is: ； The zinc-based synergist is two or more of zinc stearate, zinc citrate, and zinc acetate.

2. The halogen-free flame-retardant double-walled tube according to claim 1, characterized in that, The imidazole-based phosphorus nitrogen silicon flame retardant is prepared by the following method: S1. Under nitrogen protection, ethanolamine phosphate is dissolved in anhydrous dimethyl sulfoxide, and 2-phenylethyl silane trichloride is added dropwise through a constant pressure funnel. After the addition is completed, the temperature is raised to 50-80℃ and the reaction is continued for 8-12 hours. Then, triethylamine is added, the pH is adjusted to 7.5-8, the mixture is distilled under reduced pressure, recrystallized, filtered, and dried to obtain phosphorus-nitrogen-silicon flame retardant. S2. Add 5-aminobenzimidazole to an aqueous ethanol solution, stir for 10-20 min, then add phosphorus nitrogen silicon flame retardant, heat to 60-75℃ and continue stirring for 0.5-1 h, cool to room temperature, filter, wash and dry to obtain imidazole-based phosphorus nitrogen silicon flame retardant.

3. The halogen-free flame-retardant double-walled tube according to claim 2, characterized in that, In step S1, the ratio of 2-phenylethyl silane trichloride, ethanolamine phosphate, triethylamine, and anhydrous dimethyl sulfoxide is 10 mmol: (33-35) mmol: (33-35) mmol: (40-60) mL. In step S2, the ratio of phosphorus nitrogen silicon flame retardant, 5-aminobenzimidazole, and ethanol aqueous solution is 10 mmol: (33-35) mmol: (50-80) mL; the concentration of the ethanol aqueous solution is 60-80 wt%.

4. The halogen-free flame-retardant double-walled tube according to claim 1, characterized in that, The aminated halloysite nanotubes are obtained by modifying halloysite nanotubes with an aminosilane coupling agent.

5. The halogen-free flame-retardant double-walled tube according to claim 4, characterized in that, The aminosilane coupling agent is one or more of KH-550, KH-540, KH-791, KH-792, and KH-902.

6. The halogen-free flame-retardant double-walled tube according to claim 1, characterized in that, The plasticizer is one or more of epoxidized soybean oil, dioctyl adipate, and phthalate.

7. The halogen-free flame-retardant double-walled tube according to claim 1, characterized in that, The inner wall material comprises the following components by weight: 80-120 parts of ethylene-vinyl acetate copolymer, 6-10 parts of polyethylene, 20-60 parts of tackifier, and 0.1-1 parts of antioxidant.

8. A method for preparing a halogen-free flame-retardant double-walled tube according to any one of claims 1-7, characterized in that, Includes the following steps: Step (1): Weigh out the outer wall material and the inner wall material according to the formula; Step (2): After the outer wall raw material and the inner wall raw material are mixed evenly, they are granulated by a twin-screw extruder. Step (3): After granulation, the material is distributed to the extruder through a centralized feeding system and extruded into pipes to obtain the initial product; Step (4): After the initial product is cross-linked and expanded by irradiation, it is cooled and shaped to obtain the finished product.

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