COF nanosheet for enhancing flame retardant property of PVC (polyvinyl chloride) and PE (polyethylene) pipes as well as preparation method and application of COF nanosheet

Abstract

The invention relates to a COF nanosheet for enhancing the flame retardant property of PVC (polyvinyl chloride) and PE (polyethylene) pipes as well as a preparation method and application of the COF nanosheet. The nanosheet contains a COF framework composed of elements such as N, P and Br; in the preparation, a hydrothermal synthesis method is used, hexachlorotripolyphosphazene, 3-bromo-2, 6-dimethylpyridine and 4-pyridylaldehyde react at a high temperature to obtain a COF crystal nucleus, and the crystal nucleus is subjected to plane growth at a relatively high temperature to finally generate the COF nanosheet ZR-COF. The ZR-COF nanosheet obtained by the invention is used as a flame retardant additive to be applied to PE and PVC pipes, the flame retardant property of the PE and PVC pipes is greatly enhanced through the synergistic effect of various flame retardant elements, and the sheet structure can be in full contact with PVC and PE and can also improve the mechanical strength of the PE and PVC pipes.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer materials, and particularly relates to a preparation method and application of COF nanosheets for enhancing the flame retardant performance of PVC pipes. Background Art

[0002] Polyvinyl chloride (PVC) pipes, due to their excellent corrosion resistance, electrical insulation, processability, cost-effectiveness, and mechanical strength, are widely used in building water supply and drainage, wire and cable conduits (electrical conduits), agricultural irrigation, telecommunications sheathing, chemical fluid transportation, and mine ventilation and drainage. These applications, particularly in building interiors (wire conduits and drain pipes), power systems (cable sheathing), mines, and public transportation (subways and tunnels), place stringent requirements on the material's flame retardancy. While traditional PVC pipes offer higher flame retardancy than many plastics, they are difficult to process. Processing requires the addition of large amounts of plasticizers, which lowers the pipe's limiting oxygen index. In certain demanding applications (such as core piping in high-rise buildings and harsh power systems), their flame retardancy rating needs to be further improved to meet standards. PVC releases large amounts of smoke, as well as toxic and hazardous substances such as hydrogen chloride (HCl) gas and dioxins, during combustion or high-temperature pyrolysis. These release significant amounts of smoke, HCl gas, and dioxins, posing a serious threat to personnel safety and equipment. Therefore, improving the flame retardancy of PVC pipes is essential to meet safety regulations and market demand.

[0003] Polyethylene (PE) pipes are widely used in urban water supply systems, gas transmission, agricultural irrigation, industrial fluid pipelines, and ground-source heat pump projects due to their excellent flexibility, low-temperature impact resistance, chemical stability, non-toxicity, and long life. In scenarios such as underground integrated pipeline corridors, interlaced installations within buildings, chemical parks, and tunnel projects, PE pipes must meet strict fire safety standards. In the event of a fire, flammable PE pipes can become a carrier of fire, releasing large amounts of flammable gases and molten drippings, exacerbating the risk of disaster. Therefore, improving the flame retardant properties of PE pipes is a core technical challenge in ensuring public safety.

[0004] The flame-retardant polyethylene corrugated pipe material described in patent CN 118359862 A incorporates a modified antioxidant. The resulting phosphoric and boric acids can then combine to form highly stable boron phosphate, which coats the surface of the polyethylene resin matrix. This synergistic effect enhances the flame retardancy of the polyethylene corrugated pipe material. The flame-retardant polyvinyl chloride prepared in patent CN 119912768 A utilizes 4-carboxylphenylboric acid from a composite modified attapulgite clay to form a BOB cross-linked network structure at high temperatures. This synergistically interacts with composite metal oxides (zinc oxide and manganese oxide) to enhance the thermal stability and density of the carbon layer, thereby improving the flame retardancy of the PVC pipe. While this method improves the flame retardancy of polyethylene and polyvinyl chloride pipes, its technical benefits are limited. Summary of the Invention

[0005] The present invention addresses the shortcomings of current PE and PVC pipes, which are prone to combustion and decomposition at high temperatures, by providing a COF nanosheet, preparation method, and application thereof for enhancing the flame retardancy of PVC pipes. The nanosheets contain a COF skeleton composed of elements such as N, P, and Br. During preparation, hexachlorotriphosphazene, 3-bromo-2,6-dimethylpyridine, and 4-pyridinecarboxaldehyde are reacted at high temperatures using a hydrothermal synthesis method to form COF crystal nuclei. At a higher temperature, the crystal nuclei undergo planar growth, ultimately forming the COF nanosheets ZR-COF. The resulting ZR-COF nanosheets are used as flame-retardant additives in PE and PVC pipes. The synergistic effect of the multiple flame-retardant elements significantly enhances the flame retardancy of these pipes. Their sheet-like structure allows for full contact with PVC and PE, also improving their mechanical strength.

[0006] The technical solution of the present invention is: A COF nanosheet for enhancing the flame retardancy of PVC and PE pipes, wherein the COF nanosheet is the following compound:

[0007] The wavy lines in the formula represent repeating units (-NP- chemical bonds); the thickness of the nanosheets is 2-20 nm.

[0008] The method for preparing the COF nanosheets for enhancing the flame retardancy of PVC and PE pipes comprises the following steps: Hexachlorotriphosphazene and 3-bromo-2,6-dimethylpyridine were added to a reaction tube, and then toluene was added to the tube in an anhydrous and oxygen-free environment. After dissolution, 4-pyridine formaldehyde was dripped into the solution and reacted at a high temperature for 1-2 hours, and then at a low temperature for 65-75 hours. The reaction was then filtered, washed, dried, placed in anhydrous methanol for ultrasonication, and dried to obtain ZR-COF nanosheets. Wherein, 8-12 g of hexachlorotriphosphazene, 10-20 g of 3-bromo-2,6-dimethylpyridine, and 15-25 g of 4-pyridinecarboxaldehyde are dissolved in each liter of toluene; The high temperature range is 110-130°C in a microwave reactor, or 180-220°C in a conventional reactor; The low temperature section is 80-90° C. in a microwave reactor, or 130-180° C. in a conventional reactor.

[0009] The anhydrous and oxygen-free environment is a nitrogen atmosphere.

[0010] The ultrasonic time is 10-60 min.

[0011] The COF nanosheets for enhancing the flame retardancy of PVC and PE pipes are used as flame retardant additives in the pipes.

[0012] The steps include: Pour ZR-COF nanosheets, antioxidant 1010, thermal stabilizer and substrate into a high-speed disperser at a speed of 500-2000 r min -1 , mix for 5-15 min at room temperature and then discharge to obtain a mixture; -1 The material is fed into a twin-screw extruder at a speed of 10000 rpm, extruded through a die head, and then cooled to room temperature to obtain a pre-crosslinked PE pipe or PVC pipe.

[0013] The mass ratio is ZR-COF nanosheets: antioxidant: thermal stabilizer 1010: substrate = 1-5: 0.01-0.05: 0.01-0.05: 15-20.

[0014] The antioxidant 1010 is (pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate); the heat stabilizer is dibutyltin dilaurate; The base material is polyethylene resin or polyvinyl chloride resin.

[0015] The essential features of the present invention are: The present invention uses a heating method to chemically polymerize hexachlorotriphosphazene, 3-bromo-2,6-dimethylpyridine, and 4-pyridinecarboxaldehyde monomers. Adjusting the polymerization temperature creates and ultrasonically separates a uniquely ionically modified nanosheet structure. At high temperatures, the monomers rapidly polymerize to form a ZR-COF core. Cooling the core to a higher temperature results in the growth of ZR-COF nanosheets in a two-dimensional, layered structure. Ultrasound is then used to further separate the nanosheets. The COF nanosheets' ultrathin thickness allows for greater contact area with minimal addition, and their unique structure also results in more cavities.

[0016] This structural design enables the nanosheets to interact with the polymer matrix of PE and PVC pipes in multiple dimensions, promoting their uniform dispersion and stable presence within the resin matrix. The resulting ZR-COF, used as an additive for PE and PVC pipes, contains elements such as N, P, and Br, as well as a flame-retardant carbon layer. The synergistic effect of these multiple flame-retardant elements results in enhanced flame retardancy. The synergistic effect of the N and Br elements and NP bonds within its skeleton significantly enhances its flame retardancy. Its cavity structure also allows for the storage and absorption of toxic gases. This significantly improves the flame retardancy of PE and PVC pipes. The nanosheets' unique layered structure allows for full contact with the PVC and PE substrates, forming a cross-linked structure that significantly enhances the pipes' mechanical properties.

[0017] The beneficial effects of the present invention are: This invention addresses the shortcomings of current PE and PVC pipes in improving single properties, providing a method for preparing COF nanosheets and their application to enhance the flame retardancy of PVC and PE pipes. These ZR-COF nanosheets utilize a heating method to chemically polymerize hexachlorotriphosphazene, 3-bromo-2,6-dimethylpyridine, and 4-pyridinecarboxaldehyde monomers to form ZR-COF. The nanosheet structure is then constructed by adjusting the polymerization temperature. This structural design enables multi-dimensional interactions between the nanosheets and the polymer matrix of PE and PVC pipes, promoting uniform dispersion and stable presence within the resin matrix. The ring-shaped structure contains cavities that can absorb escaping HCl gas, significantly reducing the generation of harmful gases. A flame-retardant carbon layer within the framework prevents oxygen from contacting the substrate. Furthermore, the synergistic effect of multiple flame-retardant elements results in enhanced flame retardancy. The synergistic effects of the nitrogen and bromine elements and the NP bonds within the framework significantly enhance the flame retardancy. The unique layer structure of the nanosheets can fully contact with the PVC and PE substrates to form a cross-linked structure, greatly improving the mechanical properties of the pipe.

[0018] This invention uses hexachlorotriphosphazene, 3-bromo-2,6-dimethylpyridine, and 4-pyridinecarboxaldehyde monomers as raw materials. During the hydrothermal synthesis of COF, COF nanosheets with a specially ionically modified nanosheet structure are constructed by adjusting the polymerization temperature. These nanosheets are then added in varying proportions as additives to prepare PE and PVC plastic pipes. Compared to unadditive PE and PVC pipes, the COF nanosheets are evenly dispersed throughout the PE and PVC matrices. Due to their inherent strength and rigidity, and their strong interaction with the polymer molecular chains, the nanosheets effectively absorb and transmit stress when the pipe is subjected to tensile forces, significantly improving the pipe's tensile strength by up to 20% and 29% for PE and PVC pipes, respectively. COF nanosheets contain a flame-retardant carbon layer within their skeleton, which prevents oxygen from contacting the substrate. The synergistic effect of multiple flame-retardant elements results in a more effective flame retardant effect. The synergistic effect of the nitrogen and bromine elements and the NP bonds within their skeleton significantly enhances their flame retardancy. Their hollow structure also absorbs harmful gases. Compared to pure pipes, PE and PVC pipes containing COF nanosheets can achieve flame retardancy increases of over 70% and 25% respectively. Therefore, the COF nanosheet additive of the present invention significantly improves the overall performance of PE and PVC pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The ZR-COF nanosheets obtained in Example 1 13 C solid-state NMR spectrum.

[0020] Figure 2 This is a scanning electron microscope image of the ZR-COF nanosheets obtained in Example 1. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific implementation described herein is only used to explain the present invention and is not intended to limit the present invention.

[0022]

[0023] As shown in the above structural formula: ZR-COF nanosheets are generated by high-temperature reaction of hexachlorotriphosphazene, 3-bromo-2,6-dimethylpyridine, and 4-pyridinecarboxaldehyde, and are used as additives for PE and PVC pipes.

[0024] The resulting ZR-COF nanosheets are high-molecular polymers with a honeycomb topology, and the entire molecule circulates in this structure. The wavy lines in the formula represent the subsequent -PN- chemical bonds connected to it.

[0025] The invented COF nanosheet additive, which enhances mechanical strength and flame retardancy, is used in PE and PVC pipes. Examples 1-3 are used to prepare reinforced PE pipes, while Examples 4-6 are used to prepare reinforced PVC pipes.

[0026] Example 1: The preparation and application of the flame-retardant pipe are characterized by comprising the following steps: Synthesis of ZR-COF: 9.5 g of hexachlorotriphosphazene and 15 g of 3-bromo-2,6-dimethylpyridine were added to a reaction tube. The tube was then evacuated and then filled with nitrogen, and this process was repeated three times. Maintaining an anhydrous and oxygen-free environment, 1 L of toluene was added to the tube. Once the solution was fully dissolved, 20.5 g of 4-pyridinecarboxaldehyde was added dropwise to the solution. The reaction was then heated at 120°C in a microwave reactor for 1.5 hours to rapidly nucleate the product. The reaction was then lowered to 85°C for 70 hours. After the reaction was complete, the product was filtered and washed with large amounts of anhydrous methanol and then dichloromethane. The product was then dried, ultrasonically separated and dried in anhydrous methanol to obtain a gray-green powder, which is the ZR-COF nanosheet. The yield was approximately 88%.

[0027] We further tested and analyzed the chemical structure of ZR-COF by solid-state nuclear magnetic resonance (SSNMR). The test results are as follows: Figure 1The SSNMR spectrum shows that the signal resonance peaks at 119.6, 129.4, 135.7, and 145.7 ppm correspond to the carbon atoms on the benzene ring, and the signal resonance peaks at 124.3, 143.3, and 148.0 ppm correspond to the carbon atoms on the pyridine ring. The absence of the signal resonance peak at 190.0 ppm for the carbon atoms in the C=O bond indicates the complete progress of the reaction. The appearance of the signal resonance peak at 155.5 ppm for the carbon atoms in the C=C bond further confirms the successful preparation of ZR-COF.

[0028] like Figure 2 The scanning electron micrograph (SEM) shows that ZR-COF is a multilayered nanosheet structure, demonstrating the successful synthesis of ZR-COF nanosheets through high-temperature nucleation and low-temperature crystallization. The gaps between the multilayered nanosheets shown in the image roughly indicate a thickness of approximately 15 nm.

[0029] Preparation of PE pipe: ZR-COF, dibutyltin dilaurate, antioxidant 1010 and polyethylene were poured into a high-speed disperser at a mass ratio of 2:0.01:0.01:15 and stirred at a speed of 600 r min. -1 , kneading for 10 min at room temperature and then discharging to obtain a mixture. -1 The twin screw was fed into the twin screw extruder at a speed of 1000 rpm. The twin screw was temperature controlled in six sections: the first section was 110°C, the second section was 150°C, the third section was 160°C, the fourth section was 180°C, the fifth section was 200°C, the die temperature was 225°C, and the screw speed was 130 r min. -1 After the pipe is extruded through the die head, it is cooled to room temperature to obtain a pre-crosslinked polyethylene pipe.

[0030] The polyethylene is a powder material with a powder diameter of 45 μm.

[0031] Example 2: The other steps are the same as those in Example 1, except that the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyethylene is changed from 2:0.01:0.01:15 to 2:0.01:0.01:18; The polyethylene is a powder material with a powder diameter of 45 μm.

[0032] Example 3: The other steps are the same as those in Example 1, except that the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyethylene is changed from 2:0.01:0.01:15 to 2:0.01:0.01:20. The polyethylene is a powder material with a powder diameter of 45 μm.

[0033] Example 4: The other steps are the same as those in Example 1, except that the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyethylene is 2:0.01:0.01:15, while the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyvinyl chloride is 2:0.01:0.01:15. The polyvinyl chloride is a powder material with a powder diameter of 20 μm.

[0034] Example 5: The other steps are the same as those in Example 1, except that the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyethylene is changed from 2:0.01:0.01:15 to 2:0.01:0.01:18; The polyvinyl chloride is a powder material with a powder diameter of 20 μm.

[0035] Example 6: The other steps are the same as those in Example 1, except that the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyethylene is 2:0.01:0.01:15, while the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyvinyl chloride is 2:0.01:0.01:20. The polyvinyl chloride is a powder material with a powder diameter of 20 μm.

[0036] Comparative Example 1: Preparation of PE pipe: Pour the dibutyltin dilaurate, antioxidant 1010 and polyethylene into a high-speed disperser at a mass ratio of 0.01:0.01:15 and mix at a speed of 600 r min. -1 , kneading for 10 min at room temperature and then discharging to obtain a mixture. -1 The twin screw was fed into the twin screw extruder at a speed of 1000 rpm. The twin screw was temperature controlled in six sections: the first section was 110°C, the second section was 150°C, the third section was 160°C, the fourth section was 180°C, the fifth section was 200°C, the die temperature was 225°C, and the screw speed was 130 rmin. -1 After the pipe is extruded through the die head, it is cooled to room temperature to obtain a pre-crosslinked polyethylene pipe.

[0037] The polyethylene is a powder material with a powder diameter of 45 μm.

[0038] Comparative Example 2; The other steps were the same as those in Comparative Example 1, except that the mass ratio of dibutyltin dilaurate, antioxidant 1010, and polyethylene was 0.01:0.01:15, while the mass ratio of ZR-COF, dibutyltin dilaurate, antioxidant 1010, and polyvinyl chloride was 1:0.01:0.01:15. The polyvinyl chloride is a powder material with a powder diameter of 20 μm.

[0039] In order to verify the properties of the materials obtained in Examples 1 to 6 and Comparative Examples 1 to 2, relevant characterization and performance tests were performed.

[0040] (1) 13C solid-state NMR The prepared HUT4 was tested using an Agilent 600 M nuclear magnetic resonance spectrometer produced by Agilent. Figure 1 This is the 13C solid-state NMR spectrum of ZR-COF, showing that it was successfully synthesized.

[0041] (2) Scanning electron microscopy The prepared ZR-COF nanosheets were tested by scanning electron microscope. Figure 2 It can be clearly observed that the two materials prepared by metal salt exfoliation exhibit a nanosheet structure.

[0042] (3) Tensile strength The tensile strength of the plastic pipes prepared in Examples 1 to 6 and Comparative Examples 1 to 2 was tested according to the method specified in GB / T1040.2-2006.

[0043] (IV) Flame retardant test The limiting oxygen index (LOI) standard test (UL-94) of the plastic pipes prepared in Examples 1 to 6 and Comparative Examples 1 to 2 was conducted in accordance with the national standard GB / T2408-2008 "Determination of combustion performance of plastics - Horizontal and vertical methods" using a CZF-2 vertical combustion tester from the Nanjing Jiangning County Analytical Instrument Factory to determine the vertical combustion grade of the composite materials.

[0044]

[0045] Compared to PE pipes not doped with ZR-COF nanosheets, the addition of the nanosheets prepared in this invention significantly improves the tensile strength of PE pipes. At low concentrations, the tensile strength increases with increasing addition, while at high concentrations, the tensile strength decreases. However, increasing the addition significantly improves the flame retardancy of the pipe.

[0046]

[0047] Compared to PVC pipes without COF nanosheets, the addition of the nanosheets produced by this invention significantly improves the tensile strength of PE pipes. At low concentrations, the tensile strength increases with increasing addition, while at high concentrations, the tensile strength decreases. However, increasing the addition significantly improves the flame retardancy of the pipe.

[0048] The above descriptions are only several preferred embodiments of the present invention, but the present invention is not limited to the above-mentioned specific embodiments. The above-mentioned specific embodiments are illustrative rather than restrictive. Under the guidance of the present invention and in accordance with the spirit and principles of the present invention, researchers in this field can also make improvements and perfections, all of which fall within the scope of protection of the present invention.

[0049] Matters not covered by the present invention are known technologies.

Claims

1. A COF nanosheet for enhancing the flame retardancy of PVC and PE pipes, characterized by: The COF nanosheets are the following compounds: 。 2. The COF nanosheet for enhancing the flame retardancy of PVC and PE pipes according to claim 1, characterized in that: The thickness of the nanosheets is 2-20 nm.

3. The method for preparing COF nanosheets for enhancing the flame retardancy of PVC and PE pipes according to claim 1, wherein: The following steps are involved: Hexachlorotriphosphazene and 3-bromo-2,6-dimethylpyridine were added to a reaction tube, and then toluene was added to the tube in an anhydrous and oxygen-free environment. After dissolution, 4-pyridine formaldehyde was dripped into the solution and reacted at a high temperature for 1-2 hours, and then at a low temperature for 65-75 hours. The reaction was then filtered, washed, dried, placed in anhydrous methanol for ultrasonication, and dried to obtain ZR-COF nanosheets. Wherein, 8-12 g of hexachlorotriphosphazene, 10-20 g of 3-bromo-2,6-dimethylpyridine, and 15-25 g of 4-pyridinecarboxaldehyde are dissolved in each liter of toluene; The high temperature range is 110-130°C in a microwave reactor, or 180-220°C in a conventional reactor; The low temperature section is 80-90° C. in a microwave reactor, or 130-180° C. in a conventional reactor.

4. The method for preparing COF nanosheets for enhancing the flame retardancy of PVC and PE pipes according to claim 1, wherein: The anhydrous and oxygen-free environment is a nitrogen atmosphere; and the ultrasonication time in anhydrous methanol is 10-60 min.

5. The use of the COF nanosheets for enhancing the flame retardancy of PVC and PE pipes as claimed in claim 1, characterized in that: Used as a flame retardant additive in pipes.

6. The use of the COF nanosheets for enhancing the flame retardancy of PVC and PE pipes as claimed in claim 1, characterized in that: The steps include: Pour ZR-COF nanosheets, antioxidant 1010, thermal stabilizer and substrate into a high-speed disperser at a speed of 500-2000 r min -1 , mix for 5-15 min at room temperature and then discharge to obtain a mixture; -1 The material is fed into a twin-screw extruder at a speed of 1000 rpm, extruded through a die head, and then cooled to room temperature to obtain a PE pipe or PVC pipe. The mass ratio is ZR-COF nanosheets: antioxidant: thermal stabilizer 1010: substrate = 1-5: 0.01-0.05: 0.01-0.05: 15-20; The antioxidant 1010 is (pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate); the heat stabilizer is dibutyltin dilaurate; The base material is polyethylene resin or polyvinyl chloride resin.

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