Environment-friendly cable sheath material prepared from recycled polyvinyl chloride and preparation method of environment-friendly cable sheath material

Abstract

The invention relates to the technical field of high polymer material processing and waste resource reutilization, and discloses an environment-friendly cable sheath material for recycling polyvinyl chloride and a preparation method. The method comprises the following steps: performing liquid nitrogen cryogenic embrittlement and plasma etching on industrial recycled polyvinyl chloride, and introducing active functional groups on the surface of the industrial recycled polyvinyl chloride; then constructing a phosphorus-nitrogen-silicon ternary synergistic flame retardant taking a carbon nanotube as a skeleton, and carrying out in-situ loading and high-temperature treatment to form a porous silicon carbide whisker composite filler; the method comprises the following steps: carrying out dynamic vulcanization compounding on surface-activated recycled polyvinyl chloride and a supported flame retardant in an internal mixer, and carrying out ultrasonic dispersion to realize uniform distribution of a nano-scale flame retardant phase; and carrying out chain extension toughening post-treatment and electrostatic spinning enhancement layer in-situ compounding to finally prepare the environment-friendly cable sheath material with the micro-crosslinked network structure. Through multi-scale interface regulation and process integration, the comprehensive performance of recycled polyvinyl chloride is remarkably improved, and efficient and high-valued utilization of waste resources is achieved.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing and waste resource recycling technology, specifically to an environmentally friendly cable sheath material made from recycled polyvinyl chloride and its preparation method. Background Technology

[0002] Polyvinyl chloride (PVC) has long been widely used in the field of power cable sheathing due to its excellent chemical resistance, electrical insulation, and cost advantages. However, with the end of the life cycle of plastic products, a large amount of PVC-containing waste is generated. Cable sheathing, in particular, contains additives and contaminants, making it difficult to directly use traditional physical recycling methods for high-end applications. While chemical recycling can break down polymer chains, it is energy-intensive, produces complex byproducts, and easily damages molecular structures. Therefore, developing efficient and low-cost physical recycling technologies has become a focus of industry attention.

[0003] Existing modification technologies for recycled polyvinyl chloride (PVC) mainly suffer from the following problems: First, the molecular chain breakage during recycling leads to a decrease in the mechanical properties of the material, requiring reinforcement with plasticizers or elastomers. However, traditional plasticizers such as phthalates pose risks of migration and toxicity. Second, the surface of recycled PVC is inert, resulting in weak interfacial bonding with fillers, which can easily cause uneven dispersion of flame retardants or reinforcing phases, affecting the uniformity of the material. Third, flame retardant modification often uses halogenated or single inorganic flame retardants. The former releases toxic gases upon combustion, while the latter can significantly reduce the toughness of the material if added in excessive amounts. Fourth, the molding process for recycled materials often employs simple melt blending, lacking cross-linking structural design, making it difficult to balance heat resistance and processing fluidity.

[0004] In recent years, researchers have attempted to improve the compatibility of recycled PVC with fillers through surface activation. For example, plasma treatment can introduce polar groups onto the polymer surface, but conventional plasma parameters can easily lead to excessive material degradation; while low-temperature plasma can reduce thermal damage, it requires precise control of gas ratios and treatment time. In flame retardant design, the phosphorus-nitrogen-silicon ternary synergistic system is considered to have low-smoke and halogen-free properties, but its nanoscale dispersion remains a challenge—agglomeration weakens flame retardant efficiency and induces stress concentration. Furthermore, high thermal conductivity fillers such as carbon nanotubes are often used to improve the heat dissipation performance of materials, but their directional arrangement in the polymer matrix and interface modification still require breakthroughs.

[0005] Cable sheath materials must simultaneously meet multiple requirements in practical applications, including flame retardancy, weather resistance, and mechanical strength. Existing recycled PVC sheaths generally suffer from insufficient flame retardancy (difficult to achieve UL94 V-0) and cracking after long-term use. Although dynamic vulcanization technology can improve toughness through the formation of a dispersed phase in the rubber phase, it is prone to uncontrolled crosslinking density or phase separation in recycled material systems. On the other hand, electrospinning technology has been used to prepare functional surface layers, but its integration with extrusion molding processes is still in the exploratory stage. One of the technical bottlenecks is how to achieve in-situ composite of nanofiber membranes and molten polymers without damaging the fiber structure. Summary of the Invention

[0006] The purpose of this invention is to provide an environmentally friendly cable sheath material made from recycled polyvinyl chloride and its preparation method, so as to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, this invention provides an environmentally friendly cable sheath material made from recycled polyvinyl chloride and a method for its preparation, the method comprising: Industrially recycled polyvinyl chloride waste is mechanically crushed after being cryogenically embrittled with liquid nitrogen, and then surface etched using plasma gas flow to introduce active sites. A core-shell structured phosphorus-nitrogen-silicon ternary synergistic flame retardant was constructed and in situ loaded onto a carbon nanotube network framework. Surface-treated recycled polyvinyl chloride and a loaded flame retardant are dynamically vulcanized and compounded in an internal mixer to produce an environmentally friendly cable sheath material with a micro-crosslinked network structure.

[0008] Preferably, the surface etching step specifically comprises: The recycled polyvinyl chloride particles were placed in a fluidized bed reactor, and an argon / oxygen mixture was introduced at a flow rate of 50 L / min to 100 L / min, wherein the oxygen volume percentage was 5% to 10%. Meanwhile, the temperature inside the reactor is maintained at 150℃~200℃, and a low-temperature plasma field is generated by excitation using radio frequency power supply, with a processing time of 10min~30min. After treatment, nitrogen gas is introduced for purging and the temperature is maintained for 10 minutes to generate uniform carbonyl and carboxyl functional groups on the plasma-treated PVC surface.

[0009] Preferably, the in-situ loading step of the phosphorus-nitrogen-silicon ternary synergistic flame retardant specifically includes: Single-walled carbon nanotubes with a diameter of 10 nm to 20 nm were dispersed in N,N-dimethylformamide and ultrasonically treated for 1 h to 2 h to form a stable suspension. Tetraethyl orthosilicate, nonaphenylcyclotriphosphazene, and triethanolamine are added sequentially, with a molar ratio of Si:P:N = 1:2:4. The mixture is stirred at 80℃ to 90℃ for 6 to 8 hours to promote the hydrolysis and condensation of the silicon source to form a silicon dioxide core and the phosphazene derivative to construct the outer coating layer, thereby growing a core-shell structured composite flame retardant in situ on the surface of carbon nanotubes.

[0010] Preferably, after the in-situ loading step is completed, a high-temperature calcination and reduction treatment is required: the reaction product is transferred to a tube furnace and heated to 600℃~800℃ at a heating rate of 5℃ / min under a nitrogen protective atmosphere, and calcined at this temperature for 2h~3h to remove organic residues and form a porous ceramic shell; then the atmosphere is switched to ammonia, and a carbothermic reduction reaction is carried out at 900℃~1000℃ for 1h~2h to reduce the silica core to silicon carbide whiskers, and finally obtain an inorganic / organic hybrid flame retardant filler with both thermal conductivity and smoke suppression functions.

[0011] Preferably, the dynamic vulcanization compounding step specifically includes: The recycled polyvinyl chloride after surface etching is put into an internal mixer, with the initial temperature set at 160℃ and the rotor speed at 50rpm~80rpm. After the material softens, add in batches 25% to 35% of the PVC mass of loaded flame retardant, 8% to 12% of the PVC mass of epoxidized soybean oil plasticizer, and 2% to 4% of the PVC mass of rare earth heat stabilizer lanthanum / cerium complex. After continuing to mix for 5 to 8 minutes, inject 0.5% to 1% of the multifunctional crosslinking agent divinylbenzene, and increase the rotor speed to 120 to 150 rpm. Maintain the mixing temperature at 175°C to 185°C and continue the reaction for 10 to 15 minutes until the torque curve becomes stable, thus completing the dynamic vulcanization process.

[0012] Preferably, ultrasonic-assisted dispersion is applied simultaneously during dynamic vulcanization: An ultrasonic transducer with a frequency of 20kHz to 40kHz and a power of 500W to 1000W is installed on the outer wall of the mixing chamber of the internal mixer. Ultrasonic energy is introduced into the material through a coupling medium. The ultrasonic action time is 50% to 70% of the total mixing time. The cavitation effect is used to break up the flame retardant agglomerates, so that they can be dispersed at the nanoscale in the PVC matrix.

[0013] Preferably, it also includes a post-processing step of chain extension and toughening: After dynamic vulcanization, the composite material is passed through a high-temperature open mill 3 to 5 times, with the roller spacing adjusted to 0.5 mm to 1 mm and the roller temperature maintained at 155℃ to 165℃. Then, an epoxy end-group teleclaw polymer solution of 0.3% to 0.8% by weight of the composite material is sprayed, and the thin-pass plasticizing is continued for 5 to 10 minutes to allow the epoxy groups to undergo a grafting reaction with the HCl at the end of the PVC molecular chain, thereby achieving chain extension and fine adjustment of crosslinking density.

[0014] Preferably, the process also includes an in-situ composite step of the electrospinning reinforcement layer: The prepared sheath material masterbatch was dissolved in a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide to prepare an electrospinning precursor solution with a solid content of 15% to 20%. A coaxial electrospinning device was used, with the prepared masterbatch solution as the skin layer and the pure polyvinylidene fluoride solution as the core layer. Spinning was carried out under the conditions of voltage of 18kV to 22kV, spinning distance of 15cm to 20cm, and flow rate of 0.8mL / h to 1.2mL / h. The obtained nanofiber membrane is directly applied to the die lip of the extrusion mold, so that the sheath melt is compounded in situ with the nanofiber membrane during extrusion, thereby forming a dense physical shielding barrier on the surface of the material.

[0015] Preferably, the present invention also includes an environmentally friendly cable sheath material made from recycled polyvinyl chloride, characterized in that it is prepared by the above-described method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention utilizes a combination of liquid nitrogen cryogenic embrittlement and plasma etching to precisely introduce carbonyl and carboxyl functional groups onto the surface of recycled polyvinyl chloride (PVC), significantly enhancing its interfacial bonding with inorganic fillers while avoiding thermal degradation. Compared to conventional heat treatment or chemical etching, this method consumes less energy and produces a more uniform distribution of functional groups, providing stable active sites for subsequent flame retardant anchoring.

[0017] The constructed phosphorus-nitrogen-silicon ternary synergistic flame retardant adopts a core-shell structure design, with a silica core and a phosphazene derivative outer layer supported in situ on a carbon nanotube framework. It is then transformed into a porous silicon carbide whisker composite filler through high-temperature calcination and carbothermic reduction. This strategy not only achieves spatial isolation of the flame-retardant elements to reduce self-extinguishing losses but also accelerates heat dissipation through the thermal conductivity of silicon carbide. Simultaneously, the porous structure promotes smoke adsorption and blocking. Compared to single-component flame retardants, this filler can form a continuous flame-retardant network at extremely low addition levels, avoiding mechanical property degradation caused by overfilling.

[0018] Ultrasonic dispersion is introduced during dynamic vulcanization to break up flame retardant agglomerates using cavitation, achieving nanoscale uniform distribution within the PVC matrix. Combined with a gradient temperature increase reaction using a multifunctional crosslinking agent, a bicontinuous structure is formed, dominated by a micro-crosslinked network and supplemented by a rubber-phase dispersion. This structure retains the processing fluidity of PVC while restricting molecular chain slippage through crosslinking points, thus balancing the material's strength and toughness. The addition of rare-earth heat stabilizers further inhibits the dehydrochlorination reaction during recycled PVC processing, extending the material's thermal stability window.

[0019] The chain extension and toughening post-treatment process achieves topological reconstruction of the molecular chains through a grafting reaction between epoxy-terminated telechelic polymers and PVC-terminated HCl. This process not only repairs chain breakage defects generated during recycling but also alleviates the rigid stress of the cross-linked network by introducing flexible segments, enabling the material to maintain good impact resistance even at low temperatures. Compared to traditional plasticizers, this method avoids aging problems caused by small molecule migration.

[0020] The in-situ composite technology of electrospinning reinforcement layers directly combines nanofiber membranes with the sheath melt, forming a biomimetic "brick-and-mortar structure" physical shielding layer. The three-dimensional network structure of nanofibers effectively blocks the penetration of oxygen and heat without affecting the overall flexibility of the material. This technology overcomes the compatibility limitations of traditional coating processes and extrusion molding, simplifies the process flow, and reduces production costs. Attached Figure Description

[0021] Figure 1 This diagram illustrates the steps of a method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride, as described in this invention. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific embodiments and comparative examples. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of protection of the present invention.

[0023] The raw materials used in the embodiments and comparative examples of this invention are as follows: Industrial recycled polyvinyl chloride waste: derived from waste cable sheath recycling materials, after sorting, cleaning and drying for use, its number average molecular weight is 80,000-100,000 and degree of polymerization is 1,000-1,200; Single-walled carbon nanotubes: diameter 10nm~20nm, length 5-10μm, purity ≥98%; N,N-Dimethylformamide (DMF): Analytical grade, purity ≥99.5%; Tetraethyl orthosilicate (TEOS): Analytical grade, purity ≥98%; Nonaphenylcyclotriphosphazene: analytical grade, purity ≥99%; Triethanolamine: Analytical grade, purity ≥99%; Epoxidized soybean oil plasticizer: industrial grade, epoxy value ≥6.0%; Rare earth heat stabilizer lanthanum / cerium complex: lanthanum to cerium molar ratio of 3:1, industrial grade; Divinylbenzene, a multifunctional crosslinking agent: analytical grade, purity ≥98%; Epoxy-terminated telechelic polymers: number average molecular weight of 5000-8000, epoxy value of 0.8-1.2 mol / 100g; Tetrahydrofuran (THF): Analytical grade, purity ≥99.5%; N,N-Dimethylacetamide (DMAc): Analytical grade, purity ≥99.5%; Polyvinylidene fluoride (PVDF): Number average molecular weight 100,000-120,000, industrial grade; Argon, oxygen, nitrogen, and ammonia: all with a purity ≥ 99.99%.

[0024] The equipment used in the embodiments and comparative examples of this invention is as follows: Liquid nitrogen cryogenic embrittlement equipment: Model LD-100, cooling temperature ≤-196℃, processing capacity 100kg / h; Mechanical crusher: Model PC-400, adjustable crushing particle size, minimum discharge particle size is 0.1mm; Fluidized bed reactor: Model FBL-50, effective volume 50L, temperature control accuracy ±5℃; Radio frequency plasma generator: Model SP-1000, radio frequency power adjustable from 0-1000W, frequency 13.56MHz; Ultrasonic disperser: Model SB-5200D, power adjustable from 0-1000W, frequency 20-80kHz; Thermostatic water bath: Model HH-S6, temperature control range 0-100℃, accuracy ±0.5℃; Tube furnace: Model SK-2-12, temperature control range is 0-1200℃, heating rate is adjustable from 1-10℃ / min; Internal mixer: Model XSM-50, volume 50L, temperature control range 0-200℃, rotor speed adjustable from 0-200rpm; Ultrasonic transducer: Model CSB-500, frequency adjustable from 20-40kHz, power adjustable from 0-1000W; High-temperature open mill: Model XK-160, roller diameter 160mm, roller temperature control range 0-200℃, roller speed ratio 1:1.2; Coaxial electrospinning device: Model ET-2000, voltage control range 0-50kV, flow rate control range 0.1-5mL / h; Extruder: Model SJ-65, screw diameter 65mm, length-to-diameter ratio 28:1, temperature control range 0-200℃; Universal testing machine: Model WDW-100, testing range 0-100kN, accuracy ±0.5%; Oxygen index meter: Model JF-3, testing range 0-100%, accuracy ±0.1%; Vertical burning tester: Model CZF-3, conforms to UL94 standard; Smoke density tester: Model JCY-2, conforming to GB / T 8323-2022 standard; Fourier transform infrared spectrometer (FTIR): Model Nicolet iS50, test range 4000-400cm⁻¹; Scanning electron microscope (SEM): Model SU8010, resolution 1.0 nm; Thermogravimetric analyzer (TGA): Model TG209F3, temperature range 0-1000℃, heating rate adjustable from 1-50℃ / min.

[0025] The performance testing methods in the embodiments and comparative examples of this invention are as follows: Mechanical property testing: Tests were conducted according to GB / T 2951.1-2021 "General Test Methods for Insulation and Sheath Materials of Cables and Optical Fibers - Part 1: General Test Methods". Tensile strength and elongation at break were tested using a universal testing machine at a speed of 50 mm / min. Shore A hardness was tested using a Shore hardness tester according to GB / T 531.1-2008. Flame retardant performance test: Oxygen index (LOI) was tested according to GB / T 2406.2-2009, with sample size of 100mm×10mm×4mm; Vertical burning rating was tested according to UL94 standard, with sample size of 127mm×12.7mm×4mm. Smoke suppression performance test: Smoke density rating (SDR) was tested according to GB / T 8323-2022, with sample size of 100mm×100mm×4mm and test time of 4min; Thermal stability test: A thermogravimetric analyzer was used under a nitrogen atmosphere at a heating rate of 10℃ / min. The test range was 50-800℃. The initial decomposition temperature of the sample was recorded. Maximum decomposition rate temperature and the carbon residue at 800℃; Surface functional group testing: Fourier transform infrared spectroscopy was used to prepare KBr pellets of the samples. The testing range was 4000-400 cm⁻¹ to analyze the types of surface functional groups and the intensity of characteristic peaks. Microscopic morphology observation: After the sample was sputter-coated with gold, the dispersion of flame retardant in the PVC matrix and the cross-sectional morphology of the material were observed using a scanning electron microscope. Environmental performance testing: The content of volatile organic compounds (VOCs) in the samples was tested in accordance with GB / T 27630-2011 "Guidelines for the Evaluation of Air Quality in Passenger Cars"; the content of heavy metals in the samples was tested in accordance with GB / T 30512-2014 "Requirements for Prohibited Substances in Automobiles".

[0026] Example 1 See appendix Figure 1 A method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride, the specific steps of which are as follows: Step 1: Surface treatment of recycled polyvinyl chloride waste Take 100 kg of industrial recycled polyvinyl chloride waste and put it into a liquid nitrogen cryogenic embrittlement device. Pour liquid nitrogen into the device for cryogenic embrittlement treatment at a temperature of -196℃ for 30 minutes. After embrittlement, send the waste to a mechanical crusher for mechanical crushing until it is broken into particles with a particle size of 0.1-0.3 mm. Then, sieve it through an 80-mesh screen to remove impurities and obtain recycled polyvinyl chloride particles.

[0027] The recovered PVC particles were placed in a fluidized bed reactor. After closing the inlet and outlet valves and checking the airtightness, an argon / oxygen mixture with a flow rate of 50 L / min (oxygen volume percentage 5%) was introduced for 5 minutes to purge air from the reactor. The reactor temperature was then raised to 150°C and maintained. An RF plasma generator was activated, and the RF power was adjusted to 500 W to generate a low-temperature plasma field for surface etching of the recovered PVC particles for 10 minutes. After treatment, the RF plasma generator was turned off, the argon / oxygen mixture was stopped, and nitrogen gas was introduced at a flow rate of 30 L / min for purging. The reactor temperature was maintained at 150°C for 10 minutes to allow uniform carbonyl and carboxyl functional groups to form on the plasma-treated PVC surface. After purging, the reactor was allowed to cool naturally to room temperature. The etched PVC particles were then removed for later use.

[0028] Step 2: Preparation of supported phosphorus-nitrogen-silicon ternary synergistic flame retardant Five kilograms of single-walled carbon nanotubes were dispersed in 200 L of N,N-dimethylformamide and placed in an ultrasonic disperser. The ultrasonic power was adjusted to 800 W and the ultrasonic frequency to 40 kHz. The mixture was ultrasonically treated for 1 hour to form a stable suspension. The suspension was then transferred to a constant temperature water bath and the water bath temperature was adjusted to 80 °C. 10 mol of tetraethyl orthosilicate, 20 mol of nonaphenylcyclotriphosphazene, and 40 mol of triethanolamine (molar ratio Si:P:N=1:2:4) were added sequentially. The stirring device was turned on and the stirring speed was adjusted to 300 rpm. The mixture was stirred and reacted at 80 °C for 6 hours to promote the hydrolysis and condensation of the silicon source to form a silicon dioxide core and the phosphazene derivative to construct the outer coating layer. This resulted in the in-situ growth of a core-shell structured composite flame retardant on the surface of the carbon nanotubes.

[0029] After the reaction was completed, the reaction product was filtered, washed three times with N,N-dimethylformamide, and then washed three times with deionized water. It was then placed in a vacuum drying oven and dried under vacuum at 80°C for 12 hours to remove moisture and residual solvent. Subsequently, the dried product was transferred to a tube furnace, and nitrogen was introduced as a protective atmosphere at a flow rate of 20 L / min. The temperature was increased to 600°C at a rate of 5°C / min, and calcined at this temperature for 2 hours to remove organic residues and form a porous ceramic shell. After calcination, the atmosphere was switched to ammonia at a flow rate of 20 L / min, and a carbothermic reduction reaction was carried out at 900°C for 1 hour to partially reduce the silica core to silicon carbide whiskers. After the reaction was completed, the product was naturally cooled to room temperature, removed, and pulverized to a particle size of 50-100 nm to obtain an inorganic / organic hybrid supported flame retardant with both thermal conductivity and smoke suppression functions, which was then put into use.

[0030] Step 3: Dynamic vulcanization compounding 100 kg of recycled PVC granules obtained from step 1 after surface etching were added to an internal mixer. The initial temperature of the internal mixer was set to 160°C and the rotor speed to 50 rpm. The internal mixer was started for mixing. After the material was completely softened (about 5 min), 25 kg of loaded flame retardant (25% of PVC mass), 8 kg of epoxidized soybean oil plasticizer (8% of PVC mass), and 2 kg of rare earth heat stabilizer lanthanum / cerium complex (2% of PVC mass) were added in batches. Mixing was continued for 5 min. Then, 1.7 kg of divinylbenzene (0.5% of the total mass of the system) was injected as a multifunctional crosslinking agent. The rotor speed was increased to 120 rpm, and the mixing temperature was maintained at 175°C. The reaction was continued for 10 min until the torque curve tended to stabilize, completing the dynamic vulcanization process.

[0031] During the dynamic vulcanization process, ultrasonic-assisted dispersion is applied simultaneously: an ultrasonic transducer with a frequency of 20kHz and a power of 500W is installed on the outer wall of the mixing chamber of the internal mixer, and glycerol is used as the coupling medium to introduce ultrasonic energy into the material; the ultrasonic action time is 50% of the total mixing time (i.e., 7.5min), and the flame retardant agglomerates are broken up by cavitation effect, so that they are dispersed at the nanoscale in the PVC matrix.

[0032] Step 4: Chain extension and toughening post-treatment The composite material after dynamic vulcanization in step 3 is taken out and placed in a high-temperature open mill. The roller spacing is adjusted to 0.5 mm and the roller temperature is maintained at 155°C. The composite material is passed through a thin mill three times. Then, 0.3% of the composite material mass of epoxy end-group teleclaw polymer solution (0.41 kg, the mass ratio of epoxy end-group teleclaw polymer to DMF is 1:1) is sprayed and thin mill plasticizing is continued for 5 minutes to allow the epoxy groups to undergo a grafting reaction with the HCl at the end of the PVC molecular chain, thereby achieving chain extension and fine adjustment of crosslinking density. After plasticizing is completed, the composite material is taken out and naturally cooled to room temperature to obtain a preliminary formed cable sheath material.

[0033] Step 5: In-situ composite of electrospinning reinforcement layer Take 10 kg of the preliminary shaped cable sheath material masterbatch obtained in step 4, dissolve it in 40 L of a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide (volume ratio of tetrahydrofuran to N,N-dimethylacetamide is 1:1), and sonicate it in an ultrasonic disperser for 30 min to prepare an electrospinning precursor solution with a solid content of 15%. At the same time, take 2 kg of polyvinylidene fluoride, dissolve it in 8 L of a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide (volume ratio 1:1), and sonicate it for 30 min to obtain a pure polyvinylidene fluoride core layer solution.

[0034] A coaxial electrospinning device was used, with the prepared masterbatch solution as the sheath layer and pure polyvinylidene fluoride solution as the core layer. The voltage was adjusted to 18kV, the spinning distance to 15cm, the sheath flow rate to 0.8mL / h, and the core flow rate to 0.2mL / h to obtain a nanofiber membrane. The obtained nanofiber membrane was directly applied to the die lip of the extrusion die. Then, the preliminary shaped cable sheath material obtained in step 4 was placed into the extruder, and the extrusion temperature was adjusted to 170℃ and the screw speed to 30rpm for extrusion molding. This allowed the sheath melt to recombine with the nanofiber membrane in situ during extrusion, thereby forming a dense physical shielding barrier on the surface of the material. After extrusion, the material was cooled and shaped to obtain an environmentally friendly cable sheath material.

[0035] Example 2 A method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride, comprising the following specific steps: Step 1: Surface treatment of recycled polyvinyl chloride waste Take 100 kg of industrial recycled polyvinyl chloride waste and put it into a liquid nitrogen cryogenic embrittlement device. Pour liquid nitrogen into the device for cryogenic embrittlement treatment at a temperature of -196℃ for 40 minutes. After embrittlement, send the waste to a mechanical crusher for mechanical crushing until it is broken into particles with a particle size of 0.2-0.4 mm. Then, sieve it through an 80-mesh screen to remove impurities and obtain recycled polyvinyl chloride particles.

[0036] The recovered PVC particles were placed in a fluidized bed reactor. After closing the inlet and outlet valves and checking the airtightness, an argon / oxygen mixture with a flow rate of 75 L / min (7.5% oxygen by volume) was introduced for 5 minutes to purge air from the reactor. The reactor temperature was then raised to 175°C and maintained. An RF plasma generator was activated, and the RF power was adjusted to 750 W to generate a low-temperature plasma field for surface etching of the recovered PVC particles. The treatment time was 20 minutes. After treatment, the RF plasma generator was turned off, the argon / oxygen mixture was stopped, and nitrogen gas was introduced at a flow rate of 30 L / min for purging. The reactor temperature was maintained at 175°C for 10 minutes to allow uniform carbonyl and carboxyl functional groups to form on the plasma-treated PVC surface. After purging, the reactor was allowed to cool naturally to room temperature. The etched PVC particles were then removed for later use.

[0037] Step 2: Preparation of supported phosphorus-nitrogen-silicon ternary synergistic flame retardant Five kilograms of single-walled carbon nanotubes were dispersed in 200 L of N,N-dimethylformamide and placed in an ultrasonic disperser. The ultrasonic power was adjusted to 800 W and the ultrasonic frequency to 40 kHz. The mixture was ultrasonically treated for 1.5 h to form a stable suspension. The suspension was then transferred to a constant temperature water bath and the water bath temperature was adjusted to 85 °C. 10 mol of tetraethyl orthosilicate, 20 mol of nonaphenylcyclotriphosphazene, and 40 mol of triethanolamine (molar ratio Si:P:N=1:2:4) were added sequentially. The stirring device was turned on and the stirring speed was adjusted to 350 rpm. The mixture was stirred and reacted at 85 °C for 7 h to promote the hydrolysis and condensation of the silicon source to form a silicon dioxide core and the phosphazene derivative to construct the outer coating layer. This resulted in the in-situ growth of a core-shell structured composite flame retardant on the surface of the carbon nanotubes.

[0038] After the reaction was completed, the reaction product was filtered, washed three times with N,N-dimethylformamide, and then washed three times with deionized water. It was then placed in a vacuum drying oven and dried at 80°C for 12 hours to remove moisture and residual solvent. The dried product was then transferred to a tube furnace, and nitrogen was introduced as a protective atmosphere at a flow rate of 20 L / min. The temperature was increased to 700°C at a rate of 5°C / min and calcined at this temperature for 2.5 hours to remove organic residues and form a porous ceramic shell. After calcination, the atmosphere was switched to ammonia at a flow rate of 20 L / min, and a carbothermic reduction reaction was carried out at 950°C for 1.5 hours to partially reduce the silica core to silicon carbide whiskers. After the reaction was completed, the product was naturally cooled to room temperature, removed, and pulverized to a particle size of 50-100 nm to obtain an inorganic / organic hybrid supported flame retardant with both thermal conductivity and smoke suppression functions, for later use.

[0039] Step 3: Dynamic vulcanization compounding 100 kg of recycled PVC granules obtained from step 1 after surface etching were added to an internal mixer. The initial temperature of the internal mixer was set to 160°C and the rotor speed to 65 rpm. The internal mixer was started for mixing. After the material was completely softened (about 5 min), 30 kg of loaded flame retardant (30% of the PVC mass), 10 kg of epoxidized soybean oil plasticizer (10% of the PVC mass), and 3 kg of rare earth heat stabilizer lanthanum / cerium complex (3% of the PVC mass) were added in batches. Mixing was continued for 6.5 min. Then, 2.35 kg of divinylbenzene (0.75% of the total mass of the system) of multifunctional crosslinking agent was injected, and the rotor speed was increased to 135 rpm. The mixing temperature was maintained at 180°C and the reaction was continued for 12.5 min until the torque curve tended to be stable, thus completing the dynamic vulcanization process.

[0040] During the dynamic vulcanization process, ultrasonic-assisted dispersion is applied simultaneously: an ultrasonic transducer with a frequency of 30kHz and a power of 750W is installed on the outer wall of the mixing chamber of the internal mixer, and glycerol is used as the coupling medium to introduce ultrasonic energy into the material; the ultrasonic action time is 60% of the total mixing time (i.e., 10.8min), and the flame retardant agglomerates are broken up by cavitation effect, so that they are dispersed at the nanoscale in the PVC matrix.

[0041] Step 4: Chain extension and toughening post-treatment The composite material after dynamic vulcanization in step 3 is taken out and placed in a high-temperature open mill. The roller spacing is adjusted to 0.75 mm and the roller temperature is maintained at 160°C. The composite material is passed through a thin mill 4 times. Then, 0.81 kg of epoxy end-group teleclaw polymer solution (the mass ratio of epoxy end-group teleclaw polymer to DMF is 1:1) is sprayed at 0.55% of the mass of the composite material. The thin mill plasticizing is continued for 7.5 min to allow the epoxy groups to undergo a grafting reaction with the HCl at the end of the PVC molecular chain, thereby achieving chain extension and fine adjustment of crosslinking density. After plasticizing is completed, the composite material is taken out and allowed to cool naturally to room temperature to obtain a preliminary formed cable sheath material.

[0042] Step 5: In-situ composite of electrospinning reinforcement layer Take 10 kg of the preliminary shaped cable sheath material masterbatch obtained in step 4, dissolve it in 33.3 L of a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide (volume ratio of tetrahydrofuran to N,N-dimethylacetamide is 1:1), and place it in an ultrasonic disperser for ultrasonic treatment for 30 min to prepare an electrospinning precursor solution with a solid content of 17.5%. At the same time, take 2 kg of polyvinylidene fluoride, dissolve it in 8 L of a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide (volume ratio 1:1), and ultrasonically disperse it for 30 min to obtain a pure polyvinylidene fluoride core layer solution.

[0043] A coaxial electrospinning device was used, with the prepared masterbatch solution as the sheath and pure polyvinylidene fluoride solution as the core. The voltage was adjusted to 20kV, the spinning distance to 17.5cm, the sheath flow rate to 1.0mL / h, and the core flow rate to 0.25mL / h to obtain a nanofiber membrane. The obtained nanofiber membrane was directly applied to the die lip of the extrusion die. Then, the preliminary shaped cable sheath material obtained in step 4 was placed into the extruder, and the extrusion temperature was adjusted to 175℃ and the screw speed to 35rpm for extrusion molding. This allowed the sheath melt to recombine with the nanofiber membrane in situ during extrusion, thereby forming a dense physical shielding barrier on the surface of the material. After extrusion, the material was cooled and shaped to obtain an environmentally friendly cable sheath material.

[0044] Example 3 A method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride, comprising the following specific steps: Step 1: Surface treatment of recycled polyvinyl chloride waste Take 100 kg of industrial recycled polyvinyl chloride waste and put it into a liquid nitrogen cryogenic embrittlement device. Pour liquid nitrogen into the device for cryogenic embrittlement treatment at a temperature of -196℃ for 50 minutes. After embrittlement, send the waste to a mechanical crusher for mechanical crushing until it is broken into particles with a particle size of 0.3-0.5 mm. Then, sieve it through an 80-mesh screen to remove impurities and obtain recycled polyvinyl chloride particles.

[0045] The recovered PVC particles were placed in a fluidized bed reactor. After closing the inlet and outlet valves and checking the airtightness, an argon / oxygen mixture with a flow rate of 100 L / min (oxygen accounting for 10% of the volume) was introduced for 5 minutes to purge the air from the reactor. Subsequently, the reactor temperature was raised to 200°C and maintained. The radio frequency plasma generator was turned on and the radio frequency power was adjusted to 1000 W to generate a low-temperature plasma field for surface etching of the recovered PVC particles for 30 minutes. After the treatment, the radio frequency plasma generator was turned off and the argon / oxygen mixture was stopped. Nitrogen gas was introduced at a flow rate of 30 L / min for purging, while maintaining the reactor temperature at 200°C for 10 minutes to generate uniform carbonyl and carboxyl functional groups on the plasma-treated PVC surface. After purging, the reactor was allowed to cool naturally to room temperature. The etched PVC particles were then removed for later use.

[0046] Step 2: Preparation of supported phosphorus-nitrogen-silicon ternary synergistic flame retardant Five kilograms of single-walled carbon nanotubes were dispersed in 200 L of N,N-dimethylformamide and placed in an ultrasonic disperser. The ultrasonic power was adjusted to 800 W and the ultrasonic frequency to 40 kHz. The mixture was ultrasonically treated for 2 hours to form a stable suspension. The suspension was then transferred to a constant temperature water bath and the water bath temperature was adjusted to 90 °C. 10 mol of tetraethyl orthosilicate, 20 mol of nonaphenylcyclotriphosphazene, and 40 mol of triethanolamine (molar ratio Si:P:N=1:2:4) were added sequentially. The stirring device was turned on and the stirring speed was adjusted to 400 rpm. The mixture was stirred and reacted at 90 °C for 8 hours to promote the hydrolysis and condensation of the silicon source to form a silicon dioxide core and the phosphazene derivative to construct the outer coating layer. This resulted in the in-situ growth of a core-shell structured composite flame retardant on the surface of the carbon nanotubes.

[0047] After the reaction was completed, the reaction product was filtered, washed three times with N,N-dimethylformamide, and then washed three times with deionized water. It was then placed in a vacuum drying oven and dried under vacuum at 80°C for 12 hours to remove moisture and residual solvent. Subsequently, the dried product was transferred to a tube furnace, and nitrogen was introduced as a protective atmosphere at a flow rate of 20 L / min. The temperature was increased to 800°C at a rate of 5°C / min, and calcined at this temperature for 3 hours to remove organic residues and form a porous ceramic shell. After calcination, the atmosphere was switched to ammonia at a flow rate of 20 L / min, and a carbothermic reduction reaction was carried out at 1000°C for 2 hours to partially reduce the silica core to silicon carbide whiskers. After the reaction was completed, the product was naturally cooled to room temperature, removed, and pulverized to a particle size of 50-100 nm to obtain an inorganic / organic hybrid supported flame retardant with both thermal conductivity and smoke suppression functions, which was then put into use.

[0048] Step 3: Dynamic vulcanization compounding 100 kg of recycled PVC granules obtained from step 1 after surface etching were added to an internal mixer. The initial temperature of the internal mixer was set to 160°C and the rotor speed to 80 rpm. The internal mixer was started for mixing. After the material was completely softened (about 5 min), 35 kg of loaded flame retardant (35% of PVC mass), 12 kg of epoxidized soybean oil plasticizer (12% of PVC mass), and 4 kg of rare earth heat stabilizer lanthanum / cerium complex (4% of PVC mass) were added in batches. Mixing was continued for 8 min. Then, 3.05 kg of divinylbenzene (1% of the total mass of the system) was injected, and the rotor speed was increased to 150 rpm. The mixing temperature was maintained at 185°C and the reaction was continued for 15 min until the torque curve tended to be stable, thus completing the dynamic vulcanization process.

[0049] During the dynamic vulcanization process, ultrasonic-assisted dispersion is applied simultaneously: an ultrasonic transducer with a frequency of 40kHz and a power of 1000W is installed on the outer wall of the mixing chamber of the internal mixer, and glycerol is used as the coupling medium to introduce ultrasonic energy into the material; the ultrasonic action time is 70% of the total mixing time (i.e., 14min), and the flame retardant agglomerates are broken up by cavitation effect, so that they are dispersed at the nanoscale in the PVC matrix.

[0050] Step 4: Chain extension and toughening post-treatment The composite material after dynamic vulcanization in step 3 is taken out and placed in a high-temperature open mill. The roller spacing is adjusted to 1 mm and the roller temperature is maintained at 165℃. The composite material is passed through a thin mill 5 times. Then, 0.8% of the composite material mass of epoxy end-group teleclaw polymer solution (1.22 kg, the mass ratio of epoxy end-group teleclaw polymer to DMF is 1:1) is sprayed and thin mill plasticizing is continued for 10 min to allow the epoxy groups to undergo a grafting reaction with the HCl at the end of the PVC molecular chain, thereby achieving chain extension and fine adjustment of crosslinking density. After plasticizing is completed, the composite material is taken out and naturally cooled to room temperature to obtain a preliminary formed cable sheath material.

[0051] Step 5: In-situ composite of electrospinning reinforcement layer Take 10 kg of the preliminary shaped cable sheath material masterbatch obtained in step 4, dissolve it in 30 L of a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide (volume ratio of tetrahydrofuran to N,N-dimethylacetamide is 1:1), and sonicate it in an ultrasonic disperser for 30 min to prepare an electrospinning precursor solution with a solid content of 20%. At the same time, take 2 kg of polyvinylidene fluoride, dissolve it in 8 L of a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide (volume ratio 1:1), and sonicate it for 30 min to obtain a pure polyvinylidene fluoride core layer solution.

[0052] A coaxial electrospinning device was used, with the prepared masterbatch solution as the sheath and pure polyvinylidene fluoride solution as the core. The voltage was adjusted to 22kV, the spinning distance to 20cm, the sheath flow rate to 1.2mL / h, and the core flow rate to 0.3mL / h to obtain a nanofiber membrane. The obtained nanofiber membrane was directly applied to the die lip of an extrusion die. Then, the preliminary shaped cable sheath material obtained in step 4 was placed into an extruder, and the extrusion temperature was adjusted to 180℃ and the screw speed to 40rpm for extrusion molding. This allowed the sheath melt to recombine with the nanofiber membrane in situ during extrusion, thereby forming a dense physical shielding barrier on the surface of the material. After extrusion, the material was cooled and shaped to obtain an environmentally friendly cable sheath material.

[0053] Comparative Example 1 A method for preparing cable sheath material made from recycled polyvinyl chloride (PVC) without plasma surface etching is described below. The remaining steps are the same as in Example 2. Step 1: Processing of recycled polyvinyl chloride waste Take 100kg of industrial recycled polyvinyl chloride waste and put it into a liquid nitrogen cryogenic embrittlement device. Pour liquid nitrogen into the device for cryogenic embrittlement treatment at a temperature of -196℃ for 40 minutes. After embrittlement, the waste is sent to a mechanical crusher for mechanical crushing to particles with a diameter of 0.2-0.4mm. Then, the particles are sieved through an 80-mesh screen to remove impurities and obtain recycled polyvinyl chloride particles. These particles are not subjected to plasma surface etching treatment and are used directly.

[0054] Step 2: Preparation of supported phosphorus-nitrogen-silicon ternary synergistic flame retardant The process is exactly the same as step 2 in Example 2, yielding a loaded flame retardant for later use.

[0055] Step 3: Dynamic vulcanization compounding The dynamic vulcanization process is completed exactly as in step 3 of Example 2.

[0056] Step 4: Chain extension and toughening post-treatment The process is exactly the same as step 4 in Example 2, resulting in a preliminary formed cable sheath material.

[0057] Step 5: In-situ composite of electrospinning reinforcement layer The process is exactly the same as step 5 in Example 2, resulting in the cable sheath material.

[0058] Comparative Example 2 A method for preparing cable sheath material from recycled polyvinyl chloride (PVC) is disclosed. Instead of preparing a loaded phosphorus-nitrogen-silicon ternary synergistic flame retardant, a common phosphorus-nitrogen flame retardant is used. The remaining steps are the same as in Example 2. The specific steps are as follows: Step 1: Surface treatment of recycled polyvinyl chloride waste The process is exactly the same as step 1 in Example 2, yielding recycled polyvinyl chloride particles with surface etching, which are then set aside for later use.

[0059] Step 2: Preparation of common phosphorus-nitrogen flame retardants Take 30 kg of ordinary phosphorus-nitrogen flame retardant (melamine cyanurate), crush it to a particle size of 50-100 nm, and set it aside. No in-situ loading or high-temperature calcination reduction treatment was performed.

[0060] Step 3: Dynamic vulcanization compounding 100 kg of recycled PVC granules obtained from step 1 after surface etching were added to an internal mixer. The initial temperature of the internal mixer was set to 160°C and the rotor speed to 65 rpm. The internal mixer was started for mixing. After the material was completely softened (about 5 min), 30 kg of ordinary phosphorus and nitrogen flame retardant, 10 kg of epoxidized soybean oil plasticizer (10% of the PVC mass), and 3 kg of rare earth heat stabilizer lanthanum / cerium complex (3% of the PVC mass) were added in batches. Mixing was continued for 6.5 min. Then, 2.35 kg of divinylbenzene, a multifunctional crosslinking agent accounting for 0.75% of the total mass of the system, was injected. The rotor speed was increased to 135 rpm, and the mixing temperature was maintained at 180°C. The reaction was continued for 12.5 min until the torque curve tended to stabilize, completing the dynamic vulcanization process.

[0061] During the dynamic vulcanization process, ultrasonic-assisted dispersion is applied simultaneously, which is exactly the same as step 3 in Example 2.

[0062] Step 4: Chain extension and toughening post-treatment The process is exactly the same as step 4 in Example 2, resulting in a preliminary formed cable sheath material.

[0063] Step 5: In-situ composite of electrospinning reinforcement layer The process is exactly the same as step 5 in Example 2, resulting in the cable sheath material.

[0064] Comparative Example 3 A method for preparing cable sheath material made from recycled polyvinyl chloride (PVC) without chain extension and toughening post-treatment or in-situ composite of electrospinning reinforcement layer; the remaining steps are the same as in Example 2, and the specific steps are as follows: Step 1: Surface treatment of recycled polyvinyl chloride waste The process is exactly the same as step 1 in Example 2, yielding recycled polyvinyl chloride particles with surface etching, which are then set aside for later use.

[0065] Step 2: Preparation of supported phosphorus-nitrogen-silicon ternary synergistic flame retardant The process is exactly the same as step 2 in Example 2, yielding a loaded flame retardant for later use.

[0066] Step 3: Dynamic vulcanization compounding The dynamic vulcanization process is completed exactly as in step 3 of Example 2.

[0067] After dynamic vulcanization, the composite material is taken out, allowed to cool naturally to room temperature, and then placed in an extruder. The extrusion temperature is adjusted to 175℃ and the screw speed is 35rpm for extrusion molding. After cooling and shaping, cable sheath material is obtained without chain extension and toughening post-treatment or in-situ composite of electrospinning reinforcement layer.

[0068] The mechanical properties, flame retardant properties, smoke suppression properties, thermal stability properties, and environmental performance of the cable sheath materials prepared in Examples 1-3 and Comparative Examples 1-3 were tested. The test results are shown in Tables 1-3 below.

[0069] Table 1: Test results of mechanical properties of cable sheath materials in Examples 1-3 and Comparative Examples 1-3 As shown in Table 1, the environmentally friendly cable sheath materials prepared in Examples 1-3 all have excellent mechanical properties. Among them, Example 2 has the best mechanical properties, with a tensile strength of 21.3 MPa, an elongation at break of 358%, a Shore A hardness of 80, and a tensile strength retention rate of 92.5% after aging at 100℃ for 72 hours. The mechanical properties of Examples 1-3 differ to some extent, mainly due to the different process parameters in each example: Example 2 used intermediate-level plasma treatment parameters (temperature 175℃, time 20min), flame retardant loading (30%), ultrasonic-assisted dispersion parameters (frequency 30kHz, power 750W, action time 60%), as well as chain extension, toughening, and electrospinning parameters, which resulted in optimal compatibility and dispersion among the components, thus exhibiting the best mechanical properties; the process parameters of Example 1 were generally low, with insufficient plasma treatment, low flame retardant loading, and short ultrasonic action time, leading to uneven component dispersion and slightly lower mechanical properties than Example 2; the process parameters of Example 3 were generally high, with excessively high plasma treatment temperature and time causing slight degradation of the PVC molecular chains, excessively high flame retardant loading increasing the rigidity of the system, and excessively high ultrasonic power and action time damaging the PVC molecular chain structure, thus its mechanical properties were slightly lower than Example 2, but still better than Example 1.

[0070] Comparing Example 2 and Comparative Example 1, it can be seen that Comparative Example 1, without plasma surface etching treatment, has a tensile strength of only 14.2 MPa, an elongation at break of 268%, and a tensile strength retention rate of only 78.3%, significantly lower than Example 2. This is because plasma surface etching can introduce active sites such as carbonyl and carboxyl groups onto the PVC surface. These active sites can interact with the functional groups on the surface of the supported flame retardant, improving the compatibility and bonding force between the flame retardant and the PVC matrix, while also improving the cross-linking effect between PVC molecular chains, thereby enhancing the mechanical properties and aging stability of the material. In contrast, the PVC surface without plasma treatment lacks active sites, resulting in poor compatibility between the flame retardant and the PVC matrix, and a tendency for aggregation, leading to internal defects in the material and a significant decrease in mechanical properties and aging stability. This is consistent with the research conclusion that plasma treatment can improve the surface polarity and compatibility of PVC.

[0071] Comparing Example 2 and Comparative Example 2, it can be seen that Comparative Example 2, which uses a common phosphorus-nitrogen flame retardant instead of a supported phosphorus-nitrogen-silicon ternary synergistic flame retardant, has a tensile strength of 15.7 MPa, an elongation at break of 285%, and a tensile strength retention rate of 81.5%, which are lower than those of Example 2. This is because the supported phosphorus-nitrogen-silicon ternary synergistic flame retardant has a core-shell structure and is in situ supported on a carbon nanotube network framework. The carbon nanotubes can play a reinforcing role. At the same time, the core-shell structured flame retardant has better compatibility with the PVC matrix and can be uniformly dispersed in the PVC matrix, playing a synergistic reinforcing role. In contrast, the common phosphorus-nitrogen flame retardant does not have a core-shell structure, has poor compatibility with the PVC matrix, poor dispersibility, and lacks the reinforcing effect of carbon nanotubes. Therefore, the mechanical properties and aging stability of the material are not as good as those of Example 2.

[0072] Comparing Example 2 and Comparative Example 3, Comparative Example 3, without chain extension and toughening post-treatment and in-situ composite with electrospinning reinforcement layer, exhibits a tensile strength of 16.3 MPa, an elongation at break of 292%, and a tensile strength retention rate of 83.7%, which are lower than those of Example 2. This is because the chain extension and toughening post-treatment can achieve chain extension and fine-tuning of crosslinking density through grafting reaction between epoxy end-group telechelic polymer and HCl at the ends of PVC molecular chains, thereby improving the toughness and tensile strength of the material. The electrospinning reinforcement layer can form a dense physical shielding barrier on the material surface, which not only improves the surface strength of the material but also enhances its aging stability and reduces the damage to the internal structure of the material caused by the external environment. Therefore, Comparative Example 3, without these two treatments, shows a decrease in both mechanical properties and aging stability, which also verifies the effective role of electrospinning nanofiber membrane in improving the surface properties of materials.

[0073] Furthermore, the Shore A hardness data shows that the Shore A hardness of Examples 1-3 is between 78 and 82, which is within the suitable hardness range for cable sheath materials. This ensures both the rigidity and flexibility of the material, meeting the requirements for cable sheath use. However, the Shore A hardness of Comparative Examples 1-3 is slightly lower, mainly because their internal structure is not dense enough and the component compatibility is poor, resulting in insufficient rigidity of the material.

[0074] Table 2: Test results of flame retardant and smoke suppression properties of cable sheath materials in Examples 1-3 and Comparative Examples 1-3 As shown in Table 2, the environmentally friendly cable sheath materials prepared in Examples 1-3 all have excellent flame retardant and smoke suppression properties. Among them, Example 2 has the best flame retardant and smoke suppression properties, with an oxygen index of 35.8%, a vertical burning rating of V-0, a smoke density rating of only 32, a maximum smoke release rate of 0.062 m² / s, and a flame retardant migration rate of only 0.8%. The differences in flame retardant and smoke suppression performance between Examples 1-3 are mainly due to the different process parameters: In Example 2, the plasma treatment parameters, flame retardant loading, high-temperature calcination reduction parameters, and ultrasonic-assisted dispersion parameters are all at intermediate levels, allowing the supported phosphorus-nitrogen-silicon ternary synergistic flame retardant to fully exert its effect. The three elements of phosphorus, nitrogen, and silicon form a synergistic flame retardant effect, while the carbon nanotube network framework can promote the uniform dispersion of the flame retardant, improve the flame retardant efficiency, and reduce smoke release. In Example 1, the flame retardant loading is low (25%), and the plasma treatment is insufficient, resulting in poor compatibility and dispersibility between the flame retardant and the PVC matrix, leading to slightly lower flame retardant and smoke suppression performance than Example 2. In Example 3, the flame retardant loading is high (35%), which can improve the flame retardant performance, but the excessively high flame retardant content will cause slight agglomeration in the PVC matrix, which will affect the smoke suppression performance. Therefore, its smoke density level is slightly higher than that of Example 2, but still better than that of Example 1.

[0075] Comparing Example 2 and Comparative Example 1, Comparative Example 1, which did not undergo plasma surface etching treatment, had an oxygen index of only 28.3%, a vertical burning rating of only V-1, a smoke density rating of 56, a maximum smoke release rate of 0.132 m² / s, and a flame retardant migration rate of 3.5%, significantly inferior to Example 2. This is because the active sites introduced by plasma surface etching can enhance the bonding force between the flame retardant and the PVC matrix, reduce flame retardant migration, and promote uniform dispersion of the flame retardant in the PVC matrix, allowing the flame retardant to fully exert its effect. In contrast, the PVC surface without plasma treatment lacks active sites, resulting in weaker bonding force between the flame retardant and the PVC matrix, making it prone to migration and uneven dispersion, leading to decreased flame retardant efficiency and increased smoke release. This is consistent with the influence of flame retardant loading uniformity on flame retardant performance. Furthermore, the difference in vertical burning rating indicates that plasma treatment can improve the flame retardant stability of the material and prevent dripping during combustion, thereby achieving the V-0 standard.

[0076] Comparing Example 2 and Comparative Example 2, it can be seen that Comparative Example 2, which uses a common phosphorus-nitrogen flame retardant instead of a supported phosphorus-nitrogen-silicon ternary synergistic flame retardant, has an oxygen index of 29.7%, a vertical burning rating of V-1, a smoke density rating of 51, a maximum smoke release rate of 0.118 m² / s, and a flame retardant migration rate of 4.2%, which is lower than that of Example 2. This is because in the supported phosphorus-nitrogen-silicon ternary synergistic flame retardant, phosphorus can form a char layer during combustion, nitrogen can release inert gas to dilute combustible gas, and silicon can form a ceramic protective layer. The three elements work synergistically to significantly improve flame retardant and smoke suppression performance. At the same time, the core-shell structure and carbon nanotube network framework can effectively inhibit the migration of flame retardant and reduce its loss. In contrast, the common phosphorus-nitrogen flame retardant relies only on the effects of phosphorus and nitrogen, lacks the synergistic effect of silicon, and does not have a core-shell structure. It is prone to migration and has poor dispersibility, resulting in poor flame retardant and smoke suppression performance and a high flame retardant migration rate. This is consistent with the advantageous characteristics of the phosphorus-nitrogen-silicon ternary synergistic flame retardant system.

[0077] Comparing Example 2 and Comparative Example 3, Comparative Example 3, without chain extension and toughening post-treatment and in-situ composite with the electrospinning reinforcement layer, exhibits an oxygen index of 31.2%, a vertical burning rating of V-0, a smoke density rating of 45, a maximum smoke release rate of 0.095 m² / s, and a flame retardant migration rate of 1.5%. Both its flame retardant and smoke suppression performance are lower than that of Example 2. This is because the dense physical barrier formed by the electrospinning reinforcement layer can block the contact between oxygen and the material's interior in the early stages of combustion, slowing the combustion process and inhibiting the diffusion and release of smoke. The cross-linked network structure formed by the chain extension and toughening post-treatment can enhance the material's structural density, reduce the generation of molten droplets during combustion, further improve flame retardant stability, and inhibit the migration of the flame retardant. In contrast, Comparative Example 3, which did not undergo these two steps, lacked a physical shielding barrier and a dense cross-linked structure. During combustion, oxygen could easily penetrate and smoke could easily spread. Flame retardants were also prone to slight migration, resulting in a decrease in flame retardant and smoke suppression performance. This also verifies the important role of electrospinning reinforcement layer and chain extension toughening treatment in synergistically improving the flame retardant and smoke suppression performance of the material.

[0078] Furthermore, the flame retardant migration rate data showed that the flame retardant migration rate of Examples 1-3 was only 0.8%-1.2%, far lower than that of Comparative Examples 1-3 (1.5%-4.2%). This was mainly due to the core-shell structure of the supported phosphorus-nitrogen-silicon ternary synergistic flame retardant, the binding effect of the carbon nanotube network skeleton, and the strong interaction between the PVC surface and the flame retardant after plasma treatment. The three factors synergistically inhibited the migration of the flame retardant, ensuring the stability of the flame retardant performance during long-term use of the material. However, Comparative Example 1 lacked plasma treatment, Comparative Example 2 used ordinary flame retardant, and Comparative Example 3 lacked cross-linking and shielding structures, all of which led to an increase in the flame retardant migration rate, affecting the long-term flame retardant effect of the material.

[0079] Table 3: Test results of thermal stability and environmental performance of cable sheath materials in Examples 1-3 and Comparative Examples 1-3 As shown in Table 3, the environmentally friendly cable sheath materials prepared in Examples 1-3 all exhibit excellent thermal stability and environmental performance. Among them, Example 2 shows the best overall performance with the lowest initial decomposition temperature. The temperature reaches 312℃, the temperature of maximum decomposition rate. At 448℃ and 800℃, the residual char rate was 32.4%, the VOCs content was only 15.7 mg / m³, and no heavy metals were detected, fully meeting environmental protection standards. The differences in thermal stability between Examples 1-3 are primarily due to variations in the optimization of process parameters: In Example 2, the plasma treatment, high-temperature calcination reduction, and dynamic vulcanization parameters were within their optimal range, enabling the supported phosphorus-nitrogen-silicon ternary synergistic flame retardant to form a stable cross-linked network with the PVC matrix. The ceramicized protective layer formed by silicon blocks heat transfer, the carbon nanotube network disperses heat, and the carbon layer formed by phosphorus inhibits the degradation of PVC molecular chains. These three elements synergistically enhance the material's thermal stability. In Example 1, the process parameters were too low, resulting in insufficient uniform dispersion of the flame retardant and inadequate cross-linking density, leading to slightly lower thermal stability than Example 2. In Example 3, the process parameters were too high, causing slight degradation of the PVC molecular chains and a slight decrease in the initial decomposition temperature, resulting in slightly lower thermal stability than Example 2, but still superior to Example 1.

[0080] Comparing Example 2 and Comparative Example 1, Comparative Example 1, which did not undergo plasma surface etching treatment, had an initial decomposition temperature of only 275°C, a maximum decomposition rate temperature of 402°C, a char residue rate of only 22.3% at 800°C, and a VOC content of 25.8 mg / m³. Its thermal stability and environmental performance were significantly inferior to Example 2. This is because plasma surface etching can repair defects on the surface of recycled PVC, reduce unstable structures at the ends of molecular chains, enhance the compatibility of flame retardants with the PVC matrix, reduce interface defects, thereby reducing the degradation rate of PVC molecular chains and improving thermal stability. Furthermore, plasma treatment can promote cross-linking of PVC molecular chains, reduce the release of small molecule volatiles, and lower VOC content. In contrast, Comparative Example 1, which did not undergo plasma treatment, had more defects on the surface of recycled PVC, uneven dispersion of flame retardants, obvious interface defects, easily degraded PVC molecular chains, poor thermal stability, and a higher content of small molecule volatiles and VOCs.

[0081] Comparing Example 2 and Comparative Example 2, it can be seen that Comparative Example 2, which uses a common phosphorus-nitrogen flame retardant instead of a supported phosphorus-nitrogen-silicon ternary synergistic flame retardant, has an initial decomposition temperature of 282°C, a maximum decomposition rate temperature of 408°C, a char residue rate of 24.5% at 800°C, and a VOCs content of 23.6 mg / m³, which are lower than those in Example 2. This is because the silicon element in the supported phosphorus-nitrogen-silicon ternary synergistic flame retardant can form a dense ceramic protective layer at high temperatures, effectively blocking the penetration of heat and oxygen, and delaying the degradation of PVC. At the same time, carbon nanotubes can quickly disperse heat, further improving thermal stability. In contrast, the common phosphorus-nitrogen flame retardant does not have a ceramic protective effect and can only play a role by releasing inert gases and forming a char layer. Its thermal stability effect is limited, and the common flame retardant has poor dispersibility, which can easily lead to local overheating, accelerate PVC degradation, and increase the release of VOCs.

[0082] Comparing Example 2 and Comparative Example 3, Comparative Example 3, which did not undergo chain extension and toughening post-treatment or in-situ composite with the electrospinning reinforcement layer, had an initial decomposition temperature of 290°C, a maximum decomposition rate temperature of 415°C, a char residue rate of 26.7% at 800°C, and a VOC content of 20.2 mg / m³, lower than Example 2. This is because the cross-linked network formed by the chain extension and toughening post-treatment can enhance the structural stability of the material, reduce the movement of PVC molecular chains, and delay its degradation; the dense barrier formed by the electrospinning reinforcement layer can block heat transfer and small molecule volatilization, thus improving thermal stability and reducing VOC content. In contrast, Comparative Example 3, which did not undergo these two treatments, had insufficient material density, making the PVC molecular chains prone to degradation and small molecule volatiles easily released, resulting in a decrease in both thermal stability and environmental performance.

[0083] In terms of environmental performance, no heavy metals were detected in Examples 1-3 and Comparative Examples 1-3. This is due to the use of rare earth heat stabilizers in this invention to replace traditional heavy metal stabilizers, thus achieving environmental protection of materials from the source. However, the VOCs content of Examples 1-3 was significantly lower than that of Comparative Examples 1-3. This is mainly because the examples effectively inhibited PVC degradation and the release of small molecule volatiles through plasma treatment, uniform dispersion of the loaded flame retardant, and the formation of cross-linked networks. This meets the strict requirements of GB / T 27630-2011 "Guidelines for Air Quality Evaluation in Passenger Cars" and is suitable for the preparation of environmentally friendly cable sheaths.

[0084] Based on the performance test results of Examples 1-3 and Comparative Examples 1-3, it can be seen that the present invention successfully prepared an environmentally friendly cable sheath material of recycled polyvinyl chloride with excellent mechanical properties, flame retardant properties, smoke suppression properties, thermal stability and environmental protection properties through a five-step core process of "recycled PVC plasma surface etching treatment - preparation of loaded phosphorus nitrogen silicon ternary synergistic flame retardant - dynamic vulcanization compounding - chain extension and toughening post-treatment - in-situ composite of electrostatic spinning reinforcement layer".

[0085] The synergistic effects of each process step are as follows: Plasma surface etching treatment introduces active functional groups, improving the compatibility of recycled PVC and flame retardants, laying the foundation for subsequent compounding; the supported phosphorus-nitrogen-silicon ternary synergistic flame retardant, with the help of carbon nanotube network framework and core-shell structure, achieves synergistic flame retardancy and smoke suppression of phosphorus, nitrogen, and silicon, while inhibiting flame retardant migration; dynamic vulcanization compounding combined with ultrasonic-assisted dispersion ensures uniform dispersion of each component, forming a stable cross-linking system; chain extension and toughening post-treatment improves the toughness and tensile properties of the material through molecular chain extension and cross-linking density fine-tuning; the dense barrier formed by in-situ composite of electrospinning reinforcement layer further improves the mechanical properties, flame retardant properties, and thermal stability of the material, while reducing VOCs release.

[0086] Among them, the process parameters of Example 2 were the most optimized, and the prepared cable sheath material had the best comprehensive performance: tensile strength 21.3 MPa, elongation at break 358%, Shore A hardness 80, tensile strength retention rate after aging at 100℃ for 72h 92.5%; oxygen index 35.8%, vertical burning rating V-0, smoke density rating 32, maximum smoke release rate 0.062m² / s, flame retardant migration rate 0.8%; initial decomposition temperature 312℃, maximum decomposition rate temperature 448℃, char residue rate at 800℃ 32.4%; VOCs content 15.7mg / m³, no heavy metals detected, fully meeting the usage requirements and environmental protection standards for cable sheath materials.

[0087] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride, characterized in that, This includes the following operations: Industrially recycled polyvinyl chloride waste is mechanically crushed after being cryogenically embrittled with liquid nitrogen, and then surface etched using plasma gas flow to introduce active sites. A core-shell structured phosphorus-nitrogen-silicon ternary synergistic flame retardant was constructed and in situ loaded onto a carbon nanotube network framework. Surface-treated recycled polyvinyl chloride and a loaded flame retardant are dynamically vulcanized and compounded in an internal mixer to produce an environmentally friendly cable sheath material with a micro-crosslinked network structure.

2. The method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride according to claim 1, characterized in that, The surface etching step specifically includes: The recycled polyvinyl chloride particles were placed in a fluidized bed reactor, and an argon / oxygen mixture was introduced at a flow rate of 50 L / min to 100 L / min, wherein the oxygen volume percentage was 5% to 10%. Meanwhile, the temperature inside the reactor is maintained at 150℃~200℃, and a low-temperature plasma field is generated by excitation using radio frequency power supply, with a processing time of 10min~30min. After treatment, nitrogen gas is introduced for purging and the temperature is maintained for 10 minutes to generate uniform carbonyl and carboxyl functional groups on the plasma-treated PVC surface.

3. The method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride according to claim 1, characterized in that, The in-situ loading step of the phosphorus-nitrogen-silicon ternary synergistic flame retardant specifically includes: Single-walled carbon nanotubes with a diameter of 10 nm to 20 nm were dispersed in N,N-dimethylformamide and ultrasonically treated for 1 h to 2 h to form a stable suspension. Tetraethyl orthosilicate, nonaphenylcyclotriphosphazene, and triethanolamine are added sequentially, with a molar ratio of Si:P:N = 1:2:

4. The mixture is stirred at 80℃ to 90℃ for 6 to 8 hours to promote the hydrolysis and condensation of the silicon source to form a silicon dioxide core and the phosphazene derivative to construct the outer coating layer, thereby growing a core-shell structured composite flame retardant in situ on the surface of carbon nanotubes.

4. The method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride according to claim 3, characterized in that, After the in-situ loading step is completed, high-temperature calcination and reduction treatment is required: the reaction product is transferred to a tube furnace and heated to 600℃~800℃ at a heating rate of 5℃ / min under a nitrogen protective atmosphere, and calcined at this temperature for 2h~3h to remove organic residues and form a porous ceramic shell; then the atmosphere is switched to ammonia, and a carbothermic reduction reaction is carried out at 900℃~1000℃ for 1h~2h to reduce the silica core to silicon carbide whiskers, finally obtaining an inorganic / organic hybrid flame retardant filler with both thermal conductivity and smoke suppression functions.

5. The method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride according to claim 1, characterized in that, The dynamic vulcanization compounding step is specifically as follows: The recycled polyvinyl chloride after surface etching is put into an internal mixer, with the initial temperature set at 160℃ and the rotor speed at 50rpm~80rpm. After the material softens, add in batches 25% to 35% of the PVC mass of loaded flame retardant, 8% to 12% of the PVC mass of epoxidized soybean oil plasticizer, and 2% to 4% of the PVC mass of rare earth heat stabilizer lanthanum / cerium complex. After continuing to mix for 5 to 8 minutes, inject 0.5% to 1% of the multifunctional crosslinking agent divinylbenzene, and increase the rotor speed to 120 to 150 rpm. Maintain the mixing temperature at 175°C to 185°C and continue the reaction for 10 to 15 minutes until the torque curve becomes stable, thus completing the dynamic vulcanization process.

6. The method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride according to claim 5, characterized in that, Simultaneous ultrasonic-assisted dispersion is applied during dynamic vulcanization: An ultrasonic transducer with a frequency of 20kHz to 40kHz and a power of 500W to 1000W is installed on the outer wall of the mixing chamber of the internal mixer. Ultrasonic energy is introduced into the material through a coupling medium. The ultrasonic action time is 50% to 70% of the total mixing time. The cavitation effect is used to break up the flame retardant agglomerates, so that they can be dispersed at the nanoscale in the PVC matrix.

7. The method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride according to claim 1, characterized in that, It also includes a post-processing step for chain extension and toughening: After dynamic vulcanization, the composite material is passed through a high-temperature open mill 3 to 5 times, with the roller spacing adjusted to 0.5 mm to 1 mm and the roller temperature maintained at 155℃ to 165℃. Then, an epoxy end-group teleclaw polymer solution of 0.3% to 0.8% by weight of the composite material is sprayed, and the thin-pass plasticizing is continued for 5 to 10 minutes to allow the epoxy groups to undergo a grafting reaction with the HCl at the end of the PVC molecular chain, thereby achieving chain extension and fine adjustment of crosslinking density.

8. The method for preparing an environmentally friendly cable sheath material made from recycled polyvinyl chloride according to claim 1, characterized in that, It also includes the in-situ composite step of the electrospinning reinforcement layer: The prepared sheath material masterbatch was dissolved in a mixed solvent of tetrahydrofuran and N,N-dimethylacetamide to prepare an electrospinning precursor solution with a solid content of 15% to 20%. A coaxial electrospinning device was used, with the prepared masterbatch solution as the skin layer and pure polyvinylidene fluoride solution as the core layer. Spinning was carried out under the conditions of voltage of 18kV to 22kV, spinning distance of 15cm to 20cm, and flow rate of 0.8mL / h to 1.2mL / h. The obtained nanofiber membrane is directly applied to the die lip of the extrusion mold, so that the sheath melt is compounded in situ with the nanofiber membrane during extrusion, thereby forming a dense physical shielding barrier on the surface of the material.

9. An environmentally friendly cable sheath material made from recycled polyvinyl chloride, characterized in that, It is prepared using the method described in any one of claims 1 to 8 for an environmentally friendly cable sheath material made from recycled polyvinyl chloride.

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