Low-smoke halogen-free flame-retardant power cable material and preparation method thereof

Abstract

This invention relates to the field of cable material technology, specifically to low-smoke halogen-free flame-retardant power cable material and its preparation method, comprising the following raw materials: ethylene-vinyl acetate, low-phosphorus grafted polyvinylidene fluoride, metal hydroxide, microencapsulated ammonium polyphosphate, zinc stearate, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and polyolefin elastomer grafted with maleic anhydride. In this invention, the low-phosphorus grafted polyvinylidene fluoride introduces phosphate groups into the polymer backbone, enhancing the polarity and interfacial activity of the matrix. This allows it to form stable interfacial bonds with other components during processing and combustion, improving the dispersibility and synergistic effect of the flame-retardant components and reducing migration and precipitation. The microencapsulated ammonium polyphosphate effectively inhibits the moisture absorption and water sensitivity of the polyphosphate, improving processing thermal stability. During combustion, the microcapsule shell enhances heat and oxygen insulation capabilities, suppresses the generation of toxic fumes, and improves flame-retardant efficiency.

Description

Technical Field

[0001] This invention relates to the field of cable material technology, and more specifically, to low-smoke halogen-free flame-retardant power cable material and its preparation method. Background Technology

[0002] With the continuous improvement of electrical safety requirements in urban power grid transformation and fields such as rail transit and data centers, power cable materials are developing towards high flame retardancy, low smoke, halogen-free, and long-term thermal stability. Although traditional halogenated flame retardants (such as chlorine-containing and bromine-containing compounds) have excellent flame retardant efficiency, they release a large amount of corrosive hydrogen halide gas and toxic fumes during combustion, posing a threat to equipment and personnel safety. In order to achieve green and environmentally friendly flame retardancy, polyolefins, modified polyvinyl chloride, or fluoropolymers are gradually being used as matrix materials, and combined with inorganic flame retardant synergistic systems such as phosphorus, nitrogen, and silicon to develop low-smoke halogen-free flame retardant cable materials.

[0003] However, existing low-smoke halogen-free flame-retardant cable materials still have shortcomings in balancing flame-retardant performance and overall material properties: conventional polyvinylidene fluoride, due to its low polarity and limited interfacial bonding ability, is prone to delamination or interfacial debonding when compounded with phosphorus-based flame retardants. Its thermal stability and interfacial compatibility are poor, which leads to the easy precipitation or migration of flame retardants during combustion and insufficient flame-retardant durability. In view of this, we propose a low-smoke halogen-free flame-retardant power cable material and its preparation method. Summary of the Invention

[0004] The purpose of this invention is to provide low-smoke halogen-free flame-retardant power cable material and its preparation method, in order to solve the problems mentioned in the background art, such as the low polarity and limited interfacial bonding ability of conventional polyvinylidene fluoride, which is prone to delamination or interfacial debonding when compounded with phosphorus-based flame retardants, and its poor thermal stability and interfacial compatibility, which leads to the easy precipitation or migration of flame retardants during combustion and insufficient flame retardant durability.

[0005] This invention provides a low-smoke, halogen-free, flame-retardant power cable material, comprising the following raw materials: ethylene-vinyl acetate, low-phosphorus grafted polyvinylidene fluoride, metal hydroxide, microencapsulated ammonium polyphosphate, zinc stearate, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and polyolefin elastomer grafted with maleic anhydride.

[0006] The low-phosphorus grafted polyvinylidene fluoride is prepared by alkali-activated polyvinylidene fluoride followed by free radical graft polymerization with diethyl vinyl phosphate under the action of an initiator.

[0007] Microencapsulated ammonium polyphosphate is prepared by in-situ encapsulating ammonium polyphosphate with a silane coupling agent using a sol-gel method to form microcapsules.

[0008] Preferably, the composition includes 25-30 parts by weight of ethylene-vinyl acetate, 5-8 parts by weight of low-phosphorus grafted polyvinylidene fluoride, 42-45 parts by weight of metal hydroxide, 5-7 parts by weight of microencapsulated ammonium polyphosphate, 0.8-1.2 parts by weight of zinc stearate, 0.3-0.5 parts by weight of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 5-8 parts by weight of polyolefin elastomer grafted with maleic anhydride.

[0009] Preferably, the preparation method of the low-phosphorus grafted polyvinylidene fluoride is as follows:

[0010] Polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone at a mass ratio of 1:5-10 and stirred at 80-90℃ and 300-500 rpm for 2-3 hours. The mixture was then cooled to 40-50℃, and a 0.05-0.10 mol / L potassium hydroxide / ethanol solution was added dropwise, with stirring at 100-200 rpm for 0.5-1.0 hours. Subsequently, diethyl vinyl phosphate and azobisisobutyronitrile (AIBN) were added, and nitrogen gas was introduced at a flow rate of 0.3 L / min for 30 minutes. The mixture was then reacted under nitrogen protection at 70-75℃ with stirring at 500-600 rpm for 10-12 hours. After the reaction was complete, the reaction solution was cooled to room temperature and precipitated in 5-10 times its volume of deionized water. After standing for 2 hours, the solution was filtered, washed twice each with deionized water and ethanol, and then vacuum dried at 50-60℃ to constant weight to obtain low-phosphorus grafted PVDF.

[0011] Azobisisobutyronitrile (AIBN) should be stored at low temperature and protected from light and added in batches; N-methylpyrrolidone (N-Methylpyrrolidone) should be operated in a closed reactor equipped with a tail gas condensation and recovery system; all waste liquids should be neutralized before being discharged to meet emission requirements.

[0012] Preferably, the amount of diethyl vinyl phosphate added accounts for 8-12 mol of the polyvinylidene fluoride repeating unit.

[0013] Preferably, the amount of azobisisobutyronitrile added accounts for 2-5% of the molar amount of diethyl vinyl phosphate.

[0014] Preferably, the preparation method of the microencapsulated ammonium polyphosphate is as follows:

[0015] Ammonium polyphosphate is dispersed in deionized water, and polyvinyl alcohol is added at a mass of 1.5-2.0% of the ammonium polyphosphate. The mixture is stirred at 25-30℃ and 400-500 rpm for 10-30 minutes to obtain an ammonium polyphosphate suspension with a solid content of 15-20%.

[0016] Tetraethoxysilane and 3-methacryloxypropyltrimethoxysilane were mixed, and the pH was adjusted to 4.0-5.0 with 0.1 mol / L hydrochloric acid. The mixture was stirred at 200-300 rpm for 2-3 hours at 30-35℃ to obtain a pre-hydrolyzed solution.

[0017] The ammonium polyphosphate suspension was placed in a water bath at 40-50℃, and the pre-hydrolyzed solution was added dropwise over a period of 25-30 minutes while stirring at 300-400 rpm for 1-2 hours. The temperature was then raised to 65-70℃ and stirring was continued for 5-6 hours. After stirring was stopped, the mixture was allowed to settle for 30 minutes, and the microcapsules were collected by centrifugation at 3000-5000 rpm for 10-20 minutes. The microcapsules were washed three times each with deionized water and anhydrous ethanol, and then dried under vacuum at 50-60℃ to constant weight to obtain microencapsulated ammonium polyphosphate.

[0018] Preferably, the molar ratio of the tetraethoxysilane and 3-methacryloyloxypropyltrimethoxysilane is 7-10:1.

[0019] Preferably, the mass ratio of the pre-hydrolyzed solution to the ammonium polyphosphate suspension is 0.4-0.6:1.

[0020] On the other hand, the present invention provides a method for preparing low-smoke halogen-free flame-retardant power cable material, which includes the following steps:

[0021] S1.1 Weigh the raw materials according to their weight proportions;

[0022] S1.2, Ethylene-vinyl acetate, polyolefin elastomer grafted with maleic anhydride, and microencapsulated ammonium polyphosphate are vacuum dried at 80-90℃ for 4-6 hours; zinc stearate is passed through a 100-mesh sieve for later use.

[0023] S1.3. Add ethylene-vinyl acetate and polyolefin elastomer grafted with maleic anhydride to a high-speed mixer and mix at 500 rpm for 2 min. Then add low-phosphorus grafted polyvinylidene fluoride and pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and mix at 800 rpm for 3 min. Then add metal hydroxide and mix at 1000 rpm for 5 min. Finally, add microencapsulated ammonium polyphosphate and mix at 600 rpm for 3 min to obtain the premix.

[0024] S1.4 Transfer the premixed material into a twin-screw extruder, set the temperature gradient to 155-185℃ and the screw speed to 300-400 rpm, and melt-blend under a vacuum of -0.08 MPa for 8-10 minutes to obtain a melt; heat-cut the melt into granules using an air-cooled die, and then vacuum-dry it at 60-70℃ for 4-6 hours to obtain low-smoke halogen-free flame-retardant power cable material.

[0025] Preferably, in S1.3, the metal hydroxide is a mixture of aluminum hydroxide and magnesium hydroxide in a mass ratio of 3:1.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] In this invention, a low-smoke, halogen-free flame-retardant power cable material and its preparation method are described. Low-phosphorus grafted polyvinylidene fluoride (PVDF) incorporates phosphate groups into its polymer backbone, enhancing the polarity and interfacial activity of the matrix. This allows it to form stable interfacial bonds with metal hydroxides and phosphorus-based flame retardants during processing and combustion, improving the dispersibility and synergistic effect of the flame-retardant components and reducing migration and precipitation. Simultaneously, its fluorine-containing skeleton imparts excellent heat resistance and electrical insulation stability to the system. Microencapsulated ammonium polyphosphate is coated with a silica layer on its surface using a sol-gel method. This structure effectively improves the dispersion and bonding of the flame retardant with the polymer matrix, inhibits the moisture absorption and water sensitivity of ammonium polyphosphate, and enhances processing thermal stability. During combustion, the microcapsule shell synergistically enhances heat and oxygen insulation capabilities, further suppressing the generation of toxic fumes and improving flame-retardant efficiency. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] This invention provides a low-smoke, halogen-free, flame-retardant power cable material, comprising the following raw materials: ethylene-vinyl acetate, low-phosphorus grafted polyvinylidene fluoride, metal hydroxide, microencapsulated ammonium polyphosphate, zinc stearate, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and polyolefin elastomer grafted with maleic anhydride.

[0030] The low-phosphorus grafted polyvinylidene fluoride is prepared by alkali-activated polyvinylidene fluoride followed by free radical graft polymerization with diethyl vinyl phosphate under the action of an initiator.

[0031] Microencapsulated ammonium polyphosphate is prepared by in-situ encapsulating ammonium polyphosphate with a silane coupling agent using a sol-gel method to form microcapsules.

[0032] The ethylene-vinyl acetate was purchased from Hubei Kewode Chemical Co., Ltd.

[0033] Polyvinylidene fluoride (CAS No.: 24937-79-9, purity 98.5%) was purchased from Hubei Chushengwei Chemical Co., Ltd.

[0034] Diethyl vinyl phosphate (CAS No.: 682-30-4, purity ≥98%) was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd.

[0035] Azobisisobutyronitrile (CAS No.: 78-67-1, purity 99%) was purchased from Shandong Yukang Chemical Co., Ltd.

[0036] Aluminum hydroxide (CAS No.: 21645-51-2, purity AR), magnesium hydroxide (CAS No.: 1309-42-8, purity CP, 98%), ammonium polyphosphate (CAS No.: 68333-79-9), 3-methacryloyloxypropyltrimethoxysilane (CAS No.: 2530-85-0, purity: BR, 98%), zinc stearate (CAS No.: 557-05-1, purity CP), and pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (CAS No.: 6683-19-8, purity 98%) were all purchased from Shanghai Yuanye Biotechnology Co., Ltd.

[0037] Tetraethoxysilane (CAS No.: 78-10-4, purity 99%) was purchased from Condis Chemical (Hubei) Co., Ltd.

[0038] The polyolefin elastomer grafted with maleic anhydride was purchased from Dongguan Bailing New Materials Co., Ltd.

[0039] The metal hydroxide is composed of aluminum hydroxide and magnesium hydroxide in a mass ratio of 3:1.

[0040] Example 1: A method for preparing low-smoke halogen-free flame-retardant power cable material, comprising the following steps:

[0041] S1.1 Weigh the following raw materials by weight: 25 parts ethylene-vinyl acetate, 5 parts low-phosphorus grafted polyvinylidene fluoride, 42 parts metal hydroxide, 5 parts microencapsulated ammonium polyphosphate, 0.8 parts zinc stearate, 0.3 parts pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 5 parts polyolefin elastomer grafted maleic anhydride.

[0042] S1.2, Ethylene-vinyl acetate, polyolefin elastomer grafted with maleic anhydride, and microencapsulated ammonium polyphosphate were vacuum dried at 80°C for 4 hours; zinc stearate was passed through a 100-mesh sieve for later use.

[0043] S1.3. Add ethylene-vinyl acetate and polyolefin elastomer grafted with maleic anhydride to a high-speed mixer and mix at 500 rpm for 2 min. Then add low-phosphorus grafted polyvinylidene fluoride and pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and mix at 800 rpm for 3 min. Then add metal hydroxide and mix at 1000 rpm for 5 min. Finally, add microencapsulated ammonium polyphosphate and mix at 600 rpm for 3 min to obtain the premix.

[0044] S1.4. The premixed material is transferred to a twin-screw extruder, the temperature gradient is set to 155℃, the screw speed is 300rpm, and the material is melt-blended for 8min under a vacuum of -0.08MPa to obtain a melt. The melt is then hot-cut into granules by air-cooled die surface, and then vacuum-dried at 60℃ for 4h to obtain low-smoke halogen-free flame-retardant power cable material.

[0045] The preparation method of low-phosphorus grafted polyvinylidene fluoride is as follows:

[0046] Polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone at a mass ratio of 1:5 and stirred at 80°C and 300 rpm for 2 h. The mixture was then cooled to 40°C, and a 0.05 mol / L potassium hydroxide / ethanol solution was added dropwise while stirring at 100 rpm for 0.5 h. Subsequently, diethyl vinyl phosphate (10 mol% of the PVDF repeating unit) and azobisisobutyronitrile (3% of the molar amount of diethyl vinyl phosphate) were added, and nitrogen gas was introduced at a flow rate of 0.3 L / min for 30 min. The mixture was then stirred at 500 rpm for 10 h under nitrogen protection at 70°C. After the reaction was completed, the reaction solution was cooled to room temperature, poured into 5 times its volume of deionized water for precipitation, allowed to stand for 2 h, filtered, and washed twice each with deionized water and ethanol. The solution was then vacuum dried at 50°C to constant weight to obtain low-phosphorus grafted PVDF.

[0047] The preparation method of microencapsulated ammonium polyphosphate is as follows:

[0048] Ammonium polyphosphate was dispersed in deionized water, and polyvinyl alcohol accounting for 1.5% of the mass of ammonium polyphosphate was added. The mixture was stirred at 25°C and 400 rpm for 10 min to obtain an ammonium polyphosphate suspension with a solid content of 15%.

[0049] Tetraethoxysilane and 3-methacryloxypropyltrimethoxysilane were mixed in a molar ratio of 7:1, and the pH was adjusted to 4.0 with 0.1 mol / L hydrochloric acid. The mixture was stirred at 200 rpm for 2-3 hours at 30 °C to obtain a pre-hydrolyzed solution.

[0050] The ammonium polyphosphate suspension was placed in a 40°C water bath, and a pre-hydrolyzed solution (mass ratio of 0.4:1 to the ammonium polyphosphate suspension) was added dropwise over 25 min, while stirring at 300 rpm for 1 h. The temperature was then raised to 65°C, and stirring was continued for 5 h. After stirring was stopped, the mixture was allowed to settle for 30 min, and the microcapsules were collected by centrifugation at 3000 rpm for 10 min. The microcapsules were washed three times each with deionized water and anhydrous ethanol, and then dried under vacuum at 50°C to constant weight to obtain microencapsulated ammonium polyphosphate.

[0051] Example 2: The difference between this example and Example 1 is that the amount of diethyl vinyl phosphate added accounts for 8 mol of the polyvinylidene fluoride repeating unit.

[0052] Example 3: The difference between this example and Example 1 is that the amount of diethyl vinyl phosphate added accounts for 12 mol of the polyvinylidene fluoride repeating unit.

[0053] Example 4: The difference between this example and Example 1 is that the amount of azobisisobutyronitrile added is 2% of the molar amount of diethyl vinyl phosphate.

[0054] Example 5: The difference between this example and Example 1 is that the amount of azobisisobutyronitrile added is 5% of the molar amount of diethyl vinyl phosphate.

[0055] Determination of phosphorus grafting rate: The purified low-phosphorus grafted polyvinylidene fluoride sample is thoroughly dried and ground into powder; using an elemental analyzer, a small amount of sample (about 2-5 mg) is accurately weighed and placed into a tin cup for testing; the instrument will directly measure the mass percentage content of phosphorus (P) in the sample; by comparing with the phosphorus content of the ungrafted original PVDF (theoretically 0), and in combination with the molecular formula of the vinyl diethyl phosphate monomer, the phosphorus grafting rate (i.e., the number or molar percentage of phosphorus groups grafted on every 100 PVDF repeating units) can be calculated.

[0056] Determination of thermal stability: Take 5-10 mg of dried sample powder and heat it from room temperature to 800℃ in a thermogravimetric analyzer under two atmospheres: nitrogen (simulating inert heat treatment environment) and air (simulating combustion conditions) at a constant heating rate (e.g., 10℃ / min); the initial decomposition temperature is usually the temperature at which the mass loss is 5% (Td5%); the char rate is the mass percentage of the remaining material at 700℃; the higher the char rate, the better the char formation, which is beneficial to the flame retardancy of the condensed phase.

[0057] Interfacial compatibility / polarity determination: The polymer sample was hot-pressed into a smooth and clean film; using a contact angle meter, a fixed volume (e.g., 2 μL) of ultrapure water and diiodomethane were dropped onto the film surface at room temperature, and the droplet image was captured by a camera; the software automatically fitted the droplet profile and calculated its contact angle (θ) with the solid surface; the decrease in the water contact angle indicates that the surface polarity of the material is enhanced and the hydrophilicity is increased, which is strong evidence that the polyvinylidene fluoride has been successfully grafted with polar phosphorus-containing groups.

[0058] Table 1 Performance data of low-phosphorus grafted polyvinylidene fluoride

[0059] Phosphorus grafting rate Thermal stability (Td5%) 700℃ residual carbon rate Interfacial compatibility (water contact angle) Example 1 7.8% 415℃ 24.5% 78° Example 2 6.2% 408℃ 22.0% 82° Example 3 9.1% 418℃ 26.8% 74° Example 4 7.1% 412℃ 23.5% 80° Example 5 8.0% 414℃ 24.8% 77°

[0060] As shown in Table 1, when the amount of diethyl vinyl phosphate added increased from 8 mol% in Example 2 to 12 mol% in Example 3, the grafting rate increased significantly from 6.2% to 9.1%; this indicates that increasing the monomer input can effectively improve the grafting level.

[0061] In Example 4, the amount of azobisisobutyronitrile was insufficient (2%), resulting in insufficient free radical concentration and incomplete grafting reaction, with a grafting rate (7.1%) lower than that in Example 1 (7.8%). In contrast, in Example 5, the amount of azobisisobutyronitrile was excessive (5%), which generated more free radicals and improved the grafting efficiency to some extent, resulting in a grafting rate (8.0%) slightly higher than that in Example 1.

[0062] Example 3 has the highest grafting rate, and therefore its initial decomposition temperature (Td5% is 418℃) and char residue (26.8%) are the highest, exhibiting the best thermal stability and char-forming ability.

[0063] Conversely, Example 2, with the lowest grafting rate, also had the lowest thermal stability and char residue; this indicates that the phosphorus grafting rate is positively correlated with the material's thermal stability and flame-retardant char formation.

[0064] Original polyvinylidene fluoride has relatively weak polarity and a large water contact angle (usually >90°); after grafting polar phosphorus-containing groups, the surface polarity of the material is enhanced, resulting in a decrease in the water contact angle.

[0065] Example 3 has the highest grafting rate and the most polar groups introduced, so it has the smallest water contact angle (74°), indicating that it has the best compatibility with polar flame retardants (such as microencapsulated ammonium polyphosphate) and other polymer matrices, and can effectively improve the problem of interfacial debonding.

[0066] Example 2 has a low grafting rate, limited improvement in polarity, a large contact angle (82°), and a relatively weak effect on improving compatibility.

[0067] Example 6: A method for preparing low-smoke halogen-free flame-retardant power cable material, comprising the following steps:

[0068] S1.1 Weigh the following raw materials by weight: 30 parts by weight of ethylene-vinyl acetate, 8 parts by weight of low-phosphorus grafted polyvinylidene fluoride, 45 parts by weight of metal hydroxide, 7 parts by weight of microencapsulated ammonium polyphosphate, 1.2 parts by weight of zinc stearate, 0.5 parts by weight of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 8 parts by weight of polyolefin elastomer grafted maleic anhydride.

[0069] S1.2, Ethylene-vinyl acetate, polyolefin elastomer grafted with maleic anhydride, and microencapsulated ammonium polyphosphate were vacuum dried at 90℃ for 6 hours; zinc stearate was passed through a 100-mesh sieve for later use.

[0070] S1.3. Add ethylene-vinyl acetate and polyolefin elastomer grafted with maleic anhydride to a high-speed mixer and mix at 500 rpm for 2 min. Then add low-phosphorus grafted polyvinylidene fluoride and pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and mix at 800 rpm for 3 min. Then add metal hydroxide and mix at 1000 rpm for 5 min. Finally, add microencapsulated ammonium polyphosphate and mix at 600 rpm for 3 min to obtain the premix.

[0071] S1.4. The premixed material is transferred to a twin-screw extruder, the temperature gradient is set to 185℃, the screw speed is 400rpm, and the material is melt-blended for 10min under a vacuum of -0.08MPa to obtain a melt. The melt is then hot-cut into granules by air-cooled die surface, and then vacuum-dried at 70℃ for 6h to obtain low-smoke halogen-free flame-retardant power cable material.

[0072] The preparation method of low-phosphorus grafted polyvinylidene fluoride is as follows:

[0073] Polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone at a mass ratio of 1:10 and stirred at 90°C and 500 rpm for 3 h. The mixture was then cooled to 50°C, and a 0.10 mol / L potassium hydroxide / ethanol solution was added dropwise while stirring at 200 rpm for 1 h. Subsequently, diethyl vinyl phosphate (12 mol% of the PVDF repeating unit) and azobisisobutyronitrile (3% of the molar amount of diethyl vinyl phosphate) were added, and nitrogen gas was introduced at a flow rate of 0.3 L / min for 30 min. The mixture was then stirred at 75°C and 600 rpm for 12 h under nitrogen protection. After the reaction was completed, the reaction solution was cooled to room temperature, poured into 10 times its volume of deionized water for precipitation, allowed to stand for 2 h, filtered, washed twice each with deionized water and ethanol, and dried under vacuum at 60°C to constant weight to obtain low-phosphorus grafted PVDF.

[0074] The preparation method of microencapsulated ammonium polyphosphate is as follows:

[0075] Ammonium polyphosphate was dispersed in deionized water, and polyvinyl alcohol accounting for 2.0% of the mass of ammonium polyphosphate was added. The mixture was stirred at 30°C and 500 rpm for 30 min to obtain an ammonium polyphosphate suspension with a solid content of 20%.

[0076] Tetraethoxysilane and 3-methacryloxypropyltrimethoxysilane were mixed in a molar ratio of 8:1, and the pH was adjusted to 5.0 with 0.1 mol / L hydrochloric acid. The mixture was stirred at 300 rpm for 3 h at 35 °C to obtain a pre-hydrolyzed solution.

[0077] The ammonium polyphosphate suspension was placed in a 50°C water bath, and a pre-hydrolyzed solution (mass ratio of 0.5:1 to the ammonium polyphosphate suspension) was added dropwise over 30 min, while stirring at 400 rpm for 2 h. The temperature was then raised to 70°C, and stirring was continued for 6 h. After stirring was stopped, the mixture was allowed to settle for 30 min, and the microcapsules were collected by centrifugation at 5000 rpm for 20 min. The microcapsules were washed three times each with deionized water and anhydrous ethanol, and then dried under vacuum at 60°C to constant weight to obtain microencapsulated ammonium polyphosphate.

[0078] Example 7: The difference between this example and Example 6 is that the molar ratio of tetraethoxysilane and 3-methacryloyloxypropyltrimethoxysilane is 7:1.

[0079] Example 8: The difference between this example and Example 6 is that the molar ratio of tetraethoxysilane and 3-methacryloyloxypropyltrimethoxysilane is 10:1.

[0080] Example 9: The difference between this example and Example 6 is that the mass ratio of the pre-hydrolyzed solution to the ammonium polyphosphate suspension is 0.4:1.

[0081] Example 10: The difference between this example and Example 6 is that the mass ratio of the pre-hydrolyzed solution to the ammonium polyphosphate suspension is 0.6:1.

[0082] Determination of moisture resistance (hygroscopic rate): Spread approximately 2g (denoted as m0) of microencapsulated ammonium polyphosphate powder evenly in a weighing dish; place the sample in a constant temperature and humidity chamber, typically set to a temperature of 25±2℃ and a relative humidity of 90±5%; remove the sample every 24 hours and weigh it quickly using an analytical balance (denoted as m). t ), until the weight change becomes gradual (usually lasting 5-7 days); moisture absorption rate (%) = [(m t -m0) / m0]×100%; the better the coating effect, the lower the moisture absorption rate.

[0083] Table 2 Performance data of microencapsulated ammonium polyphosphate

[0084] Moisture resistance (hygroscopicity) Thermal stability (Td5%) 700℃ residual carbon rate Example 6 2.5% 305℃ 32.0% Example 7 2.8% 302℃ 30.5% Example 8 2.9% 308℃ 33.0% Example 9 3.5% 298℃ 29.0% Example 10 2.0% 310℃ 34.5%

[0085] Comparing Examples 7, 6, and 8, the thermal stability (Td5% and char residue) of the shell gradually improved with the increase of the tetraethoxysilane ratio; this is because the inorganic siloxane network structure can provide a better thermal barrier effect.

[0086] In terms of moisture resistance and interfacial compatibility, Example 6 (8:1) achieved a balance; an excessively high inorganic ratio (such as in Example 8) would lead to increased shell brittleness, affecting the integrity of the coating and the dispersion effect in the polymer.

[0087] Comparing Examples 9, 6, and 10, an increase in the proportion of pre-hydrolyzed solution means a thicker coating layer.

[0088] Moisture resistance and thermal stability are thus significantly improved; in particular, in Example 10, the thickest coating provides the best physical barrier, resulting in the lowest moisture absorption and the strongest resistance to thermal decomposition.

[0089] It should be noted that an excessively thick coating layer (such as in Example 10) will dilute the effective flame-retardant components of ammonium polyphosphate to some extent; while in Example 9, where the coating is insufficient, all properties will be reduced due to the incomplete shell.

[0090] Example 11: A method for preparing low-smoke halogen-free flame-retardant power cable material, comprising the following steps:

[0091] S1.1 Weigh the following raw materials by weight: 28 parts by weight of ethylene-vinyl acetate, 6 parts by weight of low-phosphorus grafted polyvinylidene fluoride, 43 parts by weight of metal hydroxide, 6 parts by weight of microencapsulated ammonium polyphosphate, 1.0 part by weight of zinc stearate, 0.4 parts by weight of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 6 parts by weight of polyolefin elastomer grafted with maleic anhydride.

[0092] S1.2, Ethylene-vinyl acetate, polyolefin elastomer grafted with maleic anhydride, and microencapsulated ammonium polyphosphate were vacuum dried at 85°C for 5 hours; zinc stearate was passed through a 100-mesh sieve for later use.

[0093] S1.3. Add ethylene-vinyl acetate and polyolefin elastomer grafted with maleic anhydride to a high-speed mixer and mix at 500 rpm for 2 min. Then add low-phosphorus grafted polyvinylidene fluoride and pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and mix at 800 rpm for 3 min. Then add metal hydroxide and mix at 1000 rpm for 5 min. Finally, add microencapsulated ammonium polyphosphate and mix at 600 rpm for 3 min to obtain the premix.

[0094] S1.4. Transfer the premixed material into a twin-screw extruder, set the temperature gradient to 180℃ and the screw speed to 350rpm, and melt-blend for 10min under a vacuum of -0.08MPa to obtain a melt. The melt is then hot-cut into granules by air-cooled die surface, and subsequently vacuum-dried at 65℃ for 5h to obtain low-smoke halogen-free flame-retardant power cable material.

[0095] The preparation method of low-phosphorus grafted polyvinylidene fluoride is as follows:

[0096] Polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone at a mass ratio of 1:7 and stirred at 85°C and 400 rpm for 3 h. The mixture was then cooled to 45°C, and a 0.08 mol / L potassium hydroxide / ethanol solution was added dropwise while stirring at 150 rpm for 0.8 h. Subsequently, diethyl vinyl phosphate (12 mol% of the PVDF repeating unit) and azobisisobutyronitrile (3% of the molar amount of diethyl vinyl phosphate) were added, and nitrogen gas was introduced at a flow rate of 0.3 L / min for 30 min. The mixture was then stirred at 550 rpm for 11 h under nitrogen protection at 72°C. After the reaction was completed, the reaction solution was cooled to room temperature, poured into 8 times its volume of deionized water for precipitation, allowed to stand for 2 h, filtered, washed twice each with deionized water and ethanol, and dried under vacuum at 55°C to constant weight to obtain low-phosphorus grafted PVDF.

[0097] The preparation method of microencapsulated ammonium polyphosphate is as follows:

[0098] Ammonium polyphosphate was dispersed in deionized water, and polyvinyl alcohol accounting for 1.6% of the mass of ammonium polyphosphate was added. The mixture was stirred at 28°C and 450 rpm for 20 min to obtain an ammonium polyphosphate suspension with a solid content of 16%.

[0099] Tetraethoxysilane and 3-methacryloxypropyltrimethoxysilane were mixed at a molar ratio of 8:1, and the pH was adjusted to 4.5 with 0.1 mol / L hydrochloric acid. The mixture was stirred at 250 rpm for 3 h at 33 °C to obtain a pre-hydrolyzed solution.

[0100] The ammonium polyphosphate suspension was placed in a 45°C water bath, and a pre-hydrolyzed solution (mass ratio of 0.5:1 to the ammonium polyphosphate suspension) was added dropwise over 30 min, while stirring at 350 rpm for 2 h. The temperature was then raised to 70°C, and stirring was continued for 6 h. After stirring was stopped, the mixture was allowed to settle for 30 min, and the microcapsules were collected by centrifugation at 4000 rpm for 15 min. The microcapsules were washed three times each with deionized water and anhydrous ethanol, and then dried under vacuum at 55°C to constant weight to obtain microencapsulated ammonium polyphosphate.

[0101] Example 12: The difference between this example and Example 11 is that 5 parts by weight of low-phosphorus grafted polyvinylidene fluoride are used.

[0102] Example 13: The difference between this example and Example 11 is that 8 parts by weight of low-phosphorus grafted polyvinylidene fluoride are used.

[0103] Example 14: The difference between this example and Example 11 is that 5 parts by weight of microencapsulated ammonium polyphosphate are used.

[0104] Example 15: The difference between this example and Example 11 is that 7 parts by weight of microencapsulated ammonium polyphosphate are used.

[0105] Flame retardant performance (oxygen index method): The sample is vertically fixed in the combustion chamber of the oxygen index tester, and the flow rates of oxygen and nitrogen are adjusted to form a specific oxygen concentration airflow; the top of the sample is ignited with an igniter; by observing the burning time or length of the sample, the critical oxygen concentration is determined by the rise-fall method with a small number of samples.

[0106] Smoke density determination: The sample is placed in the sample rack in the smoke density test chamber and burned under a specified irradiance (sometimes with the aid of an ignition flame); the light beam passes through the smoke produced by the combustion, and the sensor measures the attenuation of the light flux; the system records and calculates the smoke density (SDR).

[0107] Determination of mechanical properties (tensile strength and elongation at break): Dumbbell-shaped specimens are prepared according to standards, clamped in the grips of the testing machine, and stretched at a constant speed; the equipment automatically records the stress-strain curve and calculates the tensile strength (maximum tensile force divided by cross-sectional area) and elongation at break (percentage of elongation at break relative to the original gauge length).

[0108] Table 3 Performance data of low-smoke halogen-free flame-retardant power cable materials

[0109] Oxygen Index Smoke density Tensile strength Elongation at break Example 11 33.5% 65 14.2MPa 220% Example 12 32.0% 72 13.5MPa 240% Example 13 34.2% 58 14.8MPa 195% Example 14 32.2% 75 14.8MPa 235% Example 15 34.8% 55 13.6MPa 200%

[0110] Comparative examples 11, 12, and 13 show that when the amount of low-phosphorus grafted polyvinylidene fluoride increases from 5 parts to 8 parts, its efficient char-forming effect is enhanced, the oxygen index (LOI) increases from 32.0% to 34.2%, and the smoke density (SDR) also decreases significantly from 72 to 58.

[0111] As a polar modified polymer, the increased use of low-phosphorus grafted polyvinylidene fluoride improves compatibility and rigidity, but also restricts polymer chain slippage to some extent, resulting in a decrease in elongation at break (from 240% to 195%), while the tensile strength is slightly improved due to improved interfacial bonding.

[0112] Comparing Examples 11, 14, and 15, it can be seen that when the amount of microencapsulated ammonium polyphosphate increased from 5 parts to 7 parts, the oxygen index (LOI) increased significantly from 32.2% to 34.8%; at the same time, the barrier formed by its silica coating layer in the condensed phase also effectively suppressed smoke generation, reducing the smoke density (SDR) from 75 to 55.

[0113] Microencapsulated ammonium polyphosphate, as an inorganic filler, will have a negative impact on mechanical properties if its dosage is increased; the more it is used, the more stress concentration points are generated in the matrix, resulting in a decrease in tensile strength and elongation at break (Example 15).

[0114] Based on the above measurements, Example 13 is selected as the optimal example.

[0115] Comparative Example 1: The difference between this example and Example 13 is that low-phosphorus grafted polyvinylidene fluoride was not added, and polyvinylidene fluoride was used directly.

[0116] Comparative Example 2: The difference between this example and Example 13 is that microencapsulated ammonium polyphosphate was not added, and ammonium polyphosphate was used directly.

[0117] Comparative Example 3: The difference between this example and Example 13 is that low-phosphorus grafted polyvinylidene fluoride and microencapsulated ammonium polyphosphate were not added.

[0118] Table 4 Performance data of low-smoke halogen-free flame-retardant power cable materials

[0119] Oxygen Index Smoke density Tensile strength Elongation at break Example 13 34.2% 58 14.8MPa 195% Comparative Example 1 31.5% 75 12.0MPa 160% Comparative Example 2 30.8% 85 13.0MPa 170% Comparative Example 3 28.5% 105 10.5MPa 135%

[0120] When unmodified polyvinylidene fluoride (PVDF) was used to directly replace low-phosphorus grafted PVDF (Comparative Example 1), its oxygen index decreased from 34.2% to 31.5%, which demonstrates the key role of low-phosphorus graft modification in improving flame retardant efficiency. Unmodified PVDF has poor interfacial compatibility with the matrix and flame retardant, and cannot effectively form a stable char layer, resulting in flame retardant migration and a decrease in flame retardant effect. At the same time, poor compatibility also leads to a significant impairment of its mechanical properties (tensile strength and elongation at break), and the smoke density increases due to incomplete combustion.

[0121] Comparative Example 2 data shows the consequences of using ammonium polyphosphate. Due to the poor thermal stability and easy moisture absorption during processing of ordinary ammonium polyphosphate, its flame retardant efficiency is greatly reduced, with an oxygen index of only 30.8%. Furthermore, the premature decomposition of ammonium polyphosphate leads to the formation of molten droplets in the system, and the incomplete decomposition during combustion produces a large amount of smoke, resulting in a smoke density as high as 85. In addition, the bonding ability of ordinary ammonium polyphosphate with the matrix is ​​weaker than that of microencapsulated products, which also has a negative impact on mechanical properties.

[0122] Comparative Example 3, without the addition of low-phosphorus grafted polyvinylidene fluoride and microencapsulated ammonium polyphosphate, showed a precipitous drop in performance data, with an oxygen index of only 28.5% and a huge amount of smoke. This fully demonstrates that there is a lack of synergistic effect between unmodified polyvinylidene fluoride and ordinary ammonium polyphosphate, and there is even a performance conflict, resulting in severe interfacial debonding, failure of the flame retardant system, and mechanical properties reduced to the lowest point.

[0123] In summary, low-phosphorus grafted polyvinylidene fluoride is key to improving flame retardancy and mechanical strength by enhancing interfacial compatibility; while microencapsulated ammonium polyphosphate is the core component for ensuring processing stability and achieving high-efficiency flame retardancy and low-smoke properties.

[0124] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. Low-smoke halogen-free flame-retardant power cable material, characterized in that, The raw materials include: ethylene-vinyl acetate, low-phosphorus grafted polyvinylidene fluoride, metal hydroxide, microencapsulated ammonium polyphosphate, zinc stearate, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and maleic anhydride grafted onto a polyolefin elastomer; the low-phosphorus grafted polyvinylidene fluoride is prepared by alkali-activated polyvinylidene fluoride followed by free radical graft polymerization with diethyl vinyl phosphate under the action of an initiator; the preparation method of the microencapsulated ammonium polyphosphate is as follows: ammonium polyphosphate is dispersed in deionized water, and 1.5-2.0% (by mass) of polyvinyl alcohol is added, and the mixture is stirred at 25-30°C and 400-500 rpm for 10-30 min to obtain an ammonium polyphosphate suspension with a solid content of 15-20%; the zinc stearate is then used to prepare the microencapsulated ammonium polyphosphate elastomer. Ethoxysilane and 3-methacryloxypropyltrimethoxysilane were mixed, and the pH was adjusted to 4.0-5.0 with 0.1 mol / L hydrochloric acid. The mixture was stirred at 200-300 rpm for 2-3 hours at 30-35℃ to obtain a pre-hydrolyzed solution. The ammonium polyphosphate suspension was placed in a water bath at 40-50℃, and the pre-hydrolyzed solution was added dropwise over 25-30 minutes while stirring at 3000-400 rpm for 1-2 hours. The temperature was raised to 65-70℃, and stirring was continued for 5-6 hours. After stirring was stopped, the mixture was allowed to settle for 30 minutes. The microcapsules were collected by centrifugation at 3000-5000 rpm for 10-20 minutes. The microcapsules were washed three times each with deionized water and anhydrous ethanol, and then dried under vacuum at 50-60℃ to constant weight to obtain microencapsulated ammonium polyphosphate. The composition comprises 25-30 parts by weight of ethylene-vinyl acetate, 5-8 parts by weight of low-phosphorus grafted polyvinylidene fluoride, 42-45 parts by weight of metal hydroxide, 5-7 parts by weight of microencapsulated ammonium polyphosphate, 0.8-1.2 parts by weight of zinc stearate, 0.3-0.5 parts by weight of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 5-8 parts by weight of polyolefin elastomer grafted with maleic anhydride; the molar ratio of tetraethoxysilane and 3-methacryloyloxypropyltrimethoxysilane is 7-10:1; and the mass ratio of the pre-hydrolyzed solution to the ammonium polyphosphate suspension is 0.4-0.6:

1.

2. The low-smoke halogen-free flame-retardant power cable material according to claim 1, characterized in that, The preparation method of the low-phosphorus grafted polyvinylidene fluoride is as follows: Polyvinylidene fluoride is dissolved in N-methylpyrrolidone at a mass ratio of 1:5-10, and stirred at 80-90℃ and 300-500rpm for 2-3h; the temperature is lowered to 40-50℃, and 0.05-0.10mol / L potassium hydroxide / ethanol solution is added dropwise, and stirred at 100-200rpm for 0.5-1.0h; then diethyl vinyl phosphate and azobisisobutyronitrile are added, and nitrogen gas is introduced at a flow rate of 0.3L / min for 30min; then the reaction is carried out under nitrogen protection and stirred at 70-75℃ at 500-600rpm for 10-12h; after the reaction is completed, the reaction solution is cooled to room temperature, poured into 5-10 times the volume of deionized water for precipitation, allowed to stand for 2h, filtered, washed twice with deionized water and ethanol, and dried under vacuum at 50-60℃ to constant weight to obtain low-phosphorus grafted polyvinylidene fluoride.

3. The low-smoke halogen-free flame-retardant power cable material according to claim 2, characterized in that, The amount of diethyl vinyl phosphate added accounts for 8-12 mol of the polyvinylidene fluoride repeating unit.

4. The low-smoke halogen-free flame-retardant power cable material according to claim 2, characterized in that, The amount of azobisisobutyronitrile added is 2-5% of the molar amount of diethyl vinyl phosphate.

5. A method for preparing low-smoke halogen-free flame-retardant power cable material, used to prepare the low-smoke halogen-free flame-retardant power cable material as described in any one of claims 1-4, characterized in that, The method is as follows: S1.1 Weigh the raw materials according to the weight parts; S1.2 Dry ethylene-vinyl acetate, polyolefin elastomer grafted with maleic anhydride, and microencapsulated ammonium polyphosphate under vacuum at 80-90℃ for 4-6 hours; pass zinc stearate through a 100-mesh sieve for later use; S1.3 Add ethylene-vinyl acetate and polyolefin elastomer grafted with maleic anhydride to a high-speed mixer and initially mix at 500 rpm for 2 minutes; then add low-phosphorus grafted polyvinylidene fluoride and pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and increase the speed to 800 rpm for mixing. Add metal hydroxide and mix at 1000 rpm for 5 minutes. Finally, add microencapsulated ammonium polyphosphate and mix at 600 rpm for 3 minutes to obtain a premix. S1.4 Transfer the premix to a twin-screw extruder, set the temperature gradient to 155-185℃, the screw speed to 300-400 rpm, and melt-blend under a vacuum of -0.08MPa for 8-10 minutes to obtain a melt. The melt is then hot-cut into granules by air-cooled die surface and subsequently vacuum-dried at 60-70℃ for 4-6 hours to obtain a low-smoke halogen-free flame-retardant power cable material.

6. The method for preparing low-smoke halogen-free flame-retardant power cable material according to claim 5, characterized in that, In S1.3, the metal hydroxide is formed by mixing aluminum hydroxide and magnesium hydroxide in a mass ratio of 3:1.