Preparation method of composite fiber with multi-flame-retardant structure for cable filling

Abstract

The invention discloses a preparation method of a composite fiber with a multi-flame-retardant structure for cable filling, and belongs to the field of high polymer material and flame-retardant cable manufacturing. The fiber comprises a continuous phase formed by polyacrylonitrile and phosphate modified polyester, and a core-shell nitrogen-phosphorus flame-retardant particle dispersed phase with the particle size of 200-400 nm. The preparation method comprises the following steps: blending polyacrylonitrile and modified polyester at 210-230 DEG C at 150 rpm for 20-30 minutes, adding amino and phosphonic acid flame-retardant particles (in a mass ratio of 2-8), carrying out ultrasonic treatment at 60 DEG C for 40 minutes, and carrying out blending extrusion to obtain core layer granules; carrying out electrostatic spinning on the core layer precursor liquid and the sheath layer precursor liquid in an electric field of 15-18kV / cm at a spinning distance of 12-14cm to obtain core-sheath composite fibers; and finally, carrying out heat treatment in an ethanol solution containing an amino silane crosslinking agent at 95-110 DEG C for 20-30 minutes, so that the sheath layer is crosslinked, washed and dried, and the composite fiber with stable structure and high flame retardance is obtained. The method has the beneficial effect that the thermal stability and flame-retardant synergistic performance of the composite fiber are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high polymer materials and flame-retardant cable manufacturing, and more particularly to a preparation method of a composite fiber with multiple flame-retardant structures for cable filling. BACKGROUND

[0002] The cable filling material technology has evolved from natural fibers to synthetic fibers, especially in terms of flame-retardant performance. Early single-component filling fibers with polyester, polypropylene, etc. as the matrix have certain mechanical properties, but they are prone to melting and collapse in high-temperature environments, making it difficult to meet modern high-flame-retardant grade requirements. Subsequently, researchers introduced flame retardants, such as antimony trioxide, phosphorus-based or nitrogen-based flame-retardant particles, to improve the limiting oxygen index and carbonization residual rate of the fibers, and developed blending modification and microencapsulation technologies to improve particle dispersion and smoke suppression performance. In addition, composite technologies such as core-sheath structure and multi-component spinning have been continuously developed, enabling the fiber to have layered synergistic functions, becoming an important direction for high-performance cable flame-retardant filling materials.

[0003] Despite the progress made in the prior art, there are still many deficiencies: flame-retardant particles tend to agglomerate, leading to unstable local structure of the fiber; the interface between the core and sheath layers is weak, and peeling or delamination is prone to occur during use; there is a lack of stable cross-linking and curing mechanism, and the sheath layer still has the risk of melting and loss under high-temperature service; and some process parameters are difficult to control, making it difficult to ensure the consistency and industrial stability of the product. Therefore, there is an urgent need for a composite fiber preparation method with a clear structure, high flame-retardant efficiency, strong interface stability, and excellent form retention ability to meet the long-term service requirements of high-grade flame-retardant cables. SUMMARY

[0004] The present application aims to provide a preparation method of a composite fiber with multiple flame-retardant structures for cable filling to solve the problems raised in the background art: flame-retardant particles tend to agglomerate, leading to unstable local structure of the fiber; the interface between the core and sheath layers is weak, and peeling or delamination is prone to occur during use; there is a lack of stable cross-linking and curing mechanism, and the sheath layer still has the risk of melting and loss under high-temperature service; and some process parameters are difficult to control, making it difficult to ensure the consistency and industrial stability of the product.

[0005] Technical solution: A composite fiber with multiple flame-retardant structures for cable filling includes a continuous phase and a dispersed phase, the continuous phase is formed by blending polyacrylonitrile and phosphate-modified polyester, and the dispersed phase is composed of nitrogen-phosphorus synergistic flame-retardant particles with a particle size of 200-400 nm and a core-shell structure.

[0006] The cable filling composite fiber with multiple flame-retardant structure is a core-sheath structure, the core layer is a flame-retardant blended material formed by melt blending, extrusion and granulation, the sheath layer is a high-phosphorus-content low-molecular-weight polymer synthesized from diphenyl phosphate and pentaerythritol, and a crosslinked solid network is constructed in the outer layer, an intermediate synergistic structure layer with a thickness of 0.5-2 μm is formed between the core layer and the sheath layer through interface interpenetration, and the intermediate synergistic structure layer contains a maleic anhydride grafted polyolefin compatibilizer.

[0007] Preferably, the cable filling composite fiber with multiple flame-retardant structure has a melamine phosphate as the inner core and a polyphosphazene as the shell layer, the shell layer contains amino groups and phosphonic acid groups after treatment with an aminopropyl silane coupling agent, the amino groups and the phosphonic acid groups are dispersed in a continuous phase to form a stable three-dimensional distribution network with a distribution density of no less than 1000 particles per cubic micrometer.

[0008] Preferably, the polyacrylonitrile has an initial weight average molecular weight of 80,000-120,000, the number of passes is controlled to be 2-6, the polyacrylonitrile is pre-melted for 30 minutes before blending with the phosphate-modified polyester, the mixing ratio is controlled to be 100:15-35 by mass, and the melt blending is performed by a twin-screw extruder at a temperature of 210-230°C.

[0009] Preferably, the preparation method of the cable filling composite fiber with multiple flame-retardant structure comprises the following steps:

[0010] S1. The polyacrylonitrile and the phosphate-modified polyester are placed in a melt mixer, mixed at a temperature of 210-230°C and a shear speed of 150 rpm for 20-30 minutes to form a homogeneous blend;

[0011] S2. The amino groups and the phosphonic acid groups are added to the homogeneous blend at a mass ratio of 2-8 parts, and ultrasonic treatment is performed in an ultrasonic treatment tank at 60°C for 40 minutes, and then the homogeneous blend is blended, extruded and granulated to obtain core layer precursor granules;

[0012] S3. The core layer precursor granules and the sheath layer precursor liquid are respectively placed in the inner tube and outer tube spinning systems of a biaxial electrospinning device, and spinning is performed under the conditions of an applied electric field strength of 15-18 kV / cm and a spinning distance of 12-14 cm to obtain core-sheath composite fibers;

[0013] S4. The core-sheath composite fibers are heat treated in an ethanol solution containing amino silane crosslinking agents at 95-110°C for 20-30 minutes to crosslink and solidify the sheath layer, and then washed with an alcohol-water mixture and dried.

[0014] Preferably, the sheath precursor liquid is obtained by condensation polymerization of diphenyl phosphate, pentaerythritol and dibutyl phthalate, the viscosity is controlled at 1500-3000 mPa·s at 25℃, the concentration is controlled at 30-50 wt%, and 0.1-0.5 wt% of p-toluene sulfonic acid is added as a condensation polymerization catalyst, and after polymerization reaction, it is cooled for standby use.

[0015] Preferably, the fiber spinneret in the spinning system is arranged as a coaxial double-layer nozzle, the inner diameter of the inner tube is 0.2 mm, the inner diameter of the outer tube is 0.5 mm, the core-sheath volume ratio is controlled at 2:1 to 4:1, the obtained composite fiber diameter is controlled at 12-20 μm, and the core layer accounts for not less than 70%.

[0016] Preferably, the grafting rate of the maleic anhydride grafted polyolefin compatibilizer is controlled at 0.5-2.0 wt%, the melt index is 3-6 g / 10 min at 190℃ / 2.16 kg, and the addition amount in the core layer matrix is 1-3 wt% of the matrix mass.

[0017] Preferably, after heat treatment, not less than 90% of a three-dimensional crosslinked structure is formed in the sheath layer, the crosslinking density is 0.5-1.5 mmol / g, and a double structure of phosphorus-oxygen crosslinking bond and hydroxyl crosslinking bond is formed.

[0018] Preferably, the fiber oxygen index is not less than 38%, the carbonization residue rate is not less than 40 wt% under the condition of nitrogen at 700℃, and the thermal decomposition temperature is greater than 320℃.

[0019] Preferably, the mechanical properties of the composite fiber with multiple flame-retardant structures for cable filling meet the following requirements: dry tensile strength ≥ 3.8 cN / dtex, elongation at break 5-15%, thermal shrinkage rate < 2.5% at 150℃×30 min, and pressure density resistance ≥ 0.3 g / cm 3 .

[0020] (1) The core layer and the sheath layer are cooperatively coated by electrospinning to enhance the thermal stability and structural integrity, which is different from the traditional single-layer flame-retardant fiber.

[0021] (2) The dispersibility and synergistic flame-retardant efficiency are improved by using surface-functionalized melamine phosphate@polyphosphazene microparticles, which is superior to the conventional way of directly mixing inorganic flame retardants.

[0022] (3) The outer layer is solidified by using amino silane crosslinking agent to prevent heat loss and improve the flame-retardant stability at service temperature.

[0023] (4) The core-sheath interfacial synergistic layer is constructed by using maleic anhydride grafted polyolefin to improve the interfacial adhesion and avoid delamination.

[0024] (5) Refine the core process parameters such as jet electric field strength, fiber diameter, particle size distribution, etc., to improve fiber consistency and application controllability.

[0025] (6) On the basis of ensuring high oxygen index and carbonization rate, while maintaining excellent tensile strength and low thermal shrinkage, the problem of brittle fracture or service deformation of traditional flame-retardant fibers is improved. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a whole flow schematic diagram of the preparation method of the composite fiber with multiple flame-retardant structures for cable filling; DETAILED DESCRIPTION

[0027] EMBODIMENT

[0028] EMBODIMENT 1-4

[0029] Embodiment 1: A composite fiber with multiple flame-retardant structures for cable filling comprises a continuous phase and a dispersed phase, the continuous phase is formed by blending polyacrylonitrile and phosphate modified polyester, and the dispersed phase is composed of nitrogen-phosphorus synergistic flame-retardant particles with a particle size of 200-400 nm and a core-shell structure.

[0030] The composite fiber with multiple flame-retardant structures for cable filling is in a core-sheath structure, the core layer is a flame-retardant blended material formed by melt blending, extrusion and granulation, the sheath layer is a high-phosphorus-content low-molecular-weight polymer synthesized from diphenyl phosphate and pentaerythritol, and a cross-linked solidification network is constructed in the outer layer, an intermediate synergistic structure layer with a thickness of 0.5-2 μm is formed between the core layer and the sheath layer through interfacial interpenetration, and the intermediate synergistic structure layer contains a maleic anhydride grafted polyolefin compatibilizer.

[0031] The composite fiber with multiple flame-retardant structures for cable filling takes melamine phosphate as the inner core and polyphosphazene as the shell layer, the shell layer contains amino and phosphonic acid groups after being treated with an aminopropyl silane coupling agent, the amino and the phosphonic acid groups are dispersed in the continuous phase to form a stable three-dimensional distribution network, and the distribution density is not less than 1000 particles per cubic micrometer.

[0032] The initial weight average molecular weight of the polyacrylonitrile is 80,000-120,000, the number of passes is controlled to be 2-6, the pre-melting treatment time is 30 minutes before blending with the phosphate modified polyester, the mixing ratio is controlled to be 100:15-35 by mass, and the melt blending twin-screw extruder is operated at 210-230℃.

[0033] The preparation method of the composite fiber with multiple flame-retardant structures for cable filling comprises the following steps:

[0034] S1. The polyacrylonitrile and the phosphate-modified polyester are placed in a melt mixer, mixed at 210-230℃ for 20-30 minutes at a shear speed of 150 rpm to form a homogeneous blend;

[0035] S2. The amino group and the phosphonic acid group are added to the homogeneous blend at a mass ratio of 2-8 parts, and ultrasonic treatment is performed in an ultrasonic treatment tank at 60℃ for 40 minutes. After uniform dispersion, blending extrusion granulation is performed to obtain core layer precursor granules;

[0036] S3. The core layer precursor granules and the sheath layer precursor liquid are placed in the inner tube and outer tube spinning systems of a biaxial electrospinning device, and spinning is performed under the conditions of an applied electric field strength of 15-18 kV / cm and a spinning distance of 12-14 cm to obtain core-sheath composite fibers;

[0037] S4. The core-sheath composite fibers are heat treated in an ethanol solution containing an amino silane crosslinking agent at 95-110℃ for 20-30 minutes to crosslink and solidify the sheath layer, and then washed with an alcohol-water mixture and dried.

[0038] The sheath layer precursor liquid is obtained by polycondensation of diphenyl phosphate, pentaerythritol, and dibutyl phthalate, and the viscosity is controlled at 1500-3000 mPa·s at 25℃, the concentration is controlled at 30-50 wt%, and 0.1-0.5 wt% of p-toluenesulfonic acid is added as a polycondensation catalyst. After polymerization, it is cooled and ready for use.

[0039] The fiber spinneret in the spinning system is arranged as a coaxial double-layer nozzle, the inner diameter of the inner tube is 0.2 mm, the inner diameter of the outer tube is 0.5 mm, the core-sheath volume ratio is controlled between 2:1 and 4:1, the obtained composite fiber diameter is controlled between 12-20 μm, and the core layer proportion is not less than 70%.

[0040] The grafting rate of the maleic anhydride grafted polyolefin compatibilizer is controlled at 0.5-2.0 wt%, the melt index is 3-6 g / 10 min at 190℃ / 2.16 kg, and the addition amount in the core layer matrix is 1-3 wt% of the matrix mass.

[0041] After heat treatment, not less than 90% of the sheath layer forms a three-dimensional crosslinked structure, the crosslinking density is between 0.5-1.5 mmol / g, and a double structure of phosphorus-oxygen crosslinking bond and hydroxyl crosslinking bond is formed.

[0042] The fiber oxygen index is not less than 38%, the carbonization residue rate is not less than 40 wt% under nitrogen at 700℃, and the thermal decomposition temperature is greater than 320℃.

[0043] The mechanical properties of the cable filling composite fiber with the multiple flame-retardant structure meet the following requirements: dry tensile strength ≥ 3.8 cN / dtex, elongation at break 5-15%, heat shrinkage rate 150°C x 30 min less than 2.5%, and pressure density resistance ≥ 0.3 g / cm 3 .

[0044] Example 2. Different from Example 1, the raw materials and process parameters are as follows:

[0045] Polyacrylonitrile (weight average molecular weight 100,000) 100 parts

[0046] Phosphate-modified polyester (phosphorus content 12 wt%) 25 parts

[0047] Surface-grafted nitrogen-phosphorus flame-retardant particles (particle size 300 nm) 6 parts

[0048] Maleic anhydride grafted polyolefin compatibilizer (grafting rate 1.2 wt%) 2 parts

[0049] Preparation process: melt blending temperature: 220°C, shear rate: 150 rpm, time 25 min

[0050] Ultrasonic dispersion treatment of flame-retardant particles: 60°C, 40 min

[0051] Electrospinning conditions: electric field strength 16 kV / cm, spinning distance 13 cm, nozzle size inner diameter 0.2 mm / outer diameter 0.5 mm

[0052] Sheath viscosity: 2200 mPa·s, crosslinking temperature 100°C, crosslinking time 25 min

[0053] Test performance: oxygen index (LOI): 39.1%

[0054] Thermogravimetric initial decomposition temperature: 326°C

[0055] Char yield (700°C): 41.3%

[0056] Dry tensile strength: 3.9 cN / dtex

[0057] Heat shrinkage rate (150°C x 30 min): 2.2%

[0058] Example 3. Different from Example 1, a higher proportion of flame-retardant particles and a medium viscosity sheath liquid are used:

[0059] Polyacrylonitrile 100 parts

[0060] Modified polyester 20 parts

[0061] Flame-retardant particles (particle size 250 nm) 8 parts

[0062] Compatibilizer 3 parts

[0063] Process parameters: blending temperature: 215°C, extrusion time: 30 min

[0064] Biaxial electrospinning voltage: 18 kV / cm

[0065] Sheath viscosity: 2800 mPa-s, crosslinking temperature 110°C, crosslinking agent is aminopropyl silane 0.3 wt%

[0066] Test performance: oxygen index: 40.5%

[0067] Char yield: 43.7%

[0068] Tensile strength: 4.1 cN / dtex

[0069] Thermal shrinkage: 1.9%

[0070] Fiber average diameter: 15 μm

[0071] Example 4. The difference from Example 1 is that a high phosphorus content sheath and a low addition amount of flame-retardant particles are used:

[0072] Polyacrylonitrile 100 parts

[0073] Modified polyester 30 parts

[0074] Flame-retardant particles 4 parts

[0075] Compatibilizer 2 parts

[0076] Process parameters: electrospinning electric field strength: 17 kV / cm, distance 12.5 cm

[0077] Sheath composition: diphenyl phosphate and pentaerythritol condensate (phosphorus content 18.2%)

[0078] Sheath viscosity: 3000 mPa-s, crosslinking time 30 min, crosslinking temperature 105°C

[0079] Performance index: oxygen index: 40.0%

[0080] Char yield: 45.2%

[0081] Tensile strength: 3.7 cN / dtex

[0082] Thermal shrinkage: 2.3%

[0083] Comparative example

[0084] Comparative example 1-3

[0085] Comparative example 1. Without using core-sheath structure, only using flame-retardant particles mixed into a single matrix spinning polyacrylonitrile 100 parts

[0086] Modified polyester 20 parts

[0087] Flame retardant particles (not surface modified) 6 parts

[0088] Sheathless structure process: single nozzle single component melt spinning, temperature 220°C, fiber drawing treatment as standard procedure Test performance: oxygen index: 31.2%

[0089] Char residue rate: 33.8%

[0090] Tensile strength: 2.8 cN / dtex

[0091] Heat shrinkage: 3.8%

[0092] Fiber structure: surface rough, obvious particle aggregation

[0093] Comparative example 2. No compatibilizer added, resulting in unstable core-sheath interface

[0094] Polyacrylonitrile 100 parts

[0095] Modified polyester 25 parts

[0096] Flame retardant particles (particle size 300 nm) 6 parts

[0097] Sheath normal, no maleic anhydride grafting between core and sheath

[0098] Spinning conditions same as example 2

[0099] Result: oxygen index: 36.4%

[0100] Tensile strength: 3.2 cN / dtex

[0101] Heat shrinkage: 3.0%

[0102] Structure observation: core-sheath interface peeling, crosslinked layer discontinuous, sheath layer easy to peel off Comparative example 3. Adopting non-crosslinked sheath, no solidified network structure formed

[0103] Polyacrylonitrile 100 parts

[0104] Modified polyester 20 parts

[0105] Flame retardant particles 6 parts

[0106] Compatibilizer 2 parts

[0107] Sheath liquid not added crosslinking agent, viscosity 1900 mPa·s

[0108] Performance. Oxygen index: 33.6%

[0109] Heat shrinkage: 3.7%

[0110] The sheath layer dissolves in hot water and the flame retardant layer fails

[0111] Good apparent morphology but extremely poor service stability

[0112] To determine the thermal gravimetric performance and flame retardant performance of the examples and comparative examples at high temperature, the following comparative experiments are designed:

[0113] Preparation of materials

[0114] Sample source: composite fibers obtained from Example 2, Example 3, and Example 4

[0115] Comparative Example 1 (no core sheath structure), Comparative Example 2 (no compatibilizer), Comparative Example 3 (sheath layer not crosslinked)

[0116] Experimental equipment and materials: thermal gravimetric analyzer (TGA, nitrogen atmosphere)

[0117] Limiting oxygen index tester (LOI, according to GB / T2406.2)

[0118] Constant temperature drying oven, electronic balance, sample loading aluminum crucible, fiber cutter

[0119] After drying, take 10 mg of fiber sample from each group, and repeat three times

[0120] The specific experimental steps are as follows:

[0121] 1. Thermal stability test

[0122] 1.1 Take 10 mg of each sample and place it in a clean aluminum crucible;

[0123] 1.2 Place the sample in the thermal gravimetric analyzer and heat it from room temperature to 700°C under a pure nitrogen atmosphere at a rate of 10°C / min;

[0124] 1.3 Record the thermal gravimetric loss curve of the sample and extract the following indicators:

[0125] Initial decomposition temperature (temperature corresponding to 5% weight loss)

[0126] Maximum decomposition rate temperature (DTG peak value)

[0127] Carbon residue rate at 700°C (%)

[0128] 2. Oxygen index test (LOI)

[0129] 2.1 According to the GB / T2406.2 standard, prepare a fiber bundle sample with dimensions of 100mm x 10mm x 1mm;

[0130] 2.2 Clamp the sample into the oxygen index tester, set different oxygen concentrations, adjust step by step to determine the minimum oxygen concentration that can maintain continuous combustion for ≥3 min;

[0131] 2.3 Repeat three times, and take the average value as the oxygen index of the sample.

[0132] The experimental data are shown in Table 1:

[0133] Sample No. Initial decomposition temperature (°C) DT peak temperature (°C) Char yield (700°C, %) Oxygen index LOI (%) Example 2 326 382 41.3 39.1 Example 3 331 388 43.7 40.5 Example 4 328 384 45.2 40 Comparative Example 1 288 341 33.8 31.2 Comparative Example 2 302 352 36.1 36.4 Comparative Example 3 295 348 34.5 33.6

[0134] Table 1

[0135] Experimental conclusion and analysis

[0136] Thermal stability: The initial decomposition temperature of the composite fibers of Examples 2-4 in a nitrogen environment is higher than 325℃, which is increased by 3040℃ compared with Comparative Example 1, indicating that the core-sheath structure combined with the nitrogen-phosphorus synergistic flame-retardant system significantly inhibits the pyrolysis process. In terms of carbon residue rate, the carbonized structure of the examples is more complete, and the carbon residue rate is more than 41%, while the carbon residue rate of the comparative examples is in the range of 3436%, indicating that the sheath layer crosslinking and particle synergistically inhibit the carbon chain cleavage.

[0137] Flame-retardant performance: The oxygen index of the examples is ≥39%, which is significantly higher than the 31-36% of the comparative examples, indicating that the core-sheath structure and particle distribution form a stable thermal barrier and gas phase flame-retardant layer. In particular, Example 3 has a high amount of flame-retardant particles and a fully crosslinked sheath layer, with an LOI of 40.5%.

[0138] Observation: Comparative Example 1 does not use a sheath layer, and the particles are aggregated and the interface is uneven, resulting in uneven pyrolysis; Comparative Example 2 causes delamination of the core-sheath layer due to the lack of a compatibilizer; Comparative Example 3 has a sheath layer that is not crosslinked, which exacerbates delamination during pyrolysis and results in unstable flame-retardant performance.

[0139] To determine the compression density and thermal shrinkage behavior of the composite fibers of the examples and comparative examples in a high-temperature compaction environment, the following comparative experiments were designed:

[0140] Preparation of experimental materials

[0141] Sample source: fiber bundles obtained from Example 2, Example 3, and Example 4

[0142] Fiber bundles obtained from Comparative Example 1, Comparative Example 2, and Comparative Example 3

[0143] Experimental equipment and materials: precision hot press tester (temperature and pressure control integrated)

[0144] Constant temperature air drying oven, caliper, electronic balance

[0145] Standard mold (diameter 20 mm, thickness 5 mm) for compression test sample preparation

[0146] Specific experimental steps

[0147] 1. Hot-pressing coupled morphology retention test

[0148] 1.1 Make each sample into a loose fiber bundle of about 5 g mass, fill into a standard mold and compact into a columnar sample;

[0149] 1.2 Place the mold on a hot press, set the hot-pressing temperature to 150°C, the pressure to 0.5 MPa, and the pressure retention time to 30 minutes;

[0150] 1.3 After cooling, measure the volume of the sample before and after compression, and calculate the compression density (volume after compression / original volume x 100%);

[0151] 1.4 At the same time, measure the length of the sample before and after compression, and calculate the thermal shrinkage rate.

[0152] 2. Shrinkage morphology stability test (dry heat condition)

[0153] 2.1 Take a fiber bundle of 50 mm in length, place it in a 150°C drying oven for 30 minutes, and then take it out;

[0154] 2.2 Measure the fiber length before and after treatment, and calculate the thermal shrinkage rate:

[0155]

[0156] where L0 is the original length and L1 is the length after treatment.

[0157] The experimental data are shown in Table 2:

[0158] Sample No. Compactness after compression (%) Thermal shrinkage (heat and pressure coupled) (%) Thermal shrinkage (dry heat) (%) Example 2 73.2 2.2 2.1 Example 3 74.8 1.9 1.8 Example 4 76.1 2.3 2.2 Comparative Example 1 63.5 4.7 4.3 Comparative Example 2 66.9 3.9 3.7 Comparative Example 3 62.1 5.1 4.8

[0159] Table 2

[0160] Experimental conclusion and analysis

[0161] Compression density: The compression density of the composite fibers of Examples 2-4 under standard hot-pressing is stably above 73%, which is significantly better than that of the comparative examples (the lowest is 62.1%). This is due to the uniform distribution of internal stress under the core-sheath synergistic structure, and the fibers are not prone to collapse.

[0162] Thermal shrinkage rate comparison: The thermal shrinkage rate of the examples is controlled to be below 2.3%, which indicates that they have good high-temperature morphology stability; while the comparative examples show obvious shrinkage under dry heat and hot-pressing coupling, especially Comparative Example 3, which has the worst thermal stability due to the uncrosslinked sheath layer, with a shrinkage rate as high as 5.1%.

[0163] The structural stability is significantly improved: the embodiment still maintains good morphological retention under the combined action of high temperature and pressure, indicating that the material of the application is suitable for long-term stable filling in high-temperature cable environment. In contrast, the comparative sample shows structural deformation and collapse tendency, which is difficult to meet the harsh service conditions.

[0164] The above shows and describes the basic principles, main features and advantages of the present application; those skilled in the art should understand that the present application is not limited to the above examples, the above examples and descriptions in the specification are only preferred examples of the present application and are not intended to limit the present application; various changes and improvements can be made without departing from the spirit and scope of the present application, and these changes and improvements all fall within the scope of the claimed application; the scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing composite fibers with multiple flame-retardant structures for cable filling, characterized in that, The composite fiber for cable filling with a multi-layer flame-retardant structure comprises a continuous phase and a dispersed phase. The continuous phase is formed by blending polyacrylonitrile and phosphate-modified polyester, and the dispersed phase consists of nitrogen-phosphorus synergistic flame-retardant particles with a core-shell structure and a particle size between 200 and 400 nm. The composite fiber for cable filling with a multi-flame-retardant structure has a core-sheath structure. The core layer is a flame-retardant blend material formed by melt blending, extrusion, and granulation. The sheath layer is a high-phosphorus, low-molecular-weight polymer synthesized from diphenyl phosphate and pentaerythritol. A cross-linked and cured network is constructed on the outer layer. The core layer and the sheath layer form an intermediate synergistic structural layer with a thickness of 0.5 to 2 μm through interfacial interpenetration. The intermediate synergistic structural layer contains a maleic anhydride-grafted polyolefin compatibilizer.

2. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 1, characterized in that, The composite fiber for cable filling with a multi-flame-retardant structure uses melamine phosphate as the core and polyphosphazene as the shell. The shell is treated with aminopropylsilane coupling agent and contains amino and phosphonic acid groups. The amino and phosphonic acid groups are dispersed in a continuous phase to form a stable three-dimensional distribution network with a distribution density of not less than 1,000 particles per cubic micrometer.

3. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 1, characterized in that, The polyacrylonitrile has an initial weight-average molecular weight of 80,000 to 120,000 and a pass number controlled between 2 and 6. Before blending with the phosphate-modified polyester, it is pre-melted for 30 minutes. The mixing ratio is controlled at a mass ratio of 100:15 to 35. The mixture is run in the range of 210 to 230°C using a melt blending twin-screw extruder.

4. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 1, characterized in that, The method for preparing a composite fiber with a multi-layer flame-retardant structure for cable filling includes the following steps: S1. The polyacrylonitrile and the phosphate-modified polyester are placed in a melt mixer and mixed at 210-230°C and a shear rate of 150 rpm for 20-30 minutes to form a homogeneous blend. S2. The amino group and the phosphonic acid group are added to the homogeneous blend at a mass ratio of 2 to 8 parts, and ultrasonically treated at 60°C for 40 minutes in an ultrasonic treatment tank. After uniform dispersion, the blend is extruded and granulated to obtain core layer precursor granules. S3. The core precursor granules and the sheath precursor liquid are respectively placed into the inner tube and outer tube spinning system of the biaxial electrospinning device, and spinning is carried out under the conditions of an applied electric field strength of 15-18kV / cm and a spinning distance of 12-14cm to obtain core-sheath composite fiber. S4. The core-sheath composite fiber is heat-treated in an ethanol solution containing an aminosilane crosslinking agent at 95-110°C for 20-30 minutes to crosslink and solidify the sheath layer, and then washed with an alcohol-water mixture and dried.

5. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 4, characterized in that, The sheath precursor liquid is obtained by polycondensation of diphenyl phosphate, pentaerythritol and dibutyl phthalate, with a viscosity controlled at 1500-3000 mPa·s, 25°C, and a concentration controlled at 30-50 wt%. 0.1-0.5 wt% of p-toluenesulfonic acid is added as a polycondensation catalyst, and the mixture is cooled for later use after the polymerization reaction.

6. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 4, characterized in that, The fiber spinneret in the spinneret system is configured as a coaxial double-layer nozzle. The inner tube has an inner diameter of 0.2 mm, the outer tube has an inner diameter of 0.5 mm, the core-sheath volume ratio is controlled between 2:1 and 4:1, the diameter of the resulting composite fiber is controlled between 12 and 20 μm, and the core layer accounts for no less than 70%.

7. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 1, characterized in that, The grafting rate of the maleic anhydride-grafted polyolefin compatibilizer is controlled at 0.5-2.0 wt%, the melt index is 3-6 g / 10 min, and the amount added is 1-3 wt% of the matrix mass when dispersed in the core matrix at 190℃ / 2.16 kg.

8. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 1, characterized in that, After heat treatment, the sheath layer forms a three-dimensional cross-linked structure with a cross-linking density between 0.5 and 1.5 mmol / g, and forms a dual structure of phosphorus-oxygen cross-links and hydroxyl cross-links.

9. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 1, characterized in that, The fiber oxygen index is not less than 38%, the carbonization residue rate is not less than 40wt% under nitrogen conditions at 700℃, and the thermogravimetric initiation temperature is greater than 320℃.

10. The method for preparing a composite fiber with a multiple flame-retardant structure for cable filling according to claim 1, characterized in that, The composite fiber of the cable filling with a multi-flame-retardant structure has the following mechanical properties: dry tensile strength ≥3.8cN / dtex, elongation at break 5~15%, heat shrinkage rate ≤2.5% at 150℃×30min, and compressive density ≥0.3g / cm³.

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