Silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material for high temperature and preparation method of silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material

Abstract

The invention relates to the technical field of high polymer materials, and discloses a silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material for high temperature and a preparation method of the silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material. A phosphate ester flame retardant; a nitrogen-based compound flame retardant; a silane coupling agent; a reactive compatibilizing chain extender; a nano layered double hydroxide; a catalyst precursor; a composite antioxidant; the preparation method of the sheath material comprises the following steps: firstly, loading a catalyst precursor on a nano layered double hydroxide to prepare composite particles; carrying out melt blending on the particles, matrix resin and a flame-retardant anchoring system, and carrying out extrusion molding; and finally, triggering the catalyst through heat-moisture treatment, and completing crosslinking through pressure-assisted polycondensation. According to the invention, through dual chemical anchoring, catalyst in-situ triggering and pressure-assisted crosslinking technologies, the interface bonding and processing safety is enhanced, the cross-linked network structure is optimized, and the mechanical, flame-retardant and heat-resistant properties of the material are comprehensively improved.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material and its preparation method. Background Technology

[0002] Polyolefin materials are widely used as insulation and sheathing layers for wires and cables due to their excellent electrical insulation properties, chemical stability, and ease of processing. With the increasing safety standards in modern industrial and civil facilities, especially in critical areas such as high-rise buildings, public transportation, and data centers, cable sheathing materials are required to not only possess basic electrical and mechanical properties but also meet high standards of flame retardancy, low smoke, halogen-free properties, and excellent high-temperature resistance. Therefore, developing high-temperature, low-smoke, halogen-free, flame-retardant polyolefin sheathing materials with superior comprehensive performance has become an important development direction in this technological field.

[0003] To achieve the above objectives, existing technologies typically employ a method of adding a large amount of halogen-free flame retardant to the polyolefin matrix. A common flame retardant system is a phosphorus-nitrogen synergistic intumescent flame retardant, which is mixed with the polyolefin resin through physical blending. Simultaneously, to improve the material's heat resistance, silane grafting and hydrothermal crosslinking processes are usually introduced. This process generally involves grafting vinyl-containing silane compounds onto the polyolefin molecular chains under the action of an initiator. Then, after extrusion molding, water or steam is used as a medium, and under the action of a tin-based catalyst, the silane groups are hydrolyzed and condensed, thereby forming a chemical crosslinking network between the polymer molecular chains.

[0004] While existing technologies have improved the flame retardancy and heat resistance of polyolefin materials to some extent through the aforementioned combinations, the inherent composition and mechanism of action of these technologies also lead to some unresolved technical problems. First, there is an inherent physicochemical incompatibility between polar inorganic fillers such as flame retardants and the non-polar polyolefin matrix. Simple physical blending results in weak interfacial bonding. This weak interface typically acts as a mechanical defect in polymer materials, preventing effective stress transfer and thus limiting the improvement of the material's final mechanical properties. Simultaneously, this also makes it easy for additives to migrate and precipitate from the matrix during long-term use or thermal aging, causing a decline in material performance. Second, in traditional silane crosslinking processes, the active catalyst is already present in the system during the melt blending stage. Due to the high processing temperature at this stage, even trace amounts of moisture in the system are sufficient to trigger localized early crosslinking reactions. This phenomenon increases the instability of the production process and significantly narrows the process window. Furthermore, during the wet-heat crosslinking step, the polycondensation reaction of silanol groups releases small molecule byproducts such as alcohols or water. Under normal pressure curing conditions, the volatilization of these byproducts can easily form micropores inside the material, affecting the density of the final product and the reliability of its long-term service. Summary of the Invention

[0005] The purpose of this invention is to provide a high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material and its preparation method, which solves the problem of poor overall performance in existing low-smoke halogen-free flame-retardant polyolefin sheath materials due to poor physical compatibility of components, insufficient controllability of traditional crosslinking processes, and defects in the network structure.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material, the sheathing material comprising the following components in parts by weight: Matrix resin: 67.5-82.5 parts; phosphate ester flame retardant: 8.0-12.0 parts; nitrogen compound flame retardant: 5.0-8.0 parts; silane coupling agent: 1.5-2.5 parts; reactive compatibilizer and chain extender: 0.5-1.5 parts; nano-layered double hydroxide: 3.0-5.0 parts; catalyst precursor: 0.05-0.2 parts; composite antioxidant: 0.3-0.5 parts.

[0007] In the technical solution provided by this invention, phosphate ester flame retardants and nitrogen-based compound flame retardants together constitute a synergistic flame retardant system. During combustion, the former promotes the formation of a dense char layer that isolates heat and oxygen, while the latter releases non-combustible gases for dilution, thus jointly improving the flame retardant efficiency.

[0008] One end of the silane coupling agent undergoes a grafting reaction with the polyolefin matrix resin during processing, while the other end chemically interacts with the surface of inorganic components (flame retardants and nanolayered double hydroxides).

[0009] Meanwhile, reactive compatibilizers, through their different functional groups, graft onto the polyolefin matrix at one end and react with the surface functional groups of the inorganic components at the other end. This forms another covalent bond between the polymer matrix and the inorganic components, besides the silane coupling agent, enhancing the strength and stability of the interfacial bonding. Nanolayered double hydroxides, acting as nanoscale reinforcements, are dispersed in the matrix to improve the material's thermal stability and mechanical properties.

[0010] Preferably, the catalyst precursor is pre-loaded onto the surface of the nanolayered double hydroxide to form composite particles. This structure allows the catalyst precursor to achieve uniform nanoscale dispersion within the matrix along with the nanolayered double hydroxide, providing a structural basis for the uniform distribution of catalytic active sites in the subsequent crosslinking reaction.

[0011] Preferably, the reactive compatibilizer and chain extender is a compound having at least two different active functional groups.

[0012] More preferably, the reactive compatibilizer and chain extender is glycidyl methacrylate. Its vinyl functional groups can branch with the polyolefin backbone, and its epoxy functional groups can undergo ring-opening reactions with the hydroxyl groups on the surface of the inorganic filler.

[0013] A second aspect of this invention provides a method for preparing a high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material, comprising the following steps: S1. Preparation of functionalized nano-reinforcement: The catalyst precursor is loaded onto the surface of a nano-layered double hydroxide to obtain composite particles; S2, Integrated Dynamic Anchoring and Chain Extension: The matrix resin, the composite particles obtained in S1, the synergistic flame retardant system and the multiple anchoring system are melt-blended in an internal mixing equipment to prepare the modified masterbatch; S3, Sheath extrusion molding: The modified masterbatch is extruded into an uncrosslinked sheath using an extruder; S4. Hydrolysis and in-situ catalyst triggering: The uncrosslinked sheath is treated in a humid and hot environment; S5, Pressure-assisted polycondensation: The sheath treated in S4 is subjected to high temperature and high pressure to complete cross-linking, thereby obtaining the high-temperature silane cross-linked low-smoke halogen-free flame-retardant polyolefin sheath material.

[0014] In the preparation method provided by this invention: Step S1 involves uniformly immobilizing the catalyst precursor onto the surface of a nanolayered double hydroxide by carrying out a loading reaction in a solvent, thereby preparing composite particles that possess both nano-enhancement and potential catalytic functions.

[0015] Preferably, the step includes: ultrasonically dispersing the nano-layered double hydroxide in anhydrous ethanol solvent for 15-30 minutes; dissolving the catalyst precursor in anhydrous ethanol and slowly adding it dropwise to the suspension at a temperature of 40-60°C and under continuous stirring, and continuing the reaction for 2-4 hours; filtering the product after the reaction, washing it 2-3 times with anhydrous ethanol, and then vacuum drying it at 60-80°C for 6-8 hours.

[0016] Step S2 involves conducting multiple chemical reactions concurrently in a mixing apparatus under specific temperature (120-140°C) and high shear stress conditions. This process is not merely physical mixing, but an in-situ chemical modification process that enables the grafting of silane coupling agents and reactive compatibilizers onto the polyolefin matrix, as well as their chemical bonding with the surfaces of inorganic components.

[0017] Preferably, the step includes: setting the mixing temperature to 120-140℃ and the rotation speed to 40-60 rpm; sequentially adding the matrix resin and composite antioxidant for melt plasticization for 3-5 minutes; then adding the composite particles prepared by S1 and mixing for 3-5 minutes; finally adding the remaining additives and continuing to mix for 5-8 minutes.

[0018] Step S3 involves conventionally extruding the modified masterbatch that has undergone in-situ chemical modification.

[0019] Preferably, this step uses a single-screw extruder, with the following temperature settings for each zone: feeding zone 130-140℃, compression zone 145-155℃, melting zone 155-165℃, die head and mold 160-170℃, and screw speed set to 20-40 rpm.

[0020] Step S4 utilizes a humid and hot environment to induce two concurrent chemical processes in water molecules that have penetrated into the material: first, the silane groups grafted onto the polyolefin chain are hydrolyzed to generate reactive silanol groups; second, the catalyst precursor supported on the surface of the nanolayered double hydroxide is converted in situ into a catalytically active substance.

[0021] Preferably, the conditions for this step are: temperature 80±2℃, relative humidity 85%-90%, and processing time 2-4 hours.

[0022] Step S5 involves constructing the final three-dimensional network structure under high temperature and high pressure conditions. The high temperature (170±2℃) provides energy for the condensation reaction between silanol groups; the applied pressure (0.5-1.0MPa) is used to suppress micropores that may be caused by the volatilization of reaction byproducts and to promote the close arrangement of molecular chain segments.

[0023] Since the catalytically active material is anchored on the surface of the nanolayered double hydroxide, the cross-linking reaction preferentially occurs at the inorganic / organic interface and extends outward, eventually forming a micro-network structure with nanoparticles as the core and a gradient distribution of cross-linking density.

[0024] Preferably, the processing time for this step is 15-30 minutes.

[0025] In summary, the present invention has at least one of the following beneficial technical effects: 1. This invention introduces silane coupling agents and reactive compatibilizers and chain extenders to establish dual chemical bonds between the polymer matrix and inorganic components such as flame retardants and nano-reinforcements. This robust chemical anchoring effectively inhibits the migration and precipitation of flame retardants and other additives during long-term use or under thermal stress, ensuring the long-term stability of material properties. Simultaneously, the enhanced interfacial bonding promotes effective stress transfer between the matrix and nano-reinforcements, thereby improving key mechanical properties of the sheathing material, such as tensile strength and elongation at break.

[0026] 2. This invention pre-loads the catalyst precursor onto the surface of a nanolayered double hydroxide, forming chemically inert composite particles at the processing temperature. The catalyst is only activated in situ under specific subsequent humid heat conditions. This design completely separates the introduction of catalytic active sites from the high-temperature melt processing stage, fundamentally avoiding the risk of early cross-linking during high-temperature steps such as mixing and extrusion. This improves the controllability and stability of the process, resulting in a wider processing window for the modified masterbatch.

[0027] 3. This invention completes silane hydrolysis and catalyst activation through a first-step hydrothermal treatment, and then completes the final network construction through a second-step pressure-assisted polycondensation. The applied pressure effectively suppresses the formation of micropores caused by the volatilization of crosslinking reaction byproducts, resulting in a more dense sheath material. Simultaneously, because the catalytic active sites are anchored on the surface of LDH nanoparticles, the crosslinking reaction is preferentially guided to occur in the filler-matrix interface region, ultimately forming a micro-network structure with LDH as the core and a gradient distribution of crosslinking density from the inside out within the material. This structure is more conducive to stress dispersion, thereby improving the overall heat resistance and long-term service reliability of the material. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to preparation examples, embodiments, comparative examples and test examples.

[0029] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0030] Preparation example: This preparation example discloses a method for preparing a matrix resin, including the following steps: Ethylene-vinyl acetate copolymer (EVA, VA content 25-30%) and linear low-density polyethylene (LLDPE, melt index 1.5-2.5 g / 10 min) were physically premixed at a mass ratio of 60:40-80:20. Add the mixture to a twin-screw extruder, set the temperature of each zone of the extruder to 120-140℃ and the screw speed to 30-60rpm, and carry out melt blending extrusion; The extruded strips are cooled in a water bath, pelletized by a pelletizer, and then dried in an oven at 60-80℃ for 4-6 hours to obtain the matrix resin particles required for subsequent experiments.

[0031] The reactive compatibilizer and chain extender is glycidyl methacrylate (GMA) with a purity of not less than 97.0%.

[0032] Example: First, the sheath material of this embodiment is composed of the following components in parts by weight: matrix resin (from Preparation Example 1): 75.0 parts; phosphate ester flame retardant: 10.0 parts; nitrogen-based compound flame retardant: 6.5 parts; silane coupling agent: 2.0 parts; reactive compatibilizer and chain extender (from Preparation Example 2): 1.0 part; nano-layered double hydroxide: 4.0 parts; catalyst precursor: 0.125 parts; composite antioxidant: 0.4 parts.

[0033] The preparation method of high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material provided in this embodiment specifically includes the following steps: S1. Preparation of Functionalized Nanoreinforcement: Weigh 4.0 parts by weight of nano-layered double hydroxide and place it in anhydrous ethanol solvent. Disperse it ultrasonically for 22.5 minutes at room temperature to form a uniform suspension. Separately weigh 0.125 parts by weight of catalyst precursor, dissolve it in an appropriate amount of anhydrous ethanol, and slowly add it dropwise to the above suspension at 50°C with continuous stirring. Maintain this temperature and continue the reaction for 3 hours. After the reaction is complete, filter the product and wash it three times with anhydrous ethanol. Then, dry it in a vacuum drying oven at 70°C for 7 hours to obtain composite particles.

[0034] S2. Integrated Dynamic Anchoring and Chain Extension: In the internal mixer, the mixing temperature is set to 130℃ and the rotation speed to 50rpm. The following order of feeding and mixing is followed: First, 75.0 parts by weight of the matrix resin and 0.4 parts by weight of the composite antioxidant are added for melt plasticization and mixing for 4 minutes; then, all the composite particles prepared in step S1 are added and mixing continues for 4 minutes to achieve uniform dispersion in the matrix; finally, 10.0 parts by weight of the phosphate ester flame retardant, 6.5 parts by weight of the nitrogen-based compound flame retardant, 2.0 parts by weight of the silane coupling agent, and 1.0 part by weight of the reactive compatibilizer and chain extender are added to the internal mixer and mixing continues for 6.5 minutes to obtain the final modified masterbatch.

[0035] S3. Sheath Extrusion Molding: The modified masterbatch obtained in step S2 is extruded using a single-screw extruder. The temperature distribution in each zone of the extruder is set to increase gradually, with the feeding zone at 135℃, the compression zone at 150℃, the melting zone at 160℃, and the die and mold zone at 165℃. The screw speed is set to 30 rpm, and the modified masterbatch is extruded into standard test specimens to obtain uncrosslinked sheaths.

[0036] S4. Hydrolysis and in-situ catalyst triggering: Place the uncrosslinked sheath obtained in step S3 in a constant temperature and humidity chamber. Set the chamber temperature to 80℃ and the relative humidity to 88%. Treat under these humid and hot conditions for 3 hours.

[0037] S5. Pressure-assisted polycondensation: The sheath treated in step S4 is placed in a flat vulcanizing machine (as a pressure heat treatment device). The temperature of the device is set to 170°C, and a pressure of 0.75 MPa is applied. Under these temperature and pressure conditions, the process is carried out for 22.5 minutes to complete the polycondensation and crosslinking reaction of the silanol groups, resulting in a high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath sample.

[0038] Comparative Example 1: Compared with the examples, the difference is that: this comparative example adopts a conventional physical blending flame retardant technology, and its components do not contain silane coupling agents, reactive compatibilizers and chain extenders, nano-layered double hydroxides and catalyst precursors; its preparation method is to directly extrude and mold all components by one-step melt blending in an internal mixing equipment, without performing steps S1, S4 and S5.

[0039] Comparative Example 2: The difference between this comparative example and the example lies in the preparation method: This comparative example does not perform the functionalized nano-reinforcement preparation in step S1. Instead, in step S2, the nano-layered double hydroxide and catalyst precursor are used as independent powder raw materials, and added to the internal mixer in the final stage along with the phosphate ester flame retardant, nitrogen-based compound flame retardant, silane coupling agent, and reactive compatibilizer and chain extender. The remaining component ratios and process steps (S3, S4, S5) are the same as in the example.

[0040] Comparative Example 3: The difference between this comparative example and the examples lies in the composition: the formulation of this comparative example does not contain reactive compatibilizers or chain extenders. The weight proportions of all other components and all preparation steps are exactly the same as those in the examples.

[0041] Test Example 1: The experimental procedures were conducted in accordance with the GB / T1040.2-2006 standard.

[0042] First, the sheath material strips prepared in the examples and comparative examples 1-3 were cut into dumbbell-shaped specimens using a standard cutter.

[0043] The experiment was conducted on a universal testing machine, with the ambient temperature set at 23±2℃ and the test speed (tensile rate) at 200 mm / min. During the tensile process, the testing machine automatically recorded the maximum force value at specimen fracture and the change in the distance between the markings.

[0044] The tensile strength is calculated based on the recorded force value and the initial cross-sectional area of ​​the specimen, and the elongation at break is calculated based on the elongation at fracture and the initial gauge length.

[0045] Five valid samples were tested in each group of samples, and their arithmetic mean was taken as the final test result.

[0046] Table 1: Mechanical property test results of each sample group Sample group Tensile strength (MPa) Elongation at break (%) Example 17.6 465 Comparative Example 1 9.8 262 Comparative Example 2 14.2 395 Comparative Example 3 13.5 410 From Table 1, we can obtain: The sheath material prepared in the examples exhibits excellent overall performance in terms of tensile strength and elongation at break. Compared with Comparative Example 1, the tensile strength and elongation at break of the examples are significantly improved. This indicates that the technical solution significantly improves the mechanical properties of the material by introducing a cross-linked network structure, nano-reinforcement, and interface modification components, thus solving the performance shortcomings of conventional physical blend systems caused by poor component compatibility.

[0047] Comparing the results of the examples with those of Comparative Example 3, the tensile strength of the material decreased significantly without the addition of the reactive compatibilizer and chain extender (Comparative Example 3). This is because the interfacial bonding between the polymer matrix and the inorganic flame retardant and filler is limited by the single chemical bond provided by the silane coupling agent. In the examples, the addition of the reactive compatibilizer and chain extender constructed another covalent bond at the inorganic / organic interface, forming a more stable interfacial layer. This structure can more effectively transfer external stress, thereby enhancing the overall strength of the material.

[0048] In Comparative Example 2, the catalyst was added along with other additives in the later stage of melt blending, resulting in uneven dispersion of the catalyst in the matrix. This led to poor uniformity of the network structure formed by the subsequent crosslinking reaction, resulting in defects. In the embodiment, by pre-loading the catalyst precursor onto the surface of a nanolayered double hydroxide, the nanoscale uniform distribution of catalytic active sites in the matrix was ensured. After in-situ triggering in subsequent steps, a more uniform and complete crosslinked network was formed, which macroscopically manifested as a significant improvement in the mechanical properties of the material.

[0049] Test Example 2: The limiting oxygen index test was conducted in accordance with the GB / T2406.2-2009 standard.

[0050] The experimental steps include: cutting the sample into strips of specified size and clamping them vertically inside the combustion chamber.

[0051] The flow rates of oxygen and nitrogen introduced into the combustion chamber are precisely controlled using a mass flow meter to create airflows with different oxygen concentrations. The top of the sample is ignited using an igniter, and its combustion behavior is observed.

[0052] By gradually adjusting the oxygen concentration using the "up-down method" or the "approximation method," the lowest oxygen volume percentage that can sustain continuous combustion of the sample (combustion time exceeding 180 seconds or combustion length exceeding 50 mm) is found. This value is the limiting oxygen index of the material.

[0053] The vertical burning test was conducted in accordance with the GB / T2408-2008 standard.

[0054] The experimental procedure includes: vertically suspending a strip sample with dimensions of 125mm×13mm×3mm, and placing degreased cotton at the lower end of the sample.

[0055] Using a standard Bunsen burner, adjust the flame height to 20 mm, apply the flame to the center of the lower end of the sample, and remove the flame after 10 seconds.

[0056] Record the first flaming combustion time (t1) and the flameless combustion (glow) time.

[0057] After the flame goes out, immediately apply the flame again for 10 seconds, remove it, and record the second flaming burning time (t2) and the flameless burning time.

[0058] The UL-94 flame retardancy rating of the five samples was determined based on their burning time and whether they ignited the absorbent cotton.

[0059] Table 2: Flame retardant performance test results for each sample group Sample group Limiting Oxygen Index (LOI, %) Vertical flammability rating (UL-94) Example 35.2 V-0 Comparative Example 1 27.5 Ungraded (molten dripping) Comparative Example 2 33.8 V-0 Comparative Example 3 33.1 V-0 From Table 2, we can obtain: The sheath material prepared in this example exhibits the highest limiting oxygen index (LOI) and the best vertical burning rating. Compared to Comparative Example 1, the flame retardant performance of this example is substantially improved, with a significantly higher LOI value and no melt dripping during the vertical burning test. This is attributed to the introduction of a phosphorus-nitrogen synergistic flame retardant system and the construction of a three-dimensional cross-linked network in this technical solution. During combustion, the phosphate ester flame retardant promotes the formation of a dense, heat-insulating char layer in the condensed phase, while nitrogen compounds release non-flammable gases in the gas phase; the two work synergistically. Simultaneously, the cross-linked network structure effectively inhibits the flow of the polymer matrix at high temperatures, providing a stable framework for the formation of the char layer.

[0060] Comparing the results of the comparative examples with those of Comparative Example 3, the limiting oxygen index of the material was lower in the absence of the reactive compatibilizer and chain extender (Comparative Example 3). This indicates that the reactive compatibilizer and chain extender, by constructing dual chemical bonds, not only enhances the mechanical properties but also promotes the fine dispersion and stable bonding of inorganic components such as flame retardants and nanolayered double hydroxides in the polyolefin matrix. This uniform dispersion allows the flame retardant to function more efficiently during combustion, forming a more complete and structurally stable protective char layer, thereby improving the flame retardant efficiency of the material.

[0061] In Comparative Example 2, the uneven dispersion of the catalyst led to non-uniformity of the cross-linked network, resulting in structural weaknesses in the char layer formed during combustion, which affected its ability to insulate against heat and oxygen. In this embodiment, the catalyst precursor was uniformly distributed by loading it onto the surface of a nanolayered double hydroxide. The resulting uniform cross-linked network can act as a more effective barrier during combustion, supporting and stabilizing the composite char layer composed of the synergistic flame-retardant system and LDH thermal decomposition products (metal oxides), thus exhibiting superior flame-retardant performance.

[0062] Test Example 3: The experimental procedures were conducted in accordance with GB / T2951.21-2008 standard.

[0063] First, dumbbell-shaped specimens were cut from the finished sheath material of each sample group, and a 20mm mark (L0) was marked on the parallel part in the middle of the specimen.

[0064] The sample is suspended vertically in a dedicated aging test chamber preheated to 200±2℃. A weight is suspended from the lower end of the sample, and the applied weight must allow the sample to withstand 20 N / cm². 2 The stress.

[0065] After maintaining the temperature at 200°C and the specified load for 15 minutes, immediately measure the distance (L1) between the two markings.

[0066] The thermal elongation is calculated using the formula ((L1-L0) / L0)×100%. Three valid specimens are tested for each group of samples, and their arithmetic mean is taken.

[0067] Table 3: Test results of thermal elongation performance of each sample group Sample group Thermal elongation (200℃, %) Example 32 Comparative Example 1 (Melt fracture) Comparative Example 2 93 Comparative Example 3 51 From Table 3, we can obtain: The sample in Comparative Example 1, as an uncrosslinked physical blend, failed to maintain its structural integrity under test conditions of 200°C, resulting in melt fracture and demonstrating its inability to withstand high temperatures. In contrast, the sample in the Example exhibited extremely low thermal elongation, directly attributed to the formation of a stable three-dimensional chemical crosslinking network within it. This network structure macroscopically connects the separated polymer chains into a unified whole, effectively restricting the movement and slippage of chain segments at high temperatures, thereby endowing the material with excellent resistance to thermal deformation.

[0068] In the embodiments, the reactive compatibilizer and chain extender constructed additional chemical bridges between the polymer matrix and the inorganic fillers (nanolayered double hydroxides and flame retardants), transforming these inorganic particles into highly functional crosslinking nodes, effectively improving the crosslinking density and structural strength of the entire network. Comparative Example 3, lacking this connection, exhibited a relatively weaker binding capacity in its crosslinked network, thus displaying greater deformation under high-temperature loads.

[0069] Comparing the results of the Example and Comparative Example 2, the thermal elongation of Comparative Example 2 is significantly higher than that of the Example. This is because in the preparation process of Comparative Example 2, the direct addition of the catalyst leads to uneven dispersion in the matrix, resulting in a cross-linked network with microscopically inconsistent densities and structural defects. These defective regions become stress concentration points and structural weak points under high-temperature loading, making the material more prone to elongation. The Example, by pre-loading the catalyst precursor onto the surface of nanoparticles, achieves a uniform distribution of catalytic active sites and guides the formation of a more uniform and dense cross-linked network through specific hydrolysis-triggered and pressure-assisted polycondensation steps, thereby significantly improving the material's resistance to high-temperature creep.

[0070] Test Example 4: The experimental procedure was conducted in an NBS smoke density chamber in accordance with the GB / T8323.2-2008 standard.

[0071] First, the sheath material for each sample group was prepared into square specimens with dimensions of 75mm × 75mm and a thickness of 3mm. The specimens were then placed in a sample holder and vertically positioned inside a sealed smoke density testing chamber. A radiant heat source was used to apply a heat of 25kW / m² to the surface of the specimens. 2 The heat flux density was used to conduct a flameless combustion test.

[0072] The test chamber contains an optical system, through which a horizontal beam of light passes and illuminates a photodetector. As the sample decomposes due to heat, producing smoke, the transmittance of the light beam inside the chamber decreases. The transmittance is continuously recorded over time during the experiment, and the specific optical density (Ds) is calculated using the formula in the standard.

[0073] Smoke density rating (SDR) is a dimensionless parameter calculated based on the specific optical density values ​​at multiple time points during the test. The smaller the value, the less smoke the material produces.

[0074] Table 4: Smoke density test results for each sample group From Table 4, we can obtain: The sheath material prepared in this example exhibits the lowest smoke density rating. Compared to Comparative Example 1, the smoke density of this example is significantly reduced. This is because the linear polymer in Comparative Example 1 undergoes incomplete combustion during pyrolysis, generating a large amount of smoke particles; while in this example, the phosphorus-nitrogen synergistic flame retardant system plays a role in the condensed phase, promoting the formation of a dense and stable char layer. Simultaneously, the metal oxides resulting from the thermal decomposition of the nanolayered double hydroxides also participate in the char formation process, acting as a catalyst and providing skeletal support. This high-quality char layer, as a physical barrier, effectively inhibits the escape of combustible gases and smoke precursors, thereby significantly reducing smoke generation.

[0075] Comparing the results of the examples with those of Comparative Example 3, the smoke density rating of the material increased in the absence of the reactive compatibilizer (Comparative Example 3). This is because, in the examples, the reactive compatibilizer established a strong chemical bond between the matrix resin and the inorganic filler, resulting in more uniform dispersion of the filler particles in the matrix and a tighter bond with the matrix. During the char formation stage, this structure can more effectively bind inorganic particles within the char layer, enhancing the structural integrity and oxidation resistance of the char layer, thereby improving its ability to suppress smoke generation.

[0076] Comparing the results of the Examples and Comparative Example 2, the smoke density level of Comparative Example 2 is significantly higher than that of the Examples. This is because the Examples, by pre-loading the catalyst precursor onto the surface of a nanolayered double hydroxide, guide the formation of a chemically cross-linked network with a uniform microstructure. This uniform network can transform into a structurally complete char layer framework during combustion. In Comparative Example 2, uneven catalyst dispersion leads to defects in the cross-linked network, and the resulting char layer is easily broken under thermal shock, failing to form an effective barrier and resulting in the release of more smoke particles. Furthermore, the pressure-assisted polycondensation step used in the Examples yields a denser material, which also physically restricts the diffusion and escape of pyrolysis products.

[0077] 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 high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material, characterized in that, The sheath material comprises the following components in parts by weight: Matrix resin: 67.5-82.5 parts; Phosphate ester flame retardant: 8.0-12.0 parts; Nitrogen-based flame retardant: 5.0-8.0 parts; Silane coupling agent: 1.5-2.5 parts; Reactive compatibilizer and chain extender: 0.5-1.5 parts; Nanolayered double hydroxide: 3.0-5.0 parts; Catalyst precursor: 0.05-0.2 parts; Compound antioxidant: 0.3-0.5 parts.

2. The high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material according to claim 1, characterized in that, The matrix resin is a blend of ethylene-vinyl acetate copolymer and linear low-density polyethylene.

3. The high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material according to claim 1, characterized in that, The reactive compatibilizer and chain extender is a compound with epoxy and vinyl functional groups.

4. The high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material according to claim 1, characterized in that, The catalyst precursor is preloaded onto the surface of the nanolayered double hydroxide to form composite particles for subsequent preparation processes.

5. A method for preparing high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material, characterized in that, The preparation of the high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material according to any one of claims 1-4 includes the following steps: S1. Preparation of functionalized nano-reinforcement: The catalyst precursor is loaded onto the surface of a nano-layered double hydroxide to obtain composite particles; S2, Integrated dynamic anchoring and chain extension: The matrix resin, the obtained composite particles, the synergistic flame retardant system and the multiple anchoring system are melt-blended in an internal mixing equipment to prepare the modified masterbatch; S3, Sheath extrusion molding: The modified masterbatch is extruded into an uncrosslinked sheath using an extruder; S4. Hydrolysis and in-situ catalyst triggering: The uncrosslinked sheath is treated in a humid and hot environment; S5, Pressure-assisted polycondensation: The treated sheath is subjected to high temperature and high pressure to complete cross-linking.

6. The method for preparing high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material according to claim 5, characterized in that, In step S1, the preparation of the functionalized nano-reinforcement includes the following steps: The nano-layered double hydroxide was ultrasonically dispersed in anhydrous ethanol solvent for 15-30 minutes to form a uniform suspension. The catalyst precursor was dissolved in anhydrous ethanol and then slowly added dropwise to the suspension at a temperature of 40-60°C with continuous stirring. The reaction was continued for 2-4 hours to complete the loading. The product after reaction was filtered, washed, and washed 2-3 times with anhydrous ethanol, and then vacuum dried at 60-80℃ for 6-8 hours to obtain the composite particles.

7. The method for preparing high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material according to claim 5, characterized in that, In step S2, the integrated dynamic anchoring and chain extension steps include: In the internal mixer, set the mixing temperature to 120-140℃ and the rotation speed to 40-60 rpm; proceed with feeding and mixing in the following order: Add the base resin and composite antioxidant to the mixture for melt plasticization and knead for 3-5 minutes; Add the composite particles and continue mixing for 3-5 minutes to achieve dispersion; Add the phosphate ester flame retardant, nitrogen-based flame retardant, silane coupling agent, and reactive compatibilizer and chain extender, and continue mixing for 5-8 minutes to complete the chemical anchoring and chain extension reaction, thereby obtaining the modified masterbatch.

8. The method for preparing high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material according to claim 5, characterized in that, In step S3, the extrusion molding of the sheath includes: A single-screw extruder is used, with the temperature of each zone set to increase in a gradient: 130-140℃ in the feeding zone, 145-155℃ in the compression zone, 155-165℃ in the melting zone, and 160-170℃ in the die head and mold. Set the screw speed to 20-40 rpm; The modified masterbatch is extruded using an extruder to obtain the uncrosslinked sheath.

9. The preparation method of the high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material according to claim 5, characterized in that, In step S4, the hydrolysis and in-situ catalyst triggering steps include: The obtained uncrosslinked sheath was placed in a constant temperature and humidity chamber; The temperature of the constant temperature and humidity chamber is set at 78-82℃, and the relative humidity is set at 85%-90%. The catalyst is treated under these humid and hot conditions for 2-4 hours to complete the hydrolysis of silane groups and in-situ triggering of the catalyst.

10. The method for preparing high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheathing material according to claim 5, characterized in that, In step S5, the pressure-assisted polycondensation step includes: The treated sheath is placed in a pressurized heat treatment device; The temperature of the pressurized heat treatment device is set to 168-172℃, and a pressure of 0.5-1.0MPa is applied. Under these temperature and pressure conditions, the material is treated for 15-30 minutes to complete the silanol-based polycondensation and crosslinking reaction, thereby obtaining the high-temperature silane crosslinked low-smoke halogen-free flame-retardant polyolefin sheath material.

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