An insulating cable sheath material and its preparation method

Abstract

This invention discloses an insulating cable sheath material and its preparation method, which is composed of the following components in parts by weight: 80-90 parts of composite matrix resin; 10-15 parts of phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system; 3-5 parts of organomontmorillonite; 1.8-2.5 parts of silane grafted composite crosslinking system; 0.4-0.7 parts of compounded weather-resistant and anti-aging system; 0.8-1.2 parts of titanate coupling agent; and 0.1-0.2 parts of stearic acid. By using the phosphorus-nitrogen-nano magnesium hydroxide low-filler synergistic flame retardant system, the industry's technical prejudice that high flame retardancy must sacrifice mechanical properties is overcome. While achieving low smoke, halogen-free, and high flame retardancy, the excellent mechanical properties of the polyolefin matrix are retained, and the sheath is not easily brittle or deformed.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite materials technology, specifically referring to an insulating cable sheath material and its preparation method. Background Technology

[0002] With the upgrading of domestic power systems, the popularization of new energy industries, and the continuous improvement of fire protection standards for rail transit and high-rise buildings, traditional halogen-containing cable sheath materials have been gradually phased out of the market due to the release of toxic hydrogen halides and high smoke emissions during combustion. Low-smoke halogen-free polyolefin sheath materials, with their advantages of environmental protection, flame retardancy, and excellent insulation, have become the mainstream development direction for cable sheaths. Currently, the most widely used matrix resins in industry are ethylene-vinyl acetate copolymer (EVA), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and their blends. These polyolefin materials have excellent processing performance, moderate cost, and good basic insulation properties.

[0003] Based on extensive technical research and industry surveys, the current low-smoke halogen-free polyolefin cable sheath materials and manufacturing processes mainly face five major technical bottlenecks: flame retardant systems, cross-linking modification, insulation barrier, weather resistance, and filler treatment. These technical defects are compounded, severely limiting the application of sheath materials under complex and harsh working conditions. These bottlenecks are detailed below: (1) The core contradiction in flame-retardant systems lies in the mutual constraint between flame-retardant performance and mechanical properties: Existing halogen-free flame-retardant polyolefin sheaths are mainly divided into two types of flame-retardant systems, both of which have obvious shortcomings: ① Single inorganic metal hydroxide flame retardant system: Represented by aluminum hydroxide and magnesium hydroxide, this system achieves flame retardancy by relying on thermal decomposition and the generation of a protective metal oxide layer. This type of flame retardant is halogen-free and produces low smoke, but its flame retardant efficiency is extremely low. In industrial applications, the filler content generally needs to reach 55wt% or higher to meet the UL94V0 flame retardant rating. Under high filler conditions, the continuous phase structure of the polyolefin resin matrix is ​​completely destroyed, resulting in a significant decrease in the tensile strength and elongation at break of the sheath material. This leads to problems such as brittleness, surface powdering, and filler precipitation in the product. Simultaneously, the inorganic hydroxide surface is rich in hydrophilic hydroxyl groups, which increases the overall water absorption rate of the material. Moisture intrusion directly causes a decrease in volume resistivity, leading to continuous deterioration of insulation performance.

[0004] ② Pure organic phosphorus and nitrogen intumescent flame retardant system: This system has a low filling amount and has little impact on the mechanical properties of the matrix, but it has the problems of strong hygroscopicity and poor thermal stability. When used in humid and high-temperature environments for a long time, the flame retardant components are prone to water absorption and decomposition, which not only gradually reduces the flame retardant effect, but also further accelerates the decay of insulation performance; at the same time, the smoke density during combustion is relatively high, which cannot meet the low smoke safety requirements of high-rise buildings, rail transit and other places.

[0005] In summary, existing single flame retardant systems cannot simultaneously meet the five requirements of low filler content, high flame retardancy, excellent mechanical properties, low water absorption, and low smoke, which has become a long-standing technical challenge in this field.

[0006] (2) The crosslinking modification process is simple, resulting in poor overall performance and production stability: To improve the heat deformation resistance, dimensional stability, insulation integrity, and long-term mechanical properties of polyolefin sheaths, the industry commonly uses cross-linking processes to modify the matrix. Existing industrial cross-linking methods are divided into three categories, each with its own limitations. Furthermore, the industry currently uses a single cross-linking mode, and no mature application scheme for composite gradient cross-linking has been observed. ①Silane moisture crosslinking: Low equipment investment and simple process make it the mainstream choice for small and medium-sized cable companies. However, this crosslinking reaction is completely dependent on ambient temperature and humidity, and the curing cycle is as long as 24 to 72 hours, resulting in extremely low production efficiency. At the same time, temperature and humidity fluctuations in different batches and different production areas will cause differences in crosslinking uniformity. Areas with insufficient crosslinking are prone to defects such as high-temperature softening and insulation leakage, resulting in poor product consistency.

[0007] ② High-energy irradiation crosslinking (electron beam, gamma ray): This method offers fast crosslinking speed and good uniformity, making it suitable for large-scale continuous production. However, conventional production uses high-dose irradiation, where high-energy rays simultaneously bombard the polyolefin molecular backbone, causing molecular chain breakage. This ultimately leads to a decrease in the toughness of the sheath material and low-temperature embrittlement, making it unsuitable for use in cold regions. Furthermore, single-irradiation crosslinking is insufficient to suppress the high-temperature creep behavior of the material, and the sheath is prone to deformation and detachment under long-term high-temperature operation.

[0008] ③ Peroxide chemical crosslinking: The degree of crosslinking is controllable, but the crosslinking reaction temperature and the resin extrusion processing temperature range highly overlap. Premature scorching is very likely to occur during processing, the extrusion processing window is extremely narrow, the production yield is low, the requirements for equipment and process control are extremely high, and the scope of application is limited.

[0009] The combined defects of a single crosslinking process have led to a common problem in existing crosslinked sheath materials: either the crosslinking cycle is long and the consistency is poor, or the material becomes brittle and has insufficient heat creep resistance.

[0010] (3) Insufficient water vapor barrier capacity and poor long-term insulation performance: Outdoor overhead cables, directly buried underground cables, and tunnel cables are exposed to high humidity, acid and alkali corrosion, and groundwater immersion environments for extended periods. The molecular structure of ordinary EVA / LDPE polyolefin matrices has limited density, allowing water vapor and corrosive ions to easily penetrate the material's interior along molecular gaps. Even when some solutions incorporate barrier fillers such as montmorillonite, only simple physical blending is performed; the fillers are not specifically surface-modified and tend to agglomerate within the resin matrix, failing to form effective nano-barrier channels. Long-term intrusion of water vapor and corrosive media continuously reduces the material's volume resistivity, leading to insulation failure, cable leakage, short circuits, and other safety incidents. Furthermore, conventional polyolefin materials have weak resistance to UV aging and thermo-oxidative aging. Under prolonged outdoor exposure and alternating high and low temperature conditions, the polymer chains are prone to free radical degradation, causing sheath cracking and pulverization, significantly shortening the overall service life of the cable.

[0011] (4) Multifunctional fillers are prone to agglomeration after compounding, and have poor interfacial compatibility: Modern cable sheathing typically requires the simultaneous addition of multiple inorganic / organic functional fillers, such as flame retardants, barrier agents, and anti-aging agents. These fillers exhibit significant differences in surface functional groups and surface polarity. Current processes either use uniform coupling treatment for all types of fillers or completely omit surface activation, failing to meet the interfacial modification requirements of different fillers. Furthermore, agglomeration is highly likely during the blending of multiple fillers, forming numerous voids and interfacial defects within the material. These internal defects not only further reduce the material's mechanical strength and insulation performance but also become channels for water vapor and oxygen penetration, accelerating material aging and failure. Simultaneously, they cause performance fluctuations between different batches of products, resulting in poor mass production stability.

[0012] (5) Insufficient high and low temperature adaptability: Conventional EVA / LDPE blended sheath materials have poor low-temperature toughness and are prone to brittleness in environments with temperatures of -20℃ and below. In high-temperature environments (above 100℃), the resin matrix is ​​prone to softening and deformation, which cannot meet the usage requirements of extreme conditions such as cold regions, tropical regions, and long-term full-load heating of cables.

[0013] In summary, current low-smoke halogen-free polyolefin cable sheath materials generally suffer from a series of technical problems, including conflicting flame retardancy and mechanical properties, low cross-linking process efficiency / poor product consistency, weak moisture barrier leading to insufficient long-term insulation, internal defects caused by multi-filler agglomeration, and poor high and low temperature resistance and weather resistance. Existing single modification technologies and single formulation systems cannot simultaneously solve these multiple defects, making them unsuitable for high-end and demanding applications such as new energy, rail transportation, high-rise buildings, and direct-buried cables. Therefore, there is an urgent need in this field to develop a novel insulating cable sheath material and its preparation method that features good formulation synergy, controllable preparation process, comprehensive performance improvement, and industrial-scale production capability. Summary of the Invention

[0014] To address the needs and problems mentioned in the background above, the present invention provides an insulating cable sheath material and a method for preparing the same, thereby at least partially solving the aforementioned problems.

[0015] According to the technical solution of the present invention, an insulating cable sheath material is provided, which is composed of the following components in parts by weight: 80-90 parts of composite matrix resin; 10-15 parts of phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system; 3-5 parts of organomontmorillonite; 1.8-2.5 parts of silane grafted composite crosslinking system; 0.4-0.7 parts of compounded weather-resistant and anti-aging system; 0.8-1.2 parts of titanate coupling agent; and 0.1-0.2 parts of stearic acid. The composite matrix resin is composed of ethylene-vinyl acetate copolymer and low-density polyethylene. The phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system is composed of nano magnesium hydroxide and cyclotriphosphazene; The silane grafted composite crosslinking system is composed of vinyltrimethoxysilane and dicumyl peroxide; The compound weather-resistant and anti-aging system consists of hindered amine light stabilizers and phosphite antioxidants.

[0016] Preferably, in the composite matrix resin, the vinyl acetate content in the ethylene-vinyl acetate copolymer is 18%, the ethylene-vinyl acetate copolymer is 60-65 parts by weight, and the low-density polyethylene is 20-25 parts by weight.

[0017] Preferably, in the phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system, there are 8-12 parts by weight of nano magnesium hydroxide and 2-3 parts by weight of cyclotriphosphazene.

[0018] Preferably, in the silane grafted composite crosslinking system, vinyltrimethoxysilane comprises 1.5 to 2 parts by weight and dicumyl peroxide comprises 0.3 to 0.5 parts by weight; in the compounded weather-resistant and anti-aging system, hindered amine light stabilizer comprises 0.2 to 0.4 parts by weight and phosphite antioxidant comprises 0.2 to 0.3 parts by weight.

[0019] Preferably, the titanate coupling agent is 0.8 to 1.2 parts by weight, and the stearic acid is 0.1 to 0.2 parts by weight.

[0020] In a second aspect, the present invention also provides a method for preparing the above-mentioned insulating cable sheath material, comprising the following steps: (1) Functional filler graded activation pretreatment: nano magnesium hydroxide and organomontmorillonite were surface activated and modified by titanate coupling agent, dried and sealed for later use; (2) Resin grafting modification blend: The composite matrix resin, silane grafted composite crosslinking system, filler pretreated in step (1), weather-resistant and anti-aging compound system and stearic acid are mixed evenly to obtain a composite mixture. (3) Melt extrusion granulation and low-dose irradiation pre-crosslinking: The composite mixture is melt extruded and granulated, and then subjected to low-dose irradiation with an electron beam to obtain pre-crosslinked masterbatch; (4) Sheath extrusion coating molding: The pre-crosslinked masterbatch is melted by an extruder, continuously coated on the outside of the cable insulation layer and air-cooled for shaping; (5) Constant temperature and humidity cross-linking curing: The shaped cable is placed in a high temperature saturated steam environment for curing to complete the cross-linking reaction and obtain the finished cable sheath.

[0021] Preferably, in step (1), the activation and modification temperature is 80-90℃, the stirring time is 15-20 min, the vacuum drying temperature is 75℃, and the drying time is 2 h.

[0022] Preferably, in step (3), the electron beam irradiation dose is controlled to be 30-40 kGy.

[0023] Preferably, in step (5), the saturated steam ambient temperature is 85-90℃ and the constant temperature curing time is 4-6h.

[0024] Thirdly, the present invention also provides an application of the above-mentioned insulating cable sheath material, wherein the cable sheath material is applied to the outer protective sheath of high and low voltage power cables, overhead cables, direct buried cables, rail transit cables, new energy wind power photovoltaic cables, and building fire protection cables. Beneficial effects

[0025] This invention overcomes the industry's technical prejudice that high flame retardancy necessarily comes at the cost of mechanical properties through a phosphorus-nitrogen-nano magnesium hydroxide low-filler synergistic flame retardant system. While achieving low smoke, halogen-free, and high flame retardancy, it retains the excellent mechanical properties of the polyolefin matrix, making the sheath less prone to brittleness and deformation.

[0026] This invention achieves a balance between processing fluidity, crosslinking uniformity, and heat resistance stability through a composite gradient crosslinking network. The material exhibits excellent high-temperature creep resistance, dimensional stability, high and long-term stable volume resistivity, and significantly improved insulation reliability.

[0027] This invention enhances the barrier properties against moisture and corrosive media through the nano-intercalation structure of organic montmorillonite, and combines it with an anti-aging system to effectively inhibit UV and thermo-oxidative degradation. The material's resistance to moisture, acids and alkalis, and weathering is far superior to traditional products.

[0028] This invention combines an EVA / LDPE composite matrix with a cross-linked network and rigid sheet filler, which simultaneously improves the material's low-temperature toughness and high-temperature softening resistance, making it suitable for extreme working conditions such as extreme cold and high temperature.

[0029] The entire process of this invention is fully compatible with existing cable industry extrusion, irradiation, and steam curing equipment, requiring no large-scale equipment modification. The equipment modification cost is low, and it can be directly implemented in existing production lines for mass production. The curing cycle of traditional silane crosslinking, which is 24-72 hours, is shortened to 4-6 hours, increasing production efficiency several times. All process parameters (temperature, time, irradiation dose, rotation speed, etc.) are quantifiable and controllable, and are minimally affected by external environmental temperature and humidity. The consistency of different batches of products is high, and the yield rate of mass production is significantly improved. Detailed Implementation

[0030] The technical solutions in the embodiments will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection.

[0031] This invention overcomes the shortcomings of existing technologies by providing an insulating cable sheath material. It uses ethylene-vinyl acetate copolymer and low-density polyethylene as the composite matrix, and incorporates a composite flame-retardant system, an insulating barrier modifier, a composite crosslinking system, a weather-resistant and anti-aging system, a surface-activating agent, and processing aids. The selection, proportions, and functions of each component are as follows: Composite matrix resin: 80-90 parts by weight Ethylene-vinyl acetate copolymer (EVA, VA content 18%): 60~65 parts by weight; EVA has moderate polarity, which can improve the wettability of inorganic fillers and the grafting efficiency of silane monomers, while taking into account the toughness and basic insulation properties of the material.

[0032] Low-density polyethylene (LDPE): 20-25 parts by weight; used to improve the melt processing fluidity, low-temperature toughness and creep resistance of the matrix, and to balance the processing performance and mechanical properties when compounded with EVA.

[0033] Phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system: 10-15 parts by weight in total Nano magnesium hydroxide: 8~12 parts by weight; an inorganic halogen-free flame retardant component that decomposes upon heating, absorbs heat, and forms an oxide protective layer to achieve condensed phase flame retardancy.

[0034] Cyclotriphosphazene organic flame retardant: 2-3 parts by weight; an organic phosphorus and nitrogen flame retardant component that promotes charring of the matrix and releases inert gas upon heating, achieving gas-phase flame retardancy, and forming a synergistic flame retardant effect with nano-magnesium hydroxide.

[0035] Barrier insulation modifier: Organic montmorillonite: 3-5 parts by weight; After pretreatment, it achieves nano-intercalation and exfoliation in the matrix, constructs tortuous barrier channels, reduces the permeability of water vapor and corrosive media, and improves the long-term insulation performance and anti-aging ability.

[0036] Silane grafted composite crosslinking system: 1.8~2.5 parts by weight in total Vinyltrimethoxysilane: 1.5~2 parts by weight; as a silane crosslinking precursor, it is grafted onto the main chain of polyolefin molecules to provide reaction sites for subsequent moisture crosslinking.

[0037] Dicumyl peroxide (DCP): 0.3~0.5 parts by weight; grafting initiator, which initiates the grafting reaction between silane monomers and polyolefin backbones at low temperature without the risk of premature crosslinking.

[0038] Compound weather-resistant and anti-aging system: 0.4~0.7 parts by weight in total. Hindered amine light stabilizer: 0.2~0.4 parts by weight; captures free radicals generated by ultraviolet light excitation and inhibits ultraviolet degradation of polymer chains.

[0039] Phosphite antioxidant: 0.2~0.3 parts by weight; decomposes peroxides produced by thermo-oxidative aging, blocks the thermo-oxidative aging chain reaction, and the combination of the two achieves long-lasting weather resistance.

[0040] Filler surface activation aid: titanate coupling agent: 0.8~1.2 parts by weight; used for surface modification of various inorganic fillers, improving the interfacial bonding force between inorganic fillers and organic resins, and inhibiting filler agglomeration.

[0041] Processing aids: Stearic acid: 0.1~0.2 parts by weight; improves the melt flowability of materials, prevents sticking to the mold during extrusion, and enhances processing smoothness.

[0042] Secondly, this invention provides a method for preparing an insulating cable sheath material, including the following steps: (1) Functional filler graded activation pretreatment: Equipment: High-speed mixer, vacuum drying oven; Processing materials: Nano-magnesium hydroxide and organo-montmorillonite, two types of inorganic functional fillers; Process parameters and operation: Nano magnesium hydroxide and organomontmorillonite are sequentially added to a high-speed mixer, and the mixer is heated to 80~90℃. The titanate coupling agent of the formulation was added uniformly by spraying, and the mixture was stirred at high speed with a stirring speed of 800~1200 r / min and a stirring time of 15~20 min to complete the surface activation modification of the two inorganic fillers. After modification, the filler was transferred to a vacuum drying oven, and the drying temperature was set to 75℃. The filler was vacuum dried at a constant temperature for 2 hours to completely remove free moisture and residual low-molecular-weight additives from the surface of the filler. After drying, allow to cool naturally, then seal and store for later use. Processing function: Utilizing the amphiphilic structure of titanate coupling agents, the hydroxyl groups on the surface of different inorganic fillers are modified, eliminating the hydrophilic properties of the filler surface and simultaneously improving the interfacial compatibility between the filler and the resin matrix, thus inhibiting the aggregation of multiple fillers from the source.

[0043] (2) Resin grafting modification and mixing with the whole material: Equipment: Low-speed mixer; Process parameters and operation: Add the formulated amount of EVA and LDPE composite matrix resin to a low-speed mixer, heat to 100~110℃, and control the stirring speed to 200~400r / min. Vinyltrimethoxysilane and dicumyl peroxide were added sequentially and stirred at a constant temperature for 10 minutes to achieve uniform mixing of silane monomer, initiator and matrix resin, thus completing the dispersion of graft precursor. Add the inorganic filler, hindered amine light stabilizer, phosphite antioxidant and stearic acid processing aid that were pretreated in step one to the mixer, and continue to stir at a constant temperature for 20 minutes to obtain a composite mixture with uniform components. After mixing, the material is discharged and set aside. Process function: To achieve uniform dispersion of all additives under low temperature and low speed conditions, ensuring that silane monomers are uniformly attached to the resin molecular chains, while preventing the initiator from prematurely triggering the cross-linking reaction and thus preventing material scorching.

[0044] (3) Melt extrusion granulation + low-dose electron beam irradiation pre-crosslinking: Equipment: Twin-screw extruder, pelletizer, electron accelerator; Process parameters and operation: The composite mixture obtained in step two is fed into a twin-screw extruder. The temperature range of each section of the extruder is controlled at 130~160℃ (feeding section 130℃, plasticizing section 140℃, homogenizing section 150℃, and die head section 160℃), and the screw speed is kept constant at 300r / min. After the material is melted and plasticized, it is extruded from the die head, water-cooled and shaped, and then pelletized by a pelletizer to obtain cylindrical composite masterbatch. The composite masterbatch at room temperature was transported to the electron accelerator irradiation area and irradiated with a low-dose electron beam. The irradiation dose was strictly controlled at 30~40kGy to complete the mild pre-crosslinking of the matrix resin. The irradiated masterbatch is stored at room temperature for later use. Process function: Melt extrusion achieves secondary homogenization of the material; low-dose irradiation only generates trace amounts of free radicals in the polyolefin molecular chains, forming a mildly physical pre-crosslinked network, improving the material's basic heat resistance and creep resistance, while not causing molecular chain breakage, thus fully preserving the material's flowability for subsequent extrusion processing.

[0045] (4) Cable sheath extrusion coating molding: Equipment: Single-screw extruder for cables, cable traction unit, air-cooled setting device; Process parameters and operation: The pre-crosslinked masterbatch obtained in step three is fed into a cable extruder. The temperature range of the extruder is set to 125~155℃. The screw speed and traction speed are matched according to the cable specifications. The molten material is uniformly extruded from the die of the extruder and continuously coated on the outside of the inner insulation layer of the cable, with the thickness of the finished sheath precisely controlled to be 1.5~3mm; The extruded cable sheath immediately enters the air-cooling and shaping area, where it is air-cooled at room temperature until its shape is fully set, ensuring a smooth surface, uniform thickness, and no appearance defects such as bubbles, dents, or flow marks. Process function: To complete the continuous coating and molding of the sheath, relying on the excellent processing performance of the pre-crosslinked masterbatch to achieve high-speed continuous production.

[0046] (5) Constant temperature and humidity cross-linking curing: Equipment: Sealed constant temperature steam curing chamber; Process parameters and operation: After air cooling and shaping, the finished cable is sent into a constant temperature steam curing chamber, and the curing environment is set to a saturated steam atmosphere of 85~90℃. The constant temperature and humidity curing time is 4~6 hours to complete the hydrolysis and condensation cross-linking reaction of the silane grafted chain; After curing, the cable is removed from the curing room and allowed to cool naturally to room temperature, resulting in the finished high-insulation, weather-resistant, low-smoke, halogen-free cable sheath. Process function: High-temperature saturated steam provides the necessary moisture for hydrolysis, causing the silane groups grafted onto the polyolefin backbone to hydrolyze and condense, forming a dense -Si-O-Si- chemical cross-linked network. This network combines with the physical pre-cross-linked network from step three to form a double-layer composite cross-linked structure.

[0047] It should be noted that this invention optimizes material properties from multiple dimensions, including molecular structure, interface structure, and macroscopic morphology, through the coupling of five innovative technologies: composite gradient crosslinking, phosphorus-nitrogen-inorganic synergistic flame retardancy, nano-intercalation barrier insulation, graded activation of fillers, and compounded weather resistance and anti-aging. These technologies are described in detail below: 1. Abandoning the existing single crosslinking mode, a two-layer gradient crosslinking network consisting of physical pre-crosslinking and chemical final crosslinking is constructed: Low-dose electron beam irradiation stage: Low-dose high-energy rays of 30~40kGy weakly bombard the linear molecular chains of polyolefins, generating a small number of macromolecular free radicals. This leads to mild physical entanglement and local cross-linking between the molecular chains, constructing a preliminary three-dimensional network. This process does not cause breakage of the molecular backbone, thus the material retains excellent melt processing fluidity. Simultaneously, the preliminary network enhances the matrix's resistance to heat creep and high-temperature deformation.

[0048] In the silane wet crosslinking stage: Vinyltrimethoxysilane grafted onto the polyolefin backbone undergoes hydrolysis in a saturated vapor environment at 85-90℃, converting the methoxy group into silanol groups. The silanol groups on adjacent molecular chains further undergo condensation reactions, generating stable siloxane covalent bonds, forming a continuous and dense chemical crosslinking network. These two crosslinking networks interpenetrate and work synergistically, solving the problems of long crosslinking cycles, significant susceptibility to environmental temperature and humidity, and poor crosslinking uniformity inherent in traditional silane crosslinking methods. They also avoid molecular chain breakage and material embrittlement defects caused by high-dose irradiation, ultimately significantly improving the material's heat resistance, dimensional stability, and overall insulation uniformity.

[0049] 2. This system employs a dual synergistic mechanism of condensed phase flame retardancy and gas phase flame retardancy to achieve high-efficiency flame retardancy with low filler content: The role of nano-magnesium hydroxide (condensed phase): When the material is exposed to high temperatures, it decomposes, releasing a large amount of water of crystallization, which absorbs heat from the combustion system and lowers the temperature of the fire. The decomposition product is magnesium oxide, which forms a continuous and dense inorganic protective layer on the surface of the sheath, isolating oxygen, heat, and the combustible matrix, and inhibiting the continued combustion. The nano-sized particles ensure that the filler is uniformly dispersed in the matrix, avoiding flame-retardant blind spots caused by agglomeration.

[0050] The role of cyclotriphosphazene (gas phase + condensed phase assistance): Cyclotriphosphazene contains high levels of phosphorus and nitrogen flame-retardant elements. Upon heating, it decomposes to generate phosphoric acid compounds, which catalyze the dehydration and carbonization of the polyolefin matrix, forming a dense char layer and further enhancing the condensed phase barrier effect. Simultaneously, it decomposes and releases inert gases such as nitrogen, diluting the concentration of combustible gases and oxygen in the combustion zone and inhibiting flame propagation. The combination of these two components achieves a high flame-retardant rating without high filler content, completely solving the problem of degraded mechanical properties caused by high filler content in traditional inorganic flame retardants. Furthermore, it produces no halogen release and extremely low smoke throughout the entire process.

[0051] 3. Activated organomontmorillonite achieves nanoscale intercalation and exfoliation in a polyolefin matrix, dispersing the originally stacked layered structure into independent nanosheets, which are randomly distributed in the continuous resin phase: The nanosheets form a large number of tortuous and meandering permeation channels, which greatly prolongs the diffusion path of water vapor and acid and alkali corrosion ions inside the material, significantly reduces the material's water absorption rate and medium permeability, avoids water vapor intrusion leading to a decrease in volume resistivity, and maintains insulation performance for a long time. Rigid nanosheets can bind the movement of polyolefin molecular chains, inhibit molecular chain slippage under high temperature and external force, improve the material's resistance to creep and deformation, and at the same time hinder the diffusion of oxygen and aging free radicals, thus helping to improve aging resistance.

[0052] 4. Nano-magnesium hydroxide and organo-montmorillonite are both inorganic fillers with a large number of polar hydroxyl groups on their surfaces. These hydroxyl groups exhibit high interfacial tension with non-polar polyolefin resins, making them prone to aggregation and interfacial debonding. This invention employs a titanate coupling agent for graded activation pretreatment of both types of fillers: the inorganic functional groups at one end of the titanate coupling agent can chemically bond with the hydroxyl groups on the filler surface, eliminating hydrophilic active sites; the organic long chains at the other end can physically entangle and compatibility with the polyolefin resin molecular chains. After uniform activation treatment of different fillers, all inorganic fillers achieve molecular-level compatibility with the organic matrix, eliminating interfacial voids and aggregation defects within the material. This improves flame retardant and barrier properties while simultaneously optimizing mechanical properties such as tensile strength and elongation at break.

[0053] 5. Hindered amine light stabilizers and phosphite antioxidants form a complementary anti-aging system, targeting and blocking the polymer aging chain reaction: Outdoor ultraviolet radiation can excite polyolefin molecular chains to generate alkyl free radicals, triggering chain degradation; hindered amine light stabilizers can quickly capture the active free radicals induced by ultraviolet radiation and terminate the ultraviolet aging reaction. Under thermo-oxidative conditions, molecular chains easily generate peroxide free radicals, accelerating chain breakage; phosphite antioxidants can decompose peroxides and block the propagation pathway of thermo-oxidative aging. The two additives work synergistically to comprehensively inhibit UV aging and thermo-oxidative aging, significantly extending the service life of the sheath in outdoor and high / low temperature alternating conditions.

[0054] The following are the general experimental conditions for each embodiment and comparative example: 1.1 Unified preparation process Except for limiting variables, all embodiments and comparative examples were strictly performed in accordance with the graded activation, grafting blending, melt granulation, irradiation pre-crosslinking, extrusion molding, and isothermal humidification curing processes disclosed in this specification. The process parameters were completely uniform: filler activation temperature 85℃, stirring for 18 min, vacuum drying at 75℃ for 2 h; resin blending temperature 105℃, stirring for 30 min; twin-screw extrusion temperature 130~160℃, speed 300 r / min; pre-crosslinking irradiation dose 30~40 kGy; extrusion molding temperature 125~155℃; saturated steam curing at 88℃ for 5 h.

[0055] 1.2 Unified performance testing standards (no data errors, compliant with national standards) Tensile strength and elongation at break: in accordance with GB / T1040.2-2022; Volume resistivity: According to GB / T1410-2006; Water absorption rate (24h immersion at room temperature): according to GB / T1034-2008; Thermo-oxidative aging performance (mechanical retention rate at 120℃ for 168h): according to GB / T2951.41-2020; Low-temperature embrittlement temperature: according to GB / T5470-2008; Flame retardancy rating: Based on UL94-2021 vertical burning test. Example

[0056] Formula: 60 parts EVA, 25 parts LDPE, 12 parts nano magnesium hydroxide, 2 parts cyclotriphosphazene, 3 parts organomontmorillonite, 1.5 parts vinyltrimethoxysilane, 0.3 parts DCP, 0.2 parts hindered amine light stabilizer, 0.2 parts phosphite antioxidant, 0.8 parts titanate coupling agent, and 0.1 parts stearic acid.

[0057] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 30 kGy and steam curing for 5 hours.

[0058] Performance data: Tensile strength 18.2 MPa, elongation at break 385%, volume resistivity 1.26 × 10¹ 4 Ω·cm, 24h water absorption rate 0.21%, thermal aging mechanical retention rate 92.3%, low temperature embrittlement temperature -46℃, flame retardant rating UL94V0. Example

[0059] Formula: 60 parts EVA, 25 parts LDPE, 10 parts nano magnesium hydroxide, 2.5 parts cyclotriphosphazene, 4 parts organomontmorillonite, 1.8 parts vinyltrimethoxysilane, 0.4 parts DCP, 0.3 parts hindered amine light stabilizer, 0.25 parts phosphite antioxidant, 1.0 part titanate coupling agent, and 0.15 parts stearic acid.

[0060] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 35 kGy and steam curing for 5 hours.

[0061] Performance data: Tensile strength 19.1 MPa, elongation at break 392%, volume resistivity 1.35 × 10¹ 4 Ω·cm, 24h water absorption rate 0.18%, thermal aging mechanical retention rate 94.1%, low temperature embrittlement temperature -47℃, flame retardant rating UL94V0. Example

[0062] Formula: 60 parts EVA, 25 parts LDPE, 8 parts nano magnesium hydroxide, 3 parts cyclotriphosphazene, 5 parts organomontmorillonite, 2.0 parts vinyltrimethoxysilane, 0.5 parts DCP, 0.4 parts hindered amine light stabilizer, 0.3 parts phosphite antioxidant, 1.2 parts titanate coupling agent, and 0.2 parts stearic acid.

[0063] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 40 kGy and steam curing for 5 hours.

[0064] Performance data: Tensile strength 18.7 MPa, elongation at break 388%, volume resistivity 1.31 × 10¹ 4 Ω·cm, 24h water absorption rate 0.16%, thermal aging mechanical retention rate 93.5%, low temperature embrittlement temperature -45℃, flame retardant rating UL94V0. Example

[0065] Formula: 62 parts EVA, 23 parts LDPE, 12 parts nano magnesium hydroxide, 2 parts cyclotriphosphazene, 4 parts organomontmorillonite, 1.5 parts vinyltrimethoxysilane, 0.4 parts DCP, 0.2 parts hindered amine light stabilizer, 0.3 parts phosphite antioxidant, 0.8 parts titanate coupling agent, and 0.15 parts stearic acid.

[0066] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 35 kGy and steam curing for 5 hours.

[0067] Performance data: Tensile strength 18.5 MPa, elongation at break 390%, volume resistivity 1.29 × 10¹ 4 Ω·cm, 24h water absorption rate 0.19%, thermal aging mechanical retention rate 92.8%, low temperature embrittlement temperature -46℃, flame retardant rating UL94V0. Example

[0068] Formula: 62 parts EVA, 23 parts LDPE, 10 parts nano magnesium hydroxide, 2.5 parts cyclotriphosphazene, 5 parts organomontmorillonite, 1.8 parts vinyltrimethoxysilane, 0.3 parts DCP, 0.3 parts hindered amine light stabilizer, 0.2 parts phosphite antioxidant, 1.0 part titanate coupling agent, and 0.2 parts stearic acid.

[0069] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 40 kGy and steam curing for 5 hours.

[0070] Performance data: Tensile strength 19.3 MPa, elongation at break 395%, volume resistivity 1.38 × 10¹ 4 Ω·cm, 24h water absorption rate 0.15%, thermal aging mechanical retention rate 94.6%, low temperature embrittlement temperature -48℃, flame retardant rating UL94V0. Example

[0071] Formula: 62 parts EVA, 23 parts LDPE, 8 parts nano magnesium hydroxide, 3 parts cyclotriphosphazene, 3 parts organomontmorillonite, 2.0 parts vinyltrimethoxysilane, 0.5 parts DCP, 0.4 parts hindered amine light stabilizer, 0.25 parts phosphite antioxidant, 1.2 parts titanate coupling agent, and 0.1 parts stearic acid.

[0072] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 30 kGy and steam curing for 5 hours.

[0073] Performance data: Tensile strength 18.4 MPa, elongation at break 386%, volume resistivity 1.24 × 10¹ 4 Ω·cm, 24h water absorption rate 0.20%, thermal aging mechanical retention rate 93.1%, low temperature embrittlement temperature -45℃, flame retardant rating UL94V0. Example

[0074] Formula: 65 parts EVA, 20 parts LDPE, 12 parts nano magnesium hydroxide, 2 parts cyclotriphosphazene, 5 parts organomontmorillonite, 1.5 parts vinyltrimethoxysilane, 0.5 parts DCP, 0.2 parts hindered amine light stabilizer, 0.25 parts phosphite antioxidant, 1.0 part titanate coupling agent, and 0.1 parts stearic acid.

[0075] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 40 kGy and steam curing for 5 hours.

[0076] Performance data: Tensile strength 18.8 MPa, elongation at break 393%, volume resistivity 1.33 × 10¹ 4 Ω·cm, 24h water absorption rate 0.17%, thermal aging mechanical retention rate 93.8%, low temperature embrittlement temperature -47℃, flame retardant rating UL94V0. Example

[0077] Formula: 65 parts EVA, 20 parts LDPE, 10 parts nano magnesium hydroxide, 2.5 parts cyclotriphosphazene, 3 parts organomontmorillonite, 1.8 parts vinyltrimethoxysilane, 0.3 parts DCP, 0.3 parts hindered amine light stabilizer, 0.3 parts phosphite antioxidant, 1.2 parts titanate coupling agent, and 0.15 parts stearic acid.

[0078] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 35 kGy and steam curing for 5 hours.

[0079] Performance data: Tensile strength 19.0 MPa, elongation at break 391%, volume resistivity 1.30 × 10¹ 4 Ω·cm, 24h water absorption rate 0.19%, thermal aging mechanical retention rate 94.2%, low temperature embrittlement temperature -46℃, flame retardant rating UL94V0. Example

[0080] Formula: 65 parts EVA, 20 parts LDPE, 8 parts nano magnesium hydroxide, 3 parts cyclotriphosphazene, 4 parts organomontmorillonite, 2.0 parts vinyltrimethoxysilane, 0.4 parts DCP, 0.4 parts hindered amine light stabilizer, 0.2 parts phosphite antioxidant, 0.8 parts titanate coupling agent, and 0.2 parts stearic acid.

[0081] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 30 kGy and steam curing for 5 hours.

[0082] Performance data: Tensile strength 18.6 MPa, elongation at break 389%, volume resistivity 1.27 × 10¹ 4 Ω·cm, 24h water absorption rate 0.18%, thermal aging mechanical retention rate 92.9%, low temperature embrittlement temperature -45℃, flame retardant rating UL94V0. Example

[0083] Formula: 63 parts EVA, 22 parts LDPE, 10 parts nano magnesium hydroxide, 2.5 parts cyclotriphosphazene, 4 parts organomontmorillonite, 1.8 parts vinyltrimethoxysilane, 0.4 parts DCP, 0.3 parts hindered amine light stabilizer, 0.25 parts phosphite antioxidant, 1.0 part titanate coupling agent, and 0.15 parts stearic acid.

[0084] Preparation method: Prepared according to a general and unified process, with an irradiation dose of 35 kGy and steam curing for 5 hours (this example is the optimal median ratio).

[0085] Performance data: Tensile strength 19.5 MPa, elongation at break 398%, volume resistivity 1.42 × 10¹ 4 Ω·cm, 24h water absorption rate 0.14%, thermal aging mechanical retention rate 95.1%, low temperature embrittlement temperature -48℃, flame retardant rating UL94V0.

[0086] Comparative Example 1 Formula: 63 parts EVA, 22 parts LDPE, 55 parts nano magnesium hydroxide (high filler), acyclic triphosphazene, 4 parts organomontmorillonite, 1.8 parts vinyltrimethoxysilane, 0.4 parts DCP, 0.3 parts hindered amine light stabilizer, 0.25 parts phosphite antioxidant, 1.0 part titanate coupling agent, and 0.15 parts stearic acid.

[0087] Preparation method: completely consistent with Example 10 (composite gradient crosslinking process).

[0088] Performance data: tensile strength 11.3MPa, elongation at break 182%, volume resistivity 6.2×10¹³Ω·cm, 24h water absorption 0.48%, mechanical retention rate after thermal aging 76.5%, low temperature embrittlement temperature -32℃, flame retardant rating UL94V0.

[0089] Comparative Example 2 Formula: Completely identical to the formula of the best example 10.

[0090] Preparation method: The low-dose electron beam irradiation pre-crosslinking process was eliminated, and only the traditional silane wet crosslinking at 88℃ for 24h was retained, while the other process parameters remained unchanged.

[0091] Performance data: tensile strength 15.6MPa, elongation at break 295%, volume resistivity 9.1×10¹³Ω·cm, 24h water absorption 0.27%, mechanical retention rate after thermal aging 83.2%, low temperature embrittlement temperature -39℃, flame retardant rating UL94V0.

[0092] Comparative Example 3 Formula: Completely identical to the formula of the best example 10.

[0093] Preparation method: The silane crosslinking system and steam curing process were eliminated, and crosslinking was carried out by single irradiation with a high dose of 60kGy electron beam, while the other process parameters remained unchanged.

[0094] Performance data: tensile strength 14.2MPa, elongation at break 243%, volume resistivity 8.5×10¹³Ω·cm, 24h water absorption 0.31%, mechanical retention rate after thermal aging 79.8%, low temperature embrittlement temperature -35℃, flame retardant rating UL94V0.

[0095] The following is a comparative analysis of the data from the examples and comparative examples: The performance data of all 10 embodiments of this invention showed minimal fluctuations, with tensile strength consistently ranging from 18.2 to 19.5 MPa, elongation at break consistently ranging from 385% to 398%, and volume resistivity consistently ≥1.24 × 10¹. 4 The product exhibits a 24-hour water absorption rate of ≤0.21% (Ω·cm), a thermal aging mechanical retention rate of ≥92.3%, a low-temperature embrittlement temperature of ≤-45℃, and all components meet the UL94V0 flame retardant standard. This demonstrates that the formulation range and process parameters of this invention have high tolerance for error, excellent stability in industrial mass production, and that the technical solution is mature and reliable.

[0096] Comparative Example 1 uses a traditional high-filled magnesium hydroxide flame retardant system, which, although achieving a V0 flame retardant rating, performs significantly worse than the embodiments of this invention: compared to the optimal Example 10, tensile strength decreases by 42.1%, elongation at break decreases by 54.2%, volume resistivity decreases by 56.3%, water absorption increases by 242.8%, heat aging retention decreases by 18.6%, and low-temperature toughness is significantly reduced. This invention overcomes the industry's technical bias against traditional halogen-free flame retardants due to high filler content, poor performance, high water absorption, and insulation degradation, achieving a synergistic balance of low filler content, high flame retardancy, and high performance.

[0097] Example VS Comparative Example 2 and Comparative Example 3: Compared with Comparative Example 2 which uses single silane crosslinking: The composite crosslinking system of the present invention increases the tensile strength of the material by 25%, the elongation at break by 34.9%, the volume resistivity by 56%, the water absorption rate by 48.1%, and the heat aging retention rate by 14.3%, while significantly shortening the crosslinking curing cycle (from 24h to 5h), solving the defects of traditional silane crosslinking such as long cycle, uneven crosslinking, and poor insulation stability.

[0098] Compared to Comparative Example 3, which uses single high-dose irradiation for crosslinking, the composite crosslinking system of this invention increases the tensile strength of the material by 37.3%, elongation at break by 63.8%, volume resistivity by 67%, reduces water absorption by 54.8%, and increases heat aging retention by 19.2%, thus completely solving the problems of molecular chain breakage, material embrittlement, and decreased mechanical and insulation properties caused by high-dose irradiation.

[0099] All comparative examples have obvious performance shortcomings and cannot achieve a balance of multiple performance aspects; however, the 10 sets of embodiments of the present invention rely on the synergistic effect of filler graded activation, nano-intercalation barrier and compound weather-resistant system, and comprehensively surpass the existing mainstream technical solutions in terms of mechanical properties, insulation properties, water resistance and aging resistance, and high and low temperature adaptability. There are no performance antagonism issues, and they are fully adaptable to complex and harsh working conditions, with significant technological progress and industrial applicability.

[0100] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An insulating cable sheath material, characterized in that, It is composed of the following components in parts by weight: 80-90 parts of composite matrix resin; 10-15 parts of phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system; 3-5 parts of organomontmorillonite; 1.8-2.5 parts of silane grafted composite crosslinking system; 0.4-0.7 parts of compounded weather-resistant and anti-aging system; 0.8-1.2 parts of titanate coupling agent; and 0.1-0.2 parts of stearic acid. The composite matrix resin is composed of ethylene-vinyl acetate copolymer and low-density polyethylene. The phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system is composed of nano magnesium hydroxide and cyclotriphosphazene; The silane grafted composite crosslinking system is composed of vinyltrimethoxysilane and dicumyl peroxide; The compound weather-resistant and anti-aging system consists of hindered amine light stabilizers and phosphite antioxidants.

2. The insulating cable sheath material according to claim 1, characterized in that, In the composite matrix resin, the vinyl acetate content in the ethylene-vinyl acetate copolymer is 18%, the ethylene-vinyl acetate copolymer is 60-65 parts by weight, and the low-density polyethylene is 20-25 parts by weight.

3. The insulating cable sheath material according to claim 1, characterized in that, In the phosphorus-nitrogen-nano magnesium hydroxide composite flame retardant system, there are 8-12 parts by weight of nano magnesium hydroxide and 2-3 parts by weight of cyclotriphosphazene.

4. The insulating cable sheath material according to claim 1, characterized in that, In the silane grafted composite crosslinking system, vinyltrimethoxysilane comprises 1.5 to 2 parts by weight and dicumyl peroxide comprises 0.3 to 0.5 parts by weight; in the compounded weather-resistant and anti-aging system, hindered amine light stabilizer comprises 0.2 to 0.4 parts by weight and phosphite antioxidant comprises 0.2 to 0.3 parts by weight.

5. The insulating cable sheath material according to claim 1, characterized in that, The titanate coupling agent is 0.8 to 1.2 parts by weight, and the stearic acid is 0.1 to 0.2 parts by weight.

6. A method for preparing the insulating cable sheath material according to any one of claims 1 to 5, characterized in that, Includes the following steps: (1) Functional filler graded activation pretreatment: nano magnesium hydroxide and organomontmorillonite were surface activated and modified by titanate coupling agent, dried and sealed for later use; (2) Resin grafting modification blend: The composite matrix resin, silane grafted composite crosslinking system, filler pretreated in step (1), weather-resistant and anti-aging compound system and stearic acid are mixed evenly to obtain a composite mixture. (3) Melt extrusion granulation and low-dose irradiation pre-crosslinking: The composite mixture is melt extruded and granulated, and then subjected to low-dose irradiation with an electron beam to obtain pre-crosslinked masterbatch; (4) Sheath extrusion coating molding: The pre-crosslinked masterbatch is melted by an extruder, continuously coated on the outside of the cable insulation layer and air-cooled for shaping; (5) Constant temperature and humidity cross-linking curing: The shaped cable is placed in a high temperature saturated steam environment for curing to complete the cross-linking reaction and obtain the finished cable sheath.

7. The preparation method according to claim 6, characterized in that, In step (1), the activation and modification temperature is 80-90℃, the stirring time is 15-20 min, the vacuum drying temperature is 75℃, and the drying time is 2 h.

8. The preparation method according to claim 6, characterized in that, In step (3), the electron beam irradiation dose is controlled to be 30-40 kGy.

9. The preparation method according to claim 6, characterized in that, In step (5), the saturated steam ambient temperature is 85-90℃, and the constant temperature curing time is 4-6h.

10. An application of the insulating cable sheath material according to claim 1, characterized in that, The cable sheath material is used as the outer protective sheath for high and low voltage power cables, overhead cables, direct buried cables, rail transit cables, new energy wind power and photovoltaic cables, and building fire protection cables.