Oil-resistant halogen-free low-smoke flame-retardant polyolefin railway insulation cable material and preparation process thereof

Abstract

The present application relates to railway insulation cable material technical field, specifically, it relates to oil-resistant halogen-free low-smoke flame-retardant polyolefin railway insulation cable material and preparation process.It includes ethylene-vinyl acetate copolymer, nitrile rubber, metallocene polyethylene, intumescent flame retardant, maleic anhydride grafted metallocene polyethylene, nano montmorillonite, composite antioxidant and lubricant; by constructing the interpenetrating type elastic toughening network of nitrile rubber and metallocene polyethylene, the physical entanglement of molecular chain is used to improve the bending elastic recovery capacity of material, at the same time, the silane modified nano montmorillonite forms the " barrier maze " and prolongs the penetration path of oil molecules, the compatible interface layer of maleic anhydride grafted metallocene polyethylene and matrix is combined, so that the toughening network is uniformly dispersed, the molecular chain constraint caused by high filling of polar flame retardant is avoided, the oil resistance is improved, the flexibility is ensured, so that the structural stability and mechanical flexibility of cable in frequent bending scene are improved.

Description

Technical Field

[0001] This invention relates to the field of railway insulating cable materials, and more specifically, to oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulating cable materials and their preparation process. Background Technology

[0002] In the railway transportation sector, with continuous technological development and increasingly stringent operating environment requirements, railway cables need to possess a variety of superior properties. On the one hand, various oily substances are present in railway operations, such as locomotive lubricating oil and fuel oil. Cable insulation materials must have good oil resistance to prevent oil corrosion that could lead to decreased insulation performance and material damage, ensuring stable electrical performance during long-term use. On the other hand, cables are subjected to various bending and tensile mechanical forces during laying and use, especially in areas requiring frequent movement or bending, such as carriage connections and track bends. This necessitates that cables possess good bending performance to facilitate installation and long-term use, reducing material fatigue and damage caused by mechanical stress.

[0003] In existing technologies, polyolefin resins (such as EVA and PE) are typically used as the matrix, and related cable materials are prepared by adding inorganic flame retardants and combining them with traditional crosslinking processes. While this approach can achieve basic flame retardancy and oil resistance, it has significant shortcomings in performance synergy: the traditional high crosslinking density structure increases the rigidity of the material's molecular chains, which not only makes the cable prone to plastic deformation or cracking due to stress concentration when bent, affecting laying adaptability and long-term service reliability, but also reduces the mobility of the material's molecular chains due to excessive densification of the crosslinking network, resulting in a significant decrease in flexibility. This leads to problems such as surface roughness and internal stress residue in the insulation layer during extrusion molding. In addition, existing technologies often rely on high filler polar flame retardants or additives to improve oil resistance, which further exacerbates the binding effect of the material's molecular chains, resulting in weakened elastic recovery ability when bent, thereby reducing the cable's bending performance and mechanical flexibility.

[0004] Therefore, there is an urgent need for oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable materials and their preparation processes. Summary of the Invention

[0005] The purpose of this invention is to provide an oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material and its preparation process, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, firstly, this invention provides an oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material, comprising the following raw materials:

[0007] The composition includes: 45-55 parts ethylene-vinyl acetate copolymer (EVA), 20-30 parts nitrile rubber (NBR), 12-18 parts metallocene polyethylene (mPE), 10-15 parts intumescent flame retardant, 6-9 parts maleic anhydride-grafted metallocene polyethylene (mPE-g-MAH), 5-8 parts nano-montmorillonite, 0.8-1.2 parts composite antioxidant, and 1.0-1.5 parts lubricant; wherein:

[0008] The intumescent flame retardant includes ammonium polyphosphate, pentaerythritol and melamine in a ratio of 4:1:1. Through the synergistic effect of acid source, carbon source and gas source, an intumescent char layer is formed during combustion to block heat and oxygen.

[0009] The nitrile rubber and metallocene polyethylene form an interpenetrating elastic toughening network. The acrylonitrile groups in the nitrile rubber and the linear molecular chains of the metallocene polyethylene enhance the elastic recovery ability of the material when bent through physical entanglement, making it suitable for frequent bending scenarios of railway cables.

[0010] The maleic anhydride-grafted vinyl acetate groups in the metallocene polyethylene and ethylene-vinyl acetate copolymer form a compatible interface layer through polar interaction, which can enhance the compatibility of the metallocene polyethylene and ethylene-vinyl acetate copolymer matrix. This promotes the uniform dispersion of the interpenetrating elastic toughening network constructed by nitrile rubber and metallocene polyethylene in the ethylene-vinyl acetate copolymer matrix, thereby improving the oil resistance of the material and enhancing the bending elastic recovery ability of the cable material.

[0011] Secondly, according to Figure 1 As shown, this invention provides a preparation process for oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material, comprising the following steps:

[0012] S1. Surface modification of nano-montmorillonite: Nano-montmorillonite and silane coupling agent KH-560 are placed in a container. The amount of silane coupling agent is 3% of the mass of nano-montmorillonite. The mixture is dried in a vacuum environment at 80-90℃ for 2-2.5 hours. Through the hydrolysis and condensation reaction of the coupling agent, siloxane groups are added to the surface of the montmorillonite sheets, which improves the compatibility with the polyolefin matrix and forms a uniformly dispersed modified nano-montmorillonite.

[0013] S2. Pre-preparation of elastic toughening network: Nitrile rubber and metallocene polyethylene are fed into a two-roll mill and mixed at 100-120℃ for 15-20 minutes. Through the physical entanglement of acrylonitrile groups in nitrile rubber and linear molecular chains of metallocene polyethylene, an interpenetrating elastic toughening network is formed. The pre-dispersed masterbatch is obtained by water-cooled granulation.

[0014] S3. Synergistic blending of matrix and additives: Ethylene-vinyl acetate copolymer, maleic anhydride-grafted metallocene polyethylene and composite antioxidant are added to a high-speed mixer and premixed at 80-90℃ for 10-15 minutes. A compatible interface layer is formed by the polar interaction between the maleic anhydride groups of maleic anhydride-grafted metallocene polyethylene and the vinyl acetate groups of ethylene-vinyl acetate copolymer. Then, an intumescent flame retardant and lubricant are added and the mixture is continued for 5-10 minutes to form a uniformly dispersed matrix-additive initial mixture system.

[0015] S4. Composite System Melt Blending: The matrix-additive initial mixture, pre-dispersed masterbatch, and modified nano-montmorillonite are fed into a twin-screw extruder and melt-blended for 10-12 minutes at 160-180℃ and screw speed of 200-250rpm. Simultaneously, a four-zone gradient temperature control is adopted, which includes zone 1 (160-165℃) for melting the matrix, zone 2 (170-175℃) for dispersing the flame retardant, zone 3 (175-180℃) for promoting the exfoliation of nano-montmorillonite sheets, and zone 4 (165-170℃) for balancing the melt viscosity. This allows the interpenetrating elastic toughening network to be uniformly embedded in the ethylene-vinyl acetate copolymer matrix, and the nano-montmorillonite to form a sheet-like "barrier maze" structure. The intumescent flame retardant and inorganic filler are synergistically coated in the resin to construct a multi-scale oil-resistant and flame-retardant composite system melt blend.

[0016] S5. Melt Extrusion and Structure Control: After the blended melt is extruded through a die, it is processed in stages through a gradient temperature-controlled cooling system. It is first initially cooled and shaped in an environment of 30-35℃, and then gradually reduced to 20-25℃ for final quenching. This results in a multi-level synergistic structure of "elastic network-lamellar barrier-amorphous buffer" inside the material: the interpenetrating elastic toughening network constructed by nitrile rubber and metallocene polyethylene serves as a flexible skeleton, giving the material elastic recovery ability when bent; silane-modified nano-montmorillonite is uniformly dispersed in lamellar form to form a "barrier maze", extending the penetration path of oil molecules; the amorphous region in the ethylene-vinyl acetate copolymer matrix serves as a stress buffer layer to alleviate stress concentration during bending; then, the cooled melt strip is granulated by a pelletizing device at a speed of 800-1000 rpm to form granular finished products of 2-3 mm.

[0017] In this invention, an interpenetrating elastic toughening network is first constructed by mixing nitrile rubber and metallocene polyethylene in a two-roll mill. This network not only utilizes the physical entanglement between the acrylonitrile groups in the nitrile rubber and the linear molecular chains of the metallocene polyethylene to impart excellent flexural elastic recovery to the material, adapting to the frequent bending scenarios of railway cables, but also, through the compatibility interface layer formed by maleic anhydride-grafted metallocene polyethylene and ethylene-vinyl acetate copolymer, promotes the uniform dispersion of the elastic toughening network in the matrix. For example, silane-modified nano-montmorillonite constructs a layered "barrier maze"-like oil-resistant barrier, which can effectively improve the synergistic effect between the material's oil resistance and flexural flexibility, solving the performance imbalance problem in traditional technologies.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] In this oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material and its preparation process, an interpenetrating elastic toughening network of nitrile rubber and metallocene polyethylene is constructed. The physical entanglement of their molecular chains enhances the material's flexural elastic recovery capability, effectively solving the problem of easy cracking during bending caused by traditional high cross-linking density cables. This allows the cable to maintain structural stability even under frequent bending scenarios. Simultaneously, the "barrier maze" structure formed by silane-modified nano-montmorillonite extends the oil molecule penetration path. Combined with the compatible interface layer formed by maleic anhydride-grafted metallocene polyethylene and the matrix, the interpenetrating elastic toughening network constructed from nitrile rubber and metallocene polyethylene is uniformly dispersed, avoiding the molecular chain binding effect caused by high polar flame retardant filling. This improves both oil resistance and material flexibility. Furthermore, in terms of bending performance, the interpenetrating elastic toughening network enhances the material's flexural elastic recovery capability through the physical entanglement of molecular chains, overcoming the shortcomings of traditional high cross-linking density structures that increase molecular chain rigidity and susceptibility to cracking. This improves the structural stability and mechanical flexibility of the cable under frequent bending scenarios. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material of the present invention. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0022] 5g of nano-montmorillonite and 0.15g of silane coupling agent KH-560 were added to a container and dried in a vacuum environment at 80℃ for 2 hours to obtain modified nano-montmorillonite; 20g of nitrile rubber and 12g of metallocene polyethylene were added to a two-roll mill and mixed at 100℃ for 15 minutes, then water-cooled and granulated to obtain pre-dispersed masterbatch; 45g of ethylene-vinyl acetate copolymer, 6g of maleic anhydride-grafted metallocene polyethylene and 0.8g of composite antioxidant were added to a high-speed mixer and premixed at 80℃ for 10 minutes, then 10g of expanded polyvinyl chloride was added. Intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine = 4:1:1) and 1.0g lubricant were mixed for 5 min. The above initial mixture, pre-dispersed masterbatch, and modified nano-montmorillonite were fed into a twin-screw extruder and melt-blended at 200 rpm for 10 min at 160℃ in zone 1, 170℃ in zone 2, 175℃ in zone 3, and 165℃ in zone 4. The extruded melt was initially cooled to 30℃ and then quenched at 20℃. Finally, it was pelletized at 800 rpm to obtain 2-3 mm particles. Example

[0023] 6.5g of nano-montmorillonite and 0.195g of silane coupling agent KH-560 were added to a container and dried in a vacuum environment at 85℃ for 2.2h to obtain modified nano-montmorillonite; 25g of nitrile rubber and 15g of metallocene polyethylene were added to a two-roll mill and mixed at 110℃ for 17min, then water-cooled and granulated to obtain pre-dispersed masterbatch; 50g of ethylene-vinyl acetate copolymer, 9g of maleic anhydride-grafted metallocene polyethylene and 1.0g of composite antioxidant were added to a high-speed mixer and premixed at 85℃ for 12min, then 12g of... 5g of intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine = 4:1:1) and 1.25g of lubricant were mixed for 7 minutes. The above initial mixture, along with pre-dispersed masterbatch and modified nano-montmorillonite, was fed into a twin-screw extruder and melt-blended at 220 rpm for 11 minutes at 162℃ in zone 1, 172℃ in zone 2, 177℃ in zone 3, and 167℃ in zone 4. The extruded melt was initially cooled at 32℃ and then quenched at 22℃. Finally, it was pelletized at 900 rpm to obtain 2-3mm particles. Example

[0024] 8g of nano-montmorillonite and 0.24g of silane coupling agent KH-560 were added to a container and dried in a vacuum environment at 90℃ for 2.5h to obtain modified nano-montmorillonite; 30g of nitrile rubber and 16g of metallocene polyethylene were added to a two-roll mill and mixed at 120℃ for 20min, then water-cooled and granulated to obtain pre-dispersed masterbatch; 55g of ethylene-vinyl acetate copolymer, 7.5g of maleic anhydride-grafted metallocene polyethylene and 1.2g of composite antioxidant were added to a high-speed mixer and premixed at 90℃ for 15min, then 15g of [unspecified ingredient] was added. Intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine = 4:1:1) and 1.5g lubricant were mixed for 10 min. The above initial mixture, pre-dispersed masterbatch, and modified nano-montmorillonite were fed into a twin-screw extruder and melt-blended at 250 rpm for 12 min under the conditions of 165℃ in zone 1, 175℃ in zone 2, 180℃ in zone 3, and 170℃ in zone 4. The extruded melt was initially cooled at 35℃ and then quenched at 25℃. Finally, it was pelletized at 1000 rpm to obtain 2-3 mm particles. Example

[0025] 7g of nano-montmorillonite and 0.21g of silane coupling agent KH-560 were added to a container and dried in a vacuum environment at 88℃ for 2.3h to obtain modified nano-montmorillonite; 28g of nitrile rubber and 18g of metallocene polyethylene were added to a two-roll mill and mixed at 115℃ for 18min, then water-cooled and granulated to obtain pre-dispersed masterbatch; 48g of ethylene-vinyl acetate copolymer, 8g of maleic anhydride-grafted metallocene polyethylene and 0.9g of composite antioxidant were added to a high-speed mixer and premixed at 88℃ for 13min, then 13g of expanded polyvinyl chloride was added. Intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine = 4:1:1) and 1.3g lubricant were mixed for 8 minutes. The above initial mixture, pre-dispersed masterbatch, and modified nano-montmorillonite were fed into a twin-screw extruder and melt-blended at 230 rpm for 11.5 minutes under the conditions of 163℃ in zone 1, 173℃ in zone 2, 178℃ in zone 3, and 168℃ in zone 4. The extruded melt was initially cooled at 33℃ and then quenched at 23℃. Finally, it was pelletized at 950 rpm to obtain 2-3 mm particles.

[0026] Table 1. Amounts of each raw material used in Examples 1-4

[0027]

[0028] To verify that the insulating cable material prepared according to the embodiments of the present invention has good oil resistance and bending flexibility, the following test examples are used to illustrate the insulating cable material provided by the embodiments of the present invention.

[0029] Test case

[0030] The purpose of this test group is to investigate the effect of different component ratios on the insulating cable material, and to test the oil resistance, bending performance, flame retardancy and electrical insulation properties of the insulating cable material of this invention.

[0031] Experimental Objective: Experimental groups A, B, C, and D adopted the component ratios of the insulating cable materials provided in Examples 1-4, respectively; the control examples included control group A and control group B, wherein:

[0032] Control group A

[0033] The raw material composition is as follows: 50 parts of ethylene-vinyl acetate copolymer, 15 parts of metallocene polyethylene, 12.5 parts of intumescent flame retardant, 7.5 parts of maleic anhydride-grafted metallocene polyethylene, 6.5 parts of nano-montmorillonite, 1.0 part of composite antioxidant, and 1.25 parts of lubricant. During preparation, nano-montmorillonite and silane coupling agent KH-560 (3% of the nano-montmorillonite mass) are vacuum dried at 85℃ for 2.2h; ethylene-vinyl acetate copolymer, maleic anhydride-grafted metallocene polyethylene, and composite antioxidant are premixed at 85℃ for 12min, and then the flame retardant and lubricant are added and mixed for 7min; then, the mixture is fed into a twin-screw extruder with modified nano-montmorillonite and metallocene polyethylene, and melt-blended at 220rpm for 11min in zones 1-4 (162℃, 172℃, 177℃, and 167℃ respectively). After extrusion, the mixture is cooled from 32℃ to 22℃ and pelletized.

[0034] Control group B

[0035] The raw material composition is as follows: 50 parts ethylene-vinyl acetate copolymer, 25 parts nitrile rubber, 12.5 parts intumescent flame retardant, 2 parts dicumyl peroxide (crosslinking agent), 6.5 parts nano-montmorillonite, 1.0 part composite antioxidant, and 1.25 parts lubricant. During preparation, the nano-montmorillonite, after silane modification, is blended with the ethylene-vinyl acetate copolymer, nitrile rubber, flame retardant, antioxidant, and lubricant in a twin-screw extruder at 170°C. After extrusion, it is crosslinked by electron beam irradiation (dose 30 kGy) and then cooled and pelletized.

[0036] Control group C

[0037] The raw material composition is as follows: 50 parts of ethylene-vinyl acetate copolymer, 25 parts of nitrile rubber, 15 parts of metallocene polyethylene, 12.5 parts of intumescent flame retardant, 6.5 parts of nano-montmorillonite, 1.0 part of composite antioxidant, and 1.25 parts of lubricant. During preparation, after silane modification of the nano-montmorillonite, the nitrile rubber and metallocene polyethylene were open-milled at 110℃ for 17 min to obtain a pre-dispersed masterbatch; the ethylene-vinyl acetate copolymer and antioxidant were premixed at 85℃ for 12 min, the flame retardant and lubricant were added and mixed for 7 min, and then co-extruded with the pre-dispersed masterbatch and modified nano-montmorillonite (process as in Example 2), without the addition of maleic anhydride-grafted metallocene polyethylene.

[0038] Control group D

[0039] The raw material composition is as follows: 50 parts ethylene-vinyl acetate copolymer, 25 parts nitrile rubber, 15 parts metallocene polyethylene, 12.5 parts intumescent flame retardant, 7.5 parts maleic anhydride-grafted metallocene polyethylene, 1.0 part composite antioxidant, and 1.25 parts lubricant. During preparation, nano-montmorillonite is replaced with an equal mass of calcium carbonate. The remaining processes are the same as in Example 2, i.e., nitrile rubber and metallocene polyethylene are open-milled to prepare pre-dispersed masterbatch, and the matrix and additives are blended and then melt-extruded.

[0040] Control group E

[0041] The raw material composition is as follows: 50 parts ethylene-vinyl acetate copolymer, 25 parts nitrile rubber, 15 parts metallocene polyethylene, 12.5 parts intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine = 2:1:1), 7.5 parts maleic anhydride-grafted metallocene polyethylene, 6.5 parts nano-montmorillonite, 1.0 part composite antioxidant, and 1.25 parts lubricant. The preparation process is the same as in Example 2, only the ratio of acid source to carbon source and gas source in the intumescent flame retardant is adjusted.

[0042] control group F

[0043] The raw material composition is as follows: 50 parts linear low-density polyethylene, 25 parts nitrile rubber, 15 parts metallocene polyethylene, 12.5 parts intumescent flame retardant, 7.5 parts maleic anhydride-grafted metallocene polyethylene, 6.5 parts nano-montmorillonite, 1.0 part composite antioxidant, and 1.25 parts lubricant. In preparation, linear low-density polyethylene replaces the ethylene-vinyl acetate copolymer. Nitrile rubber and metallocene polyethylene are used to prepare pre-dispersed masterbatch through open milling. The remaining processes are the same as in Example 2, except that the melt blending temperature is increased to 180℃-200℃.

[0044] Test methods: Based on the oil resistance, bending properties, flame retardant properties, and electrical insulation properties of the insulated cable material according to this invention, tests were conducted respectively. Specific test methods are as follows:

[0045] Oil resistance: The sample (2 mm thick, 50 mm × 50 mm) was immersed in No. 15 mineral oil at 100°C for 168 hours. After immersion, the sample was removed, and the surface oil was blotted dry with filter paper. The weight change rate was calculated. ,in Weight before soaking The oil resistance is determined by the weight after immersion; a smaller absolute value of the change rate indicates better oil resistance. The oil resistance is evaluated by testing the tensile strength retention rate before and after immersion (≥80%) according to GB / T1040.3. .

[0046] Bending performance: The bending test was conducted according to GB / T2951.4-2008 "General Test Methods for Insulation and Sheath Materials of Cables and Optical Cables". The specimen was made into a 10mm wide insulating sheet and bent 100 times (bending angle 180°, frequency 2 times / s) around a cylindrical mandrel with a diameter 10 times the specimen thickness. After bending, the specimen was released, and the angle at which it returned to its original shape was measured. The calculation formula is as follows: ;in, The original angle (180°). The remaining bending angle after loosening indicates the higher the recovery rate and the better the flexibility. Surface cracking is observed using a 50x magnifying glass to check for cracks at the bent area, and is graded as "no cracks", "micro-cracks", and "obvious cracks".

[0047] Flame retardant performance: A vertical burning test (UL94V type) was conducted according to GB / T2408-2021 "Determination of the flammability of plastics - Horizontal and Vertical Methods". A sample with dimensions of 125mm × 13mm × thickness was placed in the test device, and a flame with a height of 20mm was applied for 10 seconds and then removed. The afterflame time, afterglow time, and burning drips were recorded. The flame retardant rating was required to reach V0 level (afterflame time of a single sample ≤ 10s, total afterflame time of 5 samples in each group ≤ 50s, and no burning drips igniting the cotton wool). At the same time, the oxygen index (LOI) was tested according to GB / T2406.2. The LOI was required to be ≥ 32%. The higher the oxygen index, the stronger the flame retardant performance.

[0048] Electrical insulation: Volume resistivity is tested according to GB / T1410-2006. A 2mm thick sample is prepared, a φ50mm electrode is used, a 100V DC voltage is applied, and the resistance value is measured after 1 minute. The volume resistivity should be ≥ To reflect the stability of the material's insulation performance; dielectric strength is tested according to GB / T1408.1-2016, with a sample thickness of 1 mm prepared, and the voltage is increased to breakdown at a rate of 2 kV / s. The breakdown voltage is recorded and expressed by the formula: ,in, The breakdown voltage is (kV). The sample thickness is (mm); calculate the dielectric strength, which should be ≥20kV / mm. The higher the value, the stronger the insulation withstand voltage.

[0049] Specific testing indicators are shown in Table 2.

[0050] Table 2 Detection indicators for each sample

[0051]

[0052] As shown in Table 2, the technical solution of this invention, through constructing an interpenetrating elastic toughening network of nitrile rubber and metallocene polyethylene, introducing a silane-modified nano-montmorillonite "barrier maze" structure, and a maleic anhydride grafted compatibility interface layer, outperforms existing technologies in terms of synergistic optimization of oil resistance, bending performance, flame retardancy, and electrical insulation. Specific effects are as follows:

[0053] Oil resistance: In the embodiments of this invention, the oil resistance weight change rate of test group AD was controlled at +0.5% to +1.2%, and the tensile strength retention rate reached 85% to 92%, which was better than control group A (+2.5%, 78%) and control group D (+3.2%, 75%). This is due to the physical entanglement network formed by the acrylonitrile groups in nitrile rubber and the linear molecular chains of metallocene polyethylene, combined with the layered "maze effect" of silane-modified nano-montmorillonite, which extended the oil molecule penetration path by 40% to 50%. In contrast, control group D lacked nano-montmorillonite, so oil molecules could directly penetrate through the gaps between calcium carbonate particles, resulting in a decrease in oil resistance. In the prior art, control group B used a traditional crosslinking process (diisopropylbenzene peroxide + irradiation). Although the weight change rate was -3.8%, the high crosslinking density destroyed the flexibility of the molecular chains, and the tensile strength retention rate was only 65%, which verified the contradiction between oil resistance and mechanical properties in the traditional scheme.

[0054] Bending performance: The elastic recovery rate of the embodiments of the present invention can reach 88%~95%, and there are no surface cracks after 100 bends, which is better than control group A (without the introduction of nitrile rubber toughening network, recovery rate of 75% and microcracks) and control group B (over-crosslinking, recovery rate of 60% and accompanied by obvious cracking). This is due to the "memory effect" formed by the physical entanglement of molecular chains of the interpenetrating elastic toughening network, which enables the elastic segments of nitrile rubber and the linear molecular chains of metallocene polyethylene to deform synergistically during bending and rebound quickly after unloading, avoiding stress concentration fracture caused by the rigid structure of traditional crosslinked networks. In terms of compatibility optimization, control group C has poor compatibility between nitrile rubber and EVA matrix because it does not add maleic anhydride-grafted metallocene polyethylene, and the uneven dispersion of toughening network leads to local stress concentration, with an elastic recovery rate of only 80%. In contrast, the present invention constructs a compatible interface layer through the polar interaction between maleic anhydride graft and EVA, reducing the dispersion particle size of toughening network from 5μm to below 2μm, improving uniformity and effectively ensuring the bending flexibility of the material.

[0055] Flame retardancy and insulation performance: This invention achieves improvements through multi-scale structural synergy: In terms of flame retardancy efficiency, the oxygen index reaches 34%~36%, and the vertical combustion rating reaches V0, which is superior to the control group E, which has an oxygen index of 33% and a combustion rating of V1 due to insufficient acid source ratio of intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine ≠ 4:1:1). This is due to the "char layer-lamellae" composite barrier structure formed by the intumescent flame retardant and nano-montmorillonite in this invention. During combustion, the montmorillonite lamellars insert into the char layer, improving the density, while the char layer expansion ratio of the control group E is reduced by 20%, resulting in a decrease in flame retardancy effect; In terms of electrical insulation, the volume resistivity is ≥1×10¹. 4 The dielectric strength is 22~24kV / mm, which fully meets the railway standard. In the control group F, after replacing EVA with linear low-density polyethylene, the dielectric strength dropped to 18kV / mm and the volume resistivity data was missing due to the poor compatibility between the non-polar chain of PE and nitrile rubber. This fully demonstrates that the EVA matrix is ​​irreplaceable in terms of insulation performance.

[0056] In summary, this invention, through the synergistic effect of an interpenetrating elastic toughening network, a "barrier labyrinth" structure, and a compatible interface layer, enables the cable material to achieve a weight change rate ≤ +1.2% in mineral oil at 100°C, a bending elastic recovery rate of 88%~95%, a flame retardant rating of V0, and a volume resistivity ≥ This effectively solves the problem of imbalance between oil resistance and flexibility in traditional technologies.

[0057] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation process for oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material, characterized in that, Includes the following steps: S1. Place nano-montmorillonite and silane coupling agent KH-560 into a container for drying. Through the hydrolysis and condensation reaction of the coupling agent, siloxane groups are added to the surface of the montmorillonite sheets to form modified nano-montmorillonite. S2. Nitrile rubber and metallocene polyethylene are put into a two-roll mill for mixing. Through the physical entanglement of acrylonitrile groups in nitrile rubber and linear molecular chains of metallocene polyethylene, an interpenetrating elastic toughening network is formed. Then, the pre-dispersed masterbatch is obtained by water-cooled granulation. S3. Add ethylene-vinyl acetate copolymer, maleic anhydride-grafted metallocene polyethylene and composite antioxidant to a high-speed mixer for premixing to form a compatible interface layer. Then add intumescent flame retardant and lubricant and continue mixing to form a matrix-additive initial mixture system. S4. The matrix-additive initial mixture system, pre-dispersed masterbatch and modified nano-montmorillonite are put into a twin-screw extruder for melt blending. At the same time, a four-zone gradient temperature control is adopted to make the interpenetrating elastic toughening network uniformly embedded in the ethylene-vinyl acetate copolymer matrix. The nano-montmorillonite forms a layered "barrier maze" structure. The intumescent flame retardant and inorganic filler are synergistically coated in the resin to construct a multi-scale oil-resistant and flame-retardant composite system blend melt. S5. After the blended melt is extruded through a die, it is processed in stages through a gradient temperature-controlled cooling system to form a multi-level synergistic structure of "elastic network - lamellar barrier - amorphous buffer" inside the material; then it is granulated by a pelletizing device at a speed of 800-1000 rpm to form granular finished products of 2-3 mm. In S3, the intumescent flame retardant includes ammonium polyphosphate, pentaerythritol and melamine, with a dosage ratio of 4:1:

1. The amount of the ethylene-vinyl acetate copolymer is 45-55 parts; the amount of the nitrile rubber is 20-30 parts; the amount of the metallocene polyethylene is 12-18 parts; the amount of the intumescent flame retardant is 10-15 parts; the amount of the maleic anhydride-grafted metallocene polyethylene is 6-9 parts; the amount of the nano-montmorillonite is 5-8 parts; the amount of the composite antioxidant is 0.8-1.2 parts; and the amount of the lubricant is 1.0-1.5 parts.

2. The preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material according to claim 1, characterized in that, In S1, the amount of silane coupling agent is 3% of the mass of nano-montmorillonite, and the drying treatment is carried out in a vacuum environment at 80-90℃ for 2-2.5 hours.

3. The preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material according to claim 1, characterized in that, In S2, the two-roll open mill is used to mix the materials for 15-20 minutes at 100-120℃.

4. The preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material according to claim 1, characterized in that, In step S3, the premix is ​​premixed at 80-90℃ for 10-15 minutes, and the intumescent flame retardant and lubricant are mixed for another 5-10 minutes.

5. The preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material according to claim 1, characterized in that, In S4, the twin-screw extruder is used for melt blending at 160-180℃ and 200-250rpm for 10-12 minutes.

6. The preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material according to claim 1, characterized in that, In S4, the four-zone gradient temperature control includes zone 1 (160-165℃) for melting the matrix, zone 2 (170-175℃) for dispersing the flame retardant, zone 3 (175-180℃) for promoting the exfoliation of nano-montmorillonite sheets, and zone 4 (165-170℃) for balancing the melt viscosity.

7. The preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulation cable material according to claim 1, characterized in that, In S5, the segmented processing requires initial cooling and shaping in an environment of 30-35℃, and then gradually reducing the temperature to 20-25℃ to complete the final quenching.

8. The oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulating cable material prepared by the preparation process of the oil-resistant, halogen-free, low-smoke, flame-retardant polyolefin railway insulating cable material according to any one of claims 1-7.

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