Low-smoke halogen-free flame-retardant fire-resistant power cable and production method thereof

Abstract

The application belongs to the technical field of power cables, and particularly relates to a low-smoke halogen-free flame-retardant fire-resistant power cable and a production method thereof. The production method comprises the following steps: S1, mixing ethylene-vinyl acetate copolymer and polyolefin elastomer as a base material to obtain halogen-free flame-retardant polymer composition particles; S2, blending and extruding the composition particles in a double-screw extruder to form an insulation layer on the surface of a conductor; S3, wrapping two layers of mica tapes on the outer surface of the insulation layer to form a flame-retardant fire-resistant layer; S4, extruding a sheath layer on the outer surface of the flame-retardant fire-resistant layer through a single-screw extruder; and S5, irradiating and crosslinking the extruded cable. The application uses EVA and POE as the base material, and uses magnesium hydroxide and intumescent flame retardant in combination to ensure the flame-retardant grade, control the addition amount of the halogen-free flame retardant within 80-180 parts, reduce the addition amount by more than 20% compared with the traditional process, and increase the tensile strength and elongation at break of the cable by 15%-30%.

Description

Technical Field

[0001] This invention belongs to the field of power cable technology, and in particular relates to a low-smoke halogen-free flame-retardant and fire-resistant power cable and its production method. Background Technology

[0002] Traditional power cables release large amounts of dense smoke and halogen gases during fires, seriously threatening personnel safety and corroding equipment. Current technologies for halogen-free flame-retardant cables often use aluminum hydroxide or magnesium hydroxide as flame retardants, but large amounts can lead to a decrease in mechanical properties. Fire resistance typically relies on mica tape wrapping, but the interface between the mica tape and the insulation layer is prone to cracking at high temperatures. Furthermore, current processes often use peroxide thermal cross-linking to cross-link the insulation and sheath layers, which is energy-intensive and difficult to control in terms of cross-linking uniformity.

[0003] Therefore, developing a power cable and its production method that combines low smoke, halogen-free, flame retardant, and fire-resistant properties with strong process controllability has significant engineering application value. Summary of the Invention

[0004] The main objective of this invention is to provide a low-smoke, halogen-free, flame-retardant, and fire-resistant power cable and its manufacturing method, in order to overcome the shortcomings of the prior art.

[0005] To achieve the aforementioned objectives, the technical solution adopted by this invention includes: According to a first aspect of the present invention, a method for producing a low-smoke halogen-free flame-retardant and fire-resistant power cable is provided, comprising the following steps: S1. Mix ethylene-vinyl acetate copolymer (EVA) and polyolefin elastomer (POE) at a mass ratio of 3-5:1 as the base material, add halogen-free flame retardant, charring agent, smoke suppressant, crosslinking agent and silane coupling agent, and mix in an internal mixer at 110-130℃ for 30-50 minutes to obtain halogen-free flame retardant polymer composition particles.

[0006] S2. Ethylene-vinyl acetate copolymer and conductive carbon black are blended at a mass ratio of 100:20-35 in a twin-screw extruder with an aspect ratio of 30-40:1. The mixture is then extruded at 100-130°C onto the surface of a stranded copper conductor to form a semi-conductive shielding layer with a thickness of 0.5-1.0 mm. Subsequently, in the same twin-screw extruder (after cleaning), the halogen-free flame-retardant polymer composition particles obtained in step S1 are extruded onto the surface of the semi-conductive shielding layer to form an insulating layer. The melt temperature of the material is controlled at 130-140°C during the extrusion of the insulating layer.

[0007] S3. Wrap two layers of mica tape around the outer surface of the insulation layer to form a flame-retardant and fire-resistant layer, with an overlap rate of 30-40%.

[0008] S4. A sheath layer is extruded over the flame-retardant and fire-resistant layer using a single-screw extruder. The sheath layer material is the same as the insulation layer material, and the extrusion temperature is 130-160℃.

[0009] S5. After extrusion, the cable is subjected to radiation crosslinking with an irradiation dose of 80-150 kGy, and then naturally cooled to room temperature.

[0010] Furthermore, in step S1, the halogen-free flame retardant is a compound of magnesium hydroxide and an intumescent flame retardant. The intumescent flame retardant is composed of ammonium polyphosphate, pentaerythritol, and melamine in a mass ratio of 1:0.5:0.5 to 2:1:1, and the mass ratio of magnesium hydroxide to the intumescent flame retardant is 1:(1-3.5).

[0011] Furthermore, in step S1, the char-forming agent is dipentaerythritol, the smoke suppressant is ammonium octamolate, the crosslinking agent is dicumyl peroxide, and the silane coupling agent is vinyltrimethoxysilane.

[0012] Furthermore, in step S2, the barrel temperature of the twin-screw extruder is divided into four zones: feeding zone 110-120℃, compression zone 125-135℃, homogenization zone 140-150℃, and die head zone 130-140℃, with a screw speed of 150-300 rpm.

[0013] Furthermore, in step S3, the mica tape is phlogopite mica tape, and the tension applied when wrapping the mica tape is 10-30N. The ratio of the wrapping speed to the cable travel speed is 1.2:1 to 1.5:1.

[0014] Furthermore, after step S5, a post-processing step is included: the irradiated cross-linked cable is kept in an oven at 60-80°C for 4-6 hours, and then naturally cooled to room temperature to eliminate internal stress.

[0015] According to a second aspect of the present invention, a low-smoke halogen-free flame-retardant and fire-resistant power cable produced by the above-described method is provided, comprising a conductor, an insulation layer, a flame-retardant and fire-resistant layer, and a sheath layer. The insulation layer and the sheath layer are made of a halogen-free flame-retardant polymer composition. The composition, by weight, comprises: 100 parts of base material, 140-180 parts of halogen-free flame retardant, 10-25 parts of charring agent, 2-8 parts of smoke suppressant, 0.5-3 parts of crosslinking agent, and 0.5-2 parts of silane coupling agent. The base material is a mixture of ethylene-vinyl acetate copolymer and polyolefin elastomer, wherein the vinyl acetate content of the ethylene-vinyl acetate copolymer is 18%-28%.

[0016] Furthermore, the conductor is a stranded copper conductor, and a semi-conductive shielding layer is disposed on the surface of the conductor. The semi-conductive shielding layer is made by blending ethylene-vinyl acetate copolymer and conductive carbon black at a mass ratio of 100:20-35, and the thickness of the shielding layer is 0.5-1.0 mm.

[0017] Furthermore, the flame-retardant and fire-resistant layer consists of two overlapping layers of phlogopite mica tape, each layer of mica tape being 0.10-0.15mm thick, with the two layers of mica tape wrapped in opposite directions.

[0018] Furthermore, the thickness of the insulation layer is 2.0-4.0 mm, the thickness of the sheath layer is 1.5-3.0 mm, and the ratio of the thickness of the insulation layer to the thickness of the sheath layer is (1.25-1.33):1.

[0019] Compared with the prior art, the advantages of the present invention include: (1) This invention uses EVA and POE compound as base material, combined with magnesium hydroxide and intumescent flame retardant. While ensuring the flame retardant level (such as VW-1 or UL94V-0), the amount of halogen-free flame retardant added is controlled at 140-180 parts, which is more than 20% lower than the traditional process. The tensile strength and elongation at break of the cable are increased by 15%-30%.

[0020] (2) The present invention uses a twin-screw extruder for one-step mixing and granulation, and combines the extrusion of the insulation layer with twin-screw extrusion. The screw length-to-diameter ratio is 30-40:1 and four-section temperature control is used to improve the uniformity of dispersion of each component in the composition, reduce the surface roughness of the insulation layer, and reduce local defects.

[0021] (3) In step S3, the present invention uses two layers of mica tape wrapped in opposite directions with an overlap rate of 30%-40% and a tension control of 10-30N, so that the fire-resistant layer can be continuously powered for more than 90 minutes under a flame of 950℃, which is better than the 60 minutes of the traditional single-layer wrapping process.

[0022] (4) The present invention uses irradiation crosslinking instead of peroxide thermal crosslinking, with a crosslinking dose of 80-150kGy, which avoids the melt flow of the insulation layer and the displacement of the mica tape caused by high temperature crosslinking, improves the crosslinking uniformity, and reduces energy consumption.

[0023] (5) The heat preservation step after S5 of the present invention eliminates the internal stress generated by irradiation, thereby increasing the long-term operating temperature of the cable from 70°C to 90°C. Detailed Implementation

[0024] In view of the shortcomings of the prior art, the inventors of this invention, through long-term research and extensive practice, have proposed the technical solution of this invention. The following will further explain and illustrate this technical solution, its implementation process, and its principles.

[0025] The present invention will now be described in further detail with reference to specific embodiments. The given embodiments are merely illustrative of the invention and not intended to limit its scope. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation on the invention in any way.

[0026] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials used in the following embodiments are commercially available.

[0027] This invention provides a method for producing low-smoke halogen-free flame-retardant and fire-resistant power cables, comprising the following steps: Step S1: Base material compounding and granulation Base Material Formulation: This invention uses ethylene-vinyl acetate copolymer (EVA) and polyolefin elastomer (POE) mixed at a mass ratio of 3-5:1 as the base material. EVA has good polarity and compatibility with flame-retardant fillers. Its vinyl acetate segments can form hydrogen bonds with the surface of flame retardants, promoting filler dispersion. The introduction of POE is to improve the flexibility and low-temperature performance of the material, while reducing the mixing torque. The reason for controlling the mass ratio at 3-5:1 is that if the EVA ratio is too low (less than 3:1), the dispersibility of the flame retardant decreases, and particle agglomeration easily occurs on the surface of the insulation layer; if the POE ratio is too low (greater than 5:1), the rigidity of the material increases, and stress cracking easily occurs in the insulation layer when wrapping mica tape. The vinyl acetate content of EVA is preferably 18%-28%. Within this range, the crystallinity of EVA is moderate, which can ensure compatibility with flame retardants and maintain sufficient mechanical strength.

[0028] Halogen-free flame retardant: A compound of magnesium hydroxide and an intumescent flame retardant. Magnesium hydroxide decomposes at high temperatures, releasing water vapor and acting as an endothermic cooling agent, while the resulting magnesium oxide coats the material surface. The intumescent flame retardant, composed of ammonium polyphosphate, pentaerythritol, and melamine, forms an expanded char layer during combustion, isolating oxygen and heat. The mass ratio of the two compounds is 1:(1-3.5). This is significant because an excessively high proportion of magnesium hydroxide results in insufficient density of the expanded char layer, while an excessively high proportion of the intumescent flame retardant weakens the endothermic effect of magnesium hydroxide. The total addition amount of the compound system is 140-180 parts (relative to 100 parts of the base material). This range ensures a UL94V-0 flame retardant rating while avoiding excessive damage to mechanical properties.

[0029] Charring agent and smoke suppressant: The charring agent selected is dipentaerythritol, whose molecular structure contains multiple hydroxyl groups. At high temperatures, it can react with ammonium polyphosphate to form a cross-linked char layer, and the strength of the char layer is higher than that of pentaerythritol. The smoke suppressant selected is ammonium octamolate, which promotes the formation of the char layer and captures smoke particles during combustion, reducing the smoke density by more than 30%.

[0030] Crosslinking agent and silane coupling agent: The crosslinking agent used is dicumyl peroxide, whose decomposition temperature matches the subsequent irradiation crosslinking process, enabling the formation of a three-dimensional network structure. The silane coupling agent used is vinyltrimethoxysilane, which serves two purposes: firstly, it reacts with the hydroxyl groups on the surface of the inorganic flame retardant during the mixing process, improving the interfacial bonding between the filler and the matrix; secondly, it grafts onto the polymer chains during irradiation crosslinking, improving the crosslinking efficiency.

[0031] Mixing process: The internal mixer temperature is controlled at 110-130℃, and the mixing time is 30-50 minutes. This temperature range is below the decomposition temperature of dicumyl peroxide to avoid premature cross-linking. Too short a mixing time (less than 30 minutes) will result in uneven dispersion of the flame retardant, while too long a time (more than 50 minutes) will cause thermal degradation of the polymer. The particle size of the granulated composition is controlled at 2-5 mm to facilitate subsequent extrusion feeding.

[0032] Step S2: Twin-screw extrusion of the insulation layer Step S2 involves extruding the composition particles onto the conductor surface using a twin-screw extruder to form an insulating layer.

[0033] A screw aspect ratio of 30-40:1: A sufficiently high aspect ratio (≥30:1) ensures that the composition undergoes sufficient melting, mixing, and shearing within the screw, dispersing flame retardant agglomerates to the submicron level. When the aspect ratio is less than 30:1, the dispersion effect decreases, and flame retardant particles larger than 5μm appear in the insulation layer, becoming stress concentration points. When the aspect ratio is greater than 40:1, the material residence time is too long, increasing the risk of thermal degradation.

[0034] The barrel temperature is divided into four zones: the feeding zone (110-120℃) pre-plasticizes the granules without melting them; the compression zone (125-135℃) promotes filler wetting; the homogenization zone (140-150℃) ensures complete melting and thorough mixing of the composition; and the die head zone (130-140℃) regulates the extrusion pressure to prevent melt fracture. The screw speed is 150-300 rpm, which ensures shear dispersion while avoiding excessive shear heat that could lead to premature decomposition of the crosslinking agent.

[0035] Conductor and Insulation Layer: The conductor is a stranded copper conductor, and the insulation layer thickness is controlled at 2.0-4.0mm during extrusion. A semi-conductive shielding layer is set on the conductor surface, which is made of EVA and conductive carbon black blended at a mass ratio of 100:20-35, with a thickness of 0.5-1.0mm. Its function is to uniform the electric field and reduce tip discharge.

[0036] Step S3: Wrapping the refractory layer with mica tape Step S3 involves wrapping two layers of mica tape around the outer surface of the insulation layer to form a flame-retardant and fire-resistant layer. Phlogopite mica tape is selected because it maintains electrical insulation properties better than muscovite at high temperatures.

[0037] Two layers of mica tape are wrapped in opposite directions: the first layer is wrapped clockwise, and the second layer is wrapped counterclockwise, so that the fiber directions of the two layers of mica tape intersect each other. This structure avoids the formation of crack channels in the mica tape along the same direction when wrapped in a single layer, and even if one layer cracks in a fire, the other layer can still maintain electrical integrity.

[0038] Overlap rate 30-40%: When the overlap rate is less than 30%, weak points form at the wrapping gaps, and the mica tape shrinks at high temperatures, causing the gaps to widen. When the overlap rate is greater than 40%, the wrapping layer thickness increases excessively, affecting the cable's outer diameter and bending performance. Wrapping tension 10-30N: Too little tension will cause the mica tape to loosen, while too much tension will stretch the mica tape, resulting in a reduction in thickness. The ratio of wrapping speed to cable travel speed should be 1.2:1 to 1.5:1 to ensure uniform wrapping and that the mica tape adheres tightly to the insulation surface.

[0039] Step S4: Extrusion of the sheath layer Step S4 involves extruding a sheath layer over the flame-retardant and refractory layer using a single-screw extruder. The sheath layer material is the same as the insulation layer material; using the same material is preferred to simplify formulation management, but formulations with higher flame-retardant ratings can also be used. The extrusion temperature is 130-160°C, lower than the homogenization temperature in step S2, to avoid thermal shock to the formed mica tape.

[0040] Sheath thickness: controlled between 1.5-3.0mm, with the insulation thickness to sheath thickness ratio being (1.25-1.33):1. This ratio ensures a balance of forces between the inner and outer layers when the cable is bent, reducing the risk of delamination. If the insulation layer is too thin, the fire-resistant layer is prone to detachment during bending; if the sheath layer is too thin, mechanical protection is insufficient.

[0041] Step S5: Irradiation Crosslinking Step S5 uses an electron accelerator to irradiate and crosslink the extruded cable, replacing the traditional peroxide thermal crosslinking.

[0042] Irradiation dose of 80-150 kGy: This dose range achieves a crosslinking degree of 85%-95%. Below 80 kGy, crosslinking is insufficient, and the material softens and deforms at high temperatures; above 150 kGy, polymer chains break down and degrade, leading to decreased mechanical properties. The advantage of irradiation crosslinking is that it eliminates the need for heating, avoiding the problems of mica tape displacement and melt flow in the insulating layer at high temperatures, while also reducing energy consumption by approximately 40%.

[0043] Post-treatment: After irradiation cross-linking, the cable is kept in an oven at 60-80℃ for 4-6 hours and then naturally cooled to room temperature. This step aims to eliminate residual free radicals and internal stress generated by irradiation, increasing the long-term operating temperature to 90℃. Without post-treatment, residual free radicals will cause slow degradation when the cable operates above 70℃. To better understand the technical solution of this invention, the following detailed discussion is provided in conjunction with specific embodiments.

[0044] Example 1

[0045] S1: Mix EVA (VA content 22%) and POE at a mass ratio of 4:1 as the base material. Weigh 100 parts of the base material, 60 parts of magnesium hydroxide, 80 parts of intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine = 1.5:0.8:0.8), 15 parts of dipentaerythritol, 5 parts of ammonium octamolate, 2 parts of dicumyl peroxide, and 1.5 parts of vinyltrimethoxysilane. Put them into a mixer, set the temperature to 120℃, mix for 40 minutes, and granulate.

[0046] S2: Ethylene-vinyl acetate copolymer (VA content 22%) and conductive carbon black were blended in a twin-screw extruder (length-to-diameter ratio 35:1) at a mass ratio of 100:25. The mixture was then extruded at 115°C onto the surface of a stranded copper conductor (φ2.5mm) to form a semi-conductive shielding layer with a thickness of 0.7mm. Subsequently, in the same twin-screw extruder (after cleaning), the halogen-free flame-retardant polymer composition particles obtained in step S1 were extruded onto the surface of the semi-conductive shielding layer to form an insulating layer. During the extrusion process of the insulating layer, the barrel temperature was set as follows: feeding section 115°C, compression section 130°C, homogenization section 145°C, and die head section 135°C, with a screw speed of 200rpm. The melt temperature of the material was controlled at 135°C during the extrusion of the insulating layer.

[0047] S3: Wrap two layers of phlogopite tape (each layer 0.12mm thick) around the insulation layer. The two layers are wrapped in opposite directions, with an overlap rate of 35%, a wrapping tension of 20N, and a speed ratio of 1.35:1.

[0048] S4: Extrude the sheath layer using a single-screw extruder. The sheath layer material is the same as the insulation layer. The temperature is set to 140℃, and the sheath layer thickness is 2.0mm.

[0049] S5: Crosslinked by electron accelerator irradiation at a dose of 100 kGy, then naturally cooled to room temperature. Then placed in a 70°C oven for 5 hours and naturally cooled to room temperature.

[0050] Example 2

[0051] S1: Mix EVA (VA content 18%) and POE at a mass ratio of 3:1 as the base material. Weigh 100 parts of the base material, 90 parts of magnesium hydroxide, 90 parts of intumescent flame retardant, 10 parts of dipentaerythritol, 8 parts of ammonium octamolate, 0.5 parts of dicumyl peroxide, and 2 parts of vinyltrimethoxysilane, and put them into a mixer. Set the temperature to 110℃ and mix for 50 minutes, then granulate. The intumescent flame retardant is composed of ammonium polyphosphate, pentaerythritol, and melamine at a mass ratio of 1:0.5:0.5.

[0052] S2: Ethylene-vinyl acetate copolymer (VA content 18%) and conductive carbon black were blended in a twin-screw extruder (length-to-diameter ratio 30:1) at a mass ratio of 100:20. The mixture was then extruded at 100°C onto the surface of a stranded copper conductor (φ2.5mm) to form a semi-conductive shielding layer with a thickness of 1.0mm. Subsequently, in the same twin-screw extruder (after cleaning), the halogen-free flame-retardant polymer composition particles obtained in step S1 were extruded onto the surface of the semi-conductive shielding layer to form an insulating layer. During the extrusion process of the insulating layer, the barrel temperature was set as follows: feeding section 110°C, compression section 125°C, homogenization section 140°C, and die head section 130°C, with a screw speed of 150rpm. The melt temperature of the material was controlled at 130°C during the extrusion of the insulating layer.

[0053] S3: Wrap two layers of phlogopite tape (each layer 0.10mm thick) around the insulation layer. The two layers are wrapped in opposite directions with an overlap of 30%. The wrapping tension is 10N. The ratio of the wrapping speed to the cable travel speed is 1.2:1.

[0054] S4: Extrude the sheath layer using a single-screw extruder. The sheath layer material is the same as the insulation layer. The temperature is set to 130℃, and the sheath layer thickness is 1.5mm.

[0055] S5: Crosslinked by electron accelerator irradiation at a dose of 80 kGy, then naturally cooled to room temperature. It was then placed in a 60°C oven for 6 hours and naturally cooled to room temperature.

[0056] Example 3

[0057] S1: Mix EVA (VA content 28%) and POE at a mass ratio of 5:1 as the base material. Weigh 100 parts of the base material, 40 parts of magnesium hydroxide, 140 parts of intumescent flame retardant, 25 parts of dipentaerythritol, 2 parts of ammonium octamolate, 3 parts of dicumyl peroxide, and 0.5 parts of vinyltrimethoxysilane, and put them into a mixer. Set the temperature to 130℃ and mix for 30 minutes, then granulate. The intumescent flame retardant is composed of ammonium polyphosphate, pentaerythritol, and melamine at a mass ratio of 2:1:1.

[0058] S2: Ethylene-vinyl acetate copolymer (VA content 28%) and conductive carbon black were blended in a twin-screw extruder (length-to-diameter ratio 40:1) at a mass ratio of 100:35. The mixture was then extruded at 130°C onto the surface of a stranded copper conductor (φ2.5mm) to form a semi-conductive shielding layer with a thickness of 1.0mm. Subsequently, in the same twin-screw extruder (after cleaning), the halogen-free flame-retardant polymer composition particles obtained in step S1 were extruded onto the surface of the semi-conductive shielding layer to form an insulating layer. During the extrusion process of the insulating layer, the barrel temperature was set as follows: feeding section 120°C, compression section 135°C, homogenization section 150°C, and die head section 140°C, with a screw speed of 300rpm. The melt temperature of the material was controlled at 140°C during the extrusion of the insulating layer.

[0059] S3: Wrap two layers of phlogopite tape (each layer 0.15mm thick) around the insulation layer. The two layers are wrapped in opposite directions with an overlap of 40% and a wrapping tension of 30N. The ratio of wrapping speed to cable travel speed is 1.5:1.

[0060] S4: Extrude the sheath layer using a single-screw extruder. The sheath layer material is the same as the insulation layer. The temperature is set to 160℃, and the sheath layer thickness is 3.0mm.

[0061] S5: Crosslinked by electron accelerator irradiation at a dose of 150 kGy, then naturally cooled to room temperature. It was then placed in an 80°C oven for 4 hours and naturally cooled to room temperature.

[0062] Comparative Example 1 S1: EVA (VA content 22%) was used as the base material alone (POE not added). The remaining components were exactly the same as in Example 1: 100 parts base material, 60 parts magnesium hydroxide, 80 parts intumescent flame retardant (ammonium polyphosphate: pentaerythritol: melamine = 1.5:0.8:0.8), 15 parts dipentaerythritol, 5 parts ammonium octamolate, 2 parts dicumyl peroxide, and 1.5 parts vinyltrimethoxysilane. The mixing, granulation, and subsequent steps were the same as in Example 1.

[0063] S2~S5: Same as Example 1.

[0064] Note: When using EVA alone as the base material, the lack of elastic toughening effect of POE leads to uneven dispersion of flame retardants, resulting in decreased flame retardant performance, reduced mechanical properties, insufficient density of the char layer during combustion, significantly increased smoke density, and shortened fire resistance time.

[0065] Comparative Example 2 S1: The halogen-free flame retardant uses only 140 parts of magnesium hydroxide (replacing 60 parts of magnesium hydroxide + 80 parts of intumescent flame retardant in Example 1), with the other components remaining the same. The amount of magnesium hydroxide used is consistent with the total flame retardant dosage (140 parts) in Example 1.

[0066] S2~S5: Same as Example 1.

[0067] Note: Using only magnesium hydroxide as a flame retardant lacks the synergistic effect of intumescent flame retardants in forming char, resulting in the inability to form a dense char layer at high temperatures, low flame retardant efficiency, high smoke emission, and substandard fire resistance. Although the amount of magnesium hydroxide used is the same, the effect of a single flame retardant is far inferior to that of a compound system.

[0068] Comparative Example 3 S1~S4: Same as Example 1.

[0069] S5: Perform thermal cross-linking of the extruded cable in a vulcanizing tube at 175℃ for 20 minutes, followed by natural cooling. Post-processing steps are omitted.

[0070] Explanation: During thermal cross-linking, the high temperature reduces the melt viscosity of the insulation layer, causing the mica tape to shift under pressure and weakening the integrity of the fire-resistant layer. Insufficient cross-linking uniformity leads to softening of uncross-linked areas at high temperatures, resulting in fire-resistant failure. Irradiation cross-linking can be completed at low temperatures, avoiding the above problems, resulting in more uniform cross-linking and a higher long-term operating temperature for the cable.

[0071] The test results of Example 1 and Comparative Examples 1-3 are shown in Table 1: Table 1

[0072] The above results indicate that: ① Using EVA alone as the base material results in decreased flame retardant performance, reduced mechanical properties, insufficient char density during combustion, significantly increased smoke density, and shortened fire resistance time due to the lack of elastic toughening effect from POE and uneven dispersion of the flame retardant. ② Using magnesium hydroxide alone as the flame retardant lacks the synergistic effect of intumescent flame retardants in char formation, making it impossible to form a dense char layer at high temperatures, resulting in low flame retardant efficiency, high smoke release, and substandard fire resistance. Although the amount of magnesium hydroxide used is the same, the effect of a single flame retardant is far inferior to that of a compound system. ③ High temperatures during thermal crosslinking reduce the melt viscosity of the insulation layer, causing the mica tape to shift under compression, weakening the integrity of the fire-resistant layer. Insufficient crosslinking uniformity leads to softening of uncrosslinked areas at high temperatures, resulting in fire resistance failure. Irradiation crosslinking can be completed at low temperatures, avoiding the above problems, resulting in more uniform crosslinking and a higher long-term operating temperature for the cable. The above comparative examples, from the perspectives of base material composition, flame retardant compounding, and crosslinking method, respectively demonstrate the technical effects of the three key features of this invention: EVA and POE compounded base material, synergistic effect of magnesium hydroxide and intumescent flame retardant, and radiation crosslinking. The absence of any one of these features significantly reduces the overall performance of the cable.

[0073] The above descriptions are merely some embodiments of the present invention. It should be noted that those skilled in the art can make other modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for producing a low-smoke halogen-free flame-retardant and fire-resistant power cable, characterized in that, Includes the following steps: S1. Mix ethylene-vinyl acetate copolymer and polyolefin elastomer at a mass ratio of 3-5:1 as base material, add halogen-free flame retardant, charring agent, smoke suppressant, crosslinking agent and silane coupling agent, and mix in an internal mixer at 110-130℃ for 30-50 minutes, and granulate to obtain halogen-free flame retardant polymer composition particles. S2. Ethylene-vinyl acetate copolymer and conductive carbon black are blended at a mass ratio of 100:20-35 in a twin-screw extruder with an aspect ratio of 30-40:

1. The mixture is then extruded at 100-130°C onto the surface of a stranded copper conductor to form a semi-conductive shielding layer with a thickness of 0.5-1.0 mm. Subsequently, in the same twin-screw extruder, the halogen-free flame-retardant polymer composition particles obtained in step S1 are extruded onto the surface of the semi-conductive shielding layer to form an insulating layer. The melt temperature of the material is controlled at 130-140°C during the extrusion of the insulating layer. S3. Wrap two layers of mica tape around the outer surface of the insulation layer to form a flame-retardant and fire-resistant layer, with a wrapping overlap rate of 30-40%. S4. A sheath layer is extruded over the flame-retardant and fire-resistant layer using a single-screw extruder. The sheath layer material is the same as the insulation layer material, and the extrusion temperature is 130-160℃. S5. After extrusion, the cable is subjected to radiation crosslinking with an irradiation dose of 80-150 kGy, and then naturally cooled to room temperature.

2. The method according to claim 1, characterized in that, The halogen-free flame retardant mentioned in step S1 is a compound of magnesium hydroxide and an intumescent flame retardant. The intumescent flame retardant is composed of ammonium polyphosphate, pentaerythritol and melamine in a mass ratio of 1:0.5:0.5 to 2:1:

1. The mass ratio of magnesium hydroxide to the intumescent flame retardant is 1:(1-3.5).

3. The method according to claim 1, characterized in that, The char-forming agent in step S1 is dipentaerythritol, the smoke suppressant is ammonium octamolate, the crosslinking agent is dicumyl peroxide, and the silane coupling agent is vinyltrimethoxysilane.

4. The method according to claim 1, characterized in that, The barrel temperature of the twin-screw extruder described in step S2 is divided into four sections: feeding section 110-120℃, compression section 125-135℃, homogenization section 140-150℃, and die head section 130-140℃, with a screw speed of 150-300 rpm.

5. The method according to claim 1, characterized in that, The mica tape mentioned in step S3 is phlogopite mica tape. The tension applied when wrapping the mica tape is 10-30N, and the ratio of the wrapping speed to the cable travel speed is 1.2:1 to 1.5:

1.

6. The method according to claim 1, characterized in that, Step S5 is followed by a post-processing step: the irradiated cross-linked cable is kept in an oven at 60-80°C for 4-6 hours and then allowed to cool naturally to room temperature to eliminate internal stress.

7. A low-smoke halogen-free flame-retardant and fire-resistant power cable produced by the method according to any one of claims 1-6, comprising a conductor, an insulation layer, a flame-retardant and fire-resistant layer, and a sheath layer, characterized in that, The insulation layer and sheath layer are made of a halogen-free flame-retardant polymer composition, which comprises, by weight: 100 parts of base material, 140-180 parts of halogen-free flame retardant, 10-25 parts of charring agent, 2-8 parts of smoke suppressant, 0.5-3 parts of crosslinking agent, and 0.5-2 parts of silane coupling agent; the base material is a mixture of ethylene-vinyl acetate copolymer and polyolefin elastomer, wherein the vinyl acetate content of the ethylene-vinyl acetate copolymer is 18%-28%.

8. The low-smoke halogen-free flame-retardant and fire-resistant power cable according to claim 7, characterized in that, The conductor is a stranded copper conductor, and a semi-conductive shielding layer is provided on the surface of the conductor. The semi-conductive shielding layer is made by blending ethylene-vinyl acetate copolymer and conductive carbon black at a mass ratio of 100:20-35, and the thickness of the shielding layer is 0.5-1.0 mm.

9. The low-smoke halogen-free flame-retardant and fire-resistant power cable according to claim 7, characterized in that, The flame-retardant and fire-resistant layer consists of two overlapping layers of phlogopite mica tape, each layer having a thickness of 0.10-0.15 mm, with the two layers wrapped in opposite directions.

10. The low-smoke halogen-free flame-retardant and fire-resistant power cable according to claim 7, characterized in that, The thickness of the insulation layer is 2.0-4.0 mm, the thickness of the sheath layer is 1.5-3.0 mm, and the ratio of the thickness of the insulation layer to the thickness of the sheath layer is (1.25-1.33):1.

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