High-flexibility flame-retardant composite fiber for cable and preparation process of high-flexibility flame-retardant composite fiber

Abstract

The invention discloses a high-flexibility flame-retardant composite fiber for a cable and a preparation process of the high-flexibility flame-retardant composite fiber, and belongs to the technical field of polymer composite fiber materials. The invention discloses a high-flexibility flame-retardant composite fiber for a cable and a preparation process of the high-flexibility flame-retardant composite fiber, the fiber is of a three-layer composite structure, the fiber comprises a core layer and a skin layer, the core layer is composed of ultra-high molecular weight polyethylene and a graphene / carbon fiber reinforcement phase, and the skin layer contains a low-density polyethylene-ethylene-vinyl acetate blend and a phosphorus-nitrogen halogen-free flame retardant; the interface fusion layer is an ethylene-acrylic acid copolymer; the elongation at break is larger than or equal to 600%, the limit oxygen index is larger than or equal to 35%, and the bending radius is smaller than or equal to 5 mm. The cable has the properties of high flexibility, flame retardance, bending resistance, high strength and the like, and is suitable for cable manufacturing in the fields of electric power, electronics and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high polymer composite fiber materials, and particularly relates to a high-flexibility and flame-retardant composite fiber for cables and a preparation process thereof. BACKGROUND

[0002] In modern power transmission, electronic equipment and communication systems, cables are the core carriers for energy and signal transmission, and their performance directly affects the safety and stability of the system. With the development of cable application scenarios towards complexity and high reliability (such as new energy vehicle wiring harness, intelligent building wiring, etc.), higher requirements are put forward for the fiber materials for cables: not only excellent flexibility to adapt to frequent bending, winding and other working conditions, but also strict flame retardant standards to avoid fire hazards, while taking into account mechanical strength and environmental adaptability. In the current existing technology, the following problems exist: performance contradiction is prominent, flexibility and flame retardancy are difficult to balance, the addition of flame retardant often makes the fiber hard and easy to break, which cannot meet the demand of high bending scene; structural design defects, lack of effective interface fusion layer design, the interlayer adhesion strength of multi-layer structure fiber is low, which is easy to delaminate during long-term use, resulting in mechanical property degradation; insufficient durability, large bending radius and low strength retention rate after cyclic bending, which cannot adapt to the mechanical stress during frequent movement or installation of cables; lack of environmental protection and safety, some flame retardant systems contain halogen, which releases harmful gases during combustion, and the limiting oxygen index (LOI) is less than 35%, which is difficult to pass the high flame retardant level certification.

[0003] Therefore, it is urgent to solve the problems of performance contradiction, structural defects and insufficient durability of existing materials, and to meet the stringent requirements of modern cables for safety, reliability and environmental adaptability. SUMMARY

[0004] The application provides a high-flexibility and flame-retardant composite fiber for cables with a three-layer composite structure, the core layer contains UHMWPE and graphene / carbon fiber, the skin layer is a LDPE-EVA blend and a phosphorus-nitrogen flame retardant, and the interface layer is an ethylene-acrylic acid copolymer, which is prepared by melt blending, composite spinning and two-stage heat stretching, and has a breaking elongation of ≥600%, an limiting oxygen index of ≥35%, and high flexibility, flame retardancy and bending resistance.

[0005] The application scheme is as follows: A kind of high flexible flame-retardant composite fiber for cable, it is characterized in that, the composite fiber is three-layer composite structure, including core layer, interface fusion layer and skin layer from inside to outside in sequence: the core layer is composed of ultra-high molecular weight polyethylene (UHMWPE) and reinforcing phase, the reinforcing phase is any one or combination of two of graphene, carbon fiber, and the mass ratio of reinforcing phase in core layer is 0.5% to 10%;The skin layer is composed of the blend of low-density polyethylene LDPE and ethylene-vinyl acetate copolymer EVA and phosphorus-nitrogen halogen-free flame retardant, wherein EVA content is 15% to 30%;The interface fusion layer is composed of ethylene-acrylic acid copolymer, between core layer and skin layer, 3% to 5% of the total mass of composite fiber.

[0006] Preferably, the molecular weight of the ultra-high molecular weight polyethylene is 3 million to 5 million, and the mass ratio in the core layer is 60% to 80%.

[0007] Preferably, the mass ratio of the phosphorus-nitrogen halogen-free flame retardant in the skin layer is 10% to 20%.

[0008] Preferably, the molecular weight of the ethylene-acrylic acid copolymer is 80,000 to 150,000.

[0009] Preferably, the content of acrylic monomer in the ethylene-acrylic acid copolymer is 8%-15%.

[0010] Preferably, the thickness ratio of the core layer to the skin layer is 1:1 to 3:1.

[0011] Preferably, when the reinforcing phase contains graphene, the graphene is a sheet structure, and the angle between the sheet plane and the fiber axial direction is ≤15°;When the reinforcing phase contains carbon fiber, the aspect ratio of the carbon fiber is ≥100:1;When the reinforcing phase contains a combination of graphene and carbon fiber, the above orientation and aspect ratio requirements are met simultaneously.

[0012] Preferably, the mass ratio of low-density polyethylene LDPE to ethylene-vinyl acetate copolymer EVA is 3:1 to 1:1.

[0013] Preferably, the diameter of the composite fiber is 0.2 to 0.4 mm.

[0014] A preparation process of a high flexible flame-retardant composite fiber for cable, comprising the following steps: ​S1, core layer spinning solution preparation: melt blend the ultrahigh molecular weight polyethylene, reinforcing phase and 40% to 60% of the total mass of the interface fusion layer in the composite fiber of ethylene-acrylic acid copolymer in a twin-screw extruder, temperature 180-220 DEG C; S2, skin layer spinning solution preparation: melt blend the blend of low density polyethylene LDPE and ethylene-vinyl acetate copolymer EVA, phosphorus-nitrogen halogen-free flame retardant and the remaining ethylene-acrylic acid copolymer in another twin-screw extruder, temperature 180-200 DEG C; S3, composite spinning: under the condition of 180-220 DEG C, 5-15 MPa, the core layer and the skin layer spinning solution are synchronously extruded by concentric spinning plate to form the primary fiber; S4, hot stretching: the primary fiber is stretched in two stages: first stage stretching: stretching at 70-90 DEG C, 2-3 times of stretching ratio, 10-20 m / min of stretching rate; second stage stretching: stretching at 90-110 DEG C, 5-8 times of stretching ratio, 5-10 m / min of stretching rate; S5, cooling and setting: the stretched fiber is naturally cooled in room temperature environment to prepare the composite fiber.

[0015] Compared with the prior art, the advantages of the present application are: (1) In the present application, the core layer in the three-layer composite structure is composed of ultrahigh molecular weight polyethylene and reinforcing phase, and the skin layer is adjusted in flexibility by LDPE-EVA blend system, so that the breaking elongation of the composite fiber is ≥600%, the bending radius is ≤5 mm, and the strength retention rate is ≥85% after 10 5 times of bending cycles, meeting the requirements of cable dynamic bending and wiring in narrow space; (2) In the present application, the skin layer contains phosphorus-nitrogen halogen-free flame retardant, the limiting oxygen index is ≥35%, no toxic gas is released during combustion; the overall density is 0.9-1.2 g / cm³ and the tensile strength is ≥500 MPa, having the characteristics of environmental protection and lightweight high strength; (3) In the present application, the three-layer structure is synchronously extruded by twin-screw melt blending and concentric spinning plate, and the two-stage hot stretching process optimizes the molecular orientation, the fiber diameter is controllable at 0.2-0.4 mm, and it is suitable for industrial mass production. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is a flowchart of the preparation process of a high-flexibility flame-retardant composite fiber for cable. DETAILED DESCRIPTION

[0017] The technical solutions of the embodiments of the present application will be explained and described below, but the following embodiments are only preferred embodiments of the present application, not all. Based on the embodiments in the embodiments, other embodiments obtained by those skilled in the art without creative labor also belong to the protection scope of the present application.

[0018] Example 1: composite fiber with 0.5% graphene in core layer and 15% EVA in skin layer 1. Material composition: Core layer: ultra-high molecular weight polyethylene with molecular weight of 3 million, mass ratio of 80%; graphene, mass ratio of 0.5%; additionally, 40% of the total mass of the interface fusion layer is ethylene-acrylic acid copolymer with a molecular weight of 80,000; skin layer: low-density polyethylene and ethylene-vinyl acetate copolymer blend, with EVA content of 15%; phosphorus-nitrogen halogen-free flame retardant, mass ratio of 10%; the remaining 60% is ethylene-acrylic acid copolymer; interface fusion layer: 3% of the total mass of the composite fiber is ethylene-acrylic acid copolymer with a molecular weight of 80,000; the thickness ratio of the core layer to the skin layer is 1:1.

[0019] 2. Preparation process: S1, core layer spinning solution preparation: add ultra-high molecular weight polyethylene, graphene, and 40% of the interface fusion layer to a double screw extruder for melt blending at 180°C; S2, skin layer spinning solution preparation: add LDPE and EVA blend, phosphorus-nitrogen flame retardant, and the remaining ethylene-acrylic acid copolymer to another double screw extruder for melt blending at 180°C; S3, composite spinning: simultaneously extrude the core layer and skin layer spinning solutions through a concentric spinneret at 180°C and 5 MPa to form the primary fiber; S4, hot stretching: primary stretching is performed at 70°C with a 2-fold stretching ratio and a speed of 10 m / min; secondary stretching is performed at 90°C with a 5-fold stretching ratio and a speed of 5 m / min; S5, cooling and setting: natural cooling at room temperature.

[0020] 3. Performance indicators: elongation at break 620%, limiting oxygen index 36%, bending radius 4.5 mm, 10 5 times bending cycle strength retention rate 88%, tensile strength 520 MPa.

[0021] Example 2: composite fiber with 5% graphene in core layer and 20% EVA in skin layer 1. Material composition: Core layer: ultra-high molecular weight polyethylene with a molecular weight of 4 million, mass ratio of 70%; graphene, mass ratio of 5%; 40% of the interface fusion layer is ethylene-acrylic acid copolymer with a molecular weight of 100,000; skin layer: LDPE and EVA blend, EVA content of 20%; phosphorus-nitrogen flame retardant, mass ratio of 15%; the remaining ethylene-acrylic acid copolymer; interface fusion layer: 4% of the total mass is ethylene-acrylic acid copolymer with a molecular weight of 100,000; the thickness ratio of the core layer to the skin layer is 2:1.

[0022] 2. Preparation process: S1, core layer dope preparation: twin-screw extruder temperature 200°C, remaining steps same as example 1; S2, skin layer dope preparation: twin-screw extruder temperature 190°C, remaining steps same as example 1; S3, composite spinning: temperature 200°C, pressure 10 MPa; S4, heat stretching: primary stretching 80°C, 2.5 times, 15 m / min; secondary stretching 100°C, 6 times, 8 m / min; S5, cooling setting: same as example 1.

[0023] 3. Performance indicators: elongation at break 650%, limiting oxygen index 37%, bending radius 4.0 mm, strength retention rate 89%, tensile strength 580 MPa.

[0024] Example 3: composite fiber with 10% graphene in core layer and 30% EVA in skin layer 1. Material composition: Core layer: ultra-high molecular weight polyethylene, molecular weight 5 million, mass percentage 60%; graphene, mass percentage 10%; 60% interface fusion layer ethylene-acrylic acid copolymer, molecular weight 150,000; skin layer: LDPE and EVA blend, EVA content 30%; phosphorus-nitrogen flame retardant, mass percentage 20%; remaining ethylene-acrylic acid copolymer; interface fusion layer: 5% ethylene-acrylic acid copolymer of total mass, molecular weight 150,000; Core layer to skin layer thickness ratio 3:1.

[0025] 2. Preparation process: S1, core layer dope preparation: temperature 220°C, remaining steps same as example 1; S2, skin layer dope preparation: temperature 200°C, remaining steps same as example 1; S3, composite spinning: temperature 220°C, pressure 15 MPa; S4, heat stretching: primary stretching 90°C, 3 times, 20 m / min; secondary stretching 110°C, 8 times, 10 m / min; S5, cooling setting: same as example 1.

[0026] 3. Performance indicators: elongation at break 610%, limiting oxygen index 38%, bending radius 3.5 mm, strength retention rate 87%, tensile strength 600 MPa.

[0027] Example 4: composite fiber with 1% carbon fiber in core layer and 18% EVA in skin layer 1. Material composition: Core layer: UHMWPE with molecular weight of 3.5 million, mass ratio of 75%; carbon fiber, mass ratio of 1%; 50% of the amount of ethylene-acrylic acid copolymer with molecular weight of 80,000 for interface fusion layer; skin layer: LDPE and EVA blend, EVA content of 18%; phosphorus-nitrogen flame retardant, mass ratio of 12%; the rest is ethylene-acrylic acid copolymer; interface fusion layer: 3.5% of the total mass of ethylene-acrylic acid copolymer with molecular weight of 80,000; the thickness ratio of core layer to skin layer is 1.5:1; 2. Preparation process: S1 to S5 parameters refer to Example 2, and the melting temperature is the middle value 200℃, the stretching ratio is 2.5 times for the first stage and 6 times for the second stage.

[0028] 3. Performance index: elongation at break 630%, limiting oxygen index 36%, bending radius 4.2mm, strength retention rate 86%, tensile strength 550MPa.

[0029] Example 5: composite fiber with 5% carbon fiber in core layer and 25% EVA in skin layer 1. Material composition: Core layer: UHMWPE with molecular weight of 4.5 million, mass ratio of 72%; carbon fiber, mass ratio of 5%; 50% of the amount of ethylene-acrylic acid copolymer with molecular weight of 120,000 for interface fusion layer; skin layer: LDPE and EVA blend, EVA content of 25%; phosphorus-nitrogen flame retardant, mass ratio of 18%; the rest is ethylene-acrylic acid copolymer; interface fusion layer: 4.5% of the total mass of ethylene-acrylic acid copolymer with molecular weight of 120,000; the thickness ratio of core layer to skin layer is 2.5:1.

[0030] 2. Preparation process: S1 core layer temperature 210℃, S2 skin layer temperature 195℃, S3 composite spinning temperature 210℃, pressure 12MPa; heat stretching first stage 85℃, 2.8 times, 18m / min, second stage 105℃, 7 times, 9m / min.

[0031] 3. Performance index: elongation at break 640%, limiting oxygen index 37%, bending radius 3.8mm, strength retention rate 88%, tensile strength 590MPa.

[0032] Example 6: composite fiber with 5% graphene and 5% carbon fiber in core layer and 22% EVA in skin layer 1. Material composition: Core layer: UHMWPE, molecular weight 3.8 million, mass ratio 70%; graphene, mass ratio 5%; carbon fiber, mass ratio 5%; 50% interface fusion layer ethylene-acrylic acid copolymer, molecular weight 100,000; skin layer: LDPE and EVA blend, EVA content 22%; phosphorus-nitrogen flame retardant, mass ratio 16%; the rest is ethylene-acrylic acid copolymer; interface fusion layer: 4% of the total mass of ethylene-acrylic acid copolymer, molecular weight 100,000; the thickness ratio of core layer to skin layer is 2:1.

[0033] 2. Preparation process: S1 core layer blending temperature 200℃, S2 skin layer temperature 190℃, S3 composite spinning temperature 200℃, pressure 10MPa; heat stretching first stage 80℃, 2.5 times, 15m / min, second stage 100℃, 6 times, 8m / min.

[0034] 3. Performance index: elongation at break 635%, limiting oxygen index 37%, bending radius 4.0mm, strength retention rate 89%, tensile strength 610MPa.

[0035] Example 7: composite fiber with 2% graphene in core layer, 15% EVA in skin layer and 20% flame retardant 1. Material composition: core layer: UHMWPE, molecular weight 4.2 million, mass ratio 78%; graphene, mass ratio 2%; 45% interface fusion layer ethylene-acrylic acid copolymer, molecular weight 90,000; skin layer: LDPE and EVA blend, EVA content 15%; phosphorus-nitrogen flame retardant, mass ratio 20%; the rest is ethylene-acrylic acid copolymer; interface fusion layer: 3.8% of the total mass of ethylene-acrylic acid copolymer, molecular weight 90,000; the thickness ratio of core layer to skin layer is 1:1.

[0036] 2. Preparation process: S2 skin layer temperature 200℃, the rest of the parameters refer to Example 2, stretching ratio first stage 2.5 times, second stage 5 times.

[0037] 3. Performance index: elongation at break 625%, limiting oxygen index 39%, bending radius 4.5mm, strength retention rate 87%, tensile strength 530MPa.

[0038] Example 8: composite fiber with 3% carbon fiber in core layer and 30% EVA in skin layer 1. Material composition: Core layer: UHMWPE with molecular weight of 4.8 million, mass ratio of 65%; carbon fiber, mass ratio of 3%; 55% interface fusion layer of ethylene-acrylic acid copolymer with molecular weight of 140 thousand; skin layer: LDPE and EVA blend with EVA content of 30%; phosphorus-nitrogen flame retardant, mass ratio of 10%; the rest is ethylene-acrylic acid copolymer; interface fusion layer: 4.2% of ethylene-acrylic acid copolymer with molecular weight of 140 thousand; the thickness ratio of core layer to skin layer is 3:1.

[0039] 2. Preparation process: S1 core layer temperature 210℃, S3 composite spinning temperature 210℃, pressure 12MPa; heat stretching first stage 85℃, 3 times, 18m / min, second stage 105℃, 7 times, 9m / min.

[0040] 3. Performance index: elongation at break 660%, limiting oxygen index 36%, bending radius 3.5mm, strength retention rate 88%, tensile strength 570MPa.

[0041] Comparative Example 1: composite fiber without reinforcing phase in core layer 1. Material composition: Core layer: UHMWPE with molecular weight of 3 million, mass ratio of 100%; 40% interface fusion layer of ethylene-acrylic acid copolymer with molecular weight of 80 thousand; skin layer: LDPE and EVA blend with EVA content of 20%; phosphorus-nitrogen flame retardant, mass ratio of 15%; the rest is ethylene-acrylic acid copolymer; interface fusion layer: 3% of ethylene-acrylic acid copolymer with molecular weight of 80 thousand; the thickness ratio of core layer to skin layer is 2:1.

[0042] 2. Preparation process: same as Example 2, no reinforcing phase is added in S1.

[0043] 3. Performance index: elongation at break 580%, limiting oxygen index 35%, bending radius 5.5mm, strength retention rate after 10 bending cycles 80%, tensile strength 480MPa. 5

[0044] Comparative Example 2: composite fiber with core layer graphene content exceeding 15% 1. Material composition: Core layer: UHMWPE with molecular weight of 5 million, mass ratio of 55%; graphene, mass ratio of 15%; 60% interface fusion layer of ethylene-acrylic acid copolymer with molecular weight of 150 thousand; skin layer, interface fusion layer and Example 3 are the same, the thickness ratio of core layer to skin layer is 3:1 2. Preparation process: same as Example 3.

[0045] ​3. Performance index: elongation at break 550%, limiting oxygen index 38%, bending radius 6.0 mm, strength retention rate 75%, tensile strength 520 MPa.

[0046] Comparative Example 3: composite fiber with insufficient 10% EVA content in the skin layer 1. Material composition: the core layer is the same as in Example 2, the skin layer is a blend of LDPE and EVA with an EVA content of 10%; phosphorus-nitrogen flame retardant, mass ratio 15%; ethylene-acrylic acid copolymer; the interface fusion layer is the same as in Example 2, the thickness ratio of the core layer to the skin layer is 2:1.

[0047] 2. Preparation process: same as in Example 2.

[0048] 3. Performance index: elongation at break 550%, limiting oxygen index 36%, bending radius 5.2 mm, strength retention rate 82%, tensile strength 550 MPa.

[0049] Comparative Example 4: composite fiber without interface fusion layer 1. Material composition: Core layer: ultra-high molecular weight polyethylene with a molecular weight of 4 million, mass ratio 70%; graphene, mass ratio 5%; skin layer: blend of LDPE and EVA with an EVA content of 20%; phosphorus-nitrogen flame retardant, mass ratio 15%; thickness ratio of core layer to skin layer is 2:1, without interface fusion layer.

[0050] 2. Preparation process: S1 to S2 are the same as in Example 2, S3 directly extrudes the core layer and the skin layer without adding interface fusion layer raw materials, S4 to S5 are the same as in Example 2.

[0051] 3. Performance index: elongation at break 600%, limiting oxygen index 37%, bending radius 4.0 mm, strength retention rate 80%, interface bonding strength 10 MPa.

[0052] Comparative Example 5: composite fiber with a core layer to skin layer thickness ratio of 4:1 exceeding the limit 1. Material composition: The core layer, skin layer, and interface fusion layer are the same as in Example 3, the thickness ratio of the core layer to the skin layer is 4:1.

[0053] 2. Preparation process: same as in Example 3, adjust the size of the core layer and skin layer flow channels of the spinneret.

[0054] 3. Performance index: elongation at break 590%, limiting oxygen index 38%, bending radius 7.0 mm, strength retention rate 78%, tensile strength 580 MPa.

[0055] Comparative Example 6: composite fiber without secondary stretching 1. Material composition: same as in Example 2.

[0056] 2. Preparation process: S1-S3 same as Example 2, S4 hot drawing only one-stage stretching (80℃, 2.5 times, 15m / min), without two-stage stretching, S5 cooling setting same as Example 2.

[0057] 3. Performance index: elongation at break 500%, limiting oxygen index 37%, bending radius 5.8mm, strength retention rate 75%, tensile strength 450MPa.

[0058] Performance test comparison summary Examples / Comparative Examples Elongation at break Limiting oxygen index Bending radius Strength retention rate Tensile strength Whether the whole meets the standard Standard requirement ≥600% ≥36% ≤4.5mm ≥85% ≥500MPa - Test method GB / T 1040.1-2018 GB / T 2406.2-2009 GB / T 1843-2008 GB / T 14344-2008 GB / T 1040.1-2018 - Example 1 620% (qualified) 36% (qualified) 4.5mm (qualified) 88% (qualified) 520MPa (qualified) Meets the standard Example 2 650% (qualified) 37% (qualified) 4.0mm (qualified) 89% (qualified) 580MPa (qualified) Meets the standard Example 3 610% (qualified) 38% (qualified) 3.5mm (qualified) 87% (qualified) 600MPa (qualified) Meets the standard Example 4 630% (qualified) 36% (qualified) 4.2mm (qualified) 86% (qualified) 550MPa (qualified) Meets the standard Example 5 640% (qualified) 37% (qualified) 3.8mm (qualified) 88% (qualified) 590MPa (qualified) Meets the standard Example 6 635% (qualified) 37% (qualified) 4.0mm (qualified) 89% (qualified) 610MPa (qualified) Meets the standard Example 7 625% (qualified) 39% (qualified) 4.5mm (qualified) 87% (qualified) 530MPa (qualified) Meets the standard Example 8 660% (qualified) 36% (qualified) 3.5mm (qualified) 88% (qualified) 570MPa (qualified) Meets the standard Comparative Example 1 580% (unqualified) 35% (unqualified) 5.5mm (unqualified) 80% (unqualified) 480MPa (unqualified) Does not meet the standard Comparative Example 2 550% (unqualified) 38% (qualified) 6.0mm (unqualified) 75% (unqualified) 520MPa (qualified) Does not meet the standard Comparative Example 3 550% (unqualified) 36% (qualified) 5.2mm (unqualified) 82% (unqualified) 550MPa (qualified) Does not meet the standard Comparative Example 4 600% (qualified) 37% (qualified) 4.0mm (qualified) 80% (unqualified) 550MPa (qualified) Does not meet the standard Comparative Example 5 590% (unqualified) 38% (qualified) 7.0mm (unqualified) 78% (unqualified) 580MPa (qualified) Does not meet the standard Comparative Example 6 500% (unqualified) 37% (qualified) 5.8mm (unqualified) 75% (unqualified) 450MPa (unqualified) Does not meet the standard Performance test summary I. Overall performance analysis 1. Key role of reinforcing phase: Examples 1-8 improve the comprehensive performance of composite fibers by adding graphene or carbon fiber; in Example 6, the tensile strength reaches 610MPa with 5% graphene and 5% carbon fiber, and the elongation at break is 635%, showing that the two reinforcing phases synergistically improve the mechanical properties; Comparative Example 1 does not add reinforcing phase, with a tensile strength of 480MPa and an elongation at break of 580%, and many performance indicators do not meet the standards, verifying the necessity of reinforcing phase.

[0059] 2. Effect of EVA content in skin layer: In Example 3, the EVA content is 30%, and the limiting oxygen index is 38%, the highest; in Comparative Example 3, the EVA content is 10%, and the limiting oxygen index is 36%, which meets the standard, but the elongation at break and bending performance do not meet the standard; increasing the EVA content appropriately can enhance the flame retardancy, but the relationship with the mechanical properties needs to be balanced, such as in Example 7, the EVA content is 15%, the limiting oxygen index is 39%, and the tensile strength is still 530MPa, which meets the standard.

[0060] 3. Determining role of process parameters: Comparative Example 6 does not perform two-stage stretching, with a tensile strength of 450MPa and an elongation at break of 500%, both of which are unqualified, proving the key role of two-stage hot stretching process in molecular chain orientation; by controlling the stretching temperature and times, the strength retention rate of Examples is ≥85%, verifying the accuracy of process parameters.

[0061] II. Performance differences between examples and comparative examples 1. Consistency of all examples meets the standards: all examples meet the full indicators of elongation at break ≥600%, limiting oxygen index ≥36%, bending radius ≤4.5mm, etc., among which Example 6 performs best in tensile strength 610MPa and strength retention rate 89%, and Example 7 is suitable for high flame retardant scenarios with limiting oxygen index 39%.

[0062] 2. Failure factors of the comparative examples: Comparative Example 2, graphene content 15%, reinforcing phase content exceeds the limit, resulting in elongation at break 550%, bending radius 6.0 mm unqualified; Comparative Example 4, interface fusion layer missing, although some indicators are qualified, but the strength retention rate is 80% unqualified; Comparative Example 5, thickness ratio imbalance reaches 4:1, resulting in bending radius 7.0 mm, strength retention rate 78% unqualified.

[0063] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only preferred examples of the present application and are not intended to limit the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A highly flexible and flame-retardant composite fiber for cables, characterized in that, The composite fiber has a three-layer composite structure, consisting of a core layer, an interface fusion layer, and a skin layer from the inside out: the core layer is composed of ultra-high molecular weight polyethylene (UHMWPE) and a reinforcing phase, wherein the reinforcing phase is any one or a combination of two of graphene and carbon fiber, and the mass percentage of the reinforcing phase in the core layer is 0.5% to 10%; the skin layer is composed of a blend of low-density polyethylene (LDPE) and ethylene-vinyl acetate copolymer (EVA) and a phosphorus-nitrogen halogen-free flame retardant, wherein the EVA content is 15% to 30%; the interface fusion layer is composed of ethylene-acrylic acid copolymer, located between the core layer and the skin layer, and accounts for 3% to 5% of the total mass of the composite fiber.

2. The highly flexible flame-retardant composite fiber for cables according to claim 1, characterized in that, The ultra-high molecular weight polyethylene has a molecular weight of 3 million to 5 million and accounts for 60% to 80% of the mass in the core layer.

3. The highly flexible flame-retardant composite fiber for cables according to claim 1, characterized in that, The phosphorus-nitrogen halogen-free flame retardant accounts for 10% to 20% of the mass of the skin layer.

4. The highly flexible flame-retardant composite fiber for cables according to claim 1, characterized in that, The molecular weight of the ethylene-acrylic acid copolymer is between 80,000 and 150,000.

5. The highly flexible flame-retardant composite fiber for cables according to claim 4, characterized in that, The ethylene-acrylic acid copolymer contains 8%-15% acrylic acid monomer.

6. The highly flexible flame-retardant composite fiber for cables according to claim 1, characterized in that, The thickness ratio of the core layer to the skin layer is 1:1 to 3:

1.

7. The highly flexible flame-retardant composite fiber for cables according to claim 1, characterized in that, When the reinforcing phase contains graphene, the graphene has a sheet-like structure and the angle between the sheet plane and the fiber axis is ≤15°; when the reinforcing phase contains carbon fiber, the aspect ratio of the carbon fiber is ≥100:1; when the reinforcing phase contains a combination of graphene and carbon fiber, the above orientation and aspect ratio requirements are simultaneously met.

8. The highly flexible flame-retardant composite fiber for cables according to claim 1, characterized in that, The mass ratio of the low-density polyethylene (LDPE) to the ethylene-vinyl acetate copolymer (EVA) is 3:1 to 1:

1.

9. The highly flexible flame-retardant composite fiber for cables according to claim 1, characterized in that, The diameter of the composite fiber is 0.2 to 0.4 mm.

10. A process for preparing a highly flexible and flame-retardant composite fiber for cables as described in any one of claims 1-9, comprising the following steps: S1. Core spinning solution preparation: Ultra-high molecular weight polyethylene, reinforcing phase, and ethylene-acrylic acid copolymer accounting for 40% to 60% of the total mass of the interface fusion layer in the composite fiber are melt-blended in a twin-screw extruder at a temperature of 180℃ to 220℃; S2. Sheet spinning solution preparation: A blend of low-density polyethylene (LDPE) and ethylene-vinyl acetate copolymer (EVA), a phosphorus-nitrogen halogen-free flame retardant, and the remaining ethylene-acrylic acid copolymer are melt-blended in another twin-screw extruder at a temperature of 180℃ to 200℃; S3. Composite spinning: Core and sheet spinning solutions are simultaneously extruded through concentric spinnerets at 180℃ to 220℃ and 5MPa to 15MPa to form nascent fibers; S4. Hot stretching: Nascent fibers are subjected to two-stage stretching: First-stage stretching: Stretching is performed at 70-90℃ at a stretching ratio of 2-3 times and a stretching rate of 10-20m / min; Secondary tensile test: Tensile test at 90-110℃, with a tensile ratio of 5-8 times and a tensile rate of 5-10 m / min. S5. Cooling and shaping: The stretched fibers are naturally cooled at room temperature to obtain the composite fibers.

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