Direct current cable using polypropylene insulation, and manufacturing method therefor

Abstract

The present invention relates to the field of the preparation of insulation materials for cables, and in particular to a direct current cable using polypropylene insulation, and a manufacturing method therefor. A polypropylene insulation material for the direct current cable provided by the present invention comprises the following components in parts by weight: 100 parts of PP, 5-15 parts of EPDM, and 1-5 parts of nano MgO. By improving the manufacturing process (increasing extrusion pressure and injection molding temperature, and performing stepwise cooling), the present invention mitigates poor dispersibility of EPDM particles on a PP matrix. By means of an EPDM elastomer to which a nanofiller is added, the flexibility and torsion resistance of polypropylene insulation are improved, and the interfacial polarization between the EPDM elastomer and polypropylene is mitigated, thereby ensuring that the electrical properties are also improved while the mechanical properties are improved.

Description

A polypropylene insulated DC cable and its preparation method Technical Field

[0001] This invention belongs to the field of cable insulation material preparation, specifically relating to a polypropylene insulated DC cable and its preparation method. Background Technology

[0002] In recent years, with economic development and the increasing demand for electricity, power systems have been continuously developing towards higher voltage levels to meet the needs of electricity load. High-voltage direct current (HVDC) cables are a key component of DC transmission networks, and polypropylene (PP) is a highly competitive and promising HVDC insulation cable material. It is a high-performance thermoplastic general-purpose plastic. Due to its excellent electrical insulation, light weight, chemical corrosion resistance, and heat resistance, PP is widely used in many fields such as medical devices, mechanical equipment parts, chemical containers, clothing, and automobiles. PP also exhibits better thermal stability, sufficient mechanical flexibility, and excellent electrical properties at high temperatures. Furthermore, PP is a major petrochemical product with low cost, and its thermoplastic properties make it easier to recycle and reprocess. Therefore, PP can be considered a promising recyclable and environmentally friendly DC cable insulation material. However, PP has drawbacks such as high elastic modulus, high rigidity, poor aging resistance, high low-temperature brittleness, and decreased electrical properties under high temperature and high electric field operating conditions, especially space charge accumulation under high temperature and high electric field conditions. These drawbacks make it unsuitable for the requirements of HVDC cable insulation materials.

[0003] Currently, the main methods of modification include hybridization, nanoparticle doping, nucleating agent modification, and chemical grafting. By modifying PP, its physical and chemical properties are improved to meet the requirements for use as an insulation material in high-voltage cables.

[0004] 1. Blending modification: The addition of elastomers reduces the rigidity and increases the toughness of PP, but the charge generated by the interfacial polarization between different phases and the charge injected by the electrodes will also reduce the dielectric properties of the material.

[0005] 2. Nanoparticle doping modification: When the contact between nanoparticles and polymer interfaces is poor and the interaction is weak, the composite material will separate from the polymer phase when subjected to strong external forces, thus destroying the integrity of the material.

[0006] 3. The β spherulite structure generated by adding β nucleating agent is loose and can dissipate energy by sliding when subjected to external force. In addition, the deep traps introduced by β can suppress space charge injection, thus achieving the effect of improving mechanical and dielectric properties at the same time. However, it is difficult to guarantee the dispersibility of nucleating agent in large-scale production.

[0007] Although the addition of EPDM elastomer reduces the rigidity and increases the toughness of PP, the addition of a large amount of elastomer will form an interface in the composite material. Due to the difference in relative permittivity and conductivity between the elastomer and the polymer matrix, polarization will occur at the interface. The polarization process will accumulate space charge, thereby causing electric field distortion. Furthermore, EPDM particles exhibit poor dispersibility on the PP matrix, with a large difference in particle size between particles. Summary of the Invention

[0008] This invention expands the application range of polypropylene by improving the preparation process of polypropylene and the selection of nano-doped elastomers. While improving the mechanical properties of polypropylene and ensuring that the electrical properties are not negatively affected, it also improves the problem of space charge accumulation in polypropylene insulation under high temperature and high field conditions.

[0009] To address the aforementioned technical problems, this application provides the following technical solution:

[0010] This invention provides a method for preparing a polypropylene insulated DC cable, comprising the following steps:

[0011] S11: Stranding to prepare the conductor core;

[0012] S12: A conductor shielding layer, a polypropylene insulation layer, and an insulation shielding layer are sequentially prepared from the inside to the outside of the conductor core by three-layer co-extrusion to obtain a cross-linked wire core; the polypropylene insulation layer is obtained by extruding nano-doped EPDM elastomer through an extruder; by weight, the nano-doped EPDM elastomer is obtained by mixing 100 parts of polypropylene (PP), 5-15 parts of ethylene propylene diene monomer (EPDM) rubber, and 1-5 parts of nano-magnesium oxide;

[0013] 1. Selection of elastomer:

[0014] (1) EVA

[0015] EVA molecules are interconnected within the PP matrix, excessively disrupting the regularity of PP molecules.

[0016] (2) EPDM

[0017] Although EPDM particles exhibit poor dispersibility and large particle size differences on PP matrix, this can be mitigated by nano-doping EPDM and then blending it with PP. In this case, the large amount of nano-filler incorporated into EPDM reduces the particle size difference, and the poor dispersibility of EPDM on PP matrix can be completely improved by adjusting the production process.

[0018] 2. Selection of nano-dopants:

[0019] Nano-ZnO, Al2O3, MgO, and TiO2 can all suppress space charge and improve the DC breakdown strength of insulation. However, nano-MgO can better alter the electrical properties of polypropylene (PP) and is more likely to be widely used as an insulation material for high-voltage DC cables.

[0020] Therefore, this invention uses EPDM and nano-MgO to modify polypropylene.

[0021] During the extrusion process, the temperature zones are set as follows: Zone 1 175℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 195℃, Zone 6 195℃, Zone 7 200℃, Zone 8 200℃, and Zone 9 200℃.

[0022] S13: The cross-linked core is cooled in zones through a cooling channel, with the temperature zones set as follows: Zone 1 190℃, Zone 2 160℃, Zone 3 130℃, Zone 4 120℃, Zone 5 110℃, Zone 6 100℃, Zone 7 60℃, and Zone 8 30℃.

[0023] S14: The cross-linked core surface after being cooled in sections in step S13 is subjected to metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion in sequence to obtain the polypropylene insulated DC cable.

[0024] Preferably, the extruder is selected from a 175-type screw extruder.

[0025] Preferably, the nano-doped EPDM elastomer is composed of 100 parts polypropylene, 15 parts EPDM rubber and 5 parts nano magnesium oxide by weight.

[0026] Preferably, the conductor shielding layer is obtained by extrusion using a 75-type screw extruder.

[0027] Furthermore, during the extrusion of the conductor shielding layer, the temperature zones are set as follows: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, and Zone 7 200℃.

[0028] Preferably, the insulating shielding layer is obtained by extrusion using a 90-type screw extruder.

[0029] Furthermore, during the extrusion of the insulating shielding layer, the temperature zones are set as follows: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, and Zone 6 200℃.

[0030] The present invention also provides a polypropylene insulated DC cable prepared by the above preparation method.

[0031] Through multiple experiments, the effects of factors such as extrusion temperature, cooling temperature gradient, and doping ratio on the blending effect during the preparation process were verified, thereby producing polypropylene-grade insulation with good mechanical and electrical properties.

[0032] The results show that the mechanical and electrical properties of polypropylene insulation are optimal when the ratio of 100 parts PP, 15 parts EPDM, and 5 parts MgO is used.

[0033] EPDM particles have poor dispersibility on PP matrix, and the particle size difference between particles is large.

[0034] The technical solution of the present invention has the following advantages compared with the prior art:

[0035] 1. The problem of poor dispersibility of EPDM particles on PP matrix can be solved by improving the preparation process (increasing extrusion pressure and injection temperature, and step cooling).

[0036] 2. By adding nanofillers to EPDM elastomers, the flexibility and torsional resistance of polypropylene insulation are improved, and the interfacial polarization between EPDM elastomers and polypropylene is reduced, thereby ensuring that electrical performance is improved while mechanical properties are also improved. Detailed Implementation

[0037] The present invention will be further described below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0038] Example 1

[0039] The standard procedure is as follows:

[0040] 1) By weight, 100 parts polypropylene, 15 parts EPDM rubber, and 5 parts nano magnesium oxide were used to prepare nano-doped EPDM elastomer.

[0041] 2) Stranding to prepare the conductor core;

[0042] 3) The cross-linked wire core is prepared using a three-layer co-extrusion production line; the conductor shielding layer, the insulating shielding layer, and the polypropylene insulation layer are prepared using a 75-type screw extruder, a 90-type screw extruder, and a 175-type screw extruder, respectively.

[0043] 4) The extrusion screw temperature for the conductor shielding layer is divided into 7 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, and Zone 7 200℃; 5) The polypropylene insulation layer uses modified polypropylene material, and the screw temperature is set in a stepped manner, specifically: Zone 1 175℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 195℃, Zone 6 195℃, Zone 7 200℃, Zone 8 200℃, and Zone 9 200℃;

[0044] 6) The extrusion screw temperature of the insulating shielding layer is divided into 6 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, and Zone 6 200℃.

[0045] 7) After the cross-linked core is prepared, it passes through the cooling channel. The temperature of each zone is as follows: Zone 1: 190℃, Zone 2: 160℃, Zone 3: 130℃, Zone 4: 120℃, Zone 5: 110℃, Zone 6: 100℃; Zone 7: 60℃; Zone 8: 30℃.

[0046] 8) The metal shielding, cabling, inner sheath extrusion, armoring, and outer sheath extrusion are performed sequentially to obtain the product cable.

[0047] Example 2

[0048] The standard procedure is as follows:

[0049] 1) By weight, 100 parts polypropylene, 5 parts EPDM rubber, and 1 part nano magnesium oxide were used to prepare nano-doped EPDM elastomer.

[0050] 2) Stranding to prepare the conductor core;

[0051] 3) The cross-linked wire core is prepared using a three-layer co-extrusion production line; the conductor shielding layer, the insulating shielding layer, and the polypropylene insulation layer are prepared using a 75-type screw extruder, a 90-type screw extruder, and a 175-type screw extruder, respectively.

[0052] 4) The extrusion screw temperature for the conductor shielding layer is divided into 7 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, and Zone 7 200℃; 5) The polypropylene insulation layer uses modified polypropylene material, and the screw temperature is set in a stepped manner, specifically: Zone 1 175℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 195℃, Zone 6 195℃, Zone 7 200℃, Zone 8 200℃, and Zone 9 200℃;

[0053] 6) The extrusion screw temperature of the insulating shielding layer is divided into 6 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, and Zone 6 200℃.

[0054] 7) After the cross-linked core is prepared, it passes through the cooling channel. The temperature of each zone is as follows: Zone 1: 190℃, Zone 2: 160℃, Zone 3: 130℃, Zone 4: 120℃, Zone 5: 110℃, Zone 6: 100℃; Zone 7: 60℃; Zone 8: 30℃.

[0055] 8) The metal shielding, cabling, inner sheath extrusion, armoring, and outer sheath extrusion are performed sequentially to obtain the product cable.

[0056] Example 3

[0057] The standard procedure is as follows:

[0058] 1) By weight, 100 parts polypropylene, 5 parts EPDM rubber, and 5 parts nano magnesium oxide were used to prepare nano-doped EPDM elastomer.

[0059] 2) Stranding to prepare the conductor core;

[0060] 3) The cross-linked wire core is prepared using a three-layer co-extrusion production line; the conductor shielding layer, the insulating shielding layer, and the polypropylene insulation layer are prepared using a 75-type screw extruder, a 90-type screw extruder, and a 175-type screw extruder, respectively.

[0061] 4) The extrusion screw temperature for the conductor shielding layer is divided into 7 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, and Zone 7 200℃; 5) The polypropylene insulation layer uses modified polypropylene material, and the screw temperature is set in a stepped manner, specifically: Zone 1 175℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 195℃, Zone 6 195℃, Zone 7 200℃, Zone 8 200℃, and Zone 9 200℃;

[0062] 6) The extrusion screw temperature of the insulating shielding layer is divided into 6 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, and Zone 6 200℃.

[0063] 7) After the cross-linked core is prepared, it passes through the cooling channel. The temperature of each zone is as follows: Zone 1: 190℃, Zone 2: 160℃, Zone 3: 130℃, Zone 4: 120℃, Zone 5: 110℃, Zone 6: 100℃; Zone 7: 60℃; Zone 8: 30℃.

[0064] 8) The metal shielding, cabling, inner sheath extrusion, armoring, and outer sheath extrusion are performed sequentially to obtain the product cable.

[0065] Comparative Example 1

[0066] The cooling gradient is the same as in Example 1, but decreases by 30°C for each step.

[0067] The specific steps are as follows:

[0068] 1) By weight, 100 parts polypropylene, 5 parts EPDM rubber, and 5 parts nano magnesium oxide were used to prepare nano-doped EPDM elastomer.

[0069] 2) Stranding to prepare the conductor core;

[0070] 3) The cross-linked wire core is prepared using a three-layer co-extrusion production line; the conductor shielding layer, the insulating shielding layer, and the polypropylene insulation layer are prepared using a 75-type screw extruder, a 90-type screw extruder, and a 175-type screw extruder, respectively.

[0071] 4) The extrusion screw temperature for the conductor shielding layer is divided into 7 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, and Zone 7 200℃; 5) The polypropylene insulation layer uses modified polypropylene material, and the screw temperature is set in a stepped manner, specifically: Zone 1 175℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 195℃, Zone 6 195℃, Zone 7 200℃, Zone 8 200℃, and Zone 9 200℃;

[0072] 6) The extrusion screw temperature of the insulating shielding layer is divided into 6 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, and Zone 6 200℃.

[0073] 7) After the cross-linked core is prepared, it passes through a cooling channel, and the cooling gradient decreases every 30°C.

[0074] 8) The metal shielding, cabling, inner sheath extrusion, armoring, and outer sheath extrusion are performed sequentially to obtain the product cable.

[0075] Comparative Example 2

[0076] Comparative Example 2: The above steps are exactly the same, except that EPDM without nano-MgO is added;

[0077] The specific steps are as follows:

[0078] 1) By weight, 100 parts polypropylene and 5 parts EPDM rubber were used to prepare nano-doped EPDM elastomers without the addition of nano magnesium oxide.

[0079] 2) Stranding to prepare the conductor core;

[0080] 3) The cross-linked wire core is prepared using a three-layer co-extrusion production line; the conductor shielding layer, the insulating shielding layer, and the polypropylene insulation layer are prepared using a 75-type screw extruder, a 90-type screw extruder, and a 175-type screw extruder, respectively.

[0081] 4) The extrusion screw temperature for the conductor shielding layer is divided into 7 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, and Zone 7 200℃; 5) The polypropylene insulation layer uses modified polypropylene material, and the screw temperature is set in a stepped manner, specifically: Zone 1 175℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 195℃, Zone 6 195℃, Zone 7 200℃, Zone 8 200℃, and Zone 9 200℃;

[0082] 6) The extrusion screw temperature of the insulating shielding layer is divided into 6 zones, with the following temperatures for each zone: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, and Zone 6 200℃.

[0083] 7) After the cross-linked core is prepared, it passes through a cooling channel, and the cooling gradient decreases every 30°C.

[0084] 8) The metal shielding, cabling, inner sheath extrusion, armoring, and outer sheath extrusion are performed sequentially to obtain the product cable.

[0085] Comparative Example 3

[0086] The steps are the same as in Example 1, with EPDM and MgO added directly to PP respectively.

[0087] Comparative Example 4

[0088] The steps are the same as in Example 1, except that the proportions of EPDM and MgO are changed; in this case, by weight, 100 parts of PP, 20 parts of EPDM, and 10 parts of MgO were used.

[0089] Effect Evaluation 1

[0090] This invention reveals that the crystallization behavior of EPDM and PP blends shows that the addition of EPDM increases the crystallization temperature and nucleation rate of the blend, indicating that EPDM has a nucleation effect on PP. The decreased crystal growth rate indicates a reduction in crystal size, meaning the spheres become smaller. Consequently, EPDM reduces the crystallinity of PP. Since the crystallization temperature of PP is around 115℃, this invention employs a more intensive, stepped cooling process between 100-130℃ to maintain its original crystallinity.

[0091] 1) Compared with Example 2, Example 1 shows that the preparation method of the present invention can not only improve the electrical properties of PP, but also improve the mechanical properties of PP.

[0092] 2) Comparative Example 1 shows that increasing the cooling gradient during cooling results in a certain degree of decrease in mechanical and electrical properties compared to Example 2. This is because the increased cooling temperature gradient leads to poorer crystallization of PP during crystallization, resulting in a higher proportion of amorphous regions.

[0093] 3) Comparative Example 2 shows that adding only EPDM elastomer can improve the mechanical properties of PP. However, because EPDM forms an interface with PP, polarization charges appear, which leads to a decrease in its electrical properties.

[0094] 4) Comparative Example 3 revealed that adding EPDM and MgO separately, instead of mixing them together beforehand, resulted in a certain degree of decrease in both electrical and mechanical properties compared to Example 2. This is because when the two are added separately, MgO cannot be evenly filled into the EPDM, which partially leads to the phenomenon described in 3), resulting in a decrease in electrical performance.

[0095] Table 1 Performance tests of the examples and comparative examples

[0096] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A process for the production of a polypropylene insulated direct current cable, characterized in that, Includes the following steps: S11: Stranding to prepare the conductor core; S12: A conductor shielding layer, a polypropylene insulation layer, and an insulation shielding layer are sequentially prepared from the inside to the outside of the conductor core using a three-layer co-extrusion process to obtain a cross-linked wire core; the polypropylene insulation layer is obtained by extruding nano-doped EPDM elastomer through an extruder; by weight, the nano-doped EPDM elastomer is obtained by mixing 100 parts of polypropylene, 5-15 parts of EPDM rubber, and 1-5 parts of nano-magnesium oxide; during extrusion, the temperature zones are set as follows: Zone 1 175℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 195℃, Zone 6 195℃, Zone 7 200℃, Zone 8 200℃, Zone 9 200℃; S13: The cross-linked core is cooled in zones through a cooling channel, with the temperature zones set as follows: Zone 1 190℃, Zone 2 160℃, Zone 3 130℃, Zone 4 120℃, Zone 5 110℃, Zone 6 100℃, Zone 7 60℃, and Zone 8 30℃. S14: The cross-linked core surface after being cooled in sections in step S13 is subjected to metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion in sequence to obtain the polypropylene insulated DC cable.

2. The production method according to claim 1, wherein The extruder is selected from a 175-type screw extruder.

3. The production method according to claim 1, wherein By weight, the nano-doped EPDM elastomer is composed of 100 parts polypropylene, 15 parts EPDM rubber and 5 parts nano magnesium oxide.

4. The production method according to claim 1, wherein The conductor shielding layer is obtained by extrusion using a 75-type screw extruder.

5. The production method according to claim 4, wherein When the conductor shielding layer is extruded, the temperature zones are set as follows: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, and Zone 7 200℃.

6. The production method according to claim 1, wherein The insulating shielding layer is obtained by extrusion using a 90-type screw extruder.

7. The production method according to claim 6, wherein When the insulating shielding layer is extruded, the temperature zones are set as follows: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, and Zone 6 200℃.

8. A polypropylene insulated DC cable prepared by the preparation method according to any one of claims 1-7.

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