Polypropylene insulated direct-current cable and preparation method thereof

Abstract

The invention belongs to the field of cable insulation material preparation, and particularly relates to a polypropylene insulation direct current cable and a preparation method thereof. The direct-current cable polypropylene insulation material provided by the 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. According to the invention, the problem of poor dispersibility of EPDM particles on a PP matrix is solved by improving the preparation process (increasing the extrusion molding pressure and the injection molding temperature, and carrying out stepped cooling). By adding the EPDM elastomer of the nano filler, the flexibility and distortion resistance of polypropylene insulation are improved, and the interface polarization phenomenon between the EPDM elastomer and polypropylene is improved, so that the electrical performance can be improved while the mechanical performance is improved.

Description

Technical Field

[0001] The invention belongs to the field of cable insulation material preparation, and in particular relates to a polypropylene insulated DC cable and a preparation method thereof. Background Art

[0002] In recent years, with the economic development and the growing demand for electricity, the power system has been continuously developing towards higher voltage levels to meet the needs of power load. As a key part of the DC transmission network system, high-voltage DC cable, PP (polypropylene) is a highly competitive and potential high-voltage DC insulation cable material, and is a thermoplastic general-purpose plastic with excellent performance. PP is widely used in many fields of life such as medical devices, mechanical equipment parts, chemical containers, clothing, and automobiles due to its excellent electrical insulation, light specific gravity, chemical corrosion resistance, and heat resistance. PP also exhibits better thermal stability, sufficient mechanical flexibility, and excellent electrical properties at high temperatures. In addition, PP is a major product of petrochemicals with low cost, and the thermoplastic properties of PP make it easier to recycle and reprocess. Therefore, PP can be considered as a promising, recyclable, and environmentally friendly DC cable insulation material. However, due to its large elastic modulus, strong rigidity, poor anti-aging performance, high low-temperature brittleness, and working conditions under high temperature and high electric field, PP's electrical properties will decrease, especially the accumulation of space charge under high temperature and high electric field, which does not meet the requirements of high-voltage DC cable insulation materials.

[0003] At present, it is mainly modified by mixing, nanoparticle doping, nucleating agent and chemical grafting, etc. By modifying PP, its physical and chemical properties are improved to meet the requirements of high-voltage cable insulation materials.

[0004] 1. Blending modification: Due to the addition of elastomers, the rigidity of PP is reduced and the toughness is improved, but the charge generated by the interface polarization between different phases and the charge injected by the electrode will also reduce the dielectric properties of the material.

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

[0006] 3. The β-spherulite structure generated by adding β-nucleating agent is loose, and can slip to dissipate energy when subjected to external force. The deep traps introduced by β can inhibit space charge injection, thereby achieving the effect of improving both mechanical properties and dielectric properties. However, it is difficult to ensure the dispersion of the nucleating agent during large-scale production.

[0007] Although the addition of EPDM elastomer reduces the rigidity of PP and improves its toughness, the addition of a large amount of elastomer will form an interface in the composite material. Due to the difference in relative dielectric constant and conductivity between the elastomer and the polymer matrix, polarization will form at the interface. The polarization process will accumulate space charge, thereby causing electric field distortion. In addition, EPDM particles show poor dispersion on the PP matrix, and the particle size difference between particles is large. Summary of the invention

[0008] The invention improves the preparation process of polypropylene and the selection of nano-doped elastomers to expand its application scope, improves the mechanical properties of polypropylene, ensures that the electrical properties are not negatively affected, and improves the space charge accumulation problem of polypropylene insulation under high temperature and high field.

[0009] In order to solve the above-mentioned technical problems, this application provides the following technical solutions: The present invention provides a method for preparing a polypropylene insulated DC cable, comprising the following steps: S11: preparing the conductor core by twisting; S12: preparing a conductor shielding layer, a polypropylene insulation layer and an insulation shielding layer in sequence 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 a nano-doped EPDM elastomer through an extruder; the nano-doped EPDM elastomer is obtained by mixing 100 parts of polypropylene (PP), 5-15 parts of ethylene propylene diene monomer (EPDM) and 1-5 parts of nano-magnesium oxide by weight; 1. Selection of elastomer: (1) EVA EVA is interconnected in the PP matrix, which excessively destroys the regularity of the PP molecules.

[0010] (2) EPDM Although EPDM particles show poor dispersion on the PP matrix and the particle size difference between particles is large, it can be done by nano-doping EPDM and then blending it with PP. At this time, since EPDM is doped with a large amount of nanofillers, the difference between particles can be reduced, and the problem of poor dispersion of EPDM on the PP matrix can be completely improved by adjusting the production process.

[0011] 2. Selection of nano dopants: Nano ZnO, Al2O3, MgO and TiO2 can all suppress space charge and improve the DC breakdown strength of insulation. However, nano MgO can better change the electrical properties of PP and is more likely to be widely used as an insulation material for high-voltage DC cables.

[0012] Therefore, the present invention adopts EPDM and nano-MgO to modify polypropylene.

[0013] During the extrusion, the temperature zones are set as follows: zone 1 175°C, zone 2 180°C, zone 3 190°C, zone 4 195°C, zone 5 195°C, zone 6 195°C, zone 7 200°C, zone 8 200°C, zone 9 200°C; S13: Cooling the cross-linked wire core in different zones through a cooling channel, wherein the temperature zones are set as follows: zone 1 190° C., zone 2 160° C., zone 3 130° C., zone 4 120° C., zone 5 110° C., zone 6 100° C., zone 7 60° C., zone 8 30° C.; S14: metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion are sequentially performed on the surface of the cross-linked core after the zone cooling in the step S13 to obtain the polypropylene insulated DC cable.

[0014] Preferably, the extruder is selected from a 175-type screw extruder.

[0015] Preferably, the nano-doped EPDM elastomer consists of 100 parts of polypropylene, 15 parts of ethylene propylene diene monomer rubber and 5 parts of nano-magnesium oxide, by weight.

[0016] Preferably, the conductor shielding layer is obtained by extrusion through a 75-type screw extruder.

[0017] Furthermore, when the conductor shielding layer is extruded, the temperature zones are set as: 165°C in zone one, 180°C in zone two, 190°C in zone three, 195°C in zone four, 200°C in zone five, 200°C in zone six, and 200°C in zone seven.

[0018] Preferably, the insulating shielding layer is obtained by extruding through a 90-type screw extruder.

[0019] Furthermore, when the insulating shielding layer is extruded, the temperature zones are set as: 165°C for zone one, 180°C for zone two, 185°C for zone three, 195°C for zone four, 200°C for zone five, and 200°C for zone six.

[0020] The present invention also provides a polypropylene insulated DC cable prepared by the preparation method.

[0021] 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.

[0022] The results show that the mechanical and electrical properties of polypropylene insulation reach the optimum when the ratio is 100 parts PP, 15 parts EPDM and 5 parts MgO.

[0023] The dispersion of EPDM particles on the PP matrix is ​​poor, and the particle size difference between particles is large.

[0024] The technical solution of the present invention has the following advantages over the prior art: 1. The poor dispersion of EPDM particles on the PP matrix can be solved by improving the preparation process (increasing the extrusion pressure and injection temperature, step cooling).

[0025] 2. By adding nanofiller to EPDM elastomer, the flexibility and torsion resistance of polypropylene insulation are improved, and the interface polarization phenomenon between EPDM elastomer and polypropylene is improved, thereby ensuring that the electrical performance can be improved while improving the mechanical properties. DETAILED DESCRIPTION

[0026] The present invention is further described below in conjunction with specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention. Example 1

[0027] The standard steps are as follows: 1) By weight, 100 parts of polypropylene, 15 parts of ethylene propylene diene monomer rubber, and 5 parts of nano magnesium oxide were used to prepare a nano-doped EPDM elastomer; 2) preparing the conductor core by twisting; 3) The cross-linked wire core is prepared by a three-layer co-extrusion production line; the conductor shielding layer, the insulation shielding layer and the polypropylene insulation layer are respectively produced by a 75-type screw extruder, a 90-type screw extruder and a 175-type screw extruder; 4) The extrusion screw temperature of the conductor shielding layer adopts 7 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, Zone 7 200℃; 5) The polypropylene insulation layer adopts modified polypropylene material, and the screw temperature setting is stepped, 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℃, Zone 9 200℃; 6) The extrusion screw temperature of the insulating shielding layer adopts 6 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃; 7) After the cross-linked core is prepared, it passes through the cooling channel. The temperature of each zone is: Zone 1 190℃, Zone 2 160℃, Zone 3 130℃, Zone 4 120℃, Zone 5 110℃, Zone 6 100℃; Zone 7: 60℃; Zone 8: 30℃; 8) Carry out metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion in sequence to obtain the finished cable. Example 2

[0028] The standard steps are as follows: 1) By weight, 100 parts of polypropylene, 5 parts of ethylene propylene diene monomer rubber, and 1 part of nano magnesium oxide were used to prepare a nano-doped EPDM elastomer; 2) preparing the conductor core by twisting; 3) The cross-linked wire core is prepared by a three-layer co-extrusion production line; the conductor shielding layer, the insulation shielding layer and the polypropylene insulation layer are respectively produced by a 75-type screw extruder, a 90-type screw extruder and a 175-type screw extruder; 4) The extrusion screw temperature of the conductor shielding layer adopts 7 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, Zone 7 200℃; 5) The polypropylene insulation layer adopts modified polypropylene material, and the screw temperature setting is stepped, 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℃, Zone 9 200℃; 6) The extrusion screw temperature of the insulating shielding layer adopts 6 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃; 7) After the cross-linked core is prepared, it passes through the cooling channel. The temperature of each zone is: Zone 1 190℃, Zone 2 160℃, Zone 3 130℃, Zone 4 120℃, Zone 5 110℃, Zone 6 100℃; Zone 7: 60℃; Zone 8: 30℃; 8) Carry out metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion in sequence to obtain the finished cable. Example 3

[0029] The standard steps are as follows: 1) By weight, 100 parts of polypropylene, 5 parts of ethylene propylene diene monomer rubber, and 5 parts of nano magnesium oxide were used to prepare a nano-doped EPDM elastomer; 2) preparing the conductor core by twisting; 3) The cross-linked wire core is prepared by a three-layer co-extrusion production line; the conductor shielding layer, the insulation shielding layer and the polypropylene insulation layer are respectively produced by a 75-type screw extruder, a 90-type screw extruder and a 175-type screw extruder; 4) The extrusion screw temperature of the conductor shielding layer adopts 7 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, Zone 7 200℃; 5) The polypropylene insulation layer adopts modified polypropylene material, and the screw temperature setting is stepped, 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℃, Zone 9 200℃; 6) The extrusion screw temperature of the insulating shielding layer adopts 6 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃; 7) After the cross-linked core is prepared, it passes through the cooling channel. The temperature of each zone is: Zone 1 190℃, Zone 2 160℃, Zone 3 130℃, Zone 4 120℃, Zone 5 110℃, Zone 6 100℃; Zone 7: 60℃; Zone 8: 30℃; 8) Carry out metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion in sequence to obtain the finished cable.

[0030] Comparative Example 1 The steps are the same as those in Example 1, except that the cooling gradient is decreased by 30°C; The specific steps are as follows: 1) By weight, 100 parts of polypropylene, 5 parts of ethylene propylene diene monomer rubber, and 5 parts of nano magnesium oxide were used to prepare a nano-doped EPDM elastomer; 2) preparing the conductor core by twisting; 3) The cross-linked wire core is prepared by a three-layer co-extrusion production line; the conductor shielding layer, the insulation shielding layer and the polypropylene insulation layer are respectively produced by a 75-type screw extruder, a 90-type screw extruder and a 175-type screw extruder; 4) The extrusion screw temperature of the conductor shielding layer adopts 7 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, Zone 7 200℃; 5) The polypropylene insulation layer adopts modified polypropylene material, and the screw temperature setting is stepped, 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℃, Zone 9 200℃; 6) The extrusion screw temperature of the insulating shielding layer adopts 6 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃; 7) After the cross-linked wire core is prepared, it passes through a cooling channel, and the cooling gradient decreases by 30°C; 8) Carry out metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion in sequence to obtain the finished cable.

[0031] Comparative Example 2 Comparative Example 2: The above steps are exactly the same, and EPDM without nano-MgO is added; The specific steps are as follows: 1) By weight, 100 parts of polypropylene and 5 parts of ethylene propylene diene monomer rubber were used to prepare nano-doped EPDM elastomer without adding nano-magnesium oxide; 2) preparing the conductor core by twisting; 3) The cross-linked wire core is prepared by a three-layer co-extrusion production line; the conductor shielding layer, the insulation shielding layer and the polypropylene insulation layer are respectively produced by a 75-type screw extruder, a 90-type screw extruder and a 175-type screw extruder; 4) The extrusion screw temperature of the conductor shielding layer adopts 7 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 190℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃, Zone 7 200℃; 5) The polypropylene insulation layer adopts modified polypropylene material, and the screw temperature setting is stepped, 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℃, Zone 9 200℃; 6) The extrusion screw temperature of the insulating shielding layer adopts 6 zones, and the temperature of each zone is: Zone 1 165℃, Zone 2 180℃, Zone 3 185℃, Zone 4 195℃, Zone 5 200℃, Zone 6 200℃; 7) After the cross-linked wire core is prepared, it passes through a cooling channel, and the cooling gradient decreases by 30°C; 8) Carry out metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion in sequence to obtain the finished cable.

[0032] Comparative Example 3 The steps are the same as those in Example 1, and EPDM and MgO are added directly to PP respectively.

[0033] Comparative Example 4 The steps are the same as those in Example 1, except that the proportions of EPDM and MgO are changed; wherein, by weight, 100 parts of PP, 20 parts of EPDM, and 10 parts of MgO are used.

[0034] Effect evaluation 1

[0035] The present invention found that the crystallization behavior of EPDM and PP blends showed that the crystallization temperature and crystal nucleation rate of the blends increased after adding EPDM, indicating that EPDM has a nucleation effect on PP. The decrease in the growth rate of the crystal body indicates that the crystal size becomes smaller, that is, the sphere changes from large to small. As a result, EPDM reduces the crystallinity of PP, and the crystallization temperature of PP is about 115°C. Therefore, between 100-130°C, the present invention adopts a more intensive step-by-step cooling to ensure its original crystallinity.

[0036] 1) Compared with Example 2, it can be found 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.

[0037] 2) Through comparative example 1, it is found that increasing the temperature gradient during cooling results in a certain degree of decrease in mechanical properties and electrical properties compared with example 2. This is because the increase in the cooling temperature gradient results in no better crystallization when PP is crystallized, resulting in a larger proportion of amorphous regions.

[0038] 3) Through comparative example 2, it is found that only adding EPDM elastomer can improve the mechanical properties of PP. However, because the addition of EPDM will form an interface with PP, polarized charges will appear, resulting in a decrease in its electrical properties.

[0039] 4) Through comparative example 3, it was found that when EPDM and MgO were added separately instead of mixing the two together in advance, both the electrical properties and the mechanical properties decreased to a certain extent compared with Example 2. This is because when the two were added separately, MgO could not be evenly filled into the interior of EPDM, which partially caused the phenomenon in 3), resulting in a decrease in electrical properties.

[0040] Table 1 Performance tests of embodiments and comparative examples project Example 1 (conventional preparation method, without adding any filler) Example 2 (according to the best method of the present invention) Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Hardness (Shore A) 50 52 55 55 54 58 Tear strength (N / mm) 31.3 34 32.8 32 33 30 Tensile strength (MPa) 41.9 45 43 44.5 44 44.5 Elongation at break (%) 580 630 610 590 585 590 Volume resistivity (Ω*cm) 4.8e15 4.0e15 4.7e15 5.0e15 3.9e15 4.0e15 DC breakdown strength positive electrode (kV / mm) 330 380 350 300 320 310 DC breakdown strength negative electrode (kV / mm) 320 360 340 280 315 300 <![CDATA[Spatial charge density (20 kV / mm) (C / m 3 )]]> 2.3 1.0 1.5 3.0 2.0 3.5 Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A method for preparing a polypropylene insulated DC cable, characterized in that: The steps include: S11: preparing the conductor core by twisting; S12: A conductor shielding layer, a polypropylene insulation layer and an insulation shielding layer are prepared in sequence 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 a nano-doped EPDM elastomer through an extruder; the nano-doped EPDM elastomer is obtained by mixing 100 parts of polypropylene, 5-15 parts of ethylene propylene diene monomer rubber and 1-5 parts of nano-magnesium oxide by weight; during the extrusion, the temperature zones are set to: 175°C for zone 1, 180°C for zone 2, 190°C for zone 3, 195°C for zone 4, 195°C for zone 5, 195°C for zone 6, 200°C for zone 7, 200°C for zone 8 and 200°C for zone 9; S13: Cooling the cross-linked wire core in different zones through a cooling channel, wherein the temperature zones are set as follows: zone 1 190° C., zone 2 160° C., zone 3 130° C., zone 4 120° C., zone 5 110° C., zone 6 100° C., zone 7 60° C., zone 8 30° C.; S14: metal shielding, cabling, inner sheath extrusion, armoring and outer sheath extrusion are sequentially performed on the surface of the cross-linked core after the zone cooling in the step S13 to obtain the polypropylene insulated DC cable.

2. The preparation method according to claim 1, characterized in that The extruder is selected from a 175-type screw extruder.

3. The preparation method according to claim 1, characterized in that: In parts by weight, the nano-doped EPDM elastomer consists of 100 parts of polypropylene, 15 parts of ethylene propylene diene monomer rubber and 5 parts of nano-magnesium oxide.

4. The preparation method according to claim 1, characterized in that: The conductor shielding layer is obtained by extruding through a 75-type screw extruder.

5. The preparation method according to claim 4, characterized in that: When the conductor shielding layer is extruded, the temperature zones are set as: 165° C. for zone one, 180° C. for zone two, 190° C. for zone three, 195° C. for zone four, 200° C. for zone five, 200° C. for zone six, and 200° C. for zone seven.

6. The preparation method according to claim 1, characterized in that: The insulating shielding layer is extruded by a 90-type screw extruder.

7. The preparation method according to claim 6, characterized in that: When the insulating shielding layer is extruded, the temperature zones are set as: 165° C. for zone one, 180° C. for zone two, 185° C. for zone three, 195° C. for zone four, 200° C. for zone five, and 200° C. for zone six.

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