Crosslinkable polyethylene compositions, crosslinked polyethylene and their applications in DC cables

Abstract

This invention relates to the field of cross-linked polyethylene, and discloses a cross-linkable polyethylene composition, cross-linked polyethylene, and their applications in DC cables. The cross-linkable polyethylene composition comprises a polyethylene base material, a cross-linking agent, and a nucleating agent; wherein the nucleating agent is a polyolefin extended chain crystal; based on 100 parts by weight of polyethylene base material, the amount of cross-linking agent is 0.5-1.8 parts by weight, and the amount of polyolefin extended chain crystal is 0.1-0.5 parts by weight. The cross-linked polyethylene prepared from this composition has a stable crystal structure and excellent crystallization properties.

Description

Technical Field

[0001] This invention relates to the field of cross-linked polyethylene, and more specifically to a cross-linkable polyethylene composition, cross-linked polyethylene, its applications, and DC cables. Background Technology

[0002] In the field of ultra-high voltage direct current (UHVDC) transmission, cross-linked polyethylene (XLPE) serves as the core insulation material, and its performance directly affects the stability and lifespan of the transmission system. Given the unidirectional nature of the DC electric field, the amount of additives added to XLPE insulation materials must be strictly controlled. On the one hand, small-molecule additives are prone to directional migration under the influence of the DC electric field, leading to material aging, decreased flexibility, and consequently, deterioration of insulation performance. On the other hand, additive molecules or impurities can easily become traps and release sources of space charge, causing local electric field distortion and even insulation breakdown. Furthermore, DC cables often operate under high-temperature environments for extended periods, and some additives can reduce the volume resistivity of the material, increase leakage current, and accelerate thermal aging. Therefore, reducing the amount of additives to improve the long-term reliability of the material has become the mainstream technical direction for UHVDC XLPE insulation materials.

[0003] However, XLPE insulation materials with low additive content face significant performance challenges. Due to the reduced additive content, the plasticizing and reinforcing effects on the material are weakened, resulting in a significant decrease in mechanical properties such as tensile strength and elongation at break, making it more prone to deformation and cracking during processing and service. Simultaneously, the lack of additive regulation leads to poorer microstructural stability and increased internal defect density, resulting in increased carrier mobility and exacerbated space charge accumulation, further affecting the material's electrical properties and heat aging resistance. In existing technologies, how to effectively improve the overall performance of insulation materials while maintaining low additive content has become a pressing technical challenge in the field of ultra-high voltage DC cables.

[0004] Research has found that adjusting the microcrystalline structure, increasing its crystallinity or lamellar thickness, can significantly improve the structural stability of materials. A highly ordered crystal structure can reduce the internal defect density of the material, lengthen the carrier migration path, and reduce the formation of space charge, thereby enhancing the stability of the insulation structure. Simultaneously, the optimized crystal structure can effectively improve the mechanical properties of the material, compensating for insufficient strength and toughness caused by low additive content, ultimately achieving a synergistic improvement in the electrical and mechanical properties of the insulating material.

[0005] In existing research, introducing modified particles through blending is one of the common methods to improve the crystallization behavior of polymer matrices. However, the heterogeneous particles introduced through blending often become new impurities, affecting the electrical properties of the material. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems existing in the prior art and to provide a cross-linkable polyethylene composition, cross-linked polyethylene, and its application in DC cables. The cross-linked polyethylene made from this composition has good crystallization properties and electrical properties, which can meet the requirements of high-voltage DC cables.

[0007] To achieve the above objectives, a first aspect of the present invention provides a crosslinkable polyethylene composition, the crosslinkable polyethylene composition comprising a polyethylene base material, a crosslinking agent, and a nucleating agent;

[0008] The nucleating agent is a polyolefin extended chain crystal;

[0009] Based on 100 parts by weight of polyethylene base material, the amount of crosslinking agent is 0.5-1.8 parts by weight, and the amount of polyolefin extended chain crystals is 0.1-0.5 parts by weight.

[0010] A second aspect of the present invention provides a cross-linked polyethylene, which is prepared from the cross-linkable polyethylene composition as described above.

[0011] A third aspect of the present invention provides a method for preparing cross-linked polyethylene as described above, the method comprising:

[0012] S1. Mix the polyethylene base material and the extended chain crystals of the polyolefin in the crosslinkable polyethylene composition as described above;

[0013] S2. Extrude and granulate the mixed materials to obtain a semi-finished product.

[0014] S3. The semi-finished material is mixed with the crosslinking agent in the crosslinkable polyethylene composition as described above and then absorbed.

[0015] S4. The material after post-absorption is vulcanized to obtain the cross-linked polyethylene.

[0016] A fourth aspect of the present invention provides the application of cross-linked polyethylene in DC cables as described above.

[0017] A fifth aspect of the present invention provides a DC cable, wherein the insulation material of the DC cable is cross-linked polyethylene as described above.

[0018] Through the above technical solution, the present invention can achieve at least the following beneficial effects:

[0019] The crosslinkable polyethylene composition provided by the present invention includes specific components, and each component is used in specific amounts, so that the crosslinked polyethylene prepared from the composition has excellent crystallization properties, and further, can ensure the excellent electrical properties, mechanical properties and heat aging resistance of the crosslinked polyethylene. Attached Figure Description

[0020] Figure 1 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Example 1.

[0021] Figure 2 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Example 2.

[0022] Figure 3 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Example 3.

[0023] Figure 4 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Example 4.

[0024] Figure 5 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Example 5.

[0025] Figure 6 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Comparative Example 1.

[0026] Figure 7 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Comparative Example 2.

[0027] Figure 8 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Comparative Example 3.

[0028] Figure 9 The image shows a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Comparative Example 4.

[0029] Figure 10 This is a SEM image of the crystal morphology of cross-linked polyethylene prepared from the cross-linkable polyethylene composition of Comparative Example 5. Detailed Implementation

[0030] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0031] To achieve the above objectives, a first aspect of the present invention provides a crosslinkable polyethylene composition, the crosslinkable polyethylene composition comprising a polyethylene base material, a crosslinking agent, and a nucleating agent;

[0032] The nucleating agent is a polyolefin extended chain crystal;

[0033] Based on 100 parts by weight of polyethylene base material, the amount of crosslinking agent is 0.5-1.8 parts by weight, and the amount of polyolefin extended chain crystals is 0.1-0.5 parts by weight.

[0034] In this invention, the extended chain crystals of polyolefin have good compatibility with polyethylene base material and do not introduce new impurities. Adding them as a nucleating agent can significantly improve the crystallization properties of cross-linked polyethylene prepared from the composition.

[0035] In this invention, when the amount of each raw material added to the composition is within the above range, the cross-linked polyethylene obtained from the composition has high crystallinity and excellent crystallization performance, while ensuring the electrical and mechanical properties of the cross-linked polyethylene.

[0036] More preferably, based on 100 parts by weight of polyethylene base material, the amount of crosslinking agent is 0.8-1.5 parts by weight, and the amount of polyolefin extended chain crystals is 0.2-0.4 parts by weight.

[0037] In this invention, the crosslinking agent can be any conventional crosslinking agent in the art, as long as it can crosslink polyethylene. Preferably, the crosslinking agent is an organic peroxide.

[0038] More preferably, the crosslinking agent is selected from at least one of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, dicumyl peroxide, benzoyl peroxide, and di-tert-butyl peroxide.

[0039] Preferably, the composition further includes an antioxidant.

[0040] More preferably, based on 100 parts by weight of polyethylene base material, the amount of antioxidant is 0.1-0.8 parts by weight.

[0041] More preferably, based on 100 parts by weight of polyethylene base material, the amount of antioxidant is 0.2-0.5 parts by weight.

[0042] In this invention, to enhance the antioxidant effect, the antioxidant may include a primary antioxidant and a secondary antioxidant.

[0043] Preferably, the weight ratio of the primary antioxidant to the secondary antioxidant is 1:0.1-2.

[0044] In this invention, the antioxidant can be a conventional type of antioxidant in the art. Preferably, the primary antioxidant is a hindered phenolic antioxidant.

[0045] More preferably, the primary antioxidant is selected from at least one of 2,6-di-tert-butyl-4-methylphenol, 4,6-bis(octylthiomethyl)o-cresol, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,2'-thiodiethylbis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 4,4'-thiobis(6-tert-butyl-3-methylphenol).

[0046] Preferably, the auxiliary antioxidant is a thioester antioxidant and / or a phosphite antioxidant.

[0047] More preferably, the co-antioxidant is selected from at least one of dodecayl thiodipropionate, dodecaoctadecyl thiodipropionate, pentaerythritol tetra(3-lauryl thiopropionate), (2,4-di-tert-butylphenyl) phosphite, bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, and tris(nonylphenol) phosphite.

[0048] In this invention, the extended polyolefin crystals can be purchased from commercially available products or prepared using conventional methods in the art. For example, they can be prepared by high-pressure melt crystallization, low-pressure solution mixing, melt mixing, gel spinning, or stretching.

[0049] Preferably, the method for preparing the polyolefin extended chain crystal includes:

[0050] Polyolefin resin and liquid crystal polymer are mixed and then kneaded. After kneading, the mixture is compressed into tablets, which are then crushed and etched to obtain polyolefin extended chain crystals.

[0051] In this invention, liquid crystal polymer has the conventional meaning in the art, referring to a class of polymers that can exist as a liquid crystal phase in the molten state or in solution. This invention does not impose a particular limitation on the type of liquid crystal polymer; it can be any conventional choice within the art, such as liquid crystal cellulose.

[0052] More preferably, based on the mass of the polyolefin resin, the amount of liquid crystal polymer added is 0.1-5 wt%, preferably 0.1-2 wt%.

[0053] More preferably, the mixing conditions include: a mixing temperature of 180-250℃ and a mixing time of 10-30 minutes.

[0054] More preferably, the tableting method can be as follows: the material is placed into the steel mold cavity, the mold is placed into the tablet press, the temperature is raised to 200-270℃, the temperature is held for 10-30 minutes, the pressure is raised to 80-120MPa, and the pressure is maintained while cooling is performed.

[0055] More preferably, the pulverization process can be carried out in a ball mill, and the average particle size of the extended chain crystals of polyolefin can be adjusted by controlling the ball milling time and the diameter of the grinding balls. Preferably, the ball milling time is 0.5-4 hours, and the diameter of the grinding balls is 3-10 mm.

[0056] More preferably, the etching refers to contacting the material with an etching solution.

[0057] Preferably, the etching solution is a mixed solution of potassium permanganate, concentrated sulfuric acid and concentrated phosphoric acid.

[0058] More preferably, the volume ratio of potassium permanganate, concentrated sulfuric acid and concentrated phosphoric acid is (70-80):(10-15):(10-20), wherein the potassium permanganate is a 0.5-8 wt% aqueous solution.

[0059] Preferably, the etching time is 5-15 minutes.

[0060] According to the present invention, preferably, the extended chain crystal of polyolefin is an extended chain crystal of polypropylene.

[0061] More preferably, the polyolefin extended chain crystal is an isotactic polypropylene extended chain crystal.

[0062] In this invention, isotactic polypropylene extended chain crystals have excellent self-insulation properties, stable crystal structure and very few defects. Using them as nucleating agents can provide a large number of nucleation sites for polyethylene crystallization, further increase the crystallization temperature of cross-linked polyethylene, and induce rapid crystallization of polyethylene.

[0063] In this invention, the isotacticity of the isotactic polypropylene is ≥90%. The isotacticity is determined by... 13 The test was performed using nuclear magnetic resonance (NMR).

[0064] According to the present invention, the melting temperature of the extended chain crystal of the polyolefin is Tm1℃, the melting temperature of the polyethylene base material is Tm0℃, and Tm1-Tm0≥60℃.

[0065] In this invention, when the melting temperature of the extended chain crystals of the polyolefin is within the above-mentioned range, the nucleating agent can be prevented from melting during the processing of the composition, which is more conducive to providing stable nucleation sites.

[0066] In this invention, the melting temperature is determined by differential scanning calorimetry.

[0067] More preferably, Tm1-Tm0 ≥ 65℃.

[0068] According to the present invention, preferably, the weight-average molecular weight of the polyolefin extended chain crystal is 30,000-120,000 g / mol.

[0069] In this invention, the weight-average molecular weight of the extended-chain polyolefin crystals is determined by gel permeation chromatography-light scattering (GPC). Specifically, the extended-chain crystals are dissolved in o-dichlorobenzene at 120-140°C and stirred in a reflux apparatus for 2-4 hours until the solution is clear and transparent, with no visible particles. The solution is filtered through a 0.45 μm PTFE high-temperature filter membrane, and the weight-average molecular weight of the extended-chain polyolefin crystal sample is determined using GPC-light scattering.

[0070] More preferably, the weight-average molecular weight of the polyolefin extended chain crystal is 50,000-100,000 g / mol.

[0071] According to the present invention, preferably, the average particle size of the polyolefin extended chain crystals is 200-2000 nm. For example, it can be 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, or 2000 nm, or it can be a range consisting of any two of the above values, or any intermediate value in the range.

[0072] In this invention, the average particle size of the extended chain crystals of the polyolefin being within the above-mentioned range is more conducive to their uniform dispersion in the polyethylene matrix.

[0073] In this invention, the average particle size of the extended polyolefin crystals is determined by a particle size analyzer.

[0074] More preferably, the average particle size of the polyolefin extended chain crystals is 200-850 nm.

[0075] More preferably, the average particle size of the polyolefin extended chain crystals is 350-850 nm.

[0076] According to the present invention, preferably, the density of the polyethylene base material is 0.9 g / cm³. 3 -0.96g / cm 3 .

[0077] In this invention, the density of the polyethylene base material is within the above-mentioned range, resulting in cross-linked polyethylene with better flexibility.

[0078] According to the present invention, preferably, the melt index of the polyethylene base material at 190°C and 2.16 kg is 1.5-2.5 g / 10 min.

[0079] In this invention, the melt flow index of the polyethylene base material is within the above-mentioned range, which is more conducive to extrusion processing during the preparation process.

[0080] More preferably, the polyethylene base material has a melt index of 1.7-2.2 g / 10 min at 190°C and 2.16 kg.

[0081] A second aspect of the present invention provides a cross-linked polyethylene, which is prepared from the polyethylene composition as described above.

[0082] The cross-linked polyethylene provided by this invention has a stable crystal structure and excellent crystallization properties, while ensuring the excellent electrical and mechanical properties of cross-linked polyethylene.

[0083] According to the present invention, preferably, the average particle size of the cross-linked polyethylene crystals is 500-1600 nm. For example, it can be 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm, 1050 nm, 1100 nm, 1150 nm, 1200 nm, 1250 nm, 1300 nm, 1350 nm, 1400 nm, 1450 nm, 1500 nm, 1550 nm, or 1600 nm, or it can be a range consisting of any two of the above values, or any intermediate value within that range.

[0084] In this invention, the crystal size of the cross-linked polyethylene is within the above-mentioned range, which enables the formation of a uniform three-dimensional crystal network in the system, reduces charge accumulation caused by structural defects, and effectively improves mechanical and electrical properties.

[0085] In this invention, the average particle size of cross-linked polyethylene crystals is determined by scanning electron microscopy. A field of view is randomly selected, and the average particle size of all crystals in that field of view is calculated. This operation is repeated 10 times, and the average value of the 10 measurements is taken as the average particle size.

[0086] More preferably, the average particle size of the cross-linked polyethylene crystals is 700-1200 nm.

[0087] According to the present invention, preferably, the crystallization temperature of the cross-linked polyethylene is 84-92°C.

[0088] In this invention, the crystallization temperature of the cross-linked polyethylene is within the above-mentioned range, which enables the cross-linked polyethylene to form a more stable crystal structure during the cooling process.

[0089] More preferably, the crystallization temperature of the cross-linked polyethylene is 88-92℃.

[0090] According to the present invention, preferably, the crystallinity of the cross-linked polyethylene is 26-34%.

[0091] In this invention, the crystallinity of the cross-linked polyethylene is within the above-mentioned range, and the cable insulation layer made therefrom can maintain good structural stability at higher operating temperatures.

[0092] In this invention, the crystallization temperature and crystallinity of cross-linked polyethylene are determined by differential scanning calorimetry.

[0093] More preferably, the crystallinity of the cross-linked polyethylene is 30-34%.

[0094] According to the present invention, preferably, the tensile strength of the cross-linked polyethylene is 22-26 MPa.

[0095] According to the present invention, preferably, the elongation at break of the cross-linked polyethylene is 530-620%.

[0096] In this invention, when the tensile strength and elongation at break of the cross-linked polyethylene are within this range, it is beneficial for the cable to maintain good mechanical properties during long-term use and extend its service life.

[0097] In this invention, tensile strength and elongation at break are tested at room temperature (25±3℃) in accordance with GB / T 1040.2-2022.

[0098] More preferably, the tensile strength of the cross-linked polyethylene is 24-26 MPa.

[0099] More preferably, the cross-linked polyethylene has an elongation at break of 560-620%.

[0100] According to the present invention, preferably, the cross-linked polyethylene has a DC breakdown strength ≥380kV / mm at 25±3℃.

[0101] In this invention, the breakdown strength of the cross-linked polyethylene is within the above-mentioned range, which ensures that the cable has good pressure resistance during operation.

[0102] In this invention, the DC breakdown strength is tested with reference to GB / T 1408.1-2016 using a ball-to-ball electrode at room temperature (25±3℃) and the sample thickness is 0.2mm.

[0103] More preferably, the cross-linked polyethylene has a DC breakdown strength ≥400kV / mm at 25±3℃.

[0104] More preferably, the cross-linked polyethylene has a DC breakdown strength ≥420kV / mm at 25±3℃.

[0105] According to the present invention, preferably, the cross-linked polyethylene has a volume resistivity ≥3×10⁻⁶ at 25±3℃. 13 Ω·m.

[0106] In this invention, the volume resistivity of the cross-linked polyethylene is within the above-mentioned range, which enables the insulation layer to maintain good insulation performance and reduce current transmission loss.

[0107] In this invention, the volume resistivity test is conducted at room temperature (25±3℃) in accordance with GB / T 1410-2006, and the sample thickness is 0.2mm.

[0108] More preferably, the cross-linked polyethylene has a volume resistivity ≥5×10⁻⁶ at 25±3℃. 13 Ω·m.

[0109] According to the present invention, preferably, the cross-linked polyethylene in an air atmosphere T 5wt% ≥280℃. Generally speaking, cross-linked polyethylene at T in air... 5wt% ≤330℃.

[0110] In this invention, T 5wt% This refers to the 5wt% thermogravimetric temperature of cross-linked polyethylene, that is, the temperature at which cross-linked polyethylene loses 5% of its mass during heating. The test is conducted according to GB / T 27761-2011, with air as the test atmosphere.

[0111] More preferably, the cross-linked polyethylene in an air atmosphere T 5wt% ≥290℃.

[0112] More preferably, the cross-linked polyethylene in an air atmosphere T 5wt% ≥300℃.

[0113] According to the present invention, preferably, the absolute value of the change rate of tensile strength of the cross-linked polyethylene after heat aging at 135°C for 168 hours is ≤12%. Wherein, the change rate of tensile strength (%) = [(tensile strength after heat aging - tensile strength before heat aging) / tensile strength before heat aging] × 100%.

[0114] According to the present invention, preferably, the absolute value of the change rate of elongation at break after heat aging at 135°C for 168 hours is ≤12%. Wherein, the change rate of elongation at break (%) = [(elongation at break after heat aging - elongation at break before heat aging) / elongation at break before heat aging] × 100%.

[0115] In this invention, the T of the cross-linked polyethylene 5wt% When the rate of change of tensile strength and the rate of change of elongation at break are within this range, the operational reliability of the cable under high-temperature conditions can be further improved, and maintenance costs and safety hazards can be reduced.

[0116] More preferably, the absolute value of the change rate of tensile strength of the cross-linked polyethylene after heat aging at 135°C for 168 hours is ≤8%.

[0117] More preferably, the absolute value of the change rate of elongation at break after the cross-linked polyethylene is heat-aged at 135°C for 168 hours is ≤10%.

[0118] A third aspect of the present invention provides a method for preparing cross-linked polyethylene as described above, the method comprising:

[0119] S1. Mix the polyethylene base material and the extended chain crystals of the polyolefin in the crosslinkable polyethylene composition as described above;

[0120] S2. Extrude and granulate the mixed materials to obtain a semi-finished product.

[0121] S3. The semi-finished material is mixed with the crosslinking agent in the crosslinkable polyethylene composition as described above and then absorbed.

[0122] S4. The material after post-absorption is vulcanized to obtain the cross-linked polyethylene.

[0123] In this invention, the mixing method in step S1 is not particularly limited, as long as the raw materials can be mixed evenly. For example, the mixing can be carried out in a mixer at a temperature of 35-50°C and a mixing time of 10-30 minutes.

[0124] In this invention, if the composition also includes an antioxidant, the antioxidant is added in step S1.

[0125] In this invention, the extrusion method is not particularly limited and can be any conventional choice in the art. For example, a single-screw extruder or a twin-screw extruder can be used for extrusion.

[0126] In some specific embodiments of the present invention, extrusion is performed using a twin-screw extruder with a screw speed of 180-200 r / min.

[0127] According to the present invention, preferably, the extrusion temperature is 110-170°C.

[0128] In some specific embodiments of the present invention, the temperatures of each zone of the twin-screw extruder are set sequentially to 120±10℃, 125±10℃, 135±10℃, 145±10℃, 155±10℃, 160±10℃, 155±10℃, and 150±10℃.

[0129] In this invention, the granulation method is not particularly limited and can be any conventional choice in the art.

[0130] According to the present invention, preferably, in step S3, the temperature of the post-absorption is 60-80°C and the time is 12-24h.

[0131] More preferably, in step S3, the temperature of the post-absorption is 60-70℃, and the time is 15-20h.

[0132] In this invention, the vulcanization method and conditions can be conventionally chosen within the art, as long as they provide the reaction conditions for the crosslinking reaction. For example, the vulcanization can be carried out in a flat vulcanizing machine.

[0133] In some specific embodiments of the present invention, the vulcanization conditions include: pre-vulcanization at 100-130℃ and 2-5MPa for 5-20 minutes, followed by vulcanization at 140-170℃ and 8-20MPa for 10-30 minutes.

[0134] A fourth aspect of the present invention provides the application of cross-linked polyethylene in DC cables as described above.

[0135] A fifth aspect of the present invention provides a DC cable, wherein the insulation material of the DC cable is cross-linked polyethylene as described above.

[0136] The cross-linked polyethylene provided by this invention, when used as insulation material for DC cables, enables DC cables to have excellent electrical properties, mechanical properties, and heat aging resistance.

[0137] Preferably, the DC cable carries a voltage of 220kV or higher.

[0138] The present invention will be described in detail below through embodiments. It should be understood that the following embodiments are only used to further explain and illustrate the content of the present invention, and are not intended to limit the present invention.

[0139] Unless otherwise specified, all reagents and materials used in the following examples were purchased from reputable manufacturers or chemical reagent suppliers.

[0140] In the following examples, the low-density polyethylene was purchased from Qilu Petrochemical, grade J182C, with a melting temperature of 108℃ and a density of 0.92 g / cm³. 3 The melt index at 190℃ and 2.16kg is 1.9g / 10min.

[0141] Cellulose was treated with 70±5% concentrated sulfuric acid for 2 hours to obtain liquid crystal cellulose. The cellulose was purchased from Aladdin, brand name C434461.

[0142] Preparation Example 1

[0143] S1. Take isotactic polypropylene (purchased from Qilu Petrochemical, grade T30S) and 1 wt% liquid crystal cellulose of isotactic polypropylene, mix them evenly and then add them to a mixer. The mixing temperature is 200℃. After mixing for 15 minutes, take out the mixture.

[0144] S2. Place the mixture into the steel mold cavity, put the mold into the tablet press, heat it to 230℃, keep it at that temperature for 20 minutes, then increase the pressure to 100MPa, and then slowly cool it to room temperature while maintaining the pressure.

[0145] S3. After cooling, the pressure is released and the isotactic polypropylene containing extended chain crystals is taken out. After ball milling for 3 hours (the diameter of the milling ball is 5 mm), it is etched for 10 minutes with a mixed solution of 3 wt% potassium permanganate, concentrated sulfuric acid and concentrated phosphoric acid (volume ratio 70:15:15). After washing and drying, extended chain polypropylene crystals P1 are obtained, with an isotacticity of 95%.

[0146] P1 has a melting temperature of 178℃, an average particle size of 610nm, and a weight-average molecular weight of 60,420g / mol.

[0147] Preparation Example 2

[0148] The extended chain crystal P2 was prepared according to the method of Preparation Example 1, except that the ball milling time in step S3 was 1 hour.

[0149] P2 has a melting temperature of 173℃, an average particle size of 1835nm, and a weight-average molecular weight of 65,460g / mol.

[0150] Preparation Example 3

[0151] Extended-chain crystals P3 were prepared according to the method of Preparation Example 1, except that isotactic polypropylene was replaced with syndiotactic polypropylene (purchased from Total, brand name Finaplas 1471).

[0152] P3 has a melting temperature of 155℃, an average particle size of 530nm, and a weight-average molecular weight of 52,615g / mol.

[0153] Preparation Example 4

[0154] The extended chain crystal P4 was prepared according to the method of Preparation Example 1, except that isotactic polypropylene was replaced with poly(4-methyl-1-pentene) (purchased from Mitsui Chemicals, grade MX002), the mixing temperature in step S1 was 250°C, and the temperature was raised to 260°C in step S2.

[0155] P4 has a melting temperature of 248℃, an average particle size of 652nm, and a weight-average molecular weight of 85,620g / mol.

[0156] Comparative Preparation Example 1

[0157] S1. Take polyamide 6 (purchased from Sinopec Baling Branch, brand name YH800) and 1 wt% liquid crystal cellulose of polyamide 6, mix them evenly, and then add them to a mixer. The mixing temperature is 250℃. After mixing for 15 minutes, take out the mixture.

[0158] S2. Place the well-mixed material into the steel mold cavity, put the mold into the tablet press, heat it to 270℃, keep it at that temperature for 20 minutes, then increase the pressure to 200MPa, and then slowly cool it to room temperature while maintaining the pressure.

[0159] S3. After cooling, release the pressure and take out the polyamide 6 containing extended chain crystals; after ball milling it for 3 hours (the diameter of the grinding ball is 5 mm), use a mixed solution of potassium dichromate + water + concentrated sulfuric acid (volume ratio 3:5:10) to etch it for 10 minutes, and after washing and drying, polyamide 6 extended chain crystal DP1 is obtained.

[0160] DP1 has a melting temperature of 224℃, an average particle size of 572nm, and a weight-average molecular weight of 55,860g / mol.

[0161] Example 1

[0162] S1. Place 100 parts by weight of low-density polyethylene, 0.5 parts by weight of 4,4'-thiobis(6-tert-butyl-3-methylphenol) and 0.2 parts by weight of extended chain crystals P1 in a mixer and mix at 40°C for 20 min.

[0163] S2. The above materials are added to a twin-screw extruder for melt extrusion. The screw speed is 190 r / min. The temperature of each zone of the twin-screw extruder is set to 120℃, 125℃, 135℃, 145℃, 155℃, 165℃, 155℃ and 150℃ respectively. After extrusion, the materials are granulated and dried.

[0164] S3. Add 1.5 parts by weight of dicumyl peroxide to the above materials and absorb at 65°C for 15 hours to obtain a crosslinkable polyethylene composition.

[0165] Example 2

[0166] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that the amount of low-density polyethylene was 100 parts by weight, the amount of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] was 0.2 parts by weight, the amount of (2,4-di-tert-butylphenyl)phosphite was 0.2 parts by weight, and the amount of extended chain crystal P1 was 0.3 parts by weight.

[0167] Example 3

[0168] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that the extended chain crystal P1 was replaced with P2.

[0169] Example 4

[0170] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that the extended chain crystal P1 was replaced with P3.

[0171] Example 5

[0172] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that the extended chain crystal P1 was replaced with P4.

[0173] Comparative Example 1

[0174] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that the extended chain crystal P1 was replaced with DP1.

[0175] Comparative Example 2

[0176] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that isotactic polypropylene extended chain crystals were replaced with isotactic polypropylene resin (purchased from Qilu Petrochemical, grade T30S), which had a melting temperature of 165°C and a weight-average molecular weight of 98,410 g / mol.

[0177] Comparative Example 3

[0178] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that isotactic polypropylene extended chain crystals were replaced with silica with an average particle size of 50 nm.

[0179] Comparative Example 4

[0180] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that no nucleating agent was added.

[0181] Comparative Example 5

[0182] A crosslinkable polyethylene composition was prepared according to the method of Example 1, except that the amount of extended chain crystal P1 added was 0.6 parts by weight.

[0183] Test case

[0184] The crosslinkable polyethylene compositions in the above embodiments and comparative examples were hot-pressed using a flat vulcanizing machine: first, pre-pressed at 120°C and 5MPa for 10 min, then hot-pressed at 170°C and 15MPa for 20 min. After cooling to room temperature, crosslinked polyethylene test samples were obtained. The test samples were then degassed at 70°C, and their mechanical properties, electrical properties, and heat aging resistance were determined using the methods described above. The test results are shown in Table 1.

[0185] The testing method for the crystallization parameters of the sample to be tested is as described above.

[0186] The crystal morphology of the above-mentioned cross-linked polyethylene was tested using scanning electron microscopy, and the results are as follows: Figure 1-10 As shown in the figure. Further, the average grain size was calculated using the method described above. The results are shown in Table 1.

[0187] Table 1

[0188]

[0189] As can be seen from Table 1, the crosslinkable polyethylene composition of the present invention yields crosslinked polyethylene with crystal sizes between 500-1600 nm and high crystallinity and crystallinity, indicating that the crosslinked polyethylene has a relatively stable crystal structure. This is beneficial to improving the performance of crosslinked polyethylene in terms of mechanical, electrical and heat aging resistance, resulting in excellent comprehensive performance.

[0190] As shown in Comparative Example 1, although extended-chain polyamide 6 crystals can act as nucleating agents to improve the crystallinity of cross-linked polyethylene, polyamide 6 is a polar substance and easily causes charge accumulation, thus affecting the electrical properties of cross-linked polyethylene. As shown in Comparative Example 2, ordinary isotactic polypropylene has a low melting point and melts during processing, therefore it cannot act as a nucleating agent for cross-linked polyethylene to significantly improve its crystal structure or crystallinity, resulting in poor overall performance of the obtained cross-linked polyethylene. As shown in Comparative Example 3, silica has a significant nucleating effect on cross-linked polyethylene, but silica is small in size and easily agglomerates, forming spatial defects, which also affects the overall performance of cross-linked polyethylene. In Comparative Example 5, the addition ratio of extended-chain crystals was too high, significantly increasing the number of nucleation sites, resulting in excessively small grain size in the cross-linked polyethylene (e.g., ...). Figure 10 As shown in the figure, the crystals are less than 400 nm. At the same time, the excessive straightened crystals may hinder the crystal growth process of cross-linked polyethylene, thereby affecting its crystal structure stability and significantly reducing its overall performance.

[0191] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A crosslinkable polyethylene composition, characterized in that, The crosslinkable polyethylene composition includes a polyethylene base material, a crosslinking agent, and a nucleating agent; The nucleating agent is a polyolefin extended chain crystal; Based on 100 parts by weight of polyethylene base material, the amount of crosslinking agent is 0.5-1.8 parts by weight, and the amount of polyolefin extended chain crystals is 0.1-0.5 parts by weight.

2. The crosslinkable polyethylene composition according to claim 1, wherein, The extended chain crystal of the polyolefin is an extended chain crystal of polypropylene; And / or, the melting temperature of the extended chain crystals of the polyolefin is Tm1℃, the melting temperature of the polyethylene base material is Tm0℃, and Tm1-Tm0≥60℃; And / or, the weight-average molecular weight of the polyolefin extended chain crystal is 30,000-120,000 g / mol; And / or, the average particle size of the polyolefin extended chain crystals is 200-2000 nm.

3. The crosslinkable polyethylene composition according to claim 1 or 2, wherein, The density of the polyethylene base material is 0.9 g / cm³. 3 -0.96g / cm 3 ; And / or, the polyethylene base material has a melt index of 1.5-2.5 g / 10 min at 190°C and 2.16 kg.

4. A cross-linked polyethylene, characterized in that, The cross-linked polyethylene is prepared from the cross-linkable polyethylene composition according to any one of claims 1-3.

5. The cross-linked polyethylene according to claim 4, wherein, The average grain size of the cross-linked polyethylene is 500-1600 nm; And / or, the crystallization temperature of the cross-linked polyethylene is 84-92°C; And / or, the crystallinity of the cross-linked polyethylene is 26-34%.

6. The cross-linked polyethylene according to claim 4 or 5, wherein, The tensile strength of the cross-linked polyethylene is 22-26 MPa; And / or, the elongation at break of the cross-linked polyethylene is 530-620%; And / or, the crosslinked polyethylene has a DC breakdown strength ≥380kV / mm at 25±3℃; And / or, the cross-linked polyethylene has a volume resistivity ≥3×10⁻⁶ at 25±3°C. 13 Ω·m; And / or, the cross-linked polyethylene in an air atmosphere at T 5wt% ≥280℃; And / or, the absolute value of the change rate of tensile strength of the cross-linked polyethylene after heat aging at 135°C for 168 hours is ≤12%; And / or, the absolute value of the change rate of elongation at break after the cross-linked polyethylene has been heat-aged at 135°C for 168 hours is ≤12%.

7. A method for preparing cross-linked polyethylene according to any one of claims 4-6, characterized in that, The method includes: S1. Mix the polyethylene base material and the extended chain crystals of polyolefin in any one of claims 1-3; S2. Extrude and granulate the mixed materials to obtain a semi-finished product. S3. The semi-finished material is mixed with the crosslinking agent in the crosslinkable polyethylene composition according to any one of claims 1-3 and then absorbed. S4. The material after post-absorption is vulcanized to obtain the cross-linked polyethylene.

8. The method according to claim 7, wherein, In step S2, the extrusion temperature is 110-170℃; And / or, in step S3, the temperature of the post-absorption is 60-80℃, and the time is 12-24h.

9. The use of the crosslinkable polyethylene composition according to any one of claims 1-3 or the crosslinked polyethylene according to any one of claims 4-6 in DC cables.

10. A DC cable, characterized in that, The insulation material of the DC cable is cross-linked polyethylene as described in any one of claims 4-6.

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