A water treeing resistant crosslinked polyethylene insulation composition for high voltage direct current submarine cables and a method of making and using the same

Abstract

The application belongs to the technical field of high-voltage cross-linked polyethylene composition, and particularly relates to a water-tree-resistant cross-linked polyethylene insulation composition for high-voltage direct-current submarine cable and a preparation method and application thereof. The insulation composition comprises, in terms of weight parts, 80-120 parts of low-density polyethylene resin with a melt index of 1.9-2.1 g / 10 min; 0.2-0.3 parts of composite antioxidant; 1.25-1.85 parts of composite cross-linking agent; 0.2-0.3 parts of scorch inhibitor; 8-12 parts of water-inhibiting master batch; wherein the water-inhibiting master batch comprises 80-120 parts of composite polyolefin resin, 6-9 parts of composite water-inhibiting agent, 3.6-5.4 parts of maleic anhydride graft, and 0.15-0.25 parts of antioxidant; and the composite water-inhibiting agent comprises polyethylene glycol and oleic acid amide with a mass ratio of 8-12:1. The insulation composition can inhibit insulation scorching, resist water tree growth, has high gel content, fast extrusion cable forming speed, and stable production process.

Description

Technical Field

[0001] This invention belongs to the technical field of high-voltage cross-linked polyethylene compositions, and particularly relates to a water-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables, its preparation method, and its application. Background Technology

[0002] With the continuous development of the global economy and the rapid growth of electricity demand, electricity security is crucial to national security. Cross-linked polyethylene (XLPE), due to its excellent mechanical, electrical, and heat resistance properties, is currently widely used in the manufacture of high-voltage direct current (HVDC) cable insulation materials. In recent years, my country has continuously increased its investment in offshore wind power infrastructure construction. HVDC cables with XLPE insulation, characterized by their simple structure and high current carrying capacity, have been widely used in submarine-to-land DC power transmission.

[0003] In the production and use of traditional high-voltage chemically cross-linked polyethylene insulation materials, it has been found that due to the limitations of the low-density polyethylene resin structure, increasing the cross-linking agent content is necessary to achieve better mechanical and heat resistance properties. However, the cross-linking agent produces byproducts such as water and micropores of low-molecular-weight gases after vulcanization and cross-linking decomposition. Under the combined effects of high-voltage electric fields, operating time, and high humidity at the seabed, the byproduct water in the cable insulation, or moisture that has infiltrated the cable insulation from the outside, will form dendritic channels that grow continuously during free movement. This leads to a gradual decline in the electrical performance of the cable insulation. When the water trees grow to a certain size, they will transform into electrical trees, ultimately causing cable insulation breakdown.

[0004] Meanwhile, submarine cables for long-span power transmission require long continuous extrusion cycles for cable insulation, and high crosslinking agent content will increase the risk of pre-crosslinking during the cable insulation extrusion process, reducing cable production cycle and efficiency; more importantly, these pre-crosslinking defects, combined with the pores generated by crosslinking byproducts, will further exacerbate the formation of water trees in the insulation, adversely affecting the overall performance of the cable.

[0005] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the present invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0006] To address the aforementioned technical problems, the first objective of this invention is to provide a high-voltage insulation composition that can suppress insulation scorching, resist insulation water tree growth, has a high gel content, allows for fast extrusion cabling, and ensures stable production processes.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A water-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables comprises the following components in parts by weight:

[0009] 80-120 parts of polyethylene resin;

[0010] 0.2-0.3 parts of compound antioxidant;

[0011] 1.25-1.85 parts of composite crosslinking agent;

[0012] 0.2-0.3 parts of scorch inhibitor;

[0013] 8.0-12 parts of hydrophobic granules;

[0014] The polyethylene resin is a low-density polyethylene resin with a melt index of 1.9-2.1 g / 10 min; for example, the melt index of low-density polyethylene resin is 1.9 g / 10 min, 2.0 g / 10 min, or 2.1 g / 10 min.

[0015] Further preferred options are BASF Yangzi 2220HSC or Shanghai Petrochemical J182C low-density polyethylene resin.

[0016] In this invention, the melt index and melt index range of the low-density polyethylene resin ensure the electrical properties, environmental stress cracking resistance, and extrusion processability of the high-voltage DC water-resistant cross-linked polyethylene insulation composition. The use of a single, stable-melt-index low-density polyethylene resin allows high-voltage cables, especially submarine cables, to maintain stable insulation thickness during long-term extrusion processing, overcoming the problem of large insulation thickness fluctuations caused by uneven mixing or melt index deviations in composite resins, which in turn leads to substandard insulation thickness at the thinnest point of the high-voltage cable.

[0017] Preferably, the composite crosslinking agent includes dicumyl peroxide and triallyl isocyanurate, with a mass ratio of dicumyl peroxide to triallyl isocyanurate of 1-1.5:0.25-0.35; the scorch inhibitor is 2,4-diphenyl-4-methyl-1-pentene, with a mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate of 1:4.2-5.8:1.05-1.3.

[0018] Although 2,4-diphenyl-4-methyl-1-pentene can enhance the rigidity of polyethylene molecular chains and slow down the molecular chain movement rate when used alone, it cannot form a sufficient cross-linking network in the subsequent cross-linking stage of the insulation layer due to the lack of a cross-linking agent to initiate cross-linking. This results in a serious deficiency in the thermal elongation and mechanical properties of the insulation material. Furthermore, the scorch inhibitor itself cannot suppress the localized overheating caused by the random thermal motion of molecular chains, and long-term extrusion may still indirectly lead to a tendency to scorch.

[0019] The combination of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate maximizes the anti-scorching effect. The benzene ring group in the 2,4-diphenyl-4-methyl-1-pentene molecule anchors part of the molecular chain during the initial stages of insulation absorption and extrusion, reducing its thermal activity and delaying localized overheating and premature cross-linking caused by random collisions of the molecular chains. Simultaneously, the benzene ring structure, once integrated into the molecular chain, enhances rigidity and heat resistance. During the subsequent cross-linking stage of cable insulation preparation, dicumyl peroxide gradually decomposes to generate a large number of free radicals. By this time, the pre-inhibition effect of the scorching inhibitor has been completed, ensuring that free radicals only initiate effective cross-linking after the insulation layer is formed, reducing the risk of scorching. Triallyl isocyanurate disperses free radical active sites through polyallyl groups, avoiding micro-scorching caused by localized free radical aggregation; at the same time, it promotes the formation of a uniform and dense cross-linked network, and together with the enhanced molecular chain rigidity of 2,4-diphenyl-4-methyl-1-pentene, it further enhances the heat resistance and deformation resistance of the insulating material, fundamentally reducing the risk of water treeing / electrical treeing during long-term operation.

[0020] Further preferably, the mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate is 1:5:1.15-1.25. This ratio avoids the failure of scorch inhibitors due to excessive dicumyl peroxide, and also avoids insufficient crosslinking and low gel content due to insufficient dicumyl peroxide. It meets the requirements of long-cycle extrusion while ensuring complete crosslinking. Furthermore, the number of allyl groups in triallyl isocyanurate is just right to capture and transfer most of the free radicals from the decomposition of dicumyl peroxide, reducing ineffective free radical loss, resulting in a more uniform distribution of crosslinking points and significantly improving the consistency of material properties.

[0021] The scorch inhibitor must contain ≥99% 2,4-diphenyl-4-methyl-1-pentene, have an impurity particle size ≤70μm, and have ≤5 impurities per kilogram. For example, the scorch inhibitor could be Huazhongrong's AMSD-MBL.

[0022] Dicumyl peroxide is used as a crosslinking agent; the crosslinking agent has a dicumyl peroxide content ≥99%, a total active oxygen content ≤5.92%, an impurity particle size ≤70μm, and ≤5 impurities per kilogram. For example, the crosslinking agent can be dicumyl peroxide from Arkema.

[0023] Triallyl isocyanurate is used as a co-crosslinking agent; specifically, the co-crosslinking agent has a triallyl isocyanurate content ≥99%, an impurity particle size ≤70μm, and an impurity quantity ≤5 per kilogram. For example, the co-crosslinking agent can be Fangruida's TAIC.

[0024] In this invention, because long-span submarine power transmission cables require long continuous extrusion cycles for cable insulation, a specific amount of composite crosslinking agent cannot effectively balance the thermal elongation, mechanical properties, and scorch resistance of the cable insulation. The co-use of a clean scorch inhibitor and the composite crosslinking agent introduces benzene ring groups from the scorch inhibitor into the polyethylene molecular chain during the cable insulation crosslinking process, increasing the rigidity of the polyethylene molecular chain. This effectively ensures the heat resistance and mechanical properties of the insulation composition at a specific amount of composite crosslinking agent, while simultaneously reducing the scorch of the polyethylene molecular chain during cable insulation extrusion, thus improving the cable extrusion production cycle and efficiency.

[0025] The specific addition amounts of dicumyl peroxide and triallyl isocyanurate in this invention help reduce the total crosslinking agent content in the high-voltage DC water-resistant tree-type crosslinked polyethylene insulation composition, reduce the pre-crosslinked or old rubber generated during the extrusion process of the insulation material, and reduce pores and other by-products generated during the crosslinking reaction. This avoids the formation of dendritic and continuously growing dendritic structure channels in the cable insulation due to pores or other by-products under the combined effects of high-voltage electric fields, operating time, and other factors, and is beneficial to improving the electrical performance of the cable.

[0026] The water-suppressing masterbatch is obtained by mixing and granulating composite polyolefin resin, composite water-suppressing agent, maleic anhydride graft and antioxidant in a mass ratio of 80-120:6.0-9.0:3.6-5.4:0.15-0.25; the composite water-suppressing agent includes polyethylene glycol and oleic acid amide in a mass ratio of 8-12:1.

[0027] Preferably, the total amide content of oleamide in the water-suppressing masterbatch is ≥98.5%, the impurity particle size is ≤150μm, and the number of impurities per kilogram is ≤10. For example, the oleamide can be oleamide from Zhilian New Materials. The average molecular weight of polyethylene glycol is 20,000; for example, polyethylene glycol can be Beko Chemical PEG-20000. The mass percentage of polyethylene glycol in the water-suppressing masterbatch is 5-7%.

[0028] Excessive polyethylene glycol (PEG) content can easily lead to precipitation from the matrix during processing or long-term use, forming surface blooming or internal aggregation. The precipitated PEG may undergo abnormal reactions during high-temperature extrusion, or its uneven melting due to excessively high local concentrations can increase the probability of scorching, generating old glue spots or hard particles, and disrupting the uniformity of the insulation layer. PEG's anti-water-tree effect relies on the formation of hydrogen bonds between the numerous hydroxyl groups (-OH) in its molecular chain and water molecules, thereby fixing the moisture that has invaded the insulation layer and blocking the "water source" for water tree growth. When the content is too low, the number of hydroxyl groups is insufficient to effectively capture moisture. Under the influence of an electric field, moisture migrates and aggregates along resin defects, accelerating the formation and expansion of water tree channels, resulting in a significant decrease in anti-water-tree performance. This invention controls the PEG content to 5-7%, within which the number of hydroxyl groups is sufficient to effectively fix moisture. Combined with the dispersing effect of oleamide, this ensures that PEG is uniformly distributed in the resin matrix, blocking the growth path of water trees.

[0029] Preferably, the water-suppressing masterbatch contains a composite polyolefin resin comprising low-density polyethylene resin with a melt index of 1.9-2.1 g / 10 min and a polyolefin elastomer resin with a melt index of 4.5-5.5 g / 10 min, with a mass ratio of 1-1.5:1. Specifically, the low-density polyethylene resin used has a dielectric loss factor ≤0.0003. For example, the low-density polyethylene resin can be Yangzi BASF 2220HSC. Specifically, the polyolefin elastomer resin used is an ethylene-octene copolymer; for example, the polyolefin elastomer can be Dow 8200.

[0030] Preferably, the melt index of the maleic anhydride graft in the water-suppressing masterbatch is 1.0-3.0 g / 10 min; specifically, the maleic anhydride graft is maleic anhydride-grafted polyethylene; the content of maleic anhydride MAH groups is ≥0.5%. For example, the maleic anhydride-grafted polyethylene can be Nengzhiguang MC226.

[0031] Preferably, the antioxidant in the water-suppressing masterbatch is a thiophenolic antioxidant. Thiophenolic antioxidants, such as antioxidant 300 and antioxidant 1035, contain both phenolic hydroxyl groups and sulfur atoms (-S-) in their molecules. They combine the free radical scavenging ability of hindered phenols with the hydroperoxide decomposition function of sulfur, making them a composite antioxidant, especially performing well in high-temperature or harsh environments.

[0032] For long-cycle extruded submarine DC cables, traditional chemical cross-linked insulation materials, scorch-resistant chemical cross-linked insulation materials, or nano-modified DC chemical cross-linked insulation materials cannot effectively inhibit the formation and growth of water trees in the cable insulation, which will inevitably affect the service life of high-voltage submarine cables.

[0033] In this invention, the polyolefin elastomer in the water-suppressing masterbatch has good compatibility with low-density polyethylene (LDPE) and can be effectively and uniformly dispersed in the amorphous region of LDPE, promoting the compatibility between the masterbatch matrix and the composite water-suppressing agent. The maleic anhydride groups of maleic anhydride-grafted polyethylene can undergo esterification with the hydroxyl groups of polyethylene glycol (PEG), and its polyethylene segments have excellent compatibility with the base polyethylene. Under the action of maleic anhydride-grafted polyethylene, the high molecular weight PEG in the composite water-suppressing agent is effectively linked with the LDPE resin, making it less prone to precipitation in cable insulation. The hydroxyl groups in PEG have excellent hydrophilicity, fixing water molecules in the cable insulation and greatly reducing the formation and growth of water trees in the insulation. Simultaneously, oleamide can reduce internal friction during the preparation of the water-suppressing masterbatch, improve the fluidity of the composite polyolefin resin in the molten state, and thus reduce the uneven mixing of PEG with the composite polyolefin resin due to premature melting caused by its lower melting point.

[0034] As a preferred method, the preparation of water-suppressing masterbatch includes:

[0035] S1 is prepared by mixing low-density polyethylene resin and polyolefin elastomer resin to obtain composite polyolefin resin, and by mixing polyethylene glycol and oleamide to obtain composite water inhibitor.

[0036] Specifically, the melt indexes of low-density polyethylene resin and polyolefin elastomer resin differ significantly. Premixing ensures more uniform dispersion during the initial mixing in the high-speed mixer and the twin-screw melt extrusion, avoiding stratification or uneven local performance caused by differences in density and flowability. The ratio of polyethylene glycol to oleamide is 8-12:1. Oleamide can be pulverized into fine powder first, then mixed with polyethylene glycol particles in a certain proportion to form a composite water-suppressing agent before being added to the high-speed mixer. This ensures that oleamide is uniformly dispersed in polyethylene glycol, preventing product performance fluctuations caused by local aggregation of additives in subsequent processes.

[0037] S2 puts the composite polyolefin resin, composite water suppressant, maleic anhydride graft and antioxidant into a high-speed mixer and uses a low-speed mixing mode to mix at a speed of 30±2 rpm for 40-60 seconds.

[0038] Specifically, the purpose of low-speed mixing is to allow the raw materials to be initially and evenly mixed, avoiding excessive friction and heating or stratification caused by high-speed stirring. A mixing time of 40-60 seconds ensures that the four raw materials are initially and evenly dispersed in a short time, preparing for subsequent melt extrusion.

[0039] S3 feeds the mixture after low-speed mixing in step S2 into a twin-screw extruder through a feeding device. At a shear speed of 25-35 rpm, the screw temperature is controlled to gradually increase from the feed section to the die head, with the final section temperature at 155-165℃ and the material temperature at 160-180℃. After being filtered by a multi-layer filter screen installed in the die head, the mixture is granulated by water-pull granulation and dried to obtain clean water-suppressing masterbatch. The filter screen includes at least one layer of 300-mesh high-precision filter screen.

[0040] Specifically, three layers of filters are added, including a high-precision filter with a mesh size of 300 that filters impurities larger than 50μm. The production capacity is 200-300kg / h through water-pull granulation.

[0041] Preferably, the compound antioxidant includes at least two of hindered phenolic antioxidants, thioester antioxidants, and thiophenolic antioxidants.

[0042] Specifically, hindered phenolic antioxidants, such as antioxidant 1010, contain sterically hindered phenolic hydroxyl groups (-OH) in their molecular structure, which interrupt the oxidation chain reaction by capturing free radicals. Thioester antioxidants, such as antioxidant DLTP (dilauryl thiodipropionate), decompose hydroperoxides (ROOH) to generate stable products, synergistically enhancing antioxidant efficiency with hindered phenolic antioxidants. Thiophenolic antioxidants include antioxidant 300 or antioxidant 1035.

[0043] Specifically, the antioxidants used are antioxidant 300 and antioxidant 1010 in a mass ratio of 1-2:1; or, antioxidants used are antioxidant 300, antioxidant 1010 and antioxidant DLTP in a mass ratio of 1-1.5:1-1.5:1; or antioxidants used are antioxidant 1035, antioxidant 1010 and antioxidant DLTP in a mass ratio of 0.8-1.2:1-1.5:1.

[0044] In this invention, specific antioxidants can effectively improve the heat resistance and scorch resistance of the high-voltage direct current water-resistant cross-linked polyethylene insulation composition. For the long-cycle extrusion processing of high-voltage submarine cable insulation, even excessive amounts of a single antioxidant are insufficient to protect the material's heat aging resistance and scorch resistance during production, cable insulation extrusion, and use. Instead, excessive antioxidant migration and precipitation may occur during long-term operation of the submarine cable, affecting the cable's performance, heat resistance, and electrical properties.

[0045] Another objective of this invention is to provide a method for preparing the above-mentioned water-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables. The method involves feeding polyethylene resin, a composite antioxidant, and water-suppressing masterbatch into a BUSS high-speed shearing machine, followed by plasticization, precision filtration, underwater granulation, and blending and heat preservation absorption of the composite cross-linking agent and scorch inhibitor to obtain the water-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables.

[0046] A method for preparing a water-resistant, tree-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables, comprising:

[0047] A1 feeds polyethylene resin, composite antioxidant, and water-suppressing masterbatch into the BUSS high-speed shearing machine through a vector weighing feeding system. After plasticization and impurity filtration, it is shaped by underwater granulation process. After granulation, the particles are dehydrated and heated to dry.

[0048] A2 is mixed in a mixing system with a rotation speed of 2-6 rpm, and a composite crosslinking agent and a scorch inhibitor at a temperature of 65℃-75℃ are added. The particles, composite crosslinking agent and scorch inhibitor are mixed, dried and then kept warm to absorb, thus obtaining a high voltage DC water-resistant crosslinked polyethylene insulation composition.

[0049] Preferably, in step A1, the BUSS high-speed shear has a rotation speed of 270-350 rpm, a screw temperature of 80-120℃, a barrel temperature of 100-140℃, and a material temperature of 185-205℃. After being filtered by a multi-layer filter screen installed on the die head, the material is obtained through underwater granulation and drying. The filter screen includes at least one layer of 500-mesh high-precision filter screen.

[0050] Specifically, it can be equipped with 4 layers of filter screens, including a high-precision filter screen with a mesh size of 500 mesh that filters impurities larger than 25μm, and achieves a production capacity of 2.5T / h-3.5T / h through underwater granulation.

[0051] Preferably, in step A2, the heat absorption time is 10-14 hours, and the heat absorption temperature is 70-80℃.

[0052] This invention employs high-precision filtration design in both the preparation stages of the water-suppressing masterbatch and the insulating composition to ensure the insulation performance of high-voltage DC submarine cables. Specifically, the preparation of the water-suppressing masterbatch involves three layers of filters to remove impurities larger than 50 μm, ensuring the cleanliness of the masterbatch itself and guaranteeing uniform dispersion of polyethylene glycol, laying the foundation for the water-tree resistance of the subsequent insulation layer. The preparation of the insulating composition involves four layers of filters to remove impurities larger than 25 μm. The resulting insulating composition can be directly used for forming the submarine cable insulation layer, avoiding partial discharge and electric field distortion caused by long-term operation of the high-voltage DC submarine cable insulation layer in high-electric-field and humid environments. This also improves the microscopic uniformity of the insulating material, ensuring uniform electric field distribution and reducing the risk of localized breakdown. Furthermore, the low-content design of the composite crosslinking agent further reduces porosity and byproducts, synergistically improving the electrical performance of the insulation layer.

[0053] Another object of the present invention is to provide a submarine DC power cable capable of long-term operation at 90°C. This cable is prepared from the aforementioned high-voltage DC water-resistant cross-linked polyethylene insulation composition. It should be noted that the cable includes, but is not limited to, high-voltage DC submarine cables.

[0054] Cables formed from the water-resistant tree-type cross-linked polyethylene insulation composition for high-voltage DC submarine cables of the present invention can achieve a long-term operating temperature of 90°C.

[0055] The beneficial effects of this invention are as follows:

[0056] (1) This invention uses a single, low-density polyethylene resin matrix with a stable melt index, which enables the submarine cable to maintain a stable insulation thickness during long-term extrusion processing. This overcomes the problem of large fluctuations in insulation thickness caused by uneven mixing or melt index deviation when using composite resins, which leads to the thinnest point of the high-voltage cable insulation failing to meet standards. The introduction of composite antioxidants can delay the thermo-oxidative aging of materials, avoid performance degradation during processing and long-term operation, and improve the production qualification rate and product reliability. At the same time, by limiting the synergistic use of composite crosslinking agents and scorch inhibitors, the pre-crosslinked or old rubber and the pores and other by-products generated during the extrusion of insulation materials are reduced, avoiding the formation of dendritic and continuously growing dendritic structural channels, which is beneficial to the improvement of cable electrical performance. It can also reduce the generation of polyethylene molecular chain scorch during cable insulation extrusion, and improve the cable extrusion production cycle and efficiency.

[0057] (2) The polyolefin elastomer in the water-suppressing masterbatch has good compatibility with low-density polyethylene and can be effectively and uniformly dispersed in the non-crystalline region of low-density polyethylene, promoting the compatibility between the water-suppressing masterbatch matrix and the composite water-suppressing agent. Under the action of maleic anhydride-grafted polyethylene, the high molecular weight polyethylene glycol in the composite water-suppressing agent is effectively linked with the low-density polyethylene resin, making it less likely to precipitate in the cable insulation. Through the excellent hydrophilicity of the hydroxyl groups in polyethylene glycol, water molecules in the cable insulation are fixed, which will greatly reduce the formation and growth of water trees in the insulation. At the same time, oleamide can reduce the internal friction of the water-suppressing masterbatch during the preparation process, improve the fluidity of the composite polyolefin resin in the molten state, and thus reduce the uneven mixing of polyethylene glycol with the composite polyolefin resin after premature melting due to its low melting point. This scheme introduces pre-prepared water-suppressing masterbatch to ensure that the water-suppressing components are uniformly dispersed in the resin matrix, so that the insulation material can maintain stable insulation performance in a long-term humid electric field environment and extend the service life of the cable.

[0058] (3) The water-resistant tree-type insulating composition for high-voltage DC submarine cables of the present invention has a high gel content and uniform distribution, ensuring a dense and stable cross-linked network in the insulation layer, providing solid support for electrical performance. Simultaneously, it exhibits excellent material compatibility, enabling rapid extrusion into cables, with minimal fluctuations in insulation thickness during production and strong process stability, effectively guaranteeing the long-term operational reliability of high-voltage submarine cables. The corresponding preparation method is mature and stable. Through optimized premixing, melt plasticizing, and heat preservation absorption processes, large-scale mass production has been achieved, with a yield of 2.5T / h-3.5T / h. Furthermore, the parameters at each stage are highly controllable, significantly reducing batch fluctuation risks and balancing production efficiency with product quality consistency, making it suitable for the industrial production needs of long-span submarine cables. Detailed Implementation

[0059] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0060] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0061] Example 1

[0062] This embodiment provides a water-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables, which comprises the following components in parts by weight:

[0063] 100 parts low-density polyethylene resin; 0.25 parts composite antioxidant; 1.55 parts composite crosslinking agent; 0.25 parts 2,4-diphenyl-4-methyl-1-pentene; 10 parts water-inhibiting masterbatch;

[0064] The compound antioxidant includes 0.15 parts of antioxidant 300 and 0.1 parts of antioxidant 1010;

[0065] The composite crosslinking agent comprises 1.25 parts dicumyl peroxide and 0.3 parts triallyl isocyanurate;

[0066] The water-suppressing masterbatch is obtained by granulation of composite polyolefin resin, composite water-suppressing agent, maleic anhydride-grafted polyethylene, and antioxidant 300 in a mass ratio of 100:7.5:4.5:0.25; the composite water-suppressing agent includes polyethylene glycol and oleic acid amide in a mass ratio of 10:1. The composite polyolefin resin contains low-density polyethylene resin and ethylene-octene copolymer in a mass ratio of 1.2:1.

[0067] The preparation method of water-suppressing masterbatch includes:

[0068] S1 is prepared by mixing low-density polyethylene resin and polyolefin elastomer resin to obtain composite polyolefin resin, and by mixing polyethylene glycol and oleamide to obtain composite water inhibitor.

[0069] S2 adds the composite polyolefin resin, composite water suppressant, maleic anhydride graft and antioxidant into the mixer and mixes them at 30 rpm for 50 seconds.

[0070] S3 feeds the mixture into a twin-screw extruder. At a shear speed of 30 rpm, the screw temperature is controlled to gradually increase from the feed section to the die head, with the final section temperature reaching 160℃ and the material temperature between 160-180℃. After filtration through a three-layer filter installed in the die head, the mixture is granulated using water-jet extrusion. The filter includes at least one 300-mesh high-precision filter, which filters impurities larger than 50μm.

[0071] A method for preparing a water-resistant, tree-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables, comprising:

[0072] A1 is produced using a Swiss-imported "four-screw, four-pin" BUSS production line. Polyethylene resin, composite antioxidants, and water-suppressing masterbatch are fed into a BUSS high-speed shear mill via a vector weighing feeding system. The BUSS high-speed shear mill operates at 310 rpm, with a screw temperature of 80-120℃, a barrel temperature of 100-140℃, and a material temperature of 185-205℃. After filtration through a four-layer filter installed in the die head, the product is obtained through underwater granulation and drying. The filter includes at least one 500-mesh high-precision filter. After granulation, the particles are dehydrated and heated for drying; the 500-mesh high-precision filter filters out impurities larger than 25μm.

[0073] In a mixing system at 4 rpm, a composite crosslinking agent and a scorch inhibitor at 70°C were added. The particles, composite crosslinking agent, and scorch inhibitor were mixed, dried, and then kept warm for 12 hours at a temperature of 75°C to obtain a high-voltage direct current water-resistant crosslinked polyethylene insulation composition.

[0074] Example 2

[0075] This embodiment provides a water-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its preparation method is exactly the same as in Example 1, except that its raw material composition is different. It includes the following components in parts by weight:

[0076] 80 parts low-density polyethylene resin; 0.2 parts composite antioxidant; 1.25 parts composite crosslinking agent; 0.2 parts 2,4-diphenyl-4-methyl-1-pentene; 8 parts water-suppressing masterbatch;

[0077] The compound antioxidant includes 0.075 parts of antioxidant 300, 0.075 parts of antioxidant 1010, and 0.05 parts of antioxidant DLTP.

[0078] The composite crosslinking agent comprises 1.0 part dicumyl peroxide and 0.25 parts triallyl isocyanurate;

[0079] The water-suppressing masterbatch is obtained by granulation of composite polyolefin resin, composite water-suppressing agent, maleic anhydride-grafted polyethylene, and antioxidant 1035 in a mass ratio of 90:6.5:3.8:0.2; the composite water-suppressing agent includes polyethylene glycol and oleic acid amide in a mass ratio of 9:1. The composite polyolefin resin contains low-density polyethylene resin and ethylene-octene copolymer in a mass ratio of 1.1:1.

[0080] Example 3

[0081] This embodiment provides a water-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its preparation method is exactly the same as in Example 1, except that its raw material composition is different. It includes the following components in parts by weight:

[0082] 120 parts low-density polyethylene resin; 0.3 parts composite antioxidant; 1.85 parts composite crosslinking agent; 0.3 parts 2,4-diphenyl-4-methyl-1-pentene; 12 parts water-inhibiting masterbatch;

[0083] The compound antioxidant includes 0.09 parts of antioxidant 1035, 0.12 parts of antioxidant 1010, and 0.09 parts of antioxidant DLTP.

[0084] The composite crosslinking agent comprises 1.5 parts dicumyl peroxide and 0.35 parts triallyl isocyanurate;

[0085] The water-suppressing masterbatch is obtained by granulation of composite polyolefin resin, composite water-suppressing agent, maleic anhydride-grafted polyethylene, and antioxidant 300 in a mass ratio of 110:8.5:5.2:0.28; the composite water-suppressing agent includes polyethylene glycol and oleic acid amide in a mass ratio of 11:1. The composite polyolefin resin contains low-density polyethylene resin and ethylene-octene copolymer in a mass ratio of 1.4:1.

[0086] Table 1. Physical performance test results of Examples 1-3 (refer to JB / T 10437)

[0087]

[0088]

[0089] Comparative Example 1

[0090] This comparative example provides a water-resistant tree-type cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that no composite antioxidant is used, but 0.3 parts of antioxidant 1010 are added.

[0091] Table 2 shows the physical performance test results of Comparative Example 1 (refer to JB / T 10437).

[0092]

[0093] Comparative Example 2

[0094] This comparative example provides a water-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that 0.6 parts of a composite cross-linking agent are added, and the weight ratio of the two is the same as in Example 1. At this time, the mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide and triallyl isocyanurate is 1:1.936:0.464.

[0095] Table 3 shows the physical performance test results of Comparative Example 2 (refer to JB / T 10437).

[0096]

[0097]

[0098] Comparative Example 3

[0099] This comparative example provides a water-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation and preparation method are essentially the same as in Example 1, with the only difference being the addition of 2.0 parts of a composite cross-linking agent, maintaining the same weight ratio as in Example 1. At this point, the mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate is 1:6.452:1.548.

[0100] Table 4 shows the physical performance test results of Comparative Example 3 (refer to JB / T 10437).

[0101]

[0102] Comparative Example 4

[0103] This comparative example provides a water-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that the composite cross-linking agent is a single co-cross-linking agent, with 1.55 parts of triallyl isocyanurate added.

[0104] Table 5 shows the physical performance test results of Comparative Example 4 (refer to JB / T 10437).

[0105]

[0106]

[0107] Comparative Example 5

[0108] This comparative example provides a water-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that no scorch inhibitor is added.

[0109] Table 6 shows the physical performance test results of Comparative Example 5 (refer to JB / T 10437).

[0110]

[0111] Comparative Example 6

[0112] This comparative example provides a water-resistant tree-type cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that no water-suppressing masterbatch is added.

[0113] Table 7 shows the physical performance test results of Comparative Example 6 (refer to JB / T 10437).

[0114]

[0115]

[0116] Comparative Example 7

[0117] This comparative example provides a water-resistant tree-type cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that the composite polyolefin resin in the water-suppressing masterbatch is a single low-density polyethylene.

[0118] Table 8 shows the physical performance test results of Comparative Example 7 (refer to JB / T 10437).

[0119]

[0120] Comparative Example 8

[0121] This comparative example provides a water-resistant tree-type cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that the composite water-suppressing agent in the water-suppressing masterbatch is a single polyethylene glycol.

[0122] Table 9 shows the physical performance test results of Comparative Example 8 (refer to JB / T 10437).

[0123]

[0124]

[0125] Comparative Example 9

[0126] This comparative example provides a water-resistant tree-type cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that maleic anhydride grafts are not added to the water-suppressing masterbatch.

[0127] Table 10 shows the physical performance test results of Comparative Example 9 (refer to JB / T 10437).

[0128]

[0129] Comparative Example 10

[0130] This comparative example provides a water-resistant tree-type cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are exactly the same as those in Example 1. The difference is that water-suppressing masterbatch is not prepared. Instead, its formulation components are manually weighed and put into a high-speed mixer for mixing. After mixing for a certain period of time, the material is fed into a twin-screw extruder for melt mixing and granulation.

[0131] Table 11 shows the physical performance test results of Comparative Example 10 (refer to JB / T 10437).

[0132]

[0133]

[0134] Comparative Example 11

[0135] This comparative example provides a water-resistant tree-type cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formulation components are exactly the same as those in Example 1. The preparation method adopts conventional process: the formulation components are manually weighed and put into a high-speed mixer for mixing. After mixing for a certain period of time, the material is fed into a twin-screw extruder for melt mixing and granulation.

[0136] Table 12 shows the physical performance test results of Comparative Example 11 (refer to JB / T 10437).

[0137]

[0138] As can be seen from Example 1 and Comparative Example 1, antioxidant 1010 alone cannot make the insulation material meet the requirements for thermal aging, and cannot meet the requirements for insulation materials used in high-voltage DC submarine cables. This is mainly because the hydroxyl hydrogen atoms on the benzene ring of hindered phenolic antioxidant 1010 are easily combined with peroxy radicals (ROO·), alkoxy radicals (RO·), and hydroxyl radicals (HO·), terminating the oxidation process of polyethylene molecular chains and forming antioxidant capacity. However, antioxidant 1010 has a low melting point and is rapidly consumed during high-temperature extrusion processing and long-term thermal aging, and cannot provide long-term protection for the insulation material's performance. Vulcanization tests show that the insulation material has a short vulcanization time and low scorch resistance, making it unsuitable for long-cycle extrusion production of high-voltage DC submarine cables. Furthermore, adding too much antioxidant 1010 can cause the antioxidant to precipitate and "bloom" in the insulation material or migrate to the surface of the insulation material, reducing the electrical properties of the insulation material and the service life of the cable.

[0139] As can be seen from Example 1, Comparative Examples 2 and 3, if the amount of composite crosslinking agent is too low, the material will not be crosslinked enough, the gel content will be low, the mechanical properties of the material will decrease, and the thermal elongation after crosslinking will be too large. Although the vulcanization time in the vulcanization test is relatively long, it is mainly because the amount of composite crosslinking agent is too small, the content of free radicals generated by the decomposition of peroxide is low, resulting in low crosslinking density of the material. The material cannot be fully crosslinked, and the creep of polyethylene molecular chains increases in the non-crosslinked area, making it easy for water trees to form and increasing their length. Excessive use of composite crosslinking agents leads to an excessively high content of free radicals generated by the decomposition of peroxides. On the one hand, this consumes antioxidants in the material, reducing its thermal aging performance. On the other hand, it increases the crosslinking density, resulting in lower thermal elongation of the insulation material. Furthermore, during vulcanization, the rapid crosslinking of polyethylene molecular chains reduces scorch resistance. Simultaneously, the decomposition of excessive composite crosslinking agents increases the number of micropores in the insulation material, such as water and low molecular weight gases, increasing the probability and length of water tree formation, which significantly affects the electrical properties of the material and the service life of the cable. In addition, incompletely decomposed and polar composite crosslinking agents can transform into impurities in the material, also leading to a decrease in the material's dielectric properties and volume resistivity.

[0140] As can be seen from Example 1 and Comparative Example 4, triallyl isocyanurate relies on free radicals generated from the decomposition of dicumyl peroxide to initiate the cross-linking reaction. Single triallyl isocyanurate cannot generate free radicals independently, therefore effective cross-linking of the polyethylene molecular chains is impossible, the material cannot undergo vulcanization cross-linking, and its mechanical properties, thermal aging properties, and thermal elongation properties cannot meet the requirements for insulation materials in high-voltage DC submarine cables. Furthermore, the non-cross-linked polyethylene molecular chains undergo thermal creep migration, resulting in a large water tree length in the material.

[0141] As can be seen from Example 1 and Comparative Example 5, due to the long continuous extrusion cycle required for long-span submarine cables, composite crosslinking agents cannot effectively balance the thermal elongation, mechanical properties, and scorch resistance of cable insulation. Scorch inhibitors, during the crosslinking process of cable insulation, incorporate benzene ring groups into the polyethylene molecular chain, increasing the rigidity of the polyethylene molecular chain. This effectively ensures the heat resistance and mechanical properties of the insulation composition at a specific amount of composite crosslinking agent, reduces the scorch formation of polyethylene molecular chains during cable insulation extrusion, and improves the cable extrusion production cycle and efficiency. Adding scorch inhibitors to the insulation material allows some free radicals to link scorch inhibitor molecules into the polyethylene molecular chain during the crosslinking process caused by the decomposition of the crosslinking agent. This reduces excessive crosslinking of the polyethylene molecular chain, inhibits the formation of large-molecule polyethylene gel, and improves the material's scorch resistance. Vulcanization tests show that without scorch inhibitors, the material's scorch resistance is poor, making it unsuitable for long-cycle extrusion production of high-voltage DC submarine cables.

[0142] As demonstrated in Example 1 and Comparative Example 6, byproducts from the decomposition of the crosslinking agent, such as water and micropores of low molecular weight gases, will, under the combined effects of a high-voltage electric field and operating time, generate dendritic channels in the cable insulation, leading to a gradual decline in the electrical performance of the cable insulation. Furthermore, when water trees grow to a certain size, they will transform into electrical trees, ultimately causing cable insulation breakdown. Without the addition of water tree inhibitors, water tree growth in the material is significant, severely impacting the service life of the high-voltage DC submarine cable.

[0143] As can be seen from Examples 1-3 and Comparative Example 7, the polyolefin elastomer in the water-suppressing masterbatch composite polyolefin resin has an octene chain that is longer than the ethylene chain in its molecular structure. It plays a connecting and buffering role in the composition of the insulation material, increasing the impact resistance and elongation at break of the insulation material. At the same time, the polyolefin elastomer has a narrow molecular weight distribution and good fluidity, which can improve the fluidity and melt viscosity of the insulation material blend system, enhance the compatibility of composite water-suppressing agents and other additives with the insulation material resin matrix, and improve the resistance of the insulation material and cable insulation to water tree growth.

[0144] As can be seen from Examples 1-3 and Comparative Example 8, because the oleamide in the water-suppressing masterbatch has a certain polarity, it can reduce the friction in the melt during the extrusion processing of composite polyolefin resin and insulating material, and improve the fluidity of composite polyolefin resin and insulating material in the molten state. This reduces the uneven mixing of polyethylene glycol with composite polyolefin resin due to the premature melting of polyethylene glycol at its lower melting point. At the same time, trace amounts of oleamide migrate to the surface of the insulating material to form a film structure, which hinders the migration of polyethylene glycol with strong polarity to the surface of the insulating material. This affects the electrical properties of the insulating material and the growth of water trees in the cable insulation, resulting in a reduction in the service life of the cable.

[0145] As can be seen from Examples 1-3 and Comparative Example 9, due to the poor compatibility between the polar composite water-suppressing agent and the low-density polyethylene resin matrix of the insulating material, the polyethylene glycol in the composite water-suppressing agent will migrate to the surface during the extrusion processing of insulating materials or cable insulation. This not only affects the electrical properties of the insulating material and cable insulation, but also leads to a decrease in the cable insulation's resistance to water tree growth. The polar groups (maleic anhydride groups) of the maleic anhydride-grafted polyethylene in the water-suppressing masterbatch undergo an esterification reaction with the hydroxyl groups of polyethylene glycol to achieve chemical linkage, and the non-polar polyethylene segments are effectively linked with the low-density polyethylene resin matrix of the insulating material, making it difficult for the composite water-suppressing agent to migrate in the insulating material.

[0146] As can be seen from Examples 1-3 and Comparative Example 10, without preparing water-suppressing masterbatch, all raw materials were directly weighed and fed into a high-speed mixer for mixing, followed by melt mixing and granulation using a twin-screw extruder. Due to the low melting point of the composite water-suppressing agent, some of it adhered to the inner wall or agglomerated in the high-speed mixer. Simultaneously, in the initial stage of twin-screw extrusion, the partially melted water-suppressing agent could not be effectively and uniformly mixed with the low-density polyethylene resin. Furthermore, the grafting reaction between the maleic anhydride-grafted polyethylene and the composite water-suppressing agent on the low-density polyethylene resin required a certain temperature and time. Therefore, the composite water-suppressing agent was unevenly dispersed in the insulation material, resulting in a reduced content of the composite water-suppressing agent in the insulation material. Ungrafted composite water-suppressing agent migrated to the surface of the insulation material, affecting its electrical properties and resistance to water tree growth, and reducing production efficiency.

[0147] As can be seen from Examples 1-3 and Comparative Example 11, the production capacity is greatly reduced when using the traditional twin-screw melt mixing and granulation method. Because the composite crosslinking agent and scorch inhibitor are injected into the mixing melt in liquid form, the melt temperature is low. The pressure-bearing capacity and precision of the traditional twin-screw equipment filtration system are insufficient, and it cannot meet the fine filtration requirements of the 500-mesh high-precision filter screen. As a result, the material cannot meet the requirements of high-voltage submarine cables for impurities. At the same time, the use of manual operation reduces product stability and consistency, and reduces production efficiency.

[0148] In summary, as demonstrated in the examples and comparative examples, specific antioxidants can effectively improve the heat resistance and scorch resistance of insulation materials. Furthermore, the specific amount of composite crosslinking agent added helps reduce the crosslinking agent content in the high-voltage DC water-resistant crosslinked polyethylene insulation composition, reducing pre-crosslinked or aged rubber and pores and other byproducts generated during the extrusion process. This avoids the formation of dendritic channels in the cable insulation due to pores or other byproducts under the combined effects of high-voltage electric fields, operating time, and other factors, thus improving the electrical performance of the cable. Simultaneously, because long-span submarine cables require long continuous extrusion cycles for cable insulation, composite crosslinking agents cannot effectively balance the thermal elongation, mechanical properties, and scorch resistance of the cable insulation. The co-use of clean scorch inhibitors and composite crosslinking agents allows the benzene ring groups in the scorch inhibitor to be incorporated into the polyethylene molecular chain during the cable insulation crosslinking process, increasing the rigidity of the polyethylene molecular chain. This effectively ensures the heat resistance and mechanical properties of the insulation composition at the specific amount of composite crosslinking agent added, reduces the scorch of the polyethylene molecular chain during cable insulation extrusion, and improves the cable extrusion production cycle and efficiency. Water-suppressing masterbatch can fix water molecules in cable insulation, greatly reducing the formation and growth of water trees in the insulation and improving the service life of high-voltage cables. Using the Swiss-imported "four-screw, four-pin" BUSS production line not only significantly improves material production efficiency and stability but also ensures the stability of continuous extrusion of high-voltage cable insulation.

[0149] As shown in Tables 1 to 12, the water-tree resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables produced by this invention using Swiss-imported "four-screw, four-pin" BUSS wire exhibits excellent insulation properties, including inhibiting insulation scorching, resisting water tree growth, high gel content, fast extrusion cabling speed, and stable production process. It is suitable for the production of high-voltage DC submarine cables. The preparation method for the water-tree resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables boasts high yield and stable process, with a production capacity of up to 3.5 tons per hour. Currently, this manufacturer utilizes imported "four-screw, four-pin" BUSS wire to produce insulation material for high-voltage DC submarine cables, breaking through the conventional twin-screw production process and significantly improving production efficiency. It is a promising material for future high-voltage DC submarine cables.

[0150] Those skilled in the art should understand that the above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A water-resistant, tree-type cross-linked polyethylene insulation composition for high-voltage DC submarine cables, characterized in that, The components include the following parts by weight: 80-120 parts of polyethylene resin; 0.2-0.3 parts of compound antioxidant; 1.25-1.85 parts of composite crosslinking agent; 0.2-0.3 parts of scorch inhibitor; 8.0-12 parts of hydrophobic granules; The polyethylene resin is a low-density polyethylene resin with a melt index of 1.9-2.1 g / 10 min. The water-suppressing masterbatch is obtained by granulation of composite polyolefin resin, composite water-suppressing agent, maleic anhydride graft and antioxidant in a mass ratio of 80-120:6.0-9.0:3.6-5.4:0.15-0.25; the composite water-suppressing agent includes polyethylene glycol and oleamide in a mass ratio of 8-12:1, the average molecular weight of the polyethylene glycol is 20000, and the composite polyolefin resin includes low-density polyethylene resin with a melt index of 1.9-2.1 g / 10min and polyolefin elastomer resin with a melt index of 4.5-5.5 g / 10min in a mass ratio of 1-1.5:1; The scorch inhibitor is 2,4-diphenyl-4-methyl-1-pentene, and the composite crosslinking agent includes dicumyl peroxide and triallyl isocyanurate, wherein the mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate is 1:4.2-5.8:1.05-1.3; The composite antioxidant includes at least two of hindered phenolic antioxidants, thioester antioxidants, and thiophenolic antioxidants; The preparation method of the water-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables includes: A1 feeds the polyethylene resin, the composite antioxidant, and the water-suppressing masterbatch into a BUSS high-speed shear machine, where they are plasticized and impurities are filtered out. The mixture is then granulated using an underwater granulation process. After granulation, the particles are dehydrated and heated to dry. In a mixing system with a rotation speed of 2-6 rpm, a composite crosslinking agent and a scorch inhibitor at a temperature of 65℃-75℃ are added. The particles, the composite crosslinking agent, and the scorch inhibitor are mixed, dried, and then kept warm for absorption to obtain the high voltage DC water-resistant crosslinked polyethylene insulation composition.

2. The water-resistant, tree-like cross-linked polyethylene insulation composition for high-voltage DC submarine cables according to claim 1, characterized in that, In the water-suppressing masterbatch, the total amide content of the oleic acid amide is ≥98.5%, and the mass percentage of the polyethylene glycol in the water-suppressing masterbatch is 5-7%.

3. The water-resistant, tree-like cross-linked polyethylene insulation composition for high-voltage DC submarine cables according to claim 2, characterized in that, In the water-suppressing masterbatch, the melt index of the maleic anhydride graft is 1.0-3.0 g / 10 min; the antioxidant is a thiophenolic antioxidant.

4. The water-resistant tree-type cross-linked polyethylene insulation composition for high-voltage DC submarine cables according to claim 3, characterized in that, The method for preparing the water-suppressing masterbatch includes: S1. The composite polyolefin resin is prepared by mixing low-density polyethylene resin and polyolefin elastomer resin, and the composite water inhibitor is prepared by mixing polyethylene glycol and oleamide. S2. The composite polyolefin resin, the composite water suppressant, the maleic anhydride graft and the antioxidant are put into a mixer and mixed at a speed of 30±2 rpm for 40-60 seconds. S3 feeds the mixture into a twin-screw extruder. At a shear speed of 25-35 rpm, the screw temperature is controlled to gradually increase from the feed section to the die head, with the final section temperature at 155-165℃ and the material temperature at 160-180℃. After being filtered by a multi-layer filter screen installed in the die head, the mixture is granulated by water-pull granulation and dried to obtain clean water-suppressing masterbatch. The filter screen includes at least one layer of 300-mesh high-precision filter screen.

5. A method for preparing a water-resistant, tree-reinforced cross-linked polyethylene insulation composition for high-voltage DC submarine cables according to any one of claims 1-4, characterized in that, include: A1 feeds the polyethylene resin, the composite antioxidant, and the water-suppressing masterbatch into a BUSS high-speed shear machine, where they are plasticized and impurities are filtered out. The mixture is then granulated using an underwater granulation process. After granulation, the particles are dehydrated and heated to dry. In a mixing system with a rotation speed of 2-6 rpm, a composite crosslinking agent and a scorch inhibitor at a temperature of 65℃-75℃ are added. The particles, the composite crosslinking agent, and the scorch inhibitor are mixed, dried, and then kept warm for absorption to obtain the high voltage DC water-resistant crosslinked polyethylene insulation composition.

6. The method for preparing a water-resistant, tree-linked cross-linked polyethylene insulation composition for high-voltage DC submarine cables according to claim 5, characterized in that, In step A1, the BUSS high-speed shear has a rotation speed of 270-350 rpm, a screw temperature of 80-120℃, a barrel temperature of 100-140℃, and a material temperature of 185-205℃. After being filtered by a multi-layer filter screen installed on the die head, the material is obtained through underwater granulation and drying. The filter screen includes at least one layer of 500-mesh high-precision filter screen.

7. The method for preparing a water-resistant, tree-like cross-linked polyethylene insulation composition for high-voltage DC submarine cables according to claim 5, characterized in that, In step A2, the heat absorption time is 10-14 hours, and the heat absorption temperature is 70-80℃.

8. A cable prepared from the high voltage DC water-resistant tree-type cross-linked polyethylene insulation composition according to any one of claims 1-4.

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