A crosslinked polyethylene material for high voltage direct current cables and a method for producing the same

Abstract

The application discloses a kind of crosslinked polyethylene materials for high-voltage direct-current cable and preparation method thereof, it is related to polymer material processing technical field.The application includes the following steps: modified polyethylene, linear low-density polyethylene, modified titanium dioxide, block copolymer, modified alumina, methyl acrylate, dicumyl peroxide, antioxidant 1010, synergist are mixed after molding, then gradient crosslinking is carried out under nitrogen atmosphere, and cooling is obtained, and crosslinked polyethylene material for high-voltage direct-current cable is obtained.The modified polyethylene, linear low-density polyethylene are compounded according to specific proportion in the application, which effectively improves the insulating property and high-temperature stability of the material;The introduction of modified titanium dioxide, block copolymer, synergist and other components further improves the anti-insulation aging capacity, thermal stability, insulating property and mechanical property of the material.Therefore, the application has a more extensive application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer material processing, in particular to a cross-linked polyethylene material for high-voltage direct-current cables and a preparation method thereof. BACKGROUND

[0002] A high-voltage direct-current cable is a cable for transmitting high-voltage direct-current power, which includes a conductor, an insulation layer, a shielding layer, and a sheath layer. Cross-linked polyethylene material is widely used in the insulation layer of high-voltage direct-current cables due to its excellent insulation, electrical resistance, flexibility, tensile strength, tear resistance, heat resistance, acid and alkali corrosion resistance, and water resistance. However, the cross-linked polyethylene material has problems such as strong conductivity-temperature sensitivity, space charge accumulation, and insulation aging, which limit the further development of high-voltage direct-current cables to higher voltage levels.

[0003] In order to suppress the problems such as strong conductivity-temperature sensitivity of cross-linked polyethylene material, relevant technical personnel have made numerous researches and proposed solutions such as nano-particle doping modification and organic molecule grafting. For example, patent document CN114031837A proposes a cross-linkable polyethylene insulation material for high-voltage cables, a preparation method thereof, and the use thereof. According to the mass percentage of each component, the powder described insulation material includes the following components: polyethylene base material 97.5%-99.3%, cross-linking agent 0.5%-2%, cross-linking aid 0.1%-0.5%, and antioxidant 0.1%-0.5%. The invention modifies the traditional insulation material by introducing high-branched polyethylene, reduces the polar cross-linking decomposition by-products in the insulation material that cause non-linear changes in electrical conductivity, effectively suppresses the generation and migration of charges under a direct-current electric field, and further suppresses the non-linear effect of electrical conductivity in high-voltage cables.

[0004] However, the problem of insulation aging of cross-linked polyethylene material prepared by these existing methods still needs to be further solved. SUMMARY

[0005] The purpose of the present application is to provide a cross-linked polyethylene material for high-voltage direct-current cables and a preparation method thereof, which solves the following technical problems:

[0006] The problem of insulation aging of existing cross-linked polyethylene material.

[0007] The purpose of the present application can be achieved by the following technical solutions:

[0008] A preparation method of a cross-linked polyethylene material for high-voltage direct-current cables, comprising the following steps:

[0009] Mixing modified polyethylene, linear low density polyethylene, modified titanium dioxide, block copolymer, modified aluminum oxide, methyl acrylate, dicumyl peroxide, antioxidant 1010, synergist, and then compression molding, and then gradient crosslinking under nitrogen atmosphere, and then cooling to obtain a crosslinked polyethylene material for high-voltage direct-current cable.

[0010] Preferably, the mixing ratio of the modified polyethylene, linear low density polyethylene, modified titanium dioxide, block copolymer, modified aluminum oxide, methyl acrylate, dicumyl peroxide, antioxidant 1010, and synergist is 60-70 g: 30-35 g: 0.5-2 g: 5-6 g: 5-6 g: 3-5 g: 1-2 g: 2-3 g: 1-2 g.

[0011] The gradient crosslinking is first crosslinking at 160-170℃ for 10-15 min, then crosslinking at 180-190℃ for 10-15 min, and finally crosslinking at 195-205℃ for 10-15 min.

[0012] Preferably, the preparation method of the modified polyethylene is as follows:

[0013] Mixing low density polyethylene, maleic anhydride, and dicumyl peroxide under nitrogen atmosphere, and then melt blending at 180℃ for 20-25 min, and then granulating to obtain the modified polyethylene.

[0014] The mixing ratio of the low density polyethylene, maleic anhydride, and dicumyl peroxide is 100 g: 1.5-3 g: 0.1-0.3 g.

[0015] Preferably, the preparation method of the modified titanium dioxide is as follows:

[0016] Dispersing nano titanium dioxide in an ethanol aqueous solution, then adding dopamine hydrochloride and performing ultrasonic treatment, and then centrifuging, drying to obtain the modified titanium dioxide.

[0017] The mixing ratio of the nano titanium dioxide, the ethanol aqueous solution, and dopamine hydrochloride is 1-2 g: 100 mL: 0.05-0.1 g.

[0018] Preferably, the preparation method of the block copolymer is as follows:

[0019] Step A1: under nitrogen atmosphere, adding 4,4'-diamino diphenyl ether and N,N-dimethylacetamide to 4,4'-oxybisphthalic anhydride in sequence, stirring at 0-5℃ for 6-7 h, then adding acetic anhydride and pyridine, heating to 150-180℃, and then reacting for 5-5.2 h to obtain polyimide.

[0020] Step A2: Dissolve the polyimide in N,N-dimethylformamide, add polyethyleneimine and azobisisobutyronitrile, and react at 80-90 DEG C for 7-8h to obtain the block copolymer.

[0021] Preferably, the amount of 4,4'-oxybisphthalic anhydride, 4,4'-diamino diphenyl ether, N,N-dimethylacetamide, acetic anhydride, pyridine in step A1 is 27-31g: 17.6-20g: 200mL: 15-17g: 10-11.4g;

[0022] The amount of polyimide, N,N-dimethylformamide, polyethyleneimine, azobisisobutyronitrile in step A2 is 40-45g: 150mL: 5-5.7g: 0.5-0.57g.

[0023] Preferably, the preparation method of the modified alumina is as follows:

[0024] Mix the silane coupling agent KH550 with ethanol uniformly, then add nano-alumina and ultrasonic treatment at 55-60 DEG C for 2-3h, then centrifugal separation, drying treatment, to obtain modified alumina.

[0025] Preferably, the amount of silane coupling agent KH550, ethanol, nano-alumina is 0.04-0.18g: 100mL: 2-6g.

[0026] Preferably, the preparation method of the synergist is as follows:

[0027] At 10-20 DEG C, disperse chitosan in acetic acid aqueous solution, then add hexagonal boron nitride nanosheet, graphene oxide and deionized water and ultrasonic dispersion, then add sodium dodecyl sulfate and bidirectional freeze forming, vacuum freeze drying and gradient annealing in nitrogen atmosphere, and then cool to obtain the synergist.

[0028] Preferably, the amount of chitosan, acetic acid aqueous solution, hexagonal boron nitride nanosheet, graphene oxide, deionized water, sodium dodecyl sulfate is 30-50g: 500mL: 1-2g: 0.1-0.5g: 500-600mL: 0.05-0.1g.

[0029] The gradient annealing is first stage annealing at 100 DEG C for 1h, and then second stage annealing at 200-250 DEG C for 2h.

[0030] The beneficial effects of the present application are:

[0031] The present application provides a kind of high voltage direct current cable crosslinked polyethylene material and preparation method thereof, and the present application effectively improves the anti-insulation aging ability of crosslinked polyethylene material by the following method.

[0032] (1) The addition of specific proportions of hexagonal boron nitride nanosheets and graphene oxide in the synergist of the present application can generate physical crosslinking networks and hydrogen bonding interaction forces inside the crosslinked polyethylene material and form a network skeleton, thereby effectively hindering the crack propagation path and inhibiting the occurrence of phenomena such as strain concentration, carrier migration and electrical tree initiation, improving the tensile strength, fracture toughness and impact resistance of the material, and also improving the insulation and thermal stability. The bidirectional freezing-gradient annealing process will form an ordered crosslinking structure and a layered multi-arch structure in the synergist, so that the material can disperse stress through the "arch bridge effect", further improving the mechanical properties and thermal stability of the material. The synergist will also act together with dicumyl peroxide to form a more stable three-dimensional crosslinking network, further improving the thermal stability of the material.

[0033] (2) The present application improves the polarity and crosslinking degree of low-density polyethylene through maleic anhydride grafting and crosslinking, and further enhances the interfacial bonding by blending it with linear low-density polyethylene in a specific proportion, balances the toughness and strength, crystallinity and uniformity of the crosslinked polyethylene material, and effectively improves the mechanical properties, structural stability, insulation performance and high-temperature stability of the crosslinked polyethylene material.

[0034] (3) The addition of a specific proportion of modified alumina in the present application can significantly improve the tensile strength, elastic modulus and wear resistance of the material, form physical crosslinking points and inhibit chain segment movement; the addition of modified titanium dioxide can improve toughness through interfacial stress transfer, and the two will form a composite reinforcing network inside the material to balance strength and toughness. The addition of the two can also promote the occurrence of crosslinking reactions, improve the insulation, thermal conductivity and thermal stability of the material, and inhibit the growth of electrical trees to improve the breakdown field strength.

[0035] (4) The block copolymer of the present application can be used as an interfacial compatibilizer to improve the compatibility of modified polyethylene and linear low-density polyethylene, reduce phase separation, and improve the tensile strength and elongation at break of the material. The polar groups of the block copolymer can also interact with the surface of the nanofiller, promoting uniform dispersion of the filler and forming stronger interfacial bonding, thereby improving the modulus and toughness of the composite material. The block structure of the block copolymer will inhibit the accumulation of space charge through the synergistic effect of molecular chain segments, improving the insulation performance of the material. While inhibiting chain segment movement at high temperatures, the rigid structure of the block copolymer will also act together with dicumyl peroxide to form a thermally stable three-dimensional network, improving the thermal decomposition temperature and long-term use temperature of the material. The block copolymer will also combine with the layered structure in the synergist to form a "sandwich" interface, further optimizing impedance matching and interfacial polarization loss, improving the corona resistance and breakdown strength of the material.

[0036] Therefore, the crosslinked polyethylene material for high-voltage direct-current cables prepared by the application has more excellent anti-insulation aging capacity, thermal stability, insulation, mechanical properties, and wide application prospects. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application.

[0038] The properties and sources of some raw materials in the application are as follows:

[0039] Low-density polyethylene was purchased from Shanghai Mai Rui Biochemical Technology Co., Ltd., CAS: 9002-88-4, and the product number was M58947; nano-titanium dioxide was purchased from Shanghai Yunfu Nanometer Technology Co., Ltd., CAS: 13463-67-6; nano-aluminum oxide was purchased from Hubei Xirunde Chemical Co., Ltd., model number: TAP-A20, CAS: 1344-28-1; polyethylene imine (Mn=20000) was purchased from Suzhou Yakke Technology Co., Ltd., CAS: 9002-98-6; linear low-density polyethylene was purchased from Shanghai Mai Rui Biochemical Technology Co., Ltd., CAS: 9002-88-4, and the product number was M85222.

[0040] Example 1: A preparation method of the crosslinked polyethylene material for high-voltage direct-current cables is as follows:

[0041] S1: 100 g of low-density polyethylene, 1.5 g of maleic anhydride, and 0.1 g of dicumyl peroxide were mixed under a nitrogen atmosphere, and then melt blended at 180℃ for 20 min, followed by granulation treatment to obtain modified polyethylene;

[0042] S2: 1 g of nano-titanium dioxide was dispersed in 100 mL of an ethanol aqueous solution with a mass fraction of 20%, followed by the addition of 0.05 g of dopamine hydrochloride and ultrasonic treatment at a power of 200 W and a frequency of 40 kHz for 30 min, and then centrifugal separation and drying treatment were performed to obtain modified titanium dioxide;

[0043] S3: 17.6 g of 4,4'-diamino diphenyl ether and 200 mL of N,N-dimethylacetamide were sequentially added to 27 g of 4,4'-oxybisphthalic anhydride under a nitrogen atmosphere, and stirred at 0℃ for 6 h, followed by the addition of 15 g of acetic anhydride and 10 g of pyridine, and then the temperature was increased to 150℃ for reaction for 5 h to obtain polyimide;

[0044] S4: 40 g of polyimide was dissolved in 150 mL of N,N-dimethylformamide, 5 g of polyethyleneimine and 0.5 g of azobisisobutyronitrile were added, and the mixture was reacted at 80°C for 7 h to obtain a block copolymer;

[0045] S5: 0.04 g of silane coupling agent KH550 was mixed with 100 mL of ethanol, and then 2 g of nano-alumina was added and ultrasonicated at 55°C for 2 h. The mixture was then centrifuged and dried to obtain modified alumina.

[0046] S6: At 10°C, 30 g of chitosan was dispersed in 500 mL of a 1% acetic acid aqueous solution, followed by the addition of 1 g of hexagonal boron nitride nanosheets, 0.1 g of graphene oxide, and 500 mL of deionized water. The mixture was ultrasonically dispersed at a power of 200 W, a frequency of 40 kHz, and a time of 30 min. 0.05 g of sodium dodecyl sulfate was then added and bidirectionally frozen to form the mixture. The mixture was vacuum freeze-dried and annealed at 100°C for 1 h in a first stage under a nitrogen atmosphere, followed by a second stage annealing at 200°C for 2 h. The synergist was obtained after cooling.

[0047] S7: 60g of modified polyethylene, 30g of linear low-density polyethylene, 0.5g of modified titanium dioxide, 5g of block copolymer, 2g of modified alumina, 3g of methyl acrylate, 1g of diisopropyl peroxide, 2g of antioxidant 1010, and 1g of enhancer were mixed at 170°C for 15min and then compression molded at 115°C. The mixture was then heated to 160°C under a nitrogen atmosphere for cross-linking for 10min, then heated to 180°C for cross-linking for 10min, and finally heated to 195°C for cross-linking for 10min. After cooling, a cross-linked polyethylene material for high-voltage DC cable was obtained.

[0048] Example 2: A method for preparing a cross-linked polyethylene material for a high-voltage DC cable is as follows:

[0049] S1: Under a nitrogen atmosphere, 100 g of low-density polyethylene, 2.25 g of maleic anhydride, and 0.2 g of dicumyl peroxide were melt-blended at 180°C for 23 min, and then granulated to obtain modified polyethylene;

[0050] S2: 1.5 g of nano-titanium dioxide was dispersed in 100 mL of 20% ethanol aqueous solution, followed by the addition of 0.08 g of dopamine hydrochloride and ultrasonic treatment at a power of 250 W and a frequency of 40 kHz for 30 min. The solution was then centrifuged and dried to obtain modified titanium dioxide.

[0051] S3: 29 g of 4,4'-oxybisphthalic anhydride was added with 18.8 g of 4,4'-diamino diphenyl ether and 200 mL of N,N-dimethylacetamide successively under nitrogen atmosphere, stirred at 3°C for 6.5 h, then added with 16 g of acetic anhydride and 10.7 g of pyridine, heated to 165°C and reacted for 5.1 h to obtain a polyimide;

[0052] S4: 42.5 g of polyimide was dissolved in 125 mL of N,N-dimethylformamide, 5.3 g of polyethyleneimine and 0.53 g of azobisisobutyronitrile were added, and reacted at 85°C for 7.5 h to obtain a block copolymer;

[0053] S5: 0.11 g of silane coupling agent KH550 was uniformly mixed with 100 mL of ethanol, then 4 g of nano-alumina was added and ultrasonically treated at 58°C for 2.5 h, and then centrifuged, dried and treated to obtain modified alumina;

[0054] S6: 40 g of chitosan was dispersed in 500 mL of 1% acetic acid aqueous solution at 15°C, then 1.5 g of hexagonal boron nitride nanosheet, 0.3 g of graphene oxide and 550 mL of deionized water were added and ultrasonically dispersed at a power of 300 W and a frequency of 40 kHz for 45 min, then 0.08 g of sodium dodecyl sulfate was added and bidirectional freeze formed, vacuum freeze dried, then first stage annealed at 100°C for 1 h, and second stage annealed at 225°C for 2 h, and then cooled to obtain a synergist;

[0055] S7: 65 g of modified polyethylene, 32.5 g of linear low density polyethylene, 1.75 g of modified titanium dioxide, 5.5 g of block copolymer, 4 g of modified alumina, 4 g of methyl acrylate, 1.5 g of dicumyl peroxide, 2.5 g of antioxidant 1010, 1.5 g of synergist were mixed at 175°C for 18 min, then molded at 120°C, then crosslinked at 165°C for 13 min under nitrogen atmosphere, then crosslinked at 185°C for 13 min, and finally crosslinked at 200°C for 13 min, and then cooled to obtain a crosslinked polyethylene material for high-voltage direct-current cables.

[0056] Example 3: A preparation method of a crosslinked polyethylene material for high-voltage direct-current cables is as follows:

[0057] S1: 100 g of low density polyethylene, 3 g of maleic anhydride and 0.3 g of dicumyl peroxide were mixed under nitrogen atmosphere, then melt blended at 180°C for 25 min, and then pelletized to obtain modified polyethylene;

[0058] S2: 2 g of nano-titanium dioxide was dispersed in 100 mL of 20% ethanol aqueous solution, then 0.1 g of dopamine hydrochloride was added and ultrasonic treatment was performed at a power of 300 W and a frequency of 40 kHz for 30 min, followed by centrifugal separation and drying treatment to obtain modified titanium dioxide;

[0059] S3: 31 g of 4,4'-oxybisphthalic anhydride was added with 20 g of 4,4'-diaminodiphenyl ether and 200 mL of N,N-dimethylacetamide under a nitrogen atmosphere, and stirred at 0-5 °C for 7 h, then 17 g of acetic anhydride and 11.4 g of pyridine were added, and the temperature was raised to 180 °C for 5.2 h to obtain polyimide;

[0060] S4: 45 g of polyimide was dissolved in 150 mL of N,N-dimethylformamide, 5.7 g of polyethyleneimine and 0.57 g of azobisisobutyronitrile were added, and the reaction was carried out at 90 °C for 8 h to obtain a block copolymer;

[0061] S5: 0.18 g of silane coupling agent KH550 was uniformly mixed with 100 mL of ethanol, then 6 g of nano-alumina was added and ultrasonic treatment was carried out at 60 °C for 3 h, followed by centrifugal separation and drying treatment to obtain modified alumina;

[0062] S6: 50 g of chitosan was dispersed in 500 mL of 1% acetic acid aqueous solution at 20 °C, then 2 g of hexagonal boron nitride nanosheet, 0.5 g of graphene oxide and 600 mL of deionized water were added and ultrasonic dispersion was carried out at a power of 400 W and a frequency of 40 kHz for 60 min, then 0.1 g of sodium dodecyl sulfate was added and bidirectional freeze molding was carried out, followed by vacuum freeze drying, then first stage annealing at 100 °C for 1 h and second stage annealing at 250 °C for 2 h under a nitrogen atmosphere, and the synergist was obtained after cooling;

[0063] S7: 70 g of modified polyethylene, 35 g of linear low density polyethylene, 2 g of modified titanium dioxide, 6 g of block copolymer, 6 g of modified alumina, 5 g of methyl acrylate, 2 g of dicumyl peroxide, 3 g of antioxidant 1010, and 2 g of synergist were mixed at 180 °C for 20 min, then molded at 125 °C, then crosslinked at 170 °C for 15 min, then crosslinked at 190 °C for 15 min, and finally crosslinked at 205 °C for 15 min under a nitrogen atmosphere, and the crosslinked polyethylene material for high voltage direct current cables was obtained after cooling.

[0064] Comparative Example 1:

[0065] The comparative example is compared with example 1 only by replacing the "dispersing 10 g of chitosan in 500 mL of 1% by mass acetic acid aqueous solution, then adding 0.1 g of hexagonal boron nitride nanosheet, 1 g of graphene oxide" added in the preparation process of the synergist of S6 with "dispersing 10 g of chitosan in 500 mL of 1% by mass acetic acid aqueous solution, then adding 0.1 g of hexagonal boron nitride nanosheet, 1 g of graphene oxide", and the rest of the steps and parameters are the same, which will not be repeated here. The final high-voltage direct-current cable cross-linked polyethylene material is obtained.

[0066] Comparative example 2:

[0067] The comparative example is compared with example 1 only by replacing the "first stage annealing at 100°C for 1h, then second stage annealing at 200°C for 2h" added in the preparation process of the synergist of S6 with "second stage annealing at 200°C for 3h", and the rest of the steps and parameters are the same, which will not be repeated here. The final high-voltage direct-current cable cross-linked polyethylene material is obtained.

[0068] Comparative example 3:

[0069] The comparative example is compared with example 1 only by replacing the "60 g of modified polyethylene, 30 g of linear low-density polyethylene, 0.5 g of modified titanium dioxide, 5 g of block copolymer, 2 g of modified aluminum oxide, 3 g of methyl acrylate, 1 g of dicumyl peroxide, 2 g of antioxidant 1010, 1 g of synergist" added in the preparation process of the high-voltage direct-current cable cross-linked polyethylene material of S7 with "60 g of low-density polyethylene, 30 g of linear low-density polyethylene, 0.5 g of modified titanium dioxide, 5 g of block copolymer, 2 g of modified aluminum oxide, 3 g of methyl acrylate, 1 g of dicumyl peroxide, 2 g of antioxidant 1010, 1 g of synergist", and the rest of the steps and parameters are the same, which will not be repeated here. The final high-voltage direct-current cable cross-linked polyethylene material is obtained.

[0070] Comparative example 4:

[0071] The comparative example is compared with example 1 only by replacing the "60 g of modified polyethylene, 30 g of linear low-density polyethylene, 0.5 g of modified titanium dioxide, 5 g of block copolymer, 2 g of modified aluminum oxide, 3 g of methyl acrylate, 1 g of dicumyl peroxide, 2 g of antioxidant 1010, 1 g of synergist" added in the preparation process of the high-voltage direct-current cable cross-linked polyethylene material of S7 with "90 g of low-density polyethylene, 0.5 g of modified titanium dioxide, 5 g of block copolymer, 2 g of modified aluminum oxide, 3 g of methyl acrylate, 1 g of dicumyl peroxide, 2 g of antioxidant 1010, 1 g of synergist", and the rest of the steps and parameters are the same, which will not be repeated here. The final high-voltage direct-current cable cross-linked polyethylene material is obtained.

[0072] Comparative example 5:

[0073] The comparative example is compared with example 1 only by replacing the "60g modified polyethylene, 30g linear low density polyethylene, 0.5g modified titanium dioxide, 5g block copolymer, 2g modified alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010, 1g synergist" added in the preparation process of the crosslinked polyethylene material for high voltage direct current cable of S7 into "60g modified polyethylene, 30g linear low density polyethylene, 0.5g nano titanium dioxide, 5g block copolymer, 2g nano alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010, 1g synergist", and the rest of the steps and parameters are the same, which will not be repeated here. The final crosslinked polyethylene material for high voltage direct current cable is obtained.

[0074] Comparative example 6:

[0075] The comparative example is compared with example 1 only by replacing the "60g modified polyethylene, 30g linear low density polyethylene, 0.5g modified titanium dioxide, 5g block copolymer, 2g modified alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010, 1g synergist" added in the preparation process of the crosslinked polyethylene material for high voltage direct current cable of S7 into "60g modified polyethylene, 30g linear low density polyethylene, 0.5g modified titanium dioxide, 2g modified alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010, 1g synergist", and the rest of the steps and parameters are the same, which will not be repeated here. The final crosslinked polyethylene material for high voltage direct current cable is obtained.

[0076] Comparative example 7:

[0077] The comparative example is compared with example 1 only by replacing the "60g modified polyethylene, 30g linear low density polyethylene, 0.5g modified titanium dioxide, 5g block copolymer, 2g modified alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010, 1g synergist" added in the preparation process of the crosslinked polyethylene material for high voltage direct current cable of S7 into "60g modified polyethylene, 30g linear low density polyethylene, 0.5g modified titanium dioxide, 5g block copolymer, 2g nano alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010, 1g synergist", and the rest of the steps and parameters are the same, which will not be repeated here. The final crosslinked polyethylene material for high voltage direct current cable is obtained.

[0078] Comparative example 8:

[0079] The comparative example is compared with example 1 only by replacing the "60g modified polyethylene, 30g linear low density polyethylene, 0.5g modified titanium dioxide, 5g block copolymer, 2g nano alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010, 1g synergist" added in the preparation process of the crosslinked polyethylene material for high voltage direct current cable of S7 into "60g modified polyethylene, 30g linear low density polyethylene, 0.5g modified titanium dioxide, 5g block copolymer, 2g nano alumina, 3g methyl acrylate, 1g dicumyl peroxide, 2g antioxidant 1010", and the rest of the steps and parameters are the same, and the comparative example will not be repeated here. The final crosslinked polyethylene material for high voltage direct current cable is obtained.

[0080] Comparative example 9:

[0081] The comparative example is compared with example 1 only by replacing the "firstly heated to 160℃ for crosslinking 10min, then heated to 180℃ for crosslinking 10min, finally heated to 195℃ for crosslinking 10min" in the preparation process of the crosslinked polyethylene material for high voltage direct current cable of S7 into "firstly heated to 160℃ for crosslinking 10min, then heated to 180℃ for crosslinking 10min, finally heated to 195℃ for crosslinking 10min", and the rest of the steps and parameters are the same, and the comparative example will not be repeated here. The final crosslinked polyethylene material for high voltage direct current cable is obtained.

[0082] Performance detection:

[0083] Determination of high temperature resistance:

[0084] According to the standard of GB / T2951.12-2008 "Cables and optical cables - Insulation and sheath materials - General test methods - Part 12: General test methods - Heat ageing test", the crosslinked polyethylene material for high voltage direct current cable prepared by example 1- example 3 and comparative example 1- comparative example 9 is tested for aging resistance. The tensile strength (MPa) and elongation at break (%) of the crosslinked polyethylene material before and after aging at 150℃ for 1000h are recorded. The test results (the temperature of the crosslinked polyethylene material during testing is 25℃) are shown in table 1.

[0085] Determination of volume resistivity:

[0086] According to the standard of GB / T1410-2006 "Solid insulation materials - Volume and surface resistivity - Test methods", the volume resistivity of the crosslinked polyethylene material for high voltage direct current cable prepared by example 1- example 3 and comparative example 1- comparative example 9 is determined at 30℃, 90℃ and after aging at 90℃ for 1000h (the temperature during testing is 90℃) by using ZC36 type high resistance meter with a test voltage of 1kV. The test results are shown in table 1.

[0087] Table 1: Performance test results of Examples 1-3 and Comparative Examples 1-9

[0088]

[0089] Data Analysis:

[0090] It can be seen from Table 1 that the cross-linked polyethylene material for high-voltage DC cables prepared in the present invention has more excellent mechanical properties, high temperature resistance and insulation aging resistance.

[0091] This may be due to:

[0092] (1) The addition of a specific proportion of square boron nitride nanosheets and graphene oxide in the synergist of the present invention can generate a physical cross-linking network and hydrogen bonding and other interaction forces within the cross-linked polyethylene material and form a network skeleton, thereby effectively hindering the crack propagation path and inhibiting the occurrence of phenomena such as strain concentration, carrier migration and electrical tree induction, thereby improving the tensile strength, fracture toughness and impact resistance of the material while also improving the insulation and thermal stability. The bidirectional freeze-gradient annealing process will form an ordered cross-linked structure and a layered multi-arch structure of the synergist, so that the material can disperse stress through the "arch bridge effect", further improving the mechanical properties and thermal stability of the material. The synergist will also work together with diisopropylbenzene peroxide to form a more stable three-dimensional cross-linked network, further improving the thermal stability of the material.

[0093] (2) The present invention improves the polarity and cross-linking degree of low-density polyethylene by grafting and cross-linking with maleic anhydride, and blending it with linear low-density polyethylene in a specific proportion further enhances the interfacial bonding, balances the toughness and strength, crystallinity and uniformity of the cross-linked polyethylene material, and effectively improves the mechanical properties, structural stability, insulation properties and high-temperature stability of the cross-linked polyethylene material.

[0094] (3) The addition of modified alumina in a specific proportion in the present invention can significantly improve the tensile strength, elastic modulus, and wear resistance of the material, form physical crosslinking points, and inhibit chain segment motion. The addition of modified titanium dioxide can improve toughness through interfacial stress transfer, and the two will form a composite reinforcement network within the material, balancing strength and toughness. At the same time, the addition of both can also promote the occurrence of crosslinking reactions, improve the insulation, thermal conductivity, and thermal stability of the material, and inhibit the growth of electrical dendrites, thereby increasing the breakdown field strength.

[0095] (4) The block copolymer of the present application can be used as an interfacial compatibilizer to improve the compatibility of modified polyethylene with linear low density polyethylene, reduce phase separation, and improve the tensile strength and elongation at break of the material. The polar groups of the block copolymer can also interact with the surface of the nanofiller, promoting uniform dispersion of the filler and forming a stronger interfacial bond, thereby improving the modulus and toughness of the composite material. The block structure of the block copolymer can inhibit the accumulation of space charges through the synergistic effect of molecular segments, improving the insulating properties of the material. While the block copolymer inhibits the movement of segments at high temperatures, its rigid structure also interacts with dicumyl peroxide to form a thermally stable three-dimensional network, improving the thermal decomposition temperature and long-term use temperature of the material. The block copolymer can also combine with the layered structure of the synergist to form a "sandwich" interface, further optimizing impedance matching and interfacial polarization loss, and improving the corona resistance and breakdown strength of the material.

[0096] (5) The specific amount of methyl acrylate added in the present application can reduce the crystallinity of polyethylene, increase the segment mobility, and thus improve the flexibility and impact strength of the material. Methyl acrylate can synergistically interact with dicumyl peroxide to form a more stable crosslinking structure, improving the thermal decomposition temperature and long-term heat resistance. The ester group of methyl acrylate can also synergistically inhibit thermal oxidative degradation with antioxidant 1010, extending the service life of the material at high temperatures.

[0097] (6) The gradient crosslinking of the present application can avoid intense reaction at a single high temperature, allowing the components to gradually form a synergistic effect, perfect the crosslinking structure, inhibit charge accumulation, reduce local stress concentration, internal bubbles and micropore defects, and make the crosslinking more uniform, thereby improving the tensile strength, elastic modulus, creep resistance, volume resistivity, breakdown field strength, insulation stability and thermal decomposition temperature of the material. The modified alumina, titanium dioxide and nanosheets in the synergist also act as physical crosslinking points during the crosslinking process, forming a dual enhancement with the chemical crosslinking network, further improving the mechanical properties and thermal conductivity.

[0098] The above describes one embodiment of the present application in detail, but the content described is only a preferred embodiment of the present application and cannot be considered as limiting the scope of the present application. Any equivalent changes and improvements made within the scope of the present application should still be included in the scope of the present application.

Claims

1. A process for the preparation of a crosslinked polyethylene material for high voltage direct current cables, characterized in that, It comprises the following steps: The modified polyethylene, linear low density polyethylene, modified titanium dioxide, block copolymer, modified aluminum oxide, methyl acrylate, dicumyl peroxide, antioxidant 1010, synergist are mixed and then molded, and then gradient crosslinking is carried out under a nitrogen atmosphere, and then cooling is carried out, so that a crosslinked polyethylene material for high-voltage direct-current cables is obtained; The preparation method of the synergist is as follows: At 10-20℃, the chitosan is dispersed in the acetic acid aqueous solution, then the hexagonal boron nitride nanosheet, graphene oxide and deionized water are added and ultrasonic dispersion is carried out, then sodium dodecyl sulfate is added and bidirectional freeze forming is carried out, vacuum freeze drying is carried out, then gradient annealing is carried out under a nitrogen atmosphere, and then cooling is carried out, so that the synergist is obtained; The use amount ratio of the modified polyethylene, linear low density polyethylene, modified titanium dioxide, block copolymer, modified aluminum oxide, methyl acrylate, dicumyl peroxide, antioxidant 1010 and synergist is 60-70g:30-35g:0.5-2g:5-6g:5-6g:3-5g:1-2g:2-3g:1-2g; The gradient crosslinking is that the temperature is first increased to 160-170℃ and crosslinking is carried out for 10-15min, then the temperature is increased to 180-190℃ and crosslinking is carried out for 10-15min, and finally the temperature is increased to 195-205℃ and crosslinking is carried out for 10-15min; The preparation method of the modified polyethylene is as follows: Under a nitrogen atmosphere, the low density polyethylene, maleic anhydride and dicumyl peroxide are mixed and then melt blending is carried out at 180℃ for 20-25min, then granulation treatment is carried out, so that the modified polyethylene is obtained; The use amount ratio of the low density polyethylene, maleic anhydride and dicumyl peroxide is 100g:1.5-3g:0.1-0.3g.

2. The method of producing a crosslinked polyethylene material for high-voltage direct-current cables according to claim 1, characterized in that, The preparation method of the modified titanium dioxide is as follows: The nanometer titanium dioxide is dispersed in the ethanol aqueous solution, then dopamine hydrochloride is added and ultrasonic treatment is carried out, then centrifugal separation and drying treatment are carried out, so that the modified titanium dioxide is obtained; The use amount ratio of the nanometer titanium dioxide, ethanol aqueous solution and dopamine hydrochloride is 1-2g:100mL:0.05-0.1g.

3. The method of producing a crosslinked polyethylene material for high-voltage direct-current cables according to claim 1, characterized by, The preparation method of the block copolymer is as follows: Step A1: under a nitrogen atmosphere, 4,4'-oxyphthalic anhydride, 4,4'-diamino diphenyl ether and N,N-dimethylacetamide are sequentially added to 4,4'-oxyphthalic anhydride, stirring reaction is carried out at 0-5℃ for 6-7h, then acetic anhydride and pyridine are added, the temperature is increased to 150-180℃, and reaction is carried out for 5-5.2h, so that polyimide is obtained; Step A2: the polyimide is dissolved in N,N-dimethylformamide, polyethyleneimine and azobisisobutyronitrile are added, and reaction is carried out at 80-90℃ for 7-8h, so that the block copolymer is obtained.

4. The method of producing a crosslinked polyethylene material for high-voltage direct-current cables according to claim 3, characterized in that, The use amount ratio of 4,4'-oxyphthalic anhydride, 4,4'-diamino diphenyl ether, N,N-dimethylacetamide, acetic anhydride and pyridine in step A1 is 27-31g:17.6-20g:200mL:15-17g:10-11.4g. The polyimide, N,N-dimethylformamide, polyethyleneimine, azobisisobutyronitrile in step A2 are used in a ratio of 40-45 g: 150 mL: 5-5.7 g: 0.5-0.57 g.

5. The method of producing a crosslinked polyethylene material for high-voltage direct-current cables according to claim 1, characterized by, The preparation method of the modified alumina is as follows: The silane coupling agent KH550 is mixed with ethanol uniformly, then the nano alumina is added and ultrasonic treatment is conducted at 55-60 DEG C for 2-3 h, and then centrifugal separation and drying treatment are conducted to obtain the modified alumina.

6. The method of producing a crosslinked polyethylene material for high-voltage direct-current cables according to claim 5, characterized in that, The silane coupling agent KH550, ethanol, nano alumina are used in a ratio of 0.04-0.18 g: 100 mL: 2-6 g.

7. The method of producing a crosslinked polyethylene material for high-voltage direct-current cables according to claim 1, characterized by, The chitosan, acetic acid aqueous solution, hexagonal boron nitride nanosheet, graphene oxide, deionized water, sodium dodecyl sulfate are used in a ratio of 30-50 g: 500 mL: 1-2 g: 0.1-0.5 g: 500-600 mL: 0.05-0.1 g. The gradient annealing is first stage annealing at 100 DEG C for 1 h, and then second stage annealing at 200-250 DEG C for 2 h.

8. A crosslinked polyethylene material for high voltage direct current cables, characterized in that, The preparation method of any one of claims 1-7.

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