A polypropylene electrical insulating resin for high voltage DC cable and its preparation method and application

Abstract

The present invention relates to the technical field of insulating resins, and in particular to a polypropylene electrical insulating resin for high-voltage direct current cables, and a preparation method and application thereof. The polypropylene electrical insulating resin comprises, by weight, 45-60 parts of a high-melting-point copolymerized polypropylene resin, 10-25 parts of a low-melting-point copolymerized polypropylene resin, 20-30 parts of a thermoplastic styrene elastomer, 5-15 parts of a voltage stabilizer functionalized polypropylene-polystyrene grafted resin, and 0.3-1.5 parts of an antioxidant. The sum of the weight parts of the high-melting-point copolymerized polypropylene resin, the low-melting-point copolymerized polypropylene resin, the thermoplastic styrene elastomer, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is 100. The polypropylene electrical insulating resin for high-voltage direct current cables prepared by the present invention has high direct current breakdown strength and volume resistivity, and has a good ability to suppress space charge accumulation.

Description

Technical Field

[0001] The invention relates to the technical field of insulating resins, and in particular to a polypropylene electrical insulating resin for high-voltage direct current cables and a preparation method and application thereof. Background Art

[0002] Cross-linked polyethylene has the advantages of being cheap and easy to obtain, having high electrical insulation strength, good toughness, and high temperature resistance. As an insulating material, it is widely used in the manufacture of high-voltage DC power cables of various levels. However, with the increase in power load, in order to meet the needs of large-capacity power transmission, the transmission voltage level and operating temperature of power cables are gradually increasing, and cross-linked polyethylene power cables are gradually unable to meet the needs. In addition, after the retirement of cross-linked polyethylene power cables, cross-linked polyethylene is difficult to recycle, resulting in a large amount of waste and causing certain environmental problems. Compared with cross-linked polyethylene, polypropylene has a higher operating temperature, better electrical strength, simple processing technology, and is recyclable. It is an environmentally friendly power cable insulation material. Modified polypropylene insulation materials have been used in medium and low voltage power cables. However, the application of polypropylene as a high-voltage DC power cable also has problems such as space charge accumulation and excessive stiffness. As a good DC cable insulation material, it needs to have good toughness, high breakdown strength, low DC conductivity, and low space charge accumulation.

[0003] The method of blending polypropylene with elastomers can effectively reduce the stiffness of polypropylene and improve its toughness. Elastomers that can be used for insulation modification include polyolefin elastomers, such as POE; or styrene elastomers, such as SEBS, SEPS, etc. Polyolefin elastomers have a certain compatibility with polypropylene resins, which can effectively improve the toughness of polypropylene resins, but when the elastomer content reaches a certain level, phase separation still exists. This leads to a decrease in the electrical strength of the modified polypropylene. Styrene elastomers also have a certain compatibility with polypropylene resins. In addition, due to the presence of styrene, a large number of benzene rings are introduced into the blend. Benzene rings help absorb carrier energy and improve the electrical strength of insulating materials. Therefore, the blend of polypropylene / styrene elastomer can maintain good electrical resistance while improving mechanical properties. However, polypropylene / styrene elastomer blends have serious problems with space charge accumulation. This is because there are ethylene-butylene and ethylene-propylene rubber segments in styrene elastomers, and the rubber segments help to retain charges. When the elastomer content in the blend is too high, due to the effect of phase separation, the elastomer will form a phase of its own, which can easily cause space charge accumulation. Improving the interaction between styrene elastomer and polypropylene resin and reducing the size of the dispersed phase of the elastomer can effectively reduce the accumulation of space charge.

[0004] Studies have shown that adding voltage stabilizers to insulating materials can effectively improve the electrical breakdown strength of the materials. Voltage stabilizers can absorb the energy of carriers injected under high fields, thereby delaying the destruction of polymer molecular chains and improving the electrical strength of insulating materials. At the same time, since voltage stabilizers are usually polar, the presence of polarity also has a positive effect on inhibiting the accumulation of space charges. However, voltage stabilizers are small molecules and are easy to migrate during long-term use, resulting in a reduction in cable life. Since polypropylene is easily degraded in the presence of peroxides, it is impossible to fix the voltage stabilizer through conventional grafting technology. Summary of the invention

[0005] The purpose of the present invention is to solve the shortcomings of the prior art and to propose a polypropylene electrical insulating resin for high-voltage DC cables and a preparation method and application thereof. The prepared polypropylene electrical insulating resin for high-voltage DC cables has high DC breakdown strength and volume resistivity, and has a good ability to inhibit space charge accumulation.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A polypropylene electrical insulating resin for high-voltage DC cables, comprising, by weight:

[0008]

[0009] The sum of the weight parts of the high melting point copolymer polypropylene resin, the low melting point copolymer polypropylene resin, the thermoplastic styrene elastomer, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is 100;

[0010] The voltage stabilizer functionalized polypropylene-polystyrene grafted resin is prepared by pre-irradiating polypropylene resin, voltage stabilizer monomer, and styrene monomer by suspension grafting in an aqueous phase;

[0011] The voltage stabilizer in the voltage stabilizer functionalized polypropylene-polystyrene graft resin includes but is not limited to 4-acryloxyacetophenone, 4-acryloxy-4'-acetoxybenzil, and 2-(4-benzoyl-3-hydroxyphenoxy)ethyl acrylate, and the structural formula is as follows:

[0012]

[0013]

[0014] (c) Ethyl 2-(4-benzoyl-3-hydroxyphenoxy)acrylate.

[0015] Preferably, the high melting point copolymer polypropylene resin is a copolymer resin of ethylene and propylene, and has a melting point of 150-165°C.

[0016] Preferably, the low melting point copolymer polypropylene resin is a copolymer resin of ethylene and propylene, and has a melting point of 130-140°C.

[0017] Preferably, the thermoplastic styrene elastomer includes but is not limited to SEBS, SEPS, maleic anhydride grafted SEBS and the like.

[0018] Preferably, in the voltage stabilizer functionalized polypropylene-polystyrene graft resin, the mass percentage content of the voltage stabilizer is 10-20%, and the mass percentage content of the polystyrene is 25-40%.

[0019] Preferably, the pre-irradiated polypropylene resin is made of high melting point copolymer polypropylene resin. 60 Co or electron accelerator is used as the irradiation source, and polypropylene resin is pre-irradiated in an air atmosphere, with an irradiation dose of 15-50 kGy.

[0020] Preferably, the antioxidant is one of antioxidant 1010, antioxidant 1076, antioxidant 300 and antioxidant 1035, or a mixture of two or more of the antioxidant 1010, antioxidant 1076, antioxidant 300 and antioxidant 1035 in any proportion.

[0021] The present invention also provides a method for preparing a polypropylene electrical insulating resin for a high voltage DC cable, comprising the following steps:

[0022] Step 1: Preparation of pre-irradiated polypropylene resin

[0023] Using high melting point copolymer polypropylene resin as raw material, 60 Co is used as an irradiation source, and the polypropylene resin is pre-irradiated with gamma rays in an air atmosphere, and the pre-irradiation dose range is 15-50 kGy, so as to obtain a pre-irradiated polypropylene resin; or an electron accelerator is used as an irradiation source, and the polypropylene resin is pre-irradiated with beta rays in an air atmosphere, and the pre-irradiation dose range is 15-50 kGy, so as to obtain a pre-irradiated polypropylene resin;

[0024] Step 2: Preparation of voltage stabilizer functionalized polypropylene-polystyrene graft resin

[0025] Deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer and dispersant are dispersed in a reaction kettle, and nitrogen is passed through the reactor for protection. The temperature is raised to 60° C. for dispersion for 3 hours, and then an initiator is added. The temperature is raised to 85-90° C. for reaction for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is obtained by washing with deionized water and drying. The mass ratio of deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer and dispersant is 120:20:(4-8):(10-15):0.06; the amount of initiator used is 1% of the total mass of the voltage stabilizer monomer and the styrene monomer;

[0026] Step 3: Preparation of mixed materials

[0027] Add the voltage stabilizer functionalized polypropylene-polystyrene grafted resin prepared in step 2, high melting point copolymerized polypropylene resin, low melting point copolymerized polypropylene resin, thermoplastic styrene elastomer, and antioxidant into a high-speed mixer according to proportions, and mix them evenly;

[0028] Step 4: Preparation of polypropylene electrical insulating resin for high voltage DC cables

[0029] The mixed material prepared in step 3 is added into the hopper of a twin-screw extruder, the extrusion temperature is controlled at 130-220° C., and the polypropylene electrical insulating resin for high-voltage DC cables is obtained through extrusion, drawing, cooling and granulation.

[0030] Preferably, in step 2, the dispersant is one of polyvinyl alcohol, calcium phosphate and magnesium carbonate, and the initiator is dibenzoyl peroxide.

[0031] Another object of the present invention is to provide the use of the above-mentioned polypropylene electrical insulating resin for high-voltage DC cables as an insulating material for high-voltage DC transmission cables.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. In the voltage stabilizer functionalized polypropylene-polystyrene grafted resin prepared by the present invention, the polypropylene molecular chain is the main chain, and the copolymer chain of polystyrene-voltage stabilizer is the side chain, which has both the polypropylene resin component and the polystyrene segment component in the thermoplastic styrene elastomer, so that the prepared voltage stabilizer functionalized polypropylene-polystyrene grafted resin has the function of a high-efficiency compatibilizer, the interaction between polypropylene and the thermoplastic styrene elastomer is enhanced, and a large amount of polypropylene components are mixed into the thermoplastic styrene elastomer, which effectively suppresses the problem of space charge accumulation of the insulating material, and the presence of the polar voltage stabilizer also provides a positive effect on the space charge accumulation.

[0034] 2. The present invention directly copolymerizes the voltage stabilizer with polystyrene, and while introducing the voltage stabilizer, the voltage stabilizer is macromolecularized to avoid the migration of the voltage stabilizer. The synergistic effect formed by the voltage stabilizer and the polystyrene chain segments increases the DC breakdown strength of the high-voltage resistant polypropylene electrical insulating resin finally prepared to 530-600 kV / mm. At the same time, since the benzene rings in the voltage stabilizer and the polystyrene chain segments can effectively reduce the density and transport of the injected charge, the volume resistivity of the material is increased.

[0035] 3. In the polypropylene electrical insulating resin for high-voltage DC cables of the present invention, high-melting-point copolymerized polypropylene and low-melting-point copolymerized polypropylene are selected. The high-melting-point copolymerized polypropylene resin provides the necessary mechanical strength and heat resistance for the insulating material, and the low-melting-point copolymerized polypropylene resin can effectively improve the toughness and low-temperature resistance of the insulating material. Since the high-melting-point copolymerized polypropylene resin and the low-melting-point copolymerized polypropylene resin have similar structures but different crystallization behaviors, blending the two helps to inhibit the high-melting-point copolymerized polypropylene from growing complete spherulites, forming a structure in which crystalline regions and amorphous regions are alternately distributed, which significantly improves the electrical insulation performance of the insulating material. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a spatial charge distribution diagram of Example 2 of the present invention;

[0037] Figure 2 This is a spatial charge distribution diagram of Comparative Example 1 of the present invention;

[0038] Figure 3 This is the spatial charge distribution diagram of Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the advantages and features of the present invention, thereby making a clearer definition of the protection scope of the present invention. The embodiments described in the present invention are only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0040] Reference Figure 1-Figure 3 , a polypropylene electrical insulating resin for high voltage DC cable, comprising by weight:

[0041]

[0042] The sum of the weight parts of the high melting point copolymer polypropylene resin, the low melting point copolymer polypropylene resin, the thermoplastic styrene elastomer, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is 100;

[0043] Among them, the high melting point copolymer polypropylene resin and the low melting point copolymer polypropylene resin are both copolymerized by ethylene and propylene, the high melting point copolymer polypropylene resin has a melting point range of 150-165°C, and the low melting point copolymer polypropylene resin has a melting point range of 130-140°C, and the thermoplastic styrene elastomer includes but is not limited to SEBS, SEPS, maleic anhydride grafted SEBS, etc. The voltage stabilizer functionalized polypropylene-polystyrene grafted resin is prepared by pre-irradiated polypropylene resin, voltage stabilizer monomer, and styrene monomer by aqueous phase suspension grafting, wherein the mass percentage of the voltage stabilizer is 10-20%, and the mass percentage of the polystyrene is 25-40%. The pre-irradiated polypropylene resin uses the high melting point copolymer polypropylene resin as the raw material, and adopts 60 Co or electron accelerator is used as the irradiation source, and polypropylene resin is pre-irradiated in an air atmosphere, with an irradiation dose of 15-50 kGy.

[0044] The voltage stabilizer includes but is not limited to 4-acryloxyacetophenone, 4-acryloxy-4'-acetoxybenzil, and 2-(4-benzoyl-3)ethyl acrylate, and the structural formula is as follows:

[0045]

[0046] (c) Ethyl 2-(4-benzoyl-3-hydroxyphenoxy)acrylate.

[0047] The antioxidant is one of antioxidant 1010, antioxidant 1076, antioxidant 300 and antioxidant 1035 or a mixture of two or more of them in any proportion.

[0048] The present invention also provides a method for preparing a high voltage resistant polypropylene electrical insulating resin, comprising the following steps:

[0049] Step 1: Preparation of pre-irradiated polypropylene resin

[0050] Using high melting point copolymer polypropylene resin as raw material, 60 Co is used as the irradiation source, and the polypropylene resin is pre-irradiated with γ rays in an air atmosphere, and the pre-irradiation dose range is 15-50kGy to obtain a pre-irradiated polypropylene resin; or an electron accelerator is used as the irradiation source, and the polypropylene resin is pre-irradiated with β rays in an air atmosphere, and the pre-irradiation dose range is 15-50kGy to obtain a pre-irradiated polypropylene resin.

[0051] Step 2: Preparation of voltage stabilizer functionalized polypropylene-polystyrene graft resin

[0052] Deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer, and dispersant are dispersed in a reactor, nitrogen is passed through, the temperature is raised to 60°C for dispersion for 3 hours, and then an initiator is added, the temperature is raised to 85-90°C for reaction for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is obtained by washing with deionized water and drying. The mass ratio of deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer, and dispersant is 120:20:(4-8):(10-15):0.6. The amount of initiator is 1% of the total mass of the voltage stabilizer monomer and the styrene monomer.

[0053] The dispersant is one of polyvinyl alcohol, calcium phosphate and magnesium carbonate, and the initiator is dibenzoyl peroxide.

[0054] Step 3: Preparation of mixed materials

[0055] The voltage stabilizer functionalized polypropylene-polystyrene grafted resin prepared in step 2, high melting point copolymerized polypropylene resin, low melting point copolymerized polypropylene resin, thermoplastic styrene elastomer and antioxidant are added into a high-speed mixer according to proportion and mixed evenly.

[0056] Step 4: Preparation of polypropylene electrical insulating resin for high voltage DC cables

[0057] The mixed material prepared in step 3 is added into the hopper of a twin-screw extruder, the extrusion temperature is controlled at 130-220° C., and the polypropylene electrical insulating resin for high-voltage DC cables is obtained through extrusion, drawing, cooling and granulation.

[0058] The high voltage resistant polypropylene electrical insulating resin of the present invention is used as an insulating material for high voltage direct current transmission cables.

[0059] The DC breakdown test is conducted in accordance with GB / T1408.1-2016 and GB / T1408.2-2016 standards. A linearly increasing DC voltage is applied to the insulating material (75μm thick film sample) at a rate of 1kV / mm until the sample breaks down, and the average breakdown strength of 10 samples is obtained. The DC test is conducted after the samples have been placed for 1 day and 60 days.

[0060] Volume resistivity measurement method: Apply a 30kV / mm DC electric field to the insulating material, use an electrometer to collect the leakage current, and calculate the volume resistivity based on the test current. The formula is as follows:

[0061]

[0062] Wherein, ρ is the volume resistivity, unit: Ω·m; I is the measured leakage current, unit: A; E is the electric field strength, 30 kV / mm; S is the test electrode area, and the electrode diameter is 50 mm.

[0063] The space charge test was conducted using an electroacoustic pulse space charge measuring instrument. The sample thickness was 0.25 mm, and a 30 kV / mm DC electric field was applied for 30 min.

[0064] The mechanical properties are tested in accordance with GB / T 1040.2-2022 standard using a universal tensile testing machine.

[0065] Example 1

[0066] Using high melting point copolymer polypropylene resin as raw material, melting point 163℃, 60 Co was used as the irradiation source, and the polypropylene resin was pre-irradiated with gamma rays in an air atmosphere, with a pre-irradiation dose range of 15 kGy to obtain a pre-irradiated polypropylene resin. 3.6 L of deionized water, 600 g of pre-irradiated polypropylene resin, 120 g of a voltage stabilizer monomer 4-acryloxyacetophenone, 300 g of a styrene monomer, and 0.6 polyvinyl alcohol were dispersed in a reactor, nitrogen was passed through, the temperature was raised to 60° C. and dispersed for 3 hours, and then 4.2 g of an initiator dibenzoyl peroxide was added, the temperature was raised to 85° C. and reacted for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin was obtained by washing with deionized water and drying. A low melting point copolymer polypropylene resin with a melting point of 133°C is selected, and voltage stabilizer functionalized polypropylene-polystyrene grafted resin, high melting point copolymer polypropylene resin, low melting point copolymer polypropylene resin, thermoplastic styrene elastomer SEBS, and antioxidant 1010 are added to a high-speed mixer at a mass ratio of 5:45:25:25:0.7 and mixed evenly. The prepared mixture is added to the hopper of a twin-screw extruder, and the extrusion temperature is controlled at 130-220°C. After extrusion, drawing, cooling, and granulation, a polypropylene electrical insulating resin for high-voltage DC cables is obtained.

[0067] Example 2

[0068] Using high melting point copolymer polypropylene resin as raw material, melting point 157℃, 60Co was used as the irradiation source, and the polypropylene resin was pre-irradiated with gamma rays in an air atmosphere, and the pre-irradiation dose range was 30 kGy to obtain a pre-irradiated polypropylene resin. 3.6 L of deionized water, 600 g of pre-irradiated polypropylene resin, 240 g of a voltage stabilizer monomer 4-acryloyloxy-4'-acetoxybenzil, 450 g of a styrene monomer, and 0.6 polyvinyl alcohol were dispersed in a reactor, nitrogen was passed through, the temperature was raised to 60° C. and dispersed for 3 hours, and then 6.9 g of an initiator dibenzoyl peroxide was added, the temperature was raised to 90° C. and reacted for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin was obtained by washing with deionized water and drying. A low melting point copolymer polypropylene resin with a melting point of 137°C is selected, and voltage stabilizer functionalized polypropylene-polystyrene grafted resin, high melting point copolymer polypropylene resin, low melting point copolymer polypropylene resin, thermoplastic styrene elastomer SEBS, and antioxidant 1035 are added to a high-speed mixer at a mass ratio of 15:45:10:30:0.3 and mixed evenly. The prepared mixture is added to the hopper of a twin-screw extruder, and the extrusion temperature is controlled at 130-220°C. After extrusion, drawing, cooling, and granulation, a high voltage resistant polypropylene electrical insulation resin is obtained.

[0069] Example 3

[0070] In this embodiment, other components are the same as those in Embodiment 2, except that the voltage stabilizer functionalized polypropylene-polystyrene grafted resin, high melting point copolymerized polypropylene resin, low melting point copolymerized polypropylene resin, thermoplastic styrene elastomer SEBS, and antioxidant 1035 are mixed in a mass ratio of 5:55:10:30:0.3.

[0071] Example 4

[0072] The high melting point copolymer polypropylene resin is used as raw material, the melting point is 157°C, an electron accelerator is used as an irradiation source, and the polypropylene resin is pre-irradiated with β rays in an air atmosphere, and the pre-irradiation dose range is 50 kGy to obtain a pre-irradiated polypropylene resin. 3.6L of deionized water, 600g of pre-irradiated polypropylene resin, 150g of voltage stabilizer monomer 4-acryloxy-4'-acetoxybenzil, 360g of styrene monomer, and 0.6 calcium phosphate are dispersed in a reactor, nitrogen is passed through, the temperature is raised to 60°C for dispersion for 3 hours, and then 5.1g of initiator dibenzoyl peroxide is added, the temperature is raised to 90°C for reaction for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is obtained by washing with deionized water and drying. A low melting point copolymer polypropylene resin with a melting point of 133°C is selected, and voltage stabilizer functionalized polypropylene-polystyrene grafted resin, high melting point copolymer polypropylene resin, low melting point copolymer polypropylene resin, thermoplastic styrene elastomer SEPS, and antioxidant 1076 are added to a high-speed mixer at a mass ratio of 5:45:20:30:1.2, and mixed evenly. The prepared mixture is added to the hopper of a twin-screw extruder, and the extrusion temperature is controlled at 130-220°C. After extrusion, drawing, cooling, and granulation, a polypropylene electrical insulating resin for high-voltage DC cables is obtained.

[0073] Example 5

[0074] The high melting point copolymer polypropylene resin is used as raw material, the melting point is 163°C, an electron accelerator is used as an irradiation source, and the polypropylene resin is pre-irradiated with β rays in an air atmosphere, and the pre-irradiation dose range is 25kGy to obtain a pre-irradiated polypropylene resin. 3.6L of deionized water, 600g of pre-irradiated polypropylene resin, 210g of voltage stabilizer monomer 2-(4-benzoyl-3-hydroxyphenoxy) ethyl acrylate, 360g of styrene monomer, and 0.6 magnesium carbonate are dispersed in a reactor, nitrogen is passed through, the temperature is raised to 60°C for dispersion for 3 hours, and then 5.7g of initiator dibenzoyl peroxide is added, the temperature is raised to 85°C for reaction for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is obtained by washing with deionized water and drying. A low melting point copolymer polypropylene resin with a melting point of 137°C is selected, and voltage stabilizer functionalized polypropylene-polystyrene grafted resin, high melting point copolymer polypropylene resin, low melting point copolymer polypropylene resin, thermoplastic styrene elastomer maleic anhydride grafted SEBS, and composite antioxidant (antioxidant 1010 and antioxidant 1035 are mixed in equal mass ratios) are added to a high-speed mixer in a mass ratio of 10:60:10:20:1.5 and mixed evenly. The prepared mixture is added to the hopper of a twin-screw extruder, the extrusion temperature is controlled at 130-220°C, and the polypropylene electrical insulating resin for high-voltage DC cables is obtained through extrusion, drawing, cooling, and granulation.

[0075] Example 6

[0076] The high melting point copolymer polypropylene resin is used as raw material, the melting point is 157°C, an electron accelerator is used as an irradiation source, and the polypropylene resin is pre-irradiated with β rays in an air atmosphere, and the pre-irradiation dose range is 35kGy to obtain a pre-irradiated polypropylene resin. 3.6L of deionized water, 600g of pre-irradiated polypropylene resin, 120g of voltage stabilizer monomer 4-acryloxyacetophenone, 360g of styrene monomer, and 0.6 calcium phosphate are dispersed in a reactor, nitrogen is passed through, the temperature is raised to 60°C for dispersion for 3 hours, and then 4.8g of initiator dibenzoyl peroxide is added, the temperature is raised to 90°C for reaction for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is obtained by washing with deionized water and drying. A low melting point copolymer polypropylene resin with a melting point of 137°C is selected, and voltage stabilizer functionalized polypropylene-polystyrene grafted resin, high melting point copolymer polypropylene resin, low melting point copolymer polypropylene resin, thermoplastic styrene elastomer, and antioxidant are added to a high-speed mixer at a mass ratio of 10:50:15:25:1.0, and mixed evenly. The prepared mixture is added to the hopper of a twin-screw extruder, and the extrusion temperature is controlled at 130-220°C. After extrusion, drawing, cooling, and granulation, a polypropylene electrical insulating resin for high-voltage DC cables is obtained.

[0077] Comparative Example 1: Functionalized polypropylene-polystyrene grafted resin without voltage stabilizer

[0078] High melting point copolymer polypropylene resin, low melting point copolymer polypropylene resin, thermoplastic styrene elastomer and antioxidant are added into a high-speed mixer in a mass ratio of 60:10:30:0.3 and mixed evenly. The mixture prepared in step 3 is added into the hopper of a twin-screw extruder, and the extrusion temperature is controlled at 130-220°C. After extrusion, drawing, cooling and granulation, a high voltage resistant polypropylene electrical insulating resin is obtained.

[0079] Comparative Example 2: Polypropylene-polystyrene grafted resin does not contain voltage stabilizer

[0080] In this embodiment, other aspects are the same as those of the embodiment 4, except that when polymerizing the polypropylene-polystyrene graft resin, 150 g of the voltage stabilizer monomer 4-acryloxy-4'-acetoxybenzil is replaced with a styrene monomer.

[0081] Comparative Example 3: Voltage stabilizer without grafting

[0082] In this embodiment, the other parts are the same as those in embodiment 4, except that no voltage stabilizer is added when polymerizing the polypropylene-polystyrene grafted resin. The voltage stabilizer is added to the insulating resin by blending, and the content of the voltage stabilizer in the prepared polypropylene electrical insulating resin is kept equivalent to that of the high voltage resistant polypropylene electrical insulating resin prepared in embodiment 4.

[0083]

[0084] As can be seen from the above table: By comparing Example 2 with Comparative Example 1, after replacing 15% by mass of the high melting point polypropylene resin with the voltage stabilizer functionalized polypropylene-polystyrene grafted resin, the elongation at break is significantly increased, the yield strength is not significantly increased, the space charge accumulation is suppressed, the DC breakdown strength is significantly improved, and the volume resistivity is increased. This indicates that the voltage stabilizer functionalized polypropylene-polystyrene grafted resin can improve the comprehensive electrical insulation properties of the polypropylene insulating resin but does not have a significant effect on the mechanical properties of the resin.

[0085] Comparing Example 4 with Comparative Example 2, when there is no voltage stabilizer in the polypropylene-polystyrene grafted resin, the DC breakdown strength decreases by 50 kV / mm and the volume resistivity decreases by 3x10 14 Ω·m, the space charge accumulation increases, but the mechanical properties do not change significantly. This shows that the presence of voltage stabilizer is beneficial to improve the electrical insulation properties.

[0086] Comparing Example 4 with Comparative Example 3, when the voltage stabilizer is not added to the blend by grafting, the DC breakdown strength decreases significantly over time, and the degree of decrease is more serious than when no voltage stabilizer is added, indicating that the migration of the voltage stabilizer will lead to greater performance loss.

[0087] In summary, the polypropylene electrical insulating resin for high-voltage DC cables prepared in the present invention has high DC breakdown strength and volume resistivity, and has good ability to inhibit space charge accumulation.

[0088] The description and practice disclosed in the present invention are easy to think and understand for ordinary technicians in the technical field, and several improvements and modifications can be made without departing from the principles of the present invention. Therefore, modifications or improvements made without departing from the spirit of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A polypropylene electrical insulating resin for high voltage DC cable, characterized in that: The parts by weight include: The sum of the weight parts of the high melting point copolymer polypropylene resin, the low melting point copolymer polypropylene resin, the thermoplastic styrene elastomer, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is 100; The voltage stabilizer functionalized polypropylene-polystyrene grafted resin is prepared by pre-irradiating polypropylene resin, voltage stabilizer monomer, and styrene monomer by suspension grafting in an aqueous phase; The voltage stabilizer in the voltage stabilizer functionalized polypropylene-polystyrene graft resin includes 4-acryloxyacetophenone, 4-acryloxy-4'-acetoxybenzil, and 2-(4-benzoyl-3-hydroxyphenoxy)ethyl acrylate, and the structural formula is as follows: The preparation of voltage stabilizer functionalized polypropylene-polystyrene grafted resin is as follows: Deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer and dispersant are dispersed in a reaction kettle, and nitrogen is passed through the reactor for protection. The temperature is raised to 60° C. for dispersion for 3 hours. Subsequently, an initiator is added, and the temperature is raised to 85-90° C. for reaction for 8 hours. The voltage stabilizer functionalized polypropylene-polystyrene grafted resin is obtained by washing with deionized water and drying. The mass ratio of deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer and dispersant is 120:20:(4-8):(10-15):0.

06. The amount of initiator used is 1% of the total mass of the voltage stabilizer monomer and the styrene monomer.

2. The polypropylene electrical insulating resin for high voltage DC cable according to claim 1, characterized in that: The high melting point copolymer polypropylene resin is a copolymer resin of ethylene and propylene, and has a melting point of 150-165°C.

3. The polypropylene electrical insulating resin for high voltage DC cable according to claim 1, characterized in that: The low melting point copolymer polypropylene resin is a copolymer resin of ethylene and propylene, and has a melting point of 130-140°C.

4. The polypropylene electrical insulating resin for high voltage DC cable according to claim 1, characterized in that: The thermoplastic styrene elastomer includes SEBS, SEPS, and maleic anhydride grafted SEBS.

5. The polypropylene electrical insulating resin for high voltage DC cable according to claim 1, characterized in that: In the voltage stabilizer functionalized polypropylene-polystyrene grafted resin, the mass percentage content of the voltage stabilizer is 10-20%, and the mass percentage content of the polystyrene is 25-40%.

6. The polypropylene electrical insulating resin for high voltage DC cable according to claim 1, characterized in that: The pre-irradiated polypropylene resin is made of high melting point copolymerized polypropylene resin. 60 Co or electron accelerator is used as the irradiation source, and polypropylene resin is pre-irradiated in an air atmosphere, with an irradiation dose of 15-50 kGy.

7. The polypropylene electrical insulating resin for high voltage DC cable according to claim 1, characterized in that: The antioxidant is one of antioxidant 1010, antioxidant 1076, antioxidant 300 and antioxidant 1035, or a mixture of two or more of them in any proportion.

8. A method for preparing a polypropylene electrical insulating resin for a high voltage DC cable according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step 1: Preparation of pre-irradiated polypropylene resin Using high melting point copolymer polypropylene resin as raw material, 60 Co is used as an irradiation source, and the polypropylene resin is pre-irradiated with gamma rays in an air atmosphere, and the pre-irradiation dose range is 15-50 kGy to obtain a pre-irradiated polypropylene resin; or an electron accelerator is used as an irradiation source, and the polypropylene resin is pre-irradiated with beta rays in an air atmosphere, and the pre-irradiation dose range is 15-50 kGy to obtain a pre-irradiated polypropylene resin; Step 2: Preparation of voltage stabilizer functionalized polypropylene-polystyrene graft resin Deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer and dispersant are dispersed in a reaction kettle, and nitrogen is passed through the reactor for protection. The temperature is raised to 60° C. for dispersion for 3 hours, and then an initiator is added. The temperature is raised to 85-90° C. for reaction for 8 hours, and the voltage stabilizer functionalized polypropylene-polystyrene grafted resin is obtained by washing with deionized water and drying. The mass ratio of deionized water, pre-irradiated polypropylene resin, voltage stabilizer monomer, styrene monomer and dispersant is 120:20:(4-8):(10-15):0.06; the amount of initiator used is 1% of the total mass of the voltage stabilizer monomer and the styrene monomer; Step 3: Preparation of mixed materials Add the voltage stabilizer functionalized polypropylene-polystyrene grafted resin prepared in step 2, high melting point copolymerized polypropylene resin, low melting point copolymerized polypropylene resin, thermoplastic styrene elastomer, and antioxidant into a high-speed mixer according to proportions, and mix evenly; Step 4: Preparation of polypropylene electrical insulating resin for high voltage DC cables The mixed material prepared in step 3 is added into the hopper of a twin-screw extruder, the extrusion temperature is controlled at 130-220° C., and the polypropylene electrical insulating resin for high-voltage DC cables is obtained through extrusion, drawing, cooling and granulation.

9. The method for preparing a polypropylene electrical insulating resin for a high voltage DC cable according to claim 8, characterized in that: In step 2, the dispersant is one of polyvinyl alcohol, calcium phosphate and magnesium carbonate, and the initiator is dibenzoyl peroxide.

10. Use of the polypropylene electrical insulating resin for high-voltage direct current cables according to any one of claims 1 to 7 as an insulating material for high-voltage direct current transmission cables.

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