Multilayer composite modified polypropylene electric power pipe and preparation method thereof

Abstract

The invention discloses a multi-layer composite modified polypropylene electric power pipe and a preparation method thereof, belonging to the technical field of composite electric power pipelines. The multi-layer composite modified polypropylene electric power pipe comprises an inner layer, a foaming layer and an outer layer arranged in sequence from the inside to the outside; the inner layer, the foaming layer and the outer layer all comprise modified fillers, and the modified fillers are inorganic mixtures sequentially subjected to ultrasonic cavitation treatment, cryogenic treatment and coupling agent surface treatment, and the inorganic mixture is a mixture of nano calcium carbonate, silicon carbide and nano silicon nitride. The obtained multi-layer composite modified polypropylene electric power pipe has good bonding between layers, is not easy to delaminate even under heavy pressure, and after the multi-section composite pipe is welded, the joints also have good drag strength.

Description

Technical Field

[0001] The invention belongs to the technical field of composite power pipelines, and specifically relates to a multi-layer composite modified polypropylene power pipe and a preparation method thereof. Background Art

[0002] Polypropylene pipes have the characteristics of low density, non-toxicity, odorlessness, and excellent mechanical properties such as strength, hardness and stiffness. They are widely used in the fields of electricity, chemical industry, etc. Common polypropylene pipes on the market have single-layer structures and multi-layer structures (multi-layer composite pipes). Among them, the multi-layer polypropylene pipe has a foaming layer, which is lighter and less expensive than the single-layer solid wall pipe, and has better impact resistance, sound insulation and heat insulation. For example, the Chinese invention patent with application number 202110579278.9 discloses a polypropylene ultra-quiet pipe and a preparation method thereof. By grafting metal organic framework materials on the polypropylene matrix of the middle layer and the inner layer, the metal organic framework material with a hollow porous structure can effectively improve the quiet effect of the polypropylene material, and at the same time, it can also make the pipe have good mechanical properties, so that the pipe can meet the use requirements in various complex environments. However, at present, this type of polypropylene composite pipe with a foaming layer has the problem of insufficient strength at the welding point and easy separation between layers under heavy pressure. Summary of the invention

[0003] In order to overcome the defects of the prior art, the technical problem to be solved by the present invention is: how to overcome the problem that the polypropylene composite pipe is easy to delaminate and the welding part is easy to break.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: a multi-layer composite modified polypropylene power pipe, comprising an inner layer, a foaming layer and an outer layer arranged in sequence from the inside to the outside;

[0005] The inner layer comprises the following raw materials in parts by weight:

[0006] 100 parts of polypropylene, 8-12 parts of modified filler, 15-25 parts of low-density polyethylene and 1-2 parts of lubricant;

[0007] The foaming layer comprises the following raw materials in parts by weight:

[0008] 100 parts of polypropylene, 2-5 parts of modified filler, 1-3 parts of foaming agent and 0.5-1 part of foaming aid;

[0009] The outer layer comprises the following raw materials in parts by weight:

[0010] 100 parts of polypropylene, 15-25 parts of modified filler, 8-15 parts of low-density polyethylene and 1-3 parts of antioxidant;

[0011] The modified filler is an inorganic mixture which is sequentially subjected to ultrasonic cavitation treatment, deep cryogenic treatment and coupling agent surface treatment, and the inorganic mixture is a mixture of nano calcium carbonate, silicon carbide and nano silicon nitride.

[0012] Another technical solution adopted by the present invention is: a method for preparing a multi-layer composite modified polypropylene power pipe, comprising the following steps:

[0013] S1. The raw materials in the inner layer, the middle layer and the outer layer are respectively stirred evenly to obtain an inner layer blend, a middle layer blend and an outer layer blend;

[0014] S2, the three blends are co-extruded in multiple layers, then compounded, and then sized and cooled to obtain a multi-layer composite modified polypropylene power pipe;

[0015] The inner layer comprises the following raw materials in parts by weight:

[0016] 100 parts of polypropylene, 8-12 parts of modified filler, 15-25 parts of low-density polyethylene and 1-2 parts of lubricant;

[0017] The foaming layer comprises the following raw materials in parts by weight:

[0018] 100 parts of polypropylene, 2-5 parts of modified filler, 1-3 parts of foaming agent and 0.5-1 part of foaming aid;

[0019] The outer layer comprises the following raw materials in parts by weight:

[0020] 100 parts of polypropylene, 10-15 parts of modified filler, 8-15 parts of low-density polyethylene and 1-3 parts of antioxidant;

[0021] The modified filler is an inorganic mixture which is sequentially subjected to ultrasonic cavitation treatment, deep cryogenic treatment and coupling agent surface treatment, and the inorganic mixture is a mixture of nano calcium carbonate, silicon carbide and nano silicon nitride.

[0022] The beneficial effects of the present invention are as follows: the inorganic filler is modified by ultrasonic cavitation treatment, cryogenic treatment and coupling agent surface treatment to improve the interface bonding performance; after adding a small amount of coupling agent to modify the inorganic filler, the obtained multi-layer composite modified polypropylene power pipe with a foaming layer has good bonding between layers, is not easy to delaminate even under heavy pressure, and after the multi-section composite pipe is welded, the joints also have good pulling strength. DETAILED DESCRIPTION

[0023] In order to explain the technical content, achieved objectives and effects of the present invention in detail, the following is an explanation in conjunction with the implementation modes.

[0024] The multi-layer composite modified polypropylene electric power pipe of the present invention comprises an inner layer, a foaming layer and an outer layer which are sequentially arranged from the inside to the outside;

[0025] The inner layer comprises the following raw materials in parts by weight:

[0026] 100 parts of polypropylene, 8-12 parts of modified filler, 15-25 parts of low-density polyethylene and 1-2 parts of lubricant;

[0027] The foaming layer comprises the following raw materials in parts by weight:

[0028] 100 parts of polypropylene, 2-5 parts of modified filler, 1-3 parts of foaming agent and 0.5-1 part of foaming aid;

[0029] The outer layer comprises the following raw materials in parts by weight:

[0030] 100 parts of polypropylene, 10-15 parts of modified filler, 8-15 parts of low-density polyethylene and 1-3 parts of antioxidant;

[0031] The modified filler is an inorganic mixture which is sequentially subjected to ultrasonic cavitation treatment, deep cryogenic treatment and coupling agent surface treatment, and the inorganic mixture is a mixture of nano calcium carbonate, silicon carbide and nano silicon nitride.

[0032] From the above description, it can be seen that the beneficial effects of the present invention are: the inorganic filler is modified by ultrasonic cavitation treatment, cryogenic treatment and coupling agent surface treatment to improve the interface bonding performance, so that after a small amount of coupling agent is added to modify the inorganic filler, the compatibility and dispersibility of the inorganic filler and the resin system can be significantly improved, so that the obtained composite pipe with a foaming layer has good bonding between the layers, is not easy to delaminate even under heavy pressure, and after the multi-section composite pipe is welded, the joints also have good drag strength.

[0033] Furthermore, the wall thickness ratio of the inner layer, the foamed layer and the outer layer is 1:1.2-1.8:0.8-1.

[0034] Furthermore, the particle size of the nano calcium carbonate is 100-800 nm (0.1-0.8 μm), the particle size of the silicon carbide is 500-700 nm, and the particle size of the nano silicon nitride is 20-40 nm.

[0035] Nano calcium carbonate uses rose crystal nano calcium carbonate from Fujian Wanqi Non-metallic Materials Co., Ltd. Further, the preparation method of the modified filler is: nano calcium carbonate, silicon carbide and nano silicon nitride are mixed and ball milled for 45 to 120 seconds to obtain an inorganic mixture, the inorganic mixture is subjected to ultrasonic cavitation treatment, and after drying, it is subjected to deep freezing treatment below -130°C, and then 0.1 to 0.5% of the mass of the inorganic mixture is added with a coupling agent, mixed and stirred to obtain a modified filler.

[0036] Furthermore, the inorganic mixture consists of 65-80% of nano calcium carbonate, 15-25% of silicon carbide and 5-10% of nano silicon nitride by weight.

[0037] From the above description, it can be seen that silicon carbide and nano-silicon nitride have good thermal conductivity. By selecting silicon carbide with a larger particle size and cooperating with nano-scale calcium carbonate and silicon nitride, physical modification can be first achieved during the ball milling process, so that some small-particle calcium carbonate and silicon nitride can adhere to the silicon carbide. When the polypropylene is subsequently modified, the enhancement and thermal conductivity of the pipe can be improved better than direct dispersion and mixing. At the same time, in order to ensure the mechanical properties of the pipe, the composition of the inorganic mixture should be controlled within the above ratio.

[0038] Furthermore, the power of the ultrasonic cavitation treatment is 20000-30000 Hz, and the time is 12-20 minutes.

[0039] Furthermore, the temperature of the cryogenic treatment is -130°C to -150°C, the time is 1 to 2 hours, and the cooling rate is 10 to 20°C / min, preferably 15°C / min.

[0040] From the above description, it can be seen that ultrasonic cavitation treatment can produce local high temperature and high pressure environment with large instantaneous force, which can better impact and disperse the polymerization of inorganic mixture. The inorganic mixture after ultrasonic cavitation treatment is then cryogenically treated to further improve the modification effect. The order of cryogenic treatment and ultrasonic cavitation treatment will affect the modification effect. Changes in the order will lead to a decrease in the mechanical properties of the modified pipe.

[0041] Furthermore, the polypropylene is random copolymer polypropylene, model 3003, produced by Formosa Plastics in Taiwan, China; or model T30S, produced by Zhongjing Petrochemical.

[0042] Furthermore, the low-density polyethylene is linear low-density polyethylene, model DFDA-7042, produced by Qilu Petrochemical.

[0043] Furthermore, the foaming agent is azodicarbonamide, dinitrosopentamethylenetetramine or 4,4'-oxybis(benzenesulfonylhydrazide).

[0044] Furthermore, the foaming aid is at least one of zinc oxide, zinc carbonate, zinc citrate and zinc acetate.

[0045] Furthermore, the inner layer and the outer layer also include 0.1 to 3 parts of masterbatch, model PO2-60093.

[0046] Furthermore, the silane coupling agent is selected from one or both of KH-550 and KH-570.

[0047] Furthermore, the lubricant is zinc stearate or calcium stearate, and the calcium stearate, model S100, comes from Hunan Shaoyang Tiantang Auxiliary Chemical Co., Ltd.

[0048] Furthermore, the antioxidant is antioxidant 1010 from BASF of Germany.

[0049] The method for preparing the multi-layer composite modified polypropylene power pipe comprises the following steps:

[0050] S1. The raw materials in the inner layer, the middle layer and the outer layer are respectively stirred evenly to obtain an inner layer blend, a middle layer blend and an outer layer blend;

[0051] S2. The three blends are co-extruded in multiple layers, then compounded, and then sizing and cooling are performed to obtain a multi-layer composite modified polypropylene power pipe.

[0052] Furthermore, the process parameters of the pipe extrusion production line are as follows: the temperature of barrel zone 1 is 180-200°C, the temperature of barrel zone 2 is 190-210°C, the temperature of barrel zone 3 is 190-210°C, the temperature of barrel zone 4 is 180-200°C, the temperature of barrel zone 5 is 150-180°C, the temperature of die zone 1 is 180-200°C, the temperature of die zone 2 is 190-210°C, the temperature of die zone 3 is 170-190°C, and the extrusion molding speed is 1.0-1.2 m / min.

[0053] Furthermore, the stirring rate in S1 is 200-400 r / min, and the stirring time after adding all the raw materials is 30-60 min.

[0054] Example 1

[0055] The multi-layer composite modified polypropylene power pipe comprises an inner layer, a foaming layer and an outer layer which are sequentially arranged from the inside to the outside;

[0056] The inner layer comprises the following raw materials in parts by weight:

[0057] Random copolymer polypropylene (3003) 100kg, modified filler 8kg, low density polyethylene 20kg and calcium stearate 1kg;

[0058] The foaming layer comprises the following raw materials in parts by weight:

[0059] Random copolymer polypropylene (3003) 100kg, modified filler 4kg, azodicarbonamide 1.5kg and zinc oxide 0.65kg;

[0060] The outer layer comprises the following raw materials in parts by weight:

[0061] Random copolymer polypropylene (3003) 100kg, modified filler 10kg, low density polyethylene 10kg and antioxidant 10101.5kg;

[0062] The wall thickness ratio of the inner layer, foam layer and outer layer is 1:1.5:1;

[0063] The preparation method of the modified filler is as follows: nano calcium carbonate, silicon carbide and nano silicon nitride are mixed and ball milled for 60 seconds to obtain an inorganic mixture, the inorganic mixture is subjected to ultrasonic cavitation treatment, the inorganic mixture is dried and cooled at 15° C. / min (water content is less than 8%), and deep-cold treated at -148 to -150° C. for 1.5 hours, and then a coupling agent of 0.3% by weight of the inorganic mixture is added, mixed and stirred to obtain the modified filler;

[0064] The particle size of the nano calcium carbonate is 100 to 800 nm, the particle size of the silicon carbide is 500 to 700 nm, and the particle size of the nano silicon nitride is 20 to 40 nm;

[0065] The inorganic mixture consists of 70% nano calcium carbonate, 20% silicon carbide and 10% nano silicon nitride by weight percentage;

[0066] The power of ultrasonic cavitation treatment was 30000 Hz and the time was 12 min;

[0067] The silane coupling agent is a mixture of KH-550 and KH-570 in a mass ratio of 1:1.

[0068] The method for preparing the multi-layer composite modified polypropylene power pipe of Example 1 comprises the following steps:

[0069] S1. The raw materials in the inner layer, the middle layer and the outer layer are stirred evenly to obtain the inner layer blend, the middle layer blend and the outer layer blend; the stirring rate is 350r / min, and the stirring time after adding all the raw materials to each layer is 35min;

[0070] S2, the three blends are co-extruded in multiple layers, then compounded, and then sized and cooled to obtain a multi-layer composite modified polypropylene power pipe;

[0071] The process parameters of the pipe extrusion (twin-screw extrusion) production line are as follows: the temperature of barrel zone 1 is 190°C, the temperature of barrel zone 2 is 200°C, the temperature of barrel zone 3 is 200°C, the temperature of barrel zone 4 is 190°C, the temperature of barrel zone 5 is 160°C, the temperature of die zone 1 is 190°C, the temperature of die zone 2 is 200°C, the temperature of die zone 3 is 180°C, and the extrusion molding speed is 1.0m / min.

[0072] Example 2

[0073] The multi-layer composite modified polypropylene power pipe comprises an inner layer, a foaming layer and an outer layer which are sequentially arranged from the inside to the outside;

[0074] The inner layer comprises the following raw materials in parts by weight:

[0075] Random copolymer polypropylene (T30S) 100kg, modified filler 10kg, low density polyethylene 15kg and zinc stearate 1.5kg;

[0076] The foaming layer comprises the following raw materials in parts by weight:

[0077] Random copolymer polypropylene (T30S) 100kg, modified filler 5kg, azodicarbonamide 3kg and zinc carbonate 0.8kg;

[0078] The outer layer comprises the following raw materials in parts by weight:

[0079] Random copolymer polypropylene (T30S) 100kg, modified filler 15kg, low density polyethylene 8kg and antioxidant 10101kg;

[0080] The wall thickness ratio of the inner layer, foaming layer and outer layer is 1:1.2:0.8;

[0081] The preparation method of the modified filler is as follows: nano calcium carbonate, silicon carbide and nano silicon nitride are mixed and ball milled for 45 seconds to obtain an inorganic mixture, the inorganic mixture is subjected to ultrasonic cavitation treatment, the inorganic mixture is dried and cooled at 10° C. / min (water content is less than 8%), and deep-cold treated at -135 to -138° C. for 2 hours, and then a coupling agent of 0.1% by weight of the inorganic mixture is added, mixed and stirred to obtain the modified filler;

[0082] The particle size of the nano calcium carbonate is 20 to 200 nm, the particle size of the silicon carbide is 500 to 700 nm, and the particle size of the nano silicon nitride is 20 to 40 nm;

[0083] The inorganic mixture consists of 80% nano calcium carbonate, 15% silicon carbide and 5% nano silicon nitride by weight percentage;

[0084] The power of ultrasonic cavitation treatment was 20000 Hz and the time was 20 min;

[0085] The silane coupling agent is a mixture of KH-550 and KH-570 in a mass ratio of 1:1.

[0086] The method for preparing the multi-layer composite modified polypropylene power pipe of Example 2 comprises the following steps:

[0087] S1. The raw materials in the inner layer, the middle layer and the outer layer are stirred evenly to obtain the inner layer blend, the middle layer blend and the outer layer blend; the stirring rate is 200r / min, and the stirring time after adding all the raw materials to each layer is 60min;

[0088] S2, the three blends are co-extruded in multiple layers, then compounded, and then sized and cooled to obtain a multi-layer composite modified polypropylene power pipe;

[0089] The process parameters of the pipe extrusion production line are as follows: the temperature of barrel zone 1 is 180℃, the temperature of barrel zone 2 is 190℃, the temperature of barrel zone 3 is 190℃, the temperature of barrel zone 4 is 180℃, the temperature of barrel zone 5 is 150℃, the temperature of die zone 1 is 180℃, the temperature of die zone 2 is 190℃, the temperature of die zone 3 is 180℃, and the extrusion molding speed is 1.1m / min.

[0090] Example 3

[0091] The multi-layer composite modified polypropylene power pipe comprises an inner layer, a foaming layer and an outer layer which are sequentially arranged from the inside to the outside;

[0092] The inner layer comprises the following raw materials in parts by weight:

[0093] Random copolymer polypropylene (3003) 100kg, modified filler 12kg, low density polyethylene 25kg and calcium stearate 1.8kg;

[0094] The foaming layer comprises the following raw materials in parts by weight:

[0095] Random copolymer polypropylene (3003) 100kg, modified filler 2kg, dinitrosopentamethylenetetramine 1kg and zinc citrate 0.5kg;

[0096] The outer layer comprises the following raw materials in parts by weight:

[0097] Random copolymer polypropylene (3003) 100kg, modified filler 13kg, low density polyethylene 12kg and antioxidant 10102kg;

[0098] The wall thickness ratio of the inner layer, foam layer and outer layer is 1:1.3:0.9;

[0099] The preparation method of the modified filler is as follows: nano calcium carbonate, silicon carbide and nano silicon nitride are mixed and ball milled for 100 seconds to obtain an inorganic mixture, the inorganic mixture is subjected to ultrasonic cavitation treatment, the inorganic mixture is dried and cooled at 12° C. / min (water content is less than 8%), and deep-cold treated at -138 to -142° C. for 1.2 hours, and then 0.5% of the mass of the inorganic mixture is added with coupling agent KH-550, mixed and stirred to obtain the modified filler;

[0100] The particle size of the nano calcium carbonate is 100 to 800 nm, the particle size of the silicon carbide is 500 to 700 nm, and the particle size of the nano silicon nitride is 20 to 40 nm;

[0101] The inorganic mixture consists of 65% nano calcium carbonate, 25% silicon carbide and 10% nano silicon nitride by weight percentage;

[0102] The power of ultrasonic cavitation treatment was 25000 Hz and the time was 18 min;

[0103] The method for preparing the multi-layer composite modified polypropylene power pipe of Example 3 comprises the following steps:

[0104] S1. The raw materials in the inner layer, the middle layer and the outer layer are stirred evenly to obtain the inner layer blend, the middle layer blend and the outer layer blend; the stirring rate is 300r / min, and the stirring time after adding all the raw materials to each layer is 40min;

[0105] S2, the three blends are co-extruded in multiple layers, then compounded, and then sized and cooled to obtain a multi-layer composite modified polypropylene power pipe;

[0106] The process parameters of the pipe extrusion production line are as follows: the temperature of barrel zone 1 is 200℃, the temperature of barrel zone 2 is 210℃, the temperature of barrel zone 3 is 210℃, the temperature of barrel zone 4 is 200℃, the temperature of barrel zone 5 is 180℃, the temperature of die zone 1 is 200℃, the temperature of die zone 2 is 210℃, the temperature of die zone 3 is 190℃, and the extrusion molding speed is 1.2m / min.

[0107] Example 4

[0108] The multi-layer composite modified polypropylene power pipe comprises an inner layer, a foaming layer and an outer layer which are sequentially arranged from the inside to the outside;

[0109] The inner layer comprises the following raw materials in parts by weight:

[0110] Random copolymer polypropylene (3003) 100kg, modified filler 11kg, low density polyethylene 18kg and calcium stearate 2kg;

[0111] The foaming layer comprises the following raw materials in parts by weight:

[0112] Random copolymer polypropylene (3003) 100kg, modified filler 3kg, 4,4′-oxybis(benzenesulfonylhydrazide) 2kg and zinc acetate 1kg;

[0113] The outer layer comprises the following raw materials in parts by weight:

[0114] Random copolymer polypropylene (3003) 100kg, modified filler 12kg, low density polyethylene 15kg and antioxidant 10103kg;

[0115] The wall thickness ratio of the inner layer, foam layer and outer layer is 1:1.8:1;

[0116] The preparation method of the modified filler is as follows: nano calcium carbonate, silicon carbide and nano silicon nitride are mixed and ball milled for 120 seconds to obtain an inorganic mixture, the inorganic mixture is subjected to ultrasonic cavitation treatment, the inorganic mixture is dried and cooled at 20° C. / min (water content is less than 8%), and deep-cold treated at -145 to -148° C. for 1 hour, and then 0.4% of the mass of the inorganic mixture is added with coupling agent KH-570, mixed and stirred to obtain the modified filler;

[0117] The particle size of the nano calcium carbonate is 100 to 800 nm, the particle size of the silicon carbide is 500 to 700 nm, and the particle size of the nano silicon nitride is 20 to 40 nm;

[0118] The inorganic mixture consists of 75% nano calcium carbonate, 17% silicon carbide and 8% nano silicon nitride by weight percentage;

[0119] The power of ultrasonic cavitation treatment was 22000 Hz and the time was 16 min.

[0120] The composite pipe with a diameter of DN200 and a wall thickness of 4 mm was processed according to the above formula and method.

[0121] The method for preparing the multi-layer composite modified polypropylene power pipe of Example 4 comprises the following steps:

[0122] S1. The raw materials in the inner layer, the middle layer and the outer layer are stirred evenly to obtain the inner layer blend, the middle layer blend and the outer layer blend; the stirring rate is 400r / min, and the stirring time after adding all the raw materials to each layer is 30min;

[0123] S2, the three blends are co-extruded in multiple layers, then compounded, and then sized and cooled to obtain a multi-layer composite modified polypropylene power pipe;

[0124] The process parameters of the pipe extrusion production line are as follows: the temperature of barrel zone 1 is 190℃, the temperature of barrel zone 2 is 200℃, the temperature of barrel zone 3 is 200℃, the temperature of barrel zone 4 is 190℃, the temperature of barrel zone 5 is 160℃, the temperature of die zone 1 is 190℃, the temperature of die zone 2 is 200℃, the temperature of die zone 3 is 180℃, and the extrusion molding speed is 1.0m / min.

[0125] Comparative Example 1: The difference from Example 1 is that ultrasonic cavitation treatment and cryogenic treatment are not performed.

[0126] Comparative Example 2: The difference from Example 1 is that the order of ultrasonic cavitation treatment and cryogenic treatment is exchanged.

[0127] The pipes processed in Example 1, Comparative Example 1 and Comparative Example 2 were tested. The test methods and results are shown in Table 1. For the flattening test, 300 mm pipe sections were taken from three pipes as samples. The two sections of the sample were cut vertically and placed at a constant temperature of (23±2)°C for 2 hours. The sample was taken out of the constant temperature box and immediately placed flat between the two pressing plates of the testing machine. The sample was compressed at a speed of (5±2) mm / min to observe whether the pipe was broken when the outer diameter was compressed by 50%.

[0128] Table 1

[0129]

[0130] It can be seen that by adding modified fillers, compared with the products obtained by adding unmodified fillers, the drag strength of the weld is higher and the layers are not easy to separate or delaminate.

[0131] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent transformations made using the contents of the present invention specification, or directly or indirectly applied in related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. Multi-layer composite modified polypropylene power pipe, characterized in that: It includes an inner layer, a foaming layer and an outer layer arranged in sequence from the inside to the outside; The inner layer comprises the following raw materials in parts by weight: 100 parts of polypropylene, 8-12 parts of modified filler, 15-25 parts of low-density polyethylene and 1-2 parts of lubricant; The foaming layer comprises the following raw materials in parts by weight: 100 parts of polypropylene, 2-5 parts of modified filler, 1-3 parts of foaming agent and 0.5-1 parts of foaming aid; The outer layer comprises the following raw materials in parts by weight: 100 parts of polypropylene, 10-15 parts of modified filler, 8-15 parts of low-density polyethylene and 1-3 parts of antioxidant; The preparation method of the modified filler is as follows: nano calcium carbonate, silicon carbide and nano silicon nitride are mixed and ball milled for 45 to 120 seconds to obtain an inorganic mixture, the inorganic mixture is subjected to ultrasonic cavitation treatment, dried and subjected to deep freezing treatment below -130°C, and then a coupling agent of 0.1 to 0.5% by mass of the inorganic mixture is added, mixed and stirred to obtain the modified filler.

2. The multi-layer composite modified polypropylene electric power pipe according to claim 1, characterized in that: The wall thickness ratio of the inner layer, the foaming layer and the outer layer is 1:1.2~1.8:0.8~1.

3. The multi-layer composite modified polypropylene electric power pipe according to claim 1, characterized in that: The particle size of the nano calcium carbonate is 100-800 nm, the particle size of the silicon carbide is 500-700 nm, and the particle size of the nano silicon nitride is 20-40 nm.

4. The multi-layer composite modified polypropylene electric power pipe according to claim 1, characterized in that: The polypropylene is random copolymer polypropylene.

5. The multi-layer composite modified polypropylene electric power pipe according to claim 1, characterized in that: The foaming agent is azodicarbonamide, dinitrosopentamethylenetetramine or 4,4'-oxybis(benzenesulfonylhydrazide).

6. The multi-layer composite modified polypropylene electric power pipe according to claim 1, characterized in that: The foaming aid is at least one of zinc oxide, zinc carbonate, zinc citrate and zinc acetate.

7. The multi-layer composite modified polypropylene electric power pipe according to claim 1, characterized in that: The inner layer and the outer layer also include 0.1 to 3 parts of toner.

8. The method for preparing the multi-layer composite modified polypropylene electric power pipe according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1. The raw materials in the inner layer, the middle layer and the outer layer are respectively stirred evenly to obtain an inner layer blend, a middle layer blend and an outer layer blend; S2. The three blends are co-extruded in multiple layers, then compounded, and then sizing and cooling are performed to obtain a multi-layer composite modified polypropylene electric pipe.

9. The method for preparing the multi-layer composite modified polypropylene electric power pipe according to claim 8, characterized in that: The stirring rate in S1 is 200~400 r / min, and the stirring time after adding all the raw materials is 30~60 min.

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