Polyethylene mixed material for large-caliber thick-wall pressure-resistant polyethylene pipe and preparation method of polyethylene mixed material

Abstract

The invention relates to the technical field of polyethylene mixed materials, in particular to a polyethylene mixed material for a large-caliber thick-wall pressure-resistant polyethylene pipe and a preparation method of the polyethylene mixed material, and the polyethylene mixed material comprises the following components in percentage by mass: 93-96% of high-density polyethylene copolymer, 3.5-7% of master batch and 0.5-1% of compound antioxidant system, the high-density polyethylene copolymer is obtained by copolymerization of ethylene and a vinyl benzaldehyde compound, and the molar ratio of an ethylene monomer unit to a vinyl benzaldehyde compound unit in the high-density polyethylene copolymer is (89-96): (4-11); the master batch comprises high-density polyethylene resin prepared by catalyzing modified carbon black and a Ziegler-Natta catalyst. Compared with the prior art, the melt sag resistance of the polyethylene mixture in the extrusion of large-diameter thick-wall pipes is greatly improved, and the prepared mixture can be successfully used for processing 36in DR9 pipes.

Description

Technical Field

[0001] The invention relates to the technical field of polyethylene compounding materials, in particular to a polyethylene compounding material for large-diameter, thick-walled, pressure-resistant polyethylene pipes and a preparation method thereof. Background Art

[0002] Polyethylene (PE) cross-linking technology is a key means of improving its material properties. High-density polyethylene (HDPE) theoretically resembles a linear polymer, allowing it to dissolve and melt. However, when chemical bonds are formed between the same or different molecular chains, the polymer forms a network-like structure. The size of the network depends on the number of cross-links between the polymer branches. Through cross-linking, the polymer can form independent supramolecular networks. Cross-linking significantly restricts the rotation and movement of the molecules, thereby increasing the macroscopic strength and stiffness of the polymer. Cross-linking polyethylene (PE) can significantly improve its performance, significantly enhancing its mechanical properties, environmental stress cracking resistance, chemical corrosion resistance, creep resistance, and electrical properties. Polymers such as polyethylene and polypropylene are widely used in the manufacture of pressure-resistant plastic pipes, and cross-linking technology is also used to improve pipe properties, such as crack resistance. According to ISO 9080, the minimum required strength (MRS) specifies the design stress of cross-linked polymer pipes at different temperatures—the stress at which the pipe is designed to withstand 50 years without failure. Increasing the MRS can make the pipe thinner, that is, less material can be used to manufacture the pipe, or for a certain pipe thickness, higher stress can be withstood.

[0003] Chinese patent CN117820751A discloses a large-diameter, thick-walled peroxide-cross-linked polyethylene pipe and its preparation method and application, which includes a high-density polyethylene resin, a peroxide cross-linking agent, a hindered phenol antioxidant, a phosphite antioxidant and a co-cross-linking agent. This invention patent promotes the rapid decomposition of the cross-linking agent through the use of the co-cross-linking agent, thereby improving the cross-linking degree of the pipe. The use of hindered phenol antioxidants and phosphite antioxidants prolongs the service life of the pipe, thereby obtaining a large-diameter, thick-walled peroxide-cross-linked polyethylene pipe with stable performance and high cross-linking degree. The maximum diameter of the pipe applicable to this technology is 75mm, and the maximum wall thickness is 10.4mm. Chinese patent CN110092963A discloses a micro-cross-linked polyethylene pipe and its preparation method. During the production process of polyethylene pressure-resistant pipe, fillers and sulfur-containing silane coupling agents are added. The sulfur-containing silane coupling agent helps the filler to be better dispersed in the polyethylene resin, and at the same time decomposes during the extrusion process, causing the base resin to undergo micro-cross-linking. The micro-cross-linked polyethylene pipe of this invention has excellent pressure resistance and anti-sag performance, and can be used to produce large-diameter pressure-resistant pipes. The maximum processed pipe outer diameter can reach 1600mm, but the pipe wall thickness is not clear.

[0004] Dynamic covalent bond technology brings reversible reconstruction properties to cross-linked materials. Among them, the Diels-Alder (DA) reaction system (such as the research of Dalian University of Technology) realizes the thermal reversible regulation of the cross-linked network through the [4+2] cycloaddition reaction of furan / maleimide. However, in actual applications, it is found that this type of material has problems such as sudden viscosity change at critical temperature (Petrochemical Industry, 2020) and narrow processing window. In particular, for the processing of large-diameter thick-walled pipes, the sag phenomenon caused by insufficient melt strength seriously restricts the processing accuracy. Although recent patents CN117285769A and US20220162402A1 have made some innovations in the design of dynamic cross-linkers, they mainly focus on the fields of cable materials and plastic recycling, and have not yet solved the structural defects in the processing of thick-walled pipes.

[0005] As urban civil and industrial pipe networks develop toward larger diameters (DN ≥ 800mm) and thicker walls (≥ 30mm), existing technologies face a dual challenge: traditional systems struggle to meet the demands of ultra-large pipe processing, while dynamically cross-linked materials suffer from insufficient thermal stability. In particular, existing technologies struggle to balance melt strength during processing with the network stability of the final product, leading to quality defects such as excessive pipe ovality and uneven wall thickness.

[0006] It can be seen that there is an urgent need to develop a new polymer mixing system that can broaden the processing temperature window and improve melt strength through molecular structure design while maintaining the advantages of dynamic cross-linking to meet the performance requirements of the high-end pipe market. Summary of the Invention

[0007] In response to the shortcomings of the existing technology, the present invention synthesizes a high-density polyethylene resin containing aldehyde groups on the side groups, and at the same time combines it with a carbon black masterbatch with diprimary amine groups. Vitrimer reversibly cross-linked polyethylene prepared by the imidization exchange reaction between the aldehyde groups and the amine groups is applied to the formulation of large-diameter thick-walled pipe compounding materials, thereby greatly improving the anti-"melting sag" property of the polyethylene compounding materials in the extrusion of large-diameter thick-walled pipes. The prepared compounding materials can be successfully used for the processing of 36-inch DR9 pipes (outer diameter 914.4 mm, wall thickness 101.6 mm).

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] The first aspect of the present invention provides a polyethylene mixed material for large-diameter, thick-walled, pressure-resistant polyethylene pipes. Calculated based on 100% by mass of the polyethylene mixed material, the mass contents of the components are as follows: 93% to 96% of a high-density polyethylene copolymer, 3.5% to 7% of a masterbatch, and 0.5% to 1% of a compounded antioxidant system.

[0010] The high-density polyethylene copolymer is obtained by copolymerizing ethylene and vinylbenzaldehyde compounds, and the molar ratio of the ethylene monomer unit to the vinylbenzaldehyde compound unit in the high-density polyethylene copolymer is 89-96:4-11;

[0011] The masterbatch comprises carbon black and a high-density polyethylene resin prepared by catalysis of a Ziegler-Natta catalyst.

[0012] Furthermore, the high-density polyethylene copolymer is prepared by copolymerization of ethylene and vinylbenzaldehyde or a vinylbenzaldehyde derivative;

[0013] The vinylbenzaldehyde derivative includes one of 2-vinylbenzaldehyde, 3-vinylbenzaldehyde and 4-vinylbenzaldehyde.

[0014] Furthermore, the density of the high-density polyethylene resin in the masterbatch is at least 0.940;

[0015] The carbon black in the masterbatch is modified carbon black, which is furnace carbon black and / or channel carbon black. The content of the modified carbon black in the masterbatch accounts for 36-45% of the mass of the masterbatch, and the content of the modified carbon black accounts for 1.5-3% of the mass of the polyethylene compound.

[0016] The content of primary amine groups in the modified carbon black is 5 mmol / g to 22 mmol / g.

[0017] More preferably, the density of the high-density polyethylene resin in the masterbatch is 0.940-0.970, more preferably 0.950-0.965.

[0018] Further preferably, the content of primary amine groups in the modified carbon black is preferably 9 mmol / g-17 mmol / g.

[0019] Furthermore, the compound antioxidant system is a mixture of hindered phenol antioxidant, phosphite antioxidant and acid scavenger, and the weight ratio of the three components is hindered phenol antioxidant: phosphite antioxidant: acid scavenger = 2-2.5:2-3:0.8-1.2.

[0020] The second aspect of the present invention provides a method for preparing the polyethylene mixed material for large-diameter, thick-walled, pressure-resistant polyethylene pipes as described above, the preparation method comprising the following steps: extruding a high-density polyethylene copolymer, a masterbatch, and a compounded antioxidant system through a reactive extrusion device at an extrusion temperature of 240 to 270°C, pelletizing the pellets through a pelletizer, and then drying the pellets to obtain the polyethylene mixed material.

[0021] Furthermore, the high-density polyethylene copolymer is prepared by a fully mixed gas-phase anionic coordination polymerization reaction of ethylene, vinylbenzaldehyde compounds, and hydrogen in the presence of a Ziegler-Natta catalyst system and an alkyl aluminum cocatalyst;

[0022] The molar ratio of Al to Ti in the Ziegler-Natta catalyst system is controlled to be 45-55 mol / mol.

[0023] Furthermore, during the reaction, the total pressure of the reactor is maintained at 2200-2350 kPa, the ethylene partial pressure is 1500-1650 kPa, the molar ratio of the vinylbenzaldehyde compound to ethylene is 0.05-0.13, and the molar ratio of hydrogen to ethylene is 0.0015-0.0025;

[0024] The vinylbenzaldehyde compound is selected from one of 2-vinylbenzaldehyde, 3-vinylbenzaldehyde and 4-vinylbenzaldehyde;

[0025] The polymerization temperature is 102°C to 108°C;

[0026] The aluminum co-catalyst is a combination of one or more of triethylaluminum, diethylaluminum monochloride, and tri-n-hexyl chloride.

[0027] Furthermore, the preparation process of the masterbatch includes:

[0028] The modified carbon black, high-density polyethylene resin, and antioxidant system are extruded at 230-270° C. to prepare a carbon black masterbatch uniformly dispersed in a resin carrier;

[0029] The antioxidant system in the masterbatch is a mixture of hindered phenol antioxidant and phosphite antioxidant, and the mass ratio of hindered phenol to phosphite is 0.5-1:1.5-2.5.

[0030] More preferably, the mass ratio of phosphite is 1:2.

[0031] Furthermore, the preparation process of the modified carbon black includes: reacting oxidized carbon black with polyvalent primary amines to prepare modified carbon black with primary amine groups;

[0032] The oxidized carbon black is oxidized using nitric acid as an oxidant at a reaction temperature of ≤100°C for ≤36h, so that the main functional groups on the surface of the oxidized carbon black are carboxyl groups;

[0033] The polyvalent primary amine is selected from one of ethylenediamine, propylenediamine, tris(2-aminoethyl)amine, tris(aminomethyl)amine, tris(aminoethyl)amine, tris(aminopropyl)amine, tris(aminobutyl)amine, and tris(aminohexyl)amine.

[0034] Furthermore, the reactive extrusion equipment is one of a single-screw extruder, a twin-screw extruder, an open mixer, and an internal mixer;

[0035] During the extrusion process of the reactive extruder, a reversible cross-linking reaction occurs between the aldehyde groups in the high-density polyethylene copolymer and the amine groups in the masterbatch. That is, the cross-linking is resolved when the mixed materials are melted and heated, and the melt is cross-linked when it passes through the die for sizing and cooling. This improves the sag performance of the melt and enhances the processing performance of the material for large-diameter thick-walled pipes.

[0036] The large-diameter thick-walled pipes are pipes with a diameter and wall thickness of 32in DR9 and above.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1) Good anti-sag properties: The polyethylene compound described in the present invention has a reversible cross-linked network structure. The aldehyde groups of the synthesized high-density polyethylene side groups react with the amine groups of the carbon black masterbatch to cross-link the polymer. When heated, the cross-linked bonds rearrange the network structure through a thermally stimulated associative exchange reaction. The formation of new bonds and the breaking of old bonds occur simultaneously, giving the material thermoplastic properties. Cross-linking occurs when the melt cools, giving the material good anti-sag properties during the processing of large-diameter thick-walled pipes, effectively solving the industry's pain points in processing large-diameter thick-walled pipes, especially for pipes with a diameter and wall thickness of 32in DR9 (outer diameter 812.8mm, wall thickness 90.32mm) and above.

[0039] 2) Recyclable: The present invention applies reversible cross-linking technology to the field of polyethylene pipe processing. Compared with the non-cross-linkable polyethylene pipe processing technology in the prior art, the present invention can achieve the recycling of materials. DETAILED DESCRIPTION

[0040] The polyethylene mixed material for preparing large-diameter, thick-walled, pressure-resistant polyethylene pipes provided by the present invention has the following component contents, calculated based on 100% by mass of the polyethylene mixed material: 93%-96% high-density polyethylene copolymer content, 3.5%-7% masterbatch, and 0.5%-1% compounded antioxidant system.

[0041] The high-density polyethylene copolymer is obtained by copolymerizing ethylene and vinylbenzaldehyde compounds. When the total amount of monomer units in the high-density polyethylene copolymer is 100 mol, 89 to 96 mol of the monomer units are ethylene units and 4 to 11 mol of the vinylbenzaldehyde units are included. The high-density polyethylene copolymer of the present invention is prepared by copolymerizing ethylene and vinylbenzaldehyde or its derivatives, including vinylbenzaldehyde derivatives such as 2-vinylbenzaldehyde, 3-vinylbenzaldehyde, 4-vinylbenzaldehyde and vinylbenzaldehyde compounds whose benzene rings contain one or more methyl groups.

[0042] The masterbatch is composed of carbon black and a high-density polyethylene resin prepared using a Ziegler-Natta catalyst. The density of the high-density polyethylene prepared using a Ziegler-Natta catalyst is at least 0.940, preferably 0.940 to 0.970, and more preferably 0.950 to 0.965. Other than the resin in the masterbatch being a high-density polyethylene obtained using a Ziegler-Natta catalyst, the resin in the masterbatch of the present invention has no other particularly critical characteristics. The carbon black in the masterbatch is modified carbon black, which can be modified using various types of carbon black, such as furnace black and channel black. The modified carbon black content in the masterbatch is 36 to 45% by weight of the masterbatch, preferably 38 to 42% by weight, to achieve a modified carbon black content of 1.5 to 3% by weight, preferably 2 to 2.5% by weight, in the polymer composition. The carbon black is modified by amination to obtain the modified carbon black of the present invention. The content of primary amine groups in the modified carbon black is 5 mmol / g to 22 mmol / g, and preferably 16 mmol / g to 21 mmol / g in specific implementation.

[0043] In a specific implementation, the compound antioxidant system is a mixture of hindered phenol antioxidant, phosphite antioxidant and acid scavenger, and the weight ratio thereof is: hindered phenol: phosphite: acid scavenger = 2-2.5: 2-3: 0.8-1.2.

[0044] The present invention also provides a method for preparing the polyethylene mixed material for preparing the large-diameter, thick-walled, pressure-resistant polyethylene pipe, which is specifically as follows:

[0045] 1. Preparation of high-density polyethylene copolymers: In a gas-phase polyethylene polymerization unit, anionic coordination polymerization is carried out using ethylene, comonomer, and a small amount of hydrogen as a molecular weight regulator. Ethylene, comonomer, hydrogen, and nitrogen are added to the gas-phase polyethylene reactor, and the fully mixed gas-phase polymerization reaction is carried out in the presence of a Zigler-Natta catalyst and an alkyl aluminum co-catalyst. Among them, the Al / Ti molar ratio is between 45 and 55, preferably 48 to 52 mol / mol in specific implementation; the total pressure of the reactor is 2200 kPa to 2350 kPa, preferably 2280 kPa to 2300 kPa, the ethylene partial pressure is 1500 to 1650 kPa, the molar ratio of the comonomer to the ethylene monomer is 0.05 to 0.13 mol / mol, preferably 0.06 to 0.010 mol / mol in specific implementation, and the hydrogen-ethylene molar ratio is 0.0015 to 0.0025 mol / mol, preferably 0.0018 to 0.0020 mol / mol; and the polymerization reaction temperature is 102°C to 108°C, preferably 104°C to 106°C in specific implementation. The comonomer is vinylbenzaldehyde or its derivatives which have little effect on the activity of the Zigler-Natta catalyst, including vinylbenzaldehyde derivatives such as 2-vinylbenzaldehyde, 3-vinylbenzaldehyde, 4-vinylbenzaldehyde and vinylbenzaldehyde compounds whose benzene ring contains one or more methyl groups. In specific implementation, 4-vinylbenzaldehyde and 3-vinylbenzaldehyde are preferred.

[0046] The above-mentioned Zigler-Natta catalyst is disclosed in the 17th volume of the 2nd edition of "Encyclopedia of Polymer Science and Engineering" (1989), pages 1027-1028, and the high-density polyethylene of Zigler-Natta catalysis is well known. The reason why the present invention adopts the Zigler-Natta catalyst is that olefin polymerization catalysts such as chromium-based catalysts and metallocene catalysts are usually more sensitive to impurities than the Zigler-Natta catalyst, and the aldehyde group can change the electron cloud distribution and spatial structure of the catalyst active center, which is very likely to cause catalyst poisoning, greatly reduce its activity or even completely deactivate it. However, the present invention adopts vinylbenzaldehyde monomers, and the formaldehyde group is farther away from the vinyl group, which significantly reduces the influence of the aldehyde group on the catalyst reaction activity.

[0047] In specific implementation, the alkyl aluminum co-catalyst is one or more of triethyl aluminum, diethyl aluminum chloride, and tri-n-hexyl chloride. In specific implementation, triethyl aluminum and diethyl aluminum chloride are preferably used in combination.

[0048] 2. Preparation of Modified Carbon Black: Oxidized carbon black is reacted with polyamines to produce modified carbon black with primary amine groups. The preparation of oxidized carbon black is based on Ding Wenbing's doctoral dissertation, "Preparation and Performance Research of Surface-Modified Carbon Black, Carbon Nanotubes, and Graphene" (Zhejiang University, 2016, pp. 58-59). Nitric acid is used as the oxidant, and oxidation is carried out under relatively mild reaction conditions (≤100°C) and an appropriate reaction time (≤36 hours), resulting in carboxyl groups as the primary functional groups on the surface of the oxidized carbon black. By varying the reaction time, carbon blacks with varying carboxyl group contents can be obtained. The carboxylated carbon black is reacted with polyamine groups at room temperature and pressure for a specific time, preferably 4-12 hours, to produce primary amine-modified carbon black. The polyvalent primary amine is a compound containing at least two primary amine groups, such as ethylenediamine, propylenediamine, tri(2-aminoethyl)amine, tri(aminomethyl)amine, tri(aminoethyl)amine, tri(aminopropyl)amine, tri(aminobutyl)amine, and tri(aminohexyl)amine, preferably tri(2-aminoethyl)amine and propylenediamine.

[0049] 3. Masterbatch Preparation: The masterbatch is composed of carbon black, high-density polyethylene resin prepared with a Zigler-Natta catalyst, and an antioxidant system. The modified carbon black, high-density polyethylene resin, and antioxidant system are extruded at 230-270°C, preferably 250-260°C, to produce a carbon black masterbatch uniformly dispersed in a resin carrier. The antioxidant system in the masterbatch is a mixture of a hindered phenol antioxidant and a phosphite antioxidant, with a weight ratio of hindered phenol antioxidant to phosphite antioxidant of 0.5-1:1.5-2.5, preferably 1:2.

[0050] 4. Preparation of the polyethylene compound: The high-density polyethylene copolymer prepared above is extruded with a carbon black masterbatch and a compounded antioxidant system through a reactive extruder. The pellets are then pelletized in a pelletizer and dried to obtain the polyethylene compound of the present invention. The extrusion temperature in the reactive extruder is 240-270°C, preferably 250-260°C.

[0051] In the antioxidant system, the heat stabilizer is at least one of a hindered phenolic antioxidant and a phosphite antioxidant. The hindered phenolic antioxidants include, but are not limited to, monohydric hindered phenols and polyhydric hindered phenols, such as butylated hydroxytoluene (BHT), Antioxidant 1024, Antioxidant 3114, Antioxidant 1010, and Antioxidant 1330. The phosphite antioxidants, i.e., phosphite antioxidants, include, but are not limited to, phenol-free phosphite antioxidants, low-phenol phosphite antioxidants, and phenol-containing phosphite antioxidants, such as Antioxidant 168. The acid scavenger is at least one of calcium stearate, zinc stearate, and hydrotalcite, preferably calcium stearate.

[0052] In specific implementation, preferably, the reactive processing equipment used in the method is any one of a single-screw extruder, a twin-screw extruder, an open mixer, and an internal mixer.

[0053] The polyethylene compound described in this invention utilizes a reversible crosslinking reaction between the aldehyde groups in the high-density polyethylene copolymer and the amine groups in the masterbatch. This crosslinking is resolved during melting and heating, and crosslinked during the melt's passage through the die for sizing and cooling. This significantly improves the melt's sag performance, thereby enhancing the material's processing performance for large-diameter, thick-walled pipes. Furthermore, compared to irreversible crosslinking techniques, the polyethylene compound provided by this invention for producing large-diameter, thick-walled pipes can be repeatedly processed, achieving material recycling compared to existing crosslinked polyethylene pipe processing techniques.

[0054] The present invention will be described in detail below with reference to specific embodiments. Any features such as preparation methods, materials, structures or composition ratios not explicitly described in this technical solution are considered to be common technical features disclosed in the prior art.

[0055] Introduction to related equipment used in the embodiment:

[0056] Extruder: Model 35 twin-screw extruder (Belong Keya Nanjing Machinery Co., Ltd.)

[0057] Injection molding machine: injection molding into standard specimens through an injection molding machine (Liuzhou Injection Molding Machinery Factory UN-100).

[0058] Evaluation and analysis methods are introduced in the examples:

[0059] The data related to the polymers in the examples were obtained using the following test methods:

[0060] Melt flow rate (MFR): measured in accordance with GB / T 3682.1-2018, 190°C, 21.6 kg load.

[0061] Density: Tested in accordance with GB / T 1033.2.

[0062] Flexural modulus: tested according to ASTM D790.

[0063] Tensile yield strength: tested according to ASTM D638.

[0064] Tensile break nominal strain: tested in accordance with ASTM D 638.

[0065] Oxidation induction temperature: tested in accordance with GB / T19466.6.

[0066] Melt strength: according to the operating procedures of Rheo-Tester 2000 capillary rheometer.

[0067] Example 1

[0068] Step 1: Preparation of high-density polyethylene copolymer.

[0069] A gas-phase full-density polyethylene (HDPE) reactor was used. Refined ethylene, comonomer 4-vinylbenzaldehyde, nitrogen, and hydrogen were metered into the reactor at a total reactor pressure of 2300 kPa, an ethylene partial pressure of 1550 kPa, a 4-vinylbenzaldehyde to ethylene feed molar ratio of 0.064 mol / mol, and a hydrogen to ethylene feed molar ratio of 0.0018 mol / mol. Other partial pressures were provided by nitrogen. Typical Zigler-Natta catalyst UCAT J (Ti content 2.3 wt%) and cocatalyst diethylaluminum chloride were injected through the catalyst injection port at a rate of 5.3 kg / h of UCAT J and 0.71 kg / h of diethylaluminum chloride, respectively, with an Al / Ti molar ratio of 48. The reaction was carried out at a temperature of 105°C and a fluidizing gas velocity of 0.7 m / s under fully mixed conditions to produce a high-density polyethylene copolymer.

[0070] Step 2: Preparation of modified carbon black.

[0071] 500g of carbon black was weighed, added to 10L of concentrated nitric acid, and mechanically stirred at 100°C for 24 hours. The mixture was filtered to obtain 517g of acidified carbon black. 517g of acidified carbon black was also added to 150g of tris(aminoethyl)amine. The mixture was mechanically stirred at room temperature for 6 hours. The mixture was filtered and the filter cake was dried in a vacuum oven for 2 hours to obtain 640g of aminated modified carbon black. Infrared analysis revealed a primary amine content of 16mmol / g in the modified carbon black.

[0072] Step 3: Preparation of masterbatch.

[0073] Weigh 600g of modified carbon black and 1000g of Zigler-Natta catalyst to prepare a density of 0.960g / cm 3 To the polyethylene, 500ppm hindered phenol antioxidant 1010 and 1000ppm phosphite antioxidant 168 were added, mixed at high speed and then extruded and granulated at 250℃ by an extrusion granulator to obtain carbon black masterbatch.

[0074] Step 4: Preparation of polyethylene compound.

[0075] 5000 g of the high-density polyethylene copolymer obtained in step 1 of this example, 315 g of the masterbatch obtained in step 3, 6.6 g of hindered phenolic antioxidant 1010, 5.3 g of hindered phenolic antioxidant 330, 13.2 g of phosphite antioxidant 168, and 5.6 g of calcium stearate were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at 260°C to produce the polyethylene compound of the present invention. The resulting compound was subjected to performance testing in accordance with current relevant GBT standards. Specific process conditions and compound performance test results are shown in Table 1.

[0076] Comparative Example 1

[0077] Step 1: Preparation of high-density polyethylene copolymer.

[0078] The same method as in Example 1 was used, except that hexene was used instead of 4-vinylbenzaldehyde. The molar ratio of hexene to ethylene was still 0.0040 mol / mol. Other aspects were the same as in Example 1.

[0079] Step 2, step 3, and step 4 are the same as in Example 1. The specific process conditions and the test results of the mixed material performance are shown in Table 1.

[0080] Comparative Example 2

[0081] The steps are the same as step 1 in Example 1.

[0082] Step 2: Preparation of masterbatch.

[0083] Weigh 600g of carbon black and 900g of Zigler-Natta catalyst to prepare a density of 0.957g / cm 3 To the polyethylene, 500ppm hindered phenol antioxidant 1010 and 1000ppm phosphite antioxidant 168 were added, mixed at high speed and then extruded at 250℃ through an extrusion granulator to obtain carbon black masterbatch.

[0084] Step 3: Preparation of polyethylene compound.

[0085] 5000 g of high-density polyethylene copolymer, 315 g of the masterbatch obtained in step 2 of Comparative Example 2, 6.6 g of hindered phenolic antioxidant 1010, 5.3 g of hindered phenolic antioxidant 330, 13.2 g of phosphite antioxidant 168, and 5.5 g of calcium stearate were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at 255° C. to produce the polyethylene compound of the present invention. Specific process conditions and compound performance test results are shown in Table 1.

[0086] Comparative Example 3

[0087] Step 1: Preparation of high-density polyethylene copolymer.

[0088] The same as Example 1, except that hexene was used instead of 4-vinylbenzaldehyde. The feed molar ratio of hexene to ethylene was 0.058 mol / mol. Other aspects were the same as in Example 1.

[0089] Step 2: Preparation of masterbatch.

[0090] Weigh 600g of carbon black and 800g of Zigler-Natta catalyst to prepare a density of 0.957g / cm 3 To the polyethylene, 500ppm hindered phenol antioxidant 1010 and 1000ppm phosphite antioxidant 168 were added, mixed at high speed and then extruded at 250℃ through an extrusion granulator to obtain carbon black masterbatch.

[0091] Step 3: Preparation of polyethylene compound.

[0092] 5000 g of the high-density polyethylene copolymer prepared in step 1 of Comparative Example 3, 320 g of the masterbatch obtained in step 2 of Comparative Example 3, 7 g of hindered phenol antioxidant 1010, 7.5 g of hindered phenol antioxidant 330, 13.2 g of phosphite antioxidant 168, and 5 g of calcium stearate were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at a temperature of 255° C. to produce the polyethylene compound of the present invention. Specific process conditions and compound performance test results are shown in Table 1.

[0093] Example 2

[0094] Step 1: Preparation of high-density polyethylene copolymer.

[0095] A gas-phase full-density polyethylene (HDPE) unit was used. Refined ethylene, the comonomer 4-vinylbenzaldehyde, nitrogen, and hydrogen were metered into the reactor at a total reactor pressure of 2290 kPa, an ethylene partial pressure of 1600 kPa, a 4-vinylbenzaldehyde to ethylene molar ratio of 0.087 mol / mol, and a hydrogen to ethylene molar ratio of 0.0016 mol / mol. Other partial pressures were provided by nitrogen. A typical Zigler-Natta catalyst, UCAT J (Ti content of 2.3 wt%), and a 1:1 mixture of diethylaluminum chloride and triethylaluminum (mass ratio) were injected through the catalyst injection port. The UCAT J injection rate was 6.0 kg / h, and the mixed cocatalyst was 0.78 kg / h. The Al / Ti molar ratio was 55. The reaction was carried out at a temperature of 106°C and a fluidizing gas velocity of 0.73 m / s under fully mixed conditions to produce a high-density polyethylene copolymer.

[0096] Step 2: Preparation of modified carbon black.

[0097] 500g of carbon black was weighed, added to 12L of concentrated nitric acid, and reacted at 100°C with mechanical stirring for 28 hours. The mixture was then filtered to yield 519g of acidified carbon black. 519g of acidified carbon black was also added to 160g of propylenediamine. The mixture was mechanically stirred at room temperature for 4 hours, followed by drying in a vacuum oven for 3 hours to yield 625g of aminated modified carbon black. Infrared analysis revealed a primary amine content of 18mmol / g in the modified carbon black.

[0098] Step 3: Preparation of masterbatch.

[0099] Weigh 810 g of polyethylene with a density of 0.965 g / cm3 prepared by Zigler-Natta catalyst, weigh 600 g of modified carbon black, add 600 ppm of hindered phenol antioxidant 1010, add 1000 ppm of phosphite antioxidant 168, mix well at high speed, and extrude granulate at 250°C through an extrusion granulator to obtain carbon black masterbatch.

[0100] Step 4: Preparation of polyethylene compound.

[0101] 5000 g of the high-density polyethylene copolymer obtained in step 1 of this example, 310 g of the masterbatch obtained in step 3, 7.5 g of hindered phenolic antioxidant 1010, 14 g of hindered phenolic antioxidant 330, 20 g of phosphite antioxidant 168, and 9 g of calcium stearate were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at a temperature of 265°C to produce the polyethylene compound of the present invention. The resulting compound was subjected to performance testing in accordance with current relevant GBT standards. Specific process conditions and compound performance test results are shown in Table 1.

[0102] Example 3

[0103] Step 1: Preparation of high-density polyethylene copolymer.

[0104] A gas-phase full-density polyethylene (HDPE) unit was used. Refined ethylene, comonomer 4-vinylbenzaldehyde, nitrogen, and hydrogen were metered into the reactor at a total reactor pressure of 2237 kPa, an ethylene partial pressure of 1630 kPa, a 4-vinylbenzaldehyde to ethylene molar ratio of 0.092 mol / mol, and a hydrogen to ethylene molar ratio of 0.0020 mol / mol. Other partial pressures were provided by nitrogen. A typical Zigler-Natta catalyst, UCAT J (Ti content of 2.3 wt%), and a mixed cocatalyst mixture of tri-n-hexylaluminum and triethylaluminum (1:1 by mass) were injected through the catalyst and cocatalyst inlets. UCAT J was injected at a rate of 6.5 kg / h, and the mixed cocatalyst was injected at a rate of 0.73 kg / h. The Al / Ti molar ratio was 48. The reaction was carried out at a temperature of 106°C and a fluidizing gas velocity of 0.72 m / s under fully mixed conditions to produce a high-density polyethylene copolymer.

[0105] Step 2: Preparation of modified carbon black.

[0106] 500g of carbon black was weighed, added to 13L of concentrated nitric acid, and mechanically stirred at 100°C for 28 hours. The mixture was then filtered to obtain 519g of acidified carbon black. 519g of acidified carbon black was also added to 170g of tris(aminomethyl)amine. The mixture was mechanically stirred at room temperature for 4 hours, and then dried in a vacuum oven for 2.5 hours to produce 625g of aminated modified carbon black. Infrared analysis revealed a primary amine content of 21mmol / g in the modified carbon black.

[0107] Step 3: Preparation of masterbatch.

[0108] Weigh 1050g of Zigler-Natta catalyst to prepare a density of 0.947g / cm 3 600 g of modified carbon black was weighed, 500 ppm of hindered phenol antioxidant 1010 and 1000 ppm of phosphite antioxidant 168 were added, mixed at high speed, and then extruded and granulated at 255 ° C by an extrusion granulator to obtain carbon black masterbatch.

[0109] Step 4: Preparation of polyethylene compound.

[0110] 5000 g of the high-density polyethylene copolymer obtained in step 1 of this example, 305 g of the masterbatch obtained in step 3, 5.5 g of hindered phenolic antioxidant 1010, 7 g of hindered phenolic antioxidant 330, 16.5 g of phosphite antioxidant 168, and 6 g of calcium stearate were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at a temperature of 265°C to produce the polyethylene compound of the present invention. The resulting compound was then performance tested according to current relevant GBT standards. Specific process conditions and compound performance test results are shown in Table 1.

[0111] Example 4

[0112] Step 1: Preparation of high-density polyethylene copolymer.

[0113] A gas-phase full-density polyethylene (HDPE) unit was used. Refined ethylene, the comonomer 4-vinylbenzaldehyde, nitrogen, and hydrogen were metered into the reactor at a total reactor pressure of 2235 kPa, an ethylene partial pressure of 1640 kPa, a 4-vinylbenzaldehyde to ethylene feed molar ratio of 0.079 mol / mol, and a hydrogen to ethylene feed molar ratio of 0.0015 mol / mol. Other partial pressures were provided by nitrogen. Typical Zigler-Natta catalyst UCAT J (Ti content 2.3 wt%) and cocatalyst diethylaluminum chloride were injected through the catalyst injection port at a rate of 5.7 kg / h of UCAT J and 0.69 kg / h of diethylaluminum chloride, resulting in an Al / Ti molar ratio of 48. The reaction was carried out at a temperature of 106°C and a fluidizing gas velocity of 0.72 m / s under fully mixed conditions to produce a high-density polyethylene copolymer.

[0114] Step 2: Preparation of modified carbon black.

[0115] 500g of carbon black was weighed, added to 12L of concentrated nitric acid, and reacted at 100°C with mechanical stirring for 28 hours. The mixture was then filtered to obtain 519g of acidified carbon black. 519g of acidified carbon black was also added to 156g of tris(aminopropyl)amine. The mixture was mechanically stirred at room temperature for 4 hours, and then dried in a vacuum oven for 2.5 hours to produce 625g of aminated modified carbon black. Infrared analysis revealed a primary amine content of 15mmol / g in the modified carbon black.

[0116] Step 3: Preparation of masterbatch.

[0117] Weigh 600g of modified carbon black and weigh 850g of Zigler-Natta catalyst to prepare a density of 0.947g / cm 3 To the polyethylene, 600ppm hindered phenol antioxidant 1010 and 1000ppm phosphite antioxidant 168 were added, mixed at high speed and then extruded and granulated at 250℃ by an extrusion granulator to obtain carbon black masterbatch.

[0118] Step 4: Preparation of polyethylene compound.

[0119] 5000 g of the high-density polyethylene copolymer obtained in step 1 of this example, 300 g of the masterbatch obtained in step 3, 7 g of hindered phenolic antioxidant 1010, 14 g of hindered phenolic antioxidant 330, 20 g of phosphite antioxidant 168, and 6 g of calcium stearate were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at a temperature of 265°C to produce the polyethylene compound of the present invention. The resulting compound was then performance tested according to current relevant GBT standards. Specific process conditions and compound performance test results are shown in Table 1.

[0120] Example 5

[0121] Step 1: Preparation of high-density polyethylene copolymer.

[0122] A gas-phase full-density polyethylene (HDPE) system was used. Refined ethylene, the comonomer 2-vinylbenzaldehyde, nitrogen, and hydrogen were metered into the reactor at a total reactor pressure of 2270 kPa, an ethylene partial pressure of 1640 kPa, a 2-vinylbenzaldehyde to ethylene molar ratio of 0.0125 mol / mol, and a hydrogen to ethylene molar ratio of 0.0018 mol / mol. Other partial pressures were provided by nitrogen. Typical Zigler-Natta catalyst UCAT J (Ti content 2.3 wt%) and cocatalyst diethylaluminum chloride were injected through the catalyst injection port at a rate of 6.3 kg / h of UCAT J and 0.69 kg / h of diethylaluminum chloride, resulting in an Al / Ti molar ratio of 48. The reaction was carried out at a temperature of 106°C and a fluidizing gas velocity of 0.72 m / s under fully mixed conditions to produce a high-density polyethylene copolymer.

[0123] Step 2: Preparation of modified carbon black.

[0124] 500g of carbon black was weighed, added to 12L of concentrated nitric acid, and mechanically stirred at 100°C for 28 hours. The mixture was filtered to obtain 519g of acidified carbon black. 519g of acidified carbon black was also added to 145g of tris(aminomethyl)amine. The mixture was mechanically stirred at room temperature for 4 hours, and then dried in a vacuum oven for 2.5 hours to produce 625g of aminated modified carbon black. Infrared analysis revealed a primary amine content of 15mmol / g in the modified carbon black.

[0125] Step 3: Preparation of masterbatch.

[0126] Weigh 600g of modified carbon black and 960g of Zigler-Natta catalyst to prepare a density of 0.947g / cm 3 To the polyethylene, 600ppm hindered phenol antioxidant 1010 and 1000ppm phosphite antioxidant 168 were added, mixed at high speed and then extruded and granulated at 250℃ by an extrusion granulator to obtain carbon black masterbatch.

[0127] Step 4: Preparation of polyethylene compound.

[0128] 5000 g of the high-density polyethylene copolymer obtained in step 1 of Example 4, 300 g of the masterbatch obtained in step 3, 7 g of hindered phenolic antioxidant 1010, 14 g of hindered phenolic antioxidant 330, 20 g of phosphite antioxidant 168, and 6.5 g of zinc stearate were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at a temperature of 265°C to produce the polyethylene compound of the present invention. The resulting compound was then performance tested according to current relevant GBT standards. Specific process conditions and compound performance test results are shown in Table 1.

[0129] Example 6

[0130] Step 1: Preparation of high-density polyethylene copolymer.

[0131] A gas-phase full-density polyethylene (HDPE) unit was used. Refined ethylene, the comonomer 2-methyl, 4-vinylbenzaldehyde, nitrogen, and hydrogen were metered into the reactor at a total reactor pressure of 2300 kPa, an ethylene partial pressure of 1600 kPa, a feed molar ratio of 3-vinylbenzaldehyde to ethylene of 0.0730 mol / mol, and a feed molar ratio of hydrogen to ethylene of 0.0018 mol / mol. Other partial pressures were provided by nitrogen. A typical Zigler-Natta catalyst, UCAT J (Ti content of 2.3 wt%), and a mixed cocatalyst mixture of diethylaluminum monochloride and triethylaluminum (mass ratio 0.5:1) were injected through the catalyst and cocatalyst inlets. UCAT J was injected at a rate of 6.3 kg / h, and the mixed cocatalyst was injected at a rate of 0.70 g / h. The Al / Ti molar ratio was 50. The reaction was carried out at a fully mixed reaction temperature of 107°C and a fluidizing gas velocity of 0.74 m / s to produce a high-density polyethylene copolymer.

[0132] Step 2: Preparation of modified carbon black.

[0133] 500g of carbon black was weighed, added to 12L of concentrated nitric acid, and mechanically stirred at 100°C for 28 hours. The mixture was filtered to obtain 519g of acidified carbon black. 519g of acidified carbon black was also added to 140g of tris(aminoethyl)amine. The mixture was mechanically stirred at room temperature for 5 hours, and then dried in a vacuum oven for 3 hours to produce 610g of aminated modified carbon black. Infrared analysis revealed a primary amine content of 16mmol / g in the modified carbon black.

[0134] Step 3: Preparation of masterbatch.

[0135] Weigh 600g of modified carbon black and 890g of Zigler-Natta catalyst to prepare a density of 0.947g / cm 3 To the polyethylene, 600ppm hindered phenol antioxidant 1010 and 1000ppm phosphite antioxidant 168 were added, mixed at high speed and then extruded and granulated at 250℃ by an extrusion granulator to obtain carbon black masterbatch.

[0136] Step 4: Preparation of polyethylene compound.

[0137] 5000 g of the high-density polyethylene copolymer obtained in step 1 of this example, 300 g of the masterbatch obtained in step 3, 7 g of hindered phenolic antioxidant 1010, 14 g of hindered phenolic antioxidant 330, 20 g of phosphite antioxidant 168, and 6.5 g of hydrotalcite were weighed and uniformly mixed. The mixture was then extruded through a reactive extruder at a temperature of 260°C to produce the polyethylene compound of the present invention. The resulting compound was then performance tested according to current relevant GBT standards. Specific process conditions and compound performance test results are shown in Table 1.

[0138] Example 7

[0139] 3000 g of the polyethylene compound prepared in Example 1 was added to a twin-screw extruder for melt extrusion. The extrusion temperature was 260°C. The extruded product was cooled in a circulating water bath and pelletized to obtain the final product. Specific process conditions and resin performance test results are shown in Table 1.

[0140] Example 8

[0141] 2000g of the product from Example 7 was added with 0.5g of Hindered Phenol Antioxidant 1010, 1g of Hindered Phenol Antioxidant 330, and 2g of Phosphite Antioxidant 168. After mixing thoroughly, the mixture was added to a twin-screw extruder for melt extrusion. The extrusion temperature was 260°C, and the extruded product was cooled in a circulating water bath and pelletized to obtain the final product. Specific process conditions and resin performance test results are shown in Table 1.

[0142] Table 1 Properties of polyethylene blends

[0143]

[0144]

[0145] “-” in the above table means there is no data.

[0146] Taking Example 1 and Example 7 as examples, the final prepared mixed materials were passed through a 36-inch DR9 large-diameter pipe extrusion device to successfully produce a 36-inch DR9 large-diameter thick-walled pipe. The pipe has a smooth appearance, the outer diameter and wall thickness meet the size requirements, and the out-of-roundness of the pipe can be controlled within 4%.

[0147] The beneficial effects of the present invention are specifically demonstrated as follows: First, the mixed ingredients in Comparative Examples 1, 2, and 3 do not contain crosslinkable groups. Examples 1 through 6 all undergo, to varying degrees, a reversible imidization reaction between the aldehyde groups in the high-density polyethylene copolymer and the primary amine groups of the carbon black in the masterbatch. Table 1 shows that the tensile yield strength and melt strength of the examples are significantly higher than those of the comparative examples. Melt strength is an important indicator for evaluating a material's resistance to sag, and is closely related to the entanglement between polyethylene molecular chains. High levels of entanglement and melt strength indicate strong interactions between the molecular chains, enabling the material to better retain its shape and resist deformation in the molten state. By testing melt strength, the level of entanglement in the polyethylene molecular chains can be intuitively understood, thereby assessing the material's microstructural properties. A comprehensive comparison of the performance data of the examples and comparative examples shows that the examples significantly outperform the comparative examples in terms of tensile yield strength and melt strength, fully demonstrating the positive effect of the reversible imidization crosslinking structure introduced by the present invention on improving material performance.

[0148] Secondly, since the present invention introduces reversible crosslinking groups into the polyethylene compound, the performance of the compound can be well maintained after repeated processing, as can be seen from the data comparison of Examples 1, 7 and 8.

[0149] Comparative Example 4

[0150] Peroxide crosslinking system.

[0151] Step 1: Formula preparation.

[0152] The following formulation was used, which was different from that in Example 1: high-density polyethylene (HDPE, density 0.952 g / cm 3 ) 95.5%, dicumyl peroxide (DCP) 2.0%, antioxidant 1010 1.5%, triallyl isocyanurate (TAIC) 1.0%.

[0153] Step 2: Preparation of mixed ingredients.

[0154] After mixing the raw materials at room temperature, the materials were allowed to stand for more than 5 hours to allow the HDPE to fully contact with the other components. The melt strength test value of the test formula was 0.310N (lower than 0.35N in Example 1).

[0155] Conclusion: The peroxide cross-linking system can improve the melt strength of HDPE, but it has an obvious defect that it cannot be processed again.

[0156] Comparative Example 5

[0157] Irreversible cross-linking system.

[0158] Step 1: Using the formulation of Comparative Example 4, granulation was performed by extrusion at 190° C. to obtain pellets.

[0159] Step 2: The pellets obtained in step 1 were extruded a second time at 190°C. After the second granulation, the melt flow rate of the material dropped from the initial 5.2 g / 10 min to 1.8 g / 10 min (crosslinking formed an irreversible network), and the processing performance of the material declined, which was insufficient to meet the requirements of the large-diameter pipe extrusion process.

[0160] Conclusion: Irreversible cross-linking system cannot be reprocessed.

[0161] Comparative Example 6

[0162] Dynamic Diels-Alder (DA) crosslinking system.

[0163] Step 1: Formula preparation.

[0164] Furan-modified HDPE (furan group content 5 mol%) and bismaleimide crosslinking agent (2 wt %) were used, and the antioxidant system was the same as that in Example 1.

[0165] Step 2: Processing performance test.

[0166] Rheological test: At 140°C (DA bond dissociation temperature), the melt viscosity drops sharply from 1200 Pa·s to 200 Pa·s (viscosity mutation rate 500%). The processing window is narrow (extrusion is only allowed in the range of 130-150°C), which is not suitable for the long cooling and sizing process of large-diameter pipes.

[0167] Conclusion: The DA system has a narrow processing window due to the sudden change in viscosity and cannot stably process large-diameter pipes.

[0168] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A polyethylene compound for large-diameter, thick-walled, pressure-resistant polyethylene pipes, characterized in that: Calculated based on 100% polyethylene mixed material mass, the mass content of each component is as follows: high-density polyethylene copolymer 93% to 96%, masterbatch 3.5% to 7%, and compound antioxidant system 0.5% to 1%; The high-density polyethylene copolymer is obtained by copolymerizing ethylene and vinylbenzaldehyde compounds, and the molar ratio of the ethylene monomer unit to the vinylbenzaldehyde compound unit in the high-density polyethylene copolymer is 89-96:4-11; The masterbatch comprises carbon black and a high-density polyethylene resin prepared by catalysis of a Ziegler-Natta catalyst.

2. The polyethylene compound for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 1, characterized in that: The high-density polyethylene copolymer is prepared by copolymerizing ethylene with vinylbenzaldehyde or a vinylbenzaldehyde derivative; The vinylbenzaldehyde derivative includes one of 2-vinylbenzaldehyde, 3-vinylbenzaldehyde and 4-vinylbenzaldehyde.

3. The polyethylene compound for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 1, characterized in that: The density of the high-density polyethylene resin in the masterbatch is at least 0.940; The carbon black in the masterbatch is modified carbon black, which is furnace carbon black and / or channel carbon black. The content of the modified carbon black in the masterbatch accounts for 36-45% of the mass of the masterbatch, and the content of the modified carbon black accounts for 1.5-3% of the mass of the polyethylene compound. The content of primary amine groups in the modified carbon black is 5 mmol / g to 22 mmol / g.

4. The polyethylene compound for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 1, characterized in that: The compound antioxidant system is a mixture of a hindered phenol antioxidant, a phosphite antioxidant and an acid scavenger, and the weight ratio of the three components is hindered phenol antioxidant: phosphite antioxidant: acid scavenger = 2-2.5:2-3:0.8-1.

2.

5. A method for preparing a polyethylene compound for large-diameter, thick-walled, pressure-resistant polyethylene pipes according to any one of claims 1 to 4, characterized in that: The preparation method comprises the following steps: extruding a high-density polyethylene copolymer, a masterbatch and a compound antioxidant system through a reactive extruder at a temperature of 240-270°C, pelletizing the pellets through a pelletizer, and drying the pellets to obtain a polyethylene mixed material.

6. The method for preparing a polyethylene mixed material for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 5, characterized in that: The high-density polyethylene copolymer is prepared by a fully mixed gas phase anionic coordination polymerization reaction of ethylene, vinylbenzaldehyde compounds, and hydrogen in the presence of a Ziegler-Natta catalyst system and an alkyl aluminum co-catalyst; The molar ratio of Al to Ti in the Ziegler-Natta catalyst system is controlled to be 45-55 mol / mol.

7. The method for preparing a polyethylene mixed material for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 6, characterized in that: During the reaction, the total pressure of the reactor is maintained at 2200-2350 kPa, the ethylene partial pressure is 1500-1650 kPa, the molar ratio of the vinylbenzaldehyde compound to ethylene is 0.05-0.13, and the molar ratio of hydrogen to ethylene is 0.0015-0.0025; The vinylbenzaldehyde compound is selected from one of 2-vinylbenzaldehyde, 3-vinylbenzaldehyde and 4-vinylbenzaldehyde; The polymerization temperature is 102°C to 108°C; The aluminum co-catalyst is a combination of one or more of triethylaluminum, diethylaluminum monochloride, and tri-n-hexyl chloride.

8. The method for preparing a polyethylene mixed material for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 5, characterized in that: The preparation process of the masterbatch includes: The modified carbon black, high-density polyethylene resin, and antioxidant system are extruded at 230-270° C. to prepare a carbon black masterbatch uniformly dispersed in a resin carrier; The antioxidant system in the masterbatch is a mixture of hindered phenol antioxidant and phosphite antioxidant, and the mass ratio of hindered phenol to phosphite is 0.5-1:1.5-2.

5.

9. The method for preparing a polyethylene mixed material for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 8, characterized in that: The preparation process of the modified carbon black comprises: reacting oxidized carbon black with polyvalent primary amines to prepare modified carbon black with primary amine groups; The oxidized carbon black uses nitric acid as an oxidant and is oxidized at a temperature of 100° C. or less for 36 hours, so that the main functional groups on the surface of the oxidized carbon black are carboxyl groups. The polyvalent primary amine is selected from one of ethylenediamine, propylenediamine, tris(2-aminoethyl)amine, tris(aminomethyl)amine, tris(aminoethyl)amine, tris(aminopropyl)amine, tris(aminobutyl)amine, and tris(aminohexyl)amine.

10. The method for preparing a polyethylene mixed material for large-diameter, thick-walled, pressure-resistant polyethylene pipe according to claim 5, characterized in that: The reactive extrusion equipment is one of a single-screw extruder, a twin-screw extruder, an open mixer, and an internal mixer; During the extrusion process of the reactive extruder, a reversible cross-linking reaction occurs between the aldehyde groups in the high-density polyethylene copolymer and the amine groups in the masterbatch. That is, the cross-linking is decoupled when the mixed materials are melted and heated, and the melt cross-links when it passes through the die for sizing and cooling. This improves the sag performance of the melt and enhances the processing performance of the material for large-diameter thick-walled pipes. The large-diameter thick-walled pipes are pipes with a diameter and wall thickness of 32in DR9 and above.

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