Composite sensitizer assisted irradiation crosslinking polyethylene composite material and preparation method thereof

Abstract

The application provides a composite sensitizer assisted irradiation cross-linked polyethylene composite material and a preparation method thereof. The composite material comprises, by weight parts, 100 parts of a polyethylene matrix resin, 2.0-5.0 parts of a core-shell type hybrid cross-linking agent, 0.2-0.5 parts of an antioxidant and 0.1-0.3 parts of a lubricant. The core-shell type hybrid cross-linking agent comprises, from inside to outside, a nanometer silicon dioxide core with an olefinic unsaturated bond and an epoxy group grafted on the surface, an intermediate grafting layer grafted on the surface of the core and an outer coating layer staggered and interpenetrated with the intermediate grafting layer on a nanometer scale. The high-performance irradiation cross-linked polyethylene composite material with a gel content of greater than or equal to 90%, a tensile strength of greater than or equal to 35 MPa and a heat distortion temperature of greater than or equal to 135 DEG C can be obtained at a low irradiation dose of 20-50 kGy through the structure preset of the core-shell type hybrid cross-linking agent. The processing technology is simple, a side feeding device is not needed and the application is suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of polymer radiation crosslinking technology, specifically to a composite sensitizer-assisted radiation crosslinked polyethylene composite material and its preparation method. Background Technology

[0002] Irradiated cross-linked polyethylene (XLPE) is widely used in wires and cables, heat shrink tubing, and foam materials due to its excellent heat resistance, mechanical strength, and insulation properties. However, traditional irradiation cross-linking processes typically require high irradiation doses to achieve the desired gel content. This not only increases energy consumption and production costs but may also cause excessive degradation of the polyethylene backbone, leading to material embrittlement and darkening of color. In existing technologies, adding multifunctional monomers as sensitizers can reduce the irradiation dose, but these monomers have poor dispersibility, are prone to migration, and offer limited improvement to the long-term thermal stability of the material.

[0003] To address the aforementioned issues, this invention provides a composite sensitizer-assisted irradiation crosslinked polyethylene composite material and its preparation method. Multifunctional acrylates, azide silanes, and double-modified nano-silica are pre-assembled into a core-shell structure via chemical bonds, achieving multi-layered crosslinking synergy through pre-defined structure and time-triggered crosslinking. This allows for the acquisition of high-performance crosslinked polyethylene at low irradiation doses. Summary of the Invention

[0004] To address the existing technical problems, this invention provides a composite sensitizer-assisted irradiation crosslinked polyethylene composite material and its preparation method. To achieve the above objectives, the technical solution adopted by this invention is as follows: The present invention provides a composite sensitizer-assisted irradiation crosslinking polyethylene composite material, comprising, by weight: 100 parts of polyethylene matrix resin, 2.0-5.0 parts of core-shell hybrid crosslinking agent, 0.2-0.5 parts of antioxidant, and 0.1-0.3 parts of lubricant; Core-shell hybrid crosslinking agents include a core, intermediate branching layers, and an outer coating layer; The core is nano-silica with both olefinic unsaturated bonds and epoxy groups grafted onto its surface. The nano-silica has a particle size of 20–50 nm and a molar ratio of olefinic unsaturated bonds to epoxy groups of 1:1–2. The olefinic unsaturated bonds are used to undergo free radical copolymerization with the outer coating layer, and the epoxy groups are used to undergo ring-opening reactions with the decomposition products of the intermediate branching layer. The intermediate branching layer is an azide-based silane coupling agent, which is grafted onto the surface of the core via Si-O-Si covalent bonds; The outer coating is a multifunctional acrylate, which is coated on the outside of the intermediate interbranching layer in a semi-prepolymer state and interpenetrates with the intermediate interbranching layer at the nanoscale. Some unreacted olefinic unsaturated bonds are retained in the outer coating. The mass ratio of multifunctional acrylate, azide-based silane coupling agent and nano-silica is 1:0.3-0.6:0.5-1.0.

[0005] This application achieves multi-level synergistic crosslinking through a pre-assembled core-shell hybrid crosslinking agent combined with a three-stage gradient irradiation crosslinking process. The three-layer structure of the core-shell hybrid crosslinking agent carries different reactive functional groups, which are sequentially activated under the three stages of the gradient irradiation process: In the first stage, low-temperature pre-irradiation activates the free radical polymerization of the outer coating layer and the double bond grafting of the core, achieving the first anchoring of the core; in the second stage, medium-temperature heat treatment activates the decomposition of azide groups to generate nitrides, which insert into the polyethylene backbone, the core surface, and the outer coating layer in three directions, achieving three-layer chemical crosslinking; in the third stage, high-temperature final irradiation activates the ring-opening addition of amino groups and the epoxy groups in the core, achieving the second anchoring of the core. Through this sequential matching, the core is triple-locked in the crosslinking network by the double bond pathway, the epoxy pathway, and the nitride insertion. This synergistic mechanism enables the material to achieve a gel content of 93% and a heat distortion temperature of 140℃ with a total irradiation dose of only 30 kGy, reducing the irradiation dose by more than 50% compared to the traditional TAIC system, and significantly improving overall performance.

[0006] In some embodiments, the multifunctional acrylate is selected from at least one of trimethylolpropane trimethacrylate or ethoxylated trimethylolpropane triacrylate.

[0007] This application limits the types of multifunctional acrylates. Both monomers contain more than three olefinic unsaturated bonds, which can provide a high density of crosslinking active sites under irradiation conditions, ensuring that the outer coating layer can rapidly form a dense crosslinked network during the low-temperature pre-irradiation stage. Moreover, both monomers are liquid at room temperature, with suitable viscosity and good wettability, which can fully penetrate into the gaps of the azide-silane molecular brush in the semi-prepolymer coating process, achieving uniform interpenetration of the outer coating layer and the intermediate branching layer at the nanoscale, avoiding problems of uneven coating or loose structure caused by excessively high or low monomer viscosity.

[0008] Secondly, both monomers mentioned above have moderate free radical polymerization activity, and can achieve a controllable semi-prepolymerization reaction at low temperatures of 60-80°C through thermal initiators, retaining 50%-70% of unreacted double bonds for subsequent irradiation. This ensures the integrity of the core-shell structure during the extrusion process and reserves sufficient active sites for subsequent crosslinking reactions.

[0009] Finally, the two monomers mentioned above have good compatibility with polyethylene matrix resin. Their semi-prepolymer network can undergo effective free radical grafting reaction with polyethylene matrix during irradiation to form a strong interfacial bond, avoiding phase separation or interfacial debonding problems caused by poor interfacial compatibility.

[0010] In some embodiments, the polyethylene matrix resin is selected from at least one of low-density polyethylene, linear low-density polyethylene, or high-density polyethylene.

[0011] In this application, all three polyethylene matrix resins can achieve good melt blending with core-shell hybrid crosslinking agents at conventional processing temperatures of 150–170°C without adjusting extrusion process parameters, demonstrating good process adaptability.

[0012] Specifically, low-density polyethylene (LDPE) molecules contain more long branches and have lower crystallinity. During irradiation crosslinking, the chain segments have stronger mobility, which is beneficial for the uniform dispersion of core-shell hybrid crosslinking agents and the insertion reaction of nitrogen-containing intermediates into the polyethylene backbone. This makes it suitable for applications that require high flexibility and elongation.

[0013] Linear low-density polyethylene has a linear molecular chain structure with uniformly distributed short branches, and possesses both good processing fluidity and mechanical properties. Its regular molecular chain structure is conducive to the formation of a uniform crosslinking network during the irradiation crosslinking process, and its synergistic effect with core-shell hybrid crosslinking agents is the most significant, making it the preferred matrix resin of this application.

[0014] High-density polyethylene has high crystallinity and a tightly packed molecular chain. Its high melting point matches the decomposition temperature window of 100-130℃ for the azide groups in the core-shell hybrid crosslinking agent. This allows the product to maintain dimensional stability during the heat treatment stage, making it suitable for applications requiring high heat resistance and rigidity.

[0015] It should be noted that the above three resins can also be mixed and compounded according to specific application scenarios.

[0016] In some embodiments, the antioxidant is a hindered phenolic antioxidant and / or a phosphite antioxidant, and the lubricant is calcium stearate or zinc stearate.

[0017] In some embodiments, the hindered phenolic antioxidant is selected from at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (antioxidant 1010), octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1076), and tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanate (antioxidant 3114); the phosphite antioxidant is selected from at least one of tris(2,4-di-tert-butylphenyl) phosphite (antioxidant 168) and bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite (antioxidant 626); preferably, the hindered phenolic antioxidant and the phosphite antioxidant are used in a mass ratio of 1:1-2.

[0018] Specifically, hindered phenolic antioxidants can effectively capture free radicals generated during irradiation and heat treatment, inhibiting the thermal oxidative degradation of the polyethylene matrix under high-temperature conditions, while avoiding excessive capture of active free radicals required for cross-linking reactions, thus achieving a balance between antioxidant capacity and maintaining cross-linking efficiency. Furthermore, phosphite antioxidants, as auxiliary antioxidants, can decompose hydroperoxides generated during irradiation, producing a synergistic effect with hindered phenolic antioxidants, further enhancing the stability of the material during the final high-temperature irradiation stage.

[0019] Calcium stearate or zinc stearate, as lubricants, can reduce melt viscosity during melt blending, improve the dispersion uniformity of core-shell hybrid crosslinking agents in the polyethylene matrix, and avoid uneven crosslinking density caused by local agglomeration.

[0020] Existing technologies often use multifunctional monomers as sensitizers to reduce radiation dose. However, these monomers, such as TAIC and TMPTMA, are small-molecule organic compounds with significant differences in polarity and molecular weight compared to the polyethylene matrix resin. This makes it difficult to achieve uniform dispersion at the molecular level during melt blending, leading to the formation of localized enrichment zones or aggregates within the matrix. This results in uneven distribution of crosslinking points at the microscale and poor uniformity of crosslinking density. Furthermore, these small-molecule monomers are only bonded to the polyethylene matrix by weak intermolecular forces, lacking chemical bonding and anchoring. During subsequent processing or long-term use, especially at high temperatures, unreacted or incompletely fixed monomer molecules are prone to migration, volatilization, or precipitation, causing surface fogging, performance degradation, and environmental pollution. This limits the further application of multifunctional monomers as radiation sensitizers in high-performance, long-life radiation-crosslinked polyethylene products.

[0021] To address the aforementioned issues, this application pre-assembles multifunctional acrylates into an outer coating layer of a core-shell hybrid crosslinking agent, fixing it to the surface of nano-silica in a semi-prepolymerized state. This allows the acrylate to be uniformly dispersed within the polyethylene matrix along with the core, preventing the aggregation and local enrichment of free small monomer molecules. During the first stage of irradiation, the acrylate is transformed into a crosslinked network through free radical polymerization, permanently anchoring it to the material system and completely eliminating migration. Simultaneously, an azide-based silane coupling agent is introduced as an intermediate branching layer. The nitrogen-containing compounds generated by its decomposition can insert into the polyethylene backbone to form CN crosslinks with bond energies higher than traditional CC bonds, significantly improving the thermal stability of the crosslinked network. The core nano-silica is doubly anchored to the crosslinked network through double bonds and epoxy groups, further acting as a physical crosslinking point and reinforcing filler. Thus, this application solves the problems of dispersion and migration of multifunctional monomers while increasing the heat distortion temperature of the material from 115℃ in the traditional TAIC system to 140℃, significantly improving long-term thermal stability.

[0022] A second aspect of the present invention provides a method for preparing a core-shell hybrid crosslinking agent, comprising the following steps: (a) Disperse nano-silica in an organic solvent, add a silane coupling agent containing olefinic unsaturated bonds and a silane coupling agent containing epoxy groups to react and obtain a core with olefinic unsaturated bonds and epoxy groups grafted on the surface. (b) The kernel obtained in step (a) is dispersed in an organic solvent, an azide-silane coupling agent is added, and a grafting reaction is carried out at a temperature of 40-60°C to obtain an intermediate with azide-silane grafted on its surface. (c) The intermediate obtained in step (b) is mixed with a multifunctional acrylate, a low-temperature thermal initiator is added, and a semi-prepolymer coating reaction is carried out at 60-80°C, so that the multifunctional acrylate is coated on the surface of the intermediate in a semi-prepolymer state, and a core-shell hybrid crosslinking agent is obtained.

[0023] In some embodiments, the silane coupling agent containing olefinic unsaturated bonds in step (a) is γ-methacryloyloxypropyltrimethoxysilane, and the silane coupling agent containing epoxy groups is γ-glycidoxypropyltrimethoxysilane, with a molar ratio of 1:1 to 2, a reaction temperature of 70 to 90°C, and a reaction time of 4 to 8 hours.

[0024] In some embodiments, the mass ratio of the intermediate to the polyfunctional acrylate in step (c) is 1:2 to 3, the low-temperature thermal initiator is potassium persulfate, the amount added is 0.1% to 0.3% of the mass of the polyfunctional acrylate, and the mixing time is 3 to 8 minutes.

[0025] The core-shell hybrid crosslinking agent of this application exhibits a stable three-layer core-shell structure before irradiation. Please refer to [link / reference]. Figure 2 The system comprises a core 100, specifically nano-silica with olefinic unsaturated bonds A 120 and epoxy groups 110 grafted onto its surface, with a particle size of 20–50 nm and a molar ratio of double bonds to epoxy groups of 1:1–2. The intermediate branching layer 200 is an azide-silane coupling agent, grafted onto the core surface via Si-O-Si covalent bonds, with a grafting amount of 8%–15% of the core mass. The azide groups extend outwards in a molecular brush-like pattern. The outer coating layer 300 is a multifunctional acrylate, semi-prepolymerized and coated on the outside of the intermediate branching layer, interpenetrating with the azide-silane molecular brushes at the nanoscale. The outer coating layer retains 50%–70% of unreacted olefinic unsaturated bonds B 310, which will participate in the cross-linking reaction during subsequent irradiation.

[0026] In this state, the olefinic unsaturated bonds A 120 and epoxy groups 110 on the surface of the core 100, the azide groups in the intermediate branching layer 200, and the olefinic unsaturated bonds B 310 retained in the outer coating layer 300 are all in an unreacted state, and the entire core-shell hybrid crosslinking agent is uniformly dispersed in the polyethylene matrix resin in the form of independent particles.

[0027] A third aspect of this invention provides a method for preparing a composite sensitizer-assisted irradiation crosslinked polyethylene composite material, comprising the following steps: ① After the polyethylene matrix resin, core-shell hybrid crosslinking agent, antioxidant and lubricant are mixed evenly, they are added from the main feed port of the twin-screw extruder. The mixture is melt-blended and extruded and granulated at 150-170℃, 200-300 rpm, and the residence time of the material in the extruder is 1-3 minutes to obtain masterbatch. ② The masterbatch obtained in step ① is molded into a finished product; ③ The product obtained in step ② is subjected to gradient irradiation crosslinking treatment to obtain the finished product. The gradient irradiation crosslinking treatment includes a first irradiation, heat treatment and a second irradiation in sequence.

[0028] In some embodiments, the first irradiation in step ③ is carried out in a nitrogen atmosphere, with an irradiation dose of 10–20 kGy and an irradiation temperature of 30–50°C; the heat treatment temperature is 100–130°C and the time is 30–60 minutes; the second irradiation is carried out in an air atmosphere, with an irradiation dose of 10–20 kGy and an irradiation temperature of 120–140°C; the total irradiation dose is 20–50 kGy.

[0029] The core-shell hybrid crosslinking agent of this application achieves multi-level synergistic crosslinking through the following three stages during the irradiation crosslinking process, ultimately forming a three-dimensional crosslinked structure with triple interpenetrating networks.

[0030] Please see Figure 2 and Figure 3 The first stage involves low-temperature pre-irradiation at 10–20 kGy and 30–50 °C. During this stage, the olefinic unsaturated bonds B 310 retained in the outer coating layer 300 are activated by electron beam irradiation, undergoing free radical polymerization to transform into the reacted olefinic unsaturated bonds B 311. This transforms the outer coating layer 300 from a semi-prepolymerized state into a fully cross-linked network structure. Simultaneously, this free radical polymerization extends to the polyethylene matrix resin, forming a covalent bond between the outer coating layer 300 and the polyethylene matrix, achieving initial anchoring of the core-shell hybrid crosslinking agent in the polyethylene matrix. Furthermore, the olefinic unsaturated bonds A 120 on the surface of the core 100 are also captured by the free radical reaction during this stage, transforming into the cross-linked olefinic unsaturated bonds A 121, which then undergo graft copolymerization with the outer coating layer network, allowing the core 100 to integrate into the cross-linked network for the first time via a double bond pathway.

[0031] The second stage involves a medium-temperature heat treatment at 100–130℃ for 30–60 minutes. During this stage, the azide groups in the intermediate branching layer 200 decompose upon heating, removing nitrogen and generating a highly reactive nitride intermediate. This nitride intermediate exhibits extremely high reactivity and can undergo insertion reactions simultaneously in three directions: (i) inserting into the CH bonds of the polyethylene matrix resin to form CN covalent bonds A 210, further anchoring the core-shell hybrid crosslinking agent to the polyethylene backbone; (ii) inserting into the CH bonds of the grafted segments on the core surface to form CN covalent bonds B 220, enhancing the chemical connection between the intermediate branching layer and the core; and (iii) undergoing a cycloaddition reaction with the residual olefinic unsaturated bonds in the outer coating layer to form addition linkage bonds 230, achieving chemical crosslinking between the intermediate branching layer 200 and the outer coating layer 300. Thus, the outer coating layer 300 and the intermediate branching layer 200 form an interpenetrating network structure through covalent bonds.

[0032] The third stage involves final irradiation at 10–20 kGy and 120–140 °C. In this stage, trace amounts of amino or imine groups generated during the decomposition of the azide groups are activated under the synergistic effect of high temperature and irradiation, undergoing ring-opening addition reactions with the epoxy groups on the core surface to form a CNC covalently bridged structure 111. This reaction anchors the core 100 to the cross-linked network via a second chemical pathway, forming a double-protection anchoring of the core together with the connections formed in the first stage via the double-bond pathway.

[0033] The fourth aspect of this invention provides an irradiated crosslinked polyethylene composite material having a gel content ≥90%, tensile strength ≥35 MPa, elongation at break ≥400%, heat distortion temperature ≥135℃, and volume resistivity ≥1×10⁻⁶. 15 Ω·cm.

[0034] In this application, the core-shell hybrid crosslinking agent integrates with the polyethylene matrix resin through the following three chemical bonding methods during the irradiation crosslinking process, forming an integral crosslinking network: C-C bonds are formed between the outer coating layer and the polyethylene matrix through free radical grafting; CN bonds are formed between the intermediate branched layer and the polyethylene matrix through nitride insertion; The core achieves chemical anchoring through double bond pathways and epoxy ring-opening pathways.

[0035] The resulting triple interpenetrating network structure locks the nano-silica core within the cross-linked network by chemical bonds, avoiding the defects of easy agglomeration of nanofillers and weak interfacial bonding in traditional physical blending systems. This allows for the achievement of gel content ≥90% and excellent comprehensive performance even under low irradiation doses.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention constructs a multi-level synergistic crosslinking system with pre-synthesized core-shell hybrid crosslinking agent and gradient irradiation crosslinking process, which significantly improves the comprehensive performance of composite materials while significantly reducing the irradiation dose and simplifies the processing flow.

[0037] (2) The present invention designs a core-shell hybrid crosslinking agent: the core is double bond / epoxy double modified nano silica, which transforms it from an inert filler into an active node of the crosslinking network; the middle layer is an azide-based silane coupling agent, which is grafted onto the surface of the core through Si-O-Si bonds, and avoids thermal decomposition failure under the protection of the core and shell; the outer layer is a semi-prepolymerized multifunctional acrylate, which interpenetrates with the azide-based silane molecular brush, retains 50%-70% of unreacted double bonds, and covalently connects with the polyethylene matrix during irradiation and provides cycloaddition sites for nitrobenes, thereby achieving three-layer chemical crosslinking.

[0038] (3) The present invention adopts a three-stage gradient irradiation crosslinking process of low temperature pre-irradiation in nitrogen atmosphere, medium temperature heat treatment in air atmosphere, and high temperature final irradiation in air atmosphere. This process is matched with the three-layer structure of the core-shell hybrid crosslinking agent to form a time sequence, so that the functional components of each layer are activated and synergistically act in a specific stage, avoiding the defects of disordered reaction, many side reactions, and uneven crosslinking in the traditional one-step irradiation process. Attached Figure Description

[0039] Figure 1 This is a flowchart illustrating the preparation process of a composite sensitizer-assisted irradiation crosslinked polyethylene composite material according to the present invention.

[0040] Figure 2 This is a schematic diagram of the core-shell hybrid crosslinking agent of the present invention before irradiation crosslinking treatment.

[0041] Figure 3 This is a schematic diagram of the structure of the core-shell hybrid crosslinking agent after irradiation crosslinking treatment according to the present invention.

[0042] Among them, 100 is the core, 110 is the epoxy group, 111 is the CNC covalent bridge structure, 120 is the olefinic unsaturated bond A, 121 is the cross-linked olefinic unsaturated bond A, 200 is the intermediate branch layer, 210 is the CN covalent bond A, 220 is the CN covalent bond B, 230 is the addition linker bond, 300 is the outer coating layer, 310 is the olefinic unsaturated bond B, and 311 is the reacted olefinic unsaturated bond B. Detailed Implementation

[0043] The present invention will be described below with reference to specific implementation schemes. It should be noted that the following embodiments are examples of the present invention and are used only to illustrate the invention, not to limit it. Other combinations and various modifications within the scope of the present invention can be made without departing from its spirit or scope. A composite sensitizer-assisted radiation crosslinked polyethylene composite material was prepared according to the proportions and preparation methods of the raw materials specified in the following embodiments and comparative examples.

[0044] To facilitate implementation of this invention by those skilled in the art, the following description is provided regarding the manufacturers of some raw materials used in the embodiments and comparative examples: Nano-silica particle size: 20-30 nm, or 30-50 nm; Specific surface area: 150-300 m². 2 / g; Purity: ≥99.5%, selected from Guangzhou Jibisheng GB-SiO2-30 or other commercially available products meeting particle size requirements; Azide-based silane coupling agent selected from Aladdin, analytical grade; Trimethylolpropane trimethacrylate selected from Sartoma SR350; Ethoxylated trimethylolpropane triacrylate selected from Sartoma SR415; Low-density polyethylene selected from Sinopec Q / LDPE-2426H or Borealis LE5404; Linear low-density polyethylene selected from Sinopec DFDA-7042 or ExxonMobil LL 1002YB; High-density polyethylene selected from Dow HDPE DMDA-8007 or Sinopec HDPE 5000S; All other compounds and related reagents used can be purchased from the market.

[0045] Preparation Example 1 1. Kernel preparation: 4.0 g of nano-silica (C) that had been vacuum dried at 120°C for 2 hours was added to 80 mL of anhydrous toluene and ultrasonically dispersed for 30 minutes to form a uniform suspension.

[0046] 0.48 g KH570 and 0.62 g KH560 were added to the suspension in sequence, with a molar ratio of KH570 to KH560 of approximately 1:1.2. Then, 0.1 mL of glacial acetic acid was added as a catalyst.

[0047] The reaction system was heated to 80°C and stirred for 6 hours under nitrogen protection.

[0048] After the reaction was complete, the mixture was cooled to room temperature, centrifuged at 8000 rpm for 10 minutes, and the supernatant was discarded.

[0049] The precipitate was washed three times with 50 mL of anhydrous ethanol each time and then centrifuged.

[0050] The obtained solid was vacuum dried at 60°C for 12 hours to obtain modified nano-silica with olefinic unsaturated bonds and epoxy groups grafted on the surface, denoted as C-1, with a yield of about 3.8g.

[0051] 2. Grafting in the middle branching layer: The 3.8 g C-1 obtained in step 1 was dispersed in 60 mL of anhydrous toluene and ultrasonically dispersed for 15 minutes.

[0052] Add 2.0 g of 3-azidopropyltrimethoxysilane (B), and then add 0.05 mL of glacial acetic acid.

[0053] The reaction system was heated to 55°C and stirred for 4 hours.

[0054] After the reaction was complete, the mixture was cooled and centrifuged at 8000 rpm for 10 minutes, and the supernatant was discarded.

[0055] The precipitate was washed twice with anhydrous toluene, and then twice with anhydrous ethanol, 30 mL each time, and then centrifuged.

[0056] Vacuum drying at 50°C for 8 hours yielded azide-based silane-grafted nano-silica, denoted as CB-1, with a yield of approximately 4.5 g, a weight gain of approximately 0.7 g, and a grafting rate of approximately 18.4%.

[0057] 3. Coating of the outer layer: Add 4.5 g of CB-1 obtained in step 2 to 5.0 g of TMPTMA (A) and stir at room temperature to allow TMPTMA to fully impregnate the CB-1 powder.

[0058] Add 0.01 g of potassium persulfate (0.2% of the mass of TMPTMA) and stir until well mixed.

[0059] The mixture was transferred to a high-speed mixer and stirred at 500 rpm for 5 minutes at 70°C to ensure that TMPTMA was uniformly coated on the surface of CB-1 in a semi-prepolymerized state.

[0060] The product was removed and cooled to room temperature to obtain a core-shell hybrid crosslinking agent, denoted as CBA-1, with a yield of approximately 9.3 g.

[0061] Preparation Example 2 Replace TMPTMA with ethoxylated trimethylolpropane triacrylate (SR415), and use the same conditions as in Preparation Example 1.

[0062] Mass ratio: SR415: silane azido: nano SiO2 = 5.0: 2.0: 4.0.

[0063] The core-shell hybrid crosslinking agent was obtained and designated as CBA-2.

[0064] Preparation Example 3 Adjust the mass ratio of A:B:C to 1:0.3:0.5.

[0065] Specific dosage: Nano SiO2 (C) = 3.0 g, silane azido (B) = 1.8 g, TMPTMA (A) = 6.0 g.

[0066] The preparation steps are the same as in Preparation Example 1, but the amount of solvent is scaled up proportionally to obtain a core-shell hybrid crosslinking agent, denoted as CBA-3.

[0067] Preparation Example 4 Adjust the mass ratio of A:B:C to 1:0.6:1.0.

[0068] Specific dosage: Nano SiO2 (C) = 5.0 g, silane azido (B) = 3.0 g, TMPTMA (A) = 5.0 g.

[0069] The preparation steps are the same as in Preparation Example 1, but the amount of solvent is scaled up proportionally to obtain a core-shell hybrid crosslinking agent, denoted as CBA-4.

[0070] Example 1

[0071] 1. Melt blending and granulation Add 100 parts LLDPE, 4.0 parts core-shell hybrid crosslinking agent CBA-1, 0.3 parts antioxidant 1010, 0.2 parts antioxidant 168 and 0.2 parts calcium stearate to a high-speed mixer and mix at 800 rpm for 5 minutes at room temperature to ensure that the components are fully premixed.

[0072] The premixed material is added from the main feed port of the twin-screw extruder, which has a length-to-diameter ratio of 40:1 and a screw diameter of 25 mm.

[0073] The extruder temperature settings for each section are: Zone 1 150℃, Zone 2 160℃, Zone 3 165℃, Zone 4 165℃, and Die Head 160℃.

[0074] The screw speed is set to 250 rpm, and the residence time of the material in the extruder is 1.5 to 2 minutes.

[0075] The extruded strips are cooled in a water tank, dried by an air knife, and granulated by a pelletizer to obtain masterbatch with a pellet length of about 3 mm.

[0076] 2. Molding The masterbatch obtained in step 1 was dried in an 80°C forced-air oven for 2 hours.

[0077] A sheet with a thickness of 1 mm is produced by hot pressing at 170℃ and 10 MPa for 5 minutes using a flat vulcanizing machine, followed by cold pressing for 3 minutes to set the shape.

[0078] The sheet material was cut into standard dumbbell-shaped tensile and heat deformation test strips.

[0079] 3. Gradient irradiation crosslinking First irradiation: The sample was placed on the electron accelerator conveyor belt, irradiated at an energy of 2 MeV and a beam current of 10 mA under a nitrogen atmosphere, with an irradiation dose of 15 kGy. The irradiation temperature was controlled at 40°C using circulating cooling water. The absorbed dose was calibrated using an alanine dosimeter.

[0080] Heat treatment: Transfer the pre-irradiated sample to a forced-air drying oven and heat treat it at 120℃ for 40 minutes. After removing it, allow it to cool naturally to room temperature.

[0081] Second irradiation: The heat-treated sample was placed again on the electron accelerator conveyor belt and irradiated in an air atmosphere with an irradiation dose of 15 kGy. The irradiation temperature was controlled at 130℃ using infrared heating as an auxiliary temperature control. The total irradiation dose was 30 kGy, yielding a composite sensitizer-assisted irradiated crosslinked polyethylene composite material.

[0082] Example 2

[0083] The raw materials were 100 parts LDPE, 4.0 parts CBA-2, 0.3 parts antioxidant 1010, and 0.2 parts zinc stearate. The extrusion temperatures were adjusted as follows: Zone 1 145℃, Zone 2 155℃, Zone 3 160℃, Zone 4 160℃, and die head 155℃. The remaining preparation process was the same as in Example 1, resulting in a composite sensitizer-assisted irradiation crosslinked polyethylene composite material.

[0084] Example 3

[0085] The raw materials were 100 parts HDPE, 2.0 parts CBA-3, 0.2 parts antioxidant 1010 and 0.1 parts calcium stearate. The gradient irradiation conditions were adjusted to: first irradiation 10 kGy, temperature 30℃, heat treatment 100℃×30 minutes, second irradiation 10 kGy, temperature 120℃, total dose 20 kGy. The rest of the preparation process was the same as in Example 1, and a composite sensitizer-assisted irradiation crosslinked polyethylene composite material was obtained.

[0086] Example 4

[0087] The raw materials were 100 parts HDPE, 5.0 parts CBA-4, 0.5 parts antioxidant 1076, and 0.3 parts calcium stearate. The extrusion temperatures were adjusted as follows: Zone 1 160℃, Zone 2 170℃, Zone 3 175℃, Zone 4 175℃, and die head 170℃. Gradient irradiation conditions were as follows: first irradiation 20 kGy at 50℃, heat treatment at 130℃ for 60 minutes; second irradiation 20 kGy at 140℃, total dose 40 kGy. The remaining preparation process was the same as in Example 1, resulting in a composite sensitizer-assisted irradiated crosslinked polyethylene composite material.

[0088] Comparative Example 1 100 parts LLDPE, 3.5 parts TAIC, 0.3 parts antioxidant 1010, 0.2 parts antioxidant 168, and 0.2 parts calcium stearate were added to a high-speed mixer and mixed at 800 rpm for 5 minutes at room temperature to ensure thorough premixing of the components. The above components were then fed into a twin-screw extruder through the main feed inlet, melt-blended and granulated at 160°C. After molding, the mixture was subjected to a single irradiation at a dose of 60 kGy, room temperature, and air atmosphere to obtain a cross-linked polyethylene composite material.

[0089] Comparative Example 2 Granules were prepared under the same process and raw material conditions as Comparative Example 1, and then subjected to the same gradient irradiation crosslinking process as in Example 1 to obtain crosslinked polyethylene composite material.

[0090] Comparative Example 3 100 parts LLDPE, 1.0 part modified nano-silica with surface grafted with olefinic unsaturated bonds and epoxy groups, 0.4 parts 3-azidopropyltrimethoxysilane, 2.0 parts TMPTMA, 0.3 parts antioxidant 1010, 0.2 parts antioxidant 168, and 0.2 parts calcium stearate were added to a high-speed mixer and mixed at 800 rpm for 5 minutes at room temperature to ensure thorough premixing of the components. The above components were then fed into a twin-screw extruder through the main feed port and melt-blended and granulated at 160°C. After molding, obvious bubbles appeared during extrusion, and micropores were visible inside the granules. The gradient irradiation conditions after molding were the same as in Example 1.

[0091] Comparative Example 4 CB-1 was prepared using the same steps as in Preparation Example 1; By replacing CBA-1 with CB-1 and following the same preparation process as in Example 1, irradiated crosslinked polyethylene composite material was obtained.

[0092] Comparative Example 5 1. Kernel preparation: 4.0 g of nano-silica (C) that had been vacuum dried at 120°C for 2 hours was added to 80 mL of anhydrous toluene and ultrasonically dispersed for 30 minutes to form a uniform suspension.

[0093] 0.48 g KH570 and 0.62 g KH560 were added to the suspension in sequence, with a molar ratio of KH570 to KH560 of approximately 1:1.2. Then, 0.1 mL of glacial acetic acid was added as a catalyst.

[0094] The reaction system was heated to 80°C and stirred for 6 hours under nitrogen protection.

[0095] After the reaction was complete, the mixture was cooled to room temperature, centrifuged at 8000 rpm for 10 minutes, and the supernatant was discarded.

[0096] The precipitate was washed three times with 50 mL of anhydrous ethanol each time and then centrifuged.

[0097] The obtained solid was vacuum dried at 60°C for 12 hours to obtain modified nano-silica with olefinic unsaturated bonds and epoxy groups grafted on the surface, denoted as C-1.

[0098] 2. Add 3.8 g of C-1 obtained in step 1 to 5.0 g of TMPTMA (A) and stir at room temperature to allow TMPTMA to fully impregnate the C-1 powder.

[0099] Add 0.01 g of potassium persulfate (0.2% of the mass of TMPTMA) and stir until well mixed.

[0100] The mixture was transferred to a high-speed mixer and stirred at 500 rpm for 5 minutes at 70°C to ensure that TMPTMA was uniformly coated on the C-1 surface in a semi-prepolymerized state.

[0101] The product was removed and cooled to room temperature to obtain the C@A intermediate, denoted as CA-1.

[0102] 3. Replace CBA-1 in Example 1 with a physical mixture of CA-1 and azidosilane, and follow the same preparation process as in Example 1 to obtain an irradiated crosslinked polyethylene composite material.

[0103] Comparative Example 6 Same as steps 1 and 2 of Example 1, but step 3 is changed to: irradiate the sample once in an air atmosphere at room temperature (25°C) with a dose of 30 kGy, without heat treatment and secondary irradiation, to obtain irradiated cross-linked polyethylene composite material.

[0104] Performance testing: The retention rate of residual double bonds in the core-shell hybrid crosslinking agents prepared in Examples 1-4 above was determined by iodometric titration. The specific results are shown in Table 1.

[0105] The following performance tests were performed on the materials prepared in Examples 1-4 and Comparative Examples 1-6: The gel content of the materials prepared in Examples 1-5 and Comparative Examples 1-6 was tested using xylene extraction method. The specific results are shown in Table 2.

[0106] Xylene extraction method: The cross-linked polymer sample is immersed in boiling xylene solvent. The uncross-linked linear molecular chains are dissolved and extracted, while the three-dimensional network structure (i.e., gel) formed by cross-linking is insoluble in xylene and remains insoluble. By separating the insoluble matter and weighing it, the proportion of gel in the total mass can be calculated.

[0107] According to GB / T 1040.2-2006 "Determination of tensile properties of plastics - Part 2: Injection or compression molded specimens of molded and extruded plastics", the tensile strength and elongation at break of the materials prepared in Examples 1-5 and Comparative Examples 1-6 were tested, and the specific results are shown in Table 2.

[0108] According to GB / T 1634.2-2004 "Determination of heat distortion temperature of plastics - Part 2: Plastics, hard rubber and long fiber reinforced composites", the heat distortion temperature (0.46 MPa) of the materials prepared in Examples 1-5 and Comparative Examples 1-6 was tested, and the specific results are shown in Table 2.

[0109] According to GB / T 1410-2006 "Test Methods for Volume Resistivity and Surface Resistivity of Solid Insulating Materials", the volume resistivity of the materials prepared in Examples 1-5 and Comparative Examples 1-6 was tested, and the specific results are shown in Table 2.

[0110] According to GB / T 2951.21-2008 "Test Method for Thermal Elongation of Insulating Materials for Cables and Optical Fibers", the materials prepared in Examples 1-5 and Comparative Examples 1-6 were tested for thermal elongation at 200℃ (0.2 MPa × 15 min) using the xylene extraction method. The specific results are shown in Table 2.

[0111] Table 1

[0112] Table 2

[0113] As can be seen from the performance data in Table 2, the core-shell hybrid crosslinking agent / polyethylene composite materials prepared in Examples 1-4 of this invention have excellent gel content, high tensile strength, high elongation at break and high heat distortion temperature, and outstanding comprehensive mechanical properties and heat resistance.

[0114] A comparison of Comparative Examples 1 and 2 with Example 1 shows that using traditional TAIC as a sensitizer and subjecting the composite material to a single irradiation of 60 kGy or a three-stage gradient irradiation significantly reduces the gel content, tensile strength, and heat distortion temperature. This indicates that relying solely on the physical dispersion of traditional multifunctional monomers in a polyethylene matrix not only results in low crosslinking efficiency but also makes it difficult to construct a uniform and dense three-dimensional crosslinked network, leading to a significant impairment in the material's mechanical support and heat resistance.

[0115] The comparison between Comparative Example 3 and Example 1 shows that when physically blended free monomers are used to replace the core-shell hybrid crosslinking agent, the gel content and tensile strength of the composite material further deteriorate, and the elongation at break is reduced to the minimum. This indicates that without the pre-existing protection of the core-shell structure, the functional monomers are prone to self-polymerization or agglomeration during extrusion processing, failing to effectively connect the polyethylene molecular chains, resulting in severe breakage of the crosslinking network and extremely poor overall toughness of the material.

[0116] A comparison of Comparative Example 4 and Example 1 shows that when the core-shell structure is retained but the outer coating layer is removed, the gel content, tensile strength, and heat distortion temperature of the composite material are significantly lower than those of Example 1. The outer coating layer not only acts as a physical barrier to prevent premature reaction or damage to the core during extrusion, but also provides abundant active crosslinking sites during the irradiation crosslinking stage. The absence of this layer leads to insufficient crosslinking density and weak interfacial compatibility, becoming a performance bottleneck.

[0117] The comparison between Comparative Example 5 and Example 1 shows that when the complete core-shell structure is retained but the intermediate branching layer B is omitted, the tensile strength and heat distortion temperature of the composite material decrease. This indicates that the intermediate branching layer is a bridge to achieve efficient compatibility between the inorganic core and the organic shell. The lack of this layer leads to a decrease in interfacial bonding force and obstruction of stress transmission, thereby weakening the material's ability to resist high temperature and external force deformation.

[0118] The comparison between Comparative Example 6 and Example 1 shows that when the traditional one-step irradiation process is used instead of the three-stage gradient irradiation, although the gel content is acceptable, the tensile strength and heat distortion temperature still do not reach the level of gradient irradiation. Gradually increasing the temperature to release internal stress and promote uniform cross-linking reaction can effectively avoid the generation of internal defects in the material, thereby obtaining better overall performance.

[0119] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present application in any way. Although the present application has been disclosed above with reference to preferred embodiments, it is not intended to limit the present application. For those skilled in the art, it will be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A composite sensitizer-assisted irradiation crosslinked polyethylene composite material, characterized in that, By weight, it comprises: 100 parts of polyethylene matrix resin, 2.0 to 5.0 parts of core-shell hybrid crosslinking agent, 0.2 to 0.5 parts of antioxidant, and 0.1 to 0.3 parts of lubricant; The core-shell hybrid crosslinking agent includes a core, an intermediate branching layer grafted to the surface of the core via silicon-oxygen bonds, and an outer layer coated in a semi-prepolymer state on the outside of the intermediate branching layer. The outer coating layer and the intermediate branching layer are interwoven at the nanoscale and retain some unreacted olefinic unsaturated bonds. The core is nano-silica with olefinic unsaturated bonds and epoxy groups grafted onto its surface. The molar ratio of olefinic unsaturated bonds to epoxy groups on the surface of the nano-silica is 1:1 to 2. The intermediate grafting layer is an azide-silane coupling agent, and the outer coating layer is a multifunctional acrylate.

2. The composite sensitizer-assisted radiation crosslinked polyethylene composite material according to claim 1, characterized in that, The mass ratio of the multifunctional acrylate, the azide-based silane coupling agent, and the nano-silica is 1:0.3-0.6:0.5-1.

0.

3. The composite sensitizer-assisted radiation-crosslinked polyethylene composite material according to claim 1, characterized in that, The polyethylene matrix resin is selected from at least one of low-density polyethylene, linear low-density polyethylene, and high-density polyethylene.

4. The composite sensitizer-assisted radiation crosslinked polyethylene composite material according to claim 1, characterized in that, The multifunctional acrylate is selected from at least one of trimethylolpropane trimethacrylate and ethoxylated trimethylolpropane triacrylate.

5. The composite sensitizer-assisted radiation crosslinked polyethylene composite material according to claim 1, characterized in that, Its gel content is ≥90%, tensile strength is ≥35 MPa, elongation at break is ≥400%, heat distortion temperature is ≥135℃, and volume resistivity is ≥1×10⁻⁶. 15 Ω·cm.

6. A method for preparing a core-shell hybrid crosslinking agent according to any one of claims 1-5, characterized in that, Includes the following steps: (a) Disperse nano-silica in an organic solvent, add a silane coupling agent containing olefinic unsaturated bonds and a silane coupling agent containing epoxy groups to react and obtain a core with olefinic unsaturated bonds and epoxy groups grafted on the surface. (b) The kernel obtained in step (a) is dispersed in an organic solvent, an azide-silane coupling agent is added, and a grafting reaction is carried out at a temperature of 40-60°C to obtain an intermediate with azide-silane grafted on its surface. (c) The intermediate obtained in step (b) is mixed with a multifunctional acrylate, a low-temperature thermal initiator is added, and a semi-prepolymer coating reaction is carried out at 60-80°C, so that the multifunctional acrylate is coated on the surface of the intermediate in a semi-prepolymer state, and a core-shell hybrid crosslinking agent is obtained.

7. The preparation method according to claim 6, characterized in that, The silane coupling agent containing olefinic unsaturated bonds mentioned in step (a) is γ-methacryloyloxypropyltrimethoxysilane, and the silane coupling agent containing epoxy groups is γ-glycidoxypropyltrimethoxysilane. The molar ratio of the two is 1:1 to 2, the reaction temperature is 70 to 90°C, and the reaction time is 4 to 8 hours.

8. The preparation method according to claim 7, characterized in that, In step (c), the mass ratio of the intermediate to the polyfunctional acrylate is 1:2 to 3, the low-temperature thermal initiator is potassium persulfate, and its addition amount is 0.1% to 0.3% of the mass of the polyfunctional acrylate. The mixing time is 3 to 8 minutes.

9. A method for preparing a composite sensitizer-assisted radiation-crosslinked polyethylene composite material according to any one of claims 1-5, characterized in that, Includes the following steps: ① After uniformly mixing polyethylene matrix resin, core-shell hybrid crosslinking agent as described in any one of claims 6-8, antioxidant and lubricant, add it from the main feed port of a twin-screw extruder, melt blend and extrude granulate at 150-170°C and 200-300 rpm to obtain masterbatch; The residence time of the material in the extruder is 1 to 3 minutes; ② The masterbatch obtained in step ① is molded into a finished product; ③ The product obtained in step ② is subjected to gradient irradiation crosslinking treatment to obtain the finished product. The gradient irradiation crosslinking treatment includes a first irradiation, heat treatment and a second irradiation performed in sequence.

10. The preparation method according to claim 9, characterized in that, The first irradiation in step ③ is carried out in a nitrogen atmosphere, with an irradiation dose of 10–20 kGy and an irradiation temperature of 30–50°C; the heat treatment temperature is 100–130°C and the time is 30–60 minutes; the second irradiation is carried out in an air atmosphere, with an irradiation dose of 10–20 kGy and an irradiation temperature of 120–140°C; the total irradiation dose is 20–50 kGy.

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