A gradient crosslinking core material and a preparation method thereof

Abstract

The application provides a gradient crosslinking nuclear material and a preparation method thereof, and relates to the technical field of materials.The material is an insulating material prepared from a binary ethylene-propylene rubber base material and an inorganic flame retardant; the material has a structure with a gradient decrease in crosslinking density from an outer surface to an inner surface along a radial direction, wherein an outer layer region with a thickness ratio of 20%-30% from the outer surface to the inner surface has a first crosslinking rate, the remaining inner layer region has a second crosslinking rate, and the first crosslinking rate is greater than the second crosslinking rate.The method pre-compensates the service life of the outer layer aging through a gradient crosslinking network constructed by controlled variable-energy irradiation.The structure makes the failure times of the inner and outer layers of the cable tend to be dynamically synchronized, can effectively inhibit the interface phase separation and micro-crack growth induced by radiation, and improves the comprehensive fireproof performance and the anti-thermal creep ability.

Description

Technical Field

[0001] This invention relates to the field of materials technology, and in particular to a gradient crosslinking core material and its preparation method. Background Technology

[0002] Nuclear cables, serving as the nerves and blood vessels for control signal transmission and power delivery within the nuclear island, must withstand the dual challenges of high-energy radiation and sustained high temperatures throughout their designed service life of 40 to 60 years. Currently, nuclear cables generally employ uniform irradiation or chemical cross-linking processes to ensure that the initial physical properties of the insulation layer are completely consistent from the inside out. However, when faced with uneven aging caused by the DLO effect, these uniformly cross-linked cables often exhibit the following: the inner insulation remains intact, while the outer insulation has developed through-cracks due to excessive oxidation and embrittlement. Once the outer layer cracks, the entire cable is rendered unusable, significantly reducing its designed lifespan.

[0003] Traditional uniform crosslinking technology cannot effectively constrain flame retardant debonding under long-term radiation at the microscopic level, nor can it combat non-uniform aging and cracking caused by the DLO effect at the macroscopic level. The industry urgently needs a new insulation layer structure and preparation technology to fundamentally reshape the internal stress and network distribution of insulation materials and break the technical shackles that have long restricted the lifespan of nuclear cables.

[0004] Therefore, this invention is proposed. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a gradient crosslinked nuclear material and its preparation method. This material overcomes the bottlenecks of microscopic interface failure and macroscopic non-uniform aging faced by existing nuclear cable insulation layers during long-term service, effectively extending the service life of the cable.

[0006] In order to achieve the objective of this invention, the following technical solution is adopted: This invention provides a gradient crosslinking core material, comprising: the material having a single-layer structure, and being made of an insulating material composed of a binary ethylene propylene rubber substrate and an inorganic flame retardant; The material has a crosslinking density gradient decreasing radially from the outer surface to the inner surface, wherein the outer layer region, which accounts for 20%-30% of the thickness from the outer surface to the inner surface, has a first crosslinking rate, and the remaining inner layer region has a second crosslinking rate, and the first crosslinking rate is greater than the second crosslinking rate.

[0007] Furthermore, the mass ratio of the binary ethylene propylene rubber substrate to the inorganic flame retardant is (60-40):(40-60).

[0008] Furthermore, the mass ratio of the binary ethylene propylene rubber substrate to the inorganic flame retardant is 60:40.

[0009] Furthermore, in the outer layer region, the crosslinking rate is ≥85%; in the inner layer region, the crosslinking rate is 55%~65%.

[0010] Furthermore, the particle size of the inorganic flame retardant is 500 nm to 1 μm.

[0011] Furthermore, the thickness of the material is 1.5mm to 4.0mm.

[0012] The present invention also provides a gradient cross-linked core cable, comprising: a conductor and the gradient cross-linked core material; The gradient cross-linked core material is coated on the outer layer of the conductor.

[0013] The present invention also provides a method for preparing a gradient crosslinked core material, comprising the following steps: blending ethylene propylene diene monomer (EPDM) rubber, an inorganic flame retardant, an antioxidant, and a sensitizer.

[0014] Furthermore, the sensitizer is triallyl isocyanurate.

[0015] This invention also provides a method for preparing a gradient cross-linked nuclear cable, comprising the following steps: S1. The material prepared by any of the above steps is coated onto the outside of the conductor and extruded. S2. Perform the first stage of electron beam overall penetration irradiation; S3. Conduct the second stage of controlled variable energy surface enhancement irradiation.

[0016] Furthermore, in S2, the energy of the electron beam is 1.5 MeV to 3.0 MeV, and the absorbed radiation dose is 50 kGy to 80 kGy; in S3, the energy of the electron beam is 0.3 MeV to 0.8 MeV, and the absorbed radiation dose is 80 kGy to 150 kGy.

[0017] The present invention has the following technical effects: (1) Compared with existing uniformly cross-linked nuclear cables, this method uses a gradient cross-linking network constructed by controlled variable energy irradiation to pre-compensate for the aging of the outer layer. This structure makes the failure time of the inner and outer layers of the cable tend to be dynamically synchronized. After irradiation and thermo-oxidative coupling aging during a simulated 60-year service life of a nuclear power plant, the overall elongation at break retention rate of the insulation layer is expected to be more than 30% higher than that of traditional uniformly cross-linked cables.

[0018] (2) Under long-term high-dose radiation, existing halogen-free low-smoke cables are prone to failure due to the breakage of rubber molecular chains, which can easily lead to the debonding of flame-retardant particles and the formation of micropores. This method utilizes an ultra-high-density cross-linked network locally constructed in the outer insulation layer to achieve a microscopic fixation effect on the nano flame-retardant particles. This effectively inhibits radiation-induced interfacial phase separation and microcrack growth, and improves the overall fire resistance and thermal creep resistance.

[0019] (3) This method does not require the use of a multi-layer co-extrusion process that is costly to manufacture and prone to interlayer delamination, nor does it require the introduction of complex chemical coupling modifiers into the matrix. By simply controlling the irradiation energy parameters of the electron accelerator, an ideal gradient physical structure can be constructed in a single-layer homogeneous formulation in one step. This method is fully compatible with existing cable extrusion and irradiation production lines, is easy to operate, and is extremely suitable for large-scale industrial mass production. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 Theoretical model of radial gradient distribution of crosslinking density in insulating layer; Figure 2 Schematic diagram of physical anchoring at the microscopic interface. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0023] In a first aspect, the present invention provides a gradient crosslinking core material, comprising: the material having a single-layer structure, and being made of an insulating material composed of a binary ethylene propylene rubber substrate and an inorganic flame retardant; The material has a crosslinking density gradient decreasing radially from the outer surface to the inner surface, wherein the outer layer region, which accounts for 20%-30% of the thickness from the outer surface to the inner surface, has a first crosslinking rate, and the remaining inner layer region has a second crosslinking rate, and the first crosslinking rate is greater than the second crosslinking rate.

[0024] In some embodiments, the mass ratio of the ethylene propylene diene monomer (EPDM) rubber substrate to the inorganic flame retardant is (60-40):(40-60).

[0025] In some embodiments, the mass ratio of the ethylene propylene diene monomer (EPDM) rubber substrate to the inorganic flame retardant is 60:40.

[0026] In some embodiments, the crosslinking rate is ≥85% in the outer layer region and 55%~65% in the inner layer region.

[0027] In some embodiments, the particle size of the inorganic flame retardant is 500 nm to 1 μm.

[0028] In some embodiments, the thickness of the material is 1.5 mm to 4.0 mm.

[0029] In a second aspect, the present invention also provides a gradient cross-linked core cable, comprising: a conductor and the gradient cross-linked core material; The gradient cross-linked core material is coated on the outer layer of the conductor.

[0030] Thirdly, the present invention also provides a method for preparing a gradient crosslinked core material, comprising the following steps: blending ethylene propylene diene monomer (EPDM) rubber, an inorganic flame retardant, an antioxidant, and a sensitizer.

[0031] In some embodiments, the sensitizer is triallyl isocyanurate.

[0032] Fourthly, the present invention also provides a method for preparing a gradient cross-linked nuclear cable, comprising the following steps: S1. The material prepared by any of the above steps is coated onto the outside of the conductor and extruded. S2. Perform the first stage of electron beam overall penetration irradiation; S3. Conduct the second stage of controlled variable energy surface enhancement irradiation.

[0033] In some embodiments, in S2, the energy of the electron beam is 1.5 MeV to 3.0 MeV, and the absorbed radiation dose is 50 kGy to 80 kGy; in S3, the energy of the electron beam is 0.3 MeV to 0.8 MeV, and the absorbed radiation dose is 80 kGy to 150 kGy.

[0034] This new preparation method resolves the contradiction between the macroscopic "diffusion-limited oxygen (DLO) effect" and "uniform manufacturing process," overcoming the problem of insufficient cable life. Existing nuclear cables of this type all employ a fully radially uniform cross-linking manufacturing process, resulting in completely consistent initial mechanical and physical properties between the inner and outer layers. However, in actual nuclear power environments, due to the DLO effect, the outer layer of the cable rapidly becomes brittle due to severe oxidation, while the inner layer ages slowly. Current technology cannot prevent premature cracking of the outer insulation, leading to a waste of the excellent residual properties of the inner layer, and the actual lifespan of the entire cable is far lower than theoretically expected. This method, by constructing a gradient distribution of the microscopic physical structure, "pre-compensates" for this inevitable uneven aging, enabling the inner and outer layers of the cable to reach the failure threshold simultaneously after long-term service, thereby maximizing the extraction of material potential and significantly extending the overall safe lifespan of the cable.

[0035] On the other hand, this method solves the problem of interfacial debonding between a large amount of inorganic flame retardant and the rubber matrix caused by high-dose irradiation at the microscopic level. Existing halogen-free low-smoke cables, in order to meet fire protection requirements, must fill the ethylene propylene diene monomer (EPR) rubber with a high proportion of inorganic flame retardant. Under long-term high-dose nuclear radiation, the breakage of the rubber molecular chains leads to a sharp decrease in the binding force of the matrix on the flame retardant particles, causing large-scale debonding and peeling of the flame retardant. This not only damages the macroscopic mechanical strength of the insulation layer but also leaves micropores that allow moisture intrusion. The goal of this method is to construct a high-strength "three-dimensional anchoring network" on the outside of the insulation layer, which is most vulnerable to radiation attack, without relying on complex chemical coupling agent modification. This network is purely physical, forcibly locking the highly filled flame retardant particles within the cross-linked network, ensuring that the cable maintains excellent interfacial stability and mechanical integrity even after experiencing extremely high doses of irradiation.

[0036] Traditional irradiation crosslinking aims for radial uniformity, while this invention actively utilizes the energy depth decay characteristics of electron beams in high-energy physics to construct a physical gradient. According to the Fermi-Eyges scattering theory, the penetration depth of an electron beam in a polymer is approximately positively correlated with its accelerating voltage (MeV). This invention employs a dual-step irradiation mode of high-energy penetration + low-energy surface reinforcement. In this mode, after penetrating the surface of the insulating layer, the energy deposition of the low-energy electron beam rapidly reaches its peak and decays to zero. By controlling this energy threshold, an extremely high-density three-dimensional crosslinked network can be precisely induced on the outer side of the insulating layer without altering the chemical formulation, while the inner layer retains the basic crosslinked network constructed by the high-energy electron beam, thus forming a physical modulus gradient that is rigid on the outside and flexible on the inside.

[0037] In the context of Loss-of-Water Accidents (LOCA) and long-term radiation environments at nuclear power plants, the EPR matrix experiences chain breakage, leading to debonding of the highly packed inorganic flame retardant from the matrix and triggering macroscopic mechanical collapse. This invention utilizes an ultra-high-density cross-linked network in the outer layer, resembling a microscopic three-dimensional steel mesh, which provides a strong, forced physical anchoring force (lock-in effect) to the nano-flame retardant particles encased within. Simultaneously, this over-designed outer network acts as a radiation-resistant sacrificial layer. When external oxygen and radiation preferentially attack the outer insulating layer, the remaining dense mesh is sufficient to support the structure and prevent collapse, effectively offsetting the rapid aging of the outer layer caused by oxygen diffusion limitations, and achieving dynamic synchronization of the service life of the inner and outer sides of the insulation layer.

[0038] The following is a detailed explanation using specific embodiments: Example 1 Ethylene propylene diene monomer (EPR) rubber and an inorganic flame retardant (nano-sized magnesium hydroxide inorganic flame retardant particles) were added to a mixer at a mass ratio of 60:40. Simultaneously, appropriate amounts of antioxidant (a mixture of antioxidant 1010 and antioxidant 168 in a 1:1 mass ratio) and sensitizer were added and thoroughly mixed to prepare a homogeneous insulating compound. The antioxidant was added at a rate of 1 g per 100 g of rubber substrate, and the sensitizer at a rate of 2 g per 100 g of rubber substrate. To achieve optimal interfacial anchoring, nano-sized magnesium hydroxide with a particle size of 500 nm to 1 μm, treated with a silane coupling agent, was used as the inorganic flame retardant, and triallyl isocyanurate (TAIC) was used as the sensitizer.

[0039] The above-mentioned insulating compound is evenly coated onto the outside of the metal conductor using an extruder to form the initial state of the cable insulation core, and then cooled and shaped.

[0040] The first stage involves overall electron beam penetration irradiation. The extruded cable core is fed into the electron accelerator irradiation area. This stage uses a high-energy electron beam, with the specific energy value adjusted within the range of 1.5 MeV to 3.0 MeV (2.0 MeV in this embodiment) based on the insulation thickness to ensure the electron beam penetrates deeply enough to completely cover the entire insulation layer. The absorbed dose in this stage is controlled between 50 kGy and 80 kGy (60 kGy in this embodiment). The purpose of this step is to establish a basic and relatively uniform cross-linked network throughout the insulation layer, thereby preserving the flexibility of the cable's inner layer.

[0041] The second stage involves controlled variable-energy surface enhancement irradiation. The cable is then placed in the second irradiation area and additionally irradiated with a low-energy electron beam. The electron beam energy in this stage is reduced to 0.3 MeV~0.8 MeV (0.5 MeV in this embodiment), limiting its effective penetration depth to only 20%~30% of the outer thickness of the insulation layer. Simultaneously, the additional absorbed irradiation dose is increased to 80 kGy~150 kGy (100 kGy in this embodiment). During processing, the cable must be kept in uniform rotation and traction to ensure uniform irradiation of the outer surface. This step precisely utilizes the physical characteristics of deep energy attenuation of the low-energy electron beam to induce a high-density cross-linked network only on the surface of the insulation layer.

[0042] Theoretical model of radial gradient distribution of crosslinking density in insulating layer: like Figure 1The theoretical distribution of radial crosslinking density is shown in the figure. Based on the Fermi-Eyges electron beam energy deposition model, after undergoing double-step controlled irradiation, the crosslinking network of the insulating layer exhibits a significant heterogeneous evolution. The theoretical model shows that in the region within 30% of the thickness from the outer surface, due to the absorption of superimposed high and low energy electron beam radiation, the crosslinking rate will reach over 85%, constructing an extremely high-density three-dimensional crosslinking network; while in the inner layer region within 30% of the thickness from the conductor, due to only one penetration by the high-energy electron beam, the crosslinking rate remains within the design range of 55% to 65%. This model intuitively demonstrates that the present invention successfully constructs a physical structure that is rigid on the outside and flexible on the inside without changing the chemical formula.

[0043] like Figure 2 As shown in the microscopic model of traditional uniformly cross-linked cables, the cross-linking rate of the matrix is ​​insufficient to completely bind the highly filled flame-retardant particles. After high-dose irradiation, the stress release caused by the breakage of macromolecular chains easily leads to interfacial debonding between the flame-retardant particles and the matrix, resulting in micropore failure. However, the micromechanical model established in this invention shows that the ultra-high-density cross-linked network induced on the outer side of the insulation layer provides high-strength physical coating and three-dimensional spatial locking for the nano-flame-retardant particles. This mechanism model is expected to provide strong forced anchoring force, effectively suppressing interfacial phase separation induced by high radiation flux.

[0044] Experimental Example 1: Analysis of the Physical Structure of Gradient Crosslinking and the Mechanism of Micro-interface Anchoring according to Figure 2 It is known that under conventional uniform cross-linking conditions, despite the addition of sensitizers in the formulation, the cross-linking network density of the EPDM rubber matrix is ​​relatively limited. Although the nano-magnesium hydroxide flame retardant particles are uniformly dispersed in the matrix, the matrix's encapsulation of the particles is relatively weak. When faced with high-dose radiation or long-term aging-induced molecular chain breakage and pressure release, tiny physical gaps are easily generated between the matrix and the flame retardant particles, leading to debonding of the flame retardant particles and inducing interfacial micropore failure.

[0045] Conversely, in the outer region of the insulation layer after additional irradiation with a second-stage low-energy controlled variable-energy electron beam (0.3 MeV~0.8 MeV), the cross-linking rate reaches over 85% due to the superimposed absorption of high- and low-energy double-step electron beam energy. According to the theory of polymer radiation physics cross-linking, this region will construct an extremely dense and coherent ultra-high-density three-dimensional anchoring network. The insulation matrix will completely engulf the nano-flame retardant particles and lock them in a high-strength three-dimensional space. The tightly locked flame retardant particles fundamentally inhibit interfacial phase separation and micropore growth induced by high-dose radiation, ensuring the structural integrity of the cable insulation layer under extreme environments. This microscopic physical strengthening mechanism lays a solid foundation for subsequent significant improvements in macroscopic mechanical properties, anti-aging properties, and radiation resistance.

[0046] Experiment Example 2: Verification of Service Life In subsequent experimental examples, the initial compound preparation method used was to mix ethylene propylene diene monomer (EPDM) rubber with nano-magnesium hydroxide flame retardant in a mixing mill at 140℃~160℃ in a certain proportion, and then open mill, sheeting and cooling. The uniform crosslinking used as a control sample was prepared by using the same insulating compound formulation and extrusion molding process as the initial compound. However, in the irradiation crosslinking stage, the dual-energy irradiation of the present invention was not used. Instead, conventional processes were followed, and a single overall penetration irradiation was performed using a high-energy electron beam with an energy of 1.5 MeV to 3.0 MeV (specifically 2.0 MeV). The irradiation absorbed dose was controlled at 130 kGy to 230 kGy (specifically 200 kGy) so that the insulating layer formed a uniform crosslinking network with a basically consistent crosslinking rate from the outside to the inside.

[0047] The elongation at break of the gradient crosslinked material and the uniform crosslinked material were tested after a thermo-oxidative accelerated aging test at 135℃ for 1000 hours. The experimental results are shown in Table 1.

[0048] Table 1: Comparison of Elongation at Break of Insulation Materials Before and After Accelerated Aging As shown in Table 1, compared to the uniformly crosslinked material, the initial performance was excellent, but the elongation at break decreased significantly after accelerated aging, with a retention rate of only 17.1%, which is typical of early failure. Combined with the full-field strain contour analysis using digital image correlation (DIC) technology during the tensile process, it was found that the uniformly crosslinked material could not resist the diffusion-limited oxygen (DLO) effect, leading to severe oxidation and embrittlement of the outer insulating layer. Under tensile stress, the DIC contour map showed that stress concentration and microcrack initiation occurred very early on the outer surface. These penetrating cracks rapidly propagated into the inner layer, which still possessed some flexibility, leading to macroscopic brittle fracture of the sample. This proves that the traditional uniform crosslinking process cannot meet the 60-year long-life design requirement.

[0049] The initial elongation at break of the gradient crosslinked layer was 225%, slightly lower than that of the uniform crosslinked layer. This is because the introduction of the ultra-high density crosslinked network in the outer layer slightly increases the overall initial modulus, which is a reasonable structural concession. However, after undergoing the same high-strength aging process, its elongation at break remained at 148%, with a retention rate as high as 65.8%. This proves that the gradient crosslinking process of this invention can meet the long-life design requirement of 60 years.

[0050] The full-field strain monitoring data of DIC clearly reveals the fundamental reason for its long service life. The ultra-high density cross-linked network constructed in the outer 20%-30% region of the insulation layer in this invention acts as a highly efficient antioxidant sacrificial layer during aging. This dense network significantly reduces the rate of oxygen permeation into the inner layer. In tensile tests, DIC contour plots show excellent strain compatibility between the inner and outer layers: the high-modulus outer layer did not experience brittle cracking as seen in the comparative example, while the protected inner layer maintained excellent molecular chain slippage ability, bearing the majority of the large tensile deformation. This experimental data conclusively proves that the gradient cross-linked physical structure successfully achieves lifetime pre-compensation for the DLO effect, realizing dynamic synchronization of the service life of the inner and outer layers.

[0051] Experiment Example 3: Verification of Radiation Protection Performance To verify the interfacial stability and mechanical integrity of different crosslinking structures under extreme nuclear radiation environments, uncrosslinked rubber compounds, uniformly crosslinked compounds, and gradient crosslinked compounds were introduced for comparison. The samples were subjected to high-intensity irradiation with an absorbed dose of 500 kGy of gamma rays at room temperature. The tensile strength and elongation at break of each group of samples before and after irradiation were tested. Specific data are shown in Table 2. Table 2: Comparison of mechanical properties of insulating materials before and after 500 kGy high-dose irradiation. Data analysis and failure mechanism verification: The uncrosslinked samples, having not undergone crosslinking and curing, exhibited a loose, linear structure in their matrix molecular chains. Under high-energy radiation bombardment of 500 kGy, the EPR macromolecular chains underwent severe random breakage. Due to the lack of three-dimensional network support, the matrix completely lost its binding force on the highly filled inorganic flame retardant, leading to macroscopic pulverization or mud-like phenomena in the material. Its tensile strength plummeted to 2.1 MPa, and its elongation at break was only 35%, resulting in the complete loss of its mechanical and electrical protective functions.

[0052] Although uniformly cross-linked materials possess a basic three-dimensional network, the macromolecular chains still undergo irreversible breakage and decompression after exposure to extreme radiation of 500 kGy. Data shows that the tensile strength retention rate is only 52.4%. Combined with microscopic mechanism analysis, this is because the network of conventional density is insufficient to continue anchoring the highly packed nanoparticles when the molecular chains break, leading to severe interfacial phase separation and debonding of a large number of flame retardant particles from the matrix. These microscopic debonding points evolve into a massive number of internal micropores, triggering a rapid collapse of macroscopic mechanics.

[0053] The gradient cross-linked material exhibits the highest initial tensile strength before irradiation due to the presence of an ultra-high-density cross-linked network in the outer layer. After undergoing extreme radiation with the same dose, its tensile strength reaches 11.5 MPa, with a retention rate maintained at an excellent level of 80.9%. This directly proves the correctness of the micromechanical model of this invention. The ultra-high-density three-dimensional cross-linked network constructed in the 20%-30% thickness region of the outer layer acts as a robust physical steel mesh. Even when the matrix molecular chains undergo partial breakage due to radiation attack, this dense network can still provide a strong forced physical coating force for the nano-flame-retardant particles. This structure effectively inhibits radiation-induced interfacial phase separation and micropore growth, thus maintaining excellent mechanical integrity even under extremely high radiation flux.

[0054] Experiment Example 4: Fire resistance and resistance to thermal creep To verify the influence of high-strength physical locking at the micro-interface on the macroscopic thermal behavior of the material, limiting oxygen index (LOI) and high-temperature thermal creep (thermal extension) tests were conducted on uncrosslinked rubber compounds, uniformly crosslinked compounds, and gradient crosslinked compounds. The thermal extension test conditions were set as follows: temperature 200℃, load 0.2 MPa, and load time 15 min. Specific test data are shown in Table 3. Table 3 Comparison of fire resistance and thermal creep resistance of insulating materials Data analysis and reinforcement mechanism verification: Uncrosslinked matrix macromolecular chains are highly susceptible to disentanglement and viscous flow at high temperatures. In fire resistance tests, the heated and molten rubber matrix causes severe dripping of the flame retardant, resulting in a continuously expanding burning area and an LOI value of only 28.5%. Under the thermo-coupling effect at 200°C, the sample has no creep resistance and instantly undergoes melting and tensile failure.

[0055] The LOI value of the uniformly crosslinked group was 35.2%, which is within the normal range after conventional crosslinking. However, the LOI value of the gradient crosslinked group significantly increased to 39.8%. The core mechanism lies in the fact that the ultra-high density three-dimensional crosslinked network of the insulating outer layer forms a strong physical constraint on the nano-inorganic flame retardant. In the early stages of high-temperature combustion, this high-density network effectively inhibits the thermal motion of the rubber macromolecular chains and the escape of volatile combustible gases. Simultaneously, it forces the flame retardant particles to undergo an endothermic dehydration reaction in situ, rapidly sintering on the sample surface to form an extremely dense and hard inorganic carbonized isolation layer. This physical structure effectively isolates oxygen and heat from conduction to the interior, fundamentally eliminating dripping phenomena.

[0056] In the extreme thermal elongation test at 200℃, the uniformly cross-linked group exhibited a thermal elongation of 68.5%, which meets the standard, but the deformation is still relatively large. In contrast, the gradient cross-linked group showed a significantly reduced thermal elongation of 32.4%, with a permanent deformation rate of only 1.5%. This superior resistance to thermal creep is directly attributed to the physical modulus gradient constructed in this invention. The robust outer shell with extremely high cross-linking density on the outside of the insulation layer maintains extremely high energy storage modulus and rigidity at high temperatures, effectively limiting the macroscopic thermal expansion and creep slip behavior of the internal basic cross-linked network. This data strongly demonstrates that the gradient cross-linked structure not only fixes the flame retardant at the microscopic level but also endows the material with excellent high-temperature creep resistance in terms of macroscopic thermal dimensional stability.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of the present invention.

Claims

1. A gradient crosslinking core material, characterized in that, include: The material has a single-layer structure and is made of an insulating material composed of a binary ethylene propylene rubber substrate and an inorganic flame retardant. The material has a crosslinking density gradient decreasing radially from the outer surface to the inner surface, wherein the outer layer region with a thickness of 20%-30% from the outer surface to the inner surface has a first crosslinking rate, the remaining inner layer region has a second crosslinking rate, and the first crosslinking rate is greater than the second crosslinking rate. The mass ratio of the binary ethylene propylene rubber substrate to the inorganic flame retardant is (60-40):(40-60). In the outer layer region, the crosslinking rate is ≥85%; in the inner layer region, the crosslinking rate is 55%~65%.

2. The gradient crosslinking core material according to claim 1, characterized in that, The inorganic flame retardant has a particle size of 500 nm to 1 μm.

3. The gradient crosslinking core material according to claim 1, characterized in that, The thickness of the material is 1.5mm to 4.0mm.

4. A gradient cross-linked nuclear cable, characterized in that, include: Conductor and gradient crosslinked core material as described in any one of claims 1-3; The gradient cross-linked core material is coated on the outer layer of the conductor.

5. A method for preparing a gradient crosslinked core material as described in any one of claims 1-3, characterized in that, The process includes the following steps: blending ethylene propylene diene monomer (EPDM) rubber, inorganic flame retardant, antioxidant, and sensitizer.

6. The method for preparing the gradient crosslinked core material according to claim 5, characterized in that, The sensitizer is triallyl isocyanurate.

7. A method for preparing a gradient cross-linked nuclear cable as described in claim 4, characterized in that, Includes the following steps: S1. The material prepared according to claim 6 is coated on the outside of the conductor and extruded into shape; S2. Perform the first stage of electron beam overall penetration irradiation; S3. Conduct the second stage of controlled variable energy surface enhancement irradiation.

8. The method for preparing a gradient cross-linked nuclear cable according to claim 7, characterized in that, In step S2, the energy of the electron beam is 1.5 MeV to 3.0 MeV, and the absorbed radiation dose is 50 kGy to 80 kGy; in step S3, the energy of the electron beam is 0.3 MeV to 0.8 MeV, and the absorbed radiation dose is 80 kGy to 150 kGy.

Images