A heat-aging-resistant polypropylene cable insulation material, a preparation method and application thereof

Abstract

This invention discloses a heat-resistant polypropylene cable insulation material, its preparation method, and its application, belonging to the technical field of cable insulation materials. The insulation material is made by melt blending long-chain branched polypropylene (LCBPP) with an elastomer, wherein the mass ratio of LCBPP to elastomer is 70~90:10~30. Long-chain branching modification enhances the chain entanglement between molecular chains, inhibiting phase separation between polypropylene and elastomer during heat aging. Simultaneously, the dense spherulitic structure of LCBPP hinders oxygen intrusion, reducing the oxidation rate. After heat aging at 135℃ for 1440 hours, the carbonyl index increases by only 134.2%, the average POE size increases by 29.0%, the breakdown field strength decreases by ≤11.3%, the tensile strength decreases by ≤29.1%, and the elongation at break remains above 550%, meeting cable application standards. This solves the technical problem of poor heat aging performance of traditional polypropylene blend insulation materials and can be widely used in cable insulation under high-temperature environments.

Description

Technical Field

[0001] This invention relates to the field of cable insulation materials technology, specifically to a heat-resistant aging-resistant polypropylene cable insulation material based on long-chain branched modification, its preparation method, and its application. Background Technology

[0002] Driven by the construction of new power systems, environmentally friendly and high-performance cable insulation materials have become a core demand in the industry. Traditional cross-linked polyethylene (XLPE) insulated cables are difficult to adapt to the requirements of green development due to their complex processes, non-recyclability, and high carbon emissions. Polypropylene (PP), on the other hand, has become the core material to replace XLPE due to its excellent electrical, mechanical, and processing properties. It is widely used in the field of cable insulation materials, and its long-term allowable operating temperature can reach 100~120℃, making it the mainstream environmentally friendly insulation material.

[0003] However, polypropylene (PP) lacks sufficient low-temperature toughness. The industry commonly employs melt blending with elastomers (such as polyolefin elastomer POE) to form a "sea-island" structure (PP as the continuous phase and POE as the dispersed phase) to improve toughness. However, in high-temperature service environments above 90°C, this system faces severe thermal aging problems: high temperatures intensify PP molecular chain movement, leading to the destruction of the spherulite structure. This intensified molecular chain movement causes elastomer aggregation, decreased crystallinity, and subsequent aggregation of the POE dispersed phase. The "sea-island" structure becomes unstable, resulting in phase separation, which in turn causes a significant decrease in the electrical and mechanical properties of the insulating material, limiting its long-term use in high-temperature environments.

[0004] In existing technologies, although adding additives such as antioxidants and ultraviolet absorbers can delay aging to some extent, the long-term effect is limited and may affect the electrical properties of the material; while simply adjusting the blending ratio is difficult to change the intermolecular forces and phase interface characteristics, and cannot fundamentally inhibit phase separation.

[0005] Therefore, how to provide a polypropylene cable insulation material and its preparation method that can improve thermal aging stability at the molecular structure level has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing polypropylene blend insulation materials, such as severe phase separation and significant performance degradation during thermal aging, the present invention aims to provide a heat-resistant polypropylene cable insulation material, its preparation method, and its application. By enhancing molecular chain entanglement through long-chain branching modification, and combining density-matched quantitative formulation design with a multi-stage mixing uniformity improvement process, phase separation and oxidative aging are suppressed, effectively improving the material's heat aging resistance.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: According to a first aspect of the present invention, a heat-resistant polypropylene cable insulation material is provided. The insulation material is made by melt blending long-chain branched polypropylene (LCBPP) and polyolefin elastomer (POE). The target density of the insulation material is 0.890~0.905 g / cm³, and the mass ratio of LCBPP to POE is determined by a density quantitative formula, and the mass ratio falls within the range of 70~90:10~30; the density quantitative formula is:

[0008]

[0009] in, For the target density of the insulating material, For LCBPP density, For POE density, , The mass fractions of LCBPP and POE are respectively.

[0010] Furthermore, the long-chain branched polypropylene (LCBPP) has 3 to 8 branches per molecule, a branch length of C8 to C20, and a density of 0.900 to 0.910 g / cm³.

[0011] Furthermore, the polyolefin elastomer POE has a density of 0.860~0.900 g / cm³ and a flow rate of 1~10 g / 10 min at the melt blending temperature.

[0012] Furthermore, the density difference between POE and LCBPP is ≤0.05 g / cm³.

[0013] According to a second aspect of the present invention, a method for preparing a heat-resistant polypropylene cable insulation material according to the first aspect of the present invention is provided. The method includes the following steps: LCBPP and POE were vacuum dried at 60~70℃ for 2~4h until the moisture content was ≤0.05wt%, and LCBPP was crushed into fine particles of 0.5~1mm. Weigh long-chain branched polypropylene (LCBPP) and POE according to the mass ratio determined by the density quantitative formula, add them to a high-speed mixer, mix them at 30-40°C and 500-600 r / min for 3-5 min, then raise the temperature to 50-80°C and shear and mix them at 800-1000 r / min for 5-15 min, and obtain the premix under nitrogen protection. The premixed material is added to a twin-screw extruder for melt blending, and vacuum devolatilization is carried out at a screw speed of 100~300 r / min and a pressure of -0.08~-0.1 MPa. The material is then extruded and granulated to obtain insulating material particles of 2~3 mm. The insulating material particles were statically homogenized and aged at 40-50℃ under a nitrogen atmosphere for 12-24 hours, and then cooled to room temperature. Cooled insulating material granules are added to a cable extruder, extruded onto the conductor surface, and then cooled and shaped to obtain the cable insulation layer.

[0014] Furthermore, the melt blending includes a melting stage, a shearing stage, and a homogenization stage; The temperature of the melting stage is controlled at 140~190℃, and the temperature control time accounts for 40%~60% of the total extrusion time. The temperature during the shearing stage is controlled at 180~190℃, and the temperature control time accounts for 20%~30% of the total extrusion time. The temperature during the homogenization stage is controlled at 170~180℃, and the temperature control time accounts for 20%~30% of the total extrusion time.

[0015] Furthermore, the twin-screw extruder employs a multi-stage shearing element combination, specifically: a left-hand threaded element is used in the melting stage, a meshing block shearing element is used in the shearing stage, and a right-hand threaded element is used in the homogenization stage.

[0016] Furthermore, the cable extruder adopts a three-stage temperature control, with the temperature of each stage controlled sequentially at 170~180℃, 180~200℃, and 170~190℃, respectively; the cooling rate of the cooling molding is 5~10℃ / min.

[0017] According to a third aspect of the present invention, an application of the heat-resistant polypropylene cable insulation material according to the first aspect of the present invention is provided in the field of cable insulation. The heat-resistant polypropylene cable insulation material is suitable for environments with operating temperatures of -40 to 90°C. Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Using long-chain branched polypropylene (LCBPP) as the matrix material, the LCBPP and elastomer are blended in an optimal ratio determined by a density-quantitative formula. The branched structure of the LCBPP molecular chain enhances chain entanglement, effectively suppressing phase separation during thermal aging. Data shows that after aging at 135℃ for 1440 hours, the average size increase of POE is only 29.0%, far lower than the 66.7% of traditional PP / POE materials. 2. The dense spherulitic structure of LCBPP can hinder oxygen intrusion and reduce the oxidation rate. After aging, the carbonyl index increases by only 134.2%, which is significantly better than the 259.6% of traditional materials. 3. The material exhibits excellent electrical and mechanical property stability. After aging, the breakdown field strength decreases by ≤11.3%, the tensile strength decreases by ≤29.1%, and the elongation at break remains above 550%, meeting the requirements of T / CPCIF 0230-2022 standard. 4. By limiting the density difference between POE and LCBPP to ≤0.05 g / cm³, this invention achieves synergistic optimization of compatibility, processability, thermal aging phase stability, and dielectric-mechanical properties. 5. The present invention has carried out a multi-stage refined design of the preparation method. Through steps such as raw material pretreatment, pre-shear mixing, melt blending and multi-stage shearing, granulation and homogenization aging, the mixing uniformity is significantly improved while being compatible with existing equipment. This reduces the POE dispersion particle size from 1~2μm to 0.5~1μm, and further improves the heat aging resistance of the material. Attached Figure Description

[0018] Figure 1 This is a comparison chart of the average size change of POE during the thermal aging process between the insulating material of this invention and traditional PP / POE materials; Figure 2 This is a comparison chart of the average and maximum dimensions of the insulating material of this invention and traditional PP / POE materials during thermal aging. Figure 3 This is a comparison of the infrared spectra and carbonyl index of the insulating material of this invention and traditional PP / POE materials during thermal aging. Figure 4 This is a comparison chart of the melting temperature (Tm) and crystallinity (Xc) of the insulating material of the present invention and the traditional PP / POE material during the thermal aging process; Figure 5 This is a comparison chart of the volume conductivity of the insulating material of this invention and traditional PP / POE materials during thermal aging. Figure 6 This is a comparison chart of the power frequency breakdown characteristics of the insulating material of the present invention and the traditional PP / POE material under different temperature environments during the thermal aging process. Figure 7 This is a comparison chart of the mechanical tensile strength of the insulating material of the present invention and the traditional PP / POE material during the thermal aging process as a function of aging time. Figure 8 This is a comparison chart showing the elongation at break of the insulating material of this invention and the traditional PP / POE material as a function of aging time during thermal aging. Detailed Implementation

[0019] 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 with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the protection scope of this invention.

[0020] According to a first aspect of the present invention, a heat-resistant polypropylene cable insulation material is provided. The insulation material is made by melt blending long-chain branched polypropylene (LCBPP) and polyolefin elastomer (POE). The target density of the insulation material is 0.890~0.905 g / cm³, and the mass ratio of LCBPP to POE is determined by a density quantitative formula, and the mass ratio falls within the range of 70~90:10~30; the density quantitative formula is:

[0021]

[0022] in, For the target density of the insulating material, For LCBPP density, For POE density, , The mass fractions of LCBPP and POE are respectively.

[0023] The density quantification formula above is derived based on the volume summation principle of polyolefin blends (without significant volume shrinkage). Specifically, assuming the total volume after blending equals the sum of the volumes of each component, then:

[0024] in, , and These represent the total volume after blending, and the volumes of LCBPP and POE before blending, respectively.

[0025] use , Let LCBPP and POE represent the mass fractions respectively. Then:

[0026] By defining density as V=m / ρ, and substituting this into the volume summation formula above:

[0027] The resulting quantitative formula for density is:

[0028]

[0029] In this way, by enhancing the chain entanglement between molecular chains through long-chain branching modification, the phase separation of polypropylene and elastomer during thermal aging is suppressed. Simultaneously, the dense spherulitic structure of LCBPP hinders oxygen intrusion, reducing the oxidation rate. Furthermore, by following the aforementioned density-quantitative formula and controlling the mass ratio of long-chain branched polypropylene (LCBPP) to elastomer at 70-90:10-30, the system achieves an optimal balance between mechanical properties, thermal aging resistance, and processing performance. Specifically, if the elastomer content is too low, the toughening effect is insufficient, failing to effectively suppress phase separation during thermal aging; if the elastomer content is too high, it leads to decreased material strength, accelerated elastomer migration, deterioration of thermal aging performance, and is detrimental to the extrusion and overmolding of cable insulation layers. At this specific ratio, the elastomer is uniformly dispersed in the LCBPP matrix, forming interfacial entanglement and stable anchoring with the long branches of LCBPP, resulting in a stable phase structure, no phase separation, and high performance retention during thermal aging, making it particularly suitable for heat-resistant cable insulation materials.

[0030] Preferably, the long-chain branched polypropylene (LCBPP) has 3-8 branches per molecule, a branch length of C8-C20, and a density of 0.900-0.910 g / cm³. Here, C8-C20 indicates that the number of carbon atoms in the branches is between 8 and 20. By setting an appropriate number of branches, the system can form a moderate physical entanglement and three-dimensional network structure. By setting an appropriate branch length, a strong and stable interfacial bond can be formed between LCBPP and the elastomer. By setting an appropriate density, the density of LCBPP can be well matched with that of POE (0.86-0.90 g / cm³), significantly reducing interfacial tension and improving compatibility. This effectively inhibits phase separation, phase region coarsening, and interfacial debonding during thermal aging, allowing the material to maintain a high retention rate of mechanical and dielectric properties under long-term high-temperature conditions, making it particularly suitable for heat-resistant cable insulation materials.

[0031] Preferably, the density of the polyolefin elastomer is 0.860~0.900 g / cm³, and the flow rate at 190°C is 1~10 g / 10 min. This preferred elastomer density ensures excellent compatibility with long-chain branched polypropylene (LCBPP), while also providing good toughness and phase stability. It avoids compatibility degradation due to excessively high density or thermal aging migration caused by excessively low density. Specifically, POE molecular chains with a density below 0.86 g / cm³ are too soft and prone to migration and aggregation at high temperatures; while POE chains with a density of 0.86~0.90 g / cm³ have moderate rigidity, and their chain entanglement with LCBPP effectively limits their high-temperature movement. Simultaneously, the preferred flow rate range ensures viscosity matching and uniform dispersion of the elastomer and LCBPP during melt blending, guaranteeing processability and extrusion molding properties. It also ensures phase structure stability during thermal aging, preventing phase coarsening and phase separation, ultimately achieving optimal synergistic performance in mechanical properties, electrical properties, and heat aging resistance.

[0032] Preferably, the density difference between POE and LCBPP is ≤0.05 g / cm³. Such a density difference can significantly reduce the interfacial tension of the polyolefin blend system, allowing POE to be uniformly dispersed in the LCBPP matrix at the nanoscale particle size, forming a stable "sea-island" structure.

[0033] According to a second aspect of the present invention, a method for preparing a heat-resistant polypropylene cable insulation material according to the first aspect of the present invention is provided. The method includes the following steps: Step 1: Vacuum dry LCBPP and POE at 60~70℃ for 2~4h until the moisture content is ≤0.05wt%, and then crush LCBPP into fine particles of 0.5~1mm. This step removes moisture, prevents bubbles from forming during melt blending, and increases the contact area between LCBPP and POE, improving dispersibility during the premixing stage.

[0034] Step 2: Weigh the long-chain branched polypropylene (LCBPP) and POE according to the mass ratio determined by the density quantitative formula, add them to a high-speed mixer, mix them at 30-40°C and 500-600 r / min for 3-5 minutes, then raise the temperature to 50-80°C and shear and mix them at 800-1000 r / min for 5-15 minutes, and obtain the premix under nitrogen protection.

[0035] In this step, the raw materials are initially dispersed by low-speed premixing at 500~600 r / min, and the POE agglomerates are broken up by high-speed shearing at 800~1000 r / min. At the same time, nitrogen protection can prevent the oxidation of the raw materials.

[0036] Step 3: Add the premixed material to a twin-screw extruder for melt blending, and perform vacuum devolatilization at a screw speed of 100~300 r / min and a pressure of -0.08~-0.1 MPa. Extrusion granulation yields 2~3 mm insulating material particles.

[0037] In this step, melt blending includes a melting stage, a shearing stage, and a homogenization stage.

[0038] The temperature during the melting stage is controlled at 140~190℃, and the temperature control time accounts for 40%~60% of the total extrusion time. The temperature during the shearing stage is controlled at 180~190℃, and the temperature control time accounts for 20%~30% of the total extrusion time. The temperature during the homogenization stage is controlled at 170~180℃, and the temperature control time accounts for 20%~30% of the total extrusion time.

[0039] This setup allows the material to be gently heated and fully melted during the melting stage, achieve nanoscale dispersion of POE during the shearing stage, and achieve better homogenization during the homogenization stage, stabilizing the melt and releasing stress. This fundamentally solves the problems of uneven POE dispersion and easy instability of phase structure in traditional blending processes.

[0040] More preferably, the melting stage employs a three-stage temperature control, with the temperatures of each stage controlled sequentially at 140~150℃, 160~170℃, and 170~190℃, respectively. The duration of each stage is controlled sequentially at 20%~30%, 30%~40%, and 40%~50% of the total melting stage duration, respectively. Specifically, the 140~150℃ stage corresponds to the feeding stage, as the material's primary task upon entering the extruder is rapid preheating. Setting the temperature to 140~150℃ avoids excessive screw torque or hindered material feeding due to excessively low temperatures. The 160~170℃ and 170~190℃ stages correspond to the compression stage and the formal melting stage, respectively. These two stages bear the main responsibility for the material's transformation from semi-melted to fully melted, requiring sufficient heat transfer and time to ensure uniform melt temperature and adequate relaxation of molecular chains, preventing localized overload or uneven dispersion during subsequent shearing due to insufficient melting.

[0041] More preferably, the twin-screw extruder employs a multi-stage shear element combination, specifically: In the melting stage, a left-hand threaded element is used. The left-hand threaded element is opposite to the main screw. It has a weak conveying capacity, which can increase the material residence time, ensure full melting, and avoid premature strong shearing that leads to degradation.

[0042] In the shearing stage, an interlocking block shearing element is used, which consists of multiple disks with staggered angles. This element exerts strong extrusion, stretching, and diversion effects on the melt, thus providing high shear force to break the POE dispersed phase to nanoscale particle size and enhance interfacial entanglement.

[0043] In the homogenization stage, a right-hand threaded element is used. This element is a standard feed thread with a deep groove, resulting in strong conveying capacity and weak shearing effect. Low-shear conveying homogenizes the melt temperature and composition, stabilizes the extrusion pressure, and prevents secondary degradation.

[0044] Step 4: Static homogenize and age the insulating material particles at 40~50℃ in a nitrogen atmosphere for 12~24 hours, then cool to room temperature.

[0045] This step releases the melt stress inside the particles and allows the molecular chains of LCBPP and POE to further entangle under mild conditions, improving the uniformity of the particles.

[0046] Step 5: Add the cooled insulating material granules to the cable extruder, extrude them onto the conductor surface, and cool them to form the cable insulation layer.

[0047] In this step, the cable extruder employs a three-stage temperature control system, with the temperatures of each stage controlled sequentially at 170~180℃, 180~200℃, and 170~190℃. Specifically, 170~180℃ corresponds to the feeding stage of the cable extruder; this lower temperature prevents material adhesion. 180~200℃ corresponds to the melting stage of the cable extruder; this higher temperature ensures sufficient plasticization and good flowability. 170~190℃ corresponds to the die-head stage of the cable extruder; compared to the high temperature of the melting stage, the appropriate cooling in the die-head stage prevents overheating and degradation of the melt, while also ensuring dimensional stability after extrusion.

[0048] Furthermore, the preferred cooling strategy for cooling and molding is a gradient cooling approach of 80℃ → 50℃ → room temperature, with a preferred cooling rate of 5~10℃ / min. This gradient cooling ensures uniform crystallization of the insulation layer, forming a dense spherulite structure, further hindering oxygen intrusion and improving heat aging resistance. Specifically, the crystallization behavior of polypropylene is highly sensitive to the cooling rate. Rapid cooling leads to incomplete crystallization, resulting in small spherulites with many defects; slow cooling, on the other hand, promotes the formation of a dense, complete spherulite structure, thereby improving the material's barrier properties (impeding oxygen intrusion) and heat aging resistance. The first stage at 80℃ is higher than the glass transition temperature of polypropylene (approximately 0℃) but lower than the lower limit of its crystallization temperature range (approximately 100~120℃), allowing the melt to form stable crystal nuclei in the early stages of cooling; the second stage at 50℃ enters the crystal growth range, giving the molecular chains sufficient time to arrange themselves regularly and form dense spherulites; upon reaching room temperature, the insulation material is completely cooled and solidified, releasing residual internal stress.

[0049] According to a third aspect of the present invention, an application of the heat-resistant polypropylene cable insulation material according to the first aspect of the present invention is provided in the field of cable insulation. The heat-resistant polypropylene cable insulation material is suitable for environments with operating temperatures of -40 to 90°C.

[0050] The different aspects of the present invention will be described in detail below through more specific embodiments.

[0051] Example 1 This embodiment provides a heat-resistant aging-resistant polypropylene cable insulation material and its preparation method. The raw material composition of the insulation material is as follows: based on the density quantitative formula according to the present invention, a target density is selected. ~=0.898 g / cm³, density of LCBPP =0.905 g / cm³, POE density =0.88 g / cm³, the calculated LCBPP content is 80.2 parts by mass and POE content is 19.8 parts by mass. Among them, LCBPP has 4 branches per molecular chain, the branch length is C12 (i.e., the number of carbon atoms in the branch chain is 12); the melt flow rate at the melt blending temperature is 5 g / 10 min.

[0052] In this embodiment, the preparation method of the above-mentioned polypropylene cable insulation material includes the following steps: (1) Raw material pretreatment: LCBPP and POE were vacuum dried at 65℃ for 3h until the moisture content was ≤0.05wt%, and LCBPP was crushed into fine particles of 0.5~1mm; (2) Premixing and preshearing: Weigh the raw materials according to the above mass ratio (i.e., LCBPP is 80.2 parts by mass and POE is 19.8 parts by mass), add them to a high-speed mixer, mix at low speed of 35℃ and 550r / min for 4min, then heat to 65℃ and shear at 900r / min for 10min, and obtain the premix under nitrogen protection; (3) Melt blending and multi-stage shearing granulation: The premixed material is added to a twin-screw extruder and a combination of five-stage temperature control (145℃→165℃→180℃→185℃→175℃) and multi-stage shearing elements is used (left-handed screw elements are used in the melting section, meshing block shearing elements are used in the shearing section, and right-handed screw elements are used in the homogenization section). The screw speed is 200r / min, and the vacuum devolatilization is performed at -0.09MPa. The extrusion granulation yields 2~3mm insulating material particles. (4) Homogenization and aging after granulation: The granules were statically homogenized and aged at 45°C under a nitrogen atmosphere for 18 hours, and then cooled to room temperature; (5) Cable insulation layer forming: The particles are added to the cable extruder and extruded onto the conductor surface using three-stage temperature control (175℃→190℃→180℃). The material is then cooled and formed by gradient cooling at 80℃→50℃→room temperature at a cooling rate of 8℃ / min to obtain the cable insulation layer.

[0053] Performance testing: After the polypropylene cable insulation material prepared by the above method was heat-aged at 135℃ for 1440h, the carbonyl index increased by 132.5%, the average size of POE increased by 28.3%, the breakdown field strength decreased by 10.8% at 30℃, the tensile strength was 22.8MPa, and the elongation at break was 556.3%.

[0054] Example 2 This embodiment provides a heat-resistant aging-resistant polypropylene cable insulation material and its preparation method. The raw material composition of the insulation material is as follows: based on the density quantitative formula according to the present invention, a target density is selected. =0.891g / cm³, density of LCBPP =0.905 g / cm³, POE density =0.87 g / cm³, the calculated LCBPP content is 75.3 parts by mass and POE content is 24.7 parts by mass. Specifically, LCBPP has 6 branches per molecular chain, a branch length of C16 (i.e., 16 carbon atoms in the branches), and a melt flow rate of 3 g / 10 min at the melt blending temperature.

[0055] In this embodiment, the preparation method of the above-mentioned polypropylene cable insulation material includes the following steps: (1) Raw material pretreatment: LCBPP particles and POE particles were vacuum dried at 60℃ for 4h until the moisture content was ≤0.05wt%, and LCBPP was crushed into fine particles of 0.5~1mm; (2) Premixing and preshearing: Weigh the raw materials according to the above mass ratio, add them to a high-speed mixer, mix at low speed of 500 r / min at 30℃ for 5 min, then heat to 70℃ and shear at 850 r / min for 12 min, and obtain the premix under nitrogen protection. (3) Melt blending and multi-stage shearing granulation: The premixed material is added to a twin-screw extruder and a combination of five-stage temperature control (140℃→160℃→175℃→180℃→170℃) and multi-stage shearing elements is used (left-handed screw elements are used in the melting section, meshing block shearing elements are used in the shearing section, and right-handed screw elements are used in the homogenization section). The screw speed is 180r / min, and vacuum devolatilization is performed at -0.1MPa. The extrusion granulation yields 2~3mm insulating material particles. (4) Homogenization and aging after granulation: The granules are statically homogenized and aged at 40°C under a nitrogen atmosphere for 24 hours, and then cooled to room temperature; (5) Cable insulation layer forming: The particles are added to the cable extruder and extruded onto the conductor surface using three-stage temperature control (170℃→185℃→175℃). The material is then cooled and formed by gradient cooling at 80℃→50℃→room temperature at a cooling rate of 6℃ / min to obtain the cable insulation layer.

[0056] Performance testing: After the polypropylene cable insulation material prepared by the above method was heat-aged at 135℃ for 1440h, the carbonyl index increased by 135.1%, the average size of POE increased by 29.7%, the breakdown field strength decreased by 11.1% at 30℃, the tensile strength was 22.5MPa, and the elongation at break was 548.7%.

[0057] Comparative Example This comparative example relates to a conventional melt-blended insulating material of polypropylene (PP) and elastomer. The raw material composition of this insulating material is: 80 parts by weight of linear polypropylene (PP) and 20 parts by weight of POE elastomer. The density and melt flow rate of this POE elastomer are consistent with those used in Example 1.

[0058] Meanwhile, in this comparative example, the preparation method of the insulating material is essentially the same as that in Example 1, but the optimization steps of the present invention, such as pretreatment, multi-stage shearing, and homogenization aging, are not used. The method includes the following steps: (1) Add the above raw materials to a mixer and mix at 60°C for 10 min to obtain a premix; (2) Add the premixed material to a twin-screw extruder, with an extrusion temperature of 175~185℃ and a screw speed of 200r / min. After melt blending, extrusion and granulation, insulating material particles are obtained. (3) The particles are extruded onto the conductor surface through a cable extruder at an extrusion temperature of 185°C to form an insulation layer.

[0059] Performance testing: After thermal aging at 135℃ for 1440h, the carbonyl index of the insulating material in this comparative example increased by 258.9%, the average size of POE increased by 66.2%, the breakdown field strength at 30℃ decreased by 18.0%, the tensile strength was 16.4MPa, and the elongation at break was 374.8%.

[0060] Comparative analysis of the performance of the insulating materials in Example 1 and the comparative example: The microstructure distribution of the sample is as follows Figure 1 and Figure 2As shown, in the early stages of aging, the elastomer phase of the sample in Example 1 of this invention was uniformly dispersed. After thermal aging at 135℃ for 336 h, the average size of POE in the comparative sample significantly increased to 1.49 μm (an increase of 18.3%), exhibiting a clear trend of phase aggregation. In contrast, the average size of POE in the sample of Example 1 of this invention under the same conditions was only 1.38 μm, with an increase of only 5.3%. Experimental data show that when the aging time was extended to 720 h, the material of this invention only began to show a slight increase in size, demonstrating significantly better anti-aging fluctuations than existing technologies.

[0061] After undergoing extreme thermal aging for 1440 hours, the comparative sample showed a large number of obvious large-sized pores (phase separation products) inside, with the average POE size increasing by as much as 66.7%, severely damaging the insulation integrity of the material. In contrast, the sample of this invention, under the same aging time, had extremely small internal pore size and maintained a complete "sea-island" microstructure, with the average POE size increasing by only 29.0%.

[0062] Figure 3 Functional group characteristics and crystallinity of the insulating materials from Example 1 and the comparative example before and after aging were analyzed. The wavenumber was 2962 cm⁻¹. -1 2872 cm -1 1375 cm -1 The peak at 2926 cm⁻¹ is generated by the stretching vibration of the methyl group (-CH₃), with a wavenumber of 2926 cm⁻¹. -1 2853cm -1 1460 cm -1 The peak at 1720 cm⁻¹ is generated by the stretching vibration of the methylene group (-CH₂-). After aging, the peak at 1720 cm⁻¹... -1 A significant increase in the carbonyl peak was observed in the vicinity. Furthermore, the carbonyl index of the comparative PP / POE insulation material before aging was slightly lower than that of the LCBPP / POE insulation material in Example 1; after aging for 168 h, the carbonyl index of the comparative PP / POE insulation material showed a relatively significant increase, while the carbonyl index of the LCBPP / POE in Example 1 changed very little. When the aging time reached 336 h, the carbonyl index of the LCBPP / POE insulation material in Example 1 began to show a significant increase; after aging for 1440 h, the carbonyl index of the comparative PP / POE insulation material increased from 0.089 to 0.320, an increase of 259.6%, while the carbonyl index of the LCBPP / POE insulation material in Example 1 increased from 0.111 to 0.260, an increase of 134.2%, both less than that of the PP / POE insulation material.

[0063] To investigate the thermodynamic causes of the degradation of the macroscopic mechanical and electrical properties of insulating materials under long-term thermal aging, differential scanning calorimetry (DSC) was used to analyze the melting characteristics and melting temperature of insulating material samples before and after aging. T m ) and crystallinity ( X c %) analysis, results as follows Figure 4 As shown. Before aging, the melting temperatures of the insulating materials in Examples 1 and 2 of this invention were not significantly different from those in the comparative example. However, the initial crystallinity of the material of this invention was slightly lower than that of the comparative example. As thermal aging progressed, in the early stage of aging (168 h), high temperature caused secondary rearrangement of molecular chains in the amorphous region, and the crystallinity of both samples increased due to recrystallization. However, when aging reached 336 h, the crystallinity of the PP / POE sample of the comparative example began to show a significant inflection point, indicating that its linear molecular chains had begun to undergo severe breakage and degradation under high temperature and oxidation. At this time, the crystallinity of the sample of this invention still maintained an upward trend, indicating that the barrier constructed by the long-chain branched network effectively delayed the breakage of molecular chains, and the recrystallization phenomenon in the system still dominated. After undergoing extreme long-term thermal aging for 1440 h, the crystallinity of the PP / POE sample of the comparative example dropped sharply to 36.23%, and the crystal skeleton was severely damaged. In contrast, the crystallinity of the sample of this invention remained at a relatively high level of 39.11%, with an overall decrease of very little.

[0064] Furthermore, the volumetric conductivity of the samples from Example 1 and the comparative example at different temperatures (30, 50, 70, 90 °C) and different aging times was tested using the three-electrode method at an electric field strength of 10 kV / mm. The results are as follows: Figure 5 As shown in the figure. Experimental results show that the volumetric conductivity of all samples increases with increasing test temperature. However, in the high-temperature range, the conductivity increase of the insulation sample in the example is significantly slower than that of the PP / POE insulation material in the comparative example. This excellent temperature rise stability stems from the strong chain entanglement network formed between LCBPP and POE in this invention. This network effectively maintains the integrity of the microstructure at high temperatures and suppresses abnormal carrier migration. From the perspective of aging time, in the early stage of aging (within 168 h), driven by recrystallization, the spherulite structure tends to be complete and has a restrictive effect on carriers, resulting in a temporary decrease in volumetric conductivity. After aging for 168 h, the comparative example shows a surge in insulation defects due to severe molecular chain degradation, leading to a significant rebound in conductivity. In contrast, the material of this invention, with the protection of its long-chain branched structure, effectively reduces the generation of insulation defects during thermal oxidation, thereby maintaining a lower conductivity level during long-term service and ensuring the safe operation of cable insulation under high temperature and high pressure.

[0065] Furthermore, the power frequency breakdown characteristics of samples at different aging stages under conditions of 30 ℃ and 90 ℃ were evaluated in depth, and the 63.2% characteristic breakdown field strength results are as follows: Figure 6 As shown in the figure. Experimental data shows that under a 30 ℃ test environment, the characteristic breakdown field strength of the LCBPP / POE sample in Example 1 of this invention without aging reached 143.5 kV / mm, which is better than the 139.9 kV / mm of the PP / POE in the comparative example. With the extension of aging time, the comparative example showed a significant performance decline at 336 h, and the field strength dropped to 114.5 kV / mm at 1440 h, a drop of 18.2%; while the field strength of the sample in Example 1 of this invention under the same conditions was 127.3 kV / mm, a drop of only 11.3%. Under the more challenging 90 ℃ high-temperature test environment, the advantage is further expanded: after aging for 1440 h, the field strength of the comparative example decreased sharply by 22.7% to 90.4 kV / mm, while the field strength of the sample in Example 1 of this invention was still as high as 110.2 kV / mm, a drop of only 10.6%, showing excellent electrical reliability.

[0066] Furthermore, mechanical tensile properties were tested on the samples from Example 1 and the comparative example before and after aging. The evolution of tensile strength and elongation at break is shown in the following figures. Figure 7 and Figure 8 As shown. In the unaged state, the tensile strength (31.96 MPa) and elongation at break (792.2%) of the LCBPP / POE insulation material sample in Example 1 of this invention are both superior to those of the PP / POE insulation material sample in the comparative example. The most critical difference lies in the performance retention rate after extreme aging (1440 h): the tensile strength and elongation at break of the PP / POE insulation material sample in the comparative example decreased significantly by 47.0% and 52.3%, respectively, with the elongation at break remaining at only 375.1%, which was deemed unqualified. In contrast, after undergoing the same extreme aging, the tensile strength of the LCBPP / POE insulation material sample in Example 1 of this invention decreased by only 29.1%, and the elongation at break remained at a high level of 550.1% (a decrease of only 30.6%), with all indicators exceeding the cable application industry standards.

[0067] 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 it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A heat-resistant and aging-resistant polypropylene cable insulation material, characterized in that, The insulating material is made by melt blending long-chain branched polypropylene (LCBPP) and polyolefin elastomer (POE). The target density of the insulating material is 0.890~0.905 g / cm³. The mass ratio of LCBPP to POE is determined by a density quantitative formula, and the mass ratio falls within the range of 70~90:10~30. The density quantitative formula is: in, For the target density of the insulating material, For LCBPP density, For POE density, , The mass fractions of LCBPP and POE are respectively.

2. The heat-resistant polypropylene cable insulation material according to claim 1, characterized in that, The long-chain branched polypropylene (LCBPP) has 3 to 8 branches per molecule, a branch length of C8 to C20, and a density of 0.900 to 0.910 g / cm³.

3. The heat-resistant polypropylene cable insulation material according to claim 1, characterized in that, The polyolefin elastomer POE has a density of 0.860~0.900 g / cm³ and a flow rate of 1~10 g / 10 min at the melt blending temperature.

4. The heat-resistant polypropylene cable insulation material according to claim 3, characterized in that, The density difference between POE and LCBPP is ≤0.05 g / cm³.

5. A method for preparing a heat-resistant polypropylene cable insulation material according to any one of claims 1 to 4, characterized in that, The method includes the following steps: LCBPP and POE were vacuum dried at 60-70℃ for 2-4 hours until the moisture content was ≤0.05wt%, and LCBPP was pulverized into fine particles of 0.5-1mm. Weigh long-chain branched polypropylene (LCBPP) and POE according to the mass ratio determined by the density quantitative formula, add them to a high-speed mixer, mix them at 30-40°C and 500-600 r / min for 3-5 min, then raise the temperature to 50-80°C and shear and mix them at 800-1000 r / min for 5-15 min, and obtain the premix under nitrogen protection. The premixed material is added to a twin-screw extruder for melt blending, and vacuum devolatilization is carried out at a screw speed of 100~300 r / min and a pressure of -0.08~-0.1 MPa. The material is then extruded and granulated to obtain insulating material particles of 2~3 mm. The insulating material particles were statically homogenized and aged at 40-50℃ under a nitrogen atmosphere for 12-24 hours, and then cooled to room temperature. Cooled insulating material granules are added to a cable extruder, extruded onto the conductor surface, and then cooled and shaped to obtain the cable insulation layer.

6. The method for preparing the heat-resistant polypropylene cable insulation material according to claim 5, characterized in that, The melt blending includes a melting stage, a shearing stage, and a homogenization stage; The temperature of the melting stage is controlled at 140~190℃, and the temperature control time accounts for 40%~60% of the total extrusion time. The temperature during the shearing stage is controlled at 180~190℃, and the temperature control time accounts for 20%~30% of the total extrusion time. The temperature during the homogenization stage is controlled at 170~180℃, and the temperature control time accounts for 20%~30% of the total extrusion time.

7. The method for preparing the heat-resistant polypropylene cable insulation material according to claim 6, characterized in that, The melting stage employs a three-stage temperature control system, with the temperatures of each stage controlled sequentially at 140~150℃, 160~170℃, and 170~190℃, respectively. The duration of each stage is controlled sequentially at 20%~30%, 30%~40%, and 40%~50% of the total melting stage duration, respectively.

8. The method for preparing the heat-resistant polypropylene cable insulation material according to claim 6, characterized in that, The twin-screw extruder employs a multi-stage shearing element combination, specifically: a left-hand threaded element is used in the melting stage, a meshing block shearing element is used in the shearing stage, and a right-hand threaded element is used in the homogenization stage.

9. The method for preparing the heat-resistant polypropylene cable insulation material according to claim 5, characterized in that, The cable extruder adopts a three-stage temperature control, with the temperature of each stage controlled sequentially at 170~180℃, 180~200℃, and 170~190℃ respectively; the cooling rate of the cooling molding is 5~10℃ / min.

10. The application of the heat-resistant polypropylene cable insulation material according to any one of claims 1 to 4 in the field of cable insulation, characterized in that, The heat-resistant and aging-resistant polypropylene cable insulation material is suitable for operating environments with a working temperature of -40~90℃.

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