Crosslinked polyethylene and its preparation method and application

Abstract

The application relates to the field of polymer processing, and discloses a crosslinked polyethylene as well as a preparation method and application thereof. The preparation method of the crosslinked polyethylene comprises the following steps: S1, after polyethylene base material is subjected to first powder removal and metal impurity removal treatment, the polyethylene base material is subjected to first optical detection full inspection; S2, liquid antioxidant and preheated crosslinking agent are respectively or mixedly subjected to filtration treatment to obtain an additive mixture; S3, under vacuum conditions, the additive mixture obtained in the step S2 is sprayed on the polyethylene base material after the treatment in the step S1 to obtain an intermediate material; S4, the intermediate material is subjected to heat preservation and then cooling; after the crosslinked polyethylene after cooling is subjected to second powder removal treatment, the crosslinked polyethylene is subjected to second optical detection full inspection. The preparation method of the crosslinked polyethylene can realize 100% full inspection on polyethylene raw materials and crosslinked polyethylene finished material, and the spraying mode can effectively reduce the occurrence of pre-crosslinking and scorching caused by high-temperature processing.

Description

Technical Field

[0001] This invention relates to the field of polymer processing, specifically to a cross-linked polyethylene, its preparation method, and its application. Background Technology

[0002] High-voltage cables, as carriers of electricity, play a crucial role in modern energy systems and are widely used in the field of high-capacity, long-distance power transmission. In recent years, with the upgrading and transformation of urban power grids, the integration of renewable energy into the grid, and the rapid development of smart grid technology in my country, the demand for high-voltage cables has increased significantly. Due to the excellent electrical insulation, mechanical properties, and good heat resistance and aging resistance of cross-linked polyethylene (XLPE), extruded XLPE insulated cables have become the main form of high-voltage cables.

[0003] Cross-linked polyethylene (CXLPE) is a material produced by transforming low-density polyethylene from a linear structure to a three-dimensional network structure through chemical or physical methods, thereby improving its heat resistance, mechanical strength, and chemical stability. The main methods of cross-linking include peroxide cross-linking, radiation cross-linking, and silane cross-linking, with peroxide cross-linking being the primary form of cross-linking used in high-voltage insulation materials. Peroxide-crosslinked polyethylene insulation typically employs a two-step process: first, the intermediate material is prepared by melt extrusion, granulation, and drying of the polyethylene matrix and antioxidant; second, the pre-crosslinked material is prepared by blending, absorbing, and cooling the cross-linking agent.

[0004] To improve the processing stability and heat aging resistance of products and extend the service life of cables, a certain amount of antioxidant needs to be added during the production of cross-linked polyethylene. Currently, most antioxidants in the industry use high-melting-point antioxidants, which need to be uniformly dispersed in the base material through melt extrusion. The entire production process places high demands on the technical parameters of equipment such as screw assemblies, gear pumps, and screen filters. The temperature in the core area of ​​the screw is usually high and needs to be controlled above the melting point of the antioxidant. If parameters such as screw speed, temperature, and melt pressure are not set reasonably, it can easily cause damage to the molecular structure of the base material or the generation of chemical gels and physical impurities during processing. Simultaneously, the material comes into contact with water and air during granulation, drying, and conveying. The conductivity of water and impurity ions, as well as the cleanliness of the air, directly affect the electrical properties of the insulation material; in particular, the conveying pipeline can cause fine powder and stringing phenomena, reducing the cleanliness level of the material. High-voltage insulation materials have extremely high requirements for the control of the entire production process, especially ultra-clean control. However, the above technologies involve continuous production processes and long flows, posing problems such as high risk of introducing impurities and difficulty in achieving ultra-clean control throughout the entire process.

[0005] As cable operating voltage levels continue to rise, the formulation and large-scale production processes for high-voltage insulation materials require continuous iteration and optimization. With the development of small-molecule additive synthesis technology, liquid antioxidants with enhanced functionality suitable for cable insulation materials will have a promising market prospect. However, current research and application of liquid antioxidant dispersion processes are limited. Furthermore, in terms of impurity detection in high-voltage insulation materials, the impurity detection rate for polyethylene raw materials, finished insulation materials, and intermediate materials in China is low, typically below 5%. Because extrusion granulation capacity is large, at 2.0 t / h or even higher, and factors such as finished material packaging, space constraints, and cost must be considered, improving the impurity detection rate is challenging. Improving the impurity detection rate throughout the entire process from raw materials to finished products will help improve the cleanliness control level of insulation materials. Therefore, it is urgent to optimize the entire production process technology, shorten the process flow, and improve the level of ultra-clean control. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of long production process, high risk of impurity introduction, difficulty in achieving ultra-clean control throughout the process, and high production cost of cross-linked polyethylene in the existing technology. This invention provides a cross-linked polyethylene, its preparation method, and its application. The preparation method shortens the production process and reduces the risk of impurity introduction; at the same time, it achieves 100% full inspection of raw materials and finished products, improving the level of cleanliness control; it reduces equipment investment and lowers the production cost of insulation materials; and it reduces energy consumption and achieves green and low-carbon development.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for preparing cross-linked polyethylene, the method comprising:

[0008] S1. After the polyethylene base material undergoes a first dust removal and metal impurity removal treatment, a first optical inspection is used to fully inspect the polyethylene base material; wherein, after the first dust removal, the dust content on the surface of the polyethylene base material is ≤5mg / kg; the removal of metal impurities is achieved by magnetic separation and / or electrostatic adsorption; the magnetic separation includes strong magnetic separation and high-gradient magnetic separation; wherein, the spatial uniformity of the magnetic field in strong magnetic separation is ≥85%, and the spatial uniformity of the magnetic field in high-gradient magnetic separation is ≤65%;

[0009] S2. The liquid antioxidant and the preheated crosslinking agent are filtered separately or mixed to obtain the additive mixture;

[0010] S3. Under vacuum conditions, the additive mixture obtained in step S2 is sprayed onto the polyethylene base material treated in step S1 to obtain intermediate material.

[0011] S4. After the intermediate material is kept warm, it is cooled. After cooling, the cross-linked polyethylene undergoes a second dust removal treatment and is then subjected to a second optical inspection to fully inspect the cross-linked polyethylene. After the second dust removal, the dust content on the surface of the cross-linked polyethylene is ≤2mg / kg.

[0012] A second aspect of the present invention provides cross-linked polyethylene prepared using the method described in the first aspect.

[0013] A third aspect of the present invention provides the use of cross-linked polyethylene as an insulating material in cables, as described in the second aspect.

[0014] Through the above technical solution, the present invention can achieve at least the following beneficial effects:

[0015] (1) The cross-linked polyethylene preparation method of the present invention, through the interaction of each step and the control of special parameters, enables the method to achieve 100% full inspection of both polyethylene raw materials and cross-linked polyethylene finished products, thereby improving the cleanliness control level of the materials.

[0016] (2) The preparation method of cross-linked polyethylene of the present invention uses spraying to mix the additive mixture with the polyethylene base material to achieve uniform dispersion of the additive mixture (especially antioxidant), effectively avoid the influence of melt extrusion on the performance of the base material, and reduce the occurrence of pre-crosslinking and scorching caused by high temperature processing.

[0017] (3) In the preparation method of cross-linked polyethylene of the present invention, the base material and the additives adopt a multi-material compounding scheme, which fully integrates the performance advantages of various materials, making the comprehensive performance of the finished insulation material more excellent.

[0018] (4) The method for preparing cross-linked polyethylene of the present invention effectively reduces the risk of introducing impurities into the conveying medium, while shortening the production process, reducing equipment investment costs, significantly reducing the production cost of insulation materials, and improving economic benefits. Detailed Implementation

[0019] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0020] The first aspect of this invention provides a method for preparing cross-linked polyethylene, the method comprising:

[0021] S1. After the polyethylene base material undergoes a first dust removal and metal impurity removal treatment, a first optical inspection is used to fully inspect the polyethylene base material; wherein, after the first dust removal, the dust content on the surface of the polyethylene base material is ≤5mg / kg; the removal of metal impurities is achieved by magnetic separation and / or electrostatic adsorption; the magnetic separation includes strong magnetic separation and high-gradient magnetic separation; wherein, the spatial uniformity of the magnetic field in strong magnetic separation is ≥85%, and the spatial uniformity of the magnetic field in high-gradient magnetic separation is ≤65%;

[0022] S2. The liquid antioxidant and the preheated crosslinking agent are filtered separately or mixed to obtain the additive mixture;

[0023] S3. Under vacuum conditions, the additive mixture obtained in step S2 is sprayed onto the polyethylene base material treated in step S1 to obtain intermediate material.

[0024] S4. After the intermediate material is kept warm, it is cooled. After cooling, the cross-linked polyethylene undergoes a second dust removal treatment and is then subjected to a second optical inspection to fully inspect the cross-linked polyethylene. After the second dust removal, the dust content on the surface of the cross-linked polyethylene is ≤2mg / kg.

[0025] In this invention, the method for preparing cross-linked polyethylene allows for 100% inspection of both the polyethylene raw materials and the finished cross-linked polyethylene, effectively improving the cleanliness control of the materials. The use of a spray method to mix the additives with the polyethylene base material ensures uniform dispersion of the additives (especially antioxidants), effectively avoiding the impact of melt extrusion on the base material's properties and reducing pre-crosslinking and scorching phenomena caused by high-temperature processing. Simultaneously, both the base material and additives employ multi-material compounding schemes, fully integrating the performance advantages of various materials, resulting in superior overall performance of the finished insulation material. This method for preparing cross-linked polyethylene effectively reduces the risk of impurities introduced by the transport medium, shortens the production process, reduces equipment investment costs, significantly lowers the production cost of insulation materials, and improves economic efficiency.

[0026] In a preferred embodiment of the present invention, during the compounding process of the polyethylene base material, liquid antioxidant, and crosslinking agent, it is necessary to ensure that the number of particles with a diameter ≥0.5μm in the environment during the compounding process meets the corresponding cleanliness level requirements (≤300,000 particles / m³ for a Class 1000 environment), and that the environment is maintained at a positive pressure of 0.05-0.1Pa. Simultaneously, the temperature during compounding is controlled at 22-25℃, and the relative humidity at 40-50%. During the compounding process, sensors are installed in the clean environment to monitor the cleanliness, temperature, humidity, pressure difference, and operating parameters of the compounding equipment (such as vacuum level, metering pump flow rate, etc.) in real time, and the data is transmitted to the central control system. The system can automatically adjust the equipment operating status or issue alarms based on the monitoring data.

[0027] In this invention, preferably, the polyethylene base material may include: low-density polyethylene (LDPE) and optionally linear low-density polyethylene (LLDPE); the low-density polyethylene may have a molecular weight distribution of 4-7, a melt flow rate of 1.2-2.2 g / 10 min at 190°C and 2.16 kg, and a density of 0.918-0.922 g / cm³. 3 The linear low-density polyethylene has a molecular weight distribution of 3-5, a melt flow rate of 1-3 g / 10 min at 190°C and 2.16 kg, and a density of 0.915-0.93 g / cm³. 3 .

[0028] In this invention, preferably, based on the total mass of the polyethylene base material, the content of low-density polyethylene can be 85-100 wt%, and the content of linear low-density polyethylene can be 0-15 wt%; more preferably, based on the total mass of the polyethylene base material, the content of low-density polyethylene can be 90-99.9 wt%, and the content of linear low-density polyethylene can be 0.1-10 wt%.

[0029] In this invention, preferably, the content of the antioxidant is 0.1-0.5 parts by weight, more preferably 0.2-0.4 parts by weight, relative to 100 parts by weight of polyethylene base material; the content of the crosslinking agent is 0.5-2.5 parts by weight, more preferably 1.2-2 parts by weight.

[0030] It is understood that the strong magnetic separation described in this invention refers to the use of high-performance permanent magnet materials (such as neodymium iron boron) to construct a uniform magnetic field environment with high magnetic field strength. This allows for efficient coarse removal of magnetic impurities through the magnetization and adsorption effect of the magnetic field. Its core characteristics are high magnetic field strength, uniform magnetic field distribution, and moderate magnetic field gradient. The high-gradient magnetic separation described in this invention refers to laying a magnetic medium within a 1-1.5T base magnetic field. Utilizing the local magnetic field distortion effect generated by the magnetic medium in the base magnetic field, a non-uniform magnetic field environment with an ultra-high magnetic field gradient is constructed. Its core characteristics are a high magnetic field gradient and a locally non-uniform magnetic field distribution. The magnetic field strength for both strong and high-gradient magnetic separation is measured using a triaxial vector Hall-Gauss meter. The effective sensitive center size of the probe is Φ1.0mm, the probe material is non-magnetic non-ferrite encapsulation, the range is 0-3.0T, and the resolution is 0.0001T. Before testing, the zero-magnetic space is triaxially zeroed, and a standard permanent magnet is used for single-point calibration to eliminate zero-point drift and temperature drift.

[0031] The strong magnetic separation and high-gradient magnetic separation of this invention are not simply superimposed processes, but rather a complementary and parameter-matched combination: strong magnetic separation performs a "coarse removal" function to remove large-particle magnetic impurities and prevent them from clogging subsequent high-gradient media; high-gradient magnetic separation performs a "fine capture" function to remove ultrafine impurities that strong magnetic separation cannot cover. The combination of the two forms a comprehensive removal system for magnetic impurities of all particle sizes, and both are adaptable to large-scale continuous production, with no base material retention and no introduction of secondary impurities, ensuring the electrical insulation performance and cleanliness of the insulating material from the source.

[0032] In this invention, preferably, in step S4, the heat preservation method may include: first keeping the intermediate material at 60-75℃ for 3-6 hours, and then cooling it down to 55-70℃ and keeping it at 55-70℃ for 1-3 hours; more preferably, the heat preservation method may include: first keeping the intermediate material at 65-70℃ for 4-5 hours, and then cooling it down to 60-65℃ and keeping it at 65-65℃ for 1.5-2.5 hours.

[0033] In this invention, a stepped insulation method is employed, which addresses the shortcomings of traditional constant-temperature insulation—slow penetration, poor uniformity, and high energy consumption—through "segmented temperature control + precise time matching." This, combined with vacuum conditions and additive characteristics, ensures uniform additive dispersion, accelerates absorption and penetration, and shortens the time required. The invention solves the problem of balancing additive penetration speed and uniformity through a stepped temperature design: wetting in a high-temperature segment (60-75℃) and deep penetration in a mid-temperature segment (55-70℃). Simultaneously, by precisely controlling the insulation time at each temperature segment, it ensures additive penetration efficiency while avoiding pre-crosslinking of crosslinking agents and decomposition of antioxidants, thus resolving the issue of constant-temperature insulation's inability to simultaneously guarantee additive stability and penetration efficiency.

[0034] In this invention, preferably, in step S4, the cooling method may include composite cooling or stepped cooling.

[0035] In this invention, preferably, the composite cooling method may include: after the intermediate material has been kept at a constant temperature, it is cooled to 45-55°C in a fluidized bed and then vacuum cooled to 20-30°C.

[0036] In this invention, preferably, the inlet air temperature of the fluidized bed can be 40-50℃ and the air velocity can be 1-2m / s.

[0037] In this invention, preferably, the cooling rate of the vacuum cooling can be 1-3℃ / min, and the relative pressure of the vacuum cooling can be -0.1MPa to -0.08MPa.

[0038] In this invention, preferably, the stepped cooling method may include: using a stepped gravity tower for cooling, in the first stage cooling the intermediate material to 50-65°C, in the second stage cooling to 40-50°C, and then continuing to cool to 20-40°C by cold air.

[0039] In this invention, preferably, the temperature of the cold air can be 10-20℃.

[0040] In this invention, the use of stepped cooling can prevent internal stress caused by the difference in thermal expansion coefficients between different components of the insulating material during rapid temperature drop, which would otherwise lead to the precipitation of some weakly bonded components in the form of fine powder. At the same time, stepped cooling is a dynamic process, which can make the contact between the cooling medium and the material more uniform and prevent the generation of fine powder caused by localized sudden cooling. Finally, stepped cooling can shorten the cooling time, improve production efficiency, and reduce energy consumption. In a preferred embodiment of the invention, a three-stage stepped temperature design (first stage cooling to 50-65℃, second stage cooling to 40-50℃, and cold air cooling to 20-40℃) gradually reduces the base material temperature, eliminating microscopic internal stress caused by differences in thermal expansion coefficients. This solves the problem of reduced internal stress and mechanical properties of the base material caused by traditional rapid cooling. Simultaneously, the slow, stepped cooling allows additives on the base material surface to fully penetrate into the interior, preventing unabsorbed additives from detaching and forming fine powder due to sudden temperature changes. It also reduces microstructural damage to the base material particles, controlling fine powder generation at the source and solving a series of subsequent problems caused by incomplete powder removal during cooling in the industry. Furthermore, the stepped gravity tower structure allows base material particles to fall slowly within the tower, ensuring full contact with the cooling medium. Combined with the medium gradient design of first-stage cooling + second-stage cooling + cold air cooling, uniform cooling of the base material particles is achieved under high production capacity, solving the problem of poor cooling uniformity that easily occurs with single cooling methods under high production capacity.

[0041] In this invention, preferably, the first powder removal method can be a powder removal method commonly used in the art. For example, the first powder removal method may include: vibrating and sieving the polyethylene base material to remove coarse powder, and then performing ultrasonic purging; wherein, the vibration frequency of the high-precision vibrating sieve to remove coarse powder can be 15-20Hz, and the aperture of the sieve can be 100-150μm; the frequency of the ultrasonic purging can be 20-30kHz.

[0042] In this invention, preferably, the dust content on the surface of the base material after the first dust removal is 2-5 mg / kg; when the dust content on the surface of the polyethylene base material exceeds 5 mg / kg after the first dust removal is completed, the unqualified polyethylene base material is sent back to the first dust removal for further processing.

[0043] In this invention, preferably, the conditions for strong magnetic separation may include: the magnetic field strength may be 0.8-1.2T, more preferably 0.9-1.1T; the magnetic field gradient may be 50-80T / m, more preferably 60-70T / m; and when the material flow rate is 0.5-1.5m / s, the filtration accuracy may be 5-10μm, more preferably 6-8μm.

[0044] In this invention, preferably, the spatial uniformity of the magnetic field of the strong magnetic separation can be 85-95% (for example, it can be any two values ​​formed by 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, and values ​​within that range).

[0045] In this invention, preferably, the conditions for high gradient magnetic separation may include: the magnetic field strength can be 1-1.5T, more preferably 1.2-1.4T; the magnetic field gradient can be 60-90T / m, more preferably 70-80T / m; and when the material flow rate is 0.6-1.2m / s, the filtration accuracy can be 3-7μm, more preferably 4-6μm.

[0046] In this invention, preferably, the spatial uniformity of the magnetic field for high-gradient magnetic separation can be 40-65% (for example, it can be any two values ​​formed by 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 63%, 65%, or any value within that range).

[0047] In this invention, "filtration accuracy" refers to the minimum particle size that the magnetic separation equipment can retain for magnetic impurities of a specific size under set magnetic field parameters (magnetic field strength, magnetic field gradient) and material flow rate. It is essentially the minimum particle size threshold of magnetic impurities that the equipment can effectively remove, reflecting the magnetic separation system's ability to capture fine magnetic impurities. Simultaneously, through precise design of the flow rate ranges for strong magnetic separation (0.5-1.5 m / s) and high-gradient magnetic separation (0.6-1.2 m / s), a high degree of matching between the two-stage magnetic separation flow rates is achieved, ensuring seamless connection with subsequent process flow rates. No additional buffering devices are required, solving the problem of difficulty in coordinating the magnetic field gradient and fluid flow rate during magnetic separation.

[0048] In this invention, preferably, the magnetic medium for high-gradient magnetic separation can be at least one of stainless steel wool, ferrite, and carbonyl iron powder. The use of these magnetic media in high-gradient magnetic separation creates a "microscopic high-gradient magnetic field region," enabling precise adsorption of ultrafine impurities (e.g., 3-7 μm). Simultaneously, strong magnetic separation removes large-particle magnetic impurities first, preventing them from clogging the high-gradient magnetic medium and solving the problems of easy clogging and rapid degradation of impurity removal efficiency in traditional magnetic separation media. The selection of these magnetic media in this invention is based on a comprehensive assessment of the process requirements for ultra-clean impurity removal from high-voltage cross-linked polyethylene base materials, the physical characteristics of high-gradient magnetic field construction, and the magnetic properties, chemical stability, and process compatibility of the medium itself. It is not that other substances are completely unusable, but rather that these substances form an optimal combination in terms of magnetic field gradient construction, impurity removal efficiency, and process compatibility, making it difficult for other substances to simultaneously meet the stringent requirements of high-voltage insulation material preparation.

[0049] In a preferred embodiment of the present invention, the magnetic medium for high-gradient magnetic separation can be a blend of stainless steel fibers, ferrite, and carbonyl iron powder; wherein, based on the total mass of the magnetic medium, the proportion of stainless steel fibers is 60-80 wt%, the proportion of ferrite is 10-20 wt%, and the proportion of carbonyl iron powder is 5-15 wt%; when the above three magnetic media are blended and used in the above proportions, a three-dimensional composite magnetic medium system of fiber skeleton + particle filling + powder supplementation is formed, achieving a synergistic effect of 1+1+1>3, ultimately achieving a removal rate of over 99.9% for magnetic impurities of all particle sizes above 3μm, while maintaining smooth flow throughout the process, with no base material retention and no secondary impurity introduction, fully matching the ultra-clean impurity removal requirements of high-pressure cross-linked polyethylene base material.

[0050] In this invention, the combined use of strong magnetic separation and high-gradient magnetic separation forms a two-stage demagnetization system of "coarse removal of large particles and fine capture of fine impurities." Compared to a single magnetic separation process or a single magnetic medium, this system not only achieves complementary medium functions, covering multiple types of magnetic impurities, but also enables the superposition of magnetic field gradients, improving the capture efficiency of fine impurities. This provides support for ultra-clean control of the entire process of high-voltage insulating materials and is a key process step in achieving high-quality production of cross-linked polyethylene. Existing single strong magnetic separation cannot form a local high-gradient magnetic field, resulting in insufficient magnetic attraction for ultrafine magnetic impurities with low magnetic moments, causing them to easily escape with the base material fluid. This invention constructs a local ultra-high gradient magnetic field through high-gradient magnetic separation, effectively achieving precise adsorption and capture of ultrafine magnetic impurities.

[0051] In this invention, preferably, the conditions for electrostatic adsorption may include: an electric field strength of 3000-5000V / m, more preferably 3500-4500V / m; an electrode spacing of 50-200mm, more preferably 100-150mm; and a material flow rate of 0.2-1.8m / s, more preferably 0.8-1.2m / s.

[0052] In this invention, the conditions for magnetic separation and electrostatic adsorption are within the above-mentioned range, which not only achieves full coverage of both magnetic and non-magnetic impurities without any blind spots, but also achieves high matching of parameter ranges, ensuring that there is no mutual interference between the equipment: the flow rate parameters of strong magnetic separation (material flow rate 0.5-1.5m / s), high gradient magnetic separation (material flow rate 0.6-1.2m / s), and electrostatic adsorption (base material conveying speed 0.2-1.8m / s) are highly matched, all of which are suitable for large-scale production processing, and no additional buffer tanks are required between each process, enabling continuous production.

[0053] In this invention, preferably, the first optical detection method may include: using an industrial camera with a resolution of 5-8 megapixels and light sources of white light, ultraviolet light, and infrared light to photograph the polyethylene base material from an angle of 30-60°, and using an AI image recognition algorithm to identify impurities of different particle sizes and remove the impurities.

[0054] In this invention, preferably, the liquid antioxidant may be selected from at least one of 3,5-di-tert-butyl-4-hydroxyphenylpropionate isooctyl ester, bisphenol A glycerol dimethacrylate, 2,6-diallyl-p-cresol, 4,4'-sulfonylbis[2-(2-propenyl)]phenol and m-xylenediacrylamide.

[0055] In this invention, preferably, the crosslinking agent may be selected from at least one of dicumyl peroxide, tert-butyl peroxide, 1,4-di-tert-butylperoxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and benzoyl peroxide.

[0056] In this invention, preferably, the temperature of the preheated crosslinking agent can be 60-80℃, more preferably 65-75℃; the viscosity of the preheated crosslinking agent can be 5-15 mPa·s, more preferably 8-12 mPa·s.

[0057] In this invention, preferably, the mass ratio of liquid antioxidant to preheated crosslinking agent in the additive mixture can be 1:2-10, more preferably 1:3-7.

[0058] In this invention, the aforementioned liquid antioxidant exhibits excellent dispersion uniformity, eliminates localized concentration differences, and provides a more stable antioxidant effect. It solves the problems of solid antioxidants requiring higher processing temperatures, being difficult to disperse, and exhibiting uneven dispersion. Furthermore, it demonstrates stronger compatibility with polyethylene base materials, preventing migration and precipitation. The aforementioned crosslinking agent ensures stable and controllable crosslinking of the wire core within the vulcanization channel after insulation material extrusion, resulting in high crosslinking efficiency. The stable crosslinking bonds endow the material with excellent resistance to heat and oxygen aging.

[0059] In this invention, preferably, in step S2, the filtration process may include: primary metal mesh filtration, secondary polypropylene pleated filter cartridge filtration, and tertiary polytetrafluoroethylene membrane filter cartridge filtration; wherein, the pore size of the primary metal mesh filter can be 0.2-0.3 mm, more preferably 0.25-0.28 mm; and the angle between the plane of the primary metal mesh filter and the horizontal mounting reference plane can be 45°-60°.

[0060] In this invention, preferably, the metal filter screen can be selected from at least one of stainless steel filter screen, copper filter screen and nickel filter screen.

[0061] In this invention, preferably, the effective cut-off size of the polypropylene pleated filter element can be 5-10 μm, more preferably 6-8 μm.

[0062] In this invention, preferably, the average pore size of the polytetrafluoroethylene membrane can be 0.2-0.5 μm, more preferably 0.3-0.4 μm.

[0063] In this invention, a three-stage filtration system can precisely trap various impurities, further improving the purity of the additives and providing higher-quality raw materials for subsequent permeation and absorption processes. Furthermore, the installation of the primary metal filter screen according to the aforementioned conditions facilitates smooth liquid flow while improving the interception effect on larger particle sizes. The use of a polypropylene pleated filter element not only accurately captures fine impurities in the additive mixture but also maintains a stable filtration flow rate while ensuring high retention efficiency, avoiding flow rate attenuation due to insufficient filtration area. The use of a polytetrafluoroethylene (PTFE) membrane not only deeply removes ultrafine and colloidal impurities, preventing ultrafine impurities from entering subsequent spraying stages and causing performance defects on the material surface, but also possesses excellent chemical inertness, withstanding long-term immersion in liquid antioxidants and crosslinking agents, preventing chemical reactions between the filter element material and the additives and the introduction of new impurities.

[0064] In this invention, preferably, the spraying rate can be 5-15 mL / s relative to 1000 mL of additive mixture, more preferably 8-12 mL / s.

[0065] In this invention, preferably, the spraying method may include ultrasonic atomization spraying or pulse spraying.

[0066] In this invention, preferably, the particle size of the atomized additive mixture in the ultrasonic atomizing spray is ≤5μm, and more preferably it can be 2-4μm.

[0067] In this invention, preferably, the conditions for ultrasonic atomization spraying may include: a temperature of 65-75°C, more preferably 68-72°C; and an ultrasonic frequency of 20-30Hz, more preferably 22-28Hz.

[0068] In this invention, preferably, the pulse spraying method may include: spraying the additive mixture for 3-8 seconds every 15-45 seconds, and then adjusting the relative pressure of the vacuum condition to -5 kPa to -10 kPa; more preferably, the pulse spraying method may include: spraying the additive mixture for 4-6 seconds every 20-40 seconds, and then adjusting the relative pressure of the vacuum condition to -6 kPa to -8 kPa.

[0069] In this invention, ultrasonic atomization spraying can convert liquid into an aerosol state, maximizing the contact area between the additive and the material. This avoids the uneven distribution of additives caused by localized droplet aggregation in traditional spraying, laying the foundation for uniform penetration in subsequent stepped insulation. Pulsed spraying can avoid "droplet aggregation" by using intermittent spraying, ensuring that all surfaces of the particles can contact the additive, further improving dispersion uniformity.

[0070] In this invention, the use of negative pressure spraying inside the tank can reduce the air pressure of the material system, thereby reducing the contact angle of the additives on the particle surface and significantly improving wettability. On the other hand, the air inside the particles escapes rapidly under negative pressure, forming instantaneous microchannels, providing a path for the additives to penetrate. Finally, the negative pressure inside the tank can reduce additive loss and the risk of impurities, avoid material dust caused by "droplet impact breakage", and ensure product purity.

[0071] In this invention, preferably, a protective gas is continuously introduced during the insulation stage. The protective gas can be a commonly used protective gas in the art, such as nitrogen, helium, or argon. The stepped insulation combined with the continuous introduction of a protective gas in this invention ensures a uniform temperature distribution inside the insulation cavity, and the base material particles slowly fluidize during the insulation process, avoiding excessively high or low local temperatures. This solves the problems of large temperature gradients and uneven additive penetration in the insulation cavity under high production capacity.

[0072] In this invention, preferably, the spraying amount of the additive mixture can be 1-3 wt% of the total mass of the polyethylene base material, more preferably 1.5-2.5 wt%.

[0073] In this invention, preferably, the second dust removal method includes: vibrating and sieving the cooled material to remove coarse powder, and then performing ultrasonic blowing. After the second dust removal is completed, the dust content on the surface of the cross-linked polyethylene product can be 1-2 mg / kg.

[0074] It is understood that the method for detecting the surface dust content of polyethylene base material and cross-linked polyethylene products can be a method commonly used in the art, such as laser dust detection or dry testing.

[0075] In this invention, preferably, the vibration frequency of the high-precision vibratory sieve for removing coarse powder can be 15-20Hz, the sieve aperture can be 100-150μm, and the material can be smooth stainless steel or polyurethane.

[0076] In this invention, the use of a vibrating screen with high-precision screening capabilities can effectively reduce friction between the material and the screen mesh. By precisely controlling the amplitude and frequency of the vibrating screen, it is ensured that fine powder passes smoothly through the screen mesh.

[0077] In this invention, preferably, the frequency of the ultrasonic purging can be 20-30 kHz.

[0078] In this invention, preferably, when the dust content on the surface of the polyethylene base material detected by laser dust detection exceeds 2 mg / kg, the unqualified material is sent back to the second dust removal unit for processing.

[0079] In this invention, preferably, the second optical detection method may include: using an industrial camera with a resolution of 5-8 megapixels and light sources of white light, ultraviolet light, and infrared light to photograph the material from an angle of 30-60°, and using an AI image recognition algorithm to identify impurities of different particle sizes and remove the impurities.

[0080] In a preferred embodiment of the present invention, the first optical detection, following the "first powder removal + magnetic separation," aims to capture trace impurities (such as non-metallic impurities and residual ultrafine magnetic impurities) in the base material that have not been physically removed. The industrial camera and lens configuration uses a CMOS industrial camera with a resolution of 5-8 megapixels, preferably 6 megapixels, with a pixel size of 2.4μm × 2.4μm, paired with a fixed-focus lens with a focal length of 8-12mm. The single-frame shooting area is 100mm × 100mm, and the frame rate is 20-30fps. The camera is mounted at a height of 300-400mm above the belt surface, with the lens at an angle of 30-60° to the belt surface, preferably 45°, to avoid reflective blind spots. A polarizer (polarization degree ≥ 99%) is also provided to eliminate surface reflections from the base material particles. Multi-source synergy parameters: A three-source array arrangement of "white light + ultraviolet light + infrared light" is adopted (each source is spaced 120° apart, surrounding the camera lens): White light source (wavelength 400-760nm, power 30-50W): Illuminates the surface of the base material particles, highlighting colored impurities (such as black organic debris, red metal oxide residues), with a light intensity of 800-1200 lux; Ultraviolet light source (wavelength 280-365nm, power 20-30W): Excites the fluorescence properties of transparent / light-colored impurities (such as polyethylene gel particles) (gel particles exhibit blue fluorescence under ultraviolet light), with an exposure time of 50-100μs; Infrared light source (wavelength 800-1000nm, power 25-40W): Penetrates the surface of the base material particles (penetration depth 5-10μm), identifying internally encapsulated impurities (such as micron-sized metal debris inside the particles), with the infrared light intensity adjusted to 500-800 lux. Image Acquisition and Preprocessing: The base material is evenly distributed on the conveyor belt through vibration. Three light sources are triggered simultaneously, and the camera acquires three types of images: RGB (white light), grayscale (ultraviolet light), and infrared thermal imaging (infrared light). These images are transmitted to an industrial computer for preprocessing: White light image: Gaussian filtering is used to remove noise, and colored impurity contours are extracted through adaptive threshold segmentation (threshold range 120-180). Ultraviolet light image: Threshold segmentation (threshold 80-120) is used to extract fluorescent impurity regions, and morphological dilation (3×3 structural elements) is combined to enhance weak fluorescence signals. Infrared image: Grayscale histogram equalization is used to highlight the grayscale difference between internal impurities and the base material (internal impurity grayscale values ​​are 20-30 lower than base material), and edge detection (Canny operator) is used to determine the impurity location. AI Algorithm Recognition and Particle Size Determination: An AI model trained based on a convolutional neural network (CNN) is used to fuse and recognize the three types of images. Impurity type determination: "Surface impurities / internal impurities / fluorescent impurities" are distinguished by feature matching (such as color, fluorescence intensity, and infrared transmittance); Particle size calculation: Impurity size is measured using the "minimum circumscribed rectangle method," with a particle size identification range of 0.1-100μm and an error ≤0.5μm. Impurity particle size threshold is set: ≥0.1μm is considered unqualified.Non-conforming particle rejection mechanism: When the AI ​​identifies a non-conforming particle, it triggers a pneumatic pusher (response time ≤0.1s, thrust 5-10N), pushing the particle into the waste bin when it reaches the rejection station (500-800mm from the camera position), with a rejection accuracy rate ≥99.8%. Simultaneously, the system records the "location-particle size-type" information of the non-conforming particle and generates a real-time detection report (statistically analyzing the impurity type percentage and rejection rate hourly), facilitating the tracking of anomalies in upstream processes (such as dust removal and magnetic separation).

[0081] In a preferred embodiment of the present invention, the second optical detection, located after the "second dust removal," aims to identify impurities newly introduced during cooling and conveying (such as impurities introduced by the cooling water circuit and wear debris from the conveying pipes), as well as trace impurities missed by the first detection. The specific methods and conditions are optimized based on the first detection as follows: White light source: power increased to 40-60W, light intensity 1000-1500 lux (highlighting fine dust impurities attached to the surface); Ultraviolet light source: wavelength adjusted to 300-365nm (if unabsorbed crosslinking agent remains in the finished product, it will fluoresce yellow at this wavelength and can be identified simultaneously); Infrared light source: power increased to 35-50W, penetration depth increased to 10-15μm (the finished product has a higher particle density, requiring stronger infrared light to identify internal impurities).

[0082] In a preferred embodiment of the present invention, the first and second optical inspections strictly control the size and quantity of impurities according to the differentiated requirements for the cleanliness of the insulation material at different voltage levels. For example, according to the national standard GB / T11017.1-2014 "110kV Cross-linked Polyethylene Insulated Power Cables and Accessories", the finished cable insulation should be free of opaque impurities larger than 0.125mm, and the number of opaque impurities larger than 0.05mm and smaller than 0.125mm should not exceed 10 per 16.4cm³ of insulation, thus precisely controlling the impurity parameters.

[0083] This invention provides a method for preparing cross-linked polyethylene that allows for customized grading standards for impurity size and quantity based on different voltage level requirements. In the packaging stage, it achieves seamless automation across the entire process, from impurity detection to grading, packaging, and transportation. The impurity detection system is linked to the packaging and warehousing systems; when the grading standards are adjusted, the program automatically synchronizes to the packaging stage, replacing the corresponding grade packaging box labels and notifying the AGV (Automated Guided Vehicle) to adjust the storage location, achieving full-process responsiveness.

[0084] A second aspect of the present invention provides cross-linked polyethylene prepared using the method described in the first aspect.

[0085] A third aspect of the present invention provides the use of cross-linked polyethylene as an insulating material in cables, as described in the second aspect.

[0086] The present invention will be described in detail below through examples. In the following examples, the low-density polyethylene and linear low-density polyethylene base materials are commercially available products of Sinopec Qilu Petrochemical Company with grades J182B and 7042, the antioxidant 3,5-di-tert-butyl-4-hydroxyphenylpropionate isooctyl ester is a commercially available product of Jiangsu Jiyi New Materials Co., Ltd., and the crosslinking agents tert-butyl cumene peroxide and dicumene peroxide are commercially available products of Nourion Chemicals Co., Ltd.

[0087] (1) Fine powder content test method: dry test is adopted. Weigh 5kg of the material to be tested, place it in a 500 mesh filter, and place fine powder collection paper under the filter. Start the equipment and shake for 60 minutes. After the surface powder in the sample has almost completely fallen off, weigh the powder content of the insulating material. Repeat 3 times and calculate the average value to obtain the powder content.

[0088] (2) The test method for the elongation of the insulation material under thermal extension load after the crosslinking agent is fully absorbed shall be performed in accordance with the standard GB / T2951.21-2008.

[0089] (3) Impurity detection method: The insulating material is detected by the OCS optical impurity detection system. The equipment sensitivity is 20µm and the single test volume is 3kg.

[0090] (4) Spatial uniformity of magnetic field: A three-axis vector Hall Gauss meter is used. The effective sensitive center size of the probe is Φ1.0mm. The probe material is non-magnetic non-ferrite encapsulation. The range is 0-3.0T and the resolution is 0.0001 T. Before the test, the zero magnetic space is zeroed in three axes and a standard constant magnet is used for single-point calibration to eliminate zero drift and temperature drift.

[0091] Example 1

[0092] (1) Purification of polyethylene base material: Based on the total mass of polyethylene base material, 100wt% low-density polyethylene base material (molecular weight distribution of 5, melt flow rate of 1.5g / 10min at 190℃ and 2.16kg, density of 0.92g / cm³) 3First, it passes through a vibrating screen with a vibration frequency of 18Hz and a screen aperture of 120μm. Then, it passes through an ultrasonic purging system with a frequency of 25kHz to remove surface fine powder. (After the powder removal, the surface dust content of the low-density polyethylene base material is 3mg / kg. When the surface dust content of the low-density polyethylene base material exceeds 5mg / kg, the unqualified low-density polyethylene base material is sent back to the vibrating screen for further processing.) Then, a multi-stage composite magnetic separation system is used to remove metal ion impurities. The first stage is a strong magnetic filtration device, which uses neodymium iron boron permanent magnet material with a magnetic field strength of 0.9T, a magnetic field gradient of 60T / m, a material flow rate of 0.5m / s, a filtration accuracy of 6μm, and a magnetic field spatial uniformity of 95%. The second stage is a high-gradient magnetic separation technology (the magnetic medium is a compound of 70wt% stainless steel wool, 15wt% ferrite, and 15wt% carbonyl iron powder), with a magnetic field strength of 1.2T, a magnetic field gradient of 70T / m, a material flow rate of 0.6m / s, a filtration accuracy of 4μm, and a magnetic field spatial uniformity of 65%. The third stage is electrostatic adsorption with an electric field strength of 3500V / m, an electrode spacing of 100mm, and a material flow rate of 0.8m / s. Next, the purified polyethylene base material undergoes a full inspection using the aforementioned first optical inspection method. This involves a 6-megapixel CMOS industrial camera with a pixel size of 2.4μm × 2.4μm, paired with a 10mm fixed-focus lens. Each frame captures an area of ​​100mm × 100mm at a frame rate of 25fps. The camera is mounted 350mm above the conveyor belt surface, capturing the material at a 45° angle. A three-light source array (white light + ultraviolet light + infrared light) is used (each light source spaced 120° apart, surrounding the camera lens). The white light source (wavelength 500nm, power 40W) has an intensity of 1000 lux; the ultraviolet light source (wavelength 300nm, power 25W) has an exposure time of 80μs; ​​and the infrared light source (wavelength 900nm, power 30W) has an intensity adjusted to 600 lux. This first optical inspection removes physical impurities larger than 50µm. The ultra-clean polyethylene base material is then preheated in a 70°C storage tank.

[0093] (2) Purification of antioxidants and crosslinking agents: Liquid antioxidant 3,5-di-tert-butyl-4-hydroxyphenylpropionate isooctyl ester was selected; crosslinking agent tert-butylisopropylbenzene peroxide was selected and preheated and melted at 70°C in a feeding tank. The antioxidant and the melted crosslinking agent (the viscosity of the crosslinking agent is 8 mPa·s) were respectively filtered by a multi-stage filtration system to remove chemical impurities. The first stage was a metal mesh filter made of stainless steel with a pore size of 0.25 mm and an angle of 45° between the filter screen plane and the horizontal mounting reference plane; the second stage was a polypropylene pleated filter with an effective interception size of 6 μm; and the third stage was a polytetrafluoroethylene membrane filter with an average pore size of 0.3 μm.

[0094] (3) Premixing antioxidants and crosslinking agents: The purified antioxidants and crosslinking agents are premixed in a storage tank at a mass ratio of 1:5. The mixture is stirred at a constant temperature of 70°C for 10 minutes to obtain a uniform additive mixture.

[0095] (4) Crosslinking agent penetration and absorption: In step (1), the polyethylene base material is added to the rotary drum mixing and absorption tank by a feed weigher. The temperature is controlled at 70°C. The auxiliary agent mixture in step (3) (the average particle size of the atomized auxiliary agent mixture is 2μm) is uniformly sprayed on the polyethylene base material at a rate of 1.8wt% of the total mass of the base material (the spraying rate is 8mL / s relative to 1000mL of auxiliary agent mixture). The spraying temperature is 68°C and the ultrasonic frequency is 22Hz. The polyethylene granules and auxiliary agents are uniformly mixed to ensure that the auxiliary agents are fully absorbed until the particle surface is dry, thus obtaining the pre-crosslinked material.

[0096] (5) Pour the material from the shaking tank in step (4) into the heat-insulated storage tank for full absorption. First, keep it at 65°C for 4 hours, then cool it down to 60°C and keep it at 1.5 hours. During the heat-insulation stage, nitrogen gas is continuously introduced.

[0097] (6) Using a composite cooling method, the material after heat preservation in step (5) is cooled to 45°C in a fluidized bed (wherein, the inlet air temperature of the fluidized bed is 40°C and the air velocity is 1m / s), and then vacuum cooled to 20°C. The cooling rate of vacuum cooling is 1°C / min and the relative pressure is -0.1MPa.

[0098] (7) The cooled material is vibrated and sieved to remove coarse powder. The vibration frequency for vibrating and sieving to remove coarse powder is 15Hz and the mesh size is 100μm. Then, ultrasonic purging is performed at a frequency of 20kHz. After the second dust removal, the surface dust content of the cross-linked polyethylene product is detected by laser dust detection. The dust content is 1.5mg / kg. When the surface dust content of the polyethylene base material exceeds 2mg / kg, the unqualified material is sent back to the second dust removal for processing. After the second dust removal, the cross-linked polyethylene is fully inspected by the second optical detection according to the first optical detection method. The difference between the second optical detection and the first optical detection is that the power of the white light source is 60W, the light intensity is 1500lux, the wavelength of the ultraviolet light source is 350nm, and the power of the infrared light source is 40W. Using an industrial camera with a resolution of 5 megapixels and light sources of white light, ultraviolet light, and infrared light, the material is photographed from a 30° angle to achieve 100% detection. The AI ​​image recognition algorithm (based on an AI model trained by a convolutional neural network (CNN) to fuse and recognize images from three different light sources) identifies impurities of different particle sizes and removes them.

[0099] The information on the time required for the crosslinking agent to penetrate to dryness in step (4), the time required for the thermal insulation crosslinking agent to be fully absorbed in step (5), the elongation of the insulation material under thermal extension load after the crosslinking agent is fully absorbed in step (6), the content of fine powder in each stage, and the content of impurities and gel in each stage are shown in Table 1.

[0100] Example 2

[0101] Crosslinked polyethylene was prepared according to the method of Example 1, with the difference being:

[0102] (1) Purification of polyethylene base material: The magnetic field strength of strong magnetic filtration is 1.1T, the magnetic field gradient is 70T / m, the filtration accuracy is 8μm, the magnetic field gradient of high gradient magnetic separation is 80T / m, and the electric field strength of electrostatic adsorption is 4500V / m.

[0103] (2) Purification of antioxidants and crosslinking agents: First-stage metal mesh filtration, made of stainless steel, with a pore size of 0.28 mm and an angle of 60° between the filter screen plane and the horizontal installation reference plane; Second-stage polypropylene pleated filter cartridge filtration, with an effective interception size of 8 μm; Third-stage polytetrafluoroethylene membrane filter cartridge filtration, with an average pore size of 0.4 μm;

[0104] (3) Premixing and permeation absorption of antioxidants and crosslinking agents: The purified antioxidants and crosslinking agents are premixed in a storage tank at a mass ratio of 1:7;

[0105] (4) The additive mixture in step (3) is uniformly sprayed onto the polyethylene base material at a rate of 2.5 wt% of the total mass of the base material using a pulse spraying method (the spraying rate is 10 mL / s relative to 1000 mL of additive mixture). After spraying for 5 seconds every 30 seconds, the relative vacuum pressure inside the tank is adjusted to -6 kPa.

[0106] (5) Pour the material in the shaking tank in step (4) into the heat-insulated storage tank for full absorption. First, keep it at 70°C for 5 hours, then cool it down to 65°C and keep it at 65°C for 2.5 hours. During the heat-insulation stage, nitrogen gas is continuously introduced.

[0107] (6) A stepped gravity cooling tower is used for cooling. The temperature of the first stage is controlled at 55°C, the temperature of the second stage is controlled at 45°C, and after the material is cooled to 50°C, 20°C cold air is introduced to continue cooling and cooling down to 40°C.

[0108] (7) After cooling, the material is vibrated and sieved to remove coarse powder. The vibration frequency for vibrating and sieving to remove coarse powder is 20Hz and the mesh size is 150μm. Then, ultrasonic purging is performed at a frequency of 30kHz. After the second powder removal, laser dust detection is used to determine that the dust content on the surface of the cross-linked polyethylene product is 1.9mg / kg. An industrial camera with a resolution of 8 million pixels and light sources of white light, ultraviolet light and infrared light are used to photograph the material from a 60° angle to achieve 100% detection.

[0109] All other operations are the same as in Example 1.

[0110] Example 3

[0111] Crosslinked polyethylene was prepared according to the method of Example 1, with the difference being:

[0112] (1) Purification of polyethylene base material: Based on the total mass of polyethylene base material, 95wt% low-density polyethylene base material (molecular weight distribution of 5, melt flow rate of 1.5g / 10min at 190℃ and 2.16kg, density of 0.92g / cm³) was purified. 3 5 wt% linear low-density polyethylene (molecular weight distribution 4, melt flow rate of 2 g / 10 min at 190℃ and 2.16 kg, density of 0.93 g / cm³) 3 The magnetic field strength of the strong magnetic filter is 1T, the magnetic field gradient is 65T / m, and the filtration accuracy is 7μm. The magnetic field strength of the high gradient magnetic separation is 1.3T, the magnetic field gradient is 75T / m, the electric field strength of electrostatic adsorption is 4000V / m, and the electrode spacing is 125mm.

[0113] (2) Purification of antioxidants and crosslinking agents: First-stage metal mesh filtration, made of stainless steel, with a pore size of 0.26 mm and an angle of 50° between the filter screen plane and the horizontal installation reference plane; Second-stage polypropylene pleated filter cartridge filtration, with an effective interception size of 7 μm; Third-stage polytetrafluoroethylene membrane filter cartridge filtration, with an average pore size of 0.35 μm.

[0114] (3) Premixing and permeation absorption of antioxidants and crosslinking agents: The purified antioxidants and crosslinking agents are premixed in a storage tank at a mass ratio of 1:3;

[0115] (4) Spray evenly onto the polyethylene base material at a rate of 1.5 wt% of the total base material mass;

[0116] (5) Pour the material in the shaking tank in step (4) into the heat-insulating storage tank for full absorption. First, keep it at 68°C for 4.5 hours, then cool it down to 63°C and keep it at 63°C for 2 hours. During the heat-insulating stage, nitrogen gas is continuously introduced.

[0117] (6) A stepped gravity cooling tower is used for cooling. In the first stage, the material is cooled to 50°C, and in the second stage, the material is cooled to 40°C. Then, 15°C cold air is introduced to continue cooling down to 30°C.

[0118] (7) After cooling, the material is vibrated and sieved to remove coarse powder. The vibration frequency for vibrating and sieving to remove coarse powder is 18Hz and the mesh size is 130μm. Then, ultrasonic purging is performed at a frequency of 25kHz. After the second powder removal, laser dust detection is used to determine that the dust content on the surface of the cross-linked polyethylene product is 1.6mg / kg. An industrial camera with a resolution of 7 million pixels and light sources of white light, ultraviolet light and infrared light are used to photograph the material from a 45° angle to achieve 100% detection.

[0119] All other operations are the same as in Example 1.

[0120] Example 4

[0121] Crosslinked polyethylene was prepared according to the method of Example 1, except that the conditions for strong magnetic filtration, high gradient magnetic separation, and electrostatic adsorption in step (1) were changed. Specifically:

[0122] (1) Purification of polyethylene base material: The magnetic field strength of strong magnetic filtration is 0.8T, the magnetic field gradient is 55T / m, and the filtration accuracy is 5μm; the magnetic field strength of high gradient magnetic separation is 1.1T, the magnetic field gradient is 60T / m, and the filtration accuracy is 3μm; the electric field strength of electrostatic adsorption is 3000V / m, and the electrode spacing is 80mm.

[0123] Example 5

[0124] Crosslinked polyethylene was prepared according to the method of Example 1, except that the conditions for strong magnetic filtration, high gradient magnetic separation, and electrostatic adsorption in step (1) were changed. Specifically:

[0125] (1) Purification of polyethylene base material: The magnetic field strength of strong magnetic filtration is 1.2T, the magnetic field gradient is 80T / m, and the filtration accuracy is 9μm; the magnetic field strength of high gradient magnetic separation is 1.5T, the magnetic field gradient is 90T / m, and the filtration accuracy is 7μm; the electric field strength of electrostatic adsorption is 5000V / m, and the electrode spacing is 180mm.

[0126] Example 6

[0127] Crosslinked polyethylene was prepared according to the method of Example 1, except that the conditions for strong magnetic filtration, high gradient magnetic separation, and electrostatic adsorption in step (1) were changed. Specifically:

[0128] (1) Purification of polyethylene base material: The magnetic field strength of strong magnetic filtration is 1.15T, the magnetic field gradient is 85T / m, and the filtration accuracy is 6.5μm; the magnetic field strength of high gradient magnetic separation is 1.45T, the magnetic field gradient is 85T / m, and the filtration accuracy is 6.5μm; the electric field strength of electrostatic adsorption is 4800V / m, and the electrode spacing is 200mm.

[0129] Example 7

[0130] Crosslinked polyethylene was prepared according to the method of Example 1, except that the heat preservation conditions in step (5) were changed, specifically:

[0131] (5) Pour the material in the shaking tank in step (4) into the heat-insulated storage tank for full absorption. First, keep it at 72°C for 6 hours, then cool it down to 68°C and keep it at 68°C for 3 hours. During the heat-insulation stage, nitrogen gas is continuously introduced.

[0132] Example 8

[0133] Crosslinked polyethylene was prepared according to the method of Example 1, except that the cooling conditions in step (6) were changed. Specifically:

[0134] (6) A stepped gravity cooling tower is used for cooling. In the first stage, the material is cooled to 45°C. In the second stage, the material is cooled to 35°C and then 10°C cold air is introduced to continue cooling down to 20°C.

[0135] Example 9

[0136] Crosslinked polyethylene was prepared according to the method of Example 1, except that the heat preservation method in step (5) was changed, and gradient heat preservation was not performed. Specifically:

[0137] (5) Pour the material from the shaking tank in step (4) into the heat-insulated storage tank for full absorption, keep it at 70°C for 4 hours, and continuously introduce nitrogen during the heat-insulation stage.

[0138] Example 10

[0139] Crosslinked polyethylene was prepared according to the method of Example 1, except that the cooling method in step (6) was changed, and gradient cooling was not performed. Specifically:

[0140] (6) The material after heat preservation in step (5) is vacuum cooled to 20°C. The cooling rate of vacuum cooling is 1°C / min and the relative pressure is -0.1MPa.

[0141] Comparative Example 1

[0142] (1) The polyethylene base material was purified according to the method of Example 1, except that only strong magnetic filtration was used to remove metal ion impurities and no first optical detection was performed; wherein, the magnetic field strength of the strong magnetic filtration was 0.9T, the magnetic field gradient was 60T / m, and the filtration accuracy was 6μm.

[0143] (2) The polyethylene base material and antioxidant 3,5-di-tert-butyl-4-hydroxyphenylpropionate isooctyl ester (the amount of antioxidant is 0.3wt% of the total mass of the base material) in step (1) are fed into a twin-screw extruder through a gravity-feeding weigher. After melt extrusion, the extruder speed is 350 rpm, and the temperature settings of each section of the extruder are 95℃, 175℃, 178℃, 170℃, 163℃, 163℃, and 158℃, respectively. The screw temperature is 108℃ and the material temperature is 180℃. The intermediate material is prepared by melt precision filtration (using a 500-mesh filter screen) and underwater pelleting (cylindrical particles with a diameter of 3-4 mm and a length of 2-4 mm).

[0144] (3) Select crosslinking agent tert-butyl cumene peroxide and preheat and melt it in the feeding tank at 70°C. The intermediate material in step (2) is added to the rotary drum mixing and absorption tank by a feeding weigher. The temperature is controlled at 70°C. The crosslinking agent is sprayed evenly on the surface of the intermediate material at a rate of 1.5wt% of the total mass of the base material (the spraying rate is 8mL / s relative to 1000mL of auxiliary mixture). At 68°C, the particle surface is dry, and the pre-crosslinked material is obtained.

[0145] (4) Pour the material from the shaking tank in step (3) into an insulated storage tank and keep it at 70°C until the crosslinking agent is completely absorbed.

[0146] (5) Fluidization cooling is used to cool the material after heat preservation in step (4) into a fluidized bed to 45°C, wherein the inlet air temperature of the fluidized bed is 40°C and the air velocity is 1m / s.

[0147] (6) The cooled material is vibrated and screened to remove coarse powder. The vibration frequency of the vibrating screen to remove coarse powder is 15Hz and the screen aperture is 100μm. After removing coarse powder, the material is sampled using an industrial camera with a resolution of 5 million pixels. The sampling rate is 5%.

[0148] The information on the time required for the crosslinking agent to penetrate to dryness in step (3), the time required for the thermal insulation crosslinking agent to be fully absorbed in step (4), the elongation of the insulation material under thermal extension load after the crosslinking agent is fully absorbed in step (5), the content of each stage, the content of impurities and gel is shown in Table 1.

[0149] Comparative Example 2

[0150] Crosslinked polyethylene was prepared according to the method of Example 1, except that in step (4), the polyethylene base material and additives were not sprayed, but mixed by stirring. Specifically:

[0151] (4) Crosslinking agent penetration and absorption: In step (1), the polyethylene base material is added to the rotary drum mixing and absorption tank by the material feeding scale, and the temperature is controlled at 70℃. The additive mixture in step (3) is mixed with the base material at 1.8wt% of the total mass of the base material, and the stirring speed is 8rpm / min.

[0152] Comparative Example 3

[0153] Crosslinked polyethylene was prepared according to the method of Example 1, except that the multi-stage composite magnetic separation system in step (1) does not perform a second stage of high-gradient magnetic separation to remove metal ion impurities. Specifically:

[0154] (1) The first stage is a strong magnetic filter device, which uses neodymium iron boron permanent magnet material, with a magnetic field strength of 0.9T and a magnetic field gradient of 60T / m; the second stage is electrostatic adsorption, with an electric field strength of 3500V / m and a plate spacing of 100mm.

[0155] Table 1

[0156]

[0157] As shown in Table 1, the embodiments of this invention employ an antioxidant penetration and absorption method. Compared to the comparative example, which uses a traditional melt extrusion process for antioxidant dispersion, this method reduces the content of impurities (mainly physical impurities, such as scorched particles and metal particles) and gels (mainly referring to pre-crosslinked polyethylene macromolecules) in the material. Simultaneously, the overall production process is shorter, significantly reducing fine powder (fine powder refers to fine particles of the base material with an average particle size of 20-100 μm) generated by pipeline transmission friction. 100% inspection of the polyethylene base material and finished insulation ensures better product cleanliness. The post-absorption and heat preservation methods used in the embodiments shorten the additive penetration time and heat preservation time, ensuring complete absorption of the additives and improving production efficiency. Adjusting the cooling method in the embodiments reduces the fine powder content in the pre-crosslinked material, improving cleanliness. In this invention, the elongation rate of the insulation material under thermal extension load after complete absorption of the crosslinking agent reflects the plastic flow resistance of the material at high temperatures. The lower the thermal elongation rate, the less likely the material is to undergo irreversible deformation under high-temperature stress. Furthermore, the present invention is not limited by melt extrusion, and the continuous production time is stable and controllable, while the comparative preparation method requires frequent screen changes, and the continuous production cycle of the production line is less than 15 days.

[0158] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A process for the preparation of crosslinked polyethylene, characterized in that, The method includes: S1. After the polyethylene base material undergoes a first dust removal and metal impurity removal treatment, a first optical inspection is used to fully inspect the polyethylene base material; wherein, after the first dust removal, the dust content on the surface of the polyethylene base material is ≤5mg / kg; the removal of metal impurities is achieved using magnetic separation and electrostatic adsorption; the magnetic separation includes strong magnetic separation and high-gradient magnetic separation; wherein, the spatial uniformity of the magnetic field in strong magnetic separation is ≥85%, and the spatial uniformity of the magnetic field in high-gradient magnetic separation is ≤65%; The conditions for strong magnetic separation include: magnetic field strength of 0.9-1.1T; magnetic field gradient of 60-70T / m; the conditions for high-gradient magnetic separation include: magnetic field strength of 1.2-1.4T; magnetic field gradient of 70-80T / m; the conditions for electrostatic adsorption include: electric field strength of 3500-4500V / m; electrode spacing of 100-150mm; and material flow velocity of 0.8-1.2m / s. S2. The liquid antioxidant and the preheated crosslinking agent are filtered separately or mixed to obtain the additive mixture; S3. Under vacuum conditions, the additive mixture obtained in step S2 is sprayed onto the polyethylene base material treated in step S1 to obtain intermediate material. The spraying method includes ultrasonic atomization spraying or pulse spraying; the conditions for ultrasonic atomization spraying include: temperature of 65-75℃; ultrasonic frequency of 20-30Hz; the method for pulse spraying includes: spraying the additive mixture for 3-8 seconds every 15-45 seconds, and then adjusting the relative pressure of the vacuum condition to -5kPa to -10kPa. S4. After heat preservation, the intermediate material is cooled. After cooling, the cross-linked polyethylene undergoes a second dust removal treatment and is then subjected to a second optical inspection to fully inspect the cross-linked polyethylene. The dust content on the surface of the cross-linked polyethylene is ≤2mg / kg after the second dust removal. In step S4, the heat preservation method includes: first keeping the intermediate material at 65-70℃ for 4-5 hours, and then cooling it down to 60-65℃ and keeping it at 60-65℃ for 1.5-2.5 hours; In step S4, the cooling method includes composite cooling or stepped cooling; the composite cooling method includes: after the intermediate material is kept at a constant temperature, it enters a fluidized bed and is cooled to 45-55°C, and then vacuum cooled to 20-30°C; the stepped cooling method includes: using a stepped gravity tower for cooling, in the first stage the intermediate material is cooled to 50-65°C, in the second stage it is cooled to 40-50°C, and then cooled to 20-40°C by cold air.

2. The method of claim 1, wherein, The inlet air temperature of the fluidized bed is 40-50℃, and the air velocity is 1-2m / s; And / or, the cooling rate of the vacuum cooling is 1-3℃ / min, and the relative pressure of the vacuum cooling is -0.1MPa to -0.08MPa.

3. The method of claim 1, wherein, After the first dust removal process, the dust content on the surface of the base material is 2-5 mg / kg; And / or, when the dust content on the surface of the polyethylene base material exceeds 5 mg / kg, the unqualified polyethylene base material shall be sent back to the first dust removal unit for treatment; And / or, the spatial uniformity of the magnetic field of the strong magnetic separation is 85-95%; And / or, the spatial uniformity of the magnetic field of the high-gradient magnetic separation is 40-65%; And / or, the magnetic medium for high gradient magnetic separation is at least one of stainless steel wool, ferrite and carbonyl iron powder; And / or, the first optical detection method includes: using an industrial camera with a resolution of 5-8 megapixels and light sources of white light, ultraviolet light, and infrared light to photograph the polyethylene base material from an angle of 30-60°, and using an AI image recognition algorithm to identify impurities of different particle sizes and remove the impurities.

4. The method of claim 1, wherein, The liquid antioxidant is selected from at least one of 3,5-di-tert-butyl-4-hydroxyphenylpropionate isooctyl ester, bisphenol A glycerol dimethacrylate, 2,6-diallyl-p-cresol, 4,4'-sulfonylbis[2-(2-propenyl)]phenol and m-xylenediacrylamide; And / or, the crosslinking agent is selected from at least one of dicumyl peroxide, tert-butyl cumyl peroxide, 1,4-di-tert-butylperoxypropylbenzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and benzoyl peroxide; And / or, the temperature of the preheated crosslinking agent is 60-80℃; the viscosity of the preheated crosslinking agent is 5-15 mPa·s; And / or, in the additive mixture, the mass ratio of liquid antioxidant to preheated crosslinking agent is 1:2-10.

5. The method of claim 4, wherein, The temperature of the preheated crosslinking agent is 65-75℃; the viscosity of the preheated crosslinking agent is 8-12 mPa·s; And / or, in the additive mixture, the mass ratio of liquid antioxidant to preheated crosslinking agent is 1:3-7.

6. The method of claim 1, wherein, In step S2, the filtration process includes: primary metal mesh filtration, secondary polypropylene pleated filter cartridge filtration, and tertiary polytetrafluoroethylene membrane filter cartridge filtration.

7. The method of claim 6, wherein, The mesh size of the primary metal filter is 0.2-0.3mm; the angle between the plane of the primary metal filter and the horizontal mounting reference plane is 45°-60°. And / or, the metal filter screen is selected from at least one of stainless steel filter screen, copper filter screen and nickel filter screen; And / or, the effective filter size of the polypropylene pleated filter element is 5-10 μm; And / or, the average pore size of the polytetrafluoroethylene membrane is 0.2-0.5 μm.

8. The method of claim 6, wherein, The pore size of the primary metal filter is 0.25-0.28 mm; And / or, the effective filter size of the polypropylene pleated filter element is 6-8 μm; And / or, the average pore size of the polytetrafluoroethylene membrane is 0.3-0.4 μm.

9. The method of claim 1, wherein, The spraying rate is 5-15 mL / s relative to 1000 mL of additive mixture; And / or, during the heat preservation stage, protective gas is continuously introduced; And / or, the amount of the additive mixture sprayed is 1-3 wt% of the total mass of the polyethylene base material.

10. The method of claim 9, wherein, The spraying rate is 8-12 mL / s relative to 1000 mL of additive mixture; And / or, the particle size of the atomized additive mixture in the ultrasonic atomizing spray is ≤5μm; And / or, the spraying amount of the additive mixture is 1.5-2.5 wt% of the total mass of the polyethylene base material.

11. The method of claim 9, wherein, The particle size of the atomized additive mixture in the ultrasonic atomized spray is 2-4 μm; And / or, the conditions for the ultrasonic atomizing spray include: a temperature of 68-72℃; and an ultrasonic frequency of 22-28Hz; And / or, the pulse spraying method includes: spraying the additive mixture for 4-6 seconds every 20-40 seconds, and then adjusting the relative pressure of the vacuum conditions to -6 kPa to -8 kPa.

12. The method according to claim 1, wherein, The second dust removal method includes: after the cooled material is vibrated and sieved to remove coarse powder, it is then ultrasonically purged. After the second dust removal is completed, the dust content on the surface of the cross-linked polyethylene product can be 1-2 mg / kg. And / or, the second optical detection method includes: using an industrial camera with a resolution of 5-8 megapixels and light sources of white light, ultraviolet light, and infrared light to photograph the material from an angle of 30-60°, and using an AI image recognition algorithm to identify impurities of different particle sizes and remove the impurities.

13. The method of claim 12, wherein, The vibration frequency of the vibratory sieve for removing coarse powder is 15-20Hz, and the sieve aperture is 100-150μm. And / or, the frequency of the ultrasonic purging is 20-30 kHz; And / or, when the dust content on the surface of polyethylene base material detected by laser dust detection exceeds 2mg / kg, the unqualified material is sent back to the second dust removal unit for processing.

14. Crosslinked polyethylene prepared by the method of any one of claims 1-13.

15. The use of cross-linked polyethylene as an insulating material in cables according to claim 14.

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