A granulation process for polyethylene spray-wound modified material

Abstract

The application discloses a granulation process of polyethylene spray winding modified material, relates to the technical field of polymer material processing, and comprises the following steps: high-density polyethylene is mixed with linear low-density polyethylene, and then the mixture is melt-extruded through a double-screw extruder; first to fourth temperature zones are arranged along the axial direction of the screw in sequence, so that a wave-shaped temperature curve of temperature rising, temperature falling and temperature rising again is formed; a composite antioxidant, which is composed of a hindered phenol and a phosphite antioxidant at a mass ratio of 1:2, is fed into the third temperature zone side; free radicals are captured in situ in the temperature falling and homogenizing zone to inhibit thermal-oxidative aging of the material; the extruded material is sent into an underwater cutting chamber; the cutting is performed according to the linkage relationship among the cutting temperature, the rotating speed and the cooling water temperature; the sphericity and the particle size distribution of the particles are controlled; the smoothness of the material flow and the stability of the film width are ensured; and the finished product is obtained through centrifugal drying and homogenization after cutting. The application cooperatively regulates the thermal history and the particle morphology, so that the modified material melt strength is stable, the melting is uniform, the material flow is smooth during spray construction, the film is uniform, and the comprehensive performance is improved.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, specifically to a granulation process for polyethylene spray-coating and winding modified materials. Background Technology

[0002] Modified polyethylene is widely used as a primary raw material in spray coating and winding processes, particularly in applications such as insulated pipes, corrosion-resistant pipes, and cable sheaths. The spray coating and winding process requires the modified material to exhibit stable feeding flow, suitable melt strength, and consistent film width during application. These properties are closely related to the particle morphology, thermal history, and melting behavior of the modified material.

[0003] In the prior art, invention patent publication number CN112497795A discloses a production process for high-density polyethylene (HDPE) composite spiral wound pipes and the resulting spiral wound pipes. The granulation step involves adding talc to HDPE, melting it at 160°C to 190°C, cooling it to 70°C to 100°C, adding stearic acid, extruding it through a screw extruder, cooling, and granulating. This process uses a single high-temperature range during the melting stage and does not implement segmented temperature control throughout the granulation process. Another invention patent publication number CN103275375A discloses a special material for ultra-high molecular weight polyethylene (UHMWPE) thermal spraying and its preparation method, which involves mixing the components, melting and plasticizing them through a screw extruder, granulating, and then crushing and sieving. This technology focuses on the formulation design and the final product's spraying performance, but does not address the control of the material's thermal history and particle morphology by the temperature curve during the granulation process.

[0004] The aforementioned existing technologies all employ constant temperature or simple gradient temperature granulation methods, without considering the thermo-oxidative degradation of polyethylene modified materials caused by continuous heating during melt extrusion, and without establishing the correlation between granulation process parameters and spray coating / wrapping performance. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a granulation process for polyethylene spray-coated and wound modified material. By setting four functional temperature zones along the screw axis to form a wave-shaped temperature curve, and introducing a composite antioxidant system in situ in the cooling and homogenization zone, combined with the three-parameter linkage control of the pelletizing process, the thermal history and particle morphology of the modified material can be synergistically regulated.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a granulation process for polyethylene spray-coated and wound modified material, the process comprising the following steps: S1, a mixture of high-density polyethylene and linear low-density polyethylene is melt-extruded; S2, melt extrusion uses a twin-screw extruder with four functional temperature zones set sequentially along the screw axis. The first temperature zone is 150℃ to 165℃, the second temperature zone is 175℃ to 190℃, the third temperature zone is 155℃ to 170℃, and the fourth temperature zone is 170℃ to 185℃. These four functional temperature zones form a wave-shaped temperature curve of heating, cooling, and reheating, which is used to reduce the thermo-oxidative degradation of polyethylene molecular chains during continuous heating. S3, the third temperature zone is equipped with a side feeding port, through which a composite antioxidant system of hindered phenolic antioxidant and phosphite antioxidant in a mass ratio of 1:2 is added. This composite antioxidant system captures free radicals generated by the slowing down of molecular chain movement in situ in the cooling homogenization zone, so as to inhibit the thermo-oxidative aging of the material. S4, the melt-extruded material is fed into an underwater pelletizing chamber for pelletizing. During the pelletizing process, the pelletizing temperature, pelletizing speed, and cooling water temperature meet the following requirements: ,in This refers to the pelleting temperature, expressed in °C. This refers to the pelletizing speed, expressed in r / min. The temperature is the cooling water temperature, in °C. This three-parameter linkage is used to control the sphericity of the particles above 0.85 and the particle size distribution within the range of 2mm to 4mm, so as to ensure the smoothness of the spray coating and the stability of the film width. S5, the particles cut out by the underwater pelletizing chamber are dehydrated and dried by a centrifugal dryer.

[0007] Further, in S1, the high-density polyethylene has a melt index of 0.15 g / 10 min to 0.50 g / 10 min and a density of 0.945 g / cm³ to 0.965 g / cm³, the linear low-density polyethylene has a melt index of 0.5 g / 10 min to 2.0 g / 10 min and a density of 0.918 g / cm³ to 0.935 g / cm³, and the mass ratio of the high-density polyethylene to the linear low-density polyethylene is 6:4 to 8:2.

[0008] Furthermore, in S1, the mixture of high-density polyethylene and linear low-density polyethylene is dried at 80°C to 100°C for 2 to 4 hours before being fed into the twin-screw extruder, so that the moisture content is controlled below 0.05%.

[0009] Furthermore, in step S1, carbon black masterbatch, main antioxidant, ultraviolet absorber and lubricant are added to the mixture of high-density polyethylene and linear low-density polyethylene, and the mixture is mixed for 8 min to 12 min at a rotation speed of 800 r / min to 1000 r / min and a temperature of 80°C to 100°C to obtain a premix. In step S3, the hindered phenolic antioxidants and phosphite antioxidants in the composite antioxidant system account for 0.05% to 0.10% of the total mass of the premix.

[0010] Furthermore, in S2, the heating rate of the first temperature zone is 3℃ / min to 5℃ / min, the heating rate of the second temperature zone is 2℃ / min to 4℃ / min, the temperature of the third temperature zone is 15℃ to 25℃ lower than that of the second temperature zone, and the cooling rate is 3℃ / min to 5℃ / min, and the temperature of the fourth temperature zone is 10℃ to 20℃ higher than that of the third temperature zone, and the heating rate is 2℃ / min to 3℃ / min.

[0011] Furthermore, in S2, the screw speed of the twin-screw extruder is 180 r / min to 280 r / min, the die head melt pressure is 3 MPa to 8 MPa, and the length-to-diameter ratio of the twin-screw extruder is ≥40. This length-to-diameter ratio and pressure range ensure sufficient mixing of the melt under low-temperature granulation conditions.

[0012] Furthermore, in S4, the pelletizing temperature of the underwater pelletizing chamber is 40°C to 60°C, the pelletizing speed is 500 r / min to 800 r / min, and the cooling water temperature is 15°C to 25°C.

[0013] Furthermore, in step S5, the centrifugal dryer operates at a speed of 800 r / min to 1200 r / min and a hot air temperature of 60°C to 80°C. The dried particles enter a homogenization tank and are stirred and homogenized at a speed of 10 r / min to 20 r / min at a temperature of 60°C to 80°C for 30 min to 60 min. This homogenization process is used to make the composition and properties of particles at different locations more consistent and to reduce performance fluctuations between batches.

[0014] Furthermore, in S2, the temperature fluctuation of each temperature zone of the twin-screw extruder is controlled within ±2℃. This temperature fluctuation control range is used to ensure the accuracy and repeatability of the wavy temperature curve.

[0015] Furthermore, in S4, the particles cut by the underwater pelletizing chamber have a particle size of 2mm to 4mm, a particle sphericity ≥0.85, and a bulk density of 0.58g / cm³ to 0.62g / cm³. These particle morphology parameters are used to achieve a stable feeding rate and a uniform melting rate during spray coating and wrapping operations.

[0016] Compared with existing technologies, the granulation process for this polyethylene spray-coating and wrapping modified material has the following advantages: I. This invention constructs a wave-shaped temperature curve of heating, cooling, and reheating along the axial direction of a twin-screw extruder, and introduces a hindered phenolic and phosphite composite antioxidant system in situ in the cooling homogenization zone. This allows the material to undergo a controllable, non-monotonic thermal process during extrusion, effectively slowing down the thermo-oxidative degradation rate of polyethylene molecular chains under sustained high temperatures and reducing the amount of free radicals generated during processing. The resulting modified material has a higher oxidation induction temperature and a narrower melt index fluctuation range, resulting in more stable and consistent melt strength. This fundamentally improves the problem of batch-to-batch performance differences caused by uncontrolled thermal history, providing a reliable quality foundation for subsequent spraying and wrapping construction.

[0017] Second, this invention establishes a quantitative linkage between pelletizing temperature, pelletizing speed, and cooling water temperature, and combines underwater pelletizing technology to precisely control the pellet forming process. This allows the sphericity and particle size distribution of the pellets to be stably maintained within a suitable range for spray coating and winding. The optimization of the particle morphology improves the flow behavior of the material in the hopper of the spraying machine, resulting in a more uniform feeding rate. The melting behavior of the pellets in the heating zone tends to be synchronized. This synergistic effect directly improves the stability of the film width and the uniformity of the thickness during the spray coating and winding process, reducing process interruptions and product quality defects caused by poor feeding or asynchronous melting.

[0018] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0020] Figure 1 This is a schematic diagram of the temperature zone of the twin-screw extruder of the present invention; Figure 2 This is a schematic diagram of the three-parameter linkage control principle for underwater pelletizing according to the present invention; Figure 3 This is a flowchart of the overall granulation process of the present invention. Detailed Implementation

[0021] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0022] In related technologies, the granulation process of polyethylene spray-coated modified materials mainly focuses on the selection of formulation components and the macroscopic performance testing of the final product, while research on the thermal history control of the granulation process itself and its intrinsic relationship with particle morphology and spray coating performance is relatively limited. Long-term production practice has revealed that when polyethylene melt is kept at high temperatures in a twin-screw extruder for extended periods, the molecular chains undergo a certain degree of thermo-oxidative degradation, resulting in a wider molecular weight distribution and unstable melt strength, which in turn affects the uniformity of the film during spray coating.

[0023] Through systematic experiments and analysis, a method is proposed to intervene in the thermal process of molecular chains by constructing a wave-shaped temperature field. The core idea is to allow the material to undergo a controllable cooling and reheating process during extrusion, rather than continuous heating or maintaining a constant temperature.

[0024] This non-monotonic temperature change can alter the generation kinetics of free radicals and achieve more effective capture by utilizing a specific ratio of antioxidants during the cooling phase. Furthermore, establishing a quantitative linkage between temperature, rotation speed, and cooling water temperature in the underwater pelletizing process allows for precise control of particle sphericity and size distribution. These two parameters directly affect the flow behavior and heating / melting behavior of particles within the sprayer hopper. The synergistic effect of these three aspects constitutes the complete technical solution of this process. The following detailed explanation of each step is provided in conjunction with the specific implementation process.

[0025] Step S1, Raw Material Selection and Pretreatment: The high-density polyethylene used in this embodiment is a commercially available pipe material grade product. Its melt index, tested at 190℃ and a load of 2.16 kg, is 0.15 g / 10 min to 0.50 g / 10 min, and its density is 0.945 g / cm³ to 0.965 g / cm³. This melt index range is chosen because polyethylene with a lower melt index has higher melt strength and better mechanical properties, meeting the requirements of pipe anti-corrosion layers for material toughness and durability. The linear low-density polyethylene has a melt index of 0.5 g / 10 min to 2.0 g / 10 min and a density of 0.918 g / cm³ to 0.935 g / cm³. Its main function is to improve the material's flexibility and processing fluidity, preventing melt fracture during high-speed spraying. The mass ratio of the two is between 6:4 and 8:2. If the proportion of linear low-density polyethylene is too low, the modified material will lack flexibility, and the film will easily crack at low temperatures; if the proportion is too high, the melt strength will decrease, and the film width will be difficult to control stably during winding.

[0026] In a specific implementation scenario, 70 kg of high-density polyethylene and 30 kg of linear low-density polyethylene were weighed. Both base resins underwent visual inspection before weighing to confirm the absence of impurities and moisture-induced clumping. The weighed materials were then fed into a high-speed mixer equipped with a jacketed heating system. Before mixing, the materials were dried at 80℃ to 100℃ for 2 to 4 hours. A hot air circulating drying chamber was used, with the material layer thickness ≤ 5 cm to ensure uniform hot air penetration. After drying, the moisture content was measured using a Karl Fischer moisture analyzer; a moisture content ≤ 0.05% was considered acceptable. Moisture control is crucial for the stability of the subsequent extrusion process. Excessive moisture will vaporize and generate bubbles within the extruder, causing melt pressure fluctuations and porosity defects within the particles.

[0027] After drying, carbon black masterbatch, primary antioxidant, UV absorber, and lubricant are added to the base resin. The carbon black masterbatch improves the product's weather resistance and UV aging resistance. The primary antioxidant is a conventional hindered phenolic antioxidant, providing basic thermo-oxidative stability during processing. The UV absorber is a benzophenone or benzotriazole compound. The lubricant is calcium stearate or polyethylene wax, etc., to improve processing fluidity. The addition amounts of these additives follow industry standard proportions: typically, the carbon black masterbatch is 2% to 5% of the total mass of the base resin, the primary antioxidant is 0.1% to 0.3%, the UV absorber is 0.1% to 0.5%, and the lubricant is 0.1% to 0.5%. Mixing is carried out at a speed of 800 to 1000 rpm and a temperature of 80°C to 100°C for 8 to 12 minutes. The mixing temperature is set to ensure the lubricant fully melts and evenly coats the resin particles, while avoiding excessively high temperatures that could cause premature consumption of the antioxidant. After mixing, the resulting premix is ​​transferred to a sealed storage tank for temporary storage to prevent moisture absorption.

[0028] Step S2, Construction and Control of Functional Temperature Zones of the Twin-Screw Extruder: The melt extrusion process uses a co-rotating twin-screw extruder with an aspect ratio ≥ 40. A large aspect ratio is chosen to ensure sufficient shearing and mixing action from the long stroke of the screw at relatively low overall processing temperatures, allowing for thorough plasticization of the material. Insufficient aspect ratio may lead to poor plasticization and uneven dispersion of additives at low temperatures. In terms of screw structure design, a large-lead conveying thread element is used in the first temperature zone, while kneading blocks, reverse thread elements, and toothed disc elements are sequentially configured in the second, third, and fourth temperature zones to achieve functional zones for solid conveying, melt plasticizing, dispersion mixing, and homogenization pressure building.

[0029] like Figure 1As shown, four functional temperature zones are sequentially arranged along the screw axis. The first temperature zone is located in the feeding section and melting initiation section of the extruder barrel, the second temperature zone is located in the melting and plasticizing section, the third temperature zone is located in the front part of the homogenization section, and the fourth temperature zone is located in the rear part of the homogenization section up to the die head. The temperature settings of the four temperature zones are not simply increasing or decreasing, but rather present a wave-shaped curve of first heating up, then cooling down, and then heating up again.

[0030] Specifically, the temperature in the first temperature zone is set to 150℃ to 165℃. This temperature range is below the complete melt temperature of high-density polyethylene, and the material is in a partially molten state in this section. The solid bed is gradually broken and compacted by the shearing action of the screw. The heating rate in the first temperature zone is controlled at 3℃ / min to 5℃ / min. The heating rate is controlled by adjusting the power output curve of the barrel heater. The extruder's temperature control system can set the heating time for each section. The operator inputs the heating program before starting the machine, and the system executes it automatically.

[0031] The second temperature zone is set at 175℃ to 190℃. This temperature is higher than the crystallization melting peak temperature of high-density polyethylene, allowing the material to fully melt in this zone, and the molecular chains to gain sufficient mobility for thorough mixing. The heating rate in the second zone is controlled at 2℃ / min to 4℃ / min, slightly slower than the first zone. This is because the material has transitioned from a solid to a viscous flow state in this zone, increasing its specific heat capacity and heat absorption. Appropriately reducing the heating rate helps to ensure precise temperature control and avoid overshoot. Within the second temperature zone, the kneading blocks in the screw assembly provide high shear force, allowing the carbon black masterbatch and other additives to be fully dispersed in the melt.

[0032] The third temperature zone is set at 155℃ to 170℃. The temperature in the third zone is 15℃ to 25℃ lower than that in the second zone, with a cooling rate controlled at 3℃ / min to 5℃ / min. Cooling is achieved by reducing the input power of the cylinder heater in this zone and using an air-cooled or water-cooled jacket. In this zone, the material melt temperature decreases moderately, the melt viscosity increases slightly, and the thermal motion of the molecular chains slows down. This cooling range was determined through repeated experiments. If the cooling range is too small, the slowing down of molecular chain motion is insufficient, the rate of free radical generation does not change significantly, and the in-situ capture effect of the subsequent antioxidant is greatly reduced; if the cooling range is too large, the melt viscosity is too high, the flow resistance increases sharply, resulting in insufficient die head pressure, affecting extrusion stability and the continuity of subsequent pelletizing. Functionally, the third temperature zone is defined as a cooling homogenization zone, and its core function is to create a thermodynamic environment where molecular chain motion is relatively slowed down.

[0033] The fourth temperature zone is set at 170℃ to 185℃. The temperature in the fourth zone is 10℃ to 20℃ higher than the third zone, with a heating rate controlled at 2℃ / min to 3℃ / min. The purpose of this heating is to restore the melt temperature to a suitable processing temperature window before the material enters the die head and pelletizing chamber, ensuring the melt has appropriate fluidity and viscoelasticity for subsequent extrusion molding and pelletizing. This heating amplitude is smaller than the cooling amplitude between the second and third zones, its significance being to only moderately improve fluidity without disrupting the thermal history established in the third zone. The entire temperature curve resembles a peak followed by a trough, then a small peak, hence the name "wavy temperature curve."

[0034] Throughout the entire operation, temperature fluctuations in each zone are ≤2℃. The key to achieving this precision lies in the extruder being equipped with a PID self-tuning temperature control module, and each production line undergoing temperature control system calibration before startup. Thermocouples in the temperature control system are calibrated monthly using standard thermometers to ensure accurate readings. If temperature fluctuations exceed 2℃, the wavy temperature curve will be distorted, causing the material's thermal history to deviate from the preset path, thus affecting the thermal stability of the final modified material and the repeatability of its spray application performance.

[0035] The screw speed of the twin-screw extruder is set between 180 rpm and 280 rpm. Within this speed range, the melt pressure at the die head is maintained between 3 MPa and 8 MPa. The melt pressure is monitored in real time by a pressure sensor installed at the die head and fed back to the control system. If the pressure is too low, it indicates that the melt viscosity is too low or the feed rate is insufficient, which may result in smaller pellet size or irregular shape; if the pressure is too high, it increases the load on the equipment and may cause melt fracture. The proper matching of screw speed and melt pressure allows for the production of a uniform and dense melt even at relatively low processing temperatures.

[0036] Step S3, in-situ introduction of the composite antioxidant system: A side feed port is opened at the location of the barrel corresponding to the third temperature zone, and the composite antioxidant system is introduced into the melt through this side feed port. The location of the side feed port in the cooling and homogenization zone, rather than adding it together in the feeding section, has specific technical reasons. If all the antioxidant is added in the feeding section, it will be continuously consumed throughout the high-temperature extrusion process, and by the time the material is finally formed, the residual antioxidant capacity will be greatly weakened. Adding part of the antioxidant, especially the composite antioxidant system, in-situ from the third temperature zone ensures that the antioxidant appears at the critical nodes where molecular chain movement slows down and the rate of free radical generation decreases, achieving efficient point-to-point capture.

[0037] The composite antioxidant system is composed of hindered phenolic antioxidants and phosphite antioxidants in a 1:2 mass ratio. The hindered phenolic antioxidant is a commercially available product, such as pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], whose mechanism of action is the termination of free radical chain reactions by donating hydrogen atoms from the phenolic hydroxyl group. The phosphite antioxidant is a commercially available product, such as tris(2,4-di-tert-butylphenyl) phosphite, whose mechanism of action is the decomposition of hydroperoxides into stable non-free radical products. The synergistic use of the two produces a significant synergistic effect. The 1:2 mass ratio was determined after multiple oxidation induction time tests using a rotational rheometer and differential scanning calorimeter, resulting in the longest oxidation induction time. If the ratio is 1:1 or 2:1, the oxidation induction time decreases by approximately 15% and 22%, respectively. This phenomenon is related to the action mechanism and reaction rate constant of the two antioxidants. A 1:2 ratio allows for a good kinetic match between the chain termination reaction of the primary antioxidant and the hydroperoxide decomposition reaction of the secondary antioxidant. The amount of the composite antioxidant system added is 0.05% to 0.10% of the total mass of the premix. This range ensures sufficient antioxidant protection while avoiding excessive addition that would increase costs or have adverse effects on melt rheology.

[0038] The side-feeding device employs a precision loss-in-weight side-feeder, achieving a feeding accuracy of ±0.5%. The composite antioxidant is pre-mixed uniformly in a V-type mixer before being added to the storage hopper of the side-feeder. The screw speed of the side-feeder is set in an empirical ratio with that of the main screw speed of the extruder to ensure that the mass of composite antioxidant added per unit time matches the melt mass flow rate. In the third temperature zone, the melt is in a viscous flow state, and the powdered antioxidant injected by the side-feeder is rapidly dispersed into the melt under the stirring action of the screw. Because the melt temperature is lower than that in the second temperature zone, the molecular chain movement slows down, thus reducing the number of newly generated free radicals due to thermal initiation. At this time, the composite antioxidant can effectively capture the residual free radicals and decompose the already generated hydroperoxides, thereby inhibiting the thermo-oxidative aging process under continuous heating conditions at its source.

[0039] Step S4, Underwater Pelletizing Three-Parameter Linkage Control: After the melt is extruded through the extruder head, it enters the underwater pelletizing chamber for pelletizing. The core components of the underwater pelletizer include the pelletizing die, rotary cutter, water circulation system, and pellet conveying pipeline. The melt is extruded through the die holes of the die to form several strips. The rotary cutter rotates at high speed close to the die surface to cut the strips into pellets. At the same time, circulating cooling water rapidly cools the pellets and conveys them to the subsequent dewatering device.

[0040] In this stage, the pelleting temperature, pelleting speed, and cooling water temperature are not independent of each other, but rather follow a linked relationship. Through extensive orthogonal experiments and data regression analysis, the applicant established an empirical relationship among the three: In the formula, The unit is ℃. The unit is r / min. The unit is °C. This formula is the core guideline for setting underwater pelletizing process parameters.

[0041] The working principle of this relationship can be understood as follows: pelleting temperature. This refers to the combined characteristic of the ambient temperature and the particle's own temperature at the moment of being cut. Pelletizing speed. The higher the value, the higher the shear rate of the cutter on the molten material. Due to the heat generated by shearing, the particle temperature will rise. In the formula... This reflects the positive correlation. Cooling water temperature The lower the value, the higher the efficiency of heat removal, and the lower the particle temperature. (In the formula...) The term reflects this negative correlation. (Constant term) It is the baseline offset value.

[0042] In a specific implementation scenario, the operator first selects the target pelletizing temperature and pelletizing speed based on the production task. For example, setting the pelletizing speed to 600 r / min and the cooling water temperature to 20℃, the parameters are calculated based on the formula. =50 + 0.02 × 600 − 0.5 × 20 = 50 + 12 − 10 = 52℃. This temperature falls within the suitable range of 40℃ to 60℃ and can be implemented. If the calculated... If the temperature deviates from the range of 40℃ to 60℃, the pelletizing speed or cooling water temperature needs to be adjusted and recalculated until it falls within the allowable range.

[0043] The goal of the three-parameter linkage control is to control the sphericity of particles above 0.85 and concentrate the particle size distribution within the range of 2mm to 4mm. Particle sphericity is defined as the ratio of the surface area of ​​an equal-volume sphere to the actual surface area of ​​the particle, and is measured using a dynamic image particle analyzer. If the particle sphericity is low, i.e., the particle shape is irregular, appearing as flakes, strips, or with tails, the interlocking phenomenon between particles in the hopper of the sprayer will increase, resulting in poor flowability and unstable feeding rate, directly manifesting as fluctuations in the thickness and width of the sprayed film. An excessively wide particle size distribution, with significant differences in the pneumatic conveying behavior of large and small particles in the conveying pipeline, will also cause uneven feeding. The particle size distribution is determined using the standard sieving method. A 100g sample is sieved on a vibrating screen, the residue on each sieve layer is weighed, and the mass percentage is calculated.

[0044] To achieve the aforementioned particle morphology parameters, in addition to following the linkage formula, attention must also be paid to the die orifice diameter design. The die orifice diameter should be calculated in reverse based on the target particle size and material shrinkage rate. Polyethylene undergoes volume shrinkage when cooled from the molten state to room temperature, therefore the die orifice diameter needs to be slightly larger than the target particle size. In this embodiment, the target particle size is 2mm to 4mm, and the die orifice diameter is designed to be 3.2mm to 3.5mm. Furthermore, the adhesion pressure between the cutter and the die must be uniform, and the sharpness of the cutter itself needs to be checked and replaced regularly; generally, the cutter should be replaced or re-sharpened every 100 tons of material produced.

[0045] The pelletizing temperature is determined by the circulating water temperature in the underwater pelletizing chamber and the frictional heat generated during the pelletizing process. In actual operation, temperature sensors are installed at the inlet and outlet of the pelletizing chamber for online monitoring. The pelletizing speed is adjusted by a frequency converter to regulate the frequency of the cutter motor, ranging from 500 r / min to 800 r / min. The cooling water temperature is controlled by a chiller unit in the cooling water circulation system, with a set temperature range of 15℃ to 25℃. When the cooling water temperature is <15℃, the pellets cool too quickly, potentially causing micro-cracks on the surface or brittle fracture of the melt during pelletizing, resulting in powder formation. When the cooling water temperature is >25℃, the pellets are insufficiently cooled and prone to sticking and deformation. Therefore, the cooling water system is equipped with a heating device and a chiller unit, allowing the water temperature to be adjusted to the set point in different seasons.

[0046] Step S5, Particle Post-processing and Homogenization: The mixture of particles and cooling water conveyed from the underwater pelletizing chamber enters a centrifugal dryer. The centrifugal dryer consists of a rotating drum and a fixed screen. The high-speed rotation of the drum generates centrifugal force, throwing water off the particle surface and discharging it through the screen. The dryer speed is set to 800 r / min to 1200 r / min. If the speed is too low, dehydration will be incomplete, and residual moisture on the particle surface will cause particles to stick together during subsequent homogenization; if the speed is too high, it may cause mechanical damage to the particles, reducing sphericity. A hot air system supplies hot air at 60°C to 80°C into the centrifugal dryer to assist drying and remove residual moisture. The moisture content of the dried particles is ≤0.05%, verified using a rapid moisture analyzer.

[0047] The dried granules are not directly packaged but instead enter a homogenization tank for homogenization. The homogenization tank is a stainless steel conical hopper equipped with a low-speed agitator. During a batch production process, slight fluctuations in melt temperature, pressure, and other parameters occur at the initial, middle, and later stages of extruder operation, resulting in slight differences in antioxidant content, melt index, and color among the granules cut at different times. The homogenization process achieves macroscopic uniform mixing of these granules from different times through stirring in the homogenization tank. The homogenization temperature is controlled between 60℃ and 80℃, the stirring speed is 10 to 20 rpm, and the homogenization time is 30 to 60 minutes. The homogenization tank is jacketed and can be insulated with hot water or heat transfer oil. The agitator speed should not be too high to avoid damaging the granule morphology. After homogenization, samples are taken from the top, middle, and bottom of the homogenization tank for melt index and ash content testing. A relative deviation of ≤5% between the two is considered acceptable for homogenization. After being cooled to <40℃, qualified materials are packaged in 25kg bags by an automatic packaging machine.

[0048] The following provides implementation data for three application scenarios to more specifically demonstrate the implementation methods and effects of this process under different conditions.

[0049] In Scenario 1, the production example of modified materials for standard anti-corrosion pipelines is as follows: The base resin mixture consisted of 70 kg of high-density polyethylene and 30 kg of linear low-density polyethylene, with the addition of 2.5 kg of carbon black masterbatch, 0.2 kg of primary antioxidant, 0.3 kg of UV absorber, and 0.2 kg of lubricant. The premix was mixed at 900 rpm for 10 minutes at 90°C. The extruder had a length-to-diameter ratio of 44, with four temperature zones set at 155°C, 180°C, 160°C, and 178°C. The screw speed was 220 rpm, and the die melt pressure was 4.5 MPa. A composite antioxidant was added to the feed in the third temperature zone, with a hindered phenolic to phosphite mass ratio of 1:2, totaling 0.08% of the premix. The pelleting speed was 650 rpm, and the cooling water temperature was 20°C. Based on the linkage relationship, the calculated pelleting temperature was approximately 53°C, while the actual measured temperature was 52.5°C. The centrifugal dryer was operated at 1000 rpm, with a hot air temperature of 70°C, a homogenization temperature of 70°C, a rotation speed of 15 rpm, and a homogenization time of 45 min. The resulting particles had a particle size of 2.5 mm to 3.5 mm, a sphericity of 0.89, a bulk density of 0.60 g / cm³, and a melt flow index of 0.35 g / 10 min. During the spray coating and wrapping process, the feed rate fluctuation was <3%, the film width was 560 mm, and the width fluctuation was <5 mm.

[0050] In Scenario 2, the production example of high melt index modified material for large-diameter thermal insulation pipes is as follows: The base resin ratio is 60 kg of high-density polyethylene and 40 kg of linear low-density polyethylene, with the proportion of linear low-density polyethylene increased to raise the melt flow index. 3.0 kg of carbon black masterbatch, 0.2 kg of primary antioxidant, 0.4 kg of UV absorber, and 0.3 kg of lubricant are added to the premix. Mixing conditions are the same as in scenario one. The four temperature zones are set at 152℃, 178℃, 158℃, and 175℃, with the third zone being 20℃ lower than the second. The screw speed is 250 r / min, and the die head melt pressure is 3.8 MPa. The composite antioxidant is added at 0.09% of the premix. The pelletizing speed is 720 r / min, the cooling water temperature is 18℃, the calculated pelletizing temperature is approximately 55.4℃, and the actual measured temperature is 55.1℃. The centrifugal dryer speed is 1100 r / min, the hot air temperature is 65℃, the homogenization temperature is 65℃, the speed is 18 r / min, and the homogenization time is 50 min. The resulting particles had a particle size of 2.0 mm to 3.5 mm, a sphericity of 0.87, a bulk density of 0.59 g / cm³, and a melt flow index of 0.65 g / 10 min. During construction, the feed rate fluctuation was <4%, the diaphragm width was 630 mm, and the width fluctuation was <6 mm.

[0051] In Scenario 3, the production example of high-flexibility modified material for cable sheathing is as follows: The base resin ratio is 65 kg of high-density polyethylene and 35 kg of linear low-density polyethylene. An additional 2 kg of ethylene-octene copolymer elastomer is added to further improve flexibility. The premix contains 2.0 kg of carbon black masterbatch, 0.15 kg of primary antioxidant, 0.2 kg of UV absorber, and 0.15 kg of lubricant. The four temperature zones are set at 158℃, 185℃, 163℃, and 180℃, with the third zone being 22℃ lower than the second. The screw speed is 200 r / min, and the die head melt pressure is 5.2 MPa. The composite antioxidant accounts for 0.07% of the premix. The pelletizing speed is 580 r / min, the cooling water temperature is 22℃, the calculated pelletizing temperature is approximately 50.6℃, and the actual measured temperature is 50.8℃. The centrifugal dryer speed is 950 r / min, the hot air temperature is 72℃, the homogenization temperature is 72℃, the speed is 12 r / min, and the homogenization time is 55 min. The resulting particles had a particle size of 2.5 mm to 4.0 mm, a sphericity of 0.86, a bulk density of 0.61 g / cm³, a melt flow index of 0.45 g / 10 min, and an elongation at break that was approximately 15% higher than that of the product in Scenario 1. During construction, the material was fed smoothly, the diaphragm exhibited good flexibility, a width of 540 mm, and a fluctuation of <5 mm.

[0052] To more intuitively illustrate the advantages of the wavy temperature curve in this process compared to conventional isothermal and linear heating processes, three comparative examples were set up. Comparative Example 1 used an isothermal granulation process, with all four temperature zones of the extruder set to 175℃; Comparative Example 2 used a linear heating granulation process, with the four temperature zones set to 150℃, 170℃, 180℃, and 190℃; the example used the wavy temperature curve process of Scenario 1. The raw material formulation, screw speed, and pelletizing parameters were kept consistent in all three experiments. The test items were the oxidation induction temperature of the particles, the batch-to-batch deviation of the melt index, and the sphericity of the particles. The test results are shown in the table below.

[0053] Table 1 shows a comparison of particle properties using different granulation processes:

[0054] As shown in the table, the modified material particles granulated using the wavy temperature curve process exhibited an oxidation induction temperature 20℃ higher than that obtained by the isothermal process and 12℃ higher than that obtained by the linear heating process, indicating a significant enhancement in their thermo-oxidative stability. The batch-to-batch deviation of the melt index decreased to 4.3%, signifying better material uniformity, which directly contributes to the stability of melting behavior during spray coating and winding. The particle sphericity improved from 0.78 to 0.89, and combined with three-parameter linked pelletizing control, the particle morphology became closer to spherical, resulting in substantial improvement in flowability. These results demonstrate that the synergistic effect of the wavy temperature curve, the in-situ introduction of composite antioxidants, and the three-parameter linked pelletizing effectively regulates the thermal history and particle morphology of modified polyethylene materials, thereby providing downstream spray coating and winding applications with modified material products that offer more stable performance and a wider processing window.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A granulation process for a polyethylene spray-coating and wrapping modified material, characterized in that, The process includes the following steps: S1, a mixture of high-density polyethylene and linear low-density polyethylene is melt-extruded; S2, melt extrusion uses a twin-screw extruder, with four functional temperature zones set sequentially along the screw axis. The first temperature zone is 150℃ to 165℃, the second temperature zone is 175℃ to 190℃, the third temperature zone is 155℃ to 170℃, and the fourth temperature zone is 170℃ to 185℃. S3, the third temperature zone is equipped with a side feeding port, through which a composite antioxidant system with a mass ratio of hindered phenolic antioxidant and phosphite antioxidant of 1:2 is added; S4, the melt-extruded material is fed into an underwater pelletizing chamber for pelletizing. During the pelletizing process, the pelletizing temperature, pelletizing speed, and cooling water temperature meet the following requirements: ,in This refers to the pelleting temperature, expressed in °C. This refers to the pelletizing speed, expressed in r / min. This refers to the cooling water temperature, expressed in °C. S5, the particles cut out by the underwater pelletizing chamber are dehydrated and dried by a centrifugal dryer.

2. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S1, the high-density polyethylene has a melt index of 0.15 g / 10 min to 0.50 g / 10 min and a density of 0.945 g / cm³ to 0.965 g / cm³, the linear low-density polyethylene has a melt index of 0.5 g / 10 min to 2.0 g / 10 min and a density of 0.918 g / cm³ to 0.935 g / cm³, and the mass ratio of the high-density polyethylene to the linear low-density polyethylene is 6:4 to 8:

2.

3. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S1, the mixture of high-density polyethylene and linear low-density polyethylene is dried at 80°C to 100°C for 2 to 4 hours before being fed into the twin-screw extruder, so that the moisture content is controlled below 0.05%.

4. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S1, carbon black masterbatch, main antioxidant, ultraviolet absorber and lubricant are also added to the mixture of high-density polyethylene and linear low-density polyethylene. The mixture is mixed for 8 min to 12 min at a rotation speed of 800 r / min to 1000 r / min and a temperature of 80°C to 100°C to obtain a premix. In step S3, the hindered phenolic antioxidants and phosphite antioxidants in the composite antioxidant system account for 0.05% to 0.10% of the total mass of the premix.

5. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S2, the heating rate of the first temperature zone is 3°C / min to 5°C / min, the heating rate of the second temperature zone is 2°C / min to 4°C / min, the temperature of the third temperature zone is 15°C to 25°C lower than that of the second temperature zone, and the cooling rate is 3°C / min to 5°C / min. The temperature of the fourth temperature zone is 10°C to 20°C higher than that of the third temperature zone, and the heating rate is 2°C / min to 3°C / min.

6. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In S2, the screw speed of the twin-screw extruder is 180 r / min to 280 r / min, the die head melt pressure is 3 MPa to 8 MPa, and the length-to-diameter ratio of the twin-screw extruder is ≥40.

7. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S4, the pelletizing temperature of the underwater pelletizing chamber is 40°C to 60°C, the pelletizing speed is 500 r / min to 800 r / min, and the cooling water temperature is 15°C to 25°C.

8. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S5, the centrifugal dryer rotates at a speed of 800 r / min to 1200 r / min, and the hot air temperature is 60°C to 80°C. The dried particles enter a homogenization tank and are stirred and homogenized at a speed of 10 r / min to 20 r / min at a temperature of 60°C to 80°C for 30 min to 60 min.

9. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S2, the temperature fluctuation of each temperature zone of the twin-screw extruder is controlled within ±2℃.

10. The granulation process for a polyethylene spray-coating and winding modified material according to claim 1, characterized in that, In step S4, the particles cut by the underwater pelletizing chamber have a particle size of 2 mm to 4 mm, a particle sphericity ≥ 0.85, and a bulk density of 0.58 g / cm³ to 0.62 g / cm³.

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