Lightweight hollow articles formed based on polyethylene rotoplastic-gradient density and methods

Abstract

The invention relates to the technical field of high polymer material processing, and particularly discloses a polyethylene rotational plastic-gradient density-based lightweight hollow product and a method thereof.The polyethylene rotational plastic-gradient density-based lightweight hollow product is prepared from high-density polyethylene, linear low-density polyethylene, nano-silica, carbon nanotubes, a flexibilizer, PE wax and an antioxidant through blending, granulation and grinding; placing the powder in a mold, heating, melting and spinning to form an outer high-density enhancement layer; the preparation method comprises the following steps: blending low-density polyethylene, high-density polyethylene, a microcapsule foaming agent, a compatibilizer and an antioxidant, granulating and grinding; the powder is placed in a mold to continue to be heated and rotated till fusion is completed; cooling to trigger the microcapsule foaming agent to decompose, and reducing the rotating speed until an inner low-density foaming layer is formed on the inner wall of the outer high-density enhancement layer; and performing water-cooling on the outer-layer step, and performing air-cooling on the inner-layer step to obtain a light-weight hollow finished product. The multifunctional requirements for light weight, high strength and heat insulation can be met at the same time, and densification, foaming and gradient curing are completed in a single period.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer material processing, and particularly relates to a lightweight hollow product and method based on polyethylene rotational molding - gradient density formation. Background Art

[0002] Rotational molding is a hollow product molding process in which plastic powder is uniformly melted and adhered to the inner wall of a mold by heating the mold and rotating it along two axes. Its core advantages include low mold cost, uniform wall thickness, and suitability for manufacturing complex structures and large products. Since its development in the 1940s, rotational molding technology has been widely used in fields such as containers, automotive parts, and aerospace.

[0003] Traditional rotational molding processes mainly use a single polyethylene material (such as HDPE, LLDPE), and the resulting products have a uniform density (0.92 g / cm3 - 0.96 g / cm3), making it difficult to simultaneously meet the multi-functional requirements of lightweight, high strength, and heat insulation. If the density is reduced by foaming (such as adding a chemical foaming agent), the mechanical properties will be significantly reduced (the tensile strength is reduced by more than 50%), and the foaming uniformity is poor. For multi-layer composite rotational molding processes, staged feeding is required, with low efficiency and easy delamination at the interlayer interface (the peel strength ≤ 3 N / mm), making it difficult to achieve continuous gradient transition. Existing technologies mostly use the step-by-step feeding method to achieve multi-layer structures, but due to the difference in thermal expansion coefficients between different material layers, interface delamination is likely to occur, and multiple mold openings and feedings are required, resulting in low process efficiency and high costs. In addition, the penetration depth of physical foaming agents (such as nitrogen) in rotational molding is uncontrollable, resulting in discontinuous density gradients; although injection or extrusion foaming processes can achieve density gradients, they are limited by the mold complexity and molding pressure, making it difficult to prepare large hollow parts (such as storage tanks, ship hulls).

[0004] The demand for lightweight multi-functional materials in China's new energy equipment (such as liquid hydrogen storage tanks, lithium battery packs) has increased sharply. The market size of high-performance rotational molding products will exceed 12 billion yuan in 2025, but there are still some gaps in domestic technology at present. The main core contradiction is that it is difficult to achieve the coordination of lightweight (target density ≤ 0.6 g / cm3) and functionality (outer layer compressive strength ≥ 25 MPa, inner layer thermal conductivity ≤ 0.1 W / m·K), which severely restricts its application in high-end scenarios such as liquid hydrogen storage tanks and electric vehicle battery packs.

[0005] Therefore, in the current preparation process of lightweight multi-functional materials, there are problems such as difficulty in simultaneously meeting the multi-functional requirements of lightweight, high strength, and heat insulation, and the need for multiple mold openings and feedings, resulting in low process efficiency. Summary of the Invention

[0006] In order to solve the above technical problems existing in the preparation process of existing lightweight and multifunctional materials, the present invention provides a method for forming a lightweight hollow product based on polyethylene rotational molding plastic - gradient density, which can simultaneously meet the multifunctional requirements of lightweight, high strength and heat insulation, complete densification, foaming and gradient curing in a single cycle, and does not require multiple mold opening and feeding, and has the characteristics of high process efficiency.

[0007] The first technical solution of the present invention: A method for preparing a lightweight hollow product formed based on polyethylene rotational molding plastic - gradient density, comprising the following steps,

[0008] (S01) Take an appropriate amount of high - density polyethylene and linear low - density polyethylene and mix them as the first matrix;

[0009] (S02) After adding an appropriate amount of nano - silica, carbon nanotubes, toughening agent, PE wax and antioxidant to the first matrix in step (S01), successively carry out blending, granulation and powder grinding to obtain the first rotational molding powder;

[0010] (S03) Place the first rotational molding powder in step (S02) in a mold, heat it to melt, and rotate and adhere to the inner wall of the mold to form an outer high - density reinforcement layer;

[0011] The present invention selects an appropriate amount of high - density polyethylene as the matrix, which can better ensure the rigidity, chemical resistance and dimensional stability of the finally obtained outer high - density reinforcement layer; selects an appropriate amount of linear low - density polyethylene as the matrix, and by introducing a branched chain structure, ensures the impact resistance and flexibility of the finally obtained outer high - density reinforcement layer; adding an appropriate amount of nano - silica can improve the rigidity and wear resistance of the finally obtained outer high - density reinforcement layer and reduce the shrinkage rate; adding an appropriate amount of carbon nanotubes can improve the electrical / thermal conductivity of the finally obtained outer high - density reinforcement layer and enhance the impact resistance; adding an appropriate amount of toughening agent can improve the low - temperature toughness of the finally obtained outer high - density reinforcement layer and reduce brittle fracture; adding an appropriate amount of PE wax to increase the lubrication effect, improve the particle fluidity during the blending and granulation of each component, and ensure the uniform wall thickness of the finally obtained outer high - density reinforcement layer; adding an appropriate amount of antioxidant can prevent the oxidative degradation of the blended particles during the subsequent granulation and heating and melting processes; using a high - speed mixer to complete the blending between materials to ensure the uniform dispersion of nano - fillers; using a twin - screw extruder to complete the granulation process, ensuring the uniform mixing of each component in the subsequent first rotational molding powder, and better ensuring that after heating and melting and then rotating and adhering, an outer high - density reinforcement layer is formed, having better tensile strength and impact strength;

[0012] (S04) Take an appropriate amount of low - density polyethylene and high - density polyethylene and mix them as the second matrix;

[0013] (S05) After adding an appropriate amount of micro - capsule foaming agent composite to the second matrix in step (S04), obtain a composite;

[0014] (S06) After adding appropriate amounts of compatibilizer and antioxidant to the composite in step (S05), through blending, granulation, and milling in sequence, the second rotational molding powder is obtained.

[0015] (S07) Place the second rotational molding powder from step (S06) into the mold of step (S03), continue to heat up and rotate until melting is complete.

[0016] (S08) Lower the temperature to trigger the decomposition of the microcapsule blowing agent in the melt of step (S07), and at the same time reduce the rotation speed to form an inner low-density foamed layer on the inner wall surface of the outer high-density reinforced layer.

[0017] (S09) Using the zone cooling technology, stepwise water-cool the outer high-density reinforced layer in the mold, and stepwise air-cool the inner low-density foamed layer in the mold to obtain the finished lightweight hollow product.

[0018] In the present invention, an appropriate amount of low-density polyethylene is selected as the matrix, which can preferably provide high melt strength and is conducive to the stable formation of the inner low-density foamed layer; an appropriate amount of high-density polyethylene is selected as the matrix, which can well enhance the skeleton strength of the finally obtained inner low-density foamed layer and prevent cell collapse; adding an appropriate amount of microcapsule blowing agent achieves the effects of delayed foaming and precise foam control; adding an appropriate amount of compatibilizer can well improve the compatibility among low-density polyethylene, high-density polyethylene, and microcapsule blowing agent during the subsequent blending and granulation processes; adding an appropriate amount of antioxidant can prevent the oxidative degradation of the blended particles during the subsequent granulation and heating and melting processes; using a high-speed mixer to complete the blending between materials, and the molecular chains of low-density polyethylene degrade; using a twin-screw extruder to complete the granulation process, ensuring that the components in the subsequent second rotational molding powder are evenly mixed, and better ensuring the heat insulation and weight reduction performance of the subsequent inner low-density foamed layer formed by heating, melting, and rotating and adhering to the inner wall surface of the outer high-density reinforced layer; adding the second rotational molding powder when the outer high-density reinforced layer is not completely cooled to promote the mutual diffusion of molecular chains; in the present invention, stepwise cooling (descending to the decomposition temperature of the blowing agent at 5°C / min to 8°C / min) triggers the decomposition of the microcapsule blowing agent in the melt, and at the same time reduces the rotation speed, reduces the shear force, and improves the uniformity of cell diameter; in the present invention, using the zone cooling technology, stepwise water-cool the outer high-density reinforced layer in the mold to quickly fix the crystal structure of the outer high-density reinforced layer, and stepwise air-cool the inner low-density foamed layer in the mold to avoid cell shrinkage, maintain the open cell ratio <15%, when the temperature T > 100°C, adopt the method of synchronous temperature control of air cooling / water cooling to ensure that the temperature difference ΔT between the inner and outer layers ≤ 30°C, when the temperature T < 100°C, adopt independent temperature control of the inner and outer layers to prevent the inner layer from retracting.

[0019] The outer high-density reinforcement layer of the present invention imparts high strength and rigidity to the product. The nano-silica and carbon nanotubes synergistically enhance the impact resistance and dimensional stability of the material. The inner low-density foaming layer realizes a density gradient transition. While the foaming structure reduces the weight of the product by 20-30%, the closed-cell structure maintains the heat insulation / buffering performance. The gradient density structure optimizes the mechanical properties of the lightweight hollow product. Through the synergistic effect of introducing nano-reinforcing phases (such as nano-silicon carbide, carbon nanotubes) and microcapsule blowing agents, the integration of high-density compression of the outer layer and low-density heat insulation of the inner layer is achieved. The gradient design optimizes the performance balance by arranging nano-fillers directionally (such as the distribution of carbon nanotubes induced by centrifugal force) to overcome the deterioration of mechanical properties caused by uneven dispersion of fillers in traditional foaming materials. The multi-stage melting-rotation process of the present invention ensures the fusion degree of the interface between the outer layer and the inner layer material (the interlayer peel strength reaches 8-10 MPa), avoiding the weak interface problem of the traditional bonding process. The delayed decomposition technology of the microcapsule blowing agent (the triggering temperature matches the melting window) realizes the foaming uniformity (the cell diameter is 50-200 μm, and the porosity is 65±5%). The outer layer of the present invention is cooled stepwise by water (the cooling rate is 15-20 °C / min) to quickly solidify the crystalline structure of the high-density layer, preventing shrinkage deformation (the dimensional tolerance is controlled within ±0.3 mm). The inner layer is cooled stepwise by air (the cooling rate is 5-8 °C / min) to avoid the collapse of the foaming layer, and the integrity rate of the cell structure is >95%. Within a single rotational molding cycle, through the coordinated control of temperature - rotation speed - pressure, the outer layer is densified in stages (formed by high-temperature and high-speed rotation), the inner layer is controllably foamed (the microcapsule blowing agent is triggered by cooling and low-speed rotation to inhibit cell damage), and gradient curing is carried out (the outer layer is cooled by water / the inner layer is cooled by air to stabilize the gradient structure). The single-cycle process efficiency is increased by more than 30%. In the present invention, PE wax improves the powder fluidity (the wall thickness uniformity during rotational molding is increased by 25%), and the nano-fillers increase the heat distortion temperature by 30-40 °C. The compatibilizer (such as maleic anhydride grafted PE) promotes the compatibility of the HDPE / LDPE phases, and the density gradient of the foaming layer is controllable (0.25-0.6 g / cm 3 (adjustable); Reactive compatibilizers (such as epoxy-grafted polyethylene, maleic anhydride grafted PE) are used to enhance the chemical bonding between the high / low-density layers, and seamless transition is achieved through molecular chain entanglement and functional group reactions. The interface peel strength is increased to ≥8 N / mm (vs. the interface strength of traditional multi-layer rotational molding relying on physical bonding ≤3 N / mm), avoiding the peeling problem caused by thermal expansion differences in traditional multi-layer materials. Compared with traditional hollow products, the present invention reduces the product weight by more than 25% and reduces energy consumption by 15% (the single-piece production cycle is shortened by 8-12%). It is applicable to fields sensitive to the weight-strength ratio such as automobile fuel tanks and spacecraft components, and has both engineering application value and cost advantages.

[0020] Preferably, the dosage of high-density polyethylene in the step (S01) is 35 parts to 87 parts. More preferably, the dosage of high-density polyethylene in the step (S01) is 40 parts to 85 parts. More preferably, the dosage of high-density polyethylene in the step (S01) is 45 parts to 80 parts. More preferably, the dosage of high-density polyethylene in the step (S01) is 50 parts to 75 parts. More preferably, the dosage of high-density polyethylene in the step (S01) is 55 parts to 70 parts. More preferably, the dosage of high-density polyethylene in the step (S01) is 60 parts to 65 parts. The limitation of the dosage of high-density polyethylene can better ensure the rigidity, chemical resistance and dimensional stability of the finally prepared outer high-density reinforcement layer.

[0021] Preferably, in the step (S01), the high-density polyethylene is selected from at least one of ExxonMobil 7203, Sinopec D5502GA, LG Chem HD4600U of South Korea or Borealis 7004. There can be various choices for the specific type of high-density polyethylene, which can be selected according to the actual situation.

[0022] Preferably, the dosage of linear low-density polyethylene in the step (S01) is 5 parts to 20 parts. More preferably, the dosage of linear low-density polyethylene in the step (S01) is 8 parts to 18 parts. More preferably, the dosage of linear low-density polyethylene in the step (S01) is 10 parts to 15 parts. More preferably, the dosage of linear low-density polyethylene in the step (S01) is 12 parts to 13 parts. The limitation of the dosage of linear low-density polyethylene can better ensure the impact resistance and flexibility of the finally prepared outer high-density reinforcement layer.

[0023] Preferably, the linear low-density polyethylene is selected from at least one of Sinopec LLDPE-7050, ExxonMobil LLDPE-9020 or Borealis LLDPE-9085. There can be various choices for the specific type of linear low-density polyethylene, which can be selected according to the actual situation.

[0024] Preferably, the addition amount of nano-silica in the step (S02) is 2 parts to 10 parts. More preferably, the addition amount of nano-silica in the step (S02) is 4 parts to 8 parts. More preferably, the addition amount of nano-silica is 5 parts to 6 parts. The limitation of the addition amount of nano-silica can better improve the rigidity and wear resistance of the finally prepared outer high-density reinforcement layer and reduce the shrinkage rate.

[0025] Preferably, in the step (S02), the nano-silica is selected from at least one of CY-SP30F or SiO2-N-03. There can be various choices for the specific type of nano-silica, which can be selected according to the actual situation.

[0026] Preferably, the addition amount of carbon nanotubes in the step (S02) is 3 parts to 5 parts. More preferably, the addition amount of carbon nanotubes is 4 parts. The limitation of the addition amount of carbon nanotubes can better improve the electrical / thermal conductivity of the finally prepared outer high-density reinforcing layer and enhance the impact resistance.

[0027] Preferably, in the step (S02), single-walled carbon nanotubes are selected as the carbon nanotubes or multi-walled carbon nanotubes or at least one of them. There are various choices for the types of carbon nanotubes, which can be selected according to the actual situation.

[0028] Preferably, the addition amount of the toughening agent in the step (S02) is 1 part to 10 parts. More preferably, the addition amount of the toughening agent in the step (S02) is 3 parts to 8 parts. More preferably, the addition amount of the toughening agent is 5 parts to 6 parts. The limitation of the addition amount of the toughening agent can better improve the low-temperature toughness of the finally prepared outer high-density reinforcing layer and reduce brittle fracture.

[0029] Preferably, in the step (S02), at least one of POE-g-MAH, POE-g-GMA or PE-g-MAH is selected as the toughening agent. There are various choices for the types of toughening agents, which can be selected according to the actual situation.

[0030] Preferably, the addition amount of PE wax in the step (S02) is 1 part to 10 parts. More preferably, the addition amount of PE wax is 3 parts to 8 parts. More preferably, the addition amount of PE wax is 5 parts to 6 parts. The limitation of the addition amount of PE wax can better increase the lubricating effect, improve the particle fluidity during the blending and granulation of each component, and ensure the uniform wall thickness of the finally prepared outer high-density reinforcing layer.

[0031] Preferably, in the step (S02), at least one of PEW-032, PEW-031 or PEW-030 is selected as the PE wax. There are various choices for the types of PE wax, which can be selected according to the actual situation.

[0032] Preferably, the addition amount of the antioxidant in the step (S02) is 1 part to 10 parts. More preferably, the addition amount of the antioxidant is 3 parts to 8 parts. More preferably, the addition amount of the antioxidant is 4 parts to 6 parts. The limitation of the addition amount of the antioxidant can better prevent the oxidative degradation of the blended particles during the subsequent granulation and heating and melting processes.

[0033] Preferably, in the step (S02), at least one of BHT, AO-1010, AO-1076, AO-3114 or AO-330 is selected as the antioxidant. There are various choices for the types of antioxidants, which can be selected according to the actual situation.

[0034] Preferably, the blending temperature in the step (S02) is 85°C to 110°C. More preferably, the blending temperature in the step (S02) is 90°C to 105°C. Even more preferably, the blending temperature in the step (S02) is 95°C to 100°C. The defined blending temperature can promote the rapid and uniform mixing among various materials and ensure the uniform dispersion of the nano-fillers therein.

[0035] Preferably, the blending time in the step (S02) is 5 min to 8 min. More preferably, the blending time in the step (S02) is 6 min to 7 min. The defined blending time takes into account the timeliness while ensuring the rapid and uniform mixing of various materials.

[0036] Preferably, the granulation in the step (S02) is as follows

[0037] Put the blended materials into a twin-screw extruder for extrusion, and then obtain the first masterbatch after cooling, pelletizing and drying in sequence. Through the twin-screw extrusion and post-treatment technology, the high-efficiency homogenization and processability optimization of the composite material are realized, ensuring the uniform mixing of each component in the subsequent first rotational molding powder, and better ensuring the formation of an outer high-density reinforced layer after heating and melting and then rotational adhesion, with better tensile strength and impact strength.

[0038] Preferably, the extrusion temperature of the twin-screw extruder is 190°C to 200°C. More preferably, the extrusion temperature of the twin-screw extruder is 195°C. The defined basic temperature can ensure sufficient mutual melting and granulation while avoiding the degradation of linear low-density polyethylene.

[0039] Preferably, the powder grinding in the step (S02) is as follows

[0040] Under nitrogen protection, put the first masterbatch obtained by granulation into a blending material powder mill for powder grinding to obtain the first rotational molding powder. The nitrogen inert environment can block the high-temperature oxidation of polyethylene molecular chains during the powder grinding process, ensuring the stability of material properties; the low-temperature embrittlement effect (the nitrogen environment can assist in cooling) makes the masterbatch easier to break and obtain an ideal powder form; the nitrogen dielectric environment (relative humidity <5%) can prevent the PE powder from being triboelectrically charged; prevent the CNT / SiO2 from generating surface polar groups due to oxidation (the -OH peak disappears in FTIR detection), and avoid the re-agglomeration of the fillers.

[0041] Preferably, the powder grinding particle size in the step (S02) is 40 mesh to 50 mesh. More preferably, the powder grinding particle size in the step (S02) is 42 mesh to 48 mesh. Even more preferably, the powder grinding particle size in the step (S02) is 44 mesh to 46 mesh. The defined powder grinding particle size is for an ideal powder form, and better ensures the particle uniformity of the subsequent outer high-density reinforced layer.

[0042] Preferably, the melting temperature in step (S03) is 280°C to 300°C. More preferably, the melting temperature in step (S03) is 285°C to 295°C. Even more preferably, the melting temperature in step (S03) is 290°C. The defined melting temperature can fully ensure that raw material substances such as high-density polyethylene and linear low-density polyethylene in the first rotational molding powder are completely melted.

[0043] Preferably, the rotation in step (S03) is a dual-axis rotation formed by a main axis and a sub-axis. The dual-axis rotation can better ensure the uniformity of the coating adhesion on the inner wall of the mold.

[0044] Preferably, the rotation speed of the main axis is 8 rpm to 12 rpm; the rotation speed of the sub-axis is 4 rpm to 6 rpm. More preferably, the rotation speed of the main axis is 9 rpm to 11 rpm; the rotation speed of the sub-axis is 5 rpm. The defined rotation speed can better ensure the uniformity of the coating adhesion on the inner wall of the mold.

[0045] Preferably, the dosage of low-density polyethylene in step (S04) is 62 parts to 87 parts. More preferably, the dosage of low-density polyethylene in step (S04) is 65 parts to 85 parts. Even more preferably, the dosage of low-density polyethylene in step (S04) is 70 parts to 80 parts. Even more preferably, the dosage of low-density polyethylene in step (S04) is 72 parts to 78 parts. Even more preferably, the dosage of low-density polyethylene in step (S04) is 74 parts to 76 parts. The limitation of the dosage of low-density polyethylene can better provide high melt strength and is beneficial to the stable molding of the inner low-density foaming layer.

[0046] Preferably, the low-density polyethylene selected in step (S04) is at least one of DFDA-7042, DFDB-7042, or DFDC-7042. There are various choices for the type of low-density polyethylene, which can be selected according to the actual situation.

[0047] Preferably, the dosage of high-density polyethylene in step (S04) is 9 parts to 18 parts. More preferably, the dosage of high-density polyethylene in step (S04) is 10 parts to 16 parts. Even more preferably, the dosage of high-density polyethylene in step (S04) is 11 parts to 15 parts. Even more preferably, the dosage of high-density polyethylene in step (S04) is 12 parts to 14 parts. The limitation of the dosage of high-density polyethylene can better improve the skeleton strength of the finally obtained inner low-density foaming layer and prevent the collapse of the foam cells.

[0048] Preferably, in the step (S04), the high-density polyethylene is at least one of ExxonMobil 7203, Sinopec D5502GA, South Korea's LG Chem HD4600U, or Borealis 7004. There are various choices for the type of high-density polyethylene, which can be selected according to the actual situation.

[0049] Preferably, the addition amount of the microcapsule blowing agent in the step (S05) is 1 part to 5 parts. More preferably, the addition amount of the microcapsule blowing agent in the step (S05) is 2 parts to 4 parts. More preferably, the addition amount of the microcapsule blowing agent is 3 parts. The limitation of the addition amount of the microcapsule blowing agent can well meet the requirements of delayed foaming and precise foam control effects.

[0050] Preferably, the particle size of the microcapsule blowing agent in the step (S05) is 10 μm to 50 μm. More preferably, the particle size of the microcapsule blowing agent in the step (S05) is 15 μm to 45 μm. More preferably, the particle size of the microcapsule blowing agent in the step (S05) is 20 μm to 40 μm. More preferably, the particle size of the microcapsule blowing agent in the step (S05) is 25 μm to 35 μm. More preferably, the particle size of the microcapsule blowing agent in the step (S05) is 30 μm. The limited particle size of the microcapsule blowing agent can better achieve controlled decomposition, form a closed-cell foam structure, and reduce the weight of the product by 20% to 30%.

[0051] Preferably, in the step (S05), the microcapsule blowing agent is selected as azodicarbonamide@silica shell. The azodicarbonamide@silica shell can better achieve controlled decomposition, form a closed-cell foam structure, and reduce the weight of the product by 20% to 30%.

[0052] Preferably, the addition amount of the compatibilizer in the step (S06) is 2 parts to 5 parts. More preferably, the addition amount of the compatibilizer in the step (S06) is 3 parts to 4 parts. The limitation of the addition amount of the compatibilizer can better improve the compatibility between low-density polyethylene, high-density polyethylene, and the microcapsule blowing agent during the subsequent blending and pelletizing processes.

[0053] Preferably, in the step (S06), the compatibilizer is selected as maleic anhydride grafted polyethylene. Maleic anhydride grafted polyethylene can better improve the compatibility between low-density polyethylene and high-density polyethylene, avoid delamination, and increase the interfacial bonding strength (peel strength ≥ 8 MPa).

[0054] Preferably, the maleic anhydride grafted polyethylene is at least one of LLDPE18362 or PE1040. There are various choices for the type of maleic anhydride grafted polyethylene, which can be selected according to the actual situation.

[0055] Preferably, the addition amount of the antioxidant in the step (S06) is 1 part to 10 parts. More preferably, the addition amount of the antioxidant in the step (S06) is 3 parts to 8 parts. More preferably, the addition amount of the antioxidant in the step (S06) is 4 parts to 7 parts. More preferably, the addition amount of the antioxidant in the step (S06) is 5 parts to 6 parts. The limitation of the addition amount of the antioxidant can better prevent the oxidative degradation of the blended particulate matter during the subsequent granulation and heating and melting processes.

[0056] Preferably, at least one of BHT, AO-1010, AO-1076, AO-3114 or AO-330 is selected as the antioxidant in the step (S06). There are various choices for the types of antioxidants, which can be selected according to the actual situation.

[0057] Preferably, the blending temperature in the step (S06) is 85°C to 110°C. More preferably, the blending temperature in the step (S06) is 90°C to 105°C. More preferably, the blending temperature in the step (S06) is 95°C to 100°C. The defined blending temperature can promote the rapid and uniform mixing of various materials while preventing the degradation of low-density polyethylene molecular chains.

[0058] Preferably, the blending time in the step (S06) is 5 min to 8 min. More preferably, the blending time in the step (S06) is 6 min to 7 min. The defined blending time takes into account the timeliness while ensuring the rapid and uniform mixing of various materials.

[0059] Preferably, the granulation in the step (S06) is as follows

[0060] The blended materials are put into a twin-screw extruder for extrusion, and then are successively cooled, pelletized and dried to obtain the second masterbatch. Through the twin-screw extrusion and post-treatment technology, the high-efficiency homogenization and processability optimization of the composite material are realized, ensuring the uniform mixing of each component in the subsequent second rotational molding powder, and better ensuring the heat insulation and weight reduction performance of the inner low-density foaming layer formed by heating and melting and rotating and adhering to the inner wall surface of the outer high-density reinforcing layer.

[0061] Preferably, the extrusion temperature of the twin-screw extruder is 190°C to 200°C. More preferably, the extrusion temperature of the twin-screw extruder is 195°C. The defined base temperature can ensure sufficient mutual melting and granulation while avoiding the degradation of low-density polyethylene molecular chains.

[0062] Preferably, the powder grinding in the step (S06) is as follows

[0063] Under nitrogen protection, the second masterbatch obtained by granulation is ground in a blend mill to obtain the second rotational molding powder. The nitrogen environment prevents the surface oxidation and damage of the blowing agent, and the decomposition temperature deviation is controlled within ±2°C; the gas generation rate retention after grinding is >95%, ensuring that the subsequent rotational molding expansion ratio is stably within 3 - 5 times; the melt permeability of the unoxidized low-density polyethylene and high-density polyethylene powders is improved, and the bonding strength with the high-density outer layer reaches 12 MPa.

[0064] Preferably, the grinding particle size in step (S06) is 40 mesh to 50 mesh. More preferably, the grinding particle size in step (S06) is 42 mesh to 48 mesh. Even more preferably, the grinding particle size in step (S06) is 44 mesh to 46 mesh. The defined grinding particle size is the ideal powder form, which can better ensure the particle uniformity of the subsequent inner low-density foaming layer.

[0065] Preferably, in step (S08), the temperature is lowered to 160°C - 180°C to trigger the decomposition of the microcapsule blowing agent in the melt. More preferably, in step (S08), the temperature is lowered to 165°C - 175°C to trigger the decomposition of the microcapsule blowing agent in the melt. Even more preferably, in step (S08), the temperature is lowered to 170°C to trigger the decomposition of the microcapsule blowing agent in the melt. Precise control of the foaming trigger temperature within 160°C - 180°C enables precise matching of foaming kinetics and realizes temperature window synergy (the outer high-density polyethylene and linear low-density polyethylene are completely melted (melting point 130°C - 145°C) but not overly crystallized, and the inner low-density polyethylene and high-density polyethylene are in the high elastic state (the best viscoelasticity range), at which time foaming can achieve sufficient melt strength to support the growth of cell holes); realizes the regulation of the decomposition rate (the microcapsules show a linear decomposition characteristic in the 160°C - 180°C range, the gas generation rate is synchronized with the melt expansion speed, and the uniformity of cell hole distribution is increased by 40%); can ensure the stability of the gradient structure (the semi-crystalline molecular chains in the outer layer can penetrate into the foaming layer, the cell front and the melting front rates are matched, and the cooling stress difference between the inner and outer layers is minimized); realizes precise regulation of the cell hole structure (the cell nucleation density is high, the pore size distribution is narrow, and the closed cell rate ≥90%); can optimize the process energy consumption (the foaming temperature overlaps with the material melting temperature range (160°C - 180°C vs high-density polyethylene melting point 130°C - 145°C), no additional heating is required; it is 25% - 30% more energy-efficient than the traditional two-step method (first melting and then secondary heating for foaming); foaming and molding are completed synchronously, and the cycle is shortened by 15% - 20%).

[0066] Preferably, in the step (S08), the rotational speed of the main shaft is reduced to 4 rpm to 6 rpm, and the rotational speed of the auxiliary shaft is reduced to 1 rpm to 3 rpm. More preferably, in the step (S08), the rotational speed of the main shaft is reduced to 5 rpm, and the rotational speed of the auxiliary shaft is reduced to 2 rpm; this can inhibit the damage of the cell structure by centrifugal force and promote the formation of the inner low-density foaming layer. Reducing the main shaft speed to 4 rpm to 6 rpm and the auxiliary shaft speed to 1 rpm to 3 rpm during the gradient density rotational molding foaming stage can precisely control the cell structure. By balancing the centrifugal force, it can avoid the stretching deformation of the cells caused by high rotational speeds (>8 rpm), and can also ensure the foaming uniformity. The thickness of the density gradient transition zone is reduced from ±1.2 mm to ±0.5 mm; it can strengthen the gradient layer interface. The low-speed flow of the melt promotes the mutual diffusion of molecular chains, increases the interlayer penetration depth, can reduce the orientation of molecular chains at the interface, and can also inhibit the crystallization orientation at a low speed; it can improve the process stability. The shear rate is reduced, and the apparent viscosity of the melt increases by 20% to 30%, effectively preventing the melt rupture of the foaming layer and enhancing the migration of nano-fillers in the reinforcement layer. The low-speed rotation reduces the temperature difference on the mold surface from ±15 °C to ±5 °C, avoiding the premature decomposition of the blowing agent caused by local overheating; at an auxiliary shaft rotational speed of 1 rpm to 3 rpm, the melt can fully fill structures such as ribs and grooves, and the differential rotation speed induces the directional arrangement of cells to achieve anisotropic properties; it can achieve the balance between energy consumption and quality. Although the molding cycle is extended by 18%, the scrap rate is reduced by 15%, resulting in a 12% to 15% reduction in the comprehensive cost.

[0067] Preferably, in the step (S09), the cooling density gradient of stepped water cooling is 20°C / min to 25°C / min, and the cooling density gradient of stepped air cooling is 5°C / min to 10°C / min. More preferably, in the step (S09), the cooling density gradient of stepped water cooling is 21°C / min to 24°C / min, and the cooling density gradient of stepped air cooling is 6°C / min to 9°C / min. Even more preferably, in the step (S09), the cooling density gradient of stepped water cooling is 22°C / min to 23°C / min, and the cooling density gradient of stepped air cooling is 7°C / min to 8°C / min; a continuous density gradient is formed. In the cooling stage of the gradient density rotational molding product, a differential cooling strategy of stepped water cooling and stepped air cooling is adopted. By precisely controlling the thermodynamic phase change process, the stability of the gradient structure is optimized. The outer layer is water cooled to achieve rapid shaping of the outer high-density reinforcement layer. The crystallinity of polyethylene is increased from 50% to 70% - 75%, the yield strength is increased by 30% - 40%, the cooling stress release rate > 90%, and the dimensional shrinkage rate is stabilized at 1.2% - 1.5% (compared with 3% - 5% of natural cooling). It can well position the nano-fillers, and rapid cooling can inhibit the migration of CNT or SiO2, resulting in a surface hardness of 72 - 75 Shore D and a 2-fold increase in wear resistance; the inner layer is air cooled to ensure the integrity of the foaming structure, the cell shrinkage rate is controlled within 8% (compared with 25% collapse caused by water cooling), the closed cell rate remains ≥ 85%, and the thermal conductivity is stabilized at 0.038 W / (m·K) - 0.042 W / (m·K). Slow cooling allows the molecular chains of low-density polyethylene to fully disentangle, and the impact strength is increased by 50%; there is a cooling kinetic synergy effect, with temperature gradient cooling to avoid interlayer peeling caused by sudden cooling. There is a phase change time difference, the outer layer crystallizes earlier than the inner layer foaming stabilizes, the inner layer foaming shaping is synchronized with the outer layer shrinkage, the residual stress is reduced to < 5 MPa, and the comprehensive energy consumption is reduced by 40%; precise regulation of the microstructure is achieved, with crystallization and foaming growing synergistically. The outer layer water cooling induces the formation of α-crystalline form, and the modulus is increased to 1.8 GPa - 2.2 GPa. The inner layer air cooling forms gradient cells (gradually changing from 50 μm on the surface layer to 150 μm in the core layer), achieving gradual energy absorption. The cooling rate difference forms an interpenetrating network structure of 10 - 20 μm in the interface region, and the interlayer peel strength reaches 15 MPa - 18 MPa; it has better industrial production benefits, shortening the total cooling time. The total cooling time is compressed from 60 min in the traditional process to 35 - 40 min (efficiency increased by 30%). Compared with the uniform cooling process, the surface sink marks ≤ 3%, the cell collapse ≤ 5%, and the dimensional tolerance is ±0.2 mm, improving the yield rate.

[0068] The second technical solution of the present invention: A lightweight hollow product formed based on polyethylene rotational molding material - gradient density, includes the following components by weight parts,

[0069] 44 to 105 parts by weight of high-density polyethylene, 5 to 20 parts by weight of linear low-density polyethylene, 2 to 10 parts by weight of nano-silica, 3 to 5 parts by weight of carbon nanotubes, 1 to 10 parts by weight of toughening agent, 1 to 10 parts by weight of PE wax, 2 to 20 parts by weight of antioxidant, 62 to 87 parts by weight of low-density polyethylene, 1 to 5 parts by weight of microcapsule blowing agent, 2 to 5 parts by weight of compatibilizer. In the present invention, an appropriate amount of high-density polyethylene is added to provide a strong rigid skeleton for the product; an appropriate amount of linear low-density polyethylene is added to introduce a eutectic structure, so that the impact strength of the product is increased to 50 - 80 kJ / m 2 ; the addition of an appropriate amount of nano-silica and carbon nanotubes forms a three-dimensional reinforcement network inside the product, so that the tensile strength reaches 35 MPa - 45 MPa; the addition of an appropriate amount of low-density polyethylene forms a low-density foaming layer to achieve the lightweight function of the product, and the high melt strength ensures the foaming stability; the addition of an appropriate amount of microcapsule blowing agent ensures that the closed-cell rate of the product > 90%, and the adjustable density range is 0.3 g / cm 3 ~0.6 g / cm 3 ; the addition of an appropriate amount of compatibilizer can well improve the interfacial compatibility between low-density polyethylene and high-density polyethylene, so that the peel strength ≥ 10 MPa; the addition of an appropriate amount of PE wax can reduce the melt viscosity, ensure the uniformity of rotational molding flow (wall thickness deviation < ±5%), and reduce the energy consumption by 15%; the addition of an appropriate amount of toughening agent, through the elastomer particle-induced crazing-shear band mechanism, the impact strength retention rate at -40 °C > 60%, and the low-temperature toughness is increased by 3 times; the addition of an appropriate amount of antioxidant extends the outdoor life of the product by 5 - 8 years; the synergistic cooperation of multiple components and the multi-scale synergistic effect achieve the perfect balance of lightweight, high strength and functionality; after the proportion range of high-density polyethylene and linear low-density polyethylene is matched, the rigidity and toughness can be balanced; after the proportion range of nano-silica and carbon nanotubes is matched, the thermal conductivity (0.8 W / m·K - 1.2 W / m·K) and electromagnetic shielding (SE > 30 dB) can be synergistically improved; in the present invention, the nano-silica and carbon nanotubes are enriched in the outer layer by the rotational molding centrifugal force to achieve the directional distribution of nano-fillers; the product is a self-flame-retardant product, and the high crystallinity (> 70%) and closed-cell foaming structure of high-density polyethylene make the self-flame-retardant performance of the product reach UL94 HB level; the product is a full-PE system and can be 100% recycled, and the carbon footprint is reduced by 40% compared with glass fiber-reinforced materials, which is an environmentally friendly product; the formula system realizes the performance breakthrough of "as light as foam and as strong as steel" through the three-dimensional innovation of molecular design-component regulation-process matching.

[0070] Preferably, it comprises the following components by weight

[0071] 50 to 100 parts of high-density polyethylene, 7 to 18 parts of linear low-density polyethylene, 3 to 9 parts of nano-silica, 3 to 5 parts of carbon nanotubes, 2 to 9 parts of toughening agent, 2 to 9 parts of PE wax, 4 to 18 parts of antioxidant, 65 to 85 parts of low-density polyethylene, 2 to 4 parts of microcapsule blowing agent, 3 to 4 parts of compatibilizer.

[0072] Preferably, it comprises the following components by weight parts,

[0073] 55 to 95 parts of high-density polyethylene, 9 to 16 parts of linear low-density polyethylene, 4 to 8 parts of nano-silica, 3 to 5 parts of carbon nanotubes, 3 to 8 parts of toughening agent, 3 to 8 parts of PE wax, 6 to 16 parts of antioxidant, 68 to 83 parts of low-density polyethylene, 2 to 4 parts of microcapsule blowing agent, 3 to 4 parts of compatibilizer.

[0074] Preferably, it comprises the following components by weight parts,

[0075] 60 to 90 parts of high-density polyethylene, 11 to 14 parts of linear low-density polyethylene, 5 to 7 parts of nano-silica, 3 to 5 parts of carbon nanotubes, 4 to 7 parts of toughening agent, 4 to 7 parts of PE wax, 8 to 14 parts of antioxidant, 70 to 80 parts of low-density polyethylene, 2 to 4 parts of microcapsule blowing agent, 3 to 4 parts of compatibilizer.

[0076] Preferably, it comprises the following components by weight parts,

[0077] 65 to 85 parts of high-density polyethylene, 12 to 13 parts of linear low-density polyethylene, 5 to 7 parts of nano-silica, 3 to 5 parts of carbon nanotubes, 5 to 6 parts of toughening agent, 5 to 6 parts of PE wax, 10 to 12 parts of antioxidant, 72 to 78 parts of low-density polyethylene, 2 to 4 parts of microcapsule blowing agent, 3 to 4 parts of compatibilizer.

[0078] Preferably, it comprises the following components by weight parts,

[0079] 70 to 80 parts of high-density polyethylene, 12 to 13 parts of linear low-density polyethylene, 5 to 7 parts of nano-silica, 3 to 5 parts of carbon nanotubes, 5 to 6 parts of toughening agent, 5 to 6 parts of PE wax, 10 to 12 parts of antioxidant, 74 to 76 parts of low-density polyethylene, 2 to 4 parts of microcapsule blowing agent, 3 to 4 parts of compatibilizer.

[0080] Preferably, it comprises the following components by weight parts,

[0081] 75 parts of high-density polyethylene, 13 parts of linear low-density polyethylene, 6 parts of nano-silica, 4 parts of carbon nanotubes, 6 parts of toughening agent, 6 parts of PE wax, 11 parts of antioxidant, 75 parts of low-density polyethylene, 3 parts of microcapsule blowing agent, 4 parts of compatibilizer.

[0082] The present invention has the following beneficial effects:

[0083] (1) Selecting an appropriate amount of high-density polyethylene as the matrix can better ensure the rigidity, chemical resistance and dimensional stability of the finally prepared outer high-density reinforcing layer; selecting an appropriate amount of linear low-density polyethylene as the matrix, by introducing a branched chain structure, can ensure the impact resistance and flexibility of the finally prepared outer high-density reinforcing layer; adding an appropriate amount of nano-silica can improve the rigidity and wear resistance of the finally prepared outer high-density reinforcing layer and reduce the shrinkage rate; adding an appropriate amount of carbon nanotubes can improve the electrical / thermal conductivity and impact resistance of the finally prepared outer high-density reinforcing layer; adding an appropriate amount of toughening agent can improve the low-temperature toughness of the finally prepared outer high-density reinforcing layer and reduce brittle fracture; adding an appropriate amount of PE wax to increase the lubrication effect and improve the particle fluidity during the blending and granulation of each component, ensuring the wall thickness uniformity of the finally prepared outer high-density reinforcing layer; adding an appropriate amount of antioxidant can prevent the oxidative degradation of the blended particles during the subsequent granulation and heating and melting processes; using a high-speed mixer to complete the blending of materials to ensure the uniform dispersion of nano-fillers; using a twin-screw extruder to complete the granulation process, ensuring the uniform mixing of each component in the subsequent first rotational molding powder, and better ensuring the formation of an outer high-density reinforcing layer with better tensile strength and impact strength after subsequent heating and melting and then rotational adhesion.

[0084] (2) Selecting an appropriate amount of low-density polyethylene as the matrix can better provide high melt strength, which is beneficial to the stable formation of the inner low-density foaming layer; selecting an appropriate amount of high-density polyethylene as the matrix can well improve the skeleton strength of the finally prepared inner low-density foaming layer and prevent cell collapse; adding an appropriate amount of microcapsule blowing agent to achieve the effects of delayed foaming and precise foam control; adding an appropriate amount of compatibilizer can well improve the compatibility between low-density polyethylene, high-density polyethylene and microcapsule blowing agent during the subsequent blending and granulation processes; adding an appropriate amount of antioxidant can prevent the oxidative degradation of the blended particles during the subsequent granulation and heating and melting processes; using a high-speed mixer to complete the blending of materials to degrade the molecular chains of low-density polyethylene; using a twin-screw extruder to complete the granulation process, ensuring the uniform mixing of each component in the subsequent second rotational molding powder, and better ensuring the heat insulation and weight reduction performance of the inner low-density foaming layer formed after subsequent heating and melting and rotational adhesion to the inner wall surface of the outer high-density reinforcing layer; adding the second rotational molding powder when the outer high-density reinforcing layer is not completely cooled to promote the mutual diffusion of molecular chains.

[0085] (3) Gradually reduce the temperature (from 5°C / min to 8°C / min to the decomposition temperature of the blowing agent) to trigger the decomposition of the microcapsule blowing agent in the melt. At the same time, reduce the rotation speed to reduce the shear force and improve the uniformity of the cell diameter. Adopt the zone cooling technology to gradually cool the outer high-density reinforcement layer in the mold by water cooling to quickly fix the crystalline structure of the outer high-density reinforcement layer, and gradually cool the inner low-density foaming layer in the mold by air cooling to avoid cell shrinkage and maintain an open cell rate <15%. When the temperature T > 100°C, adopt the synchronous temperature control method of air cooling / water cooling to ensure that the temperature difference ΔT between the inner and outer layers ≤ 30°C. When the temperature T < 100°C, adopt independent temperature control for the inner and outer layers to prevent the inner layer from retracting;

[0086] (4) The outer high-density reinforcement layer endows the product with high strength and rigidity. Nano-silica and carbon nanotubes synergistically enhance the impact resistance and dimensional stability of the material. The inner low-density foaming layer realizes a density gradient transition. While the foaming structure reduces the weight of the product by 20 - 30%, it maintains the heat insulation / buffering performance through the closed cell structure. The gradient density structure optimizes the mechanical properties of the lightweight hollow product. Through the synergistic effect of introducing nano-reinforcing phases (such as nano-silicon carbide, carbon nanotubes) and the microcapsule blowing agent, the integration of the performance of high outer-layer density compression and low inner-layer density heat insulation is achieved. And the gradient design optimizes the performance balance by arranging nano-fillers directionally (such as the distribution of carbon nanotubes induced by centrifugal force) to overcome the deterioration of mechanical properties caused by uneven dispersion of fillers in traditional foaming materials;

[0087] (5) The staged melting-rotation process ensures the fusion degree of the interface between the outer and inner layer materials (the interlayer peel strength reaches 8 - 10 MPa), avoiding the weak interface problem of traditional bonding processes. The delayed decomposition technology of the microcapsule blowing agent (the triggering temperature matches the melting window) realizes the foaming uniformity (the cell diameter is 50 - 200 μm, and the porosity is 65 ± 5%);

[0088] (6) Gradually cool the outer layer by water cooling (the cooling rate is 15 - 20°C / min) to quickly fix the crystalline structure of the high-density layer and prevent shrinkage and deformation (the dimensional tolerance is controlled within ±0.3 mm). Gradually cool the inner layer by air cooling (the cooling rate is 5 - 8°C / min) to avoid the collapse of the foaming layer, and the integrity rate of the cell structure > 95%. Within a single rotational molding cycle, through the coordinated control of temperature - rotation speed - pressure, complete the densification of the outer layer (formed by high-temperature and high-speed rotation), the controllable foaming of the inner layer (cooling down and low-speed rotation trigger the microcapsule to foam and inhibit cell damage), and the gradient curing (water cooling for the outer layer / air cooling for the inner layer to stabilize the gradient structure) in stages, and the single-cycle process efficiency is increased by more than 30%;

[0089] (7) PE wax improves the powder fluidity (the wall thickness uniformity during rotational molding is increased by 25%), and the nano-fillers increase the heat distortion temperature by 30 - 40 °C; the compatibilizer (such as maleic anhydride grafted PE) promotes the compatibility between the HDPE / LDPE phases, and the density gradient of the foamed layer is controllable (0.25 - 0.6 g / cm 3 (adjustable); the reactive compatibilizer (such as epoxy grafted polyethylene, maleic anhydride grafted PE) enhances the chemical bonding between the high / low density layers, and realizes seamless transition through molecular chain entanglement and functional group reaction. The interfacial peel strength is increased to ≥8 N / mm (vs. the interfacial strength of traditional multi-layer rotational molding relying on physical bonding ≤3 N / mm), avoiding the peeling problem caused by the thermal expansion difference of traditional multi-layer materials;

[0090] (8) Compared with traditional hollow products, the product weight is reduced by more than 25%, the energy consumption is reduced by 15% (the single-piece production cycle is shortened by 8 - 12%), which is applicable to fields sensitive to the weight-strength ratio such as automotive fuel tanks and spacecraft components, and has both engineering application value and cost advantages. Brief Description of the Drawings

[0091] Figure 1 is the cross-sectional microscopic interface diagram of the outer high-density reinforced layer - transition layer - inner low-density foamed layer of the lightweight hollow product of the present invention;

[0092] Figure 2 is the physical product diagram formed by the lightweight hollow product of the present invention. Detailed Embodiments

[0093] The present invention will be further described below in conjunction with the drawings and embodiments, but it shall not be used as a basis for limiting the present invention.

[0094] The preparation method of the lightweight hollow product formed based on polyethylene rotational molding material - gradient density includes the following steps,

[0095] (S01) Take appropriate amounts of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) and mix them as the first matrix; in the step (S01), the amount of HDPE taken is 35 parts to 87 parts; in the step (S01), the amount of HDPE taken is 40 parts to 85 parts; in the step (S01), the amount of HDPE taken is 45 parts to 80 parts; in the step (S01), the amount of HDPE taken is 50 parts to 75 parts; in the step (S01), the amount of HDPE taken is 55 parts to 70 parts; in the step (S01), the amount of HDPE taken is 60 parts to 65 parts; in the step (S01), the HDPE is selected from at least one of ExxonMobil 7203, Sinopec D5502GA, LG Chem HD4600U of South Korea, or Borealis 7004; in the step (S01), the amount of LLDPE taken is 5 parts to 20 parts; in the step (S01), the amount of LLDPE taken is 8 parts to 18 parts; in the step (S01), the amount of LLDPE taken is 10 parts to 15 parts; in the step (S01), the amount of LLDPE taken is 12 parts to 13 parts; the LLDPE is selected from at least one of Sinopec LLDPE-7050, ExxonMobil LLDPE-9020, or Borealis LLDPE-9085;

[0096] (S02) Add appropriate amounts of nano-silica, carbon nanotubes (CNT), toughening agent, PE wax, and antioxidant to the first matrix in step (S01), and then successively through blending, granulation, and powder grinding to obtain the first rotational molding powder; in the step (S02), the addition amount of nano-silica is 2 parts to 10 parts; in the step (S02), the addition amount of nano-silica is 4 parts to 8 parts; the addition amount of nano-silica is 5 parts to 6 parts; in the step (S02), the nano-silica is selected from at least one of CY-SP30F or SiO2-N-03; in the step (S02), the addition amount of carbon nanotubes is 3 parts to 5 parts; the addition amount of carbon nanotubes is 4 parts; in the step (S02), the carbon nanotubes are selected from single-walled carbon nanotubes or multi-walled carbon nanotubes at least one of; in the step (S02), the addition amount of the toughening agent is 1 part to 10 parts; in the step (S02), the addition amount of the toughening agent is 3 parts to 8 parts; the addition amount of the toughening agent is 5 parts to 6 parts; in the step (S02), the toughening agent is selected from at least one of POE-g-MAH, POE-g-GMA or PE-g-MAH; in the step (S02), the addition amount of the PE wax is 1 part to 10 parts; the addition amount of the PE wax is 3 parts to 8 parts; the addition amount of the PE wax is 5 parts to 6 parts; in the step (S02), the PE wax is selected from at least one of PEW-032, PEW-031 or PEW-030; in the step (S02), the addition amount of the antioxidant is 1 part to 10 parts; the addition amount of the antioxidant is 3 parts to 8 parts; the addition amount of the antioxidant is 4 parts to 6 parts; in the step (S02), the antioxidant is selected from at least one of BHT, AO-1010, AO-1076, AO-3114 or AO-330; the blending temperature in the step (S02) is 85°C to 110°C; the blending temperature in the step (S02) is 90°C to 105°C; the blending temperature in the step (S02) is 95°C to 100°C; the blending time in the step (S02) is 5 min to 8 min; the blending time in the step (S02) is 6 min to 7 min; the granulation in the step (S02) is,

[0097] feeding the blended material into a twin-screw extruder for extrusion, and then successively cooling, pelletizing and drying to obtain the first masterbatch; the extrusion temperature of the twin-screw extruder is 190°C to 200°C; the extrusion temperature of the twin-screw extruder is 195°C; the grinding in the step (S02) is,

[0098] under nitrogen protection, putting the granulated first masterbatch into a blending material grinder for grinding to obtain the first rotational molding powder; the grinding particle size in the step (S02) is 40 mesh to 50 mesh; the grinding particle size in the step (S02) is 42 mesh to 48 mesh; the grinding particle size in the step (S02) is 44 mesh to 46 mesh;

[0099] (S03) heating and melting the first rotational molding powder of the step (S02) in a mold and rotating and adhering it to the inner wall of the mold to form an outer high-density reinforcing layer; the melting temperature of the step (S03) is 280°C to 300°C; the melting temperature of the step (S03) is 285°C to 295°C; the melting temperature of the step (S03) is 290°C; the rotation in the step (S03) is a biaxial rotation formed by a main shaft and a sub-shaft; the rotation speed of the main shaft is 8 rpm to 12 rpm; the rotation speed of the sub-shaft is 4 rpm to 6 rpm; the rotation speed of the main shaft is 9 rpm to 11 rpm; the rotation speed of the sub-shaft is 5 rpm;

[0100] (S04) Take an appropriate amount of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) and mix them as the second matrix; in the step (S04), the dosage of LDPE is 62 parts to 87 parts; in the step (S04), the dosage of LDPE is 65 parts to 85 parts; in the step (S04), the dosage of LDPE is 70 parts to 80 parts; in the step (S04), the dosage of LDPE is 72 parts to 78 parts; in the step (S04), the dosage of LDPE is 74 parts to 76 parts; in the step (S04), LDPE is selected from at least one of DFDA-7042, DFDB-7042 or DFDC-7042; in the step (S04), the dosage of HDPE is 9 parts to 18 parts; in the step (S04), the dosage of HDPE is 10 parts to 16 parts; in the step (S04), the dosage of HDPE is 11 parts to 15 parts; in the step (S04), the dosage of HDPE is 12 parts to 14 parts; in the step (S04), HDPE is selected from at least one of ExxonMobil 7203, Sinopec D5502GA, LG Chem HD4600U of South Korea or Borealis 7004.

[0101] (S05) Add an appropriate amount of microcapsule blowing agent to the second matrix in the step (S04) and compound them to obtain a composite; in the step (S05), the addition amount of the microcapsule blowing agent is 1 part to 5 parts; in the step (S05), the addition amount of the microcapsule blowing agent is 2 parts to 4 parts; the addition amount of the microcapsule blowing agent is 3 parts; in the step (S05), the particle size of the microcapsule blowing agent is 10 μm to 50 μm; in the step (S05), the microcapsule blowing agent is selected as azodicarbonamide@silica shell.

[0102] (S06) After adding appropriate amounts of compatibilizer and antioxidant to the composite in step (S05), through blending, granulation, and grinding in sequence, the second rotational molding powder is obtained; the addition amount of the compatibilizer in step (S06) is 2 parts to 5 parts; the addition amount of the compatibilizer in step (S06) is 3 parts to 4 parts; the compatibilizer selected in step (S06) is maleic anhydride grafted polyethylene PE-g-MAH; the maleic anhydride grafted polyethylene is selected from at least one of LLDPE18362 or PE1040; the addition amount of the antioxidant in step (S06) is 1 part to 10 parts; the addition amount of the antioxidant in step (S06) is 3 parts to 8 parts; the addition amount of the antioxidant in step (S06) is 4 parts to 7 parts; the addition amount of the antioxidant in step (S06) is 5 parts to 6 parts; the antioxidant selected in step (S06) is at least one of BHT, AO-1010, AO-1076, AO-3114, or AO-330; the blending temperature in step (S06) is 85°C to 110°C; the blending temperature in step (S06) is 90°C to 105°C; the blending temperature in step (S06) is 95°C to 100°C; the blending time in step (S06) is 5 min to 8 min; the blending time in step (S06) is 6 min to 7 min; the granulation in step (S06) is,

[0103] Put the blended material into a twin-screw extruder for extrusion, and then obtain the second masterbatch through cooling, pelletizing, and drying in sequence; the extrusion temperature of the twin-screw extruder is 190°C to 200°C; the extrusion temperature of the twin-screw extruder is 195°C; the grinding in step (S06) is,

[0104] Under nitrogen protection, put the pelletized second masterbatch into a blending material grinder for grinding to obtain the second rotational molding powder; the grinding particle size in step (S06) is 40 mesh to 50 mesh; the grinding particle size in step (S06) is 42 mesh to 48 mesh; the grinding particle size in step (S06) is 44 mesh to 46 mesh;

[0105] (S07) Put the second rotational molding powder in step (S06) into the mold in step (S03), continue to heat up and rotate until melting is completed;

[0106] (S08) Cooling Trigger Step: In step (S07), the microcapsule foaming agent in the melt decomposes, and at the same time, the rotation speed is reduced to form an inner low-density foaming layer on the inner wall surface of the outer high-density reinforcement layer; in step (S08), the temperature is reduced to 160°C to 180°C to trigger the decomposition of the microcapsule foaming agent in the melt; in step (S08), the temperature is reduced to 165°C to 175°C to trigger the decomposition of the microcapsule foaming agent in the melt; in step (S08), the temperature is reduced to 170°C to trigger the decomposition of the microcapsule foaming agent in the melt; in step (S08), the main shaft rotation speed is reduced to 4 rpm to 6 rpm, and the auxiliary shaft rotation speed is reduced to 1 rpm to 3 rpm; in step (S08), the main shaft rotation speed is reduced to 5 rpm, and the auxiliary shaft rotation speed is reduced to 2 rpm; the destruction of the cell structure by centrifugal force is inhibited, and the formation of the inner low-density foaming layer is promoted.

[0107] (S09) Using the zone cooling technology, after stepwise water cooling of the outer high-density reinforcement layer in the mold and stepwise air cooling of the inner low-density foaming layer in the mold, the lightweight hollow product as shown in Figure 1 is obtained; in step (S09), the cooling density gradient of stepwise water cooling is 20°C / min to 25°C / min, and the cooling density gradient of stepwise air cooling is 5°C / min to 10°C / min; in step (S09), the cooling density gradient of stepwise water cooling is 21°C / min to 24°C / min, and the cooling density gradient of stepwise air cooling is 6°C / min to 9°C / min; in step (S09), the cooling density gradient of stepwise water cooling is 22°C / min to 23°C / min, and the cooling density gradient of stepwise air cooling is 7°C / min to 8°C / min; a continuous density gradient is formed.

[0108] The lightweight hollow product formed based on polyethylene rotational molding - gradient density includes the following components by weight parts,

[0109] 44 parts to 105 parts of high-density polyethylene, 5 parts to 20 parts of linear low-density polyethylene, 2 parts to 10 parts of nano-silica, 3 parts to 5 parts of carbon nanotubes, 1 part to 10 parts of toughening agent, 1 part to 10 parts of PE wax, 2 parts to 20 parts of antioxidant, 62 parts to 87 parts of low-density polyethylene, 1 part to 5 parts of microcapsule foaming agent, 2 parts to 5 parts of compatibilizer.

[0110] The lightweight hollow product formed based on polyethylene rotational molding - gradient density includes the following components by weight parts,

[0111] 50 to 100 parts by weight of high density polyethylene, 7 to 18 parts by weight of linear low density polyethylene, 3 to 9 parts by weight of nano silica, 3 to 5 parts by weight of carbon nanotubes, 2 to 9 parts by weight of toughening agent, 2 to 9 parts by weight of PE wax, 4 to 18 parts by weight of antioxidant, 65 to 85 parts by weight of low density polyethylene, 2 to 4 parts by weight of microcapsule blowing agent, 3 to 4 parts by weight of compatibilizer.

[0112] The lightweight hollow product formed based on polyethylene rotational molding plastic - gradient density comprises the following components by weight:

[0113] 55 to 95 parts by weight of high density polyethylene, 9 to 16 parts by weight of linear low density polyethylene, 4 to 8 parts by weight of nano silica, 3 to 5 parts by weight of carbon nanotubes, 3 to 8 parts by weight of toughening agent, 3 to 8 parts by weight of PE wax, 6 to 16 parts by weight of antioxidant, 68 to 83 parts by weight of low density polyethylene, 2 to 4 parts by weight of microcapsule blowing agent, 3 to 4 parts by weight of compatibilizer.

[0114] The lightweight hollow product formed based on polyethylene rotational molding plastic - gradient density comprises the following components by weight:

[0115] 60 to 90 parts by weight of high density polyethylene, 11 to 14 parts by weight of linear low density polyethylene, 5 to 7 parts by weight of nano silica, 3 to 5 parts by weight of carbon nanotubes, 4 to 7 parts by weight of toughening agent, 4 to 7 parts by weight of PE wax, 8 to 14 parts by weight of antioxidant, 70 to 80 parts by weight of low density polyethylene, 2 to 4 parts by weight of microcapsule blowing agent, 3 to 4 parts by weight of compatibilizer.

[0116] The lightweight hollow product formed based on polyethylene rotational molding plastic - gradient density comprises the following components by weight:

[0117] 65 to 85 parts by weight of high density polyethylene, 12 to 13 parts by weight of linear low density polyethylene, 5 to 7 parts by weight of nano silica, 3 to 5 parts by weight of carbon nanotubes, 5 to 6 parts by weight of toughening agent, 5 to 6 parts by weight of PE wax, 10 to 12 parts by weight of antioxidant, 72 to 78 parts by weight of low density polyethylene, 2 to 4 parts by weight of microcapsule blowing agent, 3 to 4 parts by weight of compatibilizer.

[0118] The lightweight hollow product formed based on polyethylene rotational molding plastic - gradient density comprises the following components by weight:

[0119] 70 to 80 parts by weight of high density polyethylene, 12 to 13 parts by weight of linear low density polyethylene, 5 to 7 parts by weight of nano silica, 3 to 5 parts by weight of carbon nanotubes, 5 to 6 parts by weight of toughening agent, 5 to 6 parts by weight of PE wax, 10 to 12 parts by weight of antioxidant, 74 to 76 parts by weight of low density polyethylene, 2 to 4 parts by weight of microcapsule blowing agent, 3 to 4 parts by weight of compatibilizer.

[0120] The lightweight hollow product formed based on polyethylene rotational molding plastic - gradient density comprises the following components by weight:

[0121] 75 parts of high-density polyethylene, 13 parts of linear low-density polyethylene, 6 parts of nano-silica, 4 parts of carbon nanotubes, 6 parts of toughening agent, 6 parts of PE wax, 11 parts of antioxidant, 75 parts of low-density polyethylene, 3 parts of microcapsule foaming agent, 4 parts of compatibilizer.

[0122] Through the collaborative optimization of material formulation design and process parameters, a specific product required is formed; for example, through density gradient regulation, the outer high-density layer formed provides mechanical support, while the inner low-density layer achieves heat insulation and weight reduction, and the transition layer is continuous without interfacial defects; in terms of efficient process integration, densification, foaming and gradient curing are completed in a single cycle, avoiding multiple mold opening and feeding, and improving production efficiency; through material formulation innovation (nano-enhancement / microcapsule foaming) and process optimization (dynamic temperature control / speed coordination), technical problems such as density gradient control and interlayer bonding strengthening have been overcome, providing a low-cost and high-performance domestic solution for fields such as new energy and transportation, meeting the development direction of the national new material industry, and having significant social and economic benefits.

[0123] The products formed by lightweight hollow products are as follows Figure 2 products such as the financial box, physical fitness box, bullet box, helmet box, trolley case, medical box, general box, supply unit box, fire box, missile box, long box, cultural and entertainment box group, and combat readiness box group shown.

[0124] Example 1: A preparation method of a lightweight hollow product based on polyethylene rotational molding - gradient density, including the following steps,

[0125] (S01) Take 35 parts of high-density polyethylene HDPE ExxonMobil 7203 and 5 parts of linear low-density polyethylene LLDPE Sinopec LLDPE-7050 and mix them as the first matrix;

[0126] (S02) Add 2 parts of nano-silica CY-SP30F, 3 parts of carbon nanotubes CNT single-walled carbon nanotubes 1 part of toughening agent POE-g-MAH, 1 part of PE wax PEW-032, and 1 part of antioxidant BHT to the first matrix in step (S01), and then after 85°C - 8 min of blending, granulation and grinding, the first rotational molding powder is obtained;

[0127] The granulation is to put the blended material into a twin-screw extruder for extrusion, and then obtain the first masterbatch after cooling, pelletizing and drying in sequence; the extrusion temperature of the twin-screw extruder is 190°C;

[0128] The grinding is to place the first masterbatch granulated under nitrogen protection in a co-blended material grinder and grind it into 40 - 50 mesh powder to obtain the first rotational molding powder;

[0129] (S03) Place the first rotational molding powder from step (S02) into a mold, heat it to 280 °C to melt, and rotate it to adhere to the inner wall of the mold to form an outer high-density reinforcement layer; the rotation is a dual-axis rotation formed by an 8 rpm main shaft and a 4 rpm sub-shaft;

[0130] (S04) Take 62 parts of low-density polyethylene LDPEDFDA-7042 and 9 parts of high-density polyethylene HDPE ExxonMobil 7203 and mix them as the second matrix;

[0131] (S05) Add 1 part of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 10 μm to 50 μm;

[0132] (S06) Add 2 parts of compatibilizer LLDPE18362 maleic anhydride grafted polyethylene PE-g-MAH and 1 part of antioxidant BHT to the composite in step (S05), and then successively carry out blending at 85 °C for 8 min, granulation and powder grinding to obtain the second rotational molding powder;

[0133] Granulation is to put the blended material into a twin-screw extruder and extrude it at 190 °C, and then successively carry out cooling, pelletizing and drying to obtain the second masterbatch;

[0134] Powder grinding is to place the second masterbatch obtained by granulation into a co-blending material powder grinder and grind it into 40-mesh to 50-mesh powder under nitrogen protection to obtain the second rotational molding powder;

[0135] (S07) Place the second rotational molding powder from step (S06) into the mold in step (S03), continue to heat and rotate until melting is complete;

[0136] (S08) Cool down to 160 °C to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time reduce the rotation speed, so as to form an inner low-density foaming layer on the inner wall surface of the outer high-density reinforcement layer; in step (S08), reduce the main shaft rotation speed to 4 rpm and reduce the sub-shaft rotation speed to 1 rpm;

[0137] (S09) Use the partition cooling technology to carry out stepped water cooling on the outer high-density reinforcement layer in the mold and stepped air cooling on the inner low-density foaming layer in the mold to obtain the finished lightweight hollow product; in step (S09), the cooling density gradient of the stepped water cooling is 20 °C / min, and the cooling density gradient of the stepped air cooling is 5 °C / min.

[0138] Example 2: A method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density, including the following steps,

[0139] (S01) Take 87 parts of high-density polyethylene HDPE Sinopec D5502GA and 20 parts of linear low-density polyethylene LLDPE ExxonMobil LLDPE-9020, mix them as the first matrix;

[0140] (S02) Add 10 parts of nano-silica SiO2-N-03, 5 parts of carbon nanotubes CNT multi-walled carbon nanotubes, 10 parts of toughening agent POE-g-GMA, 10 parts of PE wax PEW-031 and 10 parts of antioxidant AO-1010 to the first matrix in step (S01). After blending at 110°C for 5 minutes, granulating and grinding, the first rotational molding powder is obtained;

[0141] Granulating means putting the blended material into a twin-screw extruder for extrusion, and then obtaining the first masterbatch after cooling, pelletizing and drying in sequence; the extrusion temperature of the twin-screw extruder is 200°C;

[0142] Grinding means placing the granulated first masterbatch in a blending material grinder under nitrogen protection and grinding it into powder with a mesh size of 42 to 48 meshes to obtain the first rotational molding powder;

[0143] (S03) Place the first rotational molding powder in step (S02) into a mold, heat it to 300°C to melt, and rotate and adhere it to the inner wall of the mold to form an outer high-density reinforced layer; the rotation is a two-axis rotation formed by a main shaft of 12 rpm and a sub-shaft of 6 rpm;

[0144] (S04) Take 87 parts of low-density polyethylene LDPE DFDB-7042 and 18 parts of high-density polyethylene HDPE Sinopec D5502GA, mix them as the second matrix;

[0145] (S05) Add 5 parts of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 15μm to 45μm;

[0146] (S06) Add 5 parts of compatibilizer PE1040 maleic anhydride grafted polyethylene PE-g-MAH and 10 parts of antioxidant AO-1010 to the composite in step (S05). After blending at 110°C for 5 minutes, granulating and grinding, the second rotational molding powder is obtained;

[0147] Granulating means putting the blended material into a twin-screw extruder and extruding it at 200°C, and then obtaining the second masterbatch after cooling, pelletizing and drying in sequence;

[0148] Grinding means placing the granulated second masterbatch in a blending material grinder under nitrogen protection and grinding it into powder with a mesh size of 42 to 48 meshes to obtain the second rotational molding powder;

[0149] (S07) Place the second rotational molding powder from step (S06) into the mold from step (S03), continue heating to melt and rotate until melting is complete;

[0150] (S08) Cool down to 180 °C to trigger the decomposition of the microcapsule blowing agent in the melt from step (S07), and at the same time reduce the rotation speed, so as to form an inner low-density foamed layer on the inner wall surface of the outer high-density reinforcement layer; in step (S08), reduce the main shaft rotation speed to 6 rpm and the auxiliary shaft rotation speed to 3 rpm;

[0151] (S09) Using the partition cooling technology, perform stepped water cooling on the outer high-density reinforcement layer in the mold and stepped air cooling on the inner low-density foamed layer in the mold, and then obtain the finished lightweight hollow product; in step (S09), the cooling density gradient of the stepped water cooling is 25 °C / min, and the cooling density gradient of the stepped air cooling is 10 °C / min.

[0152] Example 3: A method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density, including the following steps,

[0153] (S01) Take 40 parts of high-density polyethylene HDPE (LG Chem Korea HD4600U) and 8 parts of linear low-density polyethylene LLDPE (Borealis LLDPE-9085), mix them as the first matrix;

[0154] (S02) Add 4 parts of nano-silica CY-SP30F, 4 parts of carbon nanotubes CNT (single-walled carbon nanotubes) 3 parts of toughening agent PE-g-MAH, 3 parts of PE wax PEW-030, and 3 parts of antioxidant AO-1076 to the first matrix in step (S01), and then through 90 °C - 7 min of blending, granulation, and powder grinding in sequence, obtain the first rotational molding powder;

[0155] Granulation is to put the blended materials into a twin-screw extruder for extrusion, and then obtain the first masterbatch through cooling, pelletizing, and drying in sequence; the extrusion temperature of the twin-screw extruder is 195 °C;

[0156] Powder grinding is to place the granulated first masterbatch into a blended material powder grinder under nitrogen protection and grind it into powder with a mesh size of 44 - 46 meshes to obtain the first rotational molding powder;

[0157] (S03) Place the first rotational molding powder from step (S02) into the mold, heat it to 285 °C to melt, and rotate it to adhere to the inner wall of the mold to form an outer high-density reinforcement layer; the rotation is a two-axis rotation formed by a 9 rpm main shaft and a 5 rpm auxiliary shaft;

[0158] (S04) Take 65 parts of low-density polyethylene LDPEDFDC-7042 and 10 parts of high-density polyethylene HDPE LG Chem Korea HD4600U, mix them and use as the second matrix;

[0159] (S05) Add 2 parts of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 20μm to 40μm;

[0160] (S06) Add 3 parts of compatibilizer LLDPE18362 maleic anhydride grafted polyethylene PE-g-MAH and 3 parts of antioxidant AO-1076 to the composite in step (S05), and then successively carry out 90°C - 7min blending, granulation and powder grinding to obtain the second rotational molding powder;

[0161] Granulation is to put the blended material into a twin-screw extruder and extrude at 195°C, and then successively carry out cooling, pelletizing and drying to obtain the second masterbatch;

[0162] Powder grinding is to place the second masterbatch obtained by granulation in a blending material powder grinder and grind it into 44 - 46 mesh powder under nitrogen protection to obtain the second rotational molding powder;

[0163] (S07) Place the second rotational molding powder in step (S06) into the mold in step (S03), continue to heat up and rotate until melting is completed;

[0164] (S08) Cool down to 165°C to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time reduce the rotation speed, so as to form an inner low-density foaming layer on the inner wall surface of the outer high-density strengthening layer; in step (S08), reduce the main shaft rotation speed to 5 rpm and reduce the sub-shaft rotation speed to 2 rpm;

[0165] (S09) Use the partition cooling technology to carry out stepped water cooling for the outer high-density strengthening layer in the mold and stepped air cooling for the inner low-density foaming layer in the mold to obtain the finished lightweight hollow product; in step (S09), the cooling density gradient of the stepped water cooling is 21°C / min, and the cooling density gradient of the stepped air cooling is 6°C / min.

[0166] Example 4: A method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density, including the following steps,

[0167] (S01) Take 85 parts of high-density polyethylene HDPE Borealis 7004 and 18 parts of linear low-density polyethylene LLDPE Sinopec LLDPE-7050, mix them and use as the first matrix;

[0168] (S02) Add 8 parts of nano-silica SiO2-N-03, 3 parts of carbon nanotubes (multi-walled carbon nanotubes CNT), 8 parts of toughening agent POE-g-MAH, 8 parts of PE wax PEW-032, and 8 parts of antioxidant AO-3114 to the first matrix in step (S01). After blending at 105 °C for 6 minutes, granulating, and grinding, the first rotational molding powder is obtained;

[0169] Granulating means putting the blended material into a twin-screw extruder for extrusion, and then obtaining the first masterbatch after cooling, pelletizing, and drying in sequence; the extrusion temperature of the twin-screw extruder is 190 °C;

[0170] Grinding means placing the granulated first masterbatch in a blending material grinder under nitrogen protection and grinding it into powder with a mesh size of 40 to 50 meshes to obtain the first rotational molding powder;

[0171] (S03) Place the first rotational molding powder in step (S02) into a mold, heat it to 295 °C to melt, and rotate and attach it to the inner wall of the mold to form an outer high-density reinforcement layer; the rotation is a biaxial rotation formed by a main shaft of 11 rpm and a sub-shaft of 5 rpm;

[0172] (S04) Take 85 parts of low-density polyethylene LDPE DFDA-7042 and 16 parts of high-density polyethylene HDPE Borealis 7004 and mix them as the second matrix;

[0173] (S05) Add 4 parts of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 25 μm to 35 μm;

[0174] (S06) Add 4 parts of compatibilizer PE1040 maleic anhydride grafted polyethylene PE-g-MAH and 8 parts of antioxidant AO-3114 to the composite in step (S05). After blending at 105 °C for 6 minutes, granulating, and grinding, the second rotational molding powder is obtained;

[0175] Granulating means putting the blended material into a twin-screw extruder and extruding at 190 °C, and then obtaining the second masterbatch after cooling, pelletizing, and drying in sequence;

[0176] Grinding means placing the granulated second masterbatch in a blending material grinder under nitrogen protection and grinding it into powder with a mesh size of 40 to 50 meshes to obtain the second rotational molding powder;

[0177] (S07) Place the second rotational molding powder in step (S06) into the mold in step (S03), continue to heat it to melt and rotate until the melting is completed;

[0178] (S08)Cool down to 175 °C to trigger the decomposition of the microcapsule foaming agent in the melt in step (S07), and at the same time reduce the rotation speed, so as to form an inner low-density foaming layer on the inner wall surface of the outer high-density reinforcing layer; in step (S08), reduce the rotation speed of the main shaft to 4 rpm and the rotation speed of the auxiliary shaft to 1 rpm;

[0179] (S09)Using the partition cooling technology, stepwise water-cool the outer high-density reinforcing layer in the mold and stepwise air-cool the inner low-density foaming layer in the mold, and then obtain the finished lightweight hollow product; in step (S09), the cooling density gradient of stepwise water-cooling is 24 °C / min, and the cooling density gradient of stepwise air-cooling is 9 °C / min.

[0180] Example 5: A method for preparing a lightweight hollow product based on polyethylene rotational molding plastic - gradient density, comprising the following steps,

[0181] (S01)Take 45 parts of high-density polyethylene HDPE ExxonMobil 7203 and 10 parts of linear low-density polyethylene LLDPE ExxonMobil LLDPE-9020 and mix them as the first matrix;

[0182] (S02)Add 5 parts of nano-silica CY-SP30F, 5 parts of carbon nanotubes CNT single-walled carbon nanotubes 5 parts of toughening agent POE-g-GMA, 5 parts of PE wax PEW-031 and 4 parts of antioxidant AO-330 to the first matrix in step (S01), and then successively carry out blending at 95 °C for 5 min, granulation and powder grinding to obtain the first rotational molding powder;

[0183] Granulation is to put the blended material into a twin-screw extruder for extrusion, and then successively carry out cooling, pelletizing and drying to obtain the first masterbatch; the extrusion temperature of the twin-screw extruder is 200 °C;

[0184] Powder grinding is to place the first masterbatch obtained by granulation in a co-blended material powder grinder under nitrogen protection and grind it into powder with a mesh size of 42 to 48 meshes to obtain the first rotational molding powder;

[0185] (S03)Place the first rotational molding powder in step (S02) into the mold, heat it up to 290 °C to melt, and rotate and adhere to the inner wall of the mold to form an outer high-density reinforcing layer; the rotation is a two-axis rotation formed by an 8-rpm main shaft and a 6-rpm auxiliary shaft;

[0186] (S04)Take 70 parts of low-density polyethylene LDPEDFDB-7042 and 11 parts of high-density polyethylene HDPE ExxonMobil 7203 and mix them as the second matrix;

[0187] (S05) Add 3 parts of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 10 μm to 50 μm;

[0188] (S06) Add 2 parts of compatibilizer LLDPE18362 maleic anhydride grafted polyethylene PE-g-MAH and 4 parts of antioxidant AO-330 to the composite in step (S05), and then successively carry out blending at 95 °C for 5 min, granulation and grinding to obtain the second rotational molding powder;

[0189] Granulation is to put the blended material into a twin-screw extruder and extrude at 200 °C, and then successively carry out cooling, pelletizing and drying to obtain the second masterbatch;

[0190] Grinding is to place the second masterbatch obtained by granulation in a blending material grinder under nitrogen protection and grind it into powder with a mesh size of 42 to 48 meshes to obtain the second rotational molding powder;

[0191] (S07) Place the second rotational molding powder in step (S06) into the mold in step (S03) and continue to heat up and rotate until melting is completed;

[0192] (S08) Cool down to 170 °C to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time reduce the rotation speed, so as to form an inner low-density foamed layer on the inner wall surface of the outer high-density reinforced layer; in step (S08), reduce the main shaft rotation speed to 6 rpm and the sub-shaft rotation speed to 3 rpm;

[0193] (S09) Using the partition cooling technology, carry out stepped water cooling on the outer high-density reinforced layer in the mold and stepped air cooling on the inner low-density foamed layer in the mold to obtain the finished lightweight hollow product; in step (S09), the cooling density gradient of the stepped water cooling is 22 °C / min, and the cooling density gradient of the stepped air cooling is 7 °C / min.

[0194] Example 6: A preparation method of a lightweight hollow product based on polyethylene rotational molding material - gradient density formation, including the following steps,

[0195] (S01) Take 80 parts of high-density polyethylene HDPE Sinopec D5502GA and 15 parts of linear low-density polyethylene LLDPE Borealis LLDPE-9085 and mix them as the first matrix;

[0196] (S02) Add 6 parts of nano-silica SiO2-N-03 and 4 parts of carbon nanotubes CNT multi-walled carbon nanotubes to the first matrix in step (S01) After adding 6 parts of toughening agent PE-g-MAH, 6 parts of PE wax PEW-030, and 6 parts of antioxidant BHT, the mixture is successively subjected to blending at 100°C for 8 minutes, granulation, and pulverization to obtain the first rotational molding powder.

[0197] The granulation process is as follows: the blended material is put into a twin-screw extruder for extrusion, and then successively cooled, pelletized, and dried to obtain the first masterbatch; the extrusion temperature of the twin-screw extruder is 195°C.

[0198] The pulverization process is as follows: under nitrogen protection, the first masterbatch obtained from granulation is placed in a blending material pulverizer and ground into powder with a mesh size of 44 to 46 meshes to obtain the first rotational molding powder.

[0199] (S03) The first rotational molding powder from step (S02) is placed in a mold and heated to 280°C for melting, and then rotated and adhered to the inner wall of the mold to form an outer high-density reinforcing layer; the rotation is a two-axis rotation formed by a main shaft with a speed of 11 rpm and a sub-shaft with a speed of 5 rpm.

[0200] (S04) Take 80 parts of low-density polyethylene LDPEDFDC-7042 and 15 parts of high-density polyethylene HDPE Sinopec D5502GA and mix them as the second matrix.

[0201] (S05) Add 1 part of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 15 μm to 45 μm.

[0202] (S06) Add 5 parts of compatibilizer PE1040 maleic anhydride grafted polyethylene PE-g-MAH and 7 parts of antioxidant BHT to the composite in step (S05), and then successively carry out blending at 100°C for 8 minutes, granulation, and pulverization to obtain the second rotational molding powder.

[0203] The granulation process is as follows: the blended material is put into a twin-screw extruder and extruded at 195°C, and then successively cooled, pelletized, and dried to obtain the second masterbatch.

[0204] The pulverization process is as follows: under nitrogen protection, the second masterbatch obtained from granulation is placed in a blending material pulverizer and ground into powder with a mesh size of 42 to 48 meshes to obtain the second rotational molding powder.

[0205] (S07) The second rotational molding powder from step (S06) is placed in the mold from step (S03) and continues to be heated for melting and rotated until the melting is completed.

[0206] (S08) Cool down to 160°C to trigger the decomposition of the microcapsule blowing agent in the melt from step (S07), and at the same time reduce the rotation speed, so as to form an inner low-density foaming layer on the inner wall surface of the outer high-density reinforcing layer; in step (S08), the rotation speed of the main shaft is reduced to 5 rpm, and the rotation speed of the sub-shaft is reduced to 2 rpm.

[0207] (S09) Using the partition cooling technology, stepwise water cooling is carried out on the outer high-density reinforcement layer in the mold, and stepwise air cooling is carried out on the inner low-density foaming layer in the mold, and then the lightweight hollow product finished product is obtained; in the step (S09), the cooling density gradient of stepwise water cooling is 23 °C / min, and the cooling density gradient of stepwise air cooling is 8 °C / min.

[0208] Example 7: A preparation method of a lightweight hollow product based on polyethylene rotational molding plastic - gradient density, including the following steps,

[0209] (S01) Take 50 parts of high-density polyethylene HDPE LG Chem Korea HD4600U and 12 parts of linear low-density polyethylene LLDPE Sinopec LLDPE-7050 and mix them as the first matrix;

[0210] (S02) Add 2 parts of nano-silica CY-SP30F, 3 parts of carbon nanotubes CNT single-walled carbon nanotubes, 1 part of toughening agent POE-g-MAH, 1 part of PE wax PEW-032 and 1 part of antioxidant AO-1010 to the first matrix in step (S01), and then successively carry out blending at 85 °C for 7 minutes, granulation and grinding, and then the first rotational molding powder is obtained;

[0211] Granulation is to put the blended material into a twin-screw extruder for extrusion, and then successively carry out cooling, pelletizing and drying to obtain the first masterbatch; the extrusion temperature of the twin-screw extruder is 190 °C;

[0212] Grinding is to place the first masterbatch obtained by granulation in a blending material grinder under nitrogen protection and grind it into 40-mesh to 50-mesh powder, and then the first rotational molding powder is obtained;

[0213] (S03) Place the first rotational molding powder in step (S02) into the mold, heat it to 300 °C to melt, and rotate and adhere to the inner wall of the mold to form an outer high-density reinforcement layer; the rotation is a two-axis rotation formed by a 9 rpm main shaft and a 4 rpm sub-shaft;

[0214] (S04) Take 72 parts of low-density polyethylene LDPE DFDA-7042 and 12 parts of high-density polyethylene HDPE LG Chem Korea HD4600U and mix them as the second matrix;

[0215] (S05) Add 5 parts of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; in the step (S05), the particle size of the microcapsule blowing agent is 20 μm to 40 μm;

[0216] (S06) Add 3 parts of compatibilizer LLDPE18362 (maleic anhydride grafted polyethylene, PE-g-MAH) and 5 parts of antioxidant AO-1010 to the composite in step (S05), and then, after blending at 85 °C for 7 minutes, granulating, and grinding successively, obtain the second rotational molding powder;

[0217] The granulation process is as follows: Feed the blended material into a twin-screw extruder and extrude at 190 °C, and then obtain the second masterbatch after cooling, pelletizing, and drying successively;

[0218] The grinding process is as follows: Under nitrogen protection, place the second masterbatch obtained by granulation in a blending material grinder and grind it into powder with a mesh size of 40 - 50 meshes to obtain the second rotational molding powder;

[0219] (S07) Place the second rotational molding powder in step (S06) into the mold in step (S03), continue to heat up and rotate until melting is completed;

[0220] (S08) Cool down to 180 °C to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time, reduce the rotation speed, so that an inner low-density foaming layer is formed on the inner wall surface of the outer high-density reinforcing layer; in step (S08), reduce the main shaft rotation speed to 4 rpm and reduce the auxiliary shaft rotation speed to 1 rpm;

[0221] (S09) Using the zone cooling technology, perform stepped water cooling on the outer high-density reinforcing layer in the mold and stepped air cooling on the inner low-density foaming layer in the mold to obtain the finished lightweight hollow product; in step (S09), the cooling density gradient of the stepped water cooling is 20 °C / min, and the cooling density gradient of the stepped air cooling is 5 °C / min.

[0222] Example 8: A method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density formation, including the following steps

[0223] (S01) Take 75 parts of high-density polyethylene (HDPE, Borealis 7004) and 13 parts of linear low-density polyethylene (LLDPE, ExxonMobil LLDPE-9020), mix them as the first matrix;

[0224] (S02) Add 10 parts of nano-silica (SiO2-N-03), 5 parts of carbon nanotubes (CNT, multi-walled carbon nanotubes), 10 parts of toughening agent (POE-g-GMA), 10 parts of PE wax (PEW-031), and 10 parts of antioxidant (AO-1076) to the first matrix in step (S01), and then, after blending at 110 °C for 6 minutes, granulating, and grinding successively, obtain the first rotational molding powder;

[0225] Pelletizing: Feed the blended material into a twin-screw extruder for extrusion at 200 °C, and then obtain the first masterbatch after cooling, pelletizing, and drying in sequence; the extrusion temperature of the twin-screw extruder is 200 °C;

[0226] Grinding: Under nitrogen protection, place the first masterbatch obtained from pelletizing into a blended material grinder to grind into powder with a mesh size of 42 to 48 meshes, and then obtain the first rotational molding powder;

[0227] (S03) Place the first rotational molding powder in step (S02) into a mold, heat it to 285 °C to melt, and rotate it to adhere to the inner wall of the mold to form an outer high-density reinforced layer; the rotation is a two-axis rotation formed by a main shaft rotating at 11 rpm and a sub-shaft rotating at 6 rpm;

[0228] (S04) Take 78 parts of low-density polyethylene LDPEDFDB-7042 and 14 parts of high-density polyethylene HDPE Borealis 7004, mix them as the second matrix;

[0229] (S05) Add 2 parts of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 25 μm to 35 μm;

[0230] (S06) Add 4 parts of compatibilizer PE1040 maleic anhydride grafted polyethylene PE-g-MAH and 6 parts of antioxidant AO-1076 to the composite in step (S05), and then obtain the second rotational molding powder after blending at 110 °C for 6 min, pelletizing, and grinding in sequence;

[0231] Pelletizing: Feed the blended material into a twin-screw extruder for extrusion at 200 °C, and then obtain the second masterbatch after cooling, pelletizing, and drying in sequence;

[0232] Grinding: Under nitrogen protection, place the second masterbatch obtained from pelletizing into a blended material grinder to grind into powder with a mesh size of 42 to 48 meshes, and then obtain the second rotational molding powder;

[0233] (S07) Place the second rotational molding powder in step (S06) into the mold in step (S03), continue to heat it to melt and rotate until the melting is completed;

[0234] (S08) Cool down to 165 °C to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time reduce the rotation speed, so as to form an inner low-density foamed layer on the inner wall surface of the outer high-density reinforced layer; in step (S08), reduce the rotation speed of the main shaft to 6 rpm and reduce the rotation speed of the sub-shaft to 3 rpm;

[0235] (S09) Using the partition cooling technology, stepwise water cooling is carried out on the outer high-density reinforcement layer in the mold, and stepwise air cooling is carried out on the inner low-density foaming layer in the mold, and then the lightweight hollow product is obtained; in the step (S09), the cooling density gradient of the stepwise water cooling is 25 °C / min, and the cooling density gradient of the stepwise air cooling is 10 °C / min.

[0236] Example 9: A method for preparing a lightweight hollow product based on polyethylene rotational molding plastic - gradient density formation, comprising the following steps,

[0237] (S01) Take 55 parts of high-density polyethylene HDPE ExxonMobil 7203 and 5 parts of linear low-density polyethylene LLDPE Sinopec LLDPE-7050 and mix them as the first matrix;

[0238] (S02) Add 4 parts of nano-silica CY-SP30F, 4 parts of carbon nanotubes CNT single-walled carbon nanotubes 3 parts of toughening agent PE-g-MAH, 3 parts of PE wax PEW-030 and 3 parts of antioxidant AO-3114 to the first matrix in step (S01), and then carry out 90 °C - 8 min blending, granulation and grinding in sequence to obtain the first rotational molding powder;

[0239] Granulation is to put the blended material into a twin-screw extruder for extrusion, and then obtain the first masterbatch through cooling, pelletizing and drying in sequence; the extrusion temperature of the twin-screw extruder is 195 °C;

[0240] Grinding is to place the granulated first masterbatch in a blending material grinder under nitrogen protection and grind it into 44 - 46 mesh powder to obtain the first rotational molding powder;

[0241] (S03) Place the first rotational molding powder in step (S02) into the mold and heat it to 295 °C for melting, and rotate and adhere to the inner wall of the mold to form an outer high-density reinforcement layer; the rotation is a two-axis rotation formed by an 8 rpm main shaft and a 5 rpm sub-shaft;

[0242] (S04) Take 74 parts of low-density polyethylene LDPE DFDC-7042 and 9 parts of high-density polyethylene HDPE ExxonMobil 7203 and mix them as the second matrix;

[0243] (S05) Add 4 parts of microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 10 μm - 50 μm;

[0244] (S06) After adding 2 parts of compatibilizer LLDPE18362 (maleic anhydride grafted polyethylene, PE-g-MAH) and 1 part of antioxidant BHT to the complex in step (S05), the mixture is successively subjected to blending at 90 °C for 8 min, granulation, and grinding to obtain the second rotational molding powder;

[0245] Granulation is carried out by feeding the blended material into a twin-screw extruder and extruding at 195 °C, followed by cooling, pelletizing, and drying to obtain the second masterbatch;

[0246] Grinding is carried out by placing the second masterbatch obtained from pelletizing in a blending material grinder under nitrogen protection and grinding it into powder with a mesh size of 44 - 46 meshes to obtain the second rotational molding powder;

[0247] (S07) Place the second rotational molding powder in step (S06) into the mold in step (S03), continue to heat up and rotate until melting is completed;

[0248] (S08) Cool down to 175 °C to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time reduce the rotation speed, so that an inner low-density foamed layer is formed on the inner wall surface of the outer high-density reinforced layer; in step (S08), the rotation speed of the main shaft is reduced to 5 rpm, and the rotation speed of the auxiliary shaft is reduced to 2 rpm;

[0249] (S09) Using the zone cooling technology, stepwise water cooling is carried out on the outer high-density reinforced layer in the mold, and stepwise air cooling is carried out on the inner low-density foamed layer in the mold to obtain the finished lightweight hollow product; in step (S09), the cooling density gradient of stepwise water cooling is 21 °C / min, and the cooling density gradient of stepwise air cooling is 6 °C / min.

[0250] Example 10: A method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density formation, comprising the following steps,

[0251] (S01) Take 70 parts of high-density polyethylene HDPE (Sinopec D5502GA) and 20 parts of linear low-density polyethylene LLDPE (Sinopec LLDPE-7050), mix them as the first matrix;

[0252] (S02) Add 8 parts of nano-silica SiO2-N-03, 3 parts of carbon nanotubes CNT (multi-walled carbon nanotubes), 8 parts of toughening agent POE-g-MAH, 8 parts of PE wax PEW-032, and 8 parts of antioxidant AO-330 to the first matrix in step (S01), and then successively carry out blending at 105 °C for 5 min, granulation, and grinding to obtain the first rotational molding powder;

[0253] Pelletizing: feeding the blended materials into a twin-screw extruder for extrusion, and then obtaining the first masterbatch after cooling, pelletizing, and drying in sequence; the extrusion temperature of the twin-screw extruder is 190°C;

[0254] Grinding: under nitrogen protection, placing the first masterbatch obtained from pelletizing into a blended material grinder to grind into powder with a mesh size of 40 to 50 meshes, and then obtaining the first rotational molding powder;

[0255] (S03) Placing the first rotational molding powder in step (S02) into a mold, heating it to 290°C for melting, and rotating it to adhere to the inner wall of the mold to form an outer high-density reinforcing layer; the rotation is a two-axis rotation formed by a main shaft with a speed of 11 rpm and a sub-shaft with a speed of 4 rpm;

[0256] (S04) Taking 76 parts of low-density polyethylene LDPEDFDA-7042 and 18 parts of high-density polyethylene HDPE Sinopec D5502GA, mixing them as the second matrix;

[0257] (S05) Adding 3 parts of a microcapsule blowing agent azodicarbonamide@silica shell composite to the second matrix in step (S04) to obtain a composite; the particle size of the microcapsule blowing agent in step (S05) is 15 μm to 45 μm;

[0258] (S06) Adding 5 parts of a compatibilizer PE1040 maleic anhydride grafted polyethylene PE-g-MAH and 10 parts of an antioxidant AO-330 to the composite in step (S05), and then obtaining the second rotational molding powder after blending at 105°C for 5 minutes, pelletizing, and grinding in sequence;

[0259] Pelletizing: feeding the blended materials into a twin-screw extruder for extrusion at 190°C, and then obtaining the second masterbatch after cooling, pelletizing, and drying in sequence;

[0260] Grinding: under nitrogen protection, placing the second masterbatch obtained from pelletizing into a blended material grinder to grind into powder with a mesh size of 40 to 50 meshes, and then obtaining the second rotational molding powder;

[0261] (S07) Placing the second rotational molding powder in step (S06) into the mold in step (S03), continuing to heat it for melting and rotating until the melting is completed;

[0262] (S08) Cooling to 170°C to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time reducing the rotation speed, so as to form an inner low-density foaming layer on the inner wall surface of the outer high-density reinforcing layer; in step (S08), reducing the rotation speed of the main shaft to 4 rpm and reducing the rotation speed of the sub-shaft to 1 rpm;

[0263] (S09) Using the partition cooling technology, after stepwise water cooling the outer high-density reinforcement layer in the mold and stepwise air cooling the inner low-density foaming layer in the mold, a lightweight hollow product is obtained; in the step (S09), the cooling density gradient of the stepwise water cooling is 24 °C / min, and the cooling density gradient of the stepwise air cooling is 9 °C / min.

[0264] The performance comparisons of Examples 1 to 10 and the comparative examples are shown in Table 1 below;

[0265] Table 1: Performance table of Examples 1 to 10

[0266]

[0267]

[0268] As can be seen from Table 1, the balance of rigidity and toughness of the outer layer performance breaks through the industry bottleneck. All examples have ultra-high rigidity, with a flexural modulus ≥ 1.9 GPa (exceeding the national standard requirement of 1.8 GPa). Among them, Example 1 reaches 1.96 GPa, approaching the level of glass fiber-reinforced PP of 2.0 GPa. Thanks to the synergistic reinforcement of nano-silica and carbon nanotubes, the nano-fillers are distributed directionally and enriched in the outer layer during high-speed rotation, forming a three-dimensional reinforcement network. High-density polyethylene has a high crystalline orientation, and the rapid water cooling process promotes a crystallinity > 70%; the toughness balance is prominent, and the notch impact strength is stable at 9.2 kJ / m 2 ~9.6 kJ / m 2 , thanks to the linear low-density polyethylene toughening phase inducing crazing shear bands, and the lubricating effect of PE wax reducing stress concentration and improving stress transfer efficiency; the lightweight and functionalization of the inner layer foaming structure are coordinated, and the density is in the range of 0.40 g / cm 3 ~0.49 g / cm 3 , thanks to the precise pore control of the microcapsule blowing agent; the closed cell rate reaches 89% - 93%, thanks to the stepwise air cooling inhibiting the coalescence of bubbles; the thermal conductivity is in the range of 0.04 - 0.07 W / (m·K), thanks to the adiabatic network constructed by the closed cell air gaps; Example 3 still maintains a 93% closed cell rate at an ultra-low density of 0.40 g / cm 3 , and the thermal conductivity is better than that of most polymer foams; the interfacial peel strength at the interface junction is 9.2 N / mm - 9.6 N / mm, thanks to the mechanical interlock formed by the rotational speed difference strategy and the promotion of molecular chain entanglement by maleic anhydride-grafted PE; the overall density is (0.91 - 0.96) g / cm 3 +(0.40 - 0.49) g / cm 3 , compared with the existing single-layer products (0.91 - 0.97) g / cm 3It can still maintain high strength with a 35% weight reduction, flexural modulus (1.91 - 1.96) GPa + notched impact strength (9.2 - 9.6) kJ / m 2 , with the synergistic effect of nano-fillers / toughening agents, having a high strength-to-toughness ratio, inner layer thermal conductivity (0.04 - 0.07) W / (m·K), having good heat insulation effect, and the closed-cell structure realizes multi-physical field coupling;

[0269] Examples 1, 2, 3, 6, and 9 can meet the requirements of the battery pack housing (modulus > 1.8 GPa + thermal conductivity ≤ 0.05 W / (m·K)) and the fuel tank penetration standard (closed-cell rate > 90%);

[0270] The density of Examples 3, 5, and 7 ≤ 0.42 g / cm 3 Density + 93% closed-cell rate achieves an industry breakthrough (traditional foamed PE closed-cell rate < 80%), and the weight reduction effect is equivalent to that of aluminum honeycomb core materials, which can meet the requirements of aerospace;

[0271] The interfacial strength ≥ 9.5 N / mm in Examples 1, 2, 4, 8, 9, and 10 is 20% higher than that of the lamination process;

[0272] The gradient material system of the present invention is designed by "nano-enhancement - foaming synergy" and precisely matched by "process - component". Its comprehensive performance exceeds the mainstream market solutions (such as micro-foamed PP, structural foam PET). It is especially suitable for extreme working conditions such as new energy vehicle battery packs and spacecraft fuel tanks. The outer layer, inner layer, and the transition between them have a density gradient. The foaming uniformity is achieved through the delayed decomposition technology of microcapsule blowing agents (the triggering temperature matches the melting window), which will not affect the mechanical properties. The staged melting - rotation process ensures the fusion degree of the interface between the outer layer and inner layer materials (the interlayer peel strength reaches 8 - 10 MPa), avoiding the weak interface problem of traditional bonding processes, and can achieve continuous gradient transition. Within a single rotational molding cycle, through the coordinated control of temperature - rotational speed - pressure, the outer layer densification (formed by high-temperature and high-speed rotation), inner layer controllable foaming (cooling and low-speed rotation trigger microcapsule foaming to inhibit cell damage), and gradient curing (outer layer water cooling / inner layer air cooling to stabilize the gradient structure) are completed in stages. The single-cycle process efficiency is increased by more than 30%, greatly reducing the cost. The penetration depth between the inner layer and outer layer is controllable, the density gradient continuity is good, and the requirements for molds are not high. It can prepare large hollow parts (such as storage tanks, ship hulls), and can simultaneously meet multi-functional requirements such as lightweight (target density ≤ 0.6 g / cm3), high strength (outer layer compressive strength ≥ 25 MPa), and heat insulation (inner layer thermal conductivity ≤ 0.1 W / m·K), and is suitable for applications in high-end scenarios such as liquid hydrogen storage tanks and electric vehicle battery packs.

Claims

1. A method for preparing a lightweight hollow product formed based on polyethylene rolling plastic - gradient density, characterized in that: including the following steps, (S01) Take appropriate amounts of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) and mix them as the first matrix; (S02) Add appropriate amounts of nano-silica, carbon nanotubes (CNT), toughening agent, PE wax, and antioxidant to the first matrix in step (S01), and then through blending, granulation, and milling in sequence to obtain the first rotational molding powder; (S03) Place the first rotational molding powder in step (S02) into a mold, heat it to melt, and rotate it to adhere to the inner wall of the mold to form an outer high-density reinforcement layer; (S04) Take appropriate amounts of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) and mix them as the second matrix; (S05) Add an appropriate amount of microcapsule blowing agent composite to the second matrix in step (S04) to obtain a composite; (S06) Add appropriate amounts of compatibilizer and antioxidant to the composite in step (S05), and then through blending, granulation, and milling in sequence to obtain the second rotational molding powder; (S07) Place the second rotational molding powder in step (S06) into the mold in step (S03), continue to heat it to melt and rotate until the melting is complete; (S08) Cool down to trigger the decomposition of the microcapsule blowing agent in the melt in step (S07), and at the same time reduce the rotation speed to form an inner low-density foaming layer on the inner wall surface of the outer high-density reinforcement layer; (S09) Use the partition cooling technology to perform stepped water cooling on the outer high-density reinforcement layer in the mold and stepped air cooling on the inner low-density foaming layer in the mold to obtain the finished lightweight hollow product.

2. The method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density according to claim 1, characterized in that: In step (S01), the dosage of high-density polyethylene is 35 parts to 87 parts; in step (S01), the high-density polyethylene is selected from at least one of ExxonMobil 7203, Sinopec D5502GA, LG Chem HD4600U of South Korea, or Borealis 7004; In step (S01), the dosage of linear low-density polyethylene is 5 parts to 20 parts; the linear low-density polyethylene is selected from at least one of Sinopec LLDPE-7050, ExxonMobil LLDPE-9020, or Borealis LLDPE-9085.

3. The method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density according to claim 1, characterized in that: In step (S02), the addition amount of nano-silica is 2 parts to 10 parts; in step (S02), the nano-silica is selected from at least one of CY-SP30F or SiO2-N-03; The addition amount of carbon nanotubes in the step (S02) is 3 parts to 5 parts; the carbon nanotubes selected in the step (S02) are single-walled carbon nanotubes or multi-walled carbon nanotubes at least one of them; In step (S02), the addition amount of toughening agent is 1 part to 10 parts; in step (S02), the toughening agent is selected from at least one of POE-g-MAH, POE-g-GMA, or PE-g-MAH; In step (S02), the addition amount of PE wax is 1 part to 10 parts; in step (S02), the PE wax is selected from at least one of PEW-032, PEW-031, or PEW-030; The addition amount of the antioxidant in the step (S02) is 1 part to 10 parts; in the step (S02), the antioxidant is selected from at least one of BHT, AO-1010, AO-1076, AO-3114 or AO-330; The blending temperature in the step (S02) is 85°C to 110°C; the blending time in the step (S02) is 5 min to 8 min.

4. The preparation method of the lightweight hollow product formed based on polyethylene rolling plastic - gradient density according to claim 1, characterized in that: The granulation in the step (S02) is as follows The blended material is put into a twin-screw extruder for extrusion, and then after cooling, pelletizing and drying in sequence, the first masterbatch is obtained; the extrusion temperature of the twin-screw extruder is 190°C to 200°C; The powder grinding in the step (S02) is as follows Under nitrogen protection, the first masterbatch obtained by pelletizing is placed in a blended material powder grinder for powder grinding to obtain the first rotational molding powder; the powder grinding particle size in the step (S02) is 40 mesh to 50 mesh.

5. The preparation method of the lightweight hollow product formed based on polyethylene rolling plastic - gradient density according to claim 1, characterized in that: The melting temperature of the step (S03) is 280°C to 300°C; the rotation in the step (S03) is a twin-shaft rotation formed by a main shaft and a sub-shaft; the rotation speed of the main shaft is 8 rpm to 12 rpm; the rotation speed of the sub-shaft is 4 rpm to 6 rpm.

6. The preparation method of the lightweight hollow product formed based on polyethylene rotational molding material - gradient density according to claim 1, characterized in that: The dosage of the low-density polyethylene in the step (S04) is 62 parts to 87 parts; in the step (S04), the low-density polyethylene is selected from at least one of DFDA-7042, DFDB-7042 or DFDC-7042; The dosage of the high-density polyethylene in the step (S04) is 9 parts to 18 parts; in the step (S04), the high-density polyethylene is selected from at least one of ExxonMobil 7203, Sinopec D5502GA, South Korea LG Chem HD4600U or Borealis 7004.

7. The preparation method of the lightweight hollow product formed based on polyethylene rolling plastic - gradient density according to claim 1, characterized in that: The addition amount of the microcapsule blowing agent in the step (S05) is 1 part to 5 parts; the particle size of the microcapsule blowing agent in the step (S05) is 10 μm to 50 μm; in the step (S05), the microcapsule blowing agent is selected from azodicarbonamide@silica shell; The addition amount of the compatibilizer in the step (S06) is 2 parts to 5 parts; in the step (S06), the compatibilizer is selected from maleic anhydride grafted polyethylene (PE-g-MAH); the maleic anhydride grafted polyethylene is selected from at least one of LLDPE18362 or PE1040; The addition amount of the antioxidant in the step (S06) is 1 part to 10 parts; in the step (S06), the antioxidant is selected from at least one of BHT, AO-1010, AO-1076, AO-3114 or AO-330.

8. The preparation method of the lightweight hollow product formed based on polyethylene rolling plastic - gradient density according to claim 1, characterized in that: The blending temperature in the step (S06) is 85°C to 110°C; the blending time in the step (S06) is 5 min to 8 min; The granulation in the step (S06) is as follows The blended material is put into a twin-screw extruder for extrusion, and then after cooling, pelletizing and drying in sequence, the second masterbatch is obtained; the extrusion temperature of the twin-screw extruder is 190°C to 200°C; The milling in step (S06) is as follows: Under nitrogen protection, the second masterbatch obtained by granulation is placed in a co - blend mill for milling to obtain the second rotational molding powder; the particle size of the milling in step (S06) is 40 mesh to 50 mesh.

9. The method for preparing a lightweight hollow product based on polyethylene rotational molding material - gradient density according to claim 1, wherein: In step (S08), the temperature is lowered to 160°C to 180°C to trigger the decomposition of the micro - capsule foaming agent in the melt; In step (S08), the rotational speed of the main shaft is reduced to 4 rpm to 6 rpm, and the rotational speed of the auxiliary shaft is reduced to 1 rpm to 3 rpm; In step (S09), the temperature - lowering density gradient for step - by - step water cooling is 20°C / min to 25°C / min, and the temperature - lowering density gradient for step - by - step air cooling is 5°C / min to 10°C / min.

10. A lightweight hollow product formed based on polyethylene rolling plastic - gradient density, characterized in that: It comprises the following components by weight: 44 parts to 105 parts of high - density polyethylene, 5 parts to 20 parts of linear low - density polyethylene, 2 parts to 10 parts of nano - silica, 3 parts to 5 parts of carbon nanotubes, 1 part to 10 parts of toughening agent, 1 part to 10 parts of PE wax, 2 parts to 20 parts of antioxidant, 62 parts to 87 parts of low - density polyethylene, 1 part to 5 parts of micro - capsule foaming agent, 2 parts to 5 parts of compatibilizer.

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