Rotational molding surface-pit-free crosslinked polyethylene composition and preparation method thereof

Abstract

The invention provides a crosslinked polyethylene composition without pits on a rotational molding surface and a preparation method of the crosslinked polyethylene composition, and belongs to the technical field of high polymer materials. The crosslinked polyethylene composition without pits on the rotational molding surface is prepared from the following components in parts by weight: 100 parts of polyethylene resin, 0.1 to 2 parts of high-temperature organic peroxide, 0.5 to 3 parts of crosslinking aid, 0.05 to 0.5 part of nitroso ether and 0.1 to 1 part of antioxidant. The nitroso ether compound is introduced, high-activity nitroxide free radicals generated by the nitroso ether compound at a high temperature are used for quenching primary free radicals generated by decomposition of peroxide, generation of low-boiling-point byproducts is inhibited, and a rolling part product without pits in the surface of a rolling part can be obtained under the rotational molding condition of high temperature or rapid temperature rise.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a cross-linked polyethylene composition with a pit-free surface formed by rotational molding and its preparation method. Background Technology

[0002] Rotational molding, along with injection molding and blow molding, is a plastic molding process. In rotational molding, plastic powder is first placed in a metal mold, then rotated and heated simultaneously. The powder gradually melts and adheres to the mold cavity, and after cooling and solidification, a hollow plastic product is formed. The main characteristic of rotational molding is pressureless molding, thus requiring less sophisticated molds, giving it a significant economic advantage for producing large products. Currently, polyethylene is the primary material used in global rotational molding. Polyethylene has advantages such as low cost, chemical stability, and easy grinding. Especially after the successful commercialization of linear low-density polyethylene in the late 1970s and early 1980s, its environmental stress cracking resistance (ESCR) improved significantly, making it the main raw material in the rotational molding industry, currently holding a market share of over 85%. To further improve the temperature resistance, mechanical properties, and environmental stress cracking resistance of polyethylene, cross-linked polyethylene is gradually gaining market favor.

[0003] The cross-linking of polyethylene by rotational molding is mainly caused by free radical cross-linking induced by peroxides. During heating, the peroxides decompose to produce free radicals in low molecular weight molecules. These free radicals then capture hydrogen from the polyethylene to generate low-boiling-point substances such as alkanes, aldehydes, and acids. Considering that rotational molding is a pressureless molding process, these low-boiling-point substances cannot be removed in time, leading to the formation of bubbles or pits on the surface of the rolled part. Patent CN104334630A discloses a cross-linked polyethylene composition with improved processing performance. By using a peroxide cross-linking agent, a cross-linking accelerator, and a free radical inhibitor, premature cross-linking is delayed, reducing bubble formation. It shows good cross-linking effect at 200°C. In current technologies, most methods effectively reduce bubble formation under low-temperature processing conditions, such as when the mold temperature is below 190°C. However, when the processing temperature increases, such as when open flame processing generates localized high temperatures, pits appear on the surface, severely affecting the appearance and performance of the product. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a cross-linked polyethylene composition with a pit-free surface for rotational molding and its preparation method. This invention introduces nitrosoether compounds, utilizing their highly reactive nitroso radicals generated at high temperatures to quench primary free radicals produced by peroxide decomposition, thus inhibiting the formation of low-boiling-point byproducts and solving the problems of surface bubbles or pits under high-temperature processing conditions, and the difficulty in reducing bubble formation. A further objective of this invention is to control the proportions of each component to form a homogeneous melt and achieve uniform cross-linking at higher temperatures, thereby solving problems such as poor cross-linking degree and gel rate, low resistance to environmental stress cracking, and localized over- or under-cross-linking.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a cross-linked polyethylene composition with a pit-free surface for rotational molding, comprising the following components in parts by weight: 100 parts of polyethylene resin, 0.1-2 parts of high-temperature organic peroxide, 0.5-3 parts of cross-linking aid, 0.05-0.5 parts of nitroso ether, and 0.1-1 parts of antioxidant.

[0006] This invention introduces nitrosyl ether compounds. These nitrosyl ethers generate highly reactive nitrosyl-oxygen radicals (TEMPO) at high temperatures, which combine with low-boiling-point free radicals generated during rotational molding to form stable, high-molecular-weight substances. This suppresses the excessive formation of bubbles or pits under high-temperature rotational molding conditions. In high-temperature rotational molding, high-temperature organic peroxides generate a large number of free radicals locally within a short period, especially when the temperature rises rapidly. Common crosslinking aids have limited activity and cannot effectively react with these free radicals. The large number of free radicals then generate a large amount of low-boiling-point substances through side reactions, leading to bubble formation. The highly reactive TEMPO free radicals generated by nitrosyl ether compounds at high temperatures can efficiently quench the primary free radicals produced by the decomposition of peroxides, generating stable, high-molecular-weight substances and suppressing the formation of low-boiling-point byproducts. This reduces bubble formation and results in a crosslinked polyethylene composition with a pit-free surface, dense interior, and uniform crosslinking.

[0007] Preferably, the crosslinking aid is one or more selected from acrylates, methacrylates, polybutadienes, and allyl compounds. This invention provides a large number of effective functional groups through the crosslinking aid, further promoting crosslinking efficiency and ensuring that the degree of crosslinking, gelation rate, and resistance to environmental stress cracking are maintained at a high level.

[0008] Preferably, the crosslinking aid is one or more of triallyl cyanurate, triallyl isocyanurate, and diallyl phthalate.

[0009] Preferably, the high-temperature organic peroxide is one or more of dicumyl peroxide, bis(2,5-dimethylhexanoyl) peroxide, di-tert-butyl peroxide, and di-tert-pentyl peroxide. The high-temperature organic peroxide of this invention is an organic peroxide with a half-life greater than 1 minute in an alkane solvent at 160°C. During high-temperature rotational molding, the decomposition rate of the high-temperature organic peroxide of this invention is controllable as the temperature rises, enabling the initiation of cross-linking reactions and preventing premature or violent cross-linking. When the temperature reaches its target value, the decomposition rate intensifies. At this point, the highly reactive nitrosamine free radicals generated by nitrosyl ether compounds begin to play a role, rapidly capturing and quenching excess free radicals generated by the organic peroxide under high-temperature conditions, generating stable high-molecular-weight substances, and inhibiting the formation of low-boiling-point byproducts.

[0010] Preferably, the antioxidant is a compound system of hindered phenolic antioxidants and phosphite antioxidants; the mass ratio of the hindered phenolic antioxidant to the phosphite antioxidant is 1:(1.5-2.5). This invention improves long-term thermal stability by capturing free radicals through the antioxidant compound system, effectively preventing the breakage of polyethylene molecular chains and ensuring the strength of the cross-linked network; it inhibits the transmission of oxidative free radicals, avoids the generation of byproducts, does not affect the primary free radicals generated by the decomposition of peroxides from nitroso ether compounds, and does not affect the occurrence of cross-linking reactions initiated by peroxides, further ensuring the controllability of the reaction system.

[0011] Preferably, the antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl)phosphite.

[0012] Preferably, the polyethylene resin contains at least 95% vinyl groups and at least one C3-C8 olefin group by mass, and has a density of 0.940-0.970 g / cm³. 3 The melt flow index is 10-30 g / 10 min.

[0013] On the other hand, the present invention provides a method for preparing a cross-linked polyethylene composition with a pit-free surface after rotational molding, comprising the following steps: S1: Polyethylene resin, antioxidant, and nitrosyl ether are melt-blended, extruded, granulated, and ground to obtain a premixed powder; S2: The premixed powder, high-temperature organic peroxide, and crosslinking aid are heated and mixed in a high-speed mixer to obtain a blended powder; S3: The blended powder is loaded into a mold, and after rotational molding in a rotational molding furnace, it is cooled to obtain a cross-linked polyethylene composition roll with a pit-free surface.

[0014] This invention first melt-blends antioxidants, nitrosyl ethers, and polyethylene resin, effectively ensuring uniform distribution of antioxidants and nitrosyl ethers within the polyethylene. Other functional additives are then dry-blended by heating, ensuring uniform distribution on the surface of the polyethylene powder. During rotational molding, before the temperature reaches the target value for the drastic decomposition of high-temperature organic peroxides, the release of free radicals remains relatively mild, and the crosslinking reaction proceeds smoothly and uniformly. However, once the temperature rises to the target value, the organic peroxides, at high temperatures or with rapid temperature increases, locally generate a large number of free radicals in a short period. Since the crosslinking aids have limited activity and cannot effectively react with these free radicals, side reactions inevitably generate a large number of low-boiling-point substances, ultimately forming bubbles. However, the highly reactive nitrosyl free radicals generated by nitrosyl ethers at high temperatures can rapidly capture and quench these low-boiling-point free radicals, generating stable high-molecular-weight substances, thereby suppressing the bubbles or pits that are easily generated under high-temperature rotational molding conditions. This invention utilizes the action of nitrosyl ethers to suppress the localized excessive cross-linking phenomenon caused by the rapid release of free radicals during high temperatures or rapid temperature rises. This results in a more uniform degree of cross-linking, reduces uneven cross-linking within the material, and yields a final product with a pit-free surface, dense interior, and uniform cross-linking. Furthermore, it exhibits high cross-linking degree, gelation rate, and resistance to environmental stress cracking.

[0015] Preferably, in step S3, the rotational molding temperature is >290℃, the rotational molding time is >1200s, and the maximum air temperature inside the mold is >210℃. This invention, by introducing nitrosyl ethers, ensures the stable progress of the crosslinking reaction even when the maximum air temperature inside the mold is above 210℃, effectively reducing bubble formation and resulting in crosslinked polyethylene composition rolls with a pit-free surface.

[0016] Preferably, in step S2, the heating and blending temperature is 60°C and the heating and blending time is 5-10 min.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention addresses the problem of insufficient crosslinking by controlling the decomposition efficiency of high-temperature organic peroxides during high-temperature rotational molding, thus preventing premature or violent crosslinking reactions. The use of nitrosoethers inhibits the rapid generation of numerous free radicals in short-term localized areas, preventing uneven crosslinking and reduced gelation rate. This solves the problem of relying solely on crosslinking aids to suppress the formation of numerous low-boiling-point substances and bubbles from side reactions. Furthermore, the invention improves long-term thermal stability through a compound antioxidant system, effectively preventing polyethylene molecular chain breakage and ensuring the strength of the crosslinked network. It also avoids the formation of byproducts, does not affect the peroxide-induced crosslinking reaction, and further ensures the controllability of the reaction system. Finally, this invention ensures melt fluidity and promotes uniform and stable crosslinking reactions even at temperatures exceeding 210°C within the mold, resulting in a crosslinked polyethylene composition with a pit-free surface, dense interior, and uniform crosslinking. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0019] General Implementation Examples A method for preparing a cross-linked polyethylene composition with a pit-free surface for rotational molding includes the following steps: S1: A cross-linked polyethylene composition with a pit-free surface after rotational molding, comprising the following components in parts by weight: 100 parts polyethylene resin, 0.1-2 parts high-temperature organic peroxide, 0.5-3 parts cross-linking aid, 0.05-0.5 parts nitroso ether, and 0.1-1 parts antioxidant.

[0020] In some preferred embodiments of the present invention, the crosslinking aid is one or more of acrylates, methacrylates, polybutadienes, and allyl compounds.

[0021] In some more preferred embodiments of the present invention, the crosslinking aid is one or more of triallyl cyanurate, triallyl isocyanurate, and diallyl phthalate.

[0022] In some preferred embodiments of the present invention, the high-temperature organic peroxide is an organic peroxide with a half-life greater than 1 min in an alkane solvent at a temperature of 160°C.

[0023] In some more preferred embodiments of the present invention, the high-temperature organic peroxide is one or more of dicumyl peroxide, bis(2,5-dimethylhexanoyl) peroxide, di-tert-butyl peroxide, and di-tert-pentyl peroxide.

[0024] In some preferred embodiments of the present invention, the antioxidant is a compound system of hindered phenolic antioxidant and phosphite antioxidant; the mass ratio of the hindered phenolic antioxidant to the phosphite antioxidant is 1:(1.5-2.5).

[0025] In some more preferred embodiments of the present invention, the antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl)phosphite.

[0026] In some preferred embodiments of the present invention, the polyethylene resin contains not less than 95% by mass of vinyl groups and at least one C3-C8 olefin group, and has a density of 0.940-0.970 g / cm³. 3 The melt flow index is 10-30 g / 10 min.

[0027] In some preferred embodiments of the present invention, the structural formula of the nitrosyl ether is: .

[0028] S2: Polyethylene resin, antioxidant, and nitroso ether are melt-blended, extruded, granulated, and ground to obtain a premixed powder.

[0029] In some preferred embodiments of the present invention, the melt blending temperature in step S2 is 170-210°C.

[0030] S3: The premixed powder, high-temperature organic peroxide, and crosslinking aid are heated and mixed in a high-speed mixer to obtain a blended powder.

[0031] In some preferred embodiments of the present invention, in step S3, the heating and blending temperature is 60°C and the heating and blending time is 5-10 min.

[0032] S4: The blended powder is loaded into a mold, and after rotational molding in a rotational molding furnace, it is cooled to obtain a cross-linked polyethylene composition roll with a pit-free surface.

[0033] In some preferred embodiments of the present invention, in step S4, the rotational molding temperature is >290°C, the rotational molding time is >1200s, and the maximum air temperature inside the mold is >210°C.

[0034] After testing, the cross-linked polyethylene composition of the present invention, when the highest air temperature in the mold during rolling is 190°C, has no pits on the surface and a cross-linking degree of over 70%; when the highest air temperature in the mold during rolling is 215°C, has no pits on the surface and a gelation rate of over 70%.

[0035] Example 1 A method for preparing a cross-linked polyethylene composition with a pit-free surface for rotational molding includes the following steps: S1: A cross-linked polyethylene composition with a pit-free surface after rotational molding, comprising the following components in parts by weight: 100 parts of polyethylene resin ZH2911, 0.5 parts of bis(2,5-dimethylhexanoyl)peroxide, 1 part of triallyl cyanurate, 0.06 parts of nitrosoether NOR 116, 0.1 parts of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.2 parts of tris(2,4-di-tert-butylphenyl)phosphite; wherein the density of polyethylene resin ZH2911 is 0.960 g / cm³. 3 The melt index is 20 g / 10 min, and the antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] and tris(2,4-di-tert-butylphenyl) phosphite in a mass ratio of 1:2; S2: Polyethylene resin, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris(2,4-di-tert-butylphenyl)phosphite, and nitrosyl ether NOR 116 are melt-blended at 200℃, extruded in a twin-screw extruder, granulated, and ground to obtain a premixed powder. S3: The premixed powder, bis(2,5-dimethylhexanoyl)peroxide, and triallyl cyanurate are heated and mixed in a high-speed mixer at a controlled temperature of 60°C for 8 minutes to obtain the blended powder. S4: The blended powder is loaded into a mold and rotated in a rotational molding furnace at a temperature of 300°C for 1300 seconds. After cooling, a cross-linked polyethylene composition roll with a pit-free surface is obtained.

[0036] Example 2 A method for preparing a cross-linked polyethylene composition with a pit-free surface for rotational molding includes the following steps: S1: A cross-linked polyethylene composition with a pit-free surface after rotational molding, comprising the following components in parts by weight: 100 parts of polyethylene resin ZH2911, 0.5 parts of bis(2,5-dimethylhexanoyl)peroxide, 1 part of triallyl cyanurate, 0.1 parts of nitrosoether NOR 116, 0.1 parts of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.2 parts of tris(2,4-di-tert-butylphenyl)phosphite; wherein the density of polyethylene resin ZH2911 is 0.960 g / cm³. 3 The melt index is 20 g / 10 min, and the antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] and tris(2,4-di-tert-butylphenyl) phosphite in a mass ratio of 1:2; S2: Polyethylene resin, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris(2,4-di-tert-butylphenyl)phosphite, and nitrosyl ether NOR 116 are melt-blended at 200℃, extruded in a twin-screw extruder, granulated, and ground to obtain a premixed powder. S3: The premixed powder, bis(2,5-dimethylhexanoyl)peroxide, and triallyl cyanurate are heated and mixed in a high-speed mixer at a controlled temperature of 60°C for 8 minutes to obtain the blended powder. S4: The blended powder is loaded into a mold and rotated in a rotational molding furnace at a temperature of 300°C for 1300 seconds. After cooling, a cross-linked polyethylene composition roll with a pit-free surface is obtained.

[0037] Example 3 A method for preparing a cross-linked polyethylene composition with a pit-free surface for rotational molding includes the following steps: S1: A cross-linked polyethylene composition with a pit-free surface after rotational molding, comprising the following components in parts by weight: 100 parts of polyethylene resin ZH2911, 0.5 parts of bis(2,5-dimethylhexanoyl)peroxide, 1 part of triallyl cyanurate, 0.4 parts of nitrosoether NOR 116, 0.1 parts of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.2 parts of tris(2,4-di-tert-butylphenyl)phosphite; wherein the density of polyethylene resin ZH2911 is 0.960 g / cm³. 3 The melt index is 20 g / 10 min, and the antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] and tris(2,4-di-tert-butylphenyl) phosphite in a mass ratio of 1:2; S2: Polyethylene resin, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris(2,4-di-tert-butylphenyl)phosphite, and nitrosyl ether NOR 116 are melt-blended at 200℃, extruded in a twin-screw extruder, granulated, and ground to obtain a premixed powder. S3: The premixed powder, bis(2,5-dimethylhexanoyl)peroxide, and triallyl cyanurate are heated and mixed in a high-speed mixer at a controlled temperature of 60°C for 8 minutes to obtain the blended powder. S4: The blended powder is loaded into a mold and rotated in a rotational molding furnace at a temperature of 300°C for 1300 seconds. After cooling, a cross-linked polyethylene composition roll with a pit-free surface is obtained.

[0038] Comparative Example 1 A method for preparing a rotationally molded cross-linked polyethylene composition includes the following steps: S1: A cross-linked polyethylene composition with a pit-free surface after rotational molding, comprising the following components in parts by weight: 100 parts of polyethylene resin ZH2911, 0.5 parts of bis(2,5-dimethylhexanoyl)peroxide, 1 part of triallyl cyanurate, 0.1 parts of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.2 parts of tris(2,4-di-tert-butylphenyl)phosphite; wherein the density of polyethylene resin ZH2911 is 0.960 g / cm³. 3 The melt index is 20 g / 10 min, and the antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] and tris(2,4-di-tert-butylphenyl) phosphite in a mass ratio of 1:2. S2: Polyethylene resin, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite are melt-blended at 200°C, extruded in a twin-screw extruder, granulated and ground to obtain a premixed powder. S3: The premixed powder, bis(2,5-dimethylhexanoyl)peroxide, and triallyl cyanurate are heated and mixed in a high-speed mixer at a controlled temperature of 60°C for 8 minutes to obtain the blended powder. S4: The blended powder is loaded into a mold and rotated in a rotational molding furnace at a temperature of 300°C for 1300 seconds. After cooling, the cross-linked polyethylene composition roll is obtained.

[0039] Comparative Example 2 A method for preparing a rotationally molded cross-linked polyethylene composition includes the following steps: S1: A cross-linked polyethylene composition with a pit-free surface after rotational molding, comprising the following components in parts by weight: 100 parts of polyethylene resin ZH2911, 0.5 parts of bis(2,5-dimethylhexanoyl)peroxide, 1 part of triallyl cyanurate, 1 part of nitrosoether NOR 116, 0.1 parts of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 0.2 parts of tris(2,4-di-tert-butylphenyl)phosphite; wherein the density of polyethylene resin ZH2911 is 0.960 g / cm³. 3 The melt index is 20 g / 10 min, and the antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid] and tris(2,4-di-tert-butylphenyl) phosphite in a mass ratio of 1:2; S2: Polyethylene resin, pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tris(2,4-di-tert-butylphenyl)phosphite, and nitrosyl ether NOR 116 are melt-blended at 200℃, extruded in a twin-screw extruder, granulated, and ground to obtain a premixed powder. S3: The premixed powder, bis(2,5-dimethylhexanoyl)peroxide, and triallyl cyanurate are heated and mixed in a high-speed mixer at a controlled temperature of 60°C for 8 minutes to obtain the blended powder. S4: The blended powder is loaded into a mold and rotated in a rotational molding furnace at a temperature of 300°C for 1300 seconds. After cooling, the cross-linked polyethylene composition roll is obtained.

[0040] The compositions obtained in Examples 1-3 and Comparative Examples 1-2 were tested for surface finish and crosslinking degree at 190°C, and surface finish and gelation rate at 215°C. The test results are shown in Table 1 below. Note: 190℃ and 215℃ are the highest air temperatures inside the mold when rolling parts.

[0041] This invention uses polyethylene as the matrix, providing a uniformly meltable system basis for the high-temperature rotational molding process. During heating, high-temperature organic peroxides decompose and release free radicals that steal hydrogen atoms from the polyethylene molecular chain, generating macromolecular free radicals that initiate cross-linking reactions. However, due to the high temperature during rotational molding, high-temperature organic peroxides locally release a large number of free radicals in a short period, leading to chain reactions and free radical propagation, producing numerous byproducts. Gases or volatiles are difficult to escape in time. Therefore, this invention introduces nitrosoether compounds into the composition formulation. Nitrosoethers can generate highly active nitroxide free radicals, capturing low-boiling-point free radicals, eliminating primary free radicals generated by the decomposition of high-temperature organic peroxides, delaying the localized and excessive initiation of the cross-linking reaction, and inhibiting the localized generation of a large number of free radicals by organic peroxides in a short time at high temperatures or rapid heating. This compensates for the limited activity of cross-linking aids, avoids the generation of large amounts of low-boiling-point substances by side reactions, reduces the formation of bubbles or pits, and yields a cross-linked polyethylene composition with a pit-free surface, dense interior, and uniform cross-linking.

[0042] The antioxidants used in this invention are a compound system of hindered phenolic antioxidants and phosphite antioxidants. First, they are compounded with polyethylene, which protects the polyethylene molecules in the early stages, preventing extensive chain breakage due to thermal oxidation during high-temperature treatment. This ensures the long-term stability of polyethylene, promotes the integrity of the subsequently formed cross-linked network, and reduces cross-linking point defects. Simultaneously, the nitrosamines are uniformly distributed in the polyethylene during compounding, providing a homogeneous matrix for the subsequent cross-linking reaction and promoting uniform reaction. Subsequently, processing aids, including high-temperature organic peroxides and cross-linking aids, are introduced only through dry mixing, promoting uniform mixing of the processing aids with the premixed powder and providing a homogeneous cross-linking reaction system for high-temperature rotational molding. The high-temperature organic peroxide of this invention is an organic peroxide with a half-life > 1 min in an alkane solvent at 160°C. Its decomposition rate is controllable as the temperature rises during rotational molding, preventing premature or violent cross-linking reactions. Before reaching the target temperature, the decomposition rate is slow, and the cross-linking reaction is relatively stable. After reaching the target temperature, decomposition intensifies, releasing a large number of free radicals, which may affect the intensification of local cross-linking reactions. However, at this time, the highly reactive nitrosyl ether compounds release highly reactive nitroxide free radicals that can capture these excess free radicals generated at high temperatures, maintaining the free radicals at a stable level. This stabilizes the cross-linking reaction, removes a large number of low-boiling-point substances generated by the side reaction, promotes the cross-linking reaction to proceed uniformly and stably in a bubble-free system, reduces local defects, and does not affect the degree of cross-linking and gel rate. Acrylic esters, methacrylates, polybutadiene, allyl compounds, etc., as cross-linking aids, through their numerous effective functional groups, can combine with free radicals in polyethylene molecules during the cross-linking reaction, improve cross-linking efficiency, further strengthen the cross-linking network, and promote the formation of internally dense, highly resistant to environmental stress cracking, and pit-free rotationally molded cross-linked polyethylene compositions.

[0043] According to the test results, the cross-linked polyethylene compositions obtained in Examples 1-3, when different amounts of nitro ether compounds are added, achieve a surface free of pits under both low-temperature processing (190°C) and high-temperature processing (above 200°C, such as 215°C), with both cross-linking degree and gel rate reaching over 70%. Increasing the amount of nitrosyl ether enhances the quenching of primary free radicals released by high-temperature organic peroxides in the initial stage of high-temperature rotational molding. The blended powder can fully melt and adhere uniformly within the mold, allowing sufficient time and space for gases or volatiles in the system to escape from the homogeneous melt with good fluidity. Under low-temperature processing conditions, the rate of free radical generation by high-temperature organic peroxides is relatively slow, preventing violent cross-linking reactions and the localized generation of large amounts of low-boiling-point substances forming bubbles or pits. Under high-temperature processing conditions, the rate of free radical generation by high-temperature organic peroxides accelerates, resulting in the localized formation of a large number of low-boiling-point substances. The highly reactive nitrosamine free radicals generated by nitrosoethers play a regulatory role, rapidly combining with these low-boiling-point free radicals to generate stable, high-molecular-weight substances, thus inhibiting the formation of bubbles and pits. In a homogeneous reaction matrix free of local defects, fewer free radicals are consumed under high-temperature processing conditions, leading to a slight reduction in crosslinking points and a slight decrease in the degree of crosslinking and gelation rate, but still maintaining a high level of stability. At this point, the high functionality of the co-agent plays a role in enhancing crosslinking during the reaction, promoting the formation of a homogeneous, compact, and stable crosslinked network. Furthermore, because gases or volatiles in the system can be fully discharged, local defects are significantly reduced, resulting in a product with no bubbles or pits on the surface and a dense internal structure.

[0044] Comparative Example 1, without the use of nitrosyl ethers, achieved a pit-free surface under low-temperature processing (190°C). However, under high-temperature processing (above 200°C, such as 215°C), obvious pits appeared on the surface. This is because low-temperature conditions are suitable for most organic peroxides, ensuring a gentle and slow release of free radicals, allowing the cross-linking reaction to proceed stably without prematurely or violently promoting it. Low temperatures provide sufficient time for the escape of gases or volatiles, promoting the formation of an ordered cross-linking network without negatively impacting the degree of cross-linking. However, when the temperature exceeds 200°C, such as 215°C, which is above the critical half-life of organic peroxides, the decomposition rate is too rapid, resulting in the rapid release of a large number of free radicals locally. The cross-linking reaction occurs quickly, and the cross-linking aid has limited activity, failing to react with the excess free radicals. The large amount of low-boiling-point substances produced by the side reactions eventually form bubbles. Therefore, bubbles or pits form on both the surface and inside. Although the cross-linking network can initially maintain a relatively stable level, the presence of local defects makes long-term stability difficult.

[0045] Comparative Example 2 uses an excess of nitrosyl ether. Regardless of whether it is processed at low temperature (190°C) or high temperature (above 200°C, such as 215°C), it can achieve the effect of no pits on the surface. However, under high temperature processing conditions, the gel rate is significantly reduced. This is because the highly active free radicals generated by the excess nitrosyl ether will consume a large number of the primary free radicals generated by the peroxide. The free radicals that promote the cross-linking reaction will also be excessively consumed, resulting in insufficient cross-linking reaction and a significant reduction in cross-linking points. Although the complete removal of gas or volatiles is ensured, the formation of the cross-linking network is affected, and the degree of cross-linking and gel rate are significantly reduced.

Claims

1. A cross-linked polyethylene composition with a pit-free surface for rotational molding, characterized in that, It includes the following components in parts by weight: 100 parts polyethylene resin, 0.1-2 parts high-temperature organic peroxide, 0.5-3 parts crosslinking aid, 0.05-0.5 parts nitroso ether, and 0.1-1 parts antioxidant.

2. The cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 1, characterized in that, The crosslinking aid is one or more of acrylates, methacrylates, polybutadienes, and allyl compounds.

3. The cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 2, characterized in that, The crosslinking aid is one or more of triallyl cyanurate, triallyl isocyanurate, and diallyl phthalate.

4. A cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 1 or 2, characterized in that, The high-temperature organic peroxide is one or more of dicumyl peroxide, bis(2,5-dimethylhexanoyl) peroxide, di-tert-butyl peroxide, and di-tert-pentyl peroxide.

5. A cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 1 or 2, characterized in that, The antioxidant is a compound system of hindered phenolic antioxidants and phosphite antioxidants; The mass ratio of the hindered phenolic antioxidant to the phosphite antioxidant is 1:(1.5-2.5).

6. The cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 5, characterized in that, The antioxidant is a compound system of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl)phosphite.

7. The cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 1, characterized in that, The polyethylene resin contains at least 95% vinyl groups and at least one C3-C8 olefin group by mass, and has a density of 0.940-0.970 g / cm³. 3 The melt flow index is 10-30 g / 10 min.

8. A method for preparing a cross-linked polyethylene composition with a pit-free surface for rotational molding, characterized in that, Includes the following steps: S1: Polyethylene resin, antioxidant, and nitrosyl ether are melt-blended, extruded, granulated, and ground to obtain a premixed powder; S2: The premixed powder, high-temperature organic peroxide, and crosslinking aid are heated and mixed in a high-speed mixer to obtain a blended powder; S3: The blended powder is loaded into a mold, and after rotational molding in a rotational molding furnace, it is cooled to obtain a cross-linked polyethylene composition roll with a pit-free surface.

9. The method for preparing a cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 8, characterized in that, In step S3, the rotational molding temperature is >290℃ and the rotational molding time is >1200s; The highest air temperature inside the mold is >210℃.

10. A method for preparing a cross-linked polyethylene composition with a pit-free surface for rotational molding according to claim 8 or 9, characterized in that, In step S2, the heating and blending temperature is 60°C, and the heating and blending time is 5-10 minutes.

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