Preparation process of composite storage box and composite storage box

Abstract

The application provides a preparation process of a composite storage tank and the composite storage tank. The preparation process of the composite storage tank comprises the following steps: preparing a tank body by using raw materials containing polyethylene through a blow molding or rotational molding process; loading the tank body into a furnace and sealing the furnace, and at least the outer surface of the tank body is exposed to the furnace; performing fluorination reaction on the tank body by vacuumizing the furnace and filling mixed gas to form a fluorinated modified polyethylene layer, the mixed gas comprising a first inert gas and fluorine gas; discharging the gas in the furnace after the fluorination is completed, cleaning the tank body by using a second inert gas, and obtaining the storage tank, and the discharged gas in the furnace is recycled. The preparation process of the storage tank can form a fluorinated modified polyethylene layer with a larger and more uniform thickness on the outer surface or the inner and outer surfaces of the polyethylene tank body through offline fluorination treatment, so that the light weight and high strength are realized, and the liquid permeation resistance and corrosion resistance are significantly and reliably improved, and the storage tank is especially suitable for the storage of methanol.

Description

Technical Field

[0001] This invention relates to the field of storage box technology, particularly to the field of polymer material box technology, and even more particularly to a preparation process of a composite storage box using polyethylene raw materials, and the composite storage box itself. Background Technology

[0002] Liquid storage tanks are specialized devices used to store various liquids. They can be categorized into different types based on their purpose, material, and storage medium, and are widely used in industrial, medical, and domestic applications. Examples include methanol storage tanks, acetone storage tanks, diethyl ether storage tanks, and benzene storage tanks.

[0003] Methanol, as a clean alternative fuel, is increasingly widely used in automobiles, industrial boilers, and other fields. Because methanol molecules are extremely small and highly permeable, and because methanol also has a certain degree of swelling and corrosiveness to many polymer materials, the requirements for the impermeability of its storage containers are far higher than those for gasoline and diesel.

[0004] Based on the characteristics of methanol, the following are the main types of fuel tanks currently available on the market for storing methanol.

[0005] (1) Metal fuel tank Early methanol fuel tanks were mostly made of stainless steel or carbon steel with a special internal coating. These metal tanks were heavy, significantly increasing vehicle weight and energy consumption; the molding process was complex, making it difficult to manufacture irregularly shaped tanks, resulting in poor space adaptability; over long-term use, electrochemical corrosion would occur, and the resulting rust would contaminate the fuel and clog the fuel lines; the internal coating process itself was complex and carried the risk of coating peeling and pinholes, leading to a decline in reliability over time. The investment in materials and processes to solve corrosion problems ultimately resulted in high costs for metal fuel tanks.

[0006] (2) Multi-layer co-extruded plastic oil tank To replace metal fuel tanks, the industry has developed plastic fuel tanks made of multiple layers of different polymer materials through a co-extrusion blow molding process. A common structure is as follows: the outermost layer is a polyethylene (PE) base layer providing strength and impact resistance; the innermost layer is an ethylene-vinyl alcohol copolymer (EVOH) or polyamide (PA) barrier layer; and the layers are bonded together using an adhesive resin. This type of fuel tank has high technological barriers, high production costs, and a narrow process window. Furthermore, the production process generates a large amount of waste, resulting in significant material waste and environmental pollution, which does not meet the requirements of a circular economy. Barrier materials such as EVOH are sensitive to humidity; their barrier performance significantly degrades in high-humidity environments, leading to a risk of structural failure.

[0007] (3) Online fluoroplastic fuel tank To simplify the process and impart barrier properties to plastic fuel tanks, the industry has developed "online fluorination" technology. This technology uses a mixture of gases containing fluorine (F2) as the blowing gas during the blow molding process, allowing it to react with the high-temperature inner wall of the tank during molding, forming a thin fluorinated layer.

[0008] Although forming a fluorinated layer can give plastic fuel tanks barrier properties, the "online fluorination" technology still has many drawbacks.

[0009] Firstly, the fluorinated layer is not only extremely thin but also uneven. The fluorination reaction time is very short and limited by the molding cycle, typically forming only a fluorinated layer of about 1-3 µm. Furthermore, due to differences in airflow distribution and cooling rate during the preform blowing process, the thickness of the fluorinated layer varies greatly in different parts of the housing (especially corners and welds), resulting in unstable anti-permeability and becoming weak points for penetration.

[0010] Secondly, it cannot perform fluorination on the outer wall or differentiate fluorination between the inside and outside. The inflation gas can only form a fluorinated layer on the inner wall, leaving the outer wall of the enclosure unprotected. During long-term use, exposure to the external environment (moisture, salt spray, de-icing agents, etc.) will cause slow aging and stress cracking, and corrosive media in the external environment will also erode the enclosure from the outside. Furthermore, this method cannot provide differentiated treatment for the inner and outer walls for different purposes.

[0011] Thirdly, the utilization rate of fluorine gas is low, posing significant safety hazards. During the inflation process, most of the fluorine gas is released with the molding exhaust, resulting in low effective utilization. The direct emission of unreacted fluorine gas under high temperature and pressure not only increases the cost of fluorine gas recovery and treatment but also brings greater safety hazards and environmental pressure.

[0012] Furthermore, existing plastic fuel tanks commonly pose a safety hazard due to static electricity buildup when carrying flammable and explosive fuels. Polyethylene (PE) is an excellent electrical insulator, with a surface resistivity typically around 10⁻⁶. 14 ~10 16 Between Ω. During the transportation, refueling, and shaking of methanol fuel, friction between the fuel and the inner wall of the tank, and between the outer wall of the tank and the surrounding air and dust, continuously generates and accumulates static electricity. When the static potential rises to a certain level, a spark discharge may occur. If a flammable mixture of methanol vapor and air is present around the tank at this time, it can easily cause a fire or explosion. Traditional metal fuel tanks can naturally discharge static electricity through the tank's own conductivity and grounding, while plastic fuel tanks, due to their overall insulation, cannot release static electricity through the tank material.

[0013] To address this issue, existing technologies have included embedding metal grounding components inside the plastic enclosure or applying a conductive coating externally. However, embedding metal components not only increases manufacturing complexity but also makes it difficult to guarantee the sealing and long-term compatibility of the metal-plastic interface, potentially creating new leakage channels. External conductive coatings, on the other hand, suffer from poor adhesion, resistance to methanol swelling and peeling, and insufficient durability, resulting in a significant degradation in conductivity after prolonged use.

[0014] Therefore, how to reliably and persistently achieve static electricity discharge while maintaining the lightweight, corrosion resistance, and excellent barrier properties of plastic fuel tanks is a long-standing but unresolved technical need in the industry. Summary of the Invention

[0015] To address the aforementioned problems, this invention provides a manufacturing process for a composite storage tank and the composite storage tank itself. As one objective, the manufacturing process of this composite storage tank utilizes offline fluorination treatment to form a thicker, more uniform, and denser fluorinated modified polyethylene layer on the outer surface, or both the inner and outer surfaces, of the polyethylene tank body. This achieves lightweighting and high strength while significantly and reliably improving liquid penetration resistance and corrosion resistance, making it particularly suitable for methanol storage. As another objective, the manufacturing process of this composite storage tank can also produce a composite storage tank with electrostatic discharge functionality. This composite storage tank can safely and reliably discharge static electricity without increasing complex structural components or affecting penetration prevention and corrosion resistance, eliminating the risk of combustion and explosion caused by static electricity accumulation.

[0016] To achieve the above objectives, the first aspect of the present invention provides a process for manufacturing a composite storage box, comprising the steps of: (I) The box body is made from raw materials including polyethylene by blow molding or rotational molding process; (II) The box body is placed in the furnace and the furnace is sealed, with at least the outer surface of the box body exposed in the furnace; (iii) The furnace is evacuated and filled with a mixed gas to fluorinate the body of the box to form a fluorinated modified polyethylene layer, wherein the mixed gas includes a first inert gas and fluorine gas; (IV) After fluorination is completed, the gas in the furnace is discharged and cleaned with a second inert gas to obtain a storage tank. The gas discharged from the furnace is recycled.

[0017] In the manufacturing process of the composite storage box of this invention, the box body, prepared from polyethylene raw materials through blow molding or rotational molding, exhibits superior structural strength and impact resistance. Through offline fluorination, a long-term reaction is carried out in a static, controlled furnace environment, ensuring uniform gas contact with exposed areas of the box body, including structurally complex corners, welds, and recesses, resulting in excellent fluorination uniformity. Compared to online fluorination via blow molding, a thicker, more uniform, and denser fluorinated modified polyethylene layer can be formed on the outer surface, or both the inner and outer surfaces of the polyethylene box body. Its thickness can reach 5-20 µm, which is 2.5-10.0 times the thickness of traditional online fluorination (approximately 2 µm). This thickness range can cover a variety of protection requirements, from storing corrosive liquids (especially methanol) for everyday use to harsh industrial conditions. Furthermore, the fluorinated modified polyethylene layer and the polyethylene box body are chemically bonded, eliminating the risk of delamination or detachment. The fluorinated modified polyethylene layer has a large thickness and high bonding strength, thus achieving both lightweight and high strength while significantly and reliably improving resistance to the penetration of corrosive liquids (especially methanol) and corrosion resistance. Offline fluorination is carried out in a closed negative pressure environment inside the furnace (vacuuming before introducing fluorine gas), and unreacted fluorine gas can be recovered and reused, which can significantly reduce raw material costs and post-processing burden.

[0018] As a technical solution of the present invention, the box body is a plurality of boxes arranged in a regular manner inside the furnace, and the distance between adjacent box bodies is not less than 5cm.

[0019] As one technical solution of the present invention, both the outer and inner surfaces of the box body are exposed in the furnace, or all openings of the box body are sealed and only the outer surface is exposed in the furnace.

[0020] As a technical solution of the present invention, the furnace is evacuated to an absolute pressure of 0.01~0.03MPa and maintained for 5~15 minutes before the mixed gas is introduced.

[0021] As one technical solution of the present invention, the volume ratio of fluorine gas in the mixed gas is 1~5%, and the absolute pressure inside the furnace is 0.10~0.15MPa when the mixed gas is introduced.

[0022] As one technical solution of the present invention, the temperature of the fluorination reaction is 20~60℃, and the time of the fluorination reaction is 5~60min.

[0023] As a technical solution of the present invention, the recycling includes collecting and neutralizing the discharged gas in sequence.

[0024] A second aspect of the present invention provides a process for preparing a composite storage tank, comprising a tank body and a fluorinated modified polyethylene layer formed at least on the outer surface of the tank body. This fluorinated modified polyethylene layer achieves both lightweight and high strength in the storage tank, while significantly and reliably improving its resistance to methanol penetration and corrosion.

[0025] As one technical solution of the present invention, the thickness of the fluorinated modified polyethylene layer is 5~20µm.

[0026] As one technical solution of the present invention, the fluorine content in the fluorinated modified polyethylene layer gradually decreases along the direction close to the box body.

[0027] A third aspect of this invention provides a process for manufacturing a composite storage box, comprising the following steps: (I) The conductive box body is prepared by blow molding or rotational molding using raw materials including polyethylene and conductive fillers; (II) The conductive box body is placed in the furnace and the furnace is sealed, with only the outer surface of the conductive box body exposed in the furnace; (iii) The furnace is evacuated and filled with a mixed gas to perform a fluorination reaction on the outer surface of the conductive box body to form a fluorinated modified polyethylene layer, wherein the mixed gas includes a first inert gas and fluorine gas; (IV) After fluorination is completed, the gas in the furnace is discharged and cleaned with a second inert gas. The gas discharged from the furnace is recycled. (V) Remove part of the fluorinated modified polyethylene layer to expose the conductive filler, and fix a metal gasket in the area where part of the fluorinated modified polyethylene layer has been removed.

[0028] In the manufacturing process of the composite storage box of this invention, the box body, prepared by blow molding or rotational molding using polyethylene and conductive filler raw materials, exhibits superior structural strength, impact resistance, and conductivity. Through offline fluorination, a long-term reaction is carried out in a static, controlled furnace environment, ensuring uniform gas contact with the outer surface of the box body, including complex corners, welds, and recesses, resulting in excellent fluorination uniformity. Compared to online fluorination via blow molding, a thicker, more uniform, and denser fluorinated modified polyethylene layer can be formed on the outer surface of the polyethylene box body. Its thickness can reach 5-20 µm, which is 2.5-10.0 times the thickness of traditional online fluorination (approximately 2 µm). This thickness range can cover various protection requirements, from the storage of corrosive liquids (especially methanol) for everyday use to harsh industrial conditions. Furthermore, the fluorinated modified polyethylene layer and the outer surface of the polyethylene box body are chemically bonded, eliminating the risk of delamination and detachment. The fluorinated modified polyethylene layer has a large thickness and high bonding strength, thus achieving both lightweight and high strength while significantly and reliably improving resistance to the penetration of corrosive liquids (especially methanol) and corrosion resistance. Offline fluorination is carried out in a closed negative pressure environment inside the furnace (vacuuming before introducing fluorine gas), and unreacted fluorine gas can be recovered and reused, which can significantly reduce raw material costs and post-processing burden.

[0029] Furthermore, in the manufacturing process of this composite storage tank, the inner wall of the conductive tank body is not covered by a fluorinated modified polyethylene layer, directly exposing it to the corrosive liquid storage space. A portion of the fluorinated modified polyethylene layer is removed to expose the conductive filler, and metal gaskets are fixed in the areas where the fluorinated modified polyethylene layer has been partially removed. When the composite storage tank obtained using this manufacturing process is connected to an external grounding device (such as vehicle frame grounding), it has a complete electrostatic discharge path: inner wall of the conductive tank body (exposed conductive substrate) → interior of the conductive tank body → metal gasket → ground. Compared to external conductive coating solutions, the conductivity originates from within the conductive tank body rather than a thin surface layer, eliminating problems such as coating peeling, swelling and delamination by corrosive liquids, or long-term aging failure. The conductivity remains stable throughout its entire lifespan. This function fundamentally eliminates the fire and explosion safety hazards caused by static electricity accumulation in the storage tank, enabling the composite storage tank to reach or even surpass the electrostatic protection level of metal oil tanks.

[0030] As one technical solution of the present invention, the conductive filler accounts for 3 to 15% of the total weight of the raw materials.

[0031] As a technical solution of the present invention, the surface resistivity of the conductive box body is 10. 3 ~10 8 Ω.

[0032] As one technical solution of the present invention, the polyethylene and the conductive filler are melt-blended and granulated by a twin-screw extruder to obtain the raw material, or the polyethylene and the conductive filler are mixed online by a metering feeder to obtain the raw material.

[0033] As a technical solution of the present invention, there are multiple conductive box bodies arranged regularly inside the furnace, and the distance between adjacent conductive box bodies is not less than 5cm.

[0034] As a technical solution of the present invention, all openings of the conductive box body are sealed, and only the outer surface is exposed to the furnace.

[0035] As a technical solution of the present invention, the furnace is evacuated to an absolute pressure of 0.01~0.03MPa and maintained for 5~15 minutes before the mixed gas is introduced.

[0036] As one technical solution of the present invention, the volume ratio of fluorine gas in the mixed gas is 1~5%, and the absolute pressure inside the furnace is 0.10~0.15MPa when the mixed gas is introduced.

[0037] As one technical solution of the present invention, the temperature of the fluorination reaction is 20~60℃, and the time of the fluorination reaction is 5~60min.

[0038] As a technical solution of the present invention, the recycling includes collecting and neutralizing the discharged gas in sequence.

[0039] A fourth aspect of this invention provides a composite storage box prepared by the aforementioned composite storage box manufacturing process, comprising a conductive box body, a fluorinated modified polyethylene layer formed on the outer surface of the conductive box body, and a metal gasket. The fluorinated modified polyethylene layer has a thickness reduction region, and the metal gasket is fixed to the thickness reduction region and contacts the conductive filler. This composite storage box can safely and reliably discharge static electricity without adding complex structural components or affecting impermeability and corrosion resistance, thus eliminating the risk of combustion and explosion caused by static electricity accumulation.

[0040] As one technical solution of the present invention, the thickness of the fluorinated modified polyethylene layer is 5~20µm.

[0041] As one technical solution of the present invention, the fluorine content in the fluorinated modified polyethylene layer gradually decreases along the direction close to the conductive box body. Attached Figure Description

[0042] Figure 1 This is a schematic diagram illustrating the change in fluorine content in the fluorinated modified polyethylene layer. Detailed Implementation

[0043] This invention provides a high-strength, highly methanol-resistant, and corrosion-resistant composite storage tank. The composite storage tank includes a tank body and a fluorinated modified polyethylene (FPE) layer formed at least on the outer surface of the tank body. The FPE layer can be formed only on the outer surface of the tank body, or it can be formed on both the inner and outer surfaces of the tank body. Forming the FPE layer only on the outer surface of the tank body is suitable for scenarios where the internal storage medium does not corrode the inner wall of the tank body or only external environmental protection is required. The composite storage tank with FPE layers on both the inner and outer surfaces provides a high level of protection, with the inner protective layer preventing the internal corrosive liquid from penetrating outwards, and the outer protective layer resisting external environmental erosion. The thickness of the FPE layer is 5-20 µm. The fluorine content in the FPE layer gradually decreases from the outermost layer towards the innermost layer, meaning the fluorine content is highest at the outermost layer, approaching PTFE properties, and decreases towards the innermost layer until it transitions to unreacted polyethylene (e.g., fluorinated polyethylene). Figure 1 As shown in the figure, this gradient structure makes it impossible for the fluorinated layer to have a clear interface with the box body, resulting in extremely strong adhesion. Even if the box body is subjected to external impact or thermal expansion and contraction, it will not fall off.

[0044] The manufacturing process of this composite storage box includes the following steps: (I) The box body is made from raw materials including polyethylene by blow molding or rotational molding process; (II) The container body is placed in the furnace and the furnace is sealed, with at least the outer surface of the container body exposed in the furnace; (iii) The furnace is evacuated and filled with mixed gas to fluorinate the body of the box to form a fluorinated modified polyethylene layer; (IV) After fluorination is completed, the gas in the furnace is discharged and cleaned with a second inert gas to obtain a storage tank. The gas discharged from the furnace is recycled.

[0045] The polyethylene used can be high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE), etc. The box body is made from polyethylene raw materials through blow molding or rotational molding processes, allowing for the fabrication of the required shape and wall thickness in a single molding process. Multiple box bodies are arranged regularly within the furnace and fixed in a dedicated fluorination carriage or frame, with a minimum spacing of 5 cm between adjacent boxes to ensure uniform gas flow. Both the outer and inner surfaces of the box body are exposed in the furnace, or all openings of the box body are sealed, exposing only the outer surface. For fluorination only on the outer surface of the box body, fluorine-resistant sealing plugs or clamps can be used to seal the box openings and all internal connections before fluorination, ensuring that fluorinated gas does not enter the box interior in subsequent steps. Depending on the needs, a fluorinated modified polyethylene layer can be flexibly formed only on the outer surface of the box body or simultaneously on both the inner and outer surfaces, thereby achieving lightweight and high strength while significantly and reliably improving methanol penetration resistance and corrosion resistance. The furnace is evacuated to an absolute pressure of 0.01~0.03 MPa (i.e., a vacuum degree of -0.070~-0.099 MPa) and maintained for 5~15 minutes before the mixed gas is introduced. The evacuation serves two purposes: first, to remove adsorbed air and moisture from the furnace interior and the furnace surface, preventing the consumption of fluorine gas and the generation of harmful hydrogen fluoride (HF) byproducts; second, to create a negative pressure environment that facilitates the uniform and rapid penetration of the mixed gas into the furnace body and ensures sufficient contact with the outer surface. Maintaining this vacuum degree for 5~15 minutes allows for sufficient desorption of moisture from the furnace surface. The mixed gas consists of a primary inert gas and fluorine gas, with fluorine gas comprising 1~5% of the volume. The mixed gas is introduced until the furnace pressure reaches 0.10~0.15 MPa, creating a slight positive pressure. This helps prevent the infiltration of outside air, and the use of slight positive pressure rather than high pressure reduces the requirements for the furnace's sealing structure and minimizes the risk of fluorine leakage. The first inert gas can be nitrogen, helium, argon, neon, etc. During the fluorination reaction, fluorine reacts with the CH bonds on the surface of the polyethylene casing to form a fluorinated protective layer composed of CF bonds. The fluorination reaction proceeds from the outside in; therefore, the fluorine content in the fluorinated modified polyethylene layer gradually decreases from the outside in. Preferably, the fluorine content gradually decreases from the outside towards the inside of the casing, meaning the fluorine content is highest at the outermost layer, approaching PTFE properties, and decreases towards the inside until it transitions to unreacted polyethylene. The fluorination reaction temperature is 20~60℃, preferably 25~35℃. Offline fluorination has a lower reaction temperature, lower energy consumption, and no risk of thermal deformation to the casing. The fluorination reaction time is 5~60 min, which can be set according to the target fluorinated layer thickness and the fluorinated layer growth rate. An appropriate fluorine concentration can be selected based on the thickness of the casing to control the growth rate of the fluorinated modified polyethylene layer. The growth rate of the fluorinated modified polyethylene layer can be 0.5~3.0 μm / min.Based on the thin-walled furnace body (<3mm), the volume percentage of fluorine gas can be controlled at 1-2%, resulting in a milder reaction and a controllable growth rate of fluorinated modified polyethylene layer of 0.5-1.0 μm / min. For large-scale rapid fluorination of thick-walled furnace bodies, the volume percentage of fluorine gas can be controlled at 3-5%, resulting in a faster reaction and a fluorinated layer growth rate of 1.5-3.0 μm / min. Recycling involves sequentially collecting and neutralizing the discharged gas. After fluorination, the supply of mixed gas is stopped, and a second inert gas is introduced into the furnace for multiple (usually 2-3) purges to replace and dilute any residual fluorine gas. The second inert gas can be nitrogen, helium, argon, neon, etc. The purged waste gas is treated in an absorption tower containing alkaline substances (such as calcium hydroxide solution or solid soda lime bed) to neutralize any trace amounts of acidic gases and ensure safe emissions. The discharged gas first passes through a fluorine recovery system, where unreacted fluorine and nitrogen are separated and recovered using pressure swing adsorption (PSA) or membrane separation technology. The recovered fluorine can be reused for gas mixing, and the clean nitrogen after recovery and separation can be directly discharged. After cleaning and neutralization with a second inert gas, and confirmation of safety of the gas composition in the furnace by an online analyzer, the furnace is opened, and the frame / chamber carrying the fluorinated container is pushed out, allowed to cool naturally to room temperature, and the container is removed to obtain the finished product. In addition, not only fluorine can be recovered, but the scrap and offcuts of the composite storage box are also easier to recycle due to their simple material (a surface-modified layer on a recyclable polyethylene substrate), achieving both environmental and economic benefits.

[0046] This invention provides a composite storage tank with high strength, high resistance to methanol penetration, and corrosion resistance. Furthermore, it also provides a composite storage tank with electrostatic discharge functionality. In practice, the presence or absence of an electrostatic discharge function can be selected based on different needs. This composite storage tank with electrostatic discharge functionality includes a conductive tank body, a fluorinated modified polyethylene layer formed on the outer surface of the conductive tank body, and a metal gasket. The fluorinated modified polyethylene layer has a thinning region, and the metal gasket is fixed to the thinning region and contacts the conductive filler. The thickness of the fluorinated modified polyethylene layer is 5~20µm. The fluorine content in the fluorinated modified polyethylene layer gradually decreases from the outer side to the inner side near the conductive tank body. That is, the fluorine content is highest in the outermost layer, approaching PTFE properties, and decreases towards the inside until it transitions to unreacted polyethylene. This gradual structure results in no clear interface between the fluorinated layer and the tank body, with extremely strong adhesion, preventing detachment even when the tank body is subjected to external impact or thermal expansion and contraction.

[0047] The manufacturing process of this composite storage box includes the following steps: (I) The conductive box body is prepared by blow molding or rotational molding using raw materials including polyethylene and conductive fillers; (II) The conductive box body is placed in the furnace and the furnace is sealed, with only the outer surface of the conductive box body exposed in the furnace; (iii) Evacuate the furnace and fill it with a mixed gas to fluorinate the outer surface of the conductive box body to form a fluorinated modified polyethylene layer. (IV) After fluorination is completed, the gas in the furnace is discharged and cleaned with a second inert gas. The gas discharged from the furnace is recycled. (V) Remove part of the fluorinated modified polyethylene layer to expose the conductive filler, and fix a metal gasket in the area where part of the fluorinated modified polyethylene layer has been removed.

[0048] The polyethylene used can be high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE), etc. The conductive filler accounts for 3-15% of the total weight of the raw materials. The conductive filler can be carbon black, carbon fiber, metal powder, etc. The surface resistivity of the conductive box body is 10 Ω·cm. 3 ~10 8Ω. Polyethylene and conductive filler are melt-blended and granulated using a twin-screw extruder to obtain the raw material, or polyethylene and conductive filler are mixed online using a metering feeder to ensure uniform dispersion of the conductive filler in the matrix, forming a continuous conductive network. This avoids uneven dispersion of the conductive filler leading to localized insulating "islands" that interrupt the static discharge path. The conductive box body is obtained from the raw material through blow molding or rotational molding processes. A conductive matrix of the required shape and wall thickness can be prepared in a single molding process. Multiple conductive box bodies are arranged regularly in the furnace and fixed within a dedicated fluorination carriage or frame. The distance between adjacent conductive box bodies is not less than 5 cm to ensure uniform gas flow. All openings of the conductive box body are sealed, exposing only the outer surface to the furnace. Specifically, PTFE sealing plugs can be used to seal all openings of the conductive box body, ensuring that subsequent fluorination gas only contacts the outer surface, while the inner surface remains unfluorinated to preserve its conductivity. The sealing surface of the sealing plug must fit tightly against the opening to prevent fluorine gas from seeping into the inner cavity. The furnace is evacuated to an absolute pressure of 0.01~0.03 MPa (i.e., a vacuum degree of -0.070~-0.099 MPa) and maintained for 5~15 minutes before the mixed gas is introduced. The vacuum serves two purposes: first, to remove adsorbed air and moisture from the furnace interior and the furnace surface, preventing the consumption of fluorine gas and the generation of harmful hydrogen fluoride (HF) byproducts; second, to create a negative pressure environment that facilitates the uniform and rapid penetration of the mixed gas into the furnace body and ensures sufficient contact with the outer surface. Maintaining this vacuum degree for 5~15 minutes allows for sufficient desorption of moisture from the furnace surface. The mixed gas consists of a primary inert gas and fluorine gas, with fluorine gas comprising 1~5% of the total volume. The mixed gas is introduced until the furnace pressure reaches 0.10~0.15 MPa. This creates a slight positive pressure, which helps prevent the infiltration of outside air. Furthermore, using a slight positive pressure, rather than high pressure, reduces the requirements for the furnace's sealing structure and minimizes the risk of fluorine leakage. The first inert gas can be nitrogen, helium, argon, neon, etc. During the fluorination reaction, fluorine reacts with the CH bonds on the surface of the polyethylene casing to form a fluorinated protective layer composed of CF bonds. The fluorination reaction proceeds from the outside in; therefore, the fluorine content in the fluorinated modified polyethylene layer gradually decreases from the outside in. Preferably, the fluorine content gradually decreases from the outside towards the inside of the casing, meaning the fluorine content is highest at the outermost layer, approaching PTFE properties, and decreases towards the inside until it transitions to unreacted polyethylene. The fluorination reaction temperature is 20~60℃, preferably 25~35℃. Offline fluorination has a lower reaction temperature, lower energy consumption, and no risk of thermal deformation to the casing. The fluorination reaction time is 5~60 min, which can be set according to the target fluorinated layer thickness and the fluorinated layer growth rate. An appropriate fluorine concentration can be selected based on the thickness of the casing to control the growth rate of the fluorinated modified polyethylene layer. The growth rate of the fluorinated modified polyethylene layer can be 0.5~3.0 μm / min.Based on the thin-walled furnace body (<3mm), the volume percentage of fluorine gas can be controlled at 1-2%, resulting in a milder reaction and a controllable growth rate of fluorinated modified polyethylene layer of 0.5-1.0 μm / min. For large-scale rapid fluorination of thick-walled furnace bodies, the volume percentage of fluorine gas can be controlled at 3-5%, resulting in a faster reaction and a fluorinated layer growth rate of 1.5-3.0 μm / min. Recycling involves sequentially collecting and neutralizing the discharged gas. After fluorination, the supply of mixed gas is stopped, and a second inert gas is introduced into the furnace for multiple (usually 2-3) purges to replace and dilute any residual fluorine gas. The second inert gas can be nitrogen, helium, argon, neon, etc. The purged waste gas is treated in an absorption tower containing alkaline substances (such as calcium hydroxide solution or solid soda lime bed) to neutralize any trace amounts of acidic gases and ensure safe emissions. The discharged gas first passes through a fluorine recovery system, where unreacted fluorine and nitrogen are separated and recovered using pressure swing adsorption (PSA) or membrane separation technology. The recovered fluorine can be reused for gas mixing, and the clean nitrogen after recovery and separation can be directly discharged. After cleaning and neutralization with a second inert gas, and confirmation of safety of the gas composition in the furnace by an online analyzer, the furnace is opened, and the frame / chamber carrying the fluorinated container is pushed out, allowed to cool naturally to room temperature, and the container is removed to obtain the finished product. In addition, not only fluorine can be recovered, but the scrap and offcuts of the composite storage box are also easier to recycle due to their simple material (a surface-modified layer on a recyclable polyethylene substrate), achieving both environmental and economic benefits.

[0049] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the invention will be further described below with reference to specific embodiments. It should be noted that the methods described below are further explanations of this invention and should not be construed as limiting it.

[0050] Example 1 This embodiment is a composite storage box, including a box body and a fluorinated modified polyethylene layer formed on the outer surface of the box body. The thickness of the fluorinated modified polyethylene layer is 8.5 µm. The fluorine content in the fluorinated modified polyethylene layer gradually decreases from the outer side to the inner side near the box body.

[0051] The manufacturing process of this composite storage box includes the following steps.

[0052] (I) High-density polyethylene (HDPE) is selected as the raw material and the box body is formed by blow molding process.

[0053] (II) Seal all openings (oil inlet, oil outlet, breather valve, etc.) of the box body with a polytetrafluoroethylene (PTFE) sealing plug. Place the sealed box body on the fluorination vehicle frame, with a distance of not less than 5cm between adjacent box bodies, push it into the fluorination furnace and seal it.

[0054] (iii) Start the vacuum pump to reduce the absolute pressure inside the furnace to 0.02 MPa and maintain this pressure for 10 minutes. Introduce a pre-prepared mixture of 2.5% fluorine and 97.5% nitrogen into the furnace until the absolute pressure reaches 0.12 MPa. Treat the furnace at a constant temperature of 30°C for 25 minutes to form an 8.5 µm fluorinated modified polyethylene layer on the outer surface of the furnace body.

[0055] (IV) The mixed gas is discharged, nitrogen is introduced to 0.1 MPa, and then the pressure is purged again. This process is repeated three times for cleaning. The discharged gas passes through an activated alumina adsorption tower to recover fluorine (80% of the fluorine can be recovered), and then through a quicklime neutralization tank before being discharged with nitrogen. After cleaning, the furnace door is opened, the sealing plug is removed, and a storage box with a fluorinated modified polyethylene layer on only the outer surface is obtained.

[0056] The fluorinated modified polyethylene layer formed on the outer surface of this storage tank can reach a thickness of 8.5µm, which is quite large. After immersing it in methanol at 40℃ for 28 days, no methanol can be detected in the storage tank, indicating that methanol has extremely high impermeability.

[0057] Example 2 This embodiment is a composite storage box, including a box body and fluorinated modified polyethylene layers formed on the inner and outer surfaces of the box body. The thickness of the fluorinated modified polyethylene layers on both the inner and outer surfaces is 14.0 µm. The fluorine content in the fluorinated modified polyethylene layers gradually decreases from the outer side towards the inner side near the box body.

[0058] The manufacturing process of this composite storage box includes the following steps.

[0059] (I) Linear low-density polyethylene (LLDPE) is selected as raw material and the box body is formed by blow molding process.

[0060] (II) Place the container body directly on the fluorination vehicle frame (without any sealing), with a distance of not less than 5 cm between adjacent container bodies, push it into the fluorination furnace and seal it. Keep all pipe openings open so that the reaction gas can freely enter and exit the interior of the container body.

[0061] (iii) Start the vacuum pump to reduce the absolute pressure inside the furnace to 0.02 MPa and maintain this pressure for 10 minutes. Then, introduce a pre-prepared mixture of 4.0% fluorine and 96.0% nitrogen into the furnace until the absolute pressure inside reaches 0.10 MPa (atmospheric pressure). Treat the furnace at 45°C for 40 minutes to form a 14.0 µm fluorinated modified polyethylene layer on both the inner and outer surfaces of the furnace body.

[0062] (IV) The mixed gas is discharged, nitrogen is introduced to 0.1 MPa, and then the pressure is purged again. This process is repeated three times for cleaning. The discharged gas is introduced into a pressure swing adsorption (PSA) fluorine recovery unit through a dedicated recovery pipeline to recover approximately 85% of the unreacted fluorine gas. The gas is then neutralized in an alkaline spray tower to meet emission standards before being discharged. After cleaning, the furnace door is opened, and the box is removed, revealing a storage box with fluorinated modified polyethylene layers on both the inner and outer surfaces.

[0063] The thickness of the fluorinated modified polyethylene layer formed on the inner and outer surfaces of this storage tank can reach 14.0µm, which is quite large. After immersing it in methanol at 40℃ for 28 days, no methanol can be detected in the storage tank, indicating that the methanol has extremely high impermeability.

[0064] Example 3 This embodiment is a conductive composite storage box, comprising a conductive box body, a fluorinated modified polyethylene layer formed on the outer surface of the conductive box body, and a metal gasket. The fluorinated modified polyethylene layer has a thickness reduction zone, and the metal gasket is fixed to the thickness reduction zone and contacts the conductive filler. The thickness of the fluorinated modified polyethylene layer is 7.0 µm. The fluorine content in the fluorinated modified polyethylene layer gradually decreases from the outer side to the inner side near the conductive box body.

[0065] The fabrication process of this conductive composite storage box includes the following steps.

[0066] (I) High-density polyethylene (HDPE) and conductive carbon black are melt-blended and granulated using a twin-screw extruder to obtain the raw material, where the conductive carbon black accounts for 8% of the total weight of the HDPE. The conductive box body is then formed by blow molding, and the surface resistivity of the conductive box body is 10 Ω·cm. 5 Ω.

[0067] (II) Seal all openings (oil inlet, oil outlet, breather valve, etc.) of the conductive box body with a polytetrafluoroethylene (PTFE) sealing plug. Place the sealed conductive box body on the fluorination vehicle frame, with a distance of not less than 5cm between adjacent conductive boxes, push it into the fluorination furnace and seal it.

[0068] (iii) Reduce the absolute pressure inside the furnace to 0.03 MPa and maintain this pressure for 15 min. Introduce a pre-prepared mixture of 2.5% fluorine and 97.5% nitrogen into the furnace until the absolute pressure inside the furnace reaches 0.11 MPa. Treat the furnace at 30°C for 20 min to form a 7.0 µm fluorinated modified polyethylene layer on the outer surface of the conductive box body.

[0069] (IV) The mixed gas is discharged, nitrogen is introduced to 0.1 MPa, and then the pressure is purged again. This process is repeated three times for cleaning. The discharged gas passes through an activated alumina adsorption tower to recover fluorine (80% of the fluorine can be recovered), and then through a quicklime neutralization tank before being discharged with nitrogen. After cleaning, the furnace door is opened, the sealing plug is removed, and a conductive storage box with a fluorinated modified polyethylene layer on only the outer surface is obtained.

[0070] (V) At the pre-set grounding protrusion on the outer surface of the conductive box, gently sand away the fluorinated modified polyethylene layer (approximately 1 cm² in area) on that point using fine sandpaper. 2 The internal conductive carbon black is exposed. The metal gasket of the grounding terminal is fastened to the polished area with bolts, thus obtaining a conductive storage box with a fluorinated modified polyethylene layer on the outer surface.

[0071] The fluorinated modified polyethylene layer formed on the outer surface of this storage tank can reach a thickness of 7.0µm, which is quite large. After immersing it in methanol at 40℃ for 28 days, no methanol can be detected in the storage tank, indicating that the methanol has extremely high impermeability.

[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A manufacturing process for a composite storage box, characterized in that, Including the following steps: (I) The box body is made from raw materials including polyethylene by blow molding or rotational molding process; (II) The box body is placed in the furnace and the furnace is sealed, with at least the outer surface of the box body exposed in the furnace; (iii) The furnace is evacuated and filled with a mixed gas to fluorinate the body of the box to form a fluorinated modified polyethylene layer, wherein the mixed gas includes a first inert gas and fluorine gas; (IV) After fluorination is completed, the gas in the furnace is discharged and cleaned with a second inert gas to obtain a storage tank. The gas discharged from the furnace is recycled.

2. The manufacturing process of the composite storage box according to claim 1, characterized in that, Includes at least one of the following features (1) to (6): (1) The box body is a plurality of them and is arranged in a regular manner inside the furnace, and the distance between adjacent box bodies is not less than 5cm; (2) Both the outer and inner surfaces of the box body are exposed in the furnace, or all openings of the box body are sealed and only the outer surface is exposed in the furnace; (3) The furnace is evacuated to an absolute pressure of 0.01~0.03MPa and held for 5~15 minutes before the mixed gas is introduced; (4) The volume percentage of fluorine gas in the mixed gas is 1-5%, and the absolute pressure inside the furnace is 0.10-0.15 MPa when the mixed gas is introduced. (5) The temperature of the fluorination reaction is 20~60℃, and the time of the fluorination reaction is 5~60min; (6) The recycling includes collecting and neutralizing the discharged gas in sequence.

3. The composite storage box prepared according to the manufacturing process of the composite storage box according to claim 1 or 2, characterized in that, It includes a box body and at least a fluorinated modified polyethylene layer formed on the outer surface of the box body.

4. The composite storage box according to claim 3, characterized in that, The thickness of the fluorinated modified polyethylene layer is 5~20µm.

5. The composite storage box according to claim 3, characterized in that, The fluorine content in the fluorinated modified polyethylene layer gradually decreases from the outer side to the inner side near the body of the box.

6. A manufacturing process for a composite storage box, characterized in that, Including the following steps: (I) The conductive box body is prepared by blow molding or rotational molding using raw materials including polyethylene and conductive fillers; (II) The conductive box body is placed in the furnace and the furnace is sealed, with only the outer surface of the conductive box body exposed in the furnace; (iii) The furnace is evacuated and filled with a mixed gas to perform a fluorination reaction on the outer surface of the conductive box body to form a fluorinated modified polyethylene layer, wherein the mixed gas includes a first inert gas and fluorine gas; (IV) After fluorination is completed, the gas in the furnace is discharged and cleaned with a second inert gas. The gas discharged from the furnace is recycled. (V) Remove part of the fluorinated modified polyethylene layer to expose the conductive filler, and fix a metal gasket in the area where part of the fluorinated modified polyethylene layer has been removed.

7. The manufacturing process of the composite storage box according to claim 6, characterized in that, Includes at least one of the following features (i) to (ix): (i) The conductive filler comprises 3-15% of the total weight of the raw materials; (ii) The surface resistivity of the conductive box body is 10. 3 ~10 8 Ω; (iii) The polyethylene and the conductive filler are melt-blended and granulated using a twin-screw extruder to obtain the raw material, or the polyethylene and the conductive filler are mixed online using a metering feeder to obtain the raw material; (iv) The conductive box body is a plurality of them and is arranged in a regular manner inside the furnace, and the distance between adjacent conductive box bodies is not less than 5cm; (v) Seal all openings of the conductive box body, leaving only the outer surface exposed to the furnace; (vi) The furnace is evacuated to an absolute pressure of 0.01~0.03MPa and maintained for 5~15 minutes before the mixed gas is introduced; (vii) The volume percentage of fluorine gas in the mixed gas is 1-5%, and the mixed gas is introduced into the furnace to an absolute pressure of 0.10-0.15 MPa; (viii) The temperature of the fluorination reaction is 20~60℃, and the time of the fluorination reaction is 5~60min; (ix) The recycling includes collecting and neutralizing the discharged gas in sequence.

8. The composite storage box prepared according to the manufacturing process of the composite storage box according to claim 6 or 7, characterized in that, The device includes a conductive box body, a fluorinated modified polyethylene layer formed on the outer surface of the conductive box body, and a metal gasket. The fluorinated modified polyethylene layer has a thickness reduction region, and the metal gasket is fixed to the thickness reduction region and contacts the conductive filler.

9. The composite storage box according to claim 8, characterized in that, The thickness of the fluorinated modified polyethylene layer is 5~20µm.

10. The composite storage box according to claim 8, characterized in that, The fluorine content in the fluorinated modified polyethylene layer gradually decreases from the outer side to the inner side closer to the conductive box body.

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