A low-defect roto-molded article and its preparation method and application

Abstract

This invention relates to a low-defect rotational molding product, its preparation method, and its application in the field of rotational molding technology. The low-defect rotational molding product has an average micropore diameter of no more than 15 μm, and its yellow index changes by less than 30% compared to the yellow index of the rotational molding raw material. The low-defect rotational molding product is prepared by rotational molding of powder raw material using a variable pressure process, which includes establishing a negative pressure environment and a positive pressure environment within the mold cavity. The technical solution of this invention reduces the requirements for the particle size of the rotational molding raw material, improves the high-temperature oxidation problem during the rotational molding process, reduces the number and size of defect sites in the molded product, and improves the overall performance and aesthetics of the product.

Description

Technical Field

[0001] This invention relates to the field of rotational molding technology, and more specifically, to a low-defect rotational molded article, its preparation method, and its application. Background Technology

[0002] Rotational molding, also known as rotomolding, refers to adding powdered raw materials into a mold, then rotating the mold along two vertical axes and heating it. Under the action of gravity and heat, the plastic raw materials inside the mold are gradually and evenly coated, melted and adhered to the entire surface of the mold cavity, forming the desired shape. After cooling and solidification, the product is demolded and finally obtained.

[0003] Rotational molding offers advantages such as high design freedom, uniform wall thickness, low residual stress, low mold cost, and the ability to produce medium to large-sized hollow parts. However, it also has drawbacks, including high requirements for raw material particle size, susceptibility to raw material oxidation, low product density, and a tendency for internal wall porosity. It cannot produce products with high gas barrier performance requirements, such as high-pressure hydrogen storage type IV cylinders and gas tank liners. Patent CN 110027150 A discloses a rotational molding process using nylon. Nylon particles are ground into powder in a mill using cryogenic liquid nitrogen in a constant temperature and humidity environment, achieving a spherical particle ratio of over 96% in the nylon powder. This grinding process improves the flowability of the nylon powder, solving problems such as poor insert coating and material shortages in narrow flow channels, and preventing pinholes and sand holes on the surface of the rotationally molded product. However, the nylon powder preparation process disclosed in this patent is complex and cannot solve the yellowing problem of nylon during rotational molding. Patent CN 110039693 A discloses a rotational molding process with a controlled molding step. The molding process includes steps such as mold installation, material feeding, heating and molding, cooling, controlled molding, air cooling, and finishing. Once the mold temperature drops to 120℃, it enters a cooling chamber where high-pressure air is injected into the mold for natural cooling for 20-40 minutes. This reduces phenomena such as bulging, collapse, and large local dimensional changes in the inner lining of the mold, thereby reducing the defect rate. However, the rotational molding process disclosed in this patent is complex, only pressurizing during the cooling stage. Specifically, air is injected to create a high-pressure environment only after rotational molding is complete, which fails to address the high particle size requirements of the raw material powder and the problems of yellowing and oxidation of the finished product. Patent CN114426719A discloses a rotational molding polyethylene composition, its preparation method, and its application. This patent uses a compound of polyethylene resin, antioxidants, acid scavengers, lubricants, and aluminum nitride. By controlling the proportions of each component and adding special lubricants and aluminum nitride, it improves the defects of excessive porosity in rotationally molded products, resulting in superior performance. However, this patent indirectly reflects the defects of the product through a density method, and cannot directly reflect the size and number of porosity defects. Summary of the Invention

[0004] To address the aforementioned problems in existing technologies, this invention proposes a low-defect rotational molding product. Specifically, it relates to a low-defect rotational molding product, its preparation method, and its application. The molded product has high density, and the average micropore diameter of its micropore defects is no greater than 15 μm. The low-defect product is formed by rotational molding using a variable pressure process, and the steps may include: powder preparation, loading, sealing, establishing negative pressure, programmed rotational molding, variable pressure, cooling, and demolding. The advantages of this invention are: reducing the requirements for raw material particle size in rotational molding, improving the oxidation and yellowing problem of rotationally molded products, reducing micropore defects in the product wall, improving the overall performance and aesthetics of the product, and being suitable for rotational molding production of low-defect polyolefin and nylon products.

[0005] One objective of this invention is to provide a low-defect rotational molding product with no visible pinhole defects, an average micropore diameter of no more than 15 μm, and a yellow index of less than 30% compared to the yellow index of the rotational molding raw material. The low-defect rotational molding product is a polyolefin (e.g., polyethylene) and / or nylon product. The raw material for the low-defect rotational molding product is preferably PE and / or PA.

[0006] The low-defect rotational molding product is prepared by rotational molding of powder raw materials using a variable pressure process.

[0007] Preferably, the pressure variation process includes establishing a negative pressure environment and a positive pressure environment within the mold cavity.

[0008] The negative pressure environment is established after the material is filled and before the mold is heated; specifically, the negative pressure environment can be established by using a vacuum pump to extract air.

[0009] The absolute pressure value of the negative pressure environment is preferably adjustable from 0.01 bar to 1.0 bar, and further optimized to be adjustable from 0.01 bar to 0.5 bar; for example, it can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.12, 0.15, 0.18, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.0 bar, or any value between the above values, or a range between any two of the above values.

[0010] And / or,

[0011] The establishment of the positive pressure environment occurs after rotational molding but before the mold begins to cool.

[0012] Specifically, a positive pressure environment can be established by injecting gas. The absolute pressure value of the positive pressure environment inside the mold cavity after modification is preferably 1.0-2.5 bar, and further optimized to 1.5-2.5 bar. For example, it can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 bar, or any value between the above values, or a range between any two of the above values.

[0013] The rotational molding process parameters may include: rotational molding time of 20-40 min, mold temperature of 150-280℃, cooling time of 15-60 min, main spindle rotation speed of robotic arm set to 5-8 rpm, secondary spindle rotation speed of 9-12 rpm, wherein the cooling method is preferably air cooling or natural cooling.

[0014] The second objective of this invention is to provide a method for preparing the low-defect rotational molding article, which may specifically include the following steps:

[0015] a. Fill the powder raw material into the rotational molding mold and seal the rotational molding mold;

[0016] b. Establish a negative pressure environment for the rotational molding mold filled with powder raw materials, and mold according to the preset rotational molding process parameters;

[0017] c. After the powder raw material has completely melted and formed, change the pressure state inside the mold cavity to a positive pressure environment and rotate to cool;

[0018] d. After adjusting the pressure inside the mold to normal pressure, open the mold and demold to obtain the molded product.

[0019] In step b,

[0020] The absolute pressure value of the negative pressure environment can be adjusted from 0.01 bar to 1.0 bar, and more preferably from 0.01 bar to 0.5 bar;

[0021] In step b,

[0022] The rotational molding process parameters may include: rotational molding time of 20-40 min, mold temperature of 150-280℃, and cooling time of 15-60 min; wherein, the heating method of the rotational molding mold may be a commonly used heating method in the art, such as heating chamber or oven heating.

[0023] The preferred cooling method is air cooling or natural cooling. The main spindle rotation speed of the robotic arm can be set to 5-8 rpm, and the secondary spindle rotation speed can be set to 9-12 rpm.

[0024] And / or,

[0025] In step c, a positive pressure environment can be established by injecting gas.

[0026] The absolute pressure value of the positive pressure environment can be 1.0-2.5 bar, and more preferably 1.5-2.5 bar.

[0027] in,

[0028] The powder raw material is a polymer material, preferably PE and / or PA raw materials; the PE raw material is more preferably at least one of PE and various PE-based modified materials, and the number average molecular weight of the PE raw material can be 15,000-50,000, for example, 15,000, 18,000, 20,000, 22,000, 25,000, 28,000, 30,000, 32,000, 35,000, 38,000, 40,000, 42,000, 45,000, 48,000, 5 The value is 0000 or any value between the above values ​​or a range between any two of the above values; the PA raw material is more preferably PA or at least one of various PA-based modified materials; the number average molecular weight of the PA raw material can be 10000-30000, for example, it can be 10000, 12000, 15000, 18000, 20000, 22000, 25000, 28000, 30000 or any value between the above values ​​or a range between any two of the above values;

[0029] The melt flow index of the PE raw material is 2-30 g / 10 min (test temperature 230℃, load 2.16 kg), preferably 2-20 g / 10 min, and more preferably 3-15 g / 10 min (test temperature 230℃, load 2.16 kg); for example, it can be 2 g / 10 min, 3 g / 10 min, 4 g / 10 min, 5 g / 10 min, 6 g / 10 min, 7 g / 10 min, 8 g / 10 min, 9 g / 10 min, 10 g / 10 min, 11 g / 10 min, 12 g / 10 min, 16 g / 10 min, 18 g / 10 min, 20 g / 10 min, 22 g / 10 min, 24 g / 10 min, 26 g / 10 min, 28 g / 10 min, 30 g / 10 min or any value between the above values ​​or a range between any two of the above values;

[0030] The melt flow index of the PA raw material is 1-40 g / 10 min (test temperature is 230℃, load is 2.16 kg), preferably 2-30 g / 10 min, and more preferably between 4-20 g / 10 min. For example, it can be 1g / 10min, 2g / 10min, 3g / 10min, 4g / 10min, 5g / 10min, 6g / 10min, 7g / 10min, 8g / 10min, 9g / 10min, 10g / 10min, 11g / 10min, 12g / 10min, 16g / 10min, 18g / 10min, 20g / 10min, 22g / 10min, 25g / 10min, 27g / 10min, 30g / 10min, 32g / 10min, 36g / 10min, 38g / 10min, 40g / 10min, or any value between the above values, or a range of values ​​between any two of the above values.

[0031] The powder raw materials may include, but are not limited to, powders directly produced by interfacial polymerization, or powders obtained by mechanical crushing; the mechanical crushing is preferably, but not limited to, at least one of impact milling, air jet milling, ball milling, etc.

[0032] The particle size of the powder raw material can be 10-150 mesh, and can be further optimized to 30-100 mesh.

[0033] Before use, i.e. before step a, the powder raw material can be optionally dried; the drying temperature can be 70-100℃, and the drying time can be 4-8 hours. The specific time can be adjusted according to actual needs.

[0034] In step c, a positive pressure environment can be established by injecting gas.

[0035] The gas is preferably selected from at least one of compressed air, N2, CO2, and helium; more preferably, it is selected from at least one of N2, CO2, and helium.

[0036] The gas is preferably heated by a heat exchanger before being injected into the mold cavity, and the temperature of the heated gas is 80-120℃.

[0037] The third objective of this invention is to provide a rotational molded article obtained by the method for preparing low-defect rotational molded articles as described in the second objective of this invention.

[0038] The fourth objective of this invention is to provide a system for the preparation method of the low-defect rotational molding article, which may include a rotational molding mold, a detachable pressure gauge, and a detachable air circuit device; it may also include a heating device, which may be selected from commonly used heating devices in the art, such as a heating chamber or a heating oven.

[0039] The rotational molding mold is connected to a detachable pressure gauge and a detachable air circuit device.

[0040] The detachable gas circuit device includes a gas circuit, a vacuum pump, and a high-pressure gas storage tank;

[0041] One end of the gas path is connected to the rotational molding mold, and the other end is connected to a vacuum pump and a high-pressure gas storage tank via a three-way valve. Preferably, the gas path contains a filter screen, with a mesh size of 150-200 mesh. The system may also include a heat exchanger connected to the gas path to increase the temperature of the gas. The rotational molding mold may be equipped with a gas path switch at the connection points with the gas path and the pressure gauge.

[0042] The pressure gauge displays the pressure inside the cavity during the vacuuming and inflation stages. The pressure gauge can be removed during the heating and rotational molding process to seal the mold.

[0043] Specifically, the rotational molding mold is closed at one end, and connected to a detachable pressure gauge and a detachable air circuit device at the other end. The detachable air circuit device includes an air circuit, a vacuum pump, and a high-pressure gas storage tank. One end of the air circuit is connected to the rotational molding mold via an air circuit switch, and the other end is connected to the vacuum pump and the high-pressure gas storage tank via three-way valves. The system may also include a heat exchanger, which can be connected to the air circuit for gas heat exchange. Preferably, a filter screen is installed in the air circuit, with a mesh size preferably between 150 and 200 mesh. The system unit connection method can be as follows: Figure 8 As shown.

[0044] When using this system, powder raw materials are filled into a rotational molding mold, which is then sealed. A removable pressure gauge and a removable air circuit device are installed. A negative pressure environment is then established in the mold. To establish this environment, a vacuum pump is connected via a three-way valve. After establishing the negative pressure environment, the removable pressure gauge and air circuit device are removed, the mold is sealed, and the rotational molding process is performed according to the preset parameters. Once the powder raw materials have completely melted and formed, the removable pressure gauge and air circuit device are installed again to change the pressure in the mold cavity to a positive pressure environment. To establish this environment, a high-pressure gas tank is connected via a three-way valve. The gas is heated by a heat exchanger and then injected into the mold cavity, thus establishing a positive pressure environment. The air circuit device is then removed, the rotational molding mold is sealed, and the mold is rotated for cooling. After cooling, the pressure inside the mold is adjusted to atmospheric pressure, and the mold is opened and demolded to obtain the molded product.

[0045] Specifically,

[0046] In the rotational molding process, the pressure variation process includes establishing a negative pressure environment and establishing a positive pressure environment. The negative pressure environment is established after material filling and before mold heating by using a vacuum pump to evacuate air. The air evacuation path can be equipped with a filter (150-200 mesh) to prevent powder material from being extracted and clogging the air path during negative pressure environment establishment. The absolute pressure value of the negative pressure environment is adjustable from 0.01 bar to 1.0 bar, and can be further optimized to 0.01 bar to 0.5 bar. The positive pressure environment is established after rotational molding and before mold cooling by injecting gas. The type of gas injected includes compressed air, N2, CO2, and helium. The gas can be heated to 80-120°C via a heat exchanger before being injected into the mold cavity. After this adjustment, the absolute pressure value within the mold cavity is 1.0-2.5 bar, and can be further optimized to 1.5-2.5 bar.

[0047] The fourth objective of this invention is to provide applications of the low-defect rotational molding articles described in the first objective of this invention, or the articles obtained by the preparation method described in the second objective of this invention, or the system described in the third objective of this invention, including but not limited to applications in automotive fuel tank liners, Type IV hydrogen storage cylinder liners, high-pressure gas storage tank liners, or various rotational molding tanks.

[0048] The technical solution described in this invention implements a variable pressure rotational molding process by sealing the rotational molding mold, thereby reducing residual air during processing, lowering the requirements for the particle size of the rotational molding raw materials, improving the high-temperature oxidation problem of raw materials during rotational molding, reducing the number and size of defect sites in the molded products, and improving the overall performance and aesthetics of the products. Attached Figure Description

[0049] Figure 1 This is a schematic flowchart of the rotational molding process in Embodiment 1 of this application;

[0050] Figure 2 This is a Micro-CT scan (XY plane) of the molded article of Embodiment 1 of this application;

[0051] Figure 3 This is a Micro-CT scan (XZ plane) of the molded article of Embodiment 1 of this application;

[0052] Figure 4 This is a Micro-CT scan (YZ plane) of the molded article of Embodiment 1 of this application;

[0053] Figure 5 This is a Micro-CT scan (XY plane) of the molded article of Comparative Example 1 of this application;

[0054] Figure 6This is a Micro-CT scan (XZ plane) of the molded article of Comparative Example 1 of this application;

[0055] Figure 7 This is a Micro-CT scan (YZ plane) of the molded article of Comparative Example 1 of this application;

[0056] Figure 8 This is a schematic diagram showing the connection between the rotational molding die and the detachable air circuit device in the system described in this application;

[0057] Among them, 1 is the rotational molding mold, 2 is the detachable pressure gauge, 3 is the air circuit switch, 4 is the heat exchanger, 5 is the vacuum pump, 6 is the three-way valve, and 7 is the high-pressure air storage tank. Detailed Implementation

[0058] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0059] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0060] Sources of raw materials and equipment

[0061] The polyethylene was purchased from Sinopec Zhenhai Refining & Chemical Company, grade PE-546U, with a yellow index of -19.8.

[0062] The number average molecular weight is 21,000, and the melt index is 9.0 g / 10 min;

[0063] The nylon was purchased from Arkema in France. Roto 11 grade, the raw material yellow index is 3.8.

[0064] The number average molecular weight is 13,000, and the melt index is 5.6 g / 10 min;

[0065] The rotational molding equipment is the FD2-1000 electric heating rotational molding machine from Yantai Fangda Machinery Manufacturing Co., Ltd.

[0066] The disc crusher is the FDM-400 model from Yantai Fangda Machinery Manufacturing Co., Ltd.

[0067] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0068] Performance testing methods

[0069] Tensile strength was determined in accordance with GB / T 1040.1-2006, "Determination of Tensile Properties of Plastics".

[0070] Bending strength was determined according to GB / T 9341-2008 Plastics Bending Properties;

[0071] Low-temperature impact strength was determined according to GB / T1843-2008, Impact Strength of Plastic Cantilever Beams.

[0072] The yellow index was determined according to GB / T 2409-1980, the test method for yellow index of plastics. The rate of change of the yellow index was calculated according to the following formula:

[0073] R(%) = |(Y1-Y2) / Y2|*100%

[0074] Where R is the rate of change of yellow index, Y1 is the yellow index of rotational molded product, and Y2 is the yellow index of raw material used for rotational molding.

[0075] The melt flow index was determined according to GB / T 3682-2000. The test temperature for polyethylene was 230℃ and the nominal load was 2.16kg; the test temperature for nylon was 230℃ and the nominal load was 2.16kg.

[0076] The microstructure and porosity defects of the product were obtained using a Micro-CT 3D imaging system (Xlab-2000 model, Micro-Crystal Technology Co., Ltd.). Test parameters were: scanning voltage 50kV, scanning current 110μA, exposure time 0.5-1.0s, scanning frequency 0.3° / frame, and resolution 15μm. The average micropore diameter was used to characterize the porosity defects. The average micropore diameter was obtained through Micro-CT testing and statistical analysis.

[0077] The number of micropores is obtained by visually observing the appearance of the sample and is described using terms such as "none", "few", or "many".

[0078] System preparation:

[0079] Prepare rotational molding molds, detachable pressure gauges, detachable air circuit devices, heating ovens, etc.

[0080] One end of the rotational molding mold is closed, and the other end is connected to a detachable pressure gauge and a detachable air circuit device. The detachable air circuit device includes an air circuit, a vacuum pump, and a high-pressure air tank. One end of the air circuit is connected to the rotational molding mold, and the other end is connected to the vacuum pump and the high-pressure air tank via a three-way valve. A 200-mesh filter is installed in the air circuit. A heat exchanger is connected to the air circuit. (For standby use.)

[0081] Example 1

[0082] The polyethylene particles are crushed using a disc crusher and then sieved through a screening device to obtain powder with a particle size of 30-100 mesh.

[0083] Add polyethylene powder into the rotational molding mold, seal the mold, install a pressure gauge and connect the air circuit device, use a vacuum pump to extract the air in the mold until the absolute pressure reaches 0.1 bar, turn off the air circuit switch, and remove the pressure gauge and air circuit device.

[0084] The heating mold is placed in the oven, and the rotational molding mold temperature is set to 180℃, the rotational molding time to 30min, the main spindle rotation speed of the robotic arm is set to 8rpm, and the secondary spindle rotation speed is set to 10rpm for rotational heating.

[0085] After the rotational heating is completed, the mold is removed from the heating chamber. The pressure gauge and air circuit device are quickly installed. N2 heated to 90°C by the heat exchanger is injected into the mold to increase the pressure in the cavity to 1.5 bar. The air circuit switch is closed to seal the rotational molding mold. The air circuit device is removed and the rotational cooling continues for 30 minutes. The air circuit switch is then opened to adjust the mold cavity environment to normal pressure. The molded product is then demolded.

[0086] Comparative Example 1

[0087] The polyethylene particles are crushed using a disc crusher and then sieved through a screening device to obtain powder with a particle size of 30-100 mesh.

[0088] Polyethylene powder is added to a rotational molding mold, the mold is sealed, and the heated mold is placed in an oven. The rotational molding mold temperature is set to 180℃, the rotational molding time to 30 minutes, the main spindle rotation speed of the robotic arm to 8 rpm, and the secondary spindle rotation speed to 10 rpm for rotational heating. After rotational heating is completed, the mold is removed from the heating chamber and continues to rotate and air-cool for 30 minutes before demolding to obtain the molded product.

[0089] Comparative Example 2

[0090] The polyethylene particles are crushed using a disc crusher and then sieved through a screening device to obtain powder with a particle size of 30-100 mesh.

[0091] Polyethylene powder is added to the rotational molding mold, the mold is sealed and connected, the heated mold is sent into the oven, and the rotational molding mold temperature is set to 180℃, the rotational molding time to 30min, the main shaft rotation speed of the robotic arm is set to 8rpm, and the secondary shaft rotation speed is set to 10rpm for rotational heating.

[0092] After the rotational heating is completed, the mold is removed from the heating chamber. The pressure gauge and air circuit device are quickly installed. N2 heated to 90°C by the heat exchanger is injected into the mold to increase the pressure in the cavity to 1.5 bar. The air circuit switch is closed to seal the rotational molding mold and the air circuit device is removed. Rotational cooling continues for 30 minutes. The air circuit switch is then opened to adjust the mold cavity environment to normal pressure. The molded product is then demolded.

[0093] Comparative Example 3

[0094] The polyethylene particles are crushed using a disc crusher and then sieved through a screening device to obtain powder with a particle size of 30-100 mesh.

[0095] Add polyethylene powder into the rotational molding mold, seal the mold, install a pressure gauge and connect the air circuit device, use a vacuum pump to extract air from the mold until the absolute pressure reaches 0.1 bar, turn off the air circuit switch, and remove the pressure gauge and air circuit device.

[0096] The heating mold is placed in the oven, and the rotational molding mold temperature is set to 180℃, the rotational molding time to 30min, the main spindle rotation speed of the robotic arm is set to 8rpm, and the secondary spindle rotation speed is set to 10rpm for rotational heating.

[0097] After the rotational heating is completed, the mold is removed from the heating chamber. The pressure gauge and air circuit device are quickly installed. N2 heated to 90°C by the heat exchanger is injected into the mold to restore the cavity to normal pressure. The air circuit switch is closed to seal the rotational molding mold and the air circuit device is removed. Rotational cooling continues for 30 minutes. The air circuit switch is then opened to adjust the mold cavity environment to normal pressure. The molded product is then demolded.

[0098] Comparative Example 4

[0099] The polyethylene granules were pulverized using a disc mill and then sieved to obtain powder with a particle size of 30-100 mesh. The polyethylene powder and composite antioxidant were mixed in a high-speed mixer for 10 minutes at a speed of 800 rpm. The mixture consisted of 98 parts by weight of polyethylene powder and 2 parts by weight of composite antioxidant (the composite antioxidant comprised of antioxidant 168 and antioxidant 1010, with a weight ratio of 1:2). The mixture was then set aside for later use.

[0100] Polyethylene powder blended with composite antioxidants was used as the rotational molding material, and the rotational molding process was the same as that of Comparative Example 1.

[0101] Example 2

[0102] The polyethylene particles are crushed using a disc crusher and then sieved through a screening device to obtain powder with a particle size of 10-50 mesh.

[0103] Add polyethylene powder into the rotational molding mold, seal the mold, install a pressure gauge and connect the air circuit device, use a vacuum pump to extract air from the mold until the absolute pressure reaches 0.1 bar, turn off the air circuit switch, and remove the pressure gauge and air circuit device.

[0104] The heating mold is placed in the oven, and the rotational molding mold temperature is set to 180℃, the rotational molding time to 30min, the main spindle rotation speed of the robotic arm is set to 8rpm, and the secondary spindle rotation speed is set to 10rpm for rotational heating.

[0105] After the rotational heating is completed, the mold is removed from the heating chamber. The pressure gauge and air circuit device are quickly installed. N2 heated to 90°C by the heat exchanger is injected into the mold to increase the pressure in the cavity to 1.5 bar. The air circuit switch is closed to seal the rotational molding mold and the air circuit device is removed. Rotational cooling continues for 30 minutes. The air circuit switch is then opened to adjust the mold cavity environment to normal pressure. The molded product is then demolded.

[0106] Example 3

[0107] PA11 particles were crushed using a disc milling device, and then sieved through a sieving device to obtain powder with a particle size of 30-100 mesh. The PA11 powder was then dried in an oven at 100℃ for 6 hours.

[0108] Add the dried PA11 powder into the rotational molding mold, seal the mold, install a pressure gauge and connect the air circuit device, use a vacuum pump to extract the air in the mold until the absolute pressure reaches 0.1 bar, turn off the air circuit switch, and remove the pressure gauge and air circuit device.

[0109] The heating mold is placed in the oven, and the rotational molding mold temperature is set to 200℃, the rotational molding time to 35min, the main spindle rotation speed of the robotic arm is set to 8rpm, and the secondary spindle rotation speed is set to 10rpm for rotational heating.

[0110] After the rotational heating is completed, the mold is removed from the heating chamber. The pressure gauge and air circuit device are quickly installed. N2 heated to 105°C by the heat exchanger is injected into the mold to increase the pressure in the cavity to 1.5 bar. The air circuit switch is closed to seal the rotational molding mold and the air circuit device is removed. Rotational cooling continues for 30 minutes. The air circuit switch is then opened to adjust the mold cavity environment to normal pressure. The molded product is then demolded.

[0111] Comparative Example 5

[0112] PA11 particles were crushed using a disc milling device, and then sieved through a sieving device to obtain powder with a particle size of 30-100 mesh. The PA11 powder was then dried in an oven at 100℃ for 6 hours.

[0113] The dried PA11 powder and composite antioxidant were mixed in a high-speed mixer for 10 minutes at a speed of 800 rpm. The mixture consisted of 98 parts by weight of PA11 powder and 2 parts by weight of composite antioxidant (the composite antioxidant comprised antioxidants 168 and 1010, with a weight ratio of 1:2). The mixture was then set aside for later use.

[0114] Nylon PA11 powder blended with composite antioxidants was added to a rotational molding mold. The mold was sealed and connected, and the heated mold was placed in an oven. The rotational molding mold temperature was set to 200℃, the rotational molding time to 35 minutes, the main spindle rotation speed of the robotic arm to 8 rpm, and the secondary spindle rotation speed to 10 rpm for rotational heating. After rotational heating was completed, the mold was removed from the heating chamber and continued to rotate and air-cool for 30 minutes. The air circuit switch was turned on to adjust the mold cavity environment to normal pressure, and the molded product was demolded.

[0115] Example 4

[0116] PA11 particles were crushed using a disc milling machine, and then sieved through a sieving device to obtain powder with a particle size of 10-50 mesh. The PA11 powder was then dried in an oven at 100℃ for 6 hours.

[0117] Add the dried PA11 powder into the rotational molding mold, seal the mold, install a pressure gauge and connect the air circuit device, use a vacuum pump to extract the air in the mold until the absolute pressure reaches 0.1 bar, turn off the air circuit switch, and remove the pressure gauge and air circuit device.

[0118] The heating mold is placed in the oven, and the rotational molding mold temperature is set to 200℃, the rotational molding time to 35min, the main spindle rotation speed of the robotic arm is set to 8rpm, and the secondary spindle rotation speed is set to 10rpm for rotational heating.

[0119] After the rotational heating is completed, the mold is removed from the heating chamber. The pressure gauge and air circuit device are quickly installed. N2 heated to 105°C by the heat exchanger is injected into the mold to increase the pressure in the cavity to 1.5 bar. The air circuit switch is closed to seal the rotational molding mold, and the air circuit device is removed. Rotational cooling continues for 30 minutes. The air circuit switch is then opened to adjust the mold cavity environment to normal pressure. The molded product is then demolded.

[0120] Product performance testing

[0121] The molded products prepared in Examples 1-4 and Comparative Examples 1-5 were subjected to performance tests. The test results are shown in Table 1.

[0122] Table 1 Physical properties of inner liners formed by rotational molding processes

[0123]

[0124] In this embodiment, the mold cavity is evacuated before rotational molding, followed by heating and rotational molding (i.e., negative pressure rotational molding). Subsequently, the negative pressure environment of the mold cavity is converted to a high-pressure environment of N2 atmosphere, followed by rotary cooling (i.e., high-pressure cooling). This variable pressure process ensures that the material has minimal contact with air throughout the rotational molding process, reducing the degree of oxidation and yellowing of the product during rotational molding and minimizing air bubble defects caused by residual air in the inner wall. Simultaneously, the negative pressure rotational molding in the variable pressure process facilitates the rotational molding of large-diameter raw materials.

[0125] Micro-CT tests were performed on the samples prepared in Example 1 and Comparative Example 1, and the results are as follows: Figures 2-7 The use of rotational molding can reduce the number of micropores in the product's appearance. In the Micro-CT test, the sample prepared in Example 1 showed uniform and flat cross-sections in all three dimensions, with no micropore defects larger than 15 μm in diameter observed. In contrast, the sample in Comparative Example 1 showed numerous pore defects of varying sizes, with an average micropore diameter of 191 μm. Rotational molding also improves the mechanical strength of the product; the tensile strength and low-temperature notched impact strength of the sample in Example 1 increased to 18.7 MPa and 6.2 kJ / m², respectively. 2 This is because the variable pressure molding process avoids air bubble defects in the inner wall and increases the density of the inner wall; the yellow index of the molded product of Comparative Example 1 is -12.3, which is higher than -18.0 of Example 1, indicating that the variable pressure rotational molding process is beneficial to reduce the degree of oxidation and yellowing of raw materials during rotational molding and reduce the yellow index of the molded product.

[0126] As can be seen from Example 1, Comparative Example 1 and Comparative Example 2, the negative pressure environment formed during the heating rotational molding process of this application can significantly reduce the yellow index of the product.

[0127] As can be seen from Examples 1 and 4, the rotational molding process is more effective than adding antioxidants in improving the yellowing of materials during rotational molding. The yellowing index change rate in Example 1 was 9.1%, while that in Comparative Example 4 was 26.8%. Furthermore, as can be seen from Examples 3 and 5, the rotational molding process in Example 3 resulted in a yellowing index change rate of 10.5%, while the conventional process in Comparative Example 5 resulted in a yellowing index change rate of 331.6%. The rotational molding process significantly improves the yellowing degree of PA11 material.

[0128] Both Examples 2 and 4 use 10-50 mesh polyethylene powder or nylon 11 powder as rotational molding raw materials. The molded products have good comprehensive performance. This is because the pressure-variable environment helps to reduce the residual air between raw materials, avoid oxidation and defects in the products, save the cost of crushing raw materials, and broaden the particle size range of rotational molding materials.

[0129] In summary, this invention has to some extent solved the problems of high requirements for raw material particle size, easy oxidation of raw materials in high-temperature environments, and easy defects in rotational molding products during the production process in traditional rotational molding processes.

[0130] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A low-defect rotational molding product, wherein the average micropore diameter of the micropore defects is no greater than 15 μm, and the yellow index of the product changes by less than 30% compared to the yellow index of the rotational molding raw material; The low-defect rotational molding article is prepared by a method including the following steps: a. Fill the powder raw material into the rotational molding mold and seal the rotational molding mold; b. Establish a negative pressure environment for the rotational molding mold filled with powder raw materials, and perform rotational molding; c. Change the pressure state inside the mold cavity to a positive pressure environment and cool it; d. After adjusting the pressure inside the mold to normal pressure, open the mold and demold to obtain the molded product; In step c, a positive pressure environment is established by injecting gas; the gas is heated and then injected into the mold cavity.

2. The low-defect rotational molding product according to claim 1, characterized in that: The low-defect rotational molding products are polyolefin and / or nylon products.

3. The low-defect rotational molding article according to claim 1 or 2, characterized in that... The low-defect rotational molding product is prepared by rotational molding of powder raw materials through a variable pressure process, wherein the variable pressure process includes establishing a negative pressure environment and establishing a positive pressure environment in the mold cavity.

4. The low-defect rotational molding product according to claim 3, characterized in that: The negative pressure environment is established after the material is filled and before rotational molding.

5. The low-defect rotational molding product according to claim 3, characterized in that: The establishment of the positive pressure environment occurs after rotational molding and before the mold begins to cool.

6. The method for preparing a low-defect rotational molding article according to any one of claims 1 to 5, characterized in that... Includes the following steps: a. Fill the powder raw material into the rotational molding mold and seal the rotational molding mold; b. Establish a negative pressure environment for the rotational molding mold filled with powder raw materials, and perform rotational molding; c. Change the pressure state inside the mold cavity to a positive pressure environment and cool it; d. After adjusting the pressure inside the mold to normal pressure, open the mold and demold to obtain the molded product.

7. The method for preparing low-defect rotational molding articles according to claim 6, characterized in that: In step b, The absolute pressure value of the negative pressure environment is 0.01 bar - 1.0 bar; And / or, In step c, The absolute pressure value of the positive pressure environment is 1.0-2.5 bar.

8. The method for preparing low-defect rotational molding articles according to claim 7, characterized in that: In step b, The absolute pressure value of the negative pressure environment is 0.01 bar to 0.5 bar.

9. The method for preparing a low-defect rotational molding article according to claim 7, characterized in that: In step c, The absolute pressure value of the positive pressure environment is 1.5-2.5 bar.

10. The method for preparing a low-defect rotational molding article according to claim 6, characterized in that: The process parameters for rotational molding include: mold temperature of 150-280℃.

11. The method for preparing a low-defect rotational molding article according to claim 6, characterized in that: The process parameters for rotational molding include: Rotational molding time is 20-40 min; cooling time is 15-60 min.

12. The method for preparing low-defect rotational molding articles according to claim 6, characterized in that: The process parameters for rotational molding include: The cooling method is either air cooling or natural cooling.

13. The method for preparing low-defect rotational molding articles according to claim 6, characterized in that: The rotational molding process parameters also include setting the main spindle rotation speed of the robotic arm to 5-8 rpm and the secondary spindle rotation speed to 9-12 rpm.

14. The method for preparing low-defect rotational molding articles according to claim 6, characterized in that: The powder raw material is a polymer material.

15. The method for preparing a low-defect rotational molding article according to claim 14, characterized in that: The powder raw material is PE raw material and / or PA raw material.

16. The method for preparing a low-defect rotational molding article according to claim 15, characterized in that: The PE raw material is at least one of PE and PE-based modified materials.

17. The method for preparing a low-defect rotational molding article according to claim 15, characterized in that: The number average molecular weight of the PE raw material is 15,000-50,000.

18. The method for preparing a low-defect rotational molding article according to claim 15, characterized in that: The PA raw material is at least one of PA and PA-based modified materials, and the number average molecular weight of the PA raw material is 10,000-30,000.

19. The method for preparing a low-defect rotational molding article according to claim 15, characterized in that: The particle size of the powder raw material is 10-150 mesh.

20. The method for preparing a low-defect rotational molding article according to claim 15, characterized in that: The particle size of the powder raw material is 30-100 mesh.

21. The method for preparing a low-defect rotational molding article according to claim 15, characterized in that: The PE raw material has a melt index of 2-30 g / 10min at a test temperature of 230℃ and a load of 2.16kg.

22. The method for preparing a low-defect rotational molding article according to claim 21, characterized in that: The PE raw material has a melt index of 3-15 g / 10min at a test temperature of 230℃ and a load of 2.16kg.

23. The method for preparing a low-defect rotational molding article according to claim 15, characterized in that: The PA raw material has a melt index of 1-40 g / 10 min at a test temperature of 230℃ and a load of 2.16 kg.

24. The method for preparing a low-defect rotational molding article according to claim 23, characterized in that: The PA raw material has a melt index of 2-30 g / 10 min at a test temperature of 230℃ and a load of 2.16 kg.

25. The method for preparing a low-defect rotational molding article according to claim 6, characterized in that: The powder raw material is a powder directly produced by interfacial polymerization and / or a powder obtained by mechanical crushing.

26. The method for preparing a low-defect rotational molding article according to claim 25, characterized in that: The mechanical pulverization includes at least one of impact mill pulverization, airflow mill pulverization, and ball mill pulverization.

27. The method for preparing a low-defect rotational molding article according to claim 6, characterized in that: The gas is selected from at least one of compressed air, N2, CO2, and helium; In step c, the temperature of the heated gas is 80~120℃.

28. The method for preparing a low-defect rotational molding article according to claim 27, characterized in that: The gas is selected from at least one of N2, CO2, and helium.

29. A rotationally molded article obtained by the method for preparing a low-defect rotationally molded article according to any one of claims 6 to 28.

30. The system used in the method for preparing a low-defect rotational molding article according to any one of claims 6 to 28 includes a rotational molding mold, a detachable pressure gauge, and a detachable air circuit device; in, The rotational molding die is connected to a detachable pressure gauge and a detachable air circuit device; The detachable gas circuit device includes a gas circuit, a vacuum pump, and a high-pressure gas storage tank; One end of the gas line is connected to the rotational molding mold, and the other end is connected to the vacuum pump and the high-pressure gas storage tank through a three-way valve.

31. The system used in the method for preparing low-defect rotational molding articles according to claim 30, characterized in that: The gas path contains a filter.

32. The system used in the method for preparing low-defect rotational molding articles according to claim 31, characterized in that: The filter screen has a mesh size of 150-200.

33. The system used in the method for preparing low-defect rotational molding articles according to claim 30, characterized in that: The system includes a heat exchanger connected to a gas path.

34. Use of the low-defect rotational molding article according to any one of claims 1 to 5 or 29, or the article obtained by the preparation method according to any one of claims 6 to 28, or the article obtained by the system according to any one of claims 30 to 33.

35. The application according to claim 34, characterized in that... Applications in automotive fuel tank liners, Type IV hydrogen storage cylinder liners, high-pressure gas storage tank liners, or rotationally molded tanks.

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