Low-defect rotational molding product as well as preparation method and application thereof

Abstract

The invention relates to a low-defect rotational molding product and a preparation method and application thereof in the technical field of rotational molding. According to the low-defect rotational molding product, the average micropore diameter of the micro-pore defects is not larger than 15 microns, and the change rate of the yellow index of the product is smaller than 30% compared with that of rotational molding raw materials. The low-defect rotational molding product is prepared by rotational molding of a powder raw material through a variable pressure process, and the variable pressure process comprises establishment of a negative pressure environment and establishment of a positive pressure environment in a mold cavity. According to the technical scheme, the requirement for the particle size of the rotational molding raw material is lowered, the problem of high-temperature oxidation in the rotational molding process of the raw material is solved, the number and size of defect sites in a molded product are reduced, and the comprehensive performance and attractiveness of the product are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of rotational molding, and more particularly to a low-defect rotational molding product and a preparation method and application thereof. Background Art

[0002] Rotomolding, also known as rotational molding, refers to adding powder raw materials into a mold, which is then rotated continuously along two vertical axes and heated, so that the plastic raw materials in the mold are gradually and evenly coated, melted and adhered to the entire surface of the mold cavity under the action of gravity and heat, and formed into the desired shape, and then cooled, shaped, and demolded to finally obtain the finished product.

[0003] Roto-molding has the advantages of large design freedom, uniform product wall thickness, low residual stress, low mold cost, and the ability to produce medium and large hollow parts. However, it also has the problems of high requirements for raw material particle size, easy oxidation of raw materials, low product density, and easy hole defects on the inner wall. It is impossible to prepare products with high gas barrier properties, such as high-pressure hydrogen storage IV-type bottles and gas tank liners. Patent CN 110027150 A discloses a roto-molding process using nylon, in which nylon particles are ground in a mill using a low-temperature liquid nitrogen process in a constant temperature and humidity environment, so that the proportion of round particles of nylon powder reaches more than 96%. The grinding process improves the fluidity of nylon powder, solves the problem of poor coating of inserts and the phenomenon of lack of material at the position of fine flow channels, and prevents the appearance of pinholes and sand holes on the surface of roto-molded products. The nylon powder preparation process disclosed in the patent is complicated and cannot solve the yellowing problem of nylon during roto-molding. Patent CN 110039693 A discloses a roto-molding process with a controlled molding process. The molding process includes the steps of installing the mold, adding materials, heating and molding, cooling, controlled molding, air cooling and finishing. When the mold temperature drops to 120°C, it enters the cooling chamber, injects high-pressure air into the mold, and cools naturally for 20 to 40 minutes, which reduces the bulging and collapse of the lining plastic in the mold, and the large local size changes, thereby reducing the defective rate. The roto-molding process disclosed in the patent is complex, and only a single pressurization is performed during the cooling stage. Specifically, air is injected to form a high-pressure environment after the roto-molding is completed. It cannot solve the high requirements for the particle size of the raw material powder and the yellowing and oxidation of the product. Patent CN114426719A discloses a roto-molded polyethylene composition and its preparation method and application. The patent is compounded with polyethylene resin, antioxidant, acid absorbent, lubricant and aluminum nitride. By controlling the addition ratio of each component, and adding special lubricant and aluminum nitride, the defect of more pores in the roto-molded product is improved, and the performance of the roto-molded product is better. However, the patent indirectly reflects the defects of the product through the density method, and cannot directly reflect the size and number of the pore defects of the product. Summary of the invention

[0004] In order to solve the above-mentioned problems existing in the prior art, the present invention proposes a low-defect rotationally molded product. Specifically, it relates to a low-defect rotationally molded product and its preparation method and application. The molded product has a high density, and the average micropore diameter of its micropore defects is not more than 15μm. The low-defect product is rotationally molded by a variable pressure process, and the steps may include: powder preparation, loading, sealing, establishing negative pressure, program rotational molding, variable pressure, cooling, and demolding. The benefits of the present invention are: reducing the requirements of rotational molding on the particle size of raw materials, improving the oxidation and yellowing problem of rotationally molded products, reducing micropore defects in the wall of the product, and improving the comprehensive performance and aesthetics of the product. It is suitable for rotational molding production of low-defect polyolefin and nylon products.

[0005] One of the purposes of the present invention is to provide a low-defect rotomolded product, which has no visible pinhole defects in appearance, an average micropore diameter of microscopic pore defects is not greater than 15 μm, and a yellow index of the product changes by less than 30% compared with the yellow index of the rotomolded raw material. The low-defect rotomolded product is a polyolefin (such as polyethylene) and / or nylon product. The raw material of the low-defect rotomolded product is preferably a PE raw material and / or a PA raw material.

[0006] The low-defect rotationally molded product is prepared by rotationally molding a powder raw material through a pressure-changing process.

[0007] Preferably, the pressure changing process includes establishing a negative pressure environment and a positive pressure environment in 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 evacuating air with a vacuum pump.

[0009] The absolute pressure value of the negative pressure environment is preferably adjustable between 0.01 bar and 1.0 bar, and is further optimized to be adjustable between 0.01 bar and 0.5 bar; for example, it may 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 numerical range between any two of the above values.

[0010] and / or,

[0011] The time for establishing the positive pressure environment is after the rotational molding is completed and before the mold begins to cool;

[0012] Specifically, a positive pressure environment can be established by injecting gas, and the absolute pressure value of the positive pressure environment in the mold cavity after the change 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 numerical range between any two of the above values.

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

[0014] The second object of the present invention is to provide a method for preparing the low-defect rotationally molded product, 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. Create 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 is completely melted and formed, change the pressure state in the mold cavity to a positive pressure environment and rotate to cool;

[0018] d. Adjust the pressure in the mold to normal pressure, then open the mold and demould to obtain the molded product.

[0019] In the step b,

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

[0021] In the step b,

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

[0023] The cooling method is preferably air cooling or natural cooling. The rotation speed of the main axis of the robot arm can be set to 5-8 rpm, and the rotation speed of the secondary axis 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 may 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 a PE raw material and / or a PA raw material; the PE raw material is more preferably at least one of PE and various modified materials based on PE, and the number average molecular weight of the PE raw material can be 15000-50000, for example, 15000, 18000, 20000, 22000, 25000, 28000, 30000, 32000, 35000, 38000, 40000, 42000, 45000, 48000, 50 ... 0000 or any value between the above values ​​or a numerical range between any two of the above values; the PA raw material is more preferably PA or at least one of various modified materials based on PA; the number average molecular weight of the PA raw material may be 10000-30000, for example, specifically 10000, 12000, 15000, 18000, 20000, 22000, 25000, 28000, 30000 or any value between the above values ​​or a numerical range between any two of the above values;

[0029] The melt index of the PE raw material is 2-30g / 10min (test temperature is 230°C, load is 2.16kg), preferably 2-20g / 10min, and further preferably between 3-15g / 10min (test temperature is 230°C, load is 2.16kg); for example, it can be 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, 24g / 10min, 26g / 10min, 28g / 10min, 30g / 10min or any value between the above values ​​or a numerical range between any two of the above values;

[0030] The melt index of the PA raw material is 1-40 g / 10 min (test temperature is 230° C., 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 numerical range between any two of the above values.

[0031] The powder raw material may include but is not limited to powder directly produced by interfacial polymerization, and may also include but is not limited to powder obtained by mechanical crushing; the mechanical crushing preferably includes but is not limited to at least one of impact mill crushing, air flow crushing, ball mill crushing and the like.

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

[0033] The powder raw material can be optionally dried before use, that is, before step a is performed; the drying temperature can be 70-100° C., and the drying time can be 4-8 hours. The specific drying time can be adjusted according to actual needs.

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

[0035] The type of the gas can be preferably selected from at least one of compressed air, N2, CO2, and helium; more preferably at least one of N2, CO2, and helium;

[0036] The gas is preferably heated by a heat exchanger and then injected into the mold cavity, and the temperature of the gas after heating is 80-120°C.

[0037] The third object of the present invention is to provide a rotationally molded product obtained by the method for preparing a low-defect rotationally molded product as described in the second object of the present invention.

[0038] The fourth object of the present invention is to provide a system used in the method for preparing the low-defect roto-molded product, which may include a roto-molding mold, a detachable pressure gauge, and a detachable gas path device; and 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] Wherein, the rotational molding mold is connected with a detachable pressure gauge and a detachable gas path 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 circuit 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; the gas circuit preferably contains a filter screen, and the mesh number of the filter screen is preferably 150-200 meshes; the system may also include a heat exchanger, which is connected to the gas circuit and is used to increase the temperature of the gas in the gas circuit. The rotational molding mold may be provided with a gas circuit switch at the connection with the gas circuit and the connection with the pressure gauge.

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

[0043] Specifically, one end of the rotomolding mold is closed, and the other end is connected to a detachable pressure gauge and a detachable gas circuit device; wherein the detachable gas circuit device includes a gas circuit, a vacuum pump and a high-pressure gas storage tank; one end of the gas circuit is connected to the rotomolding mold through a gas circuit switch, and the other end is connected to the vacuum pump and the high-pressure gas storage tank respectively through a three-way valve; the system may also include a heat exchanger, which may be connected to the gas circuit for gas heat exchange; a filter screen is preferably provided in the gas circuit, and the mesh size of the filter screen is preferably 150-200 meshes. The system unit connection method can be as follows Figure 8 shown.

[0044] When using the system, the powder raw material can be filled into the rotomolding mold, the rotomolding mold can be sealed and installed, and a detachable pressure gauge and a detachable gas path device can be installed; then a negative pressure environment can be established for the rotomolding mold filled with the powder raw material. When the negative pressure environment is established, the gas path can be connected to the vacuum pump through the three-way valve to establish a negative pressure environment in the mold cavity; after the negative pressure environment is established, the detachable pressure gauge and the detachable gas path device are removed, the mold is sealed, and the molding is performed according to the preset rotomolding process parameters; after the powder raw material is completely melted and molded, the detachable pressure gauge and the detachable gas path device are installed to change the pressure state in the mold cavity to a positive pressure environment. When the positive pressure environment is established, the gas path is connected to the high-pressure gas storage tank through the three-way valve, and the gas can be heated by the heat exchanger and then injected into the mold cavity, and a positive pressure environment is established by filling gas. Then the gas path device is removed, the rotomolding mold is sealed, and the rotation is cooled; after cooling, the pressure in the mold is adjusted to normal pressure, and then the mold is opened and demolded to obtain a molded product.

[0045] Specifically,

[0046] During the rotational molding process, the pressure change process includes establishing a negative pressure environment and establishing a positive pressure environment. The negative pressure environment is established after the material is filled and before the mold is heated. The negative pressure environment is established by means of vacuum pump exhaust. The exhaust gas path can be installed with a filter (the mesh of the filter can be 150-200 meshes) to prevent the powder raw materials from being extracted and blocking the gas path when the negative pressure environment is established; the absolute pressure value of the negative pressure environment is adjustable from 0.01bar to 1.0bar, and further optimized to be adjustable from 0.01bar to 0.5bar. The positive pressure environment is established after the rotational molding is completed and before the mold begins to cool. The positive pressure environment is established by filling gas. The types of gas added include compressed air, N2, CO2, and helium. The gas can be heated to 80-120℃ through a heat exchanger and then injected into the mold cavity. After the change, the absolute pressure value in the mold cavity is 1.0-2.5bar, which is further optimized to 1.5-2.5bar.

[0047] The fourth object of the present invention is to provide the application of the low-defect rotationally molded product described in the first object of the present invention, or the product obtained by the preparation method described in the second object of the present invention, or the system described in the third object of the present invention, including but not limited to the application in vehicle fuel tank liners, type IV hydrogen storage bottle liners, high-pressure gas storage tank liners or various types of rotationally molded tank bodies.

[0048] The technical solution of the present invention implements a variable pressure rotational molding process by sealing the rotational molding mold, reducing the residual air during the processing, lowering the requirements for the particle size of the rotational molding raw materials, improving the high temperature oxidation problem of the raw materials during the rotational molding process, reducing the number and size of defect sites in the molded products, and improving the comprehensive performance and aesthetics of the products. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 is a schematic flow chart of the rotational molding process in Example 1 of the present application;

[0050] Figure 2 is a Micro-CT scan of the molded product of Example 1 of the present application (XY plane);

[0051] Figure 3 is a Micro-CT scan (XZ plane) of the molded product of Example 1 of the present application;

[0052] Figure 4 is a Micro-CT scan of the molded product of Example 1 of the present application (YZ plane);

[0053] Figure 5 It is a Micro-CT scan image (XY plane) of the molded product of Comparative Example 1 of the present application;

[0054] Figure 6It is a Micro-CT scan image (XZ plane) of the molded product of Comparative Example 1 of the present application;

[0055] Figure 7 It is a Micro-CT scan image (YZ plane) of the molded product of Comparative Example 1 of the present application;

[0056] Figure 8 It is a schematic diagram of the connection between the rotational molding mold and the detachable gas path device in the system described in the present application;

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

[0058] The present invention is described in detail below in conjunction with specific embodiments. It is necessary to point out that the following embodiments are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the contents of the present invention still fall within the scope of protection of the present invention.

[0059] The endpoints and any values ​​of the ranges disclosed in this article 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0060] Sources of raw materials and equipment

[0061] Polyethylene was purchased from Sinopec Zhenhai Refining and Chemical Company, PE-546U grade, with a raw material yellow index of -19.8.

[0062] The number average molecular weight is 21000 and the melt index is 9.0g / 10min;

[0063] Nylon was purchased from Arkema, France. Roto 11 brand, raw material yellow index is 3.8,

[0064] The number average molecular weight is 13000 and the melt index is 5.6g / 10min;

[0065] The rotomolding equipment is the FD2-1000 electric heating rotomolding machine produced by Yantai Fangda Machinery Manufacturing Co., Ltd.

[0066] The disc crusher is FDM-400 produced by Yantai Fangda Machinery Manufacturing Co., Ltd.

[0067] The raw materials used in the examples and comparative examples, unless otherwise specified, are disclosed in the prior art, for example, they can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0068] Performance Testing Methods

[0069] The tensile strength is determined in accordance with GB / T 1040.1-2006 Determination of tensile properties of plastics;

[0070] Bending strength is measured according to GB / T 9341-2008 Plastic Bending Properties;

[0071] Low temperature impact strength is measured according to GB / T1843-2008 plastic cantilever beam impact strength;

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

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

[0074] Wherein, R is the yellow index change rate, Y1 is the yellow index of the rotomolded product, and Y2 is the yellow index of the rotomolded raw material;

[0075] The melt index is measured according to GB / T 3682-2000. The test temperature of polyethylene is 230°C and the nominal load is 2.16 kg; the test temperature of nylon is 230°C and the nominal load is 2.16 kg.

[0076] The microscopic morphology and pore defects of the product were obtained using Micro-CT three-dimensional imaging equipment (Micro-Kuang Technology Company Xlab-2000), with test parameters: scanning voltage 50kV, scanning current 110μA, exposure time 0.5-1.0s, scanning frequency 0.3° / sheet, resolution 15μm. The average micropore diameter was used to characterize the pore defects. The average micropore diameter was obtained through Micro-CT testing and statistics.

[0077] The number of micropores is obtained by observing the appearance of the sample with the naked eye, and is described using words such as "none, less, more".

[0078] System Preparation:

[0079] Prepare rotational molding molds, detachable pressure gauges, detachable gas path devices, heating ovens, etc.;

[0080] One end of the rotomolding mold is closed, and the other end is connected with a detachable pressure gauge and a detachable gas 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 circuit is connected to the rotomolding mold, and the other end is connected to the vacuum pump and the high-pressure gas storage tank through a three-way valve; a filter with a mesh number of 200 is set in the gas circuit; a heat exchanger is connected to the gas circuit. Standby.

[0081] Example 1

[0082] The polyethylene particles are crushed using a grinding disc type crushing equipment, and the powder with a particle size of 30-100 mesh is screened out by a screening device.

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

[0084] The heated mold was sent into the oven, and the rotational molding mold temperature was set to 180°C, the rotational molding time was set to 30 min, the main axis rotation speed of the robot arm was set to 8 rpm, and the secondary axis rotation speed was set to 10 rpm for rotational heating.

[0085] After the rotation heating is completed, the mold is removed from the heating chamber, the pressure gauge and gas circuit device are quickly installed, and 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 gas circuit switch is closed to seal the rotational molding mold, the gas circuit device is removed, and the rotation air cooling is continued for 30 minutes. The gas circuit switch is opened to adjust the mold cavity environment to normal pressure, and the molded product is demolded.

[0086] Comparative Example 1

[0087] The polyethylene particles are crushed using a grinding disc type crushing equipment, and the powder with a particle size of 30-100 mesh is screened out by a screening device.

[0088] Add polyethylene powder to the rotomolding mold, seal and connect the mold, send the heated mold into the oven, set the rotomolding mold temperature to 180°C, the rotomolding time to 30 minutes, the mechanical arm main axis rotation speed to 8 rpm, and the secondary axis rotation speed to 10 rpm, and perform rotary heating. After the rotary heating is completed, the mold is removed from the heating chamber, continues to rotate and air cool for 30 minutes, and demolds to obtain a molded product.

[0089] Comparative Example 2

[0090] The polyethylene particles are crushed using a grinding disc type crushing equipment, and the powder with a particle size of 30-100 mesh is screened out by a screening device.

[0091] Add polyethylene powder into the rotomolding mold, seal and connect the mold, send the heated mold into the oven, set the rotomolding mold temperature to 180°C, the rotomolding time to 30 minutes, the main axis rotation speed of the robot arm to 8 rpm, and the secondary axis rotation speed to 10 rpm for rotational heating.

[0092] After the rotational heating is completed, the mold is removed from the heating chamber, the pressure gauge and gas circuit device are quickly installed, and 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 gas circuit switch is closed to seal the rotational molding mold and the gas circuit device is removed. The rotational air cooling is continued for 30 minutes, and the gas circuit switch is opened to adjust the mold cavity environment to normal pressure, and the molded product is demolded.

[0093] Comparative Example 3

[0094] The polyethylene particles are crushed using a grinding disc type crushing equipment, and the powder with a particle size of 30-100 mesh is screened out by a screening device.

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

[0096] The heated mold was sent into the oven, and the rotational molding mold temperature was set to 180°C, the rotational molding time was set to 30 min, the main axis rotation speed of the robot arm was set to 8 rpm, and the secondary axis rotation speed was set to 10 rpm for rotational heating.

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

[0098] Comparative Example 4

[0099] Use a grinding disc crushing device to crush the polyethylene particles, and use a screening device to screen out powder with a particle size of 30-100 mesh. Take the polyethylene powder and the composite antioxidant and mix them in a high-speed mixer. The mixing time is 10 minutes and the mixing speed is 800 rpm. Among them, the dosage ratio is 98 parts by weight of polyethylene powder and 2 parts by weight of composite antioxidant (the composite antioxidant contains 168 antioxidant and 1010 antioxidant, and the weight ratio of 168 antioxidant to 1010 antioxidant is 1:2). After mixing, set aside.

[0100] The polyethylene powder blended with the composite antioxidant is used as the roto-molding material, and the roto-molding process is the same as that of Comparative Example 1.

[0101] Example 2

[0102] The polyethylene particles are crushed using a grinding disc crushing device, and the powder with a particle size of 10-50 mesh is screened out using a screening device.

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

[0104] The heated mold was sent into the oven, and the rotational molding mold temperature was set to 180°C, the rotational molding time was set to 30 min, the main axis rotation speed of the robot arm was set to 8 rpm, and the secondary axis rotation speed was set to 10 rpm for rotational heating.

[0105] After the rotational heating is completed, the mold is removed from the heating chamber, the pressure gauge and gas circuit device are quickly installed, and 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 gas circuit switch is closed to seal the rotational molding mold and the gas circuit device is removed. The rotational air cooling is continued for 30 minutes, and the gas circuit switch is opened to adjust the mold cavity environment to normal pressure, and the molded product is demolded.

[0106] Example 3

[0107] The PA11 particles were crushed using a grinding disc crushing device, and the powder with a particle size of 30-100 mesh was screened out using a screening device, and the PA11 powder was dried in an oven at 100°C for 6 hours.

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

[0109] The heated mold was sent into the oven, and the rotational molding mold temperature was set to 200°C, the rotational molding time was set to 35 min, the main axis rotation speed of the robot arm was set to 8 rpm, and the secondary axis rotation speed was set to 10 rpm for rotational heating.

[0110] After the rotational heating is completed, the mold is removed from the heating chamber, the pressure gauge and gas circuit device are quickly installed, and 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 gas circuit switch is closed to seal the rotational molding mold and the gas circuit device is removed. The rotational air cooling is continued for 30 minutes, and the gas circuit switch is opened to adjust the mold cavity environment to normal pressure, and the molded product is demolded.

[0111] Comparative Example 5

[0112] The PA11 particles were crushed using a grinding disc crushing device, and the powder with a particle size of 30-100 mesh was screened out using a screening device, and the PA11 powder was dried in an oven at 100°C for 6 hours.

[0113] Take the dried PA11 powder and the composite antioxidant and mix them in a high-speed mixer for 10 minutes at a mixing speed of 800 rpm. The dosage ratio is 98 parts by weight of PA11 powder and 2 parts by weight of the composite antioxidant (the composite antioxidant includes 168 antioxidant and 1010 antioxidant, wherein the weight ratio of 168 antioxidant to 1010 antioxidant is 1:2). Mix and set aside.

[0114] Add nylon PA11 powder blended with composite antioxidant into the rotational molding mold, seal and connect the mold, send the heated mold into the oven, set the rotational molding mold temperature to 200℃, the rotational molding time to 35min, the mechanical arm main axis rotation speed to 8rmp, and the secondary axis rotation speed to 10rmp, and perform rotational heating. After the rotational heating is completed, the mold is removed from the heating chamber, and continues to rotate and air cool for 30min. The gas circuit switch is turned on to adjust the mold cavity environment to normal pressure, and the molded product is demolded.

[0115] Example 4

[0116] The PA11 particles were crushed using a grinding disc crushing device, and the powder with a particle size of 10-50 mesh was screened out using a screening device, and the PA11 powder was dried in an oven at 100°C for 6 hours.

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

[0118] The heated mold was sent into the oven, and the rotational molding mold temperature was set to 200°C, the rotational molding time was set to 35 min, the main axis rotation speed of the robot arm was set to 8 rpm, and the secondary axis rotation speed was set to 10 rpm for rotational heating.

[0119] After the rotational heating is completed, the mold is removed from the heating chamber, the pressure gauge and gas circuit device are quickly installed, and 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 gas circuit switch is closed to seal the rotational molding mold, and the gas circuit device is removed. The rotational air cooling is continued for 30 minutes, and the gas circuit switch is opened to adjust the mold cavity environment to normal pressure, and the molded product is demolded.

[0120] Product performance testing

[0121] The performance of the molded products prepared in Examples 1 to 4 and Comparative Examples 1 to 5 was tested. The test results are shown in Table 1.

[0122] Table 1 Physical properties of rotomolded liners by different processes

[0123]

[0124] In the embodiment of the present application, the mold cavity is vacuumed before rotational molding, and then heated and rotationally molded (i.e., negative pressure rotational molding) is performed, and then the negative pressure environment of the mold cavity is converted into a high-pressure environment of N2 atmosphere, and then rotational cooling (i.e., high-pressure cooling) is performed. Through the above-mentioned pressure-changing process, it is ensured that the material is in contact with the air as little as possible during the entire rotational molding process, the degree of yellowing caused by air oxidation of the product during the rotational molding process is reduced, and the bubble defects caused by residual air in the liner wall are reduced. At the same time, the negative pressure rotational molding in the pressure-changing process helps to realize the rotational molding of large-particle raw materials.

[0125] The samples prepared in Example 1 and Comparative Example 1 were subjected to Micro-CT testing, and the results are as follows: Figures 2 to 7 The use of variable pressure rotational molding can reduce the number of micropores in the appearance of the product. In the Micro-CT test, the sample prepared in Example 1 has a uniform and flat cross-section in three dimensions, and no micropore defects with a diameter greater than 15 μm are observed. However, the scanning image of the sample in Comparative Example 1 has a large number of hole defects of different sizes, and the average micropore diameter of the hole defects reaches 191 μm. Variable pressure rotational molding improves the mechanical strength of the product. The tensile strength and low-temperature notched impact strength of the sample in Example 1 are increased to 18.7 MPa and 6.2 kJ / m respectively. 2 This is because the variable pressure molding process avoids the bubble defects of the inner liner wall and improves the density of the inner liner 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 the raw materials during the rotational molding process and reduce the yellow index of the molded products.

[0126] It can be seen from Example 1, Comparative Example 1 and Comparative Example 2 that the present application uses a negative pressure environment formed during the heating rotational molding process to significantly reduce the yellowness index of the product.

[0127] It can be seen from Example 1 and Comparative Example 4 that the variable pressure roto-molding process is more effective than adding an antioxidant in improving the oxidative yellowing of the material during roto-molding, with the yellow index change rate of Example 1 being 9.1% and the yellow index change rate of Comparative Example 4 being 26.8%. Further, it can be seen from Example 3 and Comparative Example 5 that the yellow index change rate of the product using the variable pressure roto-molding process in Example 3 is 10.5%, and the conventional process used in Comparative Example 5 is 331.6%, and the variable pressure roto-molding process has a more obvious improvement on the yellowing degree of PA11 material.

[0128] Both Example 2 and Example 4 use 10-50 mesh polyethylene powder or nylon 11 powder as rotational molding raw materials, and the products after molding have good comprehensive properties. This is because the variable pressure environment is conducive to reducing the residual air between the raw materials, avoiding oxidation and defects of the products, saving the cost of raw material crushing, and broadening the particle size range of the rotational plastic.

[0129] In summary, the present invention solves to a certain extent the problems of the traditional rotational molding process having high requirements on the particle size of raw materials, the raw materials being easily oxidized in a high temperature environment, and the rotational molding products being prone to defects during the production process.

[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 present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be considered exemplary and non-restrictive in all respects, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all variations that fall within the meaning and scope of the equivalent elements of the claims be included in the present invention.

Claims

1. A low-defect rotationally molded product, wherein the average micropore diameter of the microscopic pore defects is not greater than 15 μm, and the yellow index of the product changes by less than 30% compared with the yellow index of the rotationally molded raw material; the low-defect rotationally molded product is preferably a polyolefin and / or nylon product.

2. The low-defect rotationally molded product according to claim 1, characterized in that The low-defect rotationally molded product is prepared by rotationally molding the powder raw material through a variable pressure process, and the variable pressure process includes establishing a negative pressure environment and a positive pressure environment in the mold cavity; the time for establishing the negative pressure environment is preferably after filling the material and before rotational molding; the time for establishing the positive pressure environment is preferably after rotational molding and before the mold begins to cool.

3. The method for preparing a low-defect rotationally molded product according to claim 1 or 2, characterized in that The following steps are involved: a. Fill the powder raw material into the rotational molding mold and seal the rotational molding mold; b. Create a negative pressure environment for the rotational molding mold filled with powder raw materials and perform rotational molding; c. Change the pressure state in the mold cavity to a positive pressure environment and cool it; d. Adjust the pressure in the mold to normal pressure, then open the mold and demould to obtain the molded product.

4. The method for preparing a low-defect rotationally molded product according to claim 3, characterized in that: In the step b, The absolute pressure value of the negative pressure environment is 0.01 bar-1.0 bar, more preferably 0.01 bar-0.5 bar; and / or, In the step c, The absolute pressure value of the positive pressure environment is 1.0-2.5 bar, and more preferably 1.5-2.5 bar.

5. The method for preparing a low-defect rotationally molded product according to claim 3, characterized in that: The process parameters of the rotational molding include: mold temperature of 150-280°C, the rotational molding time is preferably 20-40 minutes; the cooling time is preferably 15-60 minutes; wherein the cooling method is preferably air cooling or natural cooling; Preferably, the process parameters of the rotational molding also include the main shaft rotation speed of the robot arm being set to 5-8 rpm and the secondary shaft rotation speed being set to 9-12 rpm.

6. The method for preparing a low-defect rotationally molded product according to claim 3, characterized in that: The powder raw material is a polymer material, and the powder raw material is preferably a PE raw material and / or a PA raw material; the PE raw material is more preferably at least one of PE and a modified material based on PE; the number average molecular weight of the PE raw material is preferably 15000-50000; the PA raw material is more preferably at least one of PA and a modified material based on PA, and the number average molecular weight of the PA raw material is 10000-30000; The particle size of the powder raw material is preferably 10-150 mesh, more preferably 30-100 mesh; The melt index of the PE raw material is preferably 2-30 g / 10 min (test temperature is 230° C., load is 2.16 kg), and more preferably 3-15 g / 10 min; The melt index of the PA raw material is preferably 1-40 g / 10 min (test temperature is 230° C., load is 2.16 kg), and more preferably 2-30 g / 10 min.

7. The method for preparing a low-defect rotationally molded product according to claim 3, characterized in that: The powder raw material is powder directly produced by interfacial polymerization and / or powder obtained by mechanical crushing; the mechanical crushing preferably includes at least one of impact mill crushing, air flow crushing and ball mill crushing.

8. The method for preparing a low-defect rotationally molded product according to claim 3, characterized in that: In the step c, a positive pressure environment is established by injecting gas; The type of gas is preferably at least one of compressed air, N2, CO2, and helium; more preferably at least one of N2, CO2, and helium; The gas is preferably heated before being injected into the mold cavity, and the temperature of the heated gas is 80-120°C.

9. A rotationally molded product obtained according to the method for preparing a low-defect rotationally molded product according to any one of claims 3 to 8.

10. A system used in the method for preparing a low-defect rotationally molded product according to any one of claims 3 to 8, comprising a rotationally molded mold, a detachable pressure gauge, and a detachable gas path device; in, The rotational molding mold is connected with a detachable pressure gauge and a detachable gas path 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 circuit 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; the gas circuit preferably contains a filter screen, and the mesh number of the filter screen is preferably 150-200 meshes; the system preferably includes a heat exchanger, and the heat exchanger is connected to the gas circuit.

11. The use of the low-defect rotationally molded product according to any one of claims 1, 2, or 9, or the product obtained by the preparation method according to any one of claims 3 to 8, or the system according to claim 10, preferably in vehicle fuel tank liners, IV type hydrogen storage bottle liners, high-pressure gas storage tank liners, or rotationally molded tank bodies.

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