Flame-retardant lead-boron modified polyethylene composite material and preparation method thereof

Abstract

The invention discloses a flame-retardant lead-boron modified polyethylene composite material and a preparation method thereof, and the flame-retardant lead-boron modified polyethylene composite material is composed of the following components: 9 to 22.5 wt% of ultrahigh molecular weight polyethylene powder, 0.5 to 8 wt% of boron carbide powder, 70 to 85 wt% of lead powder, 5 to 15 wt% of a halogen-free flame retardant, 0.1 to 0.2 wt% of an antioxidant, 0.2 to 0.5 wt% of a coupling agent, 0.1 to 0.5 wt% of a lubricant, and 0.1 to 0.5 wt% of a dispersant. The shielding performance of the prepared flame-retardant lead-boron modified polyethylene composite material is equivalent to that of a conventional lead-boron polyethylene material, the flame-retardant lead-boron modified polyethylene composite material has excellent flame-retardant performance and physical and chemical properties, the oxygen index of the flame-retardant lead-boron modified polyethylene composite material is greater than or equal to 28, and the vertical combustion method reaches V0 level.

Description

Technical Field

[0001] The invention relates to the technical field of shielding materials, in particular to a flame retardant lead-boron modified polyethylene composite material and a preparation method thereof. Background Art

[0002] Lead-boron polyethylene is a composite shielding material prepared by adding lead powder, boron carbide powder and processing aids to polyethylene. The polyethylene in the lead-boron polyethylene material has a high hydrogen content, and the hydrogen element has a good weakening ability for fast neutrons; the boron content in boron carbide is high, and the boron element has a large cross-section for absorbing thermal neutrons; the lead element has a good shielding effect on gamma radiation; therefore, lead-boron polyethylene has excellent fast neutron, thermal neutron and gamma radiation shielding effects at the same time. Compared with traditional metal materials, lead-boron polyethylene helps to simplify the shielding structure, reduce the weight of the shielding body, and reduce the volume of the shielding body. It has been widely used in the shielding protection design of nuclear technology devices.

[0003] However, the base material polyethylene of lead-boron polyethylene is a flammable material, and the oxygen index of lead-boron polyethylene itself is only 18, which brings safety hazards to engineering applications. Therefore, on the basis of not reducing the shielding performance, flame retardant modification of lead-boron polyethylene and improving the flame retardant performance are the solutions to improve its safety. However, a large amount of lead powder is added to lead-boron polyethylene, and the process difficulty of adding flame retardants is significantly increased, and the improvement of flame retardant performance is very limited. As mentioned in the invention patent CN200510116946.5, "A lead-boron polyethylene composite shielding material and its preparation method", adding 2 parts by weight of flame retardants, including magnesium hydroxide, aluminum hydroxide, and phosphorus pentoxide, the oxygen index reaches 22, while the oxygen index without flame retardants is only 19. Invention patent CN101033313A "A flame-retardant lead-boron polyethylene composite material" mentions the use of 2.8 to 3.8 parts by weight of a composite flame retardant, which is 10 to 20% zinc borate, 60 to 70% microcapsule-coated red phosphorus and 10 to 20% phenolic resin, and the flame retardant performance meets the requirements of FV-1 in GB4609-84. Invention patent CN118006018A "A new type of highly flame-retardant polyethylene-based shielding material" proposes adding lead powder, boron carbide powder, cerium oxide, dibenzoyl peroxide, etc. to an ultra-high molecular weight polyethylene substrate through twin-screw extrusion granulation, and the oxygen index of the product is as high as 31, but no performance data on vertical combustion is given. At the same time, cerium oxide is a rare earth material, which is relatively expensive and difficult to promote and apply.

[0004] The above three patents achieve a certain degree of flame retardancy by adding a small amount of flame retardant, but their flame retardancy still cannot meet the actual engineering needs. For example, the oxygen index of flame retardant materials is required to be greater than 28, and some engineering fields require materials to reach the flame retardant level, the oxygen index must be greater than 32, and the vertical combustion performance must reach the V0 level requirement. The materials mentioned in the above three inventions cannot meet the requirements, either only the oxygen index index or only the vertical combustion index is proposed. For flame retardant materials, it is difficult to meet both the oxygen index and the vertical combustion performance indicators. Summary of the invention

[0005] To this end, the technical problem to be solved by the present invention is to provide a flame retardant lead-boron modified polyethylene composite material and a preparation method thereof. The prepared flame retardant lead-boron modified polyethylene composite material has shielding performance equivalent to that of previous lead-boron polyethylene materials, and at the same time has excellent flame retardant properties and physical and chemical properties, its oxygen index is ≥28, and the vertical combustion method reaches V0 level.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0007] A flame retardant lead-boron modified polyethylene composite material consists of the following components: 9-22.5wt% of ultra-high molecular weight polyethylene powder, 0.5-8wt% of boron carbide powder, 70-85wt% of lead powder, 5-15wt% of halogen-free flame retardant, 0.1-0.2wt% of antioxidant, 0.2-0.5wt% of coupling agent, 0.1-0.5wt% of lubricant and 0.1-0.5wt% of dispersant.

[0008] In the flame-retardant lead-boron modified polyethylene composite material, the particle size of the boron carbide powder is less than or equal to 100 μm.

[0009] The lead powder particle size of the flame retardant lead-boron modified polyethylene composite material is 80 to 500 meshes.

[0010] The flame retardant particle size of the flame retardant in the flame retardant lead-boron modified polyethylene composite material is 200 to 1000 meshes.

[0011] In the flame-retardant lead-boron modified polyethylene composite material, the antioxidant is one or more of antioxidant 1010, antioxidant 7910, antioxidant FD958 and antioxidant FD658.

[0012] The coupling agent of the flame-retardant lead-boron modified polyethylene composite material is a phthalate coupling agent or / and a silane coupling agent, wherein the phthalate coupling agents include NDZ201, LD101 and LD126, etc., and the silane coupling agents include KH550, KH560 and KH570, etc.

[0013] The above flame retardant lead-boron modified polyethylene composite material, the lubricant is calcium stearate and / or zinc stearate, the dispersant is polyethylene wax or oxidized polyethylene wax, wherein the polyethylene wax includes TW-F0042 and TW-F4001, etc., the oxidized polyethylene wax includes S8115 and S0816, etc., and the melting point of the polyethylene wax is ≥132°C.

[0014] The flame-retardant lead-boron modified polyethylene composite material, the ultra-high molecular weight polyethylene powder is made of ultra-high molecular weight polyethylene with a viscosity coefficient of 1000-4500 ml / g.

[0015] The flame retardant lead-boron modified polyethylene composite material has a density of 3.0 to 5.0 g / cm 3 .

[0016] The preparation method of the flame retardant lead-boron modified polyethylene composite material comprises the following steps:

[0017] Step 1) Mixing: weighing ultra-molecular weight polyethylene powder, boron carbide powder, lead powder, flame retardant, antioxidant powder, coupling agent and lubricant and mixing them in a dry powder mixer to obtain a mixed powder, with a rotation speed of 20 to 60 r / min and a mixing time of 2 to 120 min;

[0018] Step 2) Mixing: Add the mixed powder obtained in step 1) into the rheometer chamber of an internal mixer and heat and stir for 10 to 300 seconds to obtain a mixed material. The temperature of the internal mixer is 160 to 190° C. and the speed is 10 to 40 r / min.

[0019] Step 3) Molding: The mixed material obtained in step 2) is weighed and placed into a mold for pressing to obtain a flame retardant lead-boron modified polyethylene composite material; the specific pressing operation is as follows:

[0020] Step 3-1) pre-pressing for 5 to 10 minutes, the pre-pressing pressure is 3 to 5 MPa, and the pre-pressing temperature is 130 to 150°C;

[0021] Step 3-2) two-stage formal pressing: the first stage of formal pressing has a pressing time of 5 to 10 minutes, a pressure of 10 MPa, and a temperature of 150 to 170° C.; the second stage of formal pressing has a pressing time of 10 to 20 minutes, a pressure of 15 to 20 MPa, and a temperature of 170 to 190° C.;

[0022] Step 3-3) Cool the mixture to 50-60°C with cooling water and maintain the pressure at 18-20 MPa.

[0023] The technical solution of the present invention achieves the following beneficial technical effects:

[0024] 1. It has good flame retardant properties, the oxygen index reaches the flame retardant level, vertical combustion V0 level, and the flame retardant properties are significantly improved compared with traditional lead-boron polyethylene materials.

[0025] 2. Through the coordinated use of dispersants, lubricants, flame retardants, and the coordination of the preparation process, the prepared shielding material has extremely excellent mechanical properties, which are greatly improved compared with the mechanical properties of traditional shielding materials.

[0026] 3. Through the coordinated use of coupling agents, dispersants, lubricants, flame retardants, as well as reasonable preparation process and control parameters, the polyethylene substrate and a large amount of added lead powder additives are closely combined, which significantly reduces dust pollution during cutting and processing, greatly reduces the generation of lead dust, protects the physiological health of operators, and improves the environmental safety of the production environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is an electron microscope photograph of flame-retardant lead-boron modified polyethylene composite material. DETAILED DESCRIPTION

[0028] Example 1

[0029] The flame-retardant lead-boron modified polyethylene composite material in this embodiment is composed of 22 parts by weight of ultra-high molecular weight polyethylene powder MII I with a molecular weight of 3.5 million, 70 parts by weight of lead powder with a particle size of 200 mesh and a purity greater than or equal to 99.5wt%, 1.0 parts by weight of boron carbide powder with a particle size of 325 mesh, 6.4 parts by weight of expandable phosphorus-nitrogen composite flame retardant FR8310, 0.1 parts by weight of antioxidant 7910, 0.1 parts by weight of calcium stearate CV500, 0.3 parts by weight of coupling agent LD-B and 0.1 parts by weight of polyethylene wax S3816.

[0030] The flame retardant lead-boron modified polyethylene composite material in this embodiment is prepared by the following steps:

[0031] Step 1) ultra-high molecular weight polyethylene powder MII I, boron carbide powder, lead powder, flame retardant FR8310, antioxidant 7910, coupling agent LD-B, calcium stearate CV500 and polyethylene wax S3816 are weighed and placed in a closed container, and a powder mixing device is started at a speed of 50 r / min and mixed for 60 min to obtain a mixed powder;

[0032] Step 2) The temperature of the internal mixer is set to 180-190° C. and the speed is 25 r / min. The mixed powder obtained in step 1) is poured into the internal mixer and internally mixed for 2 minutes to obtain a mixed material. During the internal mixing process, the temperature is controlled to prevent the material from agglomerating due to excessive temperature.

[0033] Step 3) Weigh the mixed material obtained in step 2) and put it into the mold while it is hot, set the mold temperature to 165°C, pre-press 3-5MPa, time 5min, temperature to 130°C; increase the pressure to 10MPa, time 10min, temperature to 150°C; increase the pressure to 20MPa, maintain the pressure for 30min, temperature to 165°C; cool with cooling water, and maintain the pressure at 18-20MPa. By gradually increasing the pressure, bubbles that may be mixed in the material can be effectively removed, promoting uniform distribution of the material; maintaining the pressure during cooling helps release stress during the cooling process of the material and maintains a flat appearance;

[0034] Step 4) After hot pressing for 10 minutes, cool the mixture to room temperature with cooling water.

[0035] Example 2

[0036] The flame-retardant lead-boron modified polyethylene composite material in this embodiment is composed of 16.0 parts by weight of ultra-high molecular weight polyethylene powder with a molecular weight of 2.5 million, 70 parts by weight of lead powder with a particle size of 200 mesh and a purity greater than or equal to 99.5wt%, 8.0 parts by weight of boron carbide powder with a particle size of 200 mesh, 5.6 parts by weight of expandable phosphorus-nitrogen composite flame retardant FR8370A, 0.1 parts by weight of antioxidant 1010, 0.1 parts by weight of zinc stearate AV301, 0.1 parts by weight of coupling agent KH550 and 0.1 parts by weight of polyethylene wax S8115.

[0037] The flame retardant lead-boron modified polyethylene composite material in this embodiment is prepared by the following steps:

[0038] Step 1) After weighing ultra-high molecular weight polyethylene powder, boron carbide powder, lead powder, flame retardant FR8370, antioxidant 1010, zinc stearate AV301, coupling agent KH550 and polyethylene wax S8115, put them into a closed container, start the powder mixing equipment, rotate at 50r / min, mix for 90min, change the rotation direction once in the middle, and mix the materials more evenly by changing the rotation direction of the stirring roller;

[0039] Step 2) The temperature of the internal mixer is set to 185°C and the speed is 25r / min. The mixed powder is poured into the internal mixer and mixed for 2.5min. The temperature is controlled during the mixing process to prevent the material from agglomerating due to excessive temperature.

[0040] Step 3) The mixed lead-boron-polyethylene mixture is weighed and placed into a mold while hot, the mold temperature is set to 170°C, the pre-pressure is 3-5MPa, the time is 5min, and the temperature is 135°C; the pressure is increased to 10MPa, the time is 10min, and the temperature is 150°C; the pressure is increased to 20MPa, the pressure is maintained for 30min, and the temperature is 170°C; cooling water is passed to cool down, and the pressure is maintained at 18-20MPa.

[0041] Step 4) After hot pressing for 10 minutes, cool the mixture to room temperature with cooling water.

[0042] Example 3

[0043] The flame-retardant lead-boron modified polyethylene composite material in this embodiment includes 13.3 parts by weight of ultra-high molecular weight polyethylene with a molecular weight of 1.5 million, 77.3 parts by weight of lead powder with a particle size of 200 mesh and a purity greater than or equal to 99.5wt%, 1.5 parts by weight of boron carbide powder with a particle size of 200 mesh, 7.3 parts by weight of expandable phosphorus-nitrogen composite flame retardant FR-T305, 0.1 parts by weight of antioxidant 7910, 0.1 parts by weight of calcium stearate G130, 0.2 parts by weight of coupling agent LD122 and 0.2 parts by weight of polyethylene wax TW-F4002.

[0044] The flame retardant lead-boron modified polyethylene composite material in this embodiment is prepared by the following steps:

[0045] Step 1) After weighing ultra-high molecular weight polyethylene powder, boron carbide powder, lead powder, intumescent phosphorus-nitrogen composite flame retardant FR-T305, antioxidant 7910, calcium stearate G130, coupling agent LD122 and polyethylene wax TW-F4002, put them into a closed container, start the powder mixing equipment, rotate at 40r / min, mix for 30min, and obtain a mixed powder;

[0046] Step 2) The temperature of the internal mixer is set to 160-190° C. and the speed is 25 r / min. The mixed powder obtained in step 1) is poured into the internal mixer and internally mixed for 3 minutes to obtain a mixed material. During the internal mixing process, the temperature is controlled to prevent the material from agglomerating due to excessive temperature.

[0047] Step 3) The mixed material obtained in step 2) is weighed and put into a mold while hot, the mold temperature is set to 160°C, the pre-pressure is 3-5MPa, the time is 5min, and the temperature is 135°C; the pressure is increased to 10MPa, the time is 10min, and the temperature is 145°C; the pressure is increased to 20MPa, the pressure is maintained for 30min, and the temperature is 160°C; cooling water is passed to cool down, and the pressure is maintained at 18-20MPa.

[0048] Step 4) After hot pressing for 10 minutes, cool the mixture to room temperature with cooling water.

[0049] Example 4

[0050] The flame-retardant lead-boron modified polyethylene composite material in this embodiment includes 9.9 parts by weight of ultra-high molecular weight polyethylene with a molecular weight of 2 million, 80 parts by weight of lead powder with a particle size of 80 mesh and a purity greater than or equal to 99.5wt%, 3.0 parts by weight of boron carbide powder with a particle size of 325 mesh, 7.0 parts by weight of expandable phosphorus-nitrogen composite flame retardant FR8300, 0.1 parts by weight of antioxidant 1010, 0.1 parts of coupling agent LD123, 0.1 parts of zinc stearate AV350S and 0.1 parts by weight of oxidized polyethylene wax S0816.

[0051] The flame retardant lead-boron modified polyethylene composite material in this embodiment is prepared by the following steps:

[0052] Step 1) ultra-high molecular weight polyethylene powder, boron carbide powder, lead powder, flame retardant FR8300, antioxidant 1010, coupling agent LD123, zinc stearate AV350S and oxidized polyethylene wax dispersant S0816 are weighed and placed in a closed container, and a powder mixing device is started at a speed of 40 r / min and mixed for 50 min to obtain a mixed powder;

[0053] Step 2) The temperature of the internal mixer is set to 180° C. and the speed is set to 30 r / min. The mixed powder obtained in step 1) is poured into the internal mixer and mixed for 25 minutes to obtain a mixed material. During the internal mixing process, the temperature is controlled to prevent the material from agglomerating due to excessively high temperature.

[0054] Step 3) The mixed material obtained in step 2) is weighed and put into a mold while hot, the mold temperature is set to 190°C, the pre-pressure is 3-5MPa, the time is 5min, and the temperature is 150°C; the pressure is increased to 10MPa, the time is 10min, and the temperature is 170°C; the pressure is increased to 20MPa, the pressure is maintained for 30min, and the temperature is 190°C; cooling water is passed to cool down, and the pressure is maintained at 18-20MPa.

[0055] Step 4) After hot pressing for 10 minutes, cool the mixture to room temperature with cooling water.

[0056] Example 5

[0057] The flame-retardant lead-boron modified polyethylene composite material in this embodiment is composed of 9.3 parts by weight of ultra-high molecular weight polyethylene powder MII I with a molecular weight of 3.5 million, 85.0 parts by weight of lead powder with a particle size of 260 mesh and a purity greater than or equal to 99.5wt%, 0.5 parts by weight of boron carbide powder with a particle size of 325 mesh, 5.0 parts by weight of expandable phosphorus-nitrogen flame retardant FR8310, 0.1 parts by weight of antioxidant 7910, 0.2 parts by weight of coupling agent LD560, 0.1 parts of calcium stearate CV550 and 0.2 parts by weight of polyethylene wax TW-F0041.

[0058] The flame retardant lead-boron modified polyethylene composite material in this embodiment is prepared by the following steps:

[0059] Step 1) After weighing ultra-high molecular weight polyethylene powder, boron carbide powder, lead powder, flame retardant FR8310, antioxidant 7910, coupling agent LD560, calcium stearate CV550 and polyethylene wax TW-F0041, put them into a closed container, start the powder mixing equipment, rotate at 50r / min, mix for 60min, and change the rotation direction once in the middle;

[0060] Step 2) The temperature of the internal mixer is set to 190°C and the speed is 25r / min. The mixed powder is poured into the internal mixer and mixed for 2.5min. The temperature is controlled during the mixing process to prevent the material from agglomerating due to excessive temperature.

[0061] Step 3) The mixed lead-boron polyethylene mixture is weighed and put into a mold while hot, the mold temperature is set to 170°C, the pre-pressure is 3-5MPa, the time is 5min, and the temperature is 140°C; the pressure is increased to 10MPa, the time is 10min, and the temperature is 155°C; the pressure is increased to 20MPa, the pressure is maintained for 30min, and the temperature is 170°C; cooling water is passed to cool down, and the pressure is maintained at 18-20MPa;

[0062] Step 4) After hot pressing for 10 minutes, cool the mixture to room temperature with cooling water.

[0063] The ×500 electron microscope photo of the high-strength lead-boron polyethylene-based shielding material prepared in Example 3 of the present invention is as follows Figure 1 shown.

[0064] Performance tests were performed on the flame-retardant lead-boron modified polyethylene composite materials obtained in Examples 1 to 5 and the lead-boron polyethylene PB202 mentioned in the document "High-efficiency Shielding Material Lead-boron Polyethylene". The test results are shown in Table 1.

[0065] Table 1 Flame retardant lead-boron modified polyethylene composites and PB202 performance test results

[0066] Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 PB202 <![CDATA[Density (g / cm 3 )]]> 3.00 3.40 4.01 4.56 5.01 3.42 Tensile strength(MPa) 43.2 45.2 44.4 42.2 47.3 12.6 Bending strength(Mpa) 39.7 40.5 28.1 34.2 42.6 19.8 Oxygen Index LOI 28.2 28.3 28.0 28.5 28.0 18 Vertical combustion performance V0 V0 V0 V0 V0 ＜V2 Neutron shielding factor (4cm) 3.75 3.61 3.55 3.41 3.42 3.58 <![CDATA[ 60 Coγ shielding coefficient (4cm)]]> 1.98 2.22 2.61 2.97 3.23 2.23

[0067] Among them, samples 1 to 5 are samples obtained from the flame-retardant lead-boron modified polyethylene composite materials obtained in Examples 1 to 5. Compared with the lead-boron polyethylene PB202 mentioned in the document "High-efficiency Shielding Material Lead-boron Polyethylene", under the condition of similar density, the flame-retardant lead-boron modified polyethylene composite material in the present invention has a significantly improved oxygen index, reaching the flame-retardant level, and the vertical combustion performance reaches V0 level, while the mechanical strength is significantly improved.

[0068] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A flame retardant lead-boron modified polyethylene composite material, characterized in that: The invention is composed of the following components: 9-22.5wt% of ultra-high molecular weight polyethylene powder, 0.5-8wt% of boron carbide powder, 70-85wt% of lead powder, 5-15wt% of halogen-free flame retardant, 0.1-0.2wt% of antioxidant, 0.2-0.5wt% of coupling agent, 0.1-0.5wt% of lubricant and 0.1-0.5wt% of dispersant.

2. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The particle size of the boron carbide powder is less than or equal to 100 μm.

3. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The lead powder has a particle size of 80-500 meshes, and the purity of the lead powder is greater than or equal to 99.5wt%.

4. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The halogen-free flame retardant is composed of a smoke suppressant and a phosphorus-nitrogen intumescent composite flame retardant and / or a phosphorus-nitrogen composite organic flame retardant. The particle size of the halogen-free flame retardant is 200-1000 meshes.

5. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The antioxidant is one or more of antioxidant 1010, antioxidant 7910, antioxidant FD958 and antioxidant FD658.

6. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The coupling agent is a phthalate coupling agent or a silane coupling agent.

7. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The lubricant is calcium stearate and / or zinc stearate, and the dispersant is polyethylene wax or oxidized polyethylene wax.

8. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The ultra-high molecular weight polyethylene powder is made of ultra-high molecular weight polyethylene with a viscosity coefficient of 1000 to 4500 ml / g.

9. The flame retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The density of flame retardant lead-boron modified polyethylene composite material is 3.0~5.0g / cm 3 .

10. The method for preparing the flame-retardant lead-boron modified polyethylene composite material according to claim 1, characterized in that: The steps include: Step 1) Mixing: weighing ultra-molecular weight polyethylene powder, boron carbide powder, lead powder, flame retardant, antioxidant powder, coupling agent, lubricant and dispersant and mixing them in a dry powder mixer to obtain a mixed powder, the speed is 20 to 60 r / min, and the mixing time is 2 to 120 min; Step 2) Mixing: Add the mixed powder obtained in step 1) into the rheometer chamber of an internal mixer and heat and stir for 10 to 300 seconds to obtain a mixed material. The temperature of the internal mixer is 160 to 190° C. and the speed is 10 to 40 r / min. Step 3) Molding: The mixed material obtained in step 2) is weighed and placed into a mold for pressing to obtain a flame retardant lead-boron modified polyethylene composite material; the specific pressing operation is as follows: Step 3-1) pre-pressing for 5 to 10 minutes, the pre-pressing pressure is 3 to 5 MPa, and the pre-pressing temperature is 130 to 150°C; Step 3-2) two-stage formal pressing: the first stage of formal pressing has a pressing time of 5 to 10 minutes, a pressure of 10 MPa, and a temperature of 150 to 170° C.; the second stage of formal pressing has a pressing time of 10 to 20 minutes, a pressure of 15 to 20 MPa, and a temperature of 170 to 190° C.; Step 3-3) Cool the mixture to 50-60°C with cooling water and maintain the pressure at 18-20 MPa.

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