Method for preparing thermal insulation pipe based on supercritical carbon dioxide foaming process

Abstract

The invention discloses a method for preparing a thermal insulation pipe based on a supercritical carbon dioxide foaming process, which comprises the following steps of: melting and extruding heat-resistant polyethylene resin to obtain an inner-layer pipe, mixing a foaming matrix, a nucleating agent, a flame retardant and an antioxidant according to the amount to obtain foaming particles, and obtaining gas-saturated single-phase mixed sol by using a supercritical carbon dioxide foaming technology; the single-phase mixed sol is used for covering the inner-layer pipe to form a foaming heat preservation layer, high-strength polyethylene resin is extruded on the outer surface of the heat preservation layer to be melted and wrapped to form an outer protection layer of a pipe body, the three layers are co-extruded to form the heat preservation pipe, and the environment-friendly heat preservation pipe can be prepared through the method.

Description

Technical Field

[0001] This invention relates to the field of thermal insulation pipe manufacturing technology, and more specifically, to a method for preparing thermal insulation pipes based on supercritical carbon dioxide foaming process. Background Technology

[0002] In numerous fields such as construction, chemical industry, energy, and food processing, thermal insulation pipes are widely used to transport heat media (such as hot water and steam) or cold media (such as liquid ammonia and liquefied natural gas). Their function is to reduce heat or cold loss, thereby improving energy efficiency and reducing operating costs. Simultaneously, in certain industrial production processes, thermal insulation pipes can also ensure the temperature stability of the medium during transportation, guaranteeing the normal operation of the production process. Currently, the main method for manufacturing thermal insulation pipes on the market is polyurethane foaming. This process is characterized by using polyols and diisocyanates as the main raw materials. However, traditional polyurethane foaming processes have several significant problems. First, using polyols and diisocyanates as the main raw materials releases harmful volatile organic compounds during production and processing, as well as in the early stages of use, which can harm human health and the environment. Second, the polyurethane foaming reaction generates some toxic waste gases, and the production process produces some scraps, substandard products, and other solid waste. These waste residues contain polyurethane foam and unreacted raw materials, which are difficult to degrade naturally. Furthermore, polyurethane insulation pipes need to be disposed of after reaching the end of their service life. Due to the high chemical stability of polyurethane materials, their natural degradation rate is extremely slow, making them a difficult-to-treat solid waste.

[0003] In view of the above problems, this invention is committed to improving the production process of thermal insulation pipes and designing a method for manufacturing thermal insulation pipes using supercritical carbon dioxide foaming technology. In this process, this invention also needs to be committed to designing special production equipment (such as equipment to solve the problem of supercritical carbon dioxide permeation and bonding foaming materials) to meet the needs of producing thermal insulation pipes using supercritical carbon dioxide foaming technology. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned technical requirements and provide a method for preparing thermal insulation pipes based on supercritical carbon dioxide foaming technology. This invention uses supercritical carbon dioxide foaming technology to produce foaming agents, which completely replace the polyol and diisocyanate components used in polyurethane foaming. This avoids the generation of toxic substances and the emission of hazardous waste during the processing. This invention belongs to a green production process. The pipes produced by this invention are easier to degrade than traditional products and can effectively meet the production requirements of environmentally friendly thermal insulation pipes.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing thermal insulation pipes based on supercritical carbon dioxide foaming technology includes the following steps:

[0007] Step S1: Heat-resistant polyethylene resin is melt-extruded through a single-screw extruder to obtain the inner tube material;

[0008] Step S2: Mix the foaming matrix, nucleating agent, flame retardant and antioxidant in the specified amounts and granulate them through a twin-screw extruder to obtain pre-foamed particles. Then, use supercritical carbon dioxide foaming technology to obtain a gas-saturated single-phase mixed sol. Use the single-phase mixed sol to cover the inner pipe to form a foamed insulation layer.

[0009] Step S3: High-strength polyethylene resin is melt-coated onto the outer surface of the insulation layer through a co-extrusion die to form the outer protective layer of the pipe body;

[0010] Step S4: Water-cooled extruded pipes are then cut into finished insulated pipes.

[0011] Furthermore, the extrusion temperature in step S1 is controlled at 180-220℃, the extrusion temperature in step S3 is controlled at 160-200℃, and the cooling temperature in step S4 is controlled below 40℃.

[0012] Furthermore, the components of the pre-foamed particles in step S2, by mass parts, include: 100 parts by mass of foaming matrix, 0.5-3 parts by mass of nucleating agent, 1-5 parts by mass of flame retardant, and 1-5 parts by mass of antioxidant.

[0013] Furthermore, the fabrication of the foamed insulation layer in step S2 includes the following steps:

[0014] Step A1: Mix the foaming matrix, nucleating agent, and flame retardant according to the mass ratio, and then melt-granulate them through a twin-screw extruder. The extrusion temperature is controlled at 160-220℃ to finally obtain pre-foamed granules.

[0015] Step A2: Place the pre-foamed granules in the reactor, turn on the stirrer and inject carbon dioxide to raise the temperature and pressure to the supercritical state, and maintain the constant temperature and pressure for 2-5 hours to form a gas-saturated single-phase mixed sol.

[0016] Step A3: The sol is introduced into the mold or extrusion die. The pressure drop is generated by the sudden reduction of the cross-sectional area of ​​the flow channel. Gas is released to form bubble nuclei. The bubble nuclei grow during the cooling process to form a closed-cell foamed insulation layer on the outer surface of the inner tube.

[0017] Furthermore, in step S2, the foaming matrix is ​​polystyrene, expandable polystyrene, or extruded polystyrene; the nucleating agent is one of nano-silica, talc, or mica; the flame retardant is microcapsule-coated red phosphorus; and the antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] or dodecyl thiodipropionate.

[0018] Furthermore, the flame retardant preparation method includes the following steps:

[0019] Step B1: Add 10-15 parts of red phosphorus powder with a particle size of 15-50 μm to 40-60 parts of aluminum sulfate solution with a concentration of 10% and stir evenly; slowly add sodium hydroxide solution to adjust the pH to 6-8, so that aluminum hydroxide is deposited on the surface of red phosphorus; stir at 80°C for 2 hours to promote the formation of the coating layer; after filtration, washing and drying, red phosphorus coated with inorganic material is obtained.

[0020] Step B2: Mix melamine and formaldehyde at a molar ratio of 1:3, adjust the pH to 8.5-9.5, stir and reflux at 80°C for 2 hours, add melamine prepolymer to the above inorganic coated red phosphorus, adjust the pH to 4, stir and reflux at 80°C for 3-4 hours to form a thermosetting organic material coating layer, filter, wash and dry to obtain double-coated red phosphorus;

[0021] Step B3: Mix double-layered red phosphorus with styrene, benzoyl peroxide and water in a mass ratio of 46:6.2:0.03:50, and react at 85°C for 3 hours to polymerize styrene and form a thermoplastic organic material coating layer. After filtration, washing and drying, multi-layered microcapsule red phosphorus is obtained.

[0022] Furthermore, the saturation pressure of the supercritical carbon dioxide in step A2 is 10-18 MPa, the temperature is 90-120℃, and the amount of gas dissolved is 8-15 wt% of the mass of the foaming matrix.

[0023] Furthermore, in step A3, the flow channel cross-sectional shrinkage ratio is 5:1 to 15:1, and the pressure drop rate is controlled at 50-150 MPa / s.

[0024] The beneficial effects of this invention are:

[0025] This invention uses supercritical carbon dioxide foaming technology to produce foaming agents, completely replacing the polyols and diisocyanates used in polyurethane foaming. This avoids the generation of toxic substances and the emission of hazardous waste during processing. This invention belongs to green production processes. The pipes produced by this invention are easier to degrade than traditional products, and can effectively meet the production needs of environmentally friendly thermal insulation pipes. Attached Figure Description

[0026] Figure 1 This is a flowchart of a method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process in this embodiment;

[0027] Figure 2 This is a flowchart of the preparation steps of a foamed insulation layer in this embodiment;

[0028] Figure 3 This is a flowchart of a flame retardant preparation method in this embodiment;

[0029] Figure 4 This is a schematic diagram of a carbon dioxide saturated permeation system in this embodiment;

[0030] Figure 5 This is a schematic diagram of the reactor structure in this embodiment;

[0031] Figure 6 This is a partial structural diagram of the distributor in this embodiment.

[0032] Figure reference numerals: 1. Reactor; 11. Feeding port; 12. Discharge port; 121. Control valve; 13. Carbon dioxide injection device; 131. Injection pipe; 132. Distributor; 133. Gas chamber; 134. Vent hole; 135. Gas outlet cap; 135. Large opening; 1351. Small opening; 1352. Nitrogen injection pipe; 14. Tail gas discharge pipe; 15. Stirring mechanism; 2. Carbon dioxide input pipeline; 3. Nitrogen input pipeline; 4. Tail gas treatment box; 5. Extrusion equipment; 6. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] like Figure 1 The method for preparing thermal insulation pipes based on supercritical carbon dioxide foaming technology, as shown, includes the following steps:

[0035] Step S1: Heat-resistant polyethylene resin (PERT II) is melt-extruded through a single screw extruder to obtain the inner tube. The extrusion temperature in step S1 is controlled at 180-220℃.

[0036] Step S2: The foaming matrix, nucleating agent, flame retardant, and antioxidant are mixed in the specified amounts and granulated using a twin-screw extruder to obtain pre-foamed particles. Then, a gas-saturated single-phase mixed sol is obtained using supercritical carbon dioxide foaming technology. This single-phase mixed sol is used to cover the inner pipe to form a foamed insulation layer. The components of the pre-foamed particles in step S2, by mass parts, include 100 parts of foaming matrix, 0.5-3 parts of nucleating agent, and 1-5 parts of flame retardant. The antioxidant is 1-5 parts by weight. The foaming matrix in step S2 is polystyrene (PS), expandable polystyrene (EPS) or extruded polystyrene (XPS). The nucleating agent is one of nano silica (particle size 20-100nm), talc (mesh size ≥1250 mesh) or mica. The flame retardant is microcapsule multilayer coated red phosphorus. The antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] or dodecyl thiodipropionate.

[0037] Step S3: High-strength polyethylene resin is melt-coated onto the outer surface of the insulation layer through a co-extrusion die to form the outer protective layer of the pipe (referring to the outer side of the insulation layer). The extrusion temperature in step S3 is controlled at 160-200℃.

[0038] In steps S4, S1, S2 and S3, the inner pipe, the foamed insulation layer and the outer protective layer are mechanically anchored together by melt co-extrusion. No adhesive is needed for pipe forming. After the pipe is extruded, it is cooled by water and then cut into finished insulation pipes. The cooling temperature in step S4 is controlled below 40°C.

[0039] The foamed insulation layer in step S2 is fabricated using a specially designed supercritical carbon dioxide foaming technology, including the following steps:

[0040] Step A1: Mix the foaming matrix, nucleating agent, and flame retardant according to the mass ratio, and then melt-granulate them through a twin-screw extruder. The extrusion temperature is controlled at 160-220℃ to finally obtain pre-foamed granules.

[0041] Step A2: Place the pre-foamed granules in a reaction vessel, start stirring and inject carbon dioxide to raise the temperature and pressure to a supercritical state, maintain constant temperature and pressure for 2-5 hours to form a gas-saturated single-phase mixed sol. The stirring speed is 50-200 r / min. The saturation pressure of the supercritical carbon dioxide mentioned in step A2 is 10-18 MPa, the temperature is 90-120℃, and the gas dissolution amount is 8-15 wt% of the mass of the foamed matrix.

[0042] Step A3: The sol is introduced into the mold or extrusion die. The pressure drop is generated by the sudden reduction of the cross-sectional area of ​​the flow channel. Gas is released to form bubble nuclei. The bubble nuclei grow during the cooling process to form a closed-cell foamed insulation layer on the outer surface of the inner tube. The flow channel cross-sectional shrinkage ratio in step A3 is 5:1 to 15:1, and the pressure drop rate is controlled at 50-150 MPa / s.

[0043] The flame retardant used in step A1 above is specially prepared. The preparation method of the flame retardant includes the following steps:

[0044] Step B1: Add 10-15 parts of red phosphorus powder with a particle size of 15-50 μm to 40-60 parts of aluminum sulfate solution with a concentration of 10% and stir evenly; slowly add sodium hydroxide solution to adjust the pH to 6-8, so that aluminum hydroxide is deposited on the surface of red phosphorus; stir at 80°C for 2 hours to promote the formation of the coating layer; after filtration, washing and drying, red phosphorus coated with inorganic material is obtained.

[0045] Step B2: Mix melamine and formaldehyde at a molar ratio of 1:3, adjust the pH to 8.5-9.5, stir and reflux at 80°C for 2 hours, add melamine prepolymer to the above inorganic coated red phosphorus, adjust the pH to 4, stir and reflux at 80°C for 3-4 hours to form a thermosetting organic material coating layer, filter, wash and dry to obtain double-coated red phosphorus;

[0046] Step B3: Mix double-layered red phosphorus with styrene, benzoyl peroxide and water in a mass ratio of 46:6.2:0.03:50, and react at 85°C for 3 hours to polymerize styrene and form a thermoplastic organic material coating layer. After filtration, washing and drying, multi-layered microcapsule red phosphorus is obtained.

[0047] The flame retardant preparation mechanism and effects of this invention utilize microencapsulation technology to encapsulate red phosphorus, reducing direct contact and reaction between red phosphorus and the polymer matrix, thus improving the material's thermal stability. This reduces flame retardant migration and precipitation, extending the material's service life. During combustion, the microencapsulated red phosphorus releases substances such as phosphoric acid, promoting the formation of a char layer on the material surface, providing heat insulation and oxygen barrier functions. This achieves excellent flame retardant effects with a relatively low total addition amount, minimizing negative impacts on the material's mechanical and processing properties.

[0048] The advantages of this invention are:

[0049] 1. Supercritical CO2 (SC-CO2) is used as the foaming agent, completely replacing the polyols and diisocyanates used in polyurethane foaming, thus avoiding the emission of toxic substances and hazardous waste during processing. This is a green production process.

[0050] 2. Nucleating agent (nano SiO2 / talc) induces non-uniform nucleation; high pressure permeation (10-18MPa) combined with rapid pressure relief (50-150MPa / s) controls cell growth, and the cells have the advantage of good uniformity (pore size 50-200μm, closed cell rate ≥95%), which effectively improves the thermal insulation performance of the pipe.

[0051] 3. The addition of halogen-free flame retardants enables the insulation layer to achieve UL94 V-0 flame retardancy, while antioxidants inhibit the oxidative degradation of the PS matrix, effectively improving the service life of the pipe.

[0052] 4. The inner layer, the middle foamed insulation layer and the outer layer are mechanically anchored together by melt co-extrusion, without the need for adhesives.

[0053] In the method of this invention, the preparation of the intermediate foamed insulation layer is an extremely important step, especially in step A2. The difficulty lies in how to prepare a single-phase mixed sol by permeating the pre-foamed particles with supercritical carbon dioxide. Existing pipe manufacturing equipment cannot meet the preparation requirements of step A2. Therefore, this invention has specially developed and designed a carbon dioxide saturated permeation system to meet production needs.

[0054] like Figures 4-6 As shown, a carbon dioxide saturated permeation system includes a reactor 1 and a carbon dioxide input pipeline 3. The top of the reactor 1 is connected to a feeding port 11 for feeding the pre-foamed particles obtained in step A1. A stirring mechanism 2 is installed inside the reactor 1 for real-time stirring of the pre-foamed particles at a stirring speed of 50-200 r / min. The bottom of the reactor 1 is connected to a discharge port 12 for discharging the obtained material, namely a gas-saturated single-phase mixed sol. To improve the carbon dioxide permeability, this invention designs to install a carbon dioxide injection device 13 at the bottom of the reactor 1. The carbon dioxide injection device 13 is connected to the carbon dioxide input pipeline 3 to continuously inject carbon dioxide during the preparation process. The carbon dioxide is injected into the reactor 1 from bottom to top. During the process of gas pressure increase, it can permeate and fuse well with the pre-foamed particles to obtain a gas-saturated single-phase mixed sol. The supercritical carbon dioxide used in this invention has a saturation pressure of 10-18 MPa, a temperature of 90-120℃, and a gas dissolution amount of 8-15 wt% of the foaming matrix mass.

[0055] like Figure 5As shown, the carbon dioxide injection device 13 includes an injection pipe 131 and a distributor 132. The distributor 132 is suspended and installed at the bottom of the inner side of the reactor 1, forming a gas cavity 133 between the distributor 132 and the bottom of the inner side of the reactor 1. One end of the injection pipe 131 is connected to the gas cavity 133, and the other end is connected to the carbon dioxide input pipe 3 to continuously input carbon dioxide gas. The distributor 132 is disc-shaped, and several vent holes 134 are opened on the disc surface to realize the flow of carbon dioxide gas. To ensure... No material will fall from the vent 134. First, the vent 134 needs to be continuously ventilated. Second, the diameter of the vent 134 is designed to be smaller than the particle size of the pre-foamed particles (the particle size of the pre-foamed particles is 3-5 mm, while the diameter of the vent 134 is less than 0.5 mm). The vent 134 is densely distributed on the surface of the distributor 132 to achieve uniform air injection. The material is stirred and prepared above the distributor 132. The discharge port 12 is inserted into the reactor 1 and is connected to the center of the distributor 132.

[0056] To ensure that the vent 134 is not blocked and that no material falls out, such as Figure 6 As shown, this invention designs a vent cap 135 installed at each vent 134 position. The vent cap 135 is installed on the disc surface of the distributor 132 facing the inside of the reactor 1. The vent cap 135 is conical and has a large opening 1351 and a small opening 1352. The large opening 1351 covers the vent 134, and the small opening 1352 forms an open vent. This invention also designs the distributor 132 as a conical disc surface, with the center of the disc surface being the lowest point, which is exactly connected to the discharge port 12. When the material (whether it is granules or sol) hits the conical wall of the vent cap 135, it will naturally slide down and move towards the discharge port 12. With the continuous injection of carbon dioxide, there will be no blockage or material entry problem at the small opening 1352 of the vent cap 135, so as to effectively ensure the normal operation of carbon dioxide permeation.

[0057] The preparation time for single-phase mixed sol in a single reactor is 2-5 hours. To ensure continuous production, the carbon dioxide saturated permeation system of this invention is designed to install two reactors 1, which are installed in parallel. Their discharge ports 12 are both connected to the subsequent extrusion equipment 6. Control valves 121 are installed on the discharge ports 12 of both reactors 1. The two control valves 121 are not opened at the same time. That is to say, the two reactors 1 are used alternately in a one-to-one production mode to ensure that the subsequent extrusion equipment 6 has no shortage of raw materials and can operate continuously.

[0058] like Figure 4As shown, the present invention also includes a nitrogen input pipeline 4. A nitrogen injection pipe 14 is connected to the top of the reactor 1. The nitrogen injection pipe 14 is connected to the nitrogen input pipeline 4. Before production, nitrogen needs to be injected three times at a nitrogen injection pressure of 0.5 MPa to replace the air in the reactor and prevent the mixing of carbon dioxide and air from causing the foaming bubbles to burst.

[0059] The exhaust gas produced by this invention is carbon dioxide. The invention has an exhaust gas pipe 15 connected to the top of the reactor 1. A carbon dioxide saturated permeation system also includes an exhaust gas treatment box 5. The exhaust gas pipe 15 is connected to the exhaust gas treatment box 5. The exhaust gas can be condensed and recovered in the exhaust gas treatment box 5 to reduce the waste of production materials. The exhaust gas pipe 15 is equipped with a valve, which can control the opening and exhaust during the preparation process to ensure that the saturation pressure in the reactor is 10-18 MPa, so that the carbon dioxide is in a supercritical state.

[0060] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing thermal insulation pipes based on supercritical carbon dioxide foaming technology, characterized in that, Includes the following steps: Step S1: Heat-resistant polyethylene resin is melt-extruded through a single-screw extruder to obtain the inner tube material; Step S2: Mix the foaming matrix, nucleating agent, flame retardant and antioxidant in the specified amounts and granulate them through a twin-screw extruder to obtain pre-foamed particles. Then, use supercritical carbon dioxide foaming technology to obtain a gas-saturated single-phase mixed sol. Use the single-phase mixed sol to cover the inner pipe to form a foamed insulation layer. Step S3: High-strength polyethylene resin is melt-coated onto the outer surface of the insulation layer through a co-extrusion die to form the outer protective layer of the pipe body; Step S4: Water-cooled extruded pipes are then cut into finished insulated pipes.

2. The method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process according to claim 1, characterized in that, The extrusion temperature in step S1 is controlled at 180-220℃, the extrusion temperature in step S3 is controlled at 160-200℃, and the cooling temperature in step S4 is controlled below 40℃.

3. The method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process according to claim 1, characterized in that, The components of the pre-foamed particles in step S2, by mass parts, include: 100 parts by mass of foaming matrix, 0.5-3 parts by mass of nucleating agent, 1-5 parts by mass of flame retardant, and 1-5 parts by mass of antioxidant.

4. The method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process according to claim 1, characterized in that, The fabrication of the foamed insulation layer in step S2 includes the following steps: Step A1: Mix the foaming matrix, nucleating agent, and flame retardant according to the mass ratio, and then melt-granulate them through a twin-screw extruder. The extrusion temperature is controlled at 160-220℃ to finally obtain pre-foamed granules. Step A2: Place the pre-foamed granules in the reactor, turn on the stirrer and inject carbon dioxide to raise the temperature and pressure to the supercritical state, and maintain the constant temperature and pressure for 2-5 hours to form a gas-saturated single-phase mixed sol. Step A3: The sol is introduced into the mold or extrusion die. The pressure drop is generated by the sudden reduction of the cross-sectional area of ​​the flow channel. Gas is released to form bubble nuclei. The bubble nuclei grow during the cooling process to form a closed-cell foamed insulation layer on the outer surface of the inner tube.

5. The method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process according to claim 1, characterized in that, The foaming matrix in step S2 is polystyrene, expandable polystyrene, or extruded polystyrene; the nucleating agent is one of nano-silica, talc, or mica; the flame retardant is microcapsule-coated red phosphorus; and the antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] or dodecyl thiodipropionate.

6. The method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process according to claim 5, characterized in that, The method for preparing the flame retardant includes the following steps: Step B1: Add 10-15 parts of red phosphorus powder with a particle size of 15-50 μm to 40-60 parts of aluminum sulfate solution with a concentration of 10% and stir evenly; slowly add sodium hydroxide solution to adjust the pH to 6-8, so that aluminum hydroxide is deposited on the surface of red phosphorus; stir at 80°C for 2 hours to promote the formation of the coating layer; after filtration, washing and drying, red phosphorus coated with inorganic material is obtained. Step B2: Mix melamine and formaldehyde at a molar ratio of 1:3, adjust the pH to 8.5-9.5, stir and reflux at 80°C for 2 hours, add melamine prepolymer to the above inorganic coated red phosphorus, adjust the pH to 4, stir and reflux at 80°C for 3-4 hours to form a thermosetting organic material coating layer, filter, wash and dry to obtain double-coated red phosphorus; Step B3: Mix double-layered red phosphorus with styrene, benzoyl peroxide and water in a mass ratio of 46:6.2:0.03:50, and react at 85°C for 3 hours to polymerize styrene and form a thermoplastic organic material coating layer. After filtration, washing and drying, multi-layered microcapsule red phosphorus is obtained.

7. The method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process according to claim 4, characterized in that, The supercritical carbon dioxide described in step A2 has a saturation pressure of 10-18 MPa, a temperature of 90-120℃, and a gas dissolution amount of 8-15 wt% of the foaming matrix.

8. The method for preparing a thermal insulation pipe based on supercritical carbon dioxide foaming process according to claim 4, characterized in that, In step A3, the flow channel cross-sectional shrinkage ratio is 5:1 to 15:1, and the pressure drop rate is controlled at 50-150 MPa / s.

Images