Polyurethane semi-pipe for cryogenic pipeline insulation and preparation method thereof

Abstract

The application relates to the technical field of polyurethane, in particular to a polyurethane half-pipe for low-temperature pipeline heat preservation and a preparation method thereof. The polyurethane half-pipe is prepared by adding polyether polyol, flame-retardant polyol, a physical foaming agent, a chemical foaming agent, a surfactant, a catalyst, stannous octoate and polymethylene polyphenyl polyisocyanate, foaming and curing, and then coating modified flame-retardant silicone resin onto the surface of the polyurethane half-pipe and curing to obtain a finished product. The finished product prepared by the application has excellent flame-retardant performance and heat preservation performance, so that the application has a wide application prospect in the technical field of polyurethane.

Description

Technical Field

[0001] This invention relates to the field of polyurethane technology, specifically to a polyurethane half-pipe for low-temperature pipeline insulation and its preparation method. Background Technology

[0002] The development of polyurethane semi-tubes stems from the need to innovate traditional on-site foaming processes. Their preparation is primarily based on molding foaming technology, involving a two-component reaction of polyols and isocyanates, followed by foaming and slitting within a mold. While this production method has achieved a degree of industrialization and standardization, it still faces significant bottlenecks in current practical applications, especially in complex industrial settings such as chemical and energy sectors. On one hand, in operating environments with potential fire risks, such as chemical pipelines, the exterior of the polyurethane semi-tube may come into contact with high-temperature equipment or accidental ignition sources. If its flame-retardant properties are insufficient, the material is prone to rapid melting and dripping upon contact with fire, accelerating flame spread and significantly increasing the fire hazard, posing a direct threat to personnel safety and facility integrity. On the other hand, the insulation performance of polyurethane semi-tubes directly determines the cold loss rate of cryogenic pipelines and the system's energy efficiency level. Achieving a lower thermal conductivity through material optimization can effectively suppress heat exchange between the inside and outside of the pipeline, thereby significantly reducing the long-term operating energy consumption of the refrigeration system.

[0003] To overcome the shortcomings of the prior art, the present invention provides a polyurethane half-pipe for low-temperature pipeline insulation and its preparation method. Summary of the Invention

[0004] The purpose of this invention is to provide a polyurethane half-pipe for low-temperature pipeline insulation and its preparation method, so as to solve the problems raised in the prior art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a polyurethane semi-tube for low-temperature pipeline insulation includes the following steps: mixing polyether polyol, flame-retardant polyol, physical foaming agent, chemical foaming agent, surfactant, catalyst, and stannous octoate, stirring, then adding polymethylene polyphenyl polyisocyanate, stirring, and adding to a mold, followed by foaming and curing to obtain a polyurethane semi-tube; then coating the surface of the polyurethane semi-tube with modified flame-retardant silicone resin, curing, to obtain the finished product; The modified flame-retardant silicone resin is obtained by mixing flame-retardant silicone resin, anhydrous ethanol, and flame-retardant silicone material; wherein Schiff base flame-retardant siloxane is loaded into mesoporous silica to obtain flame-retardant silicone material; the flame-retardant siloxane material undergoes a hydrolysis-condensation reaction to obtain flame-retardant silicone resin.

[0006] In a more optimized manner, the content of each component in the finished product is as follows: by mass parts, 60-75 parts polyether polyol, 25-30 parts flame retardant polyol, 15-20 parts physical foaming agent, 1-2 parts chemical foaming agent, 1.5-2.5 parts surfactant, 0.8-1.2 parts catalyst, 0.2-0.4 parts stannous octoate, and 100-120 parts polymethylene polyphenyl polyisocyanate; wherein the physical foaming agent is 1,1,1,3,3-pentafluoropropane; the chemical foaming agent is deionized water; the surfactant is silicone oil; and the catalyst is tris(dimethylaminomethyl)phenol.

[0007] In a more optimized manner, modified flame-retardant silicone resin is coated onto the surface of polyurethane semi-tubes, and the coating thickness formed after curing is 80-100μm.

[0008] A more optimized preparation process for flame-retardant polyols is as follows: 2,4-dihydroxybenzaldehyde and anhydrous ethanol are mixed, stirred and dissolved, and then an alcoholic solution of ethanolamine is added dropwise. After the addition is completed, the temperature is raised to 80-83℃ and refluxed for 7.5-8.0 h. After the reaction is completed, the mixture is rotary evaporated and dried to obtain the flame-retardant polyol.

[0009] The optimal reaction molar ratio of 2,4-dihydroxybenzaldehyde to ethanolamine is 1:(1.05-1.10).

[0010] The optimized preparation process of the modified flame-retardant silicone resin is as follows: Step S1: Under nitrogen atmosphere, triethylamine, eugenol and ethyl acetate are mixed and stirred at 0-2℃ to dissolve. Then, an ethyl acetate solution of phenylphosphodichloro is added dropwise. After the addition is completed, the reaction continues for 1.0-1.5h. Then, the temperature is raised to 23-25℃ and the reaction continues for 45-50h. After the reaction is completed, the mixture is filtered, washed with alkali and water, dried and rotary evaporated to obtain the modified flame retardant. Step S2: Under nitrogen atmosphere, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, γ-methacryloyloxypropyltrimethoxysilane, and azobisisobutyronitrile are mixed and heated to 80-90℃ for 18-20 hours. After the reaction, a flame retardant reaction solution is obtained. The flame retardant reaction solution, anhydrous ethanol, and deionized water are mixed and heated to 75-80℃ for 8-10 hours. After the reaction, the mixture is rotary evaporated and dried to obtain a flame retardant silicone resin. Step S3: Mix the flame-retardant silicone resin and anhydrous ethanol, stir evenly, then add the flame-retardant silicone material and continue stirring for 15-20 minutes to obtain the modified flame-retardant silicone resin.

[0011] In a more optimized manner, in step S1, the molar ratio of eugenol to phenylphosphodichloro is (2.1-2.2):1; in step S2, the molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, and γ-methacryloyloxypropyltrimethoxysilane is (6.5-7.5):1:(4-5); wherein the volume ratio of anhydrous ethanol to deionized water is (3.0-3.5):1; and in step S3, the mass ratio of flame-retardant organosilicon resin to flame-retardant silicon material is 5:(1.0-1.3).

[0012] A more optimized preparation process for flame-retardant silicon materials is as follows: Step 1: Under nitrogen atmosphere, γ-aminopropyltriethoxysilane, vanillin and anhydrous ethanol are mixed and stirred evenly. The mixture is then heated to 45-55℃ and stirred for 7.5-8.5 hours. After stirring, the mixture is rotary evaporated, precipitated, filtered, washed and dried to obtain Schiff base flame retardant siloxane. Step 2: Mix hexadecyltrimethylammonium bromide, deionized water, ethanol, and ammonia. Stir at 35-40℃ until homogeneous, then add tetraethyl orthosilicate dropwise. Continue the reaction for 20-25 hours. After the reaction is complete, wash the mixture several times. Add the product to an alcoholic solution of hydrochloric acid and stir at 60-65℃ for 3.0-3.5 hours. After stirring, wash with water, wash with alcohol, filter, and dry to obtain mesoporous silica. Step 3: Mix Schiff base flame retardant siloxane and tetrahydrofuran, stir evenly, add mesoporous silica and stir under vacuum for 1.5-2.5 hours. After stirring, filter, wash and dry to obtain flame retardant silicon material.

[0013] In a more optimized manner, in step 1, the reaction molar ratio of γ-aminopropyltriethoxysilane and vanillin is 1:(1.1-1.2); in step 2, the reaction mass-volume ratio of hexadecyltrimethylammonium bromide, deionized water, ammonia, tetraethyl orthosilicate, and hydrochloric acid solution is 3g:1000mL:(18-20)mL:20mL:3mL; the mass fraction of ammonia is 25-28wt%, and the mass fraction of hydrochloric acid is 34-36wt%; in step 3, the mass ratio of mesoporous silica and Schiff base flame-retardant siloxane is 1:(0.8-1.0).

[0014] The beneficial effects of this invention are: The key feature of this invention lies in the Schiff base reaction, which involves adding γ-aminopropyltriethoxysilane and vanillin to obtain a Schiff base flame-retardant siloxane, forming an organosilicon flame-retardant compound containing C=N bonds. Then, by adding hexadecyltrimethylammonium bromide, deionized water, ammonia, tetraethyl orthosilicate, and hydrochloric acid solution, mesoporous silica with a hollow mesoporous structure is prepared. Silica itself is an excellent thermal insulation material; its solid framework has very low thermal conductivity. Furthermore, the hollow mesoporous structure is filled with static air, which has an extremely low thermal conductivity. Therefore, the solid framework combined with static air achieves an extremely low thermal conductivity.

[0015] Therefore, by mixing Schiff base flame-retardant siloxanes and mesoporous silica, the silanol groups of the mesoporous silica undergo a hydrolysis-condensation reaction with the ethoxy groups in the Schiff base flame-retardant siloxanes, forming chemical bonds. This achieves grafting loading of the Schiff base flame-retardant siloxanes onto the surface and within the pores of the mesoporous silica, resulting in a flame-retardant silica material. The flame retardant is firmly fixed to the surface and pores of the silica framework, preventing migration and seepage during use and ensuring the durability of the flame-retardant effect. Furthermore, the mesoporous silica framework itself is a high-temperature resistant ceramic material that forms a robust physical barrier when exposed to flame, blocking heat and mass transfer. Therefore, this flame-retardant silica material exhibits excellent chemical flame-retardant activity and thermal insulation properties.

[0016] The key feature of this invention is that a modified flame retardant is obtained by adding eugenol and phenylphosphodichloro to induce a substitution reaction. The modified flame retardant with carbon-carbon double bonds, γ-methacryloyloxypropyltrimethoxysilane, and the highly efficient phosphorus-based flame retardant 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide are then mixed and subjected to a free radical addition reaction initiated by azobisisobutyronitrile (AIB), resulting in a flame-retardant reaction solution with excellent flame-retardant properties. Subsequently, a flame-retardant silicone resin is prepared through the hydrolysis-condensation reaction of the siloxane in the flame-retardant reaction solution. Furthermore, the flame-retardant silicone resin and anhydrous ethanol are mixed, stirred evenly, and then a flame-retardant silicone material is added to obtain a modified flame-retardant silicone resin. The effective blending of the flame-retardant silicone resin and the flame-retardant silicone material results in a three-dimensional synergistic flame-retardant network. Meanwhile, when flame-retardant silicone material is dispersed as a filler in the resin matrix, it is equivalent to introducing countless microscopic heat-insulating particles into the resin. These heat-insulating particles are evenly distributed in the flame-retardant silicone resin. They not only have extremely low thermal conductivity, but also greatly hinder the continuous heat conduction path of the resin matrix, thus achieving excellent overall heat insulation effect.

[0017] The invention is characterized by the addition of 2,4-dihydroxybenzaldehyde and ethanolamine to induce a Schiff base reaction, yielding a flame-retardant polyol. A mixture of polyether polyol, flame-retardant polyol, physical foaming agent, chemical foaming agent, surfactant, catalyst, stannous octoate, and polymethylene polyphenyl polyisocyanate is then foamed and cured to obtain a polyurethane semi-tube. This flame-retardant polyol, as a component of polyurethane, forms an intrinsically flame-retardant polyurethane foam matrix through chemical bonding and reaction. A modified flame-retardant silicone resin with excellent chemical flame-retardant activity and thermal insulation properties is then coated onto the surface of the polyurethane semi-tube. The cured product exhibits excellent flame retardancy and thermal insulation properties. On one hand, the flame-retardant silicone material contained in the cured coating has a mesoporous structure and is itself a highly efficient heat insulator, effectively increasing the difficulty of heat penetration; furthermore, the synergistic effect of multiple flame-retardant elements also contributes to excellent flame-retardant performance. On the other hand, the cured coating forms a tough protective film that effectively prevents external moisture and humidity from penetrating the polyurethane foam semi-tube. Moisture intrusion is one of the main factors that causes a sharp increase in the thermal conductivity of foam. Therefore, the coating ensures the long-term stability of the internal foam's thermal insulation performance by preventing moisture and moisture. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0019] Raw material source: Polyether polyol, brand name SC-450; silicone oil, provided by Jinan Changyingda Chemical Co., Ltd., model 350cs; polymethylene polyphenyl polyisocyanate, model PM-200; by weight, one part is 1g.

[0020] Example 1: Step S1: Under nitrogen atmosphere, γ-aminopropyltriethoxysilane, vanillin and anhydrous ethanol were mixed, stirred evenly and heated to 55°C. After stirring for 8.5 h, the mixture was rotary evaporated, precipitated, filtered, washed and dried to obtain Schiff base flame retardant siloxane; the molar ratio of γ-aminopropyltriethoxysilane to vanillin was 1:1.15. Step S2: Hexadecyltrimethylammonium bromide, deionized water, ethanol, and ammonia were mixed and stirred at 40°C until homogeneous. Tetraethyl orthosilicate was then added dropwise, and the reaction continued for 25 hours. After the reaction was complete, the mixture was washed several times, and the product was added to an alcoholic solution of hydrochloric acid. The mixture was stirred at 65°C for 3.5 hours. After stirring, the product was washed with water, washed with alcohol, filtered, and dried to obtain mesoporous silica. The mass-to-volume ratio of hexadecyltrimethylammonium bromide, deionized water, ammonia, tetraethyl orthosilicate, and hydrochloric acid solution was 3 g: 1000 mL: 19 mL: 20 mL: 3 mL; the mass fraction of ammonia was 27 wt%, and the mass fraction of hydrochloric acid was 35 wt%. Step S3: Mix Schiff base flame retardant siloxane and tetrahydrofuran, stir evenly, add mesoporous silica and stir under vacuum for 2.5 h. After stirring, filter, wash and dry to obtain flame retardant silicon material; the mass ratio of mesoporous silica to Schiff base flame retardant siloxane is 1:0.9. Step S4: Under nitrogen atmosphere, triethylamine, eugenol, and ethyl acetate were mixed and dissolved by stirring at 2°C. Then, an ethyl acetate solution of phenylphosphine dichloride was added dropwise. After the addition was completed, the reaction was continued for 1.5 h, and then the temperature was raised to 25°C and the reaction was continued for 50 h. After the reaction was completed, the mixture was filtered, washed with alkali, washed with water, dried, and rotary evaporated to obtain the modified flame retardant. The molar ratio of eugenol to phenylphosphine dichloride was 2.15:1. Step S5: Under nitrogen atmosphere, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, γ-methacryloyloxypropyltrimethoxysilane, and azobisisobutyronitrile are mixed and heated to 90℃ for 20 hours. After the reaction, a flame-retardant reaction solution is obtained. The flame-retardant reaction solution, anhydrous ethanol, and deionized water are mixed and heated to 80℃ for 10 hours. After the reaction, the mixture is rotary evaporated and dried to obtain a flame-retardant silicone resin. The molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, and γ-methacryloyloxypropyltrimethoxysilane is 7:1:4.5; the volume ratio of anhydrous ethanol to deionized water is 3.2:1. Step S6: Mix the flame-retardant silicone resin and anhydrous ethanol, stir evenly, then add the flame-retardant silicone material, and continue stirring for 20 minutes to obtain the modified flame-retardant silicone resin; the reaction mass ratio of flame-retardant silicone resin to flame-retardant silicone material is 5:1.1. Step S7: Mix 2,4-dihydroxybenzaldehyde and anhydrous ethanol, stir to dissolve, and then add an alcoholic solution of ethanolamine dropwise. After the addition is complete, heat to 83℃ and reflux for 8.0 h. After the reaction is complete, evaporate by rotary evaporation and dry to obtain a flame-retardant polyol. The molar ratio of 2,4-dihydroxybenzaldehyde to ethanolamine is 1:1.07. Step S8: Mix 60g of polyether polyol, 25g of flame-retardant polyol, 15g of 1,1,1,3,3-pentafluoropropane, 1g of deionized water, 1.5g of silicone oil, 0.8g of tris(dimethylaminomethyl)phenol, and 0.2g of stannous octoate. After stirring, add 100g of polymethylene polyphenyl polyisocyanate and stir again. Pour the mixture into a mold, and after foaming and curing, obtain a polyurethane half-tube. Then, coat the surface of the polyurethane half-tube with modified flame-retardant silicone resin and cure to obtain a finished product with a coating thickness of 100μm.

[0021] Example 2: Step S1: Under nitrogen atmosphere, γ-aminopropyltriethoxysilane, vanillin and anhydrous ethanol were mixed, stirred evenly and heated to 50°C. After stirring and reacting for 8 hours, the mixture was rotary evaporated, precipitated, filtered, washed and dried to obtain Schiff base flame retardant siloxane; the molar ratio of γ-aminopropyltriethoxysilane to vanillin was 1:1.15. Step S2: Hexadecyltrimethylammonium bromide, deionized water, ethanol, and ammonia were mixed and stirred at 37°C until homogeneous. Tetraethyl orthosilicate was then added dropwise, and the reaction continued for 23 hours. After the reaction was complete, the mixture was washed several times, and the product was added to an alcoholic solution of hydrochloric acid. The mixture was stirred at 62°C for 3.2 hours. After stirring, the mixture was washed with water, washed with alcohol, filtered, and dried to obtain mesoporous silica. The mass-to-volume ratio of hexadecyltrimethylammonium bromide, deionized water, ammonia, tetraethyl orthosilicate, and hydrochloric acid solution was 3 g: 1000 mL: 19 mL: 20 mL: 3 mL; the mass fraction of ammonia was 27 wt%, and the mass fraction of hydrochloric acid was 35 wt%. Step S3: Mix Schiff base flame retardant siloxane and tetrahydrofuran, stir evenly, add mesoporous silica and stir under vacuum for 2 hours. After stirring, filter, wash and dry to obtain flame retardant silicon material; the mass ratio of mesoporous silica to Schiff base flame retardant siloxane is 1:0.9. Step S4: Under nitrogen atmosphere, triethylamine, eugenol, and ethyl acetate were mixed and dissolved by stirring at 1°C. Then, an ethyl acetate solution of phenylphosphine dichloride was added dropwise. After the addition was completed, the reaction was continued for 1.3 hours, and then the temperature was raised to 24°C and the reaction was continued for 47 hours. After the reaction was completed, the mixture was filtered, washed with alkali, washed with water, dried, and rotary evaporated to obtain the modified flame retardant. The molar ratio of eugenol to phenylphosphine dichloride was 2.15:1. Step S5: Under nitrogen atmosphere, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, γ-methacryloyloxypropyltrimethoxysilane, and azobisisobutyronitrile are mixed and heated to 85℃ for 19 hours. After the reaction, a flame-retardant reaction solution is obtained. The flame-retardant reaction solution, anhydrous ethanol, and deionized water are mixed and heated to 77℃ for 9 hours. After the reaction, the mixture is rotary evaporated and dried to obtain a flame-retardant silicone resin. The molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, and γ-methacryloyloxypropyltrimethoxysilane is 7:1:4.5; the volume ratio of anhydrous ethanol to deionized water is 3.2:1. Step S6: Mix the flame-retardant silicone resin and anhydrous ethanol, stir evenly, then add the flame-retardant silicone material, and continue stirring for 17 minutes to obtain the modified flame-retardant silicone resin; the reaction mass ratio of flame-retardant silicone resin to flame-retardant silicone material is 5:1.1. Step S7: Mix 2,4-dihydroxybenzaldehyde and anhydrous ethanol, stir to dissolve, and then add an alcoholic solution of ethanolamine dropwise. After the addition is complete, heat to 82℃ and reflux for 7.7 h. After the reaction is complete, evaporate by rotary evaporation and dry to obtain a flame-retardant polyol. The molar ratio of 2,4-dihydroxybenzaldehyde to ethanolamine is 1:1.07. Step S8: Mix 60g of polyether polyol, 25g of flame-retardant polyol, 15g of 1,1,1,3,3-pentafluoropropane, 1g of deionized water, 1.5g of silicone oil, 0.8g of tris(dimethylaminomethyl)phenol, and 0.2g of stannous octoate. After stirring, add 100g of polymethylene polyphenyl polyisocyanate and stir again. Pour the mixture into a mold, and after foaming and curing, obtain a polyurethane half-tube. Then, coat the surface of the polyurethane half-tube with modified flame-retardant silicone resin and cure to obtain a finished product with a coating thickness of 100μm.

[0022] Example 3: Step S1: Under nitrogen atmosphere, γ-aminopropyltriethoxysilane, vanillin and anhydrous ethanol were mixed, stirred evenly and heated to 45°C. After stirring and reacting for 7.5 h, the mixture was rotary evaporated, precipitated, filtered, washed and dried to obtain Schiff base flame retardant siloxane; the molar ratio of γ-aminopropyltriethoxysilane to vanillin was 1:1.15. Step S2: Hexadecyltrimethylammonium bromide, deionized water, ethanol, and ammonia were mixed and stirred at 35°C until homogeneous. Tetraethyl orthosilicate was then added dropwise, and the reaction continued for 20 hours. After the reaction was complete, the mixture was washed several times, and the product was added to an alcoholic solution of hydrochloric acid. The mixture was stirred at 60°C for 3.0 hours. After stirring, the mixture was washed with water, washed with alcohol, filtered, and dried to obtain mesoporous silica. The mass-to-volume ratio of hexadecyltrimethylammonium bromide, deionized water, ammonia, tetraethyl orthosilicate, and hydrochloric acid solution was 3 g: 1000 mL: 19 mL: 20 mL: 3 mL; the mass fraction of ammonia was 27 wt%, and the mass fraction of hydrochloric acid was 35 wt%. Step S3: Mix Schiff base flame retardant siloxane and tetrahydrofuran, stir evenly, add mesoporous silica and stir under vacuum for 1.5 h. After stirring, filter, wash and dry to obtain flame retardant silicon material; the mass ratio of mesoporous silica to Schiff base flame retardant siloxane is 1:0.9. Step S4: Under nitrogen atmosphere, triethylamine, eugenol, and ethyl acetate were mixed and dissolved by stirring at 0°C. Then, an ethyl acetate solution of phenylphosphine dichloride was added dropwise. After the addition was completed, the reaction was continued for 1.0 h, and then the temperature was raised to 23°C and the reaction was continued for 45 h. After the reaction was completed, the mixture was filtered, washed with alkali, washed with water, dried, and rotary evaporated to obtain the modified flame retardant. The molar ratio of eugenol to phenylphosphine dichloride was 2.15:1. Step S5: Under nitrogen atmosphere, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, γ-methacryloyloxypropyltrimethoxysilane, and azobisisobutyronitrile are mixed and heated to 80℃ for 18 hours. After the reaction, a flame-retardant reaction solution is obtained. The flame-retardant reaction solution, anhydrous ethanol, and deionized water are mixed and heated to 75℃ for 8 hours. After the reaction, the mixture is rotary evaporated and dried to obtain a flame-retardant silicone resin. The molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, and γ-methacryloyloxypropyltrimethoxysilane is 7:1:4.5; the volume ratio of anhydrous ethanol to deionized water is 3.2:1. Step S6: Mix the flame-retardant silicone resin and anhydrous ethanol, stir evenly, then add the flame-retardant silicone material, and continue stirring for 15 minutes to obtain the modified flame-retardant silicone resin; the reaction mass ratio of flame-retardant silicone resin to flame-retardant silicone material is 5:1.1. Step S7: Mix 2,4-dihydroxybenzaldehyde and anhydrous ethanol, stir to dissolve, and then add an alcoholic solution of ethanolamine dropwise. After the addition is complete, heat to 80°C and reflux for 7.5 h. After the reaction is complete, evaporate by rotary evaporation and dry to obtain a flame-retardant polyol. The molar ratio of 2,4-dihydroxybenzaldehyde to ethanolamine is 1:1.07. Step S8: Mix 60g of polyether polyol, 25g of flame-retardant polyol, 15g of 1,1,1,3,3-pentafluoropropane, 1g of deionized water, 1.5g of silicone oil, 0.8g of tris(dimethylaminomethyl)phenol, and 0.2g of stannous octoate. After stirring, add 100g of polymethylene polyphenyl polyisocyanate and stir again. Pour the mixture into a mold, and after foaming and curing, obtain a polyurethane half-tube. Then, coat the surface of the polyurethane half-tube with modified flame-retardant silicone resin and cure to obtain a finished product with a coating thickness of 100μm.

[0023] Comparative Example 1: The flame-retardant silicone material was removed, and the rest was the same as in Example 1. The specific steps are as follows: Step S1: Under nitrogen atmosphere, triethylamine, eugenol and ethyl acetate were mixed and stirred at 2°C to dissolve. Then, an ethyl acetate solution of phenylphosphine dichloride was added dropwise. After the addition was completed, the reaction was continued for 1.5 h, and then the temperature was raised to 25°C and the reaction was continued for 50 h. After the reaction was completed, the mixture was filtered, washed with alkali, washed with water, dried and rotary evaporated to obtain the modified flame retardant. The molar ratio of eugenol to phenylphosphine dichloride was 2.15:1. Step S2: Under nitrogen atmosphere, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, γ-methacryloyloxypropyltrimethoxysilane, and azobisisobutyronitrile are mixed and heated to 90℃ for 20 hours. After the reaction, a flame-retardant reaction solution is obtained. The flame-retardant reaction solution, anhydrous ethanol, and deionized water are mixed and heated to 80℃ for 10 hours. After the reaction, the mixture is rotary evaporated and dried to obtain a flame-retardant silicone resin. The molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, and γ-methacryloyloxypropyltrimethoxysilane is 7:1:4.5; the volume ratio of anhydrous ethanol to deionized water is 3.2:1. Step S3: Mix the flame-retardant silicone resin and anhydrous ethanol, stir evenly, and obtain the flame-retardant silicone resin material. Step S4: Mix 2,4-dihydroxybenzaldehyde and anhydrous ethanol, stir to dissolve, and then add an alcoholic solution of ethanolamine dropwise. After the addition is complete, heat to 83℃ and reflux for 8.0 h. After the reaction is complete, evaporate by rotary evaporation and dry to obtain a flame-retardant polyol. The molar ratio of 2,4-dihydroxybenzaldehyde to ethanolamine is 1:1.07. Step S5: Mix 60g of polyether polyol, 25g of flame-retardant polyol, 15g of 1,1,1,3,3-pentafluoropropane, 1g of deionized water, 1.5g of silicone oil, 0.8g of tris(dimethylaminomethyl)phenol, and 0.2g of stannous octoate. After stirring, add 100g of polymethylene polyphenyl polyisocyanate and stir again. Pour the mixture into a mold, and after foaming and curing, obtain a polyurethane semi-tube. Then, coat the surface of the polyurethane semi-tube with flame-retardant silicone resin and cure to obtain a finished product with a coating thickness of 100μm.

[0024] Comparative Example 2: The modified flame-retardant silicone resin was removed, and the rest was the same as in Example 1. The specific steps are as follows: Step S1: 2,4-dihydroxybenzaldehyde and anhydrous ethanol were mixed and stirred to dissolve. Then, an alcohol solution of ethanolamine was added dropwise. After the addition was completed, the temperature was raised to 83°C and refluxed for 8.0 h. After the reaction was completed, the mixture was rotary evaporated and dried to obtain a flame-retardant polyol. The molar ratio of 2,4-dihydroxybenzaldehyde to ethanolamine was 1:1.07. Step S2: Mix 60g of polyether polyol, 25g of flame-retardant polyol, 15g of 1,1,1,3,3-pentafluoropropane, 1g of deionized water, 1.5g of silicone oil, 0.8g of tris(dimethylaminomethyl)phenol, and 0.2g of stannous octoate. After stirring, add 100g of polymethylene polyphenyl polyisocyanate, stir, and then add it to the mold. After foaming and curing, the finished product is obtained.

[0025] Comparative Example 3: The flame-retardant polyol was replaced with a polyether polyol, and the rest was the same as in Example 1. The specific steps are as follows: Step S1: Under nitrogen atmosphere, γ-aminopropyltriethoxysilane, vanillin and anhydrous ethanol were mixed, stirred evenly and heated to 55°C. After stirring and reacting for 8.5 h, the mixture was rotary evaporated, precipitated, filtered, washed and dried to obtain Schiff base flame-retardant siloxane; the molar ratio of γ-aminopropyltriethoxysilane to vanillin was 1:1.15. Step S2: Hexadecyltrimethylammonium bromide, deionized water, ethanol, and ammonia were mixed and stirred at 40°C until homogeneous. Tetraethyl orthosilicate was then added dropwise, and the reaction continued for 25 hours. After the reaction was complete, the mixture was washed several times, and the product was added to an alcoholic solution of hydrochloric acid. The mixture was stirred at 65°C for 3.5 hours. After stirring, the product was washed with water, washed with alcohol, filtered, and dried to obtain mesoporous silica. The mass-to-volume ratio of hexadecyltrimethylammonium bromide, deionized water, ammonia, tetraethyl orthosilicate, and hydrochloric acid solution was 3 g: 1000 mL: 19 mL: 20 mL: 3 mL; the mass fraction of ammonia was 27 wt%, and the mass fraction of hydrochloric acid was 35 wt%. Step S3: Mix Schiff base flame retardant siloxane and tetrahydrofuran, stir evenly, add mesoporous silica and stir under vacuum for 2.5 h. After stirring, filter, wash and dry to obtain flame retardant silicon material; the mass ratio of mesoporous silica to Schiff base flame retardant siloxane is 1:0.9. Step S4: Under nitrogen atmosphere, triethylamine, eugenol, and ethyl acetate were mixed and dissolved by stirring at 2°C. Then, an ethyl acetate solution of phenylphosphine dichloride was added dropwise. After the addition was completed, the reaction was continued for 1.5 h, and then the temperature was raised to 25°C and the reaction was continued for 50 h. After the reaction was completed, the mixture was filtered, washed with alkali, washed with water, dried, and rotary evaporated to obtain the modified flame retardant. The molar ratio of eugenol to phenylphosphine dichloride was 2.15:1. Step S5: Under nitrogen atmosphere, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, γ-methacryloyloxypropyltrimethoxysilane, and azobisisobutyronitrile are mixed and heated to 90℃ for 20 hours. After the reaction, a flame-retardant reaction solution is obtained. The flame-retardant reaction solution, anhydrous ethanol, and deionized water are mixed and heated to 80℃ for 10 hours. After the reaction, the mixture is rotary evaporated and dried to obtain a flame-retardant silicone resin. The molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, and γ-methacryloyloxypropyltrimethoxysilane is 7:1:4.5; the volume ratio of anhydrous ethanol to deionized water is 3.2:1. Step S6: Mix the flame-retardant silicone resin and anhydrous ethanol, stir evenly, then add the flame-retardant silicone material, and continue stirring for 20 minutes to obtain the modified flame-retardant silicone resin; the reaction mass ratio of flame-retardant silicone resin to flame-retardant silicone material is 5:1.1. Step S7: Mix 85g of polyether polyol, 15g of 1,1,1,3,3-pentafluoropropane, 1g of deionized water, 1.5g of silicone oil, 0.8g of tris(dimethylaminomethyl)phenol, and 0.2g of stannous octoate. Stir and then add 100g of polymethylene polyphenyl polyisocyanate. Stir and then add the mixture to a mold. After foaming and curing, a polyurethane half-tube is obtained. Then, the modified flame-retardant silicone resin is coated onto the surface of the polyurethane half-tube and cured to obtain a finished product with a coating thickness of 100μm.

[0026] Testing and experimentation: Flame retardant performance test: The finished polyurethane half-tube prepared according to this invention was cut into pieces with dimensions of 150×10×10mm. 3 The limiting oxygen index of the samples was tested using an HC-2C oxygen index meter (Nanjing Shangyuan Analytical Instrument Co., Ltd.) according to ASTM D2863-19 standard.

[0027] Moisture absorption rate: The finished polyurethane half-tube prepared according to this invention is cut into pieces with dimensions of 30×30×10mm. 3 The sample was dried in an oven until constant weight, and the mass of the sample was M1. The sample was then placed in a constant temperature and humidity chamber for 7 days with the following parameters: temperature 25℃ and humidity 85%RH. The mass of the sample at this time was M2. The moisture absorption rate was calculated using the formula: (M2-M1) / M1×100%.

[0028] Thermal conductivity test: The finished polyurethane half-tube prepared according to this invention was cut into pieces with dimensions of 30×30×10mm. 3 The thermal conductivity of the samples was tested using a Swedish Hot Disk 2500-OT thermal conductivity meter at 25℃ and -150℃. The results are shown in the table below:

[0029] Conclusion: In Examples 1-3, the dosage remained unchanged, with only some reaction parameters modified. Experimental data showed no significant fluctuations in the performance of the samples.

[0030] Comparative Example 1: The flame-retardant silicone material was removed, while the rest remained the same as in Example 1. Experimental data showed that, compared to Example 1, the oxygen index decreased to 27.8%, the moisture absorption rate increased by 0.51%, the thermal conductivity at 25℃ increased to 0.0332 W / m·K, and the thermal conductivity at -150℃ increased to 0.0210 W / m·K. The reason for this is that the flame-retardant silicone material contains various flame-retardant structures and hollow mesoporous structures, thus possessing excellent chemical flame-retardant activity and thermal insulation properties. Therefore, removing it reduces the oxygen index, increases the thermal conductivity at 25℃, and increases the thermal conductivity at -150℃. Simultaneously, the presence of the flame-retardant silicone material also prevents external moisture from penetrating the polyurethane foam half-tube, thus increasing the moisture absorption rate after its removal.

[0031] Comparative Example 2: The modified flame-retardant silicone resin was removed, and the rest was the same as in Example 1. Experimental data showed that, compared to Example 1, the oxygen index decreased to 23.1%, the moisture absorption rate increased to 1.38%, the thermal conductivity at 25℃ increased to 0.0311 W / m·K, and the thermal conductivity at -150℃ increased to 0.0191 W / m·K. The reason for this is that Comparative Example 2 further removed the flame-retardant silicone resin from Comparative Example 1. The flame-retardant silicone resin can effectively improve the flame-retardant activity of the polyurethane foam matrix and also effectively prevent external moisture and humidity from penetrating the polyurethane foam half-tube by forming a protective coating. Therefore, removing it reduced the oxygen index, increased the thermal conductivity at 25℃, increased the thermal conductivity at -150℃, and increased the moisture absorption rate.

[0032] Comparative Example 3: The flame-retardant polyol was replaced with a polyether polyol, and the rest was the same as in Example 1. The experimental data showed that the oxygen index decreased to 24.9% compared with Example 1. The reason for this is that the flame-retardant polyol contains a flame-retardant Schiff base structure, which can effectively improve the intrinsic flame retardancy of the polyurethane foam matrix. Therefore, removing it reduces the flame retardancy and decreases the oxygen index.

[0033] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process method article or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process method article or apparatus.

[0034] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a polyurethane half-pipe for low-temperature pipeline insulation, characterized in that: Includes the following steps: Polyether polyol, flame retardant polyol, physical foaming agent, chemical foaming agent, surfactant, catalyst, and stannous octoate are mixed and stirred. Then, polymethylene polyphenyl polyisocyanate is added and stirred again. The mixture is then added to a mold, foamed, and cured to obtain a polyurethane semi-tube. Modified flame retardant silicone resin is then coated onto the surface of the polyurethane semi-tube and cured to obtain the finished product. The modified flame-retardant silicone resin is obtained by mixing flame-retardant silicone resin, anhydrous ethanol, and flame-retardant silicone material; wherein Schiff base flame-retardant siloxane is loaded into mesoporous silica to obtain flame-retardant silicone material; the flame-retardant siloxane material undergoes a hydrolysis-condensation reaction to obtain flame-retardant silicone resin.

2. The method for preparing a polyurethane half-pipe for low-temperature pipeline insulation according to claim 1, characterized in that: The finished product contains the following components by mass: 60-75 parts polyether polyol, 25-30 parts flame-retardant polyol, 15-20 parts physical blowing agent, 1-2 parts chemical blowing agent, 1.5-2.5 parts surfactant, 0.8-1.2 parts catalyst, 0.2-0.4 parts stannous octoate, and 100-120 parts polymethylene polyphenyl polyisocyanate; the physical blowing agent is 1,1,1,3,3-pentafluoropropane; the chemical blowing agent is deionized water; the surfactant is silicone oil; and the catalyst is tris(dimethylaminomethyl)phenol.

3. The method for preparing a polyurethane half-pipe for low-temperature pipeline insulation according to claim 2, characterized in that: The preparation process of flame-retardant polyol is as follows: 2,4-dihydroxybenzaldehyde and anhydrous ethanol are mixed and stirred to dissolve. Then, an alcohol solution of ethanolamine is added dropwise. After the addition is completed, the temperature is raised to 80-83℃ and refluxed for 7.5-8.0 h. After the reaction is completed, the mixture is rotary evaporated and dried to obtain the flame-retardant polyol.

4. The method for preparing a polyurethane half-pipe for low-temperature pipeline insulation according to claim 3, characterized in that: The molar ratio of 2,4-dihydroxybenzaldehyde to ethanolamine is 1:(1.05-1.10).

5. The method for preparing a polyurethane half-pipe for low-temperature pipeline insulation according to claim 1, characterized in that: The preparation process of modified flame-retardant silicone resin is as follows: Step S1: Under nitrogen atmosphere, triethylamine, eugenol and ethyl acetate are mixed and stirred at 0-2℃ to dissolve. Then, an ethyl acetate solution of phenylphosphodichloro is added dropwise. After the addition is completed, the reaction continues for 1.0-1.5h. Then, the temperature is raised to 23-25℃ and the reaction continues for 45-50h. After the reaction is completed, the mixture is filtered, washed with alkali and water, dried and rotary evaporated to obtain the modified flame retardant. Step S2: Under nitrogen atmosphere, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, γ-methacryloyloxypropyltrimethoxysilane, and azobisisobutyronitrile are mixed and heated to 80-90℃ for 18-20 hours. After the reaction, a flame retardant reaction solution is obtained. The flame retardant reaction solution, anhydrous ethanol, and deionized water are mixed and heated to 75-80℃ for 8-10 hours. After the reaction, the mixture is rotary evaporated and dried to obtain a flame retardant silicone resin. Step S3: Mix the flame-retardant silicone resin and anhydrous ethanol, stir evenly, then add the flame-retardant silicone material and continue stirring for 15-20 minutes to obtain the modified flame-retardant silicone resin.

6. The method for preparing a polyurethane half-pipe for low-temperature pipeline insulation according to claim 5, characterized in that: In step S1, the molar ratio of eugenol to phenylphosphodichloro is (2.1-2.2):1; in step S2, the molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, modified flame retardant, and γ-methacryloyloxypropyltrimethoxysilane is (6.5-7.5):1:(4-5); wherein the volume ratio of anhydrous ethanol to deionized water is (3.0-3.5):1; in step S3, the mass ratio of flame-retardant organosilicon resin to flame-retardant silicon material is 5:(1.0-1.3).

7. The method for preparing a polyurethane half-pipe for low-temperature pipeline insulation according to claim 6, characterized in that: The preparation process of flame-retardant silicon materials is as follows: Step 1: Under nitrogen atmosphere, γ-aminopropyltriethoxysilane, vanillin and anhydrous ethanol are mixed and stirred evenly. The mixture is then heated to 45-55℃ and stirred for 7.5-8.5 hours. After stirring, the mixture is rotary evaporated, precipitated, filtered, washed and dried to obtain Schiff base flame retardant siloxane. Step 2: Mix hexadecyltrimethylammonium bromide, deionized water, ethanol, and ammonia. Stir at 35-40℃ until homogeneous, then add tetraethyl orthosilicate dropwise. Continue the reaction for 20-25 hours. After the reaction is complete, wash the mixture several times. Add the product to an alcoholic solution of hydrochloric acid and stir at 60-65℃ for 3.0-3.5 hours. After stirring, wash with water, wash with alcohol, filter, and dry to obtain mesoporous silica. Step 3: Mix Schiff base flame retardant siloxane and tetrahydrofuran, stir evenly, add mesoporous silica and stir under vacuum for 1.5-2.5 hours. After stirring, filter, wash and dry to obtain flame retardant silicon material.

8. The method for preparing a polyurethane half-pipe for low-temperature pipeline insulation according to claim 7, characterized in that: In step 1, the molar ratio of γ-aminopropyltriethoxysilane to vanillin is 1:(1.1-1.2); in step 2, the mass-volume ratio of hexadecyltrimethylammonium bromide, deionized water, ammonia, tetraethyl orthosilicate, and hydrochloric acid solution is 3g:1000mL:(18-20)mL:20mL:3mL; the mass fraction of ammonia is 25-28wt% and the mass fraction of hydrochloric acid is 34-36wt%; in step 3, the mass ratio of mesoporous silica to Schiff base flame-retardant siloxane is 1:(0.8-1.0).

9. A polyurethane half-pipe for low-temperature pipeline insulation, characterized in that, Prepared by the preparation method according to any one of claims 1-8.

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