A process for the synthesis of hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor

Abstract

This invention belongs to the field of polymer synthesis and microchemical technology, and discloses a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor. The preparation method includes mixing butadiene, anhydrous ethanol, 30% hydrogen peroxide, and an antioxidant under nitrogen protection to obtain a raw material liquid, which is then pumped into the microchannel reactor via a high-pressure metering pump. The mixture undergoes preheating initiation and polymerization reactions sequentially, with viscosity monitored by an online viscometer. After the reaction, the product is obtained through gas-liquid separation, sedimentation and stratification, washing, vacuum solvent removal, and filtration. This invention utilizes a microchannel reactor, which provides precise temperature control and stable pressure. Segmented temperature control and online viscosity adjustment prevent reactor blockage, resulting in high product conversion and yield, and uniform molecular weight distribution. By adding auxiliary devices to optimize the process, continuous and stable production is achieved, making it more efficient, environmentally friendly, and controllable.

Description

Technical Field

[0001] This invention belongs to the field of polymer synthesis and microchemical technology, specifically relating to a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor. Background Technology

[0002] Hydroxyl-terminated polybutadiene (HTPB) is a key prepolymer for solid rocket propellants, adhesives, coatings, and sealants; its hydroxyl value and molecular weight distribution directly affect the performance of the final product. Traditional industrial production of HTPB employs a batch reactor polymerization process: butadiene, hydrogen peroxide, and anhydrous ethanol are added in a specific ratio to react in a high-pressure polymerization reactor. This method has the following drawbacks: Limited heat and mass transfer: The polymerization reaction releases a large amount of heat, and the heat exchange area of ​​the batch reactor is small, which easily leads to local overheating, resulting in branching, cross-linking, or even gel formation, affecting product quality.

[0003] High safety risks: The system contains high-pressure butadiene and strong oxidant hydrogen peroxide, and the reaction heat risk assessment level is level 3 (relatively dangerous), with significant scale-up effect.

[0004] Long reaction time: intermittent operation has a long cycle, low production efficiency, and poor batch-to-batch consistency.

[0005] The sudden increase in viscosity makes stirring difficult: the viscosity in the later stage of the reaction is >10000mPa·s, the energy consumption of stirring is high, and dead zones are easy to occur.

[0006] Continuous flow microreactors offer advantages such as large specific surface area, high heat and mass transfer efficiency, low liquid holdup, and intrinsic safety; however, they are prone to clogging when directly used for the synthesis of high-viscosity polymers. Therefore, specialized continuous flow processes and reactor structures need to be designed specifically for the characteristics of HTPB polymerization. Summary of the Invention

[0007] This invention provides a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor to solve the problems of long reaction time and low product quality of hydroxyl-terminated polybutadiene in the prior art.

[0008] This invention provides the following technical solution: a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor, comprising the following steps: S1. Mix butadiene, anhydrous ethanol, and 30% hydrogen peroxide, add antioxidant, and stir under nitrogen protection to obtain a homogeneous or emulsion reaction raw material solution. S2. The reaction raw material liquid is pumped into the microchannel reactor through a high-pressure metering pump for preheating treatment. The system reaction pressure is maintained by a back pressure valve. The preheated raw material liquid undergoes polymerization at a set reaction temperature. The viscosity of the material is monitored in real time at the outlet of the microchannel reactor by an online viscometer. S3. Based on the monitoring results of the online viscometer, the preheated material continues to undergo polymerization in the microchannel reactor, maintaining the set reaction temperature and system pressure to ensure that the polymerization reaction is fully carried out. After the material completes the polymerization reaction, it flows out from the outlet of the microchannel reactor. S4. The reaction mixture flowing out of the microreactor is depressurized by the back pressure valve and sent to the gas-liquid separator to separate and recover unreacted butadiene. The liquid phase separated by the gas-liquid separator is sent to the settling tank for static separation. The lower layer of the adhesive is taken out, washed with hot water, and the solvent and trace amount of water are removed by vacuum. After adding antioxidant, it is filtered to obtain hydroxyl-terminated polybutadiene.

[0009] Furthermore, the mass ratio of butadiene, anhydrous ethanol, and hydrogen peroxide is 100:(66~78):(3~6); The antioxidant accounts for 0.060% to 0.078% of the total material mass; The butadiene purity is ≥99.7%; The antioxidant is 2,6-di-tert-butyl-p-cresol.

[0010] Furthermore, the microchannel reactor is one of the following: a 316L stainless steel coil, a microreactor with a built-in low-shear static mixer, a stainless steel micro-sieve structure, or a Hastelloy coil. When using 316L stainless steel coils, the inner diameter is 2mm; When a stainless steel micro-sieve structure is used, the sieve diameter is 0.3 mm, the sieve spacing is 1.5 mm, and a total of 30 layers are arranged in an alternating pattern, with an effective reaction zone volume of 45 mL. When using a microreactor with a built-in low-shear static mixer, the static mixer unit is 5cm long and the reactor diameter is 3mm. When using Hastelloy coils, the inner diameter of the Hastelloy coils is 1.6 mm and the pressure resistance is 15 MPa.

[0011] Furthermore, the back pressure of the back pressure valve is 2.0~9.0MPa.

[0012] Furthermore, the inner wall of the microchannel reactor is electrochemically polished or coated with a polytetrafluoroethylene anti-stick layer.

[0013] Furthermore, the online viscometer is a vibration-type online viscometer.

[0014] Furthermore, when the viscosity monitored by the online viscometer exceeds the set threshold, preheated anhydrous ethanol is added to the microchannel reactor to dilute and reduce the viscosity of the material online; the temperature of the added preheated anhydrous ethanol is 80°C, the addition rate is 2~6 mL / min, and the addition method is single-point addition or multi-point segmented addition; the dilution ratio of the raw material liquid to the added ethanol is 1:0.2~0.3.

[0015] Compared with the prior art, the present invention has the following beneficial effects: Using microreactors for high-temperature and high-pressure free radical polymerization of butadiene: through the excellent heat transfer performance at the microscale, the traditional reaction time is shortened, the space-time yield is increased by more than 10 times, and the gel byproducts are reduced due to the uniform temperature.

[0016] To address the issue of viscosity increasing rapidly from tens of cP to 10,000 cP during polymerization, a combined solution of "mid-stage addition of diluent solvent + variable diameter pipe + low-shear static mixer" was proposed, which effectively avoids the clogging problem of high-viscosity polymers.

[0017] The liquid holding volume of the microreactor is only one-thousandth that of the batch reactor, so even in the event of a runaway reaction, the total amount released is extremely small, reducing the reaction heat risk assessment from level 3 to level 1. At the same time, the short material residence time in the high-pressure system significantly reduces the risk of decomposition and accumulation. Attached Figure Description

[0018] Figure 1 This is a flowchart of the technology of the present invention.

[0019] Figure 2 This is a schematic diagram of the synthesis process steps of hydroxyl polybutadiene in this invention. Detailed Implementation

[0020] The present invention will be further illustrated below with reference to embodiments. These embodiments are for illustrative purposes only and are not intended to limit the invention in any way. It should be understood that the described embodiments are merely some, not all, of the embodiments described in this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0021] Example 1 Figure 1 This is a flowchart of the technology of the present invention. Figure 2 This is a schematic diagram of the synthesis process steps of hydroxyl polybutadiene in this invention; like Figure 1 and Figure 2 As shown in the figure, this embodiment provides a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor. The specific steps are as follows: Preparation of raw material solution: Take 1350g of butadiene (purity 99.7%, gaseous, liquefied under pressure), 1050g of anhydrous ethanol (purity ≥99.5%), 85g of hydrogen peroxide (mass fraction 30%), and 1.5g of 2,6-di-tert-butyl-p-cresol (BHT) as antioxidant; add the above materials to a premixing tank and stir continuously for 15 minutes under nitrogen protection to obtain a homogeneous or emulsion reaction raw material solution; The microchannel reactor uses a 20m long, 2mm inner diameter 316L stainless steel coil. The inner wall of the microchannel reactor is electrochemically polished. The coil is divided into two sections. The first section is a preheating section, 2m long, placed in an 80℃ oil bath to achieve the preheating function. The second section is a polymerization section, 18m long, placed in 118℃ heat transfer oil to maintain the polymerization reaction temperature. An ethanol injection port is set in the middle of the second section of the coil (i.e., 9m from the beginning of the coil). The back pressure valve of the microreactor is set to a pressure of 2.0MPa to ensure the stability of the reaction system pressure.

[0022] The obtained reaction feed solution was pumped into the first section of the microchannel reactor at a flow rate of 30 mL / min using a high-pressure metering pump. After preheating and initiation, the material entered the second section coil for polymerization. A vibratory online viscometer was installed at the outlet of the microchannel reactor to monitor the material viscosity in real time. Simultaneously, anhydrous ethanol preheated to 80°C was injected through the ethanol replenishment port at a flow rate of 6 mL / min, with a dilution ratio of 1:0.2 between the reaction feed solution and the replenished ethanol. The effective volume of the microchannel reactor is approximately 62.8 mL (calculated based on a total length of 20 m and an inner diameter of 2 mm), the total reaction flow rate is 36 mL / min, and the theoretical residence time is approximately 1.74 minutes. Due to slight fluctuations in material density caused by temperature and pressure changes during the reaction, the actual residence time, calibrated with a tracer, is 5–8 minutes.

[0023] The reaction ran continuously and stably for 4 hours, with the pressure difference before and after the microchannel reactor never exceeding 0.6 MPa. After the reaction, samples were taken for analysis, and the results are as follows: the butadiene conversion rate was 82% (complete conversion was not achieved because the reaction did not reach 100% equilibrium conversion); the obtained hydroxyl-terminated polybutadiene (HTPB) had a hydroxyl value of 0.50 mmol / g, a number-average molecular weight (GPC detection) of 4200, a molecular weight distribution of 2.4, a viscosity of 8.2 Pa·s at 40°C, a volatile content of 0.3%, and a peroxide content of 0.02%; based on the amount of HTPB generated from the input butadiene, the product yield in this embodiment was 88% of the theoretical yield, which is 13% higher than the traditional batch reactor process (yield approximately 75%).

[0024] Example 2 The preparation of the raw material solution in this embodiment is exactly the same as in Example 1; The structure, dimensions, materials, and back pressure valve setting of the microchannel reactor are the same as in Example 1; the difference is that the second polymerization section (18m in length) of the microchannel reactor adopts segmented temperature control, specifically: the temperature of the first 6m section is controlled at 115℃, the temperature of the middle 6m section is controlled at 116℃, and the temperature of the last 6m section is controlled at 117℃; the position of the ethanol replenishment inlet is the same as in Example 1. The obtained reaction feed solution was pumped into the first section of the microchannel reactor at a flow rate of 45 mL / min using a high-pressure metering pump. After preheating and initiation, the material entered the second section coil for polymerization. A vibratory online viscometer was installed at the outlet of the microchannel reactor to monitor the material viscosity in real time. Simultaneously, anhydrous ethanol preheated to 80°C was injected through the ethanol replenishment port at a flow rate of 9 mL / min, with a dilution ratio of 1:0.3 between the reaction feed solution and the replenished ethanol. The effective volume of the microchannel reactor was the same as in Example 1, the total reaction flow rate was 45 mL / min + 9 mL / min = 54 mL / min, and the actual residence time was approximately 4.5 minutes. The reaction ran continuously and stably for 4 hours, with the pressure difference across the microchannel reactor not exceeding 0.6 MPa (consistent with Example 1). After the reaction, samples were taken for analysis, and the results were as follows: the butadiene conversion rate was 85%; the obtained hydroxyl-terminated polybutadiene (HTPB) had a hydroxyl value of 0.51 mmol / g, a number-average molecular weight (GPC detection) of 4000, a molecular weight distribution of 2.3, and a viscosity of 7.5 Pa·s at 40°C; the volatile matter content and peroxide content were consistent with Example 1; based on the amount of HTPB generated from the input butadiene, the product yield of this example was 90%, which is 15% higher than the traditional batch reactor process (yield approximately 75%).

[0025] Example 3 The preparation of the raw material solution in this embodiment is exactly the same as in Example 1.

[0026] The microchannel reactor is a microreactor with a built-in low-shear static mixer. The inner wall of the microchannel reactor is coated with a polytetrafluoroethylene anti-stick layer. The static mixer unit is 5 cm long, the reactor inner diameter is 3 mm, and the total length is 12 m. The reactor is divided into two sections. The first section is a preheating section with the same length as in Example 1, which is placed in an 80°C oil bath to achieve the preheating function. The second section is a polymerization section, which is placed in a constant 118°C heat transfer oil to maintain the polymerization reaction temperature. An ethanol injection port is provided in the middle of the second section of the reactor. The back pressure valve of the microreactor system is set to a pressure of 2.1 MPa to ensure the stability of the reaction system pressure. At the same time, the microchannel reactor is equipped with an ultrasonic auxiliary device to help reduce drag when there is an abnormal pressure difference during the reaction process.

[0027] The obtained reaction feed solution was pumped into the first stage of the microchannel reactor at a flow rate of 20 mL / min using a high-pressure metering pump. After preheating and initiation, the material entered the second stage reactor for polymerization. A vibratory online viscometer was installed at the outlet of the microchannel reactor to monitor the material viscosity in real time. Simultaneously, anhydrous ethanol preheated to 80°C was injected through the ethanol replenishment port at a flow rate of 5 mL / min, with a dilution ratio of 1:0.25 between the reaction feed solution and the replenished ethanol. The effective volume of the microchannel reactor was calculated based on a total length of 12 m and an inner diameter of 3 mm. The total reaction flow rate was 20 mL / min + 5 mL / min = 25 mL / min, and the actual residence time was approximately 6 minutes.

[0028] The reaction ran continuously and stably for 8 hours. During the later stages, the pressure difference across the microchannel reactor slowly increased to 0.8 MPa. At this point, the ultrasonic auxiliary device was triggered, operating at a frequency of 30 kHz for 30 seconds every 10 minutes. After triggering, the pressure difference dropped back to 0.5 MPa, ensuring the reaction continued stably. Samples were taken for analysis after the reaction. The results were as follows: butadiene conversion rate was 80%; the obtained hydroxyl-terminated polybutadiene (HTPB) had a hydroxyl value of 0.49 mmol / g, a number-average molecular weight (GPC detection) of 4500, a molecular weight distribution of 2.6, and a viscosity of 9.1 Pa·s at 40°C; the volatile matter content and peroxide content were consistent with Example 1; based on the amount of HTPB generated from the input butadiene, the product yield of this example was 86% of the theoretical yield, an 11% improvement compared to the traditional batch reactor process (yield approximately 75%).

[0029] Example 4 The preparation of the raw material solution in this embodiment is exactly the same as in Example 1; Four microreactors were arranged in parallel. The structure, size, material, segmentation, and ethanol replenishment inlet position of each microreactor were the same as in Example 1. The throughput of each microreactor was 25 mL / min, and the total system throughput was 100 mL / min. At the same time, the microreactor system was equipped with an automatic backwashing bypass for online backwashing when the pressure difference was abnormal during the reaction, to ensure continuous and stable operation of the reaction. The back pressure valve setting pressure of the microreactor was the same as in Example 1, which was 2.0 MPa.

[0030] The reaction raw material liquid was pumped into four parallel microreactors through high-pressure metering pumps via branch lines. The feed flow rate of each group was matched with its processing capacity, and the overall total throughput was 100 mL / min. The preheating initiation, polymerization reaction and ethanol replenishment operations of each microreactor were consistent with those in Example 1 to ensure uniform reaction conditions.

[0031] The reaction ran continuously and stably for 24 hours. Samples were taken every 2 hours during operation for testing. The average results are as follows: the average butadiene conversion rate was 84±2%; the hydroxyl-terminated polybutadiene (HTPB) produced had a hydroxyl value of 0.50±0.02 mmol / g, a number-average molecular weight (GPC detected) of 4300±200, a molecular weight distribution of 2.5±0.1, and a viscosity of 8.0±0.5 Pa·s at 40℃; the volatile matter content and peroxide content were consistent with Example 1. During operation, a slight pressure difference increase (pressure difference > 1.0 MPa) occurred, and the system automatically switched to a bypass. After 3 minutes of backwashing, it returned to normal. The total loss of yield during the entire process was approximately 5 minutes. Based on the amount of HTPB generated from the input butadiene, the overall yield of this example was 87.5%, which is 12.5% ​​higher than the traditional batch process (yield approximately 75%).

[0032] Example 5 This embodiment provides a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microreactor. Compared with Example 1, the main changes are to the microreactor structure, temperature control method, material dosage, and ethanol replenishment method, as detailed below: In this embodiment, the raw material solution is prepared as follows: 1350g of butadiene (purity ≥99.7%, gaseous, liquefied under pressure), 1080g of anhydrous ethanol (purity ≥99.5%), 90g of hydrogen peroxide (mass fraction 30%), and 1.8g of 2,6-di-tert-butyl-p-cresol (BHT) is selected as the antioxidant. The above materials are added to a premixing tank and stirred continuously for 20 minutes under nitrogen protection to obtain a milky white mixture.

[0033] The microchannel reactor adopts a stainless steel micro-sieve structure, and the inner wall of the microchannel reactor is coated with a polytetrafluoroethylene anti-stick layer. The sieve diameter is 0.3 mm, the pore spacing is 1.5 mm, and a total of 30 layers are arranged in an alternating pattern. The effective volume of the reactor reaction zone is 45 mL. The reactor is divided into three independent temperature control sections with a length ratio of 1:1:1. Each temperature control section is connected by an ethanol replenishment port. The total back pressure is set at 2.1 MPa to ensure the stability of the reaction system pressure.

[0034] The resulting milky white mixture was pumped into the first temperature-controlled section of the microchannel reactor at a total flow rate of 24 mL / min using a high-pressure metering pump. The temperature of the first section was controlled at 115°C, and the material residence time was approximately 2 minutes. No ethanol was added in this section. After the material flowed out of the first section, anhydrous ethanol preheated to 80°C was added at a flow rate of 3 mL / min through the ethanol inlet between the first and second sections. The material then entered the second temperature-controlled section, where the temperature was controlled at 116°C, and the material residence time was approximately 2 minutes. After the material flowed out of the second section, anhydrous ethanol preheated to 80°C was added at a flow rate of 2 mL / min through the ethanol inlet between the second and third sections. The material then entered the third temperature-controlled section, where the temperature was controlled at 117°C, and the material residence time was approximately 2 minutes. The total residence time for the entire reaction process was approximately 6 minutes. A vibratory online viscometer was installed at the outlet of the third temperature-controlled section to monitor the material viscosity in real time.

[0035] The reaction ran continuously and stably for 6 hours, with the pressure difference between each temperature control section remaining stable, and the pressure difference throughout the process being <0.4MPa. Online viscometer monitoring showed that the peak viscosity of the material was 4800 mPa·s, far lower than the peak viscosity of the undiluted material (>10000 mPa·s), effectively preventing reactor blockage. After the reaction, samples were taken for analysis, and the results were as follows: the butadiene conversion rate was 87%; the obtained hydroxyl-terminated polybutadiene (HTPB) had a hydroxyl value of 0.51 mmol / g, a number-average molecular weight (detected by GPC) of 4350, a molecular weight distribution of 2.3, a viscosity of 7.8 Pa·s at 40℃, a volatile content of 0.25%, and a peroxide residue of 0.01%; based on the amount of HTPB generated from the input butadiene, the product yield in this embodiment was 91%, which is 16% higher than that of the traditional batch process (yield of approximately 75%). Compared to single-point addition, multi-point segmented addition results in a more uniform system viscosity and reduces gel formation by approximately 60% (reducing the area of ​​oligomers such as dimers as measured by liquid chromatography). This method is particularly suitable for the continuous preparation of high molecular weight (Mn>4500) HTPB.

[0036] Example 6 This embodiment provides a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microreactor. Compared with Example 1, the main changes are to the initiation system, reactor segmentation method, conversion rate monitoring method, and elimination of ethanol replenishment, as detailed below: Preparation of raw material solution: Take 1350g of butadiene (liquefied, purity ≥99.7%), 1050g of anhydrous ethanol (purity ≥99.5%), 50g of hydrogen peroxide (mass fraction 30%), and 15g of di-tert-butyl peroxide (DTBP, purity 99%) (total molar amount of peroxide is equivalent to that in Example 1). The antioxidant is 1.5g of 2,6-di-tert-butyl-p-cresol (BHT). Add the above materials to a premixing tank and stir continuously under nitrogen protection to obtain a homogeneous reaction raw material solution.

[0037] The microchannel reactor uses a 316L stainless steel coil with an inner diameter of 2mm and a total length of 15m. This coil is divided into four sections: a preheating section, an initiation section, a main polymerization section, and a post-curing section. The first section is the preheating section, 1m in length, with a controlled temperature of 60℃; the second section is the initiation section, 4m in length, with a controlled temperature of 105℃; the third section is the main polymerization section, 8m in length, with a controlled temperature of 118℃; and the fourth section is the post-curing section, 2m in length, with a controlled temperature of 118℃. A flow cell is provided at the reactor outlet, connected to a Raman spectrometer (excitation wavelength 785nm, acquisition range 400~3200 cm⁻¹) for real-time calculation of butadiene conversion. The back pressure valve is set to 2.0MPa to ensure stable reaction pressure.

[0038] The prepared reaction feed solution was pumped into the first section of the microchannel reactor at a flow rate of 36 mL / min using a high-pressure metering pump. The polymerization reaction was completed by sequentially passing through the initiation section, the main polymerization section, and the post-maturation section. The Raman spectrometer collected spectral data every 30 seconds, and the real-time conversion rate was calculated based on the ratio of the peak area of ​​butadiene at 1640 cm⁻¹ (C=C stretching vibration) and ethanol at 1450 cm⁻¹. The conversion rate target was set at 85%. When the measured conversion rate was lower than 83%, the system automatically reduced the total flow rate by 5% (i.e., extended the material residence time). When the conversion rate was higher than 87%, the system automatically increased the total flow rate by 5%. No ethanol was added during the reaction. The system water content was reduced by di-tert-butyl peroxide to slow down the increase in viscosity.

[0039] The reaction ran continuously and stably for 5 hours, with the conversion rate stabilizing between 84% and 86%. The system automatically adjusted the flow rate to 32-40 mL / min; the average residence time of the material was approximately 5-7 minutes. Samples were taken for analysis after the reaction, and the results are as follows: the average butadiene conversion rate was 85%; the obtained hydroxyl-terminated polybutadiene (HTPB) had a hydroxyl value of 0.49 mmol / g, a number-average molecular weight (GPC detection) of 4550, a molecular weight distribution of 2.5, and a viscosity of 8.5 Pa·s at 40°C; based on the amount of HTPB generated from the input butadiene, the product yield in this example was 89.5%; the residual peroxide content in the product was 0.02%, comparable to the hydrogen peroxide system in Example 1; the volatile matter content was consistent with that in Example 1.

[0040] Example 7 This embodiment provides a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microreactor. Compared with Example 1, the main changes are in the solvent dosage, reactor material and size, temperature control segments, and the addition of a liquid CO2 injection unit, as detailed below: Preparation of raw material solution: Take 1350g of butadiene (liquefied, purity ≥99.7%), 900g of anhydrous ethanol (purity ≥99.5%) (reduce the amount of ethanol to increase the butadiene concentration), 85g of hydrogen peroxide (mass fraction 30%), and 1.5g of 2,6-di-tert-butyl-p-cresol (BHT) as antioxidant; add the obtained materials to a premixing tank and stir continuously under nitrogen protection to obtain a milky white mixture; separately prepare a liquid CO2 cylinder (purity ≥99.9%) and inject it into the microchannel reactor through a high-pressure plunger pump; The microchannel reactor uses Hastelloy coils with an inner diameter of 1.6 mm and a total length of 25 m, which are pressure-resistant up to 15 MPa. The coils are divided into three independent temperature control sections. The first section is a preheating / mixing section, 3m in length, with a controlled temperature of 50℃ and a controlled pressure of 9.0MPa; a T-shaped tee is installed before the reactor inlet to mix liquid CO2 with the feed liquid; The second section is the initiation section, which is 7m long, with a controlled temperature of 105℃ and a controlled pressure of 9.0MPa. The third section is the polymerization section, which is 15m long. The temperature is controlled at 118℃ and the pressure is controlled at 9.0MPa. The back pressure valve at the reactor outlet maintains the pressure at 9.0MPa. After the reaction, the pressure is reduced to release CO2, which is then condensed and recovered. The entire reactor is placed in a constant temperature chamber, and all pipelines are heated to above 50℃ to prevent premature vaporization of CO2. The prepared milky white mixture was pumped into a T-shaped tee at a flow rate of 30 mL / min using a high-pressure metering pump. Simultaneously, liquid CO2 was injected into the T-shaped tee at a flow rate of 6 mL / min using a high-pressure plunger pump. After mixing, the mixture entered the first preheating / mixing section. The total volumetric flow rate was 36 mL / min, and the CO2 molar fraction was approximately 25%. The material sequentially passed through the initiation section and the polymerization section to complete the polymerization reaction. The effective volume inside the reactor was approximately 50.3 mL. Due to CO2 compression under high pressure, the actual residence time, as determined by tracer calibration, was approximately 6.5 minutes. The temperature in the polymerization section was stabilized at 118℃, and the system pressure was stabilized at 8.9~9.1 MPa.

[0041] The reaction ran continuously and stably for 3 hours (no longer for safety reasons due to high CO2 pressure), and the pressure difference before and after the reactor remained below 0.3 MPa, indicating a significant reduction in system viscosity. After the reaction, the material was sampled and analyzed after CO2 removal under reduced pressure. The test results are as follows: the butadiene conversion rate was 91%, which was 9 percentage points higher than that of Example 1 without CO2 (because scCO2 improved the contact between butadiene and free radicals); the obtained hydroxyl-terminated polybutadiene (HTPB) had a hydroxyl value of 0.52 mmol / g, a number-average molecular weight (detected by GPC) of 4100, a molecular weight distribution of 2.2 (narrower distribution, indicating improved polymerization uniformity), a viscosity of 7.2 Pa·s at 40°C, and a volatile content of 0.2%; based on the amount of HTPB generated from the input butadiene, the product yield of this example was 93%; the product appearance was colorless and transparent, with a color (platinum-cobalt) of 35, which was better than that of Example 1 (approximately 50); the peroxide content was consistent with that of Example 1.

[0042] Example 8 This embodiment provides a method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microreactor. Based on the fundamental process of Example 1, it adds ethanol recovery, dehydration, and reuse units. The specific steps are as follows: Based on the reaction process of Example 1, a gas-liquid separator, a sedimentation / separation tank, a micro distillation column, an online dehydration column, and a blending unit are added; Gas-liquid separator: Located downstream of the microreactor outlet, operating at a pressure of 0.5 MPa, it is used to separate unreacted butadiene (gas phase). The separated butadiene is compressed and recycled back to the raw material tank for reuse. Sedimentation / separation tank: used to separate the reaction mixture, with an operating temperature of 50℃ and a settling time of 30 minutes. The lower layer of the gel enters the washing process, and the upper layer of the ethanol-water mixture (containing a small amount of oligomers) enters the distillation column. Micro-distillation column: 1.5m high, 20mm inner diameter, filled with 3mm stainless steel θ-ring packing; the bottom temperature is controlled at 85℃, the top temperature is controlled at 78℃, the operating pressure is atmospheric pressure, and the reflux ratio is 1:1; the top product is collected in the ethanol recovery storage tank after condensation, and the water and heavy components discharged from the bottom are sent to the wastewater treatment system. Online dehydration column: A 3A molecular sieve column with an effective volume of 500 mL is used to remove water from the recovered ethanol, reducing the water content to <0.5%; Blending unit: The dehydrated recovered ethanol is mixed with the newly added fresh ethanol at a ratio of 9:1, the ethanol concentration is adjusted to ≥99.5%, and then pumped back to the raw material premixing tank for recycling.

[0043] Preparation of raw material liquid: It is basically the same as in Example 1, except that when preparing the raw material, the proportion of recycled ethanol is 90% and fresh ethanol is added to supplement 10%.

[0044] The reaction was carried out continuously for 24 hours. During the operation, samples were taken from the ethanol recovery pipeline every 2 hours to test the purity, water content and organic impurities of the recovered ethanol (using GC-FID detection). The molecular sieve column was regenerated periodically (every 8 hours) by nitrogen purging and heating at 120°C for 2 hours. The remaining reaction operations (reactor parameters, feed flow rate, temperature and pressure control, etc.) were the same as in Example 1.

[0045] The reaction ran continuously and stably for 24 hours without any reactor blockage or pressure differential abnormalities; the pressure difference across the reactor remained below 0.6 MPa. The recovered ethanol tested as follows: purity 98.5–99.0% (containing trace amounts of acetaldehyde and other byproducts), water content 0.4–0.6%, and after dehydration using molecular sieves, the water content in the recycled ethanol remained stable at approximately 0.4%, meeting the process requirements.

[0046] The polymerization effect of the feed solution prepared with recycled ethanol is as follows: the butadiene conversion rate is 81-83% (slightly lower than 82-84% of the fresh ethanol system); the obtained hydroxyl-terminated polybutadiene (HTPB) has a hydroxyl value of 0.48-0.51 mmol / g, a number-average molecular weight of 4100-4400, and a molecular weight distribution of 2.4-2.6; based on the amount of HTPB generated from the input butadiene, the product yield is 86-88% (slightly lower than 88% of the fresh ethanol system, but saving about 40% in solvent costs); no abnormal peaks introduced by ethanol impurities were detected in the product, and the product quality is stable.

[0047] Comparative Example 1 (Traditional Kettle Cooker) According to the process survey form: A 1500L polymerization reactor was used, with 400kg butadiene, 320kg ethanol, 26kg hydrogen peroxide, and 0.6kg antioxidant added. The temperature was raised to 115℃, the pressure to 1.9MPa, and the reaction time was 3.5 hours. After discharge, the material was settled, washed, and dried. Sampling results showed a conversion rate of 78%, a hydroxyl value of 0.52mmol / g, a molecular weight of 4100, a distribution coefficient of 3.2, a viscosity (40℃) of 8.5Pa·s, and a yield of 75%. The hydroxyl value fluctuated between batches by ±0.08, and each batch had a small amount of gel (approximately 1-2kg) on ​​the reactor wall, requiring regular cleaning.

[0048] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A process for the synthesis of hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor, characterized in that, Includes the following steps: S1. Mix butadiene, anhydrous ethanol, and 30% hydrogen peroxide, add antioxidant, and stir under nitrogen protection to obtain a homogeneous or emulsion reaction raw material solution. S2. The reaction raw material liquid is pumped into the microchannel reactor through a high-pressure metering pump for preheating treatment. The system reaction pressure is maintained by a back pressure valve. The preheated raw material liquid undergoes polymerization at a set reaction temperature. The viscosity of the material is monitored in real time at the outlet of the microchannel reactor by an online viscometer. S3. Based on the monitoring results of the online viscometer, the preheated material continues to undergo polymerization in the microchannel reactor, maintaining the set reaction temperature and system pressure to ensure that the polymerization reaction is fully carried out. After the material completes the polymerization reaction, it flows out from the outlet of the microchannel reactor. S4. The reaction mixture flowing out of the microreactor is depressurized by the back pressure valve and sent to the gas-liquid separator to separate and recover unreacted butadiene. The liquid phase separated by the gas-liquid separator is sent to the settling tank for static separation. The lower layer of the adhesive is taken out, washed with hot water, and the solvent and trace amount of water are removed by vacuum. After adding antioxidant, it is filtered to obtain hydroxyl-terminated polybutadiene.

2. The method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor according to claim 1, characterized in that, The mass ratio of butadiene, anhydrous ethanol, and hydrogen peroxide is 100:(66~78):(3~6); The antioxidant accounts for 0.060% to 0.078% of the total material mass; The butadiene purity is ≥99.7%; The antioxidant is 2,6-di-tert-butyl-p-cresol.

3. The method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor according to claim 1, characterized in that, The microchannel reactor is one of the following: 316L stainless steel coil, microreactor with built-in low-shear static mixer, stainless steel micro-sieve structure, or Hastelloy coil. When using 316L stainless steel coils, the inner diameter is 2mm; When a stainless steel micro-sieve structure is used, the sieve diameter is 0.3 mm, the sieve spacing is 1.5 mm, and a total of 30 layers are arranged in an alternating pattern, with an effective reaction zone volume of 45 mL. When using a microreactor with a built-in low-shear static mixer, the static mixer unit is 5cm long and the reactor diameter is 3mm. When using Hastelloy coils, the inner diameter of the Hastelloy coils is 1.6 mm and the pressure resistance is 15 MPa.

4. The method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor according to claim 1, characterized in that, The back pressure of the back pressure valve is 2.0~9.0MPa.

5. The method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor according to claim 1, characterized in that, The inner wall of the microchannel reactor is electrochemically polished or coated with a polytetrafluoroethylene anti-stick layer.

6. The method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor according to claim 1, characterized in that, The online viscometer is a vibration-type online viscometer.

7. The method for synthesizing hydroxyl-terminated polybutadiene in a continuous flow microchannel reactor according to claim 1, characterized in that, When the viscosity of the material exceeds the set threshold as monitored by the online viscometer, preheated anhydrous ethanol is added to the microchannel reactor to dilute and reduce the viscosity online. The temperature of the added preheated anhydrous ethanol is 80°C, the addition rate is 2~6 mL / min, and the addition method is single-point addition or multi-point segmented addition. The dilution ratio of the raw material liquid to the added ethanol is 1:0.2~0.3.

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