A method for continuously preparing optical-grade polymethyl methacrylate by combining a micro-reactor with light control

Abstract

This application discloses a method for the continuous preparation of optical-grade polymethyl methacrylate (PMMA) using a microreactor-photocontrolled process, belonging to the field of polymer synthesis technology. The method involves transporting a reaction solution containing PMMA monomer, a photoinitiator, and a chain transfer agent into a microreactor system consisting of at least two stages of microreactors connected in series. Polymerization occurs in each stage of the microreactor under light irradiation, wherein the hydraulic diameter of the microchannels and the light intensity of the subsequent microreactors are higher than those of the preceding stage. An online viscometer is used to monitor the material viscosity in real time at the outlet of at least one microreactor, and a closed-loop control system adjusts the light intensity and / or material flow rate based on the monitoring data. This method enables precise control of the continuous polymerization process, yielding PMMA products with a narrow molecular weight distribution and stable optical properties.

Description

Technical Field

[0001] This application relates to the field of polymer material synthesis technology, and in particular to a method for the continuous preparation of optical-grade polymethyl methacrylate using a microreactor-photocontrol system. Background Technology

[0002] Polymethyl methacrylate (PMMA), due to its excellent light transmittance, weather resistance, and processability, is an indispensable key material for the manufacture of optical lenses, light guide plates, and high-end display devices. Currently, continuous bulk polymerization is commonly used in industry to produce PMMA in large-scale batch reactors. However, this traditional batch reactor process has inherent technical limitations.

[0003] First, the bulk polymerization of methyl methacrylate is a strongly exothermic reaction. However, the small heat transfer area to volume ratio of a batch reactor results in low heat dissipation efficiency and a high risk of localized hot spots forming inside the reactor. This can not only lead to safety accidents such as explosive polymerization but also cause polymer degradation or carbonization, generating impurities that affect the optical properties of the final product. Second, the complex fluid mixing patterns within a batch reactor, with prevalent backmixing and flow dead zones, result in a wide distribution of material residence time. This can lead to underpolymerization of some materials while overpolymerization of others, resulting in a broad molecular weight distribution in the final product. Its polymer dispersion index is typically greater than 2.0, severely impacting the material's optical uniformity, transmittance, and haze.

[0004] Furthermore, although some polymerization processes have proposed improving product stability through monitoring reaction parameters and implementing feedback control, traditional thermally initiated polymerization systems are sensitive to temperature changes and respond slowly to rapid, strongly exothermic systems like the bulk polymerization of methyl methacrylate (MMA), which involves a sharp increase in viscosity. When product specifications need to be adjusted, the process switchover is lengthy, resulting in poor production flexibility and excessive transition material production. Therefore, existing technologies struggle to reliably produce the highest quality optical-grade MMA. Summary of the Invention

[0005] The purpose of this application is to provide a method for the continuous preparation of optical-grade polymethyl methacrylate using a microreactor-light control system. This method aims to solve the technical problems in the prior art of continuously preparing optical-grade polymethyl methacrylate using a batch reactor, which are caused by difficulties in polymer heat management and uneven material residence time distribution, resulting in a wide molecular weight distribution of polymers, unstable optical properties of products, poor batch consistency, low production safety, and insufficient process flexibility.

[0006] To achieve the above objectives, this application provides a method for the continuous preparation of optical-grade polymethyl methacrylate using a microreactor-photocontrol system, comprising the following steps:

[0007] a) The reaction solution containing methyl methacrylate monomer, photoinitiator and chain transfer agent is transported to a microreactor system consisting of at least two microreactors connected in series;

[0008] b) In the microreactor system, polymerization reaction is carried out by light irradiation, wherein the hydraulic diameter of the microchannel of the subsequent microreactor is larger than the hydraulic diameter of the microchannel of the previous microreactor, and the light intensity applied to the subsequent microreactor is greater than the light intensity applied to the previous microreactor.

[0009] c) At the outlet of at least one microreactor, an online viscometer is installed to monitor the material viscosity in real time. Based on a preset viscosity-molecular weight-conversion rate relationship model, the light intensity and / or material flow rate are adjusted by a closed-loop control system according to the feedback of the monitoring data.

[0010] The viscosity-molecular weight-conversion rate relationship model satisfies the following equation:

[0011]

[0012] in, The kinetic viscosity of the system is measured in real time. Monomer conversion rate The number-average molecular weight of the polymer. and All of these are constants obtained through prior sampling and calibration.

[0013] d) After the material leaves the microreactor system, use physical or chemical means to rapidly quench the active free radicals in the system in order to quickly terminate the polymerization reaction;

[0014] e) Stabilize, melt-dip, and granulate the terminated material to obtain optical grade polymethyl methacrylate.

[0015] Optionally, the online viscometer is a vibratory viscometer or a rotational viscometer, and the control system sets the target viscosity range based on the monitoring data of the online viscometer.

[0016] Optionally, when the monitored viscosity deviates from the target viscosity range, the control system adjusts the parameters of light intensity and / or material flow rate within 10 seconds.

[0017] Optionally, the physical means may be to irradiate with ultraviolet light with a wavelength of less than 300 nm; or, the chemical means may be to inject a photoactivated chemical terminator into the material and irradiate it with a light source.

[0018] Optionally, the hydraulic diameter of the microchannel in the second-stage microreactor is 1.2-3 times that of the microchannel in the first-stage microreactor, and the light intensity of the second-stage microreactor is 1.5-5 times that of the first-stage microreactor.

[0019] Optionally, a static mixer may be provided between the microreactors at each stage.

[0020] Optionally, the microreactor is made of quartz glass, sapphire, or transparent polycarbonate; the illumination wavelength is matched with the absorption wavelength of the photoinitiator, and is 365nm, 385nm, 405nm, or 450nm.

[0021] Optionally, the photoinitiator is selected from one or more of 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexylphenyl ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholino)-1-propanone, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and the amount added is 0.01%-0.5% of the mass of methyl methacrylate monomer.

[0022] Optionally, the chain transfer agent is selected from one or more of n-butanethiol, isobutanethiol, n-octanethiol, isooctanethiol, n-dodecylthiol, and tert-dodecylthiol, and the amount added is 0.05%-0.5% of the mass of methyl methacrylate monomer.

[0023] Optionally, the hydraulic diameter of the microchannels in the first-stage microreactor is 100-1000 micrometers, the reaction temperature is 20-60℃, the residence time is 0.5-10 minutes, and the conversion rate in the prepolymerization stage is 10%-30%; the hydraulic diameter of the microchannels in the second-stage microreactor is 200-2000 micrometers, the reaction temperature is 40-80℃, the residence time is 2-20 minutes, and the overall conversion rate is 60%-85%.

[0024] Compared with the prior art, this application has the following beneficial effects:

[0025] 1. This invention ensures uniform material residence time through the laminar flow effect within the microreactor. Combined with the precise initiation of photocontrolled polymerization and the constant temperature field resulting from the excellent heat transfer capability of the microreactor, the polymer chain growth environment is highly consistent. As a result, the polymethyl methacrylate product produced has an extremely narrow molecular weight distribution and a polymer dispersion index of less than 1.5, which significantly reduces light scattering within the material, giving the product ultra-high light transmittance and extremely low haze.

[0026] 2. The large specific surface area of ​​the microreactor ensures that the exothermic energy of strong polymerization is instantly discharged, and the reaction temperature fluctuation can be controlled within a very small range, thereby fundamentally eliminating the risk of local overheating and thermal runaway and greatly improving production safety.

[0027] 3. The closed-loop control system for online viscosity monitoring and feedback adjustment of light intensity introduced in this invention utilizes the instantaneous response of photopolymerization to changes in light intensity, which can correct any disturbances in the production process in real time, ensuring high stability of product molecular weight and minimal batch-to-batch differences during continuous production.

[0028] 4. By adjusting the light intensity and residence time, products with different molecular weight specifications can be quickly switched within tens of minutes to meet market demands. Furthermore, the polymerization process is carried out at a lower temperature, significantly reducing energy consumption compared to traditional high-temperature thermal polymerization. Simultaneously, the use of a metal-free organic photoinitiator system avoids the impact of residual metal ions on the product's optical properties, resulting in high product purity. Attached Figure Description

[0029] Figure 1 This application provides a process flow diagram of a method for the continuous preparation of optical-grade polymethyl methacrylate using a microreactor-photocontrol system.

[0030] Figure 2 This is a schematic diagram of a system architecture for the continuous preparation of optical-grade polymethyl methacrylate using a microreactor-photocontrol system, provided as an embodiment of this application.

[0031] Figure 3 The material flow logic block diagram of the multi-stage series microreactor provided in the embodiments of this application.

[0032] Figure 4 A block diagram illustrating the working principle of the online feedback control system provided in this application embodiment.

[0033] Figure Labeling Explanation: S10: Raw material preparation; S20: Photocontrolled prepolymerization; S30: Deep polymerization; S40: Online monitoring and feedback control; S50: Phototermination and stabilization; S60: Melt devolatilization and granulation; 10: Raw material tank; 20: Metering pump; 31: First-stage microreactor; 311: First microchannel; 32: First-stage LED light source; 40: Static mixer; 51: Second-stage microreactor; 511: Second microchannel; 52: Second-stage LED light source; 60: Online viscometer; 70: Control system; 80: Phototermination device; 90: Twin-screw extruder; SP: Target viscosity; C: Controller; A1: LED driver; A2: Pump controller; P: Polymerization process.

[0034] Unless otherwise stated, "parts" in this document refers to parts by weight.

[0035] Number average molecular weight and polymer dispersion index were determined by gel permeation chromatography; transmittance and haze were determined on a 3 mm thick sample; residual monomer content was determined by gas chromatography; and yellowness index was determined by conventional polymer optical property testing methods.

[0036] The “optical grade polymethyl methacrylate” mentioned in this article refers to a product that has one or more of the following performance indicators: polymer dispersion index not higher than 1.5, light transmittance of a 3mm thick sample not lower than 92%, haze not higher than 0.5%, and residual monomer content preferably lower than 200ppm. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and specific embodiments. It is to be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0038] Example 1

[0039] This embodiment provides a core process for the continuous preparation of optical-grade polymethyl methacrylate using a light-controlled microreactor, and compares it with the traditional batch thermal polymerization process to illustrate the beneficial effects of the technical solution of this application.

[0040] Figure 1 This is an overall process flow diagram of a microreactor-photocontrol co-processing method for the continuous preparation of optical-grade polymethyl methacrylate (PMMA) according to an embodiment of this application. Figure 1 As shown, the method mainly includes: step S10, raw material preparation; step S20, light-controlled prepolymerization; step S30, deep polymerization; step S40, online monitoring and feedback control; step S50, light termination and stabilization; and step S60, melt devolatilization and granulation.

[0041] Figure 2 The diagram shows a system architecture for implementing the above method. Specifically, the system includes a raw material tank 10, a metering pump 20 as a raw material conveying device, a microreactor system consisting of a first-stage microreactor 31 and a second-stage microreactor 51 connected in series, a first-stage LED light source 32 and a second-stage LED light source 52 corresponding to each stage of the microreactor (the two together constitute the first light source), a static mixer 40, an online viscometer 60 located at the outlet of the second-stage microreactor 51, a control system 70 electrically connected to each component, a light termination device 80 located downstream of the microreactor system, and a twin-screw extruder 90 for post-processing.

[0042] In this embodiment, the specific preparation process is described as follows:

[0043] First, the raw materials are prepared (corresponding to...) Figure 1Step S10 involves refining the industrial-grade methyl methacrylate monomer using molecular sieve adsorption and nitrogen bubbling to reduce its moisture content to below 50 mg / kg and dissolved oxygen concentration to below 1 ppm. This aims to eliminate the inhibitory effects of water and oxygen on free radical polymerization, thereby ensuring the smooth progress of the reaction and the regularity of the polymer chains. Subsequently, in a clean, nitrogen-protected raw material tank 10, 100 parts by weight of the pretreated methyl methacrylate monomer are mixed with 0.1 parts by weight of the photoinitiator 2,2-dimethoxy-2-phenylacetophenone and 0.2 parts by weight of the chain transfer agent n-dodecyl mercaptan. Mechanical stirring or circulating pumping is used to ensure that all components are uniformly mixed in the raw material tank 10, forming a clear and transparent reaction solution. It should be noted that the role of the chain transfer agent is to regulate the molecular weight of the polymer through chain transfer reactions during polymerization.

[0044] Next, light-controlled prepolymerization is performed (corresponding to...) Figure 1 (Step S20). The metering pump 20 is started to precisely pump the reaction solution from the raw material tank 10 into the first-stage microreactor 31 at a constant flow rate of 10 mL / min. This first-stage microreactor 31 is a plate-type microreactor made of quartz glass, with first microchannels 311 of specific dimensions etched inside (the material flow logic is described in [reference]). Figure 3 The quartz glass material has high transmittance for the ultraviolet light required to initiate polymerization. In this embodiment, the cross-sectional dimensions of the first microchannel 311 are 500 micrometers wide and 200 micrometers deep. A first-stage LED light source 32 is integrated outside the microreactor. This light source consists of multiple light-emitting diode arrays and can provide uniform surface illumination to the microchannel region. The control system 70 controls the first-stage LED light source 32 to emit ultraviolet light with a center wavelength of 365nm and precisely controls its light intensity at 10mW / cm². At the same time, the reaction temperature of the first-stage microreactor 31 is kept constant at 40°C by jacket temperature control. Under these conditions, the residence time of the reaction liquid in the first-stage microreactor 31 is calculated to be 5 minutes. After this stage of prepolymerization, the conversion rate of methyl methacrylate monomer reaches approximately 20%, and the viscosity of the system begins to rise initially, but still maintains good fluidity.

[0045] Then, deep aggregation is performed (corresponding to...) Figure 1 (Step S30 in the process). The prepolymer material flowing out of the first-stage microreactor 31 first passes through a static mixer 40, which eliminates radial concentration and temperature gradients that may be caused by uneven laminar velocity distribution, thus achieving higher homogeneity of the material before entering the next stage reactor. Subsequently, the material enters the second-stage microreactor 51 for deep polymerization. The structure of the second-stage microreactor 51 is similar to that of the first-stage microreactor 31 (correspondence with...). Figure 3(As shown), but its internal second microchannel 511 has a larger hydraulic diameter, specifically, its cross-sectional dimensions are 800 micrometers wide and 400 micrometers deep. It can be calculated that the hydraulic diameter of the first microchannel 311 of the first-stage microreactor 31 is approximately 285 micrometers, while the hydraulic diameter of the second microchannel 511 of the second-stage microreactor 51 is approximately 533 micrometers. The hydraulic diameter of the microchannels in the second-stage microreactor is approximately 1.87 times that of the first-stage, a ratio falling within the preferred range of 1.2 to 3 times. The use of progressively increasing channel sizes is to accommodate the rapid increase in material viscosity during polymerization, thereby effectively reducing flow resistance and preventing channel blockage due to excessive viscosity. Correspondingly, the system is equipped with an independently controllable second-stage LED light source 52. The control system 70 controls the second-stage LED light source 52 to also emit 365nm ultraviolet light, but the light intensity is increased to 50mW / cm², which is 5 times the light intensity of the first stage, a ratio falling within the preferred range of 1.5 to 5 times. The progressively increasing light intensity is to compensate for the decrease in polymerization rate caused by increased viscosity and reduced monomer concentration, ensuring a highly efficient reaction even in the deep polymerization stage. Simultaneously, the reaction temperature of the second-stage microreactor 51 is controlled at 60°C. Under these conditions, the residence time of the material in the second-stage microreactor 51 is approximately 10 minutes. After this stage, the total monomer conversion reaches approximately 75%.

[0046] Subsequently, online monitoring and feedback control are implemented (corresponding to...) Figure 1 (Step S40 in the process). An online viscometer 60 (e.g., a vibratory online viscometer or a rotational online viscometer) is installed on the outlet pipe of the second-stage microreactor 51 to monitor the viscosity of the effluent material in real time and continuously. Viscosity, as a comprehensive reflection of polymer molecular weight and conversion rate, is a key indicator for evaluating product quality. The online viscometer 60 transmits the measured real-time viscosity data (e.g., 8000 mPa·s under the target steady state) to the control system 70 via a signal line. Figure 4 The working principle of the closed-loop control system is illustrated. The control system 70 has a preset target viscosity value SP and incorporates a viscosity-molecular weight-conversion rate relationship model based on extensive experimental data. This model is established by conducting multiple offline polymerization experiments under different light intensities and flow rates. The molecular weight of the polymer at different stages is determined using gel permeation chromatography (GPC) by sampling, and the conversion rate is determined by gravimetric or gas chromatography. Combined with the viscosity values ​​recorded by an online viscometer at corresponding times, a polynomial fitting or multivariate nonlinear regression algorithm is used to obtain the corresponding relationship curve or empirical formula between viscosity and conversion rate / molecular weight at the target temperature, which is then pre-loaded into the control system 70 as a comparison benchmark.

[0047] Specifically, let the kinetic viscosity of the system measured in real time be... The monomer conversion rate is The number-average molecular weight of the polymer is The preloaded relational model satisfies the following equation:

[0048]

[0049] in, and All values ​​are constants obtained through prior sampling and calibration. Controller C will transmit the measured values ​​from the online viscometer 60. A deviation signal is generated by comparing it with the target viscosity SP. If a deviation exists, controller C will generate an adjustment command within a very short time (e.g., less than 10 seconds) based on a preset proportional-integral-derivative (PID) control algorithm. In this embodiment, the closed-loop control system prioritizes calculating the adjustment compensation amount for the light intensity. Its governing equation is:

[0050]

[0051] In the formula, , , These are the proportional, integral, and differential gain coefficients, respectively. Under the specific conditions of this embodiment, after fitting and calibration, the constants are... Example value , The value is 3.4; the controller parameter is set as follows (example setting: proportional gain) The integral gain is 0.05. The differential gain is 0.01. The flow conversion gain constant is 0.005. The value is 0.002. (Generated) The instructions are output to the LED driver A1. When the light intensity adjustment reaches the hardware limit, the system then converts the remaining deviation into a flow regulation instruction and outputs it to the pump controller A2.

[0052] When the light intensity Reaching the hardware limit At that time, the system calculates the effective viscosity residual. The flow rate regulation of the feed pump is generated using the following formula. :

[0053]

[0054] in, This is the flow conversion gain constant. The negative sign indicates that when the viscosity is high, the residence time of the material in the illuminated zone is shortened by increasing the flow rate, thereby suppressing further increases in conversion rate and viscosity.

[0055] As another embodiment of this application, an online refractometer can also be set up in the prepolymerization stage to monitor the change in refractive index of the outlet material in order to calculate the monomer conversion rate; online refractometer monitoring is preferably used for conversion rate control in the prepolymerization stage, while online viscosity monitoring is preferably used to implement closed-loop feedback control at the outlet of the final microreactor.

[0056] Subsequently, optical termination and stabilization are performed (corresponding to...) Figure 1 (Step S50). The polymer material flowing from the online viscometer 60, having reached the target conversion rate, is conveyed to the light termination device 80. This device is a flow cell with a quartz window, externally equipped with a high-intensity short-wavelength ultraviolet lamp as a second light source. In this embodiment, the ultraviolet lamp emits ultraviolet light with a center wavelength of 280 nm, a wavelength less than 300 nm, and a light intensity as high as 100 mW / cm². The material is irradiated in this device for approximately 30 seconds. It is understood that this high-energy short-wavelength ultraviolet light can efficiently quench residual active free radical chains in the system, causing the polymerization reaction to "freeze" instantaneously, thereby precisely fixing the molecular weight and molecular weight distribution of the product and preventing unwanted post-polymerization during subsequent storage and processing. After terminating the reaction, appropriate amounts of heat stabilizer and ultraviolet absorber are added online to the material to improve the service life of the final product.

[0057] Finally, melt devolatilization and granulation are carried out (corresponding to...) Figure 1 (Step S60) The stabilized polymer is fed into a twin-screw extruder 90. Inside the extruder, the material is heated to a molten state, with the temperature controlled between 200°C and 240°C. Simultaneously, a high vacuum of -0.08 MPa is established at the extruder's exhaust port using a vacuum pump. Under the influence of high temperature and high vacuum, unreacted monomers, chain transfer agents, and other low-molecular-weight volatiles remaining in the polymer melt are effectively vaporized and extracted, resulting in a residual monomer content of less than 180 ppm in the final product. The fully devolatilized molten material is extruded into strips through a die, cooled in a water bath, cut into granules by a pelletizer, and finally dried to obtain the final optical-grade polymethyl methacrylate (PMMA) granules.

[0058] The product prepared in this embodiment was subjected to performance testing, and the results are as follows: the number average molecular weight is 82,000, the polymer dispersion index is 1.38, the light transmittance of the 3mm thick sample (according to ASTM D1003 standard) is 93.2%, the haze is 0.3%, the residual monomer content is 180ppm, and the yellowness index is 1.2.

[0059] Example 2

[0060] This embodiment further adds a small amount of copolymerizable monomer to the methyl methacrylate system to illustrate the applicability of this method to polymerizable systems based on MMA. This embodiment aims to verify the flexibility of the method in preparing polymethyl methacrylate products of different specifications (specifically, higher molecular weights). As an optional implementation, the system architecture used in this embodiment is the same as that in Example 1. Figure 2 The results are basically the same, but a three-stage series microreactor system was used in the reactor configuration, and the raw material formulation and process parameters were adjusted.

[0061] Raw material preparation: 100 parts by weight of pretreated methyl methacrylate monomer, 0.05 parts by weight of photoinitiator phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, 0.1 parts by weight of chain transfer agent n-octyl mercaptan, and 5 parts by weight of comonomer methyl acrylate were mixed evenly. Compared with Example 1, this example reduces the amount of chain transfer agent because a lower concentration of chain transfer agent results in a longer average polymer chain length, which is beneficial for obtaining products with higher molecular weights. Simultaneously, the introduction of methyl acrylate as a comonomer allows for fine-tuning of the material's toughness and other physical properties. The photoinitiator was replaced with phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, a highly efficient initiator that also absorbs light in the visible region.

[0062] Process Execution: The above reaction solution was pumped into a three-stage tandem microreactor system at a low flow rate of 8 mL / min. The reduced flow rate aimed to extend the total residence time of the materials in the reactor, thereby achieving higher conversion rates and molecular weights. First-stage prepolymerization: The reactor microchannel hydraulic diameter was 300 μm, the reaction temperature was controlled at 35°C, the light intensity was 8 mW / cm², and the material residence time was approximately 6 minutes. Second-stage deep polymerization: The reactor microchannel hydraulic diameter was 600 μm, the reaction temperature was controlled at 55°C, the light intensity was increased to 40 mW / cm², and the material residence time was approximately 15 minutes. Third-stage deep polymerization: The reactor microchannel hydraulic diameter was 1000 μm, the reaction temperature was further increased to 65°C, and the light intensity was further increased to 80 mW / cm², with a material residence time of approximately 12 minutes. It should be noted that the entire polymerization process also followed the principle of gradually increasing microchannel hydraulic diameter and light intensity to accommodate the continuous increase in viscosity and changes in reaction rate. The total residence time of the materials was significantly extended to 33 minutes.

[0063] The subsequent online monitoring, photo-termination, stabilization, melt devolatilization and granulation steps are basically the same as in Example 1.

[0064] The final product performance is as follows: number average molecular weight is 125,000, polymer dispersion index is 1.42, melt flow rate (230℃, 3.8kg) is 2.5g / 10min, light transmittance is 92.8%, and haze is 0.4%.

[0065] The results of this embodiment show that by simply adjusting the raw material formulation (such as the amount of chain transfer agent) and process parameters (such as residence time and reaction order), the method provided in this application can flexibly and conveniently produce high-performance polymethyl methacrylate products with different molecular weight specifications, demonstrating extremely high production flexibility.

[0066] Example 3

[0067] This embodiment aims to verify the ability of the method of this application to prepare another specification (specifically, low molecular weight, high flowability) of polymethyl methacrylate product, and to demonstrate the potential of the process to achieve high yield through parallel scale-up.

[0068] Raw material preparation: 100 parts by weight of pretreated methyl methacrylate monomer were mixed thoroughly with 0.2 parts by weight of photoinitiator 2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholino)-1-propanone and 0.3 parts by weight of chain transfer agent isooctyl mercaptan. Compared with Example 1, this example significantly increased the amount of chain transfer agent, the purpose of which was to more effectively control the length of the polymer chain, thereby obtaining a product with lower molecular weight and higher flowability.

[0069] Process Execution: In this embodiment, a parallel scale-up strategy was adopted to improve production efficiency. Specifically, the first-stage prepolymerization stage used four sets of microreactors with the same specifications as the first-stage microreactor 31 in Example 1, operating in parallel. The feed flow rate of each set of microreactors was 15 mL / min, resulting in a total feed flow rate of 60 mL / min, thus verifying the good scale-up potential of this process. First-stage prepolymerization: The four sets of parallel microreactors operated at 45°C and 20 mW / cm² light intensity, reducing the material residence time to 3 minutes. Deep polymerization: The materials flowing out of the four sets of first-stage microreactors merged and entered a single second-stage microreactor with greater processing capacity. This reactor operated at 65°C and 100 mW / cm² light intensity, with a material residence time of 8 minutes.

[0070] The subsequent processing steps are the same as in Example 1.

[0071] The final product performance is as follows: number average molecular weight of 62,000, polymer dispersion index of 1.35, melt flow rate (230℃, 3.8kg) of up to 15g / 10min, light transmittance of 93.0%, and haze of 0.35%. This high-flow-rate product is very suitable for precision injection molding applications with high filling performance requirements.

[0072] This embodiment demonstrates that by increasing the chain transfer agent concentration and adjusting process parameters, the method of this application can effectively prepare products with low molecular weight and high flowability. Furthermore, by combining microreactors in parallel, linear scale-up of production capacity can be easily achieved, providing a feasible path for industrial production.

[0073] In other embodiments of this application, the photoinitiator may also be other types well known to those skilled in the art, such as 1-hydroxycyclohexylphenyl ketone, and the chain transfer agent may also be one or more of n-butanethiol, isobutanethiol, tert-dodecylthiol, etc.

[0074] Example 4

[0075] This embodiment aims to verify, through simulating process disturbances, the actual effectiveness of the core online monitoring and feedback control system in maintaining product quality stability.

[0076] Experimental setup: First, the entire production system was started according to the system configuration and process parameters described in Example 1. The system ran stably for 30 minutes. During this period, the target viscosity SP of the control system 70 was set to 8000 mPa·s, and the reading of the online viscometer 60 remained stable within the range of 8000 ± 200 mPa·s. At this time, a sample was collected from the outlet of the twin-screw extruder 90, and its polymer dispersion index was measured to be 1.38, consistent with the results of Example 1.

[0077] Introducing a disturbance: At the 31-minute mark of system operation, a typical disturbance was artificially introduced into the process. Specifically, the feed flow rate of metering pump 20 was instantaneously increased by 10%, from 10 mL / min to 11 mL / min, by manually adjusting pump controller A2. This disturbance simulates pump fluctuations or operational errors that might occur in actual industrial production. The increased flow rate means a shorter residence time of the material in the microreactor system; without intervention, this would lead to a decrease in polymerization conversion, thereby reducing the product's molecular weight and viscosity.

[0078] System Response: Within seconds of the increased flow rate, the online viscometer 60 located at the outlet of the second-stage microreactor 51 immediately detected a decreasing trend in the material viscosity. This viscosity measurement was transmitted to the control system 70 in real time. The controller C of the control system 70 compared this decreasing measurement with the set target viscosity of 8000 mPa·s and detected a negative deviation. The system responded immediately; within 10 seconds, the controller C calculated the required compensation amount according to its internal algorithm and sent a command to the driver A1 of the second-stage LED light source 52, automatically increasing its light intensity from the original 50 mW / cm² to approximately 58 mW / cm². The increase in light intensity instantly accelerated the polymerization reaction rate, thereby effectively compensating for the conversion loss caused by the shortened residence time.

[0079] System Stabilization: After the light intensity was automatically increased, the reading of the online viscometer 60 stopped decreasing and began to rise. After a dynamic adjustment process of approximately 1 to 2 minutes, the viscosity reading recovered and stabilized within the target range of 8000 ± 200 mPa·s. Subsequently, during the 40th to 45th minute of the system's re-stabilization period, product samples were collected again.

[0080] Results analysis: Performance testing was performed on the samples collected after the system stabilized following the disturbance. The number-average molecular weight and polymer dispersion index were not significantly different from those of the samples collected before the disturbance, and the polymer dispersion index remained at around 1.38.

[0081] As demonstrated by this embodiment, the online feedback control system proposed in this application can effectively and quickly respond to and correct external disturbances in the process. By adjusting the instantaneous closed-loop intensity of light, it ensures that even when process parameters fluctuate, the key quality indicators of the final product (such as molecular weight and molecular weight distribution) remain highly stable and consistent. This is crucial for achieving continuous and large-scale production of high-quality products.

[0082] Example 5

[0083] This embodiment aims to verify the necessity and effectiveness of the optical termination technology in step S50 of this application for stabilizing product performance.

[0084] Experimental Design: This embodiment uses a comparative experiment for illustration. First, the polymerization reaction was carried out according to the process flow and parameters of Example 1. At the outlet of the second-stage microreactor 51, before entering the light termination device 80, a portion of the polymer sample was taken out through a three-way valve. At this time, the total monomer conversion rate of the sample was approximately 75%, and the system still contained a large number of active free radicals, indicating that it was in a "living" polymerization state.

[0085] Grouping: The extracted active polymer samples were quickly divided into two groups: an experimental group and a control group. Experimental group: This group of samples was immediately pumped into an independent light-permeable cell with the same structure as the light termination device 80, and irradiated for 30 seconds with 280nm wavelength ultraviolet light at an intensity of 100mW / cm² according to the conditions of the light termination step in Example 1. Control group: This group of samples underwent no treatment and remained in its active state.

[0086] Simulated storage and analysis: The treated experimental group samples and the untreated control group samples were simultaneously placed in a sealed container at 60°C under nitrogen protection and kept at this temperature for 2 hours. This condition was designed to simulate the temporary storage process that the materials might undergo in subsequent stabilization tanks or pipelines after leaving the reactor. After 2 hours, samples from both groups were taken, and their molecular weight and polymer dispersion index were determined using gel permeation chromatography.

[0087] Experimental Results: The test results showed that the number-average molecular weight and polymer dispersion index of the experimental group (after phototermination treatment) remained essentially unchanged compared to when it was taken out 2 hours prior. This indicates that the phototermination step successfully quenched the active free radicals and effectively "frozen" the polymerization reaction. In contrast, the number-average molecular weight of the control group (without phototermination treatment) increased significantly compared to 2 hours prior, and the polymer dispersion index also widened significantly. This is because at a temperature of 60°C, the remaining active free radicals in the system continued to initiate monomer polymerization, leading to continued polymer chain growth, and due to the uncontrollability of the post-polymerization process, the molecular weight distribution further deteriorated.

[0088] Conclusion: The results of this embodiment clearly demonstrate that employing the photo-termination step described in this application is crucial after the polymerization reaction reaches the target conversion rate. It can quickly and effectively terminate the polymerization reaction, preventing uncontrollable post-polymerization of the material in subsequent processes, thereby ensuring that the properties of the product flowing out of the reactor can be precisely fixed until final granulation. This is a key step in ensuring a high degree of consistency in the performance of the final product across batches.

[0089] As an alternative approach, phototermination can also be achieved chemically. For example, after the material leaves the microreactor system, a photoactivated chemical terminator, such as 2,2,6,6-tetramethylpiperidine nitride, can be continuously injected into it at a concentration of 0.05% to 0.5% of the total mass of the reaction solution. After mixing, the mixture flows through a flow cell equipped with an external ultraviolet or near-ultraviolet light source with a center wavelength of 365 nm to 405 nm. Under light intensity of 5 mW / cm² to 30 mW / cm² for 10 to 60 seconds, the chemical terminator rapidly releases polymerization inhibitor radicals upon photoexcitation, thus achieving the same effect of rapidly terminating the polymerization reaction.

[0090] Example 6

[0091] This embodiment aims to verify the implementation effect of the method of this application when using the combination of the lower limit values ​​of the various process parameters described in this application, and to further prove the stability and applicability of this process under extreme boundary conditions.

[0092] Raw material preparation: In a clean, nitrogen-protected raw material tank, 100 parts by weight of pretreated methyl methacrylate monomer were mixed thoroughly with 0.01 parts by weight of photoinitiator 2,2-dimethoxy-2-phenylacetophenone and 0.05 parts by weight of chain transfer agent n-dodecyl mercaptan to form a clear and transparent reaction solution. Compared with Example 1, this example uses the minimum addition amounts of photoinitiator and chain transfer agent specified in this application.

[0093] Process execution: The above reaction solution is continuously pumped into a two-stage series microreactor system. To explore the shortest process cycle, this embodiment adopts the upper limit of flow rate and the lower limit of channel size set within the scope of protection of this application.

[0094] First-stage prepolymerization: The microchannel hydraulic diameter of the first-stage microreactor is 100 micrometers, and the reaction temperature is controlled at 20℃ via jacket temperature control. The control system sets the illumination intensity of the first-stage LED light source to 5mW / cm², and adjusts the feed flow rate so that the material residence time in this stage is only 0.5 minutes. After this prepolymerization stage, the monomer conversion rate is approximately 10%.

[0095] Second-stage deep polymerization: After passing through a static mixer, the material enters the second-stage microreactor. The hydraulic diameter of the microchannels in the second-stage microreactor is 200 micrometers (twice that of the first-stage microreactor, within the preferred range of 1.2-3 times), and the reaction temperature is increased and controlled at 40°C. Correspondingly, the control system sets the illumination intensity of the second-stage LED light source to 7.5 mW / cm² (1.5 times that of the first-stage illumination intensity), and the residence time of the material in this stage is 2 minutes. After this deep polymerization stage, the total conversion rate of monomers reaches approximately 60%.

[0096] The subsequent online monitoring and feedback adjustment, light termination and stabilization (using ultraviolet light irradiation with a wavelength of less than 300 nm), melt devolatilization and granulation steps are basically the same as in Example 1.

[0097] The final product performance is as follows: number average molecular weight is 158,000, polymer dispersion index is 1.45, light transmittance of 3mm thick sample is 92.5%, haze is 0.4%, and residual monomer content is 160ppm.

[0098] The results of this embodiment show that even under extremely short residence times, low reaction temperatures, and extremely low lower limit boundary conditions for initiator and chain transfer agent concentrations, the microreactor-photocontrol system provided in this application can still achieve continuous and stable operation of the polymerization process, effectively control the exothermic polymerization, and produce polymethyl methacrylate products with narrow molecular weight distribution and excellent optical properties, which fully demonstrates the reliability of the present invention under a wide process window.

[0099] Although the residence time in this embodiment is short and the conversion rate is at the lower limit, the chain transfer effect is weakened because the amount of chain transfer agent added is also at the lower limit. Therefore, the resulting polymer still exhibits a high number-average molecular weight.

[0100] Example 7

[0101] This embodiment aims to verify the implementation effect of the method of this application when using the combination of the upper limit values ​​of the various process parameters described in this application. It echoes Example 6 and fully verifies the breadth of the process operation window and the ability to handle high viscosity systems of this application.

[0102] Raw material preparation: In a clean, nitrogen-protected raw material tank, 100 parts by weight of pretreated methyl methacrylate monomer, 0.5 parts by weight of photoinitiator 1-hydroxycyclohexylphenyl ketone, and 0.5 parts by weight of chain transfer agent tert-dodecyl mercaptan were mixed thoroughly to form a clear and transparent reaction solution. Compared to Example 6, this example uses the maximum amount of photoinitiator and chain transfer agent specified in this application.

[0103] Process execution: The above reaction solution is continuously pumped into a two-stage series microreactor system. To accommodate the rapidly increasing material viscosity at high conversion rates, this embodiment employs the lower limit of flow rate and upper limit of channel size settings within the scope of this application.

[0104] First-stage prepolymerization: The microchannel hydraulic diameter of the first-stage microreactor is 1000 micrometers, and the reaction temperature is controlled at 60℃ via jacket temperature control. The control system sets the illumination intensity of the first-stage LED light source to 10mW / cm², and adjusts the feed flow rate to extend the residence time of the material in this stage to 10 minutes. After this prepolymerization stage, the monomer conversion rate reaches approximately 30%.

[0105] Second-stage deep polymerization: After passing through a static mixer, the material enters the second-stage microreactor. The hydraulic diameter of the microchannels in the second-stage microreactor is 2000 micrometers (twice that of the first-stage microreactor, within the preferred range of 1.2-3 times), and the reaction temperature is further increased and controlled at 80°C. Accordingly, due to the upper limit of the hydraulic diameter of the second-stage microchannels and the extremely high viscosity of the system, the control system sets the illumination intensity of the second-stage LED light source to a significant increase to 50 mW / cm² (five times that of the first-stage illumination intensity, reaching the upper limit of the proportion specified in this application), and the residence time of the material in this stage is 20 minutes. After this deep polymerization stage, the total conversion rate of the monomer reaches approximately 85%.

[0106] The subsequent online monitoring and feedback adjustment, light termination and stabilization, melt devolatilization and granulation steps are basically the same as in Example 1.

[0107] The final product performance is as follows: number average molecular weight is 55,000 (the molecular weight is relatively low due to the chain transfer agent addition reaching the upper limit), polymer dispersion index is 1.48, light transmittance of 3mm thick sample is 92.2%, haze is 0.45%, and residual monomer content is 140ppm.

[0108] The results of this embodiment show that under conditions of long residence time, high reaction temperature, and upper limit limits of parameters for high concentrations of initiator and chain transfer agent, the system reacts violently and has extremely high viscosity at the end. However, the microreactor system of this application, with its stepwise scaling-up design of channel size and precise matching of light intensity by the closed-loop control system, can still effectively overcome the mass and heat transfer resistance of high-viscosity fluids, avoid the risks of local overheating and channel blockage, and successfully complete the continuous preparation with high conversion rate, further confirming that the process of this invention has extremely strong robustness and a wide operating range.

[0109] Example 8

[0110] This embodiment aims to verify that the method of this application can still be stably implemented when the hydraulic diameter of the microchannel of the second-stage microreactor is 1.2 times that of the first-stage microreactor.

[0111] The steps of raw material preparation, online monitoring, phototermination, stabilization, melt devolatilization and granulation are basically the same as those in Example 1.

[0112] Process execution: The reaction solution was continuously pumped into a two-stage tandem microreactor system. The first-stage microreactor had a microchannel hydraulic diameter of 500 micrometers, a reaction temperature of 40°C, a light intensity of 10 mW / cm², and a residence time of 5 minutes. The second-stage microreactor had a microchannel hydraulic diameter of 600 micrometers, 1.2 times that of the first-stage microchannel, a reaction temperature of 60°C, a light intensity of 20 mW / cm², and a residence time of 10 minutes. After the second-stage deep polymerization, the total monomer conversion rate reached approximately 72%.

[0113] The final product performance is as follows: number average molecular weight is 86,000, polymer dispersion index is 1.40, light transmittance of 3mm thick sample is 93.0%, and haze is 0.35%.

[0114] This embodiment demonstrates that when the hydraulic diameter of the microchannel in the second-stage microreactor is 1.2 times that of the first stage, it is still possible to balance flow resistance control and polymerization efficiency, achieving continuous and stable preparation.

[0115] Example 9

[0116] This embodiment aims to verify that the method of this application can still be stably implemented when the hydraulic diameter of the microchannel of the second-stage microreactor is three times that of the first-stage microreactor.

[0117] The steps of raw material preparation, online monitoring, phototermination, stabilization, melt devolatilization and granulation are basically the same as those in Example 1.

[0118] Process execution: The reaction solution was continuously pumped into a two-stage series microreactor system. The first-stage microreactor had a microchannel hydraulic diameter of 400 micrometers, a reaction temperature of 40°C, a light intensity of 10 mW / cm², and a residence time of 5 minutes. The second-stage microreactor had a microchannel hydraulic diameter of 1200 micrometers, three times that of the first-stage microchannel, a reaction temperature of 60°C, a light intensity of 50 mW / cm², and a residence time of 10 minutes. After the second-stage deep polymerization, the total monomer conversion rate reached approximately 76%.

[0119] The final product performance is as follows: number average molecular weight is 80,000, polymer dispersion index is 1.43, light transmittance of 3mm thick sample is 92.9%, and haze is 0.38%.

[0120] This embodiment demonstrates that when the hydraulic diameter of the microchannel in the second-stage microreactor is three times that of the first-stage microreactor, it can still adapt to the changes in flow resistance caused by the increase in viscosity during the later stages of polymerization, thus achieving continuous and stable preparation.

[0121] Comparative Example 1

[0122] A traditional continuous batch thermal polymerization process was employed. 100 parts by weight of methyl methacrylate monomer, 0.05 parts by weight of the thermal initiator azobisisobutyronitrile (AIBN), and 0.2 parts by weight of n-dodecyl mercaptan were mixed and continuously pumped into a 2-liter mixed-flow reactor. The reactor was operated at a constant temperature of 130°C, with an average residence time of 2 hours. After discharge, the product underwent devolatilization and granulation. Product performance was tested, yielding a number-average molecular weight of 85,000, a polymer dispersion index as high as 2.3, a light transmittance of 91.5%, a haze of 1.2%, a residual monomer content of 500 ppm, and a yellowness index of 2.5.

[0123] As can be seen from the comparison with Comparative Example 1, the method provided in this embodiment, by utilizing the excellent heat and mass transfer characteristics of the microreactor and combining the precise start-up and shutdown of photocontrolled polymerization with the real-time correction capability of online closed-loop control, produces polymethyl methacrylate products that exhibit significant advantages in terms of molecular weight distribution (polymer dispersion index significantly reduced from 2.3 to 1.38), optical purity (higher transmittance, significantly lower haze and yellowness indices), and chemical purity (less residual monomers).

[0124] Comparative Example 2

[0125] Using the same raw material composition, total light dose, and post-treatment conditions as in Example 1, the only difference was that the two-stage tandem microreactor was replaced with a single-stage microreactor. The performance of the obtained product was tested, and the results showed: a number-average molecular weight of 71,000, a polymer dispersion index increased to 1.85, a transmittance of 92.3% for a 3mm thick sample, an increase in haze to 0.7%, and a residual monomer content of 420 ppm.

[0126] Compared with Example 1, it is evident that the single-stage microreactor cannot adapt to the rapid increase in viscosity during the later stages of polymerization. Under constant channel size, high-viscosity fluids are prone to wall slippage and retention layers, resulting in a wider distribution of material residence time, which in turn leads to a worse molecular weight distribution of the product (PDI deteriorates from 1.38 to 1.85). Furthermore, uneven local mixing leads to a decrease in conversion rate, higher residual monomer levels, and ultimately affects optical properties.

[0127] Comparative Example 3

[0128] The same system and process conditions as in Example 1 were used, but the control system did not adjust based on the online viscosity signal, only maintaining a preset constant light intensity and flow rate. During continuous operation, the same flow rate disturbance as in Example 4 was introduced (the feed flow rate was increased by 10% instantaneously). After the flow rate increased, due to the shortened residence time and the lack of light intensity compensation, the online viscometer reading at the outlet continuously decreased and stabilized at approximately 6200 mPa·s. The product collected after the disturbance and in a stable state was tested, and its number-average molecular weight decreased significantly to 65,000, while the polymer dispersion index broadened to 1.65.

[0129] Compared with Example 4, it is evident that the process lacking an online feedback closed-loop control system has extremely poor anti-interference capability. Once common production disturbances such as flow rate fluctuations occur, product specifications will rapidly deviate from the target value, generating a large amount of unqualified transitional material, making it impossible to guarantee a high degree of consistency between product batches in continuous production.

[0130] Comparative Example 4

[0131] The same polymerization conditions as in Example 1 were used, but instead of UV termination after the material left the final microreactor, it was temporarily stored at 60°C under light-protected, nitrogen-protected conditions for 2 hours before being fed into a twin-screw extruder for post-processing. The performance of the resulting product was tested, and the results showed that the number-average molecular weight abnormally increased to 108,000, the polymer dispersion index severely deteriorated to 1.92, and the residual monomer content was 130 ppm. Compared with Examples 1 and 5, it is evident that without instantaneous termination using high-energy short-wave ultraviolet light, the large number of active macromolecular free radicals in the material flowing out of the microreactor will continue to undergo uncontrollable "dark polymerization" or post-polymerization reactions during temporary storage and transportation. This disordered chain growth not only causes the product molecular weight to far exceed the set target but also leads to a severely broadened molecular weight distribution, destroying the optical uniformity obtained through the precise control of the microreactor.

[0132] The above description is merely a preferred embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for continuously preparing optical grade polymethyl methacrylate by microreactor-photoreactor combination, characterized in that, Includes the following steps: a) The reaction solution containing methyl methacrylate monomer, photoinitiator and chain transfer agent is transported to a microreactor system consisting of at least two microreactors connected in series; b) In the microreactor system, polymerization reaction is carried out by light irradiation, wherein the hydraulic diameter of the microchannel of the subsequent microreactor is larger than the hydraulic diameter of the microchannel of the previous microreactor, and the light intensity applied to the subsequent microreactor is greater than the light intensity applied to the previous microreactor. c) At the outlet of at least one microreactor, an online viscometer is installed to monitor the material viscosity in real time. Based on a preset viscosity-molecular weight-conversion rate relationship model, the light intensity and / or material flow rate are adjusted by a closed-loop control system according to the feedback of the monitoring data. The viscosity-molecular weight-conversion rate relationship model satisfies the following equation: wherein, is the kinetic viscosity measured in real time by the system, is the monomer conversion, is the number average molecular weight of the polymer, and are constants obtained by calibration with previous samplings. d) After the material leaves the microreactor system, use physical or chemical means to rapidly quench the active free radicals in the system in order to quickly terminate the polymerization reaction; e) Stabilize, melt-dip, and granulate the terminated material to obtain optical-grade polymethyl methacrylate; The online viscometer is a vibratory viscometer or a rotational viscometer, and the control system sets the target viscosity range based on the monitoring data of the online viscometer. When the monitored viscosity deviates from the target viscosity range, the control system adjusts the parameters of light intensity and / or material flow rate within 10 seconds. The physical method is to irradiate with ultraviolet light with a wavelength of less than 300 nm; or, the chemical method is to inject a photoactivated chemical terminator into the material and irradiate it with a light source. The hydraulic diameter of the microchannels in the second-stage microreactor is 1.2-3 times that of the microchannels in the first-stage microreactor, and the light intensity of the second-stage microreactor is 1.5-5 times that of the first-stage microreactor. Static mixers are installed between each stage of the microreactor.

2. The method of claim 1, wherein, The microreactor is made of quartz glass, sapphire, or transparent polycarbonate; the illumination wavelength is matched with the absorption wavelength of the photoinitiator, and is 365nm, 385nm, 405nm, or 450nm.

3. The method of claim 1, wherein, The photoinitiator is selected from one or more of 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexylphenyl ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholino)-1-propanone, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and the amount added is 0.01%-0.5% of the mass of methyl methacrylate monomer.

4. The method of claim 1, wherein, The chain transfer agent is selected from one or more of n-butanethiol, isobutanethiol, n-octanethiol, isooctanethiol, n-dodecylthiol, and tert-dodecylthiol, and the amount added is 0.05%-0.5% of the mass of methyl methacrylate monomer.

5. The method of claim 1, wherein, The hydraulic diameter of the microchannels in the first-stage microreactor is 100-1000 micrometers, the reaction temperature is 20-60℃, the residence time is 0.5-10 minutes, and the conversion rate in the prepolymerization stage is 10%-30%. The hydraulic diameter of the microchannels in the second-stage microreactor is 200-2000 micrometers, the reaction temperature is 40-80℃, the residence time is 2-20 minutes, and the total conversion rate is 60%-85%.

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