Method for preparing polymethyl methacrylate based on bulk polymerization method

Abstract

The invention belongs to the technical field of high-molecular compounds, and particularly discloses a method for preparing polymethyl methacrylate (PMMA) based on a bulk polymerization method, which comprises the following steps: S1, taking methyl methacrylate as a monomer, and mixing the monomer with an initiator under the protection of inert gas to form a homogeneous solution; s2, partially polymerizing the homogeneous solution at 60-80 DEG C until the monomer conversion rate reaches 10-30% to obtain a prepolymer; s3, continuing to polymerize the prepolymer at 80-100 DEG C until the monomer conversion rate exceeds 90%; and S4, carrying out devolatilization treatment at 100-120 DEG C under a vacuum condition to obtain PMMA with the molecular weight of more than 100,000. The problems that the polymerization process is not easy to control, the product performance is unstable, the molecular weight distribution is wide and the like in the prior art are solved, and efficient preparation of the PMMA material with high purity, narrow molecular weight distribution and high transparency can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high molecular compounds, in particular to a method for preparing polymethyl methacrylate based on bulk polymerization. BACKGROUND

[0002] Polymethyl methacrylate (PMMA) is widely used in various fields due to its excellent transparency, weather resistance and mechanical properties, and is suitable for making optical devices, or as high-performance materials such as coatings, adhesives and biomedical materials. Traditional methods for preparing PMMA mostly use the emulsion polymerization mode initiated by free radicals, which has problems such as large amount of solution used, difficult separation and purification, etc. Another commonly used synthesis route for PMMA is the bulk polymerization of methyl methacrylate (MMA) monomer, but it has problems such as difficult control of polymerization rate, wide molecular weight distribution of product, and easy production of bubbles, which will affect the performance and application range of the final product. For example, a prior art discloses a continuous bulk polymerization method for polymethyl methacrylate, which comprises: mixing methyl methacrylate with an initiator, and then sequentially performing pre-polymerization, final polymerization and extrusion devolatilization treatment to obtain a polymethyl methacrylate melt; wherein the mixing temperature is 50-60℃, the pre-polymerization reaction temperature is 60-90℃, the final polymerization reaction temperature is 90-140℃, and the extrusion devolatilization temperature is 160-180℃. The PMMA prepared by this method has undergone four stages, and the reaction temperature needs to be accurately controlled and adjusted three times. Although it is not mentioned in the patent, the heating rate cannot be too fast, otherwise local overheating and explosive polymerization are likely to occur, so the operation is relatively complicated and the temperature control requirement is relatively high. In addition, the product obtained by this method has a wide molecular weight distribution (PDI is between 1.8 and 2.0). In order to reduce PDI, a prior art proposes to adjust the molecular weight of the polymer by adding a chain transfer agent, but excessive use of the chain transfer agent may introduce impurities that reduce the light transmittance, thereby affecting the transparency and stability of the final material. In addition, the judgment of the end point of the polymerization reaction is generally carried out by sampling method, but the traditional sampling method is difficult to accurately determine whether the polymerization reaction is completed, and improper end point determination time may lead to poor batch stability of the product. SUMMARY

[0003] In view of the above-mentioned shortcomings of the prior art, the purpose of the present application is to provide a method for preparing polymethyl methacrylate based on bulk polymerization, which is used to solve the problems of difficult control of polymerization process, unstable product performance, wide molecular weight distribution, etc. in the prior art, and to realize the efficient preparation of PMMA material with high purity, narrow molecular weight distribution and high transparency.

[0004] To achieve the above object and other related objects, the present application provides a method for preparing polymethyl methacrylate based on the ontology polymerization method, which comprises the following steps: S1. A homogeneous solution is formed by mixing methyl methacrylate as a monomer and an initiator under the protection of an inert gas; S2. The homogeneous solution obtained in step S1 is partially polymerized at 60-80°C until the conversion rate of the methyl methacrylate monomer reaches 10-30%, to obtain a prepolymer; S3. The prepolymer obtained in step S2 is further polymerized at 80-100°C until the conversion rate of the methyl methacrylate monomer exceeds 90%; S4. The polymer obtained in step S3 is subjected to devolatilization treatment at 100-120°C and under vacuum conditions, to obtain polymethyl methacrylate with a molecular weight of more than 100,000.

[0005] Further, the initiator is a microcapsule-coated initiator, which comprises an inner core containing the initiator and an outer coating layer covering the inner core, and the outer coating layer contains sodium alginate and calcium chloride.

[0006] Further, a chain transfer agent is also added in step S1, and the mass ratio of the monomer to the chain transfer agent is 1:0.01-0.05.

[0007] Further, step S3 comprises: transferring the prepolymer obtained in step S2 to a mold or a casting device, and further polymerizing at 80-100°C until the conversion rate of the methyl methacrylate monomer exceeds 90%.

[0008] Further, step S3 comprises: adding a photoinitiator to the prepolymer obtained in step S2 under the conditions of 80-100°C and ultraviolet light irradiation, and further polymerizing until the conversion rate of the methyl methacrylate monomer exceeds 90%; the addition amount of the photoinitiator is 0.01%-0.03% of the mass of the prepolymer obtained in step S2.

[0009] Further, step S3 comprises: further polymerizing the prepolymer obtained in step S2 at 80-100°C and under supercritical environmental conditions until the conversion rate of the methyl methacrylate monomer exceeds 90%.

[0010] Further, the polymerization process from step S2 to step S3 adopts programmed temperature rising, and the temperature rising rate is 1-2°C / min.

[0011] Further, step S4 comprises: subjecting the polymer obtained in step S3 to devolatilization treatment at 100-120°C, under vacuum and ultrasonic conditions, to obtain polymethyl methacrylate with a molecular weight of more than 100,000.

[0012] Further, in step S4, the vacuum and ultrasonic conditions include: ultrasonic power of 10-30 kHz, vacuum degree of 10-30 mbar, and the devolatilization treatment time is less than 60 min.

[0013] Further, the method further comprises step S5: after the PMMA obtained in step S4 is cooled and solidified, it is demolded or extruded and granulated to obtain a PMMA resin finished product. Preferably, the cooling and solidification mode is natural cooling to room temperature.

[0014] Further, in step S1, sodium fluorescein is also added, and the mass ratio of the monomer to the sodium fluorescein is 1:0.001-0.005; step S5 further comprises: after the solidification is completed, the product is observed for luminescence under ultraviolet light irradiation, when the product presents uniform fluorescence, it is determined that the polymerization reaction reaches the end point; when the product presents non-uniform fluorescence, it needs to return to step S3 for reprocessing.

[0015] As described above, the method for preparing PMMA based on the bulk polymerization method of the present application has the following beneficial effects: 1. The two-stage polymerization process can effectively control the polymerization rate, reduce the generation of bubbles, improve the product transparency and thus the optical performance, enhance the product mechanical properties, and eliminate the need for separate defoaming treatment.

[0016] Further, by combining the "two-stage polymerization + microcapsule coated initiator", the temperature window can be widened to 60-120℃, the heating rate can be increased to 1-2℃ / min without polymerization explosion, and the total temperature adjustment is three stages, so the temperature adjustment is more smooth, thereby significantly reducing the temperature control difficulty and energy consumption. At the same time, the initiator is coated by microcapsule, which can realize segmented release and further reduce the bubble generation rate.

[0017] 2. The synergistic effect of the initiator and the chain transfer agent can accurately control the molecular weight (Mw = 100,000-300,000) and the distribution (PDI <1.5) of the polymer, and the use of low-dose chain transfer agent will not introduce impurities that can reduce the light transmittance, and the finished product light transmittance can be maintained above 92%. In addition, when a mold is used in the production of PMMA, the polymer obtained is covalently bonded to the mold, and the present application also finds that the addition of the chain transfer agent reduces the demolding force by more than 40%. The reduction of demolding force can reduce the damage to the product during production, thereby improving the product quality.

[0018] 3. The light initiator is added in the second stage polymerization process, and the process is carried out under ultraviolet light irradiation, which can reduce the subsequent heat treatment temperature.

[0019] Further, the second stage polymerization process is carried out in a supercritical CO2 environment. CO2 has special physical and chemical properties in supercritical state, such as good solubility and heat transfer performance. It acts as a plasticizer and heat transfer medium in the polymerization process, which helps to improve the efficiency of the polymerization reaction and the quality of the product.

[0020] 4. Vacuum-ultrasonic synergistic devolatilization can improve devolatilization efficiency, shorten devolatilization time, and improve product quality.

[0021] 5. The addition of fluorescein sodium is mainly to monitor the progress of the polymerization reaction and improve production efficiency and product quality. Fluorescein sodium emits fluorescence under the irradiation of ultraviolet light of a specific wavelength. This fluorescence characteristic can be used to monitor the progress of the polymerization reaction in real time, so as to more accurately determine whether the polymerization reaction is complete. Compared with the traditional sampling method, the addition of fluorescein sodium can shorten the end point determination time by about 15 minutes, and the batch stability is better. Fluorescein sodium has little effect on the performance of PMMA resin and has good compatibility in PMMA resin, and can be used for specific functional applications. In addition, fluorescein sodium can also be used as part of the visible light initiator to initiate the curing of unsaturated resin and promote the curing reaction of the resin. The amount of fluorescein sodium added in this application is less than 0.005%, and the addition amount is very low. Although it may have a slight effect on some physical properties of the resin, in the specific application of the present application, these effects are acceptable, and the benefits brought by the addition of fluorescein sodium far outweigh its potential negative effects.

[0022] In summary, the preparation process provided by the present application can effectively control the polymerization rate, product molecular weight and its distribution, reduce bubbles, improve product light transmittance, mechanical properties and batch stability, etc., make the product more uniform and of higher quality, and the process has strong adaptability, so it is suitable for the preparation of PMMA products in various forms such as blocks, sheets and fibers, and also does not require solvents, is green and environmentally friendly, and is suitable for continuous production. DETAILED DESCRIPTION

[0023] The embodiments of the present application are described below through specific specific examples. Those skilled in the art can easily understand other advantages and effects of the present application from the disclosure of the present specification. The present application can also be implemented or applied by means of other different specific embodiments, and various modifications or changes can be made to the details in the specification without departing from the spirit of the present application.

[0024] In the present application, the term "a plurality of" means two or more, unless otherwise specified.

[0025] The character " / " means that the front and rear objects are in an "or" relationship. For example, A / B means: A or B.

[0026] The term "and / or" is a description of the relationship between objects, which means that there can be three relationships. For example, A and / or B means: A or B, or, A and B, three relationships.

[0027] An embodiment of the present application provides a method for preparing polymethyl methacrylate based on the bulk polymerization method, comprising the following steps: S1. Using methyl methacrylate as a monomer, mixing the monomer with an initiator under the protection of an inert gas to form a homogeneous solution, preferably completing the mixing at room temperature (i.e. 20±5 ℃), without additional heating, and the inert gas is preferably nitrogen or argon, and the oxygen content is controlled to be <5 ppm to prevent polymerization inhibition; S2. Partially polymerizing the homogeneous solution obtained in step S1 at 60~80 ℃ until the conversion rate of methyl methacrylate monomer reaches 10~30%, to obtain a prepolymer, and the obtained prepolymer has a certain viscosity; S3. Continuing to polymerize the prepolymer obtained in step S2 at 80~100 ℃ until the conversion rate of methyl methacrylate monomer exceeds 90%; S4. The polymer obtained in step S3 is subjected to devolatilization treatment at 100~120 ℃ and under vacuum conditions to obtain polymethyl methacrylate with a molecular weight of more than 100,000. The devolatilization treatment is to remove unreacted monomers, oligomers and other volatile components by heat treatment.

[0028] The above embodiment adopts a two-stage polymerization process, which can effectively control the polymerization rate and reduce the generation of bubbles. Reducing the generation of bubbles can improve the transparency of the product, thereby improving the optical performance of the product; at the same time, the reduction or absence of bubbles can enhance the mechanical properties of the product, and it can be done in one step without the need for separate defoaming treatment. In addition, the total temperature adjustment in the above method for preparing PMMA is three stages, and the temperature adjustment is more smooth, which can significantly reduce the temperature control difficulty and energy consumption.

[0029] In some embodiments of the present application, the initiator is a microcapsule-coated initiator, which comprises an inner core containing the initiator and an outer coating layer covering the inner core, the outer coating layer containing sodium alginate and calcium chloride, and preferably, the mass of the outer coating layer accounts for 1-2% of the mass of the initiator. For example, the initiator is selected from the group consisting of azo initiators and / or peroxide initiators, such as azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO) or a complex system thereof, but not limited thereto; and the mass ratio of the monomer to the initiator is, for example, 1:0.4-1.5. Based on the two-stage polymerization process, the use of the microcapsule-coated initiator can not only widen the temperature window to 60-120°C, but also increase the heating rate to 1-2°C / min without polymerization explosion. At the same time, the initiator coated by the microcapsule can achieve segmented release and further reduce the bubble occurrence rate.

[0030] In some embodiments of the present application, the chain transfer agent is further added in step S1, and the mass ratio of the monomer to the chain transfer agent is 1:0.01-0.05. For example, the chain transfer agent includes, but is not limited to, 3-mercaptopropyl triethoxysilane (MPTMS), n-dodecyl mercaptan, etc. The synergistic effect of the initiator and the chain transfer agent can accurately control the molecular weight (Mw = 100,000-300,000) and the distribution (PDI < 1.5) of the polymer, and the use of a low-dose chain transfer agent will not introduce impurities sufficient to reduce the light transmittance, and the light transmittance of the finished product can be maintained above 92%. In addition, when a mold is used in the production of PMMA, the obtained polymer and the mold are covalently bonded, and it is found that the addition of the chain transfer agent reduces the demolding force by more than 40%. The reduction of the demolding force can reduce the damage to the product in production, thereby improving the product quality.

[0031] In some embodiments of the present application, step S2 comprises: placing the homogeneous solution obtained in step S1 in a reaction kettle, and partially polymerizing at 60-80°C until the conversion rate of the methyl methacrylate monomer reaches 10-30% to obtain a prepolymer. The reaction kettle used is preferably a heat-conducting pressure-resistant reaction kettle, and the heat conductivity coefficient is preferably above 20 w / m / K, and the pressure-bearing capacity is preferably above 25 MPa. The reaction liquid obtained in step S2 (i.e., the prepolymer obtained in step S2) needs to be quickly transferred to the next reaction device for continuous polymerization, and the transfer speed is, for example, 400-600 L / s. In some embodiments of the present application, step S3 comprises: transferring the prepolymer obtained in step S2 to a mold or a casting device, and continuously polymerizing at 80-100°C until the conversion rate of the methyl methacrylate monomer exceeds 90%. The material of the mold includes, but is not limited to, a polytetrafluoroethylene mold, etc., and the size thereof is selected according to the actual needs.

[0032] In some embodiments of the present application, step S3 comprises: adding a photoinitiator to the prepolymer obtained in step S2 under the condition of ultraviolet light irradiation at 80-100°C, and continuing the polymerization until the conversion rate of methyl methacrylate monomers exceeds 90%; the amount of photoinitiator added is 0.01%-0.03% of the mass of the prepolymer obtained in step S2, and the photoinitiator includes but is not limited to Irgacure 819 and the like; the ultraviolet light is preferably ultraviolet light with a wavelength of 360-380 nm, but is not limited thereto; and the ultraviolet light irradiation mode may, for example, use LED lamp irradiation. The addition of photoinitiator and ultraviolet light irradiation during the second-stage polymerization process can reduce the subsequent heat treatment temperature.

[0033] In some embodiments of the present application, step S3 comprises: continuing the polymerization of the prepolymer obtained in step S2 at 80-100°C and under the condition of supercritical CO2 environment until the conversion rate of methyl methacrylate monomers exceeds 90%. The second-stage polymerization process is carried out in a supercritical CO2 environment. CO2 in a supercritical state has special physical and chemical properties, such as good solubility and heat transfer performance, which simultaneously acts as a plasticizer and a heat transfer medium during the polymerization process, and helps to improve the efficiency of the polymerization reaction and the quality of the product. For example, the process of polymerization reaction of the prepolymer obtained in step S2 in a supercritical CO2 environment comprises: S3.1, filling CO2 to form a supercritical CO2 environment: A high-pressure reaction kettle is selected as the reaction device, which can withstand the pressure (more than 8 MPa) and temperature (more than 80°C) conditions of supercritical CO2, and has good sealing performance and pressure resistance.

[0034] After the prepolymer obtained in step S2 is transferred to the high-pressure reaction kettle, CO2 gas is filled into the reaction kettle through a gas delivery system, the amount of CO2 filled is controlled, the pressure in the reaction kettle is made to be more than 8 MPa, and the temperature is maintained at 80-100°C, so that CO2 is in a supercritical state. During the filling process, the purity of the gas must be ensured.

[0035] S3.2, polymerization reaction: In the supercritical CO2 environment, the temperature in the reaction kettle is maintained at 80-100°C, and the pressure is more than 8 MPa, so that the prepolymer is subjected to the second-stage polymerization reaction under these conditions. During the reaction, the stability of the reaction conditions must be ensured to avoid large fluctuations in temperature and pressure.

[0036] S3.3, treatment after the polymerization reaction is completed: After the polymerization reaction is completed, the pressure in the reaction kettle is slowly released, and the CO2 gas is discharged. Since the solubility of CO2 in the supercritical state is high, the speed should be controlled when the pressure is released to avoid the generation of bubbles or other defects in the polymer due to the sudden drop in pressure. After the exhaust is completed, the reaction kettle is opened and the polymer product is taken out.

[0037] In some embodiments of the present application, the polymerization process of steps S2 to S3 uses programmed temperature rise to achieve coordinated control of polymerization rate and molecular weight distribution, and the temperature rise rate is 1-2°C / min.

[0038] In some embodiments of the present application, step S4 includes: subjecting the polymer obtained in step S3 to devolatilization treatment at 100-120°C under vacuum and ultrasonic conditions to obtain polymethyl methacrylate with a molecular weight of more than 100,000; the vacuum and ultrasonic conditions include: ultrasonic power of 10-30 kHz, vacuum degree of 10-30 mbar, and devolatilization treatment time of less than 60 min. Vacuum-ultrasonic synergistic devolatilization can improve devolatilization efficiency, shorten devolatilization time, and improve product quality.

[0039] In some embodiments of the present application, the method further includes step S5: after the polymethyl methacrylate obtained in step S4 is cooled and solidified, it is demolded or extruded and granulated to obtain polymethyl methacrylate resin finished product. Preferably, the cooling and solidification method is natural cooling to room temperature.

[0040] In some embodiments of the present application, sodium fluorescein is also added in step S1, and the mass ratio of monomer to sodium fluorescein is 1:0.001-0.005; step S5 further comprises: after curing, the product is observed for luminescence under ultraviolet light, and when the product presents uniform fluorescence, it is determined that the polymerization reaction has reached the end point; when the product presents uneven fluorescence, it needs to return to step S3 for reprocessing. The ultraviolet light is preferably ultraviolet light with a wavelength of 360-380 nm, but is not limited thereto. The addition of sodium fluorescein is mainly to monitor the progress of the polymerization reaction and improve the production efficiency and product quality. Sodium fluorescein emits fluorescence under the irradiation of ultraviolet light of a specific wavelength, and this fluorescence characteristic can be used to monitor the progress of the polymerization reaction in real time, so as to more accurately determine whether the polymerization reaction is completed. Compared with the traditional sampling method, the addition of sodium fluorescein can shorten the end point determination time by about 15 minutes, and the batch stability is better. Sodium fluorescein has little effect on the performance of PMMA resin and has good compatibility in PMMA resin, and can be used for specific functional applications. In addition, sodium fluorescein can also be used as part of the visible light initiator to initiate the curing of unsaturated resin and promote the curing reaction of the resin. The amount of sodium fluorescein added in the present application is less than 0.005%, and the addition amount is very low. Although it may have a slight effect on some physical properties of the resin, in some specific applications, these effects are acceptable, and the benefits brought by the addition of sodium fluorescein far outweigh its potential negative effects.

[0041] The following specific examples are used to illustrate the present application in detail. It should also be understood that the following examples are only used to specifically illustrate the present application, and cannot be understood as limiting the scope of protection of the present application. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present application are within the scope of protection of the present application. The following examples specifically illustrate the process parameters, etc. which are only one example in the appropriate range, i.e. those skilled in the art can select within the appropriate range according to the description herein, and are not limited to the specific values of the following examples.

[0042] The molds used in the following examples and comparative examples are all polytetrafluoroethylene molds with a cavity of 100 mm × 100 mm × 2 mm. In addition, unless otherwise specified, the polymerization reaction is carried out under normal pressure N2 atmosphere. The molecular weight of the product PMMA in the following examples and comparative examples is about 3 × 10 5 g / mol, which is measured by gel permeation chromatography (GPC, THF phase, 35 ℃, polystyrene calibration) according to the rules given in GB / T 21863-2008, and the results are expressed in terms of PMMA relative molecular weight.

[0043] Example 1 The present embodiment provides a method for preparing PMMA based on bulk polymerization, and the steps are as follows: Take 100 g of MMA monomer, add 0.5 g of initiator AIBN and 0.05 g of chain transfer agent n-dodecyl mercaptan, mix thoroughly under nitrogen protection environment to form a homogeneous solution. The homogeneous solution is pre-polymerized at 70°C for 2 hours, and the conversion rate reaches about 20%. Then inject into the mold, continue to polymerize at 90°C for 6 hours, and the conversion rate exceeds 90%; then heat treat at 110°C under vacuum environment for 4 hours. After natural cooling to room temperature, the transparent PMMA plate is demolded, and the product has no bubbles and no yellowing. The tested light transmittance is 92.3%, the tensile strength is 75 MPa, the glass transition temperature is 108°C, and the molecular weight distribution index PDI is 1.42.

[0044] Comparative Example 1 This comparative example provides a method for preparing PMMA based on bulk polymerization, and the steps are as follows: Take 100 g of MMA monomer, add 0.5 g of AIBN and 0.05 g of n-dodecyl mercaptan, mix thoroughly under nitrogen protection environment to form a homogeneous solution. The homogeneous solution is injected into the mold and polymerized at 90°C for 8 hours, and the conversion rate exceeds 90%; then heat treated at 110°C under vacuum environment for 4 hours. After natural cooling to room temperature, the transparent PMMA plate is demolded.

[0045] This comparative example uses a conventional one-step polymerization method, and the other conditions are the same as in Example 1. Compared with Example 1, the product obtained has obvious bubbles, the light transmittance decreases to 89.5%, and the PDI is 1.78, indicating that the polymerization process control is poor.

[0046] Example 2 This example provides a method for preparing PMMA based on bulk polymerization, and the steps are as follows: Take 100 g of MMA as monomer, add 1.0 g of initiator BPO, mix thoroughly under nitrogen protection environment to form a homogeneous solution. The homogeneous solution is pre-polymerized at 75°C for 2.5 hours, and the conversion rate reaches about 25%. Then inject into the mold, continue to polymerize at 95°C for 5.5 hours, and the conversion rate exceeds 90%; then heat treat at 115°C under vacuum environment for 3 hours. After natural cooling to room temperature, the transparent PMMA plate is demolded, and the product has no bubbles and no yellowing. The tested light transmittance is 93.0%, the tensile strength is 80 MPa, the glass transition temperature is 110°C, and the molecular weight distribution index PDI is 1.45.

[0047] Example 3 This example adds 1.0 g of n-dodecyl mercaptan as a chain transfer agent based on Example 2, and the other conditions remain unchanged. The product has no bubbles and no yellowing, and the tested light transmittance of the product is 92.8% and the PDI is 1.38.

[0048] From the comparison between Example 2 and 3, it can be seen that the introduction of chain transfer agent can not only further refine the molecular weight of the polymer, but also effectively reduce the molecular weight distribution width; at the same time, the product also shows good optical performance, with a light transmittance of 92.8%, which confirms that the low-dose chain transfer agent has little effect on the light transmittance of the product.

[0049] Example 4 The present example provides a method for preparing PMMA based on the bulk polymerization method, and the steps are as follows: In 100 g of MMA, 0.5 g of microencapsulated AIBN (formed by coating AIBN with sodium alginate and calcium chloride, with a coating amount of 1% of the mass of AIBN) and 0.03 g of MPTMS are added, and the mixture is thoroughly mixed under a nitrogen atmosphere to form a homogeneous solution. The homogeneous solution is pre-polymerized at 70°C for 2 hours, with a conversion rate of about 25%. Then, the mixture is injected into a mold, and the temperature is raised to 100°C at a rate of 2°C / min, and the polymerization is continued for 5 hours, with a conversion rate of more than 90%. The mixture is then heat-treated at 115°C under vacuum for 3 hours. After natural cooling to room temperature, the transparent PMMA plate is demolded. The product is free of bubbles and yellowing, and the light transmittance is 92.6%, the tensile strength is 80 MPa, and the PDI is 1.30.

[0050] Example 5 The present example provides a method for preparing PMMA based on the bulk polymerization method, and the steps are as follows: In 100 g of MMA, 0.5 g of microencapsulated AIBN (formed by coating AIBN with sodium alginate and calcium chloride, with a coating amount of 1% of the mass of AIBN) and 0.05 g of n-dodecyl mercaptan are added, and the mixture is thoroughly mixed under a nitrogen atmosphere to form a homogeneous solution. The homogeneous solution is pre-polymerized at 70°C for 2 hours, with a conversion rate of about 20%. Then, the mixture is injected into a mold, and the temperature is raised to 80°C at a rate of 2°C / min, and the polymerization is continued for 5 hours, with a conversion rate of more than 90%. The mixture is then heat-treated at 100°C under vacuum for 4 hours. After natural cooling to room temperature, the transparent PMMA plate is demolded. The product is free of bubbles and yellowing, and the light transmittance is 92.1%, and the PDI is 1.45.

[0051] Example 6 The difference between the present example and Example 5 is that a 365 nm LED (40 mW cm -2 ) is turned on for 30 minutes during the second stage of polymerization, and the heat treatment temperature is 100°C, and the other conditions are the same. The obtained product is free of bubbles and yellowing, and the light transmittance is 93.0%, and the PDI is 1.32.

[0052] Compared with Example 5, the heat treatment temperature in the present example can be reduced from 110°C to 100°C, which can save energy by 9%.

[0053] Example 7 This example provides a method for preparing PMMA based on bulk polymerization, the steps are as follows: In 100 g MMA, 0.5 g microencapsulated AIBN (formed by coating AIBN with sodium alginate and calcium chloride, with a coating amount of 1% of the mass of AIBN) and 0.05 g n-dodecyl mercaptan were added and mixed thoroughly under a nitrogen protective environment to form a homogeneous solution. The homogeneous solution was pre-polymerized at 70°C for 2 hours, with a conversion rate of about 20%. Then it was injected into a mold and transferred to a high-pressure reaction kettle, filled with 8 MPa CO2 (80°C) as a supercritical medium, and continued to polymerize at 80°C for 4 hours, with a conversion rate of more than 90%. Then it was heat-treated at 100°C for 4 hours in a vacuum environment. After natural cooling to room temperature, transparent PMMA plate was demolded, and the product had no odor after demolding. Test results showed that the residual monomer content was less than 0.2%, the haze was 0.3%, and the tensile strength was 78 MPa.

[0054] Example 8 The difference between this example and Example 7 is that the devolatilization process is only under vacuum of 20 mbar without ultrasonic, and the devolatilization time is 60 min. The residual monomer content of the product obtained is 0.7%, the haze is 0.6%, and the tensile strength is 74 MPa.

[0055] Compared with Example 7, the devolatilization time of this example is longer, and the residual monomer content and haze of the product obtained are higher, which indicates that vacuum-ultrasonic synergistic devolatilization can improve the devolatilization efficiency, shorten the devolatilization time, and improve the product quality.

[0056] Example 9 This example provides a method for preparing PMMA based on bulk polymerization, the steps are as follows: In 100 g MMA, 0.5 g microencapsulated AIBN (formed by coating AIBN with sodium alginate and calcium chloride, with a coating amount of 1% of the mass of AIBN) and 0.05 g n-dodecyl mercaptan were added and mixed thoroughly under a nitrogen protective environment to form a homogeneous solution. The homogeneous solution was pre-polymerized at 70°C for 2 hours, with a conversion rate of about 20%. Then it was injected into a mold and transferred to a high-pressure reaction kettle, filled with 8 MPa CO2 (80°C) as a supercritical medium, and continued to polymerize at 80°C for 4 hours, with a conversion rate of more than 90%. Then it was heat-treated at 100°C for 4 hours in a vacuum environment. After natural cooling to room temperature, transparent PMMA plate was demolded, and the product had no odor after demolding. Test results showed that the residual monomer content was less than 0.2%, the haze was 0.3%, and the tensile strength was 78 MPa.

[0057] Example 10 The difference between this example and Example 9 is that the second stage polymerization process is carried out under normal pressure N2 atmosphere, and the rest of the conditions are the same. The product obtained is tested to have a bubble level of 1-2, a light transmittance of 91.8%, and a PDI of 1.36.

[0058] Example 11 The difference between this example and Example 5 is that 0.005 g of sodium fluorescein is also added in the premixing stage. The homogeneous solution obtained after premixing is colorless under naked eye observation. After solidification is completed, it presents uniform fluorescence under 365 nm ultraviolet lamp, and the polymerization end point can be immediately determined.

[0059] Example 5 and Example 11 are each continuously carried out for 10 batches, the offline data of conversion rate of the 10 batches are counted, and the CV value is calculated.

[0060] Example 5 does not add sodium fluorescein, and the conversion rate needs to be measured offline, the determination time is extended by 30 min, and the batch CV is 3%. Compared with the traditional sampling method (2-5 g of slurry is taken from the discharge port of the reactor every 30 min using a sampling tube.), the end point determination time of this example can be shortened by 15 min, the batch CV is less than 1% (for more than 10 batches in succession), and the batch stability is better.

[0061] Example 12 This example provides a method for preparing PMMA based on the bulk polymerization method, and the steps are as follows: 0.5 g of microencapsulated AIBN (formed by coating AIBN with sodium alginate and calcium chloride, with a coating amount of 1% of the mass of AIBN), 0.03 g of MPTMS are added in 100 g of MMA, and the mixture is thoroughly mixed under a nitrogen protection environment to form a homogeneous solution. The homogeneous solution is pre-polymerized at 70°C for 2 hours, and the conversion rate reaches about 20%. Then, the mold is injected, and the temperature is raised to 90°C at a rate of 2°C / min, and the polymerization is continued for 4 hours, and the conversion rate exceeds 90%; and then the PMMA plate is heated at 110°C for 2 hours in a vacuum environment. After natural cooling to room temperature, the PMMA plate is demolded.

[0062] Demolding force test: a universal material testing machine (Instron 5967, 5 kN sensor) is used, the test temperature is 25°C, and the tensile rate is 5 mm / min. The sample is the PMMA plate cured in a polytetrafluoroethylene mold (cavity 100 mm x 100 mm x 2 mm). The test method is as follows: (1) the upper plate of the mold is connected with the testing machine clamp; (2) vertically stretch, and record the maximum peeling load (N); (3) a total of 10 batches are tested, 5 parallel samples are taken for each batch, and the arithmetic mean value is taken. The test result is: the average demolding force is 110 N, the standard deviation is 1.1 N, and the CV is 1.0%.

[0063] Example 13 The difference between this example and example 12 is that MPTMS is not added, and the rest of the conditions are the same.

[0064] The demolding force test method is the same as example 12, and the test results are: average demolding force 184 N, standard deviation 9.2 N, CV = 5.0 %.

[0065] As can be seen from the comparison of examples 12 and 13, the addition of chain transfer agent reduces the demolding force by 40%.

[0066] The above examples only illustrate the principles and effects of the present application, and are not used to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed in the present application shall be covered by the claims of the present application.

Claims

1. A method for producing polymethyl methacrylate based on an ontological polymerization method, characterized by, The method comprises the following steps: S1. mixing a methyl methacrylate monomer with an initiator under inert gas protection to form a homogeneous solution; S2. partially polymerizing the homogeneous solution obtained in step S1 at 60-80°C until the conversion rate of the methyl methacrylate monomer reaches 10-30%, to obtain a prepolymer; S3. continuing to polymerize the prepolymer obtained in step S2 at 80-100°C until the conversion rate of the methyl methacrylate monomer exceeds 90%; S4. performing devolatilization treatment on the polymer obtained in step S3 at 100-120°C and under vacuum conditions, to obtain a polymethyl methacrylate with a molecular weight of more than 100,000.

2. The method of claim 1, wherein: The initiator is a microcapsule-coated initiator, which comprises an inner core containing the initiator and an outer coating layer covering the inner core.

3. The method of claim 1, wherein: The chain transfer agent is further added in step S1, and the mass ratio of the monomer to the chain transfer agent is 1:0.01-0.

05.

4. The method of claim 1, wherein: Step S3 comprises: transferring the prepolymer obtained in step S2 to a mold or a casting device, and continuing to polymerize at 80-100°C until the conversion rate of the methyl methacrylate monomer exceeds 90%.

5. The method of claim 1, wherein: Step S3 comprises: adding a photoinitiator to the prepolymer obtained in step S2 under the conditions of 80-100°C and ultraviolet light irradiation, and continuing to polymerize until the conversion rate of the methyl methacrylate monomer exceeds 90%. And / or, step S3 comprises: continuing to polymerize the prepolymer obtained in step S2 at 80-100°C and under supercritical CO2 environmental conditions until the conversion rate of the methyl methacrylate monomer exceeds 90%.

6. The method of claim 1, wherein: The polymerization process from step S2 to step S3 adopts programmed temperature rising, and the temperature rising rate is 1-2°C / min.

7. The method of claim 1, wherein: Step S4 comprises: performing devolatilization treatment on the polymer obtained in step S3 at 100-120°C, under vacuum and ultrasonic conditions, to obtain a polymethyl methacrylate with a molecular weight of more than 100,000.

8. The method of claim 7, wherein: In step S4, the vacuum and ultrasonic conditions comprise: an ultrasonic power of 10-30 kHz, a vacuum degree of 10-30 mbar, and a devolatilization treatment time of less than 60 min.

9. The method of any one of claims 1-8, wherein: The method further comprises step S5: after the polymethyl methacrylate obtained in step S4 is cooled and solidified, the product is demolded or extruded and granulated to obtain a polymethyl methacrylate resin finished product.

10. The method of claim 9, wherein: In step S1, fluorescein sodium is further added, and the mass ratio of the monomer to the fluorescein sodium is 1:0.001-0.005; step S5 further comprises: after solidification, the product is observed for light emission under ultraviolet light irradiation, and when the product presents uniform fluorescence, it is determined that the polymerization reaction has reached the end point.