A methacrylic polymer and a method for producing the same

Abstract

The application belongs to the technical field of polymer synthesis, and particularly relates to a preparation method of a methacrylic acid polymer, which comprises the following steps: feeding polymerization monomers, a chain transfer agent and an initiator into a polymerization reactor, and performing a polymerization reaction in the polymerization reactor; the polymerization reaction temperature is 200-260 DEG C; the polymerization reactor is a loop reactor; there is no gas phase in the polymerization reactor; the flow rate of the reaction liquid in the polymerization reactor is 0.2-0.8 m / s; and the polymer concentration at the outlet of the polymerization reactor is 70-85 wt%. The loop reactor is used, a series of operation conditions are set, and a product with high polymerization conversion rate is obtained. By controlling the polymerization reaction temperature, the reaction liquid viscosity is effectively reduced, the occurrence probability of gel effect is reduced, the polymerization conversion rate is improved, and the polymer concentration at the outlet of the polymerization reactor is 70 wt%-85 wt%.

Description

Technical Field

[0001] This invention relates to the field of polymer synthesis technology, specifically to a methacrylic acid polymer and its preparation method. Background Technology

[0002] Polymethyl methacrylate (PMMA) is an important polymer widely used in optics, electronics, and automotive industries. Continuous production methods for PMMA mainly include solution polymerization and bulk polymerization. Solution polymerization requires significant investment in equipment and specific process conditions, and unavoidable side reactions can occur during polymerization. Furthermore, solution polymerization requires solvents, and solvent recovery and treatment can be complex, increasing production costs and environmental impact. Bulk polymerization has the advantages of not using solvents, eliminating the need for solvent separation and recovery processes, and effectively improving product purity and transmittance. However, bulk polymerization suffers from disadvantages such as difficulty in controlling the heat of reaction and conversion rate. Unstable heat of reaction can lead to unstable reaction system temperatures, localized gelation effects, affecting the quality of the final product, and in severe cases, causing explosive polymerization and safety accidents. Low conversion rates increase the processing requirements for monomer recovery equipment, increasing energy consumption; furthermore, excessively high monomer content in the product negatively impacts final product quality.

[0003] Polymerization reactors are generally classified into two types: batch reactors and tubular reactors. A batch reactor is a closed container with internal heating or cooling, and its volume is typically larger than that of a tubular reactor. Due to its large volume, a batch reactor can process large quantities of material, has a wide range of applications, requires less investment, and is easy to start up. However, the reaction temperature is difficult to control, and the residence time is inconsistent. In contrast, in a tubular reactor, because the reactant molecules have equal residence times within the reactor, the reactant concentration and chemical reaction rate at any point in the reactor do not change with time, but only with the length of the tube. Because the reactants react quickly and the flow rate is high in a tubular reactor, its production capacity is high. Compared to a batch reactor, it has less backmixing, and at lower flow rates, the fluid pattern within the tube is close to that of an ideal fluid.

[0004] Due to their small footprint and large throughput, batch reactors are widely used in industrial solution polymerization and bulk polymerization. However, batch reactors cannot guarantee sufficient mixing, potentially resulting in dead zones. Because PMMA polymerization exhibits a gel effect (polymerization is automatically accelerated at higher system viscosities), the polymer content in batch reactors for PMMA bulk polymerization cannot be too high. Typically, the polymer content in batch reactors for PMMA bulk polymerization is around 30wt%–65wt% to avoid gelation. To achieve higher polymerization conversion rates and reduce devolatilization energy consumption, tubular reactors are often connected in series after the batch reactor. (Because batch reactors still suffer from uneven mixing, polymerization temperature and conversion rates are difficult to control, leading to gelation, the reactors connected in series are usually tubular reactors.) To reduce gelation, the temperature inside the tubular reactor is usually higher than that in the batch reactor, and mixing elements (such as static mixers) are typically installed in the tubular reactor to ensure thorough mixing of the reaction solution in the radial direction. Typically, the polymer content at the outlet of a straight tubular reactor connected in series after a batch reactor is around 50wt% to 80wt%.

[0005] The polymerization temperature within the polymerization reactor is crucial for the polymerization equilibrium conversion rate. However, the mechanism by which polymerization temperature affects the polymerization equilibrium conversion rate is not yet universally understood. For example, Japanese Patent JP2000026507A discloses a method for manufacturing acrylic resin by using a tubular reactor following a trough reactor to obtain acrylic resin with balanced productivity and physical properties. This patent describes an example with a maximum final polymerization rate of 72%, but does not describe methods to further increase the polymerization rate. Japanese Patent JP2003002912A discloses a method for inhibiting polymerization using the polymerization equilibrium polymerization rate, where the polymerization rate is determined by the final temperature of the polymer. This patent describes an example with a maximum final polymerization rate of 68%, but argues that a final polymerization rate exceeding this value cannot be achieved. Furthermore, it makes no mention of the factors determining the equilibrium polymerization rate. Chinese Patent CN102933610A, on the other hand, suggests that adding a prescribed amount of initiator can improve monomer conversion in polymerization within a tubular reactor. It was also learned that by increasing the amount of methyl acrylate used as a copolymer component in methacrylic acid polymers, the monomer conversion rate can be improved even with the same initiator dosage as before. This patent describes an example with a final polymerization rate as high as 83%. Chinese patent CN1576285A discloses in its specification that if the polymerization temperature is above 170°C, the polymerization reaction proceeds stably, thus increasing the polymerization rate. However, due to the increased formation of dimers, there is a tendency for the polymer's transparency and mechanical strength to decrease after the removal of volatile components. These published patent documents also indicate that the conditions for the equilibrium conversion rate of PMMA polymerization are quite complex, and the mechanism is still unclear.

[0006] There are many methods for manufacturing methacrylic polymers using a single reactor. For example, Japanese Patent JP1998087705A discloses a method for manufacturing optically superior methacrylic resins, describing in the specification that the polymerization conversion rate needs to be kept substantially constant within the range of 40% to 70%, and the polymerization reaction is carried out at a temperature of 120°C to 160°C. Japanese Patent JP2000053708A discloses a method in which the temperature is maintained at 130°C to 170°C in a single reactor while uniformly stirring and mixing, and the polymer concentration (weight fraction) in the reactor is 0.40 to 0.70. Japanese Patent JP2000053709A discloses a method in which the temperature is maintained at 130 to 170°C in the reactor while uniformly stirring and mixing, and the average residence time of the reaction liquid in the reactor is maintained at 0.5 to 1.9 hours, the ratio of the half-life of the free radical generator to the average residence time of the reaction liquid in the reactor is 5.0 × 10⁻⁴ to 0.50, and the polymer concentration (weight fraction) in the reactor is 0.40 to 0.70. Chinese patent CN1576285A discloses a method using a single fully mixed reactor at a reaction temperature of 110–170°C, where the polymer is obtained by adjusting the supply of a free radical polymerization initiator. The polymer content of the reaction mixture is substantially maintained within the range of 35–70% by mass, but its examples do not specify the polymer concentration. Chinese patent CN1102833A discloses a flooded, adiabatic stirred reactor, where polymerization is carried out at a temperature range of 120–180°C to obtain a polymer composition with a polymer content of 40–70% by weight. Chinese patent CN104379616A discloses a method for producing methacrylic acid polymer compositions using a single fully mixed reactor. The temperature of the raw material mixture supplied to the reactor is -50°C to -10°C, and the polymerization temperature is 120–150°C. This patent mentions a polymerization rate of, for example, 30 wt%–90 wt%, but provides no theoretical explanation or illustrative examples.

[0007] According to the graph of zero-shear viscosity and PMMA content of PMMA polymer solution in the literature "High-temperature radical polymerization of methylmethacrylate in a continuous pilot scale process", it can be seen that at the polymerization upper limit of 150°C disclosed in patent CN104379616A, and without considering other issues, with only a 90% polymerization rate, the viscosity will reach approximately 70,000 Pa·s, which is clearly inappropriate. The patent specification also describes setting a suitable polymerization rate as a target or standard, but does not disclose specific data. At a polymerization temperature of 150°C and a PMMA concentration of 80 wt%, the zero-shear viscosity reaches approximately 1700 Pa·s. Such a high viscosity would significantly increase the tendency for gelation, and would also make stirrer selection relatively difficult.

[0008] Because the polymerization rate of a single reactor is low, many processes employ a series connection to increase the polymerization rate. For example, Chinese Patent CN1576286A discloses a polymerization process using a complete reactor and a tubular reactor connected in series, with a reaction temperature of 110–170°C. The process specifies the relationship between the inner diameter and linear velocity of the tubular reactor, and the polymer content of the reaction mixture is substantially maintained in the range of 35–70% by mass, preferably 65% ​​or less, and more preferably 40% or more. However, its examples do not specify the polymer concentration. Japanese Patent JP2000026507A uses a completely mixed reactor followed by a plug-flow reactor and a volatile matter removal device to produce monomer polymers or copolymers of methyl methacrylate. In the completely mixed reactor, uniform stirring is performed at 110–160°C to achieve a polymer content of 35–65% by weight in the reaction zone. In the plug-flow reactor, the temperature of the inner wall of the reactor is set above but not exceeding 250°C of the polymerization temperature of the first-stage completely mixed reactor, and the polymer content at the outlet is 50–85% by weight. Japanese Patent JP2003002912A discloses a method for producing polymers or copolymers of methyl methacrylate using a fully mixed reactor (A) and a subsequent reactor (B). The fully mixed reactor (A) stirs and mixes the reaction liquid composition at a polymerization temperature of 110 to 170°C to achieve a polymer content of 35 to 65 wt% in the reaction zone. Based on the polymer content relationship provided, the reactor (B) has a reaction temperature of 180 to 200°C and a polymer content of 50 to 85 wt% (related to the polymerization temperature). Chinese patent CN108424479A discloses a continuous polymerization apparatus, which includes at least a first and a second reactor of a fully mixed type. The reaction temperature of both reactors is 120-150°C. The polymerization ratio of the output from the first reactor is, for example, 5 to 80 wt%, and the polymerization ratio of the output from the second reactor is, for example, 30 to 90 wt%. According to its description, the cooling device cools the material by 5-80°C, that is, 145°C may match 80 wt%, while 150°C in the second reactor may match 90 wt%. This presents a similar problem to that of patent CN104379616A.

[0009] The circulation tube reactor (i.e., the loop reactor) has efficient mixing performance, excellent temperature control, and the ability to adapt to different reaction stages. It is widely used in the polyethylene and polypropylene industries. "High-temperature radical polymerization of methyl methacrylate in a continuous pilot scale process" (Philip Nising) has verified the feasibility study of the combination of a circulation tube reactor + a straight tube reactor in the PMMA polymerization process at 140°C < T < 170°C. According to the article description, the polymer content at the outlet of the circulation tube reactor is 0.5, and the polymer content at the outlet of the straight tube reactor is 0.8. Chinese Patent CN101724120A discloses a method for preparing a (meth)acrylate polymer, which uses a series of circulation tube reactors and straight tube reactors to obtain a polymer product. In the circulation tube reactor, the polymerization temperature is 140 - 160°C, and the polymer content is controlled at 35 - 50%; in the straight tube reactor, the temperature is set at 140 - 160°C, and the polymer content is controlled within the range of 40 - 80%. Chinese Patent CN102311518A discloses a method for preparing a (meth)acrylate polymer, which is obtained by connecting a circulation tube reactor and a straight tube reactor in series. In the circulation tube reactor, the polymerization temperature is controlled within 130 - 150°C, and the polymerization conversion rate is controlled at 45 - 60%. In the straight tube reactor, the temperature is controlled within the range of 140 - 160°C, and the final polymerization conversion rate is controlled at 55 - 90%. Unfortunately, there is no record of polymerization at higher temperatures for the above-mentioned loop reactor. Summary of the Invention

[0010] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a method for preparing a methacrylic polymer, which can increase the polymer concentration at the outlet of a single reactor to improve the polymerization conversion rate; and reduce or eliminate the gel effect of the polymerization reactor; and reduce the generation of dimers under the condition of a relatively high reaction temperature. The inventors of the present application found in the research that by controlling parameters such as the reaction temperature of the polymerization reactor and the flow rate of the reaction solution within a certain range, the polymerization conversion rate can be increased and the gel effect can be reduced.

[0011] To achieve this purpose, the present invention adopts the following technical solutions:

[0012] On one hand, this invention provides a method for preparing a methacrylic acid polymer, comprising the following steps: supplying polymerization monomers, chain transfer agents, and initiators to a polymerization reactor, carrying out a polymerization reaction in the polymerization reactor at a polymerization temperature of 200–260°C, wherein the polymerization reactor is a loop reactor, there is no gas phase in the polymerization reactor, the circulation flow rate of the reaction liquid in the polymerization reactor is 0.2–0.8 m / s, and the polymer concentration at the outlet of the polymerization reactor is 70–85 wt%. Unless otherwise specified, the following component contents refer to mass content.

[0013] The reaction liquid in the polymerization reactor mentioned in this invention refers to the raw material liquid supplied to the polymerization reactor, including polymerization monomers, chain transfer agents, and initiators.

[0014] In this invention, the polymerization reaction temperature is the average value of the temperatures in the aforementioned loop reactor (this average value can be calculated by setting multiple temperature measuring points in the loop reactor, or it can be calculated theoretically). The specific polymerization reaction temperature can be 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, or 260℃, preferably 220℃ to 240℃.

[0015] In this invention, the circulating flow rate of the reaction liquid in the polymerization reactor is 0.2 to 0.8 m / s, specifically 0.2 m / s, 0.3 m / s, 0.4 m / s, 0.5 m / s, 0.6 m / s, 0.7 m / s, or 0.8 m / s, etc.

[0016] As the viscosity of the reaction solution decreases at high temperatures, the probability of gelation also decreases. In order to further reduce the probability of gelation and reduce the formation of dimers, thereby obtaining high-quality PMMA, the circulation flow rate of the reaction solution in the loop reactor is in the range of 0.2 to 0.8 m / s. Experiments have verified that the circulation velocity of the reaction liquid in the loop reactor is a crucial parameter. A low circulation velocity (e.g., below 0.2 m / s) leads to a thicker velocity boundary layer and a thicker heat transfer boundary layer. A thicker velocity boundary layer means more reaction liquid has excessive residence time. Furthermore, since the reaction liquid is a polymer solution, its viscosity is closely related to shear force. Prolonged exposure to high temperatures increases the dimer content, increasing the tendency for gelation in some low-shear reaction liquids, potentially leading to explosive polymerization. A thicker heat transfer boundary layer indicates deteriorated heat transfer. In this invention, some heat needs to be removed through the vaporization of saturated water in the outer jacket. A thicker heat transfer boundary layer reduces the overall heat transfer coefficient, resulting in uneven temperature distribution within the loop reactor, excessive temperature rise, and an increased tendency for gelation, potentially leading to explosive polymerization and decreased product quality. Conversely, an excessively high circulation velocity (e.g., above 0.8 m / s) causes excessive pressure drop in the reaction liquid within the loop, leading to excessive power consumption of the circulation pump. Most of this power is converted into heat and enters the reaction system, further deteriorating heat transfer.

[0017] In this invention, a variable frequency circulating pump can be selected to choose a suitable combination of circulation ratio, circulation flow rate, and circulation time. There are no restrictions on the type of circulating pump; it can be a melt pump, a gear pump, or a centrifugal pump suitable for high viscosity.

[0018] In this invention, the polymer concentration at the outlet of the polymerization reactor is 70-85 wt%, specifically 70 wt%, 72 wt%, 74 wt%, 76 wt%, 78 wt%, 80 wt%, 82 wt%, 84 wt%, or 85 wt%, preferably 75 wt% to 80 wt%.

[0019] To obtain high-quality polymers and ensure that almost no coloring components are generated even when the polymerization reactor is kept at high temperatures for extended periods, deoxygenation of the total feed is necessary. Theoretically, the lower the dissolved oxygen content, the better. However, in continuously operating units, the lower the dissolved oxygen content, the greater the cost. The dissolved oxygen in the total feed needs to be reduced to below 2 ppm by mass, preferably below 1 ppm by mass, and more preferably below 0.5 ppm by mass. The method of dissolved oxygen removal is not limited; for example, nitrogen stripping in a tower can be used to remove dissolved oxygen.

[0020] This invention combines the advantages of both batch reactors and tubular reactors, creatively proposing a process for preparing methacrylic acid polymers using a single loop reactor at high temperatures, resulting in a high-polymer-concentration reactant discharge. This invention places no special restrictions on the connection method or external layout of the loop reactor; however, long-radius bends, such as 5D bends or larger, should be used for the elbows.

[0021] Furthermore, the circulation time of the reaction liquid in the polymerization reactor is 0.5–10 min. For example, it can be 0.5 min, 1 min, 1.5 min, 2 min, 2.5 min, 3 min, 3.5 min, 4 min, 4.5 min, 5 min, 5.5 min, 6 min, 6.5 min, 7 min, 7.5 min, 8 min, 8.5 min, 9 min, 9.5 min, or 10 min, preferably 0.8–8 min, more preferably 1–5 min. Note that the circulation time mentioned here is different from the residence time. Residence time refers to the ratio of the loop reactor volume to the total feed flow rate, while circulation time refers to the ratio of the loop reactor volume to the circulation flow rate. In other words, with a fixed feed rate, the circulation time and circulation flow rate (i.e., circulation ratio × feed flow rate) are inversely proportional. Because the polymerization temperature in the loop reactor is very high and the polymerization rate is very fast, the circulation time cannot be too long. If the circulation time is too long (e.g., more than 10 min), there will be too much dimer and the viscosity of the system will increase. The thermal effect of the internal circulation pump will also be more obvious, increasing the risk of gelation. If the circulation time is too short (e.g., less than 0.5 min), the polymerization conversion rate will be too low.

[0022] Furthermore, the circulation ratio of the reaction liquid in the polymerization reactor (the ratio of the flow rate in the loop reactor to the total mass of the feed flow rate) is 25 to 80. For example, it can be 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80, etc.

[0023] Since the raw materials methyl methacrylate, alkyl acrylate, initiator, and chain transfer agent are introduced at the inlet of the circulating pump, and a static mixer is installed before entering the circulating pump, a circulation ratio that is too low (e.g., less than 25) will cause uneven temperature distribution within the loop reactor, excessive temperature rise, and an increased tendency for gelation, potentially leading to explosive polymerization and a decline in product quality. Conversely, a circulation ratio that is too high (e.g., greater than 80) will, on the one hand, result in excessive power consumption for the circulating pump within the loop reactor, increasing energy consumption. More importantly, the internal circulation pump power will be transferred to the reaction liquid as heat, making heat transfer difficult and further increasing the temperature rise. On the other hand, an excessively high circulation ratio will result in a short circulation time, ultimately leading to a decrease in polymerization conversion.

[0024] Furthermore, the minimum side pressure of polymerization in the polymerization reactor is P, and the saturated vapor pressure of the reaction liquid in the loop reactor at the polymerization reaction temperature is Ps. Therefore, Ps + 0.5 MPaG ≥ P ≥ Ps + 0.1 MPaG.

[0025] To ensure that there is no gas phase in the loop reactor, the minimum side pressure of polymerization should be greater than the saturated vapor pressure of the reaction liquid in the loop reactor. Specifically, it is determined by the saturated vapor pressure of the monomers at the polymerization reaction temperature. Let the minimum side pressure of polymerization be P (the minimum pressure is on the inlet side of the circulating pump). At the polymerization reaction temperature, the saturated vapor pressure of the reaction liquid in the loop reactor is Ps. Then, the following condition is satisfied: Ps + 0.5 MPaG ≥ P ≥ Ps + 0.1 MPaG.

[0026] Furthermore, the polymerization reactor is provided with a jacket for cooling the reaction liquid inside the polymerization reactor. The cooling medium inside the jacket is saturated water, the saturation temperature of the cooling medium is T2, and the average temperature inside the polymerization reactor (i.e. the polymerization reaction temperature) is T1, satisfying T2+2℃≤T1≤T2+10℃.

[0027] To obtain high-quality PMMA, the temperature difference within the loop reactor should not be too large. The loop reactor is equipped with jacket cooling. The temperature difference between the inside of the loop reactor and the jacket should not be too large. The cooling medium is saturated water. Let the saturation temperature of the cooling medium be T2, and the average temperature inside the loop reactor be T1. Then, the following conditions must be met: T2 + 2℃ ≤ T1 ≤ T2 + 10℃. Saturated water is used for heat transfer because saturated water has a constant vaporization temperature, a small internal and external temperature difference, and a high heat transfer coefficient. If the temperature difference is too large, it will lead to a wider molecular weight distribution and a decrease in quality of the product.

[0028] Furthermore, the polymerization monomers include methyl methacrylate and alkyl acrylate, with methyl methacrylate supplied to the polymerization reactor accounting for 80-100 wt% of the total polymerization monomers and alkyl acrylate supplied to the polymerization reactor accounting for 0-20 wt% of the total polymerization monomers.

[0029] The essential component of the polymerization monomer used in this invention is methyl methacrylate, which includes methyl methacrylate and alkyl acrylates. All methyl methacrylate supplied to the loop reactor accounts for 80-100 wt% of the total polymerization monomers, specifically 80 wt%, 82 wt%, 84 wt%, 86 wt%, 88 wt%, 90 wt%, 92 wt%, 94 wt%, 96 wt%, 98 wt%, or 100 wt%, preferably 85-95 wt%. All alkyl acrylates supplied to the loop reactor account for 0-20 wt% of the total polymerization monomers, specifically 0 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt%, 10 wt%, 12 wt%, 14 wt%, 16 wt%, 18 wt%, or 20 wt%, preferably 5-15 wt%. The alkyl acrylates can be, for example, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, etc. Among these, methyl acrylate or ethyl acrylate is preferred.

[0030] If a recycling section is involved in the subsequent process, the methyl methacrylate supplied to the loop reactor includes the sum of fresh methyl methacrylate and methyl methacrylate from the recycled material, and the alkyl acrylate supplied to the loop reactor includes the sum of fresh alkyl acrylate and alkyl acrylate from the recycled material.

[0031] Furthermore, the feed temperature of the polymerization monomer is -30 to 40°C.

[0032] Because this invention limits the circulation ratio, circulation flow rate, and circulation time, the jacket heat exchange area cannot be significantly adjusted. The cooling method is insufficient to remove the heat of reaction, and it is also necessary to remove some of the heat of reaction by lowering the feed temperature of the raw materials. For example, the feed temperature of the polymerization monomers (methyl methacrylate, alkyl acrylate) can be adjusted to -30 to 40°C, specifically -30°C, -25°C, -20°C, -15°C, -10°C, -5°C, 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, or 40°C.

[0033] Furthermore, based on 100 parts by mass of the polymerization monomer, the amount of the chain transfer agent is 0.05 to 0.5 parts by mass.

[0034] In this invention, the amount of chain transfer agent supplied is such that the methacrylic acid polymer generated through the polymerization reaction achieves the desired molecular weight and molecular weight distribution. Specifically, the amount of chain transfer agent supplied relative to 100 parts by mass of the polymerization monomer (the polymerization monomer ultimately fed to the polymerization reactor) is preferably 0.05 to 0.5 parts by mass, specifically 0.05 parts by mass, 0.10 parts by mass, 0.15 parts by mass, 0.20 parts by mass, 0.25 parts by mass, 0.30 parts by mass, 0.35 parts by mass, 0.40 parts by mass, 0.45 parts by mass, 0.50 parts by mass, etc., and more preferably 0.1 to 0.3 parts by mass.

[0035] Examples of chain transfer agents used in this invention include: n-octyl mercaptan, n-dodecyl mercaptan, tert-dodecyl mercaptan, 1,4-butanedithiol, 1,6-hexanedithiol, ethylene glycol dithiopropionate, butanediol dithioglycolate, butanediol dithiopropionate, hexanediol dithioglycolate, and hexanediol dithiopropionate. Among these, alkyl mercaptans with 12 or fewer carbon atoms are preferred, n-octyl mercaptan or n-dodecyl mercaptan are more preferred, and n-octyl mercaptan is even more preferred. These chain transfer agents can be used alone or in combination of two or more.

[0036] Furthermore, based on 100% by mass of the total monomers used in the polymerization reaction, the amount of the initiator is 50 to 400 ppm.

[0037] In this invention, since the polymerization reaction temperature in the loop reactor is very high and the cycle time is short, it is necessary to select an initiator with a short half-life, such as an initiator with a half-life of 0.05s to 600s at the polymerization reaction temperature, preferably an initiator with a half-life of 0.1s to 300s at the polymerization reaction temperature, and more preferably an initiator with a half-life of 0.5s to 100s at the polymerization reaction temperature.

[0038] The initiator can be, for example, one of tert-butyl cumene peroxide, 1,1,3,3-tetramethylbutyl hydroperoxide, di-tert-butyl cumene peroxide, isopropyl hydroperoxide, cyclohexanone peroxide, benzoyl peroxide, and di-tert-butyl peroxide.

[0039] The amount of initiator used is closely related to the polymerization temperature. The higher the polymerization temperature, the less initiator is needed. However, since the present invention requires that the cycle time not be too long, the amount of initiator used cannot be too little. For example, based on 100 parts by mass of the polymerization monomer (the polymerization monomer ultimately supplied to the polymerization reactor), the amount of initiator is preferably 50 to 400 ppm, specifically 50 ppm, 70 ppm, 90 ppm, 110 ppm, 130 ppm, 150 ppm, 170 ppm, 190 ppm, 210 ppm, 230 ppm, 250 ppm, 270 ppm, 290 ppm, 310 ppm, 330 ppm, 350 ppm, 370 ppm, 390 ppm or 400 ppm, etc., and more preferably 100 to 300 ppm.

[0040] The aforementioned chain transfer agent and initiator can be dissolved in a portion of the polymerization monomer, mixed with the polymerization monomer, and then injected into the loop reactor. This portion of the polymerization monomer needs to be included in the aforementioned mass percentage of the polymerization monomer supplied to the loop reactor.

[0041] Furthermore, the polymerization reactor is powered by a circulating pump, and the polymerization monomer, the initiator, and the chain transfer agent are introduced at the inlet of the circulating pump, with a static mixer provided before entering the circulating pump.

[0042] Because the reaction liquid circulation ratio and flow rate are high in the loop reactor, it is not appropriate to install mixing elements in all parts of the loop reactor. At least one static mixer can be installed in other locations of the loop reactor (except before the inlet of the circulation pump mentioned above) to enhance the mixing effect.

[0043] On the other hand, the present invention also provides a methacrylic polymer, which is prepared by the above-described method for preparing methacrylic polymers.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present invention uses a loop reactor and obtains a product with high polymerization conversion rate by setting a series of operating conditions. By controlling the polymerization reaction temperature, the viscosity of the reaction liquid is effectively reduced, the probability of gelation effect is reduced, thereby improving the polymerization conversion rate. The polymer concentration at the outlet of the polymerization reactor is 70wt% to 85wt%.

[0045] 2. This invention effectively reduces the formation of dimers by controlling parameters such as polymerization reaction temperature and the flow rate of the reaction liquid in the polymerization reactor. Experimental results show that at high temperatures, the viscosity of the reaction liquid decreases, and the probability of gelation also decreases. However, excessively high temperatures lead to increased dimer formation, thus affecting the quality of PMMA. The operating conditions of this invention effectively balance the viscosity of the reaction liquid and the formation of dimers, thereby obtaining high-quality PMMA. Detailed Implementation

[0046] The present invention will be described in more detail below through examples, but these examples do not limit the invention.

[0047] The present invention provides a method for preparing a methacrylic acid polymer, comprising the following steps: supplying polymerization monomers, chain transfer agents and initiators to a polymerization reactor, carrying out a polymerization reaction in the polymerization reactor, the polymerization reaction temperature being 200-260°C, the polymerization reactor being a loop reactor, the polymerization reactor having no gas phase, the flow rate of the reaction liquid in the polymerization reactor being 0.2-0.8 m / s, and the polymer concentration at the outlet of the polymerization reactor being 70-85 wt%.

[0048] This invention effectively reduces dimer formation by controlling parameters such as reaction temperature and the flow rate of the reaction liquid in the polymerization reactor. Experimental results show that at high temperatures, the viscosity of the reaction liquid decreases, and the probability of gelation also decreases. However, excessively high temperatures lead to increased dimer formation, thus affecting the quality of PMMA. The operating conditions of this invention effectively balance the reaction liquid viscosity and dimer formation, resulting in high-quality PMMA.

[0049] The formation mechanism of dimers is relatively complex, generally related to initiator concentration, monomer concentration, operating temperature, and cycle time. Dimers are polymerization byproducts that primarily affect the transparency and mechanical strength of PMMA. Excessive formation has a significant impact on product performance. They mainly arise from two unsaturated methyl methacrylates reacting via a Diels-Aldertype mechanism to form the unsaturated dimer 2-methyl-5-methylene-adipic acid-dimethyl ester. The reaction formula for the dimer is as follows:

[0050]

[0051] The gel effect, also known as autoacceleration, is a phenomenon where the polymerization rate increases significantly as the polymerization reaction progresses to a certain stage. If the reaction solution is not mixed uniformly or has dead zones, these areas will undergo molecular chain cross-linking and physical entanglement, causing localized melting states to transform into gel states, significantly increasing viscosity, further deteriorating the process, and even posing a risk of explosive polymerization. While batch reactors can only use stirring to ensure uniform mixing of the reaction solution, they obviously have many dead zones, increasing the tendency for the gel effect to occur. Furthermore, higher temperatures result in faster polymerization rates; therefore, batch reactors can only reduce the probability of the gel effect by lowering the polymerization temperature and reducing the polymerization conversion rate. This invention, however, uses a loop reactor and, by setting a series of operating parameters, obtains a product with a high polymerization conversion rate.

[0052] Based on the zero-shear viscosity and PMMA content diagram of the PMMA polymer solution in the literature "High-temperature radical polymerization of methylmethacrylate in a continuous pilot scale process", it can be approximately estimated that the zero-shear viscosity of a PMMA solution with a content of 85 wt% at 260℃ is about 10 Pa·s, while at 200℃, the zero-shear viscosity of a PMMA solution with a content of 70 wt% is about 7 Pa·s. The flow rate of the reaction liquid in the loop reactor ranges from 0.2 to 0.8 m / s, which also makes the actual fluid viscosity lower than the zero-shear viscosity. Multiple experiments have demonstrated that using the method of this invention, the reaction liquid viscosity can be kept below 30 Pa·s at the polymerization temperature, thus virtually eliminating the gel effect. As can be seen from the various examples, no abnormal polymerization was observed. Of course, the zero-shear viscosity of the solution is also related to the molecular weight of PMMA; the higher the molecular weight, the higher the zero-shear viscosity. If this process is used to obtain PMMA products with higher molecular weights, it is recommended to select a polymer concentration range of 75 wt% to 80 wt% at the reactor outlet.

[0053] The following are specific embodiments of the present invention.

[0054] Example 1

[0055] The polymerization monomers (including 85 wt% methyl methacrylate and 15 wt% methyl acrylate), 0.1 wt% n-octyl mercaptan relative to the total amount of polymerization monomers, and 250 ppm di-tert-butyl peroxide relative to the total amount of polymerization monomers (the meanings of the contents in the following examples and comparative examples are as described in Example 1) were mixed and purged with nitrogen to reduce the dissolved oxygen content in the mixture to below 0.5 ppm. A sufficient amount of the reaction mixture was prepared and maintained at -30 °C. The reaction mixture was injected into the loop reactor at a total feed rate of 20 kg / h using a feed pump. At the same time, the jacket cooling of the loop reactor was turned on, and the cooling medium was saturated water with a saturation temperature of 197 °C (saturation pressure of approximately 1.38 MPaG). The lowest side pressure of the loop reactor was maintained at 1.3 MPaG, and the circulation pump flow rate was adjusted to 0.768 m3 / h (under these polymerization conditions, the density of the reaction mixture is approximately 912 kg / m3). The loop reactor has a specification of Φ42×3 mm and a jacket length of 50 mm. The loop reactor (approximately 52m in length, with no jacket at the bend, only insulation) has temperature measuring points installed approximately every 7 meters, for a total of 8 points. The reaction product continuously flows out of the loop reactor and enters the subsequent devolatilization and extrusion process to remove volatiles, followed by granulation to obtain methyl methacrylate polymer particles. After 10 hours of continuous and stable operation, the average temperature observed from the 8 thermometers was 200.9℃. The particle appearance was directly observed. Other parameters and product information are shown in Table 1.

[0056] Parameter settings and performance testing:

[0057] Circulation rate: controlled by frequency converter of the circulation pump.

[0058] The circulation velocity (v, m / s) is calculated from parameters such as the circulation ratio (k), total feed (F, kg / h), average density of the reaction liquid (ρ, kg / m3), and reactor inner diameter (d, mm). The calculation formula is: v = k * F / ρ * 353.86 / d / d.

[0059] The circulation time (t, min) is calculated from parameters such as the circulation ratio (k), total feed (F, kg / h), average density of the reaction liquid (ρ, kg / m3), reactor inner diameter (d, mm), and total reactor length (L, m). The calculation formula is: t = 0.785 * (d / 1000) * (d / 1000) * L / (k * F / ρ) * 60.

[0060] Other parameters can be obtained or set using industry-standard methods.

[0061] Dimer and polymer content: In the extrusion devolatilization section, all volatiles are condensed and collected, and the dimer content of the collected liquid is determined by gas chromatography [the dimer boiling point in this invention is 236℃, and the viscosity is 25 cp (25℃)]; in addition, the total amount of all volatiles is measured, and the remaining non-volatiles are the polymer, from which the polymer content can be calculated.

[0062] Product weight-average molecular weight: determined by gel permeation chromatography (GPC) according to GB / T 21863-2008, using tetrahydrofuran as the eluent, and processed by a GPC data processing device.

[0063] Whether the product is foamed: Manually observe the finished particles to check for the presence of air bubbles. √ indicates the presence of air bubbles, and × indicates the absence of air bubbles.

[0064] Examples 2-7

[0065] The process is basically the same as in Example 1, except that the parameters such as feed ratio, feed amount, polymerization operation conditions, raw material feed temperature, and jacket tube temperature are different. The parameters and results of each example are shown in Table 1.

[0066] Table 1

[0067]

[0068] Comparative Examples 1-6

[0069] The process is basically the same as in Example 1, except that the parameters such as feed ratio, feed amount, polymerization operation conditions, raw material feed temperature, and jacket temperature are different. The parameters and results of each comparison are shown in Table 2.

[0070] Table 2

[0071]

[0072] As can be seen from Tables 1 and 2 above, in Examples 1-7 and Comparative Examples 1-2, no bubbles were found in the finished particles upon visual inspection, and the appearance of the molded particles was good. However, in Comparative Examples 3-6, some particles were found to contain bubbles, and some particles also showed silver streaks. Tables 1 and 2 show that under the process conditions of this invention, Examples 1-7 all achieved good results, with the polymer content at the polymerization reactor outlet meeting the requirements, and the dimer content and product quality were all satisfactory. For Comparative Examples 1 and 2, at 190℃, the polymer content was low, resulting in high energy consumption in the subsequent devolatilization process. For Comparative Examples 3 and 4, at 270℃, although the polymer content was high, the dimer content was also very high. These dimers were difficult to remove completely in subsequent processes (if the recovered monomers were recycled back into the feed, the dimer concentration would be even higher), directly affecting product quality. Comparative Example 5 used a lower recycling rate (to avoid excessive polymerization, the reaction temperature needed to be lowered, and the reaction temperature was 222.7℃), and the recycling time reached 8.9 min, resulting in a product with a high polymer content and a large molecular weight. However, the dimer content was seriously excessive. Comparative Example 6 used a higher recycling rate, resulting in a product with a slightly lower molecular weight and a higher dimer content. More importantly, the pressure on the lowest side of the reactor reached 6.7 MPaG, the pressure drop in the circulation pipeline was too high, the heating effect of the internal circulation pump was very significant, heat transfer deteriorated, and product quality declined. In Comparative Examples 3-6, some particles exhibited air bubbles, and a small number of particles also showed silver streaks, suggesting that a gelation effect may have occurred.

[0073] As can be seen from the above embodiments, under the parameter settings of reaction temperature, circulation flow rate, circulation ratio, jacket temperature, etc., as defined by the present invention, qualified PMMA products with high conversion rate can be obtained.

Claims

1. A method for producing a methacrylic polymer, characterized by, The method comprises the following steps: polymerization monomers, chain transfer agent and initiator are supplied to a polymerization reactor, polymerization is carried out in the polymerization reactor, the polymerization temperature is 200-260℃, the polymerization reactor is a loop reactor, there is no gas phase in the polymerization reactor, the circulating flow rate of the reaction liquid in the polymerization reactor is 0.2-0.8m / s, the polymer concentration at the outlet of the polymerization reactor is 70-85wt%, and the circulating ratio of the reaction liquid in the polymerization reactor is 25-80.

2. The method of preparing a methacrylic polymer according to claim 1, wherein The circulating time of the reaction liquid in the polymerization reactor is 0.5-10min.

3. The method for preparing a methacrylic polymer according to claim 1, characterized in that, The minimum side pressure of polymerization in the polymerization reactor is P, the saturated steam pressure of the reaction liquid in the polymerization reactor at the polymerization temperature is Ps, and Ps+0.5MPaG≥P≥Ps+0.1MPaG.

4. The method for preparing a methacrylic polymer according to claim 1, characterized in that, A jacket for cooling the reaction liquid in the polymerization reactor is arranged on the polymerization reactor, the cooling medium in the jacket is saturated water, the saturated temperature of the cooling medium is T2, the polymerization temperature is T1, and T2+2℃≤T1≤T2+10℃ is satisfied.

5. The method for preparing a methacrylic polymer according to claim 1, characterized in that, The feeding temperature of the polymerization monomers is -30-40℃.

6. The method for preparing a methacrylic polymer according to claim 1, characterized in that, The chain transfer agent comprises one or two or more of n-octyl mercaptan, n-dodecyl mercaptan, tert-dodecyl mercaptan, 1,4-butanediol, 1,6-hexanediol, ethylene glycol dithiopropionic acid ester, butanediol dithioethanol acid ester, butanediol dithiopropionic acid ester, hexanediol dithioethanol acid ester and hexanediol dithiopropionic acid ester.

7. The method for preparing a methacrylic polymer according to claim 1, characterized in that, The initiator comprises one of tert-butyl isopropyl benzene peroxide, 1,1,3,3-tetramethyl butyl hydrogen peroxide, di-tert-butyl isopropyl benzene peroxide, isopropyl hydrogen peroxide, cyclohexanone peroxide, benzoyl peroxide and di-tert-butyl peroxide.

8. The method for preparing a methacrylic polymer according to claim 1, characterized in that, The circulating power is provided by a circulating pump in the polymerization reactor, the polymerization monomers, the initiator and the chain transfer agent are introduced at the inlet of the circulating pump, and a static mixer is arranged before the circulating pump.

9. A methacrylic polymer, characterized by, The method is used for preparing the methacrylic polymer. The method is used for preparing the methacrylic polymer.

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