Methacrylic polymer and preparation method thereof

Abstract

The invention 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: supplying a polymerization reaction monomer, a chain transfer agent and an initiator to a polymerization reactor, and carrying out polymerization reaction in the polymerization reactor at the temperature of 200-260 DEG C, no gas phase exists in the polymerization reactor, the flow velocity 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%. According to the invention, a loop reactor is adopted, and a series of operation conditions are set, so that a product with high polymerization conversion rate is obtained. By controlling the polymerization reaction temperature, the viscosity of the reaction liquid is effectively reduced, and the occurrence probability of the gel effect is reduced, so that the polymerization conversion rate is increased, and the polymer concentration at the outlet of the polymerization reactor is 70-85wt%.

Description

Technical Field

[0001] The invention relates to the technical field of polymer synthesis, in particular to a methacrylic acid polymer and a preparation method thereof. Background Art

[0002] Polymethyl methacrylate (PMMA) is an important polymer widely used in optics, electronics, automobiles and other fields. The continuous production methods of PMMA mainly include solution polymerization and bulk polymerization. The investment in solution polymerization production equipment is large, and specific equipment and process conditions are required. In addition, unavoidable side reactions will occur during the polymerization process. In addition, solution polymerization requires the use of solvents, and the recovery and treatment process of the solvents may be more complicated, increasing production costs and environmental pressure. The advantages of bulk polymerization are: no solvent is used, no solvent separation and recovery process is required, and the purity and transmittance of the product can be effectively improved. However, bulk polymerization has the disadvantages of difficulty in controlling the heat of reaction and the conversion rate. Unstable reaction heat control can easily lead to unstable temperature of the reaction system, local gel effect, affecting the quality of the final product, and severe explosion and safety accidents; too low a conversion rate, on the one hand, increases the equipment processing requirements of the monomer recovery process and increases energy consumption, and on the other hand, too high a monomer content in the product affects the quality of the final product.

[0003] Polymerization reactors are usually of two types: kettle type and tubular type. A kettle reactor is a closed container with internal heating or cooling. Compared with a tubular reactor, a kettle reactor is usually larger in size. Due to its large size, a kettle reactor can process a large amount of materials, has a wide range of applications, requires little investment, and is easy to put into production, but the reaction temperature is difficult to control and the residence time is inconsistent; whereas in a tubular reactor, since the molecules of the reactants have an equal residence time in the reactor, the concentration of the reactants and the chemical reaction rate at any point in the reactor do not change with time, but only with the length of the tube. Since the reactants react quickly and flow quickly in a tubular reactor, its production capacity is high. Compared with a kettle reactor, it has less backmixing, and at a lower flow rate, the flow pattern of the fluid in its tube is close to that of an ideal fluid.

[0004] Tank reactors have a small footprint and a large processing capacity, so most industrial solution polymerizations or bulk polymerizations use tank reactors. However, tank reactors cannot guarantee adequate stirring, that is, there may be dead zones. Due to the gel effect in PMMA polymerization (when the system viscosity is large, the polymerization will be automatically accelerated), this also leads to the polymer content of PMMA bulk polymerization in the tank reactor cannot be too high. Usually, the polymer content of PMMA bulk polymerization in the tank reactor is about 30wt% to 65wt% to avoid the gel effect in the tank reactor. In order to achieve a higher polymerization conversion rate and reduce the energy consumption of devolatilization, people have connected a straight tube reactor in series after the tank reactor (because the tank reactor still has uneven stirring, the polymerization temperature and polymerization conversion rate are not easy to control, which leads to the gel effect, so the series reactor is usually a tubular reactor). In order to reduce the gel effect, the temperature in the tubular reactor is usually higher than that in the tank reactor, and it is usually necessary to install a mixing element (such as a static mixer) in the tubular reactor so that the reaction liquid in the tubular reactor can be fully mixed in the radial direction. Usually, the polymer content at the outlet of the straight tube reactor connected in series after the autoclave reactor is about 50wt% to 80wt%.

[0005] The polymerization temperature in the polymerization reactor is also very important for the polymerization equilibrium conversion rate. However, there is no unified understanding of the mechanism of the polymerization temperature on the polymerization equilibrium conversion rate. For example, Japanese patent JP2000026507A discloses: in the production of acrylic resin, a tubular reactor is used after a tank reactor to obtain a method for producing acrylic resin with balanced productivity and physical properties. In this patent, an example with a maximum final polymerization rate of 72% is recorded, but there is no description of how to further increase the polymerization rate. Japanese patent JP2003002912A discloses: a description of polymerization inhibition using the equilibrium polymerization rate (equilibrium polymerization), and the polymerization rate is determined by the final temperature of the polymer. In this patent, an example with a maximum final polymerization rate of 68% is recorded, but it is believed that the final polymerization rate exceeding this value cannot be achieved. In addition, there is no mention of the factors that determine the equilibrium polymerization rate. Chinese patent CN102933610A believes that in the polymerization in a tubular reactor, the monomer conversion rate can be increased by adding a specified amount of initiator. It is also known that by increasing the amount of methyl acrylate used as a copolymer component of methacrylic acid polymers, the monomer conversion rate can be increased even with the same initiator amount as before. The patent records an embodiment in which the highest final polymerization rate is 83%. Chinese patent CN1576285A discloses in the specification that if the polymerization temperature is higher than 170°C, the polymerization reaction will proceed stably, so the polymerization rate can be increased. However, due to the increase in the formation of dimers, there is a tendency for the transparency and mechanical strength of the polymer to decrease after the volatile components are removed. The above-mentioned public patent documents also illustrate that the conditions for the equilibrium conversion rate of PMMA polymerization are relatively complex and the mechanism is still unclear.

[0006] There are many methods for producing methacrylic acid polymers in a single reactor. For example, Japanese patent JP1998087705A discloses a method for producing optical quality methacrylic acid resins, and describes in the specification: 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 that in a single reactor, the temperature is maintained at 130°C to 170°C and uniform stirring and mixing is performed, and the polymer concentration (weight fraction) in the reactor is 0.40 to 0.70. Japanese patent JP2000053709A discloses that the temperature in the reactor is maintained at 130 to 170°C and uniform stirring and mixing is performed, while the average residence time of the reaction liquid in the reactor is 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 that a single complete reactor is used, the reaction temperature is 110-170°C, and the polymer is obtained by increasing or decreasing the supply amount of free radical polymerization initiator. The polymer content of the reaction liquid composition is substantially maintained in the range of 35-70% by mass, but the embodiment does not give the polymer concentration. Chinese patent CN1102833A discloses a fully liquid-operated adiabatic stirring reactor, which performs polymerization reaction in the polymerization reaction 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 a methacrylic polymer composition, which uses a single fully mixed reactor, the temperature of the raw material mixture provided to the reactor is -50°C to -10°C, and the polymerization temperature is 120-150°C. The patent mentions in the specification that the polymerization rate is, for example, 30wt%-90wt%, but no theoretical explanation or example proof is provided.

[0007] According to the zero shear viscosity and PMMA content graph of PMMA polymer solution in the document "high-temperature radical polymerization of methylmethacrylate in a continuous pilot scale process", it can be seen that at the upper limit polymerization temperature of 150°C disclosed in patent CN104379616A, and without considering other issues, the viscosity will reach about 70,000 Pa·s with only a 90% polymerization rate, which is obviously 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 80wt%, the zero shear viscosity reaches about 1700 Pa·s. Such a high viscosity will also increase the tendency of gel effect, and the selection of agitators will become relatively difficult.

[0008] Since the polymerization rate of a single reactor is low, many processes use a series connection to increase the polymerization rate. For example, Chinese patent CN1576286A discloses polymerization in a series connection of a complete reactor and a tubular reactor, the reaction temperature is 110-170°C, the relationship between the inner diameter of the tubular reactor and the linear velocity is specified, and the polymer content of the reaction liquid composition is substantially maintained in the range of 35-70% by mass, preferably 65% ​​or less, preferably 40% or more, but the embodiment does not give the polymer concentration. Japanese patent JP2000026507A uses a completely mixed reactor and a plug flow reactor and a volatile removal device arranged in series to produce a monomer polymer or copolymer of methyl methacrylate, wherein in the completely mixed reactor, uniform stirring and mixing are carried out at 110-160°C to make the polymer content of the reaction zone reach 35-65% by weight, in the plug flow reactor, the temperature of the inner wall of the reactor is set to be above the polymerization temperature of the first stage of the completely mixed reactor but not more than 250°C, and the polymer content at the outlet reaches 50-85% by weight. Japanese patent JP2003002912A discloses a method for producing a polymer or copolymer of methyl methacrylate using a completely mixed reactor (A) and a subsequent reactor (B). The completely mixed reactor (A) stirs and mixes the reaction liquid composition at a polymerization temperature of 110 to 170°C so that the polymer content in the reaction zone reaches 35 to 65wt%. According to the polymer content relationship formula provided by the reactor (B), it can be seen that the reaction temperature of the reactor (B) is 180 to 200°C, and the polymer content reaches 50 to 85wt% (related to the polymerization temperature). Chinese patent CN108424479A discloses a continuous polymerization device, which includes at least a first and a second reactor of a fully mixed type, the reaction temperature of the two reactors is 120-150°C, wherein the polymerization ratio of the discharge of the first reactor is, for example, 5 to 80 wt%, and the polymerization ratio of the discharge of the second reactor is, for example, 30 to 90 wt%; according to its specification, the cooling device will cool 5-80°C, that is, 145°C may match 80 wt%, and the second reactor 150°C may match 90 wt%, and there are also similar problems as patent CN104379616A.

[0009] The loop reactor (i.e., the loop tube reactor) has efficient mixing performance and excellent temperature control, and can 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 loop 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 loop 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 loop reactors and straight tube reactors to obtain a polymer product. In the loop 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 loop reactor and a straight tube reactor in series. In the loop 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 a higher temperature 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: On the one hand, the present invention provides a method for preparing a methacrylic acid polymer, comprising the following steps: supplying a polymerization reaction monomer, a chain transfer agent, and an initiator to a polymerization reactor, performing a polymerization reaction in the polymerization reactor, wherein the polymerization reaction temperature is 200 to 260° C., the polymerization reactor is a loop reactor, and there is no gas phase in the polymerization reactor, the circulation flow rate of the reaction liquid in the polymerization reactor is 0.2 to 0.8 m / s, and the polymer concentration at the outlet of the polymerization reactor is 70 to 85 wt %. The following component contents are all weight contents unless otherwise specified.

[0012] The reaction liquid in the polymerization reactor in the present invention refers to the raw material liquid of the polymerization reaction monomer, chain transfer agent, initiator, etc. supplied to the polymerization reactor.

[0013] The polymerization reaction temperature in the present invention is the average value of the temperature of the loop reactor (the average value can be calculated by setting a plurality of temperature measuring points in the loop reactor, or can be calculated by theoretical calculation). The polymerization reaction temperature can be 200°C, 210°C, 220°C, 230°C, 240°C, 250°C or 260°C, etc., preferably 220-240°C.

[0014] The circulation velocity of the reaction liquid in the polymerization reactor of the present invention is 0.2-0.8 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.

[0015] Since the viscosity of the reaction liquid decreases at high temperature, the probability of the gel effect also decreases. In order to further reduce the probability of the gel effect and reduce the production of dimers to obtain high-quality PMMA, the circulation flow rate of the reaction liquid in the annular reactor ranges from 0.2 to 0.8 m / s. It has been verified by experiments that the circulation velocity of the reaction liquid in the loop reactor is a very important parameter. If the circulation velocity is too low (for example, less than 0.2 m / s), the velocity boundary layer and the heat transfer boundary layer will become thicker. The thickening of the velocity boundary layer means that there is more reaction liquid retention time. In addition, the reaction liquid is a polymer solution, and the viscosity and shear force are closely related. When it is in a high temperature environment for a long time, the dimer content increases, and the tendency of some reaction liquids with smaller shear force to produce gel effect also increases, and there is a possibility of implosion; and the thickening of the heat transfer boundary layer means that the heat transfer is deteriorated. In the present invention, part of the heat needs to be taken away by the saturated water vaporization of the outer jacket. The thickening of the heat transfer boundary layer causes the total heat transfer coefficient to become smaller, thereby causing uneven temperature distribution in the loop reactor, excessive temperature rise, and increased tendency to produce gel effect. There is a possibility of implosion, resulting in a decrease in product quality; and an excessively high circulation velocity (for example, higher than 0.8 m / s) will cause the reaction liquid in the loop to drop too much, resulting in excessive power of the circulation pump, and most of the power of the circulation pump will be converted into heat and enter the reaction system, so that the heat transfer is deteriorated.

[0016] In the present invention, a circulating pump with frequency conversion can be selected to select a suitable combination of circulation ratio, circulation flow rate and circulation time. The type of the circulating pump is not limited, and can be a melt pump, a gear pump or a centrifugal pump that can adapt to high viscosity.

[0017] The polymer concentration at the outlet of the polymerization reactor in the present invention is 70-85wt%, specifically 70wt%, 72wt%, 74wt%, 76wt%, 78wt%, 80wt%, 82wt%, 84wt% or 85wt%, etc., preferably 75wt% to 80wt%.

[0018] In order to obtain high-quality polymers and ensure that almost no coloring components are produced even when the high temperature is maintained for a long time in the polymerization reactor, the total feed raw materials need to be deoxygenated. In theory, the lower the dissolved oxygen content, the better. However, in a continuously operating device, the lower the dissolved oxygen content is removed, the greater the cost. The dissolved oxygen in the total feed needs to be removed to less than 2 mass ppm, preferably less than 1 mass ppm, and more preferably less than 0.5 mass ppm. The operation of removing dissolved oxygen is not limited, for example, it can be to remove dissolved oxygen by nitrogen stripping in a tower.

[0019] The present invention combines the advantages of a kettle reactor and a tubular reactor, and creatively proposes a process for preparing a methacrylic acid polymer at a high temperature using a single loop reactor to obtain a reactant discharge with a high polymer concentration. The present invention has no special restrictions on the connection mode and shape layout of the loop reactor, and the elbow should be a long radius elbow, such as a 5D elbow or larger.

[0020] Further, the circulation time of the reaction liquid in the polymerization reactor is 0.5 to 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 to 8 min, more preferably 1 to 5 min. Note that the circulation time mentioned here is different from the residence time. The residence time refers to the ratio of the volume of the loop reactor to the total feed flow rate, while the circulation time refers to the ratio of the volume of the loop reactor to the circulation flow rate. In other words, when the feed amount is constant, the circulation time is inversely proportional to the circulation flow rate (i.e., circulation ratio × feed flow rate). Since the polymerization reaction temperature of the loop reactor is very high and the polymerization rate is very fast, the cycle time cannot be too long. If the cycle time is too long (for example, greater than 10 minutes), it will lead to excessive dimers, increase the viscosity of the system, and make the thermal effect of the internal circulation pump more obvious, thereby increasing the risk of gel effect. If the cycle time is too short (for example, less than 0.5 minutes), the polymerization conversion rate will be too low.

[0021] 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.

[0022] Since the raw materials methyl methacrylate, alkyl acrylate, initiator and chain transfer agent are introduced at the inlet of the circulation pump, and a static mixer is set before entering the circulation pump, a circulation ratio that is too low (e.g. less than 25) will cause uneven temperature distribution in the loop reactor, excessive temperature rise, increased tendency of gel effect, and the possibility of implosion, resulting in a decrease in product quality. A circulation ratio that is too high (e.g. greater than 80) will, on the one hand, make the power of the circulation pump in the loop reactor too large, which will not only increase energy consumption, but more importantly, the power of the internal circulation pump will be transferred to the reaction liquid in the form of heat, which will make it difficult to transfer heat from the reaction liquid, which will increase the temperature rise. On the other hand, a circulation ratio that is too high will result in a short circulation time, which will ultimately lead to a decrease in polymerization conversion rate.

[0023] Furthermore, the lowest polymerization side pressure in the polymerization reactor is P, and at the polymerization reaction temperature, the saturated vapor pressure of the reaction liquid in the loop reactor is Ps, then Ps+0.5MPaG≥P≥Ps+0.1MPaG.

[0024] In order to ensure that there is no gas phase in the loop reactor, the lowest side pressure of the 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 polymerization monomer at the polymerization reaction temperature. The lowest side pressure of the polymerization is denoted as P (the lowest pressure is at the inlet side of the circulation pump). At the polymerization reaction temperature, the saturated vapor pressure of the reaction liquid in the loop reactor is Ps, which satisfies Ps+0.5MPaG≥P≥Ps+0.1MPaG.

[0025] Furthermore, the polymerization reactor is provided with a jacket for cooling the reaction liquid in the polymerization reactor, the cooling medium in the jacket is saturated water, the saturation temperature of the cooling medium is T2, the average temperature in the polymerization reactor (i.e., the polymerization reaction temperature) is T1, and T2+2°C≤T1≤T2+10°C is satisfied.

[0026] In order to obtain high-quality PMMA, the temperature difference in the loop reactor should not be too large. The loop reactor is equipped with a jacket cooling. The temperature difference between the loop reactor and the jacket should not be too large. The cooling medium is saturated water. If the saturated temperature of the cooling medium is T2 and the average temperature in the loop reactor is T1, then T2+2℃≤T1≤T2+10℃ is satisfied. Saturated water is used for heat transfer because the saturated water vaporization temperature is constant, the temperature difference between the inside and outside is small, and the heat transfer coefficient is large. If the temperature difference is too large, the molecular weight distribution of the product will become wider and the quality will decrease.

[0027] Furthermore, the polymerization reaction monomers include methyl methacrylate and alkyl acrylate, the methyl methacrylate supplied to the polymerization reactor accounts for 80-100wt% of the total polymerization reaction monomers, and the alkyl acrylate supplied to the polymerization reactor accounts for 0-20wt% of the total polymerization reaction monomers.

[0028] The essential component of the polymerization monomer used in the present invention is methyl methacrylate, and the polymerization monomer includes methyl methacrylate and alkyl acrylate. All methyl methacrylate supplied to the loop reactor accounts for 80-100wt% of the total polymerization monomer, and can be specifically 80wt%, 82wt%, 84wt%, 86wt%, 88wt%, 90wt%, 92wt%, 94wt%, 96wt%, 98wt% or 100wt%, etc., preferably 85-95wt%. All alkyl acrylates supplied to the loop reactor account for 0-20wt% of the total polymerization monomer, and can be specifically 0wt%, 2wt%, 4wt%, 6wt%, 8wt%, 10wt%, 12wt%, 14wt%, 16wt%, 18wt% or 20wt%, etc., preferably 5-15wt%. The alkyl acrylate can be listed as: methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, etc. Among these, methyl acrylate or ethyl acrylate is preferred.

[0029] If there is a recovery section in the subsequent process, the methyl methacrylate supplied to the loop reactor includes the sum of fresh methyl methacrylate and methyl methacrylate in the recovered material, and the alkyl acrylate supplied to the loop reactor includes the sum of fresh alkyl acrylate and alkyl acrylate in the recovered material.

[0030] Furthermore, the feeding temperature of the polymerization reaction monomer is -30 to 40°C.

[0031] Since the present invention limits the circulation ratio, circulation flow rate and circulation time, the heat exchange area of ​​the jacket cannot be greatly adjusted, and the cooling method is not sufficient to remove the reaction heat. It is also necessary to remove part of the reaction heat by lowering the feed temperature of the raw materials. For example, the feed temperature of the polymerization reaction monomer (methyl methacrylate, alkyl acrylate) is 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, etc.

[0032] Furthermore, based on 100 parts by mass of the polymerization reaction monomer, the amount of the chain transfer agent is 0.05 to 0.5 parts by mass.

[0033] In the present invention, the amount of the chain transfer agent supplied is an amount that can achieve the desired molecular weight and molecular weight distribution of the methacrylic acid polymer produced by the polymerization reaction. Specifically, the amount of the chain transfer agent supplied is preferably 0.05 to 0.5 parts by mass relative to 100 parts by mass of the polymerization monomer (polymerization monomer finally fed to the polymerization reactor), and can be 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.

[0034] As the chain transfer agent used in the present invention, there can be mentioned: n-octyl mercaptan, n-dodecyl mercaptan, tert-dodecyl mercaptan, 1,4-butanedithiol, 1,6-hexanedithiol, ethylene glycol dithiopropionate, butanediol dithioglycolate, butanediol dithiopropionate, hexanediol dithioglycolate, hexanediol dithiopropionate, etc. Among these, alkyl mercaptans having a carbon number of 12 or less are preferred, n-octyl mercaptan or n-dodecyl mercaptan is more preferred, and n-octyl mercaptan is further preferred. These chain transfer agents can be used alone or in combination of two or more.

[0035] Furthermore, based on 100% by mass of the total amount of the polymerization reaction monomers, the amount of the initiator used is 50 to 400 ppm.

[0036] In the present 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 shorter half-life, for example, 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.

[0037] The initiator may 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.

[0038] The amount of initiator used is closely related to the polymerization reaction temperature. The higher the polymerization reaction temperature, the less the amount of initiator used. However, because the present invention requires that the cycle time cannot be too long, the amount of initiator used cannot be too small. For example, based on 100 parts by mass of the polymerization reaction monomer (the polymerization reaction monomer finally supplied to the polymerization reactor), the amount of the initiator used 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, and more preferably 100 to 300 ppm.

[0039] The chain transfer agent and initiator can be dissolved with part of the polymerization monomers, mixed with the polymerization monomers and then injected into the loop reactor. This part of the polymerization monomers needs to be included in the above-mentioned mass proportion of the polymerization monomers supplied to the loop reactor.

[0040] Furthermore, a circulation pump is used to provide circulation power in the polymerization reactor, the polymerization reaction monomers, the initiator and the chain transfer agent are introduced at the inlet of the circulation pump, and a static mixer is arranged before entering the circulation pump.

[0041] Since the reaction liquid in the loop reactor has a large circulation ratio and a high flow rate, it is not appropriate to set mixing elements in all the loop reactors. At least one static mixer can be set at other positions of the loop reactor (except before the inlet of the above-mentioned circulation pump) to enhance the mixing effect.

[0042] On the other hand, the present invention further provides a methacrylic acid polymer, which is prepared by the above-mentioned preparation method of the methacrylic acid polymer.

[0043] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention adopts a loop reactor and obtains a product with a 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 the occurrence of the gel effect is reduced, and the polymerization conversion rate is improved. The polymer concentration at the outlet of the polymerization reactor is 70wt% to 85wt%.

[0044] 2. The present invention effectively reduces the generation 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 the gel effect also decreases, but too high a temperature will increase the generation of dimers, thereby affecting the quality of PMMA. However, the operating conditions of the present invention effectively balance the viscosity of the reaction liquid and the generation of dimers, thereby obtaining high-quality PMMA. DETAILED DESCRIPTION

[0045] The present invention is described in more detail below by way of examples, but these do not limit the present invention.

[0046] The invention provides a method for preparing a methacrylic acid polymer, comprising the following steps: supplying a polymerization reaction monomer, a chain transfer agent and an initiator to a polymerization reactor, carrying out a polymerization reaction in the polymerization reactor, wherein the polymerization reaction temperature is 200 to 260° 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 to 0.8 m / s, and the polymer concentration at the outlet of the polymerization reactor is 70 to 85 wt %.

[0047] The present invention effectively reduces the generation of dimers 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 the gel effect also decreases, but too high a temperature will increase the generation of dimers, thereby affecting the quality of PMMA. However, the operating conditions of the present invention effectively balance the viscosity of the reaction liquid and the generation of dimers, thereby obtaining high-quality PMMA.

[0048] The formation mechanism of dimer is relatively complex. Generally speaking, it is related to the initiator concentration, monomer concentration, operating temperature and cycle time. Dimer is a polymerization reaction byproduct that mainly affects the transparency and mechanical strength of PMMA. Excessive production has a great impact on product performance. It is mainly composed of two unsaturated methyl methacrylates based on the Diels-Aldertype reaction mechanism to generate an unsaturated dimer 2-methyl-5-methylene-adipic acid-dimethyl ester. The following is the reaction formula of the dimer:

[0049] The gel effect, also known as the auto-acceleration phenomenon, is a phenomenon in which the polymerization rate significantly increases when the polymerization reaction is carried out to a certain extent. If the reaction solution is unevenly mixed or there are dead zones, molecular chains will crosslink and physically entangle in these areas, causing the local molten state to become a gel state, and the viscosity will significantly increase, further deteriorate, and even produce the danger of implosion. The kettle reactor can only use stirring to make the reaction solution mix evenly, but it is obvious that the kettle reactor will have many dead zones, resulting in an increased tendency for the gel effect to occur. In addition, the higher the temperature, the faster the polymerization rate, so the kettle reactor can only reduce the probability of the occurrence of the gel effect by lowering the polymerization temperature and reducing the polymerization conversion rate. The present invention, by adopting a loop reactor and setting a series of operating parameters, obtains a product with a high polymerization conversion rate.

[0050] According to the zero shear viscosity and PMMA content diagram of PMMA polymer solution in the document "high-temperature radical polymerization of methylmethacrylate in a continuous pilot scale process", it can be approximately inferred that at 260°C, the zero shear viscosity of the PMMA solution with a content of 85wt% is about 10Pa·s, and at 200°C, the zero shear viscosity of the PMMA solution with a content of 70wt% is about 7Pa·s. The flow rate of the reaction liquid in the loop reactor is in the range of 0.2-0.8m / s, which also makes the actual fluid viscosity less than the zero shear viscosity. It has been proved through multiple experiments that the method of the present invention can make the reaction liquid less than 30Pa·s at the polymerization temperature, and the gel effect will basically not occur. It can also be seen from each embodiment that no abnormal polymerization is found. Of course, the zero shear viscosity of the solution is also related to the molecular weight of PMMA. The larger the molecular weight, the greater the zero shear viscosity. If the present process is used to obtain a PMMA product with a higher molecular weight, it is recommended to select a polymer concentration of 75wt% to 80wt% at the reactor outlet.

[0051] The following are specific embodiments of the present invention.

[0052] Example 1

[0053] The polymerization monomers (including 85 wt % of methyl methacrylate and 15 wt % of methyl acrylate), 0.1 wt % of n-octyl mercaptan relative to the total amount of the polymerization monomers, and 250 ppm of di-tert-butyl peroxide relative to the total amount of the polymerization monomers were mixed (the meanings of the contents in the following examples and comparative examples are the same as those in Example 1), and replaced by nitrogen bubbling to make the dissolved oxygen content in the mixture lower than 0.5 ppm. A sufficient amount of the reaction liquid mixture was prepared and maintained at -30 ° C. The reaction liquid mixture was injected into the loop reactor with a total feed rate of 20 kg / h by a feed pump. At the same time, the jacket of the loop reactor was turned on for cooling. The cooling medium was saturated water with a saturation temperature of 197 ° C (saturated pressure of about 1.38 MPaG). The lowest side pressure of the loop reactor was maintained at 1.3 MPaG. The circulation pump flow rate was adjusted to 0.768 m3 / h (under the polymerization conditions, the reaction liquid density was about 912 kg / m3). The specifications of the loop reactor were Φ42×3 mm and the jacket length was 50 m (no jacket at the elbow, only insulation, total length of about 52m), the loop reactor is installed with temperature measuring points every about 7 meters, a total of 8 installed. The reaction liquid product flows out of the loop reactor continuously, enters the subsequent devolatilization extrusion to remove volatiles, and then granulates to obtain methyl methacrylate polymer particle products. After the device has been running stably for 10 hours, the average temperature of 8 thermometers was observed to be 200.9℃, and the appearance of the particles was directly observed. Other parameters and product information are shown in Table 1.

[0054] Parameter setting and performance test: Circulation ratio: controlled by frequency conversion of circulation pump.

[0055] Circulation velocity (v, m / s): calculated from the circulation ratio (k), total feed volume (F, kg / h), average density of the reaction solution (ρ, kg / m3) and the inner diameter of the reactor (d, mm). The calculation formula is: v=k*F / ρ*353.86 / d / d.

[0056] Cycle time (t, min): calculated from parameters such as cycle ratio (k), total feed amount (F, kg / h), average density of reaction liquid (ρ, kg / m3), reactor inner diameter (d, mm), and total length of reactor (L, m). The calculation formula is: t=0.785*(d / 1000) *(d / 1000)*L / (k*F / ρ)*60.

[0057] The remaining parameters can be obtained or set according to conventional industry methods.

[0058] Dimer and polymer content: In the extrusion devolatilization section, all volatile components are condensed and collected, and the dimer content of the collected liquid is determined by gas chromatography [the boiling point of the dimer in the present invention is 236°C and the viscosity is 25 cp (25°C)]; in addition, the total amount of all volatile components is measured, and the remaining non-volatile components are polymers, and the polymer content can be calculated.

[0059] Product weight average molecular weight: GB / T 21863-2008 gel permeation chromatography (GPC) is used with tetrahydrofuran as eluent and measured by GPC data processing device.

[0060] Whether the product is foamed: Observe the finished particles manually to see whether there are bubbles. √ means there are bubbles, and × means there are no bubbles.

[0061] Embodiment 2-7 The process is basically the same as that of Example 1, except that the feed ratio, feed amount, polymerization operating conditions, raw material feed temperature, jacket temperature and other parameters are different. The parameters and results of each embodiment are analyzed in Table 1.

[0062] Table 1

[0063] Comparative Examples 1-6 The process is basically the same as that of Example 1, except that the feed ratio, feed amount, polymerization operating conditions, raw material feed temperature, jacket temperature and other parameters are different. The parameters and results of each comparative example are shown in Table 2.

[0064] Table 2

[0065] According to Table 1 and Table 2, it can be seen that in Examples 1-7 and Comparative Examples 1-2, no bubbles were found in the finished particles, and the appearance of the formed particles was good, while in Comparative Examples 3-6, bubbles were found in some particles, and silver streaks appeared in some particles. It can be seen from Table 1 and Table 2 that under the process conditions of the present invention, Examples 1-7 all achieved good results, the polymer content at the outlet of the polymerization reactor all met the requirements, and the dimer content and product quality were all good. For Comparative Examples 1 and 2, at 190°C, the polymer content is low, and the subsequent devolatilization process consumes a lot of energy. For Comparative Examples 3 and 4, at 270°C, although the polymer content is high, the dimer content is also very high. These dimers are difficult to be completely removed in the subsequent process (if the recovered monomer is recycled and added to the feed, the dimer concentration will be higher), which directly affects the product quality; Comparative Example 5 uses a lower circulation ratio (to avoid excessive polymerization, the reaction temperature needs to be lowered, the reaction temperature is 222.7°C), and the circulation time reaches 8.9 min, and a product with a higher polymer content and a larger molecular weight is obtained, but the dimer content is seriously exceeded; Comparative Example 6 uses a higher circulation ratio, and the molecular weight of the obtained product is slightly lower, and the dimer content is also higher. More importantly, the pressure on the lowest side of the reactor has reached 6.7 MPaG, the pressure drop of the circulation pipeline is too high, the heating effect of the internal circulation pump is very significant, the heat transfer is deteriorated, and the product quality is reduced. In Comparative Examples 3-6, bubbles appeared in some particles, and silver streaks appeared in a small number of particles. It is speculated that a gel effect may have occurred.

[0066] It can be seen from the above examples that under the setting of the parameters such as reaction temperature, circulation flow rate, circulation ratio, jacket temperature, etc. specified in the present invention, a qualified PMMA product with a high conversion rate can be obtained.

Claims

1. A method for preparing a methacrylic acid polymer, comprising the following steps: The polymerization reaction monomer, chain transfer agent and initiator are supplied to the polymerization reactor, and the polymerization reaction is carried out in the polymerization reactor. The polymerization reaction temperature is 200-260° 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. The polymer concentration at the outlet of the polymerization reactor is 70-85wt%.

2. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The circulation ratio of the reaction liquid in the polymerization reactor is 25-80.

3. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The circulation time of the reaction liquid in the polymerization reactor is 0.5 to 10 minutes.

4. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The lowest polymerization side pressure in the polymerization reactor is P. At the polymerization reaction temperature, the saturated vapor pressure of the reaction liquid in the polymerization reactor is Ps, then Ps+0.5MPaG≥P≥Ps+0.1MPaG.

5. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The polymerization reactor is provided with a jacket for cooling the reaction liquid in the polymerization reactor, the cooling medium in the jacket is saturated water, the saturation temperature of the cooling medium is T2, the polymerization reaction temperature is T1, and T2+2°C≤T1≤T2+10°C is satisfied.

6. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The feeding temperature of the polymerization reaction monomer is -30 to 40°C.

7. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The chain transfer agent includes one or more of 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.

8. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The initiator includes one of tert-butyl isopropyl benzene peroxide, 1,1,3,3-tetramethylbutyl hydroperoxide, di-tert-butyl isopropyl benzene peroxide, isopropyl hydroperoxide, cyclohexanone peroxide, benzoyl peroxide and di-tert-butyl peroxide.

9. The method for preparing a methacrylic acid polymer according to claim 1, characterized in that: The polymerization reactor is provided with circulation power by a circulation pump, the polymerization reaction monomers, the initiator and the chain transfer agent are introduced into the inlet of the circulation pump, and a static mixer is arranged before the circulation pump.

10. A methacrylic acid polymer, characterized in that The methacrylic acid polymer is prepared by the method for preparing the methacrylic acid polymer according to any one of claims 1 to 9.

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