JP5160491B2 - Method for temporarily stopping methacrylic resin continuous polymerization - Google Patents

Description

  The present invention is a reaction in which a monomer mixture containing methyl methacrylate as a main component is continuously bulk-polymerized in a complete mixing type reaction tank, and when the polymerization is temporarily stopped and resumed, the retention in the reaction tank or the devolatilizing extruder The present invention relates to a method for suppressing generation of colored substances and preventing generation of colored foreign substances.

As one of the typical production methods of methacrylic resin, methyl methacrylate monomer is continuously injected into the polymerization tank, and polymerized to a certain polymerization rate is dissolved in unpolymerized monomer. A so-called continuous bulk polymerization method is known in which a solution is continuously fed to a devolatilizing extruder and unpolymerized monomers are recovered under reduced pressure to separate a product polymer (see Patent Document 1). In this method, if the devolatilizing extruder in the subsequent process is stopped during continuous operation, for example, for cleaning or replacement of parts, the temperature of the polymerization tank in the previous process that is operating normally is lowered or the reaction solution is drawn out. The raw materials and heat were wasted, and it took time and effort to resume operation. In addition, when the reaction liquid is allowed to stay in the polymerization tank without lowering the temperature during the stop time, the resin oxides and degradation products (yellow, brown, or black color change) in the methacrylic resin of the product after restarting operation. There was a problem that colored foreign matters such as gels and gels were mixed.
Therefore, when the operation is temporarily stopped, there is a method for eliminating waste of raw materials, minimizing the time required for restarting the operation and simplifying the operation, and mixing colored foreign substances into the methacrylic resin after the operation is restarted. There was a need for a way to minimize it.

Japanese Patent Laid-Open No. 11-255828

  The present invention provides a method in which continuous polymerization of methacrylic resins can be smoothly resumed by minimizing the amount of raw material that is wasted and the time required for resuming operation when the operation is temporarily stopped, and preventing the occurrence of colored foreign substances. For the purpose.

  As a result of studying the above problems, the inventors of the present invention have a melt flow rate (MFR) of a product methacrylic resin after restarting operation when the operation is temporarily stopped in continuous polymerization of a methyl methacrylate monomer. The inventors have found that the operation can be resumed smoothly if the reaction liquid is retained in the polymerization tank without lowering the temperature in the polymerization tank in such a state that it falls within a specific high range, and the present invention has been completed.

That is, the present invention provides:
While using a radical polymerization initiator having a half-life of 0.5 to 20 seconds at 140 ° C. for the methyl methacrylate monomer, while maintaining the polymerization rate at 40 to 65% by mass at 130 to 160 ° C., In a reaction that continuously polymerizes in a fully mixed reactor to produce a methacrylic resin having a melt flow rate of less than 9 g / 10 min;
A method of temporarily stopping the reaction while stopping the supply of the monomer and the initiator and maintaining the amount of the reaction liquid, the temperature and the polymerization rate in the fully mixed reaction tank;
Before stopping the supply of the methyl methacrylate monomer and the radical polymerization initiator, a chain transfer agent is added so that the melt flow rate of the polymer is 9 to 25 g / 10 minutes. .

  The present invention suppresses the generation of stagnant substances in the reaction tank or devolatilizing extruder when the polymerization is temporarily stopped and resumed in the process of continuously polymerizing methyl methacrylate monomers in a fully mixed reaction tank. Therefore, it is suitable for preventing the occurrence of colored foreign matters. In addition, the method of the present invention can quickly and easily resume operation after stopping.

  Hereinafter, preferred embodiments of the present invention will be described, but the present invention is not limited to the following descriptions.

  The methyl methacrylate monomer in the present invention is a monomer consisting of methyl methacrylate alone, or consisting of 80% by mass or more of methyl methacrylate and 20% by mass or less of a monomer copolymerizable with methyl methacrylate, Refers to a monomer mixture. Monomers copolymerizable with methyl methacrylate include, for example, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate and other alkyl acrylates, ethyl methacrylate, butyl methacrylate and other methyl methacrylate. Aromatic vinyl compounds such as alkyl methacrylate other than styrene, styrene, α-methylstyrene, and p-methylstyrene can be used. The methyl methacrylate monomer may contain a recovered monomer recovered under reduced pressure by a devolatilizing extruder in a subsequent step.

  The polymerization initiator used in the present invention is a radical polymerization initiator having a half life at 140 ° C. of 0.5 to 20 seconds, preferably 1 to 15 seconds. When an initiator having a half-life exceeding 20 seconds is used, the fluctuation range of the monomer conversion rate becomes large and it is difficult to ensure stable operation. On the other hand, if the half-life is less than 0.5 seconds, the amount of the polymerization initiator used becomes too large, causing a problem in the colorless transparency of the product polymer. By using such a short-lived polymerization initiator, in the present invention, it is possible to smoothly perform an operation for stopping the operation and an operation for decreasing the degree of polymerization (that is, an operation for increasing the melt flow rate). In addition, since the polymerization initiator is cleaved and disappears within a very short time at the temperature in the reaction vessel, the termination of the polymerization reaction in the present invention is the continuous of the monomer in which the polymerization initiator is dissolved in the reaction vessel. The state after the time when supply is stopped.

  As polymerization initiators used in the present invention, 2,2-azobisisobutyronitrile, 2,2-azobis (2,4-dimethylvaleronitrile), 2,2-azohist (2-methylbutyronitrile) Azo compounds such as dimethyl 2,2′-azobis (2-methylpropionate), and 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, t-butylperoxyisobuty Organic peroxides such as rate, benzoyl peroxide and lauroyl peroxide are preferred.

  An example of the continuous polymerization method in the present invention will be specifically described. First, a) methyl methacrylate, b) comonomer, c) if desired, the recovered monomer is mixed with a flow controller so as to have a predetermined composition, and d) the mixture (methyl methacrylate monomer) The chain transfer agent is continuously added in an amount of 0.1 to 0.4% by mass with a metering pump to automatically adjust the feed monomer solution. Next, the feed monomer solution is placed in a gas-liquid separation tank cooled to 20 ° C. or less through a static mixer together with nitrogen that is at least twice as large as the gas volume flow rate with respect to the flow rate. A monomer solution having a dissolved oxygen content of 1 ppm or less is continuously prepared by separating oxygen. When the dissolved oxygen in the preparation liquid is 1 ppm or more, the polymerization rate is affected by this, and a stable polymerization reaction cannot be maintained. The prepared solution is cooled to a constant temperature of 5 ° C. or less with a cooler, and then continuously fed to a fully mixed reaction tank (polymerization reactor) by a pump. In the continuous polymerization method in the present invention, in order to adjust the temperature and viscosity in the tank, a solvent that dissolves both the methyl methacrylate monomer and the polymer may coexist, but it is excellent in productivity. From the above, bulk polymerization without using a solvent is preferable. The fully mixed reaction tank may be divided into a plurality of reaction tanks such as a pre-polymerization tank and a post-polymerization tank as necessary. In this case, the present invention is applied to at least one of the reaction tanks. Is done.

  As a chain transfer agent used here, alkyl mercaptans, such as n-butyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, are good, and 4-18 are preferred as carbon number of this alkyl group.

On the other hand, the polymerization initiator is dissolved in a monomer containing 50 ppm or more of dissolved oxygen, and is continuously added to the monomer solution by a metering pump as a 0.5 to 5% monomer solution. The feed monomer solution to which the polymerization initiator has been added is filtered through a filter of 0.5 μm or less and then continuously charged into the polymerization reactor. By this filter, dust in the feed liquid is removed, and a methyl methacrylate copolymer with few minute foreign matters can be obtained. The polymerization initiator is preferably used in a ratio of 2 × 10 ^{−5 to} 5 × 10 ⁻⁵ mol with respect to 100 g of the methyl methacrylate monomer.

The method for temporarily stopping the continuous polymerization of the present invention is 130 to 160 ° C., preferably 130 to 150 ° C., and the melt flow rate is 9 (g / 10 minutes) by continuous polymerization while maintaining the polymerization rate at 40 to 65% by mass. ), Preferably 0.4 (g / 10 min) or more and less than 9 (g / 10 min). In the production process of methacrylic resin with a melt flow rate of 9 (g / 10 min) or higher, the reaction solution volume, temperature, and polymer concentration (or polymerization rate) in the reaction tank are maintained because the viscosity of the reaction solution is low. As long as the polymerization reaction stop time is 24 hours or less, no colored foreign matter is generated due to residence without increasing the amount of chain transfer agent added when stopping the supply of the monomer and initiator. .
The melt flow rate of the methacrylic resin in the present invention refers to a value measured under conditions of 230 ° C., 3.8 kg load and 10 minutes in accordance with JIS K7210.

In order to stop the operation, chain transfer is performed so that the MFR of the methacrylic resin dissolved in the reaction solution inside the reaction vessel is 9 to 25 (g / 10 minutes), more preferably 9 to 20 (g / 10 minutes). Adjust the supply amount of the agent. If the MFR is 9 (g / 10 min) or less, the viscosity is high, so there is a large amount of accumulated matter, and it is not preferable because it carbonizes with time and becomes colored foreign matter. If the MFR is 25 (g / 10 min) or more, the viscosity is low. Therefore, it is not preferable because the strand cannot be pulled from the devolatilizing extruder at the time of restarting.
The amount of the chain transfer agent is increased prior to stopping the supply of the monomer solution to the fully mixed reaction vessel. The timing for increasing the amount of addition depends on the residence time of the reaction solution, but when the internal volume of the reaction vessel is, for example, about 0.2 to ³⁰ m ³ and the residence time of the reaction solution is 0.5 to 5 hours, 2 It is preferably from about 0.1 hours ago, and more preferably from 1.5 to 0.2 hours ago.
The addition amount of the chain transfer agent is, for example, 1.5 to 5 times the addition amount during continuous polymerization of a methacrylic resin having an MFR of less than 9 (g / 10 minutes), thereby adjusting the melt flow rate to 9 to 25. It can be in the range of (g / 10 minutes).
The heat generated from the polymerization reaction heat disappears in the stopped reaction vessel. In order to maintain the temperature in the tank, the stirring is slowed down to reduce the amount of monomer evaporation, heat loss due to evaporation is reduced, and heating is performed to such an extent that the temperature in the reaction tank can be maintained.

  In the shutdown method of the present invention, as long as the polymerization rate of the methyl methacrylate monomer in the complete mixing reactor is in the range of 40 to 65% by mass, a slight heat in the absence of a polymerization initiator is used. The reaction liquid can be diluted with a methyl methacrylate monomer in order to offset the increase in the polymerization rate due to polymerization. The dilution amount is preferably 10% by mass or less of the reaction solution, and more preferably 5% by mass or less. The time for which the operation of the continuous polymerization is temporarily stopped by applying the method of the present invention is preferably within 24 hours.

  EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited thereto. In addition, the measurement or evaluation of the physical property value in an Example and a comparative example was performed with the following method.

(1) MFR measurement MFR was measured under the conditions of a load of 3.8 kg at 230 ° C. for 10 minutes in accordance with JIS K7210.
(2) Measurement of foreign matters in pellets Colored foreign matters in 300 g of methacrylic resin pellets were measured from a pellet / powder foreign matter inspection device (manufactured by Hue Brain).

<Example 1>
96% by mass of methyl methacrylate and 4% by mass of methyl acrylate are mixed, and 0.15% by mass of the chain transfer agent n-octyl mercaptan is mixed with the mixture by a quantitative pump, and the polymerization initiator azobisisobutyrate is mixed. Lononitrile (half-life at 140 ° C .: 8.4 seconds) continuously added at a temperature of 140 ° C. and a polymerization rate of 52% by mass in a state where 0.0008% by mass is continuously added to the fully mixed reaction vessel. Polymerized. The melt flow rate of the methacrylic resin produced during the continuous polymerization was 1 (g / 10 minutes).
n-octyl mercaptan was increased to 0.5% by mass, the supply of the initiator was stopped after 1 hour, the supply of the monomer and the extraction of the reaction liquid were stopped, and the polymerization tank was kept at 140 ° C. The reaction liquid in the reactor and the devolatilizing extruder was not extracted and left for 3 hours with low speed stirring. The MFR of the methacrylic resin contained in the reaction solution being allowed to stand was 10 (g / 10 minutes).
After 3 hours, the system was restarted and polymerized continuously. The resin thus obtained was sampled after 5, 10 and 20 hours, and the number of colored foreign matters in 300 g of methacrylic resin pellets was measured.

<Example 2>
Except for producing a methacrylic resin having a melt flow rate of 2 (g / 10 min) with an n-octyl mercaptan content of 0.26 mass% and a polymerization rate of 50 mass% during continuous polymerization, the same as in Example 1. A continuous polymerization reaction was performed.
The n-octyl mercaptan was increased to 0.4% by mass, and after 1 hour, the supply of the initiator was stopped, the supply of the monomer and the reaction liquid were stopped, and the polymerization tank was kept at 140 ° C. The reaction solution in the reactor and the devolatilizing extruder was left without being extracted for 3 hours. The MFR of the methacrylic resin contained in the reaction solution being allowed to stand was 10 (g / 10 minutes). As a result of restarting in the same manner as in Example 1 and measuring the number of colored foreign matters, it was as shown in Table 1.

<Comparative Example 1>
In Example 1, n-octyl mercaptan was allowed to stand without increasing during continuous polymerization, and as a result, the number of colored foreign substances shown in Table 1 was obtained.

<Comparative example 2>
In Example 1, n-octyl mercaptan was increased until the melt flow rate of the methacrylic resin reached 35 (g / 10 min), and the reaction solution was left as a result. It was not possible.

Claims (3)

While using a radical polymerization initiator having a half-life of 0.5 to 20 seconds at 140 ° C. for the methyl methacrylate monomer, while maintaining the polymerization rate at 40 to 65% by mass at 130 to 160 ° C., In a reaction that continuously polymerizes in a fully mixed reactor to produce a methacrylic resin having a melt flow rate of less than 9 g / 10 min;
A method of temporarily stopping the reaction while stopping the supply of the monomer and the initiator and maintaining the amount of the reaction liquid, the temperature and the polymerization rate in the fully mixed reaction tank;
A method for temporarily stopping continuous polymerization, wherein a chain transfer agent is added so that the melt flow rate of the polymer is 9 to 25 g / 10 minutes before the supply of the methyl methacrylate monomer and the radical polymerization initiator is stopped.   The temporary stop method according to claim 1, wherein the polymerization reaction is a bulk polymerization reaction.   The suspension method according to claim 1, wherein the stop time is within 24 hours.