JP3628124B2 - Method for removing impurities in continuous bulk polymerization - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for removing impurities in a continuous bulk polymerization method of a methacrylic resin.
[0002]
[Prior art]
Methacrylic resins have excellent transparency, good mechanical properties, processability, and beautiful appearance of molded products. For example, lighting parts, signboards, various decorative products, nameplates, tail lamps, and other automotive parts, tableware Widely used in Recently, it has been widely used in the optical field such as various lenses, optical disks, and light guide plates. Therefore, advanced optical characteristics are being demanded.
[0003]
As a method for producing a methacrylic resin, a suspension polymerization method is widely adopted. However, the biggest drawback of this production method is that some of the suspension stabilizer used remains in the polymer and the polymer properties, in particular the optical properties, are reduced.
As a method for improving the disadvantages of this suspension polymerization method, a bulk polymerization method (Japanese Patent Laid-Open No. 49-37993, Japanese Patent Laid-Open No. 3-111408), a solution polymerization method (Japanese Patent Laid-Open No. 63-57613, Japanese Patent Publication) 7-119259).
[0004]
According to these methods, unlike the suspension polymerization method, since a suspension stabilizer is not used, a methacrylic resin having excellent optical characteristics can be produced. Comparing bulk polymerization and solution polymerization, although there are advantages and disadvantages, bulk polymerization has performance aspects, for example, less gas generation due to decomposition of the polymer at the time of molding, so that silver streak called so-called silver streak is hard to occur and thermal decomposition resistance is improved. It has a favorable aspect such as superiority, and is adopted industrially. However, there are major problems in this bulk polymerization.
[0005]
In the case of industrially performing bulk polymerization, the bulk polymerization is performed in a continuous polymerization mode, as compared with a suspension polymerization that produces a methacrylic resin in a batch mode. That is, recovering the unreacted monomer after polymerization, making it a recycle liquid, adding a new monomer, polymerizing, devolatilizing the polymerization liquid, separating the polymer and unreacted monomer, and taking out the polymer At the same time, the unreacted monomer is recycled and continuously produced, but impurities generated during the polymerization mixed in the recycle liquid are mixed in the polymer, and the optical characteristics are deteriorated. In order to avoid this, the recycle liquid is usually purified.
[0006]
As a purification method, a partial condenser is installed in the devolatilization process for separating and collecting unreacted monomers and polymers, or a high boiling point impurity is removed, or a distillation tower is installed to remove impurities. Has been done. In the method of removing high-boiling impurities by installing a partial condenser, removal is difficult unless the boiling point of the impurities to be removed is sufficiently high compared to the boiling point of the unreacted monomer or solvent used. There are drawbacks and purification in a distillation column is preferred. However, the purification method using a distillation column in bulk polymerization generally supplies the recycle liquid to the distillation column, distills it, and extracts high-boiling impurities together with the methyl methacrylate monomer from the bottom of the distillation column. Is implemented. In this case, since the residence time of the bottom liquid is long and the temperature is high, the polymerization of the methyl methacrylate monomer proceeds at the bottom of the distillation tower, the heat exchange efficiency in the reboiler is reduced, the piping is blocked by the generated polymer, etc. Occur. In an extreme case, there is a big problem in industrially stable production such as polymerization proceeds and solidifies in the entire bottom of the distillation column. In order to avoid these problems, for example, the method of increasing the amount of extraction of the distillation column bottom liquid and shortening the residence time of the methyl methacrylate monomer at the bottom of the distillation column has been taken. Increasing the extraction amount more than necessary is not preferable because it is extremely disadvantageous economically.
[0007]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for producing a methacrylic resin having excellent optical characteristics by removing impurities generated at the time of polymerization stably industrially by continuous bulk polymerization.
[0008]
[Means for Solving the Problems]
As a result of intensive studies on a method for solving the above problems, the present inventors have made a solvent having a boiling point higher than that of a monomer used for polymerization, such as methyl methacrylate monomer, present in the bottom of the distillation column, and Polymerization progresses at the bottom of the distillation tower by distilling the recycle liquid containing the reaction monomer or the recycle liquid and the new monomer, supplying the distilled monomer to the polymerization reactor, and performing continuous polymerization. Without causing problems, it is possible to remove by-products generated during polymerization economically and efficiently, as well as impurities such as polymerization inhibitors and impurities contained in the raw materials, or polymerization inhibitors added to preserve recycled liquids. The present inventors have found that a methacrylic resin having excellent optical characteristics can be produced.
[0009]
That is, the present invention comprises (A) (a) a recycle liquid containing unreacted methyl methacrylate as a main component and monomer after polymerization, and (b) a new monomer having methyl methacrylate as a main component. Continuously supply to the distillation tower, continuously supply the distilled monomer to the polymerization reactor, polymerize, then devolatilize the polymerization liquid and take out the polymer, and at the same time recycle the unreacted monomer A continuous bulk polymerization process used or (B) (a) a recycle liquid containing a monomer mainly composed of unreacted methyl methacrylate after polymerization is continuously supplied to a distillation column, and a single unit distilled. And (b) a monomer containing methyl methacrylate as a main component is continuously supplied to the polymerization reactor, polymerized, and then the polymerization solution is devolatilized to take out the polymer and at the same time unreacted monomer In a continuous bulk polymerization process that uses recycled materials, A solvent having a boiling point higher than the boiling point of the body is present at the bottom of the distillation column, and the bottom liquid is continuously or intermittently extracted from the bottom of the distillation column, and at the same time, the same amount of solvent is continuously or intermittently distilled. By supplying to the tower bottom, impurities such as by-products generated during polymerization, polymerization inhibitors added for stabilizing the recycle liquid or polymerization inhibitors and impurities contained in the raw material are removed. It is a continuous bulk polymerization method.
[0010]
Examples of the monomer having methyl methacrylate as a main component in the present invention include methyl methacrylate alone or a monomer copolymerizable with methyl methacrylate. Examples of monomers copolymerizable with methyl methacrylate include alkyl methacrylates such as ethyl methacrylate, butyl methacrylate and cyclohexyl methacrylate, alkyl acrylates such as methyl acrylate, ethyl acrylate and butyl acrylate. , Aromatic vinyl compounds such as styrene, vinyl toluene and α-methyl styrene, vinyl cyanides such as acrylonitrile and methacrylonitrile, maleimides such as N-phenylmaleimide and N-cyclohexylmaleimide, and unsaturateds such as maleic anhydride Examples thereof include unsaturated acids such as carboxylic acid anhydrides, acrylic acid, methacrylic acid and maleic acid. Among these monomers that can be copolymerized with methyl methacrylate, acrylic esters are particularly good in thermal decomposition resistance, and methacrylic resins obtained by copolymerizing acrylic esters have fluidity during molding. Highly preferred. The amount of acrylic acid ester used when methyl methacrylate is copolymerized with methyl methacrylate is preferably 15% by weight or less. If it is 15% by weight or more, the effect of improving the heat decomposability and fluidity is high, but the heat resistance, that is, the heat distortion temperature is lowered, which is not preferable. Among the acrylate esters, particularly, methyl acrylate and ethyl acrylate are remarkably most preferable in terms of the above improvement effect even if they are copolymerized with a small amount of methyl methacrylate. The said monomer which can be copolymerized with methyl methacrylate can also be used 1 type or in combination of 2 or more types.
[0011]
The solvent in the present invention is not particularly limited as long as it dissolves the impurities to be removed and has a higher boiling point than the methyl methacrylate monomer and the monomer copolymerizable with methyl methacrylate. In general, aromatic compounds such as toluene, xylene, ethylbenzene and diethylbenzene, aliphatic compounds such as octane and decane, alicyclic compounds such as decalin, ester compounds such as butyl acetate and pentyl acetate, 1,1,1,2 -Halogen compounds such as tetrachloroethane and 1,1,2,2, -tetrachloroethane can be used. The boiling point of the solvent is higher than the boiling points of the methyl methacrylate monomer and the monomer copolymerizable with methyl methacrylate, preferably 10 ° C. or higher, more preferably 20 ° C. or higher, more preferably 30 ° C. or higher. . Among these, alkylbenzene is preferable because it does not adversely affect the polymerization and the solubility of impurities generated by the polymerization is also high. Among alkylbenzenes, toluene, xylene, ethylbenzene, especially xylene, and ethylbenzene have an appropriate boiling point, have a low load for devolatilization, and do not adversely affect the polymerization, but dissolve impurities generated by the polymerization. It is most preferable because it has high properties and can be obtained industrially at low cost.
[0012]
The polymerization inhibitor added to prevent polymerization of the recycle liquid or to prevent polymerization of the raw material monomer is dissolved in the solvent and the monomer. For example, hydroquinone, methoxyhydroquinone, t-butylcatechol, etc. Can be mentioned.
Examples of impurities contained in the raw material include impurities contained in the raw material methyl methacrylate monomer, impurities contained in a polymerization initiator used in the continuous bulk polymerization process, a chain transfer agent, and the like.
[0013]
By-products generated during the polymerization are components separated and recovered from the polymer in the devolatilization step among the products generated in the polymerization reactor, such as a catalyst decomposition product and a methyl methacrylate monomer. And oligomer components such as dimers and trimers.
An example of a typical distillation column is shown in FIG. 1, and an example of a continuous bulk polymerization process is shown in FIG. The present invention is carried out as follows using, for example, equipment as shown in the figure. That is, (A) (a) a recycled liquid containing a monomer based on unreacted methyl methacrylate after polymerization and (b) a new monomer based on methyl methacrylate in succession Supply to the distillation column, and continuously or intermittently extract impurities from the bottom of the distillation column. At the same time, add additional polymerization initiator and molecular weight regulator to the distilled monomer, and continuously supply it to the polymerization reactor. Then, polymerization is carried out in a polymerization reactor, and then the polymerization solution is devolatilized to take out the polymer, and at the same time, unreacted monomers are recycled to perform continuous bulk polymerization. Or (A) (a) A recycle liquid containing a monomer mainly composed of unreacted methyl methacrylate after polymerization is continuously supplied to the distillation column, and impurities are continuously or intermittently supplied from the bottom of the distillation column. A monomer distilled at the same time as withdrawing, (b) a monomer comprising methyl methacrylate as a main component, a polymerization initiator and a molecular weight regulator are additionally added and continuously supplied to the polymerization reactor. Then, the polymerization solution is devolatilized to take out the polymer, and at the same time, the unreacted monomer is recycled to perform continuous bulk polymerization.
[0014]
Distillation is carried out by a distillation column such as a packed column type or a plate type. The distillation method is, for example, supplying a recycle liquid containing impurities to be removed or a mixture of this and a new monomer from the middle or upper stage of the distillation tower, and heating the bottom liquid of the distillation tower with a heater such as a reboiler. The distillation is carried out by condensing in a condenser the monomer vapor composed mainly of methyl methacrylate that is distilled from the top of the distillation column. At this time, impurities having a boiling point higher than that of the monomer are concentrated at the bottom of the distillation column made of a solvent. The impurities can be continuously removed by adding the solvent in the amount removed at the same time as the impurities are continuously or intermittently removed from the bottom of the distillation column. The additional addition of the solvent is carried out by such a method that it is carried out directly or intermittently at the bottom of the distillation column, or continuously in combination with a recycle liquid before distillation or a new monomer.
[0015]
In addition, as described above, there are two methods of distillation: a method in which a new monomer and a recycled liquid containing unreacted monomers after polymerization are distilled together, and a method in which only the recycled liquid is distilled. Is more preferable because it can remove impurities contained in the new monomer. In addition, it is preferable that the distillation column has a number of stages such that a monomer mainly composed of methyl methacrylate is not mixed in the bottom liquid of the distillation column in the bulk polymerization of the present invention. However, even when the boiling point of the solvent is close to that of the monomer or the number of stages is small and a small amount of monomer enters the bottom of the distillation column, polymerization of the monomer generated at the bottom of the distillation column does not occur or If it happens, you can avoid trouble.
[0016]
In the distillation column of the present invention, the boiling point of the solvent used is higher than the boiling point of the monomer mainly composed of methyl methacrylate and the boiling point of the impurities to be removed is higher than the boiling point of the monomer. Concentrated, so the bottom of the distillation column is only the impurities and solvent to be removed. Therefore, while heating the distillation column bottom liquid using a heater such as a reboiler, a part of the bottom liquid is continuously or intermittently withdrawn from the system to prevent discharge and accumulation of impurities to be removed. I can measure. At this time, if the amount to be withdrawn is small, the impurity concentration in the bottom liquid of the distillation tower becomes high, causing deterioration of heat exchange efficiency in the heater due to an increase in liquid viscosity, and blockage of piping and heater due to impurity precipitation. there is a possibility. In addition, when the amount to be extracted is too large, there is no problem in operation, but it is not economical. Therefore, in removing impurities, it is preferable to reduce the amount of distillation column bottom liquid extracted to the extent that impurities do not precipitate as long as the heater has the capability.
[0017]
A monomer mainly composed of methyl methacrylate distilled as described above, and a monomer composed mainly of new methyl methacrylate as the case may be, or a polymerization initiator, a molecular weight regulator, etc. Supply to the polymerization reactor and polymerize. In order to improve the optical characteristics, this monomer solution is continuously supplied to, for example, a counter-current contact tower and replaced with an inert gas, so that dissolved oxygen in the monomer solution is 1 ppm or less. Further, in order to remove foreign matters such as metallic foreign matters, it is preferable to filter the monomer solution with a filter of 0.5 μm or less.
[0018]
In this case, the polymerization initiator used is one that decomposes actively at the polymerization temperature to generate radicals. For example, di-tert-butyl peroxide, di-cumyl peroxide, methyl ethyl ketone peroxide, di-tert-butyl peroxide. Phthalate, di-tert-butyl perbenzoate, tert-butyl peracetate, 2,5-dimethyl-2,5-di (tert-butylperoxy) hexane, 1,1-bis (t-butylperoxy) -3 , 3,5-trimethylcyclohexane, di-tert-amylperoxide, benzoyl peroxide, cumene hydroperoxide and organic peroxides such as lauryl peroxide, azobisisobutanol diacetate, 1,1'-azobis Cyclohexanecarbonitrile, 2-phenylazo 2,4-dimethyl An azo compound such as ru-4-methoxyvaleronitrile, 2-cyano-2-propylazoformaside and 2,2′-azobisisobutyronitrile can be used. These can be used alone or in combination of two or more. The amount of these polymerization initiators used is preferably in the range of 0.0010 to 0.03% by weight based on the weight of the total reaction mixture.
[0019]
Furthermore, mercaptans are mainly used as molecular weight regulators. Examples of mercaptans include primary and secondary alkyl groups having a substituted or alkyl group such as n-butyl mercaptan, isobutyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, sec-dodecyl mercaptan, tert-butyl mercaptan, and the like. Aromatic mercaptans such as tertiary and tertiary mercaptans, phenyl mercaptans and thiocresols, thioglycolic acid and its esters, ethylene thioglycol and the like can be used. These can be used alone or in combination of two or more. The amount of these molecular weight modifiers to be used is appropriately determined according to the molecular weight of the polymer to be produced, but is usually selected in the range of 0.01 to 0.5% by weight based on the weight of the total reaction mixture.
[0020]
As the polymerization reactor, an apparatus that is uniformly stirred by a stirring blade such as a double helical ribbon or a pitched paddle type is used. The polymerization is carried out at a temperature of 120 to 160 ° C. so that the monomer solution is continuously fed to the polymerization reactor and the polymerization conversion of the monomer is substantially constant within the range of 40 to 70%. A polymerization reaction is carried out. When the polymerization conversion rate is less than 40%, the load of the devolatilization step due to the volatile components is large, and for example, devolatilization may be insufficient due to the restriction of the heat transfer area of the preheater, which is not preferable. On the other hand, if it exceeds 70%, for example, the piping pressure loss between the polymerization reactor and the preheater becomes large, which makes it difficult to transport the polymerization solution. If the polymerization temperature is less than 120 ° C., the polymerization rate is too slow to be practical, and if it exceeds 160 ° C., the polymerization rate is too fast, making it difficult to adjust the polymerization conversion rate or reducing the thermal decomposition resistance. It is not preferable.
[0021]
The polymerization solution obtained by such a polymerization reaction is devolatilized to take out the polymer, and at the same time, a monomer mainly composed of unreacted methyl methacrylate as a volatile component is separated. Volatiles are distilled and reused as a recycle liquid to continuously produce methacrylic resins. In general, it is preferable to add a polymerization inhibitor to the recycle liquid to prevent polymerization during storage. As the devolatilizer, an extruder with a multistage vent, a devolatilization tank, or the like is used. Preferably, the polymerization solution is heated to a temperature of 200 to 290 ° C. with a pre-heater or the like, has a sufficient space in the upper part, and has a temperature of 200 to 250 ° C., 20 to 100 torr, in a devolatilization tank under vacuum. At the same time when the polymer is taken out by feeding, the volatile component composed of the monomer mainly composed of unreacted methyl methacrylate is separated and reused as a recycle liquid. The volatile content remaining in the polymer is 1% by weight or less, preferably 0.5% by weight or less, and more preferably 0.3% by weight or less. The method of introducing the polymerization liquid into the devolatilization tank held under this reduced pressure causes volatilization between the volatile components and foaming thereby, forming an extremely large evaporation area, and efficiently volatile components in a short time. Is a preferred devolatilization method that is excellent in optical properties with less residual monomer remaining in the polymer and less polymer coloring.
[0022]
Hereinafter, although not limited to this, a method for removing impurities in the distillation column will be supplementarily described with reference to FIG.
A monomer containing impurities to be removed is continuously supplied from line 1 to distillation column 2. The inside of the distillation column 2 has a structure in which distillation can be carried out efficiently by filling with a filler such as Raschig ring or by installing a shelf such as a sieve tray. The impurities and solvent accumulated in the bottom of the column are heated by the reboiler 4 via the staying liquid circulation pump 3 and then returned to the column. The reboiler 4 is generally a multi-tube heat exchanger, and the supply amount and temperature of the heat medium are controlled by installing an appropriate control mechanism so that the temperature in the distillation column and the amount of liquid staying at the bottom of the distillation column are constant. Is done. The solvent containing impurities is continuously or intermittently discharged from the extraction line 5 branched from the outlet of the circulation pump 3. The distilled monomer is cooled and condensed in the condenser 6 and then continuously supplied to the reactor. The pressure in the distillation column is controlled to a constant pressure by a control valve installed in the vacuum line 7.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Each property of the polymer was measured as follows.
(1) The intrinsic viscosity of the polymer was measured with an Ostwald viscometer at a temperature of 25 ° C. by dissolving 150 mg of the polymer in 50 ml of chloroform.
(2) The total light transmittance was measured by the ASTM D-1003 method.
[0024]
[Example 1]
It was discharged from the polymerization reactor and recovered by devolatilization in a devolatilization tank. Recycled solution of 97.8% methyl methacrylate and 2.2% by weight methyl acrylate to which 0.01 part of hydroquinone was added / methyl methacrylate 97.9% A fresh monomer solution of 2.1% by weight of methyl acrylate / 2.1% by weight of methyl acrylate / ethylbenzene was mixed at a weight ratio of 49.9 / 49.9 / 0.2 and continuously fed to the distillation column. The distillation column is made of stainless steel and has the structure shown in FIG. 1, and the inside is filled with Raschig rings. The distillation column is heated by the reboiler with steam so that the amount of the retained solution at the bottom of the distillation column is constant while being maintained at 80 Torr by a control valve installed in the vacuum line 7. The bottom portion of the distillation column was composed of ethylbenzene and impurities contained in the recycle liquid, and the same amount of ethylbenzene containing impurities from the extraction line 5 was continuously extracted. The distilled monomer solution was 97.8% by weight methyl methacrylate and 2.2% by weight methyl acrylate. To this monomer solution, 85 ppm of 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane and 2100 ppm of octyl mercaptan were added as polymerization initiators, and the polymerization temperature was 155 in a fully mixed polymerization reactor. Polymerization was conducted at 2.0 ° C. and a residence time of 2.0 hours, and the polymerization was continuously carried out to a polymerization conversion rate of 50%. This polymerization liquid was continuously taken out from the polymerization reactor, then heated to 260 ° C. with a heating plate, and dropped into a devolatilization tank through the interval between the heating plates. A devolatilization tank was maintained at 30 torr and 230 ° C. to separate the polymer and unreacted monomer. The polymer was extruded from an extrusion die, and the unreacted monomer was distilled and reused as a recycle liquid, followed by continuous operation for one week. The methyl methacrylate content in the obtained methacrylic resin pellet was 1.9% by weight, and the residual monomer was 2400 ppm. The intrinsic viscosity was 53 ml / g and the total light transmittance was 92.5%. There were no large foreign objects in the polymer and the product was not colored. Further, no clogging due to polymer formation in the distillation tower occurred at all for one week of continuous operation.
[0025]
[Example 2]
In Example 1, the same operation as in Example 1 was performed except that ethylbenzene was changed to xylene and the polymerization initiator and chain transfer agent were slightly modified so that the polymerization conversion and molecular weight were almost the same as in Example 1. . The obtained methacrylic resin pellets had a methyl acrylate content of 1.9% by weight and a residual monomer of 2300 ppm. The intrinsic viscosity was 55 ml / g and the total light transmittance was 93%. There were no large foreign objects in the polymer and the product was not colored. In addition, clogging due to polymer formation in the distillation tower did not occur at all for one week of continuous operation.
[0026]
[Comparative Example 1]
Except not using ethylbenzene, the same operation as Example 1 was performed. Three days after the start of operation, polymer was deposited on the extraction line from the liquid at the bottom of the distillation column, and the extraction line was blocked on the fifth day, so the subsequent operation was stopped.
[0027]
[Comparative Example 2]
Under the conditions of Example 1, bulk polymerization was performed without using ethylbenzene and no distillation operation, and the methacrylic resin gradually colored as the continuous operation passed.
[0028]
【The invention's effect】
According to the impurity removal method of the present invention, long-term continuous bulk polymerization can be carried out stably while preventing coloring of the product. Therefore, a high quality product can be produced with high productivity by the continuous bulk polymerization method.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a distillation column used in an impurity removal method of the present invention.
FIG. 2 is a diagram illustrating a schematic diagram of a continuous bulk polymerization apparatus using a distillation column used in the impurity removal method of the present invention.
[Explanation of symbols]
1. Vinyl monomer and optionally solvent supply line 2, distillation tower 3, circulation pump 4, reboiler 5, impurity extraction line 6, condenser 7, vacuum line 8, polymerization reaction tank 9, liquid feed pump 10, heating plate 11, devolatilization tank 12, vacuum line

Claims (8)

(A) (a) Recycled liquid containing unreacted methyl methacrylate as a main component and monomer after polymerization, and (b) New monomer containing methyl methacrylate as a main component continuously into a distillation column Supply and distill the monomer continuously to the polymerization reactor, polymerize, then devolatilize the polymerization liquid and take out the polymer, and at the same time recycle unreacted monomer In the polymerization process, a solvent having a boiling point higher than that of the monomer is present at the bottom of the distillation column, and the bottom liquid is continuously or intermittently extracted from the bottom of the distillation column, and at the same time, the same amount of solvent is continuously extracted. A continuous bulk polymerization process, characterized in that it is fed to the bottom of the distillation column periodically or intermittently. (B) (a) A recycle liquid containing a monomer mainly composed of unreacted methyl methacrylate after polymerization is continuously supplied to the distillation column, and the distilled monomer and (b) new methacrylic acid Monomer-based monomer is continuously supplied to the polymerization reactor, polymerized, and then the polymerization solution is devolatilized to take out the polymer, and at the same time, the unreacted monomer is recycled and used as a continuous mass In the polymerization process, a solvent having a boiling point higher than that of the monomer is present at the bottom of the distillation column, and the bottom liquid is continuously or intermittently extracted from the bottom of the distillation column, and at the same time, the same amount of solvent is continuously extracted. A continuous bulk polymerization process, characterized in that it is fed to the bottom of the distillation column periodically or intermittently. The continuous bulk polymerization method according to claim 1 or 2, wherein the boiling point of the solvent is 10 ° C or higher than the boiling point of the vinyl monomer. 4. The continuous bulk polymerization method according to claim 3, wherein the solvent is alkylbenzene. The continuous bulk polymerization method according to claim 4, wherein the solvent is selected from xylene and ethylbenzene. The continuous bulk polymerization method according to claim 4, wherein the solvent is selected from toluene. The continuous monomer according to claim 1 or 2, wherein the monomer comprising methyl methacrylate as a main component comprises methyl methacrylate or a methyl methacrylate monomer and 15 wt% or less of an acrylate monomer. Bulk polymerization method. The continuous bulk polymerization method according to claim 7, wherein the acrylate monomer is a methyl acrylate monomer or an ethyl acrylate monomer.

Images