JP2001316417A - Method for producing polystyrene resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a polystyrene resin which contains a reduced monomer or oligomer, is excellent in moldability and heat stability, and is also excellent in the secondary processing such as printing. SOLUTION: The method is characterized in that a polystyrene polymer solution obtained through an anionic polymerization is heated gradually under a specific degassing condition to dry so that it may be protected from the heat decomposition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polystyrene resin by anionic polymerization. More specifically, the present invention relates to a method for producing a monomer by decomposing a polymer by relaxing a heat history in a devolatilization step in a vacuum devolatilization tank. By reducing the amount of styrene-based monomers and oligomers such as dimers and trimers in the product by reducing the amount of olefins and oligomers, and efficiently performing devolatilization and drying with an extruder equipped with a reduced pressure vent. The present invention relates to a method for efficiently producing a polystyrene resin having excellent printability.

[0002]

2. Description of the Related Art Conventionally, polystyrene resins have been molded by various molding methods, for example, injection molding, extrusion molding, hollow molding, vacuum molding, injection molding, and the like. Widely used for containers, toys, etc. Polystyrene resins are mainly produced by thermal polymerization or radical polymerization using an initiator. The main production processes include a bulk polymerization method and a suspension polymerization method, but the bulk polymerization method is mainly used because impurities such as a dispersant are hard to mix and the cost is advantageous.

However, even in radical polymerization using any of these methods, such as thermal initiation or peroxide initiator, since the polymerization reaction is not completed 100%, a large amount of unreacted monomer remains in the polymer solution immediately after polymerization. I have. Further, it is well known that the radical polymerization of styrene generally involves the formation of an oligomer, and the styrene-based monomer tends to remain.

[0004] For example, Encyclopedia of Chemical Techn
ology, Kirk-Othmer, Third Edition, JohnWily & Sons, 21
According to Vol. 817, thermal polymerization at 100 ° C. or higher involves by-products of oligomers such as styrene dimer and styrene trimer, and the amount thereof is about 1 wt%. Specific oligomer components include 1-phenyl-4-
(1′-phenylethyl) tetralin, 1,2-diphenylcyclobutane, 2,4-diphenyl-1-butene,
It is said that 2,4,6-triphenyl-1-hexene and the like are present.

[0005] In the bulk polymerization process, polymerization is usually carried out at 80 to 180 ° C, and then most of the contained solvent and unreacted monomer is subjected to a devolatilization treatment in a recovery step. The recovered solvent, unreacted monomers, and the like are recycled, but some monomers, oligomers such as dimers and trimers are hardly volatilized, and some remain in the product resin.

As a result of analyzing the polystyrene resin thus produced, it was found that impurities, residues and by-products during polymerization were contained in the raw material. Specifically, styrene,
α-methylstyrene, n-propylstyrene, n-propylbenzene, iso-propylbenzene, 1,3-diphenyl-1-butene, 1,2-diphenylcyclobutane, 1-phenyltetralin, 2,4,6-triphenyl-1-hexene , 1,3,5-triphenylbenzene, 1-phenyl-4- (1′-phenylethyl) tetralin and the like were found to be included in such polystyrene resins.

[0007] Such monomers and oligomers in the styrene-based resin may cause a decrease in thermal stability, or may adhere to a mold or a molded product as an oily substance during injection molding to cause a decrease in moldability. Furthermore, since the monomer or oligomer in the styrene resin penetrates from the inside of the molded article to the surface, there is a problem that the printed surface of the molded article is easily peeled off. On the other hand, a method of polymerizing polystyrene by anionic polymerization using an organometallic compound as an initiator is also known (for example,
Journal of Organometallic Chemisty., 10 (1967) 1-
6).

[0008] According to the anionic polymerization method, the polymerization reaction is continued as long as the monomer is substantially present, so that the amount of unreacted monomer in the product can be greatly reduced. However, since anionic polymerization is generally performed by a solution polymerization method, it is removed by devolatilizing the polymerization solvent under reduced pressure using a devolatilizing drum or a devolatilizing tank after the polymerization. In this process,
When an excessive heat history is given to the styrene-based resin, oligomers such as styrene monomers and dimers and trimers are newly generated by thermal decomposition and the like.

US Pat. No. 4,725,654, US Pat. No. 4,572,819 and Japanese Patent Publication No. 6-17409 propose an anionic polymerization method of styrenes using an organometallic initiator. In these, a resin having a constant molecular weight is introduced by continuously introducing a raw material into a reaction vessel having stirring, taking out a reaction product mixture at the same speed, measuring a molecular weight of a produced resin, and controlling a flow rate of an initiator and the like. Is obtained. However, there is no disclosure or suggestion regarding the amount of oligomer in the obtained resin.

[0010]

That is, the present invention reduces the amount of monomers in products and the amount of oligomers such as dimers and trimers, which cannot be achieved by conventional polystyrene resins, and is excellent in moldability and thermal stability and printing. The object of the present invention is to provide a method for producing a styrene-based resin which is excellent in secondary workability, such as water resistance.

[0011]

According to the present invention, a polystyrene-based polymerization solution obtained by anionic polymerization using an organometallic initiator is devolatilized under reduced pressure under a specific condition. It is surprising that the amount of monomers and oligomers in the product can be significantly reduced without impairing the characteristics of anionic polymerization, and a polystyrene-based resin with excellent moldability, thermal stability, and excellent secondary processing properties such as printability can be obtained. It was made based on facts that should be.

That is, according to the present invention, a hydrocarbon-based solvent is used.
A mixed solution containing 0 to 20 parts by weight of the styrene-based monomer and 10 to 80 parts by weight of the styrene-based monomer is supplied to a polymerization tank, and an anionic polymerization is performed using an organometallic compound as an initiator so that the conversion is 99% by weight or more. Alternatively, the polystyrene anion end is deactivated with water or the like, and then the polymerization solution is
(A) Desolvation under reduced pressure in a range where the polystyrene-based polymer content satisfies the following formula (1) in the first desolvation step: 0.3A + 60 <B <95 (1) (where A represents a polymerization tank) B represents the weight part of the total amount of the styrene monomer in the mixed solution of the hydrocarbon solvent and the styrene monomer to be supplied, and B represents the weight% of the polystyrene polymer relative to the total polystyrene polymer solution at the outlet of the first desolvation step. Represents.)

(B) Next, as a second desolvation step, the mixture is subjected to melt kneading and devolatilization by an extruder equipped with a reduced pressure vent, so that the total amount of the monomer, dimer and trimer in the obtained polystyrene resin is less than 1000 ppm. The present invention relates to a method for producing a polystyrene resin. Further, the present invention relates to a method for producing a polystyrene resin, which comprises anionically polymerizing a styrene monomer solution in which the hydrocarbon solvent is toluene, ethylbenzene or xylene.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for producing a polystyrene resin according to the present invention will be described in detail. Examples of the organic metal of the anionic polymerization initiator used in the method for producing a polystyrene resin of the present invention include an organic lithium initiator. Specific examples include n-butyl lithium, sec-butyl lithium, t-butyl lithium, methyl lithium, ethyl lithium, benzyl lithium,
1,6-dilithiohexane and the like. These may be used alone or as a mixture of two or more. Preferred examples include n-butyl lithium, se
c-butyl lithium and t-butyl lithium.

These organic lithium initiators include hydrocarbon solvents, for example, aliphatic hydrocarbon solvents such as hexane, cyclohexane, pentane and heptane; aromatic hydrocarbon solvents such as benzene, toluene, ethylbenzene and xylene; It can be used by dissolving it in ethers such as tetrahydrofuran, dityl ether and dimethyl ether. The amount of organolithium initiator used depends on the molecular weight of the polymer to be obtained. That is, the molecular weight of the polymer is basically determined by the monomer amount and the composition ratio of the organolithium initiator. The amount of organolithium used per 100 g of monomer is 0.05
-20 mmol. Preferably 0.1 to 10
Mmol, particularly preferably in the range from 0.2 to 2 mmol.

There are various styrene monomers used in the method of the present invention. Specific examples include styrene, p-methylstyrene, m-methylstyrene, o-methylstyrene, 2,4-dimethylstyrene, , 5-dimethylstyrene, 3,4-dimethylstyrene, 3,5-
Examples thereof include alkyl styrenes such as dimethyl styrene, p-ethyl styrene, m-ethyl styrene, and o-ethyl styrene. These styrene monomers may be used alone or in combination of two or more. The most preferred styrene monomer includes styrene.

In the present invention, an anionic copolymerizable monomer can be used together with the above-mentioned styrene-based monomer. Copolymerizable monomers include aromatic monovinyl compounds such as α-methylstyrene, vinyltoluene, vinylethylbenzene, and vinylxylene; conjugated diene compounds such as 1,3-butadiene and isoprene; and methyl methacrylate. Can be mentioned. These copolymer monomers may be useful in improving or adjusting the heat resistance, softening temperature, impact strength, rigidity, processability, etc. of the resin.

The industrial raw materials or commercially available styrene-based monomers contain the corresponding acetylenes (phenylacetylene in the case of styrene). When phenylacetylene is polymerized with n-butyllithium using styrene as a monomer, it deactivates n-butyllithium,
And / or acts as a chain transfer agent, reducing the efficiency of the n-butyllithium used and / or producing a low molecular weight polymer. In order to obtain the polystyrene resin of the present invention, the phenylacetylene compound is preferably less than 200 ppm based on the styrene monomer. More preferably less than 150 ppm, most preferably less than 100 ppm. When a styrene-based monomer containing 200 ppm or more of phenylacetylenes is used, the above-mentioned problems become apparent, low molecular components in the product increase, and thermal stability during molding may be impaired. .

The polymerization process of the styrenic monomer of the present invention is carried out by solution polymerization. That is, polymerization is carried out by dissolving a styrene monomer in a hydrocarbon solvent such as toluene, ethylbenzene or xylene. The amounts of the styrene monomer and the hydrocarbon solvent used are 80 to 10 parts by weight for the styrene monomer and 20 to 90 parts by weight for the hydrocarbon solvent. Preferably, the styrene monomer is 60 to 10 parts by weight and the hydrocarbon solvent is 40 to 90 parts by weight. When the amount of styrene monomer exceeds 80 parts by weight, very large power is required for stirring and transporting to increase the viscosity of the polymerization solution, and when the polymerization temperature is increased to lower the viscosity, the reaction speed becomes very fast. Removal of heat becomes difficult or polymerization is stopped due to heat deactivation. If the amount is less than 10 parts by weight, a large amount of thermal energy is required for the recovery of the solvent, so that the heat history applied to the polystyrene resin in the recovery step becomes excessive and the thermal decomposition of the resin is promoted.

The polymerization process of the styrenic monomer of the present invention may be any solution polymerization, and other processes are not particularly limited.
Generally, it is carried out by the following process. For example, a complete batch polymerization method in which the entire amount of the monomer and the initiator is charged into the reaction tank and then polymerization, a semi-batch polymerization method in which the reaction tank is charged with part or all of the initiator, and then polymerization is performed while the monomer is additionally charged, and complete stirring. The raw material system (monomer and initiator) is continuously charged into the reaction tank in the state, and the same amount of the production system (polymer solution) is taken out while continuously stirring, or the reaction raw material system is started from one end of the tubular reaction tank. , And the product system is taken out from the other end. A continuous polymerization method of plug flow, or a series connection process thereof is conceivable.

In the production of the polystyrene resin of the present invention, the production can be preferably carried out by a composite process in which a continuous tank with complete stirring and then a continuous tank with plug flow are connected. Since the polymerization rate in anionic polymerization is extremely high, it is important to remove the heat of polymerization in the polymerization process. A completely stirred mixing tank equipped with a reflux condenser can be preferably used. In the continuous polymerization process with complete stirring, the molecular weight distribution of the obtained polymer is broadened, and the remaining monomer can be efficiently converted in the continuous polymerization process in the next plug flow. Broadening the molecular weight distribution helps to improve the processability of the resin, and complete conversion of the monomer can eliminate low molecular weight components derived from the monomer mixed into the resin.

The complete stirring mentioned here is not limited to a narrow meaning. It is envisaged that the initiator has a broad residence time distribution, thereby agitating such that the molecular weight distribution of the polymer is significantly broadened. Specifically, the molecular weight distribution of the obtained polymer,
That is, it is assumed that the stirring condition is such that the ratio Mw / Mn of the weight average molecular weight to the number average molecular weight is 1.5 or more. The polymerization temperature ranges from 0 to 130C. Preferably 10 to 120
° C, particularly preferably in the range of 20 to 110 ° C. When the polymerization temperature is extremely low, the reaction rate is reduced, and the method is not practical.
On the other hand, if the polymerization temperature is extremely high, the reaction rate is lowered due to decomposition of the initiator, which is not preferable. If the temperature is higher than 110 ° C., the polymer may be colored, which is not preferable depending on the application.

After the polymerization, a carbon-lithium bond remains at the polymer terminal. If this is left as it is, it may be subjected to air oxidation or the like at the finishing stage or the like, which may cause a decrease in stability or coloring of the obtained polystyrene resin. After polymerization, it is preferable to stabilize the active terminal of the polymer, that is, the carbon-lithium bond. For example, water, alcohol, phenol,
Addition of a compound having an oxygen-hydrogen bond such as carboxylic acid,
Epoxy compounds, ester compounds, ketone compounds, carboxylic anhydrides, compounds having a carbon-halogen bond, and the like can also expect similar effects. The use amount of these additives is preferably about equivalent to about 10 times equivalent to the carbon-lithium bond. If it is too large, not only is it disadvantageous in terms of cost, but also mixing of the remaining additives becomes an obstacle in many cases.

A coupling reaction with a polyfunctional compound utilizing a carbon-lithium bond can be carried out to increase the molecular weight of the polymer and to form a branched structure of the polymer chain. The polyfunctional compound used for such a coupling reaction can be selected from known compounds. Examples of the polyfunctional compound include a polyhalogen compound, a polyepoxy compound, a mono- or polycarboxylic acid ester, a polyketone compound, a mono- or polycarboxylic anhydride, and the like. Specific examples include silicon tetrachloride and di (trichlorosilyl)
Ethane, 1,3,5-tribromobenzene, epoxidized soybean oil, tetraglycidyl 1,3-bisaminomethylcyclohexane, dimethyl oxalate, tri-2-ethylhexyl trimellitate, pyromellitic dianhydride, diethyl carbonate And the like.

An alkali component derived from organolithium,
For example, lithium oxide or lithium hydroxide can be neutralized and stabilized by adding an acidic compound. Examples of such acidic compounds include carbon dioxide, boric acid, various carboxylic acid compounds, and the like. By these additions, the coloring resistance can be particularly improved in some cases.

The polymer obtained by the method for producing a polystyrene resin of the present invention may contain known stabilizers for improving its thermal or mechanical stability, antioxidant property, weather resistance and light resistance. Can be. For example, JP-A-7-292188 discloses that addition of a 2,4,6-trisubstituted phenol is advantageous as a method for stabilizing polystyrene. Preferred examples of the 2,4,6-trisubstituted phenol include 2,6-di-t-butyl-
4-methylphenol, triethylene glycol-bis- [3- (3-t-butyl-5-methyl-4-hydroxyphenyl) propionate], pentaerythritoltetrakis [3- (3,5-di-t-butyl) -4-hydroxyphenyl) propionate], octadecyl-3-
(3,5-di-t-butyl-4-hydroxyphenyl)
Propionate, 1,3,5-trimethyl-2,4,6
-Tris (3,5-di-t-butyl-4-hydroxybenzyl) benzene, n-octadecyl 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, 2-t-butyl- 6- (3-t-butyl-2-hydroxy-5-methylbenzyl) -4-methylphenyl acrylate, 2 [1- (2-hydroxy-3,5-di-t
-Pentylphenyl)]-4,6-di-t-pentylphenyl acrylate, tetrakis [methylene 3- (3,
5-di-t-butyl-4-hydroxyphenyl) propionate] methane, 3,9bis [2- {3- (t-butyl-4-hydroxy-5-methylphenyl) propynyloxy} -1,1-dimethyl Ethyl] -2,4,8,1
0-tetraoxo [5,5] undecane, 1,3,5-
Tris (3 ′, 5′-di-t-butyl-4′-hydroxybenzyl) -s-triazine-2,4,6 (1H, 2
H, 3H) -trione, 1,1,4-tris (2-methyl-4-hydroxy-5-t-butylphenyl) butane, 4,4′-butylidenebis (3-methyl-6-t-
Butylphenol) and the like.

These stabilizers can be mixed after the recovery of the polymer. However, it is preferable to add them at the stage of the solution after the polymerization because mixing is easy and deterioration in the solvent recovery step can be suppressed. In the present invention, the desolvation step (devolatilization step) in the solvent recovery step is the most important. That is, in the present invention, the recovery of the solvent is performed in two stages.
In the first desolvation step, the solvent is removed by flashing in a generally known vacuum devolatilizing drum or vacuum devolatilization tank. In the first solvent removal step, the content of the polymer component in the polymerization solution is represented by the following formula (1). Must be removed within the range.

0.3A + 60 <B <95 (1) (wherein A represents parts by weight based on the total volume of the styrene monomer in the mixed solution of the hydrocarbon solvent and the styrene monomer supplied to the polymerization tank. B Represents the weight% of the polystyrene-based polymer relative to the total polystyrene-based polymer solution at the outlet of the first desolvation step.)

When the amount of desolvation in the first desolvation step is small, that is, B <0.3A + 60 (where A is a styrene-based solvent in a mixed solution of a hydrocarbon-based solvent and a styrene-based monomer supplied to a polymerization tank). B is the weight part of the total polystyrene-based polymer solution at the outlet of the first desolvation step.
Represents In the case of (2), the heat load in the subsequent second desolvation step becomes too large, and a large amount of monomer and oligomer components are generated due to thermal decomposition of the polymer in the second desolvation step, which is not preferable. If it exceeds 95% by weight, the viscosity becomes too high, and it becomes difficult to supply the mixture to the vented extruder, which is the second desolvation step, and the heat load in the first desolvation step becomes too high. Thermal decomposition and the like of the polymer in one solvent removal step are promoted.

The operating conditions in the first desolvation step can be variously selected within a range that satisfies the above formula (1), but the temperature is preferably lower than 240 ° C. More preferably 2
It is less than 00 ° C. Most preferably it is below 180 ° C. A temperature of 240 ° C. or higher is not preferable because it leads to adding a thermal history to the polymer for a long time at a high temperature. Since the first desolvation step also serves as a buffering step for supplying to the second desolvation step, the residence time is relatively long, but preferably less than 60 minutes. More preferably less than 30 minutes.

Although the first desolvation step is performed under reduced pressure, there is no particular limitation as long as the conditions are such that devolatilization is performed within the scope of the present invention under the above conditions, but preferably less than 50 kPa. More preferably less than 30 kPa, most preferably 1 kPa.
It is less than 0 kPa. In order to satisfy the conditions of the present invention at a degree of reduced pressure exceeding 50 kPa, the temperature may exceed 240 ° C., which is not preferable. In the second desolvation step, it is essential to use an extruder with a reduced pressure vent. The extruder may be a twin-screw or single-screw extruder, but is preferably a twin-screw extruder from the viewpoint of devolatilization and drying efficiency. It is sufficient that at least one or more pressure reduction vents are provided, but it is more preferable that there are two or more pressure reduction vents. More preferably, the number is three or more.

The cylinder length / cylinder diameter (L / D) of the extruder used is in the range of 20 to 50.
The above is preferred. The temperature of the cylinder and the die of the extruder is usually in the range of 170 to 290 ° C. Preferably 2
00-260 ° C. Depressurization degree of the extruder vent is low molecular weight component in the resulting polystyrene resin pellets,
That is, the total amount of the monomer, dimer and trimer can be set to be less than 1000 ppm.
It is desirable to set so as to be less than 0 ppm.
It is preferably less than kPa. More preferably, it is less than 5 kP. Most preferably it is less than 1 kPa. When using an extruder having a plurality of decompression vents, it is desirable to make the degree of decompression inclined.

In order to promote the devolatilization of the low molecular components in the extruder, water, carbon dioxide, nitrogen and the like may be coexisted, if desired. The total amount of monomers, oligomers and trimers contained in the polystyrene resin obtained by the method of the present invention must be less than 1000 ppm. Preferably it is less than 500 ppm, more preferably less than 200 ppm. When the amount of the low molecular weight component is large, the thermal stability and the moldability are reduced.

The polystyrene resin obtained by the present invention generally has a weight average molecular weight of 100,000 to 50.
000, and various additives commonly used as needed, for example, stabilizers, dyes, pigments, fillers,
A molding material can be obtained by adding a lubricant, a release agent, a plasticizer, an antistatic agent, an antioxidant, and the like. In addition, the polystyrene resin obtained by the present invention may be used, if desired, with general-purpose polystyrene or high-impact polystyrene obtained by radical polymerization, other thermoplastic resins, for example, styrene-butadiene block copolymer or its hydride, polyphenylene ether, ABS, etc. Can be blended.

The styrenic resin of the present invention thus obtained is excellent in moldability, thermal stability and printability.
It can be widely used for household electrical appliances, office equipment, household goods, food containers, various packaging materials, toys and the like. The polystyrene resin obtained by the present invention can be molded by a conventionally known method, for example, injection molding, extrusion molding, hollow molding, vacuum molding, injection molding, or the like.

Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these. The determination of the styrene monomer, dimer and trimer in the resin was performed using gas chromatography. Here, the dimer is the total amount of 1,2-diphenylcyclobutane and 2,4-diphenyl-1-butene, and the trimer is 1
It was determined as the total amount of -phenyl-4- (1'-phenylethyl) tetralin and 2,4,6-triphenyl-1-hexene. The molecular weight and molecular weight distribution were determined at 35 ° C. by GPC using UV / RI with tetrahydrofuran as a measuring solvent and calculated in terms of polystyrene.

[0037]

Example 1 A capacity 12 with a jacket equipped with a stirrer
0.2m heat transfer area on top of L complete mixing reactor
The first reactor was equipped with a reflux condenser of No. ³ . Further, a 4 L piston flow reactor (L / D = 3.5) equipped with a stirrer was arranged in series downstream of the first reactor to form a second reactor. The insides of the first and second reactors were degassed under reduced pressure, and the inside of the tank was set to a nitrogen atmosphere.

50 parts by weight of degassed styrene from which a polymerization inhibitor (tertiary butyl catechol) and water have been removed, and 50 parts of degassed and purified ethylbenzene as a polymerization solvent.
The parts by weight were mixed in the compounding tank. In the first reactor and the second reactor, the temperature of the reaction vessel was set at 80 ° C. Add a mixture of styrene monomer and ethylbenzene 8 times from the blending tank.
Feed was performed at a flow rate of L / h. The amount held in the tank of the first reactor was controlled at 8 L. Depressurize the first reactor and reduce the pressure to 15
The polymerization temperature was kept at 80 ° C. by maintaining the pressure at kPa and refluxing ethylbenzene. n-Butyl lithium was fed to the first reactor in an amount corresponding to 0.9 mmol per 100 g of styrene monomer.

The reaction liquid flowing out of the first reactor was subsequently introduced into the second reactor and passed through a piston flow.
The conversion after passing through the second reactor was 99.99% or more. The resulting polymer solution was deactivated by adding 10 times equivalent of methanol to the amount of lithium. Then, 100 g of polymer was added to the polymer solution.
Per day, 0.05 g of antioxidant was added. Next, as a first desolvation step, degassing was performed in a flushing tank (temperature: 200 ° C.) at a reduced pressure of 5 kPa. The residence time in the flushing tank was 24 minutes. The polymer concentration in the polymer solution at the outlet of the first desolvation step was 86% by weight on average.

Subsequently, as a second desolvation step, a twin screw extruder (L / D = 25 mm) having three reduced-pressure vent ports was used.
33.5) was fed with a polymer solution. The extruder inlet temperature was controlled at 230 ° C, and the outlet temperature was controlled at 265 ° C. The degree of pressure reduction of the vent is 300, 200, 1 in order from the entrance side.
The operation was performed at 00 Pa. The molecular weight distribution and the contents of monomers and oligomers (dimers and trimers) of the polystyrene resin thus obtained were quantified. Table 1 shows the results
Shown in Furthermore, the amount of the monomer and oligomer after stagnating the obtained styrene-based resin in a melt indexer heated to 280 ° C. for 10 minutes was quantified. Table 1 shows the results
Shown in

[0041]

Example 2 Example 2 was carried out in the same manner as in Example 1 except that the composition ratio of the styrene monomer and ethylbenzene as the polymerization solvent was changed to 70/30 by weight. The polymer concentration in the polymer solution at the outlet of the first desolvation step was 89% on average. The molecular weight distribution and the contents of monomers and oligomers (dimers and trimers) of the polystyrene resin thus obtained were quantified. Table 1 shows the results. Furthermore, the amount of the monomer and oligomer after stagnating the obtained styrene-based resin in a melt indexer heated to 280 ° C. for 10 minutes was quantified. Table 1 shows the results.

[0042]

[Comparative Example 1] 5 kP without using an extruder in the second desolvation step
The procedure was carried out in the same manner as in Example 1 except that devolatilization under reduced pressure was performed in a flushing tank (temperature: 250 ° C.) in which the pressure was reduced to a. The molecular weight distribution and the contents of monomers and oligomers (dimers and trimers) of the polystyrene resin thus obtained were quantified. Table 1 shows the results. Furthermore, the amount of the monomer and oligomer after stagnating the obtained styrene-based resin in a melt indexer heated to 280 ° C. for 10 minutes was quantified. Table 1 shows the results.

[0043]

Comparative Example 2 Capacity 12 with a jacket equipped with a stirrer
L was used as the first reactor. Further, a 4 L piston flow reactor (L / D = 3.5) equipped with a stirrer was arranged in series downstream of the first reactor to form a second reactor. The insides of the first and second reactors were degassed under reduced pressure, and the inside of the tank was set to a nitrogen atmosphere. In advance, 90 parts by weight of degassed styrene from which a polymerization inhibitor (tertiary butyl catechol) and water were removed, and 10 parts by weight of degassed and purified ethylbenzene as a polymerization solvent were mixed in a mixing tank.

The temperature of the first reactor was set at 140 ° C. and the temperature of the second reactor was set at 180 ° C. A mixed solution of styrene monomer and ethylbenzene was fed at a flow rate of 1.3 L / h from the blending tank to perform thermal radical polymerization. The amount held in the tank of the first reactor was controlled at 8 L. The reaction liquid flowing out of the first reactor was subsequently introduced into the second reactor and passed through a piston flow.

Thereafter, the polymer solution was treated in the same manner as in Example 1. The molecular weight distribution and the contents of monomers and oligomers (dimers and trimers) of the polystyrene resin thus obtained were quantified. Table 1 shows the results. Further, after the obtained styrene-based resin was retained in a melt indexer heated to 280 ° C. for 10 minutes, the amounts of monomers and oligomers were quantified, and the thermal stability was evaluated.

Further, using the obtained styrene-based resin, a 1.2 mm-thick sheet was formed at a resin temperature of 240 ° C. and a mold temperature of 5 ° C.
Injection molding was performed at 0 ° C. The moldability was evaluated in the state of the flow mark on the molded sheet. In addition, the printability is as follows: printing ink for polystyrene is silk screen printed on the above-mentioned molded sheet, dried at 90 ° C. for 2 hours, left at 25 ° C. for 24 hours, and then adhered to a cellophane tape and peeled off the tape. And the peeling state of the printed surface was observed and evaluated. Table 2 shows the results.

[0047]

[Table 1]

[0048]

[Table 2]

(In the table, the moldability was evaluated as follows: ○: no flow mark was observed, Δ: slight flow mark was observed, x: flow mark was noticeably observed. No peeling of the printed surface is observed, Δ:
X: Slight peeling of the printed surface was observed. X: Remarkably peeling of the printed surface was observed. )

[0050]

The styrenic resin obtained by the method of the present invention has a total amount of monomer and oligomer in the product of less than 1000 ppm, is excellent in moldability and thermal stability, and has good printability. Excellent secondary workability.

Claims (2)

[Claims] A mixed solution containing 90 to 20 parts by weight of a hydrocarbon-based solvent and 10 to 80 parts by weight of a styrene-based monomer is supplied to a polymerization tank, and the organic metal compound is used as an initiator to achieve a conversion of 99% by weight or more. After anionic polymerization so that
The polystyrene anion end is deactivated with an alcohol or water, and then the polymerization solution is subjected to (a) desolvation under reduced pressure in the first desolvation step so that the polystyrene-based polymer content satisfies the following formula (1). 0.3A + 60 <B <95 (1) (In the formula, A represents a part by weight based on the total volume of the styrene-based monomer in the mixed solution of the hydrocarbon-based solvent and the styrene-based monomer supplied to the polymerization tank. (% By weight of the polystyrene-based polymer with respect to the total polystyrene-based polymer solution at the outlet of one desolvation step.) (B) Next, as a second desolvation step, melt kneading and devolatilization are performed by an extruder equipped with a reduced pressure vent. A method for producing a polystyrene resin in which the total amount of monomers, dimers and trimers in the obtained polystyrene resin is less than 1000 ppm. 2. The method according to claim 1, wherein the hydrocarbon solvent is toluene, ethylbenzene or xylene.