JPH0613590B2 - Method for producing rubber-modified styrene / acrylonitrile copolymer resin composition - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a styrene / acrylonitrile-based copolymer resin composition whose impact resistance is improved by a rubber-like polymer. More specifically, production of a resin composition having improved gloss and impact resistance obtained by copolymerizing a styrene monomer and an acrylonitrile monomer by a continuous bulk or solution polymerization method in the presence of a rubbery polymer. Regarding the method.

[Conventional technology]

Conventionally, a terpolymer resin composed of a rubber-like polymer and a styrene-acrylonitrile-based copolymer has been widely used as a base material for various mechanical parts and electrical products as a resin excellent in impact strength and mechanical strength. ing. Conventionally, such a terpolymer resin is mainly produced by an emulsion polymerization method.

[Problems to be Solved by the Invention]

On the other hand, the continuous bulk or solution polymerization method has production advantages such as low cost, easy process, and easy wastewater treatment as compared with the emulsion polymerization method, but on the other hand, the gloss of the resin is low, or Since the impact resistance is insufficient, the quality as an industrial product has a problem.

[Means for Solving the Problems]

As a result of intensive studies conducted by the present inventors in view of the importance of such a problem, in a continuous bulk or solution polymerization method, a specific rubber-like polymer was used, and a specific rubber-like polymer particle diameter and a mixed solution index were obtained. In addition, the inventors have found that the composition having a specific monomer composition has remarkably improved gloss and impact resistance, and is comparable to the performance balance of emulsion polymerization, and has reached the present invention.

That is, in the present invention, in the presence of 1 to 20 parts by weight of a rubber-like polymer having a solution viscosity of 20 to 50 centistokes, 65 to 92 parts by weight of a styrene-based monomer and an acrylonitrile-based monomer by a continuous bulk or solution polymerization method. 8 to 35 parts by weight of the copolymer, and 1) mixing the obtained rubber-modified styrene / acrylonitrile-based copolymer resin composition with a mixed solvent having a mixing ratio of toluene and methyl ethyl ketone of 7/3 Solution index is 6 ~
14 times, 2) the average particle size of the rubber-like polymer particles in the resin composition is 0.5 to 1.5 μ, and 3) the resin composition is mixed with methyl ethyl ketone and methanol in a ratio of 7/3. When dissolved in a solution, it is insoluble in the acrylonitrile-based monomer weight composition ( _WSA ) and styrene-based monomer weight composition ( _WSS ) of the polymer portion which is soluble in the mixed solution, and insoluble in the mixed solution. And the acrylonitrile-based monomer weight composition (W _DA ) of the polymer portion other than the rubber polymer
And the styrene-based monomer weight composition (W _DS ) have the following formula (I) (I) 1.2 ≧ X _S / X _D ≧ 0.9 The method for producing a rubber-modified styrene / acrylonitrile-based copolymer resin composition is characterized by controlling the copolymerization so as to satisfy the above condition.

A known method or a combination thereof is used as the continuous bulk or solution polymerization method in the present invention and is not particularly limited. The bulk polymerization method will be described with reference to an example. A rubber-like polymer is dissolved in a monomer, and a monomer solution of the rubber-like polymer is stirred with or without addition of a molecular weight regulator, a polymerization initiator, etc. Continuously fed to the reactor
10 or more of the monomers by one or more stirred reactors
Prepolymerization is carried out until 60% is converted to polymer, at the same time converting the rubbery polymer into the form of dispersed particles. Then, main polymerization is further continued by one or more reactors, and 50 to 99% of the total amount of the monomers used for the polymerization.
Is converted into a polymer, the polymerization liquid is introduced into a devolatilization tank to remove unreacted monomers and a part of oligomers,
Further, a granular resin composition is obtained through a granulation step. Further, in the example of continuous solution polymerization, ethylbenzene, in one or more steps of the rubber dissolving step and the reaction step described above,
A solvent such as toluene or methyl ethyl ketone is supplied, and most of it is recovered together with the monomer in the devolatilization step.
In any of the bulk and solution polymerization methods, the molecular weight regulator, the polymerization initiator, etc. are supplied at any position,
It is preferable that the conversion rate of the monomer into the polymer is supplied in the range of 0 to 50%. Further, the monomer is added in an increased amount in any step to continue the polymerization.

In the resin composition obtained in the present invention, the mixed solution index is in the range of 6 to 14 times, preferably 6.5 to 13 times, more preferably 6.5 to 11 times. If this value is less than 6 or exceeds 14, the object of the present invention is not achieved. In the present invention, the mixed solution index is 1.0 as a sample.
gr 3 ml of toluene and methyl ethyl ketone 7/3
After standing in a mixture of ratio of input, centrifuged, the soluble component was removed by decantation into a mixture, measured immediately weight of insoluble components in a swollen state with a mixture (W _S), thereafter said The weight (W _d ) of the insoluble component obtained by drying the component by vacuum drying is measured, and the value of W _s / W _d is displayed as a double. The mixed solution index is the amount of the rubber-like polymer during the polymerization, the amount of the molecular weight modifier, the amount of the solvent, the amount and type of the polymerization initiator, the residence time in the devolatilization step and the granulation step after the polymerization, and It can be adjusted by adjusting the processing temperature and the like, and those skilled in the art can adjust it to a predetermined value by the trial and error method. When the amount of the molecular weight modifier is increased, the mixed solution index becomes small, when the amount of the solvent is increased, the mixed solution index becomes large, and when the amount of the polymerization initiator is increased, the mixed solution index becomes small. Further, when the residence time in the devolatilization step and granulation step after polymerization is increased, the mixed solution index becomes small, and when the processing temperature in the devolatilization step and granulation step after polymerization is increased, the mixed solution index Is small. Thus, the mixed solution index can be controlled by adjusting these conditions. The reason why the gloss and impact resistance of the composition within the range of the mixed solution index in the present invention are improved is not clear, but since the resin performance changes extremely sharply with respect to the mixed solution index, such index is continuous. It is considered to be an index that reflects the properties of the rubber-like polymer particles in the rubber-modified styrene / acrylonitrile-based copolymer resin composition obtained by copolymerization by bulk or solution polymerization.
In the measurement of the mixed solution index in the present invention, when toluene is used instead of the mixed solvent in the present invention, no teaching about performance improvement can be obtained.

In the present invention, the rubber-like polymer particles have an average particle size of 0.5.
˜1.5 μ, preferably 0.6 to 1.3 μ, and more preferably 0.6 to 1.1 μ. This value is 0.5μ
If it is less than 1.5 μm or exceeds 1.5 μm, the effect of the present invention cannot be obtained. In the present invention, the average particle size of the rubber-like polymer particles is measured as follows. That is, in the electron micrograph of the resin obtained by the ultrathin section method, 5 of the rubber-like polymer particles
The particle diameters of 0 to 200 particles were measured and averaged by the following formula.

Average particle diameter = ΣnD ² / ΣnD (where n is the number of rubber-like polymer particles having a particle diameter D.) The average particle diameter of the rubber-like polymer is the condition of the prepolymerization step in the resin production step, for example, Amount of rubber-like polymer used in prepolymerization step, amount of monomer used, amount of molecular weight modifier, amount and type of polymerization initiator, conversion of monomer to polymer, polymerization temperature, polymerization rate, etc. Can be adjusted to a desired average particle size by a trial and error method by those skilled in the art. When the amount of the rubber-like polymer used in the prepolymerization step is increased, the average particle size of the rubber-like polymer is increased, and when the amount of the molecular weight modifier is increased, the average particle size of the rubber-like polymer is increased and the polymerization initiation When the amount of the agent is increased, the average particle size of the rubbery polymer becomes smaller. Further, when the conversion rate of the monomer to the polymer is reduced, the average particle diameter of the rubber-like polymer becomes large, and when the polymerization temperature is raised, the average particle diameter of the rubber-like polymer becomes large. Thus, the average particle size of the rubber-like polymer can be controlled by adjusting these conditions.

In the present invention, when the rubber-modified styrene / acrylonitrile copolymer resin composition is dissolved in a 7/3 mixed solution of methyl ethyl ketone and methanol, the weight of the acrylonitrile monomer of the polymer portion which is soluble in the mixed solution. Composition ( _WSA ) and styrene-based monomer weight composition ( _WSS ), and acrylonitrile-based monomer weight composition ( _WDA ) and styrene-based monomer of insoluble and polymer portion other than rubber-like polymer The relationship of the weight composition (W _DS ) satisfies the following formula (I), preferably the following formula (II).

(I) 1.2 ≧ X _S / X _D ≧ X0.9 (II) 1.05 ≧ X _S / X _D ≧ X0.95 The reason for this is not clear, but from the polymer portion which is soluble in the mixed solution of methyl ethyl ketone and methanol of 7/3 of the rubber-modified styrene / acrylonitrile copolymer resin composition and the polymer portion which is insoluble in this solvent. The composition of the polymer portion excluding the rubber component is close, that is, X _S /
It is preferred that X _D is close to 1.

W _SA , W _SS , W _DA and W _DS in the above equation are measured as follows, respectively.

That is, about 1 g of the resin was left to stand in a mixed solution of 30 ml of methyl ethyl ketone and methanol at 7/3 to achieve partial dissolution, and then centrifuged to dissolve the polymer portion insoluble in the mixed solution. A polymer portion is separated by a gradient method. Then, the solvent is removed from both parts by vacuum drying, and the weight composition of the acrylonitrile monomer constituting the polymer of each part is determined based on the elemental analysis value of N element of the dried product. In addition, the weight composition of the styrene-based monomer that constitutes the polymer of each portion is obtained by, for example, taking the balance of the weight values of the raw material and the produced polymer, and the soluble portion and the insoluble portion of the mixed solution. . The value of X _S / X _D is the concentration ratio (a) of the styrene-based monomer and the acrylonitrile-based monomer in the polymerization mixture when the conversion rate of the monomer to the polymer is in the range of 10 to 70%, and Ratio of concentration ratio (b) of styrene-based monomer and acrylonitrile-based monomer in the conversion rate range of 40 to 99%, that is, (a) / (b), polymerization temperature in both conversion rate ranges, devolatilization It is adjusted by the operation of the minute step, etc., and can be set to a desired value by a trial and error method by those skilled in the art.
(A) / a The larger _(b), the value of X S / _{X D} becomes smaller, by a 0.9 to 1.2 of (a) / (b), be adjusted in a predetermined range it can. Conversion rate is 1
A higher polymerization temperature at 0% to 70% _range, the value of X S / _{X D} becomes smaller, the conversion rate to increase the polymerization temperature in 40-99% _range, the value of X S / _{X D} increases . Conversion of monomer to polymer is 50 to 90%, preferably 50 to 8
A method of performing the devolatilization operation by controlling to 0%, more preferably 55 to 70%, is preferably used.

The rubbery polymer referred to in the present invention may be any as long as it exhibits rubbery properties at room temperature, and examples thereof include polybutadienes, styrene-butadiene copolymers, styrene-butadiene block copolymers and ethylene-propylene-based polymers. Copolymers, ethylene-propylene-non-conjugated diene terpolymers, isopropylene copolymers, ethylene-isoprene copolymers, chloroprene copolymers, butadiene-
One or more types of acrylonitrile copolymers, acrylic acid ester copolymers, silicone rubbers and the like are used. Among these rubber-like polymers, polybutadienes and styrene-butadiene copolymers are preferable, and use of polyptadiene is particularly preferable.

The rubber-like polymer used in the present invention has a solution viscosity of 20 to 50 c.
st, preferably in the range of 30 to 50 cst. However, if it is less than 20 cst or exceeds 50 cst, the performance balance of impact resistance and gloss is deteriorated. The microstructure of the rubber-like polymer is not particularly limited,
When the total butadiene component constituting the rubbery polymer is 100 parts, the 1,4 cis-bonded butadiene component is 20 to 40.
Parts, or 91 parts or more are preferably used, and those having a 1,2-vinyl-bonded butadiene component of 25 parts or less are more preferably used.

The rubber-modified styrene-acrylonitrile copolymer resin composition of the present invention is not particularly limited, but preferably contains a rubber-like polymer in an amount of 2 to 25% by weight,
Particularly preferably, it is contained in the range of 4 to 18% by weight.

Examples of the styrene-based monomer used for producing the rubber-modified styrene-acrylonitrile-based copolymer resin composition of the present invention include styrene, α-methylstyrene, o, m,
vinyl-substituted or nucleus-substituted alkylstyrenes such as p-methylstyrene, ethylstyrenes, isopropylstyrenes, and butylstyrenes, vinyl-substituted or nucleus-substituted alkylstyrenes such as α, o, m, p-bromostyrene, and chlorostyrenes One or more types of halogenated styrenes, halogenated alkyl styrenes and the like are used. Of these, styrene and paramethylstyrene are preferably used. As the acrylonitrile-based monomer, for example, one or more of acrylonitrile, methacrylonitrile, α-chloroacrylonitrile and the like are used, and acrylonitrile and methacrylonitrile are preferably used.

The composition of the polymer portion of the styrene-based monomer and the acrylonitrile-based monomer is such that the total of both is 100 polymer, and the styrene-based monomer is usually 65 to 92 polymer, preferably 70.
A range of up to 80 parts by weight is used. Further, one or more of methyl methacrylate, maleic anhydride and the like may be added as a constituent component of the polymer within a range not exceeding 20 parts by weight.

An organic peroxide may be used in the production of the resin composition obtained in the present invention. A known organic peroxide is used as such an organic peroxide. Among them, those having a 10-hour half-life temperature of 50 to 140 ° C. are preferable, and 60
Those at a temperature of up to 130 ° C are more preferably used. Further, in the production of the composition of the present invention, a known molecular weight regulator used in the production of a styrene-based polymer is similarly used, and t-dodecyl mercaptan, n-dodecyl mercaptan, alphamethylstyrene dimer and the like are used. It is preferably used.

The rubber-modified styrene / acrylonitrile-based copolymer resin composition obtained in the present invention can be used alone,
It may be used as a mixture with other resin such as polycarbonate, styrene / acrylonitrile copolymer resin, styrene / maleic anhydride copolymer resin, or the like. Further, it is preferable to add and use one or more of a stabilizer against heat, light and oxygen, a flame retardant, a plasticizer, a coloring agent, a lubricant, an antistatic agent and the like to the composition of the present invention.

[Examples] Next, the present invention will be further described with reference to Examples.

Example 1 a. Manufacturing method 75.5 parts by weight of styrene, 24.5 parts by weight of acrylonitrile, 5 parts by weight of ethylbenzene, 7 parts by weight of a rubber-like polymer (polybutadiene manufactured by Ube Industries, Ltd., Ubepol 13HB, solution viscosity 41 cst), organic peroxide Things (1,1
A raw material solution consisting of 0.05 parts by weight of -bis (t-butylperoxy) 3,3,5-trimethylcyclohexane] and 0.2 parts by weight of t-dodecyl mercaptan was prepared. Polymerization was carried out in a three-stage stirred type polymerization tank array reactor. The raw material solution was continuously supplied from the first-stage tank. The stirring number in the first-stage tank was 200 rpm. From the third tank, the polymerization solution was introduced into a devolatilization tank consisting of two preheaters in series and a vacuum chamber. The amount of the monomers at the inlet of the first-stage devolatilization tank was 35 parts by weight, and the total amount of the monomers converted into the polymer was 65 parts by weight. The residual amount of the monomer at the outlet of the first devolatilization tank is set to 10 parts by weight, and the mixture is allowed to pass through a flow region at a temperature of 90 to 140 ° C. and an average residence time of 45 minutes, and then the second devolatilization tank is used. To substantially completely remove the residual monomer and solvent,
A polymer at 75 ° C. was obtained, and a granulation step was further performed to obtain a rubber-modified styrene / acrylonitrile-based copolymer resin composition.

b. Analysis and Performance Evaluation Table 1 shows the results of analysis and performance evaluation of the obtained rubber-modified styrene / acrylonitrile copolymer resin composition. The performance evaluation was based on the following method.

Impact resistance: The impact resistance was evaluated by measuring the falling weight impact strength.

Drop heavy metal objects by gradually increasing the drop height,
The height at which cracking occurred in the test piece was determined, and the impact resistance was expressed by the product of the height and the weight of the heavy object.

Gloss: Measured according to JIS K-8741.

Flowability: In injection molding, the hydraulic pressure of the molding machine (short shot hydraulic pressure) required for the minimum injection pressure that does not cause short shot was evaluated.

In Table 1, the difference is described as a positive or negative value with reference to the reference example.

(In the case of a negative value, the hydraulic pressure is lower than that of the reference example, and it is evaluated as a material having good fluidity during molding.) Heat resistance: The Vicat softening point was measured according to ASTM-D1525.

Examples 2 and 3 and Comparative Examples 1 and 2 In Example 1, except that the rotation number of the first-stage stirring layer was changed as shown in Table 2, the same experiment as in Example 1 was conducted to carry out the rubber-like weight test. Rubber-modified styrene / acrylonitrile-based copolymer resin compositions having different coalesced particle diameters were obtained. The analytical evaluation results are shown in Table 1.

Reference Example Table 1 shows the evaluation results of a commercially available rubber-modified styrene / acrylonitrile copolymer resin composition by emulsion polymerization.

Comparative Example 3 Example 1 was repeated except that in Example 1, the amount of acrylonitrile supplied to the first-stage stirring tank was 12 parts by weight, and 12.5 parts by weight of acrylonitrile was supplied to the second-stage stirring tank. It experimented similarly to. The results are shown in Table 1.

Comparative Example 4 In Example 1, the amount of styrene supplied to the first-stage stirring tank was 60 parts by weight, and the styrene in the second-stage stirring tank was 15.5.
An experiment was conducted in the same manner as in Example 1 except that styrene was supplied in an amount of part by weight. The results are shown in Table 1. Examples 4, 5 and Comparative Examples 5, 6, 7 In Example 1, the residual amount of the monomer at the outlet of the first-stage devolatilization tank and the flow region to the second-stage devolatilization tank The experiment was conducted in the same manner as in Example 1 except that the temperature of 1 and the operating temperature of the second stage devolatilization tank were changed as shown in Table 2. The results are shown in Table 1.

Example 6 Example 1 was repeated except that as the rubber-like polymer, polybutadiene manufactured by Asahi Kasei Co., Ltd. under the trade name Asaprene 700A (solution viscosity 43 cst) was used, and the rotation speed of the first-stage stirring tank was changed as shown in Table 2. It experimented similarly to. The results are shown in Table 1.

Comparative Example 8 In Example 1, as a rubbery polymer, polybutadiene manufactured by Asahi Kasei Corporation under the trade name of diene 35 (solution viscosity 89 cs) was used.
The experiment was performed in the same manner as in Example 1 except that t) was used and the rotation speed of the first-stage stirring layer was changed as shown in Table 2. The results are shown in Table 1.

Example 7 75.5 parts by weight of styrene, 24.5 parts by weight of acrylonitrile, 5 parts by weight of ethylbenzene, a rubber-like polymer (styrene-butadiene copolymer manufactured by Asahi Kasei Corporation, trade name Tafden 2000AS, styrene content 25% by weight, Solution viscosity 5
0 cst) 7 parts by weight, organic peroxide [1,1-bis (t-
Butylperoxy) 3,3,5-trimethylcyclohexane]
A raw material solution consisting of 0.05 parts by weight and 0.2 parts by weight of t-dodecyl mercaptan was prepared. Polymerization was carried out in a three-stage stirred type polymerization tank array reactor. The raw material solution was continuously supplied from the first-stage tank. The stirring number in the first stage tank is 200 rp
m. From the third tank, the polymerization solution was introduced into a devolatilization tank consisting of two preheaters in series and a vacuum chamber. The amount of the monomers at the inlet of the first-stage devolatilization tank was 35 parts by weight, and the total amount of the monomers converted into the polymer was 65 parts by weight. The residual amount of the monomer at the outlet of the first devolatilization tank is set to 10 parts by weight, and the mixture is allowed to pass through a flow region at a temperature of 90 to 140 ° C. and an average residence time of 45 minutes, and then the second devolatilization tank is used. The residual monomer and the solvent were substantially completely removed to obtain a polymer at 275 ° C., and a granulation step was further performed to obtain a rubber-modified styrene / acrylonitrile copolymer resin composition.

The analytical evaluation results are shown in Table 1.

Example 8 75.5 parts by weight of styrene, 24.5 parts by weight of acrylonitrile, a rubbery polymer (polybutadiene manufactured by Ube Industries, Ltd., trade name Ubepol 13HB, solution viscosity 41 cst)
A raw material solution consisting of 7 parts by weight, 0.04 parts by weight of organic peroxide [1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane], and 0.3 parts by weight of t-dodecyl mercaptan was added. Created. Polymerization was carried out in a three-stage stirred type polymerization tank array reactor. The raw material solution was continuously supplied from the first-stage tank. The stirring number in the first-stage tank was 200 rpm.
From the third tank, the polymerization solution was introduced into a devolatilization tank consisting of two preheaters in series and a vacuum chamber. The amount of the monomers at the inlet of the first-stage devolatilization tank was 35 parts by weight, and the total amount of the monomers converted into the polymer was 65 parts by weight. The residual amount of the monomer at the outlet of the first devolatilization tank is set to 10 parts by weight, and the mixture is allowed to pass through a flow region at a temperature of 90 to 140 ° C. and an average residence time of 45 minutes, and then the second devolatilization tank is used. The residual monomer and the solvent were substantially completely removed to obtain a polymer at 275 ° C., and a granulation step was further performed to obtain a rubber-modified styrene / acrylonitrile copolymer resin composition.

The analytical evaluation results are shown in Table 1.

Comparative Example 9 75 parts by weight of styrene, 25 parts by weight of acrylonitrile, 7 parts by weight of a rubbery polymer (polybutadiene manufactured by Ube Industries, Ltd., Ubepol 13HB, solution viscosity 41 cst).
0.2 parts by weight of t-dodecyl mercaptan, organic peroxide (t-butylperoxy-2-ethylhexanoate)
A raw material solution consisting of 0.05 parts by weight was prepared. Polymerization was carried out in a reactor equipped with a stirrer until the conversion rate of the monomers into the polymer reached 22%. Thereafter, 0.05 part by weight of t-butylperoxy-2-ethylhexanoate was added, and further 2 parts by weight of acrylonitrile, 200 parts by weight of water, and a suspension stabilizer of 0.
2 parts by weight was added and suspension polymerization was carried out with stirring. When the conversion rate of the monomer to the polymer reached 72%, the polymerization was terminated and the residual monomer was removed by a steam distillation method. Then, a rubber-modified styrene / acrylonitrile copolymer resin composition was obtained through an extrusion process.

The analytical evaluation results are shown in Table 1.

[Effects of the Invention] The method of the present invention is significantly lower in cost than the emulsion polymerization method, and further has production advantages such as simple steps and easy wastewater treatment.

In addition, the rubber-modified styrene / acrylonitrile copolymer resin composition obtained by the continuous bulk or solution polymerization method of the present invention has extremely high gloss and impact resistance, which is not expected from the prior art. As a result, the industrial utility value is extremely high.

 ─────────────────────────────────────────────────── --Continued from the front page Examiner Yoshio Kakizaki (56) References JP 56-72010 (JP, A) JP 55-36201 (JP, A) JP 57-57058 (JP, B2)

Claims (3)

[Claims] 1. In the presence of 1 to 20 parts by weight of a rubber-like polymer having a solution viscosity of 20 to 50 centistokes, 65 to 92 parts by weight of a styrene-based monomer and an acrylonitrile-based monomer by a continuous bulk or solution polymerization method. 8 to 35 parts by weight of the copolymer, and 1) mixing the obtained rubber-modified styrene / acrylonitrile-based copolymer resin composition with a mixed solvent having a mixing ratio of toluene and methyl ethyl ketone of 7/3 Solution index is 6 ~
14 times, 2) the average particle size of the rubber-like polymer particles in the resin composition is 0.5 to 1.5 μ, and 3) the resin composition is mixed with methyl ethyl ketone and methanol in a ratio of 7/3. When dissolved in a solution, it is insoluble in the acrylonitrile-based monomer weight composition ( _WSA ) and styrene-based monomer weight composition ( _WSS ) of the polymer portion which is soluble in the mixed solution, and insoluble in the mixed solution. And the relationship between the acrylonitrile-based monomer weight composition (W _DA ) and the styrene-based monomer weight composition (W _DS ) of the polymer portion other than the rubber-like polymer is expressed by the following formulas (I) (I) 1.2 ≧ X _S / X _D ≧ 0.9 where A method for producing a rubber-modified styrene / acrylonitrile-based copolymer resin composition, which comprises controlling copolymerization so as to satisfy 2. The method for producing a rubber-modified styrene / acrylonitrile copolymer resin composition according to claim 1, wherein the copolymerization is controlled so as to satisfy the following formula (II). (II) 1.05 ≧ X _S / X _D ≧ 0.95 3. The method for producing a rubber-modified styrene / acrylonitrile copolymer resin composition according to claim 1, wherein a multistage stirred tank reactor is used in a continuous bulk or solution polymerization method.