RU2851287C2 - Method for producing impact-resistant polystyrene and impact-resistant polystyrene - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: present invention relates to a method for producing impact-resistant polystyrene (IRPS). A method for producing impact-resistant polystyrene (IRPS) is described, comprising the following steps: adding a polymerisation degree regulator and a low-molecular-weight vinyl aromatic polymer to a solution of a mixture of vinyl aromatic monomer, solvent, rubber and an auxiliary agent, and performing pre-polymerisation at a stirring speed of 20-90 rpm, after which final polymerisation and devolatilisation are performed to form IPP, in which the vinyl aromatic monomer preferably includes styrene or a mixture of styrene andα -methylstyrene; the solvent is preferably toluene, ethylbenzene, xylene or a mixture thereof; and the rubber is preferably butadiene-styrene rubber (BSR), polybutadiene rubber, polyisoprene rubber, natural rubber, or a mixture of two or more of the aforementioned rubbers, wherein the polymerisation degree regulator is used in an amount of 100-500 parts per million in the mixture solution, and the low molecular weight vinyl aromatic polymer has a mass fraction of 2-8% in the mixture solution.

EFFECT: reduction in the viscosity of the continuous phase, which in turn ensures the preservation of the morphological structure of the rubber phase, an increase in impact strength and a reduction in the effect of destruction on the morphological structure of the rubber phase by means of shear or other damage.

8 cl, 4 dwg, 1 tbl, 17 ex

Description

AREA OF TECHNOLOGY

The present invention relates to the field of polymer production and, more particularly, to a method for producing high-impact polystyrene (HIPS) and to HIPS obtained by this method.

LEVEL OF TECHNOLOGY

Polystyrene (PS) is currently one of the thermoplastic resins with the largest industrial production volumes. The molecules of PS and their aggregated structure make it a rigid and brittle material. Styrene-based copolymers, particularly PS resins modified by grafting to rubber, are being explored for use in applications requiring reduced brittleness. Rubber-modified PS is commonly referred to as HIPS, which expands the applications of PS resins.

The final physical properties of HIPS are influenced by the molecular weight of the continuous phase, the rubber particle size distribution, and the morphological structure of the rubber phase. The particle size distribution and morphological structure of rubber can be controlled by many different parameters, including shear stress, two-phase viscosity, and so on. The morphological structure of rubber typically exhibits a "cellular" morphology, which is typically blended with a certain amount of PS. Phase separation can be promoted at appropriate shear rates, although the morphological structure of rubber is easily disrupted by shearing action, leading to particle breakage. In industrial production, high impact strength is achieved by adjusting the stirring speed to ensure the rubber particle size distribution is within the optimal range. However, this optimal stirring speed is not easily achieved, as the morphological structure of the rubber is damaged to varying degrees when the stirring speed is adjusted. The particle size distribution and morphology of rubber are also affected by the viscosity of the dispersed phase relative to the continuous phase and the viscosity of the continuous phase. The viscosity of the rubber phase is determined by the level of the rubber and the viscosity of the rubber solution. The higher the viscosity of the continuous phase, the more easily the morphological structure of the rubber is destroyed.

It is generally desirable for the rubber phase in HIPS to form a suitable "cellular" morphological structure to achieve improved impact properties. Such a suitable "cellular" morphological structure can be achieved by raising the prepolymerization reaction temperature or increasing the amount of initiator to increase the PS content in the "cellular" structure. However, raising the temperature or adding too much initiator leads to an excessively high polymerization rate, which easily leads to implosion and makes production control difficult. In patent CN1860151B, the "cellular" morphological structure is improved by mixing two grafting and non-grafting initiators to increase impact strength. However, this method complicates production. In patent CN1795218A, controlled radical polymerization is achieved using a functionalized diolefin rubber to provide control over the particle size and morphology of the rubber. In this method, the raw material for functionalized diolefin rubber is difficult to obtain.

DISCLOSURE OF THE INVENTION

To produce HIPS, the present invention provides a method for producing HIPS. A suitable morphological structure of the rubber phase can be maintained by appropriately reducing the viscosity of the prepolymerized continuous phase. After appropriately reducing the viscosity of the continuous phase, the impact of shear or other damage on the morphological structure of the rubber phase is significantly reduced. Within a certain range of shear rates, the morphological structure of the produced HIPS can be maintained to improve impact strength.

In order to achieve the above objective, the present invention proposes the following technical solutions.

A method for producing HIPS is proposed, which includes the steps of adding a polymerization degree regulator and/or a low-molecular vinyl aromatic polymer to a solution of a mixture of a vinyl aromatic monomer, a solvent, rubber and an auxiliary agent and performing prepolymerization at a stirring speed of 20-90 rpm, after which final polymerization and devolatization are performed to form HIPS.

Preferably, the polymerization degree regulator is used in an amount of 100-500 parts per million in the mixture solution.

Preferably, the polymerization degree regulator is used in an amount of 150-300 parts per million in the mixture solution.

Preferably, the polymerization degree regulator is one of n-butyl mercaptan, isobutyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, t-butyl mercaptan, and α-methylstyrene dimer.

Preferably, the low molecular weight vinyl aromatic polymer has a mass fraction of 2-8% in the mixture solution.

Preferably, the low molecular weight vinyl aromatic polymer is low molecular weight PS.

Preferably, the low molecular weight PS has a weight average molecular weight (Mw) of 50,000-150,000.

Preferably, the manufacturing method also includes polymerization in the presence of an initiator.

Preferably, the prepolymerization is carried out at a temperature of 110-130°C and the final polymerization is carried out at a temperature of 130-170°C.

The present invention also provides a HIPS produced by the HIPS production method described above.

Compared with the prior art, the present invention has the following advantages.

According to the method for producing HIPS proposed by the present invention, a suitable morphological structure of the rubber phase can be maintained by appropriately reducing the viscosity of the prepolymerized continuous phase. After appropriately reducing the viscosity of the continuous phase, the impact of shear or other damage on the morphological structure of the rubber phase is significantly reduced. Within a certain range of shear rates, the morphological structure of the manufactured HIPS can be maintained to improve impact strength.

In the present invention, the viscosity of the continuous phase is reduced by the following technical means.

1) A certain amount of polymerization degree regulator is added to the solution of rubber and styrene for prepolymerization.

2) A certain amount of low molecular weight PS is added to the prepolymerization solution to regulate the viscosity of the continuous phase.

By means of the above mentioned method, it was unexpectedly discovered that the morphological structure of the produced UPPS is preserved and the impact toughness increases within a certain range of shear rates.

In prior art methods, a chain transfer agent (CTSA) is primarily used to regulate the molecular weight of the PS resin and is typically added to the reaction system after prepolymerization. Several methods also add mercaptan to the prepolymerization solution, which is not combined with other conditions, such as shear rate, to maintain the morphological structure of the rubber phase.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 - transmission electron microscope (TEM) photograph of the UPSP in Example 1;

FIG. 2 - TEM photograph of the UPSP in Example 2;

FIG. 3 - TEM photograph of the UPSP in Example 3; and

FIG. 4 - TEM photograph of the UPSP in Comparative Example 1.

DETAILED DESCRIPTION

The present invention provides a method for producing HIPS, comprising the following steps. A polymerization degree regulator and/or a low-molecular-weight vinyl aromatic polymer are added to a solution of a mixture of vinyl aromatic monomer, solvent, rubber, and auxiliary agent, and prepolymerization is performed at a stirring speed of 20-90 rpm, followed by final polymerization and devolatization to form HIPS.

The manufacturing method is preferably a continuous method or a batch method.

In the present invention, a suitable morphological structure of the rubber phase can be maintained by appropriately reducing the viscosity of the prepolymerized continuous phase. After appropriately reducing the viscosity of the continuous phase, the impact of shear or other damage on the morphological structure of the rubber phase is significantly reduced. Within a certain shear rate range, the morphological structure of the manufactured HIPS can be maintained to improve impact strength.

In the present invention, the viscosity of the continuous phase is reduced by the following technical means.

1) A certain amount of polymerization degree regulator is added to the solution of rubber and styrene for prepolymerization.

2) A certain amount of low molecular weight PS is added to the prepolymerization solution to regulate the viscosity of the continuous phase.

By means of the above mentioned methods, it was unexpectedly found that the morphological structure of the produced UPPS is preserved and the impact toughness is increased within a certain range of shear rates.

In prior art methods, a chain transfer agent (CSA) is primarily used to regulate the molecular weight of the PS resin and is typically added to the reaction system after prepolymerization. Several methods also add mercaptan to the prepolymerization solution, which is not combined with other conditions, such as shear rate, to maintain the morphological structure of the rubber phase.

In order to produce HIPS by implementing the method for producing HIPS proposed by the present invention, the rubber must be dissolved in a vinyl aromatic monomer and a solvent, i.e., a solution polymerization method is used.

The rubber may be polybutadiene rubber, styrene butadiene rubber, polyisoprene rubber, natural rubber, or a mixture of two or more of these.

The solvent used is preferably toluene, ethylbenzene, or xylene, preferably a mixture of these. The purpose of adding the solvent is to reduce the viscosity of the polymerization reaction system, enhance heat transfer, and promote the grafting reaction between the rubber and monomer.

The vinyl aromatic monomer preferably comprises styrene or a mixture of styrene and α-methylstyrene.

The excipient is preferably at least one of an antioxidant, a plasticizer (preferably white oil), a mold release agent, and a UV stabilizer. Any additives known to those skilled in the art that can be used in the manufacture of HIPS can be used in the present invention. Such additives are not limited to those mentioned above, which are merely preferred.

The morphology and particle distribution of the rubber phase of HIPS are influenced by the viscosity of the two phases. The viscosity of the rubber phase is determined by the level of the rubber and the viscosity of the rubber solution. Typically, the type of rubber selected for the rubber phase is fixed, and its viscosity is also fixed. The higher the viscosity of the continuous phase, the more easily the rubber's morphological structure is destroyed. In the present invention, by reducing the viscosity of the continuous phase, the morphological structure of the rubber can be maximally maintained, thereby increasing impact strength.

In the present invention, the viscosity of the continuous phase is reduced using two technical methods.

First, a certain amount of polymerization degree regulator is added.

Second, a certain amount of low molecular weight PS is added.

The molecular weight of the PS phase (continuous phase) decreases after the addition of a polymerization degree regulator to the prepolymerization reaction system, thereby reducing the viscosity of the PS phase. It is generally easy to understand that a decrease in molecular weight leads to a decrease in viscosity.

The polymerization degree regulator in the present invention is selected from n-butyl mercaptan, isobutyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, t-butyl mercaptan or α-methylstyrene dimer, preferably from n-dodecyl mercaptan, t-dodecyl mercaptan or α-methylstyrene dimer.

The greater the amount of chain transfer agent, the greater the reduction in the molecular weight of the continuous phase (PS phase). However, a significant reduction in the molecular weight of the PS phase will lead to a decrease in impact strength. Therefore, the present invention defines a range of chain transfer agent amounts; the chain transfer agent is used in an amount of 100-500 ppm in the prepolymerization mixture solution, preferably 150-300 ppm.

The preferred low-molecular-weight vinyl aromatic polymer is low-molecular-weight PS, and the addition of low-molecular-weight PS has a very effective effect on reducing the viscosity of the continuous phase. Low-molecular-weight PS has a mass fraction of 2-8% in the mixture solution; it can be manufactured or purchased, with a Mw range of 50,000-150,000.

Shear during stirring plays a crucial role in the properties of HIPS. Without stirring, the polymerization products form a highly branched gel; and at extremely high stirring speeds, the rubber's morphological structure is further damaged, leading to particle breakage and the formation of particles of excessively small size, thereby reducing the final impact strength. In the present invention, the stirring speed for shear is defined as 20-90 rpm, so that the morphology of the rubber phase can be better maintained in the presence of a polymerization degree regulator or low-molecular-weight PS.

The products can be polymerized by any method known to those skilled in the art for producing HIPS, including continuous and batch processes. In the present invention, prepolymerization is performed at a temperature of 110-130°C, and final polymerization is performed at a temperature of 130-170°C. The solvent and residual monomer are subsequently removed in a devolatizer, and the temperature and method for devolatization are well known to those skilled in the art and are not described in further detail here.

In the present invention, it is also possible to carry out the polymerization in the presence of an initiator. The initiator herein is not particularly limited and can be used alone or in combination. The initiator includes (but is not limited to): (1) azo compounds such as 2,2'-azodiisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile) and 2,2'-azobis-(2-dimethylvaleronitrile); (2) peroxydiacyl compounds such as dilauroyl peroxide, eicosanoyl peroxide and dibenzoyl peroxide; (3) peroxydiane compounds such as 2,5-dimethyl-2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, dicumyl peroxide and 1,3-bis(tert-butylperoxyisopropyl)benzene; (4) peroxide ester compounds such as tert-butyl peroxypivalate or 2,5-dimethyl-2,5-di(2-ethylhexanolperoxy)hexane; (5) carbonate peroxide compounds such as 2-ethylhexyl tert-amyl peroxocarbonate and 2-ethylhexyl tert-butyl peroxocarbonate; (6) dicarbonate peroxide compounds such as dimyristyl peroxydicarbonate and bis(4-tert-butylcyclohexyl) peroxydicarbonate; (7) peroxyketone acetal compounds such as 1,1-di-(tert-butylperoxy)-3,3,5-trimethylcyclohexane or 2,2-bis(4,4-di-tert-butyldioxycyclohexyl)propane; and (8) hydrogen peroxide compounds such as tert-butyl hydroperoxide or cumyl hydroperoxide. The amount of initiator should not be excessive, otherwise implosion can easily occur. Based on a total mass of the added monomer mixture of 100 parts by mass, the initiator amount used is preferably 0.01-0.03 parts by mass.

In the present invention, polymerization can be performed in the presence of an initiator. The initiator is preferably used in an amount of 50-300 ppm, more preferably 100-200 ppm.

Below, the technical solutions of the present invention are described clearly and in detail with reference to specific examples.

The starting materials used in the Examples and Comparative Examples are described below. These starting materials are merely illustrative selections, and those skilled in the art may also select other starting materials mentioned in the present invention or common excipients known to those skilled in the art that will not be described in detail. The starting materials are selected as follows.

Source materials:

The styrene has a purity of 99.8% and a moisture content below 500 ppm, produced by Jiangsu Xinyang Science and Technology Group, with the polymerization inhibitor removed before use.

Ethylbenzene with purity of 99.9% and moisture content below 500 ppm, produced by Jiangsu Xinyang Science and Technology Group.

Rubber:

255p, low cis content, has a 1.2 structure content of 8%, produced by Liaoning North-Dynasol Co., Ltd.

S1430: Solution polymerized styrene butadiene rubber (SSBR), has a total styrene content of 41% and a block styrene content of 30%.

N-dodecyl mercaptan is a pure analytical grade product manufactured by Sinopharm Chemical Reagent Co., Ltd.

α-Methylstyrene dimer is a pure analytical grade product manufactured by Sinopharm Chemical Reagent Co., Ltd.

Antioxidant 1076 is available from YiXing Tianshi Compound Chemical Co., Ltd.

Low molecular weight PS with Mm 50,000 and Mm 150,000 was manufactured independently.

In the following, the added styrene monomers, ethylbenzene, rubber 255P and antioxidant 1076 are indicated in parts by weight.

Example 1

100 parts of styrene monomer, 3 parts of ethylbenzene, 5 parts of 255P rubber, and 0.1 parts of 1076 antioxidant were added to a 5-L high-pressure reactor. After the rubber was completely dissolved under stirring at normal temperature, 100 ppm of n-dodecyl mercaptan was added to the mixed solution. After raising the temperature to 125°C, prepolymerization was performed at a stirring speed of 50 rpm until the solid content was 25%, and then the temperature was raised to 150°C for reaction until the solid content was 70%. The material was taken out and molded into pellets after removing residual solvents in a small devolatizer to obtain a PS resin. The TEM photograph is shown in FIG. 1.

Example 2

100 parts of styrene monomer, 6 parts of ethylbenzene, 7 parts of 255P rubber, and 0.1 parts of 1076 antioxidant were added to a 5-liter high-pressure reactor. After the rubber was completely dissolved under stirring at normal temperature, 500 ppm of n-dodecyl mercaptan was added to the mixed solution. After raising the temperature to 115°C, prepolymerization was performed at a stirring speed of 50 rpm until the solid content was 28%, and then the temperature was raised to 160°C for reaction until the solid content was 72%. The material was taken out and molded into pellets after removing residual solvents in a small devolatizer to obtain a PS resin. The TEM photograph is shown in FIG. 2.

Example 3

100 parts of styrene monomer, 2 parts of ethylbenzene, 6 parts of 255P rubber, and 0.1 parts of 1076 antioxidant were added into a 5L high-pressure reactor. After the rubber was completely dissolved by stirring at normal temperature, low molecular weight PS was added to the mixed solution at 2% by weight and with a Mw of 50,000. After raising the temperature to 12°C, prepolymerization was performed at a stirring speed of 50 rpm until the solid content was 29%, and then the temperature was raised to 155°C for reaction until the solid content was 73%. The material was taken out and molded into pellets after removing residual solvents in a small devolatizer to obtain a PS resin. The TEM photograph is shown in FIG. 3.

Example 4

100 parts of styrene monomer, 5 parts of toluene, 8 parts of 255P rubber and 0.1 part of 1076 antioxidant were added into a 5L high-pressure reactor. After the rubber was completely dissolved by stirring at normal temperature, low molecular weight PS was added into the mixed solution at 8% by weight and with a Mw of 150,000. After raising the temperature to 120°C, prepolymerization was performed at a stirring speed of 50 rpm until the solid content was 30%, and then the temperature was raised to 155°C for reaction until the solid content was 68%. The material was taken out and molded into pellets after removing residual solvents in a small devolatizer to obtain PS resin.

Example 5

The difference from Example 1 was that n-dodecyl mercaptan was added in an amount of 150 ppm.

Example 6

The difference from Example 1 was that n-dodecyl mercaptan was added in an amount of 200 ppm.

Example 7

The difference from Example 1 was that n-dodecyl mercaptan was added in an amount of 250 ppm.

Example 8

The difference from Example 1 was that n-dodecyl mercaptan was added in an amount of 300 ppm.

Example 9

The difference from Example 3 was that the low molecular weight PS had a mass fraction of 5%.

Example 10

In Example 1, n-dodecyl mercaptan was replaced by α-methylstyrene dimer, and other conditions were the same as in Example 1.

Example 11

The stirring speed during prepolymerization in Example 1 was changed to 90 rpm, and other conditions were the same as in Example 1.

Example 12

The rubber in Example 4 was replaced with S1430, and other conditions were the same as in Example 4.

Example 13

The difference from Example 1 was that n-dodecyl mercaptan and low molecular weight PS with Mm 100,000 were added to the mixed solution.

Example 14

The difference from Example 1 was that 100 ppm of initiator was also added when adding n-dodecyl mercaptan.

Example 15

The difference from Example 3 was that 150 ppm of initiator was also added when adding low molecular weight PS.

Example 16

The difference from Example 9 was that 200 ppm of initiator was also added when adding low molecular weight PS.

Example 17

The difference from Example 1 was that the stirring speed was 20 rpm.

Comparative example 1

The difference from Example 1 was that n-dodecyl mercaptan was not added in Comparative Example 1 to obtain the PS resin. The TEM photograph is shown in FIG. 4.

Comparative example 2

The low molecular weight PS in Example 4 was added after completion of the prepolymerization, and other conditions were the same as in Example 4.

Comparative example 3

N-dodecyl mercaptan in Example 2 was used in an increased amount of 700 ppm, and other conditions were the same as in Example 2.

Comparative example 4

The stirring speed during prepolymerization was changed to 110 rpm, and other conditions were the same as in Example 1.

Comparative example 5

The difference from Example 12 was that low molecular weight PS was not added in Comparative Example 5.

The impact strength and melt index values in Examples 1-17 and Comparative Examples 1-5 were tested according to the following standards.

The Izod notched impact strength value was tested according to ISO 180-A.

The melt index was measured according to ISO 1133 at a temperature of 200°C and with a load of 5 kg.

The test results for Examples 1-17 and Comparative Examples 1-5 are shown in Table 1 below.

Table 1. Test results

Example Melt index /(g/10 min) Impact strength/Izod (kJ/ ^m2 ) Example 1 2.8 13.0 Example 2 5.8 12 Example 3 4.0 14 Example 4 3.3 13 Example 5 3.2 13.5 Example 6 3.6 13.6 Example 7 4.1 13.2 Example 8 4.8 13.5 Example 9 3.8 13.2 Example 10 3.0 13.2 Example 11 3.1 12.8 Example 12 3.8 9 Example 13 4.0 12.8 Example 14 4.1 13.2 Example 15 3.9 14.2 Example 16 4.5 13.5 Example 17 3.5 13.2 Comparative example 1 2.6 11.5 Comparative example 2 3.0 10.5 Comparative example 3 7 9.5 Comparative example 4 3.0 10.5 Comparative example 5 3.5 4.5

As can be seen from the table above, by comparing Example 1 with Comparative Example 1, Example 4 with Comparative Example 2, and Example 4 with Comparative Example 5, the impact strength property of HIPS can be significantly improved by adding mercaptan or low-molecular PS during prepolymerization. As can be seen from Comparative Example 3 and Comparative Example 4, when the amount of mercaptan used or the stirring speed exceeds the range specified in the present invention, the impact strength of the product significantly decreases. The melt flow rate tested by the implementation method is 3 g/10 min or more, which meets the market requirement for the melt flow rate of the product. In FIG. 1-3, it can be seen that the HIPS produced in Examples 1-3 have relatively perfect cellular structures (and the drawings attached to the description of the present invention are given only for the purpose of example and illustration of the fact that relatively perfect cellular structures are obtained in the present invention), that is, the rubber phase contains more PS, and the morphological structure of the rubber is preserved. In contrast, in FIG. 4, it can be seen that the rubber phase in Comparative Example 1 does not have the perfect morphological structures shown in FIG. 1-3, and the morphology of the rubber phase is not preserved, therefore, the impact strength decreases.

Examples 5–8 exhibit the highest impact strength. Comparing Example 10 with Example 1, it can be seen that changing the types of chain transfer agents has little effect on impact strength. Comparing Example 12 with Comparative Example 5, the impact strength of Example 12 is significantly higher than that of Comparative Example 5, even though low-molecular-weight PS and low-impact S1430 rubber were added in Example 12 and low-molecular-weight PS was not added in Comparative Example 5.

Comparisons between other examples and Comparative Examples will not be described in detail.

In the present invention, for the sake of better understanding, only TEM photographs of some examples and comparative examples are given, and other examples and comparative examples are basically similar and will not be given separately.

The above description only describes a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Within the technical scope disclosed by the present invention, any equivalent substitutions or modifications made by those skilled in the art in accordance with the technical solution and idea for improving the present invention shall fall within the scope of protection of the present invention.

Claims (14)

1. A method for producing high-impact polystyrene (HIPS), comprising the following steps: adding a polymerization degree regulator and a low-molecular-weight vinyl aromatic polymer to a solution of a mixture of vinyl aromatic monomer, solvent, rubber and auxiliary agent and performing prepolymerization at a stirring speed of 20-90 rpm, after which final polymerization and devolatization are performed to form HIPS, in which the vinyl aromatic monomer preferably comprises styrene or a mixture of styrene and α-methylstyrene; the solvent is preferably toluene, ethylbenzene, xylene or a mixture thereof; and the rubber is preferably styrene butadiene rubber (SBR), polybutadiene rubber, polyisoprene rubber, natural rubber or a mixture of two or more of the mentioned rubbers, in this case, the polymerization degree regulator is used in an amount of 100-500 parts per million in the mixture solution, Low molecular weight vinyl aromatic polymer has a mass fraction of 2-8% in the mixture solution. 2. A method for producing UPSP according to paragraph 1, in which the polymerization degree regulator is used in an amount of 150-300 parts per million in the mixture solution. 3. A method for producing HIPS according to claim 1, wherein the polymerization degree regulator is one of n-butyl mercaptan, isobutyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, t-butyl mercaptan, and α-methylstyrene dimer. 4. A method for producing UPSP according to claim 1, wherein the low-molecular-weight vinyl aromatic polymer is low-molecular-weight polystyrene (PS). 5. A method for producing UPSP according to claim 4, wherein the low molecular weight PS has a weight average molecular weight (Mm) of 50,000-150,000. 6. A method for producing UPSP according to claim 1, in which polymerization is additionally carried out in the presence of an initiator. 7. A method for producing HIPS according to claim 1, in which prepolymerization is performed at a temperature of 110-130°C and final polymerization is performed at a temperature of 130-170°C. 8. UPSP obtained by the method for producing UPSP according to any one of paragraphs 1-7.