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Title:
HIGHLY PURE POLYSTYRENE, METHOD FOR PREPARING THE SAME AND DISPOSABLE FOOD CONTAINER USING THE SAME
Document Type and Number:
WIPO Patent Application WO/2000/015678
Kind Code:
A1
Abstract:
The present invention relates to a process for preparing highly pure polystyrene for food container having a number-average molecular weight of from 10,000 to 1,000,000, and a polydispersity of from 1.1 to 20, styrene dimer content of less than 50ppm and styrene trimer content of less than 500ppm. The polystyrene is produced by an anionic polymerization, and the anionic polymerization is carried out by adding a styrene monomer, a solvent whose amount is more than 100 weight part to 100 weight part of the monomer and an initiator whose amount is from 0.01 to 1 mol part to 100 mol part of the monomer to initiate polymerization of the monomer into a batch-wise or a continuous-wise reactor, and reacting them at a reaction temperature of 10 $m(k) 160 °C and, wherein the initiator is added in two or more steps.

Inventors:
Kim, Kye-hyun (Daelim Doorae Apt, Shinseong-dong 152-1 Yuseong-ku Deajeon-city 305-345, 103-1008, KR)
Cho, Jae-cheol (Daelim Doorae Apt. 101-804, Shinseong-dong 152-1 Yuseong-ku Daejeon-city 305-345, KR)
Kim, Won-seop (190-125, Oryu-dong Chung-ku Daejeon-city 301-120, KR)
Application Number:
PCT/KR1999/000536
Publication Date:
March 23, 2000
Filing Date:
September 11, 1999
Export Citation:
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Assignee:
DAELIM INDUSTRIAL CO., LTD. (1-170, Sunhwa-dong Chung-ku Seoul 100-130, KR)
Kim, Kye-hyun (Daelim Doorae Apt, Shinseong-dong 152-1 Yuseong-ku Deajeon-city 305-345, 103-1008, KR)
Cho, Jae-cheol (Daelim Doorae Apt. 101-804, Shinseong-dong 152-1 Yuseong-ku Daejeon-city 305-345, KR)
Kim, Won-seop (190-125, Oryu-dong Chung-ku Daejeon-city 301-120, KR)
International Classes:
C08F2/06; C08F4/48; C08F12/08; C08F112/08; (IPC1-7): C08F112/08; A47G29/14; B65D85/72; C08F4/48
Attorney, Agent or Firm:
Kim, Won-ho (702 Teheran Boulevard 825-33, Yoksam-dong Kangnam-ku Seoul 135-080, Yoksam-dong Kangnam-ku Seoul 135-080, KR)
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Claims:
WHAT IS CLAIMED IS:
1. A process for preparing the highly pure polystyrene for food container by an anionic polymerization, wherein the polystyrene has a numberaverage molecular weight of from 10,000 to 1,000,000, and a polydispersity of from 1.1 to 20, styrene dimer content of less than 50ppm and styrene trimer content of less than 500ppm, characterized in that the anionic polymerization is carried out by adding a styrene monomer, a solvent whose amount is more than 100 weight part to 100 weight part of the monomer and an initiator whose amount is from 0.01 to 1 mol part to 100 mol part of the monomer to initiate polymerization of the monomer into a batchwise or a continuouswise reactor, and reacting them at a reaction temperature of 10 160°C, wherein the initiator is added in two or more steps.
2. The process of preparing the highly pure polystyrene according to claim 1, wherein the anionic polymerization process further comprises the step of adding from 0.01 to 5 weight part of a coupling agent to 100 weight part of the styrene monomer.
3. The process of preparing the highly pure polystyrene according to claim 2, wherein the coupling agent is selected from the group consisting of epoxidized soybean oil, epoxidized linseed oil, divinyl benzene, epoxidized liquid polybutadiene, silicon tetrahalide, silicon tetrachloride, and mixtures thereof.
4. The process of preparing the highly pure polystyrene according to claim 1, wherein the initiator is selected from the group consisting of n butyllithium, secbutyllithium, tertbutyllithium, methyllithium, ethyllithium, phenyllithium, and mixtures thereof.
5. The process for preparing the highly pure polystyrene according to claim 1, wherein the initiator is added in multiple steps of 2 to 10 steps for time duration of less than 30 minutes for each step.
6. A food container produced by the highly pure polystyrene of claim 1.
7. A process for preparing the highly pure polystyrene for food container by an anionic polymerization wherein the polystyrene has a numberaverage molecular weight of from 10,000 to 1,000,000, and a polydispersity of from 1.1 to 20, styrene dimer content of less than 50ppm and styrene trimer content of less than 500ppm, characterized in that the anionic polymerization is carried out by adding a styrene monomer, a solvent whose amount is more than 100 weight part to 100 weight part of the monomer, an initiator whose amount is from 0.01 to 1 mol part to 100 mol part of the monomer to initiate polymerization of the monomer, and a coupling agent whose amount is from 0.01 to 5 weight part to 100 weight part of the monomer into a batchwise or a continuouswise reactor, and reacting them at a reaction temperature of 10 160°C and, wherein the initiator is added in one step.
8. The process preparing the highly pure polystyrene according to claim 7, wherein the coupling agent is selected from the group consisting of epoxidized soybean oil, epoxidized linseed oil, divinyl benzene, epoxidized liquid polybutadiene, silicon tetrahalide, silicon tetrachloride, and a mixture thereof.
9. The process of preparing the highly pure polystyrene according to claim 7, wherein the initiator is selected from the group consisting of n butyllithium, secbutyllithium, tertbutyllithium, methyllithium, ethyllithium, phenyllithium, and mixtures thereof.
10. A food container produced by the highly pure polystyrene of claim 7.
Description:
HIGHLY PURE POLYSTYRENE, METHOD FOR PREPARING THE SAME AND DISPOSABLE FOOD CONTAINER USING THE SAME.

BACKGROUND OF THE INVENTION (a) Field of the Invention The present invention relates to a highly pure polystyrene, and more particularly to a highly pure polystyrene prepared by anionic polymerization and a disposable food container using the same. The polystyrene, from which styrene dimers and trimers do not substantially migrate, is particularly useful as food containers, such as disposable noodle cups, disposable trays etc.

(b) Description of the Related Art Generally, a polystyrene, used as polystyrene paper for noodle cups and a wide variety of disposable food containers such as cups, trays etc., is produced by radical polymerization of styrene monomers. During the radical polymerization, styrene dimers and trimers, which are reaction by-products of two or more monomers, and have been recently suspected as the chemicals being responsible for endocrine disruptions, are produced due to the numerous side reactions. When using the polystyrene as the material of the food container, the styrene dimers and trimers, which were incorporated in the polystyrene, tend to migrate from the polystyrene into the food. According to F. R. Mayo, (Journal of American Chemical Society, 90,1968, p1289), when styrene monomers are allowed to stand for a day in 20°C, 30 to 40 ppm of styrene dimers and trimers are produced. The amount of the styrene dimers and trimers generally increases during the radical polymerization process carried out at a temperature above 60°C.

These chemicals having adverse endocrine effects are more specifically known as endocrine disrupting chemicals that influence animal life and humans by disrupting hormonal activities and functions of endocrine glands, which result in decreased sperm production and anormal functioning of animal reproductive organs. The Environmental Protection Agency (EPA) in U. S., which has already

established DDT insecticide and dioxin released from incinerators of industrial waste treatment facilities as endocrine disrupting chemicals, also classified the styrene dimers and trimers as being possible agents of endocrine disrupting chemicals. In addition, Worldwide Fund for Nature (WWF) in Canada classified the styrene dimers and trimers as endocrine disrupting chemicals. From various tests on the anormal influence of the styrene dimers and trimers on animal life, there is a general consensus of their adverse endocrine effects.

Meanwhile, Japan Food and Drug Agency released a report on April 25, 1998, confirming the presence of styrene dimers and trimers in the polystyrene food containers such as noodle cups. The report showed that in 25 different types of disposable food containers using polystyrene papers, 9509, ug of styrene dimers and trimers are found in a gram of polystyrene papers. Few months later, Korea Food and Drug Agency confirmed the same finding.

The polystyrene can also be produced through an anionic polymerization of styrene monomer in an organic solvent, where an initiator is used to activate styrene monomers to initiate polymerization. The activated monomer continuously combines with other monomer to form a polymer until stopped by a terminator. This anionic polymerization process for producing polystyrene are disclosed in the U. S. Patent Nos. 5,391,655; 5,089,572; 4,883,846; 4,748,222; 4,205,016; 4,200,713; 4,016,348; and 3,954,894. More particularly, U. S.

Patent'846 discloses an anionic polymerization process for producing polystyrene having a narrow molecular-weight distribution, while U. S. Patents '348,'222, and'655 disclose apparatus for continuous anionic polystyrene polymerization process rather than a batch process, and U. S. Patents'016 and '572 disclose process for preparing initiators and their use in the polystyrene polymerization process. However, none of the above prior arts recognize their adverse endocrine effects when the polystyrene paper is used as disposable food containers.

SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide a

process for preparing a highly pure polystyrene that can be used as disposable food containers which are safe to human and substantially not releasing styrene dimers and trimers.

It is an another object of the present invention to provide polystyrene for a food container which is substantially free from releasing styrene dimers and trimers.

It is an another object of the present invention to provide a food container comprising polystyrene which is substantially free from releasing styrene dimers and trimers.

To accomplish these and other advantages, the present invention provides a process for preparing highly pure polystyrene for food container by an anionic polymerization wherein the polystyrene has a number-average molecular weight of from 10,000 to 1,000,000, and a polydispersity of from 1.1 to 20, styrene dimer content of less than 50ppm and styrene trimer content of less than 500ppm, characterized in that the anionic polymerization is carried out by adding a styrene monomer, a solvent whose amount is more than 100 weight part to 100 weight part of the monomer, and an initiator whose amount is from 0.01 to 1 mol part to 100 mol part of the monomer to initiate polymerization of the monomer into a batch-wise or a continuous-wise reactor, and reacting them at a reaction temperature of 10 ~ 160tC and, wherein the initiator is added in two or more steps.

Also, to accomplis these and other advantages, the present invention provides a process for preparing highly pure polystyrene for food container by an anionic polymerization wherein the polystyrene has a number-average molecular weight of from 10,000 to 1,000,000, and a polydispersity of from 1.1 to 20, styrene dimer content of less than 50ppm and styrene trimer content of less than 500ppm, characterized in that the anionic polymerization is carried out by adding a styrene monomer, a solvent whose amount is more than 100 weight part to 100 weight part of the monomer, an initiator whose amount is from 0.01 to 1 mol part to 100 mol part of the monomer to initiate polymerization of the monomer, and a coupling agent whose amount is from 0.01 to 5 weight part to 100 weight part of

the monomer into a batch-wise or a continuous-wise reactor, and reacting them at a reaction temperature of 10-160°C and, wherein the initiator is added in one step. And the present of this invention provides food containers produced by the highly pure polystyrene.

DETAILED DESCRIPTION OF THE INVENTION For a better understanding of the present invention, reference will now be made in detail to the following disclosures and appended claims.

Generally, a polymer produced by a radical polymerization contains a lot of low-molecular weight compounds due to various side reactions. Especially, it contains a lot of styrene dimers and styrene trimers classified as being possible agents of endocrine disrupting chemicals. Therefore, it is undesirable to use the polystyrene obtained by a radical polymerization as the food container.

The present invention minimizes the above-mentioned side reaction and reduces the production of low-molecular weight compounds such as styrene dimers and styrene trimers by a batch-wise or continuous-wise anionic polymerization process.

In the present invention, styrene monomer and polymerization initiator are reacted in a solvent at a temperature of from 10 to 160°C to provide the a highly pure polystyrene having a number-average molecular weight of from 10,000 to 1,000,000, and a polydispersity of from 1.1 to 20, styrene dimer content of less than 50ppm and styrene trimer content of less than 500ppm. The amount of the initiator to initiate polymerization is preferably from 0.01 to 1 mol part to 100 mol part of the styrene monomer, and the initiator is preferably added in two or more steps, and more preferably in multiple steps of 2 to 10 steps. The amount of the solvent used is preferably more than 100 weight part to 100 weight part of the styrene monomer, and more preferably from 150 to 600 weight part to 100 weight part of the styrene monomer. Also, while adding the polymerization initiator in two or more steps, a coupling agent is preferably added to produce the highly pure polystyrene.

Alternatively, the highly pure polystyrene is produced by reacting the

styrene monomer, the solvent whose the amount is more than 100 weight part, preferably from 150 to 600 weight part to 100 weight part of the styrene monomer, the initiator whose amount is from 0.01 to 1 mol part to 100 weight part of the styrene monomer. In this process, the initiator is added in one step and a coupling agent whose amount is from 0.01 to 5 weight part to 100 weight part of the styrene monomer is added to the reactants, and the reaction is carried out at a temperature of from 10 to 160 C.

In the present invention, it is preferable that the styrene monomer is purified just before use. the purification of the styrene monomer is carried out by distilling it at the temperature of 25°C under reduced pressure in the presence of reaction inhibitor such as p-tert-butylcatecol, hydroquinone, p-benzoquinone, trinitrobenzene, or passing it through an activated neutral alumina column.

In this case, the polymerization reaction is also preferably carried out at a temperature of 10 to 160°C in the absence of air and moisture.

If the polydispersity of the polystyrene is less than 1.1, there is a processing problem due to low melt flow rate, and if the polydispersity of the polystyrene is more than 20, the physical properties of the polystyrene are deteriorated. The preferable melt-index of the polystyrene of the present invention is from 0.5 to 50. If the melt-index of the polystyrene is less than 0.5, it is difficult to manufacture the final products with the polystyrene, and if the melt-index is more than 50, the physical properties of the polystyrene are deteriorated.

The amount of the initiator used in the present invention to initiate polymerization is from 0.01 to 1 mol part to 100 mol part of the styrene monomer.

The initiator can be added in one or more steps. In the present invention, the polydispersity of the produced polystyrene can be controlled by controlling the initiator adding steps and adding time. The preferable maximum adding time (duration) of the initiator is 30 minutes for each adding step. As the initiator of the present invention, organomonoalkali metal initiator is preferably used. The more preferable initiator includes n-butyllithium, sec-butyllithium, tert-butyllithium, methyllithium, ethyllithium, phenyllithium or mixtures thereof. In the case that

the added amount of the initiator is less than 0.01 mol part, there is a processing problem due to extremely high molecular weight of the polystyrene, and in the case that the amount of the initiator is more than 1 mol part, physical properties of the polystyrene are deteriorated.

The solvent used in the present invention does not react with other reactants. The preferable amount of solvent is more than 100 weight part to 100 weight part of the styrene monomer and the more preferable amount of the solvent is from 150 to 600 weight part to 100 weight part of the styrene monomer.

In the case that the amount of solvent is less than 100 weight part, the viscosity of the produced polystyrene increases, therefore the desired molecular weight of the polystyrene cannot be obtained. The preferable solvent includes non-polar or polar hydrocarbon compounds, such as cyclohexane, n-hexane, benzene, ethyl benzene, n-heptane, toluene, tetrahydrofuran, diethylether or mixtures thereof.

A coupling agent can be further added in the present invention. The roles of the coupling agent are to shorten reaction time and broaden the polydispersity of the produced polystyrene, and also some coupling agent is used as terminator. The amount of the coupling agent used in the present invention is from 0.01 to 5 weight part to 100 weight part of the styrene monomer.

The coupling agent used in the present invention includes epoxidized soybean oil, epoxidized linseed oil, divinyl benzene, epoxidized liquid polybutadiene, silicon tetrahalide, silicon tetrachloride or mixtures thereof. If the amount of the coupling agent is less than 0.01 weight part, the preferable coupling reaction is not obtained. If the amount of the coupling agent is more than 5 weight part, the excess coupling agent deteriorates the physical properties of the produced polystyrene.

Also, a reaction terminator can be further added in the present invention.

The reaction terminator used in the present invention includes methanol, ethanol, isopropanol, and water. To efficiently neutralize the initiator remained in the solution after polymerization, CO2 can be simultaneously added with water.

The amount of the C02 in mol is preferably equivalent or more than the amount

of the initiator in mol.

The polystyrene produced in the present invention is processed into a polystyrene paper preferably by an extrusion process, and the polystyrene paper is thermoformed to disposable food containers.

The present invention also provides disposable food containers, such as noodle cups, disposable cups, food trays using the polystyrene. The disposable food containers of the present invention do not or much less release styrene dimer and styrene trimer which are classified as being possible endocrine disrupting chemicals than conventional food containers.

In order to more fully illustrate the preferred embodiments of the present invention, the following detailed examples are given but the present invention is not restricted to the following examples.

Example 1 Polymerization was carried out in pure nitrogen atmosphere in a two- gallon stainless steel jacket reactor. To the reaction chamber of the reactor, were added 3.8 liter of cyclohexane, 10cc of tetrahydrofuran, and 0.05 mole of n-butyllithium.

Then, 17 mole of the styrene monomer, which was distille at 25 C under reduced pressure in the presence of 0.005% p-tert-butylcatecol just before use, was added to the mixture and allowed to react for 30 minutes at a reaction temperature of 35°C.

A prepolymer having a number-average degree of polymerization of 340 and a polydispersity of 1.02 was obtained.

The above prepolymer was coupled by adding 0.05 mole of epoxidized soybean oil and the reaction was terminated after reacting it for 35 minutes at a temperature of 75°C, and then the product was neutralized by adding water and CO2.

A final polymer having a polydispersity of 1.74 and a melt-index of 6 was obtained. (yield 95%)

Example 2 To a two-gallon reactor, were added 2.8 liter of cyclohexane, 1 liter of n- hexane, 12cc of tetrahydrofuran, and 0.03 mole of n-butyllithium.

Then, 12 mole of the styrene monomer, which was distilled at 25 °C under reduced pressure in the presence of 0.005 % p-tert-butylcatecol just before use, was added to the mixture and allowed to react for 30 minutes at a reaction temperature of 30°C.

A prepolymer having a number-average degree of polymerization of 400 and a polydispersity of 1.02 was obtained.

To the prepolymer solution, were added 0.05 mole of n-butyllithium and 15 mole of the styrene monomer.

The mixture was allowed to react for 20 minutes at a temperature of 50 °C.

A prepolymer having a polydispersity of 1.54 was obtained.

To the above prepolymer solution, was added 0.03 mole of divinylbenzene, and the mixture was allowed to react for 20 minutes at a temperature of 75°C, then the reaction was terminated by adding water and CO2.

A final polymer having a polydispersity of 1.94 and a melt-index of 8 was obtained (yield 96%).

Example 3 To a two-gallon reactor, were added 3.8 liter of cyclohexane, 5cc of tetrahydrofuran, and 0.01 mole of sec-butyllithium. Then, 9 mole of the styrene monomer, which was passed through an activated neutral alumina column just before use, was added to the mixture and allowed to react for 25 minutes at a reaction temperature of 15°C.

A prepolymer having a number-average degree of polymerization of 900 and a polydispersity of 1.04 was obtained.

0.01 mole of n-butyllithium and 12 mole of the styrene monomer were added to the prepolymer solution and reacted for 20 minutes at a temperature of 80°C, then the reaction was terminated by adding isopropanol.

A final polymer having a polydispersity of 1.87 and a melt-index of 4.3 was

obtained (yield 94%).

Example 4 3.8 liter of cyclohexane, 20cc of tetrahydrofuran, and 0.01 mole of n- butyllithium were added to a two-gallon reactor.

Then, 16 mole of the styrene monomer, which was distille at 25 C under reduced pressure in the presence of 0.005% p-tert-butylcatecol just before use, was added to the mixture and allowed to react for 20 minutes at a reaction temperature of 38°C.

A prepolymer having a number-average degree of polymerization of 1600 and a polydispersity of 1.02 was obtained.

The above prepolymer was coupled by adding 0.02 mole of epoxidized soybean oil slowly in a period of 3 minutes and the reaction was terminated by reacting it for 20 minutes at a temperature of 75°C, and then the product was neutralized by adding CO2 and water.

A final polymer having a polydispersity of 1.46 and a melt-index of 6 was obtained (yield 96%) Example 5 To a two-gallon reactor, were added 3.8 liter of cyclohexane, 20cc of tetrahydrofuran, and 14 mole of the styrene monomer, which was distille at 25°C under reduced pressure in the presence of 0.005% p-tert-butylcatecol just before use.

0.07 mole of n-butyllithium was slowly added to the mixture in a period of 5 minutes.

Then, the mixture was allowed to react for 5 minutes at a temperature of 48°C. A prepolymer having a number-average degree of polymerization of 200 and a polydispersity of 1.03 was obtained.

6 mole of the styrene monomer was added to the prepolymer solution, and 0.02 mole of n-butyllithium was slowly added in a period of 12 minutes.

The mixture was allowed to react for 10 minutes at a temperature of 80 °C.

The reaction was terminated by adding methanol.

A final polymer having a polydispersity of 1.74 and a melt-index of 10 was obtained (yield 95%).

Example 6 To a two-gallon reactor, were added 3.6 liter of cyclohexane, 15cc of tetrahydrofuran, and 17 mole of the styrene monomer, which was distille at 25°C under reduced pressure in the presence of 0.005% p-tert-butylcatecol just before use. Then 0.05 mole of n-butyllithium was slowly added to the mixture in a period of 5 minutes.

The mixture was allowed to react for 5 minutes at a reaction temperature of 48°C. A prepolymer having a number-average degree of polymerization of 340 and a polydispersity of 1.03 was obtained.

The above prepolymer was coupled by adding 0.02 mole of epoxidized linseed oil and the reaction was terminated by reacting it for 10 minutes at a temperature of 95°C, and then the product was neutralized by adding water and C02.

A final polymer having a polydispersity of 1.47 and a melt-index of 2.5 was obtained (yield 97%).

Example 7 To a two-gallon reactor, were added 3.8 liter of ethylbenzene, 12cc of tetrahydrofuran, 10 mole of the styrene monomer which was distille at 25 °C under reduced pressure in the presence of 0.005% p-tert-butylcatecol just before use, 3 mole of alpha-methylstyrene monomer and 0.05 mole of n-butyllithium.

The mixture was allowed to react for 5 minutes at a reaction temperature of 35°C. A prepolymer having a number-average degree of polymerization of 260 and a polydispersity of 1.03 was obtained.

7 mole of the styrene monomer was added to the prepolymer solution, then 0.015 mole of n-butyllithium were slowly added in a period of 12 minutes.

The mixture was reacted for 10 minutes at a temperature of 80 °C, then the

reaction was terminated and neutralized by adding methanol, CO2, and water.

A final polymer having a polydispersity of 1.32 and a met-index of 4.0 was obtained (yield 92%).

Comparative example 1 To a two-gallon reactor, were added 50 mole of purified styrene monomer and 0.002 mole of benzoyl peroxide as an initiator for heat polymerization.

The mixture was allowed to react for 20 minutes at a reaction temperature of70°C under nitrogen atmosphere.

A final polymer was obtained by removing residual monomers under vacuum (yield 30%).

Comparative example 2 To a two-gallon reactor, was added 48 mole of purified styrene monomer.

The monomer was allowed to react without using any heat polymerization initiators for 120 minutes at a temperature of 100°C under nitrogen atmosphere.

Residual monomers in the polymerization solution were removed by vacuum, followed by addition of methanol, and the remaining solution was filtered with a filter paper.

A final polymer was obtained by drying the filtrate in an oven under vacuum at 100°C.

Comparative example 3 To a two-gallon reactor, were added 3.8 liter of cyclohexane, 20cc of tetrahydrofuran, and 16 mole of the styrene monomer which was distille at 25 °C under reduced pressure in the presence of 0.005% p-tert-butylcatecol just before use. Then 0.008 mole of n-butyllithium was slowly added to the mixture in a period of 5 minutes.

The mixture was allowed to react for 20 minutes at a reaction temperature of 38°C, then the reaction was terminated and neutralized by adding isopropanol, CO2 and water.

A final polymer having a polydispersity of 1.02 and a melt-index of 0.2 was

obtained (yield 96%).

Example 8 100 g of the polystyrene obtained from Examples 1 to 7 and Comparative examples 1 to 2 was extruded by using a twin screw extruder at a speed ranging- from 50 to 200 rpm and at the temperature of from 70 to 200°C. When the temperature of extruded polystyrene increases to from 80 to 130°C, the polystyrene was kneaded by injecting butane gas as a forming agent through side conveying path installed at the two thirds of the twin screw extruder with a injection pressure of from 10 to 100kg/cm2, and then the melted polystyrene is extruded by using T-die to form a polystyrene paper.

The polystyrene paper was thermoformed to a food container at the temperature of from 100 to 170 °C.

The polystyrene of Comparative example 3 was processed to produce a formed sheet, but the preferable food container was not obtained due to its low melting index and low melting stress.

A. Analysis of styrene dimer and styrene trimer contents The polystyrene obtained from Examples 1 to 7 and Comparative examples 1 and 2 were made into a foamed sheet and manufactured into a food container. 0.5g of the food container was pulverized and added into 10moi solution of 1: 1 cyclohexane and 2-propanol mixture, and the solution was allowed to stand for a day. Then, 5ml of the solution was concentrated down to 0.2 ml under nitrogen atmosphere. 4.5m1 of acetonitrile was added into the solution and stirred for 10 minutes. Then acetonitrile was further added into the solution until total volume of the solution is 5ml. The solution was filtered by 5 ; 1m filter and the styrene dimer and the styrene trimer contents were measured by GC-Mass. The results obtained are shown in the following Table 1.

Table 1 Styrene Styrene Total (ppm) dimer (ppm) trimer (ppm) Example 101515 Example 202728 Example 3077 Example 4089 Example 5 e501313 Example 601616 Example 7099 Comparative1,3379,46510,802 Example 1 Comparative56310,23410,797 Example 2

As shown in above table 1, the amounts of the styrene dimers and the styrene trimers which are products of side reactions of a radical polymerization process are greatly reduced by preparing the polystyrene by the anionic polymerization according to the present invention.

B. Migration tests of styrene dimer and styrene trimer The polystyrene obtained from Examples 1 to 7 and Comparative Examples 1 to 2 were made into a foamed sheet and thermoformed into a food container. The final product was subjected to an migration test by utilizing water, 20% ethanol, 50% ethanol and n-heptane.

Except for a test utilizing n-heptane which was carried out for 60 minutes at the temperature of 25 C, all other migration tests were conducted for 30 minutes at the temperature of 60 °C.

The amounts of migrated styrene dimers and trimers were analyzed and the results obtained are shown in the following Table 2.

Table 2 Migrated amount(µg/cm²) Solvents Styrene dimer Styrene trimer Total Water ND ND ND 20% ethanol ND ND ND Example 1 50% ethanol ND ND ND n-heptane ND ND ND Water ND ND ND 20% ethanol ND ND ND Example 2 50% ethanol ND ND ND n-heptane ND ND ND WaterNDNDND 20% ethanol ND ND ND Example 3 50% ethanol ND ND ND n-heptane ND ND ND Water ND ND ND 20% ethanol ND ND ND Example 4 50% ethanol ND ND ND n-heptane ND ND ND Water ND ND ND 20% ethanol ND ND ND Example 5 50% ethanol ND ND ND n-heptane ND ND ND Water ND ND ND 20% ethanol ND ND ND Example 6 50% ethanol ND ND n-heptane ND ND ND Water ND ND ND 20% ethanoi ND ND ND Example 7 50 % ethanol ND ND ND n-heptane ND ND ND WaterNDNDND Comparative 20% ethanol 0. 09 0. 12 0. 21 Example 1 50% ethanol 0. 18 1. 45 1. 63 n-heptane 0. 83 54. 3 55.13 Water ND ND ND Comparative 20% ethanol 0.08 0. 14 0. 22 Example 2 50% ethanol 0. 97 52. 3 53.27 n-heptane 1. 42 102 103.42

In above table, ND represents"non-detected"and a migrated amount below 0.01//g/oif was designated to be"non-detected".

In this disciosure, there are shown and described only the preferred examples of the invention, but, as aforementioned, it is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concepts as expressed herein.