WO2002051894A1

WO2002051894A1 - Method of preparing the new grafted copolymer having high rubber contents and high performance

Info

Publication number: WO2002051894A1
Application number: PCT/KR2001/001663
Authority: WO
Inventors: Jae Bum Park; Doo Han Ha; Yong Hwan Lim; Kyung Su Kim
Original assignee: Cheil Industires Inc.
Priority date: 2000-12-27
Filing date: 2001-10-05
Publication date: 2002-07-04
Also published as: KR100453875B1; KR20020054103A

Abstract

The present invention provides a method of preparing a graft copolymer with a high rubber content showing good mechanical properties, which comprises (i) a first step of preparing a first graft copolymer latex by graft-polymerizing a monomer mixture of styrene and acrylonitrile onto a rubber polymer using an oil-soluble initiator; and (ii) a second step of preparing a second graft copolymer latex by reacting said first graft copolymer latex using a redox catalyst system and an initiator and adding continuously a monomer mixture of styrene and acrylonitrile, a molecular weight controlling agent and an initiator over 2-6 hours.

Description

METHOD OF PREPARING THE NEW GRAFTED COPOLYMER HAVING HIGH RUBBER CONTENTS AND HIGH PERFORMANCE

Field of the Invention

The present invention relates to a method of preparing a graft copolymer with high rubber content having high impact resistance, good fluidity and excellent natural color. The present invention relates to a method of preparing a graft copolymer with high rubber content having high impact resistance, good fluidity and excellent natural color by controlling the morphology of rubber particle of the graft copolymer and optimizing the graft ratio of the graft copolymer in the emulsion polymerization.

Background of the Invention

In general, acrylonitrile-butadiene-styrene copolymer (hereinafter "ABS resin") is prepared by continuous bulk polymerization or discontinuous emulsion polymerization and has good physical properties such as toughness, processability and surface gloss. Therefore, the ABS resin has been widely used in electric and electronic parts, office equipments, automobile parts and etc.

The bulk polymerization process for preparing ABS resin has several advantages such as mass production, low manufacturing cost and good natural color in comparison with the emulsion polymerization process. However, the bulk polymerization process has some disadvantages in that the variety of the product, which is important for commercial purpose, is limited to reduce the commercial value, and in that the article produced with the ABS resin reveals deterioration of the physical properties including impact resistance and surface gloss, which results in a limitation to the application field. So, the ABS resin prepared by the bulk polymerization process has been used for manufacturing articles that have a particular property or for blending with products by the emulsion polymerization process.

Most of the ABS resin is mainly prepared by emulsion polymerization process. The ABS resin has a good balance of physical properties and excellent surface gloss. Therefore, the ABS resin may be processed with styrene-acrilonitrile copolymer (hereinafter "SAN resin") to produce a highly valued product, which will carry the characteristics of the SAN resin.

However, the ABS resin prepared in accordance with the emulsion polymerization process does not have good impact resistance and fluidity. Further, the ABS resin is inferior in natural color to that of the bulk polymerization process, and the application of the resin is limited to certain fields. For this reason, it is very difficult to satisfy both impact resistance and fluidity simultaneously. Moreover, a great amount of a white pigment is needed in preparation of a colored resin, which will increase the production cost and make it difficult to manufacture a high quality article.

Accordingly, there have been proposed various methods in order to overcome the above-mentioned problems. First, in order to improve both impact resistance and fluidity, a rubber having a large particle size was used, a SAN resin was blended with a smaller amount of the graft copolymer, and the SAN resin having a lower molecular weight was used. Second, in order to improve the natural color of the resin, the amount of the additives such as emulsifier that affects the natural color was minimized, or a polymerization initiator was adopted to minimize the additives to be used. However, these methods have been considered to be disadvantageous, because, if the rubber having larger than 0.5 μm of average rubber particle diameter is used, undesirable coagulum is abundantly formed during polymerization reaction, and because, when the graft copolymer is blended with the SAN resin, the surface gloss of the resultant resin composition becomes deteriorated. Further, if a SAN resin with a lower molecular weight is used, the impact resistance becomes poor. In addition, the method of improving natural color of the resin either by minimization of the amount of the additives or by use of a polymerization initiator that can minimize other additives cannot overcome the problems sufficiently.

Accordingly, the present inventors have developed a method of preparing a graft copolymer having a high rubber content, which has high impact resistance, good fluidity and improved natural color, through the emulsion polymerization while using a rubber polymer with a commonly used particle size.

The gist of the present invention lies in (i) a first step of polymerizing monomers into a rubber polymer to prepare a first graft copolymer latex with a good impact resistance, and (ii) a second step of preparing a second graft copolymer latex having a core-shell structure in which the rubber particles are completely surrounded by the graft monomers, resulting in good fluidity and surface gloss. In the second step, in order to keep the good natural color during agglomeration and dehydration, a color stabilizer is introduced, and, in order to remove metal ions that might affect adversely the natural color, a chelating agent is used. Accordingly, the present invention is to provide a graft copolymer with a high rubber content, which has a good balance of mechanical properties as well as excellent impact strength, fluidity and natural color when the copolymer is blended with a thermoplastic resin.

Objects of the Invention

An object of this invention is to provide a graft copolymer having excellent fluidity and improved surface gloss, and a method for preparation thereof. Another object of the invention is to provide a graft copolymer having a high amount of rubber content, which shows good natural color even when the graft copolymer is blended with a thermoplastic resin.

Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims. Summary of the Invention

The method of preparing a graft copolymer with a high rubber content according to the present invention comprises (i) a first step of preparing a first graft copolymer latex by mixing a graft monomer mixture comprising styrene and acrylonitrile with an oil-soluble initiator under stirring, adding the mixture to a rubber polymer, further adding a chain transfer agent and deionized water, heating the mixture under stirring to a temperature of 55-65 °C , further stirring the mixture for less than 10 minutes so that the graft monomer mixture and initiator are swollen completely in the rubber particles, and adding an emulsifier followed by heating the temperature to initiate the polymerization reaction; and (ii) a second step of preparing a second graft copolymer latex by adding a redox catalyst and an initiator of polymerization to the first graft copolymer latex, stirring the resulting solution for about 5-10 minutes, adding to the resulting solution a mixture of a chain transfer agent and the remaining graft monomer mixture comprising styrene and acrylonitrile continuously over 2-5 hours together with a polymerization initiator. The detailed description of the present invention is as follow.

Detailed Description of the Invention

The graft copolymer according to the present invention is prepared through (i) a first step of polymerizing monomers into a rubber polymer to prepare a first graft copolymer latex with a good impact resistance, and (ii) a second step of preparing a second graft copolymer latex having a core-shell structure in which the rubber particles are completely surrounded by the graft monomers, resulting in good fluidity and surface gloss. In the second step, in order to keep the good natural color during agglomeration and dehydration, a color stabilizer is introduced, and, in order to remove metal ions that might affect adversely the natural color, a chelating agent is used. Accordingly, the present invention is to provide a graft copolymer with a high rubber content, which has a good balance of mechanical properties as well as excellent impact strength, fluidity and natural color when the copolymer is blended with a thermoplastic resin.

The rubber polymer of the present invention has an average particle diameter of about 0.28-0.33 an and a gel content of about 75-85 w t%. As a polymerization initiator, an oil-soluble initiator, a water-soluble initiator and a redox type initiator can be used respectively in the polymerization process. In case of only using an oil-soluble initiator, the resin composition is excellent in impact strength and natural color, but poor in fluidity and appearance of the articles. In case of only using a water-soluble initiator, the resin composition is excellent in fluidity and natural color, but poor in impact strength. In case of using a redox type initiator, impact strength and fluidity are excellent, but natural color is bad.

Thus, it is understood that the impact strength and fluidity are affected by the morphology of the graft copolymer and that the morphology of the graft copolymer is affected by the type of the polymerization initiator. Also, it is understood that the natural color of the resin is affected by initiator type, temperature of the agglomeration and dehydration process, and metal ions.

Accordingly, it is preferable to divide the reaction process into two steps in order to control the morphology of the rubber and to improve the natural color. In the first step, a preferable rubber structure is obtained by using an oil-soluble initiator so that the impact resistance is improved, and, in the second step, there is provided a preferable rubber structure by using a redox type initiator so that the fluidity is improved. In addition, by using a redox type initiator so as not to affect the natural color, even though the initiator remains in the copolymer after completion of polymerization, and by introducing a chelating agent to remove metal ions, the resin composition of the present invention is excellent in impact resistance, fluidity and natural color. Also, the resin composition may have a high content of rubber.

The graft copolymer according to the present invention is obtained by graft copolymerization of about 55-70 parts by weight of a rubber polymer (on the basis of the solid content) which has an average particle diameter of about 0.28-0.33 μm and a gel content of 75-85 % by weight with a graft monomer mixture of an aromatic vinyl component and an unsaturated nitrile component. The graft copolymer is prepared by a method with two steps. In the first step, a first graft copolymer latex is prepared by mixing a certain portion of the total graft monomer comprising styrene and acrylonitrile with an oil-soluble initiator under stirring, adding the mixture to a rubber polymer, so that the graft monomer and oil-soluble initiator are swollen completely into the rubber particles except the cross-linked part of the rubber polymer, and heating the mixture until the conversion rate of the copolymer reaches to a predetermined level. The first graft copolymer latex has a good impact resistance. In the second step, a second graft copolymer latex is prepared by adding a redox type catalyst and an initiator to the first graft copolymer latex, and adding continuously the remaining graft monomer and a chain transfer agent. The radicals of the oil-soluble initiator in the first step may make the graft monomers infiltrated in the rubber particles to participate in the reaction. As the result, the SAN resin can have a good impact resistance.

The present invention provides a method of preparing a graft copolymer that minimizes coagulum during polymerization by polymerizing the graft monomers uniformly on the surface of the rubber particles in the graft polymer. This polymerization reaction can be stable. And, the graft copolymer of the present invention can have good impact resistance, fluidity and glossy appearance by forming a core-shell structure that results in a preferable balance between the inner occlusion SAN in the rubber which affects the impact resistance and the outer grafted SAN on the surface of the rubber which affects the fluidity and surface gloss. Furthermore, in the present invention, a chelating agent is used along with a redox catalyst, it is also believed that the graft copolymer shows an improved color tone, impact resistance and fluidity when it is blended with a thermoplastic resin. The each step will be described in detail as below. Step 1 : Preparing a first graft copolymer

The graft copolymer of the present invention can be obtained by polymerizing about 45-30 parts by weight of a graft monomer mixture in the presence of about 55-70 parts by weight of a rubber polymer. In the present invention, it is preferable to divide the graft monomer mixture into two portions i.e. about 10-30 % by weight of the graft monomer mixture for the first step and about 90-70 % by weight of the graft monomer mixture for the second step. The graft monomer mixture of about 10-30 % by weight and an oil-soluble initiator are mixed under stirring and then a rubber polymer is added to the mixture. The rubber polymer used in the present invention has an average particle diameter of about 0.28-0.33 μm and a gel content of about 75-85 wt.%.

Thereafter, a chain transfer agent and deionized water are added to the mixture under stirring to raise the temperature. When the temperature reaches to about 55-65 °C , the mixture is stirred for less than 10 min so that the graft monomer and initiator are swollen inside the rubber particles. Next, an emulsifier is added to the mixture followed by raising the temperature to about 70-75 °C to initiate the polymerization reaction. When the temperature is reached to about 70-75 ^°C , the temperature is maintained at that level by using a cooler and the polymerization reaction is maintained for about 30-90 minutes until the conversion rate reaches to more than about 93 %.

Step 2 : Preparing a second graft copolymer

To the reactor, a redox type catalyst is added to the first graft copolymer latex prepared in the first step and then stirred for about 5- 10 minutes. After that, a mixture of a chain transfer agent and about 90-70 % by weight of the remaining graft monomer mixture is continuously added to the reactor over about 2-5 hours and at the same time a polymerization initiator is added to the reactor over about 2-5 hours to prepare a second graft copolymer latex. The reaction temperature of the graft copolymerization is preferably about 70-75 °C . After completion of addition of the graft monomers, the temperature is maintained at that level for about 20-60 minutes. When the final polymerization conversion rate is reached to about 93-98 %, the mixture is cooled to terminate the reaction to obtain a graft copolymer latex having about 38-44 parts by weight of solid content.

The preferred rubber polymer used in the present invention is polybutadiene copolymer containing more than about 80 % by weight of butadiene, and other conventional rubber polymers used in preparation of ABS resin can be employed. The average rubber particle diameter of rubber is preferably in the range of about 0.28-0.33 μm. If the average rubber particle diameter is smaller than 0.28 μm, the fluidity, surface gloss and natural color of the resin are improved, however, the impact resistance is deteriorated. On the other hand, if the average rubber particle diameter is larger than about 0.33 μm, the impact resistance of the resin is somewhat improved, but the polymerization system becomes unstable and a large amount of coagulum occurs, becoming the fluidity and glossy appearance deteriorated.

In the present invention, it is preferable that the amount of rubber polymer is about 55-70 parts by weight on the basis of solid content. If the amount of rubber polymer is less than about 55 parts by weight, the amount of graft monomers relatively increases, so that the polymerization reaction progresses excessively, resulting in deterioration of natural color as well as increase of coagulum. On the other hand, if the amount of rubber polymer excesses about 70 parts by weight, the amount of graft monomers relatively decreases, so that the graft ratio does not increase sufficiently and the graft reaction at the inside and outside of the rubber particles progress disuniformly, which makes the preparation of graft copolymer having a core-shell structure be impossible, thereby the glossy appearance and impact resistance of the final article being deteriorated.

The graft monomers used in the first step are swollen inside the rubber latex particles and the graft monomers used in the second step participate in the reaction at the surface of the rubber latex particles, so that the grafted SAN can have a uniform balance of the mechanical properties. The content of the graft monomer mixture is about 30-45 parts by weight, and it is preferable to divide the total monomer mixture into two portions i.e. about 10-30 % by weight for the first step and about 90-70 % by weight for the second step. If the proportion of the two portions deviates from the ratio above, the balance of the grafted monomers between at the inside of the rubber and at the surface of the rubber is not quite satisfactory. Especially, if the graft monomers for the first step exceed the ratio above, the graft reaction proceeds excessively. As the result, the rubber particles are excessively swollen, and the polymerization system is unstable. A graft copolymer with good mechanical properties cannot be obtained. Furthermore, since the graft monomers used in the second step are relatively small, it is difficult to obtain a graft copolymer having a core-shell type in which grafted SAN is uniformly polymerized on the surface of the rubber particles. On the other hand, if the graft monomers for the first step are lower than the ratio above, the impact strength of the resin composition is decreased, because the SAN is not sufficiently polymerized inside the rubber particles.

It is preferable that the graft monomer mixture contains about 70-80 % by weight of styrene and about 20-30 % by weight of acrylonitrile. If the monomer mixture deviates from the ratio above, the resin composition prepared thereform shows poor physical properties.

In Addition, the graft polymerization is carried out under a proper polymerization temperature of about 70-75 °C . If the polymerization temperature is lower than the range above, the polymerization reaction proceeds slowly, which results in decrease of productivity, and if the polymerization temperature is higher than the range above, the polymerization system becomes unstable and coagulum tends to be formed abundantly. In particular, if the polymerization temperature at the first step is lower than the range above, the decomposition rate of the oil-soluble polymerization initiator decreases, so that the polymerization rate also decreases and the rubber structure according to the present invention cannot be obtained. Further, the kinds of emulsifier, chain transfer agent, color stabilizer, chelating agent, polymerization initiator and the amounts thereof as well as the rates of addition thereof are also important factors. In particular, the kinds of the polymerization initiator, method of feeding thereof and the kinds of the redox type catalyst are very important to regulate the physical properties of the resin composition, because they may affect the reaction rate and stability of polymerization.

The polymerization initiator used in the present invention is the most important factor. If either an oil-soluble initiator or a water-soluble initiator is used alone in the two steps of the present invention, it is difficult to obtain the graft copolymer having a high rubber content, and good impact resistance, fluidity and excellent natural color.

The initiator that can be used in the first step is preferably an oil-soluble initiator selected from the group consisting of acetylchlorohexylsulfonyl peroxide, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis-(2-amidinopropane) dihydrochloride, lauroyl peroxide, 2,2'-azobisisobutyronitrile, benzoyl peroxide, dimethyl-2,2'-azobisisobutyronitrile and 4,4'-azobis-4-cyanovaleric acid. Among them, 2,2'-azobisisobutyronitrile which has a relatively low decomposition temperature is preferable, and the preferable amount thereof is about 0.05-0.20 % by weight. The preferable redox type catalyst used in the second step is a mixture of about 0.0005-0.006 % by weight of ferrous sulfate, about 0.01-0.15 % by weight of sodium formaldehyde sulfoxylate and about 0.05-0.15 % by weight of sodium ethylene diamine tetraacetate. And for the polymerization initiator of the second step, about 0.1-0.25 % by weight of cumen hydroperoxide is preferably used. If the polymerization initiator is inadequately used, the core-shell type graft copolymer cannot be obtained, and if the amount of the initiator is less than the range above, unreacted monomers become increased, resulting that ungrafted polymers are excessively formed. And, if the amount thereof is more than the range above, the reaction rate becomes increased, resulting that the reaction system is unstable and the amount of coagulum becomes increased. The emulsifier used in the present invention is not particularly limited, and can be suitably selected from the commercially available ones. Specific examples include a fatty acid metal salt such as sodium laurylate, sodium oleate, potassium oleate and potassium stearate; a sodium lauryl sulfate; and a potassium rosinate. Among them, the potassium stearate, the potassium rosinate or a mixture thereof are preferable. The amount of the emulsifier is preferably about 0.3-1.5 % by weight. If the amount of the emulsifier is less than the range above, undesirable coagulum is abundantly produced in the grafted latex. On the other hand, if the amount of the emulsifier exceeds the range above, natural color and glossy appearance of the final product are deteriorated due to gas formation during the injection molding process. The chain transfer agent used in the present invention is at least one selected from the group consisting of mercaptans having 8 to 18 carbon atoms, organic halogen compounds having 8 to 18 carbon atoms, α-methylstyrene dimer, terpinolene, or-terpenes and a mixture thereof. The amount of the chain transfer agent is about 0.005-0.15 % by weight in the first step and about 0.01-0.20 % by weight in the second step. If the amount of the chain transfer agent is less than the range above, the graft reaction proceeds excessively, resulting that the latex stability and the physical properties of the final product are deteriorated. And, if the amount of the chain transfer agent exceeds the range above, the polymerization rate tends to lower, resulting that the productivity is decreased. Preferably, the chain transfer agent used in the second step is mixed with graft monomers and added together continuously.

In order to evaluate the stability of the graft copolymer latex, the weight percent of coagulum is calculated according to the following equation after filtering the graft copolymer latex through 200 mesh and drying the coagulum.

weight of coagulum coagulum(%) = x 100 weight of (rubber + monomer mixture) In addition, the graft ratio is measured by coagulating the resultant graft copolymer latex with isopropyl alcohol, dehydrating and drying to obtain white powder, followed by dissolving the powder with acetone, solid-liquid separating by means of a centrifugal separator, drying the insoluble portion and measuring the weight of the insoluble portion. The graft ratio is represented by the following equation.

weight of (insoluble portion-rubber) graft ratio = — : — _* ^{x 10}° weight of rubber

According to the present invention, the graft ratio is preferably from about

40% to 60%. When the graft ratio is lower than this range, the impact strength tends to decrease and the surface gloss becomes inferior due to an undesirable phenomenon such as fish eye, pinehole and sand surface during the injection molding process. On the other hand, if the graft ratio exceeds this range, the impact strength, fluidity and natural color are deteriorated when the graft copolymer is blended with a matrix SAN resin, because the surface adhesion between the matrix SAN resin and the graft copolymer becomes poor.

The resulting copolymer latex is coagulated with a mixed aqueous solution comprising about 0.5-1.5 % of magnesium sulfate and phosphoric acid, and dried to yield a white powder. The copolymer is mixed and extruded with a separately prepared SAN resin to obtain a thermoplastic resin composition according to the present invention. The afore-mentioned SAN resin is used as a matrix resin and can be prepared by a conventional bulk or suspension polymerization process. In the present invention, it is preferable to use a SAN resin having about 125,000 of a weight average molecular weight and about 30 % of acrylonitrile content.

At the time of mixing the graft copolymer and the SAN resin, the content of the graft copolymer is about 20-40 % by weight and the content of the SAN resin is about 80-60 % by weight. To the mixture, a stabilizer and a lubricant are added to the extrusion and injection mold to produce test specimens.

The invention may be better understood by reference to the following examples that are intended for the purpose of illustration and are not to be construed as in any way limiting the scope of the present invention, which is defined in the claims appended hereto. In the following explanation, all parts are by weight unless otherwise noted.

Examples

Example 1

(1) First step : Preparation of the first graft copolymer latex

About 65.0 parts by weight of polybutadiene rubber latex having an average particle diameter of about 0.30 μm and gel content of about 82 % by weight and about 140 parts by weight of deionized water were charged into a 30 t reactor provided with a mechanical stirrer, heating and cooling equipments, a reflux condenser and a raw material supplying feeder. Next, about 1.75 parts by weight of acrylonitrile, about 5.25 parts by weight of styrene and about 0.03 wt % of 2,2'- azobisisobutylonitrile (AIBN) were mixed in an agitating vessel and then added to the reactor together with about 0.03 wt % of t-dodecyl mercaptan. The mixture was heated to the temperature of about 60 °C .

When the temperature of the mixture reached to about 60 °C , about 0.5 wt % of potassium stearate was further added, followed by agitation over 10 minutes. The mixture was heated gradually to the temperature of about 70 °C for about 60 minutes, and polymerized for about 20 minutes. When the conversion rate reached to more than about 93%, the polymerization was terminated to obtain the first graft latex.

(2) Second step : Preparation of the second graft copolymer latex About 0.001 wt % of ferrous sulfate, about 0.07 wt % of sodium formadehyde sulfoxylate, and about 0.05 wt % of sodium ethylenediamine tetraacetate was added to the reactor in the presence of the first graft latex and the mixed solution was stirred for 10 minutes. Next, about 21.0 parts by weight of styrene, about 7.0 parts by weight of acrylonitrile and about 0.10 wt % of t-dodecyl mercaptan were mixed in an agitating vessel and then added to the reactor together with about 0.15 wt % of cumene hydroperoxide continuously for about 3 hours at 70 °C to proceed the second graft polymerization. After completion of adding the monomer mixture, the reaction was continued for 40 minutes and cooled to about 60 °C . Thereafter, about 0.5 parts of antioxidant (product name : Wingstay L produced by Goodyear Company) was added to the reaction mixture to terminate the reaction.

After completion of the first and the second polymerizations, the conversion was about 97% and the solid content of the latex was about 40.2 %. The resulting graft latex was coagulated with an aqueous solution comprising about 1 % magnesium sulfate and about 0.5 % phosphoric acid and washed and dried to yield a white powder. The reaction condition and the results are shown in Table 1.

Examples 2-3

The polymerization reaction was carried out as same as in Example 1 except the reaction condition indicated in Table 1. The reaction conditions and the test results are shown in Table 1.

Comparative Example 1

About 65.0 parts by weight of polybutadiene rubber latex having an average particle diameter of about 0.30 μm and a gel content of about 82 wt % by weight which is the same as used in Example 1 and about 140 parts by weight of deionized water, about 0.6 wt % of potassium rosinate and about 0.15 wt % of sodium pyrophosphate were charged into the same reactor as mentioned in Example 1. Next, about 1.75 parts by weight of acrylonitrile, about 5.25 parts by weight of styrene, about 0.04 wt % of t-dodecylmercaptan and about 0.03 wt % of cumene hydroperoxide were further added under stirring and the mixture was heated to the temperature of about 60 °C . When the temperature of the mixture reached to about 60 ^°C , stirring was continued for 20 minutes followed by adding about 0.005 wt % of ferrous sulfate and about 0.22 wt % of dextrose to initiate polymerization for 1 hour. After completion of the first graft polymerization, about 21.0 parts by weight of styrene, about 7.0 part by weight of acrylonitrile and about 0.11 wt % of t- dodecylmercaptan were mixed for 10 minutes in an agitating vessel and then added to the reactor together with about 0.17 wt % of cumene hydroperoxide continuously for 3 hours at 70 °C to proceed the second graft polymerization. After completion of adding the monomer mixture and polymerization initiator, the reaction was continued for 40 minutes to finish the polymerization. The second graft reaction was initiated at about 70 ^°C , and gradually heated up to 75 about °C . After completion of the polymerization, the conversion rate was about 97% and the solid content of the latex was about 40.9 %. The reaction conditions and the test results are shown in Table 1.

Comparative Example 2

About 65.0 parts by weight of polybutadiene rubber latex having an average particle diameter of about 0.30 μm and a gel content of about 82 wt % by weight which is the same as used in Example 1 and about 140 parts by weight of deionized water, about 0.6 wt % of potassium rosinate, about 1.75 wt % of acrylonitrile, about 5.25 wt % of styrene, and about 0.02 wt % t-dodecylmercaptan were charged into the same reactor as mentioned in Example 1 and the mixture was heated to the temperature of about 60 ^°C . When the temperature of the mixture reached to about 60 °C , stirring was continued for 20 minutes followed by adding about 0.35 wt % of potassium persulfate to initiate polymerization. When the temperature of the mixture reached to about 70 °C , reaction was continued for 30 minutes to obtain the first graft latex. Thereafter, about 21.0 parts by weight of styrene, about 7.0 part by weight of acrylonitrile and about 0.10 wt % of t-dodecylmercaptan were mixed for 10 minutes in an agitating vessel and then added to the reactor continuously for 3 hours at 70 ^°C . After completion of adding the monomer mixture, reaction was continued for 40 minutes to finish the polymerization. After completion of the polymerization, the conversion rate was about 95 % and the solid content of the latex was about 40.1 %. The reaction conditions and the test results are shown in Table 1.

Comparative Example 3

The same procedure as in Comparative Example 2 was conducted except that 2,2'-azobisisobutylonitrile (AIBN) was used as a polymerization initiator.

Comparative Example 4

The same procedure as in Comparative Example 1 was conducted except that a rubbery polymer with different rubber particle diameters was used.

Table 1

Examples Comparative : Exampl es

1 2 3 1 2 3 4

Average : particle diameter

0.30 0.30 0.30 0.30 0.30 0.30 0.45 (μm)

SM(l^st step) 5.25 2.6 7.9 5.25 5.25 5.25 5.25

Monomer AN(l^st step) 1.75 0.9 2.6 1.75 1.75 1.75 1.75 content SM(2^nd step) 21.0 23.6 18.4 21.0 21.0 21.0 21.0

AN(2^nd step) 7.0 7.9 6.1 7.0 7.0 7.0 7.0

Chain 1^st step 0.03 0.02 0.04 0.04 0.02 0.02 0.04 transfer agent 2^nd step 0.10 0.11 0.09 0.11 0.10 0.10 0.11

Rubber content

(based on solid 65 65 65 65 65 65 65 content, %)

Graft ratio (%) 53.2 51.5 54.3 45.7 40.2 44.5 44.8

Coagulum (%) 0.05 0.03 0.04 0.05 0.15 0.16 0.25

25 parts by weight of each of the resulting graft copolymers prepared in Examples 1-3 and Comparative Examples 1-4 and 75 parts by weight of a SAN resin were mixed and about 0.4 wt % of magnesium stearate as a stabilizer and about 1.0 wt % of ethylene bis stearylamide were further added to the mixture. The mixture was extruded by means of twin screw extruder and injection molded to prepare a test specimen for testing mechanical properties such as impact resistance, surface gloss, yellowness index, whiteness index, fluidity and surface appearance. The measured mechanical properties are shown in Table 2. Table 2

Examples Comparative Examples

4

Graft copolymer (part) 25 25 25 25 25 25 25

SAN resin (part) 75 75 75 75 75 75 75

Notched IZOD impact strength

29.1 27.8 30.4 25.2 21.7 30.7 27.5

(1/4") ASTM D256

(kg- cm/cm, 23 °C) specular gloss

93.2 92.6 90.1 93.8 94.1 77.2 79.3 (ASTM D523) yellowness index 9.1 9.8 9.9 17.8 14.5 9.8 18.5 (ASTM D 1925) whiteness index

83.7 83.2 82.4 78.3 88.2 90.6 77.5 (ASTM D 1925)

Melt Flow Index (ASTM D 1238) 2.2 2.1 2.0 2.0 2.1 1.1 2.2 (g/10min 200°C, 5kg) surface appearance 1000x70x3 mm O o o o Δ X Δ (with naked eye)

Notes: O : good , Δ : normal, X: bad

Claims

What is claimed is:

1. A method for preparing a graft copolymer comprising:

(i) a first step of preparing a first graft copolymer latex by mixing a graft monomer mixture comprising styrene and acrylonitrile with an oil-soluble initiator under stirring, adding the mixture to a rubber polymer, further adding a chain transfer agent and deionized water, heating the mixture with stirring to the temperature of about 55-65 ^°C , further stirring the mixture for less than 10 minutes so that the graft monomer mixtures and initiator are swollen completely in the rubber particles, and adding an emulsifier followed by heating the temperature to initiate the polymerization reaction; and

(ii) a second step of preparing a second graft copolymer latex by adding a redox catalyst and an polymerization initiator to the first graft copolymer latex, stirring the resulting solution for about 5-10 minutes, adding to the resulting solution a mixture of a chain transfer agent and the remaining graft monomer mixture comprising styrene and acrylonitrile continuously over 2-5 hours together with a polymerization initiator.

2. The method for preparing a graft copolymer as defined in claim 1 , wherein said rubber polymer has an average particle diameter of about 0.28-0.33 μm and a gel content of about 75-85 wt %.

3. The method for preparing a graft copolymer as defined in claim 1, wherein the amount of the rubber polymer is about 55-70 parts by weight and that of the graft monomer mixture is about 45-30 parts by weight, wherein about 10-30 % by weight of the graft monomer mixture is used in the first step and about 90-70 % by weight in the second step.

4. The method for preparing a graft copolymer as defined in claim 1, wherein said first step is conducted at the temperature of about 10-30 % by weight 70-75 ^°C to initiate the polymerization reaction after adding an emulsifier, and said polymerization reaction is maintained at the temperature for 30-90 minutes until the conversion rate reaches to more than 93 %.

5. The method for preparing a graft copolymer as defined in claim 1, wherein said second step is conducted at the temperature of about 10-30 % by weight 70- 75 ^°C and further comprises, after completion of continuous addition of a mixture, maintaining the temperature at that level for 20-60 minutes until the conversion rate reached to about 93-98%, cooling the mixture to terminate the polymerization to obtain a graft copolymer latex having the solid content of about 38-44 %.

6. The method for preparing a graft copolymer as defined in claim 1, wherein said graft monomer mixture comprises about 70-80 % by weight of styrene and about 30-20 % by weight of acrylonitrile.

7. The method for preparing a graft copolymer as defined in claim 1 , wherein said oil-soluble initiator is selected from the group consisting of acetylchlorohexylsulfonyl peroxide, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'- azobis-(2-amidinopropane) dihydrochloride, lauroyl peroxide, 2,2'- azobisisobutyronitrile, benzoyl peroxide, dimethyl-2,2'-azobisisobutyronitrile and 4,4'-azobis-4-cyanovaleric acid; said redox catalyst is selected from the group consisting of ferrous sulfate, sodium formaldehyde sulfoxylate and sodium ethylene diamine tetraacetate; said polymerization initiator is cumen hydroperoxide; said emulsifier is selected from the group consisting of a fatty acid metal salt comprising sodium laurylate, sodium oleate, potassium oleate and potassium stearate, a sodium lauryl sulfate and a potassium rosinate; said chain transfer agent is selected from the group consisting of mercaptans having 8 to 18 carbon atoms, organic halogen compounds having 8 to 18 carbon atoms, α-methylstyrene dimer, terpinolene, a- terpenes and a mixture thereof.