WO2002055572A1

WO2002055572A1 - Acrylic rubber latex having bimodal particle distribution

Info

Publication number: WO2002055572A1
Application number: PCT/KR2002/000044
Authority: WO
Inventors: Yong-Hun Lee; Chang-Sun Han; Dong-Jo Ryu
Original assignee: Lg Chem, Ltd.
Priority date: 2001-01-11
Filing date: 2002-01-10
Publication date: 2002-07-18
Also published as: EP1268589A1; US20030114597A1; DE60230089D1; US6864303B2; KR100417062B1; CN1211408C; EP1268589B1; KR20020060507A; CN1455788A

Abstract

This invention provides a method for preparing an acrylic rubber latex having a bimodal particle size distribution, comprising the steps of; a) preparing seed rubber latex containing i) alkyl acrylate, ii) saturated or unsaturated C1-C9 organic acid, and iii) an acrylic multi-unsaturated monomer; b) preparing a bimodal rubber latex by adding i) alkyl acrylate and iii) an acrylic multi-unsaturated monomer to the seed rubber latex obtained in step a): and c) preparing rubber latex by grafting iv) alkyl methacrylate to the bimodal rubber latex obtained in step b). According to this invention, an acrylic rubber latex can be provided that satisfies the important factors for the improvement of impact resistance, i.e., the particle size of the impact modifier dispersed in a matrix and the distance between impact modifiers, thereby allowing the impact modifier to improve the impact resistance and weatherability of vinyl chloride resin.

Description

Acrylic rubber latex having bimodal particle distribution

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a method for preparing an acrylic

rubber latex having a bimodal particle distribution, and more particularly, to a

method for preparing an acrylic impact modifier having two kinds of particle

distribution by means of multi-step emulsion polymerization.

(b) Description of the Related Art

Impact modifiers used for improving the impact resistance of vinyl

chloride resin include, for example, methyl methacrylate-butadiene-styrene

(MBS) based resin, chloropolyethylene (CPE) based resin, and acrylic based

resin. Among them acrylic based impact modifiers have good weatherability

and are widely used in products that are exposed to sunlight. Particularly,

for the applications in window frames where a high impact resistance and

weatherability are required, a so-called acrylic graft polymer of a core-shell

type is widely used in order to achieve weatherability, wherein the polymer is

formed by grafting an acrylic polymer, which has excellent compatibility with

vinyl chloride resin, to a rubbery core that is made by crosslinking an alkyl

acrylate monomer with a crosslinking agent.

The mechanical properties of an acrylic impact modifier having a

core-shell structure is greatly affected by the method of connecting rubbery core and shell through chemical bonding. One of the important factors on

which impact resistance and processability of resins depend is the degree of

bonding between the dispersed rubber particles and their matrix. The

diameter of rubber particles dispersed into the matrix as well as the distance

between the rubber particles acts as important factors which affect the processability and impact resistance.

Examples of conventional production methods of impact modifier for vinyl chloride resins include :

a multi-step emulsion polymerization process comprising a step of preparing seed rubber latex having a small particle size, followed by preparing a mutilayered rubbery polymer by adding a rubbery polymer-

forming monomer and a hard shell polymer-forming monomer sequentially and dividedly in multiple-steps; and a microagglomeration process comprising steps of preparing a primary core-shell rubber latex having a small size (smaller than 1000A),

making it grow to a desired size through an agglomeration process, and finally forming an encapsulated shell, and thereby impact modifiers having

core-shell structures are obtained.

Such latexes prepared by the above methods are acrylic-based

impact modifiers having a single particle size and a single type of particle

distribution, and are disclosed in U.S. Pat. No. 5,612,413 and 5,312,575.

When dispersed into a matrix polymer, polymers prepared by the multi-step emulsion polymerization process are dispersed with the state of particle size

of final latices, whereas impact modifiers prepared by the micro-

agglomeration process are re-dispersed with the state of primary particles,

that is, a state of initial particles.

Generally, impact resistance of vinyl chloride resins is known to be

satisfactory when the particle size of impact modifiers dispersed into a

matrix is about 2000A and the distances between impact modifier particles

are maintained to be under a specific value.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide the method of

preparing an impact modifier for vinyl chloride resins which satisfies essential

requirements for the improvement of an impact resistance thereof, that is, the

particle size of impact modifiers dispersed into the matrix and the distance

between the impact modifier particles. Briefly, the present invention is

provids the bimodal latex with excellent impact strength and mechanical

properties.

Another object of the present invention is to provide an acrylic rubber

latex which has excellent impact resistance and weatherability.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to achieve the object of the present invention, there is

provided a method for preparing an acrylic rubber latex having a bimodal

particle distribution, comprising the steps of: a) preparing seed rubber latex containing the following materials:

i) alkyl acrylate;

ii) saturated or unsaturated C C₉ organic acid; and

iii) an acrylic multi-unsaturated monomer, b) preparing a bimodal rubber latex by adding the following

materials to the seed rubber latex obtained in step a):

i) alkyl acrylate; iii) an acrylic multi-unsaturated monomer, and c) preparing rubber latex by grafting the following material to the

bimodal rubber latex obtained in step b): iv) alkyl methacrylate. The present invention will be explained more in detail in the following. The present invention provides a method for preparing an acrylic

rubber latex having a bimodal particle distribution by means of: a) a seed reaction for preparing rubber latex using a monomer with a

low glass transition temperature (Tg), a crosslinking agent, a polymerization initiator and deionized water; b) a core-forming reaction for preparing a

bimodal rubber latex consisting of small and large particles by adding a

monomer, an emulsifier, a crosslinking agent, an initiator and deionized

water to the seed rubber latex; and c) graft polymerization by adding mixed

monomers having good compatibility with polyvinyl chloride (PVC) to the

bimodal core latex. The acrylic rubber latex prepared by this method has improved the impact strength of vinyl chloride resins through satisfying

important factors for the improvement of impact resistance, that is, controlled

particle size of impact modifiers dispersed into the matrix and the distances

between the impact modifier particles. It has excellent impact resistance and

weatherability as expected in core-shell structures.

More specifically, it is preferable to prepare said acrylic rubber latex

by a method comprising the steps of:

a) preparing a seed rubber latex having a particle diameter in the

range of 500-2000A by adding monomers including:

i) 90 to 99.92 parts by weight of alkyl acrylate having a C₂-C₈

alkyl group;

ii) 0.03 to 5.0 parts by weight of a saturated or unsaturated

organic acid; and

iii) 0.05 to 5.0 parts by weight of an acrylic multi-unsaturated

monomer,

in such a manner that monomer, initiator, emulsifier, and deionized

water are charged as a whole; or a pre-emulsion is formed using a monomer,

initiator, emulsifier, and deionized water, and then the pre-emulsion is

charged as a whole;

b) preparing a bimodal rubber latex consisting of small particles and

large particles with diameters respectively in the range of 600-

2500A and 1500-3500A by adding 0.5 to 5.0 parts by weight of emulsifier to the seed rubber latex obtained in step a), and then

adding monomers including:

i) 95 to 99.95 parts by weight of alkyl acrylate having a C₂-C₈

alkyl group; and

iii) 0.05 to 5.0 parts by weight of an acrylic multi-unsaturated monomer,

in such a manner that monomer, initiator, emulsifier, and deionized water are charged as a whole; or a pre-emulsion is formed using monomer,

initiator, emulsifier, and deionized water, and then pre-emulsion is charged

as a whole; and c) preparing a bimodal rubber latex polymer having a core-shell structure and consisting of small particles and large particles with diameters in the range of 1000-3500A and 2000-400θΛ, respectively, by adding pre-emulsion including:

iv) 80 to 100 parts by weight of alkyl methacrylate; v) not more than 10 parts by weight of ethyl acrylate; and

vi) not more than 10 parts by weight of acrylonitrile, in such a manner that the pre-emulsion is divided into two or more

portions and charged sequentially, or the pre-emulsion is charged

continuously to the bimodal rubber latex obtained in step b), and by

performing graft polymerization.

In the present invention, a rubbery polymer consisting of seed and core is formed by emulsion polymerization, wherein the polymers consist

seed of 1 to 20 parts by weight and core-forming polymers 50 to 94 parts by

weight, based on the total weight of polymers.

The polymerization in step b) is accomplished by one or more steps. And if

two or more steps are used for the polymerization, it is preferable to conduct them in such a manner that in the first core-forming reaction, (1) a polymer- forming monomer, a polymerization initiator, an emulsifier, and deionized

water are charged as a whole; or (2) a pre-emulsion is formed using a polymer-forming monomer, a polymerization initiator, an emulsifier, and

deionized water, and then it is charged as a whole. And then, in both case, the reagents including monomers in each following step are introduced

continuously as a pre-emulsion, wherein the first core polymer comprises preferably at least 5 parts by weight of the total core-forming polymer.

In the present invention, the examples of alkyl acrylates of i) in both step a) and step b) are alky acrylates having C₂-C₈ alkyl groups such as ethyl

acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, 2-ethylhexyl

acrylate, octyl acrylate, or mixtures thereof, and preferably at least one compound is used that is selected from the group consisting of butyl acrylate,

2-ethylhexyl acrylate, and ethyl acrylate, which have relatively low glass

transition temperatures.

A rubbery polymer formed by using alkyl acrylate monomers of i) in

both step a) and b) has a glass transition temperature of -20°C or below, and for the purpose of better impact resistance and heat stability, the rubbery

polymer preferably comprises at least 80 parts by weight of units derived

from alkyl acrylate, while the remainder may comprise a monomer derived

from alkyl methacrylate or aromatic compounds. In order to achieve further

improvement in the impact resistance and heat stability, It is preferred that

the polymer comprises at least 95 parts by weight of alkyl acrylate and that the remainder includes a monomer derived from multi-unsaturated

monomers.

The C C₉ saturated or unsaturated organic acid (including aromatic

organic acids) of ii) of step a) is used in an amount of 0.03 to 5.0 parts by weight. In the case of using less than 0.03 parts by weight, particle size

control will be inefficient, whereas using more than 5.0 parts by weight will disturb the stability of reaction system. As an example, the organic acid is preferably at least one compound selected from the group consisting of

methacrylic acid, acrylic acid, itaconic acid, formic acid, acetic acid, crotonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, and cinnamic acid.

The acrylic multi-unsaturated monomer of iii) in both step a) and b) has two or more carbon-carbon double bonds, and it comprises monomers

derived from multi-unsaturated monomers which have acrylic functional

group or arylic structures without conjugated double bonds. Examples

thereof may be at least one compound selected from the group consisting of

1 ,3-butanediol acrylate, 1,3-butanediol dimethacrylate, 1 ,4-butanediol diacrylate, 1 ,4-butanediol dimethacrylate, aryl acrylate, aryl methacrylate,

tetraethylene glycol diacrylate, and tetraethylene glycol dimethacrylate.

The amount of the acrylic multi-unsaturated monomer of iii) in both

step a) and step b) is used in the range of 0.05 to 5.0 parts by weight, and if more than 5.0 parts are used, the glass transition temperature becomes high

which will result in the reduction of impact resistance; and if less than 0.05

parts are used, the degree of crosslinking is lowered which will reduce processability, and thus it can give rise to problems in processing.

The shell-forming reaction of step c) is carried out with hard polymer-

forming monomers having a glass transition temperature of at least 60°C,

and monomers containing about 80 to 100 parts by weight of units derived from alkyl methacrylate. In addition, the total polymer of step c) comprise 5 to 30 parts by weight of shell polymer.

The alkyl methacrylate of iv) in step c) may be used alone, or as a mixture with at least one monomer selected from the group consisting of

alkyl acrylate such as ethyl methacrylate and butyl acrylate, methacrylate,

styrene, methacrylic acid, and acrylonitrile. The alkyl methacrylate is preferable to use methyl methacrylate.

In the method according to the present invention, if necessary, an

emulsifier, a crosslinking agent, a polymerization initiator, and deionized

water may be added.

More specifically, the emulsifiers are preferably alkaline metal salts of C₁₄-C₂₄ saturated or unsaturated fatty acid. Furthermore, the amount of

the emulsifier used is 0.05 to 0.5 parts by weight in the seed reaction of step

a), 0.5 to 5.0 parts by weight before the core-forming reaction and 0.05 to 1.0

parts by weight in the core-forming reaction of step b), and 0.05 to 0.5 parts

by weight in the shell reaction of step c). When a smaller amount is used

than is described in each of the above steps, an excessive amount of solid

substance is formed due to reduced stability in polymerization, which results

in a reduction of productivity.

The polymerization initiator may be selected from the group

consisting of potassium persulfate, ammonium persulfate, benzoyl peroxide,

azobisbutyronitrile, butyl hydroperoxide and cumyl hydroperoxide, and it is

preferable to use potassium persulfate or butyl hydroperoxide.

The present invention will be explained more in detail with reference

to the following Examples and Comparative Examples. These examples,

however, are intended only for illustrative purpose and not for limitation of the

present invention.

EXAMPLES

Example 1

(Preparation of seed)

Experiment is carried out in a 2-liter reactor equipped with a temperature

recorder and mechanical stirrer. The reactor is surrounding by a jacket

through which passes a heat-transfer fluid for maintaining the temperature of the system.

415.8 parts by weight of deionized water were introduced, while

degassing with nitrogen, into a reactor described above and the temperature

was raised to 78°C. When the temperature of the deionized water reached

78°C, 43.56 parts of butyl acrylate, 0.72 parts of 1 ,3-butanediol diacrylate,

0.72 parts of methacrylic acid, and 24.21 parts of potassium salt of fatty acid (diluted solution to 8%) were added. Then 12.6 parts of potassium

persulfate (diluted solution to 3%) were added while maintaining the temperature of the system at 78°C, and then rubber latex was obtained.

The degassing with nitrogen was continued until the reaction was completed.

(Preparation of the first rubbery core)

276.45 parts of rubber latex obtained by the seed reaction were charged into another reactor and 9.375 parts of potassium salt of fatty acid (diluted

solution to 8%) were added to it, then the temperature was raised to 78°C

while degassing with nitrogen. 180.3 parts of deionized water, 288.9 parts of butyl acrylate, 0.35 parts of 1 ,3-butanediol diacrylate, 0.75 parts of aryl

methacrylate, 34.675 parts of potassium salt of fatty acid (diluted solution to

8%), and 8.33 parts of potassium persulfate (diluted solution to 3%) were

mixed and stirred to make pre-emulsion, and the pre-emulsion was charged

into a reactor as a whole. The temperature at the time of charging was

78°C. The degassing with nitrogen was continued until the reaction was

completed. (Preparation of the second rubbery core)

59.2 parts of deionized water, 109.2 parts of butyl acrylate, 0.15

parts of 1 ,3-butanediol diacrylate, 0.65 parts of aryl methacrylate, 15.625 parts of potassium salt of fatty acid (diluted solution to 8%), and 6.67 parts of

potassium persulfate (diluted solution to 3%) were mixed and stirred to make

pre-emulsion, and then they were charged into the reactor in which the first core-forming reaction was finished, by means of a pump, over an 1 hour

period. During this process, an internal nitrogen sweep of the reactor was continued and the temperature of the system was kept constant at 78°C.

After charging of the pre-emulsified mixture, the reaction mixture was allowed to stand for 1 hour for aging.

(Graft reaction)

97.4 parts of deionized water, 71.25 parts of methyl methacrylate, 3.75 parts of ethyl acrylate, 2.8 parts of acrylonitrile, 9.375 parts of potassium salt of fatty acid (diluted solution to 8%), and 6.33 parts of potassium

persulfate (diluted solution to 3%) were mixed and stirred to make pre-

emulsion, and then they were charged into the reactor gradually over an 1.5 hour period. A degassing with nitrogen was carried out in the same way as

in the core reaction, and the temperature of the system was kept constant at

78°C. After the charging was finished, the reactants were aged for 1 hour

to produce bimodal latices. Example 2

The same process as in Example 1 was carried out except that 0.54

parts of methacrylic acid were used in the seed reaction.

Example 3 The same process as in Example 1 was carried out except that 0.36

parts of methacrylic acid were used in the seed reaction.

Example 4

The same process as in Example 1 was carried out except that 0.18 parts of methacrylic acid were used in the seed reaction.

Example 5

The same process as in Example 1 was carried out except that 0.2 parts of acrylic acid were used in the seed reaction.

Example 6

(Preparation of seed)

The same process as in Example 1 was carried out.

(Preparation of rubbery core)

276.45 parts of rubber latex prepared by the seed reaction in

Example 1 were charged into a reactor and 12.0 parts of potassium salt of

fatty acid (diluted solution to 8%) were added, then a degassing with nitrogen

was carried out. 600 parts of deionized water, 398.1 parts of butyl acrylate,

0.50 parts of 1 ,3-butanediol diacrylate, 0.75 parts of aryl methacrylate, 50.3

parts of potassium salt of fatty acid (diluted solution to 8%), and 8.33 parts of potassium persulfate (diluted solution to 3%) were mixed and stirred at room

temperature to make pre-emulsion, and then they were charged into a

reactor at one time. The temperature at the time of charging was 65°C.

The degassing with nitrogen was continued until the reaction was completed.

(Graft reaction)

The same process as in Example 1 was carried out.

Comparative Example 1

(Preparation of seed)

336.16 parts by weight of deionized water were charged into a

reactor and the temperature was raised to 75°C while performing a

degassing with nitrogen. When the temperature of the deionized water reached 75°C, 39.36 parts of butyl acrylate, 0.64 parts of 1 ,3-butanediol

diacrylate, and 25.0 parts of potassium salt of fatty acid (diluted solution to

8%) were added. Then 11.2 parts of potassium persulfate (diluent solution to 3%) were added while maintaining the temperature at 75°C, and thus

rubber latex was obtained.

(Preparation of rubbery core)

383.12 parts of deionized water, 596.96 parts of butyl acrylate, 0.8

parts of 1 ,3-butanediol diacrylate, 2.24 parts of aryl methacrylate, 80.0 parts

of potassium salt of fatty acid (diluted solution to 8%), and 24.0 parts of

potassium persulfate (diluted solution to 3%) were mixed and stirred to make

pre-emulsion, and then the pre-emulsion was charged slowly into a reactor by means of a pump, over a 3 hour period. The reaction mixture was allowed

to age for an hour in order to remove unreacted monomers. During the

process, the temperature of the reaction mixture was kept constant at 75°C.

(Graft reaction)

The same process as in Example 1 was carried out.

Comparative Example 2

The same process as in Example 1 was carried out except that 50 parts of potassium salt of fatty acid (diluted solution to 8%) were used in the seed reaction.

Comparative Example 3

The same process as in Example 1 was carried out except that methacrylic acid was not used in the seed reaction. Comparative Example 4

The same process as in Example 6 was carried out except that methacrylic acid was not used in the seed reaction.

Small amounts of latex prepared in each of the steps of Examples 1

to 6 and Comparative Examples 1 to 4 were taken for particle size measurements. And after filtering off a small amount of solid substances

from the latex sample using a 325-mesh filter, the particle size was

measured by means of CHDF (capillary hydrodynamic fractionation) and the results are shown in Table 1. Table 1

From Table 1 , it can be seen that acrylic impact modifiers prepared by Example 1 to 6 according to the present invention have a bimodal particle size distribution, and that the reaction products of Comparative Examples 1 to 4 exhibit a single particle size distribution. Especially, in the case of

Comparative Example 3, the reaction product is not a bimodal-type latex

even though the same process as in Example 1 was used, except that

methacrylic acid was not used.

The impact modifier resins from the latex obtained in Examples 1 to

6 and Comparative Examples 1 to 4 were isolated through a procedure as

follows. Deionized water was added to the polymerized latex to lower its

solid content to 10%, and a calcium chloride solution (diluted solution to 10%) was added to the diluted latex while stirring for coagulation. The

temperature of the coagulated mixture was raised to 90°C, and kept for 10

minutes for aging and then, the mixture was cooled. The mixture was

washed with deionized water two to three times to remove residual emulsifier,

and then dehydrated. The dehydrated impact modifier was dried at 85°C for

2 hours using a fluidized bed dryer, to give impact modifier powder.

The impact strength and mechanical properties of the impact modifier obtained above were evaluated by the following steps. 100 parts

of a vinyl chloride resin (degree of polymerization: 1000), 4.0 parts of a stabilizer (lead series), 0.9 parts of an internal lubricant, 0.36 parts of an external lubricant, 1.5 parts of processing aid, and 6.0 parts of a filler

(CaCO₃) were charged into a henshel mixer at room temperature, then the temperature was raised to 115°C by increasing the rotation to lOOOrpm,

followed by cooling of the mixture to 40°C, and thereby a master batch was

prepared. 7 parts of each impact modifier used was added to each master

batch, and it was processed by a two-roll mill at 190°C for 7 minutes to make

a sheet of 0.6mm thickness, and then the sheet was cut to a size of 150mm

x 200mm. After the cut pieces were aligned to be in the same processing

direction, they were laminated in a mold having dimensions of 3mm x

170mm x 200mm. The laminated pieces were treated by preheating (0.5kg)

for 8 minutes and compressing (10kg) for 4 minutes using a heat-press, and

thereby a pressed test piece of 3mm thickness was prepared. The pressed test piece was precisely cut according to ASTM D-256 to make a test piece

for an impact test. The measurement of Izod impact strength was

performed at room temperature (25°C). The above pressed test piece was

fabricated to a test piece for mechanical properties according to ASTM D-

638, and it was subjected to tests of tensile strength and elongation, the results of which are shown in Table 2.

Table 2

Examples 1 to 6 in which the polymer had a bimodal particle distribution exhibit superior impact strength, tensile strength, and elongation

to the Comparative Examples 1 to 4 in which the polymer had single particle size distribution.

In the present invention, an acrylic rubber latex was provided that

satisfies the important factors for the improvement of impact resistance, i.e.,

the particle size of the impact modifier dispersed in a matrix and the distance

between impact modifiers, thereby improves the impact resistance and

weatherability of vinyl chloride resin.

Claims

WHAT IS CLAIMED IS:

1. A method for preparing an acrylic rubber latex having a bimodal

particle distribution, comprising the steps of:

a) preparing seed rubber latex containing the following materials:

i) alkyl acrylate;

ii) saturated or unsaturated C C₉ organic acid; and

iii) an acrylic multi-unsaturated monomer,

b) preparing a bimodal rubber latex by .adding the following materials

to the seed rubber latex obtained in step a):

i) alkyl acrylate;

iii) an acrylic multi-unsaturated monomer, and

c) preparing rubber latex by grafting the following material to the

bimodal rubber latex obtained in step b):

iv) alkyl methacrylate.

2. The method of claim 1 , comprising the steps of:

a) preparing a seed rubber latex having a particle diameter in the

range of 500-2000A by adding monomers including:

i) 90 to 99.92 parts by weight of alkyl acrylate;

ii) 0.03 to 5.0 parts by weight of saturated or unsaturated C C₉

organic acid; and

iii) 0.05 to 5.0 parts by weight of an acrylic multi-unsaturated

monomer, in such a manner that monomer, initiator, emulsifier, and deionized

water are charged as a whole; or a pre-emulsion is formed using a monomer,

initiator, emulsifier, and deionized water, and then the pre-emulsion is

charged as a whole, and by performing a seed reaction;

b) preparing a bimodal rubber latex consisting of small particles and

large particles whose diameters are respectively in the range of 600-2500A

and 1500-3500A by adding monomers to the seed rubber latex obtained in

step a) including:

i) 95 to 99.95 parts by weight of alkyl acrylate; and

iii) 0.05 to 5.0 parts by weight of an acrylic multi-unsaturated

monomer,

in such a manner that monomer, initiator, emulsifier, and deionized

water are charged as a whole; or a pre-emulsion is formed using monomer,

initiator, emulsifier, and deionized water, and then pre-emulsion is charged

as a whole, and by performing a core-forming reaction; and

c) preparing a bimodal rubber latex polymer having a core-shell

structure and consisting of small particles and large particles whose

diameters are respectively in the range of 1000-350θΛ and 2000-4000A,

respectively, by pre-emulsifying monomers including:

iv) 80 to 100 parts by weight of alkyl methacrylate;

v) not more than 10 parts by weight of ethyl acrylate; and

portions and charged sequentially, or the pre-emulsion is charged

continuously to the bimodal rubber latex obtained in step b), and by performing graft polymerization.

3. The method of claim 1 , wherein the alkyl acrylates of i) in both

steps a) and b) comprise at least one compound selected from the group consisting of C₂-C₈ alkyl acrylates including ethyl acrylate, propyl acrylate,

isopropyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and octyl acrylate, or

mixtures thereof.

4. The method of claim 1 , wherein the C C₉ saturated or unsaturated

organic acid (including aromatic organic acids) of ii) in step a) comprises at least one compound selected from the group consisting of methacrylic acid, acrylic acid, itaconic acid, formic acid, acetic acid, crotonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, and cinnamic acid.

5. The method of claim 1 , wherein the acrylic multi-unsaturated

monomers of in iii) in both steps a) and b) comprise at least one compound

selected from the group consisting of 1 ,3-butanediol acrylate, 1 ,3-butanediol dimethacrylate, 1 ,4-butanediol diacrylate, 1 ,4-butanediol dimethacrylate, aryl

acrylate, aryl methacrylate, tetraethylene glycol diacrylate, and tetraethylene

glycol dimethacrylate.

6. The method of claim 1 , the core-forming reaction of step b)

comprises multi-step polymerization which is carried out via at least one step.

7. The method of claim 1 , wherein in the case of one-step

polymerization, the polymer-forming monomers are added as a whole; and in

the case of two-step or multi-step polymerizations, the polymer-forming

monomers are added as a whole in the first core-forming step and then the

rest of polymer-forming monomers is added continuously in the following

steps.

8. The method of claim 2, wherein the difference of particle sizes

between the small particles and the large particles prepared in step c) is not less than 500A.