WO2010077094A2

WO2010077094A2 - Spherical polymer beads and preparation method thereof

Info

Publication number: WO2010077094A2
Application number: PCT/KR2009/007965
Authority: WO
Inventors: Seol-A Yu; Ho-Suk Song; Ki-Beom Eom; Young-Seo Yoon
Original assignee: Kolon Industries, Inc.
Priority date: 2008-12-31
Filing date: 2009-12-30
Publication date: 2010-07-08
Also published as: WO2010077094A3

Abstract

The present invention relates to low specific gravity polymer beads and a method for preparing the same, more particularly to spherical polymer beads that have lower specific gravity than the existing light diffusing agent polymethylmethacrylate(PMMA), and have higher solvent resistance than the existing low specific gravity polymer beads, and a preparation method of thereof using homogenized suspension polymerization.

Description

TITLE OF THE INVENTION

SPHERICAL POLYMER BEADS AND PREPARATION METHOD THEREOF

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to polymer beads and a preparation method of thereof. More particularly, the present invention relates to a method for preparing polymer beads that can improve process efficiency and product yield compared to the existing polymer beads, can prepare polymer beads having low specific gravity and maintaining spherical shape with a desired size, and can increase solvent resistance to secure coating stability and liquid stability, and spherical polymer beads prepared thereby.

(b) Description of the Related Art Spherical polymer beads are widely used in various industrial fields including optical products, a signboard, a display, lighting apparatus, cosmetics, paints, molded plastic products, etc. due to its excellent rheology property.

Particularly, as polymer beads used as light diffusing agent of an optical film, a light diffusion plate, etc. which is a core part of LCD BLU (backlight unit), polymethylmethacrylate or polystyrene has been mainly used for light diffusion effects due to refractive index difference between a film or binder, or a light diffusion plate and a matrix. Recently, the size of light diffusing agent used for exhibiting various optical properties has been diversified and light diffusing agent with a size of 20um or more has been increasingly used. Specifically, as the size of polymer bead that can be used as light diffusing agent increases, lens effect increases to contribute increase in luminance of a light diffusion sheet or light diffusion plate. Thus, large diameter beads have been recently spotlighted. However, it is difficult to prepare large diameter beads satisfying all the properties.

In addition, the polymethylmethacrylate bead, which is a light diffusing agent mixed with a binder and coated on a light diffusion film by Micro Gravure method, has specific gravity of approximately 1.2, and, after formulated into a coating liquid, it may be precipitated in a liquid state depending on the size of particle, and thus, it is generally subjected to pretreatment in a liquid tank involving agitation. Thus, product efficiency and yield may decrease, and the precipitation of light diffusing agent according to time elapse cannot be completely controlled by 100%.

For improvement, low specific gravity beads having specific gravity of 1.2 or less have been developed.

For example, a light diffusion plate of which used amount can be reduced by using inorganic light diffusing agent having low specific gravity while exhibiting identical light scattering and diffusion effects has been used. However, organic products having low specific gravity have not been developed yet. Furthermore, if inorganic beads having low specific gravity are used for preparation of low specific gravity bead, compatibility with an organic substance in a liquid is not good. Therefore, there has been a demand for development of organic beads. Accordingly, functional polymer beads having low specific gravity have been introduced from other application fields. For example, in the case of polymer bead named Expancel TM, an organic foaming agent contained in the polymer bead is foamed and expanded by increasing the temperature to form voids inside thereby making low specific gravity product. However, above material has high refractive index due to a main chain of acrylonitrile, thus decreasing diffusion effects, and, the internal voids may cause increase in free volume in the molecule to thin polymer shell, thereby causing easy swelling of the above material in a solvent of a light diffusion film. This may deteriorate liquid stability due to liquid viscosity increase to disturb smooth coating process and cause decrease in product yield. In addition, the shell layer may be easily collapsed depending on the processing temperature thus failing to maintain spherical form, consequently decreasing diffusion effects of a light diffusion plate.

Moreover, although porous polymer beads used for cosmetics have been introduced, solvent resistance may be inferior due to large voids of a few hundred nanometer, and optical properties may be inferior due to light scattering. SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a method for preparing polymer beads having lower specific gravity than the existing methyl (metha) acrylate polymer beads, by making molecular structural free volume in polymer beads, appropriately using alkyl (metha) acrylate having carbon number of 2 or more that has been used for preparing polymer beads in the prior art.

It is another aspect of the present invention to provide a method for preparing polymer beads having improved solvent resistance while maintaining low specific gravity, using a copolymer of specific fluorine-containing monomer having excellent solvent resistance and chemical resistance and hydrocarbon monomers that have been used for preparing low specific gravity beads in the prior art.

It is another aspect of the present invention to provide low spherical polymer beads with low specific gravity prepared by the above method, which has excellent physical properties such as processability, solvent resistance, chemical resistance, etc. and thus can be used for a high luminance light diffusing agent of an optical film, a light diffusion plate, etc. and molded plastic parts, etc.

According to one preferred embodiment of the invention, provided are polymer beads comprising cross-linked emulsion copolymers of a vinyl-based monomer and a multifunctional monomer, and having an average particle diameter of 2 to 200μm, coefficient of variation (CV.) of 35% or less, and true specific gravity of 1.20 or less.

The cross-linked emulsion copolymer according to one embodiment can be prepared by polymerization of an emulsion comprising the vinyl-based monomer, the multifunctional monomer, a cross-linking agent, a polymerization initiator, and a dispersion stabilizer. In addition, according to another embodiment of the invention, provided are polymer beads comprising cross-linked emulsion copolymers of a vinyl-based monomer, a fluorine-substituted vinyl-based monomer and a multifunctional monomer, and having an average particle diameter of 2 to 200μm, coefficient of variation (CV.) of 35% or less, and true specific gravity of 1.20 or less. The cross-linked emulsion copolymer according to another embodiment can be prepared by polymerization of an emulsion comprising the vinyl-based monomer, the fluorine-substituted vinyl-based monomer, the multifunctional monomer, a cross-linking agent, a polymerization initiator, and a dispersion stabilizer.

The multifunctional monomer may be selected one or more kinds from the group consisting of (metha)acrylate, trimethylol methane tetraacrylate, trimethylol methane triacrylate, trimethylol butane triacrylate, ethylene glycol dimethacrylate, and divinylbenzene. Also, different kinds of the multifunctional monomer can be used in the first and second embodiments. According to the present invention, provided are polymer beads showing excellent solvent resistance, wherein viscosity increase measured after 8 hours, based on the initial viscosity of a liquid mixing the polymer beads and toluene at a weight ratio of 1 :1, is less than 100%.

BRIEF DESCRIPTION QF THE DRAWINGS

Fig. 1 is a photo showing precipitation speed of light diffusion film liquid using the polymer beads of example 2 according to one preferred embodiment of the invention.

Fig. 2 is a graph showing the evaluation results of solvent resistance against toluene, of examples 4 to 8 and comparative examples 3 to 5 according to another preferred embodiment of the invention. Fig. 3 is a graph showing the evaluation results of solvent resistance against n-butyl acetate, of examples 4 to 8 and comparative examples 3 to 5 according to another preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS The present invention will now be explained in detail.

In the prior art, low specific gravity particles were prepared by formation of internal voids or secondary thermal expansion through foams, or by mixing monomers having low specific gravity. However, the inventors identified the problems of the above methods that the process time is very long and secondary processes are required, and the disadvantage that solvent resistance for a common solvent is very low depending on the size of the void. In addition, it was also identified that the above methods limitedly prepare large diameter while maintaining low specific gravity.

In the present invention, it was identified that a method for lowering specific gravity by preparing an emulsion by homogenized suspension polymerization using a specific vinyl-based monomer having alkyl group of carbon number of 2 or more, and forming molecular structural free volume in the polymer bead obtained by polymerizing the emulsion is very effective. In addition, it was also identified that in case a three-dimensional network structure is formed in the polymer bead using identical multifunctional monomer, more excellent solvent resistance can be obtained compared to the polymer beads prepared by the existing method.

Therefore, the first and second preferred embodiments of the invention provide spherical polymer beads having lower specific gravity than the polymer beads using methyl (metha) acrylate only, excellent solvent resistance and liquid stability, and improved coating property, and a method for preparing the same.

The first and second preferred embodiments of the invention are described in more detail.

Methylmethacrylate having Cl alkyl group which has been mainly used as a monomer has a short chain, and thus the polymer bead prepared therefrom has dense molecular structure to show specific gravity of 1.20 or more.

However, in the present invention, the specific vinyl-based monomer having alkyl group of carbon number of 2 or more is used, thereby easily providing polymer beads having true specific gravity of 1.2 or less by the effect of a longer chain than methyl having carbon number of 1 , which were difficult to realize in the prior art. More specifically, in the prior art, particles having low specific gravity were not categorized, and low specific gravity product was used for inorganic beads only. In addition, since the vinyl-based monomer having carbon number of 2 or more has ethyl, butyl group, etc., instead of methyl group, compared to methylmethacrylate, it has molecular structural free volume by the effect of the long chain, and thus specific gravity of the monomer itself is lower than methlamethacrylate. However, in case the free volume forms, solvent resistance may be low. For compensating this, it is important to effectively control the use of various cross-linking agents to maintain solvent resistance.

Therefore, the present invention provides an economical product that has excellent solvent resistance and low specific gravity by appropriate combination of various cross-linking agents and ingredients with the specific monomers, and thus can replace the existing product.

Accordingly, one preferred embodiment of the invention provides spherical polymer beads comprising cross-linked emulsion copolymers of the vinyl-based monomer and the multifunctional monomer, and having an average particle diameter of 2 to 200μm, preferably 5 to lOOjtan, coefficient of variation (CV.) of 35% or less, and true specific gravity of 1.20 or less.

The polymer bead of the present invention shows low specific gravity thus capable of reducing the amount used on the basis of weight, and thus it may be industrially useful and especially prevent precipitation.

The polymer beads comprising the cross-linked emulsion copolymers can be prepared by polymerization of an emulsion comprising the vinyl-based monomer, the multifunctional monomer, a cross-linking agent, a polymerization initiator and a dispersion stabilizer. Preferably, the polymer beads may be prepared by a method comprising the steps of (a) preparing an emulsion using the vinyl-based monomer having alkyl group of carbon number of 2 or more, the multifunctional monomer, a cross-linking agent, a polymerization initiator and a dispersion stabilizer, and (b) polymerizing the emulsion obtained in the above emulsification step to prepare polymer beads, and then, recovering the same. The emulsification step may comprise mixing the vinyl-based monomer having alkyl group of carbon number of 2 or more, the multifunctional monomer, a cross-linking agent, and a polymerization initiator, and adding the mixture to a solution comprising a dispersion stabilizer to prepare an emulsion. Wherein, a homogenized emulsion may be obtained using a homo-mixer. Although the emulsification condition is not specifically limited, preferably the emulsification may be conducted at a temperature of 10 to 30 ^°C for 5 to 60 minutes.

In the present invention, a droplet with a desired particle size distribution is obtained by the above emulsification process, and then, the emulsion is introduced into a 4-opening flask and polymerized within 6 to 40 hours considering the amount of remaining monomers under nitrogen atmosphere. At polymerization, the agitation speed should be appropriately maintained so that the produced polymer beads may not be settled down. Preferably, the agitation speed may be 100 to 300 rpm. As the polymerization method, commonly used homogenized suspension polymerization may be used, but not specifically limited. The vinyl-based monomer having carbon number of 2 or more may be selected one or more kinds from the group consisting of alkyl (metha) acrylate having C₂-C₁₆ alkyl group, preferably alkyl (metha) acrylate having C₂-C₁₂ alkyl group, more preferably lauryl methacrylate or butyl methacrylate. The vinyl-based monomer may be appropriately mixed depending on the degree of desired specific gravity and solvent resistance. The vinyl-based monomer may be preferably used in the content of 10 to 90 parts by weight, based on 100 parts by weight of the sum of the vinyl-based monomer and the multifunctional monomer. If the content is less than 10 parts by weight, the specific gravity of the prepared polymer bead is high because the specific gravity of multifunctional monomer is high. In addition, if it exceeds 90 parts by weight, a solvent resistance is low due to the long chain of the main chain.

Further, if necessary, a methylmethacryalte monomer may be further comprised in the emulsification step, wherein the methylmethacrylate monomer may be preferably used in the content of 20 to 80 parts by weight, based on 100 parts by weight of the sum of the vinyl-based monomer and the multifunctional monomer. In the first embodiment, the multifunctional monomer may be selected one or more kinds from the group consisting of trimethylolmethane tetraacrylate, trimethylolmethane triacrylate, trimethylolbutane triacrylate, ethylene glycol dimethacrylate and divinyl benzene. The multifunctional monomer may be preferably used in the content of 10 to 90 parts by weight, based on 100 parts by weight of the sum of the vinyl-based monomer and the multifunctional monomer. If the content is less than 10 parts by weight, solvent resistance is low due to low cross-linking density, and if it exceeds 90 parts by weight, true specific gravity is high due to the specific gravity of the multifunctional monomer itself.

Also, the polymerization initiator may be selected from the group consisting of an azo type initiator and a peroxide type initiator. The azo type initiator is a compound capable of initiating polymerization by pyrolysis in oil phase, and for examples, it may include 2,2-azobisisobutyronitrile, 4,4-azobis(4~cyanopentanoic acid),

2,2-azobis(2-methyl butyronitrile), 2,2'-azobis(2,4-dimethyl valeronitrile), etc. The peroxide type initiator may include benzoyl peroxide, lauryl peroxide, octanoyl peroxide, dicumyl peroxide, etc. The polymerization initiator may be preferably used in the content of 1 to 5 parts by weight, based on 100 parts by weight of the sum of the vinyl-based monomer and the multifunctional monomer. If the content is less than 1 parts by weight, unreacted monomers may be excessively generated, and if it exceeds 5 parts by weight, polymerization stability may be decreased due to rapid heat generation. The polymerization stabilizer may be selected one or more kinds from the group consisting of polyvinyl pyrrolidone, polyvinyl methylether, polyethyleneimine, polymethylmethacrylate acrylic acid copolymer, polyvinylalcohol, vinylacetate copolymer, ethylcellulose, and hydroxypropylcellulose. The polymerization stabilizer may be preferably used in the content of 1 to 10 parts by weight, based on 100 parts by weight of the sum of the vinyl-based monomer and the multifunctional monomer. If the content is less than 1 parts by weight, emulsion stability may decrease to generate a large quantity of polymerized aggregates, and if it exceeds 10 parts by weight, a desired particle diameter cannot be obtained and the it may be difficult to remove polymerization stabilizer during the washing process of polymer beads.

In addition, another embodiment of the invention provides polymer beads having excellent properties using the fluorine-substituted vinyl-based monomer. Polymer beads having low specific gravity and increased solvent resistance and chemical resistance is provided by appropriately combining various cross-linking agents and ingredients with the specific monomers. The low specific gravity bead having increased solvent resistance of the present invention has the form of a polymer, not a composition.

The polymer bead comprises the cross-linked emulsion copolymers of the vinyl-based monomer, the fluorine-substituted vinyl-based monomer and the multifunctional monomer, and has true specific gravity of 1.2 or less and average particle diameter of 2~200μm. Also, it has coefficient of variation(C.V.) of 35% or less, preferably 5-35%.

The polymer bead with excellent properties comprising the cross-linked emulsion copolymers can be prepared by polymerization of an emulsion comprising the vinyl-based monomer, the fluorine-substituted vinyl -based monomer, the multifunctional monomer, a cross-linking agent, a polymerization initiator, and a dispersion stabilizer. In the second embodiment, the multifunctional monomer different from the first embodiment may be used, and preferably, (metha)acrylate having C₄-Ci₂ alkyl group may be used. According to the present invention, the vinyl-based monomer, (metha)acrylate having C₄-C₁₂ alkyl group and the fluorine-containing monomer have excellent compatibility to each other thus preventing phase separation.

Particularly, in the second embodiment, viscosity increase of liquid and deterioration of solvent resistance may be prevented due to the use of the fluorine-substituted vinyl-based monomer, thus allowing excellent coating property, and improving yield due to increased solvent resistance. In the second embodiment, as the monomer, a compound having one or more unsaturated carbons may be used, and if necessary, methylmethacryalte may be additionally used.

A method for preparing polymer beads of the present invention is explained in more detail.

The polymer bead of the present invention can be obtained in the form of a polymer of the Chemical Formula 3, by radical polymerization of the vinyl-based monomer having one or more unsaturated carbons of the Chemical Formula 1, the fluorine-substituted vinyl-based monomer of the Chemical Formula 2, and (metha)acrylate having C₄-C₁₂ alkyl groups as shown in the following scheme 1. [Scheme 1]

(1) (2) (2') radical

(3) wherein,

R and R are independently hydrogen; acryl group; metha acryl group; amide group; linear or branched C₁-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group; C₂-C₂₀ alkylene unsubstituted or substituted with acryl, metha acryl or amide group; C₁-C₂₀ alkoxycarbonyl unsubstituted or substituted with acryl, metha acryl or amide group; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl or amide group, R³ is linear or branched C₁-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group; C₁-C₂₀ alkoxycarbonyl; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl or amide group,

R⁴ and R⁵ are independently hydrogen; fluorine; acryl, metha acryl or amide group substituted with at least one fluorine atom; acryl, metha acryl or amide group having Ci-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; linear or branched C₁-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group; or, C₅-C_2O hydrocarbon substituted with 1-41 fluorine atoms, unsubstituted or substituted with acryl, metha acryl or amide group,

R⁶ is linear or branched Cl-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group substituted with at least one fluorine atom; or, acryl, metha acryl or amide group having C₁-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; C₁-C₂₀ alkoxycarbonyl unsubstituted or substituted with a fluorine atom; or, C₅~C₂o aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl, or amide group, at least one of R⁴ to R has a fluorine substituent.

R⁷ is C₄-C₁₂ alkyl, and

1, m, and n are independently an integer of 500 ~ 100,000.

Preferably, the method of the present invention may be progressed by the following scheme 2.

[Scheme 2]

R² R⁵

Laurylmethacrylate R¹C=C + R⁴C=C + or butylmethacrylate

R³ R⁶

(D (2) (2¹) radical

(3) wherein, R¹ to R⁶, 1, m and n are as defined above and R⁷ is butyl or lauryl group.

Referring to the above schemes 1 and 2, the present invention, in order to improve low solvent resistance of low specific gravity bead, essentially uses a specific vinyl-based monomer, i.e., (metha)acrylate (Chemical Formula 2') having C₄-C₁₂ alkyl group including laurylmethacrylate, butylmethacrylate, etc., as material capable of allowing low specific gravity property, together with the fluorine-substituted vinyl-based monomer. The (metha)acrylate having C₄-C₁₂ alkyl group of the Chemical Formula 2' may be preferably laurylmethacrylate or butylmethacrylate. Therefore, according to the present invention, polymer beads having increased chemical resistance as well as solvent resistance can be prepared by appropriate combination of the monomer of the Chemical Formula 2, (metha)acrylate having C₄-C₁₂ alkyl group of the Chemical Formula 2', vinyl-based monomer of the Chemical Formula 1, a cross-linking agent, a polymerization initiator, a dispersion stabilizer, etc. More specifically, the low specific gravity polymer bead having increased solvent resistance according to the second embodiment of the present invention can be prepared by a method comprising the steps of preparing an emulsion using a) the vinyl-based monomer having one or more fluorine-substituted unsaturated carbon, b) the vinyl-based monomer having one or more unsaturated carbon, c) as the multifunctional monomer, (metha)acrylate having C₄-C₁₂ alkyl group, d) a dispersion stabilizer, e) a radical polymerization initiator, f) a cross-linking agent, and if necessary, g) a co-solvent, and conducting suspension polymerization. In addition, the above method may further comprise the step of recovering polymer beads obtained by the polymerization.

Thereby, low specific gravity spherical polymer beads having excellent solvent resistance comprising the polymer represented by the following Chemical Formula 3 can be provided.

[Chemical Formula 3]

Wherein, R¹ to R⁷, 1, m and n are as defined above.

In the present invention, first, the steps of preparing a monomer mixture and an aqueous mixture are conducted.

Specifically, a first solution of a monomer mixture is prepared by mixing the vinyl-based monomer having one or more fluorine-substituted unsaturated carbon, the vinyl-based monomer having one or more unsaturated carbon, (metha)acrylate having C4-12 alkyl group, and a cross-linking agent and stabilizing the mixture, and adding a radical polymerization initiator. In addition, a second solution of an aqueous mixture is prepared by mixing a dispersion stabilizer and ion exchange water, and if necessary, adding a co-solvent.

Then, the first and second solutions are mixed for 1~5 hours at 200rpm or more to prepare a suspension, and then, homogenized to prepare a mixed solution. Preferably, the mixing time is 1-3 hours, agitation speed is 200~2000rpm.

Subsequently, the suspension is subjected to a radical suspension polymerization at 40~95 ^°C for 2~24 hours with agitating at 50~500rpm. The radical polymerization may be preferably conducted at an agitation speed of 100~300rpm, at a temperature of 50~90^°C, for 5—18 hours. After the reaction is completed, polymer beads are separated by filtration, washed with ion exchange water 3-4 times, and dehydrated, and then, dried at 70 "C for 24 hours to obtain low specific gravity beads having increased solvent resistance.

In the first solution, the vinyl-based monomer may include a compound having one or more unsaturated carbon, and for example, the compound of the following Chemical Formula may be used. [Chemical Formula 1] R²

R¹C=C

R³

Wherein R¹ and R² are independently hydrogen; acryl group; metha acryl group; amide group; linear or branched C₁-C₂₀ alkyl unsubstitued or substituted with acryl, metha acryl or amide group; C₂-C_2O alkylene unsubstituted or substituted with acryl, metha acryl or amide group; C₁-C₂₀ alkoxycarbonyl unsubstitued or substituted with acryl, metha acryl or amide group; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl, or amide group; and

R³ is linear or branched C₁-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group; C₁-C₂₀ alkoxycarbonyl; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl or amide group.

The vinyl-based monomer of the above Chemical Formula 1 is a monomer comprising 1-6 unsaturated carbons and capable of radical polymerization, preferably an organic compound having C₂-C₂₀. In addition, acryl-based, urethane-based monomers may also be preferable, and 2 or more kinds of the monomers can be used in combination. The vinyl-based monomer of the Chemical Formula 1 may include methylmethacrylate, methyl acrylate, butyl acrylate, urethane acrylate, benzyl methacrylate, cyclohexyl acrylate, styrene, etc. Particularly, in order to allow low specific gravity, it is preferable to use one or more kinds of methylmethacrylate, butyl acrylate, benzyl methacrylate, cyclohexyl acrylate, etc. in combination.

As the multifunctional monomer used in the second embodiment, (metha)acrylate having C₄-C₁₂ alkyl group is essentially comprised in the monomer mixture to lower specific gravity of the bead. As the monomer, for example, laurylmethacrylate or butylmethacrylate may be used. The multifunctional monomer of

(metha)acrylate having C₄-C₁₂ alkyl group may be used in the content of 20 to 70 parts by weight, based on 100 parts by weight of the vinyl-based monomer of the Chemical Formula 1. If the content is less than 20 parts by weight, it may be difficult to obtain low specific gravity of 1.2 or less, and if it exceeds 70 parts by weight, solvent resistance may be deteriorated due to viscosity increase.

As the fluorine-substituted vinyl-based monomer, a vinyl-based monomer having one or more unsaturated carbon substituted with one or more fluorine may be used, and a compound of the following Chemical Formula 2 may be preferably used. [Chemical Formula 2]

R⁵ R4_C__C

R⁶ wherein, R⁴ and R⁵ are independently hydrogen; fluorine; acryl, metha acryl or amide group substituted with at least one fluorine atom; acryl, metha acryl or amide group having C₁-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; linear or branched C₁-C₂₀ alkyl unsubstitued or substituted with acryl, metha acryl or amide group; C₅-C₂₀ hydrocarbon substituted with 1-41 fluorine atoms, unsubstituted or substituted with acryl, metha acryl or amide group,

R⁶ is linear or branched Ci~C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group substituted with at least one fluorine atom; or, acryl, metha acryl or amide group having C₁-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; C₁-C₂₀ alkoxycarbonyl unsubstituted or substituted with a fluorine atom; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl, or amide group, and at least one of R⁴ to R⁶ has a fluorine substituent.

The fluorine-substituted vinyl-based monomer may be preferably used in the content of 5 to 100 parts by weight, based on 100 parts by weight of the vinyl-based monomer. If the content is less than 5 parts by weight, solvent resistance increase may be insufficient with adding the fluorine-substituted vinyl-based monomer, and if it exceeds 100 parts by weight, particle stability may be deteriorated during polymerization. As the cross-linking agent, a compound having 2 or more unsaturated carbons may be used, and for example, it may include one or more kinds selected from the group consisting of 1,2-ethanediol diacrylate, 1,3 -propanediol diacrylate, 1,3-butanediol diacrylate, 1 ,4-butanediol diacrylate, 1,5-pentanediol diacrylate, 1,6-hexanediol diacrylate, divinylbenzene, ethylene glycol diacrylate, propylene glycol diacrylate, butylenes glycol diacrylate, Methylene glycol diacrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, polybutylene glycol diacrylate, allyl acrylate, 1,2-ethanediol dimethacrylate, 1,3 -propanediol dimethacrylate, 1,3-butanediol dimethacrylate, 1,4-butanediol dimethacrylate, 1,5-pentanediol dimethacrylate, 1,6-hexanediol dimethacrylate, ethylene glycol dimethacrylate, propylene glycol dimethacrylate, butylene glycol dimethacrylate, triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, polypropylene glycol dimethacrylate, triethylene glycol methacrylate, , polybutylene glycol dimethacrylate, allyl methacrylate, and diallyl maleate.

The cross-linking agent may be used in the content of 3 to 50 parts by weight, preferably 1 to 35 parts by weight, based on 100 parts by weight of the vinyl-based monomer. If the content exceeds 50 parts by weight, it may be difficult to control heat of reaction, and to achieve low specific gravity due to decrease in free volume in the molecule, and if it is less than 3 parts by weight, solvent resistance cannot be increased due to low cross-linking even when the content of the fluorine-substituted vinyl-based monomer is 100 parts by weight.

The polymerization initiator is a radical initiator, and it may be selected from a fat soluble initiator and a water soluble initiator. The fat soluble initiator may be selected one or more kinds from the group consisting of benzoyl peroxide, azobisisobutyronitrile, azobis kenyl butyronitrile and azobis cyclohexane carbonitrile. The water soluble initiator may be selected one or more kinds from the group consisting of potassium persulfate, sodium persulfate, ammonium persulfate and azo type water soluble initiator.

The polymerization initiator may be used in the content of 0.1 to 3 parts by weight, based on 100 parts by weight of the vinyl-based monomer. If the content is less than 0.1 parts by weight, the degree of reaction completion may be lowered, and if it exceeds 3 parts by weight, it may be difficult to control heat of reaction.

In the second solution, as the dispersion stabilizer, the same dispersion stabilizer as used in the first embodiment may be used. For example, the dispersion stabilizer may include polyvinyl pyrrolidone, polyvinyl methylether, polyethyleneimine, polyacrylic acid, polymethylmethacrylate acrylic acid copolymer, polyvinylalcohol, vinylacetate copolymer, ethylcellulose, hydroxypropylcellulose, silica and siloxane, alone or in combination of 2 or more kinds.

Although the content of the dispersion stabilizer is not specifically limited, it may be used in the content of 0.1 to 5 parts by weight of the total aqueous mixture (the second solution).

The ion exchange water may be included in the remaining content of the total aqueous mixture.

The co-solvent may be used in the content of 1 to 40 parts by weight of the total aqueous mixture. In this case, the ion exchange water may be included in the remaining content. As explained, the low specific gravity polymer beads having increased solvent resistance prepared according to the second embodiment of the invention has true specific gravity of 1.2 or less, has excellent liquid stability, and may overcome solvent resistance problem which has been indicated as disadvantage of low specific gravity bead of the prior art, thus effectively preventing deterioration of coating stability due to precipitation.

The present invention is further explained in more detail with reference to the following examples. These examples, however, should not be interpreted as limiting the scope of the present invention in any manner.

Example 1

As vinyl-based monomer, 80 parts by weight of laurylmethacrylate(LMA), and as the multifunctional monomer, 20 parts by weight of ethylene glycol dimethacrylate(EGDMA) were mixed, and as a polymerization initiator, 1 parts by weight of 2,2'-azobis(2,4-dimethyl valeronitrile) (ADVN) were added thereto. Then, 5 parts by weight of a dispersion stabilizer polyvinylalcohol (PVA) were dissolved in a dispersion medium of ionized water, and the above mixture was introduced in the solution and homogenized at 8000rpm for 5 minutes using homogenizer to emulsify it. Then, the emulsion was introduced in a reaction tube and reacted at 50 ^°C for 6 hours under nitrogen atmosphere, and the temperature was elevated to 75 ^"C and the reaction was conducted for 4 hours. The polymer synthesized by the above reaction was filtered, washed with water and an ethanol aqueous solution, and then, introduced in a vacuum oven and dried overnight to prepare white inodorous spherical polymer beads. Example 2

White inodorous spherical polymer composite beads were prepared by the same method as Example 1, except that as the vinyl-based monomer, 20 parts by weight of lauryl methacrylate and 60 parts by weight of methylmethacrylate, and as the multifunctional monomer, 20 parts by weight of ethylene glycol dimethacrylate(EGDMA) were mixed and emulsified. Example 3

Spherical polymer beads were prepared by the same method as Example 1, except that as the vinyl-based monomer, 80 parts by weight of butyl methacrylate, and as the multifunctional monomer, 20 parts by weight of ethylene glycol dimethacrylate(EGDMA) were mixed and emulsified. Comparative Example 1

Spherical polymer beads were prepared by the same method as Example 1, except that as the vinyl-based monomer, 80 parts by weight of methyl methacrylate, and as the multifunctional monomer, 20 parts by weight of ethylene glycol dimethacrylate(EGDMA) were mixed and emulsified. Comparative Example 2

Spherical polymer beads having a lot of voids were prepared by the same method as Example 1, except that as the vinyl-based monomer, 40 parts by weight of methyl methacrylate, and as the multifunctional monomer, 10 parts by weight of ethylene glycol dimethacrylate(EGDMA) were mixed and emulsified using 50 parts by weight of a co-solvent toluene.

Experimental Example 1

[Comparison Analysis] In order to compare polymer beads prepared in the Examples 1-3 and

Comparative Examples 1-2, SEM, product yield, average particle diameter, true specific gravity, etc. were measured for each sample, and the results are shown in the following Table 1.

- SEM measurement: Hitachi S-4300 - product yield measurement: yield after filtering

- average particle diameter: Coulter Multisizer M3

- C.V.(%) = (standard deviation of particle diameter/average particle diameter) x 100

- True specific gravity measurement: Pycnometer(Micromeritics instrument) - solvent resistance measurement: If viscosity increase measured after 8 hours, based on the initial viscosity of the liquid mixing the polymer beads and toluene at a weight ratio of 1 :1 is less than 100%, solvent resistance was evaluated as "good", and if it is 100% or more, solvent resistance was evaluated as "bad". Viscosity was measured using DVII of Brookfield Company. [Table 1]

As shown in the Table 1 , the spherical polymer beads prepared in Examples 1 to 3 of the present invention had true specific gravity decreased maximum 15%, compared to the bead of Comparative Example 1 prepared using methylmethacrylate as a main chain. The polymer beads according to the present invention were normally polymerized without abnormal polymerization, and thus, they did not show any significant difference in product yield compared to Comparative Example 1, and has identical average particle diameter. In addition, the polymer bead having voids prepared by the method of Comparative Example 2 had similar true specific gravity, but it had inferior solvent resistance due to easy penetration of solvent into the voids.

Experimental Example 2

For liquids mixing the polymer beads of Example 2 and Comparative Example

1, binder resins, and toluene respectively at a weight ratio of 3:3:4, time-elapse changes of precipitation speed were measured indirectly from the heights of the mixed layer, and the results are shown in Fig. 1 and Table 2. The results show that in the case of Example 2, precipitation according to time-elapse was much improved. * Measurement of mixed layer height

: Immediately after introducing the mixed liquid into a vial, the vial was placed 2cm ahead of a white screen that is 10cm spaced apart from a lamp and the shadow of the vial was analyzed, and the height showing 99% of contrast ratio of the shadow at the darkest height was defined as height of mixed layer. Relative values of the mixed layer heights according to time elapse, based on the height of the mixed layer immediately after introducing the mixed liquid, are described in Table 2. (The lower the height of the mixed layer, the more precipitation formed.) [Table 2]

Experimental Example 3

Immediately after mixing the liquids of Experimental Example 2, transmittance was measured, and the results are shown in Table 3. As shown, in case low specific gravity material is used, the used amount of bead can be reduced on the basis of weight, and thus, it can be reduced maximum 40% to obtain identical optical properties. [Table 3]

Example 4

To a reactor, 100 parts by weight of methylmethacrylate, 20 parts by weight of lauryl methacrylate, 20 parts by weight of 2,2,2-trifluoroethyl methacrylate, 0.3 parts by weight of azobisisobutyronitrile as an intiator, and 10 parts by weight of 1,2-ethanediol diacrylate as a cross-linking agent were added to prepare a first solution of monomer mixture.

Then, 5 parts by weight of a dispersion stabilizer polyvinylalcohol (PVA) were dissolved in a dispersion medium of ionized water to prepare a second solution. Into the solution, the monomer mixture (the first solution) was introduced, and homogenized at 8000rpm for 5 minutes and emulsified. And then, the emulsion was introduced into a reaction tube and reacted at 50 ^°C for 6 hours under nitrogen atmosphere, and the temperature was elevated to 75 ^°C and the reaction was continued for 4 hours. The polymer synthesized by the reaction was filtered and washed with water and ethanol aqueous solution, and the filtrate was introduced into a vacuum oven and dried overnight to prepare a white inodorous spherical polymer beads.

Example 5

Spherical polymer beads were prepared by the same method as Example 4, except that 50 parts by weight of lauryl methacrylate were used when preparing the first solution of monomer mixture. Example 6

Spherical polymer beads were prepared by the same method as Example 4, except that 50 parts by weight of 2,2,2-trifluoroethyl methacrylate were used when preparing the first solution of monomer mixture. Example 7 Spherical polymer beads were prepared by the same method as Example 4, except that 80 parts by weight of 2,2,2-trifluoroethyl methacrylate were used when preparing the first solution of monomer mixture. Example 8

Spherical polymer beads were prepared by the same method as Example 5, except that 50 parts by weight of 2,2,2-trifluoroethyl methacrylate were used when preparing the first solution of monomer mixture. Comparative Example 3

Polymer beads were prepared by the same method as Example 4, except that lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate were not used. Comparative Example 4

Polymer beads were prepared by the same method as Example 5, except that 2,2,2-trifluoroethyl methacrylate was not used. Comparative Example 5

Spherical polymer beads were prepared by the same method as Example 4, except that 80 parts by weight of lauryl methacrylate were used when preparing the first solution of monomer mixture.

Experimental Example 4

[Comparison Analysis] For the polymer beads prepared in Examples 4-8 and Comparative Examples

3-5, average particle diameter and true specific gravity were measured by the method as Experimental Example 1. In addition, coefficient of variation(C.V)(%) was measured by the following Calculation Formula 1. The results are shown in Table 4.

[Calculation Formula 1] C.V.(%) = (standard deviation of particle diameter/average particle diameter) x

100 [Table 4]

As shown in the Table 4, the polymer beads prepared in Examples 4 to 8 of the present invention had maximum 15% decreased true specific gravity, compared to the bead of Comparative Example 3 prepared using only methylmethacrylate as a main chain.

To the contrary, the present invention can prepare various beads with desired specific gravity by controlling the contents of laurylmethacrylate and 2,2,2-trifluoroethyl methacrylate.

Experimental Example 5

The beads of Examples 4-8 and Comparative Example 3-5 were respectively dispersed in toluene and n-butyl acetate, and stability was evaluated after 0, 1, 2, 3, 4, 6, 8, 24 hours, and the results are shown in Figs. 2 and 3.

Stability was evaluated by measuring the viscosity of liquids at predetermined times at 50rpm using Brookfield viscometer with spindle No. 2, and the measurement value was shown as viscosity(cP). Solvent resistance was confirmed by measuring viscosity according to time elapse.

As shown in Figs. 2 and 3, solvent resistance increases as the content of the fluorine-substituted vinyl-based monomer increases, and low specific gravity product with excellent solvent resistance can be prepared by controlling the weight ratio of the fluorine-substituted vinyl-based monomer.

Particularly, comparing solvent resistance of the beads with identical specific gravity, the beads of Example 5 has superior solvent resistance to the existing low specific gravity bead of Comparative Example 4 (wherein 2,2,2-trifluoroethylmethacrylate is not used).

Meanwhile, the bead of Comparative Example 3 has equal solvent resistance. However, since it has higher true specific gravity than the present invention as shown in Experimental Example 4, precipitation may form after preparing a coating liquid and thus pre-treatment in a liquid tank may be required, thus failing to completely control precipitation of light diffusing agent according to time elapse. In addition, the bead of Comparative Example 4 has low specific gravity but inferior solvent resistance to the present invention because it does not use the fluorine-substituted vinyl-based monomer. Also, from Comparative Example 5, it can be seen that as the content of lauryl methacrylate increases, solvent resistance is deteriorated due to viscosity increase. Thus, although the beads of Comparative Examples 4 and 5 have true specific gravity, particle diameter and coefficient of variation falling within the range of the present invention, they have inferior solvent resistance to the present invention. Therefore, it can be seen that in order to obtain product with excellent solvent resistance, the kinds or contents of monomers should be controlled according to the method of the present invention.

In conclusion, the polymer beads of Examples 4 to 8 have low specific gravity, thus having excellent liquid storage and stability, and particularly have excellent solvent resistance compared to the low specific gravity bead of Comparative Example 4 prepared by the existing method, thus showing properties suitable for use as light diffusing agent for an optical film and light diffusion plate, etc. Furthermore, the spherical polymer bead of the present invention can be used as various molding additives for plastic.

As explained, the polymer beads of Example 1-8 prepared economically compared to those of the prior art have low specific gravity thus having excellent liquid storage and stability, and the used amount can be reduced if formulated on the basis of weight, thus showing properties suitable for use as light diffusing agent of an optical film and light diffusion plate, etc. Namely, since the beads of the present invention have low specific gravity, when used as light diffusing agent, even if the smaller amount is used than the existing polymer beads, they can exhibit effects equivalent to or more than those of the prior art. In addition, the spherical polymer bead of the present invention can be used as various molding additives for plastic.

Claims

WHAT IS CLAIMED IS:

1. Polymer beads comprising cross-linked emulsion copolymers of a vinyl-based monomer and a multifunctional monomer, and having an average particle diameter of 2 to 200μm, coefficient of variation (CV.) of 35% or less, and true specific gravity of 1.20 or less.

2. The polymer beads according to claim 1, wherein the cross-linked emulsion copolymers are prepared by polymerization of an emulsion comprising the vinyl-based monomer, the multifunctional monomer, a cross linking agent, a polymerization initiator and a dispersion stabilizer.

3. The polymer beads according to claim 1, wherein viscosity increase measured after 8 hours, based on the initial viscosity of a liquid mixing the polymer beads and toluene at a weight ratio of 1 : 1 , is less than 100%.

4. The polymer beads according to claim 1, wherein the vinyl-based monomer is one or more kinds selected from the group consisting of alkyl(metha)acrylate having C₂-C₁₆ alkyl group.

5. The polymer beads according to claim 4, wherein the vinyl-based monomer further comprise a methylmethacrylate monomer.

6. The polymer beads according to claim 1, wherein the vinyl-based monomer is one or more kinds selected from a compound represented by the following Chemical Formula 1:

[Chemical Formula 1] R²

R¹C=C

R³ Wherein R¹ and R² are independently hydrogen; acryl group; metha acryl group; amide group; linear or branched C₁-C₂₀ alkyl unsubstitued or substituted with acryl, metha acryl or amide group; C₂-C₂₀ alkylene unsubstituted or substituted with acryl, metha acryl or amide group; C₁-C₂₀ alkoxycarbonyl unsubstitued or substituted with acryl, metha acryl or amide group; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl, or amide group; and

7. The polymer beads according to claim 6, wherein the vinyl-based monomer is one or more kinds selected from the group consisting of methyl acrylate, methyl methacrylate, butyl acrylate, butyl methacrylate, urethane acrylate, benzyl methacrylate, cyclohexyl acrylate, and styrene.

8. The polymer beads according to claim 7, wherein the vinyl-based monomer comprises a fluorine-substituted vinyl-based monomer.

9. The polymer beads according to claim 8, wherein the fluorine-substituted vinyl-based monomer is represented by the following Chemical Formula 2:

[Chemical Formula 2]

wherein, R⁴ and R⁵ are independently hydrogen; fluorine; acryl, metha acryl or amide group substituted with at least one fluorine atom; acryl, metha acryl or amide group having C₁-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; linear or branched C₁-C₂₀ alkyl unsubstitued or substituted with acryl, metha acryl or amide group; C₅-C₂₀ hydrocarbon substituted with 1 — 41 fluorine atoms, unsubstituted or substituted with acryl, metha acryl or amide group,

R⁶ is linear or branched C₁-C₂₀ alky unsubstituted or substituted with acryl, metha acryl or amide group substituted with at least one fluorine atom; or, acryl, metha acryl or amide group having C₁-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; C₁-C₂₀ alkoxycarbonyl unsubstituted or substituted with a fluorine atom; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl, or amide group, and at least one of R to R has a fluorine substituent.

10. The polymer beads according to claim 9, wherein the polymer bead is a polymer represented by the following Chemical Formula 3: [Chemical Formula 3]

wherein,

R¹ and R² are independently hydrogen; acryl group; metha acryl group; amide group; linear or branched C₁-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group; C₂-C₂₀ alkylene unsubstituted or substituted with acryl, metha acryl or amide group; C₁-C₂₀ alkoxycarbonyl unsubstituted or substituted with acryl, metha acryl or amide group; or, C₅-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl or amide group,

R³ is linear or branched C₁-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group; C₁-C₂₀ alkoxycarbonyl; or, C₅-C_2O aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl or amide group,

R⁴ and R⁵ are independently hydrogen; fluorine; acryl, metha acryl or amide group substituted with at least one fluorine atom; acryl, metha acryl or amide group having C₁-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; linear or branched C₁-C_2O alkyl unsubstituted or substituted with acryl, metha acryl or amide group; or, C₅-C₂₀ hydrocarbon substituted with 1-41 fluorine atoms, unsubstituted or substituted with acryl, metha acryl or amide group,

R⁶ is linear or branched C₁-C₂₀ alkyl unsubstituted or substituted with acryl, metha acryl or amide group substituted with at least one fluorine atom; or, acryl, metha acryl or amide group having C₁-C₂₀ alkyl or alkylene group substituted with 1-30 fluorine atoms; C₁-C₂₀ alkoxycarbonyl unsubstituted or substituted with a fluorine atom; or, Cs-C₂₀ aromatic or alicyclic hydrocarbon unsubstituted or substituted with acryl, metha acryl, or amide group, at least one of R⁴ to R⁶ has a fluorine substituent,

R⁷ is C₄-C₁₂ alkyl, and

1, m, and n are independently an integer of 500 - 100,000.

11. The polymer beads according to claim 2, wherein the multifunctional monomer is one or more kinds selected from the group consisting of (metha)acrylate, trimethylolmethane tetraacrylate, trimethylolmethane triacrylate, trimethylolbutane triacrylate, ethylene glycol dimethacrylate, and divinylbenzene having C₄-Ci₂ alkyl group.

12. The polymer beads according to claim 2, wherein the cross-linking agent is one or more kinds selected from the group consisting of 1,2-ethanediol diacrylate, 1,3 -propanediol diacrylate, 1,3-butanediol diacrylate, 1,4-butanediol diacrylate, 1,5-pentanediol diacrylate, 1,6-hexanediol diacrylate, divinylbenzene, ethylene glycol diacrylate, propylene glycol diacrylate, butylene glycol diacrylate, triethylene glycol diacrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, polybutylene glycol diacrylate, allylacrylate, 1,2-ethanediol dimethacrylate, 1,3 -propanediol dimethacrylate, 1,3-butanediol dimethacrylate, 1,4-butanediol dimethacrylate, 1,5-pentanediol dimethacrylate, 1,6-hexanediol dimethacrylate, ethylene glycol dimethacrylate, propylene glycol dimethacrylate, butylene glycol dimethacrylate, triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, polypropylene glycol dimethacrylate, Methylene glycol methacrylate, polybutylene glycol dimethacrylate, allyl methacrylate, and diallyl maleate.

13. The polymer beads according to claim 2, wherein the dispersion stabilizer is one or more kinds selected from the group consisting of polyvinyl pyrrolidone, polyvinyl methylether, polyethyleneimine, polyacrylic acid, polymethylmethacrylate acrylic acid copolymer, polyvinylalcohol, vinylacetate copolymer, ethylcellulose, hydroxypropylcellulose, silica and siloxane.

14. The polymer beads according to claim 2, wherein the emulsion comprises 10 to 90 parts by weight of the vinyl-based monomer, based on 100 parts by weight of the sum of the vinyl-based monomer and the multifunctional monomer.

15. The polymer beads according to claim 2, wherein the emulsion comprises 20 to 70 parts by weight of the multifunctional monomer, based on 100 parts by weight of the vinyl-based monomer.

16. The polymer beads according to claim 9, wherein the fluorine-substituted vinyl-based monomer is comprised in an amount of 5 to 100 parts by weight, based on 100 parts by weight of the vinyl-based monomer.

17. The polymer beads according to claim 2, wherein the emulsion comprises 3 to 50 parts by weight of the cross-linking agent, based on 100 parts by weight of the vinyl-based monomer.