INTERNALLY CROSSLINKED POLYMER MICROPARTICLES HAVING THREE-DIMENSIONAL NETWORK STRUCTURE

Abstract: Polymer microparticles having a three-dimensional network structure and an average particle size of 0.0l to l micron are obtained by emulsion polymerization of ethylenic­ ally unsaturated monomeric composition containing (a) at least one polyfunctional monomer having a plurality of poly­ merizable sites, or (b) a combination of two ethylenically unsaturated monomers each bearing mutually reactive functional groups in an aqueous medium in the presence of a compound or resin having a betaine group. The betaine compound or resin is incorporated into the polymer microparticles by physically adhering or covalently binding thereto to enhance the dispersing stability of the microparticles over a wide range of pH.

Description: This invention relates to internally crosslinked polymer microparticles having a three-dimensional network structure. Recently, internally crosslinked polymer microparticles commonly known as polymer microgels have become interested in the coating industry. The provision of high-solids coating compositions has been demanded for social reasons from the viewpoint of saving natural resources and pollution control. Polymer microgels may be effectively used in the preparation of high-solids coating compositions without compromising their workability. Besides high-solids coating compositions, polymer microgels find a wide variety of uses such as adhesives, sealants, optical fiber coverings, printing materials, biomedical materials and the like. Several methods are known to produce polymer microgels. One such method includes the steps of emulsion polymerizing a mixture of ethylenically unsaturated monomers including at least one crosslinking comonomer in an aqueous medium, and then removing water from the resulting polymer emulsion by, for example, solvent substitution, azeotropic distillation, centrifugation, filtering or drying. US-A-4,461,870 discloses an emulsion polymerization method using, as an emulsifier and dispersant, a compound or resin having an amphoionic group of the formula: wherein R is optionally substituted alkylene or phenylene and Y is -COOH or -SO₃H. The compound or resin having such amphoionic group will be bound into the polymers constituting microgels physically or through covalent bonds. The use of said compound or resin as an emulsifier and dispersant is advantageous in that it can dispense with the step of subsequently removing emulsifier or dispersant as required when using conventional surfactants which, if remained, will adversely affect properties of coating films. Furthermore, the microgels exhibit a number of advantageous characteristics such as high stability and dispersibility in both aqueous and nonaqueous systems based on the presence of said amphoionic groups. Experiments have shown, however, that polymer microparticles bearing said amphoionic groups become unstable against pH variation, particularly in acidic ranges. This is because, whereas said amphoionic group serves to increase the stability of polymer dispersions in water in the presence of a sufficient amount of counter ions, such counter ions are entrapped by another compound when pH has varied. Experiments have also shown that when a compound or resin having said amphoionic group of the formula: is used in the emulsion polymerization to prepare internally crosslinked polymer microparticles, the proportion of polyfunctional crosslinking monomers is usually limited upto 20% of total monomer mixtures. When higher solvent resistance, weatherability, thermal deformation resistance and other physical and chemical properties are desired for microgels, the crosslinking density of microgels must be increased as high as possible by increasing the proportion of the crosslinking monomers to 50-100%. EP-A-228 879 which is a prior art document in the sense of Article 54 (3) EPC discloses a process for producing a cross-linked polymer latex having a three-dimensional network structure. The latex is produced from the emulsion polymerization of (i) an ethylenically unsaturated monomer and an unsaturated monomer having a reactive functional group in the presence of a betaine ester emulsifier or (ii) an ethylenically unsaturated monomer and, optionally, an unsaturated monomer having a reactive functional group in the presence of a polyoxyalkylene ethylenically unsaturated carboxylic acid polyester emulsifier and, optionally a betaine ester emulsifier. GB-A-1 529 378 discloses a self-cross-linking thermosetting resin which comprises units derived from (1) at least one polymerizable monomer having an ampho-ionic structure, (2) at least one polymerizable monomer containing an oxirane group and (3) at least one polymerizable monomer other than the polymerizable monomers (1) and (2). The resin does not undergo cross-linking reactions under the polymerization conditions for producing it. It is, therefore, a main object of the present invention to provide internally crosslinked polymer microparticles having a three-dimensional network structure and crosslinking density which are highly stable in a dispersed state irrespective of pH variation. This invention relates to internally crosslinked polymer microparticles having a three-dimensional network structure and an average particle size of 0.01 to 1 micron and being obtainable by emulsion polymerization of a monomer composition containing at least 10% by weight of the composition of at least one polyfunctional monomer having a plurality of polymerizable ethylenically unsaturated bonds in the molecule, whereby the emulsion polymerization of said monomer composition is conducted in an aqueous medium in the presence of a reactive betaine group-containing emulsifier compound being selected from Compounds of the formula: wherein R₁ is hydrogen or methyl, R₂ and R₃ are independently C₁-C₆ alkyl, A is -O- or -NH-, n is 1-6, ml is 1-12, and X ⊖ is COO ⊖ or SO₃ ⊖ and 3-(4-vinylpyridin-1-yl)-propanesulfonic acid betaine, 3-(2-vinylpyridin-1-yl)-propanesulfonic acid betaine, 3-(4-vinylpyridin-1-yl)-propionic acid betaine, and 3-(2-vinylpyridin-1-yl)-propionic acid betaine, and wherein said betaine group-containing emulsifier compound is covalently bonded to the polymer microparticles by copolymerization with said monomer composition. The term "betaine" as used herein also includes sulfobetaines. Compounds having a betaine structure may act as a surfactant like corresponding amino carboxylic acids or amino sulfonic acids. However contrary to the latter, they do not undergo a tautomerism but always take an amphoionic form. Therefore, they exhibit a desired level of surface activity in the absence of counter ions independently from pH levels. Accordingly, when polymerizing said monomeric composition in the presence of a compound or resin having a betaine group as emulsifier, the resulting dispersion system and polymer microparticles separated therefrom are stable over a wide pH range. The resulting polymer microparticles may be advantageously incorporated to, for example various coating compositions for rheology control purposes with increased interaction between polymer microparticles due to the presence of betaine groups. Because the polymer microparticles are highly cross-linked into a three-dimensional network structure, they exhibit higher physical and chemical properties such as higher solvent resistance, weatherability and thermal deformation resistance. MONOMER COMPOSITION Monomer compositions constituting the polymer microparticles of this invention should contain at least 10 % by weight of the composition of at least one polyfunctional monomer having a plurality of ethylenically unsaturated bonds. each bearing mutually reactive functional groups. In other words, the monomeric composition may consist of 10 to 100% of said polyfunctional monomer and 0 to 90% of at least one monofunctional ethylenically unsaturated monomer. Monomers having at least two polymerization sites may typically be represented by esters of a polyhydric alcohol with an ethylenically unsaturated monocarboxylic acid, esters of an ethylenically unsaturated monoalcohol with a polycarboxylic acid and aromatic compounds having at least two vinyl substituents. Specific examples thereof include, ethylene glycol diacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, 1,4-butanediol diacrylate, neopentyl glycol diacrylate, 1,6-hexanediol diacrylate, pentaerythritol diacrylate, pentaerythritol triacrylate, pentaerythritol tetracrylate, pentaerythritol dimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, glycerol diacrylate, glycerol allyloxy dimethacrylate, 1,1,1-tris(hydroxymethyl)ethane diacrylate, 1,1,1-tris(hydroxymethyl)ethane triacrylate, 1,1,1-tris(hydroxymethyl)ethane dimethacrylate, 1,1,1-tris(hydroxymethyl)ethane trimethacrylate, 1,1,1-tris(hydroxymethyl)propane diacrylate, 1,1,1-tris(hydroxymethyl)propane triacrylate, 1,1,1-tris(hydroxymethyl)propane dimethacrylate, 1,1,1-tris(hydroxymethyl)propane trimethacrylate, triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate, diallyl phthalate, diallyl terephthalate and divinyl benzene. Examples of monomers having one polymerization site includes: (1) carboxyl bearing monomers as, for example, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid and fumaric acid, (2) hydroxyl bearing monomers as, for example, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, allyl alcohol and methallyl alcohol, (3) nitrogen containing alkyl acrylates or methacrylates as, for example, dimethylaminoethyl acrylate, and dimethylaminoethyl methacrylate, (4) polymerizable amides as, for example, acrylic amide and methacrylic amide, (5) polymerizable nitriles as, for example, acrylonitrile and methacrylonitrile, (6) alkyl acrylates or methacrylates as, for example, methyl acrylate, methyl methacrylate, ethyl acrylate, n-butylacrylate, n-butyl methacrylate, and 2-ethylhexylacrylate, (7) polymerizable glycidyl compounds as, for example, glycidyl (meth)acrylate, (8) polymerizable aromatic compounds as, for example, styrene, α-methyl styrene, vinyl toluene and t-butylstyrene, (9) α-olefins as, for example, ethylene and propylene, (10) vinyl compounds as, for example, vinyl acetate and vinyl propionate, and (11) diene compounds, as, for example, butadiene and isoprene. These monomers are used alone or in combination. BETAINE COMPOUNDS HAVING POLYMERIZABLE UNSATURATED GROUP As a reactive emulsifier, betaine compounds having an ethylenically unsaturated group may be used in the emulsion polymerization of monomers constituting the polymer microparticles of this invention. These compounds are bonded to the polymer microparticles through a covalent bond by a copolymerization reaction with said monomers. One class of such compounds has the formula I defined above. These compounds may be synthesized by reacting corresponding aminoalkyl esters or amides of (meth)acrylic acid with lactones or sultones. Sepcific examples are: 3-(N,N-dimethyl-N-methacryloyloxyethyl)-aminopropanesulfonic acid betaine, 3-(N,N-diethyl-N-methacryloyloxyethyl)-aminopropanesulfonic acid betaine, 3-(N,N-dimethyl-N-acryloyloxyethyl)-aminopropanesulfonic acid betaine, 3-(N,N-diethyl-N-acryloyloxyethyl)-aminopropanesulfonic acid betaine, N,N-dimethyl-N-methacryloylethyl-β-alanine betaine, N,N-diethyl-N-methacryloylethyl-β-alanine betaine, N,N-dimethyl-N-acryloylethyl-β-alanine betaine, and N,N-diethyl-N-acryloylethyl-β-alanine betaine, and the corresponding N-(meth)acryloylaminoethyl compounds. Another class of polymerizable betaine compounds are the following vinylpyridine compounds which may be synthesized by reacting corresponding pyridine compounds with sultones or lactones: 3-(4-vinylpyridin-1-yl)propanesulfonic acid betaine, 3-(2-vinylpyridin-1-yl)propanesulfonic acid betaine, 3-(4-vinylpyridin-1-yl)propionic acid betaine and 3-(2-vinylpyridin-1-yl)propionic acid betaine. EMULSION POLYMERIZATION Using the above-described betaine group-containing compounds or resins as an emulsifier, the polymer microparticles of this invention are prepared from the above-described monomer composition by a conventional emulsion polymerization technique in an aqueous medium. Said betaine group-containing compounds or resins are thereby physically adhered or covalently bonded to the resulting polymer microparticles. The amount of said compounds or resins ranges from 0.5 to 100 parts, preferably from 1 to 50 parts for polymerizable betaine group-containing compounds, and from 0.3 to 400 parts, preferably from 0.5 to 100 parts for betaine group-containing resins per 100 parts of monomer compositions. If this amount is too small, the system is less stable than is desirable. Conversely, excessive amounts tend to impair the water-resistance of microgels or increase the viscosity of the system too high. The average particle size of resulting polymer microparticles may be controlled by selecting suitable conditions and preferably range 0.01 to 1 micron. After the polymerization, resulting polymer microparticles may be used for various uses either in the form of an emulsion containing dispersing medium or in a anhydrous form after removing water by solvent substitution, azeotropic distillation, centrifugation, filtration or drying. The following examples are given for illustrate purposes only. All parts and percentages therein are by weight unless otherwise specified. EXAMPLE 1 A one liter flask having stirring means, temperature control means and condenser was charged with 425 parts of deionized water and heated to 80°C. 16 parts of 3-(N,N-dimethyl-N-methacryloyloxyethyl)aminopropane sulfonic acid betaine were dissolved in 60 parts of deionized water. A monomer mixture was separately prepared by mixing 20 parts of styrene, 36 parts of n-butyl acrylate, 8 parts of 2-hydroxyethyl methacrylate and 120 parts of 1,6-hexanediol dimethacrylate. An initiator solution was separately prepared by dissolving 3 parts of azobiscyanovaleric acid in 50 parts of water containing 2 parts of dimethylethanolamine. After having confirmed the inner temperature of the flask to be 80°C, the above betaine solution, monomer mixture and initiator solution were added dropwise concurrently over 90 minutes, 90 minutes and 110 minutes, respectively. The reaction was continued for additional 60 minutes at the same temperature to bring completion. The resulting dispersion of polymer microparticles had a nonvolatile content of 26.8% and an average particle size of 210 nm. EXAMPLE 2 A one liter flask having stirring means, temperature control means and condenser was charged with 365 parts of deionizied water and heated to 80°C. 30 parts of 3-(2-vinylpyridine-1-yl)propane sulphonic acid betaine were dissolved in 120 parts of deionized water. A monomer mixture was separately prepared by mixing 10 parts of methyl methacrylate, 20 parts of n-butyl acrylate and 140 parts of neopentyl glycol dimethacrylate. An initiator solution was separately prepared by dissolving 3 parts of azobiscyanovaleric acid in 50 parts of deionizied water containing 2 parts of dimethylethanolamine. After having confirmed the inner temperature of the flask to be 80°C, the above betaine solution, monomer mixture and initiator solution were added dropwise concurrently over 90 minutes, 90 minutes and 110 minutes, respectively. The reaction was continued for additional 60 minutes at the same temperature to bring completion. The resulting dispersion of polymer microparticles had a nonvolatile content of 26.7% and an average particle size of 250 nm.

Claims: Internally crosslinked polymer microparticles haying a three-dimensional network structure and an average particle size of 0.01 to 1 micron and being obtainable by emulsion polymerization of a monomer composition containing at least 10% by weight of the composition of at least one polyfuntional monomer having a plurality of polymerizable ethylenically unsaturated bonds in the molecule, whereby the emulsion polymerization of said monomer composition is conducted in an aqueous medium in the presence of a reactive betaine group-containing emulsifier compound being selected from Compounds of the formula: wherein R₁ is hydrogen or methyl, R₂ and R₃ are independently C₁-C₆ alkyl, A is -O- or -NH-, n is 1-6, ml is 1-12, and X ⊖ is COO ⊖ or SO₃ ⊖ and 3-(4-vinylpyridin-1-yl)-propanesulfonic acid betaine, 3-(2-vinylpyridin-1-yl)-propanesulfonic acid betaine, 3-(4-vinylpyridin-1-yl)-propionic acid betaine, and 3-(2-vinylpyridin-1-yl)-propionic acid betaine, and wherein said betaine group-containing emulsifier compound is covalently bonded to the polymer microparticles by copolymerization with said monomer composition. The internally crosslinked polymer microparticles of claim 1 , wherein said betaine compound is 3-(N,N-dimethyl-N-methacryloyloxyethyl)aminopropane-sulfonic acid betaine. The internally crosslinked polymer microparticles of claim 1 , wherein said betaine compound is 3-(2-vinylpyridin-1-yl)-propanesulfonic acid betaine. The internally crosslinked polymer microparticles of anyone of claims 1-3 wherein said monomer composition contains the balance of a monofunctional monomer having an ethylenically unsaturated group. A coating composition containing the polymer microparticles of anyone of claims 1 to 4.

Inventors: ISHII, KEIZOU, KANDA, KAZUNORI, KASHIHARA, AKIO, MIZUGUCHI, RYUZO
Applicants: NIPPON PAINT CO., LTD.
Application year: 1987
Main classification: C08F246
IPC codes: C08F220, C08K5, C09D5, C09D4, C08F20, C08L101, C09D201, C08F2, C08F246, C09D157