Aqueous crosslinked polymer particles and their production; aqueous dispersions containing same; and use in aqueous paints

Abstract: Aqueous crosslinked polymer particles providing excellent properties and paint work have an average particle size (De), measured by a transmission electron microscope after dyeing with uranyl acetate, of 20 to 200 nm, and an average roundness (R) represented by the equation: R = (Lc/L)² of 0.5 to 0.9 [wherein Lc is the perimeter of a circle having the same area as that of the particle measured by the transmission electron microscope, and L is the actual perimeter of the particle]. A process for producing aqueous crosslinked polymer particles is also described.

Description: The present invention relates to aqueous crosslinked polymer particles which impart excellent properties and coating workability to a paint by formulating them in an aqueous paint, and their production. At present, water-based paint compositions are replacing solvent-based paint compositions, as a paint, particularly paints used for spraying automobiles. It is considered that this tendency will increase from a viewpoint of problems such as painting environment, working environment and environmental pollution. In general, polymer particles are often formulated in water-based (aqueous) paint compositions so as to improve paint work. However, such a formulation of polymer particles always involves problems such as agglomeration, - separation, thickening and the like. In US-A-4 403 003 it is disclosed that fine particles having a particle size of 0.01 to 10 µm are formulated in a metallic-based paint. These fine particles are produced by a two-stage emulsion polymerization wherein core resin particles are formed and then a water-soluble polymer layer is formed thereon to improve dispersion stability in an aqueous medium. However, the two-stage polymerization method is not practical because of its complicated operation. Further, stability and paint viscosity in an aqueous paint of particles thus obtained may not necessarily be sufficient. The present invention provides aqueous crosslinked polymer particles which provide excellent properties and coating workability. The present invention also provides a process for producing aqueous crosslinked polymer particles. The above advantages as well as other advantages of the present invention will become apparent to those skilled in the art from the following description. The term "aqueous crosslinked polymer particles" as used in this patent application means crosslinked polymer particles which are stable in an aqueous medium. In order to obtain polymer particles which are stable in an aqueous medium and have improved viscosity polymer particles have been intensively studied from a viewpoint of the form and physical properties thereof, and the present invention derives from such studies. According to the present invention there are provided crosslinked polymer particles which are stable in an aqueous medium having an average particle size (De), measured by a transmission electron microscope after dyeing with uranyl acetate of 20 to 200 nm; having an average roundness (R) represented by the equation: R = (Lc/L) 2 of 0.5 to 0.9 [wherein Lc is the perimeter of a circle having the same area as that of the particle measured by the transmission electron microscope, and L is the actual perimeter of the particle]; having a degree of water swelling (Dw/De) which is the ratio of the particle size (Dw) in aqueous dispersion of the particles measured in an aqueous dispersion by a laser light scattering method to the average particle size (De) measured by the transmission electron microscope, of 1.8 to 3; and further having a degree of solvent swelling (Ds/Dw) which is the ratio of the particle size in aqueous dispersion (Dw) to the particle size in organic solvent (Ds) measured by the laser light scattering method after a small amount of an aqueous dispersion of particles is dispersed in a large amount of 1-ethoxy-2-propanol, of not more than 1.5. Generally spherical polymer particles are normally formed by emulsion polymerization. However, it is considered that they are not spherical particles but irregular particles that remarkably change stability and viscosity of the paint composition of the present invention. These irregular particles can be formed by using a monomer to be formulated which causes layer separation. The aqueous crosslinked polymer particles of the present invention have an average particle size (De) measured by a transmission electron microscope after dyeing with uranyl acetate of 20 to 200 nm, preferably 20 to 80 nm. Further, it is necessary that the polymer particles of the present invention have an average roundness (R) of 0.5 to 0.9, preferably 0.7 to 0.9. The average roundness (R) is determined by the equation: R = (Lc/L) 2 [wherein Lc is the perimeter of a circle having the same area as that of a particle measured by the transmission electron microscope after dyeing with uranyl acetate, and L is the actual perimeter of the particle]. The average roundness can be easily determined by subjecting an electron micrograph of the polymer particles to computer image processing. When the average roundness is close to 1, the particles have high roundness. When it is smaller than 1, the particles have large distortion and irregularity. In the present invention, polymer particles having high roundness, which is generally spherical, are not preferred because of poor stability in an aqueous paint. In the aqueous crosslinked polymer particles of the present invention, the ratio of the underwater particle size (Dw) of the articles measured in an aqueous dispersion by a laser light scattering method to the average particle size (De) measured by the transmission electron microscope [degree of water swelling: Dw/De] is in the range of 1.8 to 3. Further, the ratio of the underwater particle size (Dw) to the undersolvent particle size (Ds) measured by the laser light scattering method after a small amount of an aqueous dispersion of particles is dispersed in a large amount of 1-ethoxy-2-propanol [degree of solvent swelling: Ds/Dw] is not more than 1.5, more preferably 1.0 to 1.5. Even if the aqueous crosslinked polymer particles are synthesized by any method, the expected advantages of the present invention can be accomplished as long as the above numerical limits are satisfied. Most preferably, the emulsion polymerization is conducted using three kinds of monomers, for example, a hydrophilic monomer, a hydrophobic monomer and a crosslinkable monomer providing crosslinking. When using a crosslinkable monomer having a hydrophilic funtional group, the hydrophilic monomer may be eliminated. Examples of hydrophilic functional groups of the hydrophilic monomers include the hydroxyl group, carboxyl group and amide group (including N-substituted groups). Examples of the hydroxyl group-containing monomers include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate and allyl alcohol. Examples of the carboxyl group-containing monomers include acrylic acid, methacrylic acid and itaconic acid, and examples of the amide group-containing monomer include acrylamide, methacrylamide and N-methylolacrylamide. As the hydrophobic monomer, for example, there are momomers having a general double bond other than the above hydrophilic monomers. Examples thereof include (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate and phenyl (meth)acrylate; aromatic vinyl monomers such as ethylene, α-methylstyrene, p-butylstyrene, p-chlorostyrene and chloromethylstyrene; nitriles such as (meth)acrylonitrile; vinyl derivatives such as vinyl acetate and vinyl propionate; dienes such as butadiene and isoprene; group-containing monomers such as vinyltrimethoxysilane, vinyltriethoxysilane and γ-methacryloxypropyltrimethoxysilane. In the present invention, since the particles used are crosslinked particles, it is necessary to use a crosslinkable monomer having at least two radical polymerizable groups in an amount of 2 to 50 % by weight based on the total monomers. By using the crosslinkable monomer, solvent resistance is improved. Further, layer separation occurs and irregularity is formed without using a large amount of styrene or acrylic long-chain alkyl esters having a high hydrophobic nature and, therefore, particles having high hydrophilic nature are produced. Examples of the crosslinkable monomers include ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-propanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, glycerol di(meth)acrylate, allyl (meth)acrylate, diallyl phthalate, divinylbenzene, methylenebis (meth)acrylamide and glyoxal di(meth)acrylamide. The polymer particles of the present invention can be prepared by subjecting the above monomer mixture to emulsion polymerization. The emulsifier used for the emulsion polymerization may be a normal emulsifier. In particular, in the present invention, by using a reactive emulsifier, the amount of water-soluble polymer formed is small even if a large amount of the hydrophilic monomer is used; thereby, viscosity can be lowered. Further, the polymer particles are not very swelled in a solvent and, therefore, dispersibility becomes high even in an aqueous paint containing a solvent. As the reactive emulsifier, an emulsifier wherein an allyl group or a methacryl group which imparts radical polymerizability is introduced in a compound having an emulsification function is normally used. As the reactive emulsifier used in the present invention, methacrylic polyoxyethylene sulfate type emulsifier is preferred. The reactive emulsifier is used in an amount of 2 to 40 % by weight based on the total weight of the above monomers. The emulsion polymerization can be conducted by a normal method, for example, it may be conducted at a polymerization temperature of 40 to 95°C for 0.5 to 10 hours. The aqueous dispersion of the particles of the present invention is obtained by emulsion polymerization. By removing water from the aqueous dispersion, aqueous crosslinked polymer particles are obtained. The average roundness can be determined by analyzing a transmission electron photograph of dried polymer particles dyed with uranyl acetate. As described above, according to the present invention there are provided polymer particles which are stable in an aqueous paint and which can improve paint workability. The reasons why particles of the present invention are stable in an aqueous paint and improve paint workability are not clear but it is considered that water swell and increase of the surface due to irregularity of the surface promote an interaction with the base resin to enhance viscosity. Further, it is also considered that a thick aqueous layer due to water swell becomes a stabilized layer, thereby, dispersibility is improved. Further, low swell characteristics towards a solvent inhibits a solvent from being absorbed by a paint and stabilizes viscosity of the paint. The following Examples, Comparative Examples and Production Example further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Hereinafter, all "parts" are by weight unless otherwise stated. Example 1 A flask equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet and a dropping funnel was charged with 150 parts of deionized water and 4 parts of a reactive emulsifier RA-1022 (as an active component) and heated to 80°C. The flask was further charged with 5 % of a monomer mixture of 54 parts of methyl methacrylate, 8 parts of n-butyl acrylate, 6 parts of 2-hydroxyethyl acrylate and 32 parts of neopentyl glycol dimethacrylate. Then, an initiator solution, wherein 0.5 parts of azobiscyanovaleric acid was neutralized with 0.32 parts of N,N-dimethylaminoethanol and the resultant was dissolved in 5 parts of deionized water, was added to initiate the reaction. After 10 minutes, a pre-emulsified product prepared from 140 parts of deionized water, 21 parts of RA-1022 (as an active component) and the remainder monomer was added dropwise over 120 minutes and, at the same time, an initiator solution, wherein 0.5 parts of azobiscyanovaleric acid was neutralized with 0.32 part of N,N-dimethylaminoethanol and the resultant was dissolved in 200 parts of deionized water, was added dropwise over 130 minutes and maintained at 80°C for 60 minutes as it was. After cooling, the reaction mixture was filtered through a #400 mesh stainless steel net to obtain an aqueous polymer fine particles dispersion having properties shown in Table 2. Examples 2, 3, 7 and 8 and Comparative Examples 1 to 3 and 5 According to the same manner as that described in Example 1 except using each formulation as shown in Table 1, an aqueous polymer fine particles dispersion having properties shown in Table 2 were obtained. Examples 4 to 6 and Comparative Example 4 N,N-dimethylaminoethanol neutralized with azobiscyanovaleric acid as the initiator of Example 1 was changed to ammonium persulfate. As the initiator for initial addition, that in which 0.1 parts of ammonium persulfate was dissolved in 5 parts of deionized water was used, based on 100 parts of the monomer. As the initiator for dropping, that in which 0.2 parts of ammonium persulfate was dissolved in deionized water (of which amount is shown in Table 1) was used. After the completion of the reaction, the reaction mixture was neutralized with an aqueous N,N-dimethylaminoethanol solution (of which the amount is shown in Table 1) to obtain an aqueous polymer fine particles solution having the properties shown in Table 2. Comparative Example 6 According to the same manner as that described in Example 1, a flask was charged with 100 parts of deionized water and 0.1 parts of an emulsifier Newcoal 707SF (as an active component) and heated to 80°C. The flask was further charged with 5 % of a monomer mixture (for forming the core part) of 39 parts of methyl methacrylate, 39 parts of n-butyl acrylate and 2 parts of ethylene glycol dimethacrylate. Then, an initiator solution, wherein 0.1 parts of ammonium persulfate was dissolved in 5 parts of deionized water, was added to initiate the reaction. After 10 minutes, a pre-emulsified product prepared from 50 parts of deionized water, 0.4 parts of Newcoal 707SF (as an active component) and the remained monomer was added dropwise over 90 minutes and, at the same time, an initiator solution, wherein 0.2 parts of ammonium persulfate was dissolved in 50 parts of deionized water, was added dropwise over 150 minutes. When 20 minutes had passed after the completion of dropping the pre-emulsified product, a monomer mixture (for forming a shell part) of 4 parts of methyl methacrylate, 8 parts of n-butyl acrylate, 3 parts of methacrylic acid and 5 parts of 2-hydroxyethyl acrylate was added dropwise over 30 minutes. After the completion of dropping, the reaction mixture was maintained at 80°C for 60 minutes as it was. Then, this was neutralized with 0.9 parts of N,N-dimethylaminoethanol and 28 parts of deionized water to obtain an aqueous polymer fine particles dispersion having the properties shown in Table 2. Production Example 1 Water-soluble acrylic resin varnish A flask equipped with a stirrer, a thermometer, a reflux condenser and a dropping funnel was charged with 76 parts of ethylene glycol monobutyl ether. To the flask, 61 parts of a monomer mixture of 45 parts of styrene, 63 parts of methyl methacrylate, 48 parts of 2-hydroxyethyl methacrylate, 117 parts of n-butyl acrylate, 27 parts of methacrylic acid and 3 parts of azobisisobutyronitrile was added and heated to 120°C with stirring. While maintaining the same temperature, 242 parts of the above monomer solution was added dropwise over 3 hours, followed by additional stirring for 1 hour. Then, 28 parts of dimethylethanolamine and 600 parts of deionized water were added to obtain an acrylic resin varnish having a solid content of 30 % and the number average molecular weight of 1200 (of resin). This resin had an OH value of 70 and an acid value of 58. Formulation Deionized water: Initial charging is an amount to be charged in a flask before a reaction, and pre-emulsification is an amount to be used for emulsifying a monomer added dropwise. Emulsifier: an amount of an active component to be used for initial charging and pre-emulsification based on 100 parts of a monomer. Monomer: an amount based on 100 parts of a monomer, and 5 % of a mixture is used for initial charging. Initiator: ACVA is used in an amount of 1 part based on 100 parts of monomer (0.5 parts thereof are used for initial charging). APS is used in an amount of 0.3 parts based on 100 parts of a monomer (0.1 parts thereof are used for initial charging). An amount of deionized water is used for adding dropwise together with pre-emulsified product for initial charging. Neutralization The amount of N,N-dimethylaminoethanol (DMEA)/deionized water for dilution is based on 100 parts of a monomer (corresponding to 30 molar % based on the amount of the acidic monomer used). In the measurement by laser light scattering method, model N4 manufactured by Coulter Electronics, Inc. was used. In the measurement by a transmission electron microscope method, JEM-2000FX-II manufactured by JEOL Ltd. was used. Roundness A transmission electron microscope photograph was subjected to image processing and roundness R was calculated from the equation: R = [Lc/L] 2 [wherein Lc is the perimeter of a circle having the same area as that of the particle, and L is the actual perimeter of the particle]. The image processing was conducted using EXCEL manufactured by Nippon Avionic Co., Ltd. Stability An aqueous melamine resin Sumimal M-50W (manufactured by Sumitomo Chemical Co., Ltd.) and each aqueous polymer particle dispersion was added in turn to an aqueous acrylic resin varnish of Production Example 1 while stirring with a mixer such that a ratio of the solid content became 70:30:15, and then viscosity was adjusted to 30 seconds (Fordcup No.4) to prepare a clear paint. This paint was allowed to stand at 40°C for 2 weeks and applied to a glass plate by a coater (4 mill) and then baked at 140°C for 30 minutes. The appearance was evaluated by the following criteria. A: clear B: slightly turbid C: extremely turbid, or agglomerates are formed Viscosity A low shear viscosity of the above viscosity-adjusted paint (E type viscometer, measured at 10 -1 sec -1 ) as well as that of a concentrated paint of which solid content was increased in an amount of 5 % by evaporating the water content and solvent content of the paint, and then the following evaluation was conducted with a view to the degree of viscosity increase. A: viscosity increase is not less than 500 poise B: viscosity increase is not less than 400 poise C: viscosity increase is less than 400 poise

Claims: Crosslinked polymer particles which are stable in an aqueous medium having an average particle size (De), measured by a transmission electron microscope after dyeing with uranyl acetate of 20 to 200 nm; having an average roundness (R) represented by the equation: R = (Lc/L) 2 of 0.5 to 0.9 [wherein Lc is the perimeter of a circle having the same area as that of the particle measured by the transmission electron microscope, and L is the actual perimeter of the particle]; having a degree of water swelling (Dw/De) which is the ratio of the particle size (Dw) in aqueous dispersion of the particles measured in an aqueous dispersion by a laser light scattering method to the average particle size (De) measured by the transmission electron microscope, of 1.8 to 3; and further having a degree of solvent swelling (Ds/Dw) which is the ratio of the particle size in aqueous dispersion (Dw) to the particle size in organic solvent (Ds) measured by the laser light scattering method after a small amount of an aqueous dispersion of particles is dispersed in a large amount of 1-ethoxy-2-propanol, of not more than 1.5. An aqueous dispersion containing crosslinked polymer particles as claimed in claim 1. A process for producing crosslinked polymer particles which are stable in an aqueous medium according to claim 1, which process comprises preparing an emulsion containing a monomer mixture comprising: (1) 2 to 50% by weight of a radical polymerizable monomer having a hydrogen bonding functional group selected from a hydroxyl group, an amide group and a carboxyl group; (2) 2 to 50% by weight of a crosslinkable monomer having at least two radical polymerizable groups in the molecule; and (3) 40 to 90% by weight of a radical polymerizable monomer other than the above monomers (1) and (2), and a methacrylic polyoxyethylene sulfate reactive emulsifier in an amount of 2 to 40% by weight based on the total amount of the monomer mixture, and then conducting an emulsion polymerization. Use of crosslinked polymer particles as claimed in claim 1 or an aqueous dispersion as claimed in claim 2 in an aqueous paint.

Inventors: IMAMURA TSUYOSHI, OHIWA MASANORI, SAITO KOICHI, YAMOTO SHUHEI, YOSHIOKA MANABU, IMAMURA, TSUYOSHI, OHIWA, MASANORI, SAITO, KOICHI, YAMOTO, SHUHEI, YOSHIOKA, MANABU
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1992
Main classification: C08F246
IPC codes: C08F246, C09D5, C08F2, C08F220, C08F20