Process for preparation of polymer particles, process for preparation of toner particles and particles prepared according to these processes

Abstract: Disclosed a process for the preparation of polymer particles, which comprises suspending a polymerizable monomer, a radical initiator and a hardly soluble inorganic dispersant in a dispersion medium and carrying out polymerization in this state, wherein a hardly soluble inorganic dispersant, the solubility of which is changed according to the pH value in the dispersion medium and which is precipitated at a pH value within a certain pH value region of the dispersion medium, is used as the dispersant, suspension of the polymerizable monomer in the dispersion medium is carried out in the state where the dispersant is dissolved in the dispersion medium, an acid or base is then added to adjust the pH value of the dispersion medium to a predetermined level to precipitate the dispersant, and the polymerization is initiated in the state where oil drops having a fine particulate size are formed. The process is effectively utilized for obtaining a toner having good flowability and blocking resistance and excellent developing and fixing properties by using a colorant together with the polymerizable monomer.

Description: The present invention relates to a process for the preparation of polymer particles having a sharp particle size distribution without formation of fine particles. Furthermore, the invention relates to a process for the preparation of toner particles useful for electrophotography or electrostatic printing. The toner is formed by incorporating a colorant into the above-mentioned polymer particles, which have a sharp particle size distribution, good flowability and blocking resistance, and excellent developing and fixing properties. In the suspension polymerization process, since the polymerization of a monomer is carried out in oil drops suspended and dispersed in an aqueous medium, the polymerization heat can be easily removed. By dint of this advantage, the suspension polymerization process is widely utilized on an industrial scale. In this suspension polymerization process, however, in order to stably suspend and disperse, for example, a monomer into an aqueous medium, it is necessary to use surface active agents or hardly soluble inorganic salts. Accordingly, these surface active agents are readily incorporated as impurities in the formed polymer and have an adverse influence on the characteristics, such as the electric properties, of the formed polymer. The formed polymer is obtained in a particulate shape in the suspension polymerization process. If it is intended to obtain particles having a small particle size, for example a particle size smaller than 10 µm, it is necessary to increase the amount used of a dispersant such as a surface active agent, and therefore emulsion polymerization particles having a particle size smaller than 1 µm are formed and are incorporated as impurities in the formed polymer. In the field of the electrophotography, a toner is used for visualizing an electrostatic image. Particles of this toner are composed of a composition formed by incorporating a colorant and, if necessary, a charge controlling agent and other additives in a resin medium, and the particle size is adjusted to a certain range, for example, from 1 to 30 µm. A resin having desirable electroscopic and binding properties, for example, a styrene resin, is used as the resin medium, and carbon black or an organic or inorganic coloring pigment is used as the colorant. As the process for the preparation of such a toner, there is known a process in which a toner is directly prepared at the polymerization step for preparing a toner resin. In general, this preparation process comprises dissolving in a water-insoluble monomer a polymerization initiation soluble in the monomer, adding a colorant and other additive to the solution, suspending the resulting composition in an aqueous solution comprising a dispersant such as a water-soluble polymer, an inorganic powder and a surface active agent under high-speed shearing agitation and effecting polymerization to prepare colored polymer particles. In this suspension polymerization process, the particle size of the final toner is determined by the suspension state of the monomer composition in the water. The particle size distribution is greatly broadened according to the state of charging of the monomer composition into the water and the stirring state, and it is very difficult to obtain particles having a uniform particle size. Moreover, coarse particles having a particle size of scores of µm to several mm are generally formed in this suspension polymerization process, and it is difficult to obtain particles having a particle size of 1 to 30 µm which are valuable for a developer toner. Of course, it is possible to reduce the particle size of the formed polymer by increasing the amount incorporated of the dispersant. In this case, however, the dispersant is included in the toner, and the toner becomes sensitive to moisture and the electrophotographic characteristics are readily degraded. A post treatment is necessary to eliminate this disadvantage. Thus the number of steps increases and the process is not practically applicable. The most serious problem of the toner prepared by the suspension polymerization process resides in the unevenness of the particle size. Namely, incorporation of fine particles at a specific ratio cannot be avoided. These fine particles are composed solely of a polymer not containing toner additives such as the colorant and have characteristics different from those of the toner particles. Accordingly, even if these fine particles are incorporated in small amounts, scattering of the toner or fogging is caused at the development step and there is a risk of reduction of the flowability of the toner. Even if these fine particles can be removed, the composition of the obtained toner particles is substantially different from the initial composition, and the desirable characteristics cannot be obtained and the cost is increased due to a reduction of the yield. Various proposals have been made to prepare a toner having improved developing and fixing properties by suspension polymerization. For example, Japanese Examined Patent Publication No. 64-11941 discloses a process in which a plurality of initiators differing in the half-value period are contained in a polymerizable monomer and the polymerization is carried out in this state. In this process, however, if an initiator having a long half-value period is used to broaden the molecular weight distribution, the time required for the polymerization is long, and the period during which oil drop particles are dispersed in the highly viscous state at the polymerization becomes long and coalescence of particles is readily caused. As a result the formed toner contains coarse particles and the particle size distribution is broadened. The thus-obtained toner shows certain effects in connection with the fixing property and offset resistance, but the durability and blocking resistance are poor, presumably because of the presence of the low-molecular-weight component on the surfaces of the formed polymer particles. Accordingly, further improvements are desired. GB-A-2 091 435 discloses a process for producing toner particles by dispersing monomer, colorant, charge controlling agent, and initiator into water, suspending the dispersed mixture in the presence of a suspended dispersing agent like tricalcium phosphate by stirring to form oil droplets and then polymerizing the mixture. The present invention seeks to provide a process for preparing polymer particles by suspension polymerization without the adverse influence of a dispersion stabilizer or surface active agent, but which can provide particles having a sharp particle size distribution without formation of fine particles. The present invention provides a process for preparing polymer particles, which comprises: (1) dissolving a hardly water-soluble inorganic dispersant which has a solubility which changes according to the pH of the dispersion medium, in an aqueous dispersion medium, by appropriate adjustment of the pH, (2) adding a radical-polymerizable monomer and a radical polymerization initiator to the aqueous dispersion medium and suspending and dispersing said monomer and initiator as oil drops in said dispersion medium, (3) adding an acid or alkali to the dispersion medium with stirring to precipitate the dispersant, and (4) carrying out polymerization. The process of the present invention may be used to prepare a toner composed of a particulate polymer in which the adverse influences of a dispersant or surface active agent are not manifested, the particles having a sharp particle size distribution, and in which scattering of the toner or reduction of the flowability does not occur. According to this process, the preparation is carried out by suspension polymerization, and a colorant is also added to the dispersion medium in step (2). The toner prepared according to this process is composed of colorant-containing polymer particles. The particles have such a particle size distribution such that the median diameter of the particles based on the volume is from 5 to 20 µm and the ratio of the 25% cumulative diameter (D25) based on the volume to the 75% cumulative diameter (D75) based on the volume is from 1.4:1 to 1.7:1. The cumulative diameter based on the volume referred to in the present invention is determined in the following manner. The diameters of particles are measured by a Coulter counter, and the measured values of the particle diameters are cumulated in the order of the diameter magnitude. The diameter of the particle cumulated when the volume of the cumulated particles reaches 25% based on the total volume is designated as the 25% cumulative diameter (D25) based on the volume, and the diameter of the particle cumulated when the volume of the cumulated particles reaches 75% of the total volume is designated as the 75% cumulated particles reaches 75% of the total volume is designated as the 75% cumulative diameter (D75) based on the volume. Brief Description of the Drawings Fig. 1 is a curve showing the molecular weight distribution, determined by GPC, of the toner obtained in Example 13. Fig. 2 is a curve showing the molecular weight distribution, determined by GPC, of the toner obtained in Comparative Example 5. Fig. 3 is a curve showing the molecular weight distribution, determined by GPC, of the toner obtained in Example 15. Fig. 4 is a curve showing the molecular weight distribution, determined by GPC, of the toner obtained in Comparative Example 6. Detailed Description of the Invention Process for Preparation of Polymer Particles Having a Uniform Particle Size In suspension polymerization processes, a monomer is dispersed and suspended in water by a suspension dispersion stabilizer, and the polymerization is carried out in the presence of a radical polymerization initiator dissolved in the monomer. It is known that a hardly soluble inorganic powder such as tricalcium phosphate may be used as the suspension dispersion stabilizer. However, this hardly soluble inorganic powder is not completely satisfactory in respect of suspension stability, the resin particles formed are coarse and have a large particle size, and the particle size distribution is very broad. On the other hand, a surface active agent is excellent in respect of suspension stability but the tendency of a part of the monomer to emulsity cannot be avoided and incorporation of fine emulsion polymerization particles into the suspension polymerization particles also cannot be avoided. In the present invention, a hardly soluble dispersant is used, the solubility of which changes according to the pH of the dispersion medium and which precipitates at a pH within a certain pH range of the dispersion medium. Tricalcium phosphate [Ca(PO 4 ) 2 ] can be used as the hardly soluble dispersant. The tricalcium phosphate is dissolved in advance in an aqueous solution of an acid, before the monomer and other additives are suspended and dispersed in the solution, and polymerization is carried out in the state where the tricalcium phosphate is precipitated. If the tricalcium phosphate is mixed with the monomer and other additives, and then the mixture is suspended and dispersed and the polymerization is carried out, the suspended and dispersed particles per se are coarse, and the resin particles formed have a large particle size and a very broad particle size distribution. According to the present invention, the tricalcium phosphate is dissolved in advance, and after the monomer and other components are suspended and dispersed in the solution, the tricalcium phosphate is precipitated. Accordingly, the tricalcium phosphate finely precipitated around the dispersed oil drops is uniformly distributed, agglomeration of the oil drops or coarsening thereof is effectively prevented and oil drops having a fine particulate size are effectively maintained, with the result that polymer particles having a fine particle size and a sharp particle size distribution can be obtained. Starting Materials (a) Radical-Polymerizable Monomer In the suspension polymerization process of the present invention, at least one monomer having an ethylenically unsaturated bond is used as the radical-polymerizable monomer. Preferred examples of the monomer are monovinyl aromatic monomers, acrylic monomers, vinyl ester monomers, vinyl ether monomers, diolefin monomers and mono-olefin monomers. As the monovinyl aromatic monomer, there can be mentioned monovinyl aromatic hydrocarbons of formula: wherein R 1 represents a hydrogen atom, a lower alkyl group or a halogen atom, and R 2 represents a hydrogen atom, a lower alkyl group, a halogen atom, an alkoxy group, an amino group, a nitro group, a vinyl group or a carboxyl group. For example, at least one member selected from styrene, α-methylstyrene, vinyltoluene, α-chlorostyrene, o-, m-and p-chlorostyrenes, p-ethylstyrene, sodium styrene-sulfonate and divinylbenzene can be used. Other monomers that can be used in the present invention are acrylic monomers of formula: wherein R 3 represents a hydrogen atom or a lower alkyl group, and R 4 represents a hydrocarbon group having up to 12 carbon atoms, a hydroxyalkyl group, a vinyl ester group or an aminoalkyl group, such as acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, phenyl acrylate, methyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, ethyl β-hydroxyacrylate, propyl γ-hydroxyacrylate, butyl δ-hydroxyacrylate, ethyl β-hydroxymethacrylate, propyl γ-aminoacrylate, propyl γ-N,N-diethylaminoacrylate, ethylene glycol dimethacrylate and tetraethylene glycol dimethacrylate, vinyl esters of formula: wherein R 5 represents a hydrogen atom or a lower alkyl group, such as vinyl formate, vinyl acetate and vinyl propionate, vinyl ethers of formula: wherein R 6 represents a monovalent hydrocarbon group having up to 12 carbon atoms, such as vinyl methyl ether, vinyl ethyl ether, vinyl n-butyl ether, vinyl phenyl ether and vinyl cyclohexyl ether, diolefins of formula: wherein R 7 , R 8 and R 9 represent a hydrogen atom, a lower alkyl group and a halogen atom, such as butadiene, isoprene and chloroprene, and monoolefins of formula: wherein R 10 and R 11 represent a hydrogen atom or a lower alkyl group, such as ethylene, propylene, isobutylene, butene-1, pentene-1 and 4-methylpentene-1. In the present invention, in order to promote the stabilization of the dispersed oil drops, it is preferred that the polarity of the polymerizable composition be controlled according to the polarity of the hardly soluble inorganic dispersant. For example, there can be mentioned a method in which an ethylenically unsaturated monomer having an anionic group or a cationic group is incorporated or is built in a polymer chain, or a method in which an unpolymerizable organic compound having an anionic group or a cationic group is incorporated. Preferred examples are ionic monomers such as acrylic acid, methacrylic acid, maleic acid anhydride, fumaric acid, itaconic acid anhydride, crotonic acid, tetrahydrophthalic acid anhydride, styrene-sulfonic acid and 2-acrylamido-2-methylpropane-sulfonic acid, cationic monomers such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, diethylaminopropyl (meth)acrylate, N-aminoethylaminopropyl (meth)acrylate, vinylpyridine, 2-vinylimidazole and 2-hydroxy-3-acryloxypropyltrimethyl ammonium chloride, anionic organic compounds such as homopolymers and copolymers containing an anionic monomer as mentioned above in the polymer chain, monoazo dye metal complexes and complexes of hydroxycarboxylic acids with metals such as salicylic acid and naphthoic acid, and cationic organic compounds such as homopolymers and copolymers having a cationic monomer as mentioned above in the polymer chain, oil-soluble dyes, for example, Nigrosine Base (CI 5045), Oil Black (CI 26150) and Spilon Black, primary, secondary and tertiary amines and quaternary ammonium salts. From the viewpoint of the adhesiveness, it is preferred that the main component of the polymerizable monomer be at least one member selected from styrene, acrylic acid esters and methacrylic acid esters, and a cationic or anionic group-containing monomer be contained at a concentration of 0.1 to 20% by weight, especially 0.5 to 10% by weight, based on the monomer composition. (b) Polymerization Initiator As the polymerization initiator, there can be used radical polymerization initiators, for example, azo compounds such as 2,2-azobis(2,4-dimethylvaleronitrile), 2,2'-azobisisobutyronitrile, 1,1'-azobis(cyclohexane-1-carbonitrile) and 2,2'-azobis-4-methoxy-2,4-dimethyl-valeronitrile, and organic peroxides such as cumene hydroperoxide, t-butyl hydroperoxides, dicumyl peroxide, di-t-butyl peroxide, benzoyl peroxide and lauroyl peroxide. These radical polymerization initiators can be used in combination with ionizing radiation such as γ-rays and accelerated electron beams. The amount incorporated of the polymerization initiator such as an azo compound or a peroxide can be a so-called catalytic amount, and it is generally preferred that the polymerization initiator be used in an amount of 0.1 to 10% by weight based on the charged monomer. (c) Dispersion Stabilizer In the present invention, a hardly soluble inorganic dispersant is used as the dispersant. For example, powders of inorganic salts such as calcium sulfate, tricalcium phosphate, magnesium carbonate, barium carbonate, calcium carbonate and aluminum hydroxides can be used. The inorganic dispersant is generally used in an amount of 0.5 to 20 parts by weight, preferably 1 to 10 parts by weight, per 100 parts by weight of water. (d) Other Additives In the present invention, in addition to the above-mentioned radical-polymerizable monomer (a) and radical polymerization initiator (b), a crosslinking agent or a chain transfer agent can be used for adjusting the molecular weight, if necessary. As the crosslinking agent, there can be used any known crosslinking agent, for example, divinyl compounds such as divinylbenzene, divinyl ether and divinylsulfone, diallyl compounds such as allyl phthalate and diallyl carbinol, and diacryl esters such as diacryl phenol. Diacryl esters are especially preferably used. As the chain transfer agent, there can be mentioned alkylmercaptans such as butylmercaptan and dodecylmercaptan, lower alkyl xanthogenates such as diisopropyl xanthogenate, and halogenated hydrocarbons such as carbon tetrachloride and carbon tetrabromide. Dispersant-Dissolving Step (1) The dispersant is used in a specific pH region determined according to the dispersant used. For example, a phosphoric acid salt is soluble in acidic conditions and insoluble in alkaline conditions. When tricalcium phosphate [Ca 3 (PO 4 ) 2 ] is used as the dispersant it is added to an aqueous solution of an acid to dissolve it. A mineral acid such as hydrochloric acid, sulfuric acid or nitric acid can be used as the acid. Hydrochloric acid is especially preferably used. For example, when hydrochloric acid is used as the acid, the reaction is represented by the following formula: Ca 3 (PO 4 ) 2 + 6HCℓ → 3CaCℓ 2 + 2H 3 PO 4 Tricalcium phosphate is an inorganic powder, and since tricalcium phosphate is dissolved in an aqueous solution of an acid in the present invention, the particle size of the tricalcium phosphate used is not particularly critical. Dissolution of tricalcium phosphate can also be accomplished by dispersing tricalcium phosphate in water and adding an acid or an aqueous solution of an acid to the dispersion. The mineral acid is used in such an amount that all of the tricalcium phosphate used is completely dissolved. Instead of the method in which the dispersant is dissolved, there can be adopted a method in which substances that can be precipitated as a dispersant at a specific pH, such as calcium chloride and phosphoric acid, are dissolved in advance. Suspending-Dispersing Step (2) The radical-polymerizable monomer and the radical polymerization initiator are suspended and dispersed in the solution of the dispersant such as tricalcium phosphate. Other additives such as the crosslinking agent and chain transfer agent can be added in this step, if necessary. The amount of monomer charged is generally 5 to 20 parts by weight, especially 10 to 100 parts by weight, per 100 parts by weight of the aqueous solution of the acid containing tricalcium phosphate dissolved therein The amount of radical initiator incorporated can be a so-called catalytic amount. In general, the amount of the radical initiator is 0.1 to 10% by weight based on the monomer. In the present invention, it is preferred that a surface active agent is added to the aqueous solution of the acid before the addition of the monomer and other components. The surface active agent acts as a dispersion stabilizer at the oil drop-forming step described hereinafter. An anionic surface active agent is advantageously used as the surface active agent. Preferable examples include fatty acid salts such as sodium oleate and a potassium soap of castor oil, higher alcohol sulfate salts such as sodium lauryl sulfate and sodium cetyl sulfate, alkylaryl sulfonate salts such as sodium dodecyl benzene-sulfonate, a sodium alkyl naphthalene-sulfonate and a sodium salt of a β-naphthalene-sulfonic acid/formalin condensate, naphthalene-sulfonic acid salt derivatives, dialkyl sulfosuccinate salts, dialkyl phosphate salts, polyoxyethylene alkyl ether sulfate salts, polyoxyethylene alkyl ether sulfate salts, polyoxyethylene alkyl ether sulfate triethanolamines and polyoxyethylene alkyl phenol ether sulfate salts. Use of a small amount of the surface active agent is sufficient. For example, the surface active agent is used in an amount of up to 0.5 part by weight, preferably up to 0.1 part by weight, per 100 parts by weight of water. Oil Drop-Forming Step (3) In the present invention a base such as LiOH, KOH or NaOH is added with stirring. By addition of the base, calcium triphosphate is precipitated to form very fine suspended oil drops. The tricalcium triphosphate-precipitating reaction is represented by the following formula: 3CaCℓ 2 + 2H 3 PO 4 + 6NaOH→Ca 3 (PO 4 ) 2 ↓ + 6NaCℓ + 6H 2 O Polymerization Step (4) The radical polymerization reaction is carried out in the state where the fine suspended oil drops have been formed. The polymerization temperature and time may be those adopted in the conventional techniques. In general, it is sufficient if the polymerization is carried out at a temperature of 40 to 100°C for 1 to 50 hours. The stirring of the reaction mixture may be gentle stirring causing a homogeneous reaction in the entire reaction mixture. In order to prevent inhibition of the polymerization by oxygen, the polymerization can be carried out while replacing the atmosphere of the reaction system by an inert gas such as nitrogen. By the reaction, the polymerization product is obtained in particulate form and has a particle size smaller than 30 µm, especially 1 to 10 µm. The particles formed are recovered by filtration, washed with water if necessary, and dried to obtain a particulate polymer. According to the above-mentioned process of the present invention, polymer particles having a fine particle size and a sharp particle size distribution can be obtained. According to the polymerization process of the present invention, by adjusting the concentration of the inorganic salt dissolved in the reaction mixture or the pH, the molecular weight of the polymer can be adjusted without changing the quantities of the radical polymerization initiator, the crosslinking agent or the chain transfer agent. For example, in the above-mentioned tricalcium phosphate-dissolving step or oil drop-forming step, the concentration in the aqueous solution of the acid or the amount of the base added may be controlled to appropriately adjust the concentration of the calcium salt dissolved in the reaction mixture, whereby the molecular weight of the polymer can be changed according to the calcium concentration. As is apparent from a comparison of Example 2 with Examples 3, 4, 5, 6 and 7, when the amount used of the acid or base used is large, that is, the concentration of the inorganic salt dissolved in the reaction mixture is low, even if the pH is the same, the molecular weight of the polymer is high. Furthermore, as is apparent from the comparison of Examples 2, 4 and 6 with Examples 3, 5 and 7, if the amount of base relative to the amount of acid is large, that is, the pH value of the reaction mixture for polymerization is high, the molecular weight of the polymer is low. As is apparent from the foregoing description, the molecular weight of the polymer can be adjusted by adjusting the amounts used of the acid and base only without adjusting the quantities of the components to be subjected to polymerization. In the present invention, if the amount of base added is adjusted so that the pH of the reaction mixture is 7 to 8, by reducing the amount of the surface active agent used, polymer particles having a volume average particle size smaller than 10 µm can be obtained, and furthermore the reaction efficiency of polymerization is increased and the amount of unreacted residual monomer can be drastically reduced. If the amount of base added is adjusted so that the pH of the reaction mixture is larger than 11, the polymer particles obtained have a very sharp particle size distribution, that is, a uniform particle size. The particles obtained can be utilized as a gap-adjusting agent for a liquid crystal display board, a filler for a column for, for example, chromatography or a filler for cosmetics. The suspension polymerization process of the present invention can be advantageously applied to the production of a toner for the electrophotography, for example, by using a colorant together with the monomer component. In the production of toner particles a colorant and optionally, other additives such as a charge-controlling agent are added in step (2). Colorant Used A known coloring pigment can be used. Appropriate examples are: (a) Black Pigments Carbon black, acetylene black, lamp black and aniline black. (b) Yellow Pigments Chrome yellow, zinc yellow, cadmium yellow, yellow iron oxide, Mineral Fast Yellow, nickel titanium yellow, naples yellow, Teftol Yellow S, Hansa Yellow 10G, Benzidine Yellow G, Quinoline Yellow Lake, Permanent Yellow NGG and Tartrazine Lake. (c) Orange Pigments Chrome orange, molybdenum orange, Permanent Orange GTR, Pyrazolone Orange, Vulcan Orange, Indanthrene Brilliant Orange, Benzidine Orange G and Indanthrene Brilliant Orange GK. (d) Red Pigments Red iron oxide, cadmium red, red lead, mercury cadmium sulfide, Permanent Orange 4R, Lithol Red, Pyrazolone Red, Watchung Red calcium salt, Lake Red D, Brilliant Carmine 6B, Eosine Lake, Rhodamine Lake B, Alizarine Lake and Brilliant Carmine 3B. (e) Violet Pigments Manganese violet, Fast Violet B and Methyl Violet Lake. (f) Blue Pigments Iron blue, cobalt blue, Alkali Blue Lake, Phthalcyanine Blue, metal-free Phthalocyanine Blue, partially chlorinated Phthalocyanine Blue, Fast Sky Blue and Indanthrene Blue BC. (g) Green Pigments Chrome green, chromium oxide, Pigment Green B, Marachite Green Lake and Final Yellow Green G. (h) White Pigments Zinc flower, titanium oxide, antimony white and zinc sulfide. (i) Extender Pigments Baryte powder, barium carbonate, clay, silica, white carbon, talc and alumina white. It is preferred that the coloring pigment be incorporated in an amount of 1 to 20 parts by weight, especially 5 to 15 parts by weight, per 100 parts by weight of the polymerizable monomer. Magnetic Material Pigments As the magnetic material pigment, there are known, for example, triiron tetroxide (Fe 3 O 4 ), diiron trioxide (γ-Fe 2 O 3 ), zinc iron oxide (ZnFe 2 O 4 ), yttrium iron oxide (Y 3 Fe 5 O 12 ), cadmium iron oxide (Cd 3 Fe 5 O 12 ), copper iron oxide (CuFe 2 O 4 ), lead iron oxide (PbFe 12 O 19 ), neodium iron oxide (NdFeO 3 ), borium iron oxide (BaFe 12 O 19 ), magnesium iron oxide (MgFe 2 O 4 ), manganese iron oxide (MnFe 2 O 4 ), lanthanum iron oxide (LaFeO 3 ), iron powder (Fe), cobalt powder (Co) and nickel powder (Ni). Fine powders of these known magnetic materials can optionally be used. The magnetic material pigment is generally used in an amount of 1 to 200 parts by weight per 100 parts by weight of the polymerizable monomer. Other additive agents Known charge controlling agents such as metal soaps of naphthenic acid, metal soaps of fatty acids and soaps of resin acids can be incorporated for adjusting the charging property of the toner. The charge controlling agent is generally used an amount of 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight, per 100 parts by weight of the polymerizable monomer. A release agent such as low-molecular-weight polyethylene, low-molecular-weight polypropylene, a wax or a silicone oil can be incorporated for imparting an offset-preventing effect to the toner. The offset-preventing agent is generally incorporated in an amount of 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight, per 100 parts by weight of the polymerizable monomer. Polymerization Step For Toner Particles (4) Known polymerization temperatures and times can be adopted. In general, it is sufficient if the polymerization is carried out at 40 to 100°C for 1 to 50 hours. The stirring of the reaction mixture can be a gentle stirring causing a uniform reaction in the entire reaction mixture. In order to prevent inhibition of the polymerization by oxygen, the polymerization can be carried out while replacing the atmosphere of the reaction system by an inert gas such as nitrogen. If the polymerization is carried out at a pH of 7 to 8, the reaction efficiency is increased and the amount of the unreacted residual monomer can be advantageously reduced. If the polymerization is carried out at a pH of greater than 11, an advantage of controlling the formation of emulsion polymerization particles having a particle size smaller than 1 µm can be attained. This adjustment of the pH can be easily accomplished by addition of the above-mentioned alkali metal hydroxide. Since the polymerization product is obtained in the form of particles having the above-mentioned particle size, colored toner particles can be obtained by recovering the formed particles by filtration, washing them with, for example, water if necessary and drying them. Carbon black or hydrophobic silica, for example, can be sprinkled on the colored toner particles to produce a final toner. According to the present invention, a toner having a particle size of 1 to 30 µm, especially a particle size smaller than 10 µm, and a sharp particle size distribution can be obtained by suspension polymerization. This toner is substantially free of emulsion polymerization particles having a very fine particle size. Accordingly, the toner obtained by the process of the present invention has excellent flowability and scattering of the toner or fogging is not caused at all. Examples of Preparation of Polymer Particles Having a Uniform Particle Size Example 1 To 400 parts of water was added 10 parts by weight of Ca 3 (PO 4 ) 2 , and 38 parts of 12N hydrochloric acid was added to the dispersion to dissolve the Ca 3 (PO 4 ) 2 . A monomer composition comprising 80 parts by weight of monomeric styrene, 18 parts by weight of 2-ethylhexyl methacrylate, 2 parts by weight ethylene glycol dimethacrylate and 3 parts by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) was added to the dispersion medium containing Ca 3 (PO 4 ) 2 dissolved therein. The dispersion was stirred at 11,000 rpm for 3 minutes by a TK homomixer (supplied by Tokushu Kika Kogyo), and 102 parts by weight of 4N sodium hydroxide was added to the dispersion to precipitate Ca 3 (PO 4 ) 2 . At this point, the pH of the aqueous phase was 9. The dispersion was further stirred at 11,000 rpm for 60 minutes to obtain a suspended dispersion. The suspended dispersion was transferred into a separable flask, and polymerization was carried out in a nitrogen atmosphere at a rotation rate of 80 rpm and a temperature of 70°C for 6 hours. After the polymerization reaction, the polymer was treated with a dilute acid, washed with water and dried to obtain spherical polymer particles. When the particle size distribution of the obtained polymer particles was measured by a Coulter counter, it was found that the average particle size was 18 µm, the proportion of the particles having a particle size smaller than 8 µm was 1.7%, and the proportion of the particles having a particle size larger than 25 µm was 2.0 %. Example 2 The procedures of Example 1 were repeated in the same manner except that the amount of Ca 3 (PO 4 ) 2 was changed to 7 parts by weight, the amount of 12N hydrochloric acid was changed to 23 parts by weight and 0.04 part by weight of sodium dodecyl benzene-sulfonate was further added. The monomer composition was added to the dispersion medium, and 72 parts by weight of 4N sodium hydroxide (NaOH) was added to precipitate Ca 3 (PO 4 ) 2 . At this point, the pH of the aqueous phase was 9. The polymerization was carried out in the same manner as described in Example 1 except that the rotation rate was changed to 10,000 rpm and the polymerization time was changed to 60 minutes, whereby spherical polymer particles were obtained. The particle size distribution of the obtained polymer particles, the molecular weight distribution determined by GPC, Tg (°C), Ti (°C) and Tm (°C) determined by a flow tester, the content (%) of the residual monomer and the state of formation of particles having a particle size smaller than 1 µm as the by-product are shown in Table 1. Example 3 The procedures of Example 2 were repeated in the same manner except that the amount added of 12N hydrochloric acid was changed to 13 parts and the amount added of 4N sodium hydroxide was changed to 41 parts by weight, and at an aqueous phase pH of 9, the suspension granulation was effected and the polymerization was carried out to obtain spherical polymer particles. The characteristics, such as the particle size distribution, of the obtained polymer particles are shown in Table 1. Example 4 The procedures of Example 2 were repeated in the same manner except that the amount added of 12N hydrochloric acid was changed to 23 parts by weight and the amount added of 4N sodium hydroxide was changed to 66 parts by weight. At an aqueous phase pH of 7, the suspension granulation was effected and the polymerization was carried out to obtain spherical particles. Characteristics, such as the particle size distribution, of the obtained polymer particles are shown in Table 1. Example 5 The procedures of Example 2 were repeated in the same manner except that the amount added of 12N hydrochloric acid was changed to 13 parts by weight and the amount added of 4N sodium hydroxide was changed to 37 parts by weight. At an aqueous phase pH of 7, the suspension granulation was effected and the polymerization was carried out to obtain spherical polymer particles. Characteristics, such as the particle size distribution, of the obtained polymer particles are shown in Table 1. Example 6 The procedures of Example 2 were repeated in the same manner except that the amount added of 12N hydrochloric acid was changed to 23 parts by weight and the amount added of 4N sodium hydroxide was changed to 91 parts by weight. At an aqueous phase pH of 14, the suspension granulation was effected, and the polymerization reaction was carried out to obtain spherical polymer particles. Characteristics, such as the particle size distribution, of the obtained polymer particles are shown in Table 1. Example 7 The procedures of Example 2 were repeated in the same manner except that the amount added of 12N hydrochloric acid was changed to 13 parts by weight and the amount added of 4N sodium hydroxide (NaOH) was changed to 52 parts by weight. At an aqueous phase pH of 14, the suspension granulation was effected, and the polymerization was carried out to obtain spherical polymer particles. Characteristics, such as the particle size distribution, of the obtained polymer particles are shown in Table 1. Comparative Example 1 To 400 parts by weight of water was added 10 parts by weight of Ca 3 (PO 4 ) 2 , and Ca 3 (PO 4 ) 2 was dispersed in water by an ultrasonic vibration. The same monomer composition as used in Example 1 was added to the obtained aqueous dispersion, and the liquid was dispersed at 11,000 rpm for 60 minute by a TK homomixer to obtain a suspended dispersion. In the same manner as described in Example 1, the polymerization was carried out to obtain spherical polymer particles. When the particle size distribution of the obtained polymer particles was measured by a Coulter counter, it was found that the average particle size was 26 µm, the proportion of the particles smaller than 10 µm was 2% and the proportion of the particles larger than 32 µm was 25%. Thus, it was confirmed that the particle size distribution was broad. Comparative Example 2 Spherical polymer particles were prepared in the same manner as described in Comparative Example 1 except that 0.04 part by weight of sodium dodecyl benzene-sulfonate was added together with Ca 3 (PO 4 ) 2 . The particle size distribution of the obtained polymer particles was measured by a Coulter counter. It was found that the average particle size was 24 µm, the proportion of the particles having a particle size smaller than 10 µm was 3% and the proportion of the particles having a particle size larger than 32 µm was 23 %. Accordingly, it was confirmed that the particle size distribution was broad. Comparative Example 3 To 400 parts by weight of water were added 10 parts by weight of Ca 3 (PO 4 ) 2 , 0.04 part by weight of sodium dodecyl benzene-sulfonate and 38 parts by weight of 12N hydrochloric acid, and the mixture was stirred by a magnetic stirrer to dissolve Ca 3 (PO 4 ) 3 . Then, 102 parts by weight of 4N sodium hydroxide was added to precipitate Ca 3 (PO 4 ) 2 . In the same manner as described in Comparative Example 1, the monomer composition was added to obtain a suspended dispersion and the polymerization was carried out to obtain spherical polymer particles. When the particle size distribution of the obtained polymer particles was measured by a Coulter counter, it was found that the average particle size was 18 µm, the proportion of the particles having a particle size smaller than 8 µm was 2.0% and the proportion of the particles having a particle size larger than 25 µm was 6.0%. Thus, it was confirmed that the particle size distribution was broad. From the foregoing Examples and Comparative Examples, it is seen that if Ca 3 (PO 4 ) 2 is dissolved in an aqueous solution of an acid, a composition comprising a polymerizable monomer is added and suspension-dispersed into the solution, an alkali metal hydroxide is added to the dispersion to precipitate the dissolved ca 3 (PO 4 ) 3 and the polymerization is carried out in this state, polymer particles having a sharp particle size distribution can be obtained while controlling formation of ultrafine particles or coarse particles. From Examples 2 through 7, it is seen that even if the pH in the aqueous phase is the same, when the salt concentration in the aqueous phase is changed according to the amount added of the acid or alkali, the molecular weight of the formed polymer greatly differs. In contrast, even if the salt concentration is made equal according to the amount added of the acid or alkali, if the pH is different, the molecular weight of the formed polymer greatly differs. Accordingly, it is seen that the molecular weight of the formed polymer can be controlled by changing the pH of the aqueous phase and the salt concentration at the polymerization. As is apparent from the foregoing description, according to the polymerization process of the present invention, polymer particles having a very sharp particle size distribution can be obtained, and the molecular weight can be effectively controlled. Examples of Preparation of Toners Example 8 Aqueous Phase To 670 parts by weight of distilled water was added 7 parts by weight of Ca 3 (PO 4 ) 3 , and while the inorganic salt was dispersed by a stirrer, 23 parts by weight of HC was added to sufficiently dissolve Ca 3 (PO 4 ) 2 . Then, 17 parts by weight of a 0.2% aqueous solution of NaDBS was further added to the solution. Styrene 82 parts by weight 2-Ethylhexyl methacrylate 18 parts by weight Grafted carbon black 5 parts by weight Spilon Black TRH 1 part by weight Divinylbenzene 0.2 part by weight Diethylene glycol dimethacrylate 1.5 parts by weight AIBN 0.32 part by weight ADVN 2.5 parts by weight The above-mentioned oil phase was added to the aqueous phase, and the mixture was stirred at 11,000 rpm for 3 minutes by a homomixer supplied by Tokushu Kika to sufficiently disperse the oil phase. Then, stirring was similarly continued and 65 parts by weight of 4N NaOH was added to adjust the pH of the aqueous phase to 9 and precipitate Ca 3 (PB 4 ) 2 . Then, stirring was further conducted for 20 minutes to form suspended oil drops having an average particle size of 10.4 µm. This suspension was subjected to polymerization at 80°C in an N 2 current for 12 hours. The precipitate was separated from the supernatant, treated with a dilute acid, washed with water and dried to obtain a toner. When the particle size of the obtained toner was measured by a Coulter counter, it was found that the volume average particle size D50 was 10.4 µm and the D25:D75 ratio was 1.5:1, and it was confirmed that the toner had a sharp particle size distribution. The residual monomer content in the obtained toner was 0.73%. When a copying operation was carried out in a commercially available electrophotographic copying machine (Model DC-111C supplied by Mita Kogyo) by using the obtained toner, a shape image having a good density in the solid color portion and a good resolution was obtained. Example 9 An aqueous phase having a pH of 7 was prepared in the same manner as described in Example 8 except that the amount added of the 0.2% solution of NaDBS was changed to 5 parts by weight and the amount added of 4N NaOH was changed to 60 parts by weight. Then, stirring was conducted at 9,800 rpm for 20 minutes to form suspended oil drops having an average particle size of 9.8 µm. Then, 5 parts by weight of 4N NaOH was added to the obtained suspension to adjust the pH of the aqueous phase to 9, and polymerization was carried out at 80°C in an N 2 current for 12 hours to obtain a polymer. The precipitate was separated from the supernatant, treated with a dilute acid, washed with water and dried to obtain a toner. The supernatant separated was substantially transparent. When the particle size of the obtained toner was measured by a Coulter counter, it was found that the volume average particle size (D50) was 9.7 µm and the D25:D75 ratio was 15:1, and it was confirmed that the toner had a sharp particle size distribution. The residual monomer content in the toner was 0.72%. It was confirmed that even if the amount of the surface active agent is much reduced as compared with the amount used in Example 8, a toner having a sharp particle size distribution can be obtained. When a copying operation was carried out in an electrophotographic copying machine (Model DC-111C supplied by Mita Kogyo) by using the obtained toner, a sharp image having a good density in the solid color portion and a good resolution was obtained. Example 10 In the same manner as described in Example 8, the pH of the aqueous phase was adjusted to 9 and stirring was conducted at 1,100 rpm for 20 minutes to form suspended oil drops. At the polymerization, 2 parts by weight of HCℓ was added to adjust the pH value to 7, and polymerization was carried out at 80°C in an N 2 current. The polymerization was completed in a short time of 7 hours. The obtained polymerization product was separated into a precipitate and a supernatant, and the precipitate was treated with a dilute acid, washed with water and dried to obtain a toner. The supernatant was substantially transparent. When the measurement of the particle size distribution and the copying test were carried out in the same manner as described in Examlpe 8, it was found that the volume average particle size (D50) was 10.3 µm and the particle size distribution was so sharp that the D25:D75 ratio was 1.50:1, the residual monomer content in the toner was 0.75%, and an image having a good density in the solid color portion and a good resolution was obtained. Example 11 The procedures of Example 8 were repeated in the same manner except that at the precipitation of Ca 3 (PO 4 ) 3 , the pH was adjusted to 14 by using 83 parts by weight of 4N NaOH and stirring was conducted at 11,000 rpm for 20 minutes. Oil drops having an average particle size of 11.2 µm and such a sharp particle size distribution that the D25:D75 ratio was 1.33:1 were obtained. Then, 6 parts by weight of HC was added to the suspension to adjust the pH value to 9, and the polymerization was carried out at 80°C in an N 2 current for 12 hours to obtain a toner. The polymerization product was separated into a precipitate and a supernatant. The precipitate was treated with a dilute acid, washed with water and dried to obtain a toner. The supernatant was substantially transparent. When the measurement of the particle size distribution and the copying test were carried out in the same manner as described in Example 8, it was found that the volume average particle size (D50) was 11.1 µm, the particle size distribution was so sharp that the D25:D75 ratio was 1.33:1, the residual toner content in the toner was 0.71%, and the obtained image had a good density in the solid color portion and a satisfactory resolution. Example 12 In the same manner as described in Example 8, suspended oil drops were formed by adjusting the pH to 9, and at the polymerization, 18 parts by weight of 4N NaOH was further added to adjust the pH value to 14 and the polymerization was carried out at 80°C in an N 2 current for 12 hours. The obtained polymerization product was separated into a precipitate and a supernatant, and the precipitate was treated with a dilute acid, washed with water and dried to obtain a toner. The supernatant was transparent and formation of particles as the by-product was hardly observed. The dilute acid treatment and water washing treatment were greatly facilitated. When the measurement of the particle size distribution and the copying test were carried out in the same manner as described in Example 8, it was found that the volume average particle size (D50) was 10.3 µm, the particle size distribution was so sharp that the D25:D75 ratio was 1.51:1, the residual monomer content in the toner was 0.73%, and a sharp image having a good density of the solid color portion and a good resolution was obtained. Comparative Example 4 To 670 parts by weight of water was added 7 parts by weight of Ca 3 (PO 4 ) 2 , and without performing the operations of dissolving and precipitating Ca 3 (PO 4 ) 2 , 40 parts by weight of a 0.2% solution of NaDBS was added with stirring by a stirrer. The same oil phase as used in Example 8 was added to the aqueous phase and the mixture was stirred at 11,000 rpm for 25 minutes to obtain suspended oil drops having an average particle size of 10.3 µm. The suspension was subjected to polymerization at 80°C in an N 2 current for 12 hours. The polymerization product was separated into a precipitate and a supernatant, and the precipitate was treated with a dilute acid, washed with water and dried to obtain a toner. Formation of particles as the by-product was conspicuous in the supernatant, and the supernatant was opaque. When the average particle size and particle size distribution of the toner were measured, it was found that the volume average particle size (D50) was 10.1 µm but the particle size distribution was so broad that the D25:D75 ratio was 3.32:1. The obtained copied image was satisfactory in the density but the resolution was low, and defacement and blurring were caused in portions of letters and fine lines. With respect to each of the toners obtained in the Examples and Comparative Examples, a developer was prepared by adding 0.2 part by weight of hydrophobic silica to 100 parts by weight of the toner to form a toner composition and mixing the toner composition with a ferrite carrier having an average particle size of 90 µm so that the toner concentration was 4.0%. A printing resistance test was carried out under normal temperature and normal humidity conditions (20°C and 60% relative humidity) and high temperature and high humidity conditions (35°C and 85% relative humidity) by using an electrophotographic copying machine (Model DC-111C supplied by Mita Kogyo). The obtained results are shown in Table 2. Incidentally, D25, D50 and D75 in the Examples and Comparative Examples represent the particle sizes observed when the volume or weight ratios reach 25%, 50% and 75%, respectively, in cumulating the particle sizes from smaller particles. As is apparent from Table 2, it is seen that the toners obtained in the examples give sharp images having a high density and an excellent resolution without fogging, and the durability is improved. Since the toner of Example 11 has a very sharp particle size distribution, the resolution is especially highly improved. Furthermore, it is seen that the toner of Example 12 shows a very good printing resistance at the printing resistance test because incorporation of by-product particles is much controlled. Example 13 Aqueous Phase To 440 parts of water was added 7 parts by weight of tricalcium phosphate, and while sufficiently dispersing the mixture by a stirrer, 23 parts by weight of 12N hydrochloric acid was added to dissolve the tricalcium phosphate, and 17 parts by weight of 2% sodium dodecyl benzene was added. The foregoing components were sufficiently mixed with stirring to form a polymerizable composition and added to the aqueous phase. While stirring the mixture at 11,000 rpm by a homomixer (supplied by Tokushu Kika), 83 parts by weight of 4N NaOH was added to adjust the pH in the aqueous phase above 11 and precipitate the dissolved tricalcium phosphate, and the dispersion was further stirred for 20 minutes to form suspended oil drops having an average particle size of 10.5 µm. The obtained suspension was transferred into a separable flask, and the polymerization was carried out at 80°C in an N 2 current. When two hours had passed from the start of the polymerization, 6 parts by weight of 12N hydrochloric acid was added to the polymerization mixture at a rate of 0.001 g/cc·min by a microfeeder to adjust the pH of the aqueous phase to 7. Then, the polymerization reaction was conducted for 7 hours to complete the polymerization. The polymerization product was subjected to solid-liquid separation, and the solid was treated with a dilute acid, washed with water and dried to obtain a powdery toner. When the particle size distribution of the obtained toner was measured by a Coulter counter, it was found that the volume average particle size (D50) was 10.4 µm, the dispersion degree (D25/D75) of the particle size was 1.4 and the particle size distribution was very sharp. When the molecular weight of the toner was measured by GPC, it was found that the weight average molecular weight (Mw) was 160,000 and the dispersion degree (Mw/Mn) was 16. Accordingly, it was confirmed that the molecular weight expanded from the low-molecular weight refion to the high-molecular-weight region (see Fig. 1). Then, 100 parts by weight of the toner was mixed with 0.2 part weight of hydrophobic silica to form a toner composition, and a developer was prepared by mixing this toner composition with a ferrite carrier having an average particle size of 90 µm so that the toner concentration was 4.0%. Various tests were carried out in a remodelled machine of electrophotographic copying machine Model DC-1205 (supplied by Mita Kogyo). The obtained results are shown in Table 3. Example 14 In the same manner as described in Example 13, suspended oil drops having an average particle size of 10.5 µm were formed, and in the same manner as described in Example 13, the polymerization was started. When 3 hours had passed from the start of the polymerization, the pH was charged to 9 and then, the polymerization was conducted for 7.5 hours to complete the polymerization reaction. The volume average molecular weight (D50) of the obtained toner was 11.0 µm, and the dispersion degree (D25/D75) of the particle size was 1.41 and the toner showed a sharp particle size distribution. When the molecular weight distribution of the toner was measured by GPC, it was found that the weight average molecular weight (Mw) was 145,000 and the dispersion degree (Mw/Mn) of the molecular weight was 15, and it was confirmed that the molecular weight expanded from the low-molecular-weight region to the high-molecular-weight region. In the same manner as described in Example 13, a developer was prepared and the copying test was carried out. The obtained results are shown in Table 3. Comparative Example 5 A polymerizable composition prepared in the same manner as described in Example 13 except that 2.5 parts by weight of ADVN (half-value period at 70°C = 40 minutes) and 0.5 part by weight of ACHN ( half-value period at 90°C = 370 minutes) were added as the polymerization initiators to the same aqueous phase as used in Example 13, and 83 parts by weight of 4N-NaOH was added to precipitate tricalcium phosphate and the dispersion was stirred at 11,000 rpm to form suspended oil drops having an average particle size of 11.2 µm. The obtained suspension was transferred into a separable flask and the polymerization was carried out at 80°C in an N 2 current for 15 hours. Since the half-value period of the second initiator was long, a long time was necessary for completion of the polymerization and coalescence of particles was conspicuous. The obtained polymerization product was subjected to soil-liquid separation, and the solid was treated with a dilute acid, washed with water and sufficiently dried to obtain a toner. The volume average molecular weight (D50) of the obtained toner was 12.3 µm and the dispersion degree (D25/D75) of the particle size was 1.78, and it was confirmed that the toner had a broad particle size distribution. When the molecular weight distribution of the toner was measured by GPC, it was found that the weight average molecular weight (Mw) was 116,000 and the dispersion degree (Mw/Mn) of the molecular weight was 6. In the same manner as described in Example 13, a developer was prepared and the copying test was carried out (the molecular weight distribution curve is shown in Fig. 2). The obtained results are shown in Table 3. The fixing property referred to in Table 3 was determined in the following manner. The temperature of a heating roller of a remodelled machine of Model DC-1205 of the heating and pressing roll fixation type was elevated stepwise by 2.5°C from 100°C, and a transfer paper having a toner image formed thereon was passed through the heating roller to effect fixation. An adhesive tape was pressed to the fixed image and then, the adhesive type was peeled. The density of the fixed image was measured before and after the peeling by a reflection densitometer (supplied by Tokyo Denshoku), and the temperature at which the fixing ratio represented by the following formula: Fixing ratio (%) = image density after peeling image density before peeling x 100 was at least 90% was determined, and the lowest fixing temperature and the high-temperature offset-causing temperature were determined. The blocking resistance was evaluated by observing the state of the toner during the copying test or in a developing device after the copying test. Example 15 Aqueous Phase To 440 parts of water was added 7 parts by weight of tricalcium phosphate, and while sufficiently dispersing the mixture by a stirrer, 23 parts by weight of 12N hydrochloric acid was added to dissolve the tricalcium phosphate, and 17 parts by weight of 2% sodium dodecyl benzene-sulfonate was added to prepare an aqueous phase. Styrene 82 parts by weight 2-Ethylhexyl methacrylate 18 parts by weight Grafted carbon black 5 parts by weight Spilon Black TRH8 (charge-controlling agent) 1 part by weight Styrene/sodium styrene-sulfonate copolymer 2.5 parts by weight Divinylbenzene 0.2 part by weight 2EG 1.5 parts by weight ADVN (half-value period at 70°C = 40 minutes) 2.5 parts by weight AIBN (half-value period at 70°C = 300 minutes) 0.32 part by weight The above-mentioned oil phase-forming materials were sufficiently mixed with stirring, and the formed oil phase was added to the aqueous phase, and while stirring the mixture by a TK homomixer supplied by Tokusho Kika at 11,000 rpm, 83 parts by weight of 4N NaOH was added to adjust the pH above 11 and precipitate the dissolved tricalcium phosphate. The dispersion was stirred for 20 minutes to form suspended oil drops having an average particle size of 11.2 µm. Then, the obtained suspension was transferred into a separable flask and the polymerization was initiated at 80°C in an N 2 current. After 5 hours, 6 parts by weight of 12N hydrochloric acid was added to the polymerization mixture at a rate of 0.001 g/cc·min by a microfeeder to adjust the pH to 7. Then, the polymerization was further conducted for 3 hours to complete the polymerization. The polymerization product was subjected to solid-liquid separation. The solid was treated with a dilute acid, washed with water and dried to obtain a toner. Incidentally, when 5 hours had passed from the start of the polymerization, the polymerization ratio was 68%. When the particle size of the obtained toner was measured by a Coulter counter, it was found that the volume average particle size D50 was 11.2 µm and the dispersion degree D25/D75 was 1.41, and the toner had a very sharp particle size distribution. When the molecular weight was measured by GPC, as shown in Fig. 3, the weight average molecular weight (Mw) was 145,000 and the dispersion degree (Mw/Mn) was 15. The flowability of the toner was very good. In an oven maintained at 60°C, 20 g of the toner was charged in a glass cylinder having an inner diameter of 26.5 mm, and 100g of a weight was placed on the toner and the toner was allowed to stand still for 30 minutes in this state. Then, the cylinder was extracted and it was checked whether or not the toner collapsed. At this heat resistance test, when the cylinder was extracted, the toner of the present example simultaneously collapsed. Accordingly, it was confirmed that blocking was not caused and the toner had an excellent heat resistance. Then, 100 parts by weight of the obtained toner was mixed with 0.2 part by weight of hydrophobic silica to form a toner composition, and the toner composition was mixed with a ferrite carrier having an average particle size of 90 µm so that the toner concentration was 4.0%. When the copying test was carried out in a remodelled machine of an electrophotographic copying apparatus (Model DC-1205 supplied by Mita Kogyo) by using the obtained toner, a sharp image having a high density in the solid color portion and a resolution of 7 lines/mm was obtained without fogging. A similar image was obtained under high temperature and high humidity conditions (35°C and relative humidity of 85%). When the fixing property was examined, it was found that the lowest fixing temperature was 120°C and the high-temperature offset-causing temperature was 180°C, and it was confirmed that the fixing-possible temperature range was broadened. Example 16 In the same manner as described in Example 15, suspended oil drops having an average particle size of 10.5 µm were formed and the polymerization reaction was initiated. The change of the pH was carried out after 3 hours from the start of the polymerization reaction, and the polymerization was further conducted for 5 hours at a pH of 7. The polymerization ratio after 3 hours from the start of the polymerization was 50%. The volume average particle size (D50) of the obtained toner was 10.5 µm and the dispersion degree (D25/D75) of the particle size was 1.43, and the particle size distribution was very sharp. When the molecular weight distribution was determined by GPC, a distribution curve as shown in Fig. 4 was obtained, and it was found that the weight average molecular weight (Mw) was 155,000 and the dispersion degree (Mw/Mn) of the molecular weight was 17. The flowability of the toner was very good, and at the heat resistance test, blocking was not caused at 60°C. When the copying test was carried out in the same manner as described in Example 15, the density of the solid color portion was sufficient, fogging was not caused and the resolution was 7 lines/mm, and the obtained image was very sharp. At the fixing test, the lowest fixing temperature was 125°C and the high-temperature offset-causing temperature was 200°C, and the fixing-possible temperature range was very broad. When the toner of the present Example was compared with a comparative toner having substantially the same molecular weight distribution curve by GPC, it was found that the toner of the present example showed a very high heat resistance at the heat resistance test. Accordingly, it was confirmed that even if the profile of the molecular weight distribution by GPC is the same, the heat resistance is highly improved in the toner of the present invention where the high-molecular-weight component is convergently present in the surface portion of the particles. Comparative Example 6 Suspended oil drops having an average particle size of 11.3 µm were formed by suspending and dispersing the same aqueous phase and oil phase as used in Example 15. Then, the polymerization was carried out at 80°C in an N 2 current for 50 minutes, and 6 parts by weight of 12N hydrochloric acid was added at a rate of 0.001 g/cc min by a microfeeder to adjust the pH of the aqueous phase to 7. Then, the polymerization reaction was further conducted for 7 hours. The polymerization product was subjected to solid-liquid separation, and the solid was treated with a dilute acid, washed with water and dried to obtain a toner. The polymerization ratio after 50 minutes from the start of the polymerization was 20%. When the particle size distribution of the toner was measured, It was found that the volume average particle size (D50) was 11.4 µm and the dispersion degree was 1.44. When the molecular weight characteristics were determined by GPC, it was found that the weight average molecular weight (Mw) was 250,000 and the dispersion degree (Mw/Mn) was 5. The obtained toner was subjected to the heat resistance test, fixing test and copying test in the same manner as described in Example 15. It was found that the heat resistance was good and blocking was not caused. However, at the fixing test, the lowest fixing temperature was 140°C and the high-temperature offset-causing temperature was 190°C, and the fixing-possible temperature was narrow and the fixing property at low temperatures was poor. The image characteristics were tolerably good. Comparative Example 7 In the same manner as described in Example 15, suspended oil drops having an average particle size of 11.2 µm were formed by using the same aqueous phase and oil phase as used in Example 15. Then, the polymerization reaction was carried out at 80°C in an N 2 current for 12 hours. When the particle size distribution of the toner was measured, it was found that the volume average particle size (D50) was 11.3 µm and the dispersion degree of the particle size was 1.43. When the molecular weight characteristics were determined, it was found that the weight average molecular weight (Mw) was 100,000. The obtained toner was subjected to the heat resistance test, fixing test and copying test in the same manner as described in Example 15. At the heat resistance test, blocking was conspicuous and the toner was coagulated. At the fixing test, the lowest fixing temperature was 120°C and the high-temperature offset-causing temperature was 150°C, and the fixing-possible temperature range was narrow and the offset resistance was poor. The image characteristics were tolerably good.

Claims: A process for preparing polymer particles, which comprises: (1) dissolving a hardly water-soluble inorganic dispersant which has a solubility which changes according to the pH of the dispersion medium, in an aqueous dispersion medium, by appropriate adjustment of the pH, (2) adding a radical-polymerizable monomer and a radical polymerization initiator to the aqueous dispersion medium and suspending and dispersing said monomer and initiator as oil drops in said dispersion medium, (3) adding an acid or alkali to the dispersion medium with stirring to precipitate the dispersant, and (4) carrying out polymerization. A process according to claim 1 for preparing toner particles wherein, in step (2), a colorant is also added to the dispersion medium. A process according to claim 2 wherein, in step (2), a magnetic material is added together with the colorant. A process according to any one of the preceding claims wherein the inorganic dispersant is calcium sulfate, tricalcium phosphate, magnesium carbonate, barium carbonate, calcium carbonate or aluminum hydroxide. A process according to claim 1 or 2, wherein the inorganic dispersant is tricalcium phosphate and polymerization is carried out when the aqueous medium has a pH of 7 to 8. A process according to any one of the preceding claims wherein the polymerizable monomer is at least one selected from monovinyl aromatic monomers, acrylic monomers, vinyl ester monomers, vinyl ether monomers, diolefin monomers and mono-olefin monomers. A process according to any one of the preceding claims wherein, in step (2), a surface active agent is also incorporated into the aqueous medium. A process according to claim 7 wherein the surface active agent is a fatty acid type surface active agent or alkyl sulfonate type surface active agent.

Inventors: HIRANO NOBUHIRO, OTA HIDEKI, SHIMOYAMA HIROSHI, TOKUNO TOSHIRO, WADA TOMONORI, HIRANO, NOBUHIRO, OTA, HIDEKI, SHIMOYAMA, HIROSHI, TOKUNO, TOSHIRO, WADA, TOMONORI
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: C08F2
IPC codes: C08F2, G03G9