SPHERICAL TONER PARTICLE

Abstract: A toner composition is particles substantially in the spherical form and comprises a binder resin and carbon black, the surface area covered with the carbon black of the toner particle being 25 percent or smaller of the entire surface area of the toner particle.

Description: The present invention relates to a method for preparing spherical toner particles for developing an electrostatically charged image in electrophotography, electrostatic recording or electrostatic printing. Up to this time, an electrostatically charged image formed on a recording medium in electrophotography, electrostatic recording or electrostatic printing has been developed by two main methods, i.e., a wet developing method using a developer comprising a fine dispersion of various pigments or dyes in an insulating liquid or a dry developing method using a finely powdered developer which is a so-called toner and prepared by dispersing a coloring material in a natural or synthetic resin. Examples of the latter method include a cascade method, manual brushing, magnetic brushing, an impression method and a powder cloud method. The present invention relates to a method preparing a toner suitable for this dry developing method. Up to this time, toner particles for developing an electrostatically charged image have been prepared by dispersing a coloring material in a soft polymer by melting and kneading and grinding the obtained polymer containing the coloring material dispersed therein. However, the powder obtained by this process has a very wide particle size distribution, so that the powder must be classified prior to the practical use as a toner. Thus, the process itself is disadvantageous in complexity and cost. Further, the toner particles prepared by the above process involving a grinding step have edges and small cracks and are poor in fluidity. During stirring in a developing device, these edges and small cracks are broken to generate dust which causes lowering the quality of an image, or scumming, thus shortening the life of the developer. On the other hand, several polymerization processes for directly preparing a colored polymer particle not involving any grinding step have been proposed in, for example, JP-B-36-10231, 47-51830 and 51-14895 and JP-A-53-17735, 53-17736 and 53-17737. These processes comprise suspending an oil phase containing a monomer, a polymerisation initiator and a coloring material in an aqueous medium and polymerizing the obtained suspension to directly obtain toner particles and relate to a so-called suspension plymerization. These processes have the advantages that the obtained toner particles are spherical and are excellent in fluidity, whereby the preparation process itself is simple and the costs are low. However, the toner particles prepared by these processes have the disadvantages in that their properties are highly dependent from humidity and therefore, are poor in humidity resistance and electrostatic chargeability, whereby the electrostatic chargeability and the maintenance of a charge are insufficient even at ordinary temperature and humidity to give a low-quality image. FR-A-2356977 discloses a toner composition in form of substantially spherical particles comprising a binder resin and carbon black which is uniformly dispersed in the toner particles. The inventors of the present invention have studied the reason for the above mentioned disadvantages and have found that since carbon black which has been uniformly dispersed among monomers at the initiation of the suspension polymerization gathers near the surface of the toner particles during the polymerization, the surface resistance of the obtained toner is lowered, so that the electrostatic chargeability and charge stability of the toner are also lowered, of which the latter is particularly lowered at high humidity. The inventors of the present invention have carried out extensive investigations to overcome the above disadvantages and have found that these disadvantages can be overcome by employing spherical toner particles which have been prepared by a method comprising the steps of: dispersing carbon black, a polymerization initiator, a charge control agent, a hydrophobic dispersant and/or a thickening agent in an α, β unsaturated monomer producing an oily phase dispersion; adding said oily phase dispersion into water containing a suspension stabilizer producing a polymerization dispersion; and polymerizing said monomer from said polymerization dispersion to produce toner particles, whereby the ratio of the area of the surface of each toner particle covered with carbon black to the whole surface area of the toner particles is not greater than 25%. Preferably the ratio of the area of the surface of each toner particle covered with carbon black is not greater than 15%, more preferably is 3%. The hydrophobic dispersant includes, for example, an inorganic dispersant such as calcium silicate, silicon carbide, magnesium silicate, an organic dispersant such as an alkenyl succinic imide, polyethyleneimine or derivatives thereof. The thickening agent includes, for example, aluminum dialkyl phosphate, aluminum stearate, 12-hydroxy-stearic acid, dibenzylidene sorbitol. The hydrophobic dispersant and/or thickening agent serves to prevent the gathering of the carbon at the surface of the toner particle. The term "spherical toner particles" used in this specification does not refer only to the one of a genuine sphere but also to the one of a distorted sphere such as cocoon-like shape. That is to say, the spherical toner particles prepared according to the method of the present invention may have edges or undulations microscopically as far as it shows no edges on its surface macroscopically. The ratio of the area of the surface covered with carbon black of each toner particle to the whole surface area of the toner particles is determined as follows: Toner particles are added to an epoxy resin. The resulting resin is cut into thin films each having a thickness of several tens of µm (several hundreds of A °). The thin film is photographed with an electron microscope of the transmission type. The obtained photograph is analyzed for the state (dispersibility, agglomeration, number of particles and the like) of carbon black with an image analyser. The ratio of the area of the surface covered with carbon black of a toner to the whole surface area of the toner is calculated by the following equation: a b x 100 (%) wherein b is the full length of a boundary line between the toner and the space, i.e., a line forming the periphery of the toner, in the cross-sectional photograph of the toner and a is the length of the part of the above line covered with carbon black. The spherical toner particles prepared according to the method of the present invention can be prepared by suspension polymerization. An oily dispersion obtained by dispersing a polymerization initiator, a charge control agent, carbon black and a hydrophobic dispersant and/or a thickening agent in an α,β-unsaturated monomer is added to an aqueous medium obtained by homogeneously dissolving a water-soluble polymer or dispersing a suspension stabilizer such as an inorganic salt which is difficultly water-soluble in water. The resulting mixture is homogenized with a homomixer or homogenizer to form an oily disperse phase of 5 to 30 µm. The weight ratio of the oily phase to the aqueous phase is between 1 : 2 and 1 : 10 and is so selected as not to cause cohesion of particles during the polymerization. The homogeneous O/W dispersion thus prepared is transferred to a separable flask fitted with a stirrer, a condenser, a thermometer and a nitrogen gas inlet tube and heated to a temperature (50 to 90°C), at which the polymerization initiator can be decomposed, in a nitrogen atmosphere to carry out the polymerization. After the completion of the polymerization, the polymerization mixture is filtered to remove the aqueous phase. When inorganic powder adheres to the surface of a product, the product is treated with a dilute acid to remove the powder. The resulting product is washed with water and dried by spray drying, vacuum drying to obtain an objective toner. The α,β-unsaturated monomer to be used in the method of the present invention includes styrene, p-chlorostyrene, p-methylstyrene, vinyl acetate, vinyl propionate, vinyl benzoate, methyl acrylate, ethyl acrylate, n-butyl acrylate, iso-butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, n-octyl acrylate, methyl methacrylate, ethyl methacrylat n-butyl methacrylate, iso-butyl methacrylate, lauryl methacrylate, diethylaminoethyl methacrylate, t-butylaminomethyl mathacrylate, acrylonitrile, 2-vinylpyridine and 4-vinylpyridine. These monomers may be used alone or as a mixture of two or more of them. In the method of the present invention, a polyfunctional monomer may be used as crosslinking agent in addition to the above monomer to thereby further enhance the endurance of a toner. The amount of the polyfunctional monomer is from 0.05 to 20 % by weight, preferably 0.5 to 5 % by weight based on the monomer. The polymerization initiator to be used in the present invention is an oil-soluble peroxide or azo initiator. Examples thereof include benzoyl peroxide, lauroyl peroxide, 2,2'-azobisiso-butyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), o-chlorobenzoyl peroxide and o-methoxybenzoyl peroxide. The polymerization initiator is used in an amount of 0.1 to 10 % by weight, preferably 0.5 to 5 % by weight based on the monomer. Examples of the suspension stabilizer to be used in the method of the present invention include water-soluble polymers such as gelatin, starch, hydroxyethylcellulose, carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl alkyl ether, polyvinyl alcohol, inorganic salts with poor solubility in water, such as barium sulfate, calcium sulfate, barium carbonate, calcium carbonate, magnesium carbonate and calcium phosphate. The suspension stabilizer is used in an amount of 0.1 to 5 % by weight, preferably 0.5 to 2 % by weight based on the water. The toner particles prepared by the method according to the present invention may further contain a low-molecular weight olefin polymer which is known as a so-called parting agent with the purpose of the inhibition of offset and the improvement in fluidity and fixability. It is preferable that this low-molecular weight olefin polymer is present in the polymerization system together with a coloring material. Examples of the low-molecular weight olefin polymer to be used in the toner particles prepared by the method of the present invention include polyethylene, polypropylene, ethylene-vinyl acetate copolymer, chlorinated polyethylene wax, polyamide, polyester, polyurethane, polyvinyl butyral, butadiene rubbers, phenolic resins, epoxy resins, rosin-modified resins, silicone oil and silicone wax. The toner particles obtained by the method of the present invention preferably has a softening point of 106 to 160°C and a glass transition temperature of 50 to 80°C. If the softening point is lower than 106°C, no sufficient non-offset range will be attained, while if the point exceeds 160°C, the minimum fixing temperature will be too high and other unfavorable phenomena will occur. On the other hand, if the glass transition temperature is lower than 50°C, the resulting toner will be poor in storage stability, while if it exceeds 80°C, the fixability will be unfavorably lowered. Although the carbon black to be used in the method of present invention is not particularly limited and may be any commercially available one, it is preferable to use a hydrophobic carbon black having a low oil-absorbing power, because the use of such carbon black enables the easy preparation of the toner particles according to the method of the present invention. Carbon black is generally present in the toner particles as a secondary agglomerate rather than in a monodisperse state. According to the method of the present invention, carbon black must be dispersed in the toner particles in such a way that no carbon black is present on the surface of the toner particles or in such a way that the ratio of the area of the surface covered with carbon black of a toner to the whole surface area of the toner is not more than 25 %, even if carbon black is present on the surface thereof. The toner particles prepared by the method according to the present invention exhibit charging characteristics which are stable against any environmental change. For example, the charging characteristics are constant at ordinary temperature and ordinary humidity (25°C, 50%), at high temperature and high humidity (35°C, 85%) and at low temperature and low humidity (15°C, 35%). Since, further, the toner is excellent in fluidity and is not broken in service, no dust generates and therefore neither scumming nor lowering in the quality of the resulting image occurs. [Examples] The method of the present invention will be described in more detail by the following Examples, though it is not limited to them. In the Examples, all parts are by weight. Example 1 85 parts of styrene, 15 parts of 2-ethylhexyl acrylate (2EHA), 2 parts of a charge controller (TRH, a product of Hodogaya Chemical Co., Ltd.), 8 parts of carbon black (Printex 150T; a product of DEGUSSA), 0.5 part of aluminium stearate and 3 parts of polyethylene wax (a product of Mitsui Petrochemical Industries, Ltd. ; 210 P) were mixed to obtain a mixture. 500 parts of water and 1 part of polyvinyl alcohol were added to 100 parts of the mixture. The obtained mixture was homogenized by stirring at a high rate of 10,000 rpm with a homomixer (TK) to obtain a fine dispersion. This dispersion was transferred to a separable flask fitted with stirring blades to carry out the suspension polymerization at 60°C for 9 hours. The polymerization mixture was washed with hot cater of 50°C and dried to obtain a toner. 0.5 g of the toner was homogeneously dispersed in a liquid mixture comprising 9.3 ml of an epoxy resin (Epoc 812) , 4.0 ml of dodecenylsuccinic anhydride (DDSA), 6.7 ml of methyl nadic anhydride (MNA) and 0.3 ml of tri(dimethylaminomethyl)phenol (DMP-30). The obtained dispersion was allowed to stand at an ordinary temperature for 2 days. The obtained toner-containing epoxy resin was cut into thin films having a thickness of several tens of µm (several hundreds of A °) with a microtome (MT2-B). The thin film sample was subjected to electron microscopy with an electron microscope of transmission type. The obtained electron microscope photograph was analyzed with an image analyzer (LUZEX-500) for the disperse state of carbon black in the cross-section of the toner. 3 % of the whole surface area of the obtained toner particle was covered with carbon black. A developer was prepared by the use of the toner and a commercially available ferrite carrier having a particle size distribution of 0.104/0.061 mm (150/250 mesh) at a toner/carrier ratio of 4/96 and applied to a duplicating machine (Ricoh FT 4060). The obtained image was evaluated. A clear image free from fogging and scumming was obtained under any environmental condition among those of low temperature and low humidity (15°C, 30%), ordinary temperature and ordinary humidity (25°C, 50%) and high temperature and high humidity (35°C, 85%). Further, the printing using the above developer was repeated at an ordinary temperature and an ordinary humidity ten thousand times. Good images were obtained until the last without any change in the quantity of charge. Example 2 85 parts of styrene, 15 parts of 2EHA, 2 parts of a charge control agent (a product of Hodogaya Chemical Co., Ltd.; TRH), 8 parts of carbon black (a product of DEGUSSA; Printex 150T), 0.5 part of silicon carbide and 3 parts of polyethylene wax (a product of Mitsui Petrochemical Industries, Ltd.; 210p) were mixed to obtain a mixture. 500 parts of water and 1 part of polyvinyl alcohol were added to 100 parts of the mixture. The obtained mixture was homogenized by stirring at a high rate of 10,000 rpm with a homomixer (TK) to obtain a fine dispersion. This dispersion was transferred to a separable flask fitted with stirring blades to carry out the suspension polymerization at 60°C for 9 hours. The polymerization mixture was washed with hot water of 50°C and dried to obtain an objective toner. 0.5 g of the toner was homogeneously dispersed in a liquid mixture comprising 9.3 ml of an epoxy resin (Epoc 812), 4.0 ml or DDSA, 6.7 ml of MNA and 0.3 ml of DMP-30. The obtained dispersion was allowed to stand at an ordinary temperature for two days. The obtained toner-containing epoxy resin was cut into thin films having a thickness of several tens of µm (several hundreds of A °) with a microtome; (MT2-B). This thin film sample was subjected to electron microscopy with an electron microscope of transmission type. The obtained electron microscope photograph was analyzed with an image analyzer; (LUZEX-500) for the disperse state of carbon black in the cross-section of the toner particles. 10 % of the whole surface area of the obtained toner particles were covered with carbon black. A developer was prepared by the use of the toner particles and a commercially available ferrite carrier having a particle size distribution of 0.104/0.061 mm (150/250 mesh) at a toner/carrier ratio of 4/96 and applied to a duplicating machine (Ricoh FT4060). The obtained image was evaluated. A clear image free from fogging and scumming was obtained under any environmental condition among those of low temperature and low humidity (15°C, 30%), ordinary temperature and ordinary humidity (25°C, 50%) and high temperature and high humidity (35°C, 85%). The printing using the above developer was repeated at an ordinary temperature and an ordinary humidity ten thousand times. Good images were obtained until the last without any change in the quantity of charge. Comparative Example 1 85 parts of styrene, 15 parts of 2EHA, 2 parts of a charge controller (a product of Hodogaya Chemical Co., Ltd.; TRH), 8 parts of carbon black (a product of Mitsubishi Chemical Industries, Ltd.; #44) and 2 parts of polyethylene wax (Mitsui Petrochemical Industries, Ltd.; 210P) were mixed to obtain a mixture. 500 parts of water and 1 part of polyvinyl alcohol were added to 100 parts of the mixture. The obtained mixture was homogenized by stirring at a high rate of 10,000 rpm with a homomixer (a product of Tokushu Kakoki Co., Ltd.; TK) to obtain a fine dispersion. This dispersion was transferred to a separable flask fitted with stirring blades to carry out the suspension polymerization at 60°C for 9 hours. The polymerization mixture was washed with hot water of 50°C and dried to obtain a control toner. 0.5 g of the toner was homogeneously dispersed in a liquid mixture comprising 9.3 ml of an epoxy resin (Epoc 812) 4.0 ml of DDSA, 6.7 ml of MNA and 0.3 ml of DMP-30. The obtained dispersion was allowed to stand at an ordinary temperature for two days. The obtained toner-containing epoxy resin was cut into thin films having a thickness of several tens of µm (several hundreds of A °) with a microtome; (MT2-B). This thin film sample was subjected to electron microscopy with an electron microscope of transmission type. The obtained electron microscope photograph was analyzed with an image analyzer (LUZEX-500) for the disperse state of carbon black in the cross-section of the toner particles. 35 % of the whole surface area of the obtained toner particles were covered with carbon black. A developer was prepared by the use of the toner and a commercially available ferrite carrier having a particle size distribution of 150/250 mesh at a toner/carrier ratio of 4/96 and applied to a duplicating machine (Ricoh FT 4060). The obtained image was evaluated. Under the condition of high temperature and high humidity, the density of the image was lowered to give a very uneven and obscure image.

Claims: A method of preparing spherical toner particles comprising the steps of: dispersing carbon black, a polymerization initiator, a charge control agent, a hydrophobic dispersant and/or a thickening agent in an α,β-unsaturated monomer producing an oily phase dispersion; adding said oily phase dispersion into water containing a suspension stabilizer producing a polymerization dispersion; and polymerizing said monomer from said polymerization dispersion to produce toner particles, whereby the ratio of the area of the surface of each toner particle covered with carbon black to the whole surface area of the toner particles is not greater than 25%. A method as claimed in claim 1, whereby the ratio of the area of the surface of each toner particle covered with carbon black to the whole surface area of the toner particles is not greater than 15%. A method as claimed in claim 1, whereby the ratio of the area of the surface of each toner particle covered with carbon black to the whole surface area of the toner particles is 3%. A method as claimed in any of the claims 1 to 3, wherein the hydrophobic dispersant is calcium silicate, magnesium silicate, silicon carbide, alkenyl succinic imide, polyethyleneimine or derivatives thereof. A method as claimed in any of the claims 1 to 4, wherein the thickening agent is aluminum dialkyl phosphate, aluminum stearate, 12-hydroxy-stearic acid, or dibenzylidene sorbitol. A method as claimed in any of the claims 1 to 5, wherein the suspension stabilizer is a difficultly water-soluble inorganic salt or a water-soluble polymer. A method as claimed in any of the claims 1 to 6, wherein the α,β-unsaturated monomer is a styrene derivative, a vinyl ester, an acrylate, a methacrylate, acrylonitrile or a vinylpyridine. A method as claimed in any of the claims 1 to 7, wherein a polyfunctional monomer is used as crosslinking agent in an amount of 0.05 to 20 % by weight, based on the monomer, in addition to the unsaturated monomer. A method as claimed in any of the claims 1 to 8, wherein the polymerization initiator is an oil-soluble peroxide or azo-compound. A method as claimed in any of the claims 1 to 9, wherein the polymerizable mixture contains a low-molecular weight olefin polymer as a parting agent.

Inventors: SHIRASAKI, YOSHITSUGU, TORIMOTO, YOSHIAKI
Applicants: KAO CORPORATION
Application year: 1987
Main classification: G03G9
IPC codes: G03G9