Toner for developing electrostatic images and a method of manufacturing the same

Abstract: Spheroidal toner for developing electrostatic images is composed of a binding resin as its main component in which at least carbon black and a dispersion stabilizer for the carbon black are contained, the conductivity of the toner being in the range of 5 × 10⁻¹¹ to 5 × 10⁻⁹ S/cm and is manufactured by the steps of: forming grafted carbon black by prepolymerization of polymerizable composition containing carbon black and a monomer forming a binding resin; adding a carbon black dispersion stabilizer to the prepolymerized composition; and suspending the composition in an aqueous medium and polymerizing the suspended particles in the presence of a polymerization initiator.

Description: The present invention relates to toner particles for use in an image forming apparatus such as an electrophotographic graphic copying machine or the like and to a method of manufacturing the same said toner being capable of retaining excellent electrostatic charge characteristics despite changes in temperature, humidity and other environmental conditions. In an image forming apparatus such as an electrophotographic copying machine or the like, a two-component developer consisting of particulate toner and magnetic carrier, or other frictionally chargeable materials, is usually used as the developer to make visible by dry development an electrostatic latent image formed on a photoconductor. A typical method of manufacturing the particulate toner used in the two-component developer is as follows: First, a binding resin which is the main component of the toner, and additives such as colorants for conferring desired properties on the toner are melted and mixed together, thereby uniformly dispersing the toner additives. The mixture is then ground by a mill and classified by a classifier to select toner having a prescribed particle size. Since the particle shape is irregular, the toner manufactured by such method generally has a low flowability and is therefore susceptible to blocking. Usually, carbon black is added to black toner. When toner containing carbon black is manufactured by the above grinding method, since the obtained toner particles have irregular shapes, carbon black often crops out at the fractured surfaces of the toner particles. This causes the resistance of the toner to be partially reduced since carbon black is a conductor. As a result, the charge amount of the toner charged through friction with the carrier is varied resulting in a variation in image density, fogging of the image, and splashing of toner. To solve the problem with the above grinding method for the manufacturing toner, there has been proposed a method known as the polymerization method wherein a polymerization reaction is performed with carbon black dispersed in a monomer forming the binding resin. With this polymerization method, however, the carbon black tends to flocculate in the monomer, and toner with carbon black uniformly dispersed in the binding resin cannot be obtained by polymerization alone. If the carbon black is not dispersed uniformly, the electrostatic charge characteristics of the toner tends to wander, causing such problems as splashing of toner and fogging of images. JP-A-56 116 044 disclose a method to produce toner particles which comprises the steps of polymerizing a monomer in the presence of grafted carbon black (of ≤ 10% rate of polymerization). Also EP-A-289624 discloses such a method but the rate of polymerization for the grafted carbon black is not mentioned. The present invention, which overcomes the above-described problems, makes possible the objects of providing toner having carbon black uniformly dispersed and therefore having stable electrostatic charge characteristics and being capable of producing high-density images without causing splashing of toner and fogging of images, and also providing a method for the manufacturing the same. According to the present invention, there is provided spheroidal toner particles for developing electrostatic images comprising a binding resin as a main component, carbon black dispersed in the binding resin, and a dispersion stabilzer for the carbon black, wherein the carbon black is contained in the form of a grafted carbon black with a graft rate of 20 to 200%, and wherein the dispersion stabilizer is selected from oil-soluble dyes and coupling agents, the conductivity of the toner being in the range of 5 x 10⁻¹¹ to 5 x 10⁻⁹ S/cm. In a preferred embodiment, the conductivity of the toner is in the range of 4 x 10⁻¹⁰ to 5 x 10⁻⁹ S/cm. It is preferred that the toner be in the range of 0.95 to 1.0 according to Wadel's practical spheroidicity. According to a second aspect of the present invention, there is provided a method of manufacturing toner particles for developing electrostatic images containing a binding resin, carbon black dispersed in the binding resin, and a dispersion stabilizer for the carbon black being selected from oil-soluble dyes and coupling agents, the method including the steps of: forming a grafted carbon black with a graft rate of 20 to 200% by prepolymerization of a polymerizable composition containing carbon black and a monomer forming a binding resin; adding said dispersion stabilizer for the carbon black to the prepolymerized composition containing grafted carbon black; suspending the composition containing the grafted carbon black, the dispersion stabilizer and the monomer in an aqueous medium; and polymerizing the suspended composition in the presence of a polymerization initiator. In a preferred embodiment, the mixing percentage of the carbon black is within the range of 2 to 10 percent by weight on the basis of the weight of the monomer. Thus, the invention herein stated enables one or more of the following objects to be met, namely (1) To provide spheroidal toner having a preferred conductivity and excellent electrostatic charge characteristics; (2) To provide toner capable of stably retaining excellent electrostatic charge characteristics despite changes in temperature, humidity and other environmental conditions, and capable of producing good images; (3) To provide toner free from such problems as fogging and splashing; and (4) To provide a method of manufacturing the toner having the above-described excellent characteristics. The inventors have found a preferred conductivity for spheroidal toner that has excellent electrostatic charge characteristics. The toner of the present invention has carbon black evenly and uniformly dispersed in a binding resin, the conductivity of the toner being set within a specific range so as to stably maintain the charge amount of the toner. Since the spheroidal toner whose conductivity is set within specific range shows a very good flowability, electric charges generated through friction are suitably moderated to provide stable electrostatic charge characteristics. Carbon black generally has polar groups such as hydroxyl groups, carboxyl groups, etc. on its surface and therefore has a poor affinity for monomers forming the binding resin which is lipophilic. This tends to cause the carbon black particles to agglomerate In the present invention, carbon black is prepolymerized in a monomer to form grafted carbon black so as to enable the carbon black to exist in the monomer in a stable condition. The grafted carbon black stays dispersed in the form of fine particles in suspension particles while the polymerization reaction is completed. When carbon black is present during prepolymerisation effected so as to achieve a graft rate (the weight percentage of polymer bound to carbon black with respect to the weight of carbon black) of preferably 20 to 200%, grafted carbon black having good dispersibility in monomers can be obtained. When the graft ratio is lower than 20%, sufficient dispersion effects cannot be guaranteed. On the other hand, when the graft ratio is higher than 200%, the graft polymers of the grafted carbon black are likely to crosslink with each other or to gel, thus deteriorating the dispersibility of the grafted carbon black. According to the present invention, a dispersion stabilizer which may be an oil-soluble dye such as Oil Black (CI 26150) or Nigrosine Base (CI 5045), or a coupling agent is added to the prepolymer containing carbon black grafted as the above specified, thereby helping to further enhance the retention of carbon black during the polymerization reaction and in the resulting toner and thus making it possible to obtain toner having the previously mentioned preferred conductivity with good reproducibility. Also, any known carbon black can be used as the carbon black to be used for the present invention, but commercially available carbon black contains agglomerates having a size ranging from 1 µm to several hundreds pm. Therefore, in the present invention, it is desirable that carbon black be predispersed in a monomer using an ultrasonic disperser, ball mill, homomixer, etc. before grafting. Since the contact areas of the carbon black with the monomer are increased as a result of the predispersion, efficient grafting occurs, thus further enhancing the dispersibility of the carbon black. It is desirable that carbon black be mixed in 2 to 10 percent by weight with respect to the monomer. If the mixing percentage is higher than 10 percent by weight, the conductivity of the obtained toner tends to become higher than 5 x 10⁻⁹ S/cm and the charge amount of the toner therefore tends to drop, leading to splashing of toner and fogging of images. On the contrary, if the percentage is lower than 2 percent by weight, it becomes difficult to adjust the toner conductivity, with carbon black uniformly dispersed therein, to the specified range, which causes the charge amount of the toner to increase and thus the image density to decrease. The above grafting can be performed by thermal polymerization. For efficient graft polymerization, it is desirable to use an azoic polymerization initiator. As the azoic polymerization initiator, any azoic initiator can be used, including 2,2′-azobis-(2,4′-dimethylvaleronitrile), 2,2′-azobis-isobutyronitrile, etc., and the initiator is used in 0.01 to 2 percent by weight with respect to the polymerizable monomer. The monomers used for the present invention should be capable of radical polymerization and should form polymers having fixing and detecting properties required for the toner. Such monomers include vinyl aromatic monomers, acrylic monomers, vinyl ester monomers, vinyl ether monomers, diolefin monomers, mono-olefin monomers, etc. The vinyl aromatic monomers used are expressed by the following general formula (1). In the formula, R₁ represents a hydrogen atom, a lower alkyl group or a halogen atom, and R₂ denotes a hydrogen atom, a lower alkyl group, a halogen atom, an alkoxy group, a nitro group or a vinyl group. Specifically, such monomers include styrene, α-methylstyrene, vinyltoluene, α-chlorostyrene, o-,m-, p-chlorostyrene, p-methylstyrene, and divinylbenzene. The acrylic monomers used are expressed by the following general formula (2). In the formula, R₃ represents a hydrogen atom or a lower alkyl group, and R₄ denotes a hydrogen atom, a hydrocarbon radical having 1 to 12 carbon atoms, a hydroxyalkyl group or a vinyl ester group. Specifically such monomers include 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-hydroxyacrylate, ethylene glycol dimethacrylate and tetraethylene glycol dimethacrylate. The vinyl ester monomers used are expressed by the following general formula (3). In the formula, R₅ represents a hydrogen atom or a lower alkyl group. Specifically, such monomers include vinyl formate, vinyl acetate, and vinyl propionate. The vinyl ether monomers used are expressed by the following general formula (4). In the formula, R₆ represents a hydrocarbon radical having 1 to 12 carbon atoms. Specifically, such monomers include vinyl-n-butylether, vinylphenylether and vinyl-cyclohexylether. The diolefin monomers used are expressed by the following general formula (5). In the formula, R₇, R₈, and R₉ respectively represent a hydrogen atom, a lower alkyl group or a halogen atom. Specifically, such monomers include butadiene, isoprene.and chloroprene. The monoolefin monomers used are expressed by the following general formula (6). In the formula, R₁₀ and R₁₁ respectively represent a hydrogen atom or a lower alkyl group. Specifically, such monomers include ethylene, propylene, isobutylene, butene-1, pentene-1 and 4-methylpentene. In the foregoing, lower alkyl preferably denotes alkyl containing 1 to 4 carbon atoms. Either one, or a combination of two or more kinds of the above-mentioned monomers can be used, but from the viewpoint of the fixing properties, it is desirable that at least one kind of monomer be selected which consists mainly of styrene, acrylic ester and methacrylic ester. After grafting the carbon black, a carbon black dispersion stabilizer is added to the prepolymer composition containing the grafted carbon black. For the carbon black dispersion stabilizer, one or more stabilizers selected from oil-soluble dyes such as Oil Black (CI 26150). Nigrosine Base (CI 5045) oil-soluble dyes, coupling agents such as aluminum, titan, etc., and surfactants are used, as previously mentioned. The dispersion stabilizer is preferably added in 0.1 to 10 percent by weight with respect to carbon black. The above mixture is then suspended in an aqueous medium. At this time, any other additives conferring desired properties on the toner may be added to the aqueous medium. Next any, further polymerizable monomer and other toner additives are added to the polymer containing the carbon black, to prepare a polymerizable composition for suspension polymerization. The additives used will generally include a polymerization initiator, a charge control agent for improving the charge characteristics of the toner, a mold release agent for conferring offset preventing effect on the toner, etc. The charge control agents used include Nigrosine Base (CI 5045), Oil Black (CI 26150), Spiron Black and other oil-soluble dyes; metal naphthenates; fatty acid metal soap; resin acid soap; and others. The mold release agents used include low molecular polyethylene, low molecular polypropylene various waxes, silicone oil. These additives should preferably be added in 0.1 to 10 parts by weight for every 100 parts by weight of the monomer. The above-described initiator includes azo compounds such as azobisisobutyronitrile and oil-soluble initiator such as cumene hydroperoxide, t-butylhydroperoxide, dicumylhydroperoxide, di-t-butylhydroperoxide, benzoyl peroxide, and lauroyl peroxide. The ratio of the monomer to water can be varied over a wide range but should generally be within the range of 1: 99 to 50: 50 by weight, and preferably within the range of 5: 95 to 30: 70. The mixing ratio of the polymerization initiator should be determined as considered appropriate as a catalyst. Generally, 0.1 to 10 percent by weight is desirable with respect to the monomer charged. The polymerization initiating temperature should generally be 40 to 100°C, and preferably 50 to 90°C, as in the case of conventional suspension polymerization. The polymerization time varies depending on the kind of monomer used, but should be selected between 2 to 20 hours according to whatever time is necessary to complete the polymerization. The resulting polymer after reaction is filtered to separate solids from liquid, and the thus separated polymer is washed and treated with dilute acid, etc. to obtain the toner of the present invention. By following the above manufacturing method, spheroidal toner having a conductivity of 5 x 10⁻⁹ to 5 x 10⁻¹¹ S/cm is obtained. The toner whose conductivity is within the above range is provided with excellent electrostatic charge characteristics. Spheroidal toner having a conductivity of 4 x 10⁻¹⁰ to 5 x 10⁻⁹ S/cm exhibits further enhanced charge characteristics. When the conductivity of the spheroidal toner is higher than 5 x 10⁻⁹ S/cm, splashing of toner and fogging of images will result because of insufficient charging of the toner. When the conductivity of the spheroidal toner is lower than 5 x 10⁻¹¹ S/cm, the image density will decrease because of a highly increased charge amount of the toner. It is further desirable that the toner be nearly spherical as in Wadel's practical spheroidicity of 0.95 to 1.0, and have a conductivity of 5 x 10⁻⁹ to 5 x 10⁻¹¹ S/cm, since the flowability of such toner is extremely stable and therefore the variation in the charge amount of toner is kept within an extremely narrow range. The following examples illustrate the present invention: Example 1 Grafting of carbon black: A composition consisting of 5 parts by weight of carbon black MA-100 (Brand name of Mitsubishi Kasei), 40 parts by weight of styrene, and 0.2 parts by weight of polymerization initiator ADVN was polymerized for 1.5 hours at 70°C for grafting. The polymerization rate was 25%, and the graft rate was 70%. Further, to the resultant compound was added 0.25 part by weight of a carbon black dispersion stabilizer "Oil Black HBB" (Brand name of Orient Chemical). Suspension polymerization To the above composition there were added 40 parts by weight of styrene, 0.7 parts by weight of divinylbenzene as a crosslinking agent, 1 part by weight of "Spiron Black TRH" as a charge control agent (Brand name of Orient Chemical), and 20 parts by weight of 2-ethylhexyl methacrylate, which were adequately mixed using a ball mill. To this mixture were added 5 parts by weight of polymerization initiator AIBN. The mixture was then put into an aqueous phase consisting of 400 parts by weight of water, 6 parts by weight of tribasic calcium phosphate, and 0.05 parts by weight of dodecyl-sodium benzensulfonic acid, the mixture being stirred for 10 minutes by a TK homomixer at 10000 rpm for dispersion and suspension in the aqueous phase. The thus prepared mixture was polymerized for 10 hours at 80°C until the reaction was completed. The resultant polymer was filtered, washed, and dried to obtain toner. The conductivity of the toner measured 7 x 10⁻¹⁰ S/cm. The toner was mixed with ferrite carrier with the toner density adjusted to 3%, to prepare a two-component developer. The thus prepared developer was subjected to copying tests on an electrophotographic copying machine DC-1205 (Model name of Mita Industrial Co., Ltd.). As a result of the tests, high-density, clear images were stably obtained without causing splashing of toner or fogging of images. Example 2 Toner was manufactured in the same manner as in Example 1, except that 0.25 part by weight of an aluminum coupling agent AL-M (Brand name of Ajinomoto) was used as the carbon black dispersion stabilizer. The conductivity of the resultant toner measured 6 x 10⁻¹⁰ S/cm. Using the toner, a developer was prepared in the same manner as in Example 1 and subjected to image producing tests. As a result of the tests, high-density, clear images were stably obtained without causing splashing of toner or fogging of images. Comparative Example 1 Toner was manufactured in the same manner as in Example 1, except that the carbon black dispersion stabilizer was not added. In the resultant toner, there was noted agglomeration of carbon black. The conductivity of the toner measured 6.2 × 10⁻⁹ S/cm, but since there were many toner particles not sufficiently charged, fogging was noted as a result of image producing tests. Comparative Example 2 Toner was manufactured in the same manner as in Example 1, except that the amount of carbon black was changed to 2 parts by weight. The conductivity of the resultant toner measured 4 x 10⁻¹¹ S/cm. Using the toner, a developer was prepared in the same manner as in Example 1 and subjected to image producing tests. As a result of the tests, there was not much splashing or fogging, but it was only possible to obtain low-density images.

Claims: Spheroidal toner particles for developing electrostatic images comprising a binding resin as a main component, carbon black dispersed in the binding resin, and a dispersion stabilizer for the carbon black, wherein the carbon black is contained in the form of a grafted carbon black with a graft rate of 20 to 200%, and wherein the dispersion stabilizer is selected from oil-soluble dyes and coupling agents, the conductivity of the toner being in the range of 5 x 10⁻¹¹ to 5 x 10⁻⁹ S/cm Toner particles for developing electrostatic images according to Claim 1, wherein the conductivity of the toner is in the range of 4 x 10⁻¹⁰ to 5 x 10⁻⁹ S/cm. Toner particles for developing electrostatic images according to claim 1 or 2, wherein according to Wadel's practical spheroidicity, the toner is in the range of 0.95 to 1.0. A method of manufacturing spheroidal toner particles for developing electrostatic images containing a binding resin, carbon black dispersed in the binding resin, and a dispersion stabilizer for the carbon black being selected from oil-soluble dyes and coupling agents, the method including the steps of: forming a grafted carbon black with a graft rate of 20 to 200% by prepolymerization of a polymerizable composition containing carbon black and a monomer forming a binding resin; adding said dispersion stabilizer for the carbon black to the prepolymerized composition containing grafted carbon black; suspending the composition containing the grafted carbon black, the dispersion stabilizer and the monomer in an aqueous medium; and polymerizing the suspended composition in the presence of a polymerization initiator. A method according to Claim 4, wherein the mixing percentage of the carbon black is within the range of 2 to 10 percent by weight on the basis of the weight of the monomer. A method as claimed in Claim 4 or 5, wherein the suspension polymerization is carried out in the presence of further binding resin-forming monomer. A method as claimed in any one of Claims 4 to 6, wherein the resin forming monomer is selected from styrene, an acrylic ester or a methacrylic ester.

Inventors: HIRANO NOBUHIRO, IMAFUKU TATSUO, OHTA HIDEKI, SHIMOYAMA HIROSHI, TOKUNO TOSHIRO, TSUJIHIRO MASAMI, HIRANO, NOBUHIRO, IMAFUKU, TATSUO, OHTA, HIDEKI, SHIMOYAMA, HIROSHI, TOKUNO, TOSHIRO, TSUJIHIRO, MASAMI
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G03G9
IPC codes: G03G9