The present invention relates to a toner and a process for producing the same that is used for developing an electrostatic latent image formed by an electrophotographic method and an electrostatic recording method, a developer for an electrostatic latent image, and a process for producing an image.
A method of visualizing image information via an electrostatic image, such as an electrophotographic method, is utilized in various field of art. In the electrophotographic method, an electrostatic static latent image is formed on a photoreceptor through charging and exposure steps, and the electrostatic latent image is developed with a developer containing a toner, which is then visualized through transfer and fixing steps.
As the developer used herein, a two-component developer containing a toner and a carrier and a one-component developer solely using a magnetic toner or a non-magnetic toner are known.
The production of the toners are generally conducted by a kneading and pulverization method, in which a thermoplastic resin is melted and kneaded with a pigment, a charge controlling agent and a releasing agent, such as a wax, and after cooling, it is finely pulverized and classified. In order to improve the fluidity and the cleaning property, inorganic or organic fine particles are sometimes added to the surface of the toner particles depending on necessity. While a toner of good quality can be produced by the process described, the following problems are involved.
The shape of the toner and the surface structure of the toner do not become constant by using the ordinary kneading and pulverization method, and they are delicately changed depending on the pulverization property of the material used and the pulverization conditions. Therefore, it is difficult to positively control the shape of the toner and the surface structure of the toner. Furthermore, there is restriction on the selection of the material in the kneading and pulverization method. Specifically, it is necessary that the dispersion body of the resin containing the coloring agent dispersed therein is sufficiently brittle and can be finely pulverized by an apparatus that can be practically employed from the standpoint of economy.
However, when the dispersion body of the resin and the coloring agent is brittle, there are cases where fine powder is formed or the shape of the toner is changed due to a mechanical sharing force in a developing device. As a result, the fine powder is fixed on the surface of the carrier to accelerate deterioration of the charging property of the developer in the two-component developer, and the particle size distribution is broadened to cause the toner-smoke and the developing property is lowered by the change of the toner shape to deteriorate the image quality in the one-component toner.
In the case of toner to which a large amount of a releasing agent such as wax is internally added, the releasing agent is often exposed on the surface of the toner depending on the combination with the thermoplastic resin, so as to influence the property of the toner. Particularly, in the case of a combination of a resin that is relatively difficult to be pulverized owing to a high elasticity due to a high molecular weight component and brittle wax such as polyethylene, exposure of the polyethylene on the toner surface is often observed. This phenomenon is advantageous for the releasing property on fixing and the cleaning of a non-transferred toner on the photoreceptor, but the polyethylene on the surface layer is easily transferred by a mechanical force to contaminate the developing roll, the photoreceptor and the carrier, which brings about reduction in reliability.
Furthermore, when the shape of the toner is irregular, the fluidity cannot be sufficiently ensured even though a fluidizing aid is added, and thus the fine particles on the surface of the toner migrate to concave parts of the toner by a mechanical sharing force upon use to lower the fluidity with the lapse of time, or the fluidizing aid is buried inside the toner to deteriorate the developing property, the transfer property and the cleaning property. Furthermore, in the case where a toner recovered by cleaning is reused by bringing back to the developing device, the image quality is liable to be lowered. When the amount of the fluidizing aid is increased to prevent the problems, other problems occur in that black spots are formed on the photoreceptor, and the particles of the aid are scattered.
In recent years, as a method for positively controlling the shape and the surface structure of the toner, a process for producing a toner by an emulsion polymerization and aggregation method has been proposed in JP-A-282752/1988 and JP-A-250439/1994. In the method, a resin fine particle dispersion is firstly prepared by emulsion polymerization, and separately, a coloring agent dispersion is prepared by dispersing a coloring agent in a solvent, followed by mixing and heating both the dispersion to produce a toner through fusing and coalescence. While the shape of the toner can be controlled in a certain extent to improve the charging property and the durability in this process, because the internal structure becomes substantially uniform, there are problems in the releasing property of a fixing sheet on fixing and in the stability of transparency on outputting on an OHP sheet.
As described in the foregoing, in the electrophotographic process, in order to stably maintain the performance of the toner even under various kinds of mechanical stress, it is necessary that the exposure of the releasing agent on the toner surface is controlled, and the surface hardness is increased without deteriorating the fixing property, so as to increase the mechanical strength of the toner itself and to sufficiently maintain both the charging property and the fixing property.
Owing to increase in demand for high image quality in recent years, there are considerable trends of reduction in particle diameter of the toner to realize an image of high definition particularly in the formation of a color image. However, when the particle diameter is simply decreased maintaining the conventional particle size distribution, problems of contamination of the carrier and the photoreceptor and the toner-smoke due to the presence of the toner as fine particles become outstanding, and it is difficult to simultaneously realize both the high image quality and the high reliability. In order to solve the problem, it becomes important that the particle diameter is decreased, and simultaneously the particle size distribution is sharpened.
In digital full-color duplicator and printer in recent years, a color image is subjected to color separation by filters of B (blue), R (red) and G (green), and a latent image formed with dots having a diameter of from 20 to 70 xcexcm corresponding to the original image is developed by using toners of Y (yellow), M (magenta), C (cyan) and Bk (black) with utilization of the subtractive color mixing. In comparison to an ordinary monochrome duplicator, it is necessary that a full-color duplicator transfers a large amount of a toner and handles a small dot diameter, and therefore the importance of the uniform charging property, the stability, the toner strength and the sharpness of the particle size distribution is increased. In order to meet the high-speed performance and the energy saving property of the apparatus, the further low temperature fixing property is demanded. Therefore, the aggregation and fusing/coalescence process which is suitable to produce such toner with sharpness of particle size distribution and with small particle diameter is preferred to impart excellent characteristics to the toner.
Because a large amount of toners is installed in a full color duplicator, it is necessary to sufficiently conduct color mixing thereof, and improvement in color reproducibility and transparency on OHP on color mixing becomes necessary.
As a component of the releasing agent, polyolefin series wax is generally used to prevent low temperature offset upon fixing, which is internally added to the toner. In addition to this, a slight amount of a silicone oil is uniformly coated on a fixing roller to improve the high temperature offset property. Therefore, there is a problem of causing uncomfortable sticky feeling due to attachment of the silicone oil to the transfer material.
JP-A-61239/1993 proposes a toner suitable for oilless fixing, in which no oil is coated on a fixing roll, by incorporating a large amount of a releasing agent component in the toner. However, when a large amount of releasing agent is added, the releasing property can be certainly improved, but the binder resin component and the releasing agent are dissolved in each other, and oozing of the releasing agent becomes non-uniform, so as to damage the stability of releasing. Furthermore, there are cases where a free component of the releasing agent causes charging inhibition.
As methods for solving the problems, a method of adding a high molecular weight component to increase the rigidity of the binder resin is proposed in JP-A-69666/1992 and JP-A-258481/1997, and a method of introducing crosslinking to the binder resin to supplement the rigidity is proposed in JP-A-218460/1984 and JP-A-218459/1984.
However, when a crosslinking component is added to the binder resin in the later case, because the rigidity of the binder resin is increased, the flexibility of a fixed image becomes poor. Particularly, when only the molecular weight of the crosslinking agent is increased in JP-A-218460/1984, the molecular weight between the crosslinking points is increased to slightly improve the flexibility of the fixed image itself, but because both the molecular weight and the crosslinking density of the binder resin are not simultaneously controlled, it is difficult that the releasing property, the gloss of the surface of the fixed image and the transparency on OHP cannot be simultaneously attained with maintaining the sufficient flexibility. In particular, a sufficient fixed image cannot be obtained when it is used in a fixing device of an energy saving type or a duplicator or a printer of a high printing speed disclosed in JP-A-221987/1998.
The invention has been made to solve the problems and to provide a toner for developing an electrostatic latent image, a process for producing the same, a developer for an electrostatic latent image, and a process for forming an image in that the adhesion property of a fixed image to a fixing sheet, the peeling property of a fixing sheet, the hot offset resisting property, the flexibility of a fixed image, the gloss of the surface of the fixed image and the transparency on OHP are excellent, the uniformity and the stability in charging are high, fogging and scattering of the toner are caused, and an image of high quality can be formed.
The inventors have been conducted earnest investigations mainly in the molecular weight between crosslinking points Mc and the crosslinking density Me, and as a result, it has been found that the problems are solved by the following configurations.
The present invention provides a toner for developing an electrostatic latent image containing a binder resin and a coloring agent, the toner having a molecular weight between crosslinking points Mc obtained by a measurement of temperature dispersion on dynamic viscoelasticity of from 3.6xc3x97106 to 7.5xc3x97108, and the molecular weight between crosslinking points Mc and a crosslinking density Me satisfying the relationship: 14.0xe2x89xa6Log10(Mc/Me)xe2x89xa616.5. The toner may have a glass transition temperature Tg of from 45 to 52xc2x0 C. The toner may have a weight average molecular weight in a range of from 25,000 to 55,000. The toner may have a volume average particle size distribution index GSDv of 1.30 or less, and a ratio of the volume average particle size distribution index GSDv and a number average particle size distribution index GSDp of 0.95 or more. The toner may have a shape factor SF1 in a range of from 100 to 140. The coloring agent contained in the toner may have a central diameter in a range of from 100 to 330 nm measured by a transmission electron microscope (TEM). An amount of the coloring agent in the toner may be in a range of from 4 to 15% by weight based on a total weight of the toner. The toner may further contains a releasing agent. An amount of the releasing agent in the toner may be in a range of from 5 to 25% by weight based on a total weight of the toner. The toner may have an absolute value of a charging amount of from 20 to 40 xcexcC/g.
The present invention also provides a process for producing a toner for developing an electrostatic latent image, the process containing a step of mixing a resin particle dispersion containing resin particles dispersed therein and a coloring agent dispersion containing a coloring agent dispersed therein to aggregate fine particles, and a step of fusing the aggregated particles by heating. The process may further contains, after the aggregationn step, a step of additionally mixing the resin particle dispersion. The step of aggregating the fine particles may contain mixing a releasing agent dispersion. A polymer of a metallic salt may be used in the step of aggregating the fine particles.
The toner may be used as a developer with a carrier particles.
The present invention also provides a process for producing an image containing a step of developing an electrostatic latent image formed on a latent image holding member with the developer, a step of transferring a toner image thus formed to a transfer material, and a step of fixing the toner image to the transfer material. The process may further contains, after transferring step, a step of recovering the toner remaining on the latent image holding member, and a recycle step of bringing the recovered toner to the developer. A cleaning step may not be conducted after the transferring step. The fixing step may be conducted by using a belt in the fixing step.