1. Field of the Invention
The present invention relates to a method for producing an electrostatic image developing toner which is useful in used electrophotographic system copying machines and printers. More particularly, the invention relates to a method for producing an electrostatic image developing toner, which comprises conducting particle size control in an agglomeration step to prepare toner particles having a sharp particle size distribution with a lower content of fine powder and coarse grains.
2. Discussion of Background
A conventional electrostatic image developing toner which has been widely used in the field of electrophotography is produced by melt-kneading a mixture comprising a coloring agent such as carbon black or a pigment, wax and/or a magnetic material with a resin (polymer) by an extruder, followed by pulverization and classification. However, by using this method, it is difficult to disperse the coloring agent and the compounding agent in the resin.
On the other hand, in recent years there has been a need for printers and copying machines having improved performance with respect to high picture quality and a high printing speed. To achieve a high picture quality, the toner is required to consist of small particle sizes ranging from 3 to 9 xcexcm and a narrow particle size distribution. Further, to achieve a high printing speed, the toner is required to have a high fixation rate, i.e. it should be able to be fixed at a low temperature, as a toner physical property. Such are also favorable from the viewpoint of providing an environmental printer or copying machine which shortens the standby time, which does not generate much heat, and which has a small electric power consumption.
In order to achieve fixation of the toner at a low temperature, the softening point of the resin should be lowered. However, the conventional toner obtained by the melt-kneading/pulverization method has an antinomic relation such that when the softening point of the resin is lowered, the fixation property at low temperature will be improved, but at the same time, the Tg of the toner tends to be low, and the blocking resistance tends to be impaired. Namely, it is impossible to control the dispersion of the materials, and the structure control which satisfies both the blocking resistance and the fixation property at low temperature is extremely difficult.
A method of mixing an additive such as wax in the toner formulation is known, but the added amount of the additive is limited to a level of from 4 to 5 parts by weight based on 100 parts by weight of the resin, and the fixation characteristic of the toner at a low temperatures tends to be inadequate when the additive is incorporated in the toner in such amounts. Further, when flakes obtained by melt-kneading are mechanically pulverized to prepare a toner, the yield tends to be poor, and the particle size distribution tends to be broad.
Particularly when it is attempted to obtain a toner having small particle sizes, this tendency is very significant.
In recent years, the known production method of producing polymer toner by-means of emulsion polymerization or suspension polymerization is expected to take the place of the melt-kneading/pulverization method. By these methods, it is possible to control the dispersion of the materials in a manner which is different from the melt-kneading/pulverization method. Further, it is also possible to prepare a toner having small particle sizes and a good particle size distribution. Particularly by the emulsion polymerization agglomeration method, it is possible to control the particle size, the particle size distribution and the shape of the toner.
In the case of producing a toner by the emulsion polymerization method, a pigment and a charge controlling agent are added to a dispersion of an emulsified resin, containing primary resin particles having particle sizes of ranging from 0.05 to 0.5 xcexcm obtained by polymerization, and also e.g. an electrolyte to agglomerate the primary particles thereby preparing a toner of a particle size ranging from 3 to 9 xcexcm. The slurry obtained containing the toner particles is washed and dried thereby preparing toner particles as a product. However, it is difficult to prepare toner particles having a uniform particle size in the agglomeration step even by this method. Stirring plays an important role in the agglomeration of the primary particles. That is, in the agglomeration step, the viscosity of the mixture changes as the particles agglomerate. Accordingly, it is necessary to control the agglomeration of particles by changing the-rate of stirring the mixture as the viscosity of the medium changes during agglomeration. However, no method has been proposed which adequately controls the agglomeration of particles. Accordingly, it is difficult prepare a uniform toner having small particle sizes even by emulsion polymerization/agglomeration, and it is necessary to further e.g. process the agglomerated particles such as by classification after the agglomeration step to achieve the desired performance of the toner.
Accordingly, one object of the present invention is to provide a method of preparing toner particles having small particle sizes and a sharp particle size distribution, suitable to achieve a high picture quality and a high printing speed, by agglomerating primary polymer particles prepared by e.g. emulsion polymerization.
It has now been found that the above objective can be achieved by controlling the stirring conditions during agglomeration of primary polymer particles under specific conditions with a view to uniformly mix the liquid containing the particles for agglomeration so that the agglomeration medium thickens during agglomeration, provides sharpness of the particle size distribution of the agglomerated particles and provides ease of particle size control.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a method for producing an electrostatic image developing toner, which comprises, stirring a dispersion containing at least primary polymer particles and coloring agent particles in a stirring tank, thereby agglomerating the particles, wherein the shape of the liquid level during the stirring operation in the agglomeration step satisfies the following formulae (I) and (II):
HCxe2x89xa60.8Hxe2x80x83xe2x80x83(I), 
1.2Hxe2x89xa6HExe2x80x83xe2x80x83(II) 
H: the distance between the bottom of the stirring tank at its center and the horizontal plane of the surface of the dispersion before the start of stirring,
HC: the distance between the bottom of the stirring tank at its center and the liquid level of the surface of the dispersion at its center during stirring, and
HE: the distance between the bottom of the stirring tank at its center and the liquid level of the surface of the dispersion at the edge of the tank during stirring.
Another aspect of the present invention is a method for producing an electrostatic image developing toner, which comprises, an agglomeration step of stirring a dispersion containing at least primary polymer particles and coloring agent particles in a stirring tank to prepare agglomerates of the primary particles and coloring agent particles, wherein a double helical ribbon blade is employed as the agitating blade within the stirring tank.