1. Field of the Invention
The present invention relates to a method of producing spherical-shaped toner by dispersing oil phase containing a resin with an acid group in an aqueous dispersion medium in the presence of base, in particular relates to a method of producing positively-charged toner particles with high chargeability.
2. Description of the Related Art
As conventional methods for producing toner, the pulverizing method, suspension polymerization method, dissolve suspension method and the like are known. The pulverizing method, however, has a disadvantage that it is difficult to obtain spherical-shaped particles with the method. In the suspension polymerization method, on the other hand, since toner is obtained by polymerizing monomer, a low-molecular weight component with low polymerization degree is generated together with predetermined or desired toner particles. Accordingly, there are problems such that the yield is low by the amount of the generated low-molecular weight component, and that additional cost is needed for recovery and disposal of low-molecular weight component. Further, in the dissolve suspension method, an emulsifying agent is used for stabilizing the liquid-droplet state in which the resin is held, washing needs to be repeated many times to remove the emulsifying agent from the toner particles. Accordingly, when the emulsifying agent cannot be successfully removed by the washing and remains in the toner particles, such remaining emulsifying agent adversely affects the physical property of the toner particles in some cases.
On the other hand, Japanese Patent Application Laid-open No. 10-319639 describes a method of obtaining spherical-shaped toner particles by dispersing a hydrophobic liquid composition containing a resin with an acid group, a hydrophobic solvent capable of dissolving the resin, and a colorant, in an aqueous dispersion medium in the presence of base so as to make the hydrophobic liquid composition into liquid droplets; and by removing the solvent from the liquid droplets to solidify the liquid droplets. In this method, the acid group possessed by the resin is neutralized by the base to be ionized in the aqueous dispersion medium. Accordingly, the hydrophilic property of the ionized portion of the hydrophobic liquid composition becomes high. As a result, the hydrophobic liquid composition become liquid droplets of which outer surface is hydrophilic and of which inner portion is hydrophobic and the liquid droplets disperse by themselves (self-disperse) in the aqueous dispersion medium. Accordingly, it is possible to obtain liquid droplets having stable spherical shape without using any emulsifying agent, and after removing the solvent from the liquid droplets by means of heating or the like, spherical-shaped toner particles can be obtained.
Thus, the method described in Japanese Patent Application Laid-open No. 10-319639 has advantages that it is possible to obtain the spherical-shaped particles, which are difficult to produce by the pulverizing method, that no low-molecular weight component is generated as in the suspension polymerization method, and that there is no need to use any emulsifying agent as in the dissolve suspension method.
The method described in Japanese Patent Application Laid-open No. 10-319639, however, still needs to be improved, specifically because of the disadvantage that it is difficult to obtain positively charged toner with this method. To provide more detailed explanation, in general, toners can be roughly classified into positively charged toner and negatively charged toner. The positively charged toner can be obtained by adding a charge control agent which is positively charged, during the production of toner.
In case of the method of Japanese Patent Application Laid-open No. 10-319639, however, there is a disadvantage that the charge amount cannot be sufficiently increased merely by adding a charge control agent which is positively charged, which results in adversely affecting the image quality of an image finally obtained by using the toner such that a phenomenon so-called “fog” is easily occurred.
In addition, an additive such as silica is sometimes added to this kind of toner so as to increase the fluidity of the toner. However, when the additive such as silica is added, the problem that the charge amount cannot be sufficiently increased as described above becomes more prominent.