1. Field of the Invention
The present invention relates to a toner, a developer, and a method for producing a toner.
2. Description of the Related Art
Hitherto, there have been disclosed electrostatic image developing toner particles each containing: a colored resin particle (core) produced by the emulsification dispersion method; and a coating layer made of resin particles produced by the emulsification polymerization method using a surfactant or by the emulsification dispersion method using a surfactant, wherein the core is made of polyester resin and the coating layer is made of vinyl resin (see, for example, Japanese Patent Application Laid-Open No. 2005-084183).
A contact heating fixing system such as a heating roller fixing system is widely employed as the fixing system for toner. In the contact heating fixing system described above, a low heating temperature means saving energy. Therefore, a resin in a toner preferably has a low melting point. However, a mechanical stress or a thermal stress is applied to a toner in the electrophotography process, which imposes limitation on thermal characteristics of the toner such as glass transition temperature to avoid blocking, or on molecular weight of the toner to prevent cracking. The resin contained in the toner is preferable to satisfy these characteristics. These two are in a trade-off relationship and balancing these two is preferable. To strike this balance, a core/shell type toner is manufactured and known. Such a toner contains a resin favorable in terms of heat fixing in the core and a resin favorable in terms of blocking in the shell that covers the core. In addition, as a material for such a resin, polyester is well known because of its advantages for toughness, heat resistance and fixability. For example, there is known a technology in which a core particle is manufactured by agglomeration and/or curing salting of a liquid dispersion of polyester resin particles using an agglomeration salt and a liquid dispersion of polyester resin particles is added to form a shell layer by agglomeration/curing salting using an agglomeration salt followed by adhesion of the core particle and the shell layer (see Japanese Patent (JP-B) No. 4033096). Furthermore, there is known a method in which both a core and a shell layer are formed by dissolving a polyester resin in an organic solvent and preparing resin particles by a phase transfer emulsification followed by addition of an electrolyte for agglomeration (see JP-A No. 2008-089670). Moreover, there is known a toner containing a non-crystalline resin as a main ingredient and a crystalline resin mixed with the non-crystalline resin, in order to improve fixability at low temperatures (see JP-A No. 2008-180938).
However, there has not been obtained a toner whose shell layer sufficiently exhibits its intrinsic function without impeding fixability of the core. Especially for one-component developing process, there has not been obtained a toner with a core-shell structure which is resistant to severe conditions in a developing device and also exhibits satisfactory fixability.