In recent years, it has been required to decrease heat energy during the toner fixing process for the purpose of higher printing speed, expansion of usable paper types, and decrease of an environmental load. In order to decrease the heat energy during the toner fixing process, it is requested a technology enabling to improve low-temperature fixability of an electrostatic image developing toner (hereafter, it is simply called as “a toner”). As one of the achieving technologies, it is known a method to introduce a crystalline resin such as a crystalline polyester resin having a sharp-melting property in a binder resin.
For example, Patent document 1 (JP-A No. 2006-251564) discloses a toner having a binder resin which contains a crystalline polyester resin and an amorphous resin. By using a binder resin mixed with a crystalline polyester resin and an amorphous resin, the crystalline component of the crystalline polyester resin will be melted when the temperature of the heat fixing process is above the melting point of the crystalline polyester resin. This melted crystalline component will move into the amorphous resin component to result in promoting heat melting of the amorphous resin. Thus, it is possible to achieve low-temperature fixing.
However, the viscosity of the toner will be rapidly decreased by the presence of the crystalline polyester resin in the toner particles at the heat fixing process. As a result, the image formed with the melted toner will be broken, and a part of the image may be transferred to a fixing member (it is called as a high-temperature offset phenomenon), or it may produce a problem of fixing-separation failure in which an image support substrate is wound to the fixing member due to the lack of releasing property.
In order to solve the above-described problems, Patent document 2 (JP-A No. 2005-234046) discloses a toner using a crystalline polyester resin together with two or more kinds of releasing agents. According to the method disclosed in Patent document 2, the toner has: a core layer containing a crystalline polyester resin and two or more kinds of releasing agents having a different melting point; and a shell layer containing an amorphous polymer. It is said that this toner enables to achieve a wide range of temperature for separation and to achieve low-temperature fixing.
However, by an increased demand for expansion of usable paper types such as thin-paper handling, a sufficient fixing-separation property has not been ensured. Further, there is a problem in which crystalline sizes of the wax and the crystalline polyester resin are diverse, and this will induce deterioration of uniform glossiness and will generate gloss unevenness.