1. Field of the Invention
The present invention relates to a toner for use in electrophotography, an image forming method for visualizing an electrostatic charge image and a toner jet.
2. Description of the Related Art
As a general electrophotographic method, a method for obtaining a copied image by forming a latent image on an image carrier (a photosensitive body), visualizing the latent image by supplying a toner thereto, transferring the resulting toner image onto a transfer material such as paper and then fixing the toner image on the transfer material with heat/pressure is known.
In order to reduce the power consumption and shorten the wait time of an electrophotographic apparatus, an on-demand type fixing apparatus obtained by combining a ceramic heater with a small thermal capacity and a film has been put to practical use as a fixing apparatus.
In such a fixing apparatus, attempts have been made to reduce a fixing nip internal pressure thereof from the viewpoint of elongation of the lifetime and applicability to a variety of media.
Furthermore, according to recent increase in a print speed, time when a toner and a medium such as paper pass through a nip of a fixing apparatus has become shorter year by year.
Moreover, there are recently increasing opportunities for a user to output, by using an image forming apparatus such as a laser beam printer (LBP), a graphic image with a high coverage rate such as image data taken through a digital camera, a portable terminal or the like, or a poster.
In this context, there is a demand for a toner capable of showing an excellent low temperature fixing property even under severer fixing conditions, for example, conditions for forming an image with a high coverage rate in a short period of time with a low nip internal fixing pressure.
In order to achieve low temperature fixation by such a fixing apparatus, it is necessary to attain better low temperature fixation of a toner than that of a conventional toner, and there are a large number of reports on use, as a binding resin, of not only an amorphous rein but also a crystalline resin for this purpose.
It is known that a crystalline resin is abruptly molten in the vicinity of its glass transition temperature and can be improved in the low temperature fixing property by increasing compatibility with an amorphous resin (Japanese Patent Application Laid-Open No. 2010-102058).
When the compatibility therebetween is too high, however, there arises a problem in which the resulting toner is degraded in a heat-resistant storage property and crystallizability on the contrary.
In contrast, when the compatibility between an amorphous resin and a crystalline resin is lowered, the crystal of the crystalline resin is liable to be easily formed, but these resins are hard to be compatible with each other even at a temperature over melting points thereof, and hence, it is difficult to improve the low temperature fixing property particularly when a fixing time is short or a nip internal pressure is low.
Meanwhile, it is known that the fixing property and the resistance to deterioration of a toner can be improved by reducing the content of a low molecular weight component in a resin (Japanese Patent Application Laid-Open No. 2005-84226).
If the fixing pressure is low, however, a sufficient fixing property cannot be attained merely by providing a sharp melt property to the resin.
In order to solve this problem, it is known that the low temperature fixing property and glossiness can be improved by causing a toner to contain amorphous polyester including a small amount of low molecular weight component and crystalline polyester (Japanese Patent Application Laid-Open No. 2007-21595).
If the toner merely contains the amorphous polyester and the crystalline polyester, however, the amorphous polyester and the crystalline polyester become compatible with each other when the toner is molten in a fixing step. As a result, the toner present in a fixed image becomes more plasticized than necessary, and the toner image resulting from fixing may be blocked in some cases under a severe environment of a high temperature and high humidity.
In this manner, there still remains a large number of technical problems for attaining both an excellent low temperature fixing performance and long-term storage stability of a fixed image under a high temperature environment, and there still is room for further improvement.