1. Field of the Invention
The present invention relates to a toner used in an electrophotographic method, an image-forming method for actualizing an electrostatic image, and a toner jet.
2. Description of the Related Art
Image-forming apparatuses using an electrophotographic method have recently been required to demonstrate faster speeds and higher reliability.
In addition, there also growing demands for improved appearance of the image as well as energy savings, shortening of wait time and improvement of image productivity, and toners are required to demonstrate superior low-temperature fixing performance in order to respond thereto.
In general, low-temperature fixing performance is related to the viscosity of the toner, and toners are required to have the property of being rapidly melted by heat during fixation (so-called sharp melt property).
Accompanying recent increases in printer speed in particular, the amount of time during which the toner and paper or other media pass through the nip of the fixing apparatus is becoming shorter each year. In addition, there are a growing number of opportunities for users to continuously output images of high quality having a high coverage rate onto heavy paper in the manner of graphic images or posters using an image-forming apparatus such as a laser printer.
In the case of using heavy paper, since the amount of heat lost to the paper during fixation increases, it becomes difficult to transfer heat from the fixing member on the paper side to the toner, thereby resulting in inadequate melting of the toner contacting the outermost layer of the paper and increased susceptibility to poor toner fixing performance. In addition, in the case of images having a high coverage rate, since a state results in which much of the toner is layered on the paper during fixation, it again becomes difficult to transfer heat from the fixing member on the toner side to the outermost layer of the paper, resulting in even an even greater likelihood of poor fixing performance.
Consequently, in the case of outputting images having a high coverage rate using heavy paper, there were many cases in which means were devised for changing the fixing conditions, such as by lowering the printing speed of the fixing apparatus to a speed lower than that of ordinary paper or by increasing the set temperature of the fixing apparatus in order to increase the amount of heat transferred to the toner.
However, image productivity is sacrificed in the case of means for lowering the printing speed of the fixing apparatus, while it becomes difficult to save energy or shorten wait time in the case of means for increasing the set temperature of the fixing apparatus.
As has been described above, although there is a need for a toner that can be fixed under the same fixing conditions as ordinary paper even when using heavy paper, a toner capable of satisfying that requirement has yet to be obtained.
Numerous toners have been proposed as examples of technology for improving toner low-temperature fixability that contain not only an amorphous resin but also a crystalline polyester resin for use as a binder resin (see, for example, Japanese Patent Application Laid-open No. 2003-337443).
Toners containing a crystalline polyester resin are able to improve low-temperature fixability as a result of the crystalline polyester resin melting and becoming compatible with amorphous resin due to the heat during fixation, and the binder resin being plasticized due to this compatibility, thereby resulting in an enhanced sharp melt property.
However, in order to print images having a high coverage rate using heavy paper at high speed, the speed at which the crystalline polyester resin plasticizes the binder resin during fixation (to also simply be referred to as the plasticizing speed during fixation) is inadequate, thereby resulting in the need for further improvement.
In addition, when low-temperature fixability is attempted to be improved, there are cases in which toner durability and the rate at which charging rises up become poor, and a toner that satisfies all of these requirements with respect to low-temperature fixability, durability and the rate at which charging rises up has yet to be obtained at the present time.
For example, if compatibility between a crystalline polyester resin and an amorphous resin is made to be excessively high in order to improve low-temperature fixability, the crystalline polyester resin ends up melting even at normal temperatures, resulting in a toner that contains plasticized, soft toner particles.
As a result, the toner has weak durability with respect to external stress such as that applied when stirring the developer, and in the case of outputting low coverage images such as half-tone images in a mode that is severe on toner deterioration in the manner of continuous output, there is increased susceptibility to a decrease in image density caused by increased attachment force of the toner surface caused by embedding external additives. In this manner, it was difficult to realize both toner low-temperature fixability and durability in toners containing a crystalline polyester resin.
In contrast, it has also been considered to enhance toner durability by providing an annealing step in the toner production process and inhibiting compatibility of a crystalline polyester resin by promoting crystallization of the crystalline polyester resin (see, for example, Japanese Patent Application Laid-open No. 2010-152102).
Although it is true that crystallization of a crystalline polyester resin can be promoted by an annealing step, due to the slow nucleation rate during crystallization, crystallization proceeds while the crystalline polyester resin aggregates, thereby resulting in an increased likelihood of the crystalline polyester resin being in a poorly dispersed state.
Due to the effects thereof, the charge on the surface of toner particles becomes disproportionate during triboelectric charging of the surface, the rate at which charging of the toner rises up ends up decreasing, and there may be increased likelihood of image fogging particularly in the case of continuous output of images having a high coverage rate.
In other words, since it was difficult for toners containing a crystalline polyester resin to realize both crystallinity and dispersibility of the crystalline polyester resin, it was also difficult to realize both toner durability and the rate at which charging rises up.
Furthermore, proposals have also been made to increase the crystallization rate of crystalline polyester resins by adding an inorganic crystal nucleating agent such as silica (see, for example, Japanese Patent Application Laid-open No. 2007-033773) or an organic crystal nucleating agent such as fatty acid amide (see, for example, Japanese Patent Application Laid-open No. 2006-113473).
However, even if these crystal nucleating agents are added, since opportunities for contact with the crystal nucleating agent are limited, crystalline polyester resin ends up remaining that has not crystallized as a result of not being acted on by the crystal nucleating agent, thereby limiting the effect of improving toner durability. In addition, the dispersibility of the crystalline polyester resin was also unable to be improved and the rate at which charging rises up easily became worse. On the other hand, proposals have also been made for improving the amorphous resin combined with the crystalline polyester resin. For example, the use of a hybrid resin containing amorphous resin in the form of a polyester unit and vinyl copolymer unit was proposed in order to improve low-temperature fixability (see, for example, Japanese Patent Application Laid-open No. 2003-173047).
However, due to the inadequate plasticization speed during fixation, further improvements were required for high-speed printing of images having a high coverage rate when using heavy paper.
In addition, since improvement of the crystallinity and dispersibility of the crystalline polyester resin in toner was inadequate, there were also problems with toner durability and the rate at which charging rises up.
As has been described above, a toner has yet to be obtained that is able to satisfy all requirements relating to low-temperature fixability, durability and the rate at which charging rises up during use of heavy paper.