1. Field of the Invention
The present invention relates to a toner and a producing method thereof, a developer, a two-component developer, a developing device and an image forming apparatus.
2. Description of the Related Art
A toner developing a latent image is used in a variety of image forming processes. As one of the examples, it is known that the toner is used in an image forming process of an electrophotographic system.
An image forming apparatus of the electrophotographic system is conventionally prevalent as a copying machine, and recently has excellent properties as an output apparatus of a computer image created by a computer. Accordingly, as the computer prevails, the image forming apparatus thereof is also widely used in a printer, a facsimile apparatus and the like. The image forming apparatus of the electrophotographic system is a apparatus for forming a desirable image on a recording medium by performing a charging step of uniformly charging, for example, a photoreceptor layer on a surface of a photoreceptor drum, an exposing step of forming an electrostatic latent image on the charged surface of the photoreceptor drum by projecting signal lights of a document image thereon, a developing step of supplying an electrophotographic toner (hereinafter, simply referred as “toner”) to the electrostatic latent image on the surface of the photoreceptor drum so as to visualize the electrostatic latent image, a transferring step of transferring the visualized image on the surface of the photoreceptor drum onto the recording medium such as sheet and viewgraph, a fixing step of fixing the visualized image on the recording medium through heating, pressurization and the like, and a cleaning step of cleaning the surface of the photoreceptor drum after transferring the visualized image by removing the toner and the like remaining thereon by means of a cleaning blade. The visualized image may also be transferred on the recording medium through an intermediate transfer medium.
Due to a further advancement of a variety of techniques as to the computer, for example, due to an advancement of high-definition of the computer image, the image forming apparatus of the electrophotographic system is also required to correctly and distinctly duplicate a fine figure, a subtle hue change and the like in the computer image, and to form a high-definition image comparable to the computer image. In order to meet the requirement, for example, reduction of toner particle size has been intended, where a variety of considerations have been carried out for producing a toner whose particle size is about 5 μm which is useful for the high-definition of the image.
Although such a reduced particle size toner is useful for forming the high-definition image, the toner encompasses a lot of fine powders so that there is a disadvantage in that transferring efficiency is low. The reduced particle size toner has high charge-accumulation property and a large specific surface area compared to a toner for example whose particle size is more than 5 μm, so that adhesiveness of the reduced particle size toner to the photoreceptor drum and the intermediate transfer medium is strong. The reduced particle size toner has the low transferring efficiency and is hard to be transformed to the recording medium, causing a problem that, when the reduced particle size toner is used, an amount of the toner remaining on the photoreceptor drum and the intermediate transfer medium after transferring the visualized images onto the recording medium is increased.
In view of such a problem, it is suggested that, by spheroidizing the toner, a contact area between the photoreceptor drum and the intermediate transfer medium, and the toner is so reduced that the transferring efficiency of the toner is enhanced (referring to Japanese Examined Patent Publication JP-B2 3253228 and Japanese Unexamined Patent Publication JP-A 2005-257976, for example).
In JP-B2 3253228, it is disclosed that, by defining the shape coefficient SF-1 indicating the degree of roundness of a toner particle as 111 to 160, the shape coefficient SF-2 indicating the degree of irregularity of a surface of the toner particle as 110 to 140, SF-2/SF-1 as 1 or less, and the content of releasing agent as 5 to 40%, the toner particle can be transferred from a photoreceptor and an intermediate photoreceptor at a high transfer ratio, and the toner which does not cause melt-bonding of the toner and filming is obtained.
In JP-A 2005-257976, it is disclosed that, by defining the degree of circular of the toner particle as 95 or more, the shape coefficient SF-2 thereof as 120 to 150, resistance thereof as 5×107 to 3×1011 Ω·cm, the toner which can achieve a balance among improvement of transferability (reduction of the transfer remaining toner), ungeneration of transfer struggle (prevention of abnormal image generation) and good cleanability, and which can form a high-definition image is obtained.
When the toner particle constituting the toner has irregularity of the surface, that is the toner particle has irregular shape, the toner particle has a tendency to be caught by the cleaning blade so that the cleanability is made good, but the transferability to the recording medium is so low that the high-definition image is not stably formed. When the toner particle becomes nearly spherical shape, the transferability is elevated, but the toner particle is so hard to be caught by the cleaning blade that the cleanability degrades. Therefore, in a spheroidizing technique of the toner, the toner design which has both the transferability and the cleanability at high level and which can respond to the high-definition of the image is required.
The toner disclosed in JP-B2 3253228 and JP-A 2005-257976 is intended to keep both the cleanability and the transferability at high level by controlling the degree of the roundness of the all toner particles and an average value of the shape coefficient indicating the degree of the irregularity of the surface thereof in a constant range so as to control an averaged shape of the all toner particles in a constant range, and further by defying both the content of the releasing agent and a resistance value of the toner.
However, behavior of the toner particles is different from each other due to the particle size, so that when the shape of the all toner particles are defined to be averaged, the smaller the size of the toner particle becomes, the worse the cleanability gets. It is good for the cleanability that the degree of the irregularity of the surface of the toner particle is larger, however, from, the point of view of the fixation, it is not preferable. The higher the degree of the irregularity of the surface of the toner particle becomes, the larger the space between the toner particles gets, so that the heat conductivity between the toner particles degrades, causing negative effect on the fixation. That is, the heat conductivity between the toner particles degrades so that the required temperature for the fixation rises. The effect of the degree of the irregularity of the surface on the heat conductivity rises as the size of the toner particle increases.
Although the so-called struggle caused when the toner flies to the portion to be white of an image bearing member such as the photoreceptor can be prevented or suppressed by controlling the shape of the toner, the struggle caused by the larger size of the toner particle is so remarkable that it is important to prevent the struggle of the large size of the toner particle.
Thus, for a toner property, the balance between the size of the toner particle and the shape thereof is important. However, in the art disclosed in JP-B2 3253228 and JP-A 2005-257976, the effect of the size of the toner particle is not evaluated.