This invention relates to a toner for electrostatic image development, particularly to toner which has a good preservation stability and can therefore provide clear developed images over a long time after its manufacture.
Generally, in electrophotography a colored resin powder, i.e., toner is contacted with a photosensitive plate or paper with an electrostatically charged image. The toner particles are deposited on the surface of the photosensitive plate or paper by the electrostatic attraction between themselves and the electrostatically charged image. In this manner a visible image is developed on the plate or paper. The developed image may be transcribed from the photosensitive plate on to a plain paper. And the toner particles are thermally fused to the surface of the photosensitive paper or plain paper, thereby to fix the visible image on the paper.
The above described is called dry development method. In this method the toner is mixed with a carrier in order to electrically charge the toner to a necessary degree and to contact the toner particles with the photosensitive plate or paper in uniform concentration. There are several dry development methods. For example, one is called cascade development in which the mixture of toner and carrier, i.e., developing agent, is applied onto a photosensitive plate or paper. Another is called magnetic brush development in which a magnetic brush is used. The magnetic brush consists of a magnet with a mass of iron fillings. To the brush, toner mixture is attached by magnetic attraction. Image development is accomplished simply by brushing the surface of the photosensitive plate or paper. During this development only the toner particles are detached from the brush and are deposited onto the surface of the photosensitive plate or paper.
In either dry development method, however, it is difficult to keep clear the surface of each carrier particle since the toner particles adhere to the carrier particles.
If the toner particles adhere to the carrier particles, the toner fails to be charged electrically to necessary degree, thus degrading the quality of the developed image. Further, the toner particles are deposited unnecessarily onto those portions of the photosensitive plate or paper where no image is electrostatically formed. If not clear of unnecessary toner particles, the photosensitive plate or paper adversely serve to degrade very much the quality of the developed electrostatic image, particularly in case it is repeatedly used and subjected to electrical charging, exposure, development and transcription many times.
Moreover, the toner contains a thermoplastic resin binder which melts or softens at a relatively low temperature. While being used or preserved, it may therefore easily turn into a block, mass or cake according to the ambient temperature. If this happens, the toner loses its function as a development toner.
To lengthen the life time of such toner or to elevate the preservation stability of such toner, many attempts have been made. For example, it has been proposed to mingle a non-adhesive filler or a non-adhesive metal soap with such toner, or to add micro powder or an oily substance to such toner. Such techniques are disclosed in Japanese Patent Publications Nos. 37-4695, 43-26716, 44-6398, 44-27879 and 44-32470 and in Japanese Patent Disclosures Nos. 48-47345 and 48-47346. But the toner does not exhibit a satisfactory preservation stability, and the quality of the developed images it provides are not good enough.