1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images, and specifically relates to a negatively chargeable toner for use in digital type image forming apparatuses.
2. Description of the Related Art
Conventional image forming apparatuses are generally analog type image forming apparatuses such as used in copiers and the like wherein a document is illuminated by a light source and the light reflected from said document irradiates the surface of a photosensitive member so as to form an electrostatic latent image on the surface of said photosensitive member. Image forming apparatuses of the digital type are known wherein digitally written electrostatic latent image is developed by supplying a developer containing a toner to said latent image. Digital type image forming apparatuses have been practicalized in the forms of electrophotographic type facsimile apparatuses, digital copiers which form images based on image information read by an image reader, and printers using the output of computer terminals.
In image forming apparatuses of the digital type, an electrostatic latent image is formed in dot units on the surface of a negatively charged organic photosensitive member by digitally writing image data via irradiation of said surface by a laser beam or the like, this latent image is reverse developed by a negatively charged toner, and the obtained toner image is transferred onto a recording member and fused thereon to form a recorded image. The toner used in such digital type processes must have excellent dot reproducibility. That is, the toner must have a true reproducibility in dot units when developing an electrostatic latent image formed on the surface of a photosensitive member, and this reproducibility must not be reduced even after repeated use. To satisfy such characteristics, a toner must have excellent charge rise characteristics as well as excellent stability relative to charging. In the case of two-component developers, a toner is mixed with a carrier within the developing device so as to be triboelectrically charged; the toner must have charging characteristics such that a desired amount of charge is attained rapidly in a short mixing time, but thereafter the charge amount drops somewhat or does not increase even with additional mixing. The toner must have excellent charge stability relative to environmental fluctuations (temperature and humidity) and a rise in temperature in the image forming apparatus. The toner must have excellent characteristics that prevent toner-film-formation on the photosensitive member and decrease of thickness of the photosensitive member. Further, the toner must have excellent transfer characteristics from the photosensitive member to a transfer member.