(1) Field of the Invention
The present invention relates to a developer for electrophotography and a process for the preparation thereof. More particularly, the invention relates to a developer for electrophotography having an improved adaptability to the developing operation, being excellent in transferring and cleaning properties, being capable of forming an image free of fog and having a good fixing property without causing an offset phenomenon, especially a composite magnetic developer having such excellent characteristics, and also to a process for the preparation thereof.
Furthermore, the present invention relates to a novel process in which a developer for electrophotography, especially a composite magnetic developer, having the above-mentioned excellent characteristics, can be prepared by a small number of steps at a high operation efficiency with various advantages.
(2) Description of the Prior Art
As one of the methods for developing electrostatic latent images formed by the electrostatic photographic process, there is widely adopted a so-called magnetic brush method. According to the early magnetic brush development method, a developer formed by mixing toner particles comprising a pigment and a resin binder imparting the fixing property and desirable electric characteristics to the pigment with a magnetic carrier such as iron powder is used, and an electrostatic latent image-carrying surface of a support is intimately contacted with a magnetic brush of this developer to visualize the electrostatic latent image with the toner particles. In this method using a mixture of the toner and magnetic carrier, however, while the developing operation is carried out, only the toner particles are consumed in the mixture, and therefore, the mixing ratio of the toner to the magnetic carrier is changed. Accordingly, in order to maintain a good balance between the toner and magnetic carrier, it is necessary to frequently supply the toner into the mixture in a copying machine during the operation.
A so-called magnetic developer is known as a developer capable of performing the development singly without the aid of a magnetic carrier. Magnetic developers customarily used are ordinarily prepared by dispersing a powdery magnetic material such as powdery triiron tetroxide, Fe.sub.3 O.sub.4 together with other pigment according to need, in a binder resin medium, forming the dispersion into granules and embedding or holding a conducting substance such as carbon black on surfaces of the granules, whereby the entire particles are rendered magnetically attractable and the surfaces of the particles are rendered electrically conductive.
We previously found that particles formed by spraying a liquid dispersion comprising a binder resin medium and a fine powder of a magnetic material into a drying atmosphere have fine convexities and concavities on the outer surfaces thereof, namely crater-like surfaces, and that when these particles are dry-blended with an electrically conductive fine powder such as carbon black, there can be obtained a developer having much improved conductivity and flowability. Based on this finding, we proposed a novel one-component type developer (see Japanese Patent Application Laid-Open Specification No. 52639/77).
This developer is very excellent in the adaptability to the developing operation and is valuable for attaining an object of obtaining a clear image free of bleeding in contours. However, a further improvement is desired when this developer is used in the reproduction process in which a developer, image formed on a photosensitive layer for electrophotography is transferred onto a transfer sheet and the photosensitive layer is used repeatedly for reproduction. Especially when a soft binder is incorporated into the developer so as to enable pressure fixation or a soft parting agent is incorporated so as to prevent occurrence of an offset phenomenon, some defects arise.
More specifically, such developer particles are ordinarily low in the efficiency of transfer from a photosensitive plate to a transfer sheet, and in the repeated reproduction process, it is often difficult to completely clean away the developer particles remaining on the surface of the photosensitive plate. The most serious defect in the developer of this type is that a soft binder or parting agent contained in the developer particles is transferred to the surface of a photosensitive plate and is gradually accumulated thereon to form an electrically insulating layer on the surface of the photosensitive plate. Accordingly, when this developer is employed, even if the photosensitive plate is used repeatedly a small number of times, fogging in prints or electric breakdown on the photosensitive plate is caused by the residual voltage, resulting in extreme shortening of the life of the photosensitive plate.
A so-called microcapsulated developer prepared by forming a shell of a hard component such as a resin on the surface of a core composed of a soft component such as wax, a magnetic material, a pigment and the like is known. This microcapsulated developer, however, is defective in that since the electric resistance of the particles per se is extremely high, fogging is readily caused and it is difficult to form a sharp image. In order to eliminate this defect, it is necessary to render the particles electrically conductive, and for this purpose, the particles should be dry-blended with a large amount of carbon black. As a result, the surface of a photosensitive plate is gradually contaminated with free carbon black or an optical system or corona discharge mechanism of a copying machine is often contaminated by free carbon scattered around. In such microcapsulated developer, utilization of the soft parting or fixing material contained in the core is less sufficient than in the ordinary uncapsulated developer, and therefore, the microcapsulated developer is relatively insufficient in the offset-preventing property and adaptability to pressure fixation. Furthermore, in production of such microcapsulated developer, in order to distribute the soft component in the core and the hard component in the shell, it is necessary to adopt troublesome operations and select strictly the process conditions for formation of developer particles, and therefore, the manufacturing cost is inevitably increased.