Electrophotography is a system in which a latent image formed on a photoconductive solid member using its photoconductivity is developed by allowing a toner in the form of colored particles to electrostatically adhere thereto, and the thus developed toner image is transferred and then fixed on a paper, etc. The electrophotographic system has been extensively used in the applications such as copying machines and printers, and further recently applied to general printing machines.
In the electrophotographic development, when using a toner having no magnetism, carrier particles called a magnetic carrier have been used together with the toner. The magnetic carrier serves not only for imparting an adequate amount of a positive or negative electrical charge to the toner owing to frictional electrification therebetween, but also for delivering the toner through a developing sleeve accommodating a magnet to near the surface of a photosensitive member on which a latent image is formed, by utilizing a magnetic force of the developing sleeve (A mixture of the magnetic carrier and the toner, etc., is a developer which is ready for immediate development of the latent image). In recent years, in the field of electrophotography, coloration of toner images developed tends to rapidly proceed. However, a color toner used for the above purpose has no magnetism, and there is therefore a rapidly increasing demand for the magnetic carrier. At the same time, there is a demand for a high quality of the resulting color images and a high copying speed thereof. To meet the requirements, the magnetic carrier is also required to have further improved functions.
Hitherto, as a material of a central portion of the magnetic carrier (hereinafter occasionally referred to as a “carrier core”) or a material of the magnetic carrier itself, there have been developed iron powder carriers, ferrite carriers or binder-type carriers, and these carriers have been put into practice.
The iron powder carriers are in the form of a carrier core prepared by pulverizing an iron powder, and have a flake shape, a sponge shape or an amorphous shape in many cases. The iron powder carriers thus prepared from an iron powder is inexpensive, but have a large true specific gravity ranging from 7 to 8 and a large bulk density ranging from 3 to 4 g/cm3. Therefore, a large driving force is required to stir the iron powder carriers in a developing device so that the iron powder carrier tends to frequently suffer from severe mechanical abrasion. For this reason, there tend to occur spent toners and deterioration in charge properties of the carrier itself, which tends to result in poor functions of the carrier for a short period of time or risk of damage to a photosensitive member used therewith.
The ferrite carriers are in the form of a magnetic carrier prepared by pulverizing ferrite having a smaller specific gravity than that of the iron powder, and frequently have a spherical shape as compared to the iron powder carriers. The ferrite carriers have a smaller true specific gravity of 4.5 to 5.5 and a smaller bulk density of 2 to 3 g/cm3 than those of the iron powder carriers owing to the ferrite material, and therefore are enhanced in durability and cause a less damage to the photosensitive member as compared to the iron powder carriers. However, in the ferrite carriers, there tend to be used metals such as copper-zinc, manganese-magnesium-strontium, lithium-magnesium-calcium, etc., which are not safe for environments and human bodies. Further, since the ferrite carriers are prepared through the pulverization step, it may be difficult to finely control a shape thereof and reduce a particle diameter thereof. Thus, the ferrite carriers are not sufficiently suitable for high-image quality development in future.
The binder-type carriers are in the form of a magnetic carrier prepared by molding magnetic fine particles with a binder such as resins, and have a good durability and cause a less damage to the photosensitive member owing to a small bulk density of about 2.5 g/cm3. The binder-type carriers are further classified into pulverized carriers and granulated carriers. The pulverized carriers tend to be hardly finely controlled in their shape, and the particle diameter tends to be hardly reduced. Therefore, the pulverized carriers are not sufficiently suitable for high-image quality development in future. The granulated carriers are likely to be adjustably controlled in their shape and formed into a spherical shape, a rice-grain shape, etc., and therefore tend to be readily controlled in fluidity or degree of contact with the toner. Further, the granulated carriers have a narrow particle size distribution, so that the particle diameter thereof tends to be readily reduced. For this reason, the granulated carriers are capable of realizing an enhanced durability and a high image quality. From these viewpoints, it is considered that the granulated binder-type carriers are extensively used in future.
The carrier cores have been coated with a resin, etc., in order to impart a good frictional electrification performance (electrical charge amount) and a good electrical resistivity thereto, and the thus coated carrier cores are used as a magnetic carrier. As the resin used for coating the carrier cores therewith or the binder resin used for the binder-type carriers, there have been employed thermoplastic resins such as vinyl-based resins and polyester-based resins, and thermosetting resins such as phenol-based resins, melamine-based resins and epoxy-based resins. Almost all of these resins are resins derived from underground sources such as petroleum and coal. However, environmental burden caused by using these underground sources has not been taken into consideration.
In recent years, the environmental problems such as exhaustion of underground sources and global warming have been noticed worldwide. For this reason, in order to realize medium- or long-term prosperity of human beings, it is required that the use of underground sources is reduced to a level as small as possible to suppress generation of carbon dioxide causing the global warming. In consequence, it is largely expected that various products are obtained from bio-based polymers which can be produced from vegetable raw materials, etc., and are therefore regenerative and reusable, and further generate a less amount of carbon dioxide. At present, in the market of carriers for electrophotography, the magnetic carriers have been used in an amount of about 9,700 tons (quoted from the “Results of Business of Japanese Makers, 2007”, Nippon Data Supply Co., Ltd.), and it is considered that the magnetic carrier market becomes more and more expanded with the progress of coloration in future. If a part of several thousand tons of resin components used in the magnetic carriers are replaced with the bio-based polymers, it is considered to be effective for reduction in environmental burden such as saving of underground sources and prevention of global warming.
In addition, it is known that the bio-based polymers have a low toxicity to human bodies and is therefore safe. Thus, from the viewpoint of enhanced safety, the use of the bio-based polymers is desirable.
Also, some of the bio-based polymers have a biodegradability (of course, there are present those bio-based polymers having no biodegradability). Since the biodegradability tends to cause deterioration in durability and strength, the use of the bio-based polymers is not necessarily recommended in the above applications. However, there are known the techniques in which the bio-based polymers having a biodegradability are used as a part of the resin components in favor of their biodegradability (Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Application Laid-Open (KOKAI) No. 7-98520    Patent Document 2: Japanese Patent Application Laid-Open (KOKAI) No. 7-295300