Electrophotography is grouped into two types; in one type, employed are both magnetic working force and electric working force for the transference of developers, while in the other type, employed is only electric working force. In the former type where both magnetic working force and electric working force are employed, any of two-component developers comprising magnetic carrier and non-magnetic toner, or one-component developers comprising only magnetic toner may be used. As opposed to this, in the latter type where only electric working force is employed, one-component developers comprising only non-magnetic toner (hereinafter referred to as non-magnetic one-component developers) may be used.
FIG. 2 shows the constitution of a developing device and its peripheral devices in a printer or a duplicator in which is used a non-magnetic one-component developer. The illustrated constitution is first described below.
On the surface of a photoreceptor 1, which is an electrostatic latent image-carrying device, a photoconductive insulating layer is formed. The photoconductive insulator for the layer is a substance which is electrically insulating in ordinary condition but, when exposed to light, becomes conductive. The photoreceptor 1 rotates in the rotating direction 1a, and its surface is uniformly charged by the charger 2.
When the surface of an original manuscript is scanned with light, the surface of the photoreceptor 1 is exposed to the reflected light via an optical system (not shown) in the direction 10, whereupon the exposed site of the surface of the photoreceptor 1 becomes conductive to lose the charge, and an electrostatic latent image is formed on the surface of the photoreceptor 1. For example, in a laser printer, the exposure 10 is effected by optical beams of which the strength is modulated in accordance with the image to be recorded. In a duplicator, the reflected light that results from the optical scanning of the surface of an original manuscript is led to the surface of the photoreceptor 1 via an optical system, by which the surface of the photoreceptor 1 is exposed as in 10. When the charged powdery toner of a non-magnetic one-component developer is transferred by the developing roller 33 onto the electrostatic latent image formed on the surface of the photoreceptor 1, the latent image is developed into a visual image.
The toner having been transferred onto the surface of the photoreceptor 1 is further transferred onto recording paper 4. In this transferring step, electrostatic attraction is applied to the back surface of the recording paper 4 by means of the transfer device 5. A cleaning device 6 such as a cleaning blade or the like is disposed around the photoreceptor 1 in the site downstream the transfer device 5 in the rotating direction 1a. The cleaning device 6 is to remove the toner having been adsorbed on the surface of the photoreceptor 1 but not transferred onto the recording paper 4, from the surface of the photoreceptor 1.
The recording paper 4 onto which the toner has been transferred is conveyed to a fixing device 7. The fixing device 7 comprises a hot roller 8 and a pressure roller 9, between which the recording paper is passed so that the transferred toner is fixed on the recording paper 4.
The developing device 3 is so constructed that the toner container 31 that contains toner 12 therein is combined with the developing roller 33, in which the surface of the developing roller 33 is contacted with the photoreceptor 1 whereby the toner 12 is transferred onto the surface of the photoreceptor 1. The developing system of this type is referred to as a contact-type developing system. The toner container 31 has therein a supply roller 34 that acts for sufficient transference of the toner 12 onto the surface of the developing roller 33, and is provided with a thin layer-forming blade 36. The blade 36 is to control the thickness of the toner layer to be on the surface of the developing roller 33, and is kept in contact with the developing roller 33.
To the developing roller 33, negative voltage relative to the earth voltage is applied, for example, by means of a negative power source 37. To the supply roller 34, applied is negative voltage relative to the earth voltage, by means of a negative power source 38. The negative voltage applied to the supply roller 34 is larger than that applied to the developing roller 33 in terms of the absolute value. In that manner, the toner having adhered to and accumulated on the surface of the developing roller 33 is charged.
Another method is known for charging toner, in which the toner 12 is charged by the friction between the controller parts such as blades and rolls, and the surface of the developing roller 33 whereby the toner 12 is carried on the surface of the developing roller 12. In this case, where the toner 12 is charged in minus electricity, the surface layer of the developing roller 33 is to be made of a material selected from plus-charging materials in frictional electrification series. On the other hand, where the toner 12 is charged in plus electricity, the surface layer of the developing roller 33 is to be made of a material selected from minus-charging materials in frictional electrification series. Therefore, as the surface layer of most developing rollers widely used at present in the art, much used are nylon resins and urethane resins. Many patent applications for selective inventions from Japanese Patent Publication (JP-B) Sho-50-13661 have heretofore been filed, including, for example, Japanese Patent Application (JP-A) Sho-63-183470 relating to a surface layer that comprises an urethane and a reactive group-having fluorine compound; JP-A Sho-63-189876 relating to an alkali metal-containing urethane surface layer; JP-A Hei-1-252979 relating to an urethane surface layer that covers a conductive layer of oil-resistant rubber; JP-A Hei-3-249675 relating to a conductive filler that comprises an ester-type urethane and carbon black; JP-A Hei-5-158341 relating to an isocyanate-processed surface layer; JP-A Hei-7-54836 relating to a polyurethane surface layer as crosslinked with an amino resin; JP-A Hei-7-199645 relating to an urethane surface layer having a small degree of water absorption; JP-A Hei-7-310732 relating to an urethane-modified acrylic resin layer, etc.
The most important one of the characteristics which the surface layers such as those mentioned above of developing rollers are required to have is that the toner filming is few on those surface layers. At present, it is not too much to say that the toner filming determines the life of developing devices. The toner filming means filmy adhesion of toner not participating in development to the surfaces of developing rollers, and this has negative influences on images formed.
To prevent the toner filming on developing rollers, the toner releasability is specifically noted. For this, fluorine-containing particles are added to the surface layers of developing rollers, or urethane resins that contain fluorine-containing components, for example, cured products of fluorine-containing polyols and polyisocyanates are used for forming the surface layers.
However, as a result of our studies that are directed to solving the problem of toner filming on the surface layers of developer rollers on the basis of the toner releasability from developer rollers, we, the present inventors have found that the surface layers made of fluorine-containing resins or the like noted above are problematic in that the image density obtained is lowered even though the toner filming could be evaded. On the other hand, when the surface layers are made of nylon or the like resins in order to increase the image density, toner filming occurs thereon, resulting in that good images could not be obtained. Even when the surface layers are made of ester-type urethane resins, toner filming still occurs thereon.
Accordingly, the current techniques for preventing toner filming on developing rollers are not always satisfactory, and the toner filming is still a difficult problem.