1. Field of the Invention
The present invention relates to an electrophotographic liquid developer which is used for developing an electrostatic latent image in an image forming apparatus such as an electrophotographic copying machine, a printer or the like, and also relates to an image forming apparatus which uses liquid developer for developing the electrostatic latent image.
2. Description of the Background Art
Electrophotographic image formation is generally performed in such a manner that an electrostatic latent image is formed on an electrostatic latent image carrier such as a photosensitive member, for example, by image exposure corresponding to an original image or image data, and is developed into a visible toner image, which is transferred and fixed to a record member for obtaining an intended image.
The developing method can be classified into a dry developing method and a wet developing method.
In the dry developing method, developer which is formed of coloring particles (toner) or is formed of toner and carrier made of particles having magnetism or the like is used. The dry toner is usually formed of pigment and binder resin as major components, and also contains, if necessary, a charging control agent or material, a conductivity control agent, a plasticizer and a mold release material or the like internally or externally added to the major components. The magnetic toner further contains magnetic powder such as Fe.sub.3 O.sub.4. Usually in the dry developing method, the toner is electrically charged by contact with a specific surface of the developing device or mutual contact of the toner particles. If the two-component developer containing carrier is used, the toner is charged by contact with the carrier or the like. Also, the toner is charged by electrostatic induction by an electric field, injection of electrons, ion absorption by discharging of ionized air and others. The toner thus charged is transported to an electrostatic latent image portion on an elctrostatic latent image carrier such as a photosensitive member by an electrostatic force, a mechanical force, a magnetic force or the like, and is developed with an electrostatic force.
Since the dry toner used in the dry development may escape into an atmosphere and may float in the air, it is impossible to employ the toner of an extremely small particle diameter. Usually, in the dry development toner having a relatively large average particle diameter of about 10 .mu.m or less is used. Due to the relatively large toner particle diameter, the development with the dry toner can increase the resolution only to a limited extent.
Meanwhile, in the wet developing method the liquid developer is used. The liquid developer which is now mainstream and available is formed of a dispersing medium (carrier liquid) having an electrically insulating property as well as coloring particles (toner), which principally comprising pigment and binder resin, and a charging control material, a dispersion stabilizer and others. It has been considered that the toner is charged by absorption of ions owing to the charging control agent, and the charged toner is used for the development according to the principle of electrophoresis.
Since there is no possibility that the toner used in the wet development escapes into an atmosphere, the toner can be made of extremely fine particles, and the toner having an average particle diameter on the order of submicrons can be practically used. This can achieve an image having a high resolution, and can also achieve easy fixing of the toner image and other advantages.
According to the wet development, a toner image on the electrostatic latent image carrier, which is formed by developing an electrostatic latent image on the electrostatic latent image carrier with the liquid developer, is transferred and fixed to a record member in such a manner (1) that the toner image is directly transferred to the record member and then is fixed by a fixing device, (2) that the toner image is transferred and fixed directly to the record member by a simultaneous thermal-transfer/fixing manner or the like, (3) that the toner image is directly transferred to the record member by the simultaneous thermal-transfer/fixing manner or the like and further is finally fixed by the fixing device, (4) the toner image is transferred to an intermediate transfer member, and then the image is transferred to the record member and is fixed by the fixing device, (5) the image on the intermediate transfer member is transferred and fixed to the record member by the simultaneous thermal-transfer/fixing manner or the like, or (6) the image is transferred from the intermediate transfer member to the record member by the simultaneous thermal-transfer/fixing manner or the like, and further is finally fixed by the fixing device.
The intermediate transfer member is used, for example, in such a case that, for color image formation, multiple toner images of various colors such as cyan and magenta are formed on the intermediate transfer member, and then is collectively transferred to the record member, and in the following case. The electrostatic latent image carrier such as a photosensitive drum has a relatively hard and smooth surface. Meanwhile, the record member such as a paper sheet has a rough surface. If the toner image, which was formed with the liquid developer containing toner of about 1 .mu.m-about 3 .mu.m in average particle diameter, is electrostatically transferred directly onto the record member, a transfer efficiency may be low, and an image may be disturbed or destroyed. Therefore, thermal transfer can provide a better transfer property when the toner image is electrostatically transferred directly to the record member. In this case, simultaneous thermal-transfer/fixing may be performed with the intermediate transfer member which can perform better electrostatic transfer of the toner image from the electrostatic latent image carrier. In this simultaneous thermal-transfer/fixing, the toner image is first electrostatically transferred onto the intermediate transfer member, and then is thermally transferred onto a final transfer member such as a paper sheet.
However, in the structure where the liquid developer is used and the toner image formed on the electrostatic latent image carrier is directly transferred onto the record member, the electrostatic latent image carrier represented by a photosensitive member as well as other members is liable to be deteriorated due to an influence by heat. This disadvantage occurs regardless of whether the fixing device is employed in the final stage or not, if simultaneous thermal-transfer/fixing is employed, or if the image is temporarily carried on the intermediate transfer member and then is thermally transferred and fixed to the record member.
The following is an example employing the intermediate transfer member. Since the intermediate transfer member is always subject to a high temperature from about 140.degree. C. to about 180.degree. C., it is liable to be deteriorated, resulting in a short lifetime. Further, the photosensitive member neighboring to the intermediate transfer member has a low heat resistance. Therefore, the photosensitive member is liable to be deteriorated due to heating by the hot intermediate transfer member, resulting in a short lifetime. Since an organic photosensitive member which is generally used as the photosensitive member has a particularly low heat resistance, it remarkably suffers the above problem.
Even if the thermal transfer is not employed for the transfer of the toner image to the record member, an independent fixing device may be employed. This fixing device is usually arranged near the electrostatic latent image carrier such as a photosensitive member usually having a low heat resistance for a present demand for a compact structure of the image forming apparatus. Therefore, the electrostatic latent image carrier is liable to be deteriorated by the heat applied from this fixing device.
The thermal transfer (thermal transfer/fixing) and/or thermal fixing by the fixing device at a relatively low temperature may be performed for suppressing thermal damages and deterioration of parts in the image forming apparatus such as an electrostatic latent image carrier. For this, it is necessary that the toner of the toner image to be transferred and fixed to the record member melts at a relatively low temperature to allow thermal transfer and/or thermal fixing.
However, if the toner has such properties that the toner melts at an excessively low transfer and fixing temperatures is used, it also has a low glass transition temperature (Tg). Consequently, when a record member carrying the toner image transferred and fixed thereto is overlapped with another record member, the toner image on the record member on or under another record member may be partially transferred to a rear surface of the underlying or overlying record member, and thus an image rear-side transfer phenomenon occurs. Also, the upper and lower record members may adhere to each other, and thus blocking of the record members may occur. Further, blocking of toner particles may occur due to influences such as an atmospheric temperature during storage of the developer. Moreover, the toner which may melt at an excessively low transfer temperature is liable to cause a cleaning failure when cleaning the untransferred residual toner on the electrostatic latent image carrier and the intermediate transfer member.
For performing the thermal transfer and thermal fixing at a relatively low temperature and avoiding problems such as toner blocking, allowed thermal characteristics of the binder resin of the toner are restricted only to a narrow range. If only one kind of binder resin is used in the toner, predetermined thermal characteristics of the resin can be set by adjusting a composition of monomer, a molecular weight of the resin and others. However, production or synthesizing of the resin may be difficult depending on resin synthesizing conditions such as a composition of the monomer and a degree of polymerization. This reduces a range of selection of the binder resin.