Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – To produce color reproduction
Reexamination Certificate
2000-03-15
2001-02-13
Martin, Roland (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
To produce color reproduction
C430S102000, C430S106100, C430S108200, C430S122520
Reexamination Certificate
active
06187490
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a carrier in a developer used for developing electrical latent images or magnetic latent images in electrophotography, electrostatic printing, etc., and particularly a resin-coated carrier with improved durability, image forming characteristic and environmental characteristic, and a two-component developer and an image forming method using the resin-coated carrier.
Carriers forming two-component developers are roughly classified into an electroconductive carrier and an insulating carrier. As the electroconductive carrier, oxidized or monoxidized iron powder has been ordinarily used. In a two-component developer using such an iron powder carrier together with a toner, however, the triboelectric chargeability of the toner is liable to be unstable, and the resultant visible images formed by using the two-component developer are liable to be accompanied with fog. More specifically, as the two-component developer is continually used, toner particles are attached onto the carrier particle surfaces to increase the electrical resistivity of the carrier particles, whereby the bias current is lowered and the triboelectric charge becomes unstable, so that the developed visible images are liable to have a lower image density and be accompanied with increased fog. Accordingly, when a two-component developer containing such an iron powder carrier is used for continuous copying in an electrophotographic copying apparatus, the developer is liable to be deteriorated in a relatively small number of cycles, and the developer has to be exchanged in a short period, thus resulting in an increased running cost.
The insulating carrier representatively has a structure comprising a carrier core of a ferromagnetic material, such as iron, nickel or ferrite, and an insulating resin layer uniformly coating the carrier core. In a two-component developer using such an insulating carrier, the toner particles are less liable to be melt-attached onto the carrier surfaces than in the case of an electroconductive carrier, and it is easy to control the triboelectric chargeability between the toner and the carrier. Thus, the two-component developer is provided with an excellent durability and a longer life, so that it is particularly suitable in a high-speed electrophotographic copying machine.
Various properties are required of an insulating carrier, inclusive of appropriate charging performance, impact resistance, anti-wearing characteristic, good adhesion between the core and the coating material and uniform charge distribution, as particularly important properties.
In view of the above-mentioned requirements, conventionally used insulating carriers still have left room for improvement, and perfect ones are not available so far. For example, the use of an acrylic resin as a carrier coating material has been disclosed in Japanese Laid-Open Patent Application (JP-A) 47-13954 and JP-A 60-208765. Particularly, the molecular weight effect of the coating material is discussed in JP-A 60-208767, which teaches that the charging performance of a coated carrier is stabilized if the molecular weight of the coating resin is controlled at constant. On the other hand, the attachment of a coating resin onto a carrier core material is remarkably affected by coating apparatus conditions and environmental conditions, particularly humidity, and even under strict control of these conditions, it is difficult to stably attach the resin onto the core material to provide sufficient charging performance and durability, so that satisfactory performances cannot be attained.
Several proposals have been made for using a coating layer of a resin showing a low surface energy for preventing spent toner accumulation on the carrier due to toner melt-sticking, etc., and silicone resin has been raised as an example of the resin having a low surface energy.
Silicone resin has advantageous features of not only low surface energy (tension) but also high water-repellency, but on the other hand, has a drawback of providing a coating layer liable to be peeled due to poor adhesiveness.
For providing a solution to the problem, various proposals have been made, including use of a resin-modified silicone resin (JP-A 55-127569), inclusion of vinylsilane for reaction with another resin (JP-A 56-32149), use of a mixture of trialkoxysilane and ethylcellulose (U.S. Pat. No. 3,840,464), and use of a mixture of organosilicone terpolymer and polyphenylene resin (U.S. Pat. No. 3,849,127). These proposals are however accompanied with problems, such as the necessity of a high temperature of 300° C. or higher for forming the coating film, and poor mutual solubility between silicone resin and another resin to result in an ununiform coating film, thus failing to achieve desired performances. It has been also proposed to form a coating film at a relatively low curing temperature (JP-A 55-127569). However, the resultant coating film is liable to show an insufficient adhesion and insufficient toughness, thus being liable to be worn. As a result, if the carrier particles are subjected to collision with the inner wall of the developing device or the photosensitive member surface or collision with each other due to strong stirring for long hours within the developing device, e.g., in a high-speed coating apparatus, the silicone resin coating layer is worn or peeled apart with time, so that the triboelectrification changes from one between the toner and the silicone resin to one between the toner and the carrier core, whereby the triboelectric charge of the developer is not made constant to result in deterioration of image qualities.
In recent years, there are increasing demands for higher resolution and higher image qualities of copying machines on the market, and the use of a smaller particle size toner has been tried for accomplishing high-quality color images. However, a smaller particle size toner is caused to have an increased surface area per unit weight and thus tends to have a larger triboelectric chargeability, which is liable to result in a lower image density and inferior continuous image forming performances.
In color copying using a chromatic toner, a continuous gradation characteristic is an important factor affecting the image quality, and the occurrence of the edge effect that images with an emphasized contour are liable to be formed after continuous copying on a large number of sheets can remarkably impair the gradation of the resultant images. A false contour can be found in the neighborhood of an actual contour, and this impairs the reproducibilities of copying inclusive of color reproducibility in color copying. The areal image percentage in conventional mono-chromatic copying is 10% or below, wherein most images reproduced are line images, such as those of letters, documents and reports. In contrast thereto, a high percentage or area of reproduced images are occupied with solid images with gradation, such as photographs, technical brochures, map and pictures giving an image areal percentage of 20% or higher at the least.
When continuous copying is performed by using an original having such a high image areal percentage, high image density copied products may be produced at the initial stage, but the toner replenishment to the two-component developer is liable to be gradually insufficient to result in difficulties, such as a lowering in image density of the resultant images, the supply of the replenished toner in an insufficiently charged state to the developing region to cause fog, and a local fluctuation in toner concentration (i.e., toner carrier mixing ratio) on the developing sleeve, leading to scartchy or fading images and nonuniform image densities. This tendency is more pronounced in the case of smaller size toners.
These difficulties may be attributable to a lower toner content (i.e., concentration) in the developer, or a poor rise in triboelectric chargeability due to failure in quick triboelectrification between the replenished toner and the carrier in the two-component deve
Ichikawa Yasuhiro
Kohtaki Takaaki
Taya Masaaki
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Martin Roland
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