1. Field of the Invention
The present invention relates to a toner and a color toner set for use in electrophotographic image forming apparatuses such as copiers, printers, and facsimiles. The present invention also relates to a process cartridge and an image forming method using the toner and/or the color toner set.
2. Discussion of the Background
In a typical electrophotographic image formation, a full-color image signal is optically or electrically separated into the subtractive primary colors (i.e., yellow, magenta, and cyan) and black, and dot images of each color are formed. The dot images of each color are superimposed on one another on paper or an intermediate transfer member and finally fixed on paper. Electrophotographic full-color image forming technique has been drastically improved in terms of image quality, however, there is still room for improvement.
Disadvantageously, the minimum dot diameter in electrophotography is still larger than that of offset printing. Also, it is likely that dot diameter is more variable as dot density increases or dot size decreases. With regard to highlight image area, there is an attempt to reduce the number of dots to be written to avoid the above problems. However, this attempt causes another problem that image granularity is variable depending on image density. Compared to offset printing, highlight image area in electrophotographic image is more grainy, because the degree of coloration per dot is high.
In high-density image area, toners of the subtractive primary colors, i.e., yellow, magenta, and cyan are superimposed on one another. In white image area (hereinafter also referred to as “non-image area” or “background area”), paper is exposed. Accordingly, as the image density increases, the amount of toner deposited on paper also increases, and vice versa. In particular, light which has been reflected diffusely at background area is absorbed in toner layers in halftone image area. As a result, the image density visually appears to be higher than the actual dot area ratio in halftone image area. This phenomenon is what is called “optical dot gain”. Optical dot gain is one of the reasons for poor reproducibility of highlight image area.
With regard to color images, preferred gloss depends on the kind of image. For example, photographic images with high descriptiveness, such as portraits and landscapes, are generally preferred to have appropriate gloss to provide magnificent texture. In electrophotography, as described above, the amount of toner deposited on paper is different between high-density image area and low-density image area, which provides a difference in surface smoothness. As a result, gloss may vary depending on image density and therefore unevenness in gloss may be observed within a single image, providing us with sense of discomfort. When the difference in gloss is large between background area and image area, text image may also be illegible.
On the other hand, ink-jet recording methods can readily produce full-color images with making little printing noise. Therefore, ink-jet recording methods have been widely used recently in accordance with rapid progress of their printing performance. For example, ink-jet recording methods have been used for recording documents which are written using word-processing software, recording digital images such as digital photographs, making copies of scanned images of beautiful printings such as silver halide photographs and books, and making display images such as posters in a relatively small amount. Recently, in the field of commercial printing that makes various kinds of products in small lots, there are more opportunities that ink-jet recording methods are employed in place of offset printing methods.
Various types of ink-jet recording media have been proposed. For example, normal paper types have been proposed for simply recording texts, on the surface of which texts are directly recorded. Coated paper types having an ink absorbing layer (i.e., a coating layer) have been also proposed, to obtain images with high resolution and high color reproducibility comparable to silver halide photographs. Particularly, cast-coated paper types in which a coating layer is formed by a cast method are preferable when the resultant image requires high gloss. Roller-coated paper types having a thick coating layer are preferable for display images such as posters.
However, it is likely that an ink-jet recording medium which is capable of producing glossy images comparable to glossy offset printings is not commercially available because of its high manufacturing cost.
Ink absorbing layers of coated paper types are required to absorb as large amount as possible of inks because images which require high color reproducibility generally use a large amount of inks. To improve ink absorbing ability, there is an attempt to include a porous substance such as synthetic amorphous silica in an ink absorbing layer. This attempt improves ink absorbing ability but has disadvantages that the resulting image has low gloss and the texture thereof is different from offset printings. The texture of an image formed on a cast-coated-paper-type recording medium is also different from offset printings because the gloss is extremely high and the thickness is very large. The manufacturing costs of the above-described ink-jet recording media are higher than recording media for offset printing because the ink-jet recording media generally include expensive raw materials such as silica, alumina, polyvinyl alcohol, ethylene vinyl acetate emulsions, and ink fixatives (e.g., polyamines, DADMACs, polyamidines) in large amounts.
Conventional ink-jet recording media, particularly glossy ink-jet recording media, are classified into swelling types and void types. The recent mainstream is void types because the drying rate is advantageously very high. A typical void-type recording medium has an ink absorbing layer including voids for incorporating inks, and optionally has a porous gloss layer. Such a void-type recording medium can be prepared by coating a base material with a single layer or multiple layers formed from a liquid in which a silica and/or an alumina hydrate are/is dispersed, and optionally further coating the layer or layers with a gloss layer including a large amount of a colloidal silica. The void-type recording media are designed so as to have affinity for dye-based inks, which are the recent mainstream of inks, and have been widely used as glossy recording media in ink-jet printing.
Glossy ink-jet recording media provide images with high gloss and high definition, however, the manufacturing cost is very higher than that of glossy coated paper for general commercial printings. This is because the raw materials are very expensive and the manufacturing process is complicated. Therefore, glossy ink-jet recording media tend to be used only for high-grade printing such as photographic printing and not to be used for commercial printing of leaflets, catalogs, and brochures which require large output at low cost. In accordance with a recent tendency that the number of color inks used in high-quality images is increased, ink absorbing ability is also required to be more increased. To increase ink absorbing ability of media, one proposed approach involves increasing the thickness of an ink absorbing layer. This approach requires a large amount of expensive raw materials, which results in increase of the price of the resulting medium.
As described above, the mainstream approach of producing high-gloss images in ink-jet recording is to subject recording media to a treatment. However, this approach has a problem of high cost and cannot take advantage of ink-jet recording methods that are capable of printing images on normal paper.
In contrast to ink-jet recording, in electrophotography, toners, more particularly binder resins of the toners, are provided with gloss to produce high-gloss images. However, a color image formed with four toners of cyan, magenta, yellow, and black may have a disadvantage that gloss may vary among image area, non-image area, half-tone image area, and image area in which the density continuously changes.
In attempting to solve the above problem, one proposed approach involves using a transparent toner in addition to yellow, magenta, cyan, and black toners so that the resulting image gloss is adjusted or controlled.
For example, Japanese Patent Application Publication No. (hereinafter JP-A) 07-248662 discloses a color image forming method in which a transparent toner is deposited accordingly in addition to yellow, magenta, cyan, and black toners so that the total amount of the deposited toners becomes always constant.
JP-A 08-106195 discloses a color image forming device in which a transparent toner is deposited first, followed by deposition of yellow, magenta, cyan, and black toners.
JP-A 09-200551 discloses a digital color copying machine in which a transparent toner image is formed with an inverted image signal of any among yellow, magenta, cyan, and black image signals.
JP-A10-123853 discloses an image forming device and method in which a transparent toner layer is formed on an intermediate transfer member and colored toner images are formed thereon.
JP-A 10-207174 discloses a multicolor image printer in which a transparent toner image is superimposed on a colored toner image.
JP-A 11-7174 discloses a multicolor image forming method in which the deposited amount of a transparent toner is adjusted according to the surface roughness of a transfer member.
JP-A 05-232840 discloses a recorder in which surface texture of an image is completely or partially controlled with a transparent toner layer.
JP-A 07-72696 discloses a method for electrostatic photographic printing in which the position and the amount of a transparent toner to be deposited are controllable.
JP-A 04-278967 discloses a method for forming color image in which a transparent toner is deposited on an intermediate transfer member first and then colored toner images are transferred thereon. The resulting layers comprised of the transparent toner and the colored toners are transferred onto a transfer paper and fixed thereon.
The above-described approaches have proposed the use of a transparent toner that is colorless and achromatic. In all of these approaches, the resulting image gloss is made uniform by depositing a transparent toner on low-density image areas or covering all over an image with a transparent toner, which may reduce our sense of discomfort. Such transparent toners produce desired effects so long as the melting properties thereof are optimized.
It is natural that a colored toner that includes a colorant in a large amount and a transparent toner that includes no colorant exhibit different dynamic melting properties when being fixed on a recoding medium. If melting properties of the toners are not controlled appropriately, a colored toner image area and a transparent toner image area may exhibit different gloss and different transparency, which results in deterioration of image quality. Additionally, if melting properties of the colored toner are not adjusted appropriately, it is likely that hot offset problem occurs and an image is not normally formed. If a transparent toner has poor durability, the surface of the resultant image may be easily abraded. As a result, the image may become cloudy and color reproducibility of the image may deteriorate.
In attempting to solve the above-described problem, another proposed approach involves combining ink-jet recording and electrophotography.
For example, JP-A 2002-326455 and Japanese Patent No. 3902733 disclose the following 2 methods:
(1) forming an image on a substrate with an ink by an ink-jet recording method, and exposing the image to a transparent toner; and
(2) exposing a substrate to a transparent toner, and forming an image on the substrate with an ink by an ink-jet recording method.
In the above methods, first, the substrate (e.g., paper) is charged. An image is then formed thereon with an ink by an ink-jet recording method so that the charge on the substrate is neutralized by the ink, resulting in formation of a latent image for a transparent toner. Accordingly, the transparent toner is directly deposited on the substrate, the mechanism of which is different from a typical electrophotographic method which includes forming an electrostatic latent image on a photoreceptor, not on paper, depositing a toner on the electrostatic latent image to form a toner image, and transferring the toner image from the photoreceptor onto paper.
More specifically, in the above method (1), the substrate (e.g., paper) is charged and an image is then formed thereon with an ink by an ink-jet recording method. The charge on the substrate is neutralized by the ink. Therefore, an area of the substrate to which the ink is adhered (i.e., an image area) has no charge, whereas an area of the substrate to which the ink is not adhered (i.e., a non-image area) keeps the charge. The transparent toner is selectively deposited on the image area owing to this charge difference between the image area and the non-image area. Accordingly, even if the transparent toner is deposited on whole surface of the substrate, gloss is made uneven because the deposited amount of the transparent toner is different between the image are and the non-image area. In halftone image area, the transparent toner may be deposited in the form of grain or bulk, which may cause local diffused reflection. In a case in which water is adhered to a boundary between the image area and the non-image area, the image area that is covered with the transparent toner may swell due to migration of the water through the paper. As a result, the image may blur and water resistance of the image may deteriorate.
In the above method (2), according to JP-A 2002-326455, a polymer having hydrophilicity and wettability is included in the transparent toner so that an ink-jet image can be formed even on the transparent toner. However, such a toner including a polymer with wettability is likely to adsorb moisture with time, disadvantageously fusing on or aggregating in a toner bottle or a developing device.
As another approach, Japanese Patent No. 3955459 discloses a protective coating which is formed on an ink-jet color image by melting a transparent toner by heat. The transparent toner includes a thermoplastic ionomer that is a polymer resin having a polar group to which a metal ion is added by ionic-biding crosslinking. Because the ionomer has high affinity for metals, the toner may fixedly adhere to a developing sleeve or a carrier with time. Consequently, images are not reliably formed for an extended period of time. Additionally, since ionic-binding substances generally have high water-solubility, the transparent toner including such an ionic-binding substance is likely to adsorb moisture in the air. As a result, disadvantageously, the properties of the transparent toner may vary with time. Ionomer resins advantageously have high strength and high transparency but are disadvantageously expensive compared to general-purpose materials used in electrophotography such as polyester resins, styrene-acrylic resins, and polyolefin resins. There is still no method which can readily produce high-gloss images at low cost, which can replace offset printing.
Accordingly, the background of the present invention can be summarized as follows.
When a transparent toner is used in electrophotography for the purpose of adjusting gloss difference within a single image, controlling the gloss of an image, or adjusting the relation between image density and the deposited amount of toner, the resulting image density and transparency may be not always uniform. The reasons for this have been considered that:
(1) the transparent toner and colored toners have different properties;
(2) a layer of the transparent toner and a combined layer of colored toners and the transparent toner exhibit different glosses; and
(3) gloss varies depending on the thickness of a layer of the transparent toner that is formed on a layer of colored toners.
With regard to ink-jet recording, there is a problem that glossy images can be produced only on expensive and exclusive paper without taking advantage of ink-jet recording which is capable of printing images on normal paper.
With regard to methods combining ink-jet recording and electrophotography, there are problems that water resistance of the resulting image is poor because of their mechanism of image forming and that wettability and cost are high because usable raw materials are hydrophilic and expensive.