1. Field of the Invention
The present invention relates to an image-forming color toner for electrophotography and electrostatic printing, a developing agent containing the same, an image-forming apparatus using the same, a toner container containing the same, an image-forming process cartridge containing the same, and image-forming process using the same.
2. Description of the Related Art
In the conventional image-forming dry process of electrophotography, electrostatic printing, electrostatic recording, and the like, a dry toner generally includes at least a binder resin and a colorant as a principal composition, and frictional electrification is applied to the dry toner by a charger. Thereafter, a latent image is developed by the charged toner so as to form a toner image, and the toner image is transferred and fixed onto a recording paper. In order to obtain a high-quality image, the most important question is how to develop a latent image formed on a photoconductor (also referred as “a latent image bearing member” hereinafter) accurately using a toner under various conditions in this process, and studies are currently being performed thereon.
The properties required of a developing agent, which can achieve a high-quality image, are storage properties (anti-blocking properties), transport properties, developing properties, transfer properties, charging properties, image fixing properties, and the like.
In general a process used to fix a dry toner on a recording material is of the contact and heating image-fixing type wherein a roller or a belt with a flat and smooth surface is heated, and pressed in contact with the toner. As a heating efficiency is high, in this method, high-speed fixing is possible, and gloss and transparency may also be imparted to a color toner. On the other hand, a so-called “offset phenomenon” may occur as the result of that the heated fixing member surface is released after the contact with the toner in the molten state under pressure. Namely, a part of the toner image adheres to the surface of the fixing roller and then transferred to another image. To prevent this offset phenomenon, it is common to employ a method that a surface of the fixing roller is formed of silicone rubber or fluorinated resin which have excellent mold releasing effects, and a mold releasing oil such as a silicone oil is coated thereon. Although this method is very effective to prevent offset there still remains a problem in that an apparatus for supplying a mold releasing oil is required so that a fixing apparatus becomes bulky and costly. Hence, with monochrome toners, there is a recent trend to increase an elasticity of a molten toner by adjusting a molecular weight distribution of a binder resin so that the molten toner does not internally fracture and to add a mold releasing agent such as wax to the toner. This method allows the image forming method not to apply the mold releasing oil to the fixing roller, or to applying only a small amount of the mold releasing oil.
In color image-formation by a full color electrophotography, however, it is generally required to use a three-color toner having the three colors of yellow, magenta and cyan, or a four-color toner with the addition of black, to laminate and reproduce all the colors, and give a clear color image with excellent color reproducibility. To obtain a clear color image, a surface of the fixed toner image must be smoothed some extent to reduce light scattering. This means that the viscoelasticity in the molten state must be reduced. Thus, when forming a full color image, offset tends to occur more easily than with a monochrome toner having no gloss, and it is less suitable to use an oilless image fixing apparatus or to use only a small amount of the oil coating. Moreover, if a mold releasing agent is contained in the toner, a transferability to transfer paper decreases as a result of an increase in a toner adherence, and charging properties are also degraded as a result of a contamination occurred to a frictional charger such as a carrier by the mold releasing agent in the toner. This led to a decline in the durability of the developing agent.
In image fixing methods, a heat roller fixing method is often used because it has a simple apparatus construction and easy handling. However, the heat roller fixing which is frequently used in the related art has the following problems:    (1) a waiting time until a heating roller reaches a predetermined temperature is long;    (2) the heating roller must be maintained at a suitable temperature to prevent fixing defects or offset due to temperature fluctuations of the heating roller resulting from a passage of recording materials or other external factors, so that an increase of the thermal capacity is required in the heating roller or the heating body; also in general, in the case of full color toners with low viscosity,    (3) due to the curvature of the roller, there is a problem of offset or the toner being taken up by the roller during paper discharge, and it is necessary to provide a mold releasing oil and install an oil tank for the mold releasing oil.
To resolve these problems, a belt heating fixing method has been proposed, as well as a belt fixing method where oil is not applied (oilless method) or only a small amount is applied. However, a pressure is small and the pressure cannot be used effectively as a means of obtaining high gloss in the belt fixing method, whereas a roller having an elastic body of a certain thickness is often used and a high pressure is often applied in the full color roller fixing method as described above.
There are following methods for obtaining high gloss in belt fixing.
Japanese Patent Application Laid-Open (JP-A) No. 02-160250 discloses a method in which an average particle diameter, a fine powder content and a rough powder content are specified, and toner surface imperfections are eliminated. However, in this case, a glossy image is not necessarily obtained even if there are few imperfections on the recording material and the toner is adhering to it.
For increasing flatness and obtaining high gloss, in JP-A No. 11-125948, a method is described for specifying a surface roughness of the image on an OHP sheet. However in this case, as the surface properties of an OHP sheet are different from those of paper, even if the surface roughness of the image on the OHP sheet is specified, offset and glossiness are not necessarily good depending on the surface properties of the paper.
Although glossy images were often desired with full color images, there is now a large demand on printer output, and it may occur that too much gloss is not desirable. Further, a non-glossy image may also be desired in some cases. Hence, it is required to be able to obtain images of differing glossiness depending on the case. To address this need, JP-A No. 04-194967 proposes a fixing method wherein a selection can be made between high gloss and no gloss with one fixing apparatus. However, conditions for obtaining high gloss are disadvantageous for offset, so depending on the conditions, they may not be sufficient tolerance to offset. Specifically, offset does not occur in the beta area, but there is a problem of offset (fine offset) in the halftone parts.
Also, depending on the fastness of the gloss under conditional changes, the properties of gloss may be affected to considerable extent. Thus the glossiness becomes unstable in changeable conditions.
Various types of toner have been discussed in the related art. For example, JP-A No. 08-220808 proposes a toner using a linear polyester resin having a softening point of 90° C. to 120° C. and carnauba wax, JP-A No. 09-106105 proposes a toner containing a resin and wax which are compatible and have different softening points, JP-A No. 09-304964 proposes a toner specifying a melt viscosity of a polyester resin and wax, JP-A No. 10-293425 proposes a toner containing a polyester resin having a softening point of 90° C. to 120° C., rice wax, carnauba wax and a silicone oil, and JP-A No. 05-61242 proposes wax occlusion polymer toner. Although all of these toners gave a suitable gloss, anti-offset properties were insufficient even if mold releasing oil was not coated or was coated in only a small amount on the fixing roller, and the toners did not have very good transfer properties, durability, charge stability relative to humidity or crushing properties.
In recent years, market demands for high-quality images are increasing, and as sufficiently high image qualities can no longer be obtained with a toner having a weight average particle diameter of 9 μm to 15 μm of the related art, a toner of still finer particle diameter is required. As the specific surface area increases the smaller particle diameter of the toner is, the powder fluidity of the parent colorant particles decreases, a large amount of external additives must be added as a surface treatment to confer fluidity. If it is attempted to obtain a desired fluidity, moreover, the aforesaid side-effects become more obvious. Also, the mold releasing agent separates due to stress and the like. In particular, toners manufactured by pulverization have a narrow molecular weight distribution and the brittle mold releasing agent easily becomes a pulverization interface, so the surface of the mold releasing agent became exposed which was frequently found in fine powder. Therefore, making the toner particles finer to obtain high image quality imposes a stricter requirement on filming. To resolve these problems, in toners using a styrene resin as the toner binder, it is known that polyolefin mold releasing agents such as low molecular weight polyethylene or low molecular weight polypropylene, or resins wherein a styrene resin has been grafted onto these polyolefin resins, are effective (Japanese Patent Application Publication (JP-B) No. 52-3304, JP-B No. 07-82255, and the like). However, as the styrene resins used here have poor low temperature fixing properties, there was a problem in lowering the fixing temperature to satisfy energy-saving demands in recent years. To improve low temperature fixing properties, studies were performed using polyester resin as the binder resin which has excellent low temperature fixing properties. However, if polyester resins are used alone, the dispersion of the mold releasing agent in the polyester resin was insufficient, and if only one type of polyester resin was used, it was difficult to maintain a sufficiently wide fixing temperature range having both low temperature fixing properties and hot offset properties.