The present invention relates to a toner used for developing electrostatic images formed in an image forming method, such as electrophotography, electrostatic recording and electrostatic printing.
Full color copying machines proposed in recent years have generally adopted a process wherein four photosensitive members and a belt-form transfer member are used, electrostatic images formed on the photosensitive members are developed with a cyan toner, a magenta toner, a yellow toner and a black toner, respectively, to form respective toner images on the photosensitive members, and the toner images are successively transferred onto a transfer(-receiving) material conveyed along a straight path between the photosensitive members and the belt-form transfer member to forma full-color image; or a process wherein a transfer(-receiving) material is wound about the circumference of a transfer member with an electrostatic force or a mechanical force exerted by e.g., a gripper, and a development-transfer cycle is repeated four times to form a full color image on the transfer material.
Toners used in such a full-color copying machine are required to exhibit an improved color reproducibility and cause sufficient color mixing in a heat-pressure fixing to provide a full color image with good transparency as required in overhead projector (OHP) images.
Compared with an ordinary black toner for mono-chromatic copying machines, a toner for full-color image formation may preferably comprise a relatively low-molecular weight binder resin exhibiting a sharp-melting characteristic. However, a toner comprising such a sharp-melting binder resin is liable to cause a problem of high-temperature offset because of low self-cohesion of the binder resin at the time of toner melting in the heat-pressure fixing step.
For an ordinary black toner for monochromatic copying machine, a relatively high-crystalline wax as represented by polyethylene wax or polypropylene wax has been used as a release agent in order to improve the anti-high-temperature offset characteristic at the time of fixation, as proposed in Japanese Patent Publication (JP-B) 52-3304, JP-B 52-3305 and JP-B 57-52574. When such a high-crystallinity wax is used in a toner for full-color image formation, however, the fixed toner image is liable to have inferior transparency, thus providing a projected image with lower saturation and brightness when projected as an OHP image, because of the high crystallinity and difference in refractive index from an OHP sheet material of the wax.
In order to solve the above problem, the use of a nucleating agent together with a wax for lowering the wax crystallinity has been proposed in Japanese Laid-Open Patent Application (JP-A) 4-149559 and JP-A 4-107467.
The use of waxes having a low crystallinity has been proposed in JP-A 4-301853 and JP-A 5-61238. Montan wax has relatively good transparency and a low-melting point, and the use of montan waxes has been proposed in JP-A 1-185660, JP-A 1-185661, JP-A 1-185662, JP-A 1-185663 and JP-A 1-238672.
However, such waxes cannot fully satisfy all the requirements of transparency for OHP use, and low-temperature fixability and anti-high temperature offset characteristic at the time of heat-pressure fixation. For this reason, it has been generally practiced to minimize or omit such a wax or release agent in an ordinary color toner and apply an oil, such as silicone oil or fluorine-containing oil onto a heat-fixing roller so as to improve the anti-high temperature offset characteristic and the transparency for OHP use.
However, according to the measure, the resultant fixed image is liable to have excessive oil on its surface, and the oil is liable to soil the photosensitive member by attachment and swell the fixing roller to shorten the life of the roller. Further, the oil has to be supplied to the fixing roller surface uniformly and at a controlled rate in order to prevent the occurrence of oil lines on the fixed image, and thus tends to require an increase in overall size of the fixing apparatus.
Accordingly, there is a strong desire for a toner which can effectively suppress the occurrence of offset when used in a heat-pressure fixing means omitting or minimizing the use of such an oil for preventing high-temperature offset, and can also provide fixed images with an excellent transparency.
JP-A 8-314300 and JP-A 8-50368 have proposed a toner comprising toner particles enclosing a wax therein formed through suspension polymerization and an image forming method not requiring the fixing oil application.
The toner can suppress the occurrence of oil lines on the fixed images but has to enclose a large amount of wax in the toner particles. Moreover, a binder principally comprising a styrene-acrylate resin is used. As a result, the resultant fixed images are liable to have surface unevennesses, to result in a lower transparency for the OHP use.
Moreover, recorded image products obtained by using the toner tend to exhibit low gloss. This is advantageous for providing graphic images including both graphic images and character images not lacking harmony therebetween but is liable to result in pictorial images with narrow reproduced color ranges because of lower secondary color mixability due to insufficient toner melting in the fixing step.
Accordingly, there is a strong desire for a toner which can exhibit excellent secondary color mixability and transparency for OHP use, a broad color reproducibility range and a broad non-offset temperature range, even when processed by a heat-pressure fixing means omitting or minimizing the use of a fixing oil.
A generic object of the present invention is to provide a toner having solved the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide a toner which can be fixed without applying a large amount of oil or by omitting the oil application at all.
Another object of the present invention is to provide a color toner which can exhibit good transparency for OHP use and a broad color reproducibility range because of good secondary color mixability.
Another object of the present invention is to provide a toner showing good flowability and developing performance.
Another object of the present invention is to provide a toner showing excellent low-temperature fixability and anti-high-temperature offset characteristic, thus showing a broad non-offset temperature range.
Another object of the present invention is to provide a toner showing excellent storability under standing in a high-temperature environment.
A further object of the present invention is to provide an image forming method for forming full-color images by using a toner as mentioned above.
According to the present invention, there is provided a toner, comprising; at least a binder resin, a colorant and a wax, wherein
the binder resin comprises a resin selected from the group consisting of (a) a polyester resin, (b) a hybrid resin having a polyester unit and a vinyl polymer unit, and (c) a mixture of these resins, and
the wax has a structural unit including a polar group and represented by any one of formulae (I)-(IV) or a structure having a polar group and represented by formula (V): 
wherein R1 denotes hydrogen or a hydrocarbon group having 1-8 carbon atoms, 
wherein R5 denotes a saturated hydrocarbon group having 2-20 carbon atoms, an unsaturated hydrocarbon group having 2-10 carbon atoms, an aromatic hydrocarbon group, or an alicyclic hydrocarbon group, and 
wherein R2, R3 and R4 independently denote hydrogen or a hydrocarbon group having 8-50 carbon atoms with the proviso that at least one of R2, R3 and R4 is a hydrocarbon group having 8-50 carbon atoms.
According to the present invention, there is further provided an image forming method, comprising:
(A) an image forming cycle including:
a step of forming an electrostatic image on an image bearing member,
a step of developing the electrostatic image with a color toner to form a color toner image on the image bearing member, and
a step of transferring the color toner image onto a transfer material via or without via an intermediate transfer member,
(B) a process of repeating the image forming cycle (A) four times by using first to fourth color toners, respectively, to form superposed first to fourth color toner images on the transfer material, and
(C) a step of fixing the superposed first to fourth color toner images on the transfer material under application of heat and pressure to form a fixed full-color image on the transfer material, wherein
the first to fourth color color toners are selected successively in an arbitrary order from the group consisting of a cyan toner, a magenta toner, a yellow toner and a black toner,
each of the cyan, magenta, yellow and black toners comprises at least a binder resin, a wax and a corresponding colorant selected from the group consisting of a cyan colorant, a magenta colorant, a yellow colorant and a black colorant,
the binder resin comprises a resin selected from the group consisting of (a) a polyester resin, (b) a hybrid resin having a polyester unit and a vinyl polymer unit, and (c) a mixture of these resins, and
the wax has a structural unit including a polar group and represented by any one of formulae (I)-(IV) or a structure having a polar group and represented by formula (V): 
wherein R1 denotes hydrogen or a hydrocarbon group having 1-8 carbon atoms, 
wherein R5 denotes a saturated hydrocarbon group having 2-20 carbon atoms, an unsaturated hydrocarbon group having 2-10 carbon atoms, an aromatic hydrocarbon group, or an alicyclic hydrocarbon group, and 
wherein R2, R3 and R4 independently denote hydrogen or a hydrocarbon group having 8-50 carbon atoms with the proviso that at least one of R2, R3 and R4 is a hydrocarbon group having 8-50 carbon atoms.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.