1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image in dry process to form images in electrophotography, electrostatic recording, electrostatic printing, and the like. The present invention also relates to a fine powdery titanium oxide, particularly to hydrophobic fine powdery titanium oxide suitable as an external additive to the toner.
2. Related Background Art
Methods for developing an electrostatic image are well known in which an electrostatic image is formed on a photoconductive surface by electrostatic means and subsequently the image is developed with a toner. Various methods have been disclosed, for example, in U.S. Pat. No. 2,297,691, JP-B-42-23910, and JP-B-4324748 ("JP-B" herein means "examined Japanese patent publication). In these methods, generally, an electrostatic latent image is formed on a photosensitive member employing a photoconductive substance and thereon a toner image is formed corresponding to the electrostatic image with the toner adhering to the latent image. Then, the toner image is transferred onto a surface of a desired image supporting medium (recording medium) such as a paper sheet, and the transferred image is fixed by heating, pressing, heat-pressing, or solvent-vapor exposure to obtain a copy. In the method involving a step of transferring the toner image, a cleaning step is usually provided to remove remaining toner as required.
The known developing methods to visualize an electrostatic image with a toner include powder cloud development described in U.S. Pat. No. 2,221,776, cascade development described in U.S. Pat. No. 2,618,552, magnetic brush development described in U.S. Pat. No. 2,874,063, and electroconductive magnetic toner development described in U.S. Pat. No. 3,909,258.
The toner used in such development methods is generally prepared by mixing a thermoplastic resin with a colorant for dispersion, and pulverizing it. Polystyrene resin is generally used as the thermoplastic resin. Polyester resin, epoxy resin, acrylic resin, urethane resin, or the like is also useful therefor. As the colorant, carbon black is widely used. In a magnetic toner, iron oxide type black magnetic powder is widely used. When a two-component developer is used, the toner is usually mixed with a particulate carrier such as glass beads, iron powder, magnetic ferrite powder, or resin-coated particles thereof.
The toner image on the final image bearing member like a paper sheet is fixed thereon by heating and/or pressing. Thermal fixation is frequently employed therefor.
In recent years, copying machines and printers have been rapidly developing from mono-color image to full-color image, and two-color copying machines and full-color copying machines are now practically used. For example, studies on color reproduction, or tone reproduction are reported in "Denshi Shashin Gakkaishi (Journal of Electrophotography Society)": Vol. 22, No.1 (1983), and Vol. 25, No.1, page 52 (1986).
Further improvement of the present full-color electrophotographic image formation method is required, since the people are accustomed to artificial color images like television images, color photographs and color prints more beautiful than actual objects and not compared directly with the actual objects.
In full-color electrophotography, all colors of an image are generally reproduced with three kinds of primary color toners, that is, yellow, magenta, and cyan. In this method, an electrostatic latent image is formed on a photoconductive layer using light from the original passing through a color separation filter which transmits only the complementary color of a toner color, and then the image is developed with the toner and transferred onto a recording member. This process is repeated for each color, and the toner images are superposed on one and the same recording member with registration, and the superposed images are fixed in one step to obtain a final full-color image.
When a two-component developer composed of a toner and a carrier is used, the toner particles are electrostatically charged by friction with the carrier particles to a required level in the desired polarity, and the electrostatic image is developed using electrostatic attraction. Therefore, the triboelectrification properties of the toner mainly determined in relation to the carrier are important for obtaining a excellent visible image.
Accordingly, to obtain satisfying triboelectrification properties, many studies have been made on carrier core materials, carrier core-coating materials, optimum coating amount, toner charge-controlling agents, fluidity-improving agents, improvement of the toner binder, and so forth.
For example, an electrification aid like an electrifiable fine particulate material is added to the toner, JP-B-52-32256 and JP-A-56-64352 ("JP-A" herein means "Unexamined Laid-Open Japanese Patent Application") propose to add to the developer a fine powdery resin having the polarity opposite to the toner; and JP-A-61-160760 discloses addition of a fluorine-containing compound to the developer to obtain stable triboelectrification properties of the toner.
The above electrification aid is generally added by adhesion of the electrification aid to the toner particle surface through electrostatic force, Van der Waals force, etc. where a stirring or mixing machine is used for it. In such a method, uniform adhesion of the electrification aid does not readily occur on the toner particle surface, and the additive not adhering to the toner form aggregates. This tendency is more remarkable with the larger electric resistivity and the smaller particle size of the electrification aid, which adversely affects the toner performance. For example, unstable triboelectrification, fluctuation of image density, and fogging tend to occur. There is also a problem that the content of the electrification aid changes so that the initial image quality cannot be maintained during continuous copying.
In another method of addition, the electrification aid is added in advance during production of the toner along with a binding resin and a colorant. However, uniform dispersion of the electrification aid is difficult. Further, only the electrification aid on or near the surface actually contributes to the electrification, and the electrification aid or the charge-controlling agent in the interior of the toner particles do not contribute to the electrification properties. Therefore, the amount of the electrification aid to be added or the amount thereof on the toner surface is difficult to control, and the triboelectrification is liable to be unstable even with the toner prepared by such a method.
In recent years, for image output of copying machines and printers, finer and higher quality is required. To improve the image quality, it has been attempted to make the toner particle diameter smaller. However, the finer particle diameter of the toner results in increase of the surface area per unit weight, thus increasing the amount of electrification, which tends to cause low image density and deterioration of durability. Additionally, the larger electric charge of the toner increases the adhesion between toner particles, lowering fluidity of the toner and causing problems in stability of toner supply and impartment of triboelectricity to the supplied toner.
A color toner, which does not contain electroconductive substance like carbon black or magnetic material to leak electric charge, tends to be charged at a higher level. This tendency is remarkable when a polyester binder is employed as the binder for the toner.
For color toners, are strongly required properties as set forth in below.
(1) The fixed toner particles should be almost completely fused so that the individual toner particles are not discriminable to reproduce the color without irregular reflection of light; PA1 (2) The color toner should be transparent not to intercept the underlying color of another color toner layer; and PA1 (3) Each color toner should have well-balanced hue and spectral reflection characteristics, and sufficient color saturation.
From such viewpoints, various binder resins are under investigation to satisfy the above requirements. At the moment, polyester resins are widely used as the binder resin for the color toner. The toner comprising a polyester resin is generally affected by temperature and humidity, electric charge is excess at low humidity, and insufficient at high humidity. Therefore, color toners are desired which have stable charge under various environmental conditions.
In JP-A-48-47345, use of powdery metal oxide is disclosed as an abrasive. Metal oxides like titanium oxide are disclosed as the fluidizing agent in JP-A-52-19535 and JP-A-56-128956. Further, a surface treated amorphous titanium oxide powder is disclosed in JP-A-4-337739, JP-A-4-348354, JP-A-4-40467, and JP-A-5-72797 for the purpose of imparting fluidity to toner particles, stabilizing electric charge and preventing the filming of the toner.
Addition of hydrophobic titanium oxide treated with alkyltrialkoxysilane to the toner is proposed in JP-A-59-52255. The added hydrophobic titanium oxide improves electrophotographic properties of the toner. However, the hydrophobicity may be still insufficient for that purpose since the surface activity of titanium oxide is inherently low in comparison with silica. Further increasing the amount of the treating agent or the treatment period to raise the hydrophobicity is liable to cause agglomeration of titanium oxide particles during the treatment or to cause non-uniformity of the hydrophobicity.
Fine powdery materials are used in various fields and their surfaces are usually treated to meet the requirements in the particular field.
As stated above, in the electrophotographic application field, fine powdery silica, fine powdery titanium oxide, or the like is used as the external additive to the toner for electrophotographic development, and the powdery material treated for hydrophobicity is also used to stabilize the electrophotographic properties of the toner in various environmental conditions.
Known titanium oxide includes those formed by neutralizing aqueous titanium sulfate solution and firing the formed precipitate; those formed by decomposing and oxidizing titanium chloride at a high temperature; and those formed by hydrolyzing or pyrolyzing titanium alkoxide. These titanium oxide are anatase, rutile, or amorphous in crystal structure. The titanium oxide is treated with a hydrophobicity-imparting agent such as the silane-coupling agent to form hydrophobic titanium oxide. Known titanium oxides, however, are not sufficiently reactive to the hydrophobicity-imparting agent, and therefore development of more reactive fine powdery titanium has been desired.