As the movement toward faster speeds and better quality images in office automation equipment continues at an increasing pace, the precise control of semiconductivity in the intermediate transfer belt is now indispensable to optimize transfer efficiency. Particularly important is the technique to precisely control the electrical resistance of the intermediate transfer belt.
A carbon black-dispersed polyamic acid solution composition, used as a feedstock of the intermediate transfer belt, is generally produced by dispersing and mixing the carbon black added to a polyamic acid solution prepared by polymerization of tetracarboxylic dianhydride and diamine. For operability such as ease of belt formation, the polyamic acid solution is generally a solution of a high-molecular polyamic acid with a weight-average molecular weight of 30,000 or more. This has limited the solubility of the polyamic acid resin in organic polar solvents, making it difficult to increase the content of the resin (for example, at most 20 weight % in terms of a solid content of the solution).
Further, because adding the carbon black to the polyamic acid solution increases the viscosity of the solution, it is difficult to grind the carbon black even with the force of impact between the balls of a disperser such as a bead mill. Formation of a uniform dispersion of carbon black in the polyamic acid solution must involve grinding of the carbon black by a disperser, and a surface phenomenon known as “wetting”, in which the pulverizing carbon black is wet by the solvent. Currently, a uniform dispersion of carbon black is obtained by adding a large amount of organic polar solvent with the carbon black. Consequently, the solid content of the resulting high-carbon black-content polyamic acid solution composition can only be increased to generally about 15 to 20 weight %.
With such a low solid content, the polyamic acid solution cannot be easily molded into a thick belt at once. Further, since a large amount of organic polar solvent is required, evaporating and removing the solvent takes a great deal of time. These factors add time and cost to complete the entire process, and accordingly, improved efficiency and economy in the process is necessary.
Patent Document 1 describes a conductive polyimide seamless belt in which conductive carbon blacks having a relatively small powder resistance, such as acetylene black and Ketjen black, are dispersed in polyimide resin.
However, because the conductive carbon blacks used in this publication greatly increase the amount of polyamic acid or solvent adsorbed or retained, there is problematic increased viscosity, reduced dispersibility, and poor dispersion stability, which lead to deficiencies such as reduced processibility.
Further, with the intermediate transfer belt containing such a conductive carbon black, non-uniform images are formed even when the volume resistivity is adjusted within a predetermined range. Conceivably, this is due to the structure formation caused by primary aggregation, or conductive chains created by secondary aggregation, both of which occur when the conductive carbon black is dispersed in the polyimide resin, and create severely impaired images.
Patent Document 2 discloses a semiconductive belt that contains 1 to 30 parts by weight of one or more kinds of carbon black having a volatile content of at least 2% and less than 30% with respect to 100 parts by weight of binder resin. According to the publication, a carbon black content in excess of 30 parts by weight is not preferable because it makes the molded semiconductive belt brittle, and destroys the characteristic mechanical properties (here, high tenacity) of the binder resin.
Patent Document 3 discloses a semiconductive polyimide belt that uses a conductive carbon black whose volatile content, primarily volatile acids, is 10% to 25%. According to the publication, a carbon black volatile content of less than 10% reduces the dispersibility of the carbon black, and cannot provide a sufficient electrical resistance.
According to a channel process, carbon black is produced in low-temperature air; the surface of the product carbon black therefore contains large numbers of oxygen-containing functional groups, such as a carboxyl group, a phenolic hydroxyl group, a quinone group, and a lactone group. These oxygen-containing functional groups are known to improve the dispersibility of the carbon black in the polyamic acid solution. However, the volatile content of the carbon black produced by a channel process contains large amounts of impurities such as sulfur and undecomposed feedstock hydrocarbons (PAH), in addition to hydrogen and oxygen. The undecomposed feedstock hydrocarbons (PAH), in particular, react with nitrogen oxide to produce nitro compounds, which are converted into highly carcinogenic, polycyclic aromatic nitro compounds. Therefore, the use of carbon black produced by a channel process is not desirable in terms of safety and environmental adaptability.
On the other hand, in an oil furnace process, the carbon black is produced by the pyrolysis of hydrocarbons in a reducing atmosphere under the heat of a high-temperature gas of combusting fuel equal to or greater than 1,400° C. The carbon black produced by an oil furnace process is preferable because it contains only small amounts of oxygen and impurity inside or on the surface of the particles, and therefore permits the formation of crystallites. Drawbacks of the carbon black produced by an oil furnace process, however, are poor dispersibility in the polyamic acid solution, aggregation during storage, and the like.
Patent Documents 4 through 7 report various oxidation methods in which carbon black produced by an oil furnace process is modified to add oxygen functional groups to the surface, in order to produce carbon black that can be suitably used in applications such as in liquid toners, ink, and coating materials.
Patent Document 1: JP-A-5-77252
Patent Document 2: JP-A-2000-309712
Patent Document 3: JP-A-2001-47451
Patent Document 4: JP-A-11-181326
Patent Document 5: JP-A-2000-7937
Patent Document 6: JP-A-2000-290529
Patent Document 7: JP-A-2001-40240