1. Field of the Invention
The present invention relates to a developer for developing latent electrostatic images for use in electrophotography, electrostatic printing and the like.
2. Discussion of Background
Two-component dry developers comprising carrier particles and toner particles have been conventionally known. In such a two-component dry developer, finely-divided toner particles are held on the surface of compartively large carrier particles by the electric force generated by the friction between the carrier particles and the toner particles. When the two-component dry developer is caused to come close to a latent electrostatic image, the attraction force generated between the toner particles and the latent electrostatic images overcomes the bonding force between the toner particles and the carrier particles, so that the toner particles are caused to be deposited on the latent electrostatic images. As a result, the latent electrostatic image is developed with the toner particles to a visible toner image. Therefore, the two-component dry developer is used as the toner particles are supplied thereto from time to time while in use in compensating for the toner particles used.
As the materials for a carrier for two-component dry developers, metallic oxides such as magnetite and ferrite are widely used. This is because such metallic oxides have a smaller apparent density than that of an iron powder carrier, so that when such metallic oxides are used as the materials for a carrier, the weight of the two-component dry developer can be reduced. Furthermore, such metallic oxides have an advantage over other materials that when such a metallic oxide is used as a carrier for a two-component dry developer, the stirring resistance of the two-component developer in a development unit is smaller than the stirring resistance of other materials employed in the carrier.
In addition, such metallic oxides have a smaller residual magnetic flux density and a smaller anti-magnetization force than those of an iron powder carrier, and accordingly have a smaller hysteresis loop area than that of an iron powder carrier. Furthermore, such metallic oxides have the characteristics that initial characteristics are always maintained against magnetic reversion and magnetization hysteresis.
Since magnetite and ferrite are oxides, they are chemically stable and hardly chemically changed in contact with ozone, NO.sub.x and the like, which are formed within a copying machine.
A carrier comprising an oxide such as ferrite or magnetite, however, has the shortcoming that a so-called spent phenomenon that a toner film is formed on the surface of carrier particles takes place by the heat generated while in use by the collision among developer particles during high speed development or during the process of making a number of copies, or by a mechanical collision between developer particles and members for a development unit while in use. Once such a spent phenomenon takes place, the charging performance of the carrier is decreased with time while in use. As a result, the toner particles are scattered and toner particles are deposited on the background of images.
In order to prevent the occurrence of such a spent phenomenon, methods of coating the surface of the core particles of carrier particles with a variety of resins have been proposed. However, none of them is satisfactory for use in practice. To be more specific, carrier particles coated with styrene-methacrylate resin or styrene polymer have excellent charging characteristics, but the critical surface tension thereof is relatively high, so that the above-mentioned spent phenomenon takes place during a repeated copy making operation and the life of the developer is shortened.
Examples of a conventional negative charge controlling agent include metal complex salts of monoazo dyes, nitrohumic acid and salts thereof, sulfonated copper phthalocyanine pigments, nitro-group- or chlorine-introduced styrene oligomers, chlorinate paraffin, and melamine resin. These compounds have a complicated structure, and therefore are unstable in the properties.
When such negative charge controlling agents are kneaded with application of heat thereto, they are easily decomposed and are caused to deteriorate, so that the charge controlling performance is lowered. Furthermore, the chargeability of many of such charge controlling agents is changed by the ambient conditions thereof.
There is a case where when a toner comprising such a conventional charge controlling agent is used for an extended period of time, the toner is deposited in the form of a film on a photoconductor because of its improper chargeability.
For instance, Japanese Laid-Open Patent Application 61-223753 discloses toners comprising aromatic hydroxy metallic salts such as a salicylic acid chromium complex. These toners, however, have the shortcomings that the chargeability is unstable, and the charging performance is greatly changed depending upon the ambient conditions thereof.
Japanese Laid-Open Patent Application 3-1162 discloses a method of using a fluorinated ammonium compound or iminium compound. However, when this method is employed, the charging stability differs depending upon a carrier employed, and it is difficult to obtain a sufficient charge-stabilizing effect on a non-coated carrier for use in practice by this method.
In the case of a styrene-acrylic copolymer coated carrier, the chargeability is stable in a continuous mixing process, but when it is repeatedly used with a toner being replenished in a development unit, the chargeability is unstable, and the initial charge-rising performance is not satisfactory for use in practice.