Electrophotography – Image formation – Charging
Reexamination Certificate
1999-02-26
2001-04-17
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Charging
C029S895320
Reexamination Certificate
active
06219510
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a charging member, which is employed in an image forming apparatus such as a copying machine or a printer, and is placed in contact with the image bearing member of the image forming apparatus to charge the image bearing member.
In a conventional electrophotographic image forming apparatus, a corona based charging device has been widely used as a charging apparatus for giving primary charge to a photosensitive member as an image bearing member. In order to charge an object with the use of a corona based charging device, an object to be charged is exposed to corona discharge induced by applying high voltage to a piece of wire, which the device comprises.
In recent years, development of a contact type charging apparatus has been in progress. In order to charge an object, with the use of a contact type charging apparatus, a charging member is placed in contact with an object to be charged, and then, voltage is applied to the charging member. A contact type charging process has some advantages over a corona based charging process, which is a noncontact type charging process. For example, the voltage required in a contact type charging process to charge the surface of an object to a predetermined potential level is lower than that required by a corona based charging process, and also the amount of ozone generated while the object is charged in the former is smaller than that in the latter. Among the contact type charging systems, a roller based charging system, which employs an electrically conductive roller, is more desirable, particularly in terms of reliability, than the other contact type charging systems. Thus, recently, a roller based charging system has come to be widely used.
Illustrated in
FIGS. 14 and 15
is an example of a charge roller as a charging member in a conventional electrophotographic image forming apparatus.
In the drawings, a charging member
1
comprises an electrically conductive shaft
2
, an elastic layer
3
, a resistance layer
4
, and a protection layer
5
. The electrically conductive layer
2
doubles as a power supply electrode. The elastic layer
3
is formed on solid rubber, or resin, for example, styrene-butadiene rubber (SBR), isopropylene rubber, silicone rubber, or the like, in which electrically conductive material such as carbon black or metallic powder is dispersed to make the elastic layer electrically conductive.
The resistance layer
4
is a layer provided for giving a proper amount of electrical resistance to the charging member
1
. As for the material for the resistance layer
4
, acrylic resin, hydrin rubber, urethane, silicone rubber, and the like can be listed. Into these materials, a proper amount of electrically conductive particles, for example, carbon black powder, metallic powder and the like, is dispersed to give these materials a proper amount of electrical conductivity.
The protection layer
5
is provided to give the charging member
1
desirable surface properties, and to prevent the surface of the image bearing member from being contaminated with the resistance layer material. As for the materials for the protection layer
5
, synthetic resin such acrylic resin, N-methoxymethylnylon, acrylic denatured urethane, and the like can be listed.
The protection layer
5
must not disturb the uniform electrical resistance given to the charging member
1
by the resistance layer
4
. However, making the protection layer
5
conductive is liable to allow electrical charge to leak, if low resistance areas such as pinholes are created in the photosensitive member surface. Thus, it is necessary that the protection layer
5
has as much electrical resistance as the resistance layer
4
. In other words, the protection layer
5
must be formed of a material which is uniform in electrical resistance, and in which the aforementioned electrically conductive particles can be desirably dispersed.
Another reason why a charging member is provided with the protection layer
5
which gives the charging member the desirable surface properties is to prevent the electrical resistance of the charging member from becoming uneven through usage. For this objective, the surface energy of the protection layer
5
is desired to be as small as possible. Thus, it has been considered to disperse particles of fluorinated substance (for example, tetrafluoroethylene powder) into the material for the protection layer
5
.
Presently, in order to form the protection layer
5
, which satisfies the above described rigid requirements, that is, a protection layer
5
which is excellent in surface properties, and is uniform in electrical resistance, a solution created by mixing the aforementioned resin, electrically conductive particles, and fluorinated substance, and the like, into organic solvent is coated on the resistance layer
4
, and then, is air-dried at a temperature of no more than 120° C.
It has been known that if the coated layer of the above described solution is air-dried at a temperature of no less than 120° C., the coated layer wrinkles or cracks as it dries, adversely affecting the surface properties of a charging member. Further, if a high temperature is used for drying the coated layers of a charging member, the underlying layers are liable to deform, which also adversely affects the surface properties of a charging member. Therefore, it is desired that relatively low temperature is used also when forming the resistance layer.
Recently, charging members of a new type have been devised. In the cases of these charging members, aqueous resin is employed as the material for the resistance layer
4
and the protection layer
5
, instead of the organic solvent soluble resin used to form charging members of the conventional type. Water-soluble resin is used for the following reason. That is, if organic solvent soluble resin is employed as the material for the resistance layer
4
formed on the elastic layer
3
. The elastic layer
3
, which is formed of foamed material, is caused to swell by the organic solvent, which makes it difficult to produce a charging member with a smooth surface. Thus, aqueous resin is employed to prevent such swelling of the elastic layer
3
caused by the organic solvent.
Employment of aqueous resin enjoys additional merits: it eliminates the need for organic solvent management, and therefore, it is advantageous in terms of cost; and using no organic solvent eliminates the concern about environmental contamination.
However, employment of aqueous material has its own problems. For example, it has been known that if the temperature and humidity of the environment in which a aqueous material based charging member
1
is used become very high, the charging member
1
is liable to adhere to an image bearing member, and if the image bearing member is driven under these conditions, the protection layer
5
is liable to be peeled away from the charging member
1
. Hereinafter, this phenomenon will be simply called “peeling”.
It has been thought that the peeling occurs through the following mechanism.
When the material for the protection layer is aqueous, the protection layer
5
absorbs humidity as the temperature and humidity of the environment in which a charging member
1
is used increase. As a result, the protection layer
5
, that is, the surface layer of the charging member, adheres to the surface of an image bearing member.
It has also been known that If the surfaces of a charging member
1
and an image bearing member are wet with water, in particular, the dew formed on the surfaces of the two members through condensation, come in contact with each other, the adhesion between the two surfaces becomes firmer, because the water is confined in the nip formed by the two surfaces, and the presence of the water between the two surfaces makes it easier for the two surfaces to adhere to each other more firmly than without the presence of water.
Further, if the protection layer
5
and the resistance layer
4
are different in swelling ratio, force is generated at th
Ando Atsutoshi
Onimura Tadashi
Sunahara Satoshi
Suwa Kouichi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Grainger Quana M.
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