Developer carrier containing electrically conductive filler present in a resin coating layer for use in dry-type image developing device

A developer carrier is used in a dry-type image developing device employing a one-component developer in an electrophotographic copier or an electrostatic recording apparatus. The developer carrier has a resin coating layer including an electrically conductive filler dispersed therein which is made of a material having a volume resistivity of at most 100 .OMEGA.cm. With this arrangement, the range in which the resistance of the surface layer varies under the ambient condition of varying temperature and humidity is small, producing a copy of stable image quality free from a smeared or contaminated background.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to a developer carrier for use in a dry-type 
image developing device which employs a one-component developer in an 
electrophotographic copier or an electrostatic recording apparatus. 
2. Description of the Prior Art: 
There is known a developer carrier such as a developing roller having an 
electrically insulative resin layer around the peripheral surface thereof, 
the developer carrier being used as a means for bringing a one-component 
developer into contact with an electrostatic latent image on an 
electrostatic latent image carrier such as a photosensitive drum to 
develop the electrostatic latent image into a visible image. In order to 
remove an electric residual image from the developer layer on the 
developer carrier after the image development, the resin layer on the 
developer carrier must be made of a material exhibiting strong polarity 
such as urethane, epoxy, or the like which is of relatively high electric 
conductivity. The electric conductivity of these materials greatly varies 
as ambient conditions such as temperature and humidity vary, failing to 
provide stable image developing characteristics. 
There is also known a developer carrier having a resin layer made of highly 
insulative acrylic resin, for example, the resin layer having a plurality 
of electrodes scattered as islands therein, each of the electrodes being 
of a size larger than the diameter of a developer particle. With this type 
of developer carrier, if an inverse bias voltage applied to a lightly 
charged developer is high, charges are moved from the electrodes to the 
developer for thereby producing a developer charged in opposite polarity, 
with the result that the amount of the developer consumed is increased. 
It has been found that the image developing characteristics vary due to 
changes in the electric resistance of the resin layer on the developer 
carrier which are caused by absorption of moisture into the resin and 
dispersion of moisture from the resin. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a developer carrier for 
use in a dry-type image developing device employing a one-component 
developer, the developer carrier being arranged to solve the aforesaid 
problems of the conventional developer carriers. 
According to the present invention, a developer carrier for use in a 
dry-type image developing device has a resin coating surface layer with an 
electrically conductive filler being dispersed therein, the electrically 
conductive filler being made of a material having a volume resistivity of 
100 .OMEGA. cm or less. With this arrangement, the range in which the 
resistance of the surface layer varies under the ambient condition of 
varying temperature and humidity is small, producing a copy of stable 
image quality free from a smeared or contaminated background. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, a dry-type image developing device has a photosensitive 
drum or latent image carrier 1 for carrying on its circumferential surface 
an electrostatic latent image formed by electrophotography. The 
electrostatic latent image is developed into a visible image by a 
one-component developer supplied by a developing roller or developer 
carrier 2. The developing roller 2 is supplied with the one-component 
developer by a sponge rubber roller 3 which is capable of 
triboelectrically charging the developer. The one-component developer used 
comprises toner having electric resistance which is high enough to be 
triboelectrically charged. 
The developing roller 2 comprises a core shaft 4, a rubber layer 5 on the 
circumferential surface of the core shaft 4, and a resin coating layer 6 
on the circumferential surface of the rubber layer 5. The core shaft 4 has 
a diameter of 8 mm. The rubber layer 5 has a thickness of 6 mm and is made 
of acrylonitrile rubber (NBR rubber). The resin coating layer 6 has a 
thickness ranging from 5 to 100 micrometers and is made of 
filler-dispersed resin comprising polyester-urethane with a filler having 
a volume average particle diameter of 1 micrometer or less being dispersed 
therein at a filler/polyester-urethane ratio in the range of from 0.10 to 
1.6. 
An electrostatic latent image on the photosensitive drum 1 is developed in 
contact with the resin coating layer 6 of the developing roller 2. The 
rubber layer 5 is effective in keeping the photosensitive drum 1 which is 
substantially a rigid body and the developing roller 2 in good contact 
with each other. 
Examples of developer carriers will be described below: 
EXAMPLE 1 
A filler of TiO (Titanblack, 20M manufactured by Mitsubishi Metal Corp.) 
having a volume average particle diameter of 1 micrometer or less and a 
volume resistivity of 100 .OMEGA. cm or less was dispersed in 
polyester-urethane at a filler/polyester-urethane ratio of 1.4 to provide 
a resin coating layer having a thickness of 30 micrometers. The resin 
coating layer thus fabricated was mounted on an NBR rubber roller to 
produce a developer carrier. 
EXAMPLE 2 
A filler of SnO.sub.2 (containing Sb.sub.2 O.sub.3) having a volume average 
particle diameter of 1 micrometer or less and a volume resistivity of 100 
.OMEGA. cm or less was dispersed in polyester-urethane at a 
filler/polyester-urethane ratio of 0.6 to provide a resin coating layer 
having a thickness of 30 micrometers. The resin coating layer thus 
fabricated was mounted on an NBR rubber roller to produce a developer 
carrier. 
The material SnO.sub.2 (containing Sb.sub.2 O.sub.3) is a resistive 
material which is a metal oxide semiconductor having a positive 
temperature coefficient (PTC) of (.OMEGA.) of the surface layer of the 
developing roller varies with respect to the partial pressure (mmHg) of 
water vapor in atmosphere when the ratio of the electrically conductive 
filler comprising SnO.sub.2 (containing Sb.sub.2 O.sub.3) and the 
polyester-urethane (F/R) changes from 0.4 to 0.8. The graph shows that the 
resistance of the roller surface layer varies sharply when the F/R ratio 
is 0 and 0.8. No desired object can be achieved when the F/R ratio is 0 
and 0.8 or higher. The F/R ratio range from 0.5 to 0.7 is preferable. 
If the F/R ratio becomes higher, the electrical properties of the 
electrically conductive filler becomes more dominant, making the resin 
coating layer lower in resistance under the ambient conditions of low 
temperature and low humidity, whereas if the F/R ratio is relatively low, 
the resin coating layer exhibits higher resistance at low temperature and 
low humidity. 
The same results can be obtained by selecting the F/R ratio to range from 
0.5 to 0.7 for combinations of the electrically conductive filler and 
other resins such as urethanes such as acrylic urethane, fluoroplastics, 
or epoxy resins. Thus, a resin coating layer which is less dependent on 
the ambient conditions can be obtained. These effects are also exhibited 
by generally known superconductive materials. However, the optimum F/R 
ratio thereof may slightly deviate from the range of 0.5 to 0.7 dependent 
on the combination of materials used. 
EXAMPLE 3 
A filler of carbon black (Blackpearl L, #400 manufactured by Cabot Plastics 
Limited) having a volume average particle diameter of 1 micrometer of less 
and a volume resistivity of 100 .OMEGA.cm or less was dispersed in 
polyester-urethane at a filler/polyester-urethane ratio of 0.13 to provide 
a resin coating layer having a thickness of 30 micrometers. The resin 
coating layer thus fabricated was mounted on an NBR rubber roller to 
produce a developer carrier. 
When the developer carriers according to the above Examples were left for 
24 hours at 10.degree. C. and 15%RH, the surface layer resistance of the 
developer carriers was about 1 .times.10.sup.12 ohms. No background 
smearing or contamination was observed in reversal development when the 
potential of an image area on the electrostatic latent image carrier was 
-200V, the potential of a nonimage area (background) on the electrostatic 
latent image carrier was -900 V, and a bias voltage of -500 V was applied 
to the developer carrier. Even when a bias voltage of -850 V was applied 
to the developer carrier and the potential difference between the 
developer carrier and the background area, i.e., the developing potential 
difference, was reduced to 50 V, no background smearing or contamination 
was caused. 
When the ratio between the electrically conductive filler and the 
polyester-urethane was lower than the values indicated in the Examples 
given above, the surface layer resistance of the developer carrier 
exceeded 10.sup.12 ohms, resulting in background smearing or 
contamination. When the ratio was higher than the values in the Examples, 
the surface layer resistance of the developer carrier was lower than 1 
.times.10.sup.9 ohms, also giving rise to background smearing or 
contamination. 
Any background smearing or contamination produced when the surface layer 
resistance of the developer carrier exceeded 1 .times.10.sup.12 ohms could 
be reduced to a certain extent by regulating the bias voltage applied to 
the developer carrier. However, it was difficult to reduce background 
smearing or contamination caused when the surface layer 
resistance of the developer carrier was below 1 .times.10.sup.9 ohms, by 
regulating the bias voltage applied to the developer carrier. This appears 
to result from the fact that electric charges move from the developer 
carrier which is of a relatively low surface layer resistance to the 
developer thereby to invert the charging polarity of the developer. 
The surface of a thin layer of the one-component developer formed on the 
developer carrier should preferably be as smooth as possible. Therefore, 
the thickness of the resin coating layer of the developer carrier should 
be greater than the volume average particle diameter of the electrically 
conductive filler. The volume average diameter of the electrically 
conductive filler should be equal to or less than the volume average 
particle diameter of the developer, or preferably equal to or less than 
1/4 thereof. 
Although a certain preferred embodiment has been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.