[0001] The present invention relates to a charging roller for charging a photoconductive
element as defined in the preamble of claim 1, in particular for a copier, printer,
facsimile transceiver or similar image forming apparatus and, more particularly, to
a charging roller for uniformly charging the surface of a photoconductive element,
or imaging carrier. This charging should preferably occur during a sequence of image
forming steps.
[0002] It has been customary with an image forming apparatus of the type described to use
a corona discharger as charging means for uniformly charging the surface of a photoconductive
element. A corona discharger effectively charges the surface of a photoconductive
element uniformly to a predetermined potential. However, the problem is that a corona
discharger needs a high tension power source and generates ozone during discharge.
Ozone generated in a great amount would not only pollute the environment but also
aggravate the deterioration of a charging member as well as the photoconductive element.
[0003] In the light of this, there has been proposed a charging device using a charging
roller in place of the corona discharger. This type of charging device has a charging
roller held in contact with and driven by a photoconductive drum. The charging roller
has a metallic core. As a voltage is applied from a power source to the core of the
charging roller, the roller charges the surface of the drum. With the charging roller,
it is possible to lower the required voltage of the power source and to reduce the
amount of ozone ascribable to charging. In addition, the charging roller prevents
dust particles from electrostatically depositing on a corona wire and eliminates the
need for a high tension power source. However, the problem with this type of charger
is that the charge distribution is apt to become irregular and, in addition, the charge
potential is extremely susceptible to the environment. In fact, such a charger is
far inferior to a charger of the type using a corona discharger in respect of the
uniformity of charge distribution.
[0004] JP-A-63-149668 teaches that the uniformity of charge is noticeably improved when
an AC voltage having a peak-to-peak voltage more than twice as high as a charge start
voltage (V
TH) is superposed in the event of application of a DC voltage. However, this kind of
scheme needs an AC power source in addition to a DC power source for superposing the
AC voltage on the DC voltage, increasing the cost of the apparatus. Moreover, since
a great amount of AC current not contributing to the charge potential of the photoconductive
element is wastefully consumed. This not only increases the running cost of the apparatus
but also generates a great amount of ozone, bringing about the previously stated critical
problems.
[0005] EP-A-0 406 834 refers to a charging member including a base layer and a surface layer.
The base layer has a resistivity of 10
0 to 10
11 Ωcm. The surface layer includes 30% by wt. or higher and more preferably 50% by wt.
or higher polyurethane resin, which is made of an isocyanate group and a hydroxyl
group and particular care is involved to satisfy a relationship between the two ingredients
of the polyurethane. The volume resistivity of the surface layer is adjusted to be
within the range of 10
6 Ωcm to 10
12 Ωcm.
[0006] From DATABASE WPI, Week 5251, Derwent Publications Ltd., AN 92-418341 and JP-A-4
311 972, an electrically conductive elastic layer and a resistance layer on an electrically
conductive base are arranged to provide a charging roller for charging a photoconductive
element. The conductive elastic layer has a resistivity of about 10
1 to 10
5 Ωcm and the surface layer has a resistivity of about 10
6 Ωcm to 10
12 Ωcm. Since this elastic layer is conductive, a relatively thick surface layer is
necessary to avoid short circuiting between the elastic layer and a body to be charged,
because the thickness of the surface layer influences the characteristics of the surface
layer against breakdown between the leastic layer and a body to be charged much more
than the resistance of the surface layer. The resistance layer comprises a mixture
of epichlorohydrin and fluorine polymer obtained by crosslinking fluorine containing
copolymer comprising fluoroolefin and hydroxyl group containing vinyl ether with isocyanate.
[0007] It is, therefore, an object of the present invention to provide a charging roller
for an image forming apparatus capable of reducing the cost of the apparatus itself,
power source cost, and generation of ozone to thereby prevent a charging member and
a photoconductive element from deteriorating and avoid environmental pollution.
[0008] It is another object of the present invention to provide a charging roller for an
image forming apparatus which, with a simple construction, insure attractive images
at all times with no regard to the varying environment.
[0009] The advantages and benefits of the invention are based on a charging roller according
to claim 1.
[0010] A preferred embodiment is defined by the features in the subclaim.
[0011] The above and other objects, features and advantages of the present invention will
become more apparent from the following detailed description taken with the accompanying
drawings in which:
FIG. 1 is a vertical section of a charging device implemented with a charging roller
embodying the present invention; and
FIGS. 2 and 3 are vertical sections each showing a charging device having a particular
conventional charging roller.
[0012] To better understand the present invention, a brief reference will be made to a charging
device having a conventional charging roller, shown in FIG. 2. As shown, the charging
device 1, generally 1, has a charging roller 2 held in contact with an image carrier
implemented as a photoconductive drum 3 by way of example. While the charging roller
2 is in rotation, a high-tension DC voltage is applied from a DC power source 7 to
the roller 2 to cause it to charge the drum 3. The charging roller 2 is made up of
a metallic core 4, a conductive elastic layer 5 formed on the core 4 and having NBR
or conductive particles dispersed therein, and a fluorine-based non-adhering film
10 provided with conductivity. The problem with this kind of charging roller 2 is
that irregularity in electric characteristic is so great, the charge deposited on
the drum 3 via the conductive elastic layer 15 is irregular. As a result, the background
of an image is contaminated at a period coincident with the roller period.
[0013] FIG. 3 shows a charging device using another conventional charging roller elaborated
to eliminate the above problem. In the figure, the same or similar constituent parts
as or to the parts shown in FIG. 2 are designated by the same reference numerals,
and a detailed description thereof will not be made in order to avoid redundancy.
As shown, the charging roller 2 has an elastic layer 25 formed on the metallic core
4 and made of, for example, NBR, urethane or EPDM. A surface layer 11 is formed on
the elastic layer 25 and has a conductive substance dispersed therein. The surface
layer 11 is held in contact with the core 4. The DC power source 7 applies an AC-biased
DC voltage to the core 4. In this configuration, to charge the drum 3 uniformly, the
AC voltage biasing the DC voltage is provided with a peak-to-peak voltage twice as
high as a charge start voltage to occur at the time when the DC voltage is applied.
However, even this type of charge roller 2 is disadvantageous in that an AC power
source 8 is necessary in addition to the DC power source 7 and results in an extra
cost, in that a great amount of AC current which does not contribute to the charge
potential of the drum 3 is wastefully consumed, and in that the AC current generates
harmful ozone, as discussed earlier.
[0014] Referring to FIG. 1, a charging roller and a charging device implemented therewith
will be described which are free from the above-described problems. In the figure,
the same or similar constituent parts as or to the parts shown in FIGS. 2 and 3 are
designated by the same reference numerals, and a detailed description thereof will
not be made in order to avoid redundancy. As shown, a charging roller 2 has a metallic
core 4, an elastic layer 5 formed on the core 4, and a surface layer 6 formed on the
elastic layer 5. A DC power source 7 applies a high-tension negative DC voltage of
1.3 kV to 1.6 kV to cause it to charge a photoconductive drum 3. The elastic layer
5 is made of epichlorohydrin rubber having a medium electric resistance and in which
conductive particles are not dispersed. The epichlorohydrin rubber may be implemented
by a binary copolymer of epichlorohydrin/ethylene oxide or a ternary copolymer of
epichlorohydrin/ethylene oxide/arylglycydil ether.
[0015] The surface layer 6 is constituted by a mixture of epichlorohydrin rubber applied
to the elastic layer 5 and non-adhering fluorine-based resin. This is to enhance the
non-adhering property of the surface of the charge roller 2 against the deposition
of toner particles. The fluorine-based resin is an amorphous polymer soluble to a
solvent and produced by the copolymerization reaction of fluoroolefin and hydrocarbon-based
vinyl ether. For details of this kind of resin, a reference may be made to Kojima
et al "Journal of the Institute of Organic Synthetic Chemical Engineers of Japan",
Vol. 42 (9), page 841, 1984 and Munakata et al "Asahi Glass Study Report (Japan)",
Vol. 34 (2), pages 205-224, 1984. The fluorine-based resin has a relatively low fluorine
content, i.e., 25 wt% to 32 wt%. However, since the resin of this kind is an alternating
copolymer in which fluoroolefin and hydrocarbon vinyl ether alternate with each other,
the fluoroolefin portions which are thermochemically stable and regularly arranged
protect the unstable hydrocarbon-based vinyl ether portions electronically and sterically.
Hence, such a resin is chemically stable and durable. The resin, or amorphous polymer,
is soluble to a solvent and, therefore, has to be crosslinked after application, thereby
providing the resulting film with resistivity to solvents. For this purpose, hydroxyl
group-containing vinyl ether which is highly reactive is copolymerized with fluoroolefin
so as to produce a resin structure which promotes easy crosslinking by isocyanate.
[0016] The above structure allows the elastic layer 5 t o function with stability and uniformity
as an electric resistance body and has a small electrostatic capacity. Therefore,
even when AC is superposed on DC, the uniform charging ability is not improved to
a noticeable degree. As a result, it is not necessary to superpose AC on DC, i.e.,
high-tension DC voltage should only be applied.
[0017] In a first example, to produce the elastic layer 5, there were mixed 100 parts by
weight of epichlorohydrin rubber which is a ternary copolymer of epichlorohydrin/ethylene
oxide/arylglycydil ether (Epichlomer CG available from Daiso Co., Ltd. (Japan)), 30
parts by weight of more volatile calcium carbonate (Tamapearl TR-222H, Okatema Industries,
Ltd. (Japan)), 10 parts by weight of Sub (Neo factice GT, available from Tenma Sub
Chemicals Ltd. (Japan)), 5 parts by weight of zinc flower (Sazex I, Sakai Chemicals,
Ltd. (Japan)), 0.5 part by weight of stearic acid (Stearic Acid SA-200, Aschi Denka
Co., Ltd. (Japan)), 1 part by weight of vulcanization accelerator (Nocceler TT available
from Ouchi Shinko Chemicals Inc. (Japan). 1.5 parts by weight of Nocceler DM also
available from Ouchi Shinko Chemicals Inc. (Japan) and 0.25 part by weight of Sulphax
H available from Tsurumi Chemicals Inc. (Japan). The mixture was kneaded to prepare
a compound having a uniform composition. Then, the mixture was applied to the periphery
of a shaft made of stainless steel and having a diameter of 6 mm. Subsequently, the
shaft was vulcanized at 170°C for 10 minutes and again vulcanized at 200°C for 2 hours.
The surface of the resulting roller was machined to have a roller diameter of 12 mm.
The roller was measured to have a medium electric resistance and, physically, a volume
resistivity of 2 x 10
8 Ω·cm, rubber hardness of 33° (JIS A), and surface roughness of 3 µm·Rz.
[0018] To form the surface layer 6, there were mixed 100 parts by weight of epichlorohydrin
rubber of ternary copolymer (Epichromer CG available from Daiso Co. Ltd. (Japan)),
0.5 part by weight of stearic acid, 5 parts by weight of zinc flower (Sazex I, Sakai
Chemicals, Ltd. (Japan)), 1 part by weight of vulcanization accelerator (Nocceler
TT available from Ouchi Shinko Chemicals Inc. (Japan)), 1.5 parts by weight of Nocceler
DM also available from Ouchi Shinko Chemicals Inc. (Japan)), and 0.25 part by weight
of Sulphax H available from Tsurumi Chemicals Inc. (Japan). The mixture was kneaded
to prepare a compound having a uniform composition. 2.5 parts by weight of the compound
was dissolved in a mixture solution of 48.8 parts by weight o f toluen and 48.8 parts
by weight of 4-methyl-2-pentanone, thereby producing an epichlorohydrin rubber solution
containing 2.5 % of solids (paint A-1). Also, to produce the non-adhering resin, 22
parts by weight of solvent-soluble fluorine resin (Lumiflon LF-601C major agent available
from Asahi Glass Co., Ltd. (Japan)) and 4.4 parts by weight of isocyanate-based hardener
(Lumiflon LF-601C hardener also available from Asahi Glass Co., Ltd. (Japan)) were
dissolved in a mixture of 36.8 parts by weight of toluene and 36.8 parts by weight
of xylene. The resulting fluorine-based resin solution contained 10 % of solids (paint
B). 40 parts by weight of paint B and 100 parts by weight of paint A-1 were mixed
(ratio in solid: paint A-1/paint B = 1.0/1.6). After the mixture of paints B and A-1
were coated on the elastic layer 5 by dipping, it was dried at 160°C for 30 minutes
to form a 20 µm thick layer. This surface layer 6 had a volume resistivity of 8 x
10
9 Ω.cm.
[0019] The charging roller 2 fabricated by the above procedure was substituted for a primary
corona charger included in a positive-to-positive development type copier (FT3300
available from Ricoh Co. Ltd. (Japan)). In this condition, the roller 2 was held in
contact with and rotated by the drum 3 while a DC voltage of 1.4 kV was applied to
the core 4 thereof as a primary charge voltage. Table 1, which is shown below, indicates
a light potential measured with the charging roller 2 together with the result of
evaluation of an image in a row labeled Ex. (Example) 1. Even after the copier was
operated to produce 5,000 copies, the potential and image were free from defects.
For the measurement of the volume resistivity of the elastic layer 5, each sample
was left in a 20°C, 60 % RH atmosphere for 16 hours, use was made of an electrometer
610C, and an electrode for measurement was implemented by a tape of copper foil (No.
1245 available from 3M). Further, to measure the volume resistivity of the surface
layer alone, the material constituting it was painted on a thin aluminum plate (0.2
mm thick) to a thickness of about 50 µm. Then, the aluminum plate was left in a 20°C,
60RH atmosphere for 16 hours; for the measurement, use was made of a resistance measuring
cell (16008A available from YHP) and above-mentioned electrometer 610C.
TABLE 1
Ex. No. |
Elastic Layer (Volume Resistivity Ω·cm) |
Surface Layer (Volume Resistivity Ω·cm) |
Surface Layer Thickness (µm) |
Potential (V) |
Image Defect |
Toner Filming |
Ex. 1 |
Epichlomer CG (2×108) |
fluorine/Epichlomer CG (8×109) |
20 |
-730 |
not occurred |
not occurred |
-725 |
not occurred |
not occurred |
Ex. 2 |
Same as above |
same as above |
90 |
-720 |
not occurred |
not occurred |
-720 |
not occurred |
not occurred |
Ex. 3 |
Epichlomer C (7×107) |
fluorine/Epichlomer C (3×109) |
20 |
-740 |
not occurred |
not occurred |
-740 |
not occurred |
not occurred |
Ex. 4 |
Epichlomer CG/Epichlomer C (1×108) |
fluorine resin/Epichlomer CG (1×1010) |
6 |
-800 |
not occurred |
not occurred |
-795 |
not occurred |
not occurred |
Ex. 5 |
Epichlomer CG (2×108) |
fluorine resin (2×1014) |
8 |
-720 |
not occurred |
not occurred |
-720 |
not occurred |
not occurred |
Ex. 6 |
Epichlomer C (7×107) |
same as above |
5 |
-740 |
not occurred |
not occurred |
-735 |
not occurred |
not occurred |
Com. Ex. 1 |
Epichlomer CG (7×108) |
― |
― |
-800 |
not occurred |
not occurred |
-740 |
occurred (irregular density) |
occurred |
Com. Ex. 2 |
Epichlomer CG (2×108) |
fluorine resin/Epichlomer CG (8×109) |
230 |
-670 |
occurred (irregular density) |
not occurred |
― |
― |
― |
Com. Ex. 3 |
Epichlomer CG/Epichlomer C (1×108) |
fluorine resin (2×1014) |
30 |
-590 |
occurred (low density) |
not occurred |
― |
― |
― |
Com. Ex. 4 |
Epichlomer CG (2×108) |
fluorine resin (2×1014) |
11 |
660 |
occurred (slightly low density) |
not occurred |
― |
― |
― |
Com. Ex. 5 |
Same as above |
same as above |
15 |
620 |
occurred (low density) |
not occurred |
― |
― |
― |
[0020] In each Example shown in Table 1, the upper and lower parts of the columns "Potential
(V)", "Image Defect", and "Toner Filming" are respectively representative of the initial
condition and the condition after 5,000 copies have been produced.
[0021] In Table 1, Example 2 is identical with Example 1 described above except that the
surface layer 6 was 90 µm thick.
[0022] In Example 3, to produce the elastic layer 5, there were mixed 100 parts by weight
of epichlorohydrin rubber of binary copolymer of epichlorohydrin/ethylene oxide (Epichromer
C available from Daiso Co. Ltd. (Japan)), 30 parts by weight of more volatile calcium
carbonate, 10 parts by weight of Sub (Neo factice GT available from Tenma Sub Chemicals
Ltd. (Japan)), 5 parts by weight of zinc flower (Sazex I, Sakai Chemicals, Ltd. (Japan)),
0.5 part by weight of stearic acid (Stearic Acid SA-200, Asahi Denka Co, Ldt. (Japan)),
1 part by weight of vulcanization accelerator (Nocceler TT available from Ouchi Shinko
Chemicals Inc. (Japan), 1.5 parts by weight of Nocceler DM also available from Ouchi
Shinko Chemicals, and 0.25 part by weight of Sulfax H available from Tsurumi Chemicals
were mixed and kneaded to prepare a compound having a uniform composition. The compound
was applied to the periphery of a shaft made of stainless steel and having a diameter
of 6 mm, vulcanized at 170°C for 10 minutes, and then vulcanized at 200°C for 2 hours.
The surface of the resulting roller was machined to provide the roller with a diameter
of 12 mm. The roller was measured to have a medium electric resistance and, physically,
a volume resistivity of 7 x 10
7 Ω·cm, rubber hardness of 32° (JIS A), and surface roughness of 3 µm·Rz.
[0023] In Example 3, to form the surface layer 6, there were mixed and kneaded 100 parts
by weight of epichlorohydrin rubber of binary copolymer (Epichlomer C available from
Daiso Co. Ltd, (Japan)), 0.5 part by weight of stearic acid, 5 parts by weight of
zinc flower, 1 part of vulcanization accelerator (Nocceler TT available from Ouchi
Shinko Chemicals Inc. (Japan)), 1.5 parts by weight of Nocceler DM also available
from Ouchi Shinko Chemicals, and 0.25 part by weight of Sulfax H available from Tsurumi
Chemicals, thereby preparing a compound having a uniform composition. 2.5 parts by
weight of the compound was dissolved in a mixture of 48.8 parts by weight of toluene
and 48.8 parts by weight of 4-methyl-2-pentanone. The resulting solution of epichlorohydrin
rubber contained 2.5 % of epichlorohydrin (paint A-2). 100 parts by weight of the
paint A-2 and 40 parts by weight of paint B were mixed (ratio in solid: paint A-2/paint
B = 1.0/1.6). The mixture was coated on the elastic layer 5 by dipping and then dried
at 160°C for 30 minutes to form a 20 µm thick layer. The charge roller 2 with such
a structure had properties shown in Example 3 of Table 1.
[0024] In Example 4, to form the elastic layer 5, the compound of Example 1 (ternary copolymer
of epichlorohydrin/ethylene oxide/acrylgrycydil ether) and the compound of Example
3 (binary copolymer of epichlorohydrin/ethylene oxide) were mixed at a ratio of 1:1.
Then, the procedure of Example 1 was repeated to fabricate a charging roller having
a medium electric resistance, diameter of 12 mm, volume resistivity of 1 x 10
8 Ω·cm, rubber hardness of 33° (JIS A), and surface roughness of 3 µm·Rz. To form the
surface layer 6, 100 parts by weight of paint A-1 and 50 parts by weight of paint
B were mixed (ratio in solid: paint A-1/paint B = 1/2). The mixture paint was coated
on the elastic layer 5 by dipping, and then dried at 160°C for 30 minutes to form
a 6 µm thick layer. The surface layer 6 had a volume resistivity of 1 x 10
10 Ω·cm. The properties of the roller 2 are shown in Example 4 of Table 1.
[0025] In Example 5, corresponding to an embodiment of the present invention only the paint
B was coated on the elastic layer 5 of Example 1 by dipping and then dried at 100°C
for 30 minutes to form an 8 µm thick surface layer 6. The layer 6 had a volume resistivity
of 2 x 10
14 Ω·cm. The properties of the resulting roller 2 are indicated in Example 5 of Table
1.
[0026] In Example 6, corresponding to another embodiment of the present invention only the
paint B was coated on the elastic layer 5 of Example 3 by dipping and then dried at
100°C for 30 minutes to form a 5 µm thick surface layer. The properties of the resulting
roller 2 are shown in Example 6 of Table 1.
[0027] In Table 1, Comparative Example (Comp. Ex.) 1 is representative of a case wherein
a roller 2 had the elastic layer 5 of Example 1 and did not have the surface layer
6. It will be seen that when 5,000 copies are produced, irregular image density and
toner filming occur.
[0028] Comparative Example 2 is identical with Example 1 except that the surface layer 6
was 230 µm thick. The roller 2 had a hardness of 48° (JIS A).
[0029] In Comparative Example 3, only the paint B was coated on the elastic layer 5 of Example
4 by dipping and then dried at 100°C for 30 minutes to form a 30 µm thick surface
layer 6. It will be seen that irregular image density and toner filming occur.
[0030] Comparative Example 4 is identical with Example 6 except that the surface layer 6
was 11 µm thick. In this example, charge potential and, therefore, image density is
slightly lowered, as Table 1 indicates.
[0031] Comparative Example 5 is identical with Example 6 except that the surface layer 6
was 15 µm thick. In this case, charge potential and, therefore, image density is further
lowered, as Table 1 also indicates.
[0032] In summary, it will be seen that the present invention provides a charging roller
having various unprecedented advantages, as enumerated below.
(1) The roller is capable of charging a photoconductive element uniformly only if
applied with a high-tension DC voltage. This eliminates the need for an AC voltage
and produces no ozone. In addition, since the surface of the roller is not adhering,
toner particles are prevented from depositing thereon over a long period of time.
(2) The toner particles are prevented from filming on the surface layer of the roller,
while the photoconductive element is uniformly charged.
(3) Even when pin holes existing in the photoconductive element are brought into contact
with the roller, breakdown due to current supply is eliminated.
(4) A non-adhering layer for eliminating the deposition of toner particles can be
easily formed on the elastic layer.
[0033] Various modifications will become possible for those skilled in the art after receiving
the teachings of the present disclosure without departing from the scope of the following
claims.