Rolls for electrostatic charge

05567494 An inexpensive roll for electrostatic charge having the simple layer structure not affected by working environmental conditions, not developing exuding phenomena such as bleeding and blooming, generating no leaks and being able to conduct a uniform, stable electrostatic charge, which comprises a core member, and a conductive elastomer layer and a surface layer laminated thereon in this order, the surface layer having a conductive material dispersed therein, wherein the surface layer has a higher electrical resistance in an outermost surface region thereof and formed of a thermosetting binder resin containing carbon black dispersed therein by electrostatic deposition.

FIELD OF THE INVENTION 
The present invention relates to rolls for electrostatic charging which are 
bought into contact with surfaces of materials to be charged to charge the 
surfaces. 
BACKGROUND OF THE INVENTION 
As a means for charging materials to be charged such as photosensitive 
materials in image forming apparatuses of the electrophotographic 
recording system, the static recording systems, i.e., the contact charging 
system wherein rolls for electrostatic charging are rotated in contact 
with the surfaces of the photosensitive material to be charged has 
recently attracted attention, because this system has the advantages of 
smaller generation of ozone and lower applied voltage than the corona 
discharging system. 
Such a roll fundamentally comprises a metallic core member and a conductive 
elastomer layer formed thereon. 
However, in the case of the rolls for electrostatically charging this 
single-layer structure, adding conductive materials lowers the elasticity 
of elastomers such as EPDM (ethylene-propylene-diene) rubber, urethane 
rubber and silicone rubber. It is therefore necessary to lower the 
hardness of these rubbers. For this reason, softening agents, such as 
paraffin oil or plasticizers or sulphur-based crosslinking agents are 
added to the elastomer members, causing them to bleed or bloom, 
respectively, thereby staining the surfaces of the photosensitive 
materials to be charged and hindering the formation of images. 
For the purpose of preventing the above-mentioned bleeding and blooming, 
for example, a roll for electrostatic charging in which a conductive 
surface layer composed of a conductive resin composition mainly comprising 
a methoxy-modified polyamide, etc. is formed on a surface of a conductive 
elastomer layer. This concept has been proposed in JP-A-64-66674 (the term 
"JP-A" as used herein means an "unexamined published Japanese patent 
application"). 
However, even when such a surface layer is provided, the conductive 
elastomer layer formed of EPDM rubber fails to have a sufficient sealing 
effect against oil-based liquid matter to prevent the exuding phenomenon 
under high temperature and high humidity. Further, the methoxy-modified 
polyamide is easily affected by humidity, and has the disadvantages of 
generating leaks due to pinholes in the photosensitive material under the 
circumstances of high humidity and of generating poor electrostatic charge 
under the circumstances of low humidity. 
Rolls for electrostatic charging provided with conductive surface layers 
(volume resistivity: 10.sup.4 to 10.sup.12 .OMEGA..cm) formed of methoxy 
modified nylon or urethane have also been proposed (JP-A-6A-20518, 
JP-A-3-233473, etc). However, their resistance is seriously affected, 
particularly by changes in atmospheric humidity, and they can increase 
three times or more in volume resistivity under low humidity, which makes 
providing a stable, uniform electrostatic charge impossible. 
Furthermore, a roll for electrostatic charging having the multi-layer 
structure in which a conductive elastomer layer, is divided into an 
elastic layer and a conductive layer and a resistive layer (volume 
resistivity: 10.sup.6 to 10.sup.12 .OMEGA..cm) is provided on the 
conductive layer has also been proposed (JP-A-1-211779). However, similar 
to the above-mentioned previous proposals, the presence of the resistive 
layer such as nylon results in preventing stable, uniform electrostatic 
charge because of a wide fluctuation in resistance due to humidity and 
fails to sufficiently prevent leaking through pinholes in photosensitive 
material under high applied voltage. 
On the other hand, in manufacturing rolls for electrostatic charging having 
the multi-layer structure consisting of 3 or more layers, as well as the 
above-mentioned rolls of the two-layer structure, the respective layers on 
the elastic material are usually formed by spray coating or dip coating. 
Using these coating processes, however, makes it difficult to ensure that 
the coated films are produced with a uniform thickness. As a result, it is 
difficult to ensure uniform electrostatic charge and stable resistance, 
and the many coating steps complicate the manufacturing procedures to make 
it difficult to reduce costs. In the case of rolls for electrostatic 
charging of the multi-layer structure in which the respective layers have 
different functions, coating may be repeated several times in each coating 
step in order to secure a definite thickness of the respective layers, or 
bonding layers may be formed between the layers in some cases in order to 
provide adhesion between them. These also cause the troubles generated by 
the non-uniformity in thickness of the layers and high cost due to an 
increase in manufacturing steps. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide inexpensive rolls for 
electrostatic charging having the simple layer structure which are not 
affected by working environmental conditions, do not develop exuding 
phenomena such as bleeding and blooming, do not generate leaks and can 
conduct a uniform, stable electrostatic charge. 
According to the present invention, there is provided a roll for 
electrostatic charging that includes a core member having laminated 
thereon, in sequence, a conductive elastomer layer and a surface layer 
having an electrical resistance of 10.sup.13 to 10.sup.16 .OMEGA./cm.sup.2 
at the outermost surface region thereof. 
Further, the present invention provides the roll for electrostatic charging 
described above, wherein the surface layer is a single layer comprising a 
binder resin and a conductive material, and the outermost surface region 
has a higher electrical resistance than the other region of the surface 
layer, or the surface layer is an electrostatically coated film formed by 
electrostatic deposition. 
Furthermore, the present invention provides the roll for electrostatic 
charging described above, wherein the conductive elastomer layer is a 
layer comprising EPDM to which a softening agent, a plasticizer and a 
sulfur-based vulcanizing agent are not added, or urethane rubber. 
Still further, the present invention provides a process for producing the 
roll for electrostatic charging described above.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, the surface layer has the single layer structure 
formed of a resin composition having a conductive material dispersed into 
a thermosetting binder resin, and has an electrical resistance of 
10.sup.13 to 10.sup.16 .OMEGA./cm.sup.2 at the outermost surface region. 
It is preferred that the outermost surface region has a higher electrical 
resistance than the other inner region of the surface layer, which has 
generally a uniform electrical resistance within the range of from 
10.sup.3 to 10.sup.9 .OMEGA..cm. The thickness of the surface layer is 100 
.mu.m or less, and preferably 10 to 60 .mu.m. The outermost surface region 
of the surface layer is a region in the vicinity of the surface of the 
roll and has a thickness of 0.1 to 20 .mu.m, preferably 0.5 to 10 .mu.m, 
and more preferably 1 to 5 .mu.m. 
Examples of the above-mentioned thermosetting binder resins include acrylic 
resins such as polymethyl methacrylate and polybutyl acrylate blended with 
curing agents such as melamine resins, methylol melamine and epoxy resins; 
and urethane resins composed of polypropylene glycol, polytetrabutylene 
glycol, polyether polyols, polyester polyols, etc. blended with isocyanate 
curing agents such as MDI (diphenylmethane diisocyanate) and TDI (tolylene 
diisocyanate) or with curing agents such as melamine resins. In addition, 
epoxy resins, acid crosslinking polyamides, unsaturated polyester resins, 
alkyl resins, phenolic resins and cellulosic resins can be used. Of these, 
resin compositions containing acrylic resins, urethane resins or polyester 
resins as main components are preferred. 
Examples of the conductive material dispersed in the thermosetting binder 
resin included metals such as zinc, aluminum, gold, silver, copper, 
chromium and iron, metal oxides such as zinc oxide, titanium oxide, 
stannic oxide, antimony oxide, indium oxide and aluminum oxide, mixtures 
or solid solutions of the metal oxides, and carbon black. Of these, carbon 
black is preferably used. Examples of the carbon black include hard carbon 
and soft carbon such as SAF, HAF, GPF, and SRF as furnace black; acetylene 
black; ketjen black; and channel black. The content of these conductive 
materials is preferably from 1 to 200 parts by weight per 100 parts by 
weight of binder resin. When the content is less than 1 part by weight, 
the electrical resistance fluctuates and the production stability is 
impaired. Conversely, if the content exceeds 200 parts by weight, poor 
kneading and high hardening result. The metal oxide as the conductive 
material is preferably used in an amount of 40 to 65 wt % of the surface 
layer. The metal of the carbon black as the conductive material is 
preferably used in an amount of 0.5 to 10 wt % of the surface layer. 
The surface layer can be formed by applying a dispersion of a conductive 
material in the above-mentioned binder resin by electrostatic coating. In 
this case, the electrical resistance of the dispersion must be set to 0.01 
M.OMEGA. or more, preferably 0.1 to 1 M.OMEGA., prior to coating. When the 
electrical resistance of the dispersion is less than 0.01 M.OMEGA., the 
coating dispersion is not charged and therefore, poor extrusion of the 
coating dispersion and non-uniformity of the resulting film occur. 
The conductive elastomer layer of the present invention is prepared by 
adding carbon black or metal oxides to an elastic material to give it 
conductivity, and the volume resistivity is adjusted to 10.sup.0 to 
10.sup.10 .OMEGA..cm and the (rubber) hardness (ASKA A hardness) to 
30.degree. to 70.degree., preferably 45.degree. to 65.degree.. The volume 
resistivity of the conductive elastomer layer is preferably adjusted to 
almost the same value as that of the region other than the outermost 
surface region in the surface layer, which is in the range or 10.sup.3 to 
10.sup.9 .OMEGA..cm. 
The thickness of the conductive elastomer layer of the present invention is 
1 to 20 mm, preferably 3 to 10 mm. 
Examples of the elastic materials include rubber such as polybutadiene, 
natural rubber, polyisoprene, butyl rubber, SBR (styrene-butadiene 
rubber), CR (chloroprene), NBR (nitrile-butadiene rubber), silicone 
rubber, and epichlorohydrin rubber; thermoplastic elastomers such as BR 
(butadiene rubber), polystyrene-based rubber such as SBS 
(styrene-butadiene-styrenelastomer), polyolefin-based rubber, 
polyester-based rubber, polyurethane-based rubber, PVC (polyvinyl 
chloride) and nitrile rubber; and polymer materials such as polyurethane, 
polystyrene, polyethylene, polypropylene, PVC, acrylic resins and 
styrene-vinyl acetate copolymers. Rubber having good resistance to ozone 
includes urethane rubber, EPDM, etc. 
In particular, these elastic materials are desirably used without adding 
additives thereto, such as softening agents, plasticizers, sulfur-based 
vulcanizing agents, insulating oils (mineral oil, silicone oil, etc.), 
processing aids (phthalic acid-based plasticizers and calcium carbonate), 
silica-based fillers (hydrous silicic acid and anhydrous silicic acid) and 
vulcanization accelerators (sulfonamides, thiurams and dithiocarbamic 
acid). Preferred examples of the elastic materials include diene-based 
rubber such as butadiene rubber or polyisoprene, containing no additive 
(EPDM), or urethane rubber. When the above-mentioned additives need to be 
used, the amount thereof to be added must be maintained as low as possible 
and the low molecular weight component thereof is preferably removed 
before use with a rubber material. 
Examples of the diene-based rubber such as butadiene rubber include those 
subjected to peroxide vulcanization by use of dicumyl peroxide and the 
like; those subjected to metallic oxide crosslinking by use of metallic 
salt monomer in combination of a monomer such as acrylic acid and 
methacrylic acid with a metallic oxide such as zinc oxide and magnesium 
oxide; those subjected to thiuram vulcanization forming disulfide 
crosslinking; and those subjected to quinoid vulcanization. 
Carbon black used in the above-mentioned surface layers can also be used in 
this case. The content of carbon black is from 1 to 100 parts by weight 
per 100 parts by weight of elastic material. The content exceeding 100 
parts by weight hardens rubber when it is used as the elastic material. 
Blends of the above-mentioned elastic materials with carbon black are used 
as the conductive elastomer layers which are formed by conventional 
methods such as extrusion molding, press molding, casting and injection 
molding. 
When both sides of the elastomer layer are made perpendicular to the core 
member, the sides are desirably formed to make curved planes. Such sides 
enable the surface layer to be formed in the vicinity of the core member. 
As a result, the generation of leaks at the sides of the roll can be 
prevented, because the side regions of the roll for electrostatic charging 
are also covered with the surface layer. 
When electrostatic charge is conducted by use of this roll, a voltage of 
100 to 5,000 V is preferably applied. 
In the present invention, an electrostatic deposition (spraying) process is 
conducted as follows. A core member having molded thereon a conductive 
elastomer layer is grounded to the earth, and a spray coating apparatus 
having placed therein the above mentioned coating solution (dispersion) is 
connected to a cathode whereby a negative charge is imparted to the 
coating solution upon spraying. The sprayed coating solution is 
efficiently applied, due to the negative charge, to the core member having 
molded thereon the conductive elastomer layer, which acts as anode. In 
order to spray the coating solution, an electrostatic atomizer which 
finely particulates a coating solution by applying an electrostatic field, 
or an air atomizer (spray gun) which atomizes a coating solution by 
application of a high pressure with a compressed air may be used. The air 
atomizer is preferably used from the viewpoint of efficiency. The voltage 
applied to the coating solution is preferably from 70 to 100 KV, more 
preferably 80 to 95 KV. In case of using the air atomizer, the air is 
compressed at the pressure of 1.0 to 5.0 Kg/cm.sup.2, preferably 1.5 to 
3.5 Kg/cm.sup.2, and the amount of the coating solution to be sprayed 
(discharge) is from 10 to 200 ml/min, preferably 30 to 100 ml/min. The gap 
between the coated material on the core member and the spray gun is from 
100 to 800 mm, preferably 200 to 500 mm. An infrared drying hearth may be 
provided directly following the coating apparatus to conduct the coating 
and drying at the same time. 
In such a roll for electrostatic charging of the present invention, the 
surface layer having the conductive material dispersed therein has an 
outermost surface layer region having a higher electrical resistance than 
the other region. The roll therefore has the single layer structure 
wherein the conductive layer and the high resistive layer coexist, so that 
uniform, satisfactory electrostatic charge can be conducted without the 
generation of leak due to a pinhole. 
Further, such a surface layer with a constant thickness can be simply 
formed by electrostatic deposition. In addition, formation of the surface 
layers by electrostatic deposition absorbs and cancels non-uniformity in 
forming and joint lines in the inner conductive elastomer layer. As a 
result, it is possible to obtain an extremely smooth surface on the roll 
for electrostatic charge which prevents poor electrostatic charge due to 
the non-uniformity in forming and the joint lines, and moreover, prevents 
deterioration in images. 
The surface layer formed by electrostatic deposition has an outermost 
surface region having a high electrical resistance and therefore, it is 
unnecessary to provide a leak-proof layer (a high resistive layer) which 
is otherwise required separately in a resistive layer formed by customary 
spray coating or dip coating. For example, the surface layer formed of a 
binder resin such as an acrylic resin generates no leak up to an AC 
applied voltage V.sub.(p-p) of about 1 to about 5 kV. The reason why the 
surface layer containing such a high resistive surface region is formed is 
probably that a comparatively large amount of the conductive material in 
the coating dispersion is coated together with the binder resin during the 
first half of electrostatic deposition operation to maldistribute the 
conductive material in the direction of film thickness (the conductive 
material exists at a higher concentration in a more inner region of the 
surface layer). 
The surface layer formed of a thermosetting binder resin can be laminated 
with the conductive elastomer layer with high adhesion, so that no 
adhesive layer may be required, the surface layer being directly adhered 
to the conductive elastomer layer. 
Further, this surface layer yields stable electrostatic charge 
characteristics that are not affected by working environmental temperature 
and moisture. In addition to providing the surface layer, the inner 
conductive elastomer layer is formed of a material that lacks any additive 
that causes bleeding and blooming, which can conduct satisfactory 
electrostatic charge without problems such as bleeding, even if it is not 
constructed of multiple layers, like the prior art. 
Furthermore, as this roll for electrostatic charging has the simple 
two-layer structure and the surface layer thereof is readily formed by a 
single static deposition step, it can be manufactured in a short period of 
time at low cost. 
The present invention will be illustrated with reference to Examples and 
Comparative Examples in more detail below. Hereinafter, "part" means "part 
by weight". 
EXAMPLE 1 
To 100 parts of raw rubber, EPDM (Mooney viscosity: 20), 1 part of a 
vulcanizing agent (dicumyl peroxide), 3 parts of a methacrylic acid ester, 
1 part of zinc oxide, and 10 parts of Ketjen black and 20 parts of furnace 
black SAF were added, and the mixture was kneaded and mixed through a 
twin-roll mill for 30 minutes to prepare starting rubber. 
The resulting starting rubber was weighed in a metal mold in which a core 
member having an outer diameter of 6 mm (a stainless steel rod, a 
nickel-plated iron rod, etc.) was set, press molded, and heated at 
160.degree. C. for 30 minutes to conduct first vulcanization. The 
vulcanized rubber was taken out of the metal mold and heated again at 
160.degree. C. for 2 hours to conduct second vulcanization. Thus, a 
conductive rubber layer was formed as the conductive elastomer layer 
around the SUS core member. The conductive rubber layer had a rubber 
hardness of 55.degree. and an electrical resistance of 1.0.times.10.sup.5 
.OMEGA. (22.degree. C., 55% RH) in the thickness direction. 
Then, 100 parts of an acrylic/melamine/epoxy resin (manufactured by 
Dainippon Ink and Chemicals, Inc.) to which 20 parts of conductive carbon 
(FW-200, manufactured by Degusa) was mixed and dispersed in 100 parts of a 
mixed solvent of xylene, MIBK, butyl cellosolve, n-butanol and Swazol 
(1800) to prepare a coating for forming a surface layer having a liquid 
resistance of 0.13M .OMEGA.. 
By use of a Bell-type electrostatic deposition machine (manufactured by 
Landsberg Industries), a surface layer was formed by electrostatic 
deposition of the above-mentioned coating for forming the surface layer on 
the conductive rubber layer so that the dry film thickness becomes 30 
.mu.m to prepare a roll for electrostatic charging 1 as shown in FIG. 1. 
The conditions in the electrostatic deposition are as follows. 
Applied voltage: 85 KV 
Air pressure: 2.8 Kg/cm.sup.2 
Discharge amount: 70 ml/min 
Gap between spray gun and coated material: 350 mm 
In FIG. 1, the reference numeral 2 is the core member, the reference 
numeral 3 is the conductive elastomer (rubber) layer, and the reference 
numeral 4 is the surface layer which is electrostatically coated film by 
electrostatic deposition. 
The resistance of the surface layer of this roll for electrostatic charging 
was measured in the following manner. For resistance of an inner region of 
the surface layer, electrodes 5 of 10 square mm were painted with silver 
paste on the core member 2 and the surface layer 4, and a direct voltage 
of 100 V was applied between both the electrodes 5 to measure the value of 
current. Then, the resistance was determined from the values of current 
and applied voltage to be 5.times.10.sup.6 .OMEGA.. The resistance of an 
outermost surface region of the surface layer was measured with a Hylester 
ohmmeter (manufactured by Mitsubishi Petrochemical Co., Ltd.) to be 
9.times.10.sup.13 .OMEGA.. 
After standing for 72 hours in an atmosphere of 32.degree. C. .times.85% 
RH, this roll for electrostatic charging was mounted so as to rotate in 
contact with a photosensitive material 6 of a laser printer (LBP-8, 
manufactured by Cannon Co., Ltd.), and a charging test was conducted under 
the circumstances of high temperature and high moisture (28.degree. 
C..times.85% RH) and of low temperature and low moisture (10.degree. 
C..times.15% RH) while applying 350 V DC voltage superposed with 350.mu.A 
AC constant current to the core member 2 from a power source 7. As a 
result, uniform, satisfactory electrostatic charge was ascertained under 
either of the circumstances, and images of high quality were formed. At 
this time, the state of strains on the surface of the photosensitive 
material 6 was examined. As a result, no stain due to exuded components 
was observed. 
Further, in order to examine the effect of preventing leaks, a similar 
charging test was conducted changing the applied voltage to an AC voltage 
of 2 kV. As a result, no leak phenomenon induced by a pinhole of the 
photosensitive material 6 occurred, and a stable charged potential was 
obtained. 
EXAMPLE 2 
To 100 parts of a polyester polyol (DDX-106, manufactured by Dainippon Ink 
and Chemicals, Inc,), 10 parts of an isocyanate (Millionate MT, 
manufactured by Nippon Polyurethane Co.) and 5 parts of Ketjen black (ECF, 
manufactured by Mitsubishi Petrochemical Co., Ltd.) were added, followed 
by polymerization with stirring to obtain an original polyurethane rubber 
fluid. The original urethane rubber fluid was poured into a mold in which 
a core member was provided, and cured at 100.degree. C. for 3 hours to 
form a conductive urethane rubber layer. This conductive urethane rubber 
layer had an electrical resistance of 6.5.times.10.sup.5 .OMEGA. 
(22.degree. C., 55% RH) in the layer thickness direction. 
Then, 100 parts of an acrylic urethane resin (Hiurethane No. 5001, 
manufactured by Nippon Oil and Fats Co., Ltd.) to which 2 parts of 
conductive carbon (S Carbon, manufactured by Colombia Carbon Co.) were 
added was mixed and dispersed in 230 parts of a mixed solvent of toluene, 
xylene, ethyl acetate, butyl cellosolve and n-butanol. Subsequently, 16 
parts of a polyisocyanate (Hiurethane curing agent HF, manufactured by 
Nippon Oil and Fats Co., Ltd.) were added thereto to prepare a coating for 
forming a surface layer having a liquid resistance of 0.20M.OMEGA.. 
The resulting coating was applied to the above-mentioned conductive 
urethane rubber layer by electrostatic deposition by use of the 
electrostatic deposition machine used in Example 1 so as to give a dry 
film thickness of 35 .mu.m to provide a roll for electrostatic charge. 
The resistance of the surface layer of this roll for electrostatic charging 
was determined in the same manner as with Example 1. As a result, the 
resistance of an inner region was 2.times.10.sup.5 .OMEGA., and the 
resistance of an outermost surface region was 1.times.10.sup.14 .OMEGA.. 
Further, using this roll for electrostatic charging, a charging test was 
conducted in the same manner as with Example 1. As a result, uniform, 
satisfactory electrostatic charge was obtained under either of the 
circumstances. At this time, no stain due to exuded components was 
observed on the surface of the photosensitive material. 
Furthermore, the desired effect of preventing leaks was evaluated. As a 
result, observed leaks were found to have been effectively prevented. 
COMATIVE EXAMPLE 1 
A roll for electrostatic charge was produced in the same manner as in 
Example 1 with the exception that a coating for forming a surface layer 
having the same composition as that used in Example 1 was applied by spray 
coating. 
The resistance of the surface layer of the resulting roll for electrostatic 
charging was determined in the same manner as in Example 1. As a result, 
no difference in resistance between an inner region and an outermost 
surface region was observed, and each was 1.times.10.sup.10 .OMEGA.. 
For this roll for electrostatic charging, a charging test was conducted in 
the same manner as with Example 1. As a result, poor electrostatic charge 
was generated here and there, and a fogging phenomenon of low-density 
image regions or a ghost phenomenon was observed in the resulting images. 
Further, the effect of preventing leak was similarly examined. As a result, 
the dielectric breakdown took place in the surface layer. 
As described above, in the roll for electrostatic charging of the present 
invention, the surface layer having a higher electrical resistance in an 
outermost surface region thereof is provided on the conductive elastomer 
layer. The roll is not therefore affected by working environmental 
conditions, and can conduct uniform, stable electrostatic charge. 
Further, such a surface layer can be simply formed on the conductive 
elastomer layer to a constant thickness with high adhesion by 
electrostatic deposition, and the high resistance skin layer having no 
uneven resistance is formed on the outer side thereof. Even if a high 
voltage is applied, therefore, a stable electrostatic charge can be 
conducted without generating a of leak. 
Furthermore, as the conductive elastomer layer, diene-based rubber to which 
a softening agent, a plasticizer and a sulfur-based vulcanizing agent are 
not added (EPDM), or urethane rubber, is used, whereby the exuding 
phenomena can be more surely solved. Accordingly, the roll can be used 
over a long period of time without staining the photosensitive material, 
and the long-term storage thereof also becomes possible. 
In addition, this roll for electrostatic charging has the simple two-layer 
structure and the surface layer thereof is easily formed by a single 
static deposition step, so that it can be manufactured in a short period 
of time at low cost. 
While the invention has been described in detail and with reference to 
specific examples thereof, it will be apparent to one skilled in the art 
that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.