CA1053968A - Moisture stable bias transfer roll - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Electrostatic transfer of charged particles to a transfer member is accomplished using a roller electrode composed of an electrically conductive core or base having an electrically relaxable primary layer, a self-levelling secondary layer superimposed thereover, said layers being substantially hermetically sealed by a third overcoat layer. The overcoat layer is composed of a polymeric material having a low water vapor permeability constant such that the lower layers of the roll will be rendered insensitive to abrupt humidity changes and consequent electrical instability.

Description

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BACXGROUND OF THE INVENTION
This invention relates to electrophotography, in particular, to an electrically stable apparatus for transferring toner images from one surface to another.
In conventional electrophotographic xerography, a photosensitive plate, which consists of a photoconductive coating placed over a conductive backing, is charged uniformly and the charge plate then exposed to a light image of an orig-inal. Under the influence of the light image, the charge on the plate is selectively dissipated to record the original input scene information on the plate in the form of a latent electro-static image. The latent image is developed, or made visible, by applying oppositely charged toner particles to the plate surface in a manner so that the toner particles are attracted into the imaged areas. The developed images are generally transferred from the photoconductor to a final support material, such as paper or the like, and affixed thereto to form a per-manent record of the original.
Historically/ the transfer of toner images between supporting surfaces is accomplished with the electrostatic transfer of either a corotron or a roller electrode biased to constant potential ~constant voltage) levels. In corona in-duced transfer as, for example, disclosed in U. S. Patent

2,836,725, issued May 27, 1958, R. ~ Vyverberg, the final support sheet is placed in direct contact with the toner image while the image is supported on the photoconductive surface.
The back of the sheet, that is, the side away from the image, is sprayed w-ith a corona discharge having a polarity opposite to that carried by the toner particle causing the toner to be electrostatically transferred to the sheet. Biased roll transfer has been tried with some limited success.

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as a means of controlling the Eorces ac-tlng on the toner during transfer. This type of transfer is disclosed by C. J. Fitch in U. S. Patent No. 2,807,233 issued September 24, 1957 and involves the use of a metal roll coated with a resilient coating having a resistivity of about 106 to 108 cm. ~ecause of the resistivity of the coating, the amount of bias that can be applied to the roll is limited to relatively low operating values because, at the higher ranges, the air in and about the transfer zone begins to break down, i. e., ionizes, causing the image to be degraded during transfer. L. E. Shelffo in U. S. Patent No. 3,520,604 issued July 14, 1970, suggests that the resilient coating have a resistivity of between 1011 -1016 ohms cm. Here, in order to give the roll the needed resiliency required in most practical applications, the coatiny must be relatively thick. ~ thick coating of high resistivity acts to build up a surface charge on the roll resulting in air breakdown in the transfer region and eventually copy degradation.
More recently, improved bias transfer members have been disclosed which overcome many of the electrical and image degrad-ation problems associated with some previous transfer techniques.U. S. Patent 3,702,432, issued November 7, 1972, D. S. Hoffman et al, discloses a multiple layer transfer roll member for trans-ferring xerographic images under controlled conditions. The member is capable of electrically cooperating with a conductive support surface to attract charged toner particles from the support surface towards the member or towards a transfer material such as paper positioned therebetween, the member having a conductive substrate for supporting a biased potential thereon, an intermediate blanket (primary layer) placed in contact with the substrate to the outer periphery of the blanket ancl a relative-~ly thin outer coating (secondary layer) placed over the blanket having an ~2--! ~` ' ' ' ~ ' ; ' ' . ~

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electrical resistivi-ty to minimize ionization of the atmosphere when the transfer member is placed in electrical cooperation with the image support surface and providing a good toner release property enabling the device to be cleaned of said toner. U. S. Patent 3,781,105, issued December 25, 1973, T.
Meagher, discloses a simulator transfer member employed in conjunction wi~h a variable electrical bias means for regula-ting automatically the electrical field levels at various points on the transfer member during the transfer operation and providing constant current control.
In the preferred embodiment, the transfer member disclosed in the aforementioned two U. S. Patents consists of a roller having a central biasable conductive core further having an intermediate blanket or electrically "relaxable"
layer (primary layer) surrounding andin electrical contact with the core, and further having a second blanket or electrical-ly "self-levelling" outer layer (secondary layer) surrounding and in electrical contact with the primary layer. Under opera-ting conditions, it is desirable for optimum transfer to main-tain a relati,vely constant current flow of less than about 30micro amps in the nip area between the transfer roll surface, transfer material, and photoconductive surface from which a developed image is to be transferred. For this condition to exist at given potentials, the resistivity of the primary and secondary layers must lie within critical values and preferably be relati~ely constant under normal anticipated extremes of operating conditions. Preferably, it has been found that the pximary layer should be a resilient elastomeric material having a volume resistivity within the range of 107 to less than 10 ohm cm, and the secondary layer should be also a resilient material having a volume resistivity within the range of 10 ; 3~
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to 10 ohm cm.
In practice it has been found that elastomer materials used in the transfer member such as polyuxethanes which exhibit resistivities within the above ranges are moisture sensitive such that resistivity may vary by as much as a factor of 50 between 10~ and 80~ R.H. as a ~unction of the amount of mois-ture absorbed from or lost to the surrounding atmosphere. For example, in the case o~ polyurethane materials which are employed as the primary layer and which have exceptionally good electrical characteristics, the volume resistivity may change from 1011 ohm cm at low moisture contents, i.e., less than about 0.1~ moisture, to 109 ohm cm at higher moisture levels, i.e., about 2.5~ moisture. Other polyurethanes suitable for use as the secondary layer exhibit resistivity variations ~rom about 1015 to 1013 ohm cm as a function of increasing moisture content. The consequent variations in resistivity will ordin-arily give rise to erratic performance of the ~ransfer member from day to day particularly in terms of transfer e~ficiency unless compensated for by a concomitant change in the voltages ' sufficient to maintain a constant nip current, as disclosed in U.S. Patent 3,781,105. '~
SUMMARY OF THE INVENTION
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In accordance with one aspect o~ this invention there is provided a trans~er member for electrically cooperating with a conductive support surface to electrically attract charge particles from the support sur~ace towards the member including: (a) a conductive support; (b3 a primary elastomeric intermediate layer overlying said conductive support, said primary layer having a relatively constant volume resistivity within the range o~ about 10 to less than 10 1 ohms cm;
(c) a secondary elastomeric intermediate layer overlying said primary layer,, said secondary layer having a relatively constant ~4-" ~ ~ . .. .
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~5396t3 volume resistivity within the range of about 1011 to 10 5 ohms cm; and (d) a polymeric overcoating layer overlying and hermet-ically sealing said layers, said overcoating layer having a substantially unlform thickness of less than about 5 mils and a moisture permeability content of less than about 1 x 10 8 at In accordance with another aspect of this invention there is provided in a method for transferring a toner image from a photoconductive insulating surface to the surface of an image support material, said method including the steps of bringing said photoconductive insulating image-bearing surface into operative communication with an electrically biased transfer member and interposing a sheet of image support material there between, said transfer member being biased to a potential sufficient to attract said toner image from said insulating surface towards said image support material, the improvement which comprises employing as a txansfer member an apparatus comprising: (a) a conductive support; (b) a primary elastomeric intermediate layer overlying said conductive support, said primary layer having a relatively constant volume resistivity within the range of about 107 to less than 1011 ohms cm; (c) a secondary elastomeric intermediate layer overlying said primary layer, said secondary layer having a relatively constant volume resistivity within the range of about 1011 to 1015 ohms cm; and (d) a polymeric overcoating layer overlying and hermetically ..
sealing said layers, said overcoating layer.having a substan-tially uniform thickness of less than about 5 mils and a mois-ture permeability content of less than about 1 x 10 8 at -30C.
By way of added explanation, in accordance with an aspect of this invention there is provided a bias transfer member comprising a conductive core, an electrically relaxable primary -~a-. .

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layer laminated thereto, a self-levelling secondary layer super-imposed thereover, and a third overcoat layer which substanti-ally hermetically seals sald primary and secondary layers. The overcoat layer is composed of a polymeric material with a low water vapor permeability constant such that: the transmission of moisture therethrough to said primary and secondary layers is severely retarded. The thickness of the layer should be such that the dielectric breakdown strength of the material is exceeded under operating transfer conditions.
DESCRIPTIO~ OF THE DR~WINGS
Figure 1 is a perspective view of a partial section of the preferxed bias transfer roll of the present invention.
Figure 2 is a graph showiny a typical relationship between resistivity and moistuxe content of an elastomeric polyurethane material~
Figure 3 is a graph comparing fluctuations in moisture content of transer rolls having no overcoat vs. overcoated rolls.
DETAILED_DESCRIPTION OF THE I~JENTION
The exemplary trans~er roller 10 shown in Figure 1 includes a central conductive core or axle 11, which is prefer-ably a hollow cylindrical tube o~ conductive aluminum, an electrically "relaxable" primary layer 12, an electrically "self-levelling" secondary layer 13 and an overcoat layer 14 composed of a polymeric material exhibiting a low water vapor permeability constant. Thicknesses of the various layers shown in the drawing are not necessarily to scale but merely ~3~

illustrative.
Polyme.ric materials suitable for use as the transfer roll overcoat layer 14 according to the present invention - broadly include those materials which exh.ibit a water vapor permeability less than the w~ter vapor permeabil.ity of the materials used in the primary and secondary transfer roll layers, eOg., polyurethane. Since the main purpose of the overcoat material is to prevent or minimize moisture trans-mission from the external environment to the moisture sensitive primary and secondary layers, and since the maximum thickness of the overcoat is limited Eor electrical reasons as herein-.after disclosed, polymeric materials exhibiting very low water vapor permeabilities are much pxeferred. l'his would encompass polymeric :films having a water vapor permeability constant (P) of less than about 1 x 10 . at about 30~C., where P is expressed .
by the relationship: P=(ml at STP) (cm)/ 2 (cm ) (sec.) (cm Hg).
In this equation, (ml at STP) refers to ml of H2O absorbed at Standard Temperature and Pressure, (cm) refers to a given length of film, (sec.) re~ers to time and (cm Eg) is pressure.
Materials satisfying the above criteria include low density po].yethylene~and other polymers which exhibit a permeabili~y constant less than l.ow density polyethylene including, but not limited to, high density polyethylene, polypropylene, butyl rubber, ethylene/propylene rubber, certain low mo.isture per-meable polyamides such as nylons, polyvinylidene chloride, polyvinylidene ~luoride, polytrifluorochloroethylene, rubber hydrochloride, copolymers and terpolymers of acrylonitrile with vinyl monomers or with monoolefins and/or diolefins such as styrene/acrylonitrile resins, acrylonit*ile/butadiene/styrene (ABS resins) or styrene/acrylonitrile/butene resins, and like : , : ~ , ~S3~
materials~
rrhe amount of moisture transmitted throu~h a given film material is a func-tion of the thickness of the film, i.e., the thicker the film, the less moisture transmitted. Thus, while relatively thick overcoat films (e.~., greater than about 10 mils) might be desirable, the electrical characteristics of such films during transfer would be undesirable. Most polymeric materials have a volume resistivity in excess of 1014 ohm cmO A thick layer of a highly resistive material would reduce the field imposed at the transfer nip thereby rendering the transfer roll substantially inoperative durin~
the transfer process unless an e~tremely high and impractical potential were imposed. Therefore the maximum thickness of an overcoat compose~ of a polymeric material of high resistivity must be such that the dielectric breal~down strength of the overcoat layer may be exceeded under operating transfer conditions thereby rendering the layer i'electrically invisible"
to the process. Where the molsture barrier polymeric overcoat layer is of relatively low volume resistivity, i.e., less than about I013 ohm-cm., the maximum thickness of the layer should be below about 5 mils, preferably below 2 ~ils. Where the resistivity of the polymeric overcoat is high, i.e., greater than about 101~ ohm-cmO, the ma~imum tnic~ness of the layer should be less than about 1 mil. In most cases, the ideal thickness of a highly resistive overcoat layer for a proper balance of moisture permeability and electrical properties is in the order of about 1/10 of the thickness of the second~ry overcoat la~er, or within the range of about 0.2 to 0~3 mils or O.Q002-0.0003 inches.
The primary relaxable layer 12 of Figure 1 comprises ~7--~S396~8 a materl~1 that f~nction~lly take~ a se7e~ted time perlod to translnit a ch~r~e from ~he conductive ~ore 11 ~o the inte:r~ace between the relaxable l~yer 12 and the self-levelling layer 13 sufficient to restore s~id interface to about the blas poten-tial appliecl to the core. This sele~ted time period is that correspondincJ to the roller surface spee~ and nip region width, i.e., roughly grea-ter than the time any point on the transfer roller is in the nip region, and is chosen to be appxoximately one quarter of the roller revolution time. Functionally, this means that the magnitude of the external electric field in-creases significantly from the pre-nip entrance toward the post-nip exit, while the field within the relaxable :Layer diminishes. Thus, a rela~able layer is one that has an ex-ternal voltage profile which is non-symmetrical about the trans-fer nip.
The primary layer is formed of an elastomeric or resilient material such as polyurethane or silicone rubber having a thickness in the range of about 0.20 to 0.30 inches~
preferably about 0.25 inch and having sufficient resiliency to allow the roll to deform when brought in moving contact with a photoconductive sur-face which may be in the ~orm of a plate, drum or belt. This i~sures an extended con-tact reyion in which toner paxticles can be trans~erred betwee~ the photo-conductive surface and transfer material. Preferably the primary layer has a durometer hardness in the range of 15-25, Shore A. Because the primary layer should be capable of re-sponding rapidly to the biasing potential to electrically impart the charge potential on the core to the outer extremi-ties of the roll surface, it preferably should have a resistivity in the order of 109 to 10 ohms cm., with about ~6~5i3~
2.0 x 10 ohms cm. a particularly desirable target.
The secondary self-le~elling layer 13 is a leaky insulator. The layer i5 selected for substantially higher resistive values, which in -the present embodiments means in the order of about 10 to 10 ohms per centimeter, more preferably in the order of 10 to lG ohm cm. In addition, the self-levelling layer includes materials, ~or is so related to the re]axable layer), such that charges applied to the outer surface of the self-levelling layer will be generally dissipa-ted within one revolution of the roller. This dissipation of char~e is desirable to prevent suppression of the transfer field in the nip.
The secondary layer is also formed oE a resilient material preferably having a durome-ter hardness in the range of about 65-75, Shore D, and a preferred thic~ness in the range of about 0.0020 to 0 0030 inches, preferably about 0.0025 inch~
It has been found that in order to minimize ionization in the atmosphere and in and about the nip con-tact region, it is preferred that the secondary layer have a target resistivity of about 3.2 ~ 10 ohms cm. A pre-Eerred material for use as the secondary layer is a polyurethane marketed by the duPont Company under the trade name "Adriprene".
The moisture barrier polymeric overcoat layer may be applied to the elastomeric surface of the transfer roll by any suitable process which will permit the formation of a relatively thin film having a substantially uniform thic~ness throughout. Suitable techniques include forming a solution of the moisture barrier polymer in a suitable solvent and applying the solutio~ to the surface of the transfer roll by spraying or dipping. Alternatively, thin films of the _9_ ~L~53~
moisture barricr polymer may be laminated directly to the transer roll surface using a suitable adhesive or using heat shrinking techni~ues.
The following example illustrates the fabrication of a transfer roll having a moisture barrier overcoating com-prising polyvinylidene chloride.
EXAMPLE I
A xerographic transfer roll as disclosed in U.S.
Patents 3,702,482 or 3,781,105 was provided. The roll consists of an aluminum core, a primary overcoating layer ~'."
comprising a polyurethane material ("Upjohn 2137-20", marXeted by the Upjohn Corp.) having a thicXness of about 0.25 inch and a room temperature valume resistivity of about 5 x 109 ohms cm at a moisture content of about 1.5% (50% RH), and a secondary overcoating layer compri.sing a polyurethane material ("Adiprene L-315", marketed by the duPont Corp.) having a thickness of about 0.0025 inch and a room temperature volume resistivity of about 3 x 1014 ohms cm at a moisture content of about 1.0% (50% RH).
A solution of polyvinylidene chloride (PVC12) was prepared by dissolving 10 parts by weight of PVC12 in 225 parts by weight of methylisobutyl~Xetone. About 0.01 parts of a hydrophobic silicon dioxide (Silanox - Cabot Corporation) was added as a levelling agent for the PVC12. This solution was then loaded into a laboratory spray gun and sprayed uniformly on the peripheral surface of the secondary overcoat layer of the above transfer roll. Spraying was continued until sufficient solution had been applied to yield, after drying, a PVC12 film having a uniform thicXness of less than about 0.3 mil~ ~le resistivity of PVC12 is such that the ~LOSi39~8 thickness of the PVC12 overcoat should not exceed about 0.3 mils so that the dielectric breakdown strength of the layer may be exceed~d during the transfer operation i~e~, when about 2500 volts is applied.
The transfer roll was thoroughly dried -to yield a three layer structure wherein the elastomeric layers are hermetically sealed by a PVC12 overcoat having a substantially uniform thickness of about 0.2 mil.
.The stability of the transfer rolls of the present invention towards moisture transmission and consequent electrical stability is demonstrated by Figure 3. A coated roll as prepared in Example 1 was evaluated vs. a roll with no moisture barrier coating over a period of 28 days at various humidity values varying from 80% R~I to 10% RH. As shown in Figure 3, the 1uctuations in moisture content (both gain and loss of moisture) were severe with the uncoated roll but relatively stable with the coated sealed roll. The resis-tivity of the uncoated roll fluctuates by large order of mag-nitude over the moisture range encountered during the test.
Variations in resistivity for the uncoated roll are.shown to ~luctuate from 9 x 101 to 3 x 109 ohm-cm. under low and high humidity conditions, while the sealed roll variation is only 1 x 101 to 9 x 109 ohm-cm.
In practice it has been found that the efficiency of Z5 transfer of ton2r to the transer substrate (paper) i5 : ~ maximlzed at nip currents within the range of about 15 to 20 micro amps. Improved stabilization of the resistivities of the primary and secondary overcoat layers o the transer roll : according to the:present invention thus allows for reasonable ~30 maintenance of this nip current under operating conditions . ~

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without the need to resort to sophisticated means ~or altering the voltage applied to the transfer roll to compensate for large fluctuations in roll resistivity due to changes in humidity conditions.
The transfer roll encompassed by the present invention may be used in conjuction with any suitable electrophotographic apparatus as a means for transferring toner particles bearing an electrostatic charge from the surface of photoconductive insulat~
ing layer to a transfer surface such as paper. Transfer is accom-plished as in the prior art by feeding a sheet of transfer materialinto the nip region formed by the surface of the transfer roll and surface of a photoconductive insulating material bearing a developed image, and imposing a potential on the transfer roll sufficient to cause the transfer of the toner material from the surface of the photoconductive insulating material to the adja-cent surface of the transfer. material. In practice, any source of electrical power connected to the cen-tral conductive core of the transfer roll and capable of placing the transfer roll member at potential sufficient to attract toner images from the photo- .
conductive insulating surface towards the roll may be employed.
A more complete discussion of t~e principles and configurations involved in bias roll transfer may be found in U. S. Patents 2,951,443, issued September 6, 1960, J~ F. Byrne, 3,620,61~, issued November 16, 1971, J. Ro Davidson et al, or 3,781,105 issued December 25, 1973r T. Maegher.

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Claims (13)

What I claim is:

1. A transfer member for electrically cooperating with a conductive support surface to electrically attract charge particles from the support surface towards the member including:
(a) a conductive support;
(b) a primary elastomeric intermediate layer over-lying said conductive support, said primary layer having a relatively constant volume resistivity within the range of about 107 to less than 1011 ohms cm;
(c) a secondary elastomeric intermediate layer overlying said primary layer, said secondary layer having a relatively constant volume resistivity within the range of about 1011 to 1015 ohms cm; and (d) a polymeric overcoating layer overlying and hermet-ically sealing said layers, said overcoating layer having a substantially uniform thickness of less than about 5 mils and a moisture permeability content of less than about 1 x 10-8 at 30°C.

2. The transfer member of claim 1 wherein said polymeric overcoating layer has a volume resistivity of greater than about 1014 ohm-cm. and a thickness of less than about 1 mil.

3. The transfer member of claim 2 wherein said primary layer has a volume resistivity within the range of about 109 to 1010 ohms-cm. and said secondary layer has a volume resisti-ity within the range of about 1013 to 1015 ohms-cm.

4. The transfer member of claim 3 wherein said primary layer has a volume resistivity of about 2 x 109 ohms-cm. and said secondary layer has a volume resistivity of about 3.2 x 1014 ohms-cm.

5. The transfer member of claim 3 wherein the primary layer has a thickness of about 0.25 inch, the secondary layer has a thickness of about 0.0025 inch, and the polymeric over-coating layer has a thickness of about 0.0025 inch.

6. The transfer member of claim 3 wherein the polymeric overcoating layer is selected from the group consisting of high or low density polyethylene, polypropylene, butyl rubber, ethylene/propylene rubber, polyamides, polyvinylidene chloride, polyvinylidene fluoride, polytrifluoroethylene, rubber hydro-chloride, and copolymers of acrylonitrile with vinyl monomers, monoolefins or diolefins.

7. The transfer member of claim 6 wherein the polymeric overcoating layer comprises polyvinylidene chloride.

8. In a method for transferring a toner image from a photoconductive insulating surface to the surface of an image support material, said method including the steps of bringing said photoconductive insulating image-bearing surface into operative communication with an electrically biased transfer member and interposing a sheet of image support material there between, said transfer member being biased to a potential sufficient to attract said toner image from said insulating surface towards said image support material, the improvement which comprises employing as a transfer member an apparatus comprising:
(a) a conductive support;
(b) a primary elastomeric intermediate layer over-lying said conductive support, said primary layer having a relatively constant volume resistivity within the range of about 107 to less than 1011 ohms cm;
(c) a secondary elastomeric intermediate layer overlying said primary layer, said secondary layer having a relatively constant volume resistivity within the range of about 1011 to 1015 ohms cm; and (d) a polymeric overcoating layer overlying and hermet-ically sealing said layers, said overcoating layer having a substantially uniform thickness of less than about 5 mils and a moisture permeability content of less than about 1 x 10-8 at 30°C.

9. The method of claim 1 wherein said polymeric overcoating layer has a volume resistivity of greater than about 1014 ohm-cm. and a thickness of less than about 1 mil.

10. The method of claim 2 wherein said primary layer has a volume resistivity of about 2 x 109 ohms-cm. and said secondary layer has a volume resistivity of about 3.2 x 1014 ohms-cm.

11. The method of claim 3 wherein the primary layer has a thickness of about 0.25 inch, the secondary layer has a thickness of about 0.0025 inch, and the polymeric over-coating layer has a thickness of about 0.0025 inch.

12. The method of claim 3 wherein the polymeric overcoating layer is selected from the group consisting of high or low density polyethylene, polypropylene, butyl rubber, ethylene/propylene rubber, polyamides, polyvinylidene chloride, polyvinylidene fluoride, polytrifluoroethylene, rubber hydro-chloride, and copolymers of acrylonitirle with vinyl monomers, monoolefins or diolefins.

13. The method of claim 6 wherein the polymeric overcoating layer comprises polyvinylidene chloride.