Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
2001-12-27
2003-09-16
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S327000, C428S423100, C428S424200, C428S500000, C428S515000, C399S174000, C399S176000
Reexamination Certificate
active
06620494
ABSTRACT:
The invention relates to a conductive article in which, around a conductive sleeve, a covering layer formed with a composition containing an intrinsic conductive polymer and a film-forming polymer is present, and to a method for fabricating such an article, comprising the application of a layer of said composition to a conductive sleeve.
Conductive articles of this type, in particular rollers, doctor blades and flat or curved plates, are generally used in electrophotographic and xerographic printing and copying equipment, faxes and other office equipment, for example as a charge transfer roller for the purpose of electrostatically charging a photosensitive-sensitive drum, as a developing roller for developing an electrostatic latent image on the surface of said drum to produce a visible toner image, as an image transfer roller for transferring the toner image to a copy, or as a doctor blade for controlling the thickness of a toner layer. In the case of hollow rollers, the inside can likewise be coated with a conductive covering layer. During operation, the rollers, blades or plates can either be in continuous contact with a cooperating element, for example a photosensitive drum, or there may be a small gap between the article and the cooperating element.
If, for example in the case of a roller, the latter is in continuous contact with, for example, a drum, then the roller generally consists of a conductive core, often a metal rod, around which a likewise conductive resilient sleeve is fitted. Said sleeve consists of a resilient material which is indented when the roller is pressed against a cooperating roller or face to which charge is to be applied or from which it is to be removed. To supply the charge that is to be transferred, a voltage is applied to the roller. In order, on the one hand, to limit the current through the roller and the cooperating elements with which the roller is brought into contact, but on the other hand to transfer the desired amount of electrical charge sufficiently rapidly, the electrical resistance of the roller needs to be within certain limits. To this end, a covering layer is usually applied to the sleeve, which imparts a desired resistance to the roller as a whole. The electrical resistance of core and sleeve, which is connected in series with the resistance of the covering layer, is preferably selected to be sufficiently small for the covering layer in fact to define the total resistance. The resistance of the roller is measured as the resistance between the location where the roller, during operation, is brought into contact with the voltage to be applied, as a rule the roller shaft, and a point of the outer circumference of the roller.
If the roller, during operation, does not come into contact with a cooperating roller or face, the sleeve may also consist of a non-resilient material, and the sleeves can, for example, also be composed of metal. This is the rule, for example, in the case of magnetic developing rollers. In that case too a covering layer is present which defines the ultimate electrical resistance of the roller.
What has been said hereinabove about rollers also applies to doctor blades and plates insofar as the mutual relationship of the various layers is concerned. At all times, at least one layer, the sleeve, having a high conductivity is present with a covering layer applied thereto which defines the ultimate electrical conductivity. Any differences reside in their shape and design. These, however, do not form part of the present invention and are known per se in the art in question. Where a roller is referred to hereinafter, the disclosure, always allowing for any differences in design, equally applies to doctor blades and flat or curved plates.
The said covering layers are formed, as a rule, from a composition comprising a nonconductive binder and a conductive material finely dispersed therein.
U.S. Pat. No. 5,597,652 discloses a composition for a covering layer of a conductive roller, which consists of nylon, urethane or rubber as a binder and metal oxides or carbon black as a conductive material. The resistance of a covering layer formed from that composition depends on the ratio of binder to conductive material.
A drawback of this known composition is the poor adjustability of the electrical resistance of a covering layer made using the composition. In fact, the resistance of the known composition is found to adopt, depending on the concentration of the conductive material, two values with in between, starting from a certain concentration which is referred to as the percolation threshold, a steep transition section. The one extreme is defined by the resistance of the binder which, as a rule, is very high. The other extreme is defined by the resistance of the conductive material which, from a certain concentration in the binder onwards, forms conductive paths therein. The difference between the two extremes, determined from the resistivity, to be defined hereinafter in more detail, of the material can be very large and often amounts to a factor of from 10
8
to 10
11
. Now those values of the resistance of the covering layer of conductive articles, for example rollers in electrophotographic equipment, for example of a charge transfer roller, which are suitable for practical use are situated precisely between these two extremes and thus in the steep transition section. This makes it particularly difficult for a covering layer having a suitable, desired electrical resistance to be fabricated reproducibly from the known composition.
The resistance of a covering layer could also be affected by its thickness. The thickness of a covering layer, however, is likewise restricted to certain narrow limits. On the one hand, shorting via pinholes or flashovers must be prevented, which imposes a lower limit on the thickness. On the other hand, certainly in the case of rollers which must be indentable, the covering layer must be sufficiently flexible to be able to follow the indentation of the resilient sleeve without becoming detached or rupturing, which imposes an upper limit on the thickness. Variations in thickness will therefore, in most cases, provide no option or only limited options to influence the resistance of the covering layer.
It is an object of the invention to provide a composition from which a covering layer on a conductive roller can be fabricated with an electrical resistance in the range suitable and desired for the covering layers described.
This object is achieved according to the invention by the composition consisting of an intrinsically conductive polymer and an electrically inert film-forming polymer.
It was found that the resistance of this composition, compared with that of the known composition, changes much more evenly as the concentration of the conductive material changes, in this case the intrinsically conductive polymer. In particular, the composition according to the invention does not exhibit the steep transition section between high and low electrical resistance. Using the composition according to the invention it is possible, in a simple and reproducible manner, to apply covering layers to rollers, whose electrical resistance has a desired value at a layer thickness, which is between the limits acceptable in practice.
From U.S. Pat. No. 5,572,294 a roller is known in which the coating layer consists of conductive particles in a binder resin and having a 10% elongation load of not more than 700 gf on a 1 cm wide section. Among the conductive particles also intrinsic conductive polymers are mentioned. Any teaching that the specific combination according to the invention of specific intrinsic conductive polymers with specific film forming resins, in contrast to the other possible combinations, results in coating layers having a controllable electric resistivity showing no steep percolation threshold is absent in this reference
EP-A-594,366 discloses a coating layer mandatorily containing, in addition to an (optional) polymer binder and charge injection enabling particles (e.g. conductive polymers), cha
Bots Thomas L.
Gilberts Joris
Van Engelshoven Bernardus J.
Will Thomas
Ahmed Sheeba
Pillsbury & Winthrop LLP
Ten Cate Enbi B.V.
Thibodeau Paul
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