Patent Application
20070279442
In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which:
A printed image is formed using an ink having electrically charged marking particles. Although ink such as typical ink jet inks including pigment particles can be used (so long as the other physical requirements of the inks as described in this disclosure are met), it is preferable that the ink includes polymeric particles. Although clear polymeric particles can be used if desired, it is generally preferable to use polymeric particles having a dye, pigment, or other colorant. In this disclosure, the term “marking particles” shall include polymeric particles whether or not they include a colorant.
The ink is deposited in an image-wise fashion using appropriate ink jet deposition methods such as a continuous ink jet stream, or drop-on-demand technology onto an electrically conducting substrate. In the preferred mode of operations the substrate is electrically grounded, although it can be electrically biased if so desired. The image is then passed through a nip formed by the image-bearing substrate and a fractionating device. A potential difference is established between the fractionating device and the image bearing substrate. This is preferably done by electrically grounding the substrate and establishing a bias on the fractionating device that would drive the charged marking particles towards the substrate and the supernatant fluid comprising counter ions towards the fractionating device. Although the voltage is not critical, it is preferred that the difference of potential between the fractionating device and the substrate be between 100 and 1,000 volts, preferably between 100 and 500 volts and more preferably between 150 and 350 volts. Lower voltages may not be sufficiently strong to drive the marking particles towards the substrate within the nip residence times. Higher voltages are limited by arcing within the nip and possible by reversing the charge on the marking particles. Such charge reversal would preclude the ability to subsequently transfer the particles. After fractionating, the image is transferred from the primary imaging member to a secondary imaging member. The secondary imaging member could be an intermediate member, a receiver such as paper or transparency stock, etc. Although any appropriate means of transfer to the secondary imaging member could be employed, it is preferred that transfer be accomplished by applying an electrostatic bias of sufficient magnitude and polarity to urge the marking particles to the secondary imaging member. When the secondary imaging member is an intermediate imaging member, transfer to the receiver can, again, be accomplished using suitable transfer technology such as the application of pressure or heat and pressure or any other suitable means. However, it is preferable to transfer the image by applying an electrostatic field of such magnitude and polarity to urge the marking particles away from the secondary imaging member to the receiver. Methods of electrostatic transfer are known in the electrophotographic literature and comprise using a biased roller that presses the receiver against the imaging member, the use of a corona, etc. It should be noted that fractionation can be done, using this same technology, on an intermediate member rather than the primary imaging member. It is not, however, desirable to attempt to fractionate from the final receiver as the receiver may absorb the solvent or a sizable fraction thereof. Moreover, the presence of the relatively dilute, thereby low viscosity, ink can run on the receiver, thereby reducing image quality.
The nip formed between the fractionator and the imaging member should have a spacing of less than 250 μm, preferably less than 50 μm and more preferably less than 25 μm. In some embodiments of this invention, it is possible for the fractionator to be in physical contact with the image-bearing primary imaging member and form a nip with a finite nip width.
As an example, a fractionator can include a wedge-shaped metallic member in which the vertex of the wedge is held in close proximity to the primary imaging member. The fractionator is electrically biased as discussed above in this disclosure and the primary imaging member is grounded. The marking particles are driven towards the primary imaging member, leaving a layer of supernatant solvent that can then be skived off by the wedge.
Referring to the accompanying drawings, the preferred embodiment of the fractionator is shown in
It is further preferred that the fractionator includes a squeegee blade to remove the supernatant liquid from the fractionating roller 10. This blade is preferably made of an elastomeric polymer that is not plasticized by the solvent. The squeegee blade 30 is mounted so as to be in contact with the fractionating roller after fractionation has occurred. The supernatant fluid is then allowed to drain into a drip tray 40, where it can be recycled or discarded.
In another embodiment of this invention, the imaging member on which fractionation occurs comprises a semiconducting polymer such as an elastomer such as polyurethane. Such materials are similar to those often used in transfer rollers in electrophotographic engines. However, in this instance, the polymer cannot be plasticizable or significantly swellable by the ink solvent. Materials such as these typically comprise a charge-conducting agent and typically have resistivities between 106 and 1011 Ω-cm.
In yet another embodiment of this invention, the fractionator can have a compliant, electrically conducting blade or roller in contact with the imaging surface on which fractionation occurs. Suitable materials include elastomeric materials such as polyurethane or silicone rubber or foams made from such materials. Such fractionating members should also comprise sufficient charge conducting agent so as to result in the fractionating member having a resistivity less than 1011 Ω-cm and preferably less than 106 Ω-cm.
For fractionation to occur, it is important that the ink possess certain physical properties. These properties are often significantly different from inks commonly used in ink jet printers that do not require electrostatic fractionation. The ink must be sufficiently electrically resistive so as to support an electric-field. The resistivity of the ink is determined by measuring the current generated by an alternating voltage (AC) having a frequency of 1 kHz. The resistance is the ratio of the root-mean-square (RMS) of the applied voltage (approximately 0.707 times the amplitude of the applied AC voltage for a voltage that is varying sinusoidally with time) to the current. The resistance is the product of the resistivity times the separation distance between the electrodes containing the ink divided by the area of the electrode. It is recognized that, for high resistance materials, it is often desirable to surround the biased or active part of the electrode with conductive material that is used to form a grounded or guard ring around the active part of the electrode in order to reduce noise. For the presently described fractionator to work, the AC electrical resistivity of the ink should be greater than 109 Ω-cm and preferably greater than 1010 Ω-cm. This precludes the use of aqueous based ink jet inks and most alcohol based ink jet inks as their resistivities are typically less than 107 Ω-cm. Rather, the ink should comprise a dispersing liquid such as mineral oils such as Isopar L or Isopar G, both sold by Exxon Corporation, silicone oil, high molecular weight alcohols, etc. While certain alkanes and other aliphatic and aromatic hydrocarbons may be suitable, their associated flammabilities and the potential health risks make them less than fully desirable. For purposes of this disclosure, the AC resistivity was determined using an AC signal with an amplitude of 0.75 VAC, at a frequency of 1 kHz. 0.4 ml of the ink was placed into a cell using a pipette. The electrode spacing between electrodes was 10 μm and the active diameter of the electrodes was 1.3 cm. A guard ring surrounded one of the electrodes.
DC resistivity was determined using the same cell, but applying a DC voltage with a magnitude of 100 V. For fractionation and transfer to occur, the resistivity of the supernatant fluid should be sufficiently high so as not to short the field in either the fractionator or transfer station. This requires that the DC resistivity be in excess of 109 Ω-cm. This high resistivity precludes the use of aqueous and many alcohol based conventional ink jet inks in this process.
The ink should also comprise electrically charged marking particles. While the exact magnitude of the charge is not critical, it should be sufficiently large as to preclude flocculation of the marking particles and enable the particles to fractionate and transfer within the time allowed by the specific engine. Moreover, it is important that the vast majority of the particles have the same charge polarity to enable fractionation and transfer to occur and to prevent flocculation. The charge and charge sign can be determined using known techniques. The marking particles can comprise a colorant, which can be either a dye or a pigment. The marking particles can also comprise a polymeric binder such as polyester, polystyrene, polystyrene butyl acrylate, etc. Alternatively, the marking particles can comprise free pigment particles provided the pigment particles meet the size and charge criteria discussed in this disclosure. However, common ink jet inks that comprise dye would not be suitable as the dye is in solution and, accordingly, could be neither fractionated nor transferred in the manner disclosed herein. The particles need to be sufficiently small so as to be jetable from an ink jet head. This limits their average diameter to less than approximately 3 μm. Conversely, it would be difficult to control the motion of the particles, even in the presence of an electrostatic field, if the average particle diameter was less than approximately 0.1. Smaller particles would be subject to random motion such as that induced by Brownian motion. Particle diameters can be determined by known techniques including laser scattering, transmission electron microscopy, and scanning electron microscopy. In the preferred embodiment, the marking particles would comprise a polymeric binder. The marking particles can be colorless if desired.
The viscosity of the ink is also important, as it must be jetable. It is preferable that the viscosity be less than 20 centipoise, preferably less than 10 centipoise, and even more preferably less than 5 centipoise. The viscosity in the cited examples was measured using a Brookfield viscometer model number DV-E. The spindle model number was 00. The spindle rotated at 100 rpm. In general this viscometer model and spindle model could be used, however, depending on the viscosity the spindle would be rotated between 20 and 100 rpm. Alternatively, the viscosity could be measured with a Brookfield model LV viscometer with a UL adaptor at approximately 12 rpm.
Commercially available ink sold as cyan colored Signature by Kodak, diluted with Isopar L, was used for this experiment. The marking particles in this ink are approximately 0.1 μm in diameter, as determined using transmission electron microscopy. The AC resistivity measured at 1 kHz with an applied voltage with an amplitude of 0.75 volts, was approximately 1.46×1011 Ω-cm. The viscosity was 1.75 cPoise. The ink was jetted onto a primary imaging member comprising nickelized polyethylene terephthalate on an aluminum support. The primary imaging member was approximately 12.5 cm wide by 20 cm long. The nickel layer was electrically grounded. The roller fractionator that was described as the preferred embodiment of this invention was used in this experiment. As the Signature marking particles are charged, the roller was biased at +300 volts to drive the marking particles towards the primary imaging member. The spacer wheels used on the fractionator established a gap of approximately 40 μm. The fractionating roller was rotated at approximately 10.5 to 11 rpm counter to the direction of movement of the primary imaging member.
The ink was jetted onto the entire primary imaging member. It was then driven over the fractionator. After fractionation, the image was transferred to a clay-coated paper (Sappi Lustro Laser) that had been wrapped around a polyurethane transfer roller similar to those used in electrophotographic printing engines. The paper was chosen because it is nonporous and represents a very stressful receiver for conventional ink jet engines. Transfer was accomplished by biasing the roller at −1,000 volts to attract the marking particles to the receiver. It should be noted that it is well known that it is extremely difficult to electrostatically transfer dry toner particles having the same size as the marking particles used in this ink in electrophotographic engines.
During the fractionation process, clear supernatant liquid was observed to flow over the roller. Immediately after transfer, it was found that the image on the receiver was dry and virtually all of the marking particles transferred from the primary imaging member to the receiver. The image was also permanently fixed after transfer without having to use any external means of fixing the image such as fusing. These are surprising results.
In order to quantify how much solvent was present on the receiver after transfer, the image-bearing receiver was placed in a microbalance and its initial mass tared out. Upon evaporation of solvent, the receiver should become lighter. No solvent loss was found, to 0.1 mg, which was the limit of the balance, over a 24 hour period. This confirms that the marking particles were predominantly dry after fractionation.
This example is similar to example 1 except that no bias was applied to the fractionator. In addition, no quantitative measurements of solvent evaporation were made. In this case there was a lot of solvent visible on the paper after transfer. Moreover, a large fraction of the marking particles were skived off the primary imaging member by the fractionator. This result shows the importance of the electrical bias applied to the fractionator.
This example is similar to example 1 except that the polarity of the bias applied to the fractionator was reversed so as to attract the marking particles to, the fractionator. In this example, there were few marking particles transferred to the receiver, as most were removed from the primary imaging member by the fractionator. Solvent was visible on the receiver after transfer.
This example is similar to example 1 except that the design of the fractionator was altered. In this case, the fractionator has an aluminum member, approximately semicircular in shape. This device was attached to the frame of the breadboard that also comprised the track on which the primary imaging member traveled. The trailing edge of this member, referenced to the direction of travel of the primary imaging member, was positioned so that there was a space between the fractionator and primary imaging member of approximately 40 μm at the leading edge of the primary imaging member. However, as the fractionator was fixed to the breadboard and its separation was not indexed to the primary imaging member, the space between the fractionator and primary imaging member varied between 40 μm and 75 μm. In this case, fractionation occurred, as was evidenced by the clear supernatant liquid on the fractionator after the fractionation process. However, the ink on the primary imaging member, although concentrated, was not concentrated to the point at which the transferred image was dry. Rather, some solvent was clearly visible on the transferred image. This example shows that, although the fractionator described in this example is within the specifications of this patent and does function, it is not the preferred mode.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
