Method of developing electrostatic images using wide latitude toner

#This invention relates to electrophotography and more particularly to an improved method and means for making visible a reverse latent image formed by negative electrostatic charges on electrophotograhic plates or surfaces as are known in electrostatic printing. In certain forms of electrophotography, a relatively conductive backing member, such as a paper or metal sheet, has deposited upon its surface a uniform electrostatic charge. The charged surface is then exposed to a radiation pattern whereby the irradiated areas become discharged in accordance with the intensity of radiation, whereas the shielded areas remain charged, and thus form a latent electrostatic image. This latent image is then rendered visible by applying a dry powder developer or toner material suspended in an insulating carrier liquid such that the toner may be attracted and held electrostatically to the charged areas. The developed image may be fixed to the surface of the photoconductive material or transferred to another surface and fixed thereon.

#In the prior art it is known to produce both positive and negative liquid developers by suspending a particulate pigment wetted with an oil or resin in an insulating carrier liquid to form the toner material to be deposited electrostatically. A positive developer contains toner material which can be attracted by negative charges and, therefore, can be used, for instance, to develop a latent image formed by such negative electrostatic charges on a charged and exposed zinc oxide coated electrophotographic paper. The resulting image is a facsimile reproduction of the original radiation pattern.

#On the other hand, a negative developer contains toner material which can be attracted by positive charges or repelled by negative charges. The negative developer can be applied to a photoconductive layer containing a negative latent image to produce a reversal reproduction of the original radiation pattern by toner repulsion from the image areas holding negative charges, and by deposition of toner particles on the exposed areas which have no or little negative charge. The charged particles depositing themselves on the uncharged and lesser charged surfaces of the photoconductive layer adhere to the layer as a result of Van der Waal forces. Van der Waal forces are the attraction of the molecular forces in the photoconductive sheet or layer to the molecular structure of the toner particles. In this instance, the rate of deposition of the negatively charged toner material is inversely proportional to the magnitude of the negative surface charge retained on the photoconductive surface.

#To further explain the functioning of the liquid developer in accordance with this invention, reference is made to the accompanying drawing in which:

#FIG. 1 is a schematic cross-sectional view of a charged photoconductive layer illustrating the deposition of negatively charged particles on uncharged areas of the layer above a cut-off point where a toner having a cut-off capability is utilized.

#FIG. 2 is a similar view of a photoconductive layer where the toner has no cut-off, illustrating the deposition of negatively charged particles on partially charged, as well as uncharged, areas of the layer.

#Referring to FIG. 1, an electrophotographic sheet, such as zinc oxide coated paper, is designated by the numeral 1. The sheet 1 has been charged to a negative potential, whereby the magnitude of the negative charge decreases from the base line 2 toward the right. The sheet 1 has, subsequently, been exposed to a radiation pattern of varying intensity, whereby the initial negative charge has been reduced in the areas, 3,4,5 and 6. The area designated 5 represents the area receiving the highest intensity of radiation, whereby the negative charge in the sheet 1 is reduced practically to zero. The areas 3,4 and 6 have been exposed to lesser magnitudes of radiation, and the original magnitude of negative charge is reduced to only a certain degree in these regions, leaving each area with a residual negative charge.

#As a toner is applied to the surface of sheet 1, negatively charged particles 7 in the toner are subject to electrostatic forces. The surface of sheet 1 contains latent images formed by negative electrostatic charges of varying density. As stated previously, there is a flux present which may be considered to exist in a plane perpendicular to that of the sheet 1. This flux is represented by the lines 8 in FIG. 1. Since the particles 7 and the sheet 1 are charged to the same polarity, particles 7 are repelled from the charged areas of the sheet. However, as the magnitude of negative charge in sheet 1 decreases in a given area, charged particles 7 begin to be attracted to uncharged, or relatively smaller charged areas under the influence of intermolecular, or Van der Waal forces. This is illustrated by the numeral 9 in FIG. 1.

#The particular illustration of particle deposition in FIG. 1 relates to the use of prior art toners which comprise a "cut-off", or ability not to deposit any particles in an area retaining a specified intensity of negative charge. Such cut-off toners are disclosed in Matkan et al, U.S. Pat. No. 3,362,907. In FIG. 1, the cut-off charge line is designated by the numeral 10, and it is apparent that no areas to the left of this line 10 (representing higher negatively charged areas) receive a deposition of negatively charged particles 7. As a result, the final reproduction will sharply reveal the image represented by the area 5, but the image represented by areas 3,4 and 6 will not appear at all. Therefore, it is evident that with prior art cut-off toners, while maximum contrast in the reproduced image is afforded, light portions of the original image will be completely lost.