US2788288A - Process and composition for developing an electrostatic image - Google Patents

Description

United PROCESS AND COMPOSKTHUN FOR BEVELOPING AN ELECTRUSTATEC IMAGE No Drawing. Application July 29, 1953, Serial No. 373,431

3 Claims. (Cl. 117-475) This invention relates in general to Xerography and, in particular, to a developer composition of improved performance in the art of xerography.

in xerography, it is usual to reproduce a master by electrical photography methods, such as, most typically, placing an electrostatic charge on a photoconductive surface, selectively dissipating such charge by exposure to an optical image corresponding to the master to be reproduced and developing the resulting electrostatic latent image by exposure to an electroscopic material. According to one practice in xerography, as disclosed in U. S. Patent 2,618,552, the development of the electrostatic latent image is accomplished by rolling or cascading across the image-bearing surface a developer composition of relatively large carrier particles having on their surfaces and electrostatically coated thereon fine powder particles known as toner particles. As the composition cascades or rolls across the image-bearing surface, these toner particles are electrostatically deposited on and secured to the charged portions of the image and are not deposited on the uncharged or background portions of the image. More than that, toner particles accidentally deposited on these background portions are physically removed therefrom by electrostatic action of the carrier particles passing thereacross whereby these toner particles are electrostatically secured to the rolling carrier particles and are picked up from the surface in this manner. The result is an excellent copy of the electrostatic latent image in the form of an image by the toner particles electrostatically clinging to the image surface and removable therefrom by any of various means, such as adhesive transfer, electrostatic transfer, or the like. Thus the image body may be transferred to a sheet in contact with the image body and applying an electrostatic charge to the paper while in such contact. When the paper is subsequently stripped from the image-bearing surface, it carries with it a substantial proportion of the image body to yield a xerographic print which thereafter may be made permanent by any desired method, such as heating, solvent fixing, or the like.

After the image is transferred from the image surface to the transfer member, or paper, the image surface may be cleaned and then is ready for use in a subsequent xerographic cycle. The xerographic plate, after being properly cleaned following a previous xerographic cycle, is in its originalcondition and is substantially unimpaired for future use. However, a problem in prior experience has been caused by the fact that cleaning of the xerographic plate between cycles is unexpectedly difficult, due apparently to the strong attraction of the residual toner particles to the plate. This is evidenced in two manners; first, by the stubborn adherence of toner particles as such to the plate, with the result that after mechanical cleaning operations substantial amounts of such powder may still remain on the image surface, and,

@atent second, by the fact that a film or layer builds up on the plate surface during repeated cycles and eventually re quires additional cleaning operations, such as, for example, solvent cleaning, or the like.

Either type of residual toner, that is, either toner in its powder form, or toner as a film on the image surface, impairs the subsequent operation of the xerographic plate. Thus, for example, if toner particles remain on the plate, they interfere with the subsequent steps of xerography causing either deletions or background deposition during subsequent steps. Perhaps the more serious trouble, however, is the building up of the film which appears to consist of a gradual accumulation of the toner material on the plate surface, apparently in the form of a smear as the toner particles are mechanically removed therefrom. The presence of this film interferes in many ways. In the first place, this film has different electrical properties from those of the photoconductive layer on the xerographic plate, and thus it tends to interfere with the charging or sensitizing step. It also has mechanical or physical properties differing from those of the photoconductive layer, particularly in that it is sub stantially more sticky or tacky than the clean plate surface. Beyond this, the toner films appear to be somewhat hygroscopic to the extent that in humid weather it detrimentally affects conductivity under exposure to light and insulating properties in the absence of light,

particularly where the relative humidity is greater than about t is presently believed that a great deal of the difficulty caused by the building up of this film and, in fact, by incomplete removal of toner particles from the residual image is a function of a physical or mechanical property of the toner particle in that they tend to be somewhat tacky and apparently adhere to the image surface by mechanical means as well as by electrostatic forces. The

problem, moreover, is complicated by the fact that the usual and presently preferred method of transferring the image body from the image surface to the transfer member, such as the paper sheet, preferably is carried out by electrostatic forces, with the result that the mechanical adhesion between toner and image surface cannot be overcome simply by increasing the mechanical transfer force.

permanentizing the xerographic print employs heat fusion to melt the toner particle onto and into the surface of the transfer sheet. Thus, the toner particle must be capable of electrostatic transfer and subsequently must be usable within temperature limits readily tolerated by convenient and economical transfer members, such as paper, or the like, and the usual methods of lowering are generally charged to positive polarity for sensitization.

and thus the toner particles must be such that they are charged to negative polarity by mixing with the carrier particles. These and other problems must be solved while achieving the necessary end results of suitability for xerography, including the ability to :form an ink receptive image useful in lithography.

Now, in accordance with the present invention, there Patented Apr. 9, 1951;

Beyond this, the difl'lcuties are compounded by. the fact that the presently preferred fixing method for.

greases is provided a new and improved xerographic toner composition comprising finely'divided pigmented resin particles having a particle size less than about 20 microns and preferably having an average particle size between about and about microns and consisting of a finely divided uniform mixture of pigment in a non-tacky lowmelting resin consisting substantially of a polymerized styrene or a blend of polymerized styrenes having a melting point of about 257 F. (ball and 'ring) and being characterized by extreme toughness as measured by nonsmearing properties and by being an extremely hard thermoplastic resin. The polymerized styrene is present in the composition in a predominating amount which is defined herein as being at least about /e. of the entire composition; optionally mixed or blended with up to about 25% of polybutyl methacrylate such as polymerized n-buty'l methacrylate. The pigment is present in the toner in a sufiicient quantity to cause it to be highly colored whereby itwill form a clearly visible image on a transfer member. Thus, for example, in the usual case where a xerographic copy of a document, or the like, is desired, the pigment will be a black pigment such as carbon black or other minutely divided carbonace'ous pigment. Desirably, the pigment is employed in an amount 'of at least about 5% based on the total weight of the toner body and generally between about 5% and about 10%.

In the preparation of the toner composition according to the present invention, the ingredients are thoroughly mixed to form a uniform dispersion of the pigment in the main body and thereafter the body is finely divided to form the desired toner or powder composition. The mixing may be done by various means, including combinations of the steps of blending, mixing, and milling, and the presently preferred method includes a step of blending in a rubber mill to assure uniform and fine dispersion of the pigment in the resin. The following example is illustrative of the manner in which the composition of this invention may be blended:

Example I A two-pound batch of the toner composition was blended in a total blending time of ten minutes on a rubber mill having two rolls, each 8 inches in diameter and 16 inches long. The particular mill used had a rollspeed ratio of 1.29 with the faster roll rotating at 28 R. P. M. The rolls were spaced 0.01 5 to 0.020-inch apart and the roll temperature was 100 F. to 150 F.

Prior to blending, the toner components were mixed on the mill at a roll temperature of about 280 F. and a. roll spacing of about 0.100 inch. The toner components were added to the mill one at a time beginning with the polymerized styrene. The entire quantity of styrene resin was placed on the rolls of the mill at one time and left there for about five minute The polybutyl methacrylate and the pigment were then added, in that order. The pigment (carbon black) was added slowly to minimize loss to the air. For example, approximately ten minutes were allowed for adding 91 grams ofcarbon bla I Experience has shown that minimum blending times result from using: 1) minimum'roll spacing, (2) maximum roll speed, (3) maximum roll speed differential, (4) maximum viscosity of the dispersion medium, (5) maximum pigm nt concentration, and (6) pigments free of hard aggregates and compacts.

One particularly important characteristic of the new composition is its easy cleanability from a surface to which it is clinging by relatively high electrostatic forces. This characteristic is due to a property generally similar to low tackiness and now believed to be attributable to hardness and toughness of the finely dividedmaterial. Thus, when rubbed across a surface or when wiped oif a surface, the tough composition'does not smear or stick to the surface.

One particular type of resin composition which has been found to be unusually well suited to the present invention is a polymerized blend of styrene and styrene homologues of the general formula where R is selected from the group consisting of hydrogen and lower alkyl. The resins of this type are prepared in a wide range of average molecular weights from crude mixtures of styrene and styrene homologues, such as those obtained from fractionation of the so-called crude solvent from light oils scrubbed out of coke-oven or gas-house gas. The resins are pale in color and are nonacid and unsaponifiable. They are substantially wholly hydrocarbon in composition. Polymerization does not advance significantly with age or heat.

A particular preferred type of resin within this class is a hard, tough resin having a ring-and-ball melting point of about 125 C. or 257 F. being medium high in polymer structure. When heated to 150 C., it is quite viscous. A resin of this type, specifically designated in the following examples, is available under the name Piccolastic D-125. It, and its blends with minor amounts of up to /3 of a blended resin (wherein the predominating amount, or at least is the styrene-type resin thus defined) are hard and tough, highly resistant-to ball milling, and melt below the char point of ordinary paper.

The general nature and scope of the invention having been set forth, the following specific embodiments are presented in illustration but not in limitation thereof, and it is to be understood that the invention is to be limited only by the appended claims.

Example 11 A mixture was prepared comprising 10% by weight of carbon black, 25% by weight polybutyl methacrylate, and by weight of a blend of polymerized styrenes. The blend of polymerized styrenes is available under the name Piccolastic Resin D and is believed to be a mixture of medium-high molecular weight polystyrenes having a ball-and-ring melting point of 257 F. at room temperature. It is a hard, horny, dry solid, and, when heated, can be blended with polybutyl methacrylate to yield a homogeneous mixture.

After melting and preliminary mixing, the composition was fed to rubber mill and thoroughly milled to yield a uniformly dispersed composition of carbon black in the resin body. The resulting mixed composition was thoroughly cooled and then finely subdivided in jet pulverizer to yield a powder composition having an average particle size of about 5 microns. Particle size was moderately uniform with substantially no particles larger than 10 microns and substantially none less than one micron. The resulting powder or toner composition-is particularly adapted for use in xerography in combination with vitreous selenium xerographic plates charged to positive polarity for sensitization and exposed to an optical image to yield a positive polarity electrostatic latent image on the selenium surfaced xerographic plate.

To illustrate the suitability and effectiveness of the material, the powder was deposited on an electrostatic latent'image' on an image surface by'mixing about 1% of the powder in a two-component developer, as described in patent Number 2,618,551 and cascading the mixture across an electrostatic image-bearing surface. The imagewas developed by deposition of the powderon the electrostatic image and the powder was transferred by electrostatic means to a transfer web, such as paper, wliereon it was fused by placing in a heated oven at a temperature of 230 F. for a period of 5 seconds. A residual image was cleaned off the image-bearing surface by the method of Patent Number 2,484,7 82, or by other suitable methods.

The melting point of the resin powder was determined under the conditions of use in xerography, namely by heating the powder composition until it is sufiiciently soft to fuse into a single mass and adhere permanently to a web, such as a sheet of paper. Its melting point in this manner was 230 F. Under these same heating conditions, the paper web is not visibly affected by heat. The residual toner image remaining on the xerographic plate after electrostatic transfer is readily removed by desired methods, such as cascading thereacross a cleaning composition, as disclosed in Patent 2,484,782, or by brushing the surface with a rapidly rotating fur brush, or by like methods. When employed in the xerographic cycle with either of these cleaning operations, the plate was readily cleaned free from detectable residual toner particles and could be recycled for at least 1,000 xerographic cycles without building up significant quantites of film on the surface of the xerographic plate.

When mixed with a carrier composition surfaced with a suitable resin, such as is disclosed in Patent Number 2,618,551, the toner prepared according to this example gave clear, sharp and extremely black images of satisfactory contrast resolution and appearance. The toner particles when mixed with this carrier composition acquired a negative polarity electrostatic charge whereby they were deposited on a positive polarity electrostatic latent image and removed by the carrier from uncharged surface areas.

Example III The procedure of Example I was repeated, employing a mixture of 90% of the same blend of polymerized styrenes and carbon black. In this case, the final composition was finely subdivided to yield a powder composition having an average particle size of about 10 microns with substantially none greater than 20 microns. The product was closely comparable in behavior and properties with the product of Example I, characterized, however, with a slightly lower melting point, namely about 210 F., whereby it melted at a temperature having a greater margin of safety below the char point of paper.

Example IV The procedure of Example I was repeated employing as the mixture 10% carbon black, 5% polybutyl methacrylate and 85% of the blended polystyrenes. The product had a melting point of about 230 F., and, like the product of Example I, could be used through at least 1,000 xerographic cycles without building up a coating of film of the Xerographic plate.

In view of the preceding disclosure and description, it will be apparent that new and useful toner compositions have been invented. These compositions are advantageous and desirable, as compared with previous toner compositions, in a number of respects. For example, it has been found that greater amounts of the present toners are transferred during the ordinary transfer process, thus eifecting a real advantage in reproducing inferior original copy. Also, the present toners are blacker than previous toners (because of their increased carbon black content). In addition, the present toners are nontacky materials, which can be fused quickly in Freon (fluorinated hydrocarbon derivatives of the short chain and small ring aliphatic series of organic compounds) vapors, a characteristic not possessed by previous toners. Finally, it has been found that the present toner composition is less apt to break down or smudge than previous compositions. Other advantages and characteristics will be apparent from preceding description and examples which are intended to be illustrative only and not to limit the invention set forth in the following claims.

What is claimed is:

1. As a new composition of matter, a xerographic toner consisting essentially of finely divided pigmented particles having about 5% to about 10% pigment in a resin blend of about 5 to 25 polymerized butyl methacrylate blended with a polymerized blend of monomers of styrene and styrene homologues of the formula where R is selected from the group consisting of hydrogen and lower alkyl, the polymerized blend of styrene monomers having a ring and ball melting point of about C., said xerographic toner consisting of particles having a size between about 1 and about 20 microns, the finely divided pigmented mixture being hard and tough and being thereby adapted to be easily cleaned, by brushing, from an insulating image surface.

2. A process of xerography wherein :an electrostatic image is made visible, comprising contacting an image surface bearing an electrostatic image thereon with a finely divided pigmented resin consisting essentially of between about 5 and 10% pigment in a resin body of about 5 to 25% polymerized butyl methacrylate and the remainder a polymerized blend of monomers of styrene and styrene homologues of the formula wherein R is selected from the group consisting of hydrogen and lower alkyl, the polymerized blend of styrene monomers having a ring and ball melting point of about 125 C.

3. A process of xerography wherein an electrostatic image is made visible comprising contacting an image surface bearing an electrostatic image thereon with a finely divided pigmented resin consisting essentially of between about 5 and 10% pigment in a resin body of about 5 to 25 polymerized n-butyl methacrylate and the remainder a polymerized blend of monomers of styrene and styrene homologues of the formula wherein R is selected from the group consisting of hyrogen and lower alkyl, the polymerized blend of styrene monomers having a ring and ball melting point of about 125 C.

References Cited in the file of this patent UNITED STATES PATENTS 1,683,402 Ostromislensky Sept. 4, 1928 2,101,061 Gordon Dec. 7, 1937 2,297,691 Carlson Oct. 6, 1942 2,463,550 Myerson et a1. Mar. 8, 1949 2,489,226 Morris et al. Nov. 22, 1949 2,618,552 Wise Nov. 18, 1952 2,638,416 Walkup et a1 May 12, 1953

Claims (1)

1. 2. A PROCESS OF XEROGRAPHY WHEREIN AN ELECTROSTATIC IMAGE IS MADE VISIBLE, COMPRISING CONTACTING AN IMAGE SURFACE BEARING AN ELECTROSTATIC IMAGE THEREON WITH A FINELY DIVIDED PIGMENTED RESIN CONSISTING ESSENTIALLY OF BETWEEN ABOUT 5 AND 10% PIGMENT IN A RESIN BODY OF ABOUT 5 TO 25% POLYMERIZED BUTYL METHACRYLATE AND THE REMAINDER A POLYMERIZED BLEND OF MONOMERS OF STYRENE AND STYRENE HOMOLOGUES OF THE FORMULA