Electrostatic developer composition and method therefor

United States Patent Office . _t^{3 079}>³⁴²

Patented Feb. 25, 1963

3,079,342 ELECTROSTATIC DEVELOPER COMPOSITION AND METHOD THEREFOR

Michael A. Isssalaco, Webster, N.Y., assignor to Xerox Corporation, a corporation of New York

No Drawing. Fifed Feb. 12, 1960, Ser. No. 8,222 8 Claims. (CI. 2S2—62.1)

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, placingwm 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 image by exposure to an electroscopic material.

According to one practice in xerography as disclosed in U.S. 2,618,552 to E. N. Wise, the development of the electrostatic 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 image in the form of an image of 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 by 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 original condition 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, second, by the fact that a film or layer builds up on the plate surface during repeated cycles and eventually requires additional cleaning operations, such as, for example, solvent cleaning or the like.

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, and that the presently preferred fixing method for 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 melting points generally 10 tend to increase tackiness. A further and occasionally nagging difficulty is the fact that the toner particles must be charged to correct polarity upon mixing with and coating on the surface of the carrier particles so that the toner will be deposited on the image areas by electro15 static attraction and removed from the non-image areas also by electrostatic attraction. At the present time, xerographic photosensitive members are generally charged to positive polarity for sensitization and thus the toner particles must be such that they are charged to negative 20 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.

. Unfortunately the formulation of xerographic toners is still an art and not a science.. While it is, of course, essential that a resin possesses the proper melting point and physical properties for use with a xerographic toner, the factors which determine the presence of suitable ³⁰ xerographic properties are neither understood nor predictable. These electrostatic properties of a toner are manifest by sharpness of deposition in the image areas, the presence of undue background deposition and the development latitude.

³⁵ A substantial advance was made in the art of formulating xerographic toners with the discovery by my coworkers of the superior properties of toners containing principally polystyrene as the resin component. Poly₄θ styrene based toners have been found to have excellent xerographic properties in these respects. It is now common procedure in xerography to modify the basic polystyrene resin by the inclusion of polymeric modifiers so as to improve the physical properties thereof without ₄₅ loss of the excellent electrostatic properties of the basic polystyrene resin. This has been done by mechanically mixing the modifying polymer into the pigmented polystyrene resin and by chemically mixing as by copolymerizing the styrene monomers and the monomer of the de_5ij sired resin modifier. Both approaches have been successful in achieving improved xerographic toners. However, in each case the resulting toner is necessarily a compromise between the desired physical properties and the desired electrostatic properties.

Now, in accordance with the present invention, there is provided a new and improved xerographic toner composition comprising divided pigmented resin particles having a particle size less than about 30 microns and preferably having an average particle size between about 5 and 15 ₆₀ microns and consisting of the finely-divided, uniform mixture of pigment and/or dye in a non-tacky low melting resin consisting substantially of a copolymerized mixture of styrene or a blend of styrene homologs with from 10 to 40% of one or more methacrylate esters selected from ₆₅ the group consisting of ethyl, propyl and butyl methacrylates. The copolymer is persent in the composition in a predominating amount which is defined herein as being at least about 80% of the resin portion of the toner and being blended with from about 1% to about 20% of ₇Q modifying polymeric material. The pigment or dye is present in the toner in a sufficient quantity to cause it to be highly colored whereby it will form a clearly visible

8,079,342 ³ image on a transfer member. Thus, for example, in the usual case where a xerographic copy of the document or •the like is desired, the pigment will be a black pigment such as carbon black or other minutely divided carbonaceous 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 3% and about 20%. In the case of the use of a dye as in formulating colored toners, substantially smaller amounts of dye may be used.

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 dispersions of the pigment in the resin.

In preparing the copolymer with butyl methacrylate either pure styrene or a mixture of styrene and styrene homologs of the general formula

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tained from Pennsylvania Industrial Chemical Corporation). To compare the suitability and effectiveness of these compositions, the toners prepared therefrom, were deposited on an electrostatic latent image on an image 5 surface by mixing about 1% of the powder in a two-component developer as described in U.S. Patent No. 2,618,551 and cascading the mixture across an electrostatic image-bearing surface. The image was developed by deposition of the powder on the electrostatic image and IQ the powder was transferred by electrostatic means to a transfer web such as paper whereon it was fused by placing in a heated oven. The residual powder image was cleaned off the image-bearing surface by the method of U.S. Patent No. 2,484,782. The toner was utilized in 15 this means in an automatic xerographic machine. As a result of extensive use in these machines, it was found that the toner No. 3 had excellent electrostatic properties but had a tendency toward cold flow causing pelletization in the machine. The toner No. 2 showed substantially ₂₀ reduced pelletization as compared to toner No. 3, but more dusting, more background deposition and less development latitude than for toner No. 3. Toner No. 1 was substantially better than toner No. 3 for cold flow and pelletization, even exceeding toner No. 2 in this respect. In addition, the electrostatic properties of the toner were substantially better than those of toner No. 2, toner No. 1 giving sharp images, good development latitude and minimal background deposition.

Examples 2-9 where R is selected from the group consisting of hydrogen and lower alkyl may be used. Such mixtures as these are obtained from fractionation of the so-called “Crude Solvent” from “Light Oils” scrubbed out of coke oven or gas house gas. The styrene or mixture of styrene and styrene homologs should comprise at least about 60% to about 90% of the copolymer composition. The other component of the copolymer may be either n-butyl methacrylate, iso-butyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate or a mixture thereof. A particularly preferred type of resin within this class is a hard, tough resin prepared by copolymerizing 35% butyl methacrylate and 65% styrene (all parts by weight) and having a melting range of about 86-104° C. as determined by Fisher-Johns apparatus and being medium high in polymer structure.

The other resin component mixed in minor amount with the copolymer is a long chain thermoplastic plasticizer which has little tendency toward agglomeration or cold flow such as polyvinyl butyral, polyethylene, shellac, waxes, polyesters, polyvinyl acetal, polyvinyl (choride acetate) etc. The resin modifier comprises from about 20% to about 1% based on the total weight of the composition. Preferably it comprises from about 2% to about 10% based on the total weight of the composition. 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. All parts are by weight unless otherwise noted.

Example 1

A series of three toners were formulated. The first consisted of 18 parts of a copolymer prepared by the bulk polymerization with a benzoyl peroxide catalyst of 35 parts of n-butyl methacrylate and 65 parts of styrene, 2 parts of carbon black pigment obtained from the Columbian Carbon Co. under the trade name Neo Spectra Mark II Powder and 2 parts of a polyvinyl butyral resin obtained from the Bakelite Company under the trade name Vinylite XYHL. The second consisted of 18 parts of the same copolymer and 2 parts of Neo Spectra Mark II. The third toner consisted of 10% of Neo Spectra Mark II, 25% of polybutyl methacrylate and 65% of Piccolastic D125 (a blend of polymerized styrene homologs obA series of eight toners were formulated as in Exampie 1. In each case the toner contained 18 parts of a; copolymer of 35% n-butyl methacrylate and 65% styrene as in Example 1. To this were added 2 parts of ³⁵ carbon black and Ά part of polyethylene sebacate .in Example 2; 1 part of carbon black and 1 part of shellac in Example 3; 2 parts of carbon black and VI part of polyethylene in Example 4; 2 parts of polyvinyl butyral and 2 parts of quinoline yellow base (a dye obtained ⁴⁰ from National Aniline) in Example 5; 2 parts of Oil Red O dye (also from National Aniline) and 2 parts of polyvinyl butyral in Example 6; 2 parts of Luxol fast blue dye and 2 parts of polyvinyl butyral in Example 7; 2 parts of chrome green oxide and 2 parts of j-5 polyvinyl butyral in Example 8; and 1 part of titanium dioxide pigment, Vs part of carbon black and 2 parts of polyvinyl butyral in Example 9. The toners of Examples 2 through 9 were all characterized by excellent electrostatic properties, reduced tendency to cause pel50 letization and little or no dusting when used in automatic xerographic machines.

The novel toner compositions of the instant invention have exceptional utility in developing electrostatic images. In the normal xerographic process such images 55 are created and developed on the surface of a photoconductive insulating member which is generally amorphous selenium. Other photoconductive insulating materials include photoconductive pigments as zinc oxide, zinccadmium sulfide, tetragonal lead monoxide, titanium di00 oxide, etc., in an insulating resin binder. Such materials may also be used as the photoconductive insulating layer.

In addition, rather than developing the electrostatic image on the photoconductive insulating layer, if de65 sired, the electrostatic image pattern may be transferred to an electrically insulating film as polyethylene terephthalate. This process is described more fully in U.S. Patent 2,825,814 to L. E. Walkup. The toner composition of the instant invention may then be used to develop 70 the electrostatic image on this insulating film to form a powder image corresponding to the electrostatic image thereon. The resulting powder image may then be either permanently affixed to the insulating film or transferred to a support member as paper, metal, plastic, etc., 75 and the insulating film cleaned and reused in the process..

3,079,342

As used in developing electrostatic images the toner composition is loosely coated on a carrier surface to which it remains loosely affixed by reason of electrostatic attraction thereto. The most widely used method of carrier development is known as cascade carrier de- 5 velopment as more fully described in U.S. 2,618,551, to L. E. Walkup; U.S. 2,618,552, to E. N. Wise; and U.S. 2,638,416, to Walkup and Wise. In this process the electroscopic toner is desirably mixed with a granular carrier, either electrically conducting or insulating, 10 magnetic or non-magnetic, provided that the particles of granular material when brought in close contact with the powder particles acquire a charge having an opposite polarity to that of the powder particles adhering to and surrounding the granular carrier particles. If a 15 positive reproduction of the electrostatic image is desired, the carrier is selected so that the toner particles acquire a charge having the opposite polarity to that of the electrostatic image. Alternatively, if a reversal reproduction of the electrostatic image is desired the 20 carrier is selected so that the toner particles acquire a charge having the same polarity as that of the electrostatic image. Thus, the materials for the granular material are selected in accordance with their triboelectric properties in respect to the electroscopic toner so that 25 when mixed or brought into contact one material is charged positively if the other is below it in a triboelectric series, and negatively if the other material is above it in a triboelectric series. By selecting materials in accordance with their triboelectric effects, the polarities of 30 their charge when mixed are such that the electroscopic toner particles adhere to and are coated on the granular carrier particles and also adhere to the electrostatic image on the plate which thus retains the electroscopic toner in the charge areas that have a greater attraction ³⁵for the toner than the granular carrier particles have.

The granular carrier particles are grossly larger than the toner particles by at least one order of magnitude of size, and are shaped to roll across the image-bearing surface. Generally speaking, the carrier particles should ⁴be of sufficient size so that their gravitational or momentum force is greater than the force of attraction of the toner in the charged areas where the toner is retained on the plate in order that the granular carrier particles will not be retained by the toner particles, while, at the ⁴⁵same time, the toner particles are attracted and held, or repelled, as the case may be, by the charged or uncharged areas of the plate since they acquire a charge of opposite polarity to the charge of both the granular carrier particles and the plate. It has been found best ⁵⁰to use granular carrier particles of a size larger than about 200 mesh, usually between about 20 and about 100 mesh, and toner particles of a size from about 1 to 30 microns. The granular carrier particles may, if de- -sired, be somewhat larger or smaller as long as the proper size relationship to the electroscopic toner is maintained so that the granular carrier particles will flow easily over the image surface by gravity when the plate is inclined without requiring additional means or meas- „„ ures to remove them.

The degree of contrast or other photographic qualities in the finished image may be varied by changing the ratio of granular carrier to electroscopic material. Successful results have been had with from about 10 to „about 200 parts by weight of granular carrier particles ° capable of being passed through a 20 mesh screen and being collected on a 60 mesh screen to 1 part of the electroscopic toner having a particle size of 1 to 30 microns. Generally speaking, carrier-to-toner ratios in the order of about 100 to 1 prove satisfactory and preferred compositions run from about 70 to 1 to about 150 to 1. In such preferred compositions the carrier acts effectively to remove any toner particles which might tend to adhere to a non-image area and the 75 ⁶ toner itself forms a dense readily transferable and fusible image.

In addition to the use of granular particles to provide the carrier surface, the bristles of a fur brush may also be used. Here also, the toner particles acquire an electrostatic charge of polarity determined by the relative position of the toner particles and the fur fibers in the triboelectric series. The toner particles form a coating on the bristles of the fur clinging thereto by reason, of the electrostatic attraction between the toner and the fur just as the toner clings to the surface of the granular carrier particles. The general process of fur brush development is described in greater detail in U.S. patent application Ser. No. 401,811, filed by L. E. Walkup on January 4, 1954.

Even more closely related to the cascade carrier development is magnetic brush development. In this process a granular carrier is selected having ferromagnetic properties and selected relative to the toner in a triboelectric series so as to impart the desired electrostatic polarity to the toner and carrier as in cascade carrier development. On inserting a magnet into such a mixture of toner and magnetic granular material the carrier particles align themselves along the lines of force of the magnet to assume a brush-like array. The toner particles are electrostatically coated on the surface of the granular powder carrier particles. Development proceeds as in regular cascade carrier development on moving the magnet over the surface bearing the electrostatic image so that the “bristles” of the magnetic brush contact the electrostatic image-bearing surface.

Still another method of carrier development is known as sheet carrier development in which the toner particles are placed on a sheet or pellicle as of paper, plastic, or metal. This process is described in U.S. 2,895,847 to C. R. Mayo. As described therein the electrostatic attraction between the sheet surface and toner particles necessary to assure electrostatic attraction therebetween may be obtained by leading the sheet through a mass of electroscopic toner particles whereby there is obtained a rubbing or sliding contact between the sheet and the toner. In general it is desirable to spray the surface of the sheet bearing the electroscopic toner particles with ions of the desired polarity as by the use of a corona charging device as described in the patent of Mayo.

In any event, as used in developing an electrostatic image the. toner composition of the instant invention is electrostatically coated on a suitable carrier surface which is then in turn contacted with the surface bearing the electrostatic image whereby the electroscopic toner particles are transferred to the surface bearing the electrostatic image to form thereon a powder image corresponding to the electrostatic image.

The novel toners of the instant invention may also be formulated to possess both electrostatic and magnetic properties by utilizing a magnetic powdered pigment as the pigment therein. Thus, magnetic iron oxide or similar materials may be used as the pigment in such toners. In this instance substantially larger amounts are used in formulating the toner than when conventional coloring pigments or dyes are used. Thus, a toner having both electroscopic and magnetic properties desirably contains up to 50%, by weight, of pigment.