US3391082A - Method of making xergographic toner compositions by emulsion polymerization - Google Patents

Description

United States Patent METHOD OF MAKHNG XEROGRAPHXC TQNER CQMPUMTEONS BY EMULSKQN POLYMEREZATEGN William N. Maclay, Monroeville, Pa., assiguor to Koppers Company, Inc, a corporation of Delaware No Drawing. Filed Apr. 6, 1965, Ser. No. 446,104 9 Claims. (Cl. 252-621) ABSTRACT OF THE DHSCLOSURE A xerographic toner powder, consisting essentially of from about 5-10% by weight of pigment, such as carbon black, and about 9095 by Weight resin, such as styrenen-butylacrylate copolymer, having a second order or glass transition temperature (Tg) of -65 C. and a limiting viscosity [7 of 0.15-0.35, is provided. The resin is prepared as a latex in the presence of an organic chain transfer agent to control the limiting viscosity, and

the monomer mixture from which the resin is made is pre selected to provide the required Tg value. The pigment is uniformly distributed within the latex and the pigment containing latex is dried to form substantially spherical powder particles having an average particle size of less than 10 microns. The toner powder. when used as a component of a xerographic developer, provides a developer capable of producing xerographich prints of excellent contrast resolution and improved background.

This invention relates to an improved xerographic developer. In one specific aspect, it relates to a novel xerographic toner powder used as an essential ingredient in a xerographic developer.

In the art of xerography a master is reproduced by placing an electrostatic charge on a photoconductive surface, selectively dissipating such charge by exposure to an optial image corresponding to the master and developing the resulting electrostatic image by exposure to an electroscopic material. In a preferred method, taught by E. N. Wise in U.S. 2,618,552, 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 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. Toner particles accidentally deposited on background portions are physically removed by electrostatic action of the cascading carrier particles. A copy of the electrostatic image is formed by the toner particles electrostatically clinging to the image surface and is removable therefrom by adhesive or electrostatic transfer. The image body may be transferred to a paper sheet in contact with the image body by applying an electrostatic charge to the paper during such contact. When the paper is subsequently stripped from the image-bearing surface, it carries with it a substantial portion of the image body to yield a xerographic print which thereafter may be made permanent by heating or solvent fixing.

After the transfer of the image from the image sur face to the paper, the xerographic plate is cleaned for use in a subsequent xerographic cycle. If the plate is thoroughly cleaned, it is substantially unimpaired for future use. However, a serious problem has been encountered in cleaning the plate between cycles, because of the strong attraction of the residual toner particles to the plate. This is evidenced by the stubborn adherence of toner particles "ice as such to the plate and by a build-up of a film or layer on the plate surface during repeated cycles, eventually requiring additional cleaning operations, such as, for example, solvent cleaning.

Residual toner, either in powder form or as a film on the image surface, impairs the subsequent operation of the xerographic plate. If toner particles remain on the plate, they interfere with the subsequent steps of xerography, causing either deletions or background deposition. The formation of toner film interferes in many ways. The film has different electrical properties from those of the photoconductive layer on the xerographic plate and thus interferes with the charging or the sensitizing step. The film also has mechanical or physical properties differing from those of the photoconductive layer, particularly in that the film is substantially more sticky or tacky than the clean plate surface. Also, toner films are hygroscopic to the extent that in humid weather they detrimentally afiect conductivity under exposure to light and insulating properties in the absence of light.

Incomplete removal of toner particles from the residual image is to a measurable extent a function of physical or mechanical properties of the toner particles. The particles tend to be somewhat tacky under conditions r of use and apparently adhere to the image surface by mechanical means as Well as by electrostatic forces. The problem of removal is complicated by the electrostatic forces used in transferring the image body from the image surface to the paper sheet, since, because of these forces, the mechanical adhesion between toner and image surface cannot be overcome simply by increasing the mechanical transfer force. Another complication is introduced by the preferred fixing method for the xerographic print, which 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 paper. The usual methods of lowering the melting point of the toner particles generally tend to increase tackiness. A further difficulty associated with the physical properties of particles is that the 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 electrostatic 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 polarity by mixing with the carrier particles.

A well-known xerographic toner is described by Chester F. Carlson in U.S. Patent Reissue 25,126. This toner consists essentially of from 5-10 percent pigment and from -95 percent of a non-tacky, low-melting resin containing at least about two-thirds polymerized styrene or styrene homologues mixed or blended with up to about 25 percent of modifying polymeric material to adjust the physical and mechanical properties of the toner particles. According to Carlson, the modifying material may be combined with the styrene portion of the resin either by mechanical mixing of the polymers or by chemical mixing through copolymerization. Mechanical mixing is accomplished by melt blending. After melt blending and preliminary mixing with the pigment, or after preliminary mixing with the pigment if a copolymer is used, the toner composition is fed into a rubber mill and thoroughly milled to yield a uniformly dispersed composition of pigment in the resin body. The uniformly mixed composition is cooled and finely sub-divided in a jet pulverizer to provide a powder having an average particle size of about five microns. The jet milling step is necessary in making the Carlson toner to provide the particle size required for use in xerography, regardless of whether the resin used is a copolymer or a melt blend of homopolymers.

Unfortunately, .jet milling provides granular toner particles that are asymmetric in shape. The asymmetry of these particles causes them to nest and bridge on the photocond-uctive layer, thus contributing measurably to the difficulty of removing residual particles from the xerographic plate. The packing of the irregularly shaped particles on the Xerographic surface appears to contribute to film formation.

I have discovered that substantially spherical toner particles can be provided by preparing the resin component of the toner as a latex, provided that the properties of the resin are carefully controlled by regulating the second order or glass transition temperature (Tg) and the intrinsic viscosity [1;],,. Latex particles ordinarily have a size range of 0.03-0.25 micron. When the latex is uniformly combined with a dispersion of a suitable pigment, such as colloidal carbon black, and dried in a convenient manner, such as, for example, by spray drying, there is thus obtained substantially spherical toner powder particles having an average particle size of 1-5 microns, there-by avoiding the costly and undesirable jet milling step.

The substantially spherical form of my novel toner particles provides a minimum surface-to-volume ratio. Thus, the particles are attracted to the photoconductive surface in those areas in which the density of the electrostatic charge is greatest. Symmetric-ally charged spheres are considerably superior to randomly charged irregular particles in providing clear, sharp images of satisfactory contrast resolution and appearance. The reduced tendency of spheres to adhere to uncharged areas of the photoconductive surface results in a markedly improved background.

It is therefore an object of the invention toprovide an improved xerographic toner composed of substantially spherical particles to be used as a component of a xerographic developer. It is a further object of the invention to provide a novel developer capable of producing xerographic prints of excellent contrast resolution and improved background.

In accordance with the invention there is provided a xerographic toner powder comprising substantially spherica-l particles having an average particle size of less than ten microns. The powder consists essentially of from about 5-10 weight percent pigment and about 90-95 percent resin having a second order or glass transition temperature (Tg) of 30-65 C. and a limiting viscosity [1 of 0.15-0.35. The resin is prepared as a latex in the presence of an organic chain-transfer agent having a chain transfer constant (C value of at least 0.1 by the catalytic polymerization in aqueous emulsion of a monomer mixture of (1) a hard monomer component, which can be styrene, methyl methacrylate, ethyl methacrylate, acrylo nitrile, vinyl acetate, vinyl chloride, or mixtures thereof; and (2) a soft monomer component, which can be alkyl acrylate wherein the alkyl group has up to 12 carbon atoms, an alkyl methacrylate wherein the alkyl group has from 2-12 carbon atoms, vinyl acetate, vinylidine chloride, or mixtures thereof. The relative proportion of the hard monomer component (=1) to the soft monomer component (2) in the monomer mixture is selected to provide a polymer having a Tg value of 30-65 C. During the polymerization, the chain-transfer agent is present in an amount suflicient to provide a polymer having an [17] value of 0.15-0.35. The pigment is uniformly distributed within the latex by either forming the latex in the presence of the pigment or by mechanical dispersion after polymerization is complete. The pigment-containing latex is dried in a convenient manner to form substantially spherical powder particles having an average particle size of less than ten microns.

The developer of the invention comprises the abovedescribed toner particles uniformly electrostatically coated on a carrier surface capable of retaining the toner powder particles by electrostatic attraction. The carrier surf-ace is adapted to make firm contact with a surface bearing an electrostatic image and the toner particles are removably coated on the carrier surface.

The selection and preparation of the resin component of my novel toner is of vital importance. In order to avoid the conventional jet milling step that results in irregular granular particles, the resin must be prepared in the form of a latex, and in order to make the resin useful for xerographic purposes, the physical properties thereof must be carefully controlled Within certain essential limits. In

conventional toner resins, regardless of Whether the principal polymeric ingredient is a copolymer or a homopolyme-r, the final adjustment of the physical properties required to make the resin acceptable for xerographic purposes is ordinarily accomplished by melt blending with a certain percentage of modifying polymeric material, such as polyvinyl butyral, polyethylene, or a polyester. Modification of a copolymer resin in this manner is described in US. Patent 3,079,342 of Michael A. Insalaco. The necessity of including a melt blending step would, of course, defeat the purpose of preparing the resin in latex form, since after rnelt blending, a milling operation would be required to achieve an average particle size of less than 10-20 microns.

In addition to the obvious requirement that the resin selected must be receptive to an electrostatic charge, the melt flow properties of the resin are most important from a xerographic standpoint. In xerography it is common practice to fix the image body transferred to the paper by heating the paper to a temperature below its char point for a few seconds. Conveniently, this is done by placing the paper in close proximity to a hot wire, thus exposing the image deposited on the paper to a temperature somewhat less than 350 F., preferably 250300 E, for a few seconds. At these temperatures, the resin must be capable of flow to the extent that it will fuse into a single mass; in other Words, it must be capable of reaching the point of incipient flow.

I have discovered that the fusion point, or point of incipient flow, of the resin is best controlled by choosing materials have a glass transition temperature (Tg) within certain critical limits, i.e., 30-65 C. The glass temperature of a resin is more precisely termed the second order transition temperature, because at this temperature a phase change takes place. The polymer loses its hardness or brittleness and becomes more flexible and soft. At the glass temperature, noticeable changes are apparent in the specific volume, thermal conductivity, refractive index, heat content, and dielectric loss. Below the glass temperature, motion in the polymer chain is restricted to small movement of individual atoms. Above the glass temperature, greater molecular motion occurs in which entire segments of the polymer chain are in motion and the polymer loses its rigidity. Glass transition temperatures can be conveniently determined by differential thermal analysis =(DTA), as described by J. J. Keavney and E. C. Eberlin, J., Appl. Sci. 3, 47 (1960).

Resins having a Tg value Within the above-indicated range are not per se useful as a component of a Xerographic toner. Unmodified resins which fuse or flow at the proper temperature for use in xerography have an incipient melting point which is low enough to make them tacky under ordinary conditions of use. Surprisingly, I have found that the temperature differential between incipient melting and incipient flow can be reduced by lowering the average molecular weight of the resin by the presence during polymerization of a chain-transfer agent. The molecular weight required to provide a non-tacky resin 100,000 and is best ranges from about 10,000 to about determined in terms of the limiting intrinsic viscosity [7 which can be calculated from standard physical meas urements made in a suitable solvent, such as toluene, tetrahydrofuran, or cyclohexanone, by well-known methods described by F. A. Bovey et al., Emulsion Polymerizaticn, Intersci., N.Y. (1955) p. 304 if, and G. A. Cline, Analytical Chemistry of Polymers, Intersci., NY. (1952) p. 12 ii". Resins suitable for the toner composition of the invention must have an [1 1 value of 0.15-0.35, as determined from physical measurements in toluene at C. or physical measurements in another solvent (if the polymer is insoluble in toluene) and correlated to the values obtained in toluene.

As I have noted above, the resin component of the toner of the invention is prepared as a latex in the presence of an organic chain-transfer agent having a chain transfer constant (Cs) of at least 0.1 by catalytic polymerization in aqueous emulsion of a preselected monomer. In most instances the starting monomer is a monomer mixture of one or more hard monomer components and one or more soft monomer components. Vinyl acetate and ethyl methacrylate have a Tg value that makes them suitable as either the hard or the soft component; thus, these monomers can be used as such to provide a homopolymer in latex form. Suitable hard monomers include styrene, methyl methacrylate, ethyl methacrylate, acrylonitrile, vinyl acetate, vinyl chloride, and mixtures thereof. The soft monomer can be an alkyl acrylate wherein the alkyl group has up to 12 carbon atoms, such as methyl acrylate, butyl acrylate, Z-ethylhexylacrylate, and the like; an alkyl methacrylate wherein the alkyl group has from 2-12 carbon atoms, such as ethyl meth-acrylate, butyl methacrylate, and lauryl methacrylate; vinyl acetate; vinylidine chl0 ride; and mixtures thereof.

The relative proportion of hard monomer to soft monomer in the monomer mixture is selected to provide a polymer having a Tg value of 30-65 0, preferably from -50 C. The Tg value can be easily predetermined without experimental measurements with the use of the Rohm & Haas Company Special Products Department Glass Temperature Analyzer, available from the Rohm & Haas Company as SP-2228/ 63. Useful monomer mixtures are given below in Table I, which shows for a number of compositions the weight parts hard monomer, weight parts monomer, and the resulting glass transition temperature. The two columns of the table indicate the approximate upper and lower limits for given combinations of hard monomer and soft monomer.

TABLE I 'lg: 30 C. (80 F.) T t=- C. (140 F.)

30 Styrene/ Methylaerylate (32 65(ftyrene/35 Methylacrylate (63 52 Styrene/50 Ethylacrylate (30 0.). 78 Styrene/22 Ethylacrylate (64 C.) 64 Styrene/36 Butylacrylate (31 0.). 83 CStyrene/U Bntylacrylate (63 (v C. 20 Styrgne/EO Butylmethacrylate p 3- i 71 Styrene/29 Laurylmethscrylate 86 CStyrene/14 Laurylrnethacrylate 20 Acrylonitrile/ Butylmethacry- 61 Acrylonitrile/39 Butylmethacrylate (33 0. late (ir 30 Methylmethacrylatelm Methyl- 65 Mcthylmethacrylate/35 Methyl- (33 0.). acrylste (6 C.

5O Methylmethacrylate/SO Ethyl- 74 1\lethylrnethaerylate/26 Ethylacrylate (30 0.). aerylate (03 0.).

63 Methylmethacrylate/W Butyl- S2 Methylmethacrylate/lS Butylacrylate (31 0.). aerylate (65 0.).

67 Methylmethaervlate/33 2-Ethyl- 84 Methylmethacrylnte/lfi 2 Ethylhexylacrylste (31 (3.). hexylacrylate (65 (1.).

16 Methylmethacrylate/M Butyl- 6O lvlethylmethacrylatel40 Butylmethaervlate (30 0.). mcthacrylate (6 0.).

70 Methylmethacrylate/fst) Lauryl- 85 Methylmethacrylate/lfi Laurylmethacrylate (31 methacrylate (6 C.). v

100 Polyvinylacetate (30 0.). 49 Methylmethacrylate/St Vinylacetate (64 (3.).

Vinylcl1l0ride/38 Vinylidine 89 Vinylchloride/ll Vinylidine Chloride (30 0.). Chloride (65 0.).

35 Vinylehloride/GS Methylacrylate 80 Vinylchloridc/2O Methylacry- (31 0.). late (65 (3.).

The average molecular weight of the desired homopolymer or copolymer having the correct Tg value must be controlled by polymerizing in the presence of an organic chain-transfer agent. A growing polymer chain possesses an unpaired electron at its end and can be prevented from growing further by removal of an atom from some substance present in the reaction mixture to give a new radical, which may, in turn, start a new chain. Substances which provide the termination atom and the new propagating radical are known as chain-transfer agents.

The activity of a particular chain-transfer agent is measured in terms of the Chain Transfer Constant (Cs). The values of chain transfer constants in the polymerization of methyl methacrylate, vinyl acetate, acrylonitrile, and styrene are given in a series of articles by I. N. Sen et al. appearing in the Jour. Indian Chem. 800., 0.1. vol. 4-0 No. 9 (1963), p. 729. A chain-transfer agent having a transfer constant (Cs) of one indicates. that the material behaves kinetically like monomer. Since the polymers having a proper Tg value require considerable reduction of their molecular weight in order to make them useful as a component in a xerographic toner, it is necessary to use during polymerization an active organic chaintransfer agent having a Cs value of at least 0.1. Particularly effective are normal or tertiary alkyl mercaptans having from 4-16 carbon atoms, such as t-dodecylmercaptan. Other useful chain-transfer agents include the lower alkyl xanthogens, such as diisopropyl xanthogen; alpha-bromoethylbenzene; alpha-ch]oroethylbenzene; and carbon tetrabromide. Sutficient chaintransfer agent must be present during polymerization to provide a resin having a molecular weight in the range of 10,000-100,000, i.e., an [1,] value of 0.15-0.35. Although the amount used varies to some extent with the activity of the particular chain-transfer agent, the particular monomer being polymerized, and the rate of polymerization, ordinarily the chain-transfer agent is present in an amount ranging between about 0.5 and 3 parts by weight per 100 parts monomer.

Any conventional emulsion polymerization system can be used to prepare the latex, although it is desirable to choose polymerization conditions that will induce rapid polymerization and thus obviate the use of excess chaintransfer agent in controlling the molecular weight. One effective polymerization method involves forming aqueous emulsion in a suitable reactor using a conventional emulsifying agent, such as alkali metal alkyl sulfates; e.g., sodium laurylsulfate, either alone or in combination with non-ionic wetting agents, and adding thereto 0.1-3 percent by weight of a suitable catalyst, such as sodium, potassium, or ammonium persulfate. The pH is adjusted to the acid side with, for example, acetic acid, and the monomer-chain-transfer agent charge is then added. The mixture is heated to 50-80 C. and maintained at this temperature for one to three hours. The resulting latex is then neutralized to pH 9.0-9.5 with ammonium hydroxide and filtered in the conventional manner. Another method of making the latex involves preparing the emulsion as described above, adding thereto a monomer-chaintransfer agent charge, and initiating the polymerization at room temperature with 0.1-3 parts per 100 parts monomer of a Redox catalyst system, such as an organic hydroperoxide, e.g., t-butyl-hydroperoxide, together with an aldehyde sulfoxylate, e.g., sodium formaldehyde sulfoxylate. The reaction is markedly exothermic. A temperature of 90-100 C. is reached within 15 minutes and this temperature is held for approximately one hour. The latex thus prepared is cooled, neutralized, and filtered in the conventional manner. The above procedure may be modified by charging half of the reactants initially, allowing polymerization to take place, and then charging the remaining half of the reactants.

The latex thus prepared can be intimately wet-blended with a suitable pigment and thereafter dried in any conventional manner that would not cause excessive agglomeration of the particles. It is particularly convenient to spray dry the wet-blended pigment and latex at a temperature that is high enough to vaporize the aqueous medium and below the decomposition temperature of the polymer. The wet-blended material ordinarily contains about 30-50 percent solids. Practical drying temperatures for rapid removal of water are in the range of 30-350 0, preferably 100-200 C.

The individual latex particles are less than about one micron in size, ordinarily in the size range of 0.03-0.25 micron. When combined with the pigment and dried, the particles are agglomerated to an average size of about 1-5 microns and thus are ready for use as xerographic toner.

One particularly effective method of incorporating the pigment involves conducting the polymerization in the presence of pigment. In this case the polymer surrounds the pigment, resulting in an encapsulated resin-containing pigment averaging in size to about 0.l5-0.5 micron. After drying in a suitable manner, such as spray drying, the particle size will average less than ten microns, ordinarily 1-5 microns.

The pigment is present in the toner in an amount sufficient to cause it to be highly colored, whereby it will form a clearly visible image on a transfer sheet. In the usual case where xerographic copy of documents is required, the pigment will be a black pigment, such as carbon black, or other finely-divided carbonaceous pigment. Organic pigments can be used in situations where a colored copy is desired. Ordinarily the pigment is used in an amount of about 5 percent, based on the total weight of the toner body, and may be used in an amount up to about percent by weight.

My novel toner particles, because of their substantially spherical shape, are outstanding in developing electrostatic images. In the usual xerographic processes such images are created and developed on the surface of a photoconductive insulating layer, which is generally amorphous selenium. Other photoconductive insulating materials include photoconductive pigments, such as Zinc oxide, zinc-cadmium sulfide, tetragonal lead monoxide, and titanium dioxide, contained in an insulating resin binder. Such materials may also be used as the photoconductive insulating layer.

In developing electrostatic images the toner is loosely coated on a carrier surface to which it remains loosely affixed by electrostatic attraction. The most widely used method of carrier development is known as cascade carrier development, which is described in detail in US. 2,618,551 of L. E. Walkup, US. 2,618,552 of E. N. Wise, and US. 2,638,416 of Walkup and Wise. In this method the electroscopic toner is mixed with a granular carrier which can be electrically conducting or insulating and magnetic or non-magnetic. The particles of granular carrier, when brought in close contact with the toner powder particles, acquire a charge having an opposite polarity to that of the powder particles, which thus adhere to and surround the granular carrier particles. If a positive reproduction of the electrostatic image is desired, the carrier is chosen 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 carrier is chosen so that the toner particles acquire a charge having the same polarity as that of the electrostatic image. Thus, materials for the granular carrier are selected in accordance with their triboelectric properties in respect of the electroscopic toner, so that when mixed or brought into mutual 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 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 retains electroscopic toner in the charge areas that have a greater attraction for the toner than the graular carrier particles.

The granular carrier particles are larger than the toner particles by at least one order of magnitude of size, and are shaped to roll across the image-bearing surface. The carrier particles should be of sufiicient size to provide a gravitation 0r momentum force greater than the force of attraction of the toner in the charged areas where the toner is retained on the plate, so that the toner can be sepa rated from the carrier in these areas. It is best to use granular carrier particles of a size larger than about 200 mesh (U.S. sieves), usually between about 20 and about mesh, and toner particles of a size from about 1 to 20 microns. The granular carrier particles may, if desired, be somewhat larger or smaller, provided that the proper size relationship to the electroscopic toner particles is maintained to permit the granular carrier particles to flow easily over the image surface by gravity when the plate is inclined.

The degree of contrast in the finished image can be varied by changing the ratio of granular carrier to electroscopic toner. Successful results are obtained using from about 10 to about 200 parts by weight of granular carrier particles having a size range of 30-60 mesh =(U.S. sieves) to one part of the electroscopic toner having a particle size of one to twenty microns. Carrier-to-toner ratios in the order of about 70:1 to about 150:1 are preferable. Using these ratios, the carrier acts effectively to remove any toner particles that might tend to adhere to a non-image area and the toner itself forms a dense, readily transferable and fusible image.

My invention is further illustrated by the following examples:

Example I A resin component of xerographic toner is prepared front the following ingredients:

The sodium formaldehyde sulfoxylate component of the Redox initiator is used as a two percent aqueous solution. Separately, a premix is prepared consisting of the monomers, mercaptan, and t-butyl hydroperoxide.

A two-liter resin kettle equipped with stirrer, reflux condenser, thermometer, and nitrogen inlet is charged with 1059 g. distilled water and g. of a thirty percent aqueous solution of sodium laurylsulfate. The charge is agitated, purged with nitrogen, and the monomer-mercaptan-t-butyl hydroperoxide premix is added thereto, followed by the addition of 1.4 g. of acetic acid to adjust the pH. The resulting emulsion is heated to 30 C. and the heating mantle is removed. The reaction is initiated by the addition of 0.1 part sodium formaldehyde sulfoxylate solution. When a temperature of 40 C. is reached (2-4 minutes after initiation), the remaining 0.1 part sodium formaldehyde sulfoxylate is added over a tenminute period. The temperature reaches a maximum of 90-95 C. and the emulsion is held at this temperature for one hour. The polymerized latex is then cooled to room temperature with an ice-water bath and neutralized to a pH of 9.0-9.5 with 28 percent ammonium hydroxide. The latex is then filtered through 50 micron Sparkle M paper. The copolymer latex thus produced has a Tg value of -40 C. and a [1 of 0.21.

Eight parts of carbon black per hundred parts polymer is added as a colloidal dispersion to the filtered latex under mild agitation until homogeneity is obtained. The resulting Wet-blend of carbon black and latex is spray dried at a temperature of 110 C. to provide a xerographic toner powder of substantially spherically shaped particles having an average particle size of less than ten microns.

The toner powder is deposited on an electrostatic latent image on an image surface by mixing about one percent of the toner powder in a two-component developer, as described in US. 2,618,551, and cascading the mixture across an electrostatic irnage-bearing surface. The image is developed by deposition of the powder on the electrostatic image and the powder is transferred by electrostatic means to paper, whereon it fuses by placing it in a heated oven at a temperature of 250 F. for a period of five seconds. Residual powder is cleaned ofi the image-bearing surface by conventional means.

Example II A resin component of xerographic toner is prepared from the following ingredients:

Potassium persulfate is used as a two percent solution. Separately, a premix of monomers and mercaptan is prepared.

A two-liter resin kettle equipped with stirrer, reflux condenser, thermometer, and nitrogen inlet is charged with 1024 g. of distilled water, 140 g. of a thirty percent aqueous solution of sodium laurylsulfate, 1.4 g. of acetic acid, and 105 g. of a two percent solution of potassium persulfate. The charge is agitated and the kettle is charged with nitrogen. The monomer-mercaptan premix is then added and the temperature of the emulsion is raised to 60 C. over a half-hour period. The 60 C. temperature is maintained for two hours, then the temperature is raised to 70 C. and held for an additional one-hour period. The polymerized latex is cooled to room temperature, neutralized to a pH of 9.0-9.5 with 28 percent ammonium hydroxide and thereafter filtered through 50 micron Sparkle M paper.

The filtered latex is homogeneously wet-blended with carbon black as described in Example I and the blend is spray dried to provide a xerographic toner powder of substantially spherically shaped particles having an aver-age particle size of 1-5 microns. When the toner is mixed with a granular carrier surfaced with a suitable resin, such as described in US. 2,618,551, the resulting developer gives extremely sharp black images of excellent contrast resolution and appearance. The background obtained using the novel developer is excellent.

Example III The procedure of Example I is repeated with the exception that five parts carbon black (based on 100 parts monomers) as a colloidal dispersion is charged to the resin kettle prior to polymerization. The carbon is thus encapsulated within the copolymer to provide a pigmentcontaining latex particle having Tg and [1 1 values corresponding to those of Example I and a particle size in the range of 0.15-0.25 micron. After spray drying, there is obtained substantially spherical xerographic toner powder particles having an average particle size of 1-5 microns.

Example IV By repeating the procedure of Example I using diisopropyl xanthogen as the chain-transfer agent, a latex copolymer having an [M value of 0.26 is obtained. When the copolymer is homogeneously admixed with eight percent carbon black, a xerographic toner having excellent properties is obtained.

Example V By repeating the procedure of Example I using a monomer charge of 87 weight parts styrene and 13 parts nbutylacrylate, there is obtained a copolymer latexhaving a T-g value of 72 C. Toner made by homogeneously combining this copolymer with eight percent carbon black does not fuse at temperatures safely below the char point of paper and thus is unsuitable for xerographic purposes.

Example VI By repeating the procedure of Example I using a monomer charge of 62 weight parts styrene and 38 weight parts n-butylacrylate, there is obtained a copolymer latex having a Tg value of 27 C. Toner made by homogeneously combining this copolymer with eight percent carbon black is somewhat tacky under conditions originally employed in xerography, and thus is unsuitable.

Example VII By repeating the procedure of Example I with the omission of the mercaptan chain-transfer agent, the re sulting copolymer latex has an [7 1 value of 0.93. Toner made by homogeneously combining this copolymer with eight percent carbon black becomes tacky under conditions of use and causes a buildup of residual toner on the xerographic plate.

Example VIII By repeating the procedure of Example I using a monomer charge of Weight parts styrene and 28 weight parts Z-ethylhexylacrylate and 3 parts per 100 t-dodecylmercaptan, there is obtained a 37 C. and an [1,1 value of 0.16. Toner made by homogeneously combining this copolymer with eight percent carbon black has excellent xerographic properties.

Example 1X By repeating the procedure of Example I using a monomer charge of weight parts styrene and 15 weight parts laurylmethacrylate and 2 parts per t-dodecylmercaptan, there is obtained a copolymer latex having a Tg value of 60 C. and an [7710 value of 0.22. Toner made by homogeneously combining this copolymer with eight percent carbon black has good xerographic prop erties.

Example X By repeating the procedure of Example I using a monomer charge of 25 weight parts styrene and 75 weight parts n-butylmethacrylate and 3 parts per 100 t-dodecylmercaptan, there is obtained a copolymer latex having a Tg value of 38 C. and an [1 1 value of 0.17. Toner made by homogeneously combining this copolymer with eight percent carbon black has excellent xerographic properties.

Example XI By repeating the procedure of Example I using a monomer charge of 25 weight parts methyl. methacrylate and 75 weight parts n-butylmethacrylate and 2 parts per 100 t-dodecylmercaptan, there is obtained a copolymer latex having a Tg value of 39 C. and an [1;] value of 0.25. Toner made by homogeneously combining this copolymer with eight percent carbon black has excellent xerographic properties.

Example XII By repeating the procedure of Example I using a monomer charge of 77 weight parts methyl methacryiate and 23 weight parts Z-ethylhexylacrylate and 1 part per 100 t-dodecylmercaptan, there is obtained a copolymer latex having a Tg value of 47 C. and an [1110 value of 0.31. Toner made by homogeneously combining this copolymer with eight percent carbon black has excellent xerographic properties.

1 1 Example XIII By repeating the procedure of Example I using a monomer charge of 100 weight parts ethyl methacrylate and 2 parts per 100 t-dodecylmercaptan, there is obtained a homopolymer latex having a Tg value of 47 C. and an [1 1 value of 0.24. Toner made by homogeneously combining this homopolymer with eight percent carbon black has excellent xerographic properties.

Example )GV By repeating the procedure of Example I using a monomer charge of 100 weight parts vinylacetate and 3 parts per 100 t-dodecylmercaptan, there is obtained a homopolymer latex having a Tg value of 30 C. and an [1 1 value of 0.15. Toner made by homogeneously combining this homopoly-mer with eight percent carbon black has good xerographic properties.

Example XV By repeating the procedure of Example I using a monomer charge of 46 weight parts methyl methacrylate and 54 weight parts vinylacetate and 2 parts per 100 carbon tetrabromide, there is obtained a copolymer latex having a Tg value of 55 C. and an [1 1 value of 0.28. Toner made by homogeneously combining this copolymer with eight percent carbon black has good xerographic properties.

Example XVI By repeating the procedure of Example I using a monomer charge of 80 weight parts vinylchloride and 20 weight parts vinylidine chloride and 2 parts per 100 alpha-bromoethylbenzene, there is obtained a copolymer latex having a Tg value of 47 C. and an average molecular weight of 60,000, corresponding to an [1 value of 0.28. Toner made by homogeneously combining this copolymer with eight percent carbon black has excellent xerographic properties.

Example XVII By repeating the procedure of Example I using a monomer charge of 55 weight parts acrylonitrile and 45 weight parts n-butylmethacrylate and 2 parts per 100 t-dodecylmercaptan, there is obtained a copolymer latex having a Tg value of 58 C. and an [7;] value of 0.23. Toner made by homogeneously combining this copolymer with eight percent carbon black has excellent xerographic properties.

Example XVIII By repeating the procedure of Example I using a monomer charge of 50 weight parts styrene, 30 weight parts butyl methacrylate and 20 weight parts Z-ethylhexylacrylate and 3 parts per 100 t-dodecylmercaptan, there is obtained a copolymer latex having a Tg value of 32 C. and an [1 1 value of 0.15. Toner made by homogeneously combining this copolymer with eight percent carbon black has good xerographic properties.

Example XIX By repeating the procedure of Example I using a monomer charge of 70 weight parts methyl methacrylate, 20 weight parts Z-ethylhexylacrylate, and 10 weight parts methylacrylate and 2 parts per 100 t-dodecylmercaptan, there is obtained a copolymer latex having a Tg value of 48 C. and an [1 1 value of 0.22. Toner made by homogeneously combining this copolymer with eight percent carbon black has excellent xerographic properties.

I claim:

1. A process of xerography wherein electrostatic latent image is developed comprising depositing on an image surface having an electrostatic latent image thereon a xerographic toner powder comprising substantially spherical particles, having an average particle size of less than 10 microns, consisting essentially of from about to weight percent pigment and from about 90 to 95 weight percent resin having a Tg value of 30 to 65 C. and an [M value of 0.15 to 0.35, said resin having been prepared as a latex in the presence of an organic chain-transfer agent having a Cs value of at least 0.1 by the catalytic polymerization in aqueous emulsion of (A) a monomer selected from the-group consisting of ethyl methacrylate and vinyl acetate or (B) a monomer mixture of (1) at least one member selected from the group consisting of styrene, methyl methacrylate, ethyl methacrylate, acrylonitrile, and vinyl chloride, and (2) at least one member selected from the group consisting of alkyl acrylates wherein the alkyl group has up to 12 carbon atoms, alkyl methacrylates wherein the alkyl group has from 3-12 carbon atoms, vinyl acetate, and vinylidine chloride, the relative proportion of 1) to (2) in said mixture being selected to provide a polymer having a Tg value of 30-65 C. and said chain-transfer agent being present in an amount sufi'icient to provide a polymer having an [0 value of 0.15-0.35; said pigment having been uniformly distributed within said latex; and the pigment-containing latex having been dried to form substantially spherical powder particles having an average particle size of less than 10 microns.

2. A process of xerography wherein electrostatic latent image is developed comprising depositing on an image surface having an electrostatic latent image thereon a xerographic toner powder comprising substantially spherical particles, having an average particle size of less than 10 microns, consisting essentially of from about 5 to 10 weight percent pigment and from about to weight percent resin having a Tg value of 30 to 65 C. and an [1 vaule of 0.15 to 0.35, said resin having been prepared as a latex in the presence of said pigment and an organic chain-transfer agent having a Cs value of at least 0.1 by the catalytic polymerization in aqueous emulsion of (A) a monomer selected from the group consisting of ethyl methacrylate and vinyl acetate or (B) a monomer mixture of (1) at least one member selected from the group consisting of styrene, methyl methacrylate, ethyl methacrylate, acrylonitrile, and vinyl chloride, and (2) at least one member selected from the group consisting of alkyl acrylates wherein the alkyl group has up to 12 carbon atoms, alkyl methacrylates wherein the alkyl group has from 3-12 carbon atoms, vinyl acetate, and vinylidine chloride, the relative proportion of (l) to (2) in said mixture being selected to provide a polymer having a Tg value of 3065 C. and said chain-transfer agent being present in an amount sufficient to provide a polymer having a [1 value of 0.15-0.35; and the pigment-containing latex having been spray dried to form substantially spherical particles having an average particle size of less than 10 microns.

3. A process of xerography wherein electrostatic latent image is developed comprising depositing on an image surface having an electrostatic latent image thereon a xerographic toner powder comprising substantially spherical particles having an average particle size of less than 10 microns, consisting essentially of from about 5 to 10 weight percent pigment and from about 90 to 95 weight percent resin having a Tg value of 35 to 50 C. and an [1 1 value of 0.15 to 0.35, said resin having been prepared as a latex in the presence of an alkyl mercaptan having from 4 to 16 carbon atoms by the catalytic polymerization in aueous emulsion of (A) a monomer selected from the group consisting of ethyl methacrylate and vinyl acetate or (B) a monomer mixture of (1) at least one member selected from the group consisting of styrene, methyl methacrylate, ethyl methacrylate, acrylonitrile, and vinyl chloride, and (2) at least one member selected from the group consisting of alkyl acrylates wherein the alkyl group has up to 12 carbon atoms, alkyl methacryl-ates wherein the alkyl group has from 3-12 carbon atoms, vinyl acetate, and vinylidine chloride, the relative proportion of (1) to (2) in said mixture being selected to provide a polymer having a Tg value of 3550 C. and said mercaptan being present in an amount sufficient to provide a polymer having an [4,1 value of 0.15-0.35; said pigment having been uniformly distribtued within said latex; and the pigment-containing latex having been dried to form substantially spherical powder particles having an average particle size of less than 10 microns.

4. A process of xerography wherein electrostatic latent image is developed comprising depositing on an image surface having an electrostatic latent image thereon a xerographic toner powder comprising substantially spherical particels, having an average particle size of less than 10 microns, consisting essentially of from about to Weight percent pigment and from about 90 to 95 weight percent resin having a Tg value of 35-50 C. and an [4 value of 0.15-0.35, said resin having been prepared as a latex in the presence of an alkyl mercaptan having from 4-16 carbon atoms by the catalytic polymerization in aqueous emulsion of a monomer mixture of styrene and n-butylacrylate in a relative proportion to provide a Tg value of 35-50 C. and said mercaptan being present in an amount of 0.5-3 weight parts per part of said monomer mixture; said pigment having been uniformly distributed within said latex; and the pigment-containing latex having been spray dried to form substantially spherical powder particels having an average particle size of less than 10 mircons.

5. A process of xerography wherein electrostatic latent image is developed comprising depositing on an image surface having an electrostatic latent image thereon a xerographic toner powder comprising substantially spherical particles having an average particle size of less than 10 microns, consisting essentially of from about 5 to 10 weight percent pigment and from about 90 to 95 weight percent resin having a Tg value of 35-50 C. and an [1 value of 0.15-0.35, said resin having been prepared as a latex in the presence of an alkyl mercaptan having from 4-16 carbon atoms by the catalytic polymerization in aqueous emulsion of a monomer mixture of styrene and 2-ethylhexylacrylate in a relative proportion to provide a Tg value of 35-50 C. and said mercaptan being present in an amount of 0.5-3 weight parts per part of said monomer mixture; said pigment having been uniformly distributed within said latex; and the pigment-containing latex having been spray dried to form substantially spherical powder particles having an average particle size of less than 10 microns.

6. A process of xerography wherein electrostatic latent image is developed comprising depositing on an image surface having an electrostatic latent image thereon a xerographic toner powder comprising substantially spherical particles, having an average particle size of less than 10 microns, consisting essentially of from about 5 to 10 weight percent pigment and from about 90 to 95 weight percent resin having a Tg value of 35-50 C. and an [7 1 value of 0.15-0.35, said resin having been prepared as a latex in the presence of an alkyl mercaptan having from 4-16 carbon atoms by the catalytic polymerization in aqueous emulsion of a monomer mixture of styrene and n-butylmethacrylate in a relative proportion to provide a Tg value of 35-50 C. and said mercaptan being present in an amount of 0.5-3 weight parts per part .of said monomer mixture; said pigment having been uniformly distributed within said latex; and the pigment-containing latex having been spray dried to form substantially spherical powder particles having an average particle size of less than 10 microns.

7. A process of xerography wherein electrostatic latent image is developed comprising depositing on an image surface having an electrosatic latent image thereon a xerographic toner powder comprising substantially spherical particles, having an average particle size of less than 10 microns, consisting essentially of from about 5 to 10 weight percent pigment and from about 90 to 95 weight percent resin having a Tg value of 34-50 C. and an [1 valueof 0.15-0.35, said resin having been prepared as a latex in the presence of an alkyl mercaptan having from 4-16 carbon atoms by the catalytic polymerization in aqueous emulsion of a monomer mixture of styrene and laurylmethacrylate in a relative proportion to provide a Tg value of 35-50 C. and said mercaptan being present in an amount of 0.5-3 weight parts per part of said monomer mixture; said pigment having been uniformly distributed within said latex; and the pigment-containing latex having been spray dried to form substantially spherical powder particles having an average particle size of less than 10 microns.

8. A process of xerography wherein electrosatic latent image is developed comprising depositing on an image surface having an electrostatic latent image thereon a xerographic toner powder comprising substantially spherical particles, having an average particle: size of less than 10 microns, consisting essentially of from about 5 to 10 weight percent pigment and from about. to weight percent resin having a Tg value of 35-50 C. and an [1 value of 0.15-0.35, said resin having been prepared as a latex in the presence of an alkyl mercaptan having from 4-16 carbon atoms by the catalytic polymerization in aqueous emulsion of a monomer mixture of methyl methacrylate and 2-ethylhexylacrylate in a relative proportion to provide a Tg value .of 35-50 C. and said mercaptan being present in an amount of 0.5-3 Weight parts per part of said monomer mixture; said pigment having been uniformly distributed within said latex; and the pigment-containing latex having been spray dried to form substantially spherical powder particles having an average particle size of less than 10 microns.

9. A xerographic developer comprising finely-divided powder particles uniformly electrostatically coated on a carrier surface capable of retaining said powder particles by electrostatic attraction; the carrier surface being adapted to make firm Contact with a surface bearing an electrostatic image and having removably coated thereon by electrostatic attraction xerographic toner powder particles substantially spherical in shape and having an average particle size of less than 10 microns; said toner powder particles consisting essentially of from about 5 to 10 weight percent pigment and from about 90 to 95 weight percent resin having a Tg value of 3065 C. and an [1 value of 0.15-0.35; said resin having been prepared as a latex in the presence of an organic chain-transfer agent having a Cs value of at least 0.1 by the catalytic polymerization in aqueous emulsion of (A) a monomer selected from the group consisting of ethyl methacrylate and vinyl acetate or (B) a monomer mixture of (1) at least one member selected from the group consisting of styrene, methyl methacrylate, ethyl methacrylate, acrylonitrile, and vinyl chloride, and (2) at least one member selected from the group consisting of alkyl acrylates wherein the alkyl group has up to 12 carbon atoms, alkyl methacrylates wherein the alkyl group has from 3 to 12 carbon atoms, vinyl acetate, and vinylidine chloride, the relative proportion of (1) to (2) in said mixture being selected to provide a polymer having a Tg value of 30-65" C. and said chain-transfer agent being present in an amount sufiicient to provide a polymer having an [1 1 value of 0.15-0.35; said pigment having been uniformly distributed within said latex; and the pigment-containing latex being dried to form substantially spherical powder particles having an average particle size ,of less than 10 microns.

References Cited UNITED STATES PATENTS 2,297,691 10/ 1942 Carlson 252--62.1 2,788,288 3/ 1957 Rheinfrank et a1. 25262.1 XR

OTHER REFERENCES Bovey, Emulsion Polymerization, Interscience Pub. (1955) pp. 13, 245-6, 251-4.

American Ink Maker, December 1963, pp. 28, 29, 31. LEON D. ROSDOL, Primary Examiner. J. D. WELSH, Assistant Examiner.