Toners Containing Positively Chargeable Modified Pigments

1. Field of the Invention

The present invention relates to toner and developer compositions containing positively chargeable modified pigments.

2. Discussion of the Related Art

Electrophotographic processes and image-forming apparatus are widespread nowadays. Particularly, aspects of the xerographic process are set forth in R. M. Schaffert "Electrography", the Focal Press, London & N.Y., enlarged and revised edition, 1975. In electrophotography, an image comprising an electrostatic field pattern (also referred to as an electrostatic latent image), usually of nonuniform strength, is formed on an insulative surface of an electrophotographic element. The insulative surface comprises a photoconductive layer and an electrically conductive substrate. The electrostatic latent image may be formed by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then visualized by contacting the latent image with an oppositely charged toner powder generally containing a colorant. This process of visualization of a latent image is known as development, and the composition containing the dry toner powder is known as the developer. The toned image is then transferred onto a transfer medium such as paper and fixed thereon by heating and/or pressure. The last step involves cleaning residual toner from the electrophotographic element.

Developer compositions used in dry electrophotography to visualize latent electrostatic images are divided into one-component systems composed of a dry toner powder, generally including a binder resin having a colorant dispersed therein, and two-component systems composed of a dry toner powder and carrier particles. Charge control agents are often melt mixed with the toner resin to control the chargeability of the toner during use. Known positive charge controlling compounds for use in dry toners are dye bases and salts thereof such as nigrosine dye base and salts. In order that toner compositions have process suitability in copying, they are required to be excellent in fluidity, anti-caking properties, fixability, chargeability, cleaning properties, and the like. To improve these properties, particularly fluidity, anti-caking properties, and chargeability, extraparticulate inorganic fine particles are frequently added to toner compositions. The components of the toner are dispersed or dissolved in the toner resin vehicle during the compounding step of the preparation process. The degree of dispersion has an effect on the performance of the toner material in the printing process. Inadequate dispersion can in many instances lead to a lack of consistency of homogeneity in the toner particle to particle. This can lead to a broad spread in charge distribution of the toner because of the dissimilarity of composition of the particulate toner. The electrostatic printing process is best performed when the toner used has a uniform charging behavior which will minimize the occurrence of print defects such as fogging, background, haloing, character spread, and dust contamination of the internal parts of the printing apparatus.

Development of a latent electrostatic image requires that a charge be developed on the toner particles prior to their deposition on the latent image, and that this charge be opposite to the charge of the latent image. All components of a toner, including binder resin, colorants, charge control agents, waxes and the like, can influence the development of charge on the toner particles. The influence of the colorants on the charging behavior of toner compositions is seldom considered, as there are few known methods to change and control the natural charging behavior of colorants such as carbon black. Thus an unmet need in dry toner technology is for pigments which have certain unique and predictable tribocharging properties.

One approach to meeting this need is to surface-modify known pigments to enhance or change their natural tribocharging properties. For example, Japanese Patent Application Hei 3›1991!-197961 relates to surface treatment of carbon blacks with amine-functional silane coupling agents which can, to some extent, overcome the natural tendency of carbon blacks to tribocharge negatively, which makes the carbon blacks more useful as pigments in positive-charging toners. However, it is believed that for such treatments to be effective, the silane coupling agents must form a covalent bond to the surface of the carbon black. The chemical groups believed to be present on the surface of normal carbon black are oxygen-containing groups. Silane coupling agents can form covalent bonds with these groups. Such groups are normally present on the surface of carbon black at low and poorly-controlled levels, making such treatment with silane coupling agents of limited scope and value.

A feature of the present invention is to provide alternative additives which impart or assist in imparting a positive charge to the toner particles in toner and developer compositions.

Another feature of the present invention is to provide a colorant for use in toner and developer compositions.

Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the written description and appended claims.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention relates to a toner composition which includes styrenic polymer-based resin particles and modified pigment particles which have at least one organic group attached to the pigment particles, wherein the organic group is positively chargeable.

The present invention also relates to a developer composition which includes carrier particles and the toner composition described above.

In addition, the present invention further relates to a method of imaging which includes the steps of formulating an electrostatic latent image on a negatively charged photoconductive imaging member, effecting the development thereof with a toner composition which includes styrenic polymer-based resin particles and modified pigment particles having attached an organic group that is positively chargeable, and thereafter transferring the developed image onto a suitable substrate.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.

FIG. 1 is a graph (Q/M vs Roll Mill Time) showing the tribocharging behavior of dry powders, including a toner of the present invention, using a standard carrier.

FIG. 2 is a graph (Q/M vs Roll Mill Time) showing the tribocharging behavior of dry powders, including a toner of the present invention, using a positive carrier.

FIG. 3 is a graph (Q/M vs Roll Mill Time) showing the tribocharging behavior of dry powders, including a toner of the present invention, using a standard carrier.

FIG. 4 is a graph (Q/M vs Roll Mill Time) showing the tribocharging behavior of dry powders, including a toner of the present invention, using a positive carrier.

The present invention relates to toner and developer compositions which include modified pigment particles having attached at least one organic group that is positively chargeable, and styrenic polymer-based resin particles.

The pigment particles that are modified can be carbon black, cyan, magenta, yellow, blue, green, brown, violet, red, or mixtures thereof. Suitable pigments are pigment particles capable of being modified with attachment of at least one organic group that is positively chargeable. Carbon black is the preferred pigment and examples include, but are not limited to, commercially available forms of carbon black, such as Regal.RTM. carbon black from Cabot Corporation. Pigments which may be capable of being modified are described, for instance, in U.S. Pat. Nos. 5,484,675; 5,571,654; 5,275,900; and EP 0 723 206 A1, all incorporated in their entirety by reference herein. As the pigment for black toner compositions, carbon black pigments alone or in combination with blue, green, magnetite or black dyes can be used.

The modified pigment has at least one organic group attached to the pigment particles and the organic group is positively chargeable. The organic group can be attached to the pigment in various amounts, i.e., low to high amounts, thus allowing fine control over charge modification. The organic group that is attached to the pigment particles can be any group which permits the modified pigment to be positively chargeable once incorporated into the toner or developer composition. Preferably, the organic group comprises an aromatic group or a C.sub.1 -C.sub.20 alkyl group, wherein either group can be substituted or unsubstituted. It is also preferred that the aromatic group or C.sub.1 -C.sub.20 alkyl group is directly attached to the pigment particles. Preferred groups of positively chargeable organic groups are nitrogen containing or phosphorus containing organic groups.

Preferred positive chargeable organic groups have the general structures: ##STR1## wherein Q represents the elements nitrogen or phosphorus; X represents a counterion such as Cl.sup.-, Br.sup.-, ArSO.sub.3.sup.-, and the like; R.sub.1 represents an alkylene group or an arylene group attached to the pigment; and R.sub.2, R.sub.3, and R.sub.4, which may be the same or different, each represent an alkyl group or an aryl group. Preferably, the alkylene or alkyl group is a C.sub.1 -C.sub.10 alkylene or alkyl group and the arylene or aryl group is a C.sub.6 -C.sub.20 arylene or aryl group. For the purposes of this invention, aryl and arylene groups include heteroaryl and heteroarylene groups, respectively.

Other preferred organic groups that can be attached to the pigment particles include, but are not limited to the following: ##STR2## in which Ar represents an aromatic group and Ar' represents an aromatic group. The aromatic group includes, but is not limited to, unsaturated cyclic hydrocarbons containing one or more rings. The aromatic group may be substituted or unsubstituted. Aromatic groups include aryl groups (for example, phenyl, naphthyl, anthracenyl, and the like), and heteroaryl groups (imidazolyl, pyrazolyl, pyridinyl, thienyl, thiazolyl, furyl, triazinyl, indolyl, and the like).

The following discussion is with reference to the modification of the preferred pigment, carbon black. However, modified pigments other than carbon black can be similarly prepared. The modified carbon black may be prepared preferably by reacting carbon with a diazonium salt in a liquid reaction medium to attach at least one organic group to the surface of the carbon. The diazonium salt may contain the organic group to be attached to the carbon. A diazonium salt is an organic compound having one or more diazonium groups. Preferred reaction media include water, any medium containing water, and any medium containing alcohol. Water is the most preferred medium. Examples of modified carbon black and various preferred methods for their preparation are described in U.S. patent application Ser. No. 08/356,660 entitled "Reaction of Carbon Black with Diazonium Salts, Resultant Carbon Black Products and Their Uses," filed Dec. 15, 1994, now abandoned, and its continuation-in-part application, U.S. patent application Ser. No. 08/572,525, filed Dec. 14, 1995, U.S. Pat. No. 5,554,739 entitled "Reaction of Carbon Materials With Diazonium Salts and Resultant Carbon Products," WO 96/18696 and WO 96/18688, all incorporated herein by reference.

In the preferred preparation of the above modified carbon black, the diazonium salt need only be sufficiently stable to allow reaction with the carbon. Thus, that reaction can be carried out with some diazonium salts otherwise considered to be unstable and subject to decomposition. Some decomposition processes may compete with the reaction between the carbon and the diazonium salt and may reduce the total number of organic groups attached to the carbon. Further, the reaction may be carried out at elevated temperatures where many diazonium salts may be susceptible to decomposition. Elevated temperatures may also advantageously increase the solubility of the diazonium salt in the reaction medium and improve its handling during the process. However, elevated temperatures may result in some loss of the diazonium salt due to other decomposition processes. The diazonium salts may be prepared in situ. It is preferred that the modified carbon black of the present invention contain no by-products or unattached salts.

With respect to suitable toner resins for use in the toner and developer compositions of the present invention, a styrenic polymer-based is used, such as a styrenated acrylic resin. Examples of preferred styrenic polymer-based resins include, but are not limited to, homopolymers and copolymers of styrene and its derivatives such as: polystyrene; poly-p-cholorostyrene; polyvinyltoluene; styrene-p-chlorostyrene copolymer; and styrene-vinyltoluene copolymer; copolymers of styrene and acrylic acid esters such as: styrenemethylacrylate copolymer; styrene-ethylacrylate copolymer; and styrene-n-butyl acrylate copolymer; copolymers of styrene and methacrylic acid esters such as: styrene-methyl methacrylate copolymer; styrene-ethyl methacrylate copolymer; styrene-n-butyl methacrylate copolymer; and multi-component copolymers of styrene, acrylic acid ester and methacrylic acid esters; copolymers of styrene and other vinyl monomers such as: styrene-acrylonitrile copolymer, styrene-methyl ether copolymer; styrene-butadienee copolymer; styrene-vinyl methyl ketone copolymer; styrene-acrylonitrileindene copolymer; styrene maleic acid ester copolymer; and the like. These binder resins may be used singly or in combination. Generally, resins particularly suitable for use in xerographic toner manufacturing have a melting point (ring and ball method) in the range of 100.degree. C. to 135.degree. C. and have a glass transition temperature (Tg) greater than about 60.degree. C. Examples of styrenic polymer-based resin particles and suitable amounts can also be found in U.S. Pat. Nos. 5,278,018; 5,510,221; 5,275,900; 5,571,654; 5,484,575; and EP 0 720 066 A1, all incorporated in their entirety by reference herein.

Generally, the modified pigment of the present invention, alone or with other pigments, is present in total amounts of from about 1% by weight to about 30% by weight of the toner or developer composition. The amount of pigment present in the toner composition is preferably from about 0.1 to about 12 wt parts per 100 wt parts of resin. However, lesser or greater amounts of the modified pigment may be used. Also, generally, the toner resin is present in amounts of from about 60% by weight to about 99% by weight of the toner or developer composition.

Optional external additives may also be mixed or blended with the toner compositions of the present invention including carrier additives; additional positive or negative charge controlling agents such as quaternary ammonium salts, pyridinum salts, sulfates, phosphates, and carboxylates; flow aid additives; silicone oils; waxes such as commercially available polypropylenes and polyethylenes; magnetite; and other known additives. Generally, these additives are present in amounts of from about 0.05% by weight to about 30% by weight, however, lesser or greater amounts of the additives may be selected depending on the particular system and desired properties. Specific examples of additives and amounts are also described in the patents and the European patent application mentioned above and incorporated herein by reference.

The toner compositions can be prepared by a number of known methods, such as admixing and heating the resin, the modified pigment particles, optional charge enhancing additives and other additives in conventional melt extrusion devices and related equipment. Other methods include spray drying and the like. Compounding of the modified pigment and other ingredients with the resin is generally followed by mechanical attrition and classification to provide toner particles having a desired particle size and particle size distribution. Conventional equipment for dry blending of powders may be used for mixing or blending the modified pigment particles with the resin. Again, conventional methods of preparing toner and developer compositions can be used and are described in the patents and European application described above and incorporated herein by reference.

In more detail, the toner material can be prepared by dry blending the binder resin with all other ingredients, including the pigment, and then melt-extruding in a high shear mixer to form a homogeneously mixed mass. During this process the components are held at a temperature above the melting point of the binder resin, and those components that are insoluble in the resin are ground so that their average particle size is reduced. This homogeneously mixed mass is then allowed to cool and solidify, after which it is pre-ground to an average particle size of about 100 microns. This material is then further subjected to particle size reduction until its average particle size meets the size range specification required for classification. A variety of classifying techniques may be used. The preferred type is an air classification type. By this method, particles in the ground material which are too large or too small are segregated from the portion of the material which is of the desired particle size range.

The toner composition of the present invention may be used alone in monocomponent developers or may be mixed with suitable carrier particles to form dual component developers. The carrier vehicles which can be used to form dual component developer compositions can be selected from various materials. Such materials typically include carrier core particles and core particles overcoated with a thin layer of film-forming resin to help establish the correct triboelectric relationship and charge level with the toner employed. Suitable carriers for two component toner compositions include iron powder, glass beads, crystals of inorganic salts, ferrite powder, nickel powder, all of which are typically coated with resin coating such as an epoxy or fluorocarbon resin. Examples of carrier particles and coatings that can be used and are described in the patents and European application described above and incorporated herein by reference.

The present invention is further directed to a method of imaging which includes formulating an electrostatic latent image on a negatively charged photoconductive imaging member, affecting the development thereof with toner composition comprising resin particles and modified pigment particles, and thereafter transferring the developed image onto a suitable substrate. Conventional methods of imaging can be used, such as shown in the patents and European patent application described above.

The present invention will be further clarified by the following examples which are intended to be purely exemplary of the present invention.

A solution of 2.83 g of sodium nitrite in about 100 g of water was added slowly to a mixture of 200 g of carbon black, 3.95 g of concentrated HCl, 4.48 g of p-phenylenediamine and 1.8 L of water that was stirring at about 70.degree. C. The carbon black, Regal.RTM.330 carbon black, had a surface area of 94 m.sup.2 /g and a DBPA of 65 mL/100 g. After stirring for about two hours, the mixture was allowed to stand overnight. The aqueous layer was decanted, and the remainder of the material was dried at 70.degree. C. The product had attached C.sub.6 H.sub.4 NH.sub.2 groups.

A solution of 2.84 g of sodium nitrite in about 100 g of water was added slowly to a mixture of 200 g of carbon black, 3.94 g of concentrated HCl, 2.22 g of p-phenylenediamine, 4.34 g of 4-aminophenylpyridinium chloride and 1.8 L of water that was stirring at about 70.degree. C. The carbon black, Regal.RTM.330 carbon black, had a surface area of 94 m.sup.2 /g and a DBPA of 65 mL/100 g. After stirring for two hours, the mixture was allowed to stand overnight. The aqueous layer was decanted, and the remainder of the material was dried at 70.degree. C. The product had attached C.sub.6 H.sub.4 NH.sub.2 and C.sub.6 H.sub.4 NC.sub.5 H.sub.5.sup.+ Cl.sup.- groups.

A black toner powder was prepared by the conventional technique of melt-mixing, extruding, pre-grinding, jetmilling and classifying. Thus, 8 wt % of Regal.RTM.330 carbon black (unmodified) (available from Cabot Corporation, Boston, Mass.) was dry blended with 92 wt % of Dialec 1601 styrenated acrylic polymer (available from Polytribo Inc, Bristol, Pa.) and melt-extruded in a B&P 19-millimeter extruder (available from B&P Process Equipment & Systems, LLC, Saginaw, Mich.) operating in a typical screw and paddle configuration. The resulting carbon black/polymer product was pre-ground in a Krups Mini Blender, then jetmilled and classified using a Majac A-12 and mini-grinder to form a black toner powder having an average particle size of about 13 microns, as determined using a Coulter Multisizer Particle Size Analyzer. This toner is referred to as Sample 2 in Table 1 of Example 5.

Developer compositions were prepared by mixing the toner composition described above either with a positive charging ferrite powder, or a standard ferrite powder (both available from Powdertech, Inc., Valparaiso, Ind.), in an amount sufficient to yield a 2.0 wt % loading.

Tribocharge measurements were made by tumble blending the above developer compositions (toner plus carrier) in stainless steel vessels on a roll mill. At blending times of 15 minutes, 30 minutes, 45 minutes, and 60 minutes, a small sample of the developer composition was removed and its charge-to-mass ratio (Q/M) was determined by the Faraday cage tribo blow-off method using a Vertex T-150 tribocharge tester (available from Vertex, Inc., Yukon, Pa.).

A black toner powder was prepared by the conventional technique of melt-mixing, extruding, pre-grinding, jetmilling and classifying. Thus 8 wt % of the modified carbon black prepared in Example 1 was dry blended with 92 wt % of Dialec 1601 styrenated acrylic polymer (available from Polytribo Inc, Bristol, Pa.) and melt-extruded in a B&P 19-millimeter extruder (available from B&P Process Equipment & Systems, LLC, Saginaw, Mich.) operating in a typical screw and paddle configuration. The resulting carbon black/polymer product was pre-ground in a Krups Mini Blender, then jetmilled and classified using a Majac A-12 and mini-grinder to form a black toner powder having an average particle size of about 12 microns, as determined using a Coulter Multisizer Particle Size Analyzer. This toner is referred to as Sample 3 in Table 1 of Example 5.

Developer compositions were prepared by mixing the toner composition described above either with a positive charging ferrite powder, or a standard ferrite powder (both available from Powdertech, Inc., Valparaiso, Ind.), in an amount sufficient to yield a 2.0 wt % loading.

Tribocharge measurements were made by tumble blending the above developer compositions (toner plus carrier) in stainless steel vessels on a roll mill. At blending times of 15 minutes, 30 minutes, 45 minutes, and 60 minutes, a small sample of the developer composition was removed and its charge-to-mass ratio (Q/M) was determined by the Faraday cage tribo blow-off method using a Vertex T-150 tribocharge tester (available from Vertex, Inc., Yukon, Pa.).

The results of tribocharge testing of Sample 3 using a standard ferrite carrier are presented in FIG. 1. Also in FIG. 1 are shown for comparative purposes the tribocharging behavior of a pseudo toner containing a jet-milled Dialec 1601 resin which was combined with a standard ferrite carrier in an amount to yield a 2.0 wt. % loading (Sample 1) and the toner of Example 3 combined with a standard ferrite carrier in an amount to yield a 2.0 wt. % loading. After 15 minutes the charge on Sample 1 was strongly negative at about -20 microcoulombs/g, and was slightly more negative after 60 minutes. In comparison the charges at 15 minutes on Samples 2 and 3 were more than 15 microcoulombs/g more positive and were nearly equal. After 60 minutes, however, the charge on sample 2 (containing unmodified Regal.RTM.330) became more negative by more than 12 microcoulombs/g while the charge on Sample 3 (containing the modified carbon black from Example 1) decreased by less than 3 microcoulombs/g.

The results of tribocharge testing of Sample 3 using a positive ferrite carrier are presented in FIG. 2. Also in FIG. 2 are shown for comparative purposes the tribocharging behavior of a pseudo toner containing a jetmilled Dialec 1601 resin (Sample 1) which was combined with a positive ferrite carrier in an amount to yield a 2.0 wt. % loading and the toner of Example 3 combined with a positive ferrite carrier in an amount to yield a 2.0 wt. % loading. (Sample 2). After 15 minutes the charge on Sample 1 was strongly negative, while by comparison the charge on Sample 2 was more positive by 20 microcoulombs/g, and the charge on Sample 3 was more positive by 30 microcoulombs/g. After 60 minutes the charge on Sample 2 (containing unmodified Regal.RTM.330) became more negative by about 10 microcoulombs/g, while the charge on Sample 3 (containing the modified carbon black from Example 1) actually became slightly more positive, by less than 3 microcoulombs/g.

A black toner powder was prepared by the conventional technique of melt-mixing, extruding, pre-grinding, jetmilling and classifying. Thus, 8 wt % of the modified carbon black prepared in Example 2 was dry blended with 92 wt % of Dialec 1601 styrenated acrylic polymer (available from Polytribo Inc, Bristol, Pa.) and melt-extruded in a B&P 19-millimeter extruder (available from B&P Process Equipment and Systems, LLC, Saginaw, Mich.) operating in a typical screw and paddle configuration. The resulting carbon black/polymer product was pre-ground in a Krups Mini Blender, then jetmilled and classified using a Majac A-12 and mini-grinder to form a black toner powder having an average particle size of about 12 microns, as determined using a Coulter Multisizer Particle Size Analyzer. This toner is referred to as Sample 4 in Table 1 of Example 5.

Developer compositions were prepared by mixing the toner composition described above either with a positive charging ferrite powder, or a standard ferrite powder (both available from Powdertech, Inc., Valparaiso, Ind.), in an amount sufficient to yield a 2.0 wt % loading.

Tribocharge measurements were made by tumble blending the above developer compositions (toner plus carrier) in stainless steel vessels on a roll mill. At blending times of 15 minutes, 30 minutes, 45 minutes and, 60 minutes, a small sample of the developer composition was removed and its charge-to-mass ratio (Q/M) was determined by the Faraday cage tribo blow-off method using a Vertex T-150 tribocharge tester (available from Vertex, Inc., Yukon, Pa.).

The results of tribocharge testing of Sample 4 against a standard ferrite carrier are presented in FIG. 3. Also in FIG. 3 are shown for comparative purposes the tribocharging behavior of a pseudo toner containing a jet-milled Dialec 1601 resin (Sample 1) which was combined with a standard ferrite carrier in an amount to yield a 2.0 wt. % loading and the toner of Example 3 (Sample 2) which was combined with a standard ferrite carrier in an amount to yield a 2.0 wt. % loading. After 15 minutes the charge on Sample 1 was strongly negative at about -20 microcoulombs/g and was slightly more negative at 60 minutes. In comparison, the charge at 15 minutes on Sample 2 was more than 15 microcoulombs/g more positive, and the charge on Sample 4 was more than 25 microcoulombs/g more positive. After 60 minutes, however, the charge on sample 2 (containing unmodified Regal.RTM.330) became more negative by more than 12 microcoulombs/g while the charge on Sample 4 (containing the modified carbon black from Example 2) decreased by less than 3 microcoulombs/g.

The results of tribocharge testing of Sample 4 against a positive ferrite carrier are presented in FIG. 4. Also in FIG. 4 are shown for comparative purposes the tribocharging behavior of a pseudo toner containing a jet-milled Dialec 1601 resin (Sample 1) which was combined with a positive ferrite carrier in an amount to uield a 2.0 wt. % loading and the toner of Example 3 (Sample 2) which was combined with a positive ferrite carrier in an amount to yield a 2.0 wt. % loading. After 15 minutes the charge on Sample 1 is strongly negative at about -30 microcoulombs/g and is slightly more negative at 60 minutes. In comparison the charge at 15 minutes on Sample 2 was more positive by about 20 microcoulombs/g, and the charge on Sample 4 was more positive by 45 microcoulombs/g. After 60 minutes the charge on Sample 2 (containing unmodified Regal.RTM.330) became more negative by about 10 microcoulombs/g, while the charge on Sample 4 (containing the modified carbon black from Example 2) remains at about the same positive level of +15 microcoulombs/g.

Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.