Toner and developer providing offset lithography print quality

Abstract: A method of forming a toner having controlled properties for use in a developer composition finding particular use in a hybrid scavengeless development scheme includes feeding at least one binder and at least one colorant into a mixing device at a feed ratio, upon exit of the mixture from the mixing device, monitoring one or more properties of the mixture with at least one monitoring device, wherein if the monitoring indicates that the one or more properties being monitored is out of specification, removing the monitored mixture from the method and adjusting the feed ratio by adjusting the feeding of the at least one binder or of the at least one colorant, thereby retaining in-specification mixture in the method, grinding the in-specification mixture, optionally together with a portion of one or more external additives to be added to the mixture, classifying the ground in-specification mixture, and mixing the classified in-specification mixture with one or more external additives to obtain the toner having controlled properties. Use of the toner obtained in a developer in a hybrid scavengeless development apparatus enables images to be achieved with properties similar to that achieved in offset lithography.

Description: The invention relates to methods of forming toners and developers having carefully controlled properties to that provide offset-like print quality when used in developing electrostatic images with a device containing a hybrid scavengeless development system. Historically, xerography has not been required to deliver prints of the same caliber as offset lithography. The offset lithography customer demands a level of print quality much higher than is available from conventional xerographic machines. U.S. Patent No. 5,545,501 describes an electrostatographic developer composition comprising carrier particles and toner particles with a toner particle size distribution having a volume average particle size (T) such that 4 µm ≤ T ≤ 12 µm and an average charge (absolute value) pro diameter in femtocoulomb/10 µm (C T ) after triboelectric contact with said carrier particles such that 1 fC/10 µm ≤ C T ≤ 10 fC/10 µm characterized in that (i) said carrier particles have a saturation magnetization value, M sat , expressed in Tesla (T) such that M sat ≥ 0.30 T, (ii) said carrier particles have a volume average particle size (C avg ) such that 30 µm ≤ C avg ≤ 60 µm, (iii) said volume based particle size distribution of said carrier particles has at least 90% of the particles having a particle diameter C such that 0.5 C avg ≤ C ≤ 2 C avg , (iv) said volume based particles size distribution of said carrier particles comprises less than b % particles smaller than 25 µm wherein b=0.35 X (M sat ) 2 X P with M sat : saturation magnetization value, M sat , expressed in T and P: the maximal field strength of the magnetic developing pole expressed in kA/m, and (v) said carrier particles comprise a core particle coated with a resin coating in an amount (RC) such that 0.2% w/w ≤ RC ≤ 2% w/w. See the Abstract. This patent describes that such developer achieves images of offset-quality in systems in which a latent image is developed with a fine hair magnetic brush. See column 4, lines 7-17. It is the object of the present invention to provide a method of forming a toner having controlled properties. The present invention provides a method of forming a toner having controlled properties for use in a developer composition, the method comprising feeding at least one binder and at least one colorant into a mixing device at a feed ratio to form a mixture, upon exit of the mixture from the mixing device, monitoring the melt rheology of the mixture with an on-line rheometer, wherein, if the monitoring indicates that the melt rheology is out-of-specification, removing the monitored mixture from the method and adjusting the feed ratio by adjusting the feeding of the at least one binder, thereby retaining an in-specification mixture in the method, grinding the in-specfication mixture, optionally together with a portion of one or more external additives to be added to the mixture, classifying the ground in-specfication mixture, and mixing the classified in-specification mixture with one or more external additives to obtain the toner having controlled properties. Preferred embodiments of the invention are set forth in the sub-claims. Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from, for example, a scanning laser beam, an LED source, etc., or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface of the photoreceptor. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. In the present invention, two-component developer materials are used in the first step of the development process. A typical two-component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. Toner particles are attracted to the latent image, forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration. The electrophotographic marking process given above can be modified to produce color images. One type of color electrophotographic marking process, called image-on-image (IOI) processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While the IOI process provides certain benefits, such as a compact architecture, there are several challenges to its successful implementation. For instance, the viability of printing system concepts such as IOI processing requires development systems that do not interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single-component development, interact with the image on the receiver, a previously toned image will be scavenged by subsequent development if interacting development systems are used. Thus, for the IOI process, there is a need for scavengeless or noninteractive development systems. Hybrid scavengeless development (HSD) technology develops toner via a conventional magnetic brush onto the surface of a donor roll. A plurality of electrode wires is closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the wires to generate a toner cloud in the development zone. This donor roll generally consists of a conductive core covered with a thin, for example 50-200 µm, partially conductive layer. The magnetic brush roll is held at an electrical potential difference relative to the donor core to produce the field necessary for toner development. The toner layer on the donor roll is then disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles. Typical AC voltages of the wires relative to the donor are 700-900 Vpp at frequencies of 5-15 kHz. These AC signals are often square waves, rather than pure sinusoidal waves. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image. In the present invention, while any suitable electrostatic image development device may be used, it is most preferred to use a device employing the hybrid scavengeless development system. Such a system is described in, for example, U.S. Patent No. 5,978,633. Satisfaction of stringent offset-like print quality requirements in a xerographic engine has been enabled in the present invention by IOI xerography of which hybrid scavengeless development is a preferred subsystem component. Both the image quality and the unique subsystem requirements result in highly constrained toner designs. This invention describes the aspects of toners that operate in this restrictive atmosphere to produce prints of near offset quality. In addition to achieving offset-like print quality, the digital imaging processes of the above-described device also enables customization of each print (such as an address, or special information for regional distribution), which is not practical with offset lithography. This invention describes a unique combination of toner, toner manufacturing process, developer, and carrier properties to enable a materials set to ideally function in the restrictive atmosphere of the device discussed above. The toners deliver prints that will delight the customer with vivid (high Chroma), reliable color rendition. Color gamut, the maximum set of colors that can be printed, is benchmark for a four-color xerographic system. Solid and halftone areas are uniform and stable in density and color. They are of uniform gloss. Pictorials contain accurate, realistic rendition. Text is crisp with well-defined edges regardless of font size or type. There is no background. Color, solids, halftones, gloss, pictorials, text and background are stable over the entire job run. The prints do not exhibit objectionable paper curl, nor are the images disturbed by handling or storage, for example when stored in contact with vinyl or other document surfaces. To meet these print quality attributes, toner materials must operate in a consistent, predictable manner. As process speed increases, dwell time through the fuser decreases, resulting in lower toner-paper interface temperatures. During the fusing event, it is necessary for toner particles to coalesce, flow and adhere to the substrate (for example, paper, transparency sheets, etc.) at temperatures that are consistent with the device process speeds. It is also necessary for the melt viscosity at the device fusing conditions to provide the required gloss level, while maintaining a high enough elasticity to prevent fuser roll hot-offset (i.e., transfer of toner to the fuser roll). Occurrence of offset results in print defects and a reduction of fuser roll life. Therefore, it is desirable to choose an appropriate toner binder resin and control its melt rheology to provide low minimum fuse temperature, broad fusing latitude and desired gloss at the machine operating conditions. It is further desirable to use an appropriate binder resin such that the toner enables long fuser roll life. The functionality required for the toners is a controlled melt rheology which provides low minimum fuse temperature, broad fusing latitude and desired gloss at the machine operating conditions. The minimum fusing temperature is generally characterized by the minimum fix temperature (MFT) of the fusing subsystem (i.e., the lowest temperature of fusing that the toner will fix to substrate paper well, as determined by creasing a section of the paper with a toned image and quantifying the degree to which the toner in the crease separates from the paper). The fusing latitude is generally determined to be the difference between the hot offset temperature (HOT) (i.e., the highest temperature of fusing that can be conducted without causing toner to offset to the fusing roll, as determined by the presence of previous images printed onto current images or the failure of the paper to release from the fuser roll) and the MFT. The gloss level of the fused toner layer (i.e., the shininess of the fused toner layer at a given fusing temperature as determined by industry standard light reflection measurement) is also dependent on the temperature at which the toner is fused, and can further restrict the fusing latitude; that is, if the gloss level of the toner becomes too high at a temperature below the HOT or too low at a temperature above the MFT this restricted range of temperatures will serve to define the fusing latitude. The melt rheology profile of the toner must be optimized to give the lowest minimum fusing temperature and broadest fusing latitude. The melt rheology profile of the toner which is enabling in the present invention has a viscosity of between 3.9x10 4 and 6.7x10 4 Poise at a temperature of 97°C, a viscosity of between 4.0x10 3 and 1.6x10 4 Poise at a temperature of 116°C, and a viscosity of between 6.1x10 2 and 5.9x10 3 Poise at a temperature of 136°C. The melt rheology profile of the toner which is enabling in the present invention further has an elastic modulus of between 6.6x10 5 and 2.4x10 6 dynes per square centimeter at a temperature of 97°C, an elastic modulus of between 2.6x10 4 and 5.9x10 5 dynes per square centimeter at a temperature of 116°C, and an elastic modulus of between 2.7x10 3 and 3.0x10 5 dynes per square centimeter at a temperature of 136°C. Both the viscosity and elastic modulus are determined by measurement using a standard mechanical spectrometer at 40 radians per second. An alternate method of characterizing the toner rheology is by measurement of the melt flow index (MFI), defined as the weight of a toner (in grams) which passes through an orifice of length L and diameter D in a 10 minute period with a specified applied load. The melt rheology profile of the toner which is enabling in the present invention has an MFI of between 1 and 25 grams per 10 minutes, most preferably between 6 and 14 grams per 10 minutes at a temperature of 117°C, under an applied load of 2.16 kilograms with an L/D die ratio of 3.8. This narrow range of melt rheology profile will provide the required minimum fix, appropriate gloss and the desired hot offset behavior, enabling long fuser roll life. The print quality requirements for the HSD product translate into toner functional properties, as discussed above. By this invention, functionality is designed into the toners with the goal of meeting the many print quality requirements. Four different color toners, cyan (C), magenta (M), yellow (Y) and black (K), are typically used in developing full color images (although other color toners may also be used). Each of theses color toners used in the present invention are preferably comprised of resin binder, appropriate colorants and an additive package comprised of one or more additives. Suitable and preferred materials for use in preparing toners that possess the properties discussed above will now be discussed. The specific formulations used to achieve the functional properties discussed above should not, however, be viewed as restricting the scope of the invention. Illustrative examples of suitable toner resins selected for the toner and developer compositions include vinyl polymers such as styrene polymers, acrylonitrile polymers, vinyl ether polymers, acrylate and methacrylate polymers; epoxy polymers; diolefins; polyurethanes; polyamides and polyimides; polyesters such as the polymeric esterification products of a dicarboxylic acid and a diol comprising a diphenol, crosslinked polyesters; and the like. The polymer resins selected for the toner compositions include homopolymers or copolymers of two or more monomers. Furthermore, the above-mentioned polymer resins may also be crosslinked. Illustrative vinyl monomer units in the vinyl polymers include styrene, substituted styrenes such as methyl styrene, chlorostyrene, styrene acrylates and styrene methacrylates; vinyl esters like the esters of monocarboxylic acids including methyl acrylate, ethyl acrylate, n-butyl-acrylate, isobutyl acrylate, propyl acrylate, pentyl acrylate, dodecyl acrylate, n-octyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, methylalphachloracrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, and pentyl methacrylate; styrene butadienes; vinyl chloride; acrylonitrile; acrylamide; alkyl vinyl ether and the like. Further examples include p-chlorostyrene vinyl naphthalene, unsaturated mono-olefins such as ethylene, propylene, butylene and isobutylene; vinyl halides such as vinyl chloride, vinyl bromide, vinyl fluoride, vinyl acetate, vinyl propionate, vinyl benzoate, and vinyl butyrate; acrylonitrile, methacrylonitrile, acrylamide, vinyl ethers, inclusive of vinyl methyl ether, vinyl isobutyl ether, and vinyl ethyl ether; vinyl ketones inclusive of vinyl methyl ketone, vinyl hexyl ketone and methyl isopropenyl ketone; vinylidene halides such as vinylidene chloride and vinylidene chlorofluaride; N-vinyl indole, N-vinyl pyrrolidone; and the like Illustrative examples of the dicarboxylic acid units in the polyester resins suitable for use in the toner compositions include phthalic acid, terephthalic acid, isophthalic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid, dimethyl glutaric acid, bromoadipic acids, dichloroglutaric acids, and the like; while illustrative examples of the diol units in the polyester resins include ethanediol, propanediols, butanediols, pentanediols, pinacol, cyclopentanediols, hydrobenzoin, bis(hydroxyphenyl)alkanes, dihydroxybiphenyl, substituted dihydroxybiphenyls, and the like. As one toner resin, there are selected polyester resins derived from a dicarboxylic acid and a diphenol. These resins are illustrated in U.S. Pat. No. 3,590,000. Also, polyester resins obtained from the reaction of bisphenol A and propylene oxide, and in particular including such polyesters followed by the reaction of the resulting product with fumaric acid, and branched polyester resins resulting from the reaction of dimethylterephthalate with 1,3-butanediol, 1,2-propanediol, and pentaerythritol may also preferable be used. Further, low melting polyesters, especially those prepared by reactive extrusion, reference U.S. Patent No. 5,227,460, can be selected as toner resins. Other specific toner resins may include styrene-methacrylate copolymers, styrenebutadiene copolymers, PLIOLITES™, and suspension polymerized styrenebutadienes (U.S. Patent No. 4,558,108). More preferred resin binders for use in the present invention comprise polyester resins containing both linear portions and cross-linked portions of the type described in U.S. Patent No. 5,227,460. The cross-linked portion of the binder consists essentially of microgel particles with an average volume particle diameter up to 0.1 µm (micron), preferably 0.005 to 0.1 µm (micron), as determined by scanning electron microscopy and transmission electron microscopy, the microgel particles being substantially uniformly distributed throughout the linear portions. This resin may be prepared by a reactive melt mixing process as known in the art. The highly cross-linked dense microgel particles distributed throughout the linear portion impart elasticity to the resin, which improves the resin offset properties, while not substantially affecting the resin minimum fix temperature. The toner resin is thus preferably a partially cross-linked unsaturated resin such as unsaturated polyester prepared by cross-linking a linear unsaturated resin (hereinafter called base resin) such as linear unsaturated polyester resin, preferably with a chemical initiator, in a melt mixing device such as, for example, an extruder at high temperature (e.g., above the melting temperature of the resin and preferably up to 150°C above that melting temperature) and under high shear. The toner resin has a weight fraction of the microgel (gel content) in the resin mixture in the range typically from 0.001 to 50 weight percent, preferably from 1 to 20 weight percent, more preferably 1 to 10 weight percent, most preferably 2 to 9 weight percent. The linear portion is comprised of base resin, preferably unsaturated polyester, in the range from 50 to 99.999 percent by weight of said toner resin, and preferably in the range from 80 to 98 percent by weight of said toner resin. The linear portion of the resin preferably comprises low molecular weight reactive base resin that did not cross-link during the cross-linking reaction, preferably unsaturated polyester resin. The molecular weight distribution of the resin is thus bimodal, having different ranges for the linear and the cross-linked portions of the binder. The number-average molecular weight (Mn) of the linear portion as measured by gel permeation chromatography (GPC) is in the range of from, for example, 1,000 to 20,000, and preferably from 3,000 to 8,000. The weight-average molecular weight (Mw) of the linear portion is in the range of from, for example, 2,000 to 40,000, and preferably from 5,000 to 20,000. The weight average molecular weight of the gel portions is, on the other hand, generally greater than 1,000,000. The molecular weight distribution (Mw/Mn) of the linear portion is in the range of from, for example, 1.5 to 6, and preferably from 1.8 to 4. The onset glass transition temperature (Tg) of the linear portion as measured by differential scanning calorimetry (DSC) is in the range of from, for example, 50°C to 70°C. This binder resin can provide a low melt toner with a minimum fix temperature of from 100°C to 200°C, preferably 100°C to 160°C, more preferably 110°C to 140°C, provide the low melt toner with a wide fusing latitude to minimize or prevent offset of the toner onto the fuser roll, and maintain high toner pulverization efficiencies. The toner resins and thus toners show minimized or substantially no vinyl or document offset. In a preferred embodiment, the cross-linked portion consists essentially of very high molecular weight microgel particles with high density cross-linking (as measured by gel content) and which are not soluble in substantially any solvents such as, for example, tetrahydrofuran, toluene and the like. The microgel particles are highly cross-linked polymers with a very small, if any, cross-link distance. This type of cross-linked polymer may be formed by reacting chemical initiator with linear unsaturated polymer, and more preferably linear unsaturated polyester, at high temperature and under high shear. The initiator molecule breaks into radicals and reacts with one or more double bond or other reactive site within the polymer chain forming a polymer radical. This polymer radical reacts with other polymer chains or polymer radicals many times, forming a highly and directly cross-linked microgel. This renders the microgel very dense and results in the microgel not swelling very well in solvent. The dense microgel also imparts elasticity to the resin and increases its hot offset temperature while not affecting its minimum fix temperature. Linear unsaturated polyesters used as the base resin are low molecular weight condensation polymers which may be formed by the step-wise reactions between both saturated and unsaturated diacids (or anhydrides) and dihydric alcohols (glycols or diols). The resulting unsaturated polyesters are reactive (e.g., cross-linkable) on two fronts: (i) unsaturation sites (double bonds) along the polyester chain, and (ii) functional groups such as carboxyl, hydroxy, etc., groups amenable to acid-base reactions. Typical unsaturated polyester base resins useful for this invention are prepared by melt polycondensation or other polymerization processes using diacids and/or anhydrides and diols. Suitable diacids and dianhydrides include saturated diacids and/or anhydrides such as for example succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, isophthalic acid, terephthalic acid, hexachloroendo methylene tetrahydrophthalic acid, phthalic anhydride, chlorendic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, endomethylene tetrahydrophthalic anhydride, tetrachlorophthalic anhydride, tetrabromophthalic anhydride, and the like and mixtures thereof; and unsaturated diacids and/or anhydrides such as for example maleic acid, fumaric acid, chloromaleic acid, methacrylic acid, acrylic acid, itaconic acid, citraconic acid, mesaconic acid, maleic anhydride, and the like and mixtures thereof. Suitable diols include for example propylene glycol, ethylene glycol, diethylene glycol, neopentyl glycol, dipropylene glycol, dibromoneopentyl glycol, propoxylated bisphenol A, 2,2,4-trimethylpentane-1,3-diol, tetrabromo bisphenol dipropoxy ether, 1,4-butanediol, and the like and mixtures thereof, soluble in good solvents such as, for example, tetrahydrofuran, toluene and the like. Preferred unsaturated polyester base resins are prepared from diacids and/or anhydrides such as, for example, maleic anhydride, fumaric acid, and the like and mixtures thereof, and diols such as, for example, propoxylated bisphenol A, propylene glycol, and the like and mixtures thereof. A particularly preferred polyester is poly(propoxylated bisphenol A fumarate). In a most preferred embodiment of the present invention, the toner binder resin comprises a melt extrusion of (a) linear propoxylated bisphenol A fumarate resin and (b) this resin cross-linked by reactive extrusion of this linear resin, with the resulting extrudate comprising a resin with an overall gel content of from 2 to 9 weight percent. Linear propoxylated bisphenol A fumarate resin is available under the tradename SPARII from Resana S/A Industrias Quimicas, Sao Paulo Brazil, or as Neoxyl P2294 or P2297 from DSM Polymer, Geleen, The Netherlands, for example. For suitable toner storage and prevention of vinyl and document offset, the polyester resin blend preferably has Tg range of from, for example, 52 to 64°C. Using resin having only the linear portion of the propoxylated bisphenol A fumarate resin does not attain the needed melt rheology profile. Chemical initiators such as, for example, organic peroxides or azo-compounds are preferred for making the cross-linked toner resins. By permitting use of low concentrations of chemical initiator and utilizing all of it in the cross-linking reaction, usually in the range from 0.01 to 1.0 weight percent, and preferably in the range from 0.1 to 4 weight percent, the residual contaminants produced in the cross-linking reaction in preferred embodiments can be minimal. Since the cross-linking can be carried out at high temperature, the reaction is very fast (e.g., less than 10 minutes, preferably 2 seconds to 5 minutes) and thus little or no unreacted initiator remains in the product. The low melt toners and toner resins may be prepared by a reactive melt mixing process wherein reactive resins are partially cross-linked. For example, low melt toner resins may be fabricated by a reactive melt mixing process comprising the steps of: (1) melting reactive base resin, thereby forming a polymer melt, in a melt mixing device; (2) initiating cross-linking of the polymer melt, preferably with a chemical cross-linking initiator and increased reaction temperature; (3) keeping the polymer melt in the melt mixing device for a sufficient residence time that partial cross-linking of the base resin may be achieved; (4) providing sufficiently high shear during the cross-linking reaction to keep the gel particles formed and broken down during shearing and mixing and well distributed in the polymer melt; (5) optionally devolatilizing the polymer melt to remove any effluent volatiles; and (6) optionally adding additional linear base resin after the cross-linking in order to achieve the desired level of gel content in the end resin. The high temperature reactive melt mixing process allows for very fast cross-linking which enables the production of substantially only microgel particles, and the high shear of the process prevents undue growth of the microgels and enables the microgel particles to be uniformly distributed in the resin. A reactive melt mixing process is a process wherein chemical reactions can be carried out on the polymer in the melt phase in a melt mixing device, such as an extruder. In preparing the toner resins, these reactions are used to modify the chemical structure and the molecular weight, and thus the melt rheology and fusing properties, of the polymer. Reactive melt mixing is particularly efficient for highly viscous materials, and is advantageous because it requires no solvents, and thus is easily environmentally controlled. As soon as the amount of cross-linking desired is achieved, the reaction products can be quickly removed from the reaction chamber. The resins are generally present in the toner in an amount of from 40 to 98 percent by weight, and more preferably from 70 to 98 percent by weight, although they may be present in greater or lesser amounts, provided that the objectives of the invention are achieved. The toner resins can be subsequently melt blended or otherwise mixed with a colorant, charge carrier additives, surfactants, emulsifiers, pigment dispersants, flow additives, embrittling agents, and the like. The resultant product can then be pulverized by known methods such as milling to form toner particles. If desired, waxes with a molecular weight of from 1,000 to 7,000, such as polyethylene, polypropylene, and paraffin waxes, can be included in, or on the toner compositions as fusing release agents. Various suitable colorants of any color without restriction can be employed in toners, including suitable colored pigments, dyes, and mixtures thereof. The colorant, preferably black, cyan, magenta and/or yellow colorant, is incorporated in an amount sufficient to impart the desired color to the toner. In general, pigment or dye is employed in an amount ranging from 2 to 60 percent by weight, and preferably from 2 to 9 percent by weight for color toner and 3 to 60 percent by weight for black toner. Any suitable surface additives may be used in the present invention. Most preferred in the present invention are one or more of SiO 2 , metal oxides such as, for example, TiO 2 and aluminum oxide, and a lubricating agent such as, for example, a metal salt of a fatty acid (e.g., zinc stearate (ZnSt), calcium stearate) or long chain alcohols such as Unilin 700, as external surface additives. In general, silica is applied to the toner surface for toner flow, tribo enhancement, admix control, improved development and transfer stability and higher toner blocking temperature. TiO 2 is applied for improved relative humidity (RH) stability, tribo control and improved development and transfer stability. The most preferred SiO 2 and TiO 2 have been surface treated with compounds including DTMS (dodecyltrimethoxysilane) or HMDS (hexamethyldisilazane). Examples of these additives are: NA50HS silica, obtained from DeGussa/Nippon Aerosil Corporation, coated with a mixture of HMDS and aminopropyltriethoxysilane; DTMS silica, obtained from Cabot Corporation, comprised of a fumed silica, for example silicon dioxide core L90 coated with DTMS; H2050EP, obtained from Wacker Chemie, coated with an amino functionalized organopolysiloxane; and SMT5103, obtained from Tayca Corporation, comprised of a crystalline titanium dioxide core MT500B, coated with DTMS. Zinc stearate is preferably also used as an external additive for the toners, the zinc stearate providing lubricating properties. Zinc stearate provides developer conductivity and tribo enhancement, both due to its lubricating nature. In addition, zinc stearate enables higher toner charge and charge stability by increasing the number of contacts between toner and carrier particles. Calcium stearate and magnesium stearate provide similar functions. Most preferred is a commercially available zinc stearate known as Zinc Stearate L, obtained from Ferro Corporation, which has an average particle diameter of about 9 µm (microns), as measured in a Coulter counter. Most preferably, the toners contain from, for example, 0.1 to 5 weight percent titania, 0.1 to 8 weight percent silica and 0.1 to 4 weight percent zinc stearate. The additives discussed above are chosen to enable superior toner flow properties, as well as high toner charge and charge stability. The surface treatments on the SiO 2 and TiO 2 , as well as the relative amounts of the two additives, can be manipulated to provide a range of toner charge. For further enhancing the positive charging characteristics of the developer compositions described herein, and as optional components there can be incorporated into the toner or on its surface charge enhancing additives inclusive of alkyl pyridinium halides, reference U.S. Patent No. 4,298,672; organic sulfate or sulfonate compositions, reference U.S. Patent No. 4,338,390; distearyl dimethyl ammonium sulfate; bisulfates, and the like and other similar known charge enhancing additives. Also, negative charge enhancing additives may also be selected, such as aluminum complexes, like BONTRON E-88, and the like. These additives may be incorporated into the toner in an amount of from 0.1 percent by weight to 20 percent by weight, and preferably from 1 to 3 percent by weight. The toner composition can be prepared by a number of known methods including melt blending the toner resin particles, and pigment particles or colorants followed by mechanical attrition. Other methods include those well known in the art such as spray drying, melt dispersion, dispersion polymerization, suspension polymerization, and extrusion. The toner is preferably made by first mixing the binder, preferably comprised of both the linear resin and the cross-linked resin as discussed above, and the colorant together in a mixing device, preferably an extruder, and then extruding the mixture. The extruded mixture is then preferably micronized in a grinder along with 0.3 to 0.5 weight percent of the total amount of silica to be used as an external additive. The toner is then classified to form a toner with the desired volume median particle size and percent fines as discussed above. Care should also be taken in the method in order to limit the coarse particles, grits and giant particles. Subsequent toner blending of the remaining external additives is preferably accomplished using a mixer or blender, for example a Henschel mixer, followed by screening to obtain the final toner product. The process is carefully controlled and monitored in order to consistently achieve toners having the necessary properties discussed above. First, the ingredients are fed into the extruder in a closed loop system from hoppers containing, respectively, the linear resin, the cross-linked resin, the predispersed pigment (i.e., the pigment dispersed in a portion of binder such as linear propoxylated bisphenol A fumarate and is as discussed above) and reclaimed toner fines. Reclaimed toner fines are those toner particles that have been removed from previously made toner during classification as being too small. As this can be a large percentage of material, it is most preferred to recycle this material back into the method as reclaimed toner fines. This material thus already contains the resins and the colorant, as well as any additives introduced into the toner at the extrusion, grinding, or classification processes. It may comprise anywhere from 5 to 50% by weight of the total material added into the extruder. As the extrudate passes through the die, it is monitored with an on-line rheometer that can provide feedback signals to control the amounts of the individual materials added into the extruder so as to carefully control the composition and properties of the toner, and thus ensure that a consistent product is obtained. This is quite significant in the present invention, where tight toner functional properties are required as discussed above. Most preferably, the extrudate is monitored with both an on-line rheometer and a near IR spectrophotometer as the monitoring devices. The on-line rheometer evaluates the melt rheology of the product extrudate and provides a feedback signal to control the amount of linear and cross-linked resin being dispensed. For example, if the melt rheology is too high, the signal indicates that the amount of linear resin added relative to the cross-linked resin should be increased. This monitoring provides control of the toner melt rheology, one of the properties that must be met in order for the performance in an HSD device to be maximized as discussed above. The near IR spectrophotometer, used in transmission mode, can distinguish between the colors as well as monitor colorant concentration. The spectrophotometer can be used to generate a signal to appropriately adjust the amount of colorant added into the extruder. This monitoring provides control over the amount of pigmentation and thereby enables the functionality of toner chroma and can also identify color cross-contamination. By this monitoring, any out-of-specification product can be intercepted at the point of monitoring and purged from the line while in-specification product can continue downstream to the grinding and classification equipment. In grinding, the addition of a portion of the total amount of silica to be added facilitates the grind and class operations. Specifically, injection into the grinder of between 0.1 and 1.0% of an silica or metal oxide flow aid decreases the level of variability in the output of the grinding operation, allowing better control of the grinding process and allowing it to operate at an optimized level. Additionally, this process enhances the jetting rate of the toner by between 10 and 20 percent. When the toner which is ground in this manner is classified to remove the fine portion of the toner particles, the classification yield and throughput rate are improved which helps control costs during the classification step where very tight control over particle size and distribution must be maintained for the toner to achieve the properties discussed above. Classified toner product is then blended with the external surface additives in a manner to enable even distribution and firm attachment of the surface additives, for example by using a high intensity blender. The blended toner achieved has the appropriate level and stability of toner flow and triboelectric properties. The resulting toner particles can then be formulated into a developer composition. Preferably, the toner particles are mixed with carrier particles to achieve a two-component developer composition. It is desirable to design toner and developer materials to have an average toner charge level that avoids failure modes of both too high and too low toner charge. This will preserve development of solids, halftones, fine lines and text, as well as prevention of background and image contamination. The distribution of developer and toner charge level must be sufficiently narrow such that the tails of the distribution do not adversely affect image quality (i.e., the low charge population is not of sufficient magnitude so as to degrade the image quality attributes known to be related to low toner charge level). Developer and toner charge level and distribution must be maintained over the full range of customer run modes (job run length and AC). As in the case of toner charge, the charge of a toner in the developer is described in terms of either the charge to particle mass, Q/M, in µC/g, or the charge/particle diameter, Q/D, in fC/µm following triboelectric contact of the toner with carrier particles. The measurement of Q/M is accomplished by the well-known Faraday Cage technique. The measurement of the average Q/D of the toner particles, as well as the full distribution of Q/D values, can be done by means of a charge spectrograph apparatus as well known in the art. In order to attain the print quality discussed above when used in an HSD developer apparatus, the Q/D of the toner particles in the developer must have an average value of from, for example, -0.1 to -1.0 fC/µm, preferably from -0.5 to -1.0 fC/µm. This charge must remain stable throughout the development process in order to insure consistency in the richness of the images obtained using the toner. Thus, the toner charge should exhibit a change in the average Q/D value of at most from, for example, 0 to 0.25 fC/µm. The charge distribution of the toner in the developer, as measured by a charge spectrograph, should be narrow, that is possessing a peak width of less than 0.5 fC/µm, preferably less than 0.3 fC/µm, and unimodal, that is, possessing only a single peak in the frequency distribution, indicating the presence of no or very little low charge toner (too little charge for a sufficiently strong coulomb attraction) and wrong sign toner. Low charge toner should comprise no more than, for example, no more than 15% of the total number of toner particles, preferably no more than 6% of the total toner, more preferably no more than 2%, while wrong sign toner should comprise no more than, for example, 5% of the total number of toner particles, preferably no more than 3% of the total toner, more preferably no more than 1%. Using the complementary well known Faraday cage measurement, the toner in the developer must also preferably exhibit a triboelectric value of from, for example, -25 to -70 µC/g, more preferably -35 to -60 µC/g. The tribo must be stable, varying at most from, for example, 0 to 15 µC/g, preferably from no more than 0 to 8 µC/g, during development with the toner, for example during development in an HSD system. The carrier core and coating, as well as the toner additives discussed above, are all chosen to enable high developer charge and charge stability. The processing conditions of the carrier, as well as the levels of toner additives selected, can be manipulated to affect the developer charging level. A hybrid scavengeless development system uses a magnetic brush of a conventional two component system in conjunction with a donor roll used in typical single component systems to transfer toner from the magnetic brush to the photoreceptor surface. As a result, the donor roll must be completely reloaded with toner in just one revolution. The inability to complete reloading of the donor roll in one revolution will result in a print quality defect called reload. This defect is seen on prints as solid areas that become lighter with successive revolutions of the donor roll, or alternately if the structure of an image from one revolution of the donor roll is visible in the image printed by the donor roll on its next revolution, a phenomenon known as ghosting in the art related to single component xerographic development. Highly conductive developers aid in the reduction of this defect. The more conductive developers allow for the maximum transfer of toner from the magnetic brush to the donor roll. Therefore, it is desirable to select developer materials which when combined, are conductive enough to reload the donor roll in a single revolution. The conductivity of the developer is primarily driven by the carrier conductivity. To achieve the most conductive carrier possible, electrically conductive carrier cores, for example atomized steel cores, with partial coatings of electrically insulating polymers to allow a level of exposed carrier core, are used. An alternative technology of using conductive polymers to coat the carrier core is also feasible. Additionally, irregularly shaped carrier cores provide valleys into which the polymer coating may flow, leaving exposed asperities for more conductive developers. Irregularly shaped carrier cores also function to allow toner particles to contact the surface of the carrier core in the valleys to provide charge to the toner while not interfering with the contact between the uncoated carrier asperities which provides the overall developer conductivity. The addition of zinc stearate to the toner additive package also assists in the lubrication of the carrier and toner, increasing the number of contacts between carrier and toner particles. Preferably, the conductivity of the developer ranges from, for example, between 10 -11 and 10 -14 (ohm-cm) -1 , at a toner concentration of between 3.5 and 5.5 percent by weight as measured, for example, across a 0.1 inch magnetic brush at an applied potential of 30 volts. At a toner concentration of between 0 and 0.5 percent, that is bare carrier or carrier that has only a small amount of residual toner on the surface, the carrier has a conductivity of between 10 -8 and 10 -12 (ohm-cm) -1 as measured under the same conditions. The requirement of the toner concentration level is determined by the requirements of machine set-up. It is therefore critical to be able to blend a developer that will meet the required toner concentration, and control, the concentration of toner to the desired level. Preferably, the toner concentration ranges from, for example, 1 to 6%, more preferably 3.5 to 5.5%, by weight of the total weight of the developer. The toners must have the appropriate color characteristics to enable broad color gamut. The choice of colorants enable the rendition of a higher percentage of standard Pantone® colors than is typically available from four-color xerography. For each toner, chroma (C*) must be maximized, and it is very important to have the color remain accurate relative to the requested color. Materials in the developer housing can cause the color of the toner to shift as a function of developer age, print area coverage, or other machine operating conditions, which is measured via the difference between the target color and the actual color, specifically as ΔE CMC , (where CMC stands for the Color Measurement Committee of the Society of Dyers and Colorists) which calculates the color change in the three dimensional L*, a*, b* CIELAB space defined in section D. The carrier may contribute to the variation in color, or chroma shift, but may only cause a shift of about ±1/3 ΔE CMC units. Therefore, it is critical to select carrier cores and carrier core coatings that do not contribute to chroma shift of the toner as a function of the state of the developer. Carrier core and coating polymers must be chosen such that they are lightly colored or colorless and are mechanically robust to the wear experienced in the developer housing. This will prevent a change in ΔE CMC performance should the carrier coating become abraded. The coating polymer and core should also be robust to mechanical wear that will be experienced in the developer housing. Robustness of the coating polymer would allow the use of darker colored additives to be utilized in the carrier coating without the risk of chroma shift. Preferably, the ΔE CMC exhibited over all machine and developer running conditions in all customer environments using the developer and toner ranges from at most, for example, 0 to 0.60, more preferably from at most, for example, 0 to 0.30. Given the small toner size discussed above, it is desirable to also move to a smaller carrier size in order to maintain a ratio of carrier volume median diameter to toner volume median diameter of about 10:1, with the toner volume median as determined by the well known Coulter counter technique and the carrier volume median diameter as determined by well known laser diffraction techniques. This ratio enables a TC 0 on the order of 1. This TC 0 of 1 translates into a greater tribo sensitivity to toner concentration. This therefore allows the machine control system to use toner concentration as a tuning knob for tribo in the housing. It is also important to maintain a low level of fines in the carrier in order to prevent bead carry-out onto the prints, which generally leads to a print quality defect known as debris-centered deletions (DCDs). Therefore, it is desirable to control the carrier particle size and limit the amount of fine carrier particles. Given the small toner size discussed above, it is desirable to also move to a smaller size carrier size in order to maintain a ratio of carrier volume median diameter to toner volume median diameter of approximately 10:1. The carrier particles thus should have an average particle size (diameter) of from, for example, 65 to 90 µm (microns), preferably from 70 to 84 µm (microns). The fine side of the carrier distribution is well controlled with only about 2.0% of the weight distribution having a size less than 38 µm (microns). In addition, the developer should exhibit consistent and stable developability, for example exhibiting a stable developed toner mass per unit area (DMA) on the photoreceptor, with a target in the range of between, 0.4 to 1.0 mg/cm 2 , as measured directly by removal of the toner in given area from the photoreceptor and subsequent weighing or as determined indirectly by a calibrated reflectance measurement from the photoreceptor, at the operational voltages of the development device (for example, at a wire voltage of 200 V in an HSD development device), and a variation of the DMA from the target value of at most 0.4 mg/cm 2 , most preferably of at most 0.2 mg/cm 2 . The developer must also exhibit high transfer efficiency to the image receiving substrate with very low residual toner left on the photoreceptor surface following transfer. The print quality requirements for the HSD product translate into developer functional properties, as discussed above. By this invention, functionality is designed into the toners and developers with the goal of meeting the many print quality requirements. Suitable and preferred materials for use as carriers used in preparing developers containing the above-discussed toners that possess the properties discussed above will now be discussed. Illustrative examples of carrier particles that can be selected for mixing with the toner composition prepared in accordance with the present invention include those particles that are capable of triboelectrically obtaining a charge of opposite polarity to that of the toner particles. Illustrative examples of suitable carrier particles include granular zircon, granular silicon, glass, steel, nickel, ferrites, iron ferrites, silicon dioxide, and the like. Additionally, there can be selected as carrier particles nickel berry carriers as disclosed in U.S. Patent No. 3,847,604, comprised of nodular carrier beads of nickel, characterized by surfaces of reoccurring recesses and protrusions thereby providing particles with a relatively large external area. Other carriers are disclosed in U.S. Patents Nos. 4,937,166 and 4,935,326. In a most preferred embodiment, the carrier core is comprised of atomized steel available commercially from, for example, Hoeganaes Corporation. The selected carrier particles can be used with or without a coating, the coating generally being comprised of fluoropolymers, such as polyvinylidene fluoride resins, terpolymers of styrene, methyl methacrylate, a silane, such as triethoxy silane, tetrafluorethylenes, other known coatings and the like. In a most preferred embodiment, the carrier core is partially coated with a polymethyl methacrylate (PMMA) polymer having a weight average molecular weight of 300,000 to 350,000 commercially available from Soken. The PMMA is an electropositive polymer in that the polymer that will generally impart a negative charge on the toner with which it is contacted. The PMMA may optionally be copolymerized with any desired comonomer, so long as the resulting copolymer retains a suitable particle size. Suitable comonomers can include monoalkyl, or dialkyl amines, such as a dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, diisopropylaminoethyl methacrylate, or t-butylaminoethyl methacrylate, and the like. The carrier particles may be prepared by mixing the carrier core with from, for example, between 0.05 to 10 percent by weight, more preferably between 0.05 percent and 3 percent by weight, based on the weight of the coated carrier particles, of polymer in until adherence thereof to the carrier core by mechanical impaction and/or electrostatic attraction. The polymer is most preferably applied in dry powder form and having an average particle size of less than 1 micrometer, preferably less than 0.5 micrometers. Various effective suitable means can be used to apply the polymer to the surface of the carrier core particles. Examples of typical means for this purpose include combining the carrier core material and the polymer by cascade roll mixing, or tumbling, milling, shaking, electrostatic powder cloud spraying, fluidized bed, electrostatic disc processing, and with an electrostatic curtain. The mixture of carrier core particles and polymer is then heated to a temperature below the decomposition temperature of the polymer coating. For example, the mixture is heated to a temperature of from 90°C to 350°C, for a period of time of from, for example, 10 minutes to 60 minutes, enabling the polymer to melt and fuse to the carrier core particles. The coated carrier particles are then cooled and thereafter classified to a desired particle size. The coating preferably has a coating weight of from, for example, 0.1 to 3.0% by weight of the carrier, preferably 0.5 to 1.3% by weight. In a further most preferred embodiment, the polymer coating of the carrier core is comprised of PMMA, most preferably PMMA applied in dry powder form and having an average particle size of less than 1 micrometer, preferably less than 0.5 micrometers, that is applied (melted and fused) to the carrier core at higher temperatures on the order of 220°C to 260°C. Temperatures above 260°C may adversely degrade the PMMA. Triboelectric tunability of the carrier and developers is provided by the temperature at which the carrier coating is applied, higher temperatures resulting in higher tribo up to a point beyond which increasing temperature acts to degrade the polymer coating and thus lower tribo. With higher tribo, longer development life and improvement in fringe field development is expected. As discussed above, it is desirable to maintain a ratio of carrier volume median diameter to toner volume median diameter of approximately 10:1. The carrier particles thus should have an average particle size (volume median diameter) of from, for example, 65 to 90 µm (microns), preferably from 70 to 89 µm (microns), most preferably from 75 to 85 µm (microns). The size distribution of the carrier particles is further defined such that no more than 10 percent of the carrier particles by weight should have a diameter of less than 50 µm (microns) and no more than 10 percent of the carrier particles by weight should have a diameter of greater than 120 µm (microns). The fine side of the carrier distribution is well controlled with only about 2.0% of the weight distribution having a size less than 38 µm (microns), preferably only 1.0% of the weight distribution having a size less than 38 µm (microns). The carrier particles can be mixed with the toner particles in various suitable combinations. However, best results are obtained when 1 part to 5 parts by weight of toner particles are mixed with from 10 to 300 parts by weight of the carrier particles, preferably when 3.4 to 5.3 parts by weight of toner particles are mixed with from 90 to 110 parts by weight of the carrier particles. The toner concentration in the developer composition is thus preferably between 3.0 and 5.5% by weight. In a still further preferred embodiment, it has been found that using a carrier core having a shape factor greater than 6 is preferred. The shape factor as used herein is defined as the ratio of BET surface area to the equivalent sphere surface area (ESSA) calculated using the volume median diameter, as measured above by standard laser diffraction techniques, of the core particle. It represents a measure of the surface morphology of the carrier core. It has been found that carrier conductivity is driven strongly by the core BET surface area, while the triboelectric properties are not strongly affected by the BET surface area. It is useful to express the surface characteristics of a carrier core not by BET surface area alone, which is specific to a particular core size and density, but by a shape factor which is calculated by dividing the BET surface area by the theoretical surface area of a carrier core assuming a smooth spherical surface. The theoretical surface area, also referred to as the equivalent sphere surface area (ESSA), calculated using the volume median diameter of the core particle is given by ESSA = surface area of bead/(volume of bead x density of bead) = 4πr 2 /((4π/3)r 3 x d) = 3/rd where r is the radius of the core based on laser diffraction measurement, using for instance a Mastersizer X, available from Malvern Instruments Ltd. and d is the density of the core. For the preferred atomized steel, the density is 7 g/cm 3 . Thus, for a carrier core having a size of, for example, 77 µm (microns), the ESSA is 55.7 cm 2 /g, derived from (3/(77 x 10 -4 µm x 7 g/cm 3 )). The core shape factor is a unitless number since it is the core BET surface area divided by the ESSA. As the core shape factor increases, the surface morphology of the core becomes more irregular. It is most preferred to use a carrier core having a shape factor of greater than 6.0, preferably greater than 6.8, and most preferably of 7.0 or more. Cores with such shape factor have not only excellent conductivity (for example, above 10 -12 mho/cm), but also superior tribo. The most preferred atomized steel available commercially from Hoeganaes Corporation has a shape factor of 7.9. Related to the shape factor of the core, it has been found that using a carrier core having an oxide level less than 0.24 percent, most preferably less than 0.15 percent by weight of the core, is preferred. In combination with a shape factor of greater than 7.0, carrier cores with oxide levels less than 0.15 percent by weight yield carriers which have not only excellent conductivity (for example, above 10 -10 mho/cm), but also superior tribo.

Claims: A method of forming a toner having controlled properties for use in a developer composition, the method comprising feeding at least one binder and at least one colorant into a mixing device at a feed ratio to form a mixture, upon exit of the mixture from the mixing device, monitoring the melt rheology of the mixture with an on-line rheometer, wherein, if the monitoring indicates that the melt rheology is out-of-specification, removing the monitored mixture from the method and adjusting the feed ratio by adjusting the feeding of the at least one binder, thereby retaining an in-specification mixture in the method, grinding the in-specification mixture, optionally together with a portion of one or more external additives to be added to the mixture, classifying the ground in-specification mixture, and mixing the classified in-specification mixture with one or more external additives to obtain the toner having controlled properties. The method of claim 1, wherein the mixing device comprises an extruder. The method of claim 1 or 2, wherein the on-line rheometer provides a feedback signal to control the feeding of the at least one binder. The method of any of claims 1 to 3, wherein the feeding further comprises feeding reclaimed toner fines into the mixing device. The method of any of claims 1 to 4, wherein the at least one binder comprises both a linear propoxylated bisphenol A fumarate and a cross-linked propoxylated bisphenol A fumarate which are fed into the mixing device from separate containers. The method of any of claims 1 to 5, wherein the grinding of the in-specification mixture is conducted along with 0.2 to 1.0 weight percent of a total amount of silicon dioxide or metal oxide to be added as an external additive. The method of any of claims 1 to 6, wherein the toner achieved has a charge per particle diameter (Q/D) of from -0.1 to -1.0 fC/µm with a variation during development of from 0 to 0.25 fC/µm and the distribution is unimodal and possesses a peak width of less than 0.5 fC/µm, and a triboelectric charge of from -25 to -70 µC/g with a variation during development of from 0 to 15 µC/g, an average particle diameter of from 6.9 to 7.9 µm (microns) and a size distribution such that 30% or less of the total number of toner particles have a size less than 5 µm (microns) and 0.7% or less of a total volume of toner particles have a size greater than 12.70 µm (microns), and a toner melt flow index (MFI) ranges from 1 to 25 grams per 10 minutes, most preferably from 6 to 14 grams per 10 minutes at a temperature of 117°C. The method of any of claims 1 to 7, wherein the method further comprises mixing the toner with carrier particles to obtain a two-component developer.

Inventors: A MORALES-TIRADO JUAN, CHANG HUI, HOLLENBAUGH WILLIAM JR, KUMAR SAMIR, LAFICA SUSAN J, LEUTE GERARDO, LI DONMING, SILENCE SCOTT M, STAMP AMY, A MORALES-TIRADO, JUAN, CHANG, HUI, HOLLENBAUGH, WILLIAM, JR., KUMAR, SAMIR, LAFICA, SUSAN J., LEUTE, GERARDO, LI, DONMING, SILENCE, SCOTT M., STAMP, AMY
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 2001
Main classification: G03G9
IPC codes: G03G9