Developer composition and method of preparing the same
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to developer compositions for electrophotograhic copying, and more particularly to a method of preparing an improved developer composition. 
     Various methods of electrophotographic development are knowns. One of the more common techniques is the magnetic brush development method. The magnetic brush development method utilizes magnetic means associated with a developing mixture of a magnetic carrier (e.g., particles or powder) carrying a number of smaller electrostatically adhering toner particles. The developer composition is maintained during the development cycle in a loose, brush-like orientation by a magnetic field surrounding, for example, a rotatable non-magnetic cylinder having a magnetic means fixedly mounted inside. The magnetic carrier is attracted to the cylinder by the described magnetic field, and the toner particles are held to the carrier by virtue of their opposite electrostatic polarity. Before and during development, the toner acquires an electrostatic charge of a sign opposite to that of the carrier due to triboelectric charging derived from their mutual frictional interaction. When this brush-like mass of magnetic carrier with adhering toner particles is drawn across the photoconductive surface bearing the electrostatic image, the toner particles are electrostatically attracted to an oppositely charged latent image and form a visible toner image corresponding to the electrostatic image. 
     Ferrite cores have been widely used as the magnetic carrier material. There are several critical parameters which are necessary and must be controlled to produce a suitable ferrite carrier core. First, the particulate shape, size, and size distribution affect the carrier coating and its ability to hold toner, thereby producing good copy quality. The volume resistivity of the ferrite must also be established in a range which will be compatible with the coating to control the developer conductivity. To utilize the developer in a magnetic brush development process, the ferrite core must have adequate saturation magnetization to control its carrier function. Further, the apparent density of the carrier materially affects the volume in the developer charge and provides proper flow characteristics. Two of the most important properties are the surface area and the surface morphology of the core; these have a direct impact on the coating characteristics of the core which in turn affect the copy quality. 
     To produce the optimum carrier characteristics, ferrites of both cubic and hexagonal structures have been used. It is preferred with the magnetic brush two-component developers to use the cubic spinel structure of the form MFe 2  O 4  or (MO) x  (Fe 2  O 3 ) y  where M is a metal selected from the group consisting of lithium, manganese, nickel, zinc, cadmium, copper, cobalt, iron, and magnesium such as described in U.S. Pat. No. 4,898,801 to Tachibana et al. Two of the most widely used systems are nickel zinc ferrite and copper zinc ferrite. Whereas these systems produce effective magnetic carriers, there is, however, growing concern regarding the environmental problems associated with the presence of various forms of nickel, zinc or copper such as found in the above carriers. 
     Other ferrites, are proposed, for example in U.S. Pat. No. 4,855,205 to Saha et al. Saha et al. proposes a two-phase cubic ferrite composition comprising 0.1 moles to 1.0 moles of a ferrite having the formula MFe 2  O 4  where M is at least one element which forms a spinel ferrite (e.g., nickel, cobalt, magnesium, manganese, copper, zinc, and iron) and about 2.5 moles of a magnetoplumbite having the formula R x  P 1-x  Fe 2  O 19  where R is a rare earth and P is selected from the group consisting of strontium, barium, calcium and lead. 
     U.S. Pat. No. 5,004,665 to Ohtani et al. proposes a toner composition for electrophotographic copying comprising a resin, a colerant, and polymeric-magnetic coordination complex, poly-bis-(2,6-pyridinediylmethylidenenitrilohexamethylenenitrilomethylidene) iron sulfate. 
     U.S. Pat. No. 3,590,000 to Palermiti proposes carrier particles comprised of various cores including steel, with a coating of fluoropolymers, and terpolymers of styrene, methacrylate, and silane compounds. 
     U.S. Pat. No. 5,162,187 to Lyons et al. proposes a carrier composition comprised of a semiconductive ferrite core with a coating of a mixture of first and second polymers that are not in close proximity in the triboelectric series (e.g., polystyrene and tetrafluoroethylene). At column 5, lines 4 through 14, a specific semiconductive ferrite core comprising magnesium copper zinc available from the assignee of the present invention, Steward, Inc., is described. 
     Other ferrite carriers are proposed in U.S. Pat. Nos. 2,846,333 to Wilson; 3,929,657 to Jones; and 4,127,667 to Jones. 
     Despite the general availability of ferrite carriers and various methods of producing and using them, there continues to be a desire for improvements in carrier performance. For example, it is an object to provide higher volume resistivity and a wider range of saturation magnetization. It is also an object to eliminate or reduce the use of ferrites having forms of nickel, zinc, and copper. These metals have come under increased scrutiny by regulatory agencies, and materials including these metals often require special, and typically expensive, disposal methods. It is a further object to produce a suitable ferrite carrier inexpensively and efficiently. 
     SUMMARY OF THE INVENTION 
     These and other objects, features and advantages are achieved by the method of the present invention relating to preparing a ferrite carrier for a developer composition. The carrier is devoid of zinc, copper, or nickel in its crystal structure. Moreover, the ferrite carrier has a high volume resistivity and a wide range of saturation magnetization. 
     The method comprises subjecting a ferric oxide and a metal oxide (e.g., manganese oxide or ferrous oxide) to conditions sufficient to form a ferrite have a spinel structure, mixing the ferrite having a spinel structure with a sintering agent, and sintering in the liquid phase the mixture at temperature conditions of less than about 1375° C. to provide a ferrite carrier devoid of zinc, copper, or nickel in its crystal structure. Sintering in the presence of a liquid phase (i.e., &#34;liquid phase sintering&#34;) is a way of bonding two or more materials, e.g., manganese oxide and ferric oxide or ferrous oxide and ferric oxide (magnetite), which have different melting points, into dense bodies. The system remains multiphase throughout the entire process. Liquid phase sintering uses a material transport mechanism involving viscous flow and diffusion. The liquid phase sintering mechanism promotes reactions between particles so that the growth of grains is promoted. 
     In another embodiment, the present invention provides a developer composition comprising a toner composition and a ferrite carrier composition. The toner composition is typically a resin (e.g., a copolymer of styrene and acrylic acid) and a colorant. Preferably, the ferrite carrier composition comprises, in the primary phase, from about 10 to 45 percent by weight manganese oxide and from about 55 to 90 percent by weight ferric oxide, and in the intergranular phase from about 0.5 to 3 percent by weight bismuth trioxide. The developer composition of the present invention has suitable performance properties such as a volume resistivity about 1.0×10 5  to 3×10 9  ohm cm and a saturation magnetization of about 40 to 95 EMU/g. Moreover, the developer composition can be formed without potentially environmentally undesirable zinc, nickel and copper. 
     In another embodiment, the carrier compositions prepared by the method of the present invention can be coated to stabilize the resistivity and triboelectric characteristics such as described in U.S. Pat. No. 5,162,187 to Lyons et al. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As summarized above, the present invention provides an improved carrier and a developer composition utilizing the carrier, and a method of preparing the same. The developer composition is particularly adapted for use in various elecrtrophotographic copying methods such as described in U.S. Pat. Nos. 2,618,552 to Wise, 2,874,063 to Greig, and 3,079,342 to Insalaco, the disclosures of which are incorporated herein by reference in their entirety. For example, the developer composition of the present invention can be used in the magnetic brush technique of developing wherein a developer composition is maintained during the development cycle in a brush-like composition by a magnetic field surrounding a rotatable nonmagnetic cylinder having a magnetic means fixedly mounted inside. 
     The carrier is prepared by forming a mixture of ferric oxide and a metal oxide. Exemplary metal oxides include oxides of manganese, iron, cobalt, lithium, magnesium and the like. The mixture is preferably pelletized to form a granulate. Water or other suitable liquids can be used to aid pelletization. The granulate is subjected to conditions sufficient to form a ferrite having a spinel structure. For example, the granulate is calcined in a rotary calciner at a temperature of about 900° C. to 1200° C. for about 10 to 30 minutes until spinel formation occurs. The saturation magnetization of the ferrite having a spinel structure (calcine) is about 10 to 60 EMU/g. The calcine is mixed with a sintering agent and with water or other suitable liquid, a dispersant, a defoamer and a binder agent. 
     The sintering agent is used as a flux. The sintering agent becomes a single liquid phase during the sintering. The metal oxide is dispersed in the ferric oxide through a mass transfer mechanism involving viscous flow and diffusion as will be known to those skilled in the art. Exemplary sintering agents include bismuth trioxide or any of the well known frit compositions. The sintering agent should have a melting temperature below that of the solids. An exemplary specific frit, Ferro Frit No. 3292 from Ferro Corporation, has the following composition: 
     
         ______________________________________                                    
       Compound                                                           
               Mole %                                                     
______________________________________                                    
       K.sub.2 O                                                          
               2.13                                                       
       Na.sub.2 O                                                         
               3.16                                                       
       Li.sub.2 O                                                         
               0.94                                                       
       CaO     12.01                                                      
       MgO     1.23                                                       
       SrO     2.96                                                       
       Al.sub.2 O.sub.3                                                   
               6.75                                                       
       B.sub.2 O.sub.3                                                    
               5.27                                                       
       SiO.sub.2                                                          
               65.55                                                      
               100%                                                       
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     The mixture is milled using a ball mill or attrition mill to a size of about 1.41 to 1.65 μm. The mixture is dried using wheel atomization or the like to a specific particle size distribution. The spray dried mixture is sintered in the liquid phase in air at less than about 1375° C. for about 2 to 5 hours and then annealed in an inert atmosphere such as nitrogen for about 1 to 6 hours at about 1000° C. to 1100° C. The saturation magnetization of the sintered mixture is about 40 to 95 EMU/g. The sintered mixture is deagglomerated and non-magnetic materials removed. The sintered mixture is screened to result in a final size distribution of about 15 to 120 μm median, by weight particle diameter. The resulting composition comprises the metal oxide and ferric oxide in the primary phase and sometimes a small percentage (i.e., 0.5 to 3.0 percent) of the sintering agent in an intergranular phase. 
     To form a developer composition, the carrier composition can be coated or blended with between about 0.05 to 5 percent of a toner composition comprising a resin and a colorant. Adherence can be accomplished by heating, mechanical impaction or electrostatic attraction. For example, the carrier composition can be provided as above, and a mixture of the carrier composition and the toner composition heated to a temperature of about 100° C. to 250° C., for 10 to 60 minutes to melt and fuse the resin followed by cooling the obtained developer composition. 
     Typically, the toner composition includes a binding resin and a colorant. Exemplary binding resins include gum; copal; gum sandarac; rosins; homopolymers of styrene and its substituted derivatives such as polystyrene, poly-p-chlorostyrene, polyvinyltoluene; styrene copolymers such as styrene-p-chlorostyrene copolymer, styrene-propylene copolymer, styrene-vinyl toluene copolymer, styrene-vinyl-naphthalene copolymer, styrene-methyl acrylate copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-butyl methacrylate copolymer, styrene-methyl α-chloromethacrylate, styrene-acrylonitrile copolymer, styrene-vinyl methyl ether copolymer, styrene-vinyl ethyl ether copolymer, styrene-vinyl methyl ketone copolymer, styrene-butadiene copolymer, styrene-isoprene copolymer, styrene-acrylonitrile-indene copolymer, styrene-maleic acid copolymer, styrene-maleic acid half ester copolymer, and styrene-maleic acid ester copolymer; polyvinyl alcohol; polymethyl methacrylate; polybutyl methacrylate; polyvinyl chloride; polyvinyl acetate; polyethylenes; polypropylenes; polyesters; polyurethanes; epoxies; polyvinyl-butyral; polyamides; polyacrylic acid resin; terpene resins; phenol resins; aromatic petroleum resins fluoropolymers, polysilanes; chlorinated paraffins; paraffins; waxes; and mixtures and blends thereof. Other suitable resins are also disclosed in U.S. Pat. Nos. 4,741,984 to Imai et al., 4,822,708 to Machida et al. , and 4,879,198 to Trevernier et al., the disclosures of which are incorporated herein by reference in their entirety. 
     Specific polymers are as follows: 
     
         ______________________________________                                    
Polymer        Tradename      Produced By                                 
______________________________________                                    
Polyvinylidene fluoride                                                   
               Kynar          Pennwalt                                    
Vinylidene chloride-                                                      
               Saran F220     Dow                                         
acrylonitrile copolymer                                                   
Vinylidene chloride-                                                      
acrylonitrile-acrylic acid                                                
terpolymer                                                                
Cellulose nitrate                                                         
Chlorotri-     Kel-F800       3M                                          
fluoroethylene vinylidene                                                 
fluoride                                                                  
A diphenylene sulfono                                                     
               Polysulfone P-1700                                         
                              Union Carbide                               
Epoxy/amino curing agent                                                  
               Epon 828/V125  Shell/                                      
                              General Mills                               
Cellulose acetate butyrate                                                
Acrylonitrile-butadiene-                                                  
               Cyclopac H-1000                                            
                              Borg Wamer                                  
styrene terpolymer                                                        
Chlorosulfonated                                                          
               Hypalon 30     DuPont                                      
polyethylene                                                              
A diphenylene sulfono                                                     
               Polysulfone P-3500                                         
                              Union                                       
                              Carbide                                     
Polyethylene   Epolene C      Eastman                                     
                              Chemical                                    
Polystyrene    Polystyrene 8X Koppers                                     
Ethyl cellulose                                                           
               Ethocel 10     Herculese                                   
Phenol formaldehyde                                                       
               Durez 510      Durez                                       
Polyester      Dupont 49000   DuPont                                      
Polyurethane   Estane 574OX1  Goodrich                                    
Alkyl-substituted polyvinyl                                               
               Ganex V816     GAF                                         
pyrrolidone                                                               
Polyvinyl formaldehyde                                                    
               Formvar 7/70   Monsanto                                    
A poly-bisphenol-A                                                        
               Lexan 105      GE                                          
carbonaic                                                                 
Alkyl-substituted polyvinyl                                               
               Ganex V804     GAF                                         
pyrrolidone                                                               
Alkyl-substituted polyvinyl                                               
               Ganex V904     GAF                                         
pyffolidone                                                               
Diallyl phthalate                                                         
               Dapon M        FMC                                         
Lucite 2041    Methyl methacrylaic                                        
                              DuPont                                      
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     See, L. B. Schein, Electrophotography and Development Physics, Springer-Verlag, Berlin-Heidelberg (1988). 
     Among them, polyvinyl alcohol, and styrene resins and acrylic resins are preferable in view of their thermal fixing characteristics, and developing durability or successive developing characteristics. The particular toner material to be employed, as will be within the skill of one in the art, depends upon the separation of the toner from treated carrier beads in the triboelectric series. These toners generally have an average particle diameter between about 1 and 30 μm. 
     Suitable colorants include a red pigment: red iron oxide, cadmium red, red lead oxide, cadmium, mercury sulfide, permanent red 4R, lithol red, pyrazolone red, watchung red, calcium salt, lake red D, brilliant carmine 6B, eosine lake, rhodamine lake B, alizalin lake, brilliant carmine 3B or the like; a green pigment: chrome green, chrome oxide green, pigment green B, malachite green lake, fanal yellow green G or the like; a blue pigment: prussian blue, cobalt blue, alkali blue lake, victoria blue lake, phthalocyanine blue, metal-free phthalocyanine blue, phthalocyanine blue particle chlorine compound, fast sky blue, indanthrene blue BC or the like; a magenta pigment: manganese violet, fast violet B, methyl violet lake or the like; a yellow pigment: chrome yellow, zinc yellow, cadmium yellow, yellow oxide, mineral fast yellow, nickel titanium yellow, nables yellow, naphthol yellow S, hansa yellow G, hansa yellow 10G, benzidine yellow G, benzidine yellow GR, quinoline yellow lake, permanent yellow, NCG, tartrazine lake or the like; an orange pigment: chrome orange, molybdenum orange, permanent orange GTR, indanthrene brilliant orange RK, vulcan orange, pyrazolone orange benzidine orange G, indanthrene brilliant orange GK or the like; and a black pigment: carbon black, acetylene black, lamp black, aniline black or the like. Suitable colorants are also disclosed in U.S. Pat. No. 3,989,648 to Lenhard et al. and U.S. Pat. No. 5,162,187 to Lyons et al., the disclosures of which are incorporated herein by reference in their entirety. 
     The developer composition can also include a charge control agent such as Nigrosine Base EX (available from Orient Kagaku K.K.), quaternary ammonium salt (P-51: available from Orient Kagaku K.K.), Nigrosine Bontron N-01 (available from Orient Kagaku K.K.), Sudatiefschwarz BB (Solvent Black 3, C.I. 26150), Fettschwarz HBN (C.I. No. 26150), Brillantspiritschwarz TN (available from Farben Fabriken Bayer A. G.), Zapanschwarz X (available from Farberke Hechist A. G.), and alkoxylated amines, alkyl amide, molybdic chelating agent and the like. 
     The carrier composition optionally can be coated to stabilize the resistivity and triboelectric characteristics of the carrier. For example, the carrier can be coated with a polymer coating comprising a mixture of polymers that are not in close proximity thereto in the triboelectric series such as described in U.S. Pat. No. 4,937,166 to Creatura, et al., the disclosure of which is incorporated herein by reference in its entirety. Specific examples include polyvinylidenefluoride with polyethylene; polymethylmethacrylate and copolyethylenevinylacetate; copolyvinylidenefluoride tetrafluoroethylene and polyethylene; polymethylmethacrylate and copolyethylene vinylacetate; polymethylmethacrylate and polyvinylidenefluoride, polystyrene and tetrafluorethylene; polyethylene and tetrafluoroethylene; polyethylene and polyvinly chloride; polyvinyl acetate and tetrafluoroethylene; polyvinyl acetate and polyvinyl chloride; polyvinyl acetate and polystyrene; and polyvinyl acetate and polymethyl methacrylate. 
     The foregoing example is illustrative of the present invention, and is not to be construed as limiting thereof. 
     Example 
     Ferric oxide and manganese oxide are mixed together to form a granulate. The free flowing mixture is then calcined in a rotary calciner until a spinel structure has been formed. Upon cooling, the calcine is contacted with water, Colloid 102 dispersant, polyvinyl alcohol binder, available from Rone-Poulenc, Foamblast 338 defoamer available from Ross Chemical and bismuth trioxide sintering aid available from Metalspecialties, Inc. The mixture is milled to approximately 1.5 microns average particle diameter. The slurry is spray dried to form free flowing spheres. These are sintered in an air atmosphere followed by a nitrogen anneal at 1090° C. to room temperature. The resulting product is deagglomerated and classified to a median, by weight, particle diameter of 50±4 μm to form the final product. The mixture has the following composition: 
     
         ______________________________________                                    
Compound        Wgt %   Mole %                                            
______________________________________                                    
Fe.sub.2 O.sub.3                                                          
                68.04   49.55                                             
MnO             30.23   49.55                                             
BiO.sub.3        1.74    0.90                                             
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     The following is a list of properties which were achieved from the above composition of the Example: 
     
         ______________________________________                                    
Ferrite Carrier Properties                                                
Property              Value                                               
______________________________________                                    
Sieve Analysis, Percent by Weight                                         
passing 140 mesh (106 μm)                                              
                      100.0                                               
passing 200 mesh (90 μm)                                               
                      98.1                                                
passing 400 mesh (38 μAm)                                              
                      3.47                                                
Median Diameter, μm                                                    
                      48                                                  
Volume Resistivity, (ohm cm) 100V                                         
                      9.2 × 10.sup.5                                
Saturation Magnetization, EMU/g                                           
                      69                                                  
Apparent Density, g/cm.sup.3                                              
                      2.3                                                 
BET Surface Area, cm.sup.2 /g                                             
                      576                                                 
passing 230 mesh (90 μm)                                               
                      98.1                                                
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     These properties are comparable to the properties of a manganese zinc ferrite such as &#34;12931&#34; available from Steward, Inc. 
     In the specification and examples, there have been disclosed preferred embodiments of the invention. Although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation, the scope of the invention being defined by the following claims.