Abstract:
The inventions concerns a new carrier core material consisting of particles of essentially pure, spherical magnetite. These particles are electrically insulated by an inorganic phosphorus containing coating.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to particulate magnetite materials useful as a carrier component in electrophotographic developers, in particular two-component developers comprising the carrier component together with a toner component.  
         BACKGROUND OF THE INVENTION  
         [0002]    In electrophotography, the electrostatic image formed on the photoconductor is developed by the magnetic brush method using either the so called “one-component” developer or “two-component” developer. Usually, the two-component developer system comprises a mixture of relatively fine particles of a toner and relatively coarse particles of a carrier. The toner particles are held on the carrier particles by the electrostatic forces of opposite polarities which are generated by friction of the particles. When the developer comes into contact with an electrostatic latent image formed on the photosensitive plate, the toner particles are attracted by the image and thus make the latter visible. The thus developed image is then transferred onto a recording medium, such as a paper sheet. In the process, therefore, the toner particles should be charged with an accurately controlled amount of static electricity so that they are preferentially attracted to the electrostatically imaged area of the photosensitive plate.  
           [0003]    This, in turn, means that the carrier which is used in combination with the toner must have an appropriate triboelectric property which enables it to electrostatically hold the toner particles and to transfer the held toner particles to the electrostatic latent image on the photosensitive plate when contacted. Additionally the carrier particles should have a sufficient mechanical strength to protect the carrier particles from breaking or cracking. These particles should also exhibit a good fluidity, be uniform in their electric and magnetic properties and be stable with respect to changes in the environmental conditions, such as humidity. The carrier particles should have a sufficient durability to ensure an acceptable lifetime.  
           [0004]    In the most recent printing technology, which permits improved quality and speed, the distance between magnetic brush and photoreceptor is smaller and currents during printing are higher, a consequence of which is that the carrier core itself must be able to carry some of the amount of current in the copying process. More specifically higher voltage breakdown of the carrier core itself is needed. Preferably this higher voltage breakdown should not be accompanied by a higher resistivity, but rather with a medium high resistivity.  
           [0005]    The carrier core materials normally used when high voltage breakdown values are required are selected from ferrites. These compounds have the chemical formula Fe 2 MO 4  wherein M can be Mn, Fe, Co, Ni, Cu, Zn, Cd, Mg. In order to meet different requirements depending on the specific type of copiers and printers used, i.a. the chemical composition of the ferrite has to be changed. A problem is thus that, in order to obtain ferrite powders having optimal properties, it is often necessary to manipulate the chemistry of these ferrite base powders so as to include different types of oxides of heavy metals. Such metals should however to the outmost possible extent be avoided as they are detrimental to the environment. Thus there is an increasing demand of a carrier core material which has a high voltage breakdown and which does not pollute the environment.  
           [0006]    The most simple of the ferrites is the compound wherein M is Fe, i.e. the compound having the formula Fe 3 O 4 , commonly called magnetite. Magnetite is not environmentally detrimental, but the voltage breakdown is low, normally between 30-50 V. This is an indication that it would not be possible to use magnetite in the most recent printing technology.  
           [0007]    It has now unexpectedly been found that by a comparatively simple process it is possible to use magnetite as a base material for the preparation of new carrier core materials having not only high voltage breakdown but which also in other respects can be tailored in order to meet different needs.  
         SUMMARY OF THE INVENTION  
         [0008]    In brief the new carrier core material essentially consists of a magnetite base powder, the particles of which are surrounded by an electrically insulating coating consisting of an inorganic, phosphorus containing material.  
           [0009]    The invention also concerns a method for the preparation of such a new carrier core material.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0010]    The spherical magnetite base powder may be produced as described in the U.S. Pat. No. 4,663,262 which is hereby incorporated by reference. According to this patent the magnetite base is produced from natural magnetite by the following general procedure:  
           [0011]    A magnetite powder is formed into agglomerates which are then calcined at a predetermined temperature under a specific atmosphere. The calcined granules are suitably cracked or dispersed and then classified into a desired size distribution. Because the agglomerates are formed with a binder material which is effective for reducing the raw magnetite (Fe 3 O 4 ) to wustite (FeO), the magnetite is partially converted to wustite during the calcination to give a product magnetite usually containing 15-20% of wustite. By controlling the temperature and the composition of the atmosphere during the cooling step after the calcinations magnetite powders containing less than 10%, preferably less than 3%, by weight of wustite may be obtained.  
           [0012]    The magnetite base material could of course be obtained from other sources such as synthetic sources. Furthermore the magnetite base preferably consists of at least 70% of magnetite. Minor amounts i.e. up to 30% by weight of other compounds, such as hematite, wustite, silicon, metallic iron, phosphorus, aluminia, titanium oxide, or inert inorganic or organic materials may be included in the particulate magnetite base material.  
           [0013]    Furthermore, according to an embodiment of the invention, powders having particles with essentially spherical shape are preferred as such powders have isotropic magnetic properties which are advantageous in many xerographic applications. The particle size of the base material used according to the present invention is normally between 15 and 200 μm. Typical examples of such substantially spherical magnetite base powders which may be used are magnetite powders of the CM series from Höganäs AB, Sweden.  
           [0014]    The coating on the particles of the ferromagnetic powder of the present invention should preferably exhibit a number of properties. Thus, the coating should be insoluble in water and organic solvents. Furthermore, the coating should not have a negative influence on powder properties, such as apparent density and flow. Thus the apparent density of the new carrier core powder should preferably vary between about 1 and 4 g/cm 3  and the flow between 20 and 25 s/50 g. Furthermore, the inorganic insulating coating should completely cover the individual ferrite base particles. The coating should be coherent, homogenous and uniform and not contain organic material. An important feature of the coating is that it does not affect the magnetic properties of base powder and thus the magnetic properties of the insulated powder particles are essentially the same as those of the base powder. Typical values for magnetic properties of suitable base powders are for saturation, σs, 90-96 emu/g, for remenence, σr, &lt;3 emu/g and for coercivity, H c &lt;30 Oe. Most importantly, the coating should impart high voltage breakdown as well as other properties to the carrier core materials required for modern xerographic applications.  
           [0015]    The inorganic coating may be obtained by mixing the magnetite base powder with an aqueous solution of phosphoric acid. The amount and concentration of the phosphorus acid is decided by the desired final properties of the insulated powder. Typically the amount of coating solution may range between 20 and 80 ml per kg magnetite of ferrite powder and the thickness may preferably vary within about 0.1 to about 5 μm. The coating solution may include other elements in order to obtain a coating layer which in addition to phosphorus also includes elements such as Ti, Al, Zr, Mg which may be advantageous for certain applications.  
           [0016]    According to the present invention insulated particles having very high voltage breakdown values, such as up to 1000 V or even higher may be obtained whereas values below about 500 V are less important for modern printing technology. The resistivity of the insulated particles preferably varies between about 10 8  and 10 10 .  
           [0017]    The insulated carrier core particles according to the present invention are subsequently coated with a thin resinous layer in order to produce a carrier material. This layer is needed e.g. in order to adjust the tribo and increase life. The amount of this organic or resinous layer is normally between about 1.5 to 6% by weight of the carrier core.  
           [0018]    The invention is further illustrated by the following non limiting examples. 
       
    
    
     EXAMPLE 1  
       [0019]    The base material in the following examples is CM 70, a spherical magnetite with a mean particle size of 70 μm available from Höganäs AB Sweden.  
         [0020]    A coating solution was obtained by dissolving various amounts of ortophosphorous acid in water. The coating solutions were thoroughly mixed just before they were added to the magnetite powders in order to avoid segregation. The coating solutions were added to the powder with a rate of 25 ml per kg powder for a period of 90 s. The obtained mixture was thoroughly mixed while the temperature was maintained between 80 and 90° C. The solution was then evaporated leaving the insulated particles as a residue. As a last step the dried powder was sieved in order to eliminated oversized particles and agglomerates.  
         [0021]    The following results were obtained:  
                                                         TABLE 1                                   Coating                       solution   Amount of       Voltage           % phosphoric   Coating   Resistivity*   Breakdown*           acid   Solution ml   (ohmm)   (V)                                        30   25   8.7*10 9     550           30   50   4.4*10 9     &gt;1000           30   75   4.3*10 9     &gt;1000           46   25   6.3*10 9     &gt;1000           46   50   6.3*10 9     &gt;1000           46   75    47*10 9     &gt;1000           —**   —     7*10 9     40                                              
 
         [0022]    As can be seen from the results the inorganic coating increases the resistivity of the carrier core material.  
       EXAMPLE 2  
       [0023]    In this example a base magnetite powder CM 40 was used. This powder was subjected to an oxidation treatment as suggested in the U.S. Pat. No. 4,663,262. Part of the obtained oxidised powder (=Sample CM40A) was provided with an inorganic coating (=Sample CM40B) according to the present invention. As can be seen from the Table 2 below the resistivity is increased by the oxidation treatment. However the voltage breakdown is considerably lower than that of the coated powder according to the present invention.  
                                                 TABLE 2                                           Voltage breakdown*               Resistivity* ohmm   (V)                                        CM40 A   2.2*10 9     425           CM40 B   1.1*10 10     700           CM40 (ref.)     7*10 7     40                                  
 
         [0024]    As can be seen from the results in the above table the electrical properties are considerably improved by using an inorganic coating according to the present invention. Thus, the voltage breakdown can reach high values which are comparable to those of ferrites. An unexpected effect is that the high voltage breakdown properties do not necessary involve high resitivity of the carrier cores. High resistivity of the carrier cores is not desired as the amount of toner per carrier is decreased when the resistivity is increased. Additionally the improvements in the electrical properties do not affect other properties such as magnetic properties of the carrier cores.