Abstract:
Highly-coercive isometric iron oxide useful in magnetic toners for electrostatic photo-copying processes or for magnetic printers is produced by 
     (a) subjecting Fe 3  O 4  pigment produced in a wet process to oxidation and 
     (b) subsequently reducing the oxidized pigment from (a) to produce highly-coercive Fe 3  O 4  pigment.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to a process for the production of highly-coercive isometric magnetic iron oxide pigments and to magnetic iron oxide pigments produced by this process and to the use thereof. 
     Magnetic iron oxide pigments of the composition FeO x , wherein 1.33≦x≦1.5, are used for the production of magnetic toners for electrostatic photo-copying processes or for magnetic printers. Isometric magnetite pigments having coercive field strengths (I H  c) of from about 80 to 130 Oe are substantially used in conventional processes for this purpose. However, greater demands are made on these pigments in modern one-component toners. The coercive field strengths thereof have therefore to be within the range of from 180 to 250 Oe or above. 
     Two processes are essentially known at the present time for obtaining the coercive forces required. Those are: 
     1. by the incorporation of cobalt into the iron oxide pigments, 
     2. by the use of needle-shaped pigments. 
     However, the incorporation of cobalt is expensive and 
     in addition causes an undesirable fall in the I H  c values as the temperature increases. Needle-shaped pigments produce directional dependence in the toner and rheological problems in the production thereof with the result that their use is accompanied by disadvantages. 
     DESCRIPTION OF PRIOR ART 
     Relatively highly-coercive Fe 3  O 4  pigments (DE-OS No. 3 513 788), which are obtained via a salt melt process at a temperature of from 700° to 1,300° C., have also been proposed for single-component toners. However, salt melt processes are associated with corrosion and waste water problems and for this reason have not found their way into chemical technology. 
     Thus, an object of this invention is to provide a straight forward inexpensive process for the production of isometric magnetic iron oxide pigments for use in toners, which do not suffer from the disadvantages of processes in the prior art. 
     The starting materials for production of the high-coercive magnetic iron oxide pigment of this invention are known isometric pigments produced in a wet process, i.e., those pigments which are produced by the nitrobenzene reduction process (DE-PS No. 46 37 73), by a precipitation process from iron salts and alkalis (DE-OS No. 2 612 798), by a Penniman process in the presence of Fe-scrap (DE-OS No. 2 617 569) or by other processes in aqueous media known from the relevant literature. The I H  c values conventionally obtainable by these processes are limited to about 150 Oe, as the commercially obtainable products show. 
     SUMMARY OF THE INVENTION 
     It has now surprisingly been found that the coercive field strengths of isometric magnetic iron oxide pigments produced by a wet process can be dramatically raised to at least 180 Oe if they are subjected to a combined oxidation and heat treatment and are subsequently reduced and re-converted into Fe 3  O 4  pigments. 
     DETAILED DESCRIPTION 
     Thus, this invention relates to a process for the production of highly-coercive isometric magnetic iron oxide pigments, having an I H  c value of at least 180 Oe, which is characterised in that 
     (a) Fe 3  O 4  pigments produced in a wet process are oxidized and 
     (b) are subsequently reduced to produce relatively highly-coercive Fe 3  O 4  pigments. 
     The relatively highly-coercive Fe 3  O 4  pigments obtained in this manner can be 
     (c) oxidized in a further step to produce relatively highly-coercive γ-Fe 2  O 3  pigments or bertholloid mixed phase pigments. 
     Oxidation according to step (a) of this invention is carried out until a substantial amount of the iron oxide is Fe 2  O 3 . Preferably, substantially complete conversion to Fe 2  O 3  is achieved in step (a). The oxidation in step (a) can conveniently be carried out using gaseous oxidizing agents, such as air, at a temperature of from 200° to 950° C. A temperature of from 400° C. to 950° C. is preferred. The oxidation time depends on the ratio of the quantity of oxide to the quantity of air and on the temperature. 
     Reduction according to step (b) takes place with a reducing gas at a temperature of from 200° to 700° C., preferably from 300° to 500° C. Gaseous organic compounds, such as natural gas or carbon monoxide or more preferably hydrogen, can be used as a reducing gas. The hydrogen can be moistened to prevent over-reduction taking place. The oxidation time depends on the ratio of the quantity of oxide to the quantity of air and on the temperature. It is therefore not necessarily appropriate to state a time. 
     The magnetic Fe 3  O 4  pigments obtained according to the invention are particularly suitable for the production of black toners or black magnetic printing dyes. The pigments which are oxidized to produce γ-Fe 2  O 3  are advantageously used for the production of dyed toners. The present invention is illustrated in the following examples without this limiting the scope thereof. The maximum measuring field strength in determining the magnetic data is 4,000 G. 
    
    
     EXAMPLE 1 
     Wet production process by the nitrobenzene reduction process. 
     An Fe 3  O 4  pigment is produced according to DE-PS No. 463 773 which is characterised by the following known values: average partical size=0.5 μm, I H  c=140 Oe, M s  =5000 G, obtained commercially under the trade name Bayferrox 8610 (BAYER AG) and used in toners. 
     Stage (a) 
     This product is oxidized for 30 mins. at 700° C. in air in a laboratory rotating furnace. 
     Stage (b) 
     The product is then reduced for 120 mins. at 400° C. with hydrogen which has been moistened at a temperature of 60° C. to produce black Fe 3  O 4  pigments. 
     The following magnetic values are determined: 
     I H  c=285 Oe. 
     M s  =4875 G. 
     average partical size 0.5 μm 
     specific BET surface area=4 m 2  /g. 
     EXAMPLE 2 
     The product from Example 1 is oxidized at a temperature of 280° C. over a period of 120 mins. in air to produce brown γ-Fe 2  O 3  pigment. 
     The following magnetic values are determined: 
     I H  c=178 Oe. 
     M s  =3815 G. 
     specific surface area=3.3 m 2  /g 
     average partical size 0.5 μm. 
     EXAMPLE 3 (COMPARATIVE EXAMPLE) 
     The magnetic values of the starting material for Examples 1 and 2 are determined. The magnetite Bayferrox 8610 has the following characteristics: 
     I H  c=132 Oe. 
     M s  =4900 G. 
     EXAMPLE 4 (COMPARATIVE EXAMPLE) 
     Bayferrox 8610 is oxidized at a temperature of 250° C. for 120 mins. The resulting gamma-iron oxide has the magnetic values: 
     I H  c=87 Oe. 
     M s  =4150 G.