SURFACE MODIFIED FERROMAGNETIC CHROMIUM DIOXIDE

Abstract: Stabilized ferromagnetic chromium dioxide with im­ proved magnetic and physical properties is produced when an iron or tin salt reacts with the chromium dioxide surface to form a nonmagnetic surface phase containing 0.01 to 0.25 atoms of iron or tin per atom of chromium.

Description: TITLE SURFACE MODIFIED FERROMAGNETIC CHROMIUM DIOXIDE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to ferromagnetic elements, and in particular to ferromagnetic chromium dioxide which is stabilized against degradation of magnetic properties. STATE OF THE ART Ferromagnetic chromium dioxide has many desirable characteristics which make it useful for certain applications in the manufacture of magnetic recording tapes, magnetic memory recorders, computers and other applications. Magnetic properties which are particularly important are the intrinsic coercive force (iHc), saturation per gram (as), retentivity or remanence per gram (ar), and the ratio of the remanence to the saturation (ar/as). These terms are further defined in Bottjer et al, U.S. 3,529,930 "Process for Improving Ferromagnetic Properties of Chromium Dioxide by Heating in an Oxidizing Environment" (1970). One problem with ferromagnetic chromium dioxide is that it reacts slowly with water and some organic materials to form nonmagnetic material. Thus, by a process of aging, unstabilized chromium dioxide will lose magnetic properties when incorporated in a magnetic element such as a tape or disk. Bottjer et al in.U.S. 3,512,930 "Stabilized Ferromagnetic Chromium Dioxide" (1970) disclosed that ferromagnetic chromium dioxide could be stabilized by treating with a reducing agent to form a surface which does not oxidize benzhydrol and has a characteristic x-ray diffraction pattern. Suitable reducing agents include H2S, sodium sulfite, SO2, and alkanols. In a preferred embodiment the ferromagnetic chromium dioxide which was reacted with the reducing agent was pretreated for use by a thermal "upgrading" process described in U.S. 3,529,930. U.S. 3,958,068, Umeki et al, discloses precipitating a ferromagnetic salt on the surface of A solid ferromagnetic oxide and reducing the precipitated metal salt to form the free metal as a surface coating. Thus, chromium dioxide particles could be stabilized by the precipitation of ferrous hydroxide on the surface and subsequent reduction to iron metal. Japanese Patent 21200/76 discloses a stabilized ferromagnetic chromium dioxide composition prepared by depositing magnetic iron oxide on the surface thereof by means of an immersion coating process. Thus, the prior art contains numerous suggestions for stabilizing ferromagnetic chromium dioxide by means of a protective outer layer, this being formed by the action of reducing agents, or by precipitation techniques or surface coatings. However, the latter two methods result in disadvantages such as sacrifice of magnetic properties and the need for costly and time consuming process steps. The use of reducing agents is commercially feasible, but nevertheless leaves open the possibility that the desired product could be produced by a simpler and less expensive process. SUMMARY OF THE INVENTION The present invention is directed to a stabilized ferromagnetic chromium dioxide composition comprising acicular particles of ferromagnetic chromium dioxide having a nonmagnetic protective surface phase composed of the adduct formed by contact between the particle surface and an aqueous solution of a metal salt, said protective surface phase being characterized by soft x-ray analysis (ESCA) as containing 0.01 to 0.25 atom of included metal per atom of chromium. The included metal is preferably iron, tin, cobalt, or manganese. Metal salts which yield Fe and Sn+2 cations in solution are particularly useful in this invention. The invention is also directed to the process of preparing that composition wherein an aqueous solution of such metal salt is contacted with acicular ferromagnetic chromium dioxide particles at a temperature of 100 to ll00C, and a pH of 1-8 for a time sufficient to chemisorb or chemically bond the metal ions of such salt to the surface of the Cr02 particles. Because this process is carried out at a pH of 1 to 8 the hydroxide ion concentration is kept sufficiently low so that the cations used in the invention are not removed by precipitation. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are graphs which set forth a comparison of magnetic and chemical properties of the surface-modified ferromagnetic chromium dioxide of the present invention (broken line) and those of a control (solid line). DETAILED DESCRIPTION OF THE INVENTION Acicular ferromagnetic chromium dioxide useful for the practice of the present invention can be produced by high temperature and high pressure techniques well known in the art such as the processes described in U.S. Patents 2,956,955, 3,117,093 and 3,278,263. A variety of metal oxides may be added to improve magnetic properties as in U.S. 3,640,871 (tellurium and iron), U.S. 3,778,373 (iron modifiers, among others), and U.S. 3,034,988 (a metal having an atomic number of 22-281). A particular useful pretreatment for ferromagnetic chromium dioxide, but one which is not essential for this invention, is the thermal 22upgrading" treatment described in U.S. 3,529,930, supra, which consists essentially of heating preformed Cr02 having a saturation of at least 75 emu./g. and a coercive force of 200-800 oersteds in an oxidizing environment at 1500C to 4500C at a pressure of 0.2-3000 atmospheres. In the present invention chromium dioxide particles gain weight proportionally to the quantity of metal cations which react with the particle surface, rather than undergoing the slight weight gain or loss which is typical of the stabilized Cr02 prepared by the process of U.S. 3;512,930. This indicates that the metal salts of the present invention do not function just as reducing agents, but serve as metal cation donors for the formation of a surface adduct by chemically bonding or chemisorbing to the surface of the acicular chromium dioxide particles. The metal cations thus become part of the stabilized surface structure and account for the net gain in weight accompanying the reaction. Also both physical and chemical properties are improved with the positioning of a new metal adduct on the particle surface. Not only does the present invention provide protection against chemical degradation of magnetic properties but in addition provides improved product durability and performance in terms of magnetic strength, stability, headwear, and signal to noise ratio. Metal salts suitable for the practice of the present invention must be compatible with the acicular chromium dioxide and probably must also be capable of undergoing oxidation within the reaction environment. In this respect a number of metallic compounds which have been found to be effective modifying elements for acicular chromium dioxide would be expected to be similarly suitable for the surface effect of the present invention, e.g., Fe, Sb, Te, Sn, Mn and Ru in particular. Preferred are Fe+2 Fe+3, +2 +2 and +2 Fe+2 , Fe+3 ,Sn+2 , Co+2 , and Co , and Mn The anion of the metal salt does not appear to be significant as long as the salt is soluble so that it can be employed in aqueous solution. Thus, sulfates, chlorides, and nitrates are typical metal salts which can be employed in the present invention. These salts may also contain a nonmetallic cation such as the ammonium ion. Typical salts thus are CoCl21 MnSO4, SbCl3, SnBr2, NiCl21 CuCl2, SnCl2, FeSO4#7H2O, (NH4) Fe2 (SO4)2 6H20, FeCl3, NH4Fe(SO4)2#12H2O, and Fe(NO3)39H2O. Salts which are particularly preferred for the present invention are FeSO4 7H2 0 and SnCl2. Comparative results of the stabilizing effects of ferrous and ferric salts indicate that the lower valence iron gives superior results. A possible explanation of this presumes that oxidation of the metal cation must occur during some stage of the chromium dioxide surface reaction. The Fe+2 to Fe oxidation would be expected to occur readily either in solution or on the chromium dioxide surface. Fe +4, Fe+5 , and Fe are known to exist in combination with oxygen, but these high oxidation states are not known in solution. Therefore Fe +3 oxidation would only be expected to occur on the chromium dioxide surface, with Cr02 acting as a strong oxidizing agent and providing oxygen atoms on the particle surface. The process of the present invention avoids the complexities required for prior art methods of producing magnetic iron or iron oxide on Cr02 particle surfaces. In the event iron salts are employed in the present invention it is not necessary to precipitate either Fe(OH)2 or Fe (OH)3 under alkaline conditions. Formation of the metal adduct in the present invention is not dependent on having the hydroxide ion concentration exceed the solubility product for precipitation of the cation of the metal salt. The process of the present invention can proceed under neutral or acid conditions where the prior art would be inoperative. A further simplification of the present invention is the absence of the requirement for any additional reducing step via the addition of a reducing agent or electrolysis. The reaction of the present invention proceeds without the requirement for either high temperature or pressures which are typical of the synthesis reaction for forming acicular chromium dioxide. By simply mixing the acicular chromium dioxide with a water solution of metal salt at a temperature of from -100C to 1100C the reaction can proceed rapidly to completion. Depending on variables such as concentration, surface area, temperature, and oxidizability, the reaction time can range from a few minutes to a few hours. For a particular system the course of the reaction can be studied by a variety of methods, for example; solution analysis for unreacted salt, pH changes, weight gain by the CrO2 particles, lower chromate liberation, or x-ray surface analysis. Generally, high acidity is undesirable in carrying out the process of the invention if the reaction to form the cation adduct is to be carried out using metal containers. Unless the reaction is carried out using special materials or alloys, acid will create corrosion problems which will limit equipment life and contaminate the reaction mixture. It has been found that a ferrous sulfate solution is effective when used at a pH of 5 to 8. This near neutral pH assures that the iron will not precipitate out of the reaction mixture, and avoids the adverse effects of corrosion and contamination. A technique called Electron Scanning Chemical Analysis (ESCA) can be used to determine that a metal adduct has been formed on the surface of the acicular chromium dioxide. In this technique, soft x-rays bombard the atoms situated on the particle surface and cause a secondary emission of electrons. The electron spectra thus produced can be used to characterize the surface atoms and their valence states. Simple weight analysis also shows that the metal adduct is produced in direct proportion to the amount of metal cation added. In a particular instance, a CrO2 sample of known weight was treated with a metal salt solution, washed to remove unreacted salt, dried, and weighed. As a control, a sample of known weight was treated with sodium sulfite, as per U.S. 3,512,930, washed to remove unreacted salt, dried, and weighed. In the first instance, as more metal salt was added the sample weight increased, whereas in the case of the control, increased sodium sulfite addition produced no observable weight difference. This shows that the weight gain of the present invention is clearly distinct from the prior art. Chromium dioxide particles can be tested for stabilization against degradation using a soluble chromate test. After being rinsed in water to remove surface impurities, a 0.840 g sample of chromium dioxide is placed in a stoppered 25 ml volumetric flask with distilled water and constantly shaken for 24 hours while being held at 650C. Unstabilized chromium dioxide will be degraded by hot water and chromate ions can be detected in the water. A magnet can be used to hold the chromium dioxide to the bottom of the flask while a sample is removed. A 0.6 ml sample is added to 4.4 ml of 10N NaOH to provide a stable chromate solution for testing in a visual spectrometer to determine the 367.5 um peak for chromate. Parts per million (ppm) values can be calculated in accordance with Beer's law. Chromium dioxide particles which have not been stabilized have soluble chromate values above 500 ppm. Even when Cr02 particles have been treated so that soluble chromate values are in the 400 to 500 ppm range, tape made with this oxide would not be expected to age well. When the chromium dioxide particles show test values below 200 ppm they would be expected to be very stable when used in tapes. The following examples serve to illustrate the present invention. Example 1 A 318 kg batch of commercial grade acicular ferromagnetic chromium dioxide was upgraded by the process described in U.S. 3,529,390. In this process the Cr02 is heated to 2800C in the presence of air moving at the rate of 2 cu.ft/min., held for 20 min., and then cooled to room temperature. A 6.8 Kg sample was removed for experimental and control tests. To 20 g. of the upgraded sample in a glass vessel was added 400 ml. of a .018 M solution of ferrous sulfate to form a slurry which was held at 230C for 30 min. while being stirred. The slurry was allowed to settle on a magnet for 7 minutes. The supernatant liquid was decanted and the Cr02 was twice washed with 350 ml portions of distilled water with 2 minutes of stirring and followed by 5 minutes of settling before decantation. The CrO2 was dried in a vacuum oven at 650C. A 20 g sample of the same upgraded CrO2 was stabilized with sodium sulfite as described in U.S. 3,512,930 and similarly twice washed, filtered, and dried and served as a control. Both the control and the sample treated with ferrous sulfate were weighed. The latter sample showed a 1.54% weight increase while the control weighed .32% less than before the stabilization treatment. Samples were also examined by X-ray diffraction, ESCA, and wet chemical analysis. X-ray diffraction of the control before and after reaction with sodium sulfite revealed the appearance of a new line with an interplanar spacing of 3.151 as taught by U.S. 3,513,930. Similar examination of the ferrous sulfate treated sample did not show this line. The absence of the 3.151 A line indicates the present invention provides stabilization by a different chemical reaction than the prior art. While ESCA revealed no significant differences between the upgraded control before or after sodium sulfite treatment, the experimental sample showed surface iron in the amount of 10.5 atomic %. Table 1 contains a comparison of properties. Table 1 Soluble Chromate Sample as ar (CrO 4) (ppm) none 79.2 38.7 600+ Na2SO3 (control) 73.6 36.3 318 FeSO4 77.4 38.6 177 The high chromate value for the upgraded but unstabilized starting material is indicative of poor stability which makes this material unsuitable for use in tape or disk manufacture. The superiority of the FeSO4-treated sample over both the starting material and the control is indicated by its superior magnetic properties in conjunction with its lower chromate values, which indicates increased stability. The control and the FeSO4-treated sample were each conventionally formulated and coated on polyester strips to produce an audio tape. The tapes showed comparable output and noise performance. A comparison of tapehead abrasion showed improvement, relative to the control, for the FeSO4-treated sample. The results are given in Table 2. Table 2 Audio Output dB it/100 Hrs. Sample 500 Hz 5KHz 10KHz 15KHz Noise Head wear Control 0 0 0 0 0 10 FeSO4- -1.1 +1.5 +3.2 +4.3 0 8 treated EXAMPLE 2 Experiments were run as in Example 1 except that the g salt/g Cr02 was varied for both the control and the invention. The results demonstrated a weight gain as metal adduct formed upon the surface of the Cr02 particles as higher amounts of metal salt were used. They also demonstrated a clear advantage in quality, in terms of better magnetic properties and lower soluble chromate. This is illustrated in FIG. 1. This graph compares Fe addition (broken line) with prior art sodium sulfite addition (solid line), in terms of weight, chromate, and retentivity (or) as a function of salt addition. As increasing amounts of soluble ferrous sulfate are reacted with the CrO2 a steady weight gain results, along with a lowering of soluble chromate. As increasing amounts of sodium sulfite react with the Cr02 a weight loss occurs and chromate values do not decrease as much or as rapidly as the present invention. The magnetic strength of the present invention remains consistently above the prior art control. EXAMPLE 3 Experiments were run as in Example 2 except that SnCl2 was used as the metal salt instead of ferrous sulfate. The results are shown in Figure 2. This graph illustrates a comparison of Sn addition (broken line) with prior art sodium sulfite addition (solid line). As in FIG. 1, the lowering of soluble chromate is superior to the control. As shown, the tin produces a slightly higher weight gain than observed for iron and does not maintain magnetic strength above the control. As compared to the iron salt, the tin salt gives poorer retentivity, but the weight gain and improved chromate values are similar. EXAMPLE 4 Experiments were run as in Example 2 except that Fecal3 was used instead of ferrous sulfate. While the weight gain was less than observed for an equimolar amount of ferrous sulfate, the FeCl3 functioned as a stabilizing metal salt without adversely lowering the magnetic properties of acicular chromium dioxide. EXAMPLE 5 Experiments were run as in Example 1, but replacing ferrous sulfate with zinc, manganese, nickel, and copper salts. Only zinc salts were found to be ineffective insofar as significantly reducing the chromate content of the ferromagnetic chromium dioxide. See Table 3. Table 3 Treated Product Cr02 Weight Stabilization Change, Sample Treatment % cs ar 4 Starting none - 79.2 38.7 600+ material Control Na2SO3 -.32 73.6 36.3 318 Experiment ZnSO4 -.87 79.7 38.8 600+ Experiment Zn (NO3)2 -.88 79.9 39.0 556 Experiment CoC12 -.77 80.2 38.9 431 Experiment MnSO4 -.41 80.1 38.3 208 Experiment NiCl2 -.77 80.1 38.3 227 Experiment CuC12 -.42 80.2 34.5 373 EXAMPLE 6 A batch of commercial grade chromium dioxide was prepared without the upgrading treatment of Example 1. Portions were reacted with sodium sulfite and served as controls. Other portions were treated with FeSO4 and SnCl2 according to the present invention. Table 4 gives comparative results. Table 4 Treatment Ratio g/ salt/ A Sample Salt g CrO wt. iHc as ar (CrO )= 2 4 Starting None - -1.80 572 79.7 39.4 600+ material Control Na2SO3 .041 -0.96 599 76.5 37.8 600+ Control Na2SO3 .080 -0.62 592 72.9 36.1 600+ Control Na2SO3 .162 -0.62 594 72.3 35.6 533 Inven- FeSO .050 -0.08 590 77.2 38.2 370 tion 4 Inven- FeSO4 .100 +1.08 593 77.3 38.0 396 tion Inven- FeSO4 .201 +3.79 592 73.6 36.5 373 tion Inven- SnCl2 .030 +0.74 590 77.2 38.2 573 tion Inven- SnCl .061 +3.13 599 74.0 36.5 552 tion 2 Inven- SnCl .121 +7.16 597 69.6 34.5 361 tion The significance of the loss in weight for the starting material is that all samples were washed to remove soluble materials including chromate ions. The 600+ chromate values are indicative of chromium dioxide as manufactured, and materials having such content would not be expected to give satisfactory results if used in a binder for tape manufacture. As the results indicate, both iron and tin gave lower chromate values than the starting material and the control, but the test samples containing iron were superior to those containing tin in preserving magnetic properties. EXAMPLE 7 Experiment 1 was repeated, with some variations. After the addition of the ferrous sulfate solution and stirring for 25 min. the pH of the mixture was measured as 2.1. The pH was then adjusted to 5.0 with NH OH. The neutralized iron salt remained in contact with the chromium dioxide particles for 5 min. under mild agitation. After decantation, washing, etc. the resulting stabilized chromium dioxide samples A and B were given a test comparison with the control, with the results shown in Table 5. Adjustment of pH did not affect the improvement in chromate values. Table 5 Stabili- pre at zation end of Treated CrO2 Sam- Treat- Treat- A ple ment ment wt. a5 ar (CrO4)= Con- Na2SO3 8.6 -.32 73.6 36.3 318 trol A FeSO4 2.2 +4,if 74.4 36.6 152 B FeSO4 5.0 +3.17 72.6 35.4 193 EXAMPLE 8 Ferric chloride, ferrous ammonium sulfate and ferric ammonium sulfate were compared with ferrous sulfate in the process described in Example 1. Results are shown in Table 6. Table 6 Treated CrO2 Stabilization A Sample Treatment wt.% as ar rO4)= Control Na2SO3 -.32 73.6 36.3 318 Experiment Fe+2 Sulfate 1.54 77.4 38.6 177 Experiment Fe+3 Chloride -.24 81.1 39.6 398 Experiment Fe+2 Am.Sulfate +1.49 77.4 38.1 227 Experiment Fe+3 Am.Sulfate + .61 81.5 40.2 287 The foregoing Examples demonstrate that the present invention uses to good advantage materials which are generally discarded, particularly ferrous sulfate, and it does not require forming or depositing a ferromagnetic metal coating on the ferromagnetic CrO2 particles. On the contrary it permits formation of a coating on the Cr02 particles which is neither metallic nor ferromagnetic; yet this coating surprisingly improves, rather than degrades, the overall ferromagnetic properties of the product.

Claims: CLAIMS 1. A stabilized ferromagnetic chromium dioxide composition comprising acicular ferromagnetic chromium dioxide particles having a nonmagnetic protective surface phase composed of the adduct formed by contact between the particle surface and an aqueous solution of a metal salt, said protective surface phase being characterized by soft x-ray analysis (ESCA) as containing 0.01 to 0.25 atoms of said included metal per atom of chromium. 2. The composition of claim 1 wherein the included metal is iron, tin, cobalt, or manganese. 3. The composition of claim 1 wherein the metal salt yields Fe or Sn cations in solution. 4. A magnetic recording member comprising a support bearing a layer containing the stabilized ferromagnetic chromium dioxide composition of claim 1. 5. A process for preparing a stabilized ferromagnetic chromium dioxide composition which comprises contacting acicular ferromagnetic chromium dioxide particles with an aqueous solution of a metal cation to form a protective surface on said particles, said process being conducted at a temperature of -100C to 1100C and at a pH of 1 to 8. 6. The process of claim 5 wherein the metal +2 cation is a member of the group consisting of Fe Foe+3, Sun 2 Co+2, and Mn 2. 7. The process of claim 5 wherein the protective surface phase imparts a weight gain to the ferromagnetic chromium dioxide particles which is directly proportional to the amount of metal cation reacted therewith. 8. The process of claim 5 wherein the ferromagnetic chromium dioxide which is contacted with the metal cation solution was made by heating preformed CrO2 having a saturation of at least 75 emu./g. and a coercive force of 200-800 oersteds in an oxidizing environment at 1500C to 4500C at a pressure of 0.2-3000 atmospheres. 9. The process of claim 5 wherein the acicular ferromagnetic chromium dioxide particles are contacted with an aqueous solution of FeSo4#7H20 or SnCl2.

Inventors: DOMBROWSKI, HENRY STEPHEN
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1982
Main classification: H01F1
IPC codes: H01F1, G11B5, C01G37