Coated acicular fine particulate materials, processes for preparing same and use thereof

An acicular fine particulate material which is coated with a compound containing at least one element selected from among elements of Groups III and IV of the Periodic Table in Periods 5 and 6, the acicular fine particulate material being one of (i) an acicular fine particulate material containing iron carbide, (ii) an acicular fine particulate material containing iron carbide, metallic iron and carbon, and (iii) an acicular fine particulate metallic iron containing carbon, is prepared by a promoted reduction reaction while being effectively prevented from sintering and consequently affords magnetic recording media having a high recording density.

TECHNICAL FIELD 
The present invention relates to coated acicular fine particulate 
materials, processes for preparing the materials, and magnetic coating 
compositions and magnetic recording media containing the material. 
BACKGROUND ART 
Presently, magnetic recording media are commercially available which have 
various magnetic characteristics. Higher recording densities are the 
properties required of magnetic media of the next generation. Magnetic 
powders of higher coercive force are required to fulfill this requirement. 
The coercive force of magnetic powders is dependent largely on magnetic 
shape anisotropy attributable to their shape and magnetic crystalline 
anisotropy attributable to their magnetic energy. The present invention 
relates to a method of giving an improved coercive force by enhanced shape 
anisotropy. More particularly, the invention relates to a method of 
obtaining a high coercive force by using fine particles having a great 
axial ratio and utilizing the shape thereof for preparing a magnetic 
powder which is excellent in acicular properties. 
Fine particles having an increased axial ratio generally deform and sinter 
under a thermal load during preparation, encountering difficulty in 
achieving the contemplated purpose. 
Conventionally, fine particles are coated with aluminum, silica or like 
sintering preventing agent and are thereby prevented from deforming or 
sintering (e.g., U.S. Pat. No. 4,956,220). However, particulate materials, 
when in the form of fine particles, are liable to sinter during reaction 
and therefore require a large amount of sintering preventing agent, which 
increases the amount of nonmagnetic component and hampers the reduction 
reaction to result in the drawback of impaired magnetic characteristics. 
An object of the present invention is to provide an acicular fine 
particulate material which is prepared by a promoted reduction reaction 
while being effectively prevented from sintering and which consequently 
affords magnetic recording media having a high recording density, and to 
provide a process for preparing the particulate material, and magnetic 
coating compositions and magnetic recording media containing the material. 
DISCLOSURE OF THE INVENTION 
The present invention provides an acicular fine particulate material which 
is coated with a compound containing at least one element selected from 
among elements of Groups III and IV of the Periodic Table in Periods 5 and 
6, the acicular fine particulate material being one of (i) an acicular 
fine particulate material containing iron carbide, (ii) an acicular fine 
particulate material containing iron carbide, metallic iron and carbon, 
and (iii) an acicular fine particulate metallic iron containing carbon. 
The invention also provides a process for preparing the coated acicular 
fine particulate material, and magnetic coating compositions and magnetic 
recording media containing the particulate material. 
The acicular fine particulate material of the present invention is 
characterized in that the material is coated with a compound containing at 
least one element selected from among those in Groups III and IV of the 
Periodic Table in Periods 5 and 6 (hereinafter referred to merely as 
"Group III or IV elements"). The Periodic Table hereinafter referred to is 
based on the table in conformity with the classification of subgroups 
prescribed by IU in 1970. 
The acicular fine particulate material of the present invention is (i) an 
acicular fine particulate material containing iron carbide, (ii) an 
acicular fine particulate material containing iron carbide, metallic iron 
and carbon, or (iii) an acicular fine particulate metallic iron containing 
carbon. 
The acicular fine particulate material (i) containing iron carbide is 
described in detail in JP-B-43683/1989 and has the features of being 
higher in coercive force and saturation magnetization than conventional 
iron oxide magnetic powders and excellent in corrosion resistance. Such 
iron carbide is prepared, for example, by (a) contacting with heating a 
reducing agent containing no carbon atom with a starting material iron 
compound selected from among iron oxyhydroxides and iron oxides when 
required and (b) thereafter contacting a reducing-carbonizing agent 
containing a carbon atom or mixture of the agent and a reducing agent 
containing no carbon atom with the resulting iron compound. 
Examples of preferred acicular iron oxyhydroxides are acicular 
.alpha.-FeOOH (goethite), .beta.-FeOOH (akaganite) and acicular 
.gamma.-FeOOH (lepidocrosite), and preferred acicular iron oxides are 
acicular .alpha.-Fe.sub.2 O.sub.3 (hematite) , acicular .gamma.-Fe.sub.2 
O.sub.3 (maghemite) and acicular Fe.sub.3 O.sub.4 (magnetite). 
Acicular .alpha.-Fe.sub.2 O.sub.3 or acicular .gamma.-Fe.sub.2 O.sub.3 are, 
for example, any of those prepared by heating acicular .alpha.-FeOOH, 
acicular .beta.-FeOOH or acicular .gamma.-FeOOH at about 200.degree. to 
about 350.degree. C., followed by dehydration, acicular .alpha.-Fe.sub.2 
O.sub.3 or acicular .gamma.-Fe.sub.2 O.sub.3 prepared by heating the 
resulting product further at about 350.degree. to about 900.degree. to 
compact the crystals, and others. 
The acicular iron oxyhydroxides or acicular iron oxides are usually at 
least 3, preferably 5 to 15, in average axial ratio and usually up to 2 
.mu.m, preferably 0.05 to 1 .mu.m, in average particle size (long axis). 
The acicular fine particulate iron carbide prepared from such a material 
is almost unchanged from the starting material in average axial ratio and 
average particle size. 
JP-A-106408/1986 describes in detail the steps (a) and (b) for preparing 
the acicular fine particulate material containing iron carbide and the 
reducing agent and reducing-carbonizing agent for use in these steps. 
Especially desirable carbon-free reducing agents are H.sub.2 and NH.sub.2 
NH.sub.2, and desirable carbon-containing reducing-carbonizing agents are 
CO, CH.sub.3 OH, HCOOCH.sub.3 and saturated or unsaturated aliphatic 
hydrocarbons having 1 to 5 carbon atoms. 
In the step (a) of the above, the reducing agent containing no carbon atom 
:can be used as it is or as diluted. Examples of diluents are N.sub.2, 
argon, helium, etc. The dilution ratio is suitably selected but is 
preferably about 1.1 to about 10 times (by volume). The contact 
temperature, contact time, gas flow rate and other conditions depend, for 
example, on the production history, average axial ratio, average particle 
size and specific surface area of the acicular iron oxyhydroxide or 
acicular iron oxide. The preferred contact temperature is about 
200.degree. to about 700.degree. C., preferably about 300.degree. to about 
400.degree. C. The preferred contact time is about 0.5 to about 6 hours. 
The preferred gas flow rate (excluding diluent) is about 1 to about 1000 
ml S.T.P./min, more preferably about 5 to about 500 ml S.T.P./min, per 
gram of the starting material. The contact pressure inclusive of that of 
the diluent is usually 1 to 2 arm. although not limitative particularly. 
In the step (b) of the above, the reducing-carbonizing agent containing 
carbon atom or a mixture thereof with the reducing agent containing no 
carbon atom can be used as it is or as diluted. When the mixture is used, 
the mixing ratio of the reducing-carbonizing agent containing carbon atom 
and the reducing agent containing no carbon atom is suitably selected but 
is preferably 1/0.05 to 1/5 by volume. Contact conditions are also 
suitably selected but the preferred contact temperature is about 
250.degree. to about 400.degree. C., more preferably about 300.degree. to 
about 400.degree. C. The preferred contact time is about 0.5 to 6 hours 
when the contact in (a) is conducted, and about 1 to about 12 hours when 
the contact in (a) is not conducted. The preferred gas flow rate 
(excluding diluent) is about 1 to about 1000 ml S.T.P./min, more 
preferably about 5 to about 500 ml S.T.P./ml, per gram of the starting 
iron compound. The contact pressure inclusive of that of the diluent is 
usually 1 to 2 atm. although not limitative particularly. 
The product thus obtained consists chiefly of iron carbide, while free 
carbon and iron oxide can also be present therein. The acicular fine 
particulate material of the invention containing iron carbide contains at 
least 20%, preferably at least 50% and more preferably at least 60% , of 
iron carbide, which is predomonantly Fe.sub.5 C.sub.2, while Fe.sub.7 
C.sub.3, FeC and Fe.sub.3 C can be present. The product can further be 
covered with a protective iron oxide coating by contacting the product 
with a gas containing oxygen. Unreacted iron oxide can also be present. 
The acicular fine particulate material (ii) of the invention containing 
iron carbide, metallic iron and carbon is described in detail in Japanese 
Patent Application No. 34086/1992, has a specific range of magnetic 
characteristics suitable for audio metal tapes, is low in noise level and 
can be prepared, for example, by contacting a gas mixture of a reducing 
agent having no carbon atom and a reduction controlling agent with the 
above acicular fine particulate material containing iron carbide. 
The reduction control agent for use in the above means an agent capable of 
controlling the velocity of reduction of iron carbide with the above 
reducing agent containing no carbon atom. Examples of such agents are 
carbon-containing compounds with one or two carbon atoms, and are more 
specifically carbon monoxide (CO), carbon dioxide (CO.sub.2), methane, 
methanol, ethanol, formic acid, methyl formate, etc., among which CO and 
CO.sub.2 are preferable. CO is especially preferable. 
The gas mixture of the reducing agent containing no carbon atom and 
reduction control agent for use in the reduction of the above comprises a 
very large amount of the reducing agent containing no carbon atom and a 
very small amount of reduction control agent. The reducing agent 
containing no carbon atom to reduction control agent ratio by volume is 
preferably 1:0.004 to 1:0.0005, more preferably 1:0.0025 to 0.0005. If the 
proportion of reduction control agent exceeds the above range, the 
reducing reaction slows down or fails to proceed, whereas when the 
proportion is smaller than the above range, the agent produces no effect. 
A part of the reaction control agent present and the hydrocarbon and 
carbon produced by the reduction of iron carbide can be separated out as 
free carbon on the surface of metallic iron or iron carbide depending on 
the conditions involved, which constitute a part of the components of the 
present acicular fine particulate material (ii). 
In the process for preparing the acicular fine particulate material (ii) of 
the present invention containing iron carbide, metallic iron and carbon, 
the preferred contact temperature is about 300.degree. to about 
400.degree. C. The preferred contact time varies with the amount of 
material to be treated, treating conditions and composition of fine 
particles of the invention to be desired, and is about 0.2 to about 6 
hours. When required, a diluting agent such as Ns, Ar or He is usable. The 
diluting ratio can be determined as desired, and may be, for example, 1.1 
to 10 times the amount (by volume) of the gas mixture. The preferred rate 
of flow of the gas mixture other than the diluting agent is about 1 to 
1000 ml S.T.P./min per gram of the acicular iron carbide material. 
Although not limited specifically, the contact pressure of the gas mixture 
inclusive of the diluting agent is usually 1 to 2 atm. 
The acicular fine particulate material (ii) of the present invention 
preferably contains 10 to 75 wt. % of iron carbide, 15 to 80 wt. % of 
metallic iron and 5 to 13 wt. % of free carbon. The carbon content of the 
acicular fine particles of the above can be determined only by elementary 
analysis, and no carbon or no graphite is detectable using X-rays. It is 
speculated that the carbon can be amorphous, but details still remain 
unknown. The carbon content determined by elementary analysis is the total 
carbon content which is the sum of the carbon content of the iron carbide 
(7.92% calculated for Fe.sub.5 C.sub.2) and free carbon content. 
The acicular fine particles of the above contain free carbon or iron 
carbide in addition to metallic iron, and are therefore relatively higher 
than the conventional metallic magnetic powder in corrosion resistance. 
The acicular fine particulate metallic iron (iii) containing carbon 
(carbon-containing metallic iron) of the invention is described in detail 
in JP-A-228502/1992 and is prepared, for example, by contacting a reducing 
agent containing no carbon atom with the foregoing acicular fine 
particulate material containing iron carbide. The carbon-containing 
metallic iron is equivalent to conventional metallic iron in coercive 
force but superior thereto in corrosion resistance and saturation 
magnetization. 
The reason why the carbon-containing metallic iron has excellent magnetic 
characteristics still remains to be clarified. The conventional acicular 
fine particulate metallic iron is obtained by directly reducing acicular 
fine particulate iron oxide, so that the reduction produces water which 
causes deformation and evolves heat which causes sintering and 
deformation, failing to afford highly dispersible particles. Further use 
of a large amount of sintering preventing agent results in lower magnetic 
characteristics relative to the density, failing to give particles of high 
saturation magnetization. In contrast, the reduction process for producing 
the carbon-containing metallic iron does not produce a large amount of 
water and therefore gives a product of high coercive force and high 
dispersibility with use of a lesser amount of sintering preventing agent, 
permitting the metallic iron to exhibit an inherent highly magnetizable 
property. Furthermore, the reduction of iron carbide to metallic iron 
forms carbon, which partly separates out (X-ray diifractometry appears to 
indicate that the separating-out carbon is not graphite but amorphous 
carbon) to effectively protect the active surface of the metallic iron and 
impart corrosion resistance, enabling the metallic iron to retain the 
excellent magnetic characteristics free of deterioration over a prolonged 
period of time. 
Examples of reducing agents having no carbon atom are, like those 
previously mentioned, H.sub.2, NH.sub.2 NH.sub.2 and the like. The 
preferred contact temperature is about 300.degree. to about 500.degree. 
C., and the preferred contact time is about 0.5 to about 6 hours. When 
required, diluents, the same as those previously mentioned, are usable. 
The preferred flow rate is about 1 to about 1000 ml S.T.F./min per gram of 
the acicular fine particulate material containing iron carbide, exclusive 
of the diluent. The contact pressure including that of the diluent is 
usually 1 to 2 atm. but is not limited particularly. 
The carbon-containing metallic iron of the present invention consists 
primarily of metallic iron and contains usually 90 to 50 wt. % of metallic 
iron and usually 2 to 20 wt. % , preferably 5 to 15 wt. % , of carbon. If 
the carbon content is greater than this range, the improvement in 
magnetization will be less, whereas when the carbon content is smaller, 
lower corrosion resistance will result. The carbon content is expressed in 
terms of a total carbon content which is the sum of the amount of carbon 
produced during the process for preparing iron carbide serving as the 
starting material and the amount of carbon formed and separating out 
during the preparation process of the invention. According to the 
invention, the content only of carbon can be determined by elementary 
analysis, while the carbon is not detectable in the form of iron carbide 
or graphite with use of X-rays. Although we speculate that the carbon 
might be amorphous, the type of the carbon still remains to be clarified. 
An iron oxide can further be present in the carbon-containing metallic 
iron of the invention. The carbon-containing metallic iron obtained 
according to the invention can be covered with a protective iron oxide 
coating over the surface by contacting the iron with an oxygen-containing 
gas. An unreacted iron oxide can also be present in the iron. 
The present invention is characterized in that the acicular fine 
particulate material is coated with a compound containing at least a Group 
I element or Group IV element. The acicular fine particulate material of 
the invention, which is coated with (A) the compound containing a Group 
III element or Group N element, may further be coated with (B) at least 
one of a nickel compound, copper compound, manganese compound and cobalt 
compound and/or (C) at least one of a silicon compound and aluminum 
compound. (These compounds will be referred to as "coating compounds" as 
the case may be.) 
Especially when a cobalt compound is used as the compound (B), the material 
is further improved in coercive force and magnetization and given higher 
corrosion resistance at the same time. 
Examples of elements from Group III (Groups III a, III b) for use in the 
present invention are indium, thallium, yttrium, lanthanum and elements of 
the lanthanide series. The elements of the lanthanide series include 
cerium, praseodymium, neodymium, promethium, samarium, europium, 
gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and 
lutetium. Examples of elements in Group IV (Groups IV a and IV b) are tin, 
lead, zirconium and hafnium. When the particulate material is to be coated 
with a compound of such an element, the compound may be in the form of a 
hydrochloride, sulfate, nitrate, acetate, oxalate, organic metal salt or 
the like, and is not limited to a specific form. 
Examples of nickel compounds useful for the invention are nickel chloride, 
nickel nitrate, nickel sulfate, nickel bromide, nickel acetate and the 
like. Examples of copper compounds are cupric sulfate, copper nitrate, 
cupric chloride, cupric bromide, copper acetate and the like. Examples of 
manganese compounds are manganese sulfate, manganese nitrate, manganese 
chloride, manganese bromide and the like. Examples of cobalt compounds are 
cobalt chloride, cobalt nitrate, cobalt sulfate, cobalt acetate and the 
like. 
Further examples of silicon compounds are sodium orthosilicate, sodium 
metasilicate, potassium metasilicate, water glasses of varying 
compositions, etc. Examples of aluminum compounds are aluminum sulfate, 
aluminum nitrate, aluminum chloride, various aluminum alums, sodium 
aluminate, potassium aluminate and the like. 
According to the present invention, the particulate material can be coated 
with the compounds (A), (B) and/or (C) each in an amount of 0.01 to 30 
atomic %, preferably 0.01 to 10 atomic %, more preferably 0.1 to 2 atomic 
%, based on the iron atoms. The amount is preferably in the range of 0.1 
to 20 atomic % in the case of the cobalt compound. It is desired that the 
total amount of costing compounds be not in excess of 100 atomic % based 
on the iron atoms because if the total amount of coating compounds is 
greater, lower magnetic characteristics will usually result relative to 
the density. 
The acicular fine particulate material of the present invention containing 
iron carbide can be prepared, for example, by (1) the step of coating a 
starting material iron compound selected from among iron oxyhydroxides and 
iron oxides with a compound containing a Group III or IV element, and (2) 
the step of contacting a reducing-carbonizing agent containing a carbon 
atom or a mixture of the agent and a reducing agent containing no carbon 
atom with the coated iron compound obtained by the step (1) with heating, 
after contacting the iron compound with a reducing agent containing no 
carbon atom with heating when required. 
The acicular fine particulate material of the invention containing iron 
carbide, metallic iron and carbon can be prepared, for example, by (1) the 
step of coating a starting material iron compound selected from among iron 
oxyhydroxides and iron oxides with a compound containing a Group III or IV 
element, (2) the step of containing a reducing-carbonizing agent 
containing a carbon atom or a mixture of the agent and a reducing agent 
containing no carbon atom with the coated iron compound obtained by the 
step (1) with heating, after contacting the iron compound with a reducing 
agent containing no carbon atom with heating when required, and (3) the 
step of contacting the reaction product obtained by the step (2) with a 
mixture of a reducing agent containing no carbon atom and a reducing 
controlling agent with heating. 
The acicular fine particulate metallic iron of the invention containing 
carbon can be prepared, for example, by (1) the step of coating a starting 
material iron compound selected from among iron oxyhydroxides and iron 
oxides with a compound containing a Group III or IV element, (2) the step 
of contacting a reducing-carbonizing agent containing a carbon atom or a 
mixture of the agent and a reducing agent containing no carbon atom with 
the coated iron compound obtained by the step (1) with heating, after 
contacting the iron compound with a reducing agent containing no carbon 
atom with heating when required, and (3) the step of contacting the 
reaction product obtained by the step (2) with a reducing agent containing 
no carbon atom with heating. 
As to the coating compounds to be used for coating, it is essential in the 
present invention to apply the compound (A) containing a Group III or IV 
element, but optional to apply other compounds, i.e., the nickel compound 
(B) and the silicon compound (C) or the like. Further such coating 
compounds are applied in a desired order. While these compounds are 
applied by a desired method, it is preferable to coat the starting 
material iron compound, for example, by: 
(1) dispersing the iron compound in water, 
(2) executing at least once the step of admixing a solution containing at 
least one of the coating compounds with the material from the preceeding 
step and the step of causing the coating compound to form a salt sparingly 
soluble in water to thereby precipitate the coating compound, and 
(3) filtering the resulting mixture, followed by washing and drying. 
According to the invention, the acicular fine particulate material is 
coated with the coating compound at least over the surface thereof, and 
the surface may be coated partly or entirely. The salt sparingly soluble 
in water can be formed by a suitably selected method, for example, usually 
by pH adjustment or addition of a compound having an anion for converting 
the coating compound to a sparingly soluble salt. The drying step is 
executed in a usual manner, for example, by air drying with heating at 
about 50.degree. to about 150.degree. C. 
The acicular fine particulate materials obtained by the present invention 
have the shape of the starting material iron compound. The acicular fine 
particulate material (i) containing iron carbide apparently remains 
unchanged from the starting material in shape, i.e., in average particle 
size and average axial ratio. On the other hand, the acicular fine 
particulate material (ii) containing iron carbide, metallic iron and 
carbon, and the acicular fine particulate metallic iron (iii) containing 
carbon tend to be smaller in particle size (long axis), but remain almost 
unchanged in axial ratio due to a diminution in short axis, and are 
generally within the foregoing ranges in shape. More particularly, they 
are usually up to 2 .mu.m, preferably 0.05 to 1 .mu.m, in average particle 
size (long axis) and 3 to 15, preferably 5 to 15, in average axial ratio. 
The magnetic coating composition of the present invention can be prepared 
by dispersing the above acicular fine particles of the present invention 
in an organic solvent together with a binder. To the composition are 
added, as required, a dispersing agent, lubricant, abrasive, antistatic 
agent and like additives. 
Hitherto known thermoplastic resins, thermosetting resins, reaction-type 
resins, or mixtures thereof, can be used as binders in the present 
invention. 
Suitable thermoplastic resins are those which have a softening point of 
about 150.degree. C. or less, an average molecular weight of about 10,000 
to 200,000, and a degree of polymerization of about 200 to 2,000, e.g., a 
vinyl chloridevinyl acetate copolymer, a vinyl chloride-vinylidene 
chloride copolymer, a vinyl chloride-acrylonitrile copolymer, an 
acrylate-acrylonitrile copolymer, an acrylate-vinylidene chloride 
copolymer, an acrylate-styrene copolymer, a methacrylate-acrylonitrile 
copolymer, a methacrylate-vinylidene chloride copolymer, a 
methacrylate-styrene copolymer, a urethane elastomer, a polyvinyl 
fluoride, a vinylidene chloride-acrylonitrile copolymer, a 
butadiene-acrylonitrile copolymer, a polyamide resin, polyvinyl butyral, 
cellulose derivatives such as cellulose acetate butyrate, cellulose 
diacetate, cellulose triacetate, cellulose propionate, cellulose nitrate, 
and the like, a styrene-butadiene copolymer, a polyester resin, a 
chlorovinyl etheracrylate copolymer, an amino resin, various synthetic 
rubber based thermoplastic resins and mixtures thereof. 
Suitable thermosetting resins or reaction-type resins have a molecular 
weight of about 200,000 or less as a coating solution, and when heated 
after coating and drying, the molecular weight becomes infinite due to 
reactions such as condensation, addition, and the like. Of these resins, 
preferred resins are those resins which do not soften or melt before the 
resin thermally decomposes. Representative examples of these resins are a 
phenol resin, an epoxy resin, a polyurethane hardening type resin, a urea 
resin, a melamine resin, an alkyd resin, a silicone resin, an acryl based 
reactive resin, an epoxy-polyamide resin, a mixture of a high molecular 
weight polyester resin and an isocyanate prepolymer, a mixture of a 
methacrylic acid copolymer and a diisocyanate-prepolymer, a mixture of a 
polyester-polyol and a polyisocyanate, a urea-formaldehyde resin, a 
mixture of a low molecular weight glycol, a high molecular weight diol and 
triphenylmethane triisocyanate, a polyamine resin, and mixtures thereof, 
etc. 
These binders can be used singly or in mixture, and the additives can be 
added to the binders. The binders are used in an amount of preferably 10 
to 400 parts by weight, more preferably 30 to 200 parts by weight per 100 
parts by weight of the acicular particles. 
The organic solvents include ketones such as acetone, methyl ethyl ketone, 
methyl isobutyl ketone and cyclohexanone; alcohols such as methanol, 
ethanol, propanol and butanol; esters such as methyl acetate, ethyl 
acetate, butyl acetate, ethyl lactate and glycol monoethyl ether acetate; 
ethers such as ether, glycol dimethyl ether, glycol monoethyl ether and 
dioxane; tars (aromatic hydrocarbons) such as benzene, toluene and xylene; 
chlorinated hydrocarbons such as methylene chloride, ethylene chloride, 
carbon tetrachloride, chloroform, ethylene chlorohydrin and 
dichlorobenzene; and the like. 
The dispersing agents used include aliphatic acids having 12 to 18 carbon 
atoms (R.sup.1 COOH wherein R.sup.1 is an alkyl group having 11 to 17 
carbon atoms) such as caprylic acid, captic acid, lauric acid, myristic 
acid, palmitic acid, stearic acid, oleic acid, elaidic acid, linolic acid, 
linolenic acid and stearolic acid; metal soaps comprising an alkali metal 
(such as Li, Na and K) or an alkaline earth metal (such as Mg, Ca and Ba) 
salt of the above aliphatic acids; lecithin, etc. In addition, higher 
alcohols having 12 or more carbon atoms and sulfuric esters can be used. 
These dispersing agents are added in an amount of 1 to 20 parts by weight 
per 100 parts by weight of the binder. 
The lubricants used include silicone oil, graphite, molybdenum disulfide, 
tungsten disulfide, aliphaticesters obtained from monobasic aliphatic 
acids having 12 to 16 carbon atoms and monohydric alcohols having 3 to 12 
carbon atoms, aliphatic esters obtained from monobasic aliphatic acids 
having 17 or more carbon atoms and monohydric alcohols (a total of the 
carbon atoms of the monobasic aliphatic acid and the carbon atoms of the 
monohydric alcohol are 21 to 23), etc. These lubricants are added in an 
amount of 0.2 to 20 parts by weight per 100 parts by weight of the binder. 
The abrasives used include those which are generally used, such as fused 
alumina, silicon carbide, chromium oxide, corundum, artificial corundum, 
diamond, artificial diamond, garnet and emery (main components: corundum 
and magnetite). The average particle diameter of these abrasives is 0.05 
to 5 .mu.m, preferably 0.7 to 2 .mu.m. These abrasives are added in an 
amount of 7 to 20 parts by weight per 100 parts by weight of the binder. 
Examples of the antistatic agents are natural surfactants such as saponin, 
nonionic surfactants such as alkylene oxide-base, glycerin-base or 
glycidol-base surfactant; cationic surfactants such as higher alkylamines, 
quaternary ammonium salts, pyridine and like heterocyclic compounds, 
phosphonium or sulfonium compounds; artionic surfactants such as those 
containing a carboxylic acid, sulfonic acid, phosphoric acid, sulfate 
group or phosphate group and like acid group; ampholytic surfactants such 
as amino acids, amino sulfonic acid, sulfate or phosphate of aminoalcohol, 
etc. These antistatic agent can be used singly or in mixture. Although the 
above compounds are used as antistatic agents, the compounds can be used 
in some cases, to improve the dispersibility, magnetic characteristics, 
lubricability or coating ability. These antistatic agents are added in an 
amount of 1 to 2 parts by weight per 100 parts by weight of the binder. 
The magnetic recording medium of the present invention are obtained by 
coating the above magnetic coating composition on a substrate (support). 
The thickness of the support is about 5 to 50 .mu.m, preferably about 70 to 
40 .mu.m. The materials used for the support include polyesters such as 
polyethylene terephthalate and polyethtylene-2,6-naphthalate, polyolefins 
such as polypropylene, cellulose derivatives such as cellulose triacetate 
and cellulose diacetate, polycarbonate, and the like. 
For preventing static discharge or preventing transfer printing, the above 
supports may have a back coat on the surface opposite the surface provided 
with the magnetic layer. 
The supports may be in any shape such as a tape, sheet, card, disc or drum, 
and various materials can be used depending upon the shape desired and end 
use contemplated. 
The magnetic coating composition can be applied on the support by various 
conventional methods including air doctor coating, blade coating, air 
knife coating, squeeze coating, impregnation coating, reverse roll 
coating, transfer roll coating, gravure coating, kiss coating, cast 
coating and spray coating. Other coating methods can also be used. 
The magnetic layer formed on the support by the above method is dried 
after, if desired, the coating has been subjected to a treatment for 
orienting the magnetic powder in the layer. If required, the magnetic 
layer can be subjected to a surface smoothening treatment, or cut to the 
desired shape, to thereby form the magnetic recording material of this 
invention. In the above orienting treatment for the magnetic layer, the 
orienting magnetic field can be either an AC or DC magnetic field with a 
field strength of about 500 to 5,000 gauss. The drying temperature can 
range from about 50.degree. to about 100.degree. C., and the drying time 
is about 3 to 10 minutes. 
BEST MODE FOR CARRYING OUT THE INVENTION 
The invention will be described below in detail by showing examples and 
comparative examples. 
In the following examples, characteristic, etc. are determined by 
the-methods given below. 
(1) Magnetic Characteristics 
Using a magnetic characteristics measuring device of the sample-vibrating 
type, the powder was tested in a magnetic field with a strength of 10 kOe 
for coercive force and saturation magnetization, and the sheet was tested 
at a field strength of 5 kOe for coercive force (Hc, Oe), saturation 
magnetic flux density (Bm, gauss), residual magnetic flux density (Br, 
gauss) and square ratio (Br/Bm). 
(2) SFD (Switching Field Distribution) 
SFD was determined by preparing a differentiation curve of coercive force 
with use of the differentiation circuit of the above device for a 
sheetlike test piece, measuring the half-value width of the curve and 
dividing the measured value by the peak value of coercive force of the 
curve. 
(3) Elementary Analysis for C, H and N 
The sample was subjected to elementary analysis in the conventional method 
using MT2 CHN CORDER Yanaco, product of Yanagimoto Mfg. Co., Ltd, with 
passage of oxygen (helium carrier) at 900.degree. C. 
The corrosion resistance of the acicular fine particulate material of the 
invention can be evaluated by the corrosion resistance test method 
described in JP-A-228502/1992. More specifically, the sample was allowed 
to stand in air at 100.degree. C. for 8 hours, and the resulting reduction 
in the amount of magnetization (.sigma. s) was calculated as the corrosion 
resistance (%), 
EQU Corrosion resistance (%)=(1- b/a).times.100 
where a and b are the amounts of magnetization (.sigma. s) of the sample 
before and after the standing, respectively. The lower the value, the 
greater is the corrosion resistance. 
The proportions of components of the acicular fine particulate materials of 
the present invention can be determined from the result of elementary 
analysis and the amount of saturation magnetization measured. Particularly 
described below is how to determine the proportion of metallic iron in the 
acicular fine particulate material (ii) containing iron carbide, metallic 
iron and carbon. 
1) Elementary analysis is made to determine the total carbon contents of 
the acicular fine particulate material (ii) of the invention containing 
iron carbide, metallic iron and carbon, iron carbide starting material and 
fine particulate metallic iron prepared by the process of Comparative 
Example 5 given below. 
2) The amounts of saturation magnetization of these three kinds of 
materials were measured. 
3) The proportion .alpha. of metallic iron was calculated from the 
following equation. 
EQU .alpha.={100y.sub.1 
-303.times.B.times.(100-x.sub.1)/279}/(A-303.times.B/279) 
A=100.times.y.sub.3 /100-x.sub.3 
B=27900x Y.sub.2 /{303.times.(100-x.sub.2)} 
x.sub.1 : total carbon content (%) of acicular fine particulate material 
(ii) of the invention 
x.sub.2 : total carbon content (%) of iron carbide starting material 
x.sub.3 : total carbon content (%) of fine particulate metallic iron 
obtained by the process Comparative Example 5 
y.sub.1 : amount of saturation magnetization (emu/g) acicular fine 
particulate material (ii) of the invention 
y.sub.2 : amount of saturation magnetization (emu/g) iron carbide starting 
material 
y.sub.3 : amount of saturation magnetization (emu/g) of fine particulate 
metallic iron obtained by the process of Comparative Example 5 
In the tables, for example, neodymium acetate will be referred to briefly 
as Nd acetate, nickel acetate as Ni acetate and sodium aluminate as Na 
aluminate as the case may be.

EXAMPLE 1 
In 7 liters of water were dispersed 250 g acicular goethite particles, 0.3 
.mu.m in average particle size (long axis) and 10 in average axial ratio, 
and the dispersion was adjusted to pH 5 by adding acetic acid thereto. 
Next, 4.77 g of Nd(CH.sub.3 COO).sub.3. H.sub.2 O for giving an Nd/Fe 
ratio of 0.5 atomic % and 1.40 g of Ni(CH.sub.3 COO).sub.2.4H.sub.2 O for 
giving an Ni/Fe ratio of 0.2 atomic % were added to the dispersion. The 
mixture was adjusted to a pH of 8.5 by dropwise addition of Na.sub.2 
CO.sub.3 aqueous solution and further to a pH of 11 by addition of NaOH 
aqueous solution. To the resulting mixture was added 3.69 g of water glass 
No. 3 (sodium silicate solution, JIS K1408, 9.about.10% Na.sub.2 O, 
28.about.30% SiO.sub.2) so that Si/FeOOH=0.2 wt. % (Si/Fe=0.63 atomic % ), 
followed by dropwise addition of an aqueous solution of acetic acid for 
the adjustment to pH 8.5 and further by filtration, washing with water and 
drying. 
The resulting goethite, which was coated with Nd/Ni/Si, was placed into a 
muffle furnace and heated at 600.degree. C. for 1 hour to obtain an 
.alpha.-Fe.sub.2 O.sub.5 powder. A 40 g quantity of the .alpha.-Fe.sub.2 
O.sub.3 powder was placed into a reactor tube and treated with CO at a 
flow rate of 5 liters/min at 370.degree. C. for 3.5 hours. With the gas 
replaced by nitrogen, the powder was cooled approximately to room 
temperature, and the gas was thereafter gradually replaced by air to 
obtain a powder containing iron carbide. The X-ray (Cu K.alpha. source) 
diffraction pattern of the product was in match with that of Fe.sub.5 
C.sub.2 on ASTM X-ray Powder Data File 20-509. The iron carbide-containing 
powder contained 88 wt. % of Fe.sub.5 C.sub.2 as iron carbide and had the 
magnetic characteristics of 979 Oe in coercive force Hc and 91.2 emu/g in 
saturation magnetization .sigma. s. 
A 5 g quantity of the powder and 1 g of modified polyvinyl chloride-acetate 
resin were admixed with and dispersed in 12.5 g of a solvent mixture of 
methyl ethyl ketone, toluene and cyclohexanone to prepare a magnetic 
coating composition, which was then applied to a polyethylene 
terephthalate (PET) film to a thickness of about 5 .mu.m (as dried) in an 
orientation magnetic field to obtain a magnetic sheet. The sheet was 922 
Oe in coercive force, 2040 G in residual magnetic flux density Br, 0.85 in 
square ratio Sq (Br/Bm) and 0.62 in switching field distribution SFD. 
EXAMPLE 2 
A 25 g quantity of the above powder containing iron carbide was placed into 
a reactor tube and treated with hydrogen at a flow rate of 10 liters/min 
at 400.degree. C. for 60 minutes. With the gas replaced by nitrogen, the 
powder was cooled approximately to room temperature, and the gas was 
thereafter gradually replaced by air to obtain a powder of metallic iron 
containing carbon. The powder had the magnetic characteristics of 1708 Oe 
in coercive force Hc and 158.8 emu/g in saturation magnetization .sigma. 
s, and was 8 wt. % in total carbon content and 60 m.sup.2/ g in BET 
specific surface area as determined by nitrogen adsorption method. 
In the same manner as above, a magnetic sheet was prepared which was 1718 
Oe in coercive force, 4300 G in residual magnetic flux density Br, 0.90 in 
square ratio Sq (Br/Bm) and 0.37 in switching field distribution SFD. 
EXAMPLES 3 AND 4 
Powders containing iron carbide were prepared in the same manner as in 
Example 1 with the exception of using the corresponding coating compounds 
listed in Table 1. Each powder was further made into a powder of metallic 
iron containing carbon in the same manner as in Example 2. Table 1 shows 
the magnetic characteristics of the powders obtained. 
COMATIVE EXAMPLE 1 
A powder of carbon-containing metallic iron was prepared in the same manner 
as in Example 2 except that compounds containing aluminum and silicon, 
respectively, were used as coating compounds as listed in Table 1 . Table 
1 shows the magnetic characteristics of the powder obtained. 
COMATIVE EXAMPLE 2 
A powder of carbon-containing metallic iron was prepared in the same manner 
as in Comparative Example 1 except that the starting material was replaced 
by one with a long axis size of 0.4 .mu.m. Table 1 shows the magnetic 
characteristics of the powder obtained. 
EXAMPLES 5 TO 14 
Powders containing iron carbide were prepared in the same manner as in 
Example 1 with the exception of using the corresponding coating compounds 
listed in Tables 2 and 3. Each of the powders was further made into a 
powder of carbon-containing metallic iron in the same manner as in Example 
2. Tables 2 and 3 show the magnetic characteristics of the powders 
obtained. 
COMATIVE EXAMPLE 3 
.alpha.-Fe.sub.2 O.sub.3 coated with the sintering preventing agent listed 
in Table 3 was reacted with hydrogen only at 400.degree. C. for 6 hours 
and then oxidized gradually to prepare fine particulate metallic iron. The 
amount of sintering preventing agent used was small to result in poor 
magnetic characteristics. The product had relatively high corrosion 
resistance because sintering reduced the surface area and because the 
material was initially low in saturation magnetization. 
COMATIVE EXAMPLES 4 
Carbon-free metallic iron was prepared in the same manner as in Comparative 
Example 3 except that the sintering preventing agent was used in the 
amount listed in Table 3. Use of the increased amount of sintering 
preventing agent resulted in diminished saturation magnetization and low 
corrosion resistance. 
COMATIVE EXAMPLE 5 
Six kg of acicular goethite particles, 0.4 .mu.m in average particle size 
(long axis) and 10 in average axial ratio, were dispersed in 194 liters of 
water, a small amount of alkali solution (20% of NaOH solution) was added 
to the dispersion to adjust the pH to at least 13, and 0.1 kg water glass 
No.3 (sodium silicate solution, about 0.26% of Si based on .alpha.-FeOOH) 
was thereafter added to the dispersion, followed by stirring. The aqueous 
dispersion was adjusted to pH 5 with IN HCl and filtered 1 hour later, 
followed by drying. The resulting powder was placed into a muffle furnace 
and heated at 600.degree. C. for 1 hour to obtain .alpha.-Fe.sub.2 O.sub.3 
powder. 
A 3 kg quantity of the .alpha.-Fe.sub.2 O.sub.3 powder was placed into a 
reactor tube and treated with CO at a flow rate of 90 liters/min at 
365.degree. C. for 8 hours. With the gas replaced by nitrogen, the powder 
was cooled approximately to room temperature, and the gas was thereafter 
gradually replaced by air to obtain an Iron carbide powder. The Iron 
carbide powder obtained was treated with H.sub.2 only at a flow rate of 5 
liters/min at 320.degree. C. for 2 hours, whereby a metallic iron powder 
(.alpha.-Fe) was obtained. 
In the Tables, NdAc stands for Nd acetate, NiAc Ni acetate, WG water glass, 
NaAlm Na aluminate, LaAc La acetate, CeAc Ce acetate, SmAc Sm acetate, 
GdAc Gd acetate, YAc Y acetate, product A particles containing iron 
carbide, product B metallic iron particles containing carbon, and product 
C metallic iron containing no carbon. 
TABLE 1 
__________________________________________________________________________ 
Example Com. Ex. 
1 2 3 4 1 2 
__________________________________________________________________________ 
size of starting material 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.4 .mu.m 
(long axis) 
coated element 
(atomic %) 
Nd/Fe 0.5 0.5 1.0 0.5 -- -- 
Ni/Fe 0.2 0.2 -- 0.5 -- -- 
Cu/Fe -- -- 0.5 -- -- -- 
Al/Fe -- -- -- -- 0.81 
0.81 
Si/Fe 0.63 
0.63 
-- 0.63 0.85 
0.85 
coated compound 
NdAc 
NdAc 
NdAc 
NdAc NaAlm 
NaAlm 
NiAc 
NiAc 
CuSO.sub.4 
NiAc 
WG WG WG WG WG 
product A B B B B B 
powder characteristics 
Hc (Oe) 979 1708 
1666 
1756 1448 
1610 
saturation magnetization 
91.2 
158.8 
157.4 
153.8 
153.6 
150.0 
(emu/g) 
corrosion resistance 
14.5 
16.4 
16.5 
16.9 16.9 
14.8 
(%) 
sheet characteristics 
Hc (Oe) 922 1718 
1685 
1776 1429 
1600 
residual magnetic 
2040 
4300 
4180 
4060 3560 
4272 
flux density (G) 
square ratio 0.85 
0.90 
0.89 
0.88 0.88 
0.89 
(Br/Bm) 
SFD 0.62 
0.37 
0.42 
0.47 0.56 
0.47 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Example 
5 6 7 8 9 10 11 
__________________________________________________________________________ 
size of starting material 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
(long axis) 
coated element 
(atomic %) 
coated III or IV 
Nd/Fe 
Nd/Fe 
La/Fe 
Ce/Fe 
Sm/Fe 
Gd/Fe 
Y/Fe 
relative to Fe 
0.5 0.5 0.5 0.5 0.5 0.5 0.5 
Ni/Fe -- -- 0.5 0.5 0.5 0.5 -- 
Cu/Fe -- -- -- -- -- -- -- 
Co/Fe 0.5 12 -- -- -- -- 12 
Al/Fe -- -- -- -- -- -- -- 
Si/Fe 0.63 
0.63 
0.63 
0.63 
0.63 
0.63 
1.27 
coated compound 
NdAc 
NdAc 
LaAc 
CeAc 
SmAc 
GdAc 
YAc 
CoSO.sub.4 
CoSO.sub.4 
NiAc 
NiAc 
NiAc 
NiAc 
CoSO.sub.4 
WG WG WG WG WG WG WG 
product B B B B B B B 
powder characteristics 
Hc (Oe) 1755 
1851 
1720 
1744 
1724 
1737 
1832 
saturation magnetization 
160.2 
169.5 
160.6 
161.9 
161.9 
161.4 
159.1 
(emu/g) 
corrosion resistance 
15.4 
12.4 
15.3 
15.8 
15.7 
15.7 
13.5 
(%) 
sheet characteristics 
Hc (Oe) 1728 
1836 
1714 
1729 
1705 
1708 
1891 
residual magnetic 
4047 
4560 
4109 
4079 
4028 
4171 
4252 
flux density (G) 
square ratio 
0.88 
0.89 
0.89 
0.89 
0.88 
0.90 
0.88 
(Br/Bm) 
SFD 0.43 
0.41 
0.41 
0.41 
0.42 
0.40 
0.42 
__________________________________________________________________________ 
TABLE 3 
______________________________________ 
Example Com. Ex. 
12 13 14 3 4 
______________________________________ 
size of starting 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
0.3 .mu.m 
material 
(long axis) 
coated element 
(atomic %) 
coated III or IV 
Zr/Fe Zr/Fe Zr/Fe -- -- 
relative to Fe 
0.5 2 2 
Ni/Fe 0.5 0.5 -- -- -- 
Cu/Fe -- -- -- -- -- 
Co/Fe -- -- 8 -- -- 
Al/Fe -- -- -- 0.81 2.2 
Si/Fe 0.63 0.63 0.63 0.85 5.2 
coated compound 
ZrCl.sub.2 
ZrCl.sub.2 
ZrCl.sub.2 
NaAlm NaAlm 
NiAc NiAc CoSO.sub.4 
WG WG WG WG WG 
product B B B C C 
powder characteristics 
Hc (Oe) 1776 1861 1862 754 1460 
saturation 161.9 141.5 150.3 127.4 126.4 
magnetization 
(emu/g) 
corrosion resistance 
16.3 16.3 14.4 12.6 34.4 
(%) 
sheet characteristics 
Hc (Oe) 1823 1866 1885 683 1444 
residual magnetic 
4100 3842 4021 2476 3338 
flux density (G) 
square ratio 0.88 0.89 0.88 0.64 0.84 
(Br/Bm) 
SFD 0.43 0.42 0.43 1.12 0.49 
______________________________________ 
Industrial Applicability 
The present invention provides an acicular fine particulate material which 
is prepared by a promoted reduction reaction while being effectively 
prevented from sintering and which consequently affords magnetic recording 
media having a high recording density, and provides a process for 
preparing the particulate material, and magnetic coating compositions and 
magnetic recording media containing the material.